The Potentiostatic EIS technique is used to characterize an electrochemical interface which is held at a fixed potential. A small-signal AC potential excitation is applied to an electrochemical cell. The phase-sensitive AC response of the interface is measured as the frequency of the excitation signal is varied.

EIS is a particularly powerful tool for the study of corrosion of coated metals. It is also useful in almost every other area of electrochemistry, including research in batteries, electrode kinetics, and industrial electrolysis.

The output of an EIS experiment is a complex impedance spectrum. The term complex is used in its mathematical sense: containing both real and imaginary terms. An EIS spectrum is usually graphed as either a Bode plot (impedance magnitude and phase plotted against frequency) or a Nyquist plot (imaginary impedance plotted against real impedance). Analysis of the impedance spectrum can give you the following information: