Electrodeposition of Magnesium in Deep Eutectic Solvents Based on Magnesium Chloride Hexahydrate and Choline Chloride with Urea Added

Abstract Magnesium (Mg) alloy has small density, large elastic modulus, good heat dissipation and corrosion resistance to organic matter and alkali. At present, magnesium alloy is more and more used in automotive industry, medical devices and aerospace industry. However, the traditional preparation method of Mg has the disadvantages of high investment, high labor intensity and great environmental pollution. Therefore, it is of great significance to develop simple, environment-friendly methods of the magnesium. In this study, urea was added to adjust the electrochemical property of the deep eutectic solvent (DES) mixed by choline chloride (ChCl) and magnesium chloride hexahydrate (MgCl2·6H2O). Cyclic Voltammetry (CV) curves reveals that the addition of urea made the reduction potential of Mg shifted from -0.9 V to -1.3 V. Among the CV curves, one was proposed as the “dividing line”, which shows that the electroactive species in the two DESs, ChCl-MgCl2·6H2O and urea-MgCl2·6H2O, are different due to the changes of the component of the DESs. Fourier transform infrared (FTIR) data shows the type of hydrogen bond had been changed with the increase of urea content. Furthermore, the Raman spectra indicates that Mg2+ was coordinated with urea chains, which did not exist in ChCl-MgCl2·6H2O. Moreover, it was found that urea changed the electrochemical performance of the ChCl-Urea-MgCl2·6H2O by changing the hydrogen bond in the system and coordination form of the electroactive species, rather than adsorbing onto the electrode surface. Combined with geometry calculations at the B3LYP/6-311++G (d, p), the most probable mechanism of electrodeposition process was deduced.