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Surface modification of soft-templated ordered mesoporous carbon for electrochemical supercapacitors
Accepted Manuscript Surface modification of soft-templated ordered mesoporous carbon for electrochemical supercapacitors Shunsuke Tanaka, Hiroki Fujimoto, Joeri F.M. Denayer, Manabu Miyamoto, Yasunori Oumi, Yoshikazu Miyake PII:
S1387-1811(15)00344-3
DOI:
10.1016/j.micromeso.2015.06.017
Reference:
MICMAT 7176
To appear in:
Microporous and Mesoporous Materials
Received Date: 16 April 2015 Revised Date:
10 June 2015
Accepted Date: 11 June 2015
Please cite this article as: S. Tanaka, H. Fujimoto, J.F.M. Denayer, M. Miyamoto, Y. Oumi, Y. Miyake, Surface modification of soft-templated ordered mesoporous carbon for electrochemical supercapacitors, Microporous and Mesoporous Materials (2015), doi: 10.1016/j.micromeso.2015.06.017. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Title: Surface modification of soft-templated ordered mesoporous carbon for electrochemical supercapacitors Authors: Shunsuke Tanaka (submitter), Hiroki Fujimoto, Joeri F. M. Denayer, Manabu Miyamoto, Yasunori Oumi, Yoshikazu Miyake
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Graphical Abstract
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Surface modification of soft-templated ordered
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mesoporous carbon for electrochemical supercapacitors Shunsuke Tanaka,a,b* Hiroki Fujimoto,a Joeri F. M. Denayer,c Manabu Miyamoto,d Yasunori Oumi,e
a
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and Yoshikazu Miyakea,b
Department of Chemical, Energy and Environmental Engineering, Faculty of Environmental and
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Urban Engineering, Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680 JAPAN b
Organization for Research and Development of Innovative Science and Technology (ORDIST), Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680 JAPAN
c
Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel,
d
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Department of Chemistry and Biomolecular Science, Gifu University, 1-1 Yanagido, Gifu 501-1193 JAPAN
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Division of Instrument Analysis, Life Science Research Center, Gifu University, 1-1 Yanagido Gifu,
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e
501-1193, JAPAN * Corresponding author
Tel: +81-6-6368-0851; Fax: +81-6-6388-8869; E-mail: [email protected] (S. Tanaka)
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ABSTRACT
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The increasing demand for energy has triggered numerous research efforts for the development of electricity storage devices consisting of templated nanoporous carbons. In this study, ordered mesoporous carbon with a modest specific surface area (520 m2/g) was prepared by a soft-templating method and surface-modified by a wet oxidation using nitric acid and ammonium peroxodisulfate.
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More oxygen-containing functional groups were introduced on the surface by oxidation using
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ammonium peroxodisulfate rather than nitric acid. Our study systematically investigates the effect of surface modification on electrochemical performance, based on comparisons with oxidized commercial activated microporous carbons. Oxygen functionalization leads to an increased capacitance for the ordered mesoporous carbon and a decreased capacitance for the activated microporous carbon at higher power densities. The ordered mesoporous carbon oxidized by nitric acid demonstrates excellent
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electrochemical performance with gravimetric capacitance of 290 F/g and volumetric capacitance of 300 F/cm3 at 0.1 A/g and retains over 70% of the capacitance at high current density of 5 A/g even though its surface area remained moderate (430 m2/g). The device demonstrates remarkable
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performance with an energy density of 17.4 Wh/kg, power density of 5.2 kW/kg, and excellent cycle
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life.
KEYWORDS
Ordered mesoporous carbon, Wet oxidation, Oxygen functionality, Water wettability, Supercapacitor
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1. INTRODUCTION Electrochemical double layer capacitors (EDLC) or supercapacitors represent a unique type of high
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power energy storage devices, where the capacitance arises from the charge separation at an electrode– electrolyte interface [1,2]. The energy storage in EDLC is based on the electro-adsorption of electrolyte ions on the large surface area of electrically conductive porous electrodes, most commonly porous
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carbons. EDLC has been widely used for several applications, such as an emergency or short-term secondary power source, copy machines, power booster for construction-equipments, since its
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commercialization in the 1970s. However, the main limitation of EDLC is their lower energy density than lithium-ion batteries. Thus, the improvement of their energy density is of significant for further expansion of their application fields.
The capacitance of electrically conductive porous electrodes should be in principle proportional to
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the surface area. However, many studies have shown an unfavorable divergence from this linear relation. The energy storage characteristics of EDLC are strongly affected by the capability of porous carbon to adsorb a large quantity of electrolyte ions under an applied potential and attract the ions
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closer to the pore walls. Therefore, aside from the surface area, the performance of EDLC strongly depends on the pore structure, pore size distribution, and surface functionality [3–9]. In addition, the
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capacitive rate performance is determined by how fast the electrolyte ions can travel within the pores. In order to achieve a higher energy density, it is important to appropriately design the pore structure. The templated nanoporous carbons are expected to draw a simple relationship between the structural characteristics of porous carbon and the EDLC performance [10,11]. Surface modification of carbon materials is effective not only for their dispersibility in various solvent [12] but also to improve the adsorptive performance as adsorbent [13]. A variety of
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functionalities can be introduced upon the surface of carbon materials by oxidative treatment involving dry or wet oxidation, plasma, and electrochemical treatments. In dry oxidation, oxygen, ozone, and
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carbon dioxide are usually used as gaseous oxidizing agents. Wet oxidation involves the use of nitric acid, sulfuric acid, phosphoric acid, ammonium peroxodisulfate, alone or in the combination with hydrogen peroxide, sodium hypochlorite, and so on [14–20].
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In this study, the ordered mesoporous carbon (OMC) with channel pore structure was prepared through the soft-templating method. The soft-templated OMC contained a small quantity of functional
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groups on its surface. Pore wettability, surface hydrophobic/hydrophilic balance, in general, depends on the surface functional groups and determine penetration of guest species into the pore system. The pore surface of the OMC was therefore modified by the wet oxidation. The effects of the wet oxidation on the structural characteristics and electrochemical performances of the OMC electrodes were
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investigated by comparisons with oxidized commercial activated carbons. The structural properties were characterized by the combination of powder X-ray diffraction (PXRD) and nitrogen sorption measurements. The surface chemical properties were characterized by the combination of Fourier
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transform infrared (FTIR) measurement, thermogravimetric analysis, and the Boehm titration. The electrochemical properties were characterized by using cyclic voltammetry (CV) and galvanostatic
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charge–discharge techniques.
2. EXPERIMENT 2.1 Materials
Resorcinol, phloroglucinol, 37 wt% formaldehyde, hydrochloric acid, nitric acid, ammonium peroxodisulfate (APS), potassium hydroxide, sodium hydroxide, sodium carbonate, sodium bicarbonate,
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and ethanol were purchased from Wako Pure Chemical Industries and used as received. Pluronic F127 was
purchased
from
Sigma-Aldrich
Chemical
Co.
and
used
as
received.
Polyflon
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polytetrafluoroethylene (F-104; PTFE) was received from Daikin Industries, Ltd. A commercial
of comparison.
2.2 Synthesis of ordered mesoporous carbon
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Maxsorb activated carbon was received from Kansai Coke & Chemical Co. Ltd. and used for the sake
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Resorcinol and phloroglucinol were completely dissolved in ethanol/water solution, hydrochloric acid was added, and the solution was stirred for 15 min. Pluronic F127 was then added, and after it was completely dissolved, formaldehyde was added. The final molar composition of the precursor solution was 3 resorcinol: 1 phloroglucinol: 9 formaldehyde: 0.02 Pluronic F127: 0.1 hydrochloric acid: 40
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water: 100 ethanol. The solutions were left for 3 days at room temperature, during which they separated into two phases. The upper clear phase was discarded; the lower dark brown phases were thermopolymerized in air at 100 °C for 3 h. The resultant dark brown deposition was carbonized in a
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tubular furnace with nitrogen flow (100 mL/min). The furnace was heated at a ramping rate of 1
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°C/min below 400 °C and 5 °C/min above 400 °C, then kept at 800 °C for 3 h.
2.3 Surface modification
Ordered mesoporous carbon was subsequently subjected to oxidation treatment with different oxidizing agents. For a typical run, 0.3 g of ordered mesoporous carbon was mixed with a 50 mL solution of 1M nitric acid or 1M APS, then heated at 90 °C for 6 h under stirring and refluxing conditions to obtain the surface-modified carbon samples, designated as OMC-x, where x denotes to the
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oxidizing agent, HNO3 or APS. The resultant sample was extensively washed with deionized water until the pH of the filtrate water became neutral, then dried at 100 °C in air. The same oxidation
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treatment was also carried out for the commercial Maxsorb activated carbon.
