Effects of Carbon Nanotube Grafting on the Performance of Electric

Nov 12, 2010 - Review on supercapacitors: Technologies and materials. Ander González , Eider Goikolea , Jon Andoni Barrena , Roman Mysyk. Renewable a...
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Energy Fuels 2010, 24, 6476–6482 Published on Web 11/12/2010

: DOI:10.1021/ef101208x

Effects of Carbon Nanotube Grafting on the Performance of Electric Double Layer Capacitors Chih-Ming Chuang,† Cheng-Wei Huang,‡ Hsisheng Teng,‡ and Jyh-Ming Ting*,†,§ †

Department of Materials Science and Engineering, ‡Department of Chemical Engineering, and §Institute of Nanotechnology and Microsystems Engineering, National Cheng Kung University, Tainan 70101, Taiwan Received July 5, 2010. Revised Manuscript Received October 29, 2010

Carbon nanotubes (CNTs) were grown on polyacrylonitrile (PAN)-based activated carbon cloth (ACC) for use as electrodes in electric double layer capacitors. The catalyst used for the growth of CNTs was sputter-deposited Ni. It was found that the use of sputter-deposited catalyst for the growth CNTs prevents normally observed surface pore blockage on the ACC. The growth of CNTs took place in a thermal chemical vapor deposition chamber. During the CNT growth, ammonia was introduced into the growth chamber for enhancing the CNT growth and limiting the formation of pyrolytic carbon. The resulting CNTs were found to follows the tip growth mode, leading to direct contacts between the CNTs and the ACC fiber surfaces. Because of this and the high electrical conductivity of CNTs, the obtained CNTgrafted ACC electrodes exhibit significantly enhanced electrical conductivity and improved capacitance retention.

high currents or high scan rates has been ascribed mainly to the following two reasons.15-24 One reason is that the “intricate” pore structures in activated carbons hinder the ion diffusion. The other is that the poor electrical contacts among the activated carbon fibers and between the ACC electrodes and the current collectors limit the electrical pathways. Furthermore, activated carbon fibers have essentially nongraphitized structures and therefore the intrinsic electric resistance is impossible to ignore. As a result, the performance of high power density EDLCs based on ACCs is limited. Enhancing the electrical conductance of ACC electrodes has thus been called for. It has been reported that one way to achieve this goal is to grow one-dimensional nanocarbons, including carbon nanotubes (CNTs) or carbon nanofibers (CNFs), on the surfaces of carbon fibers.12-14,25-28 Most of these reports focused on investigating the methods for the growth of CNTs and CNFs, such as studying the effects of catalyst composition and gas atmosphere on the growth of CNTs/CNFs.13,14,25-28 Common to these studies is that the surface areas of the resulting CNT/CNF-grafted carbons were decreased.12-14,28 Growth of CNFs onto ACCs using a thermal chemical vapor deposition

