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Part 7: Electrochemical Impedance Spectroscopy (EIS)

This is a basic introduction to the electrochemical measurement method electrochemical impedance spectroscopy (EIS).

The topics are listed below:

EIS V: Constant Phase Element Nyquist Plot

This issue will talk about the equivalent circuit element called CPE (Constant Phase Element). The semicircle in the actual measured Nyquist plot usually appears as an irregular (deformed) semicircle, rather than an ideal semicircle shape.

This phenomenon is due to the inhomogeneity of the electrodes (inhomogeneity of potential and current density due to the geometry of the electrodes, etc., non-uniform distribution of physical and chemical properties of the various surfaces, such as concavity, adsorption and degree of coverage of the electrode surface). The CPE impedance, expressed in the following equation, is often used to describe this phenomenon.

If only the bilayer capacity is available and there is no electrode active material in parallel, or if the set potential deviates so much from the redox potential of the active material that no electrode reaction occurs (ie Rct=∞), the Nyquist plot should show a straight line rising perpendicular to the real axis. However, in practical electrochemical systems, especially in porous electrodes, a line rising perpendicular to the real axis is rarely seen, and most can be seen as a straight line with a certain slope. This situation can be seen to correspond to Fig. 15-1.
When the above case is included, the Nyquist plot will appear as a deformed and flattened arc if the CPE and the resistor are connected in parallel. When n = 0.8, the shape of the curve is shown in Fig. 15-2. As the value of n decreases further, the flattening distortion becomes more severe (Fig. 15-3).