Electrochemical and in situ optical studies of supported iridium oxide films in aqueous solutions

Certain aspects of the dynamic behavior of electrochemically deposited hydrous Ir oxide (Ir Ox) films supported on Au microelectrodes during charge and discharge have been investigated by a combination of chronocoulometry and simultaneous in situ normalized reflectance spectroscopy techniques in aqueous solutions. Correlations between the reflectance spectra and the optical properties of the films in its various states of oxidation were sought from in situ transmission measurements for Ir Ox films supported on In-doped tin oxide on glass. The current transient response for Ir Ox ∫Au microelectrodes following a potential step, within the voltage region in which the films display pseudocapacitive characteristics, was found to exhibit a well-defined peak, as opposed to a monotonic decay reported by other groups. Some features of this behavior can be attributed to changes in the conductivity of the film as a function of its state of charge, as has been proposed for electronically conducting polymers. Also presented in this work are data collected over the pH range 0.3-13, which confirm the much faster charge-discharge dynamics in basic compared to acidic media. A primitive model based on proton conductivity within the hydrated oxide lattice is presented which accounts grossly for this pH-induced effect.