Supercapacitor is an energy storage device that has developed rapidly in recent years. The energy density and power density of supercapacitors fill the gap between rechargeable batteries and aluminum electrolytic capacitors. However, there is still a big gap between its power density and that of aluminum electrolytic capacitors. High-power capacitors are an important research direction in the development of capacitors. High-power supercapacitors have huge application potential in renewable energy grid integration and frequency regulation of electronic circuits. Graphene electrodes, due to their high electrical conductivity and short ion transport paths, are potentially important electrode materials for high-power capacitors. However, the preparation process of graphene is complicated and the yield is low, thus their applications are limited. Biomass has the characteristics of wide source, high yield and good renewable nature. Amorphous hydrothermal carbon can be obtained by hydrothermal carbonization of biomass (e.g., cellulose, lignin, starch, glucose, etc.). However, the pore structure of hydrothermal carbon is not developed enough and its electrical conductivity is extremely low, so its application in supercapacitors is limited. Recently, Professor Husam N. Alshareef and his team at King Abdullah University of Science and Technology in Saudi Arabia used CO2 laser to convert hydrothermal carbon balls into three-dimensional graphene (LSG) electrodes in one step. CO2 laser conversion realizes the conversion of hydrothermal carbon from sp3 carbon to sp2 carbon. The interior of the LSG skeleton is a turbostratic multi-layer graphene structure. By converting hydrothermal carbon with CO2 laser, LSG obtains a three-dimensional open macroporous structure, a higher specific surface area (87.8m2g-1) and a higher electrical conductivity (33.6Scm-1). More importantly, this direct CO2 laser conversion method is a direct preparation technology for graphene electrodes. During the laser conversion process, direct assembly of graphene and metal current collectors can be achieved. The symmetrical supercapacitor assembled with LSG electrodes has ultra-high power characteristics: in the voltage range of 1-2V, the power density of LSG is 28Wcm-3, which is 28 times that of commercial activated carbon supercapacitors. At the same time, the LSG electrode exhibits a very high frequency response (RC time constant at 120 Hz is 0.517 ms) and a long cycle life (capacity retention rate >98% for 10,000 cycles). This study provides a method for direct conversion of biomass into high-power graphene electrodes. At the same time, this method avoids the coating process from active material to electrode and can be directly used in existing organic electrolyte supercapacitor systems. Therefore, it is a potential commercialization method for preparing capacitor electrodes. The relevant paper was published online on SmallMethods (DOI: 10.1002/smtd.201900005). The first author of the paper is Dr. Zhang Wenli.

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