Electrochemical window electrodeposition

Germanium In situ STM studies on Ge electrodeposition on gold from an ionic liquid have quite recently been started at our institute [59, 60]. In these studies we used dry [BMIM][PF<3] as a solvent and dissolved Gel4 at estimated concentrations of 0.1-1 mmol 1 the substrate being Au(lll). This ionic liquid has, in its dry state, an electrochemical window of a little more than 4 V on gold, and the bulk deposition of Ge started several hundreds of mV positive from the solvent decomposition. Furthermore, distinct underpotential phenomena were observed. Some insight into the nanoscale processes at the electrode surface is given in Section 6.2.2.3. 

Recently, a eutectic mixture of choline chloride and urea (commercially known as Reline) was used as a medium from which CdS, as well as CdSe and ZnS, thin films were electrodeposited for the first time [53]. Reline is a conductive room-temperature ionic liquid (RTIL) with a wide electrochemical window. The voltammetric behavior of the Reline-Cd(II)-sulfur system was investigated, while CdS thin films were deposited at constant potential and characterized by photocurrent and electrolyte electroabsorbance spectroscopies. 

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. 

Piersma et al. demonstrated that lithium can be electrodeposited from 1-ethyl -3-methyl-imidazolium tetrachloroaluminate ionic liquid, when lithium chloride was dissolved in the melt [3], Platinum, glassy carbon and tungsten were used as working electrodes with molybdenum and platinum foils as counter electrodes. At -2.3 V a reduction peak of Li+ is observed and at about -1.6 V the stripping of lithium occurs. They noticed that the efficiency was much less than 100%. In addition, they were able to demonstrate that the addition of proton sources like triethanolamine-HCl widens the electrochemical window and allows the plating and stripping of lithium (and also sodium). 

In this chapter we have briefly discussed the high potential of air- and water-stable ionic liquids as electrolytes for metal deposition. Their extraordinary physical properties, superior to those of water or organic solvents, and their stability, open the door to the electrodeposition of many metals. Some advantages of air- and water-stable ionic liquids in electrodeposition are that they are quite easy to purify and handle and in most cases they do not decompose under environmental conditions. They can have pretty wide electrochemical windows of up to 6 V, and hence they... 

Chlorostannate and chloroferrate [110] systems have been characterized but these metals are of little use for electrodeposition and hence no concerted studies have been made of their electrochemical properties. The electrochemical windows of the Lewis acidic mixtures of FeCh and SnCh have been characterized with ChCl (both in a 2 1 molar ratio) and it was found that the potential windows were similar to those predicted from the standard aqueous reduction potentials [110]. The ferric chloride system was studied by Katayama et al. for battery application [111], The redox reaction between divalent and trivalent iron species in binary and ternary molten salt systems consisting of 1-ethyl-3-methylimidazolium chloride ([EMIMJC1) with iron chlorides, FeCb and FeCl j, was investigated as possible half-cell reactions for novel rechargeable redox batteries. A reversible one-electron redox reaction was observed on a platinum electrode at 130 °C. 

In these clusters tantalum atoms are bound to other tantalum atoms and are also edge bridged via halide. As our deposit was completely amorphous without any XRD peak we concluded that it did not consist of crystalline tantalum but rather of such clusters. We varied the electrode potential for deposition and tried deposition with very low constant current densities, but in no case was crystalline tantalum obtained. Thus, the electrochemical window of our liquid was surely wide enough, but for some reason the electrodeposition stopped before Ta(0) was obtained. When we studied the literature dealing with metal clusters we found that the cluster chemistry with fluoride seems to be less comprehensive. Consequently... 

The motivation of this chapter was to show that despite the enormous prospects of ionic liquids in electrodeposition some troublesome aspects have to be expected. Apart from the purity and price of ionic liquids the optimum temperature for any process has to be found. Furthermore, suitable additives for electrodeposition will have to be developed and cation/anion effects that can strongly alter the morphology of deposits have to be expected. Finally, the electrochemical window alone is not the only factor that needs to be considered for the deposition of reactive metals. Suitable precursors will have to be tailor-made and it is our personal opinion that the electrodeposition of metals like Mg, Ti, Ta and Mo may not be possible from metal halides but rather metal bis(trifluoromethylsulfonyl)amide salts and other ones may be more suitable. 

Over the past two decades, ionic liquids (ILs) have attracted considerable interest as media for a wide range of applications. For electrochemical applications they exhibit several advantages over the conventional molecular solvents and high temperature molten salts they show good electrical conductivity, wide electrochemical windows of up to 6 V, low vapor pressure, non-flammability in most cases, and thermal windows of 300-400 °C (see Chapter 4). Moreover, ionic liquids are, in most cases, aprotic so that the complications associated with hydrogen evolution that occur in aqueous baths are eliminated. Thus ILs are suitable for the electrodeposition of metals and alloys, especially those that are difficult to prepare in an aqueous bath. Several reviews on the electrodeposition of metals and alloys in ILs have already been published [1-4], A selection of published examples of the electrodeposition of alloys from ionic liquids is listed in Table 5.1 [5-40]. Ionic liquids can be classified into water/air sensitive and water/air stable ones (see Chapter 3). Historically, the water-sensitive chloroaluminate first generation ILs are the most intensively studied. However, in future the focus will rather be on air- and water-stable ionic liquids due to their variety and the less strict conditions under which... 

In Section 8.2 the basics of pulsed dectrodeposition (PED) will be described for the case of aqueous electrolytes which allow the deposition of comparatively noble metals like Cu, Ni, Pd, or Au less noble metals like Fe or Zn can still be electrodeposited from aqueous electrolytes because they exhibit a comparatively large overpotential for hydrogen evolution. However, the main limitation of aqueous dectrolytes, of course, is their narrow electrochemical window which adversely affects the electrodeposition of metals like A1 or Ta. Therefore, recently, the PED technique has been extended to ionic liquids as electrolytes. General electrochemical aspects of ionic liquids can be found in Ref. [44] here, in Section 8.3, we will only address the technical aspects with respect to PE D. Examples of nanometals and nanoalloys electrodeposited from chloroaluminate-based ionic liquids are given in... 

Ge(lll) bilayers can be obtained by electrodeposition in the dry ionic liquid [BMIMJPF6 containing GeT as a source of germanium [94], This ionic liquid has an electrochemical window of a little more than 4 V on Au(lll). However, stable... 

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]. 

Impurities are a concern in ionic liquids electrochemistry. Whereas even considerable amounts of impurities, like different metal ions, water or organic impurities, might not disturb a technical process (e.g. extractive distillation, organic synthesis) the wide electrochemical windows of an ionic liquid ( 3 V vs. NHE) allow the electrodeposition of even reactive metals like lithium and potassium, as well as the oxidation of halides to the respective gases. In the best case this codeposition only leads to a low level of impurities, in the worst case fundamental physicochemical studies are made impossible as the impurities are adsorbed onto the electrode surface and subsequently reduced. Furthermore, passivation or activation effects at the counter electrode have to be expected. 

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. 

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