JP2009064615A - Fuel cell - Google Patents

Abstract

An electrolyte membrane included in a polymer electrolyte fuel cell is prevented from being deficient in moisture.
A polymer electrolyte fuel cell includes an electrolyte membrane, an anode and a cathode that are electrodes formed on the electrolyte membrane, and a gas diffusion layer disposed on each electrode. In addition, a fuel gas flow path is formed between each gas diffusion layer and the gas diffusion layer disposed on the anode side so that the fuel gas containing hydrogen flows from the upstream side to the downstream side. A gas separator is provided between the gas diffusion layer disposed on the cathode side and forming an oxidizing gas flow path through which an oxidizing gas containing oxygen flows. At least one of the gas diffusion layer disposed on the anode side and the gas diffusion layer disposed on the cathode side includes an electrolyte membrane, a fuel gas flow channel, and a fuel gas flow channel in a region near the inlet where the fuel gas flows into the fuel gas flow channel. The thickness changes in the plane so that the thermal resistance between the two is smaller than the thermal resistance between the electrolyte membrane and the oxidizing gas flow path.
[Selection] Figure 1

Description

  The present invention relates to a fuel cell.

  Since the electrolyte membrane provided in the polymer electrolyte fuel cell exhibits good proton conductivity in a wet state, the water content of the electrolyte membrane must be sufficiently maintained in order to maintain the cell performance of the polymer electrolyte fuel cell. Is required. As one of the measures for maintaining the water content of the electrolyte membrane, in the gas diffusion layer disposed on the cathode side surface of the electrolyte membrane, for example, the gas diffusion layer is formed thicker toward the upstream side of the oxidizing gas flow. Thus, a configuration has been proposed in which the moisture permeability of the gas diffusion layer is lowered toward the upstream side of the oxidizing gas flow (see, for example, Patent Document 1). In this way, by lowering the moisture permeability of the gas diffusion layer toward the upstream side of the oxidizing gas flow, the evaporation of moisture in the electrolyte membrane into the oxidizing gas is suppressed in the upstream portion where the humidity in the oxidizing gas is lower. Thus, the water content of the electrolyte membrane is prevented from decreasing.

JP 2001-006708 A JP 2001-135326 A JP-A-8-124583

  However, in order to suppress a decrease in the water content of the electrolyte membrane, it may be insufficient to simply suppress the evaporation of water from the cathode side surface into the oxidizing gas. While water is generated on the cathode side of the electrolyte membrane as power is generated, the surface on the anode side tends to be deficient in water due to the power generation. The state of moisture content on the surface may have a greater influence on the battery performance. Therefore, further optimization of the adjustment of the water content in the electrolyte membrane has been desired.

  The present invention has been made to solve the above-described conventional problems, and an object of the present invention is to prevent the electrolyte membrane included in the polymer electrolyte fuel cell from being deficient in water.

In order to achieve the above object, a fuel cell as one embodiment of the present invention is a polymer electrolyte fuel cell,
An electrolyte membrane;
An anode and a cathode, which are electrodes formed on each surface of the electrolyte membrane;
A gas diffusion layer disposed on each of the electrodes and formed by a conductive porous member;
Between each gas diffusion layer and the gas diffusion layer disposed on the anode side, a fuel gas flow path in which a fuel gas containing hydrogen flows from the upstream side to the downstream side is formed. A gas separator that forms an oxidizing gas passage through which an oxidizing gas containing oxygen flows between the gas diffusion layer disposed on the cathode side;
With
At least one of the gas diffusion layer disposed on the anode side and the gas diffusion layer disposed on the cathode side includes the electrolyte membrane in a region near the entrance where the fuel gas flows into the fuel gas flow path. The thickness changes in the plane so that the thermal resistance between the fuel gas flow path is smaller than the thermal resistance between the electrolyte membrane and the oxidizing gas flow path.

  According to the fuel cell as one aspect of the present invention configured as described above, at least one of the gas diffusion layers includes the electrolyte membrane and the fuel in the vicinity of the inlet portion where the fuel gas flows into the fuel gas flow path. The thickness changes in the plane so that the thermal resistance between the gas flow path is smaller than the thermal resistance between the electrolyte membrane and the oxidizing gas flow path. Therefore, heat transfer from the electrolyte membrane to the fuel gas flow path side is promoted in the vicinity of the inlet portion, and movement of water from the cathode side to the anode side is promoted in the electrolyte membrane. Therefore, in the electrolyte membrane, membrane drying on the anode side upstream of the fuel gas flow can be suppressed, and battery performance can be improved.

  In the fuel cell as one aspect of the present invention, the gas diffusion layer disposed on the anode side may be formed thinner on the upstream side than on the downstream side in the flow direction of the fuel gas. With such a configuration, by changing the thickness of the gas diffusion layer on the anode side in the plane, the thermal resistance between the electrolyte membrane and the fuel gas flow path can be reduced in the region near the entrance portion described above. Membrane drying can be suppressed by making it smaller than the thermal resistance between the membrane and the oxidizing gas flow path.

