KR20020007582A - Method for preparing electrodes and electrodes manufactured by using same - Google Patents

Abstract

PURPOSE: Provided is a method for producing an electrode being excellent in a hydrogen ion transfer property and activity cell which includes applying a layer of carbon powder on a gas diffusion layer, applying a catalyst ink on the carbon layer to form a catalyst layer; and applying a solution of an ionomer on the catalyst layer. CONSTITUTION: The method comprises steps of (a) applying a layer of carbon powder(22) containing 10 to 50 wt% of polytetrafluoroethylene on a gas diffusion layer(21); (b) applying a catalyst ink on the carbon layer containing a polymer electrolyte ionomer, a catalyst and an organic solvent to form a catalyst layer(23); and (c) applying a solution of polymer electrolyte ionomer dispersed in a hydrophilic solvent on the catalyst layer to form an ionomer layer(24). By this method, an electrode being excellent in hydrogen ion transfer and its activity can be obtained with a small amount of catalyst.

Description

Manufacturing method of electrode and electrode manufactured by this method {METHOD FOR PREPARING ELECTRODES AND ELECTRODES MANUFACTURED BY USING SAME}

[Industrial use]

The present invention relates to a method of manufacturing an electrode and an electrode manufactured by the method, and more particularly, to apply a carbon powder layer on the gas diffusion layer, a catalyst ink is applied on the carbon layer to form a catalyst layer and The present invention relates to a method for producing an electrode having excellent activity by applying an ionomer solution on the catalyst layer.

[Prior art]

The electrode for a polymer electrolyte fuel cell uses a platinum catalyst supported on a carbon carrier at both a cathode and an anode. In general, an electrode for a polymer electrolyte fuel cell is manufactured by applying a thin coating of a mixture of a catalyst and a polymer electrolyte, that is, an ionomer, on a water repellent porous carbon paper. At this time, the performance of the electrode is determined by the performance of the catalyst, the distribution of the polymer electrolyte in the electrode layer, the platinum content, etc. In particular, the performance of the electrode is the ease of mass transfer, such as diffusion of the reactant to the catalyst layer and discharge of the reaction product, etc. It is greatly affected by.

In order to increase the reaction efficiency of the catalyst present in the electrode for the polymer electrolyte fuel cell, the polymer electrolyte must be uniformly and thinly applied to the pores and the outer surface of the catalyst particles, in which case the reaction efficiency of the catalyst is increased and a small amount of the catalyst is used. You can get improved reaction effect while using.

In addition, in the case of the cathode, if the water, which is a reaction product, is not easily removed, the catalyst is submerged in water, so that the oxygen of the reactant is not diffused into the catalyst layer, and thus the electrode activity is rapidly decreased. Therefore, in order to eliminate the flooding that interferes with the electrode reaction, it is necessary to apply water repellent material of Teflon (polytetrafluoroethylene) system to the electrode or to adjust the pore size of the catalyst layer to increase the rate of water removal. There is.

In the conventional method of manufacturing an electrode for a polymer electrolyte fuel cell, an electrode is manufactured by directly using a porous carbon paper or coating a water repellent carbon powder layer on a carbon paper, and then applying a catalyst layer on the carbon paper to prevent flooding of the catalyst. Has come. As a method of applying the catalyst, spray coating, filtration and deposition, screen printing, and the like are used.

Conventionally disclosed methods for manufacturing the electrode for a polymer electrolyte fuel cell include the following methods.

