| Summary: | 博士 === 國立臺灣科技大學 === 機械工程系 === 100 === In this study, the relationship among microstructure features, oxygen vacancy distributions and ionic conductivities of Gd3+ and Sr2+ co-doped ceria electrolytes and catalytic properties the ceria-based fiber cathodes are investigated by using transmission electron microscopy (TEM), Raman Spectrometer, X-ray diffractometer and AC impedance analyzer.
First, effect of Gd2O3 and SrO co-dopants amount on microstructural features, ionic conductivities and oxygen vacancy concentrations of two modified ceria electrolytes (Ce0.8Gd0.2-xSrxO1.9-0.5x and Ce0.8-xGd0.2SrxO1.9-x) are studied. The XRD results reveal that all specimens are cubic structure. Lattice parameters of co-doped ceria were observed to linearity increase with an increase of co-dopants amount, due to large radius of Sr2+. Grain size of Ce0.8-xGd0.2SrxO1.9-x is significantly larger than that of Ce0.8Gd0.2-xSrxO1.9-0.5x at the same Sr2+ concentration, because Ce0.8-xGd0.2SrxO1.9-x possesses more oxygen vacancy to migrate oxygen ion, defects and impurities easily based upon calculation of oxygen vacancy amount. Small addition of SrO into 20 mol.% Gd2O3 doped ceria was effective in enhancing of ionic conductivity, but a sudden decreasing of conductivity with Sr2+ heavy-doping. It is interesting that asymmetry Raman bands of the F2g and vacancy modes appear in aliovalent cations co-doped ceria at around 465 cm-1 and 566 cm-1 respectively.
Different morphologies of oxygen vacancy distribution affected by di- and tri-valent cations in two co-doped ceria systems could be separated and analyzed from Raman spectra. An decrease of intensity of the observed Raman band at 566 cm-1 and the negative frequency shifts of the F2g mode intensity with increased doping is found, indicating that the oxygen vacancy amount increases as the Gd3+ and Sr2+ concentrations increase. The peak intensity of 566 cm-1 feature increased with decreasing of oxygen vacancy amount when addition of Sr2+ in GDC, and then ionic conductivity of co-doped ceria was enhanced. But, if the F2g peak becomes increasingly asymmetric and full width at half maximum of the F2g peak significantly increases with Sr2+ content due to formation of serious distortion of lattice in co-doped ceria. It is found rapid decrease of ionic conductivity in co-doped ceria. We propose that Raman scattering is a very useful analytical tool for the study of relationship between variations of ionic conductivity and localized find structure of co-doped ceria and to estimate the variation of ionic conductivity. The details for discussing the relationship between localized fine structure and ionic conductivity in co-doped ceria systems were also discussed in this work.
Second, the influence of electrospinning parameters including polymer and applied voltage on microstructural features of Ce0.78Gd0.2Sr0.02O2-δ (GSDC) nanofiber are also investigated. Cathode catalyst of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) particle mixed nano-GSDC electrolyte fiber is coated onto GSDC electrolyte to study its catalytic property. The experimental results that a uniform GSDC nano-fiber of 150 nm diameter could be spun at the concentration of polyvinyl pyrrolidone (PVP) approximately 11.32 wt.% and applied voltage of 20 kV. Variations of microstructures in nano GSDC fiber with conventional furnace and microwave sintering were studied as well. Compared to conventional furnace sintering, significant improvement in crystallinity and density of GSDC fiber at lower sintering temperatures with microwave sintering was observed.
The exchange current density of the 80 wt% LSCF- 20 wt% GSDC (L8G2) nano-fiber composite cathode is much better than that of other LSCF-GSDC fiber cathodes. In addition, rapid heating rate and short sintering time for restricting serious fiber growth and density of cathode were observed as well. LSCF-GSDC fiber cathodes sintered by microwave furnace demonstrate the lower polarization resistances and the higher exchange current values. It is due to increasing of three phase boundary by porous fiber structure to increment oxygen reduction reaction. According to these above results, we would like to emphasize that microwave sintered LSCF-GSDC fiber cathode at low sintering temperatures provides the ability to enhance catalytic properties. The experimental results in this study confirmed that the new technology including fabrication of electrospun nano-fiber cathode by microwave sintering proposed will provide a more convenient and cheap approach to the fabrication of the composite fiber cathode.
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