Structure and properties of cathode and electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCS)

Abstract

Solid oxide fuel cell (SOFCs) is a device that used to convert from chemical energy to electrical energy. The aim of this thesis is to evaluate the correlation of structure and properties of electrode and electrolyte materials that were used for ITSOFCs. The positive electrode (cathode) materials with the composition of Ba0.5Sr0.5Co0.8-yFe0.2+yO3-δ (0 > y > 0.8) were prepared using combined EDTA citrate complexing method. Phase pure samples were obtained after the samples were heated at 900oC for 15 hours with intermittence grindings. X-ray diffraction (XRD) showed that all samples were formed full solid solution between both end-members with a cubic symmetry and the space group of Pm-3m. Furthermore, the crystal structure remained stable after heated up to 1100oC in air. Then, Rietveld refinements were performed to evaluate structural changes on the crystal symmetry by reducing cobalt contents in Ba0.5Sr0.5Co0.8-yFe0.2+yO3-δ. Results indicated that reducing Co contents decreased the lattice parameters and unit cell volume. Fe cation was remained at the 1b-site with the octahedral coordination. On the other hand, electrolyte material with the composition of Ce0.8Sm0.2O1.9 was prepared using conventional solid-state synthesis route. The prepared Ce0.8Sm0.2O1.9 was compared with the commercial sample to determine their structure, electrical properties, and grain size. Results show that the lattice parameters and unit cell volume of the prepared and commercial Ce0.8Sm0.2O1.9 were similar within errors. But crystallite size (using Scherrer‟s formula) and grain size (SEM micrograph) of the commercial Ce0.8Sm0.2O1.9 were relatively smaller than the prepared sample. Furthermore, the measured electrical conductivities of commercial and prepared Ce0.8Sm0.2O1.9 were 7 x 10-2 and 2 x 10-2 Scm-1at 600oC, respectively. The structure of commercial and prepared Ce0.8Sm0.2O1.9 are similar, however, electrical properties of commercial Ce0.8Sm0.2O1.9 is relatively much better than prepared Ce0.8Sm0.2O1.9. On the other hand, the electrochemical performance of in-house prepared half-cell Ba0.5Sr0.5Co0.4Fe0.6O3-δ | Ce0.8Sm0.2O1.9 | Ba0.5Sr0.5Co0.4Fe0.6O3-δ shows the lowest Area Specific Resistance (ASR) about 0.1257 ῼcm2 at 600oC.