An Extension of the Layer-Pore Resistance Model for Electrode Passivation

Abstract

A theoretical model for a surface reaction involving the deposition of a non-conducting porous layer by simultaneous spreading and thickening is described. The ratio of the kinetic and Ohmic contributions to the total resistance of the reaction is equal to Wa/θ^1/2, where θ is the coverage fraction and Wa is the Wagner number, a dimensionless group which includes the film thickness, electrolyte conductivity and reaction exchange current density. The corresponding contributions to the total overpotential are determined by application of a current-balance condition. Analytical expressions are given for the current-potential curves in the limit of linear polarization, and for the relation between the coverage fraction and the Wagner number at the current peak. It is shown that the linear polarization law is a good approximation if Wa/θ^1/2 < 0.1, and that the theory reduces to the layer-pore resistance model when Wa = 0. The behaviour predicted by the theory in the linear and non-linear polarization regimes is compared, and the use of the model in the analysis of experimental results is discussed.