Aluminum deposition from ionic liquids

It has also been observed that the particle size is a sensitive function of the cation. The deposition of nanostructured aluminum is a further option of ionic Uquids.  

So far an ideal interface between a metal and an electrolyte has been considered. Neither the solvent nor any other component of the electrolyte (except the metal ions) can adsorb on the metal interface. The advantage of electrochemical deposition is the possibility to modify this ideal surface by additives, which adsorb on the metal surface. The additives have different effects, which can be used as a characterizing feature like brightening or leveling the deposit. 

The general description of the action of additives is very complicated and difficult to predict in a general manner. But it is possible to extract rules from the complex nature of the absorption process, which can guide us through the chaos. The description of adsorption by adsorption isotheims has already been described in Chapter 4. The Gibbs free energy is the characteristic quantity that describes how strongly a complex molecule will interact with metal surfaces. 

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While the structure at the electrode/ionic liquid interface is uncertain it is clear that in the absence of neutral molecules the concentration of anions and cations at the interface will be potential dependent. The main difference between aqueous solutions and ionic liquids is the size of the ions. The ionic radii of most metal ions are in the range 1-2 A whereas for most ions of an ionic liquid they are more typically 3-5 A. This means that in an ionic liquid the electrode will be coated with a layer of ions at least 6-7 A thick. To dissolve in an ionic liquid most metal species are anionic and hence the concentration of metal ions close to the electrode surface will be potential dependent. The more negative the applied potential the smaller the concentration of anions. This means that reactive metals such as Al, Ta, Ti and W will be difficult to deposit as the effective concentration of metal might be too low to nucleate. It is proposed, as one explanation, that this is the reason that aluminum cannot be electrodeposited from Lewis basic chloroaluminate ionic liquids. More reactive metals such as lithium can however be deposited using ionic liquids because they are cationic and therefore... 

From the above, it is clear that almost all of the metals can be electrodeposited from ionic liquids. However, there are still some key issues that need further study. The deposited layers of metal from ionic liqnids are too thin to use for the commercial industrial production. The efficiency for Mg and Ti deposition was not very high, and most metals were deposited from the aluminum chloride-based ionic liquids as mention above. It is known that these ionic liquids can absorb significant amount of water from the air which can react with the ionic liquids based on PF or AICI3 to produce HF or HCl. Therefore, efforts may be directed to find more suitable ionic liquid and suitable precursors for a technically relevant process. Again, the mechanism of the electrodeposition of metals from ionic liquids still needs to be clarified. 

The electrodeposition of aluminum from ionic liquids has been intensively investigated. The principal rule for ionic liquids is the special design of the anion-cation combination for the metal to be deposited, hr Figure 7.20 the usual ionic liquid for aluminum deposition is shown. 

Stafford GR, Kongstein OE, Haarberg GM (2006) In situ stress measurements during aluminum deposition from AICis-EtMelmCi ionic liquid. J Eiectrochem Soc 153 C207-C212... 

Eutectic mixtures have been used extensively for applications of molten salts to reduce the operating temperature and this is where the significant area of ionic liquids developed from i.e. the quest to find aluminum-based salt mixtures. While the development of aluminum-containing ionic liquids is technologically very important for the field of metal deposition it is clear that there are many other issues that also need to be addressed and hence methods need to be developed to incorporate a wide range of other metals into ionic liquid formulations. 

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. 

In the 1990s John Wilkes and coworkers introduced air- and water-stable ionic liquids (see Chapter 2.2) which have attractive electrochemical windows (up to 3 V vs. NHE) and extremely low vapor pressures. Furthermore, they are free from any aluminum species per se. Nevertheless, it took a while until the first electrodeposition experiments were published. The main reason might have been that purity was a concern in the beginning, making reproducible results a challenge. Water and halide were prominent impurities interfering with the dissolved metal salts and/or the deposits. Today about 300 different ionic liquids with different qualities are commercially available from several companies. Section 4.2 summarizes the state-of-the-art of electrodeposition in air- and water-stable ionic liquids. These liquids are for example well suited to the electrodeposition of reactive elements such as Ge, Si, Ta, Nb, Li and others. 

The first electrodeposition of lithium from an ionic liquid was reported in 1985 by Lipsztajn and Osteryoung [2], They were able to deposit lithium from a 1-ethyl-3-methyl-imidazolium chloride/aluminum trichloride ionic liquid. They noticed that a neutral ionic liquid, a neutral basic ionic liquid (neutral + small excess of RC1) and a neutral acidic ionic liquid (neutral + small excess of AICI3) each have unique features. Both the basic and the neutral acidic ionic liquids show an extension of 1.5 V of the electrochemical window, making them interesting for electrochemical applications. 

Lay and Skyllas-Kazacos were the first to describe a deposition from imidazolium-based tetrachloroaluminate ionic liquid [8], On glassy carbon, aluminum was deposited at —0.2 V (instead of—0.43 V for the pyridinium-based system of Osteryoung and Welch). Furthermore, they were able to show that the deposition process has complicated kinetics and is not simply controlled by diffusion. Using a tungsten electrode they were able to demonstrate in chronopotentiometric measurements that initially a potential of—0.65 V is necessary due to the nucleation process, but after reaching the barrier the potential drops below —0.2 V. 

Endres et al. were able to deposit nanocrystalline aluminum from an aluminum chloride/1-butyl-3-methyl-imidazolium chloride-based ionic liquid (molar ratio 55/45 mol%) and to characterize it by using XRD and TEM [11], Figure 4.5 shows the corresponding XRD pattern. 

