KR102178369B1 - Graphite felt having multimodal porous carbon coating layer and redox flow battery using the same as electrodes - Google Patents

Abstract

The present invention relates to a graphite felt coated with multiporous distribution carbon and a redox flow battery using the same as an electrode. More specifically, a polymer solution preparation step of dissolving an intrinsic microporous polymer in a good solvent; A polymer solution coating step of coating the polymer solution on graphite felt; A surface porous structure layer forming step of forming a porous structure layer on the surface of the graphite felt through a phase transfer method of the graphite felt coated with the polymer solution; And a carbonization step of carbonizing the surface porous structure layer of graphite felt, a method for producing graphite felt having a carbon coating layer having a multiporous distribution, a redox flow battery electrode using the same, and a redox containing the same as an electrode It is about flow batteries.

Description

Graphite felt having multimodal porous carbon coating layer and redox flow battery using the same as electrodes}

The present invention relates to a graphite felt having a multimodal porous carbon coating layer and a redox flow battery using the same as an electrode.

More specifically, a polymer solution preparation step of dissolving the intrinsic microporous polymer in a good solvent; A polymer solution coating step of coating the polymer solution on graphite felt; A surface porous structure layer forming step of forming a porous structure layer on the surface of the graphite felt through a phase transfer method of the graphite felt coated with the polymer solution; And a carbonization step of carbonizing the surface porous structure layer of graphite felt, a method for producing graphite felt having a carbon coating layer having a multiporous distribution, and graphite felt produced therethrough; And an electrode for a redox flow battery, comprising a carbon coating layer having a multiporous distribution connected to each other formed through phase transition and carbonization of an intrinsic microporous polymer as a coating layer formed on the surface of the graphite felt. The present invention relates to an electrode for a dox flow battery and a redox flow battery including the same as an electrode.

Unlike conventional secondary batteries, the redox flow battery (VRB) is a system in which the active material in the electrolyte is oxidized/reduced to charge and discharge. It is an electrochemical power storage device that directly stores the chemical energy of the electrolyte as electrical energy. VRB is a system for supplying electric energy smoothly at night or during periods of no power generation after charging surplus power produced in connection with the renewable energy field that performs intermittent power generation such as wind and solar power. It can be installed directly in this difficult isolated area and has less space constraints compared to other large-capacity energy storage systems.

However, in order to improve the oxidation/reduction reaction rate of VRB, excellent physical and chemical properties, a porous structure for diffusion of ions, an increase in active point by introduction of a surface functional group, and high conductivity are essential factors. Graphite felt is mainly used as a flow cell electrode because it has high electronic conductivity and is stable against corrosion in strong acids. However, due to the low active point, low porosity, and hydrophobicity of graphite felt, the electrolyte and ions cannot interact smoothly. Therefore, it exhibits poor dynamics and electrochemical irreversibility, and has a disadvantage that its use is limited at high current density.

Meanwhile, in order to use porous carbon materials as electrochemical catalysts and electrode materials, many R&Ds are being conducted, and among them, it is a conventional technology for manufacturing porous carbon structures through carbonization of polymers of intrinsic microporosity (PIM). Korean Registered Patent No. 1485867 and Korean Registered Patent No. 1818757. However, such a conventional technique is only possible to manufacture a porous carbon structure through carbonization of an intrinsic microporous polymer or to use it as an electrode thereof, and a specific method for the development of a graphite felt with improved efficiency as a more specific electrode and an electrode for a redox flow battery using the same Could not be presented.

In addition, Korean Patent Laid-Open No. 2014-0144117 on a technology for modifying by introducing a hydrophilic functional group to the surface of a hydrophobic carbon-based electrode including carbon felt is a method for treating an electrode for a vanadium redox flow battery using a simple hydrophilic polymer. It is only a method, and there is a limitation in that it is not a technology that improves the efficiency of the electrode in a more practical sense, as it is inevitable to add an additional conductive filler in order to solve the disadvantage of reducing the conductivity of the electrode due to the coating of the hydrophilic polymer.

Korean Patent Registration No. 1485867. Korean Patent Registration No. 1818757. Korean Patent Application Publication No. 2014-0144117.

The present invention was developed to solve the above problems, and provides an electrode of a high-efficiency vanadium redox flow battery by coating graphite felt with carbon, which is a three-dimensional interconnected multi-distribution pore structure without the use of a binder through a simple and effective method. It aims to do.

In addition, without the use of a binder, the interconnected porous structure and nitrogen and oxygen atoms use doped carbon, which is more effective in absorbing the electrolyte compared to the existing electrode, and the vanadium redox flow is remarkably improved in terms of efficiency of the vanadium redox flow battery. I want to provide a battery.

In the present invention, in order to solve the above problem, an intrinsic microporous polymer containing a large amount of nitrogen groups and having micropores is used as a precursor for the production of a carbon coating layer having a three-dimensional interconnected multidistribution pore structure. After dip coating an intrinsic microporous polymer, a phase transfer method is performed using water or methanol vapor to form multi-distribution pores, and a multiporous carbon coating layer doped with nitrogen and oxygen atoms through the carbonization process. Graphite felt was prepared.

In addition, a redox flow battery was prepared by including the prepared graphite felt having a multiporous distribution carbon coating layer as a positive electrode or a negative electrode for a redox flow battery.

