Phosphoric acid doped hydrophobic ionic liquid-based composite membranes for anhydrous proton exchange membrane application

Article abstract of DOI:10.1039/c6ra25460h

In this study, phosphoric acid doped hydrophobic ionic liquid-based composite membranes are successfully synthesized and characterized. 1-Vinyl-3-butylimidazolium bis(trifluoromethylsulfonyl)-imide ([VBIm][NTf₂]) was synthesized and used as hydrophobic phase in the composite membranes. The H₃PO₄ uptake of the composite membranes increases with the increasing content of [VBIm][NTf₂] and then decreases. The resultant composite membranes showed good thermal stability, mechanical properties and high proton conductivity (up to the order of 10^-2 S cm^-1 at 180 °C) at high temperatures under anhydrous conditions. The results of this study suggest that this type of PEMs have good perspectives for high temperature proton exchange membrane fuel cell applications.

Full text of DOI:10.1039/c6ra25460h

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PAPER
Phosphoric acid doped hydrophobic ionic liquid-
based composite membranes for anhydrous proton
exchange membrane application
Cite this: RSC Adv., 2017, 7, 1056
Bencai Lin,*ab Gang Qiao, Fuqiang Chu, Shuai Zhang, Ningyi Yuan
and Jianning Ding*
a
a
a
ab
abc
In this study, phosphoric acid doped hydrophobic ionic liquid-based composite membranes are
successfully synthesized and characterized. 1-Vinyl-3-butylimidazolium bis(trifluoromethylsulfonyl)-imide
(
[VBIm][NTf
uptake of the composite membranes increases with the increasing content of [VBIm][NTf
decreases. The resultant composite membranes showed good thermal stability, mechanical properties
2
]) was synthesized and used as hydrophobic phase in the composite membranes. The H
3 4
PO
2
] and then
Received 19th October 2016
Accepted 9th November 2016
ꢀ2
ꢀ1
ꢁ
and high proton conductivity (up to the order of 10 S cm at 180 C) at high temperatures under
anhydrous conditions. The results of this study suggest that this type of PEMs have good perspectives for
high temperature proton exchange membrane fuel cell applications.
DOI: 10.1039/c6ra25460h
www.rsc.org/advances
PEMs, it also reduces the mechanical properties of membranes
because of high swelling ratio. Both proton conductivity and
Introduction
16
Proton exchange membrane fuel cells (PEMFCs), which convert mechanical properties of PEMs could be enhanced at elevated
chemical energy directly to electrical energy and avoid the temperatures by adding hydrophilic inorganic additives into
1
7,18
limitations of Carnot cycle, are considered as one of the most sulfonated polymers.
However, this type of composite PEMs
1–3
ꢁ
promising power generation technologies. Compared with still cannot maintain high proton conductivity above 130 C due
the normal PEMFCs operations at low temperatures (below 90 to the evaporation of water.
Phosphoric acid could be used as high temperature protonic
range of 120–200 C under anhydrous conditions have several conductors; it is an effective way to prepare high temperature
19
ꢁ
C), the PEMFCs operating at elevated temperatures in the
ꢁ
20–24
merits, including simplied heat and water management, PEMs by replacement of water with phosphoric acid.
The
increased fuel cell efficiency and improved catalyst tolerance to dynamic hydrogen bond networks that formed from phosphoric
4
–6
25,26
CO. However, typical proton exchange membranes (PEMs) acid, favor the transfer of protons.
On the other hand, the
such as DuPont's Naon® membranes can hardly operate at incorporation of phosphoric acid could improve the water
ꢁ
27
elevated temperature range (120–200 C) due to the dehydration uptake of PEMs at high temperatures. Therefore, phosphoric
7
of membranes. Therefore, a critical challenge in developing acid doped PEMs have attracted much attention due to the high
high temperature PEMFCs is to develop low cost PEMs with proton conductivity at elevated temperatures. Generally, more
high conductivity at elevated temperatures.
phosphoric acid uptake of the polymer systems leads to a higher
Recently, several studies have been conducted for the prep- conductivity of the membranes. However, too high phosphoric
aration of novel or functionalized commercially proton acid uptake of the PEMs oen reduces the mechanical proper-
2
8
exchange membranes with good fuel cell performance at ties and chemical stabilities of the membranes signicantly,
8–14
elevated temperatures.
with the sulfonic acid groups is an important approach to
Development of sulfonated polymers making them unsuitable for practical use.
