Electric double layer capacitance of highly pure single-walledcarbon nanotubes (HiPcoe Buckytubese) in propylene

carbonate electrolytes

Soshi Shiraishi *, Hideyuki Kurihara, Keiji Okabe, Denisa Hulicova, Asao Oya

Department of Chemistry, Faculty of Engineering, Gunma University, Tenjin-cho 1-5-1, Kiryu, Gunma 376-8515, Japan

Received 11 April 2002; received in revised form 17 May 2002; accepted 17 May 2002

Abstract

The double layer capacitance of highly pure single-walled carbon nanotubes (SWCNTs) prepared by the HiPcoe process was

measured in 1:0 mol dm�3 LiClO4/propylene carbonate solution. The unpurified SWCNT electrode was mainly composed of a

bundle structure of SWCNTs with around 1.0 nm tube diameter, small amount of amorphous carbons, and Fe catalyst particles.

The Fe catalysts in the surface of the SWCNT were removed by immersion in HClaq. The as-SPE analysis of the N2 adsorption

isotherms revealed that both the SWCNTs before and after the immersion in HClaq had relatively high specific surface areas of

� 500 m2 g�1 without microporosity although the tube ends were closed. The SWCNTs showed a gravimetric capacitance of

around 45 F g�1. Thus, the specific capacitance per unit surface area was estimated to be around 10 lF cm�2, which was higher

than that of conventional activated carbon fibers. Furthermore, the capacitance of the SWCNTs did not decrease even at high

current density. This good rate property of the SWCNTs is related to the large area of the external surface (� 400 m2 g�1) on which

ion adsorption/desorption can proceed fast because of no ion sieving. On the other hand, most of the Fe catalyst in the SWCNT

could be removed by thermal oxidation followed by immersion in HClaq. However, the gravimetric capacitance of this purified

SWCNT was not as great as that expected by the correlation of 10 lF cm�2. This is related to the formation of amorphous carbons

caused by the thermal oxidation. � 2002 Elsevier Science B.V. All rights reserved.

Keywords: Single-walled carbon nanotube; Electric double layer capacitance; Nonaqueous electrolyte; Thermal oxidation

1. Introduction

New porous carbon materials have been desired forimproving the energy density of electric double layercapacitors (EDLC) [1]. Carbon nanotubes (CNTs) haveattracted much attention as new carbon materials sincetheir discovery by Ijima [2]. CNTs are ideal porouscarbons with a cylindrical shape since the tube inside canbe considered to have a very uniform pore with a nmsize. In fact, the double layer capacitance of the CNThas already been reported in many references [3–10].Especially, the single-walled carbon nanotube(SWCNT) [11] is one of the most attractive CNTs for

EDLC electrode materials, because the SWCNT has atheoretically high surface area (the total specific surfacearea of the outside plane and the inside plane of theCNT is 2630 m2 g�1). Moreover, the well-defined/bentgraphene sheet of the SWCNT may exhibit a higherspecific capacitance compared with the other carbonmaterials such as activated carbons, graphite, etc.

According to the literature [7–10], the reported valuesfor the gravimetric double layer capacitance of theSWCNT electrode were over a wide range between 20and 300 F g�1. For example, Bard and co-workers [7]estimated the capacitance of the SWCNT in acetonitrileelectrolyte to be around 280 F g�1. On the other hand,Baughman and co-workers [8] showed a smaller capac-itance in the NaCl aqueous electrolyte (20–40 F g�1) ofthe pure SWCNT prepared by laser abrasion. Thecapacitance in a KOH aqueous electrolyte, reported byBeguin and co-workers [9], was 40 F g�1, while that by

Electrochemistry Communications 4 (2002) 593–598

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* Corresponding author. Tel.: +81-277-30-1352; fax: +81-277-30-

1353.

E-mail addresses: ssiraisi@chem.u.ac.jp, ssiraisi@gunma.u.ac.jp

(S. Shiraishi).

1388-2481/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S1388-2481 (02 )00382-X

Lee and co-workers [10] was 180 F g�1. This inconsis-tency may be due to the difference in the kinds of elec-trolytes and the low purity of the SWCNT. Therefore, itis still difficult to understand the capacitance of theSWCNT. The capacitor using a nonaqueous electrolytesuch as a propylene carbonate solution is used for en-ergy storage. Therefore, the exact specific capacitance ofthe CNT in a nonaqueous electrolyte is very importantand has to be clarified.

