Carboxyl-terminated benzimidazole-assisted cross-linked sulfonated poly(ether ether ketone)s for highly conductive PEM with low water uptake and methanol permeability

Article abstract of DOI:10.1039/b926354c

A carboxyl-terminated benzimidazole trimer was synthesized as a crosslinker by controlling the ratio of 3,3′-diaminobenzidine and isophthalic acid. Composite membranes were obtained by mixing the benzimidazole trimer and sulfonated poly(ether ether ketone) (SPEEK) together. Cross-linked membranes were obtained by heating the composite membranes at 160°C. All of the properties of the cross-linked membranes were significantly increased, including proton conductivity, methanol permeability and water uptake due to the more compact structure compared to the non-cross-linked membranes. The cross-linked SPEEK-BI7 and cross-linked SPEEK-BI11 had excellent proton conductivities (0.22 and 0.19 S cm^-1) at 80°C, which were higher than that of Nafion 117 (0.125 S cm^-1). Transmission electron microscopy (TEM) analysis revealed a clear microphase separated structure of cross-linked membranes. Other properties, such as thermal and mechanical stability, required for use as a proton exchange membrane (PEM) have been investigated. The cross-linked membranes showed improved properties over membranes without crosslinking.

Full text of DOI:10.1039/b926354c

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Carboxyl-terminated benzimidazole-assisted cross-linked sulfonated
poly(ether ether ketone)s for highly conductive PEM with low water
uptake and methanol permeability
*
Miaomiao Han, Gang Zhang, Ke Shao, Hongtao Li, Yang Zhang, Mingyu Li, Shuang Wang and Hui Na
Received 15th December 2009, Accepted 28th January 2010
First published as an Advance Article on the web 5th March 2010
DOI: 10.1039/b926354c
A carboxyl-terminated benzimidazole trimer was synthesized as a crosslinker by controlling the
ratio of 3,30-diaminobenzidine and isophthalic acid. Composite membranes were obtained by mixing
the benzimidazole trimer and sulfonated poly(ether ether ketone) (SPEEK) together. Cross-linked
ꢀ
membranes were obtained by heating the composite membranes at 160 C. All of the properties of
the cross-linked membranes were significantly increased, including proton conductivity, methanol
permeability and water uptake due to the more compact structure compared to the non-cross-linked
membranes. The cross-linked SPEEK-BI7 and cross-linked SPEEK-BI11 had excellent proton
conductivities (0.22 and 0.19 S cmꢁ1) at 80 C, which were higher than that of Nafion 117
ꢀ
(0.125 S cmꢁ1). Transmission electron microscopy (TEM) analysis revealed a clear microphase
separated structure of cross-linked membranes. Other properties, such as thermal and
mechanical stability, required for use as a proton exchange membrane (PEM) have been
investigated. The cross-linked membranes showed improved properties over membranes without
crosslinking.
and improve dimensional stability.22,23 Though these resulting
membranes exhibited enhanced proton conduction and lower
methanol crossover, this method may result in the leaching out of
small molecules during usage in fuel cells. Li et al.24–26 reported
that the synthesis of sulfonated aromatic polymers bearing
benzimidazole pendant groups, and the incorporation of benz-
imidazole groups by these methods, can improve thermal and
mechanical stability, WU, and methanol permeability. However,
these membranes showed lower proton conductivity than that of
the foregone simple SPEEKs.
