PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 High Performance Anode...

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


•Borohydride Background

•Alternative Anode Catalysts ▫Os/C, Pt/C, PtRu/C

•Advanced Electrode Structure▫Extended Reaction Zone Anodes (3D Anodes)

•Conclusion

Outline

3


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


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

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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

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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

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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


20 nm

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1Atwan, M. H. et al., J. New Mater. Electrochem. Syst., 8 (2005) 243

Os/C

Os/C


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

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Cyclic Voltammetry

Os/C


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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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


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


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•Confirmed with FC Tests

MM

E


• 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


•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


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

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Flowfield Plate


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


• 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


•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

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Bauer, A. et al., Electrochim. Acta, 51 (2006) 5356

200 μm

GF


• 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

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• 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

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•Natural Sciences and Engineering Research Council of Canada (NSERC)

•Auto 21 Network of Centres of Excellence (NCE)

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Acknowledgements

PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 High Performance Anode...

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