APPLIED PHYSICS LETTERS 92, 143108 共2008兲 Frequency response characteristic of single-walled carbon nanotubes as supercapacitor electrode material C. G. Liu,1,2 M. Liu,1 F. Li,1 and H. M. Cheng1,a兲 1 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China 2 State Key Laboratory of Materials Modification by Laser, Ion and Electron Beams and School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China 共Received 7 December 2007; accepted 19 March 2008; published online 9 April 2008兲 Impedance analysis was employed to study the frequency response characteristics of supercapacitors utilizing single-walled carbon nanotubes 共SWNTs兲 as electrode material. The knee frequency is around 631 Hz for the electrochemically oxidized SWNT supercapacitor, much higher than that of the as-prepared one 共around 316 Hz兲 and the common activated carbon supercapacitor 共⬍1 Hz兲. The good frequency response observed in this work is due to the introduction of nanosized mesopores and oxygen-containing groups formed during the electrochemical oxidization of SWNTs. The capacitance of electrochemically oxidized SWNT supercapacitor is 113 F / g, which is much higher than the value of the as-prepared one 共20 F / g兲. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2907501兴 Supercapacitors are energy storage devices with high power density due to their fast charge propagation.1–10 Single-walled carbon nanotubes 共SWNTs兲 are predicted to be a suitable candidate for the electrode of supercapacitors because of their high accessible surface area, low electric resistance, low mass density, high stability, and the network structure of entangling with each other. Niu et al.3 reported that the frequency “knee” of carbon nanotube supercapacitors was around 100 Hz, which suggests that most of their stored energy is accessible at frequencies below 100 Hz. This is unprecedented for this type of devices because the highest reported knee frequency in other capacitors is 6 Hz and the knee frequency for most commercially available supercapacitors, including those specially designed for high power applications, is ⬍1 Hz.11 A better frequency response means a better power performance. In particular, for low frequency electrical filtering device application, the demand of frequency response performance of supercapacitors is high. Therefore, to find suitable materials which own good frequency response is important. In this study, impedance characterization of SWNTs was investigated, from which the frequency response of the SWNT electrodes in supercapacitors was analyzed. The knee frequency around 631 Hz for the electrochemically oxidized SWNT supercapacitor was achieved due to the introduction of nanosized mesopores and oxygen-containing groups. The SWNTs used in this study were synthesized by a hydrogen arc discharge method,12 in which graphite was the carbon feedstock, hydrogen and argon gases were used as buffer gas, a mixture of Ni, Co, and Fe powders as catalyst, and FeS as growth promoter. The purity of the as-prepared SWNTs was estimated to be over 40 wt % based on transmission electron microscopic observations and thermogravitric analysis results. a兲 Author to whom correspondence should be addressed. Electronic mail: cheng@imr.ac.cn. Typically, 10– 80 mg of SWNTs was placed between two sheets of nickel foam in a cylindrical steel mold and a pellet with 8 mm diameter was formed by pressing the carbon material and nickel foam under 2 MPa. Supercapacitor was assembled in a cell with two identical pellets separated by a nylon cloth in 6M KOH aqueous solution at room temperature and atmospheric pressure. Some of the SWNT electrodes were at first electrochemically oxidized in 6M KOH aqueous solution at the voltage of 1.5 V for 36 h. These will be called as electrochemically oxidized SWNT electrode below. The impedance measurements were made at a dc bias of 0 V with a 10 mV amplitude sinusoidal signal over the frequency range of 100 kHz– 0.01 Hz using a Solartron model 1260 frequency response analyzer with a Solartron model 1287 potentiostat/galvanostat. The complex-plane impedance plots of the as-prepared SWNT and electrochemically oxidized SWNT supercapacitors are shown in Fig. 1. The impedance spectrum in the high frequency range being fitted exhibits semicircles but straight FIG. 1. 