Design And Performance Optimization Of Seals For Solid Oxide Fuel Cells

Solid Oxide Fuel Cell (SOFC) is a highly efficient, clean, quiet and reliable electric power generation device, which can be used as distributed or mobile power sources or the large-scale power plant, and has extensive applications in the sectors of power generation, transportation and military. For the sake of future energy strategy and environment protection, the SOFC technology has been under development in many countries for years. However, one of the most significant challenges for the planar SOFC design is to realize the effective high temperature sealing. With the Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC) in the background, novel compressive seals had been developed in the present study.In this dissertation, firstly, the uniqueness of the sealing for the planar IT-SOFC was analyzed; a series of hydromechanical models aiming at the interface leak and the infiltration leak were established; and the gas leak rates under various conditions were calculated according to the models and the influencing factors were discussed. According to the above results, ceramic sealing materials were selected as the subject of study and the microstructure of the seal was designed; accordingly, Al₂O₃ fine powder was chosen as the baseline material for the seals prepared by the tape casting process. The effect of Al₂O₃ powders, dispersant, binder, plasticizer as well as deairing and drying procedures on the tape casting process were systematically investigated, and hence, the tape casting process was optimized accordingly. With a sealing assessment facility developed in house, the effectiveness of the seals prepared with Al₂O₃ and Al₂O₃-based composites with addition of Al or BaO-B₂O₃-SiO₂ glass fine powders were evaluated against the Al₂O₃ powder characteristics, compressive stresses and the addition of Al and BaO-B₂O₃-SiO₂ particles. In order to demonstrate the applicability to the IT-SOFC, single cell tests with the developed seals were conducted. The major results obtained are described as follow:(1) Materials with high chemical stability, insulativity and chemical compatibility in the IT-SOFC environment are preferred the sealing application. The theoretic calculations indicated that a hydrogen leak rate of the ceramic seals per centimeter must be less than 7.35×10^-8 g·s^-1.Therefore, the interface leak should be eliminated, the D50 value of the ceramic raw materials should be in range of 1²Î¼m to keep the effective leak path less than 1μm, and the effective porosity of the seals should decrease and the tortuosity of leak path should increase as possible.(2) The optimized tape casting process is a correct choice for fabricating the Al₂O₃ and Al₂O₃-based composite seals with smooth surfaces, uniform thickness and excellent flexibility. These seals all have acceptable compressive strength, and their deformability is adequate to satisfy the need for IT-SOFC sealing assembly.(3) Satisfactory sealing effect is offered with the Al₂O₃ and Al₂O₃-based composite seals under slight applied loads. The leak rate of the seals decreases with increasing the compressive stress and decreasing the gauge pressure. The difference in sealing effectiveness between seals prepared with various Al₂O₃ powders is mainly correlated to their powder characteristics.(4) The addition of Al fine powders can significantly improve the sealing properties of the Al₂O₃ seals. The softening and deformation of Al particles upon heating and the volume expansion (28%) associated with Al oxidation can reduce the leak path and porosity of the seals. The reaction bonding generated from Al oxidation is expected to cluster individual Al₂O₃ particles, effectively increasing the tortuosity the leak paths.(5) The influence of the BaO-B₂O₃-SiO₂ glass fine powder addition on the Al₂O₃ seals is complex. The sealing effectiveness can be enhanced under compressive stresses that are causing the advantageous reaction cementation.(6) The sealing stability of the Al₂O₃ seals is closely related to the stability of their microstructures. The leak rate increases with time in some Al₂O₃ seals can be attributed to the occurrence of new leak paths due to localized microstructure instability. The reaction bonding from the Al oxidation and reaction cementation from the BaO-B₂O₃-SiO₂O glass can improve the stability of the seals.(7) Single cell tests with tape cast AS2-69, AS2-A10, AS2-A20 and AS4-A20 seals have confirmed the applicability of the developed seals to planar IT-SOFCs.