Polypyrrole-coated Boron-doped Nickel-Cobalt sulfide on electrospinning carbon nanofibers for high performance asymmetric supercapacitors.

PPy@B-NCS/CNF composites exhibit excellent electrochemical performance as electrode materials for asymmetric supercapacitors. [Display omitted] • Thedensity functional theory calculation shows that the doping of boron increases the conductivity and oxidation resistance of Ni-Co sulfide, reduces the adsorption energy of hydroxide ions, and thus significantly improves the electrochemical performance. • Electrospinning Carbon Nanofibers (CNF) as the framework and substrate of composites can effectively improve the specific surface area and conductivity of electrode materials, showing excellent electrochemical properties. Surface coating of polypyrrole further improves conductivity and electrochemical stability. • The PPy@B-NCS/CNF delivers high specific capacity, high rate performance and excellent cycle stability. The device has good cycle reversibility and stability after low temperature treatment back to 20 °C. Although nickel–cobalt bimetallic sulfides have been widely studied for supercapacitor electrodes, how to obtain high specific capacity and cycle stability is still an important challenge. Here, an efficient chemical redox method is used to adjust the crystal and electronic structure of cobalt–nickel sulfide (NCS) via B doping, combined with electrospinning technology and conductive polymer polypyrrole (PPy) coating to facilitate faraday redox reactions and obtain high energy density electrode materials. The resulting composite with boron-doped nickel–cobalt sulfide on electrospinned carbon nanofibers with polypyrrole-coating (PPy@B-NCS/CNF) has a high specific capacity (751.61C/g at 1 A/g) and good cycle stability (82.49 % retention after 4000 cycles at 5 A/g). With PPy@B-NCS/CNF as the positive electrode and activated carbon as the negative electrode, an asymmetric supercapacitor (ASC) is prepared. It has excellent electrochemical properties with a power density of 65.58 Wh kg−1 and an energy density of 819.72 W kg−1. The low-temperature performance test shows high reversibility, which provides the possibility for the development of low-temperature electrolytes. Finally, density functional theory (DFT) explains that B-doped NCS has better electrochemical properties from the energy band and state density. [ABSTRACT FROM AUTHOR]