Methods for the Determination of Valid Impedance Spectra in Non-stationary Electrochemical Systems: Concepts and Techniques of Practical Importance.

Electrochemical impedance spectroscopy (EIS) is a widely used technique for characterizing electrochemical systems. The term "electrochemical impedance spectroscopy" actually has a twofold meaning: it is both a measurement technique to determine impedance spectra and an analysis tool for extracting information about an electrochemical system. Contrary to some opinions, the design of valid EIS measurements is not trivial in practice, as systems under test also have to meet certain requirements to ensure the validity of the measured data. One of the most serious and underestimated problems is that many electrochemical systems (including biological, reacting, and corroding systems) are intrinsically non-stationary and are affected by time-dependent phenomena. This study provides an overview and guidance on some of the methodologies proposed in the literature to address problems related to impedance measurements in time-varying (non-stationary) systems. Selected techniques for the determination of "true" impedance spectra in non-stationary systems are briefly reviewed. To better understand the theoretical background of the different approaches, basic principles of the measuring methods and the fundamentals of data collection, validation, and correction are also discussed. [ABSTRACT FROM AUTHOR]