Physicochemical properties of tungstate-based room-temperature ionic liquids

Article abstract of DOI:10.1149/1.3462993

Tungstate-functionalized room-temperature ionic liquids (ILs), 1-alkyl-3-methylimidazolium tungstates [RMIm]₂ [WO₄] (R = C₆H₁₃ or C₁₂H₂₅), were synthesized and the IL's structure has been proved by NMR, Fourier transform IR, and elemental analysis. The physicochemical characteristics of the tungstate-based ILs have been determined by using methods of conductivity, viscosity, and cyclic voltammetry measurements, in comparison with those of the conventional ILs such as 1-hexyl-3-methylimidazolium methanesulfonate and 1-hexyl-3-methylimidazolium hexafluorophosphate. The influences of anions and the length of alkyl chain linked to the imidazolium ring on physicochemical properties of the ILs were examined in detail. All ILs owned higher conductivities and lower viscosities at higher temperatures. The strong intramolecular hydrogen bonds were found in the tungstate-based ILs. Cyclic voltammograms of the functionalized ILs illustrated that they were quite electrochemically stable. In general, the understanding of electrochemical properties of imidazolium ILs can facilitate their possible applications in electrochemistry.

Full text of DOI:10.1149/1.3462993

F124
Journal of The Electrochemical Society, 157 ͑9͒ F124-F129 ͑2010͒
0013-4651/2010/157͑9͒/F124/6/$28.00 © The Electrochemical Society
Physicochemical Properties of Tungstate-Based
Room-Temperature Ionic Liquids
Yunxiang Qiao,a Jun Hu,b Huan Li,a Li Hua,a Yu Hu,a Bo Feng,a and
Zhenshan Houa,z
aKey Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, and bLaboratory for
Advanced Material and Department of Chemistry, East China University of Science and Technology,
Shanghai 200237, People’s Republic of China
Tungstate-functionalized room-temperature ionic liquids ͑ILs͒, 1-alkyl-3-methylimidazolium tungstates ͓RMIm͔ ͓WO4͔ ͑R
2
= C6H13 or C12H25͒, were synthesized and the IL’s structure has been proved by NMR, Fourier transform IR, and elemental
analysis. The physicochemical characteristics of the tungstate-based ILs have been determined by using methods of conductivity,
viscosity, and cyclic voltammetry measurements, in comparison with those of the conventional ILs such as 1-hexyl-3-
methylimidazolium methanesulfonate and 1-hexyl-3-methylimidazolium hexafluorophosphate. The influences of anions and the
length of alkyl chain linked to the imidazolium ring on physicochemical properties of the ILs were examined in detail. All ILs
owned higher conductivities and lower viscosities at higher temperatures. The strong intramolecular hydrogen bonds were found
in the tungstate-based ILs. Cyclic voltammograms of the functionalized ILs illustrated that they were quite electrochemically
stable. In general, the understanding of electrochemical properties of imidazolium ILs can facilitate their possible applications in
electrochemistry.
© 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3462993͔ All rights reserved.
Manuscript submitted March 29, 2010; revised manuscript received June 16, 2010. Published July 21, 2010.
Ionic liquids ͑ILs͒ are receiving increasing attention from both
conventional RTILs 1-hexyl-3-methylimidazolium methane-
sulfonate ͓HMIm͔͓CH3SO3͔ and 1-hexyl-3-methylimidazolium
hexafluorophosphate ͓HMIm͔͓PF6͔. This study mainly discusses the
electrostatic interaction, the hydrogen bonding, and van der Waals
interactions in these RTILs and attempts to enable many structure–
property trends to be identified, including the effect of both cations
and anions.
industry and academics, and research involving ILs is expanding to
many different areas of knowledge.1,2 There are so many attractive
physical and chemical properties of the ILs,3-6 including a negligible
vapor pressure, low inflammability, high thermal stability, liquidity
over a wide temperature range, easy recycling, and being a good
solvent for a wide variety of organic and inorganic chemical com-
pounds.
Of all the imidazolium room-temperature ionic liquids ͑RTILs͒,
most of them are air- and water-stable and with higher thermal sta-
bility compared to other kinds of ILs. The imidazolium RTILs,
which are “designable” as structural modifications in both cations
͑especially the 1 and 3 positions of the imidazolium ring͒ and an-
ions, permit the tuning of properties such as electrochemical
stability,5 miscibility with water and/or organic solvents,7 melting
point, viscosity,4 etc. Recently, the discovery of new imidazolium
RTILs by the suitable choice of the anion initiates intensive research
efforts toward the potential application.8-12
Experimental
General remarks.— All manipulations involving air-sensitive
materials were carried out using standard Schlenk line techniques
under an atmosphere of nitrogen. CH2Cl2 and Et3N ͑analytic reagent
grade͒ were distilled from CaH2 before use. Commercially available
ethyl acetate, diethyl ether, 1-hexanol, 1-methylimidazole,
1-bromohexane, 1-bromododecane, silver nitrate, sodium tungstate,
and methanesulfonyl chloride were purchased from Sinopharm
Chemical Reagent Co. Ltd. and used without further purification.
