KR102439986B1 - Method for manufacturing kenaf-derived activated carbon - Google Patents

Abstract

The present invention relates to a kenaf-derived activated carbon that can be used as an electrode material for a supercapacitor and a method for manufacturing the same, and can exhibit high long-term cycle stability and high specific capacitance through a pre-carbonization process and a chemical activation process.

Description

Method for producing kenaf-derived activated carbon

The present invention relates to kenaf-derived activated carbon and a method for preparing the same.

Electric supercapacitors that can store electric charges through physical adsorption/desorption processes are attracting attention as the most promising energy storage equipment in various fields such as portable electronic devices and electric vehicles due to their high electric capacity and long-term cycle stability. The electrochemical performance of a supercapacitor depends on the properties of the electrode material used in its manufacture. In particular, since the capacitance of supercapacitors is proportional to the surface of the active material and inversely proportional to the thickness of electric double-layers (EDLs), improved electrode designs with high surface area and appropriate nanoporous structure EDLs are needed.

Activated carbon has a high specific surface area, low price, excellent electrical conductivity and well-defined pore structure, so it is widely used as an electrode material for supercapacitors. In addition, nanostructured carbon materials containing redoxactive heteroatoms are attracting attention as cathode materials for the storage of lithium and/or sodium ions. Several heteroatom functional groups in nanostructured carbon materials can act as redox hosts for fast and highly stable pseudocapacitive charge charging. Therefore, increasing the energy density of supercapacitors has been proposed as a strategy that can be used for both EDL capacitance and pseudocapacitance.

Korean Patent Publication No. 10-2020-0042689

Accordingly, the inventors of the present invention completed the present invention by confirming that the nanostructure and pore structure can be controlled by adjusting the KOH ratio in the preparation of kenaf-derived nanoporous activated carbon by chemical activation with KOH.

In order to achieve the above object, the present invention comprises the steps of pre-carbonizing kenaf to 400 °C at a heating rate of 1 to 5 °C min ^-1 ; and a chemical activation step of treating the pre-carbonized kenaf with KOH in a weight ratio of 1:1 to 6 while heating to 800°C at a heating rate of 1 to 10°C min ^-1 ; It provides a manufacturing method of

The present invention also provides a kenaf-derived activated carbon prepared by the manufacturing method according to the present invention.

The kenaf-derived activated carbon of the present invention is an amorphous carbon structure composed of a nanometer hexagonal carbon layer with well-developed micropores and multiple O- and N-containing species, and a large number of redox-activated heterogeneous carbons such as oxygen and nitrogen. It has atoms and shows a high specific surface area of 2,150.1 m ² g ^-1 . Due to these unique material properties, it exhibits high long-term cycle stability and specific capacitance of ~132.5 W hkg ^-1 , thereby exhibiting high electrochemical performance corresponding to a Na-ion storage cathode, which can be used as an excellent electrode material for supercapacitors.

1 shows (a) thermogravimetric and differential thermal analysis curves of kenaf fibers and their optical images (inset) and (b) FE-SEM images of kAC samples.
2 shows (a) an X-ray segment pattern of kACs and (b) a Raman spectrum.
Figure 3 shows (a) nitrogen adsorption and desorption isotherms of kACs nitrogen adsorption and desorption isotherms and (b) its pore size distribution results.
4 shows 2.5-4.5 V vs. The electrochemical performance of kACs characterized by the galvanostatic charge/discharge method over the Na+/Na voltage window is shown. (a) Secondary profile in electrolyte of 1 M NaPF ₆ dissolved in EC:DEC (1:1 v:v) at a charge-discharge rate of 0.1 A g ^-1 . (b) rate characteristics from 0.1 to 1 A g ⁻¹ and (c) cycling performance at 1 A g ⁻¹ at 1000 cycles of the kACs electrode.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, detailed descriptions of well-known techniques known to those skilled in the art may be omitted. In addition, in describing the present invention, if it is determined that a detailed description of a related well-known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description may be omitted. In addition, the terms used in this specification are terms used to properly express a preferred embodiment of the present invention, which may vary according to the intention of a user or operator or customs in the field to which the present invention belongs.