The graphite felt coated with carbon having a multiporous distribution according to the present invention has a high specific surface area, micro, meso, and macro pores having multiple distributions of pores that are interconnected, and thus electrochemical It has reactivity, wettability and excellent electrical conductivity.

Graphite felt, which has an interconnected porous structure and a carbon coating layer doped with nitrogen and oxygen without the use of a binder according to the present invention, has a unique structure that is effective in absorbing electrolytes compared to conventional electrodes, especially in terms of efficiency of a vanadium redox flow battery. There is an advantage in that there is a breakthrough improvement in

1 shows the chemical structure of an intrinsic microporous polymer and a ¹ H-NMR spectrum thereof according to an embodiment of the present invention.
2 is a conceptual diagram of a manufacturing process of a) carbon-coated graphite felt having a multiporous distribution according to an embodiment of the present invention, and b) pristine GF, c) PIMVC@GF-0.5, d) PIMVC@GF-1.0 , e) PIMVC@GF-2.0, f) It shows an electron scanning micrograph of the enlarged surface of PIMVC@GF-1.0.
3 shows a scanning electron microscope photograph of the surface of PIMVC@ GF-1.0 with high magnification according to an embodiment of the present invention.
Figure 4 shows a high-magnification electron scanning micrograph of the surface of PIMC@GF-1.0 that did not undergo phase transition according to an embodiment of the present invention.
5 shows a nitrogen adsorption/desorption isotherm of a graphite felt electrode according to an embodiment of the present invention.
6 shows Raman spectra of PIMVC@GF-X and Pristine GF according to an embodiment of the present invention.
7 shows the XPS spectrum of Pristine GF and PIMVC@GF-X according to an embodiment of the present invention.
FIG. 8 shows a result of measuring a contact angle using deionized water (DI) on the surface of a) Pristine GF, b) PIMVC@GF-1.0 according to an embodiment of the present invention.
9 is a diagram showing a measurement result of an electrical conduction path according to pressure of Pristine GF and PIMVC@GF-X according to an embodiment of the present invention.
10 is a 5 mV s ^-1 scan rate of Pristine GF and PIMVC@GF-X according to an embodiment of the present invention using Ag/AgCl as a reference electrode at 0.1 MV ³⁺ in 1M H ₂ SO ₄ The CV curves measured using and VO ²⁺ are shown. It shows the measurement results of a) a potential range of -1.1 to -0.5 V in catholyte, and b) a potential range of 0.3 to 1.3 V in anolyte.
11 is a Nyquist plot of Pristine GF and PIMVC@GF-X according to an embodiment of the present invention. (EIS in 1.2 MV ⁴⁺ electrolytes dissolved in 3M H ₂ SO ₄ ).
12 is a) CE (coulombic efficiency) b) VE in a current density range of 50 to 150 mA/cm ² of a VRFB single cell using Pristine GF and PIMVC@GF-X as electrodes according to an embodiment of the present invention. (voltage efficienc), cy) EE (energy efficiency).
13 is a test result of long-term stability at 100 mA/cm ² of PIMVC@GF-1.0 according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail. The terms or words used in this specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventor may appropriately define the concept of terms in order to describe his own invention in the best way. Based on the principle that the present invention should be interpreted as a meaning and concept consistent with the technical idea of the present invention.

In the present invention, in order to solve the above problems, a polymer solution preparation step of dissolving an intrinsic microporous polymer in a good solvent; A polymer solution coating step of coating the polymer solution on graphite felt; A surface porous structure layer forming step of forming a porous structure layer on the surface of the graphite felt through a phase transfer method of the graphite felt coated with the polymer solution; And a carbonization step of carbonizing the surface porous structure layer of the graphite felt. It provides a method of producing a graphite felt having a carbon coating layer having a multiporous distribution.

According to one embodiment of the present invention, the graphite felt is preferably a pure (pristine) graphite felt (GF) treated with ozone to remove impurities on the outer surface, and a good solvent ( Good solvent) is a polar aprotic solvent, which is more effective for coating the inherent microporous polymer on pure graphite felt (pristien GF). And oxygen atoms are advantageous for the formation of a doped carbon layer, and examples of such a typical aprotic solvent include chloroform.

According to an embodiment of the present invention, the intrinsic microporous polymer may have a chemical structure represented by Formula 1 below.

<Formula 1>

(The formula 1 is selected from the group consisting of any one of the following X1, X2, X3 and X4 and the following Y1, Y2, Y3, Y4, Y5, Y6, Y7, Y8, Y9 and Y10 It is produced by the reaction between any one Y.)

According to an embodiment of the present invention, the intrinsic microporous polymer may have a chemical structure represented by Formula 2 below.

<Formula 2>

(Z in Formula 2 is selected from the group consisting of the following Z1, Z2, Z3, Z4, and Z5.)

According to an embodiment of the present invention, the multiporous distribution of the carbon coating layer is then subjected to a carbonization step so that the intrinsic microporous polymer can have pyridinic-N, pyrrolic-N, quaternary N and terminal NO structures in the molecular structure. It is preferable to use those containing a nitrogen atom in the molecular structure.

According to one embodiment of the present invention, the surface of pure graphite felt (pristien GF) may be coated using a polymer solution in which the intrinsic microporous polymer is dissolved. At this time, a known dip coating or spray coating method may be used as a coating method.

