Evaluation of kinetic parameters of non-faradic processes in carbon-based electrodes using multisine dynamic electrochemical impedance spectroscopy

Dynamic Electrochemical Impedance Spectroscopy is a valuable tool to investigate kinetic parameters of non-Faradic processes in symmetric Electrochemical Double Layer Capacitors. Compared to classic Electrochemical Impedance Spectroscopy, the dynamic technique measures the impedance under non-equilibrium condition removing the stationarity requirement and, at the same time, giving the frequency-dependent information of a dynamic process in presence of overpotential. Despite not being a new technique, there is an inherent complexity in the signal sampling and data manipulation to obtain the impedance spectra down to low frequencies (the interesting band for energy storage devices impedance). Few methods to compute the impedance from sampled signals are reported in the literature and under development. For this work the Dynamic Multi-Frequency Analysis algorithm was used to compute the impedance and reconstruct the cyclic voltammetry profile as well. The time-dependency of the kinetic parameters, namely capacitances of double layer and adsorption phenomenon, were extracted from equivalent circuit model regression and the adsorption time constants with a graphical analysis of the impedance spectra dataset during a full charge/discharge cycle. The values of the adsorption capacitance from the model regression are, on average, compatible with the values computed with a common formula showing the reliability of the model regression. The use of a three-electrode compact cell allowed the characterization of both active electrodes independently during operation showing a strong potential dependence on the active material properties. Furthermore, the impedance showed the presence of a side reaction (likely oxygen evolution) connected to the increase of adsorption and double layer capacitances localized on the positive electrode in a clearer way than what was possible to observe from the voltametric profile.