  In such a fuel cell, a first water repellent layer comprising conductive particles and a water repellent substance is further provided between the anode and the gas diffusion layer disposed on the anode side, and the first repellent layer is provided. The water layer may be formed thicker on the upstream side than on the downstream side in the flow direction of the fuel gas. With such a configuration, the thickness of the gas diffusion layer on the anode side is changed in the plane by changing the thickness of the first water-repellent layer having a relatively small influence on the thermal resistance in the plane. The thickness of the entire structure in which the members including the gas diffusion layer and the water repellent layer are laminated can be made uniform without impairing the effect.

  In the fuel cell as one embodiment of the present invention, the gas diffusion layer disposed on the cathode side may be formed thicker on the upstream side than on the downstream side in the flow direction of the fuel gas. With such a configuration, in addition to the gas diffusion layer on the anode side or in place of the gas diffusion layer on the anode side, the thickness of the gas diffusion layer on the cathode side is changed in the plane, so that the above-described entrance In the region in the vicinity of the part, the thermal resistance between the electrolyte membrane and the fuel gas flow channel can be made smaller than the thermal resistance between the electrolyte membrane and the oxidizing gas flow channel, and membrane drying can be suppressed.

  In such a fuel cell, a second water repellent layer comprising conductive particles and a water repellent substance is further provided between the cathode and the gas diffusion layer disposed on the cathode side, and the second repellent layer is provided. The water layer may be formed so that the upstream side is thinner than the downstream side in the flow direction of the fuel gas. With such a configuration, the thickness of the gas diffusion layer on the cathode side is changed in the plane by changing the thickness of the second water-repellent layer having a relatively small influence on the thermal resistance in the plane. The thickness of the entire structure in which the members including the gas diffusion layer and the water repellent layer are laminated can be made uniform without impairing the effect.

  The present invention can be realized in various forms other than those described above. For example, the present invention can be realized in a form such as a method for suppressing drying of an electrolyte membrane in a fuel cell.

A. Fuel cell configuration:
FIG. 1 is a schematic cross-sectional view showing a schematic configuration of a single cell 10 constituting a fuel cell as an embodiment of the present invention, and FIG. 2 is an exploded perspective view. In FIG. 2, the position of the cross section shown in FIG. 1 is shown as a 1-1 cross section. The fuel cell of the present embodiment is a polymer electrolyte fuel cell, and has a stack structure in which a plurality of single cells 10 are stacked. The single cell 10 includes an MEA (Membrane Electrode Assembly) 12 containing an electrolyte, gas diffusion layers 16 and 17 that sandwich the MEA 12 from both sides to form a sandwich structure, and the sandwich structure from both sides. It forms by laminating | stacking sequentially the separators 20 and 21 to clamp. Further, in the single cell 10, water repellent layers 18 and 19 are formed between the MEA 12 and the gas diffusion layers 16 and 17.

  The MEA 12 includes an electrolyte membrane 13 and an anode 14 and a cathode 15 that are electrodes formed on the surface of the electrolyte membrane 13 with the electrolyte membrane 13 interposed therebetween. The electrolyte membrane 13 is a proton conductive ion exchange membrane formed of a solid polymer material, for example, a fluorine-based resin, and exhibits good electrical conductivity in a wet state. In this example, a Nafion membrane (manufactured by DuPont) was used. The anode 14 and the cathode 15 include a catalyst that promotes an electrochemical reaction, such as platinum or an alloy made of platinum and other metals. In order to form the anode 14 and the cathode 15, for example, carbon powder carrying platinum or an alloy made of platinum and another metal is produced, and the carbon powder carrying the catalyst is dispersed in a suitable organic solvent, and an electrolyte is prepared. An appropriate amount of a solution (for example, Aldrich Chemical, Nafion Solution) may be added to prepare a catalyst paste. By applying this catalyst paste onto the electrolyte membrane 13 by a method such as screen printing, the anode 14 and the cathode 15 can be formed.

  The gas diffusion layers 16 and 17 are configured by members having gas permeability and conductivity. In this embodiment, the gas diffusion layers 16 and 17 are formed of a carbon porous body such as carbon cloth or carbon paper. By providing the gas diffusion layers 16 and 17, the gas supply efficiency to the electrodes can be improved, the current collecting property between the separators 20 and 21 and the electrodes can be improved, and the electrolyte membrane 13 can be further protected. . The thickness of such gas diffusion layers 16 and 17 can be set to 100 to 500 μm, for example.

  The water repellent layers 18 and 19 are made of a paste containing conductive particles, for example, carbon particles and a water repellent resin such as a fluorine-based resin, on one surface (on the MEA 12) of the carbon porous body that becomes the gas diffusion layers 16 and 17. It is formed by coating on the overlapping surface). The water-repellent layer provided between the electrode and the gas diffusion layer has a function of repelling liquid water and pushing it back to the electrode side. By pushing back the liquid water in this way, the electrolyte membrane 13 becomes deficient in water. Suppressed. Further, by repelling the liquid water, the pores of the gas diffusion layers 16 and 17 are suppressed from being blocked by the liquid water, and the inhibition of the gas flow due to the blockage of the pores is suppressed. In the present embodiment, the thicknesses of the gas diffusion layer 16 and the water repellent layer 18 described above are not uniform and are characterized by the in-plane distribution of the thicknesses. These configurations will be described in detail later. The thickness of the water repellent layers 18 and 19 can be set to 20 to 80 μm, for example.

