On the impedance of porous electrodes – double-layer charging and charge transfer on an inhomogeneous inside electrode surface

A model of the impedance of porous electrodes was derived. The model generalized the de Levie model in so far that an inhomogeneous inside electrode surface is assumed. Then the charge transfer resistance and the double-layer capacity were assumed to be distributed. The width of the distribution is characterized by a distribution parameter. The inhomogeneities led to characteristic deviations of the Nyquist impedance plots compared to those for homogeneous inside electrode surfaces: the slope angle of the straight line for higher frequencies was less than π/4, the slope angle of π/2 of the straight line obtained for double-layer charging only and lower frequencies decreased and the semicircle behaviour obtained for double-layer charging and a Faradaic reaction in parallel were replaced by a depressed semicircle. The model allows the determination of the following parameters: the total effective double-layer capacity, the total effective charge transfer resistance, the distribution parameter and the total resistance of the electrolyte inside the pores. The impedance of porous zinc electrodes in weakly alkaline electrolyte can be theoretically well described with the new model. A scanning force microscopy 2-D-image of the morphology of the zinc electrode was performed and discussed.