The use of Room Temperature Ionic Liquid (RTIL) electrolytes promises to improve the energy density of Electrochemical Capacitors (ECs) by allowing for operation at higher voltages. RTIL electrolytes 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4), 1-ethyl-3-methylimidazolium dicyanamide (EMImN(CN)2), 1,2-dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide (DMPImTFSI), and 1-butyl-3-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMPyFAP) were studied. Tetraethylammonium tetrafluoroborate 1 molar solution in anhydrous propylene carbonate (Et4NBF4-PC 1M) was studied for comparison purposes. Carbon was produced from phenolic resin activated in CO2. The porosity of the carbon samples were characterised by N2 adsorption-desorption at 77 K and the relevant electrochemical behaviour was characterised by galvanostatic charge-discharge, electrochemical impedance spectroscopy and cyclic voltammetry. The highest operating voltage of 3.5 V was obtained for BMPyFAP, whilst the best capacitive performance was obtained for EMImBF4. The maximum energy density increased to 70 Wh kg-1 (carbon) for RTIL EMImBF4 from 35 Wh kg-1(carbon) for the organic electrolyte Et4NBF4-PC 1M. It was found that the performance of the RTIL electrolytes could be related to the IL viscosity and ion size whilst the electrolyte equivalent series resistances produced a linear relationship with viscosity. It was found that the capacitance performance of the RTIL electrolytes followed the order EMImBF4 > DMPImTFSI > BMPyFAP > EMImN(CN)2. The electrolyte and equivalent series resistance were in the order EMImN(CN)2 <EMImBF4 <DMPImTFSI <BMPyFAP.
Sillars, F. B., Fletcher, S. I., Mirzaeian, M., & Hall, P. J. (2012). Variation of electrochemical capacitor performance with room temperature ionic liquid electrolyte viscosity and ion size. Physical Chemistry Chemical Physics, 14, 6094-6100. https://doi.org/10.1039/C2CP40089H