The Application of Graphene-Carbon Black Hybrid
Catalyst Support in Proton Exchange Membrane
Fuel Cells
A thesis submitted to the University of Manchester for the degree of
Doctor of Philosophy
in the Faculty of Science and Engineering
2021
Zhaoqi Ji
School of Engineering
Department of Chemical Engineering and Analytical Science
Content Page 1 of 231
Contents
List of Figures ......................................................................................................................................... 7
List of Tables ........................................................................................................................................ 13
Nomenclature ........................................................................................................................................ 15
Abstract ................................................................................................................................................. 22
Declaration ............................................................................................................................................ 24
Copyright Statement ............................................................................................................................. 25
Acknowledgement ................................................................................................................................ 27
Chapter 1 ............................................................................................................................................... 28
Introduction ........................................................................................................................................... 28
1.1 Motivation and Objectives ...................................................................................................... 29
1.1.1 Motivation .................................................................................................................... 29
1.1.2 Objectives .................................................................................................................... 30
1.2 Outline of Thesis ..................................................................................................................... 31
Chapter 2 ............................................................................................................................................... 33
Literature Review of Proton Exchange Membrane Fuel Cells ............................................................. 33
2.1 Introduction ............................................................................................................................. 33
2.2 History and Development ....................................................................................................... 34
2.2.1 19th Century .................................................................................................................. 34
2.2.2 20th Century .................................................................................................................. 34
2.2.3 21st Century .................................................................................................................. 35
2.3 Proton Exchange Membrane Fuel Cells (PEMFCs) ............................................................... 37
2.3.1 Principle of Operation .................................................................................................. 37
2.3.2 Membrane Electrode Assembly (MEA) ....................................................................... 38
2.3.3 Electrochemical Performance ...................................................................................... 46
2.3.4 Cell Efficiency ............................................................................................................. 49
2.4 Challenges of PEMFCs ........................................................................................................... 51
2.4.1 Lower-Temperature Proton Exchange Membrane Fuel Cell (LT-PEMFC) ................ 51
Content Page 2 of 231
2.4.2 High-Temperature Proton Exchange Membrane Fuel Cell (HT-PEMFC) .................. 54
2.5 Summary ................................................................................................................................. 55
Chapter 3 ............................................................................................................................................... 56
Catalyst Carbon Supports ..................................................................................................................... 56
3.1 Introduction ............................................................................................................................. 56
3.2 High Surface Area Carbon Supports ....................................................................................... 57
3.2.1 Carbon Blacks and Graphite Materials ........................................................................ 57
3.2.2 Mesoporous Carbon ..................................................................................................... 59
3.2.3 Carbon Nanotubes ........................................................................................................ 60
3.3 Graphene-Based Supports ....................................................................................................... 62
3.3.1 Graphene Materials ...................................................................................................... 62
3.3.2 Electrochemically Exfoliated Graphene Oxide (EGO) ................................................ 63
3.3.3 Graphene/Carbon Hybrid Materials ............................................................................. 64
3.4 Nitrogen Doped Carbon Materials .......................................................................................... 65
3.4.1 Post-treatment Doping Method .................................................................................... 66
3.4.2 In-situ Doping Method ................................................................................................. 66
3.5 Summary ................................................................................................................................. 69
Chapter 4 ............................................................................................................................................... 71
Characterisation and Experimental Methodology ................................................................................. 71
4.1 Introduction ............................................................................................................................. 71
4.2 Materials ................................................................................................................................. 72
4.3 Physical Characterization ........................................................................................................ 73
4.3.1 Ultraviolet-visible Spectrophotometry (UV-Vis) ........................................................ 73
4.3.2 Raman Spectroscopy .................................................................................................... 74
4.3.3 X-ray Diffraction (XRD) ............................................................................................. 76
4.3.4 Thermogravimetric Analysis (TGA) ............................................................................ 78
4.3.5 Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) .................. 79
4.3.6 X-ray Photoelectron Spectroscopy (XPS) .................................................................... 80
4.3.7 Scanning Electron Microscopy (SEM) ........................................................................ 82
4.3.8 Energy-Dispersive X-ray Spectroscopy ....................................................................... 84
4.3.9 Transmission Electron Microscopy ............................................................................. 85
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4.3.10 Atomic Force Microscopy (AFM) ............................................................................. 86
4.4 Electrochemical Characterization ........................................................................................... 87
4.4.1 Cyclic Voltammetry (CV) ............................................................................................ 87
4.4.2 Linear Sweep Voltammetry (LSV) .............................................................................. 90
4.5 Membrane Electrode Assembly (MEA) Test .......................................................................... 91
4.5.1 Fabrication of MEA in LT-PEMFC ............................................................................. 91
4.5.2 Fabrication of MEA in HT-PEMFC ............................................................................ 92
4.5.3 Work Station ................................................................................................................ 92
4.5.4 Fuel cell performance test ............................................................................................ 96
4.6 Summary ................................................................................................................................. 99
Chapter 5 ............................................................................................................................................. 101
Graphene/Carbon Hybrid Support Materials ...................................................................................... 101
5.1 Introduction ........................................................................................................................... 101
5.2 Methodology ......................................................................................................................... 103
5.2.1 Synthesis of EGO ....................................................................................................... 103
5.2.2 Synthesis of NrEGO................................................................................................... 104
5.3 Physical Characterization ...................................................................................................... 105
5.3.1 SEM Analysis ............................................................................................................ 105
5.3.2 AFM Analysis ............................................................................................................ 106
5.3.3 UV-Vis Spectroscopy ................................................................................................ 108
5.3.4 TGA Analysis ............................................................................................................ 109
5.3.5 XPS Analysis ............................................................................................................. 110
5.3.6 Raman Analysis ......................................................................................................... 113
5.4 Summary ............................................................................................................................... 114
Chapter 6 ............................................................................................................................................. 116
Platinum Supported on Reduced Electrochemically Exfoliated Graphene Oxide and Carbon Black
Hybrid Catalyst Support (Pt/rEGOx-CBy) ........................................................................................... 116
6.1 Introduction ........................................................................................................................... 116
6.2 Synthesis of Pt/rEGOx-CBy Catalysts ................................................................................... 118
6.3 Physical Characterization of Catalysts .................................................................................. 119
6.3.1 ICP-OES Analysis...................................................................................................... 119
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6.3.2 TGA Analysis ............................................................................................................ 120
6.3.3 XRD Analysis ............................................................................................................ 121
6.3.4 SEM Analysis ............................................................................................................ 122
6.4 Electrochemical Characterization of Catalysts ..................................................................... 125
6.4.1 CV Analysis ............................................................................................................... 125
6.4.2 LSV Analysis ............................................................................................................. 126
6.4.3 Accelerated Stress Tests ............................................................................................. 128
6.5 Catalyst Performance in MEA .............................................................................................. 134
6.5.1 Polarization Curve Analysis ....................................................................................... 134
6.5.2 EIS Analysis ............................................................................................................... 135
6.5.3 Durability Analysis .................................................................................................... 139
6.6 Summary ............................................................................................................................... 141
Chapter 7 ............................................................................................................................................. 142
Pt Supported on Nitrogen doped Reduced Electrochemically Exfoliated Graphene Oxide and Carbon
Black Hybrid Catalyst Support (Pt/NrEGOx-CBy).............................................................................. 142
7.1 Introduction ........................................................................................................................... 142
7.2 Synthesis of Pt/NrEGOx-CBy Catalysts ................................................................................ 143
7.3 Physical Characterization ...................................................................................................... 144
7.3.1 ICP-OES Analysis...................................................................................................... 144
7.3.2 XRD Analysis ............................................................................................................ 145
7.3.3 XPS Analysis ............................................................................................................. 145
7.3.4 SEM Analysis ............................................................................................................ 147
7.3.5 TEM Combined with EDS Analysis .......................................................................... 148
7.4 Electrochemical Performance ............................................................................................... 150
7.4.1 CV Analysis ............................................................................................................... 150
7.4.2 LSV Analysis ............................................................................................................. 152
7.5 Fuel Cell Performance .......................................................................................................... 153
7.5.1 Polarization Curve Analysis ....................................................................................... 153
7.5.2 EIS Analysis ............................................................................................................... 157
7.5.3 Accelerated Stress Tests (ASTs) ................................................................................ 159
7.6 Summary ............................................................................................................................... 164
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Chapter 8 ............................................................................................................................................. 166
Doped Graphene/Carbon Hybrid Catalyst in HT-PEMFCs ................................................................ 166
8.1 Introduction ........................................................................................................................... 166
8.2 Characterization of Materials ................................................................................................ 168
8.2.1 Acid Doping Level ..................................................................................................... 168
8.2.2 TGA Analysis ............................................................................................................ 169
8.2.3 Cyclic Voltammetry ................................................................................................... 169
8.2.4 Linear Sweep Voltammetry ....................................................................................... 171
8.3 MEA Performance and Steady-State Operation ................................................................... 172
8.4 Study of Deactivation Mechanism ........................................................................................ 174
8.4.1 Pt Catalyst Degradation ............................................................................................. 174
8.4.2 Phosphoric Acid Leaching ......................................................................................... 177
8.5 Summary ............................................................................................................................... 180
Chapter 9 ............................................................................................................................................. 181
Conclusions and Recommendations for Future Work ........................................................................ 181
9.1 Conclusions ........................................................................................................................... 181
9.2 Recommendations for Future Work ...................................................................................... 183
Appendix ............................................................................................................................................. 185
Publication & Conference ................................................................................................................... 193
Bibliography ....................................................................................................................................... 195
[Type here] List of Figures Page 7 of 231
List of Figures
Figure 2.1 Operating principle of hydrogen fuel cell. ...................................................... 37
Figure 2.2 The structure of membrane electrode assembly in the low-temperature proton
exchange membrane fuel cell. .......................................................................................... 38
Figure 2.3 The structure of membrane electrode assembly in the high-temperature proton
exchange membrane fuel cell. .......................................................................................... 39
Figure 2.4 The chemical formula of a repeat unit of Nafion®. ......................................... 40
Figure 2.5 The diffusion mechanism in Nafion membrane. ............................................. 41
Figure 2.6 Grotthuss proton hopping mechanism in Nafion membrane. ......................... 42
Figure 2.7 The chemical formula of a section PBI. .......................................................... 42
Figure 2.8 Polarization curve of the PEM fuel cells......................................................... 49
Figure 2.9 Schematic illustration of the degradation of both Pt and carbon support after
being subjected to different potential cycling condition. ................................................. 52
Figure 3.1 TEM micrographs of (a) MWNTs before acid purification and (b) MWNTs
after HNO3-H2SO4 oxidation purification treatment ........................................................ 61
Figure 3.2 SEM images of Pt/RGO/CB-1 under (a) lower magnification and (b) high
magnification. ................................................................................................................... 65
Figure 3.3 Schematics of nitrogen doped GO by urea as N sources during pyrolysis. .... 68
Figure 4.1 A typical UV-Vis spectrum of graphene oxide solution. ................................ 74
Figure 4.2 The working principle of Raman spectroscopy. ............................................. 75
Figure 4.3 Raman spectra of a carbon material. ............................................................... 76
Figure 4.4 XRD equipment. ............................................................................................. 77
List of Figures Page 8 of 231
Figure 4.5 Schematic representation of Bragg’s law. ....................................................... 78
Figure 4.6 The diagram of ICP-OES. ............................................................................... 80
Figure 4.7 XPS equipment................................................................................................ 81
Figure 4.8 The principle of XPS. ...................................................................................... 82
Figure 4.9 The diagram of Scanning electron microscope. .............................................. 83
Figure 4.10 The diagram of transmission electron microscope. ....................................... 86
Figure 4.11 The schematic diagram of atomic force microscopy. ................................... 87
Figure 4.12 Cyclic voltammetry recorded for a polycrystalline Pt electrode in 0.5 M H2SO4
(298 K). ............................................................................................................................. 88
Figure 4.13 Scheme of LT-PEMFC working station for single cell MEA testing. .......... 93
Figure 4.14 Scheme of HT-PEMFC working station for single cell MEA testing. ......... 94
Figure 4.15 Schematic representation of the single fuel cell set up. ................................ 95
Figure 4.16 Parameter design for serpentine airflow channels in the graphitic compound
bipolar plate. ..................................................................................................................... 96
Figure 4.17 (a) The equivalent curve of Randles-Ershler model and (b) a typical Nyquist
plot for a single cell. ......................................................................................................... 99
Figure 5.1 Steps of experimental work for catalysts. ..................................................... 103
Figure 5.2 Two-step electrochemical intercalation and exfoliation for production of EGO.
........................................................................................................................................ 104
Figure 5.3 Synthesis routes of nitrogen-doped reduced electrochemically exfoliated
graphene oxide (NrEGO). A reaction route from the electrochemically exfoliated graphene
oxide (EGO) to the nitrogen-doped reduced EGO (NrEGO). ........................................ 105
Figure 5.4 (a) SEM image of EGO flakes and (b) statistics on lateral size distribution of
EGO flakes. .................................................................................................................... 106
List of Figures Page 9 of 231
Figure 5.5 (a, b) AFM image of EGO flakes and (c) statistics on thickness of EGO flakes.
........................................................................................................................................ 108
Figure 5.6 Colour changes of EGO and NrEGO. ........................................................... 108
Figure 5.7 Characterization of EGO and NrEGO by the whole wavelength analysis of UV-
vis.................................................................................................................................... 109
Figure 5.8 TGA analysis for the thermal stability of EGO, NrEGO and NrEGOx-CBy
hybrid catalyst supports. ................................................................................................. 110
Figure 5.9 High-resolution XPS spectra of C 1s in EGO and NrEGO. .......................... 111
Figure 5.10 High-resolution XPS spectra of N 1s in EGO and NrEGO. ....................... 113
Figure 5.11 Raman spectra of EGO and NrEGO under 785 nm. ................................... 114
Figure 6.1 Steps of experimental work for catalyst preparation. .................................... 117
Figure 6.2 The process of synthesizing catalysts. .......................................................... 119
Figure 6.3 Calibration line of Pt in 265.95 eV emission line by ICP-OES. ................... 120
Figure 6.4 TGA analysis of catalyst weight loss. ........................................................... 121
Figure 6.5 XRD patterns of Pt/CB, Pt/rEGO and Pt/rEGOx-CBy. ................................. 122
Figure 6.6 SEM images of Pt/CB, Pt/rEGOx-CBy and Pt/rEGO. ................................... 124
Figure 6.7 (a) Cyclic voltammetry and (b) ECSA distribution of all hybrid support
catalysts. ......................................................................................................................... 126
Figure 6.8 Pt/CB, Pt/rEGO and Pt/rEGOx-CBy in (a) LSV and (b) electron transfer in
oxygen reduction reaction activity. ................................................................................ 128
Figure 6.9 ECSA changes with accelerated stress test in (a) Pt/CB, (b) Pt/rEGO and (c)
Pt/rEGO2-CB3. The statistical analysis of ECSA loss for Pt/CB, Pt/rEGO, Pt/rEGO2-CB3.
........................................................................................................................................ 131
Figure 6.10 HAADF STEM images of fresh Pt nanoparticles on Pt/CB and Pt/rEGO2-CB3
before and after 30000 cycles of AST. Fresh Pt nanoparticles on Pt/CB (a) before and (b)
List of Figures Page 10 of 231
after 30000 cycles of AST. (c) Statistical analyses of PtNPs sizes on Pt/CB before (black)
and after (red) 30000 cycles of AST. HAADF STEM images of fresh and degraded Pt
nanoparticles on (d) CB and (e) rEGO for Pt/rEGO2-CB3 of AST; (f) Statistical analyses
on PtNPs sizes on Pt/rEGO2-CB3 before (black) and after (red) 30000 cycles of AST. 132
Figure 6.11 (a) LSV before and after 30000 cycles of AST; (b) Specific activities at 0.40
V before and after AST for different catalysts. .............................................................. 133
Figure 6.12 The hydrogen fuel cell performance with Pt/CB, Pt/rEGO and Pt/rEGOx-CBy.
........................................................................................................................................ 135
Figure 6.13 (a) Electrochemical impedance spectroscopy of all catalysts and (b) Rm, Ri
and Rc of all catalysts. .................................................................................................... 137
Figure 6.14 Cross-section SEM images of interfacial space between MPL, CL and
membrane for MEAs with (a), (b) of Pt/CB and (c), (d) of Pt/rEGO2-CB3 in the catalyst
layer. ............................................................................................................................... 138
Figure 6.15 The performance and durability of Pt/CB and Pt/rEGO2-CB3 within 24 hours.
........................................................................................................................................ 139
Figure 6.16 EIS analysis with Rm, Ri and Rc of (a) Pt/CB and (b) Pt/rEGO2-CB3. ........ 140
Figure 7.1 Steps of experimental work for nitrogen doping catalysts. ........................... 143
Figure 7.2 Calibration line of Pt in 265.95 eV emission line by ICP-OES. ..............Error!
Bookmark not defined.
Figure 7.3 The XRD spectra comparation of Pt/C, different ratios of hybrid catalyst
support Pt/NrEGOx-CBy and Pt/NrEGO. ....................................................................... 145
Figure 7.4 High-resolution XPS spectra of (a) N 1s and (b, c) Pt 4f in Pt/NrEGO2-CB3.
........................................................................................................................................ 146
Figure 7.5 SEM images for the catalyst structure of (a) Pt/NrEGO1-CB4, (b) Pt/NrEGO2-
CB3, (c) Pt/NrEGO3-CB2, (d) Pt/NrEGO4-CB1 and (e) Pt/C. ......................................... 148
List of Figures Page 11 of 231
Figure 7.6 STEM images of (a) Pt/NrEGO2-CB3 and (b) the statistics on Pt particle size
distribution by counting 421 particles. ........................................................................... 149
Figure 7.7 STEM-HAADF images of (a-f) HADDF of Pt/NrEGO2-CB3 and EDX
elemental mappings of C, N, and Pt in the Pt/NrEGO2-CB3. (e, f) HR-TEM images of
Pt/NrEGO2-CB3 shows lattice fringes of Pt with FFT pattern of the selected specific Pt
particles. .......................................................................................................................... 150
Figure 7.8 ECSAs measurement of all catalysts by cyclic voltammetry. ....................... 151
Figure 7.9 ORR activity at 1600 rpm and electron transfer number (inset) were measured
by different rotating speed of linear sweep voltammetry for all electrocatalysts. .......... 153
Figure 7.10 (a) Polarization curves of different ratio N-doped hybrid support catalysts in
MEAs; (b) Comparation power density with same ratio of no N doped hybrid support
catalysts (Pt/rEGOx-CBy) at 0.60 V (c) Tafel slopes of Pt/NrEGOx-CBy and Pt/C,
respectively. .................................................................................................................... 156
Figure 7.11 (a) EIS of all catalysts and (b) the specific value of the membrane resistance,
interfacial resistance and charge resistance of all catalysts; (c) The equivalent curve of fuel
cells. ................................................................................................................................ 158
Figure 7.12 (a) single-cell performance and (b) impedance of Pt/C and Pt/NrEGO2-CB3
before and after 30k cycles (0.6-1.0 V) AST; and (c) the specific resistance in terms of
membrane, interface and charge resistance. ................................................................... 161
Figure 7.13 (a) single-cell performance and (b) impedance of Pt/C and Pt/NrEGO2-CB3
before and after 5k cycles (1.0-1.5 V) AST; and (c) the specific resistance in terms of
membrane, interface and charge resistance. ................................................................... 164
Figure 8.1 Research strategies for graphene/carbon hybrid catalyst in HT-PEMFC. .... 167
Figure 8.2 PA doping levels of PBI membrane in the concentrated H2SO4 at room
temperature. .................................................................................................................... 168
List of Figures Page 12 of 231
Figure 8.3 Thermal stability of Pt/C and Pt/NrEGO2-CB3. ............................................ 169
Figure 8.4 ECSA changes with cyclic voltammetry in H3PO4 electrolyte solution during
the accelerated stress test. ............................................................................................... 171
Figure 8.5 ORR activity of Pt/C and Pt/NrEGO2-CB3 in H3PO4 electrolyte solution. ... 172
Figure 8.6 (a) Pt/C and (b) Pt/NrEGO2-CB3 catalysts performance in HT-PEMFC before
and after 100 hours steady-state operation; (c) Pt/C and Pt/NrEGO2-CB3 operation during
100 hours at a constant current density of 0.3 A cm-2. ................................................... 174
Figure 8.7 EIS analysis of (a) Pt/C and (b) Pt/NrEGO2-CB3 at 0.6 V before and after 100
hours steady-state operation. The insert shows the membrane resistance (Rm), interface
resistance (Ri) and charge resistance (Rc). ...................................................................... 176
Figure 8.8 (a) TEM images and (b) statistical analysis of Pt nanoparticles size in Pt/C; (c)
TEM images and (d) statistical analysis of Pt nanoparticles size in Pt/NrEGO2-CB3. .. 177
Figure 8.9 Cross-section SEM images of (a) new Pt/C, (c) aged Pt/C, (e) new Pt/NrEGO2-
CB3 and (g) aged Pt/NrEGO2-CB3; P elements distribution in (b) new Pt/C, (d) aged Pt/C,
(f) new Pt/NrEGO2-CB3 and (h) aged Pt/NrEGO2-CB3. ................................................ 179
List of Tables Page 13 of 231
List of Tables
Table 2.1 The development of fuel cell in bus. ................................................................ 36
Table 2.2 The recent development of fuel cell in cars. ..................................................... 36
Table 2.3 The properties of different types of Nafion® membrane. ................................. 41
Table 2.4 Fabrication process of carbon gas diffusion layer ............................................ 43
Table 3.1 Characterization of AB and FB carbon black. .................................................. 58
Table 3.2 Different methods of preparing nitrogen doped graphene materials with nitrogen
content. ............................................................................................................................. 68
Table 6.1 Pt loading on different catalyst supporting materials. .................................... 120
Table 6.2 ECSA calculation of all catalysts. .................................................................. 126
Table 6.3 Half-wave potential (E1/2) and onset potential (Eonset) characterized by the linear
sweep voltammetry. ........................................................................................................ 128
Table 6.4 EIS of different catalysts in the hydrogen fuel cell. ....................................... 138
Table 6.5 The changes of membrane, interface and charge resistance before and after 24
hours. .............................................................................................................................. 141
Table 7.1 ICP-OES analysis for Pt loading of Pt/NrEGOx-CBy. .................................... 144
Table 7.2 Electrochemical surface area of Pt/C and Pt/NrEGOx-CBy. .......................... 152
Table 7.3 The single cell power density performance at 0.6 V and the Tafel slope b
calculated by the activation loss region in all MEAs with different catalysts. ............... 157
Table 7.4 Tafel slope of Pt/C and Pt/NrEGO2-CB3 in the activation loss. ..................... 161
Table 7.5 Resistance changes before and after 30k cycles AST (catalyst degradation). 161
List of Tables Page 14 of 231
Table 8.1 Membrane resistance (Rm), interface resistance (Ri) and charge resistance (Rc)
of Pt/C in MEA-1 and Pt/NrEGO2-CB3 in MEA-2 before and after 100 h steady-state
operation. ........................................................................................................................ 176
Nomenclature Page 15 of 231
Nomenclature
Acronyms
AC Alternating current
AFM Atomic force microscopy
AST Accelerated stress test
BL Bonding layers
BSE Backscattered electrons
C Carbon
C(s)+ Carbon ions
CB Carbon black
CE Counter electrode
CL Catalyst layers
CNTs Carbon nanotubes
COads Carbon monoxide adsorbed on carbon supports
COsurf Carbon monoxide produced by carbon corrosion
CPE Constant phase element
CV Cyclic voltammetry
Nomenclature Page 16 of 231
CVD Chemical vapor deposition
DOE Department of Energy
DOMC Disordered mesoporous carbon
e- Electrons
ECSA Electrochemical surface area
EDS Energy-Dispersive X-ray Spectroscopy
EG Ethylene glycol
EGO Electrochemically exfoliated graphene oxide
EIS Electrochemical impedance spectroscopy
EM Electron microscopy
ETD Everhart-thornley detector
G Graphene
Gr Graphene
GDL Gas diffusion layers
GO Graphene oxide
H+ Hydrogen ions
H2 Hydrogen gas
H2O Water molecules
H3O+ Hydronium ion
Nomenclature Page 17 of 231
HF Hydrofluoric acid
HOR Hydrogen oxidation reaction
HT-PEMFC High-temperature proton exchange membrane fuel cell
HUPD Hydrogen underpotential deposition
ICP-OES Inductively coupled plasma-optical emission spectrometry
LSV Linear sweep voltammetry
LT-PEMFC Low-temperature proton exchange membrane fuel cell
MEA Membrane electrode assembly
MPL Microporous layers
MWNTs Multi-walled carbon nanotubes
NG Nitrogen-doped graphene
NrEGO Nitrogen doped reduced electrochemically exfoliated graphene
oxide
O2 Oxygen gas
OCP Open circuit potential
OMC Ordered mesoporous carbon
OMS Ordered mesoporous silica
ORR Oxygen reduction reaction
PA Phosphoric acid
Nomenclature Page 18 of 231
PAN Polyacrylonitrile
PBI Polybenzimidazole
PEMFCs Proton exchange membrane fuel cells
Pt Platinum
Pt/C Platinum/carbon
PtNPs Platinum nanoparticles
Pt/NrEGOx-CBy Platinum on reduced electrochemically exfoliated graphene
oxide and carbon black
Pt/rEGOx-CBy Platinum on nitrogen doped reduced electrochemically
exfoliated graphene oxide and carbon black
PTFE Polytetrafluoroethylene
Rc Charge transfer resistance
rGO Reduced graphene oxide
Rm Membrane resistance
Rmt Mass transfer resistance
Ri Interface resistance
SE Secondary electrons
SEM Scanning electron microscopy
SWNTs Single-walled carbon nanotubes
TGA Thermal Gravimetric Analyzer
Nomenclature Page 19 of 231
XPS X-ray Photoelectron Spectroscopy
XRD X-ray diffraction
TEM Transmission electron microscopy
TGA Thermogravimetric Analysis
TWh Terawatt hour
UV-Vis Ultraviolet-visible spectrophotometry
ZIFs zeolitic imidazolate frameworks
A Tafel slope
B Slope of the Levich-Koutecký curve
CB Bulk concentration of reactant
CS Concentration of reactant at the surface of the catalyst
C0 Bulk concentration of O2
d Interlayer spacing between atoms
D Size of crystal particles
D Diffusion coefficient of the reacting species
D0 Diffusion coefficient of O2 in 0.5 M H2SO4
EƟ Reversible open circuit voltage
EB Binding energy on a specific atomic orbital
Ec Actual voltage
Nomenclature Page 20 of 231
EK Kinetic energy of the emitted photoelectron
Er Theory open circuit voltage
E.L. Electrode Pt loading
F Faraday constant, which is 96485 C mol-1
hν Energy of the photon of the X-ray source
i Measured current density
i0 Exchange current density
iL Limiting current density
id Diffusion-limiting current density
ik Kinetic-limiting current density
k Shape factor, 0.9
n Order of diffraction
n Number of electrons per H2 (or O2)
q Charge (coulombs mol-1)
Qh Measured active catalyst surface area
Qm Adsorption charge for an atomically smooth surface
R Universal gas constant (8.314)
T Temperature
v Kinetic viscosity of water
Nomenclature Page 21 of 231
ω RDE rotation rate
Winitial Initial weight of samples
Wreal Test weight of samples
δ Diffusion distance
α Charge transfer coefficient
Wel Electrical work (J mol-1)
∆Vohm Ohmic losses
∆Vact Activation losses
∆Vconc Mass transport losses
∆G Gibb’s free energy
∆H Change in enthalpy
∆S Change in entropy
ηG Gibb’s efficiency
ηHHV Gibb’s efficiency in higher heating value
ηLHV Gibb’s efficiency in lower heating value
θ Angle of diffraction
λ Wavelength of X-ray
β Half-peak width
Abstract Page 22 of 231
Abstract
Proton exchange membrane fuel cells (PEMFCs), both low-temperature proton exchange
membrane fuel cell (LT-PEMFC) and high-temperature proton exchange membrane fuel cell
(HT-PEMFC), are regarded as some of the most promising new energy conversion devices due
to high energy density, high efficiency and zero-carbon emission. However, the
commercialization of LT-PEMFC is limited by platinum/carbon (Pt/C) catalysts as the decay
of Pt nanoparticles and carbon corrosion resulting in high price, low catalytic activity and poor
durability. HT-PEMFC suffers from the loss of catalytic sites and gas diffusion due to
phosphoric acid (PA) leaching from the PA doped polybenzimidazole (PBI) polymer
electrolyte membrane to catalyst layer (CL) and gas diffusion layer (GDL). Based on these,
this thesis aims to design a structured catalyst with graphene/carbon hybrid materials as
supports to improve the performance of electrochemical activity and stability both in LT-
PEMFC and HT-PEMFC.
In the graphene oxide preparation work, a two-step electrochemical exfoliation method was
used to prepare graphene oxide (EGO) with the flake size of 1.3 μm and the thickness of 2-3
layers. After hydrothermal treatment with urea at 160 for 6 hours. EGO has been reduced to
nitrogen doped reduced electrochemically exfoliated graphene oxide (NrEGO) with an addition
of 2.4% N atoms. This NrEGO show better thermal stability up to 700, which is higher than
EGO. Meanwhile, NrEGO has the ratio of D peak and G peak (ID/IG) to 1.11, which is higher
than that of 1.02 for EGO and indicates that NrEGO has more defects than that of EGO.
In the LT-PEMFC work, a hydrothermal treatment with ethylene glycol (EG) as the reducing
agent was used to synthesize Pt catalysts on different ratios of EGO and carbon black (CB)
hybrid supports (Pt/rEGOx-CBy). After optimization, Pt/rEGO2-CB3 had an electrochemical
Abstract Page 23 of 231
surface area (ECSA) of 42 m2 g-1 and a maximum power density of 0.537 W cm-2. To enhance
the power density performance and durability, NrEGO and CB were also used as supports and
Pt/NrEGO2-CB3 shows much improvement in ECSA with 67 m2 g-1. The power density is
0.934 W cm-2 at 0.60 V, which is 1.55 times higher than that of 0.601 W cm-2 in commercial
Pt/C. Cyclic voltammetry of accelerated stress tests (ASTs) in low potential region (0.6 - 1.0
V) and high potential region (1.0 - 1.5 V) were used to evaluate Pt catalyst degradation and
carbon corrosion, respectively. After 30k cycles scanning at lower potential region,
Pt/NrEGO2-CB3 shows better Pt stability with little change regarding the voltage loss 10 mV
(from 0.659 V to 0.649 V) at 0.8 A cm-2, this met the target reported by Department of Energy
(DOE) (<30 mV). This is attributed to the strong interaction between Pt and N with higher
combining energy (1.00 eV) than that of 0.18 eV of Pt and C bonds. Meanwhile, after 5k cycles
scanning at high potential region, Pt/NrEGO2-CB3 keeps 78% performance and showed 0.692
W cm-2 at 1.5 A cm-2. However, commercial Pt/C only maintained 54% performance and could
not even reach 1.5 A cm-2. This is associated with the introduction of N into graphene changing
the electronic structure.
In the HT-PEMFC work, Pt/NrEGO2-C3 achieved 0.432 W cm-2 with 0.25 mg cm-2 Pt loading,
being higher than that of 0.328 W cm-2 in commercial Pt/C. After 100 hours steady-state
operation, Pt/NrEGO2-CB3 keeps at 0.60 V with a decay rate of only 0.01 mV h-1, which is
much lower than that of 1.125 mV h-1 in commercial Pt/C. This is associated with the higher
activity of Pt/NrEGO2-CB3. Also, NrEGO flakes could act as a barrier between PBI membrane
and electrodes to decrease PA moving from PBI membrane to electrodes, which prevent PA
covering Pt active sites and moving into the pore space of electrodes.
Declaration Page 24 of 231
Declaration
I declare that no portion of the work referred to in the thesis has been submitted in support of
an application for another degree or qualification of this or any other university, or other
institute of learning.
Zhaoqi Ji
Manchester, June 2021
Copyright Statement Page 25 of 231
Copyright Statement
i) The author of this thesis (including any appendices and/or schedules to this thesis)
owns certain copyright or related rights in it (the “Copyright”) and s/he has given The
University of Manchester certain rights to use such Copyright, including for
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ii) Copies of this thesis, either in full or in extracts and whether in hard or electronic copy,
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Copyright Statement Page 26 of 231
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Acknowledgement Page 27 of 231
Acknowledgement
The four-year postgraduate journey at the University of Manchester is coming to an end. My
time in the centenary campus and Manchester city has given me a respect and thirst for
knowledge. It has also given me a love for life alongside my studies.
First and foremost, I would like to express my great gratitude to Professor Stuart Holmes
because he gave me the opportunity and the excellent experimental condition that allow me to
carry out my research freely and smoothly. His supervision and encouragement throughout the
research process have enabled me to overcome difficulties and successfully complete this PhD
project.
I would also like to thank Dr María Pérez-Page and Dr Madhumita Sahoo for their patient help
from the first time to learn fuel cells, to gradually gain a deeper understanding, and then
overcoming many difficulties and obstacles of my project.
My research work could not have been successfully done without the help of our group mates,
Rana Hassan-Naji, Jianuo Chen, Zunmin Guo, Ziyu Zhao, and the Master students, Shaun
Mckeefry, Miranda Mirzad and Ruoyin He. We can motivate each other to keep moving
forward in our studies and life, and they give me the confidence to finish the arduous path of
my PhD.
I would also like to thank Professor Sarah Haigh, Dr. Rongsheng Cai, Mr. Maxwell Rigby for
their assistance in my SEM, TEM characterization work.
Finally, I would like to thank my family and relatives for their great affirmation during many
choices in my life. I would also like to thank them for positively communicating with me when
I was negative. I will also go on to love you all more in the future.
Chapter 1 Page 28 of 231
Chapter 1
Introduction
In the 21st century, the rapid development of world economy cannot be separated from the
rapid growth of energy demand, the over-exploitation and use of fossil fuels. This has also led
to problems in terms of the shortage of energy and environmental pollution [1-3]. Nowadays,
the fossil energy that human beings rely on has caused a series of related ecological disasters
such as the greenhouse effect, global warming, and sea level rise, which affect the survival and
development of human beings [4-6]. In order to achieve the goal of global carbon neutrality by
2050 as proposed and approved by the Paris Agreement, changing the energy structure and
developing clean, renewable energy sources and technologies have become more important [7-
9]. Therefore, following the first energy revolution (firewood) and the second energy
revolution (fossil fuels), hydrogen energy has been attracting attention potentially becoming
the third energy revolution. The development of a hydrogen energy supply chain also promotes
the development of hydrogen fuel cells and new energy vehicles.
Proton exchange membrane fuel cells (PEMFCs) as the core of automotive power, include low-
temperature (LT-PEMFC) and high-temperature proton exchange membrane fuel cells (HT-
PEMFC), which have been widely studied [10, 11]. Compared with the traditional heat engine,
the energy conversion efficiency of fuel cells can reach more than 85% in theoretical energy
conversion efficiency as it is not affected by the Carnot cycle [12]. The only by-product of
hydrogen fuel cells is water, which avoids the emission of pollutants such as nitrogen and
sulphur compounds from fossil fuels. They are also favoured because they have lower noise
Chapter 1 Page 29 of 231
during operation and no moving parts [13, 14]. Nevertheless, their application still suffers from
some critical problems such as catalyst cost and deactivation, carbon support corrosion in LT-
PEMFC and phosphoric acid (PA) leaching in HT-PEMFC, which result in poor stability and
restrict their commercialization [15-17].
To overcome these problems, different catalyst supports with high surface area and good
electrical conductivity, such as carbon black, Vulcan XC-72, mesoporous carbon, and carbon
nanotubes, have been investigated to improve Pt nanoparticles distribution [18-20]. Therefore,
designing a structured catalyst to improve catalytic activity and prevent PA loss in both LT-
PEMFC and HT-PEMFC is a promising development.
1.1 Motivation and Objectives
1.1.1 Motivation
Platinum supported on high surface area carbon (Pt/C) is the most mature catalyst in PEMFCs.
However, the activity, poor durability and carbon corrosion of Pt/C have not been effectively
addressed. The advent of graphene materials offers a possible solution to these problems.
Platinum supported on graphene with high oxygen reduction reaction (ORR) activity has been
promoted, but it still faces the obstacles to achieve the commercialization. Hummers prepared
graphene oxide leads to environmental and safety issues (using strong oxidizing agents
potassium permanganate (KMnO4), and concentrated H2SO4) as well as a long preparation time
with high cost of the various chemicals [21]. In addition, the restacking of graphene flakes,
caused by van der Waals force, prevents fuel gas transport to the active sites of both LT-
PEMFC and HT-PEMFC [22].
Therefore, in this work, for LT-PEMFC, graphene oxide produced by electrochemical
exfoliation method (EGO) was used to replace Hummers GO, because it is faster (within 1
Chapter 1 Page 30 of 231
hour), more environmentally friendly (no strong oxidizing agents) and is scalable. Carbon
black (CB) was intercalated into reduced EGO (rEGO) with different ratios to prepare hybrid
catalyst supports (Pt/rEGOx-CBy) and avoid the stacking problem. The best performance was
obtained by optimizing the ratio of rEGO and CB. To further improve the performance and
durability of catalysts, nitrogen atoms were introduced into rEGO flakes (NrEGO) and the
ratios of NrEGO/CB were optimized.
In HT-PEMFC, phosphoric acid (PA) leaching into anodes and cathodes results in gas transport
pathways being blocked in the GDL and catalytic active sites loss, by PA covering, after long-
term steady-state operation. The hybrid support of NrEGO/CB (two-dimensional
flakes/spherical structure) is hoped to prevent Pt nanoparticles deactivation as well as be an
obstacle that could prevent the movement of PA.
1.1.2 Objectives
This work focuses on the development of a highly active and stable graphene-carbon black
composite-based platinum catalysts, addressing the two main challenges that of Pt catalyst
deactivation and catalyst support corrosion in LT-PEMFC and PA permeation in HT-PEMFC,
thereby improving the performance and durability of PEMFCs.
• Prepare EGO by two-step electrochemical exfoliation method.
• Synthesize NrEGO by the hydrothermal method.
• Physical characterization of EGO and NrEGO.
• Synthesize Pt on different ratios of reduced electrochemically exfoliated graphene oxide
intercalated by carbon black as hybrid support materials (Pt/rEGOx-CBy).
• Synthesize Pt on different ratios of NrEGO and CB hybrid supports (Pt/NrEGOx-CBy).
• Characterization of these catalysts in terms of their morphology and electrochemical
properties to investigate their activity and mechanism.
Chapter 1 Page 31 of 231
• Test Pt/rEGOx-CBy and Pt/NrEGOx-CBy performance and durability in LT-PEMFC and
HT-PEMFC.
• Study the mechanism of nitrogen enhanced performance in terms of activity and corrosion
resistance of supports.
• Preliminary study the effect of graphene/carbon black hybrid support materials on
redistribution of phosphoric acid in HT-PEMFC.
