Internal Activation Strain and Oxygen Mobility in a Thermally Stable Layered Fe3+ Oxide

Abstract

The behavior of the layered Fe3+ perovskite superstructure with composition Ba1.7Ca2.4Y0.9Fe5O13 (BCYF) as an intermediate temperature solid oxide fuel cell (IT-SOFC) cathode is investigated by an integrated modeling, secondary ion mass spectroscopy (SIMS), processing, and electrochemistry approach. Electronic structure calculations identify the defect chemistry and energetics of the multiple diffusion pathways possible in the superstructure. This reveals a wide distribution of activation energies with the favorable pathways corresponding to transport along the layers. In contrast, interlayer oxygen transport paths require higher activation barriers, whose limiting step corresponds to incorporation of oxide ion vacancies in the Ca2Fe2O5 sub-block of the structure with tetrahedrally coordinated Fe3+ ions, with a calculated activation barrier of 1.46 eV. The latter is similar to the diffusion barrier of 1.41 eV evaluated by direct SIMS measurement on polycrystalline BCYF samples. The experimentally determined oxygen tracer diffusion coefficients (D*) vary by a factor of 4 depending on selected location, confirming the anisotropic nature of the oxygen ion migration. The random orientations of the grains in the polycrystal also result in highly anisotropic oxygen surface exchange properties. and the surface exchange coefficient, k*, shows a 4-fold difference from 1.2 × 10–8 to 2.9 × 10–9 cm s–1 at 587 °C, depending on the grain-sized region selected. Enhanced ceramic processing lowers the area specific resistance (ASR) to 0.10 Ω cm2 at 750 °C, with a wide electrochemically active zone of 30–60 μm. The low calculated activation energies for in-plane oxygen transport are explained by the presence of structural strains between the component units of the layer superstructure that increase the equilibrium energy of the mobile ions, which we define as internal activation strain. The observation of good surface activity and bulk diffusion performance, limited by microstructure in the current ceramic samples, indicates that BCYF is a potential candidate for IT-SOFC cathode applications, open to property optimization by textural control.