WO2024205144A1 - Nanoporous graphite, precursor thereof, and electrochemical device including same - Google Patents

Abstract

The present invention pertains to a nanoporous graphite that significantly improves selective permeability to a target substance when used as a separator and can exhibit excellent durability due to the interlayer spacing being reliably maintained even under severe conditions or long-term use. The nanoporous graphite contains a plurality of nanopores on at least one surface of a graphite layer and has a diffraction peak corresponding to the (002) crystal plane of graphite, and at least one of the plurality of nanopores is located in-plane in the graphite layer.

Description

Nanoporous graphite, precursor thereof and electrochemical device comprising same

The present invention relates to nanoporous graphite comprising nano-sized pores, precursors thereof, and applications thereof.

Carbon materials are widely used in various industrial fields such as water treatment, gas separation, and energy devices.

Among these, research on membranes using two-dimensional structures such as graphene is being actively conducted in the field of membranes such as gas separation membranes, water treatment separation membranes, ion separation membranes, and secondary battery separation membranes that selectively allow ions or specific substances to pass through.

Among the membrane structures based on two-dimensional structures, research is being conducted continuously on the use of single-layer graphene oxide (GO) or multilayer graphene oxide in which multiple single layers are stacked to improve the selectivity or permeability for the target substance to be separated.

For example, research has been conducted to improve the permeation flux or selectivity for a specific gas mixture by including functionalized graphene such as graphene oxide on a porous support. However, in the case of a composite membrane simply made of laminated graphene oxide, such as when a composite membrane is manufactured by coating graphene oxide, there is a problem of low durability due to the phenomenon of easy exfoliation by external physical and/or chemical stimuli.

In addition, when using multilayer graphene oxide in the presence of an aqueous medium or solvent, there is a disadvantage in that the interlayer distance in the multilayer structure included in the membrane expands, rapidly reducing the efficiency of selective separation of the target substance, and the long-term stability of the membrane is poor, limiting its industrial use.

Accordingly, there is a need to develop a new structural material that can improve the selective penetration efficiency of the target substance while efficiently suppressing performance degradation even during long-term use due to excellent durability.

An object of the present invention is to provide nanoporous graphite in which the interlayer distance is stably maintained even under harsh conditions or for long-term use.

Another object of the present invention is to provide a nanoporous graphite precursor capable of enabling the production of the above-described nanoporous graphite.

Another object of the present invention is to provide various applications of the above-mentioned nanoporous graphite.

The present invention provides nanoporous graphite, characterized in that the nanoporous graphite includes a plurality of nanopores on at least one surface of a graphite layer, has a diffraction peak due to a (002) crystal plane of graphite in X-ray diffraction analysis, and at least one of the plurality of nanopores is located on an in-plane within the graphite layer.

In the nanoporous graphite according to the present invention, the nanoporous graphite may include a D' band in a Raman spectrum.

In the nanoporous graphite according to the present invention, the ratio of I _d /I _g intensities of the Raman spectrum of the nanoporous graphite may be 0.3 to 1.5.

In the nanoporous graphite according to the present invention, the nanoporous graphite may include a D' band having a stronger scattering intensity than the D band in a Raman spectrum.

In the nanoporous graphite according to the present invention, the nanoporous graphite may include a D' band having a stronger scattering intensity than the 2D band in a Raman spectrum.

In the nanoporous graphite according to the present invention, the FWHM of the diffraction peak due to the (002) crystal plane of graphite in the X-ray diffraction analysis of the nanoporous graphite may be 5° or less.

In the nanoporous graphite according to the present invention, the average diameter of the nanopores may be 500 nm or less.

In the nanoporous graphite according to the present invention, after the nanoporous graphite is exfoliated into graphene, five or more nanopores may be included within a 50 nm x 50 nm image observed using a transmission electron microscope.

In the nanoporous graphite according to the present invention, the nanoporous graphite can satisfy the following equation 1.

(Formula 1)

SNPG/SG ≤ 10

(SNPG stands for the BET surface area of nanoporous graphite, and SG stands for the BET surface area of pristine graphite)

In the nanoporous graphite according to the present invention, the nanoporous graphite may have a BET specific surface area of 20 m ² /g or less.

In the nanoporous graphite according to the present invention, the size of the nanopores can be controlled by a chemical oxidation reaction.

In the nanoporous graphite according to the present invention, the nanoporous graphite may have a pore volume of 0.1 cm ³ /g or less.

In the nanoporous graphite according to the present invention, the intensity of the peak of C-O-C, C=O or C(=O)O binding energy according to X-ray photoelectron spectroscopy may be lower than the intensity of the peak of C=C and C-C binding energy.

The present invention provides a separator, wherein the separator comprises the nanoporous graphite.

In the separation membrane according to the present invention, the separation membrane may be for water treatment, solvent nanofiltration, gas separation, or ion separation.

The present invention provides an electrochemical device, wherein the electrochemical device comprises the nanoporous graphite described above.

The present invention provides a graphite precursor, characterized in that the graphite precursor includes Cr on at least one side of a graphite layer, includes a D' band in a Raman spectrum, and has two or more diffraction peaks diffracted at an angle lower than a diffraction angle 2θ due to a (002) crystal plane of graphite in X-ray diffraction analysis.

In the graphite precursor according to the present invention, the graphite precursor may include a D band having a weaker scattering intensity than the 2D band in the Raman spectrum.

In the graphite precursor according to the present invention, the Cr may be included in an amount of 3 wt% or less of the total weight of the nanoporous graphite precursor.

The present invention provides a method for producing nanoporous graphite, the method for producing nanoporous graphite comprising: (S1) a step of producing a nanoporous graphite precursor by positioning one or more intercalating compounds on at least one surface included in graphite; (S2) a step of reacting the intercalating compound with an unsaturated bond between carbon atoms included in the graphite; and (S3) a step of forming nanopores due to the reaction of the step (S2); wherein the intercalating compound induces nanopores at the positions of the saturated bonds.

In the method for producing nanoporous graphite according to the present invention, the intercalating compound is CrO ₂ Cl ₂ ; Cr ₂ O ₃ ; FeCl ₃ ; and a compound comprising the same; may be selected from the group consisting of:

In the method for producing nanoporous graphite according to the present invention, the reaction in step (S2) may be an oxidation reaction.

In the method for producing nanoporous graphite according to the present invention, the reaction of step (S2) can be performed under acidic conditions.

In the method for manufacturing nanoporous graphite according to the present invention, the step (S2) can be performed by adding an oxidizing agent.

In the method for producing nanoporous graphite according to the present invention, the oxidizing agent may include KMnO ₄ .

The nanoporous graphite according to the present invention comprises a plurality of nanopores on at least one side of a graphite layer, has a diffraction peak due to a (002) crystal plane of graphite in X-ray diffraction analysis, and is characterized in that at least one of the plurality of nanopores is located on an in-plane within the graphite layer. Accordingly, when used as a separation membrane, not only can the selective permeability to a target substance be significantly improved, but also has the advantage of excellent durability in which the interlayer distance is stably maintained even under harsh conditions or for long-term use.

FIG. 1a, FIG. 1b, and FIG. 1c are drawings showing X-ray diffraction patterns, Raman spectra, and XPS spectra for graphite and GIC of Preparation Examples 1 to 3, respectively, and FIG. 1d is a drawing showing the Cr content included in Preparation Examples 1 to 3 calculated through XPS analysis.

Figure 2 is a schematic diagram illustrating a series of processes for manufacturing nanoporous graphite using graphite as a starting material.

FIG. 3a is a diagram showing a transmission electron microscope image of Example 1 and a corresponding energy dispersive spectroscopy (EDS) mapping image, and FIGS. 3b and 3c are diagrams showing a high-resolution transmission electron microscope (HR-TEM) image of Example 1 and a corresponding fast Fourier transform (FFT) pattern, respectively.

