Aligned CNT/Polymer nanocomposite membranes for hydrogen separation

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    een performed to confirmation of SWNT, MWNT and their presence

    pport

    al use

    selectivity limitations. These limitations were first identified

    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)

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

    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 thematrix.

    * Corresponding author. Tel.: 91 141 2702457; fax: 91 141 2707728.vijay@sancharnet.in (Y.K. Vijay).

    Avai lab le a t www.sc iencedi rec t .com

    w.

    i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 4 ( 2 0 0 9 ) 3 9 7 7 3 9 8 2E-mail addresses: anshushsharda@gmail.com (A. Sharma), yk_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 [310].

    highly permeable and selective membranes containing

    carbon nanotubes inside a polymermatrix that could easily be

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

    membranes consist of well dispersed SWNTs insidepotentially offer economic environmental and high perfor-

    mance benefits to process reliant on gas separations. However

    despite the ability to produce robust, large areamembranes at

    relatively low cost, a wider implementation of polymer

    membranes is hindered by their intrinsic permeability and

    strongly on the membrane pore diameter and the interaction

    of that species with themembrane 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 gasesKeywords:

    CNT/Polymer nanocomposites

    Gas permeation

    IV Characteristics and

    Surface topography

    1. Introduction

    Membrane technology provides o

    important separations with minim0360-3199/$ see front matter 2009 Interndoi:10.1016/j.ijhydene.2009.02.068electrical 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.

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

    unities to conduct

    of energy and can

    are orders of magnitude faster than in zeolites having

    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 dependsAccepted 27 February 2009

    Available online 26 March 2009in PC matrix. Gas permeability has been found to be increased in aligned CNT/polymer

    nanocomposites comparison to random dispersed CNT/polymer nanocomposites. The25 February 2009 measurements have bReceived 10 January 2009

    Received in revised form

    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 diffractionAligned CNT/Polymer nanocomhydrogen separation

    Anshu Sharma, Sumit Kumar, Balram Tripa

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

    a r t i c l e i n f o

    Article history:

    a b s t r a c t

    CNT/Polymer nanoco

    j ourna l homepage : wwational Association for Hposite membranes for

    i, M. Singh, Y.K. Vijay*

    osites have been fabricated by dispersing (0.1%) weight fraction of

    e lsev ier . com/ loca te /heydrogen Energy. Published by Elsevier Ltd. All rights reserved.

  • 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

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

    DC biaswas applied. The alignmentwas allowed to occur until

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

    Perm selectivity is the ratio of permeability of one gas to

    another and is given by aAB PA/PB where A and B refer todifferent gases. The flux was estimated by the gas flow rate

    through the membrane, measured by the flow rate meter.

    20 25 30 35 40 45 50 55 60 65 70

    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 on a l j o u r n a l o f h y d r o g e n en e r g y 3 4 ( 2 0 0 9 ) 3 9 7 7 3 9 8 23978as a solvent was completely evaporated.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, wheremeasurements.

    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, 0.540 mm length) and MWNT (10

    30 nmdiameter, 12 mm length,) in PC have been performed byFig. 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 l 1.0425 A within 2q 1070 at the scanspeed 0.5/min. For the confirmation of SWNT and MWNT asreported 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 Ficks formula

    P Flux thickness of membranepressure difference

    Angle (2Theta)

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

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

    PC/0.1% MWNT.

  • A 38 mm diameter membrane with porous support was

    placed in cell. The air was purged out 45 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. IV characteristics

    IV characteristics measurements have been performed by

    using Keithley-238model electrometer. The applied voltage in

    the dispersed samples was within the range of40 V to40 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 resolutionof the order of 1 mm.

    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 forSWNT the (022) plane is at 25.5. These results have beencompared 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

    100

    350

    400c

    b

    13

    14c

    6

    300

    Per

    ) )

    barre

    Cyc

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

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

    a b

    i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 4 ( 2 0 0 9 ) 3 9 7 7 3 9 8 2 39792 450

    100

    150

    200

    250

    Perm

    eab

    ility (

    No. of 0

    50a

    2 4 6 8 10

    350

    400

    No of cycle (Arb.)

    r)

    a 50mb 40mc 30m

    c150

    200

    250

    300

    meab

    ility (b

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

    membrane (c) Dependence of gas permeation on thickness.8 10

    c

    b

    a

    le (Arb.)5

    6

    7

    8

    9

    10

    11

    12 b

    a

    No of Cycle (Arb.)

    1 2 3 4 5

    Perm

    eab

    ility (b

    arrercomposite 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) IV 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.

  • alignedMWNT/PCalignment is therebutopentipsarenot clear

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

    /PC

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

    Fig. 7 Surface topography of (a) 0.1% SWNT3.3. Voltagecurrent characteristics

    Fig. 5(a) shows the IV characteristics of CNT/Polymer nano-

    composite membranes which are giving dramatically differ-

    ence between randomdispersed CNT/PC nanocomposites and

    aligned CNT/PC nanocomposite membranes. The total

    tunneling current has a kinkwhich 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 IV 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)nanocomposites can be used as a good separatingmedia. From

    IVcharacteristicmeasurements ithasbeenobserved thatflow

    of current across the aligned CNTs in PC is increased, itmay 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 dispersionaligned 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 inpolymernanocompositesprovideseasychannels

    or porosity for permeation of hydrogen. The gas permeation in

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

    alsoconfirmedby theoptical topographythat inalignedSWNT/

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

    without field (b) 0.1% SWNT/PC with field.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.

    r e f e r e n c e s

    [1] Robeson LM. Correlation of separation factor versuspermeability for polymeric membranes. J Memb Sci 1991;62:16585.

  • [2] Freeman BD. Basis of permeability/selectivity trade-offrelations in polymeric separation membranes.Macromolecules 1999;32:37580.

    [3] Koros WJ, Mahajan R. Pushing the limits on possi...