Single-atom catalysis for oxygen reduction, what's next?

In recent years, with the rapid development of single-atom catalysts (SACs) in the field of oxygen reduction reactions (ORR), a large number of design and improvement strategies have emerged, but a comprehensive review of the components in M-N-C compiled from a unified perspective is clearly lacking. This review mainly focuses on the structural flexibility caused by the arrangement and combination of metal atoms and heteroatoms in SACs, from the perspective of increasing the number of metal atoms and modulating the coordinated microenvironment. As the number of atoms increases, so does the availability of modifiable sites for metal atoms. In a broad sense, as the number of metal atoms increases and coordinated atoms become more abundant, the "tangram" effect can be used to arrange and combine single-atom coordinated structures, allowing for the arbitrary construction of desired atomic structures based on reaction characteristics. This can maximize the utility of metal atoms and coordinated atoms while optimizing the adsorption characteristics of reaction species and the binding free energy of each reaction step. In terms of the number of metal atoms, there are fixed differences in the adsorption strength of oxygen molecules due to the inherent atomic and electronic structure of different metal atoms. Flexibly embedding coordinated atoms enables tailored optimization of the electronic structure of metal atoms, which in turn adjusts their adsorption and desorption behavior toward reaction intermediates with metal atoms, breaking the Sabatier principle to improve ORR activity. This review comprehensively examines recent progress in the atomic configuration of SACs, outlines future avenues for their atomic design, acknowledges development bottlenecks, and highlights the bright prospects for the future.