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Aligned CNT/Polymer nanocomposite membranes forhydrogen separation

Anshu Sharma, Sumit Kumar, Balram Tripathi, M. Singh, Y.K. Vijay*

Department of Physics, University of Rajasthan, Jaipur 302004, India

a r t i c l e i n f o

Article history:

Received 10 January 2009

Received in revised form

25 February 2009

Accepted 27 February 2009

Available online 26 March 2009

Keywords:

CNT/Polymer nanocomposites

Gas permeation

I–V Characteristics and

Surface topography

* Corresponding author. Tel.: þ91 141 270245E-mail addresses: anshushsharda@gmail

0360-3199/$ – see front matter ª 2009 Interndoi:10.1016/j.ijhydene.2009.02.068

a b s t r a c t

CNT/Polymer nanocomposites have been fabricated by dispersing (0.1%) weight fraction of

SWNT and MWNT in polycarbonate matrix separately using benzene as a solvent. Align-

ment has been performed by inducing DC electric field (500 V/cm). X-ray diffraction

measurements have been performed to confirmation of SWNT, MWNT and their presence

in PC matrix. Gas permeability has been found to be increased in aligned CNT/polymer

nanocomposites comparison to random dispersed CNT/polymer nanocomposites. The

electrical conductivity in aligned CNT/polymer composite membranes indicates two

resistive regions. Experimental results exhibits here that CNT/polymer nanocomposite

membranes can be used as good hydrogen separating media. Surface morphology of

aligned CNT/polymer nanocomposites was confirmed by optical microscopy.

ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction are orders of magnitude faster than in zeolites having

Membrane technology provides opportunities to conduct

important separations with minimal use of energy and can

potentially offer economic environmental and high perfor-

mance benefits to process reliant on gas separations. However

despite the ability to produce robust, large area membranes at

relatively low cost, a wider implementation of polymer

membranes is hindered by their intrinsic permeability and

selectivity limitations. These limitations were first identified

by Robeson and characterized by Freeman [1,2]. To improve

polymeric membrane performance a considerable research

effort has focused on the addition of inorganic materials such

as zeolites or carbon molecular sieves to polymers [3–10].

Recently, computer simulations have been used to investigate

the adsorption [11], selectivity and transport properties [12] of

light gases in single walled carbon nanotubes (SWNTs). Sholl

and co-workers were the first to predict that transport diffu-

sivities of gases in single walled carbon nanotubes (SWNTs)

7; fax: þ91 141 2707728..com (A. Sharma), yk_vijaational Association for H

comparable pore sizes [13], give the high selectivities theo-

retically possible due to the precise diameter of the nano-

tubes. The transport of the permeating species depends

strongly on the membrane pore diameter and the interaction

of that species with the membrane structure. Accessible pores

are classified as micropores, mesopores and macropores

which provides approximate boundaries for different trans-

port and separation mechanisms that are relevant for gases

and liquids [14]. The main purpose of this study is to construct

highly permeable and selective membranes containing

carbon nanotubes inside a polymer matrix that could easily be

scaled up to large area membranes.[15] These nanocomposite

membranes consist of well dispersed SWNTs inside

a commercial polycarbonate (PC) matrix. However the prep-

aration of satisfactory CNT composites is still great challenges

that still need to be overcome to get their full potential.

Effective use of CNTs in composite applications depends on

the ability to disperse the CNTs uniformly through the matrix.

y@sancharnet.in (Y.K. Vijay).ydrogen Energy. Published by Elsevier Ltd. All rights reserved.

0

50

100

150

200

250

300

350

001

002

002

b

a

In

ten

sity (A

rb

.)

b MWNTa SWNT

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 3 9 7 7 – 3 9 8 23978

Due to Vander wall attraction forces between CNTs, they tend

to form agglomerates or bundles instead of individual tubes

[16]. Thus they have very low solubility in solvents and tend to

remain as entangled agglomerates .CNTs are highly aniso-

tropic in nature because of their high aspect ratio. It is

important to have aligned CNTs in polymer matrix to take

advantage of their anisotropic structure and to have improved

properties in the direction of alignment. The electric field

alignment technique is very powerful and of great importance

since nanotubes can be placed at specific locations in a more

simple way to realize functional devices and circuits. These

CNT dispersed membranes were characterized by XRD, gas

permeation, electrical conductivity and surface topography

measurements.

20 25 30 35 40 45 50 55 60 65 70Angle (2Theta)

Fig. 2 – X-ray diffraction patterns for (a) SWNT and (b)

MWNT.

2. Experimental

2.1. CNT/Polymer nanocomposite preparation

The Polycarbonate (PC), a glassy polymer (Gadra Plastic Poly-

mer Pvt. Ltd., Bharuch, Gujarat) used for the present study.

