JP2006179317A - Gas diffusion layer and fuel cell using this - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas diffusion layer further enhanced in water draining property and gas diffusion property even at a low temperature. <P>SOLUTION: The gas diffusion layer is composed by forming a water repellent layer 120 on a gas diffusion substrate 110, containing carbon particles 121 coated with a water repellent material in the water repellent layer, and continuously passing carbon particles 122 subjected to hydrophilic treatment through in the thickness direction in at least one part of the water repellent layer. By the effect of the carbon particles subjected to hydrophilic treatment, water penetration paths can be secured. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a gas diffusion layer, and more particularly to a gas diffusion layer with improved drainage.

  In recent years, in response to social demands and trends against the background of energy and environmental issues, polymer electrolyte fuel cells that can operate at room temperature and obtain high output density have attracted attention as power sources for electric vehicles and stationary power sources. . The solid polymer fuel cell is characterized by using a film-like solid polymer electrolyte membrane.

  The polymer electrolyte fuel cell generally has a structure in which a membrane-electrode assembly (hereinafter also referred to as “MEA”) is sandwiched between separators. The MEA has a structure in which two electrodes are disposed on both sides of a solid polymer electrolyte membrane and this is sandwiched between gas diffusion layers. The electrode is a porous material formed by a mixture of a solid polymer electrolyte and an electrode catalyst in which catalyst particles are supported on a conductive carrier, and is also called an electrode catalyst layer.

  In the polymer electrolyte fuel cell, electricity can be taken out through the following electrochemical reaction. First, hydrogen contained in the fuel gas supplied to the fuel electrode (anode) side is oxidized by the catalyst particles to become protons and electrons. Next, the generated protons pass through the solid polymer electrolyte contained in the fuel electrode side electrode catalyst layer and the solid polymer electrolyte membrane in contact with the electrode catalyst layer, and reach the oxygen electrode (cathode) side electrode catalyst layer. . Electrons generated in the fuel electrode side electrode catalyst layer include a conductive carrier constituting the electrode catalyst layer, a gas diffusion layer in contact with a different side of the electrode catalyst layer from the solid polymer electrolyte membrane, a separator, and It reaches the oxygen electrode side electrode catalyst layer through an external circuit. The protons and electrons that reach the oxygen electrode side electrode catalyst layer react with oxygen contained in the oxidant gas supplied to the oxygen electrode side to generate water.

  Such an electrochemical reaction mainly proceeds at a three-phase interface where catalyst particles, a solid polymer electrolyte, and a reaction gas such as a fuel gas or an oxidant gas are in contact with each other. Therefore, the gas diffusion layer is required to uniformly supply the supplied reaction gas to the electrode.

  In addition, a solid polymer electrolyte membrane in a fuel cell does not exhibit high proton conductivity unless it is wet. Therefore, it is necessary to make the humidity distribution of the solid polymer electrolyte membrane uniform by humidifying the reaction gas supplied to the solid polymer fuel cell using a gas humidifier or the like.

  Under operating conditions such as high humidification and high current density, the amount of water that moves with the protons that move through the polymer electrolyte membrane from the anode to the cathode, and the water that forms in the oxygen electrode-side electrode catalyst layer The amount of generated water increases. At this time, these produced waters particularly stay in the oxygen electrode-side electrode catalyst layer and cause a flooding phenomenon that closes the pores that have become the reaction gas supply path. As a result, the diffusion of the reaction gas is inhibited, the electrochemical reaction is hindered, and as a result, the battery performance is lowered.

  Therefore, conventionally, various attempts have been made to prevent the flooding phenomenon by improving the drainage of the gas diffusion layer. For example, Patent Literature 1 discloses a gas diffusion layer having a gradient in the content of the water repellent in the gas diffusion base material.

  Patent Document 2 discloses a gas diffusion layer having a water repellent layer containing carbon black and a fluororesin on the surface of the gas diffusion substrate. The water repellent layer has a porous structure made of an aggregate of carbon black coated with a fluororesin. Therefore, according to the water repellent layer, it is possible not only to diffuse the reaction gas supplied by humidification and supply it uniformly by the electrode catalyst layer, but also to quickly drain excess water.

Further, Patent Document 3 discloses a gas diffusion layer with improved drainage and the like by imparting a gradient to the water repellent, pore diameter, etc. in the water repellent layer from the gas inlet side to the gas outlet side. It is disclosed.
JP 2003-109604 A JP 2003-89968 A JP 2003-92112 A

  However, the conventional gas diffusion layers such as Patent Documents 1 to 3 described above may not be able to obtain sufficient drainage. Moreover, in the said patent document 1 and the said patent document 3, it is necessary to manage gradients, such as a water repellent, and a manufacturing method is complicated.

  Also, at low temperatures such as in winter, the temperature of the fuel cell is significantly lower than the proper operating temperature when stopped. In particular, in a fuel cell when the temperature is below freezing, water generated in the electrode catalyst layer may stay in the gas diffusion layer and freeze. Therefore, in the conventional gas diffusion layers such as Patent Documents 1 to 3 described above, diffusion of the reaction gas is inhibited by freezing of water, and it is difficult to restart the fuel cell in a short time. Further, in order to restart in a short time, a large amount of energy is required to raise the temperature of the fuel cell.

  Then, an object of this invention is to provide the gas diffusion layer which can improve drainage more and can exhibit the outstanding gas diffusibility also at the time of low temperature.

