Nanocrystalline Metal Oxides Dispersed Multiwalled Carbon

J. Phys. Chem. C 2007, 111, 7727-7734

7727

Nanocrystalline Metal Oxides Dispersed Multiwalled Carbon Nanotubes as Supercapacitor Electrodes A. Leela Mohana Reddy and S. Ramaprabhu* AlternatiVe Energy Technology Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai - 600 036, India ReceiVed: December 29, 2006; In Final Form: March 21, 2007

RuO2/MWNT, TiO2/MWNT, and SnO2/MWNT nanocrystalline composites for supercapacitor electrodes have been synthesized by chemical reduction method using functionalized MWNT and respective salts. MWNT have been synthesized by thermal catalytic chemical vapor deposition (CCVD) over hydrogen decrepitated Mischmetal (Mm)-based AB3 alloy hydride catalysts. Structural and morphological characterizations of metal oxide dispersed MWNT have been carried out using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM and HRTEM), energy dispersive X-ray analysis (EDAX), and Raman spectroscopy. Electrochemical performance of these electrodes has been investigated using cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy. Specific capacitance of RuO2, TiO2, and SnO2 dispersed MWNT electrodes increases compared to that pure MWNT electrode due to the pseudo capacitance of the nanocrystalline metal oxides dispersed on the functionalized MWNT. The present study shows the potential of cost-effective TiO2/MWNT novel nanocrystalline composite material for electrochemical double layer capacitor.

1. Introduction Electrochemical capacitors (supercapacitors) are promising power sources for portable systems and automotive applications.1,2 Depending on charge-storage mechanism, they are basically classified into two types: electric double layer capacitors (EDLCs) based on carbon electrodes3 and pseudocapacitors with certain metal oxides (RuO2, IrO2, NiO, CoOx, SnO2, and MnO2)4-7 or conducting polymers as electrode materials.8 While the storage mechanism in carbon based EDLCs is through electrostatic forces, fast Faradaic redox reactions are responsible for the charge-storage mechanism in pseudo-capacitors. Carbon based supercapacitor electrodes have been attractive due to their high surface area and porous nature.3,9-11 Recently, both single-walled and multiwalled carbon nanotubes (SWNT and MWNT) have been recognized as potential electrode materials for electrochemical supercapacitors, owing to their special properties such as high chemical stability, low mass density, low resistivity, narrow distribution of mesopore sizes, and large surface area.12,13 However, due to the microtexture, defects, micropore volume, and catalyst contamination of carbon nanotubes (CNTs), their specific capacitance is restricted to low values. Hence, there have been considerable efforts to improve the capacitance of CNT-based supercapacitor electrodes by various techniques such as activating CNTs with heat/acid treatment14 to improve the micropore volume of CNTs and modification of CNTs with conducting polymers15 or with certain transition-metal oxides.16,17 Thus, the hybrid of an electric double layer system and a Faradaic pseudocapacitive system could be a good candidate for a supercapacitor with high specific capacitance and energy density.3 However, the polymer electrode materials do not have long-term stability and cycle life, due to their degradation.8 Transition metal oxides * Correspondingauthor.Fax: +91-44-22574862.E-mail: [email protected]. Tel: + 91-44-22574862.

attached to CNTs have been studied recently and are expected to show improved capacitive behavior due to their enhanced stability and high conductivity.18 Among the transition metal oxides, hydrous ruthenium dioxide has been recognized as one of the most promising candidates for electrodes in electrochemical capacitors, as it can store charges reversibly by redox reaction.19-21 However, the high cost of RuO2 has prompted the research community to focus on other transition metal oxides such as MnO2, NiO, and so forth, mainly because of the involved cost-effectiveness. Of the various non-noble metals or transition metal oxides studied, TiO2 and SnO2 enjoy a special place because of its lower cost and environmentally benign nature. In the present work, we present the synthesis and supercapacitor behavior of two new nanocrystalline composites TiO2/MWNT and SnO2/MWNT (Scheme 1). MWNT have been synthesized by thermal chemical vapor deposition of acetylene over hydrogen decrepitated Mischmetal (Mm) based AB3 alloy hydride catalysts. MWNT have been functionalized with nanocrystalline RuO2, TiO2 and SnO2 by a simple chemical reduction method. The supercapacitative behavior of these nanocrystalline composite electrodes have been studied using cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy measurements, and the results have been compared with that of pure MWNT electrodes and discussed. 2. Experimental 2.1 Synthesis and Functionalization of MWNT. MWNT were synthesized using a single stage furnace thermal CVD facility by catalytic decomposition of acetylene over Mm based AB3 alloy hydride catalysts. These catalysts were prepared through hydrogen decrepitation route by performing several cycles of hydrogenation/dehydrogenation of the alloy using a Seiverts apparatus.22 Fine powders of alloy obtained after several cycles of hydrogenation/dehydrogenation were directly placed

