JP5251303B2 - Gas diffusion layer for fuel cell, membrane electrode assembly and fuel cell using the same - Google Patents

Description

  The present invention relates to a gas diffusion layer used in a membrane electrode assembly of a fuel cell and a fuel cell using the same.

  A solid polymer fuel cell is known as one form of the fuel cell. Solid polymer fuel cells have a lower operating temperature (about -30 ° C to 120 ° C) than other types of fuel cells, and can be reduced in cost and size. Expected.

  As shown in FIG. 3, the polymer electrolyte fuel cell includes a membrane electrode assembly (MEA) 50 constituting a power generation unit as main components, which are used as a fuel (hydrogen) gas channel 51 a and an air gas channel. One fuel battery cell 53 is formed by being sandwiched by separators 52a and 52b including 51b and press-fitting. The membrane electrode assembly 50 is composed of an electrolyte membrane 55 that is an ion exchange membrane, and electrodes 58 and 58 including catalyst layers 56 and 56 formed on both surfaces thereof and gas diffusion layers 57 and 57 laminated on the outside of the catalyst layer. Composed.

  The catalyst layer 56 is a mixture of a particulate catalyst substance in which a catalytic metal such as platinum is supported on the surface of a conductive material such as carbon particles and an electrolyte resin dispersed in water or an organic solvent-based solution. A solution (catalyst substance-containing ink) is prepared, and this mixed solution is applied to a solid polymer electrolyte membrane, and then heated and dried to fix.

  The gas diffusion layer 57 has both gas permeability and electronic conductivity, and conventionally, a porous carbon support such as carbon paper or carbon cloth has been mainly used. In addition, the gas diffusion layer preferably has high water repellency to improve the drainage of water generated during power generation, and the surface of carbon paper or carbon cloth is coated with a resin solution such as polytetrafluoroethylene (PTFE). Then, a water repellency treatment may be performed to dry.

  As another gas diffusion layer, Patent Document 1 discloses, for example, conductive non-fibrous carbon particles (for example, carbon black powder, graphite flakes, etc.) composed of two groups having different particle diameters on the outer peripheral surface of a flexible reticulated polymer foam. The one covered with is described. Patent Document 2 describes a carbon fiber non-woven network in which a mixture of graphite particles (for example, graphitized carbon black) and a hydrophobic polymer such as PTFE is included.

JP 2006-529054 Gazette Special table 2008-503043 gazette

  The inventors of the present invention have continuously conducted many experiments and researches on gas diffusion layers for fuel cells. In the process, the membrane electrode assembly is sandwiched between and clamped with a fuel cell. In addition, when a large number of the fuel cells thus manufactured are stacked and pressed as a single stack, a separator is provided in the thickness direction of the diffusion layer constituting the membrane electrode assembly. Concentrated stress is locally generated by the convex portion caused by the shape of the gas flow path, etc., and the concentrated stress may propagate to the electrolyte membrane and damage the electrolyte membrane, and may further promote the damage. Experienced that. The tendency was great when granular carbon such as carbon black powder was used as the material of the diffusion layer as described in Patent Document 1.

  In addition, as described in Patent Document 2, when a mixture of graphite particles and a resin material (for example, a hydrophobic polymer such as PTFE) is used as a material for the diffusion layer, the degree of damage to the electrolyte membrane is relatively small. It was. This was interpreted as a result of the resin material performing a kind of buffering function and dispersing concentrated stress generated in the diffusion layer in the surface direction. However, it was inferior in gas diffusibility compared with a diffusion layer made of granular carbon. It was understood that this was because a part of the original void was blocked by the deformation of the resin material that occurred during stress dispersion.

  The present invention has been made on the basis of the facts actually experienced by the inventors as described above, and can suppress damage to the electrolyte membrane during pressing, and at the same time maintain high gas diffusibility. It is an object of the present invention to provide a gas diffusion layer for a fuel cell, a membrane electrode assembly including the gas diffusion layer, and a fuel cell.

  The gas diffusion layer for a fuel cell according to the present invention includes, as constituent materials, at least a first particle material that is carbon particles and a second particle material obtained by granulation from a mixture of carbon particles and a resin. It is characterized by that.

