Abstract
AbstractThermochemical 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)O2.55 (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, La0.60Sr0.40MnO3. 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 (Sr1.10La0.64Ce0.26)MnO3.88 (I4/mmm, K2NiF4‐type) at reduction temperature (1350 °C) to the n = 2 Ruddlesden–Popper phase (Sr2.60La0.22Ce0.18)Mn2O6.6 (I4/mmm, Sr3Ti2O7‐type) at re‐oxidation temperature (1000 °C) after the CO2 splitting step.
Highlights
Thermogravimetric Redox Cycling ExperimentsTGA redox cycling experiments were conducted to estimate the oxygen exchange capacity (OEC) (Δδ) of the target materials
Thermodynamic studies suggest that the redox cyclability of a metal oxide is largely dependent on the oxide formation enthalpy, Solar-driven thermochemical (STC) technologies have the which in turn is dependent on the metal–oxygen bond dissopotential to generate carbon-neutral transportation fuels at the ciation energy, and the maximum reversible oxygen non-stoiglobal scale, especially for the aviation and maritime sectors.[1,2] chiometry (δ).[6]
The oxygen exchange capacity (OEC) is a trade-off between the energy required to break metal–oxygen bonds in the perovskite oxide lattice during the reduction and the energy released upon re-oxidation using CO2 to restore these bonds, thereby reducing CO2 to CO
Summary
TGA redox cycling experiments were conducted to estimate the OEC (Δδ) of the target materials. TGA cycling experiment plots of pure LSM, LSC10M90, LSC25M75, and LSC40M60 samples at various reduction and oxidation temperatures over 4 consecutive redox cycles are displayed, Supporting Information, and an additional zoomed plot of mass changes (Δm %) during the second cycle is shown, Supporting Information. Pure LSM and ceria clearly represent the boundaries of reduction and oxidation extents of the LSCM compositional range. Pure LSM represents a lower boundary with its high reduction extent (δ) but a poor re-oxidation extent largely due to the thermodynamic limitations and ceria displays an upper boundary due to its rapid re-oxidation extent, while exhibiting a lower reduction extent (δ). The LSC25M75 sample exhibits a good balance between high reducibility and rapid re-oxidation extent with a slope of its reoxidation curve almost twice as that of pure LSM.
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