Comparative study on performances of trimethyl-sulfonium and trimethyl-ammonium based ionic liquids in molecular solvents as electrolyte for electrochemical double layer capacitors

The present work reports a comparative study on the performances of two bis[(trifluoromethyl)sulfonyl]imide-based protic (PIL) and aprotic (AIL) ionic liquids, namely, trimethyl-ammonium bis[(trifluoromethyl)sulfonyl]imide ([HN₁₁₁][TFSI], PIL) and trimethyl-sulfonium bis[(trifluoromethyl) sulfonyl]imide ([S₁₁₁][TFSI], AIL), as mixtures with three molecular solvents: gamma butyrolactone (γ-BL), propylene carbonate (PC), and acetonitrile (ACN) as electrolytes for supercapacitor applications. After an analysis of their transport properties as a function of temperature, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge measurements were conducted at 25 and -30 C to investigate the performance of these mixtures as electrolytes for supercapacitors using activated carbon as the electrode material. Surprisingly, for each solvent investigated, no significant differences were observed between the electrolytes based on the PIL and AIL in their electrochemical performance due to the presence or the absence of the labile proton. Furthermore, good specific capacitances were observed in the case of γ-BL-based electrolytes even at low temperature. Capacitances up to 131 and 80 F·g^-1 are observed for the case of the [S₁₁₁][TFSI] + γ-BL mixture at 25 and -30 C, respectively. This latter result is very promising particularly for the formulation of new environmentally friendly electrolytes within energy storage systems even at low temperatures.