1
Kenichi ShimizuResearch Seminar Nov. 8 2007
Study of Electrochemical Catalysts in Fuel Cells
2
Study of Electrochemical Catalysts in Fuel Cells
I. Study of catalyst effect in anodic oxidation of organic fuels.
II. Synthesis of Pt on single-walled carbon nanotube for cathodic reduction of oxygen.
3
Fuel Cells A system that harvests electrical
energy directly from a spontaneous redox reaction.
Electricity will be generated continuously as long as fuel is supplied.
Sutton, G.W. Direct Energy Conversion. Inter-University Electronics Series, Vol.3, NY, 1966.
4
F + zH2O wCO2 + vH+ + ve- + xI (E1)
yO2 + vH+ + ve- zH2O (E2)
F + yO2 wCO2 + zH2O + xI (Ecell = E1+ E2)
Fuel Cells
Sutton, G.W. Direct Energy Conversion. Inter-University Electronics Series, Vol.3, NY, 1966.
5
Experimental design by William R. Grove
Grove, W.R. Phil. Trans. 1843, 133, 91-112.
Gas Voltaic Battery (1842)
molkJHmolkJGOHOH
molkJHmolkJGOHeHO
molkJHmolkJGeHH
rxnrxn /8.285/2.2372
1
/8.285/2.237222
1
/0.0/0.022
222
22
2
6
After >100 years of Nothing Alkaline hydroxide
fuel cell was developed for Apollo mission in the 1950’s.
Hoogers G.; Fuel cell technology handbook, CRC press, NY, 2003, pp2-1.
0
2000
4000
6000
8000
10000
# pu
blic
atio
n
200620011996199119861981
Year
9733
7
Types of Fuel Cells
Solid oxide fuel cell.
Molten carbonate fuel cell.
Polymer Electrolyte Membrane (PEM) fuel cell.
Microbial fuel cell.
Hoogers G.; Fuel cell technology handbook, CRC press, NY, 2003, 2-1.Fuel Cell Technology. Reaching Towards Commercialization, Springer, Germany, 2006, 277-293.Meier, F.; et al.; J. Membr. Sci. 2004, 241, 137.
www.news.cornell.edu
Nafion®
8
Polymer Electrolyte Membrane Fuel Cell
Nafion®
Meier, F.; et al.; J. Membr. Sci. 2004, 241, 137.
9
Zero (low) emission. No mechanical parts. Higher fuel efficiency.
GM Sequal fuel cell vehiclehttp://www.fueleconomy.gov/feg/
Automotive Application of Fuel Cell
http://www.fueleconomy.gov/feg/
10
Generates little or no air pollution.
Sustainable fuel source. Anaerobic digester gas
Quiet.
www.fuelcellenergy.com
Stationary Power Plant
11
Fuel cells could supply larger energy density than the conventional battery system.
Very quick recharge.http://pr.fujitsu.com/en/news/
http://www.physorg.com/news6542.html
Portable power sources
12
Limitations of Fuel Cells Fuel availability and storage.
Use of Hydrogen as anode fuel
Low power density. Kinetic limitations.
High cost. Pt catalyst. Polymer electrolyte membrane.
O’Hayre, R. et al. Fuel Cell Fundamentals. John Wiley & Sons. N.Y. 2006.
Part 2
Part 1
13
Study of Electrochemical Catalysts in Fuel Cells
I. Study of catalyst activity in anodic oxidation of organic fuels.
II. Synthesis of Pt on single-walled carbon nanotube for cathodic reduction of oxygen.
14
Limitations of Fuel Cells Fuel availability and storage.
Use of Hydrogen as anode fuel
Low power density. Kinetic limitations.
High cost. Pt catalyst. Polymer electrolyte membrane.
O’Hayre, R. et al. Fuel Cell Fundamentals. John Wiley & Sons. N.Y. 2006.
