In-situ methods to determine high oxygen deficits in cerium-zirconium mixed oxides

• Applicants: Professor Dr.-Ing. Holger Fritze; Professor Dr.-Ing. Ralf Moos
• Subject Area: Synthesis and Properties of Functional Materials
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 536885464

Cerium-zirconium mixed oxides (CZO) exhibit catalytic properties as well as a high oxygen deficiency, which was investigated in detail in the first project phase in situ with resonant nanobalances, a microwave-based approach, and by impedance spectroscopy on powders, ceramics and films with low, but identical impurities. Merging the non-stoichiometry and impedance data yield the clarification of defect formation and the maximum achievable oxygen non-stoichiometry, with particular reference to the sample morphology. Furthermore, the chemical expansion of CZO films was found to be higher than that of pure ceria. An extremely high oxygen storage capacity is observed, which is opposed by a high chemical expansion, possibly limiting the use of CZO under changing oxygen partial pressures as in reversible fuel cells due to crack formation. In the second funding period, the focus will be broadened by including foreign atoms (Ru) or dopants (Fe, Mn) and determining their influence on the stability and the reduction of CO2 and H2O for sustainable energy sources. Another key objective is to reduce chemical expansion or relieve mechanical stresses in CZO films by foreign atoms. It is to be clarified whether the correlation of oxygen non-stoichiometry and chemical expansion can be overcome in this way and whether foreign atoms segregated at grain boundaries allow the removal of mechanical stresses. The hypothesis, which is detailed in the application, is that the reducibility of CZO is further enhanced by targeted doping and the nanostructure, and that mechanical stresses are reduced by foreign atoms. Furthermore, a fundamental picture of the dominant defect mechanisms in doped CZO systems will be obtained. Beside nanogravimetry, the focus is on microwave-based analysis of the degree of oxidation of doped CZO and operando observation of the influence on the reduction of CO2 and H2O. Specifically, the influence of dopants will be clarified. In addition, the reduction will be observed "at work" using the high-frequency technique to derive recommendations regarding the suitability of CZO materials for e. g. solar fuels. The research strategy is to prepare common samples, measure them in both laboratories, correlate the data and use them jointly for modelling. The approach is based on the complementary expertise of the applicants and provides selective redundancies that increase the reliability of the data. The expected results deepen the understanding of the defect chemistry of doped cerium-zirconium mixed oxides at high oxygen deficiencies and at the same time provide the prerequisite for in situ monitoring of corresponding high-temperature systems.

• DFG Programme: Research Grants