  The present invention solves the above-mentioned problem by a gas diffusion layer having a water permeation path in at least a part of the water repellent layer in a gas diffusion layer having a water repellent layer on a gas diffusion base material.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the gas diffusion layer whose drainage property was improved more by having the permeation | transmission path | route of water in at least one part of a water repellent layer. Furthermore, when the temperature is below freezing point, the water freezes in the water permeation path, so that it is possible to secure a gas diffusion path in a portion other than the water permeation path. This makes it possible to provide a gas diffusion layer that can exhibit excellent gas diffusibility even at low temperatures.

  The first of the present invention is a gas diffusion layer having a water repellent layer on a gas diffusion base material, and having a water permeation path in at least a part of the water repellent layer.

  The water permeation path in the water-repellent layer refers to sucking water contained in an electrode catalyst layer adjacent to the water-repellent layer or a fuel cell when it is assembled, and guiding the water to the gas diffusion base material. It has a function of discharging.

  In the present invention, it has been found that the drainage property of the gas diffusion layer can be further improved by providing the water permeation path to at least a part of the water repellent layer. Furthermore, in the water-repellent layer, water can be actively guided into the path at the portion having the water permeation path and drained to the gas diffusion base material, whereby the gas diffusion path can be formed at the other portions. It can be secured.

  This prevents flooding in the gas diffusion layer even when a large amount of generated water is generated during the operation of the fuel cell, etc., and uniformly diffuses the gas supplied from the outside to supply it to the electrode catalyst layer, etc. Can do. Furthermore, since water can be actively taken into the water permeation path, even in an environment where the water contained therein is prone to freeze when the fuel cell is stopped, the water can flow only within the water permeation path. It is possible to freeze and secure a gas diffusion path at other sites.

  The water permeation path in the water repellent layer is preferably formed so as to penetrate in the thickness direction. Thereby, water can be more reliably discharged | emitted to a gas diffusion base material. In the water-repellent layer, the shape of the water permeation path is not particularly limited as long as it is formed so as to penetrate in the thickness direction, and may be any of a straight line shape, a mesh shape, and the like.

  In the gas diffusion layer of the present invention, the water permeation path in the water-repellent layer is preferably made of carbon particles that have been subjected to hydrophilic treatment. A water-repellent layer having at least a part of a water permeation path containing the hydrophilically treated carbon particles will be described with reference to FIG.

  In FIG. 1, for convenience of explanation, the existence form of the carbon particles in the water repellent layer is exaggerated, but the existence form of the carbon particles is not limited to the illustrated form.

  FIG. 1 is a schematic view of a gas diffusion layer according to a preferred embodiment of the present invention, which has a configuration in which a water repellent layer 120 is formed on a gas diffusion base 110, and the water repellent layer 120 is a water repellent. The carbon particles 121 are coated on each other, and at least a part of the water repellent layer 120 includes hydrophilicly treated carbon particles 122. Thus, in the water-repellent layer 120, the hydrophilically treated carbon particles 122 are continuously present, so that a water permeation path can be secured by penetrating in the thickness direction, and the hydrophilically treated carbon particles 122. It becomes possible to guide water into the path due to the hydrophilic effect.

  Specifically, the hydrophilically treated carbon particles include carbon particles coated with a hydrophilic agent; carbon whose surface has been modified to be hydrophilic by introducing hydrophilic groups such as carboxyl groups and phenol groups. Particles. Among these, carbon particles that are coated with a hydrophilic agent are preferred because the hydrophilically treated carbon particles are preferably easy to produce.

  In the carbon particles coated with the hydrophilic agent, the carbon particles may be any carbon particles having excellent electron conductivity, and examples thereof include carbon black powder, graphite powder, and expanded graphite powder. Carbon blacks such as oil furnace black, channel black, lamp black, thermal black, and acetylene black are preferred because of their excellent electron conductivity and large specific surface area.

  Commercially available products can be used as the carbon particles, such as Cabot Vulcan XC-72, Vulcan P, Black Pearls 880, Black Pearls 1100, Black Pearls 1300, Black Pearls 2000, Legal 400, Ketjen Black EC manufactured by Lion. Oil furnace black such as # 3150 and # 3250 manufactured by Mitsubishi Chemical Corporation; acetylene black such as Denka Black manufactured by Denki Kagaku Kogyo Co., Ltd. and the like. In addition to carbon black, artificial graphite or carbon obtained from organic compounds such as natural graphite, pitch, coke, polyacrylonitrile, phenol resin, and furan resin may be used. Moreover, in order to improve corrosion resistance etc., you may process the said carbon particle, such as a graphitization process.

  The particle diameter of the carbon particles is preferably 0.2 to 5 μm, more preferably 0.3 to 1 μm. If the particle size of the carbon particles exceeds 5 μm, the surface may become rough and the contact resistance may increase, and if it is less than 0.2 μm, the porosity of the water-repellent layer may decrease and the gas diffusibility may decrease. .

  Next, as the hydrophilic agent in the carbon particles coated with the hydrophilic agent, sulfone such as perfluorocarbon sulfonic acid polymer represented by Nafion (registered trademark by DuPont) and Flemion (registered trademark by Asahi Kasei Co., Ltd.) can be used. A polymer containing a hydrophilic group such as an acid group is preferred.

