相較於其他質子交換膜材料，Nafion有較優秀的穩定性及導電度，然而Nafion在高溫操作環境下水分散失嚴重造成質子傳導困難，使導電度下降。在高分子質子交換膜中摻合無機奈米物質可以增加高溫下的保水性質，在高溫下可維持高導電度甚至在低濕度的環境中能夠提升電池效能，比使用純Nafion做為交換膜的燃料電池更優秀。本研究中的新穎燃料電池交換膜是在Nafion中加入了具管狀結構的親水性二氧化鈦奈米管，並在奈米管表面以1,3-propane sultone修飾上磺酸官能基。修飾後的奈米管IEC為1.4 mmol/g。奈米管與Nafion混合形成有機/無機複合薄膜，該無機添加物具有的高保水性質及一維長距離連續性親水性界面可有效改善Nafion在高溫低濕使用下的缺點。 特別的是本研究在無機物表面修飾磺酸官能基，解決傳統方法的有機/無機複合薄膜有混合不均及相分離的問題。此外，由於酸根的親水性，與高分子上的磺酸根產生交互作用，使水分子在有機高分子及無機物間產生較強的作用力，提高薄膜在高溫下的保水能力使導電度升高。本研究由DSC實驗觀察到sTNT具有延緩水分子散失的現象，保水能力比磺酸化TiO2顆粒更佳。從TEM、SAXS及XRD等測量中觀察到sTNT穿透於數個Nafion離子集團的空間分佈造成薄膜結晶相的改變。此現象除了增加長距離質子導電度外，對薄膜的含水量及抗滲透性也造成直接的影響。實驗中以5wt % sTNT含量的sTNT / Nafion複合薄膜呈現最佳的相容性及質子導電度。在100℃、40 %相對溼度環境下5wt% sTNT複合薄膜具有接近10-2 S/cm 的導電度為純Nafion薄膜的兩倍以上。製作成MEA後的DMFC效能在60℃、70℃的效能達50 mW/cm2比未添加sTNT的純Nafion薄膜效能40 mW/cm2還要好。本研究顯示，以sTNT改良後的Nafion薄膜產生了不同的微結構型態，改變了薄膜內的水分子脫附及運動的行為，進而影響質子導電度及電池效率。 Nafion has more excellent stability and conductivity in comparison to other materials. However, Nafion shows severe loss of water and reduced proton conductivity under high temperature operating conditions. Proton exchange membrane prepared by inorganic nanomaterials composite can increase water retention at high temperatures. These composite membranes can maintain high conductivity at high temperatures and even enhances cell performance at low humidity condition. In this study, a novel organic/inorganic composite proton conducting membrane were prepared by blending Nafion with sulfonate functionalized Titania nanotube,and applied to PEMFC operating at high temperatures. Sulfonat functionalized titania nanotubes (sTNT) resolves the draw-backs of of phase separation commonly occured in organic/inorganic composite meterials. In addition, composite membran exhibit good water retention behavior at high temperatures because the high surface interaction with of sTNT. Furthermore, composite with sTNT created a highly efficient proton conducting channels, forming a continuous and long-range proton transfer behavior existed between the organic and inorganic interface which yield favorable proton conductivity at high temperature and low humidity. From TEM measurement, we observed these sTNT which cross through many ionic clusters in these composite membranes. And we also found that presence of sTNT changed the crystallinity of the membranes by XRD. This phenomenon has a direct effect in water uptake and permeability, in addition to facilitate long-range proton conductivity, The sTNT showed a much better water retention and conductivity than titania nanoparticle by DSC. The study shows 5wt% of sTNT yielded the most balanced performance of ion conductivity and water uptake property. At 100℃ and 40% relative humidity, the proton conductivity of the 5wt% sTNT composite membrane is 8 mS/cm, a great improvement over 2 mS/cm for a recast Nafion membrane. DMFC performance for this composite membrane is 50 mW/cm2 better than 40 mW/cm2 with pure Nafion at 70℃. The study shows the composite membrane formed from the addition of sTNT, created different types of membrane morphology and nano-structures which affected water ad-/desorption and proton conducting behavior. A strategy to improve fuel cell performance can be established by improving these membrane morphology and nano-structures.
