Variation of electrochemical capacitor performance with room temperature ionic liquid electrolyte viscosity and ion size

Abstract

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.