| 摘要: | 在燃料電池系統中，介於陰陽極之間的質子交換膜最急迫需要解決的問題是：(一)高溫下，薄膜失水速度過快，致使導電度急遽下降；(二)甲醇竄透問題嚴重，造成效能低落；(三)薄膜使用壽命不足，經長時間使用後薄膜易損壞。本研究構想是將具有鹼基修飾的SiO2 (f-SiO2)均勻的構建在Nafion膜的親水孔道中達成修飾Nafion目標。為避免有機-無機物產生分相本研究使用原位凝膠 (in-situ sol-gel)法製備，主要區別是本方法中無機矽氧(f-SiO2)奈米粒子會在Nafion溶液中成核並與Nafion成膜的過程中同時成長。 由SEM和Si- mapping實驗中可證實所製備的有機/無機複合薄膜奈米粒子尺寸大小相近(highly homogeneous)且均勻分散。TEM中顯示15-1p4T膜材展現了較佳的親、疏水區域的大小及分佈。XRD證明加入適量的鹼基無機物能幫助Nafion主結構的結晶度提高，尤以15 phr的添加量時有較佳的表現。本系統中最佳化的無機物含量在15phr和鹼基比例在base silane：TEOS = 1：4，在此條件下的複合薄膜(15-1p4T)的複合薄膜展現最低的Water uptake和Swelling ratio數值，導電度雖然減少了約17%、但甲醇滲透也改善了近50%；選擇性(C/P 比)上，較reNafion增進了60%~70%。另一方面，由變溫導電度中所求得的活化能，15-1p4T比reNafion減少50%以上，證明本複合膜材中已構建一條更適合質子傳導的路徑。由於無機物具有保水的性質當高溫低濕環境(20% RH)下，質子導電度仍能維持在1×10-3S/cm 以上。在70oC的直接甲醇燃料電池效更可以高達88.2mW/cm2，優於同條件下N117的結果。綜合以上各項實驗結果顯示本材料設計已成功改善Nafion薄膜的微結構，使薄膜擁有低膨潤率、質子傳導所需活化能較低、無機物高度分散、高C/P ratio等優點，提供一個更適用於高溫直接甲醇燃料電池的質子交換膜。In the fuel cell system, the most urgent issue of proton exchange membrane, which separates anode and cathode are: (1) the rapid loss of water at high temperature, which leads to a rapid decline of conductivity; (2) The cross-over of methanol fuel, which deteriorated the fuel cell performance; and (3) the mechanical and electrochemical durability that membrane disintegrated after prolonged usage, thus limited fuel cell lifetime. The membrane is designed based on the idea that Nafion channel morphology modified or tunable by incorporating a base-modified SiO2 nano-particle in-beded in the hydrophilic domains. To prevent the organic-inorganic phase separation in the composite material, the novel membrane, Nafion/ pyrrolidone silane (Py-silane) modified SiO2 composite (f-SiO2) were prepared by in-situ sol-gel method where the silica particle first nucleated in Nafion solution and growth in dimension during the Nafion membrane formation. SEM and Si-mapping confirmed the silica nano-particle is homogeneously distributed in the composite membrane. TEM results indicated the membrane 15-1p4T displayed the best morphology where the hydrophilic and hydrophobic domains maintains the most suitable size and most uniform distribution. Crystallinity provided by XRD shows the addition of alkaline-modified SiO2 has raised the crystallinity on Nafion main chain structure (PVdF), which is especially true for the 15 phr sample. The composite membrane bearing 15 phr silica content with base silane : TEOS ratio = 1:4 (15-1p4T), yielded the lowest water uptake, and smallest swelling ratio. Although the conductivity is reduced about 17%, its methanol permeability is reduced about 50%, leading to a better selectivity (C/P ratio) over recast-Nafion of 60%to 70%. In, addition, the activation energy derived from variable temperature water diffusion constant is about half of that in recast-Nafion, suggesting a novel water transport path which is more suitable for proton conduction. As moisture is preserved by the inorganic moiety, proton conductivity is not lost at low humidity (~1×10-3S/cm under 20% RH). The DMFC performance reached 88.2mW/cm2 under 70oC which is better than N117 under the same operating condition. All results indicated this material has successfully tailored the Nafion membrane micro-structure. This proton exchange membrane promises low swelling ratio, lower proton conduction activation energy, highly dispersed inorganic composite, and higher C/P ratio; suitable for direct methanol fuel cell to operate at elevated temperature. |