Recently, a highly pure SWCNT was prepared by thethermal decomposition of CO under high pressure withan Fe catalyst [12,13] and this pure SWCNT has beencommercialized by Carbon Nanotechnologies (USA)under the trade name of ‘‘HiPcoe Buckytubese’’. TheSWCNT prepared by the HiPcoe process is very dif-ferent from the other SWCNTs prepared such as the arcdischarge [11,14] or by laser abrasion [15] with respect tothe point of purity and tube diameter. The SWCNT‘‘HiPcoe Bucktubese’’ be considered as one of themost suitable materials for understanding the trueproperties of the double layer capacitance for SWCNTsbecause of its high purity. In this paper, the correlationbetween the surface area and the double layer capaci-tance of this highly pure SWCNT in nonaqueous elec-trolyte is discussed and compared with traditionalactivated carbon fibers.

2. Experimental

2.1. Sample preparation

An unpurified single-walled carbon nanotube(HiPcoe Buckytubese, Lot No. 10518-53156) wasobtained from Carbon Nanotechnologies. The HiPcoeBuckytubese contain a small amount of the metallic Fecatalyst [13]. The SWCNT electrode for the double layercapacitance measurement was prepared from the unpu-rified HiPcoe Buckytubese in the following ways. A100 mg sample of the HiPcoe Buckytubese was dis-persed in 500 ml of methanol under sonication for 2 h.The dispersed nanotubes were filtered through mem-brane of porous PTFE film to form an accumulated layerof SWCNT. The sample was dried for 1 h at 60 �C andthen was peeled off the membrane filter to obtain a pa-per-like sample, called ‘‘Bucky Paper’’. This paper-likesample is referred to as ‘‘SWCNTunp’’ hereafter. Twokinds of purified SWCNT electrodes were also preparedfrom the unpurified HiPcoe Buckytubese. The first onewas the SWCNTunp washed with HClaq. The washingprocess involved immersion of the SWCNTunp electrodein 6 M HClaq for 24 h to remove the Fe catalyst on thesurface of the SWCNT. This sample (referred to as‘‘SWCNTHCl’’) after the immersion was washed withpure water and dried in vacuum at 200 �C for 3 h. Thesecond purified sample was obtained by thermal oxida-

tion in air and the following washing process in HClaq tocompletely remove the Fe catalyst. The HiPcoeBuckytubese (100 mg) were oxidized in humid air at 225�C for 5 h, which is similar to that described in the lit-erature [13]. After oxidation, the sample were immersedin 6 M HClaq for 2 h under sonication and then filteredthrough a microporous PTFE membrane and washed inpure water and methanol. This washed sample was dis-persed in 500 ml of methanol under ultrasonics and fil-tered again using the microporous membrane to obtain apaper-like sample (referred to as ‘‘SWCNTox-HCl’’ here-after). The above preparation scheme is shown inScheme 1.

The Fe content of the above three electrodes wasestimated by TG analysis in dry air and the ICP analysisof the ash (Fe2O3). The bulk density of these electrodeswas also estimated by measuring their areas and thick-nesses. The Fe content and the bulk density are alsoshown in Scheme 1.

Activated carbon fibers (ACFs) were used as refer-ence samples for correlation between the specific sur-face area and the double layer capacitance. The ACFswere prepared by the carbonization and steam-activa-tion of phenolic resin fibers. The preparation procedureof the ACFs is described in our previous papers[16–18].

2.2. Characterization of SWCNT electrode

The SWCNT electrodes were characterized using ascanning electron microscope (SEM, JSM5100, JEOL,

Scheme 1. Preparation procedure, Fe content, and bulk density of

various SWCNT electrodes.

