【作者】 李涛；

【导师】 杨勇；

【作者基本信息】 厦门大学 ， 物理化学， 2009， 博士

【摘要】 直接甲醇燃料电池（DMFC），其燃料甲醇常温下为液体，储存和运输较为便利和安全，并且具有高的能量密度；因其直接氧化甲醇，电池系统较为简单和易于小型化。因此直接甲醇燃料电池作为新一代高效电源在便携式电子设备、电动车上具有良好的应用前景。作为直接甲醇燃料电池关键组件之一的质子交换膜，目前广泛使用的是全氟磺酸膜，如杜邦公司生产的Nafion系列产品。这种材料具有优异的化学及热稳定性，并且当其充分润湿时表现出优秀的质子电导率。但是全氟磺酸膜也存在着燃料甲醇从阳极向阴极渗透严重的问题，甲醇渗透不但浪费燃料（因渗透造成得浪费率最多可达40％），而且会在阴极造成混合电位并毒化阴极催化剂，从而大大降低了燃料电池的性能，因此甲醇渗透成为阻碍直接甲醇燃料电池实际应用的主要问题之一。要改进现有质子交换膜的性能，就必须深入了解质子交换膜中甲醇传输的机制以及影响甲醇传输的各项主要因素。基于以上研究目标，本论文对现有全氟磺酸膜进行了复合改性以降低其甲醇渗透率，尝试制备了几种新型部分氟化型复合聚合物膜体系，并对质子交换膜的结构、阻醇性能及其可能的物质传输机理进行了研究，取得的主要结果如下：1．硅氧化物表面不同基团调控全氟磺酸质子交换膜性能的研究论文选用了具有极性较强的相对亲水有机基团的硅烷偶联剂，制备了一系列Nafion／有机硅氧化物复合膜。发现当复合膜中所含的硅氧化物量基本相同时，使用有机基团中含有氨基的硅烷偶联剂改性的复合膜表现出极低的甲醇渗透率及质子电导率。在此基础上还发现，当改变与-NH₂相连的基团的电负性时，可以对生成的复合膜的甲醇渗透率及质子电导率进行调控。如利用带有脲基的有机硅烷偶联剂改性的复合膜与Nafion117膜相比，甲醇渗透率降低了89％，而质子电导率仅下降了49％；并且表现出比Nafion117更稳定的电池性能。实验结果表明为降低全氟磺酸膜的甲醇渗透率，应从甲醇的渗透通路——膜内部互相连接的离子簇入手。而离子簇是由磺酸基团充分水合并集聚形成的，因此利用与磺酸基团有相互作用的添加剂可以影响离子簇的形成，从而抑制甲醇的渗透，但由于质子传导也必须经由相互连接的离子簇，所以这种影响同时降低了膜的质子电导，对膜的综合性能的提高产生了不利影响。2．半互贯穿网络型（semi-IPN）Nafion／PVP（聚乙烯吡咯烷酮）复合膜的性能研究。通过对Nafion/有机硅氧化物复合膜的研究发现，所得复合膜的甲醇渗透率的降低总是以牺牲其质子电导率为代价的。这可能是由于在磺酸化质子交换膜中甲醇与质子有着近似的传输路径，因而很难提高此类膜对质子的选择透过性。而互贯穿聚合物网络膜（IPN）由两种或多种相互独立的交联聚合物构成，通过调节各种聚合物的比例、交联程度、交联结构有可能获得高质子选择性的质子交换膜。本论文选择4，4’-双叠氮菧-2，2’二磺酸钠（DAS）为交联试剂，以紫外光引发，使聚乙烯吡咯烷酮（PVP）在Nafion膜中发生交联，从而制备了semi-IPN型Nafion／PVP复合膜。对复合膜的性能测试发现，PVP含量不同的复合膜，均表现出较低的甲醇渗透率（为Nafion117的41～47％），而PVP含量较少的复合膜表现出比Nafion117更高的室温质子电导率（最多可增加～30％），并且复合膜还表现出比Nafion117膜更佳的热稳定性，其氟醚支链的分解温度提高了近60度。通过对复合膜广角X射线衍射及1H固体核磁共振谱的表征，认为这是因为网状的交联PVP结构限制了Nafion分子链的自由蠕动，影响了全氟磺酸膜充分水合时内部离子簇的相互连接，从而抑制了甲醇渗透；而质子具有通过氢键的跃迁传输机制，交联试剂的引入同时增加了复合膜中磺酸基团的密度，所以特定交联度的复合膜表现出更高的质子电导。研究结果表明，利用质子相对于甲醇所独有的跃迁传输机制，可能设计出质子选择性更优的高性能质子交换膜。3．互贯穿网络型（IPN）聚偏氟乙烯（PVDF）／2-丙烯酰胺基-2-甲基丙磺酸（AMPS）复合聚合物质子交换膜。针对目前全氟磺酸质子交换膜价格昂贵的问题，并为了进一步研究IPN型质子交换膜的特性，本论文还开展了IPN型部分氟化复合聚合物质子交换膜的研究。合成了以聚偏氟乙烯（PVDF）为阻醇聚合物及质子交换膜聚合物骨架，以2-丙烯酰胺基-2-甲基丙磺酸（AMPS）为离子交换载体（以乙烯基三乙氧基硅烷（VTES）、二乙烯基苯（DVB）、N，N′-亚甲基双丙烯酰胺（NMBA）等对AMPS进行交联）的IPN型PVDF／AMPS质子交换膜。通过对AMPS的预聚合，选择合适的交联试剂及用量，优化交联反应时间和温度等措施，使制备的质子交换膜具有良好的机械性能，与Nafion117相比甲醇渗透率降低了2／3，质子电导率基本不变，质子选择系数可达Nafion117膜的2．5倍。同时发现，相对于离子聚合物中离子交换位（磺酸基团密度）的多少，膜的交联程度、微观结构及表面形貌对质子交换膜的性能影响更大。在此基础上，我们对PVDF／AMPS质子交换膜进行了紫外接枝AMPS的表面修饰，实验结果发现，表面改性可进一步提高质子交换膜的质子电导，使膜的质子选择系数达到Nafion117膜的～3．0倍。更多还原

【Abstract】 The direct methanol fuel cell （DMFC） is a promising power source candidate forportable electronic devices and electric vehicles due to its high energy efficiency andenvironmental compatibility. Its fuel, methanol, which is liquid at room temperature,can be easily and safely stored and transported. In addition, the DMFC system issimple in design and can be operated without fuel reforming. So far, theperfluorosulfonic acid （PFSA） membrane （e.g. Nafion by Dupont） has been usedwidely as proton exchange membrane （PEM） for the DMFC, owing to its excellentstability both in chemical and thermal environment, as well as its high protonconductivity when contains enough water. However the main disadvantage of PFSAmembrane