EIS is a powerful technique for characterization of electrochemical systems. The fundamental approach of the EIS is to apply a small amplitude sinusoidal voltage excitation to the system under steady state and measure its linear current response at a wide range of frequencies. Analysis of the system response contains information about the interface, its structure and reactions taking place at the interface.
The value of EIS derives from the effectiveness of the technique in isolating the individual reaction and migration steps in a multi-step process. This is possible because each reaction or migration step has ideally a unique time constant associated with it. These steps can be easily separated in the frequency domain.
Generally, there are two types of EIS techniques:
1. Measurement of the impedance as a function of the frequency of a small-amplitude sinusoidal potential perturbation. This technique is called as “Electrochemical Impedance Spectroscopy” or “Impedance Voltammetry”.
2. Superimposition of a single-frequency sinusoidal potential on a scanned or stepped direct potential (open circuit potential) and measuring the sinusoidal current response as a function of the direct potential (OCP). This technique is named as "Alternating Current Polarography" or “Alternating Current Voltammetry”.
EIS cannot give all the answers. It is a complementary technique and the other methods must be used to elucidate the interfacial processes.
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