JP2014160671A - Membrane electrode assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a membrane electrode assembly which exhibits excellent power generation performance even in a low humidification condition.SOLUTION: The membrane electrode assembly of the present invention has a structure in which an anode electrode and a cathode electrode are disposed on both surfaces of a solid polymer electrolyte membrane so as to be opposite to each other and are held between a pair of gas diffusion layers. The membrane electrode assembly is manufactured by a manufacturing method including applying a cathode catalyst ink containing a catalyst carrier in which a catalyst component is carried on a conductive carrier, a polymer electrolyte, and SiOto one surface of the solid polymer electrolyte membrane, and drying the cathode catalyst ink to obtain a catalyst layer-electrolyte membrane assembly. The average size of SiOin the cathode electrode is 50-500 nm.

Description

  The present invention relates to a fuel cell electrode. More specifically, the present invention relates to a membrane / electrode assembly that is excellent in ion conductivity and that can exhibit high power generation performance even under low humidification conditions.

  In recent years, in response to social demands and trends against the background of energy and environmental problems, 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. A fuel cell is a clean power generation system in which the product of an electrode reaction is water in principle and has almost no adverse effect on the global environment. In particular, a polymer electrolyte fuel cell (PEFC) is expected as a power source for electric vehicles because it operates at a relatively low temperature. The structure of a polymer electrolyte fuel cell (hereinafter also simply referred to as “fuel cell”) generally includes a membrane electrode assembly (hereinafter also referred to as “MEA” or “membrane-electrode assembly”) as a separator. It has a structure sandwiched between. In the membrane / electrode assembly, an electrode catalyst layer (hereinafter also referred to as an “electrode”) is disposed opposite to both surfaces of a solid polymer electrolyte membrane, and a pair of gas diffusion layers are sandwiched between the electrode catalyst layers.

In the polymer electrolyte fuel cell having the membrane electrode assembly as described above, the electrode reaction represented by the following reaction formula is performed on both electrodes (cathode and anode) sandwiching the polymer electrolyte membrane according to the polarity. Proceed and get electrical energy. First, hydrogen contained in the fuel gas supplied to the anode (hydrogen electrode) side is oxidized by the catalyst component to become protons (hydrogen ions) and electrons (2H ₂ → 4H ⁺ + 4e ⁻ : reaction 1). Next, the generated proton is accompanied by several water molecules, passes through the polymer electrolyte (hereinafter also referred to as “ionomer”) contained in the electrode, and further passes through the solid polymer electrolyte membrane that is in contact with the electrode. Reach the catalyst on the (oxygen electrode) side. Further, the electrons generated at the anode electrode are transferred to the cathode electrode through the conductive carrier constituting the catalyst carrier, the gas diffusion layer in contact with the electrode on the side different from the solid polymer electrolyte membrane, the separator, and the external circuit. Reach. Then, protons and electrons that have reached the cathode electrode react with oxygen contained in the oxidant gas supplied to the cathode side to generate water (O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O: Reaction 2). In the fuel cell, electricity can be taken out through the above-described electrochemical reaction.

  The above electrochemical reaction takes place at the catalyst-electrolyte-reactive gas three-phase interface. Therefore, in order to increase the contact area between the catalyst and the electrolyte, a polymer electrolyte made of the same material as the solid polymer electrolyte membrane is usually dispersed around the catalyst. As the polymer electrolyte, perfluorocarbon-based resins having sulfonic acid groups are widely used. Such a perfluorocarbon-based resin does not have a cross-linked structure, and has a structure that easily swells and shrinks depending on the surrounding moisture concentration environment. Furthermore, perfluorocarbon-based resins have a property that proton conductivity decreases when the water content in the resin decreases. Therefore, in order to maintain good proton conductivity (hereinafter also referred to as “ion conductivity”), the water concentration in the electrode should be maintained to such an extent that the polymer electrolyte made of such a resin is sufficiently swollen. Is preferred.

  In general, in the membrane electrode assembly as described above, the anode electrode may be in a relatively low humidification condition because water molecules move to the cathode side simultaneously with the movement of protons accompanying the oxidation reaction of the fuel gas. Are known. On the other hand, the cathode electrode is relatively humidified by water generated by the electrochemical reaction. Nevertheless, it is difficult to sufficiently maintain the moisture concentration of the cathode electrode. In order to ensure the desired power generation performance, the oxidant gas (air) supplied to the cathode electrode is also subjected to some humidification means. It is necessary to supply the cathode electrode from the current state.

  Under such circumstances, various methods for maintaining the moisture concentration in the electrode have been tried. For example, Patent Document 1 describes a catalyst layer comprising catalyst particles, a polymer electrolyte, and layered silicate particles. According to this document, water is retained between the layers of the layered silicate particles, so that it has an excellent self-humidifying function, thereby enabling a low-humidifying operation without reducing power generation performance.

JP 2002-216777 A

  However, even with the catalyst layer containing the layered silicate particles described in Patent Document 1, there is a problem that desired power generation performance cannot be obtained under low humidification conditions.

  Then, an object of this invention is to provide the membrane electrode assembly which exhibits the outstanding electric power generation performance also in low humidification conditions.

The present inventors have intensively studied to solve the above problems. As a result, it has been found that by providing an ion conduction auxiliary phase containing SiO ₂ in the cathode electrode of the membrane electrode assembly, ion conductivity is remarkably improved and excellent power generation performance is exhibited even under low humidification conditions. .

