93

CHAPTER 3

RESULTS AND DISCUSSION ON QPSEBS AND ITS

COMPOSITE MEMBRANES

This chapter elucidates in detail the results of the various

characterizations and experiments of the QPSEBS membrane, Ag/C, Pd-Ni/C

catalysts as well as the different composites of QPSEBS. This section also

includes the discussion of the obtained results.

3.1 QPSEBS

The anion exchange membrane (AEM) was prepared from poly

(styrene ethylene butylene) poly styrene [PSEBS]. AEMs can be prepared by

several routes: (a) polymer blended with alkali, (b) pyridinium base type

polymer, (c) radiation-grafting and quaternization of polymer, (d)

chloromethylation and quaternization of polymer. Among these, preparation

of anion exchange membrane by chloromethylation and quaternization route

is more advantageous and important because the membrane prepared by this

method has good physical stability and relatively high chemical stability. The

commonly used chloromethylating agents such as chloromethyl methyl ether

(CMME) and bis-chloromethyl ether (BCME) provide excellent conversions

and high yields. But they are now considered to be carcinogens and their use

has been restricted since the 1970s (Laskin et al 1971, Taylor and Laughlin

1975).

94

To avoid the use of such hazardous materials, in this work

paraformaldehyde and concentrated hydrochloric acid was used as

chloromethylating agent as reported by other investigators (Fang and Shen

2006, Xiong et al 2009). The three steps in the preparation of anion exchange

membrane include (i) chloromethylation, (ii) quaternization and (iii)

alkalization. Poly (styrene ethylene butylene poly) styrene was

chloromethylated by nHCHO, conc. HCl (chloromethylating agent) and ZnCl2

(catalyst) then quaternized by triethylamine. The degree of substitution

depends on many parameters such as reaction time, temperature,

concentration of chloromethylating agent and polymer concentration. After

optimization of these parameters, the PSEBS polymer was successfully

chloromethylated to an appropriate level. The optimized parameters are,

[PSEBS] = 28.5 mmol, [nHCHO] = 180 mmol, [Conc. HCl] = 540 mmol,

[ZnCl2] = 25.3 mmol, reaction time = 48 h, reaction temperature = 60 C and

degree of chloromethylation =1.47. The thicknesses of the membranes were

found to be from 85 to 90 µm as measured by micrometer (precision ± 3 µm).

3.1.1 Fourier Transform Infra Red

PSEBS, chloromethylated PSEBS (CMPSEBS) and quaternized

PSEBS (QPSEBS) samples were characterized by FTIR. The FTIR spectra

were obtained to confirm the chloromethylation and quaternization of the

polymer and the resulting spectra is shown in Figure 3.1. The following

conclusions were drawn from the obtained spectra.

95

Figure 3.1 FTIR spectra of (a) PSEBS, (b) CMPSEBS and (c) QPSEBS

Figure 3.1 (a) shows the IR spectrum of PSEBS. Appearance of

peak around 1638 cm-1

was assigned to the aromatic ring C=C. Appearance of

peak around 1458 cm-1

and 1366 cm-1

was assigned to the bending vibration

which is due to the presence of aromatic ring backbone –CH- bending

vibration. The peak at 2924 cm-1

is due to the stretching of C-H bond of

aromatic hydrocarbon. Appearance of peak around 699 cm-1

and 747 cm-1

was

assigned to the aromatic ring out of plane C-H bending vibration. Appearance

of these peaks confirmed the structure of PSEBS.

Due to chloromethylation reaction (Figure 3.1 (b)), the IR bands of

C=C in aromatic ring from styrene unit are shifted to a lower frequency in the

range of 1601 cm-1

. As well as the IR bands of the aromatic ring backbone

CH bending vibration, are shifted to a lower frequency in the range of 1404

96

cm-1

and 1247 cm-1

. This confirms that the reaction has occurred in the

aromatic ring. Appearance of peak at 1247 cm-1

shows that the CH2Cl group is

substituted at the phenyl ring.

Due to quaternization reaction, the IR bands of aromatic ring C=C

and aromatic ring backbone –CH- bending vibration are shifted to a lower

frequency around 1600 cm-1

and 1371 cm-1

. In this reaction, triethylamine is

used to form the quaternized product. IR bands for tertiary amine cannot be

seen in the spectrum. Appearance of peak around 1371 cm-1

was assigned to

C-N stretching vibration. Due to the quaternization (Figure 3.1 (c)), a small

intense peak at 2365 cm-1

has appeared, which is the characteristic absorption

peak of quaternary ammonium groups. From all these evidences, it is clear

that chloromethylation and quaternization reactions have been successfully

carried out in the PSEBS unit at phenyl rings.

3.1.2 1H-Nuclear Magnetic Resonance

Figure 3.2 shows the 1H-NMR spectra of PSEBS, CMPSEBS and

QPSEBS. By characterizing the polymer samples of PSEBS, CMPSEBS and

QPSEBS with proton nuclear magnetic resonance spectroscopy, the following

conclusions were drawn. The base polymer PSEBS contains 29% of

polystyrene and 71% of ethylene and butylene. Since the styrene content is

only 29%, it shows a very low intensity in NMR bands. The solvent peak

appears at = 7.28 ppm. The alkyl proton bands namely CH3 proton, CH2

proton and CH proton are appearing at 0.9ppm, 1.3ppm and 1.5ppm

respectively. The bands that appear in the region between 6-8.5ppm are due to

the aromatic ring protons. Appearance of these proton bands clearly confirms

the structure of the PSEBS.

97

Figure 3.2 1H-NMR spectra of (a) PSEBS, (b) CMPSEBS and (c)

QPSEBS

Due to chloromethylation reaction, a small singlet peak is observed

at 4.5 is assigned to the presence of –CH2Cl group in the phenyl ring.

Due to quaternization reaction, some structural changes takes place

in the reactant PSEBS. Appearance of a new triplet peak at 1.3 is assigned

to the CH3 protons from N-CH2-CH3 group. The new peak appears at 4.1

is assigned to the CH2-N protons due to the presence of the phenyl group. The

quartet band formed at 3.0 is assigned to the N-CH2-CH3 group. All these

confirm the structure of QPSEBS. In summary, the changes in NMR spectra

support the chemical modification of PSEBS in to an anion exchange

polymer.

98

3.1.3 Water Absorption, Ion Exchange Capacity and Stability Test

Water uptake of PSEBS, CMPSEBS, QPSEBS and alkalized

PSEBS (ALPSEBS) membranes are shown in Figure 3.3. Water absorption

capacity depends on the presence of number of ion exchange groups present

in the membrane. The alkalized PSEBS showed the highest water absorption

(5.74%), followed by the quaternized PSEBS (4.04%), chloromethylated

PSEBS (2.25%) and the base PSEBS (0.49%). This percentage variation may

be attributed to the nature of functional group that each polymer carries.

Apparently, ALPSEBS is the most polar polymer, while PSEBS is the least

polar. Although, QPSEBS contained Cl-, this anion is much less active than

the OH- on the alkalized polymer when interacting with water (Wang et al

2009). This water absorption indicates that the introduction of quaternary

ammonium group converts the hydrophobic PSEBS in to hydrophilic.

PSEBS CMPSEBS QPSEBS ALPSEBS

0

1

2

3

4

5

6

Wate

r ab

so

rpti

on

, %

Figure 3.3 Water uptake of PSEBS, CMPSEBS, QPSEBS and ALPSEBS

Experimental ion exchange capacity value (IEC) was calculated for

the QPSEBS by titration method. IEC can provide information on the density

of ionizable functional group in the membrane. IEC of the membrane depends

on the number of ionic channels present in the membrane, which are in turn

responsible for the hydroxyl conductivity of the membrane through hopping

99

mechanism. The ion exchange capacity of the prepared membrane was found

to be 0.958 meq/g.

Hydrolytic stability of the membrane was investigated by

introducing the membrane in to boiling water. It was found that the QPSEBS

withstood the boiling water condition without any physical deformation,

which means that the structure of polymer makes the QPSEBS membrane to

have good hydrolytic stability. Hence it is inferred that the membrane can be

safely used in the fuel cell at higher temperatures (80-100 °C).

Alkaline stability test involves acceleration of the degradation

process, thereby indicating the mechanical and chemical stability. The

QPSEBS ionic membrane was introduced in to 5M NaOH solution. The ionic

membrane does not have any physical deformation and the membrane was

stable for more than two days (48 h). The property of IEC was measured after

the hydrolytic and alkaline stability test, the IEC was not significantly

changed, which means that the prepared QPSEBS membrane has good

mechanical and chemical stability.

3.1.4 Thermogravimetric Analysis

0 100 200 300 400 500 600 700 800

0

20

40

60

80

100

Weig

ht

resid

ue

(%

)

Temperature (OC)

PSEBS

CMPSEBS

QPSEBS

Figure 3.4 Thermogravimetric curves of PSEBS, CMPSEBS and

QPSEBS

100

The thermal degradation of CMPSEBS and QPSEBS was studied

with thermo gravimetric analysis. Figure 3.4 shows the thermo gravimetric

curves of PSEBS, CMPSEBS and QPSEBS. The PSEBS polymer shows a

major decomposition in TGA curve from 416 °C to

459 °C which may be due to the polymer degradation. The chloromethylated

PSEBS also shows sharp single step decomposition at 425 °C. This is due to

the main chain decomposition. The decrease in thermal stability of

CMPSEBS is due to the introduction of polar group in to the polymer chain,

whereas, the QPSEBS shows a two step degradation. The first weight loss

between 70 and 190 °C is due to the removal of quaternary ammonium groups

from the polymer and the second weight loss observed at temperature higher

than 460 °C, is due to the complete degradation of the polymer chain. The

curves demonstrate that the QPSEBS has moderate thermal stability and

higher degradation temperature. This suggests that the QPSEBS based anion

exchange membrane is suitable to work in alkaline fuel cell conditions.

3.1.5 Differential Scanning Calorimetry

50 100 150 200 250 300

Exo

(c)

(b)

(a)

mW

(mg

)

Temperature (OC)

Figure 3.5 DSC curves of (a) PSEBS, (b) CMPSEBS and (c) QPSEBS

101

The thermal behavior of the PSEBS, CMPSEBS and QPSEBS were

studied using DSC. The Tg value for PSEBS (Figure 3.5 (a)) was found to be

78 °C, whereas for CMPSEBS (Figure 3.5 (b)) it was 124 °C. The

remarkable increase in the Tg value could be attributed to the electronegative

nature of the –CH2Cl group. On the other hand, for QPSEBS (Figure 3.5 (c)),

due to the bulkiness of the quaternized group, a lowering in the Tg value (86

°C) was observed. Hence a moderate Tg value obtained for QPSEBS is an

evidence for the adequate thermal property of QPSEBS for AMFC

application. From Figure 3.5 (c) it may be inferred that the crystallization

temperature of QPSEBS was 100 °C and the melting temperature was

observed to be 210 °C.

3.1.6 Dynamic Mechanical Analysis

Figure 3.6 Storage modulus vs. temperature at 1 Hz for (a) PSEBS and

(b) QPSEBS

Figure 3.6 shows the temperature dependence of the storage

modulus E’ of the PSEBS (Figure 3.6a) and QPSEBS (Figure 3.6b)

membrane. The ionomer (QPSEBS) displays typical behavior of an

102

amorphous polymer. In the glassy state, the storage modulus E’ remains

roughly constant. The Tg value of PSEBS and QPSEBS were found to be 77

°C and 86 °C respectively which are close to each other. In the temperature

versus storage modulus plot the two curves belonging to PSEBS and QPSEBS

were found to be close to each other. This is an indication that the Tg values of

the two polymeric materials were close. The storage modulus is an indication

of the rigidity of the material. Upon increasing the temperature from 30 to 200

°C there is a decrease in the storage modulus. The storage modulus was found

to be around 5 MPa when the temperature reaches 105 C for PSEBS and

about 3 MPa for QPSEBS at 120 °C. Around 45 °C, the modulus drop is

associated with the glass-rubber transition of the ionomers. This modulus

corresponds to energy dissipation displayed in a relaxation process (Stoica et

al 2007).

Figure 3.7 Tan delta vs. temperature at 1 Hz for (a) PSEBS and (b)

QPSEBS

The properties of the polymers change with temperature. In

particular, it has been found that the coefficients of thermal expansion of

amorphous polymers undergo abrupt changes in the region of the glass

transition. The temperature at this abrupt change is defined as glass transition

103

temperature, Tg. From the Figure 3.7, it is observed that QPSEBS (Figure

3.7b) membrane shows an inflection point at 86 oC, where the internal friction

(tan ) curve goes through a maximum. This value corresponds to the glass

transition region of the QPSEBS membrane. The Tg values obtained from

DMA is more accurate. The Tg values for PSEBS (Figure 3.7a) and QPSEBS

from DMA were found to be 82 oC and 86

oC and were in good agreement

with the DSC values.

