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

摘要

Thermodynamic data for several LaMnO3-based perovskites indicates that in the high oxygen partial pressure (pO(2)) 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 pO(2). This suggests that these materials may be more optimal for isothermal operation than ceria and offers the potential for more efficient H-2/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 La1-x(Sr,Ca)(x)Mn(1-y)A1(y)O(3) 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 pO(2) 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 PO, reduction method. For example, at 1773 K, the calculated efficiencies were 35.17% for La0.6Sr0.4Mn0.6Al0.4O3 and 28.26% for ceria when implementing thermochemical oxygen pumping. Other methods of lowering the reduction pO(2) resulted in lower efficiencies, where electrochemical oxygen pumping > inert gas sweeping > vacuum pumping. Small temperature swings using inert gases to lower the PO, 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 La0.6Ca0.4MnO3 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.