Electrodepositing Iron–Cobalt–Nickel-Based (Oxy)Hydroxides to Reveal the Crucial Iron-Species for Efficient Oxygen Evolution

Iron (Fe), cobalt (Co), and nickel (Ni)-based catalysts demonstrate considerable potential for the oxygen evolution reaction (OER). However, their catalytic activities derived from the reconstructed surface (oxy)hydroxides are usually influenced by the original structures through an interfacial effect, which hinders the precise reveal of the relationship between the electronic microenvironment of the active phase and OER activity. In this paper, the bimetallic Fe-Co-Ni-based (oxy)hydroxides (including FeNiOOH, FeCoOOH, and NiCoOOH) were directly constructed by the electrodepositing method. It is found that the introduced Fe atoms in CoOOH or NiOOH induce electron transfer from Co or Ni atoms to Fe atoms in Fe-O-Ni (or -Co) bridging. Both FeNiOOH and FeCoOOH exhibit reduced potential (about 0.12 V at 300 mA cm^-2) compared with NiCoOOH and enhanced catalytic stability. Based on the in situ characterization and theoretical calculations, this electron shuttling that occurred in Fe-O-Ni (or -Co) bridging effectively reduces their charge transfer resistance and conducts the adsorption of *OOH intermediates at low potential. The Fe-species with an electron-rich state in bimetallic Fe-Co-Ni-based (oxy)hydroxides can act as efficient active sites and optimize the electronic structure of the neighboring Ni- or Co-species with the high-valence states to exhibit lower OER energy barriers, thus synergistically enhancing the catalytic activity.