共Color online兲 Complex-plane impedance of a single cell supercapacitor fabricated with the as-prepared and electrochemically oxidized SWNTs. 0003-6951/2008/92共14兲/143108/3/$23.00 92, 143108-1 © 2008 American Institute of Physics Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp 143108-2 Appl. Phys. Lett. 92, 143108 共2008兲 Liu et al. TABLE I. Elemental composition of as-prepared and electrochemically oxidized SWNT surfaces detected by XPS. SWNTs Nonoxidized Oxidized FIG. 2. 共Color online兲 Bode angle plot of a single cell supercapacitor fabricated with the as-prepared and electrochemically oxidized SWNTs. lines in the low frequency range, as shown in Fig. 1. An electrode-solution system, especially those porous materials, as in supercapacitors, has complex combinations of C and R components. The interfacial impedance of a supercapacitor is associated with a double-layer capacitance Cdl, a pseudocapacitance C p, a Faradaic charge-transfer resistance R f and the sum of the electrolyte resistance, the electrode resistance, and the contact resistance between the electrode and the current collector Rs. The presence of semicircles of SWNT supercapacitors is attributed to the Faradic reaction of oxygencontaining functional groups at the electrode surface. From the value of Rs 共the point impedance curve intersects the Z⬘ axis at high frequency range兲, the equivalent series resistance of the electrochemically oxidized SWNT and the as-prepared SWNT supercapacitor can be obtained as 0.54 and 0.62 ⍀, respectively. The electrolyte resistance and the contact resistance are identical in both SWNT supercapacitors. Therefore, a decrease of Rs indicates a decrease of the SWNT electrode resistance in electrochemically oxidized SWNT capacitor. In the complex-plane impedance plots, SWNT supercapacitors differ somewhat from those of conventional capacitors at the low frequency range. The ideal capacitor exhibits a vertical line, while the plot of SWNT supercapacitors starts with a 45° impedance line and approaching an almost vertical line at low frequencies. It is due to the diffusion process on the porous surface of SWNTs. It is consistent with the porous nature of the electrodes when saturated with electrolyte. The 45° region 共Warburg region兲 is a consequence of the distributed resistance/capacitance in a porous electrode. At higher frequency, the resistance as well as the capacitance of a porous electrode decreases because only part of the active porous layer is accessible at high frequency. The knee frequency for the electrochemically oxi- C 1s, at. % O 1s, at. % O / C ration, % 87.2 58.5 12.8 41.5 14.7 70.9 dized SWNT supercapacitor was found around 631 Hz from Fig. 1, much higher than those of the as-prepared one 共around 316 Hz兲. Figure 2 shows a Bode angle plot of the as-prepared SWNT supercapacitor and electrochemically oxidized SWNT supercapacitor. For frequency of up to 1 Hz, the phase angle of the electrochemically oxidized SWNT supercapacitor is close to −90°, while it is −50° for the asprepared one. This suggests that the electrochemically oxidized SWNT supercapacitor functions like an ideal capacitor and better than the as-prepared one. Such improved frequency response performance is mainly achieved by the introduction of nanosized mesopores and oxygen-containing groups, favoring the access of electrolyte to the surface of SWNT. After electrochemical oxidation treatment, parts of the tips and sidewall of SWNTs were etched and the amount of small mesopores was increased.13 It is considered that the small mesopores serve as channels for electrolytes and can be readily accessed in the charge/ discharge process. On the other hand, oxygen-containing groups can form during the electrochemical oxidation process, which are confirmed by x-ray photoemission spectroscopy 共XPS兲 data, as shown in Fig. 3共a兲. A strong peak of O 1s has been observed for electrochemically oxidized SWNTs, indicating a high surface oxygen concentration.14,15 Table I