Novel highly proton conductive sulfonated poly(p-phenylene) from 2,5-dichloro-4-(phenoxypropyl)benzophenone as proton exchange membranes for fuel cell applications

Article abstract of DOI:10.1039/b908140b

A novel sulfonated poly(4-phenoxypropylbenzoyl-1,4-phenylene) introducing a flexible alkyl chain was synthesized and achieved high proton conductivity even at low humidity (SPP: 0.077 S cm^-1, Nafion 112: 0.029 S cm ^-1 at 80 °C 50% RH

Full text of DOI:10.1039/b908140b

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Novel highly proton conductive sulfonated poly(p-phenylene) from
2,5-dichloro-4-(phenoxypropyl)benzophenone as proton exchange
membranes for fuel cell applicationsw
Surasak Seesukphronrarak and Akihiro Ohira*
Received (in Cambridge, UK) 23rd April 2009, Accepted 15th June 2009
First published as an Advance Article on the web 1st July 2009
DOI: 10.1039/b908140b
the hydrolytic stability and to increase the polymer chain
mobility, which had a significant influence on proton transport
resulting in an increase in proton conductivity. Therefore,
in this communication, a novel side chain type sulfonated
poly(p-phenylene) was prepared by Ni(0) catalyzed coupling
polymerization from 2,5-dichloro-4-(phenoxypropyl)benzo-
phenone. Preliminary results of proton conductivity, water
uptake, thermal stability properties, and gas permeability
were also obtained and compared with data for sulfonated
poly(ether ether ketone), S-PEEK, and Nafion.
A novel sulfonated poly(4-phenoxypropylbenzoyl-1,4-phenylene)
introducing a flexible alkyl chain was synthesized and achieved
high proton conductivity even at low humidity (SPP: 0.077 S cmÀ1
,
Nafion 112: 0.029 S cmÀ1 at 80 1C 50% RH).
The proton exchange membranes (PEM), the key material for
the operation of a proton exchange membrane fuel cell
(PEMFC), have received much attention as promising materials
for clean power sources in portable and stationary electronic
applications.1 Currently, perfluorosulfonic acids (PFSA) such
as Nafions produced by DuPont are widely used as the PEM
in PEMFC because of their good mechanical properties and
high oxidative stability as well as their high proton conductivity
compared to most hydrocarbon (HC) based membranes.
However, high cost, loss of proton conductivity at high
temperature and low humidity, and relatively high gas
permeability have limited PFSA membranes for further
commercial application.2 Therefore, the development of an
alternative non-perfluorinated polymeric materials such as
sulfonated poly(arylene ether ketone),3 sulfonated poly(ether
sulfone),4 sulfonated polyimide,5 and sulfonated derivatives of
poly(p-phenylene)3a,6 have been extensively investigated by
many research groups to achieve high proton conductivity
and application at conditions of high temperatures with lower
fuel crossover. Recently, sulfonated poly(p-phenylene) and
their derivatives, with wholly aromatic structure throughout
the backbone, have received much attraction as candidates for
PEM due to their excellent thermal and hydrolytic stabilities.
Unfortunately, some membranes composed of those polymers
exhibit low solubility in organic solvents as well as poor
flexibility (making it difficult to handle them) thereby it has
still been difficult to apply these to PEMFC. Based on the
significance of the development of novel polymers for
PEMFC, here we developed a novel side chain type monomer,
2,5-dichloro-4-(phenoxypropyl)benzophenone, which contained
an aliphatic group linked between a phenoxy unit and benzo-
phenone in order to improve the flexibility of the polymer
membrane and to increase the proton conductivity of the
membrane. The flexible aliphatic group was expected to improve
A novel monomer M1 having a propoxy linkage between
the phenoxy unit and benzophenone group, 2,5-dichloro-4-
(phenoxypropy)lbenzophenone, was designed and prepared in
three steps as shown in Scheme 1. First, 2,5-dichlorobenzoyl
chloride was prepared from 2,5-dichlorobenzoic acid with
thionyl chloride. Then, 2,5-dichloro-4-(bromopropyl)benzo-
phenone was obtained by Friedel–Crafts acylation of
benzenepropylbromide with 2,5-dichlorobenzoyl chloride
using AlCl3 as catalyst. M1 was obtained by reaction of
2,5-dichloro-4-(bromopropyl)benzophenone and phenol in
the presence of K2CO3 and KI as a base and catalyst in
acetone. The crude product was recrystallized from hexane
twice to afford M1 as white crystals in 60% yield. The
structure of the final monomer M1 was confirmed by FT-IR,
1H-, 13C-NMR, MS, and elemental analysis (see ESIw for
details); all data supported the proposed chemical structure
of M1. Homopolymer PP was synthesized by Ni(0) catalyzed
coupling polymerization, generated in situ from Ni(PPh3)2Cl2,
PPh3, Zn, and NaI in NMP.7 Polymerization was carried out
by heating at 75 1C for 3–4 h or until the polymer mixture
FC-Cubic, National Institute of Advanced Industrial Science and
Technology (AIST), Tokyo 135-0064, Japan.
