Energy Fuels 2010, 24, 3373–3377 Published on Web 04/14/2010
: DOI:10.1021/ef9015203
)
Hole Opening of Carbon Nanotubes and Their Capacitor Performance† )
Yasuhiro Yamada,‡,§ Osamu Kimizuka, Kenji Machida,‡ Shunzo Suematsu,‡ Kenji Tamamitsu,‡ Susumu Saeki, Yoshio Yamada,^ Noriko Yoshizawa,^ Osamu Tanaike,^ Junya Yamashita,^ Futaba Don,^ Kenji Hata,^ and Hiroaki Hatori*,^ Nippon Chemi-Con Corporation, 363 Arakawa, Takahagi 318-8505, Japan, §Graduate School of Engineering, Chiba University, Chiba 262-8522, Japan, Faculty of Engineering, University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan, and ^National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8561, Japan )
‡
Received December 12, 2009. Revised Manuscript Received March 29, 2010
Pore structures and electrochemical performances of mass-produced metal-free single-walled carbon nanotubes (mf-SWCNTs) heated in either CO2 gas or dry air were investigated. Pore structures, defects, and electrochemical performances of mf-SWCNTs were analyzed mainly using the N2 adsorption method, apparent density measurement, Raman spectroscopy, and cyclic voltammogram. mf-SWCNT sheets heated in CO2 gas showed a remarkable increase of the specific surface area up to 1900 m2 g-1, but sheets heated in air showed an increase of only up to 1400 m2 g-1. Propan-2-ol, 4-methyl-1,3-dioxolan-2-one, or ionic liquid was impregnated in the inner space of mf-SWCNTs to confirm whether the inner space of mf-SWCNTs is filled with each liquid. mf-SWCNTs heated in air were completely filled, but mf-SWCNTs heated in CO2 were partially filled. From these results, it is estimated that holes of the size of 0.56 nm or larger are opened on the side wall of mf-SWCNTs. A combination of two heat treatments maximized the effective specific capacitance of heated mf-SWCNTs up to 100 F g-1, which is 1.6 times higher than that of as-grown mf-SWCNTs because of the removal of the carboneous component and the creation of defects on the sidewall of mf-SWCNTs, which allow the electrolyte into the inner space of mf-SWCNTs.
Opening the end caps of CNTs and filling the inner space of CNTs has been studied computationally6-8 and experimentally.9 Methods for opening the holes on both single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) by heating in oxygen,10,11 carbon dioxide,12,13 KOH,14-16 and NaOH16 and by treating in acids17 have been reported. These different treatments result in creating defects, such as functional groups and holes. Although CNTs opened by many methods have been reported, the satisfactory qualitative analysis of the amount of liquid filled in CNTs especially with low metal content, such as supergrowth carbon nanotubes,18 has not been reported. Counting the number of opened holes and measuring the size of opened holes by transmission electron microscopy (TEM) is an obscure evaluation method unless a large number of CNTs is fairly counted. On the other hand, measurement of apparent
1. Introduction Carbon nanotubes (CNTs) have been widely studied for more than a decade because of the potential applications of CNTs, such as electrochemical double-layer capacitors (EDLCs),1-3 catalyst supports,4 and nano circuits.5 Especially, the EDLC is one of the most promising applications. Because the mass production of CNTs has been started, the speed of research has been even more accelerated. However, the mechanism of the CNTs as an EDLC electrode is still unclear. The surface area calculated from nitrogen adsorption is the only effective evaluation method if all of the electrolyte is impregnated in the inner space of CNTs. However, the complete opening and complete filling of CNTs were not confirmed prior to the electrochemical measurement for most cases. † This paper has been designated for the special section Carbon for Energy Storage and Environment Protection. *To whom correspondence should be addressed. Fax: þ81-29-8618408. E-mail:
