Thermal behavior and electrochemistry of protic ionic liquids based on triethylamine with different acids

Article abstract of DOI:10.1039/c6ra21360j

Protic ionic liquids (PILs) composed of the triethylammonium cation with dihydrogen phosphite, tosylate, and trifluoroacetate anions were synthesized. All samples were salts with melting points below 100 °C and were characterized via NMR spectroscopy, attenuated total reflection (ATR) spectroscopy, differential scanning calorimetry (DSC), and thermogravimetry (TG). The electrochemical characteristics of each protic ionic liquid were obtained using a combination of impedance spectroscopy and cyclic voltammetry. Moreover, the influence of water on the thermal behavior and conductivity of triethylammonium tosylate is studied. A linear correlation between the temperatures of melting and crystallization is established for a number of PILs, including both derivatives of triethylamine with different acids reported in the literature and the newly synthesized PILs. The optimal combination of thermal characteristics and electroconductivity was observed for triethylammonium trifluoroacetate and tosylate.

Full text of DOI:10.1039/c6ra21360j

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Thermal behavior and electrochemistry of protic ionic liquids on
the base of triethylamine with different acids
L. E. Shmukler,* M. S. Gruzdev, N. O. Kudryakova, Yu. A. Fadeeva, A. M. Kolker, and L. P. Safonova
Received 00th January 20xx,
Accepted 00th January 20xx
Protic ionic liquids (PILs) composed by triethylammonium cation with dihydrogen phosphite, tosylate, and trifluoroacetat
anions were synthesized. All samples were salts with melting points below 100 oC. Salts have been characterized by NMR –
spectroscopy, attenuated total reflection (ATR) spectroscopy, differential scanning calorimetry (DSC), and
thermogravimetry (TG). The electrochemical characteristics of each protic ionic liquid have been obtained by complex of
impedance spectroscopy and cyclic voltammetry. Moreover water influence on thermal behavior and conductivity of
triethylammonium tosylate has been studied. Linear correlation between the temperatures of melting and crystallization
was established for a number of PILs covering both derivatives of triethylamine with different acids, performed in
literature, and PILs newly synthesized.. The optimal combination of thermal characteristics and electroconductivity was
observed for triethylammonium trifluoroacetat and tosylate.
DOI: 10.1039/x0xx00000x
www.rsc.org/
conventional electrolytes in some characteristics viz. they are
more safety and highly thermostable, as well as they have a
wide range of operating temperatures. There are a lot of
Introduction
The ionic liquids (ILs), which are organic salts, attract an
extensive attention due to their unique physicochemical
properties such as low vapor pressure, high thermal stability
and ionic conductivity, wide electrochemical window, good
solvation capacity, and some others. The room temperature
ionic liquids (Tm < 25 oC) and molten salts (Tm > 100 oC) are
distinguished depending on their melting points.
works reporting the synthesis of protic ionic liquids and ionic
melts with high thermal stability in combination with high ionic
conductivity under anhydrous conditions. For example, it was
shown3
trifluoromethanesulfonate ([dema]/[TfO] = 1/1) exhibited high
that
anhydrous
diethylmethylammonium
o
o
thermal stability (360 C), low melting point (− 6 C), and high
ionic conductivity (4.3∙10-2 S∙cm-1 at 120 oC). The authors of
work5 have succeeded in synthesis of the neutral salt,
There are two main families of ILs: aprotic ionic liquids (AILs)
and protic ones (PILs). The aprotic ionic liquids have no active
protons in their chemical structure. Typical AILs, such as
imidazolium salts, are generally synthesized by the
quaternization (alkylation) of amine, followed by an anion
exchange. PILs are prepared by the proton transfer reactions
from Brønsted acids to Brønsted bases and have exchangeable
(active) protons able to form H-bond. PILs have been less
studied in comparison with AILs. Within this work our
attention is focused on the former due to their remarkable
properties, which make them potentially useable in many
industrial and technical applications. Moreover the huge
variety of substances, which are potential components of PILs,
allows synthesizing PILs with properties required.
