| 摘要: | 本研究針對平板式陽極支撐(400-μm-Ni-YSZ/3-μm-YSZ/12-μm-LSC-GDC; 5 x 5 cm2)固態氧化物燃料電池(Solid Oxide Fuel Cell, SOFC)在陽極氨燃料中添加氫氣，以測試添加氫氣是否對氮化鎳的還原反應有效果？氮化鎳的發生，主要是因為氨氣在SOFC一般操作溫度下會以兩步驟進行反應，先裂解成氫氣和氮氣，再由氫氣進行反應產電。然而，氨氣在裂解成氫氣和氮氣時是吸熱反應，這會導致電池的操作溫度降低，在操作溫度低於750oC時會造成氨氣未裂解完全，進而與陽極的鎳反應生成氮化鎳，使陽極鎳觸媒有劣化的問題。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)，在不同操作溫度(T)及壓力(p)下，量測陽極三種燃料: (1)H2/N2(540/360 sccm); (2)NH3/N2(360/180 sccm); (3) NH3/H2/N2(90/300/210 sccm)於平板型固態氧化物燃料電池的電池性能、電化學阻抗頻譜與熱循環測試(Thermal Cycle Test, TCT)，其中陰極為空氣(900 sccm)。在1大氣壓和800oC的操作溫度下，三種燃料的性能幾乎沒有差別，這是因為氨氣會在T ? 750oC時完全裂解。此外，不管在任何操作溫度下提升操作壓力皆會使OCV及電池性能上升。因為提高操作壓力會促進多孔電極中的氣體擴散及電極觸媒表面的反應物吸附速率，進而加快電化學反應速率。有關TCT，本實驗將溫度範圍設在600oC~700oC之間，以4個步驟為一循環，分別為(1)維持700oC; (2)降溫至600oC; (3)維持600oC; (4)升溫至700oC，每個步驟時間均設定為1小時，一次TCT實驗共含6個循環，總共24小時，並在每個循環間測試一組電池性能及其電化學阻抗頻譜。結果顯示，隨著循環次數的增加，電池的性能會緩慢的下降，6個循環24小時後，1大氣壓NH3/N2的性能下降24%、1大氣壓NH3/H2/N2的性能下降為10%; 而在3大氣壓時，NH3/H2/N2性能僅下降5%，顯示高壓條件下可促進氧化還原氮化鎳，使電池性能穩定性得以延長。上述結果，顯示在氨氣中添加氫氣可以有效地還原氮化鎳，特別是在高操作壓力條件下，且從電化學阻抗頻譜也得到相同的結果。本研究成果，對使用氨為燃料之SOFC在低溫操作(600 ~ 700 oC)條件時，應有所助益。;This thesis investigates the effect of doping hydrogen into ammonia as a fuel on the planar anode-supported solid oxide fuel cell (SOFC; 400-μm-Ni-YSZ/3-μm-YSZ/12-μm-LSC-GDC; 50 x 50 cm2) to see whether it is effective for the reduction reaction of nickel nitride. The occurrence of nickel nitride is mainly due to the two-step reaction of ammonia in SOFC. First, ammonia decomposes into hydrogen and nitrogen, and then hydrogen oxidation takes pace to generate electricity. However, the decomposition reaction of ammonia is an endothermic reaction, which reduces the temperature of the cell and ammonia can mot be decomposed completely when the operating temperature is less than T < 750oC. Then the remaining ammonia can react with the anode nickel to produce nickel nitride, resulting in degradation of the anode nickel catalyst. We perform measurements of the cell performance, electrochemical impedance spectra (EIS), and thermal cycle test (TCT) of three anode fuels: ((1) H2/N2(540/360 sccm), (2) NH3/N2(360/180 sccm), and (3) NH3/H2/N2(90/300/210 sccm)) on planar SOFC under different operating temperature (T) and pressure (p) conditions via our established dual-chamber high-temperature and high-pressure SOFC testing platform (including measurement of electrochemical impedance spectra measurements). At p = 1 atm and T = 800oC, there is almost no difference of the cell performance for these three cases. This is because ammonia is completely decomposed to H2 and N2 when T ? 750°C. Also, increasing p decomposed increases the open-circuit voltage and cell performance at any given T, owing to the increase of p can promote the gas diffusion in the porous electrode and the adsorption rate of reactants on the electrode catalyst surface, leading to acceleration of the electrochemical reaction rate. As to TCT, the temperature range is set at T = 600oC and T = 700oC by using 4 steps per cycle: (1) maintaining at T = 700oC; (2) cooling to T = 600oC; (3) maintaining at T = 600oC; (4) heating to T = 700oC, each step having 1 hour. As such, one TCT experiment with six cycles has a total of 24 hours; the cell performance and electrochemical impedance spectra are measured for each of six cycles. Results show that as the number of cycles increases, the cell performance slowly decreases. After 6 cycles (24 hours), the cell performance of NH3/N2 decreases by 24% at p = 1 atm, but the cell performance of NH3/H2/N2 decreases by only 10%. Moreover, when p increases to 3 atm, the decremental percentage of cell performance for the case of NH3/H2/N2 reduces to 5%. These measurements suggests that doping hydrogen into ammonia in anode can significantly reduce the nickel nitride, especially at elevated pressure condition, as to be explained by EIS date. Finally, the aforesaid results are useful for the understanding of ammonia SOFC using Ni-YSZ anode when it is operated at low temperature ranges (600 ~ 700 oC). |
