i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 3 8 5e9 3 9 7
Avai lab le a t www.sc iencedi rec t .com
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candidate for nafion substitution in fuel cellSurvey of sulfonated polyimide membrane as a good
Leila Akbarian-Feizi, Shahram Mehdipour-Ataei*, Hamid Yeganeh
Iran Polymer and Petrochemical Institute, P.O. Box 14965/115, Tehran, Iran
a r t i c l e i n f o
Article history:
Received 26 July 2009
Received in revised form
15 February 2010
Accepted 3 March 2010
Keywords:
Polyimide
Membrane
Nafion
Modification
Fuel cell
Abbreviations: s, Proton conductivity; Δtc,* Corresponding author. Tel.: þ98 21 4458000E-mail address: [email protected] (S
0360-3199/$ e see front matter ª 2010 Profedoi:10.1016/j.ijhydene.2010.03.072
a b s t r a c t
Studies in fuel cell membranes show that modification of polyimides by introduction of
aliphatic linkages in the structure of sulfonated copolyimides, synthesis of branched/
crosslinked sulfonated polyimides, and semi and fully interpenetrating polymer networks
of sulfonated polyimides restrain suitable potential for Nafion substitution.
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1. Introduction and high methanol crossover. Seeking for low cost and high
Fuel cell is an energy conversion device which directly
transforms the chemical energy of fuels such as hydrogen and
methanol into electrical energy. More attentions have been
paid to polymer electrolyte membrane fuel cells (PEMFCs)
during the past decade because they provide clean, quiet and
portable power for many applications such as vehicular
transportation, electronic devices, and home appliances. The
polymer electrolyte membrane is the key component of
a PEMFC. At present, Nafion, Aciplex, and Flemion are typical
perfluoropolymer membranes that are practically used in fuel
cells. They show unique properties including high proton
conductivity, good mechanical properties, and high thermal,
electrochemical and chemical stability. However, the major
disadvantages of these perfluoropolymers are their high cost,
low conductivity at low humidity and/or high temperature,
Thickness change; m, Mic0; fax: þ98 21 44580023.. Mehdipour-Ataei).ssor T. Nejat Veziroglu. P
performance polymers as alternative materials is the main
aim in this field. A large number of sulfonated hydrocarbon
polymers has been investigated for this purpose and among
them sulfonated polyimides (SPIs) have been identified as one
of the most promising membrane materials especially for
PEMFC application [1e25]. Sulfonated polyimides fulfill basic
properties for this application. They generally show high
amount of ionic conductivity, low gas and methanol perme-
ability, suitable mechanical properties, and unique water
uptake behavior [26,27].
Structural specifications and their modifications are the
key factors for obtaining appropriate polyimides as fuel cell
membranes. Regarding structural modifications, for instance
experiments have been performed both with phthalic and
naphthalenic sulfonated polyimides at 60 �C, at a 3 bar pres-
sure for H2 and O2 and under a constant current density of
ro; Δlc, Diameter change.
ublished by Elsevier Ltd. All rights reserved.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 3 8 5e9 3 9 79386
250 mA cm�2. Different structural stability was revealed
depending on the polyimide structure concerned. In the
similar conditions and within a few days, the phthalic struc-
ture was quickly degraded and became brittle, while naph-
thalenic polyimides showed significant stability. To
investigate the membrane aging, performance of polyimide
membranes was followed during 3000 h operation. Slow
decreasing of the fuel cell performances during the operation
could be attributed to a loss of ionic conductivity which was
resulted from either a continuous dehydration or polymer
degradation. Hydrolysis leading to the polymer chain scis-
sions could be accounted for the degradation of sulfonated
polyimide membrane. Decreasing of the molecular weight
and consequently the brittleness of the observed membrane
resulted. So according to the above facts and despite themany
advantages of polyimides, their poor hydrolytic stability is the
main drawback that prevents the use of these membranes in
fuel cell [28e30].
Several ways to improve the hydrolytic stability of
sulfonated polyimides have been reported. Diamines with
high nucleophilic character and preparation of monomers
that can import flexibility to the final polymer are reported to
provide more hydrolytic stability to the polyimides. Presence
of alkyl substituents in the ortho position of aromatic
diamines imparts flexibility to the final polyimides. In this
way, the nucleophilicity of the diamine increases because of
the positive inductive effect (þI) of the alkyl groups. Due to the
bulkiness of the substituents, the flexibility of the polymer
chain also increases, leading to increased hydrolytic stability.
Additionally, because of the increased fractional free volume,
these polymers are expected to contribute high temperature
conductivity.
