Out-of-plane-coordinated iron single atoms boosting oxygen electroreduction

Out-of-plane-coordinated metal single atoms with defined electronic structures and regulated local microenvironments exhibit higher intrinsic activity than conventional in-plane single-atom sites. Curvature engineering provides one effective way to realize the out-of-plane coordination of metals. However, controlling the nanocurvation of in-plane metal single atoms to fabricate the out-of-plane-coordinated counterpart is technically challenging, and its effect on catalytic properties remains almost completely unexplored. Herein, we report a facile metal-organic framework (MOF)-mediated synthesis of onion-like carbons decorated with high-density out-of-plane-coordinated Fe–N₄ sites for oxygen reduction reaction (ORR) catalysis. Density functional theory calculations and finite element method simulations, together with in situ spectroelectrochemical experiments, corroborate that the out-of-plane-coordinated iron single atoms can not only modulate the adsorbate binding energies but also effectively induce strong local electrostatic fields, thereby enhancing the kinetics of the proton-coupled electron transfer process. This study clarifies the advantageous role of curved configurations on catalysis.