The IL samples were dried further at 70°C under vacuum for 2 h
before conducting thermogravimetric analysis ͑TGA͒ and electro-
chemical measurements. All NMR spectra were recorded on a
Electrochemistry of imidazolium RTILs is a very attractive area
of research, which is due to their chemical and electrochemical sta-
bility, wide electrochemical windows, high electrical conductivities,
and ionic mobilities.4,5,13-16 Electrochemical applications of imida-
zolium RTILs as electrolytes are found in, e.g., fuel cells,17
electrodeposition,18 capacitors,19-21 solar cells,22,23 batteries,24 and
water electrolysis for hydrogen generation.25 These ILs usually con-
sist of an inorganic or organic anion, whereas the most commonly
used IL anions include halide ions, tetrachloroaluminate ͑AlCl−4͒,
Bruker Advance 500 instrument ͑500 MHz H, 125 MHz 13C͒ using
1
chloroform-D ͑CDCl3͒ and tetramethylsilane as solvent and refer-
ence, respectively. Chemical shifts ͑␦͒ were given in ppm and cou-
pling constants ͑J͒ in hertz. Fourier transform infrared ͑FTIR͒ spec-
tra were recorded at room temperature on a Nicolet FTIR
spectrometer ͑Magna 550͒. The elemental analysis of C, H, and N
was performed on an Elementar Vario EI III Elementa and induc-
tively coupled plasma–atomic emission spectroscopy analysis of W
on a TJA IRIS 1000 instrument. The water content of the dried ILs
was determined by Karl Fischer titration using a KF-1B Karl Fischer
coulometer ͑Shanghai, China͒. A Perkin-Elmer Pyris Diamond was
used in the current study for the TGA measurements. A constant
heating rate of 10°C min−1 and high purity nitrogen purge
͑100 mL min−1͒ was used for all the measurements. The standard
material used for calibrating the equipment was ␣-sapphire powder
͑Perkin-Elmer͒. The rheological ͑viscosity͒ measurements were per-
formed on a Physica MCR 501 ͑Anton Paar GmbH͒ rheometer with
25 cm parallel plate geometry and a gap of 1 mm. The temperature
was controlled by a Peltier plate. Conductivity experiments were
carried out on a DDS-307 conductivity meter ͑Shanghai Jingke,
China͒. Electrochemical experiments were carried out on a CHI
430A electrochemical workstation ͑Shanghai Chenhua, China͒ with
a three-electrode system, a glassy carbon electrode as the working
electrode and a platinum counter electrode as the auxiliary electrode.
tetrafluoroborate ͑BF4−͒, hexafluorophosphate ͑PF−6͒, and
bis͓͑trifluoro-methyl͒sulfonyl͔imide anion ͑CF3SO2͒2N− ͓also
known as bistriflylimide ͑Tf2N−͔͒.
Needless to say, understanding the physicochemical properties of
ILs is important to provide information about their application
scope.26,27 Herewith, novel tungstate-based RTILs 1-hexyl-3-
methylimidazolium tungstate ͓HMIm͔ ͓WO4͔ and 1-dodecyl-3-
2
methylimidazolium tungstate ͓DMIm͔ ͓WO4͔ can be obtained by a
2
simple anion exchange of imidazolium bromide with silver tung-
state, and then the physicochemical properties of these RTILs, in-
cluding the temperature dependency of the dynamic ionic conduc-
tivity, viscosity, and the electrochemical stability of these RTILs, are
determined and analyzed according to the nature of the cations
and/or anions in detail, which are also compared with those of the
z E-mail: houzhenshan@ecust.edu.cn
Downloaded on 2014-11-13 to IP 128.248.155.225 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 157 ͑9͒ F124-F129 ͑2010͒
A nonaqueous Ag/AgCl reference electrode was used in all measure-
F125
ments. It consists of a AgCl-coated Ag wire, which was immersed in
the IL in a sealed glass tubing with an ion porous Vycor glass tip.
The cyclic voltammetry ͑CV͒ measurement was carried out at room
temperature.
Na WO
2
4
3102.8
3143.5
[DMIm] [WO ]
2
4
The synthesis of 1-hexyl-3-methylimidazolium bromide.— 0.05
mol 1-methylimidazole ͑4.10 g͒ and 0.05 mol 1-bromohexane ͑8.25
g͒ were stirred at 90°C under 1.0 MPa nitrogen atmosphere in a
stainless steel autoclave ͑50 mL͒ for 24 h, cooled to room tempera-
ture and washed with ethyl acetate ͑3 ϫ 15 mL͒, and then dried
under reduced pressure to afford 1-hexyl-3-methylimidazolium bro-
mide ͓͑HMIm͔Br͒ as a colorless, viscous product ͑11.62 g, 94%͒.
[HMIm] [WO ]
2
4
[HMIm][PF ]
6
3120.9
[HMIm][CH3SO3]
3172.1
3150.5
The synthesis of silver tungstate ͑Ag WO ͒
.— The aqueous so-
3106.0
2
4
lution of 0.05 mol AgNO3 ͑8.49 g͒ was added to the solution of
0.025 mol Na2WO4 ͑8.25 g͒ in the dark, and then a pale yellow
precipitate was produced immediately and washed with deionized
water three times.
4000 3500 3000 2500 2000
1000 500
150-01
Wavenumbers (cm )
Figure 1. FTIR spectra of the different ILs.
The synthesis of ͓HMIm͔ ͓WO ͔
.— The aqueous solution of
2
4
0.047 mol ͓HMIm͔Br ͑11.62 g͒ was added to the freshly prepared
Ag2WO4. The color of the precipitate changed obviously from pale
yellow to yellow and, simultaneously, the particle size became big-
ger. The reaction mixture was stirred for an additional period of 0.5
h and filtered, and then the transparent colorless filtrate was concen-
trated and dried under reduced pressure to afford the colorless liquid
water, and finally dried with sodium carbonate. The organic solvent
was evaporated under reduced pressure to afford the desired hexyl
methanesulfonate as a colorless liquid ͑6.60 g, 92%͒. ␦H ͑500 MHz,
CDCl3͒: 1.28 ͑3H, t, CH3͒, 1.34 ͑4H, m, C2H4͒, 1.75 ͑2H, m, CH2͒,
3.00 ͑3H, s, CH3͒, 4.22 ͑2H, t, CH2͒.
