Processing and Characterization of Proton Conducting Yttrium Doped Barium Zirconate for Solid Oxide Fuel Cell Applications

Citation

Babilo, Peter (2007) Processing and Characterization of Proton Conducting Yttrium Doped Barium Zirconate for Solid Oxide Fuel Cell Applications. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/WE3X-GD14. https://resolver.caltech.edu/CaltechETD:etd-05242007-161826

Abstract

To address the wide range of reported conductivities in literature and investigate the viability of yttrium-doped barium zirconate (BaZr_1-xY_xO₃) as a membrane in electrochemical devices, the factors governing the protonic transport properties have been explored, with the aim of attaining reproducible proton conductivity in well-densified samples. It was found that a small initial particle size and high temperature sintering in the presence of excess barium were essential. By this procedure, BaZr_0.8Y_0.2O₃ with 93–99% of theoretical density and high total (bulk plus grain boundary) conductivity could be reliably prepared. Samples sintered in the absence of excess barium displayed yttria precipitates and a bulk conductivity that was reduced by more than two orders of magnitude.

Hydrogen transport across grain boundaries has been explored and the specific conductivity found to be two orders of magnitude lower than the bulk. Microstructural optimization of the total grain boundary conductivity included both decreasing total grain boundary density as well as improving intrinsic grain boundary properties.

To investigate the influence of defect chemistry on stability, proton solubility, and proton mobility; samples with yttrium dopant concentration of 30 and 40 mol % were prepared in addition to the 20 Y mol %. Lattice parameters obtained suggests the solubility of yttrium in barium zirconate to be at least 40 mol %. Thermogravimetric analysis of the barium zirconate system showed excellent chemical stability under CO₂ and protonic defects to be approaching theoretical hydrogen concentration for 20, 30, and 40 Y mol %. Significant hydroxyl-dopant associations were observed, especially at lower temperatures, which trap protons and impede transport.

To simplify processing procedures, the influence of transitional metal oxides additives (especially zinc oxide) on the densification and electrical properties of doped barium zirconate have been examined. With the use of zinc oxide as a sintering aid, BaZr_0.85Y_0.15O₃ was readily sintered to above 93% of theoretical density at 1300 °C. SEM investigations showed Zn accumulation in the intergranular regions.

Electromotive force measurements of BaZr_0.8Y_0.2O₃ showed the ionic transference number under fuel cell conditions to be at least 0.92 at 600 °C. Fuel cells based on BYZ20 were prepared and characterized.

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