Surface Properties, Crystal Morphology, and Reactivity of La 2 Zr 2 O 7 -based Cathode Infiltrate in Solid Oxide Fuel Ce
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Surface Properties, Crystal Morphology, and Reactivity of La2Zr2O7-based Cathode Infiltrate in Solid Oxide Fuel Cells: A Theoretical Study Yves A. Mantz U.S. Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Road and PO Box 880, Morgantown, WV 26507, U.S.A. ABSTRACT Selected properties of the lanthanum zirconate (La2Zr2O7, LZ) low-index faces, representing the first theoretical attempt to characterize the surfaces of a pyrochlore oxide, as well as oxygen (O2) interacting with LZ are predicted at the level of density-functional theory. All possible surface terminations formed by cleaving a perfect crystal are considered, as well as selected defective surfaces. After deriving the expression for the free energy of an LZ surface, surface free energies are computed. The most stable surfaces are identified, and it is suggested how to refine the ratios of surface free energies for comparison to experimental results obtained by the analysis of x-ray diffraction (XRD) patterns. The interaction of O2 with selected faces is examined. A strong dependence of the binding energy on surface oxygen content is predicted. INTRODUCTION LZ-based compounds[1] are of interest for catalytic applications.[2-4] For example, it was recently shown at our laboratory that LZ, when doped with Rh or Sr and Rh metals, is an effective catalyst for the partial oxidation of fuel into syngas (CnHm + O2 → H2 + COx).[2] The success of this application has led to other prospective uses for this material as a catalyst host / scaffolding in other energy applications, e.g., the dry reforming of methane (CH4 + CO2 → H2 + COx) as well as cathode infiltration in solid oxide fuel cells. Cathode infiltration is the process whereby an oxide material is deposited atop the primary cathode material to catalyze the oxygen reduction reaction (O2 + 4e- → 2O2-). In these contexts, an understanding of LZ surface properties would be useful. However, despite extensive experimental studies of LZ thin films[5] and crystallites,[2-4] our atomistic level understanding of the surfaces is limited. Thus, a motivation is provided for the present computational study, the first of a pyrochlore oxide surface. In addition, the interaction of O2 with selected faces is studied, which will influence the properties of the surfaces and is important in the context of fuel cells and cathode infiltration. THEORY Computations are performed at the level of density-functional theory (DFT) and the pseudopotential approximation, as implemented in the Vienna ab initio simulation package (VASP).[6] For bulk LZ and neat LZ surfaces, computational details are: the generalized gradient approximation (GGA) (PW91 functional), the projector augmented wave (PAW) pseudopotential method, spin-restricted DFT, a plane-wave basis set (250 eV cutoff), and a Monkhorst-Pack k-point sampling scheme (4×4×4 for bulk, 4×4×1 for surfaces). Results reported are converged with respect to the treatment of electron spin, plane-wave cutoff, and kpoint mesh density. The surfaces are modeled using
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