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

Abstract

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.

The facile synthesis of and light-driven water splitting on a hetero-metallic bismuth oxide catalyst

“…23,24 They also have high proton conductivity, high chemical stability, abundant storage, and cost-effectiveness that are required for photovoltaic cells, photocatalytic water splitting, light-driven carbon dioxide reduction, and photocatalytic degradation of pollutants. 25–27…”

“…23,24 They also have high proton conductivity, high chemical stability, abundant storage, and cost-effectiveness that are required for photovoltaic cells, photocatalytic water splitting, light-driven carbon dioxide reduction, and photocatalytic degradation of pollutants. [25][26][27] Bi 2 O 3 , a post-transition metal oxide with a gap of 2.8 eV and readily accessible two-electron redox couples has received increasing attention in recent years as the ideal alternative for large-scale applications and photocatalytic redox processes such as H 2 generation and CO 2 reduction, as well as many other interesting applications such as the degradation of organic pollutants, gas sensing, solid oxide fuel cells, ceramic glass fabrication, and optical coating, owing to its low toxicity, low cost, and abundant polymorphism. Therefore, it is anticipated that this material may also drive the four-electron water oxidation process under alkaline conditions, resulting in dualactive OER/HER catalysis.…”

Mesoporous CuFe2O4 Photoanodes for Solar Water Oxidation: Impact of Surface Morphology on the Photoelectrochemical Properties**

“…Up to now, there is only one study available in literature reporting on the PEC performance of KLEtemplated ferrites, which explores periodically ordered highentropy spinel photoanodes. [47] Furthermore, Laberty-Robert et al demonstrated for PIB-PEO templated ordered mesoporous WO 3 photoanodes that the PEC photoresponse of the mesostructured species was increased compared to dense WO 3 thin films. These data are supported by the work of Yagi et al in which the high PEC water oxidation performance of small-sized mesoporous WO 3 photoanodes was assigned to both the high Chemistry-A European Journal surface area (158 m 2 /g) which offered a large amount of catalytically active oxidation sites and the shorter carrier diffusion length which was provided by the 4-6 nm sized pore walls.…”

“…The precise evaluation of these parameters and their dependence on the surface morphology and degree of crystallinity is of great importance to understand the limiting factors of mesoporous materials applied as photoelectrodes for solar water splitting. Up to now, there is only one study available in literature reporting on the PEC performance of KLE‐templated ferrites, which explores periodically ordered high‐entropy spinel photoanodes [47] . Furthermore, Laberty‐Robert et al.…”

Self-Floating Nanoporous High-Entropy Oxides with Tunable Bandgap for Efficient Solar Seawater Desalination

“…(3) The narrowed bandgap, owing to optimally mixing different 3d transition elements, enables these materials to separate electrons and holes with greater ease and absorb full-wavelength sunlight more efficiently. This is attributed to the multiple low-intensity d–d conversions of different transition metals, whereby the charge carriers are excited from t 2g to the corresponding e g level, ultimately enhancing the solar energy absorption ability. Comparing the effects of Cu, Mo, and Cr on narrowing the bandgap, we find that Cu and Mo exhibit a better effect compared to Cr. − Consequently, the evaporation rates of AlNiCoFeCu-O x and AlNiCoFeMo-O x are higher than that of AlNiCoFeCr-O x .…”