Promoted OH- Adsorption and Electron-transfer Kinetics by Electrospinning Mono-disperse NiCo2S4 Nanocrystals within Porous CNFs for Solid Asymmetric Supercapacitors

Bimetallic sulfide NiCo2S4 has been regarded as a potential supercapacitor electrode material with excellent electrochemical performance. However, the origin of its high specific capacity is little studied, and the design of a rational structure still remains a challenge to exert its intrinsic advantage. In this work, the advantage of NiCo2S4 over NiS and CoS is explained by density functional theory calculation from the aspects of energy band, density of electronic states and OH- adsorption energy. It is proved that the synergistic effect of Ni and Co in NiCo2S4 can reduce its OH- adsorption energy and provide more active electrons near the Fermi level, thus promoting electrochemical reaction kinetics in supercapacitors. Then, a simple electrospinning method is used to in-situ load mono-disperse NiCo2S4 nanocrystals within amorphous carbon nanofibers, obtaining a porous, lotus-leaf-stem-like one-dimensional nanocomposite of NiCo2S4/CNF. Ex-situ XPS characterization confirms that the proportion of metal ions involved in electrochemical reactions and the number of transferred electrons in NiCo2S4/CNF during the redox reaction are significantly higher than those in mono-metallic sulfides (NiS/CNF and CoS/CNF), verifying the calculation results. With its boosting reaction kinetics, the NiCo2S4/CNF gives the specific capacity of 757.97 C g-1 at 1 A g-1 and the capacity retention of 95.15% after 10000 cycles at 5 A g-1, both greater than NiS/CNF and CoS/CNF. The NiCo2S4/CNF, as the positive electrode, and activated carbon, as the negative electrode, are assembled into liquid-state and solid-state asymmetric supercapacitor (ASC) devices, and both show high power density (760.6 W kg-1 for liquid-state device and 1067.4 W kg-1 for solid-state device), high energy density (52.25 Wh kg-1 for liquid-state device and 48.54 Wh kg-1 for solid-state device) and great cycle stability. Moreover, the solid-state ASC device possesses excellent low temperature capacity and reversibility, further demonstrating the wide application potential of the NiCo2S4/CNF composite.