Passage of current in electrochemical systems can be attributed to faradaic reactions and to double-layer charging. Faradaic and charging currents are usually considered separately when developing models for the impedance response [1]. Delahay [2] criticized the assumption that charging and faradaic currents are uncoupled on the grounds that part of the flux of reacting species must contribute to the charging of the interface as well as to the faradaic reaction. Nevertheless, the accepted procedure for model development has been to assume that the faradaic and charging currents are independent.

When charging and faradaic currents are considered uncoupled, the flux of reacting species contributes only to the faradaic reaction. Under the assumption that charging and faradaic currents are coupled, the reacting species are seen to contribute to both the faradaic reaction and, along with the inert species, to the charging current.

Relaxation of the assumption that faradaic and charging currents are independent requires coupling an explicit model of the double layer to the convective diffusion equations for each ionic species. Nisancioglu and Newman [3] provided an appropriate mathematical formalism, used by Wu et al.[4] to model the impedance response of a rotating disk electrode.

Through numerical simulation, the coupling of faradaic and charging currents is shown to result in impedance responses yielding frequency dispersion that resembles Constant-Phase-Element (CPE) behavior. The results show that coupling of faradaic and charging currents should be considered when modeling the impedance response for systems influenced by mass transfer, even for large concentrations of supporting electrolyte. Features of the simulations are compared to the results of experiment.

References

1. J. H. Sluyters, Recl. Trav. Chim. Pays-Bas, 79 (1960), 1092-1100.
2. P. Delahay, J. Phys. Chem., 70 (1966), 2373-2379.
3. K. Nisancioglu and J. S. Newman, J. Electrochem. Soc., 159 (2012), E59-E61.
4. S.-L. Wu, M. E. Orazem, B. Tribollet, and V. Vivier, Electrochim. Acta, 131 (2014), 3-12.