1.2 Outline of Thesis
The thesis is composed of 8 chapters and details as shown below:
Chapter 1 gives a brief background of the research. Overview of the current global energy
crisis and environmental issue necessitating PEMFCs, as an alternative technology. Meanwhile,
it also presents current issues with the challenge of developing higher activity and more stable
catalysts, corresponding to this project motivation. Based on these, it gives the objectives.
Chapter 2 gives the literature review and research background of the hydrogen fuel cell,
membrane electrode assembly (MEA) and electrochemical properties. At first, the history and
development have been discussed. Secondly, the principle of operation and components
involved in MEA layers are explained in detail with issues influencing the cell performance.
Finally, the challenge of the catalyst to be solved is described.
Chapter 3 is a review of current research on supporting materials in the hydrogen fuel cells.
After analysing different categories of supporting materials, the graphene-based materials in
the hybrid is described corresponding to this project target.
Chapter 4 describes the physical characterization techniques, electrochemical characterization
methods of graphene-based materials and as-synthesized catalysts. MEA fabrication, fuel cell
Chapter 1 Page 32 of 231
work station build up, polarization curves, electrochemical impedance spectroscopy (EIS) and
durability performance were tested to evaluate the performance in hydrogen fuel cells.
Chapter 5, Chapter 6, Chapter 7 and Chapter 8 analyse experimental data including the
property of EGO and NrEGO. Physical, electrochemical characterization of Pt/rEGOx-CBy and
Pt/NrEGOx-CBy. Finally, the appropriate ratio of EGO and carbon black as support and the
optimized catalysts in terms of performance and durability in the low-temperature and high-
temperature proton exchange membrane fuel cells is obtained.
Chapter 9 summarizes the overall achievements in this project and gives recommendations for
future work.
Chapter 2 Page 33 of 231
Chapter 2
Literature Review of Proton Exchange
Membrane Fuel Cells
2.1 Introduction
With the development of industry, the worldwide demand for energy supplies is increasing and
the consumption of fossil fuels has increased from 94314 terawatt-hour (TWh) in 2001 to
136761 TWh in 2021 [23]. The exploitation and use of fossil fuels has exacerbated the
depletion of resources and environmental pollution, which in turn threatens human survival
and development. Therefore, countries all over the world committed to achieve carbon
neutrality by 2050 at the Paris Agreement [24]. It needs development of clean energies and has
led to the development of new energy vehicles. To realize this goal, the automotive drivetrains
need new energy conversion technologies, such as fuel cells and batteries. Among these
technologies, proton exchange membrane fuel cells are recognized as one of the most attractive
devices because they are environmentally friendly and highly efficient [25].
The regular use of hydrogen-fuelled PEMFCs for automotive drivetrains is getting closer to
reality. However, the large-scale commercial application of such proton exchange membrane
fuel cells still faces many challenges, particularly reducing cost, improving performance, and
longer operating lifetime of the core membrane electrode assembly (MEA) [12, 26, 27]. The
MEA is the heart of PEMFCs stack including a sandwich structure of the anode, the proton
exchange membrane (PEM), and the cathode. Hydrogen is oxidised on the catalysts of the
Chapter 2 Page 34 of 231
anode and oxygen/air is reduced on the catalysts of the cathode. Electrons and proton are
transported through external circuits and PEM respectively to form the current with only heat
and water as by-products.
2.2 History and Development
2.2.1 19th Century
The research and development of PEMFCs dates to the early 19th century. Sir William Grove
was a chemist whose famous water electrolyser/fuel cell experiment conceived of a reverse
process that could be used to generate electricity and is therefore widely regarded as the father
of fuel cell science. Grove immersed two platinum electrodes on one end in the sulfuric acid
solution and the other two ends separately sealed in containers of oxygen and hydrogen. Then
a constant current was discovered to be flowing between two electrodes. This device came to
be known as a fuel cell [28]. After that, in 1893, Friedrich Wilhelm Ostwald, revealed the
interconnection of electrodes, electrolyte, oxidising, reducing agents, anions and cations. This
discovery laid the foundation for the further research in the field of fuel cells [29]. Until in
1896, William W. Jacques developed the first practical fuel cell [30].
2.2.2 20th Century
In early 1933, Thomas Francis Bacon advanced the development of the fuel cell. Bacon
developed the first fuel cell combining hydrogen and oxygen, which was put into practical use.
His work began with the study of the alkaline fuel cell, which converts hydrogen and air
directly into electricity through an electrochemical process. During World War II, He
developed a fuel cell that used in submarines by the Royal Navy. Then, in 1958, Bacon
presented an alkaline fuel cell with the cell diameter of 25.4 mm to the Britain’s National
Research Development Corporation and Pratt & Whitney acquired the patent of this work to
Chapter 2 Page 35 of 231
use in Apollo spacecraft [31]. In 1959, Bacon also presented a fully functional fuel cell with
40 single cells of 5 kW and 60% efficiency, which was licensed and adopted by National
Aeronautics and Space Administration (NASA) [32].
In 1950, Teflon material (Polytetrafluoroethylene, PTFE) became available and was firstly
used in the platinum electrodes and acidic electrolytes of fuel cells, which is still commonly
used in fuel cell now. In 1955, Thomas Grubb, a chemist in General Electric Company,
improved the fuel cell by a modified ion-exchange membrane made of sulphated polystyrene.
In 1958, Leonard Niedrach proposed a platinum deposition method on the membrane for
hydrogen oxidation and oxygen reduction reaction. In the 1980s, based on the flooding issue
during humidification in the lower temperature proton exchange membrane, Robert F. Savinell
and Jesse S. Wainright developed PBI/PA membranes for high-temperature proton exchange
membrane fuel cell (HT-PEMFC) at Case Western Reserve University (CWRU), which is the
start of studying HT-PEMFC [33].
2.2.3 21st Century
The 21st century is the era that dramatically accelerated research and achieved
commercialisation of PEMFCs. Vehicle manufacturers are focusing on the research and
development of PEMFC vehicles. Ballard company in Canada is a global fuel cell leader from
transportation to stationary power plants PEMFC technology. Ballard Generation System is
also considered as a world leader in the development, production, and marketing of zero-
emissions proton exchange membrane fuel cells. Many automobile manufacturers have
participated in the development of fuel cell vehicles, such as Chrysler, Ford, GM, Honda,
Nissan, Volkswagen and Volvo etc, which developed further PEMFCs technology. In 2000,
Bavaria produced the first fuel cell bus with a Ballard fuel cell. After that, the total driving
distance improved from 300 km to 563 km, and the speed of fuel cell bus improved from 80
Chapter 2 Page 36 of 231
km h-1 to 106 km h-1 within 5 years as shown in Table 2.1. Until 2007, Honda firstly developed
the FCX Clarity model in the car at the Los Angeles Auto Salon and has been on sale since
2008 as shown in Table 2.2 [34, 35]. Nowadays, improving the fuel cell vehicles stable
operation and fast speed is still attractive.
Table 2. 1 The development of fuel cell in bus.
Manufacturer Fuel cell Speed Year
Bavaria Ballard 300 km―80 km h-1 2000
Neoplan GmbH 250 km―80 km h-1 2000
Toyota Toyota 300 km―80 km h-1 2001
Volvo Ballard 563 km―106 km h-1 2005
Van Hol UTC 400 km―106 km h-1 2006
Table 2. 2 The recent development of fuel cell in cars.
Manufacturer Fuel cell Speed Year
Honda FCX Clarity Honda 570 km―160 km h-1 2007
Ford HySeries edge Ballard 491 km―137 km h-1 2007
Fiat Panda Nuvera 200 km―130 km h-1 2007
Hyundai I-Blue Fuel cell 600 km―165 km h-1 2007
Toyota FCHV-adv Hybrid (fuel cell + Battery) 830 km―155 km h-1 2008
GM Provoq GM 483 km―160 km h-1 2008
Renault Scenic FCV H2 Nissan 240 km―161 km h-1 2008
Daimler-Chrysler Hybrid (fuel cell + Battery) 483 km―185 km h-1 2008
Toyota Mirai Fuel cell 650 km―175 km h-1 2020
Chapter 2 Page 37 of 231
2.3 Proton Exchange Membrane Fuel Cells (PEMFCs)
2.3.1 Principle of Operation
The electrochemical reactions in proton exchange membrane fuel cells happen simultaneously
on both the anode and the cathode, as shown in Figure 2.1. Hydrogen is oxidized at the anode
to liberate electrons and produce protons. The electrons arrive at the cathode via the external
circuit and the protons arrive at the cathode through the membrane. The electrons and protons
react with oxygen to produce water and heat. The basic fuel cell reactions are as follows [5]:
At the anode:
𝐻2 ↔ 2𝐻+ + 2𝑒− Eq. 2.1
At the cathode:
1
2𝑂2 + 2𝐻+ + 2𝑒− ↔ 𝐻2𝑂 Eq. 2.2
Overall reaction:
𝐻2 +1
2𝑂2 ↔ 𝐻2𝑂 (+1.23 𝑉) Eq. 2.3
Figure 2.1 Operating principle of hydrogen fuel cell.
Chapter 2 Page 38 of 231
2.3.2 Membrane Electrode Assembly (MEA)
The MEA contains the gas diffusion layers (GDL), microporous layers (MPL), catalyst layers
(CL), bonding layers (BL) and the proton conducting membrane [5]. Based on the operating
temperature of PEM fuel cells, they can be divided into the low-temperature hydrogen fuel cell
(50 - 80) and the high-temperature fuel cell (120 - 200). The biggest difference
between these two cells is the different proton conducting membranes. The low-temperature
PEM fuel cell (LT-PEMFC) uses a sulfonated tetrafluoroethylene-based fluoropolymer-
copolymer (Nafion) membrane in Figure 2.2 and the high-temperature PEM fuel cell (HT-
PEMFC) uses a phosphoric acid (PA)-doped polybenzimidazole (PBI) membrane in Figure
2.3 [36-39].
Figure 2.2 The structure of membrane electrode assembly in the low-temperature proton
exchange membrane fuel cell.
Chapter 2 Page 39 of 231
Figure 2.3 The structure of membrane electrode assembly in the high-temperature proton
exchange membrane fuel cell.
2.3.2.1 Proton Exchange Membrane
i) Nafion Membrane in LT-PEMFC
The PEM fuel cell membrane must have high proton conductivity and provide a sufficient
barrier to separate the anodic fuel and the cathodic reactant gas [40]. Usually, the membrane
of a PEM fuel cell is made of perfluorosulfonic acid. At present, the most well-known
membrane material is Nafion produced by the DuPont, which is chemically and mechanically
stable during the PEM fuel cell operation [41]. Other companies also developed similar
membranes such as the Asahi chemical company, Solvay, Fumatech, 3 M, the Dow chemical
company. The difference among these materials is their respective repeat unit equivalent
weight, 1100, 1000 and 1200, and the chemical nature of the side chains. The chemical formula
of Nafion is given in Figure 2.4.
Chapter 2 Page 40 of 231
Figure 2.4 The chemical formula of a repeat unit of Nafion®.
A repeat unit of Nafion consists of a highly hydrophobic main chain-Teflon and a highly
hydrophilic side chain sulfonic acid. There are various types Nafion membranes depending on
the fabrication process and the membrane thickness as shown in Table 2.3 [42]. Teflon can
provide mechanical strength and dimensional stability and perfluorosulfonic acid is able to
absorb relatively large amounts of water to a supply proton conducting environment.
To improve proton conductivity, perfluorosulfonic acid of hydrophilic side chains should fully
absorb water molecules. The greater hydration, the higher the proton conductivity [43].
However, the mechanical strength of Nafion membrane will deteriorate with water uptake
increasing [40], because radicals produced during the electrochemical processes will attack
perfluorosulfonic acid resulting in the decomposition of membranes [44-46].
Protons transfer in Nafion membrane by both hydronium ion diffusion mechanism (rely on
H3O+ carry H+) and Grotthuss (proton hopping) [47, 48]. The hopping mechanism has little
contribution to conductivity of perflouronated sulfonic acid membranes such as Nafion [49]. It
mainly relies on diffusion mechanism, hydrated proton (H3O+) diffuses through the membrane.
The schematic design of the diffusion mechanism in proton conduction in pristine and
nanocomposite membranes has been shown in the Figure 2.5.
Chapter 2 Page 41 of 231
Table 2.3 The properties of different types of Nafion® membrane [42].
Membrane Fabrication process
Equivalent weight
(gpolymer/moleSO3H)
Thickness
(μm)
Nafion® NR-211 Dispersion cast 990-1050 25
Nafion® NR-212 Dispersion cast 990-1050 50
Nafion 115 Extruded 1100 127
Nafion 117 Extruded 1100 193
Figure 2.5 The diffusion mechanism in Nafion membrane.
ii) PBI Membrane in HT-PEMFC
Normally, Nafion membrane need water because it mainly relies on hydrated hydrogen ions
(H3O+) with humidified reaction gas. However, HT-PEMFC typically operate in a dry
condition above 120, where there is little liquid water molecules and Nafion membrane has
lower proton conductivity without wet [50-52]. Therefore, In 1804, Theodor von Grotthuss
proposed the hopping mechanism for proton transfer in chains of hydrogen-bonded water [49].
Water molecules adhere to the sulfuric groups. The proton produced by oxidation of H2 at the
anode contacts with water molecules to form the hydronium ion, and then, the proton hops
from one hydronium ion to the next water molecule [53]. The simple scheme of the hopping
mechanism is shown in Figure 2.6 [54]. The fabrication process of PBI membrane is similar
Chapter 2 Page 42 of 231
with Nafion membrane and mainly include dispersion casting and extrusion. PBI membrane
with amino groups (-NH2) replace Nafion materials in the HT-PEMFC as shown in Figure 2.7.
In order to improve proton conductivity of PBI membrane, different acids, such as phosphoric
acid (H3PO4), sulfuric acid (H2SO4), nitric acid (HNO3) and hydrochloric acid (HCl), are
introduced into membrane and form hydrogen bonds with -NH2 or free acids, and the proton
will be transferred from the anode to the cathode by Grotthuss mechanism [55-58]. Among
these acids, the proton conductivity of different acids order is: H2SO4>H3PO4>HClO4>
HNO3>HCl. However, H2SO4 needs at least 50% relative humidity to improve proton transfer,
which is similar in low-temperature PEM fuel cell [59]. Therefore, H3PO4 becomes the best
choice at high temperature without humidification.
Figure 2.6 Grotthuss proton hopping mechanism in Nafion membrane [54].
Figure 2.7 The chemical formula of a section PBI.
On the one hand, two PA molecules are immobilized with a -NH2 site via hydrogen bonds.
Protons could pass through the membrane via rearranging of hydrogen bonds between PA and
protons from the anode to the cathode [60]. On the other hand, there are some free PA
molecules in PBI membrane which could also carry protons from the anode to the cathode.
Chapter 2 Page 43 of 231
However, the excess suffers from a leaching and moving by electro-osmotic drag from the
anode to the cathode during the operation [33].
2.3.2.2 Gas Diffusion Layer
The gas diffusion layer is between the catalyst layer and the bipolar plate (gas flow channel) in
both low- and high-temperature hydrogen fuel cell. Carbon paper and carbon cloth are two
common materials adopted in the hydrogen fuel cell. These commercial carbon materials have
various fabrication routes in different companies as shown in Table 2.4 [61, 62]. The important
functions of gas diffusion layer include (i) act as the support for the electrode, (ii) provide the
pathway for the gas from the flow field to the catalyst layer and access for the generated water
from the catalyst layer to the flow field, (iii) supply electronic conductivity pathway, (iv) help
to discharge water. Based on its situation and purpose in the PEMFCs, it should have good
electronic conductivity, thermal conductivity and porous structure, hydrophobicity,
compressibility, gas or water permeability, corrosion resistance and a balance of stiffness and
flexibility [49].
Table 2.4 Fabrication process of carbon gas diffusion layer [61, 62].
Substrates Fabrication routes
Carbon-fiber paper Carbonization of polyacrylonitrile (PAN) fiber,
papermaking, resin impregnation, molding, and
graphitization.
Carbon cloth Weaving of PAN yarn and carbonization.
Wet-laid filled paper Carbonization of PAN fiber, papermaking, PTFE
binding, and heat treatment.
Dry-laid filled paper Dry-laying of PAN fiber, carbonization, impregnation,
and graphitization.
Chapter 2 Page 44 of 231
Expanded graphite Flake graphite, oxidation, thermal shock, and
perforation of graphite sheet.
2.3.2.3 Microporous Layer
The microporous layer (MPL), which normally consists of carbon black with hydrophobic
agents (e.g. PTFE), is deposited on the surface of the gas diffusion layer. Generally, these
carbon materials have 20-200 nm pores [63], which are smaller pores than that of GDL (0.05-
100 um pores) [64, 65]. This structure helps to reduce the contact resistance between the
catalyst layer and gas diffusion layer. Meanwhile, since the hydrophobic MPL forms a
relatively high capillary pressure that facilitates water discharge from catalyst layer to gas
diffusion layer facilitates water management. Therefore, the carbon materials used in the
microporous layer should have good electron conductivity and hydrophobicity. The most used
materials are carbon nanotubes (CNT), carbon nanofiber, carbon black (Vulcan XC-72, Vulcan
XC-72R), acetylene black and graphitized carbon [66-69].
2.3.2.4 Catalyst Layer
The electrochemical reactions occurring at the electrodes produce charge and proton transfer
by the external circuit and membrane, respectively. Hydrogen oxidization and oxygen
reduction take place at catalyst layer on the surface of electrodes. The catalysts require
activation to achieve ideal current density. At the anode, the most widely accepted mechanism
of hydrogen oxidized reaction is the chemical adsorption step (Tafel) or electrochemical
adsorption step (Heyrovsky), followed by the adsorbed hydrogen atom discharge step
(Volmer) , as shown below [70]:
𝐻2 + 2𝑀 2𝑀𝐻 (𝑇𝑎𝑓𝑒𝑙 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛) Eq. 2.4
And/or electrochemical:
𝐻2 + 𝑀 𝑀𝐻 + 𝐻+ + 𝑒− (𝐻𝑒𝑦𝑟𝑜𝑣𝑠𝑘𝑦 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛) Eq. 2.5
Chapter 2 Page 45 of 231
Adsorption steps, followed by a discharge path of the absorbed hydrogen atom given by:
𝑀𝐻 𝑀 + 𝐻+ + 𝑒− (𝑉𝑜𝑙𝑚𝑒𝑟 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛) Eq. 2.6
At the cathode [71], oxygen reduction reaction takes place via two alternative pathways:
(i) the direct or four-electron pathway:
𝑂2 + 4𝑒− + 4𝐻+ 2𝐻2𝑂 Eq. 2.7
(ii) or, the series or 2 electrons pathway, where hydrogen peroxide is formed as an
intermediate:
𝑂2 + 2𝑒− + 2𝐻+ 𝐻2𝑂2 Eq. 2.8
Hydrogen peroxide can thereafter be reduced further to water:
𝐻2𝑂2 + 2𝑒− + 2𝐻+ 2𝐻2𝑂 Eq. 2.9
If the catalyst has lower 4e- ORR activity, hydrogen peroxide produced by 2e- pathway and
free radicals during hydrogen oxidation reaction (HOR) and ORR will result in the membrane
degradation and catalyst support corrosion [17, 71].
High Pt catalyst usage accounts for a high proportion (34%-41%) of the cost of a fuel cell stack
[72-74]. Traditionally, platinum nanoparticles (PtNPs) supported on high surface area carbon
materials are the most widely used hydrogen fuel cell catalysts. However, these materials still
face three important challenges: (i) high cost due to the use of precious PtNPs catalysts; (ii)
Poor stability of operation due to PtNPs deactivation; (iii) Reduced lifetimes because of carbon
corrosion and PtNPs agglomeration, especially at high electrochemical potential [75-77].
Another important factor is that the catalyst layer always contains catalysts and ionomers (e.g.
Nafion). This is to allow for a better combination between electrodes and Nafion membrane
after hot pressing [78]. This improves the protons produced at the anode side transferring from
anode to the cathode.
Chapter 2 Page 46 of 231
2.3.3 Electrochemical Performance
In theory, charge and potential produce electrochemical energy in Eq. 2.10 and Eq. 2.11:
𝑊𝑒𝑙 = 𝑞𝐸 Eq. 2.10
𝑞 = 𝑛𝐹 Eq. 2.11
Where, Wel is the electrical work (J mol-1); q is the charge (coulombs mol-1); E is the potential
(V); n is the number of electrons per H2 molecule (2), and F is Faraday’s constant (96485 C
mol-1). All the electrochemical energy is equal to Gibbs free energy. Therefore, the theoretical
open cell voltage at 25 can be calculated in Eq. 2.12 and Eq. 2.13:
𝑊𝑒𝑙 = −∆𝐺 Eq. 2.12
𝐸 =−∆𝐺
𝑛𝐹=
237340
2×96485= 1.23 𝑉 Eq. 2.13
In practice, the actual operating voltage, which is affected by membrane resistance and
electrode catalyst kinetic, is less than the theoretical open cell voltage (1.23 V). This
phenomenon arises as over potential is present in the fuel cell system [79]. This can be regarded
as voltage loss due to sources of irreversibility in the hydrogen fuel cell. These sources of
irreversibility are outlined as follows:
1) Activation losses
2) Crossover losses
3) Ohmic losses
4) Mass transport losses.
2.3.3.1 Activation Losses
In a fuel cell, the electrochemical reaction taking place at the surface of the catalyst layer is
driven by the generated voltage which causes some losses in the output voltage of the fuel cell.
This type of loss is known as activation loss. This drop can be estimated by using the Tafel
equation on a single cell [80]:
Chapter 2 Page 47 of 231
∆𝑉𝑎𝑐𝑡 = 𝐸𝜃 − 𝐸𝑐 = 𝐴𝑙𝑛(𝑖
𝑖0) Eq. 2.14
𝐴 =𝑅𝑇
2𝛼𝐹 Eq. 2.15
where EƟ is reversible open circuit potential, which represents at the potential when all the
substances involved in the cell reaction being in equilibrium and there is no external current,
Ec is the actual voltage, i0 is the exchange current density, A is the Tafel slope, R is the universal
gas constant, which is 8.314, T is the temperature, α is the charge transfer coefficient. The
reaction kinetics increase with the increase of the exchange current density (i0), which results
in the decrease of the activation losses. The decrease of Tafel slope is also used to prove the
decrease of the activation loss by increasing the charge transfer coefficient, which represents
higher kinetic activity for H2/O2/air of catalysts. Activation losses are associated with sluggish
electrode kinetics and the voltage drop mainly occurs at the cathode, because 1 mole H2 could
only produce 2 electrons and 1 mole O2 reduction needs 4 electrons, which shows lower
reactive kinetics and higher over potential, especially at medium and low temperature.
Therefore, increasing the current density can decrease activation losses through the voltage.
The specific methods of reducing activation losses are to increase pressure, temperature, using
higher activity of catalysts and increasing the purity of fuels [79].
2.3.3.2 Crossover Losses
The crossover loss is mainly caused by a small amount of hydrogen diffusing through the
proton exchange membrane from the anode to the cathode, and reacting with oxygen on the
cathode side, which will reduce the cathode potential. This situation usually occurs and makes
a significant influence on the cell potential when the fuel cell is operating at open circuit
potential or at a very low current density. The permeation of hydrogen is a function of the
membrane permeability, the thickness of the membrane and the partial pressure differential of
hydrogen as the main driving force for hydrogen permeation on both sides of the membrane.
Chapter 2 Page 48 of 231
In addition, although oxygen permeability is much lower than hydrogen permeability, oxygen
can also cross the membrane. Choosing the appropriate thickness of membrane is the most
impactful way to reduce crossover loss [80].
2.3.3.3 Ohmic Losses
The ohmic loss exists due to the impedance of the electrolyte to protons, the resistance of
interface contacts and the resistance to the flow of electrons passing through the conductive
elements of PEMFCs. These losses can be defined by Ohm’s law [79]:
∆𝑉𝑜ℎ𝑚 = 𝑖𝑅 Eq. 2.16
Generally, the resistance to electrons can be ignored because of the high electron conductivity
of graphite or its composite materials.
2.3.3.4 Mass Transport Losses
The electrochemical reaction occurs at the surface of catalyst layer. As a larger amount of
current is drawn, the depletion of reactant near the catalyst becomes more severe. In addition,
the accumulation of liquid water in the fluid channel and the GDL increases the reactant
transportation loss, which is mainly due to the hydrophilic and hydrophobic nature of the
electrode material preventing reactants approaching the catalyst layer. At this region, reactant
concentration depends on the current density as follows:
𝑖 =𝑛𝑓𝐷(𝐶𝐵−𝐶𝑆)𝐴
𝛿 Eq. 2.17
where δ is diffusion distance; CB is the bulk concentration of reactant; CS is concentration of
reactant at the surface of the catalyst; D is the diffusion coefficient of the reacting species.
When the concentration is equal to zero, the current density is called the limiting current density
iL:
𝑖𝐿 =𝑛𝑓𝐷𝐶𝐵𝐴
𝛿 Eq. 2.18
Chapter 2 Page 49 of 231
The mass transport losses can be represented as follows:
∆𝑉𝑐𝑜𝑛𝑐 =𝑅𝑇
𝑛𝐹ln (
𝐶𝐵
𝐶𝑆) =
𝑅𝑇
𝑛𝐹ln (
𝑖𝐿
𝑖𝐿−𝑖) Eq. 2.19
The main solution is to increase the operating temperature of the fuel cell and increase the
evaporation of internal water in the MEAs or operate the fuel cell in relatively dry reactants.
Taking all sources of irreversibility into account, the actual voltage of the hydrogen fuel cell is
given as follows and the polarization curve is shown in Figure 2.8 [79]:
𝑉𝑐𝑒𝑙𝑙 = 𝐸𝑟 − (∆𝑉𝑎𝑐𝑡 + ∆𝑉𝑐𝑜𝑛𝑐)𝑎 − (∆𝑉𝑎𝑐𝑡 + ∆𝑉𝑐𝑜𝑛𝑐)𝑐 − ∆𝑉𝑜ℎ𝑚 Eq. 2.20
Figure 2.8 Polarization curve of the PEM fuel cells.
2.3.4 Cell Efficiency
Fuel cell efficiency (η) is defined as the ratio between generated electrical energy and
consumed hydrogen. Compared with conventional internal combustion engines, in which the
efficiency is limited by the theoretical maximum coming from the Carnot-process, the
efficiency of the hydrogen fuel cell is not limited due to thermodynamics, but by the enthalpy
of the electro-chemical processes [81]. The enthalpy of the hydrogen combustion reaction is
Chapter 2 Page 50 of 231
called the hydrogen heating value. When hydrogen is fully combusted, only liquid water is left
and the enthalpy shows that 286 kJ of heat is released, which is known as the higher heating
value (HHV). If hydrogen is combusted with excess of oxygen, the measurement of the heat
value should be lower, then the heat value is 241 kJ, which is known as lower heating value
[79]. During chemical reaction, some entropy is produced causing a portion of the HHV not
converted into electrical energy. Therefore, the Gibbs free energy could be expressed as Eq.
2.21:
∆𝐺 = ∆𝐻 − 𝑇∆𝑆 Eq. 2.21
Assuming that all Gibbs free energy can be converted into electrical energy, the Gibbs
efficiency ηG can be calculated by Eq. 2.22 [82, 83]:
𝜂𝐺 =∆𝐺
∆𝐻=
𝑛𝑓𝐸0
∆𝐻 Eq. 2.22
Taking the enthalpy of hydrogen into account and under ideal condition for higher heating
value:
𝜂𝐻𝐻𝑉 =237
286= 0.83 Eq. 2.23
On the other hand, the lower heating value condition:
𝜂𝐿𝐻𝑉 =237
241= 0.98 Eq. 2.24
Obviously, under the condition of excess oxygen (lower heating value), the hydrogen fuel cell
has high efficiency. Unfortunately, this efficiency is a theoretical value because of many losses
mentioned above when operating a fuel cell. Thus, the actual efficiency is lower than 83%.
Chapter 2 Page 51 of 231
2.4 Challenges of PEMFCs
2.4.1 Lower-Temperature Proton Exchange Membrane Fuel Cell
(LT-PEMFC)
A number of key issues that need to be overcome before the low-temperature proton exchange
membrane fuel cell can achieve commercialization on a large scale. One of the biggest
limitations is the activity and poor stability of the catalyst, especially for oxygen reduction
reactions at the cathode, and the high cost (41%) [84].
It is widely recognized that catalyst supports play an important role in the catalyst activity and
stability [85, 86]. To obtain a higher performance of fuel cells, the catalyst support should have
many important properties, such as a large specific surface area, strong corrosion resistance
and good electrical conductivity. Currently, platinum supported on high surface area carbon
are the most advanced and practical catalysts for low-temperature proton exchange membrane
fuel cells. Pt has lower Fermi energy level, which facilitates electron transfer between Pt and
O2. Unfortunately, electrochemical corrosion of the carbon seriously affects the durability and
reliability of the catalysts during its operation. In addition, the coarsening, agglomeration and
dissolution of Pt nanoparticles is another important factor, which affects catalyst activity and
stability [74, 87, 88]. Therefore, the main causes of performance degradation in low
temperature proton exchange membrane fuel cells are attributed to:
(1) the dissolution and growth of Pt nanoparticles.
(2) corrosion of the carbon carrier.
2.4.1.1 Dissolution and Agglomeration of Pt Nanoparticles
For practical benefits, metal nano-particles are widely used in proton exchange membrane fuel
cells. The nanoscale size confers a large electrochemical specific surface area to the catalyst,
Chapter 2 Page 52 of 231
while the high free energy of specific surface results in thermodynamic instability. As a result,
Pt nanoparticles are easy to dissolve and agglomerate, reducing its electrochemical specific
surface area. This has been considered as the main degradation mechanism for Pt catalysts as
shown in the Figure 2.9 [89-94].
Figure 2.9 Schematic illustration of the degradation of both Pt and carbon support after
being subjected to different potential cycling condition.
In general, the dissolution and growth rates of Pt nanoparticles accelerate with increasing
voltage and temperature [95]. Tang et al. used scanning tunnelling microscopy (STM) with ab
initio calculations to examine the behaviour of individual Pt nanoparticle dissolution. The
results showed that Pt metal was oxidised into soluble Pt2+ ions through direct electro-oxidation
[96]. Some researchers have also found that the dissolution rate of platinum begins to decrease
at electrode potentials above 1.1 V, which is caused by the formation of a passivated platinum
oxide layer [95, 97]. Thus, platinum is stable in low and high potential regions. However, it
has been demonstrated that the cyclic voltammetry process can accelerate the dissolution of
platinum compared to the constant potential process [91, 98].
Chapter 2 Page 53 of 231
Based on these results, a consensus has emerged to suggest two main mechanisms to explain
the degradation behaviour of Pt particles. One is the Ostwald ripening mechanism, where small
particles with high specific surface energy dissolve and are redeposited onto the surface of
larger particles. The other is the agglomeration mechanism, where small particles combine with
adjacent microcrystalline platinum particles by migration and form larger particles as shown
in Figure 2.9.
Based on these, it always uses start-up and shut-down (SU/SD) condition in a PEMFC stack as
test object. When start-up, H2 is fed to the anode and it creates H2/O2 or H2/Air front. During
shut-down, O2 or air will replace H2 and is fed to the anode. therefore, this results in a high
interfacial potential difference between H2 and O2/air and causes carbon corrosion and catalyst
degradation. Previous research reported different ex situ tests like 0.6-1.5V cycling test and
0.85-1.5V cycling test to study the specific degradation mechanisms characteristic for a fuel
cell stack operation [99, 100].
2.4.1.2 Carbon Corrosion
In addition to agglomeration and dissolution of the platinum nanoparticles, carbon corrosion
also significantly accelerates catalyst degradation. Generally, when the electrode voltage is
below 1.1 V, carbon corrosion is negligible due to the slow kinetics [4]. However, the presence
of platinum reduces the potential for carbon oxidation and accelerates the corrosion of the
carbon support [101-103]. According to the mechanism of carbon oxidation reaction (carbon
corrosion, COR), the effect of catalyst performance goes through the following steps when the
electrode potential reaches above 1.4 V vs. RHE: the formation of C+(s) during electrochemical
process (Eq. 2.25), the formation of carbon oxides by hydration (Eq. 2.26 and Eq. 2.27), carbon
monoxide poisoning of Pt (Eq. 2.28 and Eq. 2.29) [104-108]:
𝐶 → 𝐶(𝑠)+ + 𝑒− Eq. 2.25
Chapter 2 Page 54 of 231
𝐶(𝑠)+ + 𝐻2𝑂 → 𝐶𝑂𝑠𝑢𝑟𝑓 + 2𝐻+ + 𝑒− Eq. 2.26
𝐶𝑂𝑠𝑢𝑟𝑓 + 𝐻2𝑂 → 𝐶𝑂2 + 2𝐻+ + 2𝑒− Eq. 2.27
𝐶𝑂𝑠𝑢𝑟𝑓 + 𝑃𝑡 → 𝑃𝑡 − 𝐶𝑂𝑎𝑑𝑠 Eq. 2.28
𝑃𝑡 − 𝐶𝑂𝑎𝑑𝑠 + 𝐻2𝑂 → 𝐶𝑂2 + 2𝐻+ + 2𝑒− Eq. 2.29
2.4.2 High-Temperature Proton Exchange Membrane Fuel Cell
(HT-PEMFC)
2.4.2.1 Phosphoric Acid Leaching
Phosphoric acid-doped PBI membrane has been suggested as a good electrolyte in the high-
temperature proton exchange membrane fuel cell since 1994 [109]. However, it was shown
that a PA doped PBI membrane has a rapid decrease in terms of proton conductivity as
phosphoric acid moves from anode and cathode by the electro-osmotic drag and losing,
resulting in a sever imbalance in phosphoric acid distribution [110]. Therefore, in addition to
the activity decay of electrocatalysts during operation, another big challenge is phosphoric acid
loss, which increases the area-specific resistance [33]. It has been demonstrated that the
phosphoric acid is lost with a rate of 10 ng cm-2 h-1 during operating at 0.2 A cm-2 at 80-160
after 40000h [111]. PA leaching impedes gas pathways and leads to performance decrease and
accelerate performance deterioration. Therefore, new membrane materials and design of MEA
structure is necessary to decrease phosphoric acid loss.
2.4.2.2 High Amount of Pt Loading
Due to the PBI membrane is directly contact with the catalyst layer of electrode, PA molecules
are easy to penetrate to catalyst layer, which enables proton transport from CL to PBI
membrane. However, too much PA would also result in the loss of Pt active sites and requiring
a greater amount of platinum (Pt) (normally between 0.7 and 1.2 mg cm-2) to provide high
Chapter 2 Page 55 of 231
electrochemical surface area, which accounts for a higher proportion in the cost of a total fuel
cell stack [73]. Yilser et al [112] used graphene nanoplates (GNP) as catalyst support to
synthesize Pt/GNP by the microwave irradiation method and this catalyst achieved 0.34, 0.40
and 0.46 W cm-2 at 160, 175 and 190, respectively, with 1 mg cm-2 of Pt loading. To achieve
lower Pt loading for HT-PEMFC, Su et al [73] developed an ultrasonic spray coating technique
to reduce the requirement of Pt. The MEA successfully achieved around 0.35 W cm-2 with only
0.35 mg cm-2 of Pt loading. Therefore, designing new structure of catalyst layer has a big
influence on reducing catalyst dosage, improving the catalyst efficiency and the fuel cell
performance.
2.5 Summary
This chapter provides an overview of the history of PEMFCs from a simple model of two
electrodes in a beaker in the 19th century to the commercial production of PEMFCs in the field
of automotive engineering in the 21st century. It also introduces the principles of PEMFCs, the
structure and properties of membrane electrode assemblies. Finally, we present the current
challenges of high-temperature and low-temperature proton exchange membrane fuel cells,
which is the propose of this research work.
Chapter 3 Page 56 of 231
Chapter 3
Catalyst Carbon Supports
3.1 Introduction
The key factor that determines the commercialization of the proton exchange membrane fuel
cell is the cost and the cost predominantly depends on the amount of precious metals. In the
hydrogen fuel cell, the aim of catalysis is to lower the activation energy and/or increase the
selectivity for the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR)
[113]. Platinum (Pt) is commonly used as a catalyst in proton exchange membrane fuel cells.
It has been reported that the activity of Pt nanoparticles can be enhanced with reducing particle
size. Therefore, the catalysts are supported on a high surface area substrate to prevent
agglomeration [88]. A significant decrease of platinum requirements has been achieved from
platinum black (25 mgPt cm-2) to carbon-supported platinum catalysts (0.4-0.8 mgPt cm-2). The
Department of Energy (DOE) also set a stringent target of 0.125 mgPt cm-2 for 2020 and
durability with a decay of 30 mV h-1 [114, 115]. Therefore, researchers are working on reducing
the precious metals amount by designing better electrocatalysts through methods including
different support materials. A suitable catalyst support material should have the following
structures and characteristics:
1) High surface area and good electrical conductivity properties.
2) Good porosity allowing gas reaching electrocatalysts active sites.
3) Good corrosion resistance with high chemical and electrochemical durability.
Chapter 3 Page 57 of 231
3.2 High Surface Area Carbon Supports
3.2.1 Carbon Blacks and Graphite Materials
Carbon blacks are popular and widely used as catalyst supports in proton exchange membrane
fuel cells because of their excellent charge mobilities, large specific surface area and low cost.
They are hexagonal in crystallographic structures and manufactured by the pyrolysis of
hydrocarbons, such as oil or natural gas intermediates, produced during petroleum processing
[16]. There are two main types of carbon black, acetylene black (AB) and oil-furnace black
(FB) [116, 117]. The predominant production method is the furnace black process, where the
original materials, including petroleum products, asphalt (coal tar) and other materials with
high content of carbon, are fed into a furnace and burned in a limited air supply at a temperature
of 1400 . This results in low cost and high availability, which means carbon blacks have
been widely used as Pt catalyst supports in PEMFCs. Acetylene black is obtained by the
reaction of calcium carbide reacting with saturated sodium chloride (NaCl) solution or the
continuous pyrolysis of naphtha (crude gasoline). It has been reported that the high surface area
and small particle size of the support material are able to facilitate the uniform distribution of
Pt particles and improve the electrocatalytic activity for the fuel gas [16]. Table 3.1
summarizes the physical characterization of different carbon blacks.
Chapter 3 Page 58 of 231
Table 3.1 Characterization of AB and FB carbon black [113, 114].
Carbon blacks Surface area (m2 g-1) Particle size (nm)
Denka black AB 58 40
Exp. Sample AB 835 30
Shavinigan AB 70-90 40-50
Conductex 975 FB 250 24
Vulcan XC-72R FB 254 30
Black pearls 2000 FB 1475 15
3950 FB 1500 16
In order to get well-ordered domains in carbon black, graphitized carbon black is another
process to synthesize the high surface materials by recrystallization of carbon black at 2500-
3000 . The surface area can achieve 100-300 m2 g-1 [118, 119].