Therefore, definitions of these terms should be made based on the content throughout this specification. Throughout the specification, when a part "includes" a certain component, it means that other components may be further included, rather than excluding other components, unless otherwise stated.

The present invention relates to kenaf-derived activated carbon and a method for preparing the same.

The kenaf-derived activated carbon can be prepared by the following preparation method:

pre-carbonizing the kenaf to 400° C. at a heating rate of 1 to 5° C. min ⁻¹ ; and

A chemical activation step of treating the pre-carbonized kenaf with KOH in a weight ratio of 1:1 to 6 while heating to 800°C at a heating rate of 1 to 10°C min ⁻¹ .

The heating rate in the pre-carbonizing step may be 2° C. min ⁻¹ , but is not limited thereto. The pre-carbonizing step can heat the kenaf to 400° C. at a slow heating rate of 2° C. min ⁻¹ to form more conjugated structures that are converted to carbon structures.

In the chemical activation step, the heating rate may be 5° C. min ⁻¹ , but is not limited thereto.

In the chemical activation step, the nanostructure and pore structure of the activated carbon can be controlled by treating the pre-carbonized kenaf with KOH in a weight ratio of 1:1 to 6, for example, a weight ratio of 1:4, but is limited thereto. doesn't happen

The kenaf may be a fine powder of kenaf stems, but is not limited thereto. For example, the kenaf may be used by grinding the kenaf stem into a fine powder and then drying it, but is not limited thereto.

Heating in the pre-carbonizing step and the chemical activation step may be performed under an Ar flow, for example, may be performed under a constant Ar flow of 100 cm ³ min ⁻¹ , but is not limited thereto.

The kenaf-derived activated carbon prepared by the above manufacturing method may be used as an electrode material for a supercapacitor, but is not limited thereto.

Advantages and features of the present invention, and methods of achieving them, will become apparent with reference to the embodiments described below in detail. Hereinafter, the present invention will be described in detail by way of Examples. However, these examples are for describing the present invention in detail, and the scope of the present invention is not limited to these examples.

Example 1. Preparation of Kenaf-Derived Activated Carbons (kACs)

The stems of kenaf were crushed into fine powder using a grinder and dried in a convection oven for one day. Then, about 4.0 g of kenaf powder was pre-carbonized to 400° C. without dwelling time. The heating rate was 2° C. min ⁻¹ under a constant Ar flow of 100 cm ³ min ⁻¹ . The pre-carbonized kenaf powder was washed several times until neutralized with 0.1 M HCl solution and deionized water. After drying at 80°C for 2 days in a convention oven, a chemical activation process was performed using KOH at a weight ratio of 1:1, 1:2, 1:4, and 1:6 up to 800°C. In the chemical activation process, KOH powder was stirred and mixed with the pre-carbonized kenaf in the above weight ratio, and then activation was performed. The prepared kenaf-derived activated carbons (kACs) were named kAC-0.5, kAC-1, kAC-2, and kAC-4, respectively. The heating rate was 5° C. min ⁻¹ under a constant Ar flow of 100 cm ³ min ⁻¹ .

