N-Doped hierarchical porous carbon prepared by simultaneous-activation of KOH and NH3 for high performance supercapacitors

Abstract

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 NH₃ flow. The NH₃ flow can not only promote the activation of carbon but also bring nitrogen into the carbon framework. The simultaneous-activation of KOH and NH₃ 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 m² 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.