In addition, by varying the concentration of the polymer solution in which the intrinsic microporous polymer is dissolved, the thickness and morphology of the final multiporous distribution carbon coating layer can be adjusted by varying the thickness of the coated intrinsic microporous polymer layer. If it is too low, not only the thickness of the polymer to be coated is too thin, and it is not easy to form a porous structure layer through the subsequent phase separation step.On the other hand, if the concentration of the polymer solution is too high, the thickness of the polymer to be coated is not only too thick, but finally the coating layer Degradation of the quality may occur.

Therefore, according to an embodiment of the present invention, the concentration of the intrinsic microporous polymer solution is 0.5 to 2.0% by weight, so that stable intrinsic microporous polymer coating is possible, so that a high level of mesoporous structure and optimum content are possible after the carbonization step. Doping of nitrogen and oxygen elements is possible, and finally, excellent electrochemical performance of the electrode can be obtained.

According to an embodiment of the present invention, the phase transfer method in the step of forming the surface porous structure layer may be to use a vapor induced phase separation method.

According to one embodiment of the present invention, the step of forming the surface porous structure layer may be exposing the graphite felt coated with the polymer solution to a vapor of a non-solvent.

According to an embodiment of the present invention, the non-solvent may be water or methanol.

Conventional techniques such as thermal induce phase separation and non-solvent induces phase separation are known as a phase separation method for introducing microporous structures of polymer films, but thermal induce phase separation. In the case of phase separation), a separate solvent is required for the addition and removal of a separate plasticizer, and in the case of non-solvent-induced phase separation, the polymer film is immersed in a non-solvent to induce phase separation, and the non-solvent is dried. It is necessary to introduce additional process steps, such as the need for a process that should be done, and there is trouble.

However, the vapor induced phase separation method according to an embodiment of the present invention can not only solve the hassle of such a conventional phase separation method, but also a pure graphite felt coated with a polymer solution in which an intrinsic microporous polymer is dissolved. The surface of (pristien GF) is irradiated with a vapor of a solvent such as water or methanol, which is a non-solvent, so that a separate drying step is unnecessary, and it can quickly induce phase separation of the intrinsic microporous polymer coating layer. There is an advantage to be able to. In other words, during the vapor induced phase separation (VPS) process, the intrinsic microporous polymer solution on the surface of the graphite felt is directly exposed to water vapor, which is a polar solvent, and at this time, instantaneous precipitation and phase separation occur due to phase separation, resulting in a porous structure ( mesoporous), but mechanically, it is possible to obtain a precursor of carbon-coated graphite felt having a multiporous distribution in which an intrinsic microporous polymer surface layer is formed.

According to one embodiment of the present invention, the carbonization step is preferably carbonized at 700 to 1200° C. for 5 to 7 hours in an inert atmosphere, and vapor induced phase separation (VIPS) through this carbonization step Through the process, as the surface layer of the intrinsic microporous polymer having a porous structure is finally carbonized, a graphite felt coated with carbon having a multiporous distribution can be manufactured.

According to one embodiment of the present invention, after the carbonization step, the graphite felt having a surface porous structure layer carbonized may further include annealing in an oxygen atmosphere. The heat treatment step is preferably carried out at 250 to 400 °C for 1 to 3 hours.If the heat treatment temperature is low or the heat treatment time is short, it is impossible to introduce oxygen functional groups into the carbon coating layer having a multiporous distribution, and the heat treatment temperature is too high. When is high or the heat treatment time is long, the performance as a final redox flow battery electrode decreases due to the introduction of excessive oxygen functional groups.

According to one embodiment of the present invention, the multiporous distribution carbon coating layer may further include nitrogen atoms and oxygen atoms in the molecular structure.

According to an embodiment of the present invention, the multiporous distribution of the carbon coating layer is amorphous, and the Raman spectral spectra of the carbon coating layer of the multipore distribution are 1350 cm ^-1 and 1590 cm ^-1 , respectively, of typical carbon D and It represents the G band. In addition, according to an embodiment of the present invention, the ratio of the intensity of the D band and the D band, I _D /I _G, is preferably 0.9 or more and more preferably in the range of 0.92 to 1.16. For reference, I _D /I _G of pure graphite felt (Pristine GF) is 0.89. As the value of X increases, the value of I _D / I _G increases well shows the characteristics of the amorphousness of carbon due to PIM, which is carbonized unlike pure graphite felt (pristine GF) having a highly crystalline graphite structure. The intrinsic microporous polymer (cPIM) is meant to have an amorphous structure.

According to one embodiment of the present invention, the multiporous distribution carbon coating layer is characterized in that the nitrogen atom and the oxygen atom are doped, the nitrogen atom is derived from the intrinsic microporous polymer, the oxygen atom is doped by heat treatment in an oxygen atmosphere Is by According to an embodiment of the present invention, the nitrogen atom content is preferably in the range of 0.61 to 1.36 at%, and the oxygen atom content is in the range of 8.94 to 10.29 at%.

