A novel fuel cell membrane with high efficiency

Article abstract of DOI:10.1039/c4ra00904e

A new class of polymer electrolyte membranes containing an azo based ionic diol (30, 40 and 50 mol%) was prepared for use in a fuel cell. Its proton conductivity, hydrolytic stability, water uptake capacity, swelling behaviour and ion-exchange capacity measurement data indicated its suitability for use as a proton exchange electrolyte membrane in fuel cells. For example, the membranes containing 30, 40 and 50 mol% of azo based ionic monomers exhibit fairly good proton conductivities of 0.073 S cm^-1, 0.075 S cm^-1 and 0.079 S cm^-1, respectively at 30 °C. Moreover, the prepared membranes show a phase separated morphology and exhibit a high thermal stability up to 460°C which are important parameters for successful fuel cell design. Two types of azo based sulfonated poly(arylene ether sulfone) (SPAES) such as the SPAES-30 and SPAES-50 membrane electrode assembly have been successfully fabricated and yielded a good fuel cell performance in the whole range of current density.

Full text of DOI:10.1039/c4ra00904e

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A novel fuel cell membrane with high efficiency†
Deivanayagam Paradesi, Debasis Samanta, Asit Baran Mandal
Cite this: RSC Adv., 2014, 4, 26193
*
and Sellamuthu Nagappan Jaisankar
A new class of polymer electrolyte membranes containing an azo based ionic diol (30, 40 and 50 mol%) was
prepared for use in a fuel cell. Its proton conductivity, hydrolytic stability, water uptake capacity, swelling
behaviour and ion-exchange capacity measurement data indicated its suitability for use as a proton
exchange electrolyte membrane in fuel cells. For example, the membranes containing 30, 40 and 50
mol% of azo based ionic monomers exhibit fairly good proton conductivities of 0.073 S cmꢀ1, 0.075 S
cmꢀ1 and 0.079 S cmꢀ1, respectively at 30 ꢁC. Moreover, the prepared membranes show a phase
separated morphology and exhibit a high thermal stability up to 460 ꢁC which are important parameters
for successful fuel cell design. Two types of azo based sulfonated poly(arylene ether sulfone) (SPAES)
such as the SPAES-30 and SPAES-50 membrane electrode assembly have been successfully fabricated
and yielded a good fuel cell performance in the whole range of current density.
Received 31st January 2014
Accepted 24th April 2014
DOI: 10.1039/c4ra00904e
www.rsc.org/advances
Proton Exchange Membrane Fuel Cells (PEMFCs) are one of
the best types of fuel cells for a vehicular power source to ulti-
Introduction
mately replace fuel and diesel internal combustion engines. The
interest in fuel cells is basically due to their ability to continu-
ously convert chemical energy into electrical energy with a high
efficiency and low emission of pollutants.6–9 The proton
exchange membrane is a key component of the PEMFC that
usually acts as an electrolyte to form ionic pathways for the
protons to pass from the anode to the cathode. A fuel cell
membrane must have thermal, chemical and mechanical
stability along with a good proton conductivity. The perfor-
mance of an electrolyte membrane is controlled by various
physical parameters like the ion-exchange capacity, water
uptake, swelling behaviour and proton conductivity. The
stability and proton conductivity of the electrolyte membranes
are related to the polymer backbone and its acid moiety,
respectively.
Peruorinated ionomers have an outstanding physical and
chemical stability and show an excellent performance when
applied in fuel cells, especially in H2 fuel cells. Naon® is the
commercially available standard polymer electrolyte membrane
used in PEMFCs due to its excellent mechanical and chemical
stabilities.10,11 It is quite resistant to chemical attack and has a
highly phase separated morphology that imparts excellent
proton conductivity with moderate water uptakes.12–14 However,
the limitations of these membranes are their high cost and
production process, which may include highly toxic and envi-
ronmentally-unfriendly intermediates.15 Naon® exhibits a
high methanol permeability and a low conductivity at low
humidity or high operating temperatures, which restrict its
commercialization.16,17 These drawbacks are the main reasons
for the extensive research to develop alternative proton
exchange membranes for fuel cells.18–21
The majority of energy needs have been provided by the
combustion of fossil fuels. Due to depleting natural resources
and increasing petroleum prices, the search for alternative fuels
has become intensied. Fuel cells are reliable power generation
devices for vehicles, transportation and portable devices to save
a signicant amount of energy and reduce the emission of non-
greenhouse gases.1,2 Fuel cell systems have the ability to solve
the most challenging problems associated with current battery
systems.3 Unlike the conventional power generation technique,
fuel cells work without combustion and environmental side
effects. They produce direct current and act like continuously
fuelled batteries. Recent studies have been focused on non-
uorinated hydrocarbon-based polymer electrolyte membranes
and composite4,5 membranes due to the loss of stability and
conductivity at high temperatures in peruorosulfonic acid
membranes. The high temperature operation provides the
following advantages in fuel cells.
