JP2005174835A - Electrode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode corresponding to a wide range of humidifying condition, and to provide an MEA and a fuel cell. <P>SOLUTION: The electrode contains a catalyst layer containing catalyst metal, a conductive carrier carrying the catalyst metal, and a proton conductive member, and the catalyst layer contains hydrophobic particle-carried hydrophilic particles. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a fuel cell electrode, preferably a polymer electrolyte fuel cell electrode.

  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 an electrolyte layer made of a film-like solid polymer 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 is formed by sandwiching an electrolyte layer between catalyst supporting electrodes. Therefore, at least one surface of the catalyst layer is in contact with the electrolyte layer. Further, the catalyst-carrying electrode can include a catalyst layer in which the electrode catalyst is highly dispersed and a gas diffusion layer.

  In such a polymer electrolyte fuel cell, as shown in the chemical formula below, at the anode, the hydrogen-containing gas of the fuel is oxidized to protons, and at the cathode, oxygen contained in the oxidant gas is reduced and passed through the electrolyte layer. A chemical reaction occurs in which water is combined with protons. The polymer electrolyte fuel cell directly obtains electric energy from the reaction energy obtained by the chemical reaction.

  The solid polymer membrane does not exhibit high proton conductivity unless it is humidified. Therefore, the hydrogen-containing gas supplied to the polymer electrolyte fuel cell needs to be humidified using a gas humidifier. As a humidifying method, for example, a method of humidifying using a bubbler, a mist generator, or the like, a method of supplying moisture directly to a reaction gas channel formed inside the separator, and the like have been proposed.

  In order to humidify the solid polymer membrane using such a gas humidifier, a water tank for storing water for humidification, a humidifier, a condenser for recovering water discharged from the fuel cell, etc. Various auxiliary machines are required. Further, the humidification conditions are limited to a narrow range, and battery performance is greatly deteriorated if the humidification conditions are not met. These lead to an increase in complexity and size of the entire fuel cell system. Therefore, it is desired that the fuel cell system be made smaller, lighter, and more efficient by making the humidification conditions of the polymer electrolyte fuel cell simpler.

  The humidification condition is determined by adjusting the hydrophilicity / hydrophobicity of the catalyst layer and the gas diffusion layer in the polymer electrolyte fuel cell.

  Under operating conditions where the reaction rate of the cell reaction is relatively high, the amount of water that moves with the protons that move through the solid polymer membrane from the anode to the cathode, and the formation that forms and aggregates in the catalyst layer of the cathode The amount of water increases. At this time, these waters stay in the catalyst layer of the cathode and cause a flooding phenomenon that closes the pores that have become the reaction gas supply path. This makes it difficult to stably and sufficiently supply the reaction gas to the reaction site in the catalyst layer, and a part of the reaction site does not function. In order to suppress the flooding phenomenon, a method of imparting water repellency by using a water repellent resin or the like in the catalyst layer is used.

  According to such a method, even if a large amount of generated water is generated at the cathode, it can be efficiently drained. However, when the supply gas is low-humidified, if the amount of water supplied is insufficient, a dry-out phenomenon that reduces the proton conductivity of the solid electrolyte membrane is caused. For this reason, the humidification condition of the supply gas is limited to the high humidification condition.

Patent Document 1 attempts to impart hydrophilicity to the catalyst layer by containing 1 to 10,000 ppm of a moisturizing component made of a metal oxide. According to the method of Document 1, the dry-out phenomenon of the solid polymer film can be suppressed by the moisture retained by the metal oxide species. Therefore, the humidification condition of the supply gas can be a low humidification condition or a non-humidification condition.
JP 2002-289200 A

  However, according to the method of Document 1, since the amount of metal oxide contained in the catalyst layer is small, only a very small amount of water can be stored. For this reason, a sufficient suppression effect of the dry-out phenomenon is not obtained. Further, when a fuel cell using the electrode of Document 1 is operated at a high current density, a flooding phenomenon is likely to occur. Therefore, even when the electrode of Document 1 is used, there is a problem that the humidification condition is limited to a narrow range.

  Accordingly, an object of the present invention is to provide an electrode, an MEA, and a fuel cell that can cope with a wide range of humidification conditions.

  As a result of intensive studies in view of the above problems by the present inventors, it has been found that the above problems can be solved by optimizing the hydrophilicity and hydrophobicity in the electrode.

  That is, according to the present invention, in an electrode including a catalyst layer containing a catalyst metal, a conductive carrier carrying the catalyst metal, and a proton conductive member, the catalyst layer contains hydrophobic particles carrying hydrophilic particles. The electrode is characterized in that the above-mentioned problems are solved.

  The electrode of the present invention can optimize the hydrophilicity and hydrophobicity of the electrode by including the hydrophobic particle-supporting hydrophilic particles in which the hydrophobic particles are supported on the surface of the hydrophilic particles in the catalyst layer. That is, it becomes possible to keep the moisture contained in the humidified gas and the water generated by the electrochemical reaction in an optimal state. Therefore, the fuel cell using the electrode of the present invention including the hydrophobic particles-supporting hydrophilic particles can make the diffusibility of the reaction gas uniform, thereby not only obtaining high power generation efficiency but also the catalyst layer. It is possible to cope with a wide range of humidification conditions by keeping the water content in an optimal state. Therefore, according to the present invention, it is possible to provide an MEA and a fuel cell excellent in adaptability of operating conditions, and it is possible to reduce the size, weight and efficiency of the fuel cell system.

  The first of the present invention is an electrode including a catalyst layer containing a catalyst metal, a conductive carrier carrying the catalyst metal, and a proton conductive member, wherein the catalyst layer contains hydrophobic particles carrying hydrophilic particles. It is an electrode characterized by doing.

  First, power generation using a general polymer electrolyte fuel cell will be briefly described. The polymer electrolyte fuel cell has a structure in which a membrane-electrode assembly (hereinafter also referred to as “MEA”) is sandwiched between an anode separator and a cathode separator. Furthermore, a current collector can be disposed at the end of both separators. In the MEA, two electrodes composed of a gas diffusion layer and a catalyst layer are disposed on both sides of an electrolyte layer having proton conductivity, and other layers are arranged as necessary.

  The anode separator is formed with a flow groove for flowing hydrogen gas as fuel. The cathode side separator is formed with a flow groove for flowing oxygen gas as an oxidant. Hydrogen gas and oxygen gas are supplied from the respective flow grooves and diffused to the catalyst layer through the gas diffusion layer. By supplying the reaction gas in this way, an electrochemical reaction shown in the following formulas (1) and (2) occurs, and electrons are generated as the reaction proceeds. Electrical energy can be obtained by extracting the electrons from the electrodes to an external circuit.

  In the electrochemical reaction of the fuel cell, protons are generated on the catalyst metal of the anode side catalyst layer by oxidation of the fuel (formula (1)). The generated protons reach the electrolyte layer through the proton conductive member dispersed in the catalyst layer. The proton further passes through the electrolyte layer and moves to the cathode side. Thereafter, the catalyst metal in the cathode side catalyst layer is reached through the proton conductive member dispersed in the cathode side catalyst layer. In an electrode catalyst, it reacts with oxygen gas supplied as an oxidant and electrons that have passed through an external circuit to produce water.

  In order to advance the electrochemical reaction, it is necessary to quickly discharge water generated on the cathode side and continuously supply oxygen gas to the cathode. Further, in order for the electrolyte layer made of the solid polymer membrane to exhibit high proton conductivity, the solid polymer membrane needs to be sufficiently humidified. Therefore, the oxidizing gas and the hydrogen gas are supplied after being sufficiently humidified.

  Conventionally, an attempt has been made to impart hydrophobicity to an electrode by incorporating a water-repellent resin in the catalyst layer in order to efficiently remove generated water. However, in such an electrode, the amount of water in the catalyst layer is reduced, so that the proton conductive member tends to dry out, and the operating conditions of the fuel cell are limited to high humidification conditions. In addition, a method is disclosed in which a moisture retaining component composed of a metal oxide is contained in the catalyst layer, so that the electrolyte layer can be humidified by moisture retained in the metal oxide even when the humidity of the supply gas is reduced. . In such a method, since the metal oxide is an insulator, only a small amount of the metal oxide can be contained to obtain a high battery voltage, and a sufficient effect cannot be obtained. In particular, in operation at a high current density, a large amount of water is excessively stored not only in the metal oxide but also in the vicinity of the metal oxide, so that a flooding phenomenon is likely to occur.

