JP4186762B2 - Polymer electrolyte fuel cell - Google Patents

Description

  The present invention relates to a solid polymer fuel cell using a solid polymer electrolyte membrane, and more particularly to the configuration of a diffusion layer disposed on the outer surface of an electrode of a membrane electrode assembly.

  In a polymer electrolyte fuel cell, a solid polymer electrolyte membrane is sandwiched between a fuel electrode and an air electrode, and a diffusion layer is disposed on both outer surfaces thereof to form a membrane electrode assembly, and a separator electrode disposed on the outer surface is formed. Electric energy is obtained by an electrochemical reaction by allowing the fuel gas, which is a reactive gas, and air to flow through the gas groove and supplying them to the outer surface of the diffusion layer. At this time, each reaction gas diffuses in the diffusion layer, reaches the surface of the catalyst layer of each electrode, and causes an electrochemical reaction. Since water is generated at the air electrode along with this electrochemical reaction, air that has not contributed to the electrochemical reaction and reaction product water that has become water vapor are discharged from the gas outlet of the air electrode to the outside of the battery. In order to obtain high power generation efficiency, it is necessary to supply the reaction gas evenly to the surface of the catalyst layer, and to quickly discharge the water vapor generated in the catalyst layer. The diffusion layer built in between plays a role. Therefore, this diffusion layer is made of a conductive porous material, and generally, a carbon cloth or carbon paper having a thickness of several hundred μm made of carbon fibers having a thickness of several μm is used.

  FIG. 4 is a longitudinal sectional view showing a basic configuration of a single cell of a conventional polymer electrolyte fuel cell. As shown in the figure, this single cell comprises a membrane electrode assembly in which a solid polymer electrolyte membrane 4 is sandwiched between two catalyst layers 3 and further diffusion layers 2 are arranged on both outer surfaces thereof. The membrane electrode assembly is sandwiched between a pair of separators 5 each having a gas flow groove 6. The fuel gas is guided from the fuel gas inlet provided at one end to the gas flow groove 6 on the fuel electrode side, diffuses through the diffusion layer 2 and reaches the catalyst layer 3 to cause an electrochemical reaction. The remaining gas that has not been used for the reaction is discharged out of the cell from the fuel gas outlet provided at the opposite end. Similarly, air is led from the air inlet provided at one end to the gas flow groove 6 of the separator 5 on the air electrode side, diffuses through the diffusion layer 2 and reaches the catalyst layer 3 to cause an electrochemical reaction. . The water vapor generated by the reaction diffuses through the diffusion layer 2 to reach the gas flow groove 6 and is discharged out of the cell from the air outlet provided at the other end together with the remaining gas not used in the reaction. Therefore, in the single cell of this configuration, the water vapor retention amount increases and the water vapor partial pressure increases toward the outlet side of the diffusion layer 2.

  When the partial pressure distribution of water vapor occurs in the plane of the diffusion layer 2 in this way, the concentration of the reaction gas contributing to the electrochemical reaction decreases in the region where the partial pressure of water vapor is high, so the concentration overvoltage increases and the cell voltage decreases. there's a possibility that. Further, when condensed water of water vapor generated by the reaction stays in the catalyst layer 3, the diffusion layer 2, and the gas flow groove 6, the condensed water covers the pores and inhibits the diffusion of the reaction gas, so that the catalyst layer 3 is sufficiently reacted. There may be a situation where gas is not supplied. Therefore, the diffusion layer 2 needs to be configured in such a structure that the reaction gas can quickly move from the gas flow groove 6 to the catalyst layer 3 particularly in a region where the water vapor partial pressure is high.

As a fuel cell that meets this requirement, Patent Document 1 discloses a fuel cell that incorporates a diffusion layer formed such that the porosity or porosity increases toward the downstream side of the reaction gas. Is a fuel cell in which a porous layer for water evaporation control, for example, formed by changing the average pore diameter in the flow direction of the reaction gas is inserted between the catalyst layer and the diffusion layer, and the in-plane characteristics are made uniform. It is disclosed. Patent Document 3 discloses a fuel cell in which a diffusion layer having a through hole as a water passage and a through hole as a gas passage is incorporated in the thickness direction to improve the flow of water and reaction gas. ing.
JP-A-6-267562, etc. JP 2003-92112 A JP 2003-151585 A

