Growth and properties of oxide films on platinum I. EIS and X-ray photoelectron spectroscopy studies

Electrocatalysts supporting the oxygen electrode reaction are commonly covered with thin oxide films that may have a profound influence over the kinetics of the reaction. Little is currently known about the role of oxide films on electrocatalysts, particularly with regard to the role of the defect structure. In this work, the physical and defect structures of the oxide film that forms on platinum in 0.5 M H2SO4 under steady-state conditions have been investigated using electrochemical impedance spectroscopy (EIS) and ARXPS (angle-resolved X-ray photoelectron spectroscopy). Mott-Schottky analysis shows that the film is n-type, indicating that the defects in the film are either platinum interstitials or oxygen vacancies, or both. ARXPS analysis shows that, for potentials between 1.0 and 1.3 V(SHE) a single-layer film develops, with the principal oxidation state for Pt (in the film) being +2. However, a bi-layer structure was observed for films formed at potentials greater than 1.3 V(SHE) and ARXPS analysis shows that these bilayer films comprise an inner layer containing Pt(n) and an outer layer containing Pt(IV). Optimization of the point defect model on the EIS data yields values for various kinetic parameters that are then used to predict the steady-state thickness and current. Good agreement with experiment is obtained.