JP5055854B2 - Membrane / electrode assembly for fuel cells - Google Patents

Description

  The present invention relates to a membrane / electrode assembly for a fuel cell, and more particularly to a membrane / electrode assembly for a fuel cell with an improved anode and / or cathode gas diffusion layer.

  A fuel cell converts chemical energy directly into electrical energy by supplying fuel to one of two electrically connected electrodes and supplying an oxidant to the other to cause oxidation of the fuel electrochemically. Unlike thermal power generation, fuel cells are not subject to the Carnot cycle, and thus exhibit high energy conversion efficiency. A polymer electrolyte fuel cell is a fuel cell that uses a solid polymer electrolyte membrane as an electrolyte, and has advantages such as easy miniaturization and operation at a low temperature. It is attracting attention as a power source.

  Conventionally, in a polymer electrolyte fuel cell, as shown in FIG. 8, as a membrane-electrode assembly 210, an anode (fuel) comprising an anode side catalyst layer 202a and an anode side gas diffusion layer 203a on one side of a solid polymer electrolyte membrane 201 is provided. Electrode) 205, and a cathode (oxidant electrode) 206 comprising a cathode side catalyst layer 202b and a cathode side gas diffusion layer 203b is used on the other surface. The membrane / electrode assembly 210 is further sandwiched on both sides by separators 208 a and 208 b that also function as current collectors, thereby forming a single cell 200. A fuel gas flow path 207a is formed on the side of the anode side separator 208a in contact with the anode 205, and an oxidant gas flow path 207b is formed on the side of the cathode side separator 208b in contact with the cathode 206.

When a hydrogen-containing gas is used as the fuel gas, the reaction of the formula (1) proceeds in the anode side catalyst layer 202a. At this time, electrons generated by the equation (1) reach the cathode 206 after working with an external load via an external circuit.
H ₂ → 2H ⁺ + 2e ⁻ (1)
The proton generated in the formula (1) moves in the polymer electrolyte membrane 201 from the anode 205 side to the cathode 206 side by electroosmosis while being hydrated with water.

Further, when an oxygen-containing gas is used as the oxidant gas, the reaction of the formula (2) proceeds in the cathode side catalyst layer 202b.
2H ⁺ + (1/2) O ₂ + 2e ⁻ → H ₂ O (2)
A part of the water generated in the cathode side catalyst layer 202b evaporates through the cathode side gas diffusion layer 203b and is discharged to the outside of the fuel cell, and the remaining part is back-diffused in the anode direction due to the concentration gradient.

As described above, in a polymer electrolyte fuel cell, water needs to be hydrated with protons to move from the anode to the cathode by electroosmosis of protons. The proton conductivity of each electrode provided is affected by the amount of water contained in each of these parts. Therefore, if there is a region with a low moisture content in the membrane / electrode assembly, the proton conductivity in that region is lowered, and the power generation efficiency of the entire fuel cell is lowered. In addition, variation in moisture content in the membrane / electrode assembly also greatly affects proton conductivity.
The water distribution in the surface direction of the membrane / electrode assembly generally has a low water content on the upstream side of the gas flow path and a high water content on the downstream side. This is because the gas flowing in the gas flow path gradually flows from the upstream to the downstream along the surface of the membrane / electrode assembly, and the water taken away from the vaporized or wet solid polymer electrolyte membrane is gradually This is because it is humidified and the difference in the humidified state affects each part of the membrane-electrode assembly.

  In the polymer electrolyte fuel cell, operation in a high temperature range is desired because of the merit that the radiator for cooling the cell can be reduced in size and flooding can be prevented. However, the entire gas flow path on the anode side and the gas on the cathode side are desired. Dry-up in the upstream of the flow path is conspicuous in an operation under a high temperature condition, particularly a low air utilization rate (a state where the air flow rate is large) under a high temperature condition. As a dry-up solution, use of a device such as a bubbler forcibly supplying moisture can be considered. However, such ancillary equipment increases the size of the fuel cell system and increases the cost. Therefore, it is not used as much as possible, or it is easy to reduce the load on the system or cost on the premise that it is used as an auxiliary. It is desirable to use a new one.

  As a technique for solving such a problem, for example, a mixed layer made of fluororesin and carbon black is provided between the catalyst layer and the gas diffusion layer, and the mixed layer at the fuel gas and oxidant gas inlet side portion is provided. Has been proposed in which the thickness of the fuel cell is made larger than the thickness of the outlet side portion or the porosity of the mixed layer in the inlet side portion is smaller than the porosity of the outlet side portion (Patent Document 1).

JP 2001-135326 A

By changing the gas diffusivity in each part by changing the thickness or the porosity of the mixed layer in the fuel gas and oxidant gas inlet side part and outlet side part, This prevents partial drying or excessive wetting of the electrolyte membrane.
However, in Patent Document 1, only the magnitude relationship between the gas diffusivity in the gas inlet side portion and the gas outlet side portion in the mixed layer is defined, and the gas diffusibility between the mixed layer, the gas diffusion layer, and the catalyst layer is defined. Nothing is disclosed about this relationship, and it is difficult to sufficiently prevent the catalyst layer and the electrolyte membrane from drying.

  The present invention has been accomplished in view of the above circumstances, and in a high temperature operation, the membrane / electrode assembly is dried up over a wide range of operating conditions from a low air utilization rate region to a high air utilization rate region. An object of the present invention is to provide a membrane-electrode assembly for a fuel cell that prevents the above-described problem and exhibits good power generation efficiency.

In order to solve the above problems, a membrane / electrode assembly for a fuel cell according to the present invention includes an electrolyte membrane, and at least an anode side catalyst layer and an anode side gas diffusion layer on one surface of the electrolyte membrane. A membrane / electrode assembly for a fuel cell, comprising: an anode laminated in order; and a cathode in which at least a cathode catalyst layer and a cathode gas diffusion layer are laminated in this order on the other surface of the electrolyte membrane. The catalyst layer has the following conditions on at least one of the anode side and the cathode side:
The catalyst component-supporting carbon particles and the fluorine-based electrolyte resin are contained in a weight ratio of carbon particles (not including the catalyst component): fluorine-based electrolyte resin = 0.8: 1 to 2: 1.
The carbon particles have a particle size of 10 to 40 nm and a specific surface area of 400 to 1000 m ² / g.
-The porosity is 60 to 80%.
-Thickness is 5-20 micrometers.
And between the catalyst layer and the gas diffusion layer, carbon particles having a particle size of 10 nm to 10 μm and a specific surface area of 0.5 to 1000 m ² / g, and a fluororesin, Gas diffusion limiting layer containing particles: fluorine-based resin = 0.8: 1 to 20: 1 is provided, and gas of each layer represented by “porosity × (1 / thickness)” The diffusivity is characterized in that the gas diffusion limiting layer is smaller than the catalyst layer.

  When the gas diffusion limiting layer is provided on the anode side, the amount of fuel gas reaching the anode side catalyst layer is reduced, so that the amount of water removed from the anode side catalyst layer by the flowing fuel gas can be suppressed. Since the water is prevented from being excessively taken away from the membrane / electrode assembly, the water content on the anode side increases. Thus, when the water content on the anode side increases, the amount of water that diffuses back from the cathode side also decreases, so the water content on the cathode side of the membrane-electrode assembly also increases.

  On the other hand, when the gas diffusion limiting layer is provided on the cathode side, the amount of oxidant gas reaching the cathode side catalyst layer is reduced, so that the amount of moisture removed from the cathode side catalyst layer by the flowing oxidant gas is suppressed. can do. As a result, the reverse diffusion of water from the cathode side to the anode side is promoted, and the water content on the anode side increases. Further, by appropriately restricting the movement of water generated in the cathode side catalyst layer to the cathode side diffusion layer, an appropriate amount of water is always supplied to the cathode side diffusion layer.

  As described above, according to the membrane / electrode assembly according to the present invention, the water retention on the anode and cathode side can be increased, and in particular, the moisture balance in the membrane / electrode assembly can be reduced in a low air utilization rate region during high-temperature operation. It can be maintained well over a wide range of conditions from high air utilization rates. That is, according to the present invention, dry-up can be prevented even during high-temperature operation with a low air utilization rate.

The carbon particles contained in the gas diffusion limiting layer preferably have a particle size of 1 to 10 μm and a specific surface area of 0.5 to 10 m ² / g.
The gas diffusion limiting layer preferably contains the carbon particles and the fluorine resin in a weight ratio of carbon particles: fluorine resin = 5: 1 to 20: 1.
Further, the gas diffusion limiting layer preferably has a porosity of 50 to 70% and a thickness of 30 to 100 μm.

  As a material for forming the gas diffusion limiting layer, a specific example of the fluororesin that is a water-repellent resin is polyvinylidene fluoride. Specific examples of the carbon particles that are conductive particles include mesocarbon microbeads.

  In the present invention, the gas diffusion limiting layer is provided only between the upstream side of the gas flow path provided adjacent to the gas diffusion layer between at least one of the anode side and the cathode side and the gas diffusion layer. Or the gas flow path has a distribution with higher gas diffusivity than the upstream side of the gas flow path and is provided from the upstream side to the downstream side of the gas flow path. Also good.

