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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: s.mehdipour@ippi.ac.ir (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.

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 9389

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

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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.

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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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