Study of reduced states of Ce1−yFeyO2 and Ce1−yUyO2 for the thermochemical water splitting to hydrogen

Abstract

Solar thermochemical water splitting (TCWS) is a promising approach for sustainable hydrogen production using concentrated solar energy. Cerium oxide (CeO₂) is one of the most studied redox materials for this process due to its fast oxidation kinetics and structural stability at high temperature. However, its practical implementation remains limited by the high reduction energy required to reach significant non-stoichiometry, which restricts hydrogen yields. In this review, key fundamental aspects of ceria reduction and defect formation are discussed, with a focus on strategies to enhance reducibility through partial substitution of Ce cations by transition metals and actinides. In particular, substitution with Fe or U promotes the formation of oxygen vacancies (for different reasons, as discussed) and stabilizes reduced states, thereby increasing the extent of reduction and improving hydrogen production. Experimental and theoretical studies on Ce_1−yFe_yO_2−δ and Ce_1−yU_yO_2±δ are reviewed, including insights from temperature-programmed reduction, core- and valence-level X-ray photoelectron spectroscopy, and DFT+U calculations. Test reactions show, in line with spectroscopic results, that small fractions of Fe³⁺ or U⁴⁺ gave highest Ce³⁺ and TCWS reaction yields, while increasing the dopant fraction decreases the reaction yield. The correlations between reduction behavior, oxygen vacancy formation, charge-transfer phenomena, and hydrogen yields are discussed, with emphasis on the need for fundamental understanding of reduction energetics as a key factor for advancing solar thermochemical hydrogen generation.