product ͓HMIm͔ ͓WO4͔ ͑12.72 g, 93%͒. The as-synthesized IL con-
2
Hexyl methanesulfonate ͑6.60 g, 36.8 mmol͒ was mixed with
1-methyl-imidazole ͑3.00 g, 37 mmol͒ and the reaction mixture was
kept at room temperature for 12 h, then the reaction temperature was
raised to 60°C for 48 h with vigorous stirring. After the reaction, the
product was recrystallized at 0°C three times using ethyl acetate as
a solvent ͑20 mL͒. Then after vacuum drying, a pale yellow liquid
͓HMIm͔͓CH3SO3͔ was obtained as a slightly yellow liquid ͑8.54 g,
89%͒. The water content in the IL was 0.02% by Karl Fischer titra-
tion. ͓Found: C, 50.32; H, 8.47; N, 10.63; S, 12.25%. C11H22N2O3S
requires C, 50.36; H, 8.45; N, 10.68; S, 12.22%. ␦H ͑500 MHz,
CDCl3͒: 0.79 ͑3H, t, CH3͒, 1.21 ͑6H, m, C3H6͒, 1.80 ͑2H, m, CH2͒,
2.68 ͑3H, s, CH3͒, 3.97 ͑3H, s, CH3͒, 4.18 ͑2H, t, CH2͒, 7.41 ͑1H, s,
CH͒, 7.55 ͑1H, s, CH͒, 9.68 ͑1H, s, CH͒.͔
tained no residue of Ag+ or Br− ions by elemental analysis, and
water content in the IL is 0.8% by Karl Fischer titration. ͓Found: C,
38.25; H, 6.88; N, 8.86; W, 29.74%. C20H38N4O4W requires C,
38.84; H, 6.85; N, 9.06;W, 29.73%. ␦H ͑500 MHz, CDCl3͒: 0.79
͑3H, t, CH3͒, 1.22 ͑6H, m, C3H6͒, 1.81 ͑2H, m, CH2͒, 3.82 ͑3H, s,
CH3͒, 4.14 ͑2H, t, CH2͒, 7.41 ͑1H, s, CH͒, 7.70 ͑1H, s, CH͒, 9.90
͑1H, s, CH͒.͔
The synthesis of ͓DMIm͔ ͓WO ͔
.— The synthesis route of
2
4
1-dodecyl-3-methylimidazolium bromide ͓͑DMIm͔Br͒ was very
similar to that of ͓HMIm͔Br. The product ͓DMIm͔Br was obtained
as a colorless, viscous product in high yield ͑96%͒.
The synthesis of ͓DMIm͔ ͓WO ͔
.— The aqueous solution of
2
4
The synthesis of ͓HMIm͔͓PF ͔
.— ͓HMIm͔͓PF6͔ was simply
0.048 mol ͓DMIm͔Br ͑15.90 g͒ was added to the freshly prepared
Ag2WO4. The color of the precipitate changed immediately to yel-
low and the reaction mixture was continuously stirred at 60°C for
12 h. The precipitate was filtered. Then the transparent yellow fil-
trate was evaporated and dried under reduced pressure to afford the
6
synthesized by an anion exchange reaction between
͓HMIm͔͓CH3SO3͔ and KPF6 in water. The precipitate was washed
with water three times and then dried under vacuum, and then
͓HMIm͔͓PF6͔ was obtained as a colorless liquid. The water content
in the IL was 0.01% by Karl Fischer titration. ͓Found: C, 38.49; H,
6.16; N, 8.96; P, 9.84; F, 36.48%. C10H19N2PF6 requires C, 38.47;
H, 6.13; N, 8.97; P, 9.92; F, 36.51%. ␦H ͑500 MHz, CDCl3͒: 0.88
͑3H, t, CH3͒, 1.32 ͑4H, m, C2H4͒, 1.56 ͑2H, m, CH2͒, 1.87 ͑2H, m,
CH2͒, 3.93 ͑3H, s, CH3͒, 4.15 ͑2H, t, CH2͒, 7.25 ͑1H, s, CH͒, 7.26
͑1H, s, CH͒, 8.56 ͑1H, s, CH͒.͔
yellow liquid ͓DMIm͔ ͓WO4͔ ͑16.58 g, 92%͒. The as-synthesized
2
IL contained 17.7 ppm Ag+ and a trace of Br− by elemental analysis,
and water content in the IL was 0.7% by Karl Fischer titration.
͓Found: C, 49.58; H, 8.60; N, 7.20; W, 21.46%. C32H62N4O4W
requires C, 48.85; H, 8.46; N, 7.12; W, 23.37%. ␦H ͑500 MHz,
CDCl3͒: 0.86 ͑3H, t, CH3͒, 1.11 ͑2H, m, CH2͒, 1.23 ͑16H, m,
C8H16͒, 1.84 ͑2H, m, CH2͒, 4.11 ͑3H, s, CH3͒, 4.22 ͑2H, t, CH2͒,
7.26 ͑1H, s, CH͒, 7.52 ͑1H, s, CH͒, 9.85 ͑1H, s, CH͒.͔
Results
Characterization of ͓HMIm͔ ͓WO ͔ and ͓DMIm͔ ͓WO ͔
.—
2
4
2
4
The synthesis of ͓HMIm͔͓CH SO ͔
.— ͓HMIm͔͓CH3SO3͔ was
3
3
The structures of as-synthesized ͓HMIm͔ ͓WO4͔ and
synthesized according to our previously reported procedure.28
Briefly, the reaction of 1-methylimidazole with hexyl methane-
sulfonate afforded ͓HMIm͔͓CH3SO3͔ in high yield ͑Ͼ95%͒.
