Isotherms of electrochemical doping and cyclic voltammograms of electroactive polymer films

Various forms of the isotherm of electrochemical doping are derived in this paper both for electron-conducting and conventional redox polymer film covered electrodes. The theory is based on a lattice-gas model for all types of mobile charge species within the polymer, such as polarons or bipolarons at the macromolecules and compensating ionic countercharges, with an account of long-range interactions within the mean-field approximation. The form of the doping isotherm was found to be dependent on the prevailing type of electronic charge carriers. The potential drops at the two interfaces, metal-polymer and polymer-solution, were determined not only by the doping isotherm but also by the type of charge species within the film whose concentration is restricted in the course of doping. Drastically different mechanisms of electrochemical reactions of redox species from the solution at the surfaces of electron-conducting and conventional redox polymer films are assumed to be due to different potential distributions across these systems. The doping isotherms and corresponding voltammograms are also derived with an account of retarded electron transfer at the metal-polymer interface, and of capacitance charging of the polymer-solution boundary. The theory proposed is able to give an interpretation of various published experimental data on coulometric titration of polypyrrole and polyvinylferrocene as well as on the voltammetric behaviour of polybithiophene.