Proton transport properties in an ionic liquid having a hydroxyl group

Article abstract of DOI:10.1039/c1cp21387c

An ionic liquid having a hydroxyl group, choline bis(trifluoromethylsulfonyl)amide ([N_111(2OH)][N(Tf)₂]), was synthesized to investigate the effect of hydroxyl groups on the proton transport. 1,1,1-Trifluoro-N-(trifluoromethylsulfonyl)methanesulfoneamide (HN(Tf)₂) as a proton source was mixed with the choline derivative at various molar ratios. Their thermal properties, viscosities, and ionic conductivities were investigated. [N_111(2OH)][N(Tf)₂] showed a melting point at 27 °C, and its thermal stability was higher than 400 °C. The viscosity of [N_111(2OH)][N(Tf)₂]/HN(Tf) ₂ mixtures increased as the acid molar fraction increased. The ionic conductivity of [N_111(2OH)][N(Tf)₂] was 2.1 × 10^-3 S cm^-1 at 25 °C; the ionic conductivity monotonously decreased as the acid molar fraction increased. There was a clear correlation between the ionic conductivity and the viscosity for the mixtures of the choline derivative and the acid. PFG-NMR measurements were carried out to investigate the diffusion behavior of protons. Although the acid and the hydroxyl group were indistinguishable by ¹H NMR, the self-diffusion coefficient of the ¹H of the hydroxyl group and the acid was larger than those of other ¹H nuclei. This difference suggests that a fast intermolecular proton transfer exists between the hydroxyl group and the acid. the Owner Societies 2011.

Full text of DOI:10.1039/c1cp21387c

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PAPER
Proton transport properties in an ionic liquid having a hydroxyl group
a
a
b
b
Masahiro Yoshizawa-Fujita,* Yuri Kousa, Koh Kidena, Akihiro Ohira,
a
a
Yuko Takeoka and Masahiro Rikukawa
Received 2nd May 2011, Accepted 2nd June 2011
DOI: 10.1039/c1cp21387c
An ionic liquid having a hydroxyl group, choline bis(trifluoromethylsulfonyl)amide
]), was synthesized to investigate the effect of hydroxyl groups on the proton
transport. 1,1,1-Trifluoro-N-(trifluoromethylsulfonyl)methanesulfoneamide (HN(Tf) ) as a proton
source was mixed with the choline derivative at various molar ratios. Their thermal properties,
viscosities, and ionic conductivities were investigated. [N111(2OH)][N(Tf) ] showed a melting point
at 27 1C, and its thermal stability was higher than 400 1C. The viscosity of [N111(2OH)][N(Tf) ]/
HN(Tf) mixtures increased as the acid molar fraction increased. The ionic conductivity of
(
[N111(2OH)][N(Tf)
2
2
2
2
2
À3
À1
[
N111(2OH)][N(Tf) ] was 2.1 Â 10 S cm at 25 1C; the ionic conductivity monotonously
2
decreased as the acid molar fraction increased. There was a clear correlation between the ionic
conductivity and the viscosity for the mixtures of the choline derivative and the acid. PFG-NMR
measurements were carried out to investigate the diffusion behavior of protons. Although the acid
1
and the hydroxyl group were indistinguishable by H NMR, the self-diffusion coefficient of the
1
1
H of the hydroxyl group and the acid was larger than those of other H nuclei. This difference
suggests that a fast intermolecular proton transfer exists between the hydroxyl group and
the acid.
8
–12
13,14
acid/base pairs,
and IL/acid mixtures.
ILs having chemically active protons,
15–18
Introduction
Recently, it was reported that hydroxyl
Polymer electrolyte fuel cells (PEFCs) have attracted significant
attention because of their high power generation efficiencies
and low pollution levels. Most of the PEFCs employ hydrated
groups were effective to transport protons in the IL matrix.
Choline dihydrogen phosphate ([N111(2OH)][dhp]) and 1-butyl-
3
-methylimidazolium dihydrogen phosphate ([C
4
mim][dhp])
s
perfluoro-sulfonate ionomers, typically Nafion , and hydro-
1
carbon ionomers as the polymer electrolyte membranes.
were synthesized as proton-conducting matrices. Although
À5 À1
4
mim][dhp] was 10 S cm at
–3
the ionic conductivity of [C
0 1C, [N111(2OH)][dhp] exhibited one order of magnitude
Such materials impose water management and low temperature
operation due to the use of water for the high proton conduction.
