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High microporosity of carbide-derived carbon prepared from a vacuum-treated precursor for energy storage devices
Accepted Manuscript High microporosity of carbide-derived carbon prepared from a vacuum-treated precursor for energy storage devices Sun-Hwa Yeon, Dong-Ha Kim, Sang-Ho Lee, Seong-Sik Nam, Se-Kook Park, Jae Young So, Kyoung-Hee Shin, Chang-Soo Jin, Youngjune Park, Yun Chan Kang PII:
S0008-6223(17)30304-4
DOI:
10.1016/j.carbon.2017.03.063
Reference:
CARBON 11870
To appear in:
Carbon
Received Date: 7 July 2016 Revised Date:
2 March 2017
Accepted Date: 18 March 2017
Please cite this article as: S.-H. Yeon, D.-H. Kim, S.-H. Lee, S.-S. Nam, S.-K. Park, J.Y. So, K.-H. Shin, C.-S. Jin, Y. Park, Y.C. Kang, High microporosity of carbide-derived carbon prepared from a vacuumtreated precursor for energy storage devices, Carbon (2017), doi: 10.1016/j.carbon.2017.03.063. 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.
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High Microporosity of Carbide-Derived Carbon Prepared from a
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Vacuum-Treated Precursor for Energy Storage Devices
Sun-Hwa Yeon a,*, Dong-Ha Kim a,b, Sang-Ho Lee a,c, Seong-Sik Nam a,d, Se-Kook Park a, Jae
Energy Storage Lab., Korea Institute of Energy Research, 102, Gajeong-ro, Yuseong,
Daejeon, 305-343, Republic of Korea b
Department of Energy Engineering, Hanyang University, 222, Wangsimni-ro, Seongdong-gu,
Seoul, 133-791, Republic of Korea.
Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbu-
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c
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a
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Young So a,d, Kyoung-Hee Shin a, Chang-Soo Jin a, Youngjune Parke , Yun Chan Kang c
Gu, Seoul, 136-713, Republic of Korea.
e
Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul, 04763, Republic of Korea
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d
School of Environmental Science and Engineering, Gwangju Institute of Science and
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Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea
*
Corresponding Author : Tel +82-42-860-3763; E-mail: [email protected] (S.H. Yeon) 1
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Abstract
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Carbide-derived carbon (CDC) is an attractive electrode material for electrochemical applications because diverse pore textures and structures can be controlled by changing the properties of the precursor template and the synthesis conditions. Upon the tailoring of the
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micro-pore texture and graphitic structure of CDCs via a pre-vacuum treatment of a carbide precursor, the electrode shows a greatly high increased capacitance under a range of scan
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rates from 2 mV/s to 10 mV/s. The specific capacitance of a CDC chlorinated at 1000 oC from a pre-vacuum-treated at 1700 oC was 150 F/g at 2 mV/s, which is approximately 60 %
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higher than that of a CDC chlorinated at 1000 oC.
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1. Introduction
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Various types of carbonaceous materials have been studied extensively for use as electrodes in energy storage devices for transportation and renewable energy storage applications [1, 2]. Capacitive electrodes (CEs) are used in a broad range of systems related
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to both energy and water infrastructure across the globe. Supercapacitor electrodes, traditionally associated with portable electronics [3], are emerging in grid-level energy
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storage applications [4, 5], in capacitive deionization desalination methods [6], and in energy generation systems [7].
Supercapacitors occupy a region between batteries and dielectric capacitors on the Ragone plot, which describes the relationship between energy and power. They have served
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as a solution to the mismatch between the rapid growth in power as required by devices and the inability of batteries efficiently to discharge power at high rates [8]. This high performance requirement for high power discharge is directly related to the absence of
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charge-transfer resistances, which is a characteristic of battery Faradaic reactions and which subsequently leads to better performance at low temperatures as compared to a battery system.
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Supercapacitors have received considerable attention in recent years due to their high pulse power supply, rapid charging capabilities at high current levels, long cycleability, and safe and reliable performance compared to battery systems [9]. Supercapacitors are usually connected to high-energy batteries or fuel cells to serve as a temporary energy storage device with high-power capabilities because the discharge time is too short for them to be used in independent power sources given their inherently low energy density levels [10, 11]. Improvements have been made in cell packaging and electrolytes [12], but a lack of substantial progress in terms of carbon material designs has limited the energy density, 3
ACCEPTED MANUSCRIPT effectively preventing the wide-scale usage of supercapacitors. Moreover, a new systemic approach beyond merely finding new electrode materials has been introduced. [4, 13-15]. To overcome the limitation of the low energy density density, numerous new active materials for
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electrodes have been developed to realize high capacitance, ranging from carbon-based materials to metal- oxide composites [3, 13, 16-19]. Particularly, for the application of electrochemical double-layer capacitors (EDLCs), the study of electrochemical double layers
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is a key issue when attempting to overcome the present low capacity levels, representing a complex problem associated with the solid/electrolyte interface between the pore diameter of
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the solid electrode and the ion size of the electrolyte [20]. At high EDLC capacitance levels, the preparation of nanoporous carbon with uniform porosity suitable for the size of an ion is important, as ion-size tuned porous carbon can lead to an increase in the capacitance by increasing the confinement of the ions. For example, in the 1.0 M TEA-BF4/ACN electrolyte
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system with a single associated solvent molecule of tetraethylammonium tetrafluoroborate (TEA-BF4) that is a common electrolyte system in EDLCs owing to its high conductivity and the relatively small ion size, the carbon with micro-sized pore has more of an effect on the
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high capacitance performance than that with meso-sized pores. Therefore, the control of micro-sized pores is a more important factor than ensuring meso- or macro-pores in EDLC
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applications [21].
Carbide-derived Carbon (CDC), a carbon material suitable for supercapacitor
applications, has been investigated due to its simple preparation and easy control of structures and properties, such as its graphitization and porosity. CDCs possess high porosity levels, a tunable pore size, and a narrow pore size distribution. A chemical etching method using chlorine gas can generate micro- and mesoporosity, leading to large adsorption capacities of target molecules matching the pore sizes due to the controlled pore size and high specific surface areas [22]. In previous reports, mesoporous CDCs produced by binary and ternary 4
ACCEPTED MANUSCRIPT carbides (B4C and TiC7N3 carbide precursors, respectively) showed significantly improved electrochemical capacities as lithium negative electrodes [23]. In addition, unique cyclic performance imparted by high mesoporosity was demonstrated in an electrode through the
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post-treated CDC by O2 activation [24]. However, most post-activation processes also contribute to greater rates of mesopore development. When considering a commercial electrolyte such as TEABF4/acetonitrile in a CDC electrode, a preparation method which
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increases the degree of microporosity represents an applicable spin-off technology for high storage capacitance in EDLC systems.
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In this work, we demonstrate the significantly improved electrochemical capacitance of the CDCs through control of the micro-pore texture and amorphitization through the prevacuum and heat treatment of a carbide precursor. The origin of the changes in the properties of the CDCs by the pre-vacuum treatment of the carbide precursor was investigated by means
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supercapacitors.
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of an analysis of the structural changes and electrochemical performance capabilities of
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2. Experimental To produce CDC material, TiC was purchased from Alfa Aesar, USA. Vendorfurnished Purity is 99.5%, particle size is ~5 µm, Formula weight is 59.91 g/mol, and density
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is 4.93 g/cm3. Fig. 1 shows the schematic process used to obtain the micro/mesoporous CDC from the TiC carbide, which underwent a vacuum (10-5 torr) and heating (1500 oC and 1700 o
C) pre-treatment process before a chlorination process to form the CDC with various pores.
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The TiC carbide during the pre-vacuum anealing treatment was placed into a graphite crucible at the center of a high-vacuum furnace (Solar Atmosphere, Hermitage, PA), heated to
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1500 oC or 1700 oC, annealed at these temperatures for 4 hr, and slowly cooled down to room temperature. The heating rate and the pressure in the vacuum chamber (at the highest temperature) were 10 oC/min and 10-5 torr, respectively. The as-received TiC power with particle size ~5 µm became hard and a mass of TiC after vacuum-annealing process. The
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vacuum-treated TiC sample was crushed in a diamond mortar and then separated with stainless steel sieves into of 5~10 µm size. The vacuum-annealing process induce conglomeration of the crystalline as-recieved TiC power. As shown in Rietveld Refinement
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in next section, while the basic structure, such as space group, was not changed, there was a little change in lattice parameter after vacuum-annealing process. These prepared carbon
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samples were denoted as TiC@1500Vac and TiC@1700Vac, respectively. Carbide-derived carbon (CDC) powders were produced using the chlorine of the as-received TiC and vacuumannealed TiC powders. These TiC powders were placed into a horizontal tube furnace, purged under an argon flow, and heated to 1000 ◦C and 1200 ◦C under flowing chlorine (10–15 cm3 min-1) for 3 h. The normal CDC
has high specific surface area (SSA) with pore sizes that can be
fine-tuned by controlling the chlorination temperature [22]. In previous study, the surface 6
ACCEPTED MANUSCRIPT areas and pore sizes of CDCs increased with increasing synthesis temperature from 600 oC to 1200 oC [25]. The synthesis temperature from 600 oC to 800 oC creates only micropores, but at synthesis temperatures above this, the increase in surface area is a result of mesopores
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larger than 2 nm [25]. In this study, the high chlorination temperature of above 1000 oC, at which mesopore started to appear, was chosen, in order to examine the relationship of micropore and mesopore portion after the vacuum annealing process of precursor TiC.
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The prepared carbon samples from as-received TiC precursor was correspondingly denoted as TiC@1000Cl2 and TiC@1200Cl2. The CDC powders chlorinated at 1200 ◦C were
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then annealed at 600 ◦C for 2h under flowing hydrogen to remove residual chlorine and chloride trapped in pores. Previous studies have shown that this procedure is not necessary at higher treatment temperatures [15, 16]. The CDC obtained from vacuum-annealing TiC was denoted as {TiC@1700Vac}@1200Cl2. The electrode materials for the supercapacitor cell
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used here were prepared by adding Super-P (Imerys Graphite & Carbon, Switzerland, 10 wt%) as a conductive additive and polyvinyl fluoride (PVDF, 10 wt%) as a binder in all cases. To evaluate the electrochemical (EDLC) performance of the pre-vacuum-treated
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CDCs and MSP-20, 2032-type coin cells (MTI) were fabricated. Each prepared sample was pressed onto an aluminium substrate as a working electrode, incorporating the binder and
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conductive additives. Symmetric electrode was used as a counter electrode. For practical application, symmetric full-cell tests were used for the pre-vacuum treated CDC electrodes of the same mass. The used mass loading and thickness of the electrode is 3.9±0.5 mg/cm2 and around 40 µm, respectively. The separator was CelgardR 3501. The electrolyte was 1 M tetraethylammonium
tetrafuoroborate
in
electrochemical
grade
acetonitrile
(1.0M
TEABF4/ACN). Electrochemical measurements were carried out with a potentiostat (CH Instuments, Inc.) using cyclic voltammetry (CV) and charge/discharge instrument (Maccor 7
ACCEPTED MANUSCRIPT Series 4000) using galvanic cycling. CVs were recorded at 2 mV S-1, 5 mV S-1, and 10 mV S1
scan rate in the potential range from 0 V to + 2.7 V cell voltage for full cells.
