Fluorinated aromatic polyether ionomers containing perfluorocyclobutyl as cross-link groups for fuel cell applications

Article abstract of DOI:10.1021/cm203539m

The cross-linkable copolymers (SHQx-TFVys) with varying degrees of sulfonation (DS) from 70 to 95% were prepared from potassium-2,5- dihydroxybenzenesulfonate (SHQ), decafluorobiphenyl (DFBP), and 4-(trifluorovinyloxy)-biphenyl-2,5-diol (TFVOH) as a cross-linkable moiety. To develop a highly stable polymer electrolyte membrane (PEM) for application in polymer electrolyte fuel cells (PEFC)s, cross-linked membranes were prepared by chemical cross-linking. The cross-linked membranes were synthesized by varying the amount of TFVOH (5-30 mol %) in order to achieve desirable PEM properties. The structures of the cross-linkable monomer and polymers were investigated by ¹H and ¹⁹F NMR and FT-IR spectra. The cross-linked membranes exhibited good glass transition temperature and thermal stability up to 239-271 °C and 290-312 °C, respectively. The crosslinked membranes (DS range 80-95%) exhibited higher proton conductivity(0.098-0.151 S/cm) than Nafion 212 (0.092 S/cm). Moreover, all membranes possessed lower methanol permeability (13-132 × 10^-8 cm²/s) compared with Nafion 212 (163 ×10^-8 cm²/s) under the same measurement conditions. The H₂/O₂ single cell performance tests of the cross-linked membranes and Nafion 212 were performed. The CSHQ90-TFV10 exhibited the higher maximum power density (1.053 W/cm²) than that of Nafion 212 (0.844 W/cm²).

Full text of DOI:10.1021/cm203539m

Article
pubs.acs.org/cm
Fluorinated Aromatic Polyether Ionomers Containing
Perfluorocyclobutyl as Cross-Link Groups for Fuel Cell Applications
Kwan-Soo Lee,†,‡,# Myung-Hwan Jeong,†,# Young-Jea Kim,† Su-Bin Lee,† and Jae-Suk Lee*,†
†Department of Nanobio Materials and Electronics, School of Materials Science and Engineering, Energy Storage Laboratory,
Research Institute for Solar and Sustainable Energies (RISE), Gwangju Institute of Science and Technology (GIST),
123 Cheomdan-gwagiro (1 Oryong-dong) Buk-gu, Gwangju 500-712, Republic of Korea
‡Daedeok Research Institute, Honam Petrochemical Corporation, 24-1 Jang-dong, Yuseong-gu, Daejeon 305-726, Republic of Korea
S
* Supporting Information
ABSTRACT: The cross-linkable copolymers (SHQx-TFVys) with varying
degrees of sulfonation (DS) from 70 to 95% were prepared from potassium-
2,5-dihydroxybenzenesulfonate (SHQ), decafluorobiphenyl (DFBP), and
4-(trifluorovinyloxy)-biphenyl-2,5-diol (TFVOH) as a cross-linkable moiety.
To develop a highly stable polymer electrolyte membrane (PEM) for applica-
tion in polymer electrolyte fuel cells (PEFC)s, cross-linked membranes were
prepared by chemical cross-linking. The cross-linked membranes were
synthesized by varying the amount of TFVOH (5−30 mol %) in order
to achieve desirable PEM properties. The structures of the cross-
linkable monomer and polymers were investigated by 1H and 19F NMR
and FT-IR spectra. The cross-linked membranes exhibited good glass
transition temperature and thermal stability up to 239−271 °C and
290−312 °C, respectively. The crosslinked membranes (DS range 80−
95%) exhibited higher proton conductivity (0.098−0.151 S/cm) than Nafion 212 (0.092 S/cm). Moreover, all membranes
possessed lower methanol permeability (13−132 × 10−8 cm2/s) compared with Nafion 212 (163 × 10−8 cm2/s) under the
same measurement conditions. The H2/O2 single cell performance tests of the cross-linked membranes and Nafion 212 were
performed. The CSHQ90-TFV10 exhibited the higher maximum power density (1.053 W/cm2) than that of Nafion 212
(0.844 W/cm2).
KEYWORDS: cross-linking, fuel cell, perfluorocyclobutane (PFCB), polymer electrolyte membrane, proton conductivity
fuel permeability. Also, there is much research related to
improving proton conductivity and mechanical strength by
cross-linked polymer. The low relative humidities (<50% RH)
and high temperature (>80 °C) condition are researched to
develop the PEFCs.20−25 However, it has drawbacks, such as
rapidly decreased proton conductivity.43
PEMs usually require high proton conductivity and low
methanol permeability, and both factors contribute high selec-
tivity, which shows overall performance of the membrane. The
polymers with high degree of sulfonation (DS) are widely
used to achieve high proton conductivity, but it also has
drawbacks such as high water uptake and methanol
permeability. Therefore, some research groups have studied
the improvement of the selectivity under the high methanol
concentration condition. Besides, the polymers lose their
mechanical stability and limit their practical application in
PEFCs and DMFCs.26,40−44
INTRODUCTION
■
Polymer electrolyte fuel cells (PEFCs) are receiving increasing
attention due to their potential abilities as alternative power
sources for stationary, automobile, and portable applications.
The main features of the PEFCs are high energy density, low
operational temperature, and pollution free operation.1 Poly-
mer electrolyte membranes (PEMs) have attracted considerable
attention because they play the key role in PEFCs, acting as a
proton transport media and as a separator of fuel and oxidant.
