The influence of water on the cathodic voltammetric responses of choline chloride-urea and choline chloride-ethylene glycol deep eutectic solvents

Roy, Sudipta and Valverde Armas, Priscila Estefania and Bucko, Mihael and Bajat, Jelena B. (2017) The influence of water on the cathodic voltammetric responses of choline chloride-urea and choline chloride-ethylene glycol deep eutectic solvents. In: 12th International Workshop on Electrodeposited Nanostructures, 2017-03-16 - 2017-03-18, Hotel Serdika.

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Abstract

During the last decade, choline chloride-based deep eutectic solvents (DESs) have been successfully used for electrodeposition of different metals (Cr, Mn, Cu, Ag, Fe, Zn) and alloys (ZnCr, Zn-Sn, Zn-Ni, Zn-Mn, Ni-Co etc.) on different substrates, producing films with characteristics that are completely different from those obtained from aqueous electrolytes. Yet, the processes which occur in the blank electrolytes during the cathodic polarization of the DESs, are still not completely understood. Besides, the role of water molecules in these cathodic processes, has not been investigated in detail. It is almost impossible to avoid the water presence during the electrodeposition of metal coatings from DESs. The water is absorbed due to the high hygroscopicity of DESs, but is also added with hydrated metal salts. This work aims to compare the cyclic voltammograms (CVs) of DESs of different compositions. The blank DESs were the mixtures of choline chloride with urea or with ethylene glycol. The CVs were recorded in electrolytes with various ratios of choline chloride, urea, ethylene glycol, and water. The increase in the concentration of a certain substance in DES results in the increase in the cathodic voltammetric peak or the cathodic current. So, it is possible to determine the species that are reduced preferentially from the mixture of the two or three substances. The measurements taken until now, have led us to assume interesting conclusions. Namely, the species which originate from urea, ethylene glycol or water, are reduced at more positive potential than choline chloride. Besides, it seems that the electrode potential where the reduction starts is the same for these three substances (urea, ethylene glycol and water), in case that they are analysed separately. However, when both urea and water (or both urea and ethylene glycol) are present in DES, their reduction does not occur at the same potential. Instead, the water (or ethylene glycol) reduction occurs at more positive potential, which is seen as the cathodic peak, while the urea reduction in this case is hindered (shifted to more negative potential).