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carbon fabric composite using cetyltrimethylammonium bromide for high performance capacitor
Electrochimica Acta 60 (2012) 449–455
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrodeposition of carbon nanotube/carbon fabric composite using cetyltrimethylammonium bromide for high performance capacitor Huimin Chen, Yu Li, Yiyu Feng, Peng Lv, Peng Zhang, Wei Feng ∗ Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 2 September 2011 Received in revised form 2 November 2011 Accepted 27 November 2011 Available online 6 December 2011 Keywords: Carbon nanotubes Carbon fabric Electrophoretic deposition Cetyltrimethylammonium bromide
a b s t r a c t High performance capacitor based on multi-walled carbon nanotube/carbon fabric (MWCNT/CF) composite is fabricated by electrophoretic deposition. The deposition is effectively facilitated by employing cationic cetyltrimethylammonium bromide to charge and disperse the nanotubes in an aqueous solution. Electrochemical results indicate that MWCNT/CF shows a high specific capacitance of 102 F g−1 , rapid charge/discharge characteristic and good cycling stability, which is much better than that of MWCNTs coated on stainless steel. The excellent performance is attributed to the unique architecture formed by three dimensional and interconnected CF as the current collector, favoring the accessibility for ions and charge transfer during the electrochemical tests. The flexible MWCNT/CF with excellent capacitor behaviors can be developed to be an advanced carbon/carbon electrode material for capacitors. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Since the discovery in 1991 [1–3], carbon nanotubes (CNTs) have become one of the excellent electrode materials for electrical double layer capacitors (EDLCs) due to the high specific surface area (SSA), high electrical conductivity and excellent chemical stability [4–9]. Previous studies were focused on the effects of chemical properties or microstructures of CNT mats on the electrochemical properties [10–13], but minor studies were devoted to the natural or unique structure of current collectors. Recently, it was reported [14] that the capacitor behaviors of active materials were greatly improved using porous carbon fabric (CF) as the current collector. Wu et al. [14] reported that the specific capacitance of MnO2 nanofibers coated on CF reached 432 F g−1 , which was much higher than that of MnO2 nanofibers deposited on stainless steel (SS, 177 F g−1 ). They attributed the excellent capacitance to the combination of spaced MnO2 and conductive carbon fibers, which facilitated the electrolyte penetration and electron conduction. Horng et al. [15] successfully fabricated polyaniline nanowire/carbon cloth composite and an extremely high area capacitance (1.8 F cm−2 ) was achieved wherein the architecture was favorable for effective ion diffusion. In addition, active materials throughout the three dimensional matrix could lead to a high structural integrity and buffer volume changes of the hosted materials during the charge/discharge processes [16–18], which improved their cycling stability.
∗ Corresponding author. Tel.: +86 22 27404724; fax: +86 22 27404724. E-mail address: [email protected] (W. Feng). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.11.101
It is worth noting that electrode materials based on CNT/CF composites are usually fabricated by chemical vapor deposition (CVD) method [19–21]. Although the traditional CVD method is an efficient technique for the growth of CNTs onto CF, the use of high temperatures and predeposited catalyst usually degrade the strength and electrical conductivity of the original CF, limiting its practical applications consequently [22,23]. Recently, an electrophoretic deposition (EPD) technique with advantages of low cost, process simplicity and thickness control [24,25], has proven to be an effective technique to deposit CNTs onto CF [22,26]. It is also a binder free method to lower the contact resistance between the active materials and current collectors [27,28], facilitating the electron conduction. Furthermore, CNTs need to be charged in order to be electrodeposited onto the electrode under a specific electric field. Bekyarova et al. [22] utilized EPD for the deposition of carboxyl-functionalized CNTs onto CF anode, and Rodriguez et al. [26] deposited amine-functionalized carbon nanofibers on the CF cathode by the same method. However, the functionlization process is usually time-consuming, and the original properties of CNTs could be destroyed during this process. Therefore, a relatively mild and convenient method for the deposition of charged CNTs is required necessarily. Among several methods, the utilization of surfactants is a simple and versatile way to charge and disperse CNTs for the electrodeposition [29,30], and a high performance CNT/CF electrode could be achieved accordingly. In this study, a uniform multi-walled carbon nanotube (MWCNT) aqueous solution was obtained using two different surfactants, cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). MWCNT/CF was synthesized by EPD when CTAB was applied. In order to investigate the effects of the three dimensional CF on the electrochemical properties of MWCNT films,
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H. Chen et al. / Electrochimica Acta 60 (2012) 449–455
Fig. 1. Schematic illustration of the preparation of MWCNT/CF electrode.
MWCNTs were also deposited onto a two dimensional SS plate under the same condition for comparison. The influences of different parameters during the EPD process were discussed as well. 2. Experimental 2.1. Materials Polyacrylonitrile-based CF (15 mm × 15 mm) composed of carbon fibers about 7 m in diameter (Toray Corporation of Japan) was desized in argon at 800 ◦ C for 1 h. Both CF and SS (316 L, 15 mm × 15 mm) substrates were cleaned with acetone and
deionized water alternately more than three times and dried at 60 ◦ C for 12 h before use. MWCNTs (TsingHua University, ≥80%) were heated in argon at 700 ◦ C for 2 h, and then refluxed in hydrochloric acid for 1 h to remove the metal catalyst and amorphous carbon. Purified MWCNTs were obtained after the filtration and dried in a vacuum for 24 h. Other chemicals were used without any treatment. 2.2. Preparation of MWCNT/CF electrode All electrochemical experiments were carried out in a conventional three-electrode system at room temperature using the CF or
Fig. 2. SEM images of (a) CF substrate with the magnification view of individual carbon fiber in the left corner; MWCNT/CF synthesized using (b) SDS and (c) CTAB, respectively and (d) MWCNT/SS synthesized under the same condition as (c).
