Fluorine Engineering Induces Phase Transformation in NiCo₂O₄ for Enhanced Active Motifs Formation in Oxygen Evolution Reaction.

Abstract

Dynamic reconstruction of catalysts is key to active site formation in alkaline oxygen evolution reaction (OER), but precise control over this process remains challenging. Herein, F-doped NiCo₂O₄ (NiCo₂O₄-F_n), consisting of a NiCo₂O₄ core and a (NH₄)Ni_xCo_1-xF₃ shell is reported, which promotes the formation of a dual-metal NiCoOOH active phase. In situ Raman and X-ray absorption fine structure analyses reveal that the NiCoOOH, rich in oxygen vacancies (O_v), forms at 1.2 V versus the reversible hydrogen electrode (RHE) for NiCo₂O₄-F₁, in contrast to the NiOOH phase formation at 1.4 V versus RHE for undoped NiCo₂O₄. This is facilitated by the transformation of (NH₄)Ni_xCo_1-xF₃ into amorphous Ni_xCo_1-x(OH)₂ in the KOH electrolyte without bias. Electrochemical tests show that NiCo₂O₄-F₁ exhibits a 14-fold increase in intrinsic activity compared to NiCo₂O₄. Theoretical calculations suggest that O_v-induced unsaturated Co and Ni sites enhance electroactivity by promoting ^*OH intermediates adsorption and conversion, lowering the OER energy barrier. The oriented control of NiCoOOH active motifs in NiCo₂O₄ spinel, achieved through fluorine engineering, paves a new avenue for designing efficient OER electrocatalysts.