Unveiling Dynamic Active-Site Switching of NiFe Dual-Atom Electrocatalysts With Axial Halogen Coordination for Enhanced Oxygen Evolution

NiFe (hydr)oxides are among the most active oxygen evolution reaction (OER) catalysts, yet their local structures, active sites, and synergistic effects remain under debate. Single-atom catalysts (SACs), featuring atomically defined coordination environments, provide a model platform to address these questions, but their OER performance is limited. Here, we reveal for NiFe dual-atom catalysts that dynamic active-site switching from Fe to Fe-Ni dual sites, coupled with axial halogen coordination, synergistically enhances OER. Axial halogen coordination (i.e., F, Cl, and Br), breaking the symmetric electron distribution in the metal-N₄, favors the oxygen intermediate adsorption by stabilizing metal sites in high-spin states and increasing their valence. During OER polarization, at low overpotentials (≤ 340 mV), Fe sites drive OER via concerted proton-electron transfer, whereas at high overpotentials (> 340 mV), a non-concerted mechanism emerges where both Fe and Ni function as a dual-active site, characterized by the migration of oxygen-associated intermediates. As a result, NiFe-N-C(Cl) achieves 10 mA cm^-2 at an overpotential of just 307 mV with a Tafel slope of 57.3 mV dec^-1, outperforming benchmark SACs. This work offers a straightforward strategy for modulating the coordination environment of SACs and provides new insights into the synergistic catalysis of NiFe-based OER catalysts.