Improving Solid Oxide Fuel Cell Cathode Stability with Chemical Substitutions
New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
The chemical flexibility of the perovskite structure allows enormous opportunity to modify lanthanum strontium manganite (LSM), a material that has been demonstrated to be a practical and efficient solid oxide fuel cell cathode. Calcium and nickel substitutions on the A and B sites, respectively, have been identified as potentially stabilizing substitutions on the basis of thermodynamic stability in the perovskite structure and compatibility with other materials in the fuel cell. In this study, the phase stability of YSZ-LSM composites is evaluated experimentally. LSM materials with chemistries La0.8Sr0.2MnO3-δ (LSM-20), La0.8Sr0.1Ca0.1MnO3-δ (LSM+Ca), La0.8Sr0.2Mn0.7Ni0.3O3-δ (LSM+Ni), and La0.8Sr0.1Ca0.1Mn0.7Ni0.3O3-δ (LSM+Ca+Ni), were annealed at 850°C and 1350°C in a compact with 8 mol% yttria stabilized zirconia (YSZ) to evaluate potential reactions between the two materials. Substituting calcium for A-site strontium transformed more than half of the zirconia to a cubic calcia stabilized phase. Nickel substitutions to B-site manganese stopped this transformation, but encouraged precipitation of up to 12 weight percent lanthanum zirconate, versus about 5 weight percent for calcium modified LSM and 3 percent for the base chemistry. SEM analysis shows that the zirconate crystallizes with a fine grained microstructure, and is especially prominent in samples intentionally depleted of manganese. Formation of the zirconate phase trends closely with an expansion of the LSM unit cell, suggesting the material becomes lanthanum deficient as the reaction takes place. CO2 enriched air was shown to be the most reactive atmosphere, and incorporating 3% H2O into the atmosphere limited zirconate formation, especially for the nickel substituted sample. All LSM materials assumed R-3c symmetry after synthesis, however, calcium containing samples tended to form a Pnma phase as the material was annealed. Similarly to the observations on zirconate formation, CO2 enriched air encouraged this development and humid air slowed transformation. The samples annealed in humidified atmospheric air all adopted and remained in the R-3C phase, with no other reactions taking place. The space group symmetry is closely related to the Goldschmidt Tolerance Factor, which changes with chemical substitutions, including hydroxide from humid air, and the oxidation and spin state of transition metals in the structure.
Advisory committee members: Doreen Edwards, Dawei Liu. Dissertation completed in partial fulfillment of the requirements for the degree of Masters of Science in Ceramic Engineering at the Kazuo Inamori School of Engineering, New York State College of Ceramics at Alfred University