Physical and electrochemical properties of room-temperature dicyanamide ionic liquids based on quaternary phosphonium cations

Article abstract of DOI:10.1016/j.electacta.2010.08.106

The physicochemical and electrochemical properties of room temperature ionic liquids based on quaternary phosphonium cations together with a dicyanamide anion are presented in this report. The most dicyanamide-based phosphonium ionic liquids prepared were hydrophilic, except ionic liquids containing a long alkyl chain in the phosohonium cation. It was found that asymmetric phosphonium cations gave low-melting salts in combination with a dicyanamide anion. The dicyanamide-based phosphonium ionic liquids exhibited relatively low viscosities and high conductivities when compared to those of the corresponding ammonium ionic liquids. Particularly, the ionic liquids containing a methoxy group in the phosphonium cations indicated very low viscosities. Comparatively good electrochemical stability of the dicyanamide-based phosphonium ionic liquids was confirmed by voltammetric measurements. The thermogravimetric analysis suggested that the dicyanamide-based phosphonium ionic liquids showed higher thermal stability than those of the corresponding ammonium ionic liquids, indicating an improving effect of the phosphonium cations on the thermal stability.

Full text of DOI:10.1016/j.electacta.2010.08.106

Electrochimica Acta 56 (2010) 762–766
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Physical and electrochemical properties of room-temperature dicyanamide ionic
liquids based on quaternary phosphonium cations
Katsuhiko Tsunashimaa,∗, Shun Kodamab, Masashi Sugiyab, Yoshihito Kunugic
a Department of Materials Science, Wakayama National College of Technology, 77 Noshima, Nada-cho, Gobo, Wakayama 644-0023, Japan
b Nippon Chemical Industrial Co., Ltd., 9-11-1 Kameido, Koto-ku, Tokyo 136-8515, Japan
c Department of Applied Chemistry, Faculty of Engineering, Tokai University, 1117 Kitakaname, Hiratsuka 259-1292, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 June 2010
Received in revised form 24 August 2010
Accepted 25 August 2010
Available online 17 September 2010
The physicochemical and electrochemical properties of room temperature ionic liquids based on qua-
ternary phosphonium cations together with a dicyanamide anion are presented in this report. The most
dicyanamide-based phosphonium ionic liquids prepared were hydrophilic, except ionic liquids contain-
ing a long alkyl chain in the phosohonium cation. It was found that asymmetric phosphonium cations
gave low-melting salts in combination with a dicyanamide anion. The dicyanamide-based phosphonium
ionic liquids exhibited relatively low viscosities and high conductivities when compared to those of the
corresponding ammonium ionic liquids. Particularly, the ionic liquids containing a methoxy group in the
phosphonium cations indicated very low viscosities. Comparatively good electrochemical stability of the
dicyanamide-based phosphonium ionic liquids was confirmed by voltammetric measurements. The ther-
mogravimetric analysis suggested that the dicyanamide-based phosphonium ionic liquids showed higher
thermal stability than those of the corresponding ammonium ionic liquids, indicating an improving effect
of the phosphonium cations on the thermal stability.
Keywords:
Room temperature ionic liquids
Molten salts
Quaternary phosphonium compounds
Dicyanamide anion
Thermal stability
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
One of the most promising electrochemical applications of DCA-
based RTILs is thought to be electrolytic media for dye-sensitized
Room temperature ionic liquids (RTILs) have been developed for
a wide variety of electrochemical applications due to their unique
physicochemical properties such as favorable solubility of organic
and inorganic compounds, relatively high ionic conductivity, very
low volatility, high thermal stability, low flammability, etc. [1,2].
Another remarkable advantage of RTILs is a designability of cations
and anions. Particularly, the selection of anions offers a signifi-
cant change in the physicochemical properties of RTILs. Although
many anions are employed for RTILs, interests in RTILs based on
a dicyanamide (DCA) anion (N(CN)2−) have been increasing for
their unique physicochemical properties. In the pioneering study
published by MacFarlane et al., it is suggested that the DCA anion
gives hydrophilic and low-viscous RTILs in combination with imi-
dazolium, quaternary ammonium and pyrrolidinium cations [3,4].
