JP2005302434A - Electrode for fuel cell, manufacturing method of electrode for fuel cell, and fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for a fuel cell in which damage by freezing and expansion of water in even an environment below freezing point is prevented. <P>SOLUTION: A paste of electrode is manufactured by mixing, agitating, and vacuum deaerating a carbon particulate carrying platinum 14, carbon with small particle size carrying no platinum, and a perfluoro-sulfonic acid system polymer solution of low molecule having hydrogen ion conductivity. By this process, using the mixture in which the micropores of the first carbon particulate 1 are closed by the carbon with small particle size, the electrode for fuel cell is formed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a fuel cell electrode, a method for manufacturing a fuel cell electrode, and a fuel cell.

  In recent years, polymer electrolyte fuel cells have been developed that are used as driving sources for automobiles, as cogeneration equipment that supplies electricity and heat, and as portable power sources.

  In this polymer electrolyte fuel cell, electrodes having a catalyst layer and a gas diffusion layer formed on both sides of a solid polymer electrolyte membrane are arranged to form a membrane electrode assembly (MEA) and further reacted. A fuel cell (single cell) is formed by forming a fuel electrode side separator serving as a flow path for fuel gas that is gas, an air electrode side separator serving as a flow path for air, and the like.

Generally, carbon is used as a base material for an electrode composed of a catalyst layer and a gas diffusion layer. In addition, in a conventional polymer electrolyte fuel cell, a material using carbon particles is known as an electrode catalyst material (see, for example, Patent Document 1).
JP-A-9-167622 (page 3, FIG. 1)

  In general, fine pores (diameter of about 10 nm or less) are present on the surface of carbon particles. When water enters the micropores of the carbon particles, they may freeze and expand, and the carbon particles may be destroyed. Specifically, when water enters the fine pores due to condensation of water evaporated from the solid polymer electrolyte membrane, the water does not readily escape, and this water freezes in a sub-freezing environment (−20 ° C. In this case, water existing in pores having a pore diameter of 5 nm or more is frozen), and the carbon particles may be destroyed by expansion.

  Although it is known that the surface of the electrode catalyst is impregnated with PTFE (fluororesin: polytetrafluoroethylene) or the like, the particle diameter of PTFE is large even at a low molecular weight, and the primary particle diameter is at least about 50 nm. When PTFE is supplied as a powder, it is usually granulated, and the average particle size is about several hundred nm (several μm) to several hundred thousand nm (several hundred nm). Therefore, it is difficult for PTFE to enter inside the fine pores having a diameter of 10 nm or less present in the carbon particles. For this reason, even if it impregnates with PTFE, the micropores of the carbon particles will remain unfilled. On the other hand, since the size of the water molecule is about 0.3 nm, there is a problem that water enters the fine pores even if PTFE is present on the surface of the carbon particles.

  Accordingly, the problem to be solved by the present invention is to provide a fuel cell electrode that is prevented from being destroyed by freezing and expansion of water even in a sub-freezing environment, and a fuel cell using the same.

  The invention according to the first aspect of the present invention is an electrode for a fuel cell and a fuel cell, wherein the micropores existing on the surface of the carbon fine particles for the substrate are filled with a filling material, and the gas diffusion layer and The gist is to constitute at least one of the catalyst layers.

  The invention according to the second aspect of the present invention is an electrode for a fuel cell and a fuel cell, wherein the surface of metal fine particles or ceramic fine particles is coated with carbon, and includes at least a gas diffusion layer and a catalyst layer. The gist is to constitute one.

  Furthermore, the invention according to the third aspect of the present invention is a method for producing an electrode for a fuel cell, comprising a step of carrying a catalyst material on the surface of carbon fine particles for a substrate, and a substrate carrying the catalyst material A step of mixing a carbon fine particle for filler and a filler fine particle having a particle diameter smaller than the carbon fine particle for a base material with a solution containing an ion conductive material or a solution in which a fluororesin is dispersed, The gist is to form the material so as to form at least part of the fuel cell electrode.

  According to the invention relating to the first feature, since the carbon fine particles in which the fine pores are embedded with the filling material are used as the base material, the water is frozen when the fine pores enter the environment below freezing point. Expansion and destruction of the carbon particles can be prevented. By using this material for the gas diffusion layer of the fuel cell, it is possible to prevent the destruction of the carbon particles without impairing the electronic conductivity. In addition, by using this material as the catalyst layer of the fuel cell, it is possible to prevent the catalyst metal such as platinum to be supported from being supported in the fine pores that are likely to cause flooding. It can be improved.