2.4 Chemical activation
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The post chemical activation treatment was also performed to compare the treatment effect of oxidation with that of activation. For a typical run, 0.3 g of ordered mesoporous carbon was
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impregnated with a 15 mL solution of 1M potassium hydroxide followed by water evaporation at 120 °C. The activation process was performed with nitrogen flow (100 mL/min) at 800 °C, with a ramping rate of 10 °C/min, for 1 h. The resultant sample was washed with 0.1 M hydrochloric acid solution followed by deionized water until the pH of the filtrate water became neutral, then dried at 100
Maxsorb activated carbon.
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°C in air. The same post chemical activation treatment was also carried out for the commercial
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2.5 Structural characterization
PXRD patterns were recorded on a RINT-TTR III diffractometer (Rigaku, Japan) using Cu Kα
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radiation; the copper anode was operated at 40 kV and 20 mA. Nitrogen adsorption/desorption isotherms of the samples were measured at 77 K using a BELSORP-max instrument (Bel Japan). The samples were degassed at 200 °C under vacuum. Brunauer–Emmett–Teller (BET) model surface areas, SBET, were calculated from the nitrogen adsorption branches. A part of the nitrogen adsorption isotherm in the P/P0 range 0.10–0.30 was fitted to the BET equation to estimate the SBET. The pore size distributions were calculated from adsorption branch of isotherm using the Barrett–Joyner–Halenda
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(BJH) model. The total pore volumes, Vtotal, were calculated on the basis of the amount of nitrogen adsorbed at P/P0 = 0.99. The micropore volumes, Vmicro, were calculated from the αs-plot method. The
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mesopore volumes were calculated by subtracting the value of Vmicro from that of Vtotal. Transmission electron microscope (TEM) images of the samples were recorded on a JEM-2010 microscope (JEOL)
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at an acceleration voltage of 200 kV.
2.6 Surface chemical characterization
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FTIR spectra were recorded on an IRAffinity-1 spectrometer (Shimadzu, Japan) using KBr pellet method. Thermogravimetric curves were recorded on a DTG-60H (Shimadzu, Japan) with nitrogen flow (100 mL/min) at a ramping rate of 5 °C/min. The surface functional groups were determined according to the Boehm titration method [21,22]. For a typical run, 0.1 g of dried active materials were
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added in the vials containing 100 mL of standard base solution as follows: 10 mmol/l sodium hydroxide, 0.5 mmol/l sodium carbonate, and 0.5 mmol/l sodium bicarbonate. The vials were sealed and shaken at room temperature for 48 h. Then, the solutions were filtered and 10 mL of each solution
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was taken for the subsequent titration. The filtration procedure was quite sensitive and done with extra care. The blank samples were analysed as well. All experiments were run in triplicates. The number of
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acidic sites was determined under the assumptions that (a) sodium hydroxide neutralizes carboxylic, lactonic, and phenolic groups, (b) sodium carbonate neutralizes carboxylic and lactonic groups, and (c) sodium bicarbonate neutralizes carboxylic groups. Adsorption isotherms of water were measured at 298 K using a BELSORP-18 (Bel Japan). The samples were degassed at 250 °C under vacuum for 10 h.
2.7 Electrochemical measurements
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The electrodes were prepared by mixing 80 wt% dried active material, 10 wt% carbon black, and 10 wt% PTFE. The resulting composite paste was made into an electrode pellet of about 0.2 mm in
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thickness. The electrode was cut into 1 cm by 1 cm pieces. The electrode disk was pressed and attached with Pt collector to reduce the interfacial resistance. The electrochemical performance was characterized by CV and galvanostatic charge–discharge on an HX5000 electrochemical workstation
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(Hokuto Denko, Japan). The experiments were carried out at 25 °C within a climate chamber using a three electrode cell in an electrolyte of 1 M sulfuric acid aqueous solution. An Ag/AgCl was used as a
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reference electrode. The gravimetric capacitance, Cm (F/g), of the active material was determined from CV experiments using the following equation: (1)
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where m is the mass of active material (g), v is the potential scan rate (mV/s), E1 and E2 are the cutoff potentials (V) in CV, (E2–E1) is the potential window width, i(E) is the instantaneous response current (A). The gravimetric capacitance, Cm (F/g), of the active material was determined from galvanostatic
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charge–discharge experiments using the following equation:
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(2)
where i is the discharge current (A), ∆t is the discharge time (s), ∆E is the voltage change (V) during discharge excluding the portion of iR drop. The volumetric capacitance, Cv (F/cm3), of the active material was calculated using the following equation: (3)
where ρ is the particle density calculated by
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(4)
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where ρcarbon is the true density of carbon (2 g/cm3) [11]. The energy density (ED; Wh/kg) and power density (PD; W/kg) of the cell were calculated using the following equations:
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(5)
3. RESULTS AND DISCUSSION 3.1 Structural characteristics
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(6)
The carbonized ordered mesoporous carbon looks like rocky fragments and can be easily crushed
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into a fine powder. Since the particle size of active material can affect the inner electrical resistance, the particle size was adjusted to be almost the same by grinding and sieving process. Figure 1A shows PXRD patterns of ordered mesoporous carbon before and after oxidation. All the PXRD patterns
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exhibit a reflection peak at around 1.0°, which can be attributed to mesostructure. This result indicates
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that the ordered mesostructure was still retained after oxidation. On the other hand, the second order reflection peak became weaker after oxidation, suggesting that oxidation deteriorated ordered mesostructure to some extent. The peak position did not shift after oxidation, indicating that a structural shrinkage did not occur during oxidation. Figure 2 shows TEM images of ordered mesoporous carbon before and after oxidation. Three dimensional wormhole-like mesostructure remained even after oxidation treatment. The detailed mesostructure of ordered mesoporous carbon was described elsewhere [23,24].
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The porous structures of the ordered mesoporous carbons were investigated by nitrogen adsorption/desorption measurements shown in Figure 1B. All the samples show typical type-IV curves
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with hysteresis loops and capillary condensation, ascribed to the uniform mesopores inside the carbons. As shown in Figure 1C, the mesopore sizes of OMC-HNO3 and OMC-APS were 6.2 nm and the same as that of OMC. The PXRD and nitrogen sorption results strongly indicate that no structural shrinkage
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did occur during oxidation. However, both surface area and pore volume slightly decreased after oxidation. The formation of oxygen-containing surface functional groups may contribute to the
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reduction of surface area and pore volume, which is further discussed in combination with surface chemical characteristics (Section 3.2). Their structural characteristics are summarized in Table 1.
3.2 Surface chemical characteristics
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The wet oxidation for carbon materials can significantly affect the surface functionality. FTIR analysis was performed to investigate chemical composition changes on the channel surface of the ordered mesoporous carbon before and after oxidation. Figure 3A shows the FTIR spectra of OMC,
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OMC-HNO3, and OMC-APS. While no significant band was observed in OMC, the presence of different types of oxygen functionalities in OMC-HNO3 and OMC-APS was confirmed at 1200 cm−1
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(ν(C−O) in esters, ether or phenol groups), 1550 cm−1 (ν(C=C) in aromatics group), 1700 cm−1 (ν(C=O) in carbonyl group), and 3000−4000 cm−1 (ν(O−H) in hydroxyl group) [25]. As shown in Figure 3B, the concentration of oxygen-containing functional groups in Maxsorb is higher than that of OMC even before oxidation because of its high surface area (Table 1). For both OMC and Maxsorb, more oxygencontaining functional groups were introduced on the surface by oxidation using APS rather than HNO3.
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Thermogravimetric analysis shows a total weight loss of about 20% and 40% at 800 °C for OMC and acid-oxidized OMC samples (OMC-HNO3 and OMC-APS), respectively (Supplementary Content,
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Figure S1). The acid-oxidation led to breaking C−C bonds and formation of oxygen-containing functional groups, it is therefore not too surprising that the total weight loss of about 40% occurs for OMC-HNO3 and OMC-APS. The weight loss ranging from 150 to 450 °C (step 1 in Figure S1) can be
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mainly assigned to the dehydration of adjacent carboxyl and/or phenolic hydroxyl groups, consequently resulting in the formation of carboxylic anhydrides and lactones. Another possible contribution can be
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attributed to the decomposition of carboxylic groups. The weight loss at around 550 °C (step 2) is believed to be associated with the decomposition of carboxylic anhydrides and lactones, which are more stable than carboxyl groups. The weight loss above 600 °C (step 3) is believed to be associated with the decomposition of phenol, quinone, and ether groups.
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The Boehm titration method provides quantitative identification of the oxygen-containing surface functional groups (Figure 4). The concentrations of functional groups for OMC were in the order phenolic group (0.059 mmol/g) > lactonic group (0.011 mmol/g) > carboxylic group (0.0049 mmol/g).
