Alkaline stable imidazolium-based ionomers containing poly(arylene ether sulfone) side chains for alkaline anion exchange membranes

Article abstract of DOI:10.1039/c3ta15178f

Solvent processable anion exchange membranes (AEMs) containing imidazolium cation and poly(arylene ether sulfone) side chains were prepared via the ionization of poly(4-vinylbenzyl chloride)-graft-poly(arylene ether sulfone) copolymers, and followed by anion exchange with hydroxide ions. The ionized copolymers produced ductile, transparent membranes which possess a relatively high ionic conductivity (up to 1.6 × 10^-2 S cm^-1 at room temperature). The yielded membranes are soluble in polar aprotic solvents, such as DMSO and DMF, while insoluble in water and methanol. The synthesized AEMs show good alkaline stability in 1 M KOH at 60 °C for 20 days, as well as high dimensional and thermal stability. These results should pave way to the practical application of this kind of AEM in alkaline fuel cells.

Full text of DOI:10.1039/c3ta15178f

Journal of
Materials Chemistry A
View Article Online
View Journal | View Issue
PAPER
Alkaline stable imidazolium-based ionomers
containing poly(arylene ether sulfone) side chains
Cite this: J. Mater. Chem. A, 2014, 2,
for alkaline anion exchange membranes†
4413
*
Zhihong Si, Zhe Sun, Fenglou Gu, Lihua Qiu and Feng Yan
Solvent processable anion exchange membranes (AEMs) containing imidazolium cation and poly(arylene
ether sulfone) side chains were prepared via the ionization of poly(4-vinylbenzyl chloride)-graft-
poly(arylene ether sulfone) copolymers, and followed by anion exchange with hydroxide ions. The
ionized copolymers produced ductile, transparent membranes which possess a relatively high ionic
conductivity (up to 1.6 ꢀ 10ꢁ2 S cmꢁ1 at room temperature). The yielded membranes are soluble in
polar aprotic solvents, such as DMSO and DMF, while insoluble in water and methanol. The
synthesized AEMs show good alkaline stability in 1 M KOH at 60 ꢂC for 20 days, as well as high
dimensional and thermal stability. These results should pave way to the practical application of this
kind of AEM in alkaline fuel cells.
Received 13th December 2013
Accepted 6th January 2014
DOI: 10.1039/c3ta15178f
www.rsc.org/MaterialsA
chloromethyl ether and followed by quaternization to form
quaternary ammonium cations.13 However, quaternary ammo-
Introduction
nium cations are generally unstable in high pH aqueous solu-
tion, because of the possible degradations via Hofmann
elimination (E2),14 nucleophilic substitution(SN2),15,16 or ylide
formation.1 As potential alternative cations, tertiary sulfo-
nium,17 guanidinium,18,19 phosphonium,20,21 imidazolium,22–24
benzimidazolium,25,26 phosphatranium,27 and metal-cations28
have been investigated for AEMs. Among the cations studied,
imidazolium cation based AEMs have been recently studied by
several groups, due to their relatively high chemical stability,
which is largely attributed to the steric hindrance and the
presence of the p conjugated structure.14,24,26,29–31 However,
degradation of imidazolium cation based AEMs were also
observed under vigorous conditions (high pH environment and
elevated temperature) due to the nucleophilic attack of
hydroxide ions to the imidazole rings.32,33 Our group previously
has been engaging in the study of C2-substitutions on the
alkaline stability of imidazolium cations.34 A preliminary study
suggested that C2-substituents are effective in stabilizing the
imidazolium cations and the resulting AEMs.35 These observa-
tions inspired the preparation of imidazolium cation-based
highly stable AEMs for practical applications.
On the other hand, cross-linked AEMs, such as polystyrene
and poly(aryl ether sulfone)-based membranes have been
recently synthesized, which possess enhanced dimensional
stability and intensied fuel resistance.1,36 However, cross-
linked polymers are generally insoluble in solvents and thus can
not be used in the catalyst layer to build an efficient three-phase
boundary to improve the utilization of the catalyst particles.21,37
Therefore, the development of soluble AEMs with high
conductivity and good physicochemical properties is
Proton exchange membrane fuel cells (PEMFCs) are one of the
promising environmentally friendly power generation tech-
nologies, which possess high energy-conversion efficiency,
quiet operation and high power density.1–3 As one of the key
components of PEMFCs, polymer electrolyte membranes
could separate the fuel and oxidant streams and simulta-
neously transport protons.4,5 However, the widespread
commercialization of PEMFCs is hindered by their exclusive
dependence on precious metal-based electrocatalysts (such as
platinum).6,7 Therefore, alkaline anion exchange membrane
fuel cells (AEMFCs), in which the charge carriers are hydroxide
ions, rather than protons, are attracting growing interest
due to their enhanced oxygen reduction kinetics and the
potential use of non-precious electrocatalysts, such as silver
and nickel.8,9
As one of the key components of AEMFCs, alkaline anion
exchange membranes (AEMs) play an important role in inu-
encing the performance of AEMFCs. Great efforts have been
devoted to the development of AEMs with high-performance.