H. Chen et al. / Electrochimica Acta 60 (2012) 449–455
SS, a Pt foil (15 mm × 15 mm) and an Ag/AgCl (0.2223 V vs. SHE at 25 ◦ C) as the working electrode, counter electrode and reference electrode, respectively. A uniform MWCNT aqueous solution was obtained by mixing MWCNTs and surfactant CTAB or SDS through ultrasonication. CF was immersed into the solution and then a potential was applied. The typical condition was listed as follows: 0.1 mg ml−1 MWCNTs, 3 mM CTAB (or SDS), and 10 V cm−1 potential for 1 h. The deposition process was illustrated in Fig. 1. The resultant electrodes were washed by acetone and deionized water alternately, were cleaned in 1 M KCl aqueous solution by cyclic voltammetry (CV) to remove the remaining surfactants [31], and then were dried in a vacuum for 24 h. Before deposition, the weight of CF and SS substrates was measured by an electronic balance. The mass of MWCNTs deposited on CF and SS under the typical condition using CTAB was calculated as the difference before and after the deposition process, and the value was 0.7 mg and 1.0 mg, respectively. 2.3. Characterization The morphologies were observed by scanning electron microscope (SEM, Hatchi S-4800). The surface characteristics of different electrodes were determined by the gas desorption and an automated gas absorption apparatus (Quantachrome NOVA-2000, N20-14D) was employed. The EPD processes with the CV and galvanostatical charge/discharge tests were carried out with a potentiostat/galvanostat (TD73000, Tianjin Zhonghuan Co. Ltd.). CV measurements of electrodes were conducted in the potential range of 0–0.6 V vs. Ag/AgCl, and the scanning rates were 5, 10, 20, 50, 100 and 200 mV s−1 . The galvanostatical charge/discharge tests were measured in a constant current density of 5 A g−1 . Electrochemical impedance spectroscope (EIS) measurements were conducted on DAR STAT 2263 advanced electrochemical system (Princedon Applied Research) in the frequency range of 10 mHz to 100 kHz. All electrochemical measurements were performed in 1 M KCl aqueous
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solution at room temperature in a three-electrode system with the prepared electrode, Pt foil and Ag/AgCl as the working electrode, counter electrode and reference electrode, respectively. 3. Results and discussion SEM images of CF substrate, MWCNT/CF synthesized under the typical condition with SDS and CTAB as the surfactants are shown in Fig. 2a–c. It is known that negatively charged MWCNTs will move towards the anode when a specific electric field is applied [29]. However, few MWCNTs are deposited on the surface of CF anode, indicating the inefficient deposition using SDS. Furthermore, separated and individual carbon fiber is extensively damaged due to the strong oxidation at the anode [32]. When CTAB is employed, a uniform MWCNT layer is formed on the surface of CF. The effective deposition is attributed to the adsorption of CTAB on MWCNTs. The adsorption of cationic CTAB on the sidewalls of nanotubes can result in positive Zeta potential of MWCNTs [30]. As a consequence, the positively charged MWCNTs move towards and deposit on the cathode. Furthermore, using CF as the cathode can effectively prevent from the damage and etching due to the protection of generated hydrogen, and the remaining good electrical and mechanical properties of CF could facilitate its further practical applications. To investigate the effects of CF on the electrochemical properties of MWCNT films, MWCNTs were also deposited on SS. As can be seen in Fig. 2d, MWCNT/SS tends to agglomerate closely in comparison with MWCNT/CF. The feature may be caused by the fact that CF can hold MWCNTs both on the surface and inside. Because the three dimensional CF can provide larger contact area and broader room for the deposition of MWCNTs than SS, a porous electrode with highly accessible surface area could be achieved, which is expected to have positive effects on its capacitor behaviors. The surface characteristic of original CF, MWCNT films coated on CF and SS are listed in Table 1. MWCNT film on CF shows a much higher SSA (157 m2 g−1 ) and larger total pore volume
Fig. 3. (a) CV curves of different electrodes at scanning rate of 5 mV s−1 ; (b) normalized capacitance of MWCNT/CF and MWCNT/SS conducted at 5, 10, 20, 50, 100 and 200 mV s−1 , respectively; (c) galvanostatical charge/discharge curves and (d) Nyquist plots of MWCNT/CF and MWCNT/SS.
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Table 1 Surface characteristic of original CF, MWCNT/CF and MWCNT/SS.
CF MWCNT/CF MWCNT/SS a b c
SBET a (m2 g−1 )
Vt b (cm3 g−1 )
dc (nm)
2.13 157 106
0.003 0.476 0.223
5.75 12.1 8.09
Pore size distribution Vmicro (%)
Vmeso (%)
– – –
100 100 100
SSA analyzed by using BET method. Total pore volume estimated at a relative pressure of 0.9899. Average pore diameter.