[email protected]. (1) Futaba, D. N.; Tanaike, K.; Yamada, T.; Hiraoka, T.; Hayamizu, Y.; Kakudate, Y.; Tanaike, O.; Hatori, H.; Yumura, M.; Iijima, S. Nat. Mater. 2006, 5, 987–994. (2) Kimizuka, O.; Tanaike, O.; Yamashita, J.; Hiraoka, T.; Futaba, D. N.; Hata, K.; Machida, K.; Suematsu, S.; Tamamitsu, K.; Saeki, S.; Yamada, Y.; Hatori, H. Carbon 2008, 46, 1999–2001. (3) Yamada, Y.; Kimizuka, O.; Tanaike, O.; Machida, K.; Suematsu, S.; Tamamitsu, K.; Saeki, S.; Yamada, Y.; Hatori, H. Electrochem. Solid-State Lett. 2009, 12 (3), K14–K16. (4) Planeix, J. M.; Coustel, N.; Coq, B.; Brotons, V.; Kumbhar, P. S.; Dutartre, R.; Geneste, P.; Bernier, P.; Ajayan, P. M. J. Am. Chem. Soc. 1994, 116, 7935–7936. (5) Lu, W.; Lieber, C. M. Nat. Mater. 2007, 6 (11), 841–850. (6) Clare, B. W.; Kepert, D. L. Inorg. Chim. Acta 2003, 343, 1–17. (7) Mazzoni, M. S.; Chacham, H.; Ordej on, P.; Sanchez-Portal, D.; Soler, J. M.; Artacho, E. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 60 (4), R2208–R2211.
r 2010 American Chemical Society
(8) Mann, D. J.; Hase, W. L. Phys. Chem. Chem. Phys. 2001, 3, 4376– 4383. (9) Kitaura, R.; Imazu, N.; Kobayashi, K.; Shinohara, H. Nano Lett. 2008, 8 (2), 693–699. (10) Ajayan, P. M.; Ebbesen, T. W.; Ichihashi, T.; Iijima, S.; Tanigaki, K.; Hiura, H. Nature 1993, 362, 522–525. (11) Fan, J.; Yudasaka, M.; Miyawaki, J.; Ajima, K.; Murata, K.; Iijima, S. J. Phys. Chem. B 2006, 110 (4), 1587–1591. (12) Tsang, S. C.; Harris, P. J. F.; Green, M. L. H. Nature 1993, 362, 520–522. (13) Zhang, X.; Cao, A.; Sun, Q.; Xu, C.; Wu, D. Mater. Trans. 2002, 43 (7), 1707–1710. (14) Lee, S. M.; Lee, S. C.; Jung, J. H.; Kim, H. J. Chem. Phys. Lett. 2005, 416251–416255. (15) Niu, J. J.; Wang, J. N. Solid State Sci. 2008, 10, 1189–1193. (16) Pinero, E. R.; Azais, P.; Cacciaguerra, T.; Amoros, D. C.; Solano, A. L.; Beguin, F. Carbon 2005, 43, 786–795. (17) Reyhani, A.; Mortazavi, S. Z.; Golikand, A. N.; Moshfegh, A. Z.; Mirershadi, S. J. Power Sources 2008, 183, 539–543. (18) Hata, K.; Don, F.; Mizuno, K.; Namai, T; Yumura, M.; Iijima, S. Science 2004, 306, 1362–1365.
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: DOI:10.1021/ef9015203
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density is simple and suitable for measuring various types of liquids, such as 4-methyl-1,3-dioxolan-2-one [propylene carbonate (PC)], which are commonly used in the electrolyte of EDLC. In this work, metal-free single-walled carbon nanotubes (mf-SWCNTs) were opened by heat treatment in carbon dioxide and/or dry air. Complete opening and complete filling of heat-treated mf-SWCNTs were confirmed by the apparent density and nitrogen adsorption method, and then measurement of electrochemical performance had been conducted. From the results of the apparent density measurement, nitrogen adsorption, and electrochemical measurement, this work discusses the size of the opened holes and the reason why the capacitance was increased. Figure 1. Nitrogen adsorption/desorption isotherms of mf-SWCNTs before and after heat treatment. (9 and 0) Heated up to 36% weight loss, (2 and 4) heated up to 12% weight loss, and (b and O) as-grown (9, 2, and b, adsorption; 0, 4 and O, desorption).
2. Experimental Section SWCNTs were produced by the supergrowth method. The metal content of mf-SWCNTs was determined to be 81 ppm by inductively coupled plasma (ICP) analysis of ash, which is the residue obtained after oxidation treatment of the sample in air at 1173 K. The mf-CNT sheets were prepared by homogenizing asgrown mf-SWCNT powder in propan-2-ol [isopropyl alcohol (IPA)] for 2 min, filtering, and drying in vacuum. The sheets were heated in dry air, carbon dioxide, or nitrogen gas at the temperature between 723 and 1223 K with a rate of 10 K min-1 in a furnace and held at this temperature up to 40 h in dry air, 4 h in carbon dioxide gas, or 1 h in nitrogen gas. The pore structure was evaluated from the nitrogen adsorption measurement and analyzed by Brunauer-Emmett-Teller (BET) and Dollimore-Heal (DH) methods. The adsorption/desorption isotherms at 77 K were obtained using an automatic adsorption apparatus of BELSORP 28SA of Bell Japan, Inc. The apparent density of carbon materials including mfSWCNTs in various liquids was measured by analyzing the degree of opening of the pore and the degree of filling into the pore of carbon materials. Carbon materials used for measuring the apparent density are mf-SWCNT sheets, an activated carbon powder sheet (YP17), and a highly ordered pyrolytic graphite (HOPG) produced by NT-MDT, Moscow, Russia. Activated carbon powder was in the form of a sheet composed of 90 wt % activated carbon powder (YP17) provided by Kuraray Co., Ltd. and 10 wt % poly(tetrafluoroethylene) (7A-J) as a binder provided by DuPont-Mitsubishi Fluorochemicals Co., Ltd. These carbon materials were dried at 423 K for 12 h and impregnated in IPA, PC, or 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI) at 50 kPa and 303 K for 6 h, at 25 kPa and 333 K for 6 h, and at lower than 1 kPa and 423 K for 16 h, respectively. Apparent densities of carbon materials were measured at 298 K by an immersion method using a density accessory kit (Mettler Toledo, 00238490). Electrochemical properties of mf-SWCNTs were analyzed by a cyclic voltammogram (CV) and galvanostatic cycling with potential limitation using a VMP2 potentiostat/galvanostat (Bio-Logic). CVs were measured in 1 mold m-3 (M) tetraethylammonium tetrafluoroborate (TEABF4) dissolved in PC or EMI versus the Ag/Agþ/PC reference by three electrode cells. The galvanostatic charge/discharge was monitored at a current density of 0.5 A g-1 and at the same potential range as that of the CV. The specific capacitance was calculated from the slope of the potential-time curve. The degree of presence of defects on mf-SWCNTs was analyzed by Raman spectroscopy (T-64000, Jobin Yvon) with a 514.5 nm line of an argon ion laser at 400 mW output.
sheets heated in carbon dioxide are shown in Figure 1, together with a curve of the mf-SWCNT sheets before heat treatment as a reference. Three sets of curves in Figure 1 indicate two common features, such as a drastic rise of adsorption at near 0 of relative pressure and a little hysteresis behavior appearing between adsorption/desorption loops. The shape of the isotherm showed only a slight change after the heat treatment. Table 1 shows the BET specific surface area of the mfSWCNT sheets before and after heat treatment. The surface area of mf-SWCNT sheets without any heat treatment showed 650 m2 g-1, which is lower than that of CNTs reported by Hiraoka et al. (1358 m2 g-1).19 This is because as-grown mfSWCNTs used in this study contain some volatile and nonvolatile carbonaceous impurities. This surface area was increased up to 1400 m2 g-1 by heat treatment in dry air and 1900 m2 g-1 by heat treatment in CO2, as shown in Table 1 and Figure 2. The surface area increased remarkably as the time of heat treatment in CO2 or air increased (Figure 2). This figure indicates the degree of opened holes, whose size is large enough for nitrogen to access inside the inner space of CNTs. Figure 3 represents the pore size distribution curves analyzed by the DH method for the mf-SWCNTs before and after heat treatment in CO2. The pores with the diameter of ca. 24 nm were increased significantly with the increase of the time of heat treatment. Similar curves indicating porosity around 2-4 nm were obtained by heating mf-SWCNTs in air. Taking into account the fact that the diameter of mfSWCNTs was mainly about 2-4 nm, which was observed under TEM, it is reasonable to consider that the adsorption of nitrogen gas to the inner wall of tubes contributes to such an abrupt increase in the surface area at the beginning of heat treatment (Figure 2). Once the surface area reached the maximum value, which is 1900 m2 g-1 at the weight loss of 36% in the case of CO2 and 1500 m2 g-1 at the weight loss of 10% in the case of air, it saturated as the weight loss was increased. This result suggests that further heat treatment in CO2 induces the consumption of the active edged carbons rather than the further opening of tube ends and/or walls. The small surface area obtained from the heat treatment in air is mainly due to the remaining carboneous component, which could not be removed at 823 K.
3. Results and Discussion (19) Hiraoka, T.; Izadi-Najafabadi, A.; Yamada, T.; Futaba, D. N.; Yasuda, S.; Tanaike, O.; Hatori, H.; Yumura, M.; Iijima, S.; Hata, K. Advan. Funct. Mater. 2010, 20 (3), 422–428.
3.1. Analysis of mf-SWCNTs by the Nitrogen Adsorption Method. Nitrogen adsorption isotherm curves of mf-SWCNT 3374
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Table 1. Apparent Density and Surface Area of Carbon Materials in Either IPA or PCa apparent density (g cm-3) in various liquids sample number of electrode 1 2 3 4 5 6 7 8
sample name (remaining weight after heating)
mf-SWCNTs, as-grown mf-SWCNTs, 923 K in N2 (92 ( 1%) mf-SWCNTs, 823 K in air (88 ( 2%) mf-SWCNTs, 823 K in air (63 ( 3%) mf-SWCNTs, 1123 K in CO2 (62 ( 2%) mf-SWCNTs, 1123 K in CO2 þ 823 K in air (63 ( 4%) YP (þTeflon) HOPG
IPA 1.2 1.3 2.2 2.2 1.8 2.2 2.2 2.2
PC b
1.(2) 1.(2)b 2.1 2.1 1.7 2.0 2.1
EMI
BET surface area (m2g-1)
c c 2.1
650 920 1400 1300 1900 1900 1600
1.6 2.1 1.6
a Measured densities of IPA, PC, and EMI are 0.78, 1.2, and 1.3, respectively. b The first decimal place in the parentheses is within the error. c Densities were not measurable because of the close value between the apparent density of CNTs and density of EMI.
Figure 2. Change in specific surface area of mf-SWCNTs before and after heat treatment. (O) As-grown mf-SWCNTs, (2) mf-SWCNTs heated at 1223 K in CO2, and (9) mf-SWCNTs heat-treated at 823 K in air.
Figure 4. Relation of the BET surface area and apparent density of mf-SWCNTs before and after heat treatment in either CO2 gas or air. (9) As-grown mf-SWCNTs (IPA), ([) mf-SWCNTs heated in air (IPA), (]) mf-SWCNTs heated in air (PC), (b) mf-SWCNTs heated in CO2 (IPA), and (O) mf-SWCNTs heated in CO2 (PC).
not increase its density higher than 1.9. Instead, the mfSWCNTs heated in air showed a high apparent density of 2.2 at 1400 m2 g-1. The apparent density of mf-SWCNTs in EMI and remaining weight percent after heat treatment were listed in Table 1, along with the apparent density in IPA and PC and BET surface area. HOPG and YP showed an apparent density of 2.2 g cm-3, which is close to the density of graphite (2.27 g cm-3) and which proves correctness of this method. With the similar yield (ca. 63 wt %) after heat treatment, mf-SWCNTs heated in CO2 prior to air showed the highest surface area and highest apparent density among three comparable conditions. The apparent density of opened mf-SWCNTs was close to the apparent density of HOPG. It indicates that liquids were completely impregnated. 3.3. Analysis of mf-SWCNTs by Raman Spectroscopy. The amount of defects of mf-SWCNTs before and after heat treatment was analyzed by Raman spectroscopy, as shown in Figure 5. The relation between the ID/IG ratio and weight loss mainly depends upon the temperature rather than the type of gases used for hole opening. mf-SWCNTs heated in air at 723 K increased its ID/IG ratio with a small weight loss. This increment is due to the oxidization of the walls of CNTs, with the increment of the oxygen content confirmed by XPS. The mf-SWCNTs heated in air were heated further in nitrogen at 973 K to remove most of the functional groups including oxygen. Although the ID/IG ratio was decreased by heating in nitrogen, it is not a significant change. It is previously reported that 0-30 nm pits were created on the
Figure 3. Pore size distribution of mf-SWCNTs before and after heat treatment. (O) As-grown mf-SWCNTs, (4) mf-SWCNTs heated in CO2 until weight loss reaches 12 wt %, and (0) mfSWCNTs heated in CO2 until weight loss reaches 36 wt %.
3.2. Analysis of mf-SWCNTs by the Apparent Density Measurement. Figure 4 shows the apparent density of mfSWCNTs in IPA and PC. The apparent density of mfSWCNTs increased as the time of heat treatment in CO2 gas or air increases. This is due to the creation of holes on CNTs and impregnation of liquid into the inner space of CNTs. For all cases, IPA showed slightly higher apparent density than PC, as shown in Figure 4. Although the surface area of mf-SWCNTs heated in CO2 was higher than that in air, the apparent density of mf-SWCNTs heated in CO2 did 3375
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Figure 6. CVs of mf-SWCNTs before and after heat treatment. The curves were recorded at a scan rate of 1 mV s-1 in 1 M TEABF4/PC. (;) Sample number 1, (bold ;) sample number 3, and (- - -) sample number 6.
Figure 5. Relation between the ID/IG ratio and weight loss of mfSWCNTs before and after heat treatment and estimated sizes of holes on the side wall of mf-SWCNTs. (b) As-grown mf-SWCNTs, (0) mf-SWCNTs heated in air at 723 K, (9) mf-SWCNTs heated in air at 823 K, and (2) mf-SWCNTs heated in CO2 at 1223 K.
Table 2. Specific Capacitance of mf-SWCNTs before and after Heat Treatment at 0.5 A g-1 of Current Density and in the Sweep Range from 0.65 to -1.85 V
basal plane of HOPG by heating at 833 K for 5 min in dry air and the large holes (0-210 nm in diameter) were created by heating at 1173 K for 1 min and 15 s in dry air.20 Therefore, mf-SWCNTs, which are different in shape and thickness of layers from graphite, also opened the hole on the side wall, owing to the heat treatment in air at the low temperature. Figures 4 and 5 indicate that the sidewall of the mf-SWCNTs heated in air has holes and the size of hole, which is ca. 0.56 nm or larger, on the side wall of mf-SWCNTs is present, because the size of the IPA molecule is from 0.56 to 0.59 nm. The apparent density in PC or EMI reached up to 2.1 or lower. It means that the large size of the hole is 0.56 nm or a little larger, because the average molecular size of PC or EMI is larger than that of IPA. Heat treatment of as-grown mf-SWCNTs in CO2 or heated in N2, on the other hand, decreased the ID/IG ratio because of the removal of defects of mf-SWCNTs by shortening the length of mf-SWCNTs12 and carboneous component remaining in mf-SWCNTs. As mentioned in section 3.2, mf-SWCNTs heated in CO2 showed a low apparent density (Fapp = 1.7), although the surface area of mf-SWCNTs heated in CO2 showed 1900 m2 g-1. The high surface area can be explained by the opening of the end cap,12 and the low apparent density can be explained by the difficulty of impregnation because of the long length of mf-SWCNTs and other factors, such as deformation of mf-SWCNTs.21 In addition to the removal of the end cap, the heat treatment in CO2 shortened the length, as described in Figure 5.12 From the results, it was found that heat treatment of mfSWCNTs in CO2 at a high temperature (1223 K) is more effective at removing the carboneous component than that in air and that heat treatment of mf-SWCNTs in air at a relatively low temperature (below 823 K) is more effective at obtaining a high apparent density than that in CO2 because of the hole opening of the side wall of mf-SWCNTs. As a result, a combination of two heat treatments increases the surface area and apparent density of mass-produced mf-SWCNTs. 3.4. CV of mf-SWCNTs. The effect of heat treatment in air and that of the two-step heat treatment in air prior to CO2 on
specific capacitance (F g-1) sample number of electrode electrolyte 1 3 5 6 1 6
TEABF4/PC TEABF4/PC TEABF4/PC TEABF4/PC EMI EMI
p dope
n dope
64 88 92 100 71 95
54 89 82 98 65 98
electrochemical properties are compared, as shown in Figure 6. All three voltammograms were a butterfly shape and increased as the potential increased or decreased from -0.6 V versus an Ag/Agþ reference. This butterfly shape has been explained in previous works.2,3 CV of mf-SWCNTs heated in air prior to CO2 increased the current density on both the positive and negative sides, evenly. Heat treatment of mfSWCNTs in CO2 and air increased its capacitance 1.6 times higher than as-grown mf-SWCNTs, as shown in Table 2. An observation of mf-SWCNTs by TEM revealed that the diameter of mf-SWCNTs is 3.5 nm on average, which is large enough for TEAþ (0.7 nm) and BF4- (0.5 nm) to enter inside the CNTs from the opened end cap of mf-SWCNTs. Opened holes on the side wall of mf-SWCNTs, on the other hand, are considered to be enough or a little small for the TEA ion to pass. Because PC and the TEA ion may not completely enter inside mf-SWCNTs, it is possible that there is still some room for improving the capacitance. Table 2 also shows the specific capacitance of mf-SWCNTs in ionic liquid. Although the ionic liquid was used to eliminate the solvation effect, the capacitance with ionic liquid was similar to that with TEABF4/PC. The capacitance of sample number 6 was the largest because of the largest effective surface area, which took high surface area and high apparent density into consideration. 4. Conclusion The effects of heat treatments on opening holes on mfSWCNTs were studied, and the electrochemical performance of mass-produced mf-SWCNTs before and after heat treatment was measured. The BET surface area of mass-produced mf-SWCNTs was increased from 650 to 1900 m2 g-1 by heating in CO2 because of the removal of the carboneous
(20) Hahn, J. R. Carbon 2005, 43, 1506–1511. (21) Yang, M.; Koutsos, V.; Zaiser, M. Nanotechnology 2007, 18, 155708–155713.
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component, but propan-2-ol, 4-methyl-1,3-dioxolan-2-one, or ionic liquid were partially impregnated in mf-SWCNTs. Complete impregnation of liquids was achieved by heat treatment in dry air, but the surface area was increased only up to 1400 m2 g-1. Therefore, the combination of two heat treatments was conducted, and the high degree of impregnation of liquid inside the space of mf-SWCNTs was achieved. The combination of heat treatments in CO2 and air reduced the carboneous component, which reduced the surface area and increased the amount of holes on the sidewall of mfSWCNTs, respectively. The holes on the side wall played an important role for passing molecules inside mf-SWCNTs.
As a result of a two-step heat treatment, specific capacitance was improved 1.6 times higher than that of as-grown mfSWCNTs. The size of the largest hole created on mf-SWCNTs by heat treatment in dry air is at least 0.56 nm. The capacitance is still able to be improved by controlling the size of holes created on the side wall of mf-SWCNTs, because 4-methyl1,3-dioxolan-2-one or ionic liquid had difficulty being impregnated completely. Acknowledgment. This study was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.
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