Holey Graphene Nanosheets with Surface Functional Groups as High-Performance Supercapacitors in Ionic-Liquid Electrolyte

Pores and surface functional groups are created on graphene nanosheets (GNSs) to improve supercapacitor properties in a butylmethylpyrrolidinium-dicyanamide (BMP-DCA) ionic liquid (IL) electrolyte. The GNS electrode exhibits an optimal capacitance of 330 F g^-1 and a satisfactory rate capability within a wide potential range of 3.3 V at 25 °C. Pseudocapacitive effects are confirmed using X-ray photoelectron spectroscopy. Under the same conditions, carbon nanotube and activated carbon electrodes show capacitances of 80 and 81 F g^-1, respectively. Increasing the operation temperature increases the conductivity and decreases the viscosity of the IL electrolyte, further improving cell performance. At 60 °C, a symmetric-electrode GNS supercapacitor with the IL electrolyte is able to deliver maximum energy and power densities of 140 Wh kg^-1 and 52.5 kW kg^-1 (based on the active material on both electrodes), respectively, which are much higher than the 20 Wh kg^-1 and 17.8 kW kg^-1 obtained for a control cell with a conventional organic electrolyte.