(0.476 cm3 g−1 ) than those on SS. The average pore diameter of MWCNT/CF is 12.1 nm, which is larger than the diameter of MWCNT itself (∼10 nm). Therefore, much of the porosity in the MWCNT film comes from the interstitial spaces created by the entangled CNT network. The open porosity can provide preferable accessibility for electrolytes. However, the average pore diameter of MWCNT film on SS is only 8.09 nm. The SSA and pore volume of original CF are negligible in comparison with MWCNT film, and thus the improved properties must be attributed to the porous structure of MWCNT film formed on CF rather than the effects of CF itself. The CV curves of CF, MWCNT/SS and MWCNT/CF are shown in Fig. 3a. The CV curve of MWCNT/SS shows a narrow loop with a large oblique angle, which is a typical resistive electrode. In contrast, the CV curve of MWCNT/CF displays a rectangle like shape, indicating an ideal capacitor behavior with a fast charge/discharge process. The relatively fast electrochemical switch may be attributed to the use of CF, with pores among the interconnected carbon fibers, as the current collector, which can provide better accessibility for ions (as shown in Fig. 1) during the electrochemical tests. The specific capacitance (Cm ) values of MWCNTs are calculated from the CV curves based on the following equation: Cm
1 = m
dt i dv
(1)
where i is the current, v is the potential and m is the mass of MWCNTs [31]. The specific capacitance of MWCNT/CF achieves 102 F g−1 . However, MWCNT/SS shows a low capacitance of 55 F g−1 , which is in accordance with the previous studies that unfunctionalized MWCNTs have a low capacitance around 50–80 F g−1 [2]. The specific capacitance of original CF, calculated from the CV curve of CF, is very low (∼0.034 F g−1 ), the increased capacitance maybe arise from the improved surface characteristic of MWCNT/CF rather than CF itself. As the key to reaching high capacitance for EDLCs is in using high SSA and electronically conducting electrodes [2], the higher SSA (as shown in Table 1) and better conductive path of MWCNT/CF than that of MWCNT/SS contribute to the resultant high capacitance. Fig. 3b shows the normalized capacitance of different electrodes conducted at different scanning rates. Results indicate that the capacitance of MWCNT/CF and MWCNT/SS both decrease at high scanning rate, suggesting that accessibility inside the CNT layer mainly leads to the degradation of capacitance. At low charge rates, electrolytes have enough time to penetrate into the pores and higher surface area is accessed. As the charge rate increases, the electrolyte penetration becomes worse and less surface area is accessed [8]. However, MWCNT/CF keeps a relatively higher capacitance retention, about 62.1% capacitance remaining at 200 mV s−1 compared with the capacitance at 5 mV s−1 , than that of MWCNT/SS (∼40.9%). This high capacitance retention could be attributed the relatively large pore-size of MWCNT/CF, which enables better electrolyte penetration at high scanning rates, resulting in the high surface area, and hence the high capacitance. The galvanostatical charge/discharge curves of MWCNT/CF and MWCNT/SS conducted at a current density of 5 A g−1 are presented
in Fig. 3c. There is a noticeable IR drop for MWCNT/SS, indicating a high contact resistance between the active materials and the current collectors. While the linear curve of MWCNT/CF without any evident IR drop confirms its ideal capacitance behavior, suggesting the good contact between MWCNTs and CF. The discharge capacitance of MWCNT/CF and MWCNT/SS calculated from their discharge curves is 109 and 58 F g−1 , respectively, and is in consistence with the CV results. In addition, the high linearity and symmetry in the galvanostatical charge/discharge curve of MWCNT/CF indicates its high coulombic efficiency (∼95%). This enhanced response rate may be attributed to the special three dimensional structure, providing broad room and short diffusion distances for ion transport between electrolyte and MWCNT film. The electrochemical behaviors of these electrodes were also studied by EIS method. The Nyquist plots of MWCNT/CF and MWCNT/SS, ranging from 10 mHz to 100 kHz, are shown in Fig. 3d. Two separated patterns can be observed in the Nyquist plot of MWCNT/SS. The semicircle is obtained at the high frequency region which is related to the interfacial processes. With the decreasing of the frequency, the plot transforms to a vertical line, indicating the capacitive behavior. However, no clear semicircle is observed in the plot of MWCNT/CF, which demonstrates that its internal resistance is greatly decreased. The value of charge-transfer resistance (Rct ) can be obtained by the semicircle at high frequency. The value of Rct for MWCNT/CF is negligibly small, while that for MWCNT/SS is about 100 . This confirms that the unique pore network formed by EPD effectively enhances its electrolyte penetration and electron conduction. In addition, the imaginary part in the low frequency region of MWCNT/CF is nearly vertical with the real part in the plot, suggesting an ideal capacitor behavior again. The cycling performances of MWCNT/CF and MWCNT/SS conducted by galvanostatical charge/discharge method at a current density of 5 A g−1 are shown in Fig. 4. MWCNT/CF shows no drift of specific capacitance in the whole cycling test, and the specific capacitance decreases by only 8% (from 109 to 100.3 F g−1 ) after the 5000th cycle. Because the cycling performance was conducted in an aqueous solution, various chemical groups (e.g. OH, COOH, etc.) could covalently attach to the electrodes during the cycling test, accelerating the aging process [2,33]. As a result, MWCNT/CF lost some capacitance during the cycling. However, the capacitance of MWCNT/SS fades 22% (from 58 to 45.4 F g−1 ) under the same condition. The enhanced cycling stability of MWCNT/CF electrode could be attributed to the suitable interspaces among MWCNTs, providing good accessibility for ions. Moreover, the three dimensional structure which could buffer volume changes of the active materials during the charge/discharge processes improves the cycling
Fig. 4. Cycling performances and SEM images of MWCNT/CF and MWCNT/SS after the 5000th cycle.
H. Chen et al. / Electrochimica Acta 60 (2012) 449–455
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Fig. 5. SEM images of MWCNT/CF synthesized using (a) 0.5 mM, (b) 1 mM, (c) 5 mM CTAB electrolyte and (d) magnification view of (c).
stability as well. It can be proved by the SEM images of different electrodes after the 5000th cycle as shown in Fig. 4. From the SEM images, it can be seen that MWCNT/CF remains good morphology. But serious fracture and collapse appear in MWCNT/SS, leading to its inferior stability. The influences of different parameters during the EPD process were also studied in order to find the optimal condition for
obtaining high performance MWCNT/CF electrode. SEM images of MWCNT/CF synthesized using different concentration of CTAB are shown in Fig. 5. When the concentration of CTAB is 0.5 (Fig. 5a) or 1 mM (Fig. 5b), few MWCNTs are deposited on CF indicating the inefficient deposition of MWCNTs at low concentration of CTAB. A uniform MWCNT layer is packed on CF (Fig. 2c) when the concentration of CTAB is 3 mM. In contrast, when the concentration of
Fig. 6. SEM images of MWCNT/CF deposited for (a) 10 min, (b) 30 min, (c) 90 min and (d) magnification view of (c).
