Supercapacitive performance of porous graphene nanosheets in bis(trifluoromethylsulfony)imide and bis(fluorosulfonyl)imide ionic liquid electrolytes

A method of high-heating-rate thermal reduction is used to produce porous graphene nanosheets (PGNSs). This material is characterized by a unique holey structure, which is suitable for ionic liquid (IL) electrolyte. To study the effects of electrolyte anion, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfony)imide (EMI-TFSI) and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMI-FSI) ILs are used. Due to the smaller size of FSI⁻, EMI-FSI shows a higher conductivity and lower viscosity than those of EMI-TFSI. The specific capacitances of PGNS electrodes are 90 and 260 F g⁻¹, respectively, measured in EMI-TFSI and EMI-FSI ILs at a potential scan rate of 50 mV s⁻¹. The PGNS symmetric cell with EMI-FSI electrolyte is able to deliver a maximum energy and power densities of 110 Wh kg⁻¹ and 101 kW kg⁻¹, respectively, with a cell voltage of 3.5 V. In addition, 85% of the initial capacity can be retained after 2000 charge–discharge cycles.