High-Entropy Alloy Nanocrystals: From Synthesis to Characterization and Catalytic Application.

High-entropy alloy (HEA) nanocrystals, characterized by four core effects, including high entropy, lattice distortion, sluggish diffusion, and cocktail mixing, represent a transformative class of catalytic materials that combine compositional diversity with nanoscale complexity. By combining five or more elements into single-phase solid solutions, HEA nanocrystals offer unique opportunities to tailor local atomic environments, electronic structures, and catalytic properties. This Review offers a comprehensive overview of recent advances in the syntheses, compositional and structural controls, and catalytic applications of these materials, with a focus on achieving uniform atomic mixing and well-defined surface structures. We start with key synthetic methodologies, including both top-down and bottom-up approaches, and highlight wet-chemical synthesis that allows for precise regulation of reduction, nucleation, growth, and alloying dynamics to overcome challenges such as reduction kinetics, elemental immiscibility, and crystallographic incompatibility. We then discuss advanced characterization techniques and theoretical modeling approaches used to probe the multicomponent solid-solution phases, local coordination environments, interelement interactions, and synergistic effects that collectively govern the catalytic behaviors of HEA nanocrystals. We further highlight how these fundamental features manifest at the nanoscale to enhance performance across diverse catalytic reactions, including electro-, thermo-, and photocatalysis. Finally, we identify 15 critical aspects that provide a roadmap for the rational design of next-generation HEA catalysts for sustainable energy conversions and chemical transformations.