Comparing the solar-to-fuel energy conversion efficiency of ceria and perovskite based thermochemical redox cycles for splitting H₂O and CO₂

A thermodynamic analysis was conducted on a solar thermochemical plant for syngas generation via H₂O/CO₂-splitting redox cycles in order to determine the performance of six candidate redox materials under an array of operation conditions. The values obtained for the solar-to-fuel energy conversion efficiency are higher in relative order Zr-doped CeO₂ > undoped CeO₂ > La_0.6Ca_0.4MnO₃ > La_0.6Ca_0.4Mn_0.6Al_0.4O₃ > La_0.6Sr_0.4MnO₃ > La_0.6Sr_0.4Mn_0.6Al_0.4O₃. This ordering is attributed to their relative reducibility and re-oxidizability, where the doped and undoped ceria, that favor oxidation, outperform perovskites, that favor reduction and therefore require high flowrates of excess H₂O and CO₂ during re-oxidation. Solids-solid heat recuperation during the temperature swing between the redox steps is crucial, particularly for ceria because of its low specific oxygen exchange capacity per mole and cycle. Conversely, the efficiencies of the perovskites are more dependent on gas-gas heat recuperation due to the massive excess of H₂O/CO₂. Redox material thermodynamics and plant/reactor performance are closely coupled, and must be considered together to maximize efficiency. Overall, we find that Zr-CeO₂ is the most promising redox material, while perovskites which seem promising due to high H₂/CO production capacities under large H₂O/CO₂ flow rates, perform poorly from an efficiency perspective due to the high heating duties, especially for steam.