Tailoring single-metal atom catalysts: a strategic defect engineering approach for electrochemical reduction reactions

Single metal atom catalysts (SMACs) are gaining attention as promising materials in chemical transformation and energy conversion technologies due to their well-defined geometric structures and efficient atom utilization. The rational design and incorporation of SMACs into diverse support materials, including carbon materials, two-dimensional (2D) materials and metal oxides, is critical to increasing their active-site density. This enhancement is essential for maximizing the effectiveness and selectivity of SMACs. However, there are challenges in the preparation of single-atom metal catalysts because each active metal site exhibits a tremendous amount of free energy, leading to agglomeration. This review paper discusses the importance of defects in introducing a micro-coordination environment in a wide range of materials in order to enhance the stabilization of SMACs. The introduction of defects in the preparation of SMACs has validated its success as a successful scientific approach in the enhancement of the uniform dispersion of SMACs. In this paper, we have investigated the role of several kinds of defects, such as vacancies, heteroatom doping, and intrinsic defects, on porous carbon, graphene, two-dimensional transition metal dichalcogenide (TMD) and metal oxide substrates to accommodate and achieve optimal loading of SMACs. To provide a fresh perspective, we emphasize several non-conventional schemes regarding defect-inducing techniques, such as the electrochemical method, plasma method, ion-radiation method, and etch evaporation process. Furthermore, the mechanism of defect engineering is explored in detail by analyzing the electronic structure as well as the Fermi level of metal atoms by advanced characterization. This review addresses the future challenges in the field of SMACs and summarizes the meaningful findings on the role of defects on supports in stabilizing SMACs. According to our knowledge, a detailed review that thoroughly discusses the relationship between defects in a wide range of substrates and single atom metals (both noble and non-noble), and their collective impact on electrochemical reduction reactions, gas sensing and anti-infection therapy is limited. This study widens the opportunities for the more efficient synthesis of SMACs and maximizes its utilization across a diverse spectrum of applications encompassing catalysis, energy conversion, and environmental monitoring.