Charge/discharge cycling for the full cells was carried out from 0 V to + 2.7 V at 10 mA g-1
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and 100 mA g-1 with a holding time of 5 sec over 250 cylces and 1500 cycles. The specific capacitance (Csp) was calculated by integrating the discharge current between the discharge starting time (t0) and the end time of discharge (t) , divided by U (voltage vs. applied cell
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voltage for full-cell) via equ (1) [26]. For the full-cells, m corresponds to the mass of both electrodes divided by four.
( )
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=
(1)
A gas adsorption analysis was performed using a BELSORP-Max MP (BEL Japan Inc.) with N2 adsorbate at -196 oC for the porous carbon. Approximately 70 mg of carbon was
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evacuated at 5 mTorr at 300 oC for 16 h. The measured pressure (P/P0) was 0.05-0.999. The Brunauer-Emmett-Teller specific surface area (BET SSA) and PSD (pore size distribution) were calculated by using the BET theory based on the adsorption branches of the isotherms.
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The specific pore volumes were measured at a relative pressure of 0.95. Additionally, another PSD method, non-local density functional theory (NLDFT), was applied [27]. The NLDFT
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model assumes slit-shaped pores with uniformly dense carbon walls; the adsorbate is considered as a fluid of hard spheres [27]. TEM samples were prepared by dispersing each sample in ethanol and placing the solution over a copper mesy grid with a carbon film. A TEM study was performed using a Tecnai F20 microscope at 200 kV. SEM was performed using a Zeiss Supra 50VP scanning electron microscope equipped with an energy dispersive spectroscope (EDS). A XRD analysis was carried out using a Rigaku diffractometer with CuKα radiation (λ = 0.154 nm) operated at 300 mA and 60 kV. XRD patterns were collected using step scans with a step size of 0.01° (2θ) and a count time of 2s per step between 5 (2θ) 8
ACCEPTED MANUSCRIPT and 80 (2θ) at room temperature. All XRD data was modelled using the Rietveld refinement package, GSAS [28] with EXPGUI [29] interface. Atom position and the sII lattice parameter were allowed to vary in the refinement. For crosscheking, samples were analyzed by micro-
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Raman spectroscopy (Renishaw 1000) using an Ar ion laser (514.5 nm) at 20 X magnification (NA: 0.75: ~2 µm spot size) and <2 mW power. The particle size distribution in the powders was determined on a HORIBA Partica LA950V2 particle analyzer; the
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measurements were performed on aqueous suspensions of the powders. In addition, the mean particle size of the powders was evaluated by diffraction and by microscopy using the JEOL
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JSM 6390 LA scanning electron microscope. TSI Model 3603 Particle Size Distribution Analyzer.
3. Result and Discussions
TiC-CDC was synthesized from the a crystalline TiC carbide precursor chlorinated at
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1000 oC ~ 1200 oC, in which the precursor TiC underwent a vacuum (10-5 torr) and heating (1500 oC and 1700 oC) pre-treatment process for four hours before the chlorination step. The
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respectively.
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heating rate and the pressure in the vacuum chamber were 10 oC min-1 and 10-5 torr,
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Fig. 1 Illustration of synthetic procedure of the micro/meso-porous vacuum-treated-TiC CDC. The crystalline carbide (TiC) as a precursor for vaccum-treated-TiC CDC sample expresses the fcc crystalline structure. After vacuum (10-5 torr) and heat (1500 oC and 1700 oC)
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treatments for 4 hours, the carbide was transformed to large spherical aggregated particles. After chlorination process, the pre-treated carbide was transformed to CDC with high microporosity property.
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Fig. 1 shows an illustration of the synthetic procedure used to create the micro/meso-
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porous CDC treated by the above two processes. The crystalline carbide TiC particles were aggregated and fused into larger spherical particles after the vacuum and heat treatment. After chlorination, the CDC from the pre-treated carbide precursor showed greater microporosity and a larger surface area than normal CDC at an identical chlorination temperature.
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Fig. 2. SEM images and Particle Size Distributions (PSDs) of TiC (a, d), vacuum-treated TiC at 1500 oC (b, e), vacuum-treated TiC at 1700 oC (c, f), respectively.
In order to confirm the status of the precursor TiCs before and after the high
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temperature vacuum treatment, assessments of SEM images and particle size distributions were done. These results are presented in Fig. 2. For the as-received TiC powder, most of the particles were 2.0~10 µm in size, with finer particle also clearly visible in the SEM image of
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Fig. 2(a). The powder of the as-received TiC possesses irregular flake-like fragment appearances with a layered hexagon shapes. The observed morphology of the as-received TiC
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can be explained by the powder production route used, in which the TiC is ball milled by the manufacturer to produce the desired particle distribution. The particle size distribution of the as-received TiC present evidence of a bimodal powder, with one peak at approximately 2 µm and a broad main peak at 10 µm. The average calculated particle size was ~4.5 µm, as shown in Fig. 2(d). The SEM images in Figs. 2(b) and 2 (c) show a smooth surface without layers. The samples vacuum-annealed at 1500 oC and 1700 oC became hard during the annealing process and that the particle size increased due to the conglomeration of small particles and 11
ACCEPTED MANUSCRIPT the absence of a ball-milling process. The particle size distributions of the TiC@1500Vac and TiC@1700Vac samples shows a unimodal peak at around 10 µm with an attenuated left shoulder (peak at 2~3 µm). The average calculated particle sizes are approximately 5.24 µm
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and around 5.5 µm, respectively, as correspondingly shown in Fig. 2(e) and 2(f).
Fig. 3 XRD pattern of as-received TiC, TiC@1500Vac, and TiC@1700Vac in the each 2
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range of 5o~50o (a) and 60o~80o (b), respectively, and XRD pattern of TiC@1200Cl2, {TiC@1500Vac}@1200Cl2, and {TiC@1700Vac}@1200Cl2 in the each 2
range of 5o~50o
(c) and 60o~80o (d).
XRD patterns of a typical carbide precursor (TiC) and of the TiC@1500Vac, TiC@1700Vac,
{TiC@1500Vac}@1000Cl2,
{TiC@1500Vac}@1200Cl2,
{TiC@1700Vac}@1000Cl2, and {TiC@1700Vac}@1200Cl2 samples are shown in Fig. 3. The typical carbide (TiC) shows a FCC-cubic structure of 2θ = 35.94°, 41.74°, 60.5°, and 12
ACCEPTED MANUSCRIPT 72.4° with (111), (200), (220), (311) and (420) reflection planes, respectively, of the TiC phase. After a pre-treatment with a high-temperature vacuum, the TiC@1500Vac and TiC@1700Vac samples were shifted to the right (a larger angle of 0.1~0.04°) from the typical
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TiC standard XRD peak, indicating a smaller lattice parameter. Moreover, each peak became broader after the vacuum and heat treatment, providing evidence of a decrease in the grain size. After chlorination 1200 oC of the samples using the TiC@1500Vac and TiC@1700Vac the
carbide-derived
carbon
({TiC@1500Vac}@1200Cl2
and
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precursors,
{TiC@1700Vac}@1200Cl2) samples showed a broad (002) peak at 2θ = 25.7° and 24.4° ,
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respectively. Typical high-crystalline graphite shows a sharp (002) peak at 2θ = 26.54°. The peak of 2θ = ~43° corresponds to the diffraction from the (100) plane of graphite [30]. After a chlorination treatment at 1200 oC of the vacuum-treated samples, {TiC@1500Vac}@1200Cl2 was
shifted
to
the
right
(a
larger
angle
of
1.3°)
from
the
peak
of
the
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{TiC@1700Vac}@1200Cl2 sample. The high-vacuum- treated and chlorinated sample of {TiC@1700Vac}@1200Cl2 showed a larger lattice parameter than the low-temperaturetreated or non-treated sample. There was no 002 peak noted in the XRD patterns of the
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{TiC@1500Vac}@1000Cl2 and {TiC@1700Vac}@1000Cl2 samples, which showed more amorphous properties than those treated with the 1200oC chlorination process (Figs. 3(c) and
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3(d)).
For more detailed information about the vacuum pre-treatment of the TiC precursor,
Rietveld structural refinement was carried out using precursor TiC, TiC@1500Vac, and TiC@1700Vac from X-ray diffraction data, as given in Fig. 4. The lattice parameters of the three samples were evaluated and found to be 0.43230 nm, 0.43221 nm, and 0.43215 nm. It was noted that the lattice parameter decreased slightly with the increment of the vacuum temperature from 1500 oC to 1700 oC. These parameter measurements suggest that an increment in the vacuum temperature leads to a change in the lattice parameter. Additionally, 13
ACCEPTED MANUSCRIPT lattice strain, which is influenced by dislocations, the incorporation of impurity atoms, defects, and micro/macro stresses and small crystallite sizes, generally cause peak broadening in the diffraction pattern [31]. The FWHM obtained from the structural refinement process
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shows an increase for the samples treated with a high vacuum temperature, indicating a
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decrease of the crystallite size by peak broadening, as shown in Fig. 4(d).
Fig. 4. XRD powder diffraction pattern of (a) as-received TiC, (b) TiC@1500C Vac, and (c) TiC@1700C Vac at room temperature (cross) plus the Rietveld refinement (solid line, red), using space group Fd-3m, and difference plot (bottom, blue). (d) FWHM of as-received TiC, TiC@1500C Vac, and TiC@1700C Vac. The inset shows the refined structure with atomic parameters in Ti at (0, 0, 0) and C at (0.5, 0.5, 0.5). The refinement was characterized by (a) 14
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λ2 = 2.064 and Rwp = 25 %, (b) λ2 = 4.8 and Rwp = 22 %, and (c) λ2 = 5.4 and Rwp = 23 %.
Fig. 5. TEM images of (a) as-received TiC precursor, (b) vacuum-treated TiC at 1500 oC, (e) TiC-CDC chlorinated at 1200 oC using as-received TiC precursor, (f) vaccum-treated TiCCDC chlorinated at 1200 oC using as-received TiC precursor. In the TEM images of the TiC precursor before and after the vacuum-annealing process, the TiC@1500Vac sample (Fig. 5(b)) shows more conglomeration than the as15
ACCEPTED MANUSCRIPT received TiC particles (Fig. 5(a)), making it difficult to observe the surface morphology due to the thick particle size caused by the hard vacuum-annealed carbide. From the Rietveld refinement in Fig. 4, it is likely that the crystallite sizes of the vacuum-treated TiC became
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smaller than the as-received TiC due to the peak broadening of the vacuum-annealed samples. Fig. 3(c) and Fig. 3(d) show typical TiC-CDC chlorinated at 1000 oC and 1200 oC using the as-received TiC precusor, in which TiC@1000Cl2 is amorphous and TiC@1200 Cl2 has
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mixed properties with amorphous and graphitic characteristics. After chlorination at 1200 oC of the TiC (Fig. 5(d)), the TiC@1200Cl2 sample exhibits ordered sheets of graphite with
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interplanar distances of approximately 0.31 nm containing amorphous carbon between the
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graphite fringes.
