Synthesis and NMR Studies of Electron and Proton Conducting Mesoporous Nb, Ta and Ti Oxide Composites for Alternative Energy Applications

在介观尺度(即20-500埃范围)合成具有周期性孔洞的材料是当前研究最活跃的领域之一。通过改变介孔之间的壁结构的组成，可以为各种应用量身定做各种物理性质，例如选择性气体吸附或催化或不寻常的电、光或磁性质。通过用二次客体相浸渍这些材料的孔结构来改变介孔复合材料的性质，有可能产生纯介孔固体中没有观察到的协同效应。任何材料最基本的性质之一就是它的电荷传输行为，因为在固体中移动电子和质子是无数催化或电子应用中的关键一步。虽然具有周期性介观结构的无机材料的例子不计其数，但很少有具有高电子导电性的材料。这是因为介观结构的壁往往很薄，具有任何程度的规则结构，这导致电子在陷阱中的局域化，或者是制备导电介观结构的合成条件与导致无机相导电性能的条件不相容。这是有问题的，因为具有高导电性和可变氧化态的高比表面积多孔材料将是用于锂电池的理想材料，或者作为需要电子迁移率的反应的催化剂载体材料。质子通过固体的传导性也很重要，特别是对于控制电荷从电化学电池一侧流向另一侧的燃料电池膜，但Nafion，一种带有质子传导磺酸盐基团的氟化聚合物，是最常用的燃料电池膜材料，对脱水稳定性较差，这导致在120℃以上的温度下质子传导性下降。Nafion还容易受到酒精通过结构扩散的影响，这是一个问题，因为许多燃料电池的设计都是以酒精为原料。因此，许多研究人员转向探索介孔材料/聚合物复合膜，它有望克服Nafion的问题。这项建议中的研究着眼于用不同的试剂来修饰介孔Nb、Ta和Ti氧化物的结构，这些材料具有可变的氧化态和高酸度/质子导电性，以产生一种情况下具有高电子导电性或金属行为的复合材料，而另一种情况下具有高质子导电性，这取决于所使用的试剂。因此，介孔Nb、Ta和Ti氧化物将被试剂处理，以S或Se取代表面氧化物，从而在材料的内表面产生导电的硫化物或硒相，使电子能够传输到层下的金属氧化物。还将进行用聚苯胺或聚噻吩导电子聚合物纳米线浸渍孔，以试图利用这些单链聚合物作为电子通路到达孔通道壁上的金属氧化物中心。然后，这些材料将被测试用作锂电池的正极材料，预计通过孔洞的电子迁移率将导致优越的充放电动力学，这是锂电池经常存在的一个性能问题。该方案的第二部分涉及用具有高质子导电性的磺化聚合物浸渍介孔Nb、Ta和Ti氧化物。在这里，预计介孔通道将通过在性能温度下保持水分来帮助克服Nafion的脱水问题，同时还可以限制酒精在膜中的扩散。一旦合成，这些材料将在运行条件下作为燃料电池膜进行测试。

The synthesis of materials with periodic porosity on the mesoscale (ie 20-500 Angstrom range) is one of the most active areas of current research. By varying the composition of the structure of the walls between the mesopores it is possible to tailor in an enormous variety of physical properties for various applications such as selective gas sorption or catalysis or unusual electronic, optical or magnetic, properties. By impregnating the pore structure of these materials with secondary guest phases that modify the properties of the mesoporous composite, it is possible to create cooperative effects not observed in the pure mesoporous solid. One of the most fundamental properties of any material is its charge transport behavior because moving electrons and protons through a solid is a key step in myriad catalytic or electronic applications. While countless examples of inorganic materials with a periodic mesostructure exist, very few of them possess high electron conductivity. This is because the walls of the mesostructure are often two thin to possess any degree of regular structure, which leads to localization of the electrons in traps, or that the synthesis conditions to fabricate the conducting mesostructure are not compatible with the conditions that lead to conducting properties in the inorganic phase. This is problematic because a high surface area porous material with high conductivity and variable oxidation states would be ideal for use in a Li battery or as a catalyst support material for reactions requiring electron mobility. Proton conductivity through a solid is also important, especially for fuel cell membranes which regulate the flow of charge from one side of the electrochemical cell to the other, but Nafion, a fluorinated polymer with proton conducting sulfonate groups that is the most commonly used fuel cell membrane materials, possess low stability to dehydration, which leads to a decrease in proton conductivity at temperatures over 120 C. Nafion is also vulnerable to alcohol diffusion through the structure, which is problematic since many fuel cells are designed to use alcohol as a feedstock. For this reason many researchers have turned to the exploration of mesoporous material/polymer composite membranes, which promise to overcome the problems with Nafion. The research in this proposal sets out to modify the structure of mesoporous Nb, Ta and Ti oxides, materials possessing variable oxidation states and high acidities/proton conductivity with various agents to create composites with high electronic conductivity or metallic behavior in the one instance, and high proton conductivity in the other, depending on the agent used. Hence, mesoporous Nb, Ta and Ti oxides will be treated with reagents to replace the surface oxides with S or Se, hence creating a conducting sulfide or selenide phase on the inner surface of the material which will allow electron transport to the metal oxide under layer. Impregnation of the pores with electron conducting polymer nanowires of polyaniline or polythiophene will also be carried out to try to exploit these single strand polymers as electron pathways to the metal oxide centers on the walls of the pore channels. These materials will then be tested for use as cathode materials in Li batteries, where it is anticipated that electron mobility through the pores will lead to superior charge and discharge kinetics, an area of performance that is often a problem with Li batteries. The second part of this proposal involves the impregnation of mesoporous Nb, Ta and Ti oxides with sulfonated polymers with high proton conductivity. Here it is anticipated that the mesoporous channels will help overcome dehydration problems of Nafion by holding onto moisture at performance temperatures, while also limiting alcohol diffusion through the membrane. Once synthesized these materials will be tested as fuel cell membranes in operating conditions.