Recently, syntheses of new types of ionic polymers
preserve water and obtain high temperature PEMs. Though been pursued to obtain PEMs with high proton conductivity and
29,30
have
15
29
high degree of sulfonation results in high proton conductivity of excellent mechanical properties. Si et al. prepared phosphoric
acid doped cross-linked proton-conducting membranes, and
the resultant composite membranes showed high thermal
School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center stability and high proton conductivity (at 160 C). However, the
a
ꢁ
for Photovolatic Science and Engineering, Changzhou University, Changzhou, 213164,
membranes showed a high phosphoric acid uptake, and the
mechanical properties of the membranes were not satisfactory.
Therefore, synthesis of novel PEMs with excellent chemical and
mechanical stability as well as high proton conductivity at high
temperature is in great demand.
Jiangsu, China. E-mail: linbencai@cczu.edu.cn; dingjn@cczu.edu.cn
b
Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science
and Technology, Changzhou University, Changzhou, 213164, Jiangsu, China
c
Micro/Nano Science and Technology Center, Jiangsu University, Zhenjiang, 212013,
China
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Yan et al. reported novel composite PEMs by doping protic
RSC Advances
ionic liquid into the membranes, and the composite
membranes showed good mechanical properties and high
proton conductivity due to the microscopic phase separation
1
9
formed in the membranes. It is usually considered that
membranes with a well-dened phase separation morphology
show higher conductivity and mechanical properties than
3
1
those without microscopic phase separation structure. In
the present study, a hydrophobic ionic liquid, 1-vinyl-3-
butylimidazolium bis(triuoromethylsulfonyl)-imide, was
synthesized and used for the preparation of high-temperature
PEMs. A microscopic phase separation morphology is ex-
pected to be formed in the membranes due to the presence
of a hydrophobic ionic liquid and phosphoric acid. The
effect of the hydrophobic ionic liquid content on the
membrane properties, such as thermal stability, mechanical
properties, phosphoric acid uptake, swelling ratio, micro-
structure and proton conductivity, were systematically
investigated.
Scheme 1 Reaction scheme for the preparation of the phosphoric
acid doped hydrophobic ionic liquid-based PEMs.
Experimental
Materials
Styrene, acrylonitrile, N-vinylimidazole, divinylbenzene (DVB),
benzoin ethyl ether, 1-bromobutane, ethyl acetate and bistri-
Preparation of cross-linked phosphoric acid doped
hydrophobic ionic liquid-based composite membranes

uoromethanesulfonimide lithium salt were used as
The membranes were prepared as follows (Scheme 1B): to
a mixture containing 0.30 g styrene/acrylonitrile (weight ratio,
purchased. All of the vinyl monomers were made inhibitor-free
by passing the liquid through a column lled with basic Al O to
2
3
1
: 3), 0.04 g divinylbenzene and 0.02 g benzoin isobutyl ether in
remove the polymerization inhibitor. Distilled deionized water
was used for all experiments.
a glass bottle, 0.15 g [VBIm][NTf ] and 0.55 g N-vinylimidazole
2
were added and stirred to obtain a homogeneous oil. The
mixture was cast into a glass mold and photo-cross-linked by
irradiation with UV light of 250 nm wavelength for 30 min at
Synthesis of 1-vinyl-3-butylimidazolium bromide ([VBIm][Br])
room temperature (Scheme 1B). The prepared membranes were
1
-Vinyl-3-butylimidazolium bromide ([VBIm][Br]) was synthe-
ꢁ
32
immersed in 85 wt% of H PO solution at 80 C for 24 h. The
sized as documented in previous literature. Briey, a mixture
of N-vinylimidazole (4.70 g, 50 mmol) and 1-bromobutane
3
4
acid-doped hydrophobic ionic liquid-based PEMs were wiped
ꢁ
with lter paper and then dried in vacuum at 70 C for 36 h, and
(6.85 g, 50 mmol) was stirred at room temperature for 24 h
the produced membranes were denoted as [VBIm][NTf
VIm55. In a similar way, a series of membranes, [VBIm][NTf ]
2
]
15
-
-
under a nitrogen atmosphere. The product was washed three
times with diethyl ether and ethyl acetate and then dried under
2 x
1
y 2
VIm (x and y indicates the weight ratio of [VBIm][NTf ] and
N-vinylimidazole, respectively) was obtained by varying the
dynamic vacuum. Yield: 95%. H NMR (400 MHz, CDCl
3
): 10.89
s, 1H), 7.86 (s, 1H), 7.58 (s, 1H), 7.52–7.44 (m, 1H), 6.01–5.95
dd, 1H), 5.40–5.36 (dd, 1H), 4.43–4.38 (t, 2H), 1.97–1.87 (q, 2H),
.44–1.32 (s, 2H), 0.97–0.92 (t, 3H).
