KR20070032554A - Process for forming catalyst layers on a proton exchange membrane within membrane electrode assembly for fuel cell - Google Patents

Abstract

Provided is a process for forming a catalyst layer on a proton exchange membrane of a membrane electrode assembly for a fuel cell, which allows easy control of the amount of catalyst deposition, improves catalytic efficiency and the quality of a fuel cell, and reduces the loss of catalyst. The process for forming a catalyst layer on a proton exchange membrane of a membrane electrode assembly(MEA)(5) for a fuel cell comprises the steps of: treating a proton exchange membrane in a polar solvent containing catalyst particles to allow swelling of the proton exchange membrane; and treating the proton exchange membrane in an aprotic solvent to allow the catalyst particles to be applied onto the proton exchange membrane. The MEA for a fuel cell includes the proton exchange membrane and a fuel diffusion electrode.

Description

FIELD OF THE INVENTION A method for forming a catalyst layer on a hydrogen ion exchange membrane of a membrane electrode assembly for a fuel cell {PROCESS FOR FORMING CATALYST LAYERS ON A PROTON EXCHANGE MEMBRANE WITHIN MEMBRANE ELECTRODE ASSEMBLY FOR FUEL CELL}

1 is a schematic view showing a method of forming a catalyst layer by a bridging process on a hydrogen ion exchange membrane according to the present invention.

2 is an embodiment of a flat pack-type direct methanol fuel cell with a membrane electrode assembly comprising a hydrogen ion exchange membrane having a catalyst layer formed by a bridging process by the method of the present invention.

3 is an ESEM (e-Beam Scan Electron Microscopy) image of a catalyst layer formed on a hydrogen ion exchange membrane according to the number of breeding processes. a denotes a catalyst layer formed after performing the breathing process twice, b after performing four times, c after performing six times and d after performing eight times.

4 is a graph showing the catalyst amount of the catalyst layer formed on the hydrogen ion exchange membrane according to the number of times after performing the bridging process of the hydrogen ion exchange membrane according to the present invention.

FIG. 5 is an ESEM image of a single layer of a membrane electrode assembly including a hydrogen ion exchange membrane in which a catalyst layer is formed by a bridging process.

FIG. 6 is a graph illustrating characteristics of power density with increasing current in a fuel cell using 4 M, 6 M, and 8 M aqueous methanol solution as a fuel for performance test of a fuel cell including a membrane electrode assembly according to the present invention. It is a graph. In the graph of FIG. 6,-●-indicates the performance of the MEA to which the breathing process is applied according to the present invention, and-□-indicates the performance of the MEA in which the catalyst layer is formed using a conventional spray method.

Brief description of the symbols in the drawings

    1-fuel container

    2-battery support

    3-current collector

    4-Gasket

    5-membrane electrode assembly

The present invention relates to a method for forming a catalyst layer on a hydrogen ion exchange membrane of a membrane electrode assembly for a fuel cell. More particularly, the reaction of a catalyst with a highly dispersed catalyst layer by uniformly applying catalyst particles on a hydrogen ion exchange membrane of a fuel cell. The present invention relates to a method for forming a catalyst layer on a hydrogen ion exchange membrane of a membrane electrode assembly for a fuel cell that can increase efficiency and improve energy density and output density of a fuel cell.

Recently, the problem of environmental destruction caused by the use of fossil fuels has emerged, and the necessity of fuel cells has been increased along with the high energy efficiency, which is the concern of the fuel cells. A fuel cell generates hydrogen ions from fuels such as natural gas and methanol, and generates electric energy by generating water by reacting it with oxygen in the atmosphere, thereby eliminating pollutants. In addition, the power generation efficiency is very high, 40 to 60%, and the efficiency of the exhaust heat from the reaction process can be as high as 80%.

Due to the cleanliness and high energy efficiency of such fuel cells, fuel cells are suitable for use as energy sources of the next generation, and are expected to become energy sources that are closely related to life in the near future.

Early batteries all used alkaline aqueous electrolytes and used pure hydrogen and oxygen, but gradually, gaseous fuels using fossil fuels such as methane and natural gas in addition to hydrogen, and liquid fuels such as methanol and hydrazine were used. Various fuel cells came out.

