Impedance spectroscopy, unlike any other electrochemical method, can provide a full insight into the mechanism and kinetics of electrode processes. However, the interpretation of impedance spectra is possible only for systems having a meaningful mathematical description leading to a proper equivalent circuit.

Recently, we have provided, for the first time, an exact analytical solution in linear diffusion regime for impedance of catalytic processes involving redox mediators [1]. In this process the electrochemical step (E) is followed by a homogeneous chemical reaction regenerating depolarizer (C’). In such systems various electroinactive and quite inert molecules can be catalytically oxidized or reduced by means of redox mediators that carry electrons from or to the electrode surface. Introduced model obtained for general case provide a powerful research tool that can be used in studies of many important catalytic processes. However, in many cases microelectrodes are used and spherical diffusion needs to be taken into account.

In this work, we present an analytical solution for faradaic impedance of the catalytic EC’ processes in spherical diffusion geometry. We provide the range of electrode radius, for which spherical model should be applied. We deliver an equivalent circuit together with potential dependence of all impedance parameters, which allows to rapid and precise determination of all kinetic and thermodynamic parameters of EC’ type reactions on microelectrodes. Similarly as for the solution in linear geometry the resistance of the charge transfer, R_ct tends to a constant value at high overpotentials.

The applicability of the model in characterization of the catalytic processes was confirmed experimentally. It has been shown that all key parameters of catalytic systems may be readily obtained even from a single impedance spectrum.

References:

[1] R. Jurczakowski, P. Połczyński, J. Phys. Chem. 118 (2014) 7980–7988