(257a) Experimental Investigation of YMnO3 Perovskites with Sr a-Site and Al B-Site Doping for Solar Thermochemical Fuel Production

Two-step metal oxide redox cycles employ relatively simple chemistry and enable thermodynamically favorable reduction of H₂O or CO₂ to produce H₂ or CO, respectively, at lower temperatures than allowable by thermolysis. In a typical temperature-swing cycle, the metal-oxide is reduced at high temperatures and low oxygen partial pressure, followed by re-oxidation in the presence of H₂O or CO₂ at a lower temperature. Ceria is considered to be the current state-of-the-art metal oxide due to its rapid reaction kinetics and high entropy changes during oxygen exchange, the latter of which enables thermodynamically favorable oxidation even with small temperature swings and low oxidant concentrations. However, due to its large enthalpy change during oxygen exchange, it requires high temperatures (e.g. 1773 K) or very low oxygen activity (e.g. 10^-5 atm) to achieve notable reduction extents, thereby lowering its total fuel yield per cycle.

Many recent efforts in the thermochemical community have focused on lowering the reduction temperature through the use of emerging materials such as perovskite-oxides, of the general form ABO₃, which reduce more easily and, thus, have a higher oxygen exchange capacity. YMnO₃ perovskites with Sr²⁺ doping (Y_1-xSr_xMnO₃) have been shown to achieve higher reduction extents than both ceria and similarly doped LaMnO₃ perovskites (La_1-xSr_xMnO₃), and may produce more fuel depending on the oxidation conditions. The Sr²⁺ concentration is directly related to the thermodynamic properties of the material. Reduction extents were shown to increase with increasing Sr²⁺ incorporation; however, this improvement comes with the requirement of larger temperature swings and oxidant concentrations to perform the oxidation reaction under thermodynamically favorable conditions. Additionally, doping of Al³⁺ in La_1-xSr_xMnO₃ was shown to lead to a higher oxygen exchange capacity as a result of Mn-enrichment on the surface.

Herein, we present an investigation of Y_0.8Sr_0.2Mn_0.6Al_0.4O₃ (YSMA8264), Y_0.8Sr_0.2Mn_0.4Al_0.6O₃ (YSMA8246), and Y_0.9Sr_0.1Mn_0.6Al_0.4O₃ (YSMA9164). Assessments of the oxygen exchange capacities of these materials via temperature programmed reduction showed a significant improvement in reduction extents when compared to ceria. Equilibrium measurements were performed under conditions typifying the reduction and oxidation steps, respectively. Thermogravimetric analysis was employed to measure the nonstoichiometry of the materials at temperatures from 1173-1473 K and oxygen activity from 1.61×10^-4-3.23×10^-2 atm. To assess the viability of low temperature oxidation, an isothermal relaxation scheme was used in a custom tubular reactor. The measurements were performed at temperatures from 973-1173 K and oxygen partial pressures from 1.23×10^-20-2.24×10^-13 atm. During these experiments, the oxygen partial pressure was controlled by delivering a precise mixture of H₂O and H₂, with H₂O:H₂ ratios ranging from 1.51-72.54. Under high oxidant-to-fuel conversion conditions (i.e. H₂O:H₂ ratios approaching 1), relatively small changes in oxygen content compared to ceria were observed for each of the materials.