Supercritical CO2 assisted synthesis of highly accessible iron single atoms and clusters on nitrogen-doped carbon as efficient oxygen reduction electrocatalysts

Replacing Pt-based catalysts with metal–organic frameworks (MOFs) derived single-atom catalysts (SACs) for oxygen reduction reaction (ORR) has largely been impeded by the hidden activity of SACs due to the challenges in exposing unactivated sites and regulating their electronic states. Here we demonstrate the synthesis of highly exposed iron single atoms and clusters on imidazole frameworks derived carbon (scCO2-FeC@FeNC) by supercritical CO2 (scCO2) assisted strategy. The scCO2 fluid not only incises dodecahedron into uniform cubes with tenth of thickness but also constructs hierarchical pores in frameworks without utilization of templates, and thus in favor to exposure of inactive sites. Moreover, theoretical simulations disclose that the adjacent cluster weakens hybridization between occupied d-orbitals of iron for Fe-N moiety and p-orbitals of adsorbed oxygen, thereby optimizing adsorption–desorption process of oxygenated intermediates and accelerating ORR kinetics. With these merits, scCO2-FeC@FeNC delivers an ORR activity with half-wave potential of 0.91 V in alkaline solution. The Zn-air battery using scCO2-FeC@FeNC as air cathode has a specific capacity of 784.1 mAh gZn−1 and long-time durability of 200 h, surpassing commercial Pt/C-based Zn-air battery. This work provides a promising approach to fabricate MOF-derived metal-N-C ORR electrocatalysts with high performance.