High-Capacity Biomass-Based Bi–Bi₂O₃@C Composite Derived from Natural Cotton Fibers for Aqueous Supercapacitors

Aqueous supercapacitors possess high power density, long cycle life, and rapid charging and discharging capabilities, while their practical application is significantly limited by low energy density. The development of high-performance electrode materials, especially anodes, is crucial for enhancing the energy density of supercapacitor devices. In this work, Bi–Bi₂O₃ nanoparticles were anchored onto carbon fibers (Bi–Bi₂O₃@C) via a combined solvothermal and calcination process, using natural cotton to derive the biomass-derived carbon framework. The resulting Bi–Bi₂O₃@C composite was utilized as an anode material for aqueous supercapacitors, demonstrating excellent electrochemical performance with a specific capacity of 253.5 mAh g^–1 (912.6 F g^–1) at 1 A g^–1. Furthermore, a high-performance cathode material composed of NiS₂ nanoparticle-decorated Ni(OH)₂ nanosheets (NiS₂@Ni(OH)₂) was prepared by a simple hydrothermal method to achieve optimal matching with the anode. The obtained NiS₂@Ni(OH)₂ nanosheets cathode composite displays high specific capacitance of 1984 F g^–1 at 1 A g^–1 and remarkable rate capability, retaining 57% of its original capacity even at a high current density of 10 A g^–1. The constructed NiS₂@Ni(OH)₂//Bi–Bi₂O₃@C aqueous asymmetric supercapacitor (ASC) delivers a high energy density of 64.4 Wh kg^–1 at a power density of 798.2 W kg^–1, and maintains a high energy density of 43.1 Wh kg^–1 even at a power density of 15.9 kW kg^–1.