Spinel-type high-entropy oxides for enhanced oxygen evolution reaction activity in anion exchange membrane water electrolyzers

• High-entropy oxides-derived oxygen evolution reaction (OER) catalysts. • Optimization of the synthesis strategies and stoichiometry of the metals. • Successful obtention of single-phase spinels with homogeneous metals’ distribution. • Enhanced OER activity and durability in an alkaline electrolyte. • High performance as anion exchange membrane water electrolyzer anodes. Developing efficient and cost-effective approaches to synthesize platinum group metal-free (PGM-free) electrocatalysts with high performance toward the sluggish oxygen evolution reaction (OER) is crucial for commercializing anion exchange membrane water electrolyzers (AEMWEs) to produce green hydrogen. Here, we propose a facile method to produce an emergent family of catalysts for the OER at the anode of AEMWEs. Spinel-type high entropy oxides (HEOs) based on Mg, Ni, Co, Mn, and Fe were synthesized by different methods, room-temperature or hydrothermal-assisted coprecipitation, using different coprecipitating agents (NH 3 solution vs . urea) and calcination conditions. Furthermore, HEO composition was tailored by modulating the metal’s stoichiometry. Rietveld refinement and high-resolution transmission electron microscopy, coupled with energy-dispersive X-ray spectroscopy (HRTEM-EDX), indicated that single-phase HEOs with highly crystalline nanoparticles and homogeneous distribution of the metals were obtained by coprecipitation at room temperatures using NH 3 , combined with the rapid quenching of the HEOs after treatment at 750 °C. Notably, the catalyst’s performance was significantly enhanced (E J10 = 1.62 V vs . RHE), modulating the content of Ni, Co, and Mn, promoting their surface reconstruction and activation during OER with the formation of (oxy)hydroxides. AEMWE single-cell tests were carried out by integrating the optimized HEO as an anode catalyst of a catalyst-coated membrane, using the piperION® as a polymeric membrane and ionomer and Pt/C as a cathode catalyst. A remarkable performance was indicated with a high current density ( J = 1.57 Acm −2 ) at 1.8 V, with a maximum value ( J = 4.14 Acm −2 ) being reached at 2.2 V, outperforming highly active PGM-free catalysts reported in the literature.