The effect of O vacancies, in anodic oxide films on passive Ti, upon the kinetics of O electrode reactions was studied by means of potentiodynamic polarization and electrochemical impedance spectroscopy. Oxide films with various donor densities were prepared galvanostatically at various current densities until a potential of 20.0VSHE was achieved. The semiconductive properties of the oxide films were characterized by using electrochemical impedance spectroscopy and Mott-Schottky analysis, and the thickness was measured by using ellipsometry. The film thickness was found to be almost constant at 44.7nm, but Mott-Schottky analysis of the measured high-frequency interfacial capacitance showed that the donor (O-vacancy) density in the n-type passive film decreased sharply with increasing oxide film formation rate (current density). Passive Ti surfaces, having a wide range of donor densities, were used as substrates in order to determine relationships between the rates of O reduction/evolution and the donor density. It was found that the rates of both reactions were higher for passive films with higher donor densities. The explanations offered included enhanced film-conductivity due to the vacancies facilitating charge transfer, and surface O-vacancies acting as catalytic sites for reactions. The possible involvement of surface O-vacancies in the O electrode reaction was explored by determining the kinetic order of the O electrode reaction with respect to the donor concentration. The kinetic orders were found to be greater than zero; thus indicating that O vacancies were involved as electrocatalytic reaction centers in both O evolution and reduction reactions.

Effect of Oxygen Vacancies in Anodic Titanium Oxide Films on the Kinetics of the Oxygen Electrode Reaction. B.Roh, D.D.Macdonald: Russian Journal of Electrochemistry, 2007, 43[2], 25-135