JP2005100679A - Gas diffusion electrode, its manufacturing method, and solid polymer fuel cell using this - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas diffusion electrode for a solid polymer fuel cell in which a closing phenomenon caused by water hardly occurs even in a use for a long period and gas can be supplied stably, and a solid polymer fuel cell using the same. <P>SOLUTION: This gas diffusion electrode for the solid polymer fuel cell has carbon paper and a carbon layer that is formed on one side or both sides of the paper and is composed of carbon black and polytetrafluoroethylene. A thickness in which the carbon layer enters in the thickness direction of the carbon paper is in a range of 3-35 μm. This gas diffusion electrode can be manufactured by coating on one side or both sides of the carbon paper slurry in which carbon black particles and polytetrafluoroethylene particles having a number average particle size of 0.05-3 μm are dispersed in an alcohol system dispersion medium containing low boiling-point alcohol having a boiling point of 90°C or lower, and by calcining the coating layer. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a gas diffusion electrode for a polymer electrolyte fuel cell, a production method thereof, and a polymer electrolyte fuel cell using the same.

  A fuel cell is a power generation system that continuously supplies fuel and an oxidant and extracts chemical energy as electric power when they react. Fuel cells are roughly classified into alkaline, phosphoric acid, and solid polymer types that operate at relatively low temperatures, and molten carbonate and solid oxide electrolyte types that operate at high temperatures, depending on the type of electrolyte that uses them. The

Among these, the polymer electrolyte fuel cell has a gas diffusion electrode having a catalyst supported on both surfaces of a diaphragm acting as a solid polymer solid electrolyte, and a chamber (fuel) on the side where one gas diffusion electrode exists. Hydrogen as a fuel is supplied to the chamber), and an oxygen-containing gas such as oxygen or air as the oxidant is supplied to the chamber (oxidant chamber) where the other gas diffusion electrode is present. By connecting a load circuit, it acts as a fuel cell. In addition to hydrogen, alcohols such as methanol and ethanol may be used directly as fuel for the polymer electrolyte fuel cell. The basic structure of a single cell constituting a polymer electrolyte fuel cell is that a catalyst layer is arranged on both sides of an ion conductive electrolyte membrane, a gas diffusion electrode is arranged on both sides of the electrolyte membrane, and both sides of the outside are arranged. It is common to arrange a separator on the surface. The hydrogen introduced to the gas diffusion electrode surface through the gas flow path in the separator on the fuel electrode side is uniformly diffused by the gas diffusion electrode, is introduced to the catalyst layer on the fuel electrode side, and hydrogen is absorbed by a catalyst such as platinum. Hydrogen ions and charges are separated, and the hydrogen ions are guided to the catalyst layer in the oxygen electrode on the opposite side across the electrolyte through the electrolytic membrane. On the other hand, the electric charge generated on the fuel electrode side is led to a gas diffusion electrode on the oxygen electrode side through a circuit having a load, and further to a catalyst layer on the oxygen electrode side. At the same time, oxygen introduced from the separator on the oxygen electrode side generates water in the presence of the above charges and hydrogen ions that have reached the catalyst layer on the oxygen electrode side through the gas diffusion electrode on the oxygen electrode side. Complete the power generation cycle. Carbon paper is an example of the main member of the gas diffusion electrode used in the polymer electrolyte fuel cell.
The degree of hydrophobicity / hydrophilicity of the carbon paper used for the gas diffusion electrode is a very important design factor in the design of the fuel cell, and there are various cases such as when making the carbon paper hydrophilic or hydrophobic. is there. Further, the degree of hydrophobicity / hydrophilicity needs to be uniform in the surface direction of the gas diffusion electrode.

  One method of adjusting the degree of hydrophobicity / hydrophilicity of carbon paper is to select a solvent to be used for the carbon black slurry. For example, in the case of Patent Document 1, by applying a slurry using water as a dispersion medium on the surface of carbon paper that has been previously hydrophobized, carbon particles may enter the carbon paper. There is a device to prevent it. However, this method has a problem that when the degree of hydrophobicity of the carbon paper is uneven, a difference occurs in the degree of penetration of the carbon particles into the carbon paper. In addition, when a large amount of water repellent is used, the output voltage at high load may be low, which is not preferable. This is presumably because the pore size of the carbon paper is reduced by the water repellent and the gas supply is insufficient.

  In order to solve this problem, for example, in the case of Patent Document 2 and Patent Document 3, a device for imparting uniform hydrophobicity in a small amount is made using a solvent-soluble fluororesin. However, this method has a problem that the entire carbon paper becomes hydrophobic and the hydrophilic surface is extremely small, and therefore, particularly at the oxygen electrode, it becomes difficult to discharge moisture and excessive moisture. In addition, since most of the contact portions of carbon black and between carbon black and carbon paper are hydrophobized, there may be a problem that the resistivity of the cell increases.