Yoshida et al. have physicochemically characterized a variety of
DCA-based RTILs consisted of imidazolium and the related cations
[5,6]. The very low-viscous RTILs based on the DCA anion and tri-
alkylsulfonium cations have also been reported by Wasserscheid
and coworkers [7].
solar cells [8,9]. However, it is also known that thermal sta-
bility of the DCA-based RTILs tends to be low [10,11], so that
this feature may restrict the application to dye-sensitized solar
cells. On the other hand, RTILs based on quaternary phospho-
nium cations have been receiving a great deal of attention as
potential substitutes of the corresponding ammonium counter-
parts due to the fact that phosphonium RTILs offer chemical
and thermal stabilities as practical advantages [12,13]. We have
found that bis(trifluoromethylsulfonyl)amide (TFSA) anion based
phosphonium RTILs derived from triethylphosphine (TEP) and trib-
utylphosphine (TBP) significantly exhibited relatively high thermal
stability when compared to that of the corresponding ammo-
nium RTILs [14,15]. Therefore, the thermal property of DCA-based
RTILs containing the quaternary phosphonium cations is of partic-
ular interest from the viewpoint of improvement of the thermal
stability. However, to the best of our knowledge, DCA-based
phosphonium RTILs have been rarely investigated except tri-
hexyl(tetradecyl)phosphonium dicyanamide [12].
Here we report the physicochemical and thermal character-
izations of RTILs based on quaternary phosphonium cations in
combination with the DCA anion. As shown in Fig. 1, the cations
investigated are derived from TEP and TBP. This paper describes
the improving effect of the phosphonium cations in the DCA-based
∗
Corresponding author. Tel.: +81 738 29 8413; fax: +81 738 29 8439.
E-mail address: tsunashima@wakayama-nct.ac.jp (K. Tsunashima).
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.08.106

K. Tsunashima et al. / Electrochimica Acta 56 (2010) 762–766
763
pipette at 25 ◦C. The measurements of viscosity (CBC Materials Co.
Ltd., VM-10A Laboratory Viscometer calibrated with the Brook-
field Viscosity Standard) and conductivity (Ivumu Technologies,
CompactStat, ac impedance mode) were carried out under argon
atmosphere at 25 ◦C. The thermogravimetric trace of each RTIL
was recorded by using a thermogravimetric analyzer (Seiko Instru-
ments Inc., TG/DTA6300) at a heating rate of 10 ◦C min−1 under
nitrogen atmosphere.
C2H5
H5C2 P+ CmH(2m+1)
C2H5
C4H9
H9C4 P+ CnH(2n+1)
C4H9
+
+
P222m
P444n
(m = 4, 5, 8)
(n = 1, 8)
2.3. Voltammetric measurement
C2H5
H5C2 P+ CH2OCH3
C2H5
C2H5
The voltammetric measurements were carried out using a
conventional three-electrode cell configuration under argon atmo-
sphere at 25 ◦C. The electrode potential was controlled by an
automatic polarization system (Hokuto Denko Co., Ltd., HZ-5000).
A Pt disk electrode (BAS Inc., surface area: 0.020 cm2) was used as
a working electrode. The Pt electrode was polished with alumina
paste (d = 0.05 ␮m), and then dried in vacuo before voltammetric
measurements. A Pt wire was mounted in the cell as an auxiliary
electrode. An Ag wire immersed in 0.1 mol dm−3 AgSO3CF3/EMI-
TFSA solution (EMI: 1-ethyl-3-methylimidazolium) with a Vycor®
glass separator was employed as an Ag/Ag+ reference electrode.
