KR20180017679A - Electrode for vanadium redox flow battery and vanadium redox flow battery including the same - Google Patents

Abstract

Disclosed is a carbon felt electrode for a vanadium redox flow battery which comprises a carbon felt and an ion-adsorbing layer disposed on at least one surface of the carbon felt, wherein the ion-adsorbing layer comprises a conductive carbon and a bonding polymer resin, and wherein a carbon fiber surface of the carbon felt has at least one functional group bonded to the carbon fiber surface. The present invention provides a carbon felt electrode for a vanadium redox flow battery comprises: a carbon felt and an ion-adsorbing layer disposed on at least one surface of the carbon felt, wherein the ion-adsorbing layer comprises a conductive carbon and a carbon-based material obtained by carbonization of a boding polymer resin, and wherein a carbon fiber or the ion-adsorbing layer forming the carbon felt has at least one functional group bonded to a surface. Also a vanadium redox flow battery comprising the carbon felt electrode is provided.

Description

[0001] The present invention relates to an electrode for a vanadium redox flow battery and a vanadium redox flow battery including the same,

More particularly, the present invention relates to an electrode for a vanadium redox flow cell having improved redox reversibility and current density of vanadium and an improved electrode for a vanadium redox flow battery, To a vanadium redox flow cell.

The present invention was accomplished with the support of SME technology innovation development project (Project No. S2169246), "Development of graphite felt electrode technology with a thickness of 4.5 ± 0.45 mm for vanadium redox flow cell and surface area of more than 10 ± ㎡ / g" .

Research and development of redox flow cell is actively proceeding with large capacity secondary battery for energy storage device. The redox flow cell is an electrochemical storage device in which the active energy in the electrolyte is oxidized (reduced) and charged and discharged, and the chemical energy of the electrolyte is directly stored as electrical energy.

As the electrode of the vanadium redox flow cell, carbon felt is mainly used. The surface treatment technique of carbon felt is known to be an important technology for determining the efficiency and performance of redox flow cell by enlarging the surface area of carbon fiber felt and providing a redox reaction site of electroactive species.

The conventional carbon felt is made of a carbon precursor having a long fiber and is made into a graphite felt through an oxidation-carbonization-graphitization process. The graphite felt is formed by adding steam, carbon dioxide or ozone at a temperature higher than 400 ° C. Or carbon fiber is wetted with an alkali solution or an acid solution, washed and then treated at a high temperature to introduce a functional group onto the surface of the carbon fiber to prepare an electrode

However, when the carbon felt electrode is manufactured according to the known surface treatment method, activation of the graphitized carbon fiber is very difficult, so that it is difficult to generate functional groups on the surface of the carbon fibers constituting the carbon felt, pore is very difficult to make. As a result, the adsorption of the electrolyte to the electrode surface and the electrochemical reactivity of the reactive ion species such as vanadium are lowered, and improvement is required. Also, in order to increase the performance and power density of the flow cell, an electrode capable of realizing a high current per unit area is required. In the conventional surface treatment method, the selective adsorption and reactivity of vanadium ions are limited so that an electrode having high- it's difficult.

SUMMARY OF THE INVENTION The present invention provides a carbon felt electrode for a vanadium redox-flow battery in which the adsorbability and electrochemical reactivity of an electrolyte are improved by solving the above-described problems.

It is another object of the present invention to provide a vanadium redox flow cell including the above carbon felt electrode with improved charging / discharging efficiency and current density.

A carbon felt and an ion adsorbing layer disposed on at least one side of the carbon felt according to one aspect,

Wherein the ion-adsorbing layer comprises a conductive carbon and a binding polymer resin,

There is provided a carbon felt electrode for a vanadium redox-flow battery, wherein at least one functional group is formed on the carbon fiber surface of the carbon felt.

And an ion-adsorbing layer disposed on at least one side of the carbon felt and the carbon felt according to the other aspect,

Wherein the ion-adsorbing layer comprises a carbon-based material obtained by carbonization of conductive carbon and a bonding polymer resin,

There is provided a carbon felt electrode for a vanadium redox-flow battery, wherein at least one functional group is formed on the surface of the carbon fiber and the ion adsorption layer of the carbon felt.

According to another aspect, there is provided a vanadium redox flow cell comprising the carbon felt electrode described above.

