Charge percolation in electroactive polymer films

Abstract

A steady-state dual-electrode ring—disc technique which can be utilised to determine charge-transfer diffusion coefficients in electroactive and electronically conductive polymer films is described. The method has been applied to two different electroactive polymer films: a ruthenium containing redox polymer, [Ru(bpy)₂(PVP)Cl]Cl, and the electronically conducting polymer polyaniline. Charge-transfer diffusion coefficients obtained for the redox polymer were of the order of 10^–9 cm² s^–1. The latter value is at least three orders of magnitude greater than values obtained using cyclic voltammetry or chronoamperometry, and was presumed to correspond to electron-hopping diffusion, rather than couterion motion. In contrast, for the electronically conducting polymer little difference was observed between the diffusion coefficients obtained via the steady-state dual-electrode method (ca. 10^–9 cm² s^–1) and values obtained using transient techniques (chronoamperometry, chronocoulometry) or complex impedance spectroscopy. This observation results in the implication that the counterion and electron diffusion coefficients are equal in magnitude. This proposal has also been confirmed from complex impedance spectroscopy. The shape of the steady-state current–potential response has been examined for both classes of polymers. In particular, the waveshape analysis for polyaniline films has shown that the redox transformation corresponding to the transition from an insulating to a conductive state, involves the transfer of two electrons, but does not conform to the simple Nernst equation. A modified Nernst equation is proposed which specifically considers interactions within the polymer layer. The departure from ideal conditions is quantified in terms of an interaction coefficient, σ, which has its origin in the Brown–Anson interaction theory. Representative cyclic voltammetric data for thin polyaniline layers are also examined, and confirm that the redox switching reaction must be viewed in the context of the Brown–Anson model.