Ionic layer epitaxy synthesis of ultrathin two-dimensional high-entropy alloy electrocatalyst for highly efficient and stable oxygen evolution reaction

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 °C) 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.