  According to the second aspect of the invention, the formation of micropores can be suppressed by coating carbon around metal fine particles or ceramic fine particles.

  According to the third aspect of the invention, the filling material fine particles having a particle diameter smaller than the carbon fine particles for the base material are mixed with a solution containing an ion conductive material or a solution in which a fluororesin is dispersed to form a paste. By forming this so as to form at least a part of the electrode for the fuel cell, the filler material fine particles can be reliably bound to the micropores of the carbon particles for the substrate.

  Hereinafter, the fuel cell electrode according to the embodiment of the present invention, the manufacturing method thereof, and the details of the fuel cell will be described with reference to the drawings.

  A configuration of a fuel cell electrode according to the present embodiment and a fuel cell using the same will be described with reference to FIG. The fuel cell 100 according to the present embodiment is configured by stacking a plurality of fuel cells 10. The fuel cell 10 includes an electrolyte membrane 1, electrode catalyst layers 2, 3 disposed along both surfaces of the electrolyte membrane 1, and a carbon layer disposed along the outer surface of the electrode catalyst layers 2, 3. 4, 5, gas diffusion layers 6, 7 disposed along the outer surfaces of the carbon layers 4, 5, and separators 8, 9 disposed along the outer surfaces of the gas diffusion layers 6, 7 Are generally configured. In the present embodiment, the electrode catalyst layers 2 and 3 and the carbon layers 4 and 5 constitute an electrode (fuel cell electrode).

  Further, a plurality of groove-shaped hydrogen gas flow paths 8A are formed on one surface of the separator 8, and a plurality of groove-shaped air gas flow paths 8B are formed on the other surface. The surface of the gas diffusion layer 6 on which the hydrogen gas flow path 8A of the separator 8 is formed is joined. The surface of the separator 8 on which the air gas flow path 8B is formed is used as a separator of a fuel cell adjacent to the fuel cell 10 (upper side in FIG. 1).

  Similarly, a plurality of groove-shaped hydrogen gas flow paths 9A are formed on one surface of the separator 9, and a plurality of groove-shaped air gas flow paths 9B are formed on the other surface. The surface of the separator 9 on which the air gas flow path 9B is formed is joined to the surface of the gas diffusion layer 7 of the fuel cell 10. And the surface in which the hydrogen gas flow path 9A of the separator 9 was formed is used as a separator of an adjacent fuel cell (lower side in FIG. 1).

  As the electrolyte membrane 1, a material excellent in hydrogen ion conductivity is used. The electrode catalyst layers 2 and 3 are made of materials having excellent oxygen reducing properties and catalytic performance for hydrogen ionization reaction of hydrogen. Further, the carbon layers 4 and 5 are formed of carbon particles having submicron micropores. As the gas diffusion layers 6 and 7, those having both gas permeability and electron conductivity are used.

  In this embodiment, the thickness of the electrolyte membrane 1 is about 30 μm, the thickness of the catalyst layers 2 and 3 is about 10 μm, the thickness of the carbon layers 4 and 5 is about 10 μm, and the thickness of the separator is about 5 mm. It is set.

(Example 1)
Next, a fuel cell electrode and a fuel cell according to Example 1 of the present embodiment will be described. In this example, a perfluorosulfonic acid polymer (trademark Nafion: manufactured by DuPont) was used as the electrolyte membrane 1.

  Moreover, as a catalyst contained in the electrode catalyst layers 2 and 3, platinum (Pt) which is a material excellent in oxygen reducibility and catalytic performance of hydrogen ionization reaction is used. The platinum-supporting material includes carbon to form electrons for conduction of electrons from platinum to the gas diffusion layers 6 and 7, conduction of hydrogen ions to the electrolyte membrane 1, and holes for supplying gas. Use black.

In the present embodiment, as a manufacturing method of a carbon supporting platinum was added carbon support chloroplatinic acid _{(H 2 PtC 16 · 6H 2} O) aqueous solution. Next, a fine particle of platinum was reduced by adding a reducing agent such as formaldehyde to a chloroplatinic acid aqueous solution to which a carbon carrier was added, and was made to adsorb on the carbon carrier. In this embodiment, formaldehyde is used as the reducing agent, but other reducing agents such as sodium borohydride, formic acid, sodium citrate, and hydrazine may be used.