Among them, a fuel cell having an operating temperature of 300 ° C. or lower is a low temperature type, and a fuel cell having a higher temperature is called a high temperature type. In addition, a second-generation solid-state fuel cell that is intended to improve power generation efficiency, or a high-temperature molten carbonate fuel cell that does not use a noble metal catalyst at a higher efficiency is called a third-generation fuel cell.

The classification of fuel cells is based on the type of electrolyte used, phosphate acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC). ), Alkaline Fuel Cell (AFC) and Polymer Electrolyte Membrane Fuel Cell (PEMFC). Direct Methanol Fuel Cell (DMFC) uses methanol as fuel as one of the polymer electrolyte fuel cells.

As a small fuel cell for mobile devices, the use of a solid electrolyte is selected, and a polymer electrolyte fuel cell (PEMFC) using hydrogen as a fuel and a direct methanol fuel cell using liquid fuel are used. Research is being conducted on Cell, DMFC). Since fuel cell requirements for portable applications are determined by the size, weight, temperature, etc. of the system, other types of fuel cells are not suitable for this application.

Direct methanol fuel cell (DMFC) system is based on membrane electrode assembly (MEA), which is a basic unit, stack (MEA), balance of plant (BOP) to operate the system, and portable electric energy. It is composed of BMS (Battery Management System) to be used as a power source for electronic devices. MEA is a basic unit constituting the fuel cell, and is a conjugate of a catalyst layer where an electrochemical reaction takes place and a cation transfer membrane that transfers ions generated by the reaction. have. In other words, the performance of the fuel cell depends on the performance of the MEA.

Direct methanol fuel cells (DMFCs) use about 10 times more expensive precious metal catalysts Pt-Ru and Pt catalysts than polymer electrolyte fuel cells (PEMFC) (4-5 mg / cm ² per electrode). This is because first, the oxidation rate of methanol at the anode is very slow, and secondly, the Ru promoter used to prevent poisoning from carbon monoxide (CO), a reaction byproduct, reduces the oxidation surface area. In order to overcome the performance degradation of direct methanol fuel cells due to these problems and to reduce the amount of catalyst used, which is a key factor of commercialization, and to improve the performance of fuel cells, improvement of the method of manufacturing MEA is urgently needed.

The catalyst layer forming method, which has been commonly used to manufacture MEA, has been described as a decal method, a membrane-based formation method, and a gas diffusion layer formation method. These methods mainly produce a catalyst layer using a spraying method and a brush method.

The transfer method is a method of first applying a catalyst slurry or the like on a support such as Teflon and then transferring the resulting catalyst layer to the polymer electrolyte membrane, and forming a catalyst layer using a catalyst powder prepared by a colloidal method. . However, this method has a problem of only increasing the cost because the catalyst that is not in direct contact with the electrolyte membrane is an inert catalyst.

The method of forming a membrane base is usually a method of spraying a slurry in which a catalyst is dispersed while fixing a dried electrolyte membrane after pretreatment and applying heat enough to evaporate the solvent. Membrane base formation method has good contact between the electrolyte membrane and the catalyst, but uniform catalyst is applied by the change of the appearance of the electrolyte membrane during the spraying process when an electrolyte membrane having a swelling characteristic such as a Nafion ^® membrane is used. Has the disadvantage of being difficult.

In addition, the GDL base forming method is the method most commonly used to form the catalyst layer is a method of applying by a brush or spraying a slurry in which the catalyst is dispersed on the surface of the fuel diffusion layer (GDL). In this case, the catalyst coating method is simple and the catalyst amount can be easily adjusted by measuring the mass before and after coating the catalyst. However, when the catalyst is applied by a brush, aggregation of the catalyst may occur, and when the catalyst is applied by spraying. There is a disadvantage that the catalyst loss during the spraying process is large.

The problems of the conventional catalyst layer formation method are directly related to the performance of the MEA, which is a core part of the fuel cell, and pointed out as a part that must be improved in terms of cost due to the loss of expensive catalysts.

