Equivalent Circuit Fitting of EIS Data

The use of equivalent circuit models (ECM) has become the de-facto method of analyzing EIS data. It can be extremely helpful in understanding the electrochemical reactions and physical processes that occur in at an electrode or in a device. Equivalent circuit modeling can help you formulate a hypothesis about the processes or mechanisms that are taking place. Often, an equivalent circuit model which fits the data will suggest some chemical model, process, or mechanism which can be proposed and tested. EIS data and the equivalent circuit model provide a useful tool to help you think about your electrochemical problem.

EIS data has become easier to obtain as the cost of entry has been reduced and this allows the scientist to apply a model to their data and obtain a fit without understanding the underlying mechanisms. This often involves adding components to the model until a good fit is achieved. Some guidelines are required to avoid overcomplicating your model and hence misinterpreting your system.

In general, each element in an equivalent circuit should be supported by at least a decades worth of frequency data. Goodness of fit is a measure of the suitability of your model and when you add a new element to a model, a ten-fold decrease in the goodness of fit is a good rule of thumb for its suitability. If the ooodness of fit is not substantially improved, then the simpler model is more appropriate.

Many researchers who are new to EIS measurements try to improve their equivalent circuit fit by adding additional elements, however these should only be included if there is a clear physical interpretation for the element.