Despite the good properties, carbon black shows some shortcomings: (i) the presence of a large
number of micro-pores in carbon black makes it difficult for reactants to reach the catalyst
surface and inevitably reduces catalyst performance [120, 121]; (ii) poor connection of
micropores also result in lower electronic conductivity with high ohmic resistance [16]; (iii)
carbon black is easily oxidized especially at the cathode; (iv) carbon black shows high mass
transport limitation during fuel cell operation, which exacerbates the difficulty of water
management [122]. Pt nanoparticles will detach from the corroded carbon or aggregate to larger
particles, which decreases electrochemical surface area and subsequently results in lower
performance of PEMFCs and poor durability [18].
Chapter 3 Page 59 of 231
3.2.2 Mesoporous Carbon
Mesoporous carbon has larger pores about 2-50 nm, which is bigger than that of the micropores
(< 2 nm) in carbon black and thus reduces transport problems for the fuel [122]. Mesoporous
carbon has two categories in terms of the structure and morphology: (i) ordered mesoporous
carbon (OMC) prepared by the template strategy; and (ii) disordered mesoporous carbon
(DOMC), which has irregular interconnection, poor conductivity and wide pore size
distribution. Therefore, OMC is more suitable as a catalyst support to facilitate the high and
uniform dispersion of platinum nanoparticles. The ordered pore structure of OMC also reduces
mass transfer and charge transfer resistance [123-127]. The oxygen reduction reaction is more
likely to occur at this specific nanoscale zone which has higher surface area and is easier for
the reactants/products, electrolytes and electrons to access in this nanopore architecture
compared with a microporous structure. On the other hand, macro porous carbon with large
pore size (>50 nm) has relatively low surface area and lower conductivity, which is therefore
unsuitable as a catalyst support material for platinum deposition [16, 128].
To synthesize OMC, various preparation methods of mesoporous carbon have been developed
based on different templates such as mesoporous silicate or aluminosilicate or chemical vapor
deposition (CVD) [16, 128, 129]. The various ordered mesoporous silica (OMS), like MCM-
48, SBA-15, are mostly used at the template with a specific pore topology [128, 130]. Therefore,
the structure of OMC is very dependent on the structure of templates. The appropriate carbon
precursor, such as sucrose, furfuryl alcohol, acetylene gas, pyrrole, or acrylonitrile, is filled
into the template, which is then carbonised to achieve the silica-carbon composite, and then the
template is removed by an ethanol-water solution of hydrofluoric acid (HF) or sodium
hydroxide to obtain the mesoporous carbon.
Su et al. demonstrated that platinum supported on ordered mesoporous carbon catalysts
(Pt/OMC) have a high electrochemical specific surface area of 27.0 m2 g-1, which is higher than
Chapter 3 Page 60 of 231
that of 24.4 m2 g-1 for platinum supported on carbon black (Pt/BP2000) and 19.1 m2 g-1 for
commercial E-TEK catalysts [131]. Therefore, OMC plays an important role in electrochemical
reactions. In addition, Maiyalagan et al. found that highly ordered mesoporous carbon with
larger dimensions is beneficial for electrochemical activity, toxicity resistance and stability of
catalysts [123].
3.2.3 Carbon Nanotubes
Carbon nanotubes (CNTs), which are also an important carbon material, are in the shape of
seamless cylinders formed by the convolution of one or more layers of graphene flakes. Thus,
according to the number of layers, CNTs can be divided into two types: single-walled carbon
nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) [132, 133]. MWNTs can
exhibit a highly uniform inner diameter and have a broad pore size distribution from micropore
to mesopore [134]. CNTs used as catalyst supports are typically characterized by an outer
diameter of 10-50 nm and an inner diameter of 3-15 nm. The nanotubes are 10-50 μm in length
[16]. Typically, the total surface area of SWNTs is in the range of 400-900 m2 g-1, whereas it
is 200-400 m2 g-1 for MWNTs [135].
Chapter 3 Page 61 of 231
Wildgoose and Lee et al explored the technique of synthesizing Pt supported on carbon
nanotubes (Pt/CNTs) and the application of Pt/CNTs in proton exchange membrane fuel cells
[136, 137]. The high Pt loading on unfunctionalized CNTs tends to aggregate [138-140].
Therefore, functionalization of CNTs is generally a prime requirement for synthesizing
catalysts supports. Pre-treatments, such as acid activation, high-temperature potassium
hydroxide activation or annealing (graphitization), can enhance anchoring sites of Pt
nanoparticles [141, 142]. As shown in Figure 3.1(b), MWNTs are separate and almost no
agglomeration is observed after using HNO3-H2SO4 purification treatment compared with
Figure 3.1(a) [143]. Wang et al. synthesised Pt/MWNTs catalysts using a direct chemical
method, which avoids the complex acid treatment process. Meanwhile, when a small amount
of defect sites was introduced on the surface of MWNTs. Pt supported on this low-defect
MWNTs exhibited a higher electrochemical activity (47.4 mA cm-2) and strong resistance to
carbon monoxide poisoning than that on the high-defect SWNTs by refluxing in nitric acid (65
Figure 3.1 TEM micrographs of (a) MWNTs before acid purification and (b) MWNTs after
HNO3-H2SO4 oxidation purification treatment
Chapter 3 Page 62 of 231
wt%) for 12 hours [144]. Obviously, the complexity of process and the immaturity of the
technology limit the application of carbon nanotube catalysts.
3.3 Graphene-Based Supports
3.3.1 Graphene Materials
Graphene, a two-dimensional monolayer of graphite in the form of a hexagonal lattice, has
been widely studied by researchers since it was discovered in 2004 [145-147]. In addition to
its excellent electrical conductivity (~104.36 S cm-1), theoretically huge specific surface area
(~2629 m2 g-1), graphene has similar physicochemical properties to those of carbon nanotubes
and can be seen as a two dimensional unfolded of carbon nanotubes [145, 148]. In view of fuel
cell applications, platinum/graphene (Pt/G) is regarded as promising for the design of a new
generation of catalysts with improved electrocatalytic activity and enhanced resistance to
toxicity and carbon corrosion [11, 25, 149-154]. Platinum/graphene-based catalysts have been
synthesized by a common reduction method: graphite materials were oxidized by a strong
oxidising agent to obtain graphene oxide (GO), followed by the reduction of graphene oxide
and platinum ions by different reduction processes. Pt/G catalysts with different
electrochemical performance and platinum nanoparticle size are formed [155-158].
Li et al. prepared platinum nanoclusters on graphene by simultaneous reduction of graphene
oxide and chloroplatinic acid via sodium borohydride (NaBH4). Linear sweep voltammetry
results showed that the Pt/graphene catalysts reached a limiting mass activity of 199.6 mA mg-
1 at the voltage of 0.652 V, which is nearly twice that of the Pt/Vulcan catalyst (101.2 mA mg-
1 at 0.664 V) [159]. Li et al. prepared Pt/chemically converted graphene (Pt/CCG) using
ethylene glycol (EG) as a reducing agent and found that this Pt/CCG had a larger
electrochemical surface area (36.3 m2 g-1) than that of Pt/MWCNT (33.4 m2 g-1) and better
resistance to carbon monoxide poisoning than Pt/MWCNT as well [156]. Meanwhile, Seger et
Chapter 3 Page 63 of 231
al. synthesized graphite oxide-Pt by NaBH4 and it achieved a very low power density of 0.16
W cm-2 [160]. This is attributed to the restacked reduced graphene oxide (rGO) flakes during
the reduction of the platinum salts, through π-π interaction, blocking fuel (especially O2)
diffusion to the PtNPs active sites [161]. Therefore, this significantly limits the widespread use
of graphene materials.
3.3.2 Electrochemically Exfoliated Graphene Oxide (EGO)
Recently, electrochemical exfoliation of graphite, which is fast (within 1 hour),
environmentally friendly (no use of strong oxidising agents and causing its emission), has a
high yield and is easy to scale-up, has attracted intensive research [162]. In electrochemical
exfoliation, the graphite foil acts as the working electrode and a Pt rod acts as the counter
electrode. To allow ion transport, the electrodes are immersed in an electrolyte solution, ionic
salts such as NH4NO3, (NH4)2SO4 or their mixtures being the most commonly used [163].
During electrochemical exfoliation, the graphite foil is intercalated with anions and
functionalized by the functional groups (N, P, S) present in the electrolyte [164]. These
functional groups may facilitate Pt anchoring on graphene with additional stability. Generally,
direct exfoliation in these ionic electrolytes produces a low yield of graphene with few
functional groups and a small flake size (~5 μm) [165]. Farbod Sharif et al. reported in 2020
that EGO exfoliated with NH4NO3 as the electrolyte had more oxygen functional groups than
material exfoliated with Na2SO4, (NH4)2SO4 and (NH4)2HPO4 [163]. It has been shown that a
two-step electrochemical oxidation exfoliation method, first oxidation in concentrated acid
electrolyte then intercalation in ionic electrolyte, produces GO with almost the same prevalence
of oxygen-containing functional groups as the Hummers method but with a larger lateral flake
size [166, 167].
Chapter 3 Page 64 of 231
3.3.3 Graphene/Carbon Hybrid Materials
Graphene, as a pure catalyst support material, is unable to achieve high performance in the fuel
cells, this is mainly due to the stacking of two-dimensional structure resulting in significant
loss of Pt catalytic sites and diffusion limitation of the reaction gas especially at higher current
density [168]. Therefore, changing the structure of graphene in catalysts to avoid the restacking
issue is a significant topic of research.
Li et al. designed a Pt/rGO material intercalated with carbon black (CB) as a composite support
as shown in Figure 3.2. The results indicate that it dramatically improved the durability toward
the ORR after 20000 cycles accelerated stress test (AST) in O2-saturated HClO4 electrolyte,
suggesting that rGO flakes could act as a ‘mesh bag’ to prevent PtNPs leaching [161]. However,
this catalyst performance was not tested in a fuel cell. Sanli et al. prepared Pt/rGO/CB with
electrochemical surface area (ECSA) of 102 m2 g-1, which is larger than that of most other
similar hybrid support catalysts. However, it only achieved a power density of 0.645 W cm-2
[169]. Kim et al. studied boron doped and boron/nitrogen co-doped into graphene (Gr)
intercalated by CB as a Pt catalyst support. Although the ECSA of Pt-B-Gr/CB0.3 and Pt-BCN-
Gr/CB0.5 are quite low with only 33.60 m2 g-1 and 41.00 m2 g-1 respectively, Pt efficiency was
improved to 1.08 W mg-1 [74, 170]. However, the raw materials used for synthesizing Pt-
graphene based catalysts in these publications mostly start from GO materials produced by the
Hummers method, which is costly, has environmental and safety issues and has a long
preparation time [21]. There is no reported research of electrochemically exfoliated graphene
oxide in fuel cells.
Chapter 3 Page 65 of 231
Figure 3.2 SEM images of Pt/RGO/CB-1 under (a) lower magnification and (b) high
magnification: black circle is carbon black and yellow circle is reduced graphene oxide
flakes [158].
3.4 Nitrogen Doped Carbon Materials
Doping is an effective method for changing the properties of carbon materials for use in a wide
range of applications in energy conversion devices [171, 172]. Doping with nitrogen (N), boron
(B) and sulphur (S) can significantly influence the electronic structure, chemical reactions, pH
value and electrical conductivity of carbon materials [152, 173]. In addition to adjusting the
properties of the carbon materials, the introduction of metal-free elements (N, B or S) has
higher binding energy with Pt nanoparticles (PtNPs) and inhibit the movement of PtNPs to
agglomerate, thereby improving the electrocatalytic stability of catalysts [174, 175]. Therefore,
carbon materials doped with N, B or S bring new properties for preparing highly active catalysts.
Among these candidates, the atomic radius of the nitrogen atom is the closest to that of the C
atom, making the N atom the most popular doping species. Meanwhile, N atoms has stronger
electro-negative than that of C [176-178]. Nitrogen doped on graphene could significantly
influence p-d hybridization in terms of electronic structures and then facilitate Pt absorbing and
distributing on the surface.
Chapter 3 Page 66 of 231
Nitrogen groups introduced into carbon materials can act as a bridge between the carbon
supports and the Pt nanoparticles and enhance their interaction. This is mainly reflected in three
aspects: (i) changing the nucleation and growth kinetics of the nanoparticles to form smaller
and more uniform catalyst nanoparticles; (ii) enhancing the chemical binding energy between
Pt nanoparticles and the supports to improve the stability of catalysts; (iii) changing the
electronic structure of catalysts, which in turn improves the electrocatalytic activity of catalysts
[179]. Two methods to introduce N groups into carbon support materials have been used: the
post-treatment doping method and the in-situ doping method [179, 180]. The post-treatment
doping method uses nitrogen-containing precursors to introduce N into carbon materials [102,
174, 175, 181, 182], while the in-situ doping method is achieved by direct pyrolysis of nitrogen-
containing precursors or CVD of nitrogen-containing compounds to obtain nitrogen doped
carbon materials [183-187].
3.4.1 Post-treatment Doping Method
Nitrogen-doped carbon materials can be obtained by treating them with nitrogen-containing
precursors (e.g. nitrogen, ammonia, urea, etc.). Song et al. reported a nitrogen-doped graphene
material (NG) obtained by thermal annealing of graphene oxide with ammonia [181]. In this
NG material, three types of nitrogen species, pyridinic N, pyrrolic N and graphitic N, were
introduced into the graphene flakes. Among these, pyridinic N and graphitic N have been
demonstrated to increase the defects as well as provide electro-negativity to facilitate a uniform
Pt atoms distribution, which promote ORR activity. Pyridinic N and graphitic N can be
interconverted during the ORR process [188, 189].
3.4.2 In-situ Doping Method
There is no doubt that the introduction of N species not only allows for uniform dispersion and
strongly anchoring of Pt nanoparticles, but also improves the electrical conductivity of supports
Chapter 3 Page 67 of 231
by the restoration of sp2 carbon network after reducing functional groups in graphene oxide,
and prompting high electron percolation pathways [191]. Therefore, the electrocatalytic
activity and stability of the catalyst will be enhanced. To achieve a high content of N species
homogeneously doped into carbon materials, direct pyrolysis of nitrogen-containing precursors
or CVD of nitrogen-containing compounds can generate a higher amount of N-doped carbon
materials than that of post-treatment doping method. However, it is difficult to achieve
commercially [189].
3.4.2.1 Direct Pyrolysis Method
Su et al. successfully synthesized N-doped porous carbon nanospheres (PCNs) through a two-
step process of carbonization of poly-pyrrole nanospheres and chemical activation by
potassium hydroxide (KOH) [184]. PCNs have a microporous structure with a particle size less
than 100 nm and a high surface area with 1010 m2 g-1. This microporous structure results in
higher dispersion of smaller Pt nanoparticles to improve ORR activity. In addition, Ma et al.
synthesised carbon/nitrogen nanofibers (CNx) by high-temperature pyrolysis of poly-pyrrole
nanofibers [192]. During the pyrolysis at 800 annealing, the pyrrolic N in the poly-pyrrole
nanofibers was gradually converted to pyridinic N and graphitic N. Pyridinic N can anchor the
Pt nanoparticles strongly with better distribution and average size of ~3 nm, while the graphitic
N could improve the electrical conductivity of carbon supports. The large electrochemically
active specific surface area of Pt/CNx with 104 m2 g-1 is mainly attributed to the N modification
and the homogeneous dispersion of the Pt nanoparticles.
In addition, some researchers also studied the synthesis of nitrogen-doped graphene supports
from urea or melamine as nitrogen sources and investigated their oxygen reduction activity.
Two possible reaction pathways could result in nitrogen doping: (i) oxygen functional groups
interact with amino groups, possibly being introduced on the edges as shown in Figure 3.3
[193]. Lin et al. synthesized NG using pyrolyzed method. N content decreased from 13.81% at
Chapter 3 Page 68 of 231
400 to 6.70% at 900 [194]. Another method is hydrothermal treatment studied by Sun et
al. [195] and Guo et al. [196]. This hydrothermal route normally has less than 10% of N atom
doping. (ii) carbon nitride as intermediates to introduce N on graphene flakes [197].
Figure 3.3 Schematics of nitrogen doped GO by urea as N sources during pyrolysis.
Table 3.2 Different methods of preparing nitrogen doped graphene materials with nitrogen
content.
Name Method Preparation route N (%) Ref.
NG Pyrolyzed route GO/urea at 800 for 30 min 7.86 [194]
NGS-1 Hydrothermal route GO/urea at 180 for 12 h 10.13 [195]
NGHs-4 Hydrothermal route GO/urea at 160 for 3 h 5.86 [196]
GNS Ball-milling route GO/urea at 60 for 12 h 3.15 [198]
NGCLs Pyrolyzed route Carbon/urea at 800 for 30 min 6.39 [199]
Chapter 3 Page 69 of 231
3.4.2.2 Chemical Vapour Deposition (CVD) Method
CVD is an effective method for the preparation of nitrogen-doped carbon materials [200-202].
Yue et al. reported that nitrogen doped nanotubes with 3-5% total amount of N were
synthesised by chemical vapour deposition at 700 [200]. Then, Pt/CNx nanotube catalysts
were prepared using a microwave-assisted ethylene glycol reduction method. It was possible
to anchor the Pt nanoparticles uniformly on the surface of the CNx nanotubes without pre-
treatment of the CNx nanotubes because of the strong interaction between N and Pt
nanoparticles.
Recently, zeolitic imidazolate frameworks (ZIFs) have been proven to be very good precursors
for the preparation of nitrogen-doped carbon materials due to their remarkable microporous
structure, regular morphology and high nitrogen content of imidazolium-based ligands [203-
206]. Li et al. synthesized hierarchical porous nitrogen-doped carbon materials by direct
carbonization assembly of ZIF-8 at 1000, and the modified materials have large specific
surface area with 1105 m2 g-1 and pore volume with 0.95 cm3 g-1 [207]. Wei et al. also
demonstrated that the nano-sandwich structure of nitrogen doped ZIF/GO/ZIF has a high
specific surface area (1170 cm2 g-1) and good electrical conductivity [205]. It showed higher
onset potential at 0.92 V and lower limiting current density of 5.2 mA cm-2 at 0.6 V vs RHE,
which showed a huge potential application in the electrochemical field.
3.5 Summary
To overcome the problem of catalyst degradation, this chapter specifically described the effect
of different supports on the performance and stability of catalysts in PEMFCs. It summarized
the development of materials from the conventional high surface area carbon to the new
graphene-based materials. This chapter presented graphene intercalated by carbon black as
hybrid materials to solve graphene flakes stacking issue. The introduction of nitrogen atoms
Chapter 3 Page 70 of 231
into carbon supports would enhance the interaction between Pt and C and improve the stability
of Pt particles. Therefore, nitrogen-doped hybrid supports can solve the graphene stacking
problem as well as improve the stability of the catalyst, which is the research propose of this
work.
Chapter 4 Page 71 of 231
Chapter 4
Characterisation and Experimental
Methodology
4.1 Introduction
All as-prepared electrochemically exfoliated graphene oxide and as-synthesized catalysts need
to be analysed by physical characterization, electrochemical characterization and single cell
performance test. In the physical characterization, the defects, thermal stability, elemental
composition, flake size and flake thickness and surface morphology of graphene materials were
characterized by Ultraviolet-visible spectrophotometry (UV-Vis, Shimadzu UV-2700), Raman
spectroscopy (Renshaw inViaTM Confocal Raman microscope), Thermogravimetric Analysis
(TGA, TA instrument TGA 550), X-ray Photoelectron Spectroscopy (XPS, Thermo NEXSA
XPS), scanning electron microscopy (SEM, Quanta 650) and atomic force microscopy (AFM,
Bruker MultiMode 8). The crystal structure, thermal stability, surface morphology and particle
size of the catalysts were characterized using X-ray diffraction (XRD, PANalytical X’Pert Pro),
SEM and scanning transmission electron microscopy (STEM, Thermo Fischer Titan
ChemiSTEM) was also used to analyse the morphology of the materials. For electrochemical
characterization, cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were tested
by Autolab PGSTAT30 potentiostat-galvanostat and used to evaluate the electrochemical
surface area (ECSA) and oxygen reduction reaction (ORR) activity. Accelerated stress tests
Chapter 4 Page 72 of 231
(ASTs) were employed to test the durability of catalysts. The single cell performance
(polarization curves and electrochemical impedance spectroscopy) was tested by Gamry
Interface 5000. Polarization curves were used to test the fuel cell performance and
electrochemical impedance spectroscopy (EIS) was used to analyse the resistance by the
equivalent circuit.
4.2 Materials
To prepare electrochemically exfoliated graphene oxide (EGO) and nitrogen doped reduced
electrochemically exfoliated graphene oxide (NrEGO), graphite foil (0.5mm thick, 99.8%) was
purchased from Alfa Aesar. Urea (99.00%) was used from Sigma Aldrich. Concentrated
sulfuric acid (>95.00% H2SO4) and ammonium nitrate (NH4NO3, 98.50%) were purchased
from VWR Corporation.
To synthesize Pt/rEGOx-CBy and Pt/NrEGOx-CBy catalysts, chloroplatinic acid hexahydrate
(H2PtCl6·6H2O, ACS reagent, ≥37.50% Pt basis, Honeywell FlekaTM) was used as the
precursor and ethylene glycol (EG, ≥99.00%, Fisher Scientific) was used as the reducing
agent.
To fabricate the membrane electrode assembly (MEA), carbon paper (Toray TGPH 090, 280
μm, Fuel Cell Store); Ketjen black (EC-300J, AkzoNobel) was used in micro-porous layer. 60
wt% Polytetrafluoroethylene (PTFE, Sigma Aldrich), 5 wt% and 20 wt% Nafion ionomer
solutions (Ion Power) were used as binding chemicals; iso-propanol (IPA, 99.50%), acetone
(99.60%) and ethanol (99.98%) were purchased from Fisher Scientific and used as solvents for
micro-porous layer ink and catalyst layer ink.
Chapter 4 Page 73 of 231
4.3 Physical Characterization
4.3.1 Ultraviolet-visible Spectrophotometry (UV-Vis)
UV-Vis measures the absorption of ultraviolet light and visible light to determine the
composition and content of the substance, and then speculates on the structure of the material.
The principle of UV-Vis is that when a beam of ultraviolet-visible light passes through a
transparent substance and the energy of the photon is equal to the energy difference of the
electron level, the photons of this energy are absorbed, and the electrons transition from the
ground state to the excited state. The absorption curves are used to describe the absorption
characteristics of a substance. Taking the wavelength (λ) as the abscissa and the absorbance
(A) as the ordinate, the obtained A-λ curve is called the UV-visible absorption spectrum [208].
UV-Vis spectrum of graphene oxide in water has two featured peaks [209]. A high peak at
around 230 nm, which is due to π-π* transition of C=C bonds. A broad shoulder between 290
and 300 nm, which is assigned as n-to-π* transition of C=O bonds [210, 211]. After reduction
to form reduced graphene oxide, a red shift of the π-π* peak from 230 nm to 257 nm will be
observed and the broad shoulder gradually disappeared [212]. In this work, the whole
wavelength was scanned from 200 nm to 700 nm with a step of 1 nm for 0.01 mol mL-1
electrochemically exfoliated graphene oxide (EGO) and nitrogen doped reduced
electrochemically exfoliated graphene oxide (NrEGO) solution.
Chapter 4 Page 74 of 231
Figure 4.1 A typical UV-Vis spectrum of graphene oxide solution [206].
4.3.2 Raman Spectroscopy
Raman spectroscopy is a fast, effective and non-destructive means of characterising material
thickness, stacking patterns, defects and doping. As shown in Figure 4.2, when monochromatic
light is irradiated on a sample, the interaction of photons with molecules causes the molecules,
in the ground state, to absorb energy and cause molecular rotation, vibration or jumping of
electronic energy levels. The frequency of the elastically scattered light does not change. The
frequency and direction of the inelastically scattered light changes, which is also called Raman
scattering. The resulting spectrum plotted by the shifted frequency with the incident light and
the intensity of the scattered light is the Raman spectrum.
Chapter 4 Page 75 of 231
Figure 4.2 The working principle of Raman spectroscopy.
The characteristic peaks of Raman spectra of carbon materials are in the range of 800 to 2000
cm-1 as shown in Figure 4.3. The G-peak at 1580 cm-1, which is due to in-plane vibrations of
the carbon atoms, is the main characteristic peak of graphene and it effectively reflects the
number of layers of graphene. As the number of layers (n) of graphene increases, the G peak
shifts towards a lower wave number, i.e. a red shift occurs [213]. The D peak (~1350cm-1),
caused by activated sp2 carbon atoms or the disordered edges of the nanosheets, can be used to
characterize the defects of graphene-based materials. In this work, Raman spectroscopy was
tested by a Renishaw inViaTM Confocal Raman microscope at 785 nm. The ratio of their
intensity (ID/IG) is used to determine the defects on the surface of electrochemically exfoliated
graphene oxide (EGO) and nitrogen doped reduced electrochemically exfoliated graphene
oxide (NrEGO) [190, 214].
Chapter 4 Page 76 of 231
Figure 4.3 Raman spectra of a carbon material [212].
4.3.3 X-ray Diffraction (XRD)
In 1912, the German physicist Lauer proposed that when the wavelength of X-rays and the
spacing between the atomic planes inside a crystal are similar, the crystal can act as a spatial
diffraction grating for X-rays. In 1913, the British physicists Bragg and his son proposed the
famous formula that is the basis of crystal diffraction, the Bragg equation, which describe the
relationship between wavelength and interlayer spacing [215].
XRD or X-ray crystallography is a technique that uses the diffraction effect of X-rays in
crystalline substances to analyse the structure of the substance in Figure 4.4 [216, 217]. When
a monochromatic X-ray beam is irradiated on a crystal with regularly arranged atoms that have
similar interatomic distances as the wavelength of the incident X-rays, the X-rays were
scattered by different atoms interfere with each other and produce strong X-rays diffraction or
cancel out giving no X-rays at the specific angle. The relationship between wavelength and
interlayer spacing given by the Bragg equation in Eq. 4.1:
2𝑑𝑠𝑖𝑛𝜃 = 𝑛𝜆 Eq. 4.1
Chapter 4 Page 77 of 231
Where, d is interlayer spacing between atoms ( ); 2𝜃 is the angle of diffraction (rad); n is
order of diffraction; λ is the wavelength of X-ray (nm). λ can be determined by using a known
X-ray diffraction angle. It could give the face spacing and the regular arrangement of atoms or
ions within the crystalline state. Since the number of diffractions, intensity, lattice type and cell
size are unique for each substance, and therefore there is a unique diffraction pattern.
Figure 4.4 XRD equipment [216].
As shown in Figure 4.5, The principle of X-ray diffraction for sample composition analysis is
to use the polycrystalline diffraction pattern to determine the phase. When X-rays are incident
on a small crystal, the diffraction lines become diffuse and broadened. The smaller the grain
size of the crystal, the greater the broadening of the X-ray diffraction band. The Sherrer
equation (Eq. 4.2) describes the relationship between the crystallite diameters and the half-
peak width of the diffraction peak:
𝐷 =𝑘λ
𝛽𝑐𝑜𝑠𝜃 Eq. 4.2
Chapter 4 Page 78 of 231
Where, D is the size of crystal particles (nm); β is the half-peak width (rad); k is the shape
factor, and the value for cubic crystals is 0.9.
Figure 4.5 Schematic representation of Bragg’s law [216].
In this work, the crystalline structure of the catalysts was studied by X-ray diffraction (XRD)
with CuKα (λ=1.5406 Å) radiation and the diffraction data collected for 2θ angles are from 20°
to 100° using a PANaytical X’Pert Pro.
4.3.4 Thermogravimetric Analysis (TGA)
Thermogravimetric analysis is a method for measuring the relationship between the mass of a
substance and its temperature. It relies on the displacement of the balance caused by a change
in the weight of the sample being converted into an electromagnetic quantity, which is
amplified by an amplifier and fed to a recorder for recording. The magnitude of the electrical
signal is proportional to the amount of change in the weight of the sample. Therefore, when the
substance under test undergoes sublimation, vaporization, gas decomposition or loss of
crystalline water during heating, the mass of the substance under test will change. This is when
the thermogravimetric curve decreases. By analysing the thermogravimetric curve, it is
possible to find out to what extent the substance under test has changed. Based on the weight
lost, the percentage loss can be calculated by the ratio of the changed mass to the initial mass
as shown in Eq. 4.3 [218].
Chapter 4 Page 79 of 231
𝑤𝑡 % =𝑊𝑖𝑛𝑖𝑡𝑖𝑎𝑙−𝑊𝑟𝑒𝑎𝑙
𝑊𝑖𝑛𝑖𝑡𝑖𝑎𝑙× 100% Eq. 4.3
In this study, the thermogravimetric analysis of catalysts is carried out in TA instruments TGA
550 with the increasing temperature of 20°C/min in air atmosphere under the flow rate of 25
ml/min.
4.3.5 Inductively Coupled Plasma-Optical Emission Spectrometry
(ICP-OES)
ICP-OES is mainly used to analysis the metal elements content in the water solution. ICP-OES
include four parts in Figure 4.6: sampling system (pump, nebulizer and fog chamber), light
source (torch, coil), spectroscopy system and detection system. Prior to analysis, all aqueous
or solid samples are subjected to appropriate acid digestion to make sure all catalysts ions have
been dissolved. The high frequency generator produces a variable electromagnetic field to
supply stable energy to plasma. The torch will generate an induced current and release energy
(9000-10000k), the specimen enters from the injector to the atomisation chamber, then, the
specimen is carried into the torch by argon (Ar), the specimen is atomised, ionised, excited and
emits energy in the form of light. The atoms of different elements emit characteristic spectra
of different wavelengths, which are converted by photoelectric to electrical signal. In contrast,
the intensity of the emitted characteristic light varies depending on the content of the element,
whereby the element can be analysed quantitatively [219, 220].
Chapter 4 Page 80 of 231
Figure 4.6 The diagram of ICP-OES.
In this work, firstly, to dissolve Pt ions in the sample, 5 mg Pt catalysts was treated by
microwave digestion in a mixture acid solution with a volume ratio of concentrated HNO3 and
concentrated HCl of 1:3 at 220 for 45 mins. All samples are filtered by 0.45 μm filter
membrane to remove graphene/carbon materials and subjected to ICP-OES testing. Then, Pt
standard solution (0.01, 0.025, 0.05, 0.10 and 0.20 mg mL-1) were tested under 265.95 eV
emission line and plotted the Pt standard curve. Finally, catalyst samples were quantified by
the standard curve.
4.3.6 X-ray Photoelectron Spectroscopy (XPS)
X-ray photoelectron spectroscopy (XPS) is a surface-sensitive spectroscopic technique based
on the photoelectric effect which allows quantitative analysis of the elemental composition of
materials and their chemical state. It is also called Electron Spectroscopy for Chemical
Analysis [221, 222]. In 1887, Heinrich Rudolf Hertz discovered the photoelectric effect and
Albert Einstein explained this phenomenon in 1905 [223, 224]. After World War II, the first
Chapter 4 Page 81 of 231
high-energy and high-resolution X-ray photoelectron spectrum of sodium chloride was
obtained by Kai Sigbahn, Swedish physicist, demonstrating the powerful potential of XPS
technology. In 1981, Sigbahn received the Nobel Prize in Physics for his outstanding
contribution to the development of XPS [224].
As shown in Figure 4.7 and Figure 4.8, in this technique, X-ray (1000-1500 eV), normally are
Al(Kα) or Mg(Kα) X-rays, is irradiated on to a sample exciting the inner electrons or valence
electrons, emitting photoelectrons. According to Einstein’s law of photoelectron emission in
Eq. 4.4:
𝐸𝐾 = ℎ𝑣 − 𝐸𝐵 Eq. 4.4
where EK is the kinetic energy of the emitted photoelectron, hν is the energy of the photon of
the X-ray source, and EB is the binding energy on a specific atomic orbital (different atomic
orbitals have different binding energies). It can be seen from Eq. 4.4 that the energy of the
photoelectron is unique for a specific monochromatic excitation source and a specific atomic
orbital. It is only related to the elements and the atomic orbitals. Therefore, the elemental
species of a substance can be qualitatively analysed based on the binding energy of
photoelectrons [223].
Figure 4.7 XPS equipment.
Chapter 4 Page 82 of 231
Figure 4.8 The principle of XPS.
In this study, the chemical compositions of graphene-based materials and catalysts were probed
using X-ray photoelectron spectroscopy (XPS) on a Thermo NEXSA XPS fitted with a mono-
chromated Al(kα) X-ray source (1486.7 eV).
4.3.7 Scanning Electron Microscopy (SEM)
Scanning electron microscopy is a tool for analysing the surface morphology of samples. In
1923, Busch found that an electron beam could be focused by magnetic or electric fields. This
is the start of the development of electron microscopy (EM). The first SEM was produced by
M. von Ardenne. Zworykin in 1938. The resolution was only 50 nm. In 1965, Cambridge
Instrument Company produced the first commercial SEM with a resolution of 20-25 nm [225,
226].
When an incident electron beam irradiates the surface of the sample, elastic and inelastic
scattering occur between the electrons and the nucleus (or electrons in outer orbitals) of the
element, where elastic scattering produces backscattered electrons and inelastic scattering
produces secondary electrons [225]. It also produces X-rays. These signals could be used to
analyse different topography and composition of specimens [226, 227]. A typical SEM in
Chapter 4 Page 83 of 231
Figure 4.9 consists of three components: the microscope column, computer and ancillary
equipment.
Figure 4.9 The diagram of Scanning electron microscope [226].
The thermo-electrons emitted from the filament (Tungsten or LaB6 lamps) are accelerated
through a voltage of 2-30 kV. An electron beam with high energy and intensity will be formed
by condenser lens and objective lens, finally focus on the specimen through the horizontal and
vertical deflection of scanning coils. To minimise scattering of the electron beam, the specimen
chamber is maintained at high vacuum.
Secondary electrons (SE), backscattered electrons (BSE) and X-rays can be excited through
the interaction between the incident electron beam and the specimen. Everhart-Thornley
Chapter 4 Page 84 of 231
Detector (ETD) is more sensitive to SE. Before going into detector, SE with lower energy (2
to 50 eV) need to be collected by a cage (usually applied +200 to 300V). SE are ejected only
from the surface of specimens. Therefore, it is ideal for recording topographical information.
Solid State Diode Detector is used to attract BSE and collect topographical and compositional
information. X-rays can be used to test X-ray spectroscopy (EDS) and provide chemical
information of specimens. This electronic information is converted into an optical signal on the
scintillator, and then the photomultiplier tube converts the optical signal into the electrical
signal for analysis.
In this thesis, the size of the EGO flakes was statistically analysed by scanning electron
microscopy (SEM) on a MIRA3 Tescan operating at 5 kV. The catalyst particles and membrane
electrode assembly (MEA) cross-section (cut directly into an area of 5 mm × 5 mm by a
disposable scalpel sterile) were characterized by SEM with a Quanta 650 at 5 kV. And the EGO
flakes size was calculated by the length.
4.3.8 Energy-Dispersive X-ray Spectroscopy
Energy-Dispersive X-ray Spectroscopy (EDS) is an analytical technique that used for analysing
elements and chemical properties by collecting X-rays produced from the interaction between
electron beam and specimens, which is slightly similar to SEM [226, 228]. The difference is
that EDS requires an incident electron beam with relatively higher energy (15-20k eV) to excite
the internal electrons of the ground state atoms, which will form holes after inner electrons
leaving. Meanwhile, the electrons that are in the outer and higher energy level will fill these
holes with lower energy level. The excess energy may be released as X-rays to be collected
and transferred into electrical signals to be measured by the energy scattering X-ray
spectrometers [226].
Chapter 4 Page 85 of 231
EDS is often combined with SEM or TEM analysis. A wide range of areas can be selected for
EDS analysis for small particles on SEM or TEM images. Elemental mapping with EDS can
be used to obtain the distribution of elements on the surface [226].
4.3.9 Transmission Electron Microscopy
Transmission electron microscopy (TEM) is used to analyse the microstructure of a specimen
by irradiating the sample with an electron beam in a high vacuum chamber, and the produced
transmitted electrons are collected by objective lens and intermediate lens to amplify and
analyse the diffraction image. The resolution of TEM is much higher than that of optical
microscopy, which can analyse samples with 0.1-0.2 nm. Therefore, it could be used to analyse
catalyst particle size and crystal for catalysts in fuel cells [229-231].
The typical TEM is similar to SEM and contains these components: electron gun, electron
column, electro-magnetic lens system, detectors, specimen/sample chamber, main control
panel and operational controls, image capture system as shown in Figure 4.10. The electron
gun produces the electron beam which passes through the first short focal condensor lens,
aiming to narrow the spot. Then, the electron beam passes through a long focal condensor lens,
and the electron beam irradiates and interacts with the specimen to produce transmitted
electrons, secondary/backscattered electrons and X-rays [227, 232].
The transmitted electrons sequentially pass through the upper objective lens to focus, the lower
objective lens and intermediate lens, which is to magnify the image. Finally, the image in the
detector is magnified to 1000 to 200000 times [231-233].
The microstructure of the catalysts, the distribution of PtNPs on the supports and the Pt
nanoparticle size in this work were investigated by TEM before and after accelerated stress
tests (ASTs) using a probe side aberration corrected FEI Titan G2 80-200 STEM operated at
200 kV with a probe convergence angle of 21 mrad.
Chapter 4 Page 86 of 231
Figure 4.10 The diagram of transmission electron microscope.
4.3.10 Atomic Force Microscopy (AFM)
Atomic Force Microscopy (AFM) uses a micro-cantilever to contact and amplify the force
between a sharp thin probe on the cantilever and the atoms of the sample under test, thus
achieving detection with atomic-level resolution. AFM can observe both conductors and non-
conductors [234]. The main components of AFM in Figure 4.11 are the probe, the scanning
section, the detection section and the feedback section.
Chapter 4 Page 87 of 231
Figure 4.11 The schematic diagram of atomic force microscopy.
A micro-cantilever, which is extremely sensitive to weak forces, is fixed at one end and has a
tiny needle tip at the other end to contact with the surface of samples. Due to the weak repulsive
forces between the atoms at the needle tip and the atoms on the sample surface, the micro-
cantilever with the needle tip will undulate in the vertical direction to the sample surface. The
force is measured by the cantilever deflection amplified through an optical level, thus, the
surface information of the sample can be obtained [234, 235].
AFM is less restricted by the working conditions, and can be operated in ultra-high vacuum,
air and liquid. In this work, electrochemically exfoliated graphene oxide (EGO) dispersion was
dropped on a silicon wafer and AFM was used to evaluate the thickness of EGO flakes by
Bruker MultiMode 8.
4.4 Electrochemical Characterization
4.4.1 Cyclic Voltammetry (CV)
Cyclic voltammetry is a method for rapidly locating the redox potential of electroactive species
using the scanning voltage of a triangular potential waveform. In the field of fuel cell research,
Chapter 4 Page 88 of 231
cyclic voltammetry is primarily used to determine the electrochemical surface area of
electrocatalysts mainly by the coulombic charge of a specified surface faradaic reaction like
hydrogen underpotential deposition (HUPD). Three typical regions can be distinguished,
namely in Figure 4.12 [236]:
i) the region of hydrogen adsorption or desorption (called “hydrogen region”)
ii) the region free from faradaic processes (called “double layer region”)
iii) the region of surface oxidation and reduction (called “oxide region”)
Figure 4.12 Cyclic voltammetry recorded for a polycrystalline Pt electrode in 0.5 M
H2SO4 (298 K).