Fig. 6. TEM images of {TiC@1500C Vac} @1000Cl2 (a), {TiC@1700C Vac} @1000Cl2 (b), {TiC@1500C Vac} @1200Cl2 (c), and {TiC@1700C Vac} @1200Cl2 (d).
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between the graphite fringes. The interplanar distances of the {TiC@1500Vac} @1200Cl2 and {TiC@1700Vac}@1200Cl2 samples are 0.33 nm and 0.36 nm, respectively, for which the high temperature of 1700 oC under the vacuum causes an increase in the interplanar
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distance of the graphitic fringe. This result is in good agreement with the XRD pattern (Fig. 3(c)) of the greater left shift of the 002 peak at {TiC@1700Vac}@1200Cl2 as compared to The
{TiC@1500Vac}@1000Cl2
and
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{TiC@1500Vac}@1200Cl2.
{TiC@1700Vac}@1000Cl2 samples (Figs. 6 (a and b)) show a more amorphous phase than the 1200Cl2 sample, providing evidence of high porosity. Compared to the CDC samples using the vacuum-annealed TiC precursor, it was noted that the vacuum-annealing process of
this
are
as
follows:
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the TiC precursor has the effect of increasing the graphite interplanar distances. Example of 0.31nm
for
TiC@1200
Cl2
(Fig.
5(d)),
0.33
nm
for
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{TiC@1500Vac}1200Cl2 (Fig. 6(c)), and 0.36 nm for {TiC@1700Vac}1200Cl2 (Fig. 6(d)).
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Fig. 7 Raman spectra of (a) TiC@1200Cl2, (b) {TiC@1700C Vac}@1000Cl2, and (c) {TiC@1700C Vac}@1200Cl2.
The Raman spectra (Fig. 7) were normalized to the intensity of the G-band at 1582 cm-1. Graphitic materials are characterized by bands labeled as D (~1356 cm-1, disorder mode) 18
ACCEPTED MANUSCRIPT and G (~1579 cm-1, in-plane vibrational mode). In Fig. 7, all of the CDCs exhibit a narrow D band, typical of amorphous disordered carbon, in agreement with previous studies of CDC powders [30]. The pre-vacuum-treated CDC samples revealed broader G and D bands than
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that of TiC@1200Cl2, located at around 1569.1 and 1337.2 cm-1, respectively. The prevacuum- treated CDC in {TiC@1700Vac}@1200Cl2 and {TiC@1700Vac}@1000Cl2 becomes more disordered and amorphous after the pre-vacuum-treatment, ultimately
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exhibiting broader G and D peaks. From the Lorentzian fits, the intensity ratio of the D and G bands (ID/IG) and the G band width (FWHM) were calculated. These outcomes are presented
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in Fig. 7. The FWHMs of the G and D peak were correspondingly 41.52 cm-1 and 55.43 cm-1 in the non-vacuum-treated sample of TiC@1200Cl2. The FWHMs of the G peak were 67.82 cm-1 and 82.59 cm-1 in the {TiC@1700Vac}@1200Cl2 and {TiC@1700Vac}@1000Cl2 sample, respectively. The ratio of ID/IG has long been used to estimate La (the in-plane
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crystallite sizes) in disordered carbon materials. The ratio of the D and G band intensity levels (ID/IG) is inversely proportional to the in-plane crystallite sizes La, which were obtained from the width of the X-ray diffraction peaks. Therefore, the Raman spectra of various
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graphitic systems can be measured with the λ = 514.5 nm (Elaser = 2.41 eV) laser line; an empirical expression that allows the determination of La from the (ID/IG) ratio was derived by
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Knight and White [19, 20]. A general formula with which to determine the La value of a nano-graphite systems at any excitation laser energy in the laser line wavelength (λlaser) in nm units can be written as Eqn. (2):
() = (2.4 × 10 ) ! " #%
(2)
$
The La value obtained from the ID/IG ratios for the TiC@1200Cl2, {TiC@1700Vac}@1200Cl2, and {TiC@1700Vac}@1000Cl2 samples were 19.8 nm, 27.6 nm and 6.1 nm, respectively, demonstrating that the crystalline size of the {TiC@1700Vac}@1200Cl2 sample is larger than 19
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those of the TiC@1200Cl2 and {TiC@1700Vac}@1000Cl2 samples.
Fig. 8. N2 isotherm data and pore size distribution of as received-TiC (a and d, respectively),
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TiC@1500C Vac (b and d, respectively), and TiC@1700C Vac (c and f, respectively).
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Fig. 8 shows the N2 adsorption isotherm and the estimated pore size distributions (PSDs) of the as received-TiC, the TiC@1500C Vac, and the TiC@1700C Vac, as determined from N2
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sorption isotherms using the NLDFT model for slit pores. All isotherm curves of three TiC samples show Type III of nonporous solids, showing small BET SSA values (2.99 m2/g for the as received-TiC, 1.11 m2/g for the TiC@1500C Vac, and 0.94 m2/g for the TiC@1700C Vac). The PSD patterns of the samples caused by surficial-free porous carbon in small amounts show pore formation in the range of 0.5 to 10 nm.
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Fig. 9. N2 isotherm data (a and b) and pore size distribution (c and d) of TiC@1000Cl2, TiC@1200Cl2,
{TiC@1500Vac}@1000Cl2,
{TiC@1500Vac}@1200Cl2,
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{TiC@1700Vac}@1000Cl2, and {TiC@1700Vac}@1200Cl2.
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Fig. 9 shows the N2 adsorption isotherm and the estimated pore size distributions
(PSDs)
of
the
{TiC@1700Vac}@1200Cl2,
{TiC@1700Vac}@1000Cl2,
{TiC@1500Vac}@1000Cl2, {TiC@1500Vac}@1200Cl2, TiC@1200Cl2, and TiC@1000Cl2 samples as determined from N2 sorption isotherms using the NLDFT model for slit pores. The marked hysteresis observed in the nitrogen adsorption confirms the presence of mesoporosity. The PSD patterns of the pre-vacuum-treated CDC samples are similar to those of the CDC samples (Figs. 9 (c and d)). At nearly identical chlorination temperatures, such as those used with the 1000Cl2 and 1200Cl2 samples, the pre-vacuum treatment of CDCs 21
ACCEPTED MANUSCRIPT ({TiC@1700Vac}@1000Cl2 and {TiC@1700Vac}@1200Cl2) led to larger degree of microporosity than in the {TiC@1500Vac}@1000Cl2 and {TiC@1500Vac}@1200Cl2 or TiC@1000Cl2 and TiC@1200Cl2 samples, which can range from 0.5 to 1.5 nm. These
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findings provide evidence that the pre-vacuum treatment before chlorination led to greater microporosity in these samples. All CDCs possessed individual ratios of micro/mesopore volume ratios with some portion of micropores (<2.0 nm) and mesopores (>2.0 nm): 3.22 in
{TiC@1500Vac}@1200Cl2,
4.25
in
{TiC@1700Vac}@1000Cl2,
volumes
in
and
1.2
in
Therefore, the observed increases in the
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{TiC@1700Vac}@1200Cl2, as shown in Table 1. micropore
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TiC@1000 Cl2, 1.06 in TiC@1200Cl2, 3.58 in {TiC@1500Vac}@1000Cl2, 1.86 in
{TiC@1500Vac}@1000Cl2,
{TiC@1500Vac}@1200Cl2,
{TiC@1700Vac}@1000Cl2, and {TiC@1700Vac}@1200Cl2 can be assumed to be related to the changes of lattice parameter, the small crystallite and grain size along with various defects
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after vacuum annealing process of Fig. 3 and Fig. 4.
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Fig.10. Comparisons of N2 isotherm data (a and b) and pore size distribution (c and d) of pretreated-TiC
CDC
sample
and
post-treated
TiC-CDC
sample.
(Pre-treatment
{TiC@1200Vac}@ 1200Cl2, Post-treatment : {TiC-CDC@ 1200 oC}@550O2 [Ref. 24]).
23
:
ACCEPTED MANUSCRIPT Fig. 10 shows comparisons of the pore texture properties according to the pretreatment and post-treatment of the CDC samples. Porous carbon after a post-treatment was synthesized from TiC using a previously reported air (oxygen) post-treatment procedure [24].
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In this post air (oxygen) treatment of CDC, an increase in the mesoporosity was obtained by removing the amorphous phase of the CDCs, which is one method for tailoring mesoporosity through O2 activation. While the post-treatments using O2 gas caused the CDC to have a
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considerable degree of mesoporosity, which lead to unique results in our previous study of electrode use in lithium ion batteries [24], the pre-treatment with a vacuum and the heat
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treatment of the TiC precursor had a great influence, imparting high microporosity in the CDC sample. For evidence of the microporosity effects, the electrochemical performance was
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measured for application as a supercapacitor.
24
ACCEPTED MANUSCRIPT Table 1. Characterization results of pre-vacuum treated CDC materials by N2 gas sorption technique.
{TiC@1500Vac}
{TiC@1500Vac}
{TiC@1700Vac}
{TiC@1700Vac}
1000 Cl2
1200 Cl2
@1000Cl2
@1200Cl2
@1000Cl2
@1200Cl2
0.799
0.69
0.82
0.74
0.98
0.84
0.73
0.62
0.78
1.08
0.89
0.83
0.56
0.32
0.61
0.71
0.72
0.45
0.174
0.30
0.17
0.38
3.22
1.06
3.58
1646
1066
1757
-1
(cm g ) NLDFT Pore 3
-1
volume (cm g ) NLDFT micro pore volume < 2 nm (N2 ads)a (cm3 g-1) NLDFT mesopore
0.17
0.38
1.86
4.25
1.20
1256
2086
1433
Micro/Mesopore volume ratio BET SSA (m2 g-1)
These results have been obtained applying the NLDFT method to N2 adsorption data
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obtained at -196 oC.
