Room-temperature molten salts

Room-temperature molten salts are a relatively new subgroup of liquid nonaqueous electrolytes. They share their advantages and disadvantages. Unfortunately, until now, no useful room-temperature molten salt based on lithium cations has been available. 

Jeng EGS, Sun IW (1997) Electrochemistry of tellurium (IV) in the basic aluminum chloiide-l-methyl-3-ethylimidazolium chloride room temperature molten salt. J Electrochem Soc 144 2369-2374... 

Commonly is used a short term ionic liquid instead of room temperature ionic liquid or room temperature molten salt , which makes no distinction between salts liquid at room temperature and those liquid below 100°C. 

Mitchell, J. A., The Electrodeposition of Cobalt, Iron, Antimony and Their Alloys from Acidic Aluminum Chloride 1 -methyl-3-ethylimidazolium Chloride Room-Temperature Molten Salts, Ph.D. Dissertation, 1997, University of Mississippi University, MS. 

The electrochemistry of Cd(II) was investigated at different electrodes (GC, polycrystalline tungsten, Pt, Ni) in a basic l-ethyl-3-methylimidazolium chloride/tet-rafluoroborate, at room temperature molten salt [312], and in acidic zinc chloride-l-ethyl-3-methylimidazolium [284]. 

Figure 1.2 Captain (Dr.) John S. Wilkes at the U.S. Air Force Academy in 1979. This official photo was taken about when he started his research on ionic liquids, then called room-temperature molten salts. ...

Kubo, W. et al.. Quasi-solid-state dye-sensitized solar cells using room temperature molten salts and a low molecular weight gelator, Chem. Commun., 374, 2002. 

Watanabe, M., Yamada, S-L, and Ogata, N., Ionic conductivity of polymer electrolytes containing room temperature molten salts based on pyridinium halide and aluminium chloride, Electrochim. Acta, 40,2285,1995. 

Ohno, H. and Ito, K., Room-temperature molten salt polymers as matrix for fast ion conduction, Chem. Lett., 751,1998. 

Note that [CjCiImjCl is not a room temperature molten salt since its melting point is just above 450 K. 

Matsumoto, H., Matsuda, T., and Miyazaki, Y., Room temperature molten salts based on trialkylsulfonium cations and bis(trifluoromethylsulfonyl)imide, Chem. Lett, 12, 1430-1431,2000. 

Hussey, C. L., Room-temperature molten salt systems, Adv. Molten Salt Chem., 5, 185, 1983. 

Rhinebarger, R. R., Rovang, J. W., Popov, A. L, Multinuclear magnetic- resonance studies of macrocyclic complexation in room-temperature molten salts, Inorg. Chem., 23, 2558,1984. 

Sun, J., Forsyth, M., MacFarlane, D. R., Room-temperature molten salts based on the quaternary ammonium ion, /. Phys. Chem. B., 102,8858-8864,1998. 

Murase, K., Nitta, K., Hirato, T., Awakura, Y., Electrochemical behaviour of copper in trimethyl-n-hexylammonium bis((trifluoromethyl)sulfonyl)amide, an ammonium imide-type room temperature molten salt, /. Appl. Electrochem., 31, 1089, 2001. 

Dymek, C. J., Jr., and Stewart, J. J. R, Calculation of hydrogen-bonding interactions between ions in room-temperature molten salts, Inorg. Chem., 28, 1472-1476, 1989. 

Fuller, J., Carlin, R. T., DeLong, H. C., and Haworth, D., Structure of 1-ethyl-3-methylimidazolium hexafluorophosphate Model for room temperature molten salts, Chem. Commun., 299-300, 1994. 

Nanbu, N., Sasaki, Y., and Kitamura, R, In situ FT-IR spectroscopic observation of a room-temperature molten salt gold electrode interphase, Electrochem. Commun., 5,383-387,2003. 

De Andrade, J., Does, E. S., and Stassen, H., A force field for liquid state simulations on room temperature molten salts l-ethyl-3methylimidazolium tetra-chloroaluminate, /. Phys. Ghent. B, 106, 3546-3548,2002. 

The main consensus seems to be that the first major studies of room temperature molten salts were made in the 1940s by a group led by Frank Hurley and Tom Weir at Rice University. When they mixed and gently warmed powdered pyridinium halides with aluminum chloride, the powders reacted, giving a clear, colorless liquid [4-7]. These mixtures were meant to be used in electrochemistry, particularly in electroplahng with aluminum. 