In order to overcome these problems, many researches and developments have been made, and in US Pat. No. 6,902,757, a catalyst layer was formed on a specific film, which was heated and pressed to attach to a hydrogen ion exchange membrane to improve the uniformity of the catalyst layer. In addition, US Pat. No. 6,893,761 used a contact method of an electrode, a catalyst layer, and a hydrogen ion exchange membrane as a method of forming a catalyst layer directly on carbon paper and compressing a hydrogen ion exchange membrane as a general method of manufacturing MEA.

In addition, MEA manufacturing methods and operating conditions (Y. G. Guo, J. S. Hu, H. P. Liang, L. J. Wan, C. L. Bai, Chem. Master, 15 (2003) 4332-4336) have been reported.

SUMMARY OF THE INVENTION An object of the present invention is to provide a new method for forming a catalyst layer that can improve the difficulty of forming a uniform catalyst layer and the problem of contact between an electrode, a catalyst and a hydrogen ion exchange membrane, which has been pointed out as a problem in forming a catalyst layer of a fuel cell MEA. will be.

Another object of the present invention is to simplify the process, to easily control the thickness of the catalyst layer and the amount of deposition of the catalyst, and to uniformly apply the catalyst layer on the hydrogen ion exchange membrane to increase the utilization efficiency of the catalyst on the hydrogen ion exchange membrane. It is to provide a method for forming a catalyst layer.

It is also an object of the present invention to provide an MEA which is further improved by an efficient electrochemical reaction in the catalyst layer formed by the breathing process.

In one embodiment, the present invention is to prepare a membrane electrode assembly for a fuel cell comprising a hydrogen ion exchange membrane and a fuel diffusion electrode, first swelling by treating the hydrogen ion exchange membrane in a polar solvent containing the catalyst particles, and then again an aprotic solvent The present invention provides a method for forming a catalyst layer on a hydrogen ion exchange membrane of a membrane electrode assembly for a fuel cell, wherein the catalyst particles are coated on the hydrogen ion exchange membrane by treatment in the air.

The present invention includes a method of forming a new catalyst layer which is completely different from the method used in the prior art to form the catalyst layer on the hydrogen ion exchange membrane of the MEA for fuel cells.

According to the present invention, it is possible to form a catalyst layer in which the hydrogen ion exchange membrane is uniformly dispersed by a 'breathing process', which is one of the methods for dispersing metal nanoparticles and This is how you use your breeding characteristics. In particular, a method of dispersing nanoparticles using polyacrylamide (PAM, Polyacrylamide Hydrogel) is known.

In the present invention, the 'hydrogen ion exchange membrane' is a polymer electrolyte having physical properties of being swelled in polar solvents but shrinked in aprotic solvents, preferably perfluorosulfonic acid or perluoro. Carbon sulfonic acid ionomers, more preferably Nafion ^® 115 or Nafion ^® 117 can be used.

In the present invention, a catalyst layer is directly formed on the hydrogen ion exchange membrane by a bridging process. As shown in FIG. 1, when the hydrogen ion exchange membrane, which is a polymer electrolyte, is treated in a polar aqueous solution in which the catalyst is highly dispersed according to the bridging process, the bridging phosphorus ( The effect of breathing in causes the surface of the hydrogen ion exchange membrane to swell. When the hydrogen ion exchange membrane is retreated in an aprotic solvent, the surface of the hydrogen ion exchange membrane expanded in a polar solvent shrinks due to a bridging out effect, and thus catalyst particles are physically formed on the surface of the hydrogen ion exchange membrane. It is uniformly applied by the force of phosphorus to form a layer.

In the above breeding process, the catalyst layer is uniformly formed by a simple method of repeating the number of times of breeding in and breeding out once, thereby increasing the utilization efficiency of the catalyst and increasing the reaction efficiency of the catalyst layer in which the electrochemical reaction occurs. By improving the overall performance of the fuel cell, the energy density and power density can be improved.

In the present invention, as the polar solvent that can be used in the breeding-in process, an alcohol such as water, 1-propyl alcohol, 2-propyl alcohol, or a mixture thereof may be used, and preferably water is used. In addition, aprotic solvents usable in the breeding out process include acetone, acetonitrile, and the like, and acetone is preferably used.

