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“Polymer Electrolyte Membranes for Fuel Cell Applications”
53
2.1. Aim and objective of the present investigation
Fuel cells have been developed since 19th
century but their first use was found in the
exploration of space. After that, their development has gone through several
modifications and activities. However, from the past two decades their development
has gain significant and continuous efforts around the world to discover new materials
and fuel cell systems. These efforts demand energy efficient systems, reduced
emission of polluting gases such as CO2 and the need of high energy density system
for portable applications such as mobile phones, laptops, iPod, and digital camera.
Polymer electrolyte membrane (PEM) fuel cell systems are fulfilling these demands as
they are portable electricity producing devices for transport and portable uses and
composed of highly efficient and pollution free setup.113, 114
But, the main problem in
modern fuel cell systems is to discover novel membranes other than perfluorinated
one, such as Nafion®
(DuPont). Perfluorinated membranes show good proton
conductivity and physical and chemical stability at ≤ 80°C; but they deteriorate at
≥110 °C113
. Also, high gas permeability, high cost of production and fluorination
processes are some of the serious drawbacks of perfluorinated membranes based fuel
cells.
In spite of its (Nafion) commercial use in the present fuel cell systems, it has several
demerits against the efficient PEM in fuel cell system. To develop efficient fuel cell
membranes, various research groups are doing research in developing alternative
membranes.115
Non-fluorinated moieties with ionic content116
, acid containing
polymers117
, organic/inorganic blends118
, solid acid with super-protonic phase
transition119
, and acid/base ionic liquids120
are some of the categories in which the
present day researchers are concentrating. Moreover, processing of the materials is
“Polymer Electrolyte Membranes for Fuel Cell Applications”
54
also a very important factor in constructing membranes as a polyelectrolyte. Because
of these facts, researchers are also concentrating on synthesis of polymer membranes
based on polybenzimidazoles, polystyrenes, polysulfones, poly(ether ether ketones)s,
and polyimides. Among different polymers, every polymer is unique and has its own
advantages and disadvantages, but most of the polymers are not satisfactory to meet
the desired need of higher proton conductivity and membranes robustness under real
fuel cell operating environment. Hence, various research groups have proposed
several materials other than fluorinated membranes for PEMs, but out of them,
polyimides (PIs) have acknowledged considerable attention due to their suitable
physical and chemical properties in fuel cell environment. Entirely aromatic backbone
polyimides are high performance engineering materials which are getting wide
acceptance by different types of industries as they have a number of better features.
These features include excellent physical properties, retention at higher temperature
and in wet conditions, almost constant electrical properties over a wide range of
temperatures, chemical resistibility, and non-flammability properties. According to
this, polyimides with suitable ion-conducting moiety are better candidates for the
construction of fuel cell membranes.
As far as various better properties of polyimides are concerned, they have high proton
conductivity, high mechanical strength, low swelling and very less fuel crossover, and
higher thermal and oxidative stability. For high proton conductivity of the membranes,
a high ion exchange capacity (IEC) is needed, but high sulfonation which in turn
higher ion exchange capacity leads to more and more swelling of the membranes.
Moreover, the absorption of more water molecules will bring bad polymer chain
relaxation, which lead to a considerable loss in the mechanical strength. To reduce
these difficulties, cross-linking methods have been devised. But the common covalent
“Polymer Electrolyte Membranes for Fuel Cell Applications”
55
cross-linked membranes generally brittle in dry state and on the other hand ionic
crosslinking loses its function at elevated temperature. Sulfonated polyimide (SPI)
membranes are better candidates as PEMs and used in FCs, which was developed by a
number of research groups.
In recent time, sulfonated polyimides (SPIs) membranes have been considered to
show good physical strength as well as higher proton conductivity at higher
temperature. To increase fuel cell functioning, such as enhanced carbon monoxide
(CO) resistance of catalyst at the anode and fast O2 reduction reaction at the cathode,
and higher heat recovery efficiency, it is needed to operate PEMFCs at higher
temperatures (>80 °C). The main hurdle which prohibits the practical use of
sulfonated polyimide SPI is the water stability of their membranes, which is correlated
to its mechanical stability in highly swelled state and its robustness in fuel cell system
conditions. The aromatic imide linkage is prone to hydrolyse under high water content
and at higher temperature. Due to this de-polymerization of the polymer backbone, as
a result considerable decrease in the mechanical strength of SPIs membranes
occurred.
