Int J Plast Technol (December 2011) 15(2):97–111
DOI 10.1007/s12588-012-9018-4
R E S E A R C H A RT I C L E
SPSEBS/H3PO4 composite electrolyte membranes
for application in PEMFC and DMFC
Perumal Bhavani & Dharmalingam Sangeetha
Received: 26 August 2011 / Accepted: 2 February 2012 / Published online: 10 March 2012
# Central Institute of Plastics Engineering & Technology 2012
Abstract High proton conducting composite electrolyte membranes based on Sulfonated
Poly Styrene Ethylene Butylene Poly Styrene (SPSEBS)/Phosphoric acid (2, 4, 6, 8 and
10 % H3PO4) composites were prepared. Their water and alcohol absorption increased
with increase in H3PO4 content. The membranes were stable to aging in boiling water
for 8 h. Their thermal stability was also improved. The ion-exchange capacity as well
as the proton conductivity of composites increased significantly with the increase in
the content of H3PO4. The proton conductivity of the membranes with 10% H3PO4
was in the order of 10−2 S/cm which is appreciable for an electrolyte membrane for
application in fuel cell. The composite membrane was also tested for its performance
in PEMFC and DMFC units of 25 cm2 area designed in our lab. The maximum power
density of PEMFC with composite membranes (2 and 10% phosphoric acid) was 77.5
and 84 mW/cm2 respectively, and that of DMFC 38 and 44 mW/cm2.
Keywords SPSEBS . H3PO4 . Proton conducting membranes . PEMFC . DMFC
Introduction
The proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells
(DMFC) offer a perfect stepping stone in the commercialization of fuel cells. They
can be operated at low temperatures, thus allowing them to compete in the same
market as batteries. They can also be scaled up for larger projects, such as the Ballard
Power Systems bus. Since the membrane is a solid material, the cell can easily be
stacked, as long as proper bipolar plate designs are used [1].
The proton exchange membrane (PEM) offers a great balance between power, size
and operating temperature. PEMs are being actively pursued for use in automobiles,
buses, portable applications, and even for residential power generation [2].
P. Bhavani : D. Sangeetha (*)
Department of Chemistry, Anna University, Chennai 600 025, India
e-mail: [email protected]
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Today, industries have great hope for PEM, some of them even citing that they
have exceeded all other electrical energy generating technologies in the breadth of
scope and their possible applications. The high manufacturing cost of fuel cell stacks is
the impediment to their mass manufacture. The most commonly used perfluorinated
membrane, Nafion, is an expensive component, adding to the high manufacturing cost
of fuel cell stacks [3].
Cheap engineering thermoplastics, such as polystyrene ethylene butylene polystyrene
(PSEBS) were prepared for proton exchange membrane fuel cell [4]. The fluorinated
polymer/SPEEK blends were also reported [5]. Sulfonated amine-poly (ether sulfone)
(S-APES), prepared by nitration, reduction and sulfonation of poly (ether sulfone)
(ultrason-S6010), was also used as proton exchange membrane [6]. Polymer electrolyte membranes, fabricated by blending of Poly (2, 5-benzimidazole) (ABPBI) and
Poly (vinylphosphonic acid) (PVPA) [7], SPEEK with poly vinyl alcohol [8] for
DMFC application etc., were also evaluated as alternatives to Nafion membranes. Among
the above mentioned polymers, PSEBS exhibits good chemical stability and flexibility.
PSEBS can be converted to sulfonated poly styrene ethylene butylene poly styrene
(SPSEBS), a proton conducting polymer, by electrophilic substitution of the sulfonic acid
groups in the polymer back bone, but the proton conductivity was not adequate.
The proton conductivity of polymer electrolyte membranes in general and
SPSEBS in particular can be considerably improved by incorporating fast proton
conductors in the membrane matrix. Among several types of additives, phosphoric
acid (H3PO4) has been considered as a potential material to enhance the conductivity
of composite membranes for fuel cell application due to its high proton conducting
ability. Recently, polybenzimidazole (PBI), doped with phosphoric acid as PEMs, has
been widely studied [9–13]. Acid doped PBI exhibited good proton conductivity and
it has been proposed as an electrolyte membrane for medium temperature fuel cells
(150–200 h) and hydrogen sensors [14]. In the literature, PVA membranes doped with
H3PO3 or H3PO4 and phosphotungstic acids (PWA) were also reported [15, 16].