Polymer membranes, based on various polymer backbones,
such as polysulfone and its analogues,10,11 and poly(ether
imide)12 were prepared by a post-functionalized strategy, which
is performed by chloromethylation of the polymers with
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application,
Department of Polymer Science and Engineering, College of Chemistry, Chemical
Engineering and Materials Science, Soochow University, Suzhou, 215123, P. R.
China. E-mail: fyan@suda.edu.cn; Fax: +86 512 65880089; Tel: +86 512 65880973
† Electronic supplementary information (ESI) available: GPC, SEM, and DSC
characterizations of synthesized AEMs. See DOI: 10.1039/c3ta15178f
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. A, 2014, 2, 4413–4421 | 4413

View Article Online
Journal of Materials Chemistry A
Paper
desirable.38 Polysulfone is a well-known material, due to its
outstanding mechanical, thermal, and chemical stability, and
excellent membrane-forming capability.39–41 In addition, it
has been reported that polystyrene-based ion-containing
membranes possess relatively higher alkaline stability.42 All the
reported results prompted us to design a class of soluble AEMs,
which combine the advantages of both the membrane-forming
capability of polysulfone and the alkaline stability of imidazo-
lium cations.
In the present study, we report the preparation of polymeric
membranes via a gra-onto reaction between poly(4-vinylbenzyl
chloride) with pendent benzyl chloride groups and poly(arylene
ether sulfone) with terminal phenol groups, and followed by
ionization with 1-butyl-2-methyl imidazole. Anion exchange of
the membranes with hydroxide ions produced imidazolium-
based AEMs. For the rst time, the highly stable C2 (methyl) and
N3-(butyl) substituted imidazolium cation was selected for the
preparation of AEMs. Due to the gra-onto of poly(arylene
ether sulfone), the resultant polymeric membranes are robust,
exible, and transparent and soluble in polar aprotic solvents
such as N,N-dimethylformamide (DMF) and dimethyl
sulfoxide (DMSO), while insoluble in water and methanol. The
soluble imidazolium functionalized ionomers show relatively
high hydroxide ion conductivity, good alkaline stability and
mechanical properties, demonstrating their potential practical
applications in fuel cells.
Synthesis of 1-benzyl-2-methy-3-butylimidazolium chloride
1-Benzyl-2-methy-3-butylimidazolium chloride ([BMBIm][Cl])
was synthesized by stirring a mixture containing 1-butyl-2-
methylimidazole and an equivalent molar amount of benzyl
chloride at room temperature under a nitrogen atmosphere.
The resultant white solid was washed with ethyl ether three
times and then dried in a dynamic vacuum at room temperature
for 24 h (yield: 4.42 g, 90%). 1H NMR (400 MHz, D2O): 7.36–7.42
(m, 5H), 7.34 (ds, 2H), 5.37 (s, 2H), 4.14 (t, 2H), 2.60 (s, 3H), 1.81
(m, 2H), 1.34 (m, 2H), 0.93 (t, 3H).
Synthesis of poly(4-vinylbenzyl chloride)
Poly(4-vinylbenzyl chloride) was synthesized by the free radical
polymerization of 4-vinylbenzyl chloride (initiated by 1 wt% of
AIBN) in DMF at 65 ꢂC for 8 h under a nitrogen atmosphere. The
resultant polymer was precipitated and puried with ethanol,
1
ꢂ
and then dried at 60 C in a vacuum overnight. H NMR (400
MHz, DMSO-d6) d: 7.14 (m, 2H), 6.52 (m, 2H), 4.66 (s, 2H), 2.75
(m, 2H), 1.45 (m, 2H), 1.25 (m, 2H), 1.26 (t, 3H), 0.86 (t, 3H).
Synthesis of poly(arylene ether sulfone)
A typical synthesis procedure is as follows: a mixture containing
bisphenol A (5.0 g, 21.9 mmol), 4,40-dichlorodiphenylsulfone
(6.29 g, 21.9 mmol), and potassium carbonate (4.53 g, 32.8
mmol) were added to a three-neck round bottom ask. Then, a
solvent mixture containing NMP (65 mL) and toluene (30 mL)
was added to the reaction mixture under a nitrogen atmo-
sphere. The reaction bath was heated to 150 ꢂC for 5 h to
dehydrate the system. Aer the removal of toluene and water,
the reaction temperature was gradually increased to 180 ꢂC and
stirred for 24 h. Aer cooling to room temperature, an addi-
tional 30 mL of NMP was added, so as to dilute the highly
viscous solution, aer which the solution was dropped into
stirred deionized water. The precipitate was ltered off and
washed with deionized water and ethanol several times, prior to
being dried under a vacuum to produce the nal product.
Experimental
Materials
2-Methylimidazole, bromobutane, potassium hydroxide (KOH),
acetonitrile, dichloromethane (CH2Cl2), anhydrous magnesium
sulfate (MgSO4), 4-vinylbenzyl chloride, trimethylamine
aqueous solution (30%), 2,20-azobisisobutyronitrile (AIBN), 4,
40-dichlorodiphenylsulfone, bisphenol A, potassium carbonate,
toluene, N-methylpyrrolidone (NMP), tetrahydrofuran (THF),
dimethyl sulfoxide (DMSO), ethanol, and N,N-dimethylforma-
mide (DMF) were used as purchased. All of the vinyl monomers
were made inhibitor-free by passing the liquid through a
column lled with basic alumina to remove the inhibitor and
Gra-onto reaction between poly(arylene ether sulfone) and
poly(4-vinylbenzyl chloride)
ꢂ
then stored at ꢁ5 C before use. Distilled deionized water was
The gra-onto reaction was carried out as follows:41 0.9 g
poly(arylene ether sulfone) and 0.05 g potassium carbonate
were dissolved in 100 mL DMF in a three neck round bottom
ask at 150 ꢂC. Then, an excess amount of the synthesized
poly(4-vinylbenzyl chloride) (1 g) was added to the ask and the
mixture was reuxed for 24 h. The cooled mixture was poured
into deionized water, and the precipitate was Soxhlet extracted
with THF, to remove the ungraed poly(arylene ether sulfone).
The synthesized polymer powder was washed with deionized
used throughout the experiments.
Synthesis of 1-butyl-2-methylimidazole
1-Butyl-2-methylimidazole was synthesized as follows: a mixture
containing 2-methylimidazole (2.00 g, 0.024 mol), bromobutane
(3.34 g, 0.024 mol), and KOH (2.69 g, 0.048 mol) in acetonitrile
(30 mL) was stirred at room temperature, for 4 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
ꢂ
water and methanol and dried at 60 C under a vacuum.
Preparation of the imidazolium-based anion exchange
membrane ([PUVBIm][OH])
removed under vacuum. The resultant yellow oil was dried in a
1
dynamic vacuum at room temperature (yield: 3.05 g, 92%). H 1-Butyl-2-methylimidazole, in a slight excess amount, was
NMR (400 MHz, CDCl3) d: 7.04 (s, 1H), 6.95 (s, 1H), 3.96 (t, 2H), added to a solution of the gra copolymer in DMF (5 wt%). The
2.51(s, 3H), 1.84 (m, 2H), 1.48 (m, 2H), 1.09 (t, 3H).
mixture was stirred at 80 ꢂC for 24 h. The resulting solution was
4414 | J. Mater. Chem. A, 2014, 2, 4413–4421
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry A
then cast onto a level Teon® sheet and heated at 80 ꢂC for 24 h, samples were held at a constant temperature for at least 30 min.
to remove the solvent and the unreacted 1-butyl-2-methylimid- Repeated measurements were obtained at 10 min intervals,
azole. The resultant membranes were converted from the Clꢁ to until no more change in the conductivity was observed. The
OHꢁ by anion exchange with hydroxide ions in a 1 M KOH ionic conductivity (S cmꢁ1) of a given membrane can be
ꢂ
solution at 60 C for 48 h, to produce the imidazolium-based obtained using the following formula:
AEM ([PUVBIm][OH]). Then, the converted AEMs were washed
with deionized water until the pH of the residual water was
neutral.
d
s ¼
RA
where d is the distance (cm) between two gold electrodes, A is
the cross-sectional area (cm2) of the membrane, obtained from
Characterization
The 1H NMR spectra were recorded on a Varian 400 MHz
the membrane thickness multiplied by its width, and R is the
membrane resistance value from the AC impedance data (U).
spectrometer. The Fourier transform infrared (FT-IR) spectra
were recorded on a Varian CP-3800 spectrometer in the range
Water uptake and swelling ratio
4000–400 cmꢁ1. The thermal transition temperatures of the
A membrane sample was immersed in N2 saturated deionized
polymer membranes were measured on a Perkin-Elmer DSC
4000, at a scanning rate of 10 ꢂC minꢁ1, under a N2 atmosphere.
water at room temperature for 24 h. The hydrated membrane
was taken out of water, and the excess surface water was
The thermal analysis was performed on Universal Analysis 4000
removed by wiping with a tissue paper and the mass was quickly
thermogravimetric analyzer (TGA) system. The samples were
heated from 30 to 600 ꢂC, at a heating rate of 10 ꢂC minꢁ1, under
measured (Ww). Then, the wet membrane was dried under a
ꢂ
vacuum at 80 C, until a constant dry weight (Wd). The water
a nitrogen ow. The isothermal TGA analysis was carried out by
ꢂ
uptake W was calculated as follows:
heating the membranes to 150 C, for 12 h, under a nitrogen
ow. The scanning electron microscopy (SEM) images were
taken with a Philips XL 30 FEG microscope, with an accelerating
voltage of 10 kV. The energy-dispersive X-ray spectroscopy (EDX)
measurements were performed with the spectrometer attached
on the Hitachi Model S-4700 eld-emission scanning electron
microscopy (SEM) system. The atomic force microscopy (AFM)
images were measured with a NT-MDT Solver P47 AFM in a
tapping mode. The oscillation frequency was set to approxi-
mately 255 kHz with a Si cantilever, which had a spring constant
of about 11.5 N mꢁ1. All the AFM images were taken at room
temperature. The tensile properties of the membranes were
measured using an Instron 3365, at 25 ꢂC, at a crosshead speed
of 5 mm minꢁ1. The measurements of the membranes were
conducted on membrane samples of 3 mm width and 9 mm
length. Each sample was tested at least three times to reach an
average value. The number average molecular weight (Mn) and
molecular weight distribution (Mw/Mn) of the polymers were
determined using a Waters 1515 gel permeation chromato-
graph (GPC) equipped with a refractive index detector (Waters
2414), DMF was used as the eluent at a ow rate of 1.0 ml minꢁ1
at 30 ꢂC. The samples were injected using a Waters 717 plus
autosampler and the column was calibrated with polystyrene
standards, purchased from Waters.
