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Studies on novel heat treated sulfonated poly(ether etherketone) [SPEEK]/diol membranes for fuel cell applications

Deeksha Gupta, Veena Choudhary*

Centre for Polymer Science & Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

a r t i c l e i n f o

Article history:

Received 13 January 2011

Received in revised form

18 March 2011

Accepted 6 April 2011

Available online 12 May 2011

Keywords:

Proton conductivity

Water uptake

Thermal stability

Mechanical stability

* Corresponding author. Tel.: þ91 11 2659142E-mail address: veenach@hotmail.com (V

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.04.044

a b s t r a c t

The paper describes the preparation of membranes based on sulfonated poly(ether ether

ketone) [SPEEK] [degree of sulfonation w65%] in the presence of varying amounts of poly

(ethylene glycol) (molecular weight 200) [PEG-200] and cyclohexane dimethanol [CDM]

using water:ethanol (50:50) as solvent. After drying, the membranes were heat treated at

60 �C (2 h), 80 �C (2 h), 100 �C (2 h), 120 �C (2 h) and 135 �C for 16 h. After the heat treatment,

samples were insoluble in water:ethanol (50:50) mixture. The membranes thus obtained

were characterized by FTIR spectroscopy (structural), water uptake (hydrolytic stability),

thermal stability (TG), mechanical stability and proton conductivity. A significant increase

in the hydrolytic stability was observed, SPEEK became elastic and fragile whereas the

heat-treated SPEEK/PEG and SPEEK/CDM remained stable even after 135 days of water

immersion at 35 �C.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction However, there are crucial drawbacks associated with the use

Fuel cells convert chemical energy directly into electrical

energy very efficiently with no emission of pollutants [1].

Polymer electrolyte membrane fuel cells (PEMFCs) have the

maximum potential for mass commercialization addressing

automotive, stationary and mobile applications due to quick

start up time and high flexibility to load changes hence,

PEMFC could solve the problems associated with use of fossil

fuels in transport systems and in energy production along

with environmental concerns. In PEMFC, membrane electro-

lyte is the core component which performs dual function, by

transferring protons from anode to cathode (at the same time

free electrons at the anode travel through external circuit and

reach the cathode) and also acts as the separator between the

two compartments i.e. the cathode and the anode. The state of

the art polymer electrolyte membrane (PEM) used in the fuel

cells is Nafion� i.e. perfluorosulfonic acid (PFSA) ionomer.

3; fax: þ91 11 26591421.. Choudhary).2011, Hydrogen Energy P

of Nafion like, poor performance at elevated temperature and

high cost which hinder its mass commercialization. Thus, the

short comings of PFSA basedmembranes stimulated the need

for nonfluorinated PEMs [2e4]. In this regard, sulfonated pol-

y(ether ether ketone) [SPEEK] is one of the most widely

investigated polymers for fuel cell application due to its

excellent thermal, mechanical, chemical properties and low

cost [5e7]. However, incorporation of more number of eSO3H

group in poly(ether ether ketone) [PEEK] during sulfonation,

gives higher proton conductivity but at the same time causes

excessive swellingwhich in turn leads to poormechanical and

dimensional stability.

Various approaches have been used tomodify SPEEK i.e. (a)

addition of fillers like: Ti [8], Si [9], BPO4 [10], heteropolyacid

[11], POSS [12], etc; (b) blending with other polymers e.g. poly

benzimidazole (PBI) [7], poly (vinyl alcohol) (PVA) [13], etc. and

(c) cross-linking. Different methods of cross-linking using

ublications, LLC. Published 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 6 ( 2 0 1 1 ) 8 5 2 5e8 5 3 58526

aliphatic polyols [14,15], aromatic diols [16], UV irradiation

[17], epoxy [18], diiodomethane [19] and blending [20e23] have

been reported in the literature.

Cross-linking is thought to be a simple and powerful

method to improve PEMperformance. The first reported cross-

linking of SPEEK was carried out using suitable aromatic or

aliphatic amines and formation of imide linkages, which are

acidic and supposed to participate in proton transfer and thus

contributing to proton conductivity of the polymer [24,25].

Mikhailenko et al. [14,15] carried out detailed study on

cross-linking of SPEEK using various kinds of polyatomic

alcohols such as ethylene glycol (EG), glycerol and pentery-

thritol in different solvent systems. They reported that

thermal cross-linking of SPEEK does not occur in the absence

of cross-linker or in the presence of DMAc, DMF and NMP

solvent. The performance properties of SPEEK were found to

be dependent on the nature of cross-linker and the best

properties were obtained when ethylene glycol [EG] as cross-

linker. They proposed the cross-linking mechanism which

suggested that EGmolecules initially attached to sulfonic acid

group will preferably react with another EG molecules or its

derivative, forming EG dimers, trimers or poly functional

moieties. They also explained the role of casting solvents and

concluded that DMAc, DMF, NMP competewith diolmolecules

for interaction with eSO3H and thus preventing the cross-

linking of SPEEK. There was an optimum concentration of

diol molecules at which SPEEK achieved best combination of

properties because at lower ratio insufficient cross-linking

occurs and at higher concentration membranes were brittle.

Kerres [22] have suggested that membrane swelling can be

reduced by introducing specific interactions between the

macromolecular chains in membrane components. In poly-

mers, different kind of interactions are possible i.e. van der

Waals interactions, electrostatic interaction and dipoleedi-

pole interaction. In any case, the presence of chemical bonds

between the macromolecular chains have the significant

impact on polymer structure and properties because covalent

cross-links are fixing the polymer morphology but physical

interactions between polymer chains can be reversed with

increase in temperature.

