Highly stable N3-substituted imidazolium-based alkaline anion exchange membranes: Experimental studies and theoretical calculations

Article abstract of DOI:10.1021/ma402334t

Imidazolium cations with various N3-substituents (including methyl, butyl, heptyl, dodecyl, isopropyl, and diphenylmethyl groups) were synthesized and investigated in terms of their alkaline stability. The effect of the N3-substituent on the alkaline stab

Full text of DOI:10.1021/ma402334t

Article
pubs.acs.org/Macromolecules
Highly Stable N3-Substituted Imidazolium-Based Alkaline Anion
Exchange Membranes: Experimental Studies and Theoretical
Calculations
Fenglou Gu,† Huilong Dong,‡ Youyong Li,‡ Zhihong Si,† and Feng Yan*,†
†Jiangsu Key Laboratory of Advanced Functional Polymer Design and App‡lication, Department of Polymer Science and Engineering,
College of Chemistry, Chemical Engineering and Materials Science, and Jiangsu Key Laboratory of Carbon-Based Functional
Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM) and Collaborative Innovation Center of Suzhou
Nano Science and Technology, Soochow University, Suzhou 215123, P. R. China
S
* Supporting Information
ABSTRACT: Imidazolium cations with various N3-substitu-
ents (including methyl, butyl, heptyl, dodecyl, isopropyl, and
diphenylmethyl groups) were synthesized and investigated in
terms of their alkaline stability. The effect of the N3-substituent
1
on the alkaline stability was studied by quantitative H NMR
spectra and density functional theory (GGA-BLYP) calcu-
lations. The isopropyl substituted imidazolium cation
([DMIIm]+) with the highest LUMO energy value exhibited
the highest alkaline stability in aqueous NaOH. The [DMIIm]+ cation also exhibited higher alkaline stability than that of a
quaternary ammonium cation, benzyltrimethyl-ammonium ([BTMA]+), in CD3OD/D2O NaOH solution at elevated
temperatures. This observation inspired the preparation of [DMIIm]+-based alkaline anion exchange membranes (AEMs)
which showed high alkaline stability in alkaline solution.
tional group for AEMs.28c The prepared AEMs exhibited
excellent base stability when heated to 80 °C in 1 M NaOH
INTRODUCTION
■
The generation of clean, efficient and environmental-friendly
over a 25 day period. Furthermore, the protection of the
energy is one of the major challenges for scientists and
engineers.1 Proton exchange membrane fuel cells (PEMFCs)
cations by steric hindrance and mesomeric stabilization have
been studied and demonstarted to be effective.28a,b,32a,d
that convert chemical energy from a fuel directly into electrical
work, are currently being investigated for a variety of
applications because of their high energy-conversion efficien-
Holdcroft and co-workers recently prepared sterically crowded
benzimidazolium based AEMs.32a,d Crowding around the
cies, high power density, and low operating temperature.2−4
reactive benzimidazolium C2 position by installation of
adjacent bulky groups would hinder nucleophilic attack by
However, widespread applications of PEMFCs are limited by
OH− anions and thus improve the stability of the hydroxide
the high cost of platinum (and its alloys) catalysts and polymer
1
form. On the basis of a detailed analysis of the H NMR
electrolyte membranes (such as Nafion membranes). There-
spectra, the degradation of imidazolium cations due to ring-
fore, anion exchange membrane fuel cells (AEMFCs), which
opening reaction mainly triggered by nucleophilic attack of
use nonplatinum metal (such as silver and nickel) electro-
OH− anions at the C2 position was observed by Elabd et
catalysts and carbon-free supports have been extensively
al.21,22,33 Our group has previously reported that the substituent
groups at C2 position of imidazolium rings could enhance
alkaline stability of imidazolium cations at elevated temper-
atures.34 Results of this study suggest that imidazolium cations
might be suitable for AEM applications. Although recent
studies have demonstrated that both polymer backbone and
cation chemistry play an important role in influencing the
alkaline stability of AEMs,20c,29b,35a,b we believe that once an
alkaline stable cationic-group is identified, the proper polymer
backbones with high alkaline stability can be rationally
designed.
investigated in recent years.5−8
The anion exchange membrane (AEM) is one of the key
components of AEMFCs that transport anions from the
cathode to the anode.9−11 Recently, polymers with pendant
ammonium cations to facilitate hydroxide ion conduction have
been extensively studied.12−20 However, it has been well
demonstrated that quaternary ammonium cations are generally
sensitive toward β-hydrogen (Hofmann or E2) elimination and
direct nucleophilic substitution (SN2) under alkaline con-
ditions.19−26 To circumvent this obstacle and to achieve highly
stable alkaline AEMs, cationic groups other than quaternary
alkylammonium, including guanidinium,25,26 metal−cation,27
phosphonium,28,29a imidazolium,30,31 and benzimidazolium32
cations have been recently investigated. For example, tetrakis-
(dialkylamino)phosphonium cation was evaluated as a func-
Received: November 12, 2013
Revised: December 16, 2013
Published: December 19, 2013
© 2013 American Chemical Society
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Synthesis of 1,2-dimethyl-3-Isopropylimidazolium Bromide
([DMIIm][Br]). [DMIIm][Br] was synthesized by stirring a mixture
containing 1,2-dimethylimidazole and an equivalent molar amount of
2-bromopropane at room temperature under nitrogen atmosphere.
