Template-assisted synthesis of nanocomposites for solid oxide fuel cells
Creators: name: An, Ying
Publisher: name: Imperial College London
Year: 2010

description: In this thesis, the results of an investigation into the template-assisted synthesis of the YSZ/LSM composite for solid oxide fuel cells (SOFCs) are presented. This project is motivated by the prospective application of this technique in SOFCs. Extensive characterization of all samples formed has been carried out using a range of spectroscopic and microscopic techniques. The fabrication of the three-dimensionally ordered macroporous (3-DOM) composite mainly involves the creation of a periodic lattice of colloidal crystals via the self-assembly approach, the synthesis of 3-DOM yttria-stabilized zirconia (YSZ) frameworks using the sol-gel route and the infiltration of the lanthanum strontium manganite (LSM) precursor into the YSZ template. One objective of this project was to obtain a greater understanding of the self-assembly growth process. Since the quality of self-assembled template plays an important role in determining the quality of resulting products, different synthesis conditions that affect the crystallinity have been individually investigated. Based on the results the optimal parameters were proposed. Although cracking phenomena in the template films were observed, and they were intrinsically inevitable in the fabrication of multi-layer colloidal crystal. By carefully control the synthesis parameters, the number of cracks can be minimized. The consequently obtained films exhibited good long-range ordering. All the characterization results confirmed that the obtained polystyrene films were suitable for using as templates in the synthesis of 3-DOM materials. Another objective was the use of colloidal crystal templates for the formation of 3-DOM YSZ and LSM thin films, which have potential applications in SOFCs. The synthesis of 3-DOM materials using the template-assisted approach has been proven as a complex process. A large variety of parameters showed apparent influences on the ultimate quality of 3-DOM films. All these parameters can be classified as four types, namely the template, precursor chemistry, infiltration process and calcination. The effect of each type was investigated and is discussed in this thesis. 3-DOM films exhibited large shrinkage which is attributed to the large difference in the density of the precursor and dense ceramics. A systematic study was carried out to investigate the shrinkage mechanism during the synthesis procedure, and a “floating model” was proposed to interpret the attachment of most ceramic domains when a large shrinkage was observed. Finally 3-DOM YSZ/LSM composites were successfully fabricated using template-assisted growth. The obtained film exhibited partially filled microstructure. Since YSZ/LSM composites have potential uses as the cathode material in solid oxide fuel cells, the microstructure and electrical performance of 3-DOM composites were investigated. In order to evaluate the possible performance of the 3-DOM composite as a cathode, the electrical conductivity was assessed using AC impedance spectroscopy. The impedance spectra exhibit high frequency and low frequency arcs attributed to complex electrical responses of YSZ/LSM composites. The activation energy of the composite film was obtained according to the modified Arrenius equation, and the result showed a typical value for YSZ/LSM composites with a LSM volume fraction of 30-40%. The low conductivity of the composite film is ascribed to the poor structural contact of the 3-DOM composite. In this study, large, ultra-fine polystyrene colloidal crystals were successfully fabricated. 3-DOM YSZ and LSM thin films of high quality were prepared using the polystyrene films as templates. Finally YSZ/LSM composite films were synthesized and characterized to investigate its potential uses as the cathode in SOFCs. All the work presented in this thesis provides a better understanding on the key parameters and mechanisms involved in template-assisted growth.