3.1.7 Scanning Electron Microscopy

Figure 3.8 SEM images of (a) QPSEBS and (b) ALPSEBS

The surface morphology of the QPSEBS and ALPSEBS membrane

was studied by scanning electron microscopy. Figure 3.8 (a) and (b) shows the

surface morphology of the QPSEBS and ALPSEBS membrane. Both the SEM

photographs confirms the uniformity of the membrane and also show rough

fractured surfaces, which provide some information regarding membrane’s

mechanical properties (Xiong et al 2008).

(a)

104

3.1.8 Ionic conductivity

On introducing the quaternary ammonium groups in to the polymer

chain, the hydroxyl ion conducting ability was introduced. To examine the

conductivity, the quaternized membrane was first soaked in KOH solution for

24 h to convert in to OH- form and then it was washed several times with

deionized water. After the free KOH was completely removed, the

conductivity of the membrane was measured using electrochemical

impedance spectroscopy (EIS). The anion exchange membrane (QPSEBS)

showed conductivity with the value of 1.51 × 10-2

S/cm. Hence, this anion

exchange membrane can be potentially used for alkaline fuel cell applications.

3.1.9 Methanol Permeability and Selectivity Ratio

Methanol permeability of QPSEBS membrane was found to be 2.14

x 10-6

cm2/s in 2M methanol solution whereas the commercially available

anion exchange membrane (AMI-7001) exhibits 2.25 x 10-6

cm2/s. But,

Nafion, a cation exchange membrane possesses 2.32 x 10-6

cm2/s (Cho et al

2004) in a solution mixture of methanol and deionized water (5/95 (w/w)).

This reveals that the methanol permeability of the QPSEBS and AMI-7001

membrane was slightly lower than that of Nafion membrane. Also the

concentration of methanol will play an important role during the permeability

experiments. Hence the obtained permeability results of QPSEBS and AMI-

7001 may be negligible at the lower concentration of methanol (5/95 (w/w)).

So, it is probably that the PSEBS matrix has less methanol affinity compared

with perflourinated hydrocarbon polymer and the non-ionic blocks can act as

a barrier to methanol.

In DMAMFC applications, the ratio of ionic conductivity to

methanol permeability (selectivity ratio) is a characteristic parameter to

evaluate the fuel cell performances of membranes. In general, membranes

with higher ionic conductivity and lower methanol permeability are more

105

suitable for DMAMFCs. The selectivity ratio of QPSEBS and AMI-7001

were found to be 0.71 x 104 Ss/cm

3 and 0.76 x 10

4 Ss/cm

3 respectively.

3.2 CARBON SUPPORTED CATALYSTS

The most important criterion for the ion exchange membranes was

to evaluate the performance in real scale fuel cells. For this, membrane

electrode assembly (MEA) was fabricated using three different catalysts

namely Pt/C, Ag/C and Pd-Ni/C for the cathode side while Pt/C as a constant

anode material for the above three cathode materials. Catalysts Ag/C and Pd-

Ni/C were prepared by wet impregnation and reduction methods respectively,

while Pt/C was commercially procured. The prepared catalysts were

subjected into several characterization and the results are described in this

section.

Noble metals such as Pt, Pd, Ru and Au supported over carbon

materials have been reported to be efficient catalysts for both the PEMFC and

DMFC applications. However, the cost and methanol cross-over factors

hinder the development of such products into the market. To overcome these

factors, researchers look forward to non-noble metal catalysts like Ag, Ni and

Co etc. The recent developments in fuel cells have emphasized the use of

silver (Ag) and nickel (Ni) as a principal and necessary component to

fabricate MEA as a cathode material.

To increase the electrochemically active surface area, catalysts

supported on high surface area materials, commonly carbon based materials

are widely used in low temperature fuel cells. Recent studies have revealed

that the physical properties of the carbon based materials can greatly affect

the electrochemical properties of the fuel cell catalyst. It has been reported

that carbon materials with both high surface area and good crystallinity can

not only provide a high dispersion of metal particles, but also facilitate

electron transfer, resulting in better fuel cell performance. Supported-metal

106

catalysts are widely used in the energy-related and fine-chemical industries.

In addition, compared with other supports, carbon materials have considerable

advantages, such as (1) high specific surface area up to 3000 m2/g, (2) high

stability in acidic and basic media (Auer et al 1998), (3) easy modification of

textural properties and functional groups, and (4) easy recovery of supported

metals by burning off the catalyst. Moreover, the varieties of available forms

(e.g. graphite, carbon black (CB), activated carbon (AC), activated carbon

fibers (ACF), carbon nanotubes (CNTs), and carbon molecular sieves (CMS))

make carbon materials very attractive as catalysts or supports for metal

catalysts (Mestl et al 2001, Serp et al 2003). Thus, a resurgence of interest has

occurred to synthesize non-noble metal catalysts using activated carbon

support.

3.2.1 Ag/C Catalyst

3.2.1.1 X-Ray diffraction studies

The as prepared Ag/C catalysts were characterized by XRD the

spectra of which are shown in Figure 3.9. XRD is a bulk analysis that reveals

the crystal structure, lattice constant and crystal orientation of supported

catalysts. In the Figure, the broad peak at 2 = 25.9 is associated with (002)

plane of the graphite-like structure of the activated carbon, and the diffraction

peaks at 2 = 38 , 44 , 64 and 77 can be attributed to the (111), (200), (220)

and (311) crystalline planes respectively of the face centered cubic (fcc)

structure of Ag (Han et al 2009) (JCPDS 04-0783). A diffraction peak of

graphite at 2 = 25.9 was found to slightly decrease with increase in the

percentage of the metal loading. At one stage, in particular 10 wt % of Ag, the

peak completely vanished. This was a result of the strong interaction between

the carbon support and the incorporated silver particles indicating that the

metal particles were well distributed within the activated carbon matrix. The

average size of the metal particles was calculated based on the (111) facets of

diffraction peaks according to Scherrer’s equation (Xu et al 2008). The

107

average size of Ag particles was 5.36 nm in 2 wt %, 5.65 nm in 4 wt %, 6.24

nm in 6 wt %, 7.62 nm in 8 wt % and 10.71 nm in 10 wt %. The analysis

revealed that the size of the prepared Ag particles were in nanometer range

varying from 5-11 nm. The advantage of the nanometer sized particles is their

high surface area which would drastically reduce the amount of catalyst

required and thus becoming cost effective.

10 20 30 40 50 60 70 80

(311)

(22

0)

(200)

(11

1)

c

e

d

b

a

Inte

nsit

y (a.u

)

2 (O)

Figure 3.9 XRD patterns of (a) 2 wt %, (b) 4 wt %, (c) 6 wt %, (d) 8 wt %

and (e) 10 wt % as synthesized Ag/C catalyst

3.2.1.2 UV-visible diffuse reflectance spectra

200 300 400 500 600 700 800

a

b

Absorb

ance (a.u

)

Wavelength (nm)

Figure 3.10 UV-Vis diffuse reflectance spectra of as-prepared Ag/C

catalysts (a) 2 wt % and (b) 10 wt %

108

The UV-visible diffuse reflectance spectra of the as prepared 2 wt

% and 10 wt % Ag/C catalysts are shown in Figure 3.10. Four overlapped

bands centered at about 266, 316, 440 and 560 nm can be identified in the two

samples. There were absorption bands between 400 and 750 nm. They were

assigned to surface plasma resonance band of silver nanoparticles. The

absorption bands at 266 and 316 nm correspond to the absorption of Ag4+ and

Ag5 nanoparticles respectively, as reported by Ershov et al (1993). In addition

as suggested by Sato et al (2003), the bands in the range of 240-270 nm and

275-320 nm correspond to the absorption of Agn+ and Agm nanoparticles

respectively. The difference in absorption bands are due to different in size

and shape of Ag nanoparticles. There is an absorption band close to 316 nm, it

may be assigned to Ag in very small dimensions. In other words, the

absorption bands detected over the catalysts described in the study at about

266 nm and 316 nm can be attributed to small size cationic particles and silver

particles, respectively (Miao et al 2004).

3.2.1.3 Raman spectroscopy

Figure 3.11 Raman spectra of Ag/C (a) 2 wt % and (b) 10 wt % Ag/C

109

The Raman spectra were also used to study the surface structure of

prepared Ag/C catalysts and the results are shown in Figure 3.11. Both the

samples (2 wt % and 10 wt % Ag/C) exhibited two distinct bands appearing at

around 1350 (D-band) and 1575 (G-band) cm-1

. The D-band and G-band

reflect the structure of sp3 and sp

2 hybridized carbon atom, indicating

disordered graphite and the order state on the Ag/C surfaces, respectively

(Yao et al 1998). Therefore the degree of the graphitization of Ag/C can be

quantified by the intensity of ratio of the D to G bands. The peak intensity

ratios (ID/IG) are 0.51 and 0.46 for the 2 wt % and 10 wt % of Ag/C

respectively. High ID/IG ratio indicates that the sample contains high

quantities of amorphous carbon impurities. However, in our case, the ID/IG

ratios obtained were of low value implying that the prepared catalysts

possessed good electrical conductivity and corrosion resistance. Hence, these

materials could be potentially employed as a cathode material for alkaline

membrane fuel cell applications (Wang et al 2008, Zhao et al 2009).

3.2.1.4 Scanning electron microscopy

Figure 3.12 SEM images of 2 wt % (a, b and c) and 10 wt % (d, e and f)

carbon supported silver catalysts with different magnifications

110

Electron microscopic examination is useful in evaluating the spatial

distribution and size of the Ag crystallites in the supported catalyst, since the

densities of Ag and activated carbon are significantly different from each

other.

Figure 3.12 illustrates SEM images of 2 wt % (a, b and c) and 10 wt %

(d, e and f) of activated carbon supported Ag catalysts with different

magnifications. The crystal-grown state and surface morphology of silver on

the activated carbon surface were observed by scanning electron microscopy.

The fine particles and aggregated metallic Ag particles were observed on the

surfaces of some activated carbon as indicated in Figure 3.12. As is evident

from the Figure, the aggregations of metallic Ag particles were found to

increase with increasing amount of Ag content. From these results, one can

easily observe the heterogeneously distributed metallic Ag particles on the

carbon surface. The average size of the Ag particle distributed on the carbon

surface was 8 to 10 nm, which is in good agreement with XRD results.

3.2.1.5 Thermogravimetric analysis

0 200 400 600 800

40

60

80

100

Weig

ht

resid

ue (

%)

Temperature (OC)

2 wt% Ag/C

4 wt% Ag/C

6 wt% Ag/C

8 wt% Ag/C

10 wt% Ag/C

Figure 3.13 Thermogravimetric curves of as-synthesized Ag/C catalysts

111

Figure 3.13 shows the thermogravimetric graphs for the as

synthesized Ag/C catalysts with different weight percentage of metal loading.

From this Figure 3.13, it was found that, in all five thermograms weight loss

occurred in two stages. While the first weight loss i.e. around 90 C

correspond to the removal of moisture present in the prepared catalyst, the

second weight loss at around 580 C was mainly due to the presence of

amorphous carbon in the Ag incorporated activated carbon (Chen et al 2004).

The burning of activated carbon themselves begins at around 620 C. It was

noted that approximately 40 to 70% of the sample remained after performing

TGA up to 800 C. This residue remains mainly due to the presence of carbon

particles. There was no weight gain observed during thermal treatment, since

no oxidation of the metal particles was expected to take place (Hou et al

2001). It should be noted that the prepared Ag/C catalysts can be used

successfully for high temperature fuel cell application also.

3.2.1.6 Cyclic voltammetry

0.0 0.2 0.4 0.6 0.8 1.0

-1.0x10-4

0.0

1.0x10-4

2.0x10-4

3.0x10-4

4.0x10-4

5.0x10-4

6.0x10-4

7.0x10-4

(b)

(a)

Cu

rren

t (A

)

Potential (V vs SCE)

Figure 3.14 Cyclic voltammograms of (a) bare GCE and (b) Ag/C

modified GCE in 0.5 M KOH

1 2

3

4

5

112

The cyclic voltammograms of Ag/C catalysts in freshly prepared

0.5 M KOH solution purged with nitrogen are shown in Figure 3.14. There

are three anodic peaks appearing at 0.22, 0.76 and 0.85 V vs. SCE, and one

cathodic peak appearing at 0.38 V vs. SCE. The anodic peaks appearing

between 0.22 and 0.85 V vs. SCE are due to the formation of Ag2O layers and

the cathodic peak at 0.38 V vs. SCE is assigned to the reduction of Ag2O back

to metallic silver form. According to Cheng et al (1996), peak 0.22 is due to

silver dissolution and formation of surface monolayer of Ag2O film while

peaks 0.76 and 0.85 V are due to the formation of AgOH and Ag2O,

respectively and AgOH, the short-lived intermediate, is the transition form of

Ag2O. One cathodic peak, the counterpart of the three anodic peaks, 0.38 V,

due to reduction of Ag2O to Ag form is observed in the following negative

sweep.

The CVs were used for the estimation of the electrochemically

active surface area (ECSA) of the synthesized Ag/C catalyst in alkaline

medium. The ECSA of the Ag/C electrodes was measured by determining the

coulombic charge (Q) for the reduction of Ag2O and using the relation,

ECSA (cm2/mg) = Q (µC/cm

2) / 420 (µC/cm

2) x L (mg/cm

2)

Where, Q is the coulombic charge, 420 (µC/cm2) is assumed to be the charge

required for the reduction of Ag2O monolayer and L is the catalyst loading.

The value of ECSA for Ag/C was found to be 56 m2/g.

The stability (durability) of the Ag/C catalyst was determined using

cyclic voltammetry with 100 continuous cycles in 0.5M KOH solution. The

electrode didn’t show any significant current decrement and hence the

prepared catalyst has good stability towards long run.

113

3.2.2 Pd-Ni/C Catalyst

3.2.2.1 X-ray diffraction studies

10 20 30 40 50 60 70 80

Ni(200

)

Ni(111)

Pd

(111)

Pd(2

00)

C(0

02)

(e)

(d)

(c)

(b)

(a)

Inte

nsit

y (

a.u

)

2 (O)

Figure 3.15 XRD patterns of (a) 1:1 wt %, (b) 1:3 wt %, (c) 1:5 wt %,

(d) 1:7 wt % and (e) 1:10 wt % of as synthesized Pd-Ni/C

catalyst

The XRD patterns of Pd-Ni/C catalysts with different weight

percentage of metal loading are depicted in Figure 3.15. All of them displayed

a typical face-centered cubic (fcc) pattern. The five major diffraction peaks

have been observed at 2 values of 25.2 , 39.9 , 45.9 , 46.17 and 51.8 . The

intense C (002) diffraction peak corresponds to graphite phase (JCPDS file:

25-0284). The diffraction peaks Pd (111) and (200) corresponding to 2 =

39.9 and 45.9 , respectively indicate the presence of Pd in the Pd-Ni on the

carbon support (JCPDS file: 05-0681) while the peaks corresponding to 2 =

46.17 and 51.8 have the signature of Ni (111) and (200) planes of Ni (JCPDS

file: 04-0850) in the carbon support. The different XRD patterns in Figure (c)

to (e) at around 35 - 45 suggests that the presence of H2O in the NaBH4

reduction process will facilitate the formation of Ni(OH)2 phase. The average

particle sizes calculated based on Pd (200) peak using Debye-Scherrer

114

formula, are 3.9 nm in 1:1 wt %, 4.3 nm in 1:3 wt %, 4.8 nm in 1:5 wt %, 5.4

nm in 1:7 wt % and 6.1 nm in 1:10 wt %.

3.2.2.2 UV-visible diffuse reflectance spectra

The UV-visible diffuse reflectance spectra of the as prepared 1:1 wt

% and 1:10 wt % of Pd-Ni/C catalysts are shown in Figure 3.16. There were

no significant difference observed between the above two samples. Four

overlapped bands centered at about 224, 240, 440 and 650 nm can be

identified in the two samples. There were absorption bands between 350 and

650 nm. They were assigned to surface plasma resonance band of palladium

particles. The absorption bands at 224 and 240 nm correspond to the

absorption of palladium particles. The differences in absorption bands are due

to different size and shape of Pd and Ni particles. There is an absorption band

close to 240 nm, which may be assigned to Pd and Ni in very small

dimensions. In other words, the absorption bands detected over the catalysts

described in the study at about 224 nm and 240 nm can be attributed to small

size cationic particles and Pd-Ni particles, respectively.

200 300 400 500 600 700

(b)

(a)

Ab

so

rban

ce (

a.u

)

Wavelength (nm)

Figure 3.16 UV-Vis diffuse reflectance spectra of as-prepared Pd-Ni/C

catalysts (a) 1:1 wt % and (b) 1:10 wt %

115

3.2.2.3 Raman spectroscopy

Figure 3.17 shows the Raman spectra of 1:1 and 1:10 Pd-Ni/C

catalysts. Both the samples (1:1 wt % and 1:10 wt % Pd-Ni/C) exhibited two

distinct bands appearing at around 1350 cm-1

(D-band) and 1575 cm-1

(G-

band). The D-band and G-band reflect the structure of sp3 and sp

2 hybridized

carbon atom, indicating disordered graphite and the order state on the Pd-Ni/C

surfaces, respectively. Therefore the degree of the graphitization of Pd-Ni/C

can be quantified by the intensity of ratio of the D to G bands. The peak

intensity ratios (ID/IG) are 0.62 and 0.54 for the 1:1 wt % and 1:10 wt % of

Pd-Ni/C respectively. High ID/IG ratio indicates that the sample contains high

quantities of amorphous carbon impurities. However, in our case, the ID/IG

ratios obtained were of low value implying that the prepared catalysts

possessed good electrical conductivity and corrosion resistance.

Figure 3.17 Raman spectra of Pd-Ni/C catalyst of (a) 1:1 wt % and

(b) 1:10 wt %

116

3.2.2.4 Scanning electron microscopy

Figure 3.18 illustrates SEM images of 1:1 wt % (a, b and c) and

1:10 wt % (d, e and f) of Pd-Ni on carbon supported catalysts with different

magnifications. Both the catalysts prepared from reduction method exhibit

spherical like morphology and homogeneously dispersed on Vulcan XC-72

with no remarkable observation of agglomerations. The average size of the

particles distributed on the carbon surface was 4 to 7 nm, which is in good

agreement with XRD results.

Figure 3.18 SEM images of 1:1 wt % (a, b and c) and 1:10 wt %

(d, e and f) carbon supported Pd-Ni/C catalysts with

different magnifications

117

Ni(OH)2 NiO + H2O

3.2.2.5 Thermogravimetric Analysis

0 200 400 600 800

0

20

40

60

80

100

Weig

ht

res

idu

e (

%)

Temperature (OC)

1:1 Pd-Ni/C

1:3 Pd-Ni/C

1:5 Pd-Ni/C

1:7 Pd-Ni/C

1:10 Pd-Ni/C

Figure 3.19 Thermogravimetric curves of as-synthesized Pd-Ni/C

catalysts

Figure 3.19 shows the thermogravimetric graphs for the as

synthesized Pd-Ni/C catalysts with different weight percentage of metal

loading. From this Figure 3.19, it was found that, in all five thermograms

weight loss occurred in three stages. While the first weight loss i.e. around 90

C correspond to the removal of moisture present in the prepared catalyst, the

second weight loss at around 190 to 260 C, is attributed to the dehydration of

Ni(OH)2, according to the following equation (Freitas 2001),

The huge weight loss from 500 to 550 C is the rapid oxidation of

carbon support in air.

3.2.2.6 Cyclic voltammetry

The cyclic voltammograms of bare GCE (a) and 1:10 Pd-Ni/C on

GCE (b) were investigated in 0.5 M KOH solution and is shown in Figure

118

3.20. The bare GCE do not show any oxidation or reduction peak. However,

Pd-Ni/C exhibit anodic sweep at the region of 0.3 - 0.5 V (vs. SCE) due to the

oxidation peak of Ni(OH)2 to NiOOH, and the reduction of NiOOH to

Ni(OH)2 in the cathodic sweep at the region of 0.2- 0.4 V (vs. SCE), and the

reduction of PdO to Pd at around -0.6 V (vs. SCE). Due to the penetration of

hydrogen into the Pd- based bimetallic nanostructures, the ECSA of the Pd-

Ni/C catalyst was calculated by the charge of the reduction region of PdO to

Pd. The value of ECSA for Pd-Ni/C was found to be 62 m2/g.

The stability (durability) of the Pd-Ni/C catalyst was determined

using cyclic voltammetry with 100 continuous cycles in 0.5M KOH solution.

The electrode didn’t show any significant current decrement and hence the

prepared catalyst has good stability towards long run.

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-2.0x10-3

-1.5x10-3

-1.0x10-3

-5.0x10-4

0.0

5.0x10-4

(b)

(a)

Cu

rre

nt

(A)

Potential (V vs SCE)

Figure 3.20 Cyclic voltammograms of (a) bare GCE and (b) Pd-Ni/C

modified GCE in 0.5 M KOH

Other catalysts such as Pt/C and Pt-Ru/C purchased from Arora

Mathey were also characterized. The ECSA was found to be 74 and 68 m2/g

for Pt/C and Pt-Ru/C respectively.

119

3.3 SINGLE CELL PERFORMANCE OF AMFC WITH QPSEBS

AND AMI-7001

The electrochemical single cell (25 cm2 cell area) performance of

QPSEBS and commercially available anion exchange membrane namely,

AMI-7001 is shown in Figures 3.21 and 3.22. The optimized parameter for

alkaline membrane fuel cell performance is depicted in the Table 3.1 and was

common for all the fuel cell performance studies.

Table 3.1 Experimental conditions for MEA and AMFC operation

AMFC Anode Cathode

Catalyst Pt/C Pt/C or Ag/C or Pd-Ni/C

Catalyst loading (mg/cm2) 0.5 0.375

Active area (cm2) 25 25

Fuel/Oxidant H2 O2

Flow rate (mL/min) 20 40

Relative humidity (%) 77 77

Cell temperature ( C) 60 60

The performance of QPSEBS and AMI-7001 were investigated

using three different cathode catalysts namely, Pt/C, Ag/C and Pd-Ni/C with

Pt/C being used as a common anode catalyst for all the three cathode

catalysts. I-V curves showed the improved performance of the QPSEBS

compared to the AMI-7001 membrane, when AMFC was operated at 60 °C.

The open circuit voltage (OCV) for the QPSEBS was found to be 0.74, 0.69

and 0.72 V for Pt/C, Ag/C and Pd-Ni/C cathode catalyst respectively.

However, AMI-7001 exhibits 0.73, 0.67 and 0.72 V for the Pt/C, Ag/C and

Pd-Ni/C cathode catalyst respectively.

In terms of power density, the QPSEBS using Pt/C cathode catalyst

shows maximum power density of 115.2 mW/cm2 at a current density of 320

120

mA/cm2, followed by Pd-Ni/C and Ag/C of 108.8 and 105.6 mW/cm

2

respectively at a current density of 320 mA/cm2. While AMI-7001, exhibit

maximum power density values of 109, 105 and 99 mW/cm2

of Pt/C, Pd-Ni/C

and Ag/C respectively. As a whole in terms of OCV and maximum power

density, QPSEBS have better results when compared to commercially

available anion exchange membrane. Hence QPSEBS are promising

electrolytes for fuel cells.

0 100 200 300 400 500

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Pow

er

density (m

W/c

m2)

Cell v

oltage (V)

Current density (mA/cm2)

Pt/C-Pt/C

Pt/C-Ag/C

Pt/C-(Pd-Ni)/C

0

20

40

60

80

100

120

Figure 3.21 Fuel cell performances of QPSEBS using three different

cathode catalysts and Pt/C as the anode catalyst

0 100 200 300 400 500

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Pow

er density (m

W/c

m2

)

Cell v

oltage (V)

Current density (mA/cm2)

Pt/C-Pt/C

Pt/C-Ag/C Pt/C-(Pd-Ni/C)

0

20

40

60

80

100

120

Figure 3.22 AMFC performances of AMI-7001 using three different

cathode catalysts and Pt/C as the anode catalyst

121

3.4 SINGLE CELL PERFORMANCE OF DMAMFC WITH

QPSEBS AND AMI-7001

The electrochemical single cell (25 cm2 cell area) performance of

QPSEBS and commercially available anion exchange membrane namely

AMI-7001 is shown in Figure 3.23. The optimized parameter for DMAMFC

performance is depicted in the Table 3.2 and is common for all the fuel cell

performance studies.

Table 3.2 Experimental conditions for MEA and DMAMFC operation

DMAMFC Anode Cathode

Catalyst Pt-Ru/C Pt/C

Catalyst loading (mg/cm2) 0.5 0.375

Active area (cm2) 25 25

Fuel/Oxidant CH3OH (2M) O2

Flow rate (mL/min) 20 40

Relative humidity (%) 77 77

Cell temperature ( C) 60 60

The performance of QPSEBS and AMI-7001 were investigated

using Pt-Ru/C and Pt/C as the anode and cathode catalyst respectively. I-V

curves showed the improved performance of the QPSEBS compared to the

AMI-7001 membrane, when DMAMFC is operated at 60 °C. The OCV of

QPSEBS was found to be 0.65 V and the maximum power density of 46.5

mW/cm2 was achieved at a current density of 150 mA/cm

2, whereas AMI-

7001 possess OCV of 0.69 V and the maximum power density value of 43.75

mW/cm2 at 60 C. Hence QPSEBS was preferably used in DMAMFC too.

122

0 50 100 150 200 250

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Pow

er density (m

W/c

m2)

Cell

voltage (V

)

current density (mA/cm2)

QPSEBS

AMI-7001

0

10

20

30

40

50

Figure 3.23 DMAMFC performances of QPSEBS and AMI-7001 using

Pt-Ru/C as the anode and Pt/C as the cathode

3.5 COMPOSITES USING METAL OXIDES

The main issue with anion exchange membrane is its low stability

under the working conditions of AMFC (i.e. 60 C and high pH) due to SN2

substitution and Hofmann elimination (Zhang et al 2011). One promising

strategy is to incorporate nanometer sized inorganic fillers into AEM to

improve the thermal and mechanical stabilities.

