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Preparation and characterization of mesoporous activated carbons from nonporous hard carbon via enhanced steam activation strategy
Journal Pre-proof Preparation and characterization of mesoporous activated carbons from nonporous hard carbon via enhanced steam activation strategy Yeong-Rae Son, Soo-Jin Park PII:
S0254-0584(19)31268-4
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122454
Reference:
MAC 122454
To appear in:
Materials Chemistry and Physics
Received Date: 21 August 2019 Revised Date:
17 October 2019
Accepted Date: 13 November 2019
Please cite this article as: Y.-R. Son, S.-J. Park, Preparation and characterization of mesoporous activated carbons from nonporous hard carbon via enhanced steam activation strategy, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/j.matchemphys.2019.122454. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
Manuscript submitted to “Materials Chemistry and Physics” as an Article
Preparation and characterization of mesoporous activated carbons from nonporous hard carbon via enhanced steam activation strategy
Yeong-Rae Son and Soo-Jin Park*
Department of Chemistry, Inha University, 100 Inharo, Incheon 22212, Korea
*Corresponding author. Tel.: +82-32-876-7234; Fax: +82-32-867-5604.
E-mail addresses: [email protected] (S.-J. Park)
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Abstract Activated carbon materials have been extensively investigated as electrode materials for supercapacitors because of their low cost, high specific surface area, and high charge storage capacity. In this work, we developed activated carbon with a high specific surface area from a nonporous hard carbon (HC) material through an enhanced steam activation strategy. In general, steam activation is carried out under atmospheric pressure. The enhanced steam activation strategy introduced in this study involves controlling the pressure of the injected steam. The activated hard carbon (AHC) obtained via the proposed activation strategy exhibited enhanced textural properties compared with AHC prepared under atmospheric steam pressure. Furthermore, AHCs with a specific surface area greater than 2000 m2 g−1 were obtained in high yield within a short time. The specific capacitance of the AHCs increased by a factor of up to approximately 12.7 compared with that of the nonporous HC. Furthermore, the specific capacitance of AHCs with a higher mesopore volume was confirmed to be retained with increasing applied current, indicating high rate capability. Therefore, the proposed enhanced steam activation strategy not only improves the energy storage capacity but also is effective in forming mesopores, which is an effective approach to improving the power characteristics of activated carbon materials.
Keywords: Enhanced steam activation; Activated carbon; Textural properties; Steam pressure; Mesoporous activated carbon
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1. Introduction Porous materials have been developed for various applications such as purification of the ecosystem and medical/pharmaceutical and isolation applications. In particular, porous materials with nanoscale pores have been a point of focus because of their extremely high surface area per unit mass [1]. Nanoscale pores are categorized by their size: micropores with a diameter of 2 nm or less; mesopores with a diameter between 2 and 50 nm; or macropores with a diameter greater than 50 nm [2]. Numerous nanoporous materials occur naturally or are fabricated artificially, including zeolites, porous silicas, porous polymers, and metal– organic frameworks [3–8]. Among the nanoporous materials, carbon-based porous materials have attracted intensive attention. Such materials have been used in diverse areas such as gas adsorption and energy storage because of their extraordinary physicochemical properties, which include high electrical conductivity, good chemical durability, and high specific surface areas [1,9,10]. Research on the supply and storage of sustainable and renewable energy has attracted attention because it will likely contribute to the solution to the global warming issue, which is currently worsening because of rapid industrial development. Devices that can store energy, such as lithium-ion batteries (LiBs), capacitors, supercapacitors, and fuel cells, have attracted particular interest as worldwide demand for portable and wearable electronic devices such as smartphones, smart watches, and smart glasses has steadily increased. Among the various aforementioned power sources, supercapacitors are potential candidates to replace LiBs. Supercapacitors store charge by forming electrostatic double-layers or via a Faradaic redox reaction at the interface between an electrode surface and an electrolyte [11–14]. Recently, carbon materials such as activated carbon, graphene, and carbon nanotubes have been 3
intensively researched as supercapacitor electrode materials [11–22]. Supercapacitors based on carbon electrodes have long lifetimes because they store charge via an electrostatic charge-storage mechanism. For the carbon electrode materials, activated carbon is commercially used because of its low cost, high specific surface area, and high charge storage capacity [23,24]. The capacitance of supercapacitors with activated carbon can be improved by enhancing its porosity; thus, extensive efforts have been devoted to enhancing the porosity of carbon materials for use in supercapacitors. Activated carbons can be prepared by two major activation strategies: 1) physical activation by oxidative gases (e.g., H2O (steam) or CO2) [25–27] and 2) chemical activation by chemical reagents (e.g., KOH, NaOH, or ZnCl2) [28–30]. Chemically activated carbon materials have been reported to possess a high specific surface area and pore volume because the activating agents penetrate deeply into and react within the carbon lattice. However, because the byproducts must be removed after the chemical activation, additional washing and removal processes are required, which may result in the emission of contaminants that negatively affect the environment. By contrast, the physical activation strategy is relatively eco-friendly and cost-effective because, unlike the chemical activation approach, it does not require additional processes [3,31]. However, in many cases, activated carbon materials prepared via existing physical activation methods have inferior textural properties compared with activated carbon materials prepared via chemical activation [32]. Thus, improving the porosity of activated carbon via a physical strategy is still challenging. In this study, to form nanoscale pores in nonporous hard carbon (HC), we used an enhanced steam activation strategy that controls the pressure of injected steam. A series of activated hard carbons (AHCs) were prepared under different activation conditions, such as 4
different temperatures, injected steam pressures, and activation times. After the steam activation, the AHCs exhibited a substantially higher specific surface area than the activated carbon prepared under atmospheric conditions. Furthermore, we confirmed that the pore size of AHCs was controlled by the activation conditions. The morphologies of the activated carbon materials were analyzed by scanning electron microscopy (SEM), and their chemical characteristics were elucidated by X-ray photoelectron spectroscopy (XPS) and energydispersive spectroscopy (EDS). The electrochemical behaviors of the AHCs with different textural properties were evaluated by cyclic voltammetry (CV), galvanostatic charge and discharge (GCD) measurements, and electrochemical impedance spectroscopy (EIS).
2. Experimental 2.1. Materials HC was acquired from Aekyung Petrochemical Co., Ltd., Korea. Triply Deionized water as a steam source was obtained from an Aqua MAX Basic 360 and Ultra 370 water purification system from Young In Chromass Co., Ltd., Korea. The following chemicals were used to prepare a slurry for the fabrication of working electrodes: Ketjenblack (KB) as a conductive material (AkzoNobel); polyvinylidene fluoride (PVDF, Mw = 534,000) as a binder (SigmaAldrich, Korea); and 1-methyl-2-pyrrolidone (NMP, ≥99%) as a solvent (TCI Co.). Sodium sulfate (Na2SO4) for the electrolyte was acquired from Daejung Chemicals & Metals Co., Ltd. All chemical reagents were used without further purification.
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2.2. Preparation of activated hard carbon AHCs were prepared by enhanced steam activation strategy. A semi-cylindrical quartz boat containing hard carbon (HC) was placed in the center of the heating area of a quartz tube, which was subsequently heated to a certain temperature. Before activation, the HC was heated to a certain activation temperature at a heating rate of 5 °C min−1 under a flowing inert N2 atmosphere (150 mL min−1). When the temperature reached activation temperature, the flow rate of N2 was reduced to less than 10 mL min−1 and ultrapure water was injected at a rate of 0.8 mL min−1. After cooling to room temperature, the boat containing the AHCs was carefully removed. A series of AHCs were prepared as follows: 1) To evaluate the most effective activation temperature, steam-activated HC (AHC-Tn, where n indicates the activation temperature, e.g., the sample activated at 900 °C is represented as AHC-T9) was prepared as a function of temperature from 600 to 900 °C under a fixed steam pressure (1.4 gauge bar) and activation time (40 min). 2) After the activation temperature was evaluated (in this study, 900 °C), the steam pressure and activation time were adjusted to enhance the textural properties of the AHC. The steam pressure and activation time were controlled in the range from 1.0 (atmosphere) to 1.4 gauge bar and from 30 to 60 min, respectively. The sample prepared from adjusted activation conditions was denoted as AHC-P-t, where P is the adjusted pressure and indicates one decimal place of gauge bar and t is activation time in minutes (e.g., AHC-4-45 indicates AHC prepared at a steam pressure of 1.4 gauge bar for 45 min).
