potential membranes derived from poly (aryl hexafluoro ......high-temperature pem fuel cells p....
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Potential membranes derived from poly(aryl hexafluoro sulfone benzimidazole) and poly(aryl hexafluoro ethoxy benzimidazole) forhigh-temperature PEM fuel cells
P. Muthuraja a,1, S. Prakash a,1, A. Susaimanickam b, P. Manisankar a,*
a Department of Industrial Chemistry, Alagappa University, Karaikudi 630006, Indiab Department of Chemistry, Arumugam Pillai Seethai Ammal College, Tirupattur 630211, India
a r t i c l e i n f o
Article history:
Received 13 December 2017
Received in revised form
6 March 2018
Accepted 10 March 2018
Available online xxx
Keywords:
Poly (aryl hexafluoro sulfone
benzimidazole)
Poly (aryl hexafluoro ethoxy
benzimidazole)
Fuel cells
Proton conductivity
Stability
* Corresponding author.E-mail address: manisankarp@alagappau
1 These authors contributed equally to thihttps://doi.org/10.1016/j.ijhydene.2018.03.0580360-3199/© 2018 Hydrogen Energy Publicati
Please cite this article in press as: Muthurajand poly (aryl hexafluoro ethoxy benzimidahttps://doi.org/10.1016/j.ijhydene.2018.03.05
a b s t r a c t
Poly (aryl hexafluoro sulfone benzimidazole) and poly (aryl hexafluoro ethoxy benzimid-
azole), termed as PArF6SO2BI and PArF6OBI, are synthesized and characterized systemati-
cally. PArF6SO2BI membranes illustrate good chemical stability in terms of oxidative weight
loss due to the electron-withdrawing sulfone functional group. PArF6OPBI membranes
exhibit weak chemical stability after immersion in Fenton's solution. Many of the mem-
branes show good conductivities. Higher conductivities of 3.26 � 10�2 S cm�1 at 160 �C with
286.8 wt% acid doped level for 3:1 (2.335 mmol of 4,40-sulfonyldibenzoic acid and
7.005 mmol of 2, 2-bis(4-carboxyphenyl) hexafluoropropane) ratio of PArF6SO2BI and
7.31 � 10�2 S cm�1 with 356.9 wt% for 3:1 ratio of PArF6OBI are observed. PArF6SO2BI and
PArF6OPBI membranes exhibit good conductivity, thermal and mechanical stabilities
which are crucial requirements for high temperature fuel cells.
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction
Need for novel high temperature polymer electrolyte mem-
brane fuel cell (HTPEMFC) assumes significance today to meet
the challenges for renewable energy and climate change
problems [1]. Even though Nafion membranes, perfluorinated
sulfonic acid membranes and composite membranes are
used, the Department of Energy, US targeted only acid doped
polybenzimidazole (PBI) membranes for HTPEMFC. But the
niversity.ac.in (P. Manisas work.
ons LLC. Published by Els
a P, et al., Potential membzole) for high-temperatur8
main drawback of PBI membranes is its increased degrada-
tion. Since the membrane is the main component, it neces-
sitates synthesizing novel PBI based membranes [2e6].
In the last decade, various methods have been tried to
improve the conductivity, stability and solubility of PBI. Phos-
phoric acid doped PBI (PA-PBI) was studied extensively for
HTPEMFC due to their proton conductivity mechanism and
stability [2,7] But leaching of phosphoric acid poses main
problem and it leads to corrosion also. Tang group developed
nkar).
evier Ltd. All rights reserved.
ranes derived from poly (aryl hexafluoro sulfone benzimidazole)e PEM fuel cells, International Journal of Hydrogen Energy (2018),
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 8 ) 1e1 02
the 3D gel type framework as matrix and the resultant proton
conductivity at high temperature was decreased in compari-
son with PA doped PBI membrane [8e11]. Cross-linked porous
PBI membranes were suggested to improve the mechanical
strength and oxidative stability [11e13]. Flexible spacer groups
such as ether linkages [14], fluorines [15,16], azoles [17], pyri-
dine [18], asymmetric bulky pendants (phenyl and methyl-
phenyl) [19], 4,40,5,50-tetraaminodiphenyldiphenyl ether [20]
and4,40-bibromomethenyldiphenyl ether [21]were introduced
in the PBI backbones, to make novel PBI based polymers with
improved physicochemical properties. Yang et al. developed
the sulfone linked PBI and its copolymer with good solubility
[22]. However, these approaches have some problem with the
tedious synthetic procedures, poor solubility, cross-linking
during polymerization and poor mechanical integrity.
