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 ''oxid e-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 cy cling at 0.3 V s-1 between 0.95 V and 2.00 V show a single Tafel regio n. The value of the single slope decreases steadily as the thickness o f 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 t he 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 i ndependent of the coating thickness. This electrochemical behaviour is interpreted in terms of different oxide layer structures in which a f ast redox electrochemical system is involved. This system influences t he kinetics of the OER, probably through the formation of an intermedi ate species at the outer hydrous Pt oxide layer involving oxidation st ates of Pt higher than Pt(IV).