The principle is that hydrogen ions adsorbed to the surface of the Pt accompany with scanning
towards a negative potential from 0 V to -0.2 V. When the potential continues to decrease, the
bond between H and Pt breaks to produce hydrogen gas. At this point, the potential need to
scan to the positive and the desorption of hydrogen adsorbed onto the Pt occurs to produce
hydrogen desorption peak, which is affected by different lattices of Pt, (1001), (1110) and (111).
The different lattices adsorb different amounts of Pt. The hydrogen adsorption and desorption
peak are often used to determine the electrochemical surface area (ECSA) of catalysts [237].
Chapter 4 Page 89 of 231
At the middle potential region (0.2-0.6 V), the water molecules rotate, and the charges are
transferred to produce the current. However, no electrochemical reaction takes place. At higher
potential region (0.6-1.0 V), the oxygen atoms adsorb to the surface of platinum atoms, forming
platinum oxides (PtO, PtO2, Pt2O and PtOH). When the potential continues scanning to higher
potentials, the O-H bonds break to produce O2 and escape. However, the bond between O and
Pt break when scan back to lower potential to produce water. Platinum oxides are reduced [238].
In the LT-PEMFCs of this work, the electrochemical surface area (ECSA) was measured by
CV using a conventional three-electrode system (working electrode, reference electrode and
counter electrode) with an electrolyte solution of 0.5 M H2SO4. The choice of reference
electrode requires closing to the theoretical polarization, rapidly returning to equilibrium values
after polarization, a small temperature coefficient and good resistance to corrosion and shock.
The commonly use are the glycerine electrodes (Hg/HgSO4) and the silver chloride electrode
(Ag/AgCl). The counter electrode commonly uses Pt sheet and Pt wire. The area of counter
electrode should be bigger than that of working electrode to in case the influence of forming
capacitance. A platinum wire was used as the counter electrode and Ag/AgCl was used as the
reference electrode, while a mirror-polished glassy carbon rotating disk electrode (RDE, 3 mm
diameter) was used as the working electrode completely covered by Pt/CB, Pt/rEGO,
Pt/rEGOx-CBy and Pt/NrEGOx-CBy.
Based on the Pt loading of the catalysts determined by ICP-OES, all catalyst dispersions were
prepared to the same Pt concentration at 2.5 mgPt ml-1. 5 mg of catalyst was dispersed in a
mixture of 0.95 ml ethanol and 0.05 ml 5wt% of Nafion ionomer solution. Then, the suspension
was sonicated for 1 hour before it was dropped on to the glassy carbon rotating disk electrode
with 20 µl of suspension to give a catalyst loading of ∼3.18 mgPt cm-2. Finally, the working
electrode was dried at room temperature for 15 mins and rotated at 600 rpm for another 15
mins. CVs were tested under a potential range from -0.2 to 1.0 V at the scan rate of 50 mV s-1
Chapter 4 Page 90 of 231
under N2-saturated atmosphere rotating at a rate of 1600 rpm. After 5 cycles scanning the whole
potential region, the CV curves of all catalysts were stable and recorded. ECSAs of catalysts
were determined by integrating the hydrogen adsorption charge in Eq. 4.5 [160]:
𝐸𝐶𝑆𝐴𝑠 = 𝑄ℎ (𝑄𝑚 ∙ 𝐸. 𝐿⁄ ) Eq. 4.5
Where, Qh is the measured active catalyst surface area, calculated as the value of charges during
electro-adsorption of H2 on the PtNPs active sites. Scan rate in this work is 50 mV s-1. Qm is
the adsorption charge for an atomically smooth surface, with a value for Pt of 210 mC cm-2.
E.L represents the electrode Pt loading in gPt cm-2.
In the HT-PEMFC of this work, the ECSAs were tested under the same condition but in the
2M H3PO4 electrolyte solution, which would represent the electrochemical property of
catalysts under H3PO4 condition in the HT-PEMFC.
4.4.2 Linear Sweep Voltammetry (LSV)
LSV employs a fixed potential range and test the changes of current. The potential will scan in
one direction with starting at the initial potential and stopping at the final potential. LSV has
been used as an important tool for analysing oxygen reduction reaction (ORR) activity of
PEMFCs.
In LSV experiments, the potential of the working electrode is scanned from high potential,
which no reaction occurs, to lower potential, which results in ORR. For the same catalyst, the
reaction rate is determined by the diffusion of O2 at different rotation rate. Therefore, there is
a diffusion-limiting current (id). For different catalysts, various catalytic rates will definitely
cause different diffusion-limiting current. Meanwhile, the mass-transport-corrected kinetic
current and the electron transfer number (n) of ORR was calculated by the Levich-Koutecký
equation in Eq. 4.6 and Eq. 4.7 [160]:
1
𝑖=
1
𝑖𝑘+
1
𝑖𝑑=
1
𝐵𝜔12
+1
𝑖𝑘 Eq. 4.6
Chapter 4 Page 91 of 231
𝐵 = 0.62𝑛𝐹𝐶0(𝐷0)2
3𝑣−1
6 Eq. 4.7
Where i, ik, id and ω represent the measured current density, the kinetic-limiting current density,
the diffusion-limiting current density and RDE rotation rate, respectively. B is the slope of the
Levich-Koutecký curve. n is the electron transfer number in ORR. F is the Faraday constant
(96485 C mol-1), C0 is the bulk concentration of O2 (1.26×10-3 mol L-1), D0 is the diffusion
coefficient of O2 (1.9×10-5 cm2 s-1) in 0.5 M H2SO4, v is the kinetic viscosity of water 0.01 cm2
s-1 [75]. It is clearly in Eq. 4.6 and Eq. 4.7 that a linear relationship exists between id and w1/2.
In this work, the ORR activity was measured by the voltage scanning from 1.0 to 0 V with the
scan rate of 20 mV s-1 under O2-saturated 0.5 M H2SO4 solution at 400, 900, 1600, 2500 and
3600 rpm for the rotating disk electrode (RDE). The value of diffusion-limiting current density
(at 0.40 V) of catalysts at different rotation speeds were used to calculate electron transfer
number in the ORR.
4.5 Membrane Electrode Assembly (MEA) Test
4.5.1 Fabrication of MEA in LT-PEMFC
The Membrane electrode assembly was fabricated as follows:
1. First, 7.5 cm × 3 cm Toray carbon paper (TGPH 090, 280 mm) was cut as the gas
diffusion layer (GDL).
2. Ketjen black, mixing with 60 wt% Poly(tetrafluoroethylene) (PTFE) and iso-propanol
(IPA), was deposited on the GDL by nitrogen-spraying until reaching the target of 1
mg cm-2 loading for microporous layer (MPL).
3. GDL+MPL was followed by sintering at 300°C for 3 hours.
4. Then, catalyst layer (CL), which composed of 60 wt% Pt/C, 20 wt% Nafion and acetone
as solvent, was continuously deposited on MPL by nitrogen-spraying to form 0.25 mg
cmPt-2 Pt loading on both the anode and the cathode.
Chapter 4 Page 92 of 231
5. In order to evaporate organic solvent, the electrodes were dried at 120°C for both MPL
and CL during spraying.
6. Afterwards, 0.5 mg cm-2 20 wt% Nafion bonding layer was sprayed on the catalyst layer.
7. Finally, two electrodes with Nafion® NR-212 were used to prepare membrane
electrode assembly (MEA) by hot-pressing for 4.5 minutes at 135°C and 5.5 bar.
4.5.2 Fabrication of MEA in HT-PEMFC
Before membrane electrode assembly, commercial PBI membrane (3.5 cm × 3.5 cm) was
treated with 85% concentrated phosphoric acid at room temperature for 24 hours until the PA
doping amount had no changes and reached ~300 wt% compared with initial pure PBI
membrane. The MEAs of high-temperature hydrogen fuel cell were prepared as follows: The
1. First, a 1.5 cm × 1.5 cm carbon paper was used as the gas diffusion layer (GDL).
2. Then, microporous layer ink (90% ketjen black and 10% of 60 wt% PTFE) was sprayed
on GDL with 1 mg cm-2.
3. GDL + MPL was calcinated at 300 for 3 hours.
4. After that, 80% catalysts (Pt/C or Pt/NrEGO2-CB3) and 20% of 60 wt% PTFE were
mixed as catalyst layer (CL) ink and deposited on GDL + MPL with Pt loading of 0.25
mgPt cm-2.
5. The PA doped PBI membrane was hot-pressed with two electrodes at 140 and 5.5
bar for 4.5 min to prepare a sandwiched structure MEA.
4.5.3 Work Station
The in-situ performance test of single cell MEAs were carried out in the LT-PEMFC (Figure
4.13) and HT-PEMFC (Figure 4.14) work station. The work station mainly includes four
sections:
1. Gas supply system (hydrogen and oxygen).
Chapter 4 Page 93 of 231
2. Controller system (humidification, flowing rate and temperature controller).
3. Membrane Electrode Assembly (MEA) set up.
4. Gamry electrochemical galvanostat/potentiostats with monitor.
Figure 4.13 Scheme of LT-PEMFC working station for single cell MEA testing.
Chapter 4 Page 94 of 231
Figure 4.14 Scheme of HT-PEMFC work station for single cell MEA testing.
For the MEA performance tests, the single fuel cell was put between two same graphite plates
with 40 mm × 40 mm × 20 mm in length, width and thickness and 13 mm × 1 mm × 1 mm in
length, width and thickness for the serpentine gas channel in the centre of graphite plates. The
schematic representation of the single fuel cell used the present work to test MEAs is shown in
Figure 4.15, as well as three views of parameter design for serpentine airflow channels in the
graphitic compound bipolar plate are shown in Figure 4.16.
Chapter 4 Page 95 of 231
Figure 4.15 Schematic representation of the single fuel cell set up.
Chapter 4 Page 96 of 231
Figure 4.16 Parameter design for serpentine airflow channels in the graphitic compound
bipolar plate.
4.5.4 Fuel cell performance test
The sandwich MEAs of both LT-PEMFC and HT-PEMFC were placed in the middle of two
crossed serpentine gas flow channels of graphite and was assembled by a pressure of 0.5 N۰m
adding on both anode and cathode sides for different tests.
4.5.4.1 Polarization Curves
The polarization curve, also called voltage-current (V-I) curve, is one of the most common
methods used in fuel cell performance testing. The curve records the cell voltage behaves as a
function of current. When the current in the open circuit potential (OCP) start increasing, the
Chapter 4 Page 97 of 231
electrodes begin to polarize and produce a lower voltage as the resistance appearing. The curve
provides important information on the magnitude of polarization losses, including activation
losses, ohmic losses and mass transfer losses. It is ultimately used to calculate the change of
power density in the cells [9].
In this thesis, LT-PEMFC were tested with a gas flow rate of 100 ml min-1 for both 100 relative
humidity of H2 and O2 with no back pressure. The fuel cell was activated by scan the current
from 0 to the maximum current for 5 times at room temperature, 30, 40, 50 and 60.
The results were recorded by scanning the current from 0 with the higher OCP to the maximum
current point where the voltage is 0.2 V. The scanning step of current is 0.01 A s-1 and staying
at each point for 30s. HT-PEMFC were conducted by 10 cycles of polarization curves at 110,
130 and 150 for activation but with 100 ml min-1 of dry H2 and O2. Then, the final
polarization curve was recorded as same as that of LT-PEMFC.
4.5.4.2 Accelerated Stress Tests
It is usually impractical to test the results of long-lasting fuel cell operation under laboratory
conditions, so a rapid cycle scan of the cell is performed under caustic conditions in a specific
voltage range to observe cell decay in a short period of time according to Department of Energy
(DOE) standards [115].
In this work, the durability performance of all catalysts was firstly oxidized and reduced by CV
between 0.60 and 1.00 V for 30000 cycles in an O2-saturated 0.5 M H2SO4 solution at a scan
rate of 50 mV s-1 without rotation. After 3000, 8000, 18000 and 30000 cycles, ECSAs were
recorded under N2-saturated atmosphere with 1600 rpm. ORR activity of all catalysts was also
recorded by LSV before and after 30000 cycles [114].
The in-situ AST of MEA in two different protocols reported by Department of Energy (DOE)
were carried out to evaluate the electrocatalyst degradation and carbon support corrosion [114].
In the Pt catalyst degradation AST, MEAs were operated at 60 with 100% RH H2 (50 ml
Chapter 4 Page 98 of 231
min-1) and N2 (100 ml min-1) between 0.60 V and 1.00 V at a scan rate of 50 mV s-1 for 30k
cycles. Polarization curves and EIS were recorded before and after 30k cycles at 60 with
100% RH H2 (100 ml min-1) and O2 (100 ml min-1). In the carbon support corrosion AST,
another fresh MEAs were operated at 60 with 100% RH H2 (50 ml min-1) and N2 (100 ml
min-1) between 1.00 V and 1.50 V at a scan rate of 500 mV s-1. Polarization curves and EIS
were also recorded before and after 5k cycles.
4.5.4.3 Electrochemical Impedance Spectroscopy (EIS)
Electrochemical impedance spectroscopy (EIS) is an electrochemical technique that analyses
the resistance of materials, components, or systems by applying a small alternating current (AC)
to disturb the electrochemical system, and then, the equivalent circuit diagrams composed of
resistance, capacitors, or constant phase elements combined in parallel or in series were used
to fit the components [239-242]. Typically, Randles-Ershler model as shown in Figure 4.17 (a)
includes the analysis of the membrane resistance (Rm), the charge transfer resistance between
the membrane electrode and membrane (Rc), the mass transfer resistance (Rmt) and interface
resistance (Ri) between GDL/MPL, MPL/CL or CL/Nafion membrane [243]. As shown in
Figure 4.17 (b), In the high frequency of equivalent circuit model, the intercept at the real axis
is related to Rm, representing the proton conductivity through the membrane [244]. Two semi-
circular arcs are presented in the Nyquist plots. The tiny semi-circular arc at the high frequency
is defined as the Ri, which is influenced by the gap between catalyst layer and membrane. The
larger semi-circular arc in the middle frequency is related to Rc, which is attributed to the
activity of cathode reaction [245, 246]. For the low-frequency arc, it represents Rmt, which
affected by the flooding limiting reaction gas transport in electrode and hydration effect in the
membrane [247, 248].
In this work, Electrochemical impedance spectroscopy (EIS) measurements were conducted
by Gamry Interface 5000. The impedance spectra were recorded at a constant voltage of 0.60
Chapter 4 Page 99 of 231
V by sweeping frequency between 10 kHz and 0.1 Hz with AC signal amplitude of 100mV and
recording 10 points per decade.
Figure 4.17 (a) The equivalent curve of Randles-Ershler model and (b) a typical Nyquist
plot for a single cell.
4.6 Summary
This chapter provides an insight of the recent technologies for physical and electrochemical
characterization of support and catalyst materials. SEM combined with AFM would provide
the graphene flake size and thickness to evaluate quality of graphene-based materials and the
structure of catalysts. UV-vis, XPS and Raman could be used to analyse and quantify the
functional groups and defects on the surface of graphene flakes and catalysts. TGA would
provide information of thermal stability of graphene and catalysts. XRD combined with TEM
Chapter 4 Page 100 of 231
technique was used to characterize the crystalline form and particle size of Pt catalysts. The
ECSA and ORR activity of all catalysts were evaluated by CV and LSV, respectively.
Meanwhile, the electron transfer number for ORR could be calculated by Levich-Koutecký
equation. Polarization curves and accelerated stress tests were carried out to evaluate the
catalyst performance and durability in a single cell accompany with EIS for insight analysis of
resistance. All these technologies would give a comprehensive understanding from the support
materials to catalyst performance.
Chapter 5 Page 101 of 231
Chapter 5
Graphene/Carbon Hybrid Support
Materials
5.1 Introduction
Reduced graphene oxide (rGO), a two-dimensional (2D) material with long-range π
conjugation, has attracted great attention as a catalyst support because of its excellent electrical
conductivity (∼104.36 S cm-1), large specific surface area (∼2630 m2 g-1) and good thermal
and mechanical stability [249]. However, rGO flakes restack together during the reduction of
the platinum salts, through π-π interaction, blocking fuel (especially O2) diffusion to the PtNPs
active sites [160, 161]. To avoid restacking, Li et al designed a Pt/rGO material inserted with
carbon black (CB) as a composite support and this dramatically improved the durability toward
the ORR after 20000 cycles accelerated stress test (AST) in O2-saturated HClO4 electrolyte,
suggesting that rGO flakes could act as ‘mesh bag’ to prevent PtNPs leaching [161]. However,
this catalyst was not tested in a fuel cell. Sanli et al. prepared Pt/rGO/CB with electrochemical
surface area (ECSA) of 102 m2 g-1, which is larger than that of most other similar hybrid support
catalysts. However, it only achieved a power density of 0.645 W cm-2 with a lower Pt efficiency
of only 0.86 W mg-1 [250]. After that, Kim et al. researched boron doped and boron/nitrogen
co-doped into graphene inserted by CB as Pt catalyst support. Although the ECSA of Pt-B-
Gr/CB0.3 and Pt-BCN-Gr/CB0.5 are quite lower with only 33.60 m2 g-1 and 41.00 m2 g-1
Chapter 5 Page 102 of 231
respectively, Pt efficiency was improved to 1.08 W mg-1 [74, 251]. However, the raw materials
used for synthesizing Pt-graphene based catalysts in these publications mostly start from
graphene oxide (GO) materials produced by the Hummers method, which is costly, has
environmental and safety issues and has a long preparation time [21].
Recently, electrochemical exfoliation of graphite, which is fast, environmentally friendly, has
a high yield and is easy to scale-up, has attracted intensive research [162]. In electrochemical
exfoliation, the graphite foil acts as the working electrode and a Pt rod acts as the counter
electrode. To allow ion transport the electrodes are immersed in an electrolyte solution, ionic
salts such as NH4NO3, (NH4)2SO4 or their mixtures being the most commonly used [163].
During electrochemical exfoliation, the graphite foil is introduced by anions and functionalized
by the functional groups (N, P, S) present in the electrolyte [164]. These functional groups
could facilitate Pt anchoring on graphene. Generally, direct exfoliation in these ionic
electrolytes produces a low yield of graphene with few functional groups and a small flake size
(∼5 μm) [165]. Farbod Sharif et al. reported in 2020 that EGO exfoliated with NH4NO3 as the
electrolyte had more oxygen functional groups than material exfoliated with Na2SO4,
(NH4)2SO4 and (NH4)2HPO4 [163]. It has been shown that a two-step electrochemical
oxidation-exfoliation method, first oxidation in concentrated acid electrolyte then insertion in
ionic electrolyte, produces GO with almost the same prevalence of oxygen-containing
functional groups as the Hummers method but with a larger lateral flake size [166, 167].
However, smaller graphene oxide flakes are preferred as these can greatly increase the
proportion of graphene in the graphene-based support material, which can be expected to both
solve mass transfer problem and improve the ORR activity with enhanced durability in the fuel
cell performance.
This chapter describes the preparation of two-step electrochemically exfoliated graphene oxide
(EGO) and nitrogen doped reduced electrochemically exfoliated graphene oxide (NrEGO). The
Chapter 5 Page 103 of 231
synthesized graphene-based materials were characterized by UV-vis, TGA, Raman, AFM and
SEM. Figure 5.1 shows the steps of the experimental work in this chapter.
Figure 5.1 Steps of experimental work for catalysts.
5.2 Methodology
5.2.1 Synthesis of EGO
Electrochemically exfoliated graphene oxide (EGO) was produced according to a modified
two-step electrochemical exfoliation method as shown in Figure 5.2 [252]. The reaction was
carried out in a typical electrochemical cell with two electrodes (working and counter) and an
electrolyte. The first step is the intercalation, which takes place in the concentrated sulfuric
acid as the electrolyte. Graphite foil was cut with the area of 7.5 cm × 1.5 cm and connected
with the positive of power supply acting as a working electrode. Meanwhile, the platinum rod
was connected with the negative as the counter electrode. The galvanostatic charging current
was kept at 0.1 A until the voltage achieving 2.2 V (~15 mins). The intercalation was retained
Chapter 5 Page 104 of 231
for 10 mins at 2.2 V to allow the homogeneous intercalation process both at the surface and
inside of the graphite foil electrode.
Figure 5.2 Two-step electrochemical intercalation and exfoliation for production of EGO.
The second step is the exfoliation process using electrochemical oxidation in a 5 M NH4NO3
solution. The intercalated graphite foil and Pt sheet are connected with the positive and the
negative terminals, respectively. The intercalated graphite should be quickly transferred into
NH4NO3 solution. The cell voltage should be adjusted to 5 V to exfoliate. After the immersed
graphite was exfoliated completely, the product was washed until the filtered liquid is neutral
(pH=7). Then, the product was collected in a 250 ml bottle and sonicated for 24 hours to
disperse exfoliated graphene oxide (EGO). Finally, the EGO dispersion was stored at room
temperature with continuous stirring.
5.2.2 Synthesis of NrEGO
A modified hydrothermal method was used to prepare NrEGO as illustrated in Figure 5.3 [198].
75 mg EGO was dispersed in 30 mL DI water and sonicated for 30 minutes. Then, 2.0 g urea
was added into the dispersion. The mixture was sonicated for 3 hours and then stirred for 1
hour at room temperature. The mixture was transferred into 50 mL Teflon-lined steel autoclave
to carry out hydrothermal treatment at 160 for 6 hours. After which, the mixture was cooled
Chapter 5 Page 105 of 231
down to room temperature and the products obtained were washed with DI water until pH=7.
Finally, the material was dried at 60 in a vacuum oven and labelled as NrEGO. The reaction
process is displayed in Figure 5.3.
Figure 5.3 Synthesis routes of nitrogen-doped reduced electrochemically exfoliated
graphene oxide (NrEGO). A reaction route from the electrochemically exfoliated graphene
oxide (EGO) to the nitrogen-doped reduced EGO (NrEGO).
5.3 Physical Characterization
5.3.1 SEM Analysis
To observe the physical property of the obtained EGO, the product was analysed by SEM on a
MIRA3 Tescan operating at 5 kv as shown in Figure 5.4 (a). The flake size distribution was
Chapter 5 Page 106 of 231
statistically analysed by the amount of 82 EGO flakes and an average flake size of 1.3 μm was
obtained in Figure 5.4 (b).
Figure 5.4 (a) SEM image of EGO flakes and (b) statistics on lateral size distribution of
EGO flakes.
5.3.2 AFM Analysis
To evaluate the number of layers obtained of the EGO flakes, AFM was performed in Figure
5.5 (a), and a typical AFM image of EGO in Figure 5.5 (b) shows that EGO has a mean
thickness about 3 nm. The statistical analysis for 30 flakes thickness was estimated in Figure
Chapter 5 Page 107 of 231
5.5 (c) that approximately 67% of the EGO flakes are 3 nm in the thickness while nearly 30%
of these flakes are 2 nm and there are few flakes beyond 4 nm.
Chapter 5 Page 108 of 231
Figure 5.5 (a, b) AFM image of EGO flakes and (c) statistics on thickness of EGO flakes.
5.3.3 UV-Vis Spectroscopy
The stable aqueous dispersion of EGO by two-step electrochemical oxidation-exfoliation
process displayed in Figure 5.6 (a) was prepared with a concentration of 0.05 mg ml-1 and is
yellow brown. After the reduction of EGO by urea modification process, the dispersion
becomes black as shown in Figure 5.6 (b).
Figure 5.6 Colour changes of EGO and NrEGO.
Ultraviolet-visible (UV-vis) spectroscopy was performed to prove successful exfoliation in
Figure 5.7. UV-vis spectra shows the obvious peak for EGO at 243 nm which is a slight shift
from 230 nm of Hummer’s GO, corresponding to π-π* electronic transition (C=C) and high
EGO NrEGO
a b
Chapter 5 Page 109 of 231
absorption over the whole spectra region for EGO [252]. This indicates that EGO has less
disruption of electronic conjugation structure [253]. This peak shifts from 243 nm to 269 nm
after using urea modification. This change indicates that the p-conjugated network within the
graphene sheets was gradually restored, and EGO has been reduced [77, 254, 255]. The
shoulders at 300 nm in Hummers GO and EGO, corresponding to n-π* transition of C=O bond,
disappear after nitrogen doping by the hydrothermal treatment. This is also evidence that the
oxygen functional groups of EGO have been reduced [254].
Figure 5.7 Characterization of EGO and NrEGO by the whole wavelength analysis of UV-
vis.
5.3.4 TGA Analysis
In order to know the quantity of functional groups and thermal stability of EGO, TGA was
carried out under an air atmosphere as shown in Figure 5.8. Ignoring the weight loss of the
materials due to water evaporation before 100 , there are two typical stages of weight loss
for graphene oxide: the first stage is between 100 and 300 due to the decomposition of
unstable oxygen functional groups, while the second stage (above 500) belongs to the
decomposition of carbon [196, 256]. The weight loss of pure EGO is ~30% around 200°C,
which is much more than that of 20% in previous research [21]. But it decreases gradually in a
Chapter 5 Page 110 of 231
wide range temperature: the decomposition of unstable functional groups from 120°C to 230°C
and the desorption of more stable functional groups from 230°C to 400°C [76, 77].
Compared with EGO, TGA curves were further used to investigate the thermal stability of
NrEGO and different ratios of NrEGOx-CBy hybrid catalyst supports. All N doped catalyst
support materials do not show significant weight loss in the first stage, which also indicates
that N doped graphene oxide has been reduced. Meanwhile, the carbon decomposition of all N
doped support materials was observed after 700, which are higher than that of in EGO. This
indicates that the thermal stability of NrEGO and NrEGOx-CBy are better than EGO [255].
Specifically, the mass of NrEGO1-CB4 has a slight decrease at around 600. This might be
due to the functional groups in NrEGO and the uneven mixture of NrEGO and CB.
Figure 5.8 TGA analysis for the thermal stability of EGO, NrEGO and NrEGOx-CBy hybrid
catalyst supports.
5.3.5 XPS Analysis
X-ray photoelectron spectroscopy (XPS) was further used to confirm the atomic composition.
As can be seen in Figure 5.9, EGO contains approximately 24.6% of oxygen atoms, which is
higher than the EGO obtained by Cao et al using the two-step exfoliation in concentrated H2SO4
and (NH4)2SO4 electrolyte (17.7%) [252]. Meanwhile, the C/O ratio is 3.08, which is similar
Chapter 5 Page 111 of 231
to literature values for GO obtained by the traditional Hummers method (from 2.0 to 3.0) and
much lower than other electrochemically exfoliated graphene oxide (from 4.6 to 5.5) [162-167,
252]. Compared with three distinctive components, which are assigned to C sp2 (284.42 eV),
C–O (286.42 eV) and C=O (286.98 eV) in EGO, high-resolution C 1s of NrEGO shows five
distinctive components, including C sp2 (284.48 eV), C–O (285.92 eV), C=O (287.94 eV), O–
C=O (290.03 eV) and π-π* (291.90 eV). C sp2 increases from 65.25 % (EGO) to 79.77%
(NrEGO) and the presence of π-π* transition, which is the localization of π electrons in the
aromatic network, indicates the stacking of reduced graphene oxide flakes via conjugated
structures [252]. Meanwhile, the content of oxygen atoms decreases from 24.6% (EGO) to 4.0%
(NrEGO), which also demonstrates that the oxygen functional groups has been reduced and
agrees with TGA results.
Figure 5.9 High-resolution XPS spectra of C 1s in EGO and NrEGO.
Chapter 5 Page 112 of 231
The high-resolution N 1s spectrum is shown in Figure 5.10, 0.53 % N, including graphitic N
and p-phenyl (−NH2) groups, was successfully introduced into EGO at 401.56 eV and 399.43
eV, respectively. This is generated by the NH4NO3 electrolyte used during the second step of
the exfoliation process [249, 252]. When we exfoliate graphene oxide, NH3 is given off.
Therefore, the electrochemical oxidation of NH3 involves dehydrogenation of NH3 and
formation of N2, as shown in Eq 5.1 to Eq 5.4 below: [257]
𝑁𝐻3(𝑎𝑞) → 𝑁𝐻3,𝑎𝑑𝑠 Eq 5.1
𝑁𝐻3,𝑎𝑑𝑠 → 𝑁𝐻2,𝑎𝑑𝑠 + 𝐻+ + 𝑒− Eq 5.2
𝑁𝐻2,𝑎𝑑𝑠 → 𝑁𝐻𝑎𝑑𝑠 + 𝐻+ + 𝑒− Eq 5.3
𝑁𝐻𝑎𝑑𝑠 → 𝑁𝑎𝑑𝑠 + 𝐻+ + 𝑒− Eq 5.4
However, after using urea for nitrogen doping, the total N atoms increase to 2.40 %, and –NH2
groups are the main N species in NrEGO with 84.47 % of the total N species as the route in
Figure 5.3 [258, 259]. The high content of –NH2 can be explained with the reaction route
shown in Scheme 1, the urea continues to release NH3 at a relatively slow rate and the produced
NH3 reacts with oxygen functional groups (–OH and –COOH) on the surface of EGO to form
–NH2 [260]. These –NH2 groups and graphitic N belong to electron doping strategies (n type),
which could improve effective carrier mobilities and adjust electrical conductivity [261]. –NH2
groups increase electronic density to Fermi level and graphitic N facilitates 4e- electron transfer
for ORR [262].
Chapter 5 Page 113 of 231
Figure 5.10 High-resolution XPS spectra of N 1s in EGO and NrEGO.
5.3.6 Raman Analysis
In order to identify the structure and level of disorder in EGO and NrEGO, Raman spectroscopy
was carried out using a Renishaw inViaTM Confocal Raman microscope at 785 nm. All samples
are repeated 5 times to get the average number of ID/IG. G peak at ~1590 cm-1 represents sp2
carbon. The D peak at ~1350 cm-1 caused by activated sp2 carbon atoms could be used to
characterize the defects of graphene-based materials [263-265]. The value of ID/IG is used to
determine the defect on the surface of EGO flakes as shown in Figure 5.11. It was found that
ID/IG of NrEGO slightly increases from 1.02 (EGO) to 1.11 (NrEGO). The increased value of
ID/IG indicates that NrEGO has more defects, which are higher than that of 0.86 in Hummers
GO [255]. The increased defects can be attributed to the introduction of N species and the
smaller flake size [266, 267]. It has already been reported that N could facilitate the ORR
Chapter 5 Page 114 of 231
activity by polarizing the adjacent carbon [268]. Graphitic N have been widely introduced into
graphene flakes to increase the defects as well as to provide electro-negativity to facilitate a
uniform Pt atoms distribution, thus promoting ORR activity. Electro-negative –NH2 could also
form either an ionic bond or a covalent bond with the Pt complex ions (i.e., PtCl62-) [269].
Therefore, these defects are helpful to anchor Pt strongly and improve the stability of catalysts
during the operation of fuel cells [173, 270, 271]. Meanwhile, the G peaks of EGO and NrEGO
appeared at 1599.9 and 1595.9 cm-1, respectively. The downshift of the G peaks by 4 cm-1 from
EGO to NrEGO is attributed to the restoration of the conjugated structure during the
hydrothermal process [272]. It may also be related to the electron-donating capacity of the –
NH2 groups and graphitic N in n-type doping, as a similar downshift was observed in n-type
doped graphite and carbon nanotubes (CNTs) [266, 273].
Figure 5.11 Raman spectra of EGO and NrEGO under 785 nm.
5.4 Summary
In summary, a highly oxidised graphene oxide (EGO) with an oxygen content of 24.6% was
prepared by a two-step electrochemical exfoliation method. The graphene oxide has a low
Chapter 5 Page 115 of 231
number of layers (2-3 layers), a low flake size (0.94 ± 0.49 µm) and a significant defect ratio
of 1.02 in ID/IG.
Nitrogen was successfully introduced into EGO flakes through the hydrothermal treatment
with urea as the precursor. After modification, EGO has been reduced to NrEGO. Meanwhile,
NrEGO has further higher defects with ID/IG increasing to 1.11. NrEGO has higher thermal
stability compared with EGO, and NrEGOx-CBy show higher thermal stability with NrEGO
increasing. Based on these properties, the high defects of EGO and NrEGO are suitable
materials to anchor PtNPs. Thus, EGO and NrEGO hybrid support materials are expected to
solve gas transfer while playing an important role in both performance and durability in both
low-temperature and high-temperature proton exchange membrane fuel cells.
Chapter 6 Page 116 of 231
Chapter 6
Platinum Supported on Reduced
Electrochemically Exfoliated Graphene
Oxide and Carbon Black Hybrid Catalyst
Support (Pt/rEGOx-CBy)
6.1 Introduction
The main goal of this chapter is improving the activity and durability of the platinum catalyst
in the hydrogen fuel cell using a graphene and carbon black hybrid catalyst support. To achieve
this goal, different ratios of electrochemically exfoliated graphene oxide (EGO) and carbon
black (CB) were evaluated and optimized. Figure 6.1 shows the steps of the work in this
chapter.
Chapter 6 Page 117 of 231
Figure 6.1 Steps of experimental work for catalyst preparation.
Firstly, the chapter will describe the method used for synthesizing the Pt/rEGOx-CBy catalysts.
Then, physical characterization such as ICP-OES, XRD, TGA, XPS, SEM and TEM were
employed to evaluate the properties and structure of Pt/rEGOx-CBy. Then, electrochemical
characterization including cyclic voltammetry and linear sweep voltammetry were tested to
understand the electrochemical surface area (ECSA) and oxygen reduction reaction (ORR)
activity. Meanwhile, accelerated stress test (AST) was also used to evaluate the durability of
catalysts. Finally, all catalysts were tested for performance and long-term operation in a single
cell.
Chapter 6 Page 118 of 231
6.2 Synthesis of Pt/rEGOx-CBy Catalysts
Catalysts were synthesized by a modified polyol reduction method in an autoclave and the
synthesis steps are shown in Figure 6.2 [253]. The details of all steps to synthesize catalysts
are below:
1. EGO aqueous dispersion was first dried in the fume cupboard overnight until reach the
constant weight.
2. Then, 50 mg support materials: (1) only carbon black (CB); (2) only electrochemically
exfoliated graphene oxide (EGO) or (3) combining EGO and CB with different ratio as
hybrid catalyst supports, was sonicated in 5 ml deionised water for 30 mins to get
complete dispersion.
3. After that, 200 mg H2PtCl6·6H2O catalyst precursor was dispersed in 20 ml ethylene
glycol (EG). The catalyst precursor dispersion was added into the support dispersion
and the obtained mixture were sonicated for 2 hours and followed by stirring for 1 hour
to form homogenous catalyst mixture.
4. The homogenous catalyst mixture was transferred into a 50 ml Teflon-steel autoclave
for hydrothermal treatment at 120°C in an oil bath with continuous stirring for 24 hours.
5. After cooling down to room temperature, the catalyst dispersion was washed with
ethanol to remove all chlorides and EG by a vacuum filtration.
6. Finally, the catalyst products were dried in a vacuum oven at 60°C overnight. As-
synthesized catalysts were denoted as Pt/CB, Pt/rEGO, Pt/rEGO1-CB4, Pt/rEGO2-CB3,
Pt/rEGO3-CB2, Pt/rEGO4-CB1.
Chapter 6 Page 119 of 231
Figure 6.2 The process of synthesizing catalysts.
6.3 Physical Characterization of Catalysts
6.3.1 ICP-OES Analysis
Three types of catalysts with different support materials have been synthesised. Carbon black
(CB), reduced electrochemically exfoliated graphene oxide (rEGO) and the hybrid support
material with different ratio (rEGOx-CBy). These have been evaluated to confirm the actual Pt
loading obtained. ICP-OES was performed in 5 mg of these as-synthesized Pt/CB, Pt/rEGO
and Pt/rEGOx-CBy catalysts. Calibration and results are showed in Figure 6.3 and Table 6.1,
respectively. All of them show ~50 wt% Pt loading on catalysts.
Chapter 6 Page 120 of 231
Figure 6.3 Calibration line of Pt in 265.95 eV emission line by ICP-OES.
Table 6.1 Pt loading on different catalyst supporting materials.
Catalysts Pt concentration (mg mL-1) Pt loading (wt %)
Pt/CB 0.1048 52.10
Pt/rEGO1-CB4 0.0997 48.80
Pt/rEGO2-CB3 0.1051 51.55
Pt/rEGO3-CB2 0.1084 52.25
Pt/rEGO4-CB1 0.1082 52.10
Pt/rEGO 0.1018 51.50
6.3.2 TGA Analysis
The thermal stability of Pt/CB, Pt/rEGO and Pt/rEGOx-CBy were evaluated by TGA in an air
atmosphere displayed in Figure 6.4. The result show that Pt/CB dramatically decrease around
400°C. However, with an increase in the amount of rEGO, there is a decrease in the rate of
Chapter 6 Page 121 of 231
weight loss until 700, which indicates that rEGO may improve thermal stability. Pt/rEGO
still has some weight loss before 200°C but it lowrer than pure EGO. It could be caused by
incomplete reduction with EG reducing agent under 120. TGA results also indicate that the
Pt loading over the CB and rEGOx-CBy support material were 55%-60%. Pt/rEGO has the
lowest Pt loading. This might be because that chloroplatinic acid and graphene flakes dispersed
less well during the hydrothermal treatment. Meanwhile, TGA results show that Pt loading of
all catalysts samples tend to be higher than that in ICP-OES, which might be caused by the por
accuracy of TGA causing a relatively big analysis error. Therefore, the loading calculation that
used in the electrochemical test and the preparation of membrane electrode assembly are from
ICP-OES.
Figure 6.4 TGA analysis of catalyst weight loss.
6.3.3 XRD Analysis
XRD patterns were used to identify the Pt nanoparticles crystallite structures on different
supporting materials. XRD pattern in Figure 6.5 show that Pt/rEGO has a broad hump ranging
from 20° to 30°, corresponding to the amorphous or disordered nature of the stacked layers of
Chapter 6 Page 122 of 231
reduced graphene oxide formed in the polyol reduction process [274]. These wide peaks in all
catalysts gradually disappear as the rEGO amount decreases until Pt supported on pure CB. All
catalysts XRD patterns give peaks at 39.8°, 46.2°, 67.5° and 81.4° belonging to Pt (111), (200),
(220) and (311), respectively, which also demonstrates that Pt was successfully supported on
CB, rEGO or rEGOx-CBy by the EG reduction [275]. Meanwhile, all catalysts possess intense
peak at 39.8° Pt (111) lattice, which has higher electrochemical activity for ORR [274, 276].
Figure 6.5 XRD patterns of Pt/CB, Pt/rEGO and Pt/rEGOx-CBy.
6.3.4 SEM Analysis
The morphology of the catalysts was analysed by SEM and showed different structures of
platinum nanoparticles (PtNPs) supported on CB, rEGO and rEGOx-CBy. The structure of
Pt/CB in Figure 6.6 (a) shows that CB particles are agglomerated, which is caused by the
interaction resulting from the van der Waals forces [22]. Also, according to Lee et al, carbon
agglomerates at higher electrochemical potentials [277]. Interestingly, it can be seen comparing
Figure 6.6 (b) to (f), the ratio of rEGO and CB significantly affects the morphology of catalysts.
