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|Issue Date: ||2014-10-15 14:38:49 (UTC+8)|
離子交換膜的內部型態具有親水性的孔道結構來傳輸流體，而離子傳遞也決定於介質流體傳輸的行為，因此流道中展現的形貌(Morphology) 也深深地影響離子傳輸。本研究中藉由添加磺酸根修飾後的一維奈米管柱(sZrO2、sTiO2)至Nafion高分子中並經外加強電場下鑄造成膜。在電場誘導下，一維奈米管柱受到極化，使薄膜中的親水孔道隨著管柱方向而形成有方向性且連續的結構。此結構隨著電場誘導方向(垂直薄膜平面)，提供了一個直接且連續性的奈米結構的離子傳遞路徑，使離子傳導性大幅的增加。研究中以5 wt% sZrO2、sTiO2含量添加至Nafion中所形成的複合薄膜在室溫下且相對溼度100 RH%中，質子導電度可以高達至7.5x10-2 S/cm。在外加直流電場(DC)下施加電壓1000 V，添加5 wt%奈米管柱複合薄膜可以達到8.35x10-2 S/cm。隨著外加電場強度的增強至7000 V/cm，複合薄膜質子導電度可以更一步地提升至11.6x10-2 S/cm。與市售N117 (5.84x10-2 S/cm) 或與recast 純Nafion(5.2x10-2 S/cm)相比較皆具有大幅度的提升。製作成MEA後的DMFC效能在80℃下能達到110 mW/cm2比recast純Nafion薄膜效能80 mW/cm2還要好。
除改進導電度外，外加電場的誘導使得薄膜內部結構型態形成了更有規則且具方向性的緊密排列，在疏水的部份結晶度增加，在薄膜的機械效能測試中，拉伸強度明顯增強。更有趣的是，隨電場強度的增加，複合薄膜的水含量(water uptake)卻隨之遞減至21% (7000 V/cm)比於未施加電場的Nafion薄膜(24%)為小。另外在尺寸安定性(Swelling Ratio)上，以電場強度7000 V/cm並添加5 wt% sZrO2複合薄膜swelling ratio值最低，可以減少至11%。由小角度X光繞射中也觀察到管狀流道結構在外加電場誘導下縮小， 說明了此新穎膜材吸收較少水量少及展現高抗甲醇竄透的成因。
總結本研究顯示在薄膜中形成直接且連續的親水孔道結構與磺酸化奈米管柱在高分子中的均勻分布的微結構是提高質子導電度的主要因素(關鍵在於構建最優化奈米流管而非高水量或高磺酸化)，這也是膜材能以最少量水分達成較高質子傳遞的關鍵所在。經由電場誘導一維奈米管柱的複合薄膜可以有效提昇質子導電度由1.1x10-1 S/cm (室溫)至1.8x10-1 S/cm (80℃)，有效降低薄膜膨潤率性，提高抗甲醇竄透性，並同時提高薄膜機械強度。此一新穎方法可以廣汎用於構建其它能源領域相關的高性能膜材。
;Enhancing ion conductivity is the primary goal in membrane development in fuel cell, lithium battery, vanadium redox flow battery, or energy storage devices involving ion exchange. The challenge grows when requirements are also given to preserve high mechanical strength while improving ion transport property. A common approach proposed to make the break through is by forming inorganic–organic hybrid. Composite with inorganic nanoparticles is found to retain water/moisture through inorganic surface such that it maintains high conductivity and conserved high energy out-put at elevated temperature and at low environment humidity.
Though highly important, the study of membrane morphology has not been an issue of focus to improve ion conductivity. Membrane morphology is known to be a primary structure factor responsible for fluid transportation in semi-permeable membrane. Since ion conductivity relies heavily on fluid transport; change on channel morphology will lead to change of ion conduction. In present study, a novel approach is reported to prepare ion conducting membrane under high electric field poling condition. One dimension metal oxide (ZrO2, TiO2) nanorods and nanotubes are impregnate first in Nafion, and the whole drying and membrane forming process is carried out under electric field polled condition. The field induced dipole orients the low dimensional nanotube/nanorod thus created aligned hydrophilic morphology that was fixed during membrane formation. The ordered and oriented nano-structures formed in the direction of the applied electric field, provided a direct and continuous ion path. Proton conductivity has reached 7.5x10-2 S/cm in 100% RH condition when 5 wt% of sulfonate functionalized ZrO2 or TiO2 nanotube are composited with Nafion. Upon applying a DC voltage over 1000V, the conductivity is raised to 8.35x10-2 S/cm. With continue increasing of the electric field to 7000 V/cm, the conductivity in the composite film raised to a record value of 11.6x10-2 S/cm. This is substantially improved over that of commercially available Nafion membrane N117 (5.84x10-2 S/cm) or the locally recast Nafion (5.2x10-2 S/cm) membrane.
Diffusion tensor mapping derived from NMR micro-imagine of these membrane confirmed (1) faster water diffusion as reflected in the stronger diffusion tensor, (2) more ordered tensor orientation (narrowing of Euler angle distribution) along the Z-direction (cross-channel director), and (3) more homogeneously distributed diffusion tensor in the electric field poled membrane. These results confirmed ordered diffusion in the e-field poled ionomer is indeed responsible for the high proton conductivity. Due to the more ordered morphology originated from the e-field poling, membrane mechanical property is also enhanced. However, water uptake is gradually reduced from 24% (without poling) to 21% (with poling at 7000 V/cm). Small angle x-ray diffraction shows the tubular flow channel dimension shrunk after electric field poling. This corroborates with the results that swelling ratio is reduced from 23% (without poling) to 18% (with poling at 7000 V/cm).
The fact that electric field poling produces membrane with lower water uptake and smaller swelling ratio and yet displayed high proton conductivity is a surprising find. The results alluded to the possibility that higher proton conductivity can be achieved by more effective use of water molecules through synergistic cooperation of direct water permeation channel and well distributed and connected sulfonate groups. In the present case, the amount of water required to deliver an optimized proton conductivity can be reduced to nearly ½, provided that the membrane morphology is optimized. Enhanced fuel cell performance is also realized by employing the e-poled membrane which shows superbly high ion conductivity.
|Appears in Collections:||[化學研究所] 博碩士論文|
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