Electrochemical Depositions in Ionic Liquids

In this PhD thesis, several aspects of the electrodeposition of metals and alloys in ionic liquids were investigated. First, the deposition of brass from choline acetate was studied. Secondly, the electrodeposition of pure molybdenum from ionic liquids based on phosphonium chloride and zinc chloride was treated. In each case, the influence of water, either as a main constituent of the electrolyte or an impurity, was investigated. When comparing electrochemical processes such as electrodepositions, in different solvents, a method for reporting electrode potentials is recommended by IUPAC. This method was adapted to tackle problems specific to ionic liquids. A validation of the use of square wave voltammetry for this purpose in three ionic liquids was added in an appendix.Choline chloride mixtures with copper(II) chloride dihydrate and anhydrous zinc chloride were prepared for electroplating brass. In case of a 1:2 molar mixture of choline chloride copper(II) chloride dihydrate, the coordination sphere of copper was identified to contain three chlorides and one water molecule. Copper depositions from this ionic liquid were powdery. Therefore, it was decided to start from mixtures of choline chloride with anhydrous zinc chloride as a solvent for the copper source. An optimum was found between low viscosity and an anodic shift of the cathodic solvent limit, which is the electrodeposition of zinc, by adding 20 wt% water to the mixture. The addition of copper(I) chloride or copper(II) chloride dihydrate resulted in deposits of copper-zinc alloys consisting of 15 to 90 wt% copper. On steel wires, only poorly adhering coatings were obtained. Complexing agents did not improve the appearance of the depositions. The high chloride concentration in these ionic liquids was thought to cause this poor morphology. A switch to choline acetate, a chloride-free ionic liquid, was made. In a mixture containing 20 wt% water, very thin gold-yellow coatings were obtained. Upon addition of triethanolamine, a stronger complexing agent for copper than for zinc in aqueous solutions, well-adhering copper-zinc coatings on steel of up to 200 nm were deposited. The layers had a composition of 90 wt% copper and 10 wt% zinc, with a cathodic current efficiency of about 75%. The morphological instability, observed for deposition times longer than 300 seconds, was solved by the addition of 8 mg dm-3 polyvinyl alcohol, a primary brightener in the electroplating of zinc. Mirror-bright coatings of up to 1 μm were obtained.A verification of literature reports for the electrodeposition of refractory metals in alkali halide molten salts at 250 °C was performed. The deposition temperature was lowered to 200 °C by replacing the alkali halide part of a ZnCl2-KCl-NaCl mixture with phosphonium chlorides. A subvalent molybdenum chloride coating on a nickel substrate was obtained. Addition of an excess of KF to molybdenum chloride in a 1:2 molar mixture of tetrabutylphosphonium chloride:zinc chloride resulted in non-uniform deposits of metallic molybdenum. In an attempt to further lower deposition temperatures and simultaneously reduce the chloride activity in the electrolyte, a low-melting trihexyl(tetradecyl)phosphonium dicyanamide was added. Depositions from this diluted mixture at 150 °C containing only subvalent molybdenum chlorides, not metallic molybdenum.

Publication year:2013

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