ðWW ꢁ WdÞ
Wð%Þ ¼
ꢀ 100%
Wd
The swelling ratio was characterized by a linear expansion
ratio, which was determined by the difference between the wet
and dry dimensions of a membrane sample (4 cm in length and
1 cm in width). The calculation was based on the following
equation:
Xwet ꢁ Xdry
Swellingð%Þ ¼
ꢀ 100%
Xdry
where Xwet and Xdry are the lengths of wet and dry membranes,
respectively.
Ion exchange capacity (IEC)
The IEC was measured using a back-titration method. The
AEMs were dried at 80 C under a vacuum for 24 h before the
mass was recorded (mdry) and immersed in 100 mL of a 0.01 M
HCl standard solution for 24 h. The solutions were then titrated
with a standardized NaOH solution using phenolphthalein as
an indicator. The IEC value was calculated using the expression:
ꢂ
V0;NaOHCNaOH ꢁ Vx;NaOHCNaOH
IEC ¼
mdry
Hydroxide conductivity
where V0,NaOH and Vx,NaOH are the volumes of NaOH consumed
in the titration without and with membranes, respectively.
CNaOH is the molar concentration of NaOH, which is titrated by
the standard oxalic acid solution, and mdry is the mass of the dry
membranes. Three replicates were conducted for each sample.
The resistance of the prepared membranes was measured by
four-point probe alternating current (ac) impedance spectros-
copy over the frequency range from 1 Hz to 1 MHz, using an
electrochemical workstation (Zahner IM6 EX). All the samples
were fully hydrated in N2 saturated deionized water for at least
24 h prior to the conductivity measurements. Conductivity
measurements under fully hydrated conditions were carried out
Membrane stability in alkaline solution
in a chamber lled with a N2 saturated deionized water to The alkaline stability of the membranes was examined by
maintain the relative humidity at 100% during the experiments. immersing the membrane samples in N2-saturated 1 M KOH
ꢂ
Before the measurements at each temperature set point, the solution at 60 C. The degradation of the polymer membranes
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. A, 2014, 2, 4413–4421 | 4415

View Article Online
Journal of Materials Chemistry A
Paper
was evaluated by measuring the changes of FT-IR, IEC, and
hydroxide conductivity.
Results and discussion
Base stability of imidazolium and quaternary ammonium
cations
Scheme 1 Chemical structures of the model compounds studied in
this work.
Quaternary ammonium cation-based polymers have been
extensively studied and applied in formulating AEMs.43
However, quaternary ammonium cations are generally sensitive
towards b-hydrogen elimination and direct nucleophilic
substitution under alkaline conditions.14 It has already been
demonstrated that both cation chemistry and polymer back-
bone play important roles in inuencing the alkaline stability of
AEMs. Once a chemical stable cationic-group is identied, an
appropriate polymer backbone can then be rationally
designed.42
further synthesized and studied. The synthetic procedure and a
photograph of the prepared membranes are illustrated in
Scheme 3. It can be clearly seen that the resultant membranes
are transparent, exible and can be easily cut into any desired
size and shape, or bent with a pair of tweezers. Fig. 2 exhibits
the expected chemical shis and intensities for the produced
ionomer containing pendant imidazolium groups and poly(ar-
ylene ether sulfone) chains (inserted chemical structure and
chemical shi assignments). The chemical shi at around 5.12
Here, an imidazolium cation, 1-benzyl-2-methy-3- butylimi-
dazolium ([BMBIm]+), was synthesized and the alkaline stability
was further studied. For comparison, a typical quaternary
ammonium cation, benzyltrimethylammonium ([BnNMe3]+),
was also studied under the same experimental conditions.
Scheme 1 shows the chemical structures of [BMBIm]+ and
[BnNMe3]+ studied in this work. The alkaline stability of two
model compounds was studied by 1H NMR spectra. Fig. 1A
shows the 1H NMR spectra of [BMBIm][Cl] before and aer
ꢂ
exposure to 1 M KOH CD3OD/D2O solution at 80 C. The pres-
ence of methanol could accelerate the cation degradation and
dissolve polyatomic cations better than a pure aqueous solu-
tion.20 It can be found that the [BMBIm]+ reacts rapidly with
D2O, with hydrogen/deuterium (H/D) exchange of the ring
protons belonging to C-4, C-5, a-protons in the C-2 substituents
and position 13, where the 1H signals gradually disappeared. In
addition, the chemical shi of the protons belonging to D2O
changed along with the degradation of the cations (1,4-dioxane
as an internal standard). It should be noted that a new peak at
around 2.4 ppm and 7.5 ppm appeared in the 1H NMR spectra
aer a testing period of 120 h, indicating the degradation of
[BMBIm]+ and [BnNMe3]+, respectively. Scheme 2 shows the
possible degradation mechanism of two cations in alkaline
solution.14,20,44 The degradation degrees of the cations were
1
investigated by a detailed analysis of H NMR spectra (Fig. 1).
