Recent progress on the synthesis and oxygen reduction applications of Fe-based single-atom and double-atom catalysts

Single-atom and double-atom catalysts have emerged as a new Frontier in many fields due to their high atom-utilization efficiency, excellent catalytic properties and good durability. In this decade, Fe-based atomic catalysts have exhibited great advantages toward the oxygen reduction reaction (ORR) owing to the high utilization efficiency of their single/dual atomic sites, well-defined active sites and high selectivity toward the four-electron (4e−) pathway. To explore their catalytic behavior, many efforts have been made to synthesize Fe-based atomic catalysts. In addition, the high performance of Fe-based single/dual atoms toward ORR is mainly attributed to the critical coordination condition and the corresponding electronic structures. Currently, it is a great challenge to achieve high activity and good stability with a high content of metal atoms, while maintaining their atomic dispersion. In this review, we begin with a systematic review about the influence of the coordination environment of Fe-based single/dual-atom catalysts on their electrocatalytic properties, followed by an overview of some new strategies to improve their coordination environment. Then, we present a comprehensive summary on the advantages of their unique structures, catalytic activity for the ORR and further applications. Finally, the remaining key challenges and future opportunities in this emerging field are concluded to provide more useful clues for the development of advanced catalysts.