Synthesis and Processing of Lanthanum Silicate Electrolytes for Intermediate Temperature Solid Oxide Fuel Cells

Increased energy demands combined with depleting fossil fuel reserves has fuelled the need for energy conversion technologies such as fuel cells. Fuel cells directly and efficiently convert chemical energy to electrical energy. Of the various fuel cell types, solid-oxide fuel cells (SOFC) combine the benefits of environmentally friendly power generation with fuel flexibility. However, the necessity for high operating temperatures (800-1000 °C) has resulted in high costs and materials compatibility challenges. This doctoral research focuses on the synthesis, sintering and processing of apatite type lanthanum silicate (ATLS) electrolytes and half-cells (anode- electrolyte) based on ATLS electrolyte for solid oxide fuel cell operating at intermediate temperatures (600-800 °C). A modified sol-gel approach for the synthesis of nanometric, phase pure electrolyte powders of varying compositions was developed. Using this method five ATLS compositions - La9.33Si6O26(LSO), La9.83Al1.5Si4.5O26(LASO), La9.83Fe1.5Si4.5O26 (LFSO), La9.83Al1.0Fe0.5Si4.5O26 (LAFSO) and La9.83Mg0.9Si5.1O26 (LMSO) - were synthesized, with stoichiometric oxygen content, a lower valence cation dopant on the silicon site and vacancies on the lanthanum site. The powders were sintered to closed porosity using conventional pressureless sintering and an advanced pressure assisted sintering technique – pulsed electric current sintering (PECS). In comparison to the more conventional powder processing routes like solid state synthesis, the sol-gel method helped in reducing the sintering temperature and dwell time required. Ionic conductivity studies on dense electrolyte ceramics (> 96% of the theoretical density) with different grain size and dopants revealed that doping with lower valence cations improved the conductivity of ATLS when compared to the undoped equivalents. Ceramics with a lower grain boundary area, i.e., a larger grain size, have higher ionic conductivity when compared to fine grained ceramics of the same composition. Neutron diffraction studies carried out at different temperatures revealed the changes that occur within the unit cell as a function of temperature and doping. The structural changes within the unit cell were correlated to the observed conductivity. The atoms surrounding the conduction channel influence the ionic conductivity and the ionic conduction path depends on the composition of the electrolyte.Anode-ATLS electrolyte half-cells were processed using electrophoretic deposition (EPD) to produce green half cells on porous anode substrates using appropriate suspension parameters. Alternatively, PECS was also used to fabricate half cells which made co-sintering half cells at significantly lower temperatures and yet retaining the desired microstructure in each layer needed for operation of the fuel cell possible.status: publishe