In EIS, you measure an electrochemical cell’s complex impedance over a wide range of AC frequencies. Typically, several cell elements and cell characteristics contribute to the system’s EIS spectrum. A partial list of possible elements includes: electrode double layer capacitance, electrode kinetics, diffusion layer, and solution resistance. Unfortunately, the system’s impedance at any given frequency usually depends on more than one cell element. This greatly complicates the analysis of EIS spectra.
The most common method used to analyze EIS spectra is equivalent circuit modeling. You simulate the cell incorporating the elements mentioned above. The behavior of each element is then described in terms of “classical” electrical components (resistors, capacitors, inductors) plus a few specialized electrochemical elements.