1. Introduction Electric double layer capacitors (EDLCs) having carbonaceous electrodes are receiving increasing attention as they provide high power densities and long cycle lives.1-7 Recently, the use of activated carbon fiber cloth (ACC) electrodes has emerged.8-11 ACCs are used as the electrode materials in EDLCs for their high specific surface areas and high electrochemical stability.8-14 Although the ACC EDLCs provide superior charge/discharge rates and power densities, the discharge capacitance reduces substantially or even disappears at high currents.10 The poor performance of ACC EDLCs at *To whom correspondence should be addressed. E-mail: jting@ mail.ncku.edu.tw. (1) Kinoshita, K. Carbon: Electrochemical and Physicochemical Properties; John Wiley & Sons: New York, 1988. (2) Conway, B. E. Electrochemical Supercapacitors Scientific Fundamentals and Technological Applications; Kluwer Academic: New York, 1999. (3) Osaka, T.; Liu, X.; Nijima, M.; Momma, T. J. Electrochem. Soc. 1999, 146, 1724–1729. (4) K€ otz, R.; Carlen, M. Electrochim. Acta 2000, 45, 2483–2498. (5) Qiao, W.; Yoon, S. H.; Mochida, I. Energy Fuels 2006, 20, 1680– 1684. (6) Calvo, E. G.; Ania, C. O.; Zubizarreta, L.; Menendez, J. A.; Arenillas, A. Energy Fuels 2010, 24, 3334–3339. (7) Zhao, X. Y.; Cao, J. P.; Morishita, K.; Ozaki, J.; Takarada, T. Energy Fuels 2010, 24, 1889–1893. (8) Okajima, K.; Ikeda, A.; Kamoshita, K.; Sudoh, M. Electrochim. Acta 2005, 51, 972–977. (9) Xu, B.; Wu, F.; Chen, S.; Zhang, C.; Cao, G.; Yang, Y. Electrochim. Acta 2007, 52, 4595–4598. (10) Wang, K. P.; Teng, H. J. Electrochem. Soc. 2007, 154, A993– A998. (11) Lin, J. H.; Ko, T. H.; Lin, Y. H.; Pan, C. K. Energy Fuels 2009, 23, 4668–4677. (12) Huang, C. W.; Chuang, C. M.; Ting, J. M.; Teng, H. J. Power Sources 2008, 183, 406–410. (13) Ko, T. H.; Hung, K. H.; Tzeng, S. S.; Shen, J. W.; Hung, C. H. Phys. Scr. 2007, T129, 80–84. (14) Tzeng, S. S.; Hung, K. H.; Ko, T. H. Carbon 2006, 44, 859–865. (15) Du, C. S.; Yeh, J.; Pan, N. Nanotechnology 2005, 16, 350–353. (16) Wei, Y. Z.; Fang, B.; Iwasa, S.; Kumagai, M. J. Power Sources 2005, 141, 386–391. r 2010 American Chemical Society

(17) Du, C. S.; Pan, N. Nanotechnology 2006, 17, 5314–5318. (18) Lee, G. J.; Pyun, S. I. Electrochim. Acta 2006, 51, 3029–3038. (19) Fang, B.; Binder, L. J. Power Sources 2006, 163, 616–622. (20) Prabaharan, S. R. S.; Vimala, R.; Zainal, Z. J. Power Sources 2006, 161, 730–736. (21) An, K. H.; Lee, Y. H. J. Power Sources 2007, 173, 621–625. (22) Fang, B.; Binder, L. Electrochim. Acta 2007, 52, 6916–6921. (23) Huang, C. W.; Wu, Y. T.; Hu, C. C.; Li, Y. Y. J. Power Sources 2007, 172, 460–467. (24) Liu, C. L.; Dong, W. S.; Cao, G. P.; Song, J. R.; Liu, L.; Yang, Y. S. J. Electrochem. Soc. 2008, 155, F1–F7. (25) Sun, X.; Li, R.; Stansfield, B.; Dodelet, J. P.; De’silets, S. Chem. Phys. Lett. 2004, 394, 266–270. (26) Zhu, S.; Su, C. H.; Lehoczky, S. L.; Muntele, I.; Ila, D. Diamond Relat. Mater. 2003, 12, 1825–1828. (27) Sonoyama, N.; Ohshita, M.; Nijubu, A.; Nishikawa, H.; Yanase, H.; Hayashi, J.; Chiba, T. Carbon 2006, 44, 1754–1761. (28) Piao, Y.; An, K.; Kim, J.; Yu, T.; Hyeon, T. J. Mater. Chem. 2006, 16, 2984–2989.

