Ab initio investigation of ground-states and ionic motion in particular in zirconia-based solid-oxide electrolytes

Electrolytes with high ionic conductivity at lower temperatures are the prerequisite for
the success of Solid Oxide Fuel Cells (SOFC). One candidate is doped zirconia. In the
past, the electrical resistance of zirconia based SOFC electrolytes has mainly been decreased
by reducing its thickness. But there are limits to reducing the thickness and
one can say that nowadays the normal ways are basically exhausted to further enhance
the conductivity of well-known electrolyte materials. Hence, new approaches need to be
found to discover windows of enhanced ionic conductivity. This can be achieved by understanding
the quantum-mechanical oxygen transport in unconventional configurations
of doped zirconia. Therefore, such an understanding is of fundamental importance. In
this thesis two approaches are pursued, the investigation of the strain dependent ionic
migration in zirconia based electrolytes and the designing of an electrolyte material
structure with enhanced and strongly anisotropic ionic conductivity.
The first approach expands the elementary understanding of oxygen migration in oxide
lattices. The migration barrier of the oxygen ion jumps in zirconia is determined
by applying the Density Functional Theory (DFT) calculations in connection with the
Nudged Elastic Band (NEB) method. These computations show an unexpected window
of decreased migration barriers at high compressive strains. Similar to other publications
a decrease in the migration barrier for expansive strain is observed. But, in addition,
a migration barrier decrease under high compressive strains is found beyond a maximal
height of the migration barrier. A simple analytic model offers an explanation.
The drop of the migration barrier at high compressions originates from the elevation
of the ground-state energy. This means: Increasing ground state energies becomes an
interesting alternative to facilitate ionic mobility.
The second approach is based on the idea, that actually, only in the direction of ion
transport the ionic conductivity in SOFC electrolytes is required to be high. Using a
layering of zirconium and yttrium in the fluorite structure and applying DFT and NEB
again, a high vacancy concentration and a very low migration barrier in two dimensions
is observed, while the mobility in the third direction is sacrificed. The ionic conductivity
of this new structure at 500◦C surpasses that of the state of the art electrolyte Yttrium
Stabilized Zirconia (YSZ) at 800◦C.
Throughout the process of searching for augmented ionic conductivity, the NEB
method has particularly been used extensively and has been examined in detail. This
method has been applied to quite different systems to gain a better understanding of it.
While NEB has been applied, it has been found that a certain modification of the NEB,
the Minimum search Nudged Elastic Band (MsNEB), is able to find global minima in
a complex phase space. Furthermore, the MsNEB turns out to be complementary to
simulated annealing and the genetic algorithm. This new scheme has not been applied to
electrolyte materials, yet. However, its capabilities have been demonstrated by detecting
the most stable isomers of the phosphorus P4, P8 molecules and the corresponding
molecules of Asn, Sbn, Bin, (n = 4, 8). In the case of P8, the new MsNEB has led to a
hitherto unknown configuration, being more stable than the previously assumed ground
state.