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This order corresponds with the general trends of decomposition of functional groups on carbon
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[26,27]. Of the three types of functional group the phenolic group decomposes at the highest temperature and the carboxylic group decomposes at the lowest temperature. The oxidation using APS yielded larger concentrations of the oxygen-containing functional groups on the surface compared to the oxidation using HNO3. This indicates that APS has a higher capacity to break C−C bonds and form the oxygen-containing functional groups compared to HNO3. The concentrations of functional groups in Maxsorb samples are higher than that of OMC samples because of their high surface area. There is no large difference in the composition ratio of phenolic, lactonic, and carboxylic group between
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Maxsorb-HNO3 and Maxsorb-APS. On the other hand, of particular interest is that OMC-APS has a higher ratio of lactonic group compared with OMC-HNO3.
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Figure 5 shows the water vapor adsorption isotherms of OMC, OMC-HNO3, and OMC-APS. All the OMC samples exhibit a distinct two-step uptake. The first and second steps can be attributed to adsorption in micropores and mesopores, respectively [28,29]. The OMC shows the first- and second-
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step uptakes at a relative pressure of about 0.5 and 0.95, respectively, suggesting that the pore surface is hydrophobic or slightly hydrophilic. On the other hand, the two step uptakes occur at lower relative
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pressures after oxidation. These shifts in the relative pressure was caused by a surface hydrophilicity due to the introduction of hydrophilic oxygen-containing functional groups. There is no large difference in first-step uptake between OMC-HNO3 and OMC-APS. The second steep steps corresponding to the capillary condensation in mesopores result from the narrow pore size distributions
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estimated by nitrogen adsorption/desorption measurements. While there is no large difference in mesopore size between OMC-HNO3 and OMC-APS (Figure 1C), the pressure of second-step for OMC-APS is slightly lower than that for OMC-HNO3. Combined with Boehm titration, this result
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indicates that the mesopore surface of OMC-APS is more hydrophilic than that of OMC-HNO3. These
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porous carbons were used to investigate the effect of surface functionality on the EDLC performance.
3.3 Cyclic voltammetric behaviors The CV profiles of initial efforts to estimate the electrochemical performance of OMC samples are shown in Figure 6A. It can be seen that the CV curves exhibit a symmetric rectangular shape without obvious redox peaks for OMC, indicating typical EDLC behavior [30]. On the other hand, the CV profiles of OMC-HNO3 and OMC-APS display the pair of anodic and cathodic peaks within 0.3−0.5 V,
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which can be attributed to the pseudo-Faradic reactions involving the oxygen-containing functional groups [15–17]. The original OMC shows the gravimetric capacitance of 178 F/g at the potential scan
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rate of 1 mV/s and 58% retained ratio at 50 mV/s. After oxidation, the capacitance increased to 270 F/g and 232 F/g at 1 mV/s for OMC-HNO3 and OMC-APS, respectively, in spite of their decreasing surface area.
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Figure 6B shows the CV profiles of Maxsorb samples. The CV profile of Maxsorb had a distorted rectangular shape, suggesting that the sulfate ion has a lower mobility than that of the smaller size
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proton in the micropores. On the other hand, Maxsorb-HNO3 and Maxsorb-APS showed a better, more defined rectangular shape with pseudo-Faradic redox peaks compared to the original Maxsorb. After oxidation using HNO3, the capacitance increased from 237 F/g to 283 F/g at 1 mV/s. However, the capacitance decreased to 226 F/g at 1 mV/s for Maxsorb-APS in spite of high concentration of the
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oxygen-containing functional groups.
An improved capacitive performance of surface-oxidized carbons in aqueous electrolyte systems is believed to be caused not only by the pseudo-capacitive reactions but also by the wettability with
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electrolyte. The introduction of hydrophilic oxygen-containing functional groups improves wettability of carbon materials with aqueous electrolyte. On the other hand, these functional groups can be divided
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into two main categories, depending on the nature of the C−O bonds: (i) electrochemical inactive groups (lactonic) and (ii) surface groups with surface acidity and electrochemical redox activity (carboxylic and phenolic) as follows [19,20]:
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Thus, the capacitances of carbons oxidized using HNO3 (OMC-HNO3 and Maxsorb-HNO3) increase more than those of carbons with higher concentrations of lactonic groups (OMC-APS and Maxsorb-
increasing the capacitance, more than for microporous carbon.
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3.4 Long-term cycling stability
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APS). In addition, surface modification for ordered mesoporous carbon is shown to be effective in
The capacitance decreased with increasing potential scan rate. However, the capacitance decrease
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was less sensitive to potential for ordered mesoporous carbons as compared to microporous carbons. OMC-HNO3 and OMC-APS show up to 70% higher retained capacitance and maintain a high capacitance of 193 F/g and 166 F/g, respectively, at the scan rate of 50 mV/s. On the other hand, the microporous carbons show low retained capacitance (49%, 59%, and 35% retained ratio at 50 mV/s for
diffuse within the micropores.
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Maxsorb, Maxsorb-HNO3, and Maxsorb-APS, respectively), suggesting that the ions cannot easily
For further understanding of the electrochemical performances, the long-term cycling stability of
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OMC, OMC-HNO3, and OMC-APS was investigated by repeating a CV test at potential scan rate of 100 mV/s as shown in Figure S2 (Supplementary Content). All the OMC samples exhibit excellent
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long-term cycling stability with more than 95% retention after 5000 cycle tests.
3.5 Galvanostatic charging/discharging behaviors Galvanostatic charge–discharge measurements are commonly used to quantify the electrochemical performance. As show in Figure 7A, all the OMC electrodes can be charged and discharged smoothly without obvious iR drop, and exhibit stable capacitive performance at low current density of 0.1 A/g.
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The feature indicates the typical Coulombic efficiency and ideal capacitive behavior. The area surrounded by charge–discharge curves is apparently increased after oxidation, indicating that the
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capacitances of OMC-HNO3 and OMC-APS are higher than that of the original OMC. This indicates that the electrolyte ions penetrates the inner surface sites of OMC-HNO3 and OMC-APS electrodes more deeply than the original OMC electrode.
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On the other hand, a sharp change in potential, which is induced by an equivalent series resistance iR drop and polarization of the electrode, was clearly observed at high current density of 5 A/g for OMC-
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APS in particular. This indicates that the capacitances of OMC-HNO3 and OMC-APS electrodes are consisted of the electric double layer formation and pseudo-Faradic reaction due to the existence of oxygen-containing functional groups on the surface. The iR drop of OMC-HNO3 was much smaller than that of OMC-APS, which makes it a quite favourable candidate for the electrode of high power
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capacitor.
As shown in Figure 7B, the charge–discharge profiles of Maxsorb samples do not show typical triangle symmetrical distribution even at 0.1 A/g. Although the surface area of OMC-HNO3 is much
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smaller than that of Maxsorb-HNO3, the capacitance of OMC-HNO3 is comparable to or even higher than that of Maxsorb-HNO3 at the current densities below 0.5 A/g. This indicates that the existence of
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mesopores benefits the penetration of ions. The capacitance variation with the current density is an important issue in high-rate capacitors. Figure 8 shows the current density dependence of the gravimetric capacitance. The capacitance decreased with increasing current density. However, the capacitance decrease was less sensitive for all the OMC samples and the original Maxsorb. This indicates that surface oxidation is more effective for mesoporous OMC rather than microporous Maxsorb. The homogeneous mesopore network offers much higher mobility of ions than micropore
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network. Thus, OMC-HNO3 and OMC-APS show higher retained capacitance than the original OMC even at higher current density and scan rate (Figure 8 and Supplementary Content, Figure S3) because
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they have relatively large mesopore size, compared to previously reported mesoporous carbons, allowing fast ion transport. On the other hand, the oxidized Maxsorb electrodes show unfavorable
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capacitive performance at higher current density.
3.6 Ragone plot
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The operational performance and efficiency of electrical energy storage devices are determined by two most important parameters; the specific energy density (ED) and specific power density (PD). In order to demonstrate the overall performance of the electrodes, the Ragone plots of ED versus PD are plotted and shown in Figure 9. The ED and PD values were derived from the galvanostatic charge–
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discharge profiles at different current densities. It is seen that the electrochemical capacitors are highpower devices with limited energy storage capacity. In addition, the voltage delivered depends strongly on the state of discharge.
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For OMC, the oxidation improves both ED and PD performances. A maximum ED of 25.8 Wh/kg was achieved by the OMC-HNO3 at PD of 53.9 W/kg. In addition, the OMC-HNO3 and OMC-APS
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resulted in excellent ED retention at high PD; ED of 17.4 Wh/kg at PD of 5230 W/kg for OMC-HNO3, ED of 16.6 Wh/kg at PD of 4600 W/kg for OMC-APS. On the other hand, for Maxsorb, the oxidation decreases ED within the higher PD compared to the original Maxsorb, while the ED values of Maxsorb-HNO3 and Maxsorb-APS are higher than that of Maxsorb within the range of lower PD. These results show evidence that OMC electrodes are applicable in fabricating supercapacitor devices.