The aim of the present work was to investigate systemat-

ically the effect of incorporation of varying amounts of

aliphatic/alicyclic diol i.e. poly (ethylene glycol) having

molecular weight of 200 [PEG-200]/cyclohexane dimethanol

[CDM] on the performance properties of SPEEK. Although,

O O

PEEK

O O C

O

x

SPEE

Sulfonation Su

Scheme 1 e Sulfon

cross-linking of SPEEK using ethylene glycol, glycerol and

aromatic diol have been reported in the literature [14e16], to

the best of our knowledge, cross-linking of SPEEK using PEG-

200 and CDM has not been investigated. It was therefore

considered of interest to evaluate the effect of nature and

concentration of diol on the properties of SPEEK having degree

of sulfonation w65%.

2. Experimental

2.1. Materials

Victrex PEEK (150 XF ICI, USA), sulfuric acid (98% Merck),

ethanol and poly (ethylene glycol) with molecular weight of

200 (PEG-200) [BDH, India], 1,4-cyclohexane dimethanol

[Aldrich, USA], dimethyl acetamide [DMAc] [Qualigens, India]

were used as received without further purification.

2.2. Preparation of sulfonated poly(ether ether ketone)[SPEEK]

The sulfonation of PEEK (Victrex 150 XF) was carried out using

concentrated sulfuric acid according to the procedure repor-

ted elsewhere [26e29]. 5 g of PEEK was dissolved in 95 ml of

conc. H2SO4 (95e98%) in a reaction kettle at room temperature

for about 1 h to minimize heterogenous sulfonation. After

complete dissolution of PEEK in H2SO4, the temperature was

raised to 50 �C and polymer solution was stirred vigorously for

1e2 h to achieve the desired degree of sulfonation (DS). Poly-

mer solution was poured slowly in excess of ice-cooled

distilled water under continuous mechanical agitation to

precipitate sulfonated PEEK [SPEEK]. SPEEK was separated by

filtration andwashed several timeswith distilled water till the

filtrate was free of acid. Then, polymer was dried in vacuum

oven at 70 �C for 12 h. The sulfonation reaction of PEEK is

shown in Scheme 1. The SPEEK is a copolymer comprising of

sulfonated PEEK and non-sulfonated PEEK structural units.

Structural characterization of SPEEK was done using FTIR and1H-NMR spectroscopy.

2.3. Preparation of membrane

The details of membrane preparation are shown in Table 1.

SPEEK (DS w65%) was dissolved in water:ethanol mixture

O O C

O

SO3H

C

O

y

K

lfuric Acid

ation of PEEK.

Table 1 e Details of membrane preparation.

Diol Structure Molecularweight

SPEEK/diol(w/w)

PEG He[OCH2CH2]neOH 200 80/20, 75/25,

67/33,60/40

CDM HOeCH2eC6H8eCH2eOH 144 80/20, 75/25,

67/33,60/40

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(50:50) to prepare 5 wt% solutions. Diols were weighed sepa-

rately in calculated amount in sample bottles. SPEEK solution

was poured in sample bottles containing diol and stirred

vigorously at room temperature using magnetic stirrer to

prepare homogenous solution. Poly (ethylene glycol) with

Mw ¼ 200 [PEG-200] and cyclohexane dimethanol [CDM] was

added separately in the range varying from 20 to 40 wt% of

SPEEK to prepare membranes of varying composition.

Membranes containing 20 wt% diol (both PEG and CDM) were

too brittle to handle and membranes with 40 wt% CDM have

shown the phase separation so they were not investigated

further for fuel cell applications. The mixture (SPEEK/diols)

after homogenizationwas poured in the petri dish followed by

solvent evaporation at 80 �C for 12 h to prepare the

membranes. Finally, the cast membranes were heat treated in

vacuum oven at different temperatures for definite time

period i.e. at 60 �C for 2 h, 80 �C for 2 h, 100 �C for 2 h, 120 �C for

2 h and 135 �C for 16 h.

Neat SPEEK membrane was also prepared using water-

eethanol (50:50) as solvent but could not be evaluated because

of its fragile nature. Therefore, for comparison purpose, neat

SPEEK membrane was prepared using DMAc as solvent.

Membranes were designated as SPEEK/PEG or SPEEK/CDM

followed by numeral representing the weight percentage of

SPEEK and diol respectively. For examplemembrane prepared

by using 75% SPEEK and 25% PEG-200 and 75% SPEEK and 25%

CDMhas been designated as SPEEK/PEG 75/25 and SPEEK/CDM

75/25 respectively. Membranes after scheduled heat treat-

ment were denoted by symbol ‘X’ as prefix to the letter

designation. The details of sample preparation are given in

Table 1.

2.4. Characterization methods

2.4.1. Structural characterizationA Bruker 300 MHz spectrophotometer was used to record1H-NMR of SPEEK using DMSO-d6 as solvent and tetrame-

thylsilane as an internal standard at room temperature.

FTIR spectra of membranes (before and after heat treat-

ment) were recorded on Thermo Nicolet IR 200 spectropho-

tometer using thin membrane samples in the scanning range

of 500e4000 cm�1.

2.4.2. Thermal characterizationThe thermal stability of membranes was evaluated by

recording thermo-gravimetric (TG) and derivative thermo-

gravimetric (DTG) traces in nitrogen atmosphere (Pyris 6

TGA, Perkin Elmer). Heating rate of 20 �C/min, temperature

range from50 �C to 850 �C and sampleweight of 6� 2mg in the

membrane form was used for recording TG/DTG traces.

Dynamicmechanical analysis was performed on a TAQ800

instrument in tension mode at an oscillation frequency of

1.0 Hz. Sample dimensions were: length: 25 � 5 mm, width:

6 � 1 mm, thickness: 100 � 10 mm. The DMA scans were

recorded in the temperature range of 30e250 �C at a heating

rate of 5 �C/min. The storage modulus (E0), loss modulus (E00)and loss tangent (tan d) plots were recorded as a function of

temperature.