According to one embodiment of the present invention, the carbon coating layer of the multiporous distribution may have a nitrogen atom of pyridinic-N, pyrrolic-N, quaternary N and terminal NO structures, more preferably pyridinic-N, pyrrolic-N It may be the chemical structure of

According to XPS analysis, the nitrogen atom of the multiporous distribution carbon coating layer resulting from the intrinsic microporous polymer containing nitrogen atom could be divided into 4 sub-peaks of 398.1, 399.6, 401.2 and 403.2 eV, each of which It is derived from the structure of pyridinic-N, pyrrolic-N, quaternary N and terminal NO. Among them, pyridinic-N and pyrrolic-N have an unshared electron pair, so they can give electrons to the pi-bonding system of carbon (electron donating), and as a result, have the effect of reducing the band gap of the doped carbon, so the nitrogen element is doped. It can greatly improve the electrical conductivity of the carbon. In addition, such a nitrogen functional group can promote the reduction reaction by the presence of additional electrons of the nitrogen atom, as well as promote oxidation by a positively charged carbon atom.

In addition, in the present invention is manufactured according to the above manufacturing method, graphite felt; And a carbon coating layer formed on the surface of the graphite felt, formed through phase transition and carbonization of an intrinsic microporous polymer, and a carbon coating layer having an interconnected multiporous distribution including nitrogen and oxygen atoms. A battery electrode is provided.

In addition, the present invention provides a redox flow battery comprising the electrode for the redox flow battery as a positive electrode or a negative electrode.

Hereinafter, implementation examples and examples of the present application will be described in detail so that those with general knowledge in the technical field to which the present application pertains can be easily implemented, as shown in the accompanying drawings. In particular, the technical idea of the present invention and its core configuration and operation are not limited by this. In addition, the content of the present invention may be implemented in various different types of equipment, and is not limited to the implementation examples and examples described herein.

< Manufacturing example 1> implicit Microporosity Preparation of polymer: PIM Preparation of -1

Preparation of the intrinsic microporous polymer according to an embodiment of the present invention is 5,5'6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1' spirobisindane (TTSBI) and 2,3 It was prepared by polycondensation of ,5,6-tetrafluoroterephthalonitrile (TFTPN). More specifically, TTSBI and TFTPN were reacted under the following conditions.

First, a two-neck 500 ml round bottom flask connected to a reflux condenser was used, and after drying in an oven to remove moisture in the reactor before introducing the sample, nitrogen was flowed. TTSBI (X1, 10.21 g, 30 mmol), TFTPN (Y1, 6.00 g, 30 mmol), potassium carbonate (8.29 g, 60 mmol, 210 mL of DMF were added to the reactor.) The mixture was put in an oil bath preheated to 55 °C. After the reaction was completed, 300 mL of THF was poured into the reaction mixture in order to remove the oligomer, and the supernatant containing the oligomer was removed, and the precipitate was removed. After dissolving in 300 mL THF, the polymer solution was poured into water, reprecipitated, filtered, and dried for 24 hours in a vacuum dryer at 60° C. After reprecipitation in methanol, filtration, and drying were performed, PIM I got -1.

The number average molecular weight (Mn), weight average molecular weight (Mw), and PDI (polydispersity index) of the prepared PIM-1 were measured in size-exclusion (SEC) using Sodex columns (KF-800 series) at 40 ℃ using THF as a solvent. chromatography), the number average molecular weight (Mn) was 42,300 g/mol, and the PDI was 1.94. ¹ H-NMR spectrum was measured using a Bruker 700 MHz spectrometer. The chemical structure and ¹ H-NMR spectrum of the prepared PIM-1 are shown in FIG. 1.

< Example And Comparative example > Multiporous carbon coated Graphite felt Produce

PIM-1, an intrinsic microporous polymer prepared according to Preparation Example 1, was uniformly dissolved in chloroform at 0.5, 1.0 and 2.0 wt%, respectively, to prepare a PIM solution. Pure graphite felt (pristine GF) having a total area of 9 cm ² of 30 mm × 30 mm was treated with an ozone generator for 5 minutes to clean the external surface of impurities, and then dipped in the prepared PIM solution for 5 minutes. ). The prepared PIM-coated graphite felt was exposed to water vapor by a vapor induced phase separation (VPS) process, dried at 25° C. for 10 hours, and then carbonized at 900° C. for 5 hours in a nitrogen environment. Finally, the sample was annealed at 900° C. for 2 hours in an oxygen environment to introduce an oxygen functional group.

On the other hand, pure graphite felt (pristine GF) was used as Comparative Example 1, and the prepared graphite felt coated with PIM was not exposed to water vapor, and then carbonized and heat treated was used as Comparative Example 2. The manufacturing process of the graphite felt with the carbon coating layer having a multiporous distribution prepared according to the above conditions is summarized in Table 1 below.

Sample name PIM solution concentration
(wt%) Whether or not VIPS is fair Example 1 PIMVC@GF-0.5 0.5 O Example 2 PIMVC@GF-1.0 1.0 O Example 3 PIMVC@GF-2.0 2.0 O Comparative Example 1 Pristine GF 0 X Comparative Example 2 PIMC@GF-1.0 1.0 X

FIG. 2 shows a process of manufacturing a graphite felt having a carbonized intrinsic microporous polymer (carbonized PIM: cPIM) coating layer, that is, a carbon coating layer having a multiporous distribution. Pure graphite felt (pristine GF) is immersed in a PIM solution dissolved in chloroform, a polar aprotic solvent, at various concentrations after removing oxygen (O2) molecules adsorbed on the surface using ozone. The higher the concentration of the polymer solution used, the greater the thickness of the finally carbonized intrinsic microporous polymer.

