Nanostructured N-p-carboxy benzyl chitosan-silica-PVA ...shodhganga.inflibnet.ac.in/bitstream/10603/8406/10/10_chapter 6.pdf · 6.1.2. Results and discussion for nanostructured N-p-carboxy

Chapter VI

Nanostructured N-p-carboxy benzyl chitosan-silica-PVA

hybrid PEMs†

† This chapter has been published as: J. Phys. Chem B. 112 (2008) 15678-15690 and highlighted in nature India doi:10.1038/nindia.2008.343; Published online 28 December 2008

Chapter VI: Nanostructured N-p-carboxy benzyl chitosan-silica-PVA hybrid PEMs

~ 184 ~

6.1. Functionalized organic-inorganic nanostructured N-p-carboxy benzyl chitosan-silica-PVA hybrid polyelectrolyte complex as proton exchange membrane for DMFC applications

Today, there is growing interest in developing highly conductive with low methanol

crossover and low cost alternatives as proton exchange membranes (PEMs) for direct

methanol fuel cells (DMFCs) in order to reduce Ohmic losses and enhance their efficiencies

during operation [1-4]. Up to now, perfluorosulfonic acid polymers such as Nafion have

been the reference membrane for DMFC because of their high electrochemical properties as

well as excellent chemical resistance [2-6]. However, there is much interest in alternative

polyelectrolyte membranes because of Nafion’s reduced performance above 80 °C, high

methanol crossover, and cost [6-11]. Fluorine free materials with properties comparable to

those of Nafion are one of the directions in the development of cheaper polyelectrolytes.

PEMs based on sulfonated aromatic polymers, irradiation graft polymers, cross-linked

polymers, and blend polymers were successfully proposed [12-16]. However, to achieve

acceptable conductivities, a high degree of functionalization is required, which enhanced the

swelling of polymer due to hydration [7,17,18]. Hydrophilic material such as chitosan (N-

deacetylated derivative of chitin) based composite membrane is widely used as a PEM due

to its good chemical and thermal stabilities, nontoxicity, and biodegradability [19-23].

Furthermore, the presence of hydroxyl and amino groups on the backbone of chitosan also

affords higher membrane hydrophilicity, which benefited the fuel cell operation [23].

The conductivity of chitosan film was improved by doping the proton conductors.

Problems with these types of composite materials were associated with either excessive

swelling of the organic part due to its functionalization or leaching out of proton carriers on

prolonged use at elevated temperature [5,18,24,25]. To this date, no report is available in

which chitosan was modified by grafting less acidic/swellable aromatic -COOH groups,

while strong acidic -SO3H groups were attached with chemically bound inorganic segments

for achieving a high degree of functionalization to enhance the proton conductivity. The

presence of water in the membrane is a prerequisite for reaching high proton conductivity,

but at the same time, it affects the performance of the membranes, such as dimensional and

thermal stability [26-28]. Another consideration is methanol crossover, which is a key issue

in practical uses for DMFCs. Thus, high proton conductivity, water retention capacity, less


~ 185 ~

methanol permeability, and stable chitosan based PEMs can be achieve by introducing -

COOH groups and hydrophobic aromatic rings and developing modified chitosan based

cross-linked organic-inorganic nanostructured membranes.

Organic-inorganic nanostructured composites constitute an emerging research field,

which has opened the possibility of tailoring new materials because they combine in a single

solid both the attractive properties of a mechanically and thermally stable inorganic

backbone and the specific chemical reactivity and flexibility of the organo functional groups

[16,18,24,29-32]. Reports are available for diversified applications of chitosan inorganic

hybrid nanostructured material [33-38]. Blended PEM of chitosan with zeolite was also

reported [39], but mechanical stability and leaching out are serious problems for their

prolonged use. It was expected that grafting of aromatic ring and less acidic -COOH groups

on chitosan moiety will balance its hydrophilic-hydrophobic nature and enhance proton

conducting properties. Furthermore, attachment of strong acidic group (-SO3H)

functionalized inorganic segment was expected to show high proton conducting properties

and stabilities. In this chapter, we are reporting nanostructured N-p-carboxy benzyl chitosan-

silica-poly(vinyl alcohol) (PVA) hybrid PEMs for DMFC applications. These membranes

were prepared by the sol-gel method in aqueous media and designed to consist of two types

of proton conducting groups (strong and weak).

6.1.1. Preparation of nanostructured N-p-carboxy benzyl chitosan-silica-PVA hybrid

PEMs

Deacetylated chitosan (100% deacetylated), 4-carboxybenzaldehyde,

mercaptopropylmethyldimethoxysilane (MPDMS), tetraethoxysilane (TEOS), and

Nafion117 (perfluorinated membrane) were purchased from Sigma Aldrich Chemicals and

used as received. Poly(vinyl alcohol) (PVA; MW, 125 000; degree of polymerization, 1700;

degree of hydrolysis, 88%), methanol, acetic acid, sodium borohydride, formaldehyde,

hydrogen peroxide, etc., of AR grade were obtained from SD fine chemicals India, and used

without any further purification. All other chemicals used were of analytical reagent grade

from commercial sources. In all experiments, double distilled water was used.


~ 186 ~

6.1.1.1. Synthesis of N-p-carboxy benzyl chitosan

The method of synthesis of N-p-carboxy benzyl chitosan (NCBC) is based on the

Michael condensation reaction and is shown in Fig. 6.1.1. In a typical synthetic procedure,

0.7 wt% chitosan (high viscosity grade) was allowed to dissolve in aq. acetic acid solution at

pH 3. After complete dissolution, 3 equivalent of 4-carboxy benzaldehyde was added under

constant stirring at ambient temperature for 2-3 h. After the formation of imine (Schiff’s

base) by the condensation of aldehyde and amine functionality, the pH was slowly increased

up to 4.5 by adding aq. alkali solution. Then, aqueous solution of NaBH4 (5% w/v) was

added in a drop wise manner up to the complete reduction of imine linkage, assured by

disappearance of evolved gases. Thus, the obtained mixture was stirred for 1 h and finally

precipitated in alcohol. The precipitate was washed by ethyl alcohol and deionized water to

remove the remaining sodium borohydride. The quantitative analysis of free -NH2 groups

present in pristine chitosan and synthesized NCBC was carried out by the potentiometric

titration method [40].

6.1.1.2. Membrane preparation

The NCBC-sulfonated silica-PVA hybrid membranes were prepared in two steps

using the sol-gel method followed by oxidation of the thiol group into the sulfonic acid

group. A different wt% of N-modified chitosan (NCBC) was dissolved in 50 mL of hot

deionized water in the presence of 0.1 M HCl to obtain a highly viscous solution by constant

stirring, and PVA (10 wt%) solution was prepared in deionized water at 70 °C under

constant stirring, separately. Both of the solutions were mixed and stirred at room

temperature overnight to obtain a clear sol. To obtain sol, a predetermined amount of TEOS

was added and further stirred until uniform mixing and then different amounts of MPDMS

O

NH2

OH

O

N

OH

HC COOH

O

NH

OH

H2C COOH

n

1. Aq. CH3COOH

2. 4-Carboxy benzaldehyde

n n

1. H+

2. NaBH4/C2H

5OH

Chitosan Schiff's base N-p-carboxy benzyl chitosan

Fig. 6.1.1. Reaction scheme for synthesis of N-p-carboxy benzyl chitosan.


~ 187 ~

were added in order to obtain different weight percents of membrane with respect to PVA.

In all membranes, the composition ratio of modified chitosan and MPDMS was set as 1:2.

The solution was stirred for 12 h, and the alkoxy groups of silica were hydrolyzed at pH 2

by adding 1 M HCl that produced a highly viscous gel. The resulting gel was transformed

into thin film and dried at ambient temperature for 24 h followed by drying at 60 °C for

another 24 h. These membranes were cross-linked for different times (1, 2, and 3 h) with

formal solution (HCHO + H2SO4) at 60 °C.

The prepared membranes were further treated with 30 % H2O2 at 60 °C for 1 h to

oxidize the thiol group to a sulfonic acid group [41], after which they were rinsed with

distilled water and stored under wet conditions. Thus obtained final membranes were

designated as PCS-X-Y, where X is the weight percentage of NCBC + sulfonated silica (X

is 1, 2 and 3 where, 1 = 50%, 2 = 70% and 3 = 90%) and Y is the time of cross-linking (Y =

1, 2 and 3 h). Thus total nine membranes were prepared on the basis of composition and

cross-linking time.

6.1.2. Results and discussion for nanostructured N-p-carboxy benzyl chitosan-silica-PVA hybrid PEMs 6.1.2.1. Synthesis of NCBC and membrane preparation

NCBC was synthesized by co-condensation of the amine and aldehyde group via

Schiff’s base formation using chitosan and 4-carboxy benzaldehyde as starting materials.

The reduction of Schiff’s

base with alcoholic sodium

borohydride leads to the

desired product. Fig. 6.1.1

shows the reaction route. A

hydrogen atom from the

amino group was

substituted by a 4-carboxy

benzyl group. 1H NMR

spectra of NCBC was

recorded and shown in Fig. 6.1.2.The shift at 2.1-2.2 ppm was assigned to the ring proton

bonded with the -NH- group, and the signals at 3.0 to 5.0 ppm arose due to pyranose ring

ppm (f1)0.01.02.03.04.05.06.07.08.0

Fig. 6.1.2. 1H NMR spectrum of synthesized NCBC. ppm


~ 188 ~

protons and the -CH2- group. Moreover, the chemical shift at 4.5 and 4.8 ppm was assigned

to -N-CH2-. The shift at around 7.4-8.0 ppm was due to the aromatic protons, and a small

shift at 8.2 ppm was assigned to the aromatic carboxylic group. This demonstrates that N-

benzyl carboxyl substitution was successfully achieved. Pristine chitosan also showed the

shifts for various protons at their respective positions.

The percentages of -NH2 group in unmodified chitosan and NCBC were estimated

by their potentiometric studies and found to be 80.5 and 16.1%, respectively. The data

indicated that more than 64% of chitosan was converted into NCBC and the free -NH2

terminals were blocked.

Membrane forming material was

prepared by condensation polymerization of the

NCBC and silica precursor (MPDMS) by the

acid-catalyzed sol-gel method in aqueous

media, and a desired amount of PVA was

added to enhance the film forming capability.

The resulting dried thin films were cross-linked

with formal solution (HCHO + H2SO4). The

mercapto group of cross-linked composite

membranes was oxidized to a sulfonic acid

group using H2O2 under heated conditions. Fig.

6.1.3 shows the possible reaction route for

preparation of hybrid membranes.

The organic-inorganic composite up to molecular level was prepared by the sol-gel

method, in which both segments, inorganic and organic, were joined by covalent or

hydrogen bonding. In the acid/base-catalyzed sol-gel process, the silica precursor and water

form a one-phase solution that goes through a solution-to-gel and forms a rigid two-phase

system comprised of solid silica (SiO2) and solvent-filled pores. We observed that the nature

of the catalyst affects the properties of the resulting membranes. Both types of catalyzed

reactions were bimolecular nucleophilic substitution reactions. However, the acid-catalyzed

mechanisms were preceded by rapid protonation of the –OR or –OH substituents bonded

directly to the silicon atom, whereas under basic conditions hydroxyl or silanolate anions

OH

Si OC2H5

CH3C2H5O

SH

+O

NH

OH

H2C COOH

+n n

1. Sol-gel2. Crosslinking3. Oxidation

(H2C)3

O O O OSi

OSi

(CH2)3

O

NH

O

H2CCOOH

O

NH

O

O

H2C COOH

HO3S HO3S

Fig. 6.1.3 Schematic reaction route and structure of NCBC-Silica-PVA based PEM.


~ 189 ~

attacked the silicon atom directly. With time, sufficient numbers of interconnected Si-O-Si

bonds are formed in a region; they interact cooperatively to form colloidal particles or a sol,

and further, colloidal particles link together to form a three dimensional network or a gel

[30,42]. Acid catalysis forms linear polymers, which are weakly cross-linked due to steric

crowding, and resulted in the more flexible thin film, while base catalysis forms more and

highly branched clusters due to more rapid hydrolysis and resulted in a brittle thin film.

