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 CO2 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 m2 g–1, high volumes of micropores (0.11 cm3 g–1) and mesopores (1.89 cm3 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 CO2 activation as an efficient strategy for simultaneously keeping the double-layer capacitive behavior and improving the supercapacitive energy storage performance of the carbon structures.