During the vapor induced phase separation (VPS) process, the PIM solution on the surface of the graphite felt is directly exposed to water vapor, which is a polar non-solvent, and at this time, instantaneous precipitation and phase separation occur due to the phase separation, resulting in a mesoporous structure. , Mechanically, it is possible to obtain a precursor for producing graphite felt having a carbon coating layer having a multiporous distribution having a strong PIM surface layer formed thereon.

The precursor obtained according to an embodiment of the present invention is dried at 25° C., carbonized at 900° C. in an inert gas atmosphere, and then heat-treated at 300° C. for 2 hours in an oxygen atmosphere to introduce an oxygen functional group. The obtained multiporous distribution carbon coated graphite felt was designated as PIMVC@GF-X. In this case, X is 0.5, 1.0, and 2.0, which are the concentrations of the coated intrinsic microporous polymer solution. On the other hand, as a comparative example, after coating PIM on graphite felt at the concentration of 1.0, which is the concentration of the intrinsic microporous polymer solution, without going through vapor induced phase separation (VIPS), the sample subjected to carbonization and heat treatment under the same conditions was PIMC@ It was designated as GF-1.0.

In the formation of the porous carbon coating layer on the graphite felt, the use of a separate binder was not required, and this binder-free process can significantly lower the ohmic resistance when using the produced graphite felt as an electrode. There is an effect.

<Material property analysis>

The morphology and microstructure of the prepared sample were analyzed with an acceleration voltage of 5 kV using a SEM of a Hotachi S-5600 microscope, and the nitrogen adsorption-desorption isotherm was measured using Micromeritics ASAP 2020 at -196 °C. Electrical conductivity was measured using a self-made four-point probe.

Raman spectra were measured by WITec Micro-Raman using a 532 nm laser, and XPS analysis was performed using a Kratos Axis Nova XPS system with a 150 W momochromated Al-Ka source.

The result of observing the surface of PIMVC@GF-X, a graphite felt coated with pure graphite felt and multiporous carbon according to an embodiment of the present invention, using a field emission scanning electron microscope (FESEM) is shown in FIG. To f.

As shown in Fig. 2b, pure graphite felt has a smooth and no special shape surface, and this defect-free surface is not suitable because of low electrochemical activity when applied to VRFB. Is known.

On the other hand, in the case of PIMVC@GF-X, an example according to an embodiment of the present invention, it can be seen that a thin carbonized cPIM coating layer exists on the surface of the graphite felt fiber (Fig. 2 c to e). At this time, the concentration of PIM in the coating solution has a great influence on the uniformity of the coating and the thickness and morphology of the coating layer.In Example 1, in the case of a low concentration of 0.5 wt%, the carbonized cPIM coating layer is uniform over the entire surface of the graphite felt. It can be seen from FIG. 2C that some uncoated areas exist because they are not. However, in Example 3 in the case of a high concentration of 2.0 wt%, as a result, the coating layer of cPIM is formed thick, but this concentration is somewhat undesirable in terms of the uniformity of the film as shown in FIG. It can be seen that flakes of the same shape are formed.

However, in the case of Example 2, it can be seen from FIG. 2D that the cPIM coating layer has an appropriate thickness and a relatively uniform surface morphology as the optimum PIM concentration.

In addition, looking at f of FIG. 2 in which the surface of PIMVC@GF-1.0 according to Example 2 was further enlarged, it was confirmed that the cPIM coating layer had a porous structure (mesoporous) in which pores having a size of several tens of nanometers exist as desired. I did. In Fig. 2d, the surface is not flat and seems to be somewhat uneven, but the result of Fig. 3, which is a result of a higher magnification SEM of the same sample, can be seen that the entire surface of the graphite felt is coated with cPIM. there was.

That is, the porous structure of the cPIM coating layer was observed only in Examples 1 to 3 through a vapor induced phase separation (VIPS) process, and did not undergo vapor induced phase separation (VIPS). In the case of Comparative Example 2, it was found that no porous structure was formed as shown in FIG. 4, and PIMVC@GF-X of the present invention having a pore structure of such a multipore distribution reduces the migration resistance of vanadium ions. It is possible to achieve faster adsorption at the given potential.

The pore characteristics of the prepared graphite felt coated with a carbon layer having a multiporous distribution were analyzed through nitrogen adsorption analysis. 5 shows nitrogen adsorption/desorption isotherms of various graphite felts according to an embodiment of the present invention, and the specific surface area was calculated using the Brunauer-Emmett-Teller (BET) equation. Pure graphite felt (Pristine GF) showed negligible specific surface area (< 1 m ² /g), indicating that there were no micropores on the surface. However, PIMVC@GF-X according to an embodiment of the present invention is significantly increased according to the content of the PIM precursor, and is 157, 240 and 301 in each of PIMVC@GF-0.5, PIMVC@GF-1.0 and PIMVC@GF-2.0. The specific surface area value of m ² /g was shown, indicating that the porosity was imparted to the surface of the graphite felt by the introduction of the cPIM coating layer.In particular, in the case of PIMVC@GF-1.0, the cPIM of a relatively small weight relative to the total weight A large increase in the specific surface area could be achieved even by the coating layer. This high specific surface area of PIMVC@GF-X can be attributed to the inherent microporosity of PIM, considering that the specific surface area of PIM, which was carbonized under the same carbonization condition without using graphite felt, is high, 1,297 m ² /g. have.