Generally, SPSEBS possesses poor mechanical stability. Therefore, in the present study
in order to improve the mechanical properties, as well as to enhance the proton conductivity, H3PO4 was incorporated into it and tested. The same study was not reported in the
literature to the best of our knowledge. The fabrication of the composite membranes
and the results of various characterization performed are discussed here.
Experimental
Materials
Polystyrene–block poly (ethylene butylene)-block-polystyrene (PSEBS, Mw-89,000),
Chlorosulphonic acid (CSA), Tributyl phosphate (TBP), Tetra hydro furan (THF) and
Chloroform were obtained from Spectrochem, India, Lancaster, Merck and SRL.
Sulfonation of PSEBS
Sulfonation of polystyrene ethylene butylene polystyrene (PSEBS) was conducted in
chloroform, employing chlorosulphonic acid as the sulfonating agent. A number of
Int J Plast Technol (December 2011) 15(2):97–111
99
experiments were performed to determine the optimum conditions of sulfonation
of PSEBS by varying the solvent, polymer concentration, reaction time and the
amount of sulfonating agent. After dissolving PSEBS in chloroform, the solution was allowed to cool to 0 °C in an ice bath. Then required amount of tributyl phosphate (to moderate the reaction) is added. Then chlorosulphonic acid
was added drop wise over a period of time. Continuous stirring was maintained
during the reaction. The reaction proceeded for 3 h and was terminated by
adding a lower aliphatic alcohol. The sulfonated PSEBS was recovered after
removing all the solvents by evaporation. The product was washed several
times with water. Then the product was dried at 50 °C for 6 h [17].
Preparation of composite membrane
The composite membranes were prepared by solvent evaporation method.
Initially, a desired amount of SPSEBS was dissolved in THF. Then various
weight percentages of H3PO4 in THF solvent were added and continuously stirred
until the solutions became homogeneous. After 8 h, the polymer solutions were cast
into clean dry petridish. The weight percentage of components in the composite
membranes is given in Table 1. All the prepared membranes were treated with 3%
H2O2 and 10% H2SO4 to remove the impurities. They were subsequently washed
with boiling water.
Ion exchange capacity, water and methanol uptake
Ion exchange capacity (IEC) depends on the number of sulfonic acid groups present
in the membrane. The composite membrane was immersed in saturated potassium
chloride solution over night, to allow replacement of protons with K+ ions. Protons
released from the membrane were neutralized by 0.01 N sodium carbonate solution.
Phenolphthalein was used as the indicator. The IEC was calculated using the
following formula.
IEC ¼
Normality of sodium carbonate x Volume of sodium carbonate
meq=g
Weight of dry membrane
The water and methanol uptake was determined gravimetrically. Previously dried
membranes were weighed and immersed in respective solvents at room temperature.
After saturation, the membranes were taken out and the solvent on the surface was
Table 1 SPSEBS/H3PO4 of
composite membranes
Membrane code
Weight percentage (%) SPSEBS : H3PO4
PA 2
98: 02
PA 4
96: 04
PA 6
94: 06
PA 8
92: 08
PA 10
90: 10
100
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quickly dried and the weight was noted down. The percentage uptake was calculated
using the formula,
Percentage uptake ¼
Where
Wet M
Dry M
Wet M Dry M
100
Dry M
Weight of wet membrane
Weight of dry membrane
Leaching Test:
0.3 g of the composite membrane was allowed to rest in 50 ml of water at 80 °C for
8 hours. The membrane was then removed by filtration. The filtrate was then
analyzed with blue litmus paper to verify acid leaching.