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3.7 Gravimetric vs. volumetric capacitances Aside from the gravimetric capacitance, the volumetric capacitance of active materials is a useful
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guide to assess their quality for applications, although care must be taken in extrapolating these values to fully packaged cells including other critical components such as hermetic packaging, connectors, and so on. Figure S3 (Supplementary Content) shows the gravimetric and volumetric capacitances at
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different potential scan rates. In principle, the capacitance of EDLCs should be proportional to the specific surface area of active materials. Therefore, an activation treatment is an effective tool and
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commonly used to increase the specific surface area, leading to improvement of capacitance. Thus, in this study, the OMC was activated by using potassium hydroxide to improve its surface area and porosity. The ordered mesostructure was retained after activation, while the mass was decreased by about 30 wt%. There is no large difference in unit cell parameter, mesopore size, and mesopore volume
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between OMC and activated OMC (Table 1). The specific surface area and total pore volume of OMC largely increased from 520 m2/g to 1020 m2/g and from 0.54 cc/g to 0.81 cc/g, respectively, by the activation. Comparing with OMC, the activated OMC shows a superior gravimetric capacitance at all
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potential scan rates (Figure S3A). However, the volumetric capacitance of the activated OMC is comparable to that of the original OMC and lower than the oxidized OMC electrodes (Figure S3B). In
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fact, the density of the activated OMC electrodes is noticeably lower than that of the original and oxidized OMC electrodes, which leads to its lower volumetric capacitance. It is noted that the specific surface area is dominated by micropores, suggesting that it is important to understand how to utilize the micropore surface generated by activation. OMC-HNO3 offers superior capacity for energy storage based on both gravimetric and volumetric values.
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While the volumetric capacitance of active materials is significantly more important to precisely estimate the cell volume rather than the gravimetric capacitance, the majority of studies do not report
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volumetric capacitance of the studied materials. Figure 10 shows a comparison of the gravimetric and volumetric capacitive performances of this work with literature values. Unfortunately, the direct comparison is difficult because the other electrodes were used at different conditions. Carbon inverse
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opals prepared by using close-packed microspheres as template show comparable to or even higher gravimetric capacitances than ordered mesoporous carbons. However, their volumetric capacitances are
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significantly lower than those of ordered mesoporous carbons because of their low density due to the existence of macropores. Zeolite-templated carbons show both relatively high gravimetric and volumetric capacitances compared to the other templated carbons including carbon inverse opals and ordered mesoporous carbons, which is attributed to their high gravimetric (~4100 m2/g) and volumetric
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surface areas (~1800 m2/cm3). Another possible contribution may come from their unique structural periodicity derived from the original zeolite. On the whole it can be seen that ordered mesoporous carbons exhibit lower volumetric capacitances. This means, in practice, the improvement of
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capacitance demands much of the effort to increase the surface area. However, it is an important matter how to increase the “effective” surface area for the formation of the electric double layer. The oxidized
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OMC electrodes in this study exhibit both higher gravimetric and volumetric capacitances in spite of their modest surface areas (Table 1). By controlling the types and quantity of oxygen-containing functional groups, the capacitance of OMC may be further enhanced.
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4. CONCLUSIONS Ordered mesoporous carbon was prepared by a soft-templating method and surface-modified by wet
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oxidation to investigate the effect of surface modification on the electrochemical performance. Our study, based on comparisons with oxidized commercial activated microporous carbons, reveals that the oxygen functionalization combined with mesoporous structure improves the overall capacitive
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performance of the supercapacitor. On the other hand, oxygen functionalization leads to decreased capacitance for the activated microporous carbon at higher power densities. The OMC-HNO3 exhibits
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both excellent gravimetric (290 F/g) and volumetric (300 F/cm3) capacitances at 0.1 A/g and retains over 70% of the capacitance at high current density of 5 A/g. The device demonstrates remarkable performance with ED of 17.4 Wh/kg, PD of 5.2 kW/kg, and excellent cycle life. We envision the templated nanoporous carbons with controlled surface functionalities to be useful in a broad range
ACKNOWLEDGEMENTS
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applications which request long cycle life and high power and energy densities.
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S.T. thanks the Nippon Sheet Glass Foundation for Materials Science and Engineering, and Kansai
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University’s Overseas Research Program for the year of 2014.
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Table 1 Structural characteristics a0a
Db
SBET
Vmeso
Vmicro
Vtotal
(nm)
(nm)
(m2/g)
(cc/g)
(cc/g)
(cc/g)
OMC
10.4
6.2
520
0.42
0.12
0.54
OMC-HNO3
10.3
6.2
430
0.36
0.10
0.46
OMC-APS
10.3
6.2
400
0.36
0.07
0.43
activated OMC
10.4
6.8
1020
0.49
0.32
0.81
Maxsorb
—
—
940
—
0.35
0.35
Maxsorb-HNO3
—
—
840
—
0.35
0.35
Maxsorb-APS
—
—
730
—
0.31
0.31
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a
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Sample
a0: unit cell parameter calculated by the formula a0 = 2d/√3 assuming a hexagonal unit cell, where d is
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curves.
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the d-spacing estimated from PXRD. b D: mesopore diameter estimated from the pore size distribution
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Figure captions
Figure 2 TEM images of OMC before and after oxidation.
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(C) of OMC before and after oxidation.
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Figure 1 PXRD patterns (A), nitrogen adsorption/desorption isotherms (B), and pore size distributions
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Figure 3 FTIR spectra of OMC (A) and Maxsorb (B) before and after oxidation.
Figure 4 Concentration of oxygen-containing functional groups for OMC and Maxsorb before and
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after oxidation.
Figure 5 Water adsorption isotherms on OMC, OMC-HNO3, and OMC-APS at 298 K.
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Figure 6 CV profiles of OMC (A) and Maxsorb (B) before and after oxidation at potential scan rate of
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1 mV/s in 1 M sulfuric acid aqueous electrolyte.
Figure 7 Galvanostatic charge–discharge profiles of OMC (A) and Maxsorb (B) before and after oxidation at current density of 0.1 A/g (solid line) and 5 A/g (dashed line) in 1 M sulfuric acid aqueous electrolyte.
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Figure 8 Capacitance at different current densities for OMC and Maxsorb before and after oxidation in
galvanostatic charge–discharge experiments.
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1 M sulfuric acid aqueous electrolyte. The gravimetric capacitances were determined from
Figure 9 Ragone plots of OMC (A) and Maxsorb (B) before and after oxidation compared with survey
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data summarized by Simon et al. in 2008 [1]. ED is the capacity to do work; PD is the rate at which
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work is done. The ED and PD were determined from galvanostatic charge–discharge experiments.
Figure 10 Gravimetric and volumetric capacitive performances of our OMC electrodes compared with the other carbon electrodes reported in the literature. The literature values are at low potential scan rates below 10 mV/s or at relatively low current densities below 0.5 A/g in aqueous electrolytes. Open
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diamond: ordered mesoporous carbons [31–61], closed circle: carbon inverse opals [62,63], open circle: monodispersed carbon sphere [64], open triangle: highly activated carbon [65], open cross: zeolite-templated carbons [66], double circle: others including carbonized metal-organic framework
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[67–70]. The dashed straight line indicates that the particle density is 1 g/cm3.
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performance.
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Ordered mesoporous carbon was surface-modified by wet oxidation. Systematic investigation of EDLC performances, based on comparisons with oxidized microporous carbons. Oxygen functionalization combined with ordered mesostructure improves the EDLC
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Surface Modification of Soft-Templating Ordered Mesoporous Carbon for Electrochemical Supercapacitors Shunsuke Tanaka,*,a,b Hiroki Fujimoto,a Joeri F. M. Denayer,c Manabu Miyamoto,d Yasunori Oumi,e
a
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and Yoshikazu Miyakea,b
Department of Chemical, Energy and Environmental Engineering, Faculty of Environmental and
b
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Urban Engineering, Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680 JAPAN Organization for Research and Development of Innovative Science and Technology (ORDIST), Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680 JAPAN c
Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, BELGIUM
Department of Chemistry and Biomolecular Science, Gifu University, 1-1 Yanagido, Gifu 501-1193 JAPAN
e
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d
Division of Instrument Analysis, Life Science Research Center, Gifu University, 1-1 Yanagido Gifu,
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501-1193, JAPAN
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* Corresponding author: Tel/Fax: +81-6-6368-0851; E-mail: [email protected] (S. Tanaka)
Supplementary Data Contents Page
Figure S1. TGA profiles of OMC before and after oxidation.
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Figure S2. Cycle life of OMC electrodes before and after oxidation.
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Figure S3. Gravimetric and volumetric capacitances for OMC.
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Figure S1. TGA profiles of OMC before and after oxidation.
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Figure S2. Cycle life of OMC electrodes before and after oxidation at potential scan rate of 100 mV/s in 1 M sulfuric acid aqueous electrolyte. The gravimetric capacitances were determined from CV
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Figure S3. Gravimetric (A) and volumetric (B) capacitances at different potential scan rates for oxidized and activated OMC electrodes in 1 M sulfuric acid aqueous electrolyte. The gravimetric and
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volumetric capacitances were determined from CV experiments.
