Ternary Ni-Co-F Nanocrystal-Based Supercapacitors.

Abstract

Ternary nickel cobalt fluoride (Ni-Co-F) nanocrystals are constructed solvothermally for use as the positive electrode materials in supercapacitors. The optimal Ni-Co-F (Ni/Co=2:1) shows slight chemical shifts in X-ray diffraction (XRD) data and X-ray photoelectron spectra (XPS) compared with bare Ni-F and Co-F. The Ni-Co-F (Ni/Co=2:1) exhibits typical square nanocrystal morphology together with a mesoporous surface structure, as observed through TEM observations and nitrogen sorption measurements. Owing to the stronger synergistic effect of Ni and Co redox species originated from the richer Ni and Co surface electroactive sites, Ni-Co-F (Ni/Co=2:1) shows superior performances of specific capacitance, rate capability, and charge-transfer kinetics (564 F g-1 at 1 Ag-1 , 418 F g-1 at 16 Ag-1 , 449 Ω) compared with all the other Ni-Co-F candidates. Moreover, the activated carbon (AC)//Ni-Co-F (Ni/Co=2:1) asymmetric capacitor designed based on charge balance delivers superior energy and power densities (18.4 Wh kg-1 , 6.64 kW kg-1 ) together with longer cycle life (77 % retention after 10 000 cycles at 4 A g-1 ).