composed
by
bis(trifluoromethanesulfonyl)imide
and
benzimidazole, which was hydrophobic and electrochemical
stable. That salt decomposition temperature was higher than
350 °С and its proton conductivity was equal to 8.3∙10-3 S∙cm-1
at 140 °С. In7 two series of liquid PILs, composed by secondary
and/or tertiary ammonium cations and by trifluoroacetate,
methanesulfonate, trifluoromethanesulfonate, and tosylate
anions were synthesized. Those salts had high conductivity
o
values (1.4 – 4.9 mS∙cm-1 at 70 C) and were thermostable up
to 200 – 300 oC.
Here, we report the first results of our investigation of the
anion nature effect on thermal behavior and conductivity of
protic ionic liquids composed by triethylammonium cation and
anions of different acids.
PILs incorporation into confined geometry of polymers1 makes
the systems applicable as electrolytes in non-humidified fuel
cell systems.1-6 Polymer electrolytes based on ILs excel
Experimental Part
G. A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences,
153045, Akademicheskaya St., 1, Ivanovo, RUSSIA. E-mail: les@isc-ras.ru; Tel: +7
4932 336259; Fax: +7 4932 336237
Materials, ionic liquids preparation.
Electronic Supplementary Information (ESI) available: 1H and 13C NMR graphs of
triethylammonium trifluoroacetat, dihydrogen phosphite, and tosylate; ATR
Phosphonic acid (H2PHO3) (99%, Sigma-Aldrich), trifluoroacetic
acid (TFA) (99%) and p-toluenesulfonic acid monohydrate
spectra of both PILs
DOI: 10.1039/x0xx00000x
synthesized and initial compounds. See
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(PTSA) (99%,Tokyo Chemical Industries), triethylamine (TEA) [TEA]/[H2PHO3]. 1H NMR (CDCl3, TMS, δ, ppm): 1.22 t (9H, CH3-
(99%, ACROS) were used without additional purification. , J = 7.32 Hz); 3.02-2,99 q (6H, -CH2-, J = 7.32 Hz); 6.16 s (0.5Н,
All PILs were prepared by neutralization reaction between P-H); 7.41 s (0.5, P-H), 9.84 sbr (2H, HOacid); 11.76 sbr (1H, NH).
equimolar amounts of triethylamine (Brønsted base) and 13C NMR (CDCl3, TMS, δ, ppm): 7.61 (CH3-), 44.61 (-CH2-).
different Brønsted acids according to the following scheme:
(CH3-CH2)3N + AH ↔ (CH3-CH2)3NH+A–
Scheme 1. PILs synthesis
[TEA]/[PTSA]. 1H NMR (CDCl3, TMS, δ, ppm): 1.23 t (9H, CH3-, J
= 7.32 Hz); 2.28 s (3H, -CH3); 3.05-3.04 q (6H, -CH2-, J = 7.32
Hz); 7.1 d (2Н, Ph-H, J = 7.935 Hz); 7.69 d (2H, Ph-H, J = 7.935
The reaction was carried out in argon medium. A certain Hz), 10.02 s (1H, NH).
amount of TEA (0.07 mole approximately) was placed in 100 13C NMR (CDCl3, TMS, δ, ppm): 7.56 (CH3-); 20.3 (-CH3); 45.30 (-
ml round-bottom flask equipped with a teflon coated stir bar. CH2-); 124.87 (C-H); 127.76 (C-H); 138.87 (C-H); 141.68 (C-H).
