High-Entropy Oxide Nanocatalysts for Water Electrolysis: Structure–Activity Relationships and Device Prospects

Green hydrogen is a key energy vector for low-carbon transition, yet water electrolysis remains constrained by sluggish reaction kinetics and the high cost of noble-metal catalysts. High-entropy oxides (HEOs), which contain multiple metal cations, exhibit characteristic high-entropy, lattice-distortion, sluggish-diffusion, and cocktail effects, enabling abundant active sites and tunable electronic structures. This review systematically summarizes recent progress in HEO nanocatalysts for electrolytic hydrogen production. The fundamental characteristics and synthetic approaches of HEOs are first introduced, followed by their electrocatalytic performance toward the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting. Particular emphasis is placed on the mechanisms by which multimetal synergy, defects, interfaces, and electronic-structure regulation enhance catalytic activity, together with mechanistic insights from operando characterization and theoretical calculations. Finally, challenges in controlled synthesis, stability, and scale-up are discussed, and future directions including machine learning, operando characterization, and seawater electrolysis are outlined to guide the design of efficient and durable HEO electrodes.