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2.3. Characterization of activated carbon materials The crystal structure of the samples prepared was examined via X-ray diffraction (XRD) analysis (D2 PHASER, Bruker) using a Lynx-Eye detector and Cu-Kα radiation generated at 30 kV and 10 mA (λ = 1.5406 Å). The morphologies of the materials were observed via field-emission SEM (Hitachi S-4300). An energy dispersive X-ray spectroscopy (EDS) system mounted on the scanning electron microscope was used to measure the concentrations of elements. XPS was performed using a K-Alpha instrument (Thermo Scientific) to analyze the chemical compositions of the HC and AHCs. Raman spectroscopy was performed to identify the graphitic structure of the pristine HC and AHC samples using HORIBA LabRAM Revolution at a wavelength of 532 nm. The microstructure in nanoscale of pristine HC and AHC-4-45 was characterized using transmission electron microscopy (TEM; Philips CM200). To examine the textural properties of the HC and AHCs, N2 adsorption–desorption isotherms were recorded at 77 K using a gas adsorption analyzer (BEL, BELSORP-max, Japan). The textural properties of the samples were determined from the N2 sorption isotherms. The specific surface area of the AHCs was calculated by the Brunauer–Emmett– Teller (BET) equation over a p/p0 range from 0.01 to 0.2. The total pore volume was estimated at a relative pressure p/p0 of 0.99. The pore size distributions were determined via the nonlocal density functional theory (NLDFT) model.
2.4. Electrochemical performance measurements The electrochemical properties of the AHCs prepared under different activation times were 7
evaluated using a three-electrode configuration comprising a working electrode (AHCs on Ni foam), a reference electrode (Ag/AgCl), and a current collector (Pt coil) over a 1 V (−0.3 to 0.7 V) potential window in a 0.5 M Na2SO4/H2O aqueous electrolyte. The working electrode was fabricated by coating a slurry containing AHCs, KB, and PVDF with a weight ratio of 8:1:1 onto a 1 × 4 cm2 Ni foam current collector with dimensions of 1 × 1 cm2 and drying the coated Ni foam in a drying oven at 80 °C for 12 h. The electrochemical analysis of the asprepared AHCs was conducted via CV, GCD, and EIS measurements using an IviumStat instrument. Before the analysis, the working electrodes were dipped into 0.5 M Na2SO4 electrolyte for at least 3 h. The specific capacitance (Csp) of the AHCs was calculated using the equation Csp = I∆t/(m∆V) (F g−1) from the GCD discharge plots, where I is the applied discharge current, ∆t is the discharge time, m (g) is the mass of active material coated onto the Ni-foam working electrode, and ∆V is the potential window. The EIS measurement was conducted in the frequency range from 100 kHz to 0.01 Hz with an amplitude of 5 mV. The imaginary part (C″) of the capacitance was evaluated by the equation C″(ω) = Z1(ω)/{ω|Z2(ω)|2}, where ω is the pulsation and Z1 and Z2 are the real and imaginary parts of the impedance, defined as
1
=|
+ 2
| .
3. Results and Discussion Figure 1 presents a schematic of the preparation of activated carbons using the steam activation strategy. A semicylindrical boat containing nonporous HC was placed in a quartz tube, and the tube was inserted into a tube furnace. After the tube reached a certain activation temperature, ultrapure deionized water was injected into the tube at a constant flow rate via a 8
flow-rate-controlled pump. The injected water was evaporated in the heated tube at a high temperature, and the water vapor reacted with the HC surface to form nanoscale pores. The activation mechanism is proposed as follows [3]: ∗
+H O→C∙H O
(1)
H O →∙ OH + ∙ H
(2)
C ∙ OH + ∙ H → H + C ∙ O
(3)
C ∙ O → CO
(4)
Figure 1. Schematic of the preparation of steam-activated hard carbon materials.
Figure 2 shows the N2 adsorption–desorption isotherms of AHCs prepared at different activation temperatures. The resulting isotherms of the AHCs prepared at different activation temperatures show an intense uptake the region p/p0 < 0.1, characteristic of a microporous material. In particular, the AHC-T9 showed a type IV N2 isotherm and a type H4 hysteresis loop in the p/p0 region between 0.5 and 1.0, indicating that the AHC-T9 was a mesoporous material with limited slit-like mesopores [2]. The specific surface areas of AHC-T6, AHC-T7, 9
AHC-T8, and AHC-T9 were 200, 537, 1083, and 2371 m2 g−1, respectively, and their total pore volumes were 0.086, 0.215, 0.454, and 1.425 cm3 g−1, respectively. Thus, the most effective activation temperature was 900 °C.
Figure 2. N2 adsorption–desorption isotherms of AHCs prepared at different activation temperatures.
Figure 3 presents SEM micrographs of the AHCs prepared at different activation temperatures. The surface of pristine HC showed no pores. Some large and irregular lumps and spherical agglomerates with a diameter of 0.2 to 0.3 µm were confirmed to be present on the surface. After steam activation, the agglomerates on the surface of HC granules were deformed. The progression of the deformation was substantial and holes were created with increasing activation temperature. As shown in Figure 3(d), the agglomerates on the surface of AHC activated at 800 °C almost disappeared. Moreover, the surface of the AHC activated at 900 °C not only shows that the agglomerates were removed but also reveals a large number of holes on the surface. That is, the SEM observations indicate that, when the HC is activated 10
by steam, the agglomerates on the surface are first deformed at low temperature and then steam penetrates into the interior of the HC granules to form nanoscale pores. To further investigate why the holes were intensively formed around the agglomerates, EDS was carried out. From the O1s XPS results described later (Figure 4), oxygen functional groups present in surface of HC reacted with H2O vapor. As shown in Figure S1, oxygen is concentrated in the periphery of the agglomerates. Therefore, it is believed that the holes were intensively created around the agglomerates because active sites that react with H2O vapor were concentrated near the agglomerates.
Figure 3. SEM images of AHCs prepared at different activation temperatures: (a) pristine HC, (b) AHC-T6, (c) AHC-T7, (d) AHC-T8, and (e) AHC-T9.
XPS was used to identify the chemical characteristics of the as-prepared AHCs 11
according to their activation temperature. The C1s XPS spectra of the pristine HC are presented in Figure 4(a). Five deconvoluted peaks were observed at 284.5, 284.95, 285.7, 288.2, and 292.5 eV, corresponding to sp2-hybridized carbon, sp3-hybridized carbon, oxygenfunctionalized carbon (epoxide, hydroxyl, carbonyl, ether, and carboxylate groups), and the π→π* shake-up of conjugated carbon systems [33]. The C1s spectra of AHCs prepared at different activation temperatures do not substantially differ from the spectrum of the pristine HC. The peaks at approximately 292.5 eV decreased in intensity, possibly because the conjugated systems were deformed by steam activation.
