Electric double layer capacitors (EDLCs) store energy via ion adsorption in the electric double layer forming at the electrode/electrolyte interfaces. This charge storage mechanism is very fast and highly reversible resulting in large power density and long cycle life. Applications range from regenerative breaking for hybrid electric vehicles to the smart grid. Electrochemical impedance spectroscopy (EIS) has been used widely to determine the properties of the electrode and electrolyte materials for EDLCs. The impedance of an EDLC half-cell can be measured by applying a low-amplitude sinusoidal voltage to a steady-state potential and divide the imposed potential by the output sinusoidal current. Nyquist plots showing the imaginary part as a function of the real part of the impedance is informative to determine the properties including electrode and electrolyte resistance, charge transfer or polarization resistance at the electrode/electrolyte interface, and capacitance of the half-cell. However, the interpretation of Nyquist plots has been a “subject of controversy”, with no clear relations between the shape in high, intermediate, and low frequency regime of the plot and a particular property of the device.

This study aims to derive guidelines for characterization of EDLC electrode and electrolyte materials from EIS measurements. To do so, Nyquist plots from EIC measurements were numerically reproduced for EDLC half-cells with planar electrodes in aqueous TEABF₄ electrolyte. The continuum model used was based on the modified Poisson-Nernst-Planck model and accounted for (i) the Stern layer at the electrode/electrolyte interface, (ii) finite ion size, and (iii) binary and symmetric electrolyte. The effect of the electrode and electrolyte resistance, charge transfer or polarization resistance at the electrode/electrolyte interface, and the capacitances were separated numerically. This was done by varying the bulk ion concentration, electrolyte length, ion diffusion coefficient in the electrolyte, and/or the magnitude of steady-state potential. Therefore, the behaviour in each frequency regime in Nyquist plots that corresponded to a particular property was clarified.