Highly selective proton conductive networks based on chain-end functionalized polymers with perfluorosulfonate side groups

Article abstract of DOI:10.1039/c000044b

The copolymers of vinylidene fluoride and perfluoro(4-methyl-3,6-dioxane-7- ene) sulfonyl fluoride containing amino end-groups were synthesized for the first time. The prepared amino-terminated polymers underwent cross-linking reactions with 1,3,5-benzene

Full text of DOI:10.1039/c000044b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Highly selective proton conductive networks based on chain-end
functionalized polymers with perfluorosulfonate side groups†‡
*
Kui Xu, Chalathorn Chanthad, Michael A. Hickner and Qing Wang
Received 5th January 2010, Accepted 3rd March 2010
DOI: 10.1039/c000044b
The copolymers of vinylidene fluoride and perfluoro(4-methyl-3,6-dioxane-7-ene) sulfonyl fluoride
containing amino end-groups were synthesized for the first time. The prepared amino-terminated
polymers underwent cross-linking reactions with 1,3,5-benzene triisocyanate to form proton
conductive networks. The prepared membranes exhibited excellent thermal, hydrolytic and oxidative
stabilities. The ion exchange capacity, water uptake, the state of absorbed water, and transport
properties of the membranes were found to be highly dependent upon the chemical composition of the
copolymers. The cross-linked membranes showed extremely low methanol permeability, while
maintaining high proton conductivity at the same order of magnitude as Nafion. This unique transport
feature gave rise to exceedingly higher electrochemical selectivity in relation to Nafion. The selectivity
characteristics have been rationalized based on the formation of restrained ionic domains and the state
of the absorbed water within the membranes.
Considerable efforts have thus been devoted to the exploration
Introduction
of new approaches to PEMs exhibiting a lower intrinsic meth-
anol permeability than Nafion.7 These strategies include hydro-
carbon electrolyte polymers,8–10 incorporation of inorganic
additives into Nafion or other polymer matrix,11–13 and cross-
linked polymers and blends.14–18 Nonetheless, in these alternative
materials, a great depression in methanol permeation is accom-
panied by a severe decrease in proton conductivity. The trade-off
between reduced methanol permeability and decreased conduc-
tivity leads to a marginal improvement in electrochemical selec-
tivity, i.e. the ratio of proton conductivity over methanol
permeability, which is a well accepted figure-of-merit to evaluate
the potential of membrane performance in DMFCs. The relative
electrochemical selectivity to Nafion for most PEMs with suffi-
cient conductivity (i.e. > 0.01 S cmꢀ1) is less than 10.7
Fuel cells are considered one of the most promising energy
conversion technologies by virtue of their relatively low-
temperature operation, solid-state design, and their high effi-
ciencies across a large range of sizes.1 In particular, direct
methanol fuel cells (DMFCs) have the potential to become high
density power sources for portable and mobile applications
such as microelectronics. Methanol has a volumetric energy
density of 4800 Wh lꢀ1, which is an order of magnitude greater
than state-of-the-art lithium batteries. If this energy can be
harvested electrochemically in an efficient matter, DMFCs can
drastically increase the run-time of many portable electronics
applications and enable new power-hungry mobile devices.
However, the use of liquid methanol as fuel causes other
complications to the fuel cell system. A critical issue associated
with this is methanol crossover,2,3 namely the tendency of
methanol crossing from the anode, through the polymer elec-
trolyte membrane (PEM), to the cathode catalytic layer. The
presence of methanol at the cathode hampers oxygen reduction
by occupying a part of the active catalyst sites and leads to
a mixed potential that lowers cell voltage.4 Other concomitant
negative impacts include severely reduced fuel utilization and
aggravated water management issues.5 Nafion, the current state
of the art PEM material, is known to have a poor barrier
property to methanol permeation, and performs poorly in
DMFCs.3,6
Here, we present the preparation and characterization of the
chain-end cross-linked fluoropolymer networks showing
extraordinarily high electrochemical selectivities. Cross-linking
has been demonstrated as a promising approach to decreasing
the methanol permeability via restricting the water adsorption of
PEMs.14 Electron-beam and g-irradiation have been applied to
PEMs to induce cross-linking, which also results in chain scission
and yields membranes with poor mechanical properties.19 Cross-
linking reactions have been performed through a reaction with
the sulfonic acid groups to form inter/intra-chain bridges.20,21
However, utilization of sulfonic acid groups leads to a decrease in
ion content of the membranes, and consequently a lower proton
conductivity. Another cross-linking approach involves incorpo-
ration of co-monomers with pendant functional moieties fol-
lowed by a reaction with a multifunctional curing agent.22
Unfortunately, the resulting cross-linked structures are often
difficult to control and undesirable degrees of cross-linking are
obtained, which restricts proton diffusion rates in the
membranes.
