A first principles study of direct electrooxidation of aqueous borohydride on Au and Pt surfaces

Direct Borohydride Fuel Cells (DBFCs) have the potential to generate high power densities for use in portable power applications. The volumetric energy density of an aqueous sodium borohydride solution, the DBFC anode fuel, at its solubility limit exceeds that of pure methanol and is about half that of gasoline. Despite the potential of DBFC's, current applications are limited in part, by the lack of an effective anode electrocatalyst. 1-4 Though a number of pure metals and alloys have been tested as anodes, gold and silver electrodes are uniquely capable of completing direct oxidation at near 100% coulombic efficiency.5 Overpotentials on Au anodes, however, limit the overall cell efficiency to ~30%6 and little is known of the elementary electrocatalytic mechanisms, reaction intermediates, or limiting elementary steps . In this study, we apply density functional theory (DFT) calculations to examine borohydride electro-oxidation over the Au(111), and compare with Pt(111) surfaces for which non-selective hydrolysis paths compete with direct oxidation. Possible stable surface intermediates and limiting elementary steps are identified and results are discussed in comparison with experimental data from the literature.