Size-Controlled Hydrothermal Synthesis of Crystalline High-entropy Spinel Oxide Nanoparticles for Oxygen Evolution Electrocatalysts

High-entropy oxides offer a promising platform for oxygen evolution reaction catalysis owing to their entropy-stabilized structures and flexible design of cation metal elements. However, conventional high-temperature synthesis leads to extensive particle growth and limits access to nanoscale particles with higher density of catalytic sites. Here, a subcritical hydrothermal synthesis combined with catechol-assisted growth regulation is introduced to produce high-entropy single-phase spinel oxide nanoparticles with controlled crystallite sizes down to the single-nanometer scale. The resulting reduction in particle size markedly enhances the oxygen evolution activity. A linear dependence of the oxygen evolution current density on the inverse crystallite size highlights the critical role of nanoscale structures in accelerating catalytic reactions. In-situ X-ray absorption spectroscopy reveals electronic and structural robustness of all constituent metal elements under OER conditions, with negligible shifts in oxidation state or changes in local coordination structure, which is distinct from the pronounced reconstruction typically observed in conventional oxide catalysts. When deployed in a membrane electrode assembly, our synthesized high-entropy oxide catalysts with the smallest particle size deliver 2.0 A cm-2 at 1.96 V, demonstrating practical relevance for high-current-density electrolysis. This work establishes low-temperature hydrothermal synthesis with organic molecular regulation as a powerful and scalable route to engineer highly crystalline, compositionally homogeneous high-entropy oxide nanoparticles, offering design guidelines for next-generation oxygen evolution reaction catalysts.