The new peak at about 2.4 ppm is associated with the protons
belonging to the –NH– of the degradation products (Fig. 1A).
Meanwhile, a new peak at about 7.5 ppm is ascribed to the
protons belonging to the degradation products of benyl alcohol
(Fig. 1B). The degradation degree (in 1 M KOH, at 80 ꢂC for 120
h) estimated by the relative integrations of the indicated 1H
resonances was determined to be 5.02% and 16.05% for
[BMBIm]+ and [BnNMe3]+, respectively, suggesting that imida-
zolium cations should be another promising class of functional
group choice for AEMs.
Fig. 1 1H NMR spectra for (A) [BMBIm][Cl], and (B) [BnNMe3][Cl] in a
mixture of CD3OD, 1,4-dioxane (internal standard), KOH/D2O solution
(1 M KOH, VCD3OD : VD2O ¼ 3 : 1) at 80 ꢂC for 120 h. The red squares
Membrane preparation and characterizations
Based on the alkaline stability of the model compounds studied
above, imidazolium cation based polymer membranes were indicate the degradation peaks.
4416 | J. Mater. Chem. A, 2014, 2, 4413–4421
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry A
Scheme 2 The possible degradation mechanisms of [BnNMe3]+ (up) and [BMBIm]+ (bottom) in alkaline solution.
ppm, corresponding to the protons in Ph–CH2O was observed,
suggesting the success of the gra-onto reaction. The chemical
shis at around 7.21, 4.45, 2.91, 1.57, 1.31, and 0.92 ppm are
attributed to the attached imidazolium groups.
The successful synthesis of the imidazolium-based AEMs
was further conrmed by means of the FT-IR spectra. Fig. 3
shows the FT-IR spectra of the gra copolymer AEMs in Clꢁ
and OHꢁ forms. The peaks appear at 1148 and 1377 cmꢁ1 are
associated with a sulfone (–SO2), and the peak at 1244 cmꢁ1 is
a characteristic peak of aryl ethers. The peaks at 1485–1696
cmꢁ1 conrm the existence of phenylene. The absorption peak
at around 1586 cmꢁ1 is attributed to the vibration of imida-
zolium cations. The bands at around 2848–2965 cmꢁ1 arise
from aliphatic C–H stretching vibrations. The peak at around
Fig. 2 1H NMR spectrum of [PUVBIm][Cl] membrane.
3400 cmꢁ1 is ascribed to the stretching vibration of O–H
groups, indicating the successful anion change of Clꢁ to OHꢁ.
Fig. 4 shows the EDX spectra of the membranes, where only a
trace amount of Cl (ꢃ0.33 wt%) was le in the membrane,
indicating the successful anion exchange of the polymeric
membranes. The number average molecular weight (Mn) and
molecular weight distribution (Mw/Mn) of poly(arylene ether
sulfone)-g-poly(4-vinylbenzyl chloride) are determined to be
and atomic force microscopy (AFM). As shown in Fig. S2 (see the
ESI†) both the surface and cross-section of the membranes are
uniform, compact, smooth and without any visible pores. Fig. 5
shows the surface micrographs of the produced [PUVBIm][OH]
membrane. The hydrophilic/hydrophobic microphase separa-
14 300 g molꢁ1 and 1.81, respectively (Fig. S1, see ESI†). Based tion could be observed. The dark regions are generally attrib-
on the analysis of the 1HNMR and EDX, the average value of y,
uted to the hydrophilic domains containing 1-benzyl-2-methy-3-
z, n, was determined to be 26.4, 1.1 and 12.4, respectively
(Fig. 2).
butylimidazolium cation groups, whereas the brighter regions
are ascribed to the hydrophobic regions.44,45 In addition, the
membranes are soluble in polar aprotic solvents, such as DMF
and DMSO, while insoluble in water. These properties facilitate
The morphology of the prepared membranes in the OHꢁ
form was characterized by scanning electron microscopy (SEM)
Scheme 3 Synthetic procedure and photograph of the produced [PUVBIm][OH] membrane: (a) K2CO3, NMP, toluene; (b) poly(4-vinylbenzyl
chloride), K2CO3, DMF.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. A, 2014, 2, 4413–4421 | 4417

View Article Online
Journal of Materials Chemistry A
Paper
Fig. 3 FT-IR spectra of the produced membrane in (A) Clꢁ and (B) OHꢁ forms and (C) in OHꢁ form after immersion in 1 M KOH at 60 ꢂC for 20
days.
Fig.
4 Energy-dispersive X-ray (EDX) spectra for the produced
membrane in (A) Clꢁ and (B) OHꢁ forms.
Fig. 6 (A) TGA curves of the produced [PUVBIm][OH] membrane
under the nitrogen flow. Heating rate: 10 ꢂC minꢁ1, (B) isothermal TGA
thermograms of anion exchange membranes at 150 ꢂC under the
nitrogen flow.
Fig. 5 AFM tapping (A) phase image and (B) height image of the
produced [PUVBIm][OH] membrane. Each image frame is 2 mm ꢀ 2
sulfone)-g-poly(4-vinylbenzyl chloride) membrane was deter-
mined to be 186 ꢂC, 118 ꢂC, and 162 ꢂC, respectively, suggesting
the successful gra-onto reaction of the poly(arylene ether
sulfone).