  Such a gas diffusion limiting layer restricts gas diffusion more strongly on the upstream side of the gas flow path than on the downstream side. Therefore, the water retention on the upstream side can be increased without impairing the drainage on the downstream side of the gas flow path, and the water balance can be maintained well.

The gas diffusion limiting layer is preferably provided only between the catalyst layer on the anode side and the gas diffusion layer. In the present invention, in general, it is easier to control the water balance when the gas diffusion limiting layer is provided on the anode side than when the gas diffusion limiting layer is provided on the cathode side, and components that contribute to the cell reaction This is because a sufficient supply amount of (oxygen, hydrogen) to the catalyst layer can be secured.
Furthermore, the gas diffusion limiting layer is preferably provided only on the upstream side of the gas flow path between the catalyst layer on the anode side and the gas diffusion layer. This is because the hydrogen supply amount to the anode side catalyst layer can be further secured.

  According to the membrane-electrode assembly according to the present invention, the gas diffusion limiting layer having a gas diffusibility smaller than that of the catalyst layer adjacent between the catalyst layer and the gas diffusion layer is provided on at least one of the anode side and the cathode side. By providing, the water content on the anode side and the cathode side can be kept high by reducing the amount of water removed from the catalyst layer, and the water balance can be maintained well. In particular, according to the present invention, there is provided a membrane-electrode assembly that prevents dry-up even during high-temperature operation at a low air utilization rate and exhibits good power generation efficiency even under high-temperature operation conditions.

A membrane-electrode assembly for a fuel cell according to the present invention includes an electrolyte membrane, and an anode in which at least an anode side catalyst layer and an anode side gas diffusion layer are laminated in this order on one surface of the electrolyte membrane, A membrane / electrode assembly for a fuel cell, comprising a cathode in which at least a cathode side catalyst layer and a cathode side gas diffusion layer are laminated in this order on the other surface of the electrolyte membrane, wherein the anode side and the cathode On at least one of the sides, the catalyst layer has the following conditions:
The catalyst component-supporting carbon particles and the fluorine-based electrolyte resin are contained in a weight ratio of carbon particles (not including the catalyst component): fluorine-based electrolyte resin = 0.8: 1 to 2: 1.
The carbon particles have a particle size of 10 to 40 nm and a specific surface area of 400 to 1000 m ² / g.
-The porosity is 60 to 80%.
-Thickness is 5-20 micrometers.
And between the catalyst layer and the gas diffusion layer, carbon particles having a particle size of 10 nm to 10 μm and a specific surface area of 0.5 to 1000 m ² / g, and a fluororesin, Gas diffusion limiting layer containing particles: fluorine-based resin = 0.8: 1 to 20: 1 is provided, and gas of each layer represented by “porosity × (1 / thickness)” The diffusivity is characterized in that the gas diffusion limiting layer is smaller than the catalyst layer.

  Hereinafter, specific embodiments of the present invention will be described with reference to the drawings. In this embodiment, the fuel cell is a polymer electrolyte fuel cell. However, the membrane / electrode assembly for a fuel cell according to the present invention is another type of fuel cell, for example, a solid oxide type. It can also be applied to fuel cells.

(First embodiment)
FIG. 1 is a cross-sectional view showing a cut surface perpendicular to the flow direction of gas flow paths 7a and 7b of a single cell 100a having a membrane-electrode assembly 10a for a fuel cell according to a first embodiment of the present invention. .
As shown in FIG. 1, the unit cell 100a of the polymer electrolyte fuel cell of the present invention has a structure in which a membrane / electrode assembly 10a is sandwiched between an anode side separator 8a and a cathode side separator 8b. .

A groove for forming a fuel gas flow path 7a is provided on one surface of the anode separator 8a, and a groove for forming a cooling water flow path (not shown) is provided on the opposite surface. Further, a groove for forming an oxidant gas flow path 7b is provided on one surface of the cathode side separator 8b, and a groove for forming a cooling water flow path (not shown) is provided on the opposite surface.
The fuel gas flow path 7a is a space partitioned from the periphery by a groove formed in one surface of the anode-side separator 8a and the outermost layer surface of the anode 5, and a fuel gas (containing hydrogen or hydrogen) is supplied to the anode 5. This is a route for supplying gas).

On the other hand, the oxidant gas flow path 7b comprises a space partitioned from the periphery by a groove formed in the surface of the cathode side separator 8b and the outermost layer surface of the cathode 6 described later. This is a path for supplying (a gas containing or generating oxygen).
Further, the separators 8a and 8b are required to have conductivity for electrically connecting the single cells 100a in the stack, and to be excellent in heat resistance, workability, strength, and the like. A metal plate, a high-density carbon plate, a composite plate of carbon and resin, or the like is used, and a high-density carbon plate is preferable.

  The membrane / electrode assembly 10a includes a solid polymer electrolyte membrane 1, an anode side catalyst layer 2a, an anode side gas diffusion limiting layer 4a, and an anode side gas diffusion layer provided on one surface of the solid polymer electrolyte membrane 1. The anode 5 is composed of 3a, and the cathode 6 is composed of a cathode side catalyst layer 2b and a cathode side gas diffusion layer 3b provided on the other surface of the electrolyte membrane 1.

  FIG. 2 is an exploded plan view for explaining the laminated structure of the membrane / electrode assembly 10a. From the back of the drawing, the cathode side gas diffusion layer 3b, the cathode side catalyst layer 2b, the solid polymer electrolyte membrane 1, and the anode side catalyst layer are shown. 2a, the anode side gas diffusion limiting layer 4a, and the anode side gas diffusion layer 3a are shown stacked in this order.

  The solid polymer electrolyte membrane 1 functions as a solid electrolyte while isolating fuel gas and oxidant gas. For example, perfluorocarbon sulfonic acid polymer (for example, Nafion manufactured by DuPont, USA, Flemion manufactured by Asahi Glass), sulfone, etc. Known materials such as polystyrene-based cation exchange membranes containing acid groups can be used. A perfluorocarbon sulfonic acid polymer is preferred. These membranes have a hydrogen ion exchange group in the molecule, and function as a proton-conductive electrolyte in the presence of water. The solid polymer electrolyte membrane 1 is preferably thin from the viewpoint of improving proton conduction and reverse diffusion characteristics, but if it is too thin, the gas sequestration function is lowered and the amount of unreacted hydrogen permeation increases. From this viewpoint, the thickness of the solid polymer electrolyte membrane 1 is usually 15 to 100 μm, preferably 25 to 75 μm.

  On the surface of the electrolyte membrane as described above, electrodes (anode and cathode) in which a catalyst layer and a gas diffusion layer are laminated in this order from the membrane side are provided. In the membrane / electrode assembly of the present invention, at least one of the anode side catalyst layer and the cathode side catalyst layer provided on the surface of the electrolyte membrane satisfies all of the following conditions.

The catalyst component-supporting carbon particles and the fluorine-based electrolyte resin are contained in a weight ratio of carbon particles (not including the catalyst component): fluorine-based electrolyte resin = 0.8: 1 to 2: 1.
The carbon particles have a particle size of 10 to 40 nm and a specific surface area of 400 to 1000 m ² / g.
-The porosity is 60 to 80%.
-Thickness is 5-20 micrometers.

Between the catalyst layer satisfying all the above conditions and the gas diffusion layer, carbon particles having a particle size of 10 nm to 10 μm and a specific surface area of 0.5 to 1000 m ² / g, and fluorine-based A gas diffusion limiting layer containing resin in a weight ratio of carbon particles: fluorine resin = 0.8: 1 to 20: 1 is provided.
And the gas diffusion property of each layer represented by “porosity × (1 / thickness)” is smaller in the gas diffusion limiting layer provided adjacent to the catalyst layer than in the catalyst layer. The membrane / electrode assembly of the present invention is characteristic.

  The inventor has the configuration as described above adjacent to the catalyst layer satisfying the above conditions, and the gas diffusivity represented by “porosity × (1 / thickness)” is the catalyst layer. It has been found that by providing a smaller gas diffusion limiting layer, it is possible to prevent the membrane-electrode assembly from drying up and maintain a good water balance.

  In the membrane / electrode assembly of the present invention, the gas diffusion limiting layer provided between the catalyst layer and the gas diffusion layer on the anode side or the cathode side, or between the catalyst layer and the gas diffusion layer on both the anode side and the cathode side, Compared with the adjacent catalyst layer, the gas diffusibility represented by water vapor is low, and the performance of moving moisture in the electrolyte membrane and the catalyst layer as water vapor to the gas diffusion layer side is low. By providing such a gas diffusion limiting layer between the catalyst layer and the gas diffusion layer, moisture in the electrolyte membrane and the catalyst layer diffuses to the gas diffusion layer as water vapor, and moisture in the electrolyte membrane and the catalyst layer. It is possible to prevent the amount from decreasing. As a result, the wet state of the electrolyte membrane and the catalyst layer is easily maintained, and the moisture balance can be maintained well from low air utilization to high air utilization even under high temperature conditions. Therefore, it is possible to prevent the electrolyte membrane and the catalyst layer from drying up excessively.

