Synthesis and characterization of polysulfone/

layered double hydroxides nanocomposite

membranes for fuel cell application

Q1 M. Herrero a, A.M. Martos a, A. Varez a,*, J.C. Galvan b, B. Levenfeld a

aDepartamento de Ciencia e Ingenierıa de Materiales e Ingenierıa Quımica, Universidad Carlos III de Madrid,

Avenida de la Universidad, 30, 28911 Leganes, Madrid, SpainbCentro Nacional de Investigaciones Metalurgicas (CENIM-CSIC), Avda. Gregorio del Amo, 8, 28040 Madrid, Spain

a r t i c l e i n f o

Article history:

Received 26 December 2012

Received in revised form

6 June 2013

Accepted 10 June 2013

Available online xxx

Keywords:

PEM

Polysulfone

Layered double hydroxide

Proton conductivity

a b s t r a c t

In the present study, sulfonated polysulfone (sPSU)/layered double hydroxide (LDH)

composite membranes for use in proton exchange membrane fuel cells (PEMFCs) were

investigated. Polysulfone (PSU) was sulfonated with trimethylsilyl chlorosulfonate in 1,2

dichloroethane at room temperature.

Composite membranes were prepared by blending different amount (0, 1, 2, and 5%) of

LDH nanoparticles with sPSU in dimethylacetamide (DMAc). The membranes were pre-

pared by the casting method and the samples obtained were characterized by XRD, FTIR

spectroscopy. The thermal behavior for all samples was evaluated by thermogravimetrical

analysis (TGA). Finally electrochemical impedance spectroscopy (EIS) was used to study the

membranes electrical properties. The EIS measurements were carried out with the mem-

branes in contact with HCl solutions at different concentrations (10 3 ! c ! 10 1). Results

show a clear dependence of the membrane electrical resistance with the sulfonation

degree and the amount of the LDH added.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

In the last years, great effort has gone into the fuel cells

development in order to avoid the dependence on hydrocar-

bon fuel and the pollution release during the process which

can be cause hazardous threat to the environment and human

life. The most promising fuel cells are the polymer electrolyte

membrane fuel cells (PEMFCs) due their modularity [1] and

wide variety of applications. In PEMFCs a proton exchange

membrane (PEM) is the core, playing a key role. The PEMmust

show a high proton conductivity, a low electronic conductiv-

ity, thermal stability, low fuel permeability, good mechanical

properties and low cost [2,3]. The first and most utilized

membranes are based on expensive perfluorinated polymers,

which operate only under fully hydrated conditions [4]. Many

efforts are being done in order to replace this type of polymer,

the current alternatives are: (i) modified perfluorinated com-

posites membranes, (ii) functionalized non fluorinated poly-

mers and related composite, and (iii) acidebase composite

membrane [5,6].

The second approach includes polymers with high me-

chanical and chemical stability, relatively high proton con-

ductivity, low cost and low permeability to the fuel. Some of

them are: sulfonated polysulfone (SPSU) [7], sulfonated poly

(ether ketone) (SPEK) [8], sulfonated poly (ether ether ketone)

(SPEEK) [9], etc. Among these polymers, polysulfone is one of

* Corresponding author. Tel.: þ34 916249484.E-mail address: alvar@ing.uc3m.es (A. Varez).

Available online at www.sciencedirect.com

journal homepage: www.elsevier .com/locate/he

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i n t e rn a t i o n a l j o u rn a l o f h y d r o g e n en e r g y x x x ( 2 0 1 3 ) 1e7

HE11997_proof ■ 28 June 2013 ■ 1/7

Please cite this article in press as: Herrero M, et al., Synthesis and characterization of polysulfone/layered double hydroxidesnanocompositemembranes for fuel cell application, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.06.041

0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.ijhydene.2013.06.041

ero os rez J.C.J.C. n eldeld

a clear dependence of the membrane electrical resista clear dependence of the membrane electrical resist

e, and (iii) acide, and (iii) acid

the preferred material due to its low cost, high thermal sta-

bility, and easy availability [10]. In addition, this polymer has

been easily sulfonated with various sulfonating agents giving

excellent proton conducting membranes.

