The effect of anions on the electrochemical reduction of thick oxide films on platinum electrode in acidic aqueous solutions

The cathodic galvanostatic reduction of thick oxide films on Pt has been studied in HF, HClO₄ and H₂SO₄ solutions at 25°C as a function of the current density and the concentration of these acids.The thick oxide film was found to be completely reduced at a single potential step of 0.6 V (vs rhe in the test solution) in HF solution and also in very dilute HClO₄ and H₂SO₄ solutions. However, when the oxide was slowly reduced in these solutions except HF solution, the charging curve exhibited two well-defined potential arrests. That is, as the reduction current was decreased, the first arrest at 0.6 V shifted slightly towards positive potential, but the second arrest moved remarkably in negative direction. The same shift of the second arrest was observed when the solution was agitated or the acid concentration was increased. The potential shift of the second arrest is attributed to blocking the reduction of the inner-layer oxide by anions adsorbed on the bare surface which was formed by the reduction of the outermost monolayer oxide at the first arrest potential. The effect of HSO₄^? on the reduction of the inner-layer oxide was stronger than that of ClO₄^?, and the effect of F^? was very weak.