Colorless viscous oil (yield: 73%). 1H NMR (400 MHz, D2O): 7.34−
7.44 (d, 1H), 7.22−7.30 (d, 1H), 4.5−4.64 (m, 1H), 3.64−3.74 (s,
3H), 2.46−2.60 (s, 3H), 1.34−1.48 (m, 6H).
However, it should be noted that the synthesis of C2-
substituted imidazolium cations is more difficult if compared
with that of N3-substitutions because it needs the reaction
procedure of the carbanion and halogenoalkane in alkaline
condition, in which a number of side reactions could not be
avoided and thus considerably reduced the yield of the desired
products.35c,d Conversely, N3-substituted imidazolium cations
can be synthesized in one-step by the simple nucleophilic
substitution reaction between the lone pair electron located at
nitrogen atom with halogenoalkane.
Synthesis of 1,2-Dimethyl-3-diphenylmethylimidazolium Bro-
mide ([DMDPMIm][Br]). [DMDPMIm][Br] was synthesized by
stirring a mixture containing 1,2-dimethylimidazole and an equivalent
molar amount of bromodiphenylmethane at room temperature under
1
nitrogen atmosphere. White solid (yield: 68%). H NMR (400 MHz,
On the basis of the results reported before, the alkaline
stability of C2-substituted (with methyl group) imidazolium
cations with various substituted groups on N3 position and
their corresponding cationic polymers were studied. The
influence of N3-substituents on alkaline stability of imidazolium
cations and imidazolium-based alkaline AEMs was systemati-
cally studied by 1H nuclear magnetic resonance (NMR)
analysis and density functional theory (GGA-BLYP) calcu-
lations. The differences between quaternary ammonium- and
N3-substituted imidazolium-based AEMs with the same class of
polymer backbone and identical ion-exchange capacities were
investigated with respect to their ionic conductivity and alkaline
stability.
D2O): 7.37−7.45 (m, 6H), 7.27−7.30 (s, 1H), 7.12−7.20 (m, 4H),
6.92−6.95 (d, 1H), 6.87−6.91 (d, 1H), 3.70−3.76 (s, 3H), 2.43−2.48
(s, 3H).
Synthesis of 1-(4-Vinylbenzyl)-2-methylimidazole. 1-(4-Vinyl-
benzyl)-2-methylimidazole was synthesized by stirring: a mixture
containing 4.10 g (0.05 mol) of 2-methylimidazole, 7.63 g (0.05 mol)
of 4-vinylbenzyl chloride, and 5.61 g (0.14 mol) NaOH in acetonitrile
(40 mL) was stirred at room temperature for 36 h under an argon
atmosphere. The solvent was removed under dynamic vacuum, and
the crude product was extracted with CH2Cl2 three times. The
combined organic phase was washed with distilled water and dried
over anhydrous MgSO4, and the solvent was removed under vacuum.
The resultant yellow oil was dried in dynamic vacuum at room
temperature. 1H NMR (400 MHz, CDCl3): 7.35−7.39 (d, 2H), 6.98−
7.03 (d, 2H), 6.94−6.97 (s, 1H), 6.92−6.85 (s, 1H), 6.67−6.75 (m,
1H), 5.71−5.78 (d, 1H), 5.21−5.31 (d, 1H), 4.99−5.05 (s, 2H), 2.31−
2.34 (s, 3H).
EXPERIMENTAL SECTION
■
Materials. Styrene, acrylonitrile, 2-methylimidazole, 1,2-dimethyli-
midazole, iodomethane, benzyltrimethylammonium chloride,
(vinylbenyl)trimethylammonium chloride, divinylbenzene (DVB), 4-
vinylbenzyl chloride, benzoin ethyl ether, ethyl ether, ethyl acetate,
acetonitrile, 2-bromopropane, 1-bromobutane, dichloromethane, 1-
bromoheptane, 1-bromododecane, bromodiphenylmethane, sodium
hydroxide, and hydrochloric acid were used as purchased without
further purification. All of the vinyl monomers were made inhibitor-
free by passing the liquid through a column filled with basic alumina to
remove the inhibitor and then stored at 5 °C before use. Distilled
deionized water was used throughout the experiments.
Synthesis of 1-(4-Vinylbenzyl)-2-methyl-3-butylimidazolium Bro-
mide ([VMBIm][Br]) Monomer. [VMBIm][Br] was synthesized by
stirring a mixture containing 1-(4-vinylbenzyl)-2-methylimidazole and
an equivalent molar amount of 1-bromobutane for 24 h at 0 °C. The
resultant viscous oil was washed with ethyl ether four times and then
1
dried in dynamic vacuum at room temperature. H NMR (400 MHz,
D2O): 7.46−7.51 (d, 2H), 7.32−7.39 (d, 2H), 7.19−7.24 (d, 2H),
6.73−6.80 (m, 1H), 5.77−5.87 (d, 1H) 5.30−5.38 (d, 1H), 5.25−5.29
(s, 2H), 4.02−4.12 (m, 2H), 2.49−2.57 (s, 3H), 1.68−1.80 (m, 2H),
1.22−1.32 (m, 2H), 0.82−0.91 (m, 3H).
