Switching behaviour in lightly doped polymeric porous film electrodes. Improving distributed impedance models for mixed conduction conditions

The article is concerned with the improvement of electrochemical impedance models for analysing conducting polymer electrodes. The basic framework is that of distributed impedance models (transmission lines) which have proved quite effective in the electrical study of porous systems. Impedance measu rements of poly(thiophene-3-acetic acid) films are reported at different st ages during oxidation, varying the stationary applied potential. The main i ssues addressed are: (a) the direct observation of anomalous transport feat ures in the polymeric matrix which provides a solid explanation for the oth erwise intriguing high-frequency wing of the spectra; (b) the sharp variati on of the dc polymer resistance at a threshold potential Elh that clearly m arks the insulator to conductor transition between the polymer oxidation st ates; and (c) the observation of two different inner polarisation impedance s, namely, one parallel and the other normal to the outer macroscopic inter faces. This analytical tool allows us to extract relevant and accurate info rmation about the electrical characteristics of the polymer film electrodes . In addition, some relations between electrical and morphological properti es are suggested from the dependences of the fitted parameters with the app lied potential. (C) 2001 Elsevier Science BN. All rights reserved.