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, h ave been examined for their Tafel behavior during chlorine evolution, as we ll as their cyclic voltammetric (CV) and ac impedance response at the open- circuit potential, in chlorine-free NaCl solutions. Also 30 atom % Ru elect rodes have been electrochemically deactivated, as seen by an increase in th e anode potential and the Tafel slope for the chlorine evolution reaction d uring long-term electrolysis. A comparison of the data for the fresh and th e deactivated anodes suggests that the deactivated anodes have similar elec trochemical characteristics as freshly formed, low-Ru-content (ca. 5 atom % ) oxide films. To understand this better, the experimentally obtained ac im pedance data were compared to the calculated impedance, based on a porous f ilm 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-con trolled 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 electrochem ical behavior to become more similar to that of freshly formed low atom per cent Ru oxide films. (C) 2001 The Electrochemical Society.