594 S. Shiraishi et al. / Electrochemistry Communications 4 (2002) 593–598

Japan), X-ray diffractometer (XRD, RINT 2100V/PC,Rigaku, Japan), and transmission electron microscope(TEM, JEM-1200EXS, JEOL, Japan). In the XRDmeasurement, the paper-like sample (1 � 1 cm2) wasattached to the nonreflective sample holder. The N2

adsorption/desorption isotherms were measured by a N2

adsorption/desorption system (BELSORP28SA, BelJapan, Japan) at 77 K. The specific surface area and thepore structure parameters were estimated by analysis ofthe isotherms by the as plot-SPE method [19]. The as

plot-SPE method cannot only separate the informationabout the micropores and the other pores, but alsoeffectively prevent the overestimation problem of thespecific surface area. Kaneko and co-workers [20]have already applied the as plot-SPE method to porestructure analysis of single-walled carbon nanohorns(SWCNHs).

2.3. Electric double layer capacitance measurements

A propylene carbonate solution containing1:0 mol dm�3 LiClO4 (1.0 M LiClO4/PC, KishidaChemicals, Japan) was used as the nonaqueous elec-trolyte for the double layer capacitance measurement.The water content was less than 30 ppm. The SWCNTelectrodes were pressed into a Ti mesh as current col-lector. The SWCNT electrodes with the Ti mesh wereimmersed in the electrolyte under a reduced pressure(> 103 Pa) for 12 h for degassing. The double layer ca-pacitance of a single SWCNT electrode was measuredby using a three-electrode cell in a pure argon glove box.The capacitance was estimated from a chronopotentio-gram obtained using a galvanostatic method. Thedetailed procedure for this measurement is described inthe previous papers [16–18]. The double layer capaci-tance of the activated carbon fiber electrode was alsomeasured in the same way as in the previous papers[16–18].

3. Results and discussion

3.1. SEM observations

Fig. 1 shows the SEM images of the SWCNT elec-trodes. All electrodes were composed of only bundleswith several tens nanometer diameter, corresponding totypical bucky paper composed of highly pure SWCNT.No particles or flakes of impurities were observed evenin the unpurified SWCNT electrode, which is very dif-ferent from other unpurified SWCNTs prepared by thearc-discharge method or laser abrasion method.

3.2. XRD patterns

Fig. 2 shows the XRD patterns for the SWCNTelectrodes. The diffraction lines attributed to the trianglelattice of the SWCNT and cubic lattice of metallic Fewere observed in Fig. 2(a) for the SWCNTunp electrode.A weak and broadened line assigned to amorphouscarbon was observed around �26� in the XRD pattern,however, there are no sharp diffraction lines of graphite.Therefore, it can be said that the SWCNTunp electrode ismainly composed of SWCNT and Fe catalysts. Sup-posing that the van der Waals space between tubes is0.34 nm and close to that of graphite, the diffractionangle of the 10 line of the SWCNT suggests that the tubediameter is around 1.0 nm. This diameter is smaller thanthat of the SWCNT prepared by the arc-dischargemethod or the laser abrasion (> around 1.2 nm). The fullwidth half maximum of the Fe(1 1 1) line and theScherrer equation revealed that the lattice size of the Fecatalysts was estimated to be around 3 nm. The tubediameter and the Fe catalyst size are in good agreementwith that reported by Smalley and co-workers [13]. TheXRD pattern in Fig. 2(b) for the SWCNTHCl electrodewas almost the same as the SWCNTunp. These resultssuggested that the immersion process in HClaq did not

Fig. 1. SEM images of SWCNTs. (a) SWCNTunp, and (b) SWCNTox-HCl.

S. Shiraishi et al. / Electrochemistry Communications 4 (2002) 593–598 595

change the SWCNT structure and the crystallinity. Nointensity change of the Fe diffraction line is due to theonly removal of the Fe catalyst exposed on the surface.It is confirmed by the small decrease in the Fe contentshown in Scheme 1. On the other hand, the XRD pat-tern of the SWCNTox-HCl electrode exhibited a decreasein the intensity of the diffraction lines for the Fe catalystand the growth of the broadened line of amorphouscarbon. The purification process with the oxidation ef-fectively removed the Fe catalyst, but simultaneouslycaused damage to the tube structure. The SEM image inFig. 2(b) showed the maintenance of the tube bundlestructure in the SWCNTox-HCl. Therefore, these resultsindicate that some tubes in each bundle decomposed toamorphous carbon.