  However, the electrode of the present invention can store moisture in the hydrophilic particles by including the hydrophobic particles-supporting hydrophilic particles in the catalyst layer. In addition, since some of the hydrophobic particles-supporting hydrophilic particles are hydrophobic, the periphery thereof does not hold excessive moisture, and thus the flooding phenomenon in the catalyst layer can be prevented. According to the present invention, in the fuel cell electrode, it is possible to simultaneously supply the humidified hydrogen gas uniformly to the electrolyte layer and to achieve different characteristics of efficiently discharging the generated water to the flow channel. it can. As a result, even in a high current density operation, it is possible to secure pores for diffusing the supply gas, diffusing the supply gas uniformly, and obtaining high power generation efficiency. In addition, even when the humidity of the supply gas suddenly decreases, the water stored in the hydrophilic particles is released, so that the battery performance can be prevented from being deteriorated.

  As shown in FIG. 1, the hydrophobic particle-supporting hydrophilic particle has a form in which the hydrophobic particle 2 is supported on the surface of the hydrophilic particle 1. If only hydrophobic particles and hydrophilic particles are mixed and contained in the electrode, one of the characteristics is expressed locally, so the generated water in the catalyst layer cannot be made uniform. A lot of moisture concentrates in the groove portion, and the supply gas or the like cannot be uniformly supplied to the electrolyte layer. On the other hand, in the electrode of the present invention, the hydrophobic particles-supporting hydrophilic particles have such a form, so that the water in the catalyst layer can be uniformly maintained, and the supply gas and the like are uniformly distributed to the electrolyte layer. Can be supplied.

  Below, it demonstrates in detail regarding the electrode of this invention.

  The electrode of the present invention may include a gas diffusion layer in addition to the catalyst layer. As described above, the catalyst layer contains hydrophobic particles-supporting hydrophilic particles, a catalyst metal that promotes an electrode reaction, a conductive carrier, and a proton conductive member.

  Hydrophobic particle-supporting hydrophilic particles are formed by supporting hydrophobic particles 2 on the surface of hydrophilic particles 1 as schematically shown in FIG. The size ratio between the hydrophilic particles and the hydrophobic particles is such that the average particle size of the hydrophilic particles is 10 to 1000 times, preferably 10 to 500 times, more preferably 10 to 100 times the average particle size of the hydrophobic particles. It is better to double. When the average particle size of the hydrophilic particles exceeds 1000 times the average particle size of the hydrophobic particles, the hydrophobic particles enter the fine structure of the hydrophilic particles, and the hydrophilic particles retain a sufficient amount of water. There is a risk that you will not be able to. Further, when the average particle size of the hydrophilic particles is less than 10 times the average particle size of the hydrophobic particles, the hydrophobic particles are too large to be supported by the hydrophilic particles, so that the hydrophobic particles and the hydrophilic particles are not supported. Therefore, the desired effect may not be obtained.

  The average particle diameter of the hydrophilic particles in the hydrophobic particle-supporting hydrophilic particles is 0.1 to 10 μm, preferably 0.1 to 8 μm, more preferably 0.1 to 5 μm. Hydrophobic particle-supporting hydrophilic particles are used by being uniformly dispersed in the catalyst layer. However, when the hydrophilic particles are less than 0.1 μm, the hydrophobic particle-supporting hydrophilic particles have a fine structure of the conductive carrier. There is a risk that it will be trapped in the network and it will be difficult to express the characteristics of the hydrophobic particles-supporting hydrophilic particles. In addition, when the hydrophilic particle exceeds 10 μm, it becomes considerably larger than the conductive carrier particle described later, and it becomes difficult to disperse suitably in the electrode, and an electrode having desired characteristics cannot be obtained. There is a fear.

  The average particle diameter of the hydrophobic particles in the hydrophobic particles-supporting hydrophilic particles is 0.01 to 1.0 μm, preferably 0.05 to 0.8 μm, more preferably 0.1 to 0.5 μm. . Hydrophobic particles are used by supporting them on the surface of the hydrophilic particles, but when the hydrophobic particles are less than 0.01 μm, the hydrophobic particles enter the fine pores of the hydrophilic particles, and the hydrophilic particles May not be able to retain a sufficient amount of water. When the hydrophobic particles are larger than 1.0 μm, the hydrophobic particles are hardly supported on the hydrophilic particles, and a form close to a mixture of the hydrophobic particles and the hydrophilic particles is obtained, so that the desired effect may not be obtained. There is.

  In the present invention, the average particle size of hydrophilic particles and hydrophobic particles is measured using a light scattering method such as a laser particle size distribution measuring device.

