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High Performance Anode Catalysts for Direct Borohydride Fuel CellsPRiME 2008: Joint International MeetingPRiME 2008: Joint International MeetingHonolulu – October 16, 2008Honolulu – October 16, 2008
Vincent W.S. Lam1, Előd L. Gyenge1, and Akram Alfantazi2
The University of British Columbia1Department of Chemical and Biological Engineering2Department of Materials Engineering
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
Catalyst Selection• Catalyst cost is a large part of the fuel cell cost • Many low temperature fuel cells use platinum• Pt is expensive, prices are climbing
www.platinum.matthey.com, September 2008Carlson, E.J., et al., NREL, NREL/SR-560-39104, 2005
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
•Borohydride Background
•Alternative Anode Catalysts ▫Os/C, Pt/C, PtRu/C
•Advanced Electrode Structure▫Extended Reaction Zone Anodes (3D Anodes)
•Conclusion
Outline
3
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
Background
Sodium Borohydride Borax Na2B4O7•10H2O
▫ Major Deposits: United States, Chile, Argentina, ▫ Minor Depositis: Russia, China
Schlesinger and Brown Process (T = 498 K 548 K)
4 NaH + B(OCH3)3 → NaBH4 + 3 NaOCH3
4
Wu, Zing et al., U.S. DOE, DE-FC36-04GO14008 , 2004
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
Why Sodium Borohydride?• Non-carbonaceous fuel
▫ No CO poisoning• High standard potential• High gravimetric energy density • Competitive volumetric energy density
H2 PEMFC DMFC DBFC
Eo298 K (V) 1.23 1.21 1.64
GravimetricEnergy Density
(kWh kg-1)33.0 6.1 9.3
Volumetric Energy Density
(kWh L-1)
2.36 at 20 K(liquid)
0.75 at 300 bar4.42
1.86 20wt% NaBH4
5
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008Direct Borohydride Fuel
CellPrincipal Reactions:
Direct:
NaBH4 + 8OH- = NaBO2 + 6H2O + 8e- E = 1.24VSHE
2O2 + 4H2O + 8e- = 8OH- E = 0. 40 VSHE
NaBH4 + 2O2 = NaBO2 + 2H2O E = 1.64 V
Indirect:
Hydrolysis:NaBH4 + 2H2O = 4H2 + NaBO2
Hydrogen Electrooxidation:H2 + 2OH- = 2H2O +2e-
Lam, V. W.S., and Gyenge, E. L., J. Electrochem. Soc., 155 (2008) B1155
6
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
Flo
wfield
Plate
Diffu
sion
Layer
Catalyst L
ayer
Mem
bran
e
Catalyst L
ayerD
iffusio
n L
ayer
Flo
wfield
Plate
BH4- +NaOH O2
BO2- + H2O
e-
OO
OO
e-e-
H+O-
e-e-e-e-e-
Na+
H+O-
H+O- H+
O-
H+O-
H+O-
H+O-
H+O-
Na+
Na+Na+
Na+
Na+
Na+
Na+
H+
H+B
H+
H+
H+OH+
H+OH+
H+OH+
Na+Na+
Na+Na+
Na+
Na+Na+
H+OH+H+
OH+Na+
NaOH + H2O
OB
OH+
OH+
H+OH+
H+O-
H+O-H+
O-
H+O-
H+O-
H+O-H+
O-
H+O-Na+
Na+ Na+
Na+
Na+
Na+
Na+
Na+
H+O-Na+ H+
H+B
H+
H+
H+OH+ O
BONa+ OH-H2O BH4
- BO2-
Na+
Direct Borohydride Fuel Cell
7
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
Catalysts• Three catalysts tested: 20%
Os/ C, PtRu/ C (E-Tek), Pt/ C (E-Tek)
• Os/ C synthesized via Bönnemann method1
▫ Particle growth controlled by tetra-octylammonium tri-ethylhydroborate
Lam, V. W.S., and Gyenge, E. L., J. Electrochem. Soc., 155 (2008) B1155
20 nm
8
1Atwan, M. H. et al., J. New Mater. Electrochem. Syst., 8 (2005) 243
Os/C
Os/C
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
PtRu/C
Pt/C
• Pt▫ BH4
- oxidation within entire potential range
• PtRu▫ Enhanced hydrogen
electrooxidation with the presence of BH4
-
• Os/C ▫ One broad peak was
observed most likely due to direct BH4
- electrooxidation
▫ Number of electrons calculated to be ~7
9
Cyclic Voltammetry
Os/C
Lam, V. W.S., and Gyenge, E. L., J. Electrochem. Soc., 155 (2008) B1155
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008System Study: Fuel Cell
Tests Standard conditions unless
otherwise specified:
• Anode: 1 mg cm-2
• Cathode: 4 mg cm-2 Pt
• Anolyte: 0.5 M NaBH4 - 2 M NaOH; 10 mL min-1
• Oxidant: 1.25 L min-1; 50 psig
• Temperature 333 K and 298 K
• Separator: Nafion® 117
• Separator Conditioned 24 hrs. in 2M NaOH at 293 K
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PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
11
Single Cell Fuel Cell Tests
•Similar performances for all three catalysts
