Electrochemical impedance spectroscopy on biofilm electrodes : conclusive or euphonious?

Electrochemical impedance spectroscopy (EIS) is a versatile tool that is also exploited to study bioelectrochemical systems and biofilm electrodes. EIS can be used to examine characteristics of biofilm electrodes, which are not accessible by direct current measurements such as biofilm resistance and biofilm capacitance. EIS in microbial electrochemistry is sometimes applied superficially or evaluation of presented data is not comprehensive due to misinterpretation or missing data validation. This hinders a more widespread application of this method, not only for determination of specific biofilm electrode parameters but also from a more practical perspective, e.g. as tool for in situ condition monitoring of biofilm electrodes. We discuss how a careful choice of the experimental set-up, as well as extraordinary diligent EIS data interpretation using electrical equivalent circuit models can lead to conclusive data and meaningful insights. We illustrate the special challenges of studying biofilm electrodes on the example of graphite anodes. We provide an initial guidepost on how to use EIS on biofilm electrodes that requires several preconditions, careful choice of experimental parameters and, nearly mandatory for novices similar to us, the consultation of experienced operators of EIS.