Synergistic Coupling of Anionic Ligands to Optimize the Electronic and Catalytic Properties of Metal-Organic Framework-Converted Oxygen-Evolving Catalysts

Multimetallic oxygen evolution reaction (OER) electrocatalysts have recently gained significant attention due to their excellent intrinsic activity resulting from the synergistic interplay between multiple metal sites. However, in these multimetallic catalyst systems, the function of their bridging anionic ligands (e.g., O^2-, S^2-, and P^3-/PO₄^3-) is rarely investigated, partially due to the lack of an ideal material model system. Herein, by combining a careful electrochemical conversion of metal-organic framework (MOF) precursors with low-temperature phosphorization processes, we designed a series of NiFe-based model catalysts as a proof-of-concept platform to identify the roles of different anionic ligands in tuning the redox and electronic properties of metal sites. Our experimental and theoretical results reveal that ligands having varying electron-withdrawing/-donating ability can modulate not only the electron density of Ni²⁺/Fe³⁺ centers but also the electron transfer efficiency from Ni²⁺ to neighboring Fe³⁺ sites. Importantly, synergistically coupled ligands (e.g., S^2- and PO₄^3-) with complementary electronic properties help to optimize the chemical environments of the Ni²⁺/Fe³⁺ centers (even upon partial catalyst surface reconstruction to NiFe oxyhydroxide), thus giving rise to a remarkable OER activity. These insights open new avenues for developing highly active multimetallic OER electrocatalysts.