  The hydrophilic particles must be a material that is hydrophilic and can sufficiently retain moisture, as well as having high corrosion resistance. At least one side of the catalyst layer in the electrode of the present invention is in contact with the electrolyte layer. Since a strongly acidic ion exchange resin such as a perfluorosan-based polymer is used for the electrolyte layer, the hydrophilic particles used for the catalyst layer are required to have not only hydrophilicity but also corrosion resistance.

  Specific examples of the hydrophilic particles satisfying such a condition include at least one metal oxide selected from silicon, titanium, aluminum, zirconium, tantalum, yttrium, tin, tungsten, indium, and antimony. The hydrophilic particles are more preferably at least one metal oxide selected from silicon, titanium, aluminum, and tin from the viewpoint of corrosion resistance, material, and manufacturing cost. In the present application, the hydrophilicity in the hydrophilic particles means that the metal oxide itself has water retention.

  Hydrophobic particles must have a high hydrophobicity for a long period of time and must be capable of being finely divided and supported on the surface of hydrophilic particles in a highly dispersed manner.

  Fluorine-containing compounds are used as hydrophobic particles that satisfy such conditions. Specifically, fluorine-containing compounds such as fluoroethylene polymer and fluoroethylene-propylene-copolymer are preferably used as the hydrophobic particles. In the present application, the hydrophobicity in the hydrophobic particles means that at least the surface of the material itself has water repellency.

  In the hydrophobic particle-supporting hydrophilic particle, the supported amount of the hydrophobic particle is 0.1 to 20% by mass, preferably 0.2 to 10% by mass, more preferably 0.5 to 5% by mass. . When the amount of hydrophobic particles supported in the hydrophobic particles-supporting hydrophilic particles is less than 0.1% by mass, hydrophobic properties cannot be imparted around the hydrophilic particles, and the expected effect cannot be obtained. There is a fear. In addition, when the amount of the hydrophobic particles supported in the hydrophobic particles-supporting hydrophilic particles exceeds 20% by mass, the surface of the hydrophilic particles is covered with the hydrophobic particles, so that moisture can enter the hydrophilic particles. There is a risk that a sufficient amount of water cannot be stored. The amount of the hydrophobic particles supported on the hydrophilic particles can be quantitatively analyzed with an inductively coupled plasma emission spectrometer (ICP).

  The method for supporting the hydrophobic particles on the hydrophilic particles is not particularly limited, and a conventionally known technique can be used. Specifically, the method described in the following examples can be mentioned. Using a particle processing apparatus, while spraying an aqueous dispersion solution of hydrophobic particles while allowing hydrophilic particles to flow, this is separated. By drying, hydrophobic particle-supporting hydrophilic particles in which hydrophobic particles are highly dispersed and supported on the surface of the hydrophilic particles are obtained.

  It does not specifically limit as a particle processing apparatus, Agromaster AGM-MINI PJ (made by Hosokawa Micron Corporation) is mainly mentioned. These can prepare hydrophilic particles carrying hydrophobic particles efficiently at a low temperature by a pulse shock wave as compared with the conventional spray drying method relying only on thermal energy.

  The aqueous dispersion solution of hydrophobic particles is an aqueous dispersion obtained by dispersing the above-described fluorine-containing polymer compound in an aqueous dispersion mainly containing water. The aqueous dispersion containing water as a main component is a water mixture containing, in addition to water, a dispersion aid such as a surfactant and an organic solvent, and preferably substantially free of a dispersion aid. The content of the hydrophobic particles in the aqueous dispersion solution is preferably about 1 to 60% by mass of the hydrophobic particles with respect to the total amount of the aqueous dispersion solution.

  The aqueous dispersion solution of hydrophobic particles is not particularly limited, and examples thereof include full-on PTFE dispersion XAD911, AD936 (Asahi Glass Co., Ltd.), Polyflon D-4 (Daikin Co., Ltd.) and the like.

  What is necessary is just to determine suitably the average particle diameter, addition amount, etc. of hydrophobic particle | grains and hydrophilic particle | grains so that the hydrophobic particle carrying | support hydrophilic particle | grains which have the predetermined relationship mentioned above may be obtained.

  Examples of the drying means include known methods using vacuum drying, natural drying, a rotary evaporator, a blower dryer, and the like. The drying conditions and the like may be appropriately determined so as to obtain desired hydrophobic particle-supporting hydrophilic particles according to the drying means used, but may be performed at 60 to 180 ° C. for about 3 to 12 hours. By performing such drying, organic substances such as a surfactant remaining on the hydrophobic particles-supporting hydrophilic particles can be removed. If the organic matter contained in the aqueous dispersion solution remains in the hydrophobic particle-supporting hydrophilic particles, the electrode performance may be deteriorated, so that it may be removed.

