Electrochemical Impedance Spectroscopy (EIS) is used to characterize the interface between a metal and a conductive solution.
Most EIS experiments are performed using a potentiostat. This electronic device applies both a DC potential and a small superimposed AC excitation to a metal sample immersed in a solution. The potentiostat also requires two other electrodes that are immersed in the same solution. The first electrode supplies current and the second measures the sample's electrochemical potential. The solution and the three electrodes are collectively referred to as an electrochemical cell.
In many EIS experiments, a reference electrode is not available. In these cases, the potentiostat can operate in a two-electrode mode with one electrode used to supply current and provide a voltage signal.
EIS can also be performed in a galvanostatic mode, where cell current is controlled and cell potential is measured.
In the most common EIS experiment, AC current and AC potential are measured as the frequency of the excitation is varied over a very wide range (often six decades or more). The cell voltage and current are then converted into a complex impedance. Unlike impedance measurement in most other fields, electrochemical impedance systems also have to measure DC current and potential. The DC values are required for intelligent autoranging decisions.
Analysis of a complex impedance versus frequency curve can yield information that is not easily or accurately available from other electrochemical techniques. EIS is very 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.
EIS is not the answer to all electrochemical testing needs. In general, EIS is only useful for the study of systems at 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.