Fe Single Atoms Encased in Nanoscale Zif-8 Dodecahedra for Oxygen Reduction and Flexible Zinc–Air Batteries

The development of efficient and facile nonprecious metal catalysts for the oxygen reduction reaction (ORR) holds great significance. Among these, the single-atomic metal–nitrogen–carbon (M–N–C) catalysts are emerging candidates due to their high activity and prolonged lifespan. However, traditional preparation of M–N–C catalysts suffers from thermal aggregation and low space–time yield, limiting their application in industry. Inspired by the "chainmail protection" in carbonaceous materials, here we propose an "armor protection" strategy for the large-scale synthesis of a highly active Fe single-atom ORR catalyst (Fe-8/8-CN) by pyrolyzing a well-designed precursor Fe-ZIF-8/ZIF-8. In this precursor, the 2-methylimidazole zinc MOF (ZIF-8) loaded with iron (Fe-ZIF-8) is encased in an additional ZIF-8 armor layer (Fe-ZIF-8/ZIF-8) that effectively shields the aggregation of Fe species within the inner ZIF-8 during the pyrolysis process, ultimately yielding the Fe single-atom ORR catalyst (Fe-8/8-CN). Furthermore, the facile approaches for the construction of Fe-ZIF-8/ZIF-8 make it efficiently scale up with a high space–time yield of 18.7 kg m–3 day–1. In particular, the armor protection strategy for preparing the Fe single-atom catalyst is universally applicable to leaf-shaped ZIF-L and other ZIFs with diverse morphologies. Thanks to the presence of the outer ZIF-8 layer and the inner ZIF-8 core to dispersing Fe single-atom catalytic sites with the porous and defect-rich characteristics from the volatilization of Zn in ZIF-8, the resulting Fe-8/8-CN demonstrates superior ORR performance in alkaline electrolyte (E1/2 = 0.93 V, JL = −5.89 mA cm–2). Finally, Fe-8/8-CN, as the cathode material for flexible zinc–air batteries, exhibits a high peak power density (97 mW cm–2) and excellent durability superior to Pt/C.