Department of Materials Science and Engineering, The Pennsylvania State
University, University Park, Pennsylvania, 16802, USA. E-mail: wang@
matse.psu.edu; Fax: +1 814-865-2917; Tel: +1 814-863-0042
† This paper is part of a Journal of Materials Chemistry themed issue on
proton transport for fuel cells. Guest editors: Sossina Haile and Peter
Pintauro.
‡ Electronic supplementary information (ESI) available: Additional
characterisation and humidity dependence data. See DOI:
10.1039/c000044b
Different from the literature approaches, the present study is
based on the functionalization of polymer chain ends and
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subsequent utilization of functional end-groups in the cross-
linked reaction to yield proton conductive networks. It is antic-
ipated that the selective placement of cross-linkable groups at
polymer chain ends can keep the main-chain structure of the
fluoropolymer intact and thus preserve their unique physical
properties such as excellent electrochemical stability and superior
acidity. Moreover, the chain-end cross-linking approach would
maintain continuity of the proton conduction channel of linear
polymers to a great extent. By varying polymer molecular
weights and the relative ratio between the polymer and curing
agent, molecular structures and domain sizes of the cross-linked
networks can be systematically tuned for optimized properties.
in THF, precipitating from water containing a small amount of
NaOH (pH ꢂ 10). The precipitate was then washed thoroughly
with neutral water and dried in in vacuo at 60 ꢁC. 1H MMR (d6-
DMSO, ppm): d 7.67 (d, ArH), 6.59 (d, ArH), 6.19 (s, -NH2), 4.63
(m, -COO-CH2CF2-), 2.9 (-CF2CH2-CF2CH2-, head-to-tail), 2.3
(-CF2CH2-CH2CF2-, tail-to-tail). 19F NMR (d6-DMSO, ppm):
d ꢀ77.0 to ꢀ80.0 (-OCF2CF(CF3)OCF2CF2SO2F), ꢀ92.4 to
ꢀ95.8 (-CH2CF2CH2-), ꢀ110.2 to ꢀ110.8 (-CH2CF2-
CF2CF(ORFSO2F)), ꢀ112.3 (-OCF2CF2SO2F), ꢀ114.8 and
ꢀ116.5 (-CH2CF2CF2CH2-), ꢀ123.2 (-CF2CF(ORFSO2F)),
ꢀ127.8 (-CF(ORFSO2F)-CF2CH2-), ꢀ145.9 (-OCF2CF-
(CF3)OCF2CF2SO2F).
Membrane preparation
Experimental
A fine-grinded mixture of amino terminated P(VDF-PFSVE)
(0.4 g) and 1,3,5-benzene triisocyanate (12 mg) was heated at
180 ꢁC for 1 h. The cross-linked polymer was then placed
between two Teflon sheets and hot-pressed into a ꢂ 80 mm thick
film. To hydrolyze the sulfonyl fluoride groups, the film was
immersed in a mixture of triethylamine (5 g), methanol (75 mL)
and water (75 mL) at 35 ꢁC for 10 h. After being rinsed by
de-ionized (DI) water, the film was immersed in 6 M HCl
aqueous solution at 35 ꢁC for 12 h followed by being washed with
DI water till neutral.
1,3,5-Benzene triisocyanate was prepared according to the
literature procedures.23 Vinylidene fluoride (VDF) and
perfluoro(4-methyl-3,6-dioxane-7-ene)
sulfonyl
fluoride
(PFSVE) were purchased from SynQuest Laboratory Inc. and
used as received. All other chemicals and solvents were
purchased from VWR international, Inc. and used as received
unless otherwise mentioned.
