Identification of the Evolving Dynamics of Coordination-Unsaturated Iron Atomic Active Sites under Reaction Conditions

Understanding the nature of the catalytic active center and its evolving dynamics under operating conditions is critical for the development of efficient and highly selective catalysts. By combining synchrotron-based operando X-ray absorption and infrared spectroscopies, here we uncover at an atomic level a hydroxyl was coupled on the dynamically released coordination-unsaturated Fe-N₂ moieties to form a highly active OH-Fe-N₂ structure and then promotes the adsorption of O₂ during the catalytic oxygen reduction reaction (ORR), which greatly facilitates the fabrication of the key *OOH intermediate and simplifies the fracture of the O–O bond to accelerate the multielectron reaction kinetics. The resulting covalent organic framework-derived Fe single-site catalyst could efficiently deliver an excellent ORR catalytic activity with an extremely large kinetic current density (J_k) of 81.3 mA cm^–2 and an extra high turnover frequency of 5804 h^–1, 20 times that of the Pt-C catalyst (288 h^–1, 7.7 mA cm^–2).