The thermodynamic driving force to adsorption of the BH4- ion differs substantially between the Au(111) and Pt(111) surfaces. Over the Au(111) surface, adsorption with electron transfer becomes favorable only at large anode overpotentials. The BH4 species adsorbs molecularly over Au, followed by an activated B-H dissociation step. Initial adsorption of the reactant ion therefore limits the performance of Au anodes. Dissociative adsorption is favorable at all potentials of interest over Pt(111). However, subsequent hydrogen evolution competes with the oxidation reaction, leading to non-selective hydrolysis which limits the DBFC Coulombic efficiency. On both Pt and Au, initial dehydrogenation steps occur over small reaction barriers, and direct oxidation of adsorbed BH4 species is limited by O-H dissociation steps occurring late in 8 electron pathway.
These initial results provide insight into rational design approaches for transition metal alloy catalysts for the borohydride oxidation reaction. In addition to the direct relevance of the reported results to DBFCs, the accuracy in the various methods applied to model the electrochemical interface will be discussed. Specifically, comparison between “vacuum slab” and solvated, variable potential methods in calculating ion adsorption equilibrium constants and surface reaction activation barriers will be presented.