The d-Orbital High-Spin State of Fe Single Atomic Sites via Asymmetric Axial Ligand Effect of Cu Clusters for an Ultralow Δ E in Rechargeable Zinc–Air Batteries

The development of bifunctional catalysts to diminish the kinetic barriers associated with the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is regarded as a viable approach to augmenting the efficacy of zinc–air batteries (ZABs). This research presents a methodology that integrates metal clusters and single atoms, employing ZIF-7-NH2 as the foundational material to affix metals onto derivatives, resulting in the creation of g-SAFe-Cun. Aberration-corrected transmission electron microscopy (AC-TEM) revealed discrete bright spots corresponding to iron atoms and copper cluster sites, with their bonding interactions substantiated by extended X-ray absorption fine structure (EXAFS) analysis. Electrochemical assessments indicated that g-SAFe-Cun possesses outstanding bifunctional catalytic capabilities, as evidenced by a ΔE value of 0.578 V. When integrated into a liquid-phase zinc–air battery with g-SAFe-Cun serving as the air cathode, the system exhibited remarkable long-term stability, enduring over 2100 cycles at a current density of 10 mA·cm–2, which equates to an operational duration surpassing 350 h. Density functional theory (DFT) computations elucidated that the incorporation of Cu clusters induces substantial spin polarization of the Fe 3d orbitals, thereby increasing the quantity of unpaired electrons and facilitating the transition of Fe to a high-spin state, which aids in the activation of triplet O2 to generate active intermediates. Furthermore, the robust orbital coupling between the Cu clusters and Fe atom leads to a reduction in orbital energy levels, which in turn diminishes the adsorption of *OH and enhances catalytic activity. The economic and accessibility benefits of employing non-noble-metal materials are substantial, markedly reducing costs and offering promising prospects for widespread commercial deployment.