Inorganic nano scale building blocks include carbon nanotubes

(CNTs), layered silicates (e.g. Montmorillonite, Saponite), nanoparticles of

metals (e.g. Au, Ag), metal oxides (e.g. TiO2, SiO2 and ZrO2 (Yang et al

2010)), semiconductors (e.g. PbS and CdS) and so forth, among which metal

oxides is viewed as being very important. The addition of hydrophilic

nanocrystalline metal oxides are into the polymer matrix serves to reduce the

glass transition temperature (Tg) and the crystallinity of the QPSEBS polymer

while at the same time increases the amorphous phases of the polymer matrix,

as well as its ionic conductivity (Park et al 2008, Polynton et al 2010). Since

nanocrystalline metal oxide (TiO2, SiO2 and ZrO2) has been extensively used

123

as a composite material due to its outstanding physical and chemical

properties they were incorporated into the QPSEBS and their properties were

evaluated. Metal oxides are usually used in the form of nanoparticles due to

its high surface area, activity and excellent chemical stability. Also the

mechanical properties of the QPSEBS/metal oxides composite membrane are

greatly enhanced, when nano filler is used as a stiffener material. The thermal

property, dimensional stability and swelling ratio could also be improved.

Therefore quaternized polymer/metal oxide composites have attracted

substantial academic and industrial interest.

3.6 QPSEBS/TiO2 COMPOSITE MEMBRANES

Nanocrystalline anatase TiO2 has been successfully prepared by the

sol–gel method and the characterization results TiO2 and composites of

QPSEBS/TiO2 are discussed here.

3.6.1 Characterization of TiO2

4000 3500 3000 2500 2000 1500 1000 500

0

20

40

60

80

100

% T

ransm

itta

nce (A

.U)

Wave number (cm-1)

Figure 3.24 FTIR spectrum of as synthesized TiO2

124

FTIR spectrum of prepared TiO2 nanoparticles is shown in Figure

3.24. The spectroscopic band is observed around the 3500 cm-1

, which is

ascribed to the stretching vibration of the hydroxyl group (Ti–OH) of the TiO2

nanoparticles. A broad absorption band between 500 and 1000 cm-1

are

ascribed to the vibration absorption of the Ti–O–Ti linkages in TiO2

nanoparticles (Hashimoto et al 2006). The peak at 1640 cm1 is attributed to

adsorbed water.

10 20 30 40 50 60 70 80

0

50

100

150

200

250

300

350

(21

5)

(22

0)

(116

)

(204

)

(21

1)

(10

5)

(200

)

(101

)

(004

)

Inte

nsi

ty (

A.

U)

2 (O)

Figure 3.25 XRD pattern of as synthesized TiO2

The XRD pattern of bare TiO2 nanoparticles is shown in Figure

3.25. The peaks were rather sharp indicating that the obtained TiO2 had high

crystallinity which was attributed to the anatase phase while the crystal size

was determined to be 16 nm from the peak of 1 0 1 plane reflection using

Sherrer’s equation.

TGA profile of synthesized TiO2 is shown in Figure 3.26. The

sample was heated in nitrogen at a heating rate of 10 C min1. The TGA

125

curve exhibits three weight losses associated with endothermic and

exothermic events in the DTA curve. The first endothermic event occurring

around 120 °C corresponds to the elimination of adsorbed water. The

following two exothermic events at 220 °C and 430 °C were due to the

volatilization and combustion of residual organic species. Another peak in the

DTA curve starting at 690 °C, approximately, corresponds to the

crystallization of the amorphous residue into anatase, whereas that around 720

°C indicates the phase transition anatase-rutile. There are no associated

signals with these latter thermal events in the TGA curve confirming the

crystallization and phase transition events.

0 200 400 600 800 1000

88

90

92

94

96

98

100

Temperature (OC)

Weig

ht

resid

ue (

%)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

Deri

v. w

eig

ht

(% /

OC

)

Figure 3.26 TGA-DTA curve of TiO2

Figure 3.27 shows a TEM image of anatase TiO2 nanoparticles

calcined at 500 C. The nanoparticles have a spherical morphology with an

average diameter of 15 nm, which is in good agreement with the XRD

evaluation.

126

Figure 3.27 TEM image of as synthesized TiO2

Figure 3.28 (a) and (b) shows the SEM images of anatase TiO2

nanoparticles calcined at 500 C with two different magnifications (10 and 5

m). The nanoparticles have a spherical morphology with an average

diameter of 15 nm, which is in good agreement with the XRD evaluation.

Figure 3.28 SEM images of as synthesized TiO2

127

3.6.2 Characterization of QPSEBS/TiO2 Composite Membranes

3.6.2.1 Water absorption, ion exchange capacity and ionic

conductivity

Water uptake of QPSEBS and QPSEBS/TiO2 nano composite

membranes are shown in Table 3.3. Water absorption capacity depends on the

number of ion exchange groups and nature of nano fillers present in the

membrane. The water absorption of the composite membrane increased with

increase in titania content upto 7.5 wt %. The reason was inorganic fillers

present in a composite bind larger amount of water molecules with the

membrane via. its hygroscopic effect. The voids or cavities which are present

in the inorganic fillers influence higher amount of water absorption and

tightly pack the water molecules in its voids. The higher amount of water

molecules present in the membrane humidifies the polymeric channels which

in turn favors the lower humidity operation and also circumvents the usage of

a humidifier. Though the operational temperature of fuel cell exceeds 100 C,

a tight packing of water molecules achieved via. nanofillers influences water

retention of the composite membrane.

In principle, ion exchange capacity (IEC) in an ionic-conductive

membrane usually reflects the amount of exchangeable groups in the

membrane, and a relatively high IEC is normally correlative to a higher ionic

conductivity (Wang et al 2010). Experimental IEC values of the QPSEBS and

composite membranes were calculated by back titration method. IEC can

provide information on the density of ionizable functional group in the

membrane. The IEC (Table 3.3) decreased with increase in the content of

titania.

Ionic conductivity is defined as the capability of the transportation

of ions which determines the power generation of a fuel cell. On introducing

128

the quaternary ammonium groups in to the polymer chain, the hydroxyl ion

conducting ability was introduced. Impedance value measurement of ion

exchange membranes at room temperature and 100% humidity was taken. To

examine the conductivity, the quaternized membrane and corresponding

composite membranes were first soaked in 1M KOH solution for 24 h to

convert in to OH form and then it was washed several times with deionized

water. After the free KOH was completely removed, the conductivity of the

membrane was measured. The conductivity values are shown in Table 3.3.

The ionic conductivity of the membrane increases with increase in the

inorganic filler content. This may due to the higher amount of water

molecules adsorbed by the nanofillers that promote the Grothus and vehicle-

type mechanisms. A higher amount of adsorbed water molecules solvate the

moieties of a polymer to a greater extent and has become responsible for the

higher ionic conductivity. The composite membrane with 7.5 wt % TiO2

shows the higher conductivity of 1.78x10-2

S cm-1

at 100% hydrated

condition, among all the studied membranes.

Table 3.3 Water absorption, IEC and conductivity values of

QPSEBS/TiO2 composite membranes

Membrane Water

absorption, %

IEC, meq/g Conductivity,

S/cm

AMI-7001 17 1.3 1.72 x 10-2

QPSEBS 4.04 0.958 1.51 x 10-2

QPSEBS/2.5% TiO2 7.28 0.943 1.63 x 10-2

QPSEBS/5% TiO2 9.17 0.928 1.69 x 10-2

QPSEBS/7.5% TiO2 11.32 0.906 1.78 x 10-2

QPSEBS/10% TiO2 10.09 0.891 1.74 x 10-2

129

3.6.2.2 Methanol permeability and selectivity ratio

Table 3.4 Methanol permeability and selectivity ratio of

QPSEBS/TiO2 composite membranes

Membrane Methanol permeability,

cm2/s

Selectivity ratio,

Ss/cm3

AMI-7001 2.25 x 10-6

0.76 x 104

QPSEBS 2.14 x 10-6

0.71 x 104

QPSEBS/2.5% TiO2 2.06 x 10-6

0.79 x 104

QPSEBS/5% TiO2 1.87 x 10-6

0.90 x 104

QPSEBS/7.5% TiO2 1.76 x 10-6

1.01 x 104

QPSEBS/10% TiO2 1.75 x 10-6

0.99 x 104

Methanol permeability is the product of the diffusion coefficient

and sorption coefficient in which the diffusion coefficient reflects the effect of

a surrounding environment on the molecular motion of the permanent and the

sorption coefficient correlates the concentration of a component in the fluid

phase (Gnanakumar et al 2008; Marx et al 2002).

Ionic conductivity and methanol permeability are the two

electrochemical properties which determine the efficiency of DMAMFC.

When the quaternized polymer membrane contacts with an aqueous medium,

OH- ions can combine with water molecules and form a complex. Quaternary

ammonium group create effective path ways for the transportation of ions.

But at the mean while, methanol molecule can also permeate through the

broad hydrophilic channels which are created for the ions migration (Cho et al

2006, Kim et al 2006). So a considerable effort has to be devoted to achieve

high ionic conductivity and lower methanol permeability for the high

efficiency of DMAMFC performance.

130

Low methanol permeability is an important requirement for a

membrane in direct methanol alkaline membrane fuel cells. The results of

methanol permeability of the composite membranes based on QPSEBS/TiO2

are shown in Table 3.4. It is evident from the Table 3.4 that the methanol

permeability decreases on the incorporation of TiO2 in the quaternized

polymer matrix. It is known that methanol permeates through hydrophilic

ionic channels and that OH- ions are transported by hopping between ionic

sites. Therefore, it could be explained that the methanol permeability

decreases due to the incorporation of titania acting as material for blocking

the methanol transport while ionic conductivity is increased.

The ionic conductivity, methanol permeability and selectivity ratio

evaluates the membranes for the application of DMAMFC (Hickner et al

2006). A higher selectivity ratio is favored for the DMAMFC operation. The

membrane QPSEBS/7.5% TiO2 exhibited a high selectivity of 1.01 x 104

Ss/cm3 which is greater than that of AMI-7001 membrane and can have a

great impact on direct methanol alkaline membrane fuel cells field.

3.6.2.3 Hydrolytic and alkaline stability

The property of IEC was measured after the hydrolytic and alkaline

stability tests. The loss of IEC obtained were found to be only 1.25, 1.02,

1.18, 1.55 and 1.23% for QPSEBS, QPSEBS/2.5% TiO2, QPSEBS/5% TiO2,

QPSEBS/7.5% TiO2 and QPSEBS/10% TiO2 respectively. Hence, these

results clearly revealed that the prepared composite membranes possess good

mechanical and chemical stability for use in fuel cells.

131

3.6.2.4 Fourier transform infra red

Due to quaternization reaction, the IR bands of aromatic ring C=C

and aromatic ring backbone –CH- bending vibration were observed to a lower

frequency around 1600 cm-1

and 1371 cm-1

. In this reaction, tri ethylamine

was used to form the quaternized product. IR bands for tertiary amine cannot

be seen in the spectrum. Appearance of peak around 1371 cm-1

was assigned

to C-N stretching vibration. In addition to that (Figure 3.29 (a)), a small

intense peak at 2365 cm-1

has been appeared which is the characteristic

absorption peak of quaternary ammonium group. From all these evidences, it

is clear that chloromethylation and quaternization reactions have been

successfully carried out in the PSEBS unit at phenyl rings.

But in the case of composite membranes (Figure 3.29 (b-e)), there

is not much change in the IR spectra. However, in the spectra of

QPSEBS/TiO2 composites the peaks at 1630 cm-1

is responsible for

adsorptive peak of anatase TiO2 and the peak at 3400 cm-1

is due to the –OH

stretching vibration band, which confirms the water molecule adsorbed in

TiO2. The peak beyond 3500 cm-1

is due to moisture absorbance. It can be

clearly seen that on increasing the content of titania filler the intensity broad

peak also increased due to the hydrophilic nature of titania. These

observations clearly reveal that TiO2 has strongly bound to the polymer

matrix.

132

4000 3500 3000 2500 2000 1500 1000 500

(c)

(e)

(d)

(b)

(a)

% T

ran

sm

itta

nc

e (

a.u

)

Wave number (cm-1)

Figure 3.29 FTIR spectra of (a) QPSEBS, (b) QPSEBS/2.5% TiO2,

(c) QPSEBS/5% TiO2, (d) QPSEBS/7.5% TiO2 and

(e) QPSEBS/10% TiO2 composite membranes

3.6.2.5 Thermogravimetric analysis

Thermogravimetric analysis was carried out on the membranes

synthesized to examine their thermal stability. The TGA curves of QPSEBS

and the QPSEBS/TiO2 composite membranes in nitrogen atmosphere are

shown in Figure 3.30.

