The Composite-Template Method to Construct Hierarchical Carbon Nanocages for Supercapacitors with Ultrahigh Energy and Power Densities

3D hierarchical carbon nanocages (hCNC) are becoming new platforms for advanced energy storage and conversion due to their unique structural characteristics, especially the combination of multiscale pore structure with high conductivity of sp(2) carbon frameworks, which can promote the mass/charge synergetic transfer in various electrochemical processes. Herein, the MgO@ZnO composite-template method is developed to construct hCNC and nitrogen-doped hCNC (hNCNC), which integrates the advantages of the in situ MgO template method and ZnO self-sacrificing template method. The hierarchical MgO template provides the scaffold for depositing carbon nanocages, while the self-sacrificing ZnO template helps create abundant micropores while promoting the graphitization degree, so that the microstructures of the products are effectively regulated. The optimized hCNC and hNCNC have an increased specific surface area and conductivity, which can further boost the mass/charge synergetic transfer. As the electrode materials of supercapacitors, the optimal hCNC(hNCNC) exhibits a high specific capacitance of 281(348) F g(-1) in KOH and 276(297) F g(-1) in EMIMBF4 electrolytes at 1 A g(-1). The ultrahigh energy and power densities are realized, accompanied by a high-rate capability and long cycling stability. The record-high energy densities of 141.8-71.4 Wh kg(-1) are achieved in EMIMBF4 at power densities of 10.0-250.4 kW kg(-1).