Atomically Dispersed Iron with Densely Exposed Active Sites as Bifunctional Oxygen Catalysts for Zinc-Air Flow Batteries

Atomically dispersed iron embedded carbon is a promising bifunctional catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), but its exposed iron sites must be increased. Herein, the authors propose a double steric hindrance strategy by using zeolitic imidazolate frameworks-8 as the first barrier skeleton and encapsulated phenylboronic acid as the second space obstruction to realize densely exposed atomic iron sites. Prepared PA@Z8-Fe-N-C has the highest iron content (5.49 wt%) among reported transition-metal-based single-atom oxygen catalysts. Meanwhile, its concave surfaces, hollow structures, and hierarchical pores enable the high utilization rate of iron sites to 88.5 +/- 4.5% and exposed active site density to 5.2 +/- 0.3 x 10(20) sites g(-1). Resultantly, PA@Z8-Fe-N-C exhibits superior activity and stability to commercial Pt/C and IrO2 for the ORR and OER in half-cells and zinc-air flow batteries. This provides insight for developing densely and accessibly active sites in single-atom catalysts.