Anion exchange membranes by bromination of benzylmethyl-containing poly(fluorene ether sulfone)s

Article abstract of DOI:10.1039/c4ra03381g

Fluorene-containing anion exchange membranes were synthesized via the bromination reaction of poly(sulfone)s derived from 9,9-bis(3,5-dimethyl-4- hydroxyphenyl)fluorene (DMHPF), 4-fluorophenyl sulfone and 4,4′-biphenol, quaternization reaction using trime

Full text of DOI:10.1039/c4ra03381g

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Anion exchange membranes by bromination of
benzylmethyl-containing poly(fluorene ether
sulfone)s
Cite this: RSC Adv., 2014, 4, 27502
a
b
a
a
Wenhai Mei, Changli Lu, Jingling Yan* and Zhen Wang*
¨
Fluorene-containing anion exchange membranes were synthesized via the bromination reaction of
poly(sulfone)s derived from 9,9-bis(3,5-dimethyl-4-hydroxyphenyl)fluorene (DMHPF), 4-fluorophenyl
0
sulfone and 4,4 -biphenol, quaternization reaction using trimethylamine, and ion exchange; then the
properties of these AEMs were fully characterized, and the structure–property relationship regarding this
series of AEMs was elucidated. Brominated poly(fluorene ether sulfone)s (BrPFES) were characterized
1
using proton nuclear magnetic resonance ( H NMR), and the bromination conversion, degree of
functionalization (DF), and conversion of benzylmethyl groups were calculated. BrPFES was then
quaternized by heterogeneous amination using trimethylamine, and then converted to quaternary
ammonium bicarbonate by ion exchange. The properties of these AEMs were studied in terms of water
uptakes, conductivities, swelling ratios, and mechanical properties. Compared to their homopolymer
counterparts with similar ion exchange capacities (IEC), AEMs based on copolymers showed slightly
lower conductivities but much lower water uptakes and swelling ratios, which can be explained by the
fact that the continuation of hydrophilic domains in copolymers was interrupted by the incorporation of
Received 14th April 2014
Accepted 2nd June 2014
DOI: 10.1039/c4ra03381g
0
www.rsc.org/advances
hydrophobic 4,4 -biphenol segments.
high conductivity, good mechanical robustness, and long-term
durability. A variety of quaternary ammonium-functionalized
1
. Introduction
Fuel cells are considered promising energy conversion devices polymers have been investigated as AEM materials or the cata-
7,8
9–11
for both stationary and mobile applications. Proton exchange lyst layer ionomers, including poly(styrene), poly(sulfone),
12,13
14,15
16
membrane fuel cells (PEMFCs) have been the predominant fuel poly(imide),
poly(phenlyene oxide),
poly(norbornene),
17
cell technologies in the last two decades due to their high power poly(phenylene). In most studies, halomethyl groups were
density, high energy efficiency, low operation temperature, and introduced into the polymer backbone through either chlor-
1
–3
environmental friendliness.
However, PEMFCs are still omethylation of aromatic rings or bromination of benzylmethyl
suffering from several drawbacks such as the dependence of groups, followed by the quaternization by a tertiary amine to
precious metal catalyst (e.g. platinum) and sluggish oxygen form a pendent cation. Chloromethylation is a well-established,
reduction kinetics, which prevents their widespread facile approach to creating anion exchange groups in the
commercialization.
aromatic polymers. However, there are several undesired char-
Recently, anion exchange membrane fuel cells (AEMFCs) acteristics associated with chloromethylation, such as the
have been receiving considerable attentions from both involvement of excess highly toxic reagent, long reaction time,
18
academic and industrial communities. AEMFC can offer and low selectivity. Compared to chloromethylation, boromi-
potential advantages over PEMFC, including improved oxygen nation of benzymethly-containing polymers showed faster
reduction kinetics, the applicability of non-precious metal reaction rates (several hours versus several days), high efficiency
4–6
catalyst (e.g. nickel and cobalt), and fuel exibility. High (up to 90% based on the conversion of bromine in NBS to
performance anion exchange membranes (AEMs) are the key bromine in bromomethyl group), and high selectivity which
components in the development of AEMFCs. Since the bromine and thus quaternary ammonium groups can be
commercially available sources of AEM materials are limited, directed to specic locations. This process has been well
18,19
many research efforts have been made to develop AEMs with demonstrated by Hickner, Hibbs, and other researchers.