Pt is a metal catalyst of the catalyst layer formed on the hydrogen ion exchange membrane. In addition, Pt-Ru, Pt-Mn, Pt-Ru-Mo, Pt-Ru-W, etc. may be used to prevent catalyst poisoning by carbon monoxide (CO) generated during the catalytic reaction, and preferably Pt or Pt- Ru. However, the metal catalyst usable in the present invention is not limited thereto.

The polar solvent for forming the catalyst layer on the hydrogen ion exchange membrane may be prepared in the form of a slurry by adding about 30 times the solvent weight to the catalyst weight, and the amount of the solvent may be arbitrarily adjusted according to the amount of the catalyst added.

According to the present invention, the catalyst layer formed on the hydrogen ion exchange membrane may be arbitrarily adjusted according to the thickness and the amount of catalysts depending on the number of breeding processes. For example, the bridging process is repeated 2, 4, 6, and 8 times to produce a hydrogen ion exchange membrane. The thickness and amount of catalyst of the catalyst layer applied to the surface are controlled. Those skilled in the art can adjust the thickness and catalyst amount of the catalyst layer as necessary without departing from the technical spirit of the present invention.

In another aspect, the present invention provides a method for producing an MEA comprising forming a catalyst layer on a polymer hydrogen ion exchange membrane by a breathing process and then bonding the catalyst layer to an electrode.

The electrode includes a diffusion layer for even diffusion of fuel and air at both the anode and the cathode. For example, the diffusion layer is ultrasonically pulverized with carbon black vulcan XC-72 (E-tek), Teflon Emulsion PTFE 30, and a solvent to prepare a slurry, and then the slurry is brushed to obtain carbon paper (Toray Carbon Paper TGPH-090, Thickness: 275 μm).

The anode and cathode of the carbon paper having the diffusion layer formed thereon may be coated with a catalyst used to form a catalyst layer on the hydrogen ion exchange membrane, preferably Pt-Ru (molar ratio 1: 1), which is a commercial catalyst (Johnson Matthey Co.). Is used for the anode, and Pt black of Johnson Mattey Co. is used as catalyst for the cathode. The catalyst used for the electrode in the present invention is not limited to the above.

In order to form a catalyst layer in the diffusion layer, 5% by weight of Nafion solution (5% by weight of solids content) is added three times with respect to the weight of the catalyst, and a catalyst slurry is added by adding a solvent with a weight ratio of 30 times by weight to form a catalyst slurry. A catalyst layer is formed on the carbon paper diffusion layer.

In the above-described method, the electrode in which the catalyst layer is formed in the diffusion layer and the hydrogen ion exchange membrane in which the catalyst layer is formed are bonded under heating and pressurization to be made of MEA. The hydrogen ion exchange membrane and the electrode are joined by heating to a temperature of 120 to 150 ° C, preferably 135 ° C under a pressure of 1800 to 2500 psi, preferably 2000 psi. The overall amount of catalyst is from 1 to 5.0 mg / cm ^2, preferably from 1 to 3.0 mg / cm ^2, more preferably from 3.0 mg / cm ^2, and has from 0.01 to 0.8 mg by the breeding process proton exchange membrane on the diffusion layer of carbon paper a catalyst layer of / cm ² , preferably 0.5 mg / cm ² .

According to the method of the present invention, a polymer electrolyte membrane fuel cell (PEMFC) or a flat pack-type direct methanol fuel cell (DMFC) is prepared using a MEA including a hydrogen ion exchange membrane in which a catalyst layer is formed by a bridging process. It can manufacture. As an example, as shown in FIG. 2, in the direct methanol fuel cell according to the present invention, a non-conductive gasket 4 and a current collector 3 are disposed around the MEA 5. The support 2 is wrapped to form one unit cell. The unit cells are arranged on the left and right about the fuel container 1. Methanol or aqueous methanol solution is injected into the fuel container 1, and the left side of the MEA 5 in contact with the fuel is the anode, and the right side in contact with the air in the atmosphere is the cathode. The current collector plate may be used by coating platinum (Pt) or gold (Au) on a nickel mesh (Ni-mesh) or sus-mesh.