Japanese Patent No. 61,118 discloses a method for producing an electrode by coating a slurry obtained by mixing a catalyst and an ionomer solution on a polymer electrolyte membrane, and hot-pressing the catalyst-coated electrolyte membrane with a gas diffusion layer. U.S. Patent No. 5,561,000 discloses coating a Teflon-coated carbon layer on a porous carbon paper, coating an intermediate layer of Nafion and carbon on it, and then again coating a catalyst layer comprising a mixture of catalyst (Pt / C) and Nafion solution. A method for producing an electrode is disclosed. U.S. Patent No. 5,501,915 discloses a mixed solution of a Nafion solution and a catalyst by evaporation to dryness and pulverization to prepare a fine powder of Nafion-coated catalyst, followed by mixing Nafion-coated catalyst powder with Teflon-coated carbon powder. A method of preparing an electrode by preparing a slurry and applying the slurry on a carbon paper coated with a carbon layer is disclosed. US Pat. No. 5,607,785 discloses a catalyst powder dispersed in a Nafion solution, mixed, evaporated, dried, and ground. After obtaining the Pion-coated catalyst powder, it is possible to improve the electrode performance by changing the size of the catalyst agglomerate or ionomer concentration according to the position in the electrode by coating the catalyst layer on carbon paper using filtration. Is starting.

U.S. Patent No. 5,723,173 discloses dispersing the catalyst powder in a solvent having low Nafion solubility, adding Nafion solution and mixing the mixture, and then adding a Teflon-coated carbon powder to prepare a slurry, and stirring it by ultrasonic waves to produce catalyst particles and Teflon. A method of forming a catalyst layer by coating a slurry prepared by allowing coated carbon powders to be entangled with each other via Nafion on a carbon paper by filtration is performed.

In general, isopropanol is used as a solvent for dispersing the catalyst and the ionomer solution. However, since isopropanol is a solvent that dissolves the ionomer very well, it has a detrimental effect on electrode production. For this reason, first, since the polymer electrolyte ionomer is completely dissolved in the isopropanol solvent, the ionomer excessively enters the pores of the catalyst to block the pores, thereby preventing the reactants from entering the catalyst pores, thereby lowering the use efficiency of the catalyst or Reaction products are not easily removed from the pores, resulting in low electrode activity. Second, since the mass of catalyst particles dispersed in a solvent is very small, the catalyst penetrates into the carbon paper, which is a gas diffusion layer, and is widely distributed, thereby increasing the migration path of hydrogen ions and increasing ion transfer resistance. Is degraded. Third, the ionomer solution penetrates into the carbon paper, the gas diffusion layer, and distributes to the opposite side without the catalyst layer, thereby increasing the hydrophilicity of the gas diffusion layer, making it difficult to remove the water generated at the cathode side, thereby increasing the utilization of the catalyst and the gas diffusion layer. In order to maintain the water repellency, there is a problem in that the distribution of the catalyst and the ionomer is properly adjusted and the pore size of the catalyst layer is increased.

Teflon-coated carbon paper is used to facilitate the movement of moisture within the electrode, particularly in the case of cathodes, to effectively remove the produced water and prevent flooding, and also to coat the catalyst with Teflon or in the catalyst layer. A method of adding teflon coated carbon powder is used. However, when the catalyst itself is coated with Teflon, the metal catalyst material is covered with Teflon, which prevents it from participating in the reaction. Also, in order to prevent this, when the Teflon-coated carbon powder is added to the catalyst layer, the added carbon The powder increases the thickness of the catalyst layer, resulting in a longer transport path for hydrogen ions, thus reducing the hydrogen ion conductivity leading to a decrease in electrode performance.

The present invention has been made to solve the above problems, an object of the present invention is to adjust the distribution of the polymer electrolyte ionomer and to facilitate the production of electrodes that can facilitate the transfer of hydrogen ions and in this way the activity of the catalyst with a small amount of catalyst It is to provide a method for producing an excellent electrode.

1 is a schematic diagram of a catalyst particle impregnated with an ionomer,

2 is a schematic diagram of a catalyst layer having an ionomer layer applied to a surface thereof,

3 is a graph comparing the performance of the unit cell according to the Nafion distribution,

4 is a graph comparing the performance of a unit cell with or without a carbon layer,

5 is a graph comparing the performance of a unit cell according to the type of solvent.

