High temperature proton exchange membranes based on poly(arylene ether)s with benzimidazole side groups for fuel cells

Article abstract of DOI:10.1039/c2jm32093b

A new benzimidazole containing monomer has been synthesized for the preparation of poly(arylene ether sulfone) (PAES) and poly(arylene ether benzimidazole) (PAEB) with benzimidazole side groups by nucleophilic substitution polymerization. PAES and PAEB had inherent viscosities of 0.56 and 0.93 dL g^-1, respectively, measured in N,N-dimethylacetamide (DMAc) at a concentration of 0.5 g dL^-1. The structures of the benzimidazole containing monomer, PAES and PAEB were characterized by FTIR, ¹H NMR, and elemental analyses. These polymers showed excellent solubility in common organic solvents, such as DMAc, dimethyl sulfoxide (DMSO), and N-methyl-pyrrolidinone (NMP) at room temperature. Due to the strong intermolecular hydrogen bonding from the amide and imidazole groups in the side chains, PAES and PAEB had unusually high T_gs at 374 and 381 °C, respectively. The 5% weight loss temperatures of PAES and PAEB were around 472 and 522 °C in air, respectively. The phosphoric acid doping levels of PAES and PAEB membranes were 5.6 and 15.3. The proton conductivity of phosphoric acid doped membranes increased with increasing temperatures and reached to a range of 10^-3 to 10^-2 S cm^-1 at 160 °C. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2jm32093b

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PAPER
High temperature proton exchange membranes based on poly(arylene ether)s
with benzimidazole side groups for fuel cells
a
a
Cheng-Hsun Shen, Steve Lien-Chung Hsu, Elena Bulychevab and Natalya Belomoinab
*
Received 4th April 2012, Accepted 25th July 2012
DOI: 10.1039/c2jm32093b
A new benzimidazole containing monomer has been synthesized for the preparation of poly(arylene
ether sulfone) (PAES) and poly(arylene ether benzimidazole) (PAEB) with benzimidazole side groups
by nucleophilic substitution polymerization. PAES and PAEB had inherent viscosities of 0.56 and
0.93 dL gꢀ1, respectively, measured in N,N-dimethylacetamide (DMAc) at a concentration of 0.5 g
dLꢀ1. The structures of the benzimidazole containing monomer, PAES and PAEB were characterized
1
by FTIR, H NMR, and elemental analyses. These polymers showed excellent solubility in common
organic solvents, such as DMAc, dimethyl sulfoxide (DMSO), and N-methyl-pyrrolidinone (NMP) at
room temperature. Due to the strong intermolecular hydrogen bonding from the amide and imidazole
groups in the side chains, PAES and PAEB had unusually high Tgs at 374 and 381 ꢁC, respectively. The
5% weight loss temperatures of PAES and PAEB were around 472 and 522 ꢁC in air, respectively. The
phosphoric acid doping levels of PAES and PAEB membranes were 5.6 and 15.3. The proton
conductivity of phosphoric acid doped membranes increased with increasing temperatures and reached
to a range of 10ꢀ3 to 10ꢀ2 S cmꢀ1 at 160 ꢁC.
molecular weight PBI polymers is difficult, and its cost is quite
1. Introduction
high.11–13 On the other hand, some aromatic polymers, such as
poly(arylene ether sulfone)s (PAESs), are easy to synthesize and
have a lower cost. They also have good mechanical properties,
thermal stability and hydrolytic stability.14–17 Although the
sulfonated PAESs have proton conductivity, they do not have
conductivity at temperatures as high as does Nafionꢀ.
In this study, we combined the characteristics of PBI and
poly(arylene ether)s and synthesized new polymers containing
benzimidazole on the side chains through aromatic nucleophilic
substitution polymerization. The benzimidazole groups played
an important role in the polymer. They served as the phosphoric
acid doping site and underwent proton transfer via the Grotthuss
mechanism at high temperatures.18,19 These polymers can be used
to fabricate membranes for high temperature polymer electrolyte
membrane fuel cell applications.
Proton exchange membrane fuel cells (PEMFCs) are considered
one of the most attractive power sources for both stationary and
mobile applications, including vehicles and portable electronic
devices, due to the increasing environmental and energy issues.1,2
Most PEMFCs are operated between room temperature and up
to 80 ꢁC. Recently, high temperature (operational above 100 ꢁC)
PEMFCs have gained great interest because they offer many
advantages, such as fast electrode kinetics, high tolerance to
carbon monoxide (CO) and a simplified system design. However,
the most commonly used polymer electrolyte membrane,
ꢁ
Nafionꢀ, cannot operate above 80 C due to dehydration. The
development of high temperature polymer electrolyte
membranes has become an important issue today.
A polybenzimidazole (PBI) membrane doped with phosphoric
acid is known as a promising high temperature polymer elec-
trolyte membrane system.3–7 Although the phosphoric acid (PA)
doped PBI membrane has good high temperature conductivity,
the conventional PBI polymer is difficult to dissolve in common
organic solvents for the preparation of membranes due to its
rigid molecular structure.8–10 Moreover, the synthesis of high
2. Experimental section
Materials
2,2-Bis(3-amino-4-hydroxyphenyl)hexafluoropropane, bis(4-fluoro-
phenyl) sulfone, 1H-benzimidazole-5-carboxylic acid, 3,30-dia-
minobenzidine and 4-fluorobenzoic acid were obtained from
Sigma Aldrich. N-Methyl-pyrrolidinone (NMP), dimethyl-
formamide (DMF), and N,N-dimethylacetamide (DMAc) were
purified by distillation over calcium hydride before use. Phos-
phoric acid (PA) and sulfolane were obtained from Fluka and
used as supplied. Other reagents and solvents were used as
received.
aDepartment of Materials Science and Engineering, Research Center for
Energy Technology and Strategy, Center for Micro/Nano Science and
Technology, National Cheng-Kung University, Tainan, 701-01, Taiwan,
R.O.C. E-mail: lchsu@mail.ncku.edu.tw; Fax: +886 6 2346290; Tel:
+886 6 2757575 ext. 62904
bA. N. Nesmeyanov Institute of Organoelement Compounds, Russian
Academy of Sciences, Moscow, Russia
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Characterization
stirred for 12 h at room temperature. The organic salt was filtered
off, and the major product was collected in a round bottomed
flask. Some DMF was removed in a vacuum by using a rotary
evaporator to obtain a concentrated mixture. Flash column
chromatography (eluent: EA/EtOH, 7/3, v/v) through silica gel
was used to yield the pure product (6.80 g, 52% yield). The
benzimidazole containing monomer was characterized by 1H
NMR spectroscopy as shown in Fig. 1. 1H NMR (DMSO-d6): d
(ppm) 7.39 (d, 1H); 7.80 (s, 1H); 7.90 (d, 1H); 7.92 (d, 1H); 8.18
(d, 1H); 8.50 (s, 1H); 8.87 (s, 1H). Elemental analysis calcd for
C31H20O4N6F6$2H2O: C, 53.92%; H, 3.50%; N, 12.17%. Found:
C, 54.32%; H, 3.49%; N, 12.00%.
Intrinsic viscosity measurements were carried out by using a
1
Cannon Ubbelohde dilution viscometer in DMAc (30 ꢁC). H
NMR spectra were recorded on a Varian Bruker Avance 600
spectrometer using deuterated dimethyl sulfoxide (DMSO-d6) as
a solvent and tetramethylsilane as an internal standard. Ther-
mogravimetric analysis (TGA) was run on powdered specimens
using a TA Instrument 2050 at temperatures ranging from 100 to
800 ꢁC at a heating rate of 10 ꢁC minꢀ1 under air and nitrogen.
Tensile strengths were determined from stress–strain curves using
a Shimadzu AG-SI universal testing machine at a strain rate of
5 mm minꢀ1 in ambient atmosphere. The film specimens were
30 mm long, 4.5 mm wide, and 30–40 mm thick. The thermo-
mechanical analysis (TMA) of polymer membranes was carried
out using a TA Instrument Thermal Mechanical Analyzer
Q400EM with an extension probe under 0.05 Nꢀ1 tension force on
Synthesis of 2,20-bis(4-fluoro-phenyl)-6,60-bibenzimidazole
(BFPB)
As shown in Schemes 2 and 3, 30-diaminobenzidine (9.64 g,
45 mmol) and 4-fluorobenzoic acid (13.87 g, 99 mmol) were
dissolved in 212 mL of poly(phosphoric acid) (PPA). The solu-
ꢁ
the membranes at a heating rate of 5 C min under nitrogen.
The proton conductivities of the membranes were studied with
an Autolab PGSTAT 30 impedance analyzer over a frequency
range from 100 to 105 Hz, with an amplitude of 10 mV. Speci-
mens and measuring devices were placed in a program controlled
oven to determine the temperature dependence of the proton
conductivities. The measured data were collected at various
temperatures from 80 to 160 ꢁC. The proton conductivity (s) was
calculated by the following equation:
ꢁ
tion was heated for 18 h at 175 C with stirring under nitrogen.
After cooling to room temperature, the solution was poured into
deionized water. The precipitate that formed was collected by
filtration. It was washed with deionized water and methanol
several times to remove PPA and excess 4-fluorobenzoic acid.
The brown crude product was purified by recrystallization from
DMAc to obtain the pure product (16.8 g, 81.8% yield). The 1H
NMR spectrum was used to confirm the structure of the
monomer as shown in Fig. 2. 1H NMR (DMSO-d6): d (ppm) 7.39
(d, 1H); 7.80 (s, 1H); 7.90 (d, 1H); 7.92 (d, 1H); 8.18 (d, 1H); 8.50
(s, 1H); 8.87 (s, 1H). Elemental analysis calcd for
C26H16F2N4$2H2O: C, 68.11%; H, 4.40%; N, 12.22%. Found: C,
68.01%; H, 4.54%; N, 12.28%.
s ¼ L/RA
where L is the distance between the two electrodes, R is the
resistance of the membrane and A is the cross-sectional area of
the membrane.
Preparation of the benzimidazole containing monomer
Synthesis of poly(arylene ether sulfone) (PAES)
The benzimidazole containing monomer was synthesized by the
acylation reaction as shown in Scheme 1. 6.486 g (0.04 mol) of
1H-benzimidazole-5-carboxylic acid and 10 mL SOCl2 were
added to a 100 mL three-neck flask and stirred for 16 h at room
temperature under N2 flow. The remaining SOCl2 was removed
in a vacuum by using a rotary evaporator. The crude acyl chlo-
ride derivative was obtained as a blackish green powder and used
without further purification. A solution of 7.321 g (0.02 mol) of
2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane in 24 mL
of triethylamine was added dropwise to a stirred solution of the
The synthesis of PAES was conducted in a three-neck flask with a
nitrogen inlet, a Dean–Stark trap, a condenser, and a mechanical
stirrer. The detailed synthetic procedure is as follows. The flask
was charged with the benzimidazole containing monomer (1.3811
g, 2.0 mmol). Bis(4-fluorophenyl) sulfone (0.5085 g, 2.0 mmol)
ꢁ
acyl chloride derivative in DMF at 0 C. The mixture was then
Fig. 1 1H NMR spectrum of the benzimidazole containing monomer in
Scheme 1 Synthesis of the benzimidazole containing monomer.
19270 | J. Mater. Chem., 2012, 22, 19269–19275
DMSO-d6.
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Synthesis of poly(arylene ether benzimidazole) (PAEB)
The synthetic procedure of poly(arylene ether benzimidazoles)
was similar to that of PAES, which is mentioned above. The flask
was charged with 2,20-bis(4-fluoro-phenyl)-6,60-bibenzimidazole
(0.6877 g, 1.5 mmol), benzimidazole containing monomer
(1.0358 g, 1.5 mmol), and anhydrous potassium carbonate
(0.31 g, 2.25 mmol). Then toluene (24 mL) and sulfolane (12 mL)
were carefully added. The reaction mixture was heated to 150 ꢁC
for 4 h with stirring under nitrogen. Toluene was slowly removed
from the Dean–Stark trap. After removal of toluene, the reaction
temperature was slowly increased to 220 ꢁC, and the reaction was
continued for 10 h. The solution was cooled to room temperature
and the inorganic salts were removed by filtration. The polymer
was precipitated by coagulation in stirring deionized water. The
resulting polymer was filtered and washed with water several
times. Finally, the polymer was dried in a vacuum at 100 ꢁC
for 12 h.
Scheme 2 Synthesis of 2,20-bis(4-fluoro-phenyl)-6,60-bibenzimidazole.
Preparation of membranes
PAES powder (0.5 g) was dissolved in 5 g of DMAc to prepare a
10% (w/w) solution. Membranes were cast from the viscous
solution onto glass plates with a doctor’s knife. The membranes
were dried in a vacuum at 80 ꢁC for 24 h. The resulting
membranes were yellow and transparent. The thickness of the
PAES membranes was approximately 40 mm. The PAEB
membranes were prepared in the same method.
Acid doping level of polymer membranes
Scheme 3 Synthesis of poly(arylene ether sulfone).
All membranes were immersed in aqueous PA (9 M) for 1 day
and the absorbed water was removed in an oven at 100 ꢁC until
the membrane weight remained unchanged with time. The
amount of PA in each membrane was calculated by weight
analysis. The acid-doping level (x) of the membrane was calcu-
lated as the molar number of PA per repeat unit of polymer,
which was calculated by using the following equation:
À
Áꢀ
Wdoped ꢀ Wundoped MPA
Wundoped=Mp
À
Á
x ¼
where Wdoped and Wundoped are the weight of the doped and
undoped membranes, respectively, while MPA is the molecular
weight of PA and Mp is the molecular weight of the polymer
repeat unit.
1
Fig. 2 H NMR spectrum of 2,20-bis(4-fluoro-phenyl)-6,60-bibenzimi-
dazole in DMSO-d6.
3. Results and discussion
was carefully poured into the flask with 12 mL of NMP. Then
toluene (24 mL) and anhydrous potassium carbonate (0.41 g,
3.0 mmol) were added. The reaction mixture was heated to 150 ꢁC
for 4 h with stirring under nitrogen. Toluene was slowly removed
from the Dean–Stark trap. After removal of toluene, the reaction
temperature was slowly increased to 190 ꢁC and the reaction was
continued for 24 h. The solution was cooled to room temperature
and diluted with enough NMP to allow filtering the inorganic
salts. The polymer was precipitated by coagulation in stirring
deionized water. The resulting polymer was filtered and washed
with water several times. Finally, the polymer was dried in a
vacuum at 100 ꢁC for 12 h.
Polymer synthesis and membrane fabrication
PAES with benzimidazole side chains was prepared by aromatic
nucleophilic substitution reaction from dihalides and benzimid-
azole containing monomer in the presence of K2CO3 in NMP, as
shown in Scheme 3. K2CO3 was used to convert the benzimid-
azole containing monomer into the more reactive anion. Toluene
was used as the cosolvent to azeotrope the water generated
during the polymerization. Careful control of dehydration is
essential to obtain a high molecular weight polymer in the
condensation polymerization. The sulfone, which is an electron-
withdrawing group, activates the aromatic substitution reaction.
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Although the biphenol monomer has a bulky benzimidazole side
group, we were still able to obtain a high molecular weight
polymer. The inherent viscosity of the polymer was 0.56 dL gꢀ1
in DMAc at a concentration of 0.5 g dLꢀ1 at 30 ꢁC.
PAEB was synthesized in a similar way to PAES (Scheme 4).
However, the reaction temperature needed to be raised to 220 ꢁC
in order to obtain a high-molecular weight polymer. The inherent
viscosity of PAEB was 0.93 dL gꢀ1 in DMAc at a concentration
of 0.5 g dLꢀ1 at 30 C. The reactivity of an aromatic dihalide
ꢁ
relies upon both the electron-withdrawing group present and the
halide to be substituted. The bis(4-fluorophenyl) sulfone mono-
mer showed a higher reactivity than BFPB, which is due to the
strong electron-withdrawing ability of the sulfone group.
The formation of PAES and PAEB was identified by 1H NMR
spectroscopy, as shown in Fig. 3 and 4. In Fig. 3, the signals
assigned to the aromatic protons of PAES were at 8.20 and
7.95 ppm. The other aromatic protons and benzimidazole
protons were at 7.30–8.67 ppm. In Fig. 4, the peaks at 7.43–
8.78 ppm display the aromatic region of PAEB.
The structures of BFPB, the benzimidazole containing
monomer, PAES and PAEB were further confirmed by FTIR
spectroscopy. The FTIR spectrum of BFPB in Fig. 5(a) shows a
peak at 1243 cmꢀ1, which was attributed to the stretching mode
of C–F, and a peak at 1680 cmꢀ1, which corresponded to the
stretching vibration of the C]N group. Fig. 5(b) shows the
FTIR spectrum of the benzimidazole containing monomer.
The secondary amide groups in the monomer displayed a char-
acteristic absorption peak at 1625 cmꢀ1 caused primarily by N–
H bending. The secondary amides also exhibited an amide II
band in the region of 1550 cmꢀ1. This absorption band resulted
from the interaction between N–H bending and C–N stretching
of the C–N–H group. The characteristic bands observed in the
spectrum of phenol groups resulted from O–H and C–O
stretching. The absorption bands at 3500–3200 cmꢀ1 and
1125 cmꢀ1 could be assigned to the stretching of the O–H and
C–O groups, respectively. The FTIR spectrum of PAES is shown
in Fig. 5(c). The polymer exhibited characteristic absorption
bands at 1075 and 1250 cmꢀ1 due to the symmetrical and
asymmetrical stretching vibration of the C–O–C groups. The
peaks at 1370 and 1150 cmꢀ1 were assigned to the asymmetric
Fig. 3 1H NMR spectrum of poly(arylene ether sulfone) in DMSO-d6.
and symmetric stretching of the SO2 functional groups, respec-
tively. Fig. 5(d) shows the FTIR spectrum of PAEB. The
stretching bands of C]N and N–H for the benzimidazole group
were at 1625 and 3450–3250 cmꢀ1
.
Table 1 summarizes the qualitative solubility of the PAES and
PAEB polymers. In general, conventional PBI polymers reported
in the literature exhibited poor solubility. However, PAES and
PAEB with benzimidazole side chain groups exhibited good
solubility in solvents such as DMSO, DMAc, NMP, and DMF at
room temperature. Although, the main-chain of the PAEB
polymer has rigid structure, the solubility of PAEB is as good as
Scheme 4 Synthesis of poly(arylene ether benzimidazoles).
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Fig. 6 TGA curves of PAES and PAEB in air.
Fig. 4 1H NMR spectrum of poly(arylene ether benzimidazoles) in
backbone. Furthermore, the long-term stability of the
membranes at high temperatures was assessed by isothermal
TGA analysis at 160 ꢁC under air. As shown in Fig. 7, both
membranes had very good stability. They did not show any
weight loss for 24 h at 160 ꢁC under air.
DMSO-d6.
TMA curves of the PAES and PAEB membranes are shown in
Fig. 8. The PAES and PAEB membranes had similar in-plan
coefficient of thermal expansion (CTE) values of 48.8 and
44.7 mm per m per ꢁC, respectively, in the temperature range from
100 to 250 ꢁC. From the onset temperature of the change in
slope, the Tgs of PAES and PAEB were determined to be 374 and
ꢁ
381 C, correspondingly. The unusually high Tgs of PAES and
PAEB compared to the general poly(arylene ether)s could be due
to the strong intermolecular hydrogen bonding from the amide
and imidazole groups in the side chains. PAEB has a more rigid
backbone structure than PAES, which led to a higher Tg.
Table 2 summarizes the tensile properties of PAES and PAEB
membranes with and without phosphoric acid doping. The
pristine PAES film had a tensile modulus of 1.14 GPa, a tensile
strength of 57.6 MPa, and an elongation at break of 7.8%.
Compared to that, the pristine PAEB film showed a higher
Fig. 5 FTIR spectra of the monomers and polymers: (a) BFPB, (b) the
benzimidazole containing monomer, (c) PAES, and (d) PAEB.
Table 1 Solubility of PAES and PAEB polymers
Solubilitya
Solvent
PAES
PAEB
N-Methyl-2-pyrrolidone
Dimethylacetamide
Dimethylsulfone
Dimethylformamide
Tetrahydrofuran
Methanol
+
+
+
+
ꢀ
ꢀ
ꢀ
+
+
+
+
ꢀ
ꢀ
ꢀ
Acetone
a
+ ¼ soluble; ꢀ ¼ insoluble.
PAES. This could be due to the introduction of bulky hexa-
fluoropropylidene bridging groups and the benzimidazole side
groups in the polymer backbone, reducing the intermolecular
forces between the polymer chains.
Thermal and mechanical properties of PAES and PAEB
The thermal stability of PAES and PAEB was investigated by
TGA under air atmosphere as shown in Fig. 6. Both PAES and
PAEB exhibited high thermal stability. The 5% weight loss
temperatures of PAES and PAEB were at 472 and 522 ꢁC,
respectively. PAEB showed a higher thermal stability than
PAES. It is due to the better stability of the benzimidazole
structure compared to the sulfone linkage in the polymer
Fig. 7 Isothermal TGA thermograms of (A) PAES and (B) PAEB at
160 ꢁC under air.
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Fig. 9 Proton conductivity of PAES and PAEB membranes doped with
Fig. 8 TMA curves of PAES and PAEB membranes.
phosphoric acid at different temperatures.
4. Conclusions
Table 2 Mechanical properties of PAES and PAEB membranes with
and without acid doping
A new benzimidazole containing monomer has been synthesized
for the preparation of PAES and PAEB by nucleophilic substi-
tution polymerization. Both PAES and PAEB had good
mechanical properties, thermal stability, and excellent solubility
in common organic solvents. Due to the introduction of benzi-
midazole side groups in the polymers, their membranes had high
proton conductivity after phosphoric acid doping. The proton
conductivity of phosphoric acid doped polymer membranes
increased with increasing temperatures and reached to a range of
10ꢀ3 to 10ꢀ2 S cmꢀ1 at 160 ꢁC. In addition to that, the new PAES
and PAEB membranes were easy to prepare compared to the
conventional PBI membranes.
Tensile modulus Tensile stress Elongation at break
(%)
(GPa)
(MPa)
PAES
PAES-5$6H3PO4 0.35 ꢃ 0.08
1.14 ꢃ 0.03
57.64 ꢃ 0.6
7.79 ꢃ 0.13
14.99 ꢃ 3.4 16.66 ꢃ 6.58
PAEB 1.45 ꢃ 0.08
PAEB-15$3H3PO4 0.18 ꢃ 0.01
64.14 ꢃ 4.7
10.09 ꢃ 0.6
6.89 ꢃ 1.08
12.4 ꢃ 0.91
tensile modulus (1.45 GPa) and tensile strength (64.1 MPa). This
is attributed to PAEB having a more rigid structure in the main
chain. The tensile modulus and strength of the films decreased
with increasing doping level, which could be due to the plasti-
cizing effect from phosphoric acid.
Acknowledgements
The financial support provided by the National Science Council
(Taiwan, ROC) through project NSC100-2221-E-006-052-MY3
is greatly appreciated.
Proton conductivity of PAES and PAEB membranes
The conductivity of polymer membranes is highly reliant on the
doping level of PA. The PA-doping levels of PAES and PAEB
membranes were 5.6 and 15.3, respectively. The PA-doping level
of PAES was considerably lower than that of PAEB. The acid
doping level is dependent on the number of benzimidazole
groups on the polymer repeating unit. PAEB had four benzi-
midazole groups in the polymer repeating unit, while PAES
only had two benzimidazole groups. Therefore, the PA doping
level of PAEB membranes was significantly higher than that of
PAES membranes. Fig. 9 shows the proton conductivity of
PAES and PAEB membranes doped with PA at different
temperatures. The proton conductivity of the polymer
membranes increased with increasing temperatures. The proton
conductivity of the PAES membrane was 2.6 ꢂ 10ꢀ4 S cmꢀ1 at
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