On the other hand, unlike pure graphite felt (Pristine GF) that does not show any nitrogen adsorption/desorption isotherms and hysteresis curves, PIMVC@GF-X according to an embodiment of the present invention shows a typical H3 hysteresis curve and a type-IV isotherm. It can be seen that (micro) and meso (meso) pores are present, which is consistent with the SEM analysis results discussed above.

The specific surface area and pore volume of the samples according to the exemplary embodiment of the present invention are summarized and described in Table 2.

Sample name S _BET
(m ² g ^-1 ) V _micro
(cm ³ g ^-1 ) V _meso
(cm ³ g ^-1 ) V _total
(cm ³ g ^-1 ) Example 1 PIMVC@GF-0.5 157 0.038 0.011 0.049 Example 2 PIMVC@GF-1.0 240 0.061 0.014 0.075 Example 3 PIMVC@GF-2.0 301 0.074 0.019 0.093 Comparative Example 1 Pristine GF - - - - Comparative Example 2 PIMC@GF-1.0 265 0.065 0.001 0.066

PIMVC@GF-X according to the present invention has a significant level of meso pores, and these meso pores are adsorption/desorption through meso pores of vanadium ions. There is an advantage of improving the kinetics of the redox reaction by providing an effective movement path.

In order to confirm the crystallinity of carbon of PIMVC@GF-X according to an embodiment of the present invention, the results of Raman spectroscopy analysis are shown in FIG. 6.

As can be seen from FIG. 6, all PIMVC@GF-X according to the present invention show typical D and G bands of carbon at 1350 cm ^-1 and 1590 cm ^-1 , respectively. In addition, the increase in the intensity of the D band with the increase of the X value shows the characteristics of the amorphousness of carbon due to PIM, which is carbonized unlike pure graphite felt (pristine GF) having a high crystalline graphite structure. Intrinsic microporous polymer (cPIM) is meant to have an amorphous structure.

The chemical properties of graphite felt coated with carbonized intrinsic microporous polymer (cPIM) were analyzed using X-ray photoelectron spectroscopy (XPS) and shown in FIG. 7.

7A is a result of XPS analysis of pure graphite felt and PIMVC@GF-1.0, PIMVC@GF-1.0 gives typical peaks of C, O, and N, which gives a nitrogen valence to the coating layer of cPIM during the carbonization process, It shows that oxygen atoms were contained by heat treatment in an oxygen atmosphere.

Table 3 below summarizes the element content of samples according to an embodiment of the present invention, and it can be seen that the content of N increases as the volume of the cPIM coating layer increases. That is, PIMVC@GF-0.5, PIMVC In each case of @GF-1.0 and PIMVC@GF-2.0, the content of N was 0.61, 1.18 and 1.36 at. %.

Sample name C (at.%) O (at.%) N(at.%) Example 1 PIMVC@GF-0.5 90.45 8.94 0.61 Example 2 PIMVC@GF-1.0 89.18 9.64 1.18 Example 3 PIMVC@GF-2.0 88.35 10.29 1.36 Comparative Example 1 Pristine GF 93.36 6.64 -

For the PIMVC@GF-1.0 sample, the binding form of the N dopant was analyzed using high-resolution XPS and shown in b of FIG. As a result of deconvolution of the N peak, the binding energies could be divided into four sub-peaks of 398.1, 399.6, 401.2 and 403.2 eV, each of which was in the pyridinic-N, pyrrolic-N, quaternary N and terminal NO structures. It originated. Among them, pyridinic-N and pyrrolic-N have an unshared electron pair, so they can give electrons to the pi-bonding system of carbon (electron donating), and as a result, have the effect of reducing the band gap of the doped carbon, so the nitrogen element is doped. It can greatly improve the electrical conductivity of the carbon.

In addition, such a nitrogen functional group may not only promote the reduction reaction by the presence of additional electrons of the nitrogen atom, but may also promote oxidation by a positively charged carbon atom.

Accordingly, the electrode using PIMVC@GF-X according to an embodiment of the present invention can have remarkable activity in the redox reaction related to vanadium ions.

In particular, as shown in Fig. 7c, it can be seen that PIMVC@GF-1.0 has a significant amount of oxygen functional groups, and the oxygen content of PIMVC@GF-1.0 and pure graphite felt is 9.64 and 6.64 at.%, respectively. Looking at the deconvolution results of the oxygen peak, it can be seen that the high oxygen content of PIMVC@GF-1.0 compared to pure graphite felt is due to the increase of C=O bonds, and the C=O functional group is directly redox of vanadium ions. Since it is related to the reaction, the electrode using PIMVC@GF-X according to an embodiment of the present invention can be expected to have an excellent effect in electrochemical performance.

The contact angle was measured in order to examine the effect of the chemical and structural characteristics of the introduction of the cPIM coating layer according to the embodiment of the present invention on the hydrophilicity/wetting properties and electrical conductivity of the electrode.

Figure 8a is a pure graphite felt, b shows the water droplet contact angle of PIMVC@GF-1.0. Pure graphite felt composed of highly hydrophobic graphitic carbon showed a contact angle of 106.2 ^o , suggesting very poor wettability in aqueous electrolyte systems. On the other hand, PIMVC@GF-1.0 shows very good wettability because water droplets are absorbed instantly. This remarkable improvement in wettability as an electrode material is due to an improved interaction between nitrogen and oxygen functional groups present in the cPIM coating layer and water molecules.

