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直接甲醇燃料电池用新型质子交换膜的制备与表征
Preparation and Characterization of Novel Proton Exchange Membranes for Direct Methanol Fuel Cells
【作者】 杨武斌；
【导师】 朱红；
【作者基本信息】 北京交通大学 ， 光学， 2008， 博士
【摘要】 质子交换膜是直接甲醇燃料电池（DMFC）中的关键材料之一,阻止或减缓甲醇在质子交换膜中的渗透是DMFC研究中最富有挑战性的基础课题之一。本论文工作以提高质子交换膜的阻醇性能为目标,研制和探索了多种新型质子交换膜,考察了其结构与性能,制备了两种具有自主知识产权的阻醇型质子交换膜,获得了以下创新性成果:（1）首先建立了阻醇型质子交换膜的性能表征方法,主要包括甲醇透过系数的测定方法、电导率测定方法和复合膜中杂多酸稳定性的评价方法等,并组装了相应的测试装置,为研制阻醇型质子交换膜提供了研究手段。（2）以SiO2、TiO2、Al2O3和ZnO等纳米氧化物为改性剂,制备了的纳米氧化物/Nafio?复合膜,XRD和IR分析表明纳米氧化物嵌入了Nation?膜中。比较了这些复合膜性能,结果发现,复合膜的阻醇性能均有较大幅度的提高,且以SiO2和TiO2改性的Nafion?膜最为明显,甲醇透过系数分别从-10-6cm2/s降低到-10-7cm2/s和-10-8cm2/s数量级。比较了不同SiO2/含量的SiO2/Nafion?复合膜的性能,复合膜的质子电导率随着SiO2含量的增大而减小,但其阻醇性能却随SiO2含量的增大而增大。（3）首次制备了磷钨酸/二氧化硅/磺化聚醚醚酮（PTA/SiO2/SPEEK）复合膜,XRD和IR测试结果表明二氧化硅和磷钨酸以无定形状态均匀分散于复合膜中。稳定性实验发现磷钨酸在复合膜有较好的稳定性,复合膜具有良好质子传导性能（在80℃下电导率达0.01S/cm）和阻醇性能(甲醇透过系数-10-7cm2/s),有望作为直接甲醇燃料电池用质子交换膜材料。（4）通过磺化反应把-SO3H基团引入了聚醚砜（PES）的骨架,IR测试结果证明了磺化聚醚砜（SPES）中-SO3H基团的存在。SPES膜在室温下的电导率和甲醇透过系数随着磺化度的增大而增大,当磺化度为31.2%的SPES膜的电导率达2.3×10-3S/cm,甲醇透过系数在-10-7cm2/s;制备了PES/SPES共混膜,首次探讨了其阻醇性能,PES的加入导致甲醇透过系数降低,提高了膜的阻醇性能,当PES含量是50%时,甲醇透过系数降低到-10-9cm2/s。（5）制备了不同聚醚砜（PES）含量的聚醚砜/磺化聚醚醚酮（PES/SPEEK）共混膜。测试共混膜玻璃化转变温度发现,PES与SPEEK具有良好的相容性;共混膜的TGA测试结果显示,PES提高了SPEEK膜的热稳定性;与纯SPEEK膜相比,PES/SPEEK共混膜阻醇性能和溶胀性能有所提高,且PES含量增加,性能也逐步增加,当PES含量在30%时,甲醇透过系数在10-8-10-9cm2/s之间,预示PES/SPEEK共混膜是一种良好的DMFC用质子交换膜材料。（6）制备了多种基于SPEEK的多层阻醇复合膜,提高了SPEEK等烃类膜的抗氧化能力。Nafion?| SPEEK | Nafion?和Nafion?-TiO2 | SPEEK | Nation?-TiO2多层复合膜的性能有所改善。首次制备了Nafion?-TiO2 | PES/SPEEK |Nafion?-TiO2多层复合膜,测试结果表明与PES/SPEEK相比,多层复合膜质子传导性能变化不大,但阻醇性能增加较大(P=-10-8cm2/s),而且抗氧化能力有较大幅度提高。讨论了多层阻醇复合膜抗降解机理,认为Nation?保护层促进了中间物H2O2分解,抑制了·OH自由基的存在,从而阻止了多层复合膜的氧化降解,多层复合膜有望解决烃类膜普遍存在的降解问题。
【Abstract】 Proton exchange membrane is one of the key materials in direct methanol fuel cell （DMFC）. Perfluorosulfonate ionomer membranes( e.g. commercially available Nafion? membranes, DuPont) have successfully been used in hydrogen-oxygen type proton exchange membrane fuel cell（PEMFC.）.So Nafion? membranes were used as PEM during the early years of the DMFC research. However, Nafion? membranes have high methanol crossover for the water-dependence of proton conductivity, which has reduced cell efficiency of DMFC. These have motivated great efforts in developing new type proton exchange membranes （PEMs） based on different concept. The main aim of this work is to prepare novel PEMs with no or less methanol crossover. This thesis presents the preparation and characterization of novel PEMs for DMFC. The innovative achievements related to this aim are:（1） The methodology of membrane characterization was set up to evaluate the physical and chemical properties of the PEMs and characterize the membranes surface and cross-section morphologies. Gas chromatography was used to determinate methanol permeability through PEM according to solution diffusion mechanism. Proton conductivity measurements in normal direction were performed by the two-electrode AC impedance spectroscopy. Stability of phosphotungstic acid（PTA） in composite membrane was characterized by bledout experiment. Attenuated total reflectance Fourier transform infrared spectroscopy （ATR/FT-IR） was measured to get information about the chemical structure of membranes. Characterization, to evaluate and improve the membranes obtained, is an important part of this work.（2） Nanometer oxide/Nafion? composite membranes were prepared by solution casting. To realize good quality composite membranes, aliphatic alcohol and water in Nafion? PFSA Polymer solution was firstly replaced by DMF under vacuum evaporation. XRD and ATR/FT-IR analysis showed that SiO2、TiO2、Al2O3 and ZnO were incorporated into Nafion? polymer matrix. Compared with Nafion 112 membrane and blank Nafion? membrane, methanol permeability of composite membrane decreased by a large extent, though proton conductivity of composite membrane decreased. SiO2/Nafion? and TiO2/Nafion? membranes have excellent methanol barrier performance, which permeability were about-10-8cm2/s at 25℃. The order of the proton conductivity and methanol permeability are SiO2/Nafion?