Finding anhydrous highly proton-conducting polymer electrolytes
has therefore become an important task for high temperature
9
19
4
higher conductivity than that of [C mim][dhp]. It is thought
that the ionic conductivity of [N111(2OH)][dhp] arises from the
transport of protons via phosphate anions and/or the hydroxyl
groups of the choline cation.
4
–6
operations over 100 1C.
On the other hand, ionic liquids (ILs) are being considered
as alternatives for conventional electrolyte materials. ILs
offer unique properties such as low volatility, low flammability,
and high ionic conductivity. Some ILs that show ionic
In order to further study the proton transport properties in
choline derivatives, we synthesized choline bis(trifluoromethyl-
sulfonyl)amide ([N111(2OH)][N(Tf)
Fig. 1. Since N(Tf) -based ILs are generally hydrophobic and
thermally stable up to 400 1C,
2
]) in this study, as shown in
2
À2
À1
conductivity values of over 10 S cm at room temperature
7
have been reported. However, this value includes not
20,21
2
[N111(2OH)][N(Tf) ] will
be attractive for mid-temperature fuel cell applications.
only target ions such as protons but also other constituent
ions. In order to achieve high proton conductivity in ILs,
a variety of systems have been reported using Brønsted
N,N,N-Trimethyl-N-propylammonium bis(trifluoromethyl-
2
sulfonyl)amide ([N1113][N(Tf) ]) that has a structure similar
to that of the choline derivative except for the hydroxyl group
was also used for comparison. We chose 1,1,1-trifluoro-N-
a
Department of Materials & Life Sciences, Sophia University,
(trifluoromethylsulfonyl)methanesulfoneamide (HN(Tf) ) as
2
7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan.
E-mail: masahi-f@sophia.ac.jp
Polymer Electrolyte Fuel Cell Cutting-Edge Research Centre
an acid dopant to further improve the proton conductivity.
2
The ILs were mixed with HN(Tf) at various molar ratios, and
b
thermal properties, ionic conductivities, and viscosities of their
mixtures have been investigated.
(
2
FC-Cubic), Advanced Industrial Science and Technology,
-41-6 Aomi, Koto-ku, Tokyo 135-0064, Japan
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À1
TG-DTA200 at a heating rate of 10 1C min
nitrogen flow.
under a
Viscosity
Density measurements were conducted using an Anton Paar
DMA 38 density meter. Viscosity measurements were conducted
using an Anton Paar AMVn automated micro viscometer.
Ionic conductivity
Ionic conductivity measurements were carried out in a designed
cell made from two platinum electrodes. Cell constants were
calculated with 0.1 M KCl aqueous solution. Ionic conductivity
was obtained by measuring the complex impedance of the cell
between 5 Hz and 5 MHz using a Solartron 1260.
1
H NMR chemical shift
2 2 2
Fig. 1 Structures of [N111(2OH)][N(Tf) ], [N1113][N(Tf) ], and HN(Tf) .
NMR measurements were conducted using a NMR spectro-
1
meter (JEOL GX-270) at 21 1C. H chemical shifts were
Experimental
determined by using a double tube (SC-002, Shigemi Co.
Ltd.; inner tube, 3 mm i.d., 4.1 mm o.d.; outer tube, 4.2 mm
i.d., 4.965 mm o.d.) with the sample in the inner tube and
Materials
Choline chloride (498.0%) was purchased from Tokyo
Chemical Industry Co., Ltd. Lithium bis(trifluoromethyl-
DMSO-d containing TMS as standard in the outer tube.
6
sulfonyl)amide (LiN(Tf) ) (499.8%) and 1,1,1-trifluoro-
2
Self-diffusion coefficient
N-(trifluoromethylsulfonyl)methanesulfoneamide (HN(Tf)2)
The diffusion coefficients of ILs were evaluated by a NMR
(
499.0%) were purchased from Morita Chemical Industries
Co. Ltd. These materials were used without further purification.