Synthesis of 1-(4-Vinylbenzyl)-2-methyl-3-isopropylimidazolium
Bromide ([VMIIm][Br]) Monomer. [VMIIm][Br] was synthesized by
stirring a mixture containing 1-(4-vinylbenzyl)-2-methylimidazole and
an equivalent molar amount of 2-bromopropane for 24 h at 0 °C. The
resultant viscous oil was washed with ethyl ether four times and then
Synthesis and Characterization of Imidazolium Salts. Syn-
thesis of 1,2,3-Trimethylimidazolium Iodide ([TMIm][I]). [TMIm][I]
was synthesized by stirring a mixture of 1,2-dimethylimidazole with an
equivalent molar amount of iodomethane at room temperature under
1
1
dried in dynamic vacuum at room temperature. H NMR (400 MHz,
nitrogen atmosphere. White solid (yield: 93%). H NMR (400 MHz,
D2O): 7.50−7.53 (d, 2H), 7.31−7.37 (d, 2H), 7.16−7.25 (d, 2H),
6.73−6.76 (m, 1H), 5.83−5.88 (d, 1H), 5.36−5.43(d, 1H), 5.13−5.24
(s, 2H), 4.42−4.53 (m, 1H), 2.19−2.26 (s, 3H), 1.33−1.42 (m, 6H).
Synthesis of Imidazolium and Quaternary Ammonium
Cation-Based Polymers. The cation-based polymers were synthe-
sized via free radical polymerization using azobis(isobutyronitrile)
(AIBN) as thermal initiator. For example, poly(1-(4-vinylbenzyl)-2-
methyl-3-butylimidazolium bromide) ([PVMBIm][Br]) was synthe-
sized by stirring a mixture containing [VMBIm][Br] and 1 wt % of
AIBN dissolved in DMSO at 65 °C for 8 h under a nitrogen
atmosphere. The polymer product was precipitated twice with acetone
and then dried at 60 °C overnight. Poly(1-(4-vinylbenzyl)-2-methyl-3-
isopropylimidazolium bromide) ([PVMIIm][Br]) and poly-
(vinybenzyltrimethylammonium chloride) ([PVTMA][Cl]) were
synthesized following the same procedure (for details, see Supporting
Information).
Preparation of Alkaline Anion-Exchange Membranes. Anion-
exchange membranes were prepared via photocross-linking of cation-
based monomers with styrene and acrylonitrile using divinylbenzene as
cross-inking agent. A homogeneous solution of styrene/acrylonitrile
(1:8 weight ratio), [VMBIm][Br] (27.7 wt %), divinylbenzene (4 wt %
based on the weight of monomer) and 2 wt % of benzoin ethyl ether
(as a photoinitiator) was cast onto a glass mold and photocross-linked
by irradiation with UV light of 250 nm wavelength for 40 min at room
temperature. [VMIIm]+- and [VTMA]+-based polymeric membranes
D2O): 7.27−7.290 (s, 2H), 3.75−3.77 (s, 6H), 2.57−2.54 (s, 3H).
Synthesis of 1,2-Dimethyl-3-butylimidazolium Bromide
([DMBIm][Br]). [DMBIm][Br] was synthesized by stirring a mixture
containing 1,2-dimethylimidazole and an equivalent molar amount of
1-bromobutane at room temperature under nitrogen atmosphere.
Colorless viscous oil (yield: 88%). 1H NMR (400 MHz, D2O): 7.35−
7.37 (d, 1H), 7.31−7.33 (d, 1H), 4.09−4.15 (t, 2H), 3.76−3.79 (s,
3H), 2.58−2.61 (s, 3H), 1.75−1.85 (m, 2H), 1.30−1.40 (m, 2H),
0.92−0.97 (t, 3H).
Synthesis of 1,2-Dimethyl-3-heptylimidazolium Bromide
([DMHIm][Br]). [DMHIm][Br] was synthesized by stirring a mixture
containing 1,2-dimethylimidazole and an equivalent molar amount of
1-bromoheptane at room temperature under nitrogen atmosphere.
1
White solid (yield: 76%). H NMR (400 MHz, D2O): 7.27−7.30 (d,
1H), 7.23−7.26 (d, 1H), 4.01−4.08 (t, 2H), 3.68−3.72 (s, 3H), 2.50−
2.54 (s, 3H), 1.70−1.80 (m, 2H), 1.16−1.3 (m, 8H) 0.76−0.83 (t,
3H).
Synthesis of 1,2-Dimethyl-3-dodecylimidazolium Bromide
([DMDIm][Br]). [DMDIm][Br] was synthesized by stirring a mixture
containing 1,2-dimethylimidazole and an equivalent molar amount of
1-bromododecane at room temperature under nitrogen atmosphere.
1
Hazel solid (yield: 90%). H NMR (400 MHz, D2O): 7.40−7.42 (d,
1H), 7.43−7.45 (d, 1H), 4.10−4.17 (t, 2H), 3.75−3.80 (s, 3H),2.59−
2.63 (s, 3H), 1.73−1. 83 (m, 2H), 1.14−1.36 (m, 18H), 0.76−0.83 (t,
3H).