  In the gas diffusion layer, the amount of change in the thickness direction caused by the force in the thickness direction acting on the surface is suppressed by the first particle material. Further, the concentrated stress generated in the thickness direction is dispersed in the plane direction by the deformation of the second particle material including the resin material. As a result, while maintaining the porosity (ie, gas diffusibility) as a gas diffusion layer on the one hand, the concentrated stress from above propagates as a surface pressure to the lower layer on the other hand, and the lower layer (for example, electrolyte membrane) Damage can be prevented from occurring.

  The present invention is also a membrane electrode assembly comprising an electrolyte membrane, catalyst layers formed on both sides thereof, and gas diffusion layers laminated on the outside of each catalyst layer, wherein the gas diffusion layer is a gas diffusion layer as described above. A membrane electrode assembly that is a layer is also disclosed. Furthermore, a fuel battery cell in which the membrane electrode assembly is sandwiched between separators is also disclosed.

  In the fuel cell described above, at the time of fastening, the diffusion layer can reduce damage due to concentrated stress (film damage) applied to the electrolyte membrane while securing a gap for gas diffusion as described above. Therefore, it is possible to prevent the occurrence of gas cross leak due to the breakage of the electrolyte membrane over a long period of time. In addition, since the voids can be reliably secured, it is possible to avoid a phenomenon in which the generated water remains due to a decrease in drainage due to the decrease in the voids and radicals are generated to cause film deterioration. Thereby, the durability of the membrane electrode assembly and the fuel battery cell is greatly improved.

  By using the diffusion layer according to the present invention, it is possible to produce a membrane electrode assembly and a fuel cell excellent in durability.

  Hereinafter, the present invention will be described in more detail based on embodiments. FIG. 1 shows an example of a fuel cell using a diffusion layer according to the present invention.

  In FIG. 1A, 1 is an electrolyte membrane, and a catalyst layer 2 is laminated on both sides thereof. The electrolyte membrane 1 may be a conventionally known ion exchange membrane, may be a solid electrolyte resin thin film alone, or may be one in which a reinforcing layer made of a porous substrate such as PTFE is filled with an electrolyte resin. As the electrolyte resin, a perfluoro proton exchange resin is preferably used. Examples thereof include Nafion (trade name) manufactured by DuPont, Aciplex (trade name) manufactured by Asahi Kasei, Flemion (trade name) manufactured by Asahi Glass, and Gore-Select (trade name) manufactured by Japan Gore-Tex. The catalyst layer 2 can be formed by a conventionally known method. For example, it is formed by applying a catalyst solution to the electrolyte membrane 1. In that case, the electrolyte resin constituting the catalyst solution is preferably the same as the electrolyte resin constituting the electrolyte membrane, but may be different. The catalyst-carrying conductor that constitutes the catalyst solution is, for example, one in which a catalyst is carried on carbon particles, and a wide variety of known catalysts can be used. For example, noble metal catalysts such as platinum, gold, palladium, ruthenium and iridium are preferably used because the activation overvoltage in the catalytic reaction is small.

  Gas diffusion layers 3 and 3 are laminated on the catalyst layers 2 and 2. The gas diffusion layer 3 is obtained by granulating a first particle material 4 which is carbon particles as a constituent material, and a mixture of carbon particles a and a resin b which is schematically shown in FIG. 2 particulate material 5. Preferably, in the gas diffusion layer 3, the volume ratio of the first particle material 4 to the second particle material 5 is 70 to 30% by volume for the first particle material 4 and 30 for the second particle material 5. ~ 70% by volume. When the first particulate material exceeds 70% by volume, the second particulate material becomes too small as a result, and the required stress dispersion effect for the concentrated stress acting at the time of pressing may not be obtained. Further, if the first particle material 4 is less than 30% by volume, the second particle material 5 is too much as a result, and the porosity is too small due to the force acting at the time of the above-described pressing, so that the gas diffusion layer is required Gas diffusivity may not be obtained.

  In said gas diffusion layer 3, Preferably, the bulk density of the 1st particle material 4 is 1.5 or more, and the bulk density of the 2nd particle material 5 is 0.5 or less. When the bulk density of the first particle material 4 is less than 1.5, the first particle material 4 is deformed by the force acting at the time of pressing, and the porosity as the gas diffusion layer 3 is lower than the design value. There is a risk of doing. Further, when the bulk density of the second particulate material 5 exceeds 0.5, the amount of deformation caused by the force acting at the time of pressing described above becomes insufficient, and the concentrated stress acting can be dispersed in the plane direction as required. There is a risk of disappearing.