N,N,N-Trimethyl-N-propylammonium bis(trifluoromethyl-
sulfonyl)amide ([N1113][N(Tf) ]) was donated by Kanto
spectrometer JEOL ECA-500 using a pulsed field gradient
1
PFG) method for H nuclei. Within a pulse sequence of
(
2
3
bipolar pulsed pairs-stimulated echo, the pulse duration (d)
was set to 4 ms and the diffusion time (D) was 10 ms. The
maximum amplitude of the pulsed field gradient (G) was
2
Chemical Co., Inc. and was dried in vacuo at 80 1C for 24 h
before use. The water content of each IL was less than
À1
1
3.3 T m . The diffusion coefficient, D, was estimated from
4
00 ppm as an average of three readings.
signal reduction against the magnitude of G, using the following
equation:
Synthesis of choline bis(trifluoromethylsulfonyl)amide
])
(
[N111(2OH)][N(Tf)
2
2
2
ln(I/I
0
) = À(gGd) (2/p) 4(D À t/2 À d/8)D
(1)
[
N111(2OH)][N(Tf) ] was synthesized according to the procedure
2
where I and I correspond to the signal intensity at each G and
0
22
reported in the literature. Choline chloride (33.9 g, 0.243 mol)
was dissolved in distilled water (25 ml), and an aqueous
no G, respectively, t equals the sum of d and gradient recovery
on the two sides of the field gradient pulse (0.2 ms for this
experiment), and g is the gyromagnetic ratio.
2
solution of slightly excess LiN(Tf) (85.5 g, 0.298 mol) in 25 ml
of water was added to the solution. The mixed solution was
stirred at room temperature for several hours. The ionic liquid
phase was washed several times with water until no precipitation
was observed by means of the silver nitrate test. The ionic
liquid was dried in vacuo at 120 1C overnight to provide
Results and discussion
[
N
111(2OH)][N(Tf)
2
] is a white solid at room temperature and is
À1
water soluble up to about 0.2 mol l . This solubility was
[N111(2OH)][N(Tf) ] as a colorless viscous liquid (yield = 78.6 g,
2
much higher than those of typical N(Tf) -based ILs. Such a
2
8
1
4%). [N111(2OH)][N(Tf) ] later solidified from a molten state.
2
tendency was also observed for a dabco-based IL having a
24
tertiary amine, which can interact with water molecules. This
H NMR (300 MHz, DMSO-d , Me Si): d 5.27 (t, 1H), 3.83
6
4
(
m, 2H), 3.39 (m, 2H), 3.10 (s, 9H). MS (FAB): m/z 104.1
+
somewhat hydrophilic behaviour most likely results from the
hydrogen bonding interaction between the hydroxyl group
and the water molecule. Nockemann et al. reported that
À
[
N
111(2OH)] ; calcd. 104.1, m/z 280.0 [N(Tf)
2
] ; calcd 280.1.
7
14 2 5 6 2
Anal. calcd for C H N O F S : C, 21.88; H, 3.67; N, 7.29%;
found: C, 21.64; H, 3.55; N, 7.17%.
[
N
111(2OH)][N(Tf)
2
] formed one phase with water at temperatures
above 72 1C. They also studied the temperature-dependent
miscibility of [N111(2OH)][N(Tf) ] with water by means of
H NMR. The [N111(2OH)][N(Tf) ]/water mixtures showed
2
2
Thermal properties
2
1
Differential scanning calorimetry (DSC) was carried out with a
Seiko Instruments differential scanning calorimeter DSC7020
under a nitrogen flow. DSC traces were recorded from À100 1C
2
1
the hydroxyl proton of the [N111(2OH)] cation in the H NMR
spectrum at room temperature. The chemical shift of the
hydroxyl proton completely disappeared at 100 1C, indicating
that the protons undergo a fast exchange between the hydroxyl
À1
to 100 1C at a heating/cooling rate of 10 1C min . Thermo-
gravimetric analysis was conducted on a Seiko Instruments
1
3428 Phys. Chem. Chem. Phys., 2011, 13, 13427–13432
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Fig. 3 Arrhenius plots of ionic conductivities for [N111(2OH)][N(Tf)
2
]
2 2
Fig. 2 DSC traces of [N111(2OH)][N(Tf) ] and [N1113][N(Tf) ].
and [N111(2OH)][N(Tf) ]/HN(Tf) mixtures.
2
2
proton of the [N111(2OH)] cation and water on the NMR time
scale. This also suggested that a hydrogen bond was formed
between the hydroxyl proton and water molecules. These
results were consistent with our observations.
