Coconut shells, low-cost and renewable agro-wastes, were used as a starting material in the synthesis of hierarchical activated carbons via hydrothermal, KOH-activation, and carbonization techniques. The ratio of KOH to hydrochar was varied in a systemic manner to study how it influences the texture and electrochemical behavior of the capacitor. Coconut shell-based carbon coated on nickel foams presented a surface area of 1567 m² g⁻¹, with micropores as well as mesopores widely distributed. The sample showed superior electrochemical performance, attaining 449 F g⁻¹ at 1 A g⁻¹ in 6 M LiNO₃ aqueous solution. The solid-state symmetric supercapacitor device delivered a specific capacitance of 88 F g⁻¹ at 1 A g⁻¹ and a high energy density of 48.9 Whkg⁻¹ at a power density of 1 kW kg⁻¹. At a wide voltage window of 2.0 V, the sample was highly stable during the cycle test, showing a 92% capacitance retention at 2 A g⁻¹ after cycling for 5000 times. The superior performance is due to the sample possessing great BET surface area, a good distribution of pores, and the usage of a suitable electrolyte. This facilitates an electrical double layer that can be deployed for applications to store energy.