  The method for supporting the hydrophobic particles on the hydrophilic particles is not limited to the above-described method, and various other conventionally known methods such as chemical vapor deposition and sputtering may be used.

  In the catalyst layer included in the electrode of the present invention, the content of the hydrophobic particle-supporting hydrophilic particles is 1.0 to 30% by mass, preferably 5 to 25% by mass, more preferably 10 to 20% by mass. Is good. When the content of the hydrophobic particle-supporting hydrophilic particles is less than 1.0% by mass, the amount of water that can be stored by the hydrophilic particles is small, and the function of optimally adjusting the moisture content of the catalyst layer cannot be achieved. There is a fear. Further, when the content of the hydrophobic particle-supporting hydrophilic particles exceeds 30% by mass, the ability to store moisture is sufficient, but both the hydrophilic particles and the hydrophobic particles are compared with other constituent components of the catalyst layer. In addition, since the electronic conductivity is low, the electronic conductivity of the electrode layer is lowered, and as a result, the internal resistance of the battery is increased and the battery performance may be lowered.

  Next, the catalyst metal contained in the catalyst layer in the electrode of the present invention includes at least one metal selected from the group consisting of Pt, Ir, Au, Ag, and Pd. These metals may be used alone or in combination of two or more, or some or all of them may be used in the form of an alloy. Among these, it is preferable to use Pt because it shows high oxygen reduction activity. Platinum may be used alone, but may be used as a noble metal alloy catalyst or a noble metal-base metal mixture catalyst based on Pt in order to enhance stability and activity as a catalyst metal.

As the noble metal alloy catalyst, specifically, from a group consisting of metals of noble metals other than platinum such as ruthenium, rhodium, palladium, osmium, iridium, gold, silver, chromium, iron, titanium, manganese, cobalt, nickel and copper An alloy of one or more selected metals and platinum is preferred. Further, the noble metal - as base metal mixture catalyst, specifically, noble metals such as Pt-WO ₃ - metal oxide mixtures.

  As the particle size of the catalytic metal is smaller, the effective electrode area for the electrode reaction to proceed increases. Accordingly, since the catalytic activity is increased, the amount of catalytic metal used can be reduced. However, if the particle size of the catalyst metal is too small, it is difficult to uniformly disperse the catalyst metal, which may reduce the catalyst activity. Therefore, the average particle diameter of the catalyst metal is preferably 1 to 10 nm, particularly 2 to 5 nm. The average particle diameter of the catalyst metal indicates, for example, the crystallite diameter obtained from the half-value width of the diffraction peak of the catalyst metal in X-ray diffraction or the average value of the particle diameter of the catalyst metal determined from a transmission electron microscope image.

  The conductive carrier for supporting the catalyst metal must not only support the catalyst metal particles but also function as a current collector for taking out electrons from the external circuit or taking them in from the external circuit. Therefore, if the electrical resistance of the conductive carrier is high, the internal resistance of the battery increases, resulting in a decrease in battery performance. For this reason, the electronic conductivity of the conductive carrier contained in the electrode must be sufficiently low.

  Specific examples of the conductive carrier include conductive carbon materials such as carbon black, graphitized carbon, and activated carbon. However, any material having corrosion resistance and electronic conductivity can be used and is not particularly limited. The average particle size of the conductive carrier is not particularly limited, but may be about 10 to 100 nm.

  Various methods such as impregnation method, coprecipitation method, competitive adsorption method, microemulsion method (reverse micelle method), etc. can be used as a method for supporting the catalyst metal on the conductive support. A known technique can be used and is not particularly limited.

  The amount of the catalyst metal supported on the conductive carrier is not particularly limited, but is 10 to 80% by mass, preferably about 30 to 60% by mass, based on the total amount of the electrode catalyst in which the catalyst metal is supported on the conductive carrier. It is good to do.

  In the catalyst layer of the electrode of the present invention, it is preferable that a conductive carrier carrying a catalytic metal is coated with a proton conductive member as a binder polymer. Thereby, while being able to maintain the structure of a catalyst layer stably, the sufficient reaction site (three-phase interface) where an electrode reaction advances can be ensured, and high catalyst activity can be acquired.

