Electrodeposition ionic melt

Recently, the electrodeposition of calamitic Fe-nanostructures from the ionic melt AlCl3-l-butyl-3-methylimidazolium chloride (AlCl3/[BMIM]Q) on Au(lll) and their characterization by in situ electrochemical STM microscopy have been described [19]. 

Andriiko AA, Tchemov RV (1983) Electrodeposition of powdered germanium from oxide-fluoride melts. In Physical chemistry of ionic melts and solid electrolytes. Naukova Dumka, Kiev, pp 46-60... 

Hence, the purpose of this book is to provide a unified basis for a wide range of problems relevant to the electrochemistry of many-electron processes in ionic melts and in other media as well. Equilibria in many-electron systems, non-stationary many-electron processes, electrochemical processes in mixed conductors, aspects of the electrodeposition of polyvalence elements and anode processes are considered. No arbitrary assumptions like one-step many-electron transfers or discrete discharge of complex species are involved— the consideration is based on a few very general ideas. 

From the various methods for the preparation of Ti film, such as CVD, PVD and electrochemical processes, the electrochemical deposition in ionic melts appears to be one of the most effective, as it makes it possible to deposit Ti films, depending on the composition of the electrolyte and the operating parameters of electrolysis, that is temperature, current density, current forms and so on. The high-temperature molten electrolytes (generally above 650 C) employed in the electrodeposition of Ti film can essentially be divided... 

Such electrodeposition and synthesis methods are based on multielectron processes of metal and nonmetal deposition from ionic melts. The absence of information on the theoretical basis and principles of the control of both multielectron processes and HES processes did not allow one to conduct electrometallurgic synthesis in practice. However, systematic data accumulated at the turn of the century regarding multielectron processes of refractory metal and nonmetal electrodeposition served as the scientific basis and impact for the revived interest in the problem of electrolytic deposition and HES from melts. 

Malyshev, V.V. (2004) High-Temperature Electrochemistry and Electrodeposition of Metals of Groups IV-VIA and Their Compounds in Ionic Melts (in Russian), Izd-vo un-ta [in Ukrainian], Kiev 326. 

Chloroaluminate(III) ionic liquid systems are perhaps the best established and have been most extensively studied in the development of low-melting organic ionic liquids with particular emphasis on electrochemical and electrodeposition applications, transition metal coordination chemistry, and in applications as liquid Lewis acid catalysts in organic synthesis. Variable and tunable acidity, from basic through neutral to acidic, allows for some very subtle changes in transition metal coordination chemistry. The melting points of [EMIM]C1/A1C13 mixtures can be as low as -90 °C, and the upper liquid limit almost 300 °C [4, 6]. 

It was quite recently reported that La can be electrodeposited from chloroaluminate ionic liquids [25]. Whereas only AlLa alloys can be obtained from the pure liquid, the addition of excess LiCl and small quantities of thionyl chloride (SOCI2) to a LaCl3-sat-urated melt allows the deposition of elemental La, but the electrodissolution seems to be somewhat Idnetically hindered. This result could perhaps be interesting for coating purposes, as elemental La can normally only be deposited in high-temperature molten salts, which require much more difficult experimental or technical conditions. Furthermore, La and Ce electrodeposition would be important, as their oxides have interesting catalytic activity as, for instance, oxidation catalysts. A controlled deposition of thin metal layers followed by selective oxidation could perhaps produce cat-alytically active thin layers interesting for fuel cells or waste gas treatment. 

ZnTe The electrodeposition of ZnTe was published quite recently [58]. The authors prepared a liquid that contained ZnGl2 and [EMIM]G1 in a molar ratio of 40 60. Propylene carbonate was used as a co-solvent, to provide melting points near room temperature, and 8-quinolinol was added to shift the reduction potential for Te to more negative values. Under certain potentiostatic conditions, stoichiometric deposition could be obtained. After thermal annealing, the band gap was determined by absorption spectroscopy to be 2.3 eV, in excellent agreement with ZnTe made by other methods. This study convincingly demonstrated that wide band gap semiconductors can be made from ionic liquids. 

