Cross-linked hydroxide conductive membranes with side chains for direct methanol fuel cell applications

Article abstract of DOI:10.1039/c2jm16593g

A series of novel poly(ether ether ketone) copolymers containing methyl groups on the side chain were prepared based on a new monomer (3,4-dimethyl)phenylhydroquinone. Then a series of hydroxide exchange membranes with different IEC values were obtained through bromination and quaternary amination of the copolymers. By adjusting the contents of methyl groups in the copolymers, we could control the final structures of the membranes. The chemical structures of the monomers and copolymers were analyzed by ¹H NMR spectroscopy. After that, for the purpose of enhancing the dimensional stability and methanol resistance of the membrane, we prepared cross-linked membranes through a Friedel-Crafts reaction between bromomethyl groups and aromatic rings. The properties of the membranes related to fuel cell application were evaluated in detail. All the membranes showed good thermal and mechanical stabilities and conductivities. Moreover, the cross-linked membranes exhibit better dimensional stabilities and selectivities. Among those membranes, xPEEK-Q-100 showed a high conductivity (0.036 S cm^-1 at 80 °C), a low swelling ratio of 6.6% and a methanol permeation coefficient of 2.9 × 10^-7 cm² s^-1. The outstanding properties indicated that the application of PEEK-Q-xx membranes in fuel cells was promising.

Full text of DOI:10.1039/c2jm16593g

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PAPER
Cross-linked hydroxide conductive membranes with side chains for direct
methanol fuel cell applications
a
a
b
a
a
a
a
Shuai Xu, Gang Zhang, Yang Zhang, Chengji Zhao, Liyuan Zhang, Mingyu Li, Jing Wang, Na Zhang
a
a
and Hui Na*
Received 14th December 2011, Accepted 23rd April 2012
DOI: 10.1039/c2jm16593g
A series of novel poly(ether ether ketone) copolymers containing methyl groups on the side chain were
prepared based on a new monomer (3,4-dimethyl)phenylhydroquinone. Then a series of hydroxide
exchange membranes with different IEC values were obtained through bromination and quaternary
amination of the copolymers. By adjusting the contents of methyl groups in the copolymers, we could
control the final structures of the membranes. The chemical structures of the monomers and
1
copolymers were analyzed by H NMR spectroscopy. After that, for the purpose of enhancing the
dimensional stability and methanol resistance of the membrane, we prepared cross-linked membranes
through a Friedel–Crafts reaction between bromomethyl groups and aromatic rings. The properties of
the membranes related to fuel cell application were evaluated in detail. All the membranes showed good
thermal and mechanical stabilities and conductivities. Moreover, the cross-linked membranes exhibit
better dimensional stabilities and selectivities. Among those membranes, xPEEK–Q-100 showed a high
ꢀ1
ꢁ
conductivity (0.036 S cm at 80 C), a low swelling ratio of 6.6% and a methanol permeation coefficient
2
ꢀ7
ꢀ1
of 2.9 ꢂ 10 cm s . The outstanding properties indicated that the application of PEEK–Q-xx
membranes in fuel cells was promising.
Introduction
ethylene glycol, etc.) because of their low over-potential for
10
hydrocarbon fuel oxidation and reduced fuel crossover. To
Fuel cells have been regarded as an effective approach to resolve
the recent energy crisis and environment problems, because of
their high energy density, high conversion efficiency, and low
synthesize a HEM material with quaternary ammonium groups,
11–13
chloromethylation is widely used.
The drawbacks of this
method are the strong toxicity of the chloromethyl methyl ether
reagent and the poor controllability of the chloromethylation
14
1–4
pollution levels. Among the several kinds of fuel cells, proton
exchange membrane fuel cells (PEMFCs) have attracted the
most attention. The most commercialized PEM, Nafion
position and quantity. Hickner and Yan reported another route
to synthesize quaternized polymers by bromination, which was
15
a much safer and more controllable process.