  In the carbon particles coated with the hydrophilic agent, it is desirable that the entire surface of the carbon particles is coated with the hydrophilic agent. However, it is included in the concept of carbon particles coated with a hydrophilic agent even if part of the carbon particle surface is exposed without being coated with the hydrophilic agent or is in contact with other adjacent carbon particles. Shall be.

  Further, when carbon particles coated with a hydrophilic agent are used in the water penetration path, the mixing ratio of the hydrophilic agent in the water repellent layer and the carbon particles coated with the hydrophilic agent is preferably a mass ratio, preferably 30:70 to 70:30, more preferably about 30:70 to 50:50. If the content of the hydrophilic agent is too large, the porosity in the water permeation path may decrease, and if it is too small, the water permeation path may not be sufficiently formed.

  Preferred examples of the carbon particles subjected to the hydrophilic treatment include carbon particles obtained by heat treatment in an air atmosphere in addition to those listed above.

In this application, as a result of examining the carbon particles having hydrophilicity, it was found that the oxide film formed on the surface of the carbon particles by heat treatment in an air atmosphere has hydrophilicity. The reason why such an effect is obtained is not clear, but the following may be considered. That is, when carbon particles are manufactured, impurities such as sodium and phosphorus are mixed due to mixing of furnace impurities and the like in a firing process in a carbon raw material carbonization process or a roll process. Therefore, carbon particles contain elements such as sodium and phosphorus. Therefore, it is considered that by oxidizing such carbon particles, an oxide film containing a hydrophilic oxide containing sodium and phosphorus such as sodium phosphate (NaPO ₃ ) is formed on the surface of the carbon particles. It is done.

  The carbon particles are not particularly limited, and examples thereof include the same carbon particles as those listed for the carbon particles coated with a hydrophilic agent.

  The heat treatment of the carbon particles is preferably performed at 500 to 1500 ° C., more preferably 700 to 1000 ° C., for about 0.5 to 1 hour. If the heat treatment temperature is less than 500 ° C., a sufficient oxide film may not be formed on the surface of the carbon particles, and if it exceeds 1500 ° C., all the carbon particles may be thermally decomposed, which is not desirable.

  In the carbon particles heat-treated in the air atmosphere, the thickness of the oxide film is preferably 50 to 200 nm, more preferably about 100 to 150 nm. If the thickness of the oxide film is less than 50 nm, sufficient hydrophilicity may not be obtained, and if it exceeds 200 nm, the thermal conductivity may decrease. In addition, the thickness of the oxide film of the carbon particles can be measured by elemental analysis.

  In order to further improve the hydrophilicity of the carbon particle surface, an element that forms a substance that exhibits hydrophilicity by oxidation treatment may be added to the carbon particle surface in advance before the heat treatment.

Examples of substances that exhibit hydrophilicity by oxidation treatment include oxides containing alkali and sodium such as sodium phosphate such as sodium phosphate (NaPO ₃ ), oxides of alkali metals such as potassium and lithium, magnesium, calcium, etc. And at least one metal oxide selected from the group consisting of oxides of alkaline earth metals.

  Therefore, elements added in advance to the carbon particles include metal elements constituting the metal oxide, such as alkali metals such as phosphorus, sodium, potassium, and lithium, and alkaline earth metals such as magnesium and calcium. These metal elements can be oxidized by an oxidation treatment such as a heat treatment in an air atmosphere to form a substance that exhibits hydrophilicity.

The metal elements may be used alone or in combination of two or more. The metal element may be added in advance to the carbon particles as a compound containing the metal element, such as sodium chloride (NaCl) or phosphorous lime (Ca ₅ F (PO ₄ ) ₃ ).

  In the carbon particles heat-treated in the air atmosphere, the content of the substance that exhibits hydrophilicity by the oxidation treatment is 5 to 40 wt%, preferably 10 based on the mass of the carbon particles heat-treated in the air atmosphere. It should be -30 wt%, more preferably 15-20 wt%. Thereby, the carbon particle which has moderate hydrophilic property is obtained. Therefore, in consideration of this, it is advisable to adjust the amount of element added in advance to the carbon particle surface. Further, the content of a substance that exhibits hydrophilicity by oxidation treatment can be measured by elemental analysis.

  In the water-repellent layer of the present invention, the content of the above-mentioned hydrophilically treated carbon particles is preferably 30 to 70% by volume, more preferably 30 to 50% by volume, particularly preferably the volume of the water-repellent layer. It is good to set it as 30-40 volume%. Thereby, a water permeation path penetrating in the thickness direction can be formed in at least a part of the water repellent layer. If the content of the hydrophilically treated carbon particles is less than 30% by volume, there is a possibility that a water permeation path penetrating in the thickness direction in the water repellent layer cannot be formed sufficiently. Moreover, when it exceeds 70 volume%, there exists a possibility that a gas diffusion path | route cannot fully be ensured. The content of the hydrophilically treated carbon particles in the water repellent layer can be measured by elemental analysis.

  The water-repellent layer in the gas diffusion layer of the present invention contains carbon particles coated with a water-repellent agent except for the portion where the water permeation path is formed.

  In the carbon particles coated with the water repellent, the types and particle diameters of the carbon particles are the same as those described for the hydrophilically treated carbon particles, and thus detailed description thereof is omitted here.

  Next, as the water repellent agent in the carbon particles coated with the water repellent agent, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyhexafluoropropylene, tetrafluoroethylene-hexafluoropropylene copolymer Fluorine resin such as (FEP), polypropylene, polyethylene and the like. Of these, fluorine resins are preferably used because of their excellent hydrophobicity and corrosion resistance during electrode reaction.