Since the reaction was highly exothermic the flask was [TEA]/[TFA]. 1H NMR (CDCl3, TMS, δ, ppm): 1.31 t (9H, CH3-, J =
immerged in the ice bath and the dropwise addition of every 7.32 Hz); 3.13 m (6H, -CH2-); 12.07 s (1H, NH+ COO-).
acid to TEA was carried out. Then the mixture was being 13C NMR (CDCl3, TMS, δ, ppm): 9.74 (CH3-), 46.98 (-CH2-),
stirred for 8 h at 90 oC. The reacted mixtures were dried in 119.12-116.79 (-CF3-), 163.35 (C=O).
vacuum. Triethylammonium tosylate contained a lot of water
as p-toluenesulfonic acid monohydrate was used for its
ATR-spectroscopy.
synthesis. Therefore lyophilization of the salt was carried out
to reduce the water content.8
FT-IR spectra were recorded with a VERTEX 80v (Bruker)
spectrometer at room temperature within 4000-400 cm−1
region by averaging of 128 scans at resolution of 2 cm-1.
Thus the following resulting products were obtained: white
transparent solid, yellow liquid and white crystals in the case
of
triethylammonium
dihydrogen
phosphite,
triethylammonium trifluoroacetat, and triethylammonium
tosylate, respectively. Structural formulas of PILs synthesized
are shown below:
HO
H
P
O
O
NH+
Triethylammonium dihydrogen phosphite [TEA]/[H2PHO3]
O
O
S
CH3
NH+
O
Triethylammonium tosylate [TEA]/[PTSA]
F
F
F
O
O
NH+
Triethylammonium trifluoroacetat [TEA]/[TFA]
The water content in the ionic liquids synthesized was
measured using a Karl Fischer Titrator (V30, Mettler Toledo).
1
The PILs synthesized were characterized by H NMR, 13C NMR
and ATR spectroscopic techniques.
1H and 13C NMR Chemical Shift.
1
The structures of the ionic liquids were confirmed by H and
13C NMR spectroscopy. NMR spectra were registered relative
to Tetramethylsilane (TMS) with
spectrometer.
a Bruker AVANCE-500
2 | J. Name., 2012, 00, 1-3
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The conductivity of ionic liquids was determined by
electrochemical impedance method with impedance/gain-
phase analyzer Solartron 1260A over the frequency range of
0.1 Hz – 1 MHz with signal amplitude of 10 mV and with
accuracy higher than 0.2%. The resistance magnitude was
found from high-frequency cutoff corresponding to the bulk
resistance of the sample. All measurements were carried out
within the wide temperature range in the cell with platinum
electrodes; platinum wire was used to supply current. The
temperature was kept with thermostat HAAKE DC50-K35
within ± 0.01 K. The cell was calibrated with standards
recommended by IUPAC.12 The calibration was done at 298 K;
the cell constants at higher temperatures were calculated
according to ref.13.
Voltammetric studies.
The polarization measurements were carried out on platinum
electrode in a hermetic three-electrode electrochemical cell
Figure
1 ATR spectra of synthesized PILs along with the spectra of their source
substances within 2000 - 3750 cm-1: (a) triethylammonium dihydrogen phosphite, (b)
using
a pulse potentiostat PI 50-Pro-3 with automatic
triethylammonium tosylate, (c) triethylammonium trifluoroacetate. The spectrum of
pure TFA was taken from11
.
registration; the data were processed with PS Pack 2 software.
The measurements were performed within the temperature
range of 50 to 120 oC within accuracy of
The spectra of all PILs synthesized, pure TFA and TEA were
recorded with attenuated total reflection (ATR) technique
using MVP 2 SeriesTM (Harrick) devise with diamond crystal.
The spectra of solid samples (H2PHO3 and PTSA) were
registered under vacuum using KBr pellet technique. Every
sample was ground with spectral grade KBr to the state of
homogeneous powder. The mixtures were then pressed into
disc-pellets and then analyzed.
± 0.1 °С. The potential
scan rate was 0.01 V/s for all measurements. The
electrochemical cell was designed and made specially for
measurements of both hydrophilic and hydrophobic ionic
liquids.14 The counter and reference electrodes were made of
platinum.
