An electrical model circuit that reproduces the behaviour of conducting polymer electrodes in electrolyte solutions

Abstract

An electrical model circuit is proposed that can reproduce, with high accuracy, the small-amplitude impedance behaviour of conducting polymer electrodes in electrolyte solutions. In its most general form, the model consists of a diagonally connected discrete ladder network characterized by three impedances x, y and z. However, some special cases are considered in which x, y and z are replaced by passive circuit elements, such as resistors and capacitors, in arrangements corresponding to particular processes in real polymers. The resulting responses are analysed in the complex plane. One case is explored in detail. In it, a conducting polymer electrode in an electrolyte solution is assumed to be porous, with the pores behaving as simple resistors while the mass of the polymer behaves as a binary composite medium. Concurrently, at the cylindrical walls of the pores, the interfacial electrochemistry is modelled as a Debye-type charging process in which the capacitance depends inversely on frequency. Two semicircles are predicted in the complex plane of impedance, with diameters which have opposite dependences on polymer thickness.