  As described above, in addition to the method of water-repellent treatment of the entire carbon paper using a solvent-soluble fluororesin, for example, a fluororesin dispersion such as polytetrafluoroethylene is used, and fluorocarbon resin particles are used to form carbon fibers. There is known a method of subjecting a part of carbon black or the like to water repellent treatment. For example, Patent Document 4 and Patent Document 5 disclose a method in which isopropyl alcohol is added to an aqueous dispersion of polytetrafluoroethylene and carbon paper is immersed. However, in this method, by adding isopropyl alcohol to an aqueous dispersion of polytetrafluoroethylene, the surface active effect of the suspension stabilizer contained in the aqueous dispersion of polytetrafluoroethylene is reduced. There is a problem that the particles are aggregated and coarsened. As a result, carbon fibers and carbon black are locally water-repellent, so that gas supply and water discharge are non-uniform, which is not preferable in terms of fuel cell performance.

  Moreover, in the case of the above-mentioned patent document 5, carbon black is press-fitted into the carbon fiber layer constituting the carbon paper, but in this method, the carbon black is overfilled, so the gas supply path is There is a problem of narrowing. As a result, the gas supply is insufficient in an operating condition at a high load, which may cause a decrease in output voltage, which is not preferable.

  By the way, the degree of hydrophobicity / hydrophilicity required for carbon paper varies depending on the structure of the gas diffusion electrode. For example, Patent Document 5 discloses an electrode having a structure in which a carbon black layer and a catalyst layer are arranged on both front and back surfaces of a carbon fiber layer constituting carbon paper. In the electrode of this structure, the upper and lower sides of the voids present in the carbon fiber layer of the carbon paper are covered with the carbon black layer. Therefore, when the carbon paper is not hydrophobized, water is contained inside the carbon paper. As a result, there is a problem that the gas supply is remarkably hindered. Therefore, in the electrode of this structure, the inside of the carbon paper is hydrophobized so that the water blocking phenomenon does not easily occur. That is, in the electrode of this structure, the hydrophobic treatment of carbon paper is an essential requirement. On the other hand, the structure in which the carbon intermediate layer is disposed only on one side of the carbon paper is becoming more common. However, in the electrode of this structure, either the upper or lower part of the void existing in the carbon fiber layer of the carbon paper is caused by the carbon black layer. Since it is not covered, clogging with the water is difficult to occur, and the carbon paper may not necessarily be subjected to a hydrophobic treatment. As described above, the actual situation is that the degree of hydrophobicity and hydrophilicity required for carbon paper varies depending on the structure of the electrode.

  However, conventionally, when producing carbon paper, the properties of the slurry to be used have not always been sufficiently adapted to the degree of hydrophobicity / hydrophilicity. For example, when the hydrophobicity of carbon paper is strong, if a slurry containing only water as a dispersion medium is applied, the wettability of the slurry to the carbon paper is partially deteriorated. The thickness of the carbon black layer may be uneven. On the other hand, when similar slurry is applied to hydrophilic carbon paper, carbon black easily and randomly penetrates into the carbon paper, resulting in partial depressions and microcracks in the carbon black layer. Such a problem may occur and become non-uniform.

Patent Document 6 proposes an electrode having a structure in which hydrophobic layers are arranged on both front and back surfaces of carbon paper, and a contact layer containing carbon particles is provided on either side. The contact layer of this electrode is regarded as a so-called carbon intermediate layer, but in the structure of this gas diffusion electrode, the contact layer is not adjacent to the carbon paper, and the fixing property is due to the anchor effect of the carbon intermediate layer to the carbon fiber. Improvement cannot be expected. Further, according to the study by the present inventors, the hydrophobic layer is disposed on both the front and back surfaces of the carbon paper, so that a clogging phenomenon of generated water inside the carbon paper may occur, which is not necessarily a preferable configuration. That's not true.
JP-A-10-261421 JP 2001-351537 A JP-A-9-259893 JP 2001-236976 A JP 2001-160406 A JP 2002-117865 A

  The present invention has been made for the purpose of solving the above-described problems in the prior art. That is, an object of the present invention is to provide a gas diffusion electrode for a polymer electrolyte fuel cell that is less likely to be clogged with water even during long-term use and can stably supply gas, and a polymer electrolyte fuel cell using the same. Is to provide. Another object of the present invention is to provide a gas for a polymer electrolyte fuel cell capable of providing a uniform and uniform carbon intermediate layer regardless of the surface state, such as the degree of hydrophobicity / hydrophilicity of carbon paper. The object is to provide a method for manufacturing a diffusion electrode.

  The present inventor has found that in a gas diffusion electrode for a polymer electrolyte fuel cell, uniform surface smoothness can be obtained by entering a carbon layer in the range of 3 to 35 μm in the thickness direction of the carbon paper. The present invention has been completed.