(i) Simplication of the cooling system.
(ii) Enhanced proton conductivity.
(iii) Improved tolerance of the electrodes to carbon
monoxide.
(iv) Better kinetics of methanol oxidation.
(v) Potential use of cogeneration heat.
Polymer Division, Council of Scientic and Industrial Research (CSIR)-CLRI, Adyar,
Chennai-600020, India. E-mail: snjaio@yahoo.com; snjsankar@clri.res.in; Fax: +91-
44-24911589; Tel: +91-44-24422059
† Electronic supplementary information (ESI) available: the synthetic scheme for
azo based diol, the HPLC chromatogram of the expand region of azo based ionic
diol, the FT-IR spectrum of the azo based monomer and the inuence of IEC on
the water uptake of the SPAES membranes. See DOI: 10.1039/c4ra00904e
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Sulfonated polyimides,22–24 sulfonated polysulfones,25,26 synthesized by a diazotization reaction followed by a coupling
sulfonated poly(ether ether ketone)s,27–29 sulfonated poly(aryl- reaction. The reaction scheme for the synthesis of the azo based
ene ether sulfone)s,30–32 sulfonated polystyrenes,33,34 and poly- diol is shown in the ESI Scheme S1.† In a 250 mL beaker, 75 g of
benzimidazoles35,36 have been studied for fuel cell membranes an ice–water mixture and 10 mL of conc. HCl were added and
because of their good stability, conductivity and processability. stirred at 0–5 ꢁC. An amount of 8.65 g of sulfanilic acid
Among these polymers, sulfonated poly(arylene ether sulfone)s (50 mmol) in 90 mL of a 2.5% sodium carbonate solution was
have been the centre of attention of recent studies in search of a added to the beaker followed by the addition of 3.45 g of a
replacement for Naon®. Although improvements have been saturated solution of sodium nitrite (50 mmol). A powdery white
achieved with these types of polymers, the proton conductivity precipitate of diazonium salt was obtained within a few
at elevated temperatures is still un-satisfactory because of the minutes. An amount of 4.40 g of resorcinol (40 mmol) in 60 mL
lack of water retention at temperatures much above 100 C.32
of water was slowly added to the beaker through an addition
ꢁ
ꢁ
The post sulfonation process of the high performance funnel and stirred well for 30 min at 0–5 C. The orange col-
materials such as polysulfones, poly ether ether ketones etc., oured precipitate was collected by ltration, washed thoroughly
has suggested that these materials are unable to form dened with ice-cold water and recrystallized from deionized water. The
ꢁ
hydrophilic domains. The rigid polymeric backbones prevent product was dried at 60 C under vacuum for 12 h.
the formation of ionic clusters. So the introduction of ionic
groups in the side chain polymer via a direct copolymerization Polymerization
method improves the formation of the ionic cluster, the ion-
A typical polymerization procedure for the synthesis of SPAES-
exchange capacity and the proton conductivity. Another
50 is as follows; 4,40-dichlorodiphenylsulfone (14.35 g,
advantage of this method is that it allows for the close control of
50 mmol), bisphenol-A (5.70 g, 25 mmol) and an azo based
monomer (7.38 g, 25 mmol) were placed in a 250 mL three-
necked RB ask equipped with a magnetic stirrer, a Dean–Stark
trap and a nitrogen inlet. A volume of 60 mL of DMAc was added
sulfonation and the polymer structure.