Introduction
Direct methanol fuel cells (DMFCs)1–3 have attracted much
attention due to their high energy efficiency, low emission,
compact cell design and other benefits. Proton exchange
membranes (PEM), a key component in DMFC, transports
protons from anode to cathode and at the same time separates
the fuel and the oxidant.4,5 Perfluorosulfonic acid membranes,
such as Nafion are commercialized membranes used in DMFCs
due to their good physical, chemical stability and high proton
conductivity.6,7 However, many disadvantages, such as limited
operation temperature (T < 80 ꢀC), poor thermal properties, high
cost and high methanol permeability, limit their usage.7–10
Sulfonated aromatic polymers11–15 have recently been developed
by several groups as high performance and low cost alternative
materials. But there must be certain acidic groups in order to
achieve sufficient proton conductivity. Unfortunately, a high
degree of sulfonation induces high methanol permeability and
excessive water swelling.16–18
In this study, we propose a new strategy to introduce benz-
imidazole groups into SPEEK by crosslinking. A novel benz-
imidazole trimer has been synthesized as the crosslinker for the
first time. Cross-linked membranes were obtained by the heating
method.27–29 We prepared composite membranes and cross-
linked membranes with a majority partner, SPEEK with a high
degree of sulfonation (Ds), and a minority partner, benzimid-
azole trimer. The majority partner guaranteed good proton
conductivity of the membranes and the minority partner
improved the mechanical properties and dimensional change,
meanwhile, the appropriate ratio of benzimidazole groups as
proton donors and acceptors should help to enhance proton
conductivity. The introduction of benzimidazole groups into
SPEEK avoided basic groups leaching out from the membranes
in liquid water, as well as incompatibility behavior among the
different components of the blended membranes. The cross-
linked membranes were investigated as PEMs used in DMFCs.
All of the properties, such as proton conductivity, water swelling,
methanol permeability and mechanical property, were studied in
detail. And the results showed that this method of crosslinking is
suitable for further research of PEMs.
Polybenzimidazole is a kind of high-performance polymer that
exhibits good mechanical and thermal properties.19–21 On the
other hand, benzimidazole rings possess both donor and
acceptor hydrogen bonding sites due to its amphoteric nature. A
lot of work has focused on introducing benzimidazole groups
into sulfonated aromatic polymers for the above purpose by
different means. Introducing acid-based composite membranes
of sulfonated aromatic polymers and benzimidazole or imidazole
were considered as promising strategies to reduce water uptake
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 85168868
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inletꢀ/outlet and a mechanical stirrer. The mixture was heated at
220 C for 24 h. The resulting dark brown solution was coagu-
lated in deionized water and stirred for 24 h. The trimer was
filtered, and washed with deionized water several times until the
water was neutralized. It was dried at 120 ꢀC in vacuo for at least
24 h. Chemical structures are shown in Fig. 1. Representative 1H
NMR spectra of the benzimidazole trimer using deuterated
dimethyl sulfoxide (DMSO-d6) solutions are also shown in
Fig. 1. As can be seen, the peaks of 13.25 ppm and 7.5–9.0 ppm
were assigned to the amine protons on the benzimidazole moie-
ties and the proton on the aromatic groups, respectively. By
comparing the intensity of these signals we found their ratio
approximately was 1 : 1, this substantiated the successful
synthesis of benzimidazole trimer. FTIR (neat, cmꢁ1): 3396
(N–H stretching), 1686 (C]O), 1630 (C]N), 1585, 1447
(aromatic), 1361 (C–N stretching), 1300ꢁ1000 (C–O stretching).
Post-sulfonation PEEK with a degree of sulfonation (Ds ¼ 0.76)
was synthesized in concentrated sulfuric acid at 70 ꢀC for 4.5 h.30
Experimental section
Materials
Monomers 3,30-diaminobenzidine (DAB), isophthalic acid,
poly(phosphoric acid) (PPA) and poly(ether ether ketone)
(PEEK) were purchased from commercial vendors and used
without further purification unless specified.
Synthesis
Benzimidazole trimer was synthesized in PPA. A typical coupling
reaction was performed as follows: 2.6784 g (12.5 mmol) of DAB
and 4.1533 g (25 mmol) of isophthalic acid were mixed with 137 g
of PPA in a three-neck 250 ml flask equipped with a nitrogen
Membrane preparation and crosslinking treatment
The benzimidazole trimer was first dissolved in DMSO by
heating to afford a mass fraction of 6–10%, and then the SPEEK
was dissolved in the above solution. This final solution was cast
onto a clean glass plate followed by heating at 60 ꢀC for 12 h, and
then 100 ꢀC for 6 h to remove the solvent. The cross-linked
membranes were prepared through Friedel–Crafts reaction by
heating the membranes at 160 ꢀC for 10 h in a vacuum oven.27–29
The cross-linked membranes were peeled off from the substrate
with deionzed water. All the membranes had a thickness around
30–40 mm. The crosslinking reaction and the simulated stereo
configuration are shown in Scheme 1.