Figures 4a and 4b are drawings showing the Raman spectrum and C1s XPS spectrum for the carbon body, respectively, and Figure 4c is a drawing showing the X-ray diffraction pattern for the carbon body, respectively.

FIG. 5a is a diagram showing FT-IR spectra of graphite, Preparation Example 3, Comparative Example 1, Example 1, Example 2, Example 3, and Example 4, and FIGS. 5b and 5c are diagrams showing water contact angles and zeta potentials of Examples 1 to 4, respectively.

FIG. 6a is a drawing showing high-resolution transmission electron microscope (HR-TEM) images of Example 1, Example 2, and Comparative Example 3, and FIG. 6b is a drawing showing the pore size distribution results of Example 1 and Example 2.

FIG. 7a and FIG. 7b are diagrams showing N ₂ adsorption-desorption isotherms and BJH (Barrett-Joyner-Halenda) pore size distributions for Examples 1 and 2, respectively.

Figure 8 is a schematic diagram illustrating the pore formation process of nanoporous graphite.

FIGS. 9a, 9b, and 9c are drawings showing an optical image, a top-view scanning electron microscope (SEM) image, and a cross-section SEM image, respectively, of a membrane manufactured according to Example 5.

FIG. 10a, FIG. 10b, and FIG. 10c are schematic diagrams showing the results of changes in the interlayer structure of carbon bodies included in the membranes of Example 5 and Comparative Example 4 under conditions of aqueous solutions containing various ions, the results of changes in the interlayer structure according to the concentration of ions, and the interlayer distance expansion phenomenon under pure H ₂ O conditions, respectively.

FIG. 11a and FIG. 11b are drawings showing XRD patterns measured after the membrane of Example 5 was immersed in pure water for 1 day and 60 days, respectively, and schematically showing the interlayer distance expansion phenomenon under pure H ₂ O conditions for Comparative Example 4 and Example 5.

FIG. 12a and FIG. 12b are drawings comparing XRD patterns measured for the membranes of Examples 5 to 8 in a dry state and after immersion in pure water for 24 hours, respectively, and drawings comparing interlayer distances calculated from XRD patterns measured for the membranes after immersing each membrane in 0.1 M of each ionic solution (NaCl, Na ₂ SO ₄ , LiCl, CaCl ₂ , MgCl ₂ , K ₃ [Fe(CN) ₆ ], and AlCl ₂ ) for 24 hours.

FIG. 13a and FIG. 13b are diagrams showing ion permeability characteristics according to the ion permeability and the anion-to-cation valence ratios included in the ion solution for the membranes of Examples 5 to 8, respectively.

Figures 14a and 14b are diagrams comparing the performance results of Example 5 according to a continuous forward osmosis desalination test for 24 hours and the ion rejection rate according to a continuous forward osmosis desalination test for 1 day and 7 days, respectively.

Figures 15a and 15b are diagrams showing the results of evaluating the performance of a separation membrane according to the concentration of a NaCl solution and the results of evaluating the performance of a separation membrane according to the change in pH of a NaCl solution, respectively.

FIG. 16a and FIG. 16b are diagrams showing the performance evaluation results for the membrane of Example 5 according to a continuous forward osmosis desalination test for 30 days using a 0.1 M NaCl solution and a 1 M NaCl solution as feed solutions, respectively, and the performance evaluation results for the membrane of Example 5 according to the thickness of the membrane using a 0.1 M NaCl solution.

Figure 17 is a schematic diagram illustrating the mechanism of desalination using a membrane containing nanoporous graphite.

The nanoporous graphite of the present invention will be described in detail with reference to the attached drawings.

The drawings introduced below are provided as examples so that the idea of the present invention can be sufficiently conveyed to those skilled in the art. Therefore, the present invention is not limited to the drawings presented below and may be embodied in other forms, and the drawings presented below may be exaggerated in order to clarify the idea of the present invention.

At this time, if there is no other definition for the technical and scientific terms used, they have the meaning commonly understood by a person of ordinary skill in the art to which this invention belongs, and the description of well-known functions and configurations that may unnecessarily obscure the gist of the present invention is omitted in the following description and the attached drawings.

Additionally, the singular forms used in the specification and the appended claims are intended to include the plural forms as well, unless the context clearly dictates otherwise.

In this specification and the appended claims, the terms first, second, etc. are not used in a limiting sense but are used to distinguish one component from another.

The terms “include” or “have” in this specification and the appended claims mean that a feature or component described in the specification is present, and unless specifically limited, do not exclude in advance the possibility that one or more other features or components may be added.

In this specification and the appended claims, when a part such as a film (layer), region, component, etc. is said to be on or above another part, it includes not only the case where it is directly above and in contact with the other part, but also the case where another film (layer), another region, another component, etc. is interposed.

A numerical range as used herein includes a lower bound and an upper bound and all values within that range, increments logically derived from the shape and width of the defined range, all doubly defined values, and all possible combinations of upper and lower bounds of a numerical range defined in different shapes.

The terms “about,” “substantially,” and the like, as used in this specification and the appended claims, are used in a meaning that is at or close to the numerical value when manufacturing and material tolerances inherent in the meaning referred to are presented, and are used to prevent unscrupulous infringers from unfairly utilizing the disclosure where precise or absolute values are mentioned in order to aid in the understanding of this specification and the appended claims.

Hereinafter, the nanoporous graphite of the present invention, the nanoporous graphite precursor, the electrochemical device including the same, and the components of the electrochemical device will be described in detail.

The present invention provides nanoporous graphite, characterized in that the nanoporous graphite includes a plurality of nanopores on at least one surface of a graphite layer, has a diffraction peak due to a (002) crystal plane of graphite in X-ray diffraction analysis, and at least one of the plurality of nanopores is located on an in-plane within the graphite layer.

According to one embodiment, the nanoporous graphite may include a D' band in a Raman spectrum. The nanoporous graphite may mean that the graphite layer includes one or more nanopores on a plane, i.e., on an in-plane, within the layer, including the D' band in the Raman spectrum.

According to one embodiment, the I _d /I _g intensity ratio of the Raman spectrum of the nanoporous graphite can be from 0.3 to 1.5. The I _d /I _g intensity ratio of the Raman spectrum of the nanoporous graphite can be 0.3 or more, 0.4 or more, 0.5 or more, or 0.6 or more, and an upper limit can be 2.0 or less, 1.5 or less, 1.3 or less, or 1.2 or less. Specifically, the I _d /I _g intensity ratio of the Raman spectrum of the nanoporous graphite can be 0.3 to 2.0, 0.4 to 1.5, 0.5 to 1.3, or 0.6 to 1.2. When the ratio of I _d /I _g intensities of the Raman spectrum of the above nanoporous graphite satisfies the above-described range, it can significantly improve the selective permeability to the target substance when used as a separation membrane due to fewer defects, and can also exhibit excellent durability in which the interlayer distance is stably maintained even under harsh conditions or for long-term use.

According to one embodiment, the nanoporous graphite may include a D' band having a stronger scattering intensity than the D band in a Raman spectrum.

Specifically, the D' band may be a peak that appears when the graphite layer includes defects and nano-pores in the aromatic carbon structure. Accordingly, the single layer of the nano-porous graphite may have a D' band higher than the D band, thereby maintaining high crystallinity and indicating that nano-pores are formed.

According to one embodiment, the nanoporous graphite may include a D' band having a stronger scattering intensity than the 2D band in a Raman spectrum.

Specifically, the 2D band may be a peak that appears when the crystal structure of each layer of the nanoporous graphite is maintained. Accordingly, the nanoporous graphite may mean that the interlayer crystal structure has high crystallinity and appropriate nanopores because the 2D band is maintained and the D' band is higher than the 2D band.

