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 m(2) 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 0 (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 I 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. (C) 2017 Elsevier B.V. All rights reserved.