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Title: Structure and properties of (Ba,Sr)(Co,Fe,Mo)O3 perovskite oxides
Author: Tsiamtsouri, Maria
ISNI:       0000 0004 2744 8147
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2012
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The work presented in this thesis focuses on the effect on structure and properties of the cubic perovskite system, Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), when Mo6+ partially substitutes the B-site elements (Co, Fe). BSCF is a candidate cathode material for intermediate temperature (500-750oC) solid oxide fuel cells (IT-SOFCs), showing promising electrochemical activity for the oxygen reduction reaction (ORR). Doping of the parent material BSCF with Mo6+ targeted the retention of the cathodic performance whilst improving the reported structural thermal instability and incompatibility issues with commonly used electrolytes. A range of Ba0.5Sr0.5Co0.8-xFe0.2-yMox+yO3-δ (BSCFM) compositions were synthesised, with variation to the Mo content and the Co/Fe ratio. Characterization was carried out by a combination of diffraction and microscopy techniques. It was found that the introduction of Mo6+ into the BSCF perovskite system favored the formation of perovskite structures with double the lattice parameters of the parent BSCF compound; these are referred as double perovskite (DP) phases to distinguish them from the single perovskite (SP) counterparts. The doubling of the unit cell was attributed to B-site ordering between Mo6+ and Co2+ due to the considerable difference in cation size and charge. The SP and DP phases coexisted in all the BSCFM compositions studied, with increasing DP volume as the Mo6+ content was increased. The structural and electrochemical characterisation was focused on the composition Ba0.5Sr0.5Co0.5Fe0.125Mo0.375O3-δ, (abbreviated as BSCFMo0.375-(Co/Fe=4) which demonstrated favourable stability and compatibility properties with state the of the art electrolyte SDC (Sm0.2Ce0.8O2-δ), which was used in this study. It also displayed the best electrochemical performance among the BSCFM compositions studied by AC impedance measurements in air; an area specific resistance (ASR) of 0.13 Ω.cm2 was obtained at 650oC. As observed from AC impedance measurements under variable oxygen partial pressure (pO2) environments for this composition, the activity of the ORR was found to be limited primarily by oxygen chemical exchange. The full structural analysis of the biphasic BSCFMo0.375-(Co/Fe=4) was a great challenge and was performed by combined refinement using Neutron and X-ray data and double-checked by a range of methods including microscopy techniques, Mössbauer spectroscopy, iodometric titrations and thermogravimetric analysis. The optimal performance of BSCFMo0.375-(Co/Fe=4) among the BSCFM compositions studied was attributed to the synergistic effect of the major DP (70% by weight) phase, which was found to be an oxygen-stoichiometric Co2+,Mo6+-rich, Fe3+-poor compound, with the minor SP (30% by weight) Co3+,Fe3+-rich, Mo6+-poor component with significant oxygen vacancy concentration.
Supervisor: Rosseinsky, Matthew J.; Claridge, John B. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral