CO₂-Activated Interconnected Hollow Carbon Nano-Onions for High Performance Supercapacitors

A carbon nano-onion structure with interconnected graphitic layers has been demonstrated as a high-performance supercapacitor electrode material. However, the structure suffers from a low specific surface area (SSA), which limits its further performance improvement. Herein, we use CO 2 thermal activation to tune the pore structure of the interconnected carbon nano-onions (IHCNOs) for efficiently improving the energy storage performances. The activated IHCNOs (A-IHCNOs) exhibit a high SSA of 1087.6 m 2 g –1, high volumes of micropores (0.11 cm 3 g –1 ) and mesopores (1.89 cm 3 g –1 ), high graphitic crystallinity, and excellent wettability to ionic liquids (ILs), while maintaining the structural advantages of interconnected graphitic layers. As a result, the supercapacitor device based on A-IHCNOs in a neat IL electrolyte exhibits a high specific capacitance of 92 F g –1 at 1 A g –1, corresponding to a high energy density of 204.4 Wh kg –1 . An ultrahigh power density of 400 kW kg –1 can be achieved with a high energy density of 51.9 Wh kg –1 still maintained. Moreover, the supercapacitors show excellent cycling stabilities in different IL electrolytes. This study demonstrates a green and sustainable CO 2 activation as an efficient strategy for simultaneously keeping the double-layer capacitive behavior and improving the supercapacitive energy storage performance of the carbon structures.