A voltammetric investigation of the charge storage reactions of hydrous iridium oxide layers

Abstract

The unusual potential/pH behaviour of hydrous oxide layers grown on iridium under potential cycling conditions is interpreted on the basis of a model which describes the interaction between the acid-base and redox properties of this type of system. It is pointed out that the suggested disperse nature of the material in the film is in agreement with earlier conclusions as to the nature of transition metal hydrous oxides in general based on magnetic susceptibility data. Kinetic studies show that at low film thicknesses and slow sweep-rates in 1.0 mol dm −3 H 2SO 4 the main anodic and cathodic charge storage reactions occur in a reversible manner. However with thicker films and faster sweep-rates a marked diffusion limitation is observed for the cathodic process; slow transfer of electrons between redox sites in the dispersed oxide material is assumed to be involved here. Since, for the same film thickness, diffusion limitation during the positive sweep arises only at much higher sweep-rates (there appears to be a second mode of electron transfer in this case due to the fact that in the higher, Ir(IV), oxidation state the oxide is slightly non-stoichiometric), the anodic peak current densities for thick film/fast sweep-rate conditions are generally much large. The large displacement of the anodic peak potential from its reversible value (limiting Tafel slope value ≈200 mV decade −1) demonstrates that some other form of inhibition (possibly related to chemical rearrangement within the surface layer) is involved here.