Hierarchical polypyrrole nanotubes@NiCo2S4 nanosheets core-shell composites with improved electrochemical performance as supercapacitors

NiCo2S4 is a promising electrode material for electrochemical supercapacitors, but its poor electronic conductivity and weak electrochemical stability limit the fast charge/discharge rate and long-time reuse. Herein, we coat NiCo2S4 nanosheets on conductive polypyrrole nanotubes to form a hierarchical core-shell [email protected]2S4, aiming to enhance the electron transport and electrochemical stability of NiCo2S4. The characterization analyses confirm [email protected]2S4 is successfully synthesized and NiCo2S4 nanosheets are uniformly coated on PNTs. The [email protected]2S4 exhibits a low charge-transfer resistance (0.55 Ω) and a high specific capacitance (911 F/g at 1 A/g), which is higher than that of pure NiCo2S4 (470 F/g) and other NiCo2S4-based materials. Furthermore, 93.2% of the initial specific capacitance is remained after 4000 cycles at 5 A/g, indicating the excellent stability of [email protected]2S4. Besides, an all-solid-state symmetric supercapacitor built with [email protected]2S4 shows high energy density of 21.3 Wh/kg at 417 W/kg, high power density of 8.6 kW/kg at 11.3 Wh/kg, and good cycling stability (91.6% capacitance retention after 4000 cycles at 5 A/g). The remarkable performance of [email protected]2S4 is mainly attributed to the good conductivity of PNTs, novel core-shell structure, and synergetic effect of NiCo2S4 and PNTs. The results indicate that [email protected]2S4 is potential electrode material for supercapacitors.