2.4.3. Morphological characterizationMorphological characterization was performed using field

emission scanning electron microscopy (FESEM) model Hita-

chi S-4800 on cryogenic fractured surfaces of the membranes

operated at 1.5 kV. All samples were measured directly

without any coating.

2.4.4. Water uptakeFor water uptake studies, the measurements were carried out

in triplicate. A known weight of dry heat-treated membranes

was immersed in water at three different temperatures i.e. at

30 �C, 80 �C and 100 �C for 24 h. The films were taken out;

surface water was wiped using tissue paper and weighed

using an analytical balance. The water uptake was calculated

using Equation (1).

Water Uptake ¼ �Wwet �Wdry

��Wdry � 100 (1)

Wdry ¼ weight of dry membrane, Wwet ¼ weight of membrane

after immersion in water.

2.4.5. Proton conductivity measurementThe bulk proton conductivity of the membranes was

measured by impedance spectroscopy over a frequency range

of 500e500,000 Hz and at amplitude of 20 mV using the Elec-

trochemical Workstation IM6 (ZahnereElektrik GmbH & Co.,

KG, Germany) connected to the Membrane Conductivity and

Single Cell Test System BT-552 (BekkTech, USA). The

measurement cell used in this system was based on the four-

electrode geometry (inner electrodes ¼ platinum wire; outer

electrodes ¼ platinum gauze), that allows to exclude

membraneeelectrode contact phenomena from the bulk

conductivity. The conductivity of the sample measured in

the longitudinal direction was calculated using the relation

s ¼ d/RA, where d and A are the distance between the inner

electrodes and the cross sectional area of the membrane

respectively, and R is the resistance derived, via nonlinear fit,

from the frequency-interval (500e10,000 Hz) using a simple

parallel RC-model. The measurements were performed under

a nitrogen flow of 500 standard cubic centimeters per minute

(SCCM) and pressure of 230 kPa, at various relative humidities

(from 40 to 100%), while keeping the temperature (100 �C)constant. For each relative humidity step, the system was

equilibrated for a period of 1 h at the end of which 20 spectra

were acquired. The Thales 3.16 software (ZahnereElektrik)

was used for impedance data collection and analysis. Typi-

cally, a strip of membrane 30e50 mm thick and 12.5 mm wide

was treated with 3 N H2SO4 for 24 h and then rinsed with de-

ionized water to remove excess acid and stored in water at

room temperature for 4 h before measurement. Just before

measurement, the sample was dried on filter paper.

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 6 ( 2 0 1 1 ) 8 5 2 5e8 5 3 58528

3. Results and discussion

3.1. Characterization of SPEEK

The degree of sulfonation was determined using 1H-NMR. The

introduction of SO3H functional group resulted in a distinct

signal for HE proton at d ¼ 7.55 ppm as shown in Fig. 1. Due to

the presence of sulfonic acid group, HE proton which is ortho

to the sulfonic acid group showed a downfield shift

(w0.30 ppm) as compared to HC and HD in the hydroquinone

ring. The proton of SO3H group is labile hence; signal

recording is difficult for this particular proton. The intensity of

HE content is equivalent to SO3H group content, therefore

degree of sulfonation can be calculated by taking the ratios

between the peak area of HE to the peak areas for the rest of

the aromatic hydrogen i.e. HA,A0,B,B0C,D [27,29].

Degree of sulfonation was calculated from 1H-NMR using

the following formula

n12� 2n

¼ HEPHA;A0 ;B;B0 ;C;D

Where ‘n’ is the number of HE per repeat unit

DSð%Þ ¼ n� 100 (2)

DS was calculated w65%. From the knowledge of DS, ion

exchange capacity (IEC) can be calculated using following

equation.

DSð%Þ ¼ 288ðIECÞ1000� 80ðIECÞ � 100 (3)

This equation relates the ion exchange capacity with degree

of sulfonation for SPEEK in proton form [29]. It should be noted

that the unit molecular weight of SPEEK (in proton form) and

PEEK structural unit is 368 and 288, respectively. The number

(80) generates from the difference inmolecular weights of two

structural units (i.e. SPEEK and PEEK).

8.0 7.8 7.6 7.4 7.2 7.0

HE

HA'

HA

HC

HD

HB, HB'

ppm

Fig. 1 e 1H-NMR spectrum of SPEEK depicting various kinds

of protons.

3.2. Proposed SPEEK and diol interaction mechanismafter scheduled heat treatment

The color and solubility of XSPEEK/PEG and XSPEEK/CDM

membranes after scheduled heat treatment are given in

Table 2. The membranes prepared with both kinds of diol

significantly change their color and solubility after scheduled

heat treatment. Themembranes were blackish brown [PEG] or

orange colored [CDM] andwere insoluble in hot water (at 80 �Cfor 24 h) or in a mixture of ethanol:water [50:50] except

XSPEEK/CDM 75/25 which was soluble in ethanol:water (50:50)

mixture after heat treatment and swelled significantly in

water but did not disintegrate. Mikhailenko et al. [14] has also

reported that after heat treatment of SPEEK in the presence of

ethylene glycol or glycerol, color and solubility of membranes

changes i.e. membranes became insoluble in hot water and

color was deeper than initial color. Mikhailenko et al. [15]

proposed the cross-linking mechanism which suggested that

EG molecules initially attached to sulfonic acid group will

preferably react with another EG molecules or its derivative,

forming EG dimers, trimers or poly functional moieties. The

best combination of SPEEK properties was achieved at an

optimum diol concentration because at lower ratio insuffi-

cient cross-linking occurs and at higher concentration

membranes were brittle. In the present work, care was taken

by varying the diol concentration so that optimum cross-

linking can be achieved. Due to heat treatment, various

reactions among sulfonic acid group of SPEEK and hydroxyl

group of diol includes (i) condensation of eOH of diols and

eSO3H of SPEEK to give sulfonic ester group, imparting insol-

ubility and (ii) self condensation of diol which may result in

free eOH groups imparting hydrophilicity to the membrane

(Scheme-2). Care was taken that all the eSO3H groups were

not used up by limiting diol concentration from 40 to 25% so

that the reaction with diols and subsequent heat treatment

did not affect the proton conductivity.

