Biomass valorisation of marula nutshell waste into nitrogen-doped activated carbon for use in high performance supercapacitors

This study reports on the valorisation of marula nutshell waste into a valuable electrode material for supercapacitors. Marula nutshell waste, a brown hard residue from marula fruits was valorised into a nitrogen-doped activated carbon (N-ACs) by KOH treatment and using melamine as a nitrogen source. The microporous and mesoporous nature of N-ACs exhibited a high surface area of 1427 m² g−1 and pore volume of 0.31 cm3 g−1. Three electrode measurements were done in 6 M KOH and 2.5 M KNO3 aqueous electrolytes, to examine the materials in alkaline and neutral medium. The N-ACs exhibited a capacitance of 350 F g−1 in 6 M KOH and 248 F g−1 in 2.5 M KNO3 electrolyte. Due to low toxicity and wider operating voltage (∼1.8 V) of 2.5 KNO3, the material was further analysed in a symmetric device. The high surface area, the defects, and the high content of nitrogen (5.1%) revealed that the material produced an outstanding energy density of 17.2 Wh kg−1 and a power density of 448.7 W kg−1 in 2.5 M KNO3 electrolyte. The conversion of marula nuts waste into N-ACs thus provide a means of converting biomass waste into a useful energy storage material for supercapacitors.