Liquid electrochemical stability range

A key criterion for selection of a solvent for electrochemical studies is the electrochemical stability of the solvent [12]. This is most clearly manifested by the range of voltages over which the solvent is electrochemically inert. This useful electrochemical potential window depends on the oxidative and reductive stability of the solvent. In the case of ionic liquids, the potential window depends primarily on the resistance of the cation to reduction and the resistance of the anion to oxidation. (A notable exception to this is in the acidic chloroaluminate ionic liquids, where the reduction of the heptachloroaluminate species [Al2Cl7] is the limiting cathodic process). In addition, the presence of impurities can play an important role in limiting the potential windows of ionic liquids. 

Ionic liquids can be used in a wide range of applications owing to their unusual physical and chemical properties [21]. They show high thermal stability, negligible vapor pressure, non-flammability, excellent electrochemical stability and high ionic conductivity. This, combined with their advantages of being non-volatile and... 

Binary room-temperature ionic liquid (RTIL)-based electrolytes have attracted significant attention because of their negligible vapor pressure good thermal and electrochemical stability in the range of 5.3 V [83] good dissolution properties with many organic and inorganic compounds and lithium salts and low flammability [84]. 

Finally, as is the case for water, organic liquids have a limited range of electrochemical stability. The products of these reactions are often also electroactive and can become involved in electrochemical or chemical reactions of interest. While solution oxidation or reduction can also occur in aqueous solutions, the products are simple bases or acids (i.e., and OH"). However, the products resulting fiom electroactive liquids are usually a mixture of complex organic acids. These complex organic acids can combine with other ions in solution and form other chemicals that could be even more aggressive than the acid from which they were derived. This has been shown to be an issue in methanol by Smialowska [10], who found that galvanostatic polarization of stainless steel in pure methanol resulted in the formation of formic acid as follows ... 

And indeed, the temperature stability range as well as the electrochemical stability (both against oxidation and reduction) is very pronounced for many of the commonly used Ionic Liquids. The same is true for their high non-inflammability [1,4, 5, 7]. 

It is important to choose an electrolyte with a wide electrochemically stable range. For a solvent, the selection seems difficult due to its intrinsic electrochemical stability. For example, for an aqueous solution, the electrochemical disassociation window of water is around 1.23 V at room temperature. If water is used as a supercapacitor electrolyte solvent, the maximum cell voltage will be around 1.23 V if acetonitrile is the solvent, the electrode potential window is around 2.0 V with an ion liquid, the electrode potential window can be as high as 4.0 V. Therefore, different solvents have different potential windows. Table 2.2 lists several common solvents and their potential windows for supercapacitors. 

Ionic liquids (ILs) are probably one of the most studied chemical compounds in the last decade. This is particularly motivated by ILs imique physical-chemical properties that enable their application in a broad range of scientific fields. ILs are comprised entirely by ions and most of them exhibit a negligible vapour paessure, ionic conductivity and a high thermal, chemical and electrochemical stability. (Femicola et al., 2006 Galinski et al., 2006 Lu et al., 2002)... 

As two important copolymers of PVDF, the P(VDF-HFP) [4] and P(VDF-CTFE) [23] had been developed for gel polymer electrolyte in LIBs. The introduction of copolymer components was to reduce the crystallinity of the PVDF chain. The reduction of crystallinity could increase the ionic conductivity. Electrospun P(VDF-HFP) and P(VDF-CTFE) fibrous membranes had been proved to show high ionic conductivities in the range of several mS cm which was attributed to the easy transportation of the liquid electrolyte through the fully interconnected pore structure of the membrane. For example, the electrospun P(VDF-HFP) fibrous membrane had high ionic conductivities in the range of 4.59 mS cm", high electrolyte uptake of 425 % at room temperature, and good electrochemical stability with a potential of over 4.5 V versus Li/Li+ [29]. 

Liquid organic solutions High conductivity over a wide temperature range. Liquid state. Narrow electrochemical stability window. Safety. ... 

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]... 

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