The carbon-felt electrode according to one embodiment increases the adsorption and diffusion of reactive ion species on the carbon fiber surface. Therefore, the use of such a carbon felt electrode improves the reversibility of the redox reaction and the current density. Therefore, by using the carbon felt electrode of the present invention, a vanadium redox flow cell improved in charge / discharge capacity and voltage efficiency can be manufactured.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1A is an exemplary configuration diagram showing the configuration of a vanadium redox flow cell according to an embodiment of the present invention. FIG.
FIG. 1B shows a configuration of a vanadium redox flow cell according to another embodiment of the present invention.
Fig. 2 is a cross-sectional view showing a configuration of a unit cell in Fig. 1. Fig.
3A to 3C are photomicrographs of the surface and cross-section of the electrode for vanadium redox-carrying battery of Examples 1-2 and Comparative Example 1 of the present invention.
4 is a photograph of the wettability of the surface and the cross section of the electrode for vanadium redox battery according to Examples 1-2 and Comparative Example 1 of the present invention.
5 shows the resistance characteristics of the vanadium redox flow cell of Example 3-4 and Comparative Example 2. Fig.
6 is a graph showing the output according to the current density of the electrode for a vanadium redox battery of Examples 3-4 and Comparative Example 2 of the present invention.

Hereinafter, a carbon felt electrode for a vanadium flow cell and a carbon felt electrode including the same according to exemplary embodiments will be described in more detail.

Wherein the ion adsorbing layer comprises conductive carbon and a bonding polymer resin, and the surface of the carbon fiber constituting the carbon felt has at least one function A carbon felt electrode for a vanadium redox-flow battery is provided.

The bonding polymer resin uses a material having durability in a strongly acidic electrolytic solution having a pH of 1 to 2. For example, Nafion, fluoroelastomer, polytetrafluoroethylene (PTFE), PTFE-copolymer, polyvinylidene fluoride (PVDF), PVDF-copolymer, poly (PEEK), perfluoroalkoxy polymer (PFA), fluorinated ethylene propylene (FEP), phenol resin, epoxy resin, polyester, polyvinyl ester, polyimide, poly Acrylonitrile (PAN), fluoroelastomer, and styrene butadiene rubber. Here, Napion is a trade name of a polymer having a sulfonic acid group introduced into the skeleton of polytetrafluoroethylene (du Pont).

These bonded polymer resins bind conductive carbon to the network between the carbon fibers and can bind the conductive carbon bonds. Since they are mainly fluorinated, when a substance that is stable at low pH and is well wetted by water is selected, Lt; RTI ID = 0.0 > adsorption < / RTI > When the ionic active layer containing electroconductive carbon which adsorbs water and ions well on the surface of the carbon felt having hydrophobicity is formed, the reactivity between the carbon felt electrode and the electrolyte solution can be increased.

The above-mentioned fluorinated polymer resin is available, for example, as a Nafion solution (Dupont) under the trade name of fluorine resin.

Conductive carbon is even more advantageous if it has pores on its surface, contains functional groups, or is advantageous to contain functional groups on its surface. The conductive carbon may be, for example, activated carbon, carbon black, acetylene black, Ketjen Black, denka black, carbon whisker, Vapor Grown Carbon Fiber (VGCF), carbon aerosol, A graphite powder, a graphite powder, a graphite powder, a carbon nanohorn, a graphite, a natural graphite powder, a synthetic graphite powder and a thermally expanding graphite powder. When such an ionic active layer containing conductive carbon is formed on the surface of the carbon felt, a carbon felt electrode having improved electrical conductivity can be produced.

Bonded polymer resins increase the bond between conductive carbon and conductive carbon, and between conductive carbon and carbon fiber. The content of the binder polymer resin is 3 to 30 parts by weight, for example, 5 to 15 parts by weight, based on 100 parts by weight of the total weight of the ion active layer. When the content of the binder polymer resin is within the above range, the resistance of the electrode is not increased, and the wettability of the electrolyte is not lowered, and the ionic active layer for the carbon fiber of the carbon felt has excellent bonding strength.

Wherein the ion adsorbing layer comprises a carbonaceous material obtained by carbonization of conductive carbon and a bonding polymer resin, and the carbonaceous material of the carbonaceous felt There is provided a carbon felt electrode for a vanadium redox-flow battery wherein at least one functional group is bonded to the surface of the fiber and the ion adsorption layer.

The carbon-based material obtained by carbonization of the binder polymer resin may be, for example, a pitch.

In the present invention, the ion active layer can be formed on one surface or both surfaces of the carbon felt.

The content of the ion active layer is 1 to 30 parts by weight, for example, 2 to 10 parts by weight, based on 100 parts by weight of the total weight of the carbon felt electrode. When the content of the ionic active layer, that is, the total content of the conductive carbon and the polymeric binder resin, or the total content of the carbonaceous material obtained by carbonization of the conductive carbon and the polymer is within the above range, the reactive ion species in the electrolyte solution has high wettability to the carbon felt electrode, The concentration of the reactive ionic species adsorbed on the surface is increased. As a result, the redox reaction efficiency is increased.

The surface of the electrode was analyzed by X-ray photoelectron spectroscopy (XPS) on the carbon felt electrode of the present invention, and the content of oxygen was 3 to 20 parts by weight based on 100 parts by weight of the total weight of the carbon felt electrode. The content of oxygen functional groups formed on the surface of the carbon felt can be known as the relative ratio of carbon to oxygen when the surface of the carbon fiber is analyzed through XPS. In the present invention, the amount of oxygen functional groups can be estimated from the amount of oxygen as a result of XPS.

