Effect of coordinated water of hexahydrate on nickel platings from choline–urea ionic liquid

Impacts of coordinated water of nickel chloride hexahydrate (NiCl₂·6H₂O) on the properties of choline–urea (ChCl–2Urea) ionic liquid (IL), including viscosity and electrical conductivity, and electrodeposition behavior of Ni coatings were investigated. Results reveal that the coordinated water exhibited a profound influence on reducing viscosity, improved electrical conductivity and promoted the formation of a nanocrystalline Ni coating, while the Ni nucleation mechanism was not altered by the presence of coordinated water, proceeding via that from the progressive three-dimensional nucleation to instantaneous nucleation with hemispherical diffusion-controlled growth when the deposition potential shifts to the negative direction. When water content was maintained no more than ~ 8 wt%, a compact nanocrystalline Ni coating, with a current efficiency of almost 100%, was prepared from ChCl–2Urea–NiCl₂·6H₂O with an aid of 400 mg/L nicotinic acid (NA) at 318 K. These results indicate hydrated Ni salts can be used to replace anhydrous counterparts in preparation of Ni coating from choline–urea IL, which could reduce the impact on environment and product cost. Such a strategy may be extended to other hydrated metal salts in IL electrolyte electrodeposition for high-quality coating preparation.