Joule‐Heating Synthesis of High‐Entropy Oxide Nanoparticles as Sulfion Oxidation Catalysts for Efficient and Durable Hybrid Seawater Electrolysis

Abstract Direct seawater electrolysis for hydrogen production is hindered by high energy consumption and the competing chlorine evolution reaction (CER), which compromises efficiency and generates corrosive byproducts. Replacing oxygen evolution reaction (OER) with sulfion oxidation reaction (SOR) is promising to circumvent CER, but practical applications demand high‐efficiency powdery, supported SOR catalysts operating at high current densities. Herein, we report the synthesis of FeCoNiMnCuO high‐entropy oxide nanoparticles (HEO NPs) supported on carbon through an ultrafast Joule‐heating method, which show outstanding SOR performance in natural seawater, achieving 500 mA cm⁻ 2 at 0.545 V versus the reversible hydrogen electrode and demonstrating stability over 100 h. Operando X‐ray absorption spectroscopy and in‐situ Raman spectroscopy studies reveal that in‐situ formed metal sulfides, particularly Fe−S and Cu−S species, serve as active sites. Density functional theory calculations confirm the high‐entropy effect lowers the energy cost of the potential‐determining step by increasing intrinsic charge stability. Furthermore, a membrane electrode assembly incorporating the HEO catalysts operates stably at 500 mA cm⁻ 2 over 500 h with low energy consumption and no chlorine evolution, showcasing substantial potential for low‐cost, energy‐saving, and chlorine‐free hydrogen production from seawater.