Recently, several PBI composite membranes have been devel-
oped, i.e. PBI with SPEEK [23], 1-(3-trimethoxysilylpropyl)-3-
methylimidazolium chloride [24], dispersion of graphene
oxide [25,26], and inorganic fillers such as SiO2 [27], ZrP [28],
Fe2TiO5 nanoparticles [29], mesoporous silica [30,31], and
cerium sulfophenyl phosphate [32] composite membranes
have been demonstrated to for improved conductivity, and
stabilities properties. Moreover, perovskite-type SrCeO3 [33],
CaTiO3 [34] and BaZrO3 [35] based PBI composite membranes
exploited for enhanced electrochemical properties of high
temperature proton exchange membranes. While this earlier
works have indeed yielded improvements in conductivity and
acid doping levels of membrane, some persistent challenges
remain as lowmembrane dimensional-mechanical stability.
Theoretically, fluorinated polymers are likely to have
higher stability and increased acid strength compared with
their non-fluorinated polymers [36]. Hence in the search of a
new membrane with higher physicochemical characteristics,
we focus in this work, the synthesis and characterization of
new partially-fluorinated polymers with sulfone or ether
linkage in the PBI backbone. Thiswork reports the synthesis of
two poly (aryl hexafluoro sulfone benzimidazole) (PArF6SO2BI)
and poly (aryl hexafluoro ethoxy benzimidazole) (PArF6OBI) by
polycondensation reaction between bis(4-carboxyphenyl)
hexafluoro propane, 3,30-diamino benzidine, sulfonyl diben-
zoic acid or oxybisbenzoic acid and characterizations.
Experimental
Instrumentation
The 13C and 15N NMR spectra were taken at 300 and 75 MHz
respectively using Bruker NMR spectrometer. The membrane
morphology was examined by a high resolution scanning
electron microscopy (FESEM) (FEI Quanta 250 Microscope,
Netherland). Fourier transform infrared (FTIR) spectra were
recorded using KBr pellet using Nicolet 5700 spectrophotom-
eter (Thermo Electron Co., USA). Dynamic mechanical anal-
ysis (DMA) was used to determine glass transition
temperature for the prepared membrane using Thermal
Analysis 2980DMA in a temperature range 100e400 �C. Tensiletests of the membranes were carried out on a CMT-8500
electro-mechanical universal tester (SANS), and the samples
were directly mounted to the sample clamps and stretched at
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a speed of 10 mmmin�1. TGA experiments were carried out in
the TA instruments Inc., onmodel SDT Q600 by heating under
nitrogen condition using at 10 �C min�1. The inherent vis-
cosity of the obtained polymers was measured using an
Ubbelohde viscometer with a concentration of 5 g L�1 in 96 wt
% sulfuric acid at 30 �C. Poly[2,20-(m-phenylene)-5,50-benz-imidazole] (mPBI) was synthesized in the laboratory using the
method described earlier [37,38] and the dried polymer has an
inherent viscosity value of 1.10 dL g�1.
Synthesis of polymers and membrane fabrications
PArF6SO2BI and PArF6OBI were prepared by condensation
polymerization of 3,30-diaminobenzidine (DAB) (97% Tokyo
Kasei, TCI), 2,2-bis(4-carboxyphenyl) hexafluoropropane (95%
Alfa Aesar), and 4,40-sulfonyldibenzoic acid (97%Alfa Aesar) or
oxybisbenzoic acid (98% Alfa Aesar) in a presence of poly-
phosphoric acid (PPA, Aldrich). The synthetic reaction is
shown in Scheme 1.
