CHARGE-TRANSFER REACTIONS ON OXIDE-COVERED TI ELECTRODES - EFFECT OF DIELECTRIC-CONSTANT VARIATION AND OXIDE STABILIZATION

The electron transfer kinetics of the Fe3+/Fe2+ redox couple on the ox ide covered Ti electrodes were investigated as a function of the film thickness, film stabilization, and concentration of the redox species. The oxides were formed potentiodynamically in acidic solution of pH = 3 (Na2SO4 + H2SO4) at a scan rate of 1 mV s(-1). The cathodic and ano dic Tafel lines were also measured at the same scan rate. The logarith m of the exchange current density io shows an exponential decrease wit h the oxide film thickness. Direct tunnelling may explain the kinetics of the reaction across thin TiO2 (oxide formed at the potential regio n before the O-2-evolution). Values of the cathodic and anodic transfe r coefficients are tabulated. It was found that, the stabilization of the oxide could possibly increase its conductivity. The electron trans fer reaction showed a first order dependence on the redox species conc entration, independent of the conditions of oxide film formation.