Figure 4. C1s XPS spectra of (a) the pristine HC (with deconvoluted peaks) and (b) AHCs prepared at different activation temperatures. O1s XPS spectra of (c) AHC-T6, (d) AHC-T7, (e) AHC-T8, and (f) AHC-T9. 12
The O1s spectra of AHCs activated at different temperatures are shown in Figure 4(c) to (f). The three major deconvoluted peaks were observed at 530.5, 532.1, and 533.7 eV, corresponding to C=O groups (e.g., carboxylate, lactone, carbonyl, and quinone groups), epoxide groups, and ether groups, respectively. With increasing activation temperature, the intensity of the peak associated with the epoxide group gradually decreased, indicating that the H2O vapors reacted with the epoxide group in the HC, resulting in porous AHCs. In addition, in the case of the sample activated at 900 °C, not only the peak of the epoxide group but also that of the C=O group decreased in intensity compared with the peaks in the spectra of AHCs prepared at lower activation temperatures. Therefore, the most effective steam activation effect at 900 °C could be interpreted as the active reaction of epoxide groups and C=O groups on the HC surface. To investigate the effect of steam pressure on the textural properties of AHCs, N2 gas sorption experiments were performed. The N2 sorption isotherms and pore size distributions of AHCs prepared at different steam pressures are shown in Figure 5. As shown in Figure 5(a), the N2 isotherms of all of the AHCs showed a sharp uptake in the region p/p0 < 0.1, characteristic of microporous materials. The AHC-0-45 prepared under atmospheric steam pressure exhibited a type I isotherm with no hysteresis loop. However, for the samples prepared under increased steam pressure, a type H4 hysteresis loop was observed and the loop expanded with increasing steam pressure, indicating that large pores were generated [34,35]. As depicted in Figure 5(b) and (c), in AHC-0-45, only micropores with diameters of 0.6–0.7 nm and 1.0–1.3 nm were confirmed without mesopores. The proportion of micropores with diameters of above 0.8 nm in AHC-2-45, AHC-3-45, and AHC-4-45 was 99.2, 93.3, and 82.6%, respectively, higher than that in AHC-0-45 (35.7%). In other words, 13
the micropores with relatively large size were formed in the AHCs prepared by applying steam pressure. This indicates that the micropore size can be expanded by increasing the steam pressure during steam activation. Moreover, it can be seen that the mesopore volume with sizes of 2–3.5 nm and 3.8–4.4 nm was increased with increasing steam pressure. In the case of AHC-4-45, the specific surface area and mesopore volume were reduced due to excessive pore expansion caused by high steam pressure. In addition, as shown in Figure 5(d), the yield after the pressure-adjusted steam activation decreased with increasing steam pressure. In addition, the mesopore volume increased with increasing steam pressure without deterioration of the micropore volume, indicating that the suggested activation strategy is an effective and efficient approach for inducing mesopore formation in carbon-based materials.
Figure 5. (a) N2 adsorption–desorption isotherms, the (b) micropore and (c) mesopore size distribution, and (d) the yield and pore volume of AHCs under steam pressure.
The textural properties of AHCs prepared at different steam pressures and activation times are summarized in Table 1. The specific surface areas of AHC-0-45, AHC-2-45, AHC14
3-45, and AHC-4-45 were 1120, 1711, 2278, and 2039 m2 g−1, respectively, and the Vt (i.e., the sum of Vmicro and Vmeso) values were 0.586, 0.916, 1.569, and 1.493 cm3 g−1, respectively. The specific surface area and pore volume increased with increasing steam pressure. To compare which steam pressure condition was more efficient, AHCs were prepared with different activation times at a steam pressure of 1.3 and 1.4 gauge bar, respectively. As a result, the specific surface area, pore volume, and yield of AHC-4-30 were higher than those of AHC-3-30, indicating that the more effective steam condition was the steam pressure of 1.4 gauge bar. The lower textural properties and yield of AHC-4-45 compared with those of AHC-3-45 are attributed to excessive activation. In addition, the proportion of the mesopore volume increased with increasing steam pressure and activation time.
Table 1. Textural properties of the AHCs prepared under different conditions. 15
To characterize carbon structure of AHCs prepared with steam pressure, Raman spectroscopy was performed. As shown in Figure 6, two typical peaks observed in many carbonaceous materials were measured in both pristine HC and a series of AHCs. The D-band corresponding to disordered defects appeared at 1331 cm−1. The G-band corresponding to graphitic lattice appeared in the range of 1579 – 1600 cm−1. The G-band of AHCs was slightly blue-shifted compared with that of pristine HC. Furthermore, D- and G-band of AHCs prepared by applying steam pressure were sharpened compared with those of pristine HC and AHC-0-45. The blue-shifted G-band and sharpening of D- and G-band were attributed to the reduction of defects by removal of functional groups, such as epoxides, on the HC and the formation of sp2-double bond carbon networks [36]. As shown in TEM images (Figure S2), compared with pristine HC, micro/mesopores were formed in AHC-4-45, and partially graphitized carbon lattice was confirmed.
Figure 6. Raman spectra of pristine HC and AHCs prepared with different steam pressure. 16
In addition, the intensity ratio of D-band to G-band (ID/IG) of pristine HC and AHC-(0, 2, 3, 4)-45, which is indicator of defects in carbon materials, were 0.99, 0.96, 1.01, 0.98, and 1.08, respectively. The ID/IG ratio of AHCs except AHC-4-45 was not significantly different from that of pristine HC. The increase in the ID/IG ratio of AHC-4-45 is attributed to the increase in the edge site caused by the expansion of the pore size formed by the excessive steam activation effect [37,38]. Figure 7 shows the N2 sorption isotherms and pore size distributions of AHCs prepared with different activation times under a steam pressure of 1.4 gauge bar. The N2 isotherms of the AHCs were type IV. The hysteresis loop was evaluated and found to expand with increasing activation time. Furthermore, as the activation time increased, the mesopore volume increased and micropores with a relatively large diameter were formed. The maximum specific surface area of 2351 m2 g−1 was attained under a steam pressure of 1.4 gauge bar and an activation time of 40 min.
Figure 7. (a) N2 adsorption–desorption isotherms and (b) micropore and (c) mesopore size distributions of AHCs prepared with different activation times.
The crystalline structures of the AHCs prepared with different activation times were characterized by XRD analysis. The XRD patterns of the pristine HC and AHCs in the range 5° ≤ 2θ ≤ 90° are shown in Figure 8. Broad peaks were observed in the pattern for the 17
pristine HC at 2θ = 24.3°, indicating a typical amorphous crystalline structure in the amorphous carbon materials. The intensity of the peaks decreased after the steam activation; this effect is attributed to structural modifications resulting from the activation, indicating the formation of nonuniform lattice strain in the AHCs [39–42].
Figure 8. XRD patterns of AHCs prepared with different activation times.
Figure 9 shows the morphological structures and chemical composition of the AHCs prepared with different activation times. The size of the AHC granules observed at low magnifications decreased with increasing activation time. Numerous pores and cracks were observed in the high-magnification images of the AHCs. In Figure 9(b) and (c), pores resembling circular dots were confirmed, whereas cracks were observed between pores in Figure 9(d). These observations indicate that macropores formed on the surface because of excessive activation. The amount of oxygen in the AHCs was lower than that in the pristine HC because the surface oxygen functionalities of the HCs were eliminated by reaction with H2O vapor at high temperatures, as confirmed in the XPS analysis results (Figure 4). 18
Figure 9. SEM images and EDS results of AHCs prepared with different activation times: (a) pristine HC, (b) AHC-4-30, (c) AHC-4-40, and (d) AHC-4-45 (inset images are at high magnification).