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S1387-1811(15)00344-3
DOI:
10.1016/j.micromeso.2015.06.017
Reference:
MICMAT 7176
To appear in:
Microporous and Mesoporous Materials
Received Date: 16 April 2015 Revised Date:
10 June 2015
Accepted Date: 11 June 2015
Please cite this article as: S. Tanaka, H. Fujimoto, J.F.M. Denayer, M. Miyamoto, Y. Oumi, Y. Miyake, Surface modification of soft-templated ordered mesoporous carbon for electrochemical supercapacitors, Microporous and Mesoporous Materials (2015), doi: 10.1016/j.micromeso.2015.06.017. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Title: Surface modification of soft-templated ordered mesoporous carbon for electrochemical supercapacitors Authors: Shunsuke Tanaka (submitter), Hiroki Fujimoto, Joeri F. M. Denayer, Manabu Miyamoto, Yasunori Oumi, Yoshikazu Miyake
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Graphical Abstract
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Surface modification of soft-templated ordered
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mesoporous carbon for electrochemical supercapacitors Shunsuke Tanaka,a,b* Hiroki Fujimoto,a Joeri F. M. Denayer,c Manabu Miyamoto,d Yasunori Oumi,e
a
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and Yoshikazu Miyakea,b
Department of Chemical, Energy and Environmental Engineering, Faculty of Environmental and
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Urban Engineering, Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680 JAPAN b
Organization for Research and Development of Innovative Science and Technology (ORDIST), Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680 JAPAN
c
Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel,
d
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BELGIUM
Department of Chemistry and Biomolecular Science, Gifu University, 1-1 Yanagido, Gifu 501-1193 JAPAN
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Division of Instrument Analysis, Life Science Research Center, Gifu University, 1-1 Yanagido Gifu,
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e
501-1193, JAPAN * Corresponding author
Tel: +81-6-6368-0851; Fax: +81-6-6388-8869; E-mail: [email protected] (S. Tanaka)
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ABSTRACT
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The increasing demand for energy has triggered numerous research efforts for the development of electricity storage devices consisting of templated nanoporous carbons. In this study, ordered mesoporous carbon with a modest specific surface area (520 m2/g) was prepared by a soft-templating method and surface-modified by a wet oxidation using nitric acid and ammonium peroxodisulfate.
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More oxygen-containing functional groups were introduced on the surface by oxidation using
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ammonium peroxodisulfate rather than nitric acid. Our study systematically investigates the effect of surface modification on electrochemical performance, based on comparisons with oxidized commercial activated microporous carbons. Oxygen functionalization leads to an increased capacitance for the ordered mesoporous carbon and a decreased capacitance for the activated microporous carbon at higher power densities. The ordered mesoporous carbon oxidized by nitric acid demonstrates excellent
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electrochemical performance with gravimetric capacitance of 290 F/g and volumetric capacitance of 300 F/cm3 at 0.1 A/g and retains over 70% of the capacitance at high current density of 5 A/g even though its surface area remained moderate (430 m2/g). The device demonstrates remarkable
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performance with an energy density of 17.4 Wh/kg, power density of 5.2 kW/kg, and excellent cycle
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life.
KEYWORDS
Ordered mesoporous carbon, Wet oxidation, Oxygen functionality, Water wettability, Supercapacitor
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1. INTRODUCTION Electrochemical double layer capacitors (EDLC) or supercapacitors represent a unique type of high
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power energy storage devices, where the capacitance arises from the charge separation at an electrode– electrolyte interface [1,2]. The energy storage in EDLC is based on the electro-adsorption of electrolyte ions on the large surface area of electrically conductive porous electrodes, most commonly porous
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carbons. EDLC has been widely used for several applications, such as an emergency or short-term secondary power source, copy machines, power booster for construction-equipments, since its
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commercialization in the 1970s. However, the main limitation of EDLC is their lower energy density than lithium-ion batteries. Thus, the improvement of their energy density is of significant for further expansion of their application fields.
The capacitance of electrically conductive porous electrodes should be in principle proportional to
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the surface area. However, many studies have shown an unfavorable divergence from this linear relation. The energy storage characteristics of EDLC are strongly affected by the capability of porous carbon to adsorb a large quantity of electrolyte ions under an applied potential and attract the ions
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closer to the pore walls. Therefore, aside from the surface area, the performance of EDLC strongly depends on the pore structure, pore size distribution, and surface functionality [3–9]. In addition, the
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capacitive rate performance is determined by how fast the electrolyte ions can travel within the pores. In order to achieve a higher energy density, it is important to appropriately design the pore structure. The templated nanoporous carbons are expected to draw a simple relationship between the structural characteristics of porous carbon and the EDLC performance [10,11]. Surface modification of carbon materials is effective not only for their dispersibility in various solvent [12] but also to improve the adsorptive performance as adsorbent [13]. A variety of
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functionalities can be introduced upon the surface of carbon materials by oxidative treatment involving dry or wet oxidation, plasma, and electrochemical treatments. In dry oxidation, oxygen, ozone, and
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carbon dioxide are usually used as gaseous oxidizing agents. Wet oxidation involves the use of nitric acid, sulfuric acid, phosphoric acid, ammonium peroxodisulfate, alone or in the combination with hydrogen peroxide, sodium hypochlorite, and so on [14–20].
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In this study, the ordered mesoporous carbon (OMC) with channel pore structure was prepared through the soft-templating method. The soft-templated OMC contained a small quantity of functional
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groups on its surface. Pore wettability, surface hydrophobic/hydrophilic balance, in general, depends on the surface functional groups and determine penetration of guest species into the pore system. The pore surface of the OMC was therefore modified by the wet oxidation. The effects of the wet oxidation on the structural characteristics and electrochemical performances of the OMC electrodes were
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investigated by comparisons with oxidized commercial activated carbons. The structural properties were characterized by the combination of powder X-ray diffraction (PXRD) and nitrogen sorption measurements. The surface chemical properties were characterized by the combination of Fourier
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transform infrared (FTIR) measurement, thermogravimetric analysis, and the Boehm titration. The electrochemical properties were characterized by using cyclic voltammetry (CV) and galvanostatic
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charge–discharge techniques.
2. EXPERIMENT 2.1 Materials
Resorcinol, phloroglucinol, 37 wt% formaldehyde, hydrochloric acid, nitric acid, ammonium peroxodisulfate (APS), potassium hydroxide, sodium hydroxide, sodium carbonate, sodium bicarbonate,
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and ethanol were purchased from Wako Pure Chemical Industries and used as received. Pluronic F127 was
purchased
from
Sigma-Aldrich
Chemical
Co.
and
used
as
received.
Polyflon
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polytetrafluoroethylene (F-104; PTFE) was received from Daikin Industries, Ltd. A commercial
of comparison.
2.2 Synthesis of ordered mesoporous carbon
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Maxsorb activated carbon was received from Kansai Coke & Chemical Co. Ltd. and used for the sake
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Resorcinol and phloroglucinol were completely dissolved in ethanol/water solution, hydrochloric acid was added, and the solution was stirred for 15 min. Pluronic F127 was then added, and after it was completely dissolved, formaldehyde was added. The final molar composition of the precursor solution was 3 resorcinol: 1 phloroglucinol: 9 formaldehyde: 0.02 Pluronic F127: 0.1 hydrochloric acid: 40
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water: 100 ethanol. The solutions were left for 3 days at room temperature, during which they separated into two phases. The upper clear phase was discarded; the lower dark brown phases were thermopolymerized in air at 100 °C for 3 h. The resultant dark brown deposition was carbonized in a
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tubular furnace with nitrogen flow (100 mL/min). The furnace was heated at a ramping rate of 1
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°C/min below 400 °C and 5 °C/min above 400 °C, then kept at 800 °C for 3 h.
2.3 Surface modification
Ordered mesoporous carbon was subsequently subjected to oxidation treatment with different oxidizing agents. For a typical run, 0.3 g of ordered mesoporous carbon was mixed with a 50 mL solution of 1M nitric acid or 1M APS, then heated at 90 °C for 6 h under stirring and refluxing conditions to obtain the surface-modified carbon samples, designated as OMC-x, where x denotes to the
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oxidizing agent, HNO3 or APS. The resultant sample was extensively washed with deionized water until the pH of the filtrate water became neutral, then dried at 100 °C in air. The same oxidation
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treatment was also carried out for the commercial Maxsorb activated carbon.
2.4 Chemical activation
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The post chemical activation treatment was also performed to compare the treatment effect of oxidation with that of activation. For a typical run, 0.3 g of ordered mesoporous carbon was
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impregnated with a 15 mL solution of 1M potassium hydroxide followed by water evaporation at 120 °C. The activation process was performed with nitrogen flow (100 mL/min) at 800 °C, with a ramping rate of 10 °C/min, for 1 h. The resultant sample was washed with 0.1 M hydrochloric acid solution followed by deionized water until the pH of the filtrate water became neutral, then dried at 100
Maxsorb activated carbon.