The incorporation of uorenyl groups into ion conductive
polymers offers many desired characteristics such as higher ion
conductivity, and improved chemical, thermal and mechanical
durability. Miyatake and coworkers reported on a series of u-
orene-containing, sulfonated polyimides and poly(acrylene
a
Laboratory of Polymer Composites Engineering, Changchun Institute of Applied
Chemistry, Chinese Academy of Science, Changchun, 130022, China. E-mail: jyan@
ciac.ac.cn; Fax: +86-431-85262682; Tel: +86-431-85262682
b
Institute of Chemistry, Northeast Normal University, Changchun, 130024, China
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20
ether)s and evaluated their properties as PEM materials. They were puried by recrystallization from ethanol. All other
also demonstrated that these polymers are durable for 5000 chemicals were of analytical grade, and used as received unless
hours under fuel cell operating conditions with a mediate power specied.
density. With respect to AEMs, uorene-containing polymers
were also extensively investigated since 2010. Miyatake et al.
successfully synthesized uorene-containing AEMs by the
chloromethylation of multi-block uorene-containing poly-
2

.2 Synthesis of 9,9-bis(3,5-dimethyl-4-hydroxyphenyl)-
uorene (DMHPF)
,9-Bis(3,5-dimethyl-4-hydroxyphenyl)uorene (DMHPF) was
9
(
acrylene ether)s using chloromethyl methyl ether, and then the
26
21
prepared according to a procedure in the literature, yield:
quaternization of chloromethyl group with trimethylamine.
1
5
7
6
6%. H NMR (400 MHz, DMSO-d ) d (ppm): 8.08 (s, 2H), 7.86–
.84 (d, 2H), 7.37–7.25 (m, 6H), 6.92 (s, 4H), 2.01 (s, 12H).
The ionic conductivity was improved by phase separation
caused by the multi-block architecture with highly hydrophilic
segments. Furthermore, these membranes showed reasonable
performance in a noble metal-free alkaline fuel cell fuelled by
2.3 Synthesis of 9,9-bis(3,5-dimethyl-4-hydroxyphenyl)-
22
hydrazine. Other studies by Miyatake and Kim also reported
uroene-containg, ammonium-functionalized poly(ether
sulfone)s, and conrmed that the introduction of uorene

uorene-based poly(sulfone)s

As depicted in Scheme 1, DMHPF-based poly(sulfone)s were
prepared according to a literature procedure based on a weak
base route. The molar percentages of DMHPF in the total of
bisphenols were 100%, 80%, and 60%. Thus, the benzylmethyl-
bearing poly(sulfone)s were named by these ratios, e.g. PFES-80
23,24
groups is efficient to improve AEM properties.
Meng and
28
coworkers prepared uorene-containing AEMs by the polymer-
ization of tertiary amine-functionalized bisphenol and decaf-
25
luorobiphenyl, followed by the quaterization by iodomethane.
0
was made from a mixture of DMHPF (80%) and 4,4 -biphenol
Ion exchange capacity (IEC) can be controlled by adjusting the
feeding ratio of amine-functionalized monomer and unfunc-
tionalized monomer. Chen and Hickner synthesized uorene-
containing AEMs by bromination of benzylmethyl containing
poly(ether ether ketone), and investigated the effect of different
(20%). A typical procedure for poly(sulfone) synthesis follows.
9,9-Bis(3,5-dimethyl-4-hydroxyphenyl)uorene (1.626 g, 0.004
0
mol), 4-uorophenyl sulfone (1.671 g, 0.005 mol), 4,4 -biphenol
(
0
(
0.186 g, 0.001 mol), anhydrous potassium carbonate (0.800 g,
.006 mol), 1-methyl-2-pyrrolidinone (15 mL), and toluene
25 mL) were placed into a 100 mL, three-necked, round-
26
ammoniums on the stability of the resulting AEMs. The results
indicated that quaternary ammonium was more stable than
imidazolium in NaOH solution at elevated temperatures. More
recently, Liu and coworkers reported uorene-containing AEMs
by bromination of benzylmethyl-containing poly(sulfone)s, fol-
lowed by quaternization using N,N,N',N'-tetramethyl-1,6-dia-
bottomed ask equipped with a mechanical stirrer, a Dean–
Stark trap, and gas inlet and outlet. The mixture was heated to
ꢀ
140 C under an argon atmosphere. The water was azeotropi-
cally depleted from the solution. Aer 6 hours, toluene was
removed from the system when the temperature was raised to
27
minohexane. In general, most of uorene-containing AEMs
were prepared via chloromethylation of aromatic polymers,
which lacks precise control of IEC and distribution of ionic
groups along the polymer chain.
