Dual-Confinement Strategy Enables Highly Efficient Oxygen Reduction with Fe-N₅ Electrocatalysts

Fe single-atom catalysts with well-defined porous architectures and optimized Fe-N _x microenvironments show great potential for enhancing the oxygen reduction reaction. Herein, we report Fe-N₅ single-atom catalysts, where each Fe atom coordinates with 4 pyridinic N and 1 axial pyrrolic N, for efficient oxygen reduction. A dual-confinement strategy, combining the wood framework with Fe³⁺ coordination, directs self-assembly of cellulose nanocrystals (CNCs) into a porous wood-derived architecture. Subsequent pyrolysis yields Fe-N₅ catalysts anchored on N, S-codoped carbon with hollow, hierarchically interconnected 3-dimensional pores. Notably, coordination between CNCs and Fe³⁺ guides the formation of Fe-N _x moieties within a tailored microenvironment, enabling control over the coordination number, heteroatom doping, and the electronic structure. X-ray absorption spectroscopy and density functional theory calculations reveal that FeN₅ moieties are optimized through 3 synergistic factors: Fe coordination with 4 pyridinic N and 1 axial pyrrolic N, S doping from residual sulfate ester groups in CNCs, and adjacent micropores. Collectively, these effects lower the *OH desorption barrier, accelerating the adsorption/desorption of oxygenated intermediates. Consequently, Fe-N₅ single-atom catalysts exhibit an exceptional oxygen reduction reaction activity with a half-wave potential of 0.964 V. This dual-confinement strategy enables high-performance non-precious-metal catalysts for metal-air batteries, as evidenced by Fe-N₅-based zinc-air batteries outperforming Pt/C.