The gas diffusibility of the gas diffusion limiting layer and the catalyst layer is preferably such that the value obtained by actual measurement is gas diffusion limiting layer <catalyst layer. However, depending on the structure of the gas diffusion limiting layer or the catalyst layer, it is difficult to actually measure the gas diffusibility of the gas diffusion limiting layer or the catalyst layer. As an element that is a main indicator of gas diffusivity, first, porosity is given. When the constituent conditions of the layers are the same, the gas diffusibility tends to decrease if the porosity is small. Moreover, the thickness of a layer is mentioned as another factor used as the main parameter | index of gas diffusivity. Although depending on the value of the porosity, etc., the gas diffusibility tends to decrease as the layer becomes thicker. That is, depending on the configuration conditions of each layer, the magnitude relationship of the porosity of each layer × (1 / thickness) may be regarded as the magnitude relationship of gas diffusivity.
Therefore, in the present invention, the configurations of the catalyst layer and the gas diffusion limiting layer are specified as described above, and then the porosity of the catalyst layer and the gas diffusion limiting layer × (1 / thickness) is determined for each layer. By defining the gas diffusibility of the catalyst layer and the gas diffusion limiting layer, it was possible to provide a membrane / electrode assembly having an excellent water balance.

Here, the porosity is the ratio (%) of the volume of the void portion to the volume of the entire layer, and is calculated from the layer area and the thickness of the layer measured by SEM (scanning electron microscope) observation. The specific gravity of the layer is measured from the volume and the weight of the layer, and this is compared with the specific gravity of each material forming the layer [(specific gravity of the layer / specific gravity of each material forming the layer) × 100]. . In the measurement of the porosity with a porosimeter of a gas diffusion layer, which will be described later, since the structural destruction of the catalyst layer and the gas diffusion limiting layer is concerned, it is necessary to lower the press-fitting conditions, and accurate measurement may not be possible. Therefore, in the present invention, the porosity of the catalyst layer and the gas diffusion limiting layer is a value calculated from the specific gravity as described above.
The thickness of the layer is the thickness measured by SEM observation when determining the porosity.

  In the first embodiment, the anode side gas diffusion limiting layer 4a is provided only between the anode side catalyst layer 2a and the anode side gas diffusion layer 3a. The gas diffusion limiting layer may be provided only between the catalyst layer on the cathode side and the gas diffusion layer, or may be provided on both the anode side and the cathode side. However, when the gas diffusion limiting layer is provided on the cathode side Therefore, it is relatively difficult to control the water balance by the gas diffusion limiting layer, and in particular, it tends to be difficult to prevent flooding in the cathode side catalyst layer on the downstream side of the oxidant gas flow path under conditions where the air utilization rate is high. is there. The reason for this is that generated water is generated on the cathode side of the membrane-electrode assembly, and that drainage is required, and that air containing a large amount of inert nitrogen gas as the oxidant gas is mainly used. This is because the influence of gas diffusibility is larger than that on the anode side.

  In addition, since hydrogen gas generally used as a fuel gas supplied to the anode has a high hydrogen concentration and excellent diffusivity, the gas diffusibility in the anode is reduced by the gas diffusion limiting layer. Even if the amount of hydrogen reaching the catalyst layer is somewhat reduced, the amount of hydrogen necessary for the cell reaction is sufficiently supplied. On the other hand, air generally used as an oxidant gas contains components other than oxygen and is not as diffusive as hydrogen gas. Therefore, by providing a gas diffusion limiting layer, a sufficient amount is provided downstream. There is a risk that oxygen cannot be supplied.

  Therefore, in the present invention, from the viewpoint of controlling the water balance and supplying the components (hydrogen, oxygen) that contribute to the cell reaction, the gas diffusion is usually performed on the anode side rather than the gas diffusion limiting layer provided on the cathode side. It is convenient to provide a limiting layer, and it is preferable to provide a gas diffusion layer only on the anode side.

Hereinafter, the gas diffusion limiting layer and the catalyst layer adjacent thereto in the membrane / electrode assembly of the present invention will be described in detail.
As described above, at least one of the anode side catalyst layer and the cathode side catalyst layer of the membrane / electrode assembly of the present invention satisfies all of the following conditions.

The catalyst component-supporting carbon particles and the fluorine-based electrolyte resin are contained in a weight ratio of carbon particles (not including the catalyst component): fluorine-based electrolyte resin = 0.8: 1 to 2: 1.
The carbon particles have a particle size of 10 to 40 nm and a specific surface area of 400 to 1000 m ² / g.
-The porosity is 60 to 80%.
-Thickness is 5-20 micrometers.

The catalyst component-supporting carbon particles are those in which a catalyst component is supported on carbon particles. The catalyst component is not particularly limited as long as it has catalytic activity for the hydrogen oxidation reaction at the anode or the oxygen reduction reaction at the cathode, and is preferably platinum or a platinum-ruthenium alloy. The carbon particles supporting the catalyst component have a particle size (primary particle size) of 10 to 40 nm and a specific surface area of 400 to 1000 m ² / g.
By using carbon particles having a particle size in the above range, a catalyst layer having a porosity of 60 to 80% can be easily formed. The particle size (primary particle size) of the carbon particles is particularly preferably 10 to 20 nm. Moreover, a sufficient catalyst component can be supported by using carbon particles having a specific surface area in the above range. The specific surface area of the carbon particles is particularly preferably 800 to 1000 m ² / g from the viewpoint of the amount of the catalyst component supported on the carbon particles and the thickness of the fluorine-based electrolyte resin covering the carbon particles.

The specific surface area of the carbon particles can be measured by the BET method using nitrogen gas adsorption. As a specific specific surface area measuring apparatus, for example, Tristar 3000 manufactured by Micromillics Co., Ltd. may be mentioned.
The shape of the carbon particles is not particularly limited, and the particle size of the carbon particles is a substantially spherical diameter in the case of a substantially spherical shape. The primary particle size of the carbon particles can be measured by SEM observation.

  As the fluorine-based electrolyte resin contained in the catalyst layer, for example, all the main chain and side chain hydrogens are replaced with fluorine, such as perfluorocarbon sulfonic acid polymer (for example, Nafion manufactured by DuPont USA and Flemion manufactured by Asahi Glass Co., Ltd.). The thing which was done is mentioned. From the viewpoint of excellent proton conductivity, gas permeability and durability, a perfluorocarbon sulfonic acid polymer is preferably used.

In the catalyst layer adjacent to the gas diffusion limiting layer, the weight ratio of the catalyst component-supporting carbon particles to the fluorine-based electrolyte resin is 0.8: 1 to 2: 1 for carbon particles (not including the catalyst component): fluorine-based electrolyte resin. However, the ratio is preferably 1: 1 to 2: 1, and more preferably 1.3: 1 to 1.6: 1.
Moreover, the thickness of the said catalyst layer should just be in the range of 5-20 micrometers, and it is preferable that it is especially 10-20 micrometers.

  The porosity of the catalyst layer may be 60 to 80%, but is preferably 60 to 70% from the viewpoint of securing the supply amount of the reaction gas and controlling the water balance in the electrode.

Next, the gas diffusion limiting layer will be described in detail.
Examples of the material for forming the gas diffusion limiting layers on the anode side and the cathode side include at least fluororesin that is water-repellent resin and carbon particles that are conductive particles, and compression molding at least fluororesin and carbon particles. As a gas diffusion limiting layer, a solution containing at least a fluorine-based resin and carbon particles, dried, and fired as necessary can be used. Here, the fluororesin that is a water repellent resin is a fluororesin capable of repelling water, specifically, a fluororesin having a contact angle with water of 85 ° or more, preferably 95 ° or more. . The contact angle of fluorine resin with water can be considered as a value after 1 second when 2 μl of pure water is dropped on the surface of the film formed of fluorine resin, and can be measured according to a general method. it can.

  Specific examples of the fluororesin include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene / ethylene copolymer (ETFE), tetrafluoroethylene / perfluoroalkoxyethylene copolymer ( PFA) and the like. Of these, PVDF is preferably used from the viewpoint of low melting point and solubility in a solvent.

The carbon particles that are conductive particles are conductive carbon particles, and the shape thereof may be substantially spherical, or may be a fiber shape such as a needle shape, a rod shape, or a tube shape. The particle diameter may be in the range of 10 nm to 10 μm, but from the viewpoint of gas diffusibility, it is particularly preferably in the range of 1 μm to 10 μm, and more preferably in the range of 6 μm to 10 μm. The carbon particles may be in the range a specific surface area of 0.5~1000m ² / g, in terms of the thickness of the fluorine resin or the like to coat the carbon particles, in particular 0.5 to 10 m ² / g Furthermore, it is preferable that it is the range of 1-5 m < ² > / g.
The specific surface area of the carbon particles of the gas diffusion limiting layer can be measured in the same manner as the carbon particles of the catalyst layer. The particle size of the carbon particles can be measured by SEM observation or average particle size measurement by laser diffraction. As a measuring apparatus used for the average particle diameter measurement by laser diffraction, for example, Master Sizer 2000 (manufactured by Malvern Instruments) can be mentioned.