In order to improve the accuracy of the sample according to an embodiment of the present invention, electrical conductivity is measured using a pressure-variable cell and is shown in FIG. 9.

It can be seen that PIMVC@GF-X according to an embodiment of the present invention maintains excellent electrical conductivity as a material based on graphite felt despite various modification processes such as coating, carbonization and heat treatment. On the other hand, it is known that the conductivity of graphite felt is generally lowered by heat treatment commonly used to modify the surface properties of graphite felt. As can be seen from FIG. 9, pure graphite felt used as an embodiment of the present invention is used. In the case of heat-treated Heated-GF, the result was very markedly decreased in conductivity. However, the case of PIMVC@GF-X according to an embodiment of the present invention shows high electrical conductivity compared to the heated-GF, and in particular, the case of PIMVC@GF-0.5 and PIMVC@GF-1.0 is compared to pure graphite felt. It showed a very slight decrease in conductivity.

That is, the graphite felt coated with multiporous carbon according to the manufacturing process without the use of a binder according to an embodiment of the present invention had the effect of maintaining high electrical conductivity, and in conclusion, the graphite felt coated with multiporous carbon was It can be seen that it contains a high functional group as an electrode and has excellent electrical conductivity.

<Electrochemical Characteristic Analysis>

The electrochemical properties of the graphite felt electrode coated with multiporous carbon prepared according to an embodiment of the present invention include graphite felt coated with multiporous carbon as a working electrode and Ag/AgC as a reference electrode. , Pt wire was analyzed using a CV (Gremany, Reference 600) of a three-electrode cell with a counter electrode. The CV test was performed with a voltage of 0 to 1.6 V and a scanning speed of 1 to 10 mV/s.

EIS analysis in frequency to 0.75 mV AC voltage of ⁵ mV 10 5 to 1 ^were performed under the conditions of ¹ Hz. All electrochemical measurements for evaluating the cell's performance were performed using a Bio-Logic SP-300 potentiostat/galvaostat, and the test cell was charged in a voltage range of 1.6 to 1.0 V under the condition of a current density of 50 to 150 mA/cm2. Discharge occurred.

10 shows the CV analysis results according to an embodiment of the present invention. In CV analysis, the catalytic activity of the vanadium redox reaction is the voltage difference ( Δ V=V _pa -V _pc ), the redox peak current density (I _pa and I _pc ) and redox starting voltage (redox onset potential).

10A shows the CV curve of the negative electrode reaction of VRFB corresponding to the V ²⁺ /V ³⁺ redox couple, and the pure graphite felt electrode does not observe a reduction peak and has a very small oxidation peak. It was observed that the pure graphite felt electrode had very bad catalytic activity and irreversible oxidation of vanadium ions. On the other hand, in the case of the PIMVC@GF-X electrode, a remarkable anode peak in the range of -0.3 to -0.6 V and a reversible cathode peak in the range of -0.9 to -1.0 V are shown, so that the cPIM coating layer shows the electrochemical activity of the electrode. It can be seen that the improvement in reversibility of ion adsorption to the PIMVC@GF-X electrode is due to suppression of unwanted side reactions such as hydrogen generation.

Figure 10b shows the CV curve of the positive electrode reaction of VRFB corresponding to the VO ^{2 +} / VO ^{2 +} redox couple, and the Ipa and Ipc values of the PIMVC@GF-X electrode are pure graphite felt electrodes. Compared to this, it shows a very high value. The values of Δ V and redox onset potential of PIMVC@GF-X electrode and pure graphite felt electrode are in the order of PIMVC@GF-1.0>PIMVC@GF-0.5>PIMVC@GF-2.0> Pristine GF. As a result of observing the CV performance of PIMVC@GF-1.0 to derive the effect of the microporous structure, the electrode showed the best reversibility of the VO ²⁺ /VO2 ⁺ redox reaction. Additionally, the improvement of the onset potential of the positive and negative reactions of the PIMVC@GF-1.0 electrode implies electron transfer and energy storage performance. The superior electrochemical performance of the PIMVC@GF-1.0 electrode compared to the electrode according to this other embodiment is due to a high level of microporous structure, a stable intrinsic microporous polymer coating, and an optimal nitrogen content.

In other words, as mentioned above, in the case of the PIMVC@GF-0.5 electrode, the volume of the cPIM coating layer was small, so the active part of the electrocatalytic reaction was not sufficient. In the case of the PIMVC@GF-2.0 electrode, the cPIM coating layer was somewhat excessively thick. As a result, the mesopores volume decreases and the quality of the coating layer decreases, resulting in lower activity of the electrode compared to other embodiments. The electrochemical values of the electrode according to an embodiment of the present invention are summarized in Tables 4 and 5 below.

electrode Negative half-cell Ipa
(mA) Ipc
(mA) Vpa
(V) Vpc
(V) Vpc-Vpa Example 1 PIMVC@GF-0.5 34 -112 -0.52 -0.92 -0.4 Example 2 PIMVC@GF-1.0 49 -122 -0.52 -0.88 -0.36 Example 3 PIMVC@GF-2.0 42 -116 -0.47 -0.96 -0.49 Comparative Example 1 Pristine GF - - - - -

electrode Positive half-cell Ipa
(mA) Ipc
(mA) Vpa
(V) Vpc
(V) Vpc-Vpa Example 1 PIMVC@GF-0.5 100 -93 1.06 0.73 0.33 Example 2 PIMVC@GF-1.0 103 -97 1.04 0.77 0.27 Example 3 PIMVC@GF-2.0 87 -85 1.07 0.72 0.35 Comparative Example 1 Pristine GF 79 -45 1.19 0.64 0.55 Comparative Example 2 PIMC@GF-1.0 84 -69 1.15 0.70 0.45