Although, incorporations of inorganic fillers, such as, TiO2,

ZrO2, phosphotungistic acid, leads to an increase in some

properties of the virgin polymer [1,9,11e13], however clays are

the most promising reinforcement materials due to they can

easily be exfoliated to individual platelets inducing extremely

large surfaces and interface between the filler and the poly-

mer matrix. These lamellar solids are used to increase the

mechanical properties, decrease the fuel and water perme-

ability [14], and, due to their hygroscopic properties, maintain

humidity inside the membrane [13].

In this sense, LDHs, also called anionic clays or hydrotalcite-

like compounds, exhibit certain specific advantages (purity,

crystallinity and particle size control, easy functionalization),

which are lacking in layered silicates type nanoclays. LDHs

constitute a large family of inorganic materials, with the gen-

eral formula ½M2þ1-xM

3þx$ðOHÞ2&ðA

n-Þx=n$mH2O, where M2þ is a

divalent cation such as Mg, Ni, Zn, Cu, and M3þ is a trivalent

cation such asAl, Cr, Fe, V orGa; x is a value that determines the

layer charge density and the anion exchange capacity, ranges

between 0.2 and 0.4. An is an exchangeable anion with formal

charge n [15,16].

In this work, we disclose the preparation of LDH/SPSU

membranes by the castingmethod using dimethylacetamide as

solvent. The LDH used was Zn,Al-NO3 in order to minimize the

basic properties of this kind of compounds, and was prepared

by the coprecipitationmethod. The polysulfonewas sulfonated

using chlorotrimethylsilane as sulfonating agent, different

ratios PSU: chlorotrimethylsilane were used. The degree of

dispersion of the LDH particles and the type of the polymeric

membranes obtained were studied by X-ray diffraction (XRD).

The interactions between LDH and SPSUwere discussed on the

basis of the FTIR spectroscopy data. The Water Uptake Mea-

surements were used as a quantitative measure of membrane

performance in fuel cells application. Performance of a mem-

brane is dependent on proton conductivity, which in turn often

depends on its water content. High proton conductivity is

supported by high level of water uptake; at the same time, it is

also a sign of low-dimensional stability as water influences the

polymer microstructure and mechanical properties. The ther-

mal stability was determined by TGA analysis and electro-

chemical impedance spectroscopy (EIS) was used to study the

membranes electrical properties. The EIS measurements were

carried out with the membranes in contact with HCl solutions

at different concentrations. Results showed a clear dependence

of the membrane electrical resistivity with the sulfonation

degree and the amount of the LDH added.

2. Experimental

2.1. Materials

The reagents, Zn(NO3)2$6H2O, Al(NO3)3$9H2O, NaOH were

from Panreac. Polysulfone polymer (Mn ¼ 22,000), trime-

thylsilyl chlorosulfonate (Si(CH3)3SO3Cl, 99%) and the sol-

vents, N,N-dimethylacetamide (CH3C(O)N(CH3)2, DMAc) and

1,2-dichlorethane (C2H4Cl2, DCE), were supplied by Aldrich. All

of them were used without further purification.

2.2. Preparation of the Zn,Al-NO3 LDH

The LDH selected was [Zn1 xAlx(OH)2](NO3)x0.8H2O (x ¼ 0.33)

due to the lower basicity. The powder was prepared by the

co-precipitation method [17]. A first solution was prepared by

dissolving Zn(NO3)2$6H2O and Al(NO3)3$9H2O in 250 mL of

water with a total Zn/Al concentration of 0.6 mol/L and a

Zn:Al molar ratio of 2. A second solution was prepared by

dissolving NaOH in 800 mL of water (NaOH concentration

1 mol/L). Both solutions were added drop-wise to decarbo-

nated water in a 3-neck vessel, with stirring and the pH was

maintained at a value of 8.0. The slurry was aged for 12 h at

40 (C with stirring. In order to minimize contamination with

atmospheric CO2, the reaction was carried out under a ni-

trogen purge. Prepared powder was characterized by XRD

and FTIR techniques.