P+ CH2CH2OCH3
H5C2
C2H5
+
+
P222(1O1)
P222(2O1)
C N
-N
Anion:
(DCA)
C N
Fig. 1. Schematic illustration of DCA-based phosphonium RTILs investigated in this
work.
3. Results and discussion
RTILs on the physicochemical and thermal properties of the phos-
phonium RTILs.
3.1. Physicochemical properties
All of the DCA-based phosphonium RTILs shown in Fig. 1
were successfully prepared by the synthetic method in this
work. The miscibility of the phosphonium RTILs was similar to
those of the typical hydrophilic RTILs; most of the phosphonium
RTILs were miscible in water, methanol, acetone, acetonitrile and
dichloromethane, and were immiscible in toluene and n-hexane.
Exceptionally, P4448-DCA was miscible in toluene, which means
that the hydrophobicity is increased due to its relatively long alkyl
chains.
The physicochemical properties of DCA-based phosphonium
and the corresponding ammonium RTILs are summarized in
Table 1. The phosphonium RTILs tended to be relatively low-
melting when compared to the corresponding TFSA-based RTILs.
For example, the melting points (Tm) of P2225-DCA and P4441-DCA
(−12 and 6 ◦C, respectively) were lower than those of P2225-TFSA
and P4441-TFSA (17 and 16 ◦C, respectively) [14,15]. Even Tm of
P2224-DCA was 5 ◦C, despite the fact that the corresponding TFSA-
based RTIL (P2224-TFSA) is a solid at room temperature (Tm: 55 ◦C)
[14].
2. Experimental
2.1. Preparation
The preparation of DCA-based phosphonium RTILs was per-
formed according to the procedure described in the literature
[9]. The precursor phosphonium halides were synthesized by the
nucleophilic addition of TEP (Nippon Chemical Industrial Co., Ltd.,
20% toluene solution, trade name Hishicolin P-2) or TBP (Nippon
Chemical Industrial Co., Ltd., trade name Hishicolin P-4) to the
corresponding bromoalkanes (Tokyo Chemical Industry Co., Ltd.)
at 80 ◦C under nitrogen atmosphere. The phosphonium halides
obtained were white crystalline solids. The aqueous ion exchange
reaction of the phosphonium bromides with Na-DCA (Aldrich)
was carried out at ambient temperature to give quaternary phos-
phonium DCA salts. The resulting crude RTILs were extracted
by dichloromethane and the dichloromethane solutions contain-
ing the RTILs were passed through a silica gel column using
dichloromethane as an eluent to remove the ionic impurities. After
evaporation of the solvent, all the RTILs were dried under high
vacuum for at least 8 h at 100 ◦C and were stored in an argon atmo-
sphere glove box. The isolated yields based on the precursor halides
were in the range between 40 and 60%. The RTILs obtained were
pale yellow transparent liquids. The water content in the RTILs thus
prepared was less than 100 ppm. The products obtained were con-
firmed by 1H, 13C and 31P NMRs. The residual contents of sodium
cation (<10 ppm) were checked by an inductively coupled plasma
spectrometer (Varian, ICP-AES Liberty LR Sequential). The corre-
sponding ammonium RTILs were prepared by the same procedure
as the phosphonium RTILs.
As shown in Table 1, the viscosity of the phosphonium RTILs
was significantly decreased with decreasing the cation size. This
seems to be attributed to decrease in the Stokes radii of the phos-
phonium cations. The P2224 cation is the smallest in phosphonium
cations containing side chains without heteroatoms, so that P2224
-
DCA shows relatively low viscosity (60 mPa s at 25 ◦C). Further low
viscosities were observed in P222(1O1)-DCA and P222(2O1)-DCA (29
and 39 mPa s at 25 ◦C, respectively), which indicates that intro-
ducing a methoxy group into the phosphonium cations drastically
reduce their viscosities. Especially, P222(1O1)-DCA exhibited very
low viscosity when compared to not only the corresponding ammo-
nium IL having a methoxymethyl group, N222(1O1)-DCA (42 mPa s
at 25 ◦C), but also the corresponding TFSA-based phosphonium IL,
2.2. Measurements of physicochemical properties
P
222(1O1)-TFSA (35 mPa s at 25 ◦C [14]).
