Proton Exchange Membranes for Fuel Cells with High Proton Conductivity and Membrane Selectivity by Formation Proton Conducting Channels

本論文乃探討燃料電池質子交換薄膜的化學結構、物理形態、水和分子在高分子聚電解質內的形態與分布情形，對於甲醇燃料電池中質子、甲醇分子、水分子在質子交換薄膜內傳導速率的影響，係其能藉由這些基礎參數的探討，以開發出容易製備、較商業化Nafion®薄膜便宜，且有較佳甲醇燃料電池效能之質子交換膜。而在甲醇燃料電池系統中，質子交換膜除了須具備有高質子傳導率外，亦須保有適當的水含量及低甲醇滲透率，而這些性質均與質子交換膜本身的化學結構及親磺酸根含量有關。因此，本論文乃分兩部分： 第一部分乃是以分子設計觀點，利用臭氧導入離子團基於聚偏二氟乙烯 (Poly(vinylidene fluoride)，PVDF)高分子鏈上，同時分別利用自由基聚合法(Free radical polymerization)及原子轉移自由基聚合法(Atom transfer radical polymerization，ATRP) 於PVDF主鏈上接枝出線性及分歧狀結構的磺酸根團基，比較兩種不同結構側鏈的高分子聚電解質對於其應用及性質的影響。發現在相似離子交換容積( Ion exchange capacity，IEC)值下，具有分歧狀結構之親水側鏈的PVDF對水吸收量及鍵結水含量(bound water content)與具線性結構之親水側鏈的PVDF相近，但具有分歧狀結構之親水側鏈的PVDF有較高的質子傳導率、較低的甲醇滲透率及較高的C/P 選擇比。主要乃是因親水性分歧狀側鏈之離子團基較大，容易因與PVDF主鏈本身的親疏水性差異較大，而產生微相分離，而形成質子傳導的通道，因而有較高的質子傳導率，且有較低的甲醇滲透率。 第二部分乃是直接將奈米矽粒子(silica nanoparticles，SNP)導入到高磺酸化度(即高IEC值)的二氮萘聯苯聚醚酮(sulfonated poly(phthalazinone ether ketone, sPPEK)，以抑制磺酸化度為1.23的sPPEK在80℃下之水溶性及高甲醇滲透率。實驗中發現，奈米粒子可均勻分散在高分子間，且所有的奈米複合膜之膨潤行為及甲醇滲透率均被抑制，且有較高的熱穩定性值。複合膜中含有5 phr SNP，其在3M甲醇水溶液中之單電池效能較未改質的sPPEK及商業化Nafion®薄膜高。其開路電壓為0.6 V、最佳之電功率密度為52.9 mW/cm2，且其最佳之電流密度為 264.6 mA/cm2。 然而，在sPPEK薄膜中添加7.5 phr 磺酸化奈米矽粒子(SA-SNP)，可抑制甲醇滲透、提升鍵結水含量且其質子傳導率為未改質sPPEK薄膜的3.6倍。為了進一步地了解SA-SNP在質子交換膜中所扮演的角色，乃將SA-SNP導入到部分氟化的聚電解質中-聚芳香烴醚醚酮酮(poly(arylene ether ether ketone ketone), SPAEEKK)，觀察是否會提升其薄膜之質子傳導率。發現SA-SNP導入到高磺酸化度之聚電解質中，不僅會抑制薄膜的甲醇滲透率，同時也提升薄膜之質子傳導率。由穿透式電子顯微鏡圖發現，SA-SNP上的磺酸根與SPAEEKK上的磺酸根有強的交互作用，容易聚集成較大的離子聚集集團，同時因為SPAEEKK本身具有較疏水C-F鍵，容易因其親疏水性的差異，而導致在薄膜內產生微相分離，其親水的ionic domain即提供質子傳導所需的通道，因而增加複合膜之質子傳導率。因此，SPAEEKK/SA-SNP奈米複合薄膜有較高的C/P選擇比，且其值較Nafion®薄膜的2.79倍。故藉由具磺酸根之奈米矽粒子的添加，不僅可抑制薄膜之甲醇滲透率，更可進一步地提升其質子傳導率，若將此應用於甲醇燃料電池中，預期將可提升其電池效能。Proton conductivity is one of the major properties of proton exchange membranes (PEMs) for direct methanol fuel cells (DMFCs). This work explores an approach to effectively enhance the proton conductivities as well as other properties of cost-effective PEMs for DMFCs by understanding how the chemical structures, physical morphologies, and the state of water influence the membrane’s ability to transport protons, methanol, and water. The results would help to facilitate development of new materials with favorable transport properties for fuel cells use. The discussion in this study is divided into two parts.n the first part, the effect of chemical structures and physical morphologies of PEMs on their proton conductivities is examined using poly(vinylidene fluoride) (PVDF) possessing linear and highly branched polystyrene sulfonic acid side chains. For polymers with similar ion exchange values, both types of PVDF-g-PSSA graft copolymers show similar water uptakes and bound water contents. However, the samples with highly branched PSSA side chains exhibit higher proton conductivity, lower methanol permeability, and higher selectivity compared to the linear analogues. Incorporation of highly branched side chains effectively increases the properties of the PEMs for DMFCs because of the formation of agglomerate PSSA domains, which promote proton conducting but depress methanol permeation through the PEMs. n the second part, incorporating silica nanoparticles (SNP) and sulfonated silica nanoparticles (SA-SNP) into polyelectrolyte membranes with high degree of sulfonation permits investigation of their effect on membrane properties. Sulfonated poly(phthalazinone ether ketone) (sPPEK) with a degree of sulfonation of 1.23 and poly(arylene ether ether ketone ketone) (SPAEEKK) are used as polymer matrixes. The nanoparticles homogeneously disperse in the polymer matrixes. All of the nanocomposite membranes exhibit improved swelling behavior, enhanced thermal stability, and reduced methanol crossover through the membrane. When sPPEK membrane with 5 phr SNP is operated at a high methanol concentration in the feed (3 M) for a single cell test, it shows an open cell potential of 0.6V and an optimum power density of 52.9 mW/cm2 at a current density of 264.6 mA/cm2. The cell performance is better than that of both the pristine sPPEK membrane and the