(
(
1
weight ratio of [VBIm][NTf ] and N-vinylimidazole in the
2
membranes.
A
mixture of styrene/acrylonitrile (weight ratio, 1 : 3,
3
0 wt%), [VBIm][Bi] (15–55 wt%), N-vinylimidazole (55–15 wt%),
Synthesis of 1-vinyl-3-butylimidazolium
bis(triuoromethylsulfonyl)-imide ([VBIm][NTf ])
2
divinylbenzene (4 wt% for the formulation based on the weight
of monomer), and 2 wt% of photoinitiator benzoin isobutyl
1
(
-Vinyl-3-butylimidazolium bis(triuoromethylsulfonyl)-imide ether was stirred and ultrasonicated to obtain a homogeneous
[VBIm][NTf
]) was synthesized as follows (Scheme 1A): a solu- solution, which was then cast onto a glass mold and photo-
2
tion of bistriuoromethanesulfonimide lithium salt (15.79 g, 55 cross-linked by irradiation with UV light of 250 nm wave-
mmol) and [VBIm][Br] (11.55 g, 50 mmol) in 50 mL water was length at room temperature. The resultant membrane samples
stirred at room temperature for 12 h. The resultant viscous oil were washed with ethanol and distilled deionized water and
ꢁ
was washed with deionized water three times and then dried vacuum dried at 120 C for 24 h. The prepared membranes were
1
ꢁ
4
at 80 C for 24 h. The acid-doped
under dynamic vacuum. Yield: 92%. H NMR (400 MHz, CDCl ): soaked into 85 wt% of H
3
PO
3
9
5
2
.00 (S, 1H), 7.65 (s, 1H), 7.46 (s, 1H), 7.17–7.09 (m, 1H), 5.82– membranes were wiped with lter paper and vacuum dried
.76 (dd, 1H), 5.44–5.40 (dd, 1H), 4.25–4.30 (t, 2H), 1.92–1.82 (q, at 70 C for 36 h to remove the absorbed water before
H), 1.44–1.31 (m, 2H), 0.98–0.94 (t, 3H).
characterization.
ꢁ
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Characterization
width, and R the membrane resistance value from the AC
impedance data (U).
1
H NMR spectra were recorded on an AVANCE III 500 MHz
spectrometer. Fourier transform infrared (FT-IR) spectra of the
polymers were recorded on a Nicolet Avatar 370 spectrometer in Results and discussion
ꢀ
1
the range of 4000–400 cm . Thermal analysis was carried out
Characterization of electrolytes
by Netzsch 209 F3 thermogravimetric analyzer (TGA), and
samples were heated from 30 C to 600 C at a heating rate of
0 C min under nitrogen ow. The tensile properties of the
membranes were measured using an Instron Model 5 KN/WDT-
device at 25 C at a crosshead speed of 5 mm min . Atomic
force microscopy (AFM) images were obtained using an NT-
MDT Solver P47 in semicontact mode at room temperature.
ꢁ
ꢁ
In the present study, [VBIm][NTf ] was synthesized and used as
2
ꢁ
ꢀ1
a hydrophobic phase in the copolymer membranes, and the
synthetic procedure for [VBIm][NTf ] is shown in Scheme 1A.
