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Ionic Liquids and Electrochemical Double-Layer Capacitors: A Study of Electrolyte Structure and Capacitor Performance.

机译：离子液体和电化学双层电容器：电解质结构和电容器性能的研究。

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摘要

Ionic liquids (ILs) are salts with a melting temperature (T m) at or below room temperature. Depending on their structure, they can be nonvolatile, nonflammable, and highly stable, both thermally and electrochemically. These properties make ILs attractive alternatives to the organic solvent-based electrolytes currently in use. However, ILs have a number of drawbacks, including a relatively low ionic conductivity and a Tm greater than that of many solvents which often precludes their use at low (lower than -20 °C) temperature. This work investigates two potential means of addressing these problems: the addition of aprotic solvents to ILs and mixtures containing two ILs. The use of these mixtures in electrochemical double-layer capacitors (EDLCs) is examined and links between properties and capacitor performance quantified.;The physicochemical properties (density, viscosity, ionic conductivity, and thermal phase behavior) of the pure ILs, IL-aprotic solvent mixtures, and IL-IL mixtures containing varying ions were initially investigated. While pure ILs are often highly viscous, with a correspondingly low ionic conductivity, the IL-solvent mixtures containing small amounts (20 mol%) of aprotic solvents are much more conductive and less viscous than the pure ILs. In addition, the presence of solvent molecules inhibits the crystallization of the ILs, thus retaining the liquid phase for these electrolytes to a much lower temperature. The interactions between the ILs and solvent appear to be quite weak, however, allowing the solvent to readily volatilize at higher temperatures. Thus, IL-IL mixtures were also studied as a route to modify the properties of IL-based electrolytes without reintroducing volatility concerns. Mixing two ILs can result in significant variations in the viscosity, density, and ionic conductivity as compared to that of the pure ILs. Furthermore, the compositions with mixed anions often remain amorphous mixtures, rather than crystallizing, thereby potentially allowing IL-IL mixed electrolytes to be used at low temperatures.;In addition to modifying the properties and phase behavior of electrolytes, the presence of mixed cations and/or anions in an EDLC electrolyte may influence both the energy and power density of a device. The addition of smaller ions to IL-based electrolytes was expected to result in increased specific energy due to the utilization of smaller pores and/or increased specific power due to the higher mobility of smaller ions. However, these effects are not observed for many of the IL-IL electrolytes, suggesting that factors other than ion size influence the performance of IL-based EDLCs. In addition, the noted differences in performance did not correlate with the performance that would be predicted based upon the current understanding of electrolyte optimization for capacitors. Specifically, increases in specific energy may be accompanied by increases in viscosity and decreases in conductivity (counter to what one would expect), while increases in specific power may be observed for mixtures that have no changes in physicochemical properties. In toto, these results suggest that the relationships between electrolyte properties and EDLC performance are complex and require further attention from the research community.
机译：离子液体（ILs）是具有等于或低于室温的熔融温度（T m）的盐。根据它们的结构，它们在热和电化学方面可以是不挥发的，不易燃的并且高度稳定的。这些特性使IL成为当前使用的基于有机溶剂的电解质的有吸引力的替代品。但是，离子液体具有许多缺点，包括相对较低的离子电导率和比许多溶剂更高的Tm，这常常使离子液体无法在低温（低于-20°C）下使用。这项工作研究了解决这些问题的两种潜在方法：向离子液体中添加非质子溶剂以及含有两种离子液体的混合物。检查了这些混合物在电化学双层电容器（EDLC）中的使用并量化了性能与电容器性能之间的联系;纯ILs，非质子传递蛋白的理化特性（密度，粘度，离子电导率和热相行为）最初研究了溶剂混合物和包含各种离子的IL-IL混合物。尽管纯净离子液体通常是高粘度的，具有相应较低的离子电导率，但与纯净离子液体相比，含有少量（20摩尔％）非质子溶剂的IL-溶剂混合物的导电性更高，粘度更低。另外，溶剂分子的存在抑制了IL的结晶，因此将这些电解质的液相保持在非常低的温度。 IL与溶剂​​之间的相互作用似乎很弱，但是使溶剂在较高温度下容易挥发。因此，还研究了IL-IL混合物，作为在不重新引入挥发性问题的情况下修改基于IL的电解质性质的途径。与纯IL相比，将两种IL混合会导致粘度，密度和离子电导率发生明显变化。此外，具有混合阴离子的组合物通常保持无定形混合物，而不是结晶，从而潜在地允许在低温下使用IL-IL混合电解质。除了改变电解质的性质和相行为外，还存在混合阳离子和EDLC电解质中的阴离子可能会影响设备的能量和功率密度。期望将较小的离子添加到基于IL的电解质中会由于使用较小的孔而导致比能增加，和/或由于较小的离子具有较高的迁移率而导致比功率增加。但是，对于许多IL-IL电解质都没有观察到这些影响，这表明除离子大小以外的其他因素也会影响基于IL的EDLC的性能。另外，所指出的性能差异与根据当前对电容器的电解质优化的理解所预测的性能不相关。具体而言，比能的增加可能伴随粘度的增加和电导率的降低（与人们的预期相反），而对于物理化学性质不变的混合物，可以观察到比功率的增加。总的来说，这些结果表明电解质性能和EDLC性能之间的关系很复杂，需要研究界的进一步关注。

著录项

  • 作者

    Fox, Eric Townsend.;

  • 作者单位

    North Carolina State University.;

  • 授予单位 North Carolina State University.;
  • 学科 Engineering Chemical.
  • 学位 Ph.D.
  • 年度 2012
  • 页码 147 p.
  • 总页数 147
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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