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

Authors: 
Scheffe, J. R., University of Florida
Carrillo, R., University of Florida
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 (Y1-xSrxMnO3) have been shown to achieve higher reduction extents than both ceria and similarly doped LaMnO3 perovskites (La1-xSrxMnO3), 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 La1-xSrxMnO3 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 Y0.8Sr0.2Mn0.6Al0.4O3 (YSMA8264), Y0.8Sr0.2Mn0.4Al0.6O3 (YSMA8246), and Y0.9Sr0.1Mn0.6Al0.4O3 (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.