Keeping the above points in mind, the main objective of the present investigation is to
synthesize cross-linked sulfonated polyimide (CSPI) membrane. Flexible aromatic
triamines were used as cross-linkers to improve their hydrolytic as well as mechanical
stabilities. Moreover, we have also tried to impart the flexibility in the polyimide main
chain by incorporating the novel non-sulfonated diamine with ether linkage which in
turn provides space for the accumulation of the water molecules in main chains, which
can give better proton conductivity at fuel cell operating temperatures as well as
“Polymer Electrolyte Membranes for Fuel Cell Applications”
56
improved solubility and flexibility of the membranes. A brief account of the present
investigation is as follows:
2.2. Synthesis and Characterization of Cross-linked Sulfonated Polyimide
Membranes through Novel Stilbene Containing Triamine Cross-linker
First stilbene based novel triamine monomer was prepared in the following steps and
used as a cross linker in the synthesis of cross-linked sulfonated polyimides (SPIs) and
their membranes and compared with linear sulfonated polyimide (cf. Scheme 2.4).
Triamine cross-linker synthesis has the following steps.
2.2.1. Step I: Synthesis of 4-hydroxy-4’-nitrostilbene
The general method employed for the preparation of stilbene compound is the
condensation reaction of p-nitrophenylacetic acid and p-hydroxybenzaldehyde in the
presence of piperidine as shown in Scheme 2.1.
COOH
NO2
+
OH
CHO
HO
NO2
p-Nitrophenylacetic acid
p-Hydroxybenzaldehyde
4-Hydroxy-4'-nitrostilbene
Piperidine
140 °C, 1h
Scheme 2.1. Synthesis of 4-hydroxy-4’-nitrostilbene.
“Polymer Electrolyte Membranes for Fuel Cell Applications”
57
2.2.2. Step II: Synthesis of 2, 4-dinitro-1-(4-(4-nitrostyryl) phenoxy) benzene
After the synthesis of 4-hydroxy-4’-nitrostilbene compound, as given in Scheme 2.1,
the trinitro compound (2,4-dinitro-1-(4-(4-nitrostyryl)phenoxy)benzene) was
synthesized by aromatic nucleophilic substitution reaction as given in Scheme 2.2.
NO2O2N
Cl
+ HO
NO2
O2N
O
NO2
1-chloro-2,4-dinitrobenzene
4-Hydroxy-4'-nitrostilbene
2,4-Dinitro-1-(4-(4-nitrostyryl)phenoxy)benzene
O2N
Dry acetone,
K2CO3,
18-C-6,
24 h, RT
Scheme 2.2. Synthesis of 2, 4-dinitro-1-(4-(4-nitrostyryl)phenoxy)benzene.
O2N
O
NO2
2,4-Dinitro-1-(4-(4-nitrostyryl)phenoxy)benzene
O2N
H2N
O
NH2
H2N
4-{4-[2-(4-Amino-phenyl)-vinyl]-phenoxy}-benzene-1,3-diamine
SnCl2.2H2O/HCl, EA
Reflux, N2, 3h
Scheme 2.3. Synthesis of 4-(4-(2-(4-aminophenyl)vinyl)phenoxy)benzene-1, 3-
diamine.
“Polymer Electrolyte Membranes for Fuel Cell Applications”
58
2.2.3. Step III: Synthesis of 4-(4-(2-(4-aminophenyl)vinyl)phenoxybenzene-1, 3-
diamine
The above synthesized trinitro compound (2, 4-dinitro-1-(4-(4-nitrostyryl)phenoxy)
benzene) as given in Scheme 2.3 was reduced. Reduction of trinitro compound was
carried out by using SnCl2.2H2O and concentrated hydrochloric acid in ethyl acetate
as a solvent in N2 atmosphere.
2.2.4. Synthesis of linear sulfonated polyimide
Before using the novel triamine as a cross-linker, synthesis of linear sulfonated
copolyimides were carried out by a classical two steps thermal condensation method
of 1, 4, 5, 8-naphthalenetetracarboxylic dianhydride (NTCDA) and sulfonated
diamine, 2,2’-benzidine-disulfonic acid (BDSA) with non-sulfonated diamine, 2-
bis(4-(4-aminophenoxy)phenyl)hexafluoropropane (HFBAPP) using benzoic acid as a
catalyst. Schematic representation is shown in Scheme 2.4.
