Polymer electrolytes containing ionic liquids with acidic counteranion (DMRImH2PO4, R = ethyl, butyl and octyl)

Article abstract of DOI:10.1016/j.cplett.2006.04.119

Ionic liquids with acidic counteranion and having composition: 2,3-dimethyl-1-alkylimidazolium dihydrogenphosphate (DMRImH₂PO₄, R = ethyl, butyl, octyl) have been prepared and the effect of alkyl (R) sidechain length on the conductiv

Full text of DOI:10.1016/j.cplett.2006.04.119

Chemical Physics Letters 425 (2006) 294–300
www.elsevier.com/locate/cplett
Polymer electrolytes containing ionic liquids with acidic
counteranion (DMRImH2PO4, R = ethyl, butyl and octyl)
a
a,b,
*
Boor Singh Lalia , S.S. Sekhon
a
Department of Applied Physics, Guru Nanak Dev University, Amritsar 143 005, India
Polymer Electrolyte Fuel Cell Research Department, Korea Institute of Energy Research, Daejeon 305 343, Republic of Korea
b
Received 18 February 2006; in final form 12 April 2006
Available online 20 May 2006
Abstract
Ionic liquids with acidic counteranion and having composition: 2,3-dimethyl-1-alkylimidazolium dihydrogenphosphate
(DMRImH2PO4, R = ethyl, butyl, octyl) have been prepared and the effect of alkyl (R) sidechain length on the conductivity and vis-
cosity behavior has been studied. DMEtImH2PO4 with highest conductivity (0.07 S/cm at 120 ꢀC) has been incorporated in polyvinylid-
enefluoride-co-hexafluoropropylene (PVdF-HFP) to obtain polymer electrolytes in the membrane form. The conductivity of membranes
has been found to depend upon the concentration of ionic liquid, phosphoric acid and temperature. Polymer electrolytes containing dif-
ferent ionic liquids are thermally stable up to 225 ꢀC and can be used as high temperature membranes for fuel cells.
ꢁ 2006 Elsevier B.V. All rights reserved.
1. Introduction
phobic and hydrophilic ionic liquids based on imidazolium
cations have been reported to depend upon the alkyl chain
length of the imidazolium cation and nature of the anion
[21,22]. The thermal stability of ionic liquids has also been
reported to depend upon the nature of cation [23]. A
change in viscosity, density, ion size and degree of dissoci-
ation also affects the conductivity of ionic liquid; however
it is difficult to estimate the contribution of each parameter
separately. The higher viscosity of ionic liquids is generally
explained to be due to very strong coulombic forces
between the ionic species present in ionic liquids. However,
most of the studies are mainly on ionic liquids containing
various fluoroanions and both cations and anions have
been reported to be mobile in such ionic liquids. As electro-
lytes in the thin membrane form are generally preferred for
their use in various electrochemical applications, so these
ionic liquids have been incorporated in different polymers
to obtain electrolytes in the gel and membrane form [24–
27]. The effect of the addition of plasticizer and inert insu-
lating phase on the properties of polymer electrolytes con-
taining ionic liquids has also been reported [28–30].
Room temperature ionic liquids containing various non-
chloroaluminate anions, which are stable in air and mois-
ture, are receiving much attention due to their potential
applications in many diverse fields [1–4]. They generally
comprise entirely of ions and are characterised by large
window of electrochemical stability, high conductivity,
wide operating temperature range, negligible vapor pres-
sure, non-flammability, etc., which make them attractive
candidates for various electrochemical devices including
supercapacitors, batteries, fuel cells, dye sensitized solar
cells, actuators, etc. [5–13]. They generally consist of a
combination of organic cations such as imidazolium, pyrid-
inium, pyrrolidinium, ammonium, sulfonium and phos-
phonium and bulky and soft anions, such as CF3SOꢀ,
3
NðCF3SO2Þꢀ2 , PF6ꢀ and BFꢀ4 [1,14–20]. As a large number
of combinations of cations and anions are possible, so
the physicochemical properties of ionic liquids can be con-
trolled by a suitable choice of the composition of ionic
liquid. The physicochemical properties of different hydro-
Ionic liquids comprising of imidazolium cations and
acidic counteranions are also important due to their
capability of proton conduction by hopping mechanism.
