Characterization of the kinetic properties of La1-xSrxMnO3-δ for solar thermochemical fuel production

Abstract

Production of fuel gases (H2 and CO) by solar thermochemical cycles using metal oxide catalysts is emerging as a promising technology, which can be used for generating a transportable and dispatchable chemical fuel. The state-of-the-art material $CeO_2$ (ceria) shows extraordinary performance in these reactions. However, its extremely high reduction temperature to obtain proper amount of oxygen non-stoichiometry is impeding the reactor design and operation. To avoid this problem and lower the operating temperature, perovskite oxides are proposed recently as potential oxygen carrier materials for the solar thermochemical processes, and La1-xSrxMnO3 is now in demand. It is reported that the reduction of this material is favored than ceria at reduced temperatures. However, the oxidation reaction is very sluggish and highly dependent on Sr composition. The main reason for this is predicted to be the surface reactions, but the available data of surface kinetic properties of $La_1-x}Sr_xMnO_3$ are limited in the literature. In this study, we fabricate thin films of $La_{1-x}Sr_xMnO_3-δ$ (0.1 ≤ x ≤ 0.4) and characterize their $CO_2$ and$ H_2O$ splitting kinetics at operating conditions $(T = 675-750^\circ C, 10^{-17} ≤ pO_2 ≤ 10^{-13})$ using electrical conductivity relaxation (ECR) method. The higher surface reaction kinetics were observed in CO splitting reaction with lower strontium contents under reduced temperature. From these results, the rate limiting step was predicted to be in the forepart of overall surface oxygen exchange reaction. In addition to this, fuel production rate measured by a custom-designed isothermal reaction system showed same Sr concentration dependency as surface reaction kinetics We hereby suggest enhancement of the surface reactivity for the competitiveness of LSM materials along with optimal design of the reaction system and the material’s composition.