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Influence of Finely Dispersed Carbon Nanotubes on the Performance Characteristics of Polymer Electrolytes for Lithium Batteries Kwang-Pill Lee, Anantha Iyengar Gopalan, Kalayil Manian Manesh, Padmanabhan Santhosh, and Kyu Soo Kim

Abstract—Electrospun membranes of poly(vinylidene fluorideco-hexafluoropropylene) (PVdF-HFP)/multiwall carbon nanotube (MWCNT) composite are prepared and loaded with lithium salts from electrolyte solution. Field emission transmission electron microscopy provides evidence for the uniform distribution of MWCNTs into the matrix of PVdF-HFP. The interconnected morphology as evident from field emission scanning electron micrograph forms the path for the lithium ion conduction. Results from electrochemical impedance spectroscopy inform that the presence of MWCNTs in PVdF-HFP matrix improves interfacial stability between lithium electrode and membrane and augment conduction path in the polymer electrolyte membrane. Further results from impedance measurement reveal the contribution of MWCNTs toward conductivity. A prototype cell is fabricated with PVdF-HFP/MWCNT as polymer electrolyte. The electrospun PVdF-HFP electrolyte membrane with 2% MWCNTs content shows an ionic conductivity of about 5.85 mS cm 1 at 25 C. Also, PVdF-HFP/MWCNT electrolyte membrane exhibits good electrochemical and interfacial stability and can be potentially suitable as electrolyte in lithium ion secondary battery. Index Terms—Batteries, carbon compounds, energy storage, impedance measurements, nanotechnology.

I. INTRODUCTION

P

OLYMER electrolytes have been recognized as the most viable electrolyte for potential application in a wide variety of solid-state electrochemical devices [1]. Most of the recent studies have been focused on the gel polymer electrolytes (GPEs) in which the liquid electrolyte has been immobilized by incorporating into the porous polymer matrix because their ionic conductivity can reach 10 S cm at ambient temperature. In general, higher ionic conductivity, effective lithium ion transport, and good interfacial characteristics with lithium electrode are needed for the GPEs to have high performance in lithium batteries. Recent studies have demonstrated that the ionic conductivity and the interfacial stability of the GPE while in conManuscript received July 14, 2006; revised November 13, 2006. This work was supported by the Korean Research Foundation under Grant (KRF-2006-J02402). The review of this paper was arranged by Associate Editor G. Ramanath. K.-P. Lee and K. S. Kim are with the Department of Chemistry Graduate School, Kyungpook National University, Daegu 702-701, Korea, and also with the Nano Practical Application Center, Daegu 704-230, Korea. A. I. Gopalan is with the Department of Chemistry Graduate School, Kyungpook National University, Daegu 702-701, Korea, and also with Nano Practical Application Center, Daegu 704-230, Korea, and also with the Department of Industrial Chemistry, Alagappa University, Karaikudi-630 003, Tamil Nadu, India. K. M. Manesh and P. Santhosh are with the Department of Chemistry Graduate School, Kyungpook National University, Daegu 702-701, Korea. Digital Object Identifier 10.1109/TNANO.2007.894834