*
Corresponding author. Fax: +91 183 2258820.
E-mail address: sekhon_apd@yahoo.com (S.S. Sekhon).
0009-2614/$ - see front matter ꢁ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.04.119

B.S. Lalia, S.S. Sekhon / Chemical Physics Letters 425 (2006) 294–300
295
Polymer electrolytes containing ionic liquids with dihydro-
2.2.3. Polymer electrolytes
genphosphate ðH2POꢀÞ and bisulfate ðHSOꢀÞ anions are
Polymer electrolytes in the membrane form were pre-
pared by dissolving stoichiometric quantities of polymer,
ionic liquid and acid (H3PO4) in acetonitrile and the uni-
form solution was then poured in polypropylene dishes.
The solvent was allowed to evaporate slowly and polymer
electrolytes in the free standing film form were obtained
4
4
receiving much attention due to their suitability as poten-
tial candidates for high temperature membranes for use
in polymer electrolyte membrane fuel cells (PEMFCs)
[31–33]. However, the effect of varying the composition
of these ionic liquids on the conductivity and other physi-
cochemical properties has not been fully explored, and
the present work is a step in that direction.
2.3. Measurements
In the present study, ionic liquids comprising of 2,3-
dimethyl-1-alkylimidazolium (DMRIm+) cation and dihy-
2.3.1. NMR
drogenphosphate ðH2POꢀÞ anion have been synthesized.
1H and 13C NMR spectra of ionic liquids were
recorded at room temperature with JEOL AL-300 MHz
NMR spectrometer at 300 MHz and 75 MHz, respec-
tively, in CDCl3 with trimethylsilane (TMS) as internal
standard. Chemical shifts were expressed as d (ppm)
downfield from TMS and the abbreviations used are s –
singlet, d – doublet, t – triplet, m – multiplet, J – coupling
constant. The mass spectra were recorded on LC-Agilent
100 series mass spectrometer.
4
The effect of alkyl (R) chain length on the conductivity
and viscosity behavior of ionic liquids as well as on the
thermal stability of membranes containing ionic liquids
has been studied. Ionic liquid with highest conductivity
has been incorporated in PVdF-HFP to obtain polymer
electrolytes in the membrane form. The dependence of con-
ductivity of membranes upon the concentration of ionic
liquid, phosphoric acid and temperature has also been
studied.
2.4. Conductivity
2. Experimental
The conductivity of ionic liquids has been measured by
a.c. impedance spectroscopy with a computer interfaced
Hioki 3532-50 LCR Hi-Tester using a cell with platinum
electrodes. The conductivity of membranes was measured
by using a cell with pressure contact stainless steel elec-
trodes and the variation of conductivity with temperature
was studied by placing the cell in a temperature controlled
furnace.
2.1. Materials
Polyvinylidenefluoride-co-hexafluoropropylene (PVdF-
HFP, Mw = 130,000, Aldrich), phosphoric acid (H3PO4,
Aldrich), potassium dihydrogenphosphate (KH2PO4,
Merck) 1,2-dimethyimidazole (Merck), 1-bromoethane
(C2H5Br, Merck), 1-bromobutane (C4H9Br, Merck), 1-
bromooctane (C8H17Br, Merck), ethyl acetate (SRL) and
acetonitrile (Qualigens) were used as the starting
materials.
2.5. Viscosity
Viscosity of the ionic liquids has been measured by Fun-
gilab rotating viscometer (Visco Basic L) using a small
sample adapter assembly and temperature was controlled
within 0.1 ꢀC with a Julabo water circulator (F-12 EC).