In the case of TFSA-based RTILs, it has been generally accepted
The melting point of each RTIL was analyzed by using a dif-
ferential scanning calorimeter (Seiko Instruments Inc., DSC6200)
with a cooling and heating rate of 1 ◦C min−1. The density of each
RTIL was determined by gravimetric analysis using a calibrated
that the effect is due to an electron donation from the methoxy
group into the cation center to decrease the positive charge of the
cation [16]. Hence, an electrostatic interaction between the cation
and the anion is weakened, which results in reducing both the vis-

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K. Tsunashima et al. / Electrochimica Acta 56 (2010) 762–766
Table 1
Physicochemical properties of DCA-based phosphonium and the corresponding ammonium RTILs.
Ionic liquid
FWa
Tmb/◦C
dc/g cm−3
ꢀd/mPas
ꢁe/mS cm−1
Tdecf/◦C
P2224-DCA
P2225-DCA
241.31
255.34
297.42
229.26
243.29
283.39
381.58
238.37
280.45
212.29
266.43
5
1.00
0.99
0.97
1.06
1.05
0.96
0.95
0.99
0.99
1.05
0.98
60
72
104
29
5.7
4.0
2.0
12.8
8.2
1.2
0.45
2.8
1.0
8.2
0.57
394
393
394
278
244
387
389
270
271
154
260
−12
P2228-DCA
<−50
−11
<−50
6
P222(1O1)-DCA
P222(2O1)-DCA
39
P
P
4441-DCA
4448-DCA
167
245
121
241
42
<−50
N2225-DCA
N2228-DCA
N222(1O1)-DCA
N4441-DCA
−7
<−50
<−50
12
410
a
b
c
d
e
f
Formula weight.
Melting point.
Density at 25 ◦C.
Viscosity at 25 ◦C.
Conductivity at 25 ◦C.
Thermal decomposition temperature (10% weight loss).
cosity and the melting point of the RTILs. It seems reasonable to
give similar explanations to the DCA-based phosphonium RTILs.
Furthermore, as shown in Table 1, it is obvious that DCA-based
phosphonium RTILs were less viscous and more conductive than
those of the corresponding ammonium RTILs. Similar results have
been reported in a previously published paper describing transport
property data of TFSA-based phosphonium RTILs [14].
In order to discuss the correlation between viscosity and con-
ductivity, the plot of the molar conductivity ꢂ against the inverse
viscosity ꢀ−1 (the so-called Walden plot [17,18]) for DCA-based
phosphonium and the corresponding ammonium RTILs is depicted
in Fig. 2. All the points were positioned below the ideal Walden line
as in the case of typical RTILs. In the case of relatively low-viscous
RTILs such as P222(1O1)-DCA, P222(2O1)-DCA and P2224-DCA, there
seems to be no essential difference in the deviation of the points
from the ideal Walden line. However, the points of the phospho-
nium RTILs having long alkyl chains (e.g., P2228-DCA, P4441-DCA and
P4448-DCA) tended to be slightly deviated from those of the corre-
sponding ammonium RTILs. According to the general interpretation
of the Walden plot, the deviation from the ideal line indicates a lack
of complete ion dissociation of the salt [19,20]. Therefore, the result
in Fig. 2 suggests slightly high degree of association between the
phosphonium cations and the DCA anion.
One of the differences between phosphonium and the corre-
sponding ammonium cations is a presence of empty d-orbitals in
the phosphorus atom. It has been reported and generally accepted
that quaternary phosphonium compounds are more covalent in
nature than corresponding ammonium compounds due to empty d-
orbitals in the phosphorus atom [21,22]. This allows us to infer that
the empty d-orbitals in the phosphonium cations might more or
less interact with the counteranions, leading to reduce the charges
of cations and anions, in other words, to weaken the electrostatic
force between cations and anions. Although the improving effect of
the transport property by phosphonium cations is not sufficiently
clarified at present, such an interaction seems likely to afford a
decrease in the viscosity.