tact with metallic lithium could be significantly enhanced by the addition of filler materials [2]. The increase in ionic conductivity has been attributed to the formation of amorphous phase in the polymer structure due to the presence of homogeneous dispersion of the fine filler particles. Croce et al. [3] reported that the particle size and the nature of the supposedly the filler materials play a major role in improving the polymer electrolyte properties. Poly(vinylidenefluoride-co-hexafluoropropylene) (PVdFHFP) have been widely studied as GPEs for applications in lithium rechargeable batteries [4]. The unique structure of PVdF-HFP gives high mechanical stability and chemical inertness, when used in battery applications. It has been reported that increasing the pore size (up to micrometer size) in the PVdF-HFP matrix can help to enhance the ionic conductivity and thus improve cell rate capability. Different methodologies were developed to prepare porous polymer matrix. Electrospinning is a process by which submicrometer polymer fibers can be produced using an electrostatically driven jet of polymer solution (or polymer melt). Electrospinning can be easily used to produce polymeric fibers in the average diameter range of 100 nm to 5 m [5]. The average diameter of the fibers produced by this way is at least one or two orders of magnitude smaller than the fibers produce from melt or solution spinning [6]. This technique is currently used for processing aligned nanofibers from polymer solutions or melts [7]. Carbon nanotubes (CNTs) represent an important group of nanomaterials with attractive electronic, chemical and mechanical properties [8]. The high surface area and hollow geometry, combined with electronic conductivity and mechanical properties make CNTs, a promising material for various applications. Many efforts have been directed towards combining CNTs and polymers in order to produce functional materials with superior properties [9]. The composites of CNTs with polymers find wide range of applications that include molecular electronic and sensing devices, energy storage, nanoscale probes, catalyst supports and field emission devices [10]. So far, only inert, nonconductive fillers have been used in the formulation of polymer electrolytes. Hence, an attempt has been made in this investigation to study the influence of addition of a small volume fraction of high surface area (110 m g ) multiwall carbon nanotubes (MWCNTs) on morphology, thermal transition, and ionic conductivity of PVdF-HFP membranes. MWCNTs are uniformly distributed into PVdF-HFP matrix by preparing fibrous membrane using the electrospinning technique. Impedance measurements on

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the electrospun PVdF-HFP/MWCNT membranes are made to bring out the influence of MWCNTs on the ionic conductivity in the polymer electrolyte membrane. The results from the present study form the basis for using MWCNTs as a component in polymer electrolyte. II. EXPERIMENTAL MWCNTs were amine functionalized as detailed in [11]. The following procedure was adopted for the preparation of amine functionalized MWCNTs. Fifty milligrams of MWCNT was refluxed in 4 M HNO for 24 h and filtered through a 0.2 m pore size polycarbonate membrane. The residue, MWCNT-COOH (carboxylated MWCNTs) was washed with deionized water and dried under vacuum at 60 C for 12 h. Fifty milligrams of MWCNT-COOH was refluxed in 100 ml of thionyl chloride at 65 C for 24 h to get MWCNT-COCl. MWCNT-COCl was filtered, washed with THF, and dried under vacuum at room temperature. To prepare the amine functionalized MWCNTs, MWCNT-COCl was refluxed with poly(ethylene glycol) bis(3-aminopropyl) using THF at 60 C for 24 h. The amine functionalized MWCNTs (MWCNT-CONH ) were separated by filtration and dried under vacuum. Adequate amounts of PVdF-HFP and amine functionalized MWCNTs were dissolved in DMF/acetone mixture (7 : 3 v/v). Electrospinning of the composite solution was performed at a flow rate of 10 mL/h with a potential difference of 25 kV. A distance of 15 cm was kept between the syringe tip and collector. Membranes were accumulated on the collector (drum) over the aluminium foil. The polymer electrolytes were prepared by immersing the electrospun membranes in 1M LiClO -PC electrolyte solution for 10 h. The microstructure of PVdF-HFP/MWCNT membranes was investigated by means of a field emission transmission electron microscope (FETEM) (JEOL, JEM-2000EX) and field emission scanning electron microscopy (FESEM; Hitach-530). Differential scanning calorimetric (DSC) experiments were carried out using Shimadzu DSC at a scan rate of 10 C/min. Thermogravimetric analysis (TGA) was carried out using a TA instrument 951 under nitrogen atmosphere with a scan rate of 20 C/min. Ionic conductivity was measured by recording the ac impedance spectra in the temperature range of 25 C-80 C using EG & G PAR 283 Potentiostat/Galvanostat with FRA 1025 model. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were performed with the cell made by using SS 304 as working electrode and lithium metal as counter as well as reference electrode. A sweep rate of 1 mV s was used in both cases. III. RESULTS AND DISCUSSION The electrospun PVdF-HFP/MWCNT membrane takes up lithium ions from the 1M LiClO -PC electrolyte solution and exhibits properties of electrolytes for lithium batteries. The intake of LiClO salts, mobility of Li ions in the polymer matrix, suitable dielectric to have ionization of lithium salts, and optimal ionic conduction are the key parameters for the polymer electrolyte, and these properties are expected to be

Fig. 1. FESEM (a) and FETEM (b) images of electrospun PVdF-HFP/ MWCNT (1.0%) fibrous membranes.