In order to investigate the transfer mechanism of the electrical charge of the manufactured electrode, electrochemical impedance spectroscopy (EIS) analysis was performed under the condition of 0.49V, and the results are shown in FIG. 11. EIS parameters were estimated using a Nyquist plot using the circuit diagram shown in FIG. 11. Looking at the Nyquist plot, the shape of a semicircle was shown in the high-frequency region and a linear trend was observed in the low-frequency region, indicating that the transfer and diffusion of charges occurred at the interface between the electrode and the electrolyte. it means. The semicircle of the PIMVC@GF-X electrodes is much smaller than that of pure graphite felt, which means that the active barrier of the vanadium redox reaction is significantly reduced. In addition, the resistance of charge transfer is the lowest in the PIMVC@GF-2.0 electrode, followed by PIMVC@GF-1.0, PIMC@GF-1.0, and PIMVC@GF-0.5.

The performance of the electrode according to an embodiment of the present invention was measured while changing the current density to 50 to 150 mA/cm ² . FIG. 12 shows the coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) during 30 charge/discharge cycles of the VRFB according to an embodiment of the present invention. As can be seen from a of FIG. 12, the average CE values of all electrodes were almost the same, which means that the difference between the charging capacity and the discharging capacity of all the electrodes was almost the same. In FIG. 12B, it can be seen that the PIMVC@GF-1.0 electrode shows the highest VE value. Since the VE value means the ratio of the charging voltage and the discharge voltage, the change in the VE value is mainly due to the result of a voltage drop (potential loss) due to overpotential and internal resistance. . The high VE value of the PIMVC@GF-1.0 electrode means that the voltage drop is lower than that of other electrodes. In Fig. 12C, it can be seen that the PIMVC@GF-1.0 electrode has the highest efficiency for all current densities. On the other hand, when the current density was scanned in the reverse direction from 150 to 50 mA/cm ² , the EE and VE values of PIMVC@GF-1.0 were restored to their initial values, so that the electrode was electrochemically excellent in the vanadium electrolyte. Means chemically stable.

13 shows the results of a long-term stability test of PIMVC@GF-1.0 under the condition of a current density of 100 mA/cm ² . As can be seen from FIG. 13, changes in EE, VE and CE values are hardly observed even after 100 cycles, which shows that the electrode of PIMVC@GF-1.0 is stable even under strong acidic conditions and high current density. It depends on the structure, adequate nitrogen doping and a stable and strong coating.

As described above, the graphite felt having a carbon coating layer with a multiporous distribution according to an embodiment of the present invention has a carbon surface layer doped with nitrogen and has a multiporous pore, and can be used as an anode or a cathode of VRFB by utilizing this It is possible, and it can be seen that it has long-term durability at an excellent current density of up to 150 mA/cm ² . This improved performance is due to a carbon layer on the surface having multiple distributions of pores doped with nitrogen formed on the graphite felt without using a binder.

Claims (10)

A polymer solution preparation step of dissolving an intrinsic microporous polymer having a chemical structure represented by the following formula in a good solvent;
A polymer solution coating step of coating the polymer solution on graphite felt;
A surface porous structure layer forming step of forming a porous structure layer on the surface of the graphite felt through a phase transfer method of the graphite felt coated with the polymer solution; And
A method of manufacturing an electrode for a redox flow battery, characterized in that a carbon coating layer having a multiporous distribution is formed on the graphite felt, including a carbonization step of carbonizing the surface porous structure layer of graphite felt:
<chemical formula>

In the above formula, Z is selected from the group consisting of the following Z1, Z2, Z3, Z4, and Z5.

The method of claim 1, further comprising annealing the graphite felt having a surface porous structure layer carbonized after the carbonization step in an oxygen atmosphere. . delete delete The method according to claim 1,
The method of manufacturing an electrode for a redox flow battery, characterized in that the phase transfer method in the step of forming the surface porous structure layer is a vapor induced phase separation method. The method according to claim 1,
In the step of forming the surface porous structure layer, the method of manufacturing an electrode for a redox flow battery, characterized in that the graphite felt coated with a polymer solution is exposed to a vapor of a non-solvent. The method of claim 6,
The method of manufacturing an electrode for a redox flow battery, wherein the non-solvent is water or methanol. The method according to claim 1,
The method of manufacturing an electrode for a redox flow battery, characterized in that the carbon coating layer having a multiporous distribution contains nitrogen atoms and oxygen atoms in a molecular structure. It is manufactured according to the manufacturing method according to any one of claims 1, 2, 5 to 8,
Graphite felt; And
As a coating layer formed on the surface of the graphite felt, it is formed through phase transition and carbonization of an intrinsic microporous polymer, and comprises a carbon coating layer of interconnected multiporous distribution including nitrogen and oxygen atoms. electrode. A redox flow battery comprising the electrode for a redox flow battery of claim 9 as a positive or negative electrode.

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