FT-IR measurements were conducted to characterize the
products with an attention to the interactions between the
Brønsted acids and TEA. ATR spectra of PILs together with the
spectra of source compounds over the whole frequency range
are shown in Figures in ESI section. In general the spectrum of
every PIL can be considered as a superposition of the spectra
of pure components with some bands displacement being
observed as expected. However the appearance of some new
modes within the frequency range of 2500 - 2700 cm-1 (Figure
1) is an evidence of formation of the proton transferred salts
(or protonated amine (CH3-CH2)3NH+).9, 10
Results and Discussion
Among the most important properties of ILs are its melting
and decomposition temperatures because they are
parameters which determine the range of ILs liquid state
existence. Since many ILs tend to supercooling and glass
formation, the accurate determination of the melting
temperature is not a trivial task.15 The difference in the values
of melting points reported in various literature sources can be
caused by both high viscosities of the liquids and some
experimental factors, such as cooling and heating rates, as well
as by polymorphous crystalline states.16 Moreover it has been
Thermal Properties.
shown that dissolved CO2 decreased the melting point of ILs.17-
19
The thermal properties were measured with differential
scanning calorimetry (DSC) and thermogravimetry (TG). DSC
measurements were carried out with a NETZSCH DSC 204 F1
under N2 atmosphere within the temperature range from –100
Theoretically, the freezing temperature is equal to the melting
point. However for many ILs these two temperatures are not
the same. So the substantial supercooling is observed when
PILs are being cooled from relatively high temperature and the
supercooled melt devitrifies (crystallizes) at temperature of so-
called “cold crystallization” (Tс). Such behavior was typical for
all salts synthesized. As an example, DSC scans for [TEA]/[TFA]
are shown in Figure 2.
to 100 °С. The samples for DSC measurements were tightly
sealed by using Al pans in a glove box. The sample masses of
10-20 mg were taken for analysis. The samples were cooled
from room temperature to -100
min-1and then heated back to 100
min-1.
°
С at a cooling rate of 10
°
°
С
С
°С at a heating rate of 2
Thermochemical characteristics of PILs synthesized are shown
in Table 1. It should be noted that no glass transition was
observed for the PILs under the experiment conditions. Also
one can see that the melting point (Tm) of Triethylammonium
tosylate increases but conductivity decreases after the salt
lyophilization.
The TGA of PILs was performed using a NETZSCH TG 209 F1
analyzer in a flow of argon (20 ml∙min−1) at a heating rate of 10
°
С min−1.
Ionic Conductivity.
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Figure 2 DSC thermograms of [TEA]/[TFA] ionic liquid for second cooling and heating
steps.
Table 1 Key properties of PILs synthesized
PILs
Tc
ΔH
Tm
ΔH
(J∙g-1)
23.4
41.8
65.1
59.5
Tdec
к
water
(wt%)
(oC)
(J∙g-1)
-29.1
-33.4
-55.6
-51.7
(oC)
42.8
67.5
80.2
8.6
(oC)
(mS cm-1)
[TEA]/[H2PHO3]
[TEA]/[PTSA]
-2.7
12.0
18.5
-41.4
106.1
324.6
311.3
186.3
3.92 (100oC)
7.47 (105oC)
5.28 (105oC)
10.22 (105oC)
2
3.4
<1
<1
[TEA]/[PTSA] after lyophilization
[TEA]/[TFA]
Tc – crystallization temperature; Tm – melting temperature; Td - decomposition temperature; κ – ionic conductivity
In Figure 3 TG curves of PILs under investigation are presented The thermal properties of the synthesized PILs along with
in order to show their thermal stabilities; PILs decomposition literature data for triethylammonium salts are listed in Table 2.
temperatures, stated as a temperature corresponding to 3% Thermal properties of PILs synthesized on the base of
loss of the sample weight, are listed in Table 1.
phosphorous and trifluoroacetic acids agree well with the
literature data. The discrepancy observed can be explained,
first of all, by the experimental conditions. The parameters
presented in Table 2 for salts based on methanesulfonic and
trifluoromethanesulfonic acids show that the changes of
thermal characteristics do not correlate directly with different
water content in the salts.