The electrochemical properties of the AHCs prepared at different activation times under a steam pressure of 1.4 bar, which was the most effective steam pressure condition, were analyzed by CV and GCD. The analyses were carried out in 0.5 M Na2SO4/H2O using a three-electrode system over the potential window from −0.3 to 0.7 V. Figure 10 shows the CV curves measured at 10 mV s−1, the GCD plots measured at 1 A g−1, and the specific capacitance values measured at different current densities. The CV curves of the AHCs showed an almost rectangular configuration, indicating EDL behaviors. In addition, the CV curve areas of the AHCs were larger than that of pristine HC, meaning that the specific capacitance of AHCs was substantially increased compared with that of nonporous HC. Similar results were confirmed from the GCD plots. The triangular and symmetrical GCD curves revealed that the AHCs have low resistance. From the IR drop, equivalent series resistance (ESR) was estimated using the equation, ESR = IRdrop/2I, where I is an applied current [43]. The calculated ESR of AHC-4-(30, 40, 45) is determined as 5.75, 8.25, and 5.91 19
Ω, respectively. Among the AHCs, the AHC-4-40 exhibited the highest ESR but the smallest IR drop at the identical current density, as shown in the inset of Figure 10(b). This indicates that AHC-4-40 has the lowest energy loss compared to the other two AHCs [44,45]. The specific capacitances of pristine HC, AHC-4-30, AHC-4-40, and AHC-4-45 measured at a current density of 0.5 A g−1 were 5.6, 72.6, 79.8, and 71.0 F g−1, respectively. Sample AHC-440 exhibited the highest specific capacitance, which was greater than that of the pristine HC by a factor of approximately 12.7. This substantial increase in specific capacitance is attributed to an increase of the area accessible to the electrolyte ions by steam activation. As a result, the specific capacitance of the AHCs was not substantially different; however, AHC-440, which exhibited the largest specific surface area, was confirmed to also exhibit the largest specific capacitance. As depicted in Figure 10(c), the capacitance retention of the AHCs was measured as the range of applied current density was increased from 0.5 to 5 A g−1. The capacitance of AHC-4-30, AHC-4-40, and AHC-4-45 decreased from 72.6, 79.8, and 71.0 F g−1 to 40, 44.5, and 45 F g−1, indicating retentions of 55.1, 55.8, and 63.4%, respectively. These differences in rate capability can be interpreted by evaluating the relaxation time constant (τ0). The τ0 is determined from the maximum peak frequency (f0), defining as τ0 = 1/f0. As shown in Figure 10(d), the shorter τ0 of AHC-4-45 (20 s) than that of AHC-4-30 (33.3 s) was evaluated, demonstrating that the discharging power of AHC-4-45 is higher than that of AHC-4-30. Consequently, the shorter τ0 demonstrated that the electrolyte ions were more effectively diffused in the pores of AHC-4-45, which can be ascribed to the high proportion of mesopores in the AHC-4-45, as confirmed in Table 1 [13,30,46].
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Figure 10. Electrochemical analysis of AHCs prepared with different activation times: (a) CV curves, (b) GCD plots (the inset indicates the IR drop), (c) specific capacitance measured at different current densities, and (d) the frequency dependence of the imaginary part of the specific capacitance of AHC-4-30 and AHC-4-45.
To investigate the cycle performance of AHC-4-40, the charging-discharging cycling was carried out over 5000 cycles at a current density of 5 A g−1. As shown in Figure 11, the specific capacitance of AHC-4-40 was decreased with increasing the cycle number. After 3000 cycles, the specific capacitance was retained approximately 82.8% compared to the initial cycle, and the retention was maintained after 5000 cycles.
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Figure 11. Cycle stability of AHC-4-40 by repeated charging-discharging measurement at a current density of 5 A g−1.
The AHCs prepared via enhanced steam activation exhibited enhanced textural properties compared with the pristine HC and AHC prepared under atmospheric steam pressure. Distinctive structural and chemical properties of the AHCs were observed as the activation conditions (e.g., activation temperature, steam pressure, and activation time) were changed. The enhanced steam activation strategy proposed in the present study is an efficient and effective approach to the high-yield preparation of activated carbon with enhanced pore characteristics compared with those of activated carbons produced by conventional steam activation. In addition, because activated carbon with a high proportion of mesopores can be produced, the proposed method can be used to produce carbon-based materials with excellent power characteristics.
4. Conclusions In this research, AHCs with high porosity were prepared via an enhanced steam activation 22
strategy. The pristine HC was effectively activated at 900 °C. The specific surface areas of AHCs prepared under a steam pressure of 1.4 gauge bar were greater than 2000 m2 g−1, which is greater than the specific surface area of the AHC prepared under atmospheric steam pressure. A large number of pores were formed on the surface of the AHCs. By characterizing the surface of the AHCs, we confirmed that the epoxy groups reacted with H2O vapor and that the extent of the reaction increased with increasing activation temperature. Furthermore, at 900 °C, C=O groups also reacted with H2O vapor, indicating that the activation effect increased at 900 °C. The prepared AHCs exhibited increases in specific surface area and pore volume with increasing steam pressure and activation time. In addition, substantial increase in the pore volume of the mesopores was confirmed. The specific capacitance of developed AHCs was as much as 12.7 times greater than that before activation because of the pores generated by the enhanced steam activation. Furthermore, the AHC with a high mesopore volume was confirmed to exhibit better rate-capability performance. The enhanced activation strategy suggested in this study can be applied in various fields requiring materials with nanoscale porosity, such as the removal of contaminants (toxic molecules or nanoparticles) from a gas or liquid phase as well as energy storage applications.
Acknowledgements This work was supported by the Technology Innovation Program (or Industrial Strategic Technology Development Program (10080293, Development of carbon-based non-phenolic electrode materials with 3,000 m2/g grade surface area for energy storage device) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).
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Conflicts of Interest The authors declare no conflicts of interest.
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Figure captions Figure 1. Schematic of the preparation of steam-activated hard carbon materials. Figure 2. N2 adsorption–desorption isotherms of AHCs prepared at different activation temperatures. Figure 3. SEM images of AHCs prepared at different activation temperatures: (a) pristine HC, (b) AHC-T6, (c) AHC-T7, (d) AHC-T8, and (e) AHC-T9. Figure 4. C1s XPS spectra of (a) the pristine HC, with deconvoluted peaks, and (b) AHCs prepared at different activation temperatures. O1s XPS spectra of (c) AHC-T6, (d) AHC-T7, (e) AHC-T8, and (f) AHC-T9. Figure 5. (a) N2 adsorption–desorption isotherms, the (b) micropore and (c) mesopore size distribution, and (d) the yield and pore volume of AHCs under steam pressure. Figure 6. Raman spectra of pristine HC and AHCs prepared with different steam pressure. Figure 7. (a) N2 adsorption–desorption isotherms and (b) micropore and (c) mesopore size distributions of AHCs prepared with different activation times. Figure 8. XRD patterns of AHCs prepared with different activation times. Figure 9. SEM images and EDS results of AHCs prepared with different activation times: (a) pristine HC, (b) AHC-4-30, (c) AHC-4-40, and (d) AHC-4-45 (inset images are at high magnification). Figure 10. Electrochemical analysis of AHCs prepared with different activation times: (a) CV curves, (b) GCD plots (the inset indicates the IR drop), (c) specific capacitance measured at different current densities, and (d) the frequency dependence of the imaginary part of the specific capacitance of AHC-4-30 and AHC-4-45. Figure 11. Cycle stability of AHC-4-40 by repeated charging-discharging measurement at a current density of 5 A g−1.
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Table 1. Textural properties of the AHCs prepared under different conditions. a
b
c
d
Yield (%)
(m g )
(cm g )
(cm g )
(cm g )
Vmeso/Vt (%)
pristine HC
100
–
–
–
–
–
AHC-0-45
57.5
1120
0.586
0.586
–
–
AHC-2-45
34.7
1711
0.916
0.815
0.101
11.0
AHC-3-30
17.4
1762
1.089
0.917
0.172
15.8
AHC-3-45
9.3
2278
1.569
0.874
0.695
44.3
AHC-3-60
4
2243
1.598
0.811
0.787
49.2
AHC-4-30
27.4
2014
1.260
0.926
0.334
26.5
AHC-4-40
13.4
2351
1.609
0.994
0.615
38.2
AHC-4-45
6.2
2039
1.493
0.869
0.624
41.8
Samples
a
SBET 2
-1
Vt 3
-1
Vmicro 3
-1
Vmeso 3
SBET: specific surface area computed using BET equation at a relative pressure range of 0.01–0.2. Vt: The sum of Vmicro and Vmeso. c V d micro and Vmeso: micropore and mesopore volume determined by NLDFT.
b
-1
Highlights • Activated carbon materials prepared by enhanced steam activation strategy. • Improvement of textural properties with increasing steam pressure. • Identifying steam activation mechanism through surface characterization. • Enhancement of rate capability by developing mesopores.
