Fe Clusters Liganded Single-Atom Fe-N-C Hollow Nanosheets as Bifunctional Catalysts for Stable Zn─air/Iodide Hybrid Batteries

Single-atom Fe─N─C (Fe₁-N-C) materials represent advanced oxygen reduction reaction (ORR) catalysts in base, but insufficient oxygen evolution reaction (OER) performance severely limit their applications in rechargeable Zn─air batteries (ZABs). Herein, ultrasmall Fe cluster liganded Fe-N₄ sites (Fe_nc/Fe₁-N-C) are encapsulated within N-doped carbon hollow nanosheets through ZIF phase conversion and subsequent pyrolysis. The synergistic interplay between Fe clusters and closely surrounding Fe-N₄ active sites can collectively modulate the electronic structures and optimize adsorption energetics of reaction intermediates. Such Fe_nc/Fe₁-N-C hybrid catalysts not only exhibit excellent ORR properties but also deliver remarkable activities for low-potential iodide oxidation reaction (IOR), which can replace the high-potential and destructive OER to improve the energy efficiency and cyclability of ZABs. As a result, the Fe_nc/Fe₁-N-C hollow nanosheets achieve remarkable ORR performance with a high half-wave potential of 0.931 V versus reversible hydrogen electrode (RHE). When coupled with the IOR during charging process, the Fe_nc/Fe₁-N-C based hybrid battery exhibits an unprecedented charge/discharge voltage gap of only 0.51 V and sustains ultrastable cycling up to 450 h. Theoretical calculations reveal that the Fe cluster ligands can drive delocalization of the Fe d_z² orbitals of Fe-N₄ active sites to optimize the desorption step of the intermediates, thereby optimizing oxygen intermediate adsorption energetics.
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