  In this example, as the first carbon fine particles (carbon fine particles for base material), carbon black (Vulcan: trade name) having an average particle diameter of 50 nm and second carbon fine particles (filling carbon fine particles) having a small particle diameter are used. Carbon black (Carbolac: trade name) having a particle size of 9 nm was used. Here, as the second carbon fine particles, Black Pearls (trade name) having a particle diameter of 12 nm may be used instead of Carbolac. The second carbon fine particles (Carbolac) having a small particle size were graphitized (graphitized) by heat treatment at 2500 ° C. in order to fill the fine pores. As a result, the electronic conductivity of carbon can be increased. Mix Vulcan carrying platinum, carbon with small particle size not carrying platinum, and low molecular perfluorosulfonic acid polymer (Nafion: manufactured by DuPont) with hydrogen ion conductivity. The electrode paste was prepared by vacuum degassing with an evaporator. With such a manufacturing method, the fine pores of the first carbon fine particles (Vulcan) could be closed with carbon having a small particle diameter.

  2 and 3 are views schematically showing carbon fine particles produced by the above-described method and showing a state in which fine holes are closed. 3A shows a state in which micropores are formed on the surface of the first carbon fine particles as the carbon fine particles for the substrate, and FIG. 3B shows the second carbon fine particles as the carbon fine particles for filling. This shows a state in which ionomer adheres to the surface of the first carbon fine particles in which the micropores are blocked and platinum (Pt) is supported. As shown in FIG. 2, the fine holes 11 </ b> A formed in the first carbon fine particles 11 that are particles having a large particle diameter are blocked by the second carbon fine particles 12 that are other substances. A perfluorosulfonic acid polymer (ionomer) 13 is attached to the vicinity of the opening of the micropore 11A and the surface of the first carbon fine particles. Further, platinum 14 as a catalyst is supported on the surface of the first carbon fine particles 11 (see FIG. 3B).

  Next, the above-mentioned paste is screen-printed on the surfaces on both sides of the electrolyte membrane 1, for example.

  In the first embodiment, the gas diffusion layers 6 and 7 are configured to have both gas permeability and electron conduction. In Example 1, carbon paper (manufactured by Toray) having a pore diameter of about 50 μm was used. The carbon paper was impregnated with a PTFE (polytetrafluoroethylene) dispersion and then heat treated at 380 ° C. for 1 hour. Thereby, the gas diffusion layers 6 and 7 were given water repellency.

  As carbon layers 4 and 5, in order to form a submicron water-repellent pore structure, carbon fine particles and PTFE dispersion (D-1: manufactured by Daikin Industries) are mixed, vacuum degassed and paste The resulting product was applied to carbon paper used as a gas diffusion layer. The carbon fine particles used were carbon black (Vulcan: trade name) having an average particle diameter of 50 nm and carbon black (Carbolac: trade name) having a particle diameter as small as about 9 nm.

  Then, the carbon layers 4 and 5 formed on the gas diffusion layers 6 and 7 were joined to the corresponding catalyst layers 2 and 3, respectively, and a fuel cell A was produced.

  In Example 1, the fine holes 11A of the first carbon fine particles 11 are closed with the second carbon fine particles 12, but the fine holes 11A may be closed only with a perfluorosulfonic acid polymer. It is also conceivable to use metal fine particles instead of the second carbon fine particles 12 having a small particle diameter.

(Example 2)
Next, a fuel cell electrode and a fuel cell according to Example 2 of the present embodiment will be described. In Example 2, gold (Au) was used as another substance for the purpose of closing the micropores in the carbon fine particles.

  Hereinafter, a fuel cell electrode and a fuel cell manufacturing method according to the second embodiment will be described.

  First, carbon black (carbon fine particles) is dispersed in an aqueous solution, a chloroauric acid solution is added, and formaldehyde is added to be deposited on the surface of the carbon black. The solution containing carbon black was washed with filtered water and dried at 80 ° C. As a result, as shown in the schematic diagram of FIG. 5, the fine pores of the carbon fine particles are closed with the gold fine particles.

  Next, a platinum catalyst having a particle diameter of about 3 nm is supported on carbon fine particles whose micropores are closed by gold. After mixing the carbon microparticles prepared in this way and the perfluorosulfonic acid polymer (Nafion) solution and stirring to form a paste, catalyst layers 2 and 3 are formed by screen printing along the surface of the electrolyte membrane 1. To do.

  The carbon layers 4 and 5 were produced by adsorbing onto the carbon fine particles using a gold nitrate solution.

  Other configurations and manufacturing methods in Example 2 are the same as those in Example 1, and a fuel cell B was manufactured in Example 2.