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method (CVD) for use as EDLC electrodes was reported. Co(NO3)2 3 6H2O was used as the catalyst precursor, and the catalyst seeding was carried out by immersing the ACCs in a Co(NO3)2 aqueous solution. However, some surface pores on the ACCs were blocked during the seeding process, resulting in reduced surface areas. Surface pore blocking was also observed on CNF-grafted mesoporous carbons when the catalyst seeding was carried out using a heat flux method in which an Fe(NO3)3 aqueous solution was used as the precursor.29 Sea urchin shaped carbons were fabricated by growing CNTs on carbon spheres for fuel cell application.28 Iron oxide nanoparticles were dispersed in hexane and subsequently absorbed onto the surfaces of the carbon spheres as catalyst. After the growth of CNTs, the surface area was reduced due to pore blockage. As an attempt to avoid or reduce the pore blockage, we have used sputter deposition for the preparation of Ni catalyst in this study. For the growth of CNTs, a catalyst film is deposited on the substrate usually using a sputter deposition process.30-35 The catalyst film would crack into particles prior to the growth of CNTs. In other words, the surface coverage by the catalyst is reduced. Therefore, if a porous substrate is used, the likelihood of surface pore blockage by the catalyst or the resulting CNTs is expected to be low. In this study, we have shown that the surface area is essentially unchanged after the growth of CNTs. Furthermore, the resulting CNT-grafted ACC electrodes exhibit significantly enhanced electrical conductivity and improved capacitance retention.

Figure 1. SEM images of CNT-grafted ACCs. The growth of CNTs took place (a) without and (b) with the addition of NH3 in the feedstock.

samples were characterized using Philips XL-40 scanning electron microscope (SEM) for the surface morphology and Hitachi HF2000 and Jeol JEM 2010 high-resolution transmission electron microscopy (HRTEM) equipped with a field emission gun operated at 200 kV for the microstructure. An adsorption apparatus (Micromeritics ASAP 2010) was employed for measuring the specific surface area. A Micro Raman spectrometer from Renishaw with a He-Ne laser source at 10 mW power, a spot size of 3 μm, and wavelength 633 nm was used to determine the quality of the obtained CNTs. The surface chemical states of the specimens were analyzed using high-resolution X-ray photoelectron spectroscopy (HRXPS, ULVAC-PHI i). Two-electrode cells were assembled to examine the electrochemical performance of the capacitors having bare ACC and CNT-grafted ACC. The electrodes were supported on 1 cm2 stainless-steel foil current collectors. The separator between the electrodes was a cellulose fiber filter paper. The electrochemical measurements were carried out using a 2 M H2SO4 solution as the electrolyte at the ambient temperature. Cyclic voltammetric (CV) measurements were conducted in a potential range between -1.0 and 1.0 V at different scan rates ranging from 5 to 500 mV. Alternating current (ac) impedance measurements were carried out using a Zahner IM6e ac impedance spectrum analyzer. The measurements were conducted at 0 V with an ac potential amplitude of 5 mV and a frequency range of 2 mHz-100 kHz.

