JP2007087651A - Gas diffusion layer for solid polymer fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas diffusion layer for solid polymer fuel cell sufficiently securing gas diffusion property and preventing lowering of battery performance in advance, while preventing flooding phenomenon even when water is stagnated inside the gas diffusion layer. <P>SOLUTION: At least the gas diffusion layer 3, 6 of the solid polymer fuel cell 1 at one side has a fuel electrode and an air electrode having a catalyst layer and the gas diffusion layer 3, 6 on both sides of an electrolyte layer made of a solid polymer film. As a distribution of diameter of pores, the gas diffusion layers 3, 6 have peak of diameter of pores as that for water draining passage, and a peak of diameter of pores as that for gas diffusion passage. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

   The present invention relates to a gas diffusion layer used for a fuel electrode and an air electrode constituting a solid polymer fuel cell.

  A polymer electrolyte fuel cell has a structure in which an electrolyte layer made of a polymer electrolyte membrane is sandwiched between a pair of electrodes, and a fuel gas containing hydrogen is supplied to a fuel electrode (anode) of the pair of electrodes. At the same time, an oxidizing gas containing oxygen is supplied to the air electrode (cathode) of the pair of electrodes, and an electrochemical reaction of the following formula is caused on the surface of each electrode on the electrolyte layer side to generate electric energy from the electrodes. It is something to take out.

Fuel electrode reaction: H ₂ → 2H ⁺ + 2e ⁻ (Formula 1)
Air electrode reaction: 2H ⁺ + 2e ⁻ + (1/2) O ₂ → H ₂ O (Formula 2)

  For supplying fuel gas to the fuel electrode, a method of directly supplying a fuel gas that is pure hydrogen from a hydrogen storage device, or a method of reforming a fuel containing hydrogen and supplying it as a fuel gas is known. . The hydrogen storage device includes a high-pressure gas tank, a liquefied hydrogen tank, a hydrogen storage alloy tank, and the like, and the fuel containing hydrogen includes natural gas, methanol, gasoline, and the like. On the other hand, air is generally used as the oxidizing gas supplied to the air electrode.

  Furthermore, in the polymer electrolyte fuel cell, it is necessary to keep the electrolyte layer in a moist state, and in order to fully draw out the performance of the electrolyte layer and improve the power generation efficiency, it is necessary to keep the water content of the electrolyte layer optimal. There is. For this reason, in the polymer electrolyte fuel cell, sufficient humidification is performed in advance on the fuel gas and air to be introduced.

  By the way, in the polymer electrolyte fuel cell, water is generated on the air electrode side as the above-described electrochemical reaction proceeds. This generated water is vaporized in the oxidizing gas supplied to the air electrode and discharged together with the oxidizing gas to the outside of the fuel cell. However, when the amount of generated water is large or in the gas flow path, the temperature is partially low. If there is a region, condensation may occur in the gas diffusion layer constituting the air electrode, and water may remain in the gas diffusion layer.

  On the fuel electrode side, water produced by the electrochemical reaction does not exist, but since the fuel gas to be introduced is previously humidified as described above, if the amount of gas decreases as the fuel gas is consumed, the water content in the fuel gas is reduced. Condensate, and in particular, water may condense in the gas diffusion layer constituting the fuel electrode, and water may remain in the gas diffusion layer.

  When water stays in the gas diffusion layer as described above, a flooding phenomenon (water clogging) occurs in which the pores of the gas diffusion layer are blocked with water and gas diffusion is inhibited, and the battery performance is deteriorated. There was a thing.

Therefore, in the past, by defining the diameter of the pores (holes) of the gas diffusion layer, even when water stays, the pores are secured to enable gas diffusion, thereby suppressing the deterioration of battery performance. Has been proposed (Patent Document 1).
JP 2000-182626 A

  However, in the conventional gas diffusion layer as described above, the reason why the deterioration of the battery performance can be suppressed is not clear, and the pore diameter considered to play a role of gas diffusion is very 15 nm to 1 μm. Since it is small and its pore volume is small, it cannot be said that the gas diffusivity is sufficient, and this causes a problem that the battery performance is lowered.

  The present invention has been made paying attention to the above-mentioned conventional problems, and even when water is retained in the gas diffusion layer, it is possible to prevent the occurrence of flooding phenomenon and to diffuse the gas. It is an object of the present invention to provide a gas diffusion layer of a polymer electrolyte fuel cell that can sufficiently ensure the properties.