10.1021/jp069006m CCC: $37.00 © 2007 American Chemical Society Published on Web 05/02/2007

7728 J. Phys. Chem. C, Vol. 111, No. 21, 2007

Reddy and Ramaprabhu

SCHEME 1

in a quartz boat and kept at the center of a quartz tube, which was placed inside a tubular furnace. Hydrogen (50 sccm) was introduced into the quartz tube for 1 h at 500 °C, and then furnace was heated up to 700 °C, followed by the introduction of acetylene for 30 min with a flow rate of 70 sccm. Acetylene flow was stopped, and the furnace was cooled to room temperature. Argon flow was maintained through out the experiment. As-prepared samples were purified by air oxidation followed by refluxing with conc nitric acid for 24 h.23 The sample was washed with deionized water several times, filtered, and dried at 80 °C for 2 h. Purified MWNT were ultrasonicated in con nitric acid for 3 h. Ultrasonication causes the formation of microbubbles which cause shock waves when they collapse and there by improves the nanotube wetting. After the sonication procedure, MWNT sample was refluxed under constant agitation in 30 mL of 70% HNO3 at 110 °C for 12 h, followed by washing with deionized water several times and drying the sample in air for 30 min at 100 °C. These acid treated MWNT are thus called as functionalized MWNT. 2.1. Synthesis of RuO2/MWNT, TiO2/MWNT, and SnO2/ MWNT Nanocomposites. RuO2-supported MWNT (RuO2/ MWNT) nanocomposites were prepared by chemical reduction method using a Ru salt and pretreated MWNT. Functionalized MWNT were dispersed in a solution of 1:1 volume ratio of isopropanol and water by ultrasonic agitation. RuCl3·3H2O (2:1 MWNT to Ru wt. ratio) was added to the above mixture, and ultrasonication is continued for 1 h. The blank product of the reaction was filtered and washed repeatedly with distilled water to remove the excess chloride ions. The RuO2/MWNT was dried in a vacuum oven at 100 °C for 12 h. Part of the final product was calcined at 350 °C for 2 h. TiO2/MWNT composites were prepared using sol-gel method as follows: functionalized MWNT were dispersed in dilute nitric acid (pH 0.5) by ultrasonic agitation. This solution was then transferred to a round-bottom flask and titanium tetraisopropoxide (Aldrich, 97%) was added dropwise maintaining the volume ratio of titanium tetraisopropoxide to water at 1:4. The sol obtained was stirred for 2 days in air at RT. The obtained turbid suspension was centrifuged at 6000 rpm, and the resultant residue was washed twice with distilled water. As-synthesized