2. Experimental Section The ACCs used were PAN-based ACCs and have a surface dimension of 6 cm  6 cm. To grow CNTs on the ACCs, the nickel catalyst was first deposited on the ACCs using radio frequency (rf) sputter deposition. During the rf deposition, the working pressure, the Ar flow rate, the rf power, and the deposition time were 1  10-2 Torr, 15 standard cubic centimeters per minute (sccm), 50 W, and 3 min, respectively. CNTs were then grown on the catalyst seeded ACCs using a thermal CVD method in a horizontal tubular reactor. The catalyst was first reduced at 400 °C for 40 min under 200 sccm of H2 flow. Following the catalyst reduction, the furnace temperature was raised to 800 °C, at which methane was admitted such that the ratio of the CH4/H2 mixture was 3/17 for the growth of CNTs. The CNTs growth time was 40 min. In selected experiments, NH3 was introduced during the CNTs growth stage. The functions of NH3 are to promote the pyrolysis of CH4 and enhance CNTs growth rate.34-40 The CH4/H2/NH3 ratio was 3/16/1. The resulting (29) Chuang, C. M.; Sharma, S. P.; Ting, J. M.; Lin, H. P.; Teng, H.; Huang, C. W. Diamond Relat. Mater. 2008, 17, 606–610. (30) Liao, K. H.; Ting, J. M. Carbon 2004, 42, 509–514. (31) Teng, F. Y.; Ting, J. M.; Sharma, S. P.; Liao, K. H. Nanotechnology 2008, 19, 095607. (32) Ting, J. M.; Lin, W. C. Nanotechnology 2009, 20, 025608. (33) Ting, J. M.; Wu, W. Y.; Liao, K. H.; Wu, H. H. Carbon 2009, 47, 2671–2678. (34) Ting, J. M.; Lin, S. H. Carbon 2007, 45, 1934–1940. (35) Lin, S. H.; Mishra, D. K.; Ting, J. M. J. Nanosci. Nanotechnol. 2008, 8, 2647–2650. (36) Ren, Z. F.; Huang, Z. P.; Xu, J. W.; Wang, J. H.; Bush, P.; Siegel, M. P.; Provencio, P. N. Science 1998, 282, 1105–1107. (37) Lee, C. J.; Kim, D. W.; Lee, T. J.; Choi, Y. C.; Park, Y. S.; Lee, Y. H.; Choid, W. B.; Leed, N. S.; Parke, G. S.; Kim, J. M. Chem. Phys. Lett. 1999, 312, 461–468. (38) Jung, M.; Eun, K. Y.; Lee, J. K.; Baik, Y. T.; Lee, K. R.; Park, J. W. Diamond Relat. Mater. 2001, 10, 1235–1240. (39) Choi, K. S.; Cho, Y. S.; Hong, S. Y.; Park, J. B.; Kim, D. J. J. Eur. Ceram. Soc. 2001, 21, 2095–2098. (40) Choi, G. S.; Cho, Y. S.; Hong, S. Y.; Park, J. B.; Son, K. H.; Kima, D. J. J. Appl. Phys. 2002, 91, 3847–3854.

3. Results and Discussion The surface areas of bare ACC and CNT-grafted ACC specimens were first determined. The bare ACC has an average surface area of 1,182 m2 g-1. The average surface area of the CNT-grafted ACC obtained without the addition of NH3 is slightly reduced by 1.2% to 1 168 m2 g-1. This type of specimens is designated as ACC/CNT-0 hereafter. The surface area of the CNT-grafted ACC obtained with addition of NH3 is slightly increased by 1.8% to 1 203 m2 g-1. The type of specimens is designated as ACC/CNT-1 hereafter. Therefore, it is considered that the catalyst seeding or the CNTgrafting does not affect the surface area. The surface morphology of ACC/CNT-0 is shown in Figure 1a. Pyrolytic carbon particles were observed along with a few CNTs when 6477

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Figure 2. (a) A TEM image showing a catalyst at the tip of a CNT and (b) an HRTEM image of CNT.

no NH3 was used during the growth. However, as NH3 was used, pyrolytic carbon particles were not observed and much more CNTs were obtained as shown in Figure 1b. The elimination of the pyrolytic carbon is attributed to the addition of NH3.34-40 It was also reported that adding NH3 during CNT growth can enhance the growth rate by allowing the catalyst to maintain its surface activity.34-40 Therefore the CNTs in the ACC/CNT-1 samples are longer than that in ACC/CNT-0 samples and several micrometers in length. The diameters of the CNTs shown in parts a and b of Figure 1 are similar, ranging from ∼15 to ∼38 nm and averaging at 32 nm. The CNT growth was found to follow the tip growth mode as shown in Figure 2a. All the CNTs obtained are multiwalled, as shown in Figure 2b. The d-spacing was determined to be 0.342 nm, which is the spacing between the (002) crystalline planes of graphite. The interface between CNTs and ACC was also examined using transmission electron microscopy (TEM) as shown in Figure 3. Figure 3 is a general cross sectional view of an ACC/CNT-1 sample, which shows an intimate contact between the CNT and the ACC. This figure also shows that the growth of CNTs follows the tip-growth mode. It indicates weak interaction between the sputtered Ni catalyst and the ACC substrate. Figure 4 compares the Raman signatures of ACC, ACC/ CNT-0, and ACC/CNT-1 samples. In Figure 4, curve fittings to the original Raman spectral lines (solid lines) are represented by the dashed lines. The fitted curves are further deconvoluted into several peaks, represented by the dotted lines. All the spectra show D- and G-band peaks. The ID/IG ratios of the bare ACC, ACC/CNT-0, and ACC/CNT-1 are at 1.10, 1.31, and 0.85, respectively. The pyrolytic carbon particles found in the ACC/CNT-0 sample (Figure 1a) is believed to exhibit a disordered structure, contributing an ID/IG ratio of 1.31 that is higher than that of the bare ACC sample.41-43 On the other hand, there is nearly no pyrolytic carbon found in the ACC/CNT-1 sample and the CNTs on the sample given an ID/IG ratio that is smaller than that of the bare ACC sample. Although no systematic study has been performed to correlate the presence of the D band with different defect types in CNTs, the lowest ID/IG value is likely to represent the presence of less amorphous carbon in the CNTs than in the ACC.43 Also, the bare ACC gives the largest full width at halfmaximum (fwhm) of the G-peak, i.e., 82; while that of the ACC/CNT-0 and ACC/CNT-1 are similar, i.e., 64 and 60, respectively. This indicates that the grafted CNTs have larger