Thus, in this investigation, sol-gel reaction was catalyzed by acid for achieving highly

flexible polyelectrolyte membranes. Two types of functional groups were present in the

membrane phase, namely, the sulfonic acid group (low pKa), bonded with inorganic silica

and the carboxylic acid group (high pKa), attached with the organic chitosan.

FTIR spectra of representative membranes were recorded and presented in Fig. 6.1.4.

All three membranes show a strong -SO3H stretch at ~1070 (sym. SO3 stretch), ~1250

(asym. SO3 stretch), ~2360-1650 (due to the -OH stretching vibration) cm-1 indicating the

presence of a sulfonic acid group [43]. The absorption band at 1650-1700 cm-1 arose due to

carbonyl stretching, while the weak absorption band at ~1450 cm-1 corresponded to O-H

stretching of the -COOH group [44]. In the spectra, the peaks present in the range 1020-

1100 cm-1 indicated the presence of Si-O-Si and Si-O-C groups in the membrane, and due to

their close absorption characteristics, they are partially from both Si-O-Si and Si-O-C groups

Fig. 6.1.4 ATR FTIR of representative membranes.


~ 190 ~

[45,46]. It is clear that Si-O-Si groups are the results of condensation between hydrolyzed

silanol (Si-OH) groups and the Si-O-C groups may originate from the condensation reaction

between Si-OH groups from hydrolyzed MPDMS and TEOS and C-OH groups from PVA

and NCBC. Under acidic conditions, PVA reacted with a cross-linking agent and silanol

groups and formed the C-O-C (~1200 cm-1) and Si-O-C groups. The formation of Si-O-C

and C-O-C groups will be in favor of better compatibility between organic and inorganic

components, and a better homogeneity of inorganic and organic matrix at molecular scale

leads to highly thermally and mechanically stable membrane.

6.1.2.2. Thermal strength and dynamic mechanical behavior

Parts A and B of Fig. 6.1.5 represent the TGA curves for the different membranes

with constant and varied degree of cross-linking, respectively. All thermograms showed

three-step weight loss character, comprised of water loss (loose and bound) from the

membrane phase and decarboxylation [47] (step I), desulfonation (step II), and membrane

matrix degradation (step III). Membrane samples with varying silica content retained more

than 94% weight up to a temperature of about 250 °C. The initial weight loss occurred

basically due to the bound water loss, and the weight loss in the 180-250 °C region is

assigned to the decarboxylation of NCBC. Water content of the membranes is increased

with the increase in hydrophilic functional groups or presence of inorganics in the

membrane matrix.

Fig. 6.1.5. TGA curves of different PEMs for (A) 3 h cross-linking time and (B) with varied cross-linking time.

150 300 450 600 75020

40

60

80

100

A

PCS-3-3

PCS-2-3PCS-1-3

Wei

ght l

oss (

%)

Temperature (οC)150 300 450 600 750

B

1 = PCS-2-12 = PCS-2-23 = PCS-2-3

321


~ 191 ~

The rate of decarboxylation and water loss (70-200 °C) from the membrane

decreased with the increase in NCBC-silica content and thus hydrophilic functional groups.

This may be attributed to the ionic cross-linking and H-bonding between sulfonic acid and

carboxylic acid groups and weak esterification with silanol groups. For step I, the trend of

weight loss follows the order PCS-1-3 > PCS-2-3 > PCS-3-3, which clearly indicated a high

water retention capacity and slower decarboxylation rate with an increase in functional

groups and silica. Beyond 250 °C, decomposition of membrane samples was rapid up to 380

°C, due to thiol and sulfonic acid group degradation [48]. PCS-3-3 membrane (high -SO3H

concentration) exhibiting a slow decomposition rate may be due to enhanced cluster

formation of silica. The delayed decomposition of the -SO3H group with an increase in silica

content may be due to the presence of the -SH group because of incomplete oxidation [49].

In the last step, samples show steep decomposition up to 500 °C. In this region, a very

profound effect of silica content was observed on weight loss. The PCS-1-3 retained 25%,

while PCS-2-3 and PCS-3-3 retained 35 and 38% of the initial weight, respectively. The

effect of cross-linking time on thermal degradation is presented in Fig. 6.1.5 (B). It is clear

that cross-linking improves the thermal stability and enhances the cross-linking density.

Comparatively, membrane PCS-3-3 showed the highest thermal stability. On the basis of

these observations, we can conclude functionalization at the inorganic part (silica) improved

the thermal stabilities of membranes and enhanced cross-linking density, for better

stabilities.

Parts A and B of Fig. 6.1.6 show the DSC thermograms in the N2 environment of

representative hybrid membranes with varied silica content and cross-linking time,

respectively. The first endothermic peaks (Fig. 6.1.6 (A)) corresponding to the Tg values

were found to be around 82.65, 85.95, and 63.34 °C, respectively, for different membranes.

Incorporation of NCBC and silica in the PVA matrix had a profound effect on its Tg value.

The Tg value for PCS-1-1 and PCS-2-2 was increased with silica content, while it was

reduced for PCS-3-2. The Tg value of pristine PVA was found to be 78 °C [26,50]; these

variations in the glass transition temperature may be explained by the plasticizing effect and

degree of cross-linking.

Furthermore, hydrated membranes showed low Tg values, which may be one of the

reasons for the lower Tg value of the PCS-3-2 membrane [51,52]. Fig. 6.1.6 (B) presents the


~ 192 ~

effect of cross-linking time on the first and second Tg values. First Tg values were decreased

with the cross-linking time, while second Tg values increased initially and then decreased.

This indicated that the ordered arrangement of the PVA molecules was altered by the

amorphous NCBC, sulfonated silica, and chemical cross-linking.

Fig. 6.1.7 shows the

effect of cross-linking time and

silica content on the dynamic

mechanical properties of hybrid

membranes. All tested samples

showed elongation and were

found to reduce with the

increase in cross-linking time.