Hexyl methanesulfonate was synthesized by the following steps:
A solution of 1-hexanol ͑4.09 g, 40 mmol͒ and triethylamine ͑7.8
mL, 56 mmol͒ in dichloromethane ͑30 mL͒ was cooled in an ice
bath as methanesulfonyl chloride ͑3.6 mL, 46 mmol͒ in dichlo-
romethane ͑10 mL͒ was added dropwise. An external water bath was
used to control the reaction mixture temperature between 10 and
20°C. After addition, stirring was continued for another 2 h. Then
water ͑25 mL͒ was added; the aqueous layer containing the triethy-
lammonium chloride by-product was separated; the organic layer
was washed sequentially with a saturated solution of NaHCO3 and
2
͓DMIm͔ ͓WO4͔ were first confirmed by 1H NMR and elemental
2
analysis. In addition, FTIR and TGA have also been utilized to
prove the existence of anions and determine the thermal stability of
the ILs, respectively. The FTIR bands of ͓HMIm͔ ͓WO4͔ and
2
͓DMIm͔ ͓WO4͔ are shown in Fig. 1; obviously they were very
2
similar. The characteristic bands of alkyl groups in imidazolium
rings were observed around 2958, 2930, 2860, 1466, and
1380 cm−1. The bands around 1650 and 1570 cm−1 were due to
CvN and CvC ring vibrations of the imidazolium cation part of
two ILs.29 The typical large absorption band of the WO24− anion
30-32
from ϳ700 to 900 cm−1
,
which was reported to be ascribed to
the W–O stretching vibration in the WO4 tetrahedron, is shown in
Downloaded on 2014-11-13 to IP 128.248.155.225 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

F126
Journal of The Electrochemical Society, 157 ͑9͒ F124-F129 ͑2010͒
Table I. The chemical shifts of H2, H4, and H5 in 1H NMR
100
80
60
40
20
spectra for the different ILs.
IL
H2
H4
H5
͓HMIm͔͓CH3SO3͔
9.68
9.90
9.85
8.56
7.55
7.70
7.52
7.26
7.41
7.41
7.26
7.23
͓HMIm͔ ͓WO4͔
2
͓DMIm͔ ͓WO4͔
2
͓HMIm͔͓PF6͔
[HMIm] [WO ]
2
4
Together with the fact that the H2 proton normally moved signifi-
cantly downfield with increasing acidity,40 the hydrogen bond was
more pronounced at the H2 protons than at the H4 and H5 protons.
Considering the H2 chemical shift data in the three ILs, the hydro-
[DMIm] [WO ]
2
4
0
100
200
300
400
500
600
gen bonding decreased in the following order: ͓HMIm͔ ͓WO4͔
2
ο
T / C
Ͼ ͓HMIm͔͓CH3SO3͔ Ͼ ͓HMIm͔͓PF6͔. This can be explained by
the fact that the electrostatic interaction in ͓HMIm͔ ͓WO4͔ was
stronger than that in ͓HMIm͔͓CH3SO3͔ and ͓HMIm͔͓PF6͔ because
2
Figure 2. The TGA trace of ͓HMIm͔ ͓WO4͔ and ͓DMIm͔ ͓WO4͔.
2
2
of a higher negative charge WO24− anion, and the higher charge
density should result in a stronger hydrogen bonding interaction.41,42
Vibrational spectroscopy is another versatile tool in probing the
hydrogen bonding of both liquid and solid compounds or their so-
lutions. Hence, the IR spectra of the four ILs are given in Fig. 1 and
confirmed the structures of the ILs. The peaks between 3100 and
3200 cm−1 can be attributed to the vibration of the imidazolium ring
C–H stretch vibration, whereas those between 2800 and 3000 cm−1
can be attributed to aliphatic C–H stretches. The imidazolium ring
C–H stretch vibration of ͓HMIm͔͓PF6͔ appears in 3172.1 and
3120.9 cm−1, which are much higher frequencies than those of
3150.5 and 3106.0 cm−1 in ͓HMIm͔͓CH3SO3͔ as well as 3143.5
both ILs. Meanwhile, the pronounced bands of the 1-hexyl-3-
methylimidazolium cation of both ILs ͓HMIm͔͓PF6͔ and
͓HMIm͔͓CH3SO3͔ were clearly observed in FTIR. Also, the feature
bands of PF−6 anion33 in 839.0 cm−1 as well as the bands of CH3SO−3
anion34 in 1192.4, 1043.0, and 773.3 cm−1 significantly existed in
͓HMIm͔͓PF6͔ and ͓HMIm͔͓CH3SO3͔. Although the typical bands
of WO24− and PF−6 were overlapped in the low wavenumber region,
as shown in Fig. 1, the tungstate-based ILs were not produced from
PF−6-based ILs, so the presence of WO42− in ͓HMIm͔ ͓WO4͔ and
2
͓DMIm͔ ͓WO4͔ was confirmed very clearly. In combination with
2
and 3102.8 cm−1 in ͓HMIm͔ ͓WO4͔. Upon formation of the hydro-
NMR spectra and the elemental analysis ͑ICP͒, it can be seen that
2
the as-synthesized ILs were very pure.
gen bond, the C–H bond was weakened and, thus, the frequency of
its stretching vibration is normally decreased.39 Therefore, the
strength of the hydrogen bond between the H2 proton and anions
decreased in the order WO24− Ͼ CH3SO3− Ͼ PF−6, which agreed well
The thermal stability of the two ILs ͓͑HMIm͔ ͓WO4͔ and
2
͓DMIm͔ ͓WO4͔͒ was investigated using the TGA method. As
2
shown in Fig. 2, both ILs were decomposed by two successive
weight-loss regions: The weight-loss region above 220 and 400°C
can be attributed to the gradual decomposition of the imidazolium
1
with the H NMR characterization.
