Ionic liquid electrolytes with various constituent ions for graphene-based supercapacitors

Although activated carbon shows a higher maximum capacitance than that of graphene nanosheets (GNSs) in a conventional organic electrolyte, the latter material, characterized by high conductivity and a unique planar structure, is more suitable for use in an ionic liquid (IL) electrolyte for supercapacitors. IL electrolytes consisting of various cations (1-ethyl-3-methylimidazolium (EMI⁺) and N-butyl-N-methylpyrrolidinium (BMP⁺)) and anions (bis(trifluoromethylsulfony) imide (TFSI^-), tetrafluoroborate (BF₄^-), and dicyanamide (DCA^-)) are systematically studied. Among them, BMP-DCA IL is found to be the superior electrolyte, in which the GNS electrode exhibits a capacitance of 235 F g^-1 and a satisfactory rate capability within a potential range of 3.3 V at 25 °C. This electrolyte is even more promising for elevated-temperature applications. At 60 °C, a symmetric-electrode GNS supercapacitor with BMP-DCA IL is able to deliver maximum energy and power densities of 103 Wh kg^-1 and 43.3 kW kg^-1 (based on the active material on both electrodes), respectively, which are much higher than 19 Wh kg^-1 and 17.6 kW kg^-1 for a control cell with a conventional organic electrolyte.