  The proton conductive member can be a liquid, solid, gel-like material having at least high proton conductivity, solid acid such as phosphoric acid, sulfuric acid, antimonic acid, stannic acid, heteropoly acid, and perfluorosulfonic acid ionomer. , Hydrocarbon polymer compound doped with inorganic acid such as phosphoric acid, organic / inorganic hybrid polymer partially substituted with proton conductive functional group, phosphoric acid solution or sulfuric acid solution in polymer matrix Examples include gel proton conductivity impregnated with. The proton conductive member can also be a mixed conductor having electronic conductivity at the same time.

  As the proton conductive member, commercially available materials such as various Nafion manufactured by DuPont (registered trademark: Nafion) and ion exchange resin manufactured by Dow Chemical may be used. The content of the proton conductive member is usually 10 to 50% by mass, preferably 15 to 35% by mass with respect to the total amount of the catalyst layer. However, the content of the proton conductive member is not limited to the above range, and may be appropriately determined in consideration of gas permeability, diffusibility, proton conductivity, electron conductivity, and the like.

  Examples of the gas diffusion layer that can be used in the electrode of the present invention include, but are not particularly limited to, conductive porous substrates such as porous carbon paper and carbon cloth that support the catalyst layer.

  In the present invention, the gas diffusion layer and other layers may have a multilayer structure of two or more layers. For example, a carbon layer may be disposed between the gas diffusion layer and the catalyst layer in order to suppress damage due to compression when assembling the fuel cell and electrical resistance between members. In the present invention, the carbon layer is included in the gas diffusion layer.

  As described above, the electrode of the present invention contains hydrophobic particles carrying hydrophilic particles in which hydrophobic particles are carried on the surface of the hydrophilic particles. Therefore, not only can water be retained in the electrode by the hydrophilic particles, but also water generated excessively by the water repellent function of the hydrophobic particles can be quickly discharged. Therefore, the electrode of the present invention can maintain the water content in the electrode uniformly by optimizing the hydrophilicity and hydrophobicity, so that it has high power generation efficiency and a wide range of humidification conditions. It can correspond to.

  The second of the present invention is an MEA using the above-described electrode. Since the electrode of the present invention has the above-described various characteristics, it can be suitably used for an MEA for a fuel cell. The MEA is not particularly limited, but an electrolyte layer is sandwiched between electrodes.

  In the MEA of the present invention, the electrode containing the above-described hydrophobic particles-supporting hydrophilic particles may be used for either the anode or the cathode, but is preferably used as the cathode because the moisture content can be maintained in an optimum state. . Therefore, in the MEA of the present invention, when the above-described electrode is used for the cathode, a conventionally known anode electrode used for MEA may be used for the anode in addition to the above-described electrode.

  The electrolyte layer in the MEA is not particularly limited, and examples thereof include a solid polymer electrolyte membrane made of the same proton conductive member as the catalyst layer in the electrode described above. Further, the thickness and size of the electrolyte layer are not particularly limited, and may be appropriately determined in consideration of output characteristics of the obtained fuel cell.

  As a method for manufacturing MEA, a method of directly producing an electrode on an electrolyte layer, a method of producing an electrode on a separator and bonding it to the electrolyte layer, a method of producing an electrode on a flat plate and transferring it to the electrolyte layer, etc. The various methods are mentioned. When the electrode is formed on the separator separately from the electrolyte layer, the electrode and the electrolyte layer are preferably joined by a hot press method, an adhesive method (see Japanese Patent Laid-Open No. 7-220741) or the like.

  A third aspect of the present invention is a fuel cell using the electrode described above or the MEA for fuel cell described above. The fuel cell of the present invention uses the above-described electrode of the present invention. The type of fuel cell is not particularly limited as long as desired cell characteristics can be obtained, but it is used as a polymer electrolyte fuel cell (also simply referred to as “PEFC”) from the viewpoints of practicality and safety. Is preferred. The PEFC has a structure in which the MEA is sandwiched between separators as described above.

  A known separator such as carbon paper or carbon cloth may be used as a separator for sandwiching the MEA. The separator has a function of separating air and fuel gas, and a flow groove may be formed in order to secure the flow path, and a conventionally known technique can be appropriately used. The thickness and size of the separator and the like are 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 a desired voltage or the like can be obtained by the PEFC. The shape of the PEFC is not particularly limited, and may be determined as appropriate so that desired battery characteristics such as voltage can be obtained.

  The electrode of the present invention not only has high power generation efficiency, but can be adapted to a wide range of humidification conditions. In addition, the above-described electrode can exhibit excellent characteristics even in operation at a high current density. Therefore, the MEA and the fuel cell using such an electrode have better adaptability to operating conditions as compared with the conventional one. Therefore, the fuel cell system can be highly efficient, reduced in size, and reduced in weight, and is useful as a power source for moving bodies such as vehicles and a stationary power source.

  In addition, regarding the electrode mentioned above, MEA for fuel cells, and a fuel cell, only one Embodiment of this invention was shown, and this invention is not limited to this. Therefore, the constituent elements other than the catalyst layer using the hydrophobic particles-supporting hydrophilic particles of the present invention are not particularly limited, and conventionally known techniques relating to fuel cells can be appropriately used.

  Hereinafter, the present invention will be specifically described based on examples. In addition, this invention is not limited only to these Examples. In the examples, “%” represents a mass percentage unless otherwise specified.

<Preparation of Hydrophobic Particles-Supporting Hydrophilic Particles (Teflon (Registered Trademark) -Supported Silica)>
Here, Teflon (registered trademark) particles were used as hydrophobic particles, and silica particles were used as hydrophilic particles. Teflon (registered trademark) particles were supported on silica particles using a particle processing apparatus (Agromaster AGM-MINI PJ manufactured by Hosokawa Micron Corporation). While 9.9 g of silica powder (SiO ₂ , average particle size 4 μm, BET specific surface area 620 m ^{2 /} g) is flowed with dry air in the chamber of the particle processing apparatus, a Teflon (registered trademark) dispersion solution (Fluon PTFE Disperse manufactured by Asahi Glass) John XAD 911: Average PTFE particle size 0.25 μm diluted 10-fold with pure water 6% Teflon (registered trademark) 1.7 g sprayed to support the Teflon (registered trademark) particles on the silica surface, Teflon (registered trademark) particles were supported in a highly dispersed manner on the surface of silica particles by flowing in air and simultaneously drying. This was followed by vacuum drying at 150 ° C. in order to further remove organic substances such as surfactants contained in the Teflon (registered trademark) dispersion. As a result of quantitative analysis of this sample by ICP, it was found that 0.96% of Teflon (registered trademark) was supported.

<Preparation of Pt-supported carbon>
1 g of carbon black powder (Vulcan XC-72 manufactured by Cabot) was sufficiently dispersed in 250 g of a chloroplatinic acid aqueous solution containing 0.4% platinum using a homogenizer, and 3 g of sodium citrate was added thereto. Then, the mixture was heated to 85 ° C. using a reflux reactor, and platinum was supported for reduction. And after standing to cool to room temperature, Pt carrying | support carbon black powder was obtained by filtering carbon carrying | supporting platinum. The average Pt particle size of the Pt-supported carbon black powder was about 3.1 nm from the result of observation with a transmission electron microscope. Further, as a result of examining the amount of Pt supported by ICP, it was confirmed that 48.6% of Pt was supported.

Example Electrocatalyst 1
Teflon (registered trademark) -supported silica powder and Pt-supported carbon powder prepared by the above method were dry mixed at 15:85 (mass ratio).

(Comparative Example Electrocatalyst 1)
Only Pt-supported carbon prepared by the above method was used.

<Evaluation>
(Production of MEA)
The production of MEA (membrane-electrode assembly) was performed as follows.

  After adding 5 times the amount of purified water to the mass of Example Electrode Catalyst 1 and Comparative Example Electrocatalyst 1, 0.5 times the amount of isopropyl alcohol is added, and the weight of Nafion is 0.8 times the amount. A Nafion solution (containing 5% Nafion manufactured by Aldrich) was added. The mixed slurry was well dispersed with an ultrasonic homogenizer, followed by addition of a vacuum degassing operation to prepare a catalyst slurry. A predetermined amount of catalyst slurry is printed on one side of carbon paper (Toray TGP-H-090 manufactured by Toray Industries, Inc.), which is a gas diffusion layer (GDL), according to the desired thickness, and dried at 60 ° C. for 24 hours. Then, the surface on which the cathode catalyst layer was applied was matched with the electrolyte layer, and hot pressing was performed at 120 ° C. and 0.1 MPa for 10 minutes to produce a cathode in the electrolyte layer.

