The electrochemistry of Zn in deep eutectic solvents

Ionic liquids have generated a large amount of interest as possible replacements for aqueous electrolytes in metal and alloy electrodeposition processes. A related class of fluid, the deep eutectic solvents, have recently been shown to have equally interesting electrochemical properties whilst also being more air and moisture stable and economical to produce. The electrodeposition of Zn from the deep eutectic solvents 1: 2 ChCl: ethylene glycol and 1: 2 ChCl: urea was investigated. A theory of relative chloride activities was developed and applied to effectively account for differences in the voltammetry, chronoamperometry and morphology of deposits obtained from the two solvents. Additionally the solute concentration was determined to have an effect on the physical properties of the solvent; moreover, this effect was seen to be solvent dependent. The first EXAFS study of metal speciation in deep eutectic solvents was used to elucidate the identity of the dissolved Zn species. A novel technique, the combined in-situ AFM-EQCM, has been designed and applied, in a time resolved manner, to the study of Zn electrodeposition from 1: 2 ChCl: ethylene glycol. It was shown that the organic additives ethylene diamine and ammonia could be used to modify the Zn deposit morphology. It has been proposed that the additives alter the Zn nucleation and growth mechanism through interaction with the free chloride ions in the electrochemical double layer. The effect of surfactants has also been described and sodium dodecyl sulphate found to be an effective levelling agent. The feasibility of Zn alloy deposition from choline chloride based deep eutectic solvents has been investigated. Zn-Cu and Zn-Co alloys were successfully deposited from deep eutectic solvents for the first time. In addition the electrochemical quartz crystal microbalance has been used in an original manner to monitor Zn-Sn co-deposit composition.