  Therefore, the gas diffusion electrode for a polymer electrolyte fuel cell of the present invention has carbon paper and a carbon layer made of carbon black and polytetrafluoroethylene formed on one side or both sides thereof, The thickness of the carbon layer entering the paper in the thickness direction (hereinafter abbreviated as internal thickness) is 3 to 35 μm.

  The gas diffusion electrode for a polymer electrolyte fuel cell according to the present invention has a number average particle size of 0.05 in an alcohol-based dispersion medium containing a low-boiling alcohol having a boiling point of 90 ° C. or lower on one or both surfaces of carbon paper. It can be produced by coating a slurry in which carbon black particles and polytetrafluoroethylene particles in a range of ˜3 μm are dispersed, and calcining the coating layer.

  Further, the polymer electrolyte fuel cell of the present invention is characterized in that the catalyst layer is supported on the carbon layer of the gas diffusion electrode as a fuel electrode and an oxygen electrode, respectively, provided on both surfaces of the ion exchange membrane. And

  Since the gas diffusion electrode manufacturing method of the present invention has the above-described configuration, regardless of the hydrophobicity / hydrophilicity of the carbon paper, the gas has good gas supply properties and a gas in which a uniform carbon layer is arranged. A diffusion electrode can be produced. Therefore, the gas diffusion electrode of the present invention is less likely to be clogged with water even during long-term continuous use, and produces an excellent effect that gas can be stably supplied. Furthermore, by using this gas diffusion electrode, it is possible to produce a polymer electrolyte fuel cell having stable power generation performance with extremely little partial deterioration of the catalyst.

  In the gas diffusion electrode for a polymer electrolyte fuel cell of the present invention, the carbon layer formed on one side or both sides of the carbon paper is composed of carbon black and polytetrafluoroethylene. The film thickness is preferably in the range of 10 to 100 μm, and the thickness (hereinafter referred to as “internal thickness”) that the carbon layer enters in the thickness direction of the carbon paper needs to be in the range of 3 to 35 μm. . By setting the internal thickness within the range of 3 to 35 μm, uniform surface smoothness can be obtained. The reason is not necessarily clear, but the particles of carbon black and polytetrafluoroethylene suppress the sinking of the slurry into the carbon paper inside by filling the gaps between the carbon fibers on the coated surface of the carbon paper. As a result, it is considered that the occurrence of unevenness on the surface of the carbon layer is suppressed as much as possible. In addition, by adjusting the internal thickness within the above range, the occurrence of recessed holes and minute cracks in the carbon layer can be suppressed, but the reason why these defects do not occur is also considered to be due to the same reason as described above. As another effect, when the internal thickness of the carbon layer is within the above range, the anchoring effect of the carbon paper to the carbon fiber is expressed, so that the fixing property of the carbon layer to the carbon paper is improved. Become. On the other hand, when the internal thickness exceeds 35 μm, the carbon black particles invading into the carbon fiber layer become excessive, and the gas supply performance deteriorates. In particular, it is not preferable because the gas supply performance at high load deteriorates and the voltage decreases. On the other hand, when the internal thickness is less than 3 μm, the anchor effect of the carbon layer on the carbon paper cannot be expected.

  In the present invention, the carbon paper is preferably carbon paper hydrophobized with polytetrafluoroethylene. In that case, the impregnation amount of polytetrafluoroethylene is preferably 20% by weight or less. When the amount of polytetrafluoroethylene impregnation exceeds 20% by weight, the discharge of generated water on the oxygen electrode side becomes worse, particularly in the low temperature operating region where the generated water exists in liquid form, which adversely affects fuel cell performance. There is.

  The carbon layer in the present invention is formed using a slurry in which carbon black particles and polytetrafluoroethylene particles are dispersed. In that case, the carbon black and polytetrafluoroethylene contained in the slurry preferably have a number average particle diameter in the range of 0.05 to 3 μm. More preferably, it is the range of 0.1-3 micrometers, and it is especially preferable that it is the range of 0.2-3 micrometers. When these number average particle diameters exceed 3 μm, the carbon black has little penetration into the carbon fiber, but the surface roughness of the carbon layer becomes coarse, the adhesion with the catalyst layer decreases, and the electrical resistance at the interface decreases. Inconveniences such as rising. On the other hand, when the thickness is less than 0.05 μm, carbon black easily penetrates into the carbon paper together with the dispersion medium, and the large amount of carbon black that has penetrated causes clogging of the carbon paper, resulting in a problem in gas supply properties.