According to one embodiment, the FWHM of the diffraction peak due to the (002) crystal plane of graphite in the X-ray diffraction analysis may be 5° or less. The FWHM of the diffraction peak due to the (002) crystal plane of graphite in the X-ray diffraction analysis may be 0.1° or more, 0.5° or more, or 1° or more, and may have an upper limit of 10° or less, 5° or less, or 3° or less. Specifically, the FWHM of the diffraction peak due to the (002) crystal plane of graphite in the X-ray diffraction analysis may be 0.1 to 10°, 0.5 to 5°, or 1 to 3°. The X-ray diffraction peak in the (002) crystal plane of the nanoporous graphite may maintain high crystallinity while having a large number of pores, thereby satisfying the above-described full width at half maximum (FWHM).

According to one embodiment, the average diameter of the nanopores included in the nanoporous graphite may be 500 nm or less. The average diameter of the nanopores included in the nanoporous graphite may be 1,000 nm or less, 500 nm or less, 200 nm or less, 100 nm or less, or 50 nm or less, and a lower limit may be 0.1 nm or more, 1 nm or more, or 5 nm or more. Specifically, the average diameter of the nanopores included in the nanoporous graphite may be 0.1 to 1,000 nm, 1 to 500 nm, 1 to 200 nm, 1 to 100 nm, 1 to 50 nm, or 5 to 50 nm. The nanopores may have a diameter within the above-described range to improve material transfer and/or solvent transport properties, thereby increasing permeability to a target substance.

According to one embodiment, after exfoliating one layer of graphene included in the nanoporous graphite, a 50 nm x 50 nm image observed with a scanning electron microscope (SEM) may include 5 or more nanopores. The number of nanopores included in the 50 nm x 50 nm image observed with a transmission electron microscope after exfoliating one layer of graphene included in the nanoporous graphite may be 1 or more, 2 or more, 3 or more, 5 or more, or 10 or more, and an upper limit may be 500 or less, 200 or less, 100 or less, 50 or less, or 20 or less. Specifically, the number of nanopores in a 50 nm x 50 nm image observed by a transmission electron microscope after peeling off one layer of graphene included in the nanoporous graphite may be 1 to 500, 2 to 200, 3 to 100, 5 to 50, or 10 to 20. The number of nanopores described above refers to the density of the nanopores, and the nanopores included in the nanoporous graphite may have a density in the above-described range to improve material transfer and/or solvent transport properties, thereby increasing permeability to a target substance.

According to one embodiment, the nanoporous graphite can satisfy the following equation 1.

(Formula 1)

SNPG/SG ≤ 10

(In Equation 1, SNPG represents the BET surface area of nanoporous graphite, and SG represents the BET surface area of pristine graphite.)

The value of SNPG/SG of the above formula 1 can be 20 or less, 10 or less, 5 or less, 3 or less, or 2 or less, and as a lower limit, can be greater than 1.0. Specifically, the value of SNPG/SG of the above formula 1 can be greater than 1.0 and less than 20, greater than 1.0 and less than 10, greater than 1.0 and less than 5, greater than 1.0 and less than 3, or greater than 1.0 and less than 2.

The above pristine graphite refers to general graphite, and the nanoporous graphite has a highly stacked and dense crystal structure of graphene, and thus can satisfy the range described in Equation 1. However, despite having a highly stacked and dense structure, the porous graphite has a large number of pores, and thus can have higher permeability to a target material and higher durability than general graphite materials.

According to one embodiment, the nanoporous graphite can have a BET surface area of 20 m ² /g or less. The BET surface area of the nanoporous graphite can be 20 m ² /g or less, 10 m ² /g or less, or 5 m ² /g or less, and may be, as a lower limit, 0.1 m ² /g or more, 1 m ² /g or more, or 2 m ² /g or more. Specifically, the BET surface area of the nanoporous graphite can be 0.1 to 20 m ² /g, 1 to 10 m ² /g, or 2 to 5 m ² /g. The nanoporous graphite has a highly stacked and dense structure, and thus the BET surface area can satisfy the above-described range. Nonetheless, the above nanoporous graphite has a large number of pores and thus can have higher permeability to target substances and higher durability than general graphite materials.

According to one embodiment, the size of the nanopores can be controlled by a chemical oxidation reaction.

As a non-limiting example, CrO ₂ Cl ₂ or the like is inserted, i.e., intercalated, between each plane of graphene included in the nanoporous graphite, and the CrO ₂ Cl ₂ inserted between layers under acidic conditions can react with double-bonded carbon to cleave the double bonds, thereby inducing strain in the carbon lattice to form defects and pores. More specifically, the CrO ₂ Cl ₂ can induce local cleavage of the sp ² domain of the graphene included in the nanoporous graphite to induce the nanopores. At this time, the size of the nanopores can be determined by being affected by the concentration and treatment time of the CrO ₂ Cl _2. Meanwhile, the oxidizing agent that induces the chemical oxidation reaction may be KMnO ₄ or the like, but the present invention is not limited thereto.

In one embodiment, the nanoporous graphite can have a pore volume of 0.1 cm ³ /g or less. The pore volume of the nanoporous graphite can be 0.20 cm ³ /g or less, 0.10 cm ³ /g or less, or 0.05 cm ³ /g or less, and as a lower limit, can be 0.001 cm ³ /g or more, 0.05 cm ³ /g or more, or 0.01 cm ³ /g or more. Specifically, the pore volume of the nanoporous graphite can be 0.001 to 0.20 cm ³ /g, 0.05 to 0.10 cm ³ /g, or 0.01 to 0.05 cm ³ /g. The above nanoporous graphite can have high material transfer efficiency and durability despite having a low pore volume in the above-described range due to pores appropriately formed between planes of graphene included in the nanoporous graphite.

According to one embodiment, the intensity of a peak of C-O-C, C=O or C(=O)O binding energy according to X-ray photoelectron spectroscopy (XPS) of the nanoporous graphite may be lower than the intensity of the peaks of C=C and C-C binding energies. Specifically, since the nanopores included in the nanoporous graphite may be generated by a chemical oxidation reaction as described above, the peak including an oxygen functional group of the X-ray photoelectron spectroscopy may be greater than the carbon peak. In the case of general pristine graphite that does not satisfy this, the pores are not densely located between layers but are randomly located, and thus the remarkable effect according to the present invention may not be exhibited.

The present invention provides a separator, wherein the separator comprises nanoporous graphite according to the present invention. In describing the separator, since the separator comprises the above-described nanoporous graphite, the separator according to the present invention comprises all the contents described above with respect to the nanoporous graphite.

Hereinafter, the separation membrane of the present invention will be described in more detail.

According to one embodiment, the separation membrane can be used for water treatment, solvent nanofiltration, gas separation, or ion separation. Specifically, since the separation membrane includes the nanoporous graphite described above, it can have material transfer efficiency, permeability to a target substance, and high durability. Therefore, the separation membrane can be used for the purposes described above, but the present invention is not limited thereto.

According to one embodiment, the nanoporous graphite can be included in electrode active materials, supercapacitor electrodes, and electrochemical devices.

Specifically, the electrode active material may be included in a negative electrode for a secondary battery, a supercapacitor electrode, etc. The electrode active material includes the nanoporous graphite described above, and since the nanoporous graphite can have significantly high material transfer efficiency and durability as described above, it may be included in the negative electrode for a secondary battery, a supercapacitor electrode, etc. to improve the performance and durability of the secondary battery and the supercapacitor.

In addition, specifically, the electrochemical device may be an electrochemical sensor, an electrochemical catalyst, or an energy storage device. As described above, the nanoporous graphite has remarkable effects such as high material transfer efficiency and high durability, and thus can be applied to the electrochemical device described above.

However, the nanoporous graphite according to the present invention is not limited to the above-described uses, and it can also be applied to various industrial fields to which the remarkable effects of the nanoporous graphite according to the present invention can be applied.

The present invention provides a nanoporous graphite precursor, characterized in that the nanoporous graphite precursor includes Cr on at least one side of a graphite layer, includes a D' band in a Raman spectrum, and has two or more diffraction peaks diffracted at an angle lower than 2θ due to a (002) crystal plane of graphite in X-ray diffraction analysis.

