Electrochemical Performance and Molecular Structure of Diluted 1-Alkyl-3-methylimidazolium Tetrafluoroborate Ionic Liquids and Their Mixture as Electrolytes for Double-Layer Capacitors: An Integrated Approach by Electrochemical Characterization and Molecular Dynamics Simulation

This paper reports the electrochemical studies of two room-temperature ionic liquids (RTILs), namely, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM] [BF4]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), and their mixture in 1:1 molar ratio as electrolytes of electrochemical double-layer capacitors (EDLCs). Neat RTILs resulted in better electrochemical stability and lower charge storage. An organic solvent, acetonitrile (ACN), was further added to improve the capacitive performance of the EDLC, reducing the resistance to diffusion of ions, which ensured enhanced specific capacitance but narrowed the electrochemical window. The mixture of RTILs possessed electrochemical stability that resided in between their individual stability limits and lower specific capacitance than either of its constituents. The widest operating potential window (OPW) of 2.7 V was achieved for a 3.5 mol dm(-3) solution of [BMIM][BF4]. Nearly equal specific capacitance (similar to 70 F g(-1)) was delivered by both the RTILs in 2 and 3.5 mol dm(-3) electrolyte solutions. The longer alkyl chain length of [BMIM] [BF4] imparted better electrochemical stability and specific energy (60-70 W h kg(-1)) to it. A 2 mol dm(-3) solution of the RTIL mixture in ACN delivered the highest power of 5.7 kW kg(-1). Each system investigated in this study retained the Coulombic efficiency approaching 100%. Furthermore, a molecular dynamics simulation was performed on the RTIL + cosolvent systems to analyze the liquid structure at the microscopic level with the help of the radial pair distribution function and coordination number. Moreover, a study of diffusivity of ionic species facilitated the understanding of cosolvent effects in terms of mobility of ions.