JP2007087648A - Gas diffusion layer of solid polymer fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem of gas diffusion layer of conventional solid polymer fuel cell generating flooding phenomenon and lowering battery performance caused by the water stagnated inside. <P>SOLUTION: At least the gas diffusion layer 3, 6 of the solid polymer fuel cell 1 at one side is formed by laminating a separator 5 composed of a fuel electrode having a catalyst layer and a gas diffusion layer 3 on one side of an electrolyte layer 2, and laminating a separator 8 composed of an air electrode having a gas diffusion layer 6 and a gas flow passage 7 on the other side of the electrolyte layer 2. A part where the water is apt to stagnate is formed by a carbon cloth CC, and a part where the water hardly stagnates is formed by a carbon paper CP, by the above, even when the water is stagnated, the flooding phenomenon is prevented and lowering of battery performance is prevented in advance while maintaining gas diffusion property by the carbon cloth CC used at the part where the water is stagnated. <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, conventionally, as a measure for discharging water condensed in the gas diffusion layer, the air permeability in the downstream region of the gas diffusion layer is relatively larger than the air permeability in the upstream region of the gas diffusion layer. However, it has been proposed to improve drainage in the downstream region of the gas diffusion layer (Patent Document 1).
JP 2004-273392 A

  However, in the conventional gas diffusion layer as described above, the correlation between the gas permeability of the gas diffusion layer and the discharge property of the water staying in the gas diffusion layer is low, and the gas is still in a condition where the generated water is likely to condense. There is a problem in that water may stay in the diffusion layer, which causes a flooding phenomenon and decreases battery performance.

  The present invention has been made paying attention to the above-mentioned conventional problems, and is a solid polymer fuel capable of preventing the occurrence of flooding even when water stays in the gas diffusion layer. It aims at providing the gas diffusion layer of a battery.

  In order to analyze the flooding phenomenon in the gas diffusion layer, the present inventor quantified the amount of water staying in the gas diffusion layer using a neutron beam visualization method, and grasped the relationship between the amount of water and battery performance. As a result, it has been found that the gas diffusion layer formed of carbon cloth does not hinder the diffusion of gas even when water stays and does not deteriorate the battery performance, compared to the gas diffusion layer formed of carbon paper. It was.

  Therefore, the present invention laminates a catalyst layer, a fuel electrode having a gas diffusion layer and a separator having a gas flow path on one surface of an electrolyte layer made of a solid polymer membrane, and on the other surface of the electrolyte layer, At least one gas diffusion layer in a polymer electrolyte fuel cell formed by stacking a catalyst layer, an air electrode having a gas diffusion layer, and a separator having a gas flow path, and forming a portion where water is likely to stay with carbon cloth In addition, the portion in which water does not easily stay is formed of carbon paper, and the above configuration is a means for solving the conventional problems.

  Here, as the portion where water is likely to stay, the gas flow path in the separator has a large change in the flow direction such as a bent portion such as a bent portion or a curved portion with a large curvature, or a portion on the outlet side of the gas flow. Is mentioned. Therefore, in the gas diffusion layer of the present invention, it is assumed that a carbon cloth that can secure gas diffusibility even when water stays is used for a region including these portions where water easily stays in the gas flow path. Yes.

  Further, as the portion where water does not stay easily, the gas flow path in the separator is a part where there is no change in the flow direction, such as a straight part or a curved part with a small curvature, or a part where the change in the flow direction is small, or For example, the entrance side. Therefore, in the gas diffusion layer of the present invention, carbon paper is used for the region including the portion where these waters are likely to stay in the gas flow path.

  Since the gas diffusion layer of the present invention uses carbon cloth and carbon paper separately for the part where water stays easily and the part where water does not stay easily, even if water stays, the carbon cloth used for that part Thus, the gas diffusibility can be maintained, the flooding phenomenon can be prevented, and the deterioration of the battery performance due to the gas diffusion inhibition can be prevented.