That is, the membrane electrode assembly of the present invention for achieving the above object has a structure in which an anode electrode and a cathode electrode are arranged opposite to both surfaces of a solid polymer electrolyte membrane and are sandwiched between a pair of gas diffusion layers. Have The membrane electrode assembly includes a catalyst carrier in which a catalyst component is supported on a conductive carrier, a polymer electrolyte, and a cathode catalyst ink including SiO _2, and one of the solid polymer electrolyte membranes. It is manufactured by a manufacturing method including obtaining a catalyst layer-electrolyte membrane assembly by applying to a surface and drying, and the average size of SiO _{2 in} the cathode electrode is 50 to 500 nm.

According to the present invention, the presence of SiO _{2 in} contact with the polymer electrolyte in the cathode electrode improves the ionic conductivity in the cathode electrode, and can exhibit excellent power generation performance even under low humidification conditions.

It is the cross-sectional schematic which represented typically the electrode for fuel cells of this form. It is a figure showing independent particles. It is a figure showing the aggregated particle. It is a figure showing agglomerated particles. Dark gray particles represent one aggregated particle. It is a figure for demonstrating an average size. The length from the end of the arrow to the end represents the size. It is the cross-sectional schematic which represented typically the membrane electrode assembly of this form. 1 is a schematic cross-sectional view schematically showing a fuel cell of the present embodiment. It is a figure showing typically the fuel cell system of this form. 6 is a graph showing cell voltage and cell resistance with respect to current density in Example 1 and Comparative Example 1. 10 is a graph showing cell voltage and cell resistance with respect to current density in Comparative Example 2 and Comparative Example 3. 2 is a transmission electron micrograph of an ultrathin section of an electrode portion in Example 1. The gray part enclosed with the thick line of this photograph represents an ion conduction auxiliary phase.

  Hereinafter, preferred embodiments of the present invention will be described. The present embodiment includes a catalyst carrier in which a catalyst component is supported on a conductive carrier, a polymer electrolyte that covers the catalyst carrier, and an ion conduction auxiliary phase in contact with the polymer electrolyte. The average size relates to an electrode for a fuel cell having a size of 10 nm to 1 μm. Hereinafter, the present invention will be described with reference to the drawings. However, the technical scope of the present invention should be determined based on the description of the scope of claims, and is not limited to the following embodiments. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. In addition, the dimensional ratios in the drawings are exaggerated for convenience of explanation, and may be different from the actual ratios.

[electrode]
FIG. 1 is a schematic cross-sectional view schematically showing a fuel cell electrode of the present embodiment. In FIG. 1, the electrode 10 includes a catalyst carrier 11, a polymer electrolyte 12, and fumed silica 13 as an ion conduction auxiliary phase. The catalyst carrier 11 is formed by carrying platinum 15 as a catalyst component on a conductive carrier made of carbon black 14. The surface of the catalyst carrier 11 is covered with the polymer electrolyte 12. On the other hand, the fumed silica 13 is dispersed in the electrode 10 so as to be in contact with the polymer electrolyte 12. Hereinafter, although the member which comprises the electrode for fuel cells of this invention is demonstrated in detail, the technical scope of this invention is not restrict | limited only to the following form.

(Catalyst carrier)
The catalyst carrier is formed by carrying a catalyst component on a conductive carrier. Electrons generated by the electrode reaction on the catalyst component in the anode side electrode are led to an external circuit through the conductive carrier. On the other hand, in the cathode side electrode, electrons are supplied from an external circuit to the catalyst component through the conductive carrier and are subjected to an electrode reaction.

  The conductive carrier is a carrier that supports the catalyst component and has conductivity. Any conductive carrier may be used as long as it has a specific surface area sufficient to support the catalyst component in a desired dispersed state and sufficient electron conductivity. In the composition of the conductive carrier, the main component is preferably carbon. Specific examples of the material for the conductive carrier include carbon black, activated carbon, coke, natural graphite, and artificial graphite. “The main component is carbon” means that the main component contains carbon atoms, and is a concept that includes both carbon atoms and substantially carbon atoms. In some cases, elements other than carbon atoms may be included in order to improve the characteristics of the fuel cell. In addition, “consisting essentially of carbon atoms” means that contamination of about 2 to 3% by mass or less of impurities can be allowed.

The BET (Brunauer-Emmet-Teller) specific surface area of the conductive carrier is not particularly limited as long as it is a specific surface area sufficient to carry the catalyst component in a highly dispersed state, but is preferably 100-1500 m ² / g, more Preferably it is 600-1000 m < ² > / g. When the specific surface area of the conductive support is within such a range, the balance between the dispersibility of the catalyst component on the conductive support and the effective utilization rate of the catalyst component can be appropriately controlled.

  Although there is no restriction | limiting in particular also about the average particle diameter of an electroconductive support | carrier, Usually, it is 5-200 nm, Preferably it is about 10-100 nm. As the value of “average particle diameter of conductive carrier”, a value calculated by a primary particle diameter measurement method using a transmission electron microscope (TEM) is employed.

