Asymmetrical FeN₄-O-FeO₄ Dual-Atom Sites in Fe─N─C for Robust pH-Universal Oxygen Reduction Reaction Catalysis

M-N-C single-atom catalysts have been demonstrated to be a promising class of non-noble-metal based catalysts, but their activity is largely constrained by the symmetrical electronic density distribution of the M-N₄ moieties. Herein, a ZIF-8 derived Fe/Fe dual-center catalyst (FeN₄/FeO₅@NC) is constructed by introducing a O-FeO₄ configuration adjacent to the Fe-N₄ sites to form asymmetric dumbbell-shaped FeN₄-O-FeO₄ active sites, resulting in improved intrinsic activity of Fe─N─C catalyst. The dumbbell FeN₄-O-FeO₄ dual-center catalyst features an electron transfer pathway of "FeN₄ → bridged O → FeO₄", which breaks the symmetry of the electronic density distribution of the Fe moiety and regulates the position of the d-band center, increasing the electron transfer efficiency, reducing the desorption energy barrier of the rate-determining step, and enhancing the activation ability of O₂ molecules. The obtained FeN₄-O-FeO₄ dual-center catalyst exhibits excellent ORR performance in a pH-universal range. The liquid-state Zn-air battery (ZAB) with this FeN₄-O-FeO₄ dual-center catalyst delivers an outstanding open circuit voltage of 1.502 V and a peak power density of 246 mW cm^-2, superior to the Pt/C-cathode ZAB. This work represents an exploration of ways to improve the ORR performance of Fe─N─C catalysts and provides a paradigm for rationally designing low-cost, highly catalytic, and stable non-noble metal catalysts.