A 500 mL, three-necked, round-bottom flask was set with
an overhead mechanical stirrer. A thermal couple was con-
nected with thermosensor and two glass tubes for the nitro-
gen inlet and outlet were inserted. A 9.3mmol of DABwas first
dissolved in 60 g PPA at 120 �C. Then, three quantities of BCHFP
(4.67 mmol, 2.335 mmol, 7.005 mmol) and SDBA (4.67 mmol,
7.005 mmol, 2.335 mmol) or OBBA (4.67 mmol, 7.005 mmol,
2.335 mmol) were added independently. The mixture was
stirred at 220 �C in the nitrogen atmosphere for 24 h to follow
polymerization. After the reaction, the product was poured
into water and the polymer was separated and washed with
dilute sodium bicarbonate solution to neutralize excess acid
in the polymer. The polymer was washed thoroughly with
water and methanol, successively, followed by drying under
vacuum to obtain the polymer powder. The fabrication of
membranes was done by solution casting method by pouring
2 wt% polymer solutions in DMSO onto glass plate andmaking
film using a Gardner film applicator. The film was dried at
80 �C for 24 h. The resultant membranes were then peeled off
and soaked in distilled water at 80 �C for 2 h and further dried
at 200 �C for 1 h. The thickness of the membranes was
measured and maintained in the range 80e100 mm.
Physico-electrochemical characterizations
Synthesizedmembraneswere immersed in 14.0M phosphoric
acid (PA) at 90 �C for 12 h. After that, the membranes were
cleaned with a tissue paper to eliminate the extra acid on the
membrane surface and dried at 110 �C for 4 h. The PA doping
of the membrane (%) was determined by weight gain from the
doping and calculated according to the following Eq. (1):
PA doping levelð%Þ ¼�WPA �W
W
��100 (1)
whereWPA andW are the weight of driedmembrane after and
before doping, respectively.
Water uptake of the membranes was studied to ascertain
the stability of the membranes at RT with water. Membrane
samples were immersed in water for 24 h, subsequent to the
water absorption, the residual water from the samples surface
was removed by the absorption using paper [39]. The water
ranes derived from poly (aryl hexafluoro sulfone benzimidazole)e PEM fuel cells, International Journal of Hydrogen Energy (2018),
Scheme 1 e A synthetic method for PArF6SO2BI and PArF6OBI.
i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 8 ) 1e1 0 3
uptake of the membranes were then recorded and calculated
using the following Eq. (2):
Water uptakeð%Þ ¼�Ww �Wd
Wd
��100 (2)
where Ww and Wd are the weights of the membrane at the
swelling and dehydrated state, respectively.
The oxidative ability of the membrane was examined by
Fenton's test. The membrane was immersed in a 3.0 wt%
hydrogen peroxide aqueous solution containing 4 ppm Fe2þ at
80 �C. Every 20e24 h, the membranes were taken out, washed
completely with distilled water and dried at 110 �C for 10 h.
Then, the membrane samples were transferred to fresh Fen-
ton solutions again for continued testing.
The proton conductivity measurement was carried out
using a CHI760 electrochemical workstation (CH Instruments,
USA). The measurement of conductivity completely followed
that has been described in previous study [40,41]. The mem-
brane was kept between two in-house made stainless steel
circular electrodes (1.0 cm2) and connected with Pt wire as
current collector. Direct current (dc) and sinusoidal alter-
nating currents were applied to the above electrodes for
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monitoring the frequency at 1 mA s�1 scan rate within 105 to
1 Hz. The membrane resistance was measured using Fit and
Simulation method from Nyquist plots. The measurements
were taken at different temperatures from 90 to 160 �C. Theconductivity was calculated using the following equation.
k ¼ LA
� 1R
(3)
Where k is a proton conductivity of themembrane (S cm�1)
and R, L, and A are the measured resistance (U), thickness
(cm), and cross-sectional area of the membrane (cm2),
respectively.