Nevertheless some regularity in Tm variation with the nature of
acids is observed. So the melting temperatures of PILs
obtained on the base of carbon acids are, in general, lower as
compared with PILs based on sulfonic acids. The substitution
of one hydroxyl-group in sulfuric acid by methyl-group, i.e.
when passing from sulfuric to methanesulfonic acid, results in
the decrease of Tm. At the same time the significant increase
in Tm was observed when the aliphatic methyl-group in
methanesulfonic acid (Tm=17.4÷33 °С) was replaced by the
aromatic one (Tm=119 °С for BSu). According to ref27, such
phenomenon can be explained by the structure, rigidity,
and high packing ability of t h e aromatic ring. There is the
considerable increase in Tm when CH3COO– (-18 °С) is replaced
by CF3COO– (8.6 °С). The melting point values depend also on
the length of the perfluorinated alkyl anionic chain. So Tm of
Figure 3 TG scans of the PILs synthesized
The temperature corresponding to the beginning of weight
loss of PIL based on H2PHO3 were low due to condensation
reaction of phosphites.3
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the salts increases from 61 to 82 °С when passing from correlation is observed. The only correlation established was
perfluorobutanesulfonic acid to perfluoroctane sulfonic one. the correlation between the temperatures of crystallization
Most ionic liquids based on triethylammonium cation are and melting (Figure 4).
thermostable up to 200-350 oC. Triethylammonium formate The conductivity values of triethylamine-based ionic liquids
and acetate are the least thermostable salts. In ref.30 it was (Table 2) are within 10-4 ÷ 10-3 S∙cm-1 at 25 oC. The largest
also noted that the acetate based PILs were less stable as conductivity value was obtained for the salt containing formic
compared to protic ionic liquids based on other anions.
acid.
The onset of thermal decomposition for the ionic liquids based The ionic conductivity of PILs synthesized was evaluated from
on alkylimidazolium cation showed that the anion nature their impedance spectra.. As an example, the spectra of
determined this property substantially whereas the effect of [TEA]/[TFA] at different temperatures are show in Figure 5.
the cation was much weaker.31, 32
In some works7, 33-35 the correlation between Tm, Td, and ΔрК
values was established. For TEA-based salts (Table 2) no similar
The spectra of other PILs looked similarly.
Table 2 The thermal properties of the synthesized PILs along with literature data for triethylammonium salts.
Tg, oC
Tm, oC
Tc, oC
Td, oC
κ, mS∙cm-1
Ref.
H2O
Salts
(25 oС)
TEA Ac
TEA TFSA
0.42%
-93
-18
-0.8
2
48
20
21
22
600 ppm
50 ppm
0.42%
-29
5.64
-83
-42
190
TEA TFA
TEA TFSI
8.6
3.5
3
-41.4
186.3
350
2.45
this work
<1 ppm
10 ppm
100 ppm
50 ppm
2 mol%
32.3 (130oС)
4.4
4
23
24
22
16
3
-78.9
24.3
33
-31.6
-24
1.91
305
TEA Ms
25
-31
218
2.03
16.3 (120oС)
-62.1
-96.5
17.4
21.6
25
225
<1 ppm
0.3
50 ppm
50 ppm
269.7
25
26
27
22
TEA BF
TEA PFBSu
-62
-5
0.679
-58
-58
29
370
376
32
-20
36
TEA TfOH
2 mol%
<1 ppm
26
-13
322
4.79
2.76
16
34.3
358
3
312.5
135
25
-78.4
-70
3
this work
27
TEA H2PHO3
2%
42.8
59
60
61
62
-2.7
2
3
50
30
12.0
36
59.4
106.1
320
340
383
202
3.92 (100oC)
TEA FBSu
TEA TFBSu
TEA PFBu
TEA BA
50 ppm
50 ppm
50 ppm
50 ppm
3.4%
27
27
22
TEA PTSA
67.5
72
324.6
0.46
this work
24
-27.4
