Synergistic modulation of redox equilibria in high entropy oxides

High entropy oxides (HEOs) are an emerging class of materials with significant potential for applications in catalysis, energy conversion and storage due to their tunable properties. The unique behavior of these materials arises from the synergistic interactions between the multiple cations within the solid solution, often referred to as the "cocktail effect." However, the underlying mechanisms governing these interactions, particularly their influence on redox equilibria, remain poorly understood. This study aims to utilize in situ X-ray absorption spectroscopy (XAS) to investigate how the redox equilibria of cations are affected by the cooperative effects that emerge when multiple cations coexist in HEOs. Specifically, we focus on the prototypical HEO, Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O, and related compounds with fewer components, examining the redox behavior of Co, Ni, and Cu upon heating in progressively reducing conditions.