1
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0254-0584(19)31268-4
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122454
Reference:
MAC 122454
To appear in:
Materials Chemistry and Physics
Received Date: 21 August 2019 Revised Date:
17 October 2019
Accepted Date: 13 November 2019
Please cite this article as: Y.-R. Son, S.-J. Park, Preparation and characterization of mesoporous activated carbons from nonporous hard carbon via enhanced steam activation strategy, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/j.matchemphys.2019.122454. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
Manuscript submitted to “Materials Chemistry and Physics” as an Article
Preparation and characterization of mesoporous activated carbons from nonporous hard carbon via enhanced steam activation strategy
Yeong-Rae Son and Soo-Jin Park*
Department of Chemistry, Inha University, 100 Inharo, Incheon 22212, Korea
*Corresponding author. Tel.: +82-32-876-7234; Fax: +82-32-867-5604.
E-mail addresses: [email protected] (S.-J. Park)
1
Abstract Activated carbon materials have been extensively investigated as electrode materials for supercapacitors because of their low cost, high specific surface area, and high charge storage capacity. In this work, we developed activated carbon with a high specific surface area from a nonporous hard carbon (HC) material through an enhanced steam activation strategy. In general, steam activation is carried out under atmospheric pressure. The enhanced steam activation strategy introduced in this study involves controlling the pressure of the injected steam. The activated hard carbon (AHC) obtained via the proposed activation strategy exhibited enhanced textural properties compared with AHC prepared under atmospheric steam pressure. Furthermore, AHCs with a specific surface area greater than 2000 m2 g−1 were obtained in high yield within a short time. The specific capacitance of the AHCs increased by a factor of up to approximately 12.7 compared with that of the nonporous HC. Furthermore, the specific capacitance of AHCs with a higher mesopore volume was confirmed to be retained with increasing applied current, indicating high rate capability. Therefore, the proposed enhanced steam activation strategy not only improves the energy storage capacity but also is effective in forming mesopores, which is an effective approach to improving the power characteristics of activated carbon materials.
Keywords: Enhanced steam activation; Activated carbon; Textural properties; Steam pressure; Mesoporous activated carbon
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1. Introduction Porous materials have been developed for various applications such as purification of the ecosystem and medical/pharmaceutical and isolation applications. In particular, porous materials with nanoscale pores have been a point of focus because of their extremely high surface area per unit mass [1]. Nanoscale pores are categorized by their size: micropores with a diameter of 2 nm or less; mesopores with a diameter between 2 and 50 nm; or macropores with a diameter greater than 50 nm [2]. Numerous nanoporous materials occur naturally or are fabricated artificially, including zeolites, porous silicas, porous polymers, and metal– organic frameworks [3–8]. Among the nanoporous materials, carbon-based porous materials have attracted intensive attention. Such materials have been used in diverse areas such as gas adsorption and energy storage because of their extraordinary physicochemical properties, which include high electrical conductivity, good chemical durability, and high specific surface areas [1,9,10]. Research on the supply and storage of sustainable and renewable energy has attracted attention because it will likely contribute to the solution to the global warming issue, which is currently worsening because of rapid industrial development. Devices that can store energy, such as lithium-ion batteries (LiBs), capacitors, supercapacitors, and fuel cells, have attracted particular interest as worldwide demand for portable and wearable electronic devices such as smartphones, smart watches, and smart glasses has steadily increased. Among the various aforementioned power sources, supercapacitors are potential candidates to replace LiBs. Supercapacitors store charge by forming electrostatic double-layers or via a Faradaic redox reaction at the interface between an electrode surface and an electrolyte [11–14]. Recently, carbon materials such as activated carbon, graphene, and carbon nanotubes have been 3
intensively researched as supercapacitor electrode materials [11–22]. Supercapacitors based on carbon electrodes have long lifetimes because they store charge via an electrostatic charge-storage mechanism. For the carbon electrode materials, activated carbon is commercially used because of its low cost, high specific surface area, and high charge storage capacity [23,24]. The capacitance of supercapacitors with activated carbon can be improved by enhancing its porosity; thus, extensive efforts have been devoted to enhancing the porosity of carbon materials for use in supercapacitors. Activated carbons can be prepared by two major activation strategies: 1) physical activation by oxidative gases (e.g., H2O (steam) or CO2) [25–27] and 2) chemical activation by chemical reagents (e.g., KOH, NaOH, or ZnCl2) [28–30]. Chemically activated carbon materials have been reported to possess a high specific surface area and pore volume because the activating agents penetrate deeply into and react within the carbon lattice. However, because the byproducts must be removed after the chemical activation, additional washing and removal processes are required, which may result in the emission of contaminants that negatively affect the environment. By contrast, the physical activation strategy is relatively eco-friendly and cost-effective because, unlike the chemical activation approach, it does not require additional processes [3,31]. However, in many cases, activated carbon materials prepared via existing physical activation methods have inferior textural properties compared with activated carbon materials prepared via chemical activation [32]. Thus, improving the porosity of activated carbon via a physical strategy is still challenging. In this study, to form nanoscale pores in nonporous hard carbon (HC), we used an enhanced steam activation strategy that controls the pressure of injected steam. A series of activated hard carbons (AHCs) were prepared under different activation conditions, such as 4
different temperatures, injected steam pressures, and activation times. After the steam activation, the AHCs exhibited a substantially higher specific surface area than the activated carbon prepared under atmospheric conditions. Furthermore, we confirmed that the pore size of AHCs was controlled by the activation conditions. The morphologies of the activated carbon materials were analyzed by scanning electron microscopy (SEM), and their chemical characteristics were elucidated by X-ray photoelectron spectroscopy (XPS) and energydispersive spectroscopy (EDS). The electrochemical behaviors of the AHCs with different textural properties were evaluated by cyclic voltammetry (CV), galvanostatic charge and discharge (GCD) measurements, and electrochemical impedance spectroscopy (EIS).
2. Experimental 2.1. Materials HC was acquired from Aekyung Petrochemical Co., Ltd., Korea. Triply Deionized water as a steam source was obtained from an Aqua MAX Basic 360 and Ultra 370 water purification system from Young In Chromass Co., Ltd., Korea. The following chemicals were used to prepare a slurry for the fabrication of working electrodes: Ketjenblack (KB) as a conductive material (AkzoNobel); polyvinylidene fluoride (PVDF, Mw = 534,000) as a binder (SigmaAldrich, Korea); and 1-methyl-2-pyrrolidone (NMP, ≥99%) as a solvent (TCI Co.). Sodium sulfate (Na2SO4) for the electrolyte was acquired from Daejung Chemicals & Metals Co., Ltd. All chemical reagents were used without further purification.
5
2.2. Preparation of activated hard carbon AHCs were prepared by enhanced steam activation strategy. A semi-cylindrical quartz boat containing hard carbon (HC) was placed in the center of the heating area of a quartz tube, which was subsequently heated to a certain temperature. Before activation, the HC was heated to a certain activation temperature at a heating rate of 5 °C min−1 under a flowing inert N2 atmosphere (150 mL min−1). When the temperature reached activation temperature, the flow rate of N2 was reduced to less than 10 mL min−1 and ultrapure water was injected at a rate of 0.8 mL min−1. After cooling to room temperature, the boat containing the AHCs was carefully removed. A series of AHCs were prepared as follows: 1) To evaluate the most effective activation temperature, steam-activated HC (AHC-Tn, where n indicates the activation temperature, e.g., the sample activated at 900 °C is represented as AHC-T9) was prepared as a function of temperature from 600 to 900 °C under a fixed steam pressure (1.4 gauge bar) and activation time (40 min). 2) After the activation temperature was evaluated (in this study, 900 °C), the steam pressure and activation time were adjusted to enhance the textural properties of the AHC. The steam pressure and activation time were controlled in the range from 1.0 (atmosphere) to 1.4 gauge bar and from 30 to 60 min, respectively. The sample prepared from adjusted activation conditions was denoted as AHC-P-t, where P is the adjusted pressure and indicates one decimal place of gauge bar and t is activation time in minutes (e.g., AHC-4-45 indicates AHC prepared at a steam pressure of 1.4 gauge bar for 45 min).
