Single atomic Fe-dispersed hollow carbon spheres coated with Co3O4 synergistically catalyze oxygen reduction and oxygen evolution reactions

It is highly expected to construct an efficient electrocatalyst having dual active sites that can simultaneously catalyze both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in Zn-air batteries (ZABs). In this study, a new core-shell nanostructure (Co3O4/Fe-N4/HCS) consisting of single atomic Fe (Fe-N4)-dispersed hollow carbon spheres (Fe-N4/HCS) coated with Co3O4 nanoparticles (NPs) were successfully synthesized with SiO2 templates by a coating-polymerization-etching preparation strategy. Benefit from a synergistic effect of Co3O4 and Fe-N4 dual active species with unique HCS structure, Co3O4/Fe-N4/HCS displayed outstanding activity in both ORR (half-wave potential = 0.88 V) and OER (overpotential at 10 mA cm−2 = 0.33 V), respectively. Meanwhile, compared to Co3O4/Fe-N4/C with Co3O4 NPs on Fe-N4/C, Co3O4/Fe-N4/HCS with Co3O4 NPs on Fe-N4/HCS exhibits a higher stability during electrochemical processes depending on the stronger interaction between the Co3O4 NPs and atomic dispersed Fe-N4 sites embedded in unique HCS structure. Particularly, the ZABs fabricated by Co3O4/Fe-N4/HCS demonstrates a higher specific capacity (688.7 mAh gZn−1) and superior cycling durability over 80 h. The presence of the electron transfer from Co3O4 to Fe-N4 in Co3O4/Fe-N4/HCS, enables Fe-N4 bonded carbon electron deficiency. This promotes the deposition of Co3O4 NPs and enhances metal-HCS support interactions, which combined with a protecting effect of hollow carbon spherical shell on Co3O4 NPs inside the shell, achieving excellent catalytic stability. This research introduces a novel approach to design dual active sites as high-performance bifunctional ORR/OER catalysts.