Handbook of Solvents - George Wypych - ChemTech - Ventech!

1472 D.W. Rooney, K.R. Seddon
21.2.4 ELECTROCHEMICAL APPLICATIONS
Since ionic liquid where first developed as an adjunct to the U.S. Naval program for use as
battery electrolytes it seems only fair to begin this discussion by firstly looking at the electrochemical
properties. One of the most important electrochemical properties of a solvent is
its electrochemical window, the range of potentials over which a solvent system (including
the solvent and any supporting electrolyte) is inert. The cathodic limit results from reduction
of the solvent system; the anodic limit from its oxidation. Electrochemical windows are
somewhat arbitrary as they are dependent upon the amount of background current which
can be tolerated. Figure 21.2.11 shows the electrochemical windows of a range of ionic liquids
as well as more commonly used solvents such as water and acetonitrile. The electrochemical
windows shown in Figure 21.2.11 were determined using cyclic voltammetry at a
glassy carbon disc electrode and are referenced against the Ag(I)/Ag couple (versus
Ag(I)/Ag) and converted to the equivalent SCE potential. The reference electrode consisted
of a silver wire immersed in a 0.1 M solution of AgNO 3 dissolved in [C 6-mim][NO 3]. In
chloroaluminate ionic liquids, the most common reference electrode is an aluminum wire
immersed in neat acidic ionic liquid (versus Al(III)/Al).
It can be seen from Figure 21.2.11 that the [C 4-mim] cation has a cathodic limit of approximately
-2 V versus SCE and that this value is essentially the same for all of the
[C n-mim] cations. Given that the deposition potentials for many metals will fall positive of
this potential, it becomes possible to use ionic liquids as electrolytes for metal plating and
other similar processes. The broad electrochemical windows (in some cases, over 4 V) indicate
that a variety of organic and inorganic electrochemical oxidations and reduction should
be possible in ionic liquids.
A wide variety of metals can be electrodeposited from room-temperature ionic liquids.
The electrodeposition mechanisms of transition metals, lanthanides and Group 13 metals
have been investigated 98-103 as have cobalt, 104-107 iron, 108 manganese, 109 tin, 110 gold, 111,112 silver,
113-116 palladium, 117,118 mercury, 119 cerium, 120 and lead, 121 and the actinides. 122 In addition,
ionic liquids have been used extensively to study the electrochemistry and
spectrochemistry of metal-chloride complexes. 123-131
Chloroaluminate ionic liquids have been used in the electrodeposition of aluminum
and aluminum-transition metal alloys. 132 Transition metal-aluminum alloys are valued for
their corrosion resistance and magnetic properties. A convenient method for creating thin
alloy films is through the electrodeposition of two or more metals. The electrodeposition of
aluminum and aluminum alloys from aqueous solutions is complicated by the fact that
Table 21.2.6. Diffusion coefficients and
Stokes-Einstein products of a number of
species studied in the 40-60 mol%
[C 2-mim]Cl-AlCl 3 ionic liquids at 40ºC
Do 10 -7 cm 2 s -1 ηDo T -1 ,10 -10 gcms -2 K -1
Co(II) 6.6 2.6
Cu(I) 2 0.7
Ni(II) 10 3.5
Zn(II) 6.7 2.3
dihydrogen is evolved before aluminum is
deposited. However, these materials can be
electrodeposited from aprotic non-aqueous
solvents or high temperature molten salts.
The electrodeposition of aluminum alloys
such as Cr-Al, 133 Mn-Al, 134,135 Ni-Al 136 and
Ti-Al 137 have been demonstrated from inorganic
chloroaluminate molten salts, mainly
Franklin acidic (AlCl 3-rich) AlCl 3 - NaCl.
One unfortunate drawback to the use of alkali-chloride
based chloroaluminates such
as AlCl 3-NaCl for the electroplating of