A high-performance all solid state supercapacitor incorporating ionic liquid [BMIM][BF4] (1-butyl-3-methylimidazolium tetrafluoroborate) modified reduced graphene oxide (f-graphene) electrode is fabricated. [EMIM][BF4] (1-ethyl-3-methylimidazolium tetrafluoroborate): [PVdF-HFP] Poly(vinylidenefluoride hexafluoropropylene was used as gel polymer electrolyte. Presence of imidazolium cations over graphene sheet generated a surface charge that enables large accessible surface area wettable by gel polymer electrolyte. Supercapacitor device assembled using f-graphene electrode and [EMIM][BF4]: PVdF-HFP gel polymer electrolyte exhibit very high energy density 92 Wh/kg with a power density 33 kW/kg at 0.5 A/g. The energy density demonstrates 57% retention with power density 218 kW/kg on increasing current density to 6 A/g. The relaxation time constant (tau o) for f-graphene//gel polymer electrolyte//f-graphene cell assembly was calculated to be 2.4 ms, which is much lower than any existing EDL system, indicating fast energy delivery at relatively high power. Fabricated solid state, flexible supercapacitor demonstrate high capacitance similar to 242 F/g at 5 mV/s and showed excellent galvanostatic charge/discharge cycling stability (capacitance retention > 90% up to 12,000 cycles). (C) 2015 Elsevier Ltd. All rights reserved.