Synthesis
4,40-tert-Butoxycarbonylamino benzoyl peroxide. To a mixture
of DCC dichloromethane solution (5 mL, 1M) and H2O2
aqueous solution (5 mL, 30%), 4-butoxycarbonylaminobenzoic
acid (1.2 g, 5.1 mmol) in dichloromethane was added slowly at
Characterization
Water uptake and ion-exchange capacity (IEC). The water
uptake on mass basis was determined gravimetrically. The
membrane in acid form was dried under vacuum at 60 ꢁC for
24 h, and cooled to room temperature in a desiccator before
measuring the weight in ‘‘dry’’ state, Wd. The membrane was
then allowed to equilibrate in water at 30 ꢁC for overnight. After
removal of the surface water, the weight in ‘‘wet’’ state, Ww, was
measured. The water uptake was calculated as the percentage
increase from the ‘‘dry’’ to ‘‘wet’’ weight related to Wd. The IEC
of the membrane was determined by titration of the sulfonic acid
groups. Membranes were equilibrated in a large excess of 2 M
Na2SO4 solution for at least 12 h at room temperature prior to
titration. The protons released into solution were titrated with
0.02 M NaOH aqueous solution using phenolphthalein as an
indicator.
ꢁ
ꢁ
ꢀ10 C. The reaction mixture was stirred at 0 C for 6 h. The
reaction mixture was filtered and the filter cake was washed with
cold dichloromethane several times. The filtrates were combined
and the solvent was evaporated in vacuo. The residue was
re-dissolved in cold dichloromethane and the above step was
1
repeated twice to give a white solid (1.08 g, 90%). H NMR
(d4-THF, ppm): d 8.97 (s, 1H), 7.93 (d, 2H, ArH), 7.64 (d, 2H,
ArH), 1.51 (s, 9H).
P(VDF-PFSVE) with amine end groups. 4,40-tert-But-oxy-
carbonylamino benzoyl peroxide (335 mg, 0.75 mmol), 1,1,1,3,3-
pentafluorobutane (15 mL), acetonitrile (15 mL) and a certain
amount of PFSVE (see Table 1) were added into a 70 mL Parr
reactor with a magnetic stir. The gas-condense transfer of VDF
(15 mL) was carried out with the rigorous exclusion of oxygen
and moisture through a dual-manifold Schlenk line with 10ꢀ6
Torr high vacuum. The reactor was immersed in an oil bath at
90 ꢁC for 8 h. After residual gases were discharged, the solvents
were evaporated, and the residue was dissolved in acetone and
precipitated from cold hexane. P(VDF-PFSVE) was collected
and dried in vacuo at 60 ꢁC. 1H MMR (d6-DMSO, ppm): d 9.67
(s, -NH-), 8.17 (d, ArH), 7.51 (d, ArH), 4.63 (m, -Ph(C]O)O-
CH2CF2-), 2.9 (-CF2CH2-CF2CH2-, head-to-tail structure), 2.3
(-CF2CH2-CH2CF2-, tail-to-tail structure), 1.58 (s, -CH3).
To a mixture of tert-butoxycarbonyl amino terminated
P(VDF-PFSVE) (0.5 g) in anhydrous dichloromethane (15 mL) 3
drops of iodotrimethylsilane was added at 0 ꢁC. After being
stirred for 3 h, the reaction mixture was condensed under reduced
pressure and poured into cold hexane to precipitate the polymer.
The collected polymer solid was further purified by re-dissolving
Proton conductivity measurement. In-plane proton conduc-
tivity of the membranes was measured by a four-probe AC
impedance method. Impedance data was acquired using Solar-
tron 1260 gain phase analyzer over the frequency range of 1 Hz–
100 kHz. Conductivity measurements under fully hydrated
conditions were performed after the samples were equilibrated in
water at various temperatures for at least 4 h. Temperature was
ꢁ
varied from room temperature to 80 C.
Methanol permeability measurement. The methanol perme-
ability was determined in a standard membrane separated
diffusion cell. A glass cell containing solutions A and B in two
identical compartments separated by the test membranes was
utilized for the permeability tests. The membranes were placed
between the two compartments by a screw clamp. Solution A was
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a methanol aqueous solution with different concentrations and
solution B was DI water. Both compartments were magnetically
stirred during the permeation experiments. The concentration of
methanol in solution B was evaluated using a differential
refractometer. The methanol permeability was calculated from
the slope of the straight-line plot of methanol concentration
versus permeation time. The methanol permeability was tested at
ꢁ
temperatures ranging from 30 to 70 C.