  The separators 20 and 21 are gas impermeable thin plate members formed of a conductive material such as carbon or metal. The separators 20 and 21 are members that are arranged in the single cell 10 and constitute a wall surface of a gas flow path through which a reaction gas (a fuel gas containing hydrogen or an oxidizing gas containing oxygen) flows. A concavo-convex shape for forming a gas flow path in the single cell 10 is formed. In the single cell 10, an in-cell fuel gas flow path, which is a flow path of fuel gas, is formed between one surface of the separator 20 in which the groove 22 is formed and the anode 14. In addition, an in-cell oxidizing gas channel, which is an oxidizing gas channel, is formed between one surface of the separator 21 in which the groove 23 is formed and the cathode 15 (see FIG. 1). The separators 20 and 21 are members having the same shape in which the groove 22 is formed on one surface and the groove 23 is formed on the other surface.

  The separators 20 and 21 are provided with holes 30 to 33 at positions corresponding to each other near the outer periphery thereof. When the fuel cells are assembled by laminating the separators 20 and 21 together with the MEA 12 and the gas diffusion layers 16 and 17, the holes provided at the corresponding positions of the laminated separators overlap each other in the stacking direction of the separators. A flow path penetrating the inside of the fuel cell is formed. That is, a gas manifold is formed which is a supply / exhaust gas flow path for supplying / discharging reaction gas to / from the cell gas flow path. Specifically, the hole portion 30 and the hole portion 31 communicating with the groove 22 are discharged from the fuel gas supply manifold that distributes the fuel gas to the in-cell fuel gas passages and the in-cell fuel gas passages, respectively. A fuel gas discharge manifold is formed in which the anode exhaust gas collects. Further, the hole 32 and the hole 33 communicating with the groove 23 form an oxidizing gas supply manifold and an oxidizing gas discharge manifold, respectively.

  Although illustration is omitted, in order to adjust the internal temperature of the stack structure, a refrigerant flow path through which the refrigerant passes may be provided between the single cells or every time a predetermined number of single cells are stacked. good. The refrigerant flow path may be provided between adjacent single cells and between the separator 21 provided in one single cell and the separator 20 of the other single cell provided adjacent thereto.

  When assembling a fuel cell, each member is stacked in the order shown in FIG. 1 to assemble a single cell 10, and a predetermined number of such single cells 10 are stacked to manufacture a fuel cell having a stack structure. . Here, in the present embodiment, as described above, the thicknesses of the gas diffusion layer 16 and the water repellent layer 18 are not uniform, and in the assembly of the fuel cell, the fuel gas flows in the unit cell 10 in the flow direction. On the other hand, the gas diffusion layer 16 and the water repellent layer 18 are arranged so that the thickness distribution is in a specific state. Hereinafter, the thickness distribution of the gas diffusion layer 16 and the water repellent layer 18 will be described.

  FIG. 3 is a perspective view schematically showing the configuration of the gas diffusion layer 16. The gas diffusion layer 16 formed in a quadrangular shape gradually increases in thickness from a region near one side (side 40 in FIG. 3) to a region near the side (side 41 in FIG. 3) opposite to this side. It becomes the shape to do. Specifically, a level difference is formed on one side (the front side shown in FIG. 3) so that the thickness increases stepwise, and the other side (the back side in FIG. 3). Are formed on a flat surface without a step. In this embodiment, the water repellent layer 18 is formed by applying a paste containing carbon particles and a water repellent resin on the gas diffusion layer 16 as described above. Of the surfaces of the diffusion layer 16, this is performed on the surface on which the step is formed. And the gas diffusion layer 16 to which the water-repellent layer 18 is applied has a substantially uniform thickness as a whole by applying the paste thicker in the region where the gas diffusion layer 16 is thinner. When the paste is applied, a part of the paste enters the inside from the surface of the gas diffusion layer 16 which is a porous body. The amount of paste that enters the gas diffusion layer 16 varies depending on the composition of the paste, the concentration of solids, and the like. However, in consideration of the amount that enters the interior, the paste is made to have a substantially uniform thickness as a whole. The water-repellent layer 18 may be formed by coating. 1 and 3, the thickness of the gas diffusion layer 16 is changed in three stages, but the number of stages in which the thickness is changed may be different.

  When assembling the fuel cell, the gas diffusion layer 16 provided with the water repellent layer 18 has a direction in which the thickness of the gas diffusion layer 18 becomes thinner toward the inlet side of the fuel gas flow path in the cell, and the water repellent layer 18 is formed into the MEA 12. Arranged as a side (see FIG. 1). Specifically, the gas diffusion layer is formed such that the side 40 shown in FIG. 3 is close to the hole 30 forming the fuel gas supply manifold and the side 41 is close to the hole 31 forming the fuel gas discharge manifold. 18 is arranged. The gas diffusion layer 17 disposed on the cathode side has the same outer peripheral shape as the gas diffusion layer 16 shown in FIG. 3, but is formed to have a uniform thickness as a whole. Then, a water repellent layer 19 is formed on such a gas diffusion layer 17 by applying a paste with a uniform thickness in the same manner as the water repellent layer 18 described above.