In PVA, the hydroxyl groups

contributed to the stiffness of

linear polymeric membrane by

hydrogen bonding. When the

number of hydroxyl groups decreased either by branching or cross-linking, the hydrogen

bonding was attenuated and thus the chain stiffness reduced [53]. Fig. 6.1.7 (B) reflects the

effect of cross-linking on the elongation behavior of membranes at a constant amount of

NCBC-silica content. At higher cross-linking time, the hydroxyl group of the chitosan

moiety may also get cross-linked, and result in a three-dimensional network by inter- or

Fig. 6.1.7. Dynamic mechanical property of membranes: (A) effect of cross-linking time; (B) effect of silica content.

Fig. 6.1.6. DSC thermograms for different PEMs: (A) with varied silica content; (B) with varied cross-linking time.

100 200 300 400

3

21

1= PC S-1-2(T g1/T g2= 82.65/260.41 oC )2= PC S-2-2(T g1/T g2= 85.95/261.44 oC )3= PC S-3-2(T g1/T g2= 63.34/254.84 oC )

En

do

E

xo

T em perature (oC )

50 100 150 200 250 300

3

2

1

1= P C S-2-1(T g1/T g2= 93.87 /256.60 oC )2= P C S-2-2(T

g1/T

g2= 85.95 /261.44 oC )

3= P C S-2-3(Tg1

/Tg2

= 85.88 /250.15 oC )


~ 193 ~

intramolecular cross-linking. As the concentration of additive was increased, the elongation

was decreased, but after 70% NCBC-silica content, it was found to increase. This effect may

appear due to the better miscibility of NCBC and silica with the PVA up to only a definite

concentration and indicating that there is some miscibility between all components [54].

Cross-linking density was determined by DMA analysis according to equation (2.1.6) and

data included in Table 6.1.1.

Cross-linking density

increased with the cross-linking

time and decreased with the

NCBC-silica content in the

membrane matrix. The

variations between the storage

modulus and cross-linking

density of the hybrid

membranes with NCBC-silica

content are presented in Fig.

6.1.8. Initially, the storage

modulus increased with the

NCBC-silica content, and

beyond 70% (w/w), it reduced, while the cross-linking density was decreased with NCBC-

silica content in the membrane phase. These variations may be explained by the formation of

cohesive domains due to reaction between NCBC and silica, which was more predominant

than the formation of cross-linking with plasticizer. Therefore, with the excess NCBC-silica

content, low cross-linking density and elastic modulus were observed. Thus, it is necessary

to optimize NCBC-silica content, cross-linking density, and storage modulus for achieving

better, stable hybrid membranes.

6.1.2.3. Microscopic characterizations

Fig. 6.1.9 presents SEM (surface and cross section), TEM, and SEM-EDX

measurements of dry PCS-2-2 hybrid membrane (as a representative case). Fig. 6.1.9 (A and

B) shows aggregates on the membrane surface because of accelerated hydrolysis of silane

Fig. 6.1.8. Effect of NCBC-silica content on storage modulus and the cross-linking density of PEMs (1, 2, and 3 denote the cross-linking time).


~ 194 ~

and its cross-linking advantage of preparing hybrid membrane by the sol-gel method is

uniform homogeneous distribution. These results suggest uniform hybrid membrane with

nanosized silica and sulfonic acid clusters in the membrane phase.

6.1.2.4. Oxidative and hydrolytic stabilities

During use of PEM in fuel cells, membrane stabilities and durability are very

important. Formation of H2O2, •OH, and •OOH radicals during its decomposition is believed

to attack on hydrogen containing bonds in polyelectrolyte membranes. Losses in weight,

IEC, and proton conductivity of the prepared membranes were tested in Fenton’s reagent

(Fe2+-H2O2) for 1 h at 80 °C, and results are included in Table 6.1.1. Peroxy radical attack is

more aggressive at higher temperatures and occurs in the proximity of hydrophilic domains

[55]. Weight loss decreased with increase in NCBC-silica content in the membrane phase.

Silica blocks the hydrogen containing hydrophilic pores because it forms the cross-linked

structure in these. The lifetime of •OH or •OOH radicals is very short [56], and cannot

penetrate inside the siloxane containing domains. With increasing NCBC-silica content,

Fig. 6.1.9. Surface morphology: (A) SEM (surface view); (B) SEM (cross section); (C) SEM EDX; (D) weight% of different components; (E) TEM for PCS-2-2 hybrid membrane.


~ 195 ~

membranes were highly cross-linked and compact, thus showing lower weight loss. With the

increase in NCBC-silica content in the membrane phase, loss in IEC also increases due to

weight loss contribution. As a reference, loss in membrane conductivity was in between 1

and 3% for hybrid membranes. The highly crosslinked structure may also contribute to the

excellent oxidative stabilities of these membranes. Generally, perfluorinated membranes

exhibit better chemical stability in comparison to nonfluorinated membranes. The

membranes were also subjected to accelerated hydrolytic stability testing at 140 °C and

100% RH for 24 h. All of the membranes maintained their transparency, flexibility, and

toughness. The membranes were less hydrolytically stable in comparison to oxidative

conditions. Hydrolytic stability results in terms of weight loss, losses in IEC, and membrane

conductivity are also included in Table 6.1.1. It is evident that, with the increase in NCBC-

silica content, the loss in weight, IEC, and proton conductivity values was gradually

increased. This reveals that, under hydrolytic conditions, the hydrophilic siloxane domains

were mainly affected.

Table 6.1.1. Cross-linking density (ρ), oxidative and hydrolytic loss in weight, ion-exchange capacity (IEC), and conductivity (κm) for developed PEMs.

Membranes Cross-linking density (ρ, mol/g)

Oxidative stability (Loss %)

Hydrolytic stability (Loss %)

Weight IEC κm Weight IEC κm PCS-1-1 0.35 8.93 4.32 2.26 12.42 6.18 1.30 PCS-1-2 0.36 6.42 3.85 2.14 10.85 5.94 1.22 PCS-1-3 0.38 6.20 3.60 2.10 10.64 5.82 1.20 PCS-2-1 0.34 7.36 6.48 2.16 15.34 9.33 3.14 PCS-2-2 0.34 5.24 6.12 1.98 14.62 9.12 3.00 PCS-2-3 0.36 5.15 5.89 1.85 14.18 8.95 3.00 PCS-3-1 0.30 4.43 8.96 1.74 18.25 10.72 3.96 PCS-3-2 0.31 3.32 8.54 1.55 17.54 10.61 3.82 PCS-3-3 0.33 3.25 8.22 1.52 16.94 10.54 3.88

6.1.2.5. Solvent uptake, retention ability, and swelling properties

The presence of water in the membrane phase is a prerequisite for high membrane

conductivity. On the other hand, a high volume fraction of water in the membrane phase

reduces dimensional and thermal stabilities as well as proton concentration, and thus

enhances membrane resistance. Table 6.1.2 presents solvent uptake values (φw and φw+MeOH,


~ 196 ~

measured in water and water-methanol mixtures) for hybrid membranes. For prepared

membranes, water uptake values (φw) were larger than those obtained in water-methanol

mixtures (φw+MeOH), which indicated less extent of wetting with water-methanol mixture.

Further, solvent uptake decreased with cross-linking density, which may be due to the more

compact structure and loss of -OH groups of PVA.

Also, with the increase in NCBC-silica content, solvent uptake decreased for both

cases even after an increase in cross-linking density. It seems the cross-linking density of

prepared membranes on swelling is more dominant. Membranes with a high carboxyl group

concentration favored the formation of intermolecular hydrogen bonding which reduced the

water bonding sites. The absorption of water expanded the hydrophilic domains, and stresses

the hydrophobic domains. The stress induced by water absorption enhanced the water

uptake. Thus, intermolecular interaction opposed the stress that arose due to water

absorption and thus reduced the swelling of the membrane. The total number of water

molecules per ionic site (λw) and volume expansion values of the membranes in water or

water-methanol mixture (φw and φw+MeOH, respectively) are also presented in Table 6.1.2, as

their swelling properties. The swelling profile in both media followed a similar trend as that

observed in the case of water uptake. This supports our observations made on the basis of

water uptake studies.

Table 6.1.2. Solvent uptake (ϕw and ϕw+MeoH), swelling properties (φH2O and φH2O+MeOH), number of water molecules per ionic sites (λw) and water diffusion coefficient (D) values for different PEMsa.

Membrane ϕw (%) ϕw+MeoH (%) λw φH2O φH2O+MeOH D/10-6 (cm2/s)

PCS-1-1 38.46 34.86 15 88.5 72.6 1.26 PCS-1-2 37.65 33.21 14 66.4 56.3 1.37 PCS-1-3 37.42 30.89 14 47.6 39.5 1.43 PCS-2-1 32.12 33.03 11 41.1 36.7 1.41 PCS-2-2 31.63 30.37 11 37.6 27.3 1.59 PCS-2-3 29.45 28.73 10 28.6 19.3 1.64 PCS-3-1 35.51 17.11 12 38.4 29.8 1.71 PCS-3-2 29.76 16.69 10 37.2 24.6 1.89 PCS-3-3 26.32 16.36 9 25.7 23.4 1.96

aφH2O = Volume expansion in water, φH2O+MeOH = Volume expansion in water-methanol solution. D/10-6 (cm2/s) is the diffusion coefficient of water.

Water vapor sorption and diffusion properties of polyelectrolyte membranes are very

important for their application in fuel cells and have a significant effect on their


~ 197 ~

conductivity. The water retention capability of developed membranes was illustrated by Fig.

6.1.10 (A) ((Mt/M0)-t (time) curves), where M0 is the initial amount of water in the

membrane, Mt is the amount of water remaining in the membrane at any given time, and k is

the constant. The value of k was derived from the (Mt/M0)-t1/2 curves presented in Fig. 6.1.10

(B) based on Higuchi’s model [57] for water desorption kinetics. The obtained straight lines

for varying NCBC-silica content and constant cross-linking time were fitted to Higuchi’s

model and suggested that water desorption followed a diffusion control mechanism. With

the increase in NCBC-silica content, the rate of water desorption was decreased. Thus, both

(NCBC and silica) acted as a binder for water in the membrane matrix due to inter- and

intramolecular interaction. The free water in the ionic (-COOH and -SO3H groups)

membrane matrix is less mobile and indicated that water was more bound in the hydrophilic

domains of the membrane and less apt for its dehydration. The cross-linking of the

membrane has a profound effect on the water desorption property, and with the increase in

cross-linking density, retention of water was increased. The cross-linking of polymer

increases the hydrophobic part and it resists the release of water due to the so-called

hydrophobic effect. The diffusion coefficient of water was evaluated from a best-fit

normalized mass change and also presented in Table 6.1.2. Water diffusion coefficient

values (D) were increased with NCBC-silica content and cross-linking density. Thus,

Fig. 6.1.10. Water desorption profile of membranes with 2 h cross-linking time and varied silica content: (A) desorption behavior; (B) Higuchi’s model fit of deswelling behavior.


~ 198 ~

NCBC-silica content and cross-linking density both acted as a barrier for water release and

improved water retention capacity even at higher temperature.

6.1.2.6. State of water

The performance of a membrane in fuel cell applications is highly dependent on the

nature of water present in the membrane phase. Low temperature DSC studies and water

uptake values were used to quantify and elucidate the different types of water in the

polymeric membranes. The DSC melting thermograms of all prepared membranes were

recorded in the temperature range -50 to +50 °C. Fig. 6.1.11 shows a broad endothermic

peak with two major melting peaks.

The types of water depend on its association with the polar or ionic groups and its

confinement in hydrophilic domains. The water present in the membrane phase may be

classified into three types [26,58]: (i) free water (with the same temperature and enthalpy of

melting as bulk water), (ii) freezing bound water (weakly bound with polar or ionic groups

of polymer and shows a change in temperature and enthalpy in comparison with bulk water

and can be detected by DSC), and (iii) non-freezing water (very strong interaction with polar

or ionic groups and shows no phase transition). The enthalpy of melting (ΔHm), melting

temperature (Tm), and the full width at half-maximum of the melting peak (ΔTm) for all

composite membranes were determined by DSC curves and presented in Table 6.1.3 in

comparison with Nafion117 membrane. ΔTm values for composite membranes were low in

comparison with Nafion117 membrane due to their lower water content. Comparatively

-20 -15 -10 -5 0 5 10

Endo

Exo

Temperature (oC)

PCS-1-1 PCS-1-2 PCS-1-3

-20 -10 0 10

Temperature (oC)

PCS-2-1 PCS-2-2 PCS-2-3

-20 -10 0 10

Temperature (oC)

PCS-3-1 PCS-3-2 PCS-3-3

Fig. 6.1.11. DSC heating thermograms of PEMs indicating melting of water in fully hydrated state.