TiO2/MWNT composites were heat treated at temperatures of 350 °C for 2 h in air. SnO2/MWNT nanocrystalline composites were synthesized by dissolving 1 g of tin (II) chloride (SnCl2) in 40 mL of distilled H2O followed by the addition of 1.0 mL of HCl (38%). Subsequently, 10 mg of the functionalized MWNT were dispersed in the above solution. This mixture was sonicated for 5-10 min and then stirred for 60 min at room temperature. The precipitate was then separated from the mother liquor by centrifugation and was washed with distilled H2O for several times, and then dried at 70 °C under vacuum for 6 h. Part of the final product was calcined at 350 °C for 2 h. The samples were characterized by powder X-ray diffraction (XRD (SHIMADZU XD-D1 X-ray diffractometer, Cu-KR radiation), scanning electron microscopy (SEM; JEOL JSM 840A) energy dispersive X-ray analysis (EDAX), transmission electron microscopy (TEM; PHILIPS CM 200) and Raman spectroscopy (Renishaw Raman spectrometer excited by a 514.5 nm Ar-ion laser) measurements. 2.2. Preparation and Characterization of Supercapacitor Electrodes. The electrodes were pellets (12 mm diameter and 0.1 mm thickness) prepared by pressing the functionalized MWNT, RuO2/MWNT, TiO2/MWNT, and SnO2/MWNT using a binder (PTFE) at 500 kg/cm2 and later pellets were heated at a temperature of 420 °C in vacuum to remove the binder. Two symmetric electrodes were sandwiched in a Swagelok type stainless steel (SS) test cell with a thin polymer separator (Celgard 3400) using SS current collectors. The system was kept under pressure of 16 kg/cm2 using Swagelock. The electrochemical properties and capacitance measurements of these electrodes were studied using a two-electrode system with 1M H2SO4 aqueous solution as electrolyte. Cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy measurements have been carried out using an electrochemical workstation PSGSTAT-30 (AUTOLAB). 3. Results and Discussion Mm-based AB3 alloy, after several hydrogenation and dehydrogenation cycles, was found to be finely powdered to about 5-10 µm. These novel hydride catalysts prepared using

Dispersed Multiwalled Carbon Nanotubes

Figure 1. Powder XRD patterns of (a) as-grown MWNT, (b) purified MWNT, (c) RuO2/MWNT nanocomposite, (d) TiO2/MWNT, and (e) SnO2/MWNT nanocomposite.

hydrogen decrepitation technique provides fresh surfaces with large surface area, free from oxidation, which further increases the catalytic sites for the formation of CNTs. High hydrogen absorption, large decrepitation and low cost makes these hydrides better catalysts for large scale production of CNTs. Figure 1a shows the XRD pattern of as-prepared CNTs using alloy hydrides as catalysts. The peaks are indexed to the reflections of hexagonal graphite. A few peaks corresponding to the catalytic impurities are also seen. The XRD of purified CNTs (Figure 1b) shows the removal of metallic impurities by acid treatment. The XRD pattern of RuO2/MWNT nanocomposite material (Figure 1c) shows the reflections of RuO2 along with that for graphitic carbon, whereas Figure 1d,e gives the XRD patterns TiO2/MWNT and SnO2/MWNT composites showing the reflections of corresponding oxides along with that of graphitic carbon. The broad peaks reveal the presence of nanostructured metal oxide crystals. SEM, TEM, and HRTEM images of purified MWNT (panels a-c of Figure 2, respectively) indicate the good quality of the MWNT obtained by CCVD technique using Mm based AB3 alloy hydride catalyst. Further, HRTEM image reveals the multiwalled nature of carbon nanotubes, with each graphene layer being clearly distinguishable, since the graphene sheets with a spacing of ∼0.34 nm are stacked parallel to the growth axis of carbon nanotubes. Thermogravimetric measurements were performed using a STA 409PC TGA-DTA analyzer. Within the TGA, MWNT material to be analyzed was maintained in a flow of dry air at 100 sccm while the temperature was ramped linearly in time (5 °C/min) from 25 to 1000 °C. The weight of the sample has been recorded as a function of temperature. The weight loss between 500 and 820 °C in the TG curves of purified MWNT is attributed to the oxidation of MWNT. Final residual weights