Conductivity follows Arrhenius equation for hopping-like
conduction mechanism which depends on temperature:
In k ¼ In ko � Ea
RT(4)
where k is the proton conductivity of the membrane (S cm�1),
ko is the pre-exponential factor (S K�1 cm�1), Ea is the proton
conducting activation energy (kJ mol�1), R is the ideal gas
constant (J mol�1 K�1) and T is the temperature (Kelvin). The
minimum energy (Ea) required for proton conduction is ob-
tained from the slope of linear fit of Eq. (4).
ranes derived from poly (aryl hexafluoro sulfone benzimidazole)e PEM fuel cells, International Journal of Hydrogen Energy (2018),
Fig. 1 e FTIR spectra of PArF6SO2BI and PArF6OBI
membranes.
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 8 ) 1e1 04
Results and discussion
Solubility study
For the monomer concentration of 2,2-bis(4-carboxyphenyl)
hexafluoropropane, and 4,40-sulfonyldibenzoic acid or oxy-
bisbenzoic acid was varied from 2.335 mmol to 7.005 mmol in
PPA. Three different polymers from the monomers of 4,40-sulfonyldibenzoic acid and 2,2-bis(4-carboxyphenyl) hexa-
fluoropropane in the molar ratio of 1:1, 1:3 and 3:1 were syn-
thesized (Scheme 1) and designated as 1:1 PArF6SO2BI, 1:3
PArF6SO2BI and 3:1 PArF6SO2BI. In a similar manner, three
more polymerswere synthesized fromoxybisbenzoic acid and
2,2-bis(4-carboxyphenyl) hexafluoropropane in the ratio 1:1,
1:3 and 3:1 and designated as 1:1 PArF6OBI, 1:3 PArF6OBI and
3:1 PArF6OBI respectively. The inherent viscosities were in
between 1.45 and 1.75 dL g�1. The solubility of PArF6SO2BI and
PArF6OBI was tested with solvents such as DMAc, DMSO, and
NMP at 130 �C (Table 1). All the polymers showed good solu-
bility in DMSO and poor solubility in NMP. PArF6SO2BI showed
good solubility in DMAc while PArF6OBI were moderately
soluble [40]. The presence of sulfone linkage in the molecular
chain resulted in increased solubility in DMAc and DMSO.
Spectral investigations
FTIR spectra of the membranes were presented in Fig. 1. FTIR
spectrum of PArF6SO2BI shows a broad peak at 3362 cm�1
which is due to formation imidazole rings (N-H stretching
vibration). The peaks at 1596 cm�1 and 1462 cm�1 correspond
to the formation of C]N and C]C stretch, revealed the
presence of conjugation between benzene and the imidazole
ring. The strong peak observed at 2922 cm�1 is attributed to
the C-H stretching vibration. The characteristic band at
1249 cm�1 is assigned to the O-S-O stretching vibration. The
peak observed at 1172 cm�1 is due to the C-F asymmetric
stretching vibration. The peaks at 937 cm�1, 803 cm�1 and
731 cm�1 are due to C-C bending vibration, C-F bending vi-
bration and C- S stretching vibration, respectively. Further,
FTIR spectrum of PArF6OBI shows a broad peak at 3349 cm�1
due to N-H stretching vibration. The band at 1630 cm�1 and
1485 cm�1 correspond to the formed C]N and C]C stretching
confirming the presence of conjugation between benzene and
Table 1 e Acid doping percent, water uptake, conductivity andmembranes doped in 14.0 M H3PO4 at 90 �C.
Membranes Water uptake (wt%) Acid doping (wt%) k at 160 �
1:1 PArF6SO2BI 13.3 230.9
1:3 PArF6SO2BI 9.5 171.4
3:1 PArF6SO2BI 11.3 286.8
1:1 PArF6OBI 17.9 111.0
1:3 PArF6OBI 22.1 312.0
3:1 PArF6OBI 12.2 356.9
þþ Good solubility, þ Moderate solubility, and - Insolubility.a PArF6SO2BI and PArF6OBI without doping of acid.
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the imidazole ring. The peaks at 2958, 1230, 1152, 979, 876 and
740 cm�1 are assigned to C-H stretching, C-O-C stretching, C-F
asymmetric stretching, C-C bending, C-F bending and C- S
stretching vibrations respectively.
