Borohydride oxidation over Au(111): A first-principles mechanistic study relevant to direct borohydride fuel cells

Direct borohydride fuel cells (DBFCs) offer the potential for direct chemical to electrical energy conversion from a high-specific-energy, water-soluble fuel. The lack of effective anode materials for the electrocatalysis of borohydride has been the major limitation in advancing the application of direct borohydride fuel cells. In this study, we apply electronic structure calculations to elucidate the mechanism of borohydride oxidation over the Au(111) surface. Reaction free energies computed as a function of electrode potential are used to identify stable surface bound intermediates and likely rate-limiting steps. The results suggest that the weak adsorption of B H4- over Au(111) may limit the coverage of reactive intermediates at low overpotentials. Breaking O-H bonds on the Au(111) surface is highly endothermic and occurs over substantial reaction barriers at low overpotentials, leading to stable B (OH) 2* and BOO H* species on the surface. B-O bond formation on the Au(111) surface is facile. B-H bond cleavage has a relatively low barrier, suggesting that Au(111) is effective in breaking B-H bonds. These results indicate that effective anode catalysts with stronger B H4- adsorption and greater activity for O-H dissociation compared to gold are necessary for improving the efficiency and power density of DBFCs.