Single-walled Carbon Nanotubes as Electrode Materials for Supercapacitors

Chinese Journal of Chemistry, 2006, 24, 1505—1508 Full Paper

* E-mail: [email protected]; Tel.: 0086-10-68912508; Fax: 0086-10-68451429 Received March 11, 2006; revised June 23, 2006; accepted July 28, 2006. Project supported by the National Key Basic Research and Development 973 Program (No. 2002CB211800) and the National Key Program for Basic

Research of China (No. 2001CCA05000).

© 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Single-walled Carbon Nanotubes as Electrode Materials for Supercapacitors

XU, Bina,b(徐斌) WU, Feng*,a(吴锋) WANG, Fangc(王芳) CHEN, Shia(陈实) CAO, Gao-Pingb(曹高萍) YANG, Yu-Shengb(杨裕生)

a School of Chemical Engineering & Environment, Beijing Institute of Technology, Beijing 100081, China b Research Institute of Chemical Defense, Beijing 100083, China

c Department of Chemistry, Tsinghua University, Beijing 100084, China

Large-scale synthesized single-walled carbon nanotubes (SWNT) prepared by electric arc discharge method and a mixture of NiO and Y2O3 as catalyst have been used as electrode materials for supercapacitors. N2 adsorp-tion/desorption measurement shows that the SWNT is a microporous and mesoporous material with specific surface area 435 m2•g－1. The specific capacitance of the nitric acid treated SWNT in aqueous electrolyte reaches as high as 105 F/g, which is a combination of electric double layer capacitance and pseudocapacitance. The SWNT-based ca-pacitors also have good charge/discharge reversibility and cycling perdurability.

Keywords single-walled carbon nanotube, supercapacitor, capacitance

Introduction

Supercapacitors have many advantages compared to secondary batteries, such as high power density, long cycling life (＞100000 cycles), short charging time, and safety.1,2 In recent years, supercapacitors have attracted great attention because of their promising applications to electric vehicles and other high-power apparatuses.3-5 The maximum power density of a supercapacitor is given by Pmax＝

2iV /4R, where Vi is the initial voltage, R

is the equivalent series resistance. Therefore, the key factors determining the power of supercapacitors are the resistivity of the electrode materials and the resistivity of the electrolyte within the porous structure of the elec-trode.6 Porous carbons with high surface area have been used as electrode materials for supercapacitors.7 Among the various carbon materials, carbon nanotubes (CNT) are regarded as appropriate electrode materials for high- power supercapacitors due to their highly accessible surface area, low resistivity and chemical stability.8-12

Various multi-walled carbon nanotubes (MWNT) have been checked as electrode materials for superca-pacitors.8-11 However, the capacitance of MWNT is much lower than that of activated carbon electrodes be-cause of their low surface area, which is a drawback for their application to supercapacitors. The single-walled carbon nanotube (SWNT) is more attractive than MWNT as electrode material for supercapacitors be-cause it has a theoretically high surface area (the total specific surface area of the outside plane and the inside plane of the SWNT is 2630 m2•g－1), hence high capaci-

tance. Recently, Picó et al.12 have investigated the ca-pacitance of SWNT synthesized by arc discharge method using Ni/Y as catalyst. After oxidation in air at 350 ℃ for 1 h, the capacitance of the modified SWNT reached 140 F/g in 6 mol•L－1 KOH electrolyte.

It is known that the commercial application pros-pects of SWNT in supercapacitors rely on the develop-ment of a cost-effective and large-scale production of high quality SWNT. The production of SWNT with high quality at large scale using electric arc discharge method and metal oxides of Ni and Y as catalyst has been reported.13 In this paper, we will investigate the electrochemical performances of this SWNT as elec-trode for supercapacitors.

Experimental

The single-walled carbon nanotubes, obtained from Nankai University, were synthesized by electric arc dis-charge method using a mixture of NiO and Y2O3 as catalyst. The as-synthesized SWNT were refluxed with concentrated nitric acid at 100 ℃ for 2 h, followed by filtration, washing, and drying to yield the treated SWNT. The specific surface area and porous structure were determined with N2 adsorption/desorption iso-therms at 77 K (Quantachrome NOVA 1200).

A mixture of 87 wt% of SWNT, 10 wt% of acety-lene black and 3 wt% of PTFE binder was pressed into pellets (19 mm in diameter) as electrodes. Then the electrodes were dried under vacuum at 120 ℃ for 12 h. Button-type capacitor was assembled with two SWNT

1506 Chin. J. Chem., 2006, Vol. 24, No. 11 XU et al.


electrodes using aqueous KOH (6 mol•L－1) as electro-lyte.

The cyclic voltammetry (CV) was recorded on an electrochemical workstation Solartry 1280B. The gal-vanostatic charge/discharge (I＝0.65 mA, ca. 50 mA/g) was carried out on a Land cell tester. The specific ca-pacitance (C) of a single SWNT electrode was deter-mined with the formula C＝2It/∆Vm, where I is the dis-charge current, t the discharge time, ∆V the potential change in discharge and m the mass of the active elec-trode material. The capacitors were cycled between 0 and 1.0 V unless specified.

Results and discussion

As reported in Ref. 13, the as-prepared SWNT is made up of tangled bundles. The diameter of SWNT is in the range of 1.3—1.8 nm and the bundle size in the range from 5 to 30 nm. Ash analysis indicates that the SWNT synthesized by electric arc discharge method using Ni and Y compounds as catalyst yields an amount of ca. 30 wt% metal oxide residue. After refluxed with nitric acid at 100 ℃ for 2 h, the residue in SWNT de-creased to less than 3 wt%.