2 2
The melting points of [N111(2OH)][N(Tf) ] and [N1113][N(Tf) ]
were 27 1C and 17 1C, respectively, as shown in Fig. 2.
Although the melting points of these ILs were almost the
same, the enthalpy of fusion showed a big difference because
[
N111(2OH)][N(Tf) ] also showed a solid–solid phase transition
2
at À1 1C. Since the final entropy of fusion (DS ) for
f
À1 À1
[N111(2OH)][N(Tf) ] was 15.1 J K mol , which conformed
2
À1
À1
)
to the Timmermans criterion (DS
f
o 20 J K
for molecular plastic crystals, [N111(2OH)][N(Tf)
categorized as a new ionic plastic crystal. Further details of
the plastic crystalline behaviour of [N111(2OH)][N(Tf) ] will be
reported elsewhere. These ILs were thermally stable up to
mol
2
5
2
] can be
2
2
Fig. 4 Arrhenius plots of ionic conductivities for [N1113][N(Tf) ] and
[N1113][N(Tf) ]/HN(Tf) mixtures.
2
2
4
00 1C, which were identical to the behaviour of other
2
0,21
N(Tf) -based ILs (data not shown).
2
The thermal decom-
position of [N111(2OH)][N(Tf) ] started at 404 1C. The thermal
Fig. 3 and 4 show Arrhenius plots of the ionic conductivity
2
stability of ILs depends on the nucleophilicity of the component
for the ILs and the IL/HN(Tf) mixtures. The ionic conductivity
2
2
1
À3
À1
ions, especially the anions. The nucleophilicity of hydroxyl
groups attached on the cations did not significantly affect the
thermal stability of the IL.
of [N111(2OH)][N(Tf) ] was 2.1 Â 10 S cm at 25 1C, which
2
was equal to those of N-(n-butyl)-N-methylpyrrolidinium
26
bis(trifluoromethylsulfonyl)amide ([C mpyrr][N(Tf) ]),
2
N,N-
4
A
given amount of HN(Tf)
] or [N1113][N(Tf) ] in an argon filled glovebox
to prepare a series of acid-doped systems. The mixtures were
stirred at 50 1C for 2 hours. Thermal analysis of the IL/HN(Tf)
2
was mixed with
diethyl-N-methyl-N-(n-propyl)-ammonium bis(trifluoromethyl-
[
N111(2OH)][N(Tf)
2
2
sulfonyl)amide ([N1223][N(Tf) ]), and N,N,N-triethyl-N-(2-methoxy-
2
ethyl)ammonium bis(trifluoromethylsulfonyl)amide ([N
2
22(1O2)
À2 À1
]-
27
2
[N(Tf) ]); and the ionic conductivity achieved 1.2 Â 10 S cm
2
mixtures was also carried out by DSC measurements. The
at 80 1C. The ionic conductivity of the [N
HN(Tf)2 mixtures monotonously decreased with increasing
11(2OH)
][N(Tf)2]/
1
melting points of [N111(2OH)][N(Tf) ]/HN(Tf) mixtures decreased
2
2
with increasing the HN(Tf) content. While the mixture with
2
the acid content. On the other hand, [N
À3
][N(Tf) ] showed
2
1113
À1
3
3 mol% of the HN(Tf) content ([N
2
][N(Tf) ]/HN(Tf)
2
the ionic conductivity of 3.3 Â 10 S cm at 25 1C, which
111(2OH)
2
2
8
(
33)) showed a glass transition temperature (T ) at À63 1C and
was consistent with the value reported by Matsumoto et al.
g
a melting point (T
m
) at À7 1C, [N111(2OH)][N(Tf)
50) and [N111(2OH)][N(Tf) ]/HN(Tf) (67) mixtures showed a
only at À72 1C and À68 1C, respectively. The thermal
]/H[N(Tf) ] mixtures showed a
values decreased with the H[N(Tf)
content and disappeared when the H[N(Tf) ] content reached
3 mol%. [N1113][N(Tf) ]/H[N(Tf) ] (50) mixtures showed a T
only at À85 1C, which was slightly lower than that of the
][N(Tf) ]/HN(Tf) (50) mixtures. This result would
2
]/HN(Tf)
2
Although [N1113][N(Tf) ] showed higher ionic conductivity
2
(
2
2
than those of [N111(2OH)][N(Tf) ], the ionic conductivity of
2
T
g
[N1113][N(Tf) ]/HN(Tf) mixtures also decreased by adding
2
2
properties of [N1113][N(Tf)
similar tendency. The T
2
2
2 2
HN(Tf)2 (Fig. 4). [N111(2OH)][N(Tf) ]/HN(Tf) mixtures
m
2
]
showed lower ionic conductivity than [N1113][N(Tf) ] mixtures
2
2
in all mixing ratios. This would result from the higher viscosity
3
2
2
g
through the hydrogen-bonding interaction between the hydroxyl
groups and the acid molecules.
