| dc.description.abstract | 質子交換膜是甲醇燃料電池中的關鍵元件之一，其主要功能是做為固態電解質並傳遞質子。質子交換膜結構中的親/疏水(Hydrophilic/Hydrophobic)特性構建出親水性的水通道(Water channel)，在此水通道之中，質子以水合離子(Hydronium ion)的型態進行擴散(Self diffusion)傳導或者質子在氫鍵網絡中以接續式(Relay)的方式通過薄膜而達到傳遞質子的目的。甲醇分子和水分子的極性相接近，甲醇分子也會經由水通道穿過薄膜，導致薄膜有嚴重的甲醇滲透現象(Methanol cross-over)發生。由此可知，薄膜的親水性孔洞尺寸型態、通道連通性將影響質子、水分子的動態行為以及甲醇分子的滲透，進而影響最終薄膜的導電度和甲醇滲透率。
本研究經由製備不同孔隙度、不同官能基化程度和不同微相區紋理(Morphology)的高分子質子交換膜材料，深入討論質子、水分子的動態行為、甲醇分子的滲透及開發新的傳導機制。其目標在於製備恰當的微相結構型態開發適用於甲醇燃料電池和高溫低濕下操作的新穎薄膜材料。因此本研究薄膜材料的製作方式採取(1) 半互穿網路聚合物結構(semi-Interpenetrating Polymer Network, semi-IPN)薄膜 (2) 有機/無機奈米結構質子傳導膜方式製作，並將製備出來的薄膜材料利用電子顯微鏡(Electron microscopy)、交流阻抗分析儀(AC impedance)和固態核磁共振(Solid state NMR)等重要檢測方法來進行微結構型態、質子導電度、水分子動態行為以及甲醇滲透率等重要性質分析與討論。
在semi-IPN系統中運用具高支鏈結構(Hyper branch architecture)雙馬來醯亞胺寡合物(Bismaleimide oligomer) (mBMI)作為結構基質，穿插入於磺酸化聚二醚酮(sulfonated Poly ether ether ketone, sPEEK)高分子內，築構成半互穿網路聚合物結構。實驗中使用mBMI(30) [BMI：mBMI = 70 mol%：30 mol%]和mBMI(98) [BMI：mBMI = 2 mol%：98 mol%]兩種不同溶液製備sPEEK-IPN薄膜。溶液中BMI單體數量的多寡影響semi-IPN結構的交聯程度，所以mBMI(30)與sPEEK高分子形成緻密的semi-IPN結構 (Dense semi-IPN structure)，而sPEEK/mBMI(98)薄膜則是呈現比較鬆散的semi-IPN結構 (Loose semi-IPN)。結果顯示，此物理交聯方式不僅增加薄膜的抗膨潤能力和降低甲醇滲透率之外，薄膜在室溫下的導電度還仍維持在10-2 S/cm以上，其中sPEEK/15% mBMI(98)薄膜具有最高的C/P比率(Conductivity/Permeability ratio)。另一方面，物理交聯改變了薄膜親水性通道的型態，進而影響質子和水分子在水通道內的運動性。相較於sPEEK薄膜，Semi-IPN薄膜在高含水量狀態時，因為交聯效應而降低了水通道的尺寸，使得質子和水分子在水通道內的運動性下降。我們也發現交聯程度亦會影響親水性通道的尺寸，緻密semi-IPN(Dense semi-IPN)薄膜因為其交聯程度高，水通道的尺寸變的更窄，所以質子和水分子在緻密semi-IPN薄膜中的運動性會低於在鬆散semi-IPN(Loose semi-IPN)薄膜。因此對於semi-IPN薄膜而言，薄膜的導電度受到離子交換容量(IEC)影響，但是semi-IPN薄膜的親水性通道型態亦會減低最終的導電度。薄膜在低含水量狀態時，磺酸根和水分子會在孔洞表面構建出一連續性的氫鍵網絡，所以質子主要的傳遞路徑就在孔洞表面所構建出來的氫鍵網絡中做接力賽的傳遞。特別在鬆散semi-IPN薄膜中，mBMI結構的存在提供了另一個氫鍵網絡幫助質子傳導，但在緻密semi-IPN薄膜中卻未發現此現象。
在第二個部分的sPEEK/表面鹼基化二氧化矽(SiO2)複合膜中，是原位聚合(In-Situ Polymerization)的方法製備出有機/無機之質子交換薄膜，亦即在sPEEK高分子溶液中聚合生長出具有鹼性官能基的奈米添加物無機奈米粒子，並藉由選用不同鹼度的鹼性官能基矽烷(Amine, Pyrrolidinone 和 Imidazole)以及鹼基數量，可製備並調控出不同顆粒大小的奈米粒子。結果顯示，具有鹼性官能基的無機奈米粒子均勻分散後，其表面鹼性官能基的修飾，可增加與酸性高分子間的作用力，而抑制薄膜在甲醇水溶液中的過度膨潤現象。較強的鹼性官能基(如:Imidazole)雖可大幅降低薄膜的膨潤性和甲醇滲透率，但也因為鹼性太強而與sPEEK的磺酸根產生酸/鹼中和現象，導致薄膜導電度下降至4×10-3 S/cm；最佳的薄膜為使用弱鹼的50phr PyT-H薄膜( Py silane:TEOS=4:1)其甲醇滲透率不僅大幅下降，而且導電度還維持在2×10-2 S/cm以上， C/P比率也是全部薄膜之中最高的。酸/鹼作用力影響的另一個層面為親水性通道的型態尺寸，研究顯示，水通道的尺寸受到鹼性官能基矽烷的鹼度和鹼性官能基數量兩者所影響，亦即較強的鹼性官能基或較多的官能基數量皆會增強與sPEEK之間的酸/鹼作用力，降低水通道尺寸，導致水通道內的質子和水分子的運動性下降。研究中也同時發現因為水通道變窄使得甲醇滲透率也降低。