 If the content of oxygen functional groups is excessively increased, too much oxygen is introduced into the carbon fibers, resulting in a decrease in the mechanical strength of the carbon felt and an increase in the resistance of the electrode. On the other hand, if the content of oxygen is too low, water or reactive ion species do not get wet on the surface of carbon fiber, so reaction ion does not get wet into carbon felt but reacts only on the surface. .

In the conventional research, when the surface treatment time is increased to increase the reaction site on the carbon fiber surface, or when a large amount of reaction gas is injected at a high temperature, the carbon fiber itself is oxidized to increase the resistance of the electrode, The mechanical strength is remarkably lowered, and carbon felts are easily broken when the stack is manufactured in VRB.

In the present invention, most of the reactive ion species have a structure that is easily moved to the carbon fiber surface, and the reactive ion species moved to the surface are easily adsorbed on the carbon fiber surface and the ion adsorption layer. The adsorbed reactive ion species may react in the ion adsorption layer or may migrate toward the functional group of the carbon fiber to cause redox reaction. In the total amount, the oxygen function is greatly increased to increase the reaction site, but the structure of the carbon felt becomes stronger and the resistance of the electrode becomes lower, resulting in an effect of increasing the energy density of the stack.

The specific surface area of the carbon felt electrode is 1 to 100 m < 2 > / g.

The present invention provides a carbon felt electrode in which an ion active layer is formed on one side or both sides of a carbon felt to improve the low wettability of the reactive species ion on the electrode surface and increase the reactivity. The surface of the carbon felt is first formed with a functional group, and then a layer containing a carbon material having a large specific surface area capable of adsorbing reactive ion species and a bonding polymer resin having durability in a strongly acidic atmosphere is applied on the carbon fiber surface, An active layer is formed to form a carbon felt electrode.

As another method, conductive carbon and a bonding polymer resin capable of being carbonized are applied and dried on the surface of the carbon felt precursor when carbon felt is manufactured, and carbonized, graphitized, and surface treated through the process, and conductive carbon and a bonding polymer resin And the ionic activation layer made of the carbon-based material is formed.

As used herein, the term "oxygen functional group" means a hydroxyl group (-OH), a ketone group (-C═O), or a carboxyl group (-COOH). When such a functional group is provided, hydrophobicity is imparted to the hydrophobic carbon felt electrode to increase the reactivity between the electrode and the electrolyte solution, thereby making it possible to manufacture a vanadium redox flow cell having improved charge / discharge characteristics.

When a hydroxy group and a ketone group, which are oxygen functional groups, are introduced into a carbon felt electrode, all methods commonly used in the art can be applied. For example, the carbon felt electrode can be operated according to a reaction of injecting water vapor in a steam state at an electric furnace at 400 to 600 ° C, for example, about 500 ° C.

Carbon dioxide (CO ₂ ) gas is used when a carboxyl group as an oxygen functional group is introduced into a carbon felt electrode. The type and relative ratio of each functional unit can be controlled by the type of gas used.

Hereinafter, a method for manufacturing a carbon felt electrode for a vanadium redox-flow battery of the present invention will be described. For example, the method will be described with reference to two manufacturing methods.

First, the first manufacturing method is as follows.

The oxidized carbon felt is obtained through oxidation of the carbon felt precursor, and the carbon felt is subjected to carbonization and graphitization by heat treatment in an inert atmosphere to produce carbon felt. The carbon felt obtained here is graphitized felt.

The carbon felt precursor may be, for example, rayon fiber, polyacrylonitrile fiber, cellulosic fiber, or the like.

The step of oxidizing the carbon felt precursor is carried out at 150 to 450 占 폚, for example, at 250 to 350 占 폚, specifically at about 300 占 폚 under air or oxygen gas atmosphere. Through this oxidation process, the functional polymer fiber is changed into a structure having more benzene structure, and the carbon content is increased to more than 60%.

The carbon oxide felt is subjected to heat treatment in an inert gas atmosphere to carry out carbonization and graphitization. Here, the inert gas atmosphere is formed using a gas atmosphere such as nitrogen or helium. The heat treatment is performed at 1300 to 2200 ° C. When the heat treatment is carried out in the above-mentioned temperature range, carbonization and graphitization of the carbon oxide felt are made smoothly.

Then, an oxygen functional group is introduced on the surface of the carbon felt to prepare a surface-treated carbon felt electrode. The oxygen functional group introduces a functional group containing at least one of a functional group containing a hydroxyl group or a ketone group or a carboxyl group as described above.

The carbon felt having undergone the carbonization and graphitization described above may have a wettability to water as a part of the carbon fiber is changed into a crystal structure.