15
Fuel Availability and Storage
molkJHmolkJGOHCOOOHCH
molkJHmolekJGOHeHO
molkJHmolekJGeHCOOHOHCH
rxnrxn /4.726/9.70222
3
/4.857/1.7123662
3
/0.131/2.966
2223
22
223
Heath, C.E.; Worsham, C.H.; The Electrochemical Oxidation of Hydrocabons in a Fuel Cell. In Fuel Cell, Young, G.J. Ed.; Reihold Publishing Corp.: NY, 1963, Vol. 2;, pp 182.
CH3OH Fuel Cell H2 Fuel Cell
Fuel Storage Easy Difficult
Fuel Availability High Low
Cost ($/kWh) 0.02 0.15
Efficiency (%) 97 83
Power (kW/g-Pt) 0.2 0.6
16
Lower power density
O’Hayre, R. et al. Fuel Cell Fundamentals. John Wiley & Sons. N.Y. 2006.
17
Kinetic Limitations for Fuel Cells
0
0.2
0.4
0.6
0.8
0 0.02 0.04 0.06 0.08 0.1Ampere
Cel
l p
ote
nti
al (
V)
1. Potential drop is observed due to activation energy.
2. Ohmic resistance of the cell is proportional to the applied amperage.
3. Potential starts dropping at higher current due to mass transport.
1 2
O’Hayre, R. et al. Fuel Cell Fundamentals. John Wiley & Sons. N.Y. 2006.
3
18
Basic Operation of a Fuel Cell
O’Hayre, R. et al. Fuel Cell Fundamentals. John Wiley & Sons. N.Y. 2006.
Anode
Fuel
1 13
H2OProduct
2
Electrolytee.g. Nafion®
H+
Anode Cathode
1. Activation energy.
2. Ohmic resistance
3. Mass transport.
e- e-
3
Air
3
19
Activation Energy Activation loss of cell potential is due to the
electrochemical reactions
For hydrogen fuel cell Oxygen reduction at cathode.
For fuel cells with organic fuel Anodic oxidation of organic fuel.
20
Activation Energy Activation loss of cell potential is due to the
electrochemical reactions
For hydrogen fuel cell Oxygen reduction at cathode.
For fuel cells with organic fuel Anodic oxidation of organic fuel.
21
Activity of Catalysts in Anodic Oxidation
1. Methanol oxidation.
Overview of how catalysts are evaluated in anodic oxidation using cyclic voltammetry.
2. Formic acid oxidation.
Evaluate kinetic effects of PtRu and PtBi catalysts on anodic oxidation.
22
1. Methanol OxidationCH3OH
HCHO CO
HCOOH
CO2
H2O M-OHabs+ H+
H2O M-OHabs+ H+
Christensen, P.A. et al. J. Electroanal. Chem. 1993, 362, 207-218.
23
Cyclic Voltammetric response of Methanol Oxidation
PtRuCNT
0
0.4
0.8
1.2
1.6
0.2 0.4 0.6 0.8 1 1.2E /(V vs. NHE)
i /(m
A)
24
PtRuCNT
0
0.4
0.8
1.2
1.6
0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)
i /(m
A)
Interpretation of Forward Peak
CH3OH + H2O CO2 + 6H+ + 6e-
Slow reaction kinetics
• Deactivation of catalyst surface• Mass transport
Sufficient reaction kinetics
Onset potential
25
Interpretation of Reverse PeakPtRuCNT
0
0.4
0.8
1.2
1.6
0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)
i /(m
A) Not sufficient
potential• Reviving catalyst surface • Mass transport
CH3OH + H2O CO2 + 6H+ + 6e-
Kinetically controlled reaction
Manohara, R.; Goodenough, J.B. J. Alloy. Compd. 2001, 315, 118.
26
Guidelines for Evaluating Catalysts using Cyclic Voltammetry
Catalyst efficiency (if/ib). Current density (A/cm2)
Study of catalyst effects on an electrochemical reaction.
