The mechanism of the electrolytic evolution of oxygen on platinum

Abstract

The kinetics of the electrolytic oxygen evolution reaction on bright Pt anodes in H 2 SO 4 solutions have been studied under ultra pure solution conditions over the concentration range 10 -3 to 10 -1 N in the absence and presence of excess Na 2 SO 4 and in the current density range 10 -11 to 10 -3 A cm -2 . Investigations have also been carried out on the attainability of a reversible oxygen electrode, the time variation of oxygen overpotential at constant current density, its decay with time on open circuit, and the cathodic ionization of oxygen. The unattainability of the reversible oxygen electrode in normally purified solutions is due to the fact that the velocity of simultaneous electrode reactions between impurities in these, solutions and the electrode is much greater than the exchange current for the oxygen evolution reaction. The reversible oxygen potential could be observed and some of its properties studied in solutions which had been purified by a cathodic followed by an anodic pre-electrolysis at high current densities so that they contained less than 10 -11 mole 1. -1 of impurities. The ‘irreversible’ or ‘polarized’ oxygen electrode observed in ordinary solutions is thus a mixed electrode. The time variations of the oxygen overpotential on platinum anodes obeys the relation d nt /dInt = RT /α F' where α = 1/2 or 1, depending on t . It is due to the gradual deactivation of active centres caused by increasing oxide formation. The decay upon interruption of the current follows an analogous equation but α = 1/2 . A technique of determining the variation of the anodic current density as a function of potential has been developed and allows the evaluation of ‘ideal’ Tafel lines from the reversible potential to high current densities. Tafel lines have also been established for the cathodic ionization of oxygen. They intersect with those for anodic evolution at a value of potential which confirms that the overall anodic reaction at an oxygen electrode is 40H - —4e-> O 2 + 2H 2 O. Determination of the transfer coefficient for the reaction is shown to eliminate the combinations of oxygen atoms, the formation of H 2 O 2 or HO 2 , and other previously suggested rate-determining steps. This coefficient, together with the stoichiometric number, prove that the rate of the evolution reaction must be controlled by discharge of an OH - ion or a water molecule. Salt and pH effects show the discharging entity to be water. Discharge occurs on an oxide layer, probably PtO 2 . At high acid concentrations, specific adsorption of SO 2- 4 causes an anomalous increase in overpotential.