B. Relationship between member thickness, heat transfer and water transfer:
When an electrochemical reaction proceeds in the fuel cell, water is generated at the cathode as power is generated. Part of the generated water is vaporized into the oxidizing gas from the surface of the electrolyte membrane on which the cathode is formed, and the other part diffuses and moves to the anode side in the electrolyte membrane. A part of the water in the electrolyte membrane evaporates into the fuel gas from the anode side surface of the electrolyte membrane. Here, an electrolyte membrane made of a solid polymer electrolyte has a proton conductivity that decreases with a decrease in water content. Therefore, when the water content of the electrolyte membrane decreases, the output voltage of the fuel cell decreases. In a fuel cell, the higher the operating temperature, the higher the activity of the catalyst provided in the electrode. However, when the operating temperature rises, the amount of water evaporated from the electrolyte membrane to the reaction gas increases and the electrolyte membrane tends to dry. Thus, a voltage drop is likely to be caused as described above. Therefore, in order to improve the performance of the fuel cell, it is desired to suppress drying of the electrolyte membrane even during operation at a high temperature.

  In such a fuel cell, the knowledge that the cell performance at a high temperature can be improved by changing the configuration on the anode side and the cathode side will be described below. Specifically, the relationship between the presence or absence of a gas diffusion layer and the voltage during power generation was examined. FIG. 4 is a schematic cross-sectional view showing the configuration of a single cell constituting a fuel cell used for examining battery performance. Portions common to the fuel cell of the first embodiment shown in FIG. 1 are denoted by the same reference numerals. A single cell 510 in FIG. 4A does not include a gas diffusion layer on the anode side, but includes only a water-repellent layer 518 formed to have a uniform thickness like the water-repellent layer 19 on the cathode side. In addition, the fuel cell shown in FIG. 4A includes separators 520 and 521 in which flat surfaces having no irregularities are formed instead of the separators 20 and 21 having irregularities that form the in-cell gas flow paths. . Further, the fuel cell shown in FIG. 4A has a flow made of a flat conductive porous body between the water repellent layer, 518 and the separator 520 and between the gas diffusion layer 17 and the separator 521, respectively. A path forming portion 522, 523 is provided. In the single cell of FIG. 4 (A), the in-cell gas flow path is formed by the space formed by the pores provided in the flow path forming portions 522 and 523.

  A single cell 610 in FIG. 4B includes a gas diffusion layer 616 and a water repellent layer 518 formed in a uniform thickness in the same manner as the cathode side, instead of the gas diffusion layer 16 and the water repellent layer 18. 4A, separators 520 and 521 are provided instead of the separators 20 and 21, and a flow path forming part 522 that forms an in-cell gas flow path between the gas diffusion layer and the separator. , 523 are arranged. That is, the single cell 510 in FIG. 4A and the single cell 610 in FIG. 4B differ only in the presence or absence of the gas diffusion layer on the anode side. 4A and 4B, the flow direction of the fuel gas in the in-cell fuel gas flow path formed in the flow path forming portion 522 is indicated by an arrow.

FIG. 5 is an explanatory diagram showing an example of the result of examining the output voltage of the two types of fuel cells with different operating temperatures. Here, the temperature of the cooling water flowing through the fuel cell and discharged from the fuel cell was used as the operating temperature of the fuel cell. Further, here, for each fuel cell, the voltage drop was examined under the condition of humidifying the air supplied as the oxidizing gas to the cathode and the condition of not humidifying. Humidification was performed using a 55 ° C. bubbler. Hydrogen gas was used as the fuel gas, the output current value was set to a constant value (1.6 A / cm ² ), and the state of decrease in output voltage when the operating temperature was gradually increased was examined. The voltage drop is represented by the degree of drop from the output voltage when the cooling water outlet temperature is 75 ° C.

  In FIG. 5, graph (1) represents the results of examining the voltage drop when humidified oxidizing gas was supplied to the fuel cell shown in FIG. 4 (A), and graph (2) represents the result of FIG. The result of investigating the voltage drop when the humidified oxidizing gas is supplied to the fuel cell shown in FIG. Graph (3) shows the result of examining the voltage drop when supplying non-humidified oxidizing gas to the fuel cell shown in FIG. 4 (A). Graph (4) is shown in FIG. The result of having investigated the voltage drop when supplying the oxidizing gas which does not humidify with respect to a fuel cell is represented.

