Sol-Gel-Derived Mesoporous High-Entropy Spinel Oxide Thin Films for Electrocatalysis of the Oxygen Evolution Reaction

With the introduction of high-entropy oxides (HEO) as a novel class of materials, unexpected and interesting properties have emerged. A HEO consists of five or more ions occupying a single crystallographic site and inducing a high degree of configurational disorder, which increases the entropic contribution to the Gibbs free energy of formation, thus stabilizing their crystallographic structure. Significant efforts have been devoted to the development of new HEO phases; however, the large majority of synthetic approaches are based on solid-state, rather than sol-gel chemistry allowing only the preparation of micrometer-sized, low-surface-area particles. Sol-gel chemistry requires precise control of reaction kinetics in order to form uniform structures, which is most likely the reason why the preparation of ordered mesoporous HEO thin films by the soft-templating and evaporation induced self-assembly (EISA) approach has not been reported yet.<br/><br/>The presentation informs about sol-gel synthesis of (ordered) mesoporous (CrMnFeCoNi)₃O₄[1] and (CoNiCuZnMg)Fe₂O₄ [2] high-entropy spinel oxides prepared by dip-coating and EISA process. A synthetic route was developed, utilizing the unique copolymer (poly(ethylene-co-butylene)-block-poly(ethylene oxide), known as KLE, in order to obtain periodically ordered and 15−18 nm sized mesopores within the high-entropy ferrite (HEF) framework. [2] The meso-structured HEF electrodes were found to be crack-free on the nano- and macroscale. Time-over-flight secondary ion mass spectrometry and electron microscopy verified a homogenous distribution of all elements within the structure. The fundamental impact of a nanoscale framework on the electrocatalytic properties was investigated: mesoporous HEF applied as oxygen evolution electrocatalyst for water oxidation showed near-metallic electric conductivity, which was related to an electron hopping mechanism induced by the interaction of 3d-states of the inserted transition metals, and was found to improve OER performance. This novel high-entropy nanostructures can be considered as interesting candidate for energy conversion applications.<br/><br/>[1] M. Einert, et al. <i>ACS Applied Energy Materials, </i>5.1 (2022), 717-730.<br/>[2] M. Einert, et al. <i>Small, </i>19.14, (2023), 2205412