The addition of 20% rEGO flakes, Pt/rEGO1-CB4 in Figure 6.6 (b), starts to reduce the
agglomeration. This phenomenon is clearer still at 40% rEGO for Pt/rEGO2-CB3 in Figure 6.6
Chapter 6 Page 123 of 231
(c). Furthermore, rEGO flakes become crumpled when CB particles are inserted into rEGO
flakes, which might help to relieve the stacking issue in pure rEGO and CB agglomeration as
well. In addition, it can be seen that some PtNPs are present with poor dispersion on the surface
of stacked rEGO flakes. This is because the higher graphitization of rEGO decreases the
number of nucleation sites compared to CB, which is easy to form larger agglomerates [278,
279]. Particularly, graphene flakes size in catalysts of Figure 6.6 shows much bigger than that
counted in Figure 5.4. This should be attributed to the stacking of graphene flakes making it
become large.
Chapter 6 Page 124 of 231
Figure 6.6 SEM images of Pt/CB, Pt/rEGOx-CBy and Pt/rEGO.
Chapter 6 Page 125 of 231
6.4 Electrochemical Characterization of Catalysts
6.4.1 CV Analysis
Electrochemical characterization was carried out to evaluate the activity of the catalysts. Cyclic
voltammetry of Pt supported on the hybrid support materials are shown in Figure 6.7(a).
ECSAs distribution and the specific values are in Figure 6.7(b) and Table 6.2, respectively.
As the amount of rEGO in the hybrid increases the ECSA varies from 31.99 m2 g-1 for
Pt/rEGO1-CB4 to a maximum ECSA of 41.73 m2 g-1 for Pt/rEGO2-CB3, which is higher than
that of 26.79 m2 g-1 for Pt/CB. The ECSA then decreases to 18.49 m2 g-1 for Pt/rEGO4-CB1 and
only 18.38 m2 g-1 for Pt/rEGO. The ECSAs of the materials are summarised in order: Pt/rEGO2-
CB3 > Pt/rEGO1-CB4 > Pt/rEGO3-CB2 ≈ Pt/CB > Pt/rEGO4-CB1 ≈ Pt/rEGO. This is the result
of the stacked rEGO flakes blocking the pathway of proton transfer in H2SO4 solution [161].
Meanwhile, PtNPs are not dispersed well and grow into bigger particles, thereby decreasing
the effective ECSA.
(a)
Chapter 6 Page 126 of 231
Figure 6.7 (a) Cyclic voltammetry and (b) ECSA distribution of all hybrid support catalysts.
Table 6.2 ECSA calculation of all catalysts.
Catalysts ECSA (m2 g-1)
Pt/CB 26.79
Pt/rEGO1-CB4 32.00
Pt/rEGO2-CB3 41.73
Pt/rEGO3-CB2 27.51
Pt/rEGO4-CB1 18.49
Pt/rEGO 18.38
6.4.2 LSV Analysis
LSV was also carried out to evaluate ORR activity for all catalysts under different rotating
speed in Appendix 6.1. Pt/rEGO2-CB3 has the lowest diffusion-limiting current density as
shown in Figure 6.8 (a) at 1600 rpm. The details of half-wave potential (E1/2) and onset
potential (Eonset) are listed in Table 6.3. E1/2 of Pt/rEGO2-CB3 (0.639 V) is higher than that of
(b)
Chapter 6 Page 127 of 231
Pt/CB (0.629 V) and Pt/rEGO (0.614 V). Furthermore, the Eonset of Pt/rEGO1-CB4, Pt/rEGO2-
CB3 and Pt/CB are approximately 0.820 V and Pt/rEGO shows a lower Eonset of 0.809 V with
the increase of rEGO flakes increase. This means that the ORR is lower for Pt/rEGO. Pt/rEGO
also shows lower kinetic current density, and the wide potential region from 0.20 to 0.80 V
reaching the diffusion-limiting current density shows the lower ORR kinetic activity compared
with that of Pt/CB and Pt/rEGO2-CB3 from 0.40 to 0.80 V. Obviously, Pt/rEGOx-CBy catalysts
have two platforms: the first is around 0.55 V and the second is around 0.4 V. This is attributed
to the hybrid supports. At the first platform, it reflects 4-electron reaction of ORR. However,
due to the existence of graphene in hybrid catalyst supports compared with pure CB support,
the graphene could reduce overpotential and enhance higher kinetic current density. In 0.5 M
H2SO4 acid electrolyte, it could change 4-electron ORR pathway.
The values of electron transfer number in ORR, n, increased from 3.79 for Pt/rEGO1-CB4 to
4.19 in Pt/rEGO4-CB1, which are higher than that of 3.44 for Pt/CB in Figure 6.8 (b). This is
attributed to the electrical structure of graphene network, because the introduction of graphitic
N and −NH2 groups on the surface of graphene could facilitate electron transfer [280].
Chapter 6 Page 128 of 231
Figure 6.8 Pt/CB, Pt/rEGO and Pt/rEGOx-CBy in (a) LSV and (b) electron transfer in oxygen
reduction reaction activity.
Table 6.3 Half-wave potential (E1/2) and onset potential (Eonset) characterized by the linear
sweep voltammetry.
Catalysts E1/2 (V) Eonset (V)
Pt/CB 0.629 0.820
Pt/rEGO1-CB4 0.633 0.822
Pt/rEGO2-CB3 0.639 0.820
Pt/rEGO3-CB2 0.617 0.817
Pt/rEGO4-CB1 0.605 0.807
Pt/rEGO 0.614 0.809
6.4.3 Accelerated Stress Tests
The durability of Pt/CB, Pt/rEGO and Pt/rEGO2-CB3 was first evaluated by the ECSA changes
displayed in Figure 6.9 (a), (b) and (c). The results were also calculated on percentage of
ECSA loss as shown in Figure 6.9 (d). The ECSA of Pt/CB dropped dramatically and almost
completely deactivated with 88% loss after 30000 cycles. Pt/rEGO showed 46% loss of initial
Chapter 6 Page 129 of 231
ECSA and its slope is lower than that of Pt/CB. Interestingly, the ECSAs dropped only 29%
for Pt/rEGO2-CB3, a much smaller decrease compared with Pt/CB and Pt/rEGO. This is
probably due to PtNPs supported on CB changing from spherical particles with diameters of
5.6 ± 1.8 nm before AST in Figure 6.10, to a less homogeneous nanoparticle dispersion with
larger mean diameter (7.3 ± 3.5 nm) after 30000 cycles through Ostwald ripening as shown in
Figure 6.10 (b) [160, 161, 281]. The size distribution became wider as shown in the statistical
analysis in Figure 6.10 (c). However, there is no significant change observed for the PtNPs
supported on Pt/rEGO2-CB3, which were measured as 5.7 ± 1.8 nm before AST in Figure 6.10
(d) only to 6.0 ± 1.9 nm after AST in Figure 6.10 (e). These results confirmed that Pt supported
on a single carbon support is coarsening or agglomerated more quickly. The synergetic effect
of rEGO flakes and CB in the hybrid structure seems to help prevent PtNPs coarsening or
agglomerating to form bigger particles [161, 282]. Meanwhile, the −NH2 groups acting as a
bridge between graphene and Pt particles facilitate Pt anchoring on graphene flakes and the
stability of PtNPs size on hybrid support after AST [282]. After 18000 cycles AST, the
Pt/rEGO2-CB3 (87%) has a slightly lower retained ECSA compared with the 95% observed in
the AST results of the Pt/RGO/CB-1 catalyst reported by Li et al [161]. This is likely to be
because catalysts are degraded faster in 0.5 M H2SO4 solution compared to the 0.1 M HClO4
used by Li et al [21, 161].
Chapter 6 Page 130 of 231
Chapter 6 Page 131 of 231
Figure 6.9 ECSA changes with accelerated stress test in (a) Pt/CB, (b) Pt/rEGO and (c)
Pt/rEGO2-CB3. The statistical analysis of ECSA loss for Pt/CB, Pt/rEGO, Pt/rEGO2-CB3.
Chapter 6 Page 132 of 231
Figure 6.10 HAADF STEM images of fresh Pt nanoparticles on Pt/CB and Pt/rEGO2-CB3
before and after 30000 cycles of AST. Fresh Pt nanoparticles on Pt/CB (a) before and (b)
after 30000 cycles of AST. (c) Statistical analyses of PtNPs sizes on Pt/CB before (black)
and after (red) 30000 cycles of AST. HAADF STEM images of fresh and degraded Pt
nanoparticles on (d) CB and (e) rEGO for Pt/rEGO2-CB3 of AST; (f) Statistical analyses on
PtNPs sizes on Pt/rEGO2-CB3 before (black) and after (red) 30000 cycles of AST.
The comparison of ORR activities before (solid curves) and after (dotted curves) AST further
explains the durability of these catalysts in Figure 6.11 (a), E1/2 of Pt/CB shows an obvious
decrease from 0.629 V to 0.314 V. However, the decrease is less for Pt/rEGO from 0.614 V to
0.544 V and Pt/rEGO2-CB3 decreases only from 0.639 V to 0.624 V, almost no reduction.
Meanwhile, there is a 61.11% loss of Pt mass activity for Pt/CB (from 5.929 mA mgPt-1 to 2.306
mA mgPt-1) in Figure 6.11 (b), which is 2.4 and 8.3 times higher than that of Pt/rEGO (from
5.399 mA mgPt-1 to 4.034 mA mgPt
-1) and Pt/rEGO2-CB3 (from 6.416 mA mgPt-1 to 5.947 mA
mgPt-1). It also shows the diffusion-limiting current density at 0.40 V. Pt/CB has a huge
decrease of specific activity from 4.373 to 1.700 mA cm-2, which is nearly 2.7 times greater
than that of Pt/rEGO2-CB3 from 4.682 to 4.340 mA cm-2 after AST. In summary, Pt/rEGO2-
CB3 has higher ORR activity compared with Pt/CB and Pt/rEGO after AST. Changes in ORR
Chapter 6 Page 133 of 231
activities combined with the ECSA results indicate that the improved durability performance
of Pt/rEGO2-CB3 is a result of the strengthened interaction between rEGO and CB.
Figure 6.11 (a) LSV before and after 30000 cycles of AST; (b) Specific activities at 0.40 V
before and after AST for different catalysts.
Chapter 6 Page 134 of 231
6.5 Catalyst Performance in MEA
6.5.1 Polarization Curve Analysis
The performance of all fuel cells with different catalysts enhanced with increasing temperature
and showed little change in the performance with increasing temperature after 60 in the
Appendix 6.2. The maximum power density obtained for Pt/CB at 60 was 0.299 W cm-2 in
Figure 6.12. By adding rEGO to the CB support, the performance of the hydrogen fuel cell
increases and passes through a maximum power density of 0.537 W cm-2 for Pt/rEGO2-CB3
and giving 1.074 W mg-1 Pt efficiency, which is higher than that of 0.613 W mg-1 and 0.860 W
mg-1 in other similar hybrid support catalysts as shown in Appendix 6.3 [74, 169, 250, 281,
283]. Our fuel cell is operated at 60 , which is lower than other fuel cells. This facilitates the
start-up speed of the hydrogen fuel cell in practice. Meanwhile, less fuel consumption of 100
ml min-1 in our system is also helpful in the fuel savings. As the rEGO level increases further,
the fuel cell performance worsens until Pt supported on pure rEGO has the poorest performance.
It is observed that Pt/rEGO has a maximum power density of only 0.076 W cm-2 because of
the stacked rEGO flakes blocking humidified fuel gas (H2 and O2) and making water
management difficult at higher humidification temperature. The maximum power density
performances of all catalysts in decreasing order: Pt/rEGO2-CB3 > Pt/rEGO1-CB4 > Pt/CB >
Pt/rEGO3-CB2 > Pt/rEGO4-CB1 > Pt/rEGO, which agrees well with the order of ECSAs
presented previously. This improved power density performance indicates that not only the
mass transfer issue has been reduced by CB insertion, but also the power density of Pt/rEGO2-
CB3 has achieved a 1.8 times improvement compared with Pt/CB.
Chapter 6 Page 135 of 231
Figure 6.12 The hydrogen fuel cell performance with Pt/CB, Pt/rEGO and Pt/rEGOx-CBy.
6.5.2 EIS Analysis
EIS measurements were performed at a constant voltage of 0.60 V and the Nyquist plots of
Pt/CB and Pt/rEGOx-CBy are presented in Figure 6.13 and Table 6.4. According to
Parthasarathy et al, the equivalent electrical circuit curves are fitted by membrane resistance
(Rm), interfacial resistance (Ri) between electrode and Nafion membrane and charge resistance
(Rc) [169, 284, 285]. The voltage of Pt/rEGO is not stable and decreases rapidly; therefore, it
is difficult to record impedance results. After inserting CB, Rtotal decreases from 0.970 Ω cm2
for Pt/rEGO4-CB1 to 0.460 Ω cm2 for Pt/rEGO2-CB3. After that, Rtotal increases to 0.612 Ω cm2
for Pt/CB. Rm values of the different MEAs, displayed by the intersection of the arc and the X-
axis at high frequency, has the same trend as that of the power density. Because water vapor
carried by H2 and O2 and water produced on cathode side would wet the membrane to improve
proton conductivity [286]. Under the same voltage of 0.60 V, Pt/rEGO2-CB3 MEA produces
more water with higher power density performance, therefore, it further promotes proton
transfer in the Nafion membrane. Then, Ri is caused by grain-boundary phenomena between
Chapter 6 Page 136 of 231
the membrane and the catalysts [283]. When rEGO flakes were used as support in the hydrogen
fuel cell, it causes heterogeneities and more proton transport channels in terms of
polycrystallinity, existence of multi-phase components or a non-uniform distribution of rEGO
flakes, carbon black and the Nafion membrane [287]. In the depressed arc observed at 1 kHz
to 10 kHz, no visible Ri is recorded in Pt/CB catalyst MEA. However, Ri increases greatly from
0.035 Ω cm2 of Pt/rEGO1-CB4 to 0.245 Ω cm2 of Pt/rEGO4-CB1 as the amount of rEGO flakes
increases. According to previous SEM images, after hot-press treatment to assemble the MEA,
there is no obvious interface in the sandwich structure among the Nafion membrane, Nafion
bonding layer and Pt/CB catalyst layer as shown in Figure 6.14 (a) and (b). But the sandwich
structure of the hybrid support MEA has a larger interfacial region in Pt/rEGO2-CB3 catalyst
MEA in Figure 6.14 (c) and (d). According to Seo et al., too many interfacial spaces will
increase the difficulty of proton transport, which in turn results in lower performance of the
fuel cell [287]. In terms of charge resistance Rc, Pt/CB is 0.303 Ω cm2. As the amount of rEGO
increases, Rc decreases to 0.147 Ω cm2 for Pt/rEGO2-CB3 and then increases to 0.430 Ω cm2
for Pt/rEGO4-CB1. These results are consistent with the trends observed for ECSA and LSV.
Pt/rEGO2-CB3 has more exposed PtNPs active sites resulting in higher ECSA and higher ORR
activity with the four-electron reaction.
Chapter 6 Page 137 of 231
Figure 6.13 (a) Electrochemical impedance spectroscopy of all catalysts and (b) Rm, Ri and Rc
of all catalysts.
Chapter 6 Page 138 of 231
Table 6.4 EIS of different catalysts in the hydrogen fuel cell.
Catalysts
Resistance (Ω cm2)
Rm Ri Rc Rtotal
Pt/CB 0.309 0 0.303 0.612
Pt/rEGO1-CB4 0.299 0.055 0.150 0.504
Pt/rEGO2-CB3 0.233 0.080 0.147 0.460
Pt/rEGO3-CB2 0.275 0.150 0.208 0.633
Pt/rEGO4-CB1 0.295 0.245 0.430 0.970
Figure 6.14 Cross-section SEM images of interfacial space between MPL, CL and membrane
for MEAs with (a), (b) of Pt/CB and (c), (d) of Pt/rEGO2-CB3 in the catalyst layer.
Chapter 6 Page 139 of 231
6.5.3 Durability Analysis
To test the longer-term performance of catalysts in the fuel cell, the MEAs with Pt/CB and
Pt/rEGO2-CB3 catalysts have been evaluated by keeping them at a constant current density with
an initial voltage of 0.60 V for 24 hours. As shown in Figure 6.15, the voltage of Pt/CB
decreases from 0.60 V to 0.57 V after 24 hours. Pt/rEGO2-CB3 is more stable than Pt/CB and
remains at 0.60 V with almost no change over time. This is consistent with previous accelerated
stress test results. The synergetic effect of the hybrid structure consisting of rEGO flakes and
CB particles could prevent the growth of PtNPs. Therefore, PtNPs is less likely to coarsen in
the hybrid structure than in pure carbon black and thereby retains the long-term good
performance of the hydrogen fuel cell.
Figure 6.15 The performance and durability of Pt/CB and Pt/rEGO2-CB3 within 24 hours.
In order to further understand the internal resistance changes of Pt/CB and Pt/rEGO2-CB3, EIS
was recorded after 24 hours in Figure 6.16 (a) and (b). This shows that Ri of Pt/CB and
Pt/rEGO2-CB3 has no significant change, whereas Rm has a small increase because the
produced ·OH groups at the cathode could react with Nafion and break its chains after the
longer operation time of fuel cells, making the obvious changes in the morphology of the
Chapter 6 Page 140 of 231
Nafion 212 membrane [288]. Obviously, Pt/CB has a huge increase of Rc from 0.303 to 0.763
Ω cm2 compared with the increase of Pt/rEGO2-CB3 from 0.147 to 0.157 Ω cm2 in Table 6.5,
which is mostly caused by PtNPs supported on CB agglomerating, this agrees with the result
of AST as shown in the TEM images in Figure 6.10. However, there are no obvious changes
in particle size after AST for the hybrid support material, which maintains high ORR activity
with no significant changes in charge resistance. Therefore, the durability of Pt/rEGO2-CB3 is
better after 24 hours compared with Pt/CB.
Figure 6.16 EIS analysis with Rm, Ri and Rc of (a) Pt/CB and (b) Pt/rEGO2-CB3.
Chapter 6 Page 141 of 231
Table 6.5 The changes of membrane, interface and charge resistance before and after 24 hours.
Resistance
(Ω cm2)
Pt/CB Pt/rEGO2-CB3
Before After Before After
Rm 0.309 0.348 0.233 0.248
Ri ― ― 0.080 0.080
Rc 0.303 0.763 0.147 0.157
Rtotal 0.612 1.111 0.460 0.485
6.6 Summary
In conclusion, when EGO is inserted by carbon black as a hybrid support material for Pt
catalysts, Pt/rEGO2-CB3 shows a 1.8 times improvement in the performance of the resulting
hydrogen fuel cell and excellent durability at 0.60 V without any decline after longer-term
operation compared with Pt/CB. This excellent performance is attributed to (i) −NH2 groups
and graphitic N on graphene facilitating high ORR activity with the 4e- reaction pathway; (ii)
nitrogen species as an anchor site between Pt and graphene improving Pt stability and (iii) the
structure of the hybrid support mitigating the EGO stacking and reducing catalyst
agglomeration and coarsening. The insertion of CB into rEGO flakes produces a support
morphology consisting of crumpled 2D graphene oxide flakes with a higher ECSA of 41.73 m2
g-1 compared with 26.79 m2 g-1 of Pt/CB.
Chapter 7 Page 142 of 231
Chapter 7
Pt Supported on Nitrogen doped Reduced
Electrochemically Exfoliated Graphene
Oxide and Carbon Black Hybrid Catalyst
Support (Pt/NrEGOx-CBy)
7.1 Introduction
The aim of this chapter is improving the activity and durability of nitrogen-doped
graphene/carbon black hybrid support for platinum catalysts in the low-temperature hydrogen
fuel cell. Urea as the nitrogen precursor has proved successful introducing nitrogen into
graphene flakes by the hydrothermal treatment in Chapter 5. Different ratios of nitrogen doped
reduced electrochemically exfoliated graphene oxide and carbon black (NrEGOx-CBy) were
evaluated and optimized in the low-temperature hydrogen fuel cells. Accelerated stress tests
(AST) were used to evaluate catalyst degradation and carbon corrosion between commercial
Pt/C and the optimized nitrogen doped hybrid support catalysts. Figure 7.1 shows the process
of the work in this chapter.
Chapter 7 Page 143 of 231
Figure 7.1 Steps of experimental work for nitrogen doping catalysts.
7.2 Synthesis of Pt/NrEGOx-CBy Catalysts
NrEGO was synthesized by a modified hydrothermal reduction method in the autoclave. The
details of all process to synthesize Pt/NrEGOx-CBy as below:
1. Firstly, as-prepared NrEGO was mixed with CB in a total mass of 50 mg with different
ratio (1:4, 2:3, 3:2, 4:1) and dispersed into 5 mL DI water to sonicate for 30 minutes.
2. 200 mg of H2PtCl6·6H2O dispersed in 20 mL ethylene glycol (EG) was mixed with
supports dispersion.
3. The mixtures were sonicated for 2 hours and continuously stirred for another 1 hour at
room temperature.
Chapter 7 Page 144 of 231
4. Then, the mixtures were transferred into 50 mL Teflon-steel autoclave and put in oil
bath at 120 to do the hydrothermal reduction process for 24 hours with continuously
stirring.
5. After cooling down to room temperature, the catalysts products were washed with
ethanol to remove chloride ions.
6. Finally, all catalysts of Pt/NrEGOx-CBy were dried at 60 in the vacuum oven
overnight.
7.3 Physical Characterization
7.3.1 ICP-OES Analysis
ICP-OES was used to determine the platinum amount on the different ratios of NrEGOx-CBy
hybrid support materials. The standard curve is same with Chapter 6. The specific results are
displayed in the Table 7.1.
Table 7.1 ICP-OES analysis for Pt loading of Pt/NrEGOx-CBy.
Catalysts Pt loading (wt%)
Pt/NrEGO1-CB4 50.62 ± 1.80
Pt/NrEGO2-CB3 48.69 ± 1.65
Pt/NrEGO3-CB2 48.51 ± 2.20
Pt/NrEGO4-CB1 47.07 ± 3.41
Chapter 7 Page 145 of 231
7.3.2 XRD Analysis
The XRD pattern of all catalysts with different ratios of NrEGO and CB are displayed in the
Figure 7.2. The presence of (111), (200), (220) and (311) peaks of Pt are at 39.9, 36.4, 67.7
and 81.4, respectively. Pt (111) peak is the most significant.
Figure 7.2 The XRD spectra comparation of Pt/C, different ratios of hybrid catalyst support
Pt/NrEGOx-CBy and Pt/NrEGO.
7.3.3 XPS Analysis
After synthesizing Pt/NrEGO2-CB3, Figure 7.3 (a) shows an obvious extra N 1s peak at 398.76
eV, which can be assigned to a nitride, and indicates the presence of the Pt–N bond [289, 290].
The weak signal of N 1s should be attributed to the relatively small percentage of NrEGO in
the composite support. The Pt 4f XPS spectra of Pt/NrEGO2-CB3 in Figure 7.3 (b) was
deconvoluted into two peaks with binding energy values of 71.19 eV and 74.49 eV,
corresponding to Pt0 4f7/2 and Pt0 4f5/2 respectively, showing that Pt is metallic. A positive shift
of binding energy in Pt0 4f7/2 by 0.10 eV from 71.10 eV (Pt/NrEGO1-CB4) to 71.20 eV
(Pt/NrEGO) with NrEGO amount increasing in Figure 7.3 (c). This can be attributed to both –
Chapter 7 Page 146 of 231
NH2 groups and graphitic N being electronegative, which could enhance the interaction of
positively charged Pt particles with N atoms. This is consistent with the findings of Xin et al
and confirms the influence of –NH2 groups and graphitic N on the electronic structure of Pt
particles [282]. Xin et al also speculated that the electron density of Pt atoms delocalizing with
the adjacent –NH2 is greater than the formation of Pt–N bonds, which facilitates the formation
and stability of metallic Pt [282, 291].
Figure 7.3 High-resolution XPS spectra of (a) N 1s and (b, c) Pt 4f in Pt/NrEGO2-CB3.
Chapter 7 Page 147 of 231
7.3.4 SEM Analysis
SEM were carried out to investigate the morphology of the as-prepared Pt nitrogen-doped
hybrid support catalysts. Figure 7.4 shows SEM images of different ratios of doped
Pt/NrEGOx-CBy catalysts. Compared with commercial Pt/C in Figure 7.4 (e), the
agglomerated carbon spherical particles can be seen to be separated and dispersed to some
extent, and carbon black are successfully inserted into the interior of NrEGO flakes. The
restacked NrEGO flakes were re-opened by the post-insertion of CB spherical particles of
Pt/NrEGO2-CB3 in Figure 7.4 (b). It is clearer that the restacked NrEGO flakes have more
wrinkled edges after insertion with carbon black. However, neither lower NrEGO (NrEGO1-
CB4 in Figure 7.4 (a) nor more NrEGO flakes (NrEGO3-CB2 in Figure 7.4 (c) and NrEGO4-
CB1 Figure 7.4 (d)) resulted in a good mixing of hybrid support materials.
Chapter 7 Page 148 of 231
Figure 7.4 SEM images for the catalyst structure of (a) Pt/NrEGO1-CB4, (b) Pt/NrEGO2-
CB3, (c) Pt/NrEGO3-CB2, (d) Pt/NrEGO4-CB1 and (e) Pt/C.
7.3.5 TEM Combined with EDS Analysis
The particle size distribution of PtNPs was analysed by TEM showing that PtNPs presents
more uniform distribution on NrEGO2-CB3 in Figure 7.5. Statistical analysis of PtNPs on
Pt/NrEGO2-CB3 is about (4.88 ± 1.79) nm, which is smaller than that of the mean size
determined (5.60 ± 1.80) nm for Pt/C and (5.70 ± 1.80) nm for no N-doped Pt/rEGO2-CB3.
EDS mapping data from Figure 7.6 (a) to Figure 7.6 (d) were employed to analyse the
distribution of Pt, C and N elements in the Pt/NrEGO2-CB3. The HR-TEM image in Figure
7.6 (e) and Figure 7.6 (f) shows the Pt/NrEGO2-CB3 has a single-crystalline feature and clear
lattice fringes. The lattice spacing values of PtNPs are 2.24 Å and 1.80 Å, which are assigned
to the (111) and (200) planes of Pt crystals, respectively [282]. These crystal lattices show
higher activity in oxygen reduction reaction.
Chapter 7 Page 149 of 231
Figure 7.5 STEM images of (a) Pt/NrEGO2-CB3 and (b) the statistics on Pt particle size
distribution by counting 421 particles.
Chapter 7 Page 150 of 231
Figure 7.6 STEM-HAADF images of (a-f) HADDF of Pt/NrEGO2-CB3 and EDX elemental
mappings of C, N, and Pt in the Pt/NrEGO2-CB3. (e, f) HR-TEM images of Pt/NrEGO2-CB3
shows lattice fringes of Pt with FFT pattern of the selected specific Pt particles.
7.4 Electrochemical Performance
7.4.1 CV Analysis
Cyclic voltammetry was used to evaluate the electrochemical surface area (ECSA) of all
catalysts. Figure 7.7 displays the hydrogen desorption peaks of Pt/NrEGOx-CBy which had
two types of electrodeposited H intermediates, underpotential-deposited hydrogen (UPD-H)
and overpotential-deposited hydrogen (OPD-H) [292]. Among them, UPD-H (110) and (100)
had a higher hydrogen binding energy (HBE), which is the dominating factor for the hydrogen
oxidation reaction (HOR) activity. HOR contains a two-step reaction of the chemical
Chapter 7 Page 151 of 231
adsorption step (Tafel reaction Eq. 7.1) and the electro-transfer step (Volmer reaction Eq. 7.2):
[292-294].
Tafel:
𝐻2 + 2∗ → 2𝐻𝑎𝑑 𝐸𝑞. 7.1
Volmer:
𝐻𝑎𝑑 → 𝐻+ + 𝑒 +∗ 𝐸𝑞. 7.2
After optimizing the ratio of NrEGO and CB, Pt/NrEGO2-CB3 achieves the maximum ECSA
of 66.97 m2 gPt-1 with higher peak of (110) compared with that 50.88 m2 gPt
-1 of Pt/C in Table
7.2. These ECSAs are also comparable with values of 19.11-168.20 m2 gPt-1 reported in recent
publications concerning nitrogen doped catalysts as listed in Appendix 7.1 [295-300].
Meanwhile, Pt/NrEGO2-CB3 has a prominent oxygen reduction peak at 0.55 V, while the same
peak for Pt/C has a lower potential of only 0.47 V. This is attributed to the higher activity of
smaller PtNPs resulting from the nitrogen incorporation.
Figure 7.7 ECSAs measurement of all catalysts by cyclic voltammetry.
Chapter 7 Page 152 of 231
Table 7.2 Electrochemical surface area of Pt/C and Pt/NrEGOx-CBy.
Catalysts ECSA (m2 gPt-1)
Pt/C 50.88
Pt/NrEGO1-CB4 56.39
Pt/NrEGO2-CB3 66.97
Pt/NrEGO3-CB2 40.55
Pt/NrEGO4-CB1 24.71
7.4.2 LSV Analysis
Linear sweep voltammetry was further used to analyze oxygen reduction reaction (ORR)
activity. LSV curves at different rotating speed of all catalysts are shown in Appendix 7.2. The
onset potential (E0), when ORR starts taking place, is a vital criterion for evaluating
electrocatalytic activity. In Figure 7.8, where the LSV for the different catalyst are displayed,
the onset potentials are 0.829, 0.841, 0.861 and 0.844 V from Pt/NrEGO1-CB4 with the smallest
amount of NrEGO flakes to Pt/NrEGO4-CB1, with the highest amount of NrEGO presenting a
slightly higher onset potential than that of 0.824 V for Pt/C and 0.819 V for Pt/rEGO2-CB3.
With the amount of NrEGO flakes increasing, the voltage of the starting reaction gradually
become higher, indicating that the presence of N facilitates the reaction to start. Meanwhile,
the half-wave potential (E1/2) is 0.607 V for Pt/NrEGO2-CB3, which is higher than that of 0.629
V for Pt/CB and 0.639 V for Pt/rEGO2-CB3. Pt/NrEGO2-CB3 has a lower diffusion-limiting
current density of -6.169 mA cm-2, which is similar with that of -5.556 mA cm-2 for Pt/C and
lower than that of -4.681 mA cm-2 for Pt/rEGO2-CB3. Additionally, the kinetic current density
derived from the different rotation rates (400-4800 rpm) of LSV measurements presented in
Appendix 7.2 were used to calculate the electron transfer number (n) and the electrocatalytic
activity in ORR. it was found that the electron transfer number for N-doped hybrid support
Chapter 7 Page 153 of 231
catalysts have a trend to increase from ~3.8 to 4.2 as the NrEGO amount increases, suggesting
an efficient 4e- reaction pathway with water as the product in ORR activity. This may be related
to more –NH2 groups and graphitic N content with NrEGO increase, which facilitates
conductivity and electron transfer. Pt/NrEGO2-CB3 also has a relatively high value of ~4.15,
which is higher than that of 3.8 for Pt/C. This higher ORR activity is not only attributed to the
synergistic effect of graphitic N groups indeed reducing the diffusion-limiting current density
and –NH2 groups facilitating 4e- reaction to improve ORR activity by reducing the particle size
of Pt nanoparticles and allowing for more uniform particle distribution, but also demonstrated
that the optimized ratio of 2 to 3 in NrEGO2-CB3 could best address the stacking problems.
Figure 7.8 ORR activity at 1600 rpm and electron transfer number (inset) were measured
by different rotating speed of linear sweep voltammetry for all electrocatalysts.
7.5 Fuel Cell Performance
7.5.1 Polarization Curve Analysis
Appendix 7.3 shows the polarization curves of all N-doped hybrid support electrocatalysts at
40°C, 50°C and 60°C with 100% RH of inlet gases. Performances of all fuel cells with different
Chapter 7 Page 154 of 231
catalysts increase when the temperature is increased and show the highest performance at 60.
Figure 7.9 (a) displays their comparison of the average polarization curves performance
operating at 60°C for all electrocatalysts. As control catalysts, commercial Pt/C and Pt/rEGO2-
CB3 show maximum power densities of 0.710 W cm-2 and 0.537 W cm-2, respectively.
However, Pt/NrEGO2-CB3 reached 1.18 W cm-2 at the maximum current density of 2.22 A cm-
2 which can be detected in the instrument and achieved 4.748 W mgPt-1. Therefore, the power
density performance of all catalysts at 0.60 V, at which the cell is operated in normal practice,
were used as an indicator of evaluating the enhanced performance after N modification.
Pt/NrEGO1-CB4 (0.585 W cm-2) and Pt/NrEGO2-CB3 (0.934 W cm-2) electrocatalysts showed
significant improvement in power density compared with no N-doped Pt/rEGO1-CB4 (0.267
W cm-2) and Pt/rEGO2-CB3 (0.413 W cm-2). The optimized Pt/NrEGO2-CB3 showed 1.55 and
2.26 times higher than commercial Pt/C (0.601 W cm-2) and the optimized Pt/rEGO2-CB3
catalysts, respectively, as can be observed in Figure 7.9 (b). A higher content of NrEGO flakes
produces the power density of 0.318 W cm-2 in Pt/NrEGO3-CB2; only a slight improvement
compared with that of 0.269 W cm-2 in Pt/rEGO3-CB2. If more NrEGO is added into the catalyst,
instead of an increase of the power density, the effect obtained is the opposite and Pt/NrEGO4-
CB1 provided a maximum power density of 0.130 W cm-2 displaying lower performance than
Pt/rEGO4-CB1. This is probably due to stacking of the high quality electrochemically exfoliated
graphene oxide flakes because of the formation of π-π* transition confirmed by the XPS results
during the hydrothermal reduction process and resulting in multi-layered graphite like
structures. Therefore, both the surface area and anchoring sites of Pt are lost. On the other hand,
the reduction process of synthetic catalysts exacerbates the stacking issues and prevents the
carbon black inserting sufficiently, which results in the difficulty of fuel gas (H2 and O2)
approaching Pt active sites, thereby leading to a lower ORR activity. The polarization curves
contain three important regions: activation loss (lower current density), ohmic loss
Chapter 7 Page 155 of 231
(intermediate linear region) and mass transport loss (higher current density). Tafel slopes b in
Figure 7.9 (c), fitted by the voltage as a function of logarithms of current density (Log J) in
the activation loss region (< 0.10 A cm-2), reflects the catalysts activity. Table 7.3 shows that
Pt/NrEGO1-CB4 and Pt/NrEGO2-CB3 had lower Tafel slope in 89 and 67 mV dec-1,
respectively, which indicates a higher kinetic activity of the oxygen reduction reaction and a
better electron transfer efficiency compared with that of 90 mV dec-1 in commercial Pt/C. With
the increase of NrEGO, Tafel slope increased to 128 mV dec-1 for Pt/NrEGO3-CB2 and even
183 mV dec-1 for Pt/NrEGO4-CB1 with no linear relationship between voltage and Log J over
the same range of current density, demonstrating Pt/NrEGO4-CB1 enters ohmic loss phase at
lower current density.
Chapter 7 Page 156 of 231
Figure 7.9 (a) Polarization curves of different ratio N-doped hybrid support catalysts in
MEAs; (b) Comparation power density with same ratio of no N doped hybrid support
catalysts (Pt/rEGOx-CBy) at 0.60 V (c) Tafel slopes of Pt/NrEGOx-CBy and Pt/C,
respectively.
Chapter 7 Page 157 of 231
Table 7.3 The single cell power density performance at 0.6 V and the Tafel slope b calculated
by the activation loss region in all MEAs with different catalysts.
Catalysts Power density at 0.6 V (W cm-2) Tafel slope b (mV dec-1)
Pt/C 0.601 90
Pt/NrEGO1-CB4 0.585 89
Pt/NrEGO2-CB3 0.934 67
Pt/NrEGO3-CB2 0.318 128
Pt/NrEGO4-CB1 0.130 183
7.5.2 EIS Analysis
EIS was further used to quantify the resistance of each component inside the MEA by the
equivalent curve as shown in Figure 7.10 (c) and the results are displayed in Figure 7.10 (a).
The inflection point with the X axis is related to the membrane resistance (Rm). Two semi-
circular arcs are presented in the Nyquist plots. Among them, the tiny semi-circular arc at the
high frequency is defined as the interfacial resistance (Ri) and the larger semi-circular arc is
related to the charge resistance (Rc) [245]. The Rm of all catalysts are almost the same because
all MEAs use Nafion 212 membrane. However, Ri increases with the amount of NrEGO,
because of grain-boundary phenomena between the membrane and catalyst layer [169]. Rc of
Pt/NrEGO1-CB4 and Pt/NrEGO2-CB3 are the smallest and almost same with only 0.076 Ω cm2
and 0.088 Ω cm2, respectively, which are much lower than that of 0.108 Ω cm2 in the
commercial Pt/C, 0.207 Ω cm2 in Pt/NrEGO3-CB2 and 0.373 Ω cm2 in Pt/NrEGO4-CB1.
According to the SEM images of catalysts structure and ECSA results, this variation of Rc
occurs due to the lower ECSA caused by the loss of Pt active sites after NrEGO flakes stacking.
It increases the difficulty of H2 and O2 reaching the surface of Pt, and thus, the reaction kinetics
becomes slower.
Chapter 7 Page 158 of 231
Figure 7.10 (a) EIS of all catalysts and (b) the specific value of the membrane resistance,
interfacial resistance and charge resistance of all catalysts; (c) The equivalent curve of fuel
cells.
(c)
Chapter 7 Page 159 of 231
7.5.3 Accelerated Stress Tests (ASTs)
7.5.3.1 Pt Catalyst Degradation
To evaluate the stability of Pt catalysts and corrosion resistance of catalyst supports, ASTs have
also been carried out in-situ in the fuel cells following the procedure described in the
experimental section. Figure 7.11 display the polarization curves of Pt/C and Pt/NrEGO2-CB3
after 30k cycles to evaluate the effect of catalyst degradation after AST. After 30k low-potential
cycles (0.60 V - 1.00 V), the polarization curve of commercial Pt/C deteriorated from 0.640 V
to 0.548 V at 0.80 A cm-2, thus losing 92 mV. However, the loss observed in Pt/NrEGO2-CB3
is only 10 mV (from 0.659 V to 0.649 V), which is significantly lower than the commercial
catalyst and achieves the target set by the DOE protocols (<30 mV at 0.8 A cm-2 after 30k
cycles) [114]. The Tafel slopes increased from 144 mV dec-1 to 199 mV dec-1 in Pt/C, which
changed more than that of Pt/NrEGO2-CB3 from 124 mV dec-1 to 132 mV dec-1. It suggests
that Pt/C activity decay and the electron transfer efficiency decrease much more quickly than
in Pt/NrEGO2-CB3. This can be explained by EIS results obtained before and after AST in
Figure 7.11 (c). Rc in Pt/C increase from 0.089 Ω cm2 to 0.165 Ω cm2, which is faster than that
of Pt/NrEGO2-CB3, increasing only from 0.088 Ω cm2 to 0.151 Ω cm2. The details are listed in
Table 7.5. The lower increase of charge resistance can be attributed to the stable anchoring
sites of Pt-N and C-N. Pt hopping directly over the N need an activation energy of 1.00 eV,
which is much higher than that of 0.18 eV of Pt directly hopping over C atoms [291]. Therefore,
Pt is not easy to migrate over the N atom, which results in the higher performance for
Pt/NrEGO2-CB3 after AST.