The purity and chemical structure of the synthesized [VBIm]
1
2
ꢁ
ꢀ1
5
1
[
2
NTf ] were conrmed by H NMR measurements. Phosphoric
acid doped hydrophobic ionic liquid-based composite
membranes were prepared by photo polymerization in a glass
mold, and the preparation of membranes is shown in Scheme
3 4
H PO uptake and swelling ratio
1B. Styrene and acrylonitrile were chosen to synthesize the
The membrane samples were soaked in H PO at room copolymers due to their ease of processing, good mechanical
3
4
33
temperature for 48 h. The proton exchange membranes were properties and chemical resistance. The obtained composite
removed from the vessel and excess H PO on the surface was membranes were denoted as [VBIm][NTf ] -VIm , where x and y
eliminated by wiping with a tissue paper and the sample was indicate the weight ratio of [VBIm][NTf ] and N-vinylimidazole
3
4
2
x
y
2
weighed immediately (denoted by the variable W
uptake (W) was calculated with the following equation:
w
). The H
3
PO
4
in the membrane, respectively. As shown in Scheme 1, the
prepared membrane is transparent, exible, and can be easily
cut into any desired sizes.
ðW ꢀ W Þ
w
d
W ð%Þ ¼
ꢂ 100%
The chemical structures of [VBIm][NTf ] -VIm membranes
2
x
y
W
d
without phosphoric acid were characterized by FT-IR, and the
results are shown in Fig. 1. All the samples showed the
where W
w
and W are the weight of the doped and undoped
d
membranes, respectively.
ꢀ1
absorption bands of the cyano groups (C^N) at 2241 cm , and
The swelling ratio (SR) of the composite membranes was
determined from the dimensional differences between the wet
and dry samples. The calculation was based on the following
equation:
ꢀ1
the absorption bands at about 2934 cm
arose from the
stretching vibration of methylene and methyl. The absorption
ꢀ1
bands at 1400–1600 cm indicated the presence of polystyrene.
ꢀ1
Absorption bands at around 1553 and 704 cm were attributed
to the breathing vibration of imidazole ring. The absorption
ðL
s
ꢀ L Þ
d
SR ð%Þ ¼
ꢂ 100%
ꢀ1
bands present at 1054 and 1134 cm
correspond to the
L
d
symmetric and asymmetric stretching vibration of sulfone
groups. The absorption peaks at 1348 and 1185 cm indicate
the presence of C–F groups in the membranes. The content of
where L
respectively.
s
and L
d
are the lengths of the wet and dry membranes,
ꢀ1
[VBIm][NTf
2 2
] increases from [VBIm][NTf ]15-VIm55 to [VBIm]
Proton conductivity
[NTf ] -VIm , and this trend was conrmed by a gradual
2
55
15
increase in the intensity of the characteristic peaks of C–F
groups.
The resistance of the membrane samples was measured over
the frequency range from 0.1 Hz to 1 MHz by the AC impedance
method, using an electrode system connected with an electro-
Thermal properties of [VBIm][NTf ] -VIm were character-
2 x
y
ized by thermogravimetric analysis (TGA) as shown in Fig. 2. A
3 4
chemical workstation (CHI604). A totally dried H PO -doped
rectangular piece of membrane was sandwiched between two
gold electrodes in a glass cell and placed in a programmable
oven to measure the temperature dependence of the conduc-
ꢁ
tivity. The impedance measurement was performed from 80 C
ꢁ
to 180 C under anhydrous conditions. All the samples were
equilibrated for at least 30 min at a given temperature. Repeated
measurements were obtained at 10 min intervals until no
change in conductivity was observed. The ionic conductivity of
ꢀ1
a given membrane (s, expressed in units of S cm ) was calcu-
lated from the following equation:
l
s ¼
RA
where l is the distance between two stainless steel electrodes
(
cm), A the cross-sectional area of H
3
PO
4
-doped membrane
2
(
cm ) obtained from the membrane thickness multiplied by its Fig. 1 FT-IR spectra of [VBIm][NTf
2 x y
] -VIm .
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Fig. 2 TGA plots of [VBIm][NTf
2
] -VIm
x
y
under nitrogen flow. Heating
ꢁ
ꢀ1
rate: 10 C min
.
ꢁ
small weight loss was observed at about 100 C due to the
dehydration of [VBIm][NTf
took place at above 300 C that can be ascribed to the main-
2
]
x
-VIm
y
. The main weight loss stage
ꢁ
chain degradation of [VBIm][NTf
decomposition was observed for [VBIm][NTf
2
]
x
-VIm
y
.