  The catalyst component has a function of catalyzing the electrochemical reaction. The catalyst component supported on the conductive carrier is not particularly limited as long as it has a catalytic action for promoting an electrochemical reaction, and a conventionally known catalyst component can be appropriately employed. Specific examples of the catalyst component include platinum, ruthenium, iridium, rhodium, palladium, osmium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, and alloys thereof. Etc. Of these, the catalyst component preferably contains at least platinum from the viewpoint of excellent catalytic activity, elution resistance, and the like. The composition of the alloy in the case of using an alloy as the catalyst component varies depending on the type of metal to be alloyed and can be appropriately selected by those skilled in the art, but preferably 30 to 90 atomic% of platinum, and other metals to be alloyed It is about 10-70 atomic%. Note that an “alloy” is a general term for a metal element having one or more metal elements or non-metal elements added and having metallic properties. The alloy structure consists of a eutectic alloy, which is a mixture of the component elements as separate crystals, a component element completely melted into a solid solution, and a component element composed of an intermetallic compound or a compound of a metal and a nonmetal. There are things that are forming. Any of these forms may be sufficient as the alloy which concerns on this form. Here, the alloy composition can be specified by using an ICP emission analysis method.

  The shape and size of the catalyst component are not particularly limited, and the same shape and size as those of conventionally known catalyst components can be appropriately employed. However, the shape of the catalyst component is preferably granular. And the average particle diameter of a catalyst component particle becomes like this. Preferably it is 0.5-30 nm, More preferably, it is 1-20 nm. When the average particle size of the catalyst component particles is within such a range, the balance between the catalyst utilization rate related to the effective electrode area where the electrochemical reaction proceeds and the ease of loading can be appropriately controlled. In the present invention, the value of the “average particle diameter of the catalyst component particles” is determined from the crystallite diameter determined from the half-value width of the diffraction peak of the catalyst component particles in X-ray diffraction or a transmission electron microscope (TEM) image. It can be calculated as the average value of the particle diameter of the catalyst component.

  The ratio of the content of the conductive carrier and the catalyst component in the catalyst carrier is not particularly limited. However, the content (supported amount) of the catalyst component is preferably 110 to 300% by mass, more preferably 130 to 250% by mass, and further preferably 150 to 200% based on 100% by mass of the conductive carrier. % By mass. When the content of the catalyst component is 100% by mass or more, the catalyst performance is sufficiently exerted, which can contribute to the improvement of the power generation performance of the fuel cell. On the other hand, the catalyst component content of 300% by mass or less is preferable because aggregation of the catalyst components on the surface of the conductive support is suppressed and the catalyst components are highly dispersed and supported. In addition, as a value of the ratio of content mentioned above, the value measured by ICP emission spectrometry shall be employ | adopted.

(Polymer electrolyte)
The polymer electrolyte has a function of improving ion conductivity in the electrode. There is no restriction | limiting in particular in the specific form of the polymer electrolyte contained in an electrode, A conventionally well-known polymer electrolyte can be employ | adopted suitably in the technical field of a fuel cell. As the polymer electrolyte, for example, ion exchange resins such as a fluorine-based polymer electrolyte and a hydrocarbon-based polymer electrolyte can be used.

  Specific examples of fluoropolymer electrolytes include perfluorocarbon sulfones such as Nafion (registered trademark, manufactured by DuPont), Aciplex (registered trademark, manufactured by Asahi Kasei Corporation), Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.), and the like. Acid polymer, perfluorocarbon phosphonic acid polymer, trifluorostyrene sulfonic acid polymer, ethylene tetrafluoroethylene-g-styrene sulfonic acid polymer, ethylene-tetrafluoroethylene copolymer, polyvinylidene fluoride-perfluorocarbon sulfonic acid System polymers and the like. From the viewpoint of power generation performance such as heat resistance and chemical stability, these fluorine-based polymer electrolytes are preferably used, and more preferably perfluorocarbonsulfonic acid-based polymers are used.

  Examples of the hydrocarbon polymer electrolyte include sulfonated polyethersulfone (S-PES), sulfonated polyaryletherketone, sulfonated polybenzimidazole alkyl, phosphonated polybenzimidazole alkyl, sulfonated polystyrene, sulfonated poly Examples include ether ether ketone (S-PEEK) and sulfonated polyphenylene (S-PPP). These hydrocarbon polymer electrolytes are preferably used from the viewpoint of production such that the raw material is inexpensive, the production process is simple, and the selectivity of the material is high. In addition, only 1 type may be used independently for the polymer electrolyte mentioned above, and 2 or more types may be used together.

  Materials other than the above-described ion exchange resin may be used as the polymer electrolyte. As such materials, for example, liquids, solids, and gel materials having high proton conductivity can be used, and solid acids such as phosphoric acid, sulfuric acid, antimonic acid, stannic acid, and heteropoly acid, phosphoric acid, and the like. Hydrocarbon polymer compound doped with a non-organic acid, organic / inorganic hybrid polymer partially substituted with proton conductive functional group, gel impregnated with phosphoric acid solution or sulfuric acid solution in polymer matrix And the like proton conductive material. A mixed conductor having both proton conductivity and electron conductivity can also be used as a polymer electrolyte.

  The ion exchange capacity of the polymer electrolyte is preferably 0.8 to 1.5 mmol / g, and more preferably 1.0 to 1.5 mmol / g, from the viewpoint of excellent ion conductivity. The “ion exchange capacity” of the polymer electrolyte means the number of moles of sulfonic acid groups per unit dry mass of the polymer electrolyte. The value of “ion exchange capacity” can be calculated by removing the dispersion medium of the polymer electrolyte dispersion by heat drying or the like to obtain a solid polymer electrolyte, and performing neutralization titration.

