RETRACTED: Sol-gel-derived Ordered Mesoporous High Entropy Spinel Ferrites and Assessment of their Photoelectrochemical and Electrocatalytic Water Splitting Performance

For driving the (photo-) electrocatalytic water splitting reaction both efficient and photostable absorber materials and electrocatalysts are needed in order to make the technology economically competitive. The novel material class of high entropy oxides with their unique and unexpected physicochemical properties is a potential candidate for energy applications. Herein, we report 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 high entropy ferrites (HEF) are composed of 15 ‒ 18 nm sized and periodically ordered mesopores building a highly accessible inorganic nanoarchitecture with specific surface areas up to 170 m2/g. The mesoporous HEF thin films crystallize in the cubic spinel structure and were found to be crack-free on the meso- and macroscale. The formation of the spinel structure hosting six distinct cations was verified by means of gracing incidence X-ray diffraction, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, and transmission electron microscopy accompanied with energy dispersive X-ray spectroscopy. Photoelectron spectroscopy gave insight into chemical state of the implemented transition metals supporting the structural characterization data. Analyzed as photoanode for photoelectrochemical water splitting, the HEFs showed only low photoconductivity owing to fast surface recombination as suggested by intensity-modulated photocurrent spectroscopy. When applied as oxygen evolution reaction electrocatalyst, the HEF thin films possess overpotentials of 420 mV vs. RHE at 10 mA/cm2 in 1 M KOH. The results imply that the increase of the configurational disorder within the spinel structure enhances the electronic transport properties. The evaluation of the energy band alignment by Mott-Schottky analysis allows for an estimation which redox reactions can be driven, showing that the materials are theoretically capable of promoting overall water splitting.