| dc.description.abstract | The development and use of clean and renewable modes of power generation is
essential to address the increasing environmental and health concerns associated with
fossil fuel consumption. Solid oxide fuel cells (SOFCs) are promising technologies for
clean and efficient energy generation, as they can directly convert chemical energy into
electrical power overcoming combustion limitations and utilising a variety of fuels. High
temperatures are currently required for satisfactory ionic conduction at the electrolyte and
efficient catalytic activity for the oxygen reduction reaction at the cathode, decreasing the
lifetime and increasing the device’s cost. Hence, one of the main challenges associated
with SOFC development is to decrease their operating temperatures to the intermediate
temperature (IT) range (600-800 °C). Computational modelling is an essential tool in
developing or improving IT-SOFC components, allowing researchers to predict,
understand and explain the mechanisms behind experimentally observed properties. This
thesis focus on the investigation of defect properties of perovskite-based materials and on
the improvement of their ionic and/or electronic conduction properties, aiming their
application as IT-SOFC components.
LaGaO3-based materials have been suggested as suitable electrolytes, as they
possess high conductivities at lower temperatures when compared to that of currently
used yttria stabilized zirconia and samarium or gadolinium doped ceria. DFT calculations
were performed to investigate the potential effects of a range of divalent dopants in the
ionic conductivity and structure of LaGaO3 aiming to ascertain which dopants are best
suited for solid electrolyte applications. Quantities such as doping and association
energies were evaluated, to determine how easily the distinct dopants were accepted into
the lattice and if there is any tendency of vacancy segregation around them, which could
be detrimental to ionic transport. Sr(II), Ba(II) and Mg(II) were identified as the most
suitable dopants, with steric and electronic defects playing a role in how easily these
cations are accepted into the lattice. The local structural distortion introduced upon
doping was observed to be an indicator of the tendency of vacancies to be trapped around
dopants, possibly hindering ionic conductivity.
The ionic conductivity of Sr and Mg doped LaGaO3 (LSGM) was then examined
by means of molecular dynamics simulations, as the investigation of the diffusion and dynamics in doped LaGaO3 requires larger simulation cells. An interatomic potential was
derived from ab initio data, as the currently available force fields are empirically derived
and lack transferability, and has shown good agreement with experimental structural
results. The conduction and structural properties of LSGM systems with a range of dopant
concentrations (between 5 and 50 mol%) were then investigated. The calculated ionic
conductivity and activation energy values were in good agreement with the available
experimental data, with a total dopant concentration of around 20 mol% being concluded
to be optimal. Local structure analysis was carried out to evaluate the tendency of vacancy
clustering around dopants and vacancy ordering throughout the lattice, and the influence
of dopant content in such effects. Mg-doping was observed to be more beneficial to ionic
conduction properties, as vacancies tend to be trapped around Sr cations, deteriorating
ionic transport. Vacancy ordering occurs independently of dopant identity, more intensely
in systems with higher dopant content.
Finally, the defect chemistry of La2NiO4, a layered Ruddlesden-Popper oxide and
potential cathode material, was investigated. La2NiO4 is a mixed ionic and electronic
conductor, with A- and B-site doping affecting the material’s transport properties in
different ways. Usually, a trade-off between ionic and electronic conductivity
improvements needs to be achieved. Density functional theory calculations were carried
out to investigate the effects of a range of A- and B-site dopants in the material’s defect
chemistry and conduction properties. Defect formation energies, transition level diagrams
and preferable charge compensating mechanisms were investigated under distinct
chemical environments so the optimal dopants for intermediate temperature cathode
applications could be determined. The stability of oxygen defects was examined, as the
Ruddlesden-Popper structure can accommodate both oxygen vacancies and interstitials;
interstitials were predicted to be the dominant oxygen defect. The introduction of the
selected A-site dopants is preferably charge compensated by the formation of electron
holes, and hence could yield p-type electronic conductivity improvements. The calculated
defect binding energies indicate that A-site dopants should not act as electron hole traps.
The introduction B-site dopants, on the other hand, is preferably charge compensated with
the introduction of oxygen interstitials, and hence could result in ionic conductivity
improvements. Sr(II) and Co(III) were identified as the most suitable dopants for IT-
SOFC applications. | en |
