Single atoms/metal nanoparticles-decorated hollow carbon spheres as oxygen reduction catalyst for zinc-air batteries

Non-platinum electrocatalyst development for improving the oxygen reduction reaction (ORR) is critical in electrochemical energy conversion applications such as zinc-air batteries (ZABs). Herein, we developed dual Fe-Co single atoms and Co nanoparticles embedded N-doped hollow carbon (Fe-Co/N-HCS-x) catalysts via the carbonization of polypyrrole (PPy)@zeolitic imidazolate framework (ZIF) core–shell particles, with various PPy concentrations. PPy serves as a secondary nitrogen source that enhances nitrogen amount and promotes metal dispersion to increase active sites density. The Fe-Co/N-HCS-x catalysts with optimized PPy concentration control demonstrated improved ORR performance in alkaline media, with a half-wave potential (E_1/2) of 0.921 V and an onset potential (E_onset) of 1.019 V. Fe-Co/N-HCS-x also exhibited high ZABs performance, with a power density of 244.6 mW cm⁻² and an operating time of more than 100 h. Our research contributes to the development of optimized active sites for ORR, which can be applied to new electrochemical storage and conversion systems.