Electrooxidation Mechanisms and Discharge Characteristics of Borohydride on Different Catalytic Metal Surfaces

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Abstract
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.