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Scheme 1. Molecular Structures of Imidazolium Salts Studied in This Work
were also prepared in the same way. The resulting polymeric
membranes were immersed in 1 M NaOH solution at 60 °C to
convert the membranes from Cl− to OH−. These converted
membranes were then immersed in N2-saturated deionized water for
24 h until the pH of residual water was neutral.
AEMs were soaked in 100 mL of 0.01 M HCl standard solution for 24
h to render the membrane to Cl− form. Then the solutions were back-
titrated with a standardized NaOH solution using phenolphthalein as
an indicator. The IEC was calculated as follows:
C1V1 − C2V
1
2
Characterization. H NMR spectra were carried out on a Varian
IEC =
m
400 MHz spectrometer. Thermogravimetric analysis (TGA) was
performed under nitrogen flow by Universal Analysis 2000 at a heating
rate of 10 °C min−1 and samples were heated from 30 to 500 °C.
Hydroxide Ion Conductivity. Hydroxide conductivity (σ, S
cm−1) of each membrane sample was obtained using the following
equation:
where C1 and V1 are the concentration and volume of HCl solution
before titration, respectively, C2 and V2 are the concentration and
volume of NaOH solution consumed in the titration, m is the mass of
the dried membrane.
Alkaline Stability Measurements. Alkaline stability of imidazo-
lium and ammonium cations was studied in D2O (or CD3OD/D2O)
NaOH solutions. Test solutions were placed in polypropylene jars and
heated at 80 °C for various times. Aliquots were sampled and
imediately transferred into a standard NMR tube for 1H NMR
spectrometry. The alkaline stability of imidazolium and ammonium
cation based polymers were determined in the same way.
Computational Details and Analysis. Theoretical analysis of
N3-substituted imidazolium and quaternary-ammonium cations were
performed by the Dmol3 density functional code as implemented in
Materials Studio (Version 6.0).37,38 The GGA-BLYP functional39,40a
and DNP basis set was used for all calculations. The single molecules
were fully optimized with a self-consistent field (SCF) convergence
value of 10−6 Ha. The convergence criteria for geometry optimizations
used threshold values of 5 × 10−6 Ha for energy, 0.001 Ha/Å for
gradient, and 0.005 Å for displacement convergence, respectively.
Considering the solution environments, solvent effects were added
into the calculations using ε = 78.54 for water and ε = 32.63 for
methanol.
Frequency analysis is performed on all the optimized single
molecules to ensure there is no imaginary frequency. The complete
linear synchronous transit/quadratic synchronous transit (LST/QST)
method was then used in searching transition state (TS) structures.40b
All the transition-state structures exhibit only one imaginary frequency
along the reaction coordinate by frequency calculations. The nudged-
elastic band available in the following transition state confirmation is
carried out to ensure that the TS geometries directly connect the
corresponding reactants and products.40c
l
σ =
RA
where l is the distance (cm) between two electrodes, A is the cross-
sectional area (cm2) of the membrane. The resistance value (R) was
measured over the frequency range from 1 Hz to 1 MHz by four-point
probe alternating current (ac) impedance spectroscopy using an
electrode system connected with an electrochemical workstation
(Zahner IM6 EX). The choice of the four-probe instead of two-probe
method for the through palne conductivity measurements is because
the effect of contact resistance on the four-probe ionic conductivity is
much lower than that on two-probe method, especially in high
humidity.36 Prior to the conductivity measurement, all the membrane
samples were soaked in N2 saturated deionized water for at least 24 h
and rinsed repeatedly to remove free KOH. Conductivity measure-
ments were carried out under fully hydrated conditions in a chamber
filled with a N2 saturated deionized water to maintain the relative
humidity at 100% during the experiments. All the samples were
equilibrated for at least 30 min under a given temperature. Repeated
measurements were then taken at that given temperature with 10 min
interval until no more change in conductivity was observed.
Water Uptake and Swelling Ratio. The membrane samples were
dried under vacuum at 80 °C until a constant weight was obtained
(Wd). The samples were then immersed in N2 saturated deionized
water at room temperature for 24 h. Then the membranes were taken
out, wiped with tissue papers to remove the excess water on the
surface, and quickly weighed (Ww). The water uptake W was
calculated with the following equation:
Ww − W
RESULTS AND DISCUSSION
d
■
W(%) =
× 100%
W
Alkaline Stability of N3-Substituted Imidazolium
Cations. Scheme 1 shows chemical structure of the N3-
substituted imidazolium salts, inculding 1,2,3-trimethylimidazo-
lium iodide ([TMIm][I]), 1,2-dimethyl-3-butylimidazolium
bromide ([DMBIm][Br]), 1,2-dimethyl-3-heptylimidazolium
bromide ([DMHIm][Br]), 1,2-dimethyl-3-dodecylimidazolium
bromide ([DMDIm][Br]), 1,2-dimethyl-3-isopropylimidazo-
lium bromide ([DMIIm][Br]), 1,2-dimethyl-3-diphenylmethy-
limidazolium bromide ([DMDPMIm][Br]), 1-(4-vinylbenzyl)-
2-methylimidazolium ([VMIm]), 1-(4-vinylbenzyl)-2-methyl-3-
butylimidazolium bromide ([VMBIm][Br]), and 1-(4-vinyl-
benzyl)-2-methyl-3-isopropylimidazolium bromide ([VMIIm]-
[Br]) studied in this work. The chemical structure and purity of
d
where Wd and Ww are the weight of dry and corresponding water-
swollen membranes, respectively.