Mass activity (A/g-Pt) Study of catalyst system.
PtRuCNT
0
0.4
0.8
1.2
1.6
300 500 700 900 1100E /(mV vs. NHE)
i /(m
A)
Liu, Z. et al.; J. Phys. Chem. B 2004, 108, 8234.
if
ib
27
Cyclic Voltammetric Evaluation of Catalysts in Methanol Oxidation
Yen, C.H.; Shimizu, K.; Lin, Y.-Y.; Bailey, F.; Cheng, I.F.; Wai, C.M.; Energy & Fuels 2007, 21, 2268.
Mass activity
28
Kinetic Effects of Carbon Nanotube Supported Binary Metal Catalysts; PtRuCNT
and PtBiCNT
2. Formic Acid Oxidation
29
Application of Formic Acid as a Fuel
Fuel Cell Formic acid Methanol Hydrogen
Efficiency (%) 95 97 83
Cost ($/kWh) -- 0.02 0.15
Fuel Storage Easy Easy Difficult
Fuel Availability High High Low
Cell Potential (V) 1.45 1.21 1.23
Fuel Crossover Low High High
Heath, C.E.; Worsham, C.H.; The Electrochemical Oxidation of Hydrocabons in a Fuel Cell. In Fuel Cell, Young, G.J. Ed.; Reihold Publishing Corp.: NY, 1963, Vol. 2;, pp 182.Kang, S.; et al.; J. Phys. Chem. B, 2006, 110, 7270.
30
Fuel Crossover
DMethanol = 5 x10-6 cm2s-1. Creates short circuit.
Fu
elPEM
Ai
r
H+
(Anode) (Cathode)
Meier, F.; et al.; J. Membr. Sci. 2004, 241, 137.Mauritz, K.A.; Moore, R.B.; Chem. Rev.2004, 104, 4535.
www.news.cornell.edu
H+
H+
H+
31
Formic Acid Oxidation Direct electrochemical oxidation to CO2.
Chemical pathway involves spontaneous dissociation of formic acid to water and CO.
Rice, C.; et al.; J. Power Sources, 2003, 115, 229.
32
-1
0
1
2
3
4
5
0.1 0.6 1.1E /(V vs.NHE)
mA
/cm
2
Pt CNT
Cyclic Voltammetric Response of Formic Acid Oxidation
1. HCOOH CO2 + 2e- E0 = + 0.17 V
2. HCOOH COads + H2O
3. H2O OHads + H+ +e-
4. COads + OHads CO2 + H+ + e-
1
4
1
33
PtRu CNT and PtBi CNT
Pt42Ru58CNT Pt38Bi62CNT
20 nm 50 nmAtomic ratio of Pt:Ru is 1:1.4.Atomic ratio of Pt:Bi is 1:1.6.
Image is courtesy of Clive, H. Yen.
34
Catalytic Effect of Ru
0
2
4
6
8
10
12
0.1 0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)
i /(m
A/c
m2,
PtR
uCN
T)
0
1
2
3
4
5
i /(m
A/c
m2,
PtC
NT
)
PtRuCNTPtCNT
1 M H2SO4
0.1 M HCOOH
CO2
CO
intermediate
HCOOHRxn 1
Rxn 2
35
Evaluation of PtRu CNT using Peak Currents
-1
0
1
2
3
4
5
0.1 0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)
i /(m
A/c
m2)
mA/cm2 (1) (2) (3) (1)/(3)
PtCNT 2.84 1.61 5.31 0.53
PtCBc 3.72 0.99 11.3 0.33
PtRuCNT 11.6 N/A 8.91 1.3
PtRuCBc 6.05 N/A 6.01 1.0C Commercial Catalyst
Higher current ratio suggests higher catalytic efficiency of PtRu pair.