一般酸性電解質高分子薄膜(如:sPEEK)在高溫下因為容易脫水(Dehydration)而導致質子導電度下降，增加電池中的薄膜內電阻。而薄膜中添加具質子導電性的無機奈米粒子除了可以增加保水性之外，亦可增加薄膜的質子導電度。因此本研究第三個部分是將二氧化鈦奈米管(TiO2 nano tube, TNT)表面做磺酸官能基修飾，再與sPEEK高分子進行混摻，形成有機/無機奈米複合薄膜。由於具有酸根官能基的奈米粒子會與質子傳導高分子作用，形成高度相容之均質體，無機奈米粒子可高度均勻分散於高分子薄膜中因而輕易解決傳統技術中的相分離問題。結果顯示，薄膜中添加磺酸官能基化的二氧化鈦奈米管(sTNT)後，薄膜在高溫下的保水能力以及質子導電度皆大幅提升。其中，sPEEK/5% sTNT複合薄膜在100℃、60% 相對濕度狀態下，質子導電度高達10-2 S/cm以上。在微結構型態(Morphology)和水分子動態行為研究中，發現複合薄膜在低含水量狀態時，水分子不只分佈於sPEEK的磺酸根上，水分子亦在sTNT表面而且水分子在管子表面做快速的運動，代表sTNT固體酸表面提供了連續的質子暫存之官能基(如氫鍵)形成有效的接力式之質子傳遞路徑。在低濕度下sPEEK/sTNT薄膜展現極高的質子導電度和低質子傳遞活化能(Activation energy, Eaσ)。
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| dc.description.abstract | Proton exchange membrane is a key component for direct methanol fuel cell (DMFC), as it serves to transport protons between the two electrodes. As widely accepted, proton conduction is achieved by the relay of charged ion via hydrogen bonding network, or by the self diffusion of the hydronium within the confined water channel and usually water-swollen membrane. On the other hand, the methanol can permeate through the water channels due to the polarity of methanol is similar to water. Hence water channel morphology, proton, water and methanol transport properties dictate the proton conductivity and methanol permeability of fuel cell membranes.
In present study, the proton exchange membranes with different pore size, surface functional group and morphology were prepared. This proposal disclose the design of series of proton exchange membrane bearing suitable phase separated nano-structures arranged in long range order as the plausible candidate for novel proton conducting membrane to operate in DMFC and high temperature fuel cell. Two designs with nano-structures are explored: (1) semi-Interpenetrating Polymer Network, semi-IPN (2) Organic/inorganic composite with long range ordered inorganic nano domain. The electron microscopy, AC impedance and solid state NMR methods were used in this research, in order to understand the relationships between channel morphology, ion/molecular dynamics and proton conductivity.