In the present invention, a method of forming a catalyst layer by a bridging process of a hydrogen ion exchange membrane forms a uniform and excellent contact layer with an electrode, which overcomes the problems caused by the conventional catalyst layer. As a result, the fuel cell MEA improved the utilization efficiency of the catalyst and the electrochemical reaction efficiency in the catalyst layer to improve the MEA performance of the fuel cell and at the same time increase the efficiency of the fuel cell.

Hereinafter, the effect of the manufacturing and performance testing of the MEA using the breeding process according to the present invention will be confirmed through the examples.

Example  One

A Nafion ^® 115 membrane was used as a polymer electrolyte for preparing a hydrogen ion exchange membrane in which a catalyst layer was formed. The hydrogen ion exchange membrane is immersed in a slurry made by adding Pt-Ru catalyst to secondary distilled water and highly dispersed to perform a swelling process for 5 minutes, and then immersed in an aprotic solvent to shrink out. Was carried out for 5 minutes. The breeding process was prepared with 8 iterations performed by Nafion ^® 115 membrane surface 0.5 mg catalytic amount of a proton exchange uniformly applied to a thickness of a film of 10 ㎛ per ㎠.

In manufacturing the electrode, both the anode anode and the cathode cathode should have a diffusion layer for even diffusion of fuel and air. The diffusion layer was prepared by dispersing carbon black vulcan XC-72 (E-tek), Teflon Emulsion PTFE 30, solvents ultrasonically, and then coating the 275 ㎛ thick Toray carbon paper TGPH-090 with a brush method. 1-propyl alcohol: 2-propyl alcohol: secondary deionized water was mixed in a volume ratio of 2: 2: 1 and used as a solvent of the diffusion layer.

Pt-Ru (molar ratio 1: 1), which is a commercial catalyst (Johnson Matthey Co.), was used as an anode catalyst for carbon paper on which the diffusion layer was formed, and Pt black of Johnson Mattey Co. was used as a catalyst for the cathode. After adding 300 mg of 5 wt% Nafion solution (solid content 5 wt%) to 100 mg of catalyst, 3000 mg of the solvent of the diffusion layer was added to form a catalyst slurry to form a catalyst layer on the diffusion layer by the spray method.

The electrode having the catalyst layer formed on the diffusion layer and the hydrogen ion exchange membrane on which the catalyst layer was formed were heated to 135 ° C. and pressurized at 2000 psi to prepare an MEA. The total catalyst amount was 3.0 mg / cm ² on the diffusion layer of carbon paper and 0.5 mg / cm ^{2 on} the hydrogen ion exchange membrane.

Anode Breeding (Pt / Ru: 0.5mg / cm ² ) + Spray (Pt / Ru: 3.0mg / cm ² ) Cathode Breeding (Pt / Ru: 0.5mg / cm ² ) + Spray (Pt: 3.0mg / cm ² )

The prepared MEA was tested with a flat pack type direct methanol fuel cell shown in FIG. 2 and tested for performance. As a fuel, 4 M, 6 M, and 8 M aqueous solutions of methanol were used, and the power density was measured according to the increase in current. Each result is shown in FIG.

Comparative example

After the diffusion layer was formed on the carbon paper in the same manner as in Example 1, the catalyst slurry was applied by spraying 3.5 mg / cm ² so that the total catalyst amount was the same as the catalyst amount used in Example 1 to form a catalyst layer. The catalyst amount was equalized using the spray method, and then compared with the catalyst layer forming method by breeding.

The hydrogen ion exchange membrane was heated (135 ° C.) and pressurized (2000 psi) without a bridging process as in a conventional MEA manufacturing method to prepare a MEA.

Anode Spray (Pt / Ru: 3.5 mg / cm ² ) Cathode Spray (Pt: 3.5 mg / cm ² )

The MEA prepared by the above-described method was prepared using the same flat pack type DMFC as in Example 1, and subjected to a performance evaluation test using 4 M, 6 M, and 8 M aqueous methanol solutions, and the results are shown in FIG. 6. .

