Small amplitude perturbations and impedance methods

As for conventional electrical circuits, the electrochemical impedance of the electrochemical cell is defined as 

The key step in electrochemical impedance experiments is the construction of an equivalent circuit that mimics the electrical behaviour of the cell. For this, it is necessary to consider the different ways in which the ionic charges are involved in the passage of current. 

Between the electrodes, the solution behaves as an ionic conductor with a resistance, known as the ohmic solution resistance. At the electrode-solution interface, some of the ions are used to adjust the ionic atmosphere until the charge balance between the electrode surface and the solution is reached. Electrically, this is equivalent to the charge or discharge of a capacitor with capacitance Q, known as the double layer capacitance. Simultaneously, i.e. in parallel, redox species undergo an electron transfer with the electrode this yields the faradaic current whose magnitude reflects the rate of arrival of the reactants (the rate of mass transport) and the rate of the electron transfer process. Electrically, this is equivalent to two resistors in series one for the kinetics of the electron 

The mass transfer impedance depends on the mode of mass transport within the cell. For semi-infinite linear diffusion, is a complex number whose real and imaginary parts depend on the perturbation frequency  

Combining the impedance of the four components gives the overall impedance of the cell  

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