Physicochemical Investigation of Adiponitrile-Based Electrolytes for Electrical Double Layer Capacitor

Herein, we present the formulation and the characterization of novel adiponitrile-based electrolytes as a function of the salt structure, concentration, and temperature for supercapacitor applications using activated carbon based electrode material. To drive this study two salts were selected, namely, the tetraethylammonium tetrafluoroborate and the 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide. Prior to determination of their electrochemical performance, formulated electrolytes were first characterized to quantify their thermal, volumetric, and transport properties as a function of temperature and composition. Then, cyclic voltammetry and electrochemical impedance spectroscopy techniques were used to investigate their electrochemical properties as electrolyte for supercapacitor applications in comparison with those reported for the currently used model electrolyte based on the dissolution of 1 mol·dm^{–3 of tetraethylammonium tetrafluoroborate in acetonitrile. Surprisingly, excellent electrochemical performances were observed by testing adiponitrile-based electrolytes, especially those containing the 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide room-temperature molten salt. Differences observed on electrochemical performances between the selected adiponitrile electrolytes based on high-temperature (tetraethylammonium tetrafluoroborate) and the room-temperature (1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide) molten salts are mainly driven by the salt solubility in adiponitrile, as well as by the charge and the structure of each involved species. Furthermore, in comparison with classical electrolytes, the selected adiponitrile +1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide solution exhibits almost similar specific capacitances and lower equivalent serial resistance. These results demonstrate in fact that the adiponitrile +1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide mixture can be used for the formulation of safer electrolytes presenting a very low vapor pressure even at high temperatures to design acetonitrile-free supercapacitor devices with comparable performances.}