Methanol permeability
Experiment to evaluate methanol permeability was carried out using a two compartment glass testing cell consisting of two reservoirs separated by an electrolyte
membrane with a dense layer of SPSEBS or composite membranes to reproduce a
phenomenon of methanol crossover in DMFC system. The PEM is sandwiched
between donor (Chamber A) and receptor (Chamber B) compartments. Initially the
donor compartment was filled with 50 ml of aqueous 2 M methanol solution and the
receptor compartment with 50 ml of deionised water. The solution in each bath was
stirred using magnetic stirrer during measurement to keep uniform concentration.
Due to the presence of liquid water on either side of the cell, the membrane remains
hydrated. Equal amounts of solution in both the compartments ensure that equal
hydrostatic pressure is maintained. The change in concentration of methanol in
receptor compartment was measured as a function of time. For every one hour, few
drops of solution from receptor compartment was withdrawn by syringe and placed in
a prism of a refractometer. The permeability was determined from refractometer
readings. The refractometer directly gives the percentage of methanol present in the
solution. The methanol permeability experiments were carried out at room temperature
(~30 °C). Methanol permeability was calculated by plotting methanol concentration in
receptor compartment (CB) as a function of time using the following formula,
CB ¼ ðAP=VB LÞCA ðtÞ
and
P ¼ mxðVB =ACA Þ
Where ‘m’ is the slop of the linear plot of CB versus time, ‘P’ is the methanol
permeability (cm 2/s), ‘A’ is the membrane area (cm 2), V B is the volume of
compartment ‘B’ (ml), ‘L’ is the film thickness (cm), C A and C B are the
concentrations of methanol (mol) in Cell A and Cell B and ‘t’ is time (s). A, L and
VB are the area of membrane, the thickness of membrane and the volume of Cell B,
respectively. D and K are the methanol diffusivity and partition coefficient between
the membrane and the adjacent solution, respectively. The product DK is the
methanol permeability (P), which was calculated from the slope of the straight-line
Int J Plast Technol (December 2011) 15(2):97–111
101
plot of methanol concentration versus permeation time. The measurements were
carried out at 30 °C.
Instrumental characterization
X-ray diffraction (XRD) is a technique that is used to identify the crystalline and
amorphous materials. In the present study, XRD spectrum of membrane sample was
recorded using “X” Pert Pro diffractometer.
The FTIR spectra of SPSEBS and composite membranes were scanned using
Perkin Elmer FTIR spectrometer.
The differential scanning calorimetery (DSC) spectra of the composite membranes
were obtained on NETZSCH- Geratebu model DSC 200PC. Measurements were
done over the temperature range of 28–300°C at a heating rate of 10°C/min in
hermitically sealed aluminium pans.
Thermal stability of polymer films were examined using NETZSCH-Geratebu
GMBH with the temperature varying from 27 to 900°C and at a heating rate of 20°C/min
in nitrogen atmosphere.
The membranes were dried and the surface morphology was studied by scanning
electron microscopy (SEM) using a JOEL JSM 6360 microscope.
The proton conductivity was determined by AC impedance technique under the
frequency range of 10 HZ- 40 KHz in the hydrated condition. The conductivity of
sample (σ) was measured using the formula, σ0L/RA, where, L is the thickness of
the membrane in cm, A is the area of the membrane in cm2 and R is the resistance in
ohm and σ is in S/cm.
Tensile strength of the membranes at room temperature was measured using Universal Testing Machine having a load cell of 5 KN. The gauge length and breadth of all
membranes were 50 mm and 5 mm respectively. Tests were conducted with a constant
strain rate of 10 mm/min and continued until the failure of the sample occurred.
Preparation of membrane electrode assembly (MEA)
Diffusion layer preparation
The preparation of the diffusion slurry ink included mixing 70 wt% Vulcan XC-72,
30 wt% PTFE binder solution, and a suitable amount of double distilled water and
isopropyl alcohol. The resulting black mixtures were first ultra sonicated for one hour.
The black ink was then coated onto the carbon cloth and dried in a vacuum oven at
100 °C for 2 hours and kept in muffle furnace at 350 °C for 6 h [18].
