The electrodeposition of most of technologically important metals has been shown to be possible from a wide range of room temperature molten salts, more commonly known today as 'ionic liquids'. These liquids are currently under intense scrutiny for a wide variety of applications some of which have already been commercialized. Despite the fact that electrodeposition was the first application studied in these liquids no metal deposition processes have as yet been developed to an industrial scale. This review addresses the practical and theoretical aspects that need to be considered when choosing ionic liquids for metal deposition. It details the current understanding of the physical and chemical properties of these interesting fluids and highlights the areas that need to be considered to develop practical electroplating systems. The effect of composition and temperature on viscosity and conductivity are discussed together with the fundamental approaches required to synthesise new liquids.

Application of ionic liquids to the electrodeposition of metals

In a wider sense, ionic liquids are molten salts that melt below 100 °C. As their name suggests, they are solely composed of ions and many combinations of organic and/or inorganic cations and anions exist. Depending on the systems they can reach electrochemical windows of more than 4 V and thus they give access to a number of elements that cannot be electrodeposited from aqueous solutions, such as the light and refractory metals, as well as elemental and compound semiconductors. Presumably, ionic liquids will become important for electrochemical nanotechnology.

Ionic Liquids: Solvents for the Electrodeposition of Metals and Semiconductors

Here we describe for the first time the electrolytic deposition of copper and copper composites from a solution of the metal chloride salt in either urea–choline chloride, or ethylene glycol–choline chloride based eutectics. We show that the deposition kinetics and thermodynamics are quite unlike those in aqueous solution under comparable conditions and that the copper ion complexation is also different. The mechanism of copper nucleation is studied using chronoamperometry and it is shown that progressive nucleation leads to a bright nano-structured deposit. In contrast, instantaneous nucleation, at lower concentrations of copper ions, leads to a dull deposit. This work also pioneers the use of the electrochemical quartz crystal microbalance (EQCM) to monitor both current efficiency and the inclusion of inert particulates into the copper coatings. This technique allows the first in situ quantification or particulate inclusion. It was found that the composition of composite material was strongly dependent on the amount of species suspended in solution. It was also shown that the majority of material was dragged onto the surface rather than settling on to it. The distribution of the composite material was found to be even throughout the coating. This technology is important because it facilitates deposition of bright copper coatings without co-ligands such as cyanide. The incorporation of micron-sized particulates into ionic liquids has resulted, in one case, in a decrease in viscosity. This observation is both unusual and surprising; we explain this here in terms of an increase in the free volume of the liquid and local solvent perturbation.

Electrodeposition of copper composites from deep eutectic solvents based on choline chloride.

Measurement of the redox potentials (E1/2) of ferrocene (1), decamethylferrocene (2), decamethylcobaltocene (3), and [FeICp*(η6-C6Me6)] (4) (Cp* = η5-C5Me5) in DMF, CH3CN, THF, DME, CH2Cl2, and DMSO were recorded vs. the saturated calomel electrode (SCE), showing that the E1/2 value of 1 was more solvent dependent than those of 24, and that the difference in E1/2 values among 24 was solvent independent, unlike those found between 1 and 24. This shows that 24 are much more reliable internal references for determining redox potentials than 1, but using data from this article, one can convert the literature values reported vs. 1 into values vs. 2, 3, or 4. The electrochemistry of 1 and 2 is all the more inhibited by polyamine dendrimers as the dendrimer generation is higher. On the other hand, ferrocenes or other iron sandwich compounds containing hydroxy groups can reestablish reversible cyclic voltammograms. It is shown, using a tetra(ferrocenylmethylamine), that this phenomenon is due to hydrogen bond...

Metallocenes as references for the determination of redox potentials by cyclic voltammetry Permethylated iron and cobalt sandwich complexes, inhibition by polyamine dendrimers, and the role of hydroxy-containing ferrocenes

Four different ionic liquids, based on 1-alkyl-3-methylimidazolium or quaternary ammonium cations, were used as reaction media for several typical electrochemical reactions with a strict control of the residual water concentration. The oxidation of organic molecules (anthracene, naphthalene, durene, 1,4-dithiafulvene, and veratrole) for which the cation radicals undergo first and second-order kinetics reactions were investigated in ionic liquids and compared with their behavior in acetonitrile. From the analysis of the voltammetric current responses, the reaction mechanism was established and the thermodynamic and kinetics parameters were extracted. The most interesting result is that the nature of investigated mechanisms is almost unchanged in ionic liquids as compared with conventional organic media. A decrease of the electron-transfer kinetics from the aromatic molecules to the electrode (around 1 order of magnitude) is observed for all of the studied molecules, indicating an higher solvent reorganizat...

Reactivities of Some Electrogenerated Organic Cation Radicals in Room-Temperature Ionic Liquids: Toward an Alternative to Volatile Organic Solvents?

Resultats d'etudes par voltammetrie cyclique et en presence d'une electrode, a disque tournant, a 40°C, en fonction de la composition du melange fondu

Electrochemical studies of ferrocene and ferrocenium ion in aluminum chloride-N-(1-butyl)pyridinium chloride ionic liquid

Etude des systemes redox Cu(II)-Cu(I)-Cu dans le melange liquide donne, a l'aide d'une electrode a disque tournant (carbone vitreux) et d'une electrode a disque et anneau, ainsi que par coulometrie et potentiometrie

Electrochemical Studies of Cu(I) and Cu(II) in an Aluminum Chloride‐N‐(n‐Butyl)Pyridinium Chloride Ionic Liquid

Electrochemical Studies of Fe(II) and Fe(III) in an Aluminum Chloride‐Butylpyridinium Chloride Ionic Liquid

Der Elektronenubertragungsprozes Ferrocen/Ferrocenium-Ion wurde bei 40°C alsFunktion der Zusammensetzung der Schmelze cyclisch-voltammetrisch untersucht.

Chenniah Nanjundiah

Papers

Electrochemical studies of ferrocene and ferrocenium ion in aluminum chloride-N-(1-butyl)pyridinium chloride ionic liquid

Electrochemical Studies of Cu(I) and Cu(II) in an Aluminum Chloride‐N‐(n‐Butyl)Pyridinium Chloride Ionic Liquid

Electrochemical Studies of Fe(II) and Fe(III) in an Aluminum Chloride‐Butylpyridinium Chloride Ionic Liquid

Electrochemical studies of ferrocene and ferrocenium ion in aluminum chloride‐n‐(1‐butyl)pyridinium chloride ionic liquid