This paper, written by researchers from Beijing Aerospace University and others, discusses Large size reduced graphene oxide with low charge transfer resistance as high performance electrode of fire safety high temperature stable ionic liquid based supercapacitor. The paper is published in an important journal < CHemsusChem >. IF：7.411.

In recent years, the research work of microwave chemical instrument used in the synthesis of materials has become a hot direction of scientific research, which has been paid great attention to by many scholars!

Large size reduced graphene oxide MRG-900-10 with lateral size of several micrometers and thickness of 4-6 monosheets is prepared via combined microwave intermittent heating and extraction process. The MRG-900-10 has high C/O molar ratio (5.89) and content of sp2 C (69.0%), which lead to a fast electronic transmission in the sample. Besides, the 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 electrons and ions transports in the interfaces when used as the electrode material in supercapacitor. The MRG- 900-10 possesses a low charge transfer resistance (~0.36 Ω). Used as the electrode material of supercapacitor in 6 M KOH aqueous electrolyte, the MRG-900-10 owns a high specific capacity of 327.6 F g-1 at the current density of 0.5 A g-1. The specific capacity of 248.3 F g-1 is obtained at the high current density of 100 A g-1, indicating its high rate property. And 92% of the initial capacity can be maintained after 40,000 cycles at 5 A g-1, indicating its high cycling stability. As to the MRG-900-10 symmetric supercapacitors, the energy densities of 11.0 and 36.2 Wh kg-1 are obtained in 6 M KOH aqueous and 1 M TEABF4/ACN organic electrolytes, respectively. Importantly, the high energy density of 68.6 Wh kg-1 is achieved in the nonflammable EMIMTFSI/ACN-80 ionic liquid electrolyte, and the supercapacitor is effective from room temperature to 100oC.

Large size rGO MRG-900-10 has been ingeniously synthesized via a fast and efficient intermittent heating microwave reduction method, and was creatively separated from a stable dispersion in DMA by choosing ethyl acetate as precipitant. Large amount sp2 C and less defective nanosheets have been found for the MRG-900-10 sample, which may lead to a fast electronic transmission in the sample. Large specific surface area has been observed for the MRG-900-10 sample, which can increase the contact surface between the active material and the electrolyte, thus the electrons and ions transports in the interfaces are enhanced when used as the electrode material in supercapacitor. A low charge transfer resistance has been obtained for the MRG-900-10 sample, which may not only lead to the high performances of MRG-900-10 based supercapacitors in aqueous and organic electrolyte systems, but also make it possible for the assembly of the supercapacitor in ionic liquid electrolyte system even though the electrolyte is high viscosity and low wettability. High energy density, fire safety property and high temperature stability have been obtained in the optimized ionic liquid EMIMTFSI-80 electrolyte. The advanced electrochemical properties of the MRG-900-10 material are obviously valuable for the supercapacitor utilization especially in the extremely conditions such as fire safety and/or high temperature.

GO precursor was prepared from natural graphite powder by the modified Hummer’s method.[14] The MRG materials were obtained as follows. Firstly, 20 mL of the as-prepared GO (1.0 mg mL-1) was dispersed into 100 mL of DMA, followed by 25 kHz ultrasonic tripping accompanied by magnetic stirring (640 rpm) for 40 min on the Intelligent Temperature Control and Dual Frequency Ultrasonic Synthesizer (XH- 2008DE). Secondly, the homogeneous suspension was transferred into 250 mL three-necked flask, and then moved into the Computerized Microwave Solid-Liquid Phase Synthesizer (XH-200A). The whole heating process was carried out under N2 atmosphere. To prevent intense boiling, the Intermittent Heating Technique was chosen, specifically, heating each 1.0 min at the adopted microwave heating power followed by resting for 15 s. At last, well dispersed black suspension liquid was obtained under the microwave heating at 700, 800, 900 and 999 W, respectively, for 10 min with constant stirring. After cooling down to room temperature, the resultant liquid was then transferred to 400 mL beakers, then 100 mL of ethyl acetate was poured into each beaker, and the flocculent precipitates were separated by centrifugation and dried at 60oC in an air oven overnight. The obtained samples were named as MRG-x-y (x represents the microwave power, and y represents the reaction time). Since the MRG-900-10 sample presented the best electrochemical properties among the MRG-x-10 samples, the MRG-900-y (y = 6, 8, 12 and 14) samples were also prepared by changing experimental time from 6, 8, 12 to 14 min under the microwave heating at 900 W for comparison.