Relatively little attention has been devoted to the direct electrodeposition of transition metal-aluminum alloys in spite of the fact that isothermal electrodeposition leads to coatings with very uniform composition and structure and that the deposition current gives a direct measure of the deposition rate. Unfortunately, neither aluminum nor its alloys can be electrodeposited from aqueous solutions because hydrogen is evolved before aluminum is plated. Thus, it is necessary to employ nonaqueous solvents (both molecular and ionic) for this purpose. Among the solvents that have been used successfully to electrodeposit aluminum and its transition metal alloys are the chloroaluminate molten salts, which consist of inorganic or organic chloride salts combined with anhydrous aluminum chloride. An introduction to the chemical, electrochemical, and physical properties of the most commonly used chloroaluminate melts is given below. 

In many ways, chloroaluminate molten salts are ideal solvents for the electrodeposition of transition metal-aluminum alloys because they constitute a reservoir of reducible aluminum-containing species, they are excellent solvents for many transition metal ions, and they exhibit good intrinsic ionic conductivity. In fact, the first organic salt-based chloroaluminate melt, a mixture of aluminum chloride and 1-ethylpyridinium bromide (EtPyBr), was formulated as a solvent for electroplating aluminum [55, 56] and subsequently used as a bath to electroform aluminum waveguides [57], Since these early articles, numerous reports have been published that describe the electrodeposition of aluminum from this and related chloroaluminate systems for examples, see Liao et al. [58] and articles cited therein. 

The authors reported that they obtained Nb contents of up to 29 wt-% in the deposits, at temperatures between 90 and 140 °C. In [22], chloroaluminate liquids were employed at room temperature and AlNb films could only be obtained if NbCl5 was prereduced in a chemical reaction. The authors reported that Nb powder is the most effective reducing agent for this purpose. Similar preliminary results have been obtained for Ta electrodeposition. Although it seems to be difficult to deposit pure Nb and Ta in low-melting ionic liquids, the alloys with A1 could have quite interesting properties. 

A major breakthrough was achieved in 1951 with the report of Hurley and Wier. They noticed that a mixture of N-ethylpyridinium bromide (EtPyBr) and AICI3 with a eutectic composition of 1 2 X(AlCh) = 0.66 h of EtPyBr to AICI3 became liquid at unusually low temperatures [2], They investigated these melts with regard to their potential use in the electrodeposition of aluminum at ambient temperature [3]. Several studies were carried out on this system, however, its use was very limited since it is only liquid at a mole fraction of X(A1C13) = 0.66 and the ease of oxidation of the bromide ion limits the electrochemical stability. In the following years the main interest in ionic liquids was focused on electrochemical applications [4—6]. 

Wilkes launched the field of air- and moisture-stable ionic liquids by introducing five new materials, each containing the Tethyl-3-methylimidazolium cation [EMIMJ+ with one of five anions nitrate [NC>3], nitrite [NO2]-, sulfate [SC>4]2, methyl carbonate [CH3CO2]- and tetrafluoroborate [BF [47]. Only the last two materials had melting points lower than room temperature, and the reactive nature of the methyl carbonate would make it unsuitable for many applications. This led to the early adoption of [EMIM][BF4] as a favored ionic liquid, which has since been the subject of over 350 scientific publications. One of the first appeared in 1997 [50], reporting the investigation of [EMIM][BF4] as the electrolyte system for a number of processes, including the electrodeposition of lithium (intended for use in lithium ion batteries). 

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). 

The electrodeposition of Zn-Co and Zn-Fe alloys in an aqueous bath is classified as an anomalous codeposition [44] because the less noble Zn is preferentially deposited with respect to the more noble metal. This anomaly was attributed to the formation of Zn(OH)+ which adsorbs preferentially on the electrode surface and inhibits the effective deposition of the more noble metal. This anomaly was circumvented by using zinc chloride-n-butylpyridinium chloride ([BP]+C1 / ZnCf ) [27] or [EMIMJ+Ch/ZnCh [28] ionic liquids containing Co(II). The Zn-Co deposits can be varied from Co-rich to Zn-rich by decreasing the deposition potential or increasing the deposition current. XRD measurement reveals the presence of CosZ i in the deposited Zn-Co alloys and that the Co-rich alloys are amorphous and the crystalline nature of the electrodeposits increases as the Zn content of the alloys increases. Addition of propylene carbonate cosolvent to the ionic liquid decreases the melting temperature of the solution and allows the electrodeposition to be performed at a lower temperature. The presence of CoZn alloy is evidenced by the XRD patterns shown in Figure 5.2. 

In all of the investigated systems, one of the most important tasks to be solved is to find the proper composition of the electrolyte with regard to both the suitable physico-chemical properties and the desired character of the electrodeposited product. Both problems are closely related to the actual structure, i.e. the ionic composition of the melt. 

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