Electrochemical and optical studies of thick oxide layers on iridium and their electrocatalytic activities for the oxygen evolution reaction

Abstract

Thick oxide layers can be grown on iridium in 0.5 M H 2SO 4 by a potential multicyling treatment, mainly due to the highly irreversible reaction — the electroreduction of the lower hydroxide. The optical analysis of such layers was performed by combined ellipsometric and reflectometric measurements. The results of these measurements show that a hydroxide layer with n film=1.44–0.01 i and a thickness which may exceed 2000 Å remains even at cathodic potentials on the Ir electrode, as a result of multicycling. Application of anodic potentials close to the OER region results in an increase of the extinction coefficient to a level typical for semiconductors ( n film=1.43–0.10 i at 1.50 V). This thick semiconducting phase oxide on Ir in the OER region seems to have a high level of bulk defects and a high surface concentration of active sites, the generation of both being related to the gradual variation of the oxidation state of the Ir ions in the oxide, prior to and simultaneously with the evolution of oxygen. The oxygen evolution rates on Ir between 1.50–1.60 V are shown to increase significantly in the presence of such thick oxide layers (as compared with electrodes not coverred by thick layers); a lower Tafel slope is also observed on these thick oxides.