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Fig. 7. Specific capacitance of MWCNT/CF synthesized using different concentration of CTAB (a) and different deposition times (b).
CTAB increases further and achieves 5 mM, redundant CATB bundles (Fig. 5c and d) are observed on CF. CTAB molecules tend to lay parallel to the axis of MWCNTs at low concentration, leading to a relatively low charge density. With the increasing amount of CTAB, the surfactant molecules are packed perpendicular to the MWCNT axis due to the hydrophobic force between the surfactant tails and MWCNTs. Thus MWCNTs with high charge density can easily move towards the cathode and be collected on the CF cathode under the applied potential. However, this equilibrium could be disturbed when more surfactant molecules are added into the solution. As a consequence, individual CTAB micelles with high charge density will form big bundles on CF besides MWCNTs. To investigate the growth process, Fig. 6 shows the SEM images of MWCNT/CF prepared with different deposition times. Only a small amount of MWCNTs were deposited on CF when deposited for 10 (Fig. 6a) or 30 min (Fig. 6b) in comparison with that deposited for 60 min (Fig. 2c). This demonstrates that MWCNTs are deposited onto CF gradually with the increasing of deposition time rather than being deposited on CF all at once after the potential is applied. However, few MWCNTs on CF usually results in high resistance of the electrode due to the discontinuity of the MWCNT film and bad contact between different fibers as can be seen from the SEM images. As the deposition time increases, a good conductive network is achieved when grown for 60 min. However, the conductive network could be destroyed by continually increasing the deposition time. When the deposition time increases to 90 min, MWCNTs with big bundled CTAB are stacked on CF (Fig. 6c and d) due to the strong turbulence in the suspension caused by the long time exposure to an electrical field [34]. The insulated CTAB bundles among MWCNTs could ruin the conductive network and lead to inferior capacitor behaviors of the electrode. The electrochemical properties of MWCNT/CF synthesized using different concentration of CTAB and different deposition time are studied by CV method and the resultant specific capacitance at the scanning rate of 5 mV s−1 are shown in Fig. 7. MWCNT/CF achieves the highest capacitance of 102 F g−1 when the concentration of CTAB is 3 mM and the deposition time is 60 min. The inefficient deposition of MWCNTs onto CF at low concentration of CTAB or short deposition time leads to a low capacitance of MWCNT/CF due to the defective network as can be seen from the SEM images. Insulated and inactive CTAB bundles formed on CF due to the excessive amount of CTAB or deposition time lead to low electron conduction and prevent ions from transporting through out the film, which reduces its capacitor behaviors. 4. Conclusions A three dimensional CF was used as the current collector to investigate the effects of natural structure of current collectors
on the electrochemical properties of MWCNT films. MWCNT was charged and dispersed using two different surfactants, and a porous MWCNT film on CF cathode was obtained when CTAB was employed. Electrochemical results demonstrated that MWCNT/CF showed higher specific capacitance, better high rate performance and more stable cycling property than MWCNT/SS because of its unique pore network formed by EPD. In addition, the influences of different parameters during the deposition process were investigated, and the optimum condition for obtaining high-performance MWCNT/CF electrode was as follows: 0.1 mg ml−1 MWCNTs, 3 mM CTAB, 10 V cm−1 potential for 1 h. The flexible MWCNT/CF with excellent capacitor behaviors is supposed to be an advanced carbon/carbon electrode material for EDLCs. Acknowledgements This work was supported by the National Basic Research Program of China (2010CB934700) and the National Natural Science Foundation of China (Grant Nos. 51173127, 51073115, 51003072 and 51011140072) and the Natural Science Foundation of Tianjin City (No. 10JCZDJC22400). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2011.11.101. References [1] S. Iijima, Nature 354 (1991) 56. [2] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845. [3] Y.Y. Feng, X.H. Ju, W. Feng, H.B. Zhang, Y.W. Cheng, J. Liu, A. Fujii, M. Ozaki, K. Yoshino, Appl. Phys. Lett. 94 (2009) 123302. [4] K. Metenier, V. Bertaga, F. Beguin, Appl. Phys. Lett. 77 (2000) 2421. [5] E. Frackowiak, F. Beguin, Carbon 39 (2001) 937. [6] E. Frackowiak, F. Beguin, Carbon 40 (2002) 1775. [7] Q.F. Xiao, X. Zhou, Electrochim. Acta 48 (2003) 575. [8] A.G. Pandolfo, A.F. Hollenkamp, J. Power Sources 157 (2006) 11. [9] P.W. Ruch, R. Kota, A. Wokaun, Electrochim. Acta 54 (2009) 4451. [10] Q.L. Chen, K.H. Xue, W. Shen, F.F. Tao, S.Y. Yin, W. Xu, Electrochim. Acta 49 (2004) 4157. [11] C.T. Hsieh, W.Y. Chen, Y.S. Cheng, Electrochim. Acta 55 (2010) 5294. [12] C.T. Hsieh, H.S. Teng, W.Y. Chen, Y.S. Cheng, Carbon 48 (2010) 4219. [13] T. Akter, K.W. Hu, K. Lian, Electrochim. Acta 56 (2011) 4966. [14] M.S. Wu, Z.S. Guo, J.J. Jow, J. Phys. Chem. C 114 (2010) 21861. [15] Y.Y. Horng, Y.C. Lu, Y.K. Hsu, C.C. Chen, L.C. Chen, K.H. Chen, J. Power Sources 195 (2010) 4418. [16] J.C. Guo, A. Sun, C.S. Wang, Electrochem. Commun. 12 (2010) 981. [17] C. Arbizzani, M. Lazzari, M. Mastragostino, J. Electrochem. Soc. 152 (2005) A289. [18] C. Aribzzani, S. Beninati, M. Lazzari, M. Mastragostino, J. Power Sources 158 (2006) 635. [19] C.H. Wang, H.C. Shih, Y.T. Tsai, H.Y. Du, L.C. Chen, K.H. Chen, Electrochim. Acta 52 (2006) 1612. [20] C.W. Huang, C.M. Chuang, J.M. Ting, H.S. Teng, J. Power Sources 183 (2008) 406.
