Liquid electrolytes ionic liquids

The terms pure liquid electrolyte, ionic liquid, fused salt, and molten salt are used synonymously. 

The electrochemical conversions of solid compounds and materials that are in direct contact with electrolyte solutions or liquid electrolytes (ionic liquids), belong to the most widespread reactions in electrochemistry. Such conversions take place in a wide variety of circumstances, including the majority of primary and secondary batteries, in corrosion, in electrochemical machining, in electrochemical mineral leaching, in electrochemical refining (e.g., copper refining), and in electrochemical surface treatments (e.g., the anodization of aluminum). 

TEGDME (tetraethylene glycol dimethyl ether, tetraglyme), ionic liquids, and solid electrolyte were proposed as candidate electrolyte. As a liquid electrolyte, ionic liquid may be a solution. 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI... 

Electrolytes typically used in ECDs, as in any other electrochemical cell, belong to four main classes aqueous electrolytes, organic liquid electrolytes, ionic liquids and solid polymer electrolytes. The adoption of so-called ionic liquids, such as ethyl ammonium nitrate ([EtNH3][N03]), l-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) or hexafluorophos-phate ([BMIMjfPFg]), can result in improved lifetime and response speed for electrochromics and actuators, and are best proposed being mixed with polymer or gel matrices. " ... 

Comparing this approach with previous work - except the studies on solid electrolytes - ionic liquids have two distinct advantages over aqueous or organic solvents (i) Due to their extremely low vapor pressure ionic liquids can be used without any problem in standard plasma vacuum chambers, and the pressure and composition in the gas phase can be adjusted by mass flow controllers and vacuum pumps. As the typical DC or RF plasma requires gas pressures of the order of 1 to 100 Pa, this cannot be achieved with most of the conventional liquid solvents. If the solvent has a higher vapor pressure, the plasma will be a localised corona discharge rather than the desired extended plasma cloud, (ii) The wide electrochemical windows of ionic liquids allow, in principle, the electrodeposition of elements that cannot be obtained in aqueous solutions, such as Ge, Si, Se, A1 and many others. Often this electrodeposition leads to nanoscale products, as shown e.g. by Endres and coworkers [60]. 

Another key limiting factor for the conversion of C02 is the low mass-transfer rates of C02 at the electrode/catalyst surface, which is exacerbated by the low solubility of C02 in many electrolytes. Ionic liquids [34] may help to solve this problem, as Zhao et al. [35] originally proposed. 

UV-Vis spectroscopy — Electronic absorption in the UV-Vis range by species generated during electrochemical reactions or being present at the electrochemical interface between the electronically conducting electrode and an ionically conducting phase (electrolyte solution, molten electrolyte, ionic liquid, solid electrolyte) can be studied with in situ UV-Vis spectroscopy in various modes [i-iii] ... 

Thin layer — A layer of -+ electrolyte solution (molten salt electrolyte, - ionic liquid) of about 2 to 100 pm thickness is commonly treated as a thin layer because of particular properties and behavior. In bulk - electrolysis methods the amount of convertible species contained in a thin layer is very limited, thus exhaustive electrolysis becomes feasible. In numerous spectroelec-trochemical setups the electrolyte solution confined between the electrode surface under investigation and the... 

Quaternary phosphonium salts are organophosphorous compounds used as Wittig olefination reagents, phase transfer catalysts, electrolytes, ionic liquids, and as surface active reagents. Their preparation involves the C-P bond formation in tertiary phosphines. We envisaged that addition of phosphines to unsaturated compounds should be preferable as compared to the conventional method using a substitution reaction of organohalogen compounds (Scheme 1). In this chapter, we describe our recent study on this subject. 

One-dimensional ion conduction is achieved for columnar liquid crystalline ionic liquids 10a,b [29]. In the macroscopically ordered states of these columnar materials, ionic conductivities parallel to the columnar axis (ay) is higher than those perpendicular to the axis (ox)- For example, compound 10b shows the conductivities of 3.1 X 10 S cm (ay), 7.5 x 10 S cm (cJx), and anisotropy (ay/ Qx) of 41 at 100°C. These materials function as self-organized electrolytes. They dissolve a variety of ionic species such as lithium salts. Compound 10b containing LiBp4 (molar ratio of LiBp4 to 10b 0.25) exhibits the conductivities of... 

