Article pubs.acs.org/JPCC
Thermal-Treatment-Induced Enhancement in Effective Surface Area of Single-Walled Carbon Nanohorns for Supercapacitor Application Hwan Jung Jung,†,‡ Yong-Jung Kim,§ Jong Hun Han,∥ Masako Yudasaka,⊥ Sumio Iijima,⊥,# Hirofumi Kanoh,‡ Yoong Ahm Kim,∇ Katsumi Kaneko,¶ and Cheol-Min Yang*,† †
Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), San101, Eunha-Ri, Bongdong-Eup, Wanju-Gun, Jeollabuk-Do 565-905, Republic of Korea ‡ Department of Chemistry, Graduate School of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan § Research Institute of Industrial Science and Technology (RIST), San32, Hyoja-Dong, Nam-Gu, Pohang 790-600, Republic of Korea ∥ Department of School of Applied Chemical Engineering, Chonnam National University, 300 Yongbong-Dong, Puk-Gu, Gwangju 500-757, Republic of Korea ⊥ National Institute of Advanced Industrial Science and Technology, Central 5-2, 1-1-1 Higashi, Tsukuba 305-8565, Japan # Faculty of Science and Technology, Meijo University, Tenpaku, Nagoya 468-8502, Japan ∇ Department of Polymer & Fiber System Engineering, Chonnam National University, 300 Yongbong-Dong, Puk-Gu, Gwangju 500-757, Republic of Korea ¶ Research Center for Exotic Nanocarbon, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan S Supporting Information *
ABSTRACT: We investigated the importance of the specific effective surface area through a detailed study on the relationship between electrical conductivity of single-walled carbon nanohorns (SWCNHs) and accessibility of the electrolyte ions in the SWCNH-based supercapacitor. After heat treatment of the SWCNHs, the ratio of sp2/sp3 carbons dramatically increased, suggesting an enhanced electrical conductivity of the SWCNHs. Even though the specific surface area (SSA) slightly decreased by 16% as a result of heat treatment, the specific capacitance per SSA of the SWCNH electrode remarkably increased from 22 to 47 μF cm−2. Such a result indicates an explicit increase in accessible effective surface area by electrolyte ions. Our result clearly showed that a higher degree of utilization for the interstitial pore of SWCNHs by solvated ions is a key factor in achieving a high volumetric capacitance of SWCNH-based supercapacitors.
through the nanoscale holes.4,7,18−21 The SWCNHs are synthesized with CO2 laser ablation of a graphite target in Ar atmosphere without any metal catalyst.14 The SWCNH units grow from the core graphite and form a spherical colloid. The space between adjacent walls of SWCNHs forms interstitial pores of the slender cone shape which is fit for diffusion of molecules and ions. The interstitial pores of the SWCNH spherical bundle can be regarded as quasi one-dimensional pores with the average pore width of 0.6 nm due to the partial orientation of the SWCNH particles.22 Our previous study also showed that a large amount of solvated ions could be stored inside the narrow interstitial pores of the SWCNHs.7,23 Since SWCNHs have been well-characterized and the physical properties can be associated with the nanostructure, SWCNHs are a promising model carbon to elucidate the fundamental mechanism of supercapacitors.7,23
1. INTRODUCTION Nanostructured carbon materials such as single-walled carbon nanotubes (SWCNTs) have great attraction for their applications in energy storage fields.1 Single-walled carbon nanohorns (SWCNHs) have been actively studied as alternative materials for catalyst supports,2,3 supercritical gas storage media,4−6 and supercapacitor electrodes.7−12 As-grown SWCNHs have novel physical properties and a porous feature due to a unique assembly structure consisting of highly defective graphene walls.13 They also have unique structure which is closed graphene single-wall structures with a horn shape.14,15 Although SWCNHs have a single-wall structure similar to SWCNTs, the graphene wall structure of SWCNHs has many defects such as pentagons and heptagons; SWCNH exhibits n-type semiconductivity, being different from that of SWCNT.16 The creation of nanoscale holes on the tips or sidewalls of the closed SWCNHs could be simply achieved by oxidation treatments in O2 gas or acid owing to higher reactivity of the defect sites for oxidative agents.4,5,17 Various guest molecules could be accessible to the internal space © 2013 American Chemical Society
Received: June 13, 2013 Revised: October 18, 2013 Published: October 22, 2013 25877
dx.doi.org/10.1021/jp405839z | J. Phys. Chem. C 2013, 117, 25877−25883
The Journal of Physical Chemistry C
Article
Figure 1. (a, b) FESEM and (c, d) HRTEM images of the SWCNHs before and after heat treatment: (a, c) as-grown SWCNHs; (b, d) heat-treated SWCNHs at 1273 K.