  As shown in FIG. 5, when the humidification was performed, the degree of voltage drop could be suppressed even when the operating temperature of the fuel cell was increased. This is considered to be because the moisture content of the electrolyte membrane can be sufficiently maintained even when the operation temperature becomes high by humidifying the oxidizing gas. Further, as shown in FIG. 5, the fuel cell having no anode-side gas diffusion layer shown in FIG. 4 (A), regardless of whether or not humidification, has gas diffusion layers on both sides shown in FIG. The voltage drop during high-temperature operation could be suppressed more than the fuel cell that had it. This is because, in the fuel cell of FIG. 4 (A), since the gas diffusion layer on the anode side is not provided, the difference in thermal resistance between the anode side and the cathode side of the electrolyte membrane 13 occurs. Is considered to have improved. That is, at the time of power generation of the fuel cell, heat is generated in the MEA 12 along with the power generation, and the generated heat is transferred from the MEA 12 to the gas flow passages on both sides of the cell. It is considered that the movement of water in the electrolyte membrane 13 is affected by the difference in size. Specifically, heat transfer from the electrolyte membrane 13 to the in-cell fuel gas flow path side having a low thermal resistance because it does not have a gas diffusion layer promotes the electrolyte membrane 13 from the cathode side to the anode side. The movement of water is promoted. As a result, it is considered that the water content on the anode side surface of the electrolyte membrane where no water is generated during power generation is increased, and the voltage drop due to membrane drying is suppressed. As described above, the movement of water from the cathode side to the anode side in the electrolyte membrane can be promoted as the thermal resistance when heat is transferred to the fuel gas flow path side in the cell is reduced in the electrolyte membrane. It is thought that water moves together with the movement of the water. This relationship between heat transfer and material transfer is known as the so-called sore effect.

  In FIG. 5, as described above, the thermal resistance between the electrolyte membrane 13 and the in-cell fuel gas flow path or the in-cell oxidizing gas flow path is varied by varying the presence or absence of the gas diffusion layer. As described above, there is a water repellent layer as a member relating to the thermal resistance interposed between the electrolyte membrane 13 and the gas flow path in the cell, but the gas diffusion layer has a particularly large thermal resistance compared to the water repellent layer. It can be said that it is a contributing member. Below, the result of having compared the efficiency of heat transfer about a gas diffusion layer and a water repellent layer is shown.

  FIG. 6 is an explanatory diagram showing the results of measuring the heat transfer efficiency (specifically, thermal diffusivity) of the gas diffusion layer, and FIG. 7 shows the heat transfer efficiency (specifically, thermal diffusivity) of the water repellent layer. It is explanatory drawing showing the measurement result. Here, the result of measuring the half time for a plurality of gas diffusion layers or water repellent layers having different thicknesses using the laser flash method is shown. The laser flash method is a well-known method for measuring the thermal diffusivity. Infrared radiation temperature measurement is used to instantly uniformly heat the sample surface with a laser pulse and diffuse the heat to the back of the sample. This is a method of observation by non-contact measurement or the like. The higher the thermal diffusivity of the sample, the shorter the half time (the time required for the sample back surface to reach half of the maximum temperature rise). In the present example, the measurement related to the gas diffusion layer is performed using a porous body similar to the porous body constituting the gas diffusion layer 16, and a plurality of porous bodies having different thicknesses are respectively formed into thin plate-like jigs. It was done between. In addition, the measurement related to the water repellent layer is performed by preparing a plurality of different pastes coated with the same paste as that used for forming the water repellent layer 18 between thin plate-like jigs. It was. The horizontal axis in FIG. 6 represents the total thickness of the porous body corresponding to the gas diffusion layer 16 sandwiched between the jigs. Further, the horizontal axis in FIG. 7 represents the total thickness in which a layer corresponding to the water repellent layer 18 is formed between the jigs. The vertical axis in FIGS. 6 and 7 represents half time.

  As shown in FIG. 6, the graph representing the relationship between the thickness of the gas diffusion layer and the half time is a straight line having a certain inclination. That is, it was confirmed that the thermal resistance increases as the thickness increases because the gas diffusion layer has a longer half time as the thickness increases. On the other hand, as shown in FIG. 7, the relationship between the thickness of the water-repellent layer and the half time does not show an inclination as shown in FIG. 6, and it is confirmed that the thickness of the water-repellent layer hardly affects the thermal resistance. It was done. In this embodiment, the gas diffusion layer 16 and the water repellent layer 18 are formed so that the in-plane thickness changes are contradictory, but the thickness of the water repellent layer hardly affects the thermal resistance as described above. It can be said that the in-plane distribution of the thermal resistance depends on the in-plane distribution of the thickness of the gas diffusion layer. In this embodiment, the thickness of the gas diffusion layer 16 is made thinner on the upstream side than the downstream side in the flow direction of the fuel gas in the in-cell fuel gas flow path. It can be said that the in-plane distribution of the thermal resistance when heat is transferred to the road side is smaller toward the upstream side of the fuel gas flow. Therefore, the thermal resistance between the electrolyte membrane 13 and the in-cell fuel gas flow path is smaller in the upstream side of the fuel gas flow than the thermal resistance between the electrolyte membrane 13 and the in-cell oxidizing gas flow path. Become.

