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.