optical system (Raman RXN system) at room temperature under ambient conditions. The Raman spectra were obtained by using excitation laser with a wavelength of 532 nm (Ar ion laser). The optical power at the sample position was maintained below 5 mW. X-ray photoelectron spectra (XPS) were obtained using a JPS-9010MX (JEOL) instrument. The measurements were performed with Mg Kα excitation in a vacuum < 10−7 Pa at room temperature. The acceleration tension and emission current of the nonmonochromatized X-ray source are 10 kV and 100 mA, respectively. The electrical conductivity was measured with a powder resistivity measurement system (powder resistivity meter; Hantech Co., Ltd.). The SWCNH powder samples (200 mg) were compressed in a cylinder cavity with a diameter of 20 mm under pressure of 800 kgf cm−2 at room temperature. The electrical resistivity (ρ) was evaluated by following equation (Ω cm), ρ = (RA)/l, where R is the total resistance (Ω), A is the contact area between probe and SWCNH powder sample (cm2), and l is the thickness of pelletized SWCNH sample (cm). The pore structures were determined by N2 and CO2 adsorption at 77 and 273 K using volumetric equipment (Quantachrome AS-1-MP), after preevacuation for 2 h at 423 K, while maintaining the base pressure at 10−4 Pa. Pore structure parameters were determined by subtracting pore effect (SPE) and Dubinin−Radushkevich (DR) methods. The SPE method was performed by using high-resolution αs plots constructed for standard adsorption data on highly crystalline nonporous carbon black (Mitsubishi 4040B). The SWCNH powders were mixed with 5 wt % of poly(tetrafluoroethylene) (PTFE) binder without a conducting material. The weight of SWCNH electrode was adjusted to 40 mg. Samples were subsequently pressed into a disk shape with 13 mm in diameter and 0.4 mm in thickness. The capacitor consists of a couple of electrodes that are arranged face to face, with a separator (glass paper) inserted between these electrodes. Glassy carbon was used as current collector.
Supercapacitors have attracted much attention as power supply devices for electric vehicles and hybrid electric vehicles.24 Because of the high specific surface area (SSA) and excellent electrical conductivity of nanostructured carbons, they have been actively studied as hopeful electrode materials for energy storage. Several parameters of nanostructured carbons are recognized as key factors in terms of their application as supercapacitor electrode materials: SSA,25,26 pore size distribution,27 surface chemical state,28,29 and electrical conductivity.9,10,30 In particular, the pore size and electrical conductivity of the electrode materials are important parameters from the viewpoint of ion diffusivity and effective electrode surface area which are closely related to enhancement in the specific capacitance of supercapacitors. Here, we investigated the heat-treatment effect of the SWCNHs on their capacitive properties of supercapacitor electrode to elucidate the relationship between the electrical conductivity of the SWCNHs and the electrolyte ion diffusivity. In addition, capacitance behaviors of the SWCNH electrodes were compared for the aqueous and organic electrolytes with different solvated ion sizes.
