N-Doped hierarchical porous carbon from waste boat-fruited sterculia seed for high performance supercapacitors

N-Doped hierarchical porous carbon (NHPC) was obtained from waste boat-fruited sterculia seed by hydrothermal carbonization and KOH activation. The influence factors including the ratio of activation agent to precursor, activation temperature, and heteroatom-doping were discussed. The results showed that the NHPC-800-1 activated at 800 °C with the ratio of activation agent to precursor being 1 as an electrode in an electrical double layer capacitor (EDLC) exhibited the best electrochemical performance in 6 M KOH electrolyte. It delivered the specific capacitance up to 337 F g⁻¹ at a current density of 1 A g⁻¹, and still retained a capacitance of 181.5 F g⁻¹ at the high current density of 50 A g⁻¹. This material also shows an excellent capacitance retention of 98.8% after 5000 cycles at a current density of 20 A g⁻¹. In addition, a symmetric supercapacitor fabricated by NHPC-800-1 released an energy density of 8.05 W h kg⁻¹ at a power density of 10 kW kg⁻¹, and a high cycling stability with a capacitance retention of 98.5% over 10 000 cycles at 20 A g⁻¹ in 6 M KOH electrolyte. The superior performance of NHPC-800-1 could be attributed to these advantages of hierarchical porous structure, narrow micropore size distribution and rational micro/mesopore ratio, as well as the doping of the N atoms. These very attractive electrochemical properties indicate that the NHPC-800-1 derived from boat-fruited sterculia is a promising electrode material for supercapacitors.