Characterization and measurements
1
The H NMR spectra were collected by an AVANCE 500 at
25 ꢀC with deuterated dimethyl sulfoxide (DMSO-d6) as the
Fig. 1 Synthesis of benzimidazole trimer and 1H NMR spectra of
benzimidazole trimer (in DMSO-d6).
solvent and tetramethylsilane (TMS) as the standard. Thermal
Scheme 1 Crosslinking reaction and the simulated stereo configuration of cross-linked membranes
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stability of the crosslinking membranes was determined by
a Perkin-Elmer TGA. A temperature ramp of 10 C minꢁ1 was
carboxyl after crosslinking. Table 1 also shows the solubility of
the membranes. The SPEEK and SPEEK-BIs membranes can
easily be dissolved in common organic polar solvents such as
DMSO, while the cross-linked membranes cannot be dissolved in
the same solvents.
ꢀ
applied under nitrogen flow. The mechanical properties of the
membranes were measured by a SHIMADZU AG-I 1KN at
a speed of 2 mm minꢁ1, the size of the membranes was 1 mm ꢂ
4 mm. Each sample was tested at least four times to reach an
average value.
Thermal stability
Water uptake and dimensional change of the membranes were
determined by measuring the weight and dimensional differences
between the fully hydrated membranes and dried membranes.31,32
A titration technique was used to determine the IEC of the
membranes.31,32 Proton conductivities of membranes were
measured by the four-point probe method over a frequency range
The thermal properties of the SPEEK, SPEEK-BIs and cross-
linked SPEEK-BIs polymers were investigated by TGA. Fig. 2
shows the TGA and derivative curves (example for SPEEK,
SPEEK-BI15 and cross-linked SPEEK-BI15). The figure shows
that the plain SPEEK exhibited a typical two-step degradation
of 10–107 Hz at 25, 40, 60 and 80 C using a Princeton Applied
ꢀ
ꢀ
pattern. The first weight loss region between 250 and 400 C is
Research Model 2273A Potentiostat (Model 5210 frequency
response detector, EG&GPARC, Princeton, NJ).31,32 The
methanol permeability was measured in an isothermal bath at
believed to be associated with the elimination of sulfonic acid
groups, and the second weight loss step starting at about 450 ꢀC
corresponded to the decomposition of the main polymer chain.
The desulfonation temperatures of SPEEK-BIs and cross-linked
SPEEK-BIs are higher than those of SPEEK. The SPEEK-BIs
ꢀ
25 C using a two-chamber diffusion cell method with a 10 M
methanol solution.31,32 TEM images were determined by
a JEM-2000EX. Before the test, the SPEEK was first converted
into the Ag+ form by immersing the SPEEK in a AgNO3 solution
for 48 h. The composite membrane samples were prepared by
casting the composite solutions onto copper grids; cross-linked
membranes samples were obtained by heating the copper grids
for TEM use.
Results and discussion
Ion-exchange capacities and solubility
The IEC values of the membranes were determined by titrating the
membranes in aqueous NaCl solution with standard sodium
hydroxide solution. Table 1 shows the IEC values. It should be
noted that the IEC values of composite membranes (SPEEK-BIs)
and cross-linked membranes (cross-linked SPEEK-BIs) are
much lower than that of the SPEEK, in addition the IEC values
of cross-linked SPEEK-BIs are lower than those of SPEEK-BIs
at the same loading ratio of benzimidazole trimer. These results
implied the formation of an acid–base interaction between
sulfonic acid and benzimidazole groups, and the diminishment of
Fig. 2 TGA and derivative curves of membranes under N2 atmosphere.
