Potentiostatic intermittent titration technique and electrochemical impedance spectroscopy were investigated as methods to determine solid phase chemical diffusion coefficient (D) and electronic density of states. These techniques were then applied to iridium oxide (IrOx) and iridium-tantalum oxide (IrTaOx) thin films prepared by sputter deposition. The experiments, performed in 1M propionic acid between −0.2 and 0.8V vs Ag/AgCl, showed effects of interfacial side reactions, whose contribution to the electrochemical response could be identified and corrected for in the case of potentiostatic intermittent titration technique as well as electrochemical impedance spectroscopy. It was found that D was strongly underestimated when using the potentiostatic intermittent titration technique with the common Cottrell formalism, which follows from non-negligible interfacial charge transfer and ohmic resistances. Electrochemical impedance spectroscopy indicated an anomalous diffusion mechanism, and D was determined to be in the 10−11–10−10cm2/s range for IrOx and IrTaOx. Both the potentiostatic intermittent titration technique and electrochemical impedance spectroscopy showed that the intercalated charge as a function of potential exhibits a shape that resembles the theoretical density of states of crystalline iridium oxide, especially for IrTaOx.

Determination of Solid Phase Chemical Diffusion Coefficient and Density of States by Electrochemical Methods - Application to Iridium Oxide-Based Thin Films. J.Backholm, P.Georén, G.A.Niklasson: Journal of Applied Physics, 2008, 103[2], 023702