  In the present invention, an alcohol-based dispersion medium containing an alcohol having a boiling point of 90 ° C. or lower is used as the slurry dispersion medium. In the present invention, the alcohol-based dispersion medium is used for the following reason. That is, when only water is used as the dispersion medium of the slurry, the wettability of carbon black to water is not good, and therefore a surfactant such as Triton (manufactured by DuPont) needs to be used. In addition, when carbon black having a small particle size and a large specific surface area is made into a slurry, it is necessary to apply excessive mechanical stress in order to disperse the carbon black in water at a constant particle size. At this time, if the surfactant is mixed, bubbles are likely to be generated, which is not preferable as a coating liquid property. Therefore, in the present invention, an alcohol solvent having good wettability to carbon black is used as the dispersion medium of the slurry.

  Moreover, in this invention, alcohol, such as methanol, ethanol, propanol, etc. whose boiling point is 90 degrees C or less is used as alcohol for alcohol-type dispersion media. When an alcohol having a boiling point higher than 90 ° C. is used, the viscosity of the slurry becomes excessive and the coating property is deteriorated, which is not desirable.

  In the present invention, since an alcohol-based dispersion medium containing an alcohol having a boiling point of 90 ° C. or less is used, a smooth slurry suitable for roll coating such as an applicator can be obtained without applying excessive stress for dispersion. Can do. Such an alcohol-based dispersion medium does not require excessive mechanical stress for dispersion in order to wet carbon black well as described above. Therefore, the dispersed particle size in the slurry of carbon black is relatively large. can do. As a result, voids in the carbon layer are increased, and blockage due to gas supply and water is suppressed. Further, by using an alcohol having a boiling point of 90 ° C. or less, the solvent detachability from carbon black in coating becomes good, and a uniform carbon layer can be obtained in a short time.

  The alcohol-based dispersion medium preferably has a weight mixing ratio of ionic water and the alcohol in the range of water: alcohol = 5: 95 to 95: 5, particularly 5:95 to 50:50. When the water mixing ratio is more than 95%, the wettability of the carbon black particles to the solvent decreases, and the carbon black particles tend to aggregate with each other. As a result, the slurry becomes thixotropic, making coating difficult. There is a case. On the other hand, when the mixing ratio of the alcohol is more than 95%, the aggregates of the polytetrafluoroethylene particles become too large, and thus the hydrophobic portion is localized, which is not preferable in terms of fuel cell characteristics.

  In the present invention, aggregation of polytetrafluoroethylene is also suppressed by using water in combination while suppressing aggregation of carbon black with alcohol. Polytetrafluoroethylene tends to increase in agglomerates when the mixing ratio of alcohol increases, but when the mixing ratio of water and alcohol is in the above range, polytetrafluoroethylene and carbon black are aggregated with a particle size in the above range. By adjusting to a lump, it is possible to appropriately adjust the penetration of the slurry into the carbon paper regardless of the degree of hydrophobicity / hydrophilicity of the carbon paper.

In the method for preparing a slurry in the present invention, a mixed solvent is prepared by mixing alcohol having a boiling point of 90 ° C. or less and ion-exchanged water in the above range, and then a specified amount of carbon black is added to the obtained mixed solvent. To do. At that time, it is preferable to add carbon black as little as possible while stirring the mixed solvent, whereby generation of large aggregates of carbon black can be suppressed. After the entire amount of carbon black is added to the mixed solvent by the above operation, stirring is further continued. Next, an aqueous dispersion of polytetrafluoroethylene is added and further stirred. When the specific surface area of carbon black is as large as 200 m ² / g or more and the input amount of carbon black is large, it is desirable to apply a relatively strong mechanical stress. Examples of such a stirring device that can be applied with relatively strong mechanical stress include a dispersing device using a medium such as a ball mill, an attritor, and a paint shaker (manufactured by Red Devil), a biaxial roller, and a minute gap between the rotor and the stator. A dispersion apparatus or the like that makes the dispersion liquid finer by passing the liquid can be preferably used. Examples of the rotor-stator type dispersion device include a homomixer (manufactured by Tokushu Kika Kogyo Co., Ltd.). On the other hand, when the specific surface area of carbon black is 200 m ² / g or less, it is not necessary to apply the strong mechanical stress as described above, and for example, a blade-type stirring device such as a disper or an agitator can be used. As described above, the dispersing apparatus for making the exemplified slurry is not necessarily limited to these. Next, it is desirable to remove coarse agglomerates of 50 to 70 μm or more using a sieve having an appropriate mesh size for the purpose of suppressing the unevenness of the coated surface as much as possible.

  The slurry can be applied to the carbon paper using various commercially available coating apparatuses. As the coating method, roll coating, spray coating, bar coating, blade coating, screen printing, gravure printing, etc. can be used, but it is necessary to select an appropriate coating device depending on the surface roughness of the carbon paper. is there. When the difference between the concave and convex portions on the surface of the carbon paper exceeds 20 μm at the maximum, it is desirable to use a so-called flattening coater that is easy to obtain surface smoothness such as roll coating. When the difference is 20 μm or less, the coating method The degree of freedom of selection increases, and for example, a coater that can quantify the coating amount, such as a so-called contour coater using a die head or the like, can be used. The coating apparatus illustrated above is for illustration only, and is not necessarily limited thereto.