OO
O
OO
O
H2N NH2
SO3H
HO3S
O C O NH2
CF3
CF3
H2N+ +
NTCDA BDSA HFBAPP
m-Cresol,
TEA,
Benzoic acid
80 °C 4h,
180 °C 20h
SO3H
HO3S
O C O
CF3
CF3
NN
O
OO
O
NN
O
OO
O n
Sulfonated polyimide (SPI)
Scheme 2.4. Synthesis and chemical structure of linear sulfonated polyimide (SPI).
“Polymer Electrolyte Membranes for Fuel Cell Applications”
59
2.2.5. Synthesis of cross-linked sulfonated polyimides membranes
Synthesis of cross-linked polyimides were carried out by in-situ cross-linking by using
1, 4, 5, 8-naphthalenetetracarboxylic dianhydride (NTCDA) and sulfonated diamine,
2, 2’-benzidine-disulfonic acid (BDSA) with non-sulfonated diamine, 2-bis(4-(4-
aminophenoxy)phenyl)hexafluoro propane (HFBAPP) and triamine cross-linker
(APVPDA) by using benzoic acid as a catalyst. This is schematically is shown in
Scheme 2.5.
OO
O
OO
O
H2N NH2
SO3H
HO3S
O C O NH2
CF3
CF3
H2N+ +
NTCDA BDSA HFBAPP
m-Cresol,
TEA,
Benzoic acid,
80 °C 4h,
180 °C 20h
Triamine cross-linker
Polyimide main chain
H2N NH2
O
NH2
=
Cross-linked sulfonated polyimide
H2N
H2N ONH2
Scheme 2.5. Synthesis of cross-linked sulfonated polyimide using triamine cross-
linker.
“Polymer Electrolyte Membranes for Fuel Cell Applications”
60
2.3. Synthesis and Characterization of Cross-linked Sulfonated Polyimide
Membranes through Novel Oxy-dibenzene Containing Triamine Cross-linker
Oxy-dibenzene containing cross-linker has been synthesized in two steps and then it
was used to synthesize cross-linked sulfonated polyimides and compared with linear
sulfonated polyimide (cf. Scheme 2.4).
2.3.1. Step I: Synthesis of 2, 4-dinitro-1-(4-nitrophenoxy) benzene
It was prepared by aromatic nucleophilic substitution reaction on activated aryl halide
in the presence of K2CO3 and dry acetone as a solvent as shown in Scheme 2.6.
O2N NO2
Cl
2,4-Dinitrochlorobenzene
OH
NO2
Dry acetone,
K2CO3
N2, RT,
24h
OO2N
O2N
NO2
4-Hydroxynitrobenzene
2,4-Dinitro-1-(4-nitrophenoxy)benzene
(I)
+
Scheme 2.6. Synthesis of 2, 4-dinitro-1-(4-nitrophenoxy)benzene.
“Polymer Electrolyte Membranes for Fuel Cell Applications”
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2.3.2. Step II: Synthesis of 4-(4-aminophenoxy)benzene-1, 3-diamine
The trinitro compound which was prepared through scheme 2.6 was reduced and
reduction was carried out in the presence of anhydrous SnCl2 and concentrated
hydrochloric acid as shown in Scheme 2.7.
SnCl2/HCl, EtOH
Reflux, 4h
O
NO2O2N
NO2
2,4-Dinitro-1-(4-nitrophenoxy)benzene
O
NH2H2N
NH2
4-(4-Aminophenoxy)benzene-1,3-diamine
(II)
Scheme 2.7. Synthesis of 4-(4-aminophenoxy)benzene-1, 3-diamine (cross-linker).
2.3.3. Synthesis of cross-linked sulfonated polyimide membranes
Synthesis of cross-linked sulfonated polyimides were carried out by in-situ cross-
linking using 1, 4, 5, 8-naphthalenetetracarboxylic dianhydride (NTCDA), sulfonated
diamine, 2, 2’-benzidine-disulfonic acid (BDSA), non-sulfonated diamine, 2-
bis(4(4aminophenoxy)phenyl)hexafluoropropane (HFBAPP) and triamine cross-linker
(APBDA) as shown in Scheme 2.8.
