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Synthesis of Ordered Mesoporous Materials via Microwave Processing and Highly Heteroatom Doped Ordered Mesoporous Carbons for Energy Storage

机译:通过微波处理和高杂原子掺杂有序介孔碳的微波合成合成有序介孔材料用于储能

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摘要

Ordered mesoporous materials, with pore diameter between 2 nm to 50 nm, have drawn researchers interests due to their superb properties including large surface area, uniform and adjustable pore architecture and good mechanical property, which allow a variety of different applications of the mesoporous materials, including catalysis, separation, purification, sensing and energy storage. Researchers have also reported different functionalization strategies of these mesoporous materials to improve the properties, such as improving electrochemical property by heteroatom doping of mesoporous carbon.;One generalized synthetic strategy to synthesize ordered mesoporous materials is soft templating method. The functionalization of these mesoporous materials, such as heteroatom doping, can sometimes to lead ordered porous structure collapse, which is usually due to structure deformation during high temperature treatment. For instance, it is challenging to synthesize ordered mesoporous metal oxide with high crystalline pore wall with soft-templating method, as the growth of the crystals can deform the ordered porous structure. Meanwhile, it is difficult to synthesize ordered mesoporous carbon with high doping level (>10 at%) via soft-templating strategy as well, since the rapid doping reaction at high temperature can lead to ordered structure collapse due to the high vapor pressure generated by doping reactions. Here, we demonstrate several facile synthetic routes to fabricate highly crystalline ordered mesoporous metal oxide and heteroatom doped ordered mesoporous carbon. The application of the nitrogen doped mesoporous carbons in energy storage has also been discussed in this dissertation.;Ordered mesoporous manganese oxide has been reported to be useful in energy storage due to its good electrochemical property, as the transition metal manganese has half-filled 3d orbital, which allows it to donate or accept electrons. However, synthesizing ordered mesoporous manganese oxide with high crystallinity with soft-templating strategy is challenging, as the crystal formation during thermal treatment can deform the ordered porous structure. In this dissertation, the synthesis of ordered mesoporous crystalline manganese oxide films is achieved by microwave-assisted processing to convert manganese carbonate precursor to the oxide, remove the polymeric template and crystallize the Mn3O4, all within 1 min. The microwave heating in this case is primarily induced by the microwave cross-section of the substrate (silicon wafer), together with the absorption of microwaves by manganese oxide to provide local energy, which can provide energy for nucleation/crystallization. Conversely, heating manganese carbonate in a muffle furnace at an analogous surface temperature leads to nanostructure collapse with low crystallinity.;Apart from the synthesis of highly crystalline ordered mesoporous metal oxide, we also demonstrate the functionalization of ordered mesoporous carbon via heteroatom doping. Doped carbons can exhibit enhanced electronic, electrochemical and mechanical properties compared with un-doped carbons. These properties enable their use in applications, such as photovoltaics, catalysis and energy storage. For these applications, nano porosity and high heteroatom doping are generally desirable due to large surface area provided by nanoporous structure and improved properties contributed by heteroatom doping. However, it is challenging to achieve high content of heteroatoms doped carbons as well as maintaining the nanoporous structure. One common method of doping is to use heteroatom containing polymer precursors and carbonizing to synthesize doped carbon, where heteroatom content is limited by the number