3.3. FTIR

The FTIR spectra of diols, SPEEK/CDM and SPEEK/PEG

membranesbeforeandafterheat treatmentareshown inFig. 2a

and b. In Fig. 2a (i), neat CDM shows a characteristic hydrogen

bonded broad band at 3330 cm�1 which arise due to intermo-

lecular hydrogen bonding of eOH groups in CDM. In Fig. 2a (ii),

SPEEK/CDM membrane before heat treatment shows a broad

band at 3380 cm�1 (SPEEK shows a characteristic eOH band at

Table 2 e Color and solubility of membranes after heattreatment.

Sampledesignation

Color Solubility in water:ethanol [50:50] mixture

at 80 �C for 24 h

XSPEEK/PEG 75/25 Blackish brown Insoluble

XSPEEK/PEG 67/33 Blackish brown Insoluble

XSPEEK/PEG 60/40 Blackish brown Insoluble

XSPEEK/CDM 75/25 Orange brown Insoluble

XSPEEK/CDM 67/33 Orange brown Insoluble

On

CDM

OO C

O

OO

SO2

C

O

m n

OO C

O

OO

SO2

C

O

m n

OO C

O

OO

SO2

C

O

m n

OO C

O

OO

SO2

C

O

m n

OO C

O

OO

SO2

C

O

m n

OO C

O

OO C

SO2

O

m n

o

o

o

o

o

o

refers to

PEG -200

OH

OH

Scheme 2 e Proposed structure of heat-treated 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 3 6 ( 2 0 1 1 ) 8 5 2 5e8 5 3 5 8529

3450 cm�1) which could be due to the overlapping of eOH

vibration of SO3H in SPEEK (3450 cm�1) and eOH vibrations of

CDM (3330 cm�1). In Fig. 2a (iii), SPEEK/CDM membrane after

heat treatment shows eOH overlapping absorption band (eOH

groups from SPEEK and eOH groups from CDM) has shifted

from3380cm�1e3410 cm�1, thus itmight happen that freeeOH

groups of CDMare no longer available and this band is only due

to eOH groups of eSO3H in SPEEK. Characteristic asymmetric

and symmetric stretching vibrations of alkyl (ReCH2) group of

CDM at 2910 cm�1 and 2855 cm�1 show the incorporation of

CDM in to the SPEEK matrix.

InFig. 2b (i), PEG-200,eOHabsorptionband (hydrogenbonded)

appears at 3380 cm�1. Fig. 2b (ii), before heat treatment, SPEEK/

PEG membrane shows eOH absorption band (as a result of

overlap of eOH group of PEG and eOH group of SPEEK) at

3410cm�1 (eOHabsorptionband for SPEEKappears at 3450cm�1)

and in Fig. 2b (iii), after heat treatment, XSPEEK/PEG membrane,

eOHabsorption band appears at 3420 cm�1. From these results it

can be concluded that free eOH groups of PEG-200 are not avail-

able after heat treatment ofmembranewhich could bedue to the

reaction of eOH groups of PEG-200 with eSO3H group of SPEEK.

The characteristic alkyl (ReCH2) asymmetric and symmetric

stretching vibrations of PEG-200 appeared at 2950e2850 cm�1

which shows the incorporationof PEG-200 into the SPEEKmatrix.

3.4. Thermal characterization

3.4.1. Thermo-gravimetric analysisThermal stability of membranes is a very important param-

eter because membrane should have adequate thermal

stability under fuel cell operating conditions. Fig. 3a and

b shows TG and DTG traces of SPEEK, XSPEEK/PEG and

XSPEEK/CDMmembranes. All themembrane samples showed

three step degradation and the relative thermal stability of the

membranes was evaluated by comparing the mass loss in the

different temperature ranges i.e. below 200 �C (due to loss of

physically and chemically bound water), between 200 and

450 �C (due to the decomposition of sulfonic acid groups),

between 450 and 800 �C (due to the main chain degradation of

polymers) and char yield at 800 �C. All the membranes show

minor mass loss below 200 �C i.e. in the range of 1.7e3.3%. It

was observed that mass loss below 200 �C increased with

increasing amount of diol which could be due to the enhanced

water absorption tendency of diols leading to increased

hydrophilicity of the membranes. At the same concentration

of the diols, the XSPEEK/PEG membranes show higher mass

loss as compared to XSPEEK/CDMmembranes which could be

due to the higher hydrophilicity of PEG as compared to CDM.

Mass loss in the temperature range of 200e450 �Cdecreases with increasing amount of diols [Table 2]. Mass loss

in this temperature range is mainly due to SO3H, and lower

mass loss observed for XSPEEK/PEG could be due to the

formation of cross-links (as proposed in Scheme 2) which in

turn can restrict the mobility as well as evolution of volatiles.

Mass loss associated with the main chain degradation (in the

temperature range of 450e800 �C) increased marginally with

increasing amount of diol. Char yield at 800 �C was higher for

XSPEEK/PEG membranes as compared to XSPEEK/CDM

membranes. The temperature for onset of major mass loss

was higher than 240 �C in all the membranes except XSPEEK/

Fig. 2 e FTIR spectra of (a) (i) CDM (ii) SPEEK/CDM 75/25

(before heat treatment) (iii) XSPEEK/CDM 75/25 (after heat

treatment) (b) (i) PEG (ii) SPEEK/PEG 75/25 (before heat

treatment) (iii) XSPEEK/PEG 75/25 (after heat treatment).