Table 1 Ion-exchange capacities (IEC), water uptake (WU), dimensional change, solubility, proton conductivity and methanol permeability of
membranes
Proton
conductivity/
mS cmꢁ1
Dimensional
change (%)
Wt%
BI in
WU (wt %)
IECa
Solubility
in DMSO 25 ꢀC
Methanol permeability/
Membrane
polymer (meq gꢁ1
)
25 ꢀ
C
80 ꢀC
25 ꢀC 80 ꢀ
C
80 ꢀC ꢂ 10ꢁ7 cm2 sꢁ1
SPEEK-BI3
SPEEK-BI7
SPEEK-BI11
SPEEK-BI15
SPEEK
Cross-linked SPEEK-BI3
Cross-linked SPEEK-BI7
Cross-linked SPEEK-BI11 11%
Cross-linked SPEEK-BI15 15%
Nafion 11733
3%
7%
11%
15%
0
197
33.3
25.3
22.8
18.3
157.4
34.3
27.9
21.7
2.0
1.7
1.7
2.0
44.0
8.7
4.0
2.7
Yes
Yes
Yes
Yes
0.011
0.013
0.015
0.015
0.025
0.021
0.030
0.027
0.022
0.15
0.14
0.16
0.13
14.65
7.83
3.68
2.48
1.72
1.40
1.27
2.31 and 2.18b 40.4
soluble 8.3
soluble Yes
0.11d 19.18
3%
7%
1.87
1.63
1.29
1.23
0.90
21.4
21.1
19.0
16.3
19.0c
24.4
24.4
21.8
20.9
28.6
3.3
2.3
0.7
0
6.0
5.0
3.3
1.7
—
No
No
No
No
—
0.17
0.22
0.19
0.18
2.38
1.67
1.57
1.05
—
—
0.0571c 0.125 15.5
a
Ion exchange capacity (IEC) was determined by the titration (except b). b IEC was determined by 1H NMR. c Property of Nafion 117 was at 30 ꢀC.33
Measured at 60 ꢀC due to solubility at 80 ꢀC.
d
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also exhibited a typical two-step degradation pattern. However,
visible decomposition did not appear for cross-linked SPEEK-
117,33 while they had almost the same WU. This result illustrates
that loading of benzimidazole trimer and crosslinking improved
dimensional stability of the membranes, and suggests that there
are acid–base interactions between sulfonic acid groups and
amine groups. At the same time, crosslinking bonds between the
SPEEK and benzimidazole trimer help to hold the polymer
chains together in order to restrict the hydrophilic domains and
thus decrease dimensional change.
ꢀ
BIs before 400 C, which only exhibited a one-step degradation
ꢀ
pattern (between about 450 to 700 C). These results indicated
that the incorporation of benzimidazole groups and crosslinking
bonds can improve the thermal stability of the polymer.
Water uptake and dimensional change
Water uptake plays the most important role in proton conduc-
tivity in membranes because H2O molecules act as a trans-
portation medium of protons. However, high WU leads to large
dimensional changes in membranes which causes low mechanical
properties. So it is necessary that there are suitable water mole-
cules around the sulfonic acid groups.
Mechanical property
The mechanical properties of cross-linked membranes have been
compared to SPEEK, Nafion 11733,34 and composite membranes
without crosslinking in both the dry and wet state. As shown in
Table 2, the mechanical properties of all membranes in the wet
state are not as good as those of membranes in the dry state. The
SPEEK-BI and cross-linked SPEEK-BI membranes show higher
tensile strength and Young’s modulus, and the lower elongation
than that of SPEEK and Nafion 11733,34 in both the dry and wet
state. In the wet state, the mechanical properties of cross-linked
SPEEK-BIs are better than those of SPEEK-BIs. Fig. 4 shows
how the Young’s modulus varied with different loading ratios of
the benzimidazole trimer in membranes in the wet state. The
Young’s modulus of SPEEK-BIs and cross-linked SPEEK-BIs
increase with the increment of benzimidazole trimer ratio.
Meanwhile, Young’s modulus of cross-linked SPEEK-BIs are
even higher than that of SPEEK-BIs at same loading ratio. In the
dry state, the SPEEK-BIs and cross-linked SPEEK-BI
membranes exhibit similar mechanical properties. These results
show that the introduction of benzimidazole trimer and cross-
linking improved mechanical properties of the SPEEK, espe-
cially in the wet state. It also suggests that the mechanical
properties of membranes can been related to their water uptake
and swelling.