density of heteroatoms in the precursors. Additionally, during calcination, the nanoporous structure can collapse due to the contraction of pores. In this dissertation, a generalized synthetic strategy for highly doped ordered mesoporous carbons is reported. High concentrations of heteroatom doping in ordered mesoporous carbon is achieved by infiltration of molten dopants into silica reinforced mesoporous crosslinked polymer (phenolic resol) bi-continuous framework and subsequent carbonization. This method is demonstrated to generate ordered mesoporous carbons with high heteroatom content (> 10%) for a wide variety of elements including nitrogen, boron, sulfur and phosphorus, with ordered structure preserved.;With the generalized synthetic route, nitrogen doped multi-shell hollow carbon are fabricated as a matrix material for Lithium-sulfur batteries. The hierarchical hollow structure and chemical decoration (heteroatom doping) of carbon framework can help to suppress polysulfide shuttling, while the macropore provides large pore volume to increase sulfur loading. The nitrogen doped sample shows better cycle stability compared with un-doped sample. Given that this hollow structure with a macropore can increase sulfur loading, the specific capacity is much lower compared with other cases where the pores are much smaller because of the low conductivity of sulfur.;In addition, we also systematically investigate how nitrogen doping influences the rate that molten sulfur can infiltrate and the overall extent of pore filling of highly ordered mesoporous doped carbons via in-situ small angle x-ray scattering (SAXS). Nitrogen doping introduces polar bonds to the carbon framework, which can slow down the diffusion of nonpolar sulfur molecules into porous carbon. These in-situ SAXS measurements provide insights about kinetics information for diffusion of sulfur into mesopores and how the surface chemistry of nitrogen doped carbon can significantly hinder the infiltration of the mesopores by sulfur.;In summary, this dissertation focuses on the functionalization of ordered mesoporous material without losing the ordered structure, as well as the application of nitrogen doped carbon in energy storage.
机译:孔径介于2 nm至50 nm之间的有序介孔材料由于其卓越的性能(包括较大的表面积,均匀且可调节的孔结构以及良好的机械性能)而吸引了研究人员的兴趣,这些性能允许介孔材料的各种不同应用,包括催化,分离,纯化,感测和能量存储。研究人员还报告了这些介孔材料的不同功能化策略,以改善其性能,例如通过介孔碳的杂原子掺杂改善电化学性能。合成有序介孔材料的一种通用合成策略是软模板方法。这些介孔材料的功能化(例如杂原子掺杂)有时会导致有序的多孔结构塌陷,这通常是由于高温处理期间的结构变形所致。例如,用软模板法合成具有高结晶孔壁的有序介孔金属氧化物是有挑战性的,因为晶体的生长会使有序多孔结构变形。同时,也难以通过软模板化策略来合成具有高掺杂水平(> 10 at%)的有序介孔碳,因为高温下的快速掺杂反应可能会导致高阶蒸气压导致有序结构崩溃。掺杂反应。在这里,我们展示了几种简便的合成路线,以制造高度结晶的有序介孔金属氧化物和杂原子掺杂的有序介孔碳。本文还讨论了氮掺杂的介孔碳在储能中的应用。据报道,有序介孔氧化锰由于其良好的电化学性能而被用于储能,因为过渡金属锰已经充满了3d。轨道,使其能够捐赠或接受电子。然而,利用软模板化策略合成具有高结晶度的有序介孔锰氧化物是具有挑战性的,因为热处理期间的晶体形成会使有序多孔结构变形。本文通过微波辅助处理将碳酸锰前体转化为氧化物,去除聚合物模板并使Mn3O4结晶,实现了有序介孔晶体氧化锰薄膜的合成。在这种情况下,微波加热主要是由基板(硅晶片)的微波横截面以及氧化锰对微波的吸收所引起,以提供局部能量,该局部能量可以为成核/结晶提供能量。相反地​​,在马弗炉中以类似的表面温度加热碳酸锰会导致纳米结构的塌陷和低结晶度。;除了合成高结晶有序介孔金属氧化物外,我们还证明了通过杂原子掺杂实现有序介孔碳的功能化。与未掺杂的碳相比,掺杂的碳可以表现出增强的电子,电化学和机械性能。这些特性使其可以用于光伏,催化和能量存储等应用。对于这些应用,由于纳米孔结构提供的大表面积和杂原子掺杂所带来的改善的性能,通常需要纳米孔隙率和高杂原子掺杂。然而,实现高含量的杂原子掺杂的碳以及维持纳米孔结构是具有挑战性的。一种常见的掺杂方法是使用含杂原子的聚合物前体并碳化以合成掺杂的碳,其中杂原子的含量受到前体中杂原子数密度的限制。另外,在煅烧期间,纳米孔结构可由于孔的收缩而塌陷。本文报道了一种高掺杂有序介孔碳的通用合成策略。有序介孔碳中高浓度的杂原子掺杂是通过使熔融掺杂剂渗透到二氧化硅增强的介孔交联聚合物(酚醛酚醛)双连续骨架中并随后进行碳化来实现的。经证明,该方法可为氮,硼,硫和磷等多种元素生成具有高杂原子含量(> 10%)的有序介孔碳,并保留了有序结构;采用通用合成路线,氮掺杂多壳中空碳被制造为锂硫电池的基质材料。碳骨架的分级中空结构和化学修饰(杂原子掺杂)可以帮助抑制多硫化物的穿梭,而大孔提供大的孔体积以增加硫的负载。与未掺杂样品相比,氮掺杂样品显示出更好的循环稳定性。鉴于这种具有大孔的中空结构可以增加硫负荷,由于硫的电导率低,比容比其他孔要小得多的情况要低得多;此外,我们还系统地研究了氮掺杂如何影响熔融硫的渗透速率和孔的整体范围通过原位小角度X射线散射(SAXS)填充高度有序的介孔掺杂碳。氮掺杂将极性键引入碳骨架,这可以减慢非极性硫分子向多孔碳中的扩散。这些原位SAXS测量提供了有关硫扩散到中孔的动力学信息以及氮掺杂碳的表面化学如何能够显着阻碍硫渗透到中孔的见解。综上所述,本文主要研究有序中孔的功能化。这种材料不会失去有序结构,也不会在储能中应用氮掺杂碳。

著录项

  • 作者

    Xia, Yanfeng.;

  • 作者单位

    The University of Akron.;

  • 授予单位 The University of Akron.;
  • 学科 Materials science.
  • 学位 Ph.D.
  • 年度 2018
  • 页码 247 p.
  • 总页数 247
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

  • 入库时间 2022-08-17 11:53:02

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