Because ILs ͓HMIm͔ ͓WO4͔ and ͓DMIm͔ ͓WO4͔ gave no sig-
2
2
cation part ͑63.3% loss for ͓HMIm͔ ͓WO4͔ and 70.5% loss for
nificant differences from the characterization of the FTIR and 1H
NMR chemical shift ͑Fig. 1 and Table I͒, the length of the alkyl
chain in the imidazolium ring most possibly had no obvious effect
on the hydrogen bond formation.
2
͓DMIm͔ ͓WO4͔͒.35 No further decomposition was observed up to
2
450°C. Two ILs with different alkyl chain lengths nearly had the
same thermogravimetric–differential thermal analysis curves, which
suggested that the imidazolium cation had insignificant effects on
the thermostability of the ILs.
The conductivity of ͓HMIm͔͓PF6͔, ͓HMIm͔͓CH3SO3͔,
͓HMIm͔ ͓WO ͔, and ͓DMIm͔ ͓WO ͔
.— The ionic conductivity is
2
4
2
4
Discussion
a transport property and is governed by the degree of dissociation of
the ion, which is related to ion mobility, ionic charge, and ion num-
ber involved in the conduction process. Figure 3 shows the conduc-
Hydrogen bonding formation in ͓HMIm͔͓PF6͔, ͓HMIm͔
͓CH SO ͔, ͓HMIm͔ ͓WO ͔, and ͓DMIm͔ ͓WO ͔
.— An important
3
3
2
4
2
4
tivities of ILs ͓HMIm͔͓PF6͔, ͓HMIm͔͓CH3SO3͔, ͓HMIm͔ ͓WO4͔,
2
aspect of RTILs is the nature of the interactions among the ions
present, and questions of particular interest include how the interac-
tion affects the properties. In the present work, we first used 1H
NMR to investigate the alkyl chain length in the imidazolium ring
and the anion effects on the trend of forming hydrogen bonds in this
class of ILs. The three protons in the IL imidazolium ring were
distinguished by the commonly adopted annotation H2, H4, and H5
as reported.36-38 H2 hydrogen represented the substituent at the car-
bon between the two nitrogens. H4 denoted the hydrogen substituent
at the imidazole ring carbon close to the methyl group, whereas H5
hydrogen was the substituent at the ring carbon in the vicinity of the
long alkyl chain. The proton NMR chemical shift data of the ILs are
shown in Table I. From the listed ILs in Table I, the chemical shift
variations in H4 and H5 were not significant, but H2 exhibited a
much broader chemical shift variation than either H4 or H5, with
9.68 in the IL ͓HMIm͔͓CH3SO3͔, 8.56 in the IL ͓HMIm͔͓PF6͔, and
and ͓DMIm͔ ͓WO4͔ at different temperatures. The conductivity val-
2
ues of ͓HMIm͔͓PF6͔ measured in this work agreed well with those
reported,43 which indicated that the present data were reliable. The
same conductivity–temperature correlation was observed for all the
four ILs. Higher temperatures resulted in higher conductivities. Fur-
thermore, a small increase in conductivity was observed in the lower
temperature range and a large increase was observed in the higher
temperature range. Temperature at the intersection of the low and
high temperature range dependent on conductivity was defined as
Tis, which indicated that the transport processes were differently
affected below and above this temperature ͑Table II͒. Further, the
conductivity of ͓HMIm͔͓PF6͔ was the highest among ILs at the
same temperature, whereas the conductivities of ͓HMIm͔ ͓WO4͔
2
were higher than that of ͓DMIm͔ ͓WO4͔. The conductivity of
2
͓HMIm͔͓CH3SO3͔ was slightly lower than that of ͓HMIm͔͓PF6͔
but much higher than that of the corresponding tungstate anion-
about 9.90 in ͓HMIm͔ ͓WO4͔. This can be explained such that H2
2
was more acidic and more sensitive to anion effects than H4 or
based IL ͓HMIm͔ ͓WO4͔.
2
H5,39 which also implied that the more basic CH3SO− and WO24−
For the imidazolium ILs, the positive charge is normally delocal-
ized on the imidazolium ring, that is, the long alkyl side chain fa-
3
anions show a stronger interaction with H2 proton than PF−6 anion.
Downloaded on 2014-11-13 to IP 128.248.155.225 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 157 ͑9͒ F124-F129 ͑2010͒
F127
30
25
20
15
10
5
[DMIm]2[WO4]
[HMIm]2[CH3SO3]
[HMIm]2[WO4]
[HMIm][PF6]
0
-2
[DMIm] [WO ]
-4
-6
2
4
0
30
30
40
40
50
50
60
60
70
80
1000
800
600
400
200
0
-8
-10
[HMIm] [WO ]
2
4
-12
2.8
2.9
3.0
3.1
3.2
3.3
70
80
1000/T(K)
5000
4000
3000
2000
1000
0
Figure 4. Arrhenius conductivity plots of the ILs.
[HMIm][PF ]
6
[HMIm][CH SO ]
ln ␴ = ln A − ͑Ea/RT͒
͓1͔
3
3
where Ea is the activation energy for electrical conduction ͑the en-
ergy needed for an ion to jump to a free hole͒, calculated from the
Arrhenius formula presented in Table II. The tungstate-based ILs
show higher charge-transfer activation energies. The activation en-
30
40
50
60
70
80
T (oC)
ergies
Ea
decreased
in
the
order
͓DMIm͔ ͓WO4͔
2
Ͼ ͓HMIm͔ ͓WO4͔ Ͼ ͓HMIm͔͓CH3SO3͔ Ͼ ͓HMIm͔͓PF6͔.
Figure 3. Conductivities of the ILs at different temperatures.
