Structure-Engineered Biomass-Derived Carbon Nanosheets Hosting Atomically Dispersed Iron for Efficient Zn-air Batteries

Rechargeable zinc-air batteries (ZABs) are attractive for energy storage, but their efficiency is limited by the slow kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Here, we report a bifunctional single-atom catalyst consisting of atomically dispersed iron anchored on N-doped two-dimensional (2D) carbon nanosheets (Fe-NCS-800) for high-performance ZABs. This material was sustainably synthesized through the graphene-oxide (GO)-induced hydrothermal carbonization of hemicellulose, followed by copyrolysis with NH 4 Cl and FeCl 3 and subsequent acid leaching. The resulting architecture comprises interconnected porous nanosheets with a substantial surface area (1436 m 2 g –1 ), facilitating mass transport and exposing abundant active sites. Advanced characterization techniques, including aberration-corrected scanning transmission electron microscopy (AC-STEM) and X-ray absorption spectroscopy, confirmed the dominant presence of FeN 4 coordination sites. Under alkaline conditions, Fe-NCS-800 outperformed Pt/C in the ORR with a half-wave potential ( E 1/2 ) of 0.87 V vs RHE, while also exhibiting appreciable OER activity, resulting in a low bifunctional index (Δ E ) of 0.83 V. When used as the air cathode in Zn-air batteries, it reached a peak power density of 215 mW cm –2 and a specific capacity of 766 mA h g –1, maintaining stable operation for over 400 h. These results show a sustainable strategy for developing efficient biomass-derived single-atom electrocatalysts for next-generation metal-air batteries.