The swelling ratio of the membranes was investigated by linear
expansion ratio, determined by the difference between dry and wet
dimensions of a membrane sample (3 cm in length and 1 cm in
width). The calculation was based on the following equation:
Xwet − Xdry
swelling (%) =
× 100%
Xdry
Here Xwet and Xdry are the lengths of wet and dry samples, respectively.
Ion Exchange Capacity (IEC). Ion exchange capacities (IEC)
were measured using the conventional back-titration method. The
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Scheme 2. Ring-Opening Reaction Mechanism of [DMIIm]+ Cation in Alkaline Solution
a
Table 1. Degradation Degree of N3-Substituted Imidazolium Cations in Alkaline Solutions at 80 °C
a
Key: [a] In 1 M NaOH CD3OD/D2O solution.
these imidazolium salts were investigated by 1H NMR
measurements (as shown in Supporting Information). The
chemical stability of imidazolium cations in highly alkaline
solution was first studied by NMR spectroscopy. The effects of
N3-substitutions, alkaline concentration, and reaction temper-
ature on the chemical stability of the imidazolium cations were
evaluated.
the indicated 1H resonances. For example, the quantity 1′/(1 +
1′) provides such a measure (shown in Scheme 2 and in Table
1). The results shown in Table 1 indicate that about 36.4% of
[TMIm]+ degraded in 2 M NaOH solution over a 432 h period,
indicating poor alkaline stability. Similar results were observed
for [DMDIm]+ and [DMDPMIm]+ which degraded about
47.8% and 48.3% in 2 M NaOH solution after 72 and 48 h
testing, respectively.
1H NMR spectra of synthesized N3-substituted imidazolium
cations in 2 M NaOH solution at 80 °C were shown in Figure
S1. It can be seen that the imidazolium cations reacted quickly
with D2O undergoing hydrogen/deuterium (H/D) exchange of
the ring protons, and thus the signals of C2, C4 and C5 protons
disappeared (Figure S1A). However, a new weak peak at
around 2.10 ppm was observed after being exposed to 2 M
NaOH solution at 80 °C for 24 h, probably due to the ring-
opening reaction of imidazolium cation (Figure S1A). The
intensity of the new peak increased with increasing reaction
time, indicating a continuous degradation of the imidazolium
cation. The degradation degree of N3-substituted imidazolium
cations can be calculated by the relative integrated intensities of
Among the N3-substituted imidazolium cations studied in
this work, [DMBIm]+, [DMHIm]+, and [DMIIm]+ cations
showed relatively better alkaline stability, and less than 4%
degradation was observed after 432 h testing. These relatively
alkaline stable imidazolium cations, including [DMBIm]+,
[DMHIm]+, and [DMIIm]+ were further studied in 4 and 6
1
M NaOH aqueous solutions at 80 °C (see H NMR spectra
shown in Figure 1). It can be clearly seen that [DMHIm]+
degraded quickly in 6 M NaOH solution after 8 h testing
(about 98.5% degraded), while [DMIIm]+ showed the highest
alkaline stability (22.5% degraded in 6 M NaOH solution after
156 h).
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1
Figure 1. H NMR spectra for (A) [DMBIm]+, (B) [DMHIm]+, and (C) [DMIIm]+ in 4 and 6 M NaOH solutions at 80 °C for 50 and 156 h,
respectively.
Figure 2. 1H NMR spectra of (A) [BTMA]+, (B) [DMIIm]+, and (C) [DMBIm]+ in 1 M NaOH CD3OD/D2O solution (40 wt % of NaOH in D2O,
[NaOH]/[cation] = 10/1, molar ratio) at 80 °C at various times.
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The alkaline stability order of the N3-substituted imidazo-
lium cations studied in this work decreases in the order:
[DMIIm]+ > [DMBIm]+ > [DMHIm]+ > [TMIm]+
>
[DMDIm]+ > [DMDPMIm]+. Although the imidazolium ring
undergoes ring-opening reactions mainly due to nucleophilic
attack by OH− at the C2 position,21,33 it can be concluded that
N3-substituents significantly affect the alkaline stability of these
imidazolium cations. Once the stability of C2 substitution
group is confirmed (such as methyl groups), the proper N3-
substituents could further improve the alkaline stability of the
imidazolium cations. In addition, based on these results, it can
be concluded that the degradation mechanisms (ring-opening
reactions) of all the imidazolium cations studied in this work
are the same.