Temperature (o

C)

0 200 400 600 800

We

ig

ht %

40

60

80

100

SPEEK

XSPEEK/PEG 75/25

XSPEEK/PEG 67/33

XSPEEK/PEG 60/40

XSPEEK/CDM 75/25

XSPEEK/CDM 67/33

a

Temperature (o

C)

200 400 600 800

dT

G/d

T

SPEEK

XSPEEK/PEG 75/25

XSPEEK/PEG 67/33

XSPEEK/PEG 60/40

XSPEEK/CDM 75/25

XSPEEK/CDM 67/33

b

Fig. 3 e TG/DTG traces of SPEEK, XSPEEK/PEG and XSPEEK/

CDM membranes.

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 6 ( 2 0 1 1 ) 8 5 2 5e8 5 3 58530

PEG 75/25 due to inadequate cross-linking. The thermal

stability of XSPEEK/PEG was higher as that of XSPEEK/CDM

because PEGmembranesmay have greater extent of hydrogen

bonding than CDMmembrane. From all these results, it can be

concluded that XSPEEK/PEG 67/33 membrane shows adequate

thermal stability for fuel cell applications (Table 3).

3.4.2. Dynamic mechanical analysisIn DMA experiments, dry membrane samples were used for

measurements in the tension mode [30]. In the DMA experi-

ments, a cyclic stress was applied to a specimen and funda-

mental parameters, i.e. the storage modulus E0, loss modulus

E00 and tan d (delta) values were recorded as a function of

temperature at a constant frequency (1 Hz). Fig. 4a shows the

plot of storage modulus vs. temperature for heat-treated

membranes. Storage modulus is a measure of the stiffness of

samples at a given temperature. The values of storage

modulus for samples with varying amounts of PEG or CDM are

given in Table 4. These results clearly show that the XSPEEK/

PEG membranes have significantly higher values of storage

modulus over the whole temperature range as compared to

the XSPEEK/CDM membranes. This may be due to the

formation of compact structure brought about by hydrogen

bonding in the presence of PEG as compared to CDM. The

values of storage modulus for XSPEEK membranes were

significantly higher as compared to Nafion-112.

The plot of tan d vs. temperature for cross-linked

membranes are shown in Fig. 4b and the glass transition

temperature (Tg) was noted as the peak temperature where

tan d was maximum (Table 4). The results indicated that the

Table 3 e Results of TG/DTG traces of SPEEK, XSPEEK/PEG and XSPEEK/CDM membrane samples in nitrogen atmosphere(heating rate 20 �C/min).

Sample designation Mass loss (%) % Char yield at 800 �C

Below 200 �C 200e450 �C 450e800 �C

SPEEK 2.5 27.5 33.5 36.5

XSPEEK/PEG 75/25 2.5 22.2 34.5 40.8

XSPEEK/PEG 67/33 2.7 19.2 36.8 41.3

XSPEEK/PEG 60/40 3.3 19.1 37.5 41.6

XSPEEK/CDM 75/25 1.7 30.7 37.1 30.5

XSPEEK/CDM 67/33 2.2 29.9 37.1 30.8

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glass transition temperatures of XSPEEK/PEGmembranes was

higher than the XSPEEK/CDM membranes and the highest Tg

was noted for XSPEEK/PEG 67/33 membrane i.e. this

membrane can retain dimensional stability upto 250 �C. Thevalue of Tg depends on macromolecular characteristics

affecting chain stiffness. In case of heat treated SPEEK

membranes, presence of diols diminished chain flexibility and

glass transition temperature increased due to the presence of

ionic groups, polar side groups and alicyclic chain groups,

which tend to stiffen the molecular backbone. All the XSPEEK

membranes have much higher Tg as compared to Nafion-112

(178 �C). In the XSPEEK/PEG samples, Tg increased from

Temperature (o

C)

50 100 150 200 250 300

Sto

rag

e m

od

ulu

s (

MP

a)

0

500

1000

1500

2000

2500

3000

Nafion-112

XSPEEK/PEG 75/25

XSPEEK/PEG 67/33

XSPEEK/PEG 60/40

XSPEEK/CDM 75/25

XSPEEK/CDM 67/33

a

b

Temperature (o

C)

50 100 150 200 250

Ta

n

0.0

0.2

0.4

0.6

Nafion-112

XSPEEK/PEG 75/25

XSPEEK/PEG 67/33

XSPEEK/PEG 60/40

XSPEEK/CDM 75/25

XSPEEK/CDM 67/33

Fig. 4 e DMA scans for SPEEK, Nafion-112, XSPEEK/PEG and

XSPEEK/CDM sample (a) storage modulus vs. temperature

and (b) tan d vs temperature.

218 �C to 258 �C when PEG content increased from 25 to 33%

which may be attributed to enhanced or adequate cross-

linking. Further increase of PEG resulted in decrease of Tg

(238 �C) which may be due to the plasticizing function of

increased amount of diol, but it is still much higher than

Nafion-112. Similar variation of Tg was observed in XSPEEK/

CDM system but values were much lower as compared to the

corresponding PEG based samples. Among the various

compositions, membranes with 33% PEG exhibited the best

mechanical properties as compared to all other compositions.

3.5. Morphological characterization

Fig. 5 shows the cross sectional morphologies of XSPEEK/PEG

67/33 and XSPEEK/CDM 67/33 membranes at two different

magnifications (i.e. 10.0 k and 30.0 k) after scheduled heat

treatment. Formation of inter-connected network can be

clearly seen in the presence of both kinds of diol. However,

network structure is finer in XSPEEK/PEG 67/33 membrane

than the XSPEEK/CDM 67/33 membrane. The differences in

the network pattern could be due to the difference in the

compatibility of diols with SPEEK. On account of ether linkage

PEG is more hydrophilic than CDM and hence more sites of

hydrogen bonding are present in PEG than CDM. So number of

hydrogen bonds per PEG molecule with SPEEK is higher than

that in the CDM and SPEEK system. Norddin et al. reported

similar kind of morphology in the blend membranes of SPEEK

and charged surface modifying macromolecules [31]. He sug-

gested that network morphology increases the open space in

membranes that can facilitate the intake of water molecules.

