Sub-5 nm Ultrasmall High-entropy Alloy Nanomaterials for Electrocatalysis

High-entropy alloys (HEAs) have emerged as promising candidates for next-generation electrocatalysts due to their unique structural features and superior physicochemical properties. A key breakthrough in this field is the development of ultrasmall HEA nanomaterials (<5 nm), which establish HEAs as sufficiently advanced catalysts with extremely superior performance. Despite rapid progress, a comprehensive review of the synthesis-structure-activity relationships of sub-5 nm HEA nanomaterials is still absent. This review provides the first systematic overview specifically focusing on sub-5 nm ultrasmall HEA nanomaterials for electrocatalytic applications. In particular, this review comprehensively summarizes recent research on ultrasmall HEA nanomaterials, focusing on their synthesis strategies, characterization methods, unique properties, and electrocatalytic applications, including the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), alcohol oxidation reaction (AOR), and formic acid oxidation reaction (FAOR). Furthermore, this work originally proposes some unique advantages of ultrasmall HEA nanomaterials, including the enhanced utilization of metal atoms, optimal binding energy, and remarkable nano-size effects. Meanwhile, it discusses the challenges and future directions for ultrasmall HEA nanomaterials. This work aims to offer valuable insights for advancing the field and the development of efficient electrocatalysts for practical applications.