“polymer electrolyte membranes for fuel cell...

“Polymer Electrolyte Membranes for Fuel Cell Applications”

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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


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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


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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


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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.


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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.


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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).


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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.


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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.


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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.


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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).


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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.


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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.


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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

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.


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