TEA Sacc
74.7
84.2
82
104.3
119
270
13.1 (120oС)
3
TEA H2SO4
<1 ppm
50 ppm
<1 ppm
50 ppm
0.84%
-100.1
-48
262.8
390
296.7
330
25
27
25
27
28
29
28
20
3
TEA PFOc
TEA BF4
TEA BSu
71
106
115
TEA N
TEA F
113-114
0.19%
0.51%
-119
13.05
0.23
54
-27.8
-34.4
-32.2
-59.7
-91
175
349.2
TEA H3PO4
<1 ppm
25
TEA H2PO3F
TEA(BuO)2POOH
<1 ppm
100 ppm
25
24
Ac: acetic acid; TFSA: bis(tetrafluoromethylsulfonyl)imide; TFA: trifluoroacetic acid; TFSI: bis(trifluoromethanesulfonyl)amide; MsOH: methanesulfonic acid; BF:
heptafluorobutyrate; PFBSu: perfluorobenzenesulfonic acid; TfOH: trifluoromethanesulfonic acid; H2PHO3: phosphonic acid; FBSu: fluorobenzenesulfonic acid; TFBSu:
trifluorobenzenesulfonic acid; PFBu: perfluorobutanesulfonic acid; BA: benzoic acid; PTSA: p - toluenesulfonic acid; Sacc: saccharin; H2SO4: sulfuric acid; PFOc:
perfluoroctane sulfonic acid; BF4 – tetrafluoroborate; BSu: benzenesulfonic acid; N: nitric acid; F: formic acid; H3PO4: phosphoric acid; H2PO3F: fluorophosphoric acid;
(BuO)2POOH: di-n-butyl-phosphate.
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Figure 6 Arrhenius plot of PILs specific conductivity.
Figure
4 The correlation between melting and crystallization temperatures of
From the Arrhenius plots (Figure 6), it can be seen that the
temperature dependences of the ionic conductivity are not
strictly linear. Therefore these dependences can be fitted with
triethylamine-based PILs.
the nonlinear Vogel–Tamman–Fulcher (VTF) equation 36, 37
:


− B
T −To
κ
= κo ⋅exp




Here кo is the conductivity at infinitely high temperature, B is a
coefficient related to the activation energy Ea, and To is the
ideal glass transition temperature. The calculated values of To
are by 50 degrees lower than the glass transition temperatures
determined experimentally.2
VFT parameters calculated from the temperature
dependences of the ionic conductivities of the [TEA]/[TFA] and
[TEA]/[PTSA] salts are summarized in Table 3.
Table 3 VFT parameters for the ionic conductivity of [TEA]/[TFA] and [TEA]/[PTSA] salts
(To = Tg - 50 K).
PILs
кo (S∙cm-1)
0.07±0.02
0.06±0.01
B (K)
To (K)
193±21
246±6
R2
[TEA]/[TFA]
[TEA]/[PTSA]
360±104
285±37
0.997
0.998
Figure 5 Impedance spectra for [TEA]/[TFA] at some selected temperatures.
In Figure
6 the temperature dependences of the ionic
conductivity of PILs synthesized are shown. The conductivity
values increase according to the following order:
[TEA]/[H2PHO3] < [TEA]/[PTSA] < [TEA]/[TFA]. At temperatures
below 70 oC the protic IL synthesized on the base of p-
toluenesulfonic acid was in the supercooled state due to its
high viscosity. Therefore the temperature dependence of its
specific conductivity is shown as a dotted line within the
temperature range from 25 to 70 oC.
Electrochemical window is an important parameter of
electrochemical stability of PILs. There are a lot of factors
which affect the ECW of ionic liquids. Among other things the
electrode material has a strong effect on the value of ECW. As
PILs synthesized are expected for application in
electrochemical cells in which Pt-electrodes are the most
widely used, therefore exactly Pt was used as the electrode
material for ECW measurements. In Figure 7 one can see the
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cyclic voltammograms obtained on platinum electrode at 50 °C
for three PILs synthesized.