Fig. 3 and Table 1 show the WU and dimensional change of
the membranes. It is obvious that the WU and dimensional
change of SPEEK-BIs and cross-linked SPEEK-BIs show
a decreasing tendency with the introduction of benzimidazole
trimer. SPEEK is soluble in 80 ꢀC water, but SPEEK-BI3 and the
cross-linked SPEEK-BI3 are insoluble with a dimensional
change of 44.0% and 6.0%, respectively. The shape of SPEEK-
BIs and cross-linked SPEEK-BIs was maintained, and cross-
linked SPEEK-BI membranes had a lower dimensional change
than that of SPEEK-BIs. The dimensional changes of all the
cross-linked membranes were lower than that of the Nafion
Proton conductivity and methanol permeability
The proton conductivities, as a function of temperature for the
membranes with different loading ratios of benzimidazole trimer,
are summarized in Fig. 5. For all membranes, proton conduc-
tivity increases with increasing temperature. The proton
conductivity of the membranes before crosslinking is lower than
that of SPEEK due to the interaction between the sulfonic acid
groups and amine groups. After crosslinking, all the membranes
have good proton conductivities, and higher than that of the
SPEEK, because the cross-linked SPEEK-BI membranes can
tolerate much higher ionic contents without excessive swelling,
which makes the membranes possess highly concentrated, iso-
tropically connected ionic domains.35–37 What needs to be
emphasized is that the cross-linked SPEEK-BI7 and cross-linked
SPEEK-BI11 have excellent proton conductivities (0.22 and
0.19 S cmꢁ1) at 80 C, which are higher than that of the Nafion
ꢀ
117 (0.125 S cmꢁ1).33
Fig. 6 shows how the proton conductivity of cross-linked
SPEEK-BIs at a fixed condition (80 ꢀC and 100% RH) varied
with different loading ratios of the benzimidazole trimer. As the
figure shows, with the increasing of loading ratio, the proton
conductivity of the membranes increases to a maximum
conductivity of 0.22 S cmꢁ1 at a benzimidazole trimer loading
Fig. 3 Water uptake and dimensional change of the membranes.
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Table 2 Mechanical property of membranes
Young’s modulus
(GPa)
Tensile strength (MPa)
dry wet
Elongation at break (%)
Membranes
dry
wet
dry
wet
SPEEK-BI3
SPEEK-BI7
SPEEK-BI11
SPEEK-BI15
SPEEK
cross-linked SPEEK-BI3
cross-linked SPEEK-BI7
cross-linked SPEEK-BI11
cross-linked SPEEK-BI15
Nafion 11733,34
69.54
71.48
78.03
72.30
62.31
69.80
70.58
75.39
71.71
38.0
23.98
26.72
31.83
39.81
20.18
34.42
38.44
47.59
45.23
28.4
1.70
1.80
2.02
1.96
1.22
1.69
1.96
1.92
1.75
0.18
0.37
0.51
0.72
0.91
0.30
0.69
0.73
0.86
1.00
0.10
29.99
15.02
14.48
11.17
32.17
10.74
18.86
25.92
19.08
301.5
16.11
15.47
11.89
11.93
23.90
12.17
10.54
17.31
13.97
329.2
Fig. 4 Young’s modulus of SPEEK-BIs and cross-linked SPEEK-BIs
dependence on loading of benzimidazole trimer in membranes at room
temperature in the wet state.
Fig. 6 Proton conductivities of cross-linked SPEEK-BIs dependence on
loading of benzimidazole trimer in membranes at 80 ꢀC under 100% RH
condition.
optimum ratio is 7%, where the conductivity is maximum. The
proton conductivity of the composite based on sulfonated
poly(phentylene oxide) and benzimidazole studied by Liu et al.23
showed 8.93 ꢂ 10ꢁ4 S cmꢁ1 at optimum doping ratio.
In Fig. 7 the relative water uptake as a function of relative
proton conductivities of various membranes to Nafion 11733 at
80 ꢀC is shown. We can see that the proton conductivities of all
the cross-linked membranes and SPEEK-BI(7,11,15) are higher
than that of Nafion 117,33 while they have similar WU data
which are caused by lower dimensional change. This lower
dimensional change makes membranes possess a higher effective
concentration of sulfonic acid groups within the membranes
which is useful for enhancement of proton conductivity.35
Membranes intended for DMFC must possess an effective
barrier function to stop methanol crossover from anode to
cathode. The methanol permeabilities of the SPEEK, SPEEK-
BIs and cross-linked SPEEK-BIs with 10% methanol concen-
tration at 25 ꢀC are listed in Table 1. The methanol permeabilities
of both SPEEK-BIs (2.48–14.65 ꢂ 10ꢁ7 cm2 sꢁ1) and cross-linked
SPEEK-BIs (1.05–2.38 ꢂ 10ꢁ7 cm2 sꢁ1) decrease varied with
different loading ratio of benzimidazole trimer and both are
Fig. 5 Proton conductivity of the membranes.
