Operando Characterization of Fe in Doped Ni(Fe_1-)OH Catalysts for Electrochemical Oxygen Evolution

Iron-doped nickel oxyhydroxides, Ni(Fe_1-)OH, are among the most promising oxygen evolution reaction (OER) electrocatalysts in alkaline environments. Although iron (Fe) significantly enhances the catalytic activity, there is still no clear consensus on whether Fe directly participates in the reaction or merely acts as a promoter. To elucidate the Fe's role, we performed operando X-ray spectroscopy studies supported by DFT on Ni(Fe_1-)OH electrocatalysts. We probed the reversible changes in the structure and electronic character of Ni(Fe_1-)OH as the electrode potential is cycled between the resting (here at 1.10 V_RHE) and operational states (1.66 V_RHE). DFT calculations and XAS simulations on a library of Fe structures in various NiOH environments are in favor of a distorted local octahedral Fe(III)O₃(OH)₃ configuration at the resting state with the NiOH scaffold going from α-Ni(OH)₂ to γ-NiOOH as the potential is increased. Under catalytic conditions, EXAFS and HERFD spectra reveal changes in p-d mixing (covalency) relative to the resting state between O/OH ligands and Fe leading to a shift from octahedral to square pyramidal coordination at the Fe site. XES measurements and theoretical simulations further support that the Fe equilibrium structure remains in a formal Fe(III) state under both resting and operational conditions. These spectral changes are attributed to potential dependent structural rearrangements around Fe. The results suggest that ligand dissociation leads to the C_4v symmetry as the most stable intermediate of the Fe during OER. This implies that Fe has a weakly coordinated or easily dissociable ligand that could serve to coordinate the O-O bond formation and, tentatively, play an active role in the Ni(Fe_1-)OH electrocatalyst.