[
N
Fig. 5 shows ionic conductivities at 25 1C (a) and viscosities
1
11(2OH)
2
2
be based on a hydrogen-bonding interaction between the
hydroxyl groups of [N111(2OH)] cation and the acid molecules.
at 30 1C (b) of the IL/HN(Tf) mixtures as a function of the
HN(Tf)2 content. The viscosities of [N111(2OH)][N(Tf) ] and
2
2
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the decrease in ionic conductivity, the viscosity of [N1113][N(Tf)
2
]/
HN(Tf) mixtures did not change in the range of the acid
2
content between 0 and 67 mol%. In the case of [N1113][N(Tf)2],
assuming the carrier ion mobility is the same, the number of
carrier ions in the mixtures will decrease with increasing the
HN(Tf) content according to eqn (2). HN(Tf) will exist as
2
2
molecular HN(Tf)
2
in the mixture. Thus, the addition of
HN(Tf)
2
cannot contribute to the ionic conduction.
H NMR measurements were carried out to gain more
insight into the interaction between the ILs and HN(Tf)
1
2
.
1
Fig. 6 shows H NMR spectra of [N111(2OH)][N(Tf)
2
] and
[N
111(2OH)][N(Tf) ]/HN(Tf) mixtures obtained by using a
2
2
double tube. The use of a double tube prevented mixing of a
sample and a deuterium solvent. [N111(2OH)][N(Tf) ] showed
2
1
four signals between 3 and 4.5 ppm. A sharp H signal for the
hydroxyl group of the [N111(2OH)] cation was observed at
3
.37 ppm. When the 10 mol% HN(Tf) was added in the IL,
2
1
a new H signal appeared at 5.05 ppm, and the NMR spectrum
1
still showed four signals. The H signal at 5.05 ppm was
assigned to the protons of the hydroxyl group and HN(Tf)
2
.
Since the proton exchange reaction between the HN(Tf) and
2
1
the hydroxyl group was too fast to separate those H signals,
1
the two species could not be distinguished by NMR. The H
signal at 3.37 ppm of the hydroxyl group shifted to 5.05 ppm.
As the HN(Tf)2 content in [N111(2OH)][N(Tf) ]/HN(Tf)
2
2
Fig. 5 Ionic conductivities at 25 1C (a) and viscosities at 30 1C (b) of
] (closed square) and [N1113][N(Tf) ] (closed circle)
as a function of the acid content.
1
mixtures increased, the H chemical shifts of the hydroxyl
[
N
111(2OH)][N(Tf)
2
2
group and the acid were further directed to a lower magnetic
1
field. When the HN(Tf) content was 80 mol%, the H chemical
2
1
shift reached 9.27 ppm without any shifts of other H signals
[
N
1113][N(Tf)
respectively. While [N111(2OH)][N(Tf)
than [N1113][N(Tf) ], [N111(2OH)][N(Tf)
than [N1113][N(Tf) ].
2
] were 94 mPa s and 61 mPa s at 30 1C,
] showed higher viscosity
] had lower conductivity
of the [N111(2OH)] cation observed at 3.12, 3.47, and 4.13. Such
a shift should be based on the hydrogen-bonding interaction
2
2
2
2
between the hydroxyl group and HN(Tf) . The hydroxyl
2
group acted as a proton acceptor. Furthermore, the shift value
Both mixtures showed almost the same tendency for the
HN(Tf)2 content dependence of the ionic conductivity. As
mentioned above, the ionic conductivity gradually decreased
with increasing the HN(Tf) content. In contrast, the viscosities
2
of the ILs/HN(Tf)
tendency. The viscosity of [N111(2OH)][N(Tf)
mixtures increased with increasing the HN(Tf) content, whereas
1113][N(Tf) ]/HN(Tf) mixtures showed almost the same value
regardless of the HN(Tf) content. These results also suggest that
2
mixtures showed a totally different
2
]/HN(Tf)
2
2
[N
2
2
2
there is a hydrogen-bonding interaction between the hydroxyl
group of the [N111(2OH)] cation and the acid.