Chapter 7 Page 160 of 231
Chapter 7 Page 161 of 231
Figure 7.11 (a) single-cell performance and (b) impedance of Pt/C and Pt/NrEGO2-CB3
before and after 30k cycles (0.6-1.0 V) AST; and (c) the specific resistance in terms of
membrane, interface and charge resistance.
Table 7.4 Tafel slope of Pt/C and Pt/NrEGO2-CB3 in the activation loss.
Catalysts Pt/C (mV dec-1) Pt/NrEGO2-CB3 (mV dec-1)
Tafel slope b initial 144 199
Tafel slope b after 30k AST 124 132
Table 7.5 Resistance changes before and after 30k cycles AST (catalyst degradation).
Catalysts
Rm (Ω cm2) Ri (Ω cm2) Rc (Ω cm2)
initial AST initial AST initial AST
Pt/C 0.140 0.161 - - 0.089 0.165
Pt/NrEGO2-CB3 0.101 0.114 0.040 0.040 0.088 0.151
Chapter 7 Page 162 of 231
7.5.3.2 Carbon Corrosion
The polarization curves of Pt/C and Pt/NrEGO2-CB3 before and after carbon corrosion test is
shown in Figure 7.12 (a). The maximum power density of Pt/C significantly decreases from
0.727 W cm-2 to 0.393 W cm-2, keeping 64% of initial performance after 5k cycles. It is not
able to reach 1.5 A cm-2 and the Tafel slope increases to 222 mV dec-1. However, Pt/NrEGO2-
CB3 reduces from 0.891 W cm-2 to 0.692 W cm-2, keeping 78% at 1.5 A cm-2. The Tafel slope
of Pt/NrEGO2-CB3 is also lower than that of Pt/C. These indicate that the carbon in Pt/C is
poorly resistant to corrosion, and this has a huge influence on the activity of the Pt nanoparticles.
This is also demonstrated by the significant charge resistance of Pt/C increasing to 0.285 Ω
cm2 after ASTs in Figure 7.12 (c). The charge resistance of Pt/NrEGO2-CB3 increases slightly
only to 0.128 Ω cm2. According to the mechanism of carbon oxidation reaction (carbon
corrosion, COR), the effect of catalyst performance goes through the following steps: the
formation of C+(s) during electrochemical process (Eq. 7.3), the formation of carbon oxides by
hydration (Eq. 7.4 and Eq. 7.5), carbon monoxide poisoning of Pt (Eq. 7.6 and Eq. 7.7) [104-
108]:
𝐶 → 𝐶(𝑠)+ + 𝑒− 𝐸𝑞. 7.3
𝐶(𝑠)+ + 𝐻2𝑂 → 𝐶𝑂𝑠𝑢𝑟𝑓 + 2𝐻+ + 𝑒− 𝐸𝑞. 7.4
𝐶𝑂𝑠𝑢𝑟𝑓 + 𝐻2𝑂 → 𝐶𝑂2 + 2𝐻+ + 2𝑒− 𝐸𝑞. 7.5
𝐶𝑂𝑠𝑢𝑟𝑓 + 𝑃𝑡 → 𝑃𝑡 − 𝐶𝑂𝑎𝑑𝑠 𝐸𝑞. 7.6
𝑃𝑡 − 𝐶𝑂𝑎𝑑𝑠 + 𝐻2𝑂 → 𝐶𝑂2 + 2𝐻+ + 2𝑒− 𝐸𝑞. 7.7
Therefore, carbon oxidation reaction (COR) process results in dynamic process including
leaching, Pt agglomeration/aggregation with migration and coalescence with electrochemical
process through Ostwald ripening [169, 246, 301]. The bigger Pt particles are easily detached
with carbon corrosion and carried out with the discharged water. Subsequently, Pt or Pt clusters
Chapter 7 Page 163 of 231
will be poisoned by the produced CO during the support corrosion process. The defects created
by –NH2 groups and graphitic N is more stable to prevent carbon corrosion [302].
Chapter 7 Page 164 of 231
Figure 7.12 (a) single-cell performance and (b) impedance of Pt/C and Pt/NrEGO2-CB3
before and after 5k cycles (1.0-1.5 V) AST; and (c) the specific resistance in terms of
membrane, interface and charge resistance.
7.6 Summary
In conclusion, p-phenyl (–NH2) groups and graphitic N doped reduced electrochemically
exfoliated graphene oxide inserted by carbon black as a highly durable Pt catalyst hybrid
support for the hydrogen fuel cell was prepared through a facile hydrothermal method. The
2.4 % of nitrogen atoms introduced into reduced electrochemically exfoliated graphene oxide
achieved the platinum nanoparticles size at (4.88 ± 1.79) nm with ECSA up to 66.97 m2 gPt-1.
It was found through testing in the low-temperature hydrogen fuel cell that fuel gas (H2 and O2)
transfer problems due to stacking of graphene flakes are prevented by optimising the ratio of
NrEGO to CB. The optimized Pt/NrEGO2-CB3 MEA could achieve high Pt efficiency of 4.748
W mgPt-1, which is 2 times higher than commercial Pt/C MEA. A higher power density
performance of 0.934 W cm-2 at an operating potential of 0.60 V in the low-temperature
hydrogen fuel cell. This performance is 1.55 times higher than that of commercial Pt/C.
Chapter 7 Page 165 of 231
In terms of the durability, Pt/NrEGO2-CB3 MEA has shown excellent performance in both
catalyst degradation and support corrosion. After 30k cycles of accelerated catalyst degradation
test, Pt/NrEGO2-CB3 MEA showed only a slighter decay of 10 mV compared with 92 mV in
Pt/C MEA at 0.80 A cm-2, which could meet the target of 30 mV reported by Department of
Energy [114]. The charge resistance (Rc) of Pt/NrEGO2-CB3 had only changing from 0.089 Ω
cm2 to 0.165 Ω cm2. On the other hand, after 5k cycles of accelerated carbon corrosion test,
Pt/C MEA only maintained 54 % performance and could not even reach 1.5 A cm-2. However,
Pt/NrEGO2-CB3 MEA kept 78% performance and showed 0.692 W cm-2 at 1.5 A cm-2,
indicating high resistance to corrosion. These enhanced performance and durability of
Pt/NrEGO2-CB3 were attributed to the function of N as a bridge between Pt and C, namely: i)
–NH2 groups and graphitic N has stronger interaction to anchor Pt particles, which decrease Pt
catalyst degradation. ii) –NH2 groups and graphitic N doped electrochemical exfoliation
graphene oxide and carbon black (N-C) was more stable, which improved corrosion resistance
than a single carbon material.
Chapter 8 Page 166 of 231
Chapter 8
Doped Graphene/Carbon Hybrid Catalyst
in HT-PEMFCs
8.1 Introduction
The high-temperature proton exchange membrane fuel cells (HT-PEMFCs) based on a
phosphoric acid (PA)-doped polybenzimidazole (PBI) membrane electrolyte have attracted
attention because of better kinetics of oxygen reduction reaction, no flooding issues and
excellent carbon monoxide tolerance [52, 57]. However, PA leaching from PBI membrane into
the electrode is one of the biggest problems as the loss of phosphoric acid causes reduced proton
conductivity of PBI membrane and prevents gas transfer in the electrode [303]. Furthermore,
HT-PEMFC requires a greater amount of platinum (Pt) (normally between 0.7 and 1.2 mg cm-
2) to provide high electrochemical surface area, which accounts for a higher proportion in the
cost of a total fuel cell stack [73]. Therefore, the reduction of phosphoric acid permeation and
catalyst dosage are two challenges in HT-PEMFC.
Many studies have been conducted to prevent the permeation of PA. Currently, the
modification of PBI membrane structure is the main method by introducing materials with
more -NH2 functional groups to form stable chemical bonds with phosphoric acid molecules
and prevent their movement [304-306]. However, the composite membrane materials are not
Chapter 8 Page 167 of 231
uniform and the mixed materials (SiO2, Graphene et al) will cause crossover issues to influence
the durability of HT-PEMFC [306].
Based on the results in the Chapter 7, Pt supported on nitrogen doped reduced
electrochemically exfoliated graphene oxide/carbon black hybrid catalysts (Pt/NrEGO2-CB3)
have higher oxygen reduction reaction (ORR) activity. In this chapter, the high catalytic
activity of Pt/NrEGO2-CB3 has been used in the catalyst layer to improve the performance and
durability of HT-PEMFCs. The research programme is shown in Figure 8.1 as follows:
Figure 8.1 Research strategies for graphene/carbon hybrid catalyst in HT-PEMFC.
Chapter 8 Page 168 of 231
8.2 Characterization of Materials
8.2.1 Acid Doping Level
The PBI membranes were dipped in the concentrated phosphoric acid solution and were dried
in a vacuum oven with different interval times. The acid doping level (ADL) were calculated
as below:
𝐴𝐷𝐿 =∆𝑊𝑃𝐴/98
𝑊𝑃𝐵𝐼/118 Eq. 8.1
Where, ∆WPA is the weight of phosphoric acid doped in the membrane, WPBI is the initial weight
of PBI membrane. 98 is the relative molecular mass of H3PO4 (g mol-1). 118 is the relative
molecular mass of PBI repeating unit (g mol-1).
Figure 8.2 depicts the ADL against time, and it shows PA doping level reach a plateau of 8.39
after 4 hours, which compares with publications (5.70 ~ 11.50) [59, 307, 308]. The PA will
leach and redistribute in the microporous layer (MPL) and gas diffusion layer (GDL), which is
the most serious drawback and challenge in the operation of HT-PEMFC.
Figure 8.2 PA doping levels of PBI membrane in the concentrated H2SO4 at room
temperature.
Chapter 8 Page 169 of 231
8.2.2 TGA Analysis
The thermal stability of the catalysts has an important impact on its stable operation, especially
in the HT-PEMFCs. In Figure 8.3, ignoring the weight loss at the beginning of water
evaporation, commercial Pt/C has a sharp decrease at 300 and another jump decrease from
370 to 500, which is the decomposition of carbon [161, 255]. It is clear that the weight
of Pt/NrEGO2-CB3 decreases at higher temperature and over a wide range from 400 to 800.
This demonstrates that Pt/NrEGO2-CB3 has better thermal stability than that of commercial
Pt/C. The residue after 800 is around 55%, which represents the Pt content and is in
agreement with the ICP-OES results.
Figure 8.3 Thermal stability of Pt/C and Pt/NrEGO2-CB3.
8.2.3 Cyclic Voltammetry
The cyclic voltammetry was carried out to study the electrochemical behaviour of various
catalysts. Figure 8.4 shows the voltammograms of various cycles in Pt/C and Pt/NrEGO2-CB3.
Obviously, Pt/NrEGO2-CB3 catalysts present two significant peaks in the H2 desorption region
Chapter 8 Page 170 of 231
(-0.2 - 0.2 V), which belong to the orientation of Pt particles (100) and (111) [3]. The
degradation of the catalysts in terms of the variation of ECSA is shown in Figure 8.4 (c). ECSA
of Pt/C decrease from 61.41 m2 gPt-1 to 1.23 m2 gPt
-1, which is almost completely deactivated
after 30k cycles. However, Pt/NrEGO2-CB3 remains 86.35% of initial ECSA (from 73.92 m2
gPt-1 to 63.83 m2 gPt
-1).
Chapter 8 Page 171 of 231
Figure 8.4 ECSA changes with cyclic voltammetry in H3PO4 electrolyte solution during the
accelerated stress test.
8.2.4 Linear Sweep Voltammetry
The ORR activity of commercial Pt/C and Pt/NrEGO2-CB3 in H3PO4 solution were evaluated
by LSV. Figure 8.5 shows that the half-wave potential (E1/2) of Pt/NrEGO2-CB3 performs 0.67
V, which is better compared with that of 0.62 V in commercial Pt/C. After 30k cycles
accelerated stress test, Pt/NrEGO2-CB3 keeps better E1/2 without any changes, which is higher
than that of 0.42 V in commercial Pt/C. Therefore, Pt/NrEGO2-CB3 shows superiority over
Pt/C in terms of both electrochemical activity and durability.
Chapter 8 Page 172 of 231
Figure 8.5 ORR activity of Pt/C and Pt/NrEGO2-CB3 in H3PO4 electrolyte solution.
8.3 MEA Performance and Steady-State Operation
Polarization curves in Figure 8.6 (a) show that the initial performance of MEA-1 with Pt/C at
both electrodes is 0.371 W cm-2, which is lower than that of 0.432 W cm-2 of MEA-2 with
Pt/NrEGO2-CB3 at both electrodes in Figure 8.6 (b). These results indicate that Pt/NrEGO2-
CB3 could also enhance performance in the high-temperature proton exchange membrane fuel
cell.
Steady-state operation were performed in the in-situ MEAs with a start voltage of ~0.60 V at
150 for 100 hours. The cell voltage versus time of all MEAs were recorded in Figure 8.6
(c). It can be clearly observed that the performance of MEA-1 is worse, and the voltage
decreases from 0.585 V to 0.477 V after 96 h, which declines at a rate of 1.125 mV h-1. In
contrast, the voltage of MEA-2 keeps more stable with a decay rate of only 0.01 mV h-1 for the
steady-state operation. After 100 hours steady-state operation, the performance of MEA-1 is
terrible with the poor power density dropping to 0.134 W cm-2. However, MEA-2 decrease
Chapter 8 Page 173 of 231
slightly to 0.413 W cm-2. The aged MEA-2 performs even higher than that of the initial
performance in Pt/C MEA-1.
Chapter 8 Page 174 of 231
8.4 Study of Deactivation Mechanism
8.4.1 Pt Catalyst Degradation
To further study the effect of catalyst degradation, the EIS was used to diagnoses the resistance
within the HT-PEMFC. The curves of four MEAs consist of a small arc at high frequency and
a large arc from the mid- to the low frequency, representing the interfacial resistance (Ri) and
charge resistance (Rc), respectively, as shown in Figure 8.7. The specific values are listed in
Table 8.1. The intersection of the high frequency arc with the X-axis represents the value of
the membrane resistance (Rm). It is clear to see that Rm of all MEAs do not have significant
change. However, Ri changes differently after steady-state operation. Ri value in MEA-2
decrease from 0.283 Ω cm2 to 0.218 Ω cm2. Based on research of Ri, it is mainly caused by the
gap between electrodes and membrane, which can be regarded as a capacitor in the equivalent
circuit element [309, 310]. In HT-PEMFC, the gap is usually filled by PA, which helps to
proton transfer in turn. Ri decreases with more PA distributed but without penetrating the
Figure 8.6 (a) Pt/C and (b) Pt/NrEGO2-CB3 catalysts performance in HT-PEMFC before
and after 100 hours steady-state operation; (c) Pt/C and Pt/NrEGO2-CB3 operation during
100 hours at a constant current density of 0.3 A cm-2.
Chapter 8 Page 175 of 231
catalyst layer [311]. This might be an evidence that more PA exist in the interface of MEA-2
than that of Pt/C in MEA-1. Rc is an important indicator to evaluate the catalyst activity, which
mainly reflects ORR activity. Figure 8.7 (a) shows that there is a large increase of Rc in Pt/C
of MEA-1 from 0.219 Ω cm2 to 0.728 Ω cm2, while Pt/NrEGO2-CB3 of MEA-2 increases
slightly from 0.378 Ω cm2 to 0.560 Ω cm2 in Figure 8.7 (b). This might be caused by two
reasons. One is Pt is more stable in Pt/NrEGO2-CB3 as the strong combing energy of Pt-N
bonds. This can be proved by TEM images. The particle size of Pt/C increase from 5.60 ± 1.80
nm (Chapter 7) to 6.78 ± 1.34 nm. However, Pt/NrEGO2-CB3 increases only from 4.88 ± 1.80
nm to 5.46 ± 1.46 nm. Another reason is that Pt loss accompanied with much PA leaching to
electrodes in Pt/C of MEA-1.
Chapter 8 Page 176 of 231
Figure 8.7 EIS analysis of (a) Pt/C and (b) Pt/NrEGO2-CB3 at 0.6 V before and after 100
hours steady-state operation. The insert shows the membrane resistance (Rm), interface
resistance (Ri) and charge resistance (Rc).
Table 8.1 Membrane resistance (Rm), interface resistance (Ri) and charge resistance (Rc) of
Pt/C in MEA-1 and Pt/NrEGO2-CB3 in MEA-2 before and after 100 h steady-state operation.
Fuel cells
Rm (Ω cm2) Ri (Ω cm2) Rc (Ω cm2)
0h 100h 0h 100h 0h 100h
MEA-1 0.153 0.144 0.105 0.342 0.219 0.728
MEA-2 0.204 0.208 0.283 0.218 0.378 0.560
Chapter 8 Page 177 of 231
Figure 8.8 (a) TEM images and (b) statistical analysis of Pt nanoparticles size in Pt/C; (c)
TEM images and (d) statistical analysis of Pt nanoparticles size in Pt/NrEGO2-CB3.
8.4.2 Phosphoric Acid Leaching
Phosphoric acid leaching and redistribution in the electrode would significantly affect mass
transfer. The distribution of PA that is lost in the electrode is also dependent on the pore
structure of the electrodes [312]. Therefore, we used SEM to characterise the cross-section and
EDS was used to analysis phosphorus distribution and the results are shown in Figure 8.9.
Phosphoric acid leached out from membrane to electrode of both fresh Pt/C MEA in Figure
8.9 (b) and fresh Pt/NrEGO2-CB3 MEA in Figure 8.9 (f), and the signal intensity of the EDS
Chapter 8 Page 178 of 231
shows that phosphoric acid is mainly distributed in the catalyst layer and interface between
membrane and catalyst layer. This is because free phosphoric acid was squeezed out from PBI
membrane by the gravitational pressure during the hot pressing [313]. It is also clear to see that
phosphoric acid is evenly distributed at both the anode and the cathode and there will be a
significant amount of Pt/C passing through the catalyst layer to electrode as shown in green
circles of Figure 8.9 (b). However, phosphoric acid mainly exists in the Pt/NrEGO2-CB3
catalyst layer and there is not much movement to the micro-porous layer and gas diffusion
layer in Figure 8.9 (f). After 100-hour steady state operation, it is obvious that phosphoric acid
existing in the interface between membrane and electrodes is not significant in the Pt/C MEA
in Figure 8.9 (d), and it moves to the gas diffusion layer at a distance from the PBI membrane.
However, it still has obvious signal in the interface catalyst layer of Pt/NrEGO2-CB3 MEA.
This might be because phosphoric acid moved with the produced water vapour and loss from
the gas outlet. NrEGO flakes might provide a physical barrier and the hybrid support
(NrEGO/CB) might change the pore structure of catalyst layer to affect phosphoric acid
redistribution. However, it also showed that PA has already been distributed in electrode both
in the fresh MEAs of Pt/C (Figure 8.9(b)) and Pt/NrEGO2-CB3 (Figure 8.9(f)). This might be
caused by two reasons: first, the PA molecules were squeezed out due to the hot-pressing
process. Secondly, PA molecules were carried to electrode during cutting the cross section.
Chapter 8 Page 179 of 231
Figure 8.9 Cross-section SEM images of (a) new Pt/C, (c) aged Pt/C, (e) new Pt/NrEGO2-
CB3 and (g) aged Pt/NrEGO2-CB3; P elements distribution in (b) new Pt/C, (d) aged Pt/C,
(f) new Pt/NrEGO2-CB3 and (h) aged Pt/NrEGO2-CB3.
Chapter 8 Page 180 of 231
8.5 Summary
In summary, Pt/NrEGO2-CB3 assembled in the HT-PEMFCs (MEA-2) improves the fuel cell
performance to 0.432 W cm-2 at lower Pt loading and the mass power density achieving 0.854
W mgPt-1. The decay rate of MEA-2 is only 0.01 mV h-1, which is much lower than that of
1.125 mV h-1 in Pt/C assembled MEA-1. This should not only be attributed to the N groups
doped graphene supply more homogenous distribution of Pt particles and strong interaction
between Pt and N, which improve the stability of Pt catalysts without significant increase of Pt
particle size. The cross-section EDS of phosphoric acid distribution in Pt/C and Pt/NrEGO2-
CB3 MEAs might be associated with the NrEGO/CB hybrid supports changing the pore
structure of catalyst layer and affects the phosphoric acid moving. It requires further
characterization of the electrodes in combination with different techniques such as Brunauer-
Emmett-Teller (BET) surface area analysis.
Chapter 9 Page 181 of 231
Chapter 9
Conclusions and Recommendations for
Future Work
9.1 Conclusions
This work firstly applied the nitrogen doped reduced electrochemically exfoliated graphene
oxide and carbon black as the hybrid support material for Pt catalysts, to the mass transfer issue
caused by graphene stacking, which improved the performance of both lower-temperature and
high-temperature proton exchange membrane fuel cells. The specific results are as follows:
Two-step electrochemically exfoliated graphene oxide (EGO) was synthesized by a two-step
electrochemical exfoliation method, concentrated H2SO4 and NH4NO3 electrolyte, which is fast
(within an hour), environmentally friendly (H2SO4 can be recycled and no KMnO4 strong
oxidizing agent is used) and high yield (tens of grams), showing potential for industrial
production. This kind of EGO is 2-3 layers and has smaller flake size with only 1.3 μm than
that of ~5 μm in Hummers graphene oxide (GO). After the nitrogen doping reduction process,
the defects of nitrogen doped reduced electrochemically exfoliated graphene oxide (NrEGO)
based on the ID/IG shows increase from 1.02 of EGO to 1.11 of NrEGO. This is attributed to
the nitrogen doping increase from 0.53 % of EGO to 2.40 % of NrEGO. 85 % are the –NH2
groups, which are electro-negative and form strong interactions with Pt nanoparticles. π-π bond
Chapter 9 Page 182 of 231
in XPS of NrEGO indicates that the NrEGO flakes have van der Waals force, which results in
restacked after nitrogen doping.
In the lower-temperature proton exchange membrane fuel cell, Pt/rEGO, Pt/rEGOx-CBy and
Pt/NrEGOx-CBy synthesized by a hydrothermal treatment were evaluated. After optimization,
Pt/NrEGO2-CB3 has the highest electrochemical chemical surface area (ECSA) with 66.97 m2
gPt-1, which is attributed to N atoms as a bridge between Pt and C making Pt distribute more
homogeneously and making Pt particles with size of 4.88 ± 1.79 nm. The Pt-N bond (1.0 eV)
has higher energy than Pt-C (0.18 eV), making strong interaction between Pt and N; On the
other hand, the introduction of N could decrease carbon corrosion. When all catalysts were
tested in a single cell, the voltage of Pt supported on pure graphene flakes drops sharply.
However, both Pt/rEGOx-CBy and Pt/NrEGOx-CBy becomes better with the insertion of carbon
black. This Pt/NrEGO2-CB3 shows the highest power density with 0.934 W cm-2 at 0.6 V,
which is 1.55 times higher than that of commercial Pt/C. Catalyst degradation and carbon
corrosion accelerated stress test certificated that Pt/NrEGO2-CB3 has higher Pt stability and
better resistance to carbon corrosion.
In the high-temperature proton exchange membrane fuel cell, Pt/NrEGO2-CB3 exhibits a better
maximum power density of 0.432 W cm-2, which is 1.32 times than that of 0.328 W cm-2 of
commercial Pt/C. After 100 hours steady-state operation, Pt/NrEGO2-CB3 catalyst MEA is
much more stable at 0.6 V with almost no decrease. However, commercial Pt/C decreases
rapidly. Until 100 hours, Pt/NrEGO2-CB3 shows almost the same performance in the
polarization curves but commercial Pt/C decreases to 0.142 W cm-2, reducing by 57 %. The
durability of Pt/NrEGO2-CB3 is better than that of commercial Pt/C, which results from two
reasons: i) it is attributed to the strong interaction between Pt and N making average size of Pt
nanoparticles increasing only from 4.88 ± 1.80 nm to 5.46 ± 1.46 nm in the in-situ HT-PEMFCs
operating at 150 for 100 hours, which has higher activity and stability than that of
Chapter 9 Page 183 of 231
commercial Pt/C (from 5.60 ± 1.80 nm to 6.78 ± 1.34 nm). ii) Cross-section of MEA in SEM-
EDS shows that phosphoric acid has less loss from the membrane in the Pt/NrEGO2-CB3
electrode. While the distribution of phosphoric acid moved far away into the Pt/C electrode
after 100 hours. More permeated phosphoric acid fills the pore structure of the electrodes, thus
blocking the diffusion pathway of the reaction gases (H2 and O2) to the catalytic sites, which
results in a serious mass transfer issue.
9.2 Recommendations for Future Work
The promising results obtained by nitrogen doped electrochemically exfoliated graphene oxide
inserted by carbon black as a composite catalyst support shows a huge possibility in the catalyst
layer of both lower-temperature and high-temperature proton exchange membrane fuel cells.
Based on the progress made throughout the PhD programme, the future research prospects for
graphene/carbon composite materials are proposed as follows:
1. Optimization of hybrid catalysts
Based on the PhD programme, the nitrogen doped catalysts were synthesized at confirmed
temperature, reaction time and doping level. Therefore, it is necessary to optimize different
parameters in terms of reaction temperature and time. The nitrogen doping method is also a
research direction in order to achieve more nitrogen doping level on supports. Finally, taking
production cost into different stages including small, medium and large scale, the overall cost
of graphene/carbon hybrid catalysts should be compared with commercial Pt/C.
2. Graphene/carbon composite materials in cold start operation
There is a lot of research reported graphene/carbon composite materials, but it has not reported
its pore size distribution, which is determined by the graphene flake size, carbon size and their
proportion (graphene: carbon=x: y). Different pore size in the catalyst layer will probability
reduce the super-cooled water come out before being frozen, because different pore sizes have
Chapter 9 Page 184 of 231
different capillary force to push the water [314]. Therefore, optimize the ratio of graphene and
carbon would optimize the living time of cells in cold environment.
3. Graphene/carbon composite materials in electrode
Graphene is expected to play a unique role in reducing the weight of fuel cell electrode as being
lower density and higher electrical conductivity. However, the usual 3D structure of graphene
materials is facing gas transfer problems due to closure of lamellae or blockage of gas diffusion
channels. Therefore, graphene and carbon composites electrodes can be synthesized to improve
gas transfer problems, reduce the density, weight, and volume of electrode materials. It would
provide more possible advantages for other materials used in fuel cell components.
4. Graphene/carbon composites electrode self-blocking phosphoric acid leaching in
HT-PEMFC
Phosphoric acid leaching is still a serious problem in the durability of high-temperature proton
exchange membrane fuel cell (HT-PEMFC). Graphene/carbon composites catalyst support has
shown that it acts as a barrier to control phosphoric acid leaching from PBI membrane and
moving to gas diffusion layer in the high-temperature proton exchange membrane fuel cell. In
order to achieve commercialization, it still needs to optimize the ratio of NrEGO and CB and
different flake size.
Appendix Page 185 of 231
Appendix
Appendix 5.1
Appendix 5.1 XPS Survey of EGO and NrEGO.
Appendix Page 186 of 231
Appendix 6.1
Appendix 6.1 Linear sweep voltammetry of (a) Pt/CB; (b) Pt/rEGO1-CB4; (c) Pt/rEGO2-CB3;
(d) Pt/rEGO3-CB2; (e) Pt/rEGO4-CB1 and (f) Pt/rEGO from 40 to 60 .
Appendix Page 187 of 231
Appendix 6.2
Appendix 6.2 Polarization curves and power density of (a) Pt/CB; (b) Pt/rEGO1-CB4; (c)
Pt/rEGO2-CB3; (d) Pt/rEGO3-CB2; (e) Pt/rEGO4-CB1 and (f) Pt/rEGO from 40 to 60 .
Appendix Page 188 of 231
Appendix 6.3 A
pp
end
ix 6
.3 L
iter
ature
rev
iew
of
hyb
rid c
atal
yst
support
mat
eria
ls i
n t
he
hydro
gen
fuel
cel
l.
R
ef.
[74
]
[25
0
] [16
9
]
Cat
aly
st
effi
cien
cy
(W m
g-1
)
1.0
80
0.8
60
0.6
13
1.0
74
Po
wer
d
ensi
ty
(W c
m-2
)
0.5
40
0.6
45
0.4
90
0.5
37
Max
cu
rren
t
den
sity
(A
cm
-2)
1.5
1.6
2.0
1.8
Flo
win
g r
ate
anode-
cath
ode
(ml
min
-1)
100
-100
150
-200
250
-250
100
-100
T
()
80
60
70
60
EC
SA
(m2 g
-1)
41
.00
10
2.0
0
48
.13
41
.73
Lo
adin
g
(mg
cm
-2)
0.2
0-0
.30
0.5
0-0
.25
0.4
0-0
.40
0.2
5-0
.25
Cat
alyst
s
Pt-
BC
N-G
r/C
B0.5
Pt/
rGO
/CB
Pt/
GN
Ps/
CB
This
work
Appendix Page 189 of 231
Appendix 7.1
Appendix 7.1 Literature review for the electrochemical properties and hydrogen fuel cell
performance of N-doped carbon catalysts.
Catalysts ECSA
(m2 gPt-1)
N content
(wt%)
E1/2
(V)
Power density
(W cm-2) Ref.
Pt/CNTs 55.60 ~0.600 [315]
Pt@NC/C 98.30 [295]
Pt/NCB 168.20 0.80 [296]
Pt/NCA 19.11 [316]
Pt/NOMC 2.20 0.680 [317]
Pt/G-NCNTs-1/2 118.40 4.67 [297]
Pt/NCNT 24.90 7.50 ~0.600 [318]
Pt/NSWCNH 83.00 1.00 0.904 [319]
Pt/ED-CNT 67.50 4.74 0.790 1.040 [320]
N-CNT/Pt/NC 75.50 9.90 ~0.800 0.676 [298]
Pt/NCNC700 60.40 27.60 0.838 [321]
Pt/CPPA800 31.00 10.50 ~0.800 [322]
Pt/N-CQD/G 123.40 2.40 0.880 [299]
Appendix Page 190 of 231
Pt@N-graphene foam 84.90 7.17 0.895 0.394 [300]
This work 66.97 2.40 0.607 > 1.187
Appendix Page 191 of 231
Appendix 7.2
Appendix 7.2 Linear sweep voltammetry of all nitrogen doped catalysts at different rotating
speed for the evaluation of ORR activity and electron transfer number.
Appendix Page 192 of 231
Appendix 7.3
Appendix 7.3 Single cell performance of all nitrogen doped catalysts at 40 , 50 and
60 with fully humidified H2/O2.
Publication & Conference Page 193 of 231
Publication & Conference
Publication
(i) Zhaoqi Ji, María Pérez-Page **, Jianuo Chen, Romeo Gonzalez Rodriguez, Rongsheng
Cai, Sarah J. Haigh, Stuart M. Holmes*. A structured catalyst support combining
electrochemically exfoliated graphene oxide and carbon black for enhanced
performance and durability in low-temperature hydrogen fuel cells. Energy. 2021. 226:
120318.
(ii) Zhaoqi Ji, Jianuo Chen, María Pérez-Page**, Zunmin Guo, Ziyu Zhao, Rongsheng Cai,
Maxwell T. P. Rigby, Sarah J. Haigh, Stuart M. Holmes*. Doped graphene/carbon black
hybrid catalyst giving enhanced oxygen reduction reaction activity with high resistance
to corrosion in proton exchange membrane fuel cells. Journal of Energy Chemistry.
(prepared)
(iii) Zhaoqi Ji, Jianuo Chen, Zunmin Guo, María Pérez-Page**, Stuart M. Holmes*. p-
phenyl and graphitic nitrogen doped graphene flakes as a barrier for PA invasion to
enhance HT-PEMFC durability. (Prepared)
Conference
(i) 238th PRiME of the Electrochemical Society. Honolulu, HI, USA. October 4-9, 2020.
(Oral)
Nitrogen doped reduced electrochemically exfoliated graphene oxide inserted
carbon black as novel catalyst support for the hydrogen fuel cell.
(ii) 236th of the Electrochemical Society. Atlanta, GA, USA. October 13-17, 2019. (Oral)
Publication & Conference Page 194 of 231
Platinum supported on electrochemically-exfoliated graphene oxide as a catalyst
for improving the performance of the hydrogen fuel cell.
(iii) The RSC Electrochemistry Northwest 2019. United Kingdom. July 4, 2019. (Poster)
Platinum supported on green synthesis graphene oxide as the catalyst for the
hydrogen fuel cell.
Bibliography Page 195 of 231
Bibliography
[1] S. Chu, A. Majumdar, Opportunities and challenges for a sustainable energy future, Nature
(London) 488(7411) (2012) 294-303.
[2] A.L.M. Reddy, S.R. Gowda, M.M. Shaijumon, P.M. Ajayan, Hybrid Nanostructures for
Energy Storage Applications, Advanced materials (Weinheim) 24(37) (2012) 5045-5064.
[3] H. Wang, H. Dai, Strongly coupled inorganic-nano-carbon hybrid materials for energy
storage, Chemical Society reviews 42(7) (2013) 3088-3113.
[4] J. Wu, X.Z. Yuan, J.J. Martin, H. Wang, J. Zhang, J. Shen, S. Wu, W. Merida, A review of
PEM fuel cell durability: Degradation mechanisms and mitigation strategies, Journal of power
sources 184(1) (2008) 104-119.
[5] J.-M. Tarascon, A.S. Aricò, B. Scrosati, P. Bruce, W. van Schalkwijk, Nanostructured
materials for advanced energy conversion and storage devices, Nature materials 4(5) (2005)
366-377.
[6] S. Cavaliere, S. Subianto, I. Savych, D.J. Jones, J. Rozière, Electrospinning: designed
architectures for energy conversion and storage devices, Energy & environmental science 4(12)
(2011) 4761-4785.
[7] S. Rodrigues, C. Restrepo, E. Kontos, R. Teixeira Pinto, P. Bauer, Trends of offshore wind
projects, Renewable & sustainable energy reviews 49 (2015) 1114-1135.
[8] E.M. Natsheh, A.R. Natsheh, A. Albarbar, Intelligent controller for managing power flow
within standalone hybrid power systems, IET science, measurement & technology 7(4) (2013)
191-200.
[9] A. Albarbar, M. Alrweq, Proton Exchange Membrane Fuel Cells : Design, Modelling and
Bibliography Page 196 of 231
Performance Assessment Techniques, 1st edition. ed., Springer International Publishing, Cham,
2018.
[10] Z. Xueqiang Zhang Xinbing Cheng Qiang, Nanostructured energy materials for
electrochemical energy conversion and storage: A review, Journal of energy chemistry 25(6)
(2016) 967-984.
[11] M.Z. Iqbal, S. Siddique, A. Khan, S.S. Haider, M. Khalid, Recent developments in
graphene based novel structures for efficient and durable fuel cells, Materials research bulletin
122 (2020) 110674.
[12] M.K. Debe, Electrocatalyst approaches and challenges for automotive fuel cells, Nature
486(7401) (2012) 43-51.
[13] P. Costamagna, S. Srinivasan, Quantum jumps in the PEMFC science and technology from
the 1960s to the year 2000: Part I. Fundamental scientific aspects, Journal of power sources
102(1) (2001) 242-252.
[14] A.A. Gewirth, M.S. Thorum, Electroreduction of Dioxygen for Fuel-Cell Applications:
Materials and Challenges, Inorganic chemistry 49(8) (2010) 3557-3566.
[15] Z. Ji, M. Perez-Page, J. Chen, R.G. Rodriguez, R. Cai, S.J. Haigh, S.M. Holmes, A
structured catalyst support combining electrochemically exfoliated graphene oxide and carbon
black for enhanced performance and durability in low-temperature hydrogen fuel cells, Energy
(Oxford) 226 (2021) 120318.
[16] E. Antolini, Carbon supports for low-temperature fuel cell catalysts, Applied catalysis. B,
Environmental 88(1) (2009) 1-24.
[17] S.S. Araya, F. Zhou, V. Liso, S.L. Sahlin, J.R. Vang, S. Thomas, X. Gao, C. Jeppesen, S.K.
Kær, A comprehensive review of PBI-based high temperature PEM fuel cells, International
journal of hydrogen energy 41(46) (2016) 21310-21344.
[18] Y. Shao, J. Liu, Y. Wang, Y. Lin, Novel catalyst support materials for PEM fuel cells:
Bibliography Page 197 of 231
current status and future prospects, Journal of Materials Chemistry 19(1) (2009) 46-59.
[19] J. Liu, X. Wu, L. Yang, F. Wang, J. Yin, Unprotected Pt nanoclusters anchored on ordered
mesoporous carbon as an efficient and stable catalyst for oxygen reduction reaction,
Electrochimica Acta 297 (2019) 539-544.
[20] K. Ichihashi, S. Muratsugu, S. Miyamoto, K. Sakamoto, N. Ishiguro, M. Tada, Enhanced
oxygen reduction reaction performance of size-controlled Pt nanoparticles on polypyrrole-
functionalized carbon nanotubes, Dalton Transactions 48(21) (2019) 7130-7137.
[21] E. Bertin, A. Münzer, S. Reichenberger, R. Streubel, T. Vinnay, H. Wiggers, C. Schulz, S.
Barcikowski, G. Marzun, Durability study of platinum nanoparticles supported on gas-phase
synthesized graphene in oxygen reduction reaction conditions, Applied Surface Science 467
(2019) 1181-1186.
[22] Y.-Y. Won, S.P. Meeker, V. Trappe, D.A. Weitz, N.Z. Diggs, J.I. Emert, Effect of
temperature on carbon-black agglomeration in hydrocarbon liquid with adsorbed dispersant,
Langmuir 21(3) (2005) 924-932.
[23] D. Marta, W. Marlena, Management of Energy Sources and the Development Potential in
the Energy Production Sector—A Comparison of EU Countries, Energies (Basel) 14(685)
(2021) 685.
[24] A. Singh, Translating the Paris Agreement into action in the Pacific, Springer, Cham, 2020.
[25] X. Zhou, J. Qiao, L. Yang, J. Zhang, A Review of Graphene‐Based Nanostructural
Materials for Both Catalyst Supports and Metal‐Free Catalysts in PEM Fuel Cell Oxygen
Reduction Reactions, Advanced energy materials 4(8) (2014) 1301523-n/a.
[26] F.T. Wagner, B. Lakshmanan, M.F. Mathias, Electrochemistry and the future of the
automobile, The Journal of Physical Chemistry Letters 1(14) (2010) 2204-2219.
