Investigation Of Direct Fossil Fuel Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) is an energy conversion device with high efficiency and cleanness characteristics. As a novel technology, it has positive impact on the economizing of energy and protection of global environment. For the commercialization implementation of SOFC and improving the utilization efficiency of fossil fuel, it is critical to develop the technology of direct utilization of fossil fuel in SOFC. For the direct utilization of fossil fuel, we need to achieve anode with high coking tolerance and stability and cathode with high catalytic activity to the reduction of oxygen.This thesis aims to achieve the goal above. On one hand, to develop direct fossil fuel intermediate temperature SOFCs stack with high performance based on the study of low cost fabrication technique and segmented-in-series configuration for SOFCs, on the other hand, to develop a new cathode material La0.8Sr0.2Co0.17Mn0.83O3-δ (LSMCo) which has good electrochemical performance and high stability. Meanwhile, we develop a simple and effective method to improve the open circuit voltage (OCV) of SOFCs with samarium-doped ceria (SDC) as electrolyte. Moreover, the novel structured SOFCs with SDC as electrolyte show good electrochemical performance and high coking tolerance and stability.Dip coating technique is developed to fabricate tubular NiO-8%Y2O3-stabilized ZrO2(YSZ) anode substrates. The critical technological parameters of dip coating technique, including the preparation of Ni-YSZ suspension and the skill of the technology, are investigated in detail. Then, the electrochemical performance and microstructure of the tubular anode-supported SOFC fabricated by dip coating technique are studied. The results show that the microstructure of the anode is uniform and porous and the single cell provides maximum power density of225mW/cm2and142mW/cm2at700℃and600℃, respectively. Meanwhile, the tubular anode-supported single cell shows good thermal shock resistance. It is demonstrated that dip coating technique is available for fabricating tubular SOFCs. Ohmic resistance of dominates the energy loss of cell operation due to the long current path for tubular anode. Meanwhile, an experimental design is presented to verify the reported predictive models which are valuable guides in the design of tubular anode-supported SOFCs and a practical anode current collection strategy is proposed to improve the electrochemical performance. The results show that cell performance depends on the anode current collecting design and the increase rate of cell performance decreases with the increase in anode current collecting points. Tubular3%Y2O3-stabilized ZrO2(PSZ) supported segmented-in-series SOFC stack (SIS-SOFCs) is developed. Slip casting technique is used to fabricate tubular PSZ substrates and dip coating technique is used to prepare anode, electrolyte and cathode active layers. All the cells on the PSZ substrate are connected in series by connecting the anode and cathode successively with silver paste. The electrochemical test results show that the maximum power density of two-cell-stack is240mW/cm2while the total OCV is1.978V using humidified hydrogen as fuel and ambient air as oxidant. The polarization resistance dominates the cell performance. Thus, it is necessary to improve the microstructure and catalytic activity of electrodes. The results prove that tubular PSZ supported SIS-SOFCs configuration may be promising for SOFC stack.Cone-shaped anode substrates are fabricated by dip coating technique as mentioned above. The YSZ electrolyte layer is deposited on the anode substrate by dip coating method. A11-cell-stack based on the cone-shaped tubular anode-supported SOFC is fabricated by invaginating one by one. The results show that the maximum power densities of11-cell-stack are421.4mW/cm2and310.8mW/cm2at800℃and700℃, respectively. The volume power density is0.9W/cm3at800℃, using moist hydrogen (75ml/min) as fuel and ambient air as oxidant. SEM-EDX result shows that the coking level gradually decreases from the first cell to the last one, using moist methane as fuel. Therefore, in this segmented-in-series stack configuration, different amount of catalyst should be loaded on the anodes at different section to suppress coking.The feasibility of portable direct carbon solid oxide fuel cell (DC-SOFC) is investigated based on the coned-shaped tubular anode-supported SOFC stack. The results show that the DC-SOFC stack using activated carbon as fuel can be continuously operated without any fuel carrier gas.3-cell-stack provides maximum power of2.4W and volume power density of700mW/cm3at850℃. Stability test on the direct carbon single cells and stack shows that the output performance degraded with time, and larger operating current and smaller amount of carbon fuel resulted in shorter operation life and lower carbon utilization.La0.8Sr0.2Co0.17Mn0.83O3-δ(LSMCo) is developed based on the La1-xSrxMnO3-δ(LSM) and La0.6Sr0.4Co0.2Fe0.803-δ (LSCF) composite cathode. The results show that LSMCo is chemically compatible with YSZ electrolyte under1150℃. The performance of LSMCo is～34%higher than that of LSM when they are used as cathode in single cells operated at750℃at0.7V. No obvious degradation is observed for the LSMCo cathode after operation for100h. A new concept cell structure that using a mixed ion conductor of BaZr0.1Ce0.7Y0.1Yb0.1O3(BZCYYb) to replace samarium doped ceria (SDC) in the anode to support thin SDC film electrolyte is developed. The cell reached high performance in the temperature500-750℃with OCV values larger than～1V. It is believed that there is some inter-diffusion between BZCYYb and SDC at the interface, forming a thin layer of doped barium cerate and zirconate, which can significantly suppress the reduction of SDC. The new structure also shows excellent stability under both sulfur contaminated hydrogen and methane.