Deactivation of Thermally Formed Ru/Ti Oxide Electrodes: An AC Impedance Characterization Study

Freshly formed Ru/Ti oxide anodes containing between 5 and 40 atom % Ru, have been examined for their Tafel behavior during chlorine evolution, as well as their cyclic voltammetric (CV) and ac impedance response at the open-circuit potential, in chlorine-free NaCl solutions. Also, 30 atom % Ru electrodes have been electrochemically deactivated, as seen by an increase in the anode potential and the Tafel slope for the chlorine evolution reaction during long-term electrolysis. A comparison of the data for the fresh and the deactivated anodes suggests that the deactivated anodes have similar electrochemical characteristics as freshly formed, low-Ru-content (ca. 5 atom %) oxide films. To understand this better, the experimentally obtained ac impedance data were compared to the calculated impedance, based on a porous film model in which a one-electron surface redox reaction occurs. While the fit is good at medium-to-high frequencies, the inclusion of a diffusion-controlled process for the low atom percent Ru films is required to achieve a good fit also at low frequencies. Taken together, these results support the hypothesis that the deactivation of originally high atom percent Ru anodes is due to the depletion of Ru from the oxide film, causing its electrochemical behavior to become more similar to that of freshly formed low atom percent Ru oxide films.