Impedance of ruthenium electrodes in sulphuric acid solution

Electrochemical impedance spectroscopy was employed to characterize three different types of Ru electrodes, anodically polarized and measured at +0.85 V versus SCE in 0.5 mol dm^?3 H₂SO₄, both in their non-electrochemically treated (NET states), and also following various potential/time treatment programmes (ET states). On the basis of experimental data and results of the complex non-linear curve fitting procedure, the impedance/frequency response is for all three Ru electrodes in either the NET or ET state, modelled generally as two impedance branches in parallel. The first, an almost pure capacitative impedance is ascribed to the fast and reversible charging/discharging of the oxide film surface, proceeding through oxidation/reduction of electron sites coupled by the surface deprotonation/protonation. The second impedance consists of the resistive impedance and two transmission line elements in the series connection. This impedance is ascribed to the oxide film equilibration process which proceeds mainly through diffusion-like transport of reaction species within a heterogeneous structure of the oxide film. In dependence on the type of Ru electrode and particularly on the electrode state (NET or ET) there are significant differences in contribution of each impedance to the overall impedance/frequency response. Thus predominance of diffusion-like impedance at medium and low frequencies is observed for all three Ru electrodes in their NET state. This indicates the contribution of the oxide film equilibration process in the overall electrode response. If, however, Ru electrodes are treated by defined potential/time treatment programmes, the electrode impedance/frequency response of each electrode is changed continuously, showing increase of the extent of the surface reaction and facilitated and accelerated oxide film equilibration. For some types of Ru electrodes, pure and low capacitative impedance behaviour in almost the whole frequency region is observed after only few potential steps applied.