(1)
(2)
(3)Pt CNT
1 M H2SO4
0.1 M HCOOH
36
Catalytic Effect of Bi
0
0.3
0.6
0.9
1.2
0.1 0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)
i /(m
A/c
m2 ,
PtB
iCN
T)
0
1
2
3
4
5
i /(m
A/c
m2 ,
PtC
NT
)
PtBiCNTPtCNT
1 M H2SO4
0.1 M HCOOH
CO2
CO
intermediate
HCOOHRxn 1
Rxn 2
37
A/cm2 (1) (2) (3) (2)/(3)
PtCNT 2.84 1.61 5.31 0.30
PtCBc 3.72 0.99 11.3 0.09
PtBiCNT N/A 0.73 0.52 1.4
C Commercial catalyst
Evaluation of PtBi CNT using Peak Currents
-1
0
1
2
3
4
5
0.1 0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)
i /(m
A/c
m2)
(1)
(2)
(3)Pt CNT
1 M H2SO4
0.1 M HCOOH
38
Eac (kJ/mole) CI (90%)
PtCBa 20.4 N/A
Ptb 20.9 1.6
PtCBc 21.1 2.1
PtCNT 17.5 2.5
PtRuCBc 21.0 3.9
PtRuCNT 22.2 4.3
PtBiCNT 45.5* 3.0
PtBi(III)b 42.3* 7.2C Commercial catalyst
a Lovic, J.D.; et al.; J. Electroanal. Chem., 2005, 581, 294.b Wilson, J.R.; et al.; J. Electrochem. Soc., 1984, 2369.
90% confidence interval
Activation Energy of Formic Acid Oxidation
39
Review of PtBiCNT Peak currents
Low peak current density suggests slower kinetics.
Current ratio suggests high catalyst efficiency.
Activation energy PtBiCNT requires the highest activation energy.
Bi keeps Pt free from CO poisoning but lowers overall catalytic activity.
40
Third Body Effect
Cao, D. et.al.; J. Phys.Chem. 2005, 109, 11622.
41
Third Body Effect Replace neighboring
Pt with the secondary metal catalyst.
Does not provide the three Pt binding site for CO.
Pt stays free of CO poisoning.
Casado-Rivera, E.; et al.; Chem. Phys. Chem. 2003, 4, 193.Gojković, S.Lj.; et al. Electrochimica Acta 2003, 48, 3607-3614.
42
Tafel Analysis
Exchange current, j0;
Equilibrium Potential, Eeq;
Tafel Slope, β;
PtCNT
-16
-12
-8
0.1 0.5 0.9E (V vs. NHE)
ln l
j/(A
/cm
2 )l β
Eeq
lnlj0l
Red Ox Aif
ib
kf
kb
B
1 M H2SO4
0.1 M HCOOH
43
Tafel AnalysisTafel Slope (mV/dec)
j0
(µA/cm2)
Eeq
(V vs. NHE)
PtCB 150 -- --
PtCNT 91 ± 9 3.6 ± 0.6 0.30 ± 0.02
PtCBc 80 ± 4 5.3 ± 1.3 0.23 ± 0.05
PtRuCNT 32 ± 5 11.8 ± 5.9 0.29 ± 0.01
PtRuCBc 39 ± 3 2.9 ± 0.6 0.26 ± 0.02
PtBiCNT 96 ± 3 4.3 ± 2.5 0.25 ± 0.07C Commercial catalyst
Lovic, J.D.; et al.; J. Electroanal. Chem., 2005, 581, 294.
±: 90% confidence interval
44
Conclusion of Formic Acid Oxidation
PtRuCNT improved catalytic efficiency by enhancing the reaction kinetics. No significant change in activation energies. Highest exchange current was observed.
CO2
CO
intermediate
HCOOHRxn 1
Rxn 2
45
Conclusion of Formic Acid Oxidation Addition of Bi could suppress Pt poisoning
by CO. Improved current ratio (catalytic efficiency). Activation energy was significantly higher.