[27] J.M. Kurtz, H.N. Dinh, Fuel Cell Technology Status: Degradation, National Renewable
Energy Lab.(NREL), Golden, CO (United States), 2018.
Bibliography Page 198 of 231
[28] P. Grimes, Historical pathways for fuel cells. The new electric century, Fifteenth Annual
Battery Conference on Applications and Advances (Cat. No. 00TH8490), IEEE, 2000, pp. 41-
45.
[29] A.B. Stambouli, E. Traversa, Solid oxide fuel cells (SOFCs): a review of an
environmentally clean and efficient source of energy, Renewable and sustainable energy
reviews 6(5) (2002) 433-455.
[30] A. Appleby, From Sir William Grove to today: fuel cells and the future, Journal of Power
Sources 29(1-2) (1990) 3-11.
[31] J.M. Andújar, F. Segura, Fuel cells: History and updating. A walk along two centuries,
Renewable and sustainable energy reviews 13(9) (2009) 2309-2322.
[32] Study Findings from National Aeronautics and Space Administration Provide New
Insights into Astrobiology (Geoelectrodes and Fuel Cells for Simulating Hydrothermal Vent
Environments), Energy Weekly News (2018) 632.
[33] Q. Li, D. Aili, H.A. Hjuler, J.O. Jensen, High temperature polymer electrolyte membrane
fuel cells : approaches, status, and perspectives, Springer, Cham, 2016.
[34] F. Chen, T.R.C. Fernandes, M. Yetano Roche, M. da Graça Carvalho, Investigation of
challenges to the utilization of fuel cell buses in the EU vs transition economies, Renewable &
sustainable energy reviews 11(2) (2007) 357-364.
[35] J.M. Andújar, F. Segura, M.J. Vasallo, A suitable model plant for control of the set fuel
cell−DC/DC converter, Renewable energy 33(4) (2008) 813-826.
[36] Y. Liu, W. Lehnert, H. Janßen, R.C. Samsun, D. Stolten, A review of high-temperature
polymer electrolyte membrane fuel-cell (HT-PEMFC)-based auxiliary power units for diesel-
powered road vehicles, Journal of power sources 311 (2016) 91-102.
[37] W. Lehnert, L. Lüke, R.C. Samsun, High temperature polymer electrolyte fuel cells,
Wiley-VCH: Weinheim, Germany2016.
Bibliography Page 199 of 231
[38] J.A. Asensio, E.M. Sánchez, P. Gómez-Romero, Proton-conducting membranes based on
benzimidazole polymers for high-temperature PEM fuel cells. A chemical quest, Chemical
Society Reviews 39(8) (2010) 3210-3239.
[39] A. Bauer, K. Lee, C. Song, Y. Xie, J. Zhang, R. Hui, Pt nanoparticles deposited on TiO2
based nanofibers: Electrochemical stability and oxygen reduction activity, Journal of Power
Sources 195(10) (2010) 3105-3110.
[40] J. Topler, J. Lehmann, Hydrogen and fuel cell : technologies and market perspectives,
Springer, Berlin, 2015.
[41] R.C. Alkire, A. Kapturkiewicz, Advances in electrochemical science and engineering.
Volume 5, Wiley-VCH, Weinheim ;, 1997.
[42] J. Peron, A. Mani, X. Zhao, D. Edwards, M. Adachi, T. Soboleva, Z. Shi, Z. Xie, T.
Navessin, S. Holdcroft, Properties of Nafion ® NR-211 membranes for PEMFCs, Journal of
membrane science 356(1) (2010) 44-51.
[43] T.A. Zawodzinski, C. Derouin, S. Rodzinski, R.J. Sherman, V.T. Smith, T.E. Springer, S.
Gottesfeld, Water uptake by and transport through Nafion® 117 membranes, Journal of the
Electrochemical Society 140(4) (1993) 1041-1047.
[44] H. Liu, H.A. Gasteiger, A. Laconti, J. Zhang, Factors Impacting Chemical Degradation Of
Perfluorinated Sulfonic Acid Ionomers, 2019, pp. 283-293.
[45] Impacting Factors for Chemical Degradation of Perfluorinated Sulfonic Acid Ionomer,
Meeting abstracts (Electrochemical Society) (2005).
[46] Chemical Aspects of Membrane Degradation, Meeting abstracts (Electrochemical Society)
(2007).
[47] S.J. Peighambardoust, S. Rowshanzamir, M. Amjadi, Review of the proton exchange
membranes for fuel cell applications, International journal of hydrogen energy 35(17) (2010)
9349-9384.
Bibliography Page 200 of 231
[48] Y.-K. Choe, E. Tsuchida, T. Ikeshoji, S. Yamakawa, S.-A. H, Nature of proton dynamics
in a polymer electrolyte membrane, nafion: a first-principles molecular dynamics study,
Physical chemistry chemical physics : PCCP 11(20) (2009) 3892-3899.
[49] J. Larminie, A. Dicks, Fuel cell systems explained, 2nd ed. ed., J. Wiley, Chichester, West
Sussex, 2003.
[50] P. Kallem, N. Yanar, H. Choi, Nanofiber-based proton exchange membranes: Development
of aligned electrospun nanofibers for polymer electrolyte fuel cell applications, ACS
Sustainable Chemistry & Engineering 7(2) (2018) 1808-1825.
[51] J. Zhang, Y. Tang, C. Song, J. Zhang, Polybenzimidazole-membrane-based PEM fuel cell
in the temperature range of 120–200 C, Journal of Power Sources 172(1) (2007) 163-171.
[52] J. Lobato, H. Zamora, J. Plaza, P. Cañizares, M.A. Rodrigo, Enhancement of high
temperature PEMFC stability using catalysts based on Pt supported on SiC based materials,
Applied Catalysis B: Environmental 198 (2016) 516-524.
[53] K.-D. Kreuer, S.J. Paddison, E. Spohr, M. Schuster, Transport in Proton Conductors for
Fuel-Cell Applications: Simulations, Elementary Reactions, and Phenomenology, Chemical
reviews 104(10) (2004) 4637-4678.
[54] N.W. DeLuca, Y.A. Elabd, Polymer electrolyte membranes for the direct methanol fuel
cell: A review, Journal of polymer science. Part B, Polymer physics 44(16) (2006) 2201-2225.
[55] M.A. Haque, A.B. Sulong, K.S. Loh, E.H. Majlan, T. Husaini, R.E. Rosli, Acid doped
polybenzimidazoles based membrane electrode assembly for high temperature proton
exchange membrane fuel cell: A review, International journal of hydrogen energy 42(14) (2017)
9156-9179.
[56] H. Su, Q. Xu, J. Chong, H. Li, C. Sita, S. Pasupathi, Eliminating micro-porous layer from
gas diffusion electrode for use in high temperature polymer electrolyte membrane fuel cell,
Journal of Power Sources 341 (2017) 302-308.
Bibliography Page 201 of 231
[57] Y. Hu, Y. Jiang, J.O. Jensen, L.N. Cleemann, Q. Li, Catalyst evaluation for oxygen
reduction reaction in concentrated phosphoric acid at elevated temperatures, Journal of Power
Sources 375 (2018) 77-81.
[58] F.J. Pinar, P. Cañizares, M.A. Rodrigo, D. Úbeda, J. Lobato, Long-term testing of a high-
temperature proton exchange membrane fuel cell short stack operated with improved
polybenzimidazole-based composite membranes, Journal of Power Sources 274 (2015) 177-
185.
[59] X. Wu, K. Scott, A H2SO4 Loaded Polybenzimidazole (PBI) Membrane for High
Temperature PEMFC, Fuel cells (Weinheim an der Bergstrasse, Germany) 12(4) (2012) 583-
588.
[60] J.A. Asensio, E.M. Sánchez, P. Gómez-Romero, Proton-conducting membranes based on
benzimidazole polymers for high-temperature PEM fuel cells. A chemical quest, Chemical
Society reviews 39(8) (2010) 321-3239.
[61] M. Mathias, J. Roth, J. Fleming, W. Lehnert, Diffusion media materials and
characterisation, Handbook of fuel cells (2010).
[62] M. Yazici, Mass transfer layer for liquid fuel cells, Journal of power sources 166(2) (2007)
424-429.
[63] V. Gurau, M.J. Bluemle, E.S. De Castro, Y.-M. Tsou, T.A. Zawodzinski, J. Adin Mann,
Characterization of transport properties in gas diffusion layers for proton exchange membrane
fuel cells. 2. Absolute permeability, Journal of power sources 165(2) (2007) 793-802.
[64] M.S. Wilson, J.A. Valerio, S. Gottesfeld, Low platinum loading electrodes for polymer
electrolyte fuel cells fabricated using thermoplastic ionomers, Electrochimica acta 40(3) (1995)
355-363.
[65] X. Wang, H. Zhang, J. Zhang, H. Xu, X. Zhu, J. Chen, B. Yi, A bi-functional micro-porous
layer with composite carbon black for PEM fuel cells, Journal of power sources 162(1) (2006)
Bibliography Page 202 of 231
474-479.
[66] Z. Xie, G. Chen, X. Yu, M. Hou, Z. Shao, S. Hong, C. Mu, Carbon nanotubes grown in
situ on carbon paper as a microporous layer for proton exchange membrane fuel cells,
International journal of hydrogen energy 40(29) (2015) 8958-8965.
[67] R. Sandstrom, J. Ekspong, A. Annamalai, T. Sharifi, A. Klechikov, T. Wågberg,
Fabrication of microporous layer – free hierarchical gas diffusion electrode as a low Pt-loading
PEMFC cathode by direct growth of helical carbon nanofibers, RSC advances 8(72) (2018)
41566-41574.
[68] S.-Y. Lin, M.-H. Chang, Effect of microporous layer composed of carbon nanotube and
acetylene black on polymer electrolyte membrane fuel cell performance, International journal
of hydrogen energy 40(24) (2015) 7879-7885.
[69] F. Hendricks, J. Chamier, S. Tanaka, Membrane electrode assembly performance of a
standalone microporous layer on a metallic gas diffusion layer, Journal of power sources 464
(2020) 228222.
[70] R.M.Q. Mello, E.A. Ticianelli, Kinetic study of the hydrogen oxidation reaction on
platinum and Nafion ® covered platinum electrodes, Electrochimica acta 42(6) (1997) 1031-
1039.
[71] I. Katsounaros, W.B. Schneider, J.C. Meier, U. Benedikt, P. Ulrich Biedermann, A.A. Auer,
K.J.J. Mayrhofer, Hydrogen peroxide electrochemistry on platinum: towards understanding the
oxygen reduction reaction mechanism, Physical chemistry chemical physics : PCCP 14(20)
(2012) 7384-7391.
[72] L.N. Cleemann, F. Buazar, Q. Li, J.O. Jensen, C. Pan, T. Steenberg, S. Dai, N.J. Bjerrum,
Catalyst degradation in high temperature proton exchange membrane fuel cells based on acid
doped polybenzimidazole membranes, Fuel Cells 13(5) (2013) 822-831.
[73] H. Su, T.-C. Jao, O. Barron, B.G. Pollet, S. Pasupathi, Low platinum loading for high
Bibliography Page 203 of 231
temperature proton exchange membrane fuel cell developed by ultrasonic spray coating
technique, Journal of Power Sources 267 (2014) 155-159.
[74] W. Lee, H. Yang, K. Park, B. Choi, S. Yi, W. Kim, Synergistic effect of boron/nitrogen co-
doping into graphene and intercalation of carbon black for Pt-BCN-Gr/CB hybrid catalyst on
cell performance of polymer electrolyte membrane fuel cell, Energy 96 (2016) 314-324.
[75] G. Lu, H. Yang, Y. Zhu, T. Huggins, Z.J. Ren, Z. Liu, W. Zhang, Synthesis of a conjugated
porous Co (II) porphyrinylene–ethynylene framework through alkyne metathesis and its
catalytic activity study, Journal of Materials Chemistry A 3(9) (2015) 4954-4959.
[76] M. Hosseinpour, M. Sahoo, M. Perez-Page, S.R. Baylis, F. Patel, S.M. Holmes, Improving
the performance of direct methanol fuel cells by implementing multilayer membranes blended
with cellulose nanocrystals, International Journal of Hydrogen Energy 44(57) (2019) 30409-
30419.
[77] D. Li, M.B. Müller, S. Gilje, R.B. Kaner, G.G. Wallace, Processable aqueous dispersions
of graphene nanosheets, Nature nanotechnology 3(2) (2008) 101-105.
[78] H. Kang, S.H. Kwon, R. Lawler, J.H. Lee, G. Doo, H.-T. Kim, S.-D. Yim, S.S. Jang, S.G.
Lee, Nanostructures of Nafion Film at Platinum/Carbon Surface in Catalyst Layer of PEMFC:
Molecular Dynamics Simulation Approach, Journal of physical chemistry. C 124(39) (2020)
21386-21395.
[79] A. Dicks, D.A.J. Rand, Fuel cell systems explained, Third edition. ed., Wiley, Hoboken,
NJ, USA, 2018.
[80] R.P. O'Hayre, Fuel cell fundamentals, John Wiley & Sons, Hoboken, N.J, 2006.
[81] W. Vielstich, A. Lamm, H.A. Gasteiger, Handbook of fuel cells: fundamentals technology
and applications, Wiley New York2003.
[82] K.I. Ozoemena, S. Chen, Nanomaterials for fuel cell catalysis, Springer, Switzerland, 2016.
[83] A. Heinzel, B.C.H. Steele, Materials for fuel-cell technologies, Nature (London) 414(6861)
Bibliography Page 204 of 231
(2001) 345-352.
[84] S.T. Thompson, B.D. James, J.M. Huya-Kouadio, C. Houchins, D.A. DeSantis, R.
Ahluwalia, A.R. Wilson, G. Kleen, D. Papageorgopoulos, Direct hydrogen fuel cell electric
vehicle cost analysis: System and high-volume manufacturing description, validation, and
outlook, Journal of power sources 399(C) (2018) 304-313.
[85] Y.-L. Liu, Y.-H. Su, C.-M. Chang, Suryani, D.-M. Wang, J.-Y. Lai, Preparation and
applications of Nafion-functionalized multiwalled carbon nanotubes for proton exchange
membrane fuel cells, Journal of materials chemistry 2(21) (2010) 449-4416.
[86] K. Srinivasu, S.K. Ghosh, Transition Metal Decorated Graphyne: An Efficient Catalyst
for Oxygen Reduction Reaction, Journal of physical chemistry. C 117(49) (2013) 26021-26028.
[87] S. Park, H. Yang, D. Lee, K. Park, W. Kim, Electrochemical properties of
polyethyleneimine-functionalized Pt-PEI/carbon black as a catalyst for polymer electrolyte
membrane fuel cell, Electrochimica Acta 125 (2014) 141-148.
[88] J. Wang, G. Yin, Y. Shao, S. Zhang, Z. Wang, Y. Gao, Effect of carbon black support
corrosion on the durability of Pt/C catalyst, Journal of Power sources 171(2) (2007) 331-339.
[89] S.D. Knights, K.M. Colbow, J. St-Pierre, D.P. Wilkinson, Aging mechanisms and lifetime
of PEFC and DMFC, Journal of power sources 127(1) (2004) 127-134.
[90] A.V. Virkar, Y. Zhou, Mechanism of Catalyst Degradation in Proton Exchange Membrane
Fuel Cells, Journal of the Electrochemical Society 154(6) (2007) B540.
[91] P.J. Ferreira, G.J. la O’, Y. Shao-Horn, D. Morgan, R. Makharia, S. Kocha, H.A. Gasteiger,
Instability of Pt∕C Electrocatalysts in Proton Exchange Membrane Fuel Cells, Journal of the
Electrochemical Society 152(11) (2005) A2256.
[92] K. Yasuda, A. Taniguchi, T. Akita, T. Ioroi, Z. Siroma, Platinum dissolution and deposition
in the polymer electrolyte membrane of a PEM fuel cell as studied by potential cycling,
Physical chemistry chemical physics : PCCP 8(6) (2006) 746-752.
Bibliography Page 205 of 231
[93] E. Guilminot, A. Corcella, M. Chatenet, F. Maillard, F. Charlot, G. Berthome, C. Iojoiu,
J.Y. Sanchez, E. Rossinot, E. Claude, Membrane and Active Layer Degradation upon PEMFC
Steady-State Operation, Journal of the Electrochemical Society 154(11) (2007) B1106.
[94] M.F. Labata, G. Li, J. Ocon, P.-Y.A. Chuang, Insights on platinum-carbon catalyst
degradation mechanism for oxygen reduction reaction in acidic and alkaline media, Journal of
power sources 487 (2021).
[95] X. Wang, R. Kumar, D.J. Myers, Effect of Voltage on Platinum Dissolution,
Electrochemical and solid-state letters 9(5) (2006) A225.
[96] L. Tang, B. Han, K. Persson, C. Friesen, T. He, K. Sieradzki, G. Ceder, Electrochemical
Stability of Nanometer-Scale Pt Particles in Acidic Environments, Journal of the American
Chemical Society 132(2) (2010) 596-600.
[97] V.A.T. Dam, d.F.A. Bruijn, The stability of PEMFC electrodes : platinum dissolution vs
potential and temperature investigated by quartz crystal microbalance, Journal of the
Electrochemical Society 154(5) (2007) B494-B499.
[98] R.L. Borup, J.R. Davey, F.H. Garzon, D.L. Wood, M.A. Inbody, PEM fuel cell
electrocatalyst durability measurements, Journal of power sources 163(1) (2006) 76-81.
[99] S. Mu, C. Xu, Y. Gao, H. Tang, M. Pan, Accelerated durability tests of catalyst layers with
various pore volume for catalyst coated membranes applied in PEM fuel cells, International
journal of hydrogen energy 35(7) (2010) 2872-2876.
[100] L.C. Colmenares, A. Wurth, Z. Jusys, R.J. Behm, Model study on the stability of carbon
support materials under polymer electrolyte fuel cell cathode operation conditions, Journal of
power sources 190(1) (2009) 14-24.
[101] S.-Y. Huang, P. Ganesan, S. Park, B.N. Popov, Development of a Titanium Dioxide-
Supported Platinum Catalyst with Ultrahigh Stability for Polymer Electrolyte Membrane Fuel
Cell Applications, Journal of the American Chemical Society 131(39) (2009) 13898-13899.
Bibliography Page 206 of 231
[102] Y. Shao, G. Yin, Y. Gao, Understanding and approaches for the durability issues of Pt-
based catalysts for PEM fuel cell, Journal of power sources 171(2) (2007) 558-566.
[103] L. Li, Y. Xing, Electrochemical Durability of Carbon Nanotubes in Noncatalyzed and
Catalyzed Oxidations, Journal of the Electrochemical Society 153(10) (2006) A1823.
[104] L. Roen, C. Paik, T. Jarvi, Electrocatalytic corrosion of carbon support in PEMFC
cathodes, Electrochemical and Solid State Letters 7(1) (2003) A19.
[105] J. Willsau, J. Heitbaum, The influence of Pt-activation on the corrosion of carbon in gas
diffusion electrodes—A dems study, Journal of electroanalytical chemistry and interfacial
electrochemistry 161(1) (1984) 93-101.
[106] P. Stonehart, Carbon substrates for phosphoric acid fuel cell cathodes, Carbon 22(4-5)
(1984) 423-431.
[107] S. Maass, F. Finsterwalder, G. Frank, R. Hartmann, C. Merten, Carbon support oxidation
in PEM fuel cell cathodes, Journal of Power Sources 176(2) (2008) 444-451.
[108] L. Castanheira, W.O. Silva, F.H. Lima, A. Crisci, L. Dubau, F.d.r. Maillard, Carbon
corrosion in proton-exchange membrane fuel cells: effect of the carbon structure, the
degradation protocol, and the gas atmosphere, Acs Catalysis 5(4) (2015) 2184-2194.
[109] R. Savinell, E. Yeager, D. Tryk, U. Landau, J. Wainright, D. Weng, K. Lux, M. Litt, C.
Rogers, A polymer electrolyte for operation at temperatures up to 200°C, Journal of the
Electrochemical Society 141(4) (1994) L46-L48.
[110] K. A. Perry, K. L. More, E. Andrew Payzant, R.A. Meisner, B.G. Sumpter, B.C.
Benicewicz, A comparative study of phosphoric acid‐doped m‐PBI membranes, Journal of
polymer science. Part B, Polymer physics 52(1) (2014) 26-35.
[111] S. Yu, L. Xiao, B.C. Benicewicz, Durability Studies of PBI‐based High Temperature
PEMFCs, Fuel cells (Weinheim an der Bergstrasse, Germany) 8(3‒4) (2008) 165-174.
[112] Y. Devrim, E.D. Arıca, A. Albostan, Graphene based catalyst supports for high
Bibliography Page 207 of 231
temperature PEM fuel cell application, International journal of hydrogen energy 43(26) (2018)
11820-11829.
[113] X. Guo, H. Zhang, Performance analyses of a combined system consisting of high-
temperature polymer electrolyte membrane fuel cells and thermally regenerative
electrochemical cycles, Energy 193 (2020) 116720.
[114] S. Satyapal, DOE Hydrogen program-FY 2009 Annual progress report, Department of
Energy, USA (2009).
[115] D.D.W. Group, Rotating Disk-Electrode Aqueous Electrolyte Accelerated Stress Tests
for PGM Electrocatalyst/Support Durability Evaluation, in: D.o. Energy (Ed.) 2011.
[116] M. Uchida, Y. Aoyama, M. Tanabe, N. Yanagihara, N. Eda, A. Ohta, Influences of both
carbon supports and heat-treatment of supported catalyst on electrochemical oxidation of
methanol, Journal of the Electrochemical Society 142(8) (1995) 2572-2576.
[117] E. Antolini, R.R. Passos, E.A. Ticianelli, Effects of the carbon powder characteristics in
the cathode gas diffusion layer on the performance of polymer electrolyte fuel cells, Journal of
power sources 109(2) (2002) 477-482.
[118] P. Gallezot, S. Chaumet, A. Perrard, P. Isnard, Catalytic Wet Air Oxidation of Acetic Acid
on Carbon-Supported Ruthenium Catalysts, Journal of catalysis 168(1) (1997) 104-109.
[119] C. Rudolf, F. Abi-Ghaida, B. Dragoi, A. Ungureanu, A. Mehdi, E. Dumitriu, An efficient
route to prepare highly dispersed metallic copper nanoparticles on ordered mesoporous silica
with outstanding activity for hydrogenation reactions, Catalysis science & technology 5(7)
(2015) 3735-3745.
[120] C. Alegre, M.E. Gálvez, E. Baquedano, R. Moliner, E. Pastor, M.J.s. Lázaro, Oxygen-
Functionalized Highly Mesoporous Carbon Xerogel Based Catalysts for Direct Methanol Fuel
Cell Anodes, Journal of physical chemistry. C 117(25) (2013) 13045-13058.
[121] J. Qi, L. Jiang, Q. Tang, S. Zhu, S. Wang, B. Yi, G. Sun, Synthesis of graphitic
Bibliography Page 208 of 231
mesoporous carbons with different surface areas and their use in direct methanol fuel cells,
Carbon (New York) 50(8) (2012) 2824-2831.
[122] B. Ladewig, S.P. Jiang, Y. Yan, Materials for low-temperature fuel cells, Wiley-VCH,
Verlag GmbH & Company KGaA, Weinheim, Germany, 2015.
[123] T. Maiyalagan, T.O. Alaje, K. Scott, Highly Stable Pt–Ru Nanoparticles Supported on
Three-Dimensional Cubic Ordered Mesoporous Carbon (Pt–Ru/CMK-8) as Promising
Electrocatalysts for Methanol Oxidation, Journal of physical chemistry. C 116(3) (2012) 2630-
2638.
[124] L. Calvillo, V. Celorrio, R. Moliner, A.B. Garcia, I. Camean, M.J. Lazaro, Comparative
study of Pt catalysts supported on different high conductive carbon materials for methanol and
ethanol oxidation, Electrochimica acta 102 (2013) 19-27.
[125] Z.-B. Wang, C.-R. Zhao, P.-F. Shi, Y.-S. Yang, Z.-B. Yu, W.-K. Wang, G.-P. Yin, Effect
of a Carbon Support Containing Large Mesopores on the Performance of a Pt−Ru−Ni/C
Catalyst for Direct Methanol Fuel Cells, Journal of physical chemistry. C 114(1) (2010) 672-
677.
[126] J. Zhang, L. Ma, M. Gan, F. Yang, S. Fu, X. Li, Well-dispersed platinum nanoparticles
supported on hierarchical nitrogen-doped porous hollow carbon spheres with enhanced activity
and stability for methanol electrooxidation, Journal of power sources 288 (2015) 42-52.
[127] R. Silva, D. Voiry, M. Chhowalla, T. Asefa, Efficient Metal-Free Electrocatalysts for
Oxygen Reduction: Polyaniline-Derived N- and O‑Doped Mesoporous Carbons, Journal of the
American Chemical Society 135(21) (2013) 7823-7826.
[128] H. Chang, S.H. Joo, C. Pak, Synthesis and characterization of mesoporous carbon for
fuel cell applications, Journal of materials chemistry 17(30) (2007) 3078.
[129] F. Su, J. Zeng, X. Bao, Y. Yu, J.Y. Lee, X.S. Zhao, Preparation and Characterization of
Highly Ordered Graphitic Mesoporous Carbon as a Pt Catalyst Support for Direct Methanol
Bibliography Page 209 of 231
Fuel Cells, Chemistry of materials 17(15) (2005) 3960-3967.
[130] J. Ding, K.-Y. Chan, J. Ren, F.-s. Xiao, Platinum and platinum–ruthenium nanoparticles
supported on ordered mesoporous carbon and their electrocatalytic performance for fuel cell
reactions, Electrochimica acta 50(15) (2005) 3131-3141.
[131] F. Su, C.K. Poh, Z. Tian, G. Xu, G. Koh, Z. Wang, Z. Liu, J. Lin, Electrochemical
Behavior of Pt Nanoparticles Supported on Meso- and Microporous Carbons for Fuel Cells,
Energy & fuels 24(7) (2010) 3727-3732.
[132] H.B. Ray, A.Z. Anvar, A.d.H. Walt, Carbon Nanotubes: The Route toward Applications,
Science (American Association for the Advancement of Science) 297(5582) (2002) 787-792.
[133] F.L.D.V. Michael, H.T. Sameh, H.B. Ray, A.J. Hart, Carbon Nanotubes: Present and
Future Commercial Applications, Science (American Association for the Advancement of
Science) 339(6119) (2013) 535-539.
[134] J. Fan, C. Yu, F. Gao, J. Lei, B. Tian, L. Wang, Q. Luo, B. Tu, W. Zhou, D. Zhao, Cubic
Mesoporous Silica with Large Controllable Entrance Sizes and Advanced Adsorption
Properties, Angewandte Chemie (International ed.) 42(27) (2003) 3146-3150.
[135] P. Serp, M. Corrias, P. Kalck, Carbon nanotubes and nanofibers in catalysis, Applied
Catalysis A, General 253(2) (2003) 337-358.
[136] G.G. Wildgoose, C.E. Banks, R.G. Compton, Metal Nanoparticles and Related Materials
Supported on Carbon Nanotubes: Methods and Applications, Small (Weinheim an der
Bergstrasse, Germany) 2(2) (2006) 182-193.
[137] K. Lee, J. Zhang, H. Wang, D.P. Wilkinson, Progress in the synthesis of carbon nanotube-
and nanofiber-supported Pt electrocatalysts for PEM fuel cell catalysis, Journal of applied
electrochemistry 36(5) (2006) 507-522.
[138] C. Kim, Y.J. Kim, Y.A. Kim, T. Yanagisawa, K.C. Park, M. Endo, M.S. Dresselhaus,
High performance of cup-stacked-type carbon nanotubes as a Pt–Ru catalyst support for fuel
Bibliography Page 210 of 231
cell applications, Journal of applied physics 96(10) (2004) 5903-5905.
[139] W. Li, C. Liang, W. Zhou, J. Qiu, H. Li, G. Sun, Q. Xin, Homogeneous and controllable
Pt particles deposited on multi-wall carbon nanotubes as cathode catalyst for direct methanol
fuel cells, Carbon (New York) 42(2) (2004) 436-439.
[140] T. Matsumoto, T. Komatsu, K. Arai, T. Yamazaki, M. Kijima, H. Shimizu, Y. Takasawa,
J. Nakamura, Reduction of Pt usage in fuel cell electrocatalysts with carbon nanotube
electrodes, Chemical communications (Cambridge, England) (7) (2004) 840-841.
[141] H. Chu, Y. Shen, L. Lin, X. Qin, G. Feng, Z. Lin, J. Wang, H. Liu, Y. Li, Ionic‐Liquid‐
Assisted Preparation of Carbon Nanotube‐Supported Uniform Noble Metal Nanoparticles and
Their Enhanced Catalytic Performance, Advanced functional materials 20(21) (2010) 3747-
3752.
[142] W. Tokarz, G. Lota, E. Frackowiak, A. Czerwiński, P. Piela, Fuel cell testing of Pt–Ru
catalysts supported on differently prepared and pretreated carbon nanotubes, Electrochimica
acta 98 (2013) 94-103.
[143] W. Li, C. Liang, W. Zhou, J. Qiu, Zhou, G. Sun, Q. Xin, Preparation and Characterization
of Multiwalled Carbon Nanotube-Supported Platinum for Cathode Catalysts of Direct
Methanol Fuel Cells, The journal of physical chemistry. B 107(26) (2003) 6292-6299.
[144] H. Huang, D. Sun, X. Wang, Low-Defect MWNT–Pt Nanocomposite as a High
Performance Electrocatalyst for Direct Methanol Fuel Cells, Journal of physical chemistry. C
115(39) (2011) 19405-19412.
[145] A.K. Geim, K.S. Novoselov, The rise of graphene, Nature materials 6(3) (2007) 183-191.
[146] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V.
Grigorieva, A.A. Firsov, Electric Field Effect in Atomically Thin Carbon Films, Science
(American Association for the Advancement of Science) 306(5696) (2004) 666-669.
[147] G.H.B. Dommett, R.S. Ruoff, E.J. Zimney, S.T. Nguyen, R.D. Piner, D.A. Dikin, S.
Bibliography Page 211 of 231
Stankovich, K.M. Kohlhaas, E.A. Stach, Graphene-based composite materials, Nature (London)
442(7100) (2006) 282-286.
[148] C.N.R. Rao, A.K. Sood, K.S. Subrahmanyam, A. Govindaraj, Graphene: The New Two‐
Dimensional Nanomaterial, Angewandte Chemie (International ed.) 48(42) (2009) 7752-7777.
[149] M. Liu, R. Zhang, W. Chen, Graphene-Supported Nanoelectrocatalysts for Fuel Cells:
Synthesis, Properties, and Applications, Chemical reviews 114(10) (2014) 5117-5160.
[150] I. Do, L.T. Drzal, Ionic Liquid-Assisted Synthesis of Pt Nanoparticles onto Exfoliated
Graphite Nanoplatelets for Fuel Cells, ACS applied materials & interfaces 6(15) (2014) 12126-
12136.
[151] L. Gao, W. Yue, S. Tao, L. Fan, Novel Strategy for Preparation of Graphene-Pd, Pt
Composite, and Its Enhanced Electrocatalytic Activity for Alcohol Oxidation, Langmuir 29(3)
(2013) 957-964.
[152] C. Huang, C. Li, G. Shi, Graphene based catalysts, Energy & environmental science 5(10)
(2012) 8848.
[153] W. Shuangyin, W. Xin, J. San Ping, Self-assembly of mixed Pt and Au nanoparticles on
PDDA-functionalized graphene as effective electrocatalysts for formic acid oxidation of fuel
cells, Physical chemistry chemical physics : PCCP 13(15) (2011) 6883-6891.
[154] H.-W. Ha, I.Y. Kim, S.-J. Hwang, R.S. Ruoff, One-Pot Synthesis of Platinum
Nanoparticles Embedded on Reduced Graphene Oxide for Oxygen Reduction in Methanol Fuel
Cells, Electrochemical and solid-state letters 14(7) (2011) B70.
[155] J. Zhao, Z. Liu, H. Li, W. Hu, C. Zhao, P. Zhao, D. Shi, Development of a Highly Active
Electrocatalyst via Ultrafine Pd Nanoparticles Dispersed on Pristine Graphene, Langmuir 31(8)
(2015) 2576-2583.
[156] Y. Li, W. Gao, L. Ci, C. Wang, P.M. Ajayan, Catalytic performance of Pt nanoparticles
on reduced graphene oxide for methanol electro-oxidation, Carbon (New York) 48(4) (2010)
Bibliography Page 212 of 231
1124-1130.
[157] T. Tomai, Y. Kawaguchi, S. Mitani, I. Honma, Pt sub-nano/nanoclusters stabilized at the
edge of nanographene sheets and their catalytic performance, Electrochimica acta 92 (2013)
421-426.
[158] K. Ji, G. Chang, M. Oyama, X. Shang, X. Liu, Y. He, Efficient and clean synthesis of
graphene supported platinum nanoclusters and its application in direct methanol fuel cell,
Electrochimica acta 85 (2012) 84-89.
[159] Y. Li, L. Tang, J. Li, Preparation and electrochemical performance for methanol oxidation
of pt/graphene nanocomposites, Electrochemistry communications 11(4) (2009) 846-849.
[160] B. Seger, P.V. Kamat, Electrocatalytically active graphene-platinum nanocomposites.
Role of 2-D carbon support in PEM fuel cells, The Journal of Physical Chemistry C 113(19)
(2009) 7990-7995.
[161] Y. Li, Y. Li, E. Zhu, T. McLouth, C.-Y. Chiu, X. Huang, Y. Huang, Stabilization of high-
performance oxygen reduction reaction Pt electrocatalyst supported on reduced graphene
oxide/carbon black composite, Journal of the American Chemical Society 134(30) (2012)
12326-12329.
[162] K. Parvez, Z.-S. Wu, R. Li, X. Liu, R. Graf, X. Feng, K. Mullen, Exfoliation of graphite
into graphene in aqueous solutions of inorganic salts, Journal of the American Chemical
Society 136(16) (2014) 6083-6091.
[163] F. Sharif, A.S. Zeraati, P. Ganjeh-Anzabi, N. Yasri, M. Perez-Page, S.M. Holmes, U.
Sundararaj, M. Trifkovic, E.P. Roberts, Synthesis of a high-temperature stable
electrochemically exfoliated graphene, Carbon 157 (2020) 681-692.
[164] F. Beck, J. Jiang, H. Krohn, Potential oscillations during galvanostatic overoxidation of
graphite in aqueous sulphuric acids, Journal of Electroanalytical Chemistry 389(1-2) (1995)
161-165.
Bibliography Page 213 of 231
[165] N. Liu, F. Luo, H. Wu, Y. Liu, C. Zhang, J. Chen, One‐step ionic‐liquid‐assisted
electrochemical synthesis of ionic‐liquid‐functionalized graphene sheets directly from graphite,
Advanced Functional Materials 18(10) (2008) 1518-1525.
[166] K. Parvez, R. Li, S.R. Puniredd, Y. Hernandez, F. Hinkel, S. Wang, X. Feng, K. Müllen,
Electrochemically exfoliated graphene as solution-processable, highly conductive electrodes
for organic electronics, ACS nano 7(4) (2013) 3598-3606.
[167] C.-Y. Su, A.-Y. Lu, Y. Xu, F.-R. Chen, A.N. Khlobystov, L.-J. Li, High-quality thin
graphene films from fast electrochemical exfoliation, ACS nano 5(3) (2011) 2332-2339.
[168] A. Marinkas, F. Arena, J. Mitzel, G.M. Prinz, A. Heinzel, V. Peinecke, H. Natter,
Graphene as catalyst support: The influences of carbon additives and catalyst preparation
methods on the performance of PEM fuel cells, Carbon (New York) 58 (2013) 139-150.
[169] E. Daş, B.Y. Kaplan, S.A. Gürsel, A.B. Yurtcan, Graphene nanoplatelets-carbon black
hybrids as an efficient catalyst support for Pt nanoparticles for polymer electrolyte membrane
fuel cells, Renewable Energy 139 (2019) 1099-1110.
[170] H.N. Yang, D.C. Lee, K.W. Park, W.J. Kim, Platinum–boron doped graphene intercalated
by carbon black for cathode catalyst in proton exchange membrane fuel cell, Energy (Oxford)
89 (2015) 500-510.
[171] H. Wang, T. Maiyalagan, X. Wang, Review on recent progress in nitrogen-doped
graphene: synthesis, characterization, and its potential applications, Acs Catalysis 2(5) (2012)
781-794.
[172] H. Wang, W. Wang, Y.Y. Xu, S. Dong, J. Xiao, F. Wang, H. Liu, B.Y. Xia, Hollow
Nitrogen-Doped Carbon Spheres with Fe3O4 Nanoparticles Encapsulated as a Highly Active
Oxygen-Reduction Catalyst, ACS applied materials & interfaces 9(12) (2017) 10610-10617.
[173] Y. Shao, J. Sui, G. Yin, Y. Gao, Nitrogen-doped carbon nanostructures and their
composites as catalytic materials for proton exchange membrane fuel cell, Applied Catalysis
Bibliography Page 214 of 231
B: Environmental 79(1) (2008) 89-99.
[174] B. Xiong, Y. Zhou, Y. Zhao, J. Wang, X. Chen, R. O’Hayre, Z. Shao, The use of nitrogen-
doped graphene supporting Pt nanoparticles as a catalyst for methanol electrocatalytic
oxidation, Carbon (New York) 52 (2013) 181-192.
[175] G. Yang, Y. Li, R.K. Rana, J.-J. Zhu, Pt–Au/nitrogen-doped graphene nanocomposites
for enhanced electrochemical activities, Journal of materials chemistry. A, Materials for energy
and sustainability 1(5) (2013) 1754-1762.
[176] P. Błoński, J.i. Tucek, Z.k. Sofer, V. Mazánek, M. Petr, M. Pumera, M. Otyepka, R. Zboril,
Doping with Graphitic Nitrogen Triggers Ferromagnetism in Graphene, Journal of the
American Chemical Society 139(8) (2017) 3171-3180.
[177] P. Kannan, T. Maiyalagan, N.G. Sahoo, M. Opallo, Nitrogen doped graphene nanosheet
supported platinum nanoparticles as high performance electrochemical homocysteine
biosensors, Journal of materials chemistry. B, Materials for biology and medicine 1(36) (2013)
4655-4666.
[178] D. He, Y. Jiang, H. Lv, M. Pan, S. Mu, Nitrogen-doped reduced graphene oxide supports
for noble metal catalysts with greatly enhanced activity and stability, Applied catalysis. B,
Environmental 132-133 (2013) 379-388.
[179] Y. Zhou, K. Neyerlin, T.S. Olson, S. Pylypenko, J. Bult, H.N. Dinh, T. Gennett, Z. Shao,
R. O'Hayre, Enhancement of Pt and Pt-alloy fuel cell catalyst activity and durability via
nitrogen-modified carbon supports, Energy & environmental science 3(1) (2010) 1437-1446.
[180] Y. Shao, J. Sui, G. Yin, Y. Gao, Nitrogen-doped carbon nanostructures and their
composites as catalytic materials for proton exchange membrane fuel cell, Applied catalysis.
