Influence of the boundaries in the impedance of porous film electrodes

This paper presents an enhanced model for the impedance of porous film electrodes. The impedance of a transmission line with two transport channels, a crosswise element and arbitrary terminal loads is solved analytically. The local impedances at the boundaries represent a frequency-dependent response of the blocking of ionic and electronic charge carriers at the two faces of the electrode region. A general expression is found that contains, as particular cases, a number of models of impedance for porous electrodes used in the literature. Some examples of the generalised transmission line illustrate the use of the model in the interpretation of experimental data. First, a polarisable electrode showing low-frequency dispersion of the constant phase element (CPE) type is analysed, and diagnosis criteria are derived to recognise whether the dispersion is caused by the boundary or the inner surface. Secondly, the manifestation in the impedance of the failure of a porous electrode due to direct charge transfer between the substrate and a redox couple in solution is investigated.