2.3. Sulfonation of the polysulfone

The PSUwas sulfonated according to a procedure described by

Chao et al. [18]. The dried PSU (20 g) was dissolved in 100 mL

anhydrous 1,2-dichlorethane in a three-neck flask equipped

with mechanical stirrer, condenser and nitrogen purge inlet

which carries the gaseous HCl generated by the substitution.

The resulting solution was purged with nitrogen for 1 h and

chlorotrimethylsilane diluted in 20mL 1,2 dichloroethanewas

added to the solution, dropwise for 30 min. The resulting so-

lution was vigorously stirred for 24 h at room temperature.

After reaction was completed, SPSU was isolated from the

solution by precipitation with methanol, and then was

washed with deionized water for several times and dried to

constant weight. Two molar ratio of PSU:TMSCS were pre-

pared, 1:1 and 1:3. In case of higher TMSCS content, the sul-

fonated polymer produced was water soluble.

2.4. Membrane preparation

Dried sulfonated polysulfone was dissolved (5% wt) in dime-

thylacetamide (DMAc), and the resulting solution was filtered.

To prepare the composite membranes the LDH particles were

suspended in a SPSU-DMAc solution during 2 h with me-

chanical stirring and then in an ultrasonic bath for an addi-

tional hour. The membranes were cast on a Petri dish, using

the LDH-polymer solution, and dried at 60 (C during 12 h.

Finally, the membranes were washed with distillate water.

The amount of LDH incorporated in the membranes was from

1 to 5 wt% of the total mass (LDH þ polymer). The thickness of

the resulting membranes was around 100 mm.

2.5. Characterization and measurements

Powder X-ray diffraction patterns were recorded in a X’Pert

Philips instrument using Cu-Ka radiation (l ¼ 1.54050 �A).

The FTIR spectra were recorded in a Perkin-Elmer Spec-

trum GX instrument, using KBr pellets; 100 spectra (recorded

with a nominal resolution of 4 cm 1) were averaged to

improve the signal-to-noise-ratio.

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Please cite this article in press as: Herrero M, et al., Synthesis and characterization of polysulfone/layered double hydroxidesnanocomposite membranes for fuel cell application, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.06.041

. The dried PSU (20 g) was dissolved in 100 m. The dried PSU (20 g) was dissolved in 100 m

in 20min 20m

Thermogravimetric analyses were carried out in TG-7

instrument from Perkin-Elmer. The sample was heated

from 50 to 900 (C in flowing nitrogen, at a heating rate of

10 (C min 1.

Water uptake of sulfonated polysulfones and LDH/SPSU

composites membranes were determined from the difference

between the dry and swollen membranes. A membrane of

3.8 cm2, was weighed dry and immersed in an excess of

distilled water for 24 h at room temperature and at higher

temperatures 60, 80 (C for 2 h. Then, the membrane was

removed wiped and weighed (Wwet). Finally, the membrane

was dried overnight at 80 (C in an oven, and after wasweighed

again (Wdry). The percentage of water absorbedwas calculated

using the following equation.

Water uptake (%) ¼ (Wwet Wdry) ) 100/Wdry

For electrical characterization we have used a test cell

constituted by two half-cells separated by two O-rings where

membrane was placed. A conventional electrochemical setup

of four electrodes was used for thesemeasurements involving

two saturated Ag/AgCl electrodes as reference electrodes and

two platinum electrodes as secondary electrodes. Impedance

spectroscopy (IS) measurements were carried using an

impedance analyzer (Solartron 1260) together an electro-

chemical interface (Solartron 1287). The measurements were

carried out at room temperature in the frequencies range of

0.1e105 Hz, by applying a *10 mV amplitude sinusoidal wave

perturbation, close to the resting membrane potential. Six

different HCl solutions (1 ) 10 3 ! c ! 0.1 M) were tested. The

solutions on both sides of the membrane have the same

concentration. Before measurement, membranes were

immersed for 12 h in a solution of the appropriated HCl

concentration.

3. Results and discussion

3.1. Polysulfone sulfonation

Qualitative polysulfone sulfonation was confirmed using FTIR

spectroscopy and termogravimetrical analysis.

