Iron-doped nickel sulfide nanospheres anchored on reduced graphene oxide for high performance supercapacitors

In this work, iron-doped nickel sulfides immobilized on reduced graphene oxide aerogel were synthesised by facile solvothermal and annealing methods. The obtained Fe-Ni-S/rGO composite with a 3D porous architecture provides a specific capacitance of 1220 F g-1 at 1 A g-1 with an excellent rate capability of 75.4% at 10 A g-1, which is superior to bare nickel sulfide modified rGO (Ni-S/rGO). An asymmetric supercapacitor composed of the prepared Fe-Ni-S/rGO coupled with activated carbon exhibits an energy density of 30.5 Wh kg-1 at a power density of 800 W kg-1 with excellent cycling stability (82% capacitance retention after 10 000 cycles at 3 A g-1), suggesting the practical application potential of the composite. DFT calculations indicate that the Fe dopants can modulate the electronic structure of the nickel sulfides by increasing the density of states near the Fermi level and narrowing the gap between the valence band and the conduction band, resulting in an improved charge transfer rate and hence charge storage capability of the composite. The excellent electrochemical properties of the composite suggest that doping transition metal atoms into the main lattice of the metal sulfides is an efficient way for designing electrodes for high performance supercapacitors. Fe doping in the NiS lattice leads to an increase in the density of states near the Fermi level and thus to improved charge storage.