  As described above, a number of fuel cells have been disclosed as fuel cells that prevent reaction gas diffusion inhibition due to retention of reaction product water. Among these, the configurations described in Patent Document 1 and Patent Document 2 are disclosed. However, in this type of fuel cell, it is necessary to manufacture and incorporate a diffusion layer with a special structure and a porous layer for water evaporation control, which increases the required material cost and manufacturing cost, making it difficult to put it into practical use. is there. Further, in the fuel cell having the configuration described in Patent Document 3, an increase in manufacturing cost due to the formation of the through hole is small unless the through hole has a special shape, and the cost is within an allowable range. However, if the through holes are formed evenly in the plane of the diffusion layer as described, the discharge performance of the generated water and the diffusion performance of the reaction gas increase, but from one end to the other end as shown in FIG. In the polymer electrolyte fuel cell in which the reaction gas flows, the water vapor partial pressure of the diffusion layer is low on the upstream side as shown in FIG. Since the distribution is not taken into consideration, the difference between the upstream and downstream of the accumulated water vapor cannot be handled, and the reaction gas diffusion performance is relatively lowered on the downstream side, and the cell voltage may be lowered. There is.

  The present invention has been made in consideration of the problems of the prior art as described above. The object of the present invention is that the material cost and the manufacturing cost are low, and the water vapor and the condensed water generated by the electrochemical reaction are reduced. An object of the present invention is to provide a polymer electrolyte fuel cell that is promptly discharged and can be stably operated at a low cost for which a reaction gas is appropriately supplied to a catalyst layer.

In order to achieve the above object, in the present invention, a membrane electrode assembly comprising a catalyst layer and a diffusion layer on both sides of a solid polymer electrolyte membrane is sandwiched between a pair of separators each having a gas flow groove. In the polymer electrolyte fuel cell in which fuel gas and air are supplied to the gas flow groove to obtain electric energy by electrochemical reaction, the diffusion layer has a through-hole penetrating in the thickness direction, It is set so that the area occupied by the through-holes in the surface of the diffusion layer increases as the in-plane water vapor partial pressure increases, and diffusion occurs in a region where the water vapor partial pressure is 100% or more and less than 120%. The occupied area ratio of the through holes on the surface of the layer was 20% or more and less than 60%.

  As described above, the diffusion layer incorporated in the polymer electrolyte fuel cell is provided with a through hole penetrating in the thickness direction, and the area per surface area of the diffusion layer per unit area of the diffusion layer is the in-plane water vapor when the fuel cell is operated. If it is determined according to the distribution of partial pressures and the area of the through-holes is set to be larger as the in-plane water vapor partial pressure is higher, the water vapor generated inside is discharged to the outside through the through-holes. In particular, as in the present invention, if the in-plane water vapor partial pressure where a relatively large amount of water vapor tends to stay is higher, the area ratio is increased, and if the through holes are provided more densely, the generated water vapor is efficiently Since it is discharged to the outside and the reaction gas is properly supplied to the catalyst layer, the polymer electrolyte fuel cell can be manufactured at low cost and can be stably operated for a long time.

  In the present invention, a membrane / electrode assembly formed by arranging a catalyst layer and a diffusion layer on both surfaces of a solid polymer membrane is sandwiched by a separator having a gas flow groove to constitute a single cell, and each gas flow In a polymer electrolyte fuel cell that supplies electric energy by supplying fuel gas and air as reaction gas to the groove, a through-hole through which the diffusion layer incorporated in the membrane electrode assembly, in particular, the diffusion layer on the air electrode side penetrates in the thickness direction And the area ratio occupied by the through-holes is arranged such that the downstream side of the reaction gas in which the partial pressure of water vapor increases is the best embodiment.

  FIG. 1 is a longitudinal sectional view showing a basic configuration of a single cell of an example of a polymer electrolyte fuel cell of the present invention. FIG. 2 is a cross-sectional view of the diffusion layer on the air electrode side incorporated in the single cell of this embodiment. As shown in FIG. 1, the single cell of this example also has a solid polymer electrolyte membrane 4 sandwiched between two catalyst layers 3 and a diffusion layer 2A on both outer surfaces thereof, as in the conventional example. A membrane electrode assembly is formed, and the membrane electrode assembly is sandwiched between a pair of separators 5 each having a gas flow groove 6. Fuel gas is supplied from the fuel gas inlet to the gas flow groove 6 on the fuel electrode side. In addition, air is guided from the air inlet to the gas flow groove 6 of the separator 5 on the air electrode side, and the diffusion layer 2A is diffused to reach the catalyst layer 3 to obtain electric energy by electrochemical reaction. is there. The difference from the conventional example of the unit cell of this embodiment is that a plurality of through holes 1 penetrating in the thickness direction are provided in the diffusion layer 2A on the air electrode side and the diffusion layer 2A on the fuel electrode side.