Nafion®117, although the density of sulfonic acid groups in the nanocomposite membranes is lower than others.owever, the sPPEK membrane with 7.5 phr SA-SNP exhibits low methanol crossover, high bound-water content, and a proton conductivity 3.6-fold higher than that of the pristine sPPEK membrane. We also utilize SA-SNP as additives to modify sulfonated poly(arylene ether ether ketone ketone) (SPAEEKK) and to investigate the reasons for the increase in proton conductivity by incorporating SA-SNP into sulfonated PEMs. It is found that the interaction between the sulfonic acid groups of SA-SNP and those of SPAEEKK combined with hydrophilic–hydrophobic phase separation induces the formation of proton-conducting channels, as evidenced by TEM images, which contributes to the increase in the proton conductivity of the polymer / SA-SNP nanocomposite membrane. Therefore, the SPAEEKK/SA-SNP nanocomposite membrane shows a high selectivity, which is 2.79-fold higher than the selectivity of Nafion®117. The improved selectivity of the SPAEEKK/SNP nanocomposite membrane demonstrates potential for the approach of providing hydrocarbon-based PEMs as alternatives to Nafion in direct methanol fuel cells.口試委員會審定書……………………………………………………………………. icknowledgements….…………………………………………………………………v文摘要....................................................................................................................... viibstract ...........................................................................................................................xiable of Contents ..........................................................................................................xiiiist of Figures...............................................................................................................xviiist of Tables ...............................................................................................................xxivhapter 1: Introduction to Fuel Cells ….................................................................. 1.1 What is a Fuel Cell ……......................................................................................... 1.2 Types of Fuel Cells................................................................................................. 2.3 The Polymer Electrolyte Membrane Fuel Cell....................................................... 5.3-1 Membrane Electrode Assembly – Heart of the Fuel Cell……..……………….. 5.4 Proton Exchange membranes (PEMs) …………………………………………12.4-1 Requirements of PEMs………………………………..……………………….12.4-2 The developments of PEMs................................................................................14.4-3 Hydrocarbon Based Membranes……………………………...………………..21.4-4 Hydrocarbon PEMs with an Aromatic Backbone …………………………….32.4-5 Nanocomposite Membranes…………………………………………….……..37.5 Evaluation of PEMFCs performance ………………………………………..…..38.5-1 Cell Overpotential .............................................................................................38.5-2 Cell Polarizations and Performance ...................................................................41.6 Motivation for this study………………………………………………...……….44-7 Thesis Objectives……………………………………………………..………….48hapter 2: Study on the effect of side chain architectures on the proton conductivities and other properties of PVDF-g-PSSA……………………………51.1 Introduction ...........................................................................................................51.2 Experimental Section.............................................................................................57.2-1 Matericals……………………………….……………………………..…….…57.2-2 Preparation of PVDF-g-PSSA copolymers with linear PSSA side chains......,...57.2-3Preparation of PVDF-g-PSSA copolymers with branched PSSA side chains….58.2-4 Fabrication of polyelectrolyte Membranes…………………………….…...….59.2-5 Measurements and properties evaluation …………………………..……...…..59.2-6 Measurements of IEC values……………………………….…………...……..60.2-7 Water and Methanol Uptake………………………………………….