(
DuPont), is a kind of perfluorosulfonic polymer containing
pendant sulfonic acid side groups, showing high proton
conductivity and excellent chemical, physical, and thermal
stabilities. However, some significant deficiencies limit its large-
scale application, such as the extremely high cost and excessive
Locating the electrolyte groups on the side chain has been
proved to be a promising strategy to enhance the separation of the
16,17
hydrophilic regions and the hydrophobic polymer backbone.
However, in the hydroxide exchange membrane, when quaternary
ammonium groups are set onto the aliphatic side chains, the
degradation of ammonium groups caused by a mechanism of E1
elimination or E2 elimination (Hofmann degradation) will take
5
methanol permeability. Besides, another major weakness of all
the PEMFCs is their essential dependence on the precious metal-
6,7
based catalysts, such as Pt.
Compared with PEMFCs, hydroxide exchange membrane fuel
cells (HEMFCs) show some unique characteristics. The most
significant advantage is the inherently faster kinetics of the
oxygen reduction reaction, which allows using non-noble and
ꢀ
place easily under the attack of OH in alkali media, which seri-
18,19
ously affects the stability of the membrane.
In our previous work, we synthesized brominated poly(aryl
20
ether sulfone) and related cross-linked membranes for HEMs.
8,9
low-cost metal electrocatalysts in the HEMFCs. Furthermore,
The results showed that the Br– group could be used for the
cross-linking reaction through a Friedel–Crafts mechanism
between Br– groups and aromatic rings without any catalysts.
Moreover, the cross-linking is a practical method to enhance the
HEMFCs can offer fuel flexibility (e.g., methanol, ethanol,
a
Alan G. MacDiarmid Institute, College of Chemistry, Jilin University,
Qianjin Street 2699#, Changchun 130012, P.R. China. E-mail: huina@
jlu.edu.cn; Fax: +86 431 85168870
21–24
performances of the HEMs.
b
In this work, we synthesized a novel series of HEMs combining
the cross-linked structure and the aromatic side-chain. The
SINOPEC Beijing Research Institute of Chemical Industry, Beijing
100013, P.R. China
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cross-linking could endow the membrane outstanding dimen-
sional stability and methanol resistance. Meanwhile, the
aromatic side-chain quaternary ammonium group could enlarge
the space for the motion of hydrophilic groups, could promote
the hydroxide conduction and did not aggravate the degradation
of quaternary ammonium groups like the aliphatic side chain.
The HEMs were obtained based on a series of poly(ether ether
ketone) (PEEK) copolymers synthesized from a novel bisphenol
monomer with two methyl groups. By adjusting the ratio of the
monomer, we could control the methyl contents, the degree of
bromomethylation of the polymer and the IEC values of the
membranes. Then cross-linked membranes were prepared by the
heating method. The monomers and copolymers were charac-
terized in detail. The properties, such as hydroxide conductivity,
methanol permeability, swelling ratio and alkaline stability,
were evaluated. And the results showed that this method of
cross-linking was effective for further research of HEMs.
we illustrate the synthesis of (PEEK–Me-80): Me-HQ (3.424 g,
0
0.016 mol), HQ (0.440 g, 0.004 mol), and 4,4 -difluor-
obenzophenone (4.36 g, 0.02 mol) with K CO (3.32 g, 0.024
2
3
mol) were added into a 250 mL three-necked flask equipped with
a mechanical stirrer, a nitrogen inlet, and a Dean–Stark trap.
Then N-methyl-2-pyrrolidone (NMP) (25 mL) and toluene
(15 mL) were added into the flask. The reaction mixture was
ꢁ
heated at 140 C for 2 h under nitrogen atmosphere for dehy-
dration. After removal of toluene, the reaction temperature was
ꢁ
increased to 170 C. When the viscosity had increased dramati-
cally, the mixture was poured into 2000 mL deionized water
slowly. The copolymer was washed with hot water several times
ꢁ
and dried under vacuum at 80 C for 24 h.
