Electrochemical surface structure and electrochemical performance of Ti based Ru-La oxide coatings in 3.5% NaCl solution have been investigated by electrochemical impedance spectroscopy (EIS), together with cyclic voltammetry (CV) and scanning electron microscopy (SEM). The best-fit circuit to EIS data of the coatings in the absence of the chlorine evolution reaction is R_s(R_ctQ_dl), and that in the potential range of chlorine evolution reaction is R_s(R_fQ_f)(R_ctWQ_dl). The high-frequency region corresponds to the impedance behavior of the coating, and the low-frequency region to that of the interface between outer surface and electrolyte. Results show that the pseudo-capacitances of the oxide film and the double layer increase with increasing La content, reaching a maximum at the nominal La content of 30 mol%. This finding agrees with the CV result, suggesting that the La additive in suitable amount can improve the electrochemical activity of the RuO2 coating. The electric conductivity of the coatings, however, significantly decreases when nominal La content is larger than 30 mol% in the region of the chlorine evolution potential. A character with a Warburg impedance at low-frequency region suggests the relationship to passivation of the coating surface and specific adsorption, resulting in a dramatic decrease of electrocatalytic activity of the coatings at norminal La content of 70 mol%.

Key words: electrochemical impedance pedance spectroscopy, electrocatalysis, oxide coating, lanthanum, RuO₂