新穎性交聯聚乙烯醇質子交換膜在甲醇燃料電池上之應用

Abstract

直接甲醇燃料電池(DMFC)是一種具有前瞻性的新能源來源,並應用在車輛及其他運載工具和可攜式設備上。開發質子傳導膜在甲醇燃料電池，其同時具備所需質子導電度和低甲醇穿透度是一極重要的挑戰。質子傳導膜目前用在甲醇燃料電池主要是用全氟性高分子，例如像Nafion，雖然此膜展示出良好性能和長期穩定性，然而它的高額製造成本及嚴重的甲醇穿透現象，導致燃料使用率偏低且不實用在大規模的生產。 此研究主要在討論兩種類型的質子交換膜。其中之一種是兼具物理和化學交聯的聚乙烯醇(PVA)和六氟戊二酸(HFA)的混參物理交聯膜，之後加上BASANa (Benzenesulfonic acid sodium salt)和GA (Glutaraldehyde)為其附化學交聯反應。GA和BASANa皆接在聚乙烯醇鏈上產生acetalization反應，並分別當做交聯劑和提供磺酸根基(-SO3H)的親水基。此外，六氟戊二酸(HFA)因具有雙石炭酸，不但可以當作熱交聯劑，且可提升膜的韌性和化學穩定性。另一種膜主要是合成具有雙親性的共聚高分子在交聯的丙烯酸之氨基鉀酸酯寡聚高分子(acrylated urethane oligomer)，並利用紫外光交聯反應使兩者結合在一起。此雙親性共聚高分子乃利用TEMPO之方式進行活性聚合，將磺酸鈉鹽聚苯乙烯單體轉換成疏水性單體，第二鏈段選擇DVB (divinyl benzene)作為具備交聯之單體。此方法之優點在於可在相同疏水性溶液進行反應，可避免異相反應之缺點。之後，膜的製備乃將合成之共聚高分子(PSSTBADVB)或單體(SSTBA)與acrylated urethane oligomer (AUO)、交聯劑TAIC、最後加入光起始劑並進行紫外光聚合反應。製備出的質子交換膜(包含交聯的PVA/HFA或是交聯的acrylated urethane oligomer with sulfonated styrene)具有良好的物理化學特性。這些特性可以用傅立葉轉換紅外線光譜儀(FT-IR/ATR spectroscopy), 場發射表面掃描式電子顯微鏡 (FE-SEM)、熱重量損失分析儀 (TGA)、微差掃描式熱分析儀 (DSC)、廣角X光繞射儀 (WAXD)、小角X光散射儀 (SAXS)、原子力顯微鏡 (AFM)、接觸角測試, 溶液吸收研究、機械測試和交流阻抗分析(AC impedance)方式。這兩種膜的質子導電度和甲醇穿透度分別落在10-4至10-2 S/cm和10-8至10-7 cm2/s的範圍內，但需視其交聯密度狀況而定。另外，跟Nafion®-117比較起來也可展現出良好的selectivity (Φ)。因此利用此質子交換膜在甲傳燃料電池上是可被考慮並具有其可行性的潛力。

Direct Methanol Fuel Cells (DMFCs) are promising new power sources for vehicles and protable devices. It’s significant challenge to develop proton exchange membranes (PEMs) possing both desired proton conductivity and low methanol permeability for the DMFCs. PEMs currently used in DMFC are perfluorinated polymers such as Nafion®. Even though such membranes have demonstrated good performances and long-term stability, their high cost and methanol crossover makes them unpractical for large-scale production. In this research, I describe two kinds of membranes and discuss respectively. The one is a novel physically and chemically PVA/HFA (PVA/hexafluoroglutaric acid) blending membranes with BASANa (Benzenesulfonic acid sodium salt) and GA (Glutaraldehyde) as binary reaction agents. GA and BASANa were used as cross-linkers and a donor of hydrophilic groups (-SO3H), which they both crosslinked to PVA chains for the acetalization reaction. Besides, HFA with dicarboxylic acid not only as the thermal cross-linking agent but also enhanced the flexibility and chemical stability. The other is to synthesize ampliplilic block copolymers on crosslinked acrylated urethane oligomer using UV-curing method. The amphiphilic block copolymers are synthesized using TEMPO controlled polymerization method with tertrabutylammonium styrenesulfonate (SSTBA) and divinyl benzene (DVB) as monomers for each block. This method has advantage synthesize the copolymer in one organic phase. Crosslinking of membranes of the copolymer are made by adding photo-initiators and subsequently being initiated by using UV light. By mixing acrylated urethane oligomer (AUO) with SSTBA monomer or PSSTBA-b-DVB copolymer and then add the crosslinker TAIC for third component. We can prepare PEM with excellent physicomechanical properties. These properties of the cross-linked PVA/HFA proton-conducting membranes or cross-linked acrylated urethane oligomer with sulfonated styrene membranes are examined by FT-IR/ATR spectroscopy, field emission scanning surface microscopy (FE-SEM), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), atomic force microscopy (AFM), contact angle instrument, sorption studies, tensile test and AC impedance method. The feasibility of preparing films for DMFC application is investigated in this study. The proton conductivities and the methanol permeabilities of all two kinds of the membranes are in the range of 10-4 to 10-2 S/cm and 10-8 to 10-7 cm2/s, respectively, depending on its crosslinking density, and it shows great selectivity (Φ) compared with those of Nafion®-117. The possibility of using these PEMs for DMFC device is suggested.