Figure 7 Cyclic voltammograms of the synthesized PILs at 50 °C.
The value of the electrochemical window (ECW) of TEA-based
PILs depends considerably on the anion nature and
temperature. Anticipating things it should be mentioned that
the largest values of ECW were obtained at 50 °С for all PILs
measured. The other important observation is that
triethylammonium cation reduces at different potentials
subject to the counterion nature. So the cathodic limits for
[TEA]/[TFA], [TEA]/[PTSA] and [TEA]/[H2PHO3] are equal to -
0.75, -0.85, and -0.45 V, respectively, at 50 °С. Similar
dependence of cathodic potential on the nature of anion was
observed for ILs based on halides of metals38 and for
pyrrolidinium based PIL.39 It was connected with changes in
the medium acidity. In addition, the water impurities, detected
in all PILs synthesized, probably contribute to the value of the
40
cathodic response.38, The influence of temperature on the
electrochemical stability of TEA-based PILs is shown in Figure
8.
Figure 8 Cyclic voltammograms of the synthesized PILs at different temperatures: (a)
[TEA]/[PTSA]; (b) [TEA]/[TFA]; (c) [TEA]/[ H2PHO3].
For all PILs synthesized ECW narrowing with temperature
rising is typical. Such behavior was observed earlier for RTILs
also.41, 42 ECW decreases both in cathodic and anodic areas of
potential that is probably connected with the influence of
temperature on kinetics of redox processes.34 Here it should
be noted that the magnitude of ECW narrowing is not the
same in cathodic and anodic potential areas. So ECW for all
PILs synthesized decreases less than by 300 mV in cathodic
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Journal Name
4
5
Md. A. B. H. Susan, A. Noda, S. Mitsushima and M.
Watanabe, Chem. Commun., 2003, , 938–939.
H. Nakamoto, A. Noda, K. Hayamizu, S. Hayashi, Hiro-o
area, whereas in anodic area the decrease is about 100 mV for
[TEA]/[TFA] and 400 mV for two other salts. Thus it is obvious
that [TEA]/[H2PHO3] salt is the most sensitive to the increase in
temperature and the potential range of its electrochemical
stability decreases about by 700 mV when heating from 50 to
120 °C.
8
Hamaguchi and M. Watanabe, J. Phys. Chem. C, 2007, 111
1541-1548.
,
6
7
8
A. Fernicola, B. Scrosati and H. Ohno, Ionics, 2006, 12, 95-
102.
T. A. Siddique, S. Balamurugan, S. M. Said, N. A. Sairi and W.
M. D. W. Normazlan, RSC Adv., 2016, 6, 18266-18278.
M. Gruzdev, U. Chervonova, T. Frolova and A. Kolker, Liq.
Cryst., in press,
http://dx.doi.org/10.1080/02678292.2016.1202340.
J. Luo, J. Hu, W. Saak, R. Beckhaus, G. Wittstock, I. F. J.
Vankelecom, C. Agert and O. Conrad, J. Mater. Chem., 2011,
21, 10426-10436.
Conclusions
9
PILs on the base of triethylamine with phosphonic,
trifluoroacetic and p-toluenesulfonic monohydrate acids were
synthesized. All our samples are salts with melting points
below 100 oC. Triethylammonium dihydrogen phosphite,
tosylate and trifluoroacetat were compared with respect to
their thermal and electrochemical properties. Comparison and
analysis of the properties were carried out using the literature
data on some other PILs based on triethylamine. The
10 K. Mori, S. Hashimoto, T. Yuzuri and K. Sakakibara, Bull.
Chem. Soc. Jpn., 2010, 83, 328-334.
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13 J. Barthel, F. Feuerlein, R. Neueder and R. Wachter, J.
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Ramenskaya, Russ. J. Electrochem., 2012, 48, 1166–1170.
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16 J. L. Lebga-Nebane, S. E. Rock, J. Franclemont, D. Roy and S.