  The coated product obtained by the above operation is put into a drier set at a temperature of 30 to 80 ° C. and dried in a range of 3 to 24 hours. The amount of drying air may be determined as appropriate, but rapid drying is undesirable because it may induce micro cracks on the surface. The coated product after drying is put into a baking furnace or a high-temperature dryer, heated at 300 ° C. for 5 to 20 minutes to decompose and remove the surfactant, and then calcined at 380 ° C. for 5 to 20 minutes. Then, the polytetrafluoroethylene is melted, and the carbon blacks and the carbon black and the carbon fibers are bound. Thereby, a gas diffusion electrode is produced.

  In the polymer electrolyte fuel cell of the present invention, a catalyst layer formed on the carbon layer of the gas diffusion electrode is used as a fuel electrode and an oxygen electrode, and the catalyst layer surface is in contact with the fluorine-based ion exchange membrane. And has a structure as a membrane / electrode assembly.

Hereinafter, the present invention will be described in more detail based on examples and comparative examples. However, the present invention is not limited to these examples.
Example 1
A slurry having carbon black and polytetrafluoroethylene as solid contents was prepared by the following procedure. That is, 10 g of carbon black (VALCAN XC72B, manufactured by Cabot) was added to a dispersion medium composed of 175 g of isopropyl alcohol (boiling point 82.4 ° C.) and 173 g of ion-exchanged water, and 30 minutes with a paint shaker (manufactured by Red Devil). Stir. Next, a dispersion of polytetrafluoroethylene (30J, manufactured by Mitsui DuPont Fluorochemical Co., Ltd.) is added to the slurry so that the actual weight of polytetrafluoroethylene is 3 g, and then a 1 liter poly container is used. Then, the mixture was rotated and stirred at 100 rm for 30 minutes by a ball mill. The weight ratio of water to alcohol in this mixed solution was water: alcohol = 50: 50. Next, the obtained slurry was passed through a 200 mesh sieve to remove coarse agglomerates from the slurry to obtain a coating liquid (paint 1). When the particle size of the solid content (carbon black and polytetrafluoroethylene) contained in this paint 1 was measured with Coulter Counter TA-2 (manufactured by Coulter Counter), the number average particle size was 1.2 μm. Next, the paint 1 is applied by spraying uniformly to carbon paper (TGP-H-090, manufactured by Toray Industries, Inc.) having a thickness of 270 μm that has not been subjected to a hydrophobization treatment, and then allowed to dry at 60 ° C. for 30 minutes. did. Then, it left still for 10 minutes in 380 degreeC oven, and obtained the gas diffusion electrode. When the thickness of this gas diffusion electrode was measured with a micrometer, it was 290 μm. Since the carbon paper before coating was 270 μm in thickness, it was confirmed that a carbon layer was present on the carbon paper with a thickness of 20 μm. When the cross section of the gas diffusion electrode was observed with an electron microscope, it was confirmed that a carbon layer was present from the surface of the carbon paper to a depth of 15 μm. Further, it was confirmed that no carbon black was mixed in the lower layer. Further, when the surface of the gas diffusion electrode was observed with a microscope, it was confirmed that the formed carbon layer had a uniform surface without any depressions or microcracks being observed.

Example 2
A gas diffusion electrode was obtained in the same manner as in Example 1 except that the coating amount used by spraying was increased using the coating material 1 used in Example 1. As a result, it was confirmed that a carbon layer having a thickness of 30 μm was present on a carbon paper having a thickness of 270 μm. When the cross section of the obtained gas diffusion electrode was observed with an electron microscope, it was confirmed that a carbon layer was present from the surface of the carbon paper to a depth of 20 μm. Further, it was confirmed that no carbon black was mixed in the lower layer. Further, when the surface of the gas diffusion electrode was observed with a microscope, it was confirmed that the formed carbon layer had a uniform surface without any depressions or microcracks being observed.

Example 3
A coating solution (paint 2) was prepared in the same manner as in Example 1 except that 20 g of isopropyl alcohol and 328 g of ion-exchanged water were used in the preparation of the coating solution of Example 1, and the gas diffusion electrode was processed in the same manner. Got. The paint 2 used here was water: alcohol = 94.3: 5.7 (weight ratio). When the particle size of the solid content (carbon black and polytetrafluoroethylene) contained in the paint 2 was measured with Coulter Counter TA-2 (manufactured by Coulter Counter), the number average particle size was 2.7 μm. It was confirmed that the obtained gas diffusion electrode had a carbon layer having a thickness of 25 μm on a carbon paper having a thickness of 270 μm. According to cross-sectional observation with an electron microscope, it was confirmed that the carbon layer was present in the surface of the carbon paper at a depth of about 5 μm. Further, it was confirmed that no carbon black was mixed in the lower layer. Further, when the surface of the gas diffusion electrode was observed with a microscope, it was confirmed that the formed carbon layer had a uniform surface without any depressions or microcracks being observed.

