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Description

Capacitors in EIS experiments often do not behave ideally. Instead, they act like a Constant Phase Element (CPE) as defined below. The impedance of a CPE has the form:

 

 

For a constant phase element, the exponent α < 1.

The "double-layer capacitor" on real cells often behaves like a CPE instead of like a capacitor. Several theories have been proposed to account for the non-ideal behavior of the double layer, but none has been universally accepted. In most cases, you can safely treat a as an empirical constant and not worry about its physical basis.

 

The equation describes a capacitor if the exponent α = 1. The constant Y0 is then equal to C (the capacitance).