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 m² 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).