2. EXPERIMENTAL SECTION The dahlia-like structured SWCNHs were synthesized with CO2 laser ablation of a graphite target in Ar atmosphere (101 kPa; as-grown SWCNHs). The as-grown SWCNHs were heattreated at 1273 K under high vacuum for 5 h (HT 1273 K). The commercialized activated carbon (YP17; Kuraray Chemical Co., Ltd.) was also used to compare the capacitance behavior with the SWCNH electrodes. Field-emission scanning electron microscopy (FESEM, JEOL JSM-6330F) observations were carried out at a 15 kV accelerating voltage. High-resolution transmission electron microscope (HRTEM) images were obtained with a Topcon EM-002B instrument at 200 kV accelerating voltage. Raman spectroscopy measurements were performed using a Kaiser 25878
dx.doi.org/10.1021/jp405839z | J. Phys. Chem. C 2013, 117, 25877−25883
The Journal of Physical Chemistry C
Article
Electrolytes were an aqueous electrolyte of 30 wt % H2SO4 and an organic electrolyte of 1 M tetraethylammonium tetrafluoroborate ((C2H5)4NBF4, (TEA)BF4) with propylene carbonate (PC). The charging voltages of the aqueous and organic electrolyte systems have been limited at 0.9 and 2.5 V, respectively, to ensure the stability of the solvents. The capacitance (C) of the electrodes was calculated on the basis of the following equation, C = (IΔt)/(WΔV), where I is the current at discharge, Δt is the time variation between 40 and 60% of the initial voltage, ΔV is the voltage variation from 40 to 60% of the initial voltage, and W is the SWCNH weight sum of both electrodes. All specific capacitances were calculated from the following relationship: capacitance in a three-electrode system equals four times the capacitance in a two-electrode system.
Figure 3. (a) C1s XPS spectra and (b) the deconvoluted ones of the as-grown and heat-treated SWCNHs at 1273 K.
the deconvoluted C1s XPS spectra of the as-grown and heattreated SWCNHs. All C1s XPS spectra consist of six components. Peak 1 near 284.5 eV and peak 2 near 285.6 eV can be assigned to sp2-hybridized (CC) and sp3-hybridized (CC) carbons of SWCNH walls, respectively.31 Additionally, peaks assigned to oxygen-containing functional groups are also observed in the higher binding energy side, which are 286.1 eV (CO), 287.0 eV (CO), and 289.0 eV (COO). In order to confirm the change in the bonding state of carbons, we should consider the relative peak intensity ratio of sp2/sp3 carbons for SWCNH samples. After heat treatment of the SWCNHs, the ratio of sp2/sp3 carbons dramatically increases from 1.03 to 1.72, which is attributed to the developed graphitic structure due to the increase in proportion of sp2-hybridized carbons.31 We also measured the electrical resistivity of the pelletized SWCNH samples before and after heat treatment. After heat treatment, the electrical resistivity decreases from 4.3 to 1.9 Ω cm; the heat-treated SWCNHs exhibit a higher electrical conductivity, being more appropriate for the electrode (Table 1). It is well-known that the increment in concentration of the sp2-hybridized carbons leads to enhancement in the electrical conductivity of the carbon materials. Therefore, this result is in excellent agreement with observations from Raman and XPS spectra. Figure 4 shows N2 adsorption isotherms of SWCNH samples. The N2 adsorption isotherm of as-grown SWCNHs is of type II in IUPAC classification. As-grown SWCNHs show gradual uptake of N2 at the medium relative pressure (P/P0) and also show predominant adsorption of N2 at the higher P/ P0, which are associated with multilayer adsorption on the external surface and macropores. The N2 adsorption of asgrown SWCNHs at low P/P0 almost occurs in interstitial pores due to the closed structure of individual nanohorns. The heat treatment provides slightly decreased N2 adsorption uptake compared with as-grown SWCNHs. The pore structure parameters of the samples obtained by the SPE method using the high-resolution αs plots are summarized in Table 1.5,32 The total SSA and micropore volume decrease after the heat treatment, which is mainly contributed to a decrease in the interstitial SSA. The decrease in interstitial SSA should be related to shrinkage of interstitial spaces due to thermally induced partial orientation of adjacent SWCNH particles. We also measured CO2 adsorption at 273 K up to P/P0 = 0.029. Here the small P/P0 comes from the extremely high saturation pressure of CO2 at 273 K. CO2 adsorption has been shown to be very effective for the characterization of ultramicropores (
