NiCo₂S₄@PPy core-shell nanotube arrays on Ni foam for high-performance supercapacitors

Abstract

NiCo₂S₄@PPy core–shell nanotube arrays have been successfully prepared through three facile steps: solvothermal synthesis, solvothermal sulphurisation and subsequent in situ electropolymerisation of polypyrrole (PPy). Their structure and morphology were studied by XRD, FTIR, SEM and TEM. The results suggest that a thin layer of PPy with 8 nm in thickness is wrapped on the surface of NiCo₂S₄ nanotubes. The coating PPy strong promotes pseudo-capacitance reaction of NiCo₂S₄ nanotube arrays. NiCo₂S₄@PPy shows lower charge plateau and IR drop, higher discharge plateau and Coulombic efficiency. Rate performance is improved remarkably. Average specific capacitance at 24 mA cm⁻² is even two times that of the uncoated NiCo₂S₄. The galvanostatic cycling tests at 4 mA cm⁻² indicate NiCo₂S₄@PPy exhibit the highest specific capacitance of 1524 F g⁻¹ and improved capacitance retention to 78.5% after 3000 cycles. The corresponding values of the uncoated NiCo₂S₄ nanotube arrays are only 1410 F g⁻¹ and 22%. The improved electrochemical performance is mainly attributed to unique core–shell nanotube array structure and the key assistant effect of the coating PPy which remarkably improves the conductive property of NiCo₂S₄ long nanotubes and effectively enhances structure stability and cycling stability of NiCo₂S₄ nanotubes.

Keywords:

• NiCo₂S₄
• PPy
• core–shell nanotube
• supercapacitor