Several types of poly(arylene ether sulfone)s (PAES) are
reported in the literature, where the anionomers are located in
the main chain37–39 and the side chain40–42 of the aromatic ring.
and stirred well until all the monomers were dissolved. An
Most of the arylene polymers are functionalized by sulfonating
amount of 13.8 g of anhydrous potassium carbonate (100 mmol)
the base polymer with sulfonating agents like conc. sulfuric acid,
and 30 mL of toluene were added and reuxed at 150 ꢁC for 5 h.
Following this step, the temperature was slowly raised to 180 ꢁC
oleum, chlorosulfonic acid etc. The process of post sulfonation
suffers some disadvantages, which may be due to its high degree
aer controlled the removal of toluene from the reaction
of sulfonation, degradation and cross-linking by reaction sensi-
tivity.43 If the post sulfonation takes place at the active sites of the
benzene ring, it may lead to the risk of desulfonation under fuel
cell conditions. Based on the above drawbacks, new monomers
of novel architecture are synthesized. In this work, for the rst
time we report the syntheses of azo based SPAES membranes for
fuel cell applications. The physical and chemical properties of
these membranes are characterized using various instrumenta-
tion techniques in order to evaluate the suitability of these
membranes for fuel cells.
ꢁ
mixture. The polymerization reaction was continued at 180 C
ꢁ
for 18 h. The viscous solution was cooled to 25 C and diluted
with DMF to allow easier ltering. Then, the solution was
ltered using 100 mm pore size lter paper to remove most of
the potassium salts. The polymer solution was slowly added to a
beaker containing an excess of methanol. The precipitated azo
polymer was washed several times with ethanol and then dried
ꢁ
at 80 C under vacuum for 24 h.
Membrane preparation
The SPAES copolymer of 10 wt% was dissolved in DMAc at 25 ꢁC
under constant stirring. Aer the complete dissolution of
SPAES, the viscous solution was cast onto a at glass plate. The
cast membranes were dried at 60 ꢁC for 12 h, 90 ꢁC for 10 h and
140 ꢁC for 6 h. Following this step, the resulting membranes
were soaked in deionized water for 24 h to release the
membranes from the glass plates. The obtained membranes
were thoroughly washed with deionized water and nally dried
Experimental
Materials
4,40-Dichlorodiphenylsulfone (DCDPS), resorcinol and N,N0-
dimethylacetamide (DMAc, 99.8%) were purchased from
Aldrich Co., USA. Sulfanilic acid, sodium nitrite, sodium
carbonate, 4,40-bisphenol-A (BP-A), potassium carbonate, N,N-
dimethylformamide, methanol and toluene were purchased
from Sisco Research Laboratory Private Ltd., Mumbai, India. N-
methyl-2-pyrrolidone was purchased from S. D. Fine Chem.
Ltd., Mumbai, India. DCDPS and BP-A were dried at 80 ꢁC under
vacuum for 16 h before the usage. All other chemicals were used
as received.
ꢁ
at 80 C under vacuum for 24 h.
Measurements
For the Fourier Transform Infrarꢁed Spectra (FT-IR) analysis, the
samples were rst dried at 100 C under vacuum for 6 h. The
spectrum was collected using an ABB MB3000 FT-IR spectro-
photometer. High Performance Liquid Chromatography
(HPLC) analysis was performed using a LC-10AT-VP, Shimadzu,
Monomer synthesis
The azo based ionic diol with a sulfonic acid group, (E)-4-((2,4- Japan. The surface morphology was analyzed using a scanning
dihydroxyphenyl)diazenyl)benzene
sulfonic
acid,
was electron microscope (SEM, Hitachi-S3400N). For the SEM
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analysis, the samples were cut into sufficient sizes and sputter
15 weight% of teonized Toray-TGP-H-120 carbon paper of
coated with gold to make them electro conductive. Then the 0.37 mm thickness was used as the backing layer. To prepare
sample was analyzed under vacuum at an accelerating voltage of the gas-diffusion layer (GDL), Vulcan XC-72R was suspended in
10.0 kV. Thermogravimetric analyses of the membranes were cyclohexane and agitated in an ultrasonic water bath for 30 min.
