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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
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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: [email protected] (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
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 8527
(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.
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