  In the gas diffusion layer 3 according to the present invention, the carbon particles constituting the first particle material 4 and the carbon particles a constituting the second particle material 5 may be the same or different. Preferably, the carbon particles constituting the first particle material 4 are granular graphite, mesophase carbon microbeads, and the like, and the average particle diameter is 0.5 to 100 μm. The carbon particles a constituting the second particle material 5 are carbon black, acetylene black and the like. The resin material b constituting the second particle material 5 can be any resin material provided that it has acid resistance to an acidic environment during operation of the fuel cell and can maintain its form. Examples of the fluororesin include PTFE, PVDF, and ETFE. The average particle diameter of the second particle material 5 obtained by granulation is 0.5 to 100 μm.

  The first particle material 4 that is carbon particles can be produced by any conventionally known method. For example, a pure carbon sphere having a high hardness and a bulk density of 1.5 or more can be obtained by baking a spherical product of a thermosetting resin (for example, a phenol resin) at 2000 to 3000 ° C.

  The second particulate material 5 obtained by granulation from a mixture of carbon particles a and resin b can also be produced by any conventionally known method. For example, there is a so-called spray drying method in which a mixed solution of carbon particles a, a resin b, and a solvent is sprayed from the tip of a nozzle in the form of a spray. The second particle material 5 having a desired particle diameter can be granulated by appropriately selecting the nozzle tip diameter, blowing pressure, and the like. In addition, production methods such as tableting and dry crushing granulation can be given as examples.

  In order to form the gas diffusion layer 3 by using the first particle material 4 and the second particle material 5, as shown in FIG. A method in which a mixture 6 in which one particle material 4 and second particle material 5 are mixed is sprayed to a required thickness and then heated and pressed, or as shown in FIG. 2B, the first particle material For example, a solution (ink for diffusion layer) 8 in which a mixture of 4 and the second particulate material 5 in a solvent 7 such as an aqueous solvent containing a surfactant or an organic solvent is mixed at a required ratio, and a suitable base material with a required weighing A method of removing the solvent by heating and drying after coating on the substrate 10 can be exemplified. In that case, examples of the base material include a releasable resin sheet, a catalyst layer constituting a membrane electrode assembly, and a carbon fiber sheet. When the resin sheet is used as a base material, the formed gas diffusion layer is transferred to a catalyst layer or the like. When a carbon fiber sheet is used as a base material, the whole can be used as a gas diffusion layer, and is laminated on a catalyst layer or the like by an appropriate method.

  Thus, by forming the diffusion layer 3 on the catalyst layer 2, the membrane electrode assembly 9 according to the present invention is formed, and both surfaces of the membrane / electrode assembly 9 are the fuel (hydrogen) gas flow path 51 a and the air gas as in the conventional case. One fuel cell A is formed by sandwiching the separators 52a and 52b including the flow path 51b and pressurizing and fastening them.

  In this fuel cell A, the gas diffusion layer 3 constituting the membrane electrode assembly 9 is obtained by granulating a first particle material 4 that is carbon particles and a mixture of carbon particles and a resin. The second particle material 5 disperses the concentrated stress in the surface direction by the second particle material 5 while securing a gap for gas diffusion by the first particle material 4 at the time of fastening. Thus, damage (film damage) due to concentrated stress applied to the electrolyte membrane 1 can be reduced. Thereby, it is possible to prevent the occurrence of gas cross leak due to the breakage of the electrolyte membrane 1 over a long period of time. In addition, since the voids can be ensured reliably, a phenomenon in which radicals are generated due to residual generated water and cause film deterioration can be avoided. Thereby, the durability of the membrane electrode assembly 9 and the fuel cell A is greatly improved.

