High Performance Anode Catalysts for Direct Borohydride Fuel Cells

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Vincent W.S. Lam 1 , Előd L. Gyenge 1 , and Akram Alfantazi 2

The University of British Columbia

1 Department of Chemical and Biological Engineering 2 Department of Materials Engineering

PRiME 2008: Joint International Meeting

Honolulu – 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 Carlson, E.J., et al., NREL, NREL/SR-560-39104, 2005 www.platinum.matthey.com, September 2008

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Outline

• Borohydride Background • Alternative Anode Catalysts ▫ Os/C, Pt/C, PtRu/C • Advanced Electrode Structure ▫ Extended Reaction Zone Anodes (3D Anodes) • Conclusion 3

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Background

Sodium Borohydride  Borax Na 2 B 4 O 7 •10H 2 O ▫ Major Deposits: United States, Chile, Argentina, ▫ Minor Depositis: Russia, China

Schlesinger and Brown Process

(T = 498 K  548 K) 4 NaH + B(OCH 3 ) 3 → NaBH 4 + 3 NaOCH 3 4 Wu, Zing et al., U.S. DOE, DE-FC36-04GO14008 , 2004

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Why Sodium Borohydride?

• • • • Non-carbonaceous fuel ▫ No CO poisoning High standard potential High gravimetric energy density Competitive volumetric energy density

E o 298 K (V) Gravimetric Energy Density (kWh kg -1 ) Volumetric Energy Density (kWh L -1 ) H 2 PEMFC

1.23

33.0

2.36 at 20 K (liquid) 0.75 at 300 bar

DMFC

1.21

6.1

4.42

DBFC

1.64

9.3

1.86 20wt% NaBH 4 5

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Direct Borohydride Fuel Cell

Principal Reactions: Direct:

NaBH 4 + 8OH = NaBO 2 + 6H 2 O + 8e 2O 2 + 4H 2 O + 8e = 8OH NaBH 4 + 2O 2 = NaBO 2 + 2H 2 O

Indirect:

Hydrolysis:

NaBH 4 + 2H 2 O = 4H 2 + NaBO 2

Hydrogen Electrooxidation:

H 2 + 2OH = 2H 2 O +2e E = 1.24V

SHE E = 0. 40 V SHE E = 1.64 V 6 Lam, V. W.S., and Gyenge, E. L., J. Electrochem. Soc., 155 (2008) B1155

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Direct Borohydride Fuel Cell

e 7

Na + H + O

-

Na + H + O

-

H + O

-

H + H H B + + H Na + H + O H + O

-

Na + O

BH

Na + Na +

4 -

H + O

+NaOH

H + O

-

Na + Na O O H B H + Na Na O H + + + + + + H Na H + + H + O + H + + H + H H Na +

-

+ + O O Na +

-

O

-

Na + + +

-

+ + + O + Na +

O 2

O O O O

BO 2 + H 2 O NaOH + H Na +

Na +

H 2 O

H + O H +

OH -

H + O

BH 4 -

H + H + B H + H +

BO 2 -

O B O

2 O

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

• •

Catalysts

Three catalysts tested: 20% Os/ C, PtRu/ C (E-Tek), Pt/ C (E Tek) Os/ C synthesized via Bönnemann method 1 ▫ Particle growth controlled by tetra-octylammonium tri ethylhydroborate

Os/C

8 20 nm

Os/C

Lam, V. W.S., and Gyenge, E. L., J. Electrochem. Soc., 155 (2008) B1155 1 Atwan, M. H. et al., J. New Mater. Electrochem. Syst., 8 (2005) 243

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Cyclic Voltammetry

• Pt Pt/C PtRu/C • • ▫ BH 4 oxidation within entire potential range PtRu ▫ Enhanced hydrogen electrooxidation with the presence of BH 4 Os/C ▫ One broad peak was observed most likely due to direct BH 4 electrooxidation ▫ Number of electrons calculated to be ~7 9 Os/C Lam, V. W.S., and Gyenge, E. L., J. Electrochem. Soc., 155 (2008) B1155

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

System 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

10

333 K

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Single Cell Fuel Cell Tests

298 K 11 • • • • 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

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Stability Tests

Pt/C PtRu/C Os/C 12 Reference Electrode Lam, V. W.S., and Gyenge, E. L., J. Electrochem. Soc., 155 (2008) B1155 • • • • • • Confirmed with FC Tests Working superficial area: 1 cm 2 .

Reference Electrode: Hg/ HgO Counter Electrode: Graphite Rods Continuous fuel flow: 2 mL min -1

De-aerated with N 2

Graphite Rod Counter Electrodes Working Electrode

•

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Extended Reaction Zone Electrode (3D Electrodes)

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

I L

'

I L I L

'

I L



nFA e V e k m c



nFAk m c



A e

 

m

2  

V e m

3

A

 

m

 100 • Depending on substrate mass transport may be larger for 3D electrode than 2D electrode by 2 orders of magnitude 13

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

CCM/ GDE Electrode structure

• Three Requirements Diffusion Layer Solid Electrolyte ▫ ▫ Electronic Contact ▫ Transport to Catalyst Sites

Ionic Contact

14 Catalyst Particle Carbon Support • • Supporting electrolyte negates the need for Nafion in the catalyst layer Nafion may impede mass transport of BH 4 anion to catalyst sites

CCM

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Electrode structure comparison

Flowfield Plate 3D Electrode • • Thicker electrode (~350 μm) allows greater

electronic contact

area Diffusion layer ~ 300 μm Diffusion Layer Catalyst Layer Membrane 15 3D Electrode Diffusion Layer Membrane

CCM

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Electrode structure comparison

3D Electrode 16 • Bulk fuel flows parallel to the active layer for CCM Catalyst Layer NaBH 4 + NaOH NaBH 4 + NaOH • • 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

Bulk Fuel Flow 3D Electrode Bulk Fuel Flow

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Template Electrodeposition

• 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 = 10 4 m 2 m -3 Bauer, A., Gyenge, E. L., Oloman, C. W., Electrochim. Acta 51 (2006) 5356 Bauer, A., Gyenge, E. L., Oloman, C. W., J. Power Sources 167 (2007) 281

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Characterization of PtRu 3D Electrode

18

GF

100 nm • Particle Size

D

   cos  200 μm Bauer, A. et al., Electrochim. Acta, 51 (2006) 5356 = 3.7 to 4.5nm

• Surface Area

SA

 6

x

10 4 

PtRu D

= 82 m 2 g -1 • 58 at% Pt and 42 at% Ru  ICP 20 nm

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

19

Performance comparison to CCM

• 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

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Conclusion

• 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 20

PRiME 2008: Joint International Meeting

Honolulu – October 16, 2008

Acknowledgements

• Natural Sciences and Engineering Research Council of Canada (NSERC) • Auto 21 Network of Centres of Excellence (NCE) 21