Electrode-electrolyte interface impedance: Experiments and model

The impedance of the junction between a solid or aqueous electrolyte and a metal electrode at which no charge transfer processes occur (blocking contacts) follows closely the constant phase angle form, Z=A(jω)^-n, over a wide frequency range, where A is a constant, and the frequency exponent n is typically in the range of 0.7 to 0.95. Several models have been proposed in which the magnitude of the frequency exponent n is related by a simple expression to the fractal dimension \(\bar d\) of the rough electrode surface. But experiments with aqueous H₂SO₄ and roughened platinum and silicon electrodes show that there is no simple relationship, if any at all, between n and \(\bar d\) when \(\bar d\) is determined from the analysis of one dimensional surface profiles. Moreover, n is not a simple function of the average roughness of the electrode. In order to gain some insight into the effect of electrode topography and the interface impedance, a model for the response of the interface to a constant voltage pulse was constructed. This model is based on the idea that, following a pulse, locally concentrated regions of ions accumulate rapidly at the tips of large protrusions on the electrode surface which screens deeper regions of the electrode from the field driven flux of mobile ions. After this rapid charging, ions are able to reach the deeper, screened regions of the electrode by diffusion, and it is this diffusive process that gives rise to the observed t¹⁻ⁿ dependence of the charge collected. Computer simulations, similar to the diffusion limited aggregation model, using measured profiles as fixed (non-growing) clusters, gave exponents n in good agreement with experiment.