3.2. Electrochemical stability
Fig. 3 illustrates the linear sweep voltammograms measured
in the neat DCA-based RTILs at a Pt electrode. The shapes of the
voltammograms thus observed closely resemble those of the imida-
zolium based DCA-ILs reported by Yoshida et al. [6]. Slight oxidation
currents were observed around +1.0 V vs. Ag/Ag+, which might be
due to the electrochemical oxidation of trace amount of impurities.
It is noted that RTILs based on methoxy-substituted phosphonium
10
5
0
: P222(1O1)-DCA
: P222(2O1)-DCA
-5
: P2225-DCA
: N222(1O1)-DCA
-10
-4
-3
-2
-1
0
1
2
3
4
E vs. (Ag/Ag+) / V
Fig. 2. Plots of the molar conductivity against the inverse viscosity for DCA-based
phosphonium and the corresponding ammonium RTILs. The dotted line indicates
the ideal Walden line (dilute aqueous KCl solution).
Fig. 3. Linear sweep voltammograms for neat DCA-based phosphonium and the
corresponding ammonium RTILs at a Pt electrode with a 5 mV s−1 potential sweep
rate.

K. Tsunashima et al. / Electrochimica Acta 56 (2010) 762–766
765
3.3. Thermal stability
100
80
60
40
20
0
(a)
Thermal stability is of practical importance for various elec-
trochemical applications. Fig. 4 shows the thermogravimetric
traces of several DCA-based phosphonium and the corresponding
ammonium RTILs. Additionally, the onset thermal decomposition
temperatures (Tdec) at 10% weight loss are listed in Table 1. It
was clearly found that the Tdec of the phosphonium RTILs were
considerably high when compared to those of the corresponding
ammonium RTILs. In each case, the drastic increase in the ther-
mal stability was observed (ca. 120 ◦C). It should also be noted that
the phosphonium RTILs having no methoxyalkyl group (P2224-DCA,
:
P2225-DCA
:N2225-DCA
P2225-DCA, P2228-DCA, P4441-DCA and P4448-DCA) were stable up
0
0
0
100
200
300
400
500
500
500
600
600
600
to nearly 400 ◦C, indicating the high thermal stability comparable
with those of the TFSA-based phosphonium RTILs [14,15]. These
results reveal that the phosphonium cations favorably improve the
thermal stability of DCA-based RTILs. At present, the detailed ther-
mal decomposition mechanism for the phosphonium RTILs remains
unclear; however, the presence of empty d-orbitals might play an
essential role in a significant increase in the bond strength, thereby
resulting in the high thermal stability.
Temperature / °C
100
80
60
40
20
0
(b)
4. Conclusions
:
P
222(1O1)-DCA
New DCA-based RTILs based on quaternary phosphonium
cations were physically and thermally characterized. The phos-
phonium RTILs showed relatively low viscosities and high
conductivities when compared to the corresponding ammonium
RTILs. It was found that voltammetric measurements indicated the
comparatively good electrochemical stability of the phosphonium
RTILs. The excellent thermal stability of the phosphonium RTILs was
also confirmed by the thermogravimetric analysis. These findings
allow us to suggest that low-viscous and thermally stable DCA-
based phosphonium RTILs can be good candidates for electrolytic
media used in dye-sensitized solar cells. Further electrochemical
characterization of DCA-based phosphonium RTILs will be con-
ducted.
:N22(1O1)-DCA
100
200
300
400
Temperature / °C
100
80
60
40
20
0
(c)
Acknowledgement
:
:
P4441-DCA
4441-DCA
This work was partially supported by the Grant-in-Aid for Sci-
entific Research (No. 22550131) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
N
100
200
300
400
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