Electrooxidation mechanisms and discharge characteristics of borohydride on different catalytic metal surfaces.

The electrooxidation behavior of BH4(-) on electrocatalytic Pt, hydrolytically active Ni, and noncatalytic Au electrodes were comparatively reexamined and a more generalized reaction mechanism was proposed to explain the very different anodic properties of BH4(-) on the different metal electrodes. In this mechanism, the anodic reaction behavior of BH4(-) are determined by a pair of conjugated reactions: electrochemical oxidation and chemical hydrolysis of BH4(-), the relative rates of which depend on the anodic materials, applied potentials, and chemical states of the anodic surfaces. At Pt surface, the electron number of BH4(-) oxidation increases with the increased potential polarization, while the actual electron number of BH4(-) oxidation on Ni electrode is 4 at most due to the poor electrocatalytic activity of the oxidized Ni surface and the strong catalytic activity of metallic Ni for chemical recombination of the adsorbed H intermediate. On the hydrolytic-inactive Au surface, the anodic reaction of BH4(-) can proceed predominately through direct electrochemical oxidation, delivering a near 8e discharge capacity.