J. Phys. Chem. C, Vol. 111, No. 21, 2007 7729 of ∼5% were obtained for purified MWNT, revealing a purity of about ∼95% for the purified sample (Figure 2d). Raman spectroscopy has been used to investigate the vibrational properties of the carbon samples. Figure 3 shows the Raman spectra of as-grown, purified, and functionalized MWNT. Tangential modes corresponding to the Raman allowed optical mode E2g of two-dimensional graphite, centered on 1586.4 cm-1 (G-band) is observed. In addition, a peak centered at around 1342.7 cm-1 (D-band) is mainly due to defects and carbonaceous particles present in the sample. The intensity of D-band gives degree of disorder present along the tube. These can be pentagons, heptagonal defects, the pentagon-heptagon pairs, or line defects.24 The intensity of this peak decreases on purification of the sample, indicating efficiently removal of the amorphous carbon. This peak still remains with low intensity, implying that some degree of disorder is present along the tube even after purification. Acid treated MWNT shows a clear increase in the intensity of the D-band, which can be attributed due to the defects created along the nanotube surface during the vigorous acid treatment. The FTIR spectrum of as-grown, purified, and functionalized MWNT in the range 1000-4000 cm-1 is shown in Figure 4. A broad absorption band at 3437 cm-1 is attributed to the hydroxyl group (νOH).25,26 This band might have resulted due to water νOH and δH2O27 and also the -OH functional groups resulting due to the chemical treatment during the purification process and functionalization process respectively.28 Bands at 2927 and 2853 cm-1 are due to asymmetric and symmetric stretching of CH stretching. A small peak at 1734 cm-1 is associated with the CdO stretching of the carboxylic acid (-COOH) group.29 The peak at 1639 cm-1 is due to CdC stretching of the CNTs.30 The peak at 1384 cm-1 is due to O-H bending deformation in -COOH. A small peak at 1086 cm-1 is assigned to C-O bond stretching.29 Thus, the generation of -OH and -COOH groups on CNTs due to functionalization is observed. Damages in the graphene layers of MWNT bundles were observed after chemical treatment during functionalization.31 Figures 5a-c, 6a-c, and 7a-c show the SEM, TEM, and HRTEM images of RuO2/MWNT, TiO2/MWNT, and SnO2/ MWNT nanocomposites, respectively. TEM images of RuO2/ MWNT, TiO2/MWNT, and SnO2/MWNT indicate the uniform distribution of nano crystalline metal oxide particles of size of about 3-5 nm on the MWNT. The lattice planes of RuO2, TiO2, and SnO2 nanoparticles are seen clearly in the HRTEM image of RuO2/MWNT, TiO2/MWNT, and SnO2/MWNT indicating crystalline nature of catalytic particles. The energy dispersive analysis (EDAX) (Figures 5d, 6d, and 7d) shows that the amount of RuO2, TiO2, and SnO2 loaded on the carbon nanotube support with reference to carbon can be evaluated qualitatively as about 20 wt %. In all the samples, the EDAX peaks were corrected for relative ionization efficiencies. The electrochemical properties of pure MWNT, RuO2/ MWNT, TiO2/MWNT, and SnO2/MWNT nanocomposites were studied by symmetric assemblies of each material in a twoelectrode Swagelok test cell (each electrode is 10 mg). The cyclic voltammetry (CV) responses of the different electrode materials at a scan rate of 2 mV/s are shown in Figure 8. Voltammetry testing was carried out at potentials between -1.0 and 1.0 V using 1 M H2SO4 aqueous electrolyte solution. The MWNT present the typical box like curve, expected for an ideal capacitor. However, there are oxidation peaks observed in the CV for metal oxides dispersed MWNT electrodes, which are attributed to redox reactions due to the functional groups on nanotubes.32 There is a reasonable symmetry to the curves



Figure 2. (a) SEM, (b) TEM, (c) HRTEM images, and (d) TGA of purified MWNT.

Figure 3. FT-Raman spectra of as-grown, purified, and functionalized MWNT.

for MWNT electrodes, which may be due to the capacitance arising solely due to the double layer. However, in MWNT/ metal oxide electrodes, the lack of symmetry to the curves is probably due to combination of double layer and pseudo capacitances contributing to the total capacitance. The area of the curve also increased with RuO2, TiO2, and SnO2 functionalization, indicating an enhancement of the specific capacitance

Figure 4. FTIR spectra of as-grown, purified, and functionalized MWNT.

for these electrodes. Since the measurements are made on symmetric assemblies of materials, by the basic circuit relationship for series capacitors what is measured is actually 1/2 of the capacitance of the freestanding electrode. The specific capacitance has been obtained from the CV curve according to following equation:

Csp )

i sm

(2)


J. Phys. Chem. C, Vol. 111, No. 21, 2007 7731

Figure 5. (a) SEM, (b) TEM, (c) HRTEM images, and (d) EDAX patterns of RuO2/MWNT nanocomposite.