  As the carbon particles, for example, carbon particles such as carbon black and mesocarbon microbeads (MCMB) can be used. Among these, MCBC is preferably used from the viewpoint of high electron conductivity.

  The ratio of the fluorine-based resin and the carbon particles in the gas diffusion limiting layer may be carbon particles: fluorine-based resin (weight ratio) = 0.8: 1 to 20: 1, but particularly 5: It is preferably about 1 to 20: 1, more preferably 5: 1 to 15: 1.

The porosity and thickness of the gas diffusion limiting layer having the above-described configuration are such that the gas diffusion limiting layer has a smaller gas diffusivity than the gas diffusivity [porosity × (1 / thickness)] of the adjacent catalyst layer. Thus, it may be determined appropriately. Specifically, the gas diffusion limiting layer can have a porosity of 50 to 80% and a thickness of 5 to 100 μm. Particularly preferred are a porosity of 50 to 70% and a thickness of 30 to 100 μm.
Under the conditions that the catalyst layer has a porosity of 60 to 80% and a thickness of 5 to 20 μm, and the gas diffusion limiting layer has a porosity of 50 to 70% and a thickness of 30 to 100 μm, Porosity × (1 / thickness) ”= 3 to 16 (% · μm ⁻¹ ),“ porosity × (1 / thickness) ”of gas diffusion limiting layer = 0.5 to 2.33 (% · μm ⁻¹ ), and “porosity × (1 / thickness)” of the catalyst layer> “porosity × (1 / thickness)” of the gas diffusion limiting layer. That is, it can always be considered that the gas diffusibility of the catalyst layer> the gas diffusivity of the gas diffusion limiting layer is satisfied. In addition, the gas diffusion limiting layer satisfying such conditions does not have such a low gas diffusibility as to prevent diffusion of the reaction gas necessary for the electrode reaction into the catalyst layer, so that a sufficient amount of reaction gas is supplied to the catalyst layer. can do.

In order to maintain a good moisture balance and gas supply amount in the membrane / electrode assembly, the specific level of gas diffusion of the gas diffusion limiting layer and the catalyst layer adjacent to it depends on the design of the entire cell. The gas diffusion limiting layer differs depending on whether the gas diffusion limiting layer is provided on the anode side or the cathode side. Therefore, it is preferable to appropriately adjust the gas diffusibility of each layer within a range where the relationship of gas diffusivity of the gas diffusion limiting layer <gas diffusibility of the catalyst layer is satisfied.
In the membrane / electrode assembly of the present invention, at least one of the anode and the cathode has the above-described configuration, and has a catalyst layer and a gas diffusion limiting layer that satisfy the above-described gas diffusion magnitude relationship. If so, the other electrode may have any configuration, and a catalyst layer provided in a general membrane / electrode assembly may be provided.

Next, the gas diffusion layer will be described.
The anode side gas diffusion layer 3a is a transmission path for efficiently supplying the hydrogen gas flowing through the fuel gas passage to the anode side catalyst layer 2a, while the cathode side gas diffusion layer 3b is an oxidant gas flow This is a transmission path for efficiently supplying oxygen gas flowing through the channel to the cathode side catalyst layer 2b and discharging water generated by the electrode reaction to the oxidant gas channel. In order to efficiently supply gas and discharge water, the gas diffusion layer is usually adjusted to have a larger gas diffusibility than the adjacent catalyst layer.

  A porous conductive material is used as a material for forming these gas diffusion layers. For example, a porous carbon sheet such as carbon paper, carbon cloth or carbon felt is suitable. Also, various metal meshes, various metal felts such as nickel felt, or porous metals such as various metal foams can be used. The water permeability may be controlled by subjecting these porous bodies to a water repellent treatment with polytetrafluoroethylene or the like.

The range of the porosity of the gas diffusion layer is usually 65 to 90%, preferably 70 to 80%. As with the catalyst layer, the porosity of the gas diffusion layer is measured by comparing the specific gravity of the layer with the specific gravity of each material forming the layer [(specific gravity of the layer / specific gravity of each material forming the layer) × 100. Or can be measured by a porosimeter (for example, Autopore IV9510, an automatic porosimeter manufactured by Micromeritics, etc.).
Moreover, the thickness of a gas diffusion layer is about 200-400 micrometers normally, Preferably it is 200-300 micrometers. If the thickness of the gas diffusion layer is less than the above value, the gas diffusion capacity will be insufficient and it will be difficult to supply the fuel gas or oxidant gas uniformly to the surface of the catalyst layer, and sufficient mechanical strength will be ensured. It may not be possible. In addition, it is useless to make the thickness of the gas diffusion layer larger than the above value, and on the cathode side, the generated water is condensed in the pores, and flooding may occur.

Hereinafter, a preferred method for producing a membrane / electrode assembly will be described. Here, a case where a gas diffusion limiting layer is provided only on the anode side is taken as an example.
First, a paste for forming both catalyst layers 2a and 2b is prepared.
Anode-side catalyst layer forming paste is prepared by mixing the anode-side catalyst-supporting carbon and the fluorine-based electrolyte resin in an appropriate solvent. Similarly, cathode-side catalyst-supporting carbon and solid polymer electrolyte resin (not limited to fluorine-based electrolyte resin) are mixed in an appropriate solvent to prepare a cathode-side catalyst layer forming paste.

  Next, each of these pastes is applied onto a plastic film such as a propylene film and dried to form catalyst layers 2a and 2b on the plastic film. Using the plastic film provided with the catalyst layer 2a and the plastic film provided with the catalyst layer 2b, the solid polymer electrolyte membrane 1 is sandwiched so that both the catalyst layers 2a and 2b are inside, and the solid polymer electrolyte is provided. Hot pressing is performed at a temperature near the glass transition point of the film 1. Then, catalyst layers 2 a and 2 b are formed on the solid polymer electrolyte membrane 1 by peeling the plastic film from both surfaces of the solid polymer electrolyte membrane 1.

  Next, a dispersion liquid in which carbon particles and a fluorine-based resin are dispersed in a suitable solvent is applied on a sheet-like substrate and dried to obtain the anode-side gas diffusion limiting layer sheet of this embodiment. Here, examples of the solvent for dispersing the carbon particles and the fluororesin include organic solvents such as ethanol, propanol, propylene glycol, and N-methylpyrrolidone solution. Moreover, the method of apply | coating the said dispersion liquid on a base material is not specifically limited, For example, the screen printing method, the spray method, the doctor blade method, the gravure printing method, the die-coating method etc. are mentioned.

  In addition, a carbon sheet is immersed in an aqueous polytetrafluoroethylene dispersion, dried, fired and subjected to a water repellent treatment to obtain an anode-side gas diffusion layer sheet. The same sheet as the anode side gas diffusion layer sheet is used as the cathode side gas diffusion layer sheet.

Next, the anode side gas diffusion limiting layer sheet is overlaid so that the anode side gas diffusion limiting layer 4a is in contact with the anode side catalyst layer 2a of the solid polymer electrolyte membrane 1 provided with both the catalyst layers 2a and 2b. The anode-side gas diffusion limiting layer 4a is transferred by hot pressing (for example, at a temperature near the glass transition point of the solid polymer electrolyte membrane 1) and peeling off the sheet-like substrate.
The anode side gas diffusion layer 3a is in contact with the anode side gas diffusion limiting layer 4a, and the cathode side gas diffusion layer 3b is in contact with the cathode side catalyst layer 2b. The anode side gas diffusion limiting layer 4a, the anode side gas diffusion layer 3a, and the cathode side are obtained by stacking the diffusion layer sheets and performing hot pressing (for example, at a temperature near the glass transition point of the solid polymer electrolyte membrane 1). The gas diffusion layer 3b can be formed to manufacture the membrane / electrode assembly 10a.

In addition to this manufacturing method, for example, the catalyst layer forming paste is applied to both surfaces of the solid polymer electrolyte membrane 1 and then dried, whereby the anode side catalyst layer is formed on one surface of the solid polymer electrolyte membrane 1. The cathode side catalyst layer 2b is formed on the other surface, and the anode side gas diffusion limiting layer, the anode side gas diffusion layer, and the cathode side gas diffusion layer are formed on these surfaces in the same manner as in the above-described production method. You may produce by laminating | stacking.
Further, an anode side gas diffusion layer sheet and a cathode side gas diffusion layer sheet are prepared in advance, and the anode side gas diffusion limiting layer 4a is formed on the anode side gas diffusion layer sheet in the same manner as described above. The anode side catalyst layer forming paste is applied and dried to form an anode sheet, and the cathode side catalyst layer forming paste is applied to the cathode side gas diffusion layer sheet and dried to obtain the cathode sheet. The catalyst layers 2a and 2b may be formed and laminated with the solid polymer electrolyte membrane 1 sandwiched therebetween with the catalyst layers 2a and 2b facing the solid polymer electrolyte membrane 1.

Next, the power generation mechanism of the single cell 100a of the fuel cell provided with the membrane-electrode assembly 10a of the present embodiment, particularly the moisture content distribution in the membrane-electrode assembly 10a will be described.
In the fuel cell membrane / electrode assembly 10a according to the present embodiment, an anode-side gas diffusion limiting layer having a gas diffusivity smaller than that of the anode-side catalyst layer 2a between the anode-side catalyst layer 2a and the gas diffusion layer 3a. 4a is provided.

