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).