Figure 6. (a) SEM, (b) TEM, (c) HRTEM, and (d) EDAX patterns of TiO2/MWNT nanocomposite.

where i is the average cathodic current, s is the potential sweep rate, and m is the mass of each electrode. Figure 9 shows the galvanostatic charge-discharge behavior of the pure MWNT, RuO2/MWNT, TiO2/MWNT, and SnO2/

MWNT nanocomposite electrodes with an applied constant current of 10 mA in the potential range between 0 and + 1 V. The symmetry of the charge and discharge characteristics shows good capacitive behavior. The specific capacitance has been



Figure 7. (a) SEM, (b) TEM, (c) HRTEM, and (d) EDAX patterns of SnO2/MWNT nanocomposite.

Figure 8. Cyclic voltammograms of pure MWNT, RuO2/MWNT, TiO2/MWNT, and SnO2/MWNT nanocomposites, at a scan rate of 2 mV s-1 in 1 M H2SO4 aqueous electrolyte; mass of each electrode, 10 mg.

evaluated from the charge-discharge curves, according to the following equation: Csp )

I m(dV/dt)

(3)

where I is the applied current and m is the mass of each electrode. Electrochemical impedance spectroscopy (EIS) measurements were carried out at a dc bias of 0 V with sinusoidal signal of 10 mV over the frequency range from 40 kHz to 10 MHz. Figure 10 presents complex-plane impedance plots for the purified MWNT, RuO2/MWNT, TiO2/MWNT, and SnO2/

Figure 9. Galvanostatic charge discharge of pure MWNT, RuO2/ MWNT, TiO2/MWNT, and SnO2/MWNT nanocomposites at an applied constant current of 10 mA in 1 M H2SO4 aqueous electrolyte; mass of each electrode, 10 mg.

MWNT nanocrytalline composites. At lower frequency, the imaginary part of impedance sharply increases: this is the capacitive behavior of electrode. The impedance plot should theoretically be a vertical line, parallel to the imaginary axis. In fact, a difference between this theoretical behavior and experimental can be observed. The impedance behavior of undoped MWNT comes close to an ideal capacitor. A slight variation from the ideal capacitive behavior could be attributed to the pore size distribution of MWNT.33 However, impedance behavior of RuO2/MWNT, TiO2/MWNT, and SnO2/MWNT nanocrytalline composites electrodes show a deviation from ideal capacitor and show much lower impedance than the pure


J. Phys. Chem. C, Vol. 111, No. 21, 2007 7733

Figure 10. Complex-plane impedance spectra of pure MWNT, RuO2/ MWNT, TiO2/MWNT, and SnO2/MWNT nanocomposites in 1 M H2SO4 aqueous electrolyte; mass of each electrode, 10 mg.

MWNT. The presence of a small semicircular loop for MWNT electrodes at higher frequencies is due to charge-transfer resistance of the electrode.34 Equivalent series resistance (ESR) of 7, 4, 3, and 5.5 Ω are measured for pure MWNT, RuO2/ MWNT, TiO2/MWNT, and SnO2/MWNT nanocomposites, respectively. The loading of the metal oxides increases the conductivity of the composites and the increase is maximum for the TiO2/MWNT composites. The ac impedance method has also been used to measure the specific capacitance of the electrodes, which is influenced by the frequency, especially for porous electrodes, where almost no current flows down the pore at higher frequencies.1 The energy density (E) of the supercapacitror can be calculated by using the following equation: E)

∫V dq

4. Conclusions

q C) V q ) CV dq ) C dV + V dC ) C dV

if dC ) 0

∫

1 E ) C V dV ) C(∆V)2 2

(4)

where C is the capacitance of the capacitor, V are the operating potential window (1.0 V), and m is amount of active materials in the supercapacitor (includes positive and negative electrodes). The specific power density (P) of the supercapacitor can be calculated according to the following equation: P)

