Activating Lattice Oxygen in Non-LDH Ni–Mo Catalysts via Fe Modulation for Alkaline Water Electrolysis

Abstract

Anion exchange membrane water electrolysis （AEMWE） combines the strengths of the two established electrolysis platforms: the earth-abundant electrocatalysts permitted by the alkaline environment of conventional alkaline electrolyzers, and the compact, high-current-density operation of proton exchange membrane （PEM） electrolyzers, which depend on scarce noble-metal catalysts （Pt, Ir/Ru） that are stable only in acidic media. Realizing this potential at low-cost hinges on a single, earth-abundant bifunctional catalyst that can drive both the cathodic hydrogen evolution and the anodic oxygen evolution within the same cell.

Among non-precious catalysts, Ni-Mo alloys are outstanding hydrogen evolution reaction （HER） catalysts in alkaline media, but they are intrinsically monofunctional: they show little oxygen evolution reaction （OER） activity, and Mo dissolution under anodic operation further undermines stability. Converting such an HER-specialized material into a durable bifunctional electrode is therefore the central challenge addressed in this work.

To this end, Fe-modulated Ni-Mo/MoO2 nanoarray electrodes （NiMoxFey@NF） were synthesized by hydrothermal growth followed by thermal reduction. The central question was how Fe incorporation introduces OER activity and lattice-oxygen reactivity while preserving the intrinsic HER performance, thereby rendering a single catalyst bifunctional. Varying the Mo and Fe precursor concentrations identified NiMo0.25Fe0.0625NF （Mo:Fe = 4:1） as the optimal composition. In 1.0 M KOH it required an HER overpotential of 61.1 mV at −100 mA cm-2 （Tafel slope 50.4 mV dec-1） and an OER overpotential of 145.7 mV at 50 mA cm-2, confirming activity for both reactions in one material, and it exhibited the highest double-layer capacitance of the series. Structural characterization confirmed a Ni4Mo/MoO2 heterostructure with Fe-enriched surface regions, and XPS indicated electronic coupling among Ni, Mo, and Fe consistent with the enhanced activity.

In-situ Raman spectroscopy and isotope-labeling differential electrochemical mass spectrometry （DEMS） showed that the as-prepared catalyst lacks an intrinsic layered double hydroxide （LDH） phase. Instead, anodic operation drives surface reconstruction into a Ni（Fe）OOH-like active layer through Fe incorporation together with partial Mo leaching. The appearance of mixed 16O18O products in the isotope experiments evidences lattice-oxygen participation, indicating an adsorbate evolution mechanism （AEM） with a partial lattice oxygen mechanism （LOM） contribution enabled by this Fe-modulated reconstruction. Used as both the anode and the cathode in a 9 cm2 AEMWE cell, this single bifunctional electrode reached 1000 mA cm-2 at 1.62 V and operated stably for 600 h at industrially relevant current density.

These results show that controlled Fe modulation converts an HER-specialized, non-LDH Ni-Mo catalyst into a durable bifunctional electrode by activating lattice-oxygen chemistry while preserving its high HER activity, providing a design strategy for noble-metal-free water-splitting catalysts that operate at both electrodes.