Preparation of the anode and cathode electrodes
After the preparation of diffusion layer, catalyst slurry ink for anode and cathode were
fabricated with the help of carbon supported platinum black with platinum loading of
0.375 mg/cm2 and 0.125 mg/cm2, respectively. Then suitable amounts of double
distilled water and isopropyl alcohol were mixed with the help of ultra sonicator. After
the ultra sonication, the black catalyst slurry was coated onto the respective diffusion
layers. The prepared anode and cathode were dried in a vacuum oven at 100 °C for 2 h
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and then in a muffle furnace at 350 °C for 6 h. For PEMFC fuel cells, the platinum
loading of cathode was thrice greater than anode due to the water molecules produced at
the cathode side [19].
Hot pressing
The proton conducting membrane was sandwiched between the prepared anode and
cathode electrodes and hot pressed at 80 °C and 1.5 ton pressure for 2 minutes.
Finally MEA was ready to use in PEMFC membrane fuel [20].
MEA preparation for DMFC
Membrane electrode assembly (MEA) was obtained by sandwiching the SPSEBS/H3PO4
composite membrane between the anode and cathode. For DMFC, the electrocatalyst
used was 40 wt% Pt:Ru (1:1) on Vulcan XC-72 and 20 wt% Pt on Vulcan XC-72 in
the anode (loading 0.5 mg/cm2) and cathode (loading 0.5 mg/cm2), respectively. The
catalyst layer is obtained by mixing the catalyst, isopropyl alcohol (IPA), deionised
water and Nafion solution as binder and coated on the carbon cloth. The electrodes
were of 5 cm×5 cm (area025 cm2). The MEA was fabricated uniaxially by hot
pressing the anode and cathode onto the membrane at 100 °C with a pressure of
150 kg/cm2 for 3 min [21].
Results and discussion
Ion exchange capacity
Figure 1 shows the ion exchange capacity of composite membranes. The ion exchange
capacity is directly related to the proton exchanging ability of the membrane. As the
concentration of H3PO4 was increased, it is evident that there will be an increase in the
IEC values. This is because the acid not only acts as a Lewis acid but also acts as rich proton
carriers. These protons are capable of getting exchanged and hence the increase in the IEC
Fig. 1 Ion exchange capacity of
composite membranes
2.2
2.0
1.8
IEC meq/g
1.6
1.4
1.2
1.0
Nafion
0.8
0.6
0
2
4
6
Phosporic acid content%
8
10
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103
values with increase in the content of H3PO4 was in according to the expected trend. But
the IEC values of the composite membranes (PA 2 to PA 10) were found to be lower
than the virgin SPSEBS. Moreover, SO3− ions are in the polymeric site responsible
for the proton conductivity, since in the composite membranes the SPSEBS amount
was reduced to half, the SO3− ions amount also reduced to half. So the IEC values of
the composite membranes were lesser values than the virgin SPSEBS.
Water and methanol absorption
Figure 2 shows the water and methanol absorption. For an excellent proton conducting ability, the membranes should have some appreciable water absorbing property.
The absorbed water molecule acts like a canal for the passage of protons and the
proton conductivity dependent largely on the connectivity of the hydrated domains
which in turn increases the mobility of ions. But excessive swelling in water results in
a loss of mechanical and dimensional stability. The water uptake of the SPSEBS is
totally dependent upon the sulfonation, hence SPSEBS and their composites are
preferred. Both water and methanol absorptions increased with increase in the content
of H3PO4. The acids are known to hold a very large amount of solvent and especially
water. This also confirmed an increase in the amount of H3PO4 as we move from PA
2 to PA 10. The increase in the water absorption may be due to the incorporation of
hydrophilic H3PO4 in SPSEBS composite membranes. The increase in methanol
absorption may be due to the formation of larger ionic clusters or transport channels
in the composite membrane [22].