the symmetric supercapacitor (include positive and negative electrode). The energy density of MWNT, RuO2/MWNT, TiO2/ MWNT, and SnO2/MWNT nanocomposite electrodes were found to be 21.4, 36.8, 40.2, and 25 W h/kg, respectively, at a power density of 500 W/kg. The average specific capacitances measured using the three electrochemical techniques of the pure MWNT, RuO2/MWNT, TiO2/MWNT, and SnO2/MWNT nanocomposite electrodes are 67, 138, 160, and 93 F/g respectively (Table 1). The capacitance values thus obtained here are comparatively more than literature values (30 and 80 F/g for MWNT and RuO2/MWNT, respectively).17 The increase in the capacitance of MWNT is mainly due to the functionalization of MWNT with carboxyl groups, and for RuO2/MWNT, the increase is due to uniform dispersion of metal oxide particles over functionalized MWNT. Since the specific double layer capacitance arises from the ionic double layer at the electrode/electrolyte interface, the accessibility of the active layer depends on the diffusion of solvated ions and more precisely on the pore size distribution. The central hollow core of the CNTs is also accessible for the double layer charging at lower frequencies, and thus, the purified MWNT with opened tips along with the unique network of mesopores formed by the entanglement of CNTs would have acted as sites for the accumulation of charges. The enhancement of the specific capacitances can be attributed to the presence of RuO2, TiO2, and SnO2 attached to the surface of MWNT, which in turn modify the microstructure and morphology of MWNT, allowing the metal oxides to be available for the electrochemical reactions and improves the efficiency of the composites. The progressive redox reactions occurring at the surface and bulk of transition metal oxides through faradiac charge transfer between electrolyte and electrode results in the enhancement of the specific capacitance of metal oxide dispersed MWNT from pure MWNT. Of the various non-noble metals or transition metal oxides studied, TiO2 enjoy a special place because of its cost effectiveness and environmentally friendliness.

I∆V m

(5)

where I is the current of charge-discharge, V is the potential range of a supercapacitor, and m is mass of active materials in

MWNT, synthesized by pyrolysis of acetylene using hydrogen decrepitated Mm based AB3 alloy hydride catalysts, after purification process are well ordered ones without defects. A uniform distribution of nano crystalline metal oxide particles of size of about 3-5 nm on the MWNT is achieved by chemical method. Electrochemical measurements of functionalized MWNT electrodes give a specific capacitance of around 67 F/g. The nanocrystalline RuO2/MWNT, TiO2/MWNT, and SnO2/ MWNT composite electrodes give a specific capacitance of 138, 160, and 93 F/g respectively. The enhancement of the specific capacitance of metal oxide dispersed MWNT from pure MWNT is due to the progressive redox reactions occurring at the surface and bulk of transition metal oxides through faradaic chargetransfer due to the modification of the surface morphology of MWNT by the nanocrystalline RuO2, TiO2, and SnO2. The present results show the potential of these nanocrystalline metal oxide/MWNT materials, especially TiO2/MWNT, for developing electrochemical double layer capacitors.

TABLE 1: A Comparative Tabulation of Specific Capacitance of Supercapacitor Obtained from Different Techniques electrode material MWNT RuO2/MWNT TiO2/MWNT SnO2/MWNT

cyclic voltammetry (F/g) 68 147 166 95

galvanostatic charge-discharge (F/g) 69 133 148 91

electrochemical impedance spectroscopy (F/g) 64 134 166 93

average specific capacitance (F/g) 67 138 160 93

7734 J. Phys. Chem. C, Vol. 111, No. 21, 2007 Acknowledgment. We thank MHRD, NRB, and DRDO, Govt. of India, for the support of this work. One of the authors (ALMR) is grateful to IIT Madras for financial support. References and Notes (1) Ko¨tz, R.; Carlen, M. Electrochim. Acta 2000, 45, 2483. (2) Frackowiak, E.; Beguin, F. Carbon 2001, 39, 937. (3) Lee, Y. H.; An, K. H.; Lee, J. Y.; Lim, S. C. Encycl. Nanosci. Nanotechnol. 2004, 1, 625. (4) Liu, K. C.; Anderson, M. A. J. Electrochem. Soc. 1996, 143, 124. (5) Yoon, Y. S.; Cho, W. I.; Lim, J. H.; Choi, D. J. J. Power Sources 2001, 101, 126. (6) Conway, B. E.; Briss, V.; Wojtowicz J. Power Sources 1997, 66, 1. (7) Lin, C.; Ritter, J. A.; Popov, B. N. J. Electrochem. Soc. 1998, 145, 4097. (8) Burke, A. J. Power Sources, 2000, 91, 37. (9) Frackowiak, E.; Beguin, F. Carbon 2002, 40, 1775. (10) Du, C.; Yeh, J.; Pan, N. Nanotechnology 2005, 16, 350. (11) An, K. H.; Kim, W. S.; Park, Y. S.; Moon, J. M.; Bae, D. J.; Lim, S. C.; Lee, Y. S.; Lee, Y. H. AdV. Funct. Mater. 2001, 11, 387. (12) Portet, C.; Taberna, P. L.; Simon, P.; Flahaut, E. J. Power Sources 2005, 139, 371. (13) Beguin, F.; Szostak, K.; Lota, G.; Frackowiak, E. AdV. Mater. 2005, 17, 2380. (14) Kim, Y.; Mitani, T. J. Power Sources, in press. (15) An, K. H.; Jeon, K. K.; Heo, J. K.; Lim, S. C.; Bae, D. J.; Lee, Y. H. J. Electrochem. Soc. 2002, 149 (8), A1058. (16) Wang, G. X.; Zhang, B. L.; Yu, Z. L.; Qu, M. Z. Solid State Ionics 2005, 176, 1169. (17) Arabale, G.; Wagh, D.; Kulkarni, M.; Mulla, I. S.; Vernekar, S. P.; Vijayamohanan, K.; Rao, A. M. Chem. Phys. Lett. 2003, 376, 207. (18) Ye, J. S.; Cui, H. F.; Liu, X.; Lim, T. M.; Zhang, W. D.; Sheu, F. S. Small, 2005, 1, 560.

