Isothermal Pressure-Swing Cycling of Strontium Doped Lanthanum Manganite Oxides for Fuel Production

Water splitting for fuel production via isothermal pressure-swing cycling has recently emerged as a means of converting solar heat to a storable medium [1]. For the generation of hydrogen, the process involves the high temperature reduction of a variable valence oxide under an inert gas of low oxygen content followed by oxidation by steam at the same high temperature. Due to thermolysis, the higher the temperature the greater the oxidizing power of the steam, providing a means of converting thermal energy to chemical energy. In comparison to a more conventional temperature-swing cycle, the isothermal, pressure-swing cycle offers the potential for greater efficiency because the need for solid state heat recovery is eliminated, although the demands on gas-phase heat recovery are increased. To date, isothermal, pressure-swing process has been demonstrated only using ceria. Here, we explore the behavior of perovskite-structured oxides as candidates for this cycle.

In principle, fuel production capacity can be determined, a priori, from knowledge of the thermodynamic properties of a non-stoichiometric oxide. Accordingly, as a first step, the thermodynamics of various compositions of lanthanum strontium manganite oxides (La_xSr_1-xMnO₃) were modeled at high temperature from literature data [2]. The theoretical, equilibrium fuel production capacity at 1500°C is found, on the basis of this analysis, to exceed that of undoped ceria, by almost a factor of two (and more than a factor of three at 1400°C). Motivated by this result, samples of x = 0.1, 0.2, 0.3 and 0.4 were synthesized via solid state reaction and then experimentally evaluated for fuel productivity using in an in-house constructed thermochemical reactor. Cycling between Ar (with 10 ppm oxygen) and 20% steam (balance Ar) at 1500°C shows that indeed the series of materials offer higher fuel production capacity under conditions of long-cycle times. In addition, oxygen release rates are higher for the perovskites than they are for ceria, indicating that more fuel is produced for a given cycle with finite periods.

References:

[1] Y. Hao, C.-K. Yang and S. M. Haile, Phys. Chem. Chem. Phys., 2013, 15, 17084–17092.

[2] J. Misuzaki, et al. Solid State Ionics, 2000, 132, 167–180