  According to the fuel cell of the present embodiment configured as described above, the thermal resistance of the gas diffusion layer on the anode side is higher on the upstream side in the fuel gas flow direction in the fuel gas flow path in the cell than on the cathode side. Since it is small, the heat flow rate from the electrolyte membrane 13 to the gas flow path side in each cell is larger on the anode side than on the cathode side. Thereby, the heat transfer from the electrolyte membrane 13 to the in-cell fuel gas flow path side is promoted toward the upstream side of the fuel gas flow. As described above, the more heat transfer from the electrolyte membrane to the fuel gas flow path side in the cell is promoted, the more water is moved from the cathode side to the anode side in the electrolyte membrane. According to the battery, in particular, it is possible to suppress film drying on the anode side on the upstream side in the fuel gas flow direction.

  During power generation of the fuel cell, the moisture distribution state of the electrolyte membrane is affected by the following various factors. That is, when protons move in the electrolyte membrane during power generation, the protons are hydrated, so that water moves from the anode side to the cathode side together with the protons. Further, since water is generated at the cathode accompanying the electrochemical reaction, liquid water tends to exist on the cathode side surface of the electrolyte membrane, but moisture tends to be insufficient on the anode side. In addition, when gas flows through the gas flow path in the cell, moisture evaporates from the electrolyte membrane to the gas in the gas flow path. Become. For this reason, moisture is more easily taken from the electrolyte membrane to the gas in the upstream side in the gas flow direction in which the gas having lower humidity flows. Therefore, in the electrolyte membrane, it can be said that the surface on the upstream side of the fuel gas flow is more likely to be deficient in moisture, particularly on the anode side surface. In the present embodiment, since the water movement to the anode side is promoted in the electrolyte membrane on the upstream side in the fuel gas flow direction in such a fuel gas flow path in the cell, the electrolyte membrane is short of water. Occurrence of problems can be suppressed. As a result, when the operating temperature of the fuel cell is raised to a higher temperature, the effect of stably maintaining the output voltage, that is, the cell performance, can be enhanced.

  Further, according to the fuel cell of the present embodiment, the gas diffusion layer 16 and the water repellent layer 18 are formed thicker toward the upstream side in the fuel gas flow direction in the in-cell fuel gas flow path. The entire thickness is made uniform. As described above, since the water-repellent layer has less influence on the thermal resistance than the gas diffusion layer, even if the water-repellent layer is formed thick, the degree of increase in the thermal resistance is small, and the gas diffusion layer is formed thinly. The effect of reducing the thermal resistance is not impaired. Therefore, in the electrolyte membrane 13, it is possible to achieve both the promotion of water movement from the cathode side to the anode side and the uniform thickness of the entire single cell 10 on the upstream side in the fuel gas flow direction in the in-cell fuel gas flow path. Can do. By making the thickness of the entire single cell 10 uniform, it becomes easy to assemble the fuel cell by stacking the single cells 10. The thickness of the water repellent layer is preferably such that the entire thickness is substantially the same, but it is not necessarily required to be the same. Even if they are not the same, the thinner the gas diffusion layer, the thicker the water-repellent layer, so that the thickness of the gas diffusion layer is changed in the plane, thereby causing uneven thickness in the single cell 10. Can be suppressed.

  In addition to the movement of water accompanying proton movement as described above, the movement of water from the cathode side to the anode side is also performed inside the electrolyte membrane 13 due to the concentration gradient of water. Therefore, as a method for promoting water movement from the cathode side to the anode side, it is conceivable to reduce the thickness of the electrolyte membrane. However, the reduction in thickness of the electrolyte membrane is limited because the strength of the electrolyte membrane is reduced. According to the present embodiment, the water resistance in the electrolyte membrane 13 is promoted by changing the thermal resistance between the electrolyte membrane 13 and the in-cell fuel gas flow path, resulting in a decrease in strength of the electrolyte membrane 13. There is nothing.

  When water movement from the cathode side to the anode side is promoted in the electrolyte membrane, as described above, when the operating temperature of the fuel cell is higher, cell performance is improved by suppressing membrane drying of the electrolyte membrane. On the other hand, when the operating temperature of the fuel cell is lower (for example, during power generation at normal temperature), the amount of water vaporized from the electrolyte membrane to the gas flowing through the in-cell gas flow path is reduced. In some cases, water may become excessive. In particular, in the downstream region of the in-cell gas flow path, the water content in the gas flowing through the flow path is large, so the water content of the electrolyte membrane tends to be excessive. If liquid water stays on the surface of the electrolyte membrane, a state called so-called flooding occurs, and the liquid water may obstruct gas flow and possibly reduce battery performance. In the fuel cell of this embodiment, in the electrolyte membrane 13, upstream of the in-cell fuel gas flow path, water movement from the cathode side to the anode side is promoted to suppress membrane drying during high-temperature operation, and in-cell fuel Water movement from the cathode side to the anode side can be suppressed on the downstream side of the gas flow path. Therefore, the anode side surface of the electrolyte membrane 13 is prevented from becoming excessively water on the downstream side in the fuel gas flow direction, and flooding can be suppressed even during low temperature operation such as room temperature power generation.

C. Second embodiment:
In the first embodiment, the movement of water from the cathode side to the anode side in the electrolyte membrane 13 is promoted by changing the thicknesses of the gas diffusion layer 16 and the water repellent layer 18 disposed on the anode side in the plane. However, the thickness of the gas diffusion layer 17 and the water repellent layer 19 disposed on the cathode side may be further changed in the plane. Such a configuration will be described below as a second embodiment.