2.2. Synthesis
2.2.1. DMRImBr
2,3-Dimethyl-1-alkylimidazolium bromide (DMRImBr,
where R = ethyl, butyl, octyl) was prepared by the reac-
tion of 1,2-dimethylimidazole with 1-bromoalkane and
then 2,3-dimethyl-1-alkylimidazolium bromide was washed
2–3 times with dry ethylacetate to remove any starting
material left in the reaction. Ethylacetate was then removed
by evaporation under reduced pressure.
3. Results and discussion
The formation of ionic liquids: 2,3-dimethyl-1-alkylimi-
dazolium dihydrogenphosphate (DMRImH2PO4, where
1
R = Et, Bu, and O) was checked by H, 13C NMR and
mass spectra and the results are given below:
2.2.2. DMRImH2PO4
3.1. DMEtImH2PO4
The anion exchange reaction of 2,3-dimethyl-1-alkylim-
idazolium bromide with potassium dihydrogen phosphate
(KH2PO4) was carried out in acetonitrile and the solution
was stirred for 48 hours at room temperature; precipitates
of potassium bromide settled down and were filtered from
the solution. A quantitative yield of 2,3-dimethyl-1-alky-
limidazolium dihydrogenphosphate (DMRImH2PO4) was
obtained after evaporating acetonitrile under reduced pres-
sure. The absence of bromides in the ionic liquids was
checked by bromide ion test.
1H NMR (CDCl3, 300 MHz); d (ppm): 1.48–1.53 (t, 3 H,
–CH3); 3.37 (s, 3 H, –CH3); 3.99 (s, 3H, –NCH3); 4.29–4.34
(t, 2H, –CH2); 7.61–7.62 (d, 1H, J = 2.1 Hz, ring proton);
7.67–7.68 (d, 1H, J = 2.1 Hz, ring proton).
13C NMR (CDCl3, 75 MHz): d (in ppm) = 10.78 (CH3),
15.18 (CH3); 44.0 (–NCH3); 50.24 (–CH2); 120.5, 122.9,
143.5
Mass spectra (ESI) m/z = 125.2 (DMEtIm+), 97.3
ðH2POꢀÞ.
4

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B.S. Lalia, S.S. Sekhon / Chemical Physics Letters 425 (2006) 294–300
3.2. DMBuImH2PO4
4
3
2
1
0
-1
1H NMR (CDCl3, 300 MHz); d (ppm): 0.94–0.99 (t, 3H,
–CH3); 1.36–1.44 (m, 2H, –CH2); 1.76–1.86 (m, 2H, –CH2);
2.82 (s, 3H, –CH3); 4.02 (s, 3H, –NCH3); 4.20–4.25 (t, 2H,
–CH2); 7.50-7.51 (d, 1H, J = 2.1 Hz, ring proton); 7.72–
7.73 (d, 1H, J = 2.1 Hz, ring proton).
-2
-3
-4
13C NMR (CDCl3, 75 MHz): d (in ppm) = 10.83 (–CH3),
13.40 (–CH3); 19.45 (–CH2); 31.66 (–CH2); 36.08 (–CH2);
48.64 (–NCH3); 121.06, 122.94, 143.55
Mass spectra (ESI) m/z = 153.1 (DMBuIm+), 97.3
ðH2POꢀÞ.
4
3.3. DMOImH2PO4
1H NMR (CDCl3, 300 MHz); d (ppm): 0.85–0.90 (t, 3H,
–CH3); 1.26–1.32 (m, 8H, –CH2–CH2–CH2–CH2); 1.80–
1.89 (m, 4H, –CH2–CH2); 2.81 (s, 3H, –CH3); 4.16 (s,
3H, –NCH3); 4.16–4.21 (t, 2H, –CH2); 7.41–7.42 (d, 2H,
J = 2.1 Hz, ring proton); 7.69–7.70 (d, 1H, J = 2.1 Hz, ring
proton).
2.5
2.95
3.4
1000/T (K-1)
Fig. 1. Variation of log conductivity (solid symbols) and log viscosity
(hollow symbols) with reciprocal temperature for the ionic liquids
DMRImH2PO4 with R = ethyl (d, s), butyl (j, h) and octyl (m, Á).
