Limiting ac frequency and dc current of electrochemical double layer capacitors

The general aim of the present paper was to compare and correlate factors describing operation rate limits of electrochemical double layer capacitors (EDLCs), under both ac and dc conditions. Six laboratory-made and six commercial EDLCs were investigated with the use of electrochemical impedance spectroscopy (EIS) and galvanostatic charging/discharging. A typical EIS spectrum, taken under ac conditions, consisted of two straight lines. The intersection of both straight lines indicates the frequency (transition frequency ftr) at which the device loses its capacitive properties. Similarly, during dc charging/discharging the I(dU/dt)−1 value (often called EDLCs capacitance), expressed as a function of the current I, also shows two straight lines. The intersection of the lines indicates a transition current Itr and the corresponding (dU/dt)tr voltage changes rate. A linear correlation was found between transition parameters determined under ac and dc conditions ((Redrftr)−1 and Itr(dU/dt)tr−1), with a high correlation parameter r2 = 0.99. The correlation enables estimation of the limiting operation rate of EDLCs under dc conditions based on the analysis of ac impedance spectroscopy curves.