High Performance Anode Catalysts for Direct Borohydride Fuel Cells

High Performance Anode Catalysts for Direct Borohydride Fuel Cells PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 Vincent W.S. Lam1, Előd L. Gyenge1, and Akram Alfantazi2 The University of British Columbia 1Department of Chemical and Biological Engineering 2Department 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 3 Outline • Borohydride Background • Alternative Anode Catalysts ▫ Os/C, Pt/C, PtRu/C • Advanced Electrode Structure ▫ Extended Reaction Zone Anodes (3D Anodes) • Conclusion PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 4 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 Wu, Zing et al., U.S. DOE, DE-FC36-04GO14008 , 2004 PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 5 Why Sodium Borohydride? • Non-carbonaceous fuel • High standard potential • High gravimetric energy density • Competitive volumetric energy density H2 PEMFC Eo298 K (V) Gravimetric Energy Density (kWh kg-1) Volumetric Energy Density (kWh L-1) ▫ No CO poisoning DMFC 1.21 DBFC 1.64 1.23 33.0 6.1 9.3 2.36 at 20 K (liquid) 0.75 at 300 bar 4.42 1.86 20wt% NaBH4 PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 6 Direct Borohydride Fuel Cell Principal Reactions: Direct: NaBH4 + 8OH- = NaBO2 + 6H2O + 8e2O2 + 4H2O + 8e- = 8OHNaBH4 + 2O2 = NaBO2 + 2H2O Indirect: Hydrolysis: NaBH4 + 2H2O = 4H2 + NaBO2 Hydrogen Electrooxidation: H2 + 2OH- = 2H2O +2eE = 1.24VSHE E = 0. 40 VSHE E = 1.64 V Lam, V. W.S., and Gyenge, E. L., J. Electrochem. Soc., 155 (2008) B1155 PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 7 Direct Borohydride Fuel Cell e- Membrane Catalyst Layer Diffusion Layer Diffusion Layer Catalyst Layer Flowfield Plate Flowfield Plate O O H+ H+ Na+ + H O O O Na+ + H+ H+B Na+ H H+ H+ H+ O Na+ + NaO H+ + Na O H+ H+ Na+ - - - - BH-4- +NaOH BO2- + H2O Na+ Na+ Na+ +O NaNa++ H+ H O + NaNa++ + ONaO H + B H + H+NaO H+ O+HH+ O+ + NaNa + + H+ H Na O H+ H+ Na+ + NaO + HO + Na H O+ O O++ + NaNa+ H + H+ H H + NaOONa+ H+ + + H Na ---- O2 O O O O NaOH + H2O H2O O H+ H+ OH- O H+ H+ - BH4- H+B + H+ H BO2- O B O PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 8 Catalysts • Three catalysts tested: 20% Os/ C, PtRu/ C (E-Tek), Pt/ C (ETek) • Os/ C synthesized via Bönnemann method1 ▫ Particle growth controlled by tetra-octylammonium triethylhydroborate Os/C 20 nm Os/C Lam, V. W.S., and Gyenge, E. L., J. Electrochem. Soc., 155 (2008) B1155 1Atwan, M. H. et al., J. New Mater. Electrochem. Syst., 8 (2005) 243 PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 9 Cyclic Voltammetry 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 BH4electrooxidation ▫ Number of electrons calculated to be ~7 PtRu/C 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 10 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 PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 11 Single Cell Fuel Cell Tests 333 K 298 K • • • • 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 12 Stability Tests MME Pt/C PtRu/C Os/C Reference Electrode Lam, V. W.S., and Gyenge, E. L., J. Electrochem. Soc., 155 (2008) B1155 • Confirmed with FC Tests • • • • • Working superficial area: 1 Reference Electrode: Hg/ HgO Counter Electrode: Graphite Rods Continuous fuel flow: 2 mL min-1 De-aerated with N2 cm2. Graphite Rod Counter Electrodes Working Electrode PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 13 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  nFAeVe k m c I L  nFAk m c  m2  Ae  3 Ve m 3 ' m IL   2  100 IL Am     • Depending on substrate mass transport may be larger for 3D electrode than 2D electrode by 2 orders of magnitude PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 14 CCM/ GDE Electrode structure • Three Requirements Diffusion Layer Solid Electrolyte ▫ Electronic Contact ▫ Transport to Catalyst Sites ▫ Ionic Contact Catalyst Particle Carbon Support • 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 Honolulu – October 16, 2008 15 Electrode structure comparison Flowfield Plate CCM • Thicker electrode (~350 μm) allows greater electronic contact area • Diffusion layer ~ 300 μm Diffusion Layer Catalyst Layer 3D Electrode Membrane 3D Electrode Diffusion Layer Membrane PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 16 Electrode structure comparison CCM 3D Electrode • 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 3D Electrode Catalyst Layer Membrane NaBH4 + NaOH Membrane NaBH4 + NaOH Bulk Fuel Flow 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 = 104 m2m-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 18 Characterization of PtRu 3D Electrode GF 200 μm 100 nm Bauer, A. et al., Electrochim. Acta, 51 (2006) 5356 •Particle Size D   cos  = 3.7 to 4.5nm •Surface Area 6 x10 4 SA   PtRu D = 82 m2 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 20 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 PRiME 2008: Joint International Meeting Honolulu – October 16, 2008 21 Acknowledgements • Natural Sciences and Engineering Research Council of Canada (NSERC) • Auto 21 Network of Centres of Excellence (NCE)