Effect of activated carbon xerogel pore size on the capacitance performance of ionic liquid electrolytes

Abstract

The use of ionic liquid (IL) electrolytes promises to improve the energy density of electrochemical capacitors (ECs) by allowing for operation at higher voltages. Several studies have also shown that the pore size distribution of materials used to produce electrodes is an important factor in determining EC performance. In this research the capacitative, energy and power performance of ILs 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 (BMPyT(F₅Et)PF₃) were studied and compared with the commercially utilised organic electrolyte 1 M tetraethylammonium tetrafluoroborate solution in anhydrous propylene carbonate (Et₄NBF₄–PC 1 M). To assess the effect of pore size on IL performance, controlled porosity carbons were produced from phenolic resins activated in CO₂. The carbon samples were characterised by nitrogen adsorption–desorption at 77 K and the relevant electrochemical behaviour was characterised by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy. The best capacitance performance was obtained for the activated carbon xerogel with average pore diameter 3.5 nm, whereas the optimum rate performance was obtained for the activated carbon xerogel with average pore diameter 6 nm. When combined in an EC with IL electrolyte EMImBF₄ a specific capacitance of 210 F g⁻¹ was obtained for activated carbon sample with average pore diameter 3.5 nm at an operating voltage of 3 V. The activated carbon sample with average pore diameter 6 nm allowed for maximum capacitance retention of approximately 70% at 64 mA cm⁻².