Interstitial Oxide-Ion Conductivity in Novel Melilite-Type Solid Oxide Fuel Cells

Solid Oxide Fuel Cells (SOFCs) are electrochemical devices that convert chem-ical energy into electrical energy. However, one of the main problems of SOFCs isthat they have an exceedingly high resistivity at low temperatures (<600◦C). Thecurrent solution is to power these devices at temperatures as hot as 800-1,000◦C.Thermal cycling at such high temperatures invokes a penalty on energy efficiencyas well as exposing them to material degradation due to thermally-induced me-chanical failure. Modern day SOFCs are typically made with the ceramic Yttria-stabilised zirconia (YSZ) as the electrolyte layer. However, YSZ requires theseexceedingly high temperatures to produce energy. YSZ, as well as most otherSOFC electrolytes afford ionic conduction of oxide-ion by means of a vacancymechanism.

In contrast to this, novel-melilite ceramics such as lanthanum strontium tri-gallium heptoxide, LaSrGa3O7, are a relatively new category of ionic conductor.This family of materials perform ionic conduction through an interstitial mech-anism. The parent structure LaSrGa3O7is not a good ionic conductor, but in-troducing oxygen-excess by substituting trivalent lanthanide cations in place ofgroup two alkaline-earth metals yields La1.5Sr0.5Ga3O7.25, which introduces inter-stitial oxide-ions. The highly conductive nature of these materials have been at-tributed to oxide-ion defects. However, the exact location of the interstitial oxide-ion remains unknown and the mechanisms underpinning diffusion in melilite-type materials are still poorly understood.

The objective of this thesis is to understand how the local structure in oxygen-excess melilite rearranges itself to incorporate interstitial oxide-ion defects intoit and to establish where they are situated. Their presence will be experimen-tally assessed by X-ray absorption spectroscopy (XAS) at the Ga and Ge K-edge.This is the first XAS study done on these materials to date.Ab initioand molec-ular dynamics simulations will be used to model the structure and to study themechanism of oxide-ion diffusion. By doing this, we can elucidate how oxide-ion transport takes places in these materials, which is of great importance as itis not yet entirely understood how the transport of oxide-ions takes place. Fromthe property of diffusion, the conductivity and activation energies will also becalculated as a function of temperature and dopant.The advantageous prospect in studying melilite-type materials is that they may afford ionic conductivity at lower temperatures than current SOFC elec-trolytes. However, in order to design next-generation materials, we must firstunderstand the structure and mechanism of conduction.