Electrochemical measurements on a group of activated carbon cloths (ACCs) showed similar capacitance at low current density (50 mA•g -1 ) in 1 M TEABF4 (ACN) (Figure ), suggesting that a similar amount of energy can be stored and released upon slow charging/discharging. However, the capacitance decreases with increasing current density at rates that can dramatically vary between the different carbon materials, highlighting the necessity of characterizing and reporting electrode capacitance at a range of current densities to fully capture the performance of an electrode material (Figure ). To quantify the rate performance of electrode materials, we fitted the capacitance vs. current density data with a decaying exponential function, to obtain the rate capability, J0, which represents the current density at which 63% of the initial capacitance is lost (Figure ). Higher values of J0 indicate better supercapacitor rate performance. We found J0 values ranging from 1.6 A•g -1 for ACC-10 up to 91.1 A•g -1 for ACC-20 electrodes in 1 M TEABF4 electrolyte in acetonitrile (ACN) (Figure , Table ). Since ACC-20 has larger pores than ACC-10 (Figure ), we hypothesized that this would reduce restrictions on ionic transport during charging, leading to better rate performance. To further test this hypothesis, the electrolyte cation size was systematically varied using a fixed electrode material (ACC-15), with tetraethylammonium tetrafluoroborate (TEABF4), tetrapropylammonium tetrafluoroborate (TPABF4), and tetrabutylammonium tetrafluoroborate (TBABF4) electrolytes (1 M in acetonitrile). Increasing the cation size resulted in a reduced fast charging performance (Figure , Table ) owing to the more restricted porosity accessible to the electrolyte (Figure ) .