Exposed Edge Planes of Cup-Stacked Carbon Nanotubes for an

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Exposed Edge Planes of Cup-Stacked Carbon Nanotubes for an Electrochemical Capacitor In Young Jang,† Hiroki Ogata,† Ki Chul Park,‡ Sun Hyung Lee,† Jin Sung Park,† Yong Chae Jung,‡ Yong Jung Kim,§ Yoong Ahm Kim,*,† and Morinobu Endo†,‡ †

Faculty of Engineering, and ‡ Institute of Carbon Science and Technology, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan, and §Research Institute of Industrial Science & Technology, San 32, Hyoja-Dong, Nam-Gu, Pohang, Gyeongbuk 790-330, Korea

ABSTRACT The end sites of graphitic planes and their catalytic, chemical, physical, and electrochemical roles have been a longstanding issue in the surface chemistry of carbon science. In this study, complete exposure of the active edge sites on the outer surface of catalytically grown cup-stacked carbon nanotubes is accomplished using a conventional exfoliation method, and its intrinsic contribution to the improvement of the electrochemical behavior in an electrochemical capacitor is demonstrated. The significant enhancement in the capacitance of the nanotubes after exfoliation, occurring without a distinctive change in pore structure, was confirmed with the exposure of the electrochemically active edge sites thus being able to accumulate more charge. Such active sites make nanotubes useful in the fabrication of high-performance electrochemical capacitors, catalysts, supporting materials for catalysts, and photocurrent generators in photochemical cells. SECTION Energy Conversion and Storage

he electric double-layer capacitor (or supercapacitor) is scientifically recognized as one of the important next-generation rechargeable power sources due to its intrinsic ability to decrease the gap between the dielectric capacitor and the batteries.1,2 In general, a supercapacitor is constructed much like a battery insofar as there are two electrodes immersed in an electrolyte, with an ion-permeable separator located between the electrodes.3 Recently, carbon nanotubes have been studied as a promising electrode materials for supercapacitors because of their unique one-dimensional nanostructure, high surface area, low mass density, remarkable chemical stability, and electronic conductivity.4-8 However, it has also been demonstrated that carbon nanotubes exhibit low-capacitance density due to their poor surface accessibility for ions.1 Among the various carbon nanotubes, cup-stacked carbon nanotubes that are geometrically different from multiconcentric nanotubes have attracted substantial attention as a special and novel functional nanomaterial.9-11 They are characterized mainly by their stacked morphology consisting of truncated conical graphene layers along the tube length, with a large proportion of open edges present on the outer surface.12 Unfortunately, the unavoidable formation of amorphous carbon on the outer surface of the tubes is severely detrimental to its intrinsic electrochemical function originating at the active end planes of the truncated cone-type morphology.9 This is due to the fact that in carbon materials, the edge plane is known to have a differential capacitance 10 times higher than that of the basal plane.13,14 Thus, complete

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r 2010 American Chemical Society

exposure of the end planes that are predominantly present at the outer surface of the truncated cones is critically needed not only to fully utilize their electrochemical contribution to the capacitance in supercapacitors but also to understand their electrochemical interactions with electrolytes. In this work, using a conventional exfoliation method, we expose clean active end planes by detaching the amorphous carbon deposited on the surface of the nanotubes and evaluate their electrochemical contributions in aqueous and organic electrolyte. Pristine and thermally annealed tubes at 2800 °C were used as starting materials in order to compare the morphological and structural changes of both tubes before and after exfoliation. Following the exfoliation treatment using the Staudenmaier method, the nanotubes were structurally characterized using various analytical tools and electrochemically evaluated using cyclic voltammetry (see the Supporting Information for details of the synthetic conditions and characterization tools). The textural changes in the pristine and thermally annealed nanotubes before and after exfoliation were observed under a transmission electron microscope (TEM). As shown in Figure 1a, the pristine tube having a large inner hollow core is long and straight, and its geometry is characterized by

Received Date: May 18, 2010 Accepted Date: June 17, 2010 Published on Web Date: June 22, 2010

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DOI: 10.1021/jz1006498 |J. Phys. Chem. Lett. 2010, 1, 2099–2103