Bromination of the poly(ether ether ketone)s
A typical procedure for preparing a bromomethylated copol-
ymer PEEK–Br-80 was as follows: PEEK–Me-80 (2.0 g, the
3
amount of –CH was 8.6 mmol) and 40 mL trichloromethane
Experimental
were added to a three-necked flask equipped with a mechanical
stirrer, a nitrogen inlet and a condenser. After getting a homo-
geneous solution, NBS (3.05 g, 17.6 mmol) and BPO (0.1 g) were
Materials
3
,4-Dimethylaniline and hydroquinone (HQ) were purchased
0
added to the mixture and the reaction was carried out for 24 h at
ꢁ
from Aladdin Reagent, Shanghai, China. 4,4 -Difluor-
6
0 C. The mixture was then cooled to room temperature and
obenzophenone (DFB), 1,4-benzoquinone, N-bromosuccinimide
coagulated in acetone. The resulting polymer was washed with
ꢁ
(
NBS), and benzoyl peroxide (BPO) were purchased from Sigma-
acetone several times and dried under vacuum for 24 h at 40 C.
Aldrich Ltd. Other solvents and reagents were obtained from
Beijing Chemical Company and used without further
purification.
Preparation of the hydroxide exchange membranes (PEEK–Q-xx)
Brominated polymers (0.6 g) were dissolved in 10 mL NMP. The
solutions were filtered and cast on flat glass plates. The
Synthesis of (3,4-dimethyl)phenylbenzoquinone (Me-BQ)
ꢁ
membranes were dried at 40 C for 24 h to remove the solvent.
3
,4-Dimethylaniline (6.05 g, 0.05 mol), water (20 mL), hydro-
Then the membranes were immersed in a 33% trimethylamine
chloric acid (17 mL, 0.2 mol), were added into a 500 mL beaker.
Then, a water solution (20 mL) of sodium nitrite (3.45 g,
aqueous solution for 48 h to make the membranes quaternary-
ꢀ1
aminated. After that the membranes were kept in a 1 mol L
0
0
.05 mol) was added dropwise. The reaction system was stirred at
ꢁ
ꢀ
NaOH aqueous solution for 48 h to obtain the OH form
membrane. Finally, the membranes were washed thoroughly and
immersed in deionized water for 48 h to remove residual NaOH.
–2 C for 2 h and a brownish-yellow solution was obtained after
filtering. The solution was added dropwise into a mixture of
1
0
,4-benzoquinone (4.32 g, 0.04 mol), sodium bicarbonate (12.6 g,
ꢁ
.15 mol), and deionized water (100 mL). After stirring at 5–8 C
for 4 h, the yellow product was obtained by filtering and washing
with deionized water several times. The resulting solid was dried
Preparation of cross-linked hydroxide exchange membranes
(
xPEEK–Q-xx)
ꢁ
at 80 C in the vacuum oven for 24 h.
We chose PEEK–Br-100 and PEEK–Br-80 to prepare a cross-
linked membrane. The cross-linking reaction was performed by
Synthesis of (3,4-dimethyl)phenylhydroquinone (Me-HQ)
ꢁ
heating the brominated membranes at 150 C for 2 h in a vacuum
(
3,4-Dimethyl)phenylbenzoquinone (5.27 g, 0.025 mol), Zn
oven. Then the cross-linked membranes were treated with tri-
ꢀ
1
powder (4.875 g, 0.075 mol), and deionized water (35 mL) were
added into a 250 mL three-necked flask. When the mixture was
heated to boiling, hydrochloric acid (0.075 mol, 6.4 mL) was
added dropwise into the flask. Then the mixture was refluxed for
another 4 h. Finally the hot mixture was filtered and the obtained
precipitate was recrystallized from toluene.
methylamine aqueous solution, 1 mol L NaOH aqueous solu-
tion and deionized water as for the preparation of PEEK–Q-xx
membranes.