CO2
CO
intermediate
HCOOHRxn 1
Rxn 2
46
Study of Electrochemical Catalysts in Fuel Cells
I. Study of catalyst activity in anodic oxidation of organic fuels.
II. Synthesis of Pt on single-walled carbon nanotube for cathodic reduction of oxygen.
47
For Oxygen Reduction;
O2(g) + 4H+ + 4e- 2H2O(l)
Synthesis of Pt-SWNT
48
Catalyst Requirement Stable. Adequate electrical conductivity. High surface area. High catalytic activity.
Liebhafsky, H.A; Cairns, E.J. Fuel Cells and Fuel Batteries. John Wiley & Sons Inc., N.Y., 1968, pp384.
49
Methods of Synthesis
Direct supercritical CO2 deposition.
Water-in-hexane microemulsion.
Water-in-supercritical CO2 microemulsion.
Electro-less deposition of Pt.
50
Electro-less deposition of Pt onto SWNT
Reduction of Pt2+ in Methanol/Water Solution
51
Single-Walled Carbon Nanotube Metal impurities does
not diffract on XRD. 29 wt% Fe present. Hydrophobic in nature.
TEM
20 nm
52
Pt-SWNT Synthesis 3 to 5 mg of unpurified
SWNT. Methanol/Water (1:1
v/v). Aqueous Pt2+ salt. Inspired by Choi et al.
Choi, H.C.; et al. J. Am. Chem. Soc. 2002, 124, 9058.
53
-0.1
-0.1
0.0
0.1
0.1
0.2
0.1 0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)
i /(
Am
p/m
g P
t)
Pt-SWNT in Methanol Oxidation
Oxidation of 0.1M methanol in 1M sulfuric acid. Anodic peak current: 446 mA/mg-Pt for Pt-SWNT vs. 111 mA/mg-Pt for PtCB. Pt surface area: 351 cm2/mg-Pt for Pt-SWNT vs. 107 cm2/mg-Pt.
Pt-SWNT (1:10 C:Pt) Commercial PtCB
-0.1
0.0
0.1
0.2
0.3
0.1 0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)
i /(A
/mg-
Pt)
54
Wt% Pt and % Utilization >90 % conversion of
Pt2+/Pt0 was obtained from UV-vis analysis.
50.5 wt% corresponds to only 5 % conversion.
24.534.4
50.5
0
20
40
60
1:4 C:Pt
1:6 C:Pt
1:10C:Pt
Wt%
Pt o
n P
t-S
WN
T
55
Spontaneous Reduction of Pt(II)
Pt nanoparticles can be formed through reduction of Pt2+ by alcohols.
Wang, X. et al. Nature 2005, 437, 121-124.
56
3 to 5 mg of SWNT
Pt2+
Shaking and ultrasonic agitation
Reduction of Pt by SWNT in aqueous solution
57
Reduction of Pt by SWNT
-0.1
-0.05
0
0.05
0 0.5 1E /(V vs. NHE)
i /(
mA
)
as received SWNTPt-SWNT
Hydrogen adsorption/desorption (circled region) indicates Pt(0).
2.8 wt% Pt in Pt-SWNT.
Deposition efficiency is 16 %.
2200 cm2/mg-Pt.
107 cm2/mg-Pt from the commercial PtCB.