ꢀ
ꢀ
180 C. Then the reaction mixture was held at 180 C until the
Herein, we report the synthesis and characterization of AEMs
by bromination of benzymethyl-containing poly(uorenyl ether
sulfone)s. Poly(sulfone)s with a variety of benzylmethyl content
were synthesized by polycondensation of 4-uorophenyl
0
sulfone, 4,4 -biphenol, and 9,9-bis(3,5-dimethyl-4-hydroxy-
phenyl)uorene (DMHPF). Benzyltrimethyl ammonium func-
tionality was then introduced by bromination of benzylmethyl
group, followed by amination using trimethylamine. The
properties of uorene-containing AEMs were varied by adjust-
0
ing the ratio of 4,4 -biphenol and DMHPF, as well as the amount
of brominating reagent. The properties of these AEMs were fully
characterized, and the structure–property relationship for this
series of AEMs was elucidated.
2. Experimental
2.1 Materials
9

-Fluorenone, 2,6-dimethylphenol, mercaptopropionic acid, 4-
uorophenyl sulfone, 4,4 -biphenol, N-bromosuccinimide
0
(NBS) and benzoyl peroxide (BPO) were purchased from J&K
Chemical. All other chemicals were purchased from Sigma-
Aldrich Corporation. 4-Fluorophenyl sulfone and 4,4 -biphenol membranes.
Scheme
1
Synthetic routes to PFES-based anion exchange
0
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viscosity of the solution did not increase further. Aer cooled to
room temperature, the mixture was poured into 200 mL
deionized water. The resulting brous polymer was Soxhlet
ꢀ
extracted with water for 24 hours and dried at 100 C in vacuum
for 24 hours to afford an off-white solid (3.119 g, 95%).
2.4 Bromination of 9,9-bis(3,5-dimethyl-4-hydroxyphenyl)-

uorene-based poly(sulfone)s
The bromination of DMHPF-based poly(sulfone)s was per-
formed in 1,1,2,2-tetrachloroethane using N-bromosuccinimide
(
NBS) as the bromine source and benzyol peroxide (BPO) as the
initiator. A typical process follows. 1.0 g of polymer and 20 mL of
,1,2,2-tetrachloroethane were introduced into a 100 mL, three-
1
necked, round-bottomed ask equipped with a condenser, a
1
Fig. 1 The H NMR spectra of PFES-100 and BrPFES-100.
magnetic stirrer, and gas inlet and outlet. The mixture was
ꢀ
heated to 85 C in an argon atmosphere. Once the polymer
completely dissolved, NBS (0.356 g, 0.002 mol) and BPO (0.0242
g, 0.0001 mol) were added. The reaction was held at 85 C for 5
standard. The degree of functionalization of brominated poly-
ꢀ
(
sulfone)s was determined from the integral ratio of the
hours. Aer cooled to room temperature, the mixture was
poured into 100 mL of methanol. Then the crude product was
collected by ltration, Soxhlet extracted with ethanol for 24
0
brominated benzyl peaks (peak 9 in Fig. 1) to unbromonated
benzyl peaks (peak 8 in Fig. 1). Ion exchange capacities (IEC)
were calculated from the ratio of the H NMR peaks from the
0
1
ꢀ
hours, and dried in vacuum at 80 C to afford a yellow solid (1.09
methyl protons in the quaternary ammonium moiety (peak 2 in
Fig. 2) and the sum of signals form aromatic protons. Thermal
stability was evaluated using a Perkin-Elmer TGA 2 analysis
g, 94%). The brominated polymers were named aer the
composition of their poly(sulfone) precursor and the millimoles
of NBS per 1 gram of poly(sulfone), e.g. BrPFES-80-3 were the
product of brominating PFES-80 with 3 millimole of NBS per 1
gram of poly(sulfone).
