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 130nm to 60nm and 35nm, 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 0.90V 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 (1.514V), a greater power density (223mWcm-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.