Dopant Incorporation in Ceria for Enhanced Water-Splitting Activity during Solar Thermochemical Hydrogen Generation

Ceria has emerged as an attractive candidate for solar thermochemical hydrogen production; however, the necessary temperatures for CeO2 reduction to Ce2O3 are too high for conventional solar concentrating systems, while the reduction to nonstoichiometric CeO2−δ below 1500 °C shows restricted chemical yield. Doping ceria with another metal can improve the reactivity at lower temperatures. This study focuses on the doping of ceria with different metals such as tantalum or trivalent lanthanides (La, Sm, and Gd) to form binary oxides and on the doping of ceria–zirconia solid solutions to form ternary oxides. Ceria materials doped with tantalum show a high reducibility, but the structural evolution during thermal treatment leads to the formation of a secondary phase that hinders the water dissociation reaction. Besides, the doping with trivalent lanthanides results in an improved thermal stability during consecutive cycles, while the hydrogen production is unchanged compared to ceria. Concerning ternary oxides, the addition of 1% gadolinium to ceria–zirconia solid solutions results in the production of 338.2 μmol (7.58 mL) of hydrogen per gram during one cycle with the O2-releasing step at 1400 °C and the H2-generation step at 1050 °C. This production is higher than the one observed for undoped ceria–zirconia. The addition of lanthanum enhances the thermal stability of ceria–zirconia solid solution, thus leading to stable reactivity during repeated cycles.