In describing the above nanoporous graphite precursor, the nanoporous graphite precursor may be an intermediate in the manufacturing process of the above-described nanoporous graphite, and the D' band and the (002) crystal plane of the graphite may be the same as or similar to the terms described above in the nanoporous graphite, and therefore the nanoporous graphite precursor according to the present invention includes all the contents described above in the nanoporous graphite.

Hereinafter, the nanoporous graphite precursor according to the present invention will be described in more detail.

According to one embodiment, the above-described nanoporous graphite can be manufactured by the graphite intercalation reaction of Cr of the nanoporous graphite precursor. As in the non-limiting example described above, CrO ₂ Cl ₂ or the like is inserted, i.e., intercalated, between the planes of each graphene included in the above-described nanoporous graphite, and then a chemical oxidation reaction is performed, whereby the above-described nanoporous graphite can be manufactured.

According to one embodiment, the Cr, that is, the chromium atoms, may be included in an amount of 3 wt% or less of the total weight of the nanoporous graphite precursor. The Cr, that is, the chromium atoms, may be included in an amount of 10 wt% or less, 5 wt% or less, or 3 wt% or less of the total weight of the nanoporous graphite precursor, and as a lower limit, may be included in an amount of 0.1 wt% or more, 0.5 wt% or more, or 1 wt% or more. Specifically, the Cr, that is, the chromium atoms, may be included in an amount of 0.1 to 10 wt%, 0.5 to 5 wt%, or 1 to 3 wt% of the total weight of the nanoporous graphite precursor. The Cr included in the nanoporous graphite precursor is included within the above-described range to appropriately control the pore size and pore distribution of the nanoporous graphite described above.

In addition, it goes without saying that the Cr contained in the above-described nanoporous graphite precursor can be removed after pore creation, thereby producing the above-described nanoporous graphite.

The present invention provides a method for producing nanoporous graphite, the method for producing nanoporous graphite comprising: (S1) a step of producing a nanoporous graphite precursor by positioning one or more intercalating compounds on at least one surface included in graphite; (S2) a step of reacting the intercalating compound with an unsaturated bond between carbon atoms included in the graphite; and (S3) a step of forming nanopores due to the reaction of the step (S2); wherein the intercalating compound induces nanopores at the positions of the saturated bonds.

In describing the method for producing the above nanoporous graphite, the method for producing the above nanoporous graphite may be the method for producing the above-described nanoporous graphite; and the graphite and nanopores, etc. may be the same as or similar to the terms described above in the nanoporous graphite and the nanoporous graphite precursor, and therefore the method for producing nanoporous graphite according to the present invention includes all the contents described above in the nanoporous graphite and the nanoporous graphite precursor.

Hereinafter, the method for manufacturing nanoporous graphite of the present invention will be described in detail.

As described above, as a non-limiting example, the intercalating compound is inserted, i.e., intercalated, between each plane of graphene included in the nanoporous graphite, and the intercalating compound inserted between layers under acidic conditions can react with double-bonded carbon to cleave the double bonds, thereby inducing strain in the carbon lattice to form defects and pores. More specifically, the intercalating compound can induce local cleavage of the sp ² domain of the graphene included in the nanoporous graphite to induce the nanopores.

In one embodiment, the intercalating compound is CrO ₂ Cl ₂ ; Cr ₂ O ₃ ; FeCl ₃ ; and a compound including the same; may be selected from the group consisting of: The intercalating compound may be selected from the chemical species described above and may form the nanopores between the planes of each graphene included in the graphite.

According to one embodiment, the reaction of the step (S2) may be an oxidation reaction. As described above, in the step (S2), the intercalating compound may react with the double-bonded carbon to break the double bond included in the graphene. At this time, the process in which the double bond of an unsaturated hydrocarbon is broken to become a saturated hydrocarbon; or the process in which the double bond is removed to create pores or defects; may be performed by an oxidation reaction.

According to one embodiment, the oxidation reaction may be performed by further adding an oxidizing agent. Specifically, the intercalating compound intercalated between the planes of the graphene included in the graphite may be oxidized by an oxidizing agent under acidic conditions to generate the nanopores. As a non-limiting example, the oxidizing agent may include KMnO ₄ or the like, but the present invention is not limited thereto.

According to one embodiment, the reaction of the step (S2) can be performed under acidic conditions. As described above, the reaction of the step (S2) can be an oxidation reaction and can be performed by adding an oxidizing agent. At this time, in order to facilitate the oxidation reaction, the reaction of the step (S2) can be performed under acidic conditions of less than pH 7. As a non-limiting example, the nanoporous graphite precursor is dispersed in water (H- ₂ O), and an oxidizing agent, KMnO ₄ , and sulfuric acid (H ₂ SO ₄ ), are added to the aqueous solution in which the precursor is dispersed, so that the reaction can be performed under acidic conditions and conditions in which an oxidizing agent is added.

According to one embodiment, the amount of the oxidizing agent added may be 0.01 to 10 equivalents based on 1 equivalent of the intercalating compound included in the nanoporous graphite precursor. The amount of the oxidizing agent added may be 0.01 equivalent or more, 0.05 equivalent or more, 0.1 equivalent or more, 0.2 equivalent or more, 0.5 equivalent or more, or 1 equivalent or more based on 1 equivalent of the intercalating compound included in the nanoporous graphite precursor, and the upper limit may be 100 equivalent or less, 50 equivalent or less, 30 equivalent or less, 20 equivalent or less, 10 equivalent or less, or 5 equivalent or less. Specifically, the amount of the oxidizing agent added may be 0.01 to 100 equivalents, 0.05 to 50 equivalents, 0.1 to 30 equivalents, 0.2 to 20 equivalents, 0.5 to 10 equivalents, or 1 to 5 equivalents based on 1 equivalent of the intercalating compound included in the nanoporous graphite precursor. The amount of the oxidizing agent added may be adjusted within the above-described range, but may vary depending on various conditions such as the chemical species of the additive, the chemical species of the intercalating compound, and the pH during the manufacturing process, and therefore the present invention is not limited to the above-described amount of the oxidizing agent added.

Hereinafter, the nanoporous graphite according to the present invention will be described in more detail through examples. However, the following examples are only a reference for describing the present invention in detail, and the present invention is not limited thereto, and may be implemented in various forms.

Also, unless otherwise defined, all technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description herein is only for the purpose of effectively describing particular embodiments and is not intended to be limiting of the present invention.

(Manufacturing Example 1) Manufacturing of precursor for nanoporous graphite

For the preparation of nanoporous graphite, 1 g of graphite and 10 g of Cr ₂ O ₃ were added to a 25 mL flask, and then 10 mL of HCl was slowly added to prepare a mixture.

The mixture was stirred for 2 hours to cause an intercalation reaction to form CrO ₂ Cl ₂ between the graphite layers.

Thereafter, the obtained product in which CrO ₂ Cl ₂ was formed between graphite layers was vacuum filtered, washed with deionized water, and dried in an oven at 60° C. to produce a graphite intercalation compound (GIC), which is a precursor of nanoporous graphite.

(Manufacturing example 2)

The same procedure as Manufacturing Example 1 was followed, except that the mixture was stirred for 5 hours to perform an intercalation reaction.

(Manufacturing Example 3)

The same procedure as Manufacturing Example 1 was followed, except that the mixture was stirred for 20 hours to perform an intercalation reaction.

(Experimental Example 1) Structural Analysis of Graphite Interlayer Compound (GIC)

The structures of each manufactured GIC were compared through X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS) analysis, and Raman analysis.

FIG. 1a, FIG. 1b, and FIG. 1c are drawings showing X-ray diffraction patterns, Raman spectra, and XPS spectra for graphite and GIC of Preparation Examples 1 to 3, respectively, and FIG. 1d is a drawing showing the Cr content included in Preparation Examples 1 to 3 calculated through XPS analysis.