  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.

  3A shows the separator 8 on the fuel electrode side, FIG. 3B shows the separator 5 on the air electrode side, FIG. 3C shows the fuel electrode side gas diffusion layer 6, and FIG. It is a figure explaining the gas diffusion layer 3 by the side of an air electrode. 3A to 3D all show the surface on the membrane / electrode assembly 2 side.

  The fuel electrode side and air electrode side separators 8 and 5 and the gas diffusion layers 6 and 3 are both in the shape of a square plate, and in a state of penetrating the entire fuel cell 1, the LLC supply manifold 25A, An LLC discharge manifold 25B, a fuel gas supply manifold 26A, a fuel gas discharge manifold 26B, an oxidizing gas supply manifold 27A, and an oxidizing gas discharge 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, the gas diffusion layers 6 and 3 are overlaid on the separators 8 and 5 when stacking, so that the fuel electrode side and the air electrode are stacked. Either one of the sides is turned upside down and stacked on the membrane / electrode assembly. At this time, the flow of the fuel gas and the oxidizing gas is determined by the arrangement of the manifolds 26A and 26B for the fuel gas, the manifolds 27A and 27B for the oxidizing gas, and the gas passages 4 and 7 shown in FIGS. Counter flow.

  Here, FIG. 4 is a graph showing the result of comparing the power generation performance when carbon cloth and carbon paper are used for the gas diffusion layer. According to FIG. 4, when carbon paper is used, a flooding phenomenon occurs when the current density increases, and the voltage (cell voltage) decreases. On the other hand, when carbon cloth is used, it can be seen that the flooding phenomenon does not easily occur even when the current density is increased, and the voltage drop is clearly smaller than when carbon paper is used.

  FIG. 5 is a graph showing the moisture content in the battery measured using a neutron beam visualization technique. According to FIG. 5, when carbon cloth is used, the amount of water in the battery is larger than when carbon paper is used, and water tends to stay in the gas diffusion layer. However, as described with reference to FIG. 4, the flooding phenomenon is less likely to occur when carbon cloth is used than when carbon paper is used. That is, it can be seen that when carbon cloth is used, a passage through which gas diffuses is secured even if water stays in the gas diffusion layer.

  FIG. 6 shows the amount of water in the battery with respect to the position in the gas flow direction. According to FIG. 6, it can be seen that the amount of water increases with the flow, and particularly, a large amount of water stays in the bent portion of the gas flow path. This is because the gas flow is disturbed due to the bending of the gas flow path, making it difficult to discharge water in the gas flow path.

  FIG. 7 is a graph showing film resistance against current density. According to FIG. 7, as the current density increases, the amount of generated water increases, so the solid polymer film forming the electrolyte layer gets wet and the resistance decreases. At this time, when the influence of the gas diffusion layer is seen, it can be seen that when carbon paper is used, the membrane resistance is smaller than when carbon cloth is used, and water retention is good. Therefore, when carbon paper is used, dry out of the electrolyte layer can be prevented when the inlet humidity of the gas is lowered as compared with the case of using carbon cloth.

  As is apparent from the description based on FIGS. 4 to 7 described above, in the polymer electrolyte fuel cell 1, water is generated by an electrochemical reaction on the air electrode side, and fuel gas is generated on the fuel electrode side. The water in the gas is condensed with consumption, and these waters are likely to stay in the bent portions of the gas flow paths 4 and 7.

  Therefore, in this embodiment, as shown in phantom lines in FIGS. 3 (a) and 3 (b), and gas diffusion layers 6 and 3 on the fuel electrode side and air electrode side in FIGS. A region corresponding to a bent portion of the flow paths 7 and 4, that is, a portion where water easily stays, is formed of carbon cloth CC that can ensure gas diffusibility even if water stays, and a straight portion of the gas flow paths 7 and 4. That is, the region for the portion where water does not easily stay is formed of the carbon paper CP that can ensure the water retention of the membrane / electrode assembly (electrolyte layer) 2.

