Methanol oxidation and hydrogen reactions on NiZr in acid solution

Abstract

The electrochemical properties of a Ni 50Zr 50 (at.%) alloy have been investigated by cyclic voltammetry and steady-state polarization measurements. The alloy forms a passivating oxyhydroxide film that makes it electrochemically stable in an acid solution. The oxyhydroxide film is shown to be an electrocatalyst for the methanol oxidation reaction (MOR). The reaction proceeds at surface O 2− ions neighboring a Ni 3+ ion of a thicker passivating film; electron transfer from the surface to the electrode occurs diffusively by the nickel atoms of the film. A reaction pathway is presented that accounts for the observation of an optimum thickness for the passivating film. The NiZr alloy was also found to catalyze both hydrogen-oxidation and proton-reduction reactions (HOR and PRR) if it has a thinner surface oxyhydroxide film. The alloy appears to form mixed NiZrH and NiZrH 3− x hydrides on cycling negative of the normal hydrogen potential. The activity of the hydrogen-oxidation reaction on a hydride surface was found to increase in the presence of streaming hydrogen gas and also with increasing negative initial potential. Although the hydride is unstable in acid, it may be an attractive candidate for use as a rechargeable negative electrode in an alkaline metal/air or nickel-metal hydride secondary battery.