An electrode  electrolyte interface behaves like a capacitance called electrochemical double-layer capacitance . The equivalent circuit for the redox reaction in Fig. 2 includes the double-layer capacitance  as well as the charge transfer resistance . Another analog circuit commonly used to model the electrochemical double-layer is called a constant phase element.
The electrical impedance of this circuit is easily obtained remembering the impedance of a capacitance which is given by:
where  is the angular frequency of a sinusoidal signal (rad/s), and .
It is obtained:
Nyquist diagram of the impedance of the circuit shown in Fig. 3 is a semicircle with a diameter  and an angular frequency at the apex equal to  (Fig. 3). Other representations, Bode plots, or Black plans can be used.