inbuilt in the PVdF-HFP/MWCNT membrane. These properties are expected for the membrane if an interconnected morphology between PVdF-HFP and MWCNTs and formation of transient cross-link between PVdF-HFP and Li ions exist. This would also result in alteration changes in crystalline domains of PVdF-HFP. Keeping this in view, morphology and thermal transitions of the PVdF-HFP/MWCNT membrane are analyzed. FESEM micrograph of PVdF-HFP/MWCNT fibrous membrane [Fig. 1(a)] reveals that the membrane is composed of numerous randomly oriented continuous fibers having diameters in the range of 70–80 nm. And, there is no abnormal morphology like the presence of beads and films. Importantly, MWCNTs are distributed without any agglomeration within the PVdF-HFP matrix in straight and aligned orientations along the fiber axis [Fig. 1(b)]. The presence of MWCNTs in PVdF-HFP membrane influences the thermal transitions of PVdF-HFP and consequently the crystallinity of the polymer. Fig. 2 shows the DSC thermograms of PVdF-HFP/MWCNT fibrous membranes containing different amounts. The electrospun PVdF-HFP membrane shows a melting point around 138 C without having the of PVdF-HFP [Fig. 2(a)]. of PVdF-HFP Incorporation of MWCNTs increases the membrane [Fig. 2(b)–(d)]. We presume that the interconnected network between PVdF-HFP and MWCNTs (Fig. 1) causes interactions between the MWCNTs and the polymer chains and results the high melting complex. The morphology and

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Fig. 2. DSC thermograms of electrospun PVdF-HFP/MWCNT fibrous membranes containing MWCNTs. (a) 0%. (b) 1%. (c) 2%. (d) 4%.

Fig. 4. Impedance spectra of PVdF-HFP membranes containing different amount of MWCNTs at 25 C and 80 C. MWCNTs: 0% (a), 1% (b) 2% (c) and 4% (d). Pictorial representation of equivalent circuit is shown.

Fig. 3. (a) Thermograms of electrospun PVdF-HFP. (b) and (c) PVdF-HFP/ MWCNT membranes with different weight percentage of MWCNTs: 1% (b); 2% (c).

thermal characteristics of PVdF-HFP altered by the incorporation of MWCNTs provide electrolyte properties to electrospun PVdF-HFP/MWCNT membrane. Thermal stability of the PVdF-HFP/MWCNT electrospun membranes were determined and presented in Fig. 3. It can be seen from the figure that pristine PVdF-HFP membrane showed weight loses in the temperature range of 400-500 C. Only 10% residue was found at 800 C for the pristine PVdF-HFP membrane. Incorporation of MWCNTs increases the thermal stability of PVdF-HFP electrospun membrane and this is ev18 residual masses. ident from the existence of higher Also, a slight increase in the initial decomposition temperature of PVdF-HFP (Fig. 3) was observed for PVdF-HFP/MWCNT electrospun membranes. We presume that there can be molecular level interactions between the fluorine atoms in the PVdF-HFP and the groups in the functionalized MWCNTs. The interconnected morphology for the nanotube loaded PVdF-HFP, as noticed by FETEM image [Fig. 1(b)] supports this supposition. The electrochemical characteristics and applicability of the PVdF-HFP/MWCNT electrospun membranes in

lithium batteries were investigated and the results are presented. Fig. 4 presents the impedance plots of electrospun PVdF-HFP/MWCNT fibrous membrane with different amounts of MWCNTs at 25 C and 80 C. It is apparent from the figure that PVdF-HFP/MWCNT membranes with MWCNT content up to 2% (w/w) show the typical impedance spectrum of a polymer electrolyte [2]. At 25 C, the Nyquist plot shows a depressed semicircle starting from the origin of the plot in the high-frequency region followed by a straight line inclined at constant angle to the real axis in the low frequency range. On the contrary, at 80 C, the semicircle feature is very much suppressed and shifts to higher frequency region. At lower frequency an inclined straight line, a Warburg behavior, is observed. The absence of a semicircle can be ascribed to the limitation of measurable frequency of the instrument and to the inconsequential capacitative component of the electrolyte originating from the small dielectric relaxation within the applied frequency range. The degree of contact between the SS electrode and polymer membrane is improved in the presence of nanotubes due to dielectric polarization [3]. The observed trend in Nyquist plots could be attributed to the role of MWCNTs. The impedance behavior at low and high frequencies of PVdF-HFP/MWCNT fibrous membrane containing MWCNTs of 1 and 2% (w/w) is analyzed to bring out the role of MWCNTs. The diameter of semicircle arc is decreased with increasing

