Atomistic Simulations of Defect Structures in Solid Oxide Fuel Cell Electrolytes
Date
2012-02
Authors
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Journal ISSN
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Publisher
New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
Abstract
The defect structures in two promising intermediate temperature solid oxide fuel
cell electrolytes, gadolinia-doped ceria (GDC) and scandia-doped zirconia (SDZ), were
studied by atomistic computer simulations.
In GDC, it was found that sub nano-scale defect clusters preferred a next-nearestneighbor,
pyrochlore-type structure, and that they had a tendency to grow into larger
clusters. For nano-scaled domains, however, the C-type rare earth structure, in which the
dopants and vacancies are at nearest-neighbor sites, became more stable. It was suggested
that nano-domains served as the precursor of phase separation and they could be easily
formed during synthesis. Doping concentration limited the size of the nano-domains, and
caused GDC to favor small pyrochlore-type clusters at lower concentrations, but C-type
nano-domains at higher concentrations. As such, GDC was expected to show initially an
increase in conductivity and then a decrease with increasing doping concentration. The
lattice parameter of GDC should show the same trend and could be used as an indicator
of the predominant defect structure. The cation mobility was another important factor
limiting the size of defect clusters, and could be used to control the domain formations
and thereby improve the electrolyte performance. It was also found that the defect
structure in GDC could be modulated by strain through oxygen diffusing into or out of
the nearest neighbor sites of dopants with an activation energy estimated to be 0.82 eV.
Based on such a mechanism, an explanation of the “chemical strain/stress” phenomenon
observed in gadolinia-doped ceria, as well as the benefit of zero or moderate tensile strain
for electrolyte applications, was proposed.
The phase system of SDZ, especially the cation ordering in the three
rhombohedral phases, was studied with both classical empirical potentials and ab-initio
methods utilizing the density functional theory. Characteristics of defect structures in
SDZ at different concentrations were identified based on the structures in the phase
system. The abilities of the two methods in simulating the structures and phase stability
of the SDZ system were compared and evaluated.
Description
Advisory committee members: Doreen Edwards, Walter Schulze, Olivia Graeve, Yiquan Wu. Dissertation completed in partial fulfillment of the requirements for the degree of Doctorate of Philosophy in Ceramics at the Kazuo Inamori School of Engineering, New York State College of Ceramics at Alfred University
Type
Thesis