Impedance of Mediated Electrochemical Processes. Novel Impedance Element for Unequal Diffusivities

The faradaic impedance of the catalytic EC' processes is given in an analytical form together with the potential dependence of all impedance parameters. The exact solution derived in the rigorous treatment can be used in the analysis of catalytic systems with arbitrary kinetics. Unusual impedance characteristics have been found for systems having unequal diffusivities of the oxidized and reduced forms of the redox mediator. Depending on the ratio of diffusion coefficients, either the spectra are markedly deformed at low frequency range or an inductive loop appears at the complex plane. This behavior is related to a novel impedance element, which reduces to Gerischer impedance when both diffusion coefficients are equal. The resistance of the charge transfer, R-ct, has been found to be potential-independent at far anodic or far cathodic overpotentials for electrocatalytic oxidation and reduction, respectively. It is shown that the key parameters of catalytic systems, such as both heterogeneous and homogeneous rate constants, and the ratio of diffusion coefficients, may be readily obtained even from a single impedance spectrum.