carried out using a Thermogravimetric Analyzer (Q 50, TA A 15 weight% polytetrauroethylene (PTFE) suspension in 2 mL
Instruments, USA) at a heating rate of 20 ꢁC minꢀ1 in a nitrogen ammonia was added to this solution with continuous agitation
atmosphere. Intrinsic viscosity measurements were obtained in to form a slurry that was coated onto the backing layer
N-methyl-2-pyrrolidone at 25 ꢁC using a capillary viscometer, uniformly until the required loading of 1.5 mg cmꢀ2 carbon was
Schott Gerate, West Germany. GPC analysis was done in DMF obtaꢁined. The obtained GDL was sintered in a muffle furnace at
solvent. HR-TEM images were obtained using a FET TECHNO 350 C for 30 min. Carbon supported platinum was used as a
G2 30 (300 kV) instrument.
catalyst for both the cathode and anode. The loading of the
The concentration of the ion conducting units is generally catalyst layer on both the anode and cathode was kept at 0.5 mg
denoted as the molar equivalents of the ion conductor per mass cmꢀ2. The active area for the PEMFC was 4.0 cm2. The MEA of
of dry sample and is expressed as the ion exchange capacity (IEC). the pristine and composite membrane was prepared by sand-
The IEC of the membranes was measured using the titration wiching the membrane between the two electrodes followed by
method. The polymer membranes were soaked in 1.0 M HCl for hot-pressing at 130 ꢁC for 3 min at a pressure of 20 kg cm2. The
10 h to regenerate the protons from the salt form. All the MEA was evaluated using a conventional fuel cell xture with a
membranes were thoroughly washed with deionized water parallel serpentine ow-eld machined on graphite plates. The
several times and then soaked in a 0.1 M NaCl solution for 16 h. cells were tested at 60 ꢁC under 100% humidied conditions
The protons released due to the exchange reaction with Na+ ions with hydrogen at a ow rate of 100 mL minꢀ1 at the anode and
were titrated against a 0.01 M NaOH solution using phenol- oxygen at a ow rate of 150 mL minꢀ1 at the cathode, respec-
phthalein as an indicator. The IEC was determined from eqn (1). tively. Measurements for the cell potentials with varying current
densities were conducted galvanostatically using a Fuel Cell
IEC (meq. gꢀ1) ¼ (V ꢂ NNaOH)/weight of the polymer
(1)
Test Station (Model-LCN4-25-24/LCN 50-24) procured from
Bitrode Instruments (USA).
where V is the volume of NaOH consumed, NNaOH is the
normality of NaOH and meq. is milliequivalent.
Results and discussion
The synthesis of azo based sulfonated poly(arylene ether
For the water uptake and swelling ratio studies, the acidied
membranes were dried at 120 ꢁC until constant weights and
lengths were achieved, which are recorded as Wdry and Ldry
,
sulfone)s
respectively. Then, the dried membranes were immersed into
deionized water. The water attached to the surface was quickly
swabbed with tissue paper before measurement. The weight
and length were measured several times until constant
measurements were achieved, which are denoted as Wwet and
A series of SPAES copolymer membranes were synthesised by an
aromatic nucleophilic polycondensation reaction. The polymer
code, the mole ratio of the monomers such as bisphenol-A and
azo based ionic diol used for the synthesis of the SPAES and
their physical properties are given in Table 1. The reaction was
carried out in the presence of potassium carbonate using N,N-
dimethylacetamide (DMAc) as a solvent (Scheme 1). The
resulting polymers had intrinsic viscosity in the range between
1.41 and 1.54 dL gꢀ1. The gel permeation chromatography
(GPC) experiments (ESI, Fig. S4†) indicated the weight average
molecular weight (Mw) and polydispersity index (PDI) of SPAES-
30 to be 10 640 Daltons and 1.41, respectively, while the Mw and
PDI values for SPAES-40 were 6321 Daltons and 1.31,
respectively.
Lwet, respectively. The percentage water uptake of the
membrane was calculated using eqn (2).
% Water uptake ¼ (Wwet ꢀ Wdry)/Wdry ꢂ 100
The swelling ratio was dened as eqn (3).
