| 摘要: | 本論文使用鈕扣型固態氧化物燃料電池(Solid Oxide Fuel Cell, SOFC)， 實驗量測探討陰極氧濃度與溫度效應，對甲烷SOFC性能曲線、電化學阻抗頻譜與穩定性之影響。實驗條件含三種操作溫度(T = 750, 800, 850℃)，陽極固定使用50 sccm CH4 + 150 sccm N2，陰極則分別使用五種不同氧氣濃度：(1) 200 sccm Air (21% O2)、(2) 80 sccm O2 + 120 sccm N2 (40% O2)、(3) 120 sccm O2 + 80 sccm N2 (60% O2)、(4) 160 sccm O2 + 40 sccm N2 (80% O2)與(5) 200 sccm O2 (100% O2)五種氧氣濃度。結果顯示，在五種陰極氧濃度條件下，性能皆因溫度增加而增加，從750℃增加到850℃，性能會提高約29-37%，這是因為甲烷裂解反應會隨溫度增加而增強，使得性能有所升高。在不同陰極氧濃度中，陰極為100%氧氣條件表現出最高之性能，隨著氧濃度的減少性能會越低，這是由於氣體中越多氮氣，其會使氧氣傳輸受到阻礙所導致。在三種溫度(T = 750, 800, 850℃)下，陰極為純氧條件較空氣條件之最大功率密度分別提升59.45%、79.65%、60.58%，這是因提高陰極氧氣濃度不但能減少擴散阻抗，更能增加陰極氧分壓，故能達到性能提升之效果。
隨後探討陰極氧氣濃度對電池碳沉積與穩定性之影響，在750℃操作溫度與800 mA cm-2條件下，陽極使用50 sccm CH4 + 150 sccm N2，陰極分別使用200 sccm Air與200 sccm O2進行電池穩定性測試。結果顯示，在陰極使用空氣條件下之電池僅維持13小時，性能就會快速衰退。但是，當陰極使用純氧時，除了性能表現最佳外，並且可持續穩定運作發電，其電池性能在120小時操作時間內每小時僅衰退0.052%。這代表著陰極為純氧能有效延長電池之壽命，主因為陰極氧氣濃度的提升，能使更多的氧離子擴散至陽極表面，與鎳表面生成之碳進行反應，達到除碳之效果。我們針對上述兩種穩定性測試後之電池片進行陽極表面SEM與XRD分析，可以觀察到陰極為氧氣條件較空氣條件所發現之碳明顯較少，且前者微結構幾無可見的破壞。因此，證明陰極使用純氧之甲烷SOFC，可有抑制陽極碳沉積之效果。本研究結果，對未來SOFC應用，除了有助提高性能外，並對含碳燃料之碳沉積問題與電池壽命皆有所助益。
;This thesis investigates experimentally the effect of oxygen concentration in cathode on the cell performance, electrochemical impedance spectroscopy (EIS), and stability of an anode-supported button cell fed by methane (50 CH4 sccm + 150 N2 sccm) as a fuel at three different temperatures (T = 750, 800, 850℃). There are five different oxygen concentrations used in the cathode, respectively. (1) 200 sccm Air (21% O2), (2) 80 sccm O2 + 120 sccm N2 (40% O2), (3) 120 sccm O2 + 80 sccm N2 (60% O2), (4) 160 sccm O2 + 40 sccm N2 (80% O2), and (5) 200 sccm O2 (100% O2). Experiments are conducted in an already- established, high-temperature, high-pressure solid oxide fuel cell (SOFC) testing platform. When T increases from 750℃ to 850℃, the increment percentages of the cell performance increase about 29-37% for all five concentration cases. This is because the rate of methane decomposition increases with increasing temperature, resulting in an increase of the cell performance. The cathode applies 100% oxygen has the largest maximum power density (Pmax, 100%) among all five oxygen concentration cases, while the case of 21% O2 has the lowest maximum power density (Pmax, 21%). The percentage rates of Pmax, 100%/ Pmax, 21% are 59.45%/79.65%/60.58% at T = 750, 800, 850℃, respectively. The performance is lower as the oxygen concentration is reduced because the nitrogen contained in the gas hinders the transportation of oxygen. When pure oxygen is used, the improvement in performance is due to the removal of diffusion losses and the improvement of reaction rates in the cathode given a partial pressure of one for pure oxygen. The stability test of the methane SOFC for the pure oxygen case at 750℃ and 800 mA cm-2 shows that the cell can be stably operated for at least 120 hours with a rather small cell performance decrement of 0.052% per hour, indicating that the carbon deposition can be suppressed by using pure oxygen in cathode. On the other hand, when using air as the cathode gas, the cell can be only operated 13 hours at the same 750℃ and 800 mA cm-2 conditions, where a serious carbon deposition problem is observed. In the pure oxygen case, the cell durability can be extended, because more oxygen ions can diffuse from the electrolyte YSZ interface to the Ni-YSZ anode. Then the oxygen ions can react with carbon and thus remove carbon on the Ni surface. The scanning electron microscope (SEM) images and X-ray diffraction (XRD) show that only a little carbon deposition can be observed at the anode surface without any visible destruction of the anode microstructures for the case of pure oxygen. This substantiates that using pure oxygen can inhibit the anode carbon deposition, in which the cell performance during the stability test remains roughly unchanged. These results are useful to improve the carbon deposition problem and extend the longevity of SOFC. |
