A powerful electrical network model for the impedance of mixed conductors

Impedance of mixed conductors contacted with electrodes which are blocking for ions and reversible for electrons (or vice versa) is modelled phenomeno logically. A closed form approximation is found which is valid in a very br oad materials parameter window. The approximation is verified by the exact solution of Nernst-Planck-Poisson set of equations. The closed form results show that at low frequencies electrostatic as well as chemical relaxation occur. In particular, the closed form expression describes the transition f rom the 45 degrees Warburg impedance to a 90 degrees rise in the impedance spectra. In the former case the low frequency relaxation time bears informa tion about the chemical diffusion coefficient while in the latter example i nformation about the electrode capacitance is contained. (C) 1999 Elsevier Science Ltd. All rights reserved.