Quaternary ammonium cation-based polymers have been
extensively studied and applied in formulating AEMs. There-
fore, the alkaline stability of a typical quaternary ammonium
cation, benzyltrimethylammonium ([BTMA]+), was studied
and compared with N3-substituted imidazolium cations under
the same experimental conditions. Here, the alkaline stability
characterizations of all the cations were conducted in 1 M
NaOH CD3OD/D2O solution (40 wt % of NaOH in D2O,
[NaOH]/[cation] = 10/1, molar ratio) because the presence of
methanol could accelerate the cation degradation and dissolve
polyatomic cations better than pure aqueous solution.41
It can be clearly seen from Figure 2 that about 32.5% of
quaternary ammonium cation ([BTMA]+) degraded after 168 h
testing at 80 °C, while only 6.9% and 5.7% degradations of N3-
substituted imidazolium cations ([DMBIm]+ and [DMIIm]+)
were observed under the same experimental conditions,
respectively, indicating a better alkaline stability of N3-
substituted imidazolium cations.
DFT Calculations. Figure 3 shows the LUMO energies of
the cations and the HOMO energy of OH− in water and
methanol evaluated via DFT calculations. As we can see that
the LUMO energies of imidazolium cations, including
[TMIm]+, [DMBIm]+, [DMHIm]+, [DMDIm]+, [DMIIm]+,
and [DMDPMIm]+ in aqueous solution are determined to be
−1.547, −1.527, −1.530, −1.533, −1.493, and −1.687 eV
(Figure 3A), respectively. While the HOMO energy of OH−
(in water) is calculated to be −3.081 eV which is lower than the
LUMO energy of imidazolium cations. This leads to that the
nucleophilic attack of OH− be restricted by the LUMO energy
of imidazolium cation. The higher the LUMO energy, the more
difficult it is for imidazolium cations to be attacked by OH−
anions.42a−c To look insight of the role that LUMO energy
played in nucleophilic attack of OH−, TS search was performed
for the reaction. Figure S2 shows the calculated three reaction
energy profiles, for DMIIm, DMBIm, and TMIm. Here, we set
the relative energy of ion pair as 0 kcal/mol, and the ΔE
represents the relative energy of other species against their
corresponding initial ion pair. The sequence of activation
barriers for TS1 shows good agreement with their LUMO
energy sequence (DMIIm > DMBIm > TMIm). This result
could explain why LUMO energy determines the alkaline
stability of the imidazolium cations. The electroneutral alcohol
compound (intermediate) is unstable and easy to form the
energy-favorable carbonyl compound (product), which means
the final degradation of the imidazolium cation. Similar reaction
profile of C−N bond breaking is also reported,33 which
supports the reliability of our calculations.42d−f Among the N3-
substituted imidazolium cations studied here, [DMIIm]+ shows
the highest LUMO energy (−1.493 eV), which suggests it is the
Figure 3. Frontier molecular orbital energy of N3-substituted
imidazolium and quaternary ammonium cations in (A) water and
(B) methanol solution. The gray, blue, red and white balls represent C,
N, O and H atoms, respectively. The black arrows indicate
nucleophilic attacks by OH− on imidazolium and quaternary
ammonium cations. Considering the solution environment, solvent
effect is added into the calculations (ε = 78.54 in water, and ε = 32.63
in methanol).
most stable cation of those examined in alkaline solution at
elevated temperatures. It should be noted that the LUMO
energy value of [DMDIm]+ (−1.533 eV) is higher than that of
[TMIm]+ (−1.547 eV). However, a poorer alkaline stability for
[DMDIm]+ was observed in NMR test. It has already been
demonstrated that the hyperconjugative effect between the N3-
substituent (for example, C−H (σ bond) of methyl group) and
the π-conjugated imidazole ring could increase the electron
density of imidazolium cations.34 In the case of [DMDIm]+, the
neutral long tail groups (dodecyl chains) tend to aggregate due
to collective short-range interaction.43 This tail aggregation may
obstruct the formation of hypercongugative effects between
dodecyl chains and imidazole rings, and thus facilitates
[DMDIm]+ being more easily attacked by OH− as compared
with [TMIm]+. Among the cations studied, [DMDPMIm]+
shows the worst alkaline stability, consistent with the lowest
LUMO energy value (−1.687 eV) calculated, probably due to
the electron-withdrawing effect of two benzene rings as well as
the steric hindrance effect. In addition, two benzene rings
decrease the hypercongugative effect between the C−H (σ
bond) of the methyne group and the π-conjugated imidazole
ring, which further lowers the alkaline stability of
[DMDPMIm]+.
Figure 3B shows the calculated energy diagram of
[DMBIm]+, [DMIIm]+, and [BTMA]+ and the HOMO energy
of OH− in methanol solution. The LUMO energy values of
[DMBIm]+, [DMIIm]+, and [BTMA]+ in methanol were
calculated to be −1.633, −1.597, and −1.776 eV, respectively.
The HOMO energy of OH− (in methanol) is determined to be
−3.283 eV and is also lower than the LUMO energy of
imidazolium and quaternary ammonium cations. Therefore, the
higher the LUMO energy, the more difficult it is for
imidazolium or quaternary ammonium cations to be attacked
by OH− anions.42a−c These results indicate that both
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Macromolecules
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[DMBIm]+ and [DMIIm]+ are more stable than [BTMA]+ in
alkaline methanol solution, which is consistent with the alkaline
stability results obtained earlier above.