Example 2. Characterization and Electrochemical Characterization of Kenaf-Derived Activated Carbons (kACs)

characterization

The thermal behavior of the kenaf stems was analyzed by thermogravimetric analysis (TGA; TG 209 F3, NETZSCH, Germany) from room temperature to 1,000°C at a heating rate of 10°C min ⁻¹ under inactivated argon gas (TGA; TG 209 F3, NETZSCH, Germany). It was confirmed using scanning electron microscopy (FE-SEM, S-4300, Hitachi, Japan). Raman spectra were recorded using a linearly polarized continuous-wave laser (514.5 nm, 2.41 eV, 16 mW). The laser beam was focused using a ×100 objective, resulting in a spot with a diameter of ∼1 μm. The acquisition time and number of cycles collected in each spectrum were 10 s and 3 cycles, respectively. X-ray diffraction (XRD, Rigaku D/MAX 2500) analysis was performed using Cu Kα radiation (λ = 0.154 nm) at 40 kV and 100 mA. The chemical composition of the sample was confirmed by X-ray photoelectron spectroscopy (XPS, PHI 5700 ESCA, USA) using monochromatic Al Kα radiation (hν = 1,486.6 eV). The pore structure of the sample was analyzed using an N2 adsorption/desorption isotherm obtained at -196°C using a surface area and porosimetry analyzer (Tristar, Micromeritics, USA).

Electrochemical characterization

The electrochemical properties of kAC were confirmed using Wonatech automatic battery cycler and CR2032 type coin cell. After the coin cells were assembled in an argon-filled glove box, the samples were tested as working electrodes with metallic Na foils as reference and counter electrodes. As a working electrode, a slurry was prepared in N-methyl-2-pyrrolidone (OCI Co., 99.9%, USA) in conductive carbon (10 wt.%; Alfa Aesar Co., purity: >99%, England) and polyvinylidene fluoride (10 wt. .%; Sigma-Aldrich, USA) and an active material (80 wt.%) were mixed. The prepared slurry was uniformly coated on Al foil (Wellcos Co., Korea), dried at 80° C. for 1 hour, and then roll pressed. The mass loading of the active material was ˜1 mg cm ⁻² and the total electrode weight was about 1-2 mg. An organic electrolyte was prepared by dissolving 1M NaPF ₆ (Sigma-Aldrich, USA, purity:99.99%) in a mixture of ethylene carbonate and propylene carbonate (EC:PC = 1:1 v/v). A glass microfiber filter (GF/F, Whatman, Germany) was used as a separator.

result

The thermal stability of kenaf was confirmed by TGA and DTA as shown in FIG. 1a . Below 150°C, a slight weight loss (~2wt.%) was observed due to the removal of adsorbed moisture. The weight was kept constant up to about 200 °C, but in the temperature range between 250 and 350 °C, thermal decomposition of the kenaf started and a rapid weight loss of ~67 wt.% (~78 wt. of the total weight of the kenaf was observed. 350 °C In the above, the weight steadily decreased and a residual weight of about 14% was measured after pyrolysis to 1,000° C. When sufficient heat was applied, cellulose and lignan molecules were converted to conjugated intermediates by thermal decomposition of glycoside and ether bonds. When more heat is applied, the conjugated structures are linked together to form an sp ² carbon structure Given the significant weight loss between 250 and 350° C., the primary components such as cellulose, lignans and hemicelluloses are pyrolyzed as they become conjugated. However, due to the high temperature increase rate, it does not form a heat-stable conjugated structure and shows a continuous weight loss above 400° C. That is, more conjugated structures are formed that are converted to a carbon structure at high temperature. Therefore, in the present invention, pre-carbonization was performed at 400° C. at a slow heating rate of 2° C./min, and the pre-carbonized sample was activated using various weight ratios of KOH.

The morphology of kAC was observed using FE-SEM and shown in Fig. 1b. Activated samples with low KOH ratios (kAC-1 and Kac-2) showed random micro-scale bulk particle morphology, whereas high KOH ratios were observed. With kAC-4 and kAC-6, they showed collapsed structures with rough surfaces. The chemical activation mechanism using KOH is as follows: 1) creation of pore structure through redox reaction between potassium compound and carbon atom, 2) development of pore structure through gasification of carbon, and 3) activation into carbon matrix. Further development of the pore structure through extension of the carbon lattice caused by intercalation of metallic potassium generated during That is, the structural difference between kACs is caused by the different activation effects according to the KOH ratio, the amount of etched carbon, and the exfoliated carbon layer.