Methanol permeability
Figure 3 shows the methanol permeability of composite membranes. Methanol permeability and proton conductivity are the two transport properties, which determine fuel
cell performance in DMFC. The methanol permeability of Nafion 117 (35.2×10−7 cm2/s)
was higher than that of composite membranes (11.5 to 7.5× 10−7 cm2/s). The
methanol permeability of composite membrane decreased upon the introduction of
H3PO4, which is hydrophilic and requires no sulfonic functional groups for the formation
Fig. 2 Water and methanol
uptakes of composite membranes
250
% Absorption
200
150
water
Nafion(water)
Methanol
Nafion (methanol)
100
50
0
2
4
6
8
Phosphoric acid content %
10
104
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36
Methanol permeability(x10-7cm2/s)
Fig. 3 Methanol permeability of
composite membranes
32
Nafion 117
28
24
20
16
12
8
0
2
4
6
8
10
H3PO4 content(%)
of ion clusters and methanol transport channels. The methanol permeability of SPSEBS
membrane was 22×10−7 cm2/s at room temperature. Compared to Nafion 117,
methanol permeability of composite membranes was found to be very less. These
results reveal the better performance for DMFC application.
XRD
The XRD patterns of the parent polymer and the various composite membranes are
shown in Fig. 4. In the case of SPSEBS sharp peaks were observed at higher 2θ
values (43, 44, 50, 51 and 73°). These peaks indicate the existence of tiny crystallites
at periodic positions in the membrane. An additional broad peak was observed
Intensity
(a)
(b)
(c)
(d)
(e)
(f)
10
20
30
40
50
60
70
80
2 Theta(degree)
Fig. 4 XRD Spectra for SPSEBS and composite membrane (a)SPSEBS (b) PA 2 (c) PA 4 (d) PA 6 (e) PA
8 (f) PA 10
Int J Plast Technol (December 2011) 15(2):97–111
105
between 10 and 30°. It was due to amorphous fraction of the membrane. Similar
features were also shown by other composite membranes. Hence, phosphoric acid
might not have had any effect on the origin of tiny crystallites.
FTIR
The FTIR spectra of composite membranes are shown in Fig. 5. The OH stretching
vibration appeared as a broad band between 2000 and 3800 cm−1. It was due to O-H
stretching vibration of -SO3H groups, H2O and H3PO4. Due to broadening, the
characteristic peaks of -C-H stretching vibrations of aromatic and aliphatic groups
were not clearly seen, but the –CH2-stretching vibrations just below 3000 cm−1 was
evident. The presence of water was confirmed by its bending vibration at 1637 cm−1.
The CH2 bending modes were seen at about 1450 and 1370 cm−1. The PO4 and SO3
vibrations were seen between 1000 and 1300 cm−1, but they were weak. Above
discussions conclude the presence of H3PO4 in the membrane matrix.
DSC
The DSC results of composite membranes SPSEBS/H3PO4 are illustrated in Fig. 6.
The endotherm below 150 °C was due to desorption of water. The endotherm
between 200 and 500 °C was due to degradation of polymer. The DSC traces of
2% to 10% loaded phosphoric acid in SPSEBS showed similar features.
TGA
The TGA results of SPSEBS and composite membranes are illustrated in Fig. 7. The
initial weight loss up to 150 °C was assigned to loss of water. The weight loss between
150 and 400 °C was due to expulsion of sulphonic acid groups. The major weight loss
between 400 and 500 °C was due to degradation or desorption of polymer back bone.
The residue of 0.2% in 2% acid loaded membrane was ascribed to the presence of
Fig. 5 FTIR Spectra of SPSEBS
and composite membrane (a)
SPSEBS (b) PA 2 (c) PA 4 (d)
PA 6 (e) PA 8 (f) PA 10 (g)
H3PO4
(g)
(f)
(e)
(d)
%T
(c)
(b)
(a)
4000
3500
3000
2500
2000
1500
-1
Wave Number cm
1000
500
106
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Fig. 6 DSC curve of SPSEBS
and composite membrane (a)
SPSEBS (b) PA 2 (c) PA 4 (d)
PA 6 (e) PA 8 (f) PA10
(b)
(c)
Heat Flow (W/g)
(d)
(e)
(f)
Exo
(a)
100
200
300
400
500
600
700
o
Temperature ( C)
phosphate. The thermogram of 4% H3PO4 loaded membrane gave 3.47% residue. It
indicates more amount of phosphate than the 2% loaded membrane. The thermogram
of 6% acid loaded membrane gave 5.4% residue which was higher than the previous
sample. In 8 and 10% acid loaded membranes the weight loss was high, but the
former showed higher weight loss than the latter. The thermal stability of composite
membranes was lower than SPSEBS without any correlation with H3PO4 content.