Figure 3.30 (a) represents thermogravimetric curve of QPSEBS

membrane. One can clearly say that there are three-step weight losses during

50-550 C. The first step occurred below 100 C and may be related to the

loss of moisture. The second loss, commencing around 150 C, possibly

resulted from decomposition of quaternary ammonium group and desorption

of bound water. The third weight loss started at around 450 C indicates the

onset of polymer backbone decomposition. Figure 3.30 (b), (c), (d) and (e)

represent composites of 2.5, 5, 7.5 and 10% TiO2 respectively. Once the

inorganic component was added into the polymer matrix, the thermal stability

133

of the composite membranes was synergistically enhanced. Only one weight

loss occurred for all the weight percentage of the composite membranes and

hence the percentage content of TiO2 determines the thermal stability of the

composite membranes.

Figure 3.30 Thermogravimetric curves of (a) QPSEBS, (b) QPSEBS/2.5%

TiO2, (c) QPSEBS/5% TiO2, (d) QPSEBS/7.5% TiO2 and

(e) QPSEBS/10% TiO2 composite membranes

3.6.2.6 Differential scanning calorimetry

The DSC traces of QPSEBS and 2.5, 5, 7.5 and 10% TiO2 loaded

QPSEBS are shown in Figure 3.31 (a-e). In all the traces, there is a broad

endotherm between 30-100 C due to desorption of polymer entrapped

solvent. The Tg value for QPSEBS (Figure 3.31a) was found to be 150 C,

whereas the composite membranes (i.e. inorganic filler incorporated

QPSEBS), it was around 120 C. The decrease in the Tg value could be

attributed to the addition of titania fillers in the polymer matrix. Hence a

moderate Tg value obtained for composite membranes is an evidence for the

adequate thermal property of composite membranes for AMFC application.

134

50 100 150 200 250 300 350 400 450 500 550 600

Exo

(e)

(d)

(c)

(b)

(a)

mW

(m

g)

Temperature (OC)

Figure 3.31 DSC curves for (a) QPSEBS, (b) QPSEBS/2.5% TiO2,

(c) QPSEBS/5% TiO2, (d) QPSEBS/7.5% TiO2 and

(e) QPSEBS/10% TiO2 composite membrane

3.6.2.7 Scanning electron microscopy

The surface morphology has been analyzed from scanning electron

microscope (Figure 3.32). The average particle diameter is found to be less

than 16 nm in all composites. The inorganic particles appear to be well

dispersed in both low and high wt % composites. The filler particles are

distributed relatively in a uniform fashion and the particles are almost

spherical in shape with irregular boundaries. The filler particles are seen to be

clearly embedded in the polymer matrix, which establishes the connectivity of

the composites (Rusu et al 2005). The particle distribution and particle-

polymer matrix reinforcement play vital roles for both tensile and elongation

properties of the composites. The titania particles are dispersed

homogeneously with the interspaces filled with QPSEBS upto 7.5 wt%, while

in the case of 10 wt% TiO2 composite membranes showed formation of

agglomeration on the surface of polymer matrix. This implies that the

135

prepared composite membranes can be expected to perform consistently well

in the AMFCs.

Figure 3.32 SEM images of (a) QPSEBS, (b) QPSEBS/2.5% TiO2,

(c) QPSEBS/5% TiO2, (d) QPSEBS/7.5% TiO2 and

(e) QPSEBS/10% TiO2 composite membranes

136

0% 2.5% 5% 7.5% 10%

0

50

100

150

200

Tensile s

trength

(M

Pa)

TiO2 content

3.6.2.8 Mechanical properties

Figure 3.33 Variation of tensile strength with the addition of titania

The effect of volume fraction of TiO2 on tensile strength and

percentage elongation of the composites is given in Figure 3.33. The tensile

strength of a filled polymer is more difficult to predict because it depends

strongly on polymer filler interactions. Tensile strength is the force required

to pull the composite to the point where it breaks. Specifically the tensile

strength of a material is the maximum amount of tensile stress that it can be

subjected to before failure. The virgin QPSEBS shows a tensile strength of 31

MPa. Tensile strength of the composites increases with increasing TiO2 filler

content, which may be attributed to the titanium oxide enhancement function.

The optimum loaded filler content was found to be 7.5%. The decrease in the

tensile strength at filler concentration above 7.5% could be due to

agglomerate formation which would result in the composite membranes

contributing to initiation of catastrophic failure of the composites on the

application of stress; the evidence for this statement was clearly observed

from the SEM image (Figure 3.32 (e)). Hence this composite membrane was

preferentially used in AMFC applications.

137

3.6.2.9 X-ray diffraction studies

The X-ray diffraction measurement was performed to examine the

crystallinity of the QPSEBS and QPSEBS/TiO2 composite membranes.

Figure 3.34 illustrates the diffraction pattern of the QPSEBS/TiO2 membranes

that were prepared by a blending process with different titania content (wt %).

It is clearly shown that the QPSEBS (Figure 3.34 (b)) polymer exhibits a semi

crystalline structure with a huge peak at a 2 angle of 19-20 . The first broad

peak located at 2 value of 19-20 is noted for all the samples. In composite

membranes, apart from the main peak, a small intense peak that appeared at

2 angle of 25 are attributed to the anatase titania and are assigned to (101),

whereas other anatase peaks were vanished, which clearly indicated that the

prepared inorganic filler was thoroughly mixed with the polymer matrix.

Also, the intensity of the (101) plane increased while increasing the weight

percentage of titania.

Among various samples, the membrane i.e., QPSEBS/7.5% TiO2

shows the lowest peak intensity at a 2 angle of 19-20 , which means that this

membrane has the lowest crystallinity. The low crystallinity reveals that more

amorphous phase exists in this membrane (7.5% TiO2), indicating that the

structure of the membrane is more disordered and QPSEBS and TiO2 are

mixed more uniformly. Good mixing of QPSEBS and TiO2 is useful for the

enhancement of ionic conductivity (Wang et al 2010).

138

10 20 30 40 50 60 70 80

(f)

(e)

(d)

(c)

(b)

(a)

Inte

nsity (A.U

)

2 (O)

Figure 3.34 XRD spectra of (a) TiO2, (b) QPSEBS, (c) QPSEBS/2.5%

TiO2, (d) QPSEBS/5% TiO2, (e) QPSEBS/7.5% TiO2 and

(f) QPSEBS/10% TiO2 composite membranes

The calculated percentage of crystallinity of QPSEBS and

QPSEBS/TiO2 composites is given in Table 3.5. From the Table 3.5, it can be

seen that the percentage crystallinity of the composite decreased upto 7.5 wt

% filler. After that i.e. 10 wt %, there is an increase in percentage of

crystallinity and hence 7.5 wt % TiO2 composite membrane that possess high

amorphous behavior than the rest, has the lowest crystallinity.

Table 3.5 Percentage of crystallinity values of QPSEBS/TiO2

composite membranes

Membrane % of crystallinity

QPSEBS 3.08

QPSEBS/2.5% TiO2 1.23

QPSEBS/5% TiO2 0.79

QPSEBS/7.5% TiO2 0.54

QPSEBS/10% TiO2 0.67

139

3.6.2.10 AMFC performance study

Figure 3.35 AMFC performances of QPSEBS/TiO2 using three different

cathode catalysts namely (a) Pt/C, (b) Ag/C and (c) Pd-Ni/C

The AMFC performance of QPSEBS/TiO2 composite membranes

investigated using three different cathode catalysts namely, Pt/C, Ag/C and

Pd-Ni/C with Pt/C being used as a common anode catalyst for all the three

cathode catalysts is depicted in Figure 3.35. QPSEBS/7.5% TiO2 exhibits

high OCV when compared to the other weight percentage of TiO2 and virgin

QPSEBS. The OCV for the QPSEBS/7.5% TiO2 was found to be 0.84, 0.81

and 0.83 V for Pt/C, Ag/C and Pd-Ni/C cathode catalyst respectively at 60 C.

In terms of power density, the QPSEBS/7.5% TiO2 using Pt/C and

Pd-Ni/C cathode catalysts show maximum power density of 275 mW/cm2

followed by Ag/C of 270 mW/cm2. As a whole in terms of OCV and

maximum power density, QPSEBS/7.5% TiO2 has better results when

140

compared to the other composites and commercially available anion exchange

membrane.

3.6.2.11 DMAMFC performance study

0 50 100 150 200 250 300

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Pow

er

density (m

W/c

m2)

Cell

voltage (V

)

Current density (mA/cm2)

QPSEBS

AMI-7001

QPSEBS/2.5% TiO2

QPSEBS/5% TiO2

QPSEBS/7.5% TiO2

QPSEBS/10% TiO2

-10

0

10

20

30

40

50

60

70

Figure 3.36 DMAMFC performances of QPSEBS/TiO2

The DMAMFC performance of QPSEBS/TiO2 investigated using

Pt-Ru/C and Pt/C as the anode and cathode catalyst respectively is depicted in

Figure 3.36. I-V curves showed the improved performance of the composite

membranes when compared to virgin QPSEBS and AMI-7001 membrane,

when DMAMFC operated at 60 °C. The OCV of QPSEBS/7.5% TiO2 was

found to be 0.76 V and the maximum power density of 64 mW/cm2 was

achieved at a current density of 175 mA/cm2. Hence QPSEBS/TiO2 was

preferably used in DMAMFC.

141

3.7 QPSEBS/SiO2 COMPOSITE MEMBRANES

3.7.1 Water absorption, Ion Exchange Capacity and Ionic

Conductivity

Table 3.6 Water absorption, IEC and conductivity values of

QPSEBS/SiO2 composite membranes

Membrane Water

absorption, %

IEC, meq/g Conductivity,

S/cm

AMI-7001 17 1.3 1.72 x 10-2

QPSEBS 4.04 0.958 1.51 x 10-2

QPSEBS/2.5% SiO2 6.82 0.932 1.67 x 10-2

QPSEBS/5% SiO2 8.23 0.846 1.73 x 10-2

QPSEBS/7.5% SiO2 10.45 0.813 1.81 x 10-2

QPSEBS/10% SiO2 11.24 0.784 1.86 x 10-2

Water uptake is an important parameter in studying AMFCs,

because the water that resides in the hydrophilic domains facilitates the

transport of OH- ions. However, too much of water uptake will result in loss

of mechanical stability. The water absorption, IEC and conductivity values of

QPSEBS and their composite membranes are shown in Table 3.6. As

expected, the water uptake values increased with increase in the weight

fraction of SiO2 in the QPSEBS membrane. This increase in the water

absorption capacity is due to integrated hygroscopic (silica fillers) materials

into the QPSEBS membranes (Bourlinos et al 2004, Halla et al 2003,

Kanamura et al 2005).

The IEC of the composites decreased with increase in the weight

fraction of SiO2. The decreasing trend may be attributed to the decrease in the

concentration of QPSEBS or in other words decrease in the effective

concentration of the quaternary ammonium groups in the composites.

One of the most important parameter that governs the suitability of

a polymer electrolyte membrane for use in fuel cell is its ionic conducting

142

ability. The ionic conductivity of pristine QPSEBS was found to be 1.51x10-2

S/cm. In the case of the composites, there is an increase in the ionic

conductivity with increase in the content of SiO2. This increasing trend can be

explained due to the presence of silica content that acts as Lewis acid sites to

provide extra water to the membrane. Also, membrane ionic conductivity is

affected by the ion concentration and mobility, hydration levels, and polymer

structure or chain mobility (Kreuer et al 2004). To date, the influence of

inorganic silica component on membrane conductivity has been extensively

investigated and debated but there is still no agreement. On one hand, cross-

linked Si–O–Si network may limit the mobility of the conductive ions and

hinder the formation of conductive and hydrophilic ionic clusters and

channels (as found with the perfluorosulfonic acid polymer electrolytes), thus

decreasing the ionic conductivity (Fu et al 2008, Kato et al 2008). On the

other hand, hydroxyl groups (–Si-OH) from silica have strong bonding ability

with H2O molecules, thus favoring water retention and therefore ion transfer

(Kim et al 2004). Hence, the increase in ionic conductivity with the increase

in the content of SiO2 in the present study can be explained due to the

formation of (-Si-OH) from silica.

3.7.2 Methanol Permeability and Selectivity Ratio

Table 3.7 Methanol permeability and selectivity ratio of QPSEBS/SiO2

composite membranes

Membrane Methanol permeability,

cm2/s

Selectivity ratio,

Ss/cm3

AMI-7001 2.25 x 10-6

0.76 x 104

QPSEBS 2.14 x 10-6

0.71 x 104

QPSEBS/2.5% SiO2 2.11 x 10-6

0.79 x 104

QPSEBS/5% SiO2 2.03 x 10-6

0.85 x 104

QPSEBS/7.5% SiO2 1.89 x 10-6

0.95 x 104

QPSEBS/10% SiO2 1.82 x 10-6

1.02 x 104

143

The results of methanol permeability of the composite membranes

based on QPSEBS/SiO2 are depicted in Table 3.7. It is evident from the Table

that the methanol permeability decreases on the incorporation of silica filler in

the quaternized polymer matrix due to incorporation of silica acting as

material for blocking the methanol transport while ionic conductivity is

increased.

The membrane QPSEBS/10% SiO2 exhibited a high selectivity of

1.02 x 104

Ss/cm3 which is greater than that of AMI-7001 membrane and can

have a great impact on direct methanol alkaline membrane fuel cells field.

