Ionothermal Synthesis of Graphene-Based Hierarchically Porous Carbon for High-Energy Supercapacitors with Ionic Liquid Electrolyte

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 F g−1 and 212 F g−1 at a current density of 0.5 A g−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 A g−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.