6
2.3. Characterization of activated carbon materials The crystal structure of the samples prepared was examined via X-ray diffraction (XRD) analysis (D2 PHASER, Bruker) using a Lynx-Eye detector and Cu-Kα radiation generated at 30 kV and 10 mA (λ = 1.5406 Å). The morphologies of the materials were observed via field-emission SEM (Hitachi S-4300). An energy dispersive X-ray spectroscopy (EDS) system mounted on the scanning electron microscope was used to measure the concentrations of elements. XPS was performed using a K-Alpha instrument (Thermo Scientific) to analyze the chemical compositions of the HC and AHCs. Raman spectroscopy was performed to identify the graphitic structure of the pristine HC and AHC samples using HORIBA LabRAM Revolution at a wavelength of 532 nm. The microstructure in nanoscale of pristine HC and AHC-4-45 was characterized using transmission electron microscopy (TEM; Philips CM200). To examine the textural properties of the HC and AHCs, N2 adsorption–desorption isotherms were recorded at 77 K using a gas adsorption analyzer (BEL, BELSORP-max, Japan). The textural properties of the samples were determined from the N2 sorption isotherms. The specific surface area of the AHCs was calculated by the Brunauer–Emmett– Teller (BET) equation over a p/p0 range from 0.01 to 0.2. The total pore volume was estimated at a relative pressure p/p0 of 0.99. The pore size distributions were determined via the nonlocal density functional theory (NLDFT) model.
2.4. Electrochemical performance measurements The electrochemical properties of the AHCs prepared under different activation times were 7
evaluated using a three-electrode configuration comprising a working electrode (AHCs on Ni foam), a reference electrode (Ag/AgCl), and a current collector (Pt coil) over a 1 V (−0.3 to 0.7 V) potential window in a 0.5 M Na2SO4/H2O aqueous electrolyte. The working electrode was fabricated by coating a slurry containing AHCs, KB, and PVDF with a weight ratio of 8:1:1 onto a 1 × 4 cm2 Ni foam current collector with dimensions of 1 × 1 cm2 and drying the coated Ni foam in a drying oven at 80 °C for 12 h. The electrochemical analysis of the asprepared AHCs was conducted via CV, GCD, and EIS measurements using an IviumStat instrument. Before the analysis, the working electrodes were dipped into 0.5 M Na2SO4 electrolyte for at least 3 h. The specific capacitance (Csp) of the AHCs was calculated using the equation Csp = I∆t/(m∆V) (F g−1) from the GCD discharge plots, where I is the applied discharge current, ∆t is the discharge time, m (g) is the mass of active material coated onto the Ni-foam working electrode, and ∆V is the potential window. The EIS measurement was conducted in the frequency range from 100 kHz to 0.01 Hz with an amplitude of 5 mV. The imaginary part (C″) of the capacitance was evaluated by the equation C″(ω) = Z1(ω)/{ω|Z2(ω)|2}, where ω is the pulsation and Z1 and Z2 are the real and imaginary parts of the impedance, defined as
1
=|
+ 2
| .
3. Results and Discussion Figure 1 presents a schematic of the preparation of activated carbons using the steam activation strategy. A semicylindrical boat containing nonporous HC was placed in a quartz tube, and the tube was inserted into a tube furnace. After the tube reached a certain activation temperature, ultrapure deionized water was injected into the tube at a constant flow rate via a 8
flow-rate-controlled pump. The injected water was evaporated in the heated tube at a high temperature, and the water vapor reacted with the HC surface to form nanoscale pores. The activation mechanism is proposed as follows [3]: ∗
+H O→C∙H O
(1)
H O →∙ OH + ∙ H
(2)
C ∙ OH + ∙ H → H + C ∙ O
(3)
C ∙ O → CO
(4)
Figure 1. Schematic of the preparation of steam-activated hard carbon materials.
Figure 2 shows the N2 adsorption–desorption isotherms of AHCs prepared at different activation temperatures. The resulting isotherms of the AHCs prepared at different activation temperatures show an intense uptake the region p/p0 < 0.1, characteristic of a microporous material. In particular, the AHC-T9 showed a type IV N2 isotherm and a type H4 hysteresis loop in the p/p0 region between 0.5 and 1.0, indicating that the AHC-T9 was a mesoporous material with limited slit-like mesopores [2]. The specific surface areas of AHC-T6, AHC-T7, 9
AHC-T8, and AHC-T9 were 200, 537, 1083, and 2371 m2 g−1, respectively, and their total pore volumes were 0.086, 0.215, 0.454, and 1.425 cm3 g−1, respectively. Thus, the most effective activation temperature was 900 °C.
Figure 2. N2 adsorption–desorption isotherms of AHCs prepared at different activation temperatures.
Figure 3 presents SEM micrographs of the AHCs prepared at different activation temperatures. The surface of pristine HC showed no pores. Some large and irregular lumps and spherical agglomerates with a diameter of 0.2 to 0.3 µm were confirmed to be present on the surface. After steam activation, the agglomerates on the surface of HC granules were deformed. The progression of the deformation was substantial and holes were created with increasing activation temperature. As shown in Figure 3(d), the agglomerates on the surface of AHC activated at 800 °C almost disappeared. Moreover, the surface of the AHC activated at 900 °C not only shows that the agglomerates were removed but also reveals a large number of holes on the surface. That is, the SEM observations indicate that, when the HC is activated 10
by steam, the agglomerates on the surface are first deformed at low temperature and then steam penetrates into the interior of the HC granules to form nanoscale pores. To further investigate why the holes were intensively formed around the agglomerates, EDS was carried out. From the O1s XPS results described later (Figure 4), oxygen functional groups present in surface of HC reacted with H2O vapor. As shown in Figure S1, oxygen is concentrated in the periphery of the agglomerates. Therefore, it is believed that the holes were intensively created around the agglomerates because active sites that react with H2O vapor were concentrated near the agglomerates.
Figure 3. SEM images of AHCs prepared at different activation temperatures: (a) pristine HC, (b) AHC-T6, (c) AHC-T7, (d) AHC-T8, and (e) AHC-T9.
XPS was used to identify the chemical characteristics of the as-prepared AHCs 11
according to their activation temperature. The C1s XPS spectra of the pristine HC are presented in Figure 4(a). Five deconvoluted peaks were observed at 284.5, 284.95, 285.7, 288.2, and 292.5 eV, corresponding to sp2-hybridized carbon, sp3-hybridized carbon, oxygenfunctionalized carbon (epoxide, hydroxyl, carbonyl, ether, and carboxylate groups), and the π→π* shake-up of conjugated carbon systems [33]. The C1s spectra of AHCs prepared at different activation temperatures do not substantially differ from the spectrum of the pristine HC. The peaks at approximately 292.5 eV decreased in intensity, possibly because the conjugated systems were deformed by steam activation.
