| dc.description.abstract | The thesis tests the cell performance and electrochemical impedance spectroscopy at different operating conditions (temperature, pressure, gas concentration and ratio) using syngas as a fuel in a pressurized solid oxide fuel cell with a planar single cell (50 mm×50 mm) stack and flow distributors. The flow rates are fixed for all experiments, i.e. the anode: 500 ml/min H2 + 400 ml/min N2 and/or 175 ml/min H2 (35%) + 325 ml/min CO (65%) + 400 ml/min N2; the cathode: 900 ml/min air. When the operating temperature increases from 750oC to 850oC, the cell performance at 0.8V increases from 105 mWcm-2 to 129 mWcm-2, a 22.9% increase. EIS data show that the ohmic impedance decreases with increasing T and thus the cell performance increases. When the operating pressure increases from 1 atm to 5 atm, the cell performance has a 64.3% increase. However, we find that the carbon deposition limits the performance increase when the pressure increases from 3 atm to 5 atm due to the carbon deposition at higher pressure. The stability test of the syngas SOFC at 850oC at 0.8V shows that the cell can be stably operated for at least 25 hours at 1 atm without any degradation of the cell power density. But at 3 atm and 0.8V, the cell performance begins to decay after 5 hours of operation, because of the severe carbon deposition that can even block the anode gas pipeline, indicating that the carbon deposition is a problem when syngas is used as a fuel in pressurized environment. At 850oC, we compare cell performance of hydrogen and syngas at different fuel concentrations (60%, 40%, 20% fuel mixing with nitrogen). The results show that the OCV and the power density of hydrogen and syngas SOFCs decrease with increasing dilution. When using syngas, the concentration polarization impedance increases as the fuel dilution increases. At 1 atm and 850oC, we compare cell performance using different CO ratios (i.e. 100% CO, 65% CO + 35% H2, 50% CO + 50% H2, 100% H2). When CO is added to pure hydrogen, the cell performance will decrease with the increase of CO ratio. The cell performance of pure hydrogen is the highest and pure CO is the lowest. EIS data show that as the CO ratio increases, the total polarization impedance increases. It mainly affects the impedance of the low frequency semicircle (gas transfer impedance), which is closely related to the gas transport process. These results help us understanding of syngas SOFC operated at different conditions and associated carbon deposition phenomena which should be useful for future power generation and combination with a micro gas turbine (MGT). | en_US |