E-mail: a-oohira@aist.go.jp; Fax: +81 3 3599 8554;
Tel: +81 3 3599 8550
w Electronic supplementary information (ESI) available: Monomer
and polymer synthesis, experimental methods, measurement, results of
TGA thermogram l-relative humidity relationship and oxygen
permeability. See DOI: 10.1039/b908140b
Scheme 1 Synthesis route of (A) monomer M1 and (B) sulfonated
homopolymer poly(p-phenylene) SPP.
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could no longer be stirred. The resulting polymer showed high
viscosity. After purification and drying, the polymer was
isolated in 490% yield. The molecular weight of the PP
was determined by GPC using the polystyrene as standard.
The number-average molecular weight (Mn) of PP was 3.79 Â 104
with a polydispersity (Mw/Mn) value of 3.6. PP was then
sulfonated using concentrated sulfuric acid at room temperature
for 75 h. The successful introduction of the sulfonic groups
was confirmed by FT-IR spectra which showed the character-
istic bands for SPP at around 1160 and 1024 cmÀ1 due to the
symmetric and asymmetric stretching of sulfonic groups.
Although the IEC of SPP was rather high, we found that
SPP was stable to water at room temperature, but was soluble
in boiling water—similar to S-PEEK. SPP was readily soluble
in dipolar aprotic solvents such as NMP, DMAc, DMSO, and
DMF. SPP membranes were transparent, flexible, easier to
handle even in the water-swelling form or in the dry form
compared to that of some sulfonated poly(p-phenylene)s and
their derivatives, which were rather brittle or showed difficulty
in obtaining flexible membranes by casting.4 This might be due
to the presence of flexible aliphatic propyl on the monomer
side group increasing the mobility of the polymer side chain.
The thermal stabilities of PP and SPP were determined at
the heating rate of 10 1C minÀ1 by TGA under an air and a
nitrogen atmosphere, respectively (see Fig. S1, ESIw). PP was
a thermally stable polymer with a 10% weight loss tempera-
ture of about 384 1C. In the case of SPP, two step degradation
patterns were observed as shown in Fig. S1 (see ESIw).
Generally, the weight loss occurring below 150 1C is due to
the evaporation of water absorbed in the polymers. There was
no difference in the first stage weight loss in the TGA patterns
under nitrogen and air atmospheres of SPP and decomposition
occurred over the temperature range 220 to 400 1C caused by
the decomposition of the sulfonic acid. This indicates that the
degradation of SPP under air did not affect the significant
thermal stability of the sulfonic group on the polymer side
chain. However, the second weight loss step related to the
degradation of the main chain under an air atmosphere, SPP
degraded more quickly than those with nitrogen atmosphere in
the temperature range 410–550 1C.
The humidity dependence of water uptake also was inves-
tigated at 80 1C and found that the water uptake is increased
with increasing relative humidity and reached 61% at 95%
RH, corresponding to an absorption of 10 water molecules per
ÀSO3H group. In spite of the high ion exchange capacity
(IEC: 3.39 mequiv gÀ1), SPP membranes have acceptable
water uptake of 61%, as shown in Fig. 1A. The l values of
SPP membranes increased as the relative humidity increased.
Despite the increase in water content with IEC, there is no
significant difference between SPP and Nafion. In the case of
Nafion, continuous water channels can be formed even with
small amounts of water, but for S-PEEK, a less phase-separated
structure, enough water cannot be incorporated. Less chain
mobility prevents the formation of large water domains in the
membrane.8 For SPP, however, l values are similar to those of
Nafion. This result implies that SPP, with a higher IEC value,
is able to form larger water domains. Water uptake also results
in dimensional stability in membrane thickness and in plane
direction by comparing their hydrated state with dry state
Fig. 1 Relative humidity dependence of (A) water uptake, (B) proton
conductivity of the SPP membrane compared with Nafion 112 and
S-PEEK.
membranes. At room temperature, the results given in Table 1
show that the SPP membrane exhibited much larger swelling
in the membrane thickness direction than in the plane direction.
Although further improvement is needed, this property is
promising because the low in-plane swelling may result in
much lower stress at the interface and better stability of the
MEA.9
In general, proton conductivity is known to have a significant
role in the performance of fuel cells. Higher levels of proton
conductivity allow higher power densities to be achieved.