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Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrodeposition of carbon nanotube/carbon fabric composite using cetyltrimethylammonium bromide for high performance capacitor Huimin Chen, Yu Li, Yiyu Feng, Peng Lv, Peng Zhang, Wei Feng ∗ Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 2 September 2011 Received in revised form 2 November 2011 Accepted 27 November 2011 Available online 6 December 2011 Keywords: Carbon nanotubes Carbon fabric Electrophoretic deposition Cetyltrimethylammonium bromide
a b s t r a c t High performance capacitor based on multi-walled carbon nanotube/carbon fabric (MWCNT/CF) composite is fabricated by electrophoretic deposition. The deposition is effectively facilitated by employing cationic cetyltrimethylammonium bromide to charge and disperse the nanotubes in an aqueous solution. Electrochemical results indicate that MWCNT/CF shows a high specific capacitance of 102 F g−1 , rapid charge/discharge characteristic and good cycling stability, which is much better than that of MWCNTs coated on stainless steel. The excellent performance is attributed to the unique architecture formed by three dimensional and interconnected CF as the current collector, favoring the accessibility for ions and charge transfer during the electrochemical tests. The flexible MWCNT/CF with excellent capacitor behaviors can be developed to be an advanced carbon/carbon electrode material for capacitors. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Since the discovery in 1991 [1–3], carbon nanotubes (CNTs) have become one of the excellent electrode materials for electrical double layer capacitors (EDLCs) due to the high specific surface area (SSA), high electrical conductivity and excellent chemical stability [4–9]. Previous studies were focused on the effects of chemical properties or microstructures of CNT mats on the electrochemical properties [10–13], but minor studies were devoted to the natural or unique structure of current collectors. Recently, it was reported [14] that the capacitor behaviors of active materials were greatly improved using porous carbon fabric (CF) as the current collector. Wu et al. [14] reported that the specific capacitance of MnO2 nanofibers coated on CF reached 432 F g−1 , which was much higher than that of MnO2 nanofibers deposited on stainless steel (SS, 177 F g−1 ). They attributed the excellent capacitance to the combination of spaced MnO2 and conductive carbon fibers, which facilitated the electrolyte penetration and electron conduction. Horng et al. [15] successfully fabricated polyaniline nanowire/carbon cloth composite and an extremely high area capacitance (1.8 F cm−2 ) was achieved wherein the architecture was favorable for effective ion diffusion. In addition, active materials throughout the three dimensional matrix could lead to a high structural integrity and buffer volume changes of the hosted materials during the charge/discharge processes [16–18], which improved their cycling stability.
∗ Corresponding author. Tel.: +86 22 27404724; fax: +86 22 27404724. E-mail address: [email protected] (W. Feng). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.11.101
It is worth noting that electrode materials based on CNT/CF composites are usually fabricated by chemical vapor deposition (CVD) method [19–21]. Although the traditional CVD method is an efficient technique for the growth of CNTs onto CF, the use of high temperatures and predeposited catalyst usually degrade the strength and electrical conductivity of the original CF, limiting its practical applications consequently [22,23]. Recently, an electrophoretic deposition (EPD) technique with advantages of low cost, process simplicity and thickness control [24,25], has proven to be an effective technique to deposit CNTs onto CF [22,26]. It is also a binder free method to lower the contact resistance between the active materials and current collectors [27,28], facilitating the electron conduction. Furthermore, CNTs need to be charged in order to be electrodeposited onto the electrode under a specific electric field. Bekyarova et al. [22] utilized EPD for the deposition of carboxyl-functionalized CNTs onto CF anode, and Rodriguez et al. [26] deposited amine-functionalized carbon nanofibers on the CF cathode by the same method. However, the functionlization process is usually time-consuming, and the original properties of CNTs could be destroyed during this process. Therefore, a relatively mild and convenient method for the deposition of charged CNTs is required necessarily. Among several methods, the utilization of surfactants is a simple and versatile way to charge and disperse CNTs for the electrodeposition [29,30], and a high performance CNT/CF electrode could be achieved accordingly. In this study, a uniform multi-walled carbon nanotube (MWCNT) aqueous solution was obtained using two different surfactants, cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). MWCNT/CF was synthesized by EPD when CTAB was applied. In order to investigate the effects of the three dimensional CF on the electrochemical properties of MWCNT films,
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Fig. 1. Schematic illustration of the preparation of MWCNT/CF electrode.
MWCNTs were also deposited onto a two dimensional SS plate under the same condition for comparison. The influences of different parameters during the EPD process were discussed as well. 2. Experimental 2.1. Materials Polyacrylonitrile-based CF (15 mm × 15 mm) composed of carbon fibers about 7 m in diameter (Toray Corporation of Japan) was desized in argon at 800 ◦ C for 1 h. Both CF and SS (316 L, 15 mm × 15 mm) substrates were cleaned with acetone and
deionized water alternately more than three times and dried at 60 ◦ C for 12 h before use. MWCNTs (TsingHua University, ≥80%) were heated in argon at 700 ◦ C for 2 h, and then refluxed in hydrochloric acid for 1 h to remove the metal catalyst and amorphous carbon. Purified MWCNTs were obtained after the filtration and dried in a vacuum for 24 h. Other chemicals were used without any treatment. 2.2. Preparation of MWCNT/CF electrode All electrochemical experiments were carried out in a conventional three-electrode system at room temperature using the CF or
Fig. 2. SEM images of (a) CF substrate with the magnification view of individual carbon fiber in the left corner; MWCNT/CF synthesized using (b) SDS and (c) CTAB, respectively and (d) MWCNT/SS synthesized under the same condition as (c).