The current widely used perfluorosulfonic acid copolymer
(Nafion) is considered as the state-of-the-art PEM, because it
has excellent chemical and mechanical stability with good pro-
ton conductivity. However, the drawbacks of Nafion, which
include a low glass transition temperature (ca. −20 °C)2 and
high fuel permeability, give rise to a decrease in the
performance during the fuel cell operation for high temperature
fuel cell and direct methanol fuel cell (DMFC) applications. In
addition, the material cost and difficult synthetic procedure
limit the availability for commercial use.3−5 In order to overcome
those problems of Nafion, extensive efforts have been made to
develop alternative aromatic-based ionomers, such as poly(arylene
ether)s,6−13 polyimides (PI)14−16 and polybenzimidazoles
(PBI),17−19 due to their thermal stability, low cost, and low
Cross-linking is a promising approach to improve mechanical
property and to lower methanol permeability while maintains
Received: November 27, 2011
Revised: March 20, 2012
Published: March 26, 2012
© 2012 American Chemical Society
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higher proton conductivity. So far, there have been several
studies on the application of cross-linking.27−46 Ionic cross-
linking is an interaction between different ionomer types such
as acid−base blend membranes.27−34 It showed good mech-
anical and chemical stabilities. However, ionic cross-links break
down in an aqueous environment when the temperature is
raised above 70 °C. To overcome this instability, covalently
cross-linked membranes have also been developed.35−46 The
introduction of chemical bonds has the strongest impact on the
polymer structure, because covalent cross-links fix the polymer
morphology. In general, covalently cross-linked membranes
also have excellent chemical and thermal stability, even at
elevated temperatures.
A few chemical cross-linking systems have been developed
for the aromatic based PEMs. In general, chemically cross-
linked PEMs have been prepared by three approaches.35−46
The first uses the sulfonate (SO3) groups directly as cross-
linkable moieties.35 The second adds the cross-linking media
between (or/and among) the sulfonate groups of inter- and/or
intra-polymer electrolytes.36,37 These two methods are very
simple and easy procedures for the preparation of cross-linked
PEMs. On the other hand, controlling the ion exchange
capacity (IEC) is somewhat difficult. In addition, the IEC of the
PEM is decreased due to the elimination of sulfonate groups for
cross-linking. The third option is to use other cross-linkable
moieties regardless of the sulfonate group in the PEM.38−46
The cross-linkable components are placed in various positions
of the polymer such as “in the main chain of the polymer”,38,39
“at the end group of the polymer”,40−43 and “at the side group
of the polymer”.44−46 The membrane preparation by this
method usually suffers from the thermal- or photo-cross-linking
procedure. This approach does not result in the loss of sulfonic
acid, and therefore, the PEM has the same IEC before and after
cross-linking, meaning that the IEC controllability and
treatment are much easier than other methods.
Our group has developed cross-linkable PEMs containing an
ethynyl group at the polymer end group and side group.40−44
The appropriate curing time and temperature were investigated
to optimize the PEM having good transport properties and
dimensional stability. Cross-linking densities of the PEMs also
were studied as a function of curing time for end group cross-
linkable polymers and as controlling the cross-linkable mono-
mer ratio for the side group cross-linkable polymers. The former
exhibited the better effective degree of cross-linking (DC) than
the side group cross-linkable polymers, due to higher free volume
and more flexibility of the polymer end group, while the latter
showed more controllability of DC than the end group cross-
linkable polymers. However, for the preparation of PEMs, they all
needed a high curing temperature above 220 °C, and the
enthalpy for the cross-linking of the ethynyl cross-linkable group
is around 130 kcal/mol.44 Also, the tight packing cross-linked
membranes have reduced conductivity due to the formation of
four phenyl groups from three ethynyl groups, even though they
have chance to increase the chemical stability.42−44
Herein, we report the synthesis of novel fluorinated aromatic
polyether (SHQx-TFVy) ionomers containing a new cross-
linkable group, trifluorovinyl (TFV) group, as the polymer side
group for the improvement of the cross-linked membrane
preparation procedure, and properties which exceed those of
PEMs made using ethynyl group as a side cross-linkable moiety.
This SHQx-TFVy polymer containing the TFV group has a big
advantage for the preparation of PEMs, which is to lower the
energy needed for PEM preparation. The lower onset point
around 150 °C for the reaction between the TFVs and the lower
enthalphy value around 57 kcal/mol than those of ethynyl group
(∼220 °C, ∼130 kcal/mol) make the preparation of cross-linked
PEM easier with lowering curing temperature and/or shorter
curing time.42,47 In addition, the polymerization step is an one-
step synthetic procedure compared with the end group cross-
linkable polymer. After cross-linking, the cross-linked region,
perfluorocyclobutyl (PFCB) moieties, gives rise to a more flexible
cross-linking point and better chemical resistance due to its
perfluorinated alkyl ether linkage (PFCB group with ether
linkage), compared with the cross-linking component consisting
of four phenyl moieties formed from the ethynyl side group in
polyethers. Membrane properties of cross-linked membranes
(CSHQx-TFVys) are evaluated and compared with those of non-
cross-linked membranes (NSHQx-TFVys) and our previous side-
group cross-linked membranes (SFPEx-CMys). The CSHQx-
TFVys resulted in the improvement of mechanical properties and
good hydrogen fuel cell performance at the same DS and DC
and/or similar IEC, compared to the previous structure.44
EXPERIMENTAL SECTION
■
Materials. 4-Bromophenol, bromohydroquinone, zinc, tetrakis-
(triphenylphosphine)palladium(0), potassium-2,5-dihydroxybenzene-
sulfonate, decafluorobiphenyl, acetonitrile, diethyl ether, tert-butyl-
lithium, potassium-2,5-dihydroxybenzenesulfonate (SHQ), benzene,
and N,N-dimethylacetamide were supplied by Aldrich Chemical Co.