~ 199 ~

higher Tm values may be attributed to low freezing and high bound water percentage of the

composite membranes, which was increased with the NCBC-silica content in the membrane

matrix. The effect of cross-linking density is also reflected from the ΔHm, Tm, and ΔTm

values reported in Table 6.1.3 for different membranes. An increase in ΔHm with the cross-

linking density indicated the tight confinement of water in hydrophilic domains because of

the increase in the hydrophobic part.

The number of free water molecules per ionic site (λf) to the wet membrane was

obtained from the total melting enthalpy by integration of the peak area of the melting

curves presented in Fig. 6.1.11. The number of bound water molecules per ionic sites (λb)

was obtained from subtraction of the number of freezing water molecules from the total

number of water molecules per ionic site. The degree of bound water in percentage (χ = λb /

λw) was estimated from the ratio of the number of bound water molecules per ionic site to

the number of total water molecules per ionic site. Estimated values of λf, λb, and χ for

different membranes are also included in Table 6.1.3.

Table 6.1.3. State of water: Enthalpy of melting (ΔHm), melting temperature (Tm), full width at half-maximum of the melting peak (ΔTm), number of free water molecule per ionic sites (λf), number of bound water molecules per ionic sites (λb), degree of bound water (χ).

Membranes ΔHm (J/g) Tma (ºC) ΔTm

b (oC) λf λb χc (%)

PCS-1-1 13.4137 0.60 9.0 1.55 13.45 89.67 PCS-1-2 12.6083 0.58 3.6 1.46 12.54 89.57 PCS-1-3 11.5535 0.57 5.3 1.34 12.66 90.43 PCS-2-1 29.4297 0.32 4.1 2.98 8.02 72.91 PCS-2-2 27.3794 1.01 4.7 2.78 8.22 74.73 PCS-2-3 22.4911 0.41 6.7 2.28 7.72 77.20 PCS-3-1 36.2337 0.94 6.0 3.48 8.52 71.0 PCS-3-2 33.8887 0.62 6.0 3.27 6.73 67.3 PCS-3-3 27.2893 0.23 5.0 2.63 6.37 70.78

Nafion117 ----- -2.8 8.8 9.80 14.5 59.70 a Melting temperature of free and loosely bound water. b Full width at half-maximum of the melting peak. c Bound water degree χ (%)= λb / λw.

The free water molecules in the membrane increased, while the bound water

decreased with the increase in NCBC-silica content because of the more available ionic sites

for binding. Cross-linking of polymer increases the hydrophobic part and decreases the ionic

groups, which led to a high melting temperature of water because of the amorphous


~ 200 ~

structure. The water molecules reside in between the polymer chains and siloxanes formed

by silica in spite of the strong interaction between the ionic groups and water molecules,

resulting in high bound water content. The bound water degree of all developed membranes

was in good agreement with the Nafion117 membrane.

6.1.2.7. IEC studies

The ion-exchange capacity indicates the density of ionizable hydrophilic functional

groups, which are responsible for the proton transport. The IEC values of prepared

polyelectrolyte and Nafion117 membranes are tabulated in Table 6.1.4. IEC values for

prepared membranes ranged from 1.442 mequiv. g-1 for PCS-1-1 to 1.734 mequiv. g-1 for

PCS-3-1, while Nafion117 showed a lower (0.90 mequiv. g-1) value. IEC arises due to the

presence of -SO3H and -COOH groups on inorganic and organic parts, respectively, and thus

depends on the NCBC-silica content in the membrane matrix. IEC values were unaffected

by cross-linking density, and thus, cross-linking did not affect the functional groups in the

membrane matrix. Also, the water uptake data along with IEC values were used with the

advantage for the estimation of fixed charge concentration (Φ) using the equation (5.2.2) [1].

Φ values for different membranes are also presented in Table 6.1.4. An increase in IEC and

a decrease in water content cause a concomitant increase in fixed charge concentration. On

the other hand, fixed charge concentration was increased with cross-linking density because

of increasing compactness of the matrix and allows low water to reside.

6.1.2.8. Proton conductivity and H+ mobility

Organic-inorganic hybrid PEMs containing acidic functional groups (-SO3H and -

COOH) dissociated due to hydration and allowing transport of hydrated proton (H3O+). The

proton conductivity was measured at 30 °C for hydrated membranes, and relevant data are

presented in Table 6.1.4. It was noticed that, with the increase in NCBC-silica content, the

proton conductivity was increased because of high functional group concentration.

Polyelectrolyte membranes with high IEC and Φ showed a higher conductivity value due to

the more fixed charge carrier. It was also observed that, due to the more acidic nature of –

SO3H groups, membrane conductivity was enhanced with its concentration more dominantly

rather than less acidic –COOH groups. Proton conductivity was decreased with the increase

in cross-linking density, which hindered the proton transport process because of the


~ 201 ~

enhanced compact nature of the membrane. This observation was verified from the values of

λf and κm presented in Tables 6.1.3 and 6.1.4, respectively. Thus, proton conductivity was

highly dependent on freezing water in the membrane. PCS-3-1 membrane showed the

highest proton conductivity (5.31×10-2 S cm-1) because of the higher -SO3H concentration

and low cross-linking density among the prepared polyelectrolyte membranes. Furthermore,

relatively comparable κm values of PCS-3-1 to Nafion117 membrane revealed its suitability

for fuel cell applications.

Table 6.1.4. Ion-exchange capacity (IEC), fixed charge concentration (Φ) mobility of proton ( +Hμ ) proton conductivity (κm), energy of activation (Ea), and selectivity parameter values (SP) for different nanocomposite PEMs.