Synthesis characterization of AEMs. On the basis of the
alkaline stability results of the N3-substituted imidazolium and
quaternary ammonium cations above, polymers with pendant
imidazolium and quaternary ammonium cations (Scheme 3)
[BTMA]+ cation-based monomers were photocross-linked
with styrene and acrylonitrile using divinylbenzene as cross-
inking agent. It has been previously demonstrated that the ion-
exchange capacity (IEC) value which reflects the exchangeable
groups of the polymeric membranes is closely related to water
uptake, swelling degree, and ionic conductivity. Therefore,
polymeric membranes with identical theoretical IEC value were
synthesized for comparison. All the prepared AEMs are
transparent, flexible, and could be easily cut into desired sizes
even when dry (Figure S4, see Supporting Information).
Figure S5 shows typical thermogravimetric analysis (TGA)
curves of [PVMBIm][OH], [PVMIIm][OH], and [PVTMA]-
[OH] membranes (see Supporting Information). All three
polymeric membranes show a little weight loss due to the
evaporation of absorbed moisture. The first degradative weight
loss starts at about 150 °C and is associated with the
degradation of functional groups, while the decomposition
temperature at about 250 °C can be ascribed to the scission of
copolymer backbone bonds. These results confirm that all three
membranes possess high thermal stability, far beyond the range
of interest for application in AEMFCs.
Scheme 3. Chemical Structures of Polymers with Pendant
Cations Investigated in This Work
Table 2 shows values of IEC, water uptake, swelling degree,
and conductivity obtained for our synthesized [PVMBIm]-
[OH], [PVMIIm][OH], and [PVTMA][OH] membranes. The
IEC values measured by back-titration matched the theoretical
values very well. All the fully hydrated polymeric membranes
exhibited conductivities greater than 1.0 × 10−2 S cm−1 at 30
°C, which is competitive with typical anion conductivities in
hydrated membranes found in the literature and fulfills the
basic conductivity requirement for fuel cell applications.45
The hydroxide stability of these polymeric membranes was
examined by monitoring the change in conductivity over time
for membranes soaked in 1 M NaOH solution at 80 °C, and in
15 M NaOH solution at 30 °C, respectively (Figure 4). It can
be seen that both N3-substituted imidazolium cation based
polymeric membranes, [PVMBIm][OH] and [PVMIIm][OH],
showed a slight loss of conductivity (less than 3%) after five
days of immersion and stabilized at about 94% of the original
conductivity over a 25-day immersion period in 1 M NaOH
solution at 80 °C. However, a continuous decrease (∼41%) in
conductivity was evident for immersion up to 25 days for the
quaternary ammonium cation-based [PVTMA][OH] mem-
brane. Similar alkaline stabiliy results were observed in 15 M
NaOH solution at 30 °C. Figure S6 shows the FT-IR spectra of
[PVMIIm][OH] and [PVTMA][OH] membranes in 1 M
NaOH solution at 80 °C for various time. No obvious
degradtion peaks of [PVMIIm][OH] membrane were
observed. However, the absorption peak intensity of C−N at
1260 cm−1 of [PVTMA][OH] membrane decreased and two
new absorption peaks at 1664 and 1180 cm−1 appeared,
indicating the nucleophilic displacement of benzyltrimethylam-
monium cations. Therefore, the alkaline stabiliy of these
were synthesized via free radical polymerization of correspond-
ing monomers. The alkaline stability of the synthesized
polymers was also investigated by H NMR spectra as well in
1
1 M NaOH CD3OD/D2O solution (40 wt % of NaOH in D2O,
[NaOH]/[cation] = 10/1, molar ratio) at 80 °C.
1
Figure S3 shows the H NMR spectra of [PVMIIm][Br],
[PVMBIm][Br] and [PVTMA][Cl]PVMBImBr, PVMIImBr
and PVTMACl in CD3OD/D2O alkaline solution at 80 °C for
48 h (see Supporting Information). The proton signals
belonging to the N1, C2, C4, and C5 protons disappeared
due to the hydrogen/deuterium (H/D) exchange of the ring
protons (Figure S3). A degradation of about 19.9%, 30.3% and
39.7% was observed, respectively, for [PVMIIm][Br],
[PVMBIm][Br] and [PVTMA][Cl] in 1 M NaOH CD3OD/
D2O solution at 80 °C for 48 h. However it should be noted
that the degradation degrees of polymers were higher than
those of corresponding small molecular cations due to an effect
of the polymer backbone,35a,44 which decreases the alkaline
stability of polymers. However, only the stability of the cations
and not the backbone chain of polymers was assessed in this
work. The relative alkaline stability of these homopolymers is
the same as that of the cations investigated above. We can
conclude, therefore, that [PVMIIm][Br] has the most alkaline
stability at elevated temperatures among the polymers we have
investigated.
The preparation of cation-based polymeric membranes is
another useful tool for further evaluation of the alkaline stability
of N3-substituted imidazolium and quarternary ammonium
cation based AEMs. In order to prepare AEMs with high
hydroxide conductivity, low swelling propensity, and good
mechanical properties, N3-substituted imidazolium- and
Table 2. Ion Exchange Capacity (IEC), Water Uptake, Swelling Degree, and Conductivity of [PVMBIm][OH],
[PVMIIm][OH], and [PVTMA][OH] membranes
IEC value (meq g−1
)
conductivity (×10−2 S cm−1
)
a
b
b
membrane
theoretical
experimental
water uptake (%)
swelling degree (%)
30 °C
60 °C
[PVMBIm][OH]
[PVMIIm][OH]
[PVTMA][OH]
1.02
1.02
1.02
0.97 0.07
0.93 0.05
0.94 0.06
67.62 7.82
63.51 7.53
64.55 7.11
24.05 2.98
20.73 1.63
23.27 1.86
1.09 0.06
1.03 0.04
1.02 0.05
1.58 0.08
1.53 0.07
1.55 0.08
a
b
Calculated from monomer ratio. After immersion in water at 30 °C for 24 h; average of three trials.
