A Large-Sized Reduced Graphene Oxide with Low Charge-Transfer Resistance as a High-Performance Electrode for a Nonflammable High-Temperature Stable Ionic-Liquid-Based Supercapacitor

Large-sized reduced graphene oxide prepared was prepared by combined microwave intermittent heating and an extraction process (e.g., 900 W for 10 min (MRG-900-10)) with a lateral size of several micrometers and a thickness of 4–6 monosheets. The MRG-900-10 has a high C/O molar ratio (5.89) and a sp² C content of 69.0 %, which leads to a fast electronic transmission in the sample. In addition, MRG-900-10 possesses a large specific surface area of 568.2 m² g⁻¹, which increases the contact surface area between the active material and the electrolyte, thus enhancing the electron and ion transport in the interfaces when used as the electrode material in supercapacitors. MRG-900-10 possesses a low charge-transfer resistance (≈0.36 Ω). Used as the electrode material for a supercapacitor in 6 m KOH aqueous electrolyte, MRG-900-10 produces a high specific capacity of 327.6 F g⁻¹ at the current density of 0.5 A g⁻¹. A specific capacity of 248.3 F g⁻¹ was obtained at a high current density of 100 A g⁻¹, which indicated its high-rate ability. The initial capacity of 92 % can be maintained after 40 000 cycles at 5 A g⁻¹, which indicated its high cycling stability. As for the MRG-900-10 symmetric supercapacitors, the energy densities of 11.0 and 36.2 Wh kg⁻¹ were obtained in 6 m KOH aqueous and 1 m tetraethylammonium tetrafluoroborate ([TEA]BF₄)/acetonitrile (ACN) electrolytes, respectively. Importantly, a high energy density of 68.6 Wh kg⁻¹ was achieved in the nonflammable ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethane)sulfonimide ([EMIM][TFSI])/ACN (80 vol % [EMIM][TFSI]), and the supercapacitor was effective from room temperature to 100 °C.