Membrane electrode assembly and fuel battery cell

Abstract

This membrane electrode assembly comprises: a proton conductor (10); and a catalyst layer (20) that is provided so as to sandwich the proton conductor and that is joined to the proton conductor. Provided to the proton conductor is a recessed-and-protruding section (12) where portions of the catalyst layer penetrate a catalyst bonding surface (11) to which the catalyst layer is bonded.　In the catalyst layer, a region where the proton transport resistance increases proportionally with separation from the catalyst bonding surface is defined as a high-resistance region (HA). Furthermore, in the catalyst layer, a region where the proton transport resistance is smaller than that of the high-resistance region and the degree of increase in the proton transport resistance accompanying separation from the catalyst bonding surface is smaller than that of the high-resistance region is defined as a low-resistance region (LA). As a result, the dimensions of at least the recessed-and-protruding section are set such that the proportion of the entire catalyst layer occupied by the low-resistance region is larger than when the catalyst bonding surface is a flat surface having no recesses or protrusions.

Description

Membrane electrode assemblies, fuel cells CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Japanese Patent Application No. 2024-144560, filed on August 26, 2024, the contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a membrane electrode assembly including a proton conductor and a catalyst layer, and a fuel cell including the membrane electrode assembly.

[0003] Conventionally, a membrane electrode assembly including a proton conductor made of a polymer electrolyte membrane having a plurality of fine protrusions on its surface has been known (see, for example, Patent Document 1). Patent Document 1 describes that providing a plurality of fine protrusions on the surface of the proton conductor increases the catalytic reaction area, thereby facilitating the transport of protons at the cathode.

[0004] Patent No. 4988963

[0005] The present inventors have investigated the relationship between the distance from the surface of a proton conductor to the metal catalyst contained in the catalyst layer and the proton transport resistance. This investigation revealed that in addition to a high-resistance region in which the proton transport resistance increases proportionally with increasing distance from the surface of the proton conductor, there is also a low-resistance region in which the rate of increase in proton transport resistance is smaller than that of the high-resistance region. The low-resistance region is a region in which the proton transport resistance is smaller than that of the high-resistance region.

[0006] In the above-mentioned Patent Document 1, although the surface of the proton conductor is provided with irregularities, the existence of the low-resistance region is not taken into consideration at all, and there is still room for improvement in reducing the proton transport resistance. Such a problem arises not only in fuel cells but also in membrane electrode assemblies used in other reaction devices such as water electrolysis devices.

[0007] An object of the present disclosure is to provide a membrane electrode assembly and a fuel cell that can reduce proton transport resistance and improve reaction efficiency in a catalyst.

[0008] According to one aspect of the present disclosure, a membrane electrode assembly includes a proton conductor and catalyst layers sandwiching the proton conductor and bonded to the proton conductor, the proton conductor having an uneven portion formed on the catalyst bonding surface where the catalyst layer is bonded, the high-resistance region being a region of the catalyst layer where proton transport resistance increases proportionally with increasing distance from the catalyst bonding surface, and the low-resistance region being a region where the proton transport resistance is lower than that of the high-resistance region and where the rate of increase in proton transport resistance with increasing distance from the catalyst bonding surface is lower than that of the high-resistance region, the catalyst layer has at least a dimension of the uneven portion set so that the proportion of the low-resistance region in the entire catalyst layer is larger than when the catalyst bonding surface is a flat surface without unevenness. This ensures a larger proportion of the low-resistance region in the catalyst layer than when the catalyst bonding surface is a flat surface without unevenness. Therefore, the membrane electrode assembly of the present invention can reduce proton transport resistance and improve catalytic reaction efficiency.

