(Cr,Mn,Fe,Ni,Zn) High-Entropy Oxides as Electrocatalysts for Green Hydrogen Production via Anion Exchange Membrane Water Electrolysis.

The development of sustainable and low-cost electrocatalysts based on Earth-abundant elements is essential for the large-scale deployment of green hydrogen production via anion exchange membrane water electrolysis (AEMWE). Herein, we demonstrate the feasibility of cobalt-free high-entropy oxide (HEO) electrodes for AEMWE through a set of spinel oxides based on equimolar Cr, Mn, Fe, Ni, and Zn. The HEOs were synthesized by a scalable sol-gel route followed by calcination at different temperatures (400-800 °C). The pristine oxides were employed as oxygen evolution reaction catalysts, whereas their H2/Ar-reduced counterparts were used as hydrogen evolution reaction catalysts in symmetric membrane electrode assemblies (MEAs). Comprehensive physicochemical characterization combined with electrochemical testing revealed that the phase purity of the anodic catalyst mainly correlates with both the maximum current density and the polarization resistance of the electrolyzer. In contrast, a correlation is observed between the physicochemical features of the reduced cathodic catalyst, the iR-free potential and the polarization resistance after prolonged operation. The best-performing Co-free MEA achieved a current density of 0.51 A cm-2 at 2.2 V and exhibited stable operation for hundreds of hours under alkaline electrolysis conditions. Although the complete replacement of cobalt results in lower activity than previously reported Co-containing HEOs, the present work establishes a viable design strategy for fully Co-free electrocatalysts and highlights the critical balance between catalytic performance, long-term stability, and material sustainability in future AEMWE technologies.