Battery-type Ni-Co-Se hollow microspheres cathode materials enabled by bifunctional N-doped carbon quantum dots with ultrafast electrochemical kinetics for hybrid supercapacitors

Hybrid supercapacitors (HSCs) with large energy and power density have received increasing concern for their great potential in satisfying the requirements of energy storage systems. Nonetheless, the imbalanced kinetics between the sluggish battery-type cathode and the rapid capacitor-type anode make it difficult for HSCs to achieve high energy density at high power density. Herein, a novel N-doped carbon quantum dots/Ni-Co-Se (N-CQDs/Ni-Co-Se) hollow microspheres composite is synthesized via a facile hydrothermal approach using bifunctional CQDs as size regulators and conductive agents for the first time. Thanks to the synergism of highly conductive N-CQDs with rapid electron transfer and reduced-size hollow micro-/nanostructures contributing to the enhanced ion transport, the as-prepared battery-type N-CQDs/Ni-Co-Se hollow microspheres composite cathode exhibits admirable rate property. In-depth electrochemical kinetic analyses and density functional theory (DFT) calculations are utilized to elucidate the preeminent kinetic properties. Furthermore, a novel HSC is fabricated based on the N-CQDs/Ni-Co-Se hollow microspheres composite cathode with ultrafast electrochemical kinetics, displaying a high energy density of 23.1 Wh kg−1 at a superb power density of 38.3 kW kg−1. This encouraging work provides a good strategy to construct ultrahigh rate battery-type electrode materials with tunable size and component for simultaneously obtaining high energy/power density HSCs.