Heterostructured flower-like NiO/Co3O4 microspheres modified by bifunctional carbon quantum dots as a battery-type cathode for high energy and power density hybrid supercapacitors

Hybrid supercapacitors (HSCs) comprising a battery-type cathode and capacitive anode have recently become a research hotspot. Nevertheless, the low capacity utilization, poor kinetic behavior, and unstable structure of a single battery-type oxide cathode restrict the overall performance of the device. Herein, the carbon quantum dots (CQDs) modified NiO/Co₃O₄ heterostructured flower-like microspheres are constructed, and enhanced specific capacity, rate capability, and cycling performance are achieved when used as the cathode for HSCs. This is attributed to the fact that the modification of bifunctional CQDs as size regulators and conductive agents and the construction of heterostructure can not only improve the specific surface area and provide more electroactive sites, thereby enhancing the charge storage performance but also regulate the electronic structure and boost the interface charge transfer capability and electronic conductivity, thereby boosting the reaction kinetics and cycle stability. The enhanced electrochemical kinetic behavior is revealed by electrochemical kinetic analyses based on cyclic voltammetry, electrochemical impedance spectroscopy tests and density functional theory calculations. Meanwhile, the electrochemical reaction process and energy storage mechanism are illustrated by ex-situ X-ray diffraction and X-ray photoelectron spectroscopy characterizations. Furthermore, an HSC is further constructed using the CQDs/NiO/Co₃O₄ heterostructured flower-like microspheres as the cathode, simultaneously achieving high energy density (40.9 Wh kg⁻¹), high power density (24 kW kg⁻¹), and splendid cyclic stability (94.2% capacity retention after 5000 cycles at 10 A g⁻¹). These synergistic modification strategies of bifunctional CQDs modification and heterostructure design provide a valuable direction for the design and development of HSCs with both high energy density and high power density.