Mixed-Valence-Controlled Surface Reconstruction Unlocks Heterophase Synergy in NiFe LDH for Efficient Water Oxidation

Understanding and controlling surface reconstruction is critical for advancing oxygen evolution reaction (OER) electrocatalysis, yet precise strategies to steer this dynamic process and generate highly active hydroxyl species remain elusive. Herein, we employ NiFe layered double hydroxide (LDH) as a model system to investigate the influence of iron valence states on electrochemical reconstruction during OER. A series of NiFe LDH catalysts with tunable Fe²⁺/Fe³⁺ ratios were synthesized, revealing that Fe²⁺ and Fe³⁺ undergo distinct transformations upon activation. Notably, the mixed-valence Fe²⁺/Fe³⁺-NiFe LDH reconstructs into a heterophase α/δ-FeOOH/γ-NiOOH homojunction during OER. Density functional theory calculations indicate that this heterostructure facilitates interfacial charge transfer from α-FeOOH to δ-FeOOH, enhancing electrical conductivity and reducing the reaction energy barrier of the rate-determining step. As a result, the optimal Fe²⁺/Fe³⁺-NiFe LDH electrocatalyst exhibits excellent OER performance, achieving a low overpotential of 208 mV at 10 mA cm^–2 and robust stability over 500 h at industrial current densities. When applied as an anode in an anion exchange membrane water electrolyzer, it enables 1000 mA cm^–2 at only 1.86 V. This work highlights the pivotal role of Fe valence states in governing reconstruction pathways and offers a rational strategy for designing high-efficiency OER electrocatalysts through valence-state-induced phase engineering.