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.

Medium-entropy oxides (MEOs) with broad compositional tunability and entropy-driven structural stability, are receiving booming attention as a promising candidate for oxygen evolution reaction (OER) electrocatalysts. Meanwhile, ultrathin two-dimensional (2D) nanostructure offers extremely large specific surface area and is therefore considered to be an ideal catalyst structure. However, it remains a grant challenge to synthesize ultrathin 2D MEOs due to distinct nucleation and growth kinetics of constituent multimetallic elements in 2D anisotropic systems. In this work, an ultrathin 2D MEO (MnFeCoNi)O was successfully synthesized by a facile and low-temperature ionic layer epitaxy method. Benefiting from multi-metal synergistic effects within ultrathin 2D nanostructure, this 2D MEO (MnFeCoNi)O revealed excellent OER electrocatalytic performance with a quite low overpotential of 117 mV at 10 mA·cm⁻² and an impressive stability for 120 h continuous operation with only 6.9% decay. Especially, the extremely high mass activity (5584.3 A·g⁻¹) was three orders of magnitude higher than benchmark RuO₂ (3.4 A·g⁻¹) at the same overpotential of 117 mV. This work opens up a new avenue for developing highly efficient and stable electrocatalysts by creating 2D nanostructured MEOs.