Engineering 3D Heterostructured NiCo2S4/Co9S8-CNFs via Electrospinning and Hydrothermal Strategies for Efficient Bifunctional Energy Conversion

The rational design of multifunctional electrocatalysts requires synergistic integration of conductive scaffolds with redox-active components. Here, a hierarchical core-shell NiCo₂S₄ grown/anchored on Co₉S₈-loaded carbon nanofibers (NCS/CS/CNFs) was synthesized via an electrospinning and hydrothermal approach and systematically characterized. FESEM/TEM confirmed a core-shell nanofiber structure with a NiCo₂S₄ shell thickness of ~30-70 nm, increasing the fiber diameter to ~290 ± 30 nm, while BET analysis revealed a surface area of 24.84 m² g^-1 and pore volume of 0.042 cm³ g^-1, surpassing CS/CNFs (6.12 m² g^-1) and NCS (4.85 m² g^-1). XRD confirmed crystalline NiCo₂S₄ and Co₉S₈ phases, while XPS identified mixed Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ states with strong Ni-S/Co-S bonding, indicating enhanced electron delocalization. Electrochemical measurements in 1 M KOH demonstrated outstanding OER activity, with NCS/CS/CNFs requiring only 324 mV overpotential at 10 mA cm^-2, a Tafel slope of 125.7 mV dec^-1, and low charge-transfer resistance (0.33 Ω cm²). They also achieved a high areal capacitance of 1412.5 μF cm^-2 and maintained a stable current density for >5 h. For methanol oxidation, the composite delivered 150 mA cm^-2 at 0.1 M methanol, ~1.6 times that of CS and 1.3 times that of NCS, while maintaining stability for 18,000 s. This bifunctional activity underscores the synergy between conductive CNFs and hierarchical sulfides, offering a scalable route to durable electrocatalysts for water splitting and direct methanol fuel cells.