Oxidation State Changes and Structure of Electrochromic Iridium Oxide Films

Abstract

Oxidation state changes, completeness of oxidation, and densities of anodically formed, electrochromic iridium oxide films have been determined by combined gravimetric, coulometric, and reflection spectroscopy analyses. The results show that the oxidation state of Ir ions in the oxide is changed from III to IV during the anodic coloration process [0.25 to 1.25V (RHE)], rather than II to IV as previously postulated, and that virtually all Ir ions in the film are accessible for electrochemical oxidation and reduction. The mean density of the oxide film is 2.0g cm−3, as compared to 11.68 for bulk crystalline . The structure of the film was shown by electron microscopy to consist of oxide grains 0.05–0.1 μm in diameter, surrounded by voids. In addition, the presence of a high density of microvoids ∼25Å in diameter was detected. The highly porous structure of the film permits ready access of the electrolyte to the oxide grains throughout the entire film and facilitates the rapid coloration and bleaching (∼40 msec) observed in aqueous electrolytes. The apparent accessibility of all Ir ions in the film also implies rapid transport, within the oxide grains, of the mobile charge‐compensating ions which must be injected and ejected to preserve electroneutrality. Mechanisms of ion and electron transport are discussed.