Preparation of Carbon Electrode Material with a Large Specific Surface Area and Multiscale Pore Structure from Biowaste Kochia for Symmetrical Supercapacitor

Kochia that is used as Chinese herbal medicine is always abandoned as a biowaste after extracting useful components. However, the richness of lignocellulose and the natural tubular structure make it a potential source of biomass carbon. After high‐temperature pyrolysis at 800 °C and further etching by proper activation conditions, a graded porous structured carbon‐based supercapacitor electrode material (KAC‐5) can be obtained. The electrode material is prepared in a 1:5 carbon‐to‐base ratio, possessing a specific surface area of 1441.30 m 2 g −1 , and a graded macro‐meso‐micro structure that is rich in micropores. The capacitance retention retains at 90.31% even after 10 000 cycles in 6 m KOH electrolyte and exhibits outstanding capacitance performance with a specific capacitance of 284 F g −1 at a current density of 1 A g −1 . The symmetrical supercapacitor is assembled with KAC‐5 and also has good cycling stability in 6 m KOH aqueous electrolyte, reaching 88.83% after 10 000 cycles with an energy density of 12.3 Wh kg −1 at a power density of 300 W kg −1 . Based on the above, sustainable preparation of energy storage material from waste biomass for high capacitance and stable biomass‐based supercapacitors has been achieved.