ꢀ
ꢁ1
system with a heating rate of 10 C min from room temper-
ꢀ
ature to 700 C under a continuous nitrogen ow. Molecular
weights of poly(sulfone)s were measured using a PL-GPC 120 gel
permeation chromatography. THF was used as the eluent, and
molecular weights were determined by calibration with poly-
2.5 Membrane preparation
Anion exchange membranes were prepared by the heteroge-
neous amination of brominated poly(sulfone)s using trime-
thylamine aqueous solution. A solution of brominated polymer
in chloroform (10 wt%) was ltrated to remove insoluble
impurities, and cast onto a at glass plate. Aer the solvent was
removed by evaporation at room temperature for 12 hours, the
(
styrene) standard.
The water uptakes of AEMs were obtained according to
18
literature methods. AEMs were equilibrated in DI water at
different temperatures for 24 hours before subjected to the
measurements. The fully hydrated anion exchange membranes
were taken out of water, blotted quickly to remove surface water,
and weighed immediately. The membranes were then dried at
5
ꢀ
membrane was then dried at 60 C for 6 hours to deplete
chloroform. The dry membranes were then immersed into a
ꢀ
0 C in vacuum for 24 hours and weighed again. The water
33% trimethylamine aqueous solution for 48 hours at room
uptakes were calculated by the following equation:
temperature. The aminated membranes were soaked in 1 M
KOH for 48 hours, and washed completely with deionized water
until the pH value of the water remained about 6–7. Hydroxide
W
wet ꢁ Wdry
WUð%Þ ¼
ꢂ 100%
W
dry
is very sensitive to CO due to the rapid conversion from where Wwet and Wdry were the hydrated mass and dry mass of
2
2
ꢁ
ꢁ
hydroxide to CO3 /HCO3 , which makes the measurement of the membranes, respectively.
water uptake and conductivity in hydroxide form very difficult
Swelling ratios of AEMs were evaluated by measuring the
Thus, all the membranes were converted dimensional difference in in-plane direction between the dry
18,29,30
and less reliable.
to bicarbonate form by exposure to CO
18
2
in air for 50 hours. The membranes and fully hydrated membranes. The swelling ratio
quaternized polymers were named by their brominated was calculated by the following equation:
precursors, e.g. QPFES-80-3 was the quaternized product of
BrPFES-80-3. All the membranes were stored in DI water before
characterizations.
L
wet ꢁ Ldry
SWð%Þ ¼
ꢂ 100%
L
dry
where Lwet, Ldry, are the lengths of the wet and dry membranes,
respectively.
2.6 Characterizations
1
The membrane resistances were measured by four-probe
H NMR spectra were obtained on a Bruker-400 spectrometer.
electrochemical impedance spectroscopy (EIS) in DI water at 23,
The solvent for monomer and neutral polymers was CDCl -d ,
and DMSO-d
polymers. Tetramethylsilane (TMS) was used as an internal
3
1
ꢀ
4
0, 60, 80 C using a CHI660D electrochemical workstation. EIS
6
was used for quaternary ammonium-bearing
were performed by applying an alternating voltage of 10 mV in a
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a
Table 1 The molecular weights of plain poly(fluorenyl ether sulfone)s
ꢁ1
ꢁ1
Polymer
M
n
(kg mol
)
M
w
(kg mol
)
PDI
PFES-100
PFES-80
PFES-60
80
79
117
152
143
212
1.85
1.81
1.81
a
Measured by gel permeation chromatography (GPC) using polystyrene
as a standard.
NMR spectra of plain PFES and brominated FPES (BrPFES) were
compared in Fig. 1. Bromination resulted in the decrease of
1
Fig. 2 The H NMR spectrum of QPFES-100-3.
0
benzylmethyl proton signals at around 2.0 ppm (peak 8 in
0
Fig. 1), and instead new proton signals (peak 9 in Fig. 1) for
bromomethyl groups appeared in around 4.4 ppm. Due to the
steric hindrance caused by uorenyl segments, the two bro-
momethyl groups are congurationally different from each
other. Therefore, two peaks appeared around 4.4 ppm, which
frequency range of 0.1–1 ꢂ 105 Hz. The bicarbonate conduc-
tivities were calculated by using the following equation:
L
s ¼
26
was also observed by Hickner et al. Meanwhile, the signals for
R ꢂ S
aromatic proton adjacent to bromomethyl group shied from
about 6.8 ppm in plain PFES to around 7.2 ppm for BrPFES.
