Local Coordination Motif Compatibility Induces the Formation of Shortened Fe3+–O–Ni3+δ Moieties as Active Sites for Highly Efficient Oxygen Evolution Reaction

Abstract

NiFe-based compounds are among the most promising catalysts for the oxygen evolution reaction (OER). However, the structural reconstruction of NiFe catalysts during OER is not fully understood. Most existing studies implicitly assume the formation of a homogeneous NiFe (oxy)hydroxide lattice; however, the actual reconstruction process is more likely to generate structurally heterogeneous (oxy)hydroxide phases with local distortions due to the intrinsic mismatch between Fe3+–O and Ni3+–O bond lengths in the bulk NiFe compounds. By constructing atomically dispersed Ni and Fe active sites as precatalysts and combining them with operando spectroelectrochemical studies, we observed an unusual reconstruction pathway in which isolated Ni2+ and Fe atoms can adaptively evolve into a short-range mixed NiFe (oxy)hydroxide local structure through the formation of interconnected M-O-M′ (M/M′ = Ni3+δ, Fe3+) motifs during the OER. At 1.6 V vs RHE, the reconstructed γ-Fe3+OOH clusters are induced to integrate into the high-valent γ-Ni3+δOOH lattice, resulting in a short-range mixed NixFe1–xOOH structure. This new structure is characterized by an unusually short Fe3+–Ni3+δ distance of ∼2.86 Å, which is significantly shorter than the typical Fe3+-Fe3+ distance in Fe3+ (oxy)hydroxides (2.95–3.25 Å). Interestingly, this “induction effect” is absent in Co3+–Fe3+ catalysts, as atomically dispersed Co3+ sites directly transition into γ-Co3+OOH, which lacks structural compatibility with γ-Fe3+OOH. DFT calculations reveal that the unusually short Fe3+-O bond leads to a moderate *O adsorption strength at the Fe site in the catalyst, thereby creating the most favorable conditions for the oxygen evolution reaction (OER).