The electrodes studied above function over a wide voltage range (0.1 V 4.6 V) which requires the design of electrolytes that are perfectly resistant to oxidation and reduction. Liquid electrolytes and liquid ionic polymers have been considered, primarily to determine the best formulation by trial and error. The study of electrode/electroljde interfaces and the impact of their formiilation on the electrochemical performance of the negative or positive electrode material or even of the system is currently preferred at RS2E rather than the design of new salts or solvents. 

In some respects, as for example in the aqueous or hydrate melts, water itself may be considered a proper component of the molten mixture. Other terms used for this general group of liquids are ionic liquids, fused salts, and molten or liquid electrolytes. Particularly common are the mixed salts of the alkali metals and alkaline earth metals which find application in the metallurgical industries owing to their excellent thermal and electrochemical stabilities, and good heat capacities. 

In recent studies it has been shown that the electrolyte used for ICP-based devices including artificial muscles is critical (73). Subsequent to this, it was discovered that a new class of electrolytes, ionic liquids, have characteristics that are particularly useful for ICPs (74,75). 

Usually, when thinking of electrochemical reactions, reactions are considered at a metal/electrolyte or semiconductor/electrolyte interface, but rarely about the interface between electrolyte solutions or, more recently, the electrolyte-ionic liquid interface or even the interface between two immiscible ionic liquids. 

For liquid electrolytes, ionic conductivity, self-diffusivity, and viscosity are three key properties. Though originally based on dilute aqueous electrolyte solutions, the Walden rule [52] has been proposed as a tool to provide insight to the proton transfer and ion association. The rule suggests that the molar cmiductivity of an electrolyte, A, is proportional to the fluidity, which can be expressed as the inverse of the shear viscosity i/. In other words, the product of the molar conductivity and viscosity of an electrolyte is a constant, as shown in (3.10). 

Quasi-solid-state electrolytes include gel polymer electrolytes, ionic liquids, and plastic crystal systems. It is important to distinguish polymer electrolytes and gel polymer electrolytes. In polymer electrolytes, charged cationic or anionic groups are chemically bonded to a polymer chain, while gel polymer electrolytes are solvated by a high dielectric constant solvent and are free to move. In a classical gel electrolyte, polymer and salts are mixed with a solvent, usually having a concentration above 50 wt%, and the role of the polymer is to act as a stiffener for the solvent, creating a three-dimensional network, where cations and anions move freely in the liquid phase [88]. The solid polymer electrolyte includes poly(ethylene oxide) (PEO)-based lithium ion conductors that typically show conductivities of 10 S cm while the gel polymer electrolytes have semisolid character with much higher ionic conductivities of the order 10 —10 S cm . ... 

Ionic conductors arise whenever there are mobile ions present. In electrolyte solutions, such ions are nonually fonued by the dissolution of an ionic solid. Provided the dissolution leads to the complete separation of the ionic components to fonu essentially independent anions and cations, the electrolyte is tenued strong. By contrast, weak electrolytes, such as organic carboxylic acids, are present mainly in the undissociated fonu in solution, with the total ionic concentration orders of magnitude lower than the fonual concentration of the solute. Ionic conductivity will be treated in some detail below, but we initially concentrate on the equilibrium stmcture of liquids and ionic solutions. 

U.S. Air Force Academy in 1961. He was an early researcher in the development of low-temperature molten salts as battery electrolytes. At that time low temperature meant close to 100 °C, compared to many hundreds of degrees for conventional molten salts. His work led directly to the chloroaluminate ionic liquids. 

The allcylpyridinium cations suffer from being relatively easy to reduce, both chemically and electrochemically. Charles Hussey (Figure 1.3) and I set out a program to predict cations more resistant to reduction, to synthesize ionic liquids on the basis of those predictions, and to characterize them electrochemically for use as battery electrolytes. 

