Potential Pulsing-Induced Fe Undercoordination in NiFe (Oxy)Hydroxide Enhances Electrochemical Oxygen Evolution

Undercoordinated metal centers serve as key catalytic sites for the oxygen evolution reaction (OER) in water electrolysis. However, their inherent instability under oxidative conditions leads to rapid deactivation, limiting practical applications. Here, we demonstrate that reductive potential pulsing (RPP) is an effective strategy for generating and sustaining highly active undercoordinated Fe species in NiFe (oxy)hydroxide (NiFeOOH) during prolonged OER operation. Implementing this approach in an anion exchange membrane water electrolyzer enables a remarkable 40% enhancement in current density, achieving stable operation at 0.7 A/cm 2 for 180 h. A key insight from operando X-ray absorption characterization reveals that the OER activity of NiFeOOH is directly correlated with dynamically modulated Fe coordination, while the chemical state of Ni remains largely unchanged during OER and RPP processes. Beyond providing new insights into the long-standing debate over NiFeOOH active sites, this study introduces a practical approach to enhancing both activity and stability, with potential applicability to other electrochemical systems.