Generally, the water stability is a result of the total effect of
solubility stability, hydrolysis stability, and swelling-stress
stability of membranes and in this case, it depends on the
structures of startingmaterials need for the preparation of SPI
including dianhydride, sulfonated diamine, and non-
sulfonated diamine. Normal five-membered ring phthalic
polyimides exhibit poor hydrolytic stability under the strongly
acidic conditions of a fuel cell. In this case, chain scission
occurs and mechanical properties of membrane decrease. In
comparison to this, bis(naphthalic anhydride)s, or six-
membered ring polyimides, are less strained and present
better thermal and chemical stability, in conjunction with an
improved mechanical strength [5]. Linear configuration of the
sulfonated diamines results in high solubility stability. Also,
highly basic sulfonated diamines lead to improved hydrolysis
stability of the imide ring. In this way, Okamoto classified the
sulfonated diamines to two categories for main-chain-type
diamines. In type 1, the electron-withdrawing sulfonic acid
groups are bonded directly to the aminophenyl rings and in
type 2, sulfonic acid groups are bonded to aromatic rings other
than the aminophenyl rings. The type 2 diamines show the
higher hydrolysis stability than the type 1 diamines due to
their more basic character. Also, Okamoto mentioned side-
chain-type sulfonated diamines produce better water stability
for the SPI than main-chain-type sulfonated diamines due to
two main reasons: higher basicity of the diamine moieties
results from the presence of alkoxy groups and their electron
donating effect, and a microphase separation structure
composed of hydrophilic (sulfopropoxy groups) domains and
hydrophobic (polyimide backbones) domains. Better water
stability can be obtained by presence of flexible linkages such
as ether units in the structure of sulfonated diamine due to
simple molecular relaxation of polymer chain under
membrane swelling to reduce swelling stress. On the other
hand, flexible nonsulfonated diamines further improve the
water stability [6,27,31].
Also introduction of branching and crosslink segment in
the structure of polyimide membrane increases water
stability and proton conductivity of these membranes. Semi
and fully interpenetrating network can also improve various
properties of polyimide membrane.
This article describes sulfonated copolyimide membranes
and studies the effect of substitution, bridging groups,
branching/crosslink, aliphatic linkage and semi and fully
interpenetrating network on the properties of sulfonated
polyimide membrane. Also relation between polymer struc-
ture and some important properties of relatedmembranes for
application in fuel cell has been reviewed.
2. Effect of pendant and substituent groupson membrane properties
It has been revealed that introduction of sulfonic acid units as
side chains into the structure of aromatic polyimides not only
provides good solubility of the polymers in general aprotic
solvents, also maintains their high thermal stability due to
relative high desulfonation temperature (up to 350 �C). Theresult would be preparation of flexible and tough polyimide
membranes with practically high mechanical strength.
Increasing of hydrogen bonding and polar interaction by
sulfonic acid units led to more membrane swelling in thick-
ness than in plane. Accordingly, the displaying of reasonably
high proton conductivity in spite of low ion exchanging
capacity (IEC) could be explained. For example, the related
membrane with IEC of 1.54 mequiv/g showed s values of 81
and 11 mS/cm in water and 70% relative humidity (RH),
respectively, at 60 �C [32].
Another types of sulfonated copolyimides (co-SPIs) con-
taining pendant sulfonic acid groups were synthesized from
1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), bis
(3-sulfopropoxy) benzidines (BSPBs), and some nonsulfonated
diamines. They revealed s values of 0.05e0.16 S/cm at 50 �C in
water. Further increase in proton conductivity was observed
by increasing temperature up to 120 �C. These microphase-
separated structure membranes showed relatively good
conductivity stability during the aging behavior in water at
100 �C for 300 h [33].
One of the best groups of SPIs that have been reported so
far was based on the preparation of 4,40-bis(4-aminophenoxy)-
3,30-bis(4-sulfophenyl)biphenyl diamine as a novel sulfonated
diamine containing sulfophenyl pendant groups. The related
SPIs preserved high mechanical strength and proton
conductivity after aging in water at 130 �C for 500 h. They also
revealed high cell performances in polymer electrolyte fuel
cells operated at 90 �C and 50% RH [34].
Effect of alkyl side chains (their positions and chain length)
on the properties of sulfonated polyimide copolymers
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containing fluorenyl group has been investigated. Contribu-
tion of fluorenyl unit as a bulky group into the polymer
backbone via fluorenylidene diamine caused high proton
conductivity at high temperature. It was related to the higher
water holding tendency of this unit due to producing rigid
spaces in the polymer chain. Also, existence of sulfopropyl
and sulfobutyl groups on the polyimide structures led to
higher hydrolytic stability (in 10%MeOH aq. at 100 �C) than the
main-chain-sulfonated polyimide because they increased the
electron density of the imide nitrogen atoms through induc-
tive effects. The outcome of these combinational structural
features was preparation of tough, flexible, and ductile
membranes. High mechanical properties (>34 MPa of the
maximum stress) and proton conductivity (>0.1 S cm�1) were
observed for these fluorenyl containing sulfonated polyimide
membranes at high relative humidity. The copolyimide
membranes showed promising properties even in comparison
Fig. 1 e Structure of polyimide
with Nafion 112 which made them to be a good candidate for
polymer electrolyte membrane in fuel cells and also in direct
methanol fuel cells (DMFCs) [35].
