(745f) Tunable Thermodynamic Activity of LaxSr1-XMnyAl1-YO3-δ (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) Perovskites for Solar-Driven Splitting of CO2 and H2O

To guide the development of advanced metal oxides for solar-driven splitting of CO₂ and H₂O into CO, H₂ (syngas), and O₂, we investigate the equilibrium thermodynamics of a family of perovskites, i.e. La_xSr_1-xMn_yAl_1-yO_3-δ (La_0.6Sr_0.4Mn_0.4Al_0.6O_3-δ, La_0.6Sr_0.4Mn_0.8Al_0.2O_3-δ,La_0.4Sr_0.6Mn_0.6Al_0.4O_3-δ,La_0.4Sr_0.6Mn_0.8Al_0.2O_3-δ,La_0.2Sr_0.8Mn_0.8Al_0.2O_3-δ) and Ca-doped La_0.6Ca_0.4Mn_0.8Al_0.2O_3-δ for comparison. Oxygen nonstoichiometry measurements in the temperature range 1573 to 1773 K and oxygen partial pressure range 0.206 to 180.015 mbar O₂ showed tunable reduction extents increasing with increasing Sr content. Maximum oxygen nonstoichiometry of 0.32 is established by La_0.2Sr_0.8Mn_0.8Al_0.2O_3-δ at 1773 K and 2.369 mbar O₂. The partial molar enthalpy, entropy and Gibbs free energy were extracted from the experimental data using defect models based on the two simultaneous redox couples Mn⁴⁺/Mn³⁺ and Mn³⁺/Mn²⁺ which describe the measured oxygen nonstoichiometry of all perovskites investigated in this work. While most perovskite compositions showed decreasing partial molar enthalpy with increasing oxygen nonstoichiometry, this desirable feature is most pronounced for La_0.6Sr_0.4Mn_0.4Al_0.6O_3-δ. Compared to state-of-the-art ceria, partial molar enthalpy and entropy values were found to be lower for all investigated perovskites, resulting in thermodynamically more favorable reduction but less favorable oxidation with CO₂, implying energy penalties due to larger temperature swings or large amounts of excess CO₂ or H₂O required. Generally, we observe changing the ratios of A-cations and B-cations influences redox thermodynamics significantly, while changing the ratio of B-cations alone has an only a minor effect on the materialâ??s thermodynamics. Using electronic structure theory, we conclude with a practical methodology to estimate the thermodynamic activity of perovskites in CO₂ and H₂O splitting.