Mesoporous High-Entropy Oxide Thin Films: Electrocatalytic Water Oxidation on High-Surface-Area Spinel (Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)3O4 Electrodes

In this study, the high-entropy spinel oxide (Cr0.2Mn0.2 Co0.2Fe0.2Co0.2Ni0.2)3O4 was prepared for the first time as a mesoporous thin film with controlled thickness readily available on conductive substrates via the dip-coating technique. The spinel high-entropy oxide (HEO) electrodes were fabricated through soft-templating followed by a rapid heating step to 600 °C to achieve mesoporosity. Dense and mesoporous thin films were systematically investigated regarding their physicochemical and (photo-) electrochemical properties. The HEO electrodes were structurally characterized by means of X-ray diffraction and transmission electron microscopy together with elemental mapping verifying the formed spinel structure and the homogeneous distribution of cations. Scanning electron and atomic force microscopy proved a mesostructured surface morphology offering a highly accessible mesoporous network. X-ray photoelectron spectroscopy studies provided─for the first time─detailed information about the electronic structure of the spinel HEO thin films proposing p-type conductivity. Analyzed as a photoelectrode for solar water reduction in alkaline media, the HEO samples showed only small cathodic photocurrents. Evaluating the mesoporous HEO as an oxygen evolution catalyst, overpotentials of 390 and 350 mV were found at jgeo = 10 mA/cm2 for the dense and mesoporous HEOs, respectively. Taking the electrochemical surface area (ECSA) into account, the improved catalytic activity of the mesoporous HEO was predominantly assigned to the substantially increased number of catalytically active sites.