Influence of cations size and charge on Zn electrodeposition from deep eutectic solvents

The electrodeposition of metals using ionic liquids and deep eutectic solvents (DESs) has been extensively studied due to their wide potential window and the ability to prepare them in an anhydrous form. In this study, we analyzed DESs consisting of choline chloride or calcium chloride with ethylene glycol for zinc deposition. These systems have different cations but otherwise similar composition; nonetheless their deposition processes are completely opposite, demonstrating that supposedly inert cations play a determining role. Zinc deposition at room temperature from the calcium-based liquid was fully reversible while no deposition was obtained from using choline-based liquids. Cyclic voltammetry and electrochemical impedance spectroscopy were used to provide insights on the double layer structure at the electrode/electrolyte interface. The nucleation mechanism of Zn from the calcium-based system was investigated using chronoamperometry on glassy carbon surfaces revealing that Zn nucleation follows the progressive nucleation model. Morphology and elemental composition of the metallic coatings have been characterized together with the effect of the commonly used brightener nicotinic acid. Evidently, calcium-based liquids are better for metal deposition than Ethaline at ambient conditions, likely due to their ability to accept a chloride ligand from the zinc complex during the deposition step.