13C NMR (CDCl3, 75 MHz): d (in ppm) = 10.99 (–CH3),
14.00 (–CH3); 22.51 (–CH2); 26.33 (–CH2); 28.93 (–CH2);
28.96 (–CH2); 29.78 (–CH2); 31.61 (–CH2); 31.20 (–CH2);
49.1 (–NCH3); 120.90, 123.03, 143.74
DMRImH2PO4 (with R = ethyl, butyl and octyl) was
measured at different temperatures and the variation of
conductivity with temperature is given in Fig. 1. The con-
ductivity of ionic liquids has been observed to depend upon
the chainlength of alkyl (R) group in the imidazolium cat-
ion and shows the trend, ethyl > butyl > octyl at all tem-
peratures. The curved nature of the conductivity vs
temperature plot for all the ionic liquids suggests the highly
viscous nature of electrolytes and the conductivity varia-
tion has been explained by VTF behavior. The VTF equa-
tion for conductivity is
Mass spectra (ESI) m/z = 209.1 (DMOIm+), 97.3
ðH2POꢀÞ.
4
On the basis of above NMR results, the following struc-
ture of ionic liquids is proposed:
CH3
N
R
H3C
N
+
(where R- Ethyl (Et), Butyl (Bu), Octyl (O))
r ¼ ro exp½ꢀB=ðT ꢀ ToÞꢁ;
-
where ro (S/cm), B (K) and To (K) are constants and
adjustable parameters. The best-fit parameters were calcu-
lated and are listed in Table 2. Ionic liquid containing ethyl
group (DMEtImH2PO4) shows maximum conductivity,
which reaches a value of 7.0 · 10ꢀ2 S/cm at 120 ꢀC. As vis-
cosity of ionic liquids also depends upon the chain length
of the alkyl (R) group, so viscosity of ionic liquids was
measured at different temperatures and the variation of vis-
cosity with temperature is also given in Fig. 1. The depen-
dence of viscosity upon the alkyl chain length shows the
variation, which is opposite of that observed for conductiv-
ity and varies as octyl > butyl > ethyl at all temperatures in
the 20–120 ꢀC range. The viscosity results have been
H2PO4
DMRImH2PO4
The contaminant bromides in the ionic liquids were
checked by bromide ion test and negative test indicates
their absence. Some important physicochemical properties
of different ionic liquids are given in Table 1. The conduc-
tivity of ionic liquids decreases with an increase in the alkyl
(R) chainlength whereas viscosity shows an increase. The
conductivity of ionic liquids having composition
Table 1
Some important physicochemical properties of the ionic liquids:
DMRImH2PO4, with R = ethyl, butyl and octyl
Table 2
VTF equation parameters of ionic conductivity data for ionic liquids
DMRImH2PO4
Ionic liquid
r (mS/cm)
at 50 ꢀC
g (mPa s)
at 60 ꢀC
Density
(gm/cm3)
Ionic liquid
ro (S/cm)
B (K)
To (K)
100R2
DMEtImH2PO4
DMBuImH2PO4
DMOImH2PO4
14.5
1.9
0.2
10.4
55.2
902.9
1.26
1.17
1.08
DMEtImH2PO4
DMBuImH2PO4
DMOImH2PO4
1.28
0.18
0.17
281.9
130.6
86.22
184.9
189.9
278.8
98.7
99.7
99.9

B.S. Lalia, S.S. Sekhon / Chemical Physics Letters 425 (2006) 294–300
297
explained by the VTF behavior and the VTF equation for
viscosity is
2.1
2.0
1.9
1.8
g ¼ go exp½B=ðT ꢀ ToÞꢁ
where go (mPa s), B (K) and To (K) are adjustable param-
eters. The best-fit parameters for different ionic liquids are
listed in Table 3. As viscosity (g) and mobility (l) (and
hence conductivity (r = nql)) are inversely related to each
other (l = q/6prg, where q is the charge and r is radius of
the ion), so the higher value of viscosity leads to lower con-
ductivity and vice versa. The different behavior observed
for conductivity and viscosity results is possibly due to
the difference in bulk or macroviscosity – which is mea-
sured experimentally and local or microviscosity – which
is related to mobility and conductivity. The relation
r = k/g is not obeyed over the different alkyl (R) chains
in the salt due to an increase in the non-polar part on the
imidazole. The ionic liquid with octyl group (DMOImH2-
PO4) has maximum viscosity, whereas the ionic liquid with
ethyl group (DMEtImH2PO4) possesses maximum conduc-
tivity, as given in Fig. 1. The length of alkyl group in the
imidazolium cation thus affects the conductivity and
viscosity of the resulting ionic liquid. However, the exact
contribution of alkyl group on the modification of conduc-
tivity of ionic liquids can not be estimated, as some other
parameters such as ion size, degree of dissociation etc also
affect the conductivity of ionic liquid. The ionic liquid with
ethyl group, DMEtImH2PO4, shows maximum value of
ionic conductivity and was chosen for the preparation of
polymer electrolyte membranes.
