Amorphous to Crystalline Ni₃S₂ Nanostructures Anchored on N-Doped Carbon Nanofibers for Electrochemical Splitting of Water

Nickel-based chalcogenides have recently gained considerable interest due to their potential as efficient electrocatalysts for overall water splitting. For example, nickel sulfide (NiS_x) with suitable chemisorption energy for both oxygen- and hydrogen-containing intermediates can catalyze both half-cell reactions. Here, we report the synthesis of Ni₃S₂ nanostructures of ∼8 nm mean size, anchored over electrospun N-doped carbon nanofibers using microwave synthesis. Careful optimization of the process parameters enabled the structural synthesis of three different types of Ni₃S₂ (mostly amorphous, partially crystalline, and mostly crystalline) over carbon nanofibers. Scanning and high-resolution transmission electron microscopy, powder X-ray diffraction, and X-ray photoelectron spectroscopy were used to characterize the catalysts and electrochemical performance evaluated in an alkaline medium. The partially crystalline phase of Ni₃S₂ supported over carbon nanofibers was the best-performing catalyst for the oxygen evolution reaction (OER) due to its high electrochemically active surface area, N-doping of carbon, presence of a crystalline and amorphous mixed phase, and high conductivity of the carbon support which resulted in a low overpotential of 270 mV for the OER with a Tafel slope of only 51 mV/dec and a low charge transfer resistance of 1.08 Ω. Moreover, the catalyst was stable and yielded more than 95% Faradaic efficiency in both the oxygen evolution and hydrogen evolution reactions.