| 摘要: | 本論文對加壓型合成氣固態氧化物燃料電池(Solid Oxide Fuel Cell, SOFC)添加氨氣，以觀察氨氣是否可對合成氣(35% H2 + 65% CO) SOFC的碳沉積有抑制效果？碳沉積的發生主要是來自於合成氣的一氧化碳所進行歧化反應(Boudouard reaction)，而此反應是放熱反應，故在SOFC相對低溫(500~700oC)環境時合成氣容易發生碳沉積。由過去本實驗室的研究，顯示使用合成氣在操作溫度(T)為750oC和操作壓力(p)為1 atm的條件下，SOFC可穩定操作25小時，並無任何性能衰退的現象。但當在T = 750oC，把操作壓力提升至3 atm時，合成氣SOFC卻只能穩定操作10小時，之後產生嚴重碳沉積，造成電池性能劣化。當p = 1 atm，把操作溫度降低至700oC時，合成氣SOFC僅能穩定操作7個小時，之後即有碳沉積問題之產生。本研究針對SOFC的陽極合成氣燃料中以添加氨氣方式，讓氨可優先佔據鎳陽極觸媒的酸性位點，證實可減少陽極碳沉積形成的速率。本研究使用已建立之雙腔體高溫、高壓爐與電池性能量測平台(含電化學阻抗頻譜量測)，在不同T與p的條件下量測添加氨的合成氣鈕扣型陽極支撐電池(Anode-Supported Cell, ASC; 530-μm-Ni-YSZ/3- μm-YSZ/15-μm-LSC-GDC)之電池性能、電化學阻抗頻譜與穩定性。結果顯示，在T = 700oC和p = 1 atm的操作條件下ASC可穩定運作28小時無碳沉積，相較於過去研究，加氨可使穩定操作時間增長。 尤其是在T = 650oC和p = 1 atm時，ASC可以穩定運作長達40個小時，無任何性能衰退的現象。這是因為氨氣的裂解率(裂解出來的產物為氫氣和氮氣)會隨著操作溫度的降低而下降，而在較低溫環境下未經裂解之氨氣的含量比較多，使其足夠用於抑制碳沉積形成的速率。在T = 650oC的操作溫度下，提升操作壓力(p = 3 atm)有助於提升加氨之合成氣ASC的電池性能，可穩定操作約32小時，但之後會產生碳沉積使ASC的性能大幅的衰退。從SEM (Scanning Electron Microscope)與EDX (Energy Dispersive X-Ray)分析發現，在穩定性測試條件為T = 650oC和p = 1 atm、T = 700oC和p = 1 atm以及T = 650oC和p = 3 atm之電池陽極表面的碳原子比例(atomic ratio, At. %)分別為16.08 %、87.23 %以及68.53 %。最後，本論文有兩結論：(1)合成氣加氨有助於減少碳沉積的形成，尤其是在常壓和T = 650oC條件下，加氨抑制碳沉積的效果最為顯著；(2)在高壓操作環境條件不利加氨抑制碳沉積之效果，即使在T = 650oC，顯示以鎳基為陽極觸媒之合成氣SOFC不易於加壓操作條件下進行長時間發電之運轉。以上實驗結果，對合成氣SOFC之碳沉積問題之瞭解應有所助益。;In this study, ammonia is doped into a pressurized syngas-fueled (35% H2 + 65% CO) solid oxide fuel cell (SOFC). The objective is to explore whether can ammonia inhibit the carbon deposition of syngas SOFC? The carbon deposition can be formed when the carbon monoxide in the syngas undergoing the Boudouard reaction that is an exothermic reaction which is prone to occur at low temperature (500~700oC) condition of SOFC. Our previous study showed that the syngas-fueled SOFC can operated stably 25 hour at the operating temperature (T) of 750oC and operating pressure (p) of 1atm, without any carbon deposition. However, at T = 750oC, syngas-fueled SOFC can only operate 10 hour when p increases to 3 atm, then the occurrence of severe carbon deposition results in deterioration of cell performances. When p = 1 atm, syngas-fueled SOFC can only operate 7 hour when T decreased to 700oC, then severe carbon deposition was occurred. In this study, ammonia was doped to the anode syngas fuel, proved that the ammonia can preferentially occupy the acid sites of Ni-anode catalyst resulting in the decreased of the coke formation. Experiments were conducted in an already established high- temperature and high-pressure dual-chamber SOFC facility together with cell performance measuring equipment (including electrochemical impedance spectroscopy). We measured the cell performance, impedance spectra, and durability of the syngas-doped-ammonia-fueled button anode-supported cell (ASC; 530 μm Ni-YSZ/3 μm YSZ/15μm LSC-GDC) under different T and p conditions. Results show that at T = 700oC and p = 1 atm, the syngas-doped-ammonia-fueled ASC can operate stably for 28 hours, without any carbon deposition, also the operating time was longer than the previous study. It was found that the ASC can operate up to 40 hours at T = 650oC and p = 1 atm, without any degradation. This is because the decomposition rate of ammonia to hydrogen and nitrogen will decrease with decreasing the operating temperature, so that there was more residual ammonia left at lower operating temperature (650oC), which was sufficient to occupy the acid sites of Ni-anode catalyst for being able to inhibit the carbon deposition. At T = 650oC, the cell performance of syngas-doped-ammonia-fueled ASC was increased with elevating the operating pressure (p = 3 atm). However, the ASC became vulnerable to carbon deposition after 32 hours operation, resulting in the degradation of cell performance. Scanning electron microscope (SEM) and energy dispersive X-Ray (EDX) analysis showed the carbon atomic ratio (At. %) of the cells’ anode surface after treated at T = 650oC and p = 1 atm, T = 700oC and p = 1 atm, and T = 650oC and p = 3 atm were 16.08 %, 87.23 %, and 68.53 % respectively. In the end, this thesis obtains two conclusions: (1) the doping of ammonia to syngas fuel promotes the reduction of carbon deposition, especially at atmospheric pressure and at T = 650oC conditions, the effect of doped ammonia to the inhibition of carbon deposition was more significant. (2) The higher operating pressure condition was not conducive to the suppression of carbon deposition by doped ammonia, even at T = 650oC, showed that syngas-fueled SOFC which using nickel base as anode catalyst was difficult to work for a long period under high pressure conditions. |
