NiCo2S4/tryptophan-functionalized graphene quantum dot nanohybrids for high-performance supercapacitors

Abstract

This paper reports a new NiCo₂S₄/tryptophan-functionalized graphene quantum dot nanohybrid (NiCo₂S₄/Trp-GQDs). NiCo₂S₄ was fabricated by a one-step hydrothermal reaction and then hybridized with Trp-GQDs to form NiCo₂S₄/Trp-GQDs. The result shows that the NiCo₂S₄ offers better structural stability and specific surface area compared to NiCo₂S₄ prepared by the traditional two-step method. The introduction of Trp-GQDs greatly enhances the electron/ion conductivity of NiCo₂S₄ and the mechanical properties of the binder layer. The hybrid of NiCo₂S₄ with Trp-GQDs achieves significant electrochemical synergy. The NiCo₂S₄/Trp-GQD electrode for supercapacitors provides high specific capacitance (1453.1 F g⁻¹ at a current density of 1 A g⁻¹) and rate-capability (455.6 F g⁻¹ at a current density of 20 A g⁻¹) in a 3.0 M KOH electrolyte using a three-electrode test system. The asymmetric supercapacitor of NiCo₂S₄/Trp-GQDs/activated carbon exhibits an energy density of 157.1 W h kg⁻¹ at a power density of 800 W kg⁻¹ and 103.2 W h kg⁻¹ at a power density of 4000 W kg⁻¹ in a 3.0 M KOH electrolyte. The energy density is much higher than that of a single NiCo₂S₄ electrode (67.9 W h kg⁻¹ at a power density of 800 W kg⁻¹ and 37.5 W h kg⁻¹ at a power density of 4000 W kg⁻¹). The excellent capacitive performance means the nanohybrid can be applied in next-generation high-performance supercapacitors.