Hydrous Ir oxide films: the mechanism of the anodic prepeak reaction

The origin and significance of the anodic prepeak (peak A(0)), seen in cyclic voltammetric studies of hydrous Lr oxide films in acidic solution just prior to the large Lr(III)/(IV) redox peaks (A(1)/C-1), has been examined in this work. By also studying only very thin films, a corresponding cathodic feature (peak C-0) has also been identified. The pH dependence of the A(0)/C-0 peaks, assumed to also depict the Tr(III)/(IV) process, has been examined in sulfuric acid solutions, indicating that a ca. -80 mV shift per pH unit applies to this reaction. By also taking into consideration the measured in situ mass changes as the potential is scanned through the A(0)/C-0 peaks, it is shown that these peaks are associated with a greater involvement of anions and more extensive water exchange than is the reaction in the main A(1)/C-1 peaks. Other results also indicate that the film sites that react in the A(0)/C-0 peaks are located deep within the oxide film, likely near the Ir - oxide film interface.