Figure 4. C1s XPS spectra of (a) the pristine HC (with deconvoluted peaks) and (b) AHCs prepared at different activation temperatures. O1s XPS spectra of (c) AHC-T6, (d) AHC-T7, (e) AHC-T8, and (f) AHC-T9. 12
The O1s spectra of AHCs activated at different temperatures are shown in Figure 4(c) to (f). The three major deconvoluted peaks were observed at 530.5, 532.1, and 533.7 eV, corresponding to C=O groups (e.g., carboxylate, lactone, carbonyl, and quinone groups), epoxide groups, and ether groups, respectively. With increasing activation temperature, the intensity of the peak associated with the epoxide group gradually decreased, indicating that the H2O vapors reacted with the epoxide group in the HC, resulting in porous AHCs. In addition, in the case of the sample activated at 900 °C, not only the peak of the epoxide group but also that of the C=O group decreased in intensity compared with the peaks in the spectra of AHCs prepared at lower activation temperatures. Therefore, the most effective steam activation effect at 900 °C could be interpreted as the active reaction of epoxide groups and C=O groups on the HC surface. To investigate the effect of steam pressure on the textural properties of AHCs, N2 gas sorption experiments were performed. The N2 sorption isotherms and pore size distributions of AHCs prepared at different steam pressures are shown in Figure 5. As shown in Figure 5(a), the N2 isotherms of all of the AHCs showed a sharp uptake in the region p/p0 < 0.1, characteristic of microporous materials. The AHC-0-45 prepared under atmospheric steam pressure exhibited a type I isotherm with no hysteresis loop. However, for the samples prepared under increased steam pressure, a type H4 hysteresis loop was observed and the loop expanded with increasing steam pressure, indicating that large pores were generated [34,35]. As depicted in Figure 5(b) and (c), in AHC-0-45, only micropores with diameters of 0.6–0.7 nm and 1.0–1.3 nm were confirmed without mesopores. The proportion of micropores with diameters of above 0.8 nm in AHC-2-45, AHC-3-45, and AHC-4-45 was 99.2, 93.3, and 82.6%, respectively, higher than that in AHC-0-45 (35.7%). In other words, 13
the micropores with relatively large size were formed in the AHCs prepared by applying steam pressure. This indicates that the micropore size can be expanded by increasing the steam pressure during steam activation. Moreover, it can be seen that the mesopore volume with sizes of 2–3.5 nm and 3.8–4.4 nm was increased with increasing steam pressure. In the case of AHC-4-45, the specific surface area and mesopore volume were reduced due to excessive pore expansion caused by high steam pressure. In addition, as shown in Figure 5(d), the yield after the pressure-adjusted steam activation decreased with increasing steam pressure. In addition, the mesopore volume increased with increasing steam pressure without deterioration of the micropore volume, indicating that the suggested activation strategy is an effective and efficient approach for inducing mesopore formation in carbon-based materials.
Figure 5. (a) N2 adsorption–desorption isotherms, the (b) micropore and (c) mesopore size distribution, and (d) the yield and pore volume of AHCs under steam pressure.
The textural properties of AHCs prepared at different steam pressures and activation times are summarized in Table 1. The specific surface areas of AHC-0-45, AHC-2-45, AHC14
3-45, and AHC-4-45 were 1120, 1711, 2278, and 2039 m2 g−1, respectively, and the Vt (i.e., the sum of Vmicro and Vmeso) values were 0.586, 0.916, 1.569, and 1.493 cm3 g−1, respectively. The specific surface area and pore volume increased with increasing steam pressure. To compare which steam pressure condition was more efficient, AHCs were prepared with different activation times at a steam pressure of 1.3 and 1.4 gauge bar, respectively. As a result, the specific surface area, pore volume, and yield of AHC-4-30 were higher than those of AHC-3-30, indicating that the more effective steam condition was the steam pressure of 1.4 gauge bar. The lower textural properties and yield of AHC-4-45 compared with those of AHC-3-45 are attributed to excessive activation. In addition, the proportion of the mesopore volume increased with increasing steam pressure and activation time.
Table 1. Textural properties of the AHCs prepared under different conditions. 15
To characterize carbon structure of AHCs prepared with steam pressure, Raman spectroscopy was performed. As shown in Figure 6, two typical peaks observed in many carbonaceous materials were measured in both pristine HC and a series of AHCs. The D-band corresponding to disordered defects appeared at 1331 cm−1. The G-band corresponding to graphitic lattice appeared in the range of 1579 – 1600 cm−1. The G-band of AHCs was slightly blue-shifted compared with that of pristine HC. Furthermore, D- and G-band of AHCs prepared by applying steam pressure were sharpened compared with those of pristine HC and AHC-0-45. The blue-shifted G-band and sharpening of D- and G-band were attributed to the reduction of defects by removal of functional groups, such as epoxides, on the HC and the formation of sp2-double bond carbon networks [36]. As shown in TEM images (Figure S2), compared with pristine HC, micro/mesopores were formed in AHC-4-45, and partially graphitized carbon lattice was confirmed.
Figure 6. Raman spectra of pristine HC and AHCs prepared with different steam pressure. 16
In addition, the intensity ratio of D-band to G-band (ID/IG) of pristine HC and AHC-(0, 2, 3, 4)-45, which is indicator of defects in carbon materials, were 0.99, 0.96, 1.01, 0.98, and 1.08, respectively. The ID/IG ratio of AHCs except AHC-4-45 was not significantly different from that of pristine HC. The increase in the ID/IG ratio of AHC-4-45 is attributed to the increase in the edge site caused by the expansion of the pore size formed by the excessive steam activation effect [37,38]. Figure 7 shows the N2 sorption isotherms and pore size distributions of AHCs prepared with different activation times under a steam pressure of 1.4 gauge bar. The N2 isotherms of the AHCs were type IV. The hysteresis loop was evaluated and found to expand with increasing activation time. Furthermore, as the activation time increased, the mesopore volume increased and micropores with a relatively large diameter were formed. The maximum specific surface area of 2351 m2 g−1 was attained under a steam pressure of 1.4 gauge bar and an activation time of 40 min.
Figure 7. (a) N2 adsorption–desorption isotherms and (b) micropore and (c) mesopore size distributions of AHCs prepared with different activation times.
The crystalline structures of the AHCs prepared with different activation times were characterized by XRD analysis. The XRD patterns of the pristine HC and AHCs in the range 5° ≤ 2θ ≤ 90° are shown in Figure 8. Broad peaks were observed in the pattern for the 17
pristine HC at 2θ = 24.3°, indicating a typical amorphous crystalline structure in the amorphous carbon materials. The intensity of the peaks decreased after the steam activation; this effect is attributed to structural modifications resulting from the activation, indicating the formation of nonuniform lattice strain in the AHCs [39–42].
Figure 8. XRD patterns of AHCs prepared with different activation times.
Figure 9 shows the morphological structures and chemical composition of the AHCs prepared with different activation times. The size of the AHC granules observed at low magnifications decreased with increasing activation time. Numerous pores and cracks were observed in the high-magnification images of the AHCs. In Figure 9(b) and (c), pores resembling circular dots were confirmed, whereas cracks were observed between pores in Figure 9(d). These observations indicate that macropores formed on the surface because of excessive activation. The amount of oxygen in the AHCs was lower than that in the pristine HC because the surface oxygen functionalities of the HCs were eliminated by reaction with H2O vapor at high temperatures, as confirmed in the XPS analysis results (Figure 4). 18
Figure 9. SEM images and EDS results of AHCs prepared with different activation times: (a) pristine HC, (b) AHC-4-30, (c) AHC-4-40, and (d) AHC-4-45 (inset images are at high magnification).