3.7.3 Hydrolytic and Alkaline Stability

The property of IEC was measured after the hydrolytic and alkaline

stability tests. The loss of IEC was found to be 1 to 1.5% for all the prepared

membranes, which clearly concludes the IEC was not significantly changed,

which means that the prepared QPSEBS/SiO2 composite membranes have

good mechanical and chemical stability.

3.7.4 Fourier Transform Infra Red

In the spectra of QPSEBS/SiO2 (Figure 3.37 b-e) the peaks between

800-1100 cm-1

is responsible for the Si – O – Si chemical bonding. The IR

spectra of QPSEBS/SiO2 composites show the peaks at 1037 and 1087 cm-1

that were slightly shifted from the QPSEBS polymer, which confirms the

interaction between the quaternary ammonium group and silica and also

illustrates an electrostatic interaction, exhibited between the polymer and the

inorganic filler. The peak at 3500 cm-1

is due to moisture absorbance. It can

be clearly seen that on increasing the content of silica filler the intensity of

this peak also increased due to the hydrophilic nature of silica. These

observations clearly reveal that SiO2 has strongly bound to the polymer

matrix.

144

4000 3500 3000 2500 2000 1500 1000 500

(e)

(d)

(c)

(b)

(a)

% T

ransm

itta

nce

Wave number (cm-1)

Figure 3.37 FTIR spectra of (a) QPSEBS, (b) QPSEBS/2.5% SiO2,

(c) QPSEBS/5% SiO2, (d) QPSEBS/7.5% SiO2 and

(e) QPSEBS/10% SiO2 composite membranes

3.7.5 Thermogravimetric Analysis

0 100 200 300 400 500 600 700 800

0

20

40

60

80

100

Temperature (OC )

Weig

ht

res

idu

e (

%)

QPSEBS/SiO2 composites

QPSEBS

Figure 3.38 TGA curves QPSEBS and QPSEBS/SiO2 composite

membranes

145

Figure 3.38 represents thermogravimetric curves of QPSEBS and

its SiO2 composite membranes. QPSEBS membrane exhibits three-step

weight losses during 50-550 C. The first step occurred below 100 C and

may be related to the loss of moisture. The second loss, commencing around

150 C, possibly resulted from decomposition of quaternary ammonium group

and desorption of bound water. The third weight loss that started at around

450 C indicates the onset of polymer backbone decomposition. While in the

case of composite membranes, only one weight loss occurred and this is due

to the addition of silica filler that greatly enhanced the thermal stability.

Hence, the percentage content of SiO2 determines the thermal stability of the

composite membranes.

3.7.6 Differential Scanning Calorimetry

0 50 100 150 200 250 300

(e)

(d)

(c)

(b)

(a)

Exo

mW

/mg

Temperature (OC)

QPSEBS

2.5% SiO2

5.0% SiO2

7.5% SiO2

10.0% SiO2

Figure 3.39 DSC curves of QPSEBS and QPSEBS/SiO2 composite

membranes

146

The DSC traces of QPSEBS and 2.5, 5, 7.5 and 10% SiO2 loaded

QPSEBS are shown in Figure 3.39. In all the traces, there is a broad

endotherm between 30-100 C due to desorption of polymer entrapped

solvent. The Tg value for QPSEBS was found to be 120 C, whereas for the

composite membranes (i.e. inorganic filler incorporated QPSEBS), it was

around 80 C. The decrease in the Tg value could be attributed to the addition

of silica fillers in the polymer matrix. This clearly illustrates that the prepared

membranes can be suitable for low temperature alkaline membrane fuel cells.

3.7.7 Scanning Electron Microscopy

The surface morphology of the QPSEBS and its composite

membranes were investigated by scanning electron microscopy. Figure 3.40

shows the SEM images of QPSEBS and silica fillers incorporated QPSEBS

matrix. Uniform surface was seen in the SEM images of QPSEBS (Figure

3.40(a)) and QPSEBS/SiO2 composite membranes (Figure 3.40(b) to (e)). The

silica material was found to be dispersed uniformly in the quaternized

polymer matrix. There were some white dots in images representing the area

of the SiO2/QPSEBS membranes, and this proves that the silica fillers can be

effectively dispersed when cyclohexane was added via the described solution-

cast method. The improved dispersion is probably because of good mixing

between the inorganic fillers and the QPSEBS.

147

Figure 3.40 SEM images of (a) QPSEBS, (b) QPSEBS/2.5% SiO2,

(c) QPSEBS/5% SiO2, (d) QPSEBS/7.5% SiO2 and

(e) QPSEBS/10% SiO2 composite membranes

3.7.8 Mechanical Properties

The effect of volume fraction of SiO2 on tensile strength of the

composites is given in Figure 3.41. Tensile strength of the composites

increases with increasing SiO2 filler content (upto 7.5%), which may be

attributed to the silicon oxide enhancement function and also specific

interactions between inorganic and organic components. Compared with

previously reported poly phenylene oxide/SiO2 composite membranes (Wu et

al 2010) (tensile strength = 8.2 to 25 MPa), the membranes of the present

work have promising tensile properties with higher level of tensile strength.

An increase in silica content induces an initial increase in tensile strength

148

from 31 to 75.6 MPa. This indicates that optimal silica content enhances

strength but excessive silica contents, destroys homogeneity, and reduces the

membrane strength (Wu et al 2010). The optimum loaded filler content was

found to be 7.5%. The decrease in the tensile strength at filler concentration

above 7.5% could be due to agglomerate formation which would result in the

composite membranes contributing to initiation of catastrophic failure of the

composites on the application of stress.

0% 2.5% 5% 7.5% 10%

0

15

30

45

60

75

Tensile s

trength

(M

Pa)

SiO2 content

Figure 3.41 Variation of tensile strength with the addition of silica

3.7.9 X-Ray Diffraction Studies

The X-ray diffraction measurement was performed to examine the

crystallinity of the QPSEBS and QPSEBS/SiO2 composite membranes

(Figure 3.42). The broad peak located at 2 value of 19-20 is noted for all the

samples.

It can be seen that the peak intensity at 2 = 19-20 of the

QPSEBS/SiO2 composite membranes with different weight percentage of

SiO2 was reduced as compared with that of virgin QPSEBS polymer. Among

149

various samples, the membrane i.e., QPSEBS/10% SiO2 shows the lowest

peak intensity at a 2 angle of 19-20 , which means that this membrane has

the lowest crystallinity. The low crystallinity reveals that more amorphous

phase exists in this membrane (10% SiO2), indicating that the structure of the

membrane is more disordered and QPSEBS and SiO2 are mixed more

uniformly. Good mixing of QPSEBS and SiO2 is useful for the enhancement

of ionic conductivity (Wang et al 2010). The relative degree of amorphous

phase can be estimated from the full-width at half maximum (FWHM) from

the 2 angle of 19-20 peak. The FWHM values of QPSEBS, 2.5, 5, 7.5 and

10 wt% SiO2 membranes are 6.23, 6.98, 7.42, 8.15 and 9.46 respectively.

Apparently, a higher FWHM value corresponding to broadened peak indicates

stronger interaction between QPSEBS and silica filler, which give rise to the

perturbation of long ranged spacing between the chains. Sheng Wen et al

investigated the composite membranes with sulphonated poly ether

sulfone/SiO2. They found that strong hydrogen bonding occurs between the

sulphonated poly ether sulfone and SiO2 and an increase in the amorphous

region abruptly (Wen et al 2010), which is consistent with what we have

observed.

The calculated percentage of crystallinity of QPSEBS and

QPSEBS/SiO2 composites is given in Table 3.8. From the Table, it can be

seen that the percentage crystallinity of the composite decreased with increase

in the silica content. The QPSEBS/10% SiO2 composite membrane exhibits

lowest crystallinity among all the membranes. For fuel cell applications, the

membrane should possess amorphous behavior in other words low

crystallinity because low crystallinity reveals good ionic conductivity. Hence

poor crystallinity gives better performance in fuel cell.

150

10 20 30 40 50 60 70 80

(e)

(d)

(c)

(b)

(a)

Inte

nsit

y (

A. U

)

2 (O)

Figure 3.42 XRD spectra of (a) QPSEBS, (b) QPSEBS/2.5% SiO2,

(c) QPSEBS/5% SiO2, (d) QPSEBS/7.5% SiO2 and

(e) QPSEBS/10% SiO2 composite membranes

Table 3.8 Percentage of crystallinity values of QPSEBS/SiO2

composite membranes

Membrane % of crystallinity

QPSEBS 3.08

QPSEBS/2.5% SiO2 2.23

QPSEBS/5% SiO2 2.16

QPSEBS/7.5% SiO2 1.97

QPSEBS/10% SiO2 1.14

151

3.7.10 AMFC performance study

Figure 3.43 AMFC performances of QPSEBS/SiO2 using three different

cathode catalysts namely (a) Pt/C, (b) Ag/C and (c) Pd-Ni/C

The polarization and power density curves of QPSEBS and the

composite membranes, obtained from the alkaline membrane fuel cell using

Pt as the anode and three different cathode catalysts are presented in Figure

3.43. The measurements were made by feeding hydrogen and oxygen with

flow rate of 20 and 40 mL/min respectively at 60 C. The alkaline membrane

fuel cell with a QPSEBS/10% SiO2 gave better performance than with other

composites, pristine QPSEBS and commercially available AEM. The OCV

for the QPSEBS/10% SiO2 was found to be 0.80, 0.78 and 0.79 V for Pt/C,

Ag/C and Pd-Ni/C cathode catalyst respectively at 60 C.

152

In terms of power density, the QPSEBS/10% SiO2 using Pt/C and

Pd-Ni/C cathode catalysts show maximum power density of 210 and 191

mW/cm2 respectively, followed by Ag/C of 170 mW/cm

2. As a whole in

terms of OCV and maximum power density, QPSEBS/10% SiO2 has better

results when compared to the other composites and commercially available

anion exchange membrane.

3.7.11 DMAMFC performance study

0 50 100 150 200 250 300

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Po

we

r d

en

sit

y (

mW

/cm

2)

Ce

ll v

olt

ag

e (

V)

Current density (mA/cm2)

QPSEBS AMI-7001

QPSEBS/2.5% SiO2

QPSEBS/5% SiO2

QPSEBS/7.5% SiO2

QPSEBS/10% SiO2

0

10

20

30

40

50

60

70

80

Figure 3.44 DMAMFC performances of QPSEBS/SiO2

The performance of QPSEBS/SiO2 investigated using Pt-Ru/C and

Pt/C as the anode and cathode catalyst respectively is depicted in Figure 3.44.

I-V curves showed the improved performance of the composite membranes

when compared to virgin QPSEBS and AMI-7001 membrane, when

DMAMFC operated at 60 °C. The OCV of QPSEBS/10% SiO2 was found to

be 0.75 V and the maximum power density of 74 mW/cm2 was achieved at a

current density of 225 mA/cm2.

153

3.8 QPSEBS/ZrO2 Composite Membranes

3.8.1 Water Absorption, Ion Exchange Capacity and Ionic

Conductivity

Table 3.9 Water absorption, IEC and conductivity values of

QPSEBS/ZrO2 composite membranes

Membrane Water

absorption, %

IEC, meq/g Conductivity,

S/cm

AMI-7001 17 1.3 1.72 x 10-2

QPSEBS 4.04 0.958 1.51 x 10-2

QPSEBS/2.5% ZrO2 5.92 0.814 1.63 x 10-2

QPSEBS/5% ZrO2 7.24 0.740 1.72 x 10-2

QPSEBS/7.5% ZrO2 10.01 0.712 1.79 x 10-2

QPSEBS/10% ZrO2 10.56 0.691 1.82 x 10-2

Similar trend as observed in QPSEBS/TiO2 and QPSEBS/SiO2, was

observed in the case of QPSEBS/ZrO2 composite membranes also, i.e. water

absorption and conductivity increased with increase in the content of zirconia

while ion exchange capacity reduced with the addition of zirconia. The

maximum ionic conductivity of 1.82 x 10-2

S/cm was obtained for

QPSEBS/10% ZrO2 composite membrane.

3.8.2 Methanol Permeability and Selectivity Ratio

The methanol permeability decreased with increase in the content

of zirconia filler and the selectivity ratio increased. All the composite

membranes exhibit higher selectivity ratio values than pristine QPSEBS and

commercially purchased AMI-7001 membranes. Hence, these membranes can

be potentially applicable in direct methanol alkaline membrane fuel cells.

