Comparison of Ir oxide film redox kinetics in sulfuric and p-toluene sulfonic acid solutions

Ir oxide films were grown and studied electrochemically in 0.4 M H 2SO 4 and in 0.3 M para-toluenesulfonic acid (TsOH). The equilibrium CV characteristics for films formed in these two solutions were very similar, even though the kinetics of the Ir(+III)/Ir(+IV) charge transfer reaction were ca. 10 times more rapid for films grown and studied in H 2SO 4 versus TsOH. From the ac impedance response of these films, the same equivalent circuit was found to describe them both. In both solutions, the Ir(+III)/Ir(+IV) reaction rate was found to be inversely proportional to the square of the film charge density (film thickness) and to increase exponentially with increasing potential. Kinetic differences of up to 10 times were again found for films studied at the same dc potential and of similar film charge density, but grown and studied in the two different solutions. Analysis of the impedance data could not distinguish between an electron hopping versus a coupled electron-counter ion transport model. Differences in the nanostructure of the Ir oxide films grown and studied in H 2SO 4 and TsOH are believed to be at least partly responsible for the observed kinetic differences. Field Emission Scanning Electron Microscopy ( FESEM) studies revealed a highly ordered pore structure, with pores of ca. 20–30 nm in diameter, for Ir oxide films grown in H 2SO 4, while the pore size of films formed in TsOH must be much smaller than this.