Defect chemistry of yttrium-doped barium zirconate: a thermodynamic analysis of water uptake

Thermogravimetry has been used to evaluate the equilibrium constants of the water incorporation reaction in yttrium-doped BaZrO3 with 20-40% yttrium in the temperature range 50-1000 °C under a water partial pressure of 0.023 atm. The constants, calculated under the assumption of a negligible hole concentration, were found to be linear in the Arrhenius representation only at low temperatures (≤500 °C). Nonlinearity at high temperatures is attributed to the occurrence of electronic defects. The hydration enthalpies determined here range from -22 to -26 kJ mol^-1 and are substantially smaller in magnitude than those reported previously. The difference is a direct result of the different temperature ranges employed, where previous studies have utilized higher temperature thermogravimetric measurements, despite the inapplicability of the assumption of a negligible hole concentration. The hydration entropies measured in this work, around -40 J K^-1 mol^-1, are similarly smaller in magnitude than those previously reported and are considerably smaller than what would be expected from the complete loss of entropy of vapor-phase H2O upon dissolution. This result suggests that substantial entropy is introduced into the oxide as a consequence of the hydration. The hydration reaction constants are largely independent of yttrium concentration, in agreement with earlier reports

Processing of yttrium-doped barium zirconate for high proton conductivity

The factors governing the transport properties of yttrium-doped barium zirconate (BYZ) have been explored, with the aim of attaining reproducible proton conductivity in well-densified samples. It was found that a small initial particle size (50–100 nm) and high-temperature sintering (1600 °C) in the presence of excess barium were essential. By this procedure, BaZr0.8Y0.2O3-d with 93% to 99% theoretical density and total (bulk plus grain boundary) conductivity of 7.9 × 10^-3 S/cm at 600 °C [as measured by alternating current (ac) impedance spectroscopy under humidified nitrogen] could be reliably prepared. Samples sintered in the absence of excess barium displayed yttria-like precipitates and a bulk conductivity that was reduced by more than 2 orders of magnitude

Enhanced Power Stability for Proton Conducting Solid Oxides Fuel Cells

In order to provide the basis for a rational approach to improving the performance of Y-doped BaZrO{sub 3} electrolytes for proton conducting ceramic fuel cells, we carried out a series of coupled computational and experimental studies to arrive at a consensus view of the characteristics affecting the proton conductivity of these systems. The computational part of the project developed a practical first principles approach to predicting the proton mobility as a function of temperature and doping for polycrystalline systems. This is a significant breakthrough representing the first time that first principles methods have been used to study diffusion across grain boundaries in such systems. The basis for this breakthrough was the development of the ReaxFF reactive force field that accurately describes the structure and energetics of Y-doped BaZrO{sub 3} as the proton hops from site to site. The ReaxFF parameters are all derived from an extensive set of quantum mechanics calculations on various clusters, two dimensionally infinite slabs, and three dimensionally infinite periodic systems for combinations of metals, metal alloys, metal oxides, pure and Y-doped BaZrO{sub 3}, including chemical reaction pathways and proton transport pathways, structures. The ReaxFF force field enables molecular dynamics simulations to be carried out quickly for systems with {approx} 10,000 atoms rather than the {approx}100 or so practical for QM. The first 2.5 years were spent on developing and validating the ReaxFF and we have only had an opportunity to apply these methods to only a few test cases. However these simulations lead to transport properties (diffusion coefficients and activation energy) for multi-granular systems in good agreement with current experimental results. Now that we have validated the ReaxFF for diffusion across grain boundaries, we are in the position of being able to use computation to explore strategies to improve the diffusion of protons across grain boundaries, which both theory and experiment agree is the cause of the low conductivity of multi-granular systems. Our plan for a future project is to use the theory to optimize the additives and processing conditions and following this with experiment on the most promising systems. The experimental part of this project focused on improving the synthetic techniques for controlling the grain size and making measurements on the properties of these systems as a function of doping of impurities and of process conditions. A significant attention was paid to screening potential cathode materials (transition metal perovskites) and anode electrocatalysts (metals) for reactivity with Y-doped BaZrO{sub 3}, fabrication compatibility, and chemical stability in fuel cell environment. A robust method for fabricating crack-free thin membranes, as well as methods for sealing anode and cathode chambers, have been successfully developed. Our Pt|BYZ|Pt fuel cell, with a 100 {micro}m thick Y-doped BaZrO{sub 3} electrolyte layer, demonstrates the peak power density and short circuit current density of 28 mW/cm{sup 2} and 130mA/cm{sup 2}, respectively. These are the highest values of this type of fuel cell. All of these provide the basis for a future project in which theory and computation are combined to develop modified ceramic electrolytes capable of both high proton conductivity and excellent mechanical and chemical stability

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

To address the wide range of reported conductivities in literature and investigate the viability of yttrium-doped barium zirconate (BaZr1-xYxO3) 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, BaZr0.8Y0.2O3 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, BaZr0.85Y0.15O3 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 BaZr0.8Y0.2O3 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.</p

Enhanced sintering of yttrium-doped barium zirconate by addition of ZnO

The influence of transition 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_(0.85)V_(0.15)O_(3-δ) was readily sintered to above 93% of theoretical density at 1300 degrees C. Scanning electron microscopic investigations showed Zn accumulation in the intergranular regions. Thermogravimetric analysis of the material under flowing CO_2 showed ZnO-modified barium zirconate to exhibit excellent chemical stability. The conductivity, as measured by A.C. impedance spectroscopy under H_2O saturated nitrogen, was slightly lower than that of unmodified barium zirconate. Electromotive force measurements under fuel cell conditions revealed the total ionic transport number to be ~0.9 at 600 degrees C. The combination of electrical and chemical properties and good sinterabifity render ZnO-modified barium zirconate an excellent candidate for reduced temperature solid oxide fuel cell applications