Electron Microscopy Study of Formation of Thick Oxide Films on Ir and Ru Electrodes

Abstract

The thick (5–60 μm) oxide films that can be formed electrochemically at Ir and Ru electrodes are examined by scanning and transmission electron microscopy. Their morphologies are discussed in relation to the remarkable reversibility of redox processes associated with the thick films and the enhancement of apparent electrocatalytic effectiveness of the oxide films, which is found for anodic and evolution processes to increasing extents with growth of the oxide film. The latter effects are much larger at Ir than at Ru. This difference is likely to be associated with the different morphologies of electrochemically grown oxide films that are observable on these two metals. In the case of Ir, the apparent enhancement of electrocatalysis for anodic or evolution seems to be associated with development of a hyperextended, microporous, hydrous oxide film that is accessible to and Cl⁻ ions. Comparisons are made with thermally formed oxide films generated by decomposition of coatings.