(Example 3)
Next, a fuel cell electrode and a fuel cell according to Example 3 of the present embodiment will be described. In Example 3, an attempt was made to fill the fine holes by heat treating carbon black at 2000 to 3500 ° C. It is known that carbon black is graphitized (graphitized) by the heat treatment, the carbon crystallinity is increased, and the electron conductivity is increased. In this example, carbon fine particles (Vulcan) having an average particle diameter of 50 nm were used as carbon black. By the heat treatment, the distribution of fine pores (micropores) of 10 nm or less was reduced, and the specific surface area was reduced from 300 m ² / g to 100 m ² / g.

  The fuel cell produced in Example 3 was designated as Fuel Cell C, except for the configuration and production method of the carbon fine particles in Example 3.

Example 4
Next, a fuel cell electrode and a fuel cell according to Example 4 of the present embodiment will be described. In Example 4, the sample was heated in a hydrocarbon gas atmosphere in order to coat metal or ceramic fine particles with carbon graphite. In addition, by coating fine particles such as metal with carbon in this way, generation of micropores can be suppressed. In Example 4, γ-alumina ultrafine particles having a particle size of about 50 nm were used. This heat treatment was performed at 1100 ° C. for 30 minutes in a mixed gas atmosphere of benzene gas 1 kPa and argon gas 40 kPa. As a result, a graphitized carbon film having a thickness of about 5 nm was formed on the surface of the γ-alumina ultrafine particles.

  In the fourth embodiment, γ-alumina is used, but metal fine particles such as Ni may be used. The processing temperature in the case of using Ni is preferably 400 to 500 ° C. Below this temperature, the carbon coating is amorphous and at higher processing temperatures, it changes to cementite.

  In addition, it is conceivable to use only metal fine particles without coating ceramic fine particles with carbon, but metal fine particles such as Ni and Fe are easily corroded and deteriorated.

  The fourth embodiment is the same as the first embodiment except for the configuration of the carbon fine particles. A fuel cell produced using the carbon fine particles was designated as a fuel cell D.

(Comparative example)
In the comparative example, a fuel cell E was produced by forming a catalyst layer on which platinum was supported as it was without filling the fine pores of the carbon fine particles. Other configurations in the comparative example are the same as those in the first embodiment.

[Comparison of characteristics of Examples 1 to 4 and Comparative Example]
The carbon fine particles used in the fuel cells A to E were immersed in water, vacuum deaerated with an evaporator, soaked in water with the carbon fine particles, and then frozen and thawed, and the change in the average particle diameter was observed by a light scattering method. The result is as shown in FIG. In the carbon fine particles constituting the electrode catalyst layer of the fuel cell E, it is considered that water entered the fine pores, and this was frozen and expanded below freezing point to destroy the carbon fine particles.

Next, focusing on the fuel cell (single cell) in the fuel cells A to E, power was generated at 1 atm of hydrogen and 1 atm of air. First, power generation was performed at a current density of 40 mA / cm ² . In the fuel cells of fuel cells A, B, C, and D, performance higher than that of the fuel cell of fuel cell E by about 10 to 15 mV was confirmed. The reason for this is that in the fuel cells of fuel cells A, B, C, and D, platinum is prevented from entering the fine pores of the carbon fine particles, so that the proportion of platinum that can contribute to the catalytic reaction is increased. Conceivable.

  Thereafter, the fuel cells of the fuel cells A to E were generated at a current density of 1 A / cm 2 and the freezing and thawing of the fuel cells was repeated. FIG. 4 shows the result of the output voltage drop after repeating the initial voltage and freezing / thawing. Compared to fuel cells A to D, the fuel cell E has a large voltage drop (40 mV drop). This is thought to be due to the destruction of the carbon fine particles, and the broken carbon fills the voids in the gaps between the carbon fine particles, which makes it difficult for the gas to reach the platinum catalyst, thereby reducing the voltage.

  As mentioned above, although each Example of this invention was demonstrated, for example in the said Example 2, although the chloroauric acid solution was used, other metal salt solutions, such as chromium (Cr) and palladium (Pd), were used. Also good.

  In addition, as described above, the material that closes the micropores can be generated without being corroded by hydrogen or air supplied to the fuel cell, in addition to being capable of closing the micropores of the carbon particles. It must be a material that does not dissolve in water. A material (ionomer) having electron conductivity or hydrogen ion conductivity is more preferable because it does not hinder the movement of electrons and hydrogen generated on the fuel cell catalyst.