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SS, a Pt foil (15 mm × 15 mm) and an Ag/AgCl (0.2223 V vs. SHE at 25 ◦ C) as the working electrode, counter electrode and reference electrode, respectively. A uniform MWCNT aqueous solution was obtained by mixing MWCNTs and surfactant CTAB or SDS through ultrasonication. CF was immersed into the solution and then a potential was applied. The typical condition was listed as follows: 0.1 mg ml−1 MWCNTs, 3 mM CTAB (or SDS), and 10 V cm−1 potential for 1 h. The deposition process was illustrated in Fig. 1. The resultant electrodes were washed by acetone and deionized water alternately, were cleaned in 1 M KCl aqueous solution by cyclic voltammetry (CV) to remove the remaining surfactants [31], and then were dried in a vacuum for 24 h. Before deposition, the weight of CF and SS substrates was measured by an electronic balance. The mass of MWCNTs deposited on CF and SS under the typical condition using CTAB was calculated as the difference before and after the deposition process, and the value was 0.7 mg and 1.0 mg, respectively. 2.3. Characterization The morphologies were observed by scanning electron microscope (SEM, Hatchi S-4800). The surface characteristics of different electrodes were determined by the gas desorption and an automated gas absorption apparatus (Quantachrome NOVA-2000, N20-14D) was employed. The EPD processes with the CV and galvanostatical charge/discharge tests were carried out with a potentiostat/galvanostat (TD73000, Tianjin Zhonghuan Co. Ltd.). CV measurements of electrodes were conducted in the potential range of 0–0.6 V vs. Ag/AgCl, and the scanning rates were 5, 10, 20, 50, 100 and 200 mV s−1 . The galvanostatical charge/discharge tests were measured in a constant current density of 5 A g−1 . Electrochemical impedance spectroscope (EIS) measurements were conducted on DAR STAT 2263 advanced electrochemical system (Princedon Applied Research) in the frequency range of 10 mHz to 100 kHz. All electrochemical measurements were performed in 1 M KCl aqueous
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solution at room temperature in a three-electrode system with the prepared electrode, Pt foil and Ag/AgCl as the working electrode, counter electrode and reference electrode, respectively. 3. Results and discussion SEM images of CF substrate, MWCNT/CF synthesized under the typical condition with SDS and CTAB as the surfactants are shown in Fig. 2a–c. It is known that negatively charged MWCNTs will move towards the anode when a specific electric field is applied [29]. However, few MWCNTs are deposited on the surface of CF anode, indicating the inefficient deposition using SDS. Furthermore, separated and individual carbon fiber is extensively damaged due to the strong oxidation at the anode [32]. When CTAB is employed, a uniform MWCNT layer is formed on the surface of CF. The effective deposition is attributed to the adsorption of CTAB on MWCNTs. The adsorption of cationic CTAB on the sidewalls of nanotubes can result in positive Zeta potential of MWCNTs [30]. As a consequence, the positively charged MWCNTs move towards and deposit on the cathode. Furthermore, using CF as the cathode can effectively prevent from the damage and etching due to the protection of generated hydrogen, and the remaining good electrical and mechanical properties of CF could facilitate its further practical applications. To investigate the effects of CF on the electrochemical properties of MWCNT films, MWCNTs were also deposited on SS. As can be seen in Fig. 2d, MWCNT/SS tends to agglomerate closely in comparison with MWCNT/CF. The feature may be caused by the fact that CF can hold MWCNTs both on the surface and inside. Because the three dimensional CF can provide larger contact area and broader room for the deposition of MWCNTs than SS, a porous electrode with highly accessible surface area could be achieved, which is expected to have positive effects on its capacitor behaviors. The surface characteristic of original CF, MWCNT films coated on CF and SS are listed in Table 1. MWCNT film on CF shows a much higher SSA (157 m2 g−1 ) and larger total pore volume
Fig. 3. (a) CV curves of different electrodes at scanning rate of 5 mV s−1 ; (b) normalized capacitance of MWCNT/CF and MWCNT/SS conducted at 5, 10, 20, 50, 100 and 200 mV s−1 , respectively; (c) galvanostatical charge/discharge curves and (d) Nyquist plots of MWCNT/CF and MWCNT/SS.
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Table 1 Surface characteristic of original CF, MWCNT/CF and MWCNT/SS.
CF MWCNT/CF MWCNT/SS a b c
SBET a (m2 g−1 )
Vt b (cm3 g−1 )
dc (nm)
2.13 157 106
0.003 0.476 0.223
5.75 12.1 8.09
Pore size distribution Vmicro (%)
Vmeso (%)
– – –
100 100 100
SSA analyzed by using BET method. Total pore volume estimated at a relative pressure of 0.9899. Average pore diameter.