Thermogravimetric analysis (TGA) is a key property for AEM
applications. Fig. 6A shows the typical TGA curves of the
membranes in the OHꢁ form, which were recorded from 30 to
600 ꢂC, under a nitrogen ow, at a heating rate of 10 ꢂC minꢁ1 to
mm.
the preparation of polymer impregnated electrodes and the
membrane electrode assembly.
Thermal stability
The thermal properties of the produced membranes were assess their short-term thermal stabilities. The membranes
investigated by DSC and TGA. Fig. S3 (see the ESI†) shows the show a less than 5% weight loss below 200 ꢂC, probably due to
DSC curves of the membranes synthesized. The glass transition the evaporation of absorbed water or solvent (DMF). The main-
(Tg) temperature of poly(arylene ether sulfone), poly(4-vinyl- chain degradation of polymers may take place at around 400 ꢂC,
benzyl chloride), and the corresponding poly(arylene ether which is comparable to that of the phenyltrimethylammonium
4418 | J. Mater. Chem. A, 2014, 2, 4413–4421
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry A
1 shows the values of IEC, water uptake, and swelling degree of
the produced AEMs. The experimental IEC value of the AEM
produced in this work is 1.65 mequiv gꢁ1. It is noteworthy that
under the same experimental conditions, the produced
membranes possess lower water uptake and swelling degree
along with comparable IEC than the cross-linked AEMs repor-
ted, which is likely attributed to the presence of aromatic
structures in the membranes.
Mechanical properties
Table 2 shows the mechanical properties of the produced dry
membranes in the OHꢁ form. The tensile strength at break of
the membrane is 24.14 M Pa, with a tensile modulus of 1106.24
M Pa, and elongation-at-break values of 14.08%. It should be
noted that the produced membranes in this study possess
higher tensile modulus and tensile strength, while a lower
elongation at break, when compared with these of the Naon-
117 membranes, suggesting the membranes produced in this
work are tough enough for potential use as the AEM materials.
Fig. 7 Temperature dependence of the hydroxide conductivity for
[PUVBIm][OH] membrane before and after immersion in N2 saturated 1
M KOH solution at 60 ꢂC for 20 days.
functionalized polysulfone,46 suggesting that the membranes
indeed confer a high thermal stability, far beyond the range of
interest for application in AEMFCs. The long-term thermal
stability of the membranes was assessed by isothermal TGA
analysis at 150 ꢂC, under the nitrogen ow. It can be seen from
Long-term alkaline stability
ꢂ
Fig. 6B that less than a 2% weight loss for 12 h at 150 C was
The alkaline stability of the produced membranes was investi-
gated by immersing the membranes in a 1 M KOH solution at 60
ꢂC, for 20 days and characterized by measuring the changes of
the conductivity, IEC, 1H NMR and FT-IR spectra of the
membranes. During the testing period, none of the membranes
was broken into small pieces, and all of the samples remained
in a good membrane form. There was no signicant conduc-
tivity loss observed for the [PUVBIm][OH] membrane before and
aer the immersion in a N2 saturated 1 M KOH solution at 60 ꢂC
for 20 days (Fig. 7). The IEC value is calculated to be 1.6 mequiv
gꢁ1, which is close to the initial value (1.65 mequiv gꢁ1). While
the IEC value of IM-PFEKS was reduced to 83% of its original
IEC at 60 ꢂC.48 It should also be noted that no new chemical
shis and peaks of the tested membrane were observed in both
1HNMR (Fig. 8) and FT-IR spectra (Fig. 3), respectively. All of the
results conrmed the highly stability of the AEMs synthesized
in this work.
observed, indicating that the membranes have good thermal
stability.
Hydroxide conductivity
The hydroxide conductivity is particularly important for AEMs
used in fuel cells. Fig. 7 shows the temperature dependence of
the hydroxide conductivity of the membranes in OHꢁ form. It
can be seen that there is a positive correlation between the
conductivity and temperature, probably due to the increase of
free volume in favour of ion transport and the mobility of
anions with rising temperature. It can be clearly seen that the
conductivity of the produced membranes in the temperature
range from 30 ꢂC to 80 ꢂC are all above 10ꢁ2 S cmꢁ1, which
fulls the basic conductivity requirement of AEMFCs.