2
The viscosity of ͓HMIm͔͓PF6͔, ͓HMIm͔ ͓WO4͔, and
2
͓DMIm͔ ͓WO ͔
.— Because ILs are more viscous than conventional
2
4
solvents, this is an important property for possible application. Fig-
ure shows the dynamic viscosities of ILs ͓HMIm͔͓PF6͔,
͓HMIm͔ ͓WO4͔, and ͓DMIm͔ ͓WO4͔ at different temperatures.
cilitates dispersive interactions with the anion and intermolecular
binding. Further, van der Waals interactions become more attractive
when the length of the alkyl group is increased. Therefore, the fact
5
2
2
The viscosity values of ͓HMIm͔͓PF6͔ in this work agreed well with
those reported.44 The viscosities of either ͓HMIm͔ ͓WO4͔ or
that the conductivities of ͓HMIm͔ ͓WO4͔ were higher than
2
2
͓DMIm͔ ͓WO4͔ with the longer alkyl chain was expected.
2
͓DMIm͔ ͓WO4͔ are much higher than those of the conventional IL
2
The electrostatic interaction between the 1-alkyl-3-
methylimidazolium cation and CH3SO−3 or PF6− anion was much
weaker than that between the cation and the two negative charge
WO24− anion. Thus, CH3SO−3 and PF−6 were more mobile anions than
WO24− anion, which induced that the conductivity of
such as ͓HMIm͔͓PF6͔ ͑Fig. 5͒, or 1-alkyl-3-methylimidazolium
methanesulfonate, which was reported.45 In addition, the viscosity
of ILs was inversely correlated with conductivity at the same tem-
perature. For example, the less viscous tungstate-based IL
͓͑HMIm͔ ͓WO4͔͒ owned the higher conductivity ͑Fig. 3 and 5͒,
2
which agreed with phenomena previously reported.46,47 Because the
viscosities of dialkylimidazolium ILs are dictated by a combination
of electrostatics, van der Waals interactions, hydrogen bonding, ion
͓HMIm͔ ͓WO4͔ was lower than that of ͓HMIm͔͓CH3SO3͔ or
2
͓HMIm͔͓PF6͔. Moreover, in comparison with ͓HMIm͔͓PF6͔,
͓HMIm͔͓CH3SO3͔ shows a lower conductivity, which can be attrib-
size, and polarizability,8,48 ͓HMIm͔ ͓WO4͔ had a higher viscosity
uted to the stronger hydrogen bond interaction of CH3SO−3 with H2
proton. Figure 4 shows the Arrhenius conductivity plots of ILs
2
than conventional ILs such as hexafluorophosphate and methyl sul-
fonate anion-based ILs, which could be ascribed to a stronger elec-
trostatic interaction between imidazolium cation and WO24− anion.48
͓HMIm͔͓PF6͔,
͓HMIm͔͓CH3SO3͔,
͓HMIm͔ ͓WO4͔,
and
2
͓DMIm͔ ͓WO4͔ as a function of temperature. All experimental data
2
Moreover, ͓DMIm͔ ͓WO4͔ with a longer alkyl group possessed a
in the temperature range studied fitted well the conventional Arrhen-
ius equation 1
2
higher viscosity than ͓HMIm͔ ͓WO4͔ due to stronger van der Waals
2
Table II. Activation energy, conductivity intersection, and linearity correlation of the ILs.
a
a
b
Ea
Ea
Tis
Entry
IL
͑kJ mol−1
͒
͑eV͒
͑°C͒
R2
1
2
3
4
͓HMIm͔͓CH3SO3͔
͓HMIm͔͓PF6͔
42.9
33.9
66.5
96.5
0.445
0.351
0.689
1.000
48
43
56
57
1.000
0.953
1.000
0.982
͓HMIm͔ ͓WO4͔
2
͓DMIm͔ ͓WO4͔
2
a Activation energy calculated from the Arrhenius formula.
b Temperature at the intersection of the low and high temperature range of the temperature-dependent conductivity.
Downloaded on 2014-11-13 to IP 128.248.155.225 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

F128
Journal of The Electrochemical Society, 157 ͑9͒ F124-F129 ͑2010͒
0.3
10
8
6
4
2
6
0.2
[HMIm]2[WO4
]
0.1
0.0
4
2
-0.1
-0.2
-3
-2
-1
0
1
2
3
Potential / V
[
[HMIm] WO ]
2
4
0
0
30
30
30
40
40
40
50
50
50
60
60
60
70
80
-2
-4
-6
[HMIm][CH3SO3]
[HMIm]2[WO4]
16
12
8
]
[DMIm] [WO
2
4
-3
-2
-1
0
1
2
3
4
Potential / V
70
80
Figure 7. Cyclic voltammogram of pure ILs ͓HMIm͔͓CH3SO3͔ and
͓HMIm͔ ͓WO4͔. The vertical coordinate ͓current ͑A͔͒ was reduced 20 times
2
for IL ͓HMIm͔ ͓WO4͔ ͑inset͒.
2
0.3
0.2
0.1
[PF
[HMIm]
]
ture with a scan rate of 0.1 V/s. The electrochemical behavior of
conventional imidazolium ILs has been well documented.50 In the
present work, there was no visible shoulder of ͓HMIm͔͓CH3SO3͔ or
6
͓HMIm͔ ͓WO4͔ over the range from Ϫ2 to 2 V. The reduction of
2
70
80
imidazolium cations in the cathodic electrode may proceed initially
by the reduction of ring protons to molecular hydrogen, which usu-
ally determines the cathodic window of the IL.52 Electro-oxidation
charge-transfer processes could occur at the positive potential range,
probably involving the adsorbed species formed previously on the
anodic electrode.15 This suggested that the electrochemical potential
o
T/ ( C)
Figure 5. Dynamic viscosities as a function of temperature for different ILs.
window of two ILs ͓HMIm͔͓CH3SO3͔ and ͓HMIm͔ ͓WO4͔ at room
2
interactions. This behavior confirmed that, although the high elec-
trostatic interactions between ions determined the characteristic
physicochemical properties of ILs, dispersive van der Waals-type
forces also played an important role and determined the way the
viscosity changes with the size of the alkyl side chain. The viscosi-
ties for two tungstate-based ILs measured at different temperatures
͑30–80°C͒ gave slightly curved Arrhenius plots ͑Fig. 6͒, which has
been observed in dialkylimidazolium ILs and metal complex poly-
ether molten salts.49,50 A classical explanation for such behavior
would involve coupling of chain motions ͑alkyl and polyether͒ with
the physical transport process.51
temperature is found to be nearly 4 V, agreeing with previous
studies.4 The cyclic voltammograms of ͓DMIm͔ ͓WO4͔ displayed a
2
similar behavior to that of ͓HMIm͔ ͓WO4͔. Thus, it came to the
2
conclusion that the novel tungstate-based ILs show nearly the same
electrochemical window as the conventional ILs.
