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 measurements of poly(thiophene-3-acetic acid) films are reported at different stages during oxidation, varying the stationary applied potential. The main issues addressed are: (a) the direct observation of anomalous transport features in the polymeric matrix which provides a solid explanation for the otherwise intriguing high-frequency wing of the spectra; (b) the sharp variation of the dc polymer resistance at a threshold potential Eth that clearly marks the insulator to conductor transition between the polymer oxidation states; and (c) the observation of two different inner polarisation impedances, namely, one parallel and the other normal to the outer macroscopic interfaces. This analytical tool allows us to extract relevant and accurate information about the electrical characteristics of the polymer film electrodes. In addition, some relations between electrical and morphological properties are suggested from the dependences of the fitted parameters with the applied potential.