Water state. To characterize the state of absorbed water in the
hydrated membranes, low temperature DSC measurements were
performed. Fully hydrated membrane samples (10–13 mg) were
blotted with a lab wipe to remove surface water, and then
instantly placed into a DSC pan (O-ring model TA Instruments)
and sealed. The sample was immediately put in the calorimeter
and cooled to ꢀ80 ꢁC. In a typical run, after being held at ꢀ80 ꢁC
foꢁr 20 min, the samples were heated to 100 ꢁC at a heating rate of
5 C minꢀ1. The heat of fusion for the water in the membranes
Scheme 1 Scheme showing the synthesis of amino-terminated P(VDF-
PFSVE).
was estimated from DHf ¼ H/mH O, where DHf is the heat of
2
fusion for the water contained in the sample, mH O is the mass of
2
minimized chain transfer reactions. The presence of the Boc
moieties at polymer chain ends were confirmed by the 1H NMR
spectra. As shown in Fig. 1a, the signals at 1.58, 8.17 and 7.51
ppm are assigned to the protons of tert-butyl and phenyl groups,
respectively. Amine groups were released from Boc protecting
groups using iodotrimethylsilane, as evidenced by disappearance
of the peak at 1.58 ppm belonging to tert-butyl groups and
emergence of resonances at 6.19 ppm attributed to amine groups
as shown in Fig. 1b. The small triplet in the region of 6.2–6.6 pm
corresponds to a -CF2H group resulting from a short chain-
branching process, which involves an intramolecular 1–5
hydrogen shift analogous to similar radical reactions reported in
low-density polyethylene.
water within the sample, and H is the integrated energy from the
melting endotherm. The DHf value for bulk water is 334 J gꢀ1
.
Stability. Oxidative stability of the cross-linked membranes
was tested by immersing membrane samples in Fenton’s reagent
(3% H2O2 containing 2 ppm FeSO4) at 60 ꢁC for 1 h. Hydrolytic
stability was estimated by soaking the membrane in DI water at
80 ꢁC for 2 weeks. Before and after the stability testing, all
membranes were dried under vacuum at 60 ꢁC overnight and
weighed. Gravimetric analyses were conducted after these
treatments.
As summarized in Table 1, by manipulating the monomer feed
ratio and the reaction time, P(VDF-PFSVE) copolymers (I–IV)
Results and discussion
Synthesis and preparation of the cross-linked membranes
Chain-end functionalization of fluoropolymers is notoriously
difficult due to a lack of suitable living/controlled polymerization
techniques for fluorinated alkenes. The telomerization process
with alkyl halides as transfer agents, is a commonly used method
to produce halogen-terminated fluoropolymers, and usually
generates low molecular weight (<4,000 g molꢀ1) fluorinated
oligomers.24 More recently, we have developed a new synthetic
route toward high molecular weight fluoropolymers with well-
defined functional end-groups,25,26 in which functionalized
benzoyl peroxides (BPOs) are used as the initiator in radical
polymerization of fluorinated alkenes. The domination of the
radical coupling reaction and absence of disproportionation in
the termination process of polymerization allow for the complete
transfer of functional groups carried by the initiators into the
polymer chain ends upon polymerization.27
As illustrated in Scheme 1, BPO containing t-butoxycarbonyl
(Boc) protected amine groups was synthesized from the homo-
coupling of Boc-amino-benzoic acid in the presence of H2O2 and
N,N0-dicyclohexylcarbodiimide (DCC). The functional BPO was
subsequently utilized as the initiator for co-polymerization of
VDF and PFSVE. The polymerization was carried out in the
presence of 0.5 mol% of the functional initiator, and a mixture
solvent of acetonitrile and 1,1,1,3,3-pentafluorobutane that has
Fig. 1 1H NMR spectra of chain-end functionalized P(VDF-PFSVE)s.
