Sol-Gel-Derived Ordered Mesoporous High Entropy Spinel Ferrites and Assessment of Their Photoelectrochemical and Electrocatalytic Water Splitting Performance

With the introduction of high-entropy oxides (HEO) as a novel class of materials, unexpected and interesting properties have emerged and are currently under investigation. 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. This extra gain of energy stabilizes the formation of single-phase structures showing unique and, so far, unpredictable properties. 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.<br/><br/>With respect to sol-gel synthesis, the precise control of hydrolysis and condensation reactions of the complex precursor is important in order to form uniform structures. The whole reaction system becomes even more complex when an additional component – such as a structure-directing agent – is added. This intricate interplay of reactants, such as control of reaction kinetics, is most likely the reason why the sol-gel preparation of highly ordered mesoporous HEO thin films by the well-established soft-templating and evaporation induced self-assembly (EISA) approach has not been reported yet.<br/><br/>For the first time the design of periodically ordered mesoporous high-entropy-assisted (CoNiCuZnMg)Fe2O4 spinels prepared by dip-coating and EISA process is reported. A synthetic route was developed, utilizing the unique diblock 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) network. The mesostructured HEF electrodes were found to be crack-free on the nano- and macroscale. Time-over-flight secondary ion mass spectrometry (ToF-SIMS) and electron microscopy analysis verified a homogenous distribution of all elements within the thin film structure. The mesoporous HEF electrodes were used to study the fundamental impact of a nanoscale framework on the electrocatalytic and photoelectrochemical properties. Hence, the HEF electrodes were investigated as oxygen evolution catalyst and n-type photoanode for solar water oxidation. For both applications the near-metallic electric conductivity, which was related to an electron hopping mechanism induced by the interaction of 3d-states of the inserted transition metals, was found to improve the performance. The photoresponse of HEF photoanodes is limited owing to severe surface recombination as evidenced by intensity-modulated photocurrent spectroscopy.<br/>This novel high entropy oxide class can be considered as interesting candidate and nanostructure for energy applications where high surface-areas offering a large number of (catalytically) active reaction sites are advantageous. [1]<br/><br/>[1] Einert, Marcus, et al. "Sol-Gel-Derived Ordered Mesoporous High Entropy Spinel Ferrites and Assessment of Their Photoelectrochemical and Electrocatalytic Water Splitting Performance." <i>Small</i> 19.14 (2023): 2205412.