A- and B‑Site Ordering in the A‑Cation-Deficient Perovskite Series La2−xNiTiO6−δ (0 ≤ x < 0.20) and Evaluation as Potential Cathodes for Solid Oxide Fuel Cells

Abstract

The La2–xNiTiO6−δ (0 ≤ x < 0.2) series has been investigated in order to assess its possible use as a solid oxide fuel cell (SOFC) cathode material. These perovskite-like oxides exhibit monoclinic symmetry, as determined by a series of high-resolution structural techniques (X-ray diffraction (XRD), neuron powder diffraction (NPD), selected-area electron diffraction (SAED), and transmission electron microscopy (TEM)). Ni and Ti order over the B-site and, unusually, for x > 0, the A-site ions are also ordered along the c-axis in alternate La-rich and □-rich layers (where □ represents a vacancy). Structural determination combined with accurate compositional and magnetic characterization indicates a change in the predominant charge-compensating mechanism of A-site vacancies with composition. For x = 0.1, oxygen-vacancy formation seems to be the main-charge compensating mechanism, whereas, for x = 0.2, partial replacement of Ni by Ti in the B-substructure is dominant. In addition, a small amount of trivalent nickel is present in all samples. The composition dependence of the electrical conductivity of La2–xNiTiO6−δ (x = 0, 0.1, 0.2), investigated by impedance spectroscopy, as a function of temperature and oxygen partial pressure, is successfully interpreted on the basis of the relevant charge-compensating mechanisms and associated valence states. Thermal and chemical stability have also been studied in order to perform a preliminary electrochemical characterization as prospective cathode materials for SOFCs. The material La1.80NiTiO6-δ exhibits excellent stability under oxidizing conditions and a polarization resistance of ∼0.5 Ω cm2 at 1073 K with a yttria-stabilized zirconia (YSZ) electrolyte, slightly lower than that of the state-of-the-art La1–xSrxMnO3 (LSM)-based cathodes. A higher thermal stability and a better chemical compatibility of La1.80NiTiO6−δ with common electrolytes (e.g., YSZ), in comparison with LSM, suggests that this oxide warrants further study and optimization as a prospective improved cathode material for SOFCs.