A smart architecture of nickel-cobalt sulfide nanotubes assembled nanoclusters for high-performance pseudocapacitor

Highlights: • NiCo₂S₄ material is prepared by combining hydrothermal method and ion exchange. • NiCo₂S₄ morphology is controlled by altering the kinetics of ion-exchange process. • Well-defined structure and good pseudocapacitance performance are achieved. • Asymmetric supercapacitors are assembled and high energy density is obtained. To develop a high-performance capacitive material with both superior power density and energy density, it is very important to construct a nanomaterial with a well-controlled structure. In this work, we report the preparation of NiCo₂S₄ nanotubes-assembled nanoclusters with a combination of hydrothermal and ion exchange processes. By optimizing the ion-exchange temperature, the tubular morphology can be both achieved and optimized. By tuning the conditions of the synthesis process, the diameter of the primary 1D structure can be increased, which leads to a compact cluster with decreased surface area. In particular, the sample prepared at 180 °C (NCS2) shows the morphology of nanoclusters assembled nanotubes with a wall thickness of about 7 nm. Such an architecture shows excellent electrochemical performance as a pseudocapacitor. It shows an initial specific capacitance of 1005 F g⁻¹ at the current density of 1 A g⁻¹ and remains 896 F g⁻¹ at a current density of 20 A g⁻¹. Moreover, it displays a favorable capacitance retention of 79.34% after 5000 cycles at a current density of 10 A g⁻¹. Furthermore, the NCS2//AC asymmetric supercapacitor exhibits excellent rate performance (retaining 81% of the initial capacity when the current density increases from 1 A g⁻¹ to 20 A g⁻¹) and a high energy density of 52 Wh kg⁻¹ at a power density of 9288 W kg⁻¹. This work lays the foundation for the design and optimization of NiCo₂S₄ based nanostructured materials for energy storage.