| 摘要: | 本論文使用已建立之加壓型固態氧化物燃料電池(solid oxide fuel cell, SOFC)平板型(50mm*50mm)實驗載具與肋條式流道板，量測合成氣燃料，在不同操作條件(溫度、壓力、燃氣濃度及比例)下，其電池性能(IV-Curve)與電化學阻抗頻譜(EIS)之變化，並進行長時間性能穩定性測試。實驗條件為固定氣體流率(陰極空氣: 900 ml/min; 陽極燃料: 500 ml/min H2 + 400 ml/min N2或175 ml/min H2 (35%) + 325 ml/min CO (65%) + 400 ml/min N2)。當操作溫度從750oC提升至850oC，在固定操作電壓0.8V時，功率密度從105 mWcm-2增加到129 mWcm-2，提升了22.9%，這主要是歸因於溫度上升可有效降低歐姆阻抗，進而提升性能這可由EIS結果看出。使用合成氣在850oC時，壓力從1大氣壓升至5大氣壓時，壓力效應使電池性能增加約64.3%。但是，我們發現在從3大氣壓提升至5大氣壓時，電池性能提升會有所限制，這是因為高壓條件下容易受到碳沉積之影響。在固定電壓0.8V、1大氣壓、850oC下，電池可以穩定操作25小時以上，性能沒有衰退。但在3大氣壓時，電池操作約5小時之後，因碳沉積導致陽極管路嚴重堵塞，最後電池性能大幅衰退。這顯示在高壓環境下，使用合成氣為燃料進行長時間的操作，將會有碳沉積。在850oC條件下，進行不同燃料濃度(60%、40%、20%)的氫氣與合成氣之電池性能比較，發現氫氣與合成氣之OCV和最大功率密度會隨著稀釋度的增加而降低，而當燃料使用合成氣時，隨著燃料稀釋度的增加濃度極化阻抗會增加。合成氣在1atm、850oC，燃氣使用不同CO配比(100% CO、65% CO + 35% H2、50% CO + 50% H2、100% H2)進行電池性能比較，當純氫氣添加CO後，其電池性能會隨CO比例增加而變低，純氫的電池性能最高，而純CO的電池性能最低。從EIS上得知，當CO比例增加，其總極化阻抗會隨之增加，主要是影響低頻半圓之阻抗(氣體轉移阻抗)增加，表示總極化阻抗與氣體輸送過程密切相關。以上之實驗結果，有助於了解合成氣於SOFC在不同操作條件之電池性能變化，以及在不同操作環境之碳沉積現象，這對未來SOFC欲使用合成氣為燃料並結合微氣渦輪機(MGT)，在基礎知識上，有所助益。;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). |
