Non-intuitive features of equivalent circuits for analysis of EIS data. The example of EE reaction

•Derivation of the general structure of Faradaic impedance (Zf) for EE reaction.•Four indistinguishable equivalent circuits exhist for Zf, at equilibrium, when D1 = D2 = D3.•The concept of coupling impedance is applied to Zf, at equilibrium, when D1 ≠ D2 ≠ D3.•The Faradaic impedance may exhibit inductive features away from equilibrium.•Illustration of limitations due to the equivalent-circuits approach to EIS.

Analysis of experimental EIS data in terms of equivalent circuits is the common approach used in most commercial software dedicated to EIS method. However, one-to-one assignment of impedance elements to particular electrochemical/physical processes causes difficulty, at the exception of the impedance calculated for one-step reactions. The example of EE reaction is dealt with in this article for the purpose of illustrating non-intuitive and sometimes unexpected features of equivalent circuits derived for a two-step electrochemical reaction involving only soluble diffusing species and electrons.