Fluoride ion-mediated morphology control of fluorine-doped CoFe2O4/graphene sheet composites for hybrid supercapacitors with enhanced performance

Highlights: • Fluorine-doped CoFe₂O₄/Graphene sheet composites were prepared by one-pot facile hydrothermal synthesis. • The nanocomposite exhibits a high specific capacity of 288 C g⁻¹ at 1 A g⁻¹ in a three-electrode system. • The influence of the fluorine concentration of CoFe₂O₄/GS is studied. • A hybrid supercapacitor using the nanocomposite exhibits excellent rate capability and cycling stability. A one-pot facile hydrothermal synthesis of fluorine-doped CoFe₂O₄ sphere (FS-CoFe₂O₄)/graphene sheet (GS) composites, based on the Ostwald ripening mechanism, is presented in this work. Incorporation of fluorine in CoFe₂O₄/GS improves ion diffusion by forming a highly porous structure that serves as an electrolyte reservoir during the redox reactions and facilitates charge transfer in the electrode material. The influence of the fluorine concentration on the relationship between structure and electrochemical performance of CoFe₂O₄/GS is also studied. The FS-CoFe₂O₄/GS composite exhibits a high specific capacity of 288 C g⁻¹ at a current density of 1 A g⁻¹, along with an excellent rate capability of 80% (230 C g⁻¹ at a current density of 10 A g⁻¹). Moreover, the FS-CoFe₂O₄/GS composite is used to assemble a hybrid supercapacitor, which exhibits a high operating voltage of 1.6 V and delivers a high energy density of 28.4 Wh kg⁻¹ at a power density of 0.9 kW kg⁻¹, as well as an outstanding cycling performance (90% capacity retention after 20,000 cycles, in which the GSs support FS-CoFe₂O₄ for the whole duration of the cycling tests). The present study can open up a new avenue for exploiting fluorine as a dopant in high-performance hybrid supercapacitors.