Construction of a Hierarchical NiCo2S4@PPy Core–Shell Heterostructure Nanotube Array on Ni Foam for a High-Performance Asymmetric Supercapacitor

In this paper, a hierarchical NiCo2S4@polypyrrole core–shell heterostructure nanotube array on Ni foam (NiCo2S4@PPy/NF) was successfully developed as a bind-free electrode for supercapacitors. NiCo2S4@PPy-50/NF obtained under 50 s PPy electrodeposition shows a low charge-transfer resistance (0.31 Ω) and a high area specific capacitance of 9.781 F/cm2 at a current density of 5 mA/cm2, which is two times higher than that of pristine NiCo2S4/NF (4.255 F/cm2). Furthermore, an asymmetric supercapacitor was assembled using NiCo2S4@PPy-50/NF as positive electrode and activated carbon (AC) as negative electrode. The resulting NiCo2S4@PPy-50/NF//AC device exhibits a high energy density of 34.62 Wh/kg at a power density of 120.19 W/kg with good cycling performance (80.64% of the initial capacitance retention at 50 mA/cm2 over 2500 cycles). The superior electrochemical performance can be attributed to the combined contribution of both component and unique core–shell heterostructure. The results demonstrate that the NiCo2S4@PPy-50 core–shell heterostructure nanotube array is promising as electrode material for supercapacitors in energy storage.