Membrane IEC (mequiv. g-1)

Φ (mmol cm-3)

μH+/10-4

(cm2 s-1 V-1) κm /10-2 (S cm-1)

Ea (kJ mol-1)

SP/105 (S cm-3 s)

PCS-1-1 1.442 1.134 1.955 2.14 3.93 0.5 PCS-1-2 1.442 1.272 1.669 2.05 4.39 0.68 PCS-1-3 1.442 1.305 1.524 1.92 4.28 0.66 PCS-2-1 1.645 1.324 2.676 3.42 6.14 0.57 PCS-2-2 1.645 1.437 2.429 3.37 6.18 0.88 PCS-2-3 1.645 1.581 2.09 3.19 5.87 0.91 PCS-3-1 1.734 1.469 3.744 5.31 7.43 0.73 PCS-3-2 1.734 1.582 3.443 5.26 9.15 1.09 PCS-3-3 1.734 1.657 3.095 4.95 8.09 1.23

Nafion117 0.90 1.1 0.87 9.56 6.14 0.72

The effect of methanol on proton

conductivity was also evaluated by measuring

κm values in equilibration with water-methanol

mixtures of different composition, similar to

the DMFC environment, and relevant data are

presented in Fig. 6.1.12. It was observed that

an increase in methanol composition slightly

reduced the proton conductivity of

membranes. This observation can be

explained on the basis of the enhanced

resistance of the water-methanol mixture. In

the methanol environment, protons were highly solvated and, due to the bigger size of the

1.5

3.5

5.5

7.5

15% 25% 35% 45% 55%

Methanol conc. (%)

κm/1

0-2 (S

cm-1

)

1

2

3

4

Fig. 6.1.12. Effect of methanol concentration on proton conductivity (κm) of PEMs: 1, PCS-1-2; 2, PCS-2-2; 3, PCS-3-2; 4, Nafion117.


~ 202 ~

methylated proton, it experienced more resistance in the transport process. In comparison

with tested polyelectrolyte membranes, Nafion117 showed a higher conductivity but the

rates of decrease in conductivity for both were the same.

The proton conductivity of the polyelectrolyte membrane is highly dependent on

their mobility and fixed charge concentration in the membrane matrix. Proton conductivity

data was used with the advantage of estimation of the mobility of the proton ( +Hμ ) by

equation (5.2.3) [1]. The calculated mobility of proton ranges from 1.955×10-4 to 3.095×10-4

cm2 s-1 V-1 presented in Table 6.1.4. Estimated +Hμ values for different polyelectrolyte

membranes are also presented in Table 6.1.4. The proton conductivity was increased with

the increase in proton mobility due to enhanced hydration and free water volumes in the

proton conducting channels and clusters of the membranes. +Hμ was dependent on IEC and

wϕ values and increased with the increase in NCBC-silica content and reduced along with

the cross-linking density. The high value of free water decreases the path and tortuous

resistance for proton within the membrane. The increasing cross-linking increases the

tortuousity of the membrane and also decreases the hydrophilic groups which ultimately

decreases the mobility of proton.

Fig. 6.1.13 shows Arrhenius plots

for prepared polyelectrolyte and

Nafion117 membranes. All membranes

exhibited positive temperature-

conductivity dependencies, which

suggested a thermally activated

conduction process in the experimental

temperature range from 30 to 70 °C and

further higher temperatures. The

apparent activation energy (Ea) for

proton conduction were determined from

the slope of the Arrhenius plot and

ranged between 3.93 and 9.15 kJ mol-1

(Table 6.1.4). Ea values increased significantly with the increase in NCBC-silica content in

-4

-3.5

-3

-2.5

-2

2.8 2.9 3 3.1 3.2 3.3 3.4

1000/T (K-1)

ln κm

(S c

m-1

)

Nafion117

PCS-3-2

PCS-2-2

PCS-1-2

Fig. 6.1.13. Arrhenious plots for nanocomposite and Nafion117 membranes.


~ 203 ~

the membrane phase. In all cases, 2 h cross-linked membranes with the same compositions

showed maximum Ea values. Ea values of the prepared membranes were slightly higher than

that for Nafion117 membrane. Thus, the thermal activated conduction process for prepared

composite membranes was higher in comparison with Nafion117 membrane. At higher

temperature, fast proton and water molecule diffusion resulted in a rapid conduction process,

due to the more continuous pathway because of interlinking of hydrophilic channels.

Furthermore, comparable Ea values of prepared and Nafion117 membranes indicated a

Grotthus-type conduction mechanism and the usefulness of prepared membranes for DMFC

applications.

6.1.2.9. Selectivity parameters for DMFC applications

Fig. 6.1.14 (A) shows the methanol permeability profile for all prepared membranes

at 30 and 50% (v/v) methanol concentration in water-methanol mixture, similar to DMFC

applications. Prepared membranes showed extremely low methanol transmission (2.0-

10.0×10-7 cm2 s-1), while Nafion117 exhibited 13.10×10-7 cm2 s-1 for 30% (v/v) methanol

concentration. The mass transport behavior of hydrated membrane depends on its degree of

swelling, water uptake, and microstructure. The effect of cross-linking and NCBC-silica

content is clearly seen from Fig. 6.1.14 (A). With the increase in NCBC-silica content in the

0

2

4

6

8

10

12

P/10

-7 (c

m2 s

-1)

PC

S-1-1

PC

S-1-2

PC

S-1-3

PC

S-2-1

PC

S-2-2

PC

S-2-3

PC

S-3-1

PC

S-3-2

PC

S-3-3

Membranes

30% methanol

50% methanol

1

5

9

13

65 75 85 95χ (%)

50% methanol

30% methanol

A B

Fig. 6.1.14. Methanol permeability for PEMs: (A) at varied NCBC-silica content and cross-linking time; (B) at varied bound water degree (χ).


~ 204 ~

membrane phase, the methanol permeability values were increased, while it was decreased

along with cross-linking density. It is well-known that incorporating silica particles into

polymer membranes can dramatically alter their transport properties, because of alteration in

free void volume and ionic clusters [59]. Thus, understanding the relationship between

polymer structure and membrane performance, in terms of permeability and selectivity,

enables tailoring of the membrane structure for a specific purpose. The permeation of

liquid/gas molecules through the polymer membrane occurs via the diffusion mechanism,

and the permeability of the penetrant (methanol) is the product of its solubility and

diffusivity. The penetrant diffusivity is dependent on the free void volume in the membrane,

the size of penetrant molecules, and the segmental mobility of the polymer chain. There are

two types of pores in the polymer membranes, that is, network pores or ionic clusters and

aggregate pores [60,61]. The ionic clusters were responsible for the enhanced proton

conduction at high silica content in the membrane matrix due to the functionalization on the

inorganic segment. The aggregate pores are the large cavities surrounding the polymer

aggregates, which were responsible for the mass transport (methanol in this case). It seems

that incorporation of silica led to an increase in free void volume and thus aggregate pores.