Characterization
Chemical structure of the membranes
Copolymerization of poly(ether ether ketone)s containing methyl
groups
To analyse the molecular structures of the monomers and
1
copolymers, H NMR spectra were obtained on a Bruker 510
The polymers were named as PEEK–Me-xx, where ‘‘xx’’ repre-
sents the percentage of Me-HQ in the copolymers. The content of
methyl groups was adjusted by the ratio of Me-HQ to HQ. Here
spectrometer (500 MHz) using deuterated dimethylsulfoxide
(DMSO-d
6 3
) or deuterochloroform (CDCl ) as the solvent and
tetramethylsilane (TMS) as the standard.
1
3296 | J. Mater. Chem., 2012, 22, 13295–13302
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Gel fraction of the cross-linked membranes
Methanol permeability
2
ꢀ1
The weights of the cross-linked membranes were measured
before and after immersing in N-methyl-2-pyrrolidone (NMP)
for a long time and the ratio of the residual mass was calculated
to evaluate the degree of cross-linking.
K
The methanol permeability coefficient D (cm s ) was deter-
mined using a cell that contained two reservoirs which has been
25
described in our previous study. The two reservoirs were
separated by the measured membrane. A 10 M methanol solu-
tion was placed on one side of the cell and water was placed on
the other side. Magnetic stirrers were used on each compartment.
The concentration of methanol was measured by a Shimadzu
GC-8A chromatograph. The methanol permeability was calcu-
lated as follows:
Thermal performance
The thermogravimetric analysis (TGA) curves were obtained by
a Pyris 1 TGA (Perkin-Elmer) under a nitrogen atmosphere.
ꢁ
A DK
Each sample was preheated at 110 C for 10 min to remove the
C
BðtÞ ¼ V
C
A
ðt ꢀ t
0
Þ
(4)
solvent and water completely, and then cooled and reheated from
ꢀ1
B
L
ꢁ
ꢁ
ꢁ
8
0 C to 700 C at a heating rate of 10 C min .
2
where A (cm ) and L (cm) are the effective area and the thickness
of the membrane respectively. C and C are the methanol
A
B
concentrations in methanol reservoir and water reservoir,
3
respectively. V (cm ) is the volume of the diffusion reservoir. D
Mechanical property
B
K
2
cm s ) is the methanol permeability coefficient.
ꢀ1
(
The yield tensile strength, elongation at break and Young’s
modulus of all the membranes were evaluated using a Shimadzu
ꢀ
1
AG-I 1 KN at a test speed of 2 mm min . The size of the
2
Hydroxide conductivity
membrane sample was 15 ꢂ 4 mm . For each testing, at least
The hydroxide conductivity was measured by a four-electrode
AC impedance method from 0.1 Hz to 100 kHz, 10 mV AC
perturbation and 0.0 V DC rest voltage using a Princeton
Applied Research Model 2273 potentiostat/galvanostat/FRA.
To remove the ions attached to the membrane surface,
membranes in the hydroxide form were initially immersed in
deionized water for 24 hours before measurement. The
membranes and the electrodes were set in a Teflon cell and
the distance between the reference electrodes was 1 cm. The
hydroxide ion conductivity was calculated by the following
equation:
three samples were measured and the average values were used.
Ion exchange capacity (IEC)
The ion exchange capacity (IEC) was measured using the clas-
sical back titration method. Firstly, we obtained the weights of
ꢀ
the dried membranes. Then, the membranes in the OH form
were immersed in 100 mL of 0.01 M HCl for 48 h to undergo an
ionic exchange process. After that, the solutions were titrated
with a standardized NaOH solution using phenolphthalein as an
indicator. The IEC was calculated from the formula as follows:
L
s ¼
(5)
IECðmequiv: per gÞ ¼
RS
where L is the distance between the two electrodes, R is the
resistance of the membrane and S is the cross-sectional area of
the membrane.
ꢀ
1
consumed NaOHðmLÞ ꢂ molarity NaOHðmol mL Þ ꢂ 1000
weight of dry membraneðgÞ
(
1)
Alkaline stability
The alkaline stability of the HEMs was evaluated by two
methods: in the first one, the PEEK–Q-xx membrane samples
Water uptake and swelling ratio
ꢁ
were kept in NaOH solutions (4 M) in test tubes for 7 days at
ꢁ
The membranes were vacuum-dried at 80 C for several days
until weights and lengths were constant. Then all the samples
were immersed in deionized water at a given temperature for
6
0 C, respectively. To check whether the samples had decom-
posed into pieces, the test tubes were shaken every 8 hours.
