New Insights into the Dynamics That Control the Activity of Ceria–Zirconia Solid Solutions in Thermochemical Water Splitting Cycles

The reactivity of a ceria-rich Ce0.85Zr0.15O2 solid solution toward the thermochemical water splitting process (TWS) was studied over repeated H2/H2O redox cycles. The structural and surface modifications after treatment at high temperature under air or N2 atmospheres were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and positron annihilation lifetime spectroscopy (PALS). Samples treated under nitrogen resulted more active due to phase segregation with formation of a zirconyl oxynitride phase in catalytic amount. Insertion of N3– into the structure contributes to an increase in the numbers of oxygen vacancies that preferably arrange in large clusters, and to the stabilization of Ce3+ centers on the surface. In comparison, treatment under air resulted in a different arrangement of defects with less Ce3+ and smaller and more numerous vacancy clusters. This affects charge transfer and H-coupling processes, which play an important role in boosting the rate of H2 production. The behavior is found to be only slightly dependent on the starting ceria–zirconia composition, and it is related to the development of a similar surface heterostructure configuration, characterized by the presence of at least a ceria-rich solid solution and a (cerium-doped) zirconyl oxynitride phase, which is supposed to act as a promoter for TWS reaction. The above findings confirm the importance of a multiphase structure in the design of ceria–zirconia oxides for water splitting reaction and allow a step forward to find an optimal composition. Moreover, the results indicate that doping with nitrogen might be a novel approach for the design of robust, thermally resistant, and redox active materials.