154

Table 3.10 Methanol permeability and selectivity ratio of

QPSEBS/ZrO2 composite membranes

Membrane Methanol permeability,

cm2/s

Selectivity ratio,

Ss/cm3

AMI-7001 2.25 x 10-6

0.76 x 104

QPSEBS 2.14 x 10-6

0.71 x 104

QPSEBS/2.5%

ZrO2

2.03 x 10-6

0.82 x 104

QPSEBS/5% ZrO2 1.97 x 10-6

0.87 x 104

QPSEBS/7.5%

ZrO2

1.90 x 10-6

0.95 x 104

QPSEBS/10% ZrO2 1.79 x 10-6

1.04 x 104

3.8.3 Hydrolytic and Alkaline Stability

The property of IEC was measured after the hydrolytic and alkaline

stability tests. The loss of IEC obtained was 1.25, 1.39, 1.30, 1.48 and 1.15%

for QPSEBS, QPSEBS/2.5% ZrO2, QPSEBS/5% ZrO2, QPSEBS/7.5% ZrO2

and QPSEBS/10% ZrO2 respectively. In other words, the entire membranes

exhibit the loss of IEC range from 1.15 to 1.48% only. Hence these

membranes possess good mechanical and chemical stability and also viable

candidate to work in fuel cell applications.

3.8.4 Fourier Transform Infra Red

The FTIR spectra of QPSEBS/ZrO2 are shown in Figure 3.45. The

IR spectra of QPSEBS/ZrO2 composites show the peaks at 610 and 760 cm-1

that were slightly shifted from the QPSEBS polymer. The decrease in the

intensity of these peaks confirms the interaction between the quaternary

ammonium group and zirconia and also illustrates that an electrostatic

interaction was exhibited between the polymer and the inorganic filler. The

peak at 3500 cm-1

is due to moisture absorbance. It can be clearly seen that on

155

increasing the content of zirconia filler the intensity of this peak also

increased due to the hydrophilic nature of zirconia. These observations clearly

reveal that ZrO2 has strongly bound to the polymer matrix.

4000 3500 3000 2500 2000 1500 1000 500

(c)

(d)

(e)

(b)

(a)

% T

ransm

itta

nce

Wave number (cm-1)

Figure 3.45 FTIR spectra of (a) QPSEBS, (b) QPSEBS/2.5% ZrO2,

(c) QPSEBS/5% ZrO2, (d) QPSEBS/7.5% ZrO2 and

(e) QPSEBS/10% ZrO2 composite membranes

3.8.5 Thermogravimetric Analysis

Figure 3.46 represents thermogravimetric curves of QPSEBS and

QPSEBS/ZrO2 composite membranes. When compared to pristine QPSEBS,

composite membranes exhibit higher thermal stability. Hence, the percentage

content of ZrO2 determines the thermal stability of the composite membranes.

156

0 100 200 300 400 500 600 700 800 900

0

20

40

60

80

100

(e)(d)(c)

(b)

(a)

We

igh

t re

sid

ue (

%)

Temperature (OC)

Figure 3.46 TGA curves of (a) QPSEBS, (b) QPSEBS/2.5% ZrO2,

(c) QPSEBS/5% ZrO2, (d) QPSEBS/7.5% ZrO2 and

(e) QPSEBS/10% ZrO2 composite membranes

3.8.6 Differential Scanning Calorimetry

The DSC traces of QPSEBS and 2.5, 5, 7.5 and 10% ZrO2 loaded

QPSEBS are shown in Figure 3.47. In all the traces, there is a broad

endotherm between 30-150 C due to desorption of polymer entrapped

solvent. The Tg value for QPSEBS was found to be 120 C, whereas the

composite membranes (i.e. inorganic filler incorporated QPSEBS), it was

around 80 to 90 C. The decrease in the Tg value could be attributed to the

addition of zirconia fillers in the polymer matrix.

157

100 200 300 400 500

Exo

(e)

(d)

(c)

(b)

(a)

mW

(m

g)

Temperature (OC)

Figure 3.47 DSC traces of (a) QPSEBS, (b) QPSEBS/2.5% ZrO2,

(c) QPSEBS/5% ZrO2, (d) QPSEBS/7.5% ZrO2 and

(e) QPSEBS/10% ZrO2 composite membranes

3.8.7 Scanning Electron Microscopy

The SEM images of QPSEBS/ZrO2 composite membrane are

shown in Figure 3.48. The electron micrograph shows the adhesion between

the zirconia domains and the polymer matrix is low. In the case of composite

membranes negligible amount of agglomeration was observed on the surface

of the polymer matrix.

158

Figure 3.48 SEM images of (a) QPSEBS, (b) QPSEBS/2.5% ZrO2,

(c) QPSEBS/5% ZrO2, (d) QPSEBS/7.5% ZrO2 and

(e) QPSEBS/10% ZrO2 composite membranes

3.8.8 Mechanical Properties

The tensile strength of the composite membranes incorporated with

various percentages of ZrO2 is shown in Figure 3.49. In general, there is an

improvement in the tensile strength as the inorganic content increases. The

tensile strength of the zirconia incorporated QPSEBS showed an

improvement in the tensile strength as the inorganic content increases. This is

because of the interaction between the polymer and zirconia filler.

159

0% 2.5% 5% 7.5% 10%

0

20

40

60

80

100

Ten

sile s

tren

gth

(M

Pa)

ZrO2 content

Figure 3.49 Variation of tensile strength with the addition of zirconia

3.8.9 X-Ray Diffraction Studies

The XRD patterns of QPSEBS/ZrO2 composite membranes are

shown in Figure 3.50. It can be seen that the peak intensity at 2 = 19-20 is

the QPSEBS/ZrO2 composite membranes with different weight percentage of

zirconia was reduced as compared with that of virgin QPSEBS polymer. In

composite membranes, apart from the main peak, the sharp intense peaks

appeared at 2 angle of 28.11, 50.16 and 55.80 are referred to the zirconia and

are assigned to (111), (022) and (031) facets respectively. Among various

samples, QPSEBS/10% ZrO2 composite membrane shows the lowest peak

intensity at a 2 angle of 19-20 , which means that this membrane has the

lowest crystallinity. The low crystallinity reveals that more amorphous phase

exists in this membrane, indicating that the structure of the membrane is more

disordered and QPSEBS and ZrO2 are mixed more uniformly.

160

10 20 30 40 50 60 70 80

(c)

(e)

(d)

(b)

(a)

Inte

nsit

y (

A. U

)

2 (O)

Figure 3.50 XRD patterns of (a) QPSEBS, (b) QPSEBS/2.5% ZrO2,

(c) QPSEBS/5% ZrO2, (d) QPSEBS/7.5% ZrO2 and

(e) QPSEBS/10% ZrO2 composite membranes

The calculated percentage of crystallinity of QPSEBS and

QPSEBS/ZrO2 composites is given in Table 3.11. From the Table, it can be

seen that the percentage crystallinity of the composite decreased with

increasing content of inorganic filler i.e. ZrO2. At 10 wt % of ZrO2, the

composite became more amorphous and showed low percentage of

crystallinity. For fuel cell applications, the membrane should possess

amorphous behavior in other words low crystallinity because low crystallinity

reveals good ionic conductivity (Wang et al 2010). Hence poor crystallinity

gives better performance in fuel cell.

161

Table 3.11 Percentage of crystallinity values of QPSEBS/ZrO2

composite membranes

Membrane % of crystallinity

QPSEBS 3.08

QPSEBS/2.5% ZrO2 2.45

QPSEBS/5% ZrO2 1.85

QPSEBS/7.5% ZrO2 1.62

QPSEBS/10% ZrO2 1.37

3.8.10 AMFC performance study

Figure 3.51 AMFC performances of QPSEBS/ZrO2 using three different

cathode catalysts namely (a) Pt/C, (b) Ag/C and (c) Pd-Ni/C

162

The polarization and power density curves of QPSEBS and the

composite membranes, obtained from the alkaline membrane fuel cell using

Pt as the anode and three different cathode catalysts are presented in Figure

3.51. The alkaline membrane fuel cell with QPSEBS/ZrO2 composite

membranes gave better performance than that with pristine QPSEBS and

commercially available AEM. The OCV for the QPSEBS/10% ZrO2

composites using Ag/C catalyst exhibit 0.78 V at 60 C.

In terms of power density, the QPSEBS/10% ZrO2 using Ag/C

cathode catalyst exhibit maximum power density of 198 mW/cm2. As a whole

in terms of OCV and maximum power density, QPSEBS/ZrO2 shows better

results and hence could be a strong candidate in alkaline fuel cell applications.

3.8.11 DMAMFC performance study

0 50 100 150 200 250

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Po

wer

de

nsit

y (

mW

/cm

2)

Cell v

olt

ag

e (

V)

Current density (mA/cm2)

QPSEBS

AMI-7001

QPSEBS/2.5% ZrO2

QPSEBS/5% ZrO2

QPSEBS/7.5% ZrO2

QPSEBS/10% ZrO2

-10

0

10

20

30

40

50

60

70

80

Figure 3.52 DMAMFC performances of QPSEBS/ZrO2

163

The performance of QPSEBS/ZrO2 investigated using Pt-Ru/C and

Pt/C as the anode and cathode catalyst respectively is depicted in Figure 3.52.

I-V curves showed the improved performance of the composite membranes

when compared to virgin QPSEBS and AMI-7001 membrane, when

DMAMFC operated at 60 °C. The OCV of QPSEBS/10% ZrO2 was found to

be 0.71 V and the maximum power density of 69 mW/cm2 was achieved at a

current density of 200 mA/cm2.

3.9 QPSEBS/MWCNT COMPOSITE MEMBRANES

3.9.1 Functionalization and Confirmation of MWCNT

There are two critical issues associated with CNTs, when these

materials are used as reinforcements in polymer composites (Delozier et al

2006). The major problem concerns the aggregation of CNTs into bundles

resulting in weak interfacial bonding between CNTs and the polymer matrix

which leads to a depreciation in the electrical and mechanical properties of the

composites (Xu and Wang 2008). To overcome this problem, it is necessary

to develop a strong interface between the fillers and the polymer, and thus, the

efficiency of the load transfer could be made higher. A widely employed

approach towards this objective reported thus far in the literature is the

surface functionalization of CNTs (Koval’chuk et al 2008, McIntosh et al

2007). Generally, covalent functionalization will introduce defects on the

CNT surface, disrupting the extent of conjugation and adversely impacting

charge carrier mobility in CNTs (Bekyarova et al 2005). However, since

MWCNTs are less prone to bundle than SWCNTs and because it is possible

to functionalize the surface layers only, the conductivity will not be reduced

significantly (Hatton et al 2008).

Carbon nanotubes functionalized with carboxylic acid groups were

confirmed by FT–IR. Figure 3.53 represents FTIR spectra of MWCNT and

164

functionalized MWCNT (f-MWCNT). There are four distinct absorption

peaks at 1564, 2862, 2932 and 3440 cm-1

in Figure 3.53 (b). The spectrum

shows a band around 3440 cm-1

which can be attributed to the hydroxyl group

(-OH). Bands around 2932 and 2850 cm-1

were due to asymmetric and

symmetric C-H stretching. In the acid-functionalized MWCNTs spectrum, the

peak near 1560 cm1 corresponds to the infrared-active phonon mode of the

nanotubes and the peaks at 1740 and 1250 cm1 apparently correspond to the

stretching mode of the carboxylic acid groups. These observations are clear

indications of –COOH groups on the surface of nanotubes.

4000 3500 3000 2500 2000 1500 1000 500

15

64

1250

3440

2862

293

2 17

40

(b)

(a)

% T

ran

sm

itta

nce

Wavenumber (cm-1)

Figure 3.53 FTIR spectra of (a) MWCNT and (b) functionalized MWCNT

Figure 3.54 represents the FE-SEM photographs of as-purchased

MWCNT and f-MWCNT with their corresponding EDAX spectra. Pristine

MWCNTs which have smooth surfaces appeared as large bundles with

lengths in the order of micron and diameters around 10–20 nm. After acid

165

treatment, the f-MWCNTs were disentangled and roughed, and their lengths

were slightly reduced by both the oxidation and ultrasonic treatment. The

same was confirmed through corresponding EDAX spectrum with the

presence of other elements.

Figure 3.54 FE-SEM images of (a) MWCNT, (b) f-MWCNT and their

corresponding EDAX spectra

3.9.2 Characterization of QPSEBS/MWCNT

3.9.2.1 Water absorption, ion exchange capacity and ionic conductivity

Water uptake is an important parameter in studying AMFCs,

because the water that resides in the hydrophilic domains facilitates the

transport of OH- ions. However, too much of water uptake will result in loss

of mechanical stability. The water absorption, IEC and conductivity values of

QPSEBS and their composite membranes are shown in Table 3.12. The IEC

166

of the composites decreased with increase in the weight fraction of f-

MWCNT. The decreasing trend may be attributed to the decrease in the

concentration of QPSEBS or in other words decrease in the effective

concentration of the quaternary ammonium groups in the composites.

As expected, the water uptake values increased with increase in the

weight fraction of f-MWCNT in the membrane. This increase in the water

absorption is due to the incorporation of functionalized (-COOH) MWCNTs.

One of the most important parameter that governs the suitability of

a polymer electrolyte membrane for use in fuel cell is its ion conducting

ability. The ionic conductivity of pristine QPSEBS was found to be 1.51x10-2

S/cm. In the case of the composites, there is an increase in the ionic

conductivity with increase in the content of f-MWCNT. This increasing trend

can be explained by looking into the transportation of hydroxyl ions which

was facilitated by the presence of water molecules. The increment in ionic

conductivity was also associated with the acidic site of f-MWCNT in the

presence of water. Hence the ionic conductivity increased with increasing

content of f-MWCNTs.