The ionic conductivity can be described by the following
equation:
si = n e m
(2)
where n is the number of carrier ions, e is the elementary
charge quantity, and m is the carrier ion mobility. In this
equation, m is affected by the viscosity of the electrolyte
solution. The viscosity of [N111(2OH)][N(Tf)
mixtures reached 293 mPa s when the HN(Tf) content was
7 mol%. [N1113][N(Tf) ]/HN(Tf) mixtures showed almost
the same viscosity of ca. 60 mPa s in all HN(Tf) mixing ratios.
2 2
]/HN(Tf)
2
6
2
2
2
Although the ionic conductivity of both systems decreased
with the HN(Tf)2 content, each decrement of the ionic
conductivity would result from a different cause. While the
1
Fig. 6 H NMR spectra of [N111(2OH)][N(Tf)
2
] and [N111(2OH)]-
[N(Tf) ]/HN(Tf) mixtures obtained by using a double tube. Arrows
2
2
1
indicate the H signal for the hydroxyl group of [N111(2OH)] cation and
the acid.
increase in viscosity for [N111(2OH)][N(Tf) ] mixtures induced
2
1
3430 Phys. Chem. Chem. Phys., 2011, 13, 13427–13432
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1
Fig. 8 Arrhenius plots of self-diffusion coefficients of H nuclei for
1
Fig. 7
closed square) and [N1113][N(Tf)
as a function of the acid content.
H
NMR chemical shifts of [N111(2OH)][N(Tf)
2 2
]/HN(Tf)
2 2
[N111(2OH)][HN(Tf) ]/HN(Tf) (33 mol%).
(
2
]/HN(Tf) (closed circle) mixtures
2
of the protons of the hydroxyl group and HN(Tf)
2
(D(d)) was
the largest in the mixtures. Since the H signals of the hydroxyl
group and HN(Tf) cannot be distinguished by NMR, D(d)
appears to be dependent on the number of hydrogen-bonding
species in the mixture.
1
1
2
Fig. 7 shows the H NMR chemical shifts of [N111(2OH)]-
values seem to be an average self-diffusion coefficient of the
two species. Fig. 8 shows the self-diffusion coefficients of each
[
N(Tf)
2
]/HN(Tf)
2
2 2
mixtures and [N1113][N(Tf) ]/HN(Tf)
mixtures as a function of HN(Tf)2 content. In the case of
1
2 2
proton of [N111(2OH)][N(Tf) ]/HN(Tf) (33) mixtures as a
[
N1113][N(Tf) ]/HN(Tf) mixtures, the
2
H signal of the
2
function of temperature. D(d) values were larger than the
1
diffusion coefficients of other H nuclei at all temperatures
HN(Tf) was observed at around 11.0 ppm in the range of
2
the HN(Tf) content from 10 to 50 mol% and shifted to a
2
measured in this study. On the other hand, D(a), D(b), and D(c)
higher magnetic field in the HN(Tf)
These results indicate that the proton of HN(Tf)
had less electron density in [N1113][N(Tf) ] excess compositions.
In spite of the weak interaction between the IL and HN(Tf)
2
excess compositions.
showed identical values. These protons are not exchanged with
8a
any components but are transported as [N111(2OH)] cations.
2
molecules
2
These results indicated the existence of a fast intermolecular
2
,
proton transfer between the hydroxyl group and HN(Tf)
Therefore, the proton conduction would arise from the addition
of HN(Tf)
2
.
1
the results of H NMR chemical shifts indicated that the
proton of HN(Tf) had higher acidity than those of HN(Tf)
molecules in HN(Tf) excess compositions. The dissociation
state of the acid in the IL is still unclear.
2
2
2
.
2
Conclusions
To investigate proton transport properties in [N111(2OH)]-
[
N(Tf) ]/HN(Tf)2 mixtures, we measured the self-diffusion
2
In order to study the effect of hydroxyl groups on the proton
transport properties of ILs, we prepared [N111(2OH)][N(Tf)2],
coefficients of each proton by means of the PFG-NMR method.
