Charge Transfer Kinetics in Thin-Film Voltammetry. Theoretical Study under Conditions of Square-Wave Voltammetry

An electrode reaction controlled by charge transfer kinetics and occurring in a finite diffusion space is theoretically studied under conditions of square-wave voltammetry (SWV). A simple numerical solution is derived based on the modified step-function method (Mirceski, V. J. Electroanal. Chem. 2003, 545, 29). The SW voltammetric response is mainly controlled by the thickness parameter Lambda = Lroot(f/D), the redox kinetic parameter K = k(s)/rootDf, and the electron transfer coefficient alpha, where L is the thickness of the film, k(s) is the heterogeneous standard redox rate constant, f is the frequency of the potential modulation, and D is the diffusion coefficient of the electroactive species. The mass transport regime is predominantly determined by the thickness parameter. The typical interval for finite diffusion is log(Lambda) less than or equal to 0.3. The main properties of the electrode reaction occurring in a finite diffusion space are the quasireversible maximum and the splitting of the net SW peak. The quasireversible maximum is manifested as a parabola-like dependence of the dimensionless net peak current on the redox kinetic parameter. The maximum of this dependence, which is positioned within the quasireversible region, is selectively sensitive to the standard redox rate constant, for a given thickness of the film. For a large amplitude of the potential modulation, the net SW response splits into two peaks that are positioned symmetrically around the formal potential of the redox system. The splitting is attributed to the fast electrode reactions. The separation between the split peaks is sensitive to both the SW amplitude and the redox kinetic parameter, whereas the relative heights of the split peaks are solely determined by the electron transfer coefficient. Both properties, the quasireversible maximum and the splitting of the net SW peak, can be exploited for complete kinetic characterization of the system and accurate determination of the formal potential of the quasireversible electrode reaction by a fast and simple experimental procedure.