  The present invention is a gas diffusion layer of at least one electrode in a solid polymer fuel cell comprising a catalyst layer, a fuel electrode having a gas diffusion layer, and an air electrode on both surfaces of an electrolyte layer made of a solid polymer membrane. The pore diameter distribution of the pores has a pore diameter peak that serves as a water discharge path and a pore diameter peak that serves as a gas diffusion path. Means for solving the conventional problems with the above structure It is said.

  As a more preferred embodiment, the present invention has a pore diameter distribution between 5 μm and 50 μm in the pore diameter distribution of the pores, and has a gas diameter in the range of 100 μm and the vicinity thereof. It is characterized by having a pore diameter peak that becomes a diffusion path.

  Furthermore, as a more preferable embodiment, the present invention provides a pore diameter peak serving as a water discharge path and a pore diameter peak serving as a gas diffusion path between 5 μm and 100 μm. It is characterized by having.

  Since the gas diffusion layer of the present invention has a pore diameter peak serving as a water discharge path and a pore diameter peak serving as a gas diffusion path as the pore diameter distribution of the pores, Both gas diffusibility is ensured individually, and even when water stays in the gas diffusion layer, the water stays mainly in the drainage path and the diffusion path is secured, preventing flooding phenomenon, Sufficient gas diffusibility can be ensured.

  Therefore, the polymer electrolyte fuel cell using the gas diffusion layer can maintain good cell performance without causing flooding.

  Hereinafter, an embodiment of a gas diffusion layer of a polymer electrolyte fuel cell according to the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram showing an example of a fuel cell system. The illustrated fuel cell system includes a fuel cell main body 11 formed by stacking a plurality of polymer electrolyte fuel cells (power generation elements: cells), and auxiliary equipment for operating the fuel cell main body 11. A long life coolant (hereinafter abbreviated as “LLC”), which is an antifreeze, is circulated in the fuel cell main body 11 in order to keep the temperature optimal. For example, a mixed solution of ethylene glycol and water is used for the LLC.

  The LLC circulation system includes a tank 14 that stores LLC, a circulation drive pump 15, and a temperature sensor 16 that measures the temperature of the LLC in a feed flow path 13 that extends from the radiator 12 to the fuel cell body 11, and a fuel cell. The return flow path 17 extending from the main body 11 to the radiator 12 is provided with a bypass valve 18, and the bypass valve 18 is connected to a bypass flow path 19 extending from the return flow path 17 to the feed flow path 13. .

  Further, the fuel cell main body 11 includes, for each fuel cell, a supply path 21A and a discharge path 21B for a fuel gas containing hydrogen, and a supply path 22A and a discharge path 22B for an oxidizing gas (air) containing oxygen. The fuel gas supply system is provided with a fuel electrode side water recovery device 21C, and the oxidizing gas supply system is provided with an air electrode side water recovery device 22C. Is provided.

  These water recovery devices 21C and 22C circulate to the inlet a water recovery member comprising a membrane for moving water between gases, a plate using hollow fiber or porous material, and exhaust gas from a humidified fuel cell. It is possible to include a pump or an ejector, and water movement may be performed between the fuel gas and the oxidizing gas.

  FIG. 2 is a view showing the fuel cell 1 accommodated in the fuel cell main body 11. The illustrated fuel cell 1 includes an air electrode side gas diffusion layer 3 and a gas flow path 4 on one surface of a membrane / electrode assembly 2 composed of a solid polymer membrane forming an electrolyte layer and a catalyst electrode layer. A fuel electrode side separator (anode bipolar plate) having a fuel electrode side gas diffusion layer 6 and a gas flow path 7 on the other surface of the membrane / electrode assembly 2 while laminating an electrode side separator (cathode bipolar plate) 5. 8 is provided.

  The air electrode side separator 5 is formed with an LLC channel 9 for circulating the aforementioned LLC. The LLC flow path 9 may be provided in the fuel electrode side separator 8 or in both separators 5.8.

  FIG. 3A is a view showing the separator 8 on the fuel electrode side, and FIG. 3B is a view showing the separator 5 on the air electrode side. These separators 8 and 5 are both in the shape of a square plate and penetrate the fuel cell 1 as a whole. The supply manifold 25A, the discharge manifold 25B, the supply gas manifold 26A, the fuel gas An exhaust manifold 26B, an oxidizing gas supply manifold 27A, and an oxidizing gas exhaust manifold 27B are provided.

  The fuel electrode side separator 8 is formed with the gas flow path 7 described above from the fuel gas supply manifold 26A to the fuel gas discharge manifold 26B, while the air electrode side separator 5 has an oxidizing gas. The gas flow path 4 is formed from the supply manifold 27A to the oxidant gas discharge manifold 27B.

