Can Water be an Additive? - Towards the Understanding of the Water Content Effect on Ni and Cr Electrodeposition from Choline Chloride-Based Deep Eutectic Solvent

Thin Ni and Cr coatings are commonly electrodeposited from aqueous electrolytes [1,2]. These often contain series of additives, whose role is to improve the leveling and adhesion of deposited films to the substrate, decrease internal stress, and limit the hydrogen evolution reaction (HER). However, certain additives, such as thiourea as well as Cr(VI) compounds are highly hazardous [1,2]. Therefore, due to the increasing environmental awareness of the society, the electrolytes containing these constituents require replacement by green alternatives, such as Deep Eutectic Solvents (DESs). DESs, usually composed of quaternary ammonium salt and hydrogen bond donor, i.e. ethylene glycol, have wider electrochemical potential window than H 2 O, which in principle enables obtaining metallic coatings without the HER [3]. However, due to the low conductivity and high viscosity of DESs, it is necessary to use additives, i.e. nicotinic acid, improving these physico-chemical properties and enhancing the electrodeposition process. However, due to the unavoidable presence of water (DESs are hygroscopic), H 2 O can be considered an additive to DESs. The addition of H 2 O can be beneficial since it improves the ion mobility, and therefore increases the reduction rate [4]. The current studies focus on understanding the effect of water content on the physico-chemical properties, and the structure of DESs. However, a little is known on the influence of H 2 O on the electrodeposition of Ni and Cr, especially on the nucleation and growth, the effect of the morphology and chemical composition of deposited films. Therefore, an in-depth study on this topic is required. The electrolytic reduction of Ni and Cr cations in choline chloride – ethylene glycol (1ChCl:2EG) on glassy carbon and steel substrates was investigated in function of water content. This study was performed by means of electrochemical techniques, such as linear sweep voltammetry and chronoamperometry. The obtained deposits were analyzed with surface sensitive methods, i.e. scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. In order to bring the understanding on the influence of H 2 O as an additive on the early stage electrodeposition of Ni and Cr from DES, a combination of computational modelling with UV-vis spectroscopy was involved. This approach allowed correlating the observed electrochemical behavior of Ni and Cr cations with certain water concentrations. Establishing such relationship is of high relevance, as it may enable optimizing the process parameters for Ni and Cr electroreduction from DESs. [1] N. V. Mandich and D. L. Snyder. Modern Electroplating , chapter Electrodeposition of Chromium, pages 205–248. John Wiley & Sons, Inc., New Jersey, 5 edition, 2010. [2] G. DiBari. Modern Electroplating , chapter Electrodeposition of Nickel, pages 79-114. John Wiley & Sons, Inc., New Jersey, 5 edition, 2010. [3] E.L. Smith, A.P. Abbot, K.S. Ryder, Deep Eutectic Solvents (DESs) and Their Application, American Chemical Society Publications , 2014, pp. 11060 – 11082. [4] M. Lukaczynska, E.A. Mernissi Cherigui, A. Ceglia, K. Van den Bergh, J. De Strycker, H. Terryn, and J. Ustarroz, Influence of water content and applied potential on the electrodeposition of Ni coatings from deep eutectic solvents, Electrochimica Acta , 319: 690-704, 2019.