  There is no restriction | limiting in particular also about content of the polymer electrolyte in an electrode. However, the mass ratio of the content of the polymer electrolyte to the content of the conductive carrier (the mass ratio of the polymer electrolyte / the conductive carrier) is preferably 0.5 to 2.0, more preferably 0.6. It is -1.5, More preferably, it is 0.8-1.3. A mass ratio of the polymer electrolyte / conductive support of 0.8 or more is preferable from the viewpoint of suppressing the internal resistance value in the electrode. On the other hand, the polymer electrolyte / conductive support mass ratio is preferably 1.3 or less from the viewpoint of suppressing flooding.

  In the electrode, the polymer electrolyte exists in a form covering the above-described catalyst carrier, but it is not always necessary to coat the entire surface of the catalyst carrier.

(Ion conduction auxiliary phase)
The ion conduction auxiliary phase plays a role of improving ion conductivity in the electrode. More specifically, the ion conduction auxiliary phase is made of a substance having a water retention function, and exists in contact with the polymer electrolyte in the electrode catalyst layer, thereby improving the conductivity of protons in the electrode catalyst layer. Bear.

The material constituting the ion conduction auxiliary phase is not particularly limited as long as it is a substance having a function of assisting conduction of hydrogen ions, and conventionally known materials can be appropriately used. Among these, it is preferable to use an inorganic oxide from the viewpoint of being relatively inexpensive and easily available and having low toxicity. The inorganic oxide preferably includes at least one selected from the group consisting of SiO ₂ , TiO ₂ , Al ₂ O ₃ , and ZrO ₂ . For example, when the inorganic oxide is a particle containing SiO ₂ , a silanol group (—SiOH) exists as a surface functional group on the surface of the particle. Since silanol groups have hydrophilic properties, water molecules can be retained around the particles. Accordingly, the presence of such particles in contact with the polymer electrolyte can assist in maintaining the water concentration of the polymer electrolyte and conducting protons.

  The shape of the ion conduction auxiliary phase is not particularly limited and may be various shapes such as granular, fibrous, and flake shaped, but is preferably granular from the viewpoint of water retention performance. In this specification, “granular” is a concept including any form of independent particles, aggregated particles, and agglomerated particles as shown in FIG. More specifically, the independent particles mean a form in which substantially spherical particles are present individually as shown in FIG. 2A. On the other hand, the aggregated particles mean a form in which a plurality of independent particles are physically connected as shown in FIG. 2B. In addition, the agglomerated particles mean a form in which a plurality of aggregated particles come into contact with each other to form an aggregate as shown in FIG. 2C. In FIG. 3C, dark gray particles represent one aggregated particle. Aggregated particles and agglomerated particles are both included in the same category in that they are connected bodies formed by connecting a plurality of independent particles. Of these forms, the form of aggregated particles or agglomerated particles is preferable. This is because when the particle size of the independent particles is small, a large number of the independent particles enter between the conductive carriers, which may increase the electrical resistance of the conductive carriers. Aggregated particles are formed by physically linking a plurality of independent particles and are not decomposed into independent particles by a normal operation. Therefore, even if it is an independent particle with a small particle diameter, the possibility that the above-mentioned electrical resistance will increase by making it the form of an aggregated particle is reduced. It is also advantageous in that the surface area that contributes to improving ion conductivity is enlarged. In addition, the manufacturing method of an aggregated particle does not have a restriction | limiting in particular, It can manufacture by a conventionally well-known method. Of course, various commercial products may be used. For example, the aggregated particles of inorganic oxide particles include fumed inorganic oxides such as fumed silica, fumed titania, fumed alumina, and fumed zirconia synthesized by hydrolyzing the raw material in the gas phase. It can be suitably used. Of these, fumed silica and fumed titania are preferably used. Of course, it may be in the form of independent particles as long as it has a particle size that is difficult to enter the pores of the conductive carrier. The independent particles in this case may be porous particles in order to increase the surface area.

  The average size of the ion conduction auxiliary phase is not particularly limited as long as the power generation performance is not significantly affected. However, as described above, when the size is very small, a large number may enter between the conductive carriers and increase the electric resistance. And the lower limit of average size becomes like this. Preferably it is 50 nm or more, More preferably, it is 100 nm or more. On the other hand, the upper limit value of the average size needs to be at least 1 μm or less from the viewpoint of good dispersion in the electrode. And the upper limit of average size becomes like this. Preferably it is 500 nm or less, More preferably, it is 300 nm or less. In this specification, “average size” means ion conduction observed in several to several tens of fields when an ultrathin section of an electrode is observed using an observation means such as a transmission electron microscope (TEM). Mean average size of auxiliary phase. The “size” in this case means the maximum distance among the distances between any two points on the contour line of the ion conduction auxiliary phase, as shown in FIG.

  In addition, an ion conduction auxiliary | assistant phase may contain a thing outside the range of said average size by the dispersion condition in an electrode. Therefore, when observed by the same method as the above average size measurement method, the average size is preferably 20% by area or less, more preferably 10% by area or less of the total area of the ion conduction auxiliary phase. It is permissible for an ion-conducting auxiliary phase to deviate from this range. In other words, when observed by the above method, the total area of the ion conduction auxiliary phase is preferably 80 to 100% by area, more preferably 90 to 100% by area within the above average size range. An ionic conduction auxiliary phase is present.

When the ion conduction auxiliary phase includes aggregated particles, the average particle diameter of the aggregated particles is preferably 10 to 1 μm, more preferably 50 to 500 nm, and further preferably 100 to 300 nm. Further, the BET (Brunauer-Emmet-Teller) specific surface area of the aggregated particles is preferably 30 to 500 m ² / g, more preferably 50 to 450 m ² / g, and further preferably 100 to 400 m ² / g. is there. Such agglomerated particles have a sufficiently large surface area that contributes to improving ion conductivity, so that the power generation performance can be further improved.

