Litchi seed biowaste-derived activated carbon supporting matrix for efficient symmetric and asymmetric supercapacitors

Litchi seeds are biowaste produced in 3.5 million tons annually worldwide that take more than two years to decompose naturally. Therefore, it is necessary to find a way to transform it into a high-value material. Activated carbon (AC) is one promising candidate, as it is an essential energy storage material for supercapacitors and batteries. As a sustainable source, litchi seed is used raw material for AC based on “waste to wealth.” Supercapacitors require both high energy and power density. As AC delivers low energy density, tailoring the properties of AC to enhance its electrochemical performance is a primary goal of this study. A 3D carbon composite (ARC) was synthesized as a negative electrode by combining 1D multiwalled carbon nanotubes and 2D reduced graphene oxide sheets with 3D AC as a supporting matrix, yielding a specific capacitance of 320 F g−1 at 1 A g−1 in a 6 M KOH. A positive electrode of zinc cobalt sulfide on AC (ZCS@AC) was synthesized hydrothermally by decorating 3D AC supporting matrix with 0D ZCS particles, producing a capacity of 303.3 C g−1. An asymmetric supercapacitor, ZCS@AC//ARC, was fabricated to give a high energy density of 43.7 Wh kg−1 and a power density of 11.2 kW kg−1 in KOH. A symmetric supercapacitor, ARC//ARC, was fabricated with 1-ethyl-3-methylimidazolium-tetrafluoroborate electrolyte, delivering an excellent energy density of 72.2 Wh kg−1 and a remarkable power density of 17.5 kW kg−1. This study may be used to apply a natural waste litchi seed to energy storage materials, especially for high-performance customized supercapacitors.