Magnetic-Field-Assisted Hydrothermal Synthesis of Marigold-Like Nickel Sulfide Nanoflowers with Faradaic-Dominated Intercalation Pseudocapacitance for High-Performance Hybrid Supercapacitors

Nickel sulfide nanostructures have emerged as promising electrode materials for energy storage applications because of their high conductivity and large specific surface area. In this study, nickel sulfide nanostructures were synthesized via a magnetic-field-assisted hydrothermal method, with varying reaction durations. The key significance of this study lies in the design of the magnetic-field-assisted synthesis pathway, which enhanced the growth rate and allowed precise control over the morphology of nickel sulfide nanostructures. A 24 h reaction time efficiently stabilized the formation of marigold-shaped nanoflowers. The resulting electrode showed a Faradaic-dominated intercalation pseudocapacitance mechanism. It delivered a high specific capacitance of 2954 F/g. In addition, 90% capacity retention was maintained after 5000 galvanostatic charge–discharge cycles, confirming its outstanding long-term stability. Moreover, the assembled hybrid supercapacitor (HSC) device delivered a maximum energy density of 60.1 Wh/kg at a power density of 700 W/kg. Even at a high-power density of 4200 W/kg, it maintained an energy density of 26.83 Wh/kg. This corresponds to a 6-fold increase in power density while retaining approximately 45% of the maximum energy density, highlighting the excellent rate capability of the device. Following 10,000 cycling tests, the HSC device maintains excellent retention of 88% of its initial capacitance. The practical applicability of the device is demonstrated by the illumination of a red-colored light-emitting diode using serially connected pouch-cell-type devices over a relatively longer time, suggesting that the magnetic-field-assisted hydrothermal synthesis pathway with varying reaction durations is an effective approach and is highly essential for producing high-performance nickel sulfide nanostructures for energy storage applications.