In a solid oxide fuel cell (SOFC) whose cathode consists of Sr-doped LaMnO3 (La1−xSrxMnO3, LSM) and yttria-stabilized zirconia, the reduction of oxygen on the surfaces of the LSM is critical to its performance. To understand better the surface structure of the LSM and oxygen reduction process, the energetics of LSM surfaces need to be determined. In the literature, the surface energies of the cubic LSM low-index surfaces, which are the (001), (011), and (111) surfaces, were previously estimated by determining the surface energies of the corresponding cubic LaMnO3 surfaces. Here, the surface energies of actual cubic LSM low-index surfaces are determined. Results are obtained at the level of density-functional theory using the PW91 functional. To validate the choice of methodology, the equations of state of different magnetic phases of cubic LSM with x = 0.125 and 0.25 are predicted. They are found to be consistent with previous computational results. Next, the choice is made to model the surfaces of cubic LSM with x = 0.25, referred to as cubic LSM25. This material is similar to that used in actual SOFCs, LSM with x = 0.20, and its surfaces can be modeled at an acceptable computational cost. Using carefully chosen surface models in which the Sr dopants are uniformly distributed, the surface energies of the cubic LSM25 low-index surfaces are computed as a function of surface model thickness. The converged values obtained are compared to those recently reported in the literature for the corresponding cubic LaMnO3 surfaces at the same level of theory. The two sets of values are similar, providing support for the determination of cubic LSM surface energies by examining cubic LaMnO3 surfaces. However, the values obtained for cubic LSM25 are systematically slightly greater than those obtained for cubic LaMnO3, indicating some effect of the Sr dopant on the surface energies.

Surface energies of Sr-doped LaMnO3 obtained from density-functional theory

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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