Figure 3. TEM cross sectional view of an ACC/CNT-1 sample.

Figure 4. Raman spectra of various ACC specimens.

graphite cluster sizes than the bare ACC.42-44 For both the ACC/CNT-0 and ACC/CNT-1 samples, shoulder peaks

(41) Tamor, M. A.; Vassell, W. C. J. Appl. Phys. 1994, 76, 3823–3830. (42) Schwan, J.; Ulrich, S.; Batori, V.; Ehrhardt, H.; Silva, S. R. P. J. Appl. Phys. 1996, 80, 440–447. (43) Ferrari, A. C.; Robertson, J. Phys. Rev. B 2000, 61, 14095–14107.

(44) Li, W.; Zhang, H.; Wang, C.; Zhang, Y.; Xu, L.; Zhu, K. Appl. Phys. Lett. 1997, 70, 2684–2686.

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CNT-1 EDLC exhibits the highest specific capacitance; while the bare ACC EDLC exhibits the lowest specific capacitance. In general, functional redox responses are much slower than double-layer responses. At high scan rates, the pseudocapacitance no longer exists and the performance of bare ACC EDLCs worsens primarily due to the aforementioned reasons. In this scan range, CNT grafting apparent enhances the capacitance of EDLCs. At the highest scan rate of 500 mV s-1, the capacitances of the bare ACC, ACC/CNT-0, and ACC/ CNT-1 are 18, 48, and 61 F g-1, respectively. The enhancement of capacitance due to the CNT grafting was found to be the highest at this highest scan rate, 167% and 239% increases from the ACC/CNT-0 and ACC/CNT-1 EDLCs, respectively. The CNT-grafting is believed to have reduced the resistance, as shown later, for charge migration. Although ACC/CNT-0 and ACC/CNT-1 both have CNTs on the surfaces, the capacitances of ACC/CNT-0 are slightly lower than that of ACC/CNT-1. The ACC/CNT-0 specimen surface has pyrolytic carbon with poor electrical conductivity, as also shown later, leading to increased charge resistance and thereby lower capacitances. The lower amount of CNTs on the surface of ACC/CNT-0 specimen is responsible for the reduced capacitances. Figure 5a also shows that all the capacitances decrease with the scan rate. This is known to be a result of the energy loss during the electron transfer in the electrodes and ion diffusion in the solution and the carbon surface pores.10,17-24 Furthermore, the capacitance decay rates are also different as shown in Figure 5b. The ACC/CNT-1 EDLC has the lowest decay rate while the bare ACC EDLC has the fastest decay rate. In the capacitors having CNT-grafted ACC electrodes, the CNTs provide improved electrical conductivity and therefore better pathways for faster electron transfer at high scan rates. In addition, the outstanding grafted-CNTs on the ACC surface represent more favorite sites than the carbon fibers for the adsorption of electrolyte ions12,49 and orientated adsorption of solvent molecules on the carbon surface at high scan rates. As a result, the capacitances of the CNT-grafted ACC EDLCs decay slower than that of the bare ACC EDLC. At the highest scan rate of 500 mV s-1, the relative capacitance reduces to only 15% for the bare ACC EDLC; while the relative capacitances are 45% and 55% for the ACC/CNT-0 and ACC/CNT-1 EDLCs, respectively. The ACC/CNT-0 EDLC exhibits capacitances that are lower than that of the ACC/CNT-1 EDLC by approximately 10% due to less CNTs available and the existence of pyrolytic carbon on the surface. Figure 6 a shows the cyclic voltammograms of ACC, ACC/ CNT-0, and ACC/CNT-1 capacitors at a potential scan rate of 10 mV s-1. The shapes of all the voltammograms appear to be quite rectangular, indicating ideal capacitors were obtained.2,46,47 The capacitances are 115, 103, and 107 F g-1 for bare ACC, ACC/CNT-0, and ACC/CNT-1 capacitors, respectively. Broadened peaks are seen in the CV curves of the bare ACC and ACC/CNT-1 capacitors obtained at 10 mV s-1 and indicate the occurrence of pseudocapacitance. The occurrence of pseudocapacitance is related to the surface oxygen functional groups that provide faradic currents during an electrochemical process. Specific capacitance was also obtained from dividing the specific current by the scan rate. The results obtained at a high potential scan rate of 200 mV s-1 are shown in Figure 6b. In general, a high scan rate represents a high charge-discharge process. At a scan rate of 200 mV s-1,

Figure 5. (a) Specific and (b) normalized capacitances of various EDLCs.

(D0 peaks) at 1607.6 and 1609.2 cm-1, respectively, are seen. The D0 peak is contributed by a multiwalled feature of the CNTs.30,44,45 The use of a symmetric two-electrode system not only is for testing cyclic performance but also reflects the fact that it represents a real capacitor device.46 Figure 5a shows the specific capacitances of EDLCs made out of the above three types of carbons with different scan rates. It is seen that the bare ACC EDLC exhibits the highest specific capacitance; while the ACC/CNT-0 EDLC exhibits the lowest specific capacitance at the scan rates lower than 50 mV s-1. However, the variations are within 10%. For example, at the lowest scan rate of 5 mVs-1, the capacitances of the bare ACC, ACC/ CNT-0, and ACC/CNT-1 are 119, 107, and 111 F g-1 , respectively. As mentioned above, the performance of ACC EDLCs at low scan rates is quite respectable due to both their high surface areas and the occurrence of pseudocapacitance. Although the growth of CNTs does not change the surface areas, some of surface functionalities were removed during the growth at 800 °C. Also, the removal is more severe when there was no NH3 addition.47,48 The removal of the surface functionalities results in reduced capacitances, as shown in Figure 5a. This is attributed to the disappearance of and reduced pseudocapacitance for the ACC/CNT-0 and the ACC/CNT-1 EDLCs, respectively, at the lower scan rates. This will be further discussed later using cyclic voltammograms and X-ray photoelectron spectroscopy analysis. On the other hand, at scan rates higher than 50 mV s-1, the ACC/ (45) Liu, L.; Qin, Y.; Guo, Z. X.; Zhu, D. Carbon 2003, 41, 331–335. (46) Frackowiak, E.; Beguin, F. Carbon 2001, 39, 937–950. (47) Kosmulski, M.; Pr ochniak, P.; Saneluta, C. Adsorption 2009, 15, 172–180. (48) Levie, R. De Electrochim. Acta 1963, 8, 751–780.

(49) Austin, L. G.; Gagnon, E. G. J. Electrochem. Soc. 1973, 120, 251– 254.

