Thermodynamic Analysis of Cerium-Based Oxides for Solar Thermochemical Fuel Production

The thermodynamics of ceria-based metal oxides MxCe1-xO2, where M = Gd, Y, Sm, Ca, Sr, have been studied in relation to their applicability as reactive intermediates in solar thermochemical redox cycles for splitting H2O and CO2. Oxygen nonstoichiometry was modeled and extrapolated to high temperatures and reduction extents by applying an ideal solution model in conjunction with a defect interaction model. Subsequently, relevant thermodynamic parameters were computed and equilibrium H-2 and CO concentrations determined as a function of reduction conditions (T, P-O2) and ensuing oxidation temperature. At 1 atm and above 1673 K, the degree of reduction is negatively correlated to dopant concentration, regardless of the type of dopant considered. Consequently, at a given reduction temperature, more H-2 and CO is generated at equilibrium for pure ceria compared to any of the other doped ceria materials considered. Although the reduction enthalpy decreases as the dopant concentration increases, the overall solar-to-fuel energy conversion efficiency is greater for pure ceria (20.2% at delta = 0.1, P-O2 = 10 ppm). Only when considering heat recovery of nearly 100% are theoretical efficiencies higher for the dopants.