In Fig. 1a the FTIR spectra for the pristine polysulfone (PSU)

and sulfonated polysulfone in different amount (SPSU11 and

SPSU13) are displayed. In the sulfonated samples a new broad

band is recorded at 3400 cm 1, assigned to the stretching OeH

vibration of the eSO3H group. However, this band is not

enough to confirm the sulfonation process, because some

moieties of water bonded to the hydrophilic sulfonic groups,

whichwas not totally removed by the drying process, lead this

intense band. The sulfonation process is confirmed by the

presence of the shoulder, around 1030 cm 1, together the peak

located around 1014 cm 1 and assigned to the symmetric

stretching vibration of the diphenyl ether unit [19]. The

shoulder represent the symmetric stretching of eSO3H group.

This shoulder is more pronounced in the SPSU13 sample,

indicating the increase in the sulfonation degree. In this

polymer, the adsorption band due to the ether group overlaps

to the characteristic band associated to the asymmetric

stretching vibrations of eSO3H group, which appears at

1350e1150 cm 1 [20].

The thermogravimetric analysis of the PSU and SPSU

samples, recorded in flowing nitrogen, is shown in Fig. 1b. The

thermal degradation of PSU takes place in only one step be-

tween 525 and 600 (C, while for the SPSU at least 3 steps can be

distinguished in the TGA. This also has been reported by other

groups [7,21,22]. The first weight loss (up to 250 (C) was

assigned to evaporation of residual solvent and to the

desorption of water bonded to hydrophilic sulfonic groups,

which was not totally removed by the drying process. The

weight loss between 250 and 440 (C is associated to the loss of

sulfonic acid (eSO3H) groups. Finally the weight loss over

440 (C is associated to the polymermain chain decomposition.

As the sulfonation degree increases the onset of each loss

weight takes places at lower temperature indicating that sul-

fonation has a negative effect on the polymer thermal stability

and the increase in the sulfonation degree leads less thermal

stability in the polymer.

Fig. 1 e a. FTIR spectra of PSU, SPSU11 and SPSU13 among

the regions: 800e3100 cmL1 and 1050e950 cmL1. All

spectra were normalized for comparison (vertical bar

indicate the scale of y-axis). b. Thermogravimetric analysis

for polysulfone (PSU) and sulfonated polysulfone in

different degree (SPSU11 and SPSU13).

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Please cite this article in press as: Herrero M, et al., Synthesis and characterization of polysulfone/layered double hydroxidesnanocompositemembranes for fuel cell application, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.06.041

e

((

3.2. Physical characterization of nanocomposites

3.2.1. X-ray diffraction

The XRD patterns of the Zn,Al-NO3-LDH powder, SPSU11

membrane and the LDH/SPSU11 composite membranes with

different LDH contents (1, 2 and 5%wt) are displayed in Fig. 2a.

The very broad peak at 19.7( (2q) associated to the PSU in-

dicates the low crystallinity of the polymer. In the case of

composite membranes the main diffraction peak, d003, of the

Zn,Al-NO3-LDH (d003 ¼ 9 �A, 2q ¼ 10.1() is absent in the XRD

patterns of samples with 1( 2 wt% of LDH and only it is

observed in the highest loaded composite membrane (labeled

as arrows in Fig. 2a). This result could be expected for samples

containing low amount of LDH, due to the low concentration

and low crystallinity. When the LDH content increases up to

5 wt%, two very weak and broad diffraction peaks appear at

2q z 9.4 and 10(, one of them could correspond to the main

diffraction peak of Zn,Al-NO3-LDH (d003). On the other hand,

for composite membranes with 2 and 5% of LDH, also appears

a new peak at 2q z 5.5e6( that could be associated to the

intercalation of the part of the polymer between the interlayer

of the LDH [23,24].

