The kinetics of the electrochemical formation and reduction of monomolecular oxide layers on platinum in 0.5 M H 2SO 4: Part II. Galvanostatic pulse measurements and the model of oxide growth

Abstract

The formation and reduction of platinum oxide layers in 0.5 M H2SO4 was investigated by galvanostatic pulse measurements. The results led to the conclusion that the properties of the layer are dependent on the formation conditions of the layer, i.e. the formation rate, the ageing of the layer, and the method of the setting of the coverage (anodic/increase or cathodic/decrease). At constant formation conditions, ϑ-dependent Tafel equations, log i+=A++(ε−E+)/b+, are obtained for the anodic layer growth, and log |i−|=A−−(ε−E−)/b− for the cathodic reduction. The anodic b+-factor is dependent on the coverage according to the equation b+=b+0(1+1.0 ϑ), whereas the cathodic b−-factor is almost constant, b−≈60 mV. The double layer capacity CD decrease with oxide formation according to a similar equation, 1/CD=(1/Ci)(1+1.2 ϑ). The kinetics of the layer formation can be explained by the following model: oxygen ions are chemisorbed from the electrolyte solution in an equilibrium reaction, H2O(aq)⇌O2− (ad)+2H+ (aq). The coverage of these ions does not, however, exceed a few percent. Then the adsorbed oxygen ions are exchanged for platinum ions from the first atomic layers. This process is rate-determining and dependent on the field strength. It takes place in the inner Helmholtz layer, thereby forming an epitaxial surface oxide. Somewhat thicker oxide layers (ϑ=1–2.5) are formed in a similar place exchange reaction at the metal/oxide and oxide/electrolyte interfaces, followed by a migration of platinum and oxygen ions over vacancies or interstitial positions in a high electric field. The oxide is reduced according to the same mechanism, though only at the edges of oxide islands. The b−-factor is therefore nearly independent of ϑ. A three-layer structure has been assumed for the double layer. It is composed of the outer and inner Helmholtz layer (adsorption layer) and the charge-free oxide layer. With increasing coverage, the potential drop in the oxide layer also increases, causing an ascending anodic charging curve at constant current density, a decrease of the current density at constant potential, and a dependence of the b-factors and of the double layer capacity on the layer thickness.