The solid state 15N and 13C NMR spectra are presented in
Fig. 2A and B for PArF6SO2BI and PArF6OBI (Table S1,
supplementary information). 15N NMR spectrum shows a
single strong signal at 195 ppm which is mainly attributed to
the nitrogen atom of imidazole ring (Fig. 2B). The signal at
146 ppm (denoted as c) in the 13C NMR spectrum represents
the attachment of sulfone substituted carbon to phenylene
ring and signal at 148 ppm (denoted as a) corresponds to the
mechanical strength of PArF6SO2BI and PArF6OBI
C (�10�2 S cm�1) aSolubility test at130 �C
Maximum load/MPa
DMAc DMSO NMP
2.70 þþ þþ - 32.0
2.92 þþ þþ - 51.0
3.26 þþ þþ - 18.0
3.19 þ þþ - 22.6
5.67 þ þþ - 12.5
7.31 þ þþ - 8.2
ranes derived from poly (aryl hexafluoro sulfone benzimidazole)e PEM fuel cells, International Journal of Hydrogen Energy (2018),
Fig. 2 e (A) Solid state 13C NMR, and (B) solid state 15N NMR of synthesized polymers (Inset Fig. 2, chemical structure of
polymers and notation of m and n are 1 and 1 equivalent ratio of the reactants).
i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 8 ) 1e1 0 5
imidazole carbon connecting benzene rings in the benzimid-
azole system (Fig. 2A). The signal at 147 ppm represents the
hexafluoro propane substituted carbon attached to phenylene
ring (denoted as b). The signals at 141.9, 132 and 127 ppm are
due to the sulfone substituted carbons (denoted as e, k andm).
Hexafluoro substituted carbons are observed at 135, 128 and
120 ppm (denoted as h, l and o). The signals at 134, 119 and
117 ppm (denoted as i, q and p) are assigned to aromatic car-
bons bound to the nitrogen atoms and other peaks at 136, 133
and 125 ppm (denoted as g, j and n) are due to the other
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aromatic sulfone substituted carbons. This result suggests
that amine and carboxyl groups are involved in the conden-
sation reaction, followed by the polymerization of sulfonyl
benzene and hexafluoro propane takes place to give
PArF6SO2BI.
Fig. 2B shows a single strong signal at 188 ppm which is
mainly attributed to the nitrogen atom of imidazole group.
The signal at 156 and 152 ppm (denoted as a, b) in the 13C NMR
spectrum represents the attachment of oxy substituted car-
bon to phenylene ring and signal at 147 ppm (denoted as c)
ranes derived from poly (aryl hexafluoro sulfone benzimidazole)e PEM fuel cells, International Journal of Hydrogen Energy (2018),
Fig. 4 e DMA curves of PArF6SO2BI and PArF6OBI
membranes at different temperatures.
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 8 ) 1e1 06
corresponds to the imidazole carbon connecting benzene
rings in the benzimidazole system (Fig. 2A). The signal at 145,
120 and 119 represents the hexafluoro propane substituted
carbons attached to phenylene ring (denoted as d, k and l). The
signals at 127 and 113 ppm are due to the oxy substituted
carbons of aromatic ring (denoted as h and o). Other oxy
substituted carbons directly attached to imidazole ring are
observed at 121, 114 and 111 ppm (denoted as j, n and p). The
signals at 142 and 135 ppm (denoted as e and f) are assigned to
aromatic carbons bound to the nitrogen atoms and other
peaks at 123, 116 and 113 ppm (denoted as l, m and o) are due
to the other aromatic carbons in benzimidazole system. From
this study, the amine and carboxyl groups involved in the
condensation reaction, followed by the polymerization of oxy
benzene and hexafluoropropane to give PArF6OBI is
ascertained.
Fig. 3 shows SEM images of the surface section of all
membranes. Typically all membranes showed uniform and
dense surface. From the SEM images, it is observed that all the
membranes have homogeneous surface.
Mechanical, thermal and chemical stabilities
Dynamical mechanical analysis (DMA) is a complementary
and widely used characterization technique for polymers.
