NiCo2S4 quantum dots with high redox reactivity for hybrid supercapacitors

Abstract Despite advances in the synthetic methodology and multi-dimensional morphology engineering of ternary nickel cobalt sulfides for supercapacitors electrode materials, the intrinsic incentive mechanism of nickel cobalt sulfides in elevating charge storage property still remains cryptic. Here, we successfully synthesized high quality NiCo2S4 quantum dots (QDs) with good monodispersity and size distribution. Combining DFT theoretical calculation with ex-situ XPS characterization results, we elucidate that the synergistic modulation between reactive Ni and Co donates more active electrons near the Fermi level, as well as advantageous reaction surface and decreased energy barrier. This synergistically enhanced reactivity boosts larger proportion of Ni and Co from the NiCo2S4 QDs involved in the redox reaction, contributing to more than three times augment in specific capacity over the NiS2 or CoS2 QDs. Meanwhile, electrochemical studies indicate that the structural superiorities of NiCo2S4 QDs accelerate ions diffusion and reaction kinetics. Benefiting from these aspects, a hybrid supercapacitor constructed by the NiCo2S4 QDs as cathode and nitrogen-doped reduced graphene oxide nanosheets as anode delivers an ultrahigh energy density of 67.5 Wh·kg-1 at power density of 850 W·kg-1, as well as excellent rate performance and robust cyclic stability.