Conclusions
In summary, the synthesis of the new tungstate-based imidazo-
lium ILs, ͓HMIm͔ ͓WO4͔ and ͓DMIm͔ ͓WO4͔, was first reported
2
2
1
here. The H NMR, FTIR, conductivity, viscosity, and cyclic volta-
mmogram of the tungstate-based ILs were conducted to characterize
the physicochemical properties of the ILs, which were then com-
pared with those of conventional ILs such as ͓HMIm͔͓PF6͔ and
͓HMIm͔͓CH3SO3͔. The effects of the anions and the length of the
alkyl chain in the imidazolium ring on the electrochemical proper-
ties have been mainly investigated. The tungstate anion has a deci-
sive influence on conductivity, viscosity, and cyclic voltammogram,
as well as the intramolecular hydrogen bonding interaction, although
the length of the alkyl group in the imidazolium ring also affected to
some extent the electrochemical properties of ILs. The tungstate-
based ILs show good electrochemical stability as the conventional
ILs.
The CV of ͓HMIm͔ ͓WO ͔ and ͓HMIm͔͓CH SO ͔
.— The po-
2
4
3
3
tential windows for the tungstate-functionalized ILs have been mea-
sured by CV. Figure 7 shows the cyclic voltammograms of pure ILs
͓HMIm͔͓CH3SO3͔ and ͓HMIm͔ ͓WO4͔ recorded at room tempera-
2
1.0
0.5
[HMIm]2[WO4]
[DMIm]2[WO4]
0.0
Acknowledgments
[HMIm][PF6]
-0.5
The authors are grateful for the support from the National Natu-
ral Science Foundation of China ͑no. 20773037͒ and the Commis-
sion of Science and Technology of Shanghai Municipality, China
͑no. 07PJ14023͒. The authors also thank Professor Walter Leitner
for the helpful suggestion and Dr. Nils Theyssen for assistance.
-1.0
-1.5
-2.0
References
1. T. Welton, Chem. Rev. (Washington, D.C.), 99, 2071 ͑1999͒.
2. V. I. Pârvulescu and C. Hardacre, Chem. Rev. (Washington, D.C.), 107, 2615
͑2007͒.
3. F. Endres, D. MacFarlane, and A. Abbott, Electrodeposition from Ionic Liquids, 1st
ed., Wiley-VCH, Weinheim, Germany ͑2008͒.
2.8
2.9
3.1
3.2
3.3
31.0000/T(K)
Figure 6. Arrhenius viscosity plots of the different ILs.
Downloaded on 2014-11-13 to IP 128.248.155.225 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 157 ͑9͒ F124-F129 ͑2010͒
F129
4. P. Wasserscheid and T. Welton, Ionic Liquids in Synthesis, 1st ed., Chaps. 3 and 4,
pp. 127–171, Wiley-VCH, Weinheim, Germany ͑2002͒.
5. J. Dupont, J. Braz. Chem. Soc., 15, 341 ͑2004͒.
27. J. Dupont and P. A. Z. Suarez, Phys. Chem. Chem. Phys., 8, 2441 ͑2006͒.
28. Y. X. Qiao, Z. S. Hou, H. Li, Y. Hu, B. Feng, X. R. Wang, and L. Hua, Green
Chem., 11, 1955 ͑2009͒.
6. J. Dupont, R. F. de Souza, and P. A. Z. Suarez, Chem. Rev. (Washington, D.C.),
29. A. Bordoloi, S. Sahoo, F. Lefebvre, and S. B. Halligudi, J. Catal., 259, 232 ͑2008͒.
30. S. Takai, T. Nakanishi, K. Oikawa, S. Torii, A. Hoshikawa, T. Kamiyama, and T.
Esaka, Solid State Ionics, 170, 297 ͑2004͒.
31. B. S. Chhikara, S. Tehlan, and A. Kumar, Synlett, 2005, 63.
32. W. Li, T. B. Tang, and X. Feng, J. Appl. Phys., 87, 7692 ͑2000͒.
33. J.-M. Andanson, F. Jutz, and A. Baiker, J. Phys. Chem. B, 113, 10249 ͑2009͒.
34. M. J. Tang and T. Zhu, Sci. China. Ser. Biol. Chem., 52, 93 ͑2009͒.
35. A. B. Bourlinos, K. Raman, R. Herrera, Q. Zhang, L. A. Archer, and E. P. Gian-
nelis, J. Am. Chem. Soc., 126, 15358 ͑2004͒.
102, 3667 ͑2002͒.
7. J. G. Huddleston, A. E. Visser, W. M. Reichert, H. D. Willauer, G. A. Broker, and
R. D. Rogers, Green Chem., 3, 156 ͑2001͒.
8. P. G. Rickert, M. R. Antonio, M. A. Firestone, K. A. Kubatko, T. Szreder, J. F.
Wishart, and M. L. Dietz, J. Phys. Chem. B, 111, 4685 ͑2007͒.