Referring to Fig. 1a, in Manufacturing Examples 1 to 3, it was observed that there were two diffraction peaks diffracted at an angle lower than the diffraction angle 2θ due to the (002) crystal plane of graphite.

It was observed that both Manufacturing Examples 1 to 3 included a first peak located at a diffraction angle approximately 0.4 degrees lower than the diffraction angle 2θ due to the (002) crystal plane of graphite among the two diffraction peaks, and that the diffraction angles at which the second peak was located were different.

The second peaks of Manufacturing Examples 1, 2, and 3 were observed to be located at diffraction angles that were approximately 0.9 degrees, approximately 1.8 degrees, and approximately 2.2 degrees lower, respectively, than the diffraction angle corresponding to the (002) crystal plane of graphite.

From this, it can be seen that the interlayer space of graphite becomes larger and wider as the intercalation reaction time increases due to the formation of more CrO ₂ Cl ₂ between the graphite layers, but the crystal structure of graphite is still maintained.

These characteristics can be seen more clearly by examining the Raman spectrum in Fig. 1b.

Referring to FIG. 1b, it was observed that all of Manufacturing Examples 1 to 3 included a D band having a weaker scattering intensity than the 2D band, similar to graphite, whereas, unlike graphite, a D' band was observed to be included at a Raman shift of about 1596 cm ^-1 .

That is, it can be seen that even though CrO ₂ Cl ₂ is intercalated between graphite layers to form GIC, structural defects are practically not formed, and the crystal structure is maintained almost identically to that of graphite. In addition, it can be seen that GIC includes both a region with a wide interlayer space where CrO ₂ Cl ₂ is located and a region with a narrow interlayer space where _{CrO 2} Cl ₂ is not located due to the intercalated CrO ₂ Cl 2.

Looking at FIGS. 1c and 1d, it can be seen that as the intercalation reaction time increases, more CrO ₂ Cl ₂ is formed between the graphite layers, which increases the Cr content. As shown in FIG. 1c, in Preparation Examples 1 to 3, an O 1s peak is observed along with the Cr 2p peak, which can be seen to originate from the intercalated CrO ₂ Cl _2. In addition, it was confirmed that the GICs of Preparation Examples 1, 2, and 3 contained 1.71 wt%, 2.01 wt%, and 2.13 wt% of Cr, respectively.

From these results, GIC of Manufacturing Example 3 was used as a precursor for manufacturing nanoporous graphite.

(Example 1) Preparation of nanoporous graphite

100 mg of GIC of Manufacturing Example 3 was dissolved in 10 mL of sulfuric acid and stirred at room temperature (23°C) for 2 hours to prepare a precursor solution. Subsequently, the temperature of the precursor solution was increased to 35°C in an oil bath, and a mixture containing KMnO ₄ as an oxidizing agent was stirred for 2 hours.

At this time, KMnO ₄ was added so that the weight ratio of GIC: oxidizing agent KMnO ₄ was 1:1.

Afterwards, the stirred mixture was placed in an ice bath, and deionized water and hydrogen peroxide were slowly added to stop the reaction so that the temperature did not rise. The resulting compound was purified with 10% hydrochloric acid, and then centrifuged to obtain the product, which was washed several times using deionized water.

Next, the solution containing the washed product was sonicated for 1 hour, centrifuged at 5000 rpm for 1 hour, and the supernatant obtained was freeze-dried to produce nanoporous graphite.

A series of processes for manufacturing nanoporous graphite using pristine graphite as a starting material is schematically illustrated in Fig. 2.

(Example 2)

The same procedure as Example 1 was followed, except that KMnO ₄ was added so that the weight ratio of GIC: KMnO ₄ was 1:1.25.

(Example 3)

The same procedure as Example 1 was followed, except that KMnO ₄ was added so that the weight ratio of GIC: KMnO ₄ was 1:1.5.

(Example 4)

The same procedure as Example 1 was followed, except that KMnO ₄ was added so that the weight ratio of GIC: KMnO ₄ was 1:2.

(Example 5)

The same procedure as Example 1 was followed, except that KMnO ₄ was added so that the weight ratio of GIC: KMnO ₄ was 1:0.25.

(Example 6)

The same procedure as Example 1 was followed, except that KMnO ₄ was added so that the weight ratio of GIC: KMnO ₄ was 1:0.5.

(Comparative Example 1)

The same procedure as Example 1 was followed, except that the oxidizing agent KMnO ₄ was not added.

(Comparative Example 2)

The same procedure as in Example 1 was followed, except that graphite instead of GIC was used and KMnO ₄ was added so that the weight ratio of graphite: KMnO ₄ was 1:1.

(Comparative Example 3)

The same procedure was followed as in the conventional Hummers method, except that the mixture was stirred for 15 hours with KMnO ₄ added so that the weight ratio of graphite:KMnO ₄ was 1:3.5.

(Experimental Example 2) Structural Analysis of Nanoporous Graphite

FIG. 3a is a diagram showing a transmission electron microscope image of Example 1 and a corresponding energy dispersive spectroscopy (EDS) mapping image, and FIGS. 3b and 3c are diagrams showing a high-resolution transmission electron microscope (HR-TEM) image of Example 1 and a corresponding fast Fourier transform (FFT) pattern, respectively.

As shown in Fig. 3a, it is observed that the signal for oxygen is measured at a low level, indicating that the oxidation reaction by the oxidizing agent is effectively suppressed.

In addition, referring to FIGS. 3b and 3c, it was observed that the nanoporous graphite of Example 1 included nanopores, and mainly contained sp2 carbon domains as the main structure, and it was confirmed that sp3 carbon domains existed locally due to oxygen bonding.

Additionally, it can be seen that the graphene crystal structure is maintained from the hexagonal FFT pattern derived from the HR-TEM image.

Figures 4a and 4b are drawings showing a Raman spectrum and a C1s XPS spectrum for the carbon body, respectively, and Figure 4c is a drawing showing an X-ray diffraction pattern for the carbon body, respectively.

First, referring to the Raman spectrum of Fig. 4a, unlike the GIC of the aforementioned manufacturing example, in the case of Comparative Example 1, the intensity of the D band peak located at a Raman shift of 1340 cm ^-1 increases, and the G band peak (located at a Raman shift of 1570 cm ^-1 ) and the broad D' band peak appear, indicating that defects and nanopores were formed in the aromatic carbon structure. In addition, the presence of a single 2D peak (located at a Raman shift of 2700 cm ^-1 ) indicates that nanopores were formed while maintaining the crystal structure of graphite.

On the other hand, in the case of Example 1, it was observed that a D band peak with a sharp increase in intensity was present along with a broad 2D band peak compared to Comparative Example 1, and it was confirmed that the G band peak was red-shifted. It can be seen that this is due to the presence of a mixture of sp2 bonded carbon and sp3 bonded carbon, which is consistent with the results of the HR-TEM image analysis described above.

As the concentration of the oxidizing agent increased, the sp3 carbon domain area increased, so the intensity of the D band peak gradually decreased and the red-shifted G band, i.e., the D' band peak, appeared broad.

Additionally, the ratios (ID /IG) of the intensities of the D band peak (ID) and the G band peak (IG) of graphite, Manufacturing Example 3, Comparative Example 1, Example 1, Example 2, Example 3, Example 4, and Comparative Example 2 were confirmed to be 0.072, 0.14, 0.48, 1.09, 1.04, 0.87, 0.81, and 0.95, respectively.

Next, looking at the C1s XPS spectrum of Fig. 4b, in the case of Comparative Example 1, since no oxidizing agent was added, no oxygen-related peaks were observed, and only typical carbon bond peaks were present. From this, it can be seen that the chemical structure of graphite is maintained during the nanopore formation process.

