| 摘要: | 質子交換薄膜燃料電池在高溫下操作可以避開一氧化碳毒化觸媒的問題、提高電池的化學反應速率⋯⋯等優點｡但目前已商業化的PEMFC薄膜是基於聚全氟磺酸的材料所製備而成，例如DuPont公司的Nafion系列。而高溫下操作時，水分子容易從Nafion膜中蒸發，導致離子電導率的嚴重惡化無法提供令人滿意的性能。
磷酸摻雜的polybenzimidazole(PBI)在高溫下成為高分子電解質薄膜常用的選擇，主要是因為PBI具有良好的磷酸摻雜能力，在高溫操作下仍展現高的質子導電度｡然而在非常高的磷酸摻雜水平下，PBI膜容易膨脹造成機械性能下降。此外PBI 單價昂貴也造成廣泛使用的阻礙。開發具有高質子導電度、高化學穩定性同時不損失其機械完整性的高溫燃料電池薄膜成為亟待突破的材料科學研究議題。
為改善PBI機械性能下降的情況，本研究中使用高穩定性之磺酸化聚砜(sPSU)以及易摻雜磷酸之聚乙烯亞胺(PEI)進行複合。PEI是一種價格低廉、毒性低且帶有支狀結構的高分子聚合物，主鏈上含有大量的仲氨基和伯氨基以及叔氨基，可以通過產生氫鍵或酸鹼相互作用使磷酸附著且不容易脫離。除此之外加入官能基化磷酸鋯(ZrP)奈米片，形成有機/無機複合薄膜，磷酸鋯本身具有良好的物理特性，且為層狀結構，在同一平面上的原子以牢固的共價键相结合，而相鄰的層之間存在非共價相互作用，如凡德瓦爾力和静電作用力，使其層間距可以藉由修飾官能基的方式，加以調整；磷酸鋯本身也是Brønsted acid，可以提供質子，產生質子交換反應，應用於高溫質子交換薄膜時，可以提供新的質子傳遞的通道，以維持高導電度。在本研究中藉由官能基化，在磷酸鋯層間修飾了偶聯劑(MPTMS)，MPTMS在鹼性環境中會進行水解反應，使之間產生Si-O共價交聯，成功改善了無機物在高分子中的相容性以及分散性，突破傳統有機/無機複合薄膜常見的不均勻性及相分離的問題，希望可以在高溫/低濕度下提高膜質子傳導率同時改善了薄膜的穩定性。
在本研究中顯示PEI隨著添加的比例增加，可以與sPSU形成較完整的交聯結構，因此複合高分子薄膜不僅展現出較好的熱穩定性外，還可以有效的提高薄膜的磷酸摻雜以及保留能力，使得這些薄膜在高溫下，還得以維持導電度，其中含有40 wt%的PEI的高溫質子導電度表現最好，在160 ºC時為1.2*10-1 S/cm，比含有50 wt%PEI的薄膜提升了9*10-2 S/cm；添加ZrP-SH由於偶聯劑(MPTMS)與高分子之間的作用力，會使薄膜的結構變得更加緻密，使磷酸摻雜量下降，進而影響薄膜的質子導電度，但是當ZrP-SH添加量>5wt%時，質子導電度會有些許提升，這是因為ZrP本身具有質子傳遞的能力，透過官能基化，可以有效改善其分散性，在缺乏磷酸的情況下，能夠與高分子之間建構新的質子傳遞通道，提高其性能；氧穩定度方面，雖然透過交聯反應可以有所改善，但依然無法適用於高溫質子交換膜燃料電池當中，為了改善這項缺失，添加ZrP-SH，透過偶聯劑(MPTMS)之間的Si-O共價交聯，建構出更穩定的結構，且MPTMS其末端官能基-SH
經過氧化能夠產生SO3H基團，提高與sPSU/PEI之間的氫鍵作用力，進而提升薄膜的抗氧化性。
;Proton exchange membrane fuel cells operating at high temperatures can avoid the problems of carbon monoxide poisoning catalysts, increase the chemical reaction rate of batteries, etc. However, current commercial PEMFC membranes are prepared based on polyperfluorosulfonic acid materials, for example, the Nafion series of DuPont. But at high temperatures, water molecules tend to evaporate from the Nafion membrane, causing severe deterioration in ionic conductivity that does not provide satisfactory performance.
Phosphoric acid-doped polybenzimidazole (PBI) is a common choice for polymer electrolyte membranes at high temperatures because PBI has good phosphoric acid doping ability can exhibit high proton conductivity at high temperatures. However, in very high phosphoric acid, the PBI membrane will decrease the mechanical properties. In addition, the high price of PBI also causes obstacles to widespread use.
The development of high-temperature fuel cell membranes with high proton conductivity and high chemical stability without loss of mechanical integrity has become a material science research issue to be overcome.
In this study, it is shown that the PEI can form a relatively complete crosslinked structure with sPSU with the increase of the proportion of addition. Therefore, the composite polymer film not only exhibits better thermal stability but also can effectively improve the phosphoric acid doping of the membrane. In addition to retention ability, these membranes can maintain conductivity at high temperatures. Among these membranes, a membrane with 40 wt% PEI has the best high-temperature proton conductivity, 1.2*10-1 S/cm at 160 °C. Incresing PEI content to 50 wt% PEI decreased its proton conductivity by 9*10-2 S/cm.
Furthermore, addition of ZrP-SH causes the structure of the membrane to become denser due to the interaction between the coupling agent (MPTMS) and the polymer, so that the phosphoric acid doped amount is decreases, which in turn affects the proton conductivity of the membrane, but when the amount of ZrP-SH added >5wt%, the proton conductivity will increase slightly. This is because ZrP has the ability to proton transfer, which can be effectively improved by functionalization. In the absence of phosphoric acid, ZrP-SH can establish a new proton transfer channel with the polymer to improve its performance. |