  The content (dry mass) of the ion conduction auxiliary phase in the electrode is preferably 1 to 20% by mass and more preferably 3 to 15% by mass with respect to 100% by mass of the total mass of the conductive support. By including such an amount of the ion conduction auxiliary phase in the electrode, the ion conductivity can be improved without increasing the resistance in the electrode.

  The ion conduction auxiliary phase improves the conductivity of protons in the polymer electrolyte present in the electrode, and thus exists in contact with the polymer electrolyte. However, not all ion conduction auxiliary phases contained in the electrode are necessarily present. There is no need to contact the polymer electrolyte. However, in order to improve ion conductivity more efficiently, 20 to 100% by mass is preferably in contact with the polymer electrolyte with respect to 100% by mass of the total mass of the ion conduction auxiliary phase. Further, the ion conduction auxiliary phase may be localized in at least a part of the electrode. Therefore, when the humidification conditions are different between the inlet portion and the outlet portion of the reaction gas, it is possible to arrange more ion conduction auxiliary phases in the lower humidified portion. However, in order to improve the ionic conductivity of the entire electrode, it is preferable to uniformly arrange the ion conduction auxiliary phase.

The electrode which concerns on this form can be manufactured by referring suitably the manufacturing method of a conventionally well-known electrode. As a preferred production method, first, a polymer electrolyte dispersion (ionomer dispersion) in which a polymer electrolyte is dispersed in a solvent is prepared. Among this, the material can be a ion-conductive auxiliary layer, for example, by the addition of aggregated particles of SiO _2, stirring and mixing. Thereafter, a catalyst carrier is added, and the aggregated particles are crushed using a bead mill, a ball mill, or the like. Thereby, the aggregated particles can be appropriately dispersed in the electrode ink. Furthermore, an electrode is formed in a form adjacent to the solid polymer electrolyte membrane by applying and drying the obtained electrode ink on a solid polymer electrolyte membrane described later.

[Membrane electrode assembly]
Next, the membrane electrode assembly including the fuel cell electrode will be described. FIG. 4 is a schematic cross-sectional view schematically showing the membrane electrode assembly of the present embodiment.

  As shown in FIG. 4, in the membrane-electrode assembly 100 of the present invention, the anode electrode 10 a and the cathode electrode 10 c are disposed so as to face both surfaces of the solid polymer electrolyte membrane 110. A pair of anode side gas diffusion layer 140a and cathode side gas diffusion layer 140c is sandwiched therebetween. At this time, at least one of the anode electrode 10a or the cathode electrode 10c is the fuel cell electrode described above. Hereinafter, although each component contained in the membrane electrode assembly of this form is demonstrated, the technical scope of this invention is not restrict | limited only to the following form.

(Solid polymer electrolyte membrane)
The solid polymer electrolyte membrane is composed of a polymer electrolyte having proton conductivity, and has a function of selectively transmitting protons generated at the anode electrode during operation of the fuel cell to the cathode electrode along the film thickness direction. The solid polymer electrolyte membrane also has a function as a partition for preventing the fuel gas supplied to the anode side and the oxidant gas supplied to the cathode side from being mixed.

  The specific configuration of the solid polymer electrolyte membrane is not particularly limited, and a membrane made of a polymer electrolyte that is conventionally known in the technical field of fuel cells can be appropriately employed. As the polymer electrolyte in this case, the same polymer electrolyte as that contained in the electrode of the present embodiment described above can be used. Therefore, description of the specific form of the polymer electrolyte is omitted here. In addition, 1 type may be sufficient as the polymer electrolyte contained in a solid polymer electrolyte membrane, and 2 or more types may be sufficient as it.

  The thickness of the solid polymer electrolyte membrane is appropriately determined in consideration of the characteristics of the membrane electrode assembly and the polymer electrolyte, and is not particularly limited. However, the thickness of the solid polymer electrolyte membrane is preferably 5 to 300 μm, more preferably 5 to 200 μm, still more preferably 10 to 150 μm, and particularly preferably 15 to 50 μm. When the thickness is in such a range, the balance of strength during film formation, durability during use, and output characteristics during use can be appropriately controlled.

(Gas diffusion layer)
There are two gas diffusion layers, an anode side gas diffusion layer and a cathode side gas diffusion layer. Hereinafter, when the anode side gas diffusion layer and the cathode side gas diffusion layer are not distinguished, they are simply referred to as “gas diffusion layer”. The gas diffusion layer has a function of promoting the diffusion of a reaction gas supplied through a gas flow path of a separator described later to the catalyst layer and a function as an electron conduction path.

  The material which comprises the base material of a gas diffusion layer is not specifically limited, A conventionally well-known knowledge can be referred suitably. For example, 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. The thickness of the substrate 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 of the substrate is within such a range, the balance between mechanical strength and diffusibility such as gas and water can be appropriately controlled.

  The gas diffusion electrode may further include another member (layer) as necessary. For example, the gas diffusion layer may have a carbon particle layer containing carbon particles on the catalyst layer side of the base material in order to promote the discharge of excess moisture present in the catalyst layer and suppress the occurrence of the flooding phenomenon. Good.