Hereinafter, the present invention will be described by way of examples.
(A) Spherical phenol resin was fired at 2000 to 3000 ° C. to obtain high-hardness granular graphite having an average particle diameter of 50 μm and a bulk density of 1.5 or more. This was designated as the first particulate material A.
(B) A carbon black powder and a PTFE powder dissolved in an organic solvent were granulated by a spray drying method to obtain highly elastic particles having a bulk density of 0.5 or less and an average particle diameter of 50 μm. This was designated as the second particulate material B.
(C) As shown in Table 1, four types of materials in which the first particle material A and the second particle material B are mixed at different mixing ratios are prepared, and each material has a predetermined thickness on a base sheet. After spraying and heating and pressing, the substrate sheet was removed to obtain four types of gas diffusion layers.
(D) Four types of gas diffusion layers were placed on a horizontal base material, and an initial load of 0.5 MPa was applied from above, and the load was increased to 1.5 MPa. Table 1 shows the difference between the thickness when the initial load is applied and the thickness when the load is increased to 1.5 MPa as a thickness change amount.
(E) The above four types of gas diffusion layers were used, the other configurations and conditions were the same, and a membrane electrode assembly was prepared. . Supply hydrogen gas to the gas flow path of one separator and supply nitrogen gas to the gas flow path of the other separator at a pressure higher than the normal fuel cell operating state, and measure the time until gas cross leak occurs (Accelerated test). The results are shown in Table 1.

(F) Evaluation Compared with Comparative Example 1, both Examples 1 and 2 have a larger thickness change amount. It is presumed that in Examples 1 and 2, the second particulate material B is mixed by 30% or 70%, and as a result, the stress distribution in the surface direction progresses. In Comparative Example 1, since the mixing amount of the second particulate material B is 0%, and the thickness change due to crushing does not occur, it can be estimated that the thickness change amount is small compared. When Comparative Example 1 is used as a gas diffusion layer, concentrated stress may act on the electrolyte membrane, causing damage to the electrolyte membrane.

  On the other hand, the thickness change amount of Comparative Example 2 is larger than that of Examples 1 and 2. This is presumed that the material of Comparative Example 2 was too crushed because the second particle material B was 100% and the first particle material A having high hardness was not present. When the amount of change in thickness is increased, the amount of voids is reduced, so that the desired gas diffusibility may not be obtained in the gas diffusion layer of Comparative Example 2. In comparison, Examples 1 and 2 are understood to have the required porosity.

  Compared with Comparative Examples 1 and 2, the time until the occurrence of cross leak in the acceleration test is longer in Examples 1 and 2. In the case of Comparative Example 1, it is presumed that the concentrated stress acts on the electrolyte membrane to cause damage, and in the case of Comparative Example 2, the decrease in porosity causes a decrease in drainage, resulting in the generation of radicals. Presumed to have caused film deterioration. From this, it can be seen that the products of Examples 1 and 2 which are the products of the present invention contribute to the improvement of the durability of the fuel cell.

FIG. 1A is a schematic view showing an example of a fuel cell using a gas diffusion layer according to the present invention, and FIG. 1B is a schematic view showing a second particle material. The schematic diagram explaining two aspects in the case of manufacturing the gas diffusion layer by this invention. The schematic diagram which shows an example of a fuel cell.

Explanation of symbols

A ... fuel cell, 1 ... electrolyte membrane, 2 ... catalyst layer, 3 ... gas diffusion layer, 4 ... first particle material that is carbon particles, 5 ... obtained by granulation from a mixture of carbon particles and resin Second particle material, a ... carbon particles, b ... resin, 9 ... membrane electrode assembly, 52a, 52b ... separator

Claims (5)

A gas diffusion layer for a fuel cell, comprising as constituent materials at least a first particle material that is carbon particles and a second particle material obtained by granulation from a mixture of carbon particles and a resin , A gas diffusion layer in which the bulk density of the first particulate material is 1.5 or more and the bulk density of the second particulate material is 0.5 or less . The gas diffusion layer according to claim 1 , wherein the carbon particles constituting the first particle material are granular graphite. The gas diffusion layer according to claim 1 , wherein the carbon particles constituting the second particle material are carbon black, and the resin is a fluororesin. 4. A membrane electrode assembly comprising an electrolyte membrane, catalyst layers formed on both surfaces thereof, and gas diffusion layers laminated on the outside of each catalyst layer, wherein the gas diffusion layer is any one of claims 1 to 3. The membrane electrode assembly which is a gas diffusion layer as described in 2. A fuel cell comprising the membrane electrode assembly according to claim 4 sandwiched between separators.

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