Atomic site design and hierarchical porosity: Fe-N-C catalysts for high-efficiency oxygen depolarized cathodes

Although single-atom catalysts (SACs) deliver the merits of homogeneous and heterogeneous catalysts, challenges like metal leaching and poor mass transport limit their practical performance. Herein, we report an atomically dispersed Fe single-atom decorated on hierarchical porous carbon (hierarchical porous C) support, featuring an ultrahigh active site density (1.21 × 10 20 sites g −1 ) and mixed micro/ meso /macro-pores to overcome mass transfer limitations. Moreover, the abundant carbon defects around Fe sites synergistically enhance oxygen reduction reaction (ORR) process. Density functional theory (DFT) calculations reveal that defect-mediated FeN 4 precisely modulates the Fe d-band center via charge redistribution, effectively weakening OH* adsorption and accelerating the ORR kinetics. Thus, Fe-N-CPs@CB exhibits exceptional alkaline ORR performance than Pt/C. Fe-N-CPs@CB enables aqueous Zn-air battery (A-ZAB) with a large open-circuit voltage (OCV) of 1.51 V and remarkable cycling stability for over 800 h with high round-trip efficiency during the whole cycling process. Quasi-solid-state Zn-air battery (QSS-ZAB) enables a large peak power density of 230.5 mW cm −2 and excellent reversibility for over 106 h at 0.5, 1.0, and 2.0 mA cm −2 , significantly outperforming Pt/C. Additionally, oxygen depolarized cathodes (ODCs)-based two-electrode chlor-alkali electrolyzer using Fe-N-CPs@CB achieves a significantly lower cell voltage of 1.60 V at 300 mA cm −2 than Pt/C counterpart (1.69 V). Notably, it also shows excellent durability for over 100 h. This work provides a rational design principle for high-performance ORR catalysts by synergistically engineering atomic sites and hierarchical porosity for enhanced activity, stability, and mass transport. • Hierarchical porous Fe-N-CPs@CB has ultrahigh single atom active site density (1.21 × 10 20 g −1 ). • Vacancy-modulated Fe N 4 active site enhances ORR kinetics. • Fe-N-CPs@CB-ZAB delivers large open-circuit voltage and peak power density. • Fe-N-CPs@CB-ZAB exhibits exceptional reversibility. • Fe-N-CPs@CB||RuO 2 chlor-alkali electrolyzer shows excellent durability over 100 h at 300 mA cm −2 .