Effect of water on the electrodeposition of copper on nickel in deep eutectic solvents

Most studies of metal electrodeposition in ionic liquids dry the electrolyte thoroughly, as water is thought to be detrimental. In some cases, water has a beneficial effect on deposit morphology. The electrodeposition of copper has been studied in 1ChCl: 2EG-water mixtures. It is shown here that the presence of water increases the apparent brightness of the deposit due to changes in the electrodeposit surface feature size, up to a water content of approximately 20 wt-%. This study characterises speciation and mass transport in solution, and shows that diffusion can be controlled independently of speciation. It is shown that there is an optimal water content, which is thought to originate from formation of a bi-continuous micro-emulsion phase in DES-water mixtures. Additionally, the copper species remains in a predominantly ionic medium at low water content but moves to an aqueous environment when water is the main component.