Electrocatalysis of Single-Atom Sites: Impacts of Atomic Coordination

Single metal atoms embedded within select supporting matrices have shown great potential in the development of high-efficiency, low-cost electrocatalysts because of maximal atom utilization and mass activity. As the single metal atoms are stabilized by coordination bonds with the substrate, the strong metal–support interactions can be exploited for ready manipulation of the electrocatalytic activity and selectivity toward target reactions. However, most single-atom catalysts (SACs) are prepared by pyrolysis and contain a wide range of coordination structures. Resolving the atomic configurations of the metal coordination moieties represents a critical first step in the establishment of an unambiguous correlation between the SAC structure and activity. In this Review, we summarize recent progress in the studies of single-atom electrocatalysts, with a focus on the impacts of the coordination structure of the single-atom sites on the electrocatalytic activities toward a series of reactions that are important for various electrochemical energy technologies, such as hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, nitrogen reduction reaction, CO2 reduction reaction, and so on. The survey entails a wide range of SACs, from noble metals (e.g., Pt, Pd, Ru, Ir, Au, etc.) to non-noble metals (e.g., Fe, Co, Ni, Cu, etc.), supported on a variety of substrate materials (e.g., pristine and doped carbon, metal, metal oxide, metal sulfide, etc.). Finally, the Review concludes with a perspective highlighting the promises and challenges in the further development of SACs within the context of coordination chemistry.