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volume >2 nm (N2 ads)a (cm3 g-1)
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3
TiC@
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Total Pore volume
TiC@
25
ACCEPTED MANUSCRIPT Figs. 11~13 show the electrochemical performance capabilities of an electrode composed of CDC treated by the vacuum and heating process of the TiC precursor and 1.0M TEABF4/ACN, in which all tests were performed using full cells. The cyclic voltammetry
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results shown in Figs. 11 (a~c) demonstrate the typical rectangular shape up to a scan rate of 2 ~ 10 mV/s, indicating that the porous pre-treated CDC (Figs. 11(b) and 11(c)) acted in an ideally polarizable manner. Voltammograms of the TiC@1000 Cl2 in Fig. 11(a) demonstrate
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characteristically resistive and sloped CV shapes that describe more decay than in the pretreated CDCs shown in Fig. 11(d). The specific capacitances are 150 F/g, 120 F/g, and 90 F/g
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at 2 mV/s in the {TiC@1700Vac}@1000Cl2, {TiC@1500Vac}@1000Cl2, TiC@1000Cl2 samples, respectively. These values are approximately 10 % ~ 30 % higher than those of other reported CDC electrodes [32, 33]. For the chlorination samples treated at 1200 oC, the CV curve of TiC@1200Cl2 and {TiC@1500Vac}@1200Cl2 shown in Fig. 12(a) and (b)
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shows nearly rectangular shape associated with pure double-layer capacitance at slow scan rates. However, the CV curves of Figs. 12(a) and (b) start to deviate from rectangular shape with an increasing anodic current at high scan rates, especially for TiC@1200Cl2 of Fig.
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12(a). This observed deviation can be attributed to the nonzero time constant, in which the transient current falls exponentially with RC time constant (R=ESR), and a steady-state
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current (sC, where s is the scan rate and C is the double layer capacitance) [34-36]. The transient part lasts longer with the longer time constant, which means that more time is required to charge the capacitor, indicating the collapse of the rectangular shape of the current profiles related to the pseudocapacitance contribution [36]. On the other hand, the current rising and decaying became gradually depressed with increasing scan rate, as shown in the CV profiles of Figs. 12(a) and (b). This behaviour shows less reversible character of the electrodes due to the fact that the voltage signal could not reach the pore effectively under increased scan rate [36]. It is expected that this phenomenon may occur due to the distributed 26
ACCEPTED MANUSCRIPT capacitance effect in the porous electrodes with different pore dimensions and carbon structures. The depression of CV profiles by increased scan rate may have caused by the difference between the ohmic resistances of the electrolyte at mesopores and micropores [36].
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In the pores and structures of CDCs, while the CDC prepared at chlorination temperature of 1000 oC forms the micropore (< 2 nm) along with the presence of amorphous carbon, the CDC at 1200 oC contains the micropore (< 2 nm) plus mesopore (< 4 nm) along with less
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amorphous carbon and more ordered curved sheets of graphite. In respect of the changes in electrolyte resistance within pores, the non-uniform porosity caused by the mixed micropore
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and mesopore in the 1200 oC chlorination sample of Fig. 12(a) may have led to the quasirectangular shape by increased scan rate. In the case of {TiC@1700Vac}@1200Cl2 sample, the quasi-rectangular shape was recovered with increased scan rate, which might be caused by the increased micropore after vacuum treatment, as shown in Fig. 12(c). However, the capacitances of the CDC samples prepared by a chlorination temperature of 1200 oC showed
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lower than those of the CDC samples treated at a chlorination temperature of 1000 oC (Figs. 11(d) and 12(d)). Fig. 13 shows the retained capacitance as a function of the cycle number cycles)
and
galvanostatic
charge/discharge
curve
at
100
mA
g-1.
The
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(1200
{TiC@1500Vac}@1000Cl2 sample retains 90% of their initial capacitance levels (80.3 F g-1)
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after 1500 cycles at 100 mA g-1. The {TiC@1500Vac}@1000Cl2 exhibits a frequency response with an RC time constant of 1.87 sec and ESR resistance of 1.67 Ω at 100 mA g-1 .
27
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Fig. 11. Cyclic voltamogram (a, b, and c) of TiC@1000 Cl2, {TiC@1500Vac}@1000Cl2, and TiC@1700Vac}@1000Cl2 and specific capacitance comparison (d) with TiC@1000Cl2 and
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Pre-treated TiC-CDC chlorinated at 1000 oC.
28
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Fig. 12. Cyclic voltamogram (a, b, and c) of TiC@1200 Cl2, {TiC@1500Vac}@1200Cl2, and TiC@1700Vac}@1200Cl2 and specific capacitance comparison (d) with TiC@1200Cl2 and
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Pre-treated TiC-CDC chlorinated at 1200 oC.
29
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Fig. 13. Cyclic performance and galvanostatic charge/discharge (100 mA g-1) of {TiC@1500Vac}@1000 Cl2.
30
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4. Conclusions The application of a pre-vacuum-treated CDC as a supercapacitor electrode was investigated. The system shows higher specific capacitance with a pre-vacuum-treated CDC
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than with a normal CDC at an identical chlorination temperature. This feature was obtained by tailoring the high microporosity of CDCs by a pre-vacuum process with a carbide precursor. N2 sorption measurements and TEM images verified the increased microporosity
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caused by amorphitization from the pre-vacuum treatment. The structural rearrangement and the increased microporosity caused by the pre-vacuum treatment give rise to the synergistic
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effect of enhanced capacitance as compared to normal CDCs. In supercapacitor application of the pre-vacuum treated CDC samples, the specific capacitances are 150 F/g and 120 F/g at 2 mV s-1 in the {TiC@1700Vac}@1000Cl2, {TiC@1500Vac}@1000Cl2, TiC@1000Cl2 samples, respectively, which are approximately 10 % ~ 30 % higher than those of other
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reported CDC electrodes. These results of the pre-vacuum CDC system represent the potential of the CDC electrode material, which should be investigated further to determine its
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capability to serve as an active material in practical and capacitive energy storage systems.
31
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Acknowledgements
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This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Trade, Industry and Energy (10050391) of the Republic of Korea, the National Research Foundation (NRF-2011-CIAAA0010030538 and NRF-2015069204) and conducted under the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER, B6-2413).
32
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Figure Captions
Fig. 1 Illustration of synthetic procedure of the micro/meso-porous vacuum-treated-TiC
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CDC. The crystalline carbide (TiC) as a precursor for vaccum-treated-TiC CDC sample expresses the fcc crystalline structure. After vacuum (10-5 torr) and heat (1500 oC and 1700 o
C) treatments for 4 hours, the carbide was transformed to large spherical aggregated
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particles. After chlorination process, the pre-treated carbide was transformed to CDC with high micro-porosity property.
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Fig. 2. SEM images and Particle Size Distributions (PSDs) of TiC (a, d), vacuum-treated TiC at 1500 oC (b, e), vacuum-treated TiC at 1700 oC (c, f), respectively.
Fig. 3 XRD pattern of as-received TiC, TiC@1500Vac, and TiC@1700Vac in the each 2
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range of 5o~50o (a) and 60o~80o (b), respectively, and XRD pattern of TiC@1200Cl2, {TiC@1500Vac}@1200Cl2, and {TiC@1700Vac}@1200Cl2 in the each 2
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(c) and 60o~80o (d).
range of 5o~50o
Fig. 4. XRD powder diffraction pattern of (a) as-received TiC, (b) TiC@1500C Vac, and (c)
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TiC@1700C Vac at room temperature (cross) plus the Rietveld refinement (solid line, red), using space group Fd-3m, and difference plot (bottom, blue). (d) FWHM of as-received TiC, TiC@1500C Vac, and TiC@1700C Vac. The inset shows the refined structure with atomic parameters in Ti at (0, 0, 0) and C at (0.5, 0.5, 0.5). The refinement was characterized by (a) λ2 = 2.064 and Rwp = 25 %, (b) λ2 = 4.8 and Rwp = 22 %, and (c) λ2 = 5.4 and Rwp = 23 %. Fig. 5. TEM images of (a) as-received TiC precursor, (b) vacuum-treated TiC at 1500 oC, (e) TiC-CDC chlorinated at 1200 oC using as-received TiC precursor, (f) vaccum-treated TiCCDC chlorinated at 1200 oC using as-received TiC precursor. 33
ACCEPTED MANUSCRIPT Fig. 6. TEM images of {TiC@1500C Vac} @1000Cl2 (a), {TiC@1700C Vac} @1000Cl2 (b), {TiC@1500C Vac} @1200Cl2 (c), and {TiC@1700C Vac} @1200Cl2 (d). Fig. 7 Raman spectra of (a) TiC@1200Cl2, (b) {TiC@1700C Vac}@1000Cl2, and (c)
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{TiC@1700C Vac}@1200Cl2. Fig. 8. N2 isotherm data and pore size distribution of as received-TiC (a and d, respectively), TiC@1500C Vac (b and d, respectively), and TiC@1700C Vac (c and f, respectively).
{TiC@1500Vac}@1000Cl2,
{TiC@1500Vac}@1200Cl2,
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TiC@1200Cl2,
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Fig. 9. N2 isotherm data (a and b) and pore size distribution (c and d) of TiC@1000Cl2,
{TiC@1700Vac}@1000Cl2, and {TiC@1700Vac}@1200Cl2.
Fig.10. Comparisons of N2 isotherm data (a and b) and pore size distribution (c and d) of pretreated-TiC
CDC
sample
and
post-treated
TiC-CDC
sample.
(Pre-treatment
:
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{TiC@1200Vac}@ 1200Cl2, Post-treatment : {TiC-CDC@ 1200 oC}@550O2 [Ref. 24]). Fig. 11. Cyclic voltamogram (a, b, and c) of TiC@1000 Cl2, {TiC@1500Vac}@1000Cl2, and TiC@1700Vac}@1000Cl2 and specific capacitance comparison (d) with TiC@1000Cl2 and
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Pre-treated TiC-CDC chlorinated at 1000 oC.
Fig. 12. Cyclic voltamogram (a, b, and c) of TiC@1200 Cl2, {TiC@1500Vac}@1200Cl2, and
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TiC@1700Vac}@1200Cl2 and specific capacitance comparison (d) with TiC@1200Cl2 and Pre-treated TiC-CDC chlorinated at 1200 oC Fig. 13. Cyclic performance and galvanostatic charge/discharge (100 mA g-1) of {TiC@1500Vac}@1000 Cl2.
34
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Table Captions Table 1. Characterization results of pre-vacuum treated CDC materials by N2 gas sorption
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technique.
36
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supercapacitors, Journal of Power Sources 161(1) (2006) 730-736.
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ACCEPTED MANUSCRIPT Table 1. Characterization results of pre-vacuum treated CDC materials by N2 gas sorption technique.
{TiC@1500Vac}
{TiC@1500Vac}
{TiC@1700Vac}
1000 Cl2
1200 Cl2
@1000Cl2
@1200Cl2
@1000Cl2
@1200Cl2
0.799
0.69
0.81
0.74
0.98
0.84
0.73
0.62
0.71
1.08
0.89
0.83
0.56
0.32
0.54
0.71
0.174
0.30
0.17
3.22
1.06
1646
1066
-1
(cm g ) NLDFT Pore volume (cm3 g-1) NLDFT micro pore
0.72
0.45
0.38
0.17
0.38
3.18
1.86
4.25
1.20
1757
1256
2086
1433
NLDFT mesopore volume >2 nm (N2 ads)a (cm3 g-1) Micro/Mesopore
BET SSA (m2 g-1)
These results have been obtained applying the NLDFT method to N2 adsorption data
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obtained at -196 oC.