  FIG. 8 is a schematic cross-sectional view showing the configuration of the unit cell 110 included in the fuel cell according to the second embodiment. In the fuel cell of the second embodiment, the same reference numerals are assigned to the parts common to the fuel cell of the first embodiment, and detailed description thereof is omitted. The fuel cell of the second embodiment is constituted by a single cell 110 having a gas diffusion layer 117 and a water repellent layer 119 instead of the gas diffusion layer 17 and the water repellent layer 19 of the first embodiment. The gas diffusion layer 117 is formed of a conductive porous member similar to the gas diffusion layers 16 and 17, but is formed thicker toward the upstream side in the fuel gas flow direction in the in-cell fuel gas flow path. Specifically, the gas diffusion layer 117 has the same shape as the gas diffusion layer 16 shown in FIG. 3, the side 40 is close to the hole 31 that forms the fuel gas discharge manifold, and the side 41 Are arranged so as to be close to the hole 30 forming the fuel gas supply manifold.

  The water repellent layer 119 is formed by applying the same paste as the water repellent layers 18 and 19 on the gas diffusion layer 117. The application of such paste is performed by applying the thinner region of the gas diffusion layer 117 to the surface of the gas diffusion layer 117 where the step is formed. Thus, by forming the water repellent layer 119 so that the thickness becomes thinner toward the upstream side in the fuel gas flow direction in the fuel gas flow path in the cell, the gas diffusion layer 117 on which the water repellent layer 119 is formed As a substantially uniform thickness.

  According to the fuel cell of the second embodiment formed as described above, the gas diffusion layer 16 and the water repellent layer 18 arranged on the anode side have the same configuration as that of the first embodiment, and therefore the first embodiment. The same effect as the example can be achieved. Furthermore, according to the fuel cell of the second embodiment, the gas diffusion layer 117 arranged on the cathode side is formed thicker toward the upstream side in the fuel gas flow direction in the fuel gas flow path in the cell. The thermal resistance of the gas diffusion layer on the cathode side increases toward the upstream side in the direction, and the difference in thermal resistance between the gas diffusion layer on the cathode side and the gas diffusion layer on the anode side increases. Therefore, on the upstream side in the fuel gas flow direction, the degree to which the heat flow rate from the electrolyte membrane 13 to the gas flow path side in each cell is larger on the anode side than on the cathode side is further increased. This further promotes heat transfer from the electrolyte membrane 13 to the fuel gas flow path in the cell on the upstream side in the fuel gas flow direction, and further moves water from the cathode side to the anode side in the electrolyte membrane 13. It is promoted and the anode side membrane drying on the upstream side in the fuel gas flow direction can be suppressed. Further, according to the fuel cell of the second embodiment, the downstream side of the gas flow in the in-cell fuel gas flow path is formed with a thicker water-repellent layer 119 that has a smaller effect on heat transfer. The thickness of the single cell 110 can be made uniform without impairing the effect of changing the thickness of the single cell 110.

D. Variations:
The present invention is not limited to the above-described examples and embodiments, and can be implemented in various modes without departing from the gist thereof. For example, the following modifications are possible.

D1. Modification 1:
In the first and second embodiments, the thickness of the gas diffusion layer is changed stepwise in the plane, but may have a different configuration. For example, the thickness of the gas diffusion layer 16 in FIG. 1 or the gas diffusion layers 16 and 117 in FIG. 8 may be gradually changed steplessly. If the thickness of the gas diffusion layer is changed according to the flow direction of the fuel gas in the in-cell fuel gas flow path, the same effect as in the embodiment can be obtained.

D2. Modification 2:
In the first and second embodiments, the thickness of the entire single cell is made uniform by changing the thickness of the water repellent layer formed on the gas diffusion layer together with the gas diffusion layer. good. For example, the thickness of the water repellent layer may be made uniform, and the thickness of the gas diffusion layers on the anode side and the cathode side may be changed in the plane. FIG. 9 shows the configuration of the fuel cell according to such a modification. Here, the same parts as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  A single cell 210 provided in the fuel cell of the modification shown in FIG. 9 includes gas diffusion layers 216 and 217 instead of the gas diffusion layers 16 and 17. The gas diffusion layer 216 is formed thinner toward the upstream side in the fuel gas flow direction in the fuel gas flow path in the cell, and the gas diffusion layer 217 is formed thicker toward the upstream side in the fuel gas flow direction in the fuel gas flow path in the cell. Thus, the thickness of the entire single cell 210 is made uniform. The gas diffusion layers 216 and 217 have a shape in which the thickness gradually changes steplessly. Even in such a configuration, as in the first and second embodiments, water movement from the cathode side to the anode side is promoted on the upstream side in the fuel gas flow direction in the electrolyte membrane 13, and membrane drying is suppressed. Can do.

  As described above, the water repellent layer is provided in order to suppress film drying by pushing water back to the electrolyte membrane 13 side or to prevent liquid water from staying in the vicinity of the MEA 12. It is also possible to adopt a configuration in which no is provided. For example, in the fuel cell of the modification shown in FIG. 9, at least one of the water repellent layer 518 and the water repellent layer 19 may not be provided.