  In the water repellent layer, the mixing ratio of the carbon particles and the water repellent is such that if there are too many carbon particles, the water evaporation ability may decrease, and if there are too many water repellents, the desired pore size may not be obtained. There is. Considering these, the mixing ratio of the carbon particles coated with the water repellent in the water repellent layer and the water repellent is preferably about 90:10 to 60:40 in terms of mass ratio.

  The average pore diameter of the water repellent layer is preferably 10 nm or less, more preferably 1 to 10 nm, and particularly preferably 3 to 7 nm. By setting the average pore diameter in the water-repellent layer to 10 nm or less, it is possible to make it difficult for water vapor to condense in the water-repellent pores formed by the carbon particles coated with the water-repellent agent. It becomes possible to ensure more reliably. In addition, water can be easily condensed in the water permeation path formed by hydrophilically treated carbon particles, etc., and the surface tension of the condensed water can improve the drainage property to the gas diffusion base material in the water permeation path. It becomes possible to raise. Further, in the water repellent layer, a freezing point lowering phenomenon occurs due to the interaction between the water in the pores and the pore walls. As a result, the freezing point of water contained in the water repellent layer can be lowered, and water freezing can be suppressed even when the external temperature is below freezing point.

  In order to set the average pore diameter in the water repellent layer to 10 nm or less, it is preferable to appropriately adjust the particle diameter of the carbon particles contained in the water repellent layer, and the contents of the water repellent and hydrophilic agent. The average pore diameter of the water repellent layer is the average value of the pore diameter of the water repellent layer measured by the mercury intrusion method.

  As a gas diffusion base material used for the gas diffusion layer of the present invention, any conventionally known material can be used without particular limitation. Specifically, a sheet-like material having conductivity and porosity such as a carbon woven fabric, a paper-like paper body, a felt, and a non-woven fabric can be used. More specifically, carbon paper, carbon cloth, carbon non-woven fabric and the like are preferable. A commercial item can also be used for the said gas diffusion base material, For example, Toray Co., Ltd. carbon paper TGP series, E-TEK company carbon cloth, etc. are mentioned.

  The thickness of the gas diffusion base material may be appropriately determined in consideration of the characteristics of the obtained gas diffusion layer, but may be about 30 to 500 μm. If the thickness is less than 30 μm, sufficient mechanical strength or the like may not be obtained, and if it exceeds 500 μm, the distance through which gas or water permeates is undesirably increased.

  The gas diffusion base material preferably contains a water repellent in order to further improve drainage and prevent flooding. Although it does not specifically limit as said water repellent, It is fluorine-type, such as a polytetrafluoroethylene (PTFE), a polyvinylidene fluoride (PVDF), a polyhexafluoropropylene, a tetrafluoroethylene-hexafluoropropylene copolymer (FEP). Examples thereof include polymer materials, polypropylene, and polyethylene.

  The method for producing the gas diffusion layer of the present invention is not particularly limited, and a known method can be used.

  First, as a water repellent treatment method for a gas diffusion substrate, a general water repellent treatment method may be used. Examples include a method in which a base material used for a gas diffusion layer is immersed in an aqueous dispersion solution or an alcohol dispersion solution of a water repellent, and then heated and dried in an oven or the like. In view of ease of exhaust gas treatment during drying, it is preferable to use an aqueous dispersion solution of a water repellent.

  Next, as a method for producing the water repellent layer, first, a case where carbon particles coated with a hydrophilic agent in the water permeation path are used will be described as an example.

  A water repellent-containing slurry is prepared by dispersing carbon particles, a water repellent, and the like in a solvent such as water, alcohol solvents such as perfluorobenzene, dichloropentafluoropropane, methanol, and ethanol. Next, using a hydrophilic agent instead of the water repellent, a hydrophilic agent-containing slurry is prepared in the same manner as the preparation of the water repellent-containing slurry.

  In order to form a water repellent layer using the water repellent-containing slurry and the hydrophilic agent-containing slurry obtained as described above, (i) the water repellent-containing slurry and the hydrophilic agent-containing slurry are mixed at a predetermined volume ratio. And (ii) mixing the water-repellent-containing slurry and the hydrophilic agent-containing slurry at a predetermined volume ratio, and then mixing the resulting mixed slurry with a predetermined volume ratio. Once dried and pulverized into powder, and this is applied to the gas diffusion substrate, (iii) Each of the water repellent-containing slurry and the hydrophilic agent-containing slurry is dried and pulverized into a powder. The method of apply | coating on a gas diffusion base material after mixing, etc. can be used. As a result, a water-repellent layer formed of carbon particles coated with a water repellent agent at least partially having a water permeation path composed of carbon particles coated with a hydrophilic agent is obtained. . Further, the mixing ratio of the water repellent-containing slurry and the hydrophilic agent-containing slurry may be appropriately determined so that the obtained water repellent layer has a desired water permeation route.

  Next, in the case of using carbon particles that are heat-treated in an air atmosphere in the water permeation path, the water repellent layer is produced in the same manner as above in a water repellent-containing slurry prepared in the same manner as above. After mixing the heat-treated carbon particles, a method of applying and drying the obtained mixed slurry on the gas diffusion substrate may be used. The mixed slurry may be filtered and dried, then pulverized with a pulverizer such as a commercially available ball mill, and the powder may be dispersed and applied to a gas diffusion substrate.

