The renewable biomass material obtained from rice husk, a low-cost agricultural waste, was used as a precursor to synthesize a highly porous graphene-based carbon as electrode material for supercapacitors. Activated graphene-based carbon (AGC) was obtained by a two-step chemical procedure and exhibited a very high specific surface area (SSA) of 3292 m² g⁻¹. The surface morphology of the synthesized materials was studied using scanning and transmission electron microscopy (SEM, TEM). Furthermore, the AGC was modified with nickel hydroxide Ni(OH)₂ through a simple chemical precipitation method. It was found that the most significant increase in capacitance could be reached with Ni(OH)₂ loadings of around 9 wt.%. The measured specific capacitance of the pure AGC supercapacitor electrodes was 236 F g⁻¹, whereas electrodes from the material modified with 9 wt.% Ni(OH)₂ showed a specific capacitance of up to 300 F g⁻¹ at a current density of 50 mA g⁻¹. The increase in specific capacitance achieved due to chemical modification was, therefore 27%.