summarizes the elemental composition detected by XPS and the oxygen/carbon 共O / C兲 ration. Obviously, electrochemically oxidized SWNTs show higher O / C ration of ⬃70.9%, indicating that the surfaces were severely oxidized during the electrochemical oxidation process. Figure 3共b兲 shows O 1s XPS analysis results of the electrochemically oxidized SWNT. To identify the O 1s XPS results, a curve-fitting procedure was employed. Dots shown in Fig. 3共b兲 correspond to XPS experimental data and bold solid lines are the fitting curves for the experimental data. C u O and C v O groups on electrochemically oxidized SWNT surfaces were identified. Moreover, the detection of oxygen-containing functional groups in oxidized SWNTs is in agreement with other reports.16,17 The increased oxygencontaining groups can enhance the wettability of carbon materials in aqueous electrolyte, which is important to maximize the access of the electrolyte to the surface of carbon.10,18,19 FIG. 3. 共Color online兲 XPS survey scan of as-prepared and electrochemically oxidized SWNTs 共a兲. O 1s peaks of electrochemically oxidized SWNTs 共b兲. Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp 143108-3 Appl. Phys. Lett. 92, 143108 共2008兲 Liu et al. FIG. 4. 共Color online兲 Galvanostatic discharge curves 共a兲 and cycling performance 共b兲 of a single cell supercapacitor fabricated with the asprepared and electrochemically oxidized SWNTs. Not only own good frequency response of the electrochemically oxidized SWNT supercapacitors makes it suitable for the application of strict frequency demand but also high specific capacitance for the introduction of nanosized mesopores and oxygen-containing groups. The effect of electrochemical oxidation on the capacitance of SWNT capacitors is shown in Fig. 4共a兲. The specific capacitance was measured in galvanostatic experiments using an Arbin BT-2000 system with the discharge current density of 50 mA/ g. The capacitance was obtained from the formula C = 共I ⫻ ⌬t兲 / ⌬v, where I is the constant discharging current, ⌬t is the discharging time measured from 0.6 to 0.4 V 共60%–40% of the peak voltage兲, and ⌬v is the voltage change at a constant current discharge. From the discharge curves in Fig. 4共a兲, the specific capacitance of 113 F / g was obtained for the electrochemically oxidized SWNT supercapacitor only compared to 20 F / g for the as-prepared SWNT supercapacitor. Figure 4共b兲 shows the cycling performance of the electrochemically oxidized and the as-prepared SWNT supercapacitor. Both capacitors have good cycling performance and almost no capacitance decrease was observed after 300 cycles. Impedance analysis was employed to study the frequency response characteristics of supercapacitors utilizing SWNTs as an electrode material. It was found that the knee frequency is around 631 Hz for the electrochemically oxidized SWNT supercapacitor, much higher than those of the as-prepared one 共around 316 Hz兲 and the common activated carbon supercapacitor 共⬍1 Hz兲. The good frequency response performance is due to the introduction of nanosized mesopores and oxygen-containing groups. The former serves as effective channels for electrolytes in the charge/discharge process and the latter can improve the wettability of SWNT electrode to maximize the access of the electrolyte to the SWNT surface. Moreover, the electrochemically oxidized SWNT supercapacitor has a specific capacitance of 113 F / g, which is much higher than the specific capacitance 共20 F / g兲 of the as-prepared SWNT. From the results, we can expect the electrochemically oxidized SWNTs to be good electrode material in supercapacitor application. This work was supported by National Science Foundation of China 共Grant Nos. 50328204 and 50632040兲. 1 B. E. Conway, Electrochemical Supercapacitors-Scientific Fundamentals and Technological Applications 共Kluwer/Academic/Plenum, New York, 1999兲. 2 E. Frackowiak, K. Metenier, V. Bertagna, and F. Beguin, Appl. Phys. Lett. 77, 2421 共2000兲. 3 C. Niu, E. K. Sichel, R. Hoch, D. Moy, and H. Tennet, Appl. Phys. Lett. 70, 1480 共1997兲. 4 Y. G. Wang, L. Yu, and Y. Y. Xia, J. 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