Explanation of symbols on the main parts of the drawings

11: catalyst particles 12: ionomer film

21: gas diffusion layer 22: carbon

23 catalyst layer 24 ionomer layer

In order to achieve the above object, the present invention

a) applying a carbon powder layer containing 10 to 50% by weight of polytetrafluoroethylene on top of the gas diffusion layer;

b) applying a catalyst ink comprising a polymer electrolyte ionomer, a catalyst, and an organic solvent on top of the carbon powder layer to form a catalyst layer; And

c) applying a polymer electrolyte ionomer solution dispersed in a hydrophilic solvent on top of the catalyst layer

It provides a method for manufacturing an electrode comprising a.

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

Hereinafter, after briefly explaining the structure of the polymer electrolyte fuel cell, the electrode of the present invention used in the fuel cell will be described in detail.

In the polymer electrolyte fuel cell, anode and cathode electrodes are positioned at both sides with a hydrogen ion conductive polymer membrane therebetween. The anode and cathode are prepared by applying a thin coating of the platinum catalyst and polymer electrolyte ionomer onto the water repellent carbon paper, respectively. As the polymer electrolyte ionomer solution, a Nafion solution (Nafion, manufactured by Du Pont) is used.

Figure 1 shows the catalyst particles produced in the present invention, (11) is a catalyst particle on which platinum is supported on the porous carbon particles, (12) is an ionomer film thinly coated on the inner and outer surfaces of the pores of the catalyst particles It is shown. Figure 2 shows the structure of the electrode prepared in the present invention, ionomer distribution, etc., (21) is a gas diffusion layer made of carbon paper or carbon cloth coated with polytetrafluoroethylene and (22) is Teflon supported (23) is the ionomer-supported catalyst layer and (24) is the ionomer layer.

If the carbon paper, which is a gas diffusion layer, is used as it is, the catalyst particles are penetrated into the carbon paper when the catalyst layer is applied, so that a water repellent treated carbon powder layer is coated on the carbon paper.

In order to prepare a carbon powder layer, first, a carbon slurry is prepared. The carbon slurry was prepared by mixing a fine powder of carbon black (Vulcan XC-72R from Cabot) with a 30 nm size particle and a water repellent Teflon solution (E-Tek), and then adding glycerin to increase the viscosity thereof. , Thickeners such as butyl cellulose, ethylene glycol and 1-methoxy-2-propanol are added. Teflon content in the carbon powder is preferably 10 to 60% by weight to obtain the catalyst properties required.

It is preferable to maintain this range because the carbon powder used has a particle size of 20 to 100 nm and a surface area of 20 to 300 m ² / g, which can facilitate the chemical reaction carried out on the electrode.

The prepared carbon slurry is coated on top of the carbon paper using a screen printing or tape casting method. The amount of carbon powder applied on top of the carbon paper is preferably 0.2 to 4.0 mg / cm ² based on the electrode area. In order to prevent the carbon powder layer from peeling off from the carbon paper, the carbon powder layer should be applied and then heat treated. Heat treatment of the carbon powder layer is preferably performed for about 0.5 to 2 hours at a temperature of 320 to 380 ℃ while flowing an inert gas such as nitrogen.

In addition, in order to improve the adhesion of the carbon powder layer and to control the pore size and thickness of the carbon powder layer, it is preferable to press the carbon paper at 20 to 200 atmospheres using a press machine in the middle or after heat treatment. When pressing during heat treatment, the carbon paper is first heat treated at 200 to 300 ° C. for 0.5 to 2 hours, and then pressed at 20 to 200 atmospheres, and the carbon paper is 0.5 to 2 hours at a temperature of 320 to 380 ° C. Heat treatment during.

