N-Doped hierarchical porous carbon prepared by simultaneous-activation of KOH and NH3 for high performance supercapacitorsElectronic supplementary information (ESI) available: Schematic illustration of preparation of AKPC-750; XPS spectra and electrochemical performance of the APC-750 and APC-750; detail information of control samples. See DOI: 10.1039/c6ra22426a

Hemp stem, a biomass waste, has been successfully explored as a green carbon source to prepare advanced N-doped hierarchical porous carbon by means of KOH pre-treatment and direct pyrolysis under NH 3 flow. The NH 3 flow can not only promote the activation of carbon but also bring nitrogen into the carbon framework. The simultaneous-activation of KOH and NH 3 can enhance the specific surface area, nitrogen content and electrical conductivity, significantly affecting the electrochemical performance for supercapacitors. The as-prepared sample with a high specific surface area (1949 m 2 g −1 ), high nitrogen content (4.4 wt%), hierarchical porosity and high electrical conductivity (3.5 S cm −1 ), exhibits remarkable electrochemical performance as a supercapacitor electrode material, such as a high specific capacitance of 352 F g −1 , outstanding rate capability and excellent cycling stability (96.3% retention after 10 000 cycles at 5 A g −1 ) in 6 M KOH electrolyte. In addition, the as-assembled symmetric supercapacitor delivers an ultrahigh energy density of 99.5 W h kg −1 in an ionic liquid system. Moreover, a value of 27.7 W h kg −1 is still maintained even at a high power density of 22 000 W kg −1 . More interestingly, the supercapacitor can light up a red light-emitting-diode for more than 8 minutes after being charged for only 4.4 seconds. Hemp stem simultaneously-activated by KOH and NH 3 for high performance supercapacitors.