Room temperature molten salt systems based on methyl-hexyl-imidazolium iodide appear to afford particular advantages over organic liquids as solvents for solar cell electrolytes. Cell performance showed outstanding stability, with an estimated sensitizer turnover in excess of 50 million (Papageorgiou et al., 1996). 

It was shown that room-temperature molten salts derived from the combination of 1,3-dialkylimidazolium chloride and A1C13 can be used as solvents in two-phase catalytic dimerization of propene to give hexenes catalyzed by Ni(II) compounds (125). The effects of phosphane ligands coordinated to nickel and operating variables were also investigated (126). The dimerization products separate as an organic layer above the molten salt. This reaction has been carried out with n-butenes as the reactant and cationic nickel complex catalysts dissolved in organochloroaluminate liquids (127). 

Although the chloroaluminates are the best known room-temperature molten salts, there are several other interesting systems. For example, if one mixes the crystalline solids trielhylamnionium chloride and copper[I) chloride, an endothermic reaction takes place Lo form a light green oil. The most reasonable reaction is the coordination of a second chloride ion to the copper(l) ion34... 

Rechargeable cells employing aluminum, Al, as a negative electrode in room temperature molten salts have been investigated. 

Linear sweep voltammetry, capacitance-voltage and automated admittance measurements have been applied to characterize the n-GaAs/room temperature molten salt interphase. Semiconductor crystal orientation is shown to be an important factor in the manner in which chemical interactions with the electrolyte can influence the surface potentials. For example, the flat-band shift for (100) orientation was (2.3RT/F)V per pCl" unit compared to 2(2.3RT/F)V per pCl" for (111) orientation. The manner in which these interactions may be used to optimize cell performance is discussed. The equivalent parallel conductance method has been used to identify the circuit elements for the non-illum-inated semi conductor/electrolyte interphase. The utility of this... 

Excision reactions are sometimes accompanied by redox chemistry. For example, dissolution of the 2D solid Na4Zr6BeCli6 in acetonitrile in the presence of an alkylammonium chloride salt results in simultaneous reduction of the cluster cores (144). Here, the oxidation product remains unidentified, but is presumably the solvent itself. As a means of preventing such redox activity, Hughbanks (6) developed the use of some room temperature molten salts as excision media, specifically with application to centered zirconium-halide cluster phases. A number of these solids have been shown to dissolve in l-ethyl-2-methylimidazolium chloride-aluminum chloride ionic liquids, providing an efficient route to molecular clusters with a full compliments of terminal chloride ligands. Such molten salts are also well suited for electrochemical studies. 

Between 1980 and about 2000 most of the studies on the electrodeposition in ionic liquids were performed in the first generation of ionic liquids, formerly called room-temperature molten salts or ambient temperature molten salts . These liquids are comparatively easy to synthesize from AICI3 and organic halides such as Tethyl-3-methylimidazolium chloride. Aluminum can be quite easily be electrode-posited in these liquids as well as many relatively noble elements such as silver, copper, palladium and others. Furthermore, technically important alloys such as Al-Mg, Al-Cr and others can be made by electrochemical means. The major disadvantage of these liquids is their extreme sensitivity to moisture which requires handling under a controlled inert gas atmosphere. Furthermore, A1 is relatively noble so that silicon, tantalum, lithium and other reactive elements cannot be deposited without A1 codeposition. Section 4.1 gives an introduction to electrodeposition in these first generation ionic liquids. 

It was John Wilkes who realized that room-temperature molten salts would only experience a widespread interest and uptake if they were stable under environmental conditions. Wilkes group published details of the first such liquid in 1992 using the BF]j" and the PFj anions, the latter showing a miscibility gap with water. Thus these liquids could, in principle, be made water free. (Today we know that ionic liquids containing BFJ and PF are subject to decomposition in the presence of water.) Electrochemical studies showed that even these early ionic liquids had wide electrochemical windows of about 4 V with cathodic limits of-2 to -2.5 V. vs. NHE. This cathodic limit should, from the thermodynamic point of view, be wide enough to electrodeposit many reactive elements. 

Around 1995, Seddon realized that the expression room-temperature molten salts was counter-productive. The expression molten salt was always associated with high temperature , as also the editors (and many authors) of this book had... 

Molten salts constitute a category of liquids which is called ionic liquids or molten electrolytes. These liquids have some characteristics which are different from that of liquids at room temperature. Molten salt studies are very important for understanding of the liquid state because molten salts consist of ions, and the... 

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