Commonly, membranemechanical properties are specified by
elastic modulus, tensile strength, and ductility. Table 1 shows
the mechanical strength of PArF6SO2BI and PArF6OBI mem-
branes measured at room temperature. All the three
Fig. 3 e SEM images of PA-doped different membranes variant: (
(d) 1:1 PArF6OBI, (e) 3:1 PArF6OBI, and (f) 1:3 PArF6OBI.
Please cite this article in press as: Muthuraja P, et al., Potential memband poly (aryl hexafluoro ethoxy benzimidazole) for high-temperaturhttps://doi.org/10.1016/j.ijhydene.2018.03.058
PArF6SO2BI membranes and 1:3 PArF6OBI showed good me-
chanical strength. Increase in the concentration of hexafluo-
ride group in PArF6OBI increases the mechanical strength
substantially. The samples were also tested to a periodic
mechanical strain at constant rate and different tempera-
tures. The storage modulus E0 (elastic response) and loss
modulus E00 (viscous response) are measured as a function of
temperature for all membranes (Fig. 4). The storage modulus
values are 3800 MPa (1:3 PArF6SO2BI at 200 �C), and 3780 MPa
a) 1:1 PArF6SO2BI, (b) 3:1 PArF6SO2BI, and (c) 1:3 PArF6SO2BI,
ranes derived from poly (aryl hexafluoro sulfone benzimidazole)e PEM fuel cells, International Journal of Hydrogen Energy (2018),
100
)%(
a b c d e f g
i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 8 ) 1e1 0 7
(1:1 PArF6SO2BI at 200 �C) and 2 MPa (3:1 PArF6SO2BI at 200 �C)for PArF6SO2BI membranes, whereas ether linkedmembranes
storage modulus are 250 MPa (3:1 PArF6OBI at 200 �C), and247 MPa (1:3 PArF6OBI at 200 �C) and 3000 MPa (1:1 PArF6OBI at
200 �C). The resulted 3:1 PArF6 SO2BI, 1:1 PArF6 SO2BI, and 1:1
PArF6OBI membranes showed substantial mechanical rein-
forcement and retains the elastic nature even at increased
temperatures. Other membranes are not having mechanical
stability i.e., poor elastic nature with raised temperature.
Further, the glass transition temperature (Tg) was found from
the peak of tan (d), for all membranes and they exhibited a
well-defined relaxation peak in between 275 �C and 315 �C,which are is assigned to the glass transition temperature (Tg)
of the polymer chains. Generally, high mobility of the back-
bone, cluster networkwas showed by high value loss of tan (d).
The tan (d) value for 3:1 PArF6 SO2BI and 1:1 PArF6 SO2BI
membranes showed 0.065 and 0.045 at 275 �C, respectively,which are better than that for othermembranes [42]. Thus, the
1:3 PArF6 SO2BI and 1:1 PArF6 SO2BI membranes had more
elastic nature and can withstand higher temperatures
compared to other membranes. From these studies it is
concluded that the 1:3 PArF6 SO2BI, 1:1 PArF6 SO2BI, and 1:1
PArF6OBI membranes can very well be used in fabrication of
HTPEMFC.
TGA curves of different PArF6SO2BI, PArF6OBI membranes
andm-PBI are presented in Fig. 5. The evaporation of absorbed
water was observed at temperature around 100 �C for all
membranes. The percentage loss was found to be 10 wt% and
13 wt% for 3:1 PArF6OBI and 1:3 PArF6OBI membranes
respectively. Other polymer membranes attained 3e6%
weight loss. It suggests that the 3:1 PArF6OBI and 1:3 PArF6OBI
membranes have higher capacity for water uptake compared
to other membranes studied. In the temperature range of
150e250 �C, there is weight loss around 10.1 wt% for 1:3 PArF6SO2BI and 1:1 PArF6OBI membranes which indicates the
beginning of decomposition. Thus the thermal stability of
these membranes is up to 250 �C which is higher than that of
many membranes used in HTPEMFC. But, m-PBI were also
showed good thermal stability than the 1:3 PArF6OBI, 3:1
PArF6OBI, and 1:3 PArF6 SO2BI, and 1:3 PArF6 SO2BI mem-
branes. Overall ~23% weight loss at 250e450 �C can be
observed indicated that the membrane integrity remains un-
broken for 1:1 PArF6 SO2BI and 1:1 PArF6 OBI membranes [43].
