Improving Solid Oxide Fuel Cell Cathode Stability with Chemical Substitutions
Date
2015-02
Authors
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Journal ISSN
Volume Title
Publisher
New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
Abstract
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.
Description
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
Type
Thesis