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Table 1 Chemical composition and molecular weight characteristics of
P(VDF-PFSVE) copolymer
Table 2 The IEC, water uptake, hydration number (l) and heat of
fusion (DHf) of absorbed water of the cross-linked P(VDF-PFSVE)
membranes and Nafion 117
Monomer
ratio (mol%)
Composition
(mol%)
IEC (meq/g)
Water
Copolymer VDF PFSVE VDF PFSVE Mn, GPC/g molꢀ1 PDI
Membrane
IECa
IECb
Uptake (wt.%)
l
DHf/J gꢀ1
I
II
III
IV
94
91
88
85
6
9
12
15
94.7
91.5
89.8
86.9
5.3
8.5
10.2
13.1
19,800
20,600
17,400
16,600
1.6
1.6
1.6
1.7
I
II
III
0.63
0.89
0.99
1.15
0.45
0.87
0.97
1.14
0.91
4
4.9
8.9
10.6
12.6
17.7
10
24
61
98
14
19
26
29
IV
Nafion 117
101
a
b
Calculated from copolymer compositions. Determined by titration
method.
with the PFSVE content ranging from 5.3 to 13.1 mol% were
obtained. The copolymer compositions were determined using
1H and 19F NMR spectroscopy.28 The number-average molecular
weights of the polymers were around 16–20 kg molꢀ1 with
polydispersities of ꢂ1.7.
increase of PFSVE content. In membranes II, III and IV,
experimental IEC results were found to be in good agreement
with their corresponding theoretical values estimated from VDF/
PFSVE composition. This further indicates the quantitative
conversion of salt into acid forms of the films and the effective-
ness of the new hydrolysis procedure. The deviation shown in
membrane I is believed to be owing to low sulfonate content and
difficulty in formation of a continuous ionic phase that is
accessible for ion exchange.
The cross-linking of P(VDF-PFSVE)s was conducted through
a condensation reaction between amine end-groups and a triiso-
cyanate agent. FTIR spectra confirmed the formation of urea
linkages by the appearance of urea bands at 3348 cmꢀ1 (N–H
stretching vibration), 1632 cmꢀ1 (C–N stretching), and 1546 cmꢀ1
(N–H bending) and a lack of absorbance at 2267 cmꢀ1 from
isocyanate groups. The accomplishment of cross-linking was also
supported by the changes of solubility in organic solvents. The
cross-linked membranes became insoluble in acetone, tetrahy-
drofuran and methanol, whereas their precursors, P(VDF-
PFSVE)s, can be easily dissolved in these solvents. A further
evidence for the cross-linking reaction was provided by thermal
analysis. The cross-lꢁinked membrane showed a broader melting
transition at ꢂ 120 C, and the crystallization temperature was
found to decrease by nearly 7 ꢁC relative to its polymer precursor
(ꢂ 79 ꢁC). It is known that the formation of cross-linking struc-
tures suppresses the recrystallization and leads to a reduction in
crystal sizes and in turn a lower crystallization temperature.25
Hydrolysis of the sulfonyl fluoride groups in the membranes is
required to generate the proton conducting acid sites. Strong
bases such as sodium hydroxide and potassium hydroxide are
usually used in the hydrolysis of sulfonyl fluorides. However, they
are undesirable here because of the vulnerability of VDF units
under basic conditions.29 Lithium carbonate has been employed
as a mild catalyst for base hydrolysis30 but unfortunately, the lack
of solubility in protic solvents that are used in swelling the cross-
linked films limits its utility in this case. To this end, a new mild
hydrolysis process was developed, which is based on triethyl-
amine/methanol as a catalyst to convert sulfonyl fluoride groups
into ammonium salts.31 Ion exchange was accomplished with HCl
aqueous solution, which was confirmed by the complete disap-
pearance of S–F stretching resonance at 816 cmꢀ1 in the FTIR
spectra. Concurrently, the FTIR spectra of the sulfonic acid form
of the films showed the characteristic absorbance of -SO2F groups
at 1462 cmꢀ1 attributed to S ] O stretching and a broad peak
assigned to acid groups at 3000 to 3300 cmꢀ1. The retention of the
urea linkages in the acid membranes was indicated by the
As expected, higher IEC membranes have increased water
uptake as a result of increased amount of acid groups interacting
with water molecules. Interestingly, in comparison with Nafion
117 film that has an IEC of 0.91 mmol gꢀ1 and a water uptake of
29%, membranes II and III with similar IEC values of 0.87 and
0.97 mmol gꢀ1 exhibited much lower water uptakes, only 14%
and 19% respectively. Additionally, it is worthy of note that
negligible change of water uptake was observed in the cross-
linked membranes when temperature was increased to 60 ꢁC,
which is indicative of stabilized water domain structures by the
cross-linked networks. Analogous to water uptake, hydration
number l, i.e. the average number of water molecules per
sulfonic acid group, showed strong dependence on IEC as well.