The carbon microstructure of kAC was analyzed by XRD and Raman spectroscopy, and the results are shown in FIG. 2a. The XRD pattern of kAC-0 shows two broad peaks centered at approximately 23° and 43°, which correspond to the (002) and (100) reflective surfaces of the hexagonal graphite structure, respectively, indicating that a typical amorphous carbon structure exists. it will indicate After the activation process, the graphite (002) peaks of kACs were shifted to small angles and broadened, indicating that the average interlayer distance increased and the number of stacked layers decreased, respectively. The trend of (002) peak extinction becomes more evident as the KOH ratio increases. The XRD patterns of kAC-4 and kAC-6 show little (002) reflection, confirming the low stacking structure of the carbon layer caused by the consumption of carbon by oxygen and the passage of metallic potassium between the graphitic carbon layers. it will do

The Raman spectrum of the kAC sample shows two representative carbon bands, an E _2g vibrational model of a graphite layer with sp ² carbon at a frequency of ∼1,580 cm ⁻¹ (referred to as the G band) and a structural defect at a frequency of ∼1,350 cm ⁻¹ . showed a model of A _1g breathing (known as D band) of sp ² bonded carbons near the basal edge corresponding to Contrary to the XRD results, all kAC samples showed almost similar Raman spectra even after activation with a high KOH ratio (kAC-4). The I _D /I _G intensity ratios of kAC-0, kAC-1, kAC-2, kAC-4, and kAC-6 were estimated to be 0.89, 0.92, 0.86, 0.89, and 0.87, respectively, which represent a fraction of nanometers. This indicates that the hexagonal carbon structure having a structure was not significantly destroyed through the activation process. That is, after the chemical activation process, the kAC sample still contains a hexagonal carbon crystal structure.

The surface properties of kAC were confirmed using XPS and are shown in Table 1. As the KOH ratio increased, it was confirmed that the heteroatom content (nitrogen + oxygen) increased from 12.1 to 27.1 at.%. In particular, the oxygen content, including C=O bonds favorable for pseudocapacitive charge charging, increased significantly after chemical activation. Considering the XRD and Raman results, it can be suggested that a number of heteroatoms are mainly introduced into topological defect sites such as mono- and di-vacancies, Stone-Wales defects, adatoms, and edge defects.

The pore structure and surface area of kACs were investigated using a nitrogen adsorption/desorption isothermal curve and are shown in FIG. 3 and Table 2. As the KOH ratio increased, it was observed that the amount of nitrogen adsorption at a relative pressure of 0.1 or less significantly increased, indicating that the amount of monolayer adsorption increased. This result suggests that the adsorption of nitrogen molecules on the surface of the carbon layer increases with the increase of the insufficient stacking hindered hexagonal structure induced by chemical activation. In detail, the isothermal curves of kAC-1 and kAC-2 are representative International Union of Pure and Applied Chemistry (IUPAC) type-I shape, which represents the micropore structure. The specific surface areas calculated using the Brunauer-Emmett-Teller (BET) theory were 1,350 and 1,941 m ² g ^-1 , respectively, and the micropore surface areas corresponded to 1,048 and 1,523 m ² g ^-1 , respectively. The isothermal curves of kAC-4 and kAC-6 showed IUPAC type-I and type-II hybrid isotherms, indicating micropore and macropore structures. The specific surface areas of kAC-4 and kAC-6 were 2,128 and 2,719 m ² g ⁻¹ , respectively. Interestingly, although the specific surface area was significantly increased, their micropore surface area (1,584 and 1,616 m ² g ⁻¹ in kAC-4 and kAC-6, respectively) did not increase significantly compared to kAC-2. That is, kAC-4 and kAC-6 have more meso- and macropores, indicating that they have advantages for fast charge charging as well as high specific surface area for capacitive EDL charge charging .

