磺酸化幾丁聚醣衍生物作為離子傳導膜應用於氫能源

Abstract

質子傳導膜(proton exchange membrane, PEM)於多種電化學裝置中扮演重要的角色，像是質子交換燃料電池(PEMFCs)、直接甲醇燃料電池(DMFCs)、電解水產氫及全釩液流電池(VRFBs)等。質子傳導膜除了需要具有良好的質子傳導能力，還要能避免燃料或特定離子從陽極滲透至陰極以降低電化學反應效率。目前常使用的質子傳導膜多為碳氟系高分子，像是Nafion系列，具有優秀的質子傳導能力及化學穩定性，但是其成本過高且對環境負擔較大，不利大量生產及廣泛應用。本研究因此開發出磺酸化幾丁聚醣接枝聚乙二醇(sulfonated chitosan-graft-poly(ethylene oxide), SCS-g-PEO) 並以其為基材製備質子傳導膜，期望在材料與膜材製備上達到成本低廉及環境友善的目標。 幾丁聚醣是一種天然的多醣類，主要來自甲殼類動物，例如：蝦、蟹的外殼、昆蟲的硬殼及各種真菌及藻類的細胞壁，其含量相當豐富且成本低廉。藉由化學反應將幾丁聚醣修飾上磺酸根基團增加了其親水性及改善其傳導能力。側鏈接枝低分子量的PEO有助於改善幾丁聚醣脆硬的機械性質及改變膜材的微結構，使幾丁聚醣能在質子傳導膜應用上展現良好的傳導能力與機械性質。由於SCS-g-PEO對水的溶解性佳，在製膜過程能以水作為溶劑，能有效減少有機溶劑的使用以降低對環境的負擔。本研究藉由調控幾丁聚醣的磺酸化程度及PEO接枝率，並改變膜材組成以最佳化質子傳導膜的傳導特性與機械性質。 SCS-g-PEO膜材於室溫下飽水的狀態，其質子傳導度高達46.7 mS cm-1，接近於Nafion 117。儘管SCS-g-PEO膜材具有高度的離子交換能力(IEC > 2 mmol g-1)且與水的親和力佳，導致高吸水率(WU > 100%)，但是藉由適合的交聯系統成功抑制其膨潤程度(SR < 27%)，且改善了乾膜的機械性質(Young’s modulus > 300 MPa)。值得注意的是SCS-g-PEO膜材在19 M甲醇溶液下具有低甲醇滲透率，使其選擇度高達1.34"×" 104 S s cm-1，具有作為高濃度甲醇燃料電池之潛能。另外SCS-g-PEO的磺酸根可以交換成不同離子形式以應用於其他陽離子傳導膜系統，像是高值化產氫系統，利用碘化鉀溶液在陽極進行碘離子氧化反應(Iodide oxidation reaction, IOR)並搭配陰極進行電解水產氫(hydrogen evolution reaction, HER)，當SCS-g-PEO的磺酸根置換成SO3K形式，其傳導度於1 M碘化鉀溶液中較Nafion 117高出兩倍，陽離子遷移係數高達0.990，具有作為鉀離子傳導膜(potassium conducting membranes, PCMs)之潛力。

Proton conducting membranes are one of the most important components in a varieties of electrochemical devices such as proton exchange membrane fuel cells (PEMFCs), direct methanol fuel cells (DMFCs), hydrogen production from water splitting and vanadium redox flow batteries (VRFBs). These membranes need to effectively transport protons between two electrodes while prevent undesirable crossover of fuels and ions, and which are critical to the device performance. As far, membranes based on all fluorocarbon polymers, such as Nafion, are the most employed PEMs; nevertheless, high cost and environmental issues intrinsically limit successful commercialization of these devices. In this study, we design and fabricate novel PEMs based on custom made sulfonated chitosan grafting with poly(ethylene oxide) (SCS-g-PEO) to achieve cost effectiveness and environmental benignancy in the aspect of material itself and the membrane production. Chitosan is a biorenewable, natural abundant polysaccharide which could be extracted from crustaceans, insects, and etc. to achieve possible low cost. Modification of chitosan to incorporate sulfonic acids could improve solubility and hydrophilicity of chitosan and increase proton conductivity of the resulting PEMs. The grafted PEO side chains could further tailor the mechanical property and the microstructure of the membrane. Since SCS-g-PEO is highly water soluble, free standing PEMs could be fabricated from environmental friendly water based process by incorporating suitable crosslinkers. In this work, the degree of sulfonation of SCS, the grating ratio of PEO side chains and the composition of the membranes are systematically varied to optimize the transport properties and the mechanical properties of the PEMs. SCS-g-PEO membrane exhibited a high proton conductivity (46.7 mS cm-1) at fully hydrated state at room temperature, slightly lowered than Nafion 117. Though high ion exchange capacity (IEC > 2 mmol g-1) and high water affinity of SCS-g-PEO leads high water uptake (WU > 100%), the delicately designed crosslinking would limit the swelling ratio (SR < 27%) and allow the dry membrane to exhibit good mechanical properties with Young’s modulus (> 300 MPa). It is worthy to address that the membrane could retain good dimensional stability and low methanol crossover at high methanol concentration up to 19 M (75%) leading a high selectivity (1.34"×" 104 S s cm-1), allowing it a good candidate to serve as PEMs for high energy DMFC. By simply exchanging protons with other cations, the SCS-g-PEO membranes could extend their uses to other cation conducting/exchange membranes. To realize the application in hydrogen production with KI solution at iodine oxidation reaction (IOR) side and water at hydrogen evolution reaction (HER) side, the SCS-g-PEO membranes with SO3K form shows an ion conductivity twice higher than Nafion 117 in 1M KI solution alone with a high cation transport number of 0.990, exhibiting promising potential as potassium conducting membranes (PCMs).