Impedance of Mediated Electrochemical Processes: Fenton Reaction

Impedance is a powerful technique that provides a deep insight into the mechanism, kinetics, and thermodynamics of various physicochemical processes. Impedance of electrochemical systems coupled with homogeneous chemical reactions is seldom reported. Catalytic processes governed by the EC′ mechanism are of particular relevance since these processes are widely employed for mediated electrochemical oxidation of persistent environmental pollutants, gas sensing, catalytic hydrogen evolution, carbon dioxide electroreduction, enzyme oxidation, electrochemistry of nucleic acids, amino acids, as well as for numerous other bioanalytical applications. These system has been investigated almost exclusively by using classical transient electrochemical methods.

Recently we have developed an analytical expressions describing the faradaic impedance of the catalytic processes governed by (pseudo) first-order chemical kinetics. The solution was obtained in the rigorous treatment and it is applicable to the analysis of the experimental EC’ systems regardless the kinetics of the homogeneous and electrochemical steps.¹ We have found that unequal diffusivities of the oxidized and reduced forms of the redox mediator have a strong effect on the impedance characteristics of the catalytic EC’ systems. This behavior is related to the novel impedance element, which reduces to the Gerischer impedance when both diffusion coefficients are equal.¹

In this study we demonstrate how to fully characterize EC’ type processes by using impedance spectroscopy and our model. Provided equivalent circuit together with potential dependence of all impedance parameters allow for a rapid and precise determination of all kinetic and thermodynamic parameters of EC’ type reactions. For Fenton reaction, the formal potential, E⁰’, heterogeneous, k₀, and homogeneous, k, rate constants, electron transfer coefficients, α, β, and both diffusion coefficient of the redox mediator D_ox, D_red were precisely determined. Greater part of these parameters have been determined here for the first time while the other are in good agreement with the previous literature reports.  

[1] R. Jurczakowski and P. Polczynski, J Phys. Chem. C, 2014, 118, 7980-7988.