Synthesis and Properties of Electrolyte and Cathode Materials for Proton Conducting Fuel Cells
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Solid oxide fuel cells using proton conducting electrolytes are potentially efficient devices for energy conversion, but there is a need for improving the performance of the materials to make them economically feasible. Proton conductivity has been reported for acceptor doped LaNbO4 and a higher stability in both CO2 and H2O containing atmospheres is inferred for this material relative to traditional oxide proton conductors. The proton conductivity is, however, in the lower region required for fuel cells, and further optimisation of the ionic conductivity of LaNbO4 was the main motivation for this thesis. Due to the low dopant solubility in LaNbO4 it has been shown to be difficult to enhance the proton conductivity by further acceptor doping, which cause the formation of secondary phases when exceeding the solubility of the acceptor. Early in this work it was confirmed that LaNbO4 is a “line” compound, forming the secondary phases La3NbO7 and LaNb3O9 at minor offsets in the cation stoichiometry. To quantify the effect of variation in the nominal cation nonstoichiometry, nanocrystalline LaNbO4 powders from spray pyrolysis were impregnated with small amounts of La3+, Nb 5+ and Ca2+ aqueous precursors. The sintering properties of the modified LaNbO4 were investigated by dilatometry and the microstructure and phase composition was studied by electron microscopy and X-ray diffraction. The electrical properties of the materials were studied by 4-point DC-conductivity and 2-point 4-wire AC-conductivity at elevated temperatures in controlled atmosphere. Minor variations in the cation stoichiometry were shown to have a pronounced effect on both the sintering properties as well as the electrical conductivity. Addition of CaO, which introduced secondary phases above 0.25 at% CaO, increased the sintering temperature and improved the conductivity of the materials, providing clear evidence for a low solubility of CaO in LaNbO4. La2O3 and Nb2O5 excess materials did not possess large variation in the electrical conductivity relative to pure LaNbO4. The sintering properties were however strongly affected by the nominal La/Nb ratio in LaNbO4. The present findings demonstrate the sensitivity of cation non-stoichiometry in ionic conducting materials with limited solid solubility. Hetero-doping is an alternative doping strategy where a minority phase is intentionally introduced to modify the chemistry of the interfaces, which may also alter the space charges responsible for high grain boundary resistivity in LaNbO4. This effect is further intensified by reducing the grain size into the nm region. Powder of two phase mixtures of calcium-doped LaNbO4/La3NbO7 and LaNbO4/LaNb3O9, with 10 and 30 vol% of La3NbO7/LaNb3O9, were successfully synthesised by impregnation a LaNbO4 powder with La3+ or Nb5+ aqueous precursors. The sintering behaviour of the composite materials was analysed by dilatometry and three different sintering routes, including spark plasma sintering, hot pressing and conventional pressure-less sintering. The particle size of the starting powders was about 50 nm and the grain size of the dense materials ranged from 100 nm and upwards, depending on temperature and composition. The unit cell parameters of LaNbO4, obtained by X-ray diffraction and Rietveld refinements, were dependent on the crystallite size and approached values for tetragonal LaNbO4 with decreasing size. Microstructural characteristics were analysed by electron microscopy and revealed that the minority phases La3NbO7 or LaNb3O9 accumulated mainly at triple points and not along the grain boundaries, suggesting a large dihedral angle between La3NbO7/LaNb3O9 and LaNbO4. Electrical properties were characterised by AC impedance spectroscopy, and the modification of the microstructure and introduction of heterophase interfaces did not demonstrate significant effects on the proton conductivity. The bulk conductivities were similar in all materials, except for the material of 30 vol% LaNb3O9 with electronic contribution from the percolating minority phase. Qualitative evidence for a higher CaO concentration in the minority phase than in LaNbO4, confirmed that the solubility of CaO in LaNbO4 is below 0.5 %. Composite cathodes are of potential interest due to the lack of electrode materials with both reasonable proton and electron conductivity. A novel synthesis route to all-oxide composite cathodes by oxidation driven in-situ decomposition was developed as an alternative to conventional mechanical mixing. The phase composition in the materials was designed by controlling the oxidation state of transition metals during synthesis. Nanocrystalline powders with nominal compositions La0.8Sr0.2Nb0.4M 0.6 O3 (M = Co, Fe, Mn, Ni) were synthesised by a modified Pechini route. The evolution of the phase composition in the materials was investigated by a number of techniques as a function of thermal treatment in different atmospheres. All materials were nanocrystalline or amorphous after calcination at 400 °C in air. The materials with Fe or Mn crystallized into perovskites with possible cation ordering after annealing in 5 % H2 at 650 °C, while in the two other materials Co and Ni were reduced to the metallic state and formed La3-xSrxNbO7-δ and metal. Successive oxidation of all four materials occurred at as low as 200–350 °C, but due to slow cation mobility grain growth and crystallisation into a conventional composite material was suppressed below 850 °C. Transmission electron microscopy of the Mn material revealed semicrystalline grains and possible formation of a nano-composite with compositional gradients within the grains of the material, also suggested by conductivity measurements. The two materials with Fe and Mn were successfully sintered by hot pressing without changing the phase content, and the formation of a composite consisting of LaNbO4 and La0.67Sr0.33MnO3 or La0.67Sr0.33FeO3 were obtained upon oxidation above 850 °C. Formation of La(Sr)Nb1/3Co1/3O3 or La(Sr)Nb1/3Ni2/3O3 coexisting with a minor content of La3NbO7 was confirmed in the two other materials after oxidation. The present findings demonstrate that all-oxide cathode materials can be prepared by this method. Optimisation and further development of this bottom-up synthesis approach gives rise to new ways of controlling the microstructure of cathode materials and other composites via temperature treatments at moderate temperatures.