ratio of 7%, and then decreases with loading ratio of the benz-
imidazole trimer. The proper loading ratio of benzimidazole
should be chosen to enhance the concentration of donors and
acceptors participating in the Grotthuss mechanism. The
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Selectivity, which is the ratio of the proton conductivity to the
methanol permeability, is also a useful parameter to predict
potential DMFC performance. As shown in Fig. 8, the selectiv-
ities of the cross-linked SPEEK-BIs (8.62–21.05 ꢂ 104 S s cmꢁ3
)
are higher than that of Nafion 117 (3.68 ꢂ 104 S s cmꢁ3).33 There
is also information about activation energy of proton conduc-
tivity (Ea) in Fig. 8, which is obtained from the Arrhenius rela-
tion of proton conductivity as a function of temperature.38 Ea of
cross-linked SPEEK-BI7 is the lowest, which shows it has
potential for use in DMFCs.
Microstructure of the composite membrane and cross-linked
membranes
The microstructure of the composite membrane and cross-
linked membrane were investigated by TEM. The dark areas
represent hydrophilic domain and bright areas represent the
Fig. 7 Relative proton conductivities as a function of relative water
uptake at 80 ꢀC.
hydrophobic domain. Fig.
9 shows the morphologies of
SPEEK-BI7 and cross-linked SPEEK-BI7 membranes. In the
TEM figure of SPEEK-BI7 membrane, the black clusters are
randomly dispersed throughout the composite membrane
matrix and phase separation is not apparent, which indicated
the lack of connectivity of the ionic phase in the membrane. In
contrast, cross-linked SPEEK-BI7 membrane possesses well-
segregated morphologies of ionic-rich and lamellar structures,
which substantiated the good connectivity of the ionic phase in
membrane to give ionic channels for good conductivity.
This kind of hydrophilic/hydrophobic microphase separation
morphology is also quite similar to WU and proton conduc-
tivities.
Conclusions
Cross-linked sulfonated poly(ether ether ketone)s (SPEEKs)
have been synthesized, based on a novel crosslinker, benzimid-
azole trimer. The cross reticulation structures of SPEEK-BIs
were obtained after crosslinking by heating the composite
membranes which were composed of benzimidazole trimer and
SPEEK. The sulfonic acid groups and amine groups had an acid–
base interaction, and crosslinking bonds of the SPEEK and
benzimidazoel trimer helped to make membranes more compact.
Compared with SPEEK, all of the properties of the cross-linked
membranes were significantly improved, such as proton
Fig. 8 Activationenergyof proton conductivity (Ea) and selectivity of the
membranes (property of Nafion 117 was measured at 30 ꢀC) (sample 1–10:
SPEEK-BI(3,7,11,15), cross-linked SPEEK-BI(3,7,11,15), Nafion 117).
lower than that of SPEEK (19.18 ꢂ 10ꢁ7 cm2 sꢁ1) and Nafion 117
(15.5 ꢂ 10ꢁ7 cm2 sꢁ1).33 Meanwhile, methanol permeabilities of
membranes after crosslinking are lower than that of membranes
before crosslinking due to the more compact structure.
Fig. 9 TEM micrographs of SPEEK-BI7 and cross-linked SPEEK-BI7 membranes.
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indicated that the cross-linked membranes had more compact
structures. The proton conductivity of cross-linked SPEEK-BI7
was 0.22 S cmꢁ1, which was higher than that of SPEEK
(0.11 S cmꢁ1 at 60 ꢀC) and also higher than that of Nafion 11733
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Acknowledgements
We thank Christopher Lew from University of Minnesota for
polishing the English language in the revised manuscript. We
would like to thank the China High-Tech Development 863
program (Grant No. 2007AA03Z218) for financial support of
this work.
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