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a
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volume ratio
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volume < 2 nm (N2 ads)a (cm3 g-1)
{TiC@1700Vac}
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3
TiC@
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Total Pore volume
TiC@
S0008-6223(17)30304-4
DOI:
10.1016/j.carbon.2017.03.063
Reference:
CARBON 11870
To appear in:
Carbon
Received Date: 7 July 2016 Revised Date:
2 March 2017
Accepted Date: 18 March 2017
Please cite this article as: S.-H. Yeon, D.-H. Kim, S.-H. Lee, S.-S. Nam, S.-K. Park, J.Y. So, K.-H. Shin, C.-S. Jin, Y. Park, Y.C. Kang, High microporosity of carbide-derived carbon prepared from a vacuumtreated precursor for energy storage devices, Carbon (2017), doi: 10.1016/j.carbon.2017.03.063. 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.
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High Microporosity of Carbide-Derived Carbon Prepared from a
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Vacuum-Treated Precursor for Energy Storage Devices
Sun-Hwa Yeon a,*, Dong-Ha Kim a,b, Sang-Ho Lee a,c, Seong-Sik Nam a,d, Se-Kook Park a, Jae
Energy Storage Lab., Korea Institute of Energy Research, 102, Gajeong-ro, Yuseong,
Daejeon, 305-343, Republic of Korea b
Department of Energy Engineering, Hanyang University, 222, Wangsimni-ro, Seongdong-gu,
Seoul, 133-791, Republic of Korea.
Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbu-
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c
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a
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Young So a,d, Kyoung-Hee Shin a, Chang-Soo Jin a, Youngjune Parke , Yun Chan Kang c
Gu, Seoul, 136-713, Republic of Korea.
e
Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul, 04763, Republic of Korea
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d
School of Environmental Science and Engineering, Gwangju Institute of Science and
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Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea
*
Corresponding Author : Tel +82-42-860-3763; E-mail: [email protected] (S.H. Yeon) 1
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Abstract
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Carbide-derived carbon (CDC) is an attractive electrode material for electrochemical applications because diverse pore textures and structures can be controlled by changing the properties of the precursor template and the synthesis conditions. Upon the tailoring of the
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micro-pore texture and graphitic structure of CDCs via a pre-vacuum treatment of a carbide precursor, the electrode shows a greatly high increased capacitance under a range of scan
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rates from 2 mV/s to 10 mV/s. The specific capacitance of a CDC chlorinated at 1000 oC from a pre-vacuum-treated at 1700 oC was 150 F/g at 2 mV/s, which is approximately 60 %
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higher than that of a CDC chlorinated at 1000 oC.
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1. Introduction
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Various types of carbonaceous materials have been studied extensively for use as electrodes in energy storage devices for transportation and renewable energy storage applications [1, 2]. Capacitive electrodes (CEs) are used in a broad range of systems related
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to both energy and water infrastructure across the globe. Supercapacitor electrodes, traditionally associated with portable electronics [3], are emerging in grid-level energy
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storage applications [4, 5], in capacitive deionization desalination methods [6], and in energy generation systems [7].
Supercapacitors occupy a region between batteries and dielectric capacitors on the Ragone plot, which describes the relationship between energy and power. They have served
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as a solution to the mismatch between the rapid growth in power as required by devices and the inability of batteries efficiently to discharge power at high rates [8]. This high performance requirement for high power discharge is directly related to the absence of
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charge-transfer resistances, which is a characteristic of battery Faradaic reactions and which subsequently leads to better performance at low temperatures as compared to a battery system.
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Supercapacitors have received considerable attention in recent years due to their high pulse power supply, rapid charging capabilities at high current levels, long cycleability, and safe and reliable performance compared to battery systems [9]. Supercapacitors are usually connected to high-energy batteries or fuel cells to serve as a temporary energy storage device with high-power capabilities because the discharge time is too short for them to be used in independent power sources given their inherently low energy density levels [10, 11]. Improvements have been made in cell packaging and electrolytes [12], but a lack of substantial progress in terms of carbon material designs has limited the energy density, 3
ACCEPTED MANUSCRIPT effectively preventing the wide-scale usage of supercapacitors. Moreover, a new systemic approach beyond merely finding new electrode materials has been introduced. [4, 13-15]. To overcome the limitation of the low energy density density, numerous new active materials for
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electrodes have been developed to realize high capacitance, ranging from carbon-based materials to metal- oxide composites [3, 13, 16-19]. Particularly, for the application of electrochemical double-layer capacitors (EDLCs), the study of electrochemical double layers
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is a key issue when attempting to overcome the present low capacity levels, representing a complex problem associated with the solid/electrolyte interface between the pore diameter of
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the solid electrode and the ion size of the electrolyte [20]. At high EDLC capacitance levels, the preparation of nanoporous carbon with uniform porosity suitable for the size of an ion is important, as ion-size tuned porous carbon can lead to an increase in the capacitance by increasing the confinement of the ions. For example, in the 1.0 M TEA-BF4/ACN electrolyte
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system with a single associated solvent molecule of tetraethylammonium tetrafluoroborate (TEA-BF4) that is a common electrolyte system in EDLCs owing to its high conductivity and the relatively small ion size, the carbon with micro-sized pore has more of an effect on the
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high capacitance performance than that with meso-sized pores. Therefore, the control of micro-sized pores is a more important factor than ensuring meso- or macro-pores in EDLC
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applications [21].
Carbide-derived Carbon (CDC), a carbon material suitable for supercapacitor
applications, has been investigated due to its simple preparation and easy control of structures and properties, such as its graphitization and porosity. CDCs possess high porosity levels, a tunable pore size, and a narrow pore size distribution. A chemical etching method using chlorine gas can generate micro- and mesoporosity, leading to large adsorption capacities of target molecules matching the pore sizes due to the controlled pore size and high specific surface areas [22]. In previous reports, mesoporous CDCs produced by binary and ternary 4
ACCEPTED MANUSCRIPT carbides (B4C and TiC7N3 carbide precursors, respectively) showed significantly improved electrochemical capacities as lithium negative electrodes [23]. In addition, unique cyclic performance imparted by high mesoporosity was demonstrated in an electrode through the
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post-treated CDC by O2 activation [24]. However, most post-activation processes also contribute to greater rates of mesopore development. When considering a commercial electrolyte such as TEABF4/acetonitrile in a CDC electrode, a preparation method which
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increases the degree of microporosity represents an applicable spin-off technology for high storage capacitance in EDLC systems.
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In this work, we demonstrate the significantly improved electrochemical capacitance of the CDCs through control of the micro-pore texture and amorphitization through the prevacuum and heat treatment of a carbide precursor. The origin of the changes in the properties of the CDCs by the pre-vacuum treatment of the carbide precursor was investigated by means
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supercapacitors.
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of an analysis of the structural changes and electrochemical performance capabilities of
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2. Experimental To produce CDC material, TiC was purchased from Alfa Aesar, USA. Vendorfurnished Purity is 99.5%, particle size is ~5 µm, Formula weight is 59.91 g/mol, and density
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is 4.93 g/cm3. Fig. 1 shows the schematic process used to obtain the micro/mesoporous CDC from the TiC carbide, which underwent a vacuum (10-5 torr) and heating (1500 oC and 1700 o
C) pre-treatment process before a chlorination process to form the CDC with various pores.
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The TiC carbide during the pre-vacuum anealing treatment was placed into a graphite crucible at the center of a high-vacuum furnace (Solar Atmosphere, Hermitage, PA), heated to
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1500 oC or 1700 oC, annealed at these temperatures for 4 hr, and slowly cooled down to room temperature. The heating rate and the pressure in the vacuum chamber (at the highest temperature) were 10 oC/min and 10-5 torr, respectively. The as-received TiC power with particle size ~5 µm became hard and a mass of TiC after vacuum-annealing process. The
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vacuum-treated TiC sample was crushed in a diamond mortar and then separated with stainless steel sieves into of 5~10 µm size. The vacuum-annealing process induce conglomeration of the crystalline as-recieved TiC power. As shown in Rietveld Refinement
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in next section, while the basic structure, such as space group, was not changed, there was a little change in lattice parameter after vacuum-annealing process. These prepared carbon
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samples were denoted as TiC@1500Vac and TiC@1700Vac, respectively. Carbide-derived carbon (CDC) powders were produced using the chlorine of the as-received TiC and vacuumannealed TiC powders. These TiC powders were placed into a horizontal tube furnace, purged under an argon flow, and heated to 1000 ◦C and 1200 ◦C under flowing chlorine (10–15 cm3 min-1) for 3 h. The normal CDC
has high specific surface area (SSA) with pore sizes that can be
fine-tuned by controlling the chlorination temperature [22]. In previous study, the surface 6
ACCEPTED MANUSCRIPT areas and pore sizes of CDCs increased with increasing synthesis temperature from 600 oC to 1200 oC [25]. The synthesis temperature from 600 oC to 800 oC creates only micropores, but at synthesis temperatures above this, the increase in surface area is a result of mesopores
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larger than 2 nm [25]. In this study, the high chlorination temperature of above 1000 oC, at which mesopore started to appear, was chosen, in order to examine the relationship of micropore and mesopore portion after the vacuum annealing process of precursor TiC.
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The prepared carbon samples from as-received TiC precursor was correspondingly denoted as TiC@1000Cl2 and TiC@1200Cl2. The CDC powders chlorinated at 1200 ◦C were
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then annealed at 600 ◦C for 2h under flowing hydrogen to remove residual chlorine and chloride trapped in pores. Previous studies have shown that this procedure is not necessary at higher treatment temperatures [15, 16]. The CDC obtained from vacuum-annealing TiC was denoted as {TiC@1700Vac}@1200Cl2. The electrode materials for the supercapacitor cell
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used here were prepared by adding Super-P (Imerys Graphite & Carbon, Switzerland, 10 wt%) as a conductive additive and polyvinyl fluoride (PVDF, 10 wt%) as a binder in all cases. To evaluate the electrochemical (EDLC) performance of the pre-vacuum-treated
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CDCs and MSP-20, 2032-type coin cells (MTI) were fabricated. Each prepared sample was pressed onto an aluminium substrate as a working electrode, incorporating the binder and
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conductive additives. Symmetric electrode was used as a counter electrode. For practical application, symmetric full-cell tests were used for the pre-vacuum treated CDC electrodes of the same mass. The used mass loading and thickness of the electrode is 3.9±0.5 mg/cm2 and around 40 µm, respectively. The separator was CelgardR 3501. The electrolyte was 1 M tetraethylammonium
tetrafuoroborate
in
electrochemical
grade
acetonitrile
(1.0M
TEABF4/ACN). Electrochemical measurements were carried out with a potentiostat (CH Instuments, Inc.) using cyclic voltammetry (CV) and charge/discharge instrument (Maccor 7
ACCEPTED MANUSCRIPT Series 4000) using galvanic cycling. CVs were recorded at 2 mV S-1, 5 mV S-1, and 10 mV S1
scan rate in the potential range from 0 V to + 2.7 V cell voltage for full cells.
Charge/discharge cycling for the full cells was carried out from 0 V to + 2.7 V at 10 mA g-1
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and 100 mA g-1 with a holding time of 5 sec over 250 cylces and 1500 cycles. The specific capacitance (Csp) was calculated by integrating the discharge current between the discharge starting time (t0) and the end time of discharge (t) , divided by U (voltage vs. applied cell
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voltage for full-cell) via equ (1) [26]. For the full-cells, m corresponds to the mass of both electrodes divided by four.
( )
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=
(1)
A gas adsorption analysis was performed using a BELSORP-Max MP (BEL Japan Inc.) with N2 adsorbate at -196 oC for the porous carbon. Approximately 70 mg of carbon was
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evacuated at 5 mTorr at 300 oC for 16 h. The measured pressure (P/P0) was 0.05-0.999. The Brunauer-Emmett-Teller specific surface area (BET SSA) and PSD (pore size distribution) were calculated by using the BET theory based on the adsorption branches of the isotherms.
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The specific pore volumes were measured at a relative pressure of 0.95. Additionally, another PSD method, non-local density functional theory (NLDFT), was applied [27]. The NLDFT
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model assumes slit-shaped pores with uniformly dense carbon walls; the adsorbate is considered as a fluid of hard spheres [27]. TEM samples were prepared by dispersing each sample in ethanol and placing the solution over a copper mesy grid with a carbon film. A TEM study was performed using a Tecnai F20 microscope at 200 kV. SEM was performed using a Zeiss Supra 50VP scanning electron microscope equipped with an energy dispersive spectroscope (EDS). A XRD analysis was carried out using a Rigaku diffractometer with CuKα radiation (λ = 0.154 nm) operated at 300 mA and 60 kV. XRD patterns were collected using step scans with a step size of 0.01° (2θ) and a count time of 2s per step between 5 (2θ) 8
ACCEPTED MANUSCRIPT and 80 (2θ) at room temperature. All XRD data was modelled using the Rietveld refinement package, GSAS [28] with EXPGUI [29] interface. Atom position and the sII lattice parameter were allowed to vary in the refinement. For crosscheking, samples were analyzed by micro-
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Raman spectroscopy (Renishaw 1000) using an Ar ion laser (514.5 nm) at 20 X magnification (NA: 0.75: ~2 µm spot size) and <2 mW power. The particle size distribution in the powders was determined on a HORIBA Partica LA950V2 particle analyzer; the
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measurements were performed on aqueous suspensions of the powders. In addition, the mean particle size of the powders was evaluated by diffraction and by microscopy using the JEOL
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JSM 6390 LA scanning electron microscope. TSI Model 3603 Particle Size Distribution Analyzer.
3. Result and Discussions
TiC-CDC was synthesized from the a crystalline TiC carbide precursor chlorinated at
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1000 oC ~ 1200 oC, in which the precursor TiC underwent a vacuum (10-5 torr) and heating (1500 oC and 1700 oC) pre-treatment process for four hours before the chlorination step. The
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respectively.
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heating rate and the pressure in the vacuum chamber were 10 oC min-1 and 10-5 torr,
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Fig. 1 Illustration of synthetic procedure of the micro/meso-porous vacuum-treated-TiC CDC. The crystalline carbide (TiC) as a precursor for vaccum-treated-TiC CDC sample expresses the fcc crystalline structure. After vacuum (10-5 torr) and heat (1500 oC and 1700 oC)
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treatments for 4 hours, the carbide was transformed to large spherical aggregated particles. After chlorination process, the pre-treated carbide was transformed to CDC with high microporosity property.
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Fig. 1 shows an illustration of the synthetic procedure used to create the micro/meso-
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porous CDC treated by the above two processes. The crystalline carbide TiC particles were aggregated and fused into larger spherical particles after the vacuum and heat treatment. After chlorination, the CDC from the pre-treated carbide precursor showed greater microporosity and a larger surface area than normal CDC at an identical chlorination temperature.
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Fig. 2. SEM images and Particle Size Distributions (PSDs) of TiC (a, d), vacuum-treated TiC at 1500 oC (b, e), vacuum-treated TiC at 1700 oC (c, f), respectively.
In order to confirm the status of the precursor TiCs before and after the high
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temperature vacuum treatment, assessments of SEM images and particle size distributions were done. These results are presented in Fig. 2. For the as-received TiC powder, most of the particles were 2.0~10 µm in size, with finer particle also clearly visible in the SEM image of
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Fig. 2(a). The powder of the as-received TiC possesses irregular flake-like fragment appearances with a layered hexagon shapes. The observed morphology of the as-received TiC
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can be explained by the powder production route used, in which the TiC is ball milled by the manufacturer to produce the desired particle distribution. The particle size distribution of the as-received TiC present evidence of a bimodal powder, with one peak at approximately 2 µm and a broad main peak at 10 µm. The average calculated particle size was ~4.5 µm, as shown in Fig. 2(d). The SEM images in Figs. 2(b) and 2 (c) show a smooth surface without layers. The samples vacuum-annealed at 1500 oC and 1700 oC became hard during the annealing process and that the particle size increased due to the conglomeration of small particles and 11
ACCEPTED MANUSCRIPT the absence of a ball-milling process. The particle size distributions of the TiC@1500Vac and TiC@1700Vac samples shows a unimodal peak at around 10 µm with an attenuated left shoulder (peak at 2~3 µm). The average calculated particle sizes are approximately 5.24 µm
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and around 5.5 µm, respectively, as correspondingly shown in Fig. 2(e) and 2(f).
Fig. 3 XRD pattern of as-received TiC, TiC@1500Vac, and TiC@1700Vac in the each 2
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range of 5o~50o (a) and 60o~80o (b), respectively, and XRD pattern of TiC@1200Cl2, {TiC@1500Vac}@1200Cl2, and {TiC@1700Vac}@1200Cl2 in the each 2
range of 5o~50o
(c) and 60o~80o (d).
XRD patterns of a typical carbide precursor (TiC) and of the TiC@1500Vac, TiC@1700Vac,
{TiC@1500Vac}@1000Cl2,
{TiC@1500Vac}@1200Cl2,
{TiC@1700Vac}@1000Cl2, and {TiC@1700Vac}@1200Cl2 samples are shown in Fig. 3. The typical carbide (TiC) shows a FCC-cubic structure of 2θ = 35.94°, 41.74°, 60.5°, and 12
ACCEPTED MANUSCRIPT 72.4° with (111), (200), (220), (311) and (420) reflection planes, respectively, of the TiC phase. After a pre-treatment with a high-temperature vacuum, the TiC@1500Vac and TiC@1700Vac samples were shifted to the right (a larger angle of 0.1~0.04°) from the typical
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TiC standard XRD peak, indicating a smaller lattice parameter. Moreover, each peak became broader after the vacuum and heat treatment, providing evidence of a decrease in the grain size. After chlorination 1200 oC of the samples using the TiC@1500Vac and TiC@1700Vac the
carbide-derived
carbon
({TiC@1500Vac}@1200Cl2
and
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precursors,
{TiC@1700Vac}@1200Cl2) samples showed a broad (002) peak at 2θ = 25.7° and 24.4° ,
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respectively. Typical high-crystalline graphite shows a sharp (002) peak at 2θ = 26.54°. The peak of 2θ = ~43° corresponds to the diffraction from the (100) plane of graphite [30]. After a chlorination treatment at 1200 oC of the vacuum-treated samples, {TiC@1500Vac}@1200Cl2 was
shifted
to
the
right
(a
larger
angle
of
1.3°)
from
the
peak
of
the
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{TiC@1700Vac}@1200Cl2 sample. The high-vacuum- treated and chlorinated sample of {TiC@1700Vac}@1200Cl2 showed a larger lattice parameter than the low-temperaturetreated or non-treated sample. There was no 002 peak noted in the XRD patterns of the
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{TiC@1500Vac}@1000Cl2 and {TiC@1700Vac}@1000Cl2 samples, which showed more amorphous properties than those treated with the 1200oC chlorination process (Figs. 3(c) and
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3(d)).
For more detailed information about the vacuum pre-treatment of the TiC precursor,
Rietveld structural refinement was carried out using precursor TiC, TiC@1500Vac, and TiC@1700Vac from X-ray diffraction data, as given in Fig. 4. The lattice parameters of the three samples were evaluated and found to be 0.43230 nm, 0.43221 nm, and 0.43215 nm. It was noted that the lattice parameter decreased slightly with the increment of the vacuum temperature from 1500 oC to 1700 oC. These parameter measurements suggest that an increment in the vacuum temperature leads to a change in the lattice parameter. Additionally, 13
ACCEPTED MANUSCRIPT lattice strain, which is influenced by dislocations, the incorporation of impurity atoms, defects, and micro/macro stresses and small crystallite sizes, generally cause peak broadening in the diffraction pattern [31]. The FWHM obtained from the structural refinement process
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shows an increase for the samples treated with a high vacuum temperature, indicating a
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decrease of the crystallite size by peak broadening, as shown in Fig. 4(d).
Fig. 4. XRD powder diffraction pattern of (a) as-received TiC, (b) TiC@1500C Vac, and (c) TiC@1700C Vac at room temperature (cross) plus the Rietveld refinement (solid line, red), using space group Fd-3m, and difference plot (bottom, blue). (d) FWHM of as-received TiC, TiC@1500C Vac, and TiC@1700C Vac. The inset shows the refined structure with atomic parameters in Ti at (0, 0, 0) and C at (0.5, 0.5, 0.5). The refinement was characterized by (a) 14
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λ2 = 2.064 and Rwp = 25 %, (b) λ2 = 4.8 and Rwp = 22 %, and (c) λ2 = 5.4 and Rwp = 23 %.
Fig. 5. TEM images of (a) as-received TiC precursor, (b) vacuum-treated TiC at 1500 oC, (e) TiC-CDC chlorinated at 1200 oC using as-received TiC precursor, (f) vaccum-treated TiCCDC chlorinated at 1200 oC using as-received TiC precursor. In the TEM images of the TiC precursor before and after the vacuum-annealing process, the TiC@1500Vac sample (Fig. 5(b)) shows more conglomeration than the as15
ACCEPTED MANUSCRIPT received TiC particles (Fig. 5(a)), making it difficult to observe the surface morphology due to the thick particle size caused by the hard vacuum-annealed carbide. From the Rietveld refinement in Fig. 4, it is likely that the crystallite sizes of the vacuum-treated TiC became
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smaller than the as-received TiC due to the peak broadening of the vacuum-annealed samples. Fig. 3(c) and Fig. 3(d) show typical TiC-CDC chlorinated at 1000 oC and 1200 oC using the as-received TiC precusor, in which TiC@1000Cl2 is amorphous and TiC@1200 Cl2 has
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mixed properties with amorphous and graphitic characteristics. After chlorination at 1200 oC of the TiC (Fig. 5(d)), the TiC@1200Cl2 sample exhibits ordered sheets of graphite with
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interplanar distances of approximately 0.31 nm containing amorphous carbon between the
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graphite fringes.
