Microporous carbon from malva nut for supercapacitors: effects of primary carbonizations on structures and performances

Abstract Malva nut-derived microporous carbons are synthesized and investigated as electro-active materials for electrochemical supercapacitors. The biomass precursor is suffered primary carbonizations via hydrothermal and annealing treatments, respectively, and then the bio-chars are activated by potassium hydroxide to form amorphous microporous carbon. The porous carbons possess high specific surface area (> 1842 m2 g−1) and well-developed micro-porosity (> 88%). >2.1 at. % nitrogen and over 3.8 at. % oxygen contents are also distributed in the carbon matrixes. Supercapacitor electrode enclosed the porous carbons exhibits specific capacitance of 312 F g−1 at a specific current of 1 A g−1, and shows excellent cycling stability with capacitance retention over 93% after 7500 cycles performed on 5 A g−1. A symmetric device assembled by the carbon material delivers specific capacitance of 247 F g−1 at a current density of 1 A g−1, and achieves the specific energy of 8.4 kg−1 with specific power of 246 W kg−1. Porous carbon obtained by annealing pre‑carbonization and chemical activation has relatively superior physicochemical properties and electrochemical performance. Results demonstrate that malva nut can be used as a potential precursor in the design of porous carbon for high-performance electrochemical capacitors.