Stabilizing active β-NiFeOOH phase for ampere-level alkaline water oxidation

Nickel‑iron (oxy)hydroxides (NiFeOOH) are promising electrocatalysts for the alkaline oxygen evolution reaction (OER). However, their industrial viability is critically limited by overcharge-induced phase reconstruction, in which the active β-NiFeOOH (Ni ≤ +3) phase irreversibly transforms into the catalytically sluggish and structurally fragile γ-NiFeOOH (Ni > +3) phase under ampere-level current densities. To overcome this bottleneck, we herein design a NiFe-LDH/Fe_1-xS heterostructure through a corrosion engineering approach. Combined in situ/ex situ Raman spectroscopy, X-ray photoelectron spectroscopy, and theoretical calculations reveal that spontaneous electron transfer from the Fe_1-xS substrate to the NiFe-LDH overlayer elevates the local electron density at both Ni and Fe sites, effectively mitigating the over-oxidation of the β phase to the γ phase. Furthermore, density functional theory calculations and inductively coupled plasma mass spectrometry indicate that the enhanced FeO covalency induced by the same electron transfer reduces net Fe dissolution while promoting the dissolution-redeposition of Fe, hereby maintaining the surface availability of Fe. Consequently, the NiFe-LDH/Fe_1-xS electrocatalyst achieves 1000 h durability in both OER and overall water splitting at ≥1.0 A cm^-2. When integrated into a symmetric anion exchange membrane water electrolyzer, it can operate for 230 h at 25 °C in 1.0 M KOH (1.5 A cm^-2@2.25 V) and for 35 h at 80 °C in 6.0 M KOH (1.0 A cm^-2@1.82 V), with a total operating time of 265 h. This work provides insight into stabilizing the active phase of NiFe-based electrocatalysts under industrially relevant conditions.