An earlier report also supports our observation [59]. Thus, with an increase in silica content,

methanol permeability and proton conductivity both enhanced significantly.

Fig. 6.1.14 (B) shows the influence of bound water degree on methanol permeability

for different polyelectrolyte membranes with constant cross-linking density. The

permeability reduced with the increase in χ, which indicated that there was a blocking effect

due to the high degree of bound water. From Tm and ΔTm, the membranes with a lower Tm

and a higher ΔTm have a stronger affinity to water and hence the mobile water surrounds the

hydrophilic and polarized polymer network. This strong interaction between the water and

polymer network reduces the free water in total water uptake and thus gradually decreases

the methanol permeability.

To directly compare the applicability of polyelectrolyte membranes for DMFC

application, the ratio of proton conductivity and methanol permeability (κm/P) data was used

as the selectivity parameter (SP). SP values for synthesized membranes along with

Nafion117 membrane are also presented in Table 6.1.4. Data clearly demonstrate that

synthesized membranes with high NCBC-silica content and cross-linking density (PCS-3-3)


~ 205 ~

exhibited the highest SP values (1.23×105 S s cm-3), while Nafion117 showed a 0.72×105 S s

cm-3 SP value. It was also noticed that, by increasing the operating temperature, SP values

for synthesized membranes were also increased, and it was about 2 times higher in

comparison to Nafion117 membrane. This observation may be attributed to the relatively

low methanol permeability of the prepared membranes despite their low conductivity. These

results can be explained on the basis of the lack of significant interactions between methanol

and ionic clusters (-SO3H and -COOH groups) introduced at the inorganic and organic

segment, respectively, of the cross-linked membranes. Ionized groups hydrate strongly and

excluded organic solvents (salting-out effect), which is an essential feature of the

polyelectrolyte membranes. Furthermore, higher SP values of these membranes indicate

great advantage for DMFC applications.

6.1.2.10. DMFC performances of PCS-3-3 membranes

The performance of PCS-3-3 membrane was tested by recording current-voltage

polarization curves in DMFCs under different experimental conditions with varying

methanol concentrations (20, 30, and 50% MeOH) in feed at 70 °C (Fig. 6.15 (A)). The

variations of power density with current density are also presented in Fig. 6.15 (B) under

similar experimental conditions. A membrane suitable for DMFC application should show

slow decay of cell voltage along with increase in current density, and also, it should exhibit

0.00 0.02 0.04 0.06 0.08 0.101

2

3

4

5

6

7

8

B

50% MeOH

30% MeOH

20% MeOH

Pow

er d

ensit

y (m

W c

m-2)

Current density (A cm-2)0.00 0.02 0.04 0.06 0.08 0.10

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

A

Cel

l vol

tage

(V)

Current density (A cm-2)

20% MeOH

30% MeOH

50% MeOH

Fig. 6.1.5. Current-voltage polarization curves for PCS-3-3 membrane with varying methanol concentration at 70 oC in air mode.


~ 206 ~

high power density at a given current density. Membrane conductivity increased with

NCBC-silica content, while it was reduced by an increase in cross-linking density. It seems

fixed charge concentration in the membrane matrix played an important role for observed

polarization characteristics of the synthesized membranes. At low methanol concentration,

low methanol crossover increased the open circuit voltage (OCV) and significantly

improved the performance. In this study, all polyelectrolyte membranes showed almost

comparable OCV (0.35 V). The current density for PCS-3-3 at the different methanol

concentrations was measured at about 38, 35, and 26 mA cm-2. The approximately

comparable performance of these membranes to the Nafion117 [24] reveals their suitability

for DMFC application.

6.1.3. Conclusions for nanostructured N-p-carboxy benzyl chitosan-silica-PVA hybrid

PEMs

We report hydrophilically modified chitosan and a preparation procedure of

nanostructured organic-inorganic hybrid PEMs with -SO3H and -COOH proton conductive

groups for DMFC applications, by the sol-gel method. Novel modified chitosan (NCBC)-

silica-PVA nanocomposite membranes with varied NCBC-silica content and cross-linking

density were prepared, in which highly acidic -SO3H groups were grafted on the inorganic

segment (less swellable) while less acidic –COOH groups were grafted on the organic

segment (high swellable) for achieving highly proton conductive and stable polyelectrolyte

membranes. Cross-linking density was determined by DMA studies, and it was observed

that, for high cross-linking density and elastic modulus, optimization of NCBC-silica

content in the membrane matrix is necessary. Physicochemical and electrochemical

properties of these membranes were dependent on the NCBC-silica content in the membrane

matrix as well as the cross-linking density. Developed membranes showed good stabilities,

flexibility, and water retention capacities. It was observed that the methanol permeability

and conductivity at elevated temperature was governed by bound water concentration.

Observed proton conductivities (1.92-5.31×10-2 S cm-1) of NCBC-silica-PVA cross-linked

membrane at ambient temperature dramatically increased at high temperature, in comparison

to Nafion117 membrane, due to the activated thermal conduction process. This is a great

advantage for these nanocomposite membranes to target high temperature applications.


~ 207 ~

Comparable activation energy for proton transport, current-voltage polarization

characteristics, and relatively lower methanol permeability of these membranes compared

with Nafion117 make them applicable for DMFCs, which is also reflected from about 2

times higher SP values. Also, the results showed that, by controlling NCBC-silica content

and cross-linking density, it is possible to tailor the desired architectures of hydrophobic and

hydrophilic pathways. Furthermore, chitosan is a low cost material and utilization of

modified chitosan-silica nanocomposite is novel and challenging, as it is inexpensive,

nonhazardous, and environmentally benign. Besides its superior ion conducting properties

and water retention capability, this composite material may have the potential for application

in bio-electrochemical devices, including implantable batteries and biosensors.

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