The Ba-Pb-O system and its potential as a solid oxide fuel cell (SOFC) cathode material
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The Ba-Pb-O system was investigated as a possible cathode material in a solid oxide fuel cell (SOFC). Metallic oxides with a perovskite structure form a large family that displays a wide variety of properties such as superconductivity, ferroelectricity and catalysis. Barium plumbates have been studied extensively in recent years because of such interesting properties. BaPbO₃ is known to exhibit room temperature metallic conductivity, due to an overlap (~2 eV) of the O2p nonbonding band with the Pb-O spσ antibonding band at the Fermi level. Another phase in the Ba-Pb-O system, Ba₂PbO₄, is known to adopt the K₂NiF₄ structure and behaves as a large-gap semiconductor (valence bands are separated by a ~1.7 eV semiconductor gap). The compounds BaPbO₃ and Ba₂PbO₄ have been prepared with a view to evaluating them as fuel cell materials, in terms of compatibility with common electrolytes such as YSZ and CGO, stability under fuel cell operating conditions, and overall performance. The effect of substituting Y and Sc in the structures has also been studied using a combination of XRD and A. C. impedance spectroscopy. It was found that BaPbO₃ offers good performance compared with La₀.₈Sr₀.₂MnO₃-x (LSM) and La₀.₆Ca₀.₄MnO₃-x (LCM) in the lower temperature region for SOFC operation (700 – 800 K), giving an activation energy of 0.93 eV. The introduction of Y to the BaPbO₃ structure reduced performance. Electrode/electrolyte compatibility studies revealed BaPbO₃ and YSZ or CGO to be unstable to 800 °C – the formation of cerate or zirconate phases was observed. No reaction was observed at 700 °C. Therefore CGO was considered as an electrolyte for use with BaPbO₃ since CGO offers good performance at temperatures where the electrode is stable. In order to avoid high sintering temperatures for electrode adhesion, BaPbO₃ precursors were impregnated into porous CGO, before firing at 700 °C, thus creating a composite cathode.
Thesis, MPhil Master of Philosophy