Confined to ultrathin two-dimensional (2D) architectures, high-entropy alloys (HEAs) could leverage synergistic cocktail effects and entropy-stabilized phases to enable exceptional oxygen evolution reaction (OER) kinetics and stability via maximized specific surface areas and quantum confinement. However, synthesizing atomically precise 2D HEAs remains fundamentally challenging due to disparate metal nucleation kinetics. Herein, we demonstrate a facile low-temperature (80 ℃) ionic layer epitaxy synthesis of ultrathin 2D HEA FeCoNiMo. The OER catalytic activity exhibited a strong inverse correlation with the 2D HEA thickness, where atomic-scale dimensional reduction significantly enhanced electrocatalytic performance. Atomic-scale confinement to 1.1 nm thickness unlocks exceptional OER performance. This 1.1 nm 2D HEA FeCoNiMo revealed a record-low overpotential of 79 mV at 10 mA·cm^–2 and unprecedented stability (<10% activity decay after 1532 h operation). Especially, its benchmark mass activity (9725.2 A·g^–1) represented three orders of magnitude enhancement over IrO₂ (5.7 A·g^–1) at the same overpotential of 79 mV. Density functional theory (DFT) reveals that thickness reduction facilitates charge redistribution toward Fe active sites, optimizes the rate-determining step energy barrier, and strengthens adsorbate-catalyst interactions. This work provides fundamental insights into 2D confinement effects in HEAs and establishes a general strategy for designing highly efficient electrocatalysts for sustainable energy applications.

High-entropy oxides (HEOs) have emerged as a groundbreaking class of materials in electrocatalysis, offering unparalleled compositional flexibility, synergistic multi-element effects, and exceptional stability. This review comprehensively explores the recent advances in HEOs, focusing on their unique properties, synthesis strategies, and electrocatalytic applications. We delve into the fundamental principles of HEOs, including high-entropy effects, lattice distortion, and cocktail effects, which underpin enhanced catalytic performance. Advanced synthetic methods, such as solid-state, liquid-phase, and gas-phase techniques, were systematically analyzed to customize the morphology, crystallinity, and active sites of hydroxide ions. Furthermore, we highlight the applications of HEOs in critical electrochemical reactions, including the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO₂RR), and other emerging catalytic reactions, emphasizing their superior activity and durability over conventional catalysts. Density functional theory (DFT) insights into active site modulation and reaction mechanisms are integrated to bridge experimental observations with theoretical understanding. Finally, we address current challenges and propose future directions for optimizing HEOs. This review aims to inspire innovative strategies for developing next-generation HEO-based electrocatalysts to meet global energy and sustainability demands.