The electrochemical performance of kAC was investigated in a voltage range of 1.5-4.5 V vs Na ⁺ /Na at a current rate of 0.1 A g ^-1 and shown in FIG. 4 . The second-order galvanostatic charge/discharge curves of all samples showed a linear profile without plateaus, leading to a surface-driven capacitive charge behavior. The profiles of the electrodes except for the kAC-6 electrode showed no voltage hysteresis, indicating a reversible charge mechanism based on the electric double layer phenomenon. For kAC-1, kAC-2, and kAC-4, the reversible capacitance is about 150 mA hg ^-1 at a charge/discharge rate of 0.1 A g ^-1 , and the kAC-6 electrode has an irreversible electricity of ~110 mA hg ^-1 . capacity, which means low reversibility due to the relatively large mesopores causing the cylinder effect. As the charge/discharge rate was continuously increased from 0.1 to 10 A g ⁻¹ , a remarkable rate capability was found ( FIG. 4b and Table 3). In particular, the rate characteristic of the kAC-4 electrode demonstrates a significantly faster charge-charge kinetics, showing a clear difference with increased charge-discharge rate compared to other samples. The specific capacity of the kAC-4 electrode was 150, 145, 141, 123, 113, 95, and 83 mA hg ^-1 at charge/discharge rates of 0.1, 0.2, 0.5, 1, 2, 5, and 10 A g ^-1 , respectively. , and a reversible specific capacity of ˜149 mA hg ⁻¹ was recovered, which is noteworthy reversibility. These results suggest that charge charging is related to their microstructure and bulk density. As mentioned above, the S _EXT values (>2 nm) excluding the micropore regions of kAC-4 and kAC-6 were 554 and 1,113 m ² g ⁻¹ , respectively. This suggests that the harsh activation conditions induce pores that are relatively larger than kAC-4 and their microstructures, including pathways for charge carriers, collapse. This larger pore trend is also similar in density. The density of the kAC sample increased stepwise from 1.12 to 1.36 g cm ⁻¹ , but the density of kAC-6 (1.25 g cm ⁻¹ ) was decreased relative to that of the kAC-4 sample. The mesopore and macropore volume of kAC-6 is larger than that of kAC-4 by macropore formation by a complex of carbon microstructure units, which is induced by a strong chemical activation effect. As a result, the rate characteristics of the kAC-6 electrode showed electrochemical performance that was not suitable for the actual use of the anode. Another striking result is the very stable cycling performance of kAC-4 as shown in Fig. 4c. Galvanostatic charge/discharge cycling at a charge-discharge rate of 1 A g ^-1 was very stable even over 1000 cycles and had an average cycle retention of 82%.

So far, with respect to the present invention, the preferred embodiments have been looked at. Those of ordinary skill in the art to which the present invention pertains will understand that the present invention may be implemented in a modified form without departing from the essential characteristics of the present invention. Therefore, the disclosed embodiments are to be considered in an illustrative rather than a restrictive sense. The scope of the present invention is indicated in the claims rather than the foregoing description, and all differences within the scope equivalent thereto should be construed as being included in the present invention.

Claims (7)

pre-carbonizing the kenaf to 400° C. at a heating rate of 1.9-2.1° C. min ⁻¹ ; and
a chemical activation step of treating the pre-carbonized kenaf with KOH in a weight ratio of 1:3.9-4.1 while heating to 800°C at a heating rate of 4.9-5.1°C min ⁻¹ ;
A method for producing a kenaf-derived activated carbon comprising a. The method according to claim 1, wherein the heating rate in the pre-carbonizing step is 2° C. min ⁻¹ . The method according to claim 1, wherein the pre-carbonized kenaf is treated with the KOH in a weight ratio of 1:4. The method according to claim 1, wherein the kenaf is a fine powder of kenaf stems. The method according to claim 1, wherein the heating in the pre-carbonizing step and the chemical activation step is carried out under an Ar flow. delete delete

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