Study On Zirconia Electrolyte Film Based Fuel Cell And The Optimization Of Its Electrodes

Solid oxide fuel cell (SOFC) is an energy conversion device that produces electricity power by combination of a fuel with an oxidant. It offers several advantages, such as high energy conversion efficiency and environment-friendliness. In order to minimize the temperature-dependent internal reisistance and enhance the power output, the conventional SOFC with an electrolyte supported configuration is usually operated at a high temperature around 1000oC. However, such a high operating temperature can cause some serious problems, such as shorter cell span and higher manufacturing and materials costs. It is therefore necessary to operate the SOFC at an intermediate temperature below 800oC, which could effectively mitigate the problems associated with the high operating temperature. To obtain high cell performance at a reduced temperature, great efforts should be made to minimize the polarizations of a cell, including electrolyte ohmic polarization, activation polarization and concentration polarization of both a cathode and an anode.Dense 8 mol% yttria-stabilized zirconia (YSZ) films are successfully fabricated using a slurry spin coating method proposed to reduce the electrolyte ohmic polarization. Effects of the heat-treating temperature of green films and the number of spinning on the performance of electrolyte films are investigated. Experimental results demonstrate that a gas-tight YSZ film could be obtained by repeating three consecutive spinning/heat-treating cycles with each layer of a green film heat-treated at 400oC. A single cell with a 14-μm-thick YSZ film exhibits excellent performance at a low-to-intermediate temperature range, e.g., the open-circuit voltage is 1.14 V at 800°C and the maximum power density is 0.49 W/cmP2P at 650°C. The mechanical strength of 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) is higher than that of YSZ, but the ionic conductivity of 3Y-TZP is lower than that of YSZ. It is possible to enhance the mechanical strength of an electrolyte film and increase the reliability of an SOFC by using 3Y-TZP as a raw material. Thus we use slurry spin coating to fabricate dense 3Y-TZP films and a single cell with a 7-μm-thick 3Y-TZP film exhibits maximum power density of 1.28 W/cmP2 Pat 850 oC. The experimental result demonstrates that a 3Y-TZP based SOFC can yield the cell performance comparable to a YSZ based single cell. BaB0.5BSrB0.5BCoB0.8BFeB0.2BOB3-δB (BSCF) is a high performance new cathode material for intermediate temperature SOFC. To avoid the harmful interaction between BSCF and YSZ, a SmB0.2BCeB0.8BOB1.9 B(SDC) protective layer is prepared using slurry spin coating onto a YSZ film. A dense YSZ/SDC bi-layer film is successfully obtained by using a single co-sintering step. The single cell exhibits maximum power density of 0.64 W/cmP2P at 800°C. Sr doped LaMnOB3B (LSM) is a classic high temperature SOFC cathode. SDC impregnated LSM cathodes are investigated to minimize the polarization of LSM at a low-to-intermediate temperature. Experimental results show that the electrochemical activity for reduction reaction of LSM is enhanced with increasing SDC amount; however, the mass transfer process of oxygen across the cathode is limited by the lower porosity of the cathode caused by the increase in the SDC amount. The optimum amount of SDC is determined by the competition between the increase in three-phase boundary (TPB) and the decrease in porosity. In addition, the optimum amount of SDC is also affected by the operating temperature and the oxygen partial pressure at the cathode. A comparative study on effects of the performance of LSM, LSM/YSZ and LSM/SDC cathodes on cell performance is conducted. Experimental results show that when the cathode is exposed to the stationary air, the rate determining step (rds) is charge transfer process, and the LSM/SDC composite exhibits the best electrochemical performance. When oxygen flow is introduced to the cathode, both charge transfer process and mass transfer process are the co-rds at a temperature above 700 oC, and the LSM/YSZ composite exhibits the best electrochemical performance. However, the rds is charge transfer process at a temperature blow 700 oC, and the LSM/SDC composite exhibits the best electrochemical performance.The development of impregnation process leads to a significant reduction in the polarization of LSM cathodes. Thus, the limitation of anodic polarization in cell performance cannot be neglected any more. Effect of compaction pressure on the particle size distributions of the anode materials and anode performance is investigated to improve the anode performance. The particle sizes of both NiO and YSZ remain when the apparent compaction pressure is below 200 MPa. When the apparent compaction pressure is higher than 500 MPa, the larger NiO particles at 8μm are broken into smaller ones at 0.6μm, while larger YSZ particles at 9μm are formed due to the bonding of parts of the YSZ particles at 0.2μm. The anode substrate compacted at 500 MPa exhibits the best combined performance in porosity, three phase boundary and interfacial contact conditions. Anode functional layers (AFLs) are prepared onto anode substrates to further improve the anode performance. Effect of the thickness of AFL on anode performance is investigated. Experimental results demonstrate that the AFL not only improves the gas-tightness of a cell, but also increases the amount of TPB. The optimum thickness of the AFL is 5μm, and the critical thickness of the YSZ film is reduced to 6μm with the fabrication of the AFL.It can be concluded in this thesis that slurry spin coating is proposed to fabricate high-quality electrolyte films; the performance of the cathode in intermediate temperature is significantly enhanced by impregnating SDC particles into the LSM matrixes; the performance of anode is improved by using a suitable compaction pressure Abstract and an AFL layer with a fine microstructure. Finally, a single cell fabricated using the combined optimum parameters above exhibits excellent performance, e.g., the maximum power density of the cell with the cathode exposed to the stationary air is 1.72 W/cmP2P at 800 oC; the maximum power density of the cell with an oxygen flow at 100 ml/min introduced to the cathode is 2.84 W/cm P2P at 800 oC.