Another study in this field was attributed to the substitu-
tion effect on diaminodiphenylmethane (MDA) as a como-
nomer for preparation of related copolyimide membranes.
The membrane properties versus increasing of substitutions
were examined (Fig. 1). Due to the flexibility of the substitu-
ents and reducing close packing of polyimide chains, by
increasing of substitution the solubility of the membranes
increased whereas their thermal stability decreased. Also,
Ion-exchange capacity (IEC) and water uptake (WU) reduced
with increase in substitution because of the low sulfonic acid
content at a particular weight due to the increased molecular
weight of the repeating unit. Substituted diamines showed
higher conductivity than the unsubstituted diamines at
higher temperature in spite of low IEC capacity and water
with different substitution.
Fig. 2 e Polyimide with different bridging groups.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 3 8 5e9 3 9 79388
uptake. In comparison with Nafion 115, by increasing the
temperature these polyimides showed more rapid increase in
conductivity. As mentioned previously, increasing of free
volume in the nonsulfonated segment by more bulky
substituents in diamine led to higher water uptake and in turn
conductivity of the polymer. Also, inductive effect caused
more hydrolytic stability of the polyimides with substitution
in assessment with unsubstituted diamines [36e38].
3. Effect of bridging groups on membraneproperties
Hydrolytic and oxidative stability of the polyimide
membranes could be improved by incorporation of flexible
ether bridging group into their structures (Fig. 2). Introduction
of flexible units increases the extent of plastic behavior and
consequently standing on bending operation in mechanical
test. As a comparison to Nafion 117, maintaining of mechan-
ical properties for such polyimides after soaking in water at
80 �C for 200 h, while keeping proton conductivity at a high
level have been reported [39].
Fig. 3 e Polyimide wit
Besides the flexibility, the basicity of the sulfonated
diamine moieties by increasing the electron density of the
imide nitrogen atoms also had great effect on water stability
of the polyimide membranes. For example, new sulfonated
copolyimides (SPIs) derived from 1,4,5,8-naphthalenete-
tracarboxylic dianhydride (NTDA), sulfonated diamines of
4,40-bis(4-aminophenoxy)biphenyl-3,30-disulfonic acid ( p-
BAPBDS), and nonsulfonated diamines was investigated by
Okamoto et al. Major properties of the membranes including
viscosity, mechanical strength, proton conductivity, weight
loss, and hydrolytic stability were studied. Different experi-
mental studies showed good results in which the p-BAPBDS-
based SPI membranes had the high water stability enough for
PEFC and DMFC applications below 80 �C and even promising
result up to 100 �C [27,31].
In an another instance, 9,9-bis(4-aminophenyl)fluorene-
2,7-disulfonic acid (BAPFDS)-based polyimide membranes
displayed higher water stability than the corresponding 2,20-bendizine sulfonic acid (BDSA)-based ones with similar IEC. It
was related to the fact that in the former the unfavorable
effect of the rigid structure was balanced by the favorable
effect due to the high basicity and flexibility of BAPFDS (Fig. 3).
h fluorene group.
Fig. 4 e Structures of NTDAeBDSA(X)e6FAP and
NTDAeBDSA.
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Proton conductivities of these polyimide membranes were
measured as the functions of relative humidity and temper-
ature. The resulting homopolyimide, NTDAeBAPFDS,
revealed quite comparable proton conductivities to those of
Nafion 117 in the complete humidity range (RH<100%). All the
BAPFDS-based polyimide membranes showed proton
conductivities similar to or higher than those of Nafion 117 at
100% relative humidity [40].
In order to study the effect of bulkiness of the bridging
groups, some sulfonated polyimides with different bridging
groups in the nonsulfonated diamine monomer moiety have
been prepared. It was observed that the conductivity increased
byenlargingsizeofbulky groupsand therefore freevolume.The
bulky bridging groups showed increased conductivity even at
high temperature regardless of low water uptake and IEC, and
also under similar experimental conditions higher conductivity
was practiced for them in comparison to Nafion 115. This was
attributed to the increase in the free volume in the non-
sulfonated polyimide moiety which increases the proton
conductivity at high temperature due to confine of water
molecules andsuitablewaterholding evenathigh temperature.
It isworth tomention that increasing thenumberandbulkiness
of the bridging group had an adverse effect on the thermal
stability of the polyimides. Also, it was specified that the
degradation behaviors of aromatic sulfonyl groups around
300 �C for all the polymers were almost similar regardless of
their chemical backbone and therefore the degradation of
sulfonic groups was not influenced by the polymer backbone.
Table 1 e Proton conductivity and IEC of blend and copolyimid
Membrane X(mol%)
NTDA-BDSA/NTDA-BDSA(X )-6FAP
Blend 1 20 1.67/1
Blend 2 30 1.33/1
Blend 3 40 1/1
Blend 4 50 0.67/1
Copolyimide 70 0/1
Nafion 117 e e
a At 98% relative humidity.