(0.476 cm3 g−1 ) than those on SS. The average pore diameter of MWCNT/CF is 12.1 nm, which is larger than the diameter of MWCNT itself (∼10 nm). Therefore, much of the porosity in the MWCNT film comes from the interstitial spaces created by the entangled CNT network. The open porosity can provide preferable accessibility for electrolytes. However, the average pore diameter of MWCNT film on SS is only 8.09 nm. The SSA and pore volume of original CF are negligible in comparison with MWCNT film, and thus the improved properties must be attributed to the porous structure of MWCNT film formed on CF rather than the effects of CF itself. The CV curves of CF, MWCNT/SS and MWCNT/CF are shown in Fig. 3a. The CV curve of MWCNT/SS shows a narrow loop with a large oblique angle, which is a typical resistive electrode. In contrast, the CV curve of MWCNT/CF displays a rectangle like shape, indicating an ideal capacitor behavior with a fast charge/discharge process. The relatively fast electrochemical switch may be attributed to the use of CF, with pores among the interconnected carbon fibers, as the current collector, which can provide better accessibility for ions (as shown in Fig. 1) during the electrochemical tests. The specific capacitance (Cm ) values of MWCNTs are calculated from the CV curves based on the following equation: Cm
1 = m
dt i dv
(1)
where i is the current, v is the potential and m is the mass of MWCNTs [31]. The specific capacitance of MWCNT/CF achieves 102 F g−1 . However, MWCNT/SS shows a low capacitance of 55 F g−1 , which is in accordance with the previous studies that unfunctionalized MWCNTs have a low capacitance around 50–80 F g−1 [2]. The specific capacitance of original CF, calculated from the CV curve of CF, is very low (∼0.034 F g−1 ), the increased capacitance maybe arise from the improved surface characteristic of MWCNT/CF rather than CF itself. As the key to reaching high capacitance for EDLCs is in using high SSA and electronically conducting electrodes [2], the higher SSA (as shown in Table 1) and better conductive path of MWCNT/CF than that of MWCNT/SS contribute to the resultant high capacitance. Fig. 3b shows the normalized capacitance of different electrodes conducted at different scanning rates. Results indicate that the capacitance of MWCNT/CF and MWCNT/SS both decrease at high scanning rate, suggesting that accessibility inside the CNT layer mainly leads to the degradation of capacitance. At low charge rates, electrolytes have enough time to penetrate into the pores and higher surface area is accessed. As the charge rate increases, the electrolyte penetration becomes worse and less surface area is accessed [8]. However, MWCNT/CF keeps a relatively higher capacitance retention, about 62.1% capacitance remaining at 200 mV s−1 compared with the capacitance at 5 mV s−1 , than that of MWCNT/SS (∼40.9%). This high capacitance retention could be attributed the relatively large pore-size of MWCNT/CF, which enables better electrolyte penetration at high scanning rates, resulting in the high surface area, and hence the high capacitance. The galvanostatical charge/discharge curves of MWCNT/CF and MWCNT/SS conducted at a current density of 5 A g−1 are presented
in Fig. 3c. There is a noticeable IR drop for MWCNT/SS, indicating a high contact resistance between the active materials and the current collectors. While the linear curve of MWCNT/CF without any evident IR drop confirms its ideal capacitance behavior, suggesting the good contact between MWCNTs and CF. The discharge capacitance of MWCNT/CF and MWCNT/SS calculated from their discharge curves is 109 and 58 F g−1 , respectively, and is in consistence with the CV results. In addition, the high linearity and symmetry in the galvanostatical charge/discharge curve of MWCNT/CF indicates its high coulombic efficiency (∼95%). This enhanced response rate may be attributed to the special three dimensional structure, providing broad room and short diffusion distances for ion transport between electrolyte and MWCNT film. The electrochemical behaviors of these electrodes were also studied by EIS method. The Nyquist plots of MWCNT/CF and MWCNT/SS, ranging from 10 mHz to 100 kHz, are shown in Fig. 3d. Two separated patterns can be observed in the Nyquist plot of MWCNT/SS. The semicircle is obtained at the high frequency region which is related to the interfacial processes. With the decreasing of the frequency, the plot transforms to a vertical line, indicating the capacitive behavior. However, no clear semicircle is observed in the plot of MWCNT/CF, which demonstrates that its internal resistance is greatly decreased. The value of charge-transfer resistance (Rct ) can be obtained by the semicircle at high frequency. The value of Rct for MWCNT/CF is negligibly small, while that for MWCNT/SS is about 100 . This confirms that the unique pore network formed by EPD effectively enhances its electrolyte penetration and electron conduction. In addition, the imaginary part in the low frequency region of MWCNT/CF is nearly vertical with the real part in the plot, suggesting an ideal capacitor behavior again. The cycling performances of MWCNT/CF and MWCNT/SS conducted by galvanostatical charge/discharge method at a current density of 5 A g−1 are shown in Fig. 4. MWCNT/CF shows no drift of specific capacitance in the whole cycling test, and the specific capacitance decreases by only 8% (from 109 to 100.3 F g−1 ) after the 5000th cycle. Because the cycling performance was conducted in an aqueous solution, various chemical groups (e.g. OH, COOH, etc.) could covalently attach to the electrodes during the cycling test, accelerating the aging process [2,33]. As a result, MWCNT/CF lost some capacitance during the cycling. However, the capacitance of MWCNT/SS fades 22% (from 58 to 45.4 F g−1 ) under the same condition. The enhanced cycling stability of MWCNT/CF electrode could be attributed to the suitable interspaces among MWCNTs, providing good accessibility for ions. Moreover, the three dimensional structure which could buffer volume changes of the active materials during the charge/discharge processes improves the cycling
Fig. 4. Cycling performances and SEM images of MWCNT/CF and MWCNT/SS after the 5000th cycle.
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Fig. 5. SEM images of MWCNT/CF synthesized using (a) 0.5 mM, (b) 1 mM, (c) 5 mM CTAB electrolyte and (d) magnification view of (c).
stability as well. It can be proved by the SEM images of different electrodes after the 5000th cycle as shown in Fig. 4. From the SEM images, it can be seen that MWCNT/CF remains good morphology. But serious fracture and collapse appear in MWCNT/SS, leading to its inferior stability. The influences of different parameters during the EPD process were also studied in order to find the optimal condition for
obtaining high performance MWCNT/CF electrode. SEM images of MWCNT/CF synthesized using different concentration of CTAB are shown in Fig. 5. When the concentration of CTAB is 0.5 (Fig. 5a) or 1 mM (Fig. 5b), few MWCNTs are deposited on CF indicating the inefficient deposition of MWCNTs at low concentration of CTAB. A uniform MWCNT layer is packed on CF (Fig. 2c) when the concentration of CTAB is 3 mM. In contrast, when the concentration of
Fig. 6. SEM images of MWCNT/CF deposited for (a) 10 min, (b) 30 min, (c) 90 min and (d) magnification view of (c).
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Fig. 7. Specific capacitance of MWCNT/CF synthesized using different concentration of CTAB (a) and different deposition times (b).