3.2.2. FT-IR spectroscopy

The FT-IR was, also, used in order to determinate the specific

interaction of LDH/sPSU. The position of the shoulder corre-

sponding symmetric stretching vibration of the sulfonate

group band is very sensitive to the local environment of the

sulfonate anion [19,25], e.g. hydration by moisture or

hydrogen bonding by a second component in a polymer blend,

so that any interaction would shift the anion symmetric

stretching vibration to lower frequencies (higher wave-

number). In Fig. 2b FT-IR spectra of the SPSU11, SPSU11-1%

LDH and SPSU11-5%LDH are displayed in the region

1050e950 cm 1. The shoulder is shifted to higher wave-

number, indicating weak interactions between the hydroxyl

LDH layer and the sulfonic group, probably hydrogen bonding.

Furthermore the intensity of this shoulder increases as the

content of LDH, indicatingmajor interaction between the LDH

and the sPSU.

3.2.3. Thermal stability

In order to investigate the effect of the LDH in the thermal

stability of the LDH/PSU composites TGA was performed

(Fig. 3). Two behaviors can be observed depending on the

sulfonation degree. The presence of LDH in the SPSU11

membrane basically does not modify the thermal behavior of

the film (Fig. 3a). However in the case of the SPSU13 (Fig. 3b)

the main effect of introducing the LDH nanoparticles was a

significant displacement of, basically, all degradation peaks,

improving the thermal stability of the polymer. This effect is

associated to the thermal characteristic of the LDH which

could be ascribed to the hindering role of the LDH particles on

the diffusion of oxygen and volatile products throughout the

composite materials [26]. The thermal stability improves with

the LDH, but an optimum fraction exists beyond which the

thermal stability begins to deteriorate. In the SPSU13 when

the LDH content increases (samples with 5%) the thermal

degradation temperature seems decrease. This behavior has

been previously reported for other LDH-polymer system

[27,28] and is due to the fact that the high content of LDH can

catalyze the degradation of polymer.

3.2.4. Water uptake measurements

In Fig. 4, thewater uptake at different temperatures (25, 60 and

80 (C) of the different membrane tested is compared. After

sulfonation of PSU, the water uptake increases as a conse-

quence of the hydrophilic character of acid groups. Unlike

other system, the amount of water absorbed did not increase

with the temperature [13]. In the case of composite mem-

branes, and at the same sulfonic acid content, water uptake of

the different SPSU/LDH composite membranes is lower than

Fig. 2 e (a) PXRD patterns from the LDH, SPSU11 and the different LDH/SPSU11 membranes. (b) FT-IR spectra of SPSU11 and

composites with 1 and 5% (1100e950 cmL1 range). Vertical bar indicate the scale of y-axis.

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HE11997_proof ■ 28 June 2013 ■ 4/7

Please cite this article in press as: Herrero M, et al., Synthesis and characterization of polysulfone/layered double hydroxidesnanocomposite membranes for fuel cell application, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.06.041

d in the highest loaded composite membrane (d in the highest loaded composite membrane (

e

SPSUmembrane. This behavior was found in other composite

membrane system [29,30] and it was attributed to the reduc-

tion of the free volume and swelling ability, however this

argument cannot be admitted in this case because the water

uptake increases with the amount of LDH. This situation is

compatible with the formation of hydrogen bonding between

sulfonic acid group and the hydrotalcite type compound.

Regarding to the evolution of water uptake of composite

membranes with the temperature we observed, as expected,

an increases of water absorbed with the temperature.

3.2.5. Electrochemical impedance spectroscopy

The study of the electric properties of the membranes was

performed using electrochemical impedance spectroscopy

(EIS). Prepared membranes were placed in contact with the

different acid solutions and they were equilibrated during a

specific time period in contact with the electrolyte. After this,

impedance spectroscopy measurements were carried out in a

four electrode electrochemical cell. Fig. 5a shows the Nyquist

plot (Z0 vs Z00) for SPSU11 with 1% of LDH in contact with

different HCl solutions. All the spectra present clearly two

semicircles, which change with the HCl concentration of so-

lutions. The first semicircle (high frequency arc (HFA)) could

be associated to the membrane capacitance (or constant

phase element of the membrane) acting in parallel with its

resistance. The low frequency arc (LFA) is clearly deformed

and is associated to diffusion of electroactive species through

the membrane [31]. In all the cases, the high frequency

semicircle does not intercept the origin of the plot, indicating

the presence of a resistive element in serieswith the other two

processes. This resistive element should be related to the

electrolyte.