  The water repellent layer obtained by each method described above is preferably subjected to heat treatment at about 250 to 400 ° C. using a muffle furnace or a firing furnace. Thereby, the strength of the water repellent layer can be increased.

  As described above, the water repellent layer in the gas diffusion layer of the present invention can be produced by a simple method. Furthermore, the gas permeation path and the water permeation path in the water repellent layer can be easily controlled by adjusting the addition amount of the water repellent, the hydrophilic agent, and the carbon particles heat-treated in the air atmosphere. .

  A second aspect of the present invention is a fuel cell MEA having the gas diffusion layer described above. As described above, the first gas diffusion layer of the present invention not only improves drainage by having a water permeation path, but also ensures a gas diffusion path even when the external temperature is below freezing point. Is possible. Therefore, by using the gas diffusion layer of the present invention as the MEA for fuel cells, it is possible to realize the start-up in a short time even under a low external temperature condition such as below freezing while preventing the flooding phenomenon. It is possible to provide a fuel cell MEA that is excellent in power generation performance (also simply referred to as “MEA”).

  A preferred embodiment of the MEA of the present invention will be described with reference to a schematic cross-sectional view shown in FIG. In FIG. 2, MEA 200 is arranged on both sides of solid polymer electrolyte membrane 210 with anode side electrode catalyst layer 231 and anode side gas diffusion layer 232, cathode side electrode catalyst layer 241 and cathode side gas diffusion layer 242, respectively. Have a configuration.

  The first gas diffusion layer of the present invention needs to be disposed so that the water repellent layer and the electrode catalyst layer are in contact with each other. Thus, excess water contained in the electrode catalyst layer can be absorbed by the water repellent layer and discharged to the outside through the gas diffusion base material, and flooding phenomenon in the electrode catalyst layer can be prevented.

  The configuration of the MEA is not particularly limited except that the first gas diffusion layer of the present invention is used, and various conventionally known techniques may be appropriately referred to. Therefore, the detailed description regarding the anode side electrode catalyst layer, the cathode side electrode catalyst layer, and the solid polymer electrolyte membrane is omitted here.

  In the MEA, the first gas diffusion layer of the present invention may be used for at least one of the anode-side gas diffusion layer and the cathode-side gas diffusion layer. Therefore, it is preferably used for both the anode-side gas diffusion layer and the cathode-side gas diffusion layer.

  The MEA may be produced according to a conventionally known method. For example, a method of hot pressing after forming an electrode catalyst layer on the gas diffusion layer described above, sandwiching the solid polymer electrolyte membrane using two of them, and the like can be mentioned.

  A third aspect of the present invention is a fuel cell using the above-described gas diffusion layer or MEA. According to the above-described gas diffusion layer or MEA of the present invention, the flooding phenomenon can be suppressed even under operating conditions in which a large amount of water such as high current density and high humidification is likely to be generated. Even under low temperature conditions, it is possible to secure a gas diffusion path and realize startup in a short time, and to provide a fuel cell having excellent power generation performance.

  The type of fuel cell is not particularly limited as long as desired cell characteristics can be obtained, but it is preferably used as a polymer electrolyte fuel cell (PEFC) from the viewpoints of practicality and safety. Thereby, a highly reliable fuel cell can be obtained as a power source for moving bodies and a stationary power source.

  The structure of the fuel cell is not particularly limited, and examples thereof include a structure in which MEA is sandwiched between separators.

  The separator has a function of separating air and fuel gas, and any conventional separator can be used without particular limitation. As the separator, conventionally known separators such as those made of carbon such as dense carbon graphite and a carbon plate and those made of metal such as stainless steel can be used without particular limitation. Moreover, in order to ensure the flow path of air and fuel gas, a gas distribution groove | channel may be formed and a conventionally well-known technique can be utilized suitably. The shape of the separator is not particularly limited, and may be appropriately determined in consideration of the output characteristics of the obtained fuel cell.

  Furthermore, a stack in which a plurality of MEAs are stacked via a separator and connected in series may be formed so that the fuel cell can obtain a desired voltage or the like. The shape of the fuel cell is not particularly limited, and may be determined as appropriate so that desired battery characteristics such as voltage can be obtained.

  A fourth aspect of the present invention is a method for producing hydrophilic carbon particles. In the present invention, as a result of examining the hydrophilicity of the carbon particles, it was found that an oxide film having hydrophilicity is formed on the surface of the carbon particles by heat treatment in an air atmosphere, whereby hydrophilic carbon particles are obtained. The reason why such an effect is obtained is as described in the first aspect of the present invention.

  Specific examples of the method for producing the hydrophilic carbon particles include a method including a step of heat-treating the carbon particles at 500 to 1500 ° C. in an air atmosphere.

  The carbon particles used for the production of the hydrophilic carbon particles may be any conventionally known carbon particles, specifically, the same as the carbon particles listed in the carbon particles coated with the hydrophilic agent in the first of the present invention. Is used.

By heat-treating the carbon particles in an air atmosphere, an oxide film containing a hydrophilic oxide is formed on the carbon particle surfaces. The oxide exhibiting hydrophilicity may be an oxide containing sodium and phosphorus, such as sodium phosphate (NaPO ₃ ).

