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The Electrochemical Impedance package provides a comprehensive set of tools for Electrochemical Impedance Spectroscopy (EIS).
During EIS experiments, the potentiostat applies both a DC signal and a small, superimposed AC excitation to a metal sample immersed in a solution. The frequency of the excitation is varied over a very wide range (often six decades or more). In potentiostatic mode, an AC voltage signal is applied and the AC current is measured. In galvanostatic mode, an AC current signal is applied and the resulting AC voltage is measured.
The output of an EIS experiment is a complex impedance spectrum. Unlike impedance measurement in most other fields, electrochemical impedance systems also have to measure both DC current and DC potential which are required for intelligent auto-ranging. 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 versus frequency) or a Nyquist plot (imaginary impedance plotted versus real impedance).
Analysis of the impedance spectrum can give you the following information:
•Mechanistic and kinetic information
Analysis of a complex impedance versus frequency curve can yield information that is not easily or accurately available from other electrochemical techniques. EIS is especially useful in evaluations of coatings, analysis of electrochemical mechanisms and rates, and evaluation of battery performance. Mass-transfer-limited systems have a well-defined EIS behavior that can be separated from kinetically controlled behavior. When used with dual reference electrodes, EIS can be used to study transport across membranes.
However, EIS is not the answer to all electrochemical testing needs. In general, EIS is only useful for the study of systems at the equilibrium. Dynamic or stochastic systems, such as electrode fouling or localized corrosion phenomena, are often better studied using swept or stepped potential techniques. In systems that exhibit multiple states (for example bare metal and metal covered with oxide), a single EIS spectrum only measures one of the states. In corrosion testing using standard practice, EIS can be used to measure a corroding system's polarization resistance. It cannot measure the Tafel constants required to convert this polarization resistance to a corrosion rate.