Imidazolium Based Ionic Liquids as Electrolytes for Energy Efficient Electrical Double Layer Capacitor: Insights from Molecular Dynamics and Electrochemical Characterization

Ionic liquids (ILs) have attracted considerable interest as electrolytes for electrical double layer capacitors (EDLC) bringing in enhancement of energy efficiency. This work studied three imidazolium based ILs mixed with a co-solvent as the electrolytes for EDLC. A combined study involving molecular dynamics (MD) and electrochemical experiments was carried out to interpret the potential of the electrolyte solution. Initially, MD simulation was employed to compute ionic conductivity and viscosity of pure ILs, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf2N]), 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([PMIM][Tf2N]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][Tf2N]) and the study was further extended to 1 mol center dot dm(-3) solutions of these three ILs in acetonitrile (ACN). The MD results were sequentially validated by experiments. Based on the ionic conductivity and viscosity values obtained from MD and experiments, 0.5, 1.0 and 2.0 mol center dot dm(-3) solutions of the ILs in ACN were further investigated as electrolytes for carbon based EDLC. Cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge discharge techniques were employed. From cyclic voltammetry, the observed highest value of the operating potential window was 3 V. The nearly rectangular and symmetric shape of cyclic voltammograms and vertical line of Nyquist plot at lower frequencies indicated good capacitive behavior of the system. The highest specific capacitance of 122 F center dot g(-1) was achieved for the 1 mol center dot dm(-3) solution of [PMIM][Tf2N] at 0.5 A center dot g(-1). The highest energy density values were found to be 152 and 149 W center dot h center dot kg(-1) for 1 mol center dot dm(-3) solutions of [PMIM][Tf2N] and [BMIM][Tf2N], respectively. Overall, 1 mol center dot dm(-3) solutions of the less explored [PMIM][Tf2N] and [BMIM][Tf2N] provided better electrochemical stability, energy and power density.