EDS
Count
s
58
Methanol Oxidation
Pt-SWNT
-3.0E-05
-1.0E-05
1.0E-05
3.0E-05
5.0E-05
0.1 0.3 0.5 0.7 0.9 1.1E /(V vs. NHE)
i /(
Am
p)
w/o Methanolw/ Methanol
59
Reduction of OxygenPt-SWNT
-0.6
-0.4
-0.2
-0
0.2
0 0.5 1E /(V vs. NHE)
i /(
A/m
g-P
t)
w/ Nitrogen
w/ Oxygen
Commertial PtCB
-0.1
-0.05
0
0 0.5 1E /(V vs. NHE)
i /(A
/mg
-Pt)
Ep
(V)
ip
(A/mg-Pt)
Pt-SWNT 0.68 0.52
PtCB 0.57 0.05
60
Chronoamperometry
0200400600800
100012001400
0 10 20 30Time (s)
i /(A
/mg
-Pt)
Commercial PtCB w/ NitrogenCommertial PtCB w/ OxygenPt-SWNT w/ NitrogenPt-SWNT w/ Oxygen
@ 600 mV
14 times higher catalytic activity
61
Synthesis Pt-CB and Pt-MWNT
-11
-8
-5
-2
1
0 0.5 1E /(V vs. NHE)
i /(m
A)
w/ Nitrogen
w/ Oxygen-200
-150
-100
-50
0
50
0 0.5 1E /(V vs. NHE)
i /(m A
)
This synthetic method was applicable to other carbon supports.
Carbon black substrate (80-100 mesh, 100 % carbon)
Unpurified multi-walled carbon nanotube (95 % purity)
62
Conclusion for Pt-SWNT Direct deposition of Pt onto SWNT
without added reducing agent. Prepared catalyst was 14 times more
active towards O2 reduction. Inactive towards methanol oxidation.
Will not be affected by methanol crossover.
Applicable to other carbon substrates.
63
Current/Future work Investigate reaction mechanism for Pt-
SWNT synthesis in aqueous solution. Possible improvement on Pt utilization.
Currently 16 %.
Synthesis of bimetallic catalysts. Application to fuel cell.
64
Acknowledgement Dr. Frank Cheng Dr. Chien Wai and his research group
Clive Yen, Shaofen Wang, Byunghoon Yoon, Dinesh Thanu Dr. Peter Griffiths and his research group Dr. Garry Knerr Department of chemistry and office staff Cheng group
Tina Noraduon, Derek Laine, Simon McAllister, Rubha Ponraj, Yu Qun Xie, and Chris Roske Department of Agricultural engineering for power press Department of Forest Product for heat press Tom Williams and Franklin Barely for XRD, TEM, SEM, and EDS. Maria Paulina Viteri Espinel
Financial support Electric Power Research Institute (EPRI) Innovative Small Grants Program Dr. and Mrs. Renfrew Summer scholarship
65
Additional References1. Cheng, S.; Liu, H.; Logan, B.; Environ. Sci. Technol. 2006, 40,
364.
2. Zhou, L.; Gunther, S.; Imbihl, R.; J. Catal. 2005, 230,166.
3. Park, K.W.; Choi, J.H.; Sung, Y.E.; J. Phys. Chem. 2003, 107, 5851.
4. Matsumoto, T.; et al.; Chem. Comm. 2004, 840.
5. Park, K.-W.; Choi, J.-H.; sung, Y.-E.; J. Phys. Chem. 2003, 107, 5851.
6. Tang, H.; Chen, J.H.; Wang, M.Y.; Nie, L.H.; Kuang, Y.F. Yao, S.Z.; Appl. Catal. A 2004, 275, 43.
7. Casado-Rivera, E.; Volpe, D.J.; Alden, L.; Lind, C.; Downie, C.; Vázquez-Alvarez, T.; Angelo, A.C.D.; DiSalvo, F.J.; Abruña, H.D.; J. Am. Chem. Soc. 2004, 126, 4043.
8. Huang, J.; Yang, H.; Huang, Q.; Tang, Y.; Lu, T.; Akins, D.L.; J. Electrochem. Soc. 2004, 151(11), A1815.
9. Roychowdhury, C.; Matsumoto, F.; Zeldovich, V.B.; Warren S.C.; Mutolo, P.F.; Ballesteros, M.; Wiesner, U.; Abruña, H.D.; DiSalvo, F.J.; Chem. Mater. 2006, 18, 3365.