Generally, to achieve a high proton conductivity of PEMs
composed of sulfonated random aromatic polymers, a high
ion exchange capacity (IEC) is required.10 The relative humidity
dependence (10–95% RH) of the proton conductivities (s) of
SPP membrane, S-PEEK, and Nafion 112 (for comparison)
were measured at 80 1C and are plotted in Fig. 1B. SPP
exhibited higher conductivity than S-PEEK and Nafion 112,
even in the low relative humidity range, and the proton
conductivity increased with increasing relative humidity, even-
tually reaching the highest value of 0.657 S cmÀ1 at 80 1C
and 95% RH, which is 4–5 times higher than Nafion 112 or
S-PEEK. Even at lower humidity, the conductivity is
0.013 S cmÀ1 at 30% RH at 80 1C, which is comparable to
that of Nafion. As far as we are aware, the proton conductivity
of SPP was also higher than those of other sulfonated random
aromatic polymer systems reported in the literature. The high
conductivity is thought to be due to the higher water uptake at
low relative humidity, an effect of the aliphatic propyl group
that could increase the flexibility of the polymer chain resulting
in a decrease of the distance between sulfonic acid groups on
the same polymer backbone, which makes it possible to
develop a hydrogen-bonded network for proton transport
channels.11
Since the crossover of hydrogen and oxygen between the
anode and cathode in membrane electrode assembly (MEA)
has a negative impact on the efficiency of PEMFC, the PEM
should be a good gas barrier.12 Fig. 2 shows the humidity
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Table 1 IEC, water uptake, proton conductivity and dimension change of the SPP membrane
s (80 1C)/S cmÀ1
50% RH
Dimensional changec
lb (H2O/SO3H)
WUb/wt%
95% RH
Dt
Dl
Samples
IECa/mequiv gÀ1
SPP
S-PEEK
Nafion112
3.39
2.07
0.91
10.0
7.7
8.2
61.5
28.8
13.3
0.077
0.001
0.029
0.657
0.13
0.127
1.018
0.351
0.008
0.071
0.160
0.07
a
b
c
Measured by titration with 0.01M NaOH. Measured at 80 1C, 95%RH. Sample was measured in water at RT.
undertaking an investigation of the effect of flexible alkyl side
groups on the physical properties such as water diffusion,
mechanical properties and membrane morphology.
This work was financially supported by the Ministry of
Economy, Trade and Industry (METI) and New Energy and
Industrial Technology Development Organization (NEDO),
Japan. We are grateful to Prof. Rikukawa of Sophia University
for fruitful discussion.
Notes and references
Fig. 2 Relative humidity dependence of hydrogen permeability of
SPP membrane compared with Nafion 112 and S-PEEK.
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dependence of hydrogen permeability of the SPP membrane at
80 1C. Permeabilities of SPP and S-PEEK are 10 times lower
than that of Nafion in the dried state. With increasing relative
humidity, permeabilities of all membranes increase, these
increase particularly remarkably for SPP and S-PEEK at
higher relative humidity, but the permeability of SPP is still
significant lower than that of Nafion and almost the same as
that of S-PEEK over 80% RH. Although SPP takes the largest
amount of water in those membranes, the H2 permeability of
SPP was able to be suppressed almost equally with that of
S-PEEK. Similar behavior was observed in oxygen
permeability (see Fig. S3, ESIw). We would like to emphasize
here that higher proton conduction can be achieved with lower
gas permeation in the SPP membrane. Stronger intermolecular
interaction between backbones may provide higher gas barrier
properties, and the flexible side chain gives higher proton
conductivity.
In summary, a novel side chain type sulfonated poly-
(p-phenylene) containing an aliphatic group was prepared by
Ni(0) catalyzed coupling polymerization of 2,5-dichloro-4-
(phenoxypropyl)benzophenone and sulfonated using concen-
trated sulfuric acid. The synthesized polymer showed good
solubility and was easily turned into flexible and self-standing
membranes. Although SPP possessed higher IEC value
(3.39 mequiv gÀ1), the membrane was stable to water at room
temperature but dissolved in boiling water. The SPP
membrane displayed an excellent proton conductivity even
at low humidity (0.077 S cmÀ1, 80 1C; 50% RH), while
retaining acceptable water uptake. We have demonstrated that
significant performance such as higher proton conductivity
and gas barrier with dimensional stability can be optimized by
the simple introduction of an aliphatic alkyl pendent chain.
Further investigation and development of this class of polymer
will be needed as a part of blends or copolymers to improve
the solubility of the membrane in boiling water but still retain
high proton conductivity. In our laboratory we are currently
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This journal is The Royal Society of Chemistry 2009
4746 | Chem. Commun., 2009, 4744–4746