Growth and properties of anodic oxide films on platinum

by 1974- Sun, Adan

iii 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 in 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 acid and alkaline solutions under steady state conditions has been investigated using Electrochemical Impedance Spectroscopy (EIS) and ARXPS (Angle-Resolved X-Ray Photoelectron Spectroscopy). Mott-Schottky analysis shows that the film in both acid and alkaline solutions is n-type, indicating that the defects in the film are either platinum interstitials or oxygen vacancies, or both. ARXPS analysis of films formed in acid solution shows that, for potentials between 1.0 and 1.3 VSHE, 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 VSHE and ARXPS analysis shows that these bi-layer films comprise an inner layer containing Pt(II) and an outer layer containing Pt(IV). Optimization of the Point Defect Model on the EIS data obtained in acid solution yields values for various kinetic parameters that are then used to predict the steady state thickness and current. Good agreement with experiment is obtained. The kinetics of growth of the oxide film on platinum has been studied in alkaline solution as a function of pH. The experimental data confirm the existence of steady states in the current and thickness. Both transient and steady state currents are found to increase with pH. The donor density in the film calculated according to Mott-Schottky iv analysis increases with pH. Diagnostic criteria for the dependence of the steady state current and thickness on applied potential and pH are derived from the PDM, in which the principal point defects in the barrier layer are postulated to be cation interstitials and oxygen vacancies, thereby accounting for the n-type electronic character. The steady state thickness is then predicted to vary linearly with the applied potential and pH. These predictions are in good agreement with the experimental findings. The PDM also predicts a linear relationship between the logarithm of steady state current and applied formation potential with a non-zero slope, but only if ???. If ?=? and the film is n-type, then ln(Iss) is predicted to be independent of the applied voltage. The experimentally observed of diagnostic criteria derived from the PDM are employed as the constraints in the optimization of the PDM on EIS data obtained in alkaline solutions. Good agreement between the experimental data and the calculated data is observed.