Zirconium Phosphate-Sulfonated Polystyrene Composite Particles and Their Uses in Proton Exchange Membranes

為進一步提升直接甲醇燃料電池(DMFC)與高溫氫氣燃料電池(PEMFC)的效率，近年來有機無機複合質子傳導膜的研究因而蓬勃發展，因其可有效降低質子傳導膜的甲醇滲透率並維持高溫低濕環境下的質子傳導特性。然而，摻混於複合膜內的無機奈米粒子通常具有低質子傳導度及分散不易的缺點；因此，製備具有良好質子傳導度及良好分散特性的無機奈米顆粒實為進一步提升複合膜特性的重要課題。在眾多無機材料當中，磷酸鋯(Zirconium phosphate, ZrP)為一近年來常被研究的添加物，因其具有些許的質子傳導度、良好的化學安定性及合成容易的特性。 為避免磷酸鋯奈米顆粒的聚集並提升其複合膜的質子傳導度，我們嘗試將磷酸鋯以磺酸化聚苯乙烯進行修飾，利用磺酸根以提升質子傳導度，同時以磺酸化聚苯乙烯在水中及鹼性有機溶劑的良好溶解度及本身的立體障礙而幫助奈米粒子的分散。製備方法是將帶有磷酸根的原子轉移自由基聚合(atom transfer radical polymerization, ATRP)起始劑或穩定自由基聚合(nitroxide mediated polymerization, NMP)起始劑和ZrOCl2與磷酸以原位化學法(in-situ method)反應得到起始劑修飾的磷酸鋯，接著以 ATRP或是NMP進行苯乙烯的活性聚合反應得到複合奈米粒子，並將此奈米複合材料進行磺酸化以得到磺酸化聚苯乙烯修飾的磷酸鋯。在本論文中，我們改變反應條件以得到含有不同組成及不同尺寸的起始劑修飾的磷酸鋯，並控制單體之進料比及反應時間以得到不同高分子鏈段長度的複合奈米粒子；這些奈米粒子會因聚苯乙烯分子量的不同而形成核殼(core-shell)結構或脫層(exfoliated)結構。 將磺酸化程度為10 mol%，聚苯乙烯分子量為85kg/mol的核殼結構複合粒子添加於Nafion基材內以形成複合膜，可發現複合膜的吸水率、質子傳導度及離子交換性質(IEC)隨著複合粒子之添加比例的增加有先升後降的趨勢，且隨著添加比例越高則抵擋甲醇穿透的能力越佳。我們發現在複合粒子添加比例為2.5 wt%時，其複合膜表現出最好的選擇性(selectivity)，為Nafion® 117選擇性之3倍。In order to improve the efficiency of direct methanol fuel cells (DMFC) and proton exchange membrane fuel cells (PEMFC), the researches on the organic/inorganic composite proton exchange membranes were dramatically increased recently for the capability to lower the methanol permeability and retain high proton conductivity at high temperature and low relative humidity. However, many of the inorganic additives exhibited low proton conductivity and difficulty in homogeneous dispersion of the nanoparticles within the membrane. Therefore, the preparation of inorganic nanofillers having good proton conductivity and capability of homogeneous dispersion is critical for the improvement of the composite membranes. Among various inorganic fillers, zirconium phosphate (ZrP) has shown good potential due to its moderate proton conductivity, excellent thermal and chemical stability as well as feasible syntheses. To further suppress the aggregation of the zirconium phosphate nanoparticles and to simultaneously enhance the proton conductivity of the corresponding composite membranes, ZrC was modified with sulfonated polystyrene (sPS) to form ZrC-sPS nano-composites since the sulfonic acids in sPS could enhance the proton conductivity and promote the dispersion owing to the good solubility of sPS in water and basic solvents and the steric repulsion of sPS. Atom transfer radical polymerization (ATRP) initiators or nitroxide-mediated polymerization (NMP) initiators decorated with phosphonic acids in-situ reacted with ZrOCl2 and H3PO4 to afford initiator-modified ZrC. Living radical polymerization via NMP or ATRP of styrene was proceeded to afford ZrC-PS; and followed by post-sulfonation to obtain ZrC-sPS. In this thesis, we varied the reaction parameters to obtain the initiator-modified ZrC with different chemical compositions and sizes. We also controlled the feed ratios between the initiator-modified ZrP and the monomers as well as reaction times to obtain ZrC-PS composites carrying different polymer chain lengthes. These ZrC-PS composites showed morphologies in either core-shell particles or exfoliated layer arrangements. Composite membranes containing Nafion® and ZrC-sPS with 10 mol% of degree of sulfonation were prepared via solvent casting. The water uptake, the proton conductivity and the ion exchange capacity of the composite membranes increased first and then decreased with the loading of ZrC-sPS while the methanol permeability was suppressed. The composite membrane with 2.5wt% of ZrC-sPS exhibited the highest selectivity as triple as that of Nafion