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17 J. M. Lopes, F. A. Sanchez, S. B. R. Reartes, M. D. Bermejo, A.
Martin and M. J. Cocero, J. Supercrit. Fluids, 2016, 107, 590–
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18 A. Serbanovic, Z. Petrovski, M. Manic, C. S. Marques, Gonc.
alo V.S.M. Carrera, L. C. Branco, C. A. M. Afonso and M.
Nunes da Ponte, Fluid Phase Equilibria, 2010, 294, 121–130.
crystallization of compounds under consideration was
established. The ionic conductivity of the salts synthesized was
found to be within the range from 10-2 to 10-3 S cm-1 and
increased in the following order: [TEA]/[H2PHO3]
<
[TEA]/[PTSA] < [TEA]/[TFA]. The Vogel-Tamman-Fulcher
equation was used to fit the temperature dependences of the
ionic conductivity of PILs synthesized. The maximum values of
electrochemical window were observed at 50 °С: 2.7, 1.7, and
1.4 V for [TEA]/[PTSA], [TEA]/[H2PHO3], and [TEA]/[TFA],
respectively. Electrochemical measurements of all PILs
19 A. M. Scurto and W. Leitner, Chem. Commun., 2006, 35
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,
20 Ch. Zhao, G. Burrell, A. A. J. Torriero, F. Separovic, N. F.
Dunlop, D. R. MacFarlane and A. M. Bond, J. Phys. Chem. B.,
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21 L. Timperman, P. Skowron, A. Boisset, H. Galiano, D.
Lemordant, E. Frackowiak, F. Beguin and M. Anouti, Phys.
Chem. Chem. Phys., 2012, 14, 8199–8207.
22 M. Martinez, Y. Molmeret, L. Cointeaux, C. Iojoiu, J.-C.
Lepretre, N. El Kissi, P. Judeinstein and J.-Y. Sanchez, J. Power
Sources, 2010, 195, 5829–5839.
23 H. Matsumoto, H. Sakaebe and K. Tatsumi, Journal of Power
Sources, 2005, 146, 45–50.
synthesized
demonstrate
the
decreasing
of
the
electrochemical window values with temperature rising. The
optimal combination of thermal characteristics and
conductivity values was observed for triethylammonium
trifluoroacetat and tosylate. Thus triethylammonium tosylate
and triethylammonium trifluoroacetat are expected to be
promising PIls as potential electrolytes for electrochemical
applications.
24 G. L. Burrell, I. M. Burgar, F. Separovic and N. F. Dunlop,
Phys. Chem. Chem. Phys., 2010, 12, 1571–1577.
Acknowledgements
25 J.-Ph. Belieres and C. A. Angell, J. Phys. Chem. B., 2007, 111
4926-4937.
,
This work was supported by the Russian Science Foundation
(№ 16-13-10371). All thermal (DSC and TG) and spectroscopic
(NMR, ATR, and impedance) measurements were carried out
at the center for joint use of scientific equipment (the Upper
Volga Regional Center for Physico–Chemical Research).
26 Y. Shen, D. F. Kennedy, T. L. Greaves, A. Weerawardena, R. J.
Mulder, N. Kirby, G. Song and C. J. Drummond, Phys. Chem.
Chem. Phys., 2012, 14, 7981–7992.
27 M. Martinez, C. Iojoiu, P. Judeinstein, L. Cointeaux, J.-C.
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28 T. L. Greaves, A. Weerawardena, I. Krodkiewska and C. J.
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29 T. L. Cottrell and J. E. Gill, J. Chem. Soc., 1951, 1798-1800.
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Khan, RSC Adv., 2015,
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Thermal behavior and electrochemistry of protic ionic liquids on the base of
triethylamine with different acids
L. E. Shmukler, M. S. Gruzdev, N. O. Kudryakova, Yu. A. Fadeeva,
A. M. Kolker and L. P. Safonova
CH3
CH3
H3C
H3C
+
N
acids
HN
CH3
CH3