Thermodynamic analysis of isothermal CO2 splitting and CO2-H2O co-splitting for solar fuel production

Isothermal solar thermochemistry is a promising approach for deriving solar fuels from concentrated solar energy with potential advantages in reactor design and heat recovery. Previous work has demonstrated the feasibility of isothermal water splitting with ceria, but the solar-to-fuel efficiency and operating temperature range are relatively undesirable. In this study, thermodynamic analysis is performed on the isothermal solar thermochemical splitting of CO2 as well as co-splitting of CO2 and H2O. The cycling reactions are found to exhibit considerably higher solar-to-fuel efficiencies than that of H2O due to the favorable Gibbs free energy change in the CO2 splitting reaction at elevated temperatures and the lower thermal energy requirement for gas heating due to the lack of phase change in CO2. Through the comparison of the conventional temperature-swing (Tswing) cycling strategy, the advantages and disadvantages of both methodologies, as well as indications on system and reactor design, are discussed. Strategies for controlling the composition of solar-syngas derived from isothermal co-splitting of CO2 and H2O mixtures are also proposed.