Hence H3PO4 might not be uniformly distributed in the membrane matrix.
SEM
The SEM images of SPSEBS and composite membranes (2 and 10%) are shown in
Fig. 8. The SEM image of SPSEBS showed uniform surface. The SEM image of 2%
acid loaded composite membrane showed smooth surface where as the SEM image of
Fig. 7 TGA curve of SPSEBS
and composite membrane (a)
SPSEBS (b) PA 2 (c) PA 4 (d)
PA 6 (e) PA 8 (f) PA10
(f)
100
(e)
Weight Loss %
80
(a)
(c)
(b)
60
(d)
40
20
0
100
200
300
400
500
600
0
Temperature( C)
700
800
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107
Fig. 8 SEM analysis (a) SPSEBS (b) PA 2 (c) PA 10
10% acid loaded showed uniform surface, in other words H3PO4 might be uniformly
distributed over the SPSEBS matrix which facilitates a desirable and efficient conductivity of protons through the larger interfacial areas between the sulphonated
polymer and the phosphoric acid.
Proton conductivity
The results of proton conductivity of the composite membranes are illustrated in
Fig. 9. The proton conductivity of the membranes increased with the increase in the
content of phosphoric acid. It supports the assistance of phosphoric acid in transporting protons across the membrane between the electrodes. In the membrane, the
entrapped water might facilitate the ionization of phosphoric acid for high proton
conductivity. Formation of H2PO4− was also reported [23]. Hence, increase in the
proton conductivity with the increase in the H3PO4 content indicates improved
ionization in the matrix. It reported lower activation energy for proton conductivity
of H3PO4 loaded membranes [24]. Hence it might also be an additional contributing
factor for higher proton conductivity. When the density of acid groups is high, these
ionic clusters become crowded in the hydrophilic membrane. As a result, the proton
conductivity of the composite membrane increased compared to Nafion.
Fig. 9 Proton conductivity of
composite membrane
11
10
-2
Conductivity10 S/cm
9
8
7
6
5
4
Com posite m em brane
Nafion
3
2
1
0
0
2
4
6
H 3PO 4 Content%
8
10
108
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Selectivity ratio
The selectivity ratio defined as the ratio of proton conductivity and methanol
permeability of SPSEBS and composite membranes, and is shown in Fig. 10.
The proton conductivity of composite membrane was lower than that of SPSEBS
membrane. The virgin SPSEBS membrane and all the composite membranes showed
higher selectivity ratio than Nafion 117. Out of all the composite membranes, PA 10
(1.18×105 Ss/cm2) showed highest selectivity. Hence the incorporation of H3PO4
into the SPSEBS membranes had more impact on the reduction of methanol than
proton conductivity, and therefore, the composite membranes are attractive for
DMFCs. The practical usage for membranes in DMFC must possess high proton
conductivity and low methanol permeability. The transport of methanol in membrane
also requires channels with good connectivity formed by the hydrophilic clusters. The
selectivity of SPSEBS/H3PO4 composite membranes, which is based on their
conductivities and methanol permeability, was measured at room temperature. The
selectivity ranged from 0.40 to 1.18×105 Ss/cm2, which is attractive for DMFC
performance.
Mechanical properties
The tensile strength of composite membranes incorporated with various percentage of
phosphoric acid is shown in Fig. 11. In general, there was an improvement in the
tensile strength as the H3PO4 content increased. The tensile strength of composite
membranes increased with the increased in the percentage of phosphoric acid.
