New ionic and mixed conducting materials for fuel cell applications.

The project has dealt with the synthesis and characterisation of new materials for use in solid oxide fuel cells. In terms of potential electrolytes we have been studying a range of apatite based materials. High oxide ion conductivity has been recently reported in the apatite phases La10-yM6O26+/-z (M = Si, Ge). Preliminary work focused on the preparation and characterisation of single phase samples of previously reported phases, e.g. La9.33(Si/Ge)6O26 and La8A2(Si/Ge)6O26 (A = alkaline earth). Neutron diffraction studies revealed significant disorder within the oxygen channels for systems showing high oxide ion conductivity, namely non-stoichiometric systems containing either cation vacancies, e.g. La9.33Si6O26, or excess oxygen, e.g. La9SrSi6O26.5. Compositions containing excess oxygen showed the highest conductivities, e.g. 0.01 S cm-1 at 800 °C for La9SrSi6O26.5. Conversely, fully stoichiometric systems, e.g. La8Sr2Si6O26, showed poor oxide ion conduction, which appears to be associated with oxygen ordering within the channels. A range of doping studies followed in order to optimise the oxide ion conductivity, i.e. La9.33+x/3Si6-xMxO26 (M = Al, B, Ga) and (La/M)10-xSi6O26+/-y (M = Mg, Ca, Sr, Ba). The sample La9BaSi6O26.5 showed the highest conductivity, with a value of 6 x 10-3 S cm-1 at 500 °C, which is significantly higher than that of YSZ at this temperature (1 x 10-3 S cm-1). This sample is therefore a highly promising candidate material for use as an electrolyte in intemiediate temperature SOFCs (500 °C - 700 °C), as well as other technological applications. Similar studies were performed for samples with germanium in place of silicon. High oxide ion conductivity was observed for these systems, although germanium volatility was identified as a significant problem in these cases. In terms of cathode materials research has involved the preparation of a range of perovskite type phases based on YBa2Cu3O7-x (YBCO). The compositions tested were of formula type YSr2Cu3-xMxO7-y (M = Ga, Co, Fe), and the results indicated that these samples were not promising candidates as replacement cathode materials since, they all showed only moderately high conductivity. Furthermore, the phases were shown not to be chemically compatible with most current or prospective electrolyte materials, with significant impurity phases found to be produced when the electrolyte and cuprate were heated together at SOFC operating temperatures (900 °C - 1000 °C)