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

• Published in: Small (Weinheim an der Bergstrasse, Germany) Germany), 2023-04, Vol.19 (14), p.e2205412-n/a
• Main Authors: Einert, Marcus, Waheed, Arslan, Lauterbach, Stefan, Mellin, Maximilian, Rohnke, Marcus, Wagner, Lysander Q., Gallenberger, Julia, Tian, Chuanmu, Smarsly, Bernd M., Jaegermann, Wolfram, Hess, Franziska, Schlaad, Helmut, Hofmann, Jan P.
• Format: Article
• Language: English
• Subjects: Electrocatalysts; Electrochemical analysis; Electron transport; Entropy; Ferrites; high entropy oxides; Immersion coating; mesoporous; Nanotechnology; oxygen evolution reaction (OER); Oxygen evolution reactions; Photoconductivity; Photoelectric effect; photoelectrochemical; Photoelectrons; Sol-gel processes; Spectroscopy; Spectrum analysis; Spinel; Structural analysis; Thin films; Transition metals; Transport properties; Water splitting
• ISSN: 1613-6810; 1613-6829
• DOI: 10.1002/smll.202205412

The novel material class of high entropy oxides with their unique and unexpected physicochemical properties is a candidate for energy applications. Herein, it is reported for the first time about the physico‐ and (photo‐) electrochemical properties of ordered mesoporous (CoNiCuZnMg)Fe2O4 thin films synthesized by a soft‐templating and dip‐coating approach. The A‐site high entropy ferrites (HEF) are composed of periodically ordered mesopores building a highly accessible inorganic nanoarchitecture with large specific surface areas. The mesoporous spinel HEF thin films are found to be phase‐pure and crack‐free on the meso‐ and macroscale. The formation of the spinel structure hosting six distinct cations is verified by X‐ray‐based characterization techniques. Photoelectron spectroscopy gives insight into the chemical state of the implemented transition metals supporting the structural characterization data. Applied as photoanode for photoelectrochemical water splitting, the HEFs are photostable over several hours but show only low photoconductivity owing to fast surface recombination, as evidenced by intensity‐modulated photocurrent spectroscopy. When applied as oxygen evolution reaction electrocatalyst, the HEF thin films possess overpotentials of 420 mV at 10 mA cm−2 in 1 m KOH. The results imply that the increase of the compositional disorder enhances the electronic transport properties, which are beneficial for both energy applications. High entropy assisted ferrites are prepared for the first time as highly ordered mesoporous thin films by soft‐templating and investigated as both oxygen evolution catalyst and photoanode for solar water splitting. The preservation of an ordered mesoporous framework and the compositional disorder improve the electronic transport properties of the ferrites resulting in enhanced oxygen evolution reaction activity.