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.