However, in the present invention, as described later, an ion active layer is formed on the surface of the carbon felt to improve the adsorption and wettability with respect to reactive ion species such as vanadium ions as compared with the carbon felt without the ion active layer.

A composition for forming an ionic active layer containing conductive carbon, a binder polymer resin, and a solvent is supplied to the surface-treated carbon felt and dried to prepare a carbon felt electrode for a vanadium redox-flow battery.

The solvent may be alcohols of ethanol and methanol, water, or an organic solvent. The content of the solvent is 10 to 100 parts by weight based on the total content of the conductive carbon and the bonding polymer resin. When the content of the solvent is within the above range, the components constituting the composition for forming an ion-active layer are evenly dispersed, and the ion-activating layer can be uniformly formed on the carbon felt.

The method for supplying the composition for forming an ionic active layer to the surface-treated carbon felt is not particularly limited. For example, a method of supplying using a spray nozzle or an impregnation method may be used.

The drying is carried out, for example, at 80 to 150 ° C, for example at 120 ° C.

A second method for manufacturing the carbon felt electrode of the present invention will be described.

First, the carbon felt precursor is oxidized to produce carbon oxide felt. Here, the step of oxidizing the carbon felt precursor is carried out in the same manner as in the first production method. Alternatively, it is possible to produce carbon oxide felt in a nonwoven fabric process by using oxidized staple carbon fiber, but either method can be used. A composition for forming an ion-sensitive active layer containing conductive carbon, a polymerizable binder resin capable of being carbonized and a solvent is supplied to the carbon oxide felt and dried to form an ion active layer over at least one surface or entire surface of the carbon oxide felt.

The carbonaceous felt having the ion active layer disposed thereon is heat-treated in an inert gas atmosphere to carry out the carbonization and the graphitization process to produce the carbon felt having the double structure in which the ion adsorption layer is formed.

When the carbonization and the graphitization process are performed through the above-described heat treatment, the organic polymer is carbonized and converted to carbon, and the remainder is discharged as gas. Through this process, the carbon felt is composed of carbon fibers, conductive carbon widely distributed between the skeletons of the carbon fibers, and an ion active layer containing carbon-based material obtained by carbonization of the polymer.

An oxygen functional group is introduced into the surface of the carbon felt on which the ion adsorption layer is formed to produce a surface-treated carbon felt. The method of introducing oxygen functional groups is as described in the first production method.

The second manufacturing method of the carbon felt electrode of the present invention can provide a reaction site of more vanadium ions as the bonding polymer resin for bonding the conductive carbon is carbonized. And the ion activation layer is composed of only pure carbon as compared with the case of the first manufacturing method, so that more ion adsorption sites are distributed. Also, since the second production method introduces an oxygen functional group on the surface of the carbon felt on which the ion adsorption layer is formed, it is easier to introduce the oxygen functional group than the first manufacturing method described above, and it becomes possible to introduce a relatively increased oxygen functional group. Accordingly, the carbon felt electrode having high ion adsorptivity and high electrical conductivity characteristics can be manufactured as compared with the first manufacturing method.

In the method for producing a carbon felt electrode according to the present invention, the oxygen functional group content of the carbon felt is measured by XPS analysis on the surface of the carbon felt electrode in the step of preparing the surface-treated carbon felt by introducing oxygen functional groups on the surface of the carbon felt electrode, And 3 to 20 atomic% based on the total content of oxygen. When the oxygen content is in the above range, the adsorption of reactive ion species on the electrode surface is further increased, and the reactivity of the electrode and the electrolyte solution is greatly improved, so that the current density characteristics can be improved.

The carbon felt electrode of the present invention can improve the affinity between the electrolyte solution and the carbon felt electrode and accelerate the oxidation and reduction of vanadium to produce a vanadium redox flow cell having improved charging and discharging efficiency.

The carbon felt electrode has a porous structure and a porosity of 70 to 98%.

Hereinafter, FIG. 1 illustrates a structure of a vanadium redox flow cell having a carbon felt electrode according to one embodiment.

1, the vanadium redox flow battery 10 includes a stack 30 in which unit cells 20 are stacked in series, and a tank 40, 50 And pumps 42 and 52 for circulating the active material during charging and discharging. The electrolyte of the anode and the cathode uses an acidic aqueous solution in which an active material such as vanadium (V), iron (Fe), and chromium (Cr) and a transition metal such as tin (Sn) are dissolved in a strong acid aqueous solution. The stack 30 is composed of unit cells 20 and two end plates 60 and 62. The unit cells 20 and the end plates 60 and 62 are fastened by a plurality of tie rods 64.

2, the unit cells 20 are basically disposed on both sides of a membrane 22, a pair of electrodes 24 disposed on both sides of the membrane, and electrodes 24 And a pair of separation plates 26 are provided.