Furthermore, the relative intensities for bromomethyl proton
signals increased with the increasing amount of NBS. Degree of
functionalization (DF) was calculated from the integral ratios
between unreacted benzylmethyl groups and bromomethyl
groups. DF of BrPFES spanned a range of 0.91–2.24. DF for
BrPFES polymers could be controlled by the feeding amount of
NBS. Furthermore, the distribution of bromomethyl groups
where L is the distance between the two electrodes, R is the
resistance of the membrane, and S is the conducting area. The
activation energies for bicarbonate conduction using an
Arrhenius relationship were determined according to a method
18
in the literature.
Tensile measurements were performed on an Instron 5982
system at room temperature with a stretching speed of 20 mm
ꢁ
1
min . Membrane samples were cut into rectangular shape
with a test area of 20 ꢂ 5 mm. For the measurements in wet
state, the fully hydrated samples were taken out from DI water,
swept with tissues to remove the surface water, and immediately
subjected to the tests. For measurements in dry state, the
0
could be tuned by varying the ratios of 4,4 -biphenol. The
0
brominated polymers with high 4,4 -biphenol content had more
clustered bromomethyl groups for a given DF. As summarized
in Table 2, bromination conversion, determined from the
feeding bromine in NBS and the resulting bromine in BrPFES,
was higher than 90% when the benzylmethyl conversion was
low. Generally, the bromination conversion decreased with the
increase of NBS amount, which can be attributed to the
decreasing benzylmethyl contents. However, the bromination
conversion was higher than 70% even if high feeding ratio of
NBS was applied. Benzylmethyl conversion was lower for
BrPFES-100 polymers due to their high benzylmethyl contents,
while the conversion for BrPFES-60 was much higher since
much less benzylmethyl groups were available for bromination.
ꢀ
samples were dried at 80 C for 12 hours in vacuum before tests.
Five samples were measured for each membrane, and the
averages were reported.
3. Results and discussion
3.1 Synthesis of polymers
The benzylmethyl-containing bisphenol, 9,9-bis(3,5-dimethyl-4-
hydroxyphenyl)uorene (DMHPF), was prepared according to a
26
reported procedure, using 9-uorenone and 2,6-dimethyl-
phenol as staring materials, and mercaptopropionic acid and
concentrated sulfuric acid as the catalysts. As depicted in
Scheme 1, benzylmethyl-containing poly(uorenyl ether
sulfone)s (PFES) were synthesized by polycondensing 4-uo-
0
Since 4,4 -biphenol residue can't be brominated by NBS, the
bromomethyl and quaternary ammonium groups can be
selectively directed to DMHPF residue. In contrast, only
randomly functionalized polymers can be produced via chlor-
omethylation unless a major structural modication was
rophenyl sulfone with varied molar ratios (1 : 0, 0.8 : 0.2, and
18,31
0
applied to the polymer backbones.
These results demon-
0
.6 : 0.4) of DMHPF and 4,4 -biphenol. The molecular weights
strated the high efficiency and selectivity of bromination of
benzylmethyl groups over the chloromethylation of aromatic
rings in the introduction of halomethyl groups into aromatic
polymers for anion exchange membranes.
of PFES were measured by gel permeation chromatography
(
GPC), and the results are listed in Table 1. PFES showed high
ꢁ1
w
molecular weight (M > 143 kg mol ) and reasonable poly-
dispersity index, which conrmed the successful synthesis of
PFES.
BrFPES was then quaternized by soaking the membranes in
trimethylamine aqueous solution for 48 hours. Due to the
limited solubility in polar aprotic solvents (such as NMP), the
homogeneous amination of BrPFES was unsuccessful. As shown
Bromination of the benzylmethyl groups in PFES was then
ꢀ
performed in 1,1,2,2-tetrachloroethane at 85 C. The reaction
proceeded via a free radical mechanism with NBS as the
1
1
in Fig. 2, the peak near 3.1 ppm in the H NMR spectrum of
bromine source and BPO as the initiator. The representative H
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Table 2 The bromination conversion, degree of functionalization and quaternary ammonium groups. The third step of weight loss
benzylmethyl conversion of brominated PFES
ꢀ
was above 380 C and assignable to the decomposition of poly-
sulfone) backbone.
