Functional Oxide Materials exhibiting Ionic Conductivity for Future Energy Conversion Needs

Doctoral thesis, 2013

Fuel cells are electrochemical devices that transform the chemical energy in hydrogen and oxygen into electrical energy at high efficiencies and produce only water vapour emissions. Materials with the perovskite or fluorite structure types are commonly employed as the ion conducting ceramic electrolyte membranes in intermediate and high temperature fuel cells. However, due to the desire for lower operating temperatures of ~ 600 °C, there exists several challenges, namely: i) insufficient ionic conductivity (minimum required ~ 10-2 Scm-1), ii) poor densification and iii) poor conductivity across grain boundaries. Another problem for proton conducting ceramic fuel cells is the lack of suitable cathode materials with appropriate mixed protonic-electronic conduction. In-situ cells to characterize these materials with concurrent techniques are also not available. The works herein try to address these challenges by exploring the structure property interplay of several candidate materials. The main techniques used were: Thermogravimetric Analysis, powder X-ray and neutron diffraction and electrochemical impedance spectroscopy. The proton conductivity of In3+-BaZrO3 was improved through co-doping with Yb3+ compared to individually doped In3+-BaZrO3 and Yb3+-BaZrO3 samples. Spark plasma sintering of In3+-BaZrO3 achieved high densities (92 %) samples, and the grain boundary conductivity was boosted in comparison to conventionally sintered samples. The oxygen deficient perovskite system, Ba3In2ZrO8, substituted with Ga3+ and Y3+ and Gd3+ and Y3+ combinations was shown to posses’ mixed ionic-electronic conduction with the Ga3+ containing sample having the greatest electron hole contribution. The crystal structure and conductivity of an alternative system to BaZrO3, Sc3+ substituted BaSnO3, was explored and BaSn0.6Sc0.4O3-δ found to have a proton conductivity as high as the current leading materials, i.e., 1.07×10-3 S cm-1 at 600 °C. Disorder in the anion sub-lattice of the pyrochlore-fluorite, Y2(Ti1–xZrx)2O7 system, studied using advanced analysis of neutron diffraction data, was found to significantly enhance O2- ion conductivity; in comparison disorder in the cation sub-lattice did not greatly influence the conductivity. This work also demonstrates two cells developed for in-situ conductivity and hydration studies coupled to neutron diffraction using In3+-BaZrO3 samples, and new insights into the hydration behaviour with respect to temperature and the thermal parameters of the oxygen anions were gained.