Ion exchange capacity (IEC), water uptake and swelling ratio
The ion-exchange capacity (IEC), water uptake and swelling
degree are all important properties for AEMs. The IEC value of
the membranes reects the exchangeable groups in the poly-
Conclusions
mer, which plays a key role in determining the conductivity of In summary, soluble imidazolium functionalized AEMs were
AEMs. Water uptake and physical swelling are generally successfully synthesized via the ionization of poly(4-vinylbenzyl
considered to be essential factors which inuence the chloride)-gra-poly(arylene ether sulfone) gra copolymers, and
morphologic and mechanical stability of the membranes. Table followed by anion-exchange with hydroxide. The conductivities
Table 1 Ion-exchange capacity, water uptake, and swelling degree of [PUVBIm][OH] membrane
IEC/mequiv gꢁ1
Sample
Water uptake (%)
Swelling degree (%)
Experimental
Theoreticala
[PUVBIm][OH]
37.25 ꢄ 2.54
86.39
15.00
12.28 ꢄ 1.23
20.83
11.00
1.65 ꢄ 0.16
1.58
1.85
1.80
1.70
1.90
[PMVBIm][OH]47
PSQNOH-50 (ref. 39)
a
1
Calculated from H NMR and EDX spectra.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. A, 2014, 2, 4413–4421 | 4419

View Article Online
Journal of Materials Chemistry A
Paper
Table 2 Mechanical properties of the [PUVBIm][OH] membrane (dry state)
Sample
Tensile strength (MPa)
Tensile modulus (MPa)
Elongation at break (%)
[PUVBIm][OH]
Naon-117 (ref. 6)
24.14 ꢄ 2.26
21.11 ꢄ 1.22
1106.24 ꢄ 64.34
6.60 ꢄ 1.01
14.08 ꢄ 1.29
370.62 ꢄ 20.82
5 X. Li, Y. Yu, Q. Liu and Y. Meng, ACS Appl. Mater. Interfaces,
2012, 4, 3627–3635.
6 B. Lin, L. Qiu, B. Qiu, Y. Peng and F. Yan, Macromolecules,
2011, 44, 9642–9649.
7 R. Borup, J. Meyers, B. Pivovar, Y. S. Kim, R. Mukundan,
N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood,
P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella,
J. E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota,
Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto,
K. Yasuda, K.-i. Kimijima and N. Iwashita, Chem. Rev.,
2007, 107, 3904–3951.
8 M. Tanaka, K. Fukasawa, E. Nishino, S. Yamaguchi,
K. Yamada, H. Tanaka, B. Bae, K. Miyatake and
M. Watanabe, J. Am. Chem. Soc., 2011, 133, 10646–
10654.
Fig. 8 1HNMR spectra of the ionomer membranes before and after
immersion in 1 M KOH solution at 60 ꢂC for 20 days.
9 S. Gu, R. Cai and Y. Yan, Chem. Commun., 2011, 47, 2856–
2858.
10 M. R. Hibbs, M. A. Hickner, T. M. Alam, S. K. McIntyre,
C. H. Fujimoto and C. J. Cornelius, Chem. Mater., 2008, 20,
2566–2573.
11 M. Tanaka, M. Koike, K. Miyatake and M. Watanabe, Polym.
Chem., 2011, 2, 99–106.
12 G. Wang, Y. Weng, D. Chu, D. Xie and R. Chen, J. Membr. Sci.,
2009, 326, 4–8.
13 M. Zhang, H. K. Kim, E. Chalkova, F. Mark, S. N. Lvov and
T. C. M. Chung, Macromolecules, 2011, 44, 5937–5946.
14 Y. Ye and Y. A. Elabd, Macromolecules, 2011, 44, 8494–
8503.
of produced membranes at the temperature range from 30 ꢂC to
80 ꢂC are all above 10ꢁ2 S cmꢁ1, which fulls the basic
conductivity requirement of AEMFCs. The yielded membranes
showed a relatively good alkaline stability in a 1 M KOH
aqueous solution at 60 ꢂC, as well as high dimensional and
thermal stability, demonstrating the potential practical appli-
cation in alkaline anion exchange membranes fuel cells. The
results of this work suggested that it is possible to optimize the
membrane performance by adopting a varied backbone and
cation group structures, which should pave the way for high
performance AEM design and fabrication.
15 S. Chempath, B. R. Einsla, L. R. Pratt, C. S. Macomber,
J. M. Boncella, J. A. Rau and B. S. Pivovar, J. Phys. Chem. C,
2008, 112, 3179–3182.
16 N. Li, M. D. Guiver and W. H. Binder, ChemSusChem, 2013, 6,
1376–1383.
17 B. Zhang, S. Gu, J. Wang, Y. Liu, A. M. Herring and Y. Yan,
RSC Adv., 2012, 2, 12683–12685.
18 D. S. Kim, A. Labouriau, M. D. Guiver and Y. S. Kim, Chem.
Mater., 2011, 23, 3795–3797.
Acknowledgements
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 (BK2011274), the Research
Fund for the Doctoral Program of Higher Education
(20103201110003), and the Project Funded by the Priority
Academic Program Development of Jiangsu Higher Education
Institutions.
19 Q. Zhang, S. Li and S. Zhang, Chem. Commun., 2010, 46,
7495–7497.
20 K. J. T. Noonan, K. M. Hugar, H. A. Kostalik, E. B. Lobkovsky,
˜
H. D. Abruna and G. W. Coates, J. Am. Chem. Soc., 2012, 134,
Notes and references
18161–18164.
1 J. Ni, C. Zhao, G. Zhang, Y. Zhang, J. Wang, W. Ma, Z. Liu and 21 S. Gu, R. Cai, T. Luo, Z. Chen, M. Sun, Y. Liu, G. He and
H. Na, Chem. Commun., 2011, 47, 8943–8945.
2 B. C. H. Steele and A. Heinzel, Nature, 2001, 414, 345–352.
3 Z. Si, F. Gu, J. Guo and F. Yan, J. Polym. Sci., Part B: Polym.
Phys., 2013, 51, 1311–1317.
