| Solid oxide fuel cells (SOFCs) are energy conversion devices that electrochemically convert chemical energy of fuels such as hydrogen and natural gas directly to electricity with high efficiency, very low greenhouse gas (e.g., NOx and SOx) emissions, and quiet operation. The actual efficiency of an SOFC stack is, however, critically dependent on the electrical conductivity, chemical and thermal stability and electrochemical performance of its components. In particular, for SOFCs operating at intermediate temperature ranges of 500-800℃, the power output of the cell depends strongly on the polarization resistance at the electrode/electrolyte interface and the electro-catalytic activity and stability of the cathode for the O2 reduction reaction and of the anode for the oxidation reactions of fuel cells such as hydrogen and methane. Thus, one of the main challenges is to develop electrode materials with high electro-catalytic activity and stability and to develop electrolyte materials with high oxygen ionic conductivity for intermediate temperature solid oxide fuel cells or IT-SOFCs.To develop a low cost and scalable synthesis process for the SOFCs powders and components is important as the cost is an important barrier for the commercial viability for the SOFCs technology. The powder synthesis process plays an important role as the electro-catalytic activities of electrode materials for SOFCs depend strongly on the electrode microstructure and areas of the three phase boundaries for the electrochemical reactions. This in turn depends on the characteristics of the starting raw materials and the synthesis process.The main objective of this project is to apply the gel-casting technique, a novel ceramic forming method, in the development of IT-SOFCs. The gel-casting technique was investigated, modified and optimized to synthesize strontium doped lanthanum manganate (La0.72Sr0.18MnO3, LSM), lanthanum strontium manganese chromite ((La0.75Sr0.25)1-x(Cr0.5Mn0.5)O3-δ, LSCM), LaCoO3-based nano powders and lanthanum silicate apatite electrolyte powders. The thermal decomposition process, phase formation, morphology, conductivity and electro-catalytic activity of the electrode and electrolyte powders were extensively investigated using, X-ray diffraction (XRD), differential thermal analysis (DTA), thermo-gravimetric analysis (TGA), electrochemical impedance spectroscopy and galvanostatic polarization techniques. The c haracteristics of the electrode and e lectrolyte powders were compared to that prepared by the conventional solid-state reaction method.The results showed that for both LSM and LSCM perovsiktes the phase formation temperature of the oxide powders synthesized by gel-casting technique was considerably lower than that synthesized by the conventional solid-state reaction method. The significantly reduced phase formation temperature in the gel-casting oxide powders is most likely due to the much better homogeneity, achieved through the immobilization of the precursor particles by the cross-linked polymeric network formed during the cross-linking and gel formation steps. The homogeneously distributed and immobilized particles in a polymeric network were in close vicinities of each other, enhancing the sintering and phase formation process. The electro-catalytic performance of LSM and LSCM electrodes prepared by gel-casting powders was significantly better than that prepared by solid-state reaction powders. In the case of LSCM, the A-site deficiency should be kept to a minimum or at least small than 0.1 in order to avoid the formation of the secondary spinel phase,MnCr2O4.The open circuit potential (OCP) behaviour for the methane oxidation reaction on LSCM and LSCM/YSZ composite anodes synthesized by solid-state reaction was also investigated. The oscillation behaviour of OCP is found to be strongly related to the thickness of the composite anode, the fuel composition and the DC bias. Based on the results, the OCP oscillation may be originated from the exchange reaction between the adsorbed oxygen and lattice oxygen species. Adding a small amount of water greatly enhances the OCP oscillation due to the significant increase of the adsorbed oxygen species as the result of water dissociation at high temperature.A novel symmetric LSCM/YSZ anode and LSCM/YSZ cathode-supported thin electrolyte SOFC was successfully assembled. In order to improve the performance of the cell, both LSCM/YSZ anode and LSCM/YSZ cathode were impregnated with GDC solution. Under testing conditions of humidified H2 (97%H2/3%H2O) and air, the maximum power density of the cell was 275mWcm-2 at 850℃. The relatively low power output was found to be related to the thick electrolyte film (55μm) with many closed pores. However, this is the first time to demonstrate the feasibility of the symmetric cathode-supported thin electrolyte cells based on gel-casting LSCM oxide powders, showing great promises in the development of LSM-free cathode-supported SOFCs.In addition, the gel-casting technique was modified and optimized to fabricate Ni/YSZ anode-supported SOFC. The effect of pH, the content of dispersant and pore former (graphite) on the viscosity of NiO/YSZ slurry was investigated in detail. The viscosity of NiO/YSZ/C slurry reached a minimum value by adjusting the pH value of the slurry to 10 with the addition of 1.5wt% (for NiO and YSZ) dispersant to the slurry. Under testing conditions of humidified H2 (97%H2/3%H2O) as fuel and air as oxidant, the power density of the gel-casting anode-supported thin electrolyte SOFC reached 1.07Wcm-2 at 900℃.LaCoO3 (LC), La0.6Sr0.4CoO3-δ (LSC), La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) and Sm0.5Sr0.5CoO3-δ (SSC) IT-SOFCs cathode nano powders were successfully synthesized by gel-casting technique. The phase formation temperature and morphology of synthesized nano powders were determined by XRD technique, and transmission electron microscope (TEM), the microstructure of wet gel was detected in Modular Confocal Microscope System. The results showed that: the morphology of the nano powders were spherical, the average size of nano powders decreased with the ratio of organic to nitrates, on the contrary, the crystallization of the nano powders increased with the ratio of organic to nitrates. The wet gel was homogenous with three dimension network structure. Based on these results, the principle of particle size was investigated. The three d imension network formed by the polymerization and cross linking reaction of monomer. Furthermore, the solution content including in the organic unit cell decreased with the ratio of organic to nitrates, so the particle size can be controlled by controlling the organic unit cell and the solution content inside the organic unit cell.Similar to LSM and LSCM perovskite oxide powders, La10Si6O27 apatite electrolyte powders synthesized by gel-casting showed lower phase formation temperature, higher purity and higher density than that synthesized by solid-state reaction. Most significant, the conductivity of the La10Si6O27 powder synthesized by gel-casting was higher than that of La10Si6O27 synthesized by solid-state reaction under identical sintering and testing conditions. In addition, selected doped apatite, La9BaSi6O26.5, La9SrSi6O26.5 and La10Si5AlO26.5, was also synthesized by gel-casting. The thermal expansion coefficient (TEC) of La10Si6O27, La9BaSi6O26.5, La9SrSi6O26.5 and La10Si5AlO26.5 was in the range of 9.7-11.3×10-6 K-1 . The conductivity of gel-casting La10Si6O27, La9BaSi6O26.5, La9SrSi6O26.5 and La10Si5AlO26.5 ceramic and sintered at 1550℃ was 2.56×10-4, 3.62×10-4, 2.66×10-4 and 5.48×10-4 S cm-1 at 500℃, respectively. These conductivity values are compatible with that of the well-known YSZ electrolyte and can be increased by further optimization of the composition, microstructure and gel-casting process.
|
PDF Full Text Request