Fig. 6. TEM images of {TiC@1500C Vac} @1000Cl2 (a), {TiC@1700C Vac} @1000Cl2 (b), {TiC@1500C Vac} @1200Cl2 (c), and {TiC@1700C Vac} @1200Cl2 (d).
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ACCEPTED MANUSCRIPT Figs. 6 show various TEM images of vacuum-treated (1500 oC and 1700 oC) TiC-CDC chlorinated at 1000 oC and 1200 oC. All TiC-CDC samples using vacuum-annealed TiC also exhibit ordered sheets of graphite with interplanar distances containing amorphous carbon
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between the graphite fringes. The interplanar distances of the {TiC@1500Vac} @1200Cl2 and {TiC@1700Vac}@1200Cl2 samples are 0.33 nm and 0.36 nm, respectively, for which the high temperature of 1700 oC under the vacuum causes an increase in the interplanar
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distance of the graphitic fringe. This result is in good agreement with the XRD pattern (Fig. 3(c)) of the greater left shift of the 002 peak at {TiC@1700Vac}@1200Cl2 as compared to The
{TiC@1500Vac}@1000Cl2
and
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{TiC@1500Vac}@1200Cl2.
{TiC@1700Vac}@1000Cl2 samples (Figs. 6 (a and b)) show a more amorphous phase than the 1200Cl2 sample, providing evidence of high porosity. Compared to the CDC samples using the vacuum-annealed TiC precursor, it was noted that the vacuum-annealing process of
this
are
as
follows:
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the TiC precursor has the effect of increasing the graphite interplanar distances. Example of 0.31nm
for
TiC@1200
Cl2
(Fig.
5(d)),
0.33
nm
for
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{TiC@1500Vac}1200Cl2 (Fig. 6(c)), and 0.36 nm for {TiC@1700Vac}1200Cl2 (Fig. 6(d)).
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Fig. 7 Raman spectra of (a) TiC@1200Cl2, (b) {TiC@1700C Vac}@1000Cl2, and (c) {TiC@1700C Vac}@1200Cl2.
The Raman spectra (Fig. 7) were normalized to the intensity of the G-band at 1582 cm-1. Graphitic materials are characterized by bands labeled as D (~1356 cm-1, disorder mode) 18
ACCEPTED MANUSCRIPT and G (~1579 cm-1, in-plane vibrational mode). In Fig. 7, all of the CDCs exhibit a narrow D band, typical of amorphous disordered carbon, in agreement with previous studies of CDC powders [30]. The pre-vacuum-treated CDC samples revealed broader G and D bands than
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that of TiC@1200Cl2, located at around 1569.1 and 1337.2 cm-1, respectively. The prevacuum- treated CDC in {TiC@1700Vac}@1200Cl2 and {TiC@1700Vac}@1000Cl2 becomes more disordered and amorphous after the pre-vacuum-treatment, ultimately
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exhibiting broader G and D peaks. From the Lorentzian fits, the intensity ratio of the D and G bands (ID/IG) and the G band width (FWHM) were calculated. These outcomes are presented
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in Fig. 7. The FWHMs of the G and D peak were correspondingly 41.52 cm-1 and 55.43 cm-1 in the non-vacuum-treated sample of TiC@1200Cl2. The FWHMs of the G peak were 67.82 cm-1 and 82.59 cm-1 in the {TiC@1700Vac}@1200Cl2 and {TiC@1700Vac}@1000Cl2 sample, respectively. The ratio of ID/IG has long been used to estimate La (the in-plane
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crystallite sizes) in disordered carbon materials. The ratio of the D and G band intensity levels (ID/IG) is inversely proportional to the in-plane crystallite sizes La, which were obtained from the width of the X-ray diffraction peaks. Therefore, the Raman spectra of various
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graphitic systems can be measured with the λ = 514.5 nm (Elaser = 2.41 eV) laser line; an empirical expression that allows the determination of La from the (ID/IG) ratio was derived by
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Knight and White [19, 20]. A general formula with which to determine the La value of a nano-graphite systems at any excitation laser energy in the laser line wavelength (λlaser) in nm units can be written as Eqn. (2):
() = (2.4 × 10 ) ! " #%
(2)
$
The La value obtained from the ID/IG ratios for the TiC@1200Cl2, {TiC@1700Vac}@1200Cl2, and {TiC@1700Vac}@1000Cl2 samples were 19.8 nm, 27.6 nm and 6.1 nm, respectively, demonstrating that the crystalline size of the {TiC@1700Vac}@1200Cl2 sample is larger than 19
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those of the TiC@1200Cl2 and {TiC@1700Vac}@1000Cl2 samples.
Fig. 8. N2 isotherm data and pore size distribution of as received-TiC (a and d, respectively),
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TiC@1500C Vac (b and d, respectively), and TiC@1700C Vac (c and f, respectively).
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Fig. 8 shows the N2 adsorption isotherm and the estimated pore size distributions (PSDs) of the as received-TiC, the TiC@1500C Vac, and the TiC@1700C Vac, as determined from N2
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sorption isotherms using the NLDFT model for slit pores. All isotherm curves of three TiC samples show Type III of nonporous solids, showing small BET SSA values (2.99 m2/g for the as received-TiC, 1.11 m2/g for the TiC@1500C Vac, and 0.94 m2/g for the TiC@1700C Vac). The PSD patterns of the samples caused by surficial-free porous carbon in small amounts show pore formation in the range of 0.5 to 10 nm.
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Fig. 9. N2 isotherm data (a and b) and pore size distribution (c and d) of TiC@1000Cl2, TiC@1200Cl2,
{TiC@1500Vac}@1000Cl2,
{TiC@1500Vac}@1200Cl2,
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{TiC@1700Vac}@1000Cl2, and {TiC@1700Vac}@1200Cl2.
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Fig. 9 shows the N2 adsorption isotherm and the estimated pore size distributions
(PSDs)
of
the
{TiC@1700Vac}@1200Cl2,
{TiC@1700Vac}@1000Cl2,
{TiC@1500Vac}@1000Cl2, {TiC@1500Vac}@1200Cl2, TiC@1200Cl2, and TiC@1000Cl2 samples as determined from N2 sorption isotherms using the NLDFT model for slit pores. The marked hysteresis observed in the nitrogen adsorption confirms the presence of mesoporosity. The PSD patterns of the pre-vacuum-treated CDC samples are similar to those of the CDC samples (Figs. 9 (c and d)). At nearly identical chlorination temperatures, such as those used with the 1000Cl2 and 1200Cl2 samples, the pre-vacuum treatment of CDCs 21
ACCEPTED MANUSCRIPT ({TiC@1700Vac}@1000Cl2 and {TiC@1700Vac}@1200Cl2) led to larger degree of microporosity than in the {TiC@1500Vac}@1000Cl2 and {TiC@1500Vac}@1200Cl2 or TiC@1000Cl2 and TiC@1200Cl2 samples, which can range from 0.5 to 1.5 nm. These
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findings provide evidence that the pre-vacuum treatment before chlorination led to greater microporosity in these samples. All CDCs possessed individual ratios of micro/mesopore volume ratios with some portion of micropores (<2.0 nm) and mesopores (>2.0 nm): 3.22 in
{TiC@1500Vac}@1200Cl2,
4.25
in
{TiC@1700Vac}@1000Cl2,
volumes
in
and
1.2
in
Therefore, the observed increases in the
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{TiC@1700Vac}@1200Cl2, as shown in Table 1. micropore
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TiC@1000 Cl2, 1.06 in TiC@1200Cl2, 3.58 in {TiC@1500Vac}@1000Cl2, 1.86 in
{TiC@1500Vac}@1000Cl2,
{TiC@1500Vac}@1200Cl2,
{TiC@1700Vac}@1000Cl2, and {TiC@1700Vac}@1200Cl2 can be assumed to be related to the changes of lattice parameter, the small crystallite and grain size along with various defects
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after vacuum annealing process of Fig. 3 and Fig. 4.
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Fig.10. Comparisons of N2 isotherm data (a and b) and pore size distribution (c and d) of pretreated-TiC
CDC
sample
and
post-treated
TiC-CDC
sample.
(Pre-treatment
{TiC@1200Vac}@ 1200Cl2, Post-treatment : {TiC-CDC@ 1200 oC}@550O2 [Ref. 24]).
23
:
ACCEPTED MANUSCRIPT Fig. 10 shows comparisons of the pore texture properties according to the pretreatment and post-treatment of the CDC samples. Porous carbon after a post-treatment was synthesized from TiC using a previously reported air (oxygen) post-treatment procedure [24].
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In this post air (oxygen) treatment of CDC, an increase in the mesoporosity was obtained by removing the amorphous phase of the CDCs, which is one method for tailoring mesoporosity through O2 activation. While the post-treatments using O2 gas caused the CDC to have a
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considerable degree of mesoporosity, which lead to unique results in our previous study of electrode use in lithium ion batteries [24], the pre-treatment with a vacuum and the heat
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treatment of the TiC precursor had a great influence, imparting high microporosity in the CDC sample. For evidence of the microporosity effects, the electrochemical performance was
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measured for application as a supercapacitor.
24
ACCEPTED MANUSCRIPT Table 1. Characterization results of pre-vacuum treated CDC materials by N2 gas sorption technique.
{TiC@1500Vac}
{TiC@1500Vac}
{TiC@1700Vac}
{TiC@1700Vac}
1000 Cl2
1200 Cl2
@1000Cl2
@1200Cl2
@1000Cl2
@1200Cl2
0.799
0.69
0.82
0.74
0.98
0.84
0.73
0.62
0.78
1.08
0.89
0.83
0.56
0.32
0.61
0.71
0.72
0.45
0.174
0.30
0.17
0.38
3.22
1.06
3.58
1646
1066
1757
-1
(cm g ) NLDFT Pore 3
-1
volume (cm g ) NLDFT micro pore volume < 2 nm (N2 ads)a (cm3 g-1) NLDFT mesopore
0.17
0.38
1.86
4.25
1.20
1256
2086
1433
Micro/Mesopore volume ratio BET SSA (m2 g-1)
These results have been obtained applying the NLDFT method to N2 adsorption data
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obtained at -196 oC.
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volume >2 nm (N2 ads)a (cm3 g-1)
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3
TiC@
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Total Pore volume
TiC@
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ACCEPTED MANUSCRIPT Figs. 11~13 show the electrochemical performance capabilities of an electrode composed of CDC treated by the vacuum and heating process of the TiC precursor and 1.0M TEABF4/ACN, in which all tests were performed using full cells. The cyclic voltammetry
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results shown in Figs. 11 (a~c) demonstrate the typical rectangular shape up to a scan rate of 2 ~ 10 mV/s, indicating that the porous pre-treated CDC (Figs. 11(b) and 11(c)) acted in an ideally polarizable manner. Voltammograms of the TiC@1000 Cl2 in Fig. 11(a) demonstrate
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characteristically resistive and sloped CV shapes that describe more decay than in the pretreated CDCs shown in Fig. 11(d). The specific capacitances are 150 F/g, 120 F/g, and 90 F/g
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at 2 mV/s in the {TiC@1700Vac}@1000Cl2, {TiC@1500Vac}@1000Cl2, TiC@1000Cl2 samples, respectively. These values are approximately 10 % ~ 30 % higher than those of other reported CDC electrodes [32, 33]. For the chlorination samples treated at 1200 oC, the CV curve of TiC@1200Cl2 and {TiC@1500Vac}@1200Cl2 shown in Fig. 12(a) and (b)
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shows nearly rectangular shape associated with pure double-layer capacitance at slow scan rates. However, the CV curves of Figs. 12(a) and (b) start to deviate from rectangular shape with an increasing anodic current at high scan rates, especially for TiC@1200Cl2 of Fig.
