A further step towards µ-SOFCs: high pressure structural characterization of doped ceria electrolytes

In the last decades the growing need to produce energy in a sustainable way has led to an increased interest towards Solid Oxide Fuel Cells (SOFCs), devices that combine high efficiency and considerably low pollutants emissions. In the present framework, the possibility of obtaining fuel cells of reduced dimensions (µ-SOFCs) that are portable power sources for laptops, small medical or industrial devices and so on is highly desirable. To date, our research group is evaluating the possible use of doped ceria electrolytes in µ-SOFCs. In fact, when a small fraction of trivalent rare earth ions (RE) is inserted into the fluorite-like (F) structure of pure ceria, the occurrence of not associated oxygen vacancies that are free to move through the lattice is observed, and the Ce1-xRExO2-x/2 systems become good conductors of O2- ions in the intermediate temperature range (673 – 973 K). However, when the RE amount increases, the F structure can no longer incorporate the doping ions, and different structures characterized by a lower ionic conductivity appear. In µ-SOFCs, due to the reduced dimensions, the electrolyte is deposited as a thin film on a proper substrate, thus experiencing a tensile strain, due to the oxide-substrate lattice mismatch, which promotes the passage of O2- ions through the F lattice, thus increasing the ionic conductivity of the system. However, if the strain is too high, it is released with the formation of dislocations, that reduce the oxygen ions mobility. Since a more compressible electrolyte should be able to better tolerate strain without creating dislocations, our research group undertook a high-pressure x-ray diffraction study on different RE-doped ceria systems (RE= Lu, Sm, Nd/Tm and Gd/Sm) to evaluate their compressibility, and thus the most suitable doped ceria electrolytes to be used in µ-SOFCs. Starting from the present experiments, our group also developed a novel approach to evaluate the amount and composition of the C phase defects, clusters having the typical RE2O3 cubic structure that are responsible for the drop in the ionic conductivity of these systems, which is generally observed at quite low dopant amount. A full overview of the performed high pressure structural studies will be presented.