However, still the sulfonated polyimides were thermally stable
and suitable for high-temperature operations.
Also, the water uptake and IEC values reduced with
enhancing the bulkiness of the bridging group. All the poly-
imides exhibited lower conductivity than Nafion 115 at low
temperatures and similar conductivity at temperatures above
50 �C. The increasing of conductivity in the polyimides at high
temperature was faster than Nafion 115 [41].
4. Proton-conductive membranes based onblends of polyimides
Novel types of proton-conductivity membranes based on
blends of sulfonated polyimides have been reported [42]. A
sulfonated homopolyimide and a sulfonated copolyimide
using a solvent-casting method were applied for preparation
of blend membranes (Fig. 4).
As it was expected, the proton conductivities of the blend
membranes were strongly influenced by the sulfonated
homopolyimide contents and increased with an increase in
the content, while almost similar IEC values were observed
(Table 1). Higher proton conductivity for the blended
membranes in respect to Nafion at 80 �C has been reported,
and this indicated that the proton transfer in the blend
membranes was as a result of the ionic channels induced by
the hydrophilicehydrophobic phase separation which is an
important factor for evaluation of membranes performance.
Different types of polyimide blends that are acidebase
blends have been prepared using sulfonated polyimide as the
acid polyimide and using acrydine as the source for tertiary
nitrogen in the main chain as the base polymer (Fig. 5). The
properties of polyimide blends were evaluated by changing
the content of base versus acid molar ratio. By increasing the
base polyimide contents, the thermal stability of blends
increased because of the stabilization of sulfonic acid groups
for the poly salt formation. Also by the same trend,
a remarkable increase in hydrolytic stability was observed.
Increasing of ionic crosslinkswhich opposes polymer swelling
in water and thus increases the mechanical stability of the
membrane was the main reason for this behavior. On the
other hand, by increasing of the base polyimide contents,
strong interaction of acrydine group with the sulfonic acid
group which decreases the free sulfonic acid group in the
polymer blends, reduced IEC and water uptake. By the same
e membranes.
Proton Conductivity (S/cm)a ExperimentalIEC/TheoreticalIEC (mequiv/g)80 �C 30 �C
1.9 � 10�1 3.8 � 10�2 2.31/2.44
2.5 � 10�1 4.6 � 10�2 2.39/2.44
1.6 � 10�1 3.6 � 10�2 2.36/2.44
1.4 � 10�1 2.3 � 10�2 2.37/2.44
1.4 � 10�1 2.1 � 10�2 2.40/2.44
1.2 � 10�1 4.6 � 10�2 e
Fig. 5 e Acidebase segments of blend polyimide.
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trend, the proton conductivity decreased because proton
conductivity is depended to the IEC and water uptake [43].
5. Branched/crosslinked sulfonatedpolyimide membranes
Obtaining high proton conductivity in the range of low relative
humidity (compared to Nafion) is one of the main challenges
of SPI membranes. Although increasing of the sulfonation
contents increases IEC and consequently proton conductivity,
it leads to high swelling of the membrane and loss of
mechanical properties. For improving proton conductivity in
low humidity, different methods have been suggested [34].
Some crosslinking methods such as covalent crosslinking and
ionic crosslinking have been reported [44]. However, the
common covalent crosslinking membranes usually undergo
brittleness in the dry state and the ionic crosslinking loses its
efficiency at high temperatures [45].
There are some recent works in this regard have been
focused on it. Using in-situ crosslinking technique, new types
of branched/crosslinked sulfonated polyimide (B/C-SPI)
membranes were synthesized and their properties were
measured for fuel cell membrane application. They were
prepared from chemical branching and crosslinking reaction
of sulfonated polyimide (SPI) oligomers and 1,3,5-tris(4-ami-
nophenoxy)benzene (TAPB) as a crosslinker. They showed
high water stability and mechanical properties under an
accelerated aging treatment in water at 130 �C, while they
revealed high ion-exchange capacities in the range of
2.2e2.6 mequiv g�1. The proton conductivities of these
membranes at 120 �C in water and at 50% relative humidity
were also high about 0.2e0.3 S cm�1 and 0.02e0.03 S cm�1,
respectively.
These membranes displayed comparable result in a single
H2/O2 fuel-cell system at 90 �C and better performance in
a direct methanol fuel cell with methanol concentrations as
high as 50 wt% in respect to Nafion 112 [45].
In another research, branching of sulfonated polyimide
copolymers was accomplished by using 2 mol% of melamine
in the polymerization reaction and crosslinking was done by
electron beam irradiation on the membranes. Their oxidative
stability and mechanical strength were improved without too
much sacrifice of conductivity properties. Their mechanical
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 3 8 5e9 3 9 7 9391
properties were much higher than Nafion 112, while their
proton conductivity at 30e140 �C and 100% RH was 0.2 S cm�1
that was almost comparable to that of Nafion 112 [46,47].