In the second method, we immersed one membrane into
2
4 h. We took out and wiped the membranes quickly with tissue
ꢁ
a moderate alkaline solution (1 M NaOH) at 60 C for 30 days
paper and measured their weights and lengths as soon as
possible. The water uptake and swelling ratio were calculated by
the following equations:
and measured the conductive performance of the membrane
every six days.
W
wet ꢀ Wdry
WUð%Þ ¼
ꢂ 100%
(2)
(3)
Results and discussion
W
dry
Structure characterization
Lwet ꢀ Ldry
SWð%Þ ¼
ꢂ 100%
As shown in Scheme 1, the bisphenol monomer was synthesized
by a three-step synthetic process. The structures of Me-BQ and
L
dry
1
Me-HQ were identified by H NMR spectra in Fig. 1. In
where Wwet and Wdry are the weights of wet and dry samples
respectively, Lwet and Ldry are the lengths of the wet and dry
membranes respectively.
Fig. 1(b), the proton peaks at 8.62 and 8.71 ppm were assigned to
the protons of hydroxyl groups, which could not be observed in
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the spectrum of the quinolone form (Fig. 1(a)). And the peak at
.25 ppm corresponding to the hydrogen atoms of methyl groups
2
is obvious in Fig. 1. This result proved that we have obtained the
Me-HQ monomer successfully.
Scheme 2 shows the process to synthesize the copolymers and
1
prepare hydroxide exchange membranes. We also used the H
NMR spectrum to analyse the structures of the polymers. Fig. 2
1
shows the H NMR spectra of PEEK–Me-100 and PEEK–Br-
1
00. It should be noticed that the peaks of bromomethyl groups,
which were located at about 4.60 ppm and 4.55 ppm, appeared in
the spectrum of the brominated copolymer, and they could not
be found in the pristine polymers. The spectra of other copoly-
mers were similar, which proved that we had synthesized the
brominated polymers successfully. The degrees of the bromina-
tion (DB) of the copolymers are listed in Table 2.
Gel fraction of the cross-linked membranes
After cross-linking treatment, the membranes could not be dis-
solved in any common solvent. The gel fraction can be considered
as an indirect measurement of the cross-linking density. The gel
fractions of the two cross-linked brominated membrane were
4
7.4% (xPEEK–Br-100) and 42.2% (xPEEK–Br-80). The slight
difference could be due to the low content of –Br in PEEK–Br-80.
Scheme 2 Syntheses of the copolymers.
Thermal stability
Fig. 3 shows the thermal performance of the PEEK–XX-100
polymers by the TGA curves. PEEK–Me-100 exhibited the
ꢁ
highest 5% weight loss temperature of 502.8 C, which meant the
main chain decomposition. After bromination, the polymers
Scheme 1 Synthesis of the monomer Me-HQ.
1
Fig. 2 The H NMR spectra of PEEK–Me-100 (a) and PEEK–Br-
00 (b).
1
ꢁ
showed an obvious weight loss at about 160 C, which was
associated with the elimination of the –Br groups on the side
chains of the macromolecules. The degradation of quaternary
ꢁ
26
ammonium groups started at about 200 C. The weight loss of
the cross-linked membranes xPEEK–Br-100 and xPEEK–Q-100
was lagging behind that of PEEK–Br-100 and PEEK–Q-100,
respectively. It could be attributed to the consumption of –Br
during the cross-linking process (Scheme 3). Because of the same
reason, the cross-linked membranes displayed a slightly lower
final thermal decomposition than the pristine ones. Table 1 lists
the 5% weight loss temperature of the membranes.
1
Fig. 1 The H NMR spectra of Me-BQ (a) and Me-HQ (b).