  Thus, by providing the anode side gas diffusion limiting layer 4a having a gas diffusibility smaller than that of the catalyst layer 2a so as to be adjacent to the catalyst layer 2a, the amount of fuel gas reaching the catalyst layer 2a is reduced. The amount of moisture removed from the catalyst layer 2a by the flowing fuel gas can be suppressed. That is, the movement of water from the anode side catalyst layer 2a to the anode side gas diffusion layer 3a is blocked, and the fuel gas flowing through the fuel gas flow path 7a is prevented from excessively depriving the membrane / electrode assembly 10a. Therefore, the water content of the anode 5 and the membrane increases.

Further, when the water content on the anode 5 side increases, the amount of water that reversely diffuses from the cathode side to the anode side decreases, so the water content on the cathode 6 side of the membrane-electrode assembly 10a also increases.
When the anode-side gas diffusion limiting layer 4a is provided between the anode-side catalyst layer 2a and the anode-side gas diffusion layer 3a, the above-described action can be used even under conditions where the air utilization rate is low (when a large amount of air is fed). It is estimated that dry-up on the anode 5 side and the cathode 6 side of the membrane / electrode assembly 10 a, particularly dry-up on the upstream side of the anode 5, hardly occurs. Furthermore, it is possible to prevent such dry-up even under high temperature conditions.

  On the other hand, in this embodiment, since the gas diffusion limiting layer is provided only on the anode side and not on the cathode side, when the generated water on the cathode 6 side becomes excessive, even under operating conditions with a particularly high air utilization rate, The gas diffusion limiting layer does not serve as a drainage barrier to the oxidant gas flow path. Therefore, excess water generated on the cathode 6 side is quickly discharged to the oxidant gas flow path.

  In this way, the gas diffusion limiting layer 4a provided on the anode 5 side can increase the water retention on the anode side and the cathode side without impairing the drainage on the cathode side, so that a good water balance can be maintained. Can do. According to the present invention, the water content in the membrane-electrode assembly can be favorably maintained over a wide range of conditions from a low air utilization rate region to a high air utilization rate region during high temperature operation. In particular, according to the present embodiment, the membrane / electrode assembly 10a showing good power generation efficiency that prevents dry-up even during high-temperature operation at a low air utilization rate and maintains drainage even at a high air utilization rate. Is provided.

In this embodiment, the gas diffusion limiting layer 4a is provided only on the anode 5 side. However, as described above, the gas diffusion limiting layer is provided on both the anode 5 side and the cathode 6 side, or only on the cathode 6 side. It is also possible to maintain a good water balance in the membrane / electrode assembly by providing the film.
When a gas diffusion limiting layer is provided between the catalyst layer 2b on the cathode side and the gas diffusion layer 3b, the amount of oxidant gas reaching the catalyst layer 2b is reduced, so that moisture from the catalyst layer 2b by the flowing oxidant gas is reduced. The removal amount can be suppressed. That is, since the movement of the water generated in the cathode side catalyst layer 2b to the cathode side diffusion layer 3b is appropriately limited, the water content of the cathode 6 can be kept high. As a result, the reverse diffusion of water from the cathode side to the anode side is promoted, and the water content on the anode 5 side also increases. In the case where the cathode side gas diffusion limiting layer is provided, it is presumed that the dryness in the membrane / electrode assembly can be prevented and the moisture balance can be adjusted by such an action.

(Second Embodiment)
Next, a fuel cell membrane / electrode assembly according to a second embodiment of the present invention will be described with reference to FIGS. 3a, 3b and 4. FIG. In the following drawings and description, the same members as those in the membrane / electrode assembly of the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

  FIG. 3a is a cross-sectional view showing a cut surface perpendicular to the flow direction of the gas flow paths 7a and 7b of the single cell 100b having the membrane-electrode assembly 10b for a fuel cell according to the second embodiment. The cut surface within the upstream half of the part adjacent to the gas diffusion layer of the agent gas flow path 7b is shown. FIG. 3b is also a cross-sectional view of the single cell 100b, but shows a cut surface within half of the downstream side of the portion adjacent to the gas diffusion layer of the oxidant gas flow path 7b.

FIG. 4 is an exploded plan view for explaining the state of lamination of the membrane / electrode assembly 10b, and the upper side in the drawing corresponds to the upstream side of the oxidant gas flow path. Further, the front side of the solid polymer electrolyte membrane 1 is the anode 5 side, and the back side of the paper surface is the cathode 6 side. Then, in order from the back of the page of FIG. 4, the cathode side gas diffusion layer 3b, the cathode side gas diffusion limiting layer 4b, the cathode side catalyst layer 2b, the solid polymer electrolyte membrane 1, the anode side catalyst layer 2a, and the anode side gas A diffusion layer 3a is laminated. However, the cathode-side gas diffusion limiting layer 4b is provided only in a portion corresponding to the upstream half of the channel length from the entrance to the outlet of the oxidant gas channel 7b to the gas diffusion layer 3b.
Further, the region defined by the phantom line (dotted line) on the left side of the figure on the cathode side gas diffusion layer 3b is the leftmost oxidant gas flow path (when the single cell 100b is manufactured using this membrane / electrode assembly 10b) 7b) is the place where it is formed at the back of the page.

In the cathode side gas diffusion limiting layer 4b according to the second embodiment, the conditions or elements (characteristics and mixing ratio, thickness, porosity, etc. of the constituent materials) other than the area and the installation position are the same as those in the first embodiment. The anode side gas diffusion limiting layer 4a can be formed in exactly the same manner. Further, the cathode side catalyst layer 2b adjacent to the cathode side gas diffusion limiting layer can also be formed in the same manner as the anode side catalyst layer 2a in the first embodiment.
In the present invention, as shown in the second embodiment, between the catalyst layer and the gas diffusion layer on the anode side or the cathode side, or between the catalyst layer and the gas diffusion layer on both the anode side and the cathode side, The gas diffusion limiting layer may be provided locally only on the upstream side of the gas flow path provided adjacent to the gas diffusion layer.

  In the second embodiment, the cathode side gas diffusion limiting layer 4b is provided only between the cathode side catalyst layer 2b and the cathode side gas diffusion layer 3b. Even when the gas diffusion limiting layer is provided on the cathode side, when the gas diffusion limiting layer is provided only on the upstream side of the cathode 6 as in the second embodiment, uniform gas diffusion from the upstream side to the downstream side of the cathode 6 is possible. This is one of the preferred embodiments because it is easier to control the water balance than the case where a gas diffusion limiting layer having the property is provided and the amount of oxygen supplied to the catalyst layer can be sufficiently secured.

  Here, when the gas diffusion limiting layer on the anode side and / or the cathode side is locally provided at a position corresponding to the upstream side of the gas flow path, in order to obtain the maximum effect by limiting the installation position, It is preferable to provide the gas flow path in the region of the upper third of the flow path length upstream from the entrance to the gas diffusion layer to the outlet, preferably in the half. However, the gas diffusion limiting layer does not need to be provided immediately below the inlet of the gas flow path, and may be provided from a position corresponding to the middle of the flow path. For example, the gas diffusion limiting layer is not provided up to the upper quarter of the gas flow path, but the gas is positioned at the position corresponding to the upper quarter to the second third of the flow path length of the gas flow path. A diffusion limiting layer may be provided.

When the gas diffusion limiting layer is provided only on the upstream side of the gas flow path, the degree of gas diffusibility should be determined as appropriate according to the design of the entire cell, as in the first embodiment. It differs depending on whether the gas diffusion limiting layer is provided on either the anode side or the cathode side.
The specific balance between the gas diffusibility of the gas diffusion limiting layer and the gas diffusibility of the catalyst layer is the same as in the first embodiment. Specifically, the catalyst layer on the anode side and / or the cathode side has all the conditions (weight ratio, particle size, specific surface area, porosity, thickness) of the components as described in the first embodiment. The gas diffusion limiting layer that is satisfied and is provided locally only on the upstream side of the gas flow path has the conditions as described in the first embodiment (weight ratio, particle diameter, specific surface area, further porosity, thickness of constituent components) ) And the gas diffusibility of the gas diffusion limiting layer is made smaller than that of the adjacent catalyst layer, so that a good water balance can be obtained and the supply amount of oxygen and hydrogen can be secured.
When the gas diffusion limiting layer is locally provided only on the upstream side of the gas flow path, it is not necessary to consider the influence on the flooding by the gas diffusion limiting layer. It is possible to provide a gas diffusion limiting layer with limited properties, and it is possible to set the gas diffusivity of the gas diffusion limiting layer to be smaller.

Next, the power generation mechanism of the single cell 100b of the fuel cell provided with the membrane / electrode assembly 10b of the second embodiment, particularly the moisture content distribution in the membrane / electrode assembly 10b will be described.
As described in the first embodiment, when the gas diffusion limiting layer is provided between the catalyst layer on the cathode side and the gas diffusion layer, the amount of the oxidant gas that reaches the catalyst layer 2b is reduced, so that it flows. The amount of moisture removed from the catalyst layer 2b by the oxidant gas can be suppressed. That is, since the movement of the water generated in the cathode side catalyst layer 2b to the cathode side diffusion layer 3b is appropriately limited, the water content of the cathode 6 can be kept high. As a result, the amount of water back-diffused from the cathode side to the anode side increases, and the water content on the anode 5 side also increases.