  The carbon particles contained in the carbon particle layer are not particularly limited, and conventionally known materials such as carbon black, graphite, and expanded graphite can be appropriately employed. Among them, it is preferable to use carbon black such as oil furnace black, channel black, lamp black, thermal black, and acetylene black because of excellent electron conductivity and a large specific surface area. The average particle diameter of the carbon particles is preferably about 10 to 100 nm. Thereby, while being able to obtain the high drainage property by capillary force, it becomes possible to improve contact property with a catalyst layer.

  The carbon particle layer may contain a water repellent. Examples of the water repellent include fluorine-based polymer materials such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyhexafluoropropylene, and tetrafluoroethylene-hexafluoropropylene copolymer (FEP). Examples include polypropylene and polyethylene. Among these, it is preferable to use a fluorine-based polymer material because it is excellent in water repellency, corrosion resistance during electrode reaction, and the like.

  In the membrane electrode assembly of the present embodiment, at least one of the anode electrode and the cathode electrode may be the above-described fuel cell electrode. In other words, any one of the anode electrode and the cathode electrode can be configured by the electrode of the present embodiment, and the remaining one can be configured by an electrode other than the electrode of the present embodiment. In this case, it is preferable that the cathode electrode is an electrode of this embodiment. Compared with the anode electrode, the cathode electrode is likely to be relatively humidified by water generated by the electrochemical reaction. In practice, however, the air taken from the outside as the oxidant gas has a low humidity, so it cannot be used as it is, and has to be subjected to an electrochemical reaction after passing through some humidifying means. However, since the electrode of this embodiment has remarkably superior ionic conductivity as compared with the conventional electrode, when this is used for the cathode electrode, the air taken from outside is directly used as the oxidizing agent without passing through the humidifying means. Can be used as gas. This eliminates the need for an oxidant gas humidifier, which has been indispensable until now, so that the entire fuel system can be reduced in size and cost. Needless to say, in order to improve ion conductivity in both the anode electrode and the cathode electrode, it is more preferable to use both the anode electrode and the cathode electrode as electrodes of this embodiment.

[Fuel cell]
Next, a fuel cell including the membrane electrode assembly will be described. FIG. 5 is a schematic cross-sectional view schematically showing the fuel cell of the present embodiment. FIG. 5 shows a single cell of the fuel cell. A fuel cell 200 shown in FIG. 5 has the membrane-electrode assembly 100 described above. In the polymer electrolyte fuel cell 200, the membrane-electrode assembly 100 is sandwiched between a pair of separators including an anode side separator 150a and a cathode side separator 150c. Here, the anode side gas diffusion layer 140a side surface of the anode side separator 150a is provided with a fuel gas flow path 152a through which fuel gas flows during operation, and the coolant flows through the opposite surface during operation. A cooling flow path (not shown) is provided. On the other hand, an oxidant gas flow path 152c through which an oxidant gas flows during operation is provided on the cathode side gas diffusion layer 140c side surface of the cathode side separator 150c, and a coolant is provided on the opposite side surface during operation. A cooling flow path (not shown) through which is circulated is provided. A gasket 160 is disposed around the polymer electrolyte fuel cell 200 so as to surround the pair of gas diffusion electrodes. Hereinafter, although each component included in the fuel cell of the present embodiment will be described, the technical scope of the present invention is not limited to the following embodiments.

(Separator)
The separator has a function of electrically connecting each cell in series when a plurality of single cells of a fuel cell such as a polymer electrolyte fuel cell are connected in series to form a fuel cell stack. The separator also functions as a partition wall that separates the fuel gas, the oxidant gas, and the coolant from each other. Therefore, as described above, each separator is provided with a gas flow path and a cooling flow path. As a material constituting the separator, conventionally known materials such as dense carbon graphite, carbon such as a carbon plate, and metal such as stainless steel can be appropriately employed without limitation. The thickness and size of the separator and the shape and size of each flow path provided are not particularly limited, and can be appropriately determined in consideration of the desired output characteristics of the obtained fuel cell.

(gasket)
The gasket is disposed around the fuel cell so as to surround the pair of electrodes and the gas diffusion layer, and has a function of preventing the gas supplied to the electrodes from leaking to the outside. A gas diffusion electrode refers to a joined body of a gas diffusion layer and an electrode. The material constituting the gasket is not particularly limited, but rubber materials such as fluoro rubber, silicon rubber, ethylene propylene rubber (EPDM), polyisobutylene rubber, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), Examples thereof include fluorine polymer materials such as polyhexafluoropropylene and tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and thermoplastic resins such as polyolefin and polyester. Moreover, there is no restriction | limiting in particular also in the thickness of a gasket, Preferably it is 50 micrometers-2 mm, More preferably, what is necessary is just to be about 100 micrometers-1 mm.

[Fuel cell system]
Next, a fuel cell system including the fuel cell will be described. FIG. 6 is a schematic view schematically showing the fuel cell stack of the present embodiment. A fuel cell system 300 shown in FIG. 3 has a fuel cell stack 310 formed by stacking polymer electrolyte fuel cells. In the fuel cell, the cathode electrode is the above-described fuel cell electrode. The fuel cell stack 310 is provided with fuel gas supply means for supplying fuel gas and oxidant gas supply means for supplying oxidant gas. The fuel gas supply means includes a hydrogen cylinder 320 for supplying hydrogen as a fuel gas, a flow rate adjusting valve 330a for adjusting the gas supply flow rate, and a humidifier 340a for humidifying the fuel gas. On the other hand, the oxidant gas supply means includes a compressor 350 for supplying external air, and a flow rate adjusting valve 330c for adjusting the gas supply flow rate. In addition, an inverter 360 and a load 370 are provided as an electrical system. In addition, a control system (not shown) for controlling power to be transmitted to the load, gas supply amount, humidification amount, fuel cell stack temperature, and the like is provided. A fuel cell using the fuel cell electrode of this embodiment as a cathode electrode has excellent ion conductivity and can exhibit excellent power generation performance even under low humidification conditions. Therefore, as shown in FIG. 6, the fuel cell can be operated and sufficient electric power can be supplied even if the oxidant gas supply means does not include means for humidifying the oxidant gas.