  Further, each of the gas flow paths 4 and 7 of this embodiment has a plurality (five in the illustrated example) arranged in parallel and has a generally S-shape as a whole. For this reason, the gas flow paths 4 and 7 have two bent portions in the middle when viewed roughly, and the other portions are straight portions.

  3 shows the surface on the membrane / electrode assembly 2 side, and the arrangement of the manifolds 26A and 26B for the fuel gas, the manifolds 27A and 27B for the oxidizing gas, and the gas flow paths 4 and 7 are shown. Thus, the flow of the fuel gas and the oxidizing gas becomes a counter flow.

  Here, FIG. 4 is a graph showing the power generation performance of two types of gas diffusion layers. One gas diffusion layer GDL-A is formed of carbon cloth as an example, and the other gas diffusion layer GDL-B is formed of carbon paper as an example. According to FIG. 4, in the other gas diffusion layer GDL-B, when the current density increases, a flooding phenomenon occurs, and the voltage (cell voltage) decreases greatly. On the other hand, it can be seen that in one gas diffusion layer GDL-A, the flooding phenomenon hardly occurs even when the current density is increased, and the voltage drop is small compared to the other gas diffusion layer GDL-A.

  FIG. 5 is a graph showing the moisture content in the battery measured using a neutron beam visualization technique. According to FIG. 5, one gas diffusion layer GDL-A using carbon cloth has a larger amount of water in the battery than the other gas diffusion layer GDL-B using carbon paper, and water is retained. It becomes easy. However, as described with reference to FIG. 4, the flooding phenomenon is less likely to occur in one gas diffusion layer GDL-A than in the other gas diffusion layer GDL-B. That is, it can be seen that one gas diffusion layer GDL-A has a hole for diffusing gas even if water stays inside.

  FIG. 6 is a graph showing the pore diameter distribution obtained by the mercury intrusion method. The small peak observed near the pore diameter of 0.1 μm is due to the microlayer. Looking at the macro layer which is the base material of the gas diffusion layer, the other gas diffusion layer GDL-B formed of carbon paper has only one peak of the pore diameter.

  On the other hand, one gas diffusion layer GDL-A formed of carbon cloth has a first peak with a pore diameter of 5 μm to 50 μm, and a second peak in the range of the pore diameter of 100 μm and the vicinity thereof. The holes having the first peak pore diameter function as a gas diffusion path, and the holes having the second peak pore diameter function as a water discharge path.

  FIG. 7 is a diagram schematically showing a cross section of the macro layer of one gas diffusion layer GDL-A. In one gas diffusion layer GDL-A, a water discharge path indicated by a thick arrow in the figure and a gas diffusion path indicated by a thin arrow in the figure exist independently between the carbon fibers CF. . For this reason, even if water is flowing in the water discharge path, that is, water is staying in the discharge path, the gas can diffuse to the reaction site of the catalyst via the gas diffusion path. This is the result shown in FIG. 5, that is, in one gas diffusion layer GDL-A, the flooding phenomenon does not occur even if water stays inside.

  As described above, one gas diffusion layer GDL-A has both a water discharge path and a gas diffusion path independently due to the pore diameter distribution of the pores, thereby preventing the flooding phenomenon. However, it has sufficient gas diffusivity.

  Therefore, in the present invention, the gas diffusion layers 6 and 3 on the fuel electrode side and the air electrode side shown in FIG. 2 have, as the pore diameter distribution, the first peak of the pore diameter that becomes the water discharge path, and the gas And having a second peak with a pore diameter that becomes a diffusion path of the gas, more specifically, like the one gas diffusion layer GDL-A, the first peak is between 5 μm and 50 μm, The second peak is assumed to be 100 μm and a range in the vicinity thereof.

  As a result, in the gas diffusion layers 6 and 3 on the fuel electrode side and the air electrode side, both drainage and gas diffusibility are individually secured, and even when water stays inside, the water is mainly used as a drainage path. Since the diffusion path is secured by staying in the gas, the gas diffusibility can be sufficiently secured while preventing the flooding phenomenon.

  Further, the polymer electrolyte fuel cell 1 using the gas diffusion layers 6 and 3 can maintain good battery performance without causing a flooding phenomenon.

  4 to 6 exemplify one gas diffusion layer GDL-A using carbon cloth and the other gas diffusion layer GDL-B using carbon paper, and one gas diffusion layer GDL-A includes Although it has been described as having a pore size distribution defined in the present invention, not only carbon cloth but also a gas diffusion layer using carbon paper or other materials has the above pore size distribution. Of course, it can be applied if it exists, and the same effect can be obtained.

