Fabrication And Characterization Of Cathode Materials For Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are a forward looking technology for a highly efficient, environmental friendly power generation. The traditional SOFCs are operated at high temperature. The high operating temperature promotes rapid reaction kinetics, eliminates the need of precious metal catalysts, allows internal reforming of hydrocarbon fuels, and produces high quality byproduct heat suitable for co-generation However, SOFCs are currently not economically competitive to the existing power generation technologies due to problems associated mainly with high temperature (>800℃) operations, including performance degradation of cell components due to high temperature oxidation, corrosion, chemical interdiffusion and reaction. One approach to cost reduction is lowering the SOFC operating temperature to below 700℃,However, reducing the operation temperature of SOFCs to the intermediate temperature range of 500-700℃is still a challenge in the SOFC development. The overall electrochemical performance of an SOFC will decrease with the reduction in the operating temperature due to increased polarization resistances of the electrode reactions and decreased electrolyte conductivity. The problem coming from electrolyte has been solved to a certain extent by using new electrolyte materials or adopting film technique. Therefore, the shift in emphasis has been driven from the electrolyte to the electrodes where the electrodes show a higher percentage of the voltage loss for IT-SOFCs. The cathode has been the center of the focus in the electrode development largely because oxygen reduction is the more difficult reaction to activate in SOFCs operated at reduced temperatures. Consequently, development of cathode with high electrocatalytic activity becomes critical for IT-SOFCs.This p.h. D thesis aims to develop new cathodes with high catalytical activity and long life-time.In chapter 1, the principle of SOFCs and key component materials such as electrolytes, cathodes, anodes and connector, were generally introduced. In addition, the situation and the trend of SOFCs' development were also briefly reviewed. Based on the trend in intermediate-temperature operation for SOFCs, development of new mixed ionic and electronic conductors (MIECs) and composite cathodes with novel microstructures was selected as the object of this thesis.In chapter 2, three reaction paths were briefly introduced to understand the oxygen reduction process. Since the MIECs are the most potential cathodes, it is needed to get an insight into the oxygen reduction process in porous MIEC.An image of oxygen reduction process in MIEC was set up through the systematic analysis and discussion on a continuum model.In chapter 3, composites consisting of La_2-xSr_xCo_0.8Ni_0.2O_4+δ (LSCN, x=0,0.4, 0.8, 1.2, 1.6) and Ce_0.9Gd_0.1O_1.95 (GDC) have been investigated as the cathodes for low-and-intermediate temperature solid oxide fuel cells (SOFCs).K₂NiF₄ structured La₂Co_0.8Ni_0.2O_4+δ is an oxygen overstoichiometric oxide with high oxygen diffusion and oxygen surface exchange coefficients in the temperature range from 450 to 650℃, resulting in a potential MIEC cathode material according to the conclusion drawn from the continuum model which is mentioned in chapter 2. A series of experiments were conducted to characterize the composites. The main achievements are summarized as follows:1) AC impedance spectroscopy on symmetric cells indicated that among the series of LSCN-GDC composites, La_1.2Sr_0.8Co_0.8Ni_0.2O_4+δbased electrode had the lowest interfacial polarization resistance, which was 1.36Ωcm² at 600℃when the electrode was not activated.2) Significant activation effect was observed with a single cell when current treatment was performed at 200mAcm^-2 within 30min. The single cell with La_1.2Sr_0.8Co_0.8Ni_0.2O_4+δ-GDC as the cathode generated a power density up to 350mWcm^-2 at 600℃.3) In addition, the performance was pretty stable when a constant output voltage of 0.5 V was set for 36 h.These results suggest that La_2-xSr_xCo_0.8Ni_0.2O_4+δ could be promising materials as the cathodes for SOFCs that operated at low-and-intermediate temperatures. Further work will focus on improving the electrochemical activity of the LSCN-GDC composites by changing the GDC content, by decreasing the fabrication temperature leading to greater surface area, and by optimizing the amount of Sr-dopant.In chapter 4, a highly stable electrode based on La_0.6Sr_0.4Co_3-δ (LSC) was developed for intermediate temperature solid oxide fuel cells (IT-SOFCs). The electrode was prepared by impregnating