  In the second embodiment, the cathode-side gas diffusion limiting layer having such an action is provided only on the upstream side of the gas flow path, so that even when the air utilization rate is low (when a large amount of air is fed), the membrane It is estimated that dry-up on the anode 5 side and the cathode 6 side of the electrode assembly 10 b, particularly dry-up on the upstream side of the cathode 6 and the anode 5, hardly occurs. Moreover, air generally used as oxidant gas is not as high in gas diffusivity and concentration as hydrogen gas generally used as fuel gas, but in this embodiment, the oxygen concentration in oxidant gas is low. Since the gas diffusion limiting layer is provided only on the upstream side of the high gas flow path, oxygen required for the battery reaction can be sufficiently supplied to the catalyst layer.

  On the other hand, flooding usually tends to occur downstream of the oxidant gas flow path 7b on the cathode 6 side. In the second embodiment, the cathode side gas diffusion limiting layer 4b is provided only on the upstream side of the oxidant gas flow path 7b, and on the downstream side, the gas diffusion limiting layer is interposed between the catalyst layer and the gas diffusion layer. Therefore, the water generated on the downstream side of the cathode side catalyst layer 2b can easily move to the cathode side diffusion layer 3b without being restricted by the gas diffusion limiting layer. Therefore, it is presumed that even under conditions where the air utilization rate is high, excess water in the cathode is quickly discharged into the oxidant gas flow path 7b, and flooding can be prevented.

  Thus, in the second embodiment, by providing the gas diffusion limiting layer 4b only on the upstream side of the cathode 6, the water retention on the anode side and the cathode side is not impaired without impairing the drainage property and oxygen supply amount on the cathode side. Therefore, the water balance can be maintained well. Also in the second embodiment, the moisture balance in the membrane-electrode assembly can be well maintained over a wide range of conditions from a low air utilization rate region to a high air utilization rate region during high temperature operation.

  In this embodiment, the gas diffusion limiting layer 4b is provided only on the upstream side of the cathode 6. However, even when the gas diffusion limiting layer is provided on the upstream side of the anode 5, as described in the first embodiment. By the action, the water content on the anode side and the cathode side can be increased and dry-up can be prevented. Moreover, by providing the gas diffusion limiting layer only on the upstream side of the fuel gas having a high hydrogen concentration in the fuel gas flowing through the gas flow path, it is possible to further ensure the amount of hydrogen supplied to the catalyst layer. Furthermore, there is no possibility of impairing drainage due to the gas diffusion limiting layer.

  As described above, by providing a localized gas diffusion limiting layer on the upstream side of the gas flow path on both the anode 5 side and the cathode 6 side or only on the anode 5 side, the drainage on the cathode side is impaired. Therefore, it is possible to increase the water retention on the anode side and the cathode side and maintain a good water balance in the membrane-electrode assembly. Even when the gas diffusion limiting layer is provided only on the upstream side of the oxidant gas on the cathode side as in the second embodiment, or when the gas diffusion limiting layer is provided on both the anode 5 side and the cathode 6 side. However, it is possible to maintain the water balance and secure the oxygen supply amount. However, in view of the supply amount of the component contributing to the cell reaction to the catalyst layer, the prevention of flooding, etc., in particular, the upstream side of the anode side It is preferable to provide a gas limiting layer only.

(Third embodiment)
Next, a third embodiment of a membrane-electrode assembly for a fuel cell according to the present invention will be described with reference to FIGS. 5a, 5b and 6. FIG. In the following drawings and description, the same members as those in the membrane / electrode assembly of the first embodiment are denoted by the same reference numerals, and description thereof is omitted.
FIG. 5a is a cross-sectional view showing a cut surface perpendicular to the flow path direction of the gas flow paths 7a, 7b of the single cell 100c having the membrane-electrode assembly 10c for a fuel cell according to the third embodiment. The cut surface within the upstream half of the part adjacent to the gas diffusion layer of the agent gas flow path 7b is shown. FIG. 5b is also a cross-sectional view of the single cell 100c, but shows a cut surface within the downstream half of the portion adjacent to the gas diffusion layer of the oxidant gas flow path 7b.

  FIG. 6 is an exploded plan view for explaining the state of lamination of the membrane / electrode assembly 10c, and the upper side in the drawing corresponds to the upstream side of the oxidant gas flow path. The front side of the solid polymer electrolyte membrane 1 is the anode 5 side, and the back side is the cathode 6 side. Then, in order from the back of FIG. 6, the cathode side gas diffusion layer 3b, the cathode side gas diffusion limiting layer 4b (upstream side: 4b (1), downstream side: 4b (2)), the cathode side catalyst layer 2b, solid The polymer electrolyte membrane 1, the anode side catalyst layer 2a, and the anode side gas diffusion layer 3a are laminated. However, in the cathode side gas diffusion limiting layer 4b, the portion corresponding to the upstream half of the channel length from the inlet to the outlet of the oxidant gas channel 7b to the gas diffusion layer 3b is the cathode upstream side gas diffusion limiting layer. 4b (1), the portion corresponding to the downstream half is divided into the cathode downstream gas diffusion limiting layer 4b (2), and the upstream gas diffusion limiting layer 4b (1) is the downstream gas diffusion limiting layer 4b. Gas diffusivity is smaller than (2).

  The region defined by the phantom line (dotted line) on the left side of the figure on the cathode side gas diffusion layer 3b is the leftmost oxidant gas flow path (when the single cell 100c is fabricated using this membrane / electrode assembly 10c) 7b) is the place where it is formed at the back of the page.

In the cathode side gas diffusion limiting layer 4b according to the third embodiment, conditions or elements other than the area and the installation position (characteristics and mixing ratios of constituent materials, thickness, porosity, etc.) are the same as those in the first embodiment. The anode side gas diffusion limiting layer 4a can be formed in exactly the same manner. Further, the cathode side catalyst layer 2b adjacent to the cathode side gas diffusion limiting layer can also be formed in the same manner as the anode side catalyst layer 2a in the first embodiment.
In the present invention, as shown in the third embodiment, it is adjacent between the catalyst layer and the gas diffusion layer on the anode side or the cathode side, or between the catalyst layer and the gas diffusion layer on both the anode side and the cathode side. A gas diffusion limiting layer having a distribution with a larger gas diffusivity downstream than the upstream side of the gas flow path within a range smaller than the gas diffusivity of the catalyst layer to be moved from the upstream side to the downstream side of the gas flow path. You may provide across.

In the third embodiment, the cathode-side gas diffusion restriction is distributed only between the cathode-side catalyst layer 2b and the cathode-side gas diffusion layer 3b and has a distribution in which the gas diffusivity is larger on the downstream side than on the upstream side of the gas flow path. The layer 4b is provided from the upstream side to the downstream side of the gas flow path.
In the third embodiment, the gas diffusion limit by the gas diffusion limiting layer is stronger on the upstream side of the gas flow path than on the downstream side. Therefore, according to the third embodiment, similarly to the second embodiment, the water retention on the anode side and the cathode side can be increased without greatly impairing the drainage performance and oxygen supply amount on the cathode side. Good balance can be maintained.

  In addition, as in the third embodiment, when the gas diffusion limit by the gas diffusion limiting layer is stronger on the upstream side of the gas flow path than on the downstream side, as in the second embodiment, the cathode is the same as in the second embodiment. Although the gas diffusion limiting layer is provided on the side, the water balance can be easily controlled, and furthermore, the oxygen supply amount can be secured, which is one of the preferred embodiments.

As long as the gas diffusivity distribution state of the gas diffusion limiting layer is a distribution in which the gas diffusibility is larger on the downstream side than on the upstream side, the gas diffusibility distribution state may be changed stepwise or in a continuous linear shape. It may change. From the viewpoint of ease of production, the gas diffusion limiting layer is divided into about 2 to 5 regions equally or non-equally from the upstream side to the downstream side of the gas flow path. It is preferable to set so that the downstream side is larger than the upstream side.
When the gas diffusion limiting layer has a distribution in which the gas diffusivity is larger on the downstream side than on the upstream side of the gas flow path, how much the gas diffusibility is to be determined is the same as in the first embodiment. It should be determined as appropriate according to the design of the entire cell, and differs depending on whether the gas diffusion limiting layer is provided on either the anode side or the cathode side.

  The specific balance between the gas diffusibility of the gas diffusion limiting layer and the gas diffusibility of the catalyst layer is the same as in the first embodiment. Specifically, the catalyst layer on the anode side and / or the cathode side has all the conditions (weight ratio, particle size, specific surface area, porosity, thickness) of the components as described in the first embodiment. Gas upstream of the gas diffusion limiting layer so that the gas diffusion limiting layer satisfies the conditions (weight ratio, particle size, specific surface area, porosity, thickness) of the constituents as described in the first embodiment. By providing an in-plane distribution with greater gas diffusivity downstream than diffusivity, and making gas diffusivity smaller than that of the adjacent catalyst layer, a good moisture balance can be obtained, and the supply amount of oxygen and hydrogen Can be secured.