9. T. I. Leong, I. W. Sun, M. J. Deng, C. M. Wu, and P. Y. Chen, J. Electrochem. Soc.,
155, F55 ͑2008͒.
10. G. T. Cheek, W. E. O’Grady, S. Zein El Abedin, E. M. Moustafa, and F. Endres, J.
Electrochem. Soc., 155, D91 ͑2008͒.
11. Q. X. Liu, S. Zein El Abedin, and F. Endres, J. Electrochem. Soc., 155, D357
͑2008͒.
36. M. Gjikaj, W. Brockner, J. Namyslo, and A. Adam, Cryst. Eng. Comm., 10, 103
͑2008͒.
37. Y. Jeon, J. Sung, D. Kim, C. Seo, H. Cheong, Y. Ouchi, R. Ozawa, and H. Hamagu-
12. L. S. Plashnitsa, E. Kobayashi, Y. Noguchi, S. Okada, and J. Yamaki, J. Electro-
chem. Soc., 157, A536 ͑2010͒.
13. C. H. Yang, Q. J. Sun, J. Qiao, and Y. F. Li, J. Phys. Chem. B, 107, 12981 ͑2003͒.
14. M. Salanne, C. Simon, and P. Turq, J. Phys. Chem. B, 111, 4678 ͑2007͒.
15. M. V. Migliorini, R. K. Donato, M. A. Benvegnu, J. Dupont, R. S. Gonçalves, and
H. S. Schrekker, Catal. Commun., 9, 971 ͑2008͒.
chi, J. Phys. Chem. B, 112, 923 ͑2008͒.
38. B. C. Guo, J. J. Wan, Y. D. Lei, and D. M. Jia, J. Phys. D, 42, 145307 ͑2009͒.
39. H. S. Schrekker, D. O. Silva, M. A. Gelesky, M. P. Stracke, C. M. L. Schrekker, R.
S. Goncalves, and J. Dupont, J. Braz. Chem. Soc., 19, 426 ͑2008͒.
40. A. G. Avent, P. A. Chaloner, M. P. Day, K. R. Seddon, and T. Welton, J. Chem.
Soc. Dalton Trans., 1994, 3405.
16. Y. J. Tang and X. Q. Zeng, J. Electrochem. Soc., 155, F82 ͑2008͒.
17. R. F. de Souza, J. C. Padilha, R. S. Gonçalves, and J. Dupont, Electrochem. Com-
mun., 5, 728 ͑2003͒.
18. S. Zein El Abedin and F. Endres, ChemPhysChem, 7, 58 ͑2006͒.
19. A. Lewandowski and A. Świderska, Appl. Phys. A: Mater. Sci. Process., 82, 579
͑2006͒.
41. L. M. Epstein and E. S. Shubina, Coord. Chem. Rev., 231, 165 ͑2002͒.
42. Y. Wu, T. T. Zhang, and J. R. Li, Acta Chim. Sin., 67, 1851 ͑2009͒.
43. T. Umecky, M. Kanakubo, and Y. Ikushima, J. Mol. Liq., 119, 77 ͑2005͒.
44. A. B. Pereiro, J. L. Legido, and A. Rodrıguez, J. Chem. Thermodyn., 39, 1168
͑2007͒.
45. M. Blesic, M. S. Kwasny, T. Belhocine, H. Q. N. Gunaratne, J. N. C. Lopes, M. F.
C. Gomes, A. A. H. Padua, K. R. Seddon, and L. P. N. Rebelo, Phys. Chem. Chem.
Phys., 11, 8939 ͑2009͒.
20. Q. Zhu, Y. Song, X. F. Zhu, and X. L. Wang, J. Electroanal. Chem., 601, 229
͑2007͒.
21. A. B. McEwen, H. L. Ngo, K. LeCompte, and J. L. Goldman, J. Electrochem. Soc.,
146, 1687 ͑1999͒.
22. E. Stathatos, P. Lianos, V. Jovanovski, and B. Orel, J. Photochem. Photobiol., A,
46. C. Rey-Castro and L. F. Vega, J. Phys. Chem. B, 110, 14426 ͑2006͒.
47. W. Xu, E. I. Cooper, and C. A. Angell, J. Phys. Chem. B, 107, 6170 ͑2003͒.
48. C. Chiappe and D. Pieraccini, J. Phys. Org. Chem., 18, 275 ͑2005͒.
49. H. Tokuda, K. Hayamizu, K. Ishii, M. A. B. H. Susan, and M. Watanabe, J. Phys.
Chem. B, 109, 6103 ͑2005͒.
50. W. Wang, R. Balasubramanian, and R. W. Murray, J. Phys. Chem. C, 112, 18207
͑2008͒.
51. F. M. Gary, Solid Polymer Electrolytes, Fundamentals and Technological Applica-
tions, VCH Publishers, New York ͑1991͒.
169, 57 ͑2005͒.
23. N. Papageorgiou, Y. Athanassov, M. Armand, P. Bonhote, H. Pettersson, A. Azam,
and M. Gratzel, J. Electrochem. Soc., 143, 3099 ͑1996͒.
24. E. Markevich, V. Baranchugov, and D. Aurbach, Electrochem. Commun., 8, 1331
͑2006͒.
25. R. F. de Souza, J. C. Padilha, R. S. Gonçalves, and J. Rault-Berthelot, Electrochem.
Commun., 8, 211 ͑2006͒.
52. A. A. J. Torriero, A. I. Siriwardana, A. M. Bond, I. M. Burgar, N. F. Dunlop, G. B.
Deacon, and D. R. MacFarlane, J. Phys. Chem. B, 113, 11222 ͑2009͒.
26. F. Endres and S. Zein El Abedin, Phys. Chem. Chem. Phys., 8, 2101 ͑2006͒.
Downloaded on 2014-11-13 to IP 128.248.155.225 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).