The Application of Graphene-Carbon Black Hybrid Catalyst ...

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


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


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


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


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


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)


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


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



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


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


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


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


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


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


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


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

administrative purposes.

ii) Copies of this thesis, either in full or in extracts and whether in hard or electronic copy,

may be made only in accordance with the Copyright, Designs and Patents Act 1988

(as amended) and regulations issued under it or, where appropriate, in accordance with

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iii) The ownership of certain Copyright, patents, designs, trademarks and other

intellectual property (the “Intellectual Property”) and any reproductions of copyright

works in the thesis, for example graphs and tables (“Reproductions”), which may be

described in this thesis, may not be owned by the author and may be owned by third

parties. Such Intellectual Property and Reproductions cannot and must not be made

available for use without the prior written permission of the owner(s) of the relevant

Intellectual Property and/or Reproductions.

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Thesis restriction declarations deposited in the University Library, The University

Copyright Statement Page 26 of 231

Library’s regulations (see http://www.manchester.ac.uk/library/aboutus/regulations)

and in The University’s policy on Presentation of Thesis.

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


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


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.


• 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


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

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


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


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


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


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.


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.


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.


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.


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


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.


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.


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


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.


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]:


∆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.


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


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


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%.


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,


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].


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


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


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

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.


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.


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].


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


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].


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


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


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].


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.


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.


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


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


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]


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


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

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


(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.


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.


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.


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].


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


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


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].


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].


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


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.


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


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


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].


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.


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.


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,


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].


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


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


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.


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).


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.


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.


Figure 4.15 Schematic representation of the single fuel cell set up.


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


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


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


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


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

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


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


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


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


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).


5.3.1 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


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


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.


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


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


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


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.


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].


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


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


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

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.


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.


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.


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.


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


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


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


(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.


Figure 6.6 SEM images of Pt/CB, Pt/rEGOx-CBy and Pt/rEGO.


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)


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)


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].


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


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].



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.


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


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.


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.


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


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.


Figure 6.13 (a) Electrochemical impedance spectroscopy of all catalysts and (b) Rm, Ri and Rc

of all catalysts.


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.


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


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.


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

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.


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.


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.


7.3.1 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


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 –


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.


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.


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.


Figure 7.5 STEM images of (a) Pt/NrEGO2-CB3 and (b) the statistics on Pt particle size

distribution by counting 421 particles.


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


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.


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


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


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


(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.


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.


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.


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)


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.





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


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


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].




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.


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

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


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.


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.


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


(-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).


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.


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


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.


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.


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.


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


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


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.


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.


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

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


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


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


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 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 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 6.3 A

pp

end

ix 6

.3 L

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(ml

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This

work


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]


Pt@N-graphene foam 84.90 7.17 0.895 0.394 [300]

This work 66.97 2.40 0.607 > 1.187


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 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.

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