  FIG. 8 is a graph showing the pore size distribution in the gas diffusion layer of another embodiment of the present invention. The peak of the pore diameter seen in the vicinity of 0.1 μm is due to the microlayer. Looking at the macro layer which is the base material of the gas diffusion layer, there is only one peak of the pore diameter, and the peak is widely present in the range from near 5 μm to over 100 μm.

  FIG. 9 shows the measurement of the gas diffusion layer GDL-C of the example having the pore size distribution shown in FIG. 8 and the other gas diffusion layer GDL-B described in FIGS. It is a graph which shows the moisture content in a battery. According to FIG. 9, it can be seen that the gas diffusion layer GDL-C of the example has a larger amount of moisture in the battery than the other gas diffusion layer GDL-B, and water tends to stay inside.

  FIG. 10 is a graph showing the power generation performance of the two types of gas diffusion layers. According to FIG. 10, in the other gas diffusion layer GDL-B, when the current density increases, a flooding phenomenon occurs and the voltage (cell voltage) decreases. On the other hand, it can be seen that the gas diffusion layer GDL-C of the example hardly causes the flooding phenomenon even when the current density is increased as compared with the other gas diffusion layer GDL-B.

  Thus, although the gas diffusion layer GDL-C of the example has only one peak of the pore diameter, the water discharge path and the gas diffusion path depend on the existence area of the wide pore diameter. Both are secured.

  Therefore, in the same manner as the gas diffusion layer GDL-C in the above embodiment, for example, the pore size distribution of the gas diffusion layers 6 and 3 on the fuel electrode side and the air electrode side shown in FIG. It is assumed that both the peak of the pore diameter that becomes the water discharge path and the peak of the pore diameter that becomes the gas diffusion path are included in the range exceeding, more preferably 5 μm to 100 μm.

  As a result, in the gas diffusion layers 6 and 3 on the fuel electrode side and the air electrode side, both drainage and gas diffusibility are individually secured, and even when water stays inside, the water is mainly used as a drainage path. Since the diffusion path is secured by staying in the gas, the gas diffusibility can be sufficiently secured while preventing the flooding phenomenon.

It is explanatory drawing which shows an example of a fuel cell system. It is sectional drawing explaining the polymer electrolyte fuel cell concerning this invention. In one Example of this invention, it is the top view (a) which shows a fuel electrode side separator, and the top view (b) which shows an air electrode side separator. It is a graph which shows the electric power generation performance of two types of gas diffusion layers. It is a graph which shows the moisture content in a battery measured using the neutron beam visualization method. It is a graph which shows the pore diameter distribution of the hole calculated | required by the mercury intrusion method. It is a figure which shows typically the cross section of the macro layer of one gas diffusion layer. It is a graph which shows the pore diameter distribution of the gas diffusion layer in the other Example of this invention. It is a graph which shows the moisture content in a battery measured using the neutron beam visualization method about two types of gas diffusion layers. It is a graph which shows the electric power generation performance of two types of gas diffusion layers.

Explanation of symbols

1 Fuel cell 2 Membrane / electrode assembly (electrolyte layer / catalyst layer)
3 Air electrode side gas diffusion layer 6 Fuel electrode side gas diffusion layer GDL-A Gas diffusion layer GDL-B Gas diffusion layer GDL-C Gas diffusion layer

Claims (4)

  A gas diffusion layer of at least one electrode in a polymer electrolyte fuel cell having a catalyst layer, a fuel electrode having a gas diffusion layer, and an air electrode on both surfaces of an electrolyte layer made of a solid polymer membrane, A gas diffusion layer of a polymer electrolyte fuel cell, characterized by having a pore diameter peak serving as a water discharge path and a pore diameter peak serving as a gas diffusion path as a pore diameter distribution.   The pore diameter distribution of pores has a pore diameter peak between 5 μm and 50 μm as a water discharge path, and a pore diameter peak as a gas diffusion path in the range of 100 μm and the vicinity thereof. The gas diffusion layer of a polymer electrolyte fuel cell according to claim 1,   The pore diameter distribution of pores has a pore diameter peak serving as a water discharge path and a pore diameter peak serving as a gas diffusion path between 5 μm and 100 μm. A gas diffusion layer of the polymer electrolyte fuel cell according to 1.   A solid polymer fuel cell, wherein the gas diffusion layer according to any one of claims 1 to 3 is used for at least one of a fuel electrode and an air electrode.

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