Another branched/crosslinked (B/C) sulfonated polyimides
(SPIs) have been prepared from the polycondensation reaction
of 1,4,5,8-naphathlenetetracarboxylic dianhydride (NTDA)
with 4,40-bis(4-aminophenoxy)biphenyl-3,303-disulfonic acid
(BAPBDS), 2,20- or 3,30-bis(3-sulfopropoxy)benzidine (BSPB)
and 1,3,5-tris(4-aminophenoxy)benzene (TAPB) (Fig. 6). The
membranes showed high mechanical strength and proton
conductivity (about 0.02e0.25 S cm�1 at 50e100% relative
humidity at 120 �C) and also low membrane swelling. Totally,
their fuel cell performances were comparable to that of Nafion
112 in a single H2/O2 PEFC system [48].
Among these, the BSPB-derived B/C SPI membranes
exhibited more attractive mechanical properties. Also, it was
found that more hydrolytic stability for SPIs could be obtained
if acidic groups append as pendant side groups instead of
direct attach to the main chain. As shown in Table 2, the
NTDA-3,303- BSPB/TAPB(6/5) membrane displayed an original
Young’smodulus of 2.3 GPa, whichwas about twice as large as
that of the BAPBDS-based B/C SPI membrane. This may be
attributed to the different orientation behavior of polymer
chains for these two kinds of SPIs. Therefore, more favorable
oriented structure and consequently high mechanical
strength in the in-plane direction of the membrane could be
obtained if flexible aliphatic side chains exist.
Fig. 6 e Schematic diagram of the c
New sulfonated polyimides end-capped with maleic
anhydride have been synthesized from 1,4,5,8-naph-
thalenetetracarboxylic dianhydride, 4,40-diaminobiphenyl-
2,20-disulfonic acid, 2-bis [4-(4-aminophenoxy)phenyl] hexa-
fluropropane andmaleic anhydride in order to study the effect
of crosslinking by a hydrophilic group on a sulfonated poly-
imide electrolyte membrane. These end-capped sulfonated
polyimides with various ratios of sulfonated polyimide and
poly(ethylene glycol) diacrylate were crosslinked. All the
crosslinked sulfonated polyimides were stable for over 200 h
at 80 �C in deionized water. The proton conductivity of them
was increased with the increase in poly(ethylene glycol) dia-
crylate content despite the fact that the ion-exchange capacity
was decreased [49].
6. Sulfonated copolyimides containingaliphatic linkages as proton-exchangemembranes
The effect of aliphatic units both in the main chain and in the
side chain of sulfonated polyimide ionomers was investigated
by Watanabe et al. The membranes revealed high hydrolytic
and oxidative stability and also comparable or even better
proton conductivity (0.18 S cm�1 at 140 �C) than that of Nafion
112 at high temperature and at high relative humidity condi-
tions. These preliminary results proved their potential
hemical structure of B/C SPIs.
Table 2 eWater uptake, size change, conductivity, and effect of soaking in water at 130 �C onmechanical properties for SPImembranes.
SPIs IEC WU Sizechangea
sb Soakingtime
Young’smodulus
Maximumstress
Elongationat break
mequiv g�1 wt% Dtc Dlc S cm�1 h GPa MPa %
NTDA-BAPBDS/BAPB (2/1) 1.89 51 0.20 0.04 0.090 0 1.22 104 126
48 0.81 40 5.6
96 0.61 34 6.2
NTDA-BAPBDS/TAPB (5/4) 2.29 77 0.23 0.08 0.095 0 1.22 98 64
48 0.90 5.S 10
96 0.83 57 15
NTDA-3.3’-BSPB/TAPB (5/4) 2.49 114 0.68 0.02 0.13 0 2.3 130 18
48 2.0 120 16
96 1.9 110 17
196 2.0 90 12
NTDA-2.2’-BSPB/TAPB (6/5) 2.57 104 0.59 0.01 0.10 0 e e e
48 2.4 122 8.8
96 2.3 83 4.6
Nation 117 0.91 25 e e 0.080 e e e e
a Measured in water at room temperature.
b Measured at 50 �C and 90% relative humidity.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 3 8 5e9 3 9 79392
availability as an electrolyte for high-temperature PEFCs [50].
Comparable mechanical properties of membranes to those of
the wholly aromatic polyimide ionomers and much better
than the Nafion membrane were observed [51].