Potassium carbonate was dried at 120 °C before use. The compounds,
4-(2-bromotetrafluoroethoxy)bromobenzene (BTB), 4-(trifluoroviny-
loxy) bromobenzene (TFB), and 4-(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane-yl)-phenyl trifluorovinyl ether (TBT) were prepared
according to the reported procedure.47,48
4-(Trifluorovinyloxy)biphenyl-2,5-diol (TFVOH). To a stirred
solution of bromohydroquinone (2.88 g, 15.0 mmol), 4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolane-yl)-phenyl trifluorovinyl ether (5.00 g,
16.0 mmol) in THF (100 mL), K2CO3 solution (2M) in water (50
mL), and Pd(PPh3)4 (0.400 g, 0.350 mmol) as a catalyst were added
(Scheme 1).12 The reaction mixture was heated at 80 °C under a
nitrogen atmosphere for 12 h. The solution was extracted with diethyl
ether and washed with H2O. The extracts were dried over MgSO4. The
crude product was purified by chromatography on silica gel using
hexane as the eluent. The yield of product, TFVOH, was above 80%
after purification. 1H NMR (CDCl3): δ = 6.85 (3H, m), 7.19 (2H, d),
7.53 (2H, dd). 19F NMR (CDCl3): δ = −118.3 (1F, dd, cis-CFCF2),
−126.2 (1F, dd, trans-CFCF2), −135.3 (1F, dd, CFCF2). IR
(KBr, cm−1): v 1833 (w, CFCF2), 1483 (st, Ar). MS (EI, m/z): 282
(M+, 100%). Anal. calcd for C22H10F12O2: C, 59.58; H, 3.21. Found: C,
59.41; H, 3.19.
Cross-Linkable Copolymer Synthesis (SHQx-TFVy). The cross-
linkable copolymers (SHQx-TFVys) were synthesized directly via an
aromatic nucleophilic reaction between potassium-2,5-dihydroxyben-
zenesulfonate (SHQ) and 4-(trifluorovinyloxy) biphenyl-2,5-diol
(TFVOH) in the presence of potassium carbonate in DMAc, with
benzene as an azeotropic agent. The molar ratio of SHQ/TFVOH was
varied to achieve desired target compositions of the copolymer, as
shown in Scheme 2. The polymerizations were conducted in a 250 mL
two-necked round-bottom flask with a nitrogen inlet and a Dean−
Stark trap with reflux condenser. A 10 mmol concentration of SHQ,
TFVOH, and 1.15 equivalent of potassium carbonate were added to
the flask, followed by N,N-dimethylacetamide (DMAc), which was
introduced to afford about 15 wt % solids concentration. Benzene was
used as an azeotropic agent in a 70/30 volume ratio of DMAc/
benzene. The reaction mixture was heated under reflux at 110 °C for 6
h in a nitrogen atmosphere to dehydrate the system. The most of the
benzene was removed and cooled to room temperature. The dihalide
monomer, decafluorobiphenyl (DFBP), was added using additional
DMAc. The temperature was increased slowly to 130 °C and kept
there for 20 h to achieve high molecular weight. The solution became
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Scheme 1. Synthesis of the New Cross-Linkable Monomer, (4-(Trifluorovinyloxy) Biphenyl-2,5-diol) (TFVOH), through 4-(2-
Bromotetrafluoroethoxy)bromobenzene (BTB), 4(Trifluorovinyloxy) Bromobenzene (TFB), and 4-(4,4,5,5-Tetramethyl-1,3,2-
dioxaborolane-yl)-phenyl Trifluorovinyl Ether (TBT)
Scheme 2. Synthesis of the Cross-Linkable Copolymers (CSHQx-TFVy)
very viscous and was cooled to room temperature. It was filtered to
remove most of the salt, and all polymers were precipitated into ethanol,
filtered, and washed extensively with deionized water two times to
remove the remaining salt.
Membrane Preparation. Membranes were prepared by casting
solutions of SHQx-TFVy ionomers dissolved in DMAc. The solution
was filtered to remove particles using a disposable syringe. The
solution placed on a hot plate with a gradually increasing temperature
from 50 to 150 °C over 1 h. Then, the hot plate temperature was
increased to 200 °C for 30 min for cross-linking through the reaction of
trifluorovinyl ethers.47−49 The membranes were acidified by immersing
in boiling 0.5 M H2SO4 aqueous solution for 2 h, followed by
treatment with boiling deionized water for 2 h.50
Characterization and Measurements. Structural Character-
izations. 1H and 19F NMR spectra were measured on a JEOL
JNM-LA 300 WB FT-NMR and were used to obtain the
chemical composition of the monomers and polymers. The
membranes were analyzed by FTIR-ATR (Jasco 460 Plus, Japan)
spectra, with spectra recorded at a 4 cm−1 resolution.
Thermal Characterizations. The thermal behavior of the
membranes in acid form was determined on a 2100 series TA Instru-
ment. Thermal degradations (Td) were determined by TGA in the range
from 50 to 800 °C at a heating rate of 10 °C/min under nitrogen
atmosphere. The glass transition temperatures (Tg) were determined by
DSC in the range from 50 to 350 °C at a heating rate of 10 °C/min
under nitrogen atmosphere.
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1
Figure 1. H NMR and 19F NMR spectra of (a) 4-(2-bromotetrafluoroethoxy)bromobenzene (BTB), (b) 4(trifluorovinyloxy) bromobenzene
(TFB), (c) 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-yl)-phenyl trifluorovinyl ether (TBT), and (d) (4-(trifluorovinyloxy) biphenyl-2,5-diol)
(TFVOH).