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°C in air. The same post chemical activation treatment was also carried out for the commercial
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2.5 Structural characterization
PXRD patterns were recorded on a RINT-TTR III diffractometer (Rigaku, Japan) using Cu Kα
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radiation; the copper anode was operated at 40 kV and 20 mA. Nitrogen adsorption/desorption isotherms of the samples were measured at 77 K using a BELSORP-max instrument (Bel Japan). The samples were degassed at 200 °C under vacuum. Brunauer–Emmett–Teller (BET) model surface areas, SBET, were calculated from the nitrogen adsorption branches. A part of the nitrogen adsorption isotherm in the P/P0 range 0.10–0.30 was fitted to the BET equation to estimate the SBET. The pore size distributions were calculated from adsorption branch of isotherm using the Barrett–Joyner–Halenda
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(BJH) model. The total pore volumes, Vtotal, were calculated on the basis of the amount of nitrogen adsorbed at P/P0 = 0.99. The micropore volumes, Vmicro, were calculated from the αs-plot method. The
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mesopore volumes were calculated by subtracting the value of Vmicro from that of Vtotal. Transmission electron microscope (TEM) images of the samples were recorded on a JEM-2010 microscope (JEOL)
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at an acceleration voltage of 200 kV.
2.6 Surface chemical characterization
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FTIR spectra were recorded on an IRAffinity-1 spectrometer (Shimadzu, Japan) using KBr pellet method. Thermogravimetric curves were recorded on a DTG-60H (Shimadzu, Japan) with nitrogen flow (100 mL/min) at a ramping rate of 5 °C/min. The surface functional groups were determined according to the Boehm titration method [21,22]. For a typical run, 0.1 g of dried active materials were
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added in the vials containing 100 mL of standard base solution as follows: 10 mmol/l sodium hydroxide, 0.5 mmol/l sodium carbonate, and 0.5 mmol/l sodium bicarbonate. The vials were sealed and shaken at room temperature for 48 h. Then, the solutions were filtered and 10 mL of each solution
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was taken for the subsequent titration. The filtration procedure was quite sensitive and done with extra care. The blank samples were analysed as well. All experiments were run in triplicates. The number of
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acidic sites was determined under the assumptions that (a) sodium hydroxide neutralizes carboxylic, lactonic, and phenolic groups, (b) sodium carbonate neutralizes carboxylic and lactonic groups, and (c) sodium bicarbonate neutralizes carboxylic groups. Adsorption isotherms of water were measured at 298 K using a BELSORP-18 (Bel Japan). The samples were degassed at 250 °C under vacuum for 10 h.
2.7 Electrochemical measurements
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The electrodes were prepared by mixing 80 wt% dried active material, 10 wt% carbon black, and 10 wt% PTFE. The resulting composite paste was made into an electrode pellet of about 0.2 mm in
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thickness. The electrode was cut into 1 cm by 1 cm pieces. The electrode disk was pressed and attached with Pt collector to reduce the interfacial resistance. The electrochemical performance was characterized by CV and galvanostatic charge–discharge on an HX5000 electrochemical workstation
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(Hokuto Denko, Japan). The experiments were carried out at 25 °C within a climate chamber using a three electrode cell in an electrolyte of 1 M sulfuric acid aqueous solution. An Ag/AgCl was used as a
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reference electrode. The gravimetric capacitance, Cm (F/g), of the active material was determined from CV experiments using the following equation: (1)
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where m is the mass of active material (g), v is the potential scan rate (mV/s), E1 and E2 are the cutoff potentials (V) in CV, (E2–E1) is the potential window width, i(E) is the instantaneous response current (A). The gravimetric capacitance, Cm (F/g), of the active material was determined from galvanostatic
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charge–discharge experiments using the following equation:
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(2)
where i is the discharge current (A), ∆t is the discharge time (s), ∆E is the voltage change (V) during discharge excluding the portion of iR drop. The volumetric capacitance, Cv (F/cm3), of the active material was calculated using the following equation: (3)
where ρ is the particle density calculated by
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(4)
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where ρcarbon is the true density of carbon (2 g/cm3) [11]. The energy density (ED; Wh/kg) and power density (PD; W/kg) of the cell were calculated using the following equations:
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(5)
3. RESULTS AND DISCUSSION 3.1 Structural characteristics
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(6)
The carbonized ordered mesoporous carbon looks like rocky fragments and can be easily crushed
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into a fine powder. Since the particle size of active material can affect the inner electrical resistance, the particle size was adjusted to be almost the same by grinding and sieving process. Figure 1A shows PXRD patterns of ordered mesoporous carbon before and after oxidation. All the PXRD patterns
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exhibit a reflection peak at around 1.0°, which can be attributed to mesostructure. This result indicates
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that the ordered mesostructure was still retained after oxidation. On the other hand, the second order reflection peak became weaker after oxidation, suggesting that oxidation deteriorated ordered mesostructure to some extent. The peak position did not shift after oxidation, indicating that a structural shrinkage did not occur during oxidation. Figure 2 shows TEM images of ordered mesoporous carbon before and after oxidation. Three dimensional wormhole-like mesostructure remained even after oxidation treatment. The detailed mesostructure of ordered mesoporous carbon was described elsewhere [23,24].
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The porous structures of the ordered mesoporous carbons were investigated by nitrogen adsorption/desorption measurements shown in Figure 1B. All the samples show typical type-IV curves
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with hysteresis loops and capillary condensation, ascribed to the uniform mesopores inside the carbons. As shown in Figure 1C, the mesopore sizes of OMC-HNO3 and OMC-APS were 6.2 nm and the same as that of OMC. The PXRD and nitrogen sorption results strongly indicate that no structural shrinkage
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did occur during oxidation. However, both surface area and pore volume slightly decreased after oxidation. The formation of oxygen-containing surface functional groups may contribute to the
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reduction of surface area and pore volume, which is further discussed in combination with surface chemical characteristics (Section 3.2). Their structural characteristics are summarized in Table 1.
3.2 Surface chemical characteristics
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The wet oxidation for carbon materials can significantly affect the surface functionality. FTIR analysis was performed to investigate chemical composition changes on the channel surface of the ordered mesoporous carbon before and after oxidation. Figure 3A shows the FTIR spectra of OMC,
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OMC-HNO3, and OMC-APS. While no significant band was observed in OMC, the presence of different types of oxygen functionalities in OMC-HNO3 and OMC-APS was confirmed at 1200 cm−1
AC C
(ν(C−O) in esters, ether or phenol groups), 1550 cm−1 (ν(C=C) in aromatics group), 1700 cm−1 (ν(C=O) in carbonyl group), and 3000−4000 cm−1 (ν(O−H) in hydroxyl group) [25]. As shown in Figure 3B, the concentration of oxygen-containing functional groups in Maxsorb is higher than that of OMC even before oxidation because of its high surface area (Table 1). For both OMC and Maxsorb, more oxygencontaining functional groups were introduced on the surface by oxidation using APS rather than HNO3.
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Thermogravimetric analysis shows a total weight loss of about 20% and 40% at 800 °C for OMC and acid-oxidized OMC samples (OMC-HNO3 and OMC-APS), respectively (Supplementary Content,
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Figure S1). The acid-oxidation led to breaking C−C bonds and formation of oxygen-containing functional groups, it is therefore not too surprising that the total weight loss of about 40% occurs for OMC-HNO3 and OMC-APS. The weight loss ranging from 150 to 450 °C (step 1 in Figure S1) can be
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mainly assigned to the dehydration of adjacent carboxyl and/or phenolic hydroxyl groups, consequently resulting in the formation of carboxylic anhydrides and lactones. Another possible contribution can be
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attributed to the decomposition of carboxylic groups. The weight loss at around 550 °C (step 2) is believed to be associated with the decomposition of carboxylic anhydrides and lactones, which are more stable than carboxyl groups. The weight loss above 600 °C (step 3) is believed to be associated with the decomposition of phenol, quinone, and ether groups.
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The Boehm titration method provides quantitative identification of the oxygen-containing surface functional groups (Figure 4). The concentrations of functional groups for OMC were in the order phenolic group (0.059 mmol/g) > lactonic group (0.011 mmol/g) > carboxylic group (0.0049 mmol/g).
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This order corresponds with the general trends of decomposition of functional groups on carbon
AC C
[26,27]. Of the three types of functional group the phenolic group decomposes at the highest temperature and the carboxylic group decomposes at the lowest temperature. The oxidation using APS yielded larger concentrations of the oxygen-containing functional groups on the surface compared to the oxidation using HNO3. This indicates that APS has a higher capacity to break C−C bonds and form the oxygen-containing functional groups compared to HNO3. The concentrations of functional groups in Maxsorb samples are higher than that of OMC samples because of their high surface area. There is no large difference in the composition ratio of phenolic, lactonic, and carboxylic group between
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Maxsorb-HNO3 and Maxsorb-APS. On the other hand, of particular interest is that OMC-APS has a higher ratio of lactonic group compared with OMC-HNO3.