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■
●
▲
Figure 4. Conductivity of [PVMBIm][OH] ( ), [PVMIIm][OH] (red ), and [PVTMA][OH] (blue ) membranes as a function of time after
immersion in N2 saturated (A) 1 M NaOH solution at 80 °C, and in (B) 15 M NaOH solution at 30 °C.
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polymeric AEMs is consistent with the stability discussed above
for the N3-substituted imidazolium cations, indicating that N3-
substituted imidazolium cation-based polymeric AEMs are
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CONCLUSIONS
̈
■
(6) Unlu, M.; Zhou, J.; Kohl, P. A. Angew. Chem., Int. Ed. 2010, 49,
̈
The alkaline stability of imidazolium cations with various N3-
substituents were investigated via quantitative 1H NMR spectra
and DFT calculations. Compared with traditional quaternary
ammonium cations (such as [BTMA]+), the N3-substituted
imidazolium cations, [DMIIm]+ and [DMBIm]+ show higher
alkaline stability in CH3OH/H2O NaOH solution. In addition,
[DMIIm]+ and [DMBIm]+ cation-based polymeric membranes
show conductivities comparable to that of [BTMA]+ cation-
based membranes. The stability of [DMIIm]+ and [DMBIm]+
cation-based polymeric membranes in NaOH solutions further
confirms the high alkaline stability of N3-substituted
imidazolium cations. These results should impact the design
of alkaline AEMs based on highly stable imidazolium cations
and provide improved longevity and performance.
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ASSOCIATED CONTENT
* Supporting Information
(12) Pan, J.; Lu, S.; Li, Y.; Huang, A.; Zhuang, L.; Lu, J. Adv. Funct.
Mater. 2010, 20, 312−319.
■
S
Synthesis, 1H NMR characterization of the synthesized
compounds, reaction energy profiles, photographs of the
membranes, TGA analysis of prepared AEMs, and FT-IR
spectra. This material is available free of charge via the Internet
at http://pubs.acs.org/.
(13) (a) Wang, J.; Zhao, Z.; Gong, F.; Li, S.; Zhang, S.
Macromolecules 2009, 42, 8711−8717. (b) Li, N.; Leng, Y.; Hickner,
M. A.; Wang, C. -Y. J. Am. Chem. Soc. 2013, 135, 10124−10133.
(14) Varcoe, J. R. Phys. Chem. Chem. Phys. 2007, 9, 1479−1486.
(15) Varcoe, J. R.; Slade, R. C. T.; Yee, E. L. H.; Poynton, S. D.;
Driscoll, D. J.; Apperley, D. C. Chem. Mater. 2007, 19, 2686−2693.
(16) Danks, T. N.; Slade, R. C. T.; Varcoe, J. R. J. Mater. Chem. 2002,
12, 3371−3373.
AUTHOR INFORMATION
Corresponding Author
*E-mail: (F.Y.) fyan@suda.edu.cn.
Notes
■
(17) Danks, T. N.; Slade, R. C. T.; Varcoe, J. R. J. Mater. Chem. 2003,
13, 712−721.
(18) Wu, Y.; Wu, C.; Varcoec, J. R.; Poynton, S. D.; Xu, T.; Fu, Y. J.
Power Sources 2010, 195, 3069−3076.
The authors declare no competing financial interest.
(19) Wang, G.; Weng, Y.; Chu, D.; Xie, D.; Chen, R. J. Membr. Sci.
2009, 326, 4−8.
ACKNOWLEDGMENTS
■
(20) (a) Xiong, Y.; Liu, Q.; Zhu, A.; Huang, S.; Zeng, Q. J. Power
Sources 2009, 186, 328−333. (b) Zhang, M.; Kim, H. K.; Chalkova, E.;
Mark, F.; Lvov, S. N.; Chung, T. M. Macromolecules 2011, 44, 5937−
5946. (c) Fujimoto, C.; Kim, D. -S.; Hibbs, M.; Wrobleski, D.; Kim, Y.
J. Membr. Sci. 2012, 423, 438−449. (d) Tsai, T. H.; Maes, A. M.;
Vandiver, M. A.; Versek, C.; Seifert, S.; Tuominen, M.; Liberatore, M.
W.; Herring, A. M.; Coughlin, E. B. J. Polym. Sci., Part B: Polym. Phys.
2013, 51, 1751−1760.
This work was supported by the Natural Science Foundation of
China (No. 21274101), the National Basic Research Program
of China (973 Program) (No. 2012CB825800), the Natural
Science Foundation of Jiangsu Province (No. BK2011274), and
by project funding from the Priority Academic Program
Development of Jiangsu Higher Education Institutions.
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