From Soybean residue to advanced supercapacitors

Supercapacitor technology is an extremely timely area of research with fierce international competition to develop cost-effective, environmentally friendlier EC electrode materials that have real world application. Herein, nitrogen-doped carbons with large specific surface area, optimized micropore structure and surface chemistry have been prepared by means of an environmentally sound hydrothermal carbonization process using defatted soybean (i.e., Soybean meal), a widely available and cost-effective protein-rich biomass, as precursor followed by a chemical activation step. When tested as supercapacitor electrodes in aqueous electrolytes (i.e. H(2)SO(4) and Li(2)SO(4)), they demonstrate excellent capacitive performance and robustness, with high values of specific capacitance in both gravimetric (250–260 and 176 F g(−1) in H(2)SO(4) and Li(2)SO(4) respectively) and volumetric (150–210 and 102 F cm(−3) in H(2)SO(4) and Li(2)SO(4) respectively) units, and remarkable rate capability (>60% capacitance retention at 20 A g(−1) in both media). Interestingly, when Li(2)SO(4) is used, the voltage window is extended up to 1.7 V (in contrast to 1.1 V in H(2)SO(4)). Thus, the amount of energy stored is increased by 50% compared to H(2)SO(4) electrolyte, enabling this environmentally sound Li(2)SO(4)-based supercapacitor to deliver ~12 Wh kg(−1) at a high power density of ~2 kW kg(−1).