  When the gas diffusion limiting layer is divided into several parts from the upstream side to the downstream side of the gas flow path and the porosity and thickness are changed stepwise within the above ranges, for example, the catalyst layer and the gas diffusion layer When the limiting layer satisfies the conditions as described in the first embodiment and the thickness of the gas diffusion limiting layer is constant within the above range, the numerical range (50 to 70) of the porosity of the gas diffusion limiting layer is as follows. %) May be equally divided by the number of stages to be changed, and the distribution may be set by a method in which the equally divided porosity values are cumulatively added from upstream to downstream regions. Specifically, the entire length of the gas diffusion limiting layer is divided into three equal parts, and the above-mentioned range of 50 to 70% of the porosity is divided into three equal parts (that is, 50%, 60%, 70%), The upstream one-third region has a porosity of 50%, the middle third region has a porosity of 60%, and the downstream third region has a porosity of 70%. It is good. However, the porosity distribution need not change with a constant slope over the entire region.

Regarding the membrane / electrode assembly 10c according to the third embodiment, only the method for producing the cathode-side gas diffusion limiting layer 4b will be described.
The manufacturing method of the gas diffusion limiting layer having a distribution in which the gas diffusibility on the downstream side is larger than the upstream side of the gas flow path is not particularly limited, but is contained in the upstream gas diffusion limiting layer and the downstream gas diffusion limiting layer. By changing the ratio (weight ratio) between the carbon particles (conductive particles) and the fluororesin (water repellent resin), it is easy to adjust the gas diffusivity of each gas diffusion limiting layer. For example, by preparing dispersions with different ratios of carbon particles and fluororesin, applying each dispersion to the position where each gas diffusion restriction layer should be provided on the sheet-like substrate, and drying it, gas diffusion restriction What is necessary is just to make it a sheet | seat for layers and to join a catalyst layer and a gas diffusion layer. The ratio of the carbon particles and the fluorine-based resin in the upstream gas diffusion limiting layer and the downstream gas diffusion limiting layer is not particularly limited, but in the upstream gas diffusion limiting layer, carbon particles: fluorine resin = 70: 30 to 90:10. (Weight ratio) is preferable, and about 80:20 is particularly preferable. Further, the downstream gas diffusion limiting layer is preferably carbon particles: fluorinated resin = 80: 20 to 90:10, particularly preferably about 90:10 (weight ratio).

  The membrane / electrode assembly according to the third embodiment can be obtained by treating the upstream and downstream gas diffusion limiting layer sheets thus produced in the same manner as the gas diffusion limiting layer sheet in the first embodiment. Can be produced.

Instead of providing the cathode side gas diffusion limiting layers 4b (1) and 4b (2) divided into the upstream side and the downstream side, a gas diffusion limiting layer whose gas diffusivity gradually changes may be provided. For example, the gas diffusivity can be continuously changed by gradually increasing the porosity of the gas diffusion limiting layer from the upstream side to the downstream side of the fuel gas flow path.
Also in the third embodiment, the moisture balance in the membrane-electrode assembly can be favorably maintained over a wide range of conditions from a low air utilization rate region to a high air utilization rate region during high temperature operation.

  In the present embodiment, the gas diffusion limiting layer 4b is provided only on the cathode 6. However, even when the gas diffusion limiting layer is provided on the anode 5, the action of the anode side and It is possible to increase the water content on the cathode side and prevent dry-up. Moreover, since the gas diffusivity on the downstream side of the fuel gas, where the hydrogen concentration in the fuel gas is lower than that on the upstream side, is made larger than that on the upstream side, it is possible to further secure the supply amount of hydrogen to the catalyst layer. it can. Furthermore, there is no possibility of impairing drainage due to the gas diffusion limiting layer.

  As described above, the gas diffusion limiting layer having a distribution in which the gas diffusivity is larger on the downstream side than on the upstream side of the gas flow path on both the anode 5 side and the cathode 6 side or only on the anode 5 side. By providing, it is possible to improve the water retention on the anode side and the cathode side without impairing the drainage on the cathode side, and to maintain a good water balance in the membrane-electrode assembly. Even when the gas diffusion limiting layer is provided only on the cathode side as in the third embodiment, or when the gas diffusion limiting layer is provided on both the anode 5 side and the cathode 6 side, Although it is possible to maintain the balance and secure the oxygen supply amount, it is particularly possible to provide a gas limiting layer only on the anode side from the viewpoints of supplying the components contributing to the cell reaction to the catalyst layer, preventing flooding, etc. preferable.

(Production Example 1: Production of anode-side and cathode-side gas diffusion limiting layer sheets)
92.5 g of mesocarbon microbeads (MCMB) (manufactured by Osaka Gas Chemical, specific surface area: 3 m ² / g, particle size: 7 μm) and 62.5 g of polyvinylidene fluoride (PVDF) / N-methylpyrrolidone solution (Kureha) (Chemical KF polymer) was added so that the weight ratio of MCMB to PVDF was 12: 1 and stirred well. The obtained dispersion was applied onto a Teflon (registered trademark) sheet by a doctor blade method and dried under reduced pressure at 130 ° C. in a nitrogen gas atmosphere. The thickness of the gas diffusion limiting layer formed on the Teflon (registered trademark) sheet was 50 μm. The gas diffusion limiting layer was cut into an outer shape of 5 cm × 5 cm together with the Teflon (registered trademark) sheet to obtain a gas diffusion limiting layer sheet.

(Production Example 2: Preparation of anode-side and cathode-side catalyst layer sheets)
1 g of perfluorocarbon sulfonic acid polymer solution (Aldrich, 5 wt% Nafion solution) and 0.1 g of platinum-supported carbon particles (platinum support ratio 50 wt%, carbon particle specific surface area: 800 m ² / g, carbon particle particle size: 20 nm ) Were mixed using an ultrasonic generator to prepare a catalyst paste (carbon particles: perfluorocarbon sulfonic acid polymer = 1: 1). This catalyst paste was applied onto a polypropylene film and dried at 80 ° C. for 24 hours to form a catalyst layer sheet having a thickness of 10 μm on the polypropylene film.

Example 1
As the solid polymer electrolyte membrane, a perfluorocarbon sulfonic acid polymer membrane (DuPont Nafion membrane, EW = 1100, hereinafter referred to as Nafion membrane) having an outer diameter of 6 cm × 6 cm and a thickness of 50 μm was used.
The anode side catalyst layer sheet and the cathode side catalyst layer sheet prepared in Production Example 2 were used and hot pressed at 140 ° C. with the Nafion membrane sandwiched so that the respective catalyst layers were inside. And the catalyst film of the both electrodes was formed on the said Nafion membrane | film | coat by peeling the polypropylene film of the laminated | stacked anode side and cathode side catalyst layer sheet | seat.

The gas diffusion limiting layer sheet prepared in Production Example 1 is overlaid on the anode side catalyst layer and the cathode side catalyst layer of the solid polymer electrolyte membrane provided with the catalyst layers of both electrodes, with the positions of the end portions thereof overlapped, Hot pressed at 140 ° C. for 10 minutes. The laminated Teflon (registered trademark) sheets for the anode side and cathode side gas diffusion limiting layer were peeled off, and the anode side gas diffusion limiting layer and the cathode side gas diffusion limiting layer were transferred.
Furthermore, carbon paper (CARBEL CFP-200 manufactured by Japan Gore-Tex) with an outer diameter of 5 cm × 5 cm and a thickness of 200 μm as the anode-side gas diffusion layer and the cathode-side gas diffusion layer respectively on the anode-side gas diffusion limiting layer and the cathode side. The membrane / electrode assembly of Example 1 was obtained by overlaying on the gas diffusion limiting layer and hot pressing at 140 ° C. for 10 minutes.
Further, a single cell was obtained by sandwiching both sides with a carbon separator.

(Example 2)
The same procedure as in the case of the single cell of Example 1 was performed except that the gas diffusion limiting layer was formed only between the anode side catalyst layer and the anode side gas diffusion layer without forming the cathode side gas diffusion limiting layer. The membrane-electrode assembly and unit cell of Example 2 were obtained.

(Example 3)
A fuel gas between the anode side catalyst layer and the anode side gas diffusion layer using the gas diffusion limiting layer sheet prepared in Production Example 1 cut into a 5 cm × 2.5 cm outer shape together with a Teflon (registered trademark) sheet. Except that the gas diffusion limiting layer was formed only at the position corresponding to the upstream portion of the flow path, the same procedure as in the case of the single cell of Example 2 was performed, and the membrane / electrode assembly and single cell of Example 3 were thus obtained. Got.

(Comparative Example 1)
The membrane / electrode assembly and single cell of Comparative Example 1 were subjected to the same procedure as that of the single cell of Example 1 except that no gas diffusion limiting layer was formed on either the anode side or the cathode side. Got.