The electrochemical properties of the AHCs prepared at different activation times under a steam pressure of 1.4 bar, which was the most effective steam pressure condition, were analyzed by CV and GCD. The analyses were carried out in 0.5 M Na2SO4/H2O using a three-electrode system over the potential window from −0.3 to 0.7 V. Figure 10 shows the CV curves measured at 10 mV s−1, the GCD plots measured at 1 A g−1, and the specific capacitance values measured at different current densities. The CV curves of the AHCs showed an almost rectangular configuration, indicating EDL behaviors. In addition, the CV curve areas of the AHCs were larger than that of pristine HC, meaning that the specific capacitance of AHCs was substantially increased compared with that of nonporous HC. Similar results were confirmed from the GCD plots. The triangular and symmetrical GCD curves revealed that the AHCs have low resistance. From the IR drop, equivalent series resistance (ESR) was estimated using the equation, ESR = IRdrop/2I, where I is an applied current [43]. The calculated ESR of AHC-4-(30, 40, 45) is determined as 5.75, 8.25, and 5.91 19
Ω, respectively. Among the AHCs, the AHC-4-40 exhibited the highest ESR but the smallest IR drop at the identical current density, as shown in the inset of Figure 10(b). This indicates that AHC-4-40 has the lowest energy loss compared to the other two AHCs [44,45]. The specific capacitances of pristine HC, AHC-4-30, AHC-4-40, and AHC-4-45 measured at a current density of 0.5 A g−1 were 5.6, 72.6, 79.8, and 71.0 F g−1, respectively. Sample AHC-440 exhibited the highest specific capacitance, which was greater than that of the pristine HC by a factor of approximately 12.7. This substantial increase in specific capacitance is attributed to an increase of the area accessible to the electrolyte ions by steam activation. As a result, the specific capacitance of the AHCs was not substantially different; however, AHC-440, which exhibited the largest specific surface area, was confirmed to also exhibit the largest specific capacitance. As depicted in Figure 10(c), the capacitance retention of the AHCs was measured as the range of applied current density was increased from 0.5 to 5 A g−1. The capacitance of AHC-4-30, AHC-4-40, and AHC-4-45 decreased from 72.6, 79.8, and 71.0 F g−1 to 40, 44.5, and 45 F g−1, indicating retentions of 55.1, 55.8, and 63.4%, respectively. These differences in rate capability can be interpreted by evaluating the relaxation time constant (τ0). The τ0 is determined from the maximum peak frequency (f0), defining as τ0 = 1/f0. As shown in Figure 10(d), the shorter τ0 of AHC-4-45 (20 s) than that of AHC-4-30 (33.3 s) was evaluated, demonstrating that the discharging power of AHC-4-45 is higher than that of AHC-4-30. Consequently, the shorter τ0 demonstrated that the electrolyte ions were more effectively diffused in the pores of AHC-4-45, which can be ascribed to the high proportion of mesopores in the AHC-4-45, as confirmed in Table 1 [13,30,46].
20
Figure 10. Electrochemical analysis of AHCs prepared with different activation times: (a) CV curves, (b) GCD plots (the inset indicates the IR drop), (c) specific capacitance measured at different current densities, and (d) the frequency dependence of the imaginary part of the specific capacitance of AHC-4-30 and AHC-4-45.
To investigate the cycle performance of AHC-4-40, the charging-discharging cycling was carried out over 5000 cycles at a current density of 5 A g−1. As shown in Figure 11, the specific capacitance of AHC-4-40 was decreased with increasing the cycle number. After 3000 cycles, the specific capacitance was retained approximately 82.8% compared to the initial cycle, and the retention was maintained after 5000 cycles.
21
Figure 11. Cycle stability of AHC-4-40 by repeated charging-discharging measurement at a current density of 5 A g−1.
The AHCs prepared via enhanced steam activation exhibited enhanced textural properties compared with the pristine HC and AHC prepared under atmospheric steam pressure. Distinctive structural and chemical properties of the AHCs were observed as the activation conditions (e.g., activation temperature, steam pressure, and activation time) were changed. The enhanced steam activation strategy proposed in the present study is an efficient and effective approach to the high-yield preparation of activated carbon with enhanced pore characteristics compared with those of activated carbons produced by conventional steam activation. In addition, because activated carbon with a high proportion of mesopores can be produced, the proposed method can be used to produce carbon-based materials with excellent power characteristics.
4. Conclusions In this research, AHCs with high porosity were prepared via an enhanced steam activation 22
strategy. The pristine HC was effectively activated at 900 °C. The specific surface areas of AHCs prepared under a steam pressure of 1.4 gauge bar were greater than 2000 m2 g−1, which is greater than the specific surface area of the AHC prepared under atmospheric steam pressure. A large number of pores were formed on the surface of the AHCs. By characterizing the surface of the AHCs, we confirmed that the epoxy groups reacted with H2O vapor and that the extent of the reaction increased with increasing activation temperature. Furthermore, at 900 °C, C=O groups also reacted with H2O vapor, indicating that the activation effect increased at 900 °C. The prepared AHCs exhibited increases in specific surface area and pore volume with increasing steam pressure and activation time. In addition, substantial increase in the pore volume of the mesopores was confirmed. The specific capacitance of developed AHCs was as much as 12.7 times greater than that before activation because of the pores generated by the enhanced steam activation. Furthermore, the AHC with a high mesopore volume was confirmed to exhibit better rate-capability performance. The enhanced activation strategy suggested in this study can be applied in various fields requiring materials with nanoscale porosity, such as the removal of contaminants (toxic molecules or nanoparticles) from a gas or liquid phase as well as energy storage applications.
Acknowledgements This work was supported by the Technology Innovation Program (or Industrial Strategic Technology Development Program (10080293, Development of carbon-based non-phenolic electrode materials with 3,000 m2/g grade surface area for energy storage device) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).
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Conflicts of Interest The authors declare no conflicts of interest.
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Figure captions Figure 1. Schematic of the preparation of steam-activated hard carbon materials. Figure 2. N2 adsorption–desorption isotherms of AHCs prepared at different activation temperatures. Figure 3. SEM images of AHCs prepared at different activation temperatures: (a) pristine HC, (b) AHC-T6, (c) AHC-T7, (d) AHC-T8, and (e) AHC-T9. Figure 4. C1s XPS spectra of (a) the pristine HC, with deconvoluted peaks, and (b) AHCs prepared at different activation temperatures. O1s XPS spectra of (c) AHC-T6, (d) AHC-T7, (e) AHC-T8, and (f) AHC-T9. Figure 5. (a) N2 adsorption–desorption isotherms, the (b) micropore and (c) mesopore size distribution, and (d) the yield and pore volume of AHCs under steam pressure. Figure 6. Raman spectra of pristine HC and AHCs prepared with different steam pressure. Figure 7. (a) N2 adsorption–desorption isotherms and (b) micropore and (c) mesopore size distributions of AHCs prepared with different activation times. Figure 8. XRD patterns of AHCs prepared with different activation times. Figure 9. SEM images and EDS results of AHCs prepared with different activation times: (a) pristine HC, (b) AHC-4-30, (c) AHC-4-40, and (d) AHC-4-45 (inset images are at high magnification). Figure 10. Electrochemical analysis of AHCs prepared with different activation times: (a) CV curves, (b) GCD plots (the inset indicates the IR drop), (c) specific capacitance measured at different current densities, and (d) the frequency dependence of the imaginary part of the specific capacitance of AHC-4-30 and AHC-4-45. Figure 11. Cycle stability of AHC-4-40 by repeated charging-discharging measurement at a current density of 5 A g−1.
25
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Table 1. Textural properties of the AHCs prepared under different conditions. a
b
c
d
Yield (%)
(m g )
(cm g )
(cm g )
(cm g )
Vmeso/Vt (%)
pristine HC
100
–
–
–
–
–
AHC-0-45
57.5
1120
0.586
0.586
–
–
AHC-2-45
34.7
1711
0.916
0.815
0.101
11.0
AHC-3-30
17.4
1762
1.089
0.917
0.172
15.8
AHC-3-45
9.3
2278
1.569
0.874
0.695
44.3
AHC-3-60
4
2243
1.598
0.811
0.787
49.2
AHC-4-30
27.4
2014
1.260
0.926
0.334
26.5
AHC-4-40
13.4
2351
1.609
0.994
0.615
38.2
AHC-4-45
6.2
2039
1.493
0.869
0.624
41.8
Samples
a
SBET 2
-1
Vt 3
-1
Vmicro 3
-1
Vmeso 3
SBET: specific surface area computed using BET equation at a relative pressure range of 0.01–0.2. Vt: The sum of Vmicro and Vmeso. c V d micro and Vmeso: micropore and mesopore volume determined by NLDFT.
b
-1
Highlights • Activated carbon materials prepared by enhanced steam activation strategy. • Improvement of textural properties with increasing steam pressure. • Identifying steam activation mechanism through surface characterization. • Enhancement of rate capability by developing mesopores.
1
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: