Supergrowth of Aligned Carbon Nanotubes Directly on Carbon Papers

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J. Phys. Chem. C 2010, 114, 15223–15227

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Supergrowth of Aligned Carbon Nanotubes Directly on Carbon Papers and Their Properties as Supercapacitors Byungwoo Kim, Haegeun Chung, and Woong Kim* Department of Materials Science and Engineering, Korea UniVersity, Seoul 136-713, Republic of Korea ReceiVed: June 15, 2010; ReVised Manuscript ReceiVed: July 31, 2010

We demonstrate that vertically aligned carbon nanotubes can be synthesized directly on conductive carbon papers and used as excellent electrochemical capacitors. The carbon nanotubes were synthesized with use of an Al/Fe catalyst via water-assisted chemical vapor deposition. They grew as fast as ∼100 µm/ min and approximately 70% of them had double walls with an average diameter of ∼6 nm. Interestingly, the carbon nanotube forest showed microscale patterns defined by the structure of underlying carbon papers. The nanotubes were attached well to the carbon papers and maintained their adhesion under mild ultrasonication in solution. Owing to the direct integration, naturally patterned structure, and good alignment, the carbon nanotubes showed excellent performance as supercapacitors. In aqueous 1 M H2SO4 solution, specific capacitance, energy, and power measured at the current density of 20 A/g were ca. 200 F/g, 20 Wh/kg, and 40 kW/kg, respectively. A specific energy of >100 Wh/kg was achieved when organic electrolyte was used. Demonstrated facile and direct integration of carbon nanotubes on conductive substrates and their excellent electrochemical properties may hold great promise for electrochemical energy storage applications. Introduction Electrochemical capacitors or supercapacitors have been recognized as one of the promising energy storage devices.1-5 Owing to their high power density, the supercapacitors may complement or replace batteries especially in applications that require fast and high power energy storage capability such as portable or remote devices and hybrid electric vehicles. Moreover, the supercapacitors are generally made of inexpensive and environmentally friendly materials and have long-term operation stability. Carbon nanotubes (CNTs) have been explored as candidate electrode materials which may overcome the drawbacks of conventional materials.6-9 Traditionally, porous activated carbons with high specific surface area have been used as electrode materials. However, irregular pore structures of the activated carbons can limit fast ion diffusion.9,10 Moreover, activated carbons require the use of a conductive binding agent that can cause high contact resistance between the activated carbons and current collectors.11 On the other hand, CNTs have relatively regular pore structures in addition to excellent material properties such as high conductivity, large specific surface area, and chemical stability. Also, the high aspect ratio of the CNTs provides continuous conductive paths to efficiently distribute charge to current collectors. Therefore, fabrication of a binder-free CNT electrode is important in enhancing the performance of CNT-based supercapacitors. Along this line, direct growth of CNTs on conductive substrates has been explored.11-14 However, in most cases, an insulating layer such as Al2O3 or SiO2, or a polymer layer had to be used on the conductive substrates to ensure successful growth of high-quality CNTs.12,14,15 Reports on the synthesis of CNTs directly on conductive substrates without using an insulating layer are very rare. Only recently the direct growth * To whom correspondence should be addressed. Tel: +82 2 3290 3266. Fax: +82 2 928 3584. E-mail: [email protected].

of CNTs on conductive substrates has been demonstrated by using ferrocene and xylene vapor as catalyst precursor and carbon feedstock via a chemical vapor deposition (CVD). However, this process is complicated and requires a separated heating chamber to vaporize the ferrocene/xyelene solution.11,13 In this work, we demonstrate a facile synthesis of vertically aligned CNTs on conductive carbon papers via water-assisted CVD without using an insulating layer, and their excellent electrochemical characteristics as supercapacitors. The synthesized CNTs were mostly double walled (>70%) with an average diameter of 5.7 ( 1.8 nm. Microscale patterns were observed in the CNT forest and this was attributed to the structure of underlying carbon paper. The CNTs were attached well to the carbon paper substrates leading to low contact resistance. Excellent supercapacitor properties were obtained by using the directly integrated, naturally patterned, and vertically aligned CNTs. In aqueous 1 M H2SO4 solution, specific capacitance, energy, and power measured at the current density of 20 A/g were ca. 200 F/g, 20 Wh/kg, and 40 kW/kg, respectively. The rectangular shape of the cyclic voltamogram was maintained even at a scan rate as high as ∼1000 mV/s. When organic electrolyte was used, the specific energy was increased by an order of magnitude. Demonstrated facile integration of the CNTs with conductive substrates could be an important cornerstone for the development of diverse electrochemical energy storage devices. Experimental Section Ten nanometers of Al and 1 nm of Fe were e-beam evaporated on carbon papers (Toray TGP-H-090) and used as a catalyst for CNT growth. The carbon paper has a thickness of 280 µm and an in-plane resistivity of 5.8 × 10-3 Ω · cm. The Al/Fe coated carbon paper was placed in the middle of a 1 in. diameter quartz tube in a furnace (Lindberg, TF55030A) for CVD. The substrate was heated to 800 °C in the flow of Ar

10.1021/jp105498d  2010 American Chemical Society Published on Web 08/18/2010

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(99.999%), hydrogen (99.999%), and water vapor at the flow rate of 125, 100, and 0.75 sccm, respectively. To introduce water vapor, Ar gas was passed through a bubbler containing deionized water. Once the temperature reached 800 °C, C2H4 (99.95%) was introduced as a carbon feedstock at 50 sccm. After 1-10 min of reaction, the furnace was cooled to room temperature in a stream of Ar. Morphologies of the CNTs were characterized with scanning electron microscopy (SEM) (Hitachi S-4700 and FEI Nova nano200) and transmission electron microscopy (TEM) (Philips Tecnai 20). The quality of the CNTs was evaluated with Raman spectroscopy (Horiba, HR 800). Electrochemical measurements were performed in a three-electrode system with an electrochemistry analyzer (Ivium technologies, CompactStat). In aqueous solution, a CNT-grown carbon paper, platinum gauze (99.9%), and Ag/AgCl electrode (CH Instruments, CHI111, 3 M KCl) were used as a working, counter, and reference electrode, respectively. In organic solution, silver wire was used instead of the Ag/AgCl electrode. To evaluate supercapacitor characteristics, cyclic voltammetry (CV) and chronopotentiometry were carried out either in 1 M H2SO4 in deionized water or 1 M tetraethylammonium tetrafluoroborate (TEABF4) in propylene carbonate (PC). Electrochemical oxidation of the CNTs was carried out in 0.25 M HNO3 solution in the potential range of 1.0 to 2.0 V at a sweep rate of 50 mV/s for 10 cycles.16 Once the CNT electrodes were soaked in electrolyte solution, they were never dried during the electrochemical characterization process to prevent any deformation of the CNT structure by capillary force.17 For adhesion test, CNTs grown on carbon papers and SiO2 were sonicated by using a probe-tip sonicator (Sonics, VCX 130) in methyl ethyl ketone for 90 s at a power of 52 W.

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Figure 1. Scanning electron microscopy (SEM) images of tilted views of (a) a carbon paper (CP) and (b) carbon nanotubes (CNTs) synthesized on the CP (scale bars ) 500 µm). (c) SEM images of a side view of the CNTs on a CP (scale bar ) 300 µm) and (d) the CNTs grown on both sides of a CP (scale bar ) 1 mm). (e) Zoom-in SEM image of aligned CNTs (scale bar ) 2 µm). (f) A transmission electron microscopy (TEM) image of the CNTs (scale bar ) 40 nm) (inset shows double walled CNTs, scale bar ) 5 nm). (g) Raman spectra of asgrown and electrochemically oxidized CNTs.

Results and Discussion The double walled CNTs were synthesized directly on a carbon paper via water-assisted CVD without using an insulating layer. A carbon paper is a conductive sheet of carbon fibers (d ≈ 7 µm) (Figure 1a). Once the CNTs were grown on the carbon papers as described in the Experimental Section, interestingly, a microscale pattern was observed on the CNT film over the entire area of the substrate (Figure 1b). We suggest that this is because the CNT growth was guided by the structures of the underlying carbon papers. Such microscale patterns can increase the accessibility of electrolyte ion to the nanoscale pores of CNT electrodes, and thus improve electrochemical capacitor properties.18 Growth rate was as high as ∼100 µm/min. Figure 1c shows an SEM image of the side view of approximately 500µm-long CNTs that were grown for 5 min. These CNTs were vertically aligned to the substrates as shown in Figure 1c-e. Electrolyte ion diffusion in supercapacitor electrodes can be facilitated by the alignment of CNTs.19 Figure 1d shows that the CNTs can also be grown on both sides of the carbon paper by depositing Al/Fe catalyst layers on both sides. The average diameter of the CNTs was ∼6 nm and approximately 70% of them had double walls as shown in the TEM images (Figure 1f). The Raman spectrum shows that the G-band to D-band intensity ratio (IG/ID) of the as-grown CNTs is about ∼1.7, indicating decent quality of the CNTs. This ratio decreases to 0.7 after an electrochemical oxidation process used to improve supercapacitor properties (Experimental Section). The CNTs were relatively well adhered to the carbon paper substrates, and this ensures reliable binder-free integration. We compared their adhesion properties with those of CNTs grown on SiO2 substrates via the same CVD process by simply using

Figure 2. Photographs of (a) CNTs grown on a CP and (b) CNTs grown on a SiO2 substrate immediately after being exposed to mild sonication (52 W).

tweezers. When CNTs attached to the substrates were lifted with the tweezers, the carbon papers remained attached to the CNTs while the SiO2 substrates were separated from the CNTs. Better adhesion of the CNTs to the carbon papers was also confirmed by sonication test. Most of the CNTs on the carbon papers remained attached when sonicated for 90 s at the power of 52 W (Figure 2a), while the CNTs on the SiO2 substrates detached under the same condition (Figure 2b). The photos were taken immediately after the sonicator bar was removed from the vials. We speculate that good adhesion properties might be due to the formation of aluminum carbide (Al4C3) at the interfaces of Al and CNTs and of Al and carbon papers. It has been observed that the thin Al4C3 layer can be formed as a result of the reaction between Al and carbon-based materials at the CNT growth temperature (∼800 °C).20 Strong adhesion of CNTs grown with Al/Fe catalyst to other carbon-based materials was observed by other research groups as well.21 The result indicates that reliable binder-free integration between the CNTs and the carbon papers can be achieved by the direct synthesis.

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Figure 3. Electrochemical properties measured in 1 M H2SO4 aqueous solution. Cyclic voltammograms of (a) as-grown CNTs and (b) electrochemically oxidized CNTs measured at the scan rate of 100 and 1000 mV/s (insets: Nyquist plots). (c) Galvanostatic charge/discharge curves of the as-grown and electrochemically oxidized CNTs measured at the current density of 10 A/g. (d) Specific capacitance vs current density plot.

Owing to the pattern, alignment, and direct integration of the CNTs, excellent electrochemical capacitor properties were observed. Figure 3a shows CV curves measured with an asgrown CNT electrode as a working electrode in 1 M H2SO4 aqueous solution. Specific capacitance of the as-grown CNT supercapacitor was estimated to be ∼50 F/g, and the rectangular cyclic voltammogram shape was obtained at the scan rate of 100 mV/s. The mass of the CNTs of the working electrode was ∼0.24 mg. The shape of the CV plot was maintained with only a slight distortion even when the scan rate was increased to 1000 mV/s. This rapid current response to the change of voltage sweep direction can be explained by fast ion diffusion and low equivalent series resistance (ESR).22 Fast ion diffusion was facilitated by the patterned and aligned structure of the CNTs and low ESR was achieved by the direct integration of CNTs.19 The ESR was below 5 Ω as revealed by electrochemical impedance spectroscopy as shown in the inset of Figure 3a,b. However, further reduction of the ESR is still necessary for better performance. The performance of the CNT supercapacitors can be improved by electrochemically oxidizing the CNTs (Figure 3b).16 This treatment generates oxygenated groups on the CNTs which lead to pseudocapacitance by the Faradaic process. The redox reaction can be explained by the following equation to which the redox peaks in the CV plots can be ascribed;23

>CsOH S >CdO + H+ + eMoreover, tips of the CNTs can be opened by this oxidation making the inner surface of the CNTs available for ion adsorption.16 Owing to the pseudocapacitance and the increased specific surface area, the specific capacitance was increased by a factor of 4-5 to ∼200 F/g. Main CV characteristics such as rectangular shape, fast current response, and low ESR were maintained after the electrochemical oxidation as shown in Figure 3b.

The improvement of the supercapacitor properties is confirmed by the galvanostatic charge/discharge measurement (Figure 3c,d). The specific capacitance can be calculated from the chronopotentiogram based on the equation Csp ) I/(dV/ dt) · m, where I is the applied current, dV/dt is the slope of discharge curve after IR drop, and m is the mass of CNTs. Figure 3d shows the Csp values measured in a range of current density up to ∼150 A/g. Unlike activated carbon where severe degradation in Csp is usually observed at a high current density,17,19 our aligned CNTs maintained high capacitance (∼180 F/g) even at 150 A/g. As-grown CNTs show almost the same specific capacitance regardless of the current density we applied. Specific capacitance of oxidized CNTs show an initial drop from 250 F/g to 200 F/g at a current density below 20 A/g, and the value remained almost the same even when the current density was increased to 150 A/g. Retention of Csp at such a high current density is due to the rapid ion diffusion, which was facilitated by patterned structure and good alignment of the CNTs as well as low ESR.10,14 Specific energy and specific power can be calculated from the galvanostatic charge/discharge curves by using the following equations: Esp ) CspV2/2 and Psp ) Esp/t, respectively. At the current density of 20 A/g, Esp and Psp were 20 Wh/kg and 40 kW/kg, respectively. The specific energy of the supercapacitor can be increased by using organic electrolyte because it can be operated over a wider voltage range without electrolysis of the solvent.2 Figure 4a shows CV curves measured in the range of -1.25 to 1.25 V at 100 mV/s. The CNT mass was 0.57 mg. The capacitance was also increased by electrochemical oxidation of the CNTs in organic electrolyte by a factor of 2-3 (Figure 4a,b). However, rate performance of the CNT supercapacitors was degraded in organic solution due to increased ESR. When the scan rate was further increased, the rectangular shape of the cyclic voltammogram was significantly distorted. Specific capacitance also decreased as the current density increased (Figure 4c). This was because ESR was increased to 20-30

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Figure 4. Electrochemical properties measured in 1 M tetraethylammoniium tetrafluoroborate (TEABF4) propylene carbonate (PC) solution. (a) Cyclic voltammetry (CV) curves of as-grown and electrochemically oxidized CNTs measured at the scan rate of 100 mV/s (inset: Nyquist plot for as-grown CNTs). (b) Galvanostatic charge/discharge curves of the as-grown and electrochemically oxidized CNTs measured at the current density of 10 A/g. (c) Specific capacitance vs current density plot. (d) Ragone plots of as-grown and oxidized CNTs in aqueous and organic solution.

Ω in organic solution as revealed by impedance spectroscopy (inset in Figure 4a). Consistently, increased IR drop was observed in galvanostatic charge/discharge curves (Figure 4b). We attribute this ESR increase to the low mobility of organic electrolyte ions. Specific energies and powers obtained from the as-grown and electrochemically oxidized CNTs measured both in aqueous and organic solution were presented in a Ragone plot (Figure 4d). The plot clearly shows that specific energy was increased by electrochemical oxidation in both aqueous and organic solutions. Specific energy of CNTs in organic solution was higher compared to those in aqueous solution due to the increased operation voltage range. However, increased ESR in organic solution limited the power performance of the supercapacitors. Therefore, performance of the CNT-based supercapacitors could be further improved by reducing the ESR to a lower level. Conclusions We have demonstrated a facile synthesis of vertically aligned double walled CNTs directly on carbon papers by water-assisted CVD, and characterized their properties as supercapacitors. The CNTs were naturally patterned following the network structures of carbon fibers composing the carbon papers. The CNTs were attached well to the carbon papers. This direct integration led to an ESR as low as a few ohms. Owing to the vertical alignment, pattern structure, and low ESR, excellent supercapacitor properties were obtained. In aqueous solution, rectangular cyclic voltammogram shapes were obtained even at a scan rate as fast as ∼1000 mV/s. High specific capacitance (∼180 F/g) was maintained even at a current density of ∼150 A/g. The specific capacitance, energy, and power of the oxidized CNTs at a current density of 20 A/g were ca. 200 F/g, 20 Wh/kg, and 40 kW/kg,

respectively. Specific energy of >100 Wh/kg was achieved when the supercapacitor was operated in organic solution. Direct integration of the CNTs on the conductive substrates we have demonstrated may serve as an important basis for further development of supercapacitors as well as fuel cells, batteries, field emitters, and biosensors. Acknowledgment. This work was supported by the Korea Research Council of Fundamental Science & Technology (KRCF) and Korea Institute of Science & Technology (KIST) for the “National Agenda Project” program, and by the Korea Research Foundation (KRF-2008-331-D00254). References and Notes (1) Burke, A. Electrochim. Acta 2007, 53, 1083. (2) Kotz, R.; Carlen, M. Electrochim. Acta 2000, 45, 2483. (3) Pandolfo, A. G.; Hollenkamp, A. F. J. Power Sources 2006, 157, 11. (4) Winter, M.; Brodd, R. J. Chem. ReV. 2004, 104, 4245. (5) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845. (6) An, K. H.; Kim, W. S.; Park, Y. S.; Choi, Y. C.; Lee, S. M.; Chung, D. C.; Bae, D. J.; Lim, S. C.; Lee, Y. H. AdV. Mater. 2001, 13, 497. (7) Frackowiak, E.; Metenier, K.; Bertagna, V.; Beguin, F. Appl. Phys. Lett. 2000, 77, 2421. (8) Liu, C. Y.; Bard, A. J.; Wudl, F.; Weitz, I.; Heath, J. R. Electrochem. Solid-State Lett. 1999, 2, 577. (9) Pan, H.; Li, J. Y.; Feng, Y. P. Nanoscale Res. Lett. 2010, 5, 654. (10) Korenblit, Y.; Rose, M.; Kockrick, E.; Borchardt, L.; Kvit, A.; Kaskel, S.; Yushin, G. ACS Nano 2010, 4, 1337. (11) Shah, R.; Zhang, X. F.; Talapatra, S. Nanotechnology 2009, 20, 395202. (12) Hiraoka, T.; Yamada, T.; Hata, K.; Futaba, D. N.; Kurachi, H.; Uemura, S.; Yumura, M.; Iijima, S. J. Am. Chem. Soc. 2006, 128, 13338. (13) Talapatra, S.; Kar, S.; Pal, S. K.; Vajtai, R.; Ci, L.; Victor, P.; Shaijumon, M. M.; Kaur, S.; Nalamasu, O.; Ajayan, P. M. Nat. Nanotechnol. 2006, 1, 112. (14) Zhang, H.; Cao, G. P.; Wang, Z. Y.; Yang, Y. S.; Gu, Z. N. Carbon 2008, 46, 822. (15) Kim, H. S.; Kim, B.; Lee, B.; Chung, H.; Lee, C. J.; Yoon, H. G.; Kim, W. J. Phys. Chem. C 2009, 113, 17983.

Supergrowth of Aligned Carbon Nanotubes (16) Ye, H. S.; Liu, X.; Cui, H. F.; Zhang, W. D.; Sheu, F. S.; Lim, T. M. Electrochem. Commun. 2005, 7, 249. (17) Futaba, D. N.; Hata, K.; Yamada, T.; Hiraoka, T.; Hayamizu, Y.; Kakudate, Y.; Tanaike, O.; Hatori, H.; Yumura, M.; Iijima, S. Nat. Mater. 2006, 5, 987. (18) Das, R. N.; Liu, B.; Reynolds, J. R.; Rinzler, A. G. Nano Lett. 2009, 9, 677. (19) Zhang, H.; Cao, G. P.; Yang, Y. S.; Gu, Z. N. J. Electrochem. Soc. 2008, 155, K19.

J. Phys. Chem. C, Vol. 114, No. 35, 2010 15227 (20) He, C. N.; Zhao, N. Q.; Shi, C. S.; Song, S. Z. Carbon 2010, 48, 931. (21) Liu, X.; Baronian, K. H. R.; Downard, A. J. Carbon 2009, 47, 500. (22) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications; Kluwer Academic/Plenum Publishers: New York, 1999. (23) Andreas, H. A.; Conway, B. E. Electrochim. Acta 2006, 51, 6510.

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