These electrodes are in direct contact with the electrolyte flowing in the redox flow cell 10 to provide an active site where the electroactive species dissolved in the electrolyte reacts during charging and discharging and provide a path for electrons to move. Therefore, the material used as the electrode should have high electrical conductivity, good durability in the strongly acidic solution of the supporting electrolyte used, and provide a reaction site where the redox reaction of the electroactive species can take place. In order to lower the cell resistance, the electrode structure should be provided to reduce the interface between the electrode and the graphite separator and the interface resistance between the electrode and the membrane.

The oxygen functional groups include, for example, one or more functional groups including a hydroxyl group and a ketone group carboxyl group. The formation of the oxygen functional group on the surface of the carbon felt electrode can be confirmed by FT-IR.

The carbon fiber constituting the carbon felt electrode of the present invention has an average fiber diameter of 5 to 20 mu m and an average length of 30 to 100 mm. And the carbon felt electrode is formed as a nonwoven fabric of carbon fibers.

If the electrode of the vanadium oxidation flow cell contains an additional ion adsorption layer other than carbon fiber, the oxygen content of the graphite felt surface increases from 0.5 to 20 parts by weight of the total weight of the functional group, , And the specific surface area increases from 1 m 2 / g or less to 100 m 2 / g. As a result, the amount of vanadium oxidation-reduction ion adsorbed on the surface of the graphite felt increases, thereby increasing the amount of ions participating in the oxidation-reduction reaction as the reaction site increases. Therefore, the charging / discharging capacity is increased and the efficiency of charging and discharging is increased. If the conventional graphite felt was a reaction by mass transfer of reactive ion species, the reaction of the present invention is higher because diffusion at the graphite felt surface is also increased.

The oxygen content of the oxygen functional group in the carbon felt electrode according to an embodiment is adjusted to be in the range of 5 to 20 atomic% based on the total content of carbon and oxygen in the ion active layer of the carbon felt electrode.

A vanadium redox flow cell according to another aspect of the present invention uses the electrode of the present invention as a cathode or an anode of a battery.

1B is a schematic view of a redox flow battery according to an embodiment of the present invention.

Referring to FIG. 1B, a redox flow cell according to an embodiment of the present invention includes a cathode cell 1, an anode cell 2, an ion exchange membrane 100 for isolating the two cells 1 and 2, And tanks 21 and 22, respectively, which communicate with the first and second tanks 1 and 2, respectively.

The cathode cell (1) includes a cathode (13) and a cathode electrolyte (11).

The anode cell (2) includes an anode (14) and an anode electrolyte (12).

Charging and discharging occur depending on the redox reaction occurring in the cathode 13 and the anode 14.

The cathode 13 and the anode 14 may each include the carbon felt electrode of the present invention. 1B, reference numerals 41 and 420 denote tubes, and reference numerals 31 and 32 denote pumps.

The operation principle of the redox flow battery is disclosed in Korean Patent Publication No. 2011-0088881. Korean Patent Publication No. 2011-0088881 is incorporated herein by reference in its entirety.

The redox flow battery is suitable for applications requiring a high capacity and high output such as an electric vehicle in addition to the use of existing mobile phones and portable computers. The redox flow battery can be combined with existing internal combustion engines, fuel cells, supercapacitors, It can also be used for hybrid vehicles. In addition, the redox flow battery can be used for all other applications requiring high output and high voltage.

Hereinafter, the present invention will be described in more detail with reference to the following examples. The present invention is not limited to the following examples.

Example  One: Carbon felt  Manufacture of electrodes

PAN felt (900 g / m < ² >) was made using PAN (polyacrylic nitrile) fiber using a nonwoven production process. PAN pellets manufactured in a roll state were continuously introduced into an oven at 300 ° C. in an air atmosphere and subjected to an oxidation process for 1 hour and then subjected to a carbonization-graphitization process in an N ² atmosphere electric furnace having a temperature gradient of about 1300 ° C. to 2200 ° C. To produce a graphite felt in the form of a roll. Since the weight of graphite felt produced at this time is about 400 g / m < ² > and shrinkage occurs during the graphitization process, a portion of the carbon fiber is converted into a crystal structure, Adsorption and wettability to species are low and are not suitable for direct use in electrodes.

The graphite felt obtained according to the above procedure was placed in an electric furnace at about 500 ° C, and water vapor in a steam state was injected into the furnace at a constant rate to obtain a graphite felt in which a hydroxy group and a ketone group were introduced on the carbon fiber surface of the graphite felt.

Separately, 100 g of activated carbon (Kethjene black 600JD, Azo) was mixed with 100 g of a 10% Nafion solution (Dupont) and 2 kg of ethanol to prepare a composition for forming an ionic active layer.

The composition for forming the ionic active layer was applied on the upper surface of the graphite felt having the functional group introduced on the surface of the carbon fiber by using a spray nozzle and dried in an oven at 120 캜 to form the upper surface of the graphite felt, To form an ion-adsorbing layer on the surface. At this time, the ion adsorbing layer is partially impregnated into the graphite felt to be applied to the network between the carbon fibers and to be bonded to the fiber surface. The coating amount of the composition for forming the ionic activation layer was adjusted to about 5 parts by weight based on 100 parts by weight of the graphite felt.