  The heat treatment is preferably performed at 500 to 1500 ° C., more preferably 700 to 1000 ° C. for about 0.5 to 1 hour. If the heat treatment temperature is less than 500 ° C., a sufficient oxide film may not be formed on the surface of the carbon particles, and if it exceeds 1500 ° C., all the carbon particles may be thermally decomposed, which is not desirable.

  The heat treatment may be performed in a furnace such as a muffle furnace or a firing furnace, or may be performed by flame treatment using a gas burner using an inert gas as a fuel, such as a hydrogen gas burner or an acetylene gas burner.

  In the method of the present invention, in order to further improve the hydrophilicity of the obtained hydrophilic carbon particles, carbon particles previously added with an element that forms a substance that exhibits hydrophilicity by oxidation treatment may be used. That is, before the heat treatment step, a step of previously adding an element that forms a substance that exhibits hydrophilicity by oxidation treatment to the carbon particles may be included.

Examples of the substance which exhibits hydrophilicity by the oxidation treatment include oxides containing sodium and phosphorus such as sodium phosphate (NaPO ₃ ), alkali metal oxides such as potassium and lithium, and alkalis such as magnesium and calcium. And at least one metal oxide selected from the group consisting of oxides of earth metals.

Therefore, examples of the element added in advance to the carbon particles include metal elements constituting the metal oxide, such as alkali metals such as phosphorus, sodium, potassium, and lithium, and alkaline earth metals such as magnesium and calcium. . These metal elements can be oxidized by an oxidation treatment such as a heat treatment in an air atmosphere to form a substance that exhibits hydrophilicity. As the metal element, only one kind may be added, or two or more kinds may be added in consideration of the hydrophilicity of the obtained hydrophilic carbon particles. The metal element may be added in advance to the carbon particles as a compound containing the metal element, such as sodium chloride (NaCl) or phosphorous lime (Ca ₅ F (PO ₄ ) ₃ ).

  As a method of adding the metal element to the carbon particles, a conventionally known method may be appropriately used. For example, a method of impregnating carbon particles with a solution containing the metal element is used.

The solution containing the metal element can be prepared by adding the compound containing the metal element constituting the metal oxide described above to the solvent, and then stirring and heating. Examples of the compound include sodium chloride (NaCl) and phosphorous lime (Ca ₅ F (PO ₄ ) ₃ ).

  The solvent used in the solution containing the metal element is not particularly limited, and water and / or an organic solvent is used. It does not specifically limit as said organic solvent, Ethanol, methanol, propanol, etc. are mentioned, These may be used individually by 1 type and may be used in mixture of 2 or more types.

  The carbon particles can be impregnated with the solution containing the metal element by dispersing the carbon particles in the solution and then sufficiently stirring.

  As the stirring means, a conventionally known method such as a homogenizer or an ultrasonic stirrer may be used, and these stirring means may be appropriately combined. Further, if necessary, means for allowing the solution to penetrate into the details of the carbon particles by ultrasonic irradiation or vacuum degassing may be further added.

  The addition amount of the element that forms a substance that exhibits hydrophilicity by the oxidation treatment added in advance to the carbon particles is not particularly limited, but is 5 to 50% by mass, preferably 10 to 30% by mass with respect to the carbon particles. It is good to do. If the addition amount is less than 5% by mass, the effect obtained by adding the element in advance may be small, and if it exceeds 50% by mass, the thermal conductivity may decrease. In order to make the addition amount within the above range, the concentration of the metal element in the solution containing the metal element, the amount of the carbon particle impregnated with the solution containing the metal element, and the like may be appropriately adjusted.

  Thereafter, the carbon particles impregnated with the solution containing the metal element may be filtered using a known separation means such as suction filtration, and the heat treatment in the air atmosphere described above may be performed.

  Further, the carbon particles impregnated with the solution containing the metal element are heated while appropriately stirring the mixed solution of the solution containing the metal element and the carbon particles with a rotary evaporator or the like in addition to suction filtration. The carbon particles to which the metal element has been added in advance can also be obtained by removing the solvent contained in the mixed solution by using a method such as the above.

  The method of adding the metal element to the carbon particles is not limited to the above method using the solution containing the metal element, and other physical methods such as heating vapor deposition, electron beam, sputtering, ion plating, etc. Conventionally known methods such as vapor deposition can be referred to as appropriate.

  According to the method of the present invention, the carbon particle surface can be modified to be hydrophilic by a simple method. The hydrophilic carbon particles obtained by the above method can be used as carbon particles that have been subjected to hydrophilic treatment in the water-repellent layer included in the first gas diffusion layer of the present invention. In addition, since the hydrophilic carbon particles obtained by the above method are conductive hydrophilic carbon and have the ability to retain water, the electrode catalyst used for the electrode catalyst layer for fuel cells is used. It is also suitably used for the carbon support of the above.

  Hereinafter, the present invention will be described more specifically with reference to examples. In addition, this invention is not limited to the following Example.

Example 1
(1) Water repellent treatment of gas diffusion base material Carbon paper (TGP-H-060 manufactured by Toray Industries, Inc., thickness 180 μm) was used as the gas diffusion base material, and after punching it into 100 mm × 100 mm square, PTFE aqueous A dispersion solution (D1 manufactured by Daikin Industries, Ltd., containing 60% by mass of PTFE) was immersed in a solution diluted to a predetermined concentration with pure water for 2 minutes, and then dried in an oven at 60 ° C. for 10 minutes to give a gas. The diffusion substrate was subjected to a water repellent treatment. At this time, the PTFE content in the carbon paper was 20% by mass.