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TABLE I EQUIVALENT CIRCUIT PARAMETERS CALCULATED FROM THE NONLINEAR FIT OF THE IMPEDANCE MEASUREMENTS

Fig. 5. Temperature dependence of ionic conductivity of electrospun PVdF-HFP/MWCNT fibrous membranes containing MWCNTs. (a) 0%. (b) 2%. (c) 4%.

MWCNT content. This observation informs the increase in ionic conductivity of the electrolyte membrane on increasing the content of MWCNTs. When the MWCNT content is increased further to 4%, the impedance response completely changes. At 25 C, the Nyquist plot of PVdF-HFP/MWCNT (4%) (figure not shown) appear as points on the real axis, i.e., the membranes act as simple resistors. This may be due to the formation of continuous carbon path throughout the membrane. However, at 80 C, the impedance spectrum of PVdF-HFP/MWCNT (4%) shows the semicircle response [Fig. 4(d)]. An equivalent circuit taking into account of all possible conduction mechanisms is designed and validated by fitting the impedance data to a model equivalent circuit with the help of a nonlinear least-square fit. The corresponding equivalent circuit is shown in Fig. 4. It consists of a parallel circuit of CPE1 and in series with a parallel circuit of CPE2 the bulk resistance and Warburg resistance. and the charge transfer resistance The constant phase elements (CPE) are introduced to account for the nonideality of the interface behavior between the electrode and electrolyte, specifically for the interface having rough geometry. The CPE1 and CPE2 are associated with the geometrical and double-layer capacitances, respectively. The equivalent circuit parameters calculated from the nonlinear fit of the impedance measurements are presented in Table I. The ionic conductivity of the membranes is determined from ac impedance measurements as a function of MWCNT content and presented in Fig. 5. A Vogel–Tamman–Fulcher (VTF) relationship for ion transport is observed for PVdF-HFP/MWCNT which indicates that ion transport is coupled with polymer segmental motion. Perusal of Fig. 5 informs us that increasing

Fig. 6. Linear sweep voltammogram of PVdF-HFP/ MWCNT (2%) fibrous membrane.

content of MWCNTs in the membrane increases the ionic conductivity. The increase of ionic conductivity can be attributed to the distinctive MWCNT morphology in the membrane (Fig. 1). An easy path could emerge to favor the Li ion conduction at the interfaces between polymer and MWCNTs in the PVdF-HFP/MWCNT. At 80 C, the ionic conductivity of PVdF-HFP/MWCNT (4%) is several orders higher than the simple PVdF-HFP membrane electrolyte. However, the membrane shows a substantial electronic conductivity that restricts usefulness as polymer electrolyte. The PVdF-HFP/MWCNT membrane with MWCNT content of 2% shows an ionic conductivity of about 5.85 mS cm with negligible electronic conductivity. The electrochemical stability window of a polymer electrolyte is generally determined by linear sweep voltammetry (LSV) of an inert electrode in the selected electrolyte [12]. The onset of the current in the anodic high voltage is assumed to result from a decomposition process associated with the electrode [13] and this onset voltage is taken as the upper limit of the electrolyte stability range. This voltage is generally located at the point of intersection of the extrapolated linear current in the high voltage region with the voltage axis. The electrochemical stability window of the PVdF-HFP/ MWCNT membrane is determined by LSV and is presented in Fig. 6. The potential was scanned from 1.0 to 6.0 V (versus Li) at a sweep rate of 1 mV s . Two important features can be recognized from the trend of this curve. First, the current V versus Li, suggesting that the polymer onset occurs at membrane has a high anodic stability. The second one is the

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may be due to the formation of a passive film on surface in of Li electrode caused by the reactivity of the electrode with stabilizes after 75 days of polymer membrane. However, storage. This indicates that the presence of MWCNTs is helpful to quickly form a compact and stable passive film and thereby, stabilizes the interface. This can be correlated to the ability of MWCNTs to trap water or oxygen traces that would eventually reach the Li electrode to form the passive layer. IV. CONCLUSION

Fig. 7. Cyclic voltammogram of PVdF-HFP/MWCNT (2%) fibrous membrane.

