Biomass based N-doped hierarchical porous carbon nanosheets for all-solid-state supercapacitors

Biomass is regarded as sustainable and low-cost precursors of porous carbons for electrochemical energy storage and conversion due to their varied architecture and plentiful heteroatom containing. We herein developed a hierarchical N-doped porous carbon nanosheet material from soybean milk by employing simultaneous hard-template and KOH activation strategy. Hard-template of CaCO3 nanospheres, KOH activation and the melt template of potassium species respectively result in the formation of macropores, micropores and nanosheet-like morphology. Moreover, abundant nitrogen and oxygen elements in precursor bring a high heteroatom percentage in resultants, which don't contribute psuedocapacitance but also promote electron transfer and conductivity. The obtained carbons exhibit good charge storage capacity with a specific capacitance of 240.7 F g(-1) (1 A g(-1)) and the excellent retention of initial specific capacitance (92.2% to 20 A g(-1)) that is much higher than those of biomass-based porous carbons. A symmetric all-solid-state supercapacitor using KOH/PVA as electrolyte has a specific capacitance of 149.3 F g(-1) (0.5 A g(-1)), and good cycling stability with capacitance retention of 89.3% after 5000 cycles. The device displays energy density of 10.2 Wh kg(-1) at a power density of 351 W kg(-1) and retains 8.2 Wh kg(-1) even at a high power density of 19,600 W kg(-1 )with a wide voltage window of 1.4 V.