The experimental data for each membrane and each HCl

concentration were fitted with commercial software [32] to

different equivalent circuits. The best fitting was obtained for

the equivalent circuit showed in the inset of Fig. 5. In this

circuit, Rs represents the resistance of the electrolytic solu-

tion, Ri usually is related to the ionic resistance of the mem-

brane, Cg is assigned to the geometric capacitance of the

membrane, and Zws theWarburg impedance in diffusion layer

with finite thickness. From the fitting, the values of the

different circuit parameterswere determined. As expected the

resistivity associated to the membrane decreases with

amount of HCl (Fig. 5b). This behavior has been also observed

in poly(ether ether sulfone) [33] and is attributed to the con-

centration dependence of the electrolyte solution embedded

in the membrane matrix.

Fig. 6a shows the evolution of the impedance plots as a

function of the amount of LDH added to the membrane. The

diameter of the high frequency arcs decreases with the

amount of LDH, indicating an improving of the proton con-

ductivity with the amount of inorganic powder. The SPSU/

LDH nanocomposite membranes have a better electrical

behavior than those membrane based on the SPSU matrix

alone (blank membrane), showing the lowest electrical

Fig. 3 e TGA curves of the sulfonated polysulfone in different degree (PSU11 and PSU13) and their respective LDH

composites.

Fig. 4 e Water uptake measurements at different

temperatures (25, 60 and 80 (C) for SPSU and SPSU

composites.

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HE11997_proof ■ 28 June 2013 ■ 5/7

Please cite this article in press as: Herrero M, et al., Synthesis and characterization of polysulfone/layered double hydroxidesnanocompositemembranes for fuel cell application, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.06.041

resistivity the SPSU with a 5% wt of LDH (Fig. 6b). Even these

values are lower than that of Nafion. These results indicate

that this kind of nanocomposites membranes is very prom-

ising for being applied as proton-exchange membranes.

4. Conclusions

A series of sulfonated polysulfone/layered double hydroxide

nanocomposite membranes were prepared using solvent

casting technique. The Zn,Al-NO3 LDH powder was obtained by

the coprecipitation method. Polysulfone was sulfonated with

trimethylsilyl chlorosulfonate in 1,2 dichloroethane at room

temperature and different sulfonation degrees were obtained.

The sulfonation of PSU reduces the thermal stability of the

polymer. However the introduction of LDH powders improved

the thermal behavior, especially in the case of the samples

with higher sulfonation degree. The introduction of LDH clearly

affects the properties of SPSU membranes, particularly their

water uptake, electrical and transport parameters. Electro-

chemical characterization of the membranes by EIS shows

behaviors that depend on the concentration of both the elec-

trolyte and membrane. The electrical resistivity of membrane

(inversely connected to the proton conductivity) is closely

related with both the concentration of HCl in contact with

membrane and with the layered double hydroxide nano-

composite. This work demonstrates that LDH-composites

membranes are promising materials for proton-exchange pro-

cess, due to membrane transport are quite similar to those of

the best PEM system.

Acknowledgments

Authors thanks financial support received from the regional

government (Comunidad de Madrid through MATERYENER

S2009 PPQ-1626), and Spanish Government, MICINN (MAT2010-

19837-CO6).

Fig. 5 e (a) Complex Impedance plot for the membrane composite with 1% of LDH at different HCl concentration. The

equivalent circuit used for fitting is also displayed in the inset. (b) Evolution of the membrane resistivity with the HCl

concentration.

(a)

(b)

Fig. 6 e (a) Nyquist plot for three composite membranes with different percentage of LDH (1, 2 and 5%) (c [ 0.05 M HCl). (b)

Resistivity of the membrane vs %LDH. Values are also compared with nafion membrane.

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 x x x ( 2 0 1 3 ) 1e76

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HE11997_proof ■ 28 June 2013 ■ 7/7

Please cite this article in press as: Herrero M, et al., Synthesis and characterization of polysulfone/layered double hydroxidesnanocompositemembranes for fuel cell application, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.06.041