LSC into a porous samaria-doped ceria (SDC, Sm_0.2Ce_0.8O_1.9) frame, which was deposited to an SDC electrolyte using screen-printing and co-firing techniques. The electrode frame (SDC) was well-connected with SDC electrolyte, and the fine LSC particles coated the SDCframe. This well-connected structure makes a pathway for oxygen ion transport between the cathode (through the SDC frame) and the electrolyte (the SDC substrate) since SDC is an excellent oxide conductor. Connect among LSC particles results in a pathway for electron transport from electron collector to the reactive sites. Optimization of the LSC impregnated SDC composite was performed and its electrochemical properties were characterized. The main achievements are summarized as follows:1) Both the loading of LSC and the firing temperature have significant influence on the performance of the LSC-impregnated SDC composite electrode. At a certain operating temperature, the lowest interfacial resistance of the electrode corresponded to an optimal loading. Lower or higher the loading resulted in a larger resistance. And it is the same case in the firing temperature. The optimal firing temperature which is also the lowest phase-forming temperature is 800℃for 2h.2) The electrochemical properties of the composite electrode were investigated by impedance spectroscopy. High stability upon thermal cycle was demonstrated for this composite electrode although LSC and SDC have significant difference in thermal expansion. After 20 times of 500-to-800℃thermal cycles and 10 times of room-temperature-to-800℃thermal cycles, no increase in area specific resistance (ASR) was observed for such electrodes. And the long-term stability is also exciting. The resistance of the impregnated electrode kept constant when it was held at 600℃for 72 days. So the LSC impregnated SDC composite electrode had high resistance to thermal shock/cycles and good long-term stability despite significant TEC mismatch exists between LSC catalyst and SDC electrolyte.3) The performance of the cell with the LSC-impregnated SDC as cathode was investigated. With 55wt% LSC loading and firing at 800℃for 2h, the impregnated cathode and single cell showed excellent performance. The cell showed the peak power output of 0.815 W cm^-2 at 650℃after running with a constant voltage output of 0.5V for 89h. After four thermal cycling tests, the cell performance measured at 650℃reduced about 20% mainly due to the delamination between the cathode and Ag current collecting layer.The present study has demonstrated that the LSC-impregnated electrode shows remarkable performance. The high resistance to thermal cycles and thermal shock has been achieved. In addition, very low ASR has been achieved. These results imply that a reliable electrode with high thermal resistance and high performance has beendeveloped for IT-SOFCs. It should be noted that the well-connected SDC frame can be used as a general template which can be impregnated by other electronic conductors (with/witout special microstructures) to achieve more composite electrodes with high performance.In chapter 5, nano-network structured Sm_0.5Sr_0.5CoO_3-δcathodes for low-temperature SOFCs have been successfully fabricated by simply increasing the rate to heat the precursor nitrates deposited from a well-developed ion-impregnation process. The main achievements are summarized as follows:1) The cathodes are consisted of oxide nanowires formed from the nanobeads of less than 50 nm in diameter thus exhibiting large surface area and high porosity, forming straight path for ion and electron conduction, and consequently showing remarkably low interfacial polarization resistances.2) Change in the firing rate has found to be a highly effective approach to the fabrication of high-performance nano-network electrodes for low temperature SOFCs, producing the lowest interfacial polarization resistances (0.21Ωcm² at 500℃and 0.052Ωcm² at 600℃) ever reported for the SSC cathode materials. An anode supported cell with 10-μm-thick SDC electrolyte demonstrated a peak power density of 0.44 Wcm^-2 at 500℃,which is also the highest ever reported for the SSC electrodes.3) Durability test showed that the cathode performance increased with the operating time probably due to the cathode microstructure evolution to higher porosity to optimize the gas diffusion and well-connected SSC nanowires to strengthen ionic and electronic conducting path.Although the long-term stability and formation mechanism of the nano-network electrodes are yet to be further determined, the results indicate a new direction to significantly improve the performance of low temperature SOFCs.