The capacitive performance of carbon electrode is generally determined by their microstructure, such as surface area, pore volume, pore size distribution and surface chemistry. Figure 1 shows the N2 adsorption/ desorption isotherm of the treated SWNT. It is found that the isotherm is a combination of type I and IV according to the IUPAC classification. The sharp knees at ultra low relative pressure (＜0.05) and the flat in the range of p/p0＝0.1—0.4 indicate the SWNT is a microporous material, while the typical hysteresis loop at high relative pressure characterizes a mesoporous material.

Figure 1 N2 adsorption/desorption isotherms of the treated SWNT.

The main parameters of the microtexture of the SWNT calculated from nitrogen adsorption/desorption data are shown in Table 1. The specific surface area calculated with conventional BET method reaches 435 m2•g－1, which is a relatively high value for carbon nanotubes. The mesopore volume is 0.235 cm3•g－1, which is 63% of the total pore volume.

Table 1 Porous texture of SWNT

Parameter SWNT

SBET/(m2•g－1)

Smic/(m2•g－1)

Smes/(m2•g－1)

Vtot/(cm3•g－1)

Vmic/(cm3•g－1)

Vmes/(cm3•g－1)

D/nm

435

218

217

0.374

0.139

0.235

3.44

SBET: BET specific surface area. Smic: microporous surface area. Smes: mesoporous surface area. Vtot: total pore volume. Vmic: mi-cropore volume. Vmes: mesopore volume. D: average pore diame-ter.

The pore size distribution of the SWNT calculated with density functional theory (DFT) is presented in Figure 2. It can be seen that the SWNT carries major pore around 1.4 nm in diameter, which is similar to the internal diameter of SWNT from HRTEM observation. The SWNT also carried a broad minor pore size distri-bution in the range of 4—25 nm, which is similar to the bundle size. That is to say, the curve of pore size distri-bution indicates that the SWNT is a microporous and mesoporous material, which corresponded with the re-sult of isotherm analysis. It is well-known that the pores in carbon nanotubes come from the tube entanglement as well as from the central canal of the open tubes. As for this nitric acid treated SWNT, the micropores (ca. 1.4 nm) come from the central canal, while the mesopores (4—25 nm) come from the tube entangle-ment.

Figure 2 Pore size distribution of the treated SWNT.

Figure 3 shows the cyclic voltammetry of the ca-pacitor built from SWNT electrodes between 0 and 1.0 V at 1 mV/s scan rate. Instead of a typical rectangle shape, a well remarkable region of reversible pseudo-faradic reaction was observed at ca. 0.2 V, which was attributed to the surface functional groups.11,14 Besides removal of the residue metal analyst, the treatment of SWNT with hot nitric acid also introduced a large amount of oxygenous functional group on the surface of

Single-walled carbon nanotube Chin. J. Chem., 2006 Vol. 24 No. 11 1507


SWNT, which can make an additional enhancement of pseudocapacitance. The infrared spectral analysis (Fig-ure 4) indicates that the functional groups introduced on the surface of SWNT by nitric acid treatment are mainly composed of OH (3441 cm－1) and COOH (1576 cm－1). The following reactions7,11 of electroactive surface functional groups should be considered:

Figure 3 Cyclic voltammetry of a capacitor built from treated SWNT electrodes, scan rate: 1 mV/s.

Figure 4 Infrared spectra of SWNT (a) and nitric acid treated SWNT (b).

Galvanostatic charge/discharge was performed to determine the capacitance of the active materials. The voltage range of charge/discharge is limited between 1.0 and 0 V. The voltage-time curve (Figure 5) is of basi-cally linear shape, demonstrating the capacitive behav-ior of the cell. However, it is necessary to pointed out that the curve has a little deviation from the line at the lower voltage of 0—0.5 V, ascribed to the pseudoca-pacitance of the surface functional groups. The specific capacitance based on SWNT weight was determined to be 105 F/g, which is a combination of electric double layer capacitance and pseudocapacitance. The high ca-pacitance can be attributed to the highly accessible sur-face area of SWNT and the large amount of functional

groups on its surface. The practical application of these surface functional

groups was determined by their charge-discharge re-versibility and cycling perdurability.11 It was found that the reversibility and cycling perdurability of the nitric acid treated SWNT-based capacitor were acceptable. The coulombic efficiency of the capacitor was estimated as ∆td/∆tc×100%, where ∆td and ∆tc represent the dis-charge and charge time, respectively. The coulombic efficiency reached 96%, implying good charge/dis-charge reversibility for the capacitor. Figure 6 shows the cycling performance of the capacitor between 0 and 1.0 V at current density of 100 mA/g. A high capacitance retention was obtained over prolonged cycling test. The capacity decay was about 10% of the initial discharge capacity after 1000 cycles, comparable to the pure electric double layer capacitors.

Figure 5 Charge-discharge curve of a capacitor built from treated SWNT electrodes.

Figure 6 Cycle test of a capacitor built from treated SWNT electrodes.

Conclusion

Single-walled carbon nanotubes with high quality synthesized at large scale have been used as electrode materials for supercapacitors. Besides removal of the residue metal catalyst, the treatment of SWNT with hot nitric acid also introduced a large amount of oxygenous functional group on the surface of SWNT, which made an additional enhancement of pseudocapacitance. The

1508 Chin. J. Chem., 2006, Vol. 24, No. 11 XU et al.


specific capacitance of the treated SWNT reached 105 F/g, which is a combination of electric double layer ca-pacitance and pseudocapacitance. The SWNT-based capacitors have good charge/discharge reversibility and cycling perdurability. The preliminary results have demonstrated that the single-walled carbon nanotubes are promising electrode materials for supercapacitors.

Acknowledgement

The authors thank Prof. Y. S. Huang and Dr. Y. Huang, Nankai University for providing the SWNT sample.

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Single-walled Carbon Nanotubes as Electrode Materials for Supercapacitors