In the Electrochemical impedance spectra, I have seen many papers reporting an equivalent circuit and corresponding values for solution resistance Rs, double layer capacitance Cdl, charge transfer resistance Rct, warburg impedance Zw and Pseudocapacitance Cp. However, some of them are frequency dependent e.g., Zw. How can we fit EIS with fix values of all the parameters since in Nyquist plot all points are at different frequency?
Any explanation?
Usually the fitting procedures reproduce experimental data through the use of equivalent circuits mimicking the electrochemical system you are studying. Typically this procedure allows you to obtain capacitance and resistance values which are not depending on the frequency of the signal.
I think that you misunderstood something. Fitting procedures in EIS use some electrical components in order to produce an equivalent electrical circuit that best fits/mathches your experimental data.
The values that you are refered to represent specific values of Rs, Rct etc which are characteristic for a Nyquist plot and not the latter values for every frequency.
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