% Swelling ratio ¼ (Lwet ꢀ Ldry)/Ldry ꢂ 100
(2)
(3)
The proton conductivity measurements were performed in a
two-probe cell using an AC impedance spectroscopic technique.
The AC impedance spectra of the membranes were recorded in
the frequency range of 1.0 MHz to 10.0 Hz using an Autolab
Spectral analyses
PGSTAT 30. The detailed procedure for the conductivity The high performance liquid chromatography (HPLC) analysis
measurements was reported in our paper.32 The ꢁmeasurements conrms the absence of the starting material in the azo based
were taken in the temperature range of 30–130 C. The tensile ionic diol. The chromatograms of the starting material and azo
strength and Young's modulus of the completely hydrated SPAES based ionic diol are given in Fig. 1. The reverse-phase HPLC
membranes were measured using a Universal Testing Machine analysis was performed by using the mobile phase in the
(UTM), Instron, USA. Samples with a length of 50 mm and a width proportion of 70 : 30 methanol and water, pH 2.0 adjusted with
of 20 mm were tested using a 250 N load cell that was pulled at formic acid. The ow rate was set to 0.75 mL minꢀ1. The HPLC
50 mm minꢀ1 within a 21 cm gauge length. Triplicate analyses analysis was carried out by LC-10AT-VP, with a photo diode
were carried out and the average values for the determination of array (PDA) detector, in which we can collect the chromatogram
the tensile strength and Young's modulus were taken.
at different wavelengths.
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Table 1 The mole ratio and physical properties of the SPAES copolymers
Mole ratio of BP-A/ionic
diolin copolymers
Tensile strength
(MPa)
Young's modulus
(MPa)
Activation energy
Polymer code
hinta(dL gꢀ1
)
(kJ molꢀ1
)
SPAES-30
SPAES-40
SPAES-50
1 : 0.6
1 : 0.8
1 : 1.0
1.41
1.46
1.54
43
40
33
513
468
402
7.48
7.33
7.01
a
The intrinsic viscosity measured at a concentration of 1.0 g dLꢀ1 in NMP at 25 ꢁC.
Fig. 2 The FT-IR spectrum of the azo based SPAES-50 copolymer.
Scheme 1 The synthesis of azo based SPAES.
Fig. 1 The HPLC chromatograms of the starting material (a) and the
azo based ionic diol (b).
Fig. 3 The 1H NMR spectra of the azo monomer (a) and the SPAES-50
copolymers (b).
A volume of 20 mL (0.10 mg mLꢀ1 concentration) of both the
starting material and the product in methanol medium was
injected through a rheodine setup. The lmax of resorcinol was ionic diol at a wavelength of 270 nm with the retention time of
found to be 270 nm, whereas the lmax of product was found to 2.91 min clearly shows the absence of the starting material.
be 350 nm. The chromatograms were collected at a wavelength
The FT-IR spectrum of the azo based ionic diol is given in the
of 350 nm for the azo compound and 270 nm for resorcinol. The ESI Fig. S2.† A broad band appeared at 3475 cmꢀ1 which is
retention time of resorcinol was obtained at 2.91 min and the attributed to the presence of hydroxyl groups. The aromatic C–C
retention time of the product was obtained at 3.96 min. In ESI band appeared at 1120 cmꢀ1. The peaks at 1166 and 1033 cmꢀ1
Fig. S1,† the expand view of the chromatogram of the azo based are obtained because of the stretching vibration (O]S]O) of
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Table 2 The IEC values, water uptake and swelling ratio of the SPAES
copolymers
The proton NMR spectra of the azo based ionic monomer and
the SPAES polymer are shown in Fig. 3. In the azo based
monomer, the resonance signals that appear at 8.06 to 6.90 ppm
are due to the aromatic protons and the sulfonic acid proton
appears at 2.00 ppm (see Fig. 3a). The polymer (SPAES-50)
shows resonance signals appearing at 8.10 to 7.00 ppm for the
aromatic protons. The methyl protons in SPAES-50 appear at
1.65 ppm conrming the formation of the polymer.18,44
Water uptake
(%)
Swelling ratio
(%)
IEC
Polymer code
(meq. gꢀ1
)
30 ꢁC
90 ꢁ
C
30 ꢁC
90 ꢁ
C
SPAES-30
SPAES-40
SPAES-50
1.63
1.69
1.86
26.2
28.0
35.1
33.6
38.4
45.6
4.75
5.52
6.69
6.06
6.63
8.07
Ion exchange capacity
The ion exchange capacity (IEC) is an essential parameter for
the evaluation of a polymer electrolyte membrane as it inu-
ences the water uptake and proton conductivity. The IEC of the
polymer is directly dependent on the amount of sulfonic acid
substituted in the polymer. Generally the proton conductivity
and the water uptake of the nal membrane directly depend on
the amount of sulfonic acid groups and the IEC of the polymer.