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Figure 5 shows the water vapor adsorption isotherms of OMC, OMC-HNO3, and OMC-APS. All the OMC samples exhibit a distinct two-step uptake. The first and second steps can be attributed to adsorption in micropores and mesopores, respectively [28,29]. The OMC shows the first- and second-
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step uptakes at a relative pressure of about 0.5 and 0.95, respectively, suggesting that the pore surface is hydrophobic or slightly hydrophilic. On the other hand, the two step uptakes occur at lower relative
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pressures after oxidation. These shifts in the relative pressure was caused by a surface hydrophilicity due to the introduction of hydrophilic oxygen-containing functional groups. There is no large difference in first-step uptake between OMC-HNO3 and OMC-APS. The second steep steps corresponding to the capillary condensation in mesopores result from the narrow pore size distributions
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estimated by nitrogen adsorption/desorption measurements. While there is no large difference in mesopore size between OMC-HNO3 and OMC-APS (Figure 1C), the pressure of second-step for OMC-APS is slightly lower than that for OMC-HNO3. Combined with Boehm titration, this result
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indicates that the mesopore surface of OMC-APS is more hydrophilic than that of OMC-HNO3. These
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porous carbons were used to investigate the effect of surface functionality on the EDLC performance.
3.3 Cyclic voltammetric behaviors The CV profiles of initial efforts to estimate the electrochemical performance of OMC samples are shown in Figure 6A. It can be seen that the CV curves exhibit a symmetric rectangular shape without obvious redox peaks for OMC, indicating typical EDLC behavior [30]. On the other hand, the CV profiles of OMC-HNO3 and OMC-APS display the pair of anodic and cathodic peaks within 0.3−0.5 V,
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which can be attributed to the pseudo-Faradic reactions involving the oxygen-containing functional groups [15–17]. The original OMC shows the gravimetric capacitance of 178 F/g at the potential scan
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rate of 1 mV/s and 58% retained ratio at 50 mV/s. After oxidation, the capacitance increased to 270 F/g and 232 F/g at 1 mV/s for OMC-HNO3 and OMC-APS, respectively, in spite of their decreasing surface area.
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Figure 6B shows the CV profiles of Maxsorb samples. The CV profile of Maxsorb had a distorted rectangular shape, suggesting that the sulfate ion has a lower mobility than that of the smaller size
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proton in the micropores. On the other hand, Maxsorb-HNO3 and Maxsorb-APS showed a better, more defined rectangular shape with pseudo-Faradic redox peaks compared to the original Maxsorb. After oxidation using HNO3, the capacitance increased from 237 F/g to 283 F/g at 1 mV/s. However, the capacitance decreased to 226 F/g at 1 mV/s for Maxsorb-APS in spite of high concentration of the
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oxygen-containing functional groups.
An improved capacitive performance of surface-oxidized carbons in aqueous electrolyte systems is believed to be caused not only by the pseudo-capacitive reactions but also by the wettability with
EP
electrolyte. The introduction of hydrophilic oxygen-containing functional groups improves wettability of carbon materials with aqueous electrolyte. On the other hand, these functional groups can be divided
AC C
into two main categories, depending on the nature of the C−O bonds: (i) electrochemical inactive groups (lactonic) and (ii) surface groups with surface acidity and electrochemical redox activity (carboxylic and phenolic) as follows [19,20]:
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Thus, the capacitances of carbons oxidized using HNO3 (OMC-HNO3 and Maxsorb-HNO3) increase more than those of carbons with higher concentrations of lactonic groups (OMC-APS and Maxsorb-
increasing the capacitance, more than for microporous carbon.
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3.4 Long-term cycling stability
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APS). In addition, surface modification for ordered mesoporous carbon is shown to be effective in
The capacitance decreased with increasing potential scan rate. However, the capacitance decrease
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was less sensitive to potential for ordered mesoporous carbons as compared to microporous carbons. OMC-HNO3 and OMC-APS show up to 70% higher retained capacitance and maintain a high capacitance of 193 F/g and 166 F/g, respectively, at the scan rate of 50 mV/s. On the other hand, the microporous carbons show low retained capacitance (49%, 59%, and 35% retained ratio at 50 mV/s for
diffuse within the micropores.
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Maxsorb, Maxsorb-HNO3, and Maxsorb-APS, respectively), suggesting that the ions cannot easily
For further understanding of the electrochemical performances, the long-term cycling stability of
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OMC, OMC-HNO3, and OMC-APS was investigated by repeating a CV test at potential scan rate of 100 mV/s as shown in Figure S2 (Supplementary Content). All the OMC samples exhibit excellent
AC C
long-term cycling stability with more than 95% retention after 5000 cycle tests.
3.5 Galvanostatic charging/discharging behaviors Galvanostatic charge–discharge measurements are commonly used to quantify the electrochemical performance. As show in Figure 7A, all the OMC electrodes can be charged and discharged smoothly without obvious iR drop, and exhibit stable capacitive performance at low current density of 0.1 A/g.
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The feature indicates the typical Coulombic efficiency and ideal capacitive behavior. The area surrounded by charge–discharge curves is apparently increased after oxidation, indicating that the
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capacitances of OMC-HNO3 and OMC-APS are higher than that of the original OMC. This indicates that the electrolyte ions penetrates the inner surface sites of OMC-HNO3 and OMC-APS electrodes more deeply than the original OMC electrode.
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On the other hand, a sharp change in potential, which is induced by an equivalent series resistance iR drop and polarization of the electrode, was clearly observed at high current density of 5 A/g for OMC-
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APS in particular. This indicates that the capacitances of OMC-HNO3 and OMC-APS electrodes are consisted of the electric double layer formation and pseudo-Faradic reaction due to the existence of oxygen-containing functional groups on the surface. The iR drop of OMC-HNO3 was much smaller than that of OMC-APS, which makes it a quite favourable candidate for the electrode of high power
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capacitor.
As shown in Figure 7B, the charge–discharge profiles of Maxsorb samples do not show typical triangle symmetrical distribution even at 0.1 A/g. Although the surface area of OMC-HNO3 is much
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smaller than that of Maxsorb-HNO3, the capacitance of OMC-HNO3 is comparable to or even higher than that of Maxsorb-HNO3 at the current densities below 0.5 A/g. This indicates that the existence of
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mesopores benefits the penetration of ions. The capacitance variation with the current density is an important issue in high-rate capacitors. Figure 8 shows the current density dependence of the gravimetric capacitance. The capacitance decreased with increasing current density. However, the capacitance decrease was less sensitive for all the OMC samples and the original Maxsorb. This indicates that surface oxidation is more effective for mesoporous OMC rather than microporous Maxsorb. The homogeneous mesopore network offers much higher mobility of ions than micropore
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network. Thus, OMC-HNO3 and OMC-APS show higher retained capacitance than the original OMC even at higher current density and scan rate (Figure 8 and Supplementary Content, Figure S3) because
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they have relatively large mesopore size, compared to previously reported mesoporous carbons, allowing fast ion transport. On the other hand, the oxidized Maxsorb electrodes show unfavorable
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capacitive performance at higher current density.
3.6 Ragone plot
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The operational performance and efficiency of electrical energy storage devices are determined by two most important parameters; the specific energy density (ED) and specific power density (PD). In order to demonstrate the overall performance of the electrodes, the Ragone plots of ED versus PD are plotted and shown in Figure 9. The ED and PD values were derived from the galvanostatic charge–
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discharge profiles at different current densities. It is seen that the electrochemical capacitors are highpower devices with limited energy storage capacity. In addition, the voltage delivered depends strongly on the state of discharge.
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For OMC, the oxidation improves both ED and PD performances. A maximum ED of 25.8 Wh/kg was achieved by the OMC-HNO3 at PD of 53.9 W/kg. In addition, the OMC-HNO3 and OMC-APS
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resulted in excellent ED retention at high PD; ED of 17.4 Wh/kg at PD of 5230 W/kg for OMC-HNO3, ED of 16.6 Wh/kg at PD of 4600 W/kg for OMC-APS. On the other hand, for Maxsorb, the oxidation decreases ED within the higher PD compared to the original Maxsorb, while the ED values of Maxsorb-HNO3 and Maxsorb-APS are higher than that of Maxsorb within the range of lower PD. These results show evidence that OMC electrodes are applicable in fabricating supercapacitor devices.
