PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 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 September 2008 Carlson, E.J., et al., NREL, NREL/SR , 2005

PRiME 2008: Joint International Meeting Honolulu – 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 Honolulu – October 16, 2008 Background Sodium Borohydride  Borax Na 2 B 4 O 710H 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 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 H 2 PEMFCDMFCDBFC E o 298 K (V) Gravimetric Energy Density (kWh kg -1 ) Volumetric Energy Density (kWh L -1 ) 2.36 at 20 K (liquid) 0.75 at 300 bar wt% 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 - E = 1.24V SHE 2O 2 + 4H 2 O + 8e - = 8OH - E = V SHE NaBH 4 + 2O 2 = NaBO 2 + 2H 2 OE = 1.64 V Indirect: Hydrolysis: NaBH 4 + 2H 2 O = 4H 2 + NaBO 2 Hydrogen Electrooxidation: H 2 + 2OH - = 2H 2 O +2e - Lam, V. W.S., and Gyenge, E. L., J. Electrochem. Soc., 155 (2008) B1155 6

PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 Flowfield Plate Diffusion Layer Catalyst Layer Membrane Catalyst Layer Diffusion Layer Flowfield Plate BH 4 - +NaOH O2O2 BO H 2 O e-e- OOOO e-e- e-e- H+H+ O - e-e- e-e- e-e- e-e- e-e- Na + H+H+ O - H+H+ O - H+H+ O - H+H+ O - H+H+ O - H+H+ O - H+H+ O - H+H+ H+H+ B H+H+ H+H+ H+H+ O H+H+ H+H+ O H+H+ H+H+ O H+H+ H+H+ O H+H+ H+H+ O H+H+ NaOH + H 2 O O B O H+H+ O H+H+ H+H+ O H+H+ H+H+ O - H+H+ O - H+H+ O - H+H+ O - H+H+ O - H+H+ O - H+H+ O - H+H+ O - Na + H+H+ O - H+H+ H+H+ B H+H+ H+H+ H+H+ O H+H+ O B O OH - H2OH2OBH 4 - BO 2 - Na + Direct Borohydride Fuel Cell 7

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

PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 PtRu/C Pt/C Pt ▫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 Cyclic Voltammetry 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

PRiME 2008: Joint International Meeting Honolulu – October 16, 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 Honolulu – October 16, 2008 Pt/C PtRu/C Os/C Stability 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 Working Electrode Graphite Rod Counter Electrodes Reference Electrode Lam, V. W.S., and Gyenge, E. L., J. Electrochem. Soc., 155 (2008) B Confirmed with FC Tests MME

PRiME 2008: Joint International Meeting Honolulu – 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) 13

PRiME 2008: Joint International Meeting Honolulu – 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 14 Supporting electrolyte negates the need for Nafion in the catalyst layer Nafion may impede mass transport of BH 4 - anion to catalyst sites

PRiME 2008: Joint International Meeting Honolulu – 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 Honolulu – October 16, 2008 Electrode structure comparison NaBH 4 + NaOH Bulk Fuel Flow 3D Electrode Membrane NaBH 4 + NaOH Bulk Fuel Flow CCM Membrane 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 16 Catalyst Layer 3D Electrode

PRiME 2008: Joint International Meeting Honolulu – 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 = 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 Template Electrodeposition

PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 Particle Size = 3.7 to 4.5nm Surface Area= 82 m 2 g at% Pt and 42 at% Ru  ICP Characterization of PtRu 3D Electrode 20 nm 100 nm 18 Bauer, A. et al., Electrochim. Acta, 51 (2006) μm GF

PRiME 2008: Joint International Meeting Honolulu – 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 Honolulu – 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 Honolulu – October 16, 2008 Natural Sciences and Engineering Research Council of Canada (NSERC) Auto 21 Network of Centres of Excellence (NCE) 21 Acknowledgements