[0009] According to another aspect of the present disclosure, a membrane electrode assembly includes a proton conductor and catalyst layers sandwiching the proton conductor and bonded to the proton conductor, the proton conductor having an uneven portion formed on the catalyst bonding surface where the catalyst layer is bonded, into which a portion of the catalyst layer penetrates, and the dimensions of the uneven portion and the catalyst layer are set so that the size of the low resistance region in the catalyst layer is equal to or larger than the size of the high resistance region, where the high resistance region is a region in the catalyst layer where the proton transport resistance increases proportionally with increasing distance from the catalyst bonding surface, and the low resistance region is a region where the proton transport resistance is smaller than that of the high resistance region and where the rate of increase in proton transport resistance with increasing distance from the catalyst bonding surface is smaller than that of the high resistance region. This ensures a sufficient proportion of the low resistance region in the catalyst layer. Therefore, the membrane electrode assembly of the present invention can reduce proton transport resistance and improve reaction efficiency in the catalyst.

[0010] According to another aspect of the present disclosure, a membrane electrode assembly includes a proton conductor and catalyst layers sandwiching the proton conductor and bonded to the proton conductor, the proton conductor having an uneven portion formed on the catalyst bonding surface where the catalyst layer is bonded, the high-resistance region being a region of the catalyst layer where proton transport resistance increases proportionally with increasing distance from the catalyst bonding surface, and the low-resistance region being a region where the proton transport resistance is lower than that of the high-resistance region and where the rate of increase in proton transport resistance with increasing distance from the catalyst bonding surface is lower than that of the high-resistance region. Furthermore, when the stacking direction of the catalyst layer relative to the proton conductor is the thickness direction of the catalyst layer, the dimensions of the uneven portion and the catalyst layer are set so that a first range in the thickness direction of the catalyst layer where the low-resistance region exists is equal to or larger than a second range in which the low-resistance region does not exist. This ensures a sufficient proportion of the catalyst layer occupied by the low-resistance region. Therefore, the membrane electrode assembly of the present invention can reduce proton transport resistance and improve catalytic reaction efficiency.

[0011] If the surface of a proton conductor is provided with irregularities without considering the thickness of the catalyst layer, a catalyst layer with a large thickness will have a higher proton transport resistance throughout the entire catalyst layer than a catalyst layer with a small thickness. In contrast, as in the present invention, if the dimensions of the irregularities and the catalyst layer are set so that the first range, where a low-resistance region exists, is equal to or larger than the second range, where no low-resistance region exists, the proton transport resistance throughout the entire catalyst layer can be reduced regardless of the thickness of the catalyst layer. This is particularly effective in improving the reaction efficiency of the catalyst.

[0012] According to another aspect of the present disclosure, a membrane electrode assembly includes a proton conductor and catalyst layers that are disposed on either side of the proton conductor and bonded to the proton conductor, the proton conductor having an uneven portion into which a portion of the catalyst layer enters the catalyst bonding surface to which the catalyst layer is bonded, the high-resistance region being a region of the catalyst layer in which the proton transport resistance increases proportionally with increasing distance from the catalyst bonding surface, and the low-resistance region being a region in which the proton transport resistance is smaller than that of the high-resistance region and in which the rate of increase in proton transport resistance with increasing distance from the catalyst bonding surface is smaller than that of the high-resistance region, and the dimensions of the uneven portion and the catalyst layer are set so that, when the stacking direction of the catalyst layer relative to the proton conductor is the thickness direction of the catalyst layer, the size of the low-resistance region in a low-resistance range in which the low-resistance region exists in the thickness direction of the catalyst layer is equal to or larger than the size of the high-resistance region in the low-resistance range.

[0013] This ensures that the proportion of the low-resistance region in the catalyst layer is sufficient, and therefore the membrane electrode assembly of the present invention can reduce proton transport resistance and improve the reaction efficiency in the catalyst.

[0014] According to another aspect of the present disclosure, a fuel cell that outputs electric energy through an electrochemical reaction between an oxidant gas and a reducing gas, which are reactant gases, comprises: the membrane electrode assembly according to claims 1, 2, 3, and 5; a pair of diffusion layers disposed on both sides of the membrane electrode assembly for diffusing the reactant gas; and a pair of separators disposed on both sides of the pair of diffusion layers, in which a gas flow path for the reactant gas is formed.