0
20
40
60
80
100
120
50 150 250 350 450 550 650 750 850 950
m-PBI
Temperature (oC)
Wei
ght l
oss
(%)
1:1 PArF6OBI
1:3PArF6OBI
ab
cdef
g
Fig. 5 e TGA curves of PArF6SO2BI and PArF6OBI
membranes.
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The oxidative stability of the membranes affects the long-
termoperation of fuel cells and it is of great importance for the
life time and the performance. Well known that the peroxy
radicals can attack themembranes and cause the degradation
of membranes. The tolerance nature against the peroxy
radical induced degradation is called oxidative stability and
tested by Fentons reagent. One of the major drawbacks of the
m-PBI membranes used in fuel cells is its poor oxidative
chemical stability compared to other [44,45]. PArF6SO2BI and
PArF6OBI membranes have better radical resistance because
they have aromatic rings and chemically strong bonding be-
tween carbon, sulfur and oxygen [46]. Fig. 6 shows the test
results of different membranes and m-PBI in Fenton reagent
(3%H2O2 containing 4.0 ppm FeSO4) at 80 �C for comparison. It
is seen that the all PArF6SO2BI membranes displayed signifi-
cant mass retention than the m-PBI and other membranes.
Thismay be attributed to the electron-withdrawing properties
of the sulfone group, since radicals predominantly attract
electron-rich compounds [39]. 1:1 PArF6OBI membrane also
showed good mass retention. However, 3:1 and 1:3 PArF6OBI
membranes have showed poor oxidative stability in compared
to the m-PBI after about 120 h Fenton test. The presence of
sulfone/oxygen and fluorine in the membranes leads to sig-
nificant oxidative stability [36,47].
Proton conductivity and Arrhenius equation
The acid doping level acts a main role in proton conductivity
for PBI membranes, thus all membranes were immersed in
14.0 M PA and kept at different temperatures for 12 h. The
proton conductivity of membranes at different temperature
and doping level are presented in Fig. 7A. For the PArF6SO2BI
and PArF6OBI membranes, the temperature, and PA doping
level influenced the proton conductivity significantly. At 90 �C,the conductivities of membranes are 1.52 � 10�2 S cm�1 (1:1
PArF6SO2BI with 230.9 wt%), 2.14� 10�2 S cm�1 (3:1 PArF6SO2BI
with 286.8 wt%), and 1.91 � 10�2 S cm�1 (1:3 PArF6SO2BI with
171.4 wt%), and 1.78 � 10�2 S cm�1 (1:1 PArF6OBI with 111.0 wt
%), 1.94 � 10�2 S cm�1 (1:3 PArF6OBI with 312.0 wt%), and
0 20 40 60 80 100 120 1400
20
40
60
80noitneterssa
M
Time (hour)
Fig. 6 e Fenton test results of PA-doped different
membranes at in 3 wt% H2O2 solution containing 4 ppm
Fe2þ at 80 �C: (a) 1:1 PArF6 SO2BI, (b) 3:1 PArF6 SO2BI, and (c)
1:3 PArF6 SO2BI, (d) 1:1 PArF6OBI, (e) 3:1 PArF6OBI, and (f) 1:3
PArF6OBI.
ranes derived from poly (aryl hexafluoro sulfone benzimidazole)e PEM fuel cells, International Journal of Hydrogen Energy (2018),
Fig. 7 e The proton conductivities for the PA-doped
different membranes (A) at temperatures variant, and (B)
Arrhenius plot.