Inherent Viscosity (ηinh). The inherent viscosity (ηinh) of sulfonated
copolymers was measured at 25 °C in DMAc using an Ubbelohde
viscometer, thermostatically controlled in a water bath.40
(RI750F, Younglin Instrument Co., Korea) through a 1 mm diameter
silicon tube with 1.0 mL/min constant flow driven by a Masterflex
pump. The output signal was converted by a data module (Autochro,
Younglin Instrument Co., Korea) and recorded by a personal
computer.43,44
Ion Exchange Capacity (IEC). Ion exchange capacity (IEC) was
determined using the classical titration method. After acidifying and
washing the membranes, they were immersed into 0.1 M NaCl solu-
tion for 24 h to replace H+ with Na+. The remaining liquid was titrated
with 0.1 M NaOH solution using phenolphthalein as an indicator.
Gel Fraction. Gel fractions of the networks were measured by
solvent extraction. An ∼0.3 g sample was extracted by Soxhlet extractor
using dimethyl sulfoxide (DMSO) as extraction solvent at room
temperature. The samples were placed in an excess of DMSO, and the
solvent was replaced for 24 h until no further extractable polymer could
be detected. The extracted samples were dried until constant weight.43,44
Proton Conductivity. Proton conductivity measurement was
performed on fully hydrated samples (in water) and was measured
using a four-point probe electrochemical impedance analyzer (Solatron
1280Z) over a frequency range from 1 Hz to 1 MHz at room tem-
perature. Using a Bode plot, the frequency region over which the
impedance had a constant value was checked, and the resistance was
obtained from a Nyquist plot. The impedance of each sample was
measured at least five times to ensure data reproducibility.43,44
Methanol Permeability. The methanol permeability of the cross-
linked membranes was determined using a diffusion cell consisting of
two compartments, each compartment containing deionized water
(150 mL) and 2 M methanol solution (150 mL). The test membrane
was placed between the two compartments, and the diameter of the
diffusion area was 1.0 cm. During the test, both compartments were con-
tinuously stirred and were maintained at room temperature. Methanol
permeates across the membrane by the concentration gradient between
the two compartments. The methanol concentration in the deionized
water compartment was monitored using a refractive index detector
Single Cell Performance. Gas diffusion layers (GDL, from Sigracet)
were sprayed with Pt/C catalyst slurry (40 wt % Pt), and the loading
of Pt catalyst was 0.4 mg/cm2. The membrane-electrode assembly
(MEA) was obtained by sandwiching the membrane between two
sheets of GDL without hot pressing. The MEA was then set into a fuel
cell station with an effective area of 5 cm2. The test was performed
under atmospheric pressure with the cell temperature of 60 °C. The
hydrogen for anode and the oxygen for cathode, both at a flow rate
of 100 mL/min, were prebubbled in distilled water at 60 °C for
humidification. The current−voltage (I−V) relation of the cell was
recorded at a current sweep rate of 100 mA/min using an electronic
loader. For comparison, Nafion 212 was tested under the same
conditions.
RESULTS AND DISCUSSION
■
Synthesis of New Cross-Linkable Monomer (TFVOH).
A new type of fluorinated and cross-linkable monomer, TFVOH,
designed for reducing the curing temperature for the membrane
preparation procedure and for a more flexible cross-linking point
in the cross-linked network membrane with high chemical resis-
tance. Figure 1 shows the 1H NMR and 19F NMR spectra of the
new cross-linkable monomer and its raw chemicals. After Suzuki
cross-coupling reaction, a new peak was observed at 6.85 ppm in
1H NMR spectra of TFVOH. The 19F NMR spectra for TFVOH
are dominated by peaks centered at −118, −125, and −134 ppm,
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corresponding to the trifluorovinyl of TFVOH. The integration
ratio of each proton and fluorine peak was completely matched.
Synthesis of Cross-Linkable Copolymers (SHQx-
TFVys). The cross-linkable sulfonated copolymers (SHQx-
TFVys) were prepared by copolymerization of the perfluorinated
monomer (DFBP; decafluorobiphenyl) with SHQ and TFVOH,
depicted in Scheme 2. The polymer synthesis involved con-
densing a controlled amount of SHQ, TFV, and DFBP in DMAc,
which contained benzene as an azeotropic agent. To avoid
serious branching or cross-linking side reactions that have been
observed in several polymer syntheses under traditional polymer-
ization conditions, the polymerizations were therefore conducted
under very mild reaction conditions.
Table 1. Inherent Viscosity, Gel Fraction, and Thermal
Properties of SHQx-TFVy Depending on the Concentration
of Cross-Linking Moiety
c
IV
(dL/
g)
gel
fraction
(%)
a
b
d
d
d
DS
TFV
Tg
Td5%
(°C)
membrane
(%) (mol %)
(°C)
SHQ95-TFV05
SHQ90-TFV10
SHQ85-TFV15
SHQ80-TFV20
SHQ75-TFV25
SHQ70-TFV30
95
91
86
82
78
73
5
9
1.75
1.67
1.42
1.18
0.95
0.80
64.1
75.8
79.0
87.5
91.2
97.3
239
247
253
258
266
271
290
298
303
308
310
312
4
18
22
27
a
b
1
Determined by H NMR. The TFV means the contents of TFV in
The polymer structures were confirmed by 1H NMR and 19
F
c
1
SHQx-TFVys and was determined by H NMR. Inherent viscosities
NMR spectra, giving good agreement for the designed struc-
tures (Figure 2). The degree of sulfonation (DS) was
were measured at a concentration of 0.5 g/dL in DMAc at 25 °C.
d
Measured using cross-linked SHQx-TFVys (CSHQx-TFVys).
