| 摘要: | 本論文針對合成氣固態氧化物燃料電池(Solid Oxide Fuel Cell, SOFC)添加二氧化碳，以觀察二氧化碳是否可以抑制合成氣(35%H2+65%CO)所產生的碳沉積。碳沉積的發生源自於一氧化碳所進行的歧化反應(Boudouard Reaction)，以及氫氣與一氧化碳進行的還原反應。當合成氣SOFC在較低溫度(T = 500~700oC)操作時，會提高歧化反應發生的機率，導致碳容易形成於電極表面，使陽極阻塞進而影響電化學反應和使電池性能劣化。本研究在陽極合成氣燃料中所添加之二氧化碳，可提高一氧化碳之歧化逆反應發生的次數，達到抑制碳沉積的效果。我們透過已建立之高壓高溫SOFC測試平台，使用鈕扣型陽極支撐電池(Anode-Supported Cell; 530-μm-Ni-YSZ/3-μm-YSZ/15-μm-LSC-GDC)，在不同操作溫度的條件下，於陽極端使用三種燃料：(1) H2 (200 sccm)；(2) H2/CO (70/130 sccm)；(3) H2/CO/CO2 (35/65/100 sccm)，並量測其電池性能、電化學阻抗頻譜。另外，在T = 700oC時使用合成氣、合成氣添加氮氣與合成氣添加二氧化碳進行穩定性測試。結果顯示，電池因碳沉積開始劣化之時間，會隨著添加氮氣和二氧化碳而延長。明確地說，使用合成氣、合成氣添加氮氣與合成氣添加二氧化碳，其相對應之電池性能嚴重劣化前的操作時間，分別為3小時、7小時與25小時，證實添加二氧化碳是個有助延長穩定性的方法。從合成氣與合成氣添加二氧化碳之掃描電子顯微鏡(Scanning Electron Microscope)可以發現，在添加二氧化碳的電池陽極未觀察到明顯的碳沉積，而能量散射X射線譜(Energy Dispersive X-Ray)分析結果顯示，在電池陽極表面碳原子比例(Atomic Ratio, At.%)分別為19.2%(合成氣)與11.5%(合成氣添加二氧化碳)。本論文有兩個結論：(1)當在較低溫度操作時(T ? 700oC)，在陽極合成氣燃料中添加二氧化碳可抑制碳沉積的形成；(2)雖然添加二氧化碳可抑制碳的形成，但少量的碳沉積仍會造成陽極表面鎳微結構的損壞，顯示以鎳基為陽極觸媒之SOFC，若使用合成氣為燃料將無法以添加二氧化碳來完全消除碳沉積。以上實驗結果，應對了解合成氣SOFC之碳沉積問題有所助益。;In this thesis, carbon dioxide is doped into a syngas solid oxide fuel cell (SOFC) to investigate whether carbon dioxide can inhibit the carbon deposition produced by syngas (35%H2+65%CO). Carbon deposition originates from the Boudouard reaction of carbon monoxide and the reduction of hydrogen and carbon monoxide. When the syngas SOFC is operated at lower temperature (T = 500~700oC), the probability of the occurrence of Boudouard reaction is increased, causing carbon to form easily on the electrode surface, blocking the anode and affecting the electrochemical reaction, and resulting in the cell performance degradation. This study adds carbon dioxide into the anode syngas fuel to increase the occurrence probability of the reverse Boudouard reaction of carbon dioxide in attempt to inhibit carbon deposition. The performance and electrochemical impedance spectra of a button-type anode supported cell (Anode-Supported Cell; 530-μm-Ni-YSZ/3-μm-YSZ/15-μm-LSC-GDC) are measured using an established high-pressure and high-temperature test platform under different operating temperature conditions. Three fuels are used in the anode: (1) H2 (200 sccm); (2) H2/CO (70/130 sccm); (3) H2/CO/CO2 (35/65/100 sccm). In addition, the stability tests are conducted using syngas, syngas with nitrogen, and syngas with carbon dioxide at T = 700oC. Results show that the deterioration time of cell performance due to carbon deposition can be extended by adding nitrogen and carbon dioxide. Specifically, using syngas, syngas with nitrogen, and syngas with carbon dioxide, the corresponding operating times before the occurrence of severe degradation of cell performance are 3 hours, 7 hours and 25 hours, respectively. This confirms that the addition of carbon dioxide is a beneficial way to extend the cell stability. From the SEM (Scanning Electron Microscope) images of using syngas and syngas with carbon dioxide, no significant carbon deposition is observed on the anode of the cell with the addition of carbon dioxide. However, the EDX (Energy Dispersive X-Ray) analysis shows that the atomic ratio (At.%) of carbon on the anode surface of the cell is 19.2% (syngas) and 11.5% (syngas with carbon dioxide), respectively. There are two conclusions: (1) Adding carbon dioxide to the anode syngas fuel is a useful method to inhibit carbon deposition, when the operating temperature of syngas SOFC is 700oC or lower. (2) Although the addition of carbon dioxide can inhibit the formation of carbon, even a small amount of carbon deposition can still cause some damage to the nickel microstructures, resulting in the degradation of cell performance. This suggests that the carbon deposition of syngas SOFC using the nickel-based anode cannot be completely eliminated by adding carbon dioxide. These aforesaid experimental results should be useful to the understanding of carbon deposition problem of syngas SOFC. |
