Electrochemical impedance spectroscopy (EIS) has been used widely to determine the properties of pseudocapacitive electrodes. It consists of applying a low-amplitude sinusoidal voltage to a steady-state potential and measuring the output sinusoidal current. Using complex notations, the impedance of the electrodes can be determined by dividing the oscillating potential by the oscillating current. The impedance spectra, including the Nyquist plots and Bode plots, are informative to determine the properties of the electrodes. However, the interpretation of impedance spectra has been a “subject of controversy”.

This study aims to provide physical insights in interpreting the impedance spectra of pseudocapacitive electrodes by numerically reproducing EIS measurements in a three-electrode setup. The continuum model used was based on the modified Poisson-Nernst-Planck model combined with the Butler-Volmer equation and accounted for (i) charge transport in both electrodes and electrolyte, (ii) the dynamics of the electric double layer, (iii) steric repulsion due to finite ion sizes, (iv) reversible redox reactions, and (v) intercalation. The effect of the electrode and electrolyte resistance, charge transfer resistance at the electrode/electrolyte interface were separated numerically by varying the bulk ion concentration, electrolyte length, redox reaction rate constant, and the steady-state potential, i.e., the biased potential. Moreover, the faradaic and capacitive contributions of the impedance can be interpreted separately. Therefore, the behaviour in each frequency regime in the impedance spectra can be related to specific physical processes.