| 摘要: | 本研究實驗量測氨SOFC之電池性能與電化學阻抗頻譜。我們使用自製鈕扣型全電池測試載具於實驗室已建立之雙腔體高壓高溫SOFC實驗平台，分別針對陽極支撐全電池(anode-supported cell, ASC)和電解質支撐全電池(electrolyte-supported cell, ESC)，量測以氨為燃料之ASC和ESC的電池性能曲線(I-V curve)及其電化學阻抗頻譜(electrochemical impedance spectra, EIS)，探討溫度、壓力和加濕效應對ASC和ESC之影響。ASC和ESC使用相同之實驗條件: (1)固定氣體流率，陰極: 100 sccm O2，陽極: 60 sccm H2 + 40 sccm N2或40 sccm NH3 + 20 sccm N2 (為了作H2和NH3之電池性能比較); (2)三個不同操作溫度: 750oC, 800oC, 850oC (溫度效應研究); (3)在每個操作溫度，陽極分別使用乾和加濕(2% H2O)之氨燃料(加濕效應研究); (4) ASC使用氨燃料在800oC和850oC下，分別進行1 atm和3 atm之實驗(加壓效應研究)。結果顯示，在750~850oC條件下，不管是ASC和ESC，使用氨為燃料之電池性能僅略小於使用氫燃料之電池性能，這是因為氨氣於750oC或以上即會先裂解為氫和氮，隨後氫和氧反應成水。因氨於常溫加壓到10大氣壓即為液態，故比氫燃料更具實用性。另外，ASC電池性能明顯優於ESC，氨ASC之功率密度(在0.7V和1 atm)從約370 mW/cm2增加到約700 mW/cm2，當溫度從750oC增加到850oC，相對應之氨ESC則從約175 mW/cm2增加到485 mW/cm2，顯示溫度效應（溫度愈高，性能愈好）。以上電池性能，可由EIS結果來解釋。ASC無論是使用氫氣或氨氣，主要是由低頻之濃度極化所主導，而ESC則由高頻之活化極化所主導，提高溫度可減少活化極化。再者，我們發現加濕，反而會使氨ASC和ESC之電池性能在高負載時下降，溫度越高，下降幅度越多，此乃因過多水分子可能使燃氣在三相邊界中之擴散受到阻礙。最後，無論在800或850oC，3大氣壓之氨ASC電池性能比1大氣壓有明顯地提升，顯示加壓效應會有效地提升電池性能。以上結果，顯示氨SOFC極具應用之價值。
;In this study, we experimentally measure the cell performance and electrochemical impedance spectra (EIS) of ammonia solid oxide fuel cells (SOFCs) using a homemade button-type full cell in an already-established dual-chamber high-pressure and high-temperature SOFC testing platform. Measurements of both anode-supported cell (ASC) and electrolyte-supported cell (ESC) using ammonia as a fuel are carried out to investigate various effects of temperature, pressure, and humidification on the cell performance of ASC and ESC. Both ASC and Esc apply the same experimental conditions: (1) Constant gas flow rates, cathode: 100 sccm O2, anode: 60 sccm H2 + 40 sccm N2 or 40 sccm NH3 + 20 sccm N2 (for cell performance comparison purpose between H2 and NH3); (2) three different operating temperatures: 750oC, 800oC, 850oC (temperature effect); (3) at each operating temperature, both dry and humidified (2% H2O) ammonia fuels are separately used in anode (humidified effect); (4) for the pressure effect, 1 atm and 3 atm of ammonia ASC at both 800oC and 850oC are conducted and compared. Results show that at 750~850oC, both ASC and ESC using ammonia as a fuel have slightly smaller cell performance than that of hydrogen-fuelled ASC and ESC. This is because ammonia decomposes first into H2 and N2 at 750oC or above, then following by H2 oxidation reaction to form H2O. The ammonia fuel gas can be easily become liquid at room temperature and at a pressure equal to 10 atm and it can be easily stored in a pressurized stainless-steel bottle. Therefore, ammonia is much more practical than hydrogen fuel. Further, the cell performance of ASC is obviously better than ESC. When the temperature increases from 750oC to 850oC, the ammonia ASC power densities increase from 370 mW/cm2 to 700 mW/cm2, while the ammonia ESC power densities increase from 175 mW/cm2 to 485 mW/cm2, showing the importance of the temperature effect. The aforesaid cell performance characteristics can be explained by EIS results. Either using hydrogen or ammonia, the dominated polarization of ASC is the low-frequency concentration polarization in the Nyquist plot of EIS data, while ESC is dominated by the high-frequency activation polarization. The latter decreases with increasing temperature. Moreover, it is found that the humidification decreases the cell performance of ammonia ASC and ESC at higher loadings. The higher the temperature, the more decrease of cell performance at higher loads. This is attributed to excess water molecules in the triple-phase boundary making diffusion more difficult to go through. Finally, at 800oC and 850oC conditions, the cell performance of ammonia ASC at 3 atm is clearly higher than that at 1 atm, showing the effect of pressurization on the enhancement of cell performance. These results reveal that pressurized ammonia SOFC has great potential for practical applications deserving further studies. |