[0015] This allows the low resistance region in the catalyst layer to be expanded, thereby suppressing the proton transport resistance and improving the power generation efficiency of the fuel cell.

[0016] 1 is a schematic cross-sectional view of a fuel cell according to an embodiment; FIG. 2 is an enlarged view of a portion of a cross section of a membrane electrode assembly according to an embodiment; FIG. 3 is an explanatory view illustrating the shape of concave-convex portions on the surface of a proton conductor; FIG. 4 is an explanatory view illustrating characteristics of proton transport resistance in a catalyst layer; FIG. 5 is an explanatory view illustrating a low resistance region and a high resistance region in a catalyst layer of a membrane electrode assembly; FIG. 6 is an explanatory view illustrating a relationship between a low resistance region and a high resistance region in the thickness direction of a catalyst layer; FIG. 7 is an explanatory view illustrating a relationship between a low resistance region and a high resistance region in a first range of a catalyst layer; FIG. 8 is an explanatory view illustrating a method for manufacturing a membrane electrode assembly; FIG. 9 is an explanatory view illustrating a first modified example of the shape of concave-convex portions on the surface of a proton conductor; FIG. 10 is an explanatory view illustrating a second modified example of the shape of concave-convex portions on the surface of a proton conductor; FIG. 11 is an explanatory view illustrating a third modified example of the shape of concave-convex portions on the surface of a proton conductor; FIG. 12 is an explanatory view illustrating a fourth modified example of the shape of concave-convex portions on the surface of a proton conductor; and FIG. 13 is an explanatory view illustrating a fifth modified example of the shape of concave-convex portions on the surface of a proton conductor.

[0017] An embodiment of the present disclosure will be described below with reference to Figures 1 to 8. In this embodiment, an example in which a membrane electrode assembly MEA of the present disclosure is applied to a fuel cell FC will be described.

[0018] The fuel cell FC is constructed of a polymer electrolyte fuel cell. The fuel cell FC outputs electrical energy, for example, through the electrochemical reaction of hydrogen and oxygen shown in the following reaction formulas F1 and F2. In the fuel cell FC, hydrogen becomes a reducing gas and oxygen becomes an oxidizing gas. Anode: H ２ →2H ＋ +2e － ... (F1) Cathode: 2H ＋ +1 / 2O ２ +2e － →H ２ 1, the fuel cell FC outputs electric energy to the power converter PCE via an external circuit. The fuel cell FC includes a membrane electrode assembly MEA, a gas diffusion layer GDL, and a separator SP.

[0019] The membrane electrode assembly MEA is formed by joining catalyst layers 20, which constitute a part of the electrodes, to both sides of a proton conductor 10. Note that "MEA" is an abbreviation for Membrane Electrode Assembly.

[0020] The proton conductor 10 conducts protons between the anode and the cathode. The proton conductor 10 is composed of a three-dimensional structure formed by an electrolyte membrane and an ionomer. Examples of the electrolyte membrane include a polymer membrane having at least one of sulfonic acid groups, carboxylic acid groups, phosphoric acid groups, phosphonic acid groups, and dipolar ionic functional groups in the polymer chain. Examples of the electrolyte membrane include a polymer membrane containing an acidic substance such as sulfuric acid, sulfonic acids, phosphoric acids, carboxylic acids, phosphonic acid groups, or solid acid particles. Examples of the three-dimensional structure formed by an ionomer include Nafion (registered trademark), a highly oxygen-permeable ionomer, PFAS, polybenzimidazole (PBI), and a covalent organic framework (COF).

