Beyond Ceria: Theoretical Investigation of Isothermal and Near-Isothermal Redox Cycling of Perovskites for Solar Thermochemical Fuel Production

Thermodynamic data for several LaMnO₃-based perovskites indicates that in the high oxygen partial pressure (pO₂) range (e.g., 10^–7 to 10^–3 atm), where isothermal thermochemical redox cycling is viable, they can undergo larger changes in oxidation state than ceria for a given change in pO₂. This suggests that these materials may be more optimal for isothermal operation than ceria and offers the potential for more efficient H₂/CO production via thermochemical splitting of H₂O/CO₂. To investigate this hypothesis, we developed a thermodynamic process model to predict the solar-to-fuel efficiencies of La_1–x(Sr,Ca)_xMn_1–yAl_yO₃ perovskites and compared results to ceria and Zr-doped ceria. The calculations were performed for isothermal or near-isothermal cycling from 1473 to 1773 K. Four methods of lowering the reduction pO₂ were considered: inert gas sweeping, mechanical vacuum pumping, electrochemical oxygen pumping, and thermochemical oxygen pumping. Considering a reduction pO₂ of 10^–6 atm and a gas-phase heat recovery effectiveness of 95%, the calculations showed that the perovskites outperformed ceria and Zr-doped ceria during isothermal operation in terms of fuel production and efficiency regardless of the pO₂ reduction method. For example, at 1773 K, the calculated efficiencies were 35.17% for La_0.6Sr_0.4Mn_0.6Al_0.4O₃ and 28.26% for ceria when implementing thermochemical oxygen pumping. Other methods of lowering the reduction pO₂ resulted in lower efficiencies, where electrochemical oxygen pumping > inert gas sweeping > vacuum pumping. Small temperature swings using inert gases to lower the pO₂ resulted in the highest efficiencies overall. For example, with a reduction temperature of 1773 K and a temperature swing of 100 K, the efficiency of the ceria-based cycle was 35.18% and with a temperature swing of 300 K, the efficiency of the La_0.6Ca_0.4MnO₃ cycle was 40.75%. Importantly, in the case of inert gas sweeping, the efficiency of the ceria-based cycle exceeds that of the candidate materials when the temperature swing is low. The theoretical calculations within this work show that perovskites have the potential for improved solar-to-fuel efficiencies during isothermal or near-isothermal redox cycling beyond those achievable by ceria.