We had no good way to predict if they would be liquid, but we were lucky that many were. The class of cations that were the most attractive candidates was that of the dialkylimidazolium salts, and our particular favorite was l-ethyl-3-methylimid-azolium [EMIM]. [EMIMJCl mixed with AICI3 made ionic liquids with melting temperatures below room temperature over a wide range of compositions [8]. We determined chemical and physical properties once again, and demonstrated some new battery concepts based on this well behaved new electrolyte. We and others also tried some organic reactions, such as Eriedel-Crafts chemistry, and found the ionic liquids to be excellent both as solvents and as catalysts [9]. It appeared to act like acetonitrile, except that is was totally ionic and nonvolatile. 

The pyridinium- and the imidazolium-based chloroaluminate ionic liquids share the disadvantage of being reactive with water. In 1990, Mike Zaworotko (Eigure 1.4) took a sabbatical leave at the Air Eorce Academy, where he introduced a new dimension to the growing field of ionic liquid solvents and electrolytes. 

For a review of salts formerly thought of as low-temperature ionic liquids, see Mamantov, G., Molten salt electrolytes in secondary batteries, in Materials for Advanced Batteries (Murphy, D. W., Broadhead, J., and Steele, B.C. H. eds.). Plenum Press, New York, 1980,... 

It should be emphasized that ionic liquids are simply organic salts that happen to have the characteristic of a low melting point. Many ionic liquids have been widely investigated with regard to applications other than as liquid materials as electrolytes, phase-transfer reagents [12], surfactants [13], and fungicides and biocides [14, 15], for example. 

Interest in using ionic liquid (IL) media as alternatives to traditional organic solvents in synthesis [1 ], in liquid/liquid separations from aqueous solutions [5-9], and as liquid electrolytes for electrochemical processes, including electrosynthesis, primarily focus on the unique combination of properties exhibited by ILs that differentiate them from molecular solvents. 

The early history of ionic liquid research was dominated by their application as electrochemical solvents. One of the first recognized uses of ionic liquids was as a solvent system for the room-temperature electrodeposition of aluminium [1]. In addition, much of the initial development of ionic liquids was focused on their use as electrolytes for battery and capacitor applications. Electrochemical studies in the ionic liquids have until recently been dominated by work in the room-temperature haloaluminate molten salts. This work has been extensively reviewed [2-9]. Development of non-haloaluminate ionic liquids over the past ten years has resulted in an explosion of research in these systems. However, recent reviews have provided only a cursory look at the application of these new ionic liquids as electrochemical solvents [10, 11]. 

The behavior of ionic liquids as electrolytes is strongly influenced by the transport properties of their ionic constituents. These transport properties relate to the rate of ion movement and to the manner in which the ions move (as individual ions, ion-pairs, or ion aggregates). Conductivity, for example, depends on the number and mobility of charge carriers. If an ionic liquid is dominated by highly mobile but neutral ion-pairs it will have a small number of available charge carriers and thus a low conductivity. The two quantities often used to evaluate the transport properties of electrolytes are the ion-diffusion coefficients and the ion-transport numbers. The diffusion coefficient is a measure of the rate of movement of an ion in a solution, and the transport number is a measure of the fraction of charge carried by that ion in the presence of an electric field. 

Transport numbers are intended to measure the fraction of the total ionic current carried by an ion in an electrolyte as it migrates under the influence of an applied electric field. In essence, transport numbers are an indication of the relative ability of an ion to carry charge. The classical way to measure transport numbers is to pass a current between two electrodes contained in separate compartments of a two-compartment cell These two compartments are separated by a barrier that only allows the passage of ions. After a known amount of charge has passed, the composition and/or mass of the electrolytes in the two compartments are analyzed. Erom these data the fraction of the charge transported by the cation and the anion can be calculated. Transport numbers obtained by this method are measured with respect to an external reference point (i.e., the separator), and, therefore, are often referred to as external transport numbers. Two variations of the above method, the Moving Boundary method [66] and the Eiittorff method [66-69], have been used to measure cation (tR+) and anion (tx ) transport numbers in ionic liquids, and these data are listed in Table 3.6-7. 

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