  Moreover, it is preferable that the contact angle between the surface of the fine pores of the carbon powder to be used and the embedded material is 90 ° or less. For that purpose, you may add the process of lowering a contact angle using surfactant, such as Triton X-100. In the case of a material having a contact angle with carbon in the micropores of 90 ° or less (a material having water repellency), as shown in FIG. 6A, the material enters the pores due to surface tension (Washburn Formula P = −4γcos θ / d (P: pressure, γ: surface tension, θ: contact angle, d: pore diameter)). Further, the material can be more easily penetrated into the fine holes by vacuum degassing with an evaporator or the like. On the other hand, when the contact angle is 90 ° or more, as shown in FIG. 6B, the embedding material is difficult to enter into the fine hole, so that it is necessary to pressurize from the outside.

  Furthermore, as other substances, perfluorosulfonic acid polymer (Nafion), carbon fine particles, metal fine particles, ceramic fine particles or a solution in which they are dispersed can be used. Low molecular weight perfluorosulfonic acid polymer (trade name: Nafion) material was used as a material for fixing these other substances to carbon fine particles. In addition, polytetrafluoroethylene (PTFE) and polyvinylidene fluoride were used. By using a solution in which a fluororesin is first dispersed in combination with the fine particles, or by using these fixatives, the entrance portion of the micropores can be reliably closed.

  In Example 2, since the catalyst metal is supported by a technique of oxidizing or reducing the catalyst metal salt, a binder is not required or the amount used can be reduced.

  Furthermore, in the above-described embodiment, the thickness of the electrolyte membrane 1, the thickness of the catalyst layers 2 and 3, the thickness of the carbon layers 4 and 5, and the thickness of the separator are set, but the thickness is limited to these. The design can be changed.

  The fuel cell electrode, the fuel cell electrode manufacturing method, and the fuel cell according to the present invention can also be applied to fuel cells used in various applications other than in-vehicle fuel cells.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory cross-sectional view showing the configuration of a fuel cell electrode according to an embodiment of the present invention and a fuel cell using the same. It is a figure which shows typically the carbon fine particle which shows the state which block | closed the micropore. (A) is a diagram schematically showing a state in which fine pores are formed on the surface of the first carbon fine particles, and (b) is an ionomer on the surface of the first carbon fine particles closed by the second carbon fine particles. It is a figure which shows typically the state by which platinum (Pt) is carry | supported while adhering. It is a figure which shows the test result of an Example and a comparative example. In Example 2, it is a figure which shows typically the state by which the micropore of the carbon microparticle was block | closed with the gold microparticle. (A) is a figure explaining the effect | action which acts on the embedding material when the contact angle with the carbon in a micropore is 90 degrees or less, (b) is the case where the contact angle with the carbon in a micropore is 90 degrees or less. It is a figure explaining the effect | action which acts on an embedding material.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electrolyte membrane 2,3 Catalyst layer 4,5 Carbon layer 6,7 Gas diffusion layer 8,9 Separator 10 Fuel cell 11 First carbon fine particle 11A Fine hole 12 Second carbon fine particle 14 Platinum 100 Fuel cell

Claims (10)

  An electrode for a fuel cell, characterized in that at least one of a gas diffusion layer and a catalyst layer is formed of a material in which fine pores existing on the surface of carbon fine particles for a substrate are closed with a filling material.   2. The fuel cell electrode according to claim 1, wherein the filling material is selected from carbon fine particles, metal fine particles, and ceramic fine particles having a smaller particle diameter than the carbon fine particles for the substrate.   3. The fuel cell electrode according to claim 2, wherein the filling material is fixed to the carbon fine particles for the base material with an ion conductive material.   The fuel cell electrode according to claim 2, wherein the filling material is fixed to the carbon fine particles for the base material with a fluororesin.   The electrode for a fuel cell according to any one of claims 1 to 4, wherein a catalyst material is supported on the surface of the carbon fine particles for the base material.   3. The fuel cell electrode according to claim 2, wherein the metal fine particles are embedded in the micropores of the base carbon fine particles by oxidation or reduction of a metal salt of the metal.   7. The fuel cell electrode according to claim 1, wherein the carbon fine particles are fine powder of carbon graphite.   An electrode for a fuel cell, comprising a metal fine particle or ceramic fine particle coated with carbon and constituting at least one of a gas diffusion layer and a catalyst layer.   A fuel cell comprising the fuel cell electrode according to any one of claims 1 to 8. A method for producing an electrode for a fuel cell, comprising:
A step of supporting a catalyst material on the surface of the carbon fine particles for the substrate;
The base material carbon fine particles supporting the catalyst material and the filling material fine particles having a particle diameter smaller than the base material carbon fine particles are dispersed in a solution containing an ion conductive material or a solution in which a fluororesin is dispersed. A process of mixing and pasting,
A method for producing a fuel cell electrode, comprising forming the paste material to form at least a part of a fuel cell electrode.

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