Controllable synthesis of superparamagnetic NiCo-graphene quantum dot-graphene composite with excellent dispersion for high performance magnetic field-controlled electrochemical flow hybrid supercapacitor

The in-situ formation, fixation and structural control of metal nanoparticles on graphene sheets are crucial issue to fabricate metal-graphene composite with high electrochemical performance. The study reports the synthesis of superparamagnetic NiCo- graphene quantum dot-graphene composite (NiCo-GQD-G). Histidine-functionalized graphene quantum dots (GQDs) were immobilized on graphene sheets of graphite oxide (GO) to form GQD-GO hybrid via pi-pi stacking. The hybrid are combined with Ni2+ and Co2+ to produce NiCo-GQD-GO complex. Followed by thermal reduction and partial oxidation. The resulting NiCo-GQD-G has a well-defined three-dimensional structure, monodispersity and superparamagnetic. NiCo nanocrystals were covered by NiO and CoO to produce core-shell structure with average particle size of 7.2 nm. The nanostructure can be effectively adjusted by varying the mass ratios of GQD/GO and NiCo/GO. The unique architecture promotes enhanced electron/ion conductivity, catalytic activity and magnetofluid property. The NiCo-GQD- G was adsorbed on nickel foam by magnet to make an electrode without any binder and conductive agent. The electrode exhibits high capacity (1808 F g(-1) at 1 A g(-1)), rate-capacity (1525 F g(-1) at 10 A g(-1) and 900 F g(-1) at 200 A g(-1)) and cycling stability (99.2% of capacity retention after 5000 cycles at 10 A g(-1)) in 3.0 M KOH electrolyte. The flow hybrid supercapacitor provides energy density of 247.4Wh kg(-1) at power density of 6596Wkg(-1) and 111.2Wh kg(-1) at power density of 160100 W kg(-1). The synthesis can be also used for construction of other metalgraphene composites with excellent dispersion and controllable nanostructure. (c) 2020 Elsevier Ltd. All rights reserved.