D3. Modification 3:
Only the thickness of the gas diffusion layer on the cathode side may be changed in the plane without changing the thickness of the gas diffusion layer on the anode side. In this case, the gas diffusion layer on the cathode side may be formed thicker toward the upstream side of the fuel gas flow in the fuel gas flow path in the cell. Even with this configuration, the thermal resistance between the electrolyte membrane and the in-cell fuel gas flow path is relatively small compared to the thermal resistance between the electrolyte membrane and the in-cell oxidizing gas flow path. The same effect as the example can be obtained. Even in such a case, the thickness of the entire single cell can be made uniform by adjusting the thickness of the water-repellent layer that has less influence on heat transfer.

  At least one of the anode side gas diffusion layer and the cathode side gas diffusion layer is changed in thickness in accordance with the flow direction of the fuel gas, and between the electrolyte membrane and the in-cell fuel gas flow path toward the upstream side in the fuel gas flow direction. If the thermal resistance is made smaller than the thermal resistance between the electrolyte membrane and the in-cell oxidizing gas flow path, the same effect can be obtained.

D4. Modification 4:
If the thickness of at least one gas diffusion layer is changed in accordance with the flow direction of the fuel gas in the in-cell fuel gas flow path, various modifications can be made to the oxidant gas flow in the in-cell oxidant gas flow path. In the embodiment, the flow direction of the fuel gas in the in-cell fuel gas flow path and the flow direction of the oxidant gas in the in-cell oxidation gas flow path are orthogonal to each other, but may flow in parallel, for example. Further, the in-cell gas flow path may be formed by a space formed between the concavo-convex shape provided in the separator and the MEA as in the embodiment, or the pores in the porous body as shown in FIG. You may form by the space which forms.

2 is a schematic cross-sectional view showing a schematic configuration of a single cell 10. FIG. 2 is an exploded perspective view showing a schematic configuration of a single cell 10. FIG. 2 is a perspective view illustrating an outline of a configuration of a gas diffusion layer 16. FIG. It is the cross-sectional schematic diagram of the single cell used in order to investigate battery performance. It is explanatory drawing which shows an example of the result of having investigated the output voltage by varying operating temperature. It is explanatory drawing showing the result of having measured the heat transfer efficiency of the gas diffusion layer. It is explanatory drawing showing the result of having measured the heat transfer efficiency of the water repellent layer. 2 is a schematic cross-sectional view showing a configuration of a single cell 110. FIG. 3 is a schematic cross-sectional view showing a configuration of a single cell 210. FIG.

Explanation of symbols

10, 110, 210, 510, 610 ... single cell 12 ... MEA
DESCRIPTION OF SYMBOLS 13 ... Electrolyte membrane 14 ... Anode 15 ... Cathode 16, 17, 117, 216, 217, 616 ... Gas diffusion layer 18, 19, 119, 518 ... Water-repellent layer 18 ... Gas diffusion layer 20, 21, 520, 521 ... Separator 22, 23 ... Groove 30-33 ... Hole 40, 41 ... Side 522, 523 ... Flow path forming part

Claims (5)

A polymer electrolyte fuel cell,
An electrolyte membrane;
An anode and a cathode, which are electrodes formed on each surface of the electrolyte membrane;
A gas diffusion layer disposed on each of the electrodes and formed by a conductive porous member;
Between each gas diffusion layer and the gas diffusion layer disposed on the anode side, a fuel gas flow path in which a fuel gas containing hydrogen flows from the upstream side to the downstream side is formed. A gas separator that forms an oxidizing gas passage through which an oxidizing gas containing oxygen flows between the gas diffusion layer disposed on the cathode side;
With
At least one of the gas diffusion layer disposed on the anode side and the gas diffusion layer disposed on the cathode side includes the electrolyte membrane in a region near the entrance where the fuel gas flows into the fuel gas flow path. The thickness of the fuel cell is changed so that a thermal resistance between the fuel gas channel and the fuel gas channel is smaller than a thermal resistance between the electrolyte membrane and the oxidizing gas channel. The fuel cell according to claim 1, wherein
The fuel cell, wherein the gas diffusion layer disposed on the anode side is formed thinner on the upstream side than on the downstream side in the flow direction of the fuel gas. The fuel cell according to claim 2, further comprising:
A first water-repellent layer comprising conductive particles and a water-repellent substance is provided between the anode and the gas diffusion layer disposed on the anode side,
The first water-repellent layer is formed thicker on the upstream side than on the downstream side in the flow direction of the fuel gas. A fuel cell according to any one of claims 1 to 3,
The gas diffusion layer disposed on the cathode side is formed thicker on the upstream side than on the downstream side in the flow direction of the fuel gas. The fuel cell according to claim 4, further comprising:
Between the cathode and the gas diffusion layer disposed on the cathode side, a second water repellent layer comprising conductive particles and a water repellent material is provided,
The second water-repellent layer is formed so that the upstream side is thinner than the downstream side in the flow direction of the fuel gas.

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