Graphene nanosheets are synthesized via the carbonization of brown-rice husks followed by a one-stage KOH-activation process for the design of a sustainable electrochemical energy-storage electrode. The graphene nanosheets exhibit an ultra-thin crumpled-silk-veil-wave, sheet-like structure with a high surface area of ∼1225 m² g⁻¹ and a high porosity. The graphene-nanosheet electrode shows a specific capacitance of 115 F g⁻¹ at 0.5 mA cm⁻² and a high energy density of 36.8 W h kg⁻¹ at a power density of 323 W kg⁻¹, with an excellent cyclic stability of 88% over 2000 cycles. The observed good electrochemical energy-storage performance of the biomaterial-derived graphene-nanosheet electrode is due to the synergistic effect of the intrinsically large electrochemically active surface area, an enhanced ion diffusion, and an improved electrical conductivity.