The sulfonic acid groups not only make the polymer proton
conducting but also make it hydrophilic. The IEC values of the
SPAES copolymers are given in Table 2.
As the proportion of azo monomer with an ionic group in the
SPAES membrane is increased, the IEC value also increases. It
can be seen that the order increased in the following manner:
SPAES-30 < SPAES-40 < SPAES-50. The IEC value of the
membranes clearly shows that the sulfonated monomers were
well incorporated into the copolymers.
Fig. 4 The TGA thermograms of the SPAES copolymers.
Water uptake and swelling ratio
The water uptake capacity and swelling behaviour are very
important parameters in PEMFC applications because the proton
conductivity and hydrolytic stability of polymer electrolyte
membranes are strongly related to the presence of water. A
polymer electrolyte membrane with a good water management
ability would be anticipated for fuel cell applications. In this
work, the water uptake capacity and swelling ratio of the
ꢁ
membranes were analyzed at temperatures between 30 C and
90 ꢁC. The inuence of the IEC on the water uptake of the SPAES
membranes is depicted in the ESI Fig. S3.† Here, the water uptake
follows the same trend as the IEC and the values are given in
Table 2. At 90 ꢁC, the SPAES copolymers showed a moderate water
uptake and swelling. This may be attributed to the construction
of a large and continuous ion linkage in the sulfonated polymers.
The water uptake increased with the sulfonate content due to the
strong hydrophilicity of the sulfonic acid group. Also the
bisphenol structure had a remarkable inuence on the water
uptake and swelling ratio of the membranes.
Fig. 5 The Arrhenius plots of proton conductivity for the SPAES
membranes.
Thermal and hydrolytic stability
the sulfonate salt. The FT-IR spectrum of the azo based SPAES-
50 copolymers is shown in Fig. 2. The peaks at 1585 cmꢀ1 and The thermal stability of the SPAES membranes was determined
1488 cmꢀ1 are attributed to the vibrations of the aromatic ring using TGA measurements and the thermograms are shown in
skeleton. The symmetric stretching band of the sulfone group Fig. 4. All of the membranes showed a three step degradation
appeared at 1153 cmꢀ1 and 1012 cmꢀ1. The peak at 1242 cmꢀ1 pattern. The rst step of mass loss (5%) up to 200 ꢁC in the
proves the aromatic ether linkage of the SPAES. The aromatic SPAES copolymers is due to the loss of physically and chemically
C–N stretching vibration is observed at around 1325 cmꢀ1
Proton NMR analysis was performed for the azo based ionic range of 200–460 C which corresponds to the splitting of the
diol and the polymer (SPAES-50) using DMSO-d6 as the solvent. aromatic sulfonyl groups as well as the degradation of the azo
.
bound water. The second step of degradation was found in the
ꢁ
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Table 3 The IEC and proton conductivity values of sulfonated polymers which have ionic groups as side chains
IEC
Proton conductivity
(S cmꢀ1
Membrane type
(meq. gꢀ1
)
)
Reference
Cardo poly(aryl ether sulfone)s with pendant –OH groups
SPES containing hexabenzocoronene
SPES containing DHTPEa
Sulfonated poly(phenylene oxide)
Concentrated SPES
1.41
1.61
1.68
1.62
1.51
1.86
0.06 (20 ꢁC)
0.11 (80 ꢁC)
0.10 (80 ꢁC)
0.07 (25 ꢁC)
0.06 (20 ꢁC)
0.08 (30 ꢁC)
52
53
54
55
56
SPAES having azo based ionic diols
This work
a
DHTPE – (4,4-(2,2-diphenylethenylidene)bisphenol).