(
Bromination
Degree of
Benzylmethyl
a
b
c
d
Polymer
conversion (%) functionalization conversion (%)
3
.3 IEC, water uptake and swelling ratio
BrPFES-100-2 94
BrPFES-100-3 99
BrPFES-100-4 85
1.17
1.86
2.10
1.12
1.59
1.73
2.24
0.91
1.50
1.72
1.82
29
46
52
35
50
54
70
38
62
72
76
18,29,30
ꢁ
As suggested by previous reports,
converted to CO
water and air. Consequently, the measurements of water uptake
and conductivity in OH form are difficult and less reliable.
Therefore, all the membranes were converted to bicarbonate
form by exposure to CO
bonate is not sensitive to CO
uptake as a function of IEC is plotted in Fig. 4. As expected, the
water uptakes of PFES-based AEMs increased with the increase
of IEC in a non-monotonic style. For AEMs based QPFES-100
and QPFES-80, water uptake increased rapidly when IEC was
OH can be rapidly
2ꢁ
ꢁ
3
/HCO
3
due to the absorption of CO in
2
BrPFES-80-2
BrPFES-80-3
BrPFES-80-4
BrPFES-80-5
BrPFES-60-2
BrPFES-60-3
BrPFES-60-4
BrPFES-60-5
97
92
76
78
86
95
82
70
ꢁ
2
since quaternary ammonium bicar-
in water or air. The bulk water
2
a
The rst and second numbers stand for the molar percentage of
DMHPF in the total of bisphenols, and the millimolar equivalents of
NBS used per gram PFES, respectively. Calculated from the bromine
b
1
in the feeding NBS and the resulting BrPFES determined by H NMR. higher than 2.2 meq per g, which can be regarded as a critical
c
1
Number of bromomethyl groups per repeat unit by H NMR.
Calculated by H NMR.
ion content to achieve good connectivity between hydrophilic
domains. There was no clear critical IEC observed for water
uptakes for QPFES-60 AEMs. Due to the high content of
hydrophobic domains, it was difficult to get adequate connec-
tion between hydrophilic domains even when IEC was high. For
a given IEC, the water uptake of three series of AEMs followed
the trend of QPFES-100 > QPFES-80 > QPFES-60, which can be
explained in terms of the more isolated hydrophilic domains in
copolymers. The swelling ratios of QPFES AEMs were also
measured in the in-plane direction at room temperature.
Generally, the swelling ratios of QPFES showed similar behav-
iour to that of bulk water uptake. QPFES-60 AEMs showed the
best dimensional stability for a given IEC, which is desired for
fuel cell applications.
The water uptakes for QPFES-based AEMs were measured at
different temperatures, and the results are listed in Table 3.
Fig. 5 shows the temperature dependence of water uptakes for
QPFES-60-based AEMs. The water uptakes for all AEMs
increased linearly with the increasing temperatures. The slopes
strongly correlated with IEC and the hydrophobicity of the
backbones. The high fraction of hydrophobic segments and low
ion exchange content suppressed the swelling of AEMs at high
d
1
quaternized polymers (peak 2 in Fig. 2) were assigned to the
methyl protons of in the quaternary ammonium groups. IEC
was calculated from the integral of this peak to the sum of the
aromatic peaks.
3.2 Thermal stability
The thermal stability of quaternized PFES (QPFES) and their
precursors, PFES and BrPFES were assessed by thermogravi-
metric analysis (TGA) in nitrogen atmosphere. As shown in
Fig. 3, there was no detectable weight loss below 380 C for
PFES. BrPFES showed two-stage weight loss prole in TGA
curve. The weight loss between 120 and 350 C were assigned to
the decomposition of bromomethyl groups. Then the degrada-
tion of polymer backbone happened when the temperature was
higher than 400 C. As for QPFES, the initial weight loss
between room temperature and 270 C was attributed to the
ꢀ
ꢀ
ꢀ
ꢀ
evaporation of absorbed water in ionic polymers. The rapid
weight loss at around 270 C was due to the elimination of the
ꢀ
Fig. 4 Water uptakes as a function of ion exchange capacity (IEC) for
Fig. 3 TGA traces of PFES-80, BrPFES-80-3 and QPFES-80-3.