Single cell performance of composite membranes in PEMFC
To check the functioning of the composite membrane in a real device, the electrochemical performance of the membranes was tested in a PEMFC single cell. Figure 12
showed the polarisation and power density curves for the composite membrane in a
PEMFC test at room temperature and the results were compared with Nafion 117
Fig. 10 Tensile strength of
composite membrane
10.5
10.0
Tensile strength(MPa)
9.5
9.0
8.5
8.0
7.5
7.0
6.5
2
4
6
8
Phosphoric acid content(%)
10
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Fig. 11 Selectivity ratio of
composite membrane
109
3
0.8
Selectivity ratio(x10 Ss/cm )
1.0
5
1.2
Nafion 117
0.6
0.4
0.2
0.0
0
2
4
6
H3PO4 (%)
8
10
membrane. The open circuit voltage (OCV) for the SPSEBS (0.980 V) was higher
than that of Nafion 117 (0.790 V). The OCV value of PA 2 was 0.805 V, whereas the
OCV value of PA 10 was 0.852 V.
In the whole voltage range investigated, the current values of the SPSEBS
membrane were always larger than the values obtained with Nafion 117 membrane. The maximum power density value reached at room temperature with the
SPSEBS was 50 mW/cm−2, whereas the maximum power density of Nafion 117
was 32 mW/cm−2 with the same operating condition. In the case of composite
membrane, the maximum power density values were 77.5 and 84 mW/cm−2 for PA
2 and PA 10 respectively. Power densities of composite membranes were higher than
Nafion117. These results indicate that SPSEBS and composite membranes are
promising electrolyte for fuel cell.
100
PA 2
PA 10
0.8
Voltage(V)
2
Power density(mW/cm )
80
0.6
60
0.4
40
0.2
20
0.0
0
0
100
200
300
400
2
Current density(mA/cm )
Fig. 12 Polarisation and power density curves of composite membrane
500
110
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Single cell performance of composite membrane in DMFC
Figure 13 represents the cell performance based on the composite membrane containing 2 and 10% of phosphoric acid at room temperature. The open circuit voltage
(OCV) for the SPSEBS was 0.62 V. The OCVs of composite membranes was higher
than that of Nafion 117 (0.64 V). The OCV value of PA 2 was 0.65 V which was
higher than that of Nafion 117. Similarly, the OCV value of PA10 was 0.689 V which
was higher than that of Nafion 117. In the whole voltage range investigated, the
current values of the composite membrane were larger than the values obtained with
Nafion 117 membrane. The maximum power density reached at room temperature
with the SPSEBS was 13 mW/cm2 whereas the maximum power density of Nafion
117 was 27 mW/cm2 under the same operating condition. In the case of composite
membranes (PA 2 and PA 10) the maximum power density values were 38 and
44 mW/cm2 respectively. These results indicate that the performance of the cell based
on composite membranes is comparable to that of Nafion 117 membrane. This is mainly
attributed to the good compatibility of the membrane with the electrode and high
dimensional stability of the membrane [25, 26].
Conclusions
SPSEBS was loaded with H3PO4 (2, 4, 6, 8 and 10%) to obtain composite membranes. In the composite membrane H3PO4 was expected to increase conductance and
mechanical properties. As expected, the conductivity increased from 2 to 10%
loading. The mechanical properties also increased with increase in H3PO4 loading.
The increase in conductance is attributed to ionization H3PO4 assisted by water in the
membrane matrix. Methanol permeability was also significantly reduced. Based on
high conductivity and increased mechanical properties along with reduced methanol
50
SPSEBS
Nafion 117
PA 2
PA 10
40
2
0.6
Power density(mW/cm )
0.7
Voltage(V)
0.5
30
0.4
20
0.3
0.2
10
0.1
0
0.0
0
30
60
90
120
150
180
210
240
2
270
300
330
Current density(mA/cm )
Fig. 13 Polarization and power density curves of SPSEBS, Nafion 117 and composite membranes (PA 2
and PA 10)
Int J Plast Technol (December 2011) 15(2):97–111
111
permeability, the composite membranes render a convenient substitute to SPSEBS for
application in DMFC. Accordingly, the power density of composite membranes was
also found to be higher than SPSEBS and Nafion. Because of such features the same
membranes can also find applications in PEMFC.
Acknowledgement The authors would like to thank the University Grant commission (UGC), India for
funding this project.
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