Example  2: Carbon felt  Manufacture of electrodes

The PAN felt was produced at 900 g / m ² using a nonwoven production process using PAN (polyacrylonitrile) staple fiber. The PAN felt produced in the roll state was continuously put into an oven at 300 캜 in an air atmosphere and oxidized for 1 hour to obtain an oxide felt. Oxidized carbon felt is brittle and less flexible than PAN felt, and oxidized fiber has a brittle characteristic.

Separately, a composition for forming an ion adsorption layer was prepared by mixing 100 g of activated carbon (Azo Co.), 10 g of conductive carbon CNT (Cheil Industries), 100 g of a 40 wt% phenol resin (Dow Co.) and 2 kg of ethanol.

The composition for forming an ion adsorption layer was applied on the upper surface of the oxidized felt obtained by the above process using a spray nozzle and dried in an oven at 120 ° C to form an ion adsorption layer on the upper surface and the inside and the bottom surface of the oxidized felt, Respectively. At this time, the ion adsorbing layer is evenly impregnated into the oxide felt to be applied to the network between the carbon fibers and bonded to the fiber surface. The content of the ion adsorption layer-forming composition was adjusted to be about 10 parts by weight based on 100 parts by weight of the oxidized felt. The oxidized felt coated with the composition for forming an ion adsorption layer was again subjected to carbonization-graphitization in an N ₂ atmosphere electric furnace having a temperature gradient of 1300 to 2200 ° C to prepare a graphite felt in the form of a roll. The weight of the graphite felt thus produced was about 400 g / m 2 to 500 g / m 2. The phenolic resin in the conductive ion adsorption layer applied to the oxidized felt during the carbonization of the oxidized felt serves to maintain the bond between the carbon fiber, the activated carbon and the CNT. During the carbonization process, the phenolic resin is transformed into amorphous carbon and some shrinkage occurs. In graphite felts subjected to graphitization process, ion adsorption paper is formed between the surface of the carbon fibers and the network over the whole area, where the graphite felts have a structure made entirely of carbon.

The graphite felt having the ion adsorbing layer formed therein was placed in an electric furnace at 500 ° C, and water vapor in a steam state was injected into the furnace at a constant rate to obtain a graphite felt having functional groups introduced into the surfaces of the carbon fibers and the ion adsorption layer.

Comparative Example  One: Carbon felt  Manufacture of electrodes

(JNTG Co., Ltd., GF050B) having a thickness of 4.5 mm and a thickness of 400 g / m < 2 > were placed in an electric furnace at 500 DEG C, and water vapor in a steam state was injected into the furnace at a constant rate to obtain a carbon felt electrode into which functional groups were introduced.

Example  3: vanadium Redox  Manufacture of flow cell

Nafion 117 was used as an ion exchange membrane and the electrode prepared according to Example 1 was inserted into a flow frame made of polypropylene. The collector plate was plated with gold on the surface of the copper plate, and a carbon composite plate and anodized aluminum plate were placed on both ends.

The electrolytic solution used was a mixed aqueous solution of VOSO ₄ and H ₂ SO ₄ , and the mixed state was VOSO ₄ 1.5MH ₂ SO ₄ 3M to prepare a vanadium redox flow cell.

Example  4: vanadium Redox  Manufacture of flow cell

A vanadium redox flow cell was prepared in the same manner as in Example 3 except that the electrode of Example 2 was used instead of the electrode of Example 1.

Comparative Example  2: Vanadium redox flow cell  Produce

A vanadium redox flow cell was produced in the same manner as in Example 3 except that the electrode of Comparative Example 1 was used instead of the electrode of Example 1.

Evaluation example  1: Analysis of electrode structure and water wettability

The structure of the electrode was measured using a microscope on the surface and cross-section of the electrode obtained in Example 1 and Comparative Example 1.

Microscopic photographs of the structure of each electrode are shown in Figs. 3a to 3d. FIGS. 3A and 3B are surface and cross-sectional photographs of the electrode manufactured according to Example 1, and FIGS. 3C and 3D are a surface and cross-sectional photographs of the electrode of Comparative Example 1, respectively.

Referring to this graph, in the graphite felt of Example 1, the ion adsorption layer is well adhered between the surface of the carbon fiber and the network, and the ion adsorption layer is uniformly formed over the entire area of the graphite felt in the cross- . The ion adsorption layer not only adsorbs ions, but also connects the carbon fibers physically contacting with the electrically conductive carbon, thereby increasing the electrical conductivity. On the other hand, in the graphite felt of Comparative Example 1, the surface of the carbon fiber is very smooth and the carbon fiber is weakly contacted. It can be seen that the electron transport is difficult in the high current region due to the low electric conductivity. This difference in structure leads to a difference in the time of staying in the reaction site of the active material, that is, the redox reaction time, thereby causing a difference in the performance of the electrode.

