Sustainable Low-Cost Green Electrodes with High Volumetric Capacitance for Aqueous Symmetric Supercapacitors with High Energy Density

Low-cost and high performance electrode materials are critical for rapid development and wide applications of supercapacitors. Herein, we report a cost-effective approach to prepare nitrogen-doped active carbon/graphene (N-AC/Gr) composites using renewable corn straw and soy protein as precursors, respectively. The as-prepared composites have moderate specific surface areas in a range of 1233.6-1412.9 m(2) g(-1) and abundant porosity. Furthermore, the composites demonstrate high single electrode gravimetric and volumetric specific capacitances of 378.9 F g(-1) and 257.7 F cm(-3) at 0.05 A g(-1) and of 321.1 F g(-1) and 213.2 F cm(-3) at 20 A g(-1), accounting for an excellent rate capability with a retention of 66.4% in 6 M KOH electrolyte. The assembled N-AC/Gr-based aqueous symmetric supercapacitor exhibits superior cycling durability with 93% retention after 10,000 cycles at 2 A g(-1) and delivers a high energy density of 13.1 Wh kg(-1) (11.1 Wh L-1) with a power density of 12.5 W kg(-1) (10.6 W L-1).