Applying a carbon powder layer on top of the carbon paper and when the heat treatment is completed, a catalyst ink is prepared and applied to the carbon powder layer to form a catalyst layer. The catalyst ink is composed of a mixture obtained by adding and stirring a polymer electrolyte ionomer solution to a dispersion in which a catalyst is dispersed in an organic solvent. The catalyst may generally be dispersed using isopropanol as a solvent, but has a small hydrophilicity instead of isopropanol. By using an organic solvent, it is preferable to increase the cohesion of the catalyst particles and the ionomer so that the macroparticles are formed, and to prevent the ionomer from excessively entering the pores of the catalyst to block the catalyst pores. As the polymer electrolyte ionomer solution, a Nafion solution is used, and the catalyst may be a catalyst in which platinum or platinum-ruthenium alloy, which is commonly used in a fuel cell, is supported on carbon.

In addition, n-butyl acetate (n-butyl acetate) is used as the organic solvent used.

The method of forming the catalyst layer by applying the prepared catalyst ink to the carbon powder layer may be various, preferably using a spray coating method or a screen printing method using a screen of 50 to 200 mesh (mesh) will be.

The ionomer content in the formed catalyst layer is 20 to 100% by weight based on the total amount of the catalyst, and 0.1 to 1.5 mg / cm ² is preferable based on the electrode area. In this case, in order for the hydrogen ions formed at the electrode to move to the electrolyte membrane well, the contact between the electrode and the electrolyte membrane should be made good. The contact area between the electrolyte membrane and the electrolyte should be increased. However, if the amount of ionomer in the catalyst layer of the electrode is too large (more than 100% by weight based on the total amount of the catalyst), the performance of the electrode is reduced. Therefore, in this study, the performance of the electrode can be improved by changing the relative amount of ionomer present on the catalyst layer of the electrode and the outer surface of the catalyst layer to have the amount of ionomers in the above range.

In order to achieve the above object, in the present invention, a solution obtained by dispersing a polymer electrolyte ionomer in a solvent having high hydrophilicity is applied to the upper portion of the catalyst layer. As the coating method, a spray coating method or a screen printing method using a 50 to 200 mesh screen, which is a method of applying the catalyst ink to the carbon powder layer, may be used. That is, by this method, a part of the ionomer is added to the solution for preparing the catalyst layer and distributed inside the catalyst layer, and the remaining part of the ionomer is further applied on the catalyst layer after the catalyst layer is formed. For example, 70% by weight of the total amount of ionomers contained in the electrode is present inside the catalyst layer, and the remaining 30% by weight is present on the surface of the catalyst layer, i.e., the outer surface of the electrode, so that hydrogen ions generated at the electrode are smoothly transferred to the electrolyte membrane. Can be delivered.

Therefore, in order to facilitate the transfer of the hydrogen ions to the electrolyte membrane, the amount of ionomers present in the catalyst layer is 10 to 95% by weight based on the total amount of ionomers present in the electrode, and the amount of ionomers present on the outer surface of the catalyst layer is 5 to 90% by weight. %, Preferably 5 to 70% by weight.

As the hydrophilic solvent, one or more selected from compounds having 2 to 8 carbon atoms including an ester group, an ether group, a carboxyl group, a carbonyl group or an amino group can be used, and most preferably isopropanol is used.

In addition, the present invention may further include the step of drying at a temperature of 60 to 100 ℃ between the step of forming the catalyst layer and coating the polymer electrolyte ionomer on the catalyst layer as necessary.

EXAMPLE

The following presents preferred examples and comparative examples to aid in understanding the invention. However, the following examples are merely provided to more easily understand the present invention, and the present invention is not limited to the following examples.

