Exploring the Redox Properties of Ce_1-xU_xO_2±δ (x ≤ 0.5) Oxides for Energy Applications

The Ce-U-O system, forming a solid solution in the fluorite structure, has gained much attention due to its unique properties. Mixed fluorite oxide powders of Ce_1-xU_xO_2±δ compositions were found to be particularly active for H₂ production through thermochemical water splitting. In the present work, we explore the reduction-oxidation properties of the mixed oxides with x = 0.1, 0.25, and 0.5. We report a particularly high oxygen storage capacity (OSC) for x ≥ 0.25 and show that the oxygen extracted from these mixed oxides is of a different origin than that extracted from CeO₂. While in ceria, oxygen is extracted from the tetrahedral sites, leading to the formation of oxygen vacancies, the extracted oxygen in Ce_1-xU_xO_2±δ (x ≥ 0.25) is essentially excess oxygen in the fluorite lattice (which spontaneously penetrates the oxide under ambient or oxidative conditions). This property, which is clearly related to the change in the valency of the U cations, is apparently responsible for the higher OSC and the lower activation energy for oxygen extraction from the mixed oxides compared to ceria. The mixed oxide powders are shown to be structurally stable, retaining their fluorite structure following reduction under Ar-5%H₂ or oxidation in air until 1000 °C. The presented results provide new insights into the Ce-U-O system which may be exploited for future technical applications, as a catalyst for thermochemical water splitting, or as a solid electrolyte in solid oxide fuel cells.