Y. Yan, Angew. Chem., Int. Ed., 2009, 48, 6499–6502.
22 B. Lin, L. Qiu, J. Lu and F. Yan, Chem. Mater., 2010, 22, 6718–
6725.
23 W. Li, J. Fang, M. Lv, C. Chen, X. Chi, Y. Yang and Y. Zhang,
J. Mater. Chem., 2011, 21, 11340–11346.
4 T. J. Clark, N. J. Robertson, H. A. Kostalik Iv, E. B. Lobkovsky,
˜
P. F. Mutolo, H. c. D. Abruna and G. W. Coates, J. Am. Chem. 24 F. Zhang, H. Zhang and C. Qu, J. Mater. Chem., 2011, 21,
Soc., 2009, 131, 12888–12889.
12744–12752.
4420 | J. Mater. Chem. A, 2014, 2, 4413–4421
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry A
25 X. Lin, X. Liang, S. D. Poynton, J. R. Varcoe, A. L. Ong, J. Ran, 36 B. Qiu, B. Lin, Z. Si, L. Qiu, F. Chu, J. Zhao and F. Yan, J.
Y. Li, Q. Li and T. Xu, J. Membr. Sci., 2013, 443, 193–200. Power Sources, 2012, 217, 329–335.
26 O. D. Thomas, K. J. W. Y. Soo, T. J. Peckham, M. P. Kulkarni 37 J. Pan, C. Chen, L. Zhuang and J. Lu, Acc. Chem. Res., 2012,
and S. Holdcro, J. Am. Chem. Soc., 2012, 134, 10753–10756. 45, 473–481.
27 X. Kong, K. Wadhwa, J. G. Verkade and K. Schmidt-Rohr, 38 H. A. Kostalik, T. J. Clark, N. J. Robertson, P. F. Mutolo,
˜
Macromolecules, 2009, 42, 1659–1664.
J. M. Longo, H. c. D. Abruna and G. W. Coates,
28 Y. Zha, M. L. Disabb-Miller, Z. D. Johnson, M. A. Hickner and
G. N. Tew, J. Am. Chem. Soc., 2012, 134, 4493–4496.
29 O. D. Thomas, K. Soo, T. J. Peckham, M. P. Kulkarni and
S. Holdcro, Polym. Chem., 2011, 2, 1641–1643.
Macromolecules, 2010, 43, 7147–7150.
39 J. Wang, Z. Zhao, F. Gong, S. Li and S. Zhang,
Macromolecules, 2009, 42, 8711–8717.
40 F. Wang, M. Hickner, Y. S. Kim, T. A. Zawodzinski and
J. E. McGrath, J. Membr. Sci., 2002, 197, 231–242.
30 W. H. Awad, J. W. Gilman, M. Nyden, R. H. Harris Jr,
T. E. Sutto, J. Callahan, P. C. Trulove, H. C. DeLong and 41 H. K. Kim, M. Zhang, X. Yuan, S. N. Lvov and T. C. M. Chung,
D. M. Fox, Thermochim. Acta, 2004, 409, 3–11. Macromolecules, 2012, 45, 2460–2470.
31 Y. Ye and Y. A. Elabd, in Polymers for Energy Storage and 42 S. A. Nunez and M. A. Hickner, ACS Macro Lett., 2013, 2, 49–
˜
Delivery: Polyelectrolytes for Batteries and Fuel Cells,
American Chemical Society, 2012, pp. 233–251.
32 O. M. M. Page, S. D. Poynton, S. Murphy, A. Lien Ong,
52.
43 N. Li, Y. Leng, M. A. Hickner and C.-Y. Wang, J. Am. Chem.
Soc., 2013, 135, 10124–10133.
D. M. Hillman, C. A. Hancock, M. G. Hale, D. C. Apperley 44 X. Lin, J. R. Varcoe, S. D. Poynton, X. Liang, A. L. Ong, J. Ran,
and J. R. Varcoe, RSC Adv., 2013, 3, 579–587. Y. Li and T. Xu, J. Mater. Chem. A, 2013, 1, 7262–7269.
33 O. I. Deavin, S. Murphy, A. L. Ong, S. D. Poynton, R. Zeng, 45 J. Ran, L. Wu and T. Xu, Polym. Chem., 2013, 4, 4612–4620.
H. Herman and J. R. Varcoe, Energy Environ. Sci., 2012, 5, 46 N. Li, Q. Zhang, C. Wang, Y. M. Lee and M. D. Guiver,
8584–8597.
34 B. Lin, H. Dong, Y. Li, Z. Si, F. Gu and F. Yan, Chem. Mater., 47 B. Qiu, B. Lin, L. Qiu and F. Yan, J. Mater. Chem., 2012, 22,
2013, 25, 1858–1867. 1040–1045.
35 J. Wang, S. Gu, R. B. Kaspar, B. Zhang and Y. Yan, 48 D. Chen and M. A. Hickner, ACS Appl. Mater. Interfaces, 2012,
Macromolecules, 2012, 45, 2411–2419.
ChemSusChem, 2013, 6, 2079–2082.
4, 5775–5781.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. A, 2014, 2, 4413–4421 | 4421