Size-dependent activity of Fe-N-doped mesoporous carbon nanoparticles towards oxygen reduction reaction

The rational design of Fe-N-C catalysts that possess easily accessible active sites and favorable mass transfer, which are usually determined by the structure of catalyst supports, is crucial for the oxygen reduction reaction (ORR). In this study, an oleic acid-assisted soft-templating approach is developed to synthesize size-controlled nitrogen-doped carbon nanoparticles (ranging from 130 nm to 60 nm and 35 nm, respectively) that feature spiral mesopores on their surface (SMCs). Next, atomically dispersed Fe-Nx sites are fabricated on the size-tunable SMCs (Fe1/SMC-x, where x represents the SMC size) and the size-dependent activity toward ORR is investigated. It is found that the catalytic performance of Fe1/SMCs is significantly influenced by the size of SMCs, where the Fe1/SMC-60 catalyst shows the highest ORR activity with a half-wave potential of 0.90 V vs. RHE in KOH electrolyte, indicating that the gas-liquid-solid three-phase interface on the Fe1/SMC-60 enhances the accessibility of Fe-Nx sites. In addition, when using Fe1/SMC-60 as the cathode catalyst in aqueous zinc-air batteries (ZABs), it delivers a higher open-circuit voltage (1.514 V), a greater power density (223 mW cm-2), and a larger specific capacity/energy than Pt/C-based counterparts. These results further highlight the potential of Fe1/SMC-60 for practical energy devices associated with ORR and the importance of size-controlled synthesis of SMCs.