Tips for a (simple) interpretation of the impedance response of an electrochemical cell

In electrochemical experiments much of information is enclosed in the impedance response. It is usually represented by means of an equivalent electrical circuit, whose elements are able to describe the most general physico-chemical phenomena which characterize the system. To this aim, some standard circuits have been designed and commercial software is available to perform the best fit of the experimental data. However, the search of the most appropriate equivalent circuit is sometimes not a simple task due to experimental constraints and also due to a limited knowledge of the phenomenon. Therefore, a blind fitting procedure could give unsatisfactory results. Graphical representations of impedance, in particular the phase plot, offer useful tips in guessing the most appropriate equivalent circuit and estimating the values of the circuit parameters. Here we present a procedure in three steps: a. the analysis of some graphical representations of impedance for the selection of a suitable equivalent circuit, b. the formulation of a minimal set of equations useful for guessing the circuit parameters; c. a numerical test. An application to a set of data referring to quite different experimental conditions complete the study.