(2) Production of water-repellent layer First, carbon black (product name Vulcan XC72R, average particle diameter 0.7 μm), the same aqueous PTFE dispersion solution as in the above step (1), and water were mixed with carbon black and PTFE. Was mixed and dispersed with a homogenizer for 3 hours to form a slurry, thereby obtaining a PTFE-containing slurry. Next, carbon black (product name Vulcan XC72R, average particle diameter 0.7 μm), Nafion solution (registered trademark DuPont DE520, Nafion content 5 wt%), and 2-propanol are mixed in a mass ratio of carbon particles to Nafion. The mixture was dispersed and slurried with a homogenizer for 3 hours to obtain a Nafion-containing slurry.

  PTFE-containing slurry and Nafion-containing slurry were mixed at a volume ratio of 30:70 for 1 hour with a homogenizer, and the resulting mixed slurry 1 was applied onto a gas diffusion substrate that had been previously water-repellent treated using a bar coater. After drying in an oven at 80 ° C. for 1 hour, firing was further performed at 350 ° C. in an muffle furnace for 1 hour. As a result, a water repellent layer having a water permeation route at least in part was produced on the gas diffusion substrate. The water repellent layer had a thickness of 50 μm, an area of 100 mm × 100 mm, the content of carbon particles coated with Nafion was 30% by volume, and the average pore diameter was 100 nm.

(Example 2)
Hydrophilic carbon particles with an oxide film formed on the surface by heat treatment of carbon black (product name Vulcan XC72R, average particle size 0.7 μm) in a muffle furnace at 700 ° C. for 1 hour in an air atmosphere 1 was obtained. The hydrophilic carbon particles 1 are added to the same PTFE-containing slurry prepared in the step (2) of Example 1 so that the volume ratio of the PTFE-containing carbon particles to the hydrophilic carbon particles is 30:70. Then, the slurry was mixed and dispersed with a homogenizer for 3 hours to obtain a mixed slurry 2.

  Except that the obtained mixed slurry 2 was used instead of the mixed slurry 1, in the same manner as in Example 1, after applying onto the gas diffusion substrate that had been subjected to the water-repellent treatment previously, drying and firing, A water repellent layer having a water permeation route at least partially was prepared on the gas diffusion substrate. The water repellent layer had a thickness of 50 μm, an area of 100 mm × 100 mm, the content of hydrophilic carbon particles was 30% by volume, and the average pore diameter was 100 nm.

(Example 3)
After adding 5 g each of sodium chloride (NaCl) and phosphorous lime (Ca ₅ F (PO ₄ ) ₃ ) as compounds to 500 ml of benzene, the mixture was stirred at a temperature of 80 ° C. to obtain Na and A solution containing P was prepared. After adding 5 g of carbon black (product name Vulcan XC72R, average particle size 0.7 μm) to 500 ml of the solution, the mixture was stirred for 1 hour with a homogenizer and filtered to obtain carbon black impregnated with the solution. . The carbon black impregnated with the solution was heat-treated in a muffle furnace in an air atmosphere at 700 ° C. for 1 hour to obtain hydrophilic carbon particles 2 having an oxide film formed on the surface. The hydrophilic carbon particles 2 are added to the same PTFE-containing slurry prepared in the step (2) of Example 1 so that the volume ratio of the PTFE-containing carbon particles to the hydrophilic carbon particles is 30:70. Then, the mixture was dispersed for 3 hours with a homogenizer to obtain a mixed slurry 3.

  Except that the obtained mixed slurry 3 was used in place of the mixed slurry 1, in the same manner as in Example 1, after applying on the gas diffusion substrate that had been subjected to the water-repellent treatment previously, drying and firing, A water repellent layer having a water permeation route at least partially was prepared on the gas diffusion substrate. The water repellent layer had a thickness of 50 μm, an area of 100 mm × 100 mm, the content of hydrophilic carbon particles was 30% by volume, and the average pore diameter was 100 nm.

Example 4
Example except that carbon black (product name Vulcan XC72R, average particle size 0.7 μm) used for the PTFE-containing slurry and Nafion-containing slurry was previously pulverized with a planetary ball mill to an average particle size of 0.2 μm. In the same manner as in No. 1, a water-repellent layer having a water permeation route at least partially was prepared on the gas diffusion substrate. The water repellent layer had a thickness of 50 μm, an area of 100 mm × 100 mm, the content of carbon particles coated with Nafion was 30% by volume, and the average pore diameter was 8 nm.

(Comparative Example 1)
First, carbon black (product name Vulcan XC72R, average particle diameter 0.7 μm), the same PTFE aqueous dispersion solution as in step (1) of Example 1, and water, and carbon black and PTFE in a mass ratio of 80. : PTFE-containing slurry was obtained by mixing and dispersing in a homogenizer for 3 hours to form a slurry. The PTFE-containing slurry was applied using a bar coater on the gas diffusion substrate previously treated for water repellency in the same manner as in Example 1, then dried in an oven at 80 ° C. for 1 hour, and then further muffled. A water repellent layer composed of PTFE and carbon particles was produced on a gas diffusion substrate by firing at 350 ° C. for 1 hour in a furnace. The water repellent layer had a thickness of 50 μm, an area of 100 mm × 100 mm, and an average pore diameter of 100 nm.