•Os kinetically favourable•Mass transport issues w/ Pt and PtRu•Confirms previous claims that the direct
borohydride oxidation is preferred on Os
333 K 298 K
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
Pt/C
PtRu/C
Os/C
Stability Tests
• Working superficial area: 1 cm2.• Reference Electrode: Hg/ HgO• Counter Electrode: Graphite Rods• Continuous fuel flow: 2 mL min-1
• De-aerated with N2
Working Electrode
Graphite Rod Counter Electrodes
Reference Electrode
Lam, V. W.S., and Gyenge, E. L., J. Electrochem. Soc., 155 (2008) B1155
12
•Confirmed with FC Tests
MM
E
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
• Shown to improve performance in DMFC with electrolyte
• High electrode area per unit electrode volume
• Higher residence time (normalized space velocity)
• Promotes turbulence increase in mass transport
• Depending on substrate mass transport may be larger for 3D electrode than 2D electrode by 2 orders of magnitude
Extended Reaction Zone Electrode(3D Electrodes)
100
2
33
2
'
'
mA
mVmm
A
I
I
cnFAkI
ckVnFAI
ee
L
L
mL
meeL
13
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
•Three Requirements▫Electronic
Contact ▫Transport to
Catalyst Sites▫Ionic
Contact
CCM/ GDE Electrode structure
Solid Electrolyte
Diffusion Layer
Catalyst Particle
Carbon Support
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• Supporting electrolyte negates the need for Nafion in the catalyst layer
• Nafion may impede mass transport of BH4- anion to catalyst
sites
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
Electrode structure comparison
CCM
3D Electrode
Diffusion Layer
Catalyst Layer
Membrane
3D Electrode
Membrane
Diffusion Layer
• Thicker electrode (~350 μm) allows greater electronic contact area
• Diffusion layer ~ 300 μm
15
Flowfield Plate
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
Electrode structure comparison
NaBH4
+NaOH
Bulk Fuel Flow
3D Electrode
Mem
bra
ne
NaBH4
+NaOH
Bulk Fuel Flow
CCM
Mem
bra
ne
• Bulk fuel flows parallel to the active layer for CCM
• CCM Catalyst Layer = ~15-50 μm vs. 350 μm 3D electrode
• Bulk fuel flows through the active layer in for the 3D electrode▫ Better Mass
Transport
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Catalyst Layer
3D Electrode
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
• Control deposition morphology with non-ionic surfactant
Conditions• Pt and Ru in microemulsion• Constant Current 5 mA cm-2
• Time = 1.5 hrs.• Temperature = 333 K
GF-S3• Thickness = 350 μm• Porosity = 0.95• Specific surface area = 104 m2m-3
Bauer, A., Gyenge, E. L., Oloman, C. W., Electrochim. Acta 51 (2006) 5356Bauer, A., Gyenge, E. L., Oloman, C. W., J. Power Sources 167 (2007) 281
Template Electrodeposition
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
•Particle Size = 3.7 to 4.5nm
•Surface Area = 82 m2 g-1
•58 at% Pt and 42 at% Ru ICP
Characterization of PtRu 3D Electrode
cos
D
D
xSA
PtRu
4106
20 nm
100 nm
18
Bauer, A. et al., Electrochim. Acta, 51 (2006) 5356
200 μm
GF
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
• Conditions of experiments as before. T = 333 K
• Better kinetics
• Better mass transport
• Comparable catalyst load
• Performance attributed to:▫ Pt:Ru ratio (3:2)▫ Properties of electrode
structure
Performance comparison to CCM
19
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
• There is a high potential to reduce DBFC system cost through anode material selection
• Osmium is a promising anode catalyst ▫ Fraction of the price of platinum▫ Improved kinetics▫ Lower hydrolysis of borohydride
• 3D electrode structure can further enhance anode performance▫ Increase in kinetics▫ Increase in mass transport▫ Increase in electrical contact
• Future work to incorporate Os catalyst with 3D electrode
Conclusion
20
PRiME 2008: Joint International Meeting PRiME 2008: Joint International Meeting Honolulu – October 16, 2008Honolulu – October 16, 2008
•Natural Sciences and Engineering Research Council of Canada (NSERC)
•Auto 21 Network of Centres of Excellence (NCE)
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Acknowledgements