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3.7 Gravimetric vs. volumetric capacitances Aside from the gravimetric capacitance, the volumetric capacitance of active materials is a useful
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guide to assess their quality for applications, although care must be taken in extrapolating these values to fully packaged cells including other critical components such as hermetic packaging, connectors, and so on. Figure S3 (Supplementary Content) shows the gravimetric and volumetric capacitances at
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different potential scan rates. In principle, the capacitance of EDLCs should be proportional to the specific surface area of active materials. Therefore, an activation treatment is an effective tool and
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commonly used to increase the specific surface area, leading to improvement of capacitance. Thus, in this study, the OMC was activated by using potassium hydroxide to improve its surface area and porosity. The ordered mesostructure was retained after activation, while the mass was decreased by about 30 wt%. There is no large difference in unit cell parameter, mesopore size, and mesopore volume
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between OMC and activated OMC (Table 1). The specific surface area and total pore volume of OMC largely increased from 520 m2/g to 1020 m2/g and from 0.54 cc/g to 0.81 cc/g, respectively, by the activation. Comparing with OMC, the activated OMC shows a superior gravimetric capacitance at all
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potential scan rates (Figure S3A). However, the volumetric capacitance of the activated OMC is comparable to that of the original OMC and lower than the oxidized OMC electrodes (Figure S3B). In
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fact, the density of the activated OMC electrodes is noticeably lower than that of the original and oxidized OMC electrodes, which leads to its lower volumetric capacitance. It is noted that the specific surface area is dominated by micropores, suggesting that it is important to understand how to utilize the micropore surface generated by activation. OMC-HNO3 offers superior capacity for energy storage based on both gravimetric and volumetric values.
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While the volumetric capacitance of active materials is significantly more important to precisely estimate the cell volume rather than the gravimetric capacitance, the majority of studies do not report
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volumetric capacitance of the studied materials. Figure 10 shows a comparison of the gravimetric and volumetric capacitive performances of this work with literature values. Unfortunately, the direct comparison is difficult because the other electrodes were used at different conditions. Carbon inverse
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opals prepared by using close-packed microspheres as template show comparable to or even higher gravimetric capacitances than ordered mesoporous carbons. However, their volumetric capacitances are
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significantly lower than those of ordered mesoporous carbons because of their low density due to the existence of macropores. Zeolite-templated carbons show both relatively high gravimetric and volumetric capacitances compared to the other templated carbons including carbon inverse opals and ordered mesoporous carbons, which is attributed to their high gravimetric (~4100 m2/g) and volumetric
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surface areas (~1800 m2/cm3). Another possible contribution may come from their unique structural periodicity derived from the original zeolite. On the whole it can be seen that ordered mesoporous carbons exhibit lower volumetric capacitances. This means, in practice, the improvement of
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capacitance demands much of the effort to increase the surface area. However, it is an important matter how to increase the “effective” surface area for the formation of the electric double layer. The oxidized
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OMC electrodes in this study exhibit both higher gravimetric and volumetric capacitances in spite of their modest surface areas (Table 1). By controlling the types and quantity of oxygen-containing functional groups, the capacitance of OMC may be further enhanced.
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4. CONCLUSIONS Ordered mesoporous carbon was prepared by a soft-templating method and surface-modified by wet
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oxidation to investigate the effect of surface modification on the electrochemical performance. Our study, based on comparisons with oxidized commercial activated microporous carbons, reveals that the oxygen functionalization combined with mesoporous structure improves the overall capacitive
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performance of the supercapacitor. On the other hand, oxygen functionalization leads to decreased capacitance for the activated microporous carbon at higher power densities. The OMC-HNO3 exhibits
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both excellent gravimetric (290 F/g) and volumetric (300 F/cm3) capacitances at 0.1 A/g and retains over 70% of the capacitance at high current density of 5 A/g. The device demonstrates remarkable performance with ED of 17.4 Wh/kg, PD of 5.2 kW/kg, and excellent cycle life. We envision the templated nanoporous carbons with controlled surface functionalities to be useful in a broad range
ACKNOWLEDGEMENTS
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applications which request long cycle life and high power and energy densities.
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S.T. thanks the Nippon Sheet Glass Foundation for Materials Science and Engineering, and Kansai
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University’s Overseas Research Program for the year of 2014.
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Table 1 Structural characteristics a0a
Db
SBET
Vmeso
Vmicro
Vtotal
(nm)
(nm)
(m2/g)
(cc/g)
(cc/g)
(cc/g)
OMC
10.4
6.2
520
0.42
0.12
0.54
OMC-HNO3
10.3
6.2
430
0.36
0.10
0.46
OMC-APS
10.3
6.2
400
0.36
0.07
0.43
activated OMC
10.4
6.8
1020
0.49
0.32
0.81
Maxsorb
—
—
940
—
0.35
0.35
Maxsorb-HNO3
—
—
840
—
0.35
0.35
Maxsorb-APS
—
—
730
—
0.31
0.31
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a
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Sample
a0: unit cell parameter calculated by the formula a0 = 2d/√3 assuming a hexagonal unit cell, where d is
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curves.
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the d-spacing estimated from PXRD. b D: mesopore diameter estimated from the pore size distribution
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Figure captions
Figure 2 TEM images of OMC before and after oxidation.
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(C) of OMC before and after oxidation.
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Figure 1 PXRD patterns (A), nitrogen adsorption/desorption isotherms (B), and pore size distributions
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Figure 3 FTIR spectra of OMC (A) and Maxsorb (B) before and after oxidation.
Figure 4 Concentration of oxygen-containing functional groups for OMC and Maxsorb before and
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after oxidation.
Figure 5 Water adsorption isotherms on OMC, OMC-HNO3, and OMC-APS at 298 K.
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Figure 6 CV profiles of OMC (A) and Maxsorb (B) before and after oxidation at potential scan rate of
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1 mV/s in 1 M sulfuric acid aqueous electrolyte.
Figure 7 Galvanostatic charge–discharge profiles of OMC (A) and Maxsorb (B) before and after oxidation at current density of 0.1 A/g (solid line) and 5 A/g (dashed line) in 1 M sulfuric acid aqueous electrolyte.
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Figure 8 Capacitance at different current densities for OMC and Maxsorb before and after oxidation in
galvanostatic charge–discharge experiments.
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1 M sulfuric acid aqueous electrolyte. The gravimetric capacitances were determined from
Figure 9 Ragone plots of OMC (A) and Maxsorb (B) before and after oxidation compared with survey
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data summarized by Simon et al. in 2008 [1]. ED is the capacity to do work; PD is the rate at which
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work is done. The ED and PD were determined from galvanostatic charge–discharge experiments.
Figure 10 Gravimetric and volumetric capacitive performances of our OMC electrodes compared with the other carbon electrodes reported in the literature. The literature values are at low potential scan rates below 10 mV/s or at relatively low current densities below 0.5 A/g in aqueous electrolytes. Open
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diamond: ordered mesoporous carbons [31–61], closed circle: carbon inverse opals [62,63], open circle: monodispersed carbon sphere [64], open triangle: highly activated carbon [65], open cross: zeolite-templated carbons [66], double circle: others including carbonized metal-organic framework
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[67–70]. The dashed straight line indicates that the particle density is 1 g/cm3.
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15 (2009) 5355–5363.
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[9] Y. Liang, D. Wu, R. Fu, Langmuir, 25 (2009) 7783–7785.
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Ordered mesoporous carbon was surface-modified by wet oxidation. Systematic investigation of EDLC performances, based on comparisons with oxidized microporous carbons. Oxygen functionalization combined with ordered mesostructure improves the EDLC
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Surface Modification of Soft-Templating Ordered Mesoporous Carbon for Electrochemical Supercapacitors Shunsuke Tanaka,*,a,b Hiroki Fujimoto,a Joeri F. M. Denayer,c Manabu Miyamoto,d Yasunori Oumi,e
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and Yoshikazu Miyakea,b
Department of Chemical, Energy and Environmental Engineering, Faculty of Environmental and
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Urban Engineering, Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680 JAPAN Organization for Research and Development of Innovative Science and Technology (ORDIST), Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680 JAPAN c
Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, BELGIUM
Department of Chemistry and Biomolecular Science, Gifu University, 1-1 Yanagido, Gifu 501-1193 JAPAN
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Division of Instrument Analysis, Life Science Research Center, Gifu University, 1-1 Yanagido Gifu,
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501-1193, JAPAN
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* Corresponding author: Tel/Fax: +81-6-6368-0851; E-mail: [email protected] (S. Tanaka)
Supplementary Data Contents Page
Figure S1. TGA profiles of OMC before and after oxidation.
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Figure S2. Cycle life of OMC electrodes before and after oxidation.
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Figure S3. Gravimetric and volumetric capacitances for OMC.
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Figure S1. TGA profiles of OMC before and after oxidation.
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Figure S2. Cycle life of OMC electrodes before and after oxidation at potential scan rate of 100 mV/s in 1 M sulfuric acid aqueous electrolyte. The gravimetric capacitances were determined from CV
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Figure S3. Gravimetric (A) and volumetric (B) capacitances at different potential scan rates for oxidized and activated OMC electrodes in 1 M sulfuric acid aqueous electrolyte. The gravimetric and
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