From these figures, it is evident that free volume is more in

case of XSPEEK/CDM 67/33 membrane than XSPEEK/PEG 67/33

membrane. These results can be explained with the help of

water uptake and proton conductivity which will be discussed

later in next section.

It is also clear from figures that there is no phase separa-

tion between diol and SPEEK [27]. Diol is forming connected

continuous path all over the matrix, suggesting the possibility

of chemical reaction between diol and SPEEK which is also

evident from FTIR spectra where number of eOH groups

reduced after heat treatment. Thus prepared membranes

were dense and homogenous in nature.

3.6. Water uptake and hydrolytic stability

Fig. 6 shows the water uptake as a function of temperature for

all the samples after 24 h of water immersion. It is clearly seen

Table 4 e Results of storage modulus at different temperatures and Tg of membranes from DMA experiment.

Sample designation Storage modulus (MPa) at temperature (�C) Tg (�C)a

30 60 100 120 150

Nafion-112 405 334 248 182 65 178

XSPEEK/PEG 75/25 1754 1936 2149 2188 2073 218

XSPEEK/PEG 67/33 2054 2032 2019 2007 2006 258

XSPEEK/PEG 60/40 1380 1464 1545 1562 1519 238

XSPEEK/CDM 75/25 1193 1110 992 942 707 194

XSPEEK/CDM 67/33 901 848 717 671 573 212

a Represent peak temperature obtained from tan delta plots.

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 6 ( 2 0 1 1 ) 8 5 2 5e8 5 3 58532

that water uptake increased with increase in temperature

because the mobility of polymer chains and water molecules

increase with temperature. Water uptake reduced with

increased diol concentration because large amount of eOH

groups in the diol could bind larger number of eSO3H groups

in the SPEEK so that the eSO3H groups are not free to interact

with water molecules. A significant increase in water uptake

was observed in all the samples as the temperature increased

from 80 to 100 �C. This could be due to enhanced diffusion of

water molecules in the polymer chains due to enhanced free

volume in the polymer at increased temperature.

Heat treated membranes were evaluated for hydrolytic

stability by immersing the films in water at 35 �C for 3months.

The water stability was characterized by the loss of mechan-

ical properties which was tested by bending the membrane

slightly and observing their strength and dimensional

stability. Themembranes did not break and lose their strength

and dimensional stability after 3 months of water immersion.

Fig. 5 e FESEM cross sectional images: (i) and (ii) XS

3.7. Proton conductivity

Water uptake and temperature significantly affect the proton

conductivity of membranes. Kreuer [3] studied the proton

conduction mechanism and suggested that proton transfer

occurs through water-mediated pathways between ionic

clusters containing polar groups. As number of ionic cluster

increases, water content also increases in the membranes,

hence proton conductivity also increases. Increase of

temperature enhances the mobility of polymer chain, water

molecules and protons which in turn increases the proton

conductivity.

Fig. 7 shows the plot of proton conductivity vs. relative

humidity at 100 �C for Nafion-112, SPEEK, XSPEEK/PEG and

XSPEEK/CDM membranes. It is clearly seen that proton

conductivity of XSPEEK/PEG membranes is higher than the

XSPEEK/CDM membranes over the whole humidity range

which could be attributed to the higher flexibility of PEG

PEEK/PEG 67/33 (iii) and (iv) SPEEK/CDM 67/33.

Sample name

a b c d e

Wa

te

r u

pta

ke

(%

)

0

50

100

150

200

30 C

80 C

100 C

Fig. 6 e Effect of temperature and diol content on the water

uptake of XSPEEK membranes: (a) XSPEEK/PEG 75/25 (b)

XSPEEK/PEG 60/40 (c) XSPEEK/PEG 67/33 (d) XSPEEK/CDM

75/25 (e) XSPEEK/CDM 67/33.

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 6 ( 2 0 1 1 ) 8 5 2 5e8 5 3 5 8533

molecules than CDM molecules which in turn gives faster

proton movement in the membranes. In the range from 70 to

100% RH, XSPEEK/PEG membranes have proton conductivity

comparable to SPEEKmembrane but the values are lesser than

Nafion over the whole humidity range i.e. ranging from 20 to

100%. The highest proton conductivity (56.69 mS/cm) was

achieved in XSPEEK/PEG 67/33 which is comparable to neat

SPEEK (55.76 mS/cm). The proton conductivity can be

explained with the help of water uptake and morphological

results. Water uptake is higher for the XSPEEK/CDM 67/33

membrane than the XSPEEK/PEG 67/33 membrane [Fig. 6].

However, proton conductivity is higher for XSPEEK/PEG 67/33

membrane than that of XSPEEK/CDM 67/33membrane (Fig. 7).

This is well known that not only the size of water channels is

responsible for higher proton conductivity but also the

connectivity of water channels is equally important. This

phenomenon is depicted from FESEM images where PEG

Relative humidity (% RH)

20 40 60 80 100

Pro

to

n co

nd

uctiv

ity (m

S/cm

)

0.1

1

10

100

Nafion-112

XSPEEK/PEG-75/25

XSPEEK/PEG 67/33

XSPEEK/PEG 60/40

XSPEEK/CDM 75/25

XSPEEK/CDM 67/33

Neat SPEEK

Fig. 7 e Plot of proton conductivity at 100 �C vs. relative

humidity for membrane samples.

membrane has finer network morphology than CDM

membranes. In these samples, connectivity of water channels

is playing a crucial role and better channel connectivity is

found in case of XSPEEK/PEG 67/33 membrane accounting for

its higher proton conductivity.