Evaluation example  2: Analysis of wettability against water

The electrodes prepared according to Examples 1 and 2 and Comparative Example 1 were cut into a size of 5 cm x 5 cm and then placed in a beaker containing distilled water. The degree of wetting of the electrodes was measured, and the electrodes were cut in half in the thickness direction The wettability to water was measured by immersing the cut electrode in distilled water in the same manner.

The results of water wettability analysis of the electrodes of Examples 1, 2 and Comparative Example 1 are shown in Figs. 4A to 4F. FIGS. 4A and 4B are photographs showing the surface and cross-sectional states of the electrodes of Example 1, and FIGS. 4C and 4D show the surface and cross-sectional states of the electrodes of Example 2, respectively. Figs. 4E and 4F show the surface and cross-sectional states of the electrode of Comparative Example 1. Fig.

As shown in Fig. 4, the graphite felt of Example 1 and Example 2 were uniformly wetted not only on the surface but also on the inner surface of the shaved product, and no air bubbles were observed on the graphite felt surface. There is an area that is not wetted by water. That is, the graphite felt in which the ion adsorption layer is formed in the VRB is uniformly and easily wetted by the electrolytic solution comprising the vanadium ion and the strong acid, so that the reactivity and the efficiency can be increased.

Evaluation example  3: Measurement of electrode resistance according to electrode pressure

The electrode prepared according to Example 1-2 and Comparative Example 1 was cut into a predetermined size, inserted into an electric conductivity measuring jig of a universal testing machine, and the penetration resistance of the electrode was measured while applying pressure (0 to 30 N / cm ² ). The measurement results are shown in Fig.

Referring to FIG. 5, the electric resistance was decreased with the application pressure increased, but the electrode of Example 1 exhibited a low electric resistance of about 30% as compared with the electrode of Comparative Example 1. In particular, the electrode resistance of Example 2 was about 10% lower than that of Example 1. In the case of Example 2, the phenol resin undergoes a carbonization-graphitization process, and the structure is changed to carbon having good electrical conductivity, so that all of the electrode materials are carbon. In Example 1, however, the Nafion resin is contained in the ion- And thus acts as an electric resistance component. The lower the resistance of the electrode, the smaller the charge transfer resistance in the charge / discharge reaction, so the current flow may be easier.

The electrons generated in the ion adsorption reaction site flow along the electrode. The comparative example made of carbon fiber only flows through the surface of the carbon fiber, so that the electric conductivity is low. This may be a leakage of some current during charge / discharge, It also causes efficiency.

Evaluation example  3: vanadium Redox flow  In the battery Charging and discharging  efficiency

Charge and discharge at a current density of 80 mA / cm 2 at a flow rate of 1.5 ml / min / cm 2 was carried out on the vanadium redox flow cell produced according to Examples 3 and 4 and Comparative Example 2, , Voltage efficiency and energy efficiency.

The charge and discharge characteristics of each cell are shown in FIG. 5, and the voltage change of each cell is shown in FIG.

The charging / discharging efficiency (current efficiency), the voltage efficiency and the energy efficiency of each of the batteries were calculated by the following formulas 1 to 3, respectively, and the calculation results are shown in Table 1 below.

[Formula 1]

Charge / discharge efficiency (current efficiency) (%) = (discharge capacity / charge capacity) X100

[Formula 2]

Voltage efficiency (%) = (average discharge voltage / average charge voltage) X100

[Formula 3]

Energy efficiency (%) = voltage efficiency X charge / discharge efficiency

division unit Example 3 Example 4 Comparative Example 2 charge Current capacity Ah 4.2 4.3 3.2 Discharge Current capacity Ah 4.15 4.2 3.11 Current efficiency
(Columb Efficiency) % 98.8 97.6 97.2 Voltage efficiency
(Voltage Efficiency) % 89.0 89.9 85.2 Energy efficiency
(Energy Efficiency) % 87.3 87.7 82.8

As shown in FIG. 6 and Table 1, the vanadium redox flow cell of Examples 3 and 4 had an ion adsorption layer applied to the charge / discharge capacity of Comparative Example 2 (using a carbon nonwoven fabric electrode made only of carbon fiber) It was confirmed that the charging capacity and the discharging capacity of a graphite felt electrode increased to about 1.3 times. In the cell of Comparative Example 2, only the surface of the carbon fiber having the functional group was reacted, whereas in the cells of Examples 3 and 4, the carbon fiber surface as well as the ion adsorption layer participated in the reaction to increase the capacity of about 30 to 35% . The functional group of the ion-adsorbing layer is more reversible than that of the functional group on the surface of the carbon fiber. This is because the voltage efficiency value is increased. Also, the interface between the film and the electrode, and the interface between the electrode and the current collector showed better interfacial bonding due to the ion adsorption layer, lowered interfacial resistance, and improved current efficiency.