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 8 ) 1e1 08
2.19 � 10�2 S cm�1 (3:1 PArF6OBI with 356.9 wt%). It is found
that the 3:1 PArF6OBI membrane exhibits highest conductivity
at 90 �C. The conductivity increases with increasing in the
temperature due to speed up of protonmobility. At 160 �C, theproton conductivities of membranes are 2.70 � 10�2 S cm�1
(1:1 PArF6SO2BI with 286.8 wt%), 3.26 � 10�2 S cm�1 (3:1
PArF6SO2BI with 286.8 wt%), and 2.92 � 10�2 S cm�1 (1:3
PArF6SO2BI with 171.4 wt%), 3.19 � 10�2 S cm�1 (1:1 PArF6OBI
with 111.0 wt%), 5.67� 10�2 S cm�1 (1:3 PArF6OBI with 312.0 wt
%), and 7.31 � 10�2 S cm�1 (3:1 PArF6OBI with 356.9 wt%). This
study reveals that PArF6SO2BI membranes showed lower
proton conductivity compared to the PArF6OBI membranes.
The conductivity values from this work are higher than that of
other PBI membranes reported [48e55]. The high conductivity
of 3:1 PArF6OBI and 1:3 PArF6SO2BI was aroused by the more
fluorine structure in the matrix. In conclusion, prepared
membranes could efficiently enhance the proton conductiv-
ity; displayed excellent combined properties and may become
a candidate PEM for HT-PEMFC applications.
For hopping-like conduction mechanism dependent on
temperature, conductivity follows Arrhenius equation [56].
The Arrhenius plot represented through temperature depen-
dence of proton conductivity of the all membranes in Fig. 7B.
The minimum energy (Ea) required for proton conduction is
Please cite this article in press as: Muthuraja P, et al., Potential memband poly (aryl hexafluoro ethoxy benzimidazole) for high-temperaturhttps://doi.org/10.1016/j.ijhydene.2018.03.058
obtained from the slope of linear fit of Eq. (3). Fig. 7B shows
Arrhenius plots of ionic conductivity for different membranes
and the activation energies calculated in the temperature
range 90e160 �C. The data fit fairly well the Arrhenius equa-
tion which implies that the proton transport is mainly regu-
lated by Grotthus mechanism occurring as a result of proton
hopping between protonated part of polymer chains and non-
protonated part of the modifier chains or vice versa. The
activation energies, calculated from the slope of the curves,
are 10.8 kJ mol�1 (1:1 PArF6SO2BI), 7.40 kJ mol�1 (3:1 PArF6-SO2BI), 7.69 kJ mol�1 (1:3 PArF6SO2BI), and10.97 kJ mol�1 (1:1
PArF6OBI), 20.28 kJ mol�1 (1:3 PArF6OBI), 22.8 kJ mol�1 (3:1
PArF6OBI). It indicates that the activation energy of conduc-
tion is dependent on the membrane properties, such as hy-
drophilicity, which was slightly higher than the previous
reports [57e59].
Conclusions
We presented the synthesis of PArF6SO2BI and PArF6OBI
membranes via condensation reaction. Most of the mem-
branes exhibited good thermal, chemical and mechanical
stabilities. Synthesized membranes showed good conductiv-
ity. Higher conductivity of 3.26 � 10�2 S cm�1 at 160 �C with
acid doped level of 286.8 wt% for 3:1 PArF6SO2BI and
7.31 � 10�2 S cm�1 at 160 �C with 356.9 wt% for 3:1 PArF6OBI is
observed. The as-synthesized PArF6SO2BI and PArF6OBI
membranes exhibit improved proton conductivity. Thus it is
concluded that the PArF6SO2BI and PArF6OBI membranes
posses better characteristics to be used as polymer electrolyte
membrane in the fabrication of HTPEMFC.
Acknowledgements
One of the authors, Dr. S. Prakash (File no: 201516-PDFSS-2015-
17-TAM-10968) is thankful to University Grants Commission
(UGC), New Delhi, for providing Post Doctoral Research
Fellowship. Authors acknowledge to University Grants Com-
mission (UGC), New Delhi, for providing the BSR Faculty
Fellowship (No. F. 18-1/2011(BSR)). Authors also thank to Dr. K.
Krishnamoorthy (Scientist, CSIR-NCL, Pune) and Dr. Tushar
Jana (Associate Professor, University of Hyderabad) for sup-
porting the analysis.
Appendix A. Supplementary data
Supplementary data related to this article can be found at
https://doi.org/10.1016/j.ijhydene.2018.03.058.
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