The probable reasons for higher proton conductivity in the

presence of PEGediol could be due to higher flexibility, better

connectivity of water channels and increased hydrogen

bonding among SPEEK, PEGediol and water molecules which

in turn facilitate water assisted proton conduction.

4. Discussion

It has been reported that after cross-linking of SPEEK, an

increase in hydrolytic stability was observed with subsequent

decrease in proton conductivity [14e16]. To cross-link SPEEK,

Hande et al. [16] used aromatic diols whereas Mikhailenko

et al. employed aliphatic diols [14,15]. In the present paper,

two diols, differing in their molecular weight as well as

structure are chosen to evaluate their effect on the perfor-

mance properties of SPEEK. PEG-200 is linear aliphatic diol

with ether linkages whereas CDM is alicyclic diol. At all

concentrations PEG proves to be the better cross-linker than

CDM which could be due to easier mobility of PEG molecules

(because of the ether linkage) than CDMwhich is having cyclic

structure suppose to be little sluggish than ether linkage. Thus

the PEG molecules could easily approach sulfonic acid group

of SPEEK. PEG is more flexible and hydrophilic than CDM so

this structural difference reflects clearly in producing finer

network (morphological), enhanced storage modulus and

glass transition temperature (mechanical) and proton

conductivity results. In literature, reduction of proton

conductivity was reported upon cross-linking [15e17]

however, in the present study the proton conductivity does

not change upon cross-linking.

At 80 �C, large difference has been observed betweenwater

uptake values of PEG membranes (w50%) and membranes

reported in literature [14,15] i.e. 2100% for EG and 2900% for

glycerol. We have also observed similar results for

membranes cross-linked with EG and glycerol.

This analysis showed that PEG is very effective cross-linker

in the sense that it maintains hydrolytic stability even at

higher temperatures with no compromise in proton

conductivity.

In literature [32], the use of PEG is mentioned but it did not

play a role of cross-linker because it was used in DMAc, DMF

or NMP solvent system and it has been already proved that no

cross-linking occur between hydroxyl groups of diol and

sulfonic acid groups of the SPEEK in the presence of above

mentioned solvents. PEG of different molecular weight

(200,400, 600, 2000 Da) as spacer between two silica particles to

control the nanostructure of composite membrane. In their

work, the structure of nanocomposite membrane is consid-

ered to be an interpenetrating network of nanosized silica

skeleton and polymer base, in which each silica domain

seems to have a distance of a few nanometers by bound

interior polymer domain. But these nanocomposite polymer

electrolyte membranes offer no significant advantage over

Nafion membrane, so as proton conductivity is concerned.

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 6 ( 2 0 1 1 ) 8 5 2 5e8 5 3 58534

Thesemembranes were useful for other application like water

electrolysis, ion separation and electrochemical sensors.

The novelty of our system is that we have used PEG and

CDMas cross-linker which is not reported yet and have results

which are different from those reported in literature.

5. Conclusion

Heat treated SPEEK/diol membranes were successfully

prepared using two different kinds of diol (PEG-200 and CDM).

XSPEEK/PEG membranes were hydrolytically stable with

proton conductivity comparable to neat SPEEK membrane.

Change in membrane’s color and solubility after scheduled

heat treatment resulted in membranes which did not dissolve

in water at high temperature or in a mixture of ethanol:water

[50:50]. The glass transition temperature (Tg) of XSPEEK/diol

membranes increased significantly and was dependent on the

nature and concentration of diol. Maximum Tg (258 �C) was

observed for XSPEEK/PEG 67/33 membrane. Storage modulus

was in the range of 2000 MPa for XSPEEK/PEG membranes.

Except for XSPEEK/PEG 75/25, all othermembranes showmajor

two step degradation andXSPEEK/PEG 67/33 exhibits very good

thermal stability. SEM images show the formation of network

like structure for bothkindsofdiol containingmembranes. The

finernetworknetworkwasobserved in case of PEGmembranes

which accounts for better connectivity of channels hence

giving higher proton conductivity PEG containingmembranes.

This study reveals that XSPEEK/PEGmembrane can be used as

high performance membranes for fuel cell applications.

Acknowledgments

The authors wish to thank Naval Research Board (NRB)

Ministry of Defence, India, IIT Delhi and DAAD for financial

support and Dr. Rostislav Vinokur at DWI, RWTH Aachen,

Germany for their kind support for proton conductivity

measurement. Special thanks to Prof. Martin Moller and Dr.

Xiaomin Zhu at DWI, RWTH Aachen, Germany for giving the

permission to work at DWI, RWTH Aachen, Germany.

r e f e r e n c e s

[1] Steele BCH, Heinzel A. Materials for fuel-cell technologies.Nature 2001;414:345e52.

[2] Ahmad H, Kamarudin SK, Hasran UA, Daud WRW. Overviewof hybrid membranes for direct-methanol fuel-cellapplications. Int J Hydrogen Energy 2010;35:2160e75.

[3] Kreuer KD. On the development of proton conductingpolymer membranes for hydrogen and methanol fuel cells.J Membr Sci 2001;185:29e39.

[4] Hickner MA, Ghassemi H, Kim YS, Einsla BR, McGrath JE.Alternative polymer systems for proton exchangemembranes (PEMs). Chem Rev 2004;104:4587e612.

[5] Rikukawa M, Sanui K. Proton-conducting polymer electrolytemembranes based on hydrocarbon polymers. Prog Polym Sci2000;25:1463e502.

[6] Xing P, Robertson GP, Guiver MD, Mikhailenko SD, Wang K,Kaliaguine S. Synthesis and characterization of sulfonatedpoly(ether ether ketone) for proton exchange membranes.J Membr Sci 2004;229:95e106.

[7] Pasupathi S, Ji S, Bladergroen BJ, Linkov V. High DMFCperformance output using modified acidebase polymerblend. Int J Hydrogen Energy 2008;33:3132e6.