3.3. TEM observations

Fig. 3 shows the TEM images of the SWCNTunp andthe SWCNTox-HCl. The bundle structure consisting ofSWCNT and many dark spots of the Fe catalyst parti-cles are observed in the TEM image of the SWCNTunp.For the SWCNTox-HCl, there are only a few Fe catalystparticles, but amorphous carbon was contained in theouter part of the SWCNT bundle shown in the blackcircle. The results corresponded to the XRD pattern inFig. 2(c) and suggested that the HiPcoe Buckytubesewere unstable to the thermal oxidation even at a lowtemperature such as 225 �C. The oxidation stability ofthe HiPcoe Buckytubese is very different from that ofthe SWCNTs prepared by the arc-discharge or the laserabrasion [13]. This may be due to the small diameter ofthe HiPcoe Buckytubese.

3.4. Surface and pore structure analysis

The N2 adsorption isotherms (not shown here) of theSWCNTunp and the SWCNTHCl were similar and clas-sified as type II, which indicate nonporous materials.The SWCNTox-HCl also showed a kind of type IIisotherm, but the amount of N2 adsorption at thelow relative pressure was greater than those ofthe SWCNTunp and the SWCNTHCl. This means theSWCNTox-HCl has some microporosity. In the otherhand, the Ref-ACFs showed type I isotherms of a highlymicroporous carbon. The surface area and the porestructure were estimated by these N2 adsorption iso-therms.

Fig. 2. XRD (CuKa) patterns of various SWCNTs. (a) SWCNTunp,

and (b) SWCNTHCl, and (c) SWCNTox-HCl.

Fig. 3. TEM images of SWCNTs. (a) SWCNTunp, and (b) SWCNTox-HCl.

596 S. Shiraishi et al. / Electrochemistry Communications 4 (2002) 593–598

The specific surface area, micropore volume, and av-erage micropore width for the SWCNT samples and thereference ACF samples are summarized in Table 1. Theas-SPE method provides separate structure informationabout the micropores and the external surface (contain-ing mesopores and macopores surface). The total specificsurface area (Stotal) and the micropore volume (Vmicro) ofthe SWCNT electrodes were lower than those of the Ref-ACFs. In the case of the Ref-ACFs, the micropore spe-cific surface area (Smicro) was much greater than that ofthe external specific surface area (Sext), while the micro-pore surface area of the SWCNTs was smaller than theexternal one. Especially, the external surface area of theSWCNTunp and the SWCNTHCl accounted for most ofthe total surface area accessible for gas adsorption. In thecase of the SWCNTox-HCl, changes in the microporesurface and the micropore volume were observed, whichare derived from the opening of the tube end or theformation of amorphous carbons with some micropo-rosity. However, the micropore surface area of theSWCNTox-HCl, was much smaller than the theoreticaltube inside surface area (� 1315 m2 g�1). Probably,many tube ends are still closed or inside the opened tubeis not accessible for gas adsorption. The difficulty in theaccessibility of gas adsorption may be concerned withthe surface functionalitiy formation on the tube end orthe self-closing of the tube end during the curing processafter the thermal oxidation, rather than the stability ofthe tube end-cap to thermal oxidation.

The average micropore width (wmicro) of the SWCNTsbefore the purification mainly indicates the size of theinter-space between each tube, which was smaller thanthat of the ACFs such as Ref-ACF(10) or Ref-ACF(60).The average micropore width of the SWCNTox-HCl wascomparable with the Ref-ACFs, but probably influencedby the micropore size of the amorphous carbon parts.

3.5. Double layer capacitance behavior of the SWCNTs

Fig. 4 shows the correlations between the total surfacearea and the gravimetric capacitance for the SWCNTs

and Ref-ACFs. The capacitance of the well-activatedACFs such as Ref-ACF(120), (240), and (480) seemed tobe linear with respect to the total surface area. This lin-earity suggested that the specific capacitance per surfacearea of the Ref-ACF was around 5:5 lF cm�2. The ca-pacitance of the ACFs such as Ref-ACF(10) and (60)were much smaller than that expected from the gradientof 5:5 lF cm�2. According to our previous research [16–18], this deviation is due to the ion sieving effect of themicropores that prevents an ion from going into themicropore network. The gravimetric capacitance ofthe SWCNTunp and the SWCNTHCl was around45 F g�1 and smaller than that of the well-activatedACFs. However, the specific capacitance per surface areawas about 10 lF cm�2, which is greater than that of not