PFES-based AEMs.
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Table 3 Properties of QPFES-based AEMs
c
ꢁ1
Water uptake (%)
s
bicarbonate (mS cm
)
Swelling ratio
(%)
a
b
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
d
ꢁ1
Membrane
IEC
23 C
40 C
60 C
23 C
40 C
60 C
80 C
s
hydroxide (mS cm
)
QPFES-100-2
QPFES-100-3
QPFES-100-4
QPFES-80-2
QPFES-80-3
QPFES-80-4
QPFES-80-5
QPFES-60-2
QPFES-60-3
QPFES-60-4
QPFES-60-5
1.54
2.21
2.41
1.65
2.19
2.35
2.84
1.53
2.12
2.64
2.77
49
61
110
35
54
87
268
32
50
75
86
59
72
117
37
57
98
296
36
56
89
96
61
76
143
43
10
18
31
9
15
29f
—
6
4.2
6.5
14.9
2.6
6.6
11.1
20.8
3.1
9.4
17.5
29.7
4.6
11.0
24.6
31.1
4.4
12.0
26.1
46.9
7.0
21.1
36.1
68.7
6.9
16.0
24.7
56.6
9.9
21.3
41.0
55.9
6.8
64
5.6
8.3
121
10.8
14.7
1.8
4.8
8.9
15.5
21.9
2.9
5.5
13.1
16.8
e
—
38
59
103
135
15
26f
—
10.0
20.6
23.2
15.8
32.1
35.8
18.2
33.8
43.7
11.5
a
The rst and second numbers stand for the molar percentage of DMHPF in the total of bisphenols, and the millimolar equivalents of NBS used per
b
1
c
ꢀ
d
gram PFES, respectively. Calculated from H NMR. Measured in DI water at 23, 40, 60, and 80 C. Estimated from bicarbonate conductivity
according to the method in the literature. Not measured because this membrane is too brittle at 60 C. Not measured because these
membranes are too brittle in dry state.
e
ꢀ
f
temperatures, which led to lower increasing rate of water were randomly distributed along the polymer backbone in
uptakes.
homo-polymers. With the increasing IEC, the hydrophilic
domains became more and more connected. Consequently,
the conductivity and water uptake simultaneously increased.
3.4 Conductivity
0
In copolymers, 4,4 -biphenol residue can't be functionalized
The conductivities of QPFES-based AEMs were measured at
various temperatures in bicarbonate form. For a given poly-
mer backbone, the conductivity increased with increasing
IEC. For AEMs with similar IEC but different backbone
composition, the bicarbonate conductivity followed the trend
of QPFES-100 > QPFES-80 > QPFES-60. Generally, the trend of
conductivity for all AEMs was similar to that of water uptake,
which indicated that the bicarbonate conduction in these
AEMs was strongly correlated to the bulk water uptakes. This
observation agreed with the results in the literature. AEM
QPFES-100-4 shows the highest conductivity among all the
AEMs in this study. Meanwhile, their water uptakes were also
higher than other AEMs. QPFES-60-5 and QPFES-80-4 exhibi-
ted a combination of reasonably high conductivities and
moderate water uptakes. The quaternary ammonium groups
over bromination and amination, and the quaternary
ammonium groups were selectively located in the DMHPF
segments. Therefore, 4,4 -biphenol residue acted as hydro-
0
phobic segments in copolymers. The continuation of hydro-
philic segments with quaternary ammonium groups was
0
interrupted by the incorporation of hydrophobic 4,4 -biphe-
nol segments, which suppressed the swelling of AEMs.
Meanwhile, the conductivity was still kept at a reasonable
level due to the higher ionic concentration, for a given IEC.
The properties of these AEMs could be tuned over a wide
range of water uptakes and conductivities by varying IEC and
the content of hydrophobic domains. AEMs based on copol-
ymers are more suitable for alkaline fuel cell applications
considering their reasonably high conductivities and
moderate water uptakes (Fig. 6).
1
8
Fig. 5 Water uptakes of AEMs based on QPFES-60 as a function of Fig. 6 Bicarbonate conductivities as a function of ion exchange
000/T. capacity (IEC) for PFES-based AEMs.