In conclusion, degree of sulfonation (DS = x) of SHQx-TFVy
is given at eq 2, where S1 and S2 are integration ratios of H-1
and H-6 areas, respectively, as shown in Figure 2.
2S
S + 2S
2
DS(x) = 100X =
(2)
1
2
The experimentally determined DS values were found to be
almost identical to the calculated DS values. Also, the cross-
linking moiety content for SHQx-TFVy was determined by
the eq 3, where X is the ratio of SHQ moiety.
ratio of cross‐linking moiety (TFV) = 1 − X
(3)
The successful introduction of the sulfonate (SO3) groups
was also confirmed by the FT-IR spectra as shown in Figure 3.
Figure 3. FT-IR spectra of CSHQx-TFVys containing TFV10, TFV20,
and TFV30.
Figure 2. 1H NMR and 19F NMR spectra of SHQx-TFVys containing
TFV10, TFV20, and TFV30.
The characteristic absorption peaks of the sulfonate groups in
CSHQx-TFVy membranes were observed at 1023, 1139, and
1233 cm−1, corresponding to the symmetric, asymmetric, and
wagging peaks, respectively. The intensity of these absorption
peaks increases with the increase of SHQ (Figure 3). All of
the results clearly indicate that the sulfonate groups were
quantitatively incorporated into the copolymer without any
side reactions and that the DS could be easily controlled by
changing the ratio of monomers of SHQ/TFVOH.
The inherent viscosity of SHQx-TFVy was in range from
0.80 to 1.75 dL/g, measured in DMAc at 25 °C. These results
show that the high molecular weight copolymers were suc-
cessfully synthesized (Table 1). However, the higher the molar
ratio of the TFVOH to the repeating unit of the copolymer is,
determined by the 1H NMR spectra for the SHQx-TFVy. The
peak H-6 at 7.45−7.54 ppm is assigned to the protons
adjacent to the sulfonic acid group, and the DS is estimated
by the integration ratio of the peak H-1 at 7.66−7.73 ppm
over the peak at S2. That is, the intensity of the H-6 signal
shows a value that is equal to the sulfonic acid group
contents.12 The DS could be calculated by comparing the
integral of S1 with the integral of S2 using eq 1, where S1
(7.66−7.73 ppm) is the H-1 area (2 × (1 − X) = 2 − 2X) and
S2 (7.45−7.54 ppm) is the H-6 area (X, X = ratio of SHQ
(Table 1)),
S
2(1 − X)
1
=
S
X
(1)
2
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the lower the inherent viscosities of the copolymer are, as a
result of the decrease of reactivity of TFVOH.
This indicates that the increase of TFV group in the polymer
improves the DC of polymer membrane. These results support
the previous results of the side-group cross-linking system.44
Overall, the gel fraction of CSHQx-TFVys was higher than that
of SFPEx-CMys.
Cross-Linking Reaction of Cross-Linkable Copolymers
(SHQx-TFVys). The maximum exothermic peak for the PFCB
formation through the thermal reaction between TFV groups in
DSC measurement appears at about 230 °C.47−49 In this study,
cross-linked membranes (CSHQx-TFVys) were prepared by
curing at 200 °C for 30 min.48 This curing temperature and
time for CSHQx-TFVy membranes provides an advantage for
PEM preparation by saving energy, compared with the previous
cross-linking system which used ethynyl groups as a cross-
linkable moiety.40−44 Also of note were the single cell
performances and mechanical properties of CSHQx-TFVy
membranes, which exhibited better performances and proper-
ties compared with the previous side-group cross-linked
membrane (SFPEx-CMy) (see Figure S1 in the Supporting
Information).44 These are discussed in detail later in this work.
The FT-IR spectra of membranes before and after curing are
shown in Figure 4. The R-CFCF2 vibration peak (1833 cm−1)
Thermal Properties. The thermal behavior of CSHQx-
TFVy membranes in acid form was evaluated by thermogravi-
metric analysis (TGA) and differential scanning calorimetry
(DSC) and was compared with that of SFPEx-CMy membranes
containing rigid cross-linking component which consists of four
phenyl group.44 All the CSHQx-TFVy samples were first
preheated to 120 °C for 15 min in the TGA furnace to remove
any moisture and followed by dynamic TGA experiments which
were run from 50 to 800 °C, at a heating rate of 10 °C/min
under nitrogen. The influence of the DS on both 5% weight
loss temperature is shown in Figure 5. All films displayed a two-
step degradation profile. The initial weight loss was observed at
290 °C, which was assigned to the decomposition of sulfonic
acid groups on the copolymer. The second decomposition,
which was observed at around 500 °C, was attributed to degrada-
tion of the copolymer backbone.
The glass transition temperatures (Tg) of the CSHQx-TFVy
copolymers in acid forms were analyzed by DSC through
heating the polymers from 50 to 350 °C at a rate of 10 °C/min
under a nitrogen atmosphere (Figure 6). The DSC results show
that the Tg of CSHQx-TFVy copolymer occurs in the range
239−271 °C, depending on the cross-linking density (Table 1).
The introduction of the cross-linking moiety (TFV) into the
copolymer raises the Tg by decreasing the flexibility of the
polymer chains because cross-linking hinders the rotational
ability of the chains, thereby leading to the increased Tg for the
sulfonated copolymers.
CSHQx-TFVys showed lower Tg and Td5% than SFPEx-CMy
at the same DS and DC, due to the two different cross-linking
components of each cross-linked membrane. This means that
the PFCB group as a cross-linking component in CSHQx-
TFVys gives more flexibility in the cross-linked membranes.
This flexible component in cross-linked membranes affects the
transport properties and single cell performances of the mem-
branes. These properties are explained in detail later in mechani-
cal properties and single cell performance sections in this work.