>TiO2/Nafion?>Al2O3/Nafion?>ZnO/Nafion? and TiO2/Nafion?>SiO2/ Nation?>Al2O3/Nafion?>ZnO/Nafion ?, respectively. It can be noticed that TiO2/Nafion? and SiO2/Nafion? have higher selectivity than Nafion?, Al2O3/Nafion? and ZnO/Nafion? membranes.（3） Novel PTA/SiO2/SPEEK composite membranes were firstly prepared, which was one of innovations in this thesis. ATR/FT-IR spectrographic measurements indicated that the Keggin structure characteristics of PW12O403- anion was present in PTA/SiO2/SPEEK composite membranes. XRD analysis of the composite membranes confirmed that PTA and SiO2 were amorphous. It was found that the composite membranes had good proton conductivity（σ=0.01S/cm at 80℃） and low methanol permeability(P≈-10-7cm2/s).Due to enhancement of the PTA stability in composite membranes by doped SiO2,PTA/SiO2/SPEEK composite membranes have a potential to be considered for direct methanol fuel cells （DMFC） applications.（4） Sulfonated poly （ether sulfone） （SPES） was synthesized with chlorosulfonic acid as sulfonating agent and methylene chloride as solvent at room temperature. The presence of sulfonic acid groups in SPES was identified by FT-IR Spectra. SPES Membranes and PES/SPES Blend Membranes were cast from solutions using N,N-dimethyllformamide（DMF）. Both proton conductivity and methanol permeability of SPES membranes increased with the degree of sulfonation（DS）, when DS=31.2%,σ=2.3×10-3 S/cm,P≈-10-7cm2/s. It was found that with the increasing of the PES content in PES/SPES blend membranes, methanol permeability of blend membranes decreased(P≈-10-9cm2/s when PES content is 50 %).Proton conductivity of blend membranes decreased due to the content of sulfonic acid groups decreased resulted from addition of PES. It was concluded that both SPES and PES/SPES blend membranes had lower methanol permeability and high proton conductivity.（5） PES/SPEEK blend membranes with various PES content were prepared by casting from solutions. The glass transition temperatures of blend membranes indicated the molecular miscibility between PES and SPEEK. It was found that methanol permeability of blend membranes decreased with the increase of PES content due to the formation of H bond between O=S=O group in PES and -SO3H group in SPEEK. Though the proton conductivity of PES/SPEEK blend membranes decreased to some extent, their performance for barrier methanol(P≈10-8-10-9cm2/s when PES content is below 30%) had remarkably enhanced, which showed the feasibility for direct methanol fuel cell.（6） SPEEK-Nafion? multilayered composite membranes were also prepared by spraying two Nafion? coated layers on both sides of basic SPEEK membranes to prevent SPEEK membranes from oxidating and degradating in fuel cells. Compared to the blank SPEEK membrane,the properties of Nafion? | SPEEK |Nafion? and Nafion?- TiO2 | SPEEK | Nafion?- TiO2 multilayered composite membranes were improved. Nafion?-TiO2 | PES/SPEEK | Nafion?-TiO2 multilayered composite membranes were firstly prepared, which was another important innovation in this thesis. The Fenton experiments indicated that the longevity of multilayered composite membranes was enhanced for Nafion? coated layers protect the SPEEK membrane from degradation due to Nafion? layer prevented·OH radical from attacking Ar-O-Ar band, which suggested that these novel membranes were excellent candidate membrane materials for DMFCs applications.There are 85 pictures,10 tables and 168 references in this thesis.