When an oxidizing agent is added, it is observed that in addition to the CO peak located at a binding energy of 286.2 eV, the C=O bond peak (287.8 eV) and C=O(O) (288.9 eV) bond peaks begin to appear, indicating the additional presence of epoxide groups, hydroxyl groups, and carboxyl groups.

Specifically, in all of Examples 1 to 4 and Comparative Example 2, it was confirmed that the intensity of the peak of C-O-C, C=O or C(=O)O bond energy was smaller than the intensity of the peak of C=C and C-C bond energy, and it was confirmed that the intensity of the bond peak related to the functional group containing oxygen increased as the concentration of the oxidizing agent increased, and in particular, it was confirmed that it was the strongest in Comparative Example 2.

More specifically, it was confirmed that the ratio of the intensity of the C-O-C bond peak in Example 1 to the intensity of the C=C and C-C bond peaks was 1:4.5, and the ratios of the intensities for the same bond peaks in Examples 2, 3, 4, and Comparative Example 2 were 1:2.4, 1:1.5, 1:1.4, and 1:1.2, respectively.

When comparing Example 1 and Comparative Example 2, it was confirmed that oxidation was inhibited by the GIC of Example 1 even when the same concentration of oxidizer was added.

These results were confirmed to be consistent with the results of Fourier Transform Infrared Spectroscopy (FT-IR) analysis.

Meanwhile, referring to FIG. 4c, the results of X-ray diffraction analysis of carbon bodies according to Comparative Example 1, Manufacturing Example 3, Example 1, Example 5, and Example 6 were examined, and it was confirmed that the carbon bodies according to Comparative Example 1, Manufacturing Example 3, Example 1, Example 5, and Example 6 had a diffraction peak due to the graphite (002) crystal plane. Therefore, it was confirmed that Comparative Example 1, Manufacturing Example 3, Example 1, Example 5, and Example 6 well maintained the layered structure of graphite.

In addition, it was confirmed that the oxidation peak of graphite located at 10 to 15° increased as the amount of the oxidizing agent, KMnO ₄ , increased. This means that the nano pores contained in the graphite, which are generated as the graphite is oxidized, increased, and it was confirmed that the carbon bodies according to Manufacturing Example 3 and Comparative Example 1, which did not use an oxidizing agent, did not show a diffraction peak located at 10 to 15°.

FIG. 5a is a diagram showing FT-IR spectra of graphite, Preparation Example 3, Comparative Example 1, Example 1, Example 2, Example 3, and Example 4, and FIGS. 5b and 5c are diagrams showing water contact angles and zeta potentials of Examples 1 to 4, respectively.

Referring to the FT-IR spectrum of Fig. ^5a , in the case of Manufacturing Example 3, it can be seen that CrO ₂ Cl ₂ intercalant is inserted between the layers from the peaks corresponding to the C-Cl vibration mode at wavenumbers of 1000 cm ^-1 and 1130 cm -1 and a specific aromatic carbon peak at wavenumber of 1560 cm ^-1 . In addition, referring to the cases of Examples 1 to 4, it can be seen that the intensity of peaks containing oxygen functional groups such as OH (600 cm ^-1 and 1074 cm ^-1 ), COC (1230 cm ^-1 ), and COOH (1675 cm ^-1 ) becomes stronger as the concentration of the added oxidizing agent increases.

As is known, the wettability and hydrophilic properties of carbon bodies such as graphene are affected by the content of oxygen functional groups included in the carbon body. As shown in Fig. 5b, the water contact angle of Example 1, in which oxygen functional groups are limited, was 75°, whereas Example 4, in which oxygen functional groups are relatively more included, was found to exhibit a water contact angle characteristic of 33°.

In addition, when comparing the zeta potential values as shown in Fig. 5c, it was observed that Example 1 still exhibited a zeta potential characteristic of -53 mV despite containing less oxygen functional groups. This characteristic can be advantageously applied when dispersing nanoporous graphite in water or a solvent for application, and can also further enhance the ion rejection rate effect when applied to the treatment of solutions with low ionic strength.

FIG. 6a is a drawing showing high-resolution transmission electron microscope (HR-TEM) images of Example 1, Example 2, and Comparative Example 3, and FIG. 6b is a drawing showing the pore size distribution results of Example 1 and Example 2.

As shown in Fig. 6a, it was confirmed that a large number of nanopores were present in Examples 1 and 2, whereas in the case of Comparative Example 3, almost no nanopores were formed.

That is, it can be seen that the formation of nano-pores is caused by the CrO ₂ Cl ₂ intercalant inserted between the layers. In addition, as shown in Fig. 6b, it was confirmed that the oxidizing agent added during the process of manufacturing nano-porous graphite can additionally etch the nano-pores to form larger pores.

FIG. 7a and FIG. 7b are diagrams showing N ₂ adsorption-desorption isotherms and BJH (Barrett-Joyner-Halenda) pore size distributions for Examples 1 and 2, respectively, and the pore characteristics calculated using the BET (Brunauer-Emmett-Teller) method from the analysis results of the diagrams related to the pores are summarized in Table 1 below.

Sample S _BET (m ² /g) S _external (m ² /g) S _micro (m ² /g) V _total (cm ³ /g) V _external (cm ³ /g) V _macro (cm ³ /g) Example 2 2.28 2.22 0.06 0.0074 0.0072 0.0002 Example 1 2.86 2.61 0.25 0.0098 0.0097 0.0001

Referring to FIG. 7a, FIG. 7b and Table 1, it can be seen that the pores are exposed due to the dense crystal structure of the highly stacked nanoporous graphite with graphene containing pores.

Additionally, when compared to the BET surface area of pristine graphite, the ratio of the BET surface area of nanoporous graphite / the BET surface area of pristine graphite was confirmed to be 3.59 m ² /g.

As described above, it can be seen that the nanopores contained in the nanoporous graphite are formed by CrO ₂ Cl ₂ inserted between the layers.

Specifically, referring to Fig. 8 which schematically illustrates the pore formation process of nanoporous graphite, after the formation of graphite interlayer compound (GIC) in which the interlayer distance is increased by CrO ₂ Cl ₂ inserted into the interlayer, under acidic conditions, the CrO ₂ Cl ₂ inserted into the interlayer reacts with double-bonded carbon to break the double bonds, thereby inducing strain in the carbon lattice to form defects or to favor oxidation by an oxidizing agent. However, since oxidation by an oxidizing agent is carried out by a diffusive-controlled process, the CrO ₂ Cl ₂ inserted into the interlayer can control the content of oxygen functional groups by oxidation by suppressing the diffusion of the oxidizing agent.

In addition, it can be seen that the pore size can be controlled by a chemical oxidation reaction by an oxidizing agent.

(Examples 5 to 8) Preparation of membrane

The nanoporous graphites of Examples 1, 2, 3, and 4 were used to manufacture membranes of Examples 5, 6, 7, and 8, respectively.

Each nanoporous graphite dispersion (0.01 mg/mL) in deionized water was coated on a polyethersulfone (PES) porous polymer support (47 mm, pore size: 200 nm, Sterlitech) using a vacuum filtration method and then dried at 25°C to produce each membrane.

(Comparative Example 4)

The same process as for manufacturing a membrane in Example 5 was carried out, except that the membrane was manufactured using the carbon body of Comparative Example 2.

(Experimental Example 3) Analysis of changes in the interlayer structure of the membrane

FIGS. 9A, 9B, and 9C are diagrams showing an optical image, a top-view scanning electron microscope (SEM) image, and a cross-section SEM image, respectively, of a membrane manufactured according to Example 5.

Referring to FIGS. 9a, 9b, and 9c, it can be seen that the manufactured separator is flexible and has excellent bending resistance characteristics, and it was confirmed that an active layer including nanoporous graphite with a thickness of 50 nm was formed.

FIG. 10a, FIG. 10b, and FIG. 10c are schematic diagrams showing the results of changes in the interlayer structure of carbon bodies included in the membranes of Example 5 and Comparative Example 4 under conditions of aqueous solutions containing various ions, the results of changes in the interlayer structure according to the concentration of ions, and the interlayer distance expansion phenomenon under pure H ₂ O conditions, respectively.