1
3298 | J. Mater. Chem., 2012, 22, 13295–13302
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xPEEK–Q-80 showed a lower IEC compared with the PEEK–Q-
1
00 and PEEK–Q-80 respectively. We considered that there are
three effects on the IEC in a cross-linking process: first, the
consumption of bromomethyl groups led to a decreasing intro-
duction of quaternary ammonium groups, which made the IEC
decline. Second, the cross-linked structure hindered the quater-
nization to a certain extent. Third, the elimination of Br atoms
(
whose atomic weight is 79.90) reduced the average molecular
weight of the structure unit, which led to a higher IEC indirectly.
And the first two aspects were the main influence in our
membranes.
Water uptake affects the conducting performance of inter-
mediate fuel cells to a great extent, but an exorbitant water
uptake induces unacceptable dimensional changes and a decrease
2
7
Fig. 3 The TGA curves of PEEK–Me-100, PEEK–Br-100, PEEK–Q-
00, xPEEK–Br-100, and xPEEK–Q-100.
in the mechanical strength. As is shown in Fig. 4, the water
uptake of the uncross-linked membranes increased with the IEC
and temperature naturally. After cross-linking, the water uptake
1
ꢁ
showed a sharp decline: at 80 C, the PEEK–Q-100 had a water
Mechanical properties
uptake of 41.7%, and for the xPEEK–Q-100, the value was only
Table 1 lists the yield tensile strength, elongation at break and
Young’s modulus of all the copolymers. As shown in the table,
the uncross-linked quaternized membranes with higher IEC
1
4.2%. The changing rule of the swelling ratio (shown in Fig. 5)
was similar to that of water uptake: the xPEEK–Q-80 showed the
ꢁ
lowest swelling ratio of 4.7% (20 C), which was only about one-
(
PEEK–Q-100 and PEEK–Q-80) were much more brittle than
third of the value of PEEK–Q-80. This result indicated that the
cross-linking improved the dimensional stability of the
membranes. That is one of the main reasons to do the cross-
linking treatment.
PEEK–Q-60, which could be illustrated by the yield tensile
strength and the elongation at break values.
After cross-linking, the mechanical properties of the
membranes were enhanced significantly. Both the yield tensile
strength and the elongation at break values were improved. This
meant that the cross-linked membranes had better resistance to
deformation or fracture.
Hydroxide conductivity, methanol permeability, and selectivity
The performance of a fuel cell is extremely dependent on the
28
conductivity of the membrane material. As shown in Fig. 6, the
IEC, water uptake and swelling ratio
membranes exhibited hydroxide conductivities in the range from
ꢀ1
ꢀ1
ꢁ
The IEC value is very important for HEMs, it responds to the
number of conductive groups in the polymer, and a higher IEC
usually leads to a higher conductivity. Table 2 lists the IEC values
of the membranes. The IEC values of the uncross-linked
membranes showed a decline from 1.43 to 0.90 mequiv. per g
with the decrease of the methyl group content. It illustrated that
the IEC values of the membranes could be well controlled by our
strategy. The cross-linked membranes xPEEK–Q-100 and
0.032 S cm to 0.041 S cm at 80 C. For the uncross-linked
membranes, the conductivity showed a similar trend to the IEC
value and water uptake. The cross-linked membranes had lower
conductivities compared with the uncross-linked membranes.
However, the downward tendency was not as sharp as that of
water uptake or swelling ratio. And the hydroxide conductivities
were still usable for the HEMFCs. The xPEEK–Q-100
ꢀ1
ꢁ
membrane still had a conductivity of 0.036 S cm at 80 C. Also,
Scheme 3 The mechanism of the cross-linking reaction of the membranes.
