Reversible Phase Transformations in Novel Ce‐Substituted Perovskite Oxide Composites for Solar Thermochemical Redox Splitting of CO₂

Abstract Thermochemical splitting of CO₂ and H₂O via two‐step metal oxide redox cycles offers a promising approach to produce solar fuels. Perovskite‐type oxides with the general formula ABO₃ have recently gained attention as an attractive redox material alternative to the state‐of‐the‐art ceria, due to their high structural and thermodynamic tunability. A novel Ce‐substituted lanthanum strontium manganite perovskite‐oxide composite, La³⁺_0.48Sr²⁺_0.52(Ce⁴⁺_0.06Mn³⁺_0.79)O_2.55 (LSC25M75) is introduced, aiming to bridge the gap between ceria and perovskite oxide‐based materials by overcoming their individual thermodynamic constraints. Thermochemical CO₂ splitting redox cyclability of LSC25M75 evaluated with a thermogravimetric analyzer and an infrared furnace reactor over 100 consecutive redox cycles demonstrates a twofold higher conversion extent to CO than one of the best Mn‐based perovskite oxides, La_0.60Sr_0.40MnO₃. Based on complementary in situ high temperature neutron, synchrotron X‐ray, and electron diffraction experiments, unprecedented structural and mechanistic insight is obtained into thermochemical perovskite oxide materials. A novel CO₂ splitting reaction mechanism is presented, involving reversible temperature induced phase transitions from the n = 1 Ruddlesden–Popper phase (Sr_1.10La_0.64Ce_0.26)MnO_3.88 (I4/mmm, K₂NiF₄‐type) at reduction temperature (1350 °C) to the n = 2 Ruddlesden–Popper phase (Sr_2.60La_0.22Ce_0.18)Mn₂O_6.6 (I4/mmm, Sr₃Ti₂O₇‐type) at re‐oxidation temperature (1000 °C) after the CO₂ splitting step.