CONTRIBUTION TO THE THEORY OF CONDUCTING-POLYMER ELECTRODES IN ELECTROLYTE-SOLUTIONS

This paper consists of two parts. First, an electrical model circuit is proposed that can reproduce, with high accuracy, the small-amplitude impedance behaviour of conducting-polymer electrodes in electrolyte solutions. Secondly, the electronic structure of the metal/polymer/solution interphase is discussed in terms of a band model. From the outset it is emphasized that a conducting polymer grown in an electrolyte solution is essentially a porous electrode. As a result, the electrical model circuit has the form of a diagonally connected discrete ladder network characterized by three impedances, x, y and z After showing how these generalized impedances can be replaced by particular arrangements of passive circuit elements corresponding to real processes in the polymer, two limiting behaviours are derived corresponding to readily accessible experimental conditions: these behaviours are the high-frequency impedance response of the reduced polymer, and the low-frequency impedance response of the oxidized polymer. Both responses are identified in experimental data from polypyrrole. Based on the same porous electrode model, it is next proved that, at low frequencies, the total impedance of the polymer is dominated by that of the pore walls. This opens a 'back door' to the analysis of conducting-polymer behaviour in the complex plane of impedance and, in particular, brings to light the importance of charge leakage across the pore walls. This section is followed by a discussion of the electronic structure of the polymer interior, from which an idealized band model is developed showing how the applied potential is dropped across the entire metal/polymer/Solution interphase. Finally, critical analysis of the band model reveals the importance of the flatband potential in determining the polymer behaviour. This seems to have been universally neglected in the past. One result of including the flatband potential in the model is that it becomes necessary to postulate the existence of electrostatically stabilized polarons: if real, these would also explain the asymmetry of voltammograms in dilute electrolyte solutions.