As outlined in Table 2, when IEC was increased to or more than
0.87 mmol gꢀ1, l attained a value of 8.9 or higher, which suggests
sufficiently high hydration levels in the membranes to facilitate
dissociation of sulfonic acid and proton transportation.
Thermal and chemical stability
Thermal stability was evaluated by thermal-gravimetric analysis.
As shown in Table 3, the on-set decomposition temperatures of
the cross-linked membranes were independent of IEC and stayed
in the narrow range 265–270 ꢁC, which is comparable with
Nafion. Accelerated hydrolytic and oxidative stability tests
combined with gravimetric and tensile measurements were also
executed to assess the chemical stability of the membranes. It was
found that the cross-linked membranes remained intact and
retained more than 95% of their original weight and modulus
after the tests, indicating their excellent hydrolytic and oxidative
stabilities. We postulate that the urea linkages, the most
susceptible segment to hydrophilic attack by water molecules and
oxidative attack by radical species, is protected by the
surrounding hydrophobic fluorinate chains that exclude water
molecules and OH radicals in aqueous solution.
unchanged absorption band at 1632 cmꢀ1
.
IEC and water uptake
Table
2 compares the characteristics of the cross-linked
membranes. The IEC of the membranes increases with the
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a significant role in determining the transport properties.32,33 It is
speculated that the permeation of methanol through ionic
membranes is strongly correlated to the unbound or free water.34
Free water is defined as water that is not intimately bound to
polymer backbone or ionic groups and exhibits similar thermal
transitions to bulk water. The low temperature DSC measure-
ments were carried out to elucidate the nature of water confined
in the membranes. Fig. 3 illustrates DSC curves of the fully
hydrated membranes, and the calculated DHf of the abosrbed
water is tabulated in Table 2. Since the strongly bound water
does not display thermal transitions and contribute to the
melting endotherm, the DHf value is generally proportional to
the fraction of free and weakly bound water. Consistent with the
trend in l and water uptake, DHf has been found to increase
simultaneously with increasing IEC. Membranes I and II con-
tained primarily bound water as indicated by indiscernible
melting endotherms and low DHf values, 10 and 24 J gꢀ1
respectively. Membranes III and IV showed higher DHf values of
61 and 98 J gꢀ1 respectively, indicating an increased fraction of
unbound and weakly bound water, but still less than Nafion with
Table 3 Thermal, hydrolytic and oxidative stability of the cross-linked
P(VDF-PFSVE) membranes and Nafion 117
Residue after
hydrolytic testing
(wt.%)
Residue after
oxidative testing
(wt.%)
Membrane
Td/ꢁC
I
II
III
270
265
267
265
275
98
97
97
95
99
98
98
98
96
98
IV
Nafion 117
Membrane morphology
Morphological structures in the cross-linked membranes and
Nafion were examined by TEM. Fig. 2 presents cross-sectional
TEM images of the membranes stained with lead acetate, in
which ionic domains appear as dark areas due to their affinity
with lead ions and the bright domains correspond to the
hydrophobic phases. It was found that the sizes of the ionic
aggregates in the cross-linked membranes displayed gradual
increases from I to IV due to the increased sulfonic acid
a DHf of 101 J gꢀ1
.
concentrations.
A microphase-separated morphology with
Proton conductivity and methanol permeability
hydrophilic domain size at the scale of 5–10 nm was evident in
Nafion. Contrastingly, phase separation in the cross-linked
membranes (Fig. 2b-e) was not as distinct as in Nafion. The size
of the ionic regions in the cross-linked membranes is markedly
smaller than that of Nafion, e.g. an average of only 2–3 nm in
membranes III and IV with IECs even higher than Nafion. It
appears that self-assembly behaviors in these membranes are
hindered by the formation of cross-linked structure, leading to
small ionic aggregates and in turn limited water absorption.
