Promoted OH– adsorption and electron-transfer kinetics by electrospinning mono-disperse NiCo2S4 nanocrystals within porous CNFs for solid asymmetric supercapacitors.

The Lotus-leaf-stem-like NiCo 2 S 4 /CNF composite is prepared with mono-disperse NiCo 2 S 4 nanocrystals in-situ electrospun within porous CNFs for improved supercapacitive performance. [Display omitted] • DFT calculation results show that NiCo 2 S 4 has higher electronic conductivity and lower adsorption energy of OH– than NiS and CoS. • One-step electrospinning of metal ions and carbon sources by a very simple method. • Ex-situ XPS characterization test verifies that NiCo 2 S 4 /CNF has higher proportion of metal ions involved in electrochemical reactions and larger number of transferred electrons. • l -ASC and S-ASC devices have high-performance energy storage. S-ASC devices have excellent low-temperature performance and stability. Bimetallic sulfide NiCo 2 S 4 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 NiCo 2 S 4 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 NiCo 2 S 4 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 NiCo 2 S 4 nanocrystals within amorphous carbon nanofibers, obtaining a porous, lotus-leaf-stem-like one-dimensional nanocomposite of NiCo 2 S 4 /CNF. Ex-situ XPS characterization confirms that the proportion of metal ions involved in electrochemical reactions and the number of transferred electrons in NiCo 2 S 4 /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 NiCo 2 S 4 /CNF gives the specific capacity of 757.97C g−1 at 1 A/g and the capacity retention of 95.15 % after 10,000 cycles at 5 A/g, both greater than NiS/CNF and CoS/CNF. The NiCo 2 S 4 /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 NiCo 2 S 4 /CNF composite. [ABSTRACT FROM AUTHOR]