[0021] The catalyst layer 20 has an anode catalyst layer AN that constitutes the anode and a cathode catalyst layer CA that constitutes the cathode. The anode catalyst layer AN and the cathode catalyst layer CA have the same configuration. Therefore, hereinafter, the anode catalyst layer AN and the cathode catalyst layer CA may be simply referred to as the catalyst layer 20 without distinguishing between them.

[0022] The catalyst layer 20 promotes the electrochemical reaction between hydrogen and oxygen. The catalyst layer 20 can be obtained, for example, by drying a catalyst ink to evaporate the solvent. The catalyst ink is composed of a mixture of a carbon-based powder support, such as Ketjen black or mesoporous carbon, carrying a noble metal or noble metal alloy (e.g., Pt, Ru, Co, Au, Ag, Rh, or Ir), and an ionomer solution. The catalyst ink may also use a non-noble metal catalyst, such as a metal-free carbon-based catalyst, instead of a noble metal. The ionomer solution may contain, for example, at least one of sulfonic acid, carboxylic acid, phosphoric acid, phosphonic acid, or zwitterionic functional group in the polymer chain, or an acidic substance such as sulfuric acid, sulfonic acids, phosphoric acids, carboxylic acids, phosphonic acids, or fine particles of a solid acid. Specifically, the ionomer solution may be one composed of Nafion (registered trademark), a highly oxygen-permeable ionomer, PFAS, polybenzimidazole (so-called PBI), a covalent organic framework (so-called COF), or the like.

[0023] In the membrane electrode assembly MEA configured as described above, a catalyst bonding surface 11 of the proton conductor 10 to which the catalyst layer 20 is bonded is provided with an uneven portion 12 into which a part of the catalyst layer 20 is embedded. The uneven portion 12 is formed by protrusions, grooves, a combination of protrusions and grooves, or the like. The uneven portion 12 of this embodiment is provided on the catalyst bonding surface 11 with regularity so that the intervals between the protrusions that make up the uneven portion 12 are uniform. Details of the uneven portion 12 will be described later.

[0024] The gas diffusion layer GDL, together with the catalyst layer 20, constitutes an electrode substrate. The gas diffusion layer GDL has a pair of diffusion layers DL1, DL2 arranged on both sides of the membrane electrode assembly MEA to diffuse the reactant gas. One of the pair of diffusion layers DL1, DL2 is arranged adjacent to the anode catalyst layer AN, and the other is arranged adjacent to the cathode catalyst layer CA. Each of the diffusion layers DL1, DL2 is made of, for example, a carbon cloth having electrical conductivity and liquid retention properties. "GDL" is an abbreviation for Gas Diffusion Layer.

[0025] The separators SP are disposed on the outer sides of the diffusion layers DL1 and DL2. Gas flow channels through which reactant gases flow are formed in the separators SP at locations facing the diffusion layers DL1 and DL2. The separators SP are electrically connected to the membrane electrode assembly MEA via the gas diffusion layers GDL.

[0026] When fuel gas and oxidant gas are supplied to the fuel cell FC configured as described above, the fuel cell FC generates electricity through the electrochemical reactions shown in the above-mentioned reaction formulas F1 and F2. In order to improve the power generation efficiency of the fuel cell FC, the performance of the catalyst and the reaction area of ​​the catalyst are important factors.

[0027] In response to this, in a fuel cell FC, it is effective to provide a catalyst bonding surface 11 of a proton conductor 10 to which a catalyst layer 20 is bonded with an uneven portion 12 into which a part of the catalyst layer 20 enters, thereby expanding the catalytic reaction area, as shown in Figures 2 and 3. In order to further improve the performance of the catalyst, the inventors examined the magnitude of the proton transport resistance on the side of the catalyst layer 20 closer to the proton conductor 10 and on the side farther from the proton conductor 10. Initially, the inventors assumed that the proton transport resistance increases proportionally with increasing distance from the surface of the proton conductor 10 throughout the catalyst layer 20. However, as a result of this examination, it became clear that the rate of increase in proton transport resistance with increasing distance from the surface of the proton conductor 10 becomes smaller on the side of the catalyst layer 20 closer to the proton conductor 10.