[vehicle]
A vehicle equipped with the above-described fuel cell or fuel cell stack of the present invention is also included in the technical scope of the present invention. The fuel cell and fuel cell stack of the present invention are extremely excellent in output performance particularly under low humidification conditions, and are suitable for vehicle applications that require high output.

[Operating method of fuel cell]
Furthermore, the present invention provides a method for operating the fuel cell. As described above, since the electrode of this embodiment has water retention, the ion conductivity is excellent. Therefore, the fuel cell having this electrode has excellent power generation performance, particularly under low humidification conditions. Therefore, by operating the fuel cell by supplying fuel gas or oxidant gas to the fuel cell in which the anode electrode or the cathode electrode is an electrode of this embodiment under low humidification conditions, the effect of the present invention is more remarkable. Become.

  When the anode electrode is the electrode of this embodiment, the fuel cell operating method of the present invention supplies the fuel gas at a relative humidity of 20 to 100% and the oxidation is performed when the cathode electrode is the electrode of this embodiment. The agent gas is supplied under the condition of relative humidity of 0 to 40%. The relative humidity of the fuel gas is preferably 30% or more, but if the electrode of the present invention is used, it is possible to generate power well even when the relative humidity is 50% or less. On the other hand, unless the relative humidity of the oxidant gas is conventionally high, power generation cannot be performed satisfactorily, but by using the electrode of the present invention, power generation is performed well even at a relative humidity of 40% or less, and even at a relative humidity of 30% or less. be able to. The relative humidity of the fuel gas or oxidant gas is the relative humidity in the vicinity of the entrance to the cell of the gas flow path.

  Other conditions such as the type of fuel gas and oxidant gas, supply amount, supply temperature, and operating temperature of the fuel cell are not particularly limited, and conventionally known operating methods in the technical field of fuel cells can be appropriately employed.

  Of these operating methods, the cathode electrode is preferably the electrode of this embodiment, and the oxidant gas is supplied under conditions of relative humidity of 0 to 40%. “Supplying the relative humidity of the oxidant gas at 0 to 40%” means that it is possible to “supply the external air as the oxidant gas without being humidified”. The electrode of this embodiment can exhibit excellent power generation performance even under more severe low humidification conditions as compared with the conventional electrode. Therefore, the effect of the present invention becomes even more remarkable when an oxidizing gas having a relative humidity in such a range is supplied.

  The effect of this invention is demonstrated using a following example and a comparative example. However, the technical scope of the present invention is not limited only to the following examples.

  In the following Examples and Comparative Examples, the power generation performance under low humidification conditions and the ionomer resistance in the catalyst layer were evaluated depending on the presence or absence of an ion conduction auxiliary phase. Specifically, the fuel cell electrodes of Example 1 and Comparative Examples 1 to 3 were produced as follows.

[Example 1]
(1) Preparation of catalyst ink for anode Catalyst support in which platinum is supported as a catalyst component on carbon black (Ketjen Black (registered trademark) EC) as a conductive carrier (platinum content: 46 mass%, specific surface area: 314 m ² / G) 7 g, Nafion (registered trademark, manufactured by DuPont) dispersion D-2020 (ion exchange capacity 1.0 mmol / g, electrolyte content 20% by mass) 15.3 g (conductive carrier) The mass ratio of the polymer electrolyte when the mass was 1 was 0.9), 78.3 g of ion-exchanged water, and 48.6 g of 1-propanol were dispersed and mixed using a bead mill to obtain an anode catalyst ink. .

(2) Preparation of Cathode Catalyst Ink 0.19 g of SiO ₂ fine particles (independent particle diameter of 100 nm, BET specific surface area of 300 m ^{2 /} g) are added to the same material as that used for the anode side catalyst ink, and a bead mill is prepared. By using and dispersing and mixing, a cathode catalyst ink was obtained. At this time, the weight of the SiO ₂ fine particles when the conductive carrier mass is 1 is 0.05.

(3) Application of catalyst ink to electrolyte membrane The catalyst ink for anode is applied to one surface of Nafion (registered trademark, manufactured by DuPont) NR-211 (thickness 25 μm) which is a solid polymer electrolyte membrane. Cathode catalyst ink was applied to the surface in a size of 5 cm × 5 cm using a spray-type coating device, and dried to form a catalyst layer to obtain a catalyst layer-electrolyte membrane assembly. The amount of platinum in the catalyst layer at this time was 0.35 mg / cm ² for both the anode and the cathode.

(4) Assembly of single cell for evaluation Two gas diffusion layers (GDL25BC, manufactured by SGL Carbon Japan Co., Ltd.) are stacked with the catalyst layer-electrolyte membrane assembly sandwiched therebetween so that the carbon particle layer faces each other, and the membrane-electrode A joined body was formed, and this was sandwiched in the order of two graphite separators, two gold-plated stainless steel current collector plates, and two stainless steel end plates to form a single cell for evaluation.