Polymer electrolytes in the membrane form were pre-
pared by incorporating the ionic liquid with ethyl group
(DMEtImH2PO4) in PVdF-HFP and the variation of con-
ductivity (at 30 ꢀC) as a function of the concentration of
ionic liquid is given in Fig. 2. The addition of inert polymer
to the ionic liquid results in a decrease in conductivity by
one order of magnitude, as the polymer does not contribute
to the conductivity of electrolyte [30]. The conductivity of
membranes has been found to increase by a small factor
with an increase in the concentration of ionic liquid and
membranes with different weight ratios of the polymer
and ionic liquid show conductivity of the order of
10ꢀ4 S/cm at 30 ꢀC. However, the presence of high concen-
tration of ionic liquid can lead to deterioration in the
mechanical properties of membranes. Membranes having
composition PVdF-HFP + DMEtImH2PO4 in equal
weight ratio and possessing conductivity of the same order
as other membranes along with reasonably good mechani-
cal properties were used for further studies. Phosphoric
0.8
1.3
1.8
2.3
2.8
weight ratio of DMEtImH2PO4
Fig. 2. Dependence of conductivity upon the concentration of ionic liquid
for PVdF-HFP + DMEtImH2PO4 polymer electrolytes.
acid (H3PO4), which has the same anion as the ionic liquids
(DMRImH2PO4) used in the present work, was added to
polymer electrolytes containing PVdF-HFP and DME-
tImH2PO4 in equal weight ratio and the variation of con-
ductivity (at 30 ꢀC) as a function of H3PO4 concentration
is given in Fig. 3. The addition of phosphoric acid results
in an increase in conductivity of the membranes and mem-
branes containing 1 M H3PO4 show conductivity of
2.32 · 10ꢀ4 S/cm at 30 ꢀC. Phosphoric acid upon dissocia-
tion provides free H+ ions, which contribute to the conduc-
tivity of these electrolytes. An improvement in the physical
stability of the membranes containing ionic liquid was also
observed with the addition of phosphoric acid. However,
the concentration of phosphoric acid, ionic liquid and
2.6
2.2
1.8
Table 3
VTF equation parameters of viscosity data for ionic liquids
DMRImH2PO4
0.0
0.5
1.0
1.5
2.0
R2
conc. of H3PO4 (M)
Ionic liquid
go (mPa s)
B (K)
To (K)
DMEtImH2PO4
DMBuImH2PO4
DMOImH2PO4
1.01
0.85
3.51
106.2
194.0
84.5
227.1
229.1
283.5
99.6
99.4
99.7
Fig. 3. Dependence of conductivity upon the concentration of phosphoric
acid for polymer electrolytes having composition PVdF-HFP:DME-
tImH2PO4 (1:1) + x (M) H3PO4.

298
B.S. Lalia, S.S. Sekhon / Chemical Physics Letters 425 (2006) 294–300
polymer have to be optimized so that membranes with best
combination of ionic conductivity and mechanical strength
are obtained.
important for their future use as proton conducting mem-
branes in polymer electrolyte membrane fuel cells. How-
ever, more experimental studies are required to establish
the nature of mobile species in these membranes containing
ionic liquids.
