On the rate capability of supercapacitors characterized by a constant-phase element

The capacitances of electrochemical systems determined from cyclic voltammetry (CV) are difficult to compare due to the unavoidable constant-phase element (CPE) behavior of such systems. These difficulties can be illustrated by simulated voltammograms of simplified circuits composed of a resistor in series with a CPE (R-CPE circuits). The capacitance extracted from CV of R-CPE circuits follows a power law relationship with sweep rate. By comparing the CV-obtained capacitance with that obtained from electrochemical impedance spectroscopy (EIS) data, a critical sweep rate can be defined as the rate where these capacitances intersect. This critical rate can be used to formulate an effective time constant for R-CPE circuits. Sweep rates at least two orders of magnitude below the critical rate should be used in CV to mitigate the influence of the system's time constant on the measured capacitance. To allow for more accurate comparisons of supercapacitors, the CPE parameters R, Q, and α should be reported, rather than the capacitance vs. sweep rate data from CV measurements. •Capacitance of electrochemical capacitors exhibits a power law with test rate.•Constant phase element behavior of electrochemical capacitors is responsible.•Constant phase element parameters can be determined from multiple methods.•These parameters can be used to estimate the rate capability of capacitors.