Medium‐ and High‐Entropy Spinel Ferrite Nanoparticles via Low‐Temperature Synthesis for the Oxygen Evolution Reaction

Abstract High‐entropy oxides are a material class that is currently receiving rapidly increasing attention due to the large variety in composition and the adjustable properties. Cooperative effects between different metal cations in the crystal structure in addition to entropic phase stabilization have proven beneficial for electrocatalytic applications. Most synthesis methods, however, require high synthesis temperatures and long times, and additionally only yield selected samples in good phase‐purity. Furthermore, toxic or scarce elements are often present in large amounts. Herein, a non‐aqueous microwave‐assisted solvothermal synthesis is presented as a fast and low‐temperature alternative for the fabrication of a wide range of earth‐abundant ferrites (AFe₂O₄). Directly crystalline, phase‐pure spinel ferrites of various compositions ranging from one to seven different A‐ions are successfully obtained after only 30 min at 225 °C. A detailed characterization of their properties in relation to their composition is performed, and they are also employed for the alkaline oxygen evolution reaction. A partial replacement of Fe by Co moreover shows the high versatility of the synthesis that also allows for the simultaneous variation of the B‐ion.