Tailoring asymmetric atomic strain of FeN4 sites for enhanced acidic oxygen reduction reaction

Single-atom catalysts with FeN_x centers are hitherto recognized as the promising alternative to Pt for oxygen reduction reaction (ORR). Nevertheless, the undesirable bonding strength to intermediates on FeN_x generated by their symmetrical electronic structure still limits the desorption of intermediates, thus resulting in sluggish kinetics. Herein, an asymmetric atomic strain modulation strategy of Fe sites by constructing selective C- N cleavage around Fe- N₄ is proposed. Experimental investigations and theoretical calculations reveal that asymmetric atomic strains of Fe- N₄ induce an asymmetric electronic distribution of Fe-centers. The resultant widened d-band broadening of Fe and the increased Fe-*OH antibonding-orbital occupancy render modest adsorption between Fe sites and intermediates, thus boosting ORR intrinsic activity. The engineered asymmetric atomic strain environment accelerates the *OH desorption as verified by in-situ Raman spectroscopy. Impressively, the constructed strain-modulated FeN₄ catalyst (FeN₄-SM) delivers superior activity and stability in the acidic medium, as evidenced by the half-wave potential (E_1/2) with 0.83 V and a loss of only 16 mV in E_1/2 after 60, 000 potential cycles, as well as an exceptional peak power density of 1.086 W cm⁻² in H₂/O₂ fuel cell. This work provides a foundational understanding of regulating electronic structure in asymmetric strain environment and guides the rational engineering of overadsorbed single-atomic catalysts.