Influence of water content and applied potential on the electrodeposition of Ni coatings from deep eutectic solvents

Ni coatings were electrodeposited from 1:2 choline chloride (ChCl) - urea (U) deep eutectic solvents (DESs) on low carbon steel. We report on the inter-related influence of water content in the electrolyte and applied potential on the formation of Ni films and their chemical composition and morphology. This was investigated by cyclic voltammetry (CV) and chronoamperometry (CA) in combination with ex-situ characterization techniques (FE-SEM, EDS, XPS and Raman spectroscopy). Ni electrodeposition from DES is shown to be highly complex: Ni+2 reduction is followed by water reduction, which triggers electrolyte decomposition. A water content higher than 4.5 % w t and/or performing electrodeposition at potentials more negative than E =  − 0.90 V vs Ag quasi-reference electrode enhances the decomposition of the solvent. This breakdown appears via either an electrochemical reaction or triggered by water splitting. In both cases, it leads to the incorporation of DESs decomposition products, such as trimethylamine and acetaldehyde within the Ni film. Under these conditions, the films are composed of metallic Ni and NiO x (OH)2(1−x).