At this time, the result of the change in the interlayer structure of Fig. 10a was compared using the interlayer distance calculated from the measured XRD pattern after immersing each membrane in the dried membrane and each of the pure and 0.1 M ionic solutions (NaCl, Na ₂ SO ₄ , LiCl, CaCl ₂ , MgCl ₂ , K ₃ [Fe(CN) ₆ ], and AlCl ₂ ) for 24 hours, and the result of the change in the interlayer structure of Fig. 10b was compared using the interlayer distance calculated from the measured XRD pattern after immersing each membrane in the respective ionic solutions (NaCl, Na ₂ SO _{4 ,} and MgCl ₂ ) with concentrations of 0.01 M, 0.1 M, and 1 M for 24 hours.

First, referring to FIG. 10a, the interlayer distance of the membrane of Example 5 was observed to increase by 1.4 Å from the interlayer distance of the dry membrane (7.5 Å) to 8.9 Å after being immersed in pure water, whereas the interlayer distance of the membrane of Comparative Example 4 was observed to increase by 3.6 Å from 7.9 Å to 11.5 Å after being immersed in pure water. This shows that the interlayer distance of the membrane of Comparative Example 4 expanded by about 2.6 times more than that of the membrane of Example 5, and thus the interlayer distance stability significantly deteriorates when immersed in pure water.

In addition, the degree of change in the interlayer structure is affected by the ions inserted between the layers, and it can be seen that the interlayer distance change of Example 5 and Comparative Example 4 occurs the most in the MgCl2 ion solution. In the case of Example 5, it was confirmed that the interlayer distance expanded by a maximum of 2.91 Å from 7.5 Å to 10.41 Å, whereas in the case of Comparative Example 4, the interlayer distance expanded by a maximum of 6.02 Å from 7.9 Å to 13.92 Å, and it was confirmed that the interlayer distance expanded to a maximum of about 2.1 times more than that of Example 5.

In general, in the field of water treatment, ion rejection capacity can be defined by the interlayer distance of the separation membrane, and stabilizing the interlayer distance has the advantage of improving the ion rejection rate and enhancing reliability.

Additionally, referring to Fig. 10b, it was confirmed that the interlayer distance of the membrane of Example 5 was suppressed to less than 10 Å even under various concentrations of ion solutions, but in the case of the membrane of Comparative Example 4, the interlayer distance was expanded to a maximum of 14 Å, and it was confirmed that the stability according to the concentration of the ion solution also decreased.

FIG. 11a and FIG. 11b are drawings showing XRD patterns measured after the membrane of Example 5 was immersed in pure water for 1 day and 60 days, respectively, and schematically showing the interlayer distance expansion phenomenon under pure H ₂ O conditions for Comparative Example 4 and Example 5.

As shown in Fig. 11a, the nanoporous graphite included in the membrane of Example 5 was not exfoliated even after immersion in pure water for 60 days, and the interlayer distance based on the XRD pattern was also maintained, confirming that the stability of the interlayer distance was maintained for a long period of time.

These results, as shown in Fig. 11b, indicate that the membrane of Example 5 can effectively maintain a narrow capillary shape (interlayer distance) by enhancing the interaction between layers through pi-pi (ππ interaction) because the functional groups containing oxygen contained in the nanoporous graphite are limitedly included in a pure or ionic solution.

FIG. 12a and FIG. 12b are drawings comparing XRD patterns measured for the membranes of Examples 5 to 8 in a dry state and after immersion in pure water for 24 hours, respectively, and drawings comparing interlayer distances calculated from XRD patterns measured for the membranes after immersing each membrane in 0.1 M of each ionic solution (NaCl, Na ₂ SO ₄ , LiCl, CaCl ₂ , MgCl ₂ , K ₃ [Fe(CN) ₆ ], and AlCl ₂ ) for 24 hours.

Referring to FIGS. 12a and 12b, it can be seen that the degree of change due to expansion of the interlayer distance increases as the number of oxygen-containing functional groups included in the nanoporous graphite included in the separation membrane increases.

Thus, controlled oxidation is essential for the production of nanoporous graphite with limited oxygen-containing functional groups along with pore formation in graphite, especially in the field of desalination under high-salinity conditions, because the ion rejection rate is enhanced depending on the steric effect between the interlayer distance and the hydrated shell.

(Experimental Example 4) Comparative Analysis of Membrane Performance

To compare the performance of each manufactured membrane, ion permeation tests and forward osmosis desalination tests were performed.

At this time, ion permeation tests and forward osmosis desalination tests were performed using an H-type cell divided into a feed region and a permeation region, and each membrane was positioned in the middle of each region divided so as to face the feed region.

The ion permeation test was performed by filling the feed zone and permeation zone with 80 mL of deionized water and each ion solution (KCl, NaCl, Na ₂ SO ₄ , LiCl, CaCl ₂ , MgCl ₂ , and K ₃ [Fe(CN) ₆ ]) for 24 hours, and then measuring the ion conductivity in the permeation zone to compare and evaluate the ion permeation rate.

FIG. 13a and FIG. 13b are diagrams showing ion permeability characteristics according to the ion permeability and the anion-to-cation valence ratios included in the ion solution for the membranes of Examples 5 to 8, respectively.

Referring to FIGS. 13a and 13b, in the case of Examples 7 and 8, monovalent ions (K ⁺ , Na ⁺ , and Li ⁺ ) and polyvalent ions ((SO ₄ ^2- , Ca ²⁺ , Mg ²⁺ , Al ³⁺ , It was observed that relatively high ion permeability was observed for all of Fe(CN) ₆ ^3- ), resulting in poor selectivity for ion exclusion.

On the other hand, in the case of the membrane of Example 5 containing nanoporous graphite with limited functional groups containing oxygen by controlled oxidation, it can be seen that the permeability for monovalent ions and polyvalent ions is reduced by 3 to 5 times, which can significantly improve the efficiency of ion exclusion present in the aqueous solution.

The forward osmosis desalination test was performed using an H-type cell divided into feed and draw regions similar to the ion permeation test, with the membrane of Example 5 positioned in the middle of each region. 80 mL of 0.1 M each ion solution (KCl, NaCl, Na ₂ SO ₄ , LiCl, MgCl ₂ ) was used as the feed solution, and 80 mL of 2 M sucrose solution was used as the draw solution.

At this time, the ion rejection rate R(%) was calculated using the following equation 2.

(Formula 2)

R(%) = 1 - (C_ep/C_f) x 100

(In Equation 2, C _ep is the concentration of the drawn salt and C _f is the ion concentration in the feed solution.)

The concentration of the salt drawn here was calculated using Equation 3 below.

(Formula 3)

C _ep = (C _p (V _draw +ΔV))/ΔV

(In Equation 3, C _p is the salt concentration in the draw solution, V _draw is the volume of the draw solution, and ΔV is the change in volume of the draw solution.)

Figures 14a and 14b are diagrams comparing the performance results of Example 5 according to a continuous forward osmosis desalination test for 24 hours and the ion rejection rate according to a continuous forward osmosis desalination test for 1 day and 7 days, respectively.

As shown in Fig. 14a, it was confirmed that the ion rejection rate for monovalent ions and polyvalent ions was 99.5% or higher regardless of the type of ion, and the water flux was also 0.75 to 0.92 LMH, showing excellent membrane performance. As shown in Fig. 14b, it was found that the ion rejection rate characteristic of 99.5% or higher was still maintained even after 7 days of continuous forward osmosis desalination, confirming that the durability of the membrane was also excellent.

Additionally, the performance of the membrane was compared and evaluated according to the concentration and pH conditions of the ion solution.