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vapor phase.17 Upon high-temperature thermal annealing at 2800 °C in argon, the amorphous carbon was structurally converted into well-aligned graphitic multilayers, whereas the end planes of the truncated cones were converted into more energetically stable multiloops by forming covalent bonds between the unstable (or chemically active) graphenes.17 However, the exfoliation treatment (e.g., the formation of the intercalation compound by immersion of the tubes in a mixture of nitric acid and sulfuric acid, followed by exfoliation upon abrupt thermal treatment up to 1000 °C) allows the pristine nanotubes to have completely exposed end planes yet without distinctive structural damage to the truncated cones (Figure 1c). In the case of the thermally treated tubes at 2800 °C, we observed a cone-type inner part with a zigzag outer surface morphology (Figure 1d and inset), as well as partial exfoliation with rippled graphenes (Figure S1, Supporting Information). The structural conversions and chemical compositions of each sample were characterized by Raman and X-ray photoemission spectroscopy. From the Raman spectrum of the pristine tubes (Figure 2a), we observed the D band (defectinduced mode) at 1333 cm-1, the G band (the graphite E2g2 mode) at 1580 cm-1, as well as a shoulder peak at 1620 cm-1 arising from the edge plane of graphite.18,19 Upon hightemperature thermal annealing, there was a remarkable decrease in the value of R (the intensity of the D band divided by the intensity of the G band) as well as a narrowing of the half width at half-maximum (HWHM) of both bands.20,21 These results are due to the structural evolution from the amorphous carbon to the crystalline multilayer carbons. However, the pristine tubes following the exfoliation treatment showed a higher R value and a larger HWHM of the D

regularly stacked truncated conical layers along the tube length.15-17 However, the end planes of the truncated cones were covered with amorphous carbon inevitably deposited on the outer surface of the nanotubes catalytically grown in the

Figure 1. TEM images of the pristine tubes before (a) and after (c) exfoliation and of the thermally annealed tubes before (b) and after (d) exfoliation. Insets represent their corresponding schematic models before and after exfoliation. The yellow lines indicate the catalytically grown truncated cones stacked along the tube length, whereas the red circle and lines indicate the deposited amorphous carbon and the thermally annealed crystalline layer on the outer surface, respectively.

Figure 2. (a) Raman and (b) XPS C1s spectra of the pristine and thermally annealed tubes before and after exfoliation.


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Table 1. Composition and Pore Structure Parameters of CSCNTs and Heat-Treated CSCNTs, before and after Exfoliation capacitance (F/g)a

pore structureb

aprotic electrolyte

protic electrolyte

(m /g)

(cm /g)

VDAe (cm3/g)

fmicro f (%)

APDg (nm)

12.9

10.9

30

0.175

0.019

11.1

23.19

2.0

2.1

20

0.160

0.008

5.3

31.88

pristine tube after exfoliation

55.7

28.4

32

0.269

0.016

6.0

33.43

thermally annealed tube after exfoliation

28.9

28.5

180

0.642

0.094

14.7

14.31

sample pristine tube thermally annealed tube

SBETc 2

VTotd 3

a

The capacitance was calculated according to C = Q/Em, where Q is the charge obtained by current integration during the forward scan from the CV curves, E is the potential window, and m is the mass of a single electode.29 The scan rates of CV in an aprotic and protic electrolyte were 10 and 1 mV/s, respectively. b Pore parameters were obtained from N2 adsorption. c Specific surface area using the BET method. d Total pore volume. e Pore volume obtained using the DA method, which implies the micropore volume. f fmicro, micropore fraction = VDA/VTot 100. g Average pore diameter.

Figure 3. Cyclic voltammograms of the pristine and thermally annealed tubes before and after exfoliation in (a) 1 M H2SO4 at a scan rate of 10 mV/s and (b) 1 M Et4NBF4/PC at a scan rate of 1 mV/s.

increase following exfoliation treatment.25,26 However, even though the deposited amorphous carbon was clearly detached from catalytically grown truncated stacked cups, as verified by TEM observation (see the inset in Figure 1c) in the present study, there is no distinctive change in the SSA of the pristine tubes before and after exfoliation. In contrast, the largely increased SSA in the thermally annealed tubes after the exfoliation treatment can be explained by the formation of graphene sheets from the outside graphitic multilayer (Figure S1, Supporting Information). In order to study the effect of the exposed edge planes on the electrochemical activity, cyclic voltammograms (CVs) were measured with a potential window of 1.0 V and a scan rate of 10 mV/s in 1 M H2SO4 aqueous electrolyte (Figure 3a). The pristine tube showed a deviated CV response because of the redox peak caused by the surface functional group in the vicinity of 0.5 V (versus Ag/AgCl).27 The CV profile of the thermally annealed tubes was rectangular, but it exhibited a much lower capacitance than the pristine tubes because of the decreased surface area caused by the formation of multilayer basal planes upon high-temperature thermal annealing. In contrast, pristine tubes after the exfoliation treatment exhibited a highly increased CV profile with a weak redox peak. Thus, such improved electrochemical performance in the