(Example 1)

A catalyst ink was prepared by dispersing 0.1 g of a Pt / C catalyst supported on carbon at 20% by weight of platinum in 50 g of isopropanol, and then slowly adding 0.17 g of a 5% Nafion solution (manufactured by Du Pont) to the solution. It was. The catalyst ink was applied on the Teflon-coated carbon paper (TGPH) by spray coating using an air brush gun to form a catalyst layer, and the electrode on which the catalyst layer was formed was dried in a drying furnace having a temperature of 70 ° C. for about 1 hour. After the reaction, a solution prepared by mixing 0.5 g of Nafion solution and 1.5 g of isopropanol was applied to the upper surface of the catalyst layer to complete electrode production. The platinum content of the prepared electrode was 0.7 mg / cm ² in both the positive electrode and the negative electrode, and the total Nafion content in the electrode was 0.6 mg / cm ² . The prepared electrode and the Nafion 115 membrane were pressed for 90 seconds at a temperature of 140 ° C. and a pressure of 200 atm to prepare an electrolyte / electrode assembly (MEA). In order to determine the optimal distribution of Nafion applied to the inner and outer surfaces of the electrode, the operating temperature of 80 ℃, The performance of the electrode was measured using hydrogen and oxygen as the pressure 1 atm and the reaction gas, and the results are shown in FIG. 2.

(Example 2)

10 g of fine carbon powder (Vulcan XC-72R) was dispersed in isopropanol (200 g), followed by stirring while slowly dropping a 60% Teflon solution (10 g), and 100 g of glycerol was added thereto, followed by sufficient stirring to prepare a teflon / carbon slurry. . The concentration was adjusted by evaporation to dryness at a temperature of 60 to 90 ℃ to match the viscosity of the slurry. The prepared slurry was coated on top of carbon paper (20 wt% of Teflon content) by a tape casting method using a doctor blade to form a carbon layer. At this time, the carbon content was 2 mg / cm ² based on the electrode area. After drying the carbon paper coated with the carbon layer in a drying furnace at a temperature of 70 ° C. for at least about 1 hour, the first heat treatment was carried out at 260 ° C. for about 1 hour while flowing nitrogen, an inert gas, in a high temperature sintering furnace and pressed at 100 atm. Then, heat treatment was performed at a temperature of about 360 ° C. for about 1 hour while flowing nitrogen in a high temperature sintering furnace. An electrode was prepared by supporting a catalyst on the carbon paper on which the carbon layer was formed in the same manner as in Example 1. The platinum content of the prepared electrode was 0.7 mg / cm ² , the electrode area was 200 cm ² . The reaction temperature was 80 ° C. and the reaction gas measured the performance of the prepared electrode using hydrogen and oxygen, and the results are shown in FIG. 3.

(Example 3)

0.1 g of Pt / C catalyst supported on carbon at 20% by weight of platinum was dispersed in 50 g of n-butyl acetate, and then 0.17 g of 5% Nafion solution was slowly added to the catalyst by stirring. Ink was prepared, wherein the amount of Nafion contained in the catalyst ink was 1/12 of the catalyst weight. A catalyst layer was formed by spraying a catalyst ink on 50% by weight of Teflon-coated carbon paper using an air brush gun. The electrode on which the catalyst layer was formed was dried in a drying furnace at a temperature of 70 ° C. for about 1 hour, and then spray coating a Nafion solution dispersed in isopropanol on the upper surface of the catalyst layer to complete electrode production. The amount of Nafion further sprayed on top of the catalyst bed was 1/4 of the catalyst bed weight. The platinum content of the prepared electrode was 0.4 mg / cm ² in both the positive electrode and the negative electrode, and the total Nafion content in the electrode was 0.6 mg / cm ² . Thus prepared electrode and Nafion 115 membrane was pressed for 90 seconds at 140 ℃ and 200 atm conditions to prepare an electrolyte / electrode assembly (MEA) to measure the performance of the electrode of 25 cm ² , and the results 4 is shown.

