Abstract
In this work, the electrochemical behavior of Mg and Fe in ionic liquids (IL) were studied. We performed a series of cyclic voltammetry experiments to improve the understanding of Mg behavior in an IL containing the bis(trifluoromethanesulfonylimide) ([Tf2N]) anion. The results show an irreversible deposition/dissolution of Mg at a high water concentration (ca. 1300 ppm, 50 mmol L-1) and very low reversibility (7.3%) at a moderate water concentration (ca. 65 ppm, 5 mmol L-1). The formation of a film on the electrode surface and the presence of Mg were confirmed by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS). The process irreversibility indicates the formation of a passivating film. Because the presence of water affects the reversibility of the process, studies of Fe deposition/dissolution were conducted in two different ILs and with microelectrodes to evaluate how the water modifies the reversibility and the diffusion of ions. Water plays an important role in the reversibility of Fe deposition/dissolution being that deposition is less reversible when water is absent. The Fe diffusion is also modified because the Fe ion coordination sphere is strongly affected by the presence or absence of water; the Fe diffusion was also shown to depend on the coordination ability of the cation.
Highlights
Over the past few years, ionic liquids (ILs) have attracted increasing attention as electrolytes in electrochemical devices such as Li-ion batteries,[1,2,3,4,5] supercapacitors[6,7,8,9,10] and electrochromic devices.[11,12] Because of their wide electrochemical window, the ILs are promising electrolytes for the electrodeposition of metals and alloys that, because of their highly negative reduction potential, cannot be deposited from aqueous or organic solutions.[13,14,15] In addition, ILs are liquids and are stable over a wide temperature range, with upper temperature stability limits as high as 400 °C, at least for a short time
To study the electrochemical behavior of Mg in ionic liquids (IL), we determined the electrochemical window of the ILs [BMMI] [Tf2N] and [BMP][Tf2N] using linear sweep voltammetry starting from open-circuit potential (OCP) and sweeping positive and negative potentials in two different experiments for each IL (Figure 2)
Zhao et al claimed that the deposition/ dissolution of Mg using the anion [Tf2N] cannot be achieved,[35] it is important to understand the electrochemical behavior of Mg in the ILs with [Tf2N] anion
Summary
Over the past few years, ionic liquids (ILs) have attracted increasing attention as electrolytes in electrochemical devices such as Li-ion batteries,[1,2,3,4,5] supercapacitors[6,7,8,9,10] and electrochromic devices.[11,12] Because of their wide electrochemical window, the ILs are promising electrolytes for the electrodeposition of metals and alloys that, because of their highly negative reduction potential, cannot be deposited from aqueous or organic solutions.[13,14,15] In addition, ILs are liquids and are stable over a wide temperature range, with upper temperature stability limits as high as 400 °C, at least for a short time. Some groups have reported that the addition of Li+ salts stabilizes ILs with imidazolium cations and [Tf2N] and [BF4] anions, leading to a wider electrochemical window in the cathodic limit,[36,37,38,39] this fact is noticed when Mg2+ is added to the [BMP][Tf2N], as can be seen from Figure 3 and Figure 4a, and should enable the reduction of Mg2+ at a higher overpotential than that of the neat IL degradation.
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