鹼性燃料電池是近年來研究的重點之一，這是因為鹼性中可使用非白金觸媒， 有望降低價格。但在薄膜上卻仍欠缺足夠功能的商用材料。薄膜最主要功能為傳 遞離子和阻礙燃料竄透，目前有多種膜材系統廣為研究，如陰離子交換膜(Anion Exchange Membrane AEM)、陽離子交換膜(Cathion Exchange Membrane , CEM) 和 聚苯咪唑膜 (Alkali doped PBI)。如果使用陰離子交換樹酯因化學穩定因素無法在高溫燃料電池使用。本研究選用在高溫下化性及機械性穩定的陽離子交換膜進行開發，因為導電度不佳的缺點可以透過改質添加另一材料來改善的。本研究的設計是在 Nafion 薄膜中以 K+ 陽離子交換，並在膜材中加入物理高分岐的雙馬來醯亞胺的聚合物。帶有鹼基的結構可以將水分和 OH- 保留在膜中，進而在高溫時仍保有導電度，另外，高分歧結構也可以在孔道中有效阻絕燃料的竄透；此膜的設計同時改善了乙醇竄透和高溫下低濕度的導電度。研究中製備兩種系列的K-Nafion 膜其中所包含的單體雙馬來醯亞胺（BMI）和寡聚物（mBMI）的摩爾比為 28:72（mBMI（72））與 2:98（MBMI（98））。不同添加量和不同的組成對膜材的物性所造成的影響為本研究探討的重點。 實驗上先使用 SEM、TEM、XRD、TGA 和 DSC 比較不同分岐長度雙馬來醯 亞胺 mBMI(72))和 mBMI(98)的 K-Nafion 改質膜的結構分析，以及薄膜導電度和竄透性能比較。離子導電度在去離子水中可達約 0.025 S/cm，在 1M KOH 約 0.43 S/cm。由實驗結果得知 mBMI 複合膜的離子導電度明顯增加，且在低濕度條件下(低於 40％RH)的導電度甚至可高於 K-Nafion。含有 mBMI 的陽離子交換膜(5%-mBMI(72)/K-Nafion) 顯 示 乙 醇 滲 透 率 降 低 到 2.38×10-7cm2/s 。 特 別 是5%-mBMI(72)的膜材其選擇性（C/P）高於 K-Nafion 薄膜。此膜在鹼性溶液(KOH)濃度增加到 8M 時仍保持高度化學穩定性。此一複合膜(5%-mBMI(72)/K-Nafion)使用於鹼性乙醇燃料電池在 90℃下效能為 65 mW/cm2 優於 K-Nafion 膜(60 mW/cm2)。從實驗的結果可以證明此類型的薄膜可有效運用於鹼性燃料電池中。 Alkaline fuel cell is one of the major thrust of fuel cell research in recent years, because it offers better opportunity to use non-platinum based catalyst, with potential of lowering the prices. But a suitable membrane for alkali fuel cell is still absent from the market. A main function of the membrane allows ions to pass but hinders fuel permeability. Variety of membranes systems are being explored which include anion exchange membranes (AEM), cation exchange membranes (CEM) and polyphenylene imidazole membranes (Alkali doped PBI). In the case of anion exchange membranes, its low thermal stability and the tendency to loss alkalie functional group precludes its application in fuel cells to operate at high temperature. In this study we will focus on modification of Nafion-based cation exchange membrane, because it offers both durability and mechanical stability at high temperature. Its main disadvantage, i.e. the low electrical conductivity, can be improved relatively easily by blending with a alkali functionalized moiety. Our design is to physically cross-linke Alkalie (K+ ) exchanged Nafion with bismaleic imide hyper-branched polymer. This composite membrane shows good conductivity at elevated high temperatures because the hyper-branched oligomer bears base functions augments OH- transport in the membrane effectively blocked the fuel permeation while preserving water at elevated temperature to sustain high ion conductivity. As a result, the composite membranes simultaneously improved both the conductivity in low humidity (or at elevated temperature) and reduce permeability of ethanol. In this study, physical and electronic properties of the two series of K-Nafion samples containing Bismaleimide (BMI) monomer and modi?ed Bismaleimide oligomers (mBMI) with mole ratios of 28:72 (mBMI(72)) and 2:98 (mBMI(98)) are compared in detail by SEM, TEM, XRD and TGA analyses. The ionic conductivity works upward to 0.25 S/cm in DI water and 0.43 S/cm in 1M KOH at room temperature. The improvement of ion conductivity is more pronounced at low humidity conditions where the conductivity maintains about one order higher than that of recast K-Nafion below 40% RH and shows continuous increase when temperature is raised above 80oC. These series of cation exchange membranes contain mBMI show alcohol barrier property where ethanol permeability is substantially reduced to 2.38×10-7 cm2/s in the case of mBMI(72)/K-Nafion. Selectivity(C/P ratio) of the membrane with mBMI is also higher than the recast K-Nafion membrane, especially in the case of 5%-mBMI(72). Under air atmosphere, these membranes are stable in basic media with KOH concentration up to 8 M. Performance of composite membrane (5%-mBMI (72) / K-Nafion) in the ethanol fuel cell at 90℃ is about 65 mW/cm2 which is higher than K-Nafion membrane (60 mW/cm2). The results shows the hyperbranch composite membranes can be effectively applied to alkaline fuel cells with more pronounced effects at elevated temperature.