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and ACC/CNT-1 samples are presented in Figure 7. CdO centered at 531 eV, C-OH centered at 532 eV, and H2O centered at 535 eV50,51 were found on the surfaces of the bare ACC and ACC/-CNT-1 samples; while only C-OH and H2O were found on the surface of the ACC/CNT-0 sample. The concentrations of these surface functional groups are also given in Figure 7. The concentrations of CdO (quinone groups) in the bare ACC, ACC/CNT-0, and ACC/CNT-1 samples are 23.9%, 0%, and 10%, respectively. The ACC sample has an abundant CdO. This was reduced during the growth of ACC/CNT-0 samples by the hydrogen.52-54 However, in the presence of ammonia, the reduction is limited.55,56 Therefore there are 0% and 10% of CdO on the surfaces of the ACC/CNT-0 and ACC/CNT-1 specimens, respectively. It has been reported that the quinone group leads to a redox reaction that gives faradic currents, leading to enhanced capacitance.2,57,58 Therefore, pseudocapacitance occurs for the bare ACC and ACC/CNT-1 capacitors. Furthermore, the difference in the CdO concentration also explains the aforementioned observation that the bare ACC EDLC exhibits the highest specific capacitance; while the ACC/CNT-0 EDLC exhibits the lowest specific capacitance at the scan rates lower than 50 mV s-1. Alternating current impedance measurements were also performed, and the results are given in Figure 8. In a typical Nyquist plot of porous carbons, as the one shown in the insert in Figure 8, the extrapolation of the nearly vertical line intercepts the Re(Z) axis at value that represents the total resistance Rt, which equals Rs þ Rc þ Rp, where Rs is the electrolyte resistance, Rc the contact resistance between the electrode and the current collector, and Rp the equivalent distributed pore resistance.4 It is seen that the three types of capacitors show different resistances. The ACC/CNT-1 capacitor show no contact resistance; while the ACC/CNT-0 and bare ACC capacitor show different magnitudes of contact resistances. The total resistance decreases as CNTs present and the decrease are more significant for the ACC/CNT-1 sample, as shown in the table inserted in Figure 8. The decreases are primarily due to the decrease in the contact resistance Rc. The contact resistances are 0.96, 0.35, and 0 Ω for the ACC, ACC/CNT-0, and ACC/CNT-1 EDLCs, respectively. It is obvious that CNT grafting largely reduces such contact resistance. The three capacitors also have different Rp values. At higher frequency, the diffusion of the electrolyte ions into surface pores of ACC become limited or hindered, giving a high Rp. In the ACC/CNT-1 sample, this disadvantage is compensated as the CNTs provide external surfaces for double layers formation. As a result, the ACC/ CNT-1 capacitor appears to have a lower Rp than the bare ACC capacitor. On the other hand, there are pyrolytic carbons on the surface of the ACC/CNT-0 specimen. These

Figure 6. (a) Cyclic voltammograms obtained at a scan rate of 10 mV s-1 and (b) specific capacitances obtained at a scan rate of 200 mV s-1 vs potential plots of ACC, ACC/CNT-0, and ACC/ CNT-1 capacitors in a 2 M H2SO4 electrolyte.

the specific capacitance profile of the bare ACC capacitor shows serious distortion as compared to that of ACC/CNTbase capacitors. As mentioned above, a rectangular voltammogram indicates the domination of double-layer formation in the energy storage process. The distortion of voltammograms at high scan rates or charge-discharge processes is due to the resistance of ion migration in the micropores of ACC electrodes, since increasing the scan rate aggravates the delay of the current to reach a horizontal value after reversal of the potential scan.48,49 On the other hand, the ACC/CNT-base capacitors have higher specific capacitances at high scan rates and exhibit a more rectangular shape. The much less distortion indicates that the slow ion migration in the micropores no longer plays a role. Furthermore, the CNTs provide added surface areas as additional absorption sites for the creation of extra double layers. As a result, at high scan rates, the ACC/ CNT-based capacitors show not only higher capacitance but also much less distorted voltammograms. It is also noted from Figure 8 that the solution resistance of the three capacitor systems are very similar, indicating the similarity of the ion diffusion. Surface analysis using XPS was performed. Detailed O1s XPS survey spectra of bare ACC, ACC/CNT-0,