Reddy and Ramaprabhu (19) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699. (20) Long, J. W.; Swider, K. E.; Merzbacher, C. I.; Rolison, D. R. Langmuir 2003, 19 (6), 2532. (21) Sugimoto, W.; Iwata, H.; Yokoshima, K.; Murakami, Y.; Takasu, Y. J. Phys. Chem. B 2005, 109, 7330. (22) Shaijumon, M. M.; Rajalakshmi, N.; Ryu, H.; Ramaprabhu, S. Nanotechnology 2005, 16, 518. (23) Rinzler, A. G.; Liu, J.; Dai, H.; Nikolaev, P.; Huffman, C. B.; Rodriguez-Macias, F. J.; Boul, P. J.; Lu, A. H.; Heymann, D.; Colbert, D. T.; Lee, R. S.; Fischer, J. E.; Rao, A. M.; Eklund, P. C.; Smalley, R. E. Appl. Phys. A 1998, 67, 29-37. (24) Qian, W.; Liu, T.; Fei Wei; Wang, Z.; Luo, G.; Yu, H.; Li, Z. Carbon 2003, 41, 2613. (25) Kovtyukhova, N. I.; Malouk, T. E.; Pan, L.; Dickey, E. C. J. Am. Chem. Soc. 2003, 125, 9761. (26) Colthup, N. B.; Daly, L. H.; Wiberlay, S. E. Introduction to Infrared and Raman Spectroscopy; Acadamic Press: Boston, 1990. (27) Holzinger, M.; Vostrovsky, O.; Hirsch, A.; Hennrich, F.; Kappes, M.; Weiss, R.; Jellen, F. Angew. Chem., Int. Ed. 2001, 40, 4002. (28) Kim, U. J.; Furtado, C. A.; Liu, X.; Chen, G.; Eklund, P. C. J. Am. Chem. Soc. 2005, 127, 15437. (29) Kuznetsova, A.; Douglas, D.; Namneko, V.; Yates, J. T., Jr.; Liu, J.; Smalley, R. E. Chem. Phys. Lett. 2000, 321, 292. (30) Lu, K. L.; Lago, R. M.; Chen, Y. K.; Green, M. L. H.; Harris, P. J. F.; Tsang, S. C. Carbon 1996, 34, 814. (31) Ramaprabhu, S.; Shaijumon, M. M. Carbon nanotubes and their synthesis. U.S. Patent Appl. 1129/CHE/05. (32) Frackowiak, E.; Metenier, K.; Bertagna, V.; Beguin, F. Appl. Phys. Lett. 2000, 77, 2421. (33) Song, H. K.; Hwang, H. Y.; Lee, K. H.; Dao, L. H. Electrochim. Acta. 2000, 45, 2241. (34) Gamby, J.; Taberna, P. L.; Simon, P.; Fauvarque, J. F.; Chesneau, M. J. Power Sources 2001, 101, 109.

Nanocrystalline Metal Oxides Dispersed Multiwalled Carbon

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