Fig. 4a depicts the effect of IEC on the proton conductivity and
activation energy. In accordance with water uptake and l, the
proton conductivity rose progressively from 0.005 S cmꢀ1 for
membrane I to 0.063 S cmꢀ1 for membrane IV with the increase
of IEC. In comparison with Nafion, the cross-linked membranes
exhibited reduced proton conductivities. The difference in
conductivity between Nafion and the cross- linked membranes is
probably due to the concomitant higher content of water sorp-
tion and corresponding enhancement of phase separation in
Nafion. It is important to note that membranes II, III and IV
The state of absorbed water
exhibited appreciably high conductivities of above 0.02 S cmꢀ1
,
In addition to membrane morphology, the state of the absorbed
water within the membranes has also been suggested to play
attributable to their high concentrations of PFSVE and strong
acidity of perfluorosulfonic acid groups. Activation energy for
Fig. 2 Cross-sectional TEM images of (a) Nafion 117 and the cross-linked membranes, (b) I, (c) II, (d) III and (e) IV. The scale bar denotes 10 nm.
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temperatures (Fig. 4b). The highest value of Ea,s was found with
membrane I followed by membranes II, III and then IV. Nafion
exhibited the lowest value of Ea,s. This order mirrors that of
proton conductivity and correlates well with the amount of water
contained in the membrane and the domain sizes of hydrophilic
phases.
Methanol permeability and related activation energy data are
plotted as a function of IEC in Fig. 5a. Extremely low methanol
permeabilities of 1.12 ꢃ 10ꢀ9 and 8.36 ꢃ 10ꢀ9 cm2 sꢀ1 were found
with membranes I and II respectively, which are more than two
orders of magnitude less than Nafion with a methanol perme-
ability of 1.96 ꢃ 10ꢀ6 cm2 sꢀ1. The high fraction of strongly
bound water as revealed in low DHf values from DSC
measurements and small size of ionic domains are likely
responsible for the observed excellent methanol barrier proper-
ties in the membranes. Further increase in IEC leads to
a monotonic increase in methanol permeability. Membranes III
and IV exhibit methanol permeabilities of 4.98 ꢃ 10ꢀ8 and 1.35 ꢃ
10ꢀ6 cm2 sꢀ1, respectively, which are still much lower than Nafion.
Fig. 3 DSC curves of the melting endotherms for the absorbed water
within the cross-linked membranes and Nafion 117.
proton conduction, Ea,s, is a measure of the barrier to proton
movement occurring in the membrane. Ea,s was calculated from
the Arrhenius plot, where the proton conductivity of the
membranes was measured in liquid water at various
The activation energy for methanol permeation, Ea,methanol
,
estimated from the Arrhenius plot of methanol permeability
Fig. 5 (a) Methanol permeability and activation energy of the cross-
linked membranes as a function of IEC at 30 ꢁC. (b) Temperature
dependence of methanol permeability of the membranes under fully
hydrated conditions.
Fig. 4 (a) Proton conductivity and activation energy of the cross-linked
membranes as a function of IEC at 30 ꢁC. (b) Temperature dependence of
proton conductivity of the membranes under fully hydrated conditions.
6296 | J. Mater. Chem., 2010, 20, 6291–6298
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domains has been observed with increasing IEC, which in turn
leads to significant variation in transport properties. Membranes
with relatively low IECs exhibit extremely low methanol
permeability, which are ascribed to a large portion of tightly
bound water and small sized ionic aggregates. Membranes II and
III exhibit unparalleled electrochemical selectivities that are two
orders of magnitude higher than Nafion, while maintaining
sufficient proton conductivity (i.e. > 0.02 S mꢀ1). The lower
methanol crossover of the cross-linked membranes has also been
demonstrated in the preliminary membrane-electrode assembly
(MEA) studies. The open circuit voltages of membranes II and
ꢁ
III under 3 M methanol/air conditions at 50 C were found in
a range of 0.75 to 0.85 V, which is much greater than that of
Nafion 117 (ꢂ0.65 V).
Acknowledgements
Fig. 6 Electrochemical selectivity of the cross-linked membranes and
This work was supported by the National Science Foundation
(CBET-0932740).
Nafion 117.
(Fig. 5b), follows the same trend with Ea,s. Higher values of
Ea,methanol reiterate elevated barriers to methanol transport in the
cross-linked membranes when compared to Nafion. Consider-
able changes in methanol permeability upon IEC is presumably
associated with significant variation in the water state and
domain structure. Expanded ionic channels and the increased
portion of unbound and weakly bound water give rise to greater
methanol diffusion across the membrane.
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This journal is ª The Royal Society of Chemistry 2010