<Evaluation>
(1) Measurement of porosity The gas diffusion limiting layer (the Teflon (registered trademark) sheet was peeled from the gas diffusion limiting layer sheet), the gas diffusion layer, and the catalyst layer ( The porosity of the polypropylene layer peeled from the catalyst layer sheet was calculated from the specific gravity of each layer and the specific gravity of the material constituting each layer. The results are shown in Table 1. Further, gas diffusivity [porosity × (1 / thickness)] was calculated from the porosity and thickness of each layer obtained.

(2) Single-cell performance Each single cell was supplied with hydrogen on the anode side and air on the cathode side, and the output voltage was measured under the condition of constant output current, and the relationship with the air utilization rate was investigated. The operating conditions were set as follows.

(Operating conditions)
Current density: 0.5 A / cm ²
Cell temperature: 95 ° C
Fuel gas utilization rate: 67%
Air utilization rate: 25% to 67%
Humidifier temperature: 50 ° C for both anode and cathode

(Evaluation results of single cell performance)
FIG. 7 shows the measurement results of the fuel cell characteristics of the membrane / electrode assembly.
From FIG. 7, the output voltage of the membrane / electrode assembly of Comparative Example 1 decreased as the air utilization rate decreased. In particular, when the air utilization rate is less than about 33%, the output voltage rapidly decreases. When the air utilization rate is 25%, the output voltage becomes zero. This is because, due to the large air flow rate, water is excessively taken from the membrane / electrode assembly, and excessive dry-up occurs on the upstream side of the air.

  On the other hand, the membrane / electrode assembly of Example 1 exhibited a certain output voltage in the entire air utilization rate of 25 to 67% under a high temperature condition of 95 ° C. In particular, under a condition where the air utilization rate was low, a sufficient amount of water could be retained in the membrane / electrode assembly despite the large air flow rate, and thus a high output voltage was exhibited. The reason why the output voltage decreased as the air utilization rate increased was considered to be that the amount of moisture removed from the membrane-electrode assembly decreased due to the decrease in the air flow rate, and local flooding occurred. Another possible cause is a decrease in the amount of oxygen supplied to the catalyst layer through the gas diffusion limiting layer due to a decrease in the air flow rate.

  As can be seen from FIG. 7, the membrane / electrode assembly of Example 2 in which the gas diffusion limiting layer is provided only on the anode side, is operated at a high temperature of 95 ° C., from a low air utilization rate to a high air utilization rate. High output voltage was shown. This is because a sufficient amount of moisture can be retained in the membrane-electrode assembly even under conditions where the air utilization rate is low, and the drainage on the cathode side is maintained even under conditions where the air utilization rate is high. This is because.

  The membrane / electrode assembly of Example 3 in which the gas diffusion limiting layer is provided only on the upstream side of the gas flow path on the anode side is also operated at a high temperature of 95 ° C., from a low air utilization rate to a high air utilization rate. High output voltage was shown. This is because a sufficient amount of moisture can be retained in the membrane-electrode assembly even under conditions where the air utilization rate is low, and the drainage on the cathode side is maintained even under conditions where the air utilization rate is high. This is because.

1 is a cross-sectional view taken along a plane perpendicular to a flow direction of a gas flow path, schematically showing a single cell structure of a polymer electrolyte fuel cell according to a first embodiment of the present invention. 1 is an exploded plan view of a membrane / electrode assembly of a polymer electrolyte fuel cell according to a first embodiment of the present invention. 3a schematically shows a single cell structure of the polymer electrolyte fuel cell according to the second embodiment of the present invention, which corresponds to the upstream half of the oxidant gas flow path, and in the flow path direction. It is sectional drawing in a perpendicular | vertical surface, 3b is equivalent to within the downstream half of an oxidant gas flow path which shows typically the single cell structure of the polymer electrolyte fuel cell which concerns on 2nd Embodiment of this invention. Sectional drawing in the surface which is a part to perform and is perpendicular | vertical to a flow-path direction. The disassembled top view of the membrane electrode assembly of the polymer electrolyte fuel cell which concerns on 2nd Embodiment of this invention. 5a schematically shows a single cell structure of the polymer electrolyte fuel cell according to the third embodiment of the present invention, which corresponds to the upstream half of the oxidant gas flow path, and in the flow path direction. It is sectional drawing in a perpendicular | vertical surface, 5b is equivalent to within the downstream half of an oxidant gas flow path which shows typically the single cell structure of the polymer electrolyte fuel cell which concerns on 3rd Embodiment of this invention. Sectional drawing in the surface which is a part to perform and is perpendicular | vertical to a flow-path direction. The disassembled top view of the membrane electrode assembly of the polymer electrolyte fuel cell which concerns on 3rd Embodiment of this invention. Output of a single cell (Examples 1 to 3) using the membrane / electrode assembly for a polymer electrolyte fuel cell according to the present invention and a conventional single cell (Comparative Example 1) under the condition of constant output current The graph which shows the relationship between a voltage and an air utilization factor. Sectional drawing in the surface perpendicular | vertical to the flow path direction of a gas flow path which shows typically the single cell structure of the conventional polymer electrolyte fuel cell.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Solid polymer electrolyte membrane 2a ... Anode side catalyst layer 2b ... Cathode side catalyst layer 3a ... Anode side gas diffusion layer 3b ... Cathode side gas diffusion layer 4a ... Anode side gas diffusion restriction layer 4b ... Cathode side gas diffusion restriction layer 4b (1) ... Cathode upstream side gas diffusion limiting layer 4b (2) ... Cathode downstream side gas diffusion limiting layer 5 ... Anode 6 ... Cathode 7a ... Fuel gas channel 7b ... Oxidant gas channel 8a ... Anode side separator 8b ... Cathode Side separator 10a ... membrane / electrode assembly 10b ... membrane / electrode assembly 10c ... membrane / electrode assembly 100a ... single cell 100b ... single cell 100c ... single cell 200 ... single cell 201 ... solid polymer electrolyte membrane 202a ... anode side Catalyst layer 202b ... Cathode side catalyst layer 203a ... Anode side gas diffusion layer 203b ... Cathode side gas diffusion layer 205 ... A Over de 206 ... cathode 207a ... fuel gas passage 207b ... oxidizing gas passage 208a ... anode separator 208b ... cathode-side separator 210 ... membrane electrode assembly

Claims (9)

An electrolyte membrane, an anode in which at least an anode side catalyst layer and an anode side gas diffusion layer are laminated in this order on one surface of the electrolyte membrane, and at least a cathode side catalyst layer on the other surface of the electrolyte membrane A membrane-electrode assembly for a fuel cell comprising a cathode in which a cathode side gas diffusion layer is laminated in this order,
On at least one of the anode side and the cathode side, the catalyst layer has the following conditions:
The catalyst component-supporting carbon particles and the fluorine-based electrolyte resin are contained in a weight ratio of carbon particles (not including the catalyst component): fluorine-based electrolyte resin = 0.8: 1 to 2: 1.
The carbon particles have a particle size of 10 to 40 nm and a specific surface area of 400 to 1000 m ² / g.
-The porosity is 60 to 80%.
-Thickness is 5-20 micrometers.
Satisfy all
Between the catalyst layer and the gas diffusion layer, carbon particles having a particle size of 10 nm to 10 μm and a specific surface area of 0.5 to 1000 m ² / g, and a fluorine-based resin, carbon particles: fluorine-based A gas diffusion limiting layer containing a resin in a weight ratio of 0.8: 1 to 20: 1 is provided,
The membrane / electrode assembly is characterized in that the gas diffusion property of each layer represented by “porosity × (1 / thickness)” is smaller in the gas diffusion limiting layer than in the catalyst layer. ^2. The membrane / electrode assembly according to claim 1, wherein the carbon particles contained in the gas diffusion limiting layer have a particle size of 1 to 10 μm and a specific surface area of 0.5 to 10 m ² / g.   3. The membrane / electrode according to claim 1, wherein the gas diffusion limiting layer contains the carbon particles and the fluorine resin in a weight ratio of carbon particles: fluorine resin = 5: 1 to 20: 1. Joined body.   The membrane / electrode assembly according to any one of claims 1 to 3, wherein the gas diffusion limiting layer has a porosity of 50 to 70%.   The membrane / electrode assembly according to claim 1, wherein the gas diffusion limiting layer has a thickness of 30 to 100 μm.   The gas diffusion limiting layer is provided only between the catalyst layer and the gas diffusion layer on the anode side and the cathode side, and only on the upstream side of the gas flow path provided adjacent to the gas diffusion layer. Or has a distribution in which the gas diffusibility is greater on the downstream side than the upstream side of the gas flow path and is provided from the upstream side to the downstream side of the gas flow path. 6. The membrane / electrode assembly according to any one of 5 above.   The membrane / electrode assembly according to claim 1, wherein the gas diffusion limiting layer is provided only between the catalyst layer on the anode side and the gas diffusion layer. The membrane / electrode assembly according to claim 6 , wherein the gas diffusion limiting layer is provided only on the upstream side of the gas flow path.   The membrane / electrode assembly according to any one of claims 1 to 8, wherein, in the gas diffusion limiting layer, the fluororesin is polyvinylidene fluoride and the carbon particles are mesocarbon microbeads.

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