Electrospun PVdF-HFP/MWCNT fibrous membrane electrolytes were prepared and characterized. The presence of MWCNTs in PVdF-HFP/MWCNT improves the electrochemical performance over pristine PVdF-HFP membrane. PVdF-HFP/MWCNT (2%) membrane shows ionic conductivity of about 5.85 mS cm at 25 C and an anodic stability 5 V versus Li. Further, these electrolyte memof about branes exhibit adequate interfacial characteristics with lithium electrode and proved to have the practical requirements as electrolyte in lithium batteries. ACKNOWLEDGMENT The authors would like to thank Kyungpook National University Center for Scientific Instruments.

Fig. 8. Impedance response of a symmetrical Li/PVdF-HFP/MWCNT(2%)/Li cell at different time upon storage at 25 C in open circuit condition; (a) fresh, (b) 5, (c) 15, (d) 25, (e) 35, (f) 45, (g) 55 and (h) 75 days.

very low current (0.32 cm ) prior to the onset of anodic breakdown. Above this cell voltage, the current increased steeply as the applied voltage increased. This low residual current level up to 5.0 V with the absence of any peak on the low voltage range confirms the high purity of the electrolyte. In particular, it is relevant to notice that no peak is observed at 4.25 V versus Li, namely, at the voltage expected for the oxidation of water or any impurities. Further, cyclic voltammetry was performed to study the kinetics of Li deposition-stripping process at PVdF-HFP/ MWCNT environment (Fig. 7). On cathodic sweeping of 0.87 V versus Li, correpotential, a peak is observed at sponds to the deposition of lithium on the SS electrode. On the anodic scan, stripping of Li is observed at 0.22 V versus Li. This indicates that the process at the lithium interface is reversible in PVdF-HFP/MWCNT membrane. The cyclability is good and the recovery of lithium is high. The reversible process in the electrolyte membrane demonstrates that the PVdF-HFP/MWCNT is electrochemically stable and hence it can be safely used as polymer electrolyte in rechargeable lithium batteries. The interfacial stability of Li electrode/polymer electrolyte assembly was evaluated by measuring the ac impedance of Li/PVdF-HFP/MWCNT/Li cells at 25 C under an open circuit potential condition (Fig. 8). As it is well known, the resistance and of the cell system is composed of the bulk resistance, which reflects the interfacial situation interfacial resistance, between the electrolyte and the electrode. The initial increase