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Table 1 Mechanical and thermal stability of the membranes
ꢁ
Polymers
Yield tensile strength (MPa)
Elongation at break (%)
Young’s modulus (GPa)
T
d 5% ( C)
PEEK–Me-100
PEEK–Me-80
PEEK–Me-60
PEEK–Br-100
PEEK–Br-80
PEEK–Br-60
PEEK–Q-100
PEEK–Q-80
56.02
42.69
60.29
48.16
43.06
46.43
27.11
20.87
76.07
61.82
55.76
48.95
49.53
5.01
6.20
5.09
3.08
5.33
4.40
2.27
2.78
5.84
3.52
3.27
5.72
5.18
1.42
1.82
1.83
1.51
1.27
1.18
1.39
1.06
2.08
2.20
2.03
1.21
1.36
502.8
514.6
515.6
183.6
184.0
181.0
257.6
305.9
264.5
197.1
200.3
262.9
286.9
PEEK–Q-60
xPEEK–Br-100
xPEEK–Br-80
xPEEK–Q-100
xPEEK–Q-80
a
Table 2 DB , IEC, water uptake, swelling ratio, hydroxide conductivity, methanol permeability, selectivity, and alkaline stability of PEEK–Q-xx/
b
xPEEK–Q-xx membranes
Water
uptake (%)
Swelling
ratio (%)
ꢀ
1
s (mS cm )
IEC
D
K
Selectivity
4
(ꢂ10 S s cm
Time of breaking
into pieces (days)
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢀ7
2
ꢀ1
ꢀ3
Polymers
DB (%) (mequiv. per g) 20 C 80 C 20 C 80 C 20 C 80 C (ꢂ10 cm s
)
)
PEEK–Q-100
PEEK–Q-80
PEEK–Q-60
xPEEK–Q-100
xPEEK–Q-80
45.1
50.2
40.7
—
1.43
1.23
0.90
1.18
1.01
32.1
28.9
24.6
13.5
8.2
41.7
33.2
26.6
14.2
8.6
15.3
13.8
11.9
6.2
21.2
18.8
14.7
6.6
17
15
12
14
13
41
37
33
36
32
6.1
5.2
4.9
2.9
2.1
2.7
2.9
2.4
4.8
6.2
>7 days
>7 days
>7 days
>7 days
>7 days
—
4.7
5.0
a
The degree of bromination, defined as the percentage of the methyl which converted to bromomethyl during the bromination reaction. It is calculated
1
b
by the spectra of the H NMR. Considering the consumption of –Br in cross-linking, we did not show the DB of cross-linked membranes. The alkaline
stability was measured by the time of breaking into pieces in 4 M NaOH at 60 C.
ꢁ
Fig. 5 The swelling ratios of the membranes.
Fig. 4 The water uptakes of the membranes.
the PEEK–Q-80 and xPEEK–Q-100 membranes showed similar
IEC values and conductivities. This phenomenon could also be
found in the comparison between PEEK–Q-60 and xPEEK–Q-
Methanol permeability is another key issue in the application
29
of DMFCs. Diffusion or leakage of the fuel from the anode
across the membrane to the cathode leads to not only a fuel loss
but also some other undesirable consequences, such as compli-
8
0. We could conclude that the decline was mainly caused by the
31
decrease of IEC after cross-linking. The influence of the limita-
tion from cross-linking bonds was not obvious due to the side-
chain-type structure. For fuel cell operation using hydroxide
exchange membrane materials, there is a requirement for the
cated water and thermal management, etc. Table 2 lists the
methanol permeation coefficients. Each membrane exhibited
a coefficient lower than 7 ꢂ 10ꢀ cm s , and the cross-linked
membranes showed a significant decrease in methanol perme-
ability. As previously reported, water transportation and meth-
anol permeation take place through the same pathway in the
7
2
ꢀ1
ꢀ1 30
hydroxide conductivity to be above 10 mS cm . All the
membranes in this work fulfilled the requirement.
1
3300 | J. Mater. Chem., 2012, 22, 13295–13302
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Fig. 7 The selectivity of the membranes.
Fig. 6 The conductivities of the membranes.
32
membrane. So it is easy to understand that the cross-linking
results in a more compact membrane structure, which restricts
the hydrophilic domains and suppresses the swelling of the
membranes, and further effectively resists methanol penetration.
