CHANGES IN THE KINETICS OF THE OXYGEN EVOLUTION REACTION INDUCED BY OXIDE-FILMS AT PLATINUM-ELECTRODES

The oxygen evolution reaction (OER) was studied on different types of oxide-coated Pt electrodes in 1 M H2SO4. Polarization curves at ''oxide-free'' Pt electrodes show two Tafel regions with slopes of ca. 0.120 V per decade and 0.160 V per decade, whereas the polarization curves at Pt electrodes coated with an oxide film accumulated by potential cycling at 0.3 V s-1 between 0.95 V and 2.00 V show a single Tafel region. The value of the single slope decreases steadily as the thickness of the oxide film is increased to reach a limiting value of ca. 0.090 V per decade for oxide coating thicknesses greater than 2 nm. Otherwise, hydrous Pt oxide coated electrodes show two Tafel regions; the first appears between 1.6 and 1.8 V with a Tafel slope which increases as the oxide coating thickness is increased; the second region in the 1.9-2.1 V range involves a Tafel slope of ca 0.088 V per decade which is independent of the coating thickness. This electrochemical behaviour is interpreted in terms of different oxide layer structures in which a fast redox electrochemical system is involved. This system influences the kinetics of the OER, probably through the formation of an intermediate species at the outer hydrous Pt oxide layer involving oxidation states of Pt higher than Pt(IV).