High-entropy oxide (Mg0.2Fe0.2Co0.2Cu0.2Zn0.2)O with rocksalt-to-spinel transformation and its electrocatalytic activity for the oxygen evolution reaction

Since the recent application of the concept of high entropy to multicomponent oxide systems, there has been an active pursuit of fabricating high-entropy oxides (HEOs) and exploring their applications across various fields. Given the relatively recent discovery of this novel material system, additional studies are necessary to enhance our understanding of structures and properties in a broader range of compositions. In this study, a rocksalt HEO with a novel composition, (Mg0.2Fe0.2Co0.2Cu0.2Zn0.2)O, is prepared through solid-state reactions at various calcination temperatures within the range of 1000–1400 °C. The formation of single-phased rocksalt solid solutions is observed under low-temperature calcination, while a spinel-related metastable phase and a spinel phase separate from the rocksalt phase during calcination at 1300 °C and 1400 °C, respectively. These rocksalt-to-spinel structural transformations (i.e., phase separations), depending on calcination temperature, are carefully characterized using scanning transmission electron microscopy (STEM) and energy-dispersive spectroscopy (EDS) mapping techniques. Subsequently, it is discovered that the microstructure in the HEO significantly influences the kinetics of the electrocatalytic oxygen evolution reaction (OER) in alkaline water oxidation. These findings offer valuable insights into the structural phase transformation and the structure–activity relationship within HEO systems.