The methanol permeation coefficient of the xPEEK–Q-100
membrane was only about one-eighth of that of Nafion
ꢀ
6
2
ꢀ1
(
1.67 ꢂ 10 cm s ), which meant a good methanol resistance.
Table 3 lists some recent studies of HEMs to compare the
hydroxide conductivities and methanol diffusion coefficients of
our membranes with those of other HEM materials which were
designed for DMFCs. Among these materials, our membranes,
especially the xPEEK–Q-100 membrane, exhibited good overall
performances.
To evaluate the further performance of each membrane in the
fuel cell, we calculated its selectivity, which was defined as the
ratio of hydroxide conductivity to the methanol permeability
coefficient. The selectivity could reflect the comprehensive
properties of a membrane. And a higher selectivity always means
Fig. 8 Time courses of hydroxide conductivity of the membrane in 1 M
ꢁ
NaOH at 60 C.
all the selectivity of the cross-linked membranes improved
remarkably, which translates to improved overall performance.
39
better performance in DMFCs. As shown in Fig. 7, the two
ꢀ3
4
cross-linked membranes showed selectivities of 4.8 ꢂ 10 S s cm
4
ꢀ3
and 6.2 ꢂ 10 S s cm , respectively, which were much higher
4
ꢀ3
Alkaline stability
than those of the uncross-linked membranes (2.4 ꢂ 10 S s cm
4
ꢀ3
to 2.9 ꢂ 10 S s cm ). Compared to hydroxide conductivity, the
We evaluated the alkaline stability in two aspects: first, we
measured the breaking time of the samples in a strong alkali
solution to test the stability of the polymer backbones. As listed
in Table 2, all the membranes remained intact during the testing
time (7 days). The results illustrate that the copolymers had good
tolerance ability in a strong alkaline environment. For the second
test, our purpose was to investigate the conductivity performance
methanol permeability declined more significantly. As a result,
Table 3 Hydroxide conductivity and methanol diffusion coefficient of
HEMs reported
loss in fuel cell applications. We set a condition of 1 M NaOH at
ꢁ
ꢀ
1
2 ꢀ1
(cm s
HEMs
s (mS cm
)
D
K
)
References
6
0 C to evaluate the stability of quaternary ammonium groups
ꢁ
ꢀ7
ꢁ
PEEK–Q-100
xPEEK–Q-100
CPPO-40
PBI (alkali doped)
QPVA
SPES–2OH
QSEBS
QPPEK–OH
PF2
17 (20 C)
14 (20 C)
6.1 ꢂ 10 (20 C)
This work
This work
Ref. 33
Ref. 34
Ref. 35
Ref. 20
Ref. 36
Ref. 37
Ref. 38
in operating fuel cells. The curve in Fig. 8 is related to the decline
of conductivity of the PEEK–Q-100 membrane. We can see that
in the testing time, the decrease in hydroxide conductivity was
not very serious, and the descending trend got more and more
ꢁ
ꢀ7
ꢁ
2.9 ꢂ 10 (20 C)
ꢁ
ꢀ7 ꢁ
15.3 (25 C)
18.4 (RT)
10.6 (30 C)
18.8 (20 C)
1.12 ꢂ 10 (25 C)
ꢀ
7
2.6 ꢂ 10 (RT)
ꢁ
ꢀ7
ꢁ
6.68 ꢂ 10 (25 C)
ꢁ
ꢀ9
ꢁ
ꢀ1
1.92 ꢂ 10 (20 C)
smooth. The conductivity remained at a level of 0.025 S cm
th
after the 12 day. It proved that the aromatic-side-chain type
ꢁ
ꢀ7
ꢁ
5 (30 C)
ꢁ
4.45 ꢂ 10 (30 C)
ꢀ7
ꢁ
1.2 (22 C)
ꢁ
7.21 ꢂ 10 (22 C)
ꢀ8 ꢁ
could keep the quaternary ammonium groups with a reasonable
stability.
4.8 (60 C)
7.5 ꢂ 10 (25 C)
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 13295–13302 | 13301

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1
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A novel bisphenyl monomer and a series of hydroxide conductive
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