  FIG. 3 is a characteristic diagram showing the relationship between the through hole occupation area ratio on the surface of the diffusion layer and the local water vapor partial pressure during operation, which was used when determining the area of the through hole 1 provided in the diffusion layer 2A. The water vapor partial pressure during operation of the diffusion layer 2A on the air electrode side is as shown in FIG. The ratio of the area occupied by the through-holes is selected to increase as the water vapor partial pressure increases so that the reaction gas, air, water vapor and condensed water can be moved quickly, and can be seen in the downstream region of FIG. Thus, if the water vapor partial pressure exceeds 100%, the risk that water vapor becomes condensed water and hinders air diffusion increases. Therefore, in the region where the water vapor partial pressure is 100%, the through-hole occupation area ratio is 20%. In the region where the water vapor partial pressure is 120%, the through hole occupation area ratio is selected to be 20%. The through-hole 1 schematically shown in FIG. 2 is formed by determining the occupation area ratio from the characteristics of FIG. 3 based on the distribution of FIG. No through hole is provided in the upstream region where the partial pressure of water vapor is 60% or less, but in the middle flow region where the partial pressure of water vapor is 80%, there is a through hole having an occupied area ratio of about 5%, and the partial pressure of water vapor is 100%. A through-hole having an occupied area ratio of 20% or more is provided in a downstream region exceeding%.

  The size of the through-hole can be selected arbitrarily, but if the diameter of the through-hole is too small, the number will increase and the number of manufacturing steps will increase and the manufacturing cost may increase, and the diameter of the through-hole will increase. If it is too large, the separator may be in direct contact with the catalyst layer through the through hole to damage the catalyst layer or the solid polymer electrolyte membrane. Therefore, the diameter of the through hole is preferably selected in the range of 100 μm to 10 mm.

  In this embodiment, as shown in FIG. 1, through holes are provided in both the air electrode side diffusion layer and the fuel electrode side diffusion layer. The selection may be based on the partial pressure, or may be selected based on the water vapor partial pressure on the air electrode side having a higher water vapor partial pressure. It is also effective to provide a through hole only in the diffusion layer on the air electrode side.

If the polymer electrolyte fuel cell is configured as in the present invention, the water vapor, condensed water, and reaction gas generated by the electrochemical reaction move quickly without staying in the diffusion layer, so a stable output can be obtained for a long time. can get. Further, since it is only necessary to add a simple process for forming a through hole in the diffusion layer, the manufacturing cost can be reduced. Therefore, the polymer electrolyte fuel cell according to the present invention can be used as a fuel cell power generator as a household power source or a fuel cell used in a fuel cell power generator as an in-vehicle power source.

1 is a longitudinal sectional view showing the basic configuration of a single cell of an embodiment of a polymer electrolyte fuel cell of the present invention Cross-sectional view of the diffusion layer on the air electrode side incorporated in the single cell of this example The characteristic view which shows the relationship between the through-hole occupation area ratio of the surface of a diffused layer, and the local water vapor partial pressure at the time of operation used for determination of the area of the through-hole 1 provided in 2 A of diffused layers of the single cell of a present Example A longitudinal sectional view showing the basic structure of a single cell of a conventional polymer electrolyte fuel cell Characteristic chart showing in-plane distribution of water vapor partial pressure in the diffusion layer on the air electrode side of a single cell of a polymer electrolyte fuel cell

Explanation of symbols

1 Through hole
2A diffusion layer
3 catalyst layer
4 Solid polymer electrolyte membrane
5 Separator
6 Gas distribution channel

Claims (1)

A membrane electrode assembly in which a catalyst layer and a diffusion layer are arranged on both sides of a solid polymer electrolyte membrane is sandwiched between a pair of separators having gas flow grooves, and fuel gas and air are supplied to the gas flow grooves for electricity. In polymer electrolyte fuel cells that obtain electrical energy through chemical reactions,
The diffusion layer has a through-hole penetrating in the thickness direction,
The higher the in-plane water vapor partial pressure during the fuel cell operation, the larger the occupied area ratio of the through-holes in the surface of the diffusion layer is set,
A solid polymer fuel characterized in that the occupied area ratio of the through-holes in the surface of the diffusion layer in the region where the water vapor partial pressure is 100% or more and less than 120% is selected to be 20% or more and less than 60% battery.

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