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 O-2 evolution on nickel hydrous oxide films, NiOx(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 O-2 generated at the NiOx(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 O-2 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 NiOx(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 O-2 evolution, the NiOx(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 O-2 evolution on NiOx(hydr.) grown on Ni electrodes. Implications of these results to the mechanism of self-discharge of NiOx(hydr.) electrodes for alkaline secondary batteries are discussed. (C) 1999 Elsevier Science S.A. All rights reserved.