The understanding, rational design, and application of high-entropy alloys as excellent electrocatalysts: A review

High-entropy alloys (HEAs) are attracting considerable attention in the field of electrocatalysis. In many cases, HEAs exhibit excellent activity and selectivity toward several catalytic reactions, which is often attributed to their four "core effects": the high entropy effect, the lattice distortion effect, the sluggish diffusion effect and the cocktail effect. However, the understanding and rational design of HEA electrocatalysts lack a systematic summarization. In this review, a systematic summary of HEA electrocatalysts' characteristics and applications, as well as a clarification of their design principles, is provided, which has guiding importance for HEA development. First, the reason why HEAs could be excellent electrocatalysts is illustrated from several aspects, including their outstanding mechanical properties, optimized structure and composition, abundant active sites with high intrinsic activity, and ultrahigh stability. To deepen the understanding of HEA electrocatalysts, the rational design of HEAs is carefully demonstrated in terms of design principles, element selection, and the use of computation methods for property prediction. Second, the latest advances in HEA electrocatalysts in the fields of water electrolysis, fuel cells, small organic molecule electrochemical oxidation, and carbon- and nitrogen-based conversion are discussed in detail. Importantly, theoretical calculations and in situ characterizations for an understanding of HEAs' mechanism are carefully illustrated. Finally, we propose the challenges and perspectives in the future design and application of HEA electrocatalysts.