………...61.2-8 Methanol permeation measurement by pervaporation process……..........….....61.2-9 Proton Conductivity…………………………………………………….….......62.3 Results and Discussion ..........................................................................................65.3-1 Preparation of linear and branched graft copolymer membranes.......................65.3-2 -Thermal stability of linear and branched graft copolymer membranes…….....72.3-3 Water uptakes and the states of water of LG and BG copolymer membranes…73.3-4 Methanol crossovers and proton conductivities of LG and BG copolymerembrane……………………………………………………………………...75.3-5 Selectivity of PVDF copolymer membranes for DMFC applications…………84.4 Summary…………………………………………………………………………86hapter 3: Using silica nanoparticles for improving stability of sulfonated oly(phthalazinone ether ketone) membrane……………………….87.1 Introduction ...........................................................................................................87.2 Experimental..........................................................................................................93.2-1 Materials……….................................................................................................93.2-2 Preparation of nanocomposite membranes…………….....................................93-2.3 Instrumental analysis & single cell DMFC test ................................................94.3 Results and discussion…………………………………..……………………….95.3-1 Preparation of sPPEK/silica nanocomposite membranes………………...……95.3-2 Thermal stability of sPPEK/silica nanocomposite membranes………………..98.3-3 Water and methanol absorption properties and permeability of sPPEK/silica anocomposite membranes……………………………………………....……993.3-4 Proton conductivity of sPPEK/ silica nanocomposite membranes………...…..105.3-5 Cell performance of a single DMFC test ……………………...………………107.4 Summary……………………..……………………………………..…………….112hapter 4: Improvement on the properties of PEMs by incorporating sulfonated ilica nanoparticles...............................................................................113.1 Introduction .........................................................................................................113.2 Experimental........................................................................................................116.2-1 Materials………………………………………….…………………………..116.2-2 Preparation of SPP/SA-SNP and SPA/SA-SNP nanocomposite membranes...117.2-3 Measurements and properties evaluation ……………………………….……118-3 The effect of SA-SNP content on sPPEK membrane properties………………..119.3- 1 Preparation of sPP/SA-SNP nanocomposite membranes……..……….……..119.3-2 Thermal stability of SPP/ SA-SNP nanocomposite membranes…..………......124.3-3 Water and methanol fuel uptakes and permeation properties………………....127.3-4 The state of water in the SPP/SA-SNP nanocomposite membranes………….1324.3-5 Proton conductivity of sPP/SA-SNP nanocomposite membranes…………....134.4 Investigate the reasons for the increase in proton conductivity by incorporating SA-SNP into sulfonated PEMs…………………………...……………..……..138.4-1 Preparation of SPAEEKK membranes and nanocomposite membranes……..139.4-2 Ion exchange capacity, water uptake and fixed ion concentration in the PAEEKK membranes ……………………………...……………………….142.4-3 Methanol permeability of SPAEEKK membranes and nanocomposite embranes........................................................................................................145.4-4 The state of water in the membrane…………………………………………..148.4-5 Proton conductivity of the membranes……………………………………….151.5 Summary………………………………………………………………………..157hapter 5: Conclusions and Future Work .............................................................159.1 Conclusions ...........................................................................................................159.2 Future work............................................................................................................161eferences …..............................................................................................................16