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

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 (NH3 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 NH3, combined with the rapid quenching of the HEOs after treatment at 750 °C. Notably, the catalyst's performance was significantly enhanced (EJ10 = 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.