(780b) Thermodynamic Characterization of Charge Compensating Double Doped Ceria for Improved Redox Performance of Solar Thermochemical H2O/CO2 Splitting Cycles

Solar-driven thermochemical redox cycles utilizing nonstoichiometric metal oxides are promising for splitting H₂O and CO₂ to produce H₂ and CO (syngas), the precursor of synthetic liquid hydrocarbon fuels. The state of the art redox material is pure ceria but it has been suggested that charge compensating double doped ceria might be a novel doping strategy to expand the range of viable dopants. In this work, charge compensating double doped ceria was fabricated using 3+ and 5+ dopants, namely Ce_0.9A_0.05Nb_0.05O₂ (A=Y, La, Sc). These materials, along with Ce_xLa_(1-x)/2Nb_(1-x)/2O₂ (x=0.75,0.95), were investigated by thermodynamic characterization and compared to pure and single doped ceria (Ce_0.9B_0.1O₂ where B=Y, La, Nb, Hf). Oxygen nonstoichiometry was measured by thermogravimetric analysis in the temperature range T = 1173-1773 K and oxygen partial pressure range p_O2 = 10^-15-10^-1 atm. At a given T and p_O2, the reduction extent of the double doped materials is between that of Ce_0.9Hf_0.1O₂ and pure ceria as predicted by DFT calculations, with increasing values for decreasing dopant concentrations. We found that the experimental data is accurately described by a defect model based on a combined point and cluster defect. Thermodynamic properties, namely the partial molar enthalpy and entropy were extracted from the defect model. The calculated partial molar enthalpy of the charge compensating double doped materials is within the range of 360-410 kJ/mol and varies based on composition and dopant concentration. These novel redox materials exhibit promising behavior and their properties can be tuned for improved redox performance of solar thermochemical H₂O/CO₂ splitting cycles.