Theoretical and experimental studies of imidazolium based ionic liquids for supercapacitors application

Abstract

The theoretical study of neutral ion pairs of 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf2]) has been performed. The formation of the neutral ion pairs was considered through interionic reactions involving the [EMIM]+ cation and two anions, [BF4] – and [NTf2] – . Computations were performed by the DFT/6-311++G(d,p) method with B3LYP5 and CAM–B3LYP functionals. The enthalpies of the interionic reactions ΔrH°(0) have been determined theoretically for both ion pairs. In addition, following the available experimental data for [EMIM][NTf2], the enthalpy of the reaction based on experiment was obtained. The recommended values of ΔrH°(0) are as follows: –359 ± 15 kJ mol–1 for [EMIM][BF4] and – 323 ± 9 kJ mol–1 for [EMIM][NTf2]. The thermodynamic stability of the ion pairs was interpreted through analysis of the frontier molecular orbitals as well as simple ionic model. Experimental test on influence of imidazolium-based ionic liquids in electrochemical system was investigated, with the potential window of 2.8 V for both ionic liquids. The specific capacitances were found to be 4.1 F g–1 and 4.3 F g–1 for [EMIM][BF4] and [EMIM][NTf2] respectively, at the lower scan rate of 5 mV s–1 . The maximum current densities reached were 87 mA g–1 and 88 mA g–1 for [EMIM][BF4] and [EMIM][NTf2], respectively. Electrochemical measurements by cyclic voltammetry and electrochemical impedance spectroscopy have been conducted on three electrodes system with the IILs under study. A supercapacitor prototype has been fabricated; conventional charge-discharge tests were done. Keywords: Ionic liquids, density functional theory, thermodynamic functions, supercapacitor, cyclic voltammetry, electrochemical impedance spectroscopy