Breaking Atomic Fe-N₄ Symmetry in Aerogel Catalysts by Nitrogen and Chlorine Doping for Enhancing Oxygen Reduction

The growing demand for metal-air batteries and fuel cells has spurred extensive research into low-cost, highly efficient, noble-metal-free electrocatalysts to overcome the sluggish oxygen reduction reaction (ORR) at the cathode. Herein, we propose a chemical assembly strategy to engineer an asymmetrically structured Fe-N₄ single-atom active site densely embedded within a hierarchical micro-nanoporous aerogel. The asymmetric Fe-N₄ single-atom moiety, modulated by N and Cl codopants, enhances intrinsic ORR activity, while the porous aerogel geometry facilitates rapid electron and mass transport. As a result, the resulting catalyst demonstrates high ORR performance, achieving half-wave potentials of 0.92 V in alkaline media and 0.82 V in acidic media, in stark contrast to conventional Fe catalysts with planar coordination symmetries. When used in the H₂-O₂ fuel cell, a peak power density of 755 mW cm^-2 is achieved. Furthermore, Zn-air batteries utilizing this catalyst deliver high peak power densities of 395 mW cm^-2 and 161 mW cm^-2 for liquid- and solid-state batteries, respectively, while maintaining excellent stability under repeated cycles and various mechanical deformations. Complementing these experimental results, we introduced an explainable XGBoost machine-learning model to accurately predict battery power density, uncovering critical performance trends driven by voltage, catalyst atomistic architecture, and device configurations. This work not only presents a method for fabricating high-performance single-atom aerogel catalysts but also offers valuable design principles for advancing the commercial viability of electrocatalysis-based energy systems.