The carbon nanotubes used in this work was purchased from

Helix material solution Richardson, Texas. Dispersion of

SWNT (w1.3 nm diameter, 0.5–40 mm length) and MWNT (10–

30 nm diameter, 1–2 mm length,) in PC have been performed by

using ultrasonicator (220 W, 20 kHz). Benzene has been used

as a solvent. The sonication has been done for 1 h. These CNT/

polymer nanocomposites have been prepared by solution cast

method [17].

2.2. Alignment of CNT in polymer

Fig. 1, shows the electric field alignment setup. It is two elec-

trodes geometry, the separation between the electrodes is

10 cm and the applied voltage between these electrodes is

5 kV. The net electric field produced by this setup is 500 V/cm,

which was applied during the casting of these nano-

composites. The prepared mixture of SWNT/PC and MWNT/

PC after sonication was spread over flat bottom Petrie dishes

floating on Hg between two parallel plate electrodes, where

DC bias was applied. The alignment was allowed to occur until

the placed MWNT/PC and SWNT/PC suspended and benzene

as a solvent was completely evaporated.

Fig. 1 – Electric field alignment setup.

2.3. X-ray diffraction

X-ray diffraction measurements have been performed by

using P analytical system having Cu Ka, as a radiation source

of wavelength l¼ 1.0425 A within 2q¼ 10–70� at the scan

speed 0.5�/min. For the confirmation of SWNT and MWNT as

reported in the literature The analysis has been performed by

using Powder X software [18].

2.4. Gas permeation

The permeability of gas was calculated by the Fick’s formula

P ¼ Flux� thickness of membrane

pressure difference

Perm selectivity is the ratio of permeability of one gas to

another and is given by aAB¼ PA/PB where A and B refer to

different gases. The flux was estimated by the gas flow rate

through the membrane, measured by the flow rate meter.

Fig. 3 – X-ray diffraction patterns for (a) PC/0.1% SWNT, (b)

PC/0.1% MWNT.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 3 9 7 7 – 3 9 8 2 3979

A 38 mm diameter membrane with porous support was

placed in cell. The air was purged out 4–5 times with the

experimental gas to avoid impurities. The gas was fed

through a regulator and pressure was applied on the high

pressure side. The permeate side of the diffusion cell was

connected to a glass capillary of 2 mm diameter. The

membrane area exposed to high-pressure gas was

506 mm2. Several readings were taken till a constant flow

rate was obtained [19].

2.5. I–V characteristics

I–V characteristics measurements have been performed by

using Keithley-238 model electrometer. The applied voltage in

the dispersed samples was within the range of�40 V toþ40 V.

Aluminium has been deposited on both side of the CNT/

Polymer nanocomposites for electrical contacts.

2.6. Optical microscopy

Surface topography has been performed by using Labomed

optical microscope at magnification 10� 40 having resolution

of the order of 1 mm.

0

50

100

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200

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350

400c

b

a

2 4 6 8 10

2 4 650

100

150

200

250

300

350

400

Perm

eab

ility (b

arrer)

No of cycle (Arb.)

Perm

eab

ility (b

arrer)

No. of Cyc

a PC(pristine)b PC/0.1% SWNT without fieldc PC/0.1% SWNT with field

a 50µmb 40µmc 30µm

a

c

Fig. 4 – Permeation of hydrogen gas through (a) PC/0.1% SWNT

membrane (c) Dependence of gas permeation on thickness.

3. Results and discussions

3.1. XRD

Fig. 2, shows the X-ray diffraction patterns of pristine SWNT

and MWNT and Fig. 3 shows X-ray diffraction patterns of PC/

0.1% SWNT, PC/0.1% MWNT respectively. The analysis has

been performed by using powder X software. It is found that

for pristine MWNT the (002) plane is observed at 26� while for

SWNT the (022) plane is at 25.5�. These results have been

compared to available references in the literature for the

confirmation of SWNT and MWNT. Fig. 3 shows the presence

of SWNT and MWNT in the PC matrix separately.

3.2. Gas permeation

Fig. 4, shows the gas permeability of SWNT/PC and MWNT/PC

membranes (40 mm) respectively. From Fig. 4(a) it is clear that

permeability in the aligned SWNT is 350 barrer while in

random dispersed case it is below 50 barrer. It shows that

aligned SWNT in polycarbonate matrix provides the easy

channel to permeate the hydrogen fastly. From Fig. 4(b), it is

observed that permeability also increases in case of aligned

MWNT and it is of the order of 13 barrer, while for random

5

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9

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12

13

14c

b

a

No of Cycle (Arb.)

