Synthesis, Alloying Process, and Enhanced Oxygen Evolution Activity of Highly Stable Ni–Fe–Mo Nanoparticles with a Face‐Centered Cubic Phase

Abstract Real‐time visualization of alloying evolution is crucial yet challenging for designing and synthesizing face‐centered cubic (fcc) nanoalloy catalysts for the alkaline oxygen evolution reaction (OER). Here, a trimetallic fcc NiFeMo nanoalloy is synthesized via topological reduction of NiFeMo layered double hydroxide (LDH) under an Ar/H 2 atmosphere. In situ heating transmission electron microscopy reveals the transforamtion of non‐hexagonal NiFeMo LDH nanosheets into irregular inverse‐spinel Ni 1‐x Mo x Fe 2 O 4 nanoparticles, eventually forming spherical fcc NiFeMo nanoalloy particles. Furthermore, the NiFeMo nanoalloy demonstrates a Faradaic efficiency of 99.3% and superior intrinsic OER activity compared to NiFe and Ni nanoalloys at a consistent mass loading, due to synergistic metal interactions. The NiFeMo nanoalloy‐modified nickel foam electrode exhibits excellent electrochemical stability, with structural and elemental analyses confirming that the original fcc structure remains intact after long‐term OER testing. Operando differential electrochemical mass spectrometry coupled with isotope labelling experiment illustrates that the OER on NiFeMo nanoalloy proceeds via an adsorbate evolution mechanism pathway. Theoretical calculations suggest that Mo species reduce the potential barriers for *OH intermediate formation, thereby enhancing the inherent activity of Ni active sites in the NiFeMo nanoalloy. This work presents a novel approach for developing advanced trimetallic and high‐entropy nanoalloy OER catalysts.