Abstract
The anodic behavior of Pt electrodes in O 2-saturated sulfuric acid solution is shown to be critically dependent on the treatment of the electrode. The open circuit potentials, 1.48, 1.13 and 0.98 V, observed in purified solution, are related to the thermodynamic platinum-oxide potentials. The erratic behavior of Pt electrodes in non-purified solutions is also explained. The thermodynamic value of 1.23 V for the H 2O-O 2 reaction was obtained when certain conditions were fulfilled. The Tafel (b) parameter for the anodic oxygen evolution reaction was o.113, and i o = 2.5 × 10 −10 A/cm 2. Chemisorption of oxygen leads to a dipole potential of 0.98 V. An oxide film is produced by anodic oxidation. This was confirmed by an optical method (thin film ellipsometer) which showed that the growth of a film commences only above 1 V. Cathodic transient technique was used to measure the oxygen coverage ( Q) of the Pt electrode as a function of the applied potentiostatic voltage ( V). A linear relationship between Q and V was found. This relationship, Q = f( V), is explained on the basis of a modified Cabrera-Mott model. The evidence suggests that growth takes place on certain sites of the first layer only.
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