Review—Electrochemical Surface Finishing and Energy Storage Technology with Room-Temperature Haloaluminate Ionic Liquids and Mixtures

Abstract

In this article, we review the progress in the area of electrochemical technology with Lewis acidic haloaluminate room-temperature ionic liquids (RTILs), such as AlCl 3 -1-ethyl-3-methylimidazolium chloride and AlBr 3 -1-ethyl-3-methylimidazolium bromide, and novel chloroaluminate mixtures consisting of AlCl 3 and polarizable molecules, e.g., dimethylsulfone and urea, during this decade. The number of researchers in the field seems to increase steadily, because now we can handle haloaluminate RTILs and their mixtures with other solvents and materials more easily than we could in the past. In this review, we have categorized the electrochemical technology based on these RTILs into two topics: electroplating and energy storage. In fact, much of the current research is based on work begun during the period from ∼1970 until the 1990's. Room-temperature ionic liquids (RTILs), which is a name synonymous with room-temperature molten salts, have received considerable attention as novel reaction media, liquid materials, and starting materials for functional carbon material. (The name "ionic liquid" was arbitrarily chosen to represent what are actually salts that liquefy at temperatures below 373 K.) The interest in these materials stems from their favorable physicochemical properties, such as low-flammability, negligible vapor pressure, relatively high ionic conductivity, and high electrochemical stability. [1][2][3][4][5][6][7] It is now common knowledge in the chemistry community that many RTILs are stable in air or even water. The "first generation ionic liquids" were difficult to handle because the typical anions known at that time, e.g., AlCl 4 − and Al 2 Cl 7 − , reacted strongly with moisture in the air. Only a very limited number of laboratories with special skills for handling them could effectively use these RTILs. No doubt, the flourishing interest in RTIL chemistry was heavily abetted by the discovery of air and water stable systems, i.e., the so-called "second generation ionic liquids".However, we should not devalue the first generation systems such as those based on quaternary organic halide salts and aluminum halides, e.g., AlBr 3 and AlCl 3 , known as haloaluminates. The reactivity of the haloaluminates as well as their adjustable Lewis acidity is precisely what makes them interesting and very versatile. By contrast, the second-generation "inert" ionic liquids, are just that: inert and nonreactive, both chemically and electrochemically. Furthermore, the ionic conductivity and viscosity of the haloaluminates, which are important factors for the electrochemical applications described herein, exceed those for the comparable second-generation RTILs. For example, the ionic conductivity and viscosity for 60.0-40.0 mole percent (or 60 mol%) AlCl 3 -1-ethyl-3-methylimidazolium chloride ([C 2 mim]Cl) are 17.1 mS cm −1 and 15.35 mPa s, respectively, at 298 K.8 (In the interest of brevity, we will refer to the composition of these ionic liquids by simply stating only the amount of the aluminum halide.) On the other ...

Robust Aluminum Electrodeposition from Ionic Liquid Electrolytes Containing Light Aromatic Naphta as Additive

“…All electrochemical studies with the ionic liquid electrolyte were carried out in a dry room using an Autolab model PGSTAT30 potentiostat/galvanostat fitted with a conventional three-electrode glass cell (50 mL baker, solution volume 20 mL for voltammetry and 150 mL beaker, solution volume 150 mL for electrodeposition essays). Voltammetry essays were conducted in a three-electrode system in non-stirred solutions at a temperature of 25 water, and dried prior to all measurements. Electroplating experiments were carried out in a two-electrode system in well stirred solutions under argon bubbling at 50°C temperature, j = 1.5 Acm À 2 for 2 hours.…”

“…Regarding electrochemical process parameters, on one hand it is known that electrodeposition in these electrolytes is facilitated at high temperatures as they enhance ions movement. On the other hand, it turned out that low content of aluminum chloride in the melt and high voltages gave a uniform particle size distribution in the aluminum deposits . This favourable surface morphology is attributed to changes in the Lewis acidity of the melt resulting from the depletion of Al 2 Cl 7 − species at the electrode surface during the reduction reaction.…”

“…On the other hand, it turned out that low content of aluminum chloride in the melt and high voltages gave a uniform particle size distribution in the aluminum deposits. [25] This favourable surface morphology is attributed to changes in the Lewis acidity of the melt resulting from the depletion of Al 2 Cl 7 À species at the electrode surface during the reduction reaction. Concerning the electrolyte composition, species evolution/degradation caused by the cation reduction reactions seems to be another key factor.…”

“…The mechanism whereby toluene influences the electrodeposition is related with benzene ring structure and the presence of double bonds. Xylene , [23,25] toluene [27] (30 % v/v) and benzene (50 % v/v) [28,29] have been also considered as co-solvents for obtaining homogeneous and mirror-bright Al deposits. The presence of the co-solvent promotes lower viscosity and higher conductivity in the electrolyte, in comparison with bare melts, facilitating the plating process.…”

Emerging Nonaqueous Aluminum‐Ion Batteries: Challenges, Status, and Perspectives

“…Alternative electrolytes are strongly needed for the further development of rechargeable nonaqueous AIBs because typical AlCl 3 ‐containing imidazole‐based ILs are viscous, moisture sensitive, and corrosive, with a low oxidation voltage. In addition, the diverse ILs recently used in electroplating Al 3+ , such as 1‐ethyl‐3‐methylimidazolium (EMIm) [Tf 2 N]/AlCl 3 , 1‐(2‐methoxyethyl)‐3‐methylimidazolium chloride ([MoeMIm]Cl)/AlCl 3 , mixed systems of BMIC/AlCl 3 and EMIC/AlCl 3 or [BMP] Tf 2 N and [EMIm] Tf 2 N/AlCl 3 , 1‐butyl‐1‐sulfonate‐ethylpyrrolidiniumtrifluoromethyl ([EMIm]TfO), piperidinium‐alkyl halides ([C 3 mpip][NTf 2 ]), or triethylamine hydrochloride, might have potential for application as electrolytes in AIBs. Stimulated by this possibility, several novel organic electrolytes have been introduced in aluminum battery systems as substitutes for traditional chloroaluminate ILs, and these novel electrolytes can be divided into two groups. (1)Chloride Substitution…”