Achieving Complete Conversion from Nickel Foam to Nickel Sulfide Foam for a Freestanding Hybrid-Supercapacitor Electrode

We present a unique, one-step, hydrothermal process to prepare nickel sulfide (Ni₃S₂) foam by a simple and direct conversion from nickel foam, which contributes both as a scaffold for the reaction and as reactant. The Ni₃S₂ foam exhibits remarkable mechanical stability, retaining the structural integrity of the foam and excellent crystallinity even after ultrasonication at 200 W for 30 mins. We document the transformation of the nickel foam template into a Ni₃S₂ foam, highlighting the role of synthesis duration on the phase evolution and unique morphology of Ni₃S₂. PXRD and SEM analyses reveal a complete transformation after 24 hours from the nickel foam to a pure Ni₃S₂ foam, which has a highly porous and interconnected ultra-thin nanosheet architecture. This significantly enhances the surface area and provides many electrochemical reaction sites. In a three-electrode cell, the capacity of the Ni₃S₂ foam electrode is 3.9 C cm⁻² at 8 mA cm⁻², which is higher than previous reports for Ni₃S₂. In a hybrid supercapacitor device, the Ni₃S₂ foam demonstrates significant increase in capacitance through 500 cycles and the capacitance plateaus after 2000 cycles. Even after 8500 continued charge-discharge cycles, the device exhibits excellent cycle stability indicating improvement with age.