[8] Dou Z, Zhong S, Zhao C, Li X, Fu T, Na H. Synthesis andcharacterization of a series of SPEEK/TiO2 hybridmembranes for direct methanol fuel cell. J App Pol Sci 2008;109:1057e62.

[9] Baias M, Demco DE, Colicchio I, Blumich B, Moller M. Protonexchange in hybrid sulfonated poly(ether ether ketone)esilicamembranes by 1H solid-state NMR. Chem Phys Lett 2008;456:227e30.

[10] Cho E, Park JS, Park SH, Choi YW, Yang TH, Yoon YG, et al. Astudy on the preferable preparation method of SPEEK/BPO4composite membranes via an in situ solegel process.J Membr Sci 2008;318:355e62.

[11] Colicchio I, Wen F, Keula H, Simon U, Moeller M. Sulfonatedpoly(ether ether ketone)esilica membranes doped withphosphotungstic acid. Morphology and proton conductivity.J Membr Sci 2009;326:45e57.

[12] Pezzina SH, Stock N, Shishatskiy S, Nunes SP. Modification ofproton conductive polymer membranes with phosphonatedpolysilsesquioxanes. J Membr Sci 2008;325:559e69.

[13] YangT. Preliminary study of SPEEK/PVAblendmembranes forDMFC applications. Int J Hydrogen Energy 2008;33:6772e9.

[14] Mikhailenko SD, Wang K, Kaliaguine S, Xing P, Robertson GP,Guiver MD. Proton conducting membranes based on cross-linked sulfonated poly(ether ether ketone) (SPEEK). J MembrSci 2004;233:93e9.

[15] Mikhailenko SD, Robertson GP, Guiver MD, Kaliaguine S.Properties of PEMs based on cross-linked sulfonatedpoly(ether ether ketone). J Membr Sci 2006;285:306e16.

[16] Hande VR, Rao S, Rath SK, Thakur A, Patri M. Crosslinkingof sulphonated poly(ether ether ketone) using aromaticbis(hydroxymethyl) compound. J Membr Sci 2008;322:67e73.

[17] Zhong S, Liu C, Na H. Preparation and properties of UVirradiation-induced crosslinked sulfonated poly(ether etherketone) proton exchange membranes. J Membr Sci 2009;326:400e7.

[18] Zhang Y, Fei X, Zhang G, Li H, Shao K, Zhu J, et al. Preparationand properties of epoxy-based cross-linked sulfonatedpoly(arylene ether ketone)proton exchange membrane fordirect methanol fuel cell applications. Int J Hydrogen Energy2010;35:6409e17.

[19] Luo H, Vaivars G, Mathe M. Cross-linked PEEK-WC protonexchange membrane for fuel cell. Int J Hydrogen Energy 2009;34:8616e21.

[20] Dogan H, Inan TY, Unveren E, Kaya M. Effect of cesium salt oftungstophosphoric acid (Cs-TPA) on the properties ofsulfonated polyether ether ketone (SPEEK) compositemembranes for fuel cell applications. Int J Hydrogen Energy2010;35:7784e95.

[21] Yang T, Liu C. SPEEK/sulfonated cyclodextrin blendmembranes for direct methanol fuel cell, doi:10.1016/j.ijhydene.2011.01.164.

[22] Kerres JA. Blended and cross-linked ionomer membranes forapplication in membrane fuel cells. Fuel Cells 2005;5(2):230e47.

[23] Zhao C, Lin H, Na H. Novel crosslinked sulfonated poly(etherether ketone) membranes for direct methanol fuel cell. Int JHydrogen Energy 2010;35:2176e82.

[24] Metzmann FH, Osan F, Schneller A, Ritter H, Ledjeff K,Nolte R, et al. Polymer electrolyte membrane, and process forthe production thereof, US Patent 5,438,082; 1995.

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 6 ( 2 0 1 1 ) 8 5 2 5e8 5 3 5 8535

[25] Mao SS, Hamrock SJ, Ylitalo DA. Crosslinked ion conductivemembranes, US Patent 6,090,895; 2000.

[26] Bishop MT, Karasz FE, Russo PS, Langley KH. Solubility andproperties of a poly(aryl ether ketone) in strong acids.Macromolecules 1985;18(1):86e93.

[27] Zaidi SMJ, Mikhailenko SD, Robertson GP, Guiver MD,Kaliaguine S. Proton conducting composite membranes frompolyether ether ketone and heteropolyacids for fuel cellapplications. J Membr Sci 2000;173:17e34.

[28] Muthu Laxmi RTS, Choudhary V, Varma IK. Sulfonated poly(ether ether ketone): synthesis and characterization. J MaterSci 2005;40:629e36.

[29] Huang RYM, Shao P, Burns CM, Feng X. Sulfonation of poly(ether ether ketone) (PEEK): kinetic study andcharacterization. J App Pol Sci 2001;82:2651e60.

[30] Sgreccia E, Di Vona ML, Licoccia S, Sganappa M, Casciola M,Chailan JF, et al. Self-assembled nanocomposite organiceinorganicprotonconductingsulfonatedpoly-ether-ether-ketone(SPEEK)-basedmembranes: optimizedmechanical, thermal andelectrical properties. J Power Sources 2009;192:353e9.

[31] Norddin Mohd MNA, Ismail AF, Rana D, Matsuura T,Mustafa A, Tabe-Mohammadi A. Characterization andperformance of proton exchange membranes for directmethanol fuel cell: blending of sulfonated poly(Ether etherketone) with charged surface modifying macromolecule.J Membr Sci 2008;323:404e13.

[32] Saxena A, Tripathi BP, Shahi VK. Sulfonated poly (styrene-co-maleic anhydride)-poly (ethylene glycol)-silicananocomposite polyelectrolyte membranes for fuel cellapplications. J Phys Chem B 2007;111:12454e61.