Evaluation example  4: vanadium Redox  Evaluation of cell performance of flow cell

In the evaluation of the vanadium redox flow cell, the concentration of the active species participating in the electrode reaction continuously changes during charging and discharging as the anode and cathode electrolytes are circulated in the closed system, It is difficult to confirm the characteristics of the electrode alone. Therefore, in order to evaluate the electrode performance of the vanadium redox battery, the concentration of V ⁵⁺ and V ²⁺ of the anode electrolyte and the cathode electrolyte flowing into the battery was fixed at 2M, and the current density was changed while supplying it into the battery. At this time, the anode electrolytic solution and the cathode electrolytic solution passed through the cell were not mixed with the electrolytic solution collected and supplied to the other container. That is, an experimental method for measuring the voltage while changing the current density in the discharge process is as shown in FIG.

The results of the above experiment are shown in FIG. The current densities at 1.2 V of the vanadium redox flow cells of Examples 3-4 and Comparative Example 2 are summarized in Table 2.

division unit Example 3 Example 4 Comparative Example 2 Electrode Performance (@ 1.2V) mA / cm2 140 155 107

As shown in FIG. 8, in the vanadium redox flow cell of Comparative Example 2, the current density was about 107 mA / cm 2 at 1.2 V, whereas the vanadium redox flow battery of Examples 3 and 4 had current density of 140 mA / ㎠ and 155mA / ㎠, the battery performance was improved by 1.3 ~ 1.4 times.

Evaluation example  5: Oxygen Functional  content

In order to measure the content of oxygen functional groups on the electrode surface in the carbon felt electrodes prepared according to Examples 1-2 and Comparative Example 1, atomic% of carbon and oxygen were evaluated using XPS.

As a result of the evaluation, it was confirmed that the oxygen functional group of the carbon felt electrode of Example 1-2 was higher than that of Comparative Example 1.

The atomic ratios of C and O in each case were measured using XPS. When the carbon felt is not surface-treated, the content of C is 99.9% or more, and the oxygen atom shows a very low value of 0.1% or less. On the contrary, Comparative Example 1 showed that the content of C was 97% and the content of oxygen was 3% or less. The oxygen content of Example 1 and Example 2 were 12% and 15%, respectively, indicating that the functional group content was further increased in Example 2. This is due to the formation of functional groups on the carbon surface, not only on the carbon fiber surface and the conductive carbon, but also over the entire area from the carbonized polymer resin surface.

10: vanadium redox flow cell 20, 40: unit cell
24: electrode 26: separator plate
30: stack 42, 53: tank
42, 52: pump 60, 62: end plate

Claims (5)

A carbon felt and an ion adsorbing layer disposed on at least one side of the carbon felt,
Wherein the ion-adsorbing layer comprises a conductive carbon and a binding polymer resin,
Wherein at least one functional group is bonded to the surface of the carbon fibers constituting the carbon felt. A carbon felt and an ion adsorbing layer disposed on at least one side of the carbon felt,
Wherein the ion-adsorbing layer comprises a conductive carbon and a carbon-based material obtained by carbonization of a bonding polymer resin,
Wherein at least one functional group is bonded to the surface of the carbon fiber and the ion adsorption layer of the carbon felt. 3. The method according to claim 1 or 2,
The binder polymer resin
Nafion, fluoroelastomer, polytetrafluoroethylene (PTFE), PTFE-copolymer, polyvinylidene fluoride (PVDF), PVDF-copolymer, polyetheretherketone (PEEK), perfluoroalkoxy polymer (PFA), fluorinated ethylene propylene (FEP), phenol resin, epoxy resin, polyester, polyvinyl ester, polyimide, polyacrylonitrile (PAN), fluoroelastomer and styrene butadiene rubber, and the conductive carbon is at least one selected from the group consisting of activated carbon, carbon black, acetylene black, Ketjen black, denka black, carbon whisker, Carbon nanofiber, carbon nanohorn, graphene, natural graphite powder, synthetic graphite powder and thermal expansion black (VGCF), carbon aerosol, carbon nanotube, carbon nanofiber, carbon nanohorn, One or more selected from the group consisting of powder and
Wherein the content of the polymer resin is 3 to 30 parts by weight based on 100 parts by weight of the total weight of the ionic active layer. 3. The method according to claim 1 or 2,
The total content of the ionic active layer is 1 to 30 parts by weight based on 100 parts by weight of the carbon felt electrode,
Wherein the content of oxygen on the electrode surface in the carbon felt electrode is 3 to 20 parts by weight based on 100 parts by weight of the total weight of the carbon felt electrode. A vanadium redox flow cell comprising the carbon felt electrode of claim 1 or claim 2.

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