is methanol crossover, and over 40% of methanol could across themembrane at most. Methanol crossover causes loss of fuel, reduced cathode voltageand cell performance. Such disadvantage is also one of the major problems in DMFCapplication.In order to improve the performance of the existing PEM, it’s important tounderstand the methanol crossover mechanism and the effects of PEM properties onthe methanol transport in PEM. Therefore, the thesis mainly concentrated onmodifying the PFSA membrane to lower its methanol permeability, trying to designpartial fluorizated PEM candidate, as well as investigating the structure, themethanol-blocking performance, methanol and proton transport mechanism of thePEM. The main results of the thesis are as follows:1.The effects of organic silica bearing different organic groups on the performancesof the Nafion/organic silica composite membranes.A series of Nafion/organic silica hybrid membranes has been prepared by usingorganic silane coupling agents （SCA） bearing different hydrophilic functional groupsWith approximate silica content, the composite membrane modified by SCA bearingaliphatic amino groups （SILCPM3） was found to exhibit extremely low protonconductivity and methanol permeability compared with other composite membranes. Moreover, by controlling the electronegativity of the group connected to the aminogroup, the methanol permeability and proton conductivity of the compositemembrane could also be adjusted. E.g. the composite membrane modified by SCAbearing urea group was found to exhibit 89% lower methanol permeability, 49%lower proton conductivity, as well as improved cell performance compared withNafion 117 membrane. The results showed that, in order to decrease methanolcrossover in the films, we should modify the PFSA membrane aiming at themethanol transfer path（the connected ion clusters formed by terminal -SO₃Hgroups）. Thus the additive which has interaction with sulfonic group could alsoaffect the formation of ion clusters, and restricts the methanol permeability as well.However, proton transportation also involves in the connected clusters, the effect ofadditive on sulfonic group may also depress the proton conductivity, and worsen thecell performance of the composite membranes.2. The study of Nafion/cross-linked PVP semi-interpenetrating polymer network（IPN） membrane.From the study of Nafion/organic silica composite membranes, it can be seen thatthe transfer of proton and methanol decreases simultaneously under modifying. Thissuggests that protons and methanol have similar molecular transport mechanisms insulfonic acid containing PEMs, which makes it difficult to improve selectivity forthe DMFC application. Interpenetrating polymer networks （IPNs） consists of twoindependent cross-linked polymer, the IPNs may obtain high selectivity in theory byadjusting the structure, the molar ratio, and the cross-linking degree of the twopolymers.The