Physical Interpretations of Nyquist Plots for EDLC Electrodes and Devices

Electrochemical impedance spectroscopy (EIS) consists of plotting so-called Nyquist plots representing negative of the imaginary versus the real parts of the complex impedance of individual electrodes or electrochemical cells. To date, interpretations of Nyquist plots have been based on physical intuition and/or on the use of equivalent RC circuits. However, the resulting interpretations are not unique and have often been inconsistent in the literature. This study aims to provide unequivocal physical interpretations of electrochemical impedance spectroscopy (EIS) results for electric double layer capacitor (EDLC) electrodes and devices. To do so, a physicochemical transport model was used for numerically reproducing Nyquist plots accounting for (i) electric double layer (EDL) formation at the electrode/electrolyte interface, (ii) charge transport in the electrode, and (iii) ion electrodiffusion in binary and symmetric electrolytes. Typical Nyquist plots of EDLC electrodes were reproduced numerically for different electrode conductivity and thickness, electrolyte domain thickness, as well as ion diameter, diffusion coefficient, and concentrations. The electrode resistance, electrolyte resistance, and the equilibrium differential capacitance were identified from Nyquist plots without relying on equivalent RC circuits. The internal resistance retrieved from the numerically generated Nyquist plots was comparable to that retrieved from the "IR drop" in numerically simulated galvanostatic cycling. Furthermore, EIS simulations were performed for EDLC devices, and similar interpretations of Nyquist plots were obtained. Finally, these results and interpretations were confirmed experimentally using EDLC devices consisting of two identical activated-carbon electrodes in both aqueous and nonaqueous electrolytes.