[0028] 4 shows an example of the relationship between the volume ratio of the surface of the proton conductor 10 to the entire catalyst layer 20 and the proton transport resistance. In FIG. 4, the weight ratio of the ionomer to the carbon (i.e., I / C) is 0.6, and the weight of platinum, which is the catalyst metal, is 0.2 mg / cm. ２ 1 shows the relationship between the volume ratio and the proton transport resistance when using a catalyst layer 20 in which the volume ratio is 0. This volume ratio is minimum at the position in contact with the proton conductor 10 and increases with increasing distance from the surface of the proton conductor 10. The volume ratio is maximum at the position in the catalyst layer 20 farthest from the surface of the proton conductor 10, which is 1.

[0029] 4 , the catalyst layer 20 has a region where the proton transport resistance increases proportionally with an increase in the volume ratio, as well as a region where the rate at which the proton transport resistance increases relative to the increase in the volume ratio is smaller than that of the region. This remains the same even if the constituent material of the catalyst layer 20 is changed. Hereinafter, the region of the catalyst layer 20 where the proton transport resistance increases proportionally with increasing distance from the catalyst bonding surface 11 will be referred to as the high-resistance region HA. Furthermore, the region of the catalyst layer 20 where the proton transport resistance is smaller than that of the high-resistance region HA and where the rate at which the proton transport resistance increases with increasing distance from the catalyst bonding surface 11 is smaller than that of the high-resistance region HA will be referred to as the low-resistance region LA.

[0030] Here, in this specification, "proportional" does not only mean that the rate at which the proton transport resistance increases with increasing volume ratio is strictly constant, but also means that the rate is approximately constant. "Approximately constant" can be interpreted as, for example, when the relationship between the volume ratio and the proton transport resistance is determined by regression, the difference between the predicted value corresponding to the regression line and the actual measured value is within α% (for example, within 5%) of the predicted value.

[0031] Specifically, in the example shown in FIG. 4 (I / C: 0.6, basis weight: 0.2), in the region where the volume ratio exceeds approximately 0.6, the proton transport resistance increases in proportion to the increase in the volume ratio. Therefore, in the example shown in FIG. 4, the region of the catalyst layer 20 where the volume ratio exceeds approximately 0.6 is the high-resistance region HA. Also, in the example shown in FIG. 4, in the region where the volume ratio is approximately 0.6 or less, the rate at which the proton transport resistance increases with the increase in the volume ratio is smaller than in the high-resistance region. Therefore, in the example shown in FIG. 4, the region of the catalyst layer 20 where the volume ratio is approximately 0.6 or less is the low-resistance region LA. Note that FIG. 4 is merely an example, and the high-resistance region HA and the low-resistance region LA are not limited to those shown in FIG. 4.

[0032] In the membrane electrode assembly MEA, an increase in the area occupied by the low resistance region LA in the catalyst layer 20 means a decrease in the proton transport resistance in the catalyst layer 20. Therefore, by increasing the area occupied by the low resistance region LA in the catalyst layer 20, it is possible to reduce the proton transport resistance in the catalyst layer 20. The area occupied by the low resistance region LA in the catalyst layer 20 changes depending on the dimensions of the concave-convex portion 12 in the proton conductor 10 and the dimensions of the catalyst layer 20. Therefore, the area occupied by the low resistance region LA in the catalyst layer 20 can be adjusted by adjusting the dimensions of the concave-convex portion 12 and the catalyst layer 20.

[0033] Based on the above findings, in the membrane electrode assembly MEA, the dimensions of at least the uneven portion 12 in the proton conductor 10 are set so that the area occupied by the low-resistance region LA in the catalyst layer 20 is larger than when the catalyst bonding surface 11 is a flat surface without any unevenness.