REFERENCES [1] B. Scrosati, Applications of Electroactive Polymers. London, U.K.: Chapman & Hall, 1993. [2] G. Jiang, S. Maeda, Y. Saito, S. Tanase, and T. Sakai, “Ceramic-polymer electrolytes for all-solid-state lithium rechargeable batteries,” J. Electrochem. Soc., vol. 152, pp. A767–A773, Mar. 2005. [3] F. Croce, G. B. Appetecchi, L. Persi, and B. Scrosati, “Nanocomposite polymer electrolytes for lithium batteries,” Nature, vol. 394, pp. 456–458, Jul. 1998. [4] H. Kataoka, Y. Saito, Y. Miyazaki, and S. Deki, “Ionic mobilities of PVDF-based polymer gel electrolytes as studied by direct current NMR,” Solid State Ionics, vol. 152–153, pp. 175–179, Dec. 2002. [5] D. Adam, “A fine set of threads,” Nature, vol. 411, p. 236, May 2001. [6] P. Gibson, H. S. Gibson, and D. Rivin, “Transport properties of porous membranes based on electrospun nanofibers,” Colloids Surf. A, vol. 187–188, pp. 469–481, Aug. 2001. [7] W. Salalha, Y. Dror, R. L. Khalfin, Y. Cohen, A. L. Yarin, and E. Zussman, “Single-walled carbon nanotubes embedded in oriented polymeric nanofibers by electrospinning,” Langmuir, vol. 20, pp. 9852–9855, 2004. [8] M. S. Dresselhaus, G. Dresselhaus, and Ph. Avouris, Eds., “Carbon nanotubes: Synthesis, structure, properties and applications,” in Topics in Applied Physics. Berlin, Germany: Springer, 2001, vol. 80. [9] R. H. Baughman, A. A. Zakhidov, and W. A. Heer, “Carbon nanotubes-the route toward applications,” Science, vol. 297, pp. 787–792, Aug. 2002. [10] W. Fang, H. Y. Chu, W. K. Hsu, T. W. Cheng, and N. H. Tai, “Polymerreinforced, aligned multiwalled carbon nanotube composites for micro electromechanical systems applications’’,” Adv. Mater., vol. 17, pp. 2987–2992, Dec. 2005. [11] P. Santhosh, A. Gopalan, and K. P. Lee, “Gold nanoparticles dispersed polyaniline grafted multiwall carbon nanotubes as newer electrocatalysts: Preparation and performances for methanol oxidation,” J. Catal., vol. 238, pp. 177–185, Feb. 2006. [12] P. Santhosh, A. Gopalan, T. Vasudevan, and K. P. Lee, “Evaluation of a cross-linked polyurethane acrylate as polymer electrolyte for lithium batteries,” Mater. Res. Bull., vol. 41, pp. 1023–1037, Jun. 2006. [13] P. Santhosh, T. Vasudevan, A. Gopalan, and K. P. Lee, “Preparation and properties of new cross-linked polyurethane acrylate electrolytes for lithium batteries,” J. Power Sources, vol. 160, pp. 609–620, Sep. 2006.

LEE et al.: INFLUENCE OF FINELY DISPERSED CARBON NANOTUBES ON THE PERFORMANCE CHARACTERISTICS OF POLYMER ELECTROLYTES

Kwang-Pill Lee received the M.S. degree in applied chemistry and the Ph.D. degree from Nagoya University, Japan, in 1985 and 1988, respectively. He started his career as Associate Professor in 1994 and is currently a Professor in the Department of Chemistry Education, Kyungpook National University, Daegu, Korea. He did postdoctoral work at the Japan Atomic Energy Research Institute and was Principal Researcher at Korea Research Institute of Standards and Science, and was Visiting Researcher at Lawrence Berkeley National Laboratory, among others. His current research activities involve synthesis of nanomaterials and composites, nanofibers and applications of nanomaterials as sensor, battery, separation science, etc.

Anantha Iyengar Gopalan received the M.S. degree in 1979 with specialization in physical chemistry and the Ph.D. degree in 1985 from Madurai Kamaraj University, Madurai, India. He has been working in the Department of Industrial Chemistry, Alagappa University, Karaikudi, India, since 1986. He did postdoctoral work at National Cheng Kung University, Taiwan, R.O.C., and was Visiting Researcher at Kyungpook National University, Korea, and Lawrence Berkeley National Laboratory, among others. He is recently focusing his research attention on interdisciplinary topics covering synthesis of conducting polymers, nanostructuring of materials, electrochemistry, and device applications.

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Kalayil Manian Manesh received the M.S. degree in industrial chemistry specialized in electrochemistry from Alagappa University, India, in 2004. He is currently working toward the Ph.D. degree at Kyungpook National University, Daegu, Korea. His research field includes development of nanomaterials for device applications.

Padmanabhan Santhosh was born in Erode, India, in 1976. He received the Ph.D. degree in industrial chemistry, specializing in the field of polymer electrolytes for lithium batteries, from Alagappa University, Karaikudi, India, in 2005. He moved to Prof. K. P. Lee’s group at Kyungpook National University, Daegu, Korea, for a postdoctoral stay. He is currently at Max-Planck-Institute for Solid State Research, Stuttgart, Germany. Besides polymer electrolytes, his research interests include the chemical and electrochemical modifications of carbon nanotubes and development of the modified carbon nanotubes for device applications. Kyu Soo Kim received the M.Sc. degree in chemistry from Kyungpook National University, Daegu, Korea. His research interest in development of nanocomposites based on multiwall carbon nanotubes and conducting polymer.