| 摘要: | 本論文針對使用甲烷燃料之鈕扣型陽極支撐固態氧化物燃料電池(Solid Oxide Fuel Cell, SOFC)，研究在陽極端燃料加氨對碳沉積之影響效應。我們量測相關電池極化曲線和電化學阻抗頻譜，以及進行120小時穩定性測試。實驗後並使用X光繞射(X-Ray Diffraction, XRD) 、能量色散X射線譜 (Energy Dispersive X-Ray, EDX)及掃描式電子顯微鏡(Scanning Electron Microscope, SEM)，來探討甲烷加氨對碳沉積和電池微結構之影響。實驗條件含三種操作溫度T = 700, 750, 800℃，陰極固定使用200 sccm的空氣，陽極分別獨立通入兩種燃料：(1) 50 sccm CH4 + 50 sccm NH3 + 50 sccm N2和(2) 50 sccm CH4 + 75 sccm H2 + 75 sccm N2。其中(2)加氫燃料僅進行750℃之實驗，目的是與(1)加氨燃料在750℃之結果做對比，因文獻上認為鎳基陽極觸媒(Ni catalyst)在750℃時可100%裂解氨成氫和氮，故可探討氨氣是否完全裂解?以及加氨或加氫對甲烷燃料電池性能是如何地影響?結果顯示：(1)甲烷加氨，在750℃時，可以有效抑制碳沉積，其電池性能於穩定性測試期間(120小時)，在固定800 mAcm^-2電流負載下，可輸出穩定運行約680 mWcm^-2之功率密度。XRD及EDX檢測到的碳元素多寡會受溫度高低及操作時間長短所影響，溫度越高及操作時間越長觀測到之碳元素就越多。由SEM影像，顯示甲烷之碳沉積會破壞陽極微結構，進而使電池壽命明顯劣化，劣化程度於800℃最嚴重，而於750℃無劣化(在120小時穩定性測試期間，電池輸出功率為常數)。當操作溫度750℃和在0.68V條件下，甲烷混氨及甲烷混氫的電池性能分別為767 mW cm^-2和696 mW cm^-2，前者優於後者。這應是因為甲烷及氨氣進行反應時會吸熱，因此推測氨氣並未100%裂解成氫和氮，還有少量氨氣未完全裂解，進而佔據陽極酸性位點，發揮抑制甲烷碳沉積之影響，使甲烷混氨的性能高於甲烷混氫。綜上，所得結果顯示加氨在750℃時，相對於700℃和800℃條件下，對甲烷碳沉積有最佳之改善效果。;This thesis experimentally investigates the carbon deposition problem of a button methane-fueled anode-supported solid oxide fuel cell (SOFC) by using ammonia as a blending fuel. We measure the fuel cell polarization curves and electrochemical impedance spectra; a 120-hour stability test is also performed. Post-experiment analyses are conducted using X-ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDX), and Scanning Electron Microscopy (SEM) to understand the effects of blending ammonia on carbon deposition and the variations of cell microstructures. The experimental conditions included three operating temperatures: 700°C, 750°C, and 800°C, where the cathode is supplied with a constant 200 sccm of air and the anode is independently fed by two types of fuel. (1) 50 sccm CH₄ + 50 sccm NH₃ + 50 sccm N₂ and (2) 50 sccm CH₄ + 75 sccm H₂ + 75 sccm N₂. The second fuel mixture (methane-hydrogen) is tested only at 750°C to compare with the methane-ammonia results at the same temperature, since the literature suggests that the Ni-based anode catalyst can fully decompose ammonia into hydrogen and nitrogen at 750°C. This allowed us to investigate whether ammonia is fully decomposed or not and how ammonia or hydrogen affects the performance of the methane-fueled SOFC. Results show that the blending ammonia at 750°C can effectively suppress the carbon deposition. During the 120-hour stability test, the cell exhibits a stable performance at approximately 680 mW cm⁻² under a constant current load of 800 mA cm⁻². The amount of carbon detected by XRD and EDX increases with increasing temperature and operating time. SEM images reveal that the carbon deposition mainly produced by the methane cracking reaction can deteriorate the anode microstructure, significantly shortening the cell longevity, with the most severe degradation occurring at 800°C, while no degradation is observed at 750°C (the power output remains constant throughout the 120-hour stability test). At 750°C and 0.68 V, the power densities for methane-ammonia and methane-hydrogen mixtures are respectively 767 mW cm⁻² and 696 mW cm⁻². The former mixture has a better power density than the latter. This may be due to the endothermic reaction of methane-ammonia mixture, where ammonia does not 100% decompose into hydrogen and nitrogen, leaving a small amount of ammonia that could occupy Lewis acid sites of the anode and suppress the carbon deposition. In conclusion, the aforesaid results show that blending ammonia at 750°C offers the best improvement in mitigating the carbon deposition problem in comparison with the conditions at 700°C and 800°C. |
