On the Analysis of Non-stationary Impedance Spectra

In this work we study the possibility to analyze the non-stationary impedance spectra by employing standard equivalent circuits. For this purpose, the dynamic multi-frequency analysis (DMFA) is introduced and compared with a set of consecutive stationary impedance spectra. In order to prove the hypothesis, the data are obtained by the simulation in the time domain of the electron transfer process between an electrode and a free-diffusing redox couple in solution. During the simulation, the system is perturbed with a cyclic voltammetry super-imposed to a small multisine perturbation (DMFA) or with a series of stationary impedance spectroscopies. Also, a new fitting algorithm, which takes into account the correlation between consecutive spectra, is proposed and tested. Although the Randles circuit can be used to fit both dynamic and stationary impedance spectra, the values of the fitting parameters are different and depend on the direction of the scan and on the rate. This is related to the influence of the diffusion profile on the fitting parameters.