Nitrogen-doped two-dimensional porous carbon sheets derived from clover biomass for high performance supercapacitors

Highly porous carbon sheets were prepared from fresh clover stems under air atmosphere via a facile potassium chloride salt-sealing technique, which not only avoids using the high cost inert gas protection but also spontaneously introduce multi-level porosity into the carbon structure taking advantage of the trace of oxygen in the molten salt system. The as-obtained porous carbon sheets possess high specific surface area of 2244 m2 g−1 and interconnected hierarchical pore structures from micro-to macro-scale, which provide abundant storage active sites and fast ion diffusion channels. In addition, the spontaneously formed N (2.55 at%) and O (6.94 at%) doping sites not only improve the electron conductivity of the electrode but also enhance the specific capacitance by introducing pseudocapacitance. When employed as supercapacitor electrodes, a high specific capacitance of 436 F g−1 at 1 A g−1 and an excellent rate capacity with capacitance remaining 290 F g−1 at 50 A g−1 are demonstrated. Furthermore, the assembled symmetric supercapacitor delivers a high specific capacitance of 420 F g−1 at 0.5 A g−1, excellent energy density of 58.4 Wh kg−1 and good cycling stability which retains 99.4% of the initial capacitance at 5 A g−1 after 30,000 cycles.