In situ formation of nanocrystalline Ni(OH)₂ in alkaline electrolyte explains superior capacitance and cycling stability of Ni₃S₂/NF electrodes

The free-standing Ni₃S₂ electrode was fabricated via a simple one-step hydrothermal method at 160 °C through the sulfidization of nickel foam (NF). The as-synthesized samples exhibited X-ray diffraction (XRD) patterns and Raman spectra consistent with the formation of Ni₃S₂ Although the XRD reflections of the Ni₃S₂ lattice remained unchanged after short-term electrochemical activation in 3.5 M KOH electrolyte, the Raman spectrum evolved to resemble that of oxidized nickel species, indicating a surface transformation of the electrode under electrochemical conditions. Prolonged redox cycling of the electrode for more than 10,000 cycles resulted in noticeable modifications of the XRD pattern. In addition to the characteristic reflections of the Ni₃S₂ phase, the formation of β-Ni(OH)₂ nanoparticles was observed, with an average crystallite size of approximately 2.8 nm, as estimated using the Scherrer equation. The resulting heterostructured Ni(OH)₂/Ni₃S₂/NF electrode exhibited an areal capacitance of 4.1 C cm^–2 at a current density of 20 mA cm^–2 when evaluated in a three-electrode configuration. The electrode demonstrated progressively enhanced capacitance stability with continued cycling. Capacitance retention increased from 19% after the first 10,000 cycles to 41% after the second 10,000 cycles, 66% after the third 8,000 cycles, and ultimately 79% after the fourth 6,000 cycles. Taking into account the electrochemical activation occurring between cycling intervals, the overall capacitance retention reached 72% after more than 30,000 cycles. Although the electrode capacitance decreased during galvanostatic charge–discharge (GCD) cycling, it could be partially recovered through subsequent cyclic voltammetry (CV) activation. Concurrently, a broad XRD reflection attributable to the α-Ni(OH)₂ phase emerged, providing valuable insight into the structural evolution of the electrode and the underlying electrochemical energy-storage mechanism. The Ni(OH)₂/Ni₃S₂/NF electrode aged through 10,000 GCD cycles was subsequently employed as the positive battery-type electrode in a hybrid supercapacitor, paired with an activated carbon negative electrode. The assembled device, with a Ni3S2/NF electrode area of 6 cm², delivered an areal capacitance of 4.63 C cm^–2 at a current density of 4 mA cm^–2 and maintained 83.5% capacitance retention after more than 20,000 charge–discharge cycles.