Figures 15a and 15b are diagrams showing the results of evaluating the performance of a separation membrane according to the concentration of a NaCl solution and the results of evaluating the performance of a separation membrane according to the change in pH of a NaCl solution, respectively.

Referring to Fig. 15a, it was observed that the ion rejection characteristic was greater than 99.4% even in the case of 0.01 to 1 M NaCl solutions, and it was observed that the ion rejection characteristic was maintained at greater than 90.5% even in the case of a highly concentrated 2 M NaCl solution.

In the case of the water flow rate, it can be seen that it gradually decreases as the concentration of the ion solution increases. This is because the concentration difference in the feed solution and draw solution areas decreases, thereby reducing the osmotic pressure effect. In particular, when the concentration of the ion solution is the same or higher than that of the draw solution, the driving force that enables the movement of water is low, so that the ion rejection rate characteristics appear low in the case of highly concentrated ion solutions.

As shown in Fig. 15b, it was confirmed that an ion rejection rate characteristic of more than 99.5% was maintained even under harsh conditions such as alkaline and acidic conditions during continuous forward osmosis desalination tests for 7 days.

Furthermore, the long-term durability of the membrane was evaluated by performing a forward osmosis desalination test for 30 days, and the performance according to the thickness of the membrane was additionally compared and analyzed.

At this time, the thickness of the membrane refers to the thickness of the nanoporous graphite layer coated on the PES porous polymer support.

FIG. 16a and FIG. 16b are diagrams showing the performance evaluation results for the membrane of Example 5 according to a continuous forward osmosis desalination test for 30 days using a 0.1 M NaCl solution and a 1 M NaCl solution as feed solutions, respectively, and the performance evaluation results for the membrane of Example 5 according to the thickness of the membrane using a 0.1 M NaCl solution.

As shown in Fig. 16a, even after a long-term test for 30 days, the ion rejection rates in the feed solutions of 0.1 M NaCl and 1 M NaCl were excellently maintained at the levels of 99.8% and 98%, respectively, and the water flow rate was also confirmed to be stable.

Referring to Fig. 16b, when the thickness of the membrane is 20 nm and 10 nm, the water flow rate increases to 2.21 LMH and 4.42 LMH, respectively, and the ion rejection rate is 96% and 84%, respectively. It is judged that the decrease in the ion rejection rate is due to the increase in defects due to the decrease in the membrane thickness, but the ion rejection rate characteristic of 84% still shows excellent performance based on the two-dimensional membrane.

As described above, the excellent ion rejection performance of the membrane according to the present invention is due to the modulation of the interlayer distance due to the nanopores contained in the nanoporous graphite and limited oxidation.

Figure 17 is a schematic diagram illustrating the mechanism of desalination using a membrane containing nanoporous graphite.

As illustrated in Fig. 17, through controlled oxidation, nanoporous graphite can include sp3 regions with expanded interlayer distances while containing nanopores. In this way, nanoporous graphite can include sp3 regions with expanded interlayer distances and sp2 regions with relatively narrow interlayer distances, along with an interconnected two-dimensional pore network.

At this time, the nanoporous graphite provides resistance to ion permeation due to the steric effect by including a structure including pores having a size smaller than or similar to the size of the hydrated ion and an sp3 region and an sp2 region having different interlayer distances, thereby increasing the permeation selectivity of the ion. In addition, water that is limitedly introduced into a relatively narrow interlayer distance region can shorten the movement path of the water or effectively reduce the tortuosity of the movement path by the pore structure, thereby further enhancing the selective exclusion effect of the ion.

Although the present invention has been described through specific matters and limited examples as above, these have been provided only to help a more general understanding of the present invention, and the present invention is not limited to the above examples, and those skilled in the art to which the present invention pertains can make various modifications and variations from this description.

Therefore, the idea of the present invention should not be limited to the described embodiments, and all things that are equivalent or equivalent to the claims described below as well as the claims are included in the scope of the idea of the present invention.

Claims (25)

Nanoporous graphite comprising a plurality of nanopores on at least one surface of a graphite layer, and having a diffraction peak due to a (002) crystal plane of graphite in X-ray diffraction analysis, wherein at least one of the plurality of nanopores is located on an in-plane within the graphite layer. In the first paragraph, The above nanoporous graphite is a nanoporous graphite containing a D' band in a Raman spectrum. In the first paragraph, Nanoporous graphite, wherein the ratio of I _d /I _g intensities of the Raman spectrum of the nanoporous graphite is 0.3 to 1.5. In the first paragraph, The above nanoporous graphite is a nanoporous graphite including a D' band having a stronger scattering intensity than the D band in a Raman spectrum. In the first paragraph, The above nanoporous graphite is a nanoporous graphite including a D' band having a stronger scattering intensity than the 2D band in a Raman spectrum. In the first paragraph, Nanoporous graphite, wherein the FWHM of the diffraction peak due to the (002) crystal plane of graphite in the X-ray diffraction analysis of the above nanoporous graphite is 5° or less. In the first paragraph, Nanoporous graphite having an average diameter of the above nanopores of 500 nm or less. In the first paragraph, Nanoporous graphite, wherein at least five nanopores are included in a 50 nm x 50 nm image observed with a transmission electron microscope after the nanoporous graphite is exfoliated into graphene. In the first paragraph, The above nanoporous graphite is nanoporous graphite satisfying the following equation 1. (Formula 1) SNPG/SG ≤ 10 (SNPG stands for the BET surface area of nanoporous graphite, and SG stands for the BET surface area of pristine graphite) In the first paragraph, The above nanoporous graphite is a nanoporous graphite having a BET surface area of 20 m ² /g or less. In the first paragraph, Nanoporous graphite, wherein the size of the nanopores is controlled by a chemical oxidation reaction. In the first paragraph, The above nanoporous graphite is nanoporous graphite having a pore volume of 0.1 cm ³ /g or less. In the first paragraph, The above nanoporous graphite is a nanoporous graphite in which the intensity of the peak of C-O-C, C=O or C(=O)O binding energy according to X-ray photoelectron spectroscopy is lower than the intensity of the peak of C=C and C-C binding energy. A membrane comprising nanoporous graphite according to claim 1. In Article 14, The above separation membrane is a separation membrane for water treatment, solvent nanofiltration, gas separation or ion separation. An electrochemical device comprising nanoporous graphite according to any one of claims 1 to 13. Contains Cr on at least one side of the graphite layer, and includes a D' band in the Raman spectrum, A nanoporous graphite precursor characterized by having two or more diffraction peaks diffracted at angles lower than the diffraction angle 2θ due to the (002) crystal plane of graphite in X-ray diffraction analysis. In Article 17, A nanoporous graphite precursor comprising a D band having a weaker scattering intensity than the 2D band in the above Raman spectrum. In Article 17, A nanoporous graphite precursor, wherein the Cr is contained in an amount of 3 wt% or less of the total weight of the nanoporous graphite precursor. (S1) A step of preparing a nanoporous graphite precursor by positioning one or more intercalating compounds on at least one surface included in graphite; (S2) a step in which the intercalating compound reacts with the carbon-carbon unsaturated bonds contained in the graphite; and (S3) a step in which nanopores are formed due to the reaction of the step (S2); A method for producing nanoporous graphite, wherein the intercalating compound induces nanopores at the location of the saturated bond. In Article 20, The above intercalating compound is CrO ₂ Cl ₂ ; Cr ₂ O ₃ ; FeCl ₃ ; A method for producing nanoporous graphite, wherein at least one compound is selected from the group consisting of: and a compound comprising the same. In Article 20, A method for producing nanoporous graphite, wherein the reaction in the above step (S2) is an oxidation reaction. In Article 20, A method for producing nanoporous graphite, wherein the reaction of the above step (S2) is performed under acidic conditions. In Article 20, A method for producing nanoporous graphite, wherein the step (S2) is performed by adding an oxidizing agent. In Article 24, A method for producing nanoporous graphite, wherein the oxidizing agent comprises KMnO ₄ .

Images