Mechanical Properties. Good mechanical properties of
PEMs are one of necessary requirements for their
applications because the mechanical strength of membranes
affects the fabrication conditions of the MEA and durability
of the PEFC. The membranes with various amounts of the
cross-linking moiety (TFV) were tested in the dry state to
evaluate the effect of cross-linking density on the mechanical
properties. The mechanical properties of CSHQx-
TFVy were investigated with those of the previous cross-
linked membrane (SFPEx-CMy) for the comparison of the
effect of the cross-linking component on the properties
(Figure 7).
Table 3 shows the tensile strength, Young’s modulus, and
elongation at break of the CSHQx-TFVy membranes.
CSHQx-TFVy membranes showed excellent tensile strength
ranging from 51.4 to 85.9 MPa and reasonable elongation at
break ranging from 6.01 to 10.9%. Young’s modulus for the
membranes was from 1.33 to 2.10 GPa with the increase of
the cross-linking density. On the other hand, tensile strength
and elongation at break of CSHQx-TFVy membranes decreased
with an increase in cross-linking density. The result could
be explained from the increase of brittleness with increasing
Figure 4. FT-IR spectra of CSHQ70-TFV30 (cross-linked membrane)
and NSHQ70-TFV30 (non-cross-linked membrane).
disappeared after curing, indicating that the TFV groups formed
PFCB groups, as shown in Scheme 3.47,48
The solubilities of CSHQx-TFVy and NSHQx-TFVy
membranes were tested in various solvents at room temper-
ature, and the cross-linked membranes were not soluble in any
solvent. The solubility of the cross-linked membrane is different
from that of the non-cross-linked membrane at room tem-
perature. For example, CSHQ70-TFV30 (cross-linked mem-
brane) is insoluble in all solvents, but NSHQ70-TFV30 (non-
cross-linked membrane) is soluble in DMSO, DMAc, and NMP
(Table 2). It confirms that the cross-linked membrane forms
cross-linking network structure by thermal curing and has good
chemical resistance.40
The degree of cross-linking (DC, the cross-linking density)
of cross-linkable polymers affects various PEM properties, such
as water uptake, swelling ratio, fuel permeability, and chemical
and mechanical stability. The DC of the cross-linkable copoly-
mers was determined through the heat flow calculation using
differential scanning calorimetry (DSC) analysis,42 the effect of
cross-linking time on the various properties,40 and the gel
fraction measurement of controlled cross-linkable moiety using
the various cross-linkable monomer feed ratios for the poly-
merization.44 In this study, the gel fraction measurement of the
cross-linked membranes was used for evaluating the DC of
membranes.44,46 The gel fraction values of CSHQx-TFVys were
in the range 64.1−97.3%, as shown in Table 1. The gel fractions
of the membranes increased with the molar ratio of TFVOH.
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Scheme 3. Postulated Cross-Linking Mechanism: (a) Cross-Linking Reaction of Trifluorovinyl Ether after Thermal Curing and
(b) Effect of Amount of TFV in CSHQx-TFVy
a
Table 2. Solubility of Non-Cross-Linked and Cross-Linked Membrane in Various Common Organic Solvents
b
c
d
e
sample
DMSO
DMAc
NMP
THF
methanol
I
water
NSHQ70-TFV30
CSHQ70-TFV30
S
I
S
I
S
I
I
I
I
I
I
a
b
c
d
S = soluble, I = insoluble at room temperature. DMSO: dimethylsulfoxide. DMAc: N,N-dimethylacetamide. NMP: N-methylpyrrolidone.
e
THF: tetrahydrofurane.
Figure 5. TGA curves of CSHQx-TFVys as a function of TFV mole
ratio.
Figure 6. Glass transition temperatures of cross-linked SHQx-TFVys
as a function of TFV mole ratio.
cross-linking component, which induces a decrease of toughness.
Figure 7 shows the effect of cross-linking component on the
mechanical properties as a function of the feed ratio of the
cross-linked moiety. Tensile strength and elongation at break of
CSHQx-TFVy membranes were higher than those of SFPEx-
CMy membranes at the same feed ratio of cross-linkable
moiety. These results indicated that the CSHQx-TFVy
membranes exhibited better mechanical properties compared
with SFPEx-CMy membranes and were strong and tough
enough for use as PEMs.
Swelling Ratio and Water Uptake. The swelling ratio of
the membranes was measured at room temperature (Table 4).
As expected, the swelling ratio of the CSHQx-TFVy
membranes increased with DS, because the sulfonic acid
groups on the polymer backbone increase the ionic nature of
the sulfonated copolymer. The cross-linking increased the
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can result in electrode flooding and the morphological
instability of the membrane. The water uptake also increases
with the DS of membrane, due to the strong hydrophilicity of
the sulfonic acid groups. On the other hand, the cross-linked
membranes had lower water uptake than non-cross-linked
membranes and the water uptake decreased with the increase of
the cross-linking density (Figure 8b) and the decrease of the
DS. Table 4 displays the water uptake of both cross-linked and
non-cross-linked membranes as changing DS and DC of the
membranes. Similar to the tendency of swelling ratio, the water
uptake difference of membranes between before and after
cross-linking was very huge at the same DS. For example, the
water uptake of CSHQ80-TFV20 (42%) exhibits about 6.5
times lower than that of NSHQ80-TFV20 (271%). In case of
90% DS or 95% DS membranes, the difference of water uptake
between them is presumably over 10 times. The previous
research about end-group cross-linked membranes supports
this expectation.42 In the study, the water uptake difference
between cross-linked and non-cross-linked membranes at 90−
95% DS was over 13 times.
Proton Conductivity. Proton conductivity of the mem-
branes is the most important factor for the fuel cell applications.
In general, the cross-linked membrane shows lower proton
conductivity than the non-cross-linked membrane because of
the reduced water uptake in the membrane. Figure 8c shows
the proton conductivity as a function of the TFVOH feed ratio.
As expected, the proton conductivity of CSHQx-TFVy
membranes linearly decreases with increase of the TFV feed
ratio (cross-linking density). The proton conductivity of
CSHQx-TFVy membranes is in the range 0.055−0.151 S/cm.
In particular, the proton conductivity of CSHQx-TFVy
membranes with higher than 80% DS exhibited excellent
values (0.098−0.151 S/cm), compared with Nafion 212 (0.092
S/cm). The proton conductivity of CSHQx-TFVys was
compared with the previous side-group cross-linked mem-
branes, SFPEx-CMys.42 CSHQx-TFVys (0.130 S/cm for
CSHQ90-TFV10 and 0.098 S/cm for CSHQ80-TFV20)
exhibited higher proton conductivity than SFPEx-CMys
(0.106 S/cm for SFPE90-CM10 and 0.058 S/cm for SFPE80-
CM20) at the same DS and cross-linking density. The proton
conductivity of CSHQ90-TFV10 also was higher than that of
SFPE100-CM10 at the same cross-linking density and at even
higher IEC and water uptake. These results indicate that the
more flexible PFCB groups (the cross-linked components) of
CSHQx-TFVy membranes are better than the four phenyl
groups of SFPEx-CMy membranes for improving of proton
conductivity of membranes as the cross-linked networking
system.
Methanol Permeability and Selectivity. The methanol
permeability at room temperature is plotted in Figure 8d as a
function of the feed ratio of TFVOH. The methanol
permeability of CSHQx-TFVy membranes is in the range
13−132 × 10−8 cm2/s, which was lower than that of Nafion 212
(163 × 10−8 cm2/s). The methanol permeability was
significantly reduced by the cross-linking. CSHQ75-TFV25,
for example, showed the methanol permeability (26 × 10−8
cm2/s), which is about one-fourth of that of the NSHQ75-
TFV25.
The ratio of proton conductivity to methanol permeability
(selectivity) is often used to evaluate the overall performance of
the membranes and the higher the selectivity, the better the
performance is in direct methanol fuel cell (DMFC). All the
cross-linked membranes exhibited better selectivity than Nafion
Figure 7. (a) Tensile strength (MPa) and (b) elongation at break (%)
of CSHQx-TFVys and SFPEx-CMys in dry state as a function of feed
ratio of cross-linked moiety.
a
Table 3. Mechanical Properties of CSHQx-TFVys
tensile strength Young’s modulus
elongation at
break (%)
membrane
(MPa)
(GPa)
CSHQ95-TFV05
CSHQ90-TFV10
CSHQ85-TFV15
CSHQ80-TFV20
CSHQ75-TFV25
CSHQ70-TFV30
Nafion 212
85.9
81.3
76.5
68.0
57.1
51.4
27.7
1.33
1.47
1.54
1.60
1.86
2.10
0.25
10.9
9.71
9.03
7.83
6.76
6.01
157
a
Mechanical properties of the thin dry membranes were evaluated at
room temperature on an Instron instrument at a strain rate of 10 mm/min,
and a 500 N load cell was used.
interaction of the copolymers, and the cross-linking restrained
the swelling volume to hold water; therefore, the cross-linked
membranes showed a smaller swelling ratio than the virgin
membranes (Table 4). For example, the swelling ratio of
CSHQ80-TFV20 is 16%, while that of NSHQ80-TFV20 is
79%. The cross-linking methodology can effectively improve
the dimensional stability of the membranes. As shown in Figure
8a, the swelling ratio of CSHQx-TFVy films decreased linearly
from 50 to 7%.
Appropriate membrane hydration is critical for fuel cell
applications, because of its profound effect on the proton
conductivity and mechanical property. Water can influence the
interaction between the proton and the fixed sulfonic acid
groups within the membrane. Moreover, water can change the
structure of the ionic cluster and improve proton trans-
portation. A lack of water in the electrolyte membrane leads to
poor proton conductivity, on the other hand, an excess of water
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Table 4. Membrane Properties of NSHQx-TFVy and CSHQx-TFVy
IEC (meq g−1
)
a
b
membranes
cal. IEC
2.05
exp. IEC
water uptake (%) swelling ratio (%) proton conductivity (S/cm) methanol permeability (10−8cm2/s)
c
NSHQ95-TFV05
NSHQ90-TFV10
NSHQ85-TFV15
NSHQ80-TFV20
NSHQ75-TFV25
NSHQ70-TFV30
CSHQ95-TFV05
CSHQ90-TFV10
CSHQ85-TFV15
CSHQ80-TFV20
CSHQ75-TFV25
CSHQ70-TFV30
Nafion 212
c.m.
c.m.
c.m.
c.m.
c.m.
c.m.
c.m.
c.m.
c.m.
c.m.
c.m.
c.m.
2.03
c.m.
c.m.
1.93
1.89
1.86
2.02
1.99
1.95
1.93
1.89
1.86
0.97
c.m.
2.01
c.m.
1.99
271
79
0.180
0.122
0.081
0.151
0.130
0.114
0.098
0.079
0.055
0.092
1.97
88
34
35
13
50
36
24
16
11
7
101
1.95
40
132
108
64
2.05
158
94
2.03
2.01
69
1.99
42
41
1.97
31
26
1.95
19
13
d
0.95−1.01
33
16
163
a
b
c
d
Calculated ionic exchange capacity (IEC). Measured ionic exchange capacity (IEC). c.m.: cannot be measured. Data obtained from ref 52.
Figure 8. Influence of the TFV feed ratios on membrane properties for CSHQx-TFVy.
212, indicating better performance of the cross-linked
membranes (Figure 9). For example, CSHQ80-TFV20 showed
a selectivity (239 × 103 S·s/cm3). It is approximately 4 times
higher than the selectivity of Nafion 212 (56 × 103 S·s/cm3).
These results indicate that CSHQx-TFVy membranes are
promising candidates for DMFC.
Single Cell Performance. The fabrication of a mem-
brane−electrode assembly (MEA) with newly synthesized
cross-linked membranes requires considerable effort to identify
the proper electrode/binder composition and attachment con-
ditions. Often, an MEA preparation using hot pressing method
was failed, not because the membrane performs poorly, but
rather due to poor adhesion of the electrodes to the membrane
and insufficient catalyst utilization due to problems associated
with the catalyst binder and electrode/membrane interfacial
microstructure. To avoid such difficulties, cross-linked mem-
branes and Nafion 212 were prepared without hot pressing
under same condition and sufficient catalyst was supplied
(Pt catalyst loading of 0.400 mg/cm2).
Figure 10 shows the single cell performance of CSHQ90-
TFV10, CSHQ85-TFV15, and Nafion 212 at 60 °C. The
maximum power densities of CSHQ90-TFV10 and CSHQ85-
TFV15 were 1.05 and 0.889 W/cm2, respectively. CSHQ90-
TFV10 and CSHQ85-TFV15 showed better performance than
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CONCLUSIONS
■
Novel cross-linkable polymers (SHQx-TFVys) were synthe-
sized containing the trifluorovinyl ether (TFV) group at the
polymer side chain as a cross-linkable moiety. A thermal cross-
linking reaction was conducted between the TFV groups at
lower temperature and for shorter time (200 °C and 30 min)
compared with the previous method for ethynyl groups
(250 °C and 2 h). No IEC change was observed after comple-
tion of the cross-linking reaction. The CSHQx-TFVy mem-
branes showed a huge reduction of proton conductivity, water
uptake, swelling ratio, and methanol permeability compared
with those of the NSHQx-TFVy membranes, and the difference
in properties increased with the DS. The proton conductivity of
the membrane ranged between 0.055 and 0.151 S/cm depend-
ing on DS and DC. These copolymers have relatively lower
methanol permeability with comparable proton conductivity to
Nafion 212. This cross-linked membrane (CSHQx-TFVy)
exhibited better mechanical properties and single cell perform-
ances than our previous cross-linked membrane (SFPEx-CMy).
Tensile strength and elongation at break of CSHQx-TFVy were
higher than those of SFPEx-CMy, indicating that the flexibility
of cross-linking point (PFCB group for CSHQx-TFVy and four
phenyl group for SFPEx-CMy) affects those mechanical pro-
perties. CSHQx-TFVy membranes showed excellent single cell
performance (power density) as compared to SFPEx-CMy mem-
branes and Nafion 212. These results indicate that CSHQx-TFVy
series is a promising candidate for optimizing the structure of
cross-linked membrane and for a polymer electrolyte membrane
for fuel cells (DMFCs and PEMFCs).
Figure 9. Selectivity of CSHQx-TFVys with various concentrations of
TFV moiety.
ASSOCIATED CONTENT
■
S
* Supporting Information
Additional figures and tables. This material is available free of
charge via the Internet at http://pubs.acs.org.
Figure 10. Single cell performance of CSHQ90-TFV10, CSHQ85-
TFV15, and Nafion 212.
AUTHOR INFORMATION
■
Nafion 212 (0.844 W/cm2) due to a lower membrane
resistance. The difference of the maximum power density
between CSHQ90-TFV10 and CSHQ85-TFV15 is bigger than
that between CSHQ85-TFV15 and Nafion 212 compared with
the differences of their proton conductivity values. That is, the
difference of the maximum power density between the cross-
linked membranes and Nafion 212 can also be explained by
Nafion binder solution that was used for the electrode
fabrication, because the interaction between the Nafion binder
solution and cross-linked membranes is incompatibility. That is,
both Nafion 212 and CSHQx-TFVy membranes have excellent
proton conductivity and good mechanical and chemical pro-
perties, while fuel cell performance using the CSHQx-TFVy
membranes often suffers from their poor interfacial compati-
bility with Nafion-bonded electrodes. Poor interfacial compat-
ibility results in difficulties in membrane−electrode fabrication,
increased cell resistance (reduced cell performance), local
flooding, and gradual performance degradation over time.51
The single cell performance of CSHQx-TFVy membranes
was compared to that of SFPEx-CMy. Membranes that were
used for these cell performance comparisons have similar IEC
(1.91−1.99 meq g−1) and DS values. Figure 10 and Figure S2
(see the Supporting Information) show the cell performances
of CSHQx-TFVy, SFPEx-CMy, and Nafion 212. CSHQx-TFVy
membranes exhibited higher cell performance than SFPEx-
CMy and Nafion 212.
Corresponding Author
*Phone: +82-62-970-2306. Fax: +82-62-970-2304. E-mail:
jslee@gist.ac.kr.
Author Contributions
#These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by New & Renewable
Energy R&D program (20093020030030-11-1-000) under the
Ministry of Knowledge Economy, World Class University
(WCU) Program (Project Number: R31-20008-000-10026-0),
the Research Foundation of Korea(NRF) funded by the
Korean government (MEST) (Grant No. 2010-0027330), and
the Core Technology Development Program for Next-
Generation Energy Storage of Research Institute for Solar
and Sustainable Energies (RISE), GIST.
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