  The anode is prepared by bonding only the Pt-supported carbon black prepared by the above-described method for preparing Pt-supported carbon as an electrode catalyst using the same method as that for the cathode, and joining it to the surface opposite to the cathode joint surface of the electrolyte membrane. MEA. The thicknesses of the anode and cathode catalyst layers were 7 μm and 11 μm, respectively.

In these MEAs, the amount of Pt used was 0.3 mg for the anode and 0.5 mg for the cathode per 1 cm ^{2 of the} apparent electrode area, and the electrode area was 300 cm ² . In addition, Nafion 112 (thickness: about 50 μm) was used as the electrolyte membrane.

(Single cell evaluation)
A fuel cell single cell was constructed using the produced MEA, and the cell voltage change with respect to the gas humidification temperature of the cathode was evaluated by the following method. Hydrogen was supplied as fuel to the anode side of the fuel cell, and air was supplied to the cathode side. The supply pressure for both gases is atmospheric pressure, the temperature of the fuel cell body is set to 70 ° C., the hydrogen utilization rate is 67%, the air utilization rate is 40%, and the cathode humidification temperature when the current density is 1 A / cm ^2. The change of the cell voltage when changing was examined.

FIG. 2 is a graph showing the results. In the battery using the comparative example electrode catalyst 1 that does not include the hydrophilic particles carrying the conventional hydrophobic particles, the cell at 1 A / cm ² as the cathode humidification temperature decreases. The voltage decreased, and a difference of about 50 mV was observed between the humidification temperatures of 70 ° C. and 50 ° C. On the other hand, in the battery using the example electrode catalyst 1, the decrease in the cell voltage with respect to the decrease in the cathode humidification temperature was small, and only a decrease of about 20 mV was observed at the humidification temperatures of 70 ° C and 50 ° C.

  This is presumably because the battery using the comparative example electrode catalyst 1 did not exhibit the function of maintaining an appropriate amount of water in the electrode, and flooding occurred on the high humidification side. Also from this result, it can be said that the form in which the hydrophobic particles are supported on the hydrophilic particles is important in providing the function of keeping the moisture content of the electrode in an appropriate state.

It is a schematic diagram of the hydrophilic particle which carry | supported the hydrophobic particle in this invention. When the cathode gas humidification temperature and the current density of the fuel cell using the fuel cell electrode catalyst including the hydrophobic particle-supported hydrophilic particles of the present invention and the fuel cell using the comparative example electrode catalyst are 1 A / cm ² It is the graph which showed the relationship of the cell voltage in comparison.

Explanation of symbols

  1 ... hydrophilic particles, 2 ... hydrophobic particles.

Claims (12)

  An electrode comprising a catalyst layer containing a catalyst metal, a conductive carrier carrying the catalyst metal, and a proton conductive member, wherein the catalyst layer contains hydrophilic particles carrying hydrophobic particles .   2. The electrode according to claim 1, wherein in the hydrophobic particles-supporting hydrophilic particles, the average particle diameter of the hydrophilic particles is 10 to 1000 times the average particle diameter of the hydrophobic particles.   The electrode according to claim 1 or 2, wherein in the hydrophobic particle-supporting hydrophilic particles, the hydrophilic particles have an average particle size of 0.1 to 10 µm.   The electrode according to any one of claims 1 to 3, wherein in the hydrophobic particle-supporting hydrophilic particles, the hydrophobic particles have an average particle diameter of 0.01 to 1.0 µm.   In the hydrophobic particle-supporting hydrophilic particle, the hydrophilic particle is at least one metal oxide selected from the group consisting of silicon, titanium, aluminum, zirconium, tantalum, yttrium, tin, tungsten, indium, and antimony. Item 5. The electrode according to Items 1-4.   The electrode according to any one of claims 1 to 5, wherein in the hydrophobic particle-supporting hydrophilic particle, the hydrophobic particle is a fluorine-containing polymer compound.   The electrode according to claim 1, wherein the catalytic metal includes at least one selected from the group consisting of Pt, Ir, Au, Ag, and Pd.   The electrode according to claim 1, wherein the catalyst metal has an average particle size of 1 to 10 nm.   The electrode according to any one of claims 1 to 8, wherein in the hydrophobic particle-supporting hydrophilic particle, the amount of the hydrophobic particle supported is 0.1 to 20% by mass.   The electrode according to any one of claims 1 to 9, wherein the content of the hydrophobic particle-supporting hydrophilic particles in the catalyst layer is 1.0 to 30% by mass.   An MEA comprising the electrode according to claim 1.   A fuel cell using the electrode according to claim 1.

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