Designing, Fabrication And Properties Of Zirconate/Salt Composite Proton Conductors

Protonic conductor of Y-doped zirconates exhibits good chemical stability and high bulk conductivity. However, its low grain boundary conductivity and high refractory nature, hence a low total conductivity and high sintering temperature, remain the main obstacles in its high-drain applications as electrolytes of solid state fuel cells (SOFC). In this study, we investigated the sintering behaviors and DC electrical conductivities of Y₂O₃ doped SrZrO₃ and BaZrO₃. The influences of sintering additive ZnO on the properties of Y-doped zirconates were thoroughly studied. Furthermore, heterogeneous composites were designed and fabricated with Y-doped BaZrO₃ as matrix and some inorganic salts as dispersants by a conventional powder processing. The electrical conductivities of the composites were investigated, and the enhancements were interpreted from the point of view of microstructure.SrZrxY1-x O₃-δ(SZY) and BaZrxY1-x O₃-δ(BZY) (x = 0.05, 0.10, 0.15) powders were synthesized by conventional solid-state reaction. Densified protonic conductors (95 % theoretical density) were achieved after sintering at 1600°C for more than 6 h. DC electrical measurements indicated that the protonic conductivities of SZY and BZY increased with increasing the Y₂O₃ content. The DC conductivity of SZY₁₀ (with 10 % Y₂O₃) is 1.2×10-3 S/cm at 800°C in wet hydrogen, while BZY₁₀ only achieved 5.32×10-4 S/cm under the same conditions. SZY has higher DC electrical activation energy than BZY, which closely relates to its deviation of orthorhombic lattice from cubic.Transition metal oxide ZnO is an effective sintering additive for zirconates. Addition of more than 2 mol% ZnO in SZY and BZY lowered their sintering temperatures by 250°C. XRD analysis confirmed the formation of solid-solution upon ZnO addition into SZY and BZY. The increased lattice defects by Zn substitution on Zr-site may account for the enhanced sintering ability. As for electrical properties, ZnO impart different impacts on SZY₁₀ and BZY₁₀. In BZY₁₀, addition of ZnO leads to a monotonically decreased DC electrical conductivity. In SZY₁₀, addition of 2⁵ mol% ZnO had benecial effects on its protonic conduction. Excess ZnO content (>5 mol%) in SZY₁₀ led to a decreased protonic conduction. Ionic transport number more than 0.85 at 800°C by electromotive force (EMF) measurements under fuel cell conditions revealed that SZY₁₀ and BZY₁₀ with low ZnO contents (less than 5 mol%) are nearly pure protonic conductors. The different influences of ZnO addition on the properties of SZY₁₀ and BZY₁₀ are discussed according to defect chemistry.With BZY₁₀ as the main phase, sulphates, carbonates and sodium oxyhydrate as the second phase respectively, several kinds of barium zirconate/salt composites were designed and fabricated at sintering temperature of 1320¹350 ?C and keeping time of 4 h, aiming to improve the total electrical conductivity of single phase barium zirconate. The sintering behavior, microstructure and electrical conductivity of BaZr_0.9Y_0.1O_2.95/Na2SO4 composites are thoroughly studied and compared with those of the single phase BZY₁₀. The BaZr_0.9Y_0.1O_2.95/Na₂SO₄ composites exhibit multi-phase microstructures, with BZY₁₀ grains as the main phase and small amounts of sulfates distributed at the grain boundaries, which is consistent with the expected microstructure. The electrical conductivity of BZY₁₀ increased by more than one order of magnitude, close to 10-2 S/cm, upon introducing Na2SO4 at the grain boundary. The ionic transport number of more than 0.9 indicates a pure protonic conductor at temperature range of interest. The proton transport mechanism is discussed based on the different mechanisms of zirconate and sulphates.Key factors of influencing the electrical conduction of the barium zirconate/salt composites involve sintering temperature, keeping time, ZnO content and species of salts. Low sintering temperature and short sintering time would help to avoid the evaporation of salts from the composite, and result in high conductivity. ZnO content should be minimized because of its detrimental influence on the proton conductivity of the composites. Of the salts in this study, we observed an improvement of electrical conductivity of barium zirconate in the sequence of sulphates > carbonates > sodium hydroxide.