新穎高性能燃料電池質子交換膜之研究

Abstract

近年有許多不同種類的質子交換膜(proton exchange membrane)應用於燃料電池(Fuel cells)，其中以非含氟的高分子材料為基材之質子交換膜最被廣泛研究。經由導入高含量磺酸根於高分子，非含氟材料的質子交換膜可以達到高的質子導電度(proton conductivity)，但此方式也同時損耗交換膜的機械性質，並且引起了甲醇穿透(methanol crossover)的問題。此外這些聚集的磺酸根離子也將會造成質子交換膜有過度澎潤甚至溶在甲醇水溶液中。其中交聯結構的應用是最有效率而且簡便的方法來克服這些問題。但是交聯結構的導入通常也會導致質子導電度的犧牲。此外交聯結構往往會造成空間的內縮或是產生親水和疏水相(hydrophilic/hydrophobic)的不相容，因此最終造成相分離的情況產生。所以研究如何改善質子導電度且降低甲醇穿透的問題時，同時也不犧牲交換膜的機械性質和化學穩定性仍然具有相當大的挑戰。 第一篇研究中，我們的目標在於合成新型態具磺酸根的benzoxazine，此單體在聚醚醚酮(sulfonated poly ether ether ketnoe; SPEEK)中不但可視為交聯劑，同時也當成一個連結離子叢集(ionic cluster)的橋樑。分別導入benzoxazine和含磺酸根benzoxazine於聚醚醚酮中，此兩種方式皆可提升聚醚醚酮交換膜應用性。在相近的交聯度，導入含磺酸根的benzoxazine更能達到更高的效能且更適用於燃料電池的應用。 第二篇研究中，我們成功的製備出尿嘧啶(uracil)封端磺化的聚亞醯胺(sulfonated polyimide; SPI)，並且導入以腺嘌呤(adenine)為基質的交聯劑，利用生物識別的方式建構成非共價鍵的交聯結構。末端型態的物理交聯結構有效的限制離子叢集的聚集，產生較均勻親疏水的微相分離。此外腺嘌呤為基質的交聯劑是以疊氮雜環結構(N-heterocycles)組成，在交換膜中能當成質子傳導的中介質，因而在低濕度的情況下更能提高質子跳躍(proton hopping)的能力。而這些特性將有利於交聯型超分子膜應用於甲醇燃料電池中。 在最後，我們成功利用修飾炔丙基官能基(propargyl groups)的黏土(clay)經由點擊化學(click chemistry)，以原位聚合(in situ polymerization)的方式製備出脫層型態(exfoliated)磺化polytriazole奈米複合材料。經由導入少量的黏土於磺化polytriazole中，黏土層達到脫層型態且均勻的分散，因而有效改善交換膜的熱性質、機械性質、甲醇穿透、保水性、離子通道的大小和離子叢集的分散性。導入少量的黏土於磺化polytriazole中展現高的選擇率(selectivity)，也表示此方法有潛能應用於燃料電池質子交換膜。

Among various types of proton exchange membranes (PEMs) for fuel cells, several nonfluorinated polymeric materials are attracting more attention as alternatives to perfluorinated polymer membranes. The nonfluorinated PEMs can achieve high proton conductivities by introducing high extent of sulfonic acid groups, but tend to deteriorate the mechanical strength and permeability of PEMs simultaneously. The aggregation of conductive sites will cause these PEMs highly swollen or dissolved in aqueous/alcoholic solutions. Crosslinking appears to be an efficient and simple approach to overcome these problems, however, it usually leads to a sacrifice in proton conductivity. In addition, microphase tends separation tends to occur in such crosslinking structure due to contraction of space or incompatibility between hydrophilic (sulfonic acid groups) and hydrophobic (crosslinker) components. The development of more efficient membranes with improved proton conductivity and reduced methanol crossover without detrimentally mechanical and chemical stabilities remains an important challenge. (Part 1) Our aim was to synthesize a novel benzoxazine derivative (SBa) that contains sulfonic acid groups to serve as a cross-linker and also a bridge for ionic clusters in SPEEK membrane. The incorporation of benzoxazine (Ba) or sulfonic acid containing benzoxazine (SBa) as a crosslinking agent in SPEEK proton exchange membrane (PEM) can substantially improve the SPEEK membrane performance. The SPEEK-SBa membranes give higher effective selectivity than corresponding SPEEK-Ba membranes under same crosslinker loading and thus are more suitable to be used in direct methanol fuel cells. (Part 2) The object of this work is the preparation of uracil-termminated telechelic SPI (SPI-U), followed by transforming into noncovalent network membranes by biocomplementary hydrogen bonding recognition in the presence of adenine-based crosslinking agent (SMA-A). The physically minimized network structure suppresses the formation of ionic cluster formation and results in better hydrophilic/hydrophobic distribution. Furthermore, the adenine-based crosslinking agent comprising N-heterocycles structure in the PEMs provides better proton conduction medium and promotes proton hopping under low humidity conditions. These favorable properties allow the application of the crosslinked supramolecular membrane in direct methanol fuel cells (DMFCs). (Part 3) Sulfonated polytriazole-clay (SPTA-clay) nanocomposites have been successfully prepared by in situ polymerization of SPTA using click chemistry in the presence of propargyl-functionality modified clay. The clay layers were found to be exfoliated and well dispersed in the SPTA matrix which resulted in improvement of thermal stability, mechanical strength, methanol permeatbility, water retention, ion channel size, and ionic cluster distribution by the incorporation of a small amount of clay (SPTA 1 and 3). The SPTA-clay nanocomposite membranes by incorporating a small amount of clay in SPTA matrix possess higher selectivity defined as ratio of proton conductivity to methanol permeability, therefore, it had potential usage of a proton exchange membrane (PEM) for direct methanol fuel cells (DMFCs).