  Thereby, in the gas diffusion layers 3 and 6, while preventing the dry-out of the membrane-electrode assembly 2, it is possible to prevent the occurrence of the flooding phenomenon and prevent the voltage drop due to the gas diffusion inhibition, In the fuel cell 1 using these gas diffusion layers 3 and 6, good cell performance can be maintained.

  FIG. 8 is a view for explaining another embodiment of the gas diffusion layer of the polymer electrolyte fuel cell according to the present invention. The configuration of the fuel cell and the power generation system are basically the same as in the previous embodiment. Moreover, the same components as those of the previous embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  The fuel electrode side and air electrode side separators 38.35 shown in FIGS. 8 (a) and 8 (b) and the fuel electrode side and air electrode side gas diffusion layers 36 and 33 shown in FIGS. Both have a rectangular plate shape. Further, a plurality of gas flow paths 7 and 4 of the separators 38 and 35 are arranged in parallel along the long side (seven in the illustrated example), and the whole is linear.

  FIG. 8 shows the surfaces of the separators 38 and 35 on the membrane / electrode assembly 2 side. Therefore, at the time of stacking, the gas diffusion layers 36 and 33 are stacked on the separators 38 and 35, and either the fuel electrode side or the air electrode side is turned upside down and stacked on the membrane / electrode assembly. At this time, the flow of the fuel gas and the oxidizing gas is a counter flow as in the previous embodiment.

  Here, FIG. 9 shows the moisture content in the battery with respect to the position in the gas flow direction. According to FIG. 9, it can be seen that the amount of water increases with flow.

  Therefore, the gas diffusion layers 36 and 33 of this embodiment ensure a gas diffusibility even if water stays in a predetermined region on the outlet side of the gas flow paths 7 and 4, that is, a region with respect to a portion where water easily stays. A region for the other portion, that is, a portion where water does not easily stay, is formed of carbon paper CP that can ensure water retention of the electrolyte layer.

  Thereby, in the gas diffusion layers 36 and 33, it is possible to prevent the occurrence of flooding phenomenon while preventing dry-out of the electrolyte layer, and to prevent voltage drop due to gas diffusion inhibition. In the fuel cell 1 using the layers 36 and 33, good cell performance can be maintained.

  FIG. 10 is a view for explaining still another embodiment of the gas diffusion layer of the polymer electrolyte fuel cell according to the present invention. The configuration of the fuel cell and the power generation system are basically the same as those of the previous embodiment, and the same components as those of the previous embodiment are denoted by the same reference numerals and detailed description thereof is omitted.

  In this embodiment, the entire fuel electrode side gas diffusion layer 36 is formed of carbon paper CP. In the air electrode side gas diffusion layer 33, a predetermined region on the outlet side of the gas flow path 4 is formed of carbon cloth CC. The other parts are made of carbon paper CP.

  As described above, when the gas flow paths 7 and 4 are linear as in this embodiment, water tends to stay in the latter half of the gas flow paths 7 and 4. Moreover, the occurrence of the flooding phenomenon is conspicuous on the air electrode side where water produced by the electrochemical reaction is generated.

  On the other hand, since the flow rate of off gas discharged from the fuel cell is small on the fuel electrode side, it is difficult to exchange water between the off gas and the inlet gas of the fuel electrode side water recovery device (see FIG. 1). For this reason, in the fuel electrode, it is difficult to ensure the humidity of the inlet gas, and the electrolyte layer tends to dry out. Further, hydrogen contained in the fuel gas has a higher gas diffusibility than oxygen. Accordingly, the fuel electrode is less prone to flooding than the air electrode.

