Emerging Metal Single-Atom Materials: From Fundamentals to Energy Applications

With the development of nanotechnology and characterization techniques, it has been realized that the reactivity of metal nanoparticles mainly depends on some unsaturated coordination atoms on the surface. However, only a small fraction of the surface exposed atoms can access the reactants and act as reactive sites, resulting in low utilization of metal atoms. Moreover, due to the complex structure of metal nanoparticles, the metal atoms exposed on the surface are likely to be in different chemical environments and may act as multiple active centers to catalyze the reactants, which brings great difficulties in the establishment of the structure- activity relationship of metal nanoparticles. Reducing the size of metal nanoparticles to increase the fraction of atoms on the surface is usually regarded as a straightforward and effective approach to enhancing their activity. When the metal nanoparticles are transformed into metal subnanoclusters or even metal single atoms (MSAs), discrete energy-level distribution and distinctive lowest unoccupied molecular orbital-highest occupied molecular orbital (LUMO- HOMO) gap are generated due to the unique quantum size effect. Because of the uniform active sites, maximum metal utilization, unsaturated coordination environment, and strong metal-support interaction, MSA materials often show distinct reactivity, selectivity, and stability from traditional metal nanomaterials. In the past decade, MSA materials have rapidly become the research frontier in catalysis, battery, sensing, and other fields. MSA materials not only provide a new solution for understanding the mechanism of reaction from atomic and molecular scales and studying the structure-activity relationship but also are expected to become a new family of functional materials with large-scale application potential. This Account highlights the recent advancements in the preparation and applications of MSAs in our group. The Account begins with a brief introduction to the structural properties of MSAs. After understanding the basic structural characteristics of MSAs, we summarize three common stabilization strategies for MSAs, including spatial confinement, coordination, and defect/vacancy design strategies. The controllable fabrication of stable and efficient MSAs is important for the investigation of the structure-performance relationship and a prerequisite for various applications. Some typical bottom-up and top-down strategies for the synthesis of MSAs are also briefly introduced. Furthermore, we review our recent advancements in the design and synthesis of MSAs and their applications, including the oxygen reduction reaction (ORR), the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), photocatalytic H2 production, lithium-sulfur (Li-S) batteries, and the electrocatalytic carbon dioxide reduction reaction (CO2RR). Finally, the current challenges and our perspectives on the preparation and applications of advanced MSAs have been provided.