1
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For a given polymer backbone, the tensile strength and
modulus values for wet membranes decreased as the increase of
IEC, which can be rationalized in terms of the plasticizing effect
of absorbed water. Meanwhile, similar trend was observed for
dry samples, which indicated that the introduction of quater-
nary ammonium groups lowered the mechanical properties of
PFES-based AEMs. In general, the mechanical properties for
PFES-based AEMs were acceptable for alkaline fuel cell
applications.
4. Conclusions
In this work, a series of uorene-containing AEMs were
prepared by bromination of benzylmethyl-containg poly-
Fig. 7 Arrhenius plot of bicarbonate conductivity for AEMs based on
QPFES-60 in water.
(uorene ether sulfone)s, followed by quaternization with tri-
methylamine, and ion exchange. Bromination showed several
advantages over chloromethylation such as relatively shorter
reaction time, higher efficiency, and higher selectivity. Conse-
quently, the properties of QPFES-based AEMs, such as water
uptake and conductivity, can be easily controlled by the
amounts of bromination reagent and the ratios of 9,9-bis(3,5-
Table 4 Mechanical properties of QPFES-based AEMs in both wet and
dry states
Tensile
strength
MPa)
Modulus
(GPa)
Elongation
at break (%)
(
0
dimethyl-4-hydroxyphenyl)uorene to 4,4 -biphenol. For a given
a
b
polymer backbone, water uptakes and ion conductivities
increased with the increasing IEC. Furthermore, a critical IEC
around 2.2 meq per g was found for QPFES-100 and QPFES-80
based AEMs, while no critical ion content existed in AEMs based
on QPFES-60. For a given IEC, copolymers, including QPFES-80
and QPFES-60-based AEMs, showed slightly lower conductivi-
ties but much lower water uptakes and swelling ratios than
homo-polymers, which can be attributed to the interruption of
Membrane
IEC
Wet
Dry
Wet
Dry
Wet
Dry
QPFES-100-2
QPFES-100-3
QPFES-100-4
QPFES-80-2
QPFES-80-3
QPFES-80-4
QPFES-80-5
QPFES-60-2
QPFES-60-3
QPFES-60-4
QPFES-60-5
1.54
2.21
2.41
1.65
2.19
2.35
2.84
1.53
2.12
2.64
2.77
38
30
16
37
23
9
58
50
41
42
39
32
1.1
1.7
1.4
1.2
30
23
13
87
75
25
24
127
83
44
42
15
16
5.5
50
60
8c.2
—
0.67
0.46
0.98
0.48
0.23
0.09
0.99
0.44
0.22
0.19
0.98
0.94
0.81
c
c
5
—
—
44
19
11
10
54
24
20
1.85
0.54
0.45
27
40
17
0
the continuous hydrophilic domains by the hydrophilic 4,4 -
biphenol residues. AEMs based on copolymer showed better
potentials for alkaline fuel cell applications due to a combina-
tion of reasonably high conductivities and improved dimen-
sional stability. This report provided our community some
insights on the structure–property relationships on uorene-
containing AEMs.
c
c
c
—
—
—
a
The rst and second numbers stand for the molar percentage of
DMHPF in the total of bisphenols, and the millimolar equivalents of
NBS used per gram PFES, respectively. Calculated from H NMR.
Not measured because these membranes are too brittle in dry state.
b
1
c
Acknowledgements
As shown in Table 3 and Fig. 7, the bicarbonate conductivi-
ties for QPFES-60-based AEMs increased gradually with The authors express their thanks to the National Science
increasing temperatures in an Arrhenius mechanism. The Foundation of China (no. 51173178 and 21304093) for their
activation energies for bicarbonate conduction were around 20 nancial supports.
ꢁ
1
18
kJ mol , which is typical for AEMs in bicarbonate form.
The hydroxide conductivities of QPFES-based AEMs were
Notes and references
30
estimated by a method validated by Varcoe et al.: multiplying
bicarbonate conductivities with a factor of 3.8. This approach is
based on the difference of ion mobility between hydroxide and
bicarbonate. As listed in Table 3, the hydroxide conductivities
ranged from 6.8 to 56.6 mS cm . The reasonably high
hydroxide conductivities further demonstrated the good
potential of these AEMs for alkaline fuel cell applications.
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RSC Adv., 2014, 4, 27502–27509 | 27509