which facilitate the transport of protons.47 The SPAES
membrane has a high proton conductivity because the sulfonic
acid groups have covalent bonding (S–O) and intermolecular
hydrogen bonding. The principle of proton conduction is
demonstrated by the Grotthuss mechanism, when the protons
are transferred by forming and breaking hydrogen bonds with
proton accepting media such as water or sulphonic acid.48
Several researchers have reported on different kinds of proton
conducting materials as an alternative to the Naon®
membranes.49 The water uptake capacity of the sulfonated
polymers have a profound effect on the proton conductivity and
mechanical properties.50 For the conductivity experiments, all
of the membranes were initially immersed in deionized water
for 24 h at 25 ꢁC to hydrate. The proton conductivity of the
membranes based on the monomer which have more sulfonic
acid content possess a higher conductivity. The conductivities
of the SPAES membranes with increasing temperature from
Fig. 6 The electrochemical single cell performance of the SPAES-50
membrane at 60 ꢁC and 100% RH.
ꢁ
ꢁ
30 C to 130 C were determined and the values were found to
markedly increased in the order of SPAES-30 < SPAES-40 <
SPAES-50. The increase in the proton conductivity of the
membranes is due to more water retention and the increase in
the formation of the ionic clusters.
ꢁ
groups. The third weight loss which occurred at about 460 C
was due to the decomposition of the polymer backbone.45,46
SPAES-50 shows a higher stability than SPAES-30 and SPAES-40.
In general the azo polymers are found to exhibit a better
thermal stability, the rigidity being enhanced by the presence of
azo groups. These membranes showed slight differences in the
thermal stability according to their mole ratio.
Fig. 5 shows an Arrhenius plots of the proton conductivity as
a function of temperature for the SPAES membranes and the Ea
values are reported in Table 1. The activation energies of the
membranes decreased in the order of SPAES-30 > SPAES-40 >
SPAES-50. The increase in the percentage water uptake of the
SPAES membranes lead to the lowering of their activation
energy as consequence of the increase in proton conductivity. A
similar trend of the increasing proton conductivity with the
increase in ionic content was reported by Pei et al.51 Membranes
with ionic groups in their side chain exhibit an excellent proton
conductivity. Table 3 describes the compilation of some of
the reported values of sulfonated polymers with side chain
ionic groups.
The azo based SPAES membranes had reasonably good
physical properties. It is very important for a polymer electrolyte
membrane to have good mechanical properties to withstand
high temperatures, the high pressure fabrication process for the
membrane electrode assembly and the extreme conditions of a
fuel cell. The mechanical properties of the dry SPAES
membranes are summarised in Table 1. All membranes exhibit
tensile strength in the range from 33.6 to 43.3 MPa and
modulus from 402 to 513 MPa which are exible for the usage of
polymer electrolyte membranes. An increase in the sulfonic acid
content in the polymer matrix resulted in the reduction of the
tensile strength. From the mechanical testing, it can be
concluded that the side chain substituted SPAES membranes
are exible for fuel cell applications.
It can be seen from the above compilation that the IEC and
proton conductivity of the azo based SPAES membranes are
good and can be considered for PEMFC applications.
Electrochemical study
The SPAES-30 and SPAES-50 membranes were taken for MEA
studies. Fig. 6 shows the cell polarization of MEA assembled
Proton conductivity
For a good proton conductivity solid electrolyte membranes with the synthesised SPAES membranes as the electrolyte at
should have xed charged sites surrounded by water molecules 60 ꢁC and 100% RH. The best performance under these
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conditions was obtained for the SPAES-50 membrane, which
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.
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57
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Acknowledgements
The authors are thankful to Dr S. Pitchumani, CSIR-CECRI for 28 R. Kumar, M. Mamlouk and K. Scott, RSC Adv., 2014, 4,
the proton conductivity experiments and Dr Sengupta, CSIR- 617.
NCL for the GPC experiments. The authors acknowledge the 29 V. S. Rangasamy, S. Thayumanasundaram, N. D. Greef,
funding from CSIR cross cluster project ZERIS (WP-13 and WP-
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