Abstract: Atomically precise metal nanocluster and ultrasmall nanoparticle catalysts have garnered significant interest in electrocatalysis applications due to their unique geometric and electronic structures. As an intermediate state between single-atom catalysts (SACs) and nanoparticles in size, nanoclusters with specific low nuclearity provide designated metallic states with multiple atoms or surface sites for the adsorption and transformation of reactants/intermediates. The unique catalytic properties of nanoclusters offer a novel platform for designing effective and efficient electrocatalysts, potentially surpassing the SACs in certain catalytic reactions. This review summarizes and discusses the latest progress of nanoclusters and ultrasmall nanoparticles for various electrocatalysis applications, including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), CO₂ reduction reaction (CO₂RR), nitrogen reduction reaction (NRR), hydrogen evolution reaction (HER), various chemicals oxidation reaction (COR), etc. Specifically, this review highlights surface/interface chemical modification strategies and structure-properties relationships, drawing from the atomic-level insights to determine electrocatalytic performance. Lastly, we present the challenges and opportunities associated with nanocluster or ultrasmall nanoparticle electrocatalysts.

Key words: Electrocatalysts, Nanoclusters, Ultrasmall nanoparticles, Surface, Interface engineering, Single-atom catalysts (SACs)