Bridging the Gap Between Single Atoms, Atomic Clusters and Nanoparticles in Electrocatalysis: Hierarchical Structured Heterogeneous Catalysts

Abstract In the past decade, remarkable progress has been made in advancing heterogeneous single‐atom catalysts (SACs), propelled by their extraordinary properties such as unique activities, high selectivity, and efficient metal utilization. In contrast to their nanoparticle counterparts, SACs showcase distinct advantages, yet they are not exempt from limitations. A primary constraint lies in their relatively low metal content and lack of ensemble sites, potentially impacting overall catalytic activity. Stability concerns also arise, as isolated metal atoms may be prone to aggregation, oxidation, and other structural changes. To surmount these challenges, there is a burgeoning exploration of synergies between SACs and other active species, such as single atoms, atomic clusters, or nanoparticles. This approach holds promise for augmenting the efficiency of complex catalytic reactions. However, a systematic examination of their integration and potential synergy has been notably absent. This review strives to fill that gap by comprehensively exploring key interactions, complementary effects, and bifunctional roles of single atoms, atomic clusters and nanoparticles when combined in different compositions and configurations, especially for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) applications. Additionally, the review sheds new light on the underlying catalytic mechanisms by offering insights through specific case studies. In particular, it emphasizes the dynamic nature of each active species and their interactions within these innovative catalytic entities, specifically in the context of heterogeneous electrocatalytic processes, backed by compelling in situ and operando evidence. Concluding the discussion, the review addresses the current challenges and provides a glimpse into the prospects of these integrated catalytic systems in the future. The contents can serve as a valuable guide for the design of advanced catalysts, promoting the deliberate combination and synergy of binary or multiple active species to further advance the field of catalysis with a clear roadmap and easy assessments.