A mechanistic study on the intrinsic Ni-Fe synergy in a geometrical-bias-removed MOF-derived catalytic interface for the OER.

Abstract

The origin of the synergistic enhancement of oxygen evolution reaction (OER) activity in Ni-Fe catalysts under alkaline conditions remains a subject of active debate, particularly regarding the respective roles of proton-coupled surface chemistry and lattice oxygen participation. Here, we investigate this synergy using a structurally well-defined metal-organic framework (MOF) platform, comparing a bimetallic NiFe metal-organic framework (NiFe_MOF) with its monometallic Ni_MOF and Fe_MOF analogues constructed from an identical 1,4-benzene dicarboxylic acid linker. In 1.0 M KOH, the NiFe_MOF exhibits superior OER activity and faster kinetics than either of its monometallic counterparts, despite showing no correlation with the electrochemically accessible surface area, indicating an intrinsic molecular-level origin of the enhancement. Mechanistic interrogation through pH-dependent kinetics and chemical probe experiments reveals distinct reaction regimes across the three catalysts. The Ni-MOF displays signatures of proton-coupled oxygen redox chemistry accompanied by sluggish kinetics, while the Fe_MOF and NiFe MOF follow a proton-decoupled lattice oxygen mechanism. These findings indicate that the synergistic interaction between Fe and Ni plays a crucial role in altering the reaction pathway, independent of their individual concentrations. Nevertheless, the optimized compositional ratio plays a key role in enhancing charge transfer kinetics.

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