Ultrathinning Nickel Sulfide with Modulated Electron Density for Efficient Water Splitting

Abstract Developing nonprecious electrocatalysts via a cost‐effective methods to synergistically achieve high active sites exposure and optimized intrinsic activity remains a grand challenge. Here a low‐cost and scaled‐up chemical etching method is developed for transforming nickel foam (NF) into a highly active electrocatalyst for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). The synthetic method involves a Na₂S‐induced chemical etching of NF in the presence of Fe, leading to a growth of ultrathin Fe‐doped Ni₃S₂ arrays on the NF substrate (Fe_xNi_3‐xS₂ @ NF). The combined experimental and theoretical investigations reveal that the incorporated Fe cations significantly modulate the morphology and the surface electron density of Ni₃S₂, and thus significantly boost the electrochemically active surface area, electron transfer, and optimize the hydrogen/water absorption free energy. The developed Fe_0.9Ni_2.1S₂ @ NF requires overpotentials of only 72 mV at 10 mA cm⁻² for HER and 252 mV at 100 mA cm⁻² for OER in 1.0 m KOH, respectively, enabling an alkaline electrolyzer at a low cell voltage of 1.51 V to drive 10 mA cm⁻² for overall water splitting. More broadly, this synthetic approach is very versatile and can be used to synthesize other ultrathin metal sulfides (e.g., Fe–Cu–S, Fe–Al–S, and Fe–Ti–S).