This method is quite useful to discuss each component ion in
2 2 2
[N1113][N(Tf) ], and their HN(Tf) mixtures. HN(Tf) as a
8a,18b,29–31
ILs.
of each proton for [N111(2OH)][N(Tf)
HN(Tf) (33) mixtures at 40 1C. The values for [N111(2OH)]-
Table 1 summarizes the self-diffusion coefficients
proton source was mixed with the ILs at various molar ratios.
Although the mixtures were homogeneous liquids at all mixing
2
] and [N111(2OH)][N(Tf) ]/
2
2
ratios, the interaction between [N111(2OH)][N(Tf)
2
]
and
À11 À1
2
[
N(Tf)
2
] determined in this study were ca. 6.0 Â 10
m s at
HN(Tf) would be stronger than those of [N1113][N(Tf)
2
2
]/acid
4
0 1C and were in agreement with the values for other
mixtures due to the presence of the hydroxyl group. The
decrease in ionic conductivity of [N111(2OH)][N(Tf) ]/HN(Tf)
2
9–31
[
N(Tf)
2
]-based ILs reported elsewhere.
The self-diffusion
] were larger
2
2
coefficients of the protons of [N111(2OH)][N(Tf)
2
mixtures arose from the increase in viscosity with increasing
than those of the mixtures. This difference confirmed that the
decrease in ionic conductivity of [N111(2OH)][N(Tf) ]/HN(Tf)
the HN(Tf) content. There was a clear correlation between
2
2
2
the ionic conductivity and the viscosity for [N111(2OH)][N(Tf)2]/
HN(Tf)2 mixtures. Such behaviour was not observed for
mixtures was induced by the decrease in carrier ion mobility, m,
as mentioned above. Interestingly, the self-diffusion coefficient
[N1113][N(Tf) ]/HN(Tf) mixtures. For this reason, the protons
2 2
of HN(Tf)
2
and the hydroxyl group of the [N111(2OH)] cation in
]/HN(Tf) mixtures were indistinguishable
Table
[
1
Self-diffusion coefficients for [N111(2OH)][N(Tf)
2
]
and
[N
111(2OH)][N(Tf)
1
2
2
N
111(2OH)][N(Tf) ]/HN(Tf) mixtures at 40 1C
2
2
by H NMR. The hydroxyl group acted as a proton acceptor.
The self-diffusion coefficients of the protons of the hydroxyl
group and the acid were larger than those of other protons in
the mixtures. This difference suggested the existence of a fast
intermolecular proton transfer between the hydroxyl groups
a
À11
2
m s
À1
D
(x) /10
Acid content/mol%
0
(a)
(b)
(c)
(d)
5.9
3.8
6.4
3.8
5.9
3.8
6.2
4.2
33
and the HN(Tf) molecules.
2
a
D
(x) denotes the self-diffusion coefficients of each proton for
Hydroxyl groups would be a good candidate for realizing a
fast proton transport in IL matrices. In order to achieve high
[
N
111(2OH)][N(Tf) ] and HN(Tf) , as shown in Fig. 1.
2
2
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Phys. Chem. Chem. Phys., 2011, 13, 13427–13432 13431

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proton conductivity, it is necessary to construct proton transport
networks among hydroxyl groups by optimizing the number
of hydroxyl groups, alkyl chain lengths, etc. ILs having
hydroxyl groups are attractive as anhydrous proton transport
electrolytes with high operational temperatures.
14 A. B. Bourlinos, K. Raman, R. Herrera, Q. Zhang, L. A. Archer
and E. P. Giannelis, J. Am. Chem. Soc., 2004, 126, 15358.
1
1
1
5 J. Sun, D. R. MacFarlane and M. Forsyth, Electrochim. Acta,
001, 46, 1673.
6 Z. Li, H. Liu, Y. Liu, P. He and J. Li, J. Phys. Chem. B, 2004,
108, 17512.
7 M. A. Navara, S. Penero and B. Scrosati, Electrochem. Solid-State
Lett., 2005, 8, A324.
2
Acknowledgements
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b) M. Yoshizawa-Fujita, N. Byrne, M. Forsyth, D. R. MacFarlane
and H. Ohno, J. Phys. Chem. B, 2010, 114, 16373.
(
This work was financially supported by the New Energy and
Industrial Technology Development Organization (NEDO).
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3432 Phys. Chem. Chem. Phys., 2011, 13, 13427–13432
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