Evaluation results

First, the surface of the Nafion ^® 115 membrane used as a hydrogen ion exchange membrane was subjected to surface analysis using ESEM according to the cycles of the catalyst layer formed by the bridging process as shown in FIG. 1 (cycles) (FIG. 3). The breeding process was repeated 2, 4, 6 and 8 times, and 0.21 mg / cm ² , 0.35 mg / cm ² , 0.46 mg / cm ² and 0.51 mg / cm ² catalyst were applied to the surface of the hydrogen ion exchange membrane. As shown in Fig. 3, it can be seen that the catalyst layer was formed fairly uniformly on the surface of the electrolyte.

The graph of Figure 4 shows the amount of catalyst according to the number of breeding process performed. As shown in the results of FIG. 3 and FIG. 4, the amount of catalyst can be adjusted according to the number of times of performing the bridging process, and catalyst particles are uniformly coated on the surface of the electrolyte membrane by the bridging process. In addition, a breeding process was carried out to form a catalyst layer on the Nafion ^® 115 membrane, which was then hot pressed to make the MEA. At this time, as shown in FIG. 5, the catalyst layer inside the MEA was kept uniform even after being heated and pressed using a hot press. The photo of FIG. 5 shows that the inner catalyst layer is uniform without change in shape after fabrication of MEA.

In the performance evaluation of the power density of the flat-pack type direct methanol fuel cell using the MEA prepared by the method of Example 1 and Comparative Example, the direct methanol of Example 1 as the current density increases as shown in FIG. The power density of the fuel cell is higher than that of the comparative example, indicating that the performance of the MEA is improved. These results are the same in 4 M, 6 M and 8 M methanol aqueous solution. MEA using a hydrogen ion exchange membrane having a catalyst layer formed by a bridging process has excellent contact between the electrode, the catalyst layer and the hydrogen ion exchange membrane, Since the dispersion is uniform, it can be seen that the electrochemical reactivity in the catalyst layer is improved.

The catalyst layer forming method utilizing the breeding characteristics of the hydrogen ion exchange membrane according to the present invention is a novel catalyst layer forming method applicable to the polymer electrolyte membrane fuel cell and the polymer electrolyte membrane for direct methanol fuel cell.

The method according to the present invention is completely different from the method of forming a catalyst layer by the spray method or the brush method, which is mainly used in the manufacture of membrane electrode assemblies (MEA), such as a polymer electrolyte fuel cell (PEMFC) or a direct methanol fuel cell (DMFC). In addition, since highly dispersed catalyst particles form a uniform catalyst layer, it is easy to control the deposition amount of the catalyst and increase the efficiency of using the catalyst to improve the performance of the fuel cell.

Claims (11)

In preparing a membrane electrode assembly for a fuel cell comprising a hydrogen ion exchange membrane and a fuel diffusion electrode, the hydrogen ion exchange membrane is first swelled by treating the hydrogen ion exchange membrane in a polar solvent containing catalyst particles, and then treated in an aprotic solvent to form a phase on the hydrogen ion exchange membrane. A method of forming a catalyst layer on a hydrogen ion exchange membrane of a membrane electrode assembly for a fuel cell, wherein the catalyst particles are coated on the catalyst particles. The method according to claim 1, wherein the hydrogen ion exchange membrane is a polymer electrolyte having physical properties that expand in a polar solvent and shrink in an aprotic solvent. The method of claim 2 wherein the polymeric material is perfluorosulfonic acid or perfluorocarbon sulfonic acid ionomer. The method of claim 1 wherein the hydrogen ion exchange membrane is a Nafion ^ㄾ 115 membrane or a Nafion ^ㄾ 117 membrane. The method of claim 1 wherein the polar solvent is selected from water, alcohols and mixtures thereof. The method of claim 1 wherein the aprotic solvent is acetone or acetonitrile. The method of claim 1 wherein the catalyst is selected from Pt, Pt-Ru, Pt-Mn, Pt-Ru-Mo and Pt-Ru-W. 8. The process of claim 7, wherein the catalyst is Pt or Pt-Ru. A membrane electrode assembly comprising a hydrogen ion exchange membrane having a catalyst layer formed by the method according to any one of claims 1 to 8. A direct methanol fuel cell comprising the membrane electrode assembly according to claim 9. A polymer electrolyte membrane fuel cell comprising the membrane electrode assembly according to claim 9.

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