No noticeable
ꢁ
2
]
x
-VIm
y
till 300 C,
Fig. 3 AFM tapping height and phase images of [VBIm][NTf
2
]15-VIm55
conrming that this type of hydrophobic ionic liquid-based
PEMs indeed confers a high thermal stability for their appli-
cations in high temperature PEMFCs.
(
A, B) and [VBIm][NTf 35-VIm35 (C, D).
2
]
uptake of the membranes, and [VBIm][NTf
the highest H PO uptake (134.74 wt%) among the produced
membranes. Just as expected, the change in the trends of the
swelling ratio of [VBIm][NTf -VIm is similar to that of H PO
uptake. Therefore, the H PO uptake and swelling ratio of the
2
]35-VIm35 showed
The H
are listed in Table 1. An imidazolium salt could be obtained
from the reaction of N-vinylimidazole and H PO (Scheme 1B).
Generally, the H PO uptake and swelling ratio in H PO of the
3 4 2 x y
PO uptake and swelling ratio of [VBIm][NTf ] -VIm
3
4
3
4
2
]
x
y
3
4
3
4
3
4
membranes decrease upon decreasing the content of N-vinyl-
3
4
membranes could be adjusted by varying the content of [VBIm]
imidazole and increasing the content of [VBIm][NTf ] in the
2
[
NTf ] in the membranes.
membranes. However, in the present study, the H PO uptake of
2
3
4
2 x y
The mechanical properties of [VBIm][NTf ] -VIm were
the membranes increases with an increase in the content of
VBIm][NTf ], and then decreases. This is probably because of
loose structure formed by the increase of the [VBIm][NTf
measured using a universal testing machine at room tempera-
[
2
ꢀ
1
ture with a crosshead speed of 5 mm min . Table 2 lists the
tensile strength, tensile modulus and elongation at the break of
2
]
content due to its abundant pendent imidazolium cations and
[
[
VBIm][NTf
2 x y
] -VIm membranes. The tensile strength of [VBIm]
bistriuoromethanesulfonimide anion, which results in a more
34
NTf ] -VIm is in the range of 11.75–18.01 MPa, with the tensile
free volume for adopting H
3
PO
4
molecules, and this can be
2 x
y
modulus of 400–947 MPa, and the values of elongation at break
conrmed by the height images shown in Fig. 3A and Fig. 3C.
Microscopic phase separation structure formed in the
membranes can be conrmed by the AFM phase images. The
darker areas represent the copolymer composed of styrene,
acrylonitrile and N-vinylimidazole, and the brighter areas are
associated with hard hydrophobic domain composed of [VBIm]
of 8–24%. In the present study, [VBIm][NTf ] acts as a hydro-
2
phobic phase and endows the membranes good mechanical
properties just like the backbone of Naon membranes.
Therefore, the tensile strength and tensile modulus of
membranes increase with increasing [VBIm][NTf
2
] content in
the membranes due to its hydrophobic nature. The results
[
NTf
2
] (Fig. 3B and D). However, [VBIm][NTf
2
] showed poor
compatibility with H
3
PO due to its hydrophobic nature. Addi-
4
tion of excess [VBIm][NTf ] resulted in a decrease of H PO
2
3
4
2 x y
Table 2 Mechanical properties of [VBIm][NTf ] -VIm
Table 1 The H
3
PO
4
uptake and swelling ratio of [VBIm][NTf
2
]
x
-VIm
y
Tensile
strength
(MPa)
Tensile
modulus
(MPa)
Elongation at
break (%)
H
3
PO
4
uptake
Swelling ratio in Membranes
PO (%)
Membrane
(%)
H
3
4
[
VBIm][NTf
2
]
15-VIm55
11.75 ꢃ 0.3
12.82 ꢃ 1.1
14.44 ꢃ 0.5
15.30 ꢃ 0.8
18.01 ꢃ 0.4
400.94 ꢃ 20.0
553.70 ꢃ 33.1
631.97 ꢃ 34.3
841.52 ꢃ 26.5
947.23 ꢃ 17.8
8.56 ꢃ 1.0
13.64 ꢃ 0.6
24.36 ꢃ 0.9
17.52 ꢃ 0.5
10.77 ꢃ 1.2
[VBIm][NTf
[VBIm][NTf
[VBIm][NTf
[VBIm][NTf
[VBIm][NTf
2
2
2
2
2
]
]
]
]
]
15-VIm55
25-VIm45
35-VIm35
45-VIm25
55-VIm15
74.18
98.10
134.74
107.85
84.62
37
49
63
54
41
[VBIm][NTf2]25-VIm45
[VBIm][NTf2]35-VIm35
[VBIm][NTf2]45-VIm25
[VBIm][NTf2]55-VIm15
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The conductivity activation energy (E
a 2 x y
) of [VBIm][NTf ] -VIm
can be obtained from the proton conductivity of the
33
membranes. Fig. 5 displays the plots of ln(s) vs. 1000/T (T is
the absolute temperature) for [VBIm][NTf ] -VIm . The E
a
2
x
y
obtained from the slope of the line ranged from 14.67 to
ꢀ
1
1
7.27 kJ mol . The changing trend of E
related to the H PO uptake of membranes, and [VBIm][NTf
VIm35 showed the lowest E
a
values is strongly
3
4
2 35
] -
ꢀ1
a
(14.67 kJ mol ), indicating that
proton transfer needs the least energy among the produced
membranes. It could be concluded that the proton transport
mechanism of [VBIm][NTf ] -VIm is not changed under 80–
2
x
y
ꢁ
1
80 C with no change observed in the slope of the Arrhenius
29
Fig. 4 Proton conductivity of [VBIm][NTf
2
]
x y
-VIm .
