Ceria-base fluorite and rare-earth perovskite are promising candidates for the production of synthetic H2+CO gas (syngas), in a
two-step thermochemical cycle for H2O/CO2 splitting due to their high thermal stability and auspicious redox kinetics. In contrast
to conventional fossil resources, syngas represents a renewable fuel. Both examined groups of metal oxides exhibit their specific
strength for solar-to-fuel application: While Ceria shows extraordinary fast redox kinetics, it requires a high reduction
temperature of at least 1500°C. On the other hand, oxygen release of rare earth perovskite of the La-Sr-Mn-O base system
revealed high oxygen carrier capacity and lower required reduction temperatures. Further, in the essential CO-oxidation of H2
purification from syngas, CuO-doped Ceria represents a promising catalyst candidate with high activity. One concept for the
optimisation of the solar-to-fuel process is increased efficiency by composition refinement of the chosen functional materials. For
this purpose, predictive understanding of defect evolution and charge disproportionation reactions as function of temperature and
oxygen partial pressure changes is necessary.
A predictive concept of thermodynamic stabilities and defect chemistry evaluation of complex functional oxides, rare earth
perovskite, and doped Ceria, based on optimized compound energies is presented. We apply an extension of a thermodynamic
model for nonstoichiometric ionic solid solutions  and optimise thermodynamic model parameters by a wide variety of
experimental data from calorimetry, thermogravimetry, differential thermal analysis, electric conductivity, and X-ray absorption
spectroscopy. We provide consistent physical models of phase stabilities, thermodynamic state variables and related phase
properties, including defect formation as function of temperature, oxygen partial pressure and solute doping concentrations.
Oxygen deficiency of Ceria is reproduced by the model, which is adopted for a proper extension to the Ce-Cu-Oxide system. The
evaluated defect chemistry of La-Sr-Mn-Cr-oxide perovskite is presented and discussed. Usability of our approach in solar fuel
production technology is demonstrated by thermal efficiency calculations based on computational thermodynamic results
obtained by the modeling .