Single atomic Fe-based honeycomb catalysts with hierarchically ordered meso- and macropore for the oxygen reduction reaction

Abstract

Metal-organic frameworks (MOFs) have attracted attention as promising supports for the realization of single-atomic catalysts (SACs). However, MOFs have significant limitations in terms of mass transfer and the formation of tri-phase interfaces owing to their large number of micropores. In this study, we introduce a one-pot synthesis of a Fe-N-C catalyst with hierarchically ordered mesoporous and macroporous structures via the in-verse self-templating of a zeolitic imidazolate framework-8 (ZIF-8). A mixture solution of pyrrolic nitrogen-rich carbon quantum dots and Fe precursor is coated onto the surface of the ZIF-8 particles. During carbonization, the hard shells of ZIF-8 with a coating layer were maintained and adhered to each other, whereas the soft core of the particles decomposed. Significantly, the SACs exhibited a honeycomb-like morphology, and the pore size of the SACs is proportional to the size of the ZIF-8 particles. Our SAC with a pore size of 206 nm showed a half-wave potential of 0.889 V for the oxygen reduction reaction (ORR) in 0.1 M KOH solution, which is 21 mV higher than that of the 20 wt% Pt/C catalyst. Remarkably, membrane electrode assemblies prepared using the SAC achieve a high current density of 1.31 A cm-2 at 0.4 V, primarily due to the advances in mass transport. In addition, the catalyst showed remarkable activity in an acidic medium, after accelerated durability tests, and even under methanol crossover conditions.