Table 1

Specific surface area, micropore volume, and average inicropore width of SWCNTs and Ref-ACFs

Sample Stotal ðm2 g�1Þ Sext ðm2 g�1Þ Smicro ðm2 g�1Þ Vmicro ðml g�1Þ Wmicro (nm)

SWCNTunp 462 387 75 0.015 0.40

SWCNTHCl 489 401 88 0.023 0.52

SWCNTox–HCl, 782 545 241 0.094 0.78

Ref-ACF(10)a 870 30 840 0.27 0.64

Ref-ACF(60) 1119 22 1097 0.36 0.66

Ref-ACF(120) 1341 36 1305 0.44 0.67

Ref-ACF(240) 1664 26 1638 0.59 0.72

Ref-ACF(480) 1938 44 1894 0.75 0.79

Stotal : Sext þ Smicro; Sext: specific surface area of external surface (containing mesopore and macropore) estimated by as-SPE method; Smicro:

specific surface area of micropore estimated by as-SPE method; Vmicro: pore volume of micropore estimated by as-SPE method; Wmicro: average

micropore width estimated by as-SPE method (Slit like-pore model).a Number in parenthesis means the activation duration (min).

Fig. 4. Correlation between total specific surface area (estimated by

as-SPE method) and double layer capacitance in 1.0 M LiClO4/PC

(galvanostatic: 40 mA g�1, positive process, 2 ! 4 Vvs:Li=Liþ) for

various SWCNTs and Ref-ACFs.

S. Shiraishi et al. / Electrochemistry Communications 4 (2002) 593–598 597

only the Ref-ACFs but also the other activated carbons[21] or the basal plane of HOPG [22]. This suggests ahigh potential for the double layer capacitance of theSWCNTs. Both SWCNTs showed almost the same ca-pacitive curves on the chronopotentiograms (not shownhere), so the Fe catalyst on the surface of the SWCNTdid not affect the double layer capacitance properties.The gravimetric capacitance of the SWCNTox-HCl wasnot as great as that expected by the 10 lF cm�2 line. Thisis due to amorphous carbons being formed by the de-struction of tube structure by the thermal oxidation.

Fig. 5 is the dependence of the double layer capa-citance on current density for the SWCNTs and theRef-ACFs. The SWCNTs maintained the double layercapacitance even at a high current density although thecapacitance was lower than that of the microporousACFs with a higher specific surface area. The high rateproperty of the SWCNT electrodes means that adsorp-tion/desorption did not take more time on the carbonnanotube structure than on the activated carbons. ForSWCNTs, the external surface, which provides the fastadsorption/desoption of ions due to no ion sieving effect,was much greater than that of the Ref-ACFs. Therefore,the fast ion adsorption/desoprion of the SWCNTs isconcerned with the high ratio of the external surfacearea to the total surface area.

Thus, the SWCNT electrodes exhibit the possibility amuch better performance for EDLC, especially for theviewpoint of the rate property. However, the gravi-metric capacitance of the SWCNTs is still not highenough for application to EDLC although the specificcapacitance is greater than conventional activated car-bons. Modification of the tube structure is necessary forpractical use. A high surface area close to the theoret-ical one must be achieved by a structure fabrication

such as tube diameter control, complete opening of thetube end, etc.

Acknowledgements

A part of this study is financially supported by In-dustrial Technology Research Grant Program in ’01from NEDO of Japan and Grant-in-Aid for Encour-agement of Young Scientists (2001, No. 13750770) inMEXT of Japan. The authors express special thanksto Dr. T. Umemura of Gunma University for ICPanalysis.

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Fig. 5. Dependence of double layer capacitance on current density for

various SWCNTs and Ref-ACFs in 1.0 M LiClO4/PC. Galvanostatic

method (10, 40, 80, 160 mA g�1), negative process; 4 V ! 2 V

vs:Li=Liþ. Each capacitance was calculated from each chronopoten-

tiogram in the region of 2.25–3.75 V vs.Li/Liþ to eliminate the influ-

ence of IR drop.

598 S. Shiraishi et al. / Electrochemistry Communications 4 (2002) 593–598