Electrochemical double layer capacitors (EDLCs), storing charges by electrostatic attraction of electrolyte ions to the surface of charged electrodes, require an improved energy density to broaden their applications. Here a high-energy-density EDLC is reported by employing ionic liquids not only as the solvent for material synthesis but also as electrolyte. Graphene-based hierarchically porous carbon (GPC) are synthesized via ionothermal method, and exhibits high specific capacitance of 313 Fg (1) and 212 Fg (1) at a current density of 0.5 Ag (1) in aqueous and ionic liquid electrolytes, respectively. The obtained GPC electrode retains about 94.2% of the initial capacitance after 10000 charge/discharge cycles at a current density of 2 Ag (1) in aqueous electrolyte, strongly reflecting an excellent long-term cycling stability and rate capability of the electrode. A promoted energy density of 90.4 W h kg (1) can be obtained with neat ionic liquid electrolyte, which is about 8 times higher than that with aqueous electrolyte. The ionothermal process guarantees the acquisition of the hierarchical porous structure and high surface area, while the ionic liquid electrolyte assures a broader operating voltage, which both helps to increase the energy density of EDLCs. (C) 2017 Elsevier Ltd. All rights reserved.