If one is interested in a specific cathodic reaction, should not the cathode be held at the potential seen in the CV scan where the reaction occurs, while the EIS system scans through frequencies to measure the impedance?
You can held your applied potential while measuring EIS according to your CV and for that, recommended system/electrolyte will be the same as it was in your CV study, may be buffer etc. So frequency dependent impedance scan will be appear on the given applied potential.