High-Entropy Spinel Oxide Nanoparticles with Surface Anionic Mo Species for Seawater Oxidation

The advancement of seawater electrolysis requires highly active and durable nanostructured anode catalysts to enhance the oxygen evolution reaction (OER) while suppressing chloride oxidation (ClOR) and corrosion. High-entropy spinel oxide nanoparticles (HESO NPs) have emerged as promising OER catalysts in alkaline media due to their tunable electronic structures and synergistic multimetal effects. In this study, three nanostructured high-entropy spinel oxides─(CrFeMnNiCo) 3 O 4, (CrFeMnNiCoMo) 3 O 4, and (CrFeMnNiCoRu) 3 O 4 ─were synthesized via a scalable hydrothermal–calcination method, featuring well-defined spinel phases, uniform elemental distribution, and nanoscale morphology. Notably, the Mo-doped (CrFeMnNiCoMo) 3 O 4 NPs exhibited superior OER performance, achieving low overpotentials of 184 and 229 mV at 10 mA cm –2 in 1 M KOH and alkaline simulated seawater (1 M KOH + 1 M NaCl), respectively, outperforming even the Ru-containing (CrFeMnNiCoRu) 3 O 4 NPs. Furthermore, the nanostructured (CrFeMnNiCoMo) 3 O 4 demonstrated exceptional durability, maintaining stability for 20 h at 50 mA cm –2 in 1 M KOH and 5 h in simulated seawater, significantly surpassing the (CrFeMnNiCo) 3 O 4 NPs. This enhanced stability is attributed to the protective Mo-rich surface layer, which mitigates alkaline corrosion and chloride adsorption. The nanoscale engineering of high-entropy spinel oxides, combined with Mo doping, provides a synergistic strategy to optimize OER activity and ClOR resistance, making them highly suitable for seawater electrolysis applications. This work highlights the potential of high-entropy oxide nanomaterials as efficient and robust electrocatalysts for sustainable hydrogen production.