Example 4
In the preparation of the coating liquid of Example 1, a coating liquid (paint 3) was prepared in the same manner as in Example 1 except that the amount of alcohol was 320 g and the amount of ion-exchanged water was 28 g. A gas diffusion electrode was obtained. The coating material 3 used here was water: alcohol = 8.6: 91.4 (weight ratio). When the particle size of the solid content (carbon black and polytetrafluoroethylene) contained in the paint 3 was measured with Coulter Counter TA-2 (manufactured by Coulter Counter), the number average particle size was 0.2 μm. It was confirmed that the obtained gas diffusion electrode had a carbon layer having a thickness of 15 μm on a carbon paper having a thickness of 270 μm. After peeling and removing only the carbon layer on the carbon paper, a cross-sectional observation of the carbon layer that entered the carbon paper with an electron microscope revealed that the carbon layer was present in the surface of the carbon paper at a depth of about 27 μm. It was confirmed. Further, it was confirmed that no carbon black was mixed in the lower layer. Further, when the surface of the gas diffusion electrode was observed with a microscope, it was confirmed that the formed carbon layer had a uniform surface without any depressions or microcracks being observed.

Example 5
Using the coating solution of Example 1, coating was performed with an applicator instead of spraying. The gap of the applicator was adjusted to 500 μm. Other operations were performed in the same manner as in Example 1 to obtain a gas diffusion electrode. In the obtained gas diffusion electrode, it was confirmed that a carbon layer was present with a thickness of 20 μm on carbon paper. According to cross-sectional observation with an electron microscope, it was confirmed that the carbon layer was present in the surface of the carbon paper at a depth of 33 μm. Further, it was confirmed that no carbon black was mixed in the lower layer. Further, when the surface of the gas diffusion electrode was observed with a microscope, it was confirmed that the formed carbon layer had a uniform surface without any depressions or microcracks being observed.

Example 6
The carbon paper used in Example 1 was impregnated with a dispersion having a polytetrafluoroethylene content of 1% by weight, dried at 80 ° C. for 3 hours, and then allowed to stand in an oven at 380 ° C. for 10 minutes. The carbon paper was hydrophobized. It was confirmed that 1.2% by weight of polytetrafluoroethylene was present in the obtained hydrophobic carbon paper from the weight ratio with respect to the untreated carbon paper. Using this hydrophobic carbon paper, a gas diffusion electrode was obtained in the same manner as in Example 1. In the obtained gas diffusion electrode, it was confirmed that a carbon layer was present with a thickness of 20 μm on carbon paper. According to cross-sectional observation with an electron microscope, it was confirmed that the carbon layer was present in the surface of the carbon paper at a depth of 19 μm. Further, it was confirmed that no carbon black was mixed in the lower layer. Further, when the surface of the gas diffusion electrode was observed with a microscope, it was confirmed that the formed carbon layer had a uniform surface without any depressions or microcracks being observed.

Example 7
The carbon paper used in Example 1 was impregnated with a dispersion having a polytetrafluoroethylene content of 15% by weight, dried at 80 ° C. for 3 hours, and then allowed to stand in an oven at 380 ° C. for 10 minutes. The carbon paper was hydrophobized. The obtained hydrophobized carbon paper was confirmed to contain 18.2% by weight of polytetrafluoroethylene based on the weight ratio with respect to the untreated carbon paper. Using this hydrophobic carbon paper, a gas diffusion electrode was obtained in the same manner as in Example 1. The obtained gas diffusion electrode was confirmed to have a carbon layer having a thickness of 23 μm on the carbon paper. According to cross-sectional observation with an electron microscope, it was confirmed that the carbon layer was present in the surface of the carbon paper at a depth of 10 μm. Further, it was confirmed that no carbon black was mixed in the lower layer. Further, when the surface of the gas diffusion electrode was observed with a microscope, it was confirmed that the formed carbon layer had a uniform surface without any depressions or microcracks being observed.

Comparative Example 1
In Example 1, a gas diffusion electrode of Comparative Example 1 was produced in the same manner as in Example 1 except that the screen printing method was used instead of the spray spray application. The obtained gas diffusion electrode was confirmed to have a carbon layer having a thickness of 10 μm on the carbon paper. According to cross-sectional observation with an electron microscope, it was confirmed that the carbon layer was present in the surface of the carbon paper at a depth of 51 μm. Further, when the surface of the carbon layer was observed with a microscope, it was confirmed that the upper layer portion of the carbon layer was peeled off by the screen mesh and the surface state was non-uniform.