thesis chose 4, 4’-diazostilbene-2, 2’-disulfonic acid disodium salt （DAS）as cross-linking agent to crosslink poly（vinyl pyrrolidone） （PVP） in commercialNafion117 membranes under the irradiation of ultraviolet. The semi-IPNNafion/PVP membranes exhibited decreasing methanol permeability with theincreasing of the content of PVP. However, with appropriate amount of PVP, thesemi-IPN membrane showed even higher proton conductivity than Naifonl 17（～30% higher）. It was also found that the decomposition temperature of the fluorinated etherside-chain of semi-IPN membranes was 60℃higher than that of Nafion117, whichmeans that the semi-IPN membranes had better thermal stability than pristineNafion1 17. From the characterization of wide angle XRD and 1 H solid-state NMRspectra, it can be concluded that the introduction of cross-linked PVP networkrestricted the free squirm of Nafion side-chain, which influenced the hydration ofterminal sulfonic acid group and weakened the connection of ion clusters, thusdepressed the methanol crossover. Howerver, the extra sulfonic acid groups suppliedby crosslinking agent DAS as well as the acid-base interaction produced by aminogroup and sulfonic acid group strengthened the hopping transfer of proton, thus thesemi-IPN membranes exhibited improved proton conductivity with appropriateamount of PVP. Furthermore, the unique Grotthuss transfer mechanism of protoncompared with methanol moleculer should be taken into account for designing andsynthesizing more suitable proton exchange membrane candidates for DMFC.3. The preliminary study of PVDF/AMPS interpenetrating polymer networkmembrane.Aiming at reducing the cost of PEM, and the further exploration of new IPNmembranes with good performance, a novel partial fluorizated PEM candidate wasalso been prepared. Poly （vinylidene fluoride） （PVDF） was chosen as polymermatrix, 2-acryamido-2- methyl-1-propane sulfonic acid （AMPS） was chosen asionomer. AMPS was cross-linked by different cross-linking agents （e.g.vinyltriethoxysilane （VTES）, divinylbenzene （DVB）, andN,N’-Methylenebisacrylamide （NMBA）） to form PVDF/ AMPS IPN membranes.Through the pre-polymerization of AMPS, selecting proper cross-linking agent, aswell as optimizing molar ratio, reaction time and temperature, the performance of thePVDF/AMPS IPN membranes had been optimized. Compared with Naifonl17, thenew membrane exhibited～2/3 lower methanol permeability but similar protonconductivity, and～1.5 time higher proton selectivity. It was also found that thecross-linking degree, the micro structure and the surface morphology of the IPN membranes was more important to proton selectivity than the amount of ionomer.The thesis also tried to do some surface modification of PVDF/AMPS IPNmembrane via photoinitiated graft polymerisation with AMPS. The results showedthat, after surface modification, the proton selectivity of the new membrane canreach 3 times of that of Naifon117.更多还原