[Comparative Example 1]
(2) A single cell for evaluation was produced in the same manner as in Example 1 except that no SiO ₂ fine particles were added in the preparation of the catalyst ink for cathode.

[Comparative Example 2]
(2) In the preparation of the cathode catalyst ink, SiO ₂ independent particles (independent particle diameter of 2 to 3 nm) were used as the ion conduction auxiliary phase. In addition, (3) in the application of the catalyst ink to the electrolyte membrane, an electrolyte membrane (thickness 15 μm) prepared by casting Nafion (registered trademark, manufactured by DuPont) dispersion D-2020 as the solid polymer electrolyte membrane is used. It was. Except for these changes, an evaluation unit cell was produced in the same manner as in Example 1.

[Comparative Example 3]
(2) A single cell for evaluation was produced in the same manner as in Comparative Example 2 except that the independent particles of SiO ₂ were not added in the preparation of the cathode catalyst ink.

<Measurement of ionomer resistance in catalyst layer>
In the single cell for evaluation, the cell temperature was set to 30 ° C., hydrogen having a relative humidity of 100% was supplied to the anode side, and nitrogen having a relative humidity of 100% was supplied to the cathode side, and ionomer resistance was measured. Then, ionomer resistance was derived from the results obtained by Cole-Cole plot analysis of the results obtained by EIS (electrochemical impedance spectrum method). The results of Example 1 and Comparative Example 1 are shown in Table 1 below, and the results of Comparative Example 2 and Comparative Example 3 are shown in Table 2 below.

From Table 1, it was shown that the ionomer resistance was reduced in Example 1 including the ion conduction auxiliary phase as compared with Comparative Example 1 not including the ion conduction auxiliary phase. In Table 2, the ionomer resistance increased in Comparative Example 2 containing fine independent particles of SiO ₂ compared to Comparative Example 3. This was thought to be due to the non-conductive SiO ₂ fine particles entering the pores of the conductive support and increasing the electrical resistance of the conductive support.

<Evaluation of power generation performance>
In the single cell for evaluation, the cell temperature was set to 80 ° C., hydrogen having a relative humidity of 20% was supplied to the anode side, and air having a relative humidity of 20% was supplied to the cathode side. The power generation performance was evaluated by supplying hydrogen from the anode side at 4 liters / minute and air from the cathode side at 8 liters / minute under a pressurized condition of 200 kPa absolute pressure. The results of Example 1 and Comparative Example 1 are shown in FIG. 3, and the results of Comparative Example 2 and Comparative Example 3 are shown in FIG.

According to FIG. 3, Example 1 including the ion conduction auxiliary phase showed that the cell voltage was improved as compared with Comparative Example 1 not including the ion conduction auxiliary phase. The cell resistance was almost the same between Example 1 and Comparative Example 1. On the other hand, according to FIG. 4, since the cell resistance increased in Comparative Example 2 containing fine independent particles of SiO ₂ as compared with Comparative Example 3, a decrease in cell voltage was observed.

<Observation of catalyst layer>
An ultrathin section of the catalyst layer portion prepared in Example 1 was cut out and observed using a transmission electron microscope (TEM). A TEM photograph is shown in FIG. According to FIG. 9, carbon black carrying platinum particles observed as dark gray fine particles can be confirmed. And the light gray ion conduction auxiliary phase can be confirmed in the gap part of carbon black. Although the polymer electrolyte cannot be confirmed in this photograph, it is considered that the polymer electrolyte is present in a state where carbon black and an ion conduction auxiliary phase are coated.

10 electrodes,
10a anode electrode,
10c cathode electrode,
11 catalyst carrier,
12 Polyelectrolyte,
13 Ionic conduction auxiliary phase,
14 conductive carrier,
15 catalyst components,
100 membrane-electrode assembly,
110 solid polymer electrolyte membrane,
140a anode side gas diffusion layer,
140c cathode side gas diffusion layer,
150a anode side separator,
150c cathode side separator,
152a fuel gas flow path,
152c oxidant gas flow path,
160 gasket,
200 fuel cells,
300 fuel cell system,
310 fuel cell stack,
320 hydrogen cylinder,
330a, 330c Flow rate adjusting valve,
340a humidifier,
350 compressor,
360 inverter,
370 Load.

Claims (4)

In the membrane electrode assembly in which the anode electrode and the cathode electrode are disposed opposite to both surfaces of the solid polymer electrolyte membrane and sandwiched between the gas diffusion layers,
A cathode catalyst ink containing a catalyst carrier in which a catalyst component is supported on a conductive carrier, a polymer electrolyte, and SiO ₂ is applied to one surface of the solid polymer electrolyte membrane and dried. Manufactured by a manufacturing method including obtaining a catalyst layer-electrolyte membrane assembly,
The membrane electrode assembly in which the average size of the SiO ₂ in the cathode electrode is 50 to 500 nm. A membrane electrode assembly according to claim 1;
A fuel cell having a pair of separators each including an anode side separator having a fuel gas flow path through which fuel gas flows and a cathode side separator having an oxidant gas flow path through which oxidant gas flows, sandwiching the membrane electrode assembly .   The method of operating a fuel cell according to claim 2, wherein the oxidant gas is supplied to the gas flow path under a condition of a relative humidity of 0 to 40%. A fuel cell stack formed by stacking the fuel cells according to claim 2;
Fuel gas supply means for supplying the fuel gas to the fuel cell stack;
An oxidant gas supply means for supplying the oxidant gas to the fuel cell stack,
The oxidant gas supply means does not include means for humidifying the oxidant gas.