Rotating ring-disk studies of oxidized nickel hydrous oxide: Oxygen evolution and pseudocapacitance

A rotating ring-disk electrode (RRDE) method is herein described for studies of O₂ evolution on nickel hydrous oxide films, NiO_x(hydr.), electrodeposited on the gold disk of a Au-Au RRDE in aqueous 1.0 M KOH. This technique relies on the quantitative detection of O₂ generated at the NiO_x(hydr.)|Au disk electrode during a linear potential scan, by the concentric, bare Au-ring electrode, which can then be used to determine contributions to the disk current (I_disk) derived solely from O₂ evolution. Subtraction of such contributions from I_disk in the potential range positive to the trailing edge of the peak ascribed to the oxidation of the NiO_x(hydr.) film revealed a constant, positive current when the voltage was scanned in the positive direction, and a constant, negative current, albeit of smaller magnitude, in the subsequent scan in the negative direction. This observation suggests that once account is made for O₂ evolution, the NiO_x(hydr.)|Au-electrolyte interface in that potential range (0.5-0.65 V vs. Hg|HgO,OH^-), behaves as a (pseudo) capacitor, a model that was further confirmed by monitoring the current as a function of the scan rate. The actual values of this pseudocapacitance were found to be on the order of ca. 80 kF mol^-1 Ni sites in the film (or equivalently, ca. 0.1 e^- per Ni site within the potential range specified above) and, thus, very similar to those reported earlier based on current interruption-potential decay and impedance measurements for O₂ evolution on NiO_x(hydr.) grown on Ni electrodes. Implications of these results to the mechanism of self-discharge of NiO_x(hydr.) electrodes for alkaline secondary batteries are discussed.