Oxide Ion Conductors within Hexagonal Perovskite Family for Solid Oxide Fuel Cells
Solid oxide fuel cells (SOFCs) are one of the most promising types of fuel cells having unique physiochemical properties i.e., fuel flexibility, tolerance to poisoning, noise-free, use of costeffective catalysts, heat and power combination which enhances their efficiency and negligible electrolyte management. The study of oxide ion conductors has recently gained great attention owing to their use in electrolytes for solid oxide fuel cells (SOFC) and solid oxide electrolyzer cells (SOEC), oxygen separation membranes, oxygen sensors, and oxygen pumps. Currently, high working temperatures (> 700 °C) for SOFCs is a major hurdle that leads to poor durability of components, increased costs, slow start-up times, and issues with selection of compatible materials. Significant research efforts are being devoted to the development of oxide ion conductors that can operate at intermediate temperatures (400- 600 °C). Substantial oxide ionic conductivity has recently been reported in cation-deficient hexagonal perovskite Ba3M’M’’O8.5 derivatives, with disordered hybrid 9R-palmierite average structures, which has opened a new avenue for researchers to search novel oxide ionic conductors within this family.
In the first part of the dissertation, the phase pure Ba3VWO8.5 hexagonal perovskite derivative has been synthesized and the crystal structure and electrical properties investigated using various analytical techniques. The electrical characterizations demonstrate that Ba3VWO8.5 is a good oxide ion conductor with total conductivity of 6.2 × 10-4 S cm-1 in air at 900 °C. While previously reported Ba3NbMoO8.5 and Ba3NbWO8.5 present random distribution of cationic vacancies, X-ray and neutron diffraction experiments demonstrate that the cationic vacancies are ordered on the M2 sites in Ba3VWO8.5 , resulting in a structure where M1Ox palmierite-like layers are separated by empty octahedral cavities. Bond-valence site energy (BVSE) analysis on different phases demonstrates that ordering of the cationic vacancies hinders long-range oxygen diffusivity parallel to the c-axis in Ba3VWO8.5 , thus explaining the reduced ionic conductivity of this compound.
To further investigate the structural properties of Ba3VWO8.5 , variable temperature neutron diffraction experiments were performed to determine the structural factors at the basis of the oxide ion conduction. In Ba3VWO8.5 hexagonal perovskite derivatives the presence of 50% V5+ on the M1 site, which has a strong preference for tetrahedral geometry, is enough to disrupt the flexibility of the cation sublattice resulting in the ordering of the cations exclusively on the M1 site. The results demonstrate that cation order is retained for temperatures up to 800 °C and the loss of cation disorder between the M1 and M2 sites further results in no long-range oxygen diffusion along the c axis. So, Ba3VWO8.5 has the lowest oxide ionic conductivity of all previously reported Ba3M’M’’O8.5 hexagonal perovskite derivatives.
Another hexagonal perovskite derivative Ba3-xVMoO8.5-x has been synthesized and its crystal structure is elucidated. Rietveld refinement from neutron and X-ray diffraction data show that like Ba3VWO8.5 , the cation vacancies are ordered on the M2 site leading to a structure consisting of palmierite-like layers of M1Ox polyhedra separated by vacant octahedral layers. In contrast to other members of the Ba3M’M’’O8.5 family, Ba3-xVMoO8.5- x is not stoichiometric and both barium and extra oxygen vacancies are present. Ba3- xVMoO8.5-x is unstable in the air above 400 °C, which precludes measurement of the electrical properties.
Finally, all these results from this study reveal that the new oxide ion conductors can be designed by varying the metals at M’ and M’’ sites in hexagonal perovskite derivative Ba3M’M’’O8.5 with substantial oxide ion conductivity at a lower temperature.
Gilane, A.; Fop, S.; Sher, F.; Smith, R. I.; Mclaughlin, A. C., The relationship between oxideion conductivity and cation vacancy order in the hybrid hexagonal perovskite Ba3VWO8.5 . Journal of Materials Chemistry A 2020, 8 (32), 16506-16514.
Tawse, D. N.; Gilane, A.; Fop, S.; Martinez-Felipe, A.; Sher, F.; Smith, R. I.; Mclaughlin, A. C., Investigation of the Crystal Structure, and Ionic Pathways of the Hexagonal Perovskite Derivative Ba3–xVMoO8.5–x . Inorganic Chemistry 2021, 60 (17), 13550-13556.
Gilane, A.; Fop, S.; Tawse, D. N.; Ritter, C.; Mclaughlin, A. C. J. I. C., Variable Temperature Neutron Diffraction Study of the Oxide Ion Conductor Ba3VWO8.5 . Inorganic Chemistry 2022.