plot for the membranes (Fig. 5).
Conclusion
show that [VBIm][NTf ] -VIm membranes possess sufficient
2
x
y
mechanical strength for fuel cell applications, and the
mechanical properties of [VBIm][NTf -VIm could be adjusted
In summary, a series of phosphoric acid doped cross-linked
copolymer membranes with different hydrophobic ionic
liquid contents were synthesized and characterized. The resul-
tant hydrophobic ionic liquid-based PEMs had excellent
thermal stability and high conductivity at elevated temperatures
under anhydrous conditions. The introduction of proper
2
]
x
y
easily by changing the content of [VBIm][NTf
membranes.
2
] in the
Proton conductivity is a key property for PEM materials.
Herein, anhydrous proton conductivities of the membranes
ꢁ
containing a variety amount of H PO under 80–180 C are
3
4
amount of [VBIm][NTf ] can be benecial to increase the
2
depicted in Fig. 4, and enhancement in proton conductivity
upon rising temperature has been observed for all the samples.
conductivity of PEMs. The resultant hydrophobic ionic liquid-
based PEMs demonstrated great potential for proton exchange
membrane fuel cell applications based on its good thermal
stability, sufficient mechanical properties and high proton
conductivity at high temperature. This study on hydrophobic
ionic liquid-based PEMs will have an impact on further inves-
tigations of high temperature proton exchange membranes for
fuel cells.
For example, [VBIm][NTf
2
]
15-VIm55 shows the conductivity of
ꢀ3
ꢀ1
ꢁ
ꢀ2
5
.6 ꢂ 10 S cm at 80 C and the value increases to 1.9 ꢂ 10
ꢀ1
ꢁ
S cm at 180 C. Similar to the H
ratio, the conductivity of the membranes at a given temperature
increased with increasing content of [VBIm][NTf ] at rst, and
3 4
PO uptake and swelling
2
then decreased. No decay in proton conductivity was observed
ꢁ
even at 180 C for [VBIm][NTf ] -VIm , which further conrms
2
x
y
the excellent thermal stability of the membranes. Generally,
higher H PO
membranes. Among the [VBIm][NTf
VBIm][NTf 35-VIm35 showed the highest
Furthermore, a well-dened phase separation morphology was
formed in [VBIm][NTf 35-VIm35, which favors the transmission
It is not surprising that [VBIm][NTf ] -VIm
3
4
uptake, leads to higher conductivity of the Acknowledgements
2
x
] -VIm
y
membranes,
PO uptake.
This work was supported by the National Natural Science
Foundation of China (No. 51303017 and 21476031) and the
Jiangsu Natural Science Foundation (BK20151187), and
a project funded by the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions.
[
2
]
H
3
4
2
]
3
5–38
of proton.
showed the highest conductivity (4.14 ꢂ 10 S cm at 180 C).
2
35
35
ꢀ
2
ꢀ1
ꢁ
Notes and references
1
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3
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5
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2 x y
Fig. 5 Arrhenius plots for [VBIm][NTf ] -VIm .
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