The conductivity of different membranes having compo-
sition PVdF-HFP:DMEtImH2PO4 (1:1) + x(M) H3PO4
with x = 0 and 1 was measured at different temperatures
and the variation of conductivity with temperature is given
in Fig. 4. The conductivity variation for the neat ionic
liquid (DMEtImH2PO4) has also been included in Fig. 4
for comparison. The curved nature of plots has been
explained by the VTF behavior and the best-fit parameters
are given in Table 4. The conductivity of membranes con-
taining H3PO4 is higher than for the corresponding mem-
branes without H3PO4, but is less than for the neat ionic
liquid by nearly one order of magnitude. Membranes con-
taining the ionic liquid and with conductivity higher than
10ꢀ3 S/cm above 50 ꢀC can find potential use in various
electrochemical devices. However, for their use in PEM-
FCs, the conductivity has to be improved by more than
one order of magnitude and secondly protons should be
the mobile species. Although cations and anions are the
main mobile species in ionic liquids yet the presence of
The effect of alkyl chain length on the thermal stability
of membranes containing ionic liquids with acidic count-
eranion was also studied. The simultaneous DSC/TGA/
DTG thermograms of polymer electrolyte membranes
containing DMRImH2PO4 (R = ethyl, butyl, octyl) and
PVdF-HFP in equal weight ratio, recorded in the 20-
600 ꢀC temperature range under nitrogen atmosphere
are given in Fig. 5. The DSC thermogram of membrane
containing ionic liquid with ethyl group shows an endo-
thermic peak at 62 ꢀC, which corresponds to the transi-
tion of polymer (PVdF-HFP) to the polar form [34] and
this peak shifts to 61 and 57 ꢀC for membranes containing
ionic liquid with butyl and octyl group, respectively. The
TGA thermograms for all the membranes show a weight
loss by 3% only up to 225 ꢀC, which indicates that these
membranes are thermally stable up to this temperature.
The onset of weight loss takes place between 225 and
250 ꢀC for all the electrolytes. In the case of electrolytes
containing ionic liquid with ethyl group two endothermic
peaks at 253 and 300 ꢀC in the DSC thermogram have
accompanied this weight loss. This is also reflected in
the different values of the rate of weight loss observed
at 255 ꢀC (0.65 mg/min) and 299 ꢀC (0.53 mg/min) in the
DTG plot for this electrolyte. This weight loss is due to
the decomposition/evaporation of electrolyte and the rate
is slow and takes place over a wide (225–400 ꢀC) temper-
ature range. However, for electrolytes containing ionic
liquid with butyl and octyl group, the DSC thermogram
shows only a single peak at 290 and 283 ꢀC, respectively,
and this is accompanied by a single rate of weight loss
observed at 265 (1.05 mg/min) and 270 ꢀC (1.46 mg/
min), respectively. The rate of weight loss has been found
to increase with an increase in the chain length of alkyl
group (R) and it takes place over relatively narrow tem-
perature ranges of 225–325 ꢀC and 225–300 ꢀC, respec-
tively, for electrolytes containing ionic liquids with butyl
and octyl groups. The char weight remaining at 600 ꢀC
also decreases with an increase in the chain length, from
above 30% to 25% and 22% for membranes containing
ionic liquids with ethyl, butyl and octyl group, respec-
tively. The rate of weight loss has been observed to
depend upon the length of alkyl group (R) and increases
with an increase in the length of alkyl chain group in the
imidazolium cation of ionic liquid. The above results
show that these polymer electrolytes containing ionic liq-
uids with different alkyl groups are thermally stable up to
225 ꢀC and can be safely used in different electrochemical
devices, including proton exchange membrane fuel cells,
up to 175–200 ꢀC. The thermal stability of membranes
containing phosphoric acid may be slightly lower as phos-
phoric acid is unstable above 200 ꢀC, and further studies
in this direction are in progress.
acidic counteranion ðH2POꢀÞ and phosphoric acid
4
(H3PO4) suggests that protons also contribute to the con-
ductivity of these polymer electrolyte membranes. It is
-1
-2
a
-3
c
b
-4
2.50
2.95
3.40
1000/T(K-1)
Fig. 4. Variation of log conductivity with reciprocal temperature for
electrolytes having composition: (a) DMEtImH2PO4, (b) PVdF-
HFP:DMEtImH2PO4 (1:1) and (c) PVdF-HFP:DMEtImH2PO4
(1:1) + 1 M H3PO4.
