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 BnormalH4− 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 BOOnormalH* 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 BnormalH4− adsorption and greater activity for O–H dissociation compared to gold are necessary for improving the efficiency and power density of DBFCs.

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Developing cost-effective electrocatalysts for the multi-electron borohydride oxidation reaction (BOR) is mandatory to deploy direct borohydride fuel cell (DBFC) systems to power portable and mobile devices. Currently DBFCs rely on noble metal electrocatalysts, and are not capable to fully profit from the high theoretical DBFC voltage, due to the competing hydrogen evolution reaction. Here, highly-efficient noble metal-free BOR electrocatalysts based on carbon-supported Ni nanoparticles considerably outperform Pt/C at overpotentials as low as 0.2 V, both in half-cell and in unit fuel cell configurations. Precise control of the oxidation state of surface Ni is determines the electrocatalytic activity. Density functional theory (DFT) calculations ascribe the exceptional activity of Ni compared to Pt, Pd or Au to a better balance in the adsorption energies of Had, OHad and B-containing reactive intermediates. These new findings suggest design principles for efficient noble metal-free BOR electrocatalysts for DBFCs.

Direct borohydride fuel cells (DBFCs) convert an aqueous soluble, high specific energy density borohydride fuel directly to electrical energy. The lack of effective anode electrocatalysts for the anodic oxidation of borohydride limits the efficiency and power density attainable in these devices. The complexity of the eight electron reaction makes experimental determination of the reaction mechanism extremely challenging, thereby hampering the development of a rationale for optimizing catalyst composition. Computational quantum mechanical methods provide a unique tool for evaluating elementary step reaction kinetics in this system, and can be applied to guide a rational catalyst design procedure. In this perspective, we review the experimental literature on borohydride oxidation catalysis and discuss the usefulness of quantum mechanical methods towards electrode design. Mechanistic insights provided by these computational methods are discussed as well as the prospects of applying a computationally guided design procedure towards developing novel catalyst compositions.