Comparative Example 2
In the preparation of the coating liquid of Example 1, a coating liquid (paint 4) was prepared and treated in the same manner as in Example 1 except that the amount of alcohol was 348 g and the amount of ion-exchanged water was not used. Thus, a gas diffusion electrode was obtained. Since the water contained in the polytetrafluoroethylene dispersion was mixed in the coating material 4 used here, the ratio of water: alcohol was 0.6: 99.4 (weight ratio). When the particle size of the solid content (carbon black and polytetrafluoroethylene) contained in the paint 4 was measured by Coulter Counter TA-2 (manufactured by Coulter Counter), the number average particle size was 0.03 μm. In the obtained gas diffusion electrode, it was confirmed that a carbon layer was present on the carbon paper with a thickness of 7 μm. According to cross-sectional observation by an electron microscope, it was confirmed that the carbon layer was present up to a depth of 65 μm in the surface of the carbon paper, and that carbon black was also randomly present in the lower layer.

Comparative Example 3
In the preparation of the coating liquid of Example 1, a coating liquid (paint 5) was prepared in the same manner as in Example 1 except that alcohol was not used and ion-exchanged water was changed to 348 g. Obtained. The dispersion medium of the paint 5 used here was 100% water. When the particle size of the solid content (carbon black and polytetrafluoroethylene) contained in this paint 5 was measured with Coulter Counter TA-2 (manufactured by Coulter Counter), the number average particle size was 3.5 μm. In the obtained gas diffusion electrode, it was confirmed that a carbon layer was present on the carbon paper with a thickness of 40 μm. According to cross-sectional observation with an electron microscope, it was confirmed that the carbon layer was present up to a depth of 50 μm in the surface of the carbon paper. When the surface of this gas diffusion electrode was observed with a microscope, it was confirmed that there were some carbon agglomerates of about 30 to 40 μm in some places and the surface state was non-uniform.

Comparative Example 4
The carbon paper used in Example 1 was impregnated with a dispersion having a polytetrafluoroethylene content of 25% by weight, then dried at 80 ° C. for 3 hours, and then allowed to stand in an oven at 380 ° C. for 10 minutes. The carbon paper was hydrophobized. It was confirmed that 29.3% by weight of polytetrafluoroethylene was present in the obtained hydrophobic carbon paper from the weight ratio with respect to the untreated carbon paper. Other operations were performed in the same manner as in Example 1 to obtain a gas diffusion electrode. It was confirmed that a carbon layer having a thickness of 22 μm was present on the carbon paper. According to cross-sectional observation with an electron microscope, it was confirmed that the carbon layer was present up to a depth of 48 μm in the surface of the carbon paper.

Comparative Example 5
A gas diffusion electrode was obtained in the same manner as in Example 1 except that isobutanol (boiling point 107 ° C.) was used instead of isopropyl alcohol used in Example 1. It was confirmed that a carbon layer having a thickness of 23 μm was present on the carbon paper of the obtained gas diffusion electrode. According to cross-sectional observation with an electron microscope, it was confirmed that the carbon layer was present up to a depth of 48 μm in the surface of the carbon paper. Further, when the surface was observed with a microscope, it was confirmed that minute cracks were generated and the surface state was uneven.

Comparative Example 6
In Example 1, the gas diffusion electrode was obtained by reducing the spray coating amount. It was confirmed that a carbon layer having a thickness of 9 μm was present on the carbon paper of the obtained gas diffusion electrode. According to cross-sectional observation with an electron microscope, it was confirmed that the carbon layer was present at a depth of 1 μm in the surface of the carbon paper. When the surface was observed with a microscope, it was confirmed that many recessed holes in the carbon layer were generated and the surface state was uneven. In this gas diffusion electrode, the carbon layer was easily peeled off from the carbon paper, and the characteristic evaluation as a fuel cell was impossible.

Using the gas diffusion electrodes of the above Examples and Comparative Examples, polymer electrolyte fuel cells were produced by the following method and the characteristics were evaluated. 10 parts by weight of a catalyst supporting 20% by weight of platinum and 90 parts by weight of butyl acetate were mixed with 80% by weight of carbon black (Ketchin EC, manufactured by Lion Akzo) and dispersed by an ultrasonic cleaner for 10 minutes. With respect to 100 parts by weight of the obtained dispersion, 200 parts by weight of a solution obtained by dissolving 5% by weight of perfluorosulfonic acid resin (Nafion, manufactured by DuPont) in a mixed solvent of water and ethyl alcohol, and a tetrafluoroethylene disperser John (30J, Mitsui DuPont Fluoro Chemical Co., Ltd.) 5 parts by weight was added and mixed, and further dispersed with an ultrasonic cleaner for 30 minutes to obtain a catalyst ink. The obtained catalyst ink was applied on the carbon layers of the gas diffusion electrodes of Examples and Comparative Examples so that the amount of platinum was 0.8 mg / cm ² and dried. The two gas diffusion electrodes of the example and the comparative example in which the catalyst layer is formed by performing the above-described treatment are used as a fuel electrode and an oxygen electrode, respectively, so that the catalyst layer surface is in contact with an ion exchange membrane (Nafion 117 manufactured by DuPont). And bonded with a hot press (140 ° C.) to prepare an electrode / membrane assembly. The electrode area was 25 cm ² . This electrode / membrane assembly was incorporated into a cell to form a single cell.

