| 摘要: | 本研究使用實驗室已建立的固態氧化物燃料電池(solid oxide fuel cell, SOFC)測試實驗平台，探討在不同實驗條件下，添加氨氣至合成氣燃料中，對鈕扣型陽極支撐電池的性能曲線(I-V curve)、電化學阻抗頻譜(electrochemical impedance spectra, EIS)以及性能穩定性的影響。在I-V curve和EIS測量方面，於三個不同的操作溫度(650°C、700°C、750°C)下進行，共使用了五種不同的燃料比例(單位為每分鐘毫升，sccm)，分別為(1) 200 H2、(2) 70 H2+130 CO、(3) 56 H2+104 CO+20 NH3、(4) 47 H2+86 CO+34 NH3以及(5) 35 H2+65 CO+50 NH3，陰極則皆通以200 sccm的空氣。結果顯示，這五種燃料的性能皆會隨溫度上升而提高。其中，純氫的性能最好，而H2/CO和H2/CO/NH3則在不同溫度下，有不同的性能表現。在650°C時，H2/CO的性能會高於H2/CO/NH3，但在750°C時，則是H2/CO/NH3的性能會比H2/CO高。這是因為在低溫時(650°C)，氨氣的裂解率會下降，故造成性能較大幅度地降低，反之在溫度為750°C時，氨氣的裂解率幾近100%。
在電池穩定性測試方面，於650°C、750°C下進行，並固定電流負載為350 mA cm-2，對陽極通以H2/CO (35/65 sccm)和H2/CO/NH3 (35/65/50 sccm)為燃料，測試電池性能可穩定運作的時間。結果顯示，在兩個溫度下，以合成氣加氨(H2/CO/NH3)為燃料的電池皆能穩定運作至少120小時，其中在650°C時，電池性能完全沒下降，而在750°C時，性能也僅有3.22%的降幅。相反地，以合成氣(H2/CO)為燃料時，在650°C、750°C下，電池僅分別運行了2小時和20.5小時後，其電壓便掉至0.2 V以下。添加氨氣之所以能抑制碳沉積，是因為NH3分子上具有孤對電子(lone pair of electrons)，其相較於CO分子，更容易與Ni觸媒上的酸性位點結合，進而減少碳沉積；這也解釋了在650°C時，因氨裂解率較低，有足夠的氨與Ni的酸性位點結合，使得電池性能沒有下降。
以上結果顯示，當操作溫度為650°C，氨氣的添加對於合成氣SOFC在抑制碳沉積的方面，有良好的效果，對於推廣合成氣SOFC的實際應用應有重要的助益。
;This thesis investigates the effects of adding ammonia to the syngas fuel on the performance, electrochemical impedance spectra (EIS), and operational stability of the button anode-supported solid oxide fuel cells (SOFCs) under different experimental conditions by using the SOFC test platform established in our laboratory. Current-voltage curves and EIS are measured under three different operating temperatures (650°C, 700°C, 750°C) using five different fuels: (1) 200 H2, (2) 70 H2 + 130 CO, (3) 56 H2 + 104 CO + 20 NH3, (4) 47 H2 + 86 CO + 34 NH3, and (5) 35 H2 + 65 CO + 50 NH3 (units in standard cubic centimeter per minute, sccm). The cathode is supplied with 200-sccm air for all five cases. Results show that the cell performance increases with increasing temperature. As expected, the hydrogen-fueled SOFC has the best performance. At 650°C, the performance of the syngas-fueled SOFC is higher than the syngas-ammonia-fueled SOFC, but at 750°C the performance of the syngas-ammonia-fueled SOFC is higher than the syngas-fueled SOFC. This is because the ammonia decomposition rate decreases at low temperature (650°C), resulting in a significant reduction in performance. On the other hand, the ammonia decomposition rate at 750°C is almost 100%.
The stability tests of the cells are conducted at both 650°C and 750°C where the current density is fixed at 350 mA cm-2 using H2/CO (35/65 sccm) and H2/CO/NH3 (35/65/50 sccm) as the fuels. Results show that the cells fueled by syngas and ammonia (H2/CO/NH3) can operate stably for at least 120 hours at both temperatures. At 650°C, the H2/CO/NH3 cell performance does not decrease at all, while at 750°C, there is only a 3.22% decrease in the cell performance after 120-hour operation. Conversely, when using syngas (H2/CO) as the fuel, the cell voltages drop below 0.2 V after only 2 hours and 20.5 hours at 650°C and 750°C, respectively. The reason why the addition of ammonia can inhibit carbon deposition is because NH3 molecule has a lone pair of electrons. Compared with CO molecule, it is easier to occupy the acidic sites of the Ni catalyst and then reduce carbon deposition. This also explains that the cell performance in the stability test does not decrease owing to sufficient ammonia molecules occupying the acidic sites of Ni catalyst under the low ammonia decomposition rate at 650°C.
The aforementioned results show that the addition of ammonia has a positive effect on suppressing carbon deposition in syngas SOFC when it is operated at low temperature (650°C), and it should be useful for the promotion of the practical application of syngas SOFCs. |