B, Environmental 79(1) (2008) 89-99.
[181] L.-S. Zhang, X.-Q. Liang, W.-G. Song, Z.-Y. Wu, Identification of the nitrogen species
on N-doped graphene layers and Pt/NG composite catalyst for direct methanol fuel cell,
Bibliography Page 215 of 231
Physical chemistry chemical physics : PCCP 12(38) (2010) 12055-12059.
[182] Y. Xin, J.-g. Liu, X. Jie, W. Liu, F. Liu, Y. Yin, J. Gu, Z. Zou, Preparation and
electrochemical characterization of nitrogen doped graphene by microwave as supporting
materials for fuel cell catalysts, Electrochimica acta 60 (2012) 354-358.
[183] H. Chen, Y. Yang, Z. Hu, K. Huo, Y. Ma, Y. Chen, X. Wang, Y. Lu, Synergism of C5N
Six-Membered Ring and Vapor−Liquid−Solid Growth of CNxNanotubes with Pyridine
Precursor, The journal of physical chemistry. B 110(33) (2006) 16422-16427.
[184] F. Su, Z. Tian, C.K. Poh, Z. Wang, S.H. Lim, Z. Liu, J. Lin, Pt Nanoparticles Supported
on Nitrogen-Doped Porous Carbon Nanospheres as an Electrocatalyst for Fuel Cells,
Chemistry of materials 22(3) (2010) 832-839.
[185] R. Liu, D. Wu, X. Feng, K. Müllen, Nitrogen‐Doped Ordered Mesoporous Graphitic
Arrays with High Electrocatalytic Activity for Oxygen Reduction, Angewandte Chemie
(International ed.) 49(14) (2010) 2565-2569.
[186] M. Zhang, M.M. Zhang, Y.S. Wei, X.Y. Dong, S.Q. Zang, Facile Synthesis of a
Heteroatoms′ Quaternary‐Doped Porous Carbon as an Efficient and Stable Metal‐Free Catalyst
for Oxygen Reduction, ChemistrySelect (Weinheim) 2(21) (2017) 6129-6134.
[187] S.U. Fabing, X.S. Zhao, W. Yong, W. Likui, J.Y. Lee, Hollow carbon spheres with a
controllable shell structure, Journal of materials chemistry 16(45) (2006) 4413-4419.
[188] H. Kim, K. Lee, W.O.O. Seong Ihl, Y. Jung, On the mechanism of enhanced oxygen
reduction reaction in nitrogen-doped graphene nanoribbons, Physical chemistry chemical
physics : PCCP 13(39) (2011) 17505-17510.
[189] X. Cui, S. Yang, X. Yan, J. Leng, S. Shuang, P.M. Ajayan, Z. Zhang, Pyridinic‐Nitrogen‐
Dominated Graphene Aerogels with Fe–N–C Coordination for Highly Efficient Oxygen
Reduction Reaction, Advanced Functional Materials 26(31) (2016) 5708-5717.
[190] S. Liu, C. Zhao, Z. Wang, H. Ding, H. Deng, G. Yang, J. Li, H. Zheng, Urea-assisted
Bibliography Page 216 of 231
one-step fabrication of a novel nitrogen-doped carbon fiber aerogel from cotton as metal-free
catalyst in peroxymonosulfate activation for efficient degradation of carbamazepine, Chemical
engineering journal (Lausanne, Switzerland : 1996) 386 (2020) 124015.
[191] N.P.D. Ngidi, M.A. Ollengo, V.O. Nyamori, Effect of Doping Temperatures and Nitrogen
Precursors on the Physicochemical, Optical, and Electrical Conductivity Properties of
Nitrogen-Doped Reduced Graphene Oxide, Materials 12(20) (2019) 3376.
[192] Y. Ma, S. Jiang, G. Jian, H. Tao, L. Yu, X. Wang, X. Wang, J. Zhu, Z. Hu, Y. Chen, CNx
nanofibers converted from polypyrrole nanowires as platinum support for methanol oxidation,
Energy & environmental science 2(2) (2009) 224-229.
[193] M. Seredych, D. Hulicova-Jurcakova, G.Q. Lu, T.J. Bandosz, Surface functional groups
of carbons and the effects of their chemical character, density and accessibility to ions on
electrochemical performance, Carbon (New York) 46(11) (2008) 1475-1488.
[194] Z. Lin, G. Waller, Y. Liu, M. Liu, C.P. Wong, Facile Synthesis of Nitrogen‐Doped
Graphene via Pyrolysis of Graphene Oxide and Urea, and its Electrocatalytic Activity toward
the Oxygen‐Reduction Reaction, Advanced energy materials 2(7) (2012) 884-888.
[195] L. Sun, L. Wang, C. Tian, T. Tan, Y. Xie, K. Shi, M. Li, H. Fu, Nitrogen-doped graphene
with high nitrogen level via a one-step hydrothermal reaction of graphene oxide with urea for
superior capacitive energy storage, RSC advances 2(10) (2012) 4498.
[196] H.-L. Guo, P. Su, X. Kang, S.-K. Ning, Synthesis and characterization of nitrogen-doped
graphene hydrogels by hydrothermal route with urea as reducing-doping agents, Journal of
Materials Chemistry A 1(6) (2013) 2248-2255.
[197] L.I.N. Ziyin, M.-K. Song, D. Yong, L.I.U. Yan, L.I.U. Meilin, C.-P. Wong, Facile
preparation of nitrogen-doped graphene as a metal-free catalyst for oxygen reduction reaction,
Physical chemistry chemical physics : PCCP 14(10) (2012) 3381-3387.
[198] C. Liu, X. Liu, J. Tan, Q. Wang, H. Wen, C. Zhang, Nitrogen-doped graphene by all-
Bibliography Page 217 of 231
solid-state ball-milling graphite with urea as a high-power lithium ion battery anode, Journal
of Power Sources 342 (2017) 157-164.
[199] B. Li, Z. Li, Q. Pang, J.Z. Zhang, Core/shell cable-like Ni3S2 nanowires/N-doped
graphene-like carbon layers as composite electrocatalyst for overall electrocatalytic water
splitting, Chemical engineering journal (Lausanne, Switzerland : 1996) 401 (2020) 126045.
[200] Y.U.E. Bing, M.A. Yanwen, T.A.O. Haisheng, Y.U. Leshu, J. Guoqiang, W. Xizhang, W.
Xiaoshu, L.U. Yinong, H.U. Zheng, CNx nanotubes as catalyst support to immobilize platinum
nanoparticles for methanol oxidation, Journal of materials chemistry 18(15) (2008) 1747-1750.
[201] S. Jiang, L. Zhu, Y. Ma, X. Wang, J. Liu, J. Zhu, Y. Fan, Z. Zou, Z. Hu, Direct
immobilization of Pt–Ru alloy nanoparticles on nitrogen-doped carbon nanotubes with superior
electrocatalytic performance, Journal of power sources 195(22) (2010) 7578-7582.
[202] S. Jiang, Y. Ma, G. Jian, H. Tao, X. Wang, Y. Fan, Y. Lu, Z. Hu, Y. Chen, Facile
Construction of Pt–Co/CNx Nanotube Electrocatalysts and Their Application to the Oxygen
Reduction Reaction, Advanced materials (Weinheim) 21(48) (2009) 4953-4956.
[203] X. Wang, J. Zhou, H. Fu, W. Li, X. Fan, G. Xin, J. Zheng, X. Li, MOF derived catalysts
for electrochemical oxygen reduction, Journal of materials chemistry. A, Materials for energy
and sustainability 2(34) (2014) 14064-14070.
[204] B. You, N. Jiang, M. Sheng, W.S. Drisdell, J. Yano, Y. Sun, Bimetal–Organic Framework
Self-Adjusted Synthesis of Support-Free Nonprecious Electrocatalysts for Efficient Oxygen
Reduction, ACS catalysis 5(12) (2015) 7068-7076.
[205] J. Wei, Y. Hu, Y. Liang, B. Kong, J. Zhang, J. Song, Q. Bao, G.P. Simon, S.P. Jiang, H.
Wang, Nitrogen‐Doped Nanoporous Carbon/Graphene Nano‐Sandwiches: Synthesis and
Application for Efficient Oxygen Reduction, Advanced functional materials 25(36) (2015)
5768-5777.
[206] X. Wang, H. Zhang, H. Lin, S. Gupta, C. Wang, Z. Tao, H. Fu, T. Wang, J. Zheng, G. Wu,
Bibliography Page 218 of 231
X. Li, Directly converting Fe-doped metal–organic frameworks into highly active and stable
Fe-N-C catalysts for oxygen reduction in acid, Nano energy 25(C) (2016) 110-119.
[207] J. Li, Q.-L. Zhu, Q. Xu, Pd nanoparticles supported on hierarchically porous carbons
derived from assembled nanoparticles of a zeolitic imidazolate framework (ZIF-8) for
methanol electrooxidation, Chemical communications (Cambridge, England) 51(54) (2015)
10827-10830.
[208] L. Sommer, Analytical absorption spectrophotometry in the visible and ultraviolet : the
principles, Elsevier, Amsterdam ;, 1989.
[209] Z. Lei, L. Lu, X.S. Zhao, The electrocapacitive properties of graphene oxide reduced by
urea, Energy & environmental science 5(4) (2012) 6391-6399.
[210] Z. Luo, Y. Lu, L.A. Somers, A.T.C. Johnson, High Yield Preparation of Macroscopic
Graphene Oxide Membranes, Journal of the American Chemical Society 131(3) (2009) 898-
899.
[211] T. Zhang, G.-Y. Zhu, C.-H. Yu, Y. Xie, M.-Y. Xia, B.-Y. Lu, X. Fei, Q. Peng, The UV
absorption of graphene oxide is size-dependent: possible calibration pitfalls, Mikrochimica
acta (1966) 186(3) (2019) 1-7.
[212] J. Zhang, H. Yang, G. Shen, P. Cheng, J. Zhang, S. Guo, Reduction of graphene oxide
via L-ascorbic acid, Chemical communications (Cambridge, England) 46(7) (2010) 1112-1114.
[213] M.S. Dresselhaus, G. Dresselhaus, M. Hofmann, Raman spectroscopy as a probe of
graphene and carbon nanotubes, Philosophical transactions of the Royal Society of London.
Series A: Mathematical, physical, and engineering sciences 366(1863) (2008) 231-236.
[214] M. Matsumoto, Y. Saito, C. Park, T. Fukushima, T. Aida, Ultrahigh-throughput
exfoliation of graphite into pristine 'single-layer' graphene using microwaves and molecularly
engineered ionic liquids, Nature chemistry 7(9) (2015) 730-736.
[215] Y. Waseda, E. Matsubara, K. Shinoda, X-Ray Diffraction Crystallography : Introduction,
Bibliography Page 219 of 231
Examples and Solved Problems, Springer Berlin Heidelberg, Berlin, Heidelberg, 2011.
[216] M. Ladd, R. Palmer, Structure Determination by X-ray Crystallography : Analysis by X-
rays and Neutrons, 5th ed. 2013. ed., Springer US, Boston, MA, 2013.
[217] R.J. MacDonald, E.C. Taglauer, K. Wandelt, Surface Science : Principles and Current
Applications, 1st edition 1996. ed., Springer Berlin Heidelberg, Berlin, Heidelberg, 1996.
[218] P. Gabbott, Principles and applications of thermal analysis, WILEY, Hoboken, 2008.
[219] I.M. Hwang, E.W. Moon, H.-W. Lee, N. Jamila, K. Su Kim, J.-H. Ha, S.H. Kim,
Discrimination of Chili Powder Origin Using Inductively Coupled Plasma-Mass Spectrometry
(ICP-MS), Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), and Near
Infrared (NIR) Spectroscopy, Analytical letters 52(6) (2019) 932-947.
[220] P.W.J.M. Boumans, Inductively coupled plasma emission spectroscopy, Wiley, New
York ;, 1987.
[221] J.F. Watts, J. Wolstenholme, An introduction to surface analysis by XPS and AES,
(2003).
[222] T. Bertaud, M. Sowinska, D. Walczyk, S. Thiess, A. Gloskovskii, C. Walczyk, T.
Schroeder, In-operando and non-destructive analysis of the resistive switching in the
Ti/HfO2/TiN-based system by hard x-ray photoelectron spectroscopy, Applied Physics Letters
101(14) (2012) 143501.
[223] P. Van der Heide, X-ray photoelectron spectroscopy an introduction to principles and
practices, Wiley-Blackwell, Hoboken, N.J, 2012.
[224] J.F. Watts, J. Wolstenholme, An introduction to surface analysis by XPS and AES, Second
edition. ed., Wiley, Hoboken, 2019.
[225] O.C. Wells, Scanning electron microscopy, McGraw-Hill, New York ;, 1974.
[226] J.I. Goldstein, D.E. Newbury, J.R. Michael, N.W.M. Ritchie, J.H.J. Scott, D.C. Joy,
Scanning Electron Microscopy and X-Ray Microanalysis, 4th ed. 2018. ed., Springer New York,
Bibliography Page 220 of 231
New York, NY, 2018.
[227] S.L. Flegler, J.W. Heckman, K.L. Klomparens, Scanning and transmission electron
microscopy : an introduction, Oxford University Press, New York, 1995.
[228] T.J.A. Slater, Three dimensional chemical analysis of nanoparticles using energy
dispersive X-ray spectroscopy, University of Manchester, 2015.
[229] J. Ayache, L. Beaunier, J. Boumendil, G. Ehret, D. Laub, Sample Preparation Handbook
for Transmission Electron Microscopy: Methodology, 1. Aufl. ed., Springer Science + Business
Media, New York, NY, 2010.
[230] P.D. Nellist, Scanning Transmission Electron Microscopy, Springer International
Publishing, Cham, 2019, pp. 49-99.
[231] S. Kohjiya, A. Kato, Y. Ikeda, Principle and Practice of Three-Dimensional Transmission
Electron Microscopy (3D-TEM), Springer Singapore, Singapore, 2020, pp. 49-56.
[232] X. Jiang, T. Higuchi, H. Jinnai, Transmission Electron Microscopy, Springer Japan,
Tokyo, 2019, pp. 147-153.
[233] N. Tanaka, Advanced Transmission Electron Microscopy, Springer Japan, Tokyo, 2017,
pp. 111-145.
[234] B. Voigtlander, Scanning probe microscopy : atomic force microscopy and scanning
tunneling microscopy, Springer, Heidelberg, 2015.
[235] N.J. van der Heijden, D. Smith, G. Calogero, R.S. Koster, D. Vanmaekelbergh, M.A. van
Huis, I. Swart, Recognizing nitrogen dopant atoms in graphene using atomic force microscopy,
Physical review. B 93(24) (2016).
[236] J. Wang, Analytical Electrochemistry, 3. Aufl. ed., Wiley-VCH, Hoboken, 2006.
[237] J.M.D. Rodriguez, J.A.H. Melian, J.P. Pena, Determination of the Real Surface Area of
Pt Electrodes by Hydrogen Adsorption Using Cyclic Voltammetry, Journal of chemical
education 77(9) (2000) 1195.
Bibliography Page 221 of 231
[238] M. Aliofkhazraei, A.S.H. Makhlouf, Handbook of nanoelectrochemistry :
electrochemical synthesis methods, properties, and characterization techniques, Springer
Reference, Cham, 2016.
[239] H.H. Girault, Analytical and physical electrochemistry, 1. ed. ed., EPFL, Lausanne] ;,
2004.
[240] V.S. Silva, J. Schirmer, R. Reissner, B. Ruffmann, H. Silva, A. Mendes, L.M. Madeira,
S.P. Nunes, Proton electrolyte membrane properties and direct methanol fuel cell performance.
II. Fuel cell performance and membrane properties effects, Journal of power sources 140(1)
(2005) 41-49.
[241] T.E. Springer, T.A. Zawodzinski, M.S. Wilson, S. Gottesfeld, Characterization of
polymer electrolyte fuel cells using AC impedance spectroscopy, Journal of the
Electrochemical Society 143(2) (1996) 587-599.
[242] N. Wagner, W. Schnurnberger, B. Müller, M. Lang, Electrochemical impedance spectra
of solid-oxide fuel cells and polymer membrane fuel cells, Electrochimica acta 43(24) (1998)
3785-3793.
[243] X.-Z. Yuan, C. Song, H. Wang, J. Zhang, Electrochemical Impedance Spectroscopy in
PEM Fuel Cells : Fundamentals and Applications, Springer London, London, 2010.
[244] G.J.M. Janssen, E.F. Sitters, A. Pfrang, Proton-exchange-membrane fuel cells durability
evaluated by load-on/off cycling, Journal of power sources 191(2) (2009) 501-509.
[245] H. Liang, H. Su, B.G. Pollet, V. Linkov, S. Pasupathi, Membrane electrode assembly with
enhanced platinum utilization for high temperature proton exchange membrane fuel cell
prepared by catalyst coating membrane method, Journal of Power Sources 266 (2014) 107-113.
[246] D. Malevich, E. Halliop, B.A. Peppley, J.G. Pharoah, K. Karan, Investigation of Charge-
Transfer and Mass-Transport Resistances in PEMFCs with Microporous Layer Using
Electrochemical Impedance Spectroscopy, Journal of the Electrochemical Society 156(2)
Bibliography Page 222 of 231
(2009) B216.
[247] T.J.P. Freire, E.R. Gonzalez, Effect of membrane characteristics and humidification
conditions on the impedance response of polymer electrolyte fuel cells, Journal of
electroanalytical chemistry (Lausanne, Switzerland) 503(1) (2001) 57-68.
[248] B. Andreaus, A.J. McEvoy, G.G. Scherer, Analysis of performance losses in polymer
electrolyte fuel cells at high current densities by impedance spectroscopy, Electrochimica acta
47(13) (2002) 2223-2229.
[249] M. Perez‐Page, M. Sahoo, S.M. Holmes, Single Layer 2D Crystals for electrochemical
applications of ion exchange membranes and hydrogen evolution catalysts, Advanced
Materials Interfaces 6(7) (2019) 1801838.
[250] L.I. Şanlı, V. Bayram, S. Ghobadi, N. Düzen, S.A. Gürsel, Engineered catalyst layer
design with graphene-carbon black hybrid supports for enhanced platinum utilization in PEM
fuel cell, International Journal of Hydrogen Energy 42(2) (2017) 1085-1092.
[251] H. Yang, D. Lee, K. Park, W. Kim, Platinum–boron doped graphene intercalated by
carbon black for cathode catalyst in proton exchange membrane fuel cell, Energy 89 (2015)
500-510.
[252] J. Cao, P. He, M.A. Mohammed, X. Zhao, R.J. Young, B. Derby, I.A. Kinloch, R.A. Dryfe,
Two-step electrochemical intercalation and oxidation of graphite for the mass production of
graphene oxide, Journal of the American Chemical Society 139(48) (2017) 17446-17456.
[253] L. Dong, R.R.S. Gari, Z. Li, M.M. Craig, S. Hou, Graphene-supported platinum and
platinum–ruthenium nanoparticles with high electrocatalytic activity for methanol and ethanol
oxidation, Carbon 48(3) (2010) 781-787.
[254] Z. Lei, L. Lu, X. Zhao, The electrocapacitive properties of graphene oxide reduced by
urea, Energy & Environmental Science 5(4) (2012) 6391-6399.
[255] Z.-J. Fan, W. Kai, J. Yan, T. Wei, L.-J. Zhi, J. Feng, Y.-m. Ren, L.-P. Song, F. Wei, Facile
Bibliography Page 223 of 231
synthesis of graphene nanosheets via Fe reduction of exfoliated graphite oxide, ACS nano 5(1)
(2011) 191-198.
[256] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu,
S.T. Nguyen, R.S. Ruoff, Synthesis of graphene-based nanosheets via chemical reduction of
exfoliated graphite oxide, carbon 45(7) (2007) 1558-1565.
[257] F. Lou, M.E.M. Buan, N. Muthuswamy, J.C. Walmsley, M. Rønning, D. Chen, One-step
electrochemical synthesis of tunable nitrogen-doped graphene, Journal of materials chemistry.
A, Materials for energy and sustainability 4(4) (2016) 1233-1243.
[258] J. Zhang, Z. Zhao, Z. Xia, L. Dai, A metal-free bifunctional electrocatalyst for oxygen
reduction and oxygen evolution reactions, Nature nanotechnology 10(5) (2015) 444-452.
[259] K. Cheng, M. Jiang, B. Ye, I.S. Amiinu, X. Liu, Z. Kou, W. Li, S. Mu, Three-
dimensionally costabilized metal catalysts toward an oxygen reduction reaction, Langmuir
32(9) (2016) 2236-2244.
[260] R. Arrigo, M. Havecker, S. Wrabetz, R. Blume, M. Lerch, J. McGregor, E.P. Parrott, J.A.
Zeitler, L.F. Gladden, A. Knop-Gericke, Tuning the acid/base properties of nanocarbons by
functionalization via amination, Journal of the American Chemical Society 132(28) (2010)
9616-9630.
[261] B. Russ, M.J. Robb, B.C. Popere, E.E. Perry, C.-K. Mai, S.L. Fronk, S.N. Patel, T.E.
Mates, G.C. Bazan, J.J. Urban, Tethered tertiary amines as solid-state n-type dopants for
solution-processable organic semiconductors, Chemical science 7(3) (2016) 1914-1919.
[262] L. Lai, J.R. Potts, D. Zhan, L. Wang, C.K. Poh, C. Tang, H. Gong, Z. Shen, J. Lin, R.S.
Ruoff, Exploration of the active center structure of nitrogen-doped graphene-based catalysts
for oxygen reduction reaction, Energy & Environmental Science 5(7) (2012) 7936-7942.
[263] C. Backes, D. Campi, B.M. Szydlowska, K. Synnatschke, E. Ojala, F. Rashvand, A.
Harvey, A. Griffin, Z. Sofer, N. Marzari, Equipartition of Energy Defines the Size–Thickness
Bibliography Page 224 of 231
Relationship in Liquid-Exfoliated Nanosheets, ACS nano 13(6) (2019) 7050-7061.
[264] V. Nagyte, D.J. Kelly, A. Felten, G. Picardi, Y. Shin, A. Alieva, R.E. Worsley, K. Parvez,
S. Dehm, R. Krupke, Raman Fingerprints of Graphene Produced by Anodic Electrochemical
Exfoliation, Nano Letters 20(5) (2020) 3411-3419.
[265] X. Gao, J. Zhang, P. Ju, J. Liu, L. Ji, X. Liu, T. Ma, L. Chen, H. Li, H. Zhou, Shear‐
Induced Interfacial Structural Conversion of Graphene Oxide to Graphene at Macroscale,
Advanced Functional Materials (2020) 2004498.
[266] D. Deng, X. Pan, L. Yu, Y. Cui, Y. Jiang, J. Qi, W.-X. Li, Q. Fu, X. Ma, Q. Xue, Toward
N-doped graphene via solvothermal synthesis, Chemistry of Materials 23(5) (2011) 1188-1193.
[267] Z. Lin, Y. Yao, Z. Li, Y. Liu, Z. Li, C.-P. Wong, Solvent-assisted thermal reduction of
graphite oxide, The Journal of Physical Chemistry C 114(35) (2010) 14819-14825.
[268] Z. Luo, S. Lim, Z. Tian, J. Shang, L. Lai, B. MacDonald, C. Fu, Z. Shen, T. Yu, J. Lin,
Pyridinic N doped graphene: synthesis, electronic structure, and electrocatalytic property,
Journal of Materials Chemistry 21(22) (2011) 8038-8044.
[269] E. Ticianelli, C. Derouin, A. Redondo, S. Srinivasan, Methods to advance technology of
proton exchange membrane fuel cells, Journal of The Electrochemical Society 135(9) (1988)
2209.
[270] P.H. Matter, L. Zhang, U.S. Ozkan, The role of nanostructure in nitrogen-containing
carbon catalysts for the oxygen reduction reaction, Journal of Catalysis 239(1) (2006) 83-96.
[271] V. Strelko, V. Kuts, P. Thrower, On the mechanism of possible influence of heteroatoms
of nitrogen, boron and phosphorus in a carbon matrix on the catalytic activity of carbons in
electron transfer reactions, Carbon (New York, NY) 38(10) (2000) 1499-1503.
[272] K.N. Kudin, B. Ozbas, H.C. Schniepp, R.K. Prud'homme, I.A. Aksay, R. Car, Raman
Spectra of Graphite Oxide and Functionalized Graphene Sheets, Nano letters 8(1) (2008) 36-
41.
Bibliography Page 225 of 231
[273] D. Wei, Y. Liu, Y. Wang, H. Zhang, L. Huang, G. Yu, Synthesis of N-doped graphene by
chemical vapor deposition and its electrical properties, Nano letters 9(5) (2009) 1752-1758.
[274] S.W. Lee, S. Chen, J. Suntivich, K. Sasaki, R.R. Adzic, Y. Shao-Horn, Role of surface
steps of Pt nanoparticles on the electrochemical activity for oxygen reduction, The Journal of
Physical Chemistry Letters 1(9) (2010) 1316-1320.
[275] M. He, G. Fei, Z. Zheng, Z. Cheng, Z. Wang, H. Xia, Pt Nanoparticle-Loaded Graphene
Aerogel Microspheres with Excellent Methanol Electro-Oxidation Performance, Langmuir
35(10) (2019) 3694-3700.
[276] I.E. Stephens, A.S. Bondarenko, F.J. Perez-Alonso, F. Calle-Vallejo, L. Bech, T.P.
Johansson, A.K. Jepsen, R. Frydendal, B.P. Knudsen, J. Rossmeisl, Tuning the activity of Pt
(111) for oxygen electroreduction by subsurface alloying, Journal of the American Chemical
Society 133(14) (2011) 5485-5491.
[277] S.W. Lee, S.R. Choi, J. Jang, G.-G. Park, S.H. Yu, J.-Y. Park, Tolerance to carbon
corrosion of various carbon structures as catalyst supports for polymer electrolyte membrane
fuel cells, Journal of Materials Chemistry A 7(43) (2019) 25056-25065.
[278] J. Ma, A. Habrioux, C. Morais, A. Lewera, W. Vogel, Y. Verde-Gomez, G. Ramos-
Sanchez, P.B. Balbuena, N. Alonso-Vante, Spectroelectrochemical probing of the strong
interaction between platinum nanoparticles and graphitic domains of carbon, ACS Catalysis
3(9) (2013) 1940-1950.
[279] C. Galeano, J.C. Meier, V. Peinecke, H. Bongard, I. Katsounaros, A.A. Topalov, A. Lu,
K.J. Mayrhofer, F. Schüth, Toward highly stable electrocatalysts via nanoparticle pore
confinement, Journal of the American Chemical Society 134(50) (2012) 20457-20465.
[280] D. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Active sites of nitrogen-
doped carbon materials for oxygen reduction reaction clarified using model catalysts, Science
351(6271) (2016) 361-365.
Bibliography Page 226 of 231
[281] S. Cho, H. Yang, D. Lee, S. Park, W. Kim, Electrochemical properties of Pt/graphene
intercalated by carbon black and its application in polymer electrolyte membrane fuel cell,
Journal of power sources 225 (2013) 200-206.
[282] L. Xin, F. Yang, S. Rasouli, Y. Qiu, Z.-F. Li, A. Uzunoglu, C.-J. Sun, Y. Liu, P. Ferreira,
W. Li, Understanding Pt nanoparticle anchoring on graphene supports through surface
functionalization, Acs Catalysis 6(4) (2016) 2642-2653.
[283] A. Parthasarathy, B. Dave, S. Srinivasan, A.J. Appleby, C.R. Martin, The platinum
microelectrode/Nafion interface: an electrochemical impedance spectroscopic analysis of
oxygen reduction kinetics and Nafion characteristics, Journal of the Electrochemical Society
139(6) (1992) 1634.
[284] E. Fontananova, F. Trotta, J.C. Jansen, E. Drioli, Preparation and characterization of new
non-fluorinated polymeric and composite membranes for PEMFCs, Journal of membrane
science 348(1) (2010) 326-336.
[285] S. Asghari, A. Mokmeli, M. Samavati, Study of PEM fuel cell performance by
electrochemical impedance spectroscopy, International journal of hydrogen energy 35(17)
(2010) 9283-9290.
[286] J. Yan, T. Wei, B. Shao, F. Ma, Z. Fan, M. Zhang, C. Zheng, Y. Shang, W. Qian, F. Wei,
Electrochemical properties of graphene nanosheet/carbon black composites as electrodes for
supercapacitors, Carbon 48(6) (2010) 1731-1737.
[287] S.-J. Seo, J.-J. Woo, S.-H. Yun, H.-J. Lee, J.-S. Park, T. Xu, T.-H. Yang, J. Lee, S.-H.
Moon, Analyses of interfacial resistances in a membrane-electrode assembly for a proton
exchange membrane fuel cell using symmetrical impedance spectroscopy, Physical Chemistry
Chemical Physics 12(46) (2010) 15291-15300.
[288] A.C. Fernandes, E.A. Ticianelli, A performance and degradation study of Nafion 212
membrane for proton exchange membrane fuel cells, Journal of Power Sources 193(2) (2009)
Bibliography Page 227 of 231
547-554.
[289] A. Adenier, M.M. Chehimi, I. Gallardo, J. Pinson, N. Vila, Electrochemical oxidation of
aliphatic amines and their attachment to carbon and metal surfaces, Langmuir 20(19) (2004)
8243-8253.
[290] P.-L. Kuo, W.-F. Chen, H.-Y. Huang, I.-C. Chang, S.A. Dai, Stabilizing effect of pseudo-
dendritic polyethylenimine on platinum nanoparticles supported on carbon, The Journal of
Physical Chemistry B 110(7) (2006) 3071-3077.
[291] C.L. Muhich, J.Y. Westcott IV, T.C. Morris, A.W. Weimer, C.B. Musgrave, The effect of
N and B doping on graphene and the adsorption and migration behavior of Pt atoms, The
Journal of Physical Chemistry C 117(20) (2013) 10523-10535.
[292] J. Zheng, W. Sheng, Z. Zhuang, B. Xu, Y. Yan, Universal dependence of hydrogen
oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen
binding energy, Science advances 2(3) (2016) e1501602.
[293] W. Sheng, Z. Zhuang, M. Gao, J. Zheng, J.G. Chen, Y. Yan, Correlating hydrogen
oxidation and evolution activity on platinum at different pH with measured hydrogen binding
energy, Nature communications 6(1) (2015) 1-6.
[294] X. Yang, J. Nash, N. Oliveira, Y. Yan, B. Xu, Understanding the pH Dependence of
Underpotential Deposited Hydrogen on Platinum, Angewandte Chemie International Edition
58(49) (2019) 17718-17723.
[295] J. Liu, W. Li, R. Cheng, Q. Wu, J. Zhao, D. He, S. Mu, Stabilizing Pt nanocrystals
encapsulated in N-doped carbon as double-active sites for catalyzing oxygen reduction reaction,
Langmuir 35(7) (2019) 2580-2586.
[296] S. Zhang, S. Chen, Enhanced-electrocatalytic activity of Pt nanoparticles supported on
nitrogen-doped carbon for the oxygen reduction reaction, Journal of power sources 240 (2013)
60-65.
Bibliography Page 228 of 231
[297] L.-M. Zhang, X.-L. Sui, L. Zhao, G.-S. Huang, D.-M. Gu, Z.-B. Wang, Three-
dimensional hybrid aerogels built from graphene and polypyrrole-derived nitrogen-doped
carbon nanotubes as a high-efficiency Pt-based catalyst support, Carbon 121 (2017) 518-526.
[298] Q. Zhang, X. Yu, Y. Ling, W. Cai, Z. Yang, Ultrathin nitrogen doped carbon layer
stabilized Pt electrocatalyst supported on N-doped carbon nanotubes, International Journal of
Hydrogen Energy 42(15) (2017) 10354-10362.
[299] K.K. Karuppanan, A.V. Raghu, M.K. Panthalingal, V. Thiruvenkatam, P. Karthikeyan, B.
Pullithadathil, 3D-porous electrocatalytic foam based on Pt@ N-doped graphene for high
performance and durable polymer electrolyte membrane fuel cells, Sustainable Energy & Fuels
3(4) (2019) 996-1011.
[300] J. Cao, Y. Hu, L. Chen, J. Xu, Z. Chen, Nitrogen-doped carbon quantum dot/graphene
hybrid nanocomposite as an efficient catalyst support for the oxygen reduction reaction,
International Journal of Hydrogen Energy 42(5) (2017) 2931-2942.
[301] C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic
strength of monolayer graphene, science 321(5887) (2008) 385-388.
[302] M. Yan, Q. Jiang, T. Zhang, J. Wang, L. Yang, Z. Lu, H. He, Y. Fu, X. Wang, H. Huang,
Three-dimensional low-defect carbon nanotube/nitrogen-doped graphene hybrid aerogel-
supported Pt nanoparticles as efficient electrocatalysts toward the methanol oxidation reaction,
Journal of materials chemistry. A, Materials for energy and sustainability 6(37) (2018) 18165-
18172.
[303] G. Liu, H. Zhang, J. Hu, Y. Zhai, D. Xu, Z.-g. Shao, Studies of performance degradation
of a high temperature PEMFC based on H3PO4-doped PBI, Journal of Power Sources 162(1)
(2006) 547-552.
[304] F. Liu, S. Wang, J. Li, X. Tian, X. Wang, H. Chen, Z. Wang, Polybenzimidazole/ionic-
liquid-functional silica composite membranes with improved proton conductivity for high
Bibliography Page 229 of 231
temperature proton exchange membrane fuel cells, Journal of membrane science 541 (2017)
492-499.
[305] K.-J. Peng, J.-Y. Lai, Y.-L. Liu, Nanohybrids of graphene oxide chemically-bonded with
Nafion: Preparation and application for proton exchange membrane fuel cells, Journal of
membrane science 514 (2016) 86-94.
[306] C.-L. Lu, C.-P. Chang, Y.-H. Guo, T.-K. Yeh, Y.-C. Su, P.-C. Wang, K.-L. Hsueh, F.-G.
Tseng, High-performance and low-leakage phosphoric acid fuel cell with synergic composite
membrane stacking of micro glass microfiber and nano PTFE, Renewable energy 134 (2019)
982-988.
[307] W. Zhang, Z. Cao, J. Zhang, K. Peng, Q. Ma, Q. Xu, H. Su, Enhanced Durability of Pt-
Based Electrocatalysts in High-Temperature Polymer Electrolyte Membrane Fuel Cells Using
a Graphitic Carbon Nitride Nanosheet Support, ACS sustainable chemistry & engineering 8(24)
(2020) 9195-9205.
[308] J.S. Yang, L.N. Cleemann, T. Steenberg, C. Terkelsen, Q.F. Li, J.O. Jensen, H.A. Hjuler,
N.J. Bjerrum, R.H. He, High Molecular Weight Polybenzimidazole Membranes for High
Temperature PEMFC, Fuel cells (Weinheim an der Bergstrasse, Germany) 14(1) (2014) 7-15.
[309] D. Úbeda, P. Cañizares, P. Ferreira-Aparicio, A.M. Chaparro, J. Lobato, M.A. Rodrigo,
Life test of a high temperature PEM fuel cell prepared by electrospray, International journal of
hydrogen energy 41(44) (2016) 20294-20304.
[310] F. Mack, K. Aniol, C. Ellwein, J. Kerres, R. Zeis, Novel phosphoric acid-doped PBI-
blends as membranes for high-temperature PEM fuel cells, Journal of materials chemistry. A,
Materials for energy and sustainability 3(2) (2015) 1864-1874.
[311] K. Kwon, T.Y. Kim, D.Y. Yoo, S.-G. Hong, J.O. Park, Maximization of high-temperature
proton exchange membrane fuel cell performance with the optimum distribution of phosphoric
acid, Journal of Power Sources 188(2) (2009) 463-467.
Bibliography Page 230 of 231
[312] H. Chen, S. Wang, F. Liu, D. Wang, J. Li, T. Mao, G. Liu, X. Wang, J. Xu, Z. Wang,
Base-acid doped polybenzimidazole with high phosphoric acid retention for HT-PEMFC
applications, Journal of membrane science 596 (2020) 117722.
[313] Y.H. Jeong, K. Oh, S. Ahn, N.Y. Kim, A. Byeon, H.-Y. Park, S.Y. Lee, H.S. Park, S.J.
Yoo, J.H. Jang, Investigation of electrolyte leaching in the performance degradation of
phosphoric acid-doped polybenzimidazole membrane-based high temperature fuel cells,
Journal of Power Sources 363 (2017) 365-374.
[314] H. Liu, D. Si, H. Ding, S. Wang, J. Zhang, Y. Liu, Cold start capability and durability of
electrospun catalyst layer for proton exchange membrane fuel cell, International journal of
hydrogen energy 46(19) (2021) 11140-11149.
[315] M.S. Saha, R. Li, X. Sun, S. Ye, 3-D composite electrodes for high performance PEM
fuel cells composed of Pt supported on nitrogen-doped carbon nanotubes grown on carbon
paper, Electrochemistry Communications 11(2) (2009) 438-441.
[316] G.-P. Kim, M. Lee, Y.J. Lee, S. Bae, H.D. Song, I.K. Song, J. Yi, Polymer-mediated
synthesis of a nitrogen-doped carbon aerogel with highly dispersed Pt nanoparticles for
enhanced electrocatalytic activity, Electrochimica Acta 193 (2016) 137-144.
[317] S.-H. Liu, M.-T. Wu, Y.-H. Lai, C.-C. Chiang, N. Yu, S.-B. Liu, Fabrication and
electrocatalytic performance of highly stable and active platinum nanoparticles supported on
nitrogen-doped ordered mesoporous carbons for oxygen reduction reaction, Journal of
Materials Chemistry 21(33) (2011) 12489-12496.
[318] G. Vijayaraghavan, K.J. Stevenson, Synergistic assembly of dendrimer-templated
platinum catalysts on nitrogen-doped carbon nanotube electrodes for oxygen reduction,
Langmuir 23(10) (2007) 5279-5282.
[319] L. Zhang, N. Zheng, A. Gao, C. Zhu, Z. Wang, Y. Wang, Z. Shi, Y. Liu, A robust fuel cell
cathode catalyst assembled with nitrogen-doped carbon nanohorn and platinum nanoclusters,
Bibliography Page 231 of 231
Journal of Power Sources 220 (2012) 449-454.
[320] D.C. Higgins, D. Meza, Z. Chen, Nitrogen-doped carbon nanotubes as platinum catalyst
supports for oxygen reduction reaction in proton exchange membrane fuel cells, The Journal
of Physical Chemistry C 114(50) (2010) 21982-21988.
[321] S. Shanmugam, J. Sanetuntikul, T. Momma, T. Osaka, Enhanced oxygen reduction
activities of Pt supported on nitrogen-doped carbon nanocapsules, Electrochimica Acta 137
(2014) 41-48.
[322] S. Shrestha, S. Asheghi, J. Timbro, W.E. Mustain, Temperature controlled surface
chemistry of nitrogen-doped mesoporous carbon and its influence on Pt ORR activity, Applied
Catalysis A: General 464 (2013) 233-242.