Hydrogen and oxygen were used as the supply gas to each cell. All the supply gases were humidified by bubbling and pressurized, and then operated in the stable operation region. Table 1 shows the results of examining the voltage at a current density of 1 A / cm ² .

As is clear from the results in Table 1, all of the gas diffusion electrodes of Examples 1 to 7 of the present invention have a small voltage drop during continuous operation. This is considered because the gas diffusion electrodes of Examples 1 to 7 have both high gas permeability and uniform surface properties. On the other hand, in the case of Comparative Examples 1 to 5, the voltage drop during the continuous operation is large, and the initial performance is lowered except in the case of Comparative Example 3. In the case of Comparative Example 1, since the carbon layer was pressed deeply and densely into the carbon paper by the screen printing squeegee, the gap between the carbon black and between the carbon black and the carbon fiber was small, and the gas supply performance was deteriorated. It is thought that the initial voltage was lowered. Moreover, it is considered that the generated water is easily clogged, and the voltage drop is larger than that in the example. Further, the fact that the surface condition is worse than in the case of the example is considered to be a factor causing partial deterioration of the catalyst. In the case of Comparative Example 2, since the dispersed particle diameter of carbon is small, carbon black is excessively and deeply infiltrated between the carbon fibers of the carbon paper, so that the gas supply performance is deteriorated and clogging with generated water occurs. It is easy to think that the initial and long-term performances were degraded. In the case of Comparative Example 3, since the dispersed particle size of the carbon black was large, the gas supply property at the initial stage was relatively good, but the surface property was non-uniform, so that the partial deterioration of the catalyst progressed. It is easy to assume that the voltage drop during long-term use became prominent. In the gas diffusion electrode of Comparative Example 4, since the amount of polytetrafluoroethylene impregnated in the carbon paper was excessive, and the porous fiber layer in the carbon paper with polytetrafluoroethylene became close to the closed state, the gas supply property It seems that the initial and long-term performance deteriorated. However, since the hydrophobicity of the carbon paper was good, no clogging phenomenon due to water was observed, and the voltage was stable at a low value.
As described above, as a phenomenon common to the cases of Comparative Examples 1 to 5, it is considered that the gas supply performance is lowered and the initial performance is lowered due to the paint penetrating deeply into the carbon paper. With the increase, it is considered that the ability to discharge generated water to the outside of the fuel cell system decreased, and the voltage decreased during long-time operation.

Claims (6)

In a gas diffusion electrode for a polymer electrolyte fuel cell having carbon paper and a carbon layer made of carbon black and polytetrafluoroethylene formed on one or both surfaces thereof, the carbon layer is disposed in the thickness direction of the carbon paper. A gas diffusion electrode for a polymer electrolyte fuel cell, wherein the thickness of the gas diffusion electrode is in the range of 3 to 35 μm. 2. The gas diffusion electrode for a polymer electrolyte fuel cell according to claim 1, wherein the carbon paper is a hydrophobized carbon paper containing polytetrafluoroethylene having an impregnation amount of 20% by weight or less. A slurry in which carbon black particles and polytetrafluoroethylene particles having a number average particle diameter of 0.05 to 3 μm are dispersed in one or both surfaces of carbon paper in an alcohol-based dispersion medium containing a low-boiling point alcohol having a boiling point of 90 ° C. or less. The method for producing a gas diffusion electrode for a polymer electrolyte fuel cell according to claim 1, wherein the coating layer is calcined. The method for producing a gas diffusion electrode for a polymer electrolyte fuel cell according to claim 3, wherein the alcohol-based dispersion medium is a mixture of ion-exchanged water and an alcohol having a boiling point of 90 ° C or lower. 5. The method for producing a gas diffusion electrode for a polymer electrolyte fuel cell according to claim 4, wherein the weight mixing ratio of the ion-exchanged water and the alcohol having a boiling point of 90 ° C. or less is 5:95 to 95: 5. In the polymer electrolyte fuel cell in which the fuel electrode and the oxygen electrode are respectively provided on both surfaces of the ion exchange membrane, the fuel electrode and the oxygen electrode are made of carbon paper and carbon black and polytetrafluoroethylene formed on one or both surfaces thereof. A gas diffusion electrode having a carbon layer and a catalyst layer provided on the carbon layer, the thickness of the carbon layer entering in the thickness direction of the carbon paper is in the range of 3 to 35 μm. A polymer electrolyte fuel cell characterized by the above.