Revealing Structural Evolution of Nickel Phosphide-Iron Oxide Core–Shell Nanocatalysts in Alkaline Medium for the Oxygen Evolution Reaction

Metal phosphide-containing materials have emerged as a potential candidate of nonprecious metal-based catalysts for alkaline oxygen evolution reaction (OER). While it is known that metal phosphide undergoes structural evolution, considerable debate persists regarding the effects of dynamics on the surface activation and morphological stability of the catalysts. In this study, we synthesize NiP_x-FeO_x core–shell nanocatalysts with an amorphous NiP_x core designed for enhanced OER activity. Using ex situ X-ray absorption spectroscopy, we elucidate the local structural changes as a function of the cyclic voltammetry cycles. Our studies suggest that the presence of corner-sharing octahedra in the FeO_x shell improves structural rigidity through interlayer cross-linking, thereby inhibiting the diffusion of OH^–/H₂O. Thus, the FeO_x shell preserves the amorphous NiP_x core from rapid oxidation to Ni₃(PO₄)₂ and Ni(OH)₂. On the other hand, the incorporation of Ni from the core into the FeO_x shell facilitates absorption of hydroxide ions for OER. As a result, Ni/Fe(OH)_x at the surface oxidizes to the active γ-(oxy)hydroxide phase under the applied potentials, promoting OER. This intriguing synergistic behavior holds significance as such a synthetic route involving the FeO_x shell can be extended to other systems, enabling manipulation of surface adsorption and diffusion of hydroxide ions. These findings also demonstrate that nanomaterials with core–shell morphologies can be tuned to leverage the strength of each metallic component for improved electrochemical activities.