Table 4
VTF equation parameters of conductivity data for polymer electrolytes
Polymer electrolyte
ro (S/cm)
B (K)
To (K)
R2
PVdF-HFP:DMEtImH2PO4
(1:1)
0.215
152.1
233.4
99.9
PVdF-HFP:DMEtImH2PO4
(1:1) + 1 M H3PO4
0.214
131.3
240.6
99.8

B.S. Lalia, S.S. Sekhon / Chemical Physics Letters 425 (2006) 294–300
299
255 Cel
0.65 mg/min
299 Cel
0.53 mg/min
(a)
DTG
62 Cel
253 Cel
300 Cel
22 Cel
100.0%
200 Cel
97. 2%
100 Ce l
98.2%
224 Cel
97.0%
DSC
TGA
67 Cel
98.9%
151 Cel
98.0%
275 Cel
73.3%
250 Cel
90.6%
325 Cel
47.3%
401 Cel
37.2%
300 Cel
58.2%
475 Cel
33.4%
524 Cel
32.1%
350 Cel
43.7%
265 Cel
1.05 mg/min
(b)
DTG
DSC
TGA
61 Cel
290 Cel
250 Cel
20 Cel
100 Cel
150 Cel
97.2%
225 Cel
97.0%
100.0%
97.7%
91.6%
66 Cel
98.3%
267 Cel
74.2%
284 Cel
61.8%
300 Cel
45.3%
350 Cel
34.8%
425 Cel
28.4%
474 Cel
26.1%
525 Cel
25.2%
325 Cel
37.4%
270 Cel
1.46 mg/min
(c)
DTG
57 Cel
283Cel
19 Cel
216Cel
96.7%
100 Cel
98.2%
100.0%
150 Cel
97.9%
250 Cel
92.2%
65 Cel
99.1%
DSC
TGA
266 Cel
75.2%
278 Cel
61.5%
325 Cel
400 Cel
28.2%
31.8%
475 Cel
24.3%
525 Cel
300 Cel
38.9%
23.3%
50
100
150
200
250
300
Te mp C)
350
400
450
500
550
Fig. 5. DSC/TGA/DTG plots for polymer electrolytes having composition PVdF-HFP:DMRImH2PO4 (1:1) with R = Et (a), Bu (b) and O (c).

300
B.S. Lalia, S.S. Sekhon / Chemical Physics Letters 425 (2006) 294–300
[11] R. Kawano, H. Matsui, C. Matsuyama, A. Sato, M.A.B.H. Susan, N.
4. Conclusions
Tanabe, M. Watanabe, J. Photochem, Photobiol. A: Chem. 164
(2004) 87.
The conductivity and viscosity of ionic liquids contain-
ing acidic counteranion: 2,3-dimethyl-1-alkylimidazolium
dihydrogenphosphate (DMRImH2PO4) have been found
to depend upon the chain length of alkyl (R) group and
ionic liquid with ethyl group (DMEtImH2PO4) show max-
imum conductivity of 0.07 S/cm at 120 ꢀC. The conductiv-
ity of polymer electrolytes containing DMEtImH2PO4in a
polymer (PVdF-HFP) matrix has been found to depend
upon the concentration of ionic liquid, phosphoric acid
and temperature. Polymer electrolytes containing different
ionic liquids are thermally stable up to 225 ꢀC and are suit-
able as high temperature membranes in proton exchange
membrane fuel cells under non-humid conditions.
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