Han and coworker synthesized a series of six-membered
sulfonated polyimides with aliphatic linkages (SPIAs) using
reaction of 1,4,5,8-naphthalenetetracarboxylic dianhydride
(NTDA) with 4,40-diaminobiphenyl-2,20-disulfonic acid (BDSA)
as the sulfonated diamine, and aliphatic diamines H2N
(CH2)nNH2 where n ¼ 6, 8, 10, 12. In addition to high thermal
stability that was observed for all the SPIAs, the other prop-
erties of these SPIAs including ion-exchange capacity, water
uptake, proton conductivity, and hydrolytic stability were
studied. The IEC and water uptake of the SPIAs increased with
decreasing the chain length of the aliphatic diamine. On the
other hand, substitution of aliphatic diamine with aromatic
diamine i.e., 4,40-methylenedianiline (MDA) was achieved to
prepare fully aromatic polyimide (MDA-SPI) and compare
their properties. The SPIAs showed higher proton conductivity
than Nafion 117 at high temperatures and also higher proton
conductivity than MDA-SPI at all temperatures. Hydrolytic
Table 3 e Water absorption, hydrolytic stability, and IEC of the
Membrane Water Absorption (wt
48 h 120 h 240 hsta
2,4-DABS/DAH/BTDA (20/80/100) 3.2 6.5 9.1
2,4-DABS/DAH/BTDA (30/70/100) 9.7 112 15.1
2,4-DABS/DAH/BTDA (40/60/100) 13.9 18.8 19.2
2,4-DABS/DAH/BTDA (50/50/100) 35.2 74.9 Cracked
2,4-DABS/DAH/BTDA (70/30/100) 54.3 Cracked Cracked
2,4-DABS/BTDA (100/100) Cracked Cracked Cracked
Nafion 117 15.1
a Shrinkage was observed.
stability of all SPIAs was remarkably higher than MDA-SPI
membrane [52].
In another experiment, 2,4-diaminobenzene sulfonic acid
(2,4-DABS) and 1,6-diaminohexane (DAH) have been used as
sulfonated aromatic diamine and nonsulfonated aliphatic
diamine compounds, respectively to prepare some random
sulfonated copolyimides containing aliphatic linkages (co-
SPIAs). Different ratios of two diamines were reacted with
benzophenone tetracarboxylic dianhydride (BTDA) to prepare
the co-SPIAs with controlled degrees of sulfonation (up to
70 mol% sulfonic acid). Also, fully aromatic sulfonated
homopolyimide (homo-SPI) was prepared similarly using
condensation of BTDA with 2,4-DABS. Since desulfonation
temperatures were in the range 200e350 �C, high thermal
stability of the sulfonic acid groups of these polymers was
concluded. The co-SPIAswith 40e70mol% 2,4-DABS displayed
higher proton conductivity than Nafion 117 in water. Also,
improved hydrolytic stability was observed for these
membranes. Another isomeric structure of the membrane
namely 2,5-DABS-Based SPI membranes was prepared for
comparison. Similar to 2,4-DABS, when more amounts of 2,5-
homo-SPI and co-SPIA membranes.
%) Theoretical IEC(mequiv/g of simple)
Experimental IEC(mequiv/g of simple)
Hydrolyticbility at 80 �C
>100 h 0.48 0.38
>100 h 0.71 0.49
17 h 0.93 0.90
3 h 1.14 0.97
3 h 1.55 1.52
3 h 2.11 2.07
>100 ha e 0.89
Fig. 7 e A multiblock copolymer composed of a polyimide block and a poly(arylene ether sulfone) block.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 3 8 5e9 3 9 7 9393
DABS were used in the copolymer composition lower hydro-
lytic stability was found (Table 3).
As a result, the structure of the sulfonated diamine moie-
ties and also IEC are two main factors greatly affecting the
hydrolytic stability of SPImembranes. Flexibility, basicity, and
linear configuration of sulfonated diamine units had profound
effect on hydrolytic stability of the SPI membranes. On the
other hand, usually higher IEC values cause larger water
uptakes and consequently decrease the hydrolytic stabilities.
Accordingly, proper molecular design of sulfonated diamine
in the copolymer composition is the key factor for obtaining
high IECs and hydrolytic stabilities. In these membranes, the
optimum condition from the viewpoint of proton conduc-
tivity, IEC, and hydrolytic stability was obtained when 40 mol
% concentration of 2,4-DABS in the SPIs was used [53].
7. Effect of block copolymerization and blockchain length on polyimide properties
One effective method for improving proton conductivity of
membranes is formation of interconnected hydrophilic
channels. Even in the absence of water, proton transfer
through these interconnected channels could be possible and
therefore reduces the morphological barrier for proton
transport. Nano-phase separated block copolymerswith built-
in ionic units have been used for the preparation of inter-
connected hydrophilic domains in polyimides. Self diffusion
coefficient values of water revealed that increasing the block
lengths increased the degree of interconnectivity. However,
for the random copolymers the value was intensively
decreased with time that was an indication of strong
morphological barrier for proton transfer. Also, for the
random copolymers increasing of IEC, water uptake, and self
diffusion coefficient of water led to higher methanol perme-
ability. On the contrary, Nafion and phase separated block
copolymers showed a constant value for the diffusion coeffi-
cient after certain diffusion time. Additionally, methanol
permeability of multi-block copolymers was not affected by
self diffusion coefficient of water. This was related to the
hydrophobic segments present in the block copolymers that
resist the diffusion of methanol and thus decrease the
methanol permeability. Therefore, due to the presence of
hydrophilicehydrophobic phase separation in ionomeric
block copolymers, the “hard” hydrophobic segments decrease
the methanol permeability while the “soft” hydrophilic
segments provide channels for water and proton transfer
[12,54].
