Charge-storage process in IrO2-SnO2 mixed-oxide electrodes. Role of coating composition, solution pH and Temperature

The effects of both pH and temperature on the kinetics of redox processes that involve the charging/discharging of electrocatalytic sites, at dimensionally stable anodes, have been investigated. At first, electrodes based on Ir and Sn oxides, having different molar compositions, were studied by X-ray diffraction in order to characterize the system under investigation, and with the four-probe method to evaluate how the film resistivity varies with the oxide composition. Then, cyclic voltammetric experiments were carried out at different temperatures to estimate the apparent activation energy (Wa) of the charging/discharging process of active sites. The Wa values obtained were rather low (about 0.5 kcal mol−1) and thus realistically not attributable to any chemical reaction. By excluding a role for the various physicochemical parameters that could influence the process, we suggest that the “double injection/double ejection” mechanism is related to a Grotthuss-type movement of protons, which takes its advantage from an electrode-electrolyte interface extending for a few monolayers within the oxide coating. The interface thus represents a relatively rigid and organized structure in which protons can move without any hindrance.