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12(a). This observed deviation can be attributed to the nonzero time constant, in which the transient current falls exponentially with RC time constant (R=ESR), and a steady-state
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current (sC, where s is the scan rate and C is the double layer capacitance) [34-36]. The transient part lasts longer with the longer time constant, which means that more time is required to charge the capacitor, indicating the collapse of the rectangular shape of the current profiles related to the pseudocapacitance contribution [36]. On the other hand, the current rising and decaying became gradually depressed with increasing scan rate, as shown in the CV profiles of Figs. 12(a) and (b). This behaviour shows less reversible character of the electrodes due to the fact that the voltage signal could not reach the pore effectively under increased scan rate [36]. It is expected that this phenomenon may occur due to the distributed 26
ACCEPTED MANUSCRIPT capacitance effect in the porous electrodes with different pore dimensions and carbon structures. The depression of CV profiles by increased scan rate may have caused by the difference between the ohmic resistances of the electrolyte at mesopores and micropores [36].
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In the pores and structures of CDCs, while the CDC prepared at chlorination temperature of 1000 oC forms the micropore (< 2 nm) along with the presence of amorphous carbon, the CDC at 1200 oC contains the micropore (< 2 nm) plus mesopore (< 4 nm) along with less
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amorphous carbon and more ordered curved sheets of graphite. In respect of the changes in electrolyte resistance within pores, the non-uniform porosity caused by the mixed micropore
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and mesopore in the 1200 oC chlorination sample of Fig. 12(a) may have led to the quasirectangular shape by increased scan rate. In the case of {TiC@1700Vac}@1200Cl2 sample, the quasi-rectangular shape was recovered with increased scan rate, which might be caused by the increased micropore after vacuum treatment, as shown in Fig. 12(c). However, the capacitances of the CDC samples prepared by a chlorination temperature of 1200 oC showed
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lower than those of the CDC samples treated at a chlorination temperature of 1000 oC (Figs. 11(d) and 12(d)). Fig. 13 shows the retained capacitance as a function of the cycle number cycles)
and
galvanostatic
charge/discharge
curve
at
100
mA
g-1.
The
EP
(1200
{TiC@1500Vac}@1000Cl2 sample retains 90% of their initial capacitance levels (80.3 F g-1)
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after 1500 cycles at 100 mA g-1. The {TiC@1500Vac}@1000Cl2 exhibits a frequency response with an RC time constant of 1.87 sec and ESR resistance of 1.67 Ω at 100 mA g-1 .
27
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Fig. 11. Cyclic voltamogram (a, b, and c) of TiC@1000 Cl2, {TiC@1500Vac}@1000Cl2, and TiC@1700Vac}@1000Cl2 and specific capacitance comparison (d) with TiC@1000Cl2 and
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Pre-treated TiC-CDC chlorinated at 1000 oC.
28
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Fig. 12. Cyclic voltamogram (a, b, and c) of TiC@1200 Cl2, {TiC@1500Vac}@1200Cl2, and TiC@1700Vac}@1200Cl2 and specific capacitance comparison (d) with TiC@1200Cl2 and
AC C
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Pre-treated TiC-CDC chlorinated at 1200 oC.
29
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Fig. 13. Cyclic performance and galvanostatic charge/discharge (100 mA g-1) of {TiC@1500Vac}@1000 Cl2.
30
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4. Conclusions The application of a pre-vacuum-treated CDC as a supercapacitor electrode was investigated. The system shows higher specific capacitance with a pre-vacuum-treated CDC
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than with a normal CDC at an identical chlorination temperature. This feature was obtained by tailoring the high microporosity of CDCs by a pre-vacuum process with a carbide precursor. N2 sorption measurements and TEM images verified the increased microporosity
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caused by amorphitization from the pre-vacuum treatment. The structural rearrangement and the increased microporosity caused by the pre-vacuum treatment give rise to the synergistic
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effect of enhanced capacitance as compared to normal CDCs. In supercapacitor application of the pre-vacuum treated CDC samples, the specific capacitances are 150 F/g and 120 F/g at 2 mV s-1 in the {TiC@1700Vac}@1000Cl2, {TiC@1500Vac}@1000Cl2, TiC@1000Cl2 samples, respectively, which are approximately 10 % ~ 30 % higher than those of other
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reported CDC electrodes. These results of the pre-vacuum CDC system represent the potential of the CDC electrode material, which should be investigated further to determine its
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capability to serve as an active material in practical and capacitive energy storage systems.
31
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Acknowledgements
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This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Trade, Industry and Energy (10050391) of the Republic of Korea, the National Research Foundation (NRF-2011-CIAAA0010030538 and NRF-2015069204) and conducted under the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER, B6-2413).
32
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Figure Captions
Fig. 1 Illustration of synthetic procedure of the micro/meso-porous vacuum-treated-TiC
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CDC. The crystalline carbide (TiC) as a precursor for vaccum-treated-TiC CDC sample expresses the fcc crystalline structure. After vacuum (10-5 torr) and heat (1500 oC and 1700 o
C) treatments for 4 hours, the carbide was transformed to large spherical aggregated
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particles. After chlorination process, the pre-treated carbide was transformed to CDC with high micro-porosity property.
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Fig. 2. SEM images and Particle Size Distributions (PSDs) of TiC (a, d), vacuum-treated TiC at 1500 oC (b, e), vacuum-treated TiC at 1700 oC (c, f), respectively.
Fig. 3 XRD pattern of as-received TiC, TiC@1500Vac, and TiC@1700Vac in the each 2
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range of 5o~50o (a) and 60o~80o (b), respectively, and XRD pattern of TiC@1200Cl2, {TiC@1500Vac}@1200Cl2, and {TiC@1700Vac}@1200Cl2 in the each 2
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(c) and 60o~80o (d).
range of 5o~50o
Fig. 4. XRD powder diffraction pattern of (a) as-received TiC, (b) TiC@1500C Vac, and (c)
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TiC@1700C Vac at room temperature (cross) plus the Rietveld refinement (solid line, red), using space group Fd-3m, and difference plot (bottom, blue). (d) FWHM of as-received TiC, TiC@1500C Vac, and TiC@1700C Vac. The inset shows the refined structure with atomic parameters in Ti at (0, 0, 0) and C at (0.5, 0.5, 0.5). The refinement was characterized by (a) λ2 = 2.064 and Rwp = 25 %, (b) λ2 = 4.8 and Rwp = 22 %, and (c) λ2 = 5.4 and Rwp = 23 %. Fig. 5. TEM images of (a) as-received TiC precursor, (b) vacuum-treated TiC at 1500 oC, (e) TiC-CDC chlorinated at 1200 oC using as-received TiC precursor, (f) vaccum-treated TiCCDC chlorinated at 1200 oC using as-received TiC precursor. 33
ACCEPTED MANUSCRIPT Fig. 6. TEM images of {TiC@1500C Vac} @1000Cl2 (a), {TiC@1700C Vac} @1000Cl2 (b), {TiC@1500C Vac} @1200Cl2 (c), and {TiC@1700C Vac} @1200Cl2 (d). Fig. 7 Raman spectra of (a) TiC@1200Cl2, (b) {TiC@1700C Vac}@1000Cl2, and (c)
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{TiC@1700C Vac}@1200Cl2. Fig. 8. N2 isotherm data and pore size distribution of as received-TiC (a and d, respectively), TiC@1500C Vac (b and d, respectively), and TiC@1700C Vac (c and f, respectively).
{TiC@1500Vac}@1000Cl2,
{TiC@1500Vac}@1200Cl2,
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TiC@1200Cl2,
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Fig. 9. N2 isotherm data (a and b) and pore size distribution (c and d) of TiC@1000Cl2,
{TiC@1700Vac}@1000Cl2, and {TiC@1700Vac}@1200Cl2.
Fig.10. Comparisons of N2 isotherm data (a and b) and pore size distribution (c and d) of pretreated-TiC
CDC
sample
and
post-treated
TiC-CDC
sample.
(Pre-treatment
:
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{TiC@1200Vac}@ 1200Cl2, Post-treatment : {TiC-CDC@ 1200 oC}@550O2 [Ref. 24]). Fig. 11. Cyclic voltamogram (a, b, and c) of TiC@1000 Cl2, {TiC@1500Vac}@1000Cl2, and TiC@1700Vac}@1000Cl2 and specific capacitance comparison (d) with TiC@1000Cl2 and
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Pre-treated TiC-CDC chlorinated at 1000 oC.
Fig. 12. Cyclic voltamogram (a, b, and c) of TiC@1200 Cl2, {TiC@1500Vac}@1200Cl2, and
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TiC@1700Vac}@1200Cl2 and specific capacitance comparison (d) with TiC@1200Cl2 and Pre-treated TiC-CDC chlorinated at 1200 oC Fig. 13. Cyclic performance and galvanostatic charge/discharge (100 mA g-1) of {TiC@1500Vac}@1000 Cl2.
34
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Table Captions Table 1. Characterization results of pre-vacuum treated CDC materials by N2 gas sorption
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technique.
36
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supercapacitors, Journal of Power Sources 161(1) (2006) 730-736.
39
ACCEPTED MANUSCRIPT Table 1. Characterization results of pre-vacuum treated CDC materials by N2 gas sorption technique.
{TiC@1500Vac}
{TiC@1500Vac}
{TiC@1700Vac}
1000 Cl2
1200 Cl2
@1000Cl2
@1200Cl2
@1000Cl2
@1200Cl2
0.799
0.69
0.81
0.74
0.98
0.84
0.73
0.62
0.71
1.08
0.89
0.83
0.56
0.32
0.54
0.71
0.174
0.30
0.17
3.22
1.06
1646
1066
-1
(cm g ) NLDFT Pore volume (cm3 g-1) NLDFT micro pore
0.72
0.45
0.38
0.17
0.38
3.18
1.86
4.25
1.20
1757
1256
2086
1433
NLDFT mesopore volume >2 nm (N2 ads)a (cm3 g-1) Micro/Mesopore
BET SSA (m2 g-1)
These results have been obtained applying the NLDFT method to N2 adsorption data
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obtained at -196 oC.
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a
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volume ratio
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volume < 2 nm (N2 ads)a (cm3 g-1)
{TiC@1700Vac}
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3
TiC@
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Total Pore volume
TiC@