By using an in-situ polymerization scheme, bismaleimide oligomers (mBMI) and bismaleimide (BMI) monomers forms interpenetrating network structure which encapsulated sulfonated poly ether ether ketone (sPEEK) polymer matrix. Different curing conditions on the two sPEEK samples containing BMI monomer and mBMI mole ratios of 70:30 (mBMI(30)) and 2:98 (mBMI(98)) are present. As the amount of BMI monomer increases, the branched structure and their degree of entanglement with sPEEK polymer matrix also increase. More rigid and more compact membrane is found in the case of mBMI(30)[dense semi-IPN]. In contrast, relatively loose entangled network is found for mBMI(98) sample [loose semi-IPN] where the mBMI unit remains far apart and mostly un-connected, until high concentration of mBMI(98) is present. The results shows physical cross-linking with highly branched mBMI is effective in reducing water uptake, lower methanol permeability with reduced sPEEK membrane swelling. All membranes displayed fair proton conductivity above 10-2 S/cm at room temperature. The best membrane is arrived with 15% mBMI(98), where high C/P ratio (Conductivity/Permeability ratio) can be established. In comparison, the water channel dimension in sPEEK-IPN system is strongly dependent on the cross-linking reaction condition. Compared to pure sPEEK, the proton mobility and water diffusion in all sPEEK-IPN membranes are reduced due to the presence of semi-IPN structure which leads to further constrained water channel. We also observed that proton mobility and water diffusion in dense sPEEK-IPN membranes are smaller than in loose sPEEK-IPN membranes because the water channel dimension is strongly constrained by densely semi-IPN structure. On the other hand, at low water uptake, the water migration activation energy becomes much lower than either the bulk value or that of the fully saturated sample. In this case, the water and proton motion is no-longer controlled by the viscosity of the medium but determined by the efficiency of relay motion by interconnected hydrogen bonding network between the sulfonated group and the residual water or between the imide group of mBMI and the retained water. The fact that loose sPEEK-IPN membrane shows the most narrow line width (1H NMR) and the lowest Eaσ in proton conductivity indicated there are cooperative activity between sPEEK and the imide. This mechanism is absent in the case of tight-sPEEK-IPN membrane.
The sPEEK/functionalized SiO2 nano-composite membranes were prepared by using an in-situ polymerization scheme. Present method utilizes functionalized silanes as the precursor to form a homogeneous solution with the polymer host. The alkalinity of the functional groups (Amine, Pyrrolidinone, and Imidazole) at the silane chain ends governs the degrees of association with the acidic polymer matrix, and changed the size and distribution of the surface functionalized silica in the membrane. As a result of tailoring the water channel dimensions through maneuvering both the alkalinity and amounts of base functional group on the SiO2 surface, membranes morphology, and acid group distribution, the water and methanol solvent uptake and methanol permeability are effectively reduced with slightly sacrificing the proton conductivity. The best membrane is arrived with 50phr silica (composed of Pyrrolidinone-silane : TEOS at 4:1 mole ratio), where high proton conductivity (2×10 2 S/cm), good membrane film property, lowest methanol permeability (4×10-7 cm2/s, in 50 vol% methanol solution) and high C/P ratio can all be established. On the other hand, water channel dimension is strongly dependent upon both the alkalinity of functional groups (Amine, pyrrolidinone and Imidazole) at the silane chain ends or the fraction of base functionalized on the SiO2 surface. In the case of the stronger base attached at silane chain ends, the membranes are found to yield the smallest SiO2 particle size and tightest water channel compared to that in pure sPEEK. This is due to the stronger acid/base interaction between -SO3H of sPEEK and base functional group on the SiO2 surface. Since the presence of stronger acid/base interaction lead to further constrained water channel, the mobility of proton and water are reduced when membranes are in fully hydrated state. In contrast, the membrane with weak base at silane chain ends gives the largest SiO2 particle size and wider water channel, thus the proton and water are exhibiting rapid motion.
These studies disclosed two membrane architecting designs to reach good balance between high conductivity, minimal water and methanol permeability. The key appears to lie on the water channel morphology.
The output power of PEMFC using state of art proton exchange membrane is dramatically reduced due to increase of internal resistance from the loss of water from these membranes above 80℃. Sulfonated TiO2 nano-tube (sTNT) is a proton conductor and display excellent water retention capability which preserved certain amount of water in temperature higher than the normal boiling temperature. Hence the nano-composite membranes were prepared from sPEEK and physically blended with various amounts of sTNT. The results show the proton conductivity of the membrane containing sTNT is higher than pristine sPEEK membrane. This is especially the case with 5wt% sTNT composite, where the most homogeneous morphology is observed. For sPEEK/5wt% sTNT membrane the proton conductivity reached above 10-2 S/cm at 100℃, 60%RH. In morphology and water dynamic studies, at low water content condition, we found the water is exhibiting rapid motion on the surface of TiO2 tube. This is the reason that sPEEK/sTNT display excellent proton conductivity and the smaller activation energy (Eaσ) at low humidity condition.
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