In another research, novel sulfonated block copolyimides
with various diamine compositions and block chain lengths
have been synthesized by chemical imidization of 1,4,5,8-
naphthalenetetracarboxylic dianhydride (NTDA), 4,40-dia-minobiphenyl-2,20-disulfonic acid (BDSA), and 2,20-bis(4-ami-
nophenoxy) hexafluoropropane (6FAP) using a two-pot
procedure. It was seen that there is a direct and intense rela-
tion between block chain lengths and proton conductivity.
The proton conductivity of the block copolyimide membrane
(NTDA-BDSA-b-6FAP) was about 0.35 S cm�1 which was
higher than in respect to Nafion� (0.15 S cm�1) [55].
New group of block poly(sulfonated phenylene)-block-
polyimide copolymers, namely, poly(2-(3-sulfo)benzoyl-1,4-
phenylene)-block-polynaphthalimide (PSP-b-PI) copolymers
were synthesized by Ni-catalyzed copolymerization of 2,5-
dichloro-30-sulfobenzophenone and dichloro-terminated
naphthalimide oligomer. The IEC of these microphase-sepa-
rated structure membranes was about 1.5 meq g�1 and
because of the high through-plane conductivity and good
hydrolytic stability at 130 �C they exhibited high PEFC
performance and therefore high potential for PEFC applica-
tions [56].
It has been verified that at low RH, membrane morphology
plays a significant function in water sorption kinetics and
proton conduction. In fact, phase connectivity of hydrophilic
domains is a key factor in this regard. For this study, a multi-
block copolymer composed of a hydrophobic polyimide block
and a poly(arylene ether sulfone) hydrophilic block was
prepared (Fig. 7). The fuel cell performance under high
temperature and reduced humidity showed superior
conductivity of themultiblock copolymer. The performance of
a single cell using this multiblock copolymer at 40% RH was
comparable to that of Nafion [57].
It is worth mentioning that many SPI membranes show
anisotropic membrane swelling and anisotropic proton
conductivity behavior which means different behaviors in
Fig. 8 e Synthesis of sulfonated polyimide and semi-IPNs; semi-IPN of sulfonated polyimide and networked PEGDA (- - - -).
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 3 8 5e9 3 9 79394
membrane thickness direction than in plane direction mainly
due to the polymer chain alignment in plane direction. So it
has high degree of significance for evaluation as PEMs to study
the cross-sectional morphology of membrane in addition to
the proton conductivity and membrane swelling in both the
thickness and plane directions for SPI membranes including
sulfonated polyimides bearing pendant groups. However, this
is basically required for multiblock co-SPIs, because the
random and sequenced co-SPIs have been reported to reveal
the strong anisotropic membrane swelling behavior [58e60].
8. Semi and fully interpenetrating polymernetworks based on sulfonated polyimide for fuelcell applications
Semi-interpenetrating polymer network (semi-IPN)
membranes based on sulfonated polyimide and poly(ethylene
glycol) diacrylate (PEGDA) have been prepared (Fig. 8) and their
Table 4 e Proton conductivity of semi-IPN films.
Polymer code Proton conductivity (S/cm)
30 �C 50 �C 70 �C 90 �C
Semi-IPN-0 0.051 (1.093 0.132 0.166
Semi-IPN-10 0.053 0.099 0.153 0.170
Semi-IPN-20 0.056 0.101 0.160 0.177
Semi-IPN-30 0.060 0.109 0.165 0.183
Semi-IPN-40 0.061 0.116 0.171 0.189
Semi-IPN-50 0.069 0.124 0.176 0.194
Nafion� 117 0.061 0.104 0.147 0.187
properties compared with pure sulfonated polyimide
membrane and Nafion 117. By increasing of poly(ethylene
glycol) diacrylate contents, the proton conductivity increased
while ion-exchange capacity (an essential element for
improving the proton conductivity) decreased. Hydrolytic
stability of the semi-IPNmembranes was higher than the pure
sulfonated polyimidemembrane. Also, the amorphous nature
of the films was increased with the poly(ethylene glycol) dia-
crylate contents (crosslink agent) and almost fully amorphous
polyimide obtained. For these semi-IPN membranes the best
results were obtained when poly(ethylene glycol) diacrylate
content was about 20e50% and in these conditions they dis-
played comparable proton conductivity to Nafion 117 [61]. The
semi-IPN membranes confine water molecules more effi-
ciently than the ordinary linear polymers, resulting in suitable
water trapping capability of hydrophilic segments and there-
fore high proton conductivity at high temperature.
