Ammonia is considered a promising hydrogen carrier due to its high hydrogen density and liquefaction temperature. Considering that the energy efficiency generally decreases as chemical conversion is repeated, it is more efficient to directly use ammonia as a fuel for fuel cells. However, catalysts in direct ammonia fuel cells have the critical issues of sluggish ammonia oxidation reaction (AOR) rate and poisoning of reaction intermediates. In particular, the use of precious metal as cathodic catalysts has been limited due to ammonia crossover and poisoning. In this study, we introduce Fe-based single-atom catalysts with selective activity for the oxygen reduction reaction (ORR) even in the presence of ammonia. As the Fe content increased, the single-atom structure of the catalysts changed into Fe nanoparticles or carbides. Among our Fe–N–C catalysts, FeNC-50 with a Fe loading amount of 0.34 wt% showed the highest ORR performance regardless of the ammonia concentration. In particular, the difference in activity between the catalysts increased as the concentration increased. The FeNC-50 catalyst showed remarkable stability after 1000 cycles. Therefore, we believe that single-atom dispersion is an important factor in the development of stable non-precious catalysts with high activity and inactivity for the ORR and AOR, respectively.

Metal–organic frameworks (MOFs) are widely used as supports for single-atom catalysts (SACs) owing to their high specific surface area, porosity, and ordered metal–ligand structure. Their activity can be increased by increasing the number of electrochemically accessible active sites via the formation of atomically dispersed metal catalysts (M–Nx) that coordinate with nitrogen atoms on the MOF. Herein, we introduce the relationship between the size of the MOF as a starting material and the catalytic activity for the oxygen reduction reaction in alkaline media. The morphology and features of the MOFs are critically dependent on their size. Remarkably, cage-like MOFs below 33 nm are converted into collapsed structures and are connected between each MOF, even carbon fiber- or tube-like features, after carbonization. SACs derived from medium-sized MOFs exhibit excellent activity and are comparable to commercial Pt/C catalysts owing to their porous structure. Therefore, we believed that controlling the size of MOFs containing active atoms is an effective method of modulating the morphological properties of the support and even the number of active sites that are closely related to the activity.