The electrodeposition of silver from chloroaluminate ionic liquids has been studied by several authors [45-47], Katayama et al. [48] reported that the room-temperature ionic liquid l-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF4) is applicable as an alternative electroplating bath for silver. The ionic liquid [EMIM]BF4 is superior to the chloroaluminate systems since the electrodeposition of silver can be performed without contamination of aluminum. Electrodeposition of silver in the ionic liquids 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) and l-butyl-3-methylimidazoliumhexafluorophosphate was also reported [49], Recently we showed that isolated silver nanoparticles can be deposited on the surface of the ionic liquid Tbutyl-3-methylimidazolium trifluoromethylsulfonate ([BMIMJTfO) by electrochemical reduction with free electrons from low-temperature plasma [50] (see Chapter 10). This unusual reaction represents a novel electrochemical process, leading to the reproducible growth of nanoscale materials. In our experience silver is quite easy to deposit in many air- and water-stable ionic liquids. 

Because of its high reactivity (—1.67 V vs. NHE), the electrodeposition of aluminum from aqueous solutions is not possible. Therefore, electrolytes for A1 deposition must be aprotic, such as ionic liquids or organic solvents. The electrodeposition of aluminum in organic solutions is commercially available (SIGAL-process [56, 57]) but due to volatility and flammability there are some safety issues. Therefore, the development of room-temperature ionic liquids in recent years has resulted in another potential approach for aluminum electrodeposition. Many papers have been published on the electrodeposition of aluminum from chloroaluminate (first... 

Mo are single phase, supersaturated solid solutions having an fee structure very similar to that of pure Al. Broad reflection indicative of an amorphous phase appears in deposits containing more than 6.5 atom% Mo. As the Mo content of the deposits is increased, the amount of fee phase in the alloy decreases whereas that of the amorphous phase increases. When the Mo content is more than 10 atom%, the deposits are completely amorphous. As the Mo atom has a smaller lattice volume than Al, the lattice parameter for the deposits decreases with increasing Mo content. Potentiodynamic anodic polarization experiments in deaerated aqueous NaCl revealed that increasing the Mo content for the Al-Mo alloy increases the pitting potential. It appears that the Al-Mo deposits show better corrosion resistance than most other aluminum-transition metal alloys prepared from chloroaluminate ionic liquids. 

Therefore it can be seen that metal dissolution is easier in Lewis basic melts. The zinc and aluminum deposition processes, which are by far the most frequently studied, are almost totally reversible. Since these metals have no other stable oxidation states the deposition and dissolution processes are very efficient [3-6]. This has the distinct advantage that the composition of the ionic liquid remains constant and the process becomes the removal of metal from one electrode and its deposition on the other electrode. 

No literature has been published in this area but, as a rule of thumb, metals which dissolve to give complexes that have linear or tetrahedral geometries, e.g. Cu, Ag, Zn, Sn, Pb, can be reversibly deposited and etched. Those with octahedral geometries, e.g. Fe, Ni, Co and Cr, are less reversible. The exceptions to this are the very electronegative metals, most notably A1 which is difficult to electrodeposit from some ionic liquids. The reversibility is also dependent upon the type of ionic liquid and the metal being deposited. Endres has shown that the adhesion of aluminum to mild steel is greatly enhanced by an anodic pulse prior to deposition. It has been shown that this alloy was formed between the steel substrate and the aluminum coating [1],... 

In this chapter we would like to present some plating protocols for the electrodeposition of aluminum, lithium, tantalum and zinc from different ionic liquids. These recipes have been elaborated in our laboratories and should allow the beginner to perform his first electrodeposition experiments. For aluminum we give four different recipes in order to show that the ionic liquid itself can strongly influence the deposition of metals. In the case of tantalum the deposition of the metallic phase is not straightforward as, in unstirred solutions, the more nonstoichiometric tantalum halides form the higher the current density for electrodeposition. Apart from the zinc deposition all experiments should be performed at least under dry air. 

The first electrodeposition of aluminum from an ionic liquid was reported in 1994 by Carlin etal. [157], Two years later, Zhao et al. [158] smdied the aluminum deposition processes on tungsten electrodes in trimethylphenylanunonium chlo-ride/aluminum chloride with mole ratio 1 2. It was shown that the deposition of aluminum was instantaneous as a result of three-dimensional nucleation with hemispherical diffusion-controlled growth, underpotential deposition of aluminum, corresponding to several monolayers. Liao et al. investigated the constant current electrodeposition of bulk aluminum on copper substrates was in 1-methyl-... 

Jiang et al. studied the electrodeposition and surface morphology of aluminum on tungsten (W) and aluminum (Al) electrodes from 1 2 M ratio of [Emim]CI/AlCl3 ionic liquids [165,166]. They found that the deposition process of aluminum on W substrates was controlled by instantaneous nucleation with diffusion-controlled growth. It was shown that the electrodeposits obtained on both W and Al electrodes between -0.10 and -0.40 V (vs. AI(III)/A1) are dense, continuous, and well adherent. Dense aluminum deposits were also obtained on Al substrates using constant current deposition between 10 and 70 mA/cm. The current efficiency was found to be dependent on the current density varying from 85% to 100%. Liu et al. showed in similar work that the 20-pm-thick dense smooth aluminum deposition was obtained with current density 200 A/m for 2 h electrolysis [167],... 

Organic solvents provide suitable potential windows for some important metals such as aluminum and lithium that cannot be deposited from aqueous electrolyte. Some metals are deposited from molten salts. The so-called room temperature molten salts or ionic liquids are a further group of solvents used to deposit metals like aluminum again that cannot be deposited from aqueous electrolytes. 

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