CTAB increases further and achieves 5 mM, redundant CATB bundles (Fig. 5c and d) are observed on CF. CTAB molecules tend to lay parallel to the axis of MWCNTs at low concentration, leading to a relatively low charge density. With the increasing amount of CTAB, the surfactant molecules are packed perpendicular to the MWCNT axis due to the hydrophobic force between the surfactant tails and MWCNTs. Thus MWCNTs with high charge density can easily move towards the cathode and be collected on the CF cathode under the applied potential. However, this equilibrium could be disturbed when more surfactant molecules are added into the solution. As a consequence, individual CTAB micelles with high charge density will form big bundles on CF besides MWCNTs. To investigate the growth process, Fig. 6 shows the SEM images of MWCNT/CF prepared with different deposition times. Only a small amount of MWCNTs were deposited on CF when deposited for 10 (Fig. 6a) or 30 min (Fig. 6b) in comparison with that deposited for 60 min (Fig. 2c). This demonstrates that MWCNTs are deposited onto CF gradually with the increasing of deposition time rather than being deposited on CF all at once after the potential is applied. However, few MWCNTs on CF usually results in high resistance of the electrode due to the discontinuity of the MWCNT film and bad contact between different fibers as can be seen from the SEM images. As the deposition time increases, a good conductive network is achieved when grown for 60 min. However, the conductive network could be destroyed by continually increasing the deposition time. When the deposition time increases to 90 min, MWCNTs with big bundled CTAB are stacked on CF (Fig. 6c and d) due to the strong turbulence in the suspension caused by the long time exposure to an electrical field [34]. The insulated CTAB bundles among MWCNTs could ruin the conductive network and lead to inferior capacitor behaviors of the electrode. The electrochemical properties of MWCNT/CF synthesized using different concentration of CTAB and different deposition time are studied by CV method and the resultant specific capacitance at the scanning rate of 5 mV s−1 are shown in Fig. 7. MWCNT/CF achieves the highest capacitance of 102 F g−1 when the concentration of CTAB is 3 mM and the deposition time is 60 min. The inefficient deposition of MWCNTs onto CF at low concentration of CTAB or short deposition time leads to a low capacitance of MWCNT/CF due to the defective network as can be seen from the SEM images. Insulated and inactive CTAB bundles formed on CF due to the excessive amount of CTAB or deposition time lead to low electron conduction and prevent ions from transporting through out the film, which reduces its capacitor behaviors. 4. Conclusions A three dimensional CF was used as the current collector to investigate the effects of natural structure of current collectors
on the electrochemical properties of MWCNT films. MWCNT was charged and dispersed using two different surfactants, and a porous MWCNT film on CF cathode was obtained when CTAB was employed. Electrochemical results demonstrated that MWCNT/CF showed higher specific capacitance, better high rate performance and more stable cycling property than MWCNT/SS because of its unique pore network formed by EPD. In addition, the influences of different parameters during the deposition process were investigated, and the optimum condition for obtaining high-performance MWCNT/CF electrode was as follows: 0.1 mg ml−1 MWCNTs, 3 mM CTAB, 10 V cm−1 potential for 1 h. The flexible MWCNT/CF with excellent capacitor behaviors is supposed to be an advanced carbon/carbon electrode material for EDLCs. Acknowledgements This work was supported by the National Basic Research Program of China (2010CB934700) and the National Natural Science Foundation of China (Grant Nos. 51173127, 51073115, 51003072 and 51011140072) and the Natural Science Foundation of Tianjin City (No. 10JCZDJC22400). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2011.11.101. References [1] S. Iijima, Nature 354 (1991) 56. [2] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845. [3] Y.Y. Feng, X.H. Ju, W. Feng, H.B. Zhang, Y.W. Cheng, J. Liu, A. Fujii, M. Ozaki, K. Yoshino, Appl. Phys. Lett. 94 (2009) 123302. [4] K. Metenier, V. Bertaga, F. Beguin, Appl. Phys. Lett. 77 (2000) 2421. [5] E. Frackowiak, F. Beguin, Carbon 39 (2001) 937. [6] E. Frackowiak, F. Beguin, Carbon 40 (2002) 1775. [7] Q.F. Xiao, X. Zhou, Electrochim. Acta 48 (2003) 575. [8] A.G. Pandolfo, A.F. Hollenkamp, J. Power Sources 157 (2006) 11. [9] P.W. Ruch, R. Kota, A. Wokaun, Electrochim. Acta 54 (2009) 4451. [10] Q.L. Chen, K.H. Xue, W. Shen, F.F. Tao, S.Y. Yin, W. Xu, Electrochim. Acta 49 (2004) 4157. [11] C.T. Hsieh, W.Y. Chen, Y.S. Cheng, Electrochim. Acta 55 (2010) 5294. [12] C.T. Hsieh, H.S. Teng, W.Y. Chen, Y.S. Cheng, Carbon 48 (2010) 4219. [13] T. Akter, K.W. Hu, K. Lian, Electrochim. Acta 56 (2011) 4966. [14] M.S. Wu, Z.S. Guo, J.J. Jow, J. Phys. Chem. C 114 (2010) 21861. [15] Y.Y. Horng, Y.C. Lu, Y.K. Hsu, C.C. Chen, L.C. Chen, K.H. Chen, J. Power Sources 195 (2010) 4418. [16] J.C. Guo, A. Sun, C.S. Wang, Electrochem. Commun. 12 (2010) 981. [17] C. Arbizzani, M. Lazzari, M. Mastragostino, J. Electrochem. Soc. 152 (2005) A289. [18] C. Aribzzani, S. Beninati, M. Lazzari, M. Mastragostino, J. Power Sources 158 (2006) 635. [19] C.H. Wang, H.C. Shih, Y.T. Tsai, H.Y. Du, L.C. Chen, K.H. Chen, Electrochim. Acta 52 (2006) 1612. [20] C.W. Huang, C.M. Chuang, J.M. Ting, H.S. Teng, J. Power Sources 183 (2008) 406.
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