Methanol oxidation in acid on ordered NiTi

Abstract

A 50–50 ordered NiTi alloy is shown to form a passivating layer that makes it electrochemically stable in acid solution and to be an electrocatalyst for the methanol oxidation reaction. To elucidate the reaction pathways, the electrochemical properties of Ti and Ni electrodes were compared with those of NiTi and Pt electrodes. Alloying Ti with Ni suppresses the corrosion reaction on Ni electrodes in acid by shifting the potential for Ni²⁺ion dissolution to a value above that at which the Ni²⁺ ions at the surface are converted to Ni³⁺ ions. NiTi electrodes, like Ni and Ti electrodes, form oxyhydroxide films at anodic potentials, and the thickness of the film increases with applied anodic voltage. Methanol oxidation occurs on the oxyhydroxide films. Dissociative chemisorption requires a potential sufficiently anodic of the flat-band potential of the film composition at the surface, or threshold potential E_th for the creation of surface Ni³⁺ ions, for electron transfer to take place via tunnelling across a Schottky barrier to either a conduction band, in the case of Ti, or an Ni^3+/2+ redox band in the case of Ni. In the case of NiTi, the former mechanism appears to be operative at lower potentials, the latter at higher potentials. The reaction is completed with an attack by water on a surface carboxy group, which also requires electron tunnelling across a depletion layer. Methanol oxidation on NiTi is more active than on Ti or Ni, and it is completed at a potential E < 1.23 V. A distinction was found between p-type and n-type oxyhydroxide layers. The n-type layers, formed only occasionally, gave cyclic voltammograms which resembled those obtained on elemental platinum.