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J. Phys. Chem. C 2008, 112, 11552–11555
Synthesis and Electrochemical Characterization of Vanadium Oxide/Carbon Nanotube Composites for Supercapacitors Wei-Chuan Fang* Materials and Chemical Research Laboratories, Industrial Technology Research Institute, Chutung 310, Taiwan ReceiVed: February 8, 2008; ReVised Manuscript ReceiVed: May 5, 2008
Capacitive properties of vanadium oxides on carbon nanotubes (CNTs) have been investigated. In structural properties, the Raman spectrum shows the characteristic peaks of V2O5, and XPS analysis predicts that the atomic ratio of as-prepared vanadium oxide is approximately the same as that of the stoichiometric V2O5. Cross-sectional scanning electron microscope images show that CNTs provide good support for uniform distribution of vanadium pentoxides. In capacitive behaviors, CNTs covered with uniformly dispersed oxides lead to a significantly improved capacitive performance, as compared with bare oxide films. Therefore, such V2O5/CNT nanocomposites are a potential material for power backup of hybrid electrical vehicles or portable electronics. Introduction Electrochemical supercapacitors are currently being widely investigated due to their interesting characteristics in terms of power and energy densities. These power storage devices display much larger power densities than batteries and energy densities in comparison to conventional capacitors.1 This makes them very attractive for applications requiring quick bursts of energy, as is the case, for instance, for electronic devices. Electrochemical supercapacitors make use of three main classes of materials: (i) carbon,2–5 (ii) electronically conducting polymers,6–8 and (iii) metal oxides.9–12 The two last kinds of systems involve pseudo Faradic reactions, unlike carbon systems, which use the double layer capacitance arising from the separation of charge at the interface between the solid electrode and an electrolyte. In general, the most widely investigated metal oxide is unequivocally ruthenium oxide, which displays a fairly high specific capacitance, but its use is severely limited by its high cost.13–15 Consequently, alternative less costly materials are being studied. In addition, recent research efforts have also been aimed at using a more environmentally friendly electrolyte than concentrated sulfuric acid. To this end, materials such as vanadium oxide are being synthesized and tested in the presence of a neutral electrolytic solution.16,17 As a result of the multiple valence state of vanadium, vanadium pentoxide has versatile redoxdependent properties and finds wide applications in catalysis and electrochemistry. Compared with other metal oxides, vanadium oxides can provide good capacitive performance in neutral solutions. This is the reason why they are used as electrode materials.16 Recently, hybrid nanocomposites containing CNTs have attracted much attention when each constituent component provides different functions for specific applications.18 In our earlier publication,19 we provide a simple and efficient route to prepare functional nanocomposites with well-dispersed RuO2 nanoparticles (NPs) on a vertically aligned CNT array directly grown on an Si substrate. It shows that CNTs * Corresponding author. Phone: 886-3-5916844. Fax: 886-3-5820207. E-mail:
[email protected].
Figure 1. Raman spectrum of as-prepared vanadium oxide.
incorporated with N not only have superior dispersion ability to support oxides accompanied by fast charging-discharging property but also improve the current efficiency of the electrode.20,21 To get fast and reversible supercapacitor performances for practical applications, V2O5 on N-doped and undoped CNTs is prepared in this work. From the reported data, EC performances of V2O5 on CNTs are significantly improved as compared with bare oxide films and pristine CNTs. Such CNT-V2O5 nanocomposites with prompt charging-discharging capability provide promise in practical supercapacitor applications. Experimental Section Synthesis of Nanocomposites. A 200-nm-thick Ti film acting as the conducting layer was deposited on Si substrates by sputtering. For CNT array preparation, a 10-nm-thick Ni film was deposited on Si substrates by e-beam evaporation as catalysts. The CNT growth was carried out in the inductively coupled plasma chemical-vapor deposition (ICPCVD) system.22 Prior to CNT growth, hydrogen-containing plasma treatment (1000 W, 5 min) was employed for cleaning the substrate, and the catalysts were encapsulated at the CNT tips. For CNT growth, the chamber was fed with
10.1021/jp8011602 CCC: $40.75 2008 American Chemical Society Published on Web 07/04/2008
V2O5/CNTs Composites for Supercapacitors
J. Phys. Chem. C, Vol. 112, No. 30, 2008 11553
Figure 2. (a) V 2p and (b) O 1s X-ray photoelectron spectroscopy spectrum of as-prepared vanadium oxide.
