Lanthanum strontium cobaltite ferrite, La0.6Sr0.4Co0.2Fe0.8O3-δ or LSCF6428 is a mixed ionic and electronic conductivity (MIEC) cathode material suitable for use in solid oxide fuel cells (SOFCs) operating at intermediate temperatures (ITs) between 600 - 800 oC. In this work, LSCF6428 and rare earth doped LSCF i.e. LPSCF33428, LNSCF33428 and NSCF6428 were synthesised using the sodium alginate mediated, sol gel method and the resulting structural, porous and performance properties of sintered ceramics investigated as cathode materials for IT-SOFCs. Dried, 3 wt% alginate + metal oxide sol-gel beads were calcined at 800 oC for LSCF6428, LPSCF33428, LNSCF33428 and 775 oC for NSCF6428. XRD analysis revealed nanopowders (of ~ 40 nm crystallite size) to be in the expected rhombohedral structure for LSCF and to have between 94-96% phase purity. SEM/EDX suggest Sr deficiency and Fe enrichment of the rhombohedral phase (apart from NSCF6428) due to the presence of second phase SrCO3 and probably α-Fe2O3 (likely to be retained after calcination in air). SEM and XCT reveal that nanoparticle agglomeration results in significant microporosity. The size of the pores range from 30-50 nm, based on SEM and N2 adsorption analysis, and are similar for both the undoped and rare earth doped LSCFs. The specific surface area for the calcined nanopowders meet target requirements of between 5-7 m2 /g for SOFCs.
The calcined nanopowders were sintered at 1000 oC for 1 h to produce LSCF6428 and rare earth doped LSCF microstructures suitable for IT-SOFCs cathode materials i.e. of 70-90 % density. Microstructural analysis by nano-XCT and serial sectioning FIB-SEM reveal grain size and phase inhomogeneity not detected by XRD (the latter suggesting 70 nm crystallites in a single phase). A nanocrystalline, porous grain structure plus areas of dense, micrometre size grains and separate iron rich areas are all detected by FIB-SEM (the latter likely being the α-Fe2O3 predicted by thermal analysis of the calcined material and by magnetometry of the sintered ceramics). Overall, the sintered ceramics meet the target criteria of 25-40% porosity, with < 100 nm average crystallite size and < 100 nm average pore size by bulk measures.
Electrical symmetrical cell testing of LSCF doped with praseodymium in a half-cell format (LPSCF3342/YSZ) showed a decrease in total polarization resistance compared to an equivalent LSCF6428 half-cell, suggesting that oxygen adsorption, desorption and surface diffusion for oxygen reduction reactions at the interface of a cathode and electrolyte in a SOFC would all be improved. There is potential for further improvement if more homogeneous microstructures were to be produced. However, it is clear from this thesis that rare earth dopants such as Pr can enhance the performance of a LSCF based cathode in an IT-SOFCs.