(Evaluation)
The gas diffusion layers produced in Examples 1 to 4 and Comparative Example 1 were evaluated by measuring the power generation characteristics of MEAs produced using the respective gas diffusion layers.

(1) Production of MEA Preparation of Electrode Catalyst Layer Platinum-supported carbon (TEC10E50E manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., platinum content 46.5 wt%) 10 g, Nafion solution (registered trademark DuPont DE520, Nafion content 5 wt%) 90 g, pure water 25 g, 2-propanol 10 g (special grade reagent manufactured by Wako Pure Chemical Industries, Ltd.) was mixed and dispersed for 3 hours using a homogenizer in a glass container in a water bath set to be kept at 20 ° C. to obtain a catalyst ink.

  After applying the catalyst ink prepared previously using a screen printer on one side of a 200 μm thick PTFE sheet (Naflon (registered trademark) sheet manufactured by NICHIAS) and drying in an oven at 100 ° C. for 30 minutes And cut into a 100 mm square.

2. Assembly of MEA and single cell Electrocatalyst of two sheets of PTFE sheet formed earlier, sandwiching a solid polymer electrolyte membrane (Nafion NR-112 made by DuPont) with a square of 150 mm on each side and a thickness of 30 μm The electrode catalyst layer is placed on the solid polymer electrolyte membrane by stacking so that the layer forming sides face each other, hot-pressing at 130 ° C. for 10 minutes at a pressure of 3 MPa per one side PTFE sheet, and peeling off only the PTFE sheet after cooling. Transfer was performed to obtain a joined body. At this time, the transfer rate of the electrode catalyst layer from the PTFE sheet to the solid polymer electrolyte membrane was 100%, and the weight of platinum per 1 cm ² of the electrode catalyst layer area was 0.40 mg. Moreover, the thickness of the electrode catalyst layer was 10 μm, respectively.

  The obtained joined body is sandwiched by using two gas diffusion layers prepared in advance so that the water-repellent layer faces each other to form an MEA, which is sandwiched between graphite separators, and further gold-plated stainless steel collection. A single cell for evaluation was obtained by sandwiching with an electric plate.

(2) Evaluation Using hydrogen for the anode and air for the cathode of each single cell for evaluation, gas flow rate anode / cathode SR = 1.5 / 2.5, relative humidity anode 100% RH / cathode Power generation was performed at 100% RH and a cell temperature of 70 ° C. “S.R.” (Stoichiometric Ratio) means the ratio of the amount of hydrogen or oxygen required to flow a predetermined amount of current, and “anode S.R. = 1.5” This means that hydrogen is supplied at a flow rate 1.5 times the amount of hydrogen necessary for supplying a predetermined amount of current. The output voltage with respect to the current density of the single cell at this time was measured. The evaluation results are shown in FIG.

  The power generation conditions are such that flooding is likely to occur when the relative humidity is 100% RH anode / 100% RH cathode, but the MEA using the gas diffusion layers of Examples 1 to 4 is generated. It can be seen that the drainage of the generated water is promoted because the voltage is high up to a high current density at which a large amount of water is generated, and the water repellent layer has a water permeation path. In contrast, Comparative Example 1 shows a voltage drop due to flooding at a high current density.

  According to the gas diffusion layer of the present invention, it is possible to realize a fuel cell that can suppress the flooding phenomenon and is excellent in startability below freezing point.

It is a schematic diagram of the gas diffusion layer of this invention. The cross-sectional schematic diagram of MEA which is one suitable embodiment of this invention is shown. The evaluation result of MEA using the gas diffusion layer of Examples 1-4 and Comparative Example 1 is shown.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 110 ... Gas diffusion base material, 120 ... Water-repellent layer, 121 ... Carbon particle coated with water-repellent agent, 122 ... Carbon particle subjected to hydrophilic treatment, 200 ... MEA, 210 ... Solid polymer electrolyte membrane, 231 ... Anode Side electrode catalyst layer, 232... Anode side gas diffusion layer, 241... Cathode side electrode catalyst layer, 242.

Claims (10)

In the gas diffusion layer having a water repellent layer on the gas diffusion substrate,
A gas diffusion layer having a water permeation path in at least a part of the water repellent layer.   The gas diffusion layer according to claim 1, wherein the water permeation path is made of carbon particles subjected to hydrophilic treatment.   The gas diffusion layer according to claim 2, wherein the hydrophilically treated carbon particles are carbon particles coated with a hydrophilic agent.   The gas diffusion layer according to claim 2, wherein the hydrophilically treated carbon particles are carbon particles that are heat-treated in an air atmosphere.   The gas diffusion layer according to any one of claims 2 to 4, wherein a content of the carbon particles subjected to the hydrophilic treatment in the water repellent layer is 30 to 70% by volume.   The gas diffusion layer according to any one of claims 1 to 5, wherein an average pore diameter of the water repellent layer is 10 nm or less.   A fuel cell MEA using the gas diffusion layer according to claim 1.   A fuel cell using the gas diffusion layer according to claim 1 or the MEA for fuel cell according to claim 7.   A method for producing hydrophilic carbon particles, comprising a step of heat-treating the carbon particles at 500 to 1500 ° C. in an air atmosphere.   The manufacturing method of the hydrophilic carbon particle of Claim 9 including the process of adding the element which forms the substance which expresses hydrophilicity by an oxidation process to the said carbon particle previously.

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