Phase Engineering Facilitates O-O Coupling via Lattice Oxygen Mechanism for Enhanced Oxygen Evolution on Nickel-Iron Phosphide.

Nickel-iron-based catalysts are recognized for their high efficiency in the oxygen evolution reaction (OER) under alkaline conditions, yet the underlying mechanisms that drive their superior performance remain unclear. Herein, we revealed the molecular OER mechanism and the structure-intermediate-performance relationship of OER on a phosphorus-doped nickel-iron nanocatalyst (NiFeP). NiFeP exhibited exceptional activity and stability with an overpotential of only 210 mV at 10 mA cm in 1 M KOH and a cell voltage of 1.68 V at 1 A cm in anion exchange membrane water electrolyzers. The evolution of active sites and intermediates during OER on NiFeP was in situ probed and correlated using shell-isolated nanoparticle-enhanced Raman spectroscopy, complemented by differential electrochemical mass spectrometry and density functional theory. These results provide direct evidence that OER proceeds via the lattice oxygen-mediated mechanism. Remarkably, phosphorus doping plays a critical role in stabilizing the active β-Ni(Fe)OOH phase, which facilitates the *OH deprotonation and the subsequent O-O coupling to form *OO intermediates. Our findings offer a deeper understanding of the OER mechanism, providing a clear pathway for designing next-generation OER catalysts with improved efficiency and durability.