Formulating N-Doped Carbon Hollow Nanospheres with Highly Accessible Through-Pores to Isolate Fe Single-Atoms for Efficient Oxygen Reduction

It is challenging yet promising to design highly accessible N-doped carbon skeletons to fully expose the active sites inside single-atom catalysts. Herein, mesoporous N-doped carbon hollow spheres with regulatable through-pore size can be formulated by a simple sequential synthesis procedure, in which the condensed SiO2 is acted as removable dual-templates to produce both hollow interiors and through-pores, meanwhile, the co-condensed polydopamine shell is served as N-doped carbon precursor. After that, FeNC hollow spheres (HSs) with highly accessible active sites can be obtained after rationally implanting Fe single-atoms. Microstructural analysis and X-ray absorption fine structure analysis reveal that high-density FeN4 active sites together with tiny Fe clusters are uniformly distributed on the mesoporous carbon skeleton with abundant through-pores. Benefitted from the highly accessible FeN4 active sites arising from the unique through-pore architecture, the FeNC HSs demonstrate excellent oxygen reduction reaction (ORR) performance in alkaline media with a half-wave potential up to 0.90 V versus RHE and remarkable stability, both exceeding the commercial Pt/C. When employing FeNC HSs as the air-cathode catalysts, the assembled Zn-air batteries deliver a high peak power density of 204 mW cm-2 and stable discharging voltage plateau over 140 h. Using two types of orthosilicates with different hydrolysis rates as dual-silica sources and dopamine as the N-doped carbon sources, a sequential synthesis procedure referred to the classic Stober method is developed to fabricate N-doped carbon hollow spheres with abundant unusual through-pores. The formed N-doped carbon hollow spheres can act as a favorable host to isolate Fe single-atoms.image