  Therefore, in this embodiment, the gas diffusion layer 36 formed of carbon paper CP is used for the fuel electrode. In the gas diffusion layer 33 of the air electrode, a carbon cloth CC that can ensure gas diffusibility even if water stays in the latter half portion where water is likely to stay, and the first half portion where water hardly stays is used. Carbon paper CP is used to ensure the water retention of the electrolyte layer.

  Thus, as in the previous embodiment, while preventing the electrolyte layer from drying out, it is possible to prevent the occurrence of flooding phenomenon and to prevent voltage drop due to gas diffusion inhibition. In the fuel cell using the layers 36 and 33, good cell performance can be maintained.

  The polymer electrolyte fuel cell and the gas diffusion layer according to the present invention are not limited to the details of the embodiments described above, and the gas flow path, the portion formed by the carbon cloth in the gas diffusion layer, and About the formation part etc. by carbon paper, its shape and size can be changed as appropriate, and it is configured to combine the above embodiments, or it corresponds to the part where water tends to stay and the part that is difficult to retain, It is also possible to subdivide the formation part by carbon cloth and the formation by carbon paper.

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, each is a top view (a)-(d) which shows the film | membrane and electrode assembly side surface of the separator of fuel electrode side and air electrode side, and the gas diffusion layer of fuel electrode side and air electrode side. It is. It is a graph which shows the relationship between a current density and a cell voltage. It is a graph which shows the relationship between a current density and the moisture content in a battery. It is a graph which shows the relationship between a gas flow direction position and the moisture content in a battery. It is a graph which shows the relationship between the current density and the membrane resistance of an electrolyte layer. In other embodiments of the present invention, the respective plane views (a) to (d) showing the fuel electrode side and air electrode side separators, and the surfaces of the fuel electrode side and air electrode side gas diffusion layers on the membrane / electrode assembly side, respectively. ). It is a graph which shows the relationship between a gas flow direction position and the moisture content in a battery, when a gas flow path is linear. In still another embodiment of the present invention, the fuel electrode side and air electrode side separators, and the planes (a) to (a) showing the surfaces of the fuel electrode side and air electrode side gas diffusion layers on the membrane / electrode assembly side, respectively. d).

Explanation of symbols

1 Fuel cell 2 Membrane / electrode assembly (electrolyte layer / catalyst layer)
3 33 Air electrode side gas diffusion layer 4 Gas flow path 5 35 Air electrode side separator 6 36 Fuel electrode side gas diffusion layer 7 Gas flow path 8 38 Fuel electrode side separator CC Carbon cloth CP Carbon paper

Claims (5)

  A fuel electrode having a catalyst layer and a gas diffusion layer and a separator having a gas flow path are laminated on one surface of an electrolyte layer made of a solid polymer membrane, and a catalyst layer and a gas diffusion layer are formed on the other surface of the electrolyte layer. In the polymer electrolyte fuel cell formed by laminating an air electrode having a gas flow path and a separator having a gas flow path, at least one gas diffusion layer is formed of carbon cloth, and the water is retained. A gas diffusion layer of a polymer electrolyte fuel cell, characterized in that a difficult part is formed of carbon paper.   The gas flow path of the separator has a bent portion in the middle, and a carbon cloth is provided for the area including the bent portion of the gas flow path, and the carbon paper is provided for the area including the straight portion of the gas flow path. The gas diffusion layer of a polymer electrolyte fuel cell according to claim 1, wherein the gas diffusion layer is provided.   The gas diffusion layer of a polymer electrolyte fuel cell according to claim 1 or 2, wherein a carbon cloth is provided in a predetermined region on the outlet side of the gas flow path in the separator.   4. The gas diffusion layer of a polymer electrolyte fuel cell according to claim 3, wherein a carbon cloth is provided in a predetermined region on the outlet side of the gas flow path in the separator on the air electrode side.   A solid polymer fuel cell, wherein the gas diffusion layer according to any one of claims 1 to 4 is used for at least one of a fuel electrode and an air electrode.

Images