(51) Montes-Moran, M. A.; Suarez, D.; Menendez, J. A.; Fuente, E. Carbon 2004, 42, 1219–1225. (52) Menendez, J. A.; Xia, B.; Phillips, J.; Radovic, L. R. Langmuir 1997, 13, 3414–3421. (53) Oliveira, L. C. A.; Silva, C. N.; Yoshida, M. I.; Lago, R. M. Carbon 2004, 42, 2279–2284. (54) Huang, C. W.; Teng, H. J. Electrochem. Soc. 2008, 155, A739–A744. (55) Swiatkowski, A.; Pakula, M.; Biniak, S.; Walczyk, M. Carbon 2004, 42, 3057–3069. (56) Pakula, M.; Biniak, S.; Swiatkowski, A. Adsorpt. Sci. Technol. 2002, 20, 583–594. (57) Andreas, H. A.; Conway, B. E. Electrochimi. Acta 2006, 51, 6510–6520. (58) Nian, Y. R.; Teng, H. J. Electrochem. Soc. 2002, 149, A1008– A1014.

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Figure 7. O1s XPS spectra of (a) bare ACC, (b) ACC/CNT-0, and (c) ACC/CNT-1 specimens.

CNTs may also increase the overlapping potential of the carbon surface and thus enhance the affinity of the electrode surface to the electrolyte solution.10,54 Therefore, the ACC/ CNT-1 capacitor exhibits the lowest Rs. The ACC/CNT-0 capacitor has CNTs, but there are also pyrolytic carbons on the ACC surfaces. As mentioned above, ACC/CNT-0 exhibits the highest ID/IG ratio. This indicates that the pyrolytic carbons have more aromatic groups on the surface.42 The aromatic groups could be naphthalene and/or its derivatives which often occur due to incomplete pyrolysis of CH4.59,60 The existence of aromatic groups gives the electrode poor affinity of electrolyte ions and therefore the highest Rs. 4. Conclusions Conventional ACC-based electrodes used in EDLCs suffer from their low electrical conductivity. Attempts have been made to grow CNT/CNF on the surfaces of carbon fibers to improve the electrical properties. However, common to these studies is that the surface areas of the resulting CNT/CNFgrafted carbons were decreased. In this study, we have shown that the use of sputter deposited catalyst for the growth CNTs prevents observed surface pore blockage on the ACC. The CNT growth was found to follow the tip growth mode, thus

Figure 8. Nyquist plots of the EDLCs having ACC and CNTgrafted electrodes.

pyrolytic carbons exhibit more pores due to their low densities, leading to additional pore resistance and therefore the highest Rp. Furthermore, the values of Rs are also different, although the differences are less than that among the Rp values. The CNT grafting has led to a more intimate contact of the electrolyte solution with carbon fiber to facilitate the transport of ions and solvent molecules. The presence of

(59) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. Rev. 2006, 106, 1105–1136. (60) Zhang, J.; Huang, Z. H.; Lv, R.; Yang, Q. H.; Kang, F. Langmuir 2009, 25, 269–274.

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creating a direct contact and therefore a low contact resistance between the CNTs and the ACC fiber surfaces. This, coupled with the high electrical conductivity of CNTs and no surface pore blockage on the ACCs, results in CNT-grafted ACC electrodes exhibiting significantly enhanced electrical conductivity. Electrochemical analysis also shows that the CNT-grafted ACC electrode exhibit improved capacitance retention, which is important for operation at high rate charge-discharge.

Acknowledgment. This work was supported by the National Science Council in Taiwan under Grant No. NSC 97-2120-M006-005.

Note Added after ASAP Publication. Units were corrected in the paragraphs after Figure 5 and before Figure 6 in the version of this paper published November 12, 2010. The correct version published November 18, 2010.

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