Physical Properties of Ionic Liquids for Electrochemical Applications

Physical Properties of Ionic Liquids for Electrochemical Applications 

In spite of the explosion in studies on ionic liquids (ILs), there is only a small number of studies of their basic characteristics. There are limitless possibilities for the design of ILs by changing their component ion structures. However, the chance of succes s is not very great without accurate information on the structure-properties relationship. Physico-chemical property data for ILs are therefore very important for the present and future ofthe field of ILs. In this chapter, some basic properties of air-stable ILs have been summarized. Some are not directly related to electrochemistry but are very important and useful for a wide range of science and technology related to ILs. 

ILs are defined as organic salts having a melting point (Tm) below 100°C [1-5]. In order to use these ILs as non-volatile electrolyte solutions, it is necessary to maintain the liquid phase over a wide temperature range. Consequently, Tm and the thermal degradation temperature (Tfj of ILs are important properties for ILs as electrochemical media. In this section, the thermal properties of ILs, especially of imidazolium salts, are summarized. The difference between ILs and general electrolyte solutions based on molecular solvents is clarified. Recent results on the correlation between the structure and properties of ILs will also be mentioned. 

Electrodeposition from Ionic Liquids. Edited by F. Endres, D. MacFarlane, A. Abbott Copyright 2008 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31565-9 

Ion radius is given in parentheses, a Decomposition temperature, [Nuu]+, tetramethylammonium cation [N2222]+, tetraethylammonium cation [EMIM]+, l-ethyl-3-methylimidazolium cation. 

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Physical Properties of Ionic Liquids for Electrochemical Applications Table 3.4 Heat capacity and thermal conductivity of ionic liquids... 

Imidazolium-based Ionic Liquids are of considerable interest for electrochemical applications. The hydrogen of the imidazolium group can be substituted easily and therefore a variety of properties are achievable. Probably the most widely used imidazolium-based ILs are the 1,3-aIkyl substituted derivatives, due to their low melting points, relatively low viscosity and high conductivities. Their physical and... 

While much work has been devoted to the wide range of applications of ILs, the basic understanding and study of their structure-property relationship is of equivalent importance but has lagged behind. More specifically, studies on how the structure of the ions in the IL influences their physical properties are rare. Knowledge of the structure-property relationship is important for assessing the suitability of ILs for specific applications as well as the design of new ILs. Very few works have systematically studied the qualitative and/or quantitative relationships between the structures of ILs and their fundamental properties[116], such as melting point, viscosity, density, surface tension, thermal and electrochemical stability, solvent properties, and speed of sound. At present, however, data for many other physico-chemical properties of ionic liquids are in short supply, or too unreliable to allow similar structure-property relationship studies. [117]... 

Electrochemical methods can be powerful tools. They can be used to reveal the chemical and physical properties of room-temperature ionic liquids. Most of existing electrochemical techniques [1] developed in aqueous solutions are applicable for the ionic liquids, as demonstrated in the chloroaluminate ionic liquids. However, there are several procedures that must be observed if one is to obtain reliable data in electrochemical measurements. This section describes the procedures that are important for the ionic liquids. 

A recent review by Tang et al. (2012) describes methods of preparation of ionic liquids in which either the cation or the anion beas functional groups such as hydroxyl, ethers (including PEG -(CH2-CH2-0) -CH3 with up to 16), thiols, and others. They discuss their physical, including electrochemical, properties, toxicity and biodegradability, and applications as solvents for organic and enzymatic reactions, as media for capture of CO and SO and for separation of gaseous mixtures. They list 406 references to literature. 

In the field of organic electrolytes for lithium secondary batteries, other phosphonium-based ionic liquids were proposed by Tsunashima et al These authors observed that cations bearing a methojqrmethylene substituent bound to the phosphorus atom had very low viscosity when coupled with the NTf anion. These compounds exhibited very high thermal and electrochemical stability and were also proposed as electrolytes for dye-sensitized solar cells. The physical properties of [Pg g e 14][Cl] were studied with the addition of both water and metal salts (MgCla and LiCl) to verify their applicability as electrolyte for a Mg-air battery. The addition of water to the IL [Pg g g i4][Cl] both reduced the viscosity and increased the conductivity. Hayyan et demonstrated that... 

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