Coconut Shell-Derived Activated Carbon For High-Performance Solid-State Supercapacitors

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 m(2) 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.