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Title: Interstitial oxide-ion conductivity in novel melilite-type solid oxide fuel cells
Author: Bertuzzo, Marcus
ISNI:       0000 0004 7659 7245
Awarding Body: University of Kent
Current Institution: University of Kent
Date of Award: 2018
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Solid Oxide Fuel Cells (SOFCs) are electrochemical devices that convert chemical energy into electrical energy. However, one of the main problems of SOFCs is that they have an exceedingly high resistivity at low temperatures (< 600°C). The current 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 efficiency as well as exposing them to material degradation due to thermally-induced mechanical failure. Modern day SOFCs are typically made with the ceramic Yttria-stabilised zirconia (YSZ) as the electrolyte layer. However, YSZ requires these exceedingly high temperatures to produce energy. YSZ, as well as most other SOFC electrolytes afford ionic conduction of oxide-ion by means of a vacancy mechanism. 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 mechanism. The parent structure LaSrGa3O7 is not a good ionic conductor, but introducing oxygen-excess by substituting trivalent lanthanide cations in place of group two alkaline-earth metals yields La1.5Sr0.5Ga3O7.25, which introduces inter-stitial oxide-ions. The highly conductive nature of these materials have been attributed 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 into it and to establish where they are situated. Their presence will be experimentally 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 initio and molecular dynamics simulations will be used to model the structure and to study the mechanism 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 it is not yet entirely understood how the transport of oxide-ions takes place. From the property of diffusion, the conductivity and activation energies will also be calculated 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 electrolytes. However, in order to design next-generation materials, we must first understand the structure and mechanism of conduction.
Supervisor: Alfredsson, Maria Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics