Anomalous transport effects in the impedance of porous film electrodes

The behavior of the ac conductivity in disordered materials where the charge carriers (electrons, holes, polarons, ions) proceed by discrete transitions between localized states exhibits a universal pattern consisting of a power-law domain at high frequencies and a constant conductivity in the low-frequency wing. Such behavior is likely to occur in several types of new electrode materials such as nanostructured semiconductors and electroactive polymers. We suggest new models for the analysis of these systems by impedance. The standard double-channel transmission line for the impedance of porous electrodes is endowed with a description of anomalous transport effects. The modeling of transport in a disordered medium is based on a macroscopic phenomenological theory that accounts for the retarded nature of the response to a locally acting force due to the localization of the carriers. The influence of the power-law behavior in the solid matrix gives rise to a bending of the 'Warburg' part of the impedance spectra, or to an are at high frequency, among other possible responses. (C) 1999 Elsevier Science S.A. All rights reserved.