band as the exposed edge sites directly contributed to the intensified D band. In addition, the 1620 cm-1 peak was relatively evolved in the tubes after the exfoliation treatment and was associated with the edge-enriched planes due to the complete removal of amorphous carbon.18,19 In order to determine the changes in the chemical composition of the tubes before and after exfoliation, we measured the XPS C1s spectra (Figure 2b). The deconvolution peaks at 284.3 and 285.1 eV originate from graphite-like sp2 and diamond-like sp3 hybridized carbon atoms, respectively.22,23 Thus, the intensification of sp3 peaks observed in the pristine tubes after exfoliation is closely related to the exposure of openedge planes on the outer surface of carbon nanotubes.24 The change in the specific surface area (SSA) of the tubes via the exfoliation was examined using nitrogen adsorption. The adsorption and desorption isotherms as well as the pore size distribution were calculated for all samples by using the density functional theory (Figure S2, Supporting Information). The Brunauer-Emmett-Teller (BET) surface area and other related parameters characterizing pore structure are summarized in Table 1. It should be noted that the micropore fraction to total pore volume and to SSA was decreased significantly after the heat treatment. It is reported that the surface area and pore size of graphitic carbon materials


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pristine tubes after the exfoliation could be explained by the predominant presence of the exposed edge planes on the outer surface of the tubes as no change in tube porosity is observed before and after exfoliation. This result clearly provides experimental evidence that the edge planes of the tubes contributed to the double-layer capacitance since the edge sites have the ability to accumulate more changes than the basal plane.28 We also observed an increased capacitance in the thermally annealed tubes after exfoliation. This increase can be attributed to the presence of graphene sheets that increase the specific surface area of the thermally treated tubes after exfoliation. The evolution of redox peaks in the pristine tube and the exfoliated pristine and thermally treated tubes can be ascribed to the presence of the oxygen-containing functional groups. To prevent the pseudocapacitance by proton interaction in a protic electrolyte, the prepared samples were examined using an aprotic electrolyte. CV measurements were performed using a three-electrode system from -1.25 to 1.25 V (versus Ag/Agþ) and a scan rate of 1 mV/s in 1 M Et4NBF4/PC organic electrolyte (Figure 3b). For all of the samples in an organic electrolyte, the CV responses exhibited a rectangular shape without any redox peak. The highly improved capacitance in the exfoliated tubes suggests that the end planes confer superior capacitance to the carbon nanotubes through electrostatic attraction, as occurs in EDLC capacitors. Our findings indicate that the exfoliation of cupstacked carbon nanotubes leads to an improved capacitance due to the better accessibility of electrolyte ions on the edge planes. In conclusion, the pristine and thermally annealed tubes were used to expose the edge planes present on the outer surface of cup-stacked tubes via a conventional exfoliation process. Following exfoliation, the pristine tubes were found to have dominant edge planes on the surface by detaching amorphous carbon, whereas the thermally annealed tubes showed zigzag-like multiloops. The CV responses have shown that both tubes exhibited highly enhanced capacitance after exfoliation in aqueous and organic electrolyte. This can be related to the more complete exposure of the electrochemically active end planes on the outer surface of carbon nanotubes. As such, we have demonstrated the importance of the exposed (or open) edge planes as accessible ion sites on the surface of the tubes to increasing the energy density of supercapacitors.

REFERENCES

SUPPORTING INFORMATION AVAILABLE

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Experimental procedures, HR-TEM images, and nitrogen isotherms and pore size distribution. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION (15)

Corresponding Author: *To whom correspondence should be addressed. Tel: þ81-26-269-5212. Fax: þ81-26-269-5208. E-mail: [email protected].

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ACKNOWLEDGMENT This research was supported by the (17)

Regional Innovation Cluster Program of Nagano and Specially Promoted Research (M.E, Y.A.K.) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 19002007).


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Exposed Edge Planes of Cup-Stacked Carbon Nanotubes for an

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