Table 3.12 Water absorption, IEC and conductivity values of

QPSEBS/f-MWCNT composite membranes

Membrane Water

absorption, %

IEC, meq/g Conductivity,

S/cm

AMI-7001 17 1.3 1.72 x 10-2

QPSEBS 4.04 0.958 1.51 x 10-2

QPSEBS/2.5% MWCNT 5.78 0.941 1.59 x 10-2

QPSEBS/5% MWCNT 6.56 0.908 1.66 x 10-2

167

3.9.2.2 Methanol Permeability and Selectivity Ratio

Table 3.13 Methanol permeability and selectivity ratio of QPSEBS/f-

MWCNT composite membranes

Membrane Methanol permeability,

cm2/s

Selectivity ratio,

Ss/cm3

AMI-7001 2.25 x 10-6

0.76 x 104

QPSEBS 2.14 x 10-6

0.71 x 104

QPSEBS/2.5% MWCNT 1.97 x 10-6

0.80 x 104

QPSEBS/5% MWCNT 1.86 x 10-6

0.89 x 104

Table 3.13 shows the methanol permeability and selectivity ratio of

AMI-7001, QPSEBS and composite membranes. When compared with

QPSEBS, f-MWCNT incorporated QPSEBS shows a remarkable decrement

in the methanol permeability. The resistance to the flow of methanol became

better than QPSEBS and AMI-7001. The pristine QPSEBS and AMI-7001

electrolyte membranes exhibited a selectivity ratio of 0.71 x104 Ss/cm

3 and

0.76 x104 Ss/cm

3 respectively which are lower when compared to QPSEBS/f-

MWCNT composite membranes. It indicates that the QPSEBS/f-MWCNT

composite membranes can be successfully used in DMAMFCs.

3.9.2.3 Hydrolytic and alkaline stability

The property of IEC was measured after the hydrolytic and alkaline

stability tests. The loss of IEC was found to be 1.25 to 1.56% for all the

prepared membranes, which clearly concludes the IEC was not significantly

changed, which means that the prepared QPSEBS/MWCNT composite

membranes have good mechanical and chemical stability.

168

3.9.2.4 Fourier transform infra red

FTIR spectra of QPSEBS, QPSEBS/2.5% MWCNT and

QPSEBS/5% MWCNT are presented in Figure 3.55. The change in the

intensity ratio of the quaternary ammonium peaks was observed between the

bare polymer and composites. This result may be due to the discrepancy in the

polymer structures on the surface of carbon nanotubes. Also the intensity of

peak was broadened, as evidenced by the reduction in the relative strength of

IR signals at 1600 cm1 and 1371 cm

1.

4000 3500 3000 2500 2000 1500 1000 500

(c)

(b)

(a)

% T

ran

sm

itta

nce

Wavenumber (cm-1)

Figure 3.55 FTIR spectra of (a) QPSEBS, (b) QPSEBS/2.5% f-MWCNT

and (c) QPSEBS/5% f-MWCNT

169

3.9.2.5 UV-Visible Spectroscopy

200 300 400 500 600 700 800

0

2

4

6

8

10

(b)

(a)

Ab

so

rban

ce

Wavelength (nm)

Figure 3.56 UV-Visible spectra of (a) QPSEBS and (b) QPSEBS/5%

f-MWCNT. Inset is expanded view of (b)

To elucidate the interactions between f-MWCNTs and quaternized

polymer matrix, UV-visible spectra of QPSEBS and QPSEBS/ f-MWCNT

dispersion in cyclohexane solvent were measured (Figure 3.56). UV-Visible

spectroscopy was effectively used for quantitative characterization of the

colloidal stability of f-MWCNT dispersions (Jiang et al 2003) and for the

investigation of dispersion behavior which have correlations with solubility

parameters based on the interactions between filler and polymer (Ham et al

2005). By comparing the relative heights for the peaks at 215 nm and 260 nm,

we can see that the band at 260 nm in the nanocomposite showed a higher

absorption than that in the bare polymer which indicates that there is an

interaction between f-MWCNT and QPSEBS mainly via - bonding

(Baskaran et al 2005, Frehill et al 2005).

250 260 270 280 290 300 310 320 330 340 350

0

1

2

260 nm

Absorb

ance

Wavelength, nm

170

3.5.4.2.6 Thermogravimetric analysis

0 100 200 300 400 500 600 700 800

30

40

50

60

70

80

90

100

110

(c)

(b)

(a)

Weig

ht re

sid

ue (%

)

Temperature (OC)

Figure 3.57 TGA curves of (a) QPSEBS, (b) f-MWCNT and

(c) QPSEBS/5% f-MWCNT

The effect of f-MWCNTs presence on the thermal stability of

QPSEBS composites was studied by means of TGA under N2 atmosphere, as

shown in Figure 3.57. Figure 3.57 (a) represents thermogravimetric curve of

QPSEBS membrane. One can clearly say there are three-step weight losses

during 50-550 C. The first step occurred below 100 C and may be related to

the loss of moisture. The second loss, commencing around 150 C, possibly

resulted from decomposition of quaternary ammonium group and desorption

of bound water. The third weight loss that started around 450 C indicates the

onset of polymer backbone decomposition. Figure 3.57 (b) and (c) represent

pristine MWCNT and 5% f-MWCNT composite respectively. Once the f-

MWCNT was added into the polymer matrix, the thermal stability of the

composite membranes was synergistically enhanced. Only one weight loss

171

occurred for the composite membrane. Hence, the addition of f-MWCNTs

determines the thermal stability of the composite membranes.

3.9.2.7 Scanning electron microscopy

The surface morphology of the QPSEBS and composite membrane

was investigated by scanning electron microscopy. Figure 3.58 shows the

SEM images of 5% f-MWCNT reinforced QPSEBS. The f-MWCNTs were

found to be dispersed uniformly in the quaternized polymer matrix. There

were some white dots in images representing the area of the f-

MWCNTs/QPSEBS membranes, and this proves that the multiwalled carbon

nanotubes can be effectively dispersed when cyclohexane was added via the

described solution-cast method with the f-MWCNTs. The improved

dispersion is probably because of covalent or ionic bond formation between

the carbon nanotubes and the QPSEBS.

Figure 3.58 SEM images of QPSEBS/5% f-MWCNT

3.9.2.8 Mechanical properties

One of the primary reasons for adding inorganic fillers to the

polymer matrix is to improve their mechanical performance; excellent

mechanical properties of polymer nanocomposites are vital for the

172

performance in fuel cell (Mammeri et al 2005). The composite membrane

displayed remarkable improvements in mechanical strength, exhibiting tensile

strength values of 69.1 MPa for the QPSEBS/5% f-MWCNT and 54.6 MPa

for the QPSEBS/2.5% f-MWCNT, which are 221% and 175% higher than

that of the virgin QPSEBS membrane (31.23 MPa). It is well known that the

mechanical property and the structural integrity of the composite materials are

strongly affected by interfacial bonding and properties between fillers and

polymer matrices (Hull and Clyne 1996). The enhanced mechanical strength

data indicate that the modified f-MWCNTs are homogeneously dispersed

within the QPSEBS matrix without any significant aggregation of the

MWCNTs (Joo et al 2008).

3.9.2.9 X-ray diffraction studies

10 20 30 40 50 60 70

(002)

(d)

(c)

(b)

(a)

Inte

nsity (A

.U)

2 (O)

Figure 3.59 XRD patterns of (a) QPSEBS, (b) MWCNT,

(c) QPSEBS/2.5% f-MWCNT and (d) QPSEBS/5% f-MWCNT

173

In order to confirm the influence of multiwalled carbon nanotubes

on the crystal structure of QPSEBS, XRD spectra of MWCNTs, bare

polymer, and nanocomposites of 2.5% f-MWCNTs and 5% f-MWCNTs were

studied (Figure 3.59). Diffraction pattern of MWCNTs is characterized by a

strong peak at 2 = 25.9 which is ascribed to the graphite-like structure. In

the spectrum of bare polymer, a broad peak at 2 = 19-20 represents the

crystalline nature of the polymer. The addition of f-MWCNTs into the

polymer matrix did not cause a discernible change in the lattice structure of

QPSEBS as can be seen in Figure 3.59. There was only a small reduction in

the sharpness of the crystalline peaks, as the MWCNTs were present in the

polymer matrix and it only slightly changed the crystalline nature of the bare

polymer i.e. low crystallinity. The low crystallinity reveals that more

amorphous phase exists in the composite membrane, indicating that the

structure of the membrane is more disordered and QPSEBS and MWCNTs

are mixed more uniformly. Good mixing of QPSEBS and f-MWCNTs is

useful for the enhancement of ionic conductivity.

The calculated percentage of crystallinity of QPSEBS and

QPSEBS/MWCNT composites is given in Table 3.14. From the Table, it can

be seen that the percentage crystallinity of 5 wt % f-MWCNT exhibits lower

percentage of crystallinity (1.67%). For alkaline membrane fuel cell

applications, the membrane should possess amorphous behavior in other

words, low crystallinity because low crystallinity reveals good ionic

conductivity.

Table 3.14 Percentage of crystallinity values of QPSEBS/f-MWCNT

composite membranes

Membrane % of crystallinity

QPSEBS 3.08

QPSEBS/2.5% MWCNT 2.02

QPSEBS/5% MWCNT 1.67

174

3.9.2.10 Raman spectroscopy

Raman spectroscopy has been widely used to evaluate the

interaction between polymers and CNTs in CNT based composites (Gao et al

2010). Generally, the interaction between f-MWCNTs and matrix can be

reflected by the Raman shifts of the characteristic peaks (Schadler et al 1998).

There are several main peaks for the QPSEBS film, located at 998, 1027,

1190, 1303, and 1440 cm1

as seen in Figure 3.60. In particular, the peak at

998 cm1 is due to the C–N stretching; the peak of 1303 cm

1 could be due to

the C–H deformation. It is a proof that PSEBS polymer was successfully

quaternized by triethylamine via a quaternization process. The most

prominent Raman spectra modes of f-MWCNTs are the G-band (1580 cm–1

)

and the D-band (1340 cm–1

) which appeared in the composite membrane

(Figure 3.60(b)). The other characterization peaks present in the bare

membrane were absent in the composite membrane and also the intensity of

the main peaks were greatly decreased, which clearly illustrates that the f-

MWCNTs are well blended with QPSEBS matrix and it could be preferably

used in fuel cell applications.

600 800 1000 1200 1400

(a)

Inte

nsit

y (

a.u

)

Wave number (cm-1)

800 1000 1200 1400 1600

(b)

Inte

ns

ity

(a

.u)

Wave number (cm-1)

Figure 3.60 Raman spectra of (a) QPSEBS and (b) QPSEBS/5% f-

MWCNT

175

3.9.2.11 AMFC performance study

Figure 3.61 AMFC performances of QPSEBS/ f-MWCNT using three

different cathode catalysts namely (a) Pt/C, (b) Ag/C and

(c) Pd-Ni/C

The polarization and power density curves of QPSEBS and the

composite membranes, obtained from the alkaline membrane fuel cell using

Pt as the anode and three different cathode catalysts are presented in Figure

3.61. The measurements were made by feeding hydrogen and oxygen with

flow rate of 20 and 40 mL/min respectively at 60 C. The alkaline membrane

fuel cell with a QPSEBS/5% f-MWCNT gave better performance than that

with other composites, pristine QPSEBS and commercially available AEM.

The OCV for the QPSEBS/5% f-MWCNT was found to be 0.76, 0.73 and

0.75 V for Pt/C, Ag/C and Pd-Ni/C cathode catalyst respectively at 60 C.

In terms of power density, the QPSEBS/5% f-MWCNT using Pt/C

and Ag/C cathode catalysts show maximum power density of 172.8 and 187.2

mW/cm2 respectively, followed by Pd-Ni/C of 156 mW/cm

2. As a whole in

176

terms of OCV and maximum power density, QPSEBS/5% f-MWCNT has

better results when compared to the other composites and commercially

available anion exchange membrane.

3.9.2.12 DMAMFC performance study

0 50 100 150 200 250

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Po

wer

den

sit

y (

mW

/cm

2)

Cell v

olt

ag

e (

V)

Current density (mA/cm2)

QPSEBS

AMI-7001

QPSEBS/2.5% MWCNT

QPSEBS/5% MWCNT

0

10

20

30

40

50

60

Figure 3.62 DMAMFC performances of QPSEBS/MWCNT composites

The performance of QPSEBS/MWCNT was investigated using Pt-

Ru/C and Pt/C as the anode and cathode catalyst respectively is depicted in

Figure 3.62. I-V curves showed the improved performance of the composite

membranes when compared to virgin QPSEBS and AMI-7001 membrane,

when DMAMFC operated at 60 °C. The OCV of QPSEBS/5% f-MWCNT

was found to be 0.76 V and the maximum power density of 59.5 mW/cm2 was

achieved at a current density of 175 mA/cm2.