a gas mixture of C2H2, NH3, H2, and Ar at a total pressure typically below 50 mTorr. The ICPCVD reactor was set at the following process conditions: flow rates C2H2/NH3/H2/ Ar ) 8/0.6/24/0.5 sccm, total pressure 20 mTorr, substrate temperature 550 °C, ICP power 500 W, bias power 300 W, and growth time 6 min. For V2O5 deposition, arc-ion plating under Ar gas flow was performed at a deposition time of 30 min while the arc current was kept at 40 A.23 Characterization. For material analyses, a JEOL 6700 field-emission scanning electron microscope, an X-ray photoelectron spectroscopy (XPS) (VG Scientific ESCALAB 250) were utilized. EC measurements were carried out using an Autolab potentiostat system in a three-electrode setup using a Pt wire and Ag/AgCl/3 M KCl (207 mV vs SHE at 25 °C) as the counter and reference electrode, respectively. The electrolyte used was 2 M KCl and 1 M KHSO4 at room temperature. For BET measurements, the BET surface area could not be obtained due to the fact that the amount of directly grown CNTs is so insufficient that it cannot be measured (∼0.1-0.3 mg). According to the report of Niu,24 the surface area of carbon nanotubes could be reached up to 430 m2/g but it depends on the experimental conditions. The conducting behavior of the CNTs can be found, as is evident from the earlier publications from our laboratory.25,26 For the proportion of CNTs and vanadium oxides in the electrode, the ratio with respect to weight is ∼1:1000. The surface area and the weight of the electrode are 0.5 cm2 and 0.1 mg, respectively. Results and Discussion To examine the structure of as-prepared vanadium oxide, the Raman spectrum has been performed, as shown in Figure
Figure 3. Cross-sectional scanning electron microscopy images of (a) pristine carbon nanotubes, (b) bare vanadium oxide films, and (c) V2O5-CNT nanocomposites.
1. The well-resolved spectrum exhibits several peaks at about 149, 201, 285, 306, 405, 482, 533, 698, and 993 cm-1, corresponding to the sequence observed for V2O5 single crystals and polycrystalline films.27,28 The vibrational Raman active modes of V2O5 can be described in terms of vanadium-oxygen stretching modes, vanadium-oxygenvanadium bending vibrations, and translational modes. The peak centered at 993 cm-1 is assigned to the stretching mode related to the shortest vanadium oxygen bond VdO. The prominent peak at 149 cm-1 is attributed to the skeleton bent vibration. It has been related to a deformation of the bond between different molecular units in the plane of the layers. As displayed in Figure 2a and 2b, the ratio of the O atom and the V derived from the V 2p and O 1s XPS spectrum is approximately equivalent to 2.4; therefore, it suggests that the as-prepared vanadium oxides on CNTs should be presented in the form of V2O5.29 From the above measured results, the fabricated metal oxide is vanadium pentoxide.
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Figure 5. (a) The CV curve of V2O5-CNT nanocomposites under different scan rates and (b) the capacitance of V2O5-CNT nanocomposites as a function of the scan rate.
Figure 4. Capacitance-voltage diagrams of (a) bare V2O5 films, (b) pristine CNTs, and (c) V2O5-CNT nanocomposites.
Pristine well-aligned CNTs and bare vanadium pentoxide are shown in Figure 3a and 3b, respectively. After the CNTs are covered with V2O5 films, the surface morphology is significantly different as compared with the as-grown samples. In Figure 3c, V2O5 films are homogeneously coated on CNTs to form nanocomposites, which give rise to a high surface area for the charging-discharging capacitive behavior obtained. The CNTs in Figure 3c are invisible because of the coverage of the vanadium oxides. Accordingly, this is more beneficial in supercapacitor applications. Figure 4a-c exhibits capacitance-voltage (CV) curves of bare vanadium oxides, pristine CNTs, and V2O5-CNT nanocomposites at a sweep rate of 600 mV/s in 2 M KCl and 1 M KHSO4. In Figure 4a, the CV curve of bare V2O5 films shows quite small specific capacitance. This is owing to the fact that the vanadium oxide has poor conductivity and a dense two-dimensional structure. Such an oxide would lead to the formation of inferior capacitive performances. For pristine CNTs, the CV curve in Figure 4b depicts the typical electrical double-layer capacitance property; nevertheless, the capacitance is so small due to the presence of micropores on the CNTs. After the V2O5-CNT nanocomposites are prepared,
the CV property is significantly improved by a difference of 3 orders of magnitude, as evidenced from Figure 4c. Figure 5a shows the CV curve of V2O5-CNT nanocomposites at the various of scan rates, and the ideal square capacitive behavior is found. In Figure 5b, the scan-rate-dependent CV curve demonstrates the typical charging-discharging performance. It reveals that CNTbased nanocomposites do efficiently enhance the capacitive performance of bare V2O5 thin films. After the study examined, the effect of V2O5-CNT nanocomposites on capacitive performances has been thoroughly elucidated. Such a methodology can efficiently capture extrinsic vanadium pentoxides and put them on the side walls of CNTs uniformly. This greatly helps the enhancement of energy-storage efficiency. Therefore, it provides a simple way to have prompt response ability in supercapacitor applications. Conclusions Enhanced capacitive behaviors of vanadium oxides on CNTs have been demonstrated. Raman spectrum and XPS results reveal that the atomic ratio of as-prepared vanadium oxide is approximately that of stoichiometric V2O5. Moreover, CNTs provide good support for a uniform dispersion of oxides, as shown in SEM images. For capacitive measurements, CNTs deposited with uniformly dispersed oxides give rise to significant improvement in capacitive performance as compared with V2O5 films. Hence, the V2O5/CNT nanocomposites show promise as an auxiliary power backup of hybrid electrical systems or consumer electronics. Acknowledgment. The author is grateful for the support of the Industrial Technology Research Institute (Grant No. 6101QV3210).
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