(Example 4)

In the same manner as in Example 2, after coating a carbon layer loaded with 50% by weight of Teflon on the carbon paper on the top of the carbon paper, and heat-treated, using n-butyl acetate (NBA) as a solvent in the same manner as in Example 3 The catalyst layer was coated with catalyst ink. An electrode was prepared by coating Nafion dispersed in isopropanol on the catalyst layer. The platinum content of the prepared electrode was 0.4 mg / cm ² for both the anode and the cathode, and the performance of the electrode was measured using an operating temperature of 80 ° C., a pressure of 1 atm, and hydrogen and oxygen as the reaction gas, and the results are shown in FIG. 4. Shown.

As shown in FIG. 2, the change in performance of the 25 cm ² electrode according to the Nafion distribution of Example 1 shows that the amount of Nafion present in the inside and outside of the electrode is 1 to 3 or 1 to 1 Or it can be seen that better performance than when there is an excessive amount of Nafion only inside.

Figure 3 shows the performance of the electrode manufactured in Example 2, the electrode having a carbon layer showed better performance than the electrode without a carbon layer.

Figure 4 shows the performance of the 25 cm ² electrode prepared in Example 3, comparing the electrode prepared using isopropanol as a solvent in the preparation of the catalyst ink and the electrode using n-butyl acetate, In this case, all Nafion sprayed on the surface of the catalyst layer is dissolved in isopropanol. As shown in FIG. 4, the electrode using n-butyl acetate showed higher performance than the electrode using isopropanol.

The curve (carbon layer + NBA) of FIG. 4 shows that the performance of the electrode prepared by preparing the catalyst ink using n-butyl acetate and coating Nafion on the catalyst layer is superior to that of the electrode manufactured by other methods. Can be.

The electrode manufacturing method that can be used as the electrode for the polymer electrolyte fuel cell of the present invention can control the distribution of the polymer electrolyte ionomer in the electrode catalyst layer, thereby facilitating the transfer of hydrogen ions, thereby producing an electrode having excellent activity with a small amount of catalyst.

Claims (11)

a) applying a carbon powder layer containing 10 to 50% by weight of polytetrafluoroethylene on top of the gas diffusion layer; b) applying a catalyst ink comprising a polymer electrolyte ionomer, a catalyst, and an organic solvent on top of the carbon powder layer to form a catalyst layer; And c) applying a polymer electrolyte ionomer solution dispersed in a hydrophilic solvent on top of the catalyst layer Method for producing an electrode comprising a. The method of claim 1, The gas diffusion layer is a carbon paper or carbon cloth containing 10 to 50% by weight of polytetrafluoroethylene. The method of claim 1, The carbon powder has a particle size of 20 to 100 nm and a surface area of 20 to 300 m ² / g. The method of claim 1, A method of producing an electrode, the content of the carbon powder layer is 0.2 to 4 mg / cm ² based on the electrode area. The method of claim 1, The amount of the polymer electrolyte ionomer is a method for producing an electrode of 20 to 100% by weight based on the total amount of the catalyst based on the amount applied to the electrode. The method of claim 1, The polymer electrolyte ionomer is present in the catalyst 10 to 95% by weight and the method for producing a polymer electrolyte fuel cell electrode is present in the catalyst surface 5 to 90% by weight. The method of claim 1, A method for producing an electrode for a polymer electrolyte fuel cell, wherein the organic solvent is at least one selected from a compound consisting of n-butyl acetate. The method of claim 1, The method of manufacturing an electrode for a polymer electrolyte fuel cell, wherein the hydrophilic solvent is at least one selected from a compound consisting of 2 to 8 carbon atoms including an ester group, ether group, carboxyl group, carbonyl group or amino group. The method of claim 1, The catalyst layer forming method and the method of applying the polymer electrolyte ionomer solution is a method of manufacturing an electrode for a polymer electrolyte fuel cell, the spray coating method or the screen printing method using a screen of 50 to 200 mesh. The method of claim 1, The method of manufacturing a polymer electrolyte fuel cell electrode further comprising the step of drying the catalyst layer at a temperature of 60 to 100 ℃ between steps c) and d). An electrode made by the method of claim 1.