An Investigation On The Preparing Method And Performance Of The Anode And Electrolyte Materials For Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are electrochemical energy conversion devices that convert chemical energy in fuel directly into electricity, and they exhibit many advantages such as high efficiency, low pollution, high reliability and convenience for solid modular design. Many researches are focused on promoting the peformance and reducing the cost of SOFCs.In order to modify the preparation method and promote the peformance of SOFC, we have focused on three aspects in this thesis, namely, modifying the screen printing technique to prepare the eletrolyte layer, studying the effects of Gd2Ti2O7(GT) addition in Gdo.1Ceo.901.95(GDC) electrolyte and studying the effects of Co addition in SOFC anode funtional layer (AFL) on the structure and performance. According to our researches, the main results are obtained as follows:A technique of preparing the electrolyte layer that directly screen printing the YSZ (yttria doped zirconia) slurry on the anode green tape is developed by studying the effects of screen meshes and printing times. This technique is suitable for preparing the electrolyte of planar SOFCs. Meanwhile, the electrolyte shrinks as the anode green tape shrinks during sintering, hence the densification of the electrolyte layer is promoted. Moreover, most of the methods used in our experiments, such as wet powder spraying, tape casting and screen printing are low in cost, easy to operate and highly efficient, suggesting that this screen printing technique is suitable for mass production of SOFCs. Based on this technique, we have optimized the concentration of binder and surfactant in the slurry when screen printing GDC electrolyte layer. Nextly, GDC electrolyte layer with enhanced densification is prepared by adding GT in the corresponding slurry.The effects of GT as sintering aid on the composition, densification, ionic conductivity and thermal expansion of GDC electrolyte are examined. Samples added with TiO2sintering aid are also tested for comparison. It is found that by sintering at a moderate temperature of1400℃for5h, the relative density of the GT-added GDC can reach over97%as the molar ratio of GT/GDC reaches0.02or higher. X-ray diffraction (XRD) analysis indicates that GT does not react with GDC, while TiO2reacts with Gd in GDC to form GT. The ionic conductivities of the GT-added and the TiO2-added GDC are analyzed by AC impedance spectroscopy. The result shows that although the ionic conductivity of the GT-added GDC decreases as the GT is added, it is still in the acceptable range and is much higher than that of the GDC added with an equivalent amount of TiO2. It is also found that the thermal expansion coefficient of GDC decreases as the amount of GT increases. These results show that GT is an excellent sintering aid for GDC, and the optimal molar ratio of GT/GDC is0.02in terms of densification and ionic conductivity. Besides, the promotion mechanism of GT on the densification of GDC is also discussed.The effects of Co addition in the Ni-YSZ anode functional layer (AFL) on the structure and electrochemical performance of SOFC are investigated. XRD analyses confirmed that the active metallic phase is a Ni1-xCox alloy under the operation conditions of the SOFC. Scanning electron microscope (SEM) observations indicate that the grain size of Ni1-xCox increases with the increasing Co content. Thermogravimetric (TG) analyses on the reduction of the Ni1-xCoxO-YSZ powders show that there are two processes:the chemical reaction control process and the diffusion control process. It is found that the reduction peak corresponding to the chemical reaction control process in the differential thermogravimetric (DTG) curves moves toward lower temperatures with the increasing Co content, suggesting that the catalysis activity of the Ni1-xCox is enhanced by the doping of Co. It is observed that the SOFC shows the best performance at x=0.03, and the corresponding maximum power densities are445,651and815mW cm-2at700,750and800℃, respectively. The dependence of the SOFC performance on the Co content can be attributed to the competing results between the decreased three phase boundary (TPB) length in the AFL and the enhanced catalytic activity of the Ni1-xCox phase with the increasing Co content.