Modelling the impedance properties of electrodes coated with electroactive polymer films

The response of a metal/electroactive polymer film/solution system to a low-amplitude variation of electrode polarization is analysed, taking account of both the electrodiffusion transport of electronic and ionic species in the bulk film and non-equilibrium heterogeneous charge transfer, namely electrons at the metal-film boundary and ions at the film-solution boundary. Exact analytical solutions are obtained for concentration and electric field profiles inside the film, interfacial potential drops, and complex impedance. Their dependence on frequency and the ratio of diffusion coefficients, D_e and D_i, are analysed. For unequal values, D_e ≠ D_i, the electric field inside the film is shown to be non-uniform and the instantaneous concentration profile is non-symmetrical with respect to the middle-plane of the film. The impedance of the system contains (in addition to the interfacial charging and solution contributions) a frequency-independent resistivity due to ohmic potential drops in the bulk film and at both interfaces as well as distributed capacitive and resistive elements.