The proton conductivity of the semi-IPN films at different
temperatures was shown in Table 4. The proton conductivity
increased with the increasing temperature up to 70 �C. As the
PEGDA increased, IEC decreased so it was expected that
proton conductivity would decrease with increasing PEGDA.
Surprisingly, proton conductivity increased with increasing
PEGDA due to the fact that, apart from IEC, water uptake also
plays major role in proton conductivity of the semi-IPNs.
Presence of more hydrophilic PEGDA in semi-IPN films led to
more water uptake and consequently more proton conduc-
tivity. Existence of two hydrophilic PEG and sulfonic acid
groups in semi-IPNs caused more proton conductivity
compared to Nafion 117.
In another attempt, sulfonated polyimide (SPI) inter-
penetratingpolymernetwork (IPN)esilica (SiO2) nanocomposite
membranes have been reported as proton-conducting solid
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 3 8 5e9 3 9 7 9395
electrolytes for fuel cells. In these membranes dispersing of
homogeneously distributed nano SiO2 together with cross-
linking to induce IPN formation was achieved using urethane
acrylate non-ionomers (UANs). High proton conductivity and
hydrolytic stability, and low methanol permeability were the
result of preparation of such nanocomposite membranes in
comparison topure SPI.Highproton conductivity and improved
single cell performances deduced from the presence of long
polyethylene oxide chains crosslinkers [62].
9. Fuel cell performance of sulfonatedpolyimides
A proton-conducting membrane with high conductivity and
good membrane stability is one of the most important factors
in relation to high PEFC performance. On the other hand, to
improve fuel cell performance, it is desirable to operate PEFCs
at higher temperatures (above 90 �C) and as it was mentioned,
one of the main deficiencies of Nafion is reduction in
conductivity at elevated temperatures. Faster oxygen reduc-
tion reactions at the cathode, enhanced CO tolerance of the
catalyst at the anode, and higher efficiency of heat recovery
are the major benefits of operating PEFCs at higher tempera-
tures. However, a main problem prohibiting the practical use
of SPIs is the water stability of their membranes, which is
related to the mechanical stability of highly hydrated
membranes and thus the fuel-cell lifetime. Fortunately, some
recent works have been achieved in this field and good results
for fuel cell performance reported [33,34,45,63e66].
10. Conclusion
Finding new polymeric material for substitution of Nafion in
fuel cells is one of the main challenges for widespread use of
fuel cell as an energy resource. High methanol permeability
and a major reduction in conductivity at elevated tempera-
tures and low humidity, in addition to the high cost are the
main disadvantages of Nafion.
Polyimides are well known as polymer materials of high
performance for excellent thermal and oxidative stabilities
and balanced mechanical and electric properties for long
period of operation. Properties which are typically identified
with polyimides are heat resistance, solvent resistance, good
mechanical strength, good toughness, excellent dimensional
stability, low coefficient of friction, outstanding radiation
resistance, high dielectric strength, low outgassing, and
resistance to creep and wear. According to these facts, poly-
imides with appropriate ion-conducting sites (sulfonated
polyimides) are one of the best candidates for fuel cell
membrane. Sulfonated polyimides contain most of the
essential properties for this application, including a high
degree of ionic conductivity, low gas permeability, low
methanol permeability, and good mechanical properties.
In this article, modified structure of sulfonated polyimide
membranes and comparison of their properties with Nafion
were studied. Accordingly, it can be concluded that the ionic
conductivity of highly charged SPIs is comparable to that of
Nafion. These sulfonated polyimides showed higher
conductivity in high temperature condition and also lower
methanol permeability in comparison with Nafion. However,
fuel cell tests showed low hydrolytic stability in spite of suit-
able performance in PEMFCs and DMFCs. Branching, cross-
linking, semi-interpenetrating, introducing flexible and
aliphatic linkages or using highly basic sulfonated diamines in
the structure of sulfonated copolyimides are among the
different methods have been proposed for the compensation
of the disadvantage of water stability of the SPIs. In this way,
some novel SPI membranes showed improved water stability
which is comparable or even higher than Nafion.
Therefore, design and synthesis of new monomers for the
preparation of related sulfonated polyimide membranes with
optimized proton conductivity, IEC, hydrolytic stability,
thermal stability, mechanical properties, water uptake, and
morphology are the current research trend in this field. Also,
low stability and therefore degradation of sulfonated poly-
imide membranes which results from the hydrolysis of the
imide units are the main problem of polyimides in the
circumstances of fuel cell operation. So, the future trend is
improving the chemical and hydrolytic stability of the
sulfonated polyimide membranes while maintaining fuel cell
performance through designing and preparation of new
monomers and related polyimides considering that increasing
flexibility of polymer chain, using more basic sulfonated
diamines and superior microphase separation lead to high
water stability of the SPIs.
Acknowledgements
The authors would like to appreciate Renewable Energy
Organization of Iran for collaboration and partial support of
this research.
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