“Polymer Electrolyte Membranes for Fuel Cell Applications”
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OO
O
OO
O
H2N NH2
SO3H
HO3S
O C O NH2
CF3
CF3
H2N+ +
NTCDA BDSA HFBAPP
m-Cresol,
TEA,
Benzoic acid,80 °C 4h,
180 °C 20h
Triamine cross-linker
Polyimide main chainH2N NH2
O
NH2
Cross-linked sulfonated polyimide
H2N
H2N O NH2
=
Scheme 2.8. Synthesis of cross-linked sulfonated polyimide by using triamine cross-
linker.
2.4. Synthesis and Characterization of Sulfonated Polyimide Membranes through
Novel Stilbene Containing Diamine
Stilbene containing novel diamine has been prepared in the following steps and then it
was used in the synthesis of sulfonated flexible polyimides and compared with linear
sulfonated polyimide (cf. Scheme 2.4).
“Polymer Electrolyte Membranes for Fuel Cell Applications”
63
2.4.1. Step I: Synthesis of 4-(2-nitrophenoxy)-4’-nitrostilbene
It is prepared by aromatic nucleophilic substitution reaction on activated aryl halide as
shown in Scheme 2.9.
Cl
NO2
+ HO
NO2
K2CO3,Dry acetone,
18-C-6,
RT, 24h1-Chloro-2-nitrobenzene
4-Hydroxy-4'-nitrobenzene
O
4-(2-Nitrophenoxy)-4'-nitrostilbene
NO2
N2
NO2
Scheme 2.9. Synthesis of 4-(2-nitrophenoxy)-4’-nitrostilbene.
2.4.2. Step II: Synthesis of 4-(2-aminophenoxy)-4’-aminostilbene
Reduction of dinitro compound which was prepared through Scheme 2.9 was carried
out in the presence of SnCl2/HCl as given in Scheme 2.10.
“Polymer Electrolyte Membranes for Fuel Cell Applications”
64
O2N
O
O2N
SnCl2/HCl EtOH,
Reflux,
3h
H2N
O
H2N
4-(2-Nitrophenoxy)-4'-nitrostilbene
4-(2-Aminophenoxy)-4'-aminostilbene
Scheme 2.10. Synthesis of 4-(2-aminophenoxy)-4’-aminostilbene (APAS).
2.4.3. Synthesis of Sulfonated Polyimide Membranes by Using Novel Stilbene
Containing Diamine
Synthesis of linear sulfonated polyimide was carried out by using 1, 4, 5, 8-
Naphthalenetetracarboxylic dianhydride (NTCDA), sulfonated diamine 2, 2’-
benzidine-disulfonic acid (BDSA), non-sulfonated diamine 2-bis(4-(4-
aminophenoxy)phenyl)hexafluoro propane (HFBAPP) and novel diamine (APAS) (cf.
Scheme 2.10) using benzoic acid as catalyst. This is schematically shown in Scheme
2.11.
“Polymer Electrolyte Membranes for Fuel Cell Applications”
65
OO
O
OO
O
H2N NH2
SO3H
HO3S
O C O NH2
CF3
CF3
H2N+ +
NTCDA BDSA HFBAPP
m-Cresol,
TEA,
Benzoic acid
80 °C 4h,
180 °C 20h
O
NH2
NH2
APAS
+
SO3H
HO3S
NN
O
OO
O
O C OCF3
CF3ONN
O
OO
O n
Sulfonated linear polyimide
Scheme 2.11. Synthesis of linear Sulfonated Polyimide.
Hence, the aim is to prepare covalent cross-linked sulfonated polyimide membranes
with improved properties such as water or hydrolytic stability, thermal stability,
oxidative stability with comparable proton conductivity of the resulted sulfonated
polyimides membranes. But, covalently cross-linked membranes generally show
brittleness in the dry state. But in our present investigation we have used very low
mole ratio of the cross-linker without compromising flexibility of the membranes.
In addition, we have synthesized novel non-sulfonated diamine with flexible ether
linkage and stilbene moiety with improved flexibility of the polyimide membranes in
dry state as well as the solubility in common aprotic high boiling organic solvents.
“Polymer Electrolyte Membranes for Fuel Cell Applications”
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