1 2 3 4 5

8 10

c

b

a

Perm

eab

ility (b

arrer)

le (Arb.)

a Pristine PCb PC/0.1% MWNT without fieldc PC/0.1% MWNT with field

b

composite membrane (b) PC/0.1% MWNT composite

-35-30-25-20-15-10-505

10152025

c

b

aCu

rren

t (m

icro

-am

p)

Voltage (Volts)

a PC/MWNT or PC/SWNT without fieldb PC/MWNT (with field)c PC/SWNT (with field)

0.05

0.06

0.07

0.08

0.09

0.10

dI/d

V

V

-40 -30 -20 -10 0 10 20 30 40

-30 -20 -10 0 10 20 30

-30 -20 -10 0 10 20 30

-0.006

-0.004

-0.002

0.000

0.002

0.004

0.006

0.008

0.010

0.012

d2I/d

V2

V

a b

c

Fig. 5 – (a) I–V characteristics for CNT/Polymer nanocomposites, (b) Corresponding voltage (V) versus dI/dV, (c)

Corresponding voltage (V) versus d2I/dV2 plot.

Fig. 6 – Surface topography of (a) pristine PC (b) 0.1% MWNT/PC without field (c) 0.1% MWNT/PC with field.

Fig. 7 – Surface topography of (a) 0.1% SWNT/PC without field (b) 0.1% SWNT/PC with field.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 3 9 7 7 – 3 9 8 2 3981

dispersed it is of the order of 11 barrer. A significant

enhancement in flow rate of hydrogen gas and flow of current

through membranes confirmed the improved alignment of

carbon nanotubes in polymer matrix. It may be due to the

precise diameters of the nanotubes or the new available

nanoporosity in the polymers having good permeation

potential [20]. It is found that the permeability in aligned

SWNT/PC is higher than the MWNT/PC nanocomposites may

be due to better alignment of SWNT in PC comparison to the

MWNT in PC due to the agglomeration of MWNT. The align-

ment mechanism can understood that the dipole moments

are induced in the nanotubes by applied electric field and

subsequently the nanotubes move towards the electrodes for

the alignment due to coulomb force [15].Owing to strong

dipole moment in the axis parallel to the length of the nano-

tubes they attempt to align perpendicular to the parallel

electrodes and along the electric field direction. Therefore

several nanotubes align in polymer matrix by linking up one

to another forming an interconnecting rope like structure

since the length of CNT is smaller than the 40 mm, thickness of

the polymer membrane. Fig. 4(c) shows the dependence of gas

permeation on the thickness of the nanocomposites. It is

clearly observed that gas permeation is higher for lower

thickness comparison to the higher thickness of the nano-

composite membranes. Thickness versus permeation

measurement has been performed to select the thickness of

the nanocomposites.

3.3. Voltage–current characteristics

Fig. 5(a) shows the I–V characteristics of CNT/Polymer nano-

composite membranes which are giving dramatically differ-

ence between random dispersed CNT/PC nanocomposites and

aligned CNT/PC nanocomposite membranes. The total

tunneling current has a kink which is a function of the applied

voltage. This kink becomes a step in differential conductance

(dI/dV) plot and a peak in the d2I/dV2 plot [21]. This nonline-

arity of I–V curves indicates the semi conducting behavior of

aligned carbon nanotubes and their ability to be used for the

fabrication of electronic nanodevices.

3.4. Surface topography

Fig. 6(a, b & c), shows the (a) surface topography of pristine PC,

(b) random dispersed MWNT in polycarbonate (PC) and (c)

aligned MWNT in Polycarbonate (PC). Fig. 7(a) shows the

surface topography of random dispersed SWNT in PC matrix

and Fig. 7(b) shows the aligned SWNT in PC matrix. It is clear

from these figures that aligned MWNT and SWNT in PC looks

perpendicular to the base PC. In case of aligned SWNT/PC the

open tips at the surface are more clear than the aligned

MWNT/PC. The scale for all figures is 10 mm.

4. Conclusions

It is concluded from the above study that gas permeability in

aligned SWNT/PC and MWNT/PC nanocomposites have been

found to be increased. This confirms that the aligned carbon

nanotubes in polymer nanocomposites provides easy channels

or porosity for permeation of hydrogen. The gas permeation in

aligned SWNT/PC is higher than the aligned MWNT/PC, it is

also confirmed by theoptical topography that in alignedSWNT/

PC, open tips of SWNT in PC are clearly shown, while in case of

aligned MWNT/PC alignment is there but open tips are not clear

as well as for MWNT/PC. It is suggested that CNT/polymer

nanocomposites can be used as a good separating media. From

I–V characteristic measurements it has been observed that flow

of current across the aligned CNTs in PC is increased, it may be

due to available easy conducting channels in PC provided by

CNTs. Therefore, by aligning the carbon nanotubes in polymer

one can improve mass transport property as well as electrical

conduction. Surface topography also confirms the dispersion

as well as alignment of CNT in polycarbonate.

Acknowledgements

The authors are thankful to MNRE (Ministry of new and

renewable energy resources) New Delhi for providing funding

assistance and DSA, Department of Physics, University of

Rajasthan, Jaipur for providing experimental facilities.

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