[0034] As shown in Figures 4 and 5, the dimensions of the concave-convex portion 12 and the dimensions of the catalyst layer in the membrane electrode assembly MEA are set so that the size of the low-resistance region LA in the catalyst layer 20 is equal to or larger than the size of the high-resistance region HA. Specifically, as shown in Figure 6, the dimensions of the concave-convex portion 12 and the dimensions of the catalyst layer 20 in the membrane electrode assembly MEA are set so that a first range in the thickness direction of the catalyst layer 20, in which the low-resistance region LA exists, is equal to or larger than a second range in which the low-resistance region LA does not exist. Note that the first range corresponds to the "low-resistance range." The thickness direction of the catalyst layer 20 refers to the stacking direction of the catalyst layer 20 relative to the proton conductor 10. The thickness direction of the catalyst layer 20 is also perpendicular to the surface direction of the catalyst bonding surface 11.

[0035] The first range is the range obtained by adding the height dimension of the convexities that make up the uneven portion 12 to the thickness of the region assumed to be the low-resistance region LA. The second range is the range obtained by excluding the thickness of the region assumed to be the low-resistance region LA from the thickness dimension of the catalyst layer 20. The thickness of the region assumed to be the low-resistance region LA is correlated with the constituent material of the catalyst layer 20 and the height and width dimensions of the uneven portion 12, and can be estimated based on the constituent material of the catalyst layer 20 and the dimensions of the uneven portion 12.

[0036] In light of these, in this embodiment, the thickness of the region assumed to be the low-resistance region LA is estimated, and based on the estimation result, the dimensions of the concave-convex portion 12 in the proton conductor 10 and the dimensions of the catalyst layer 20 are set so that the first range is equal to or greater than the second range. This makes it possible to obtain a membrane electrode assembly MEA in which the first range is equal to or greater than the second range.

[0037] In addition, in the membrane electrode assembly MEA of this embodiment, as shown in Figures 5 and 7, the dimensions of the uneven portion 12 and the dimensions of the catalyst layer 20 are set so that the size of the low resistance region LA in the first range is equal to or greater than the size of the high resistance region HA in the first range.

[0038] Specifically, after specifying the volume of the first range in the catalyst layer 20, the volume of the low-resistance region present near the catalyst bonding surface 11 of the proton conductor 10 is estimated. Then, the dimensions of the uneven portion 12 and the dimensions of the catalyst layer 20 are set so that the volume of the estimated low-resistance region is equal to or greater than the volume of the first range minus the volume of the low-resistance region. Note that the volume of the region assumed to be the low-resistance region LA is correlated with the constituent material of the catalyst layer 20 and the height and width dimensions of the uneven portion 12, and can be estimated based on the constituent material of the catalyst layer 20 and the dimensions of the uneven portion 12.

[0039] Here, the specific shape of the uneven portion 12 will be described with reference to Fig. 3. As shown in Fig. 3, the convexities that make up the uneven portion 12 include an upper end portion 121 that forms the apex, a pair of side wall portions 122 and 123 that are continuous with the upper end portion 121 and that are inclined so as to approach each other toward the upper end portion 121, and a bottom portion 124 that is continuous with each of the side wall portions 122 and 123.

[0040] The width WU of the upper end 121 of each of the convexes constituting the uneven portion 12 is smaller than the width WL of the base of the convexes. The pair of sidewalls 122, 123 are inclined so that the inclination angle θα relative to the upper end 121 is an obtuse angle (e.g., 110°). The distance between the upper end 121 of adjacent convexes is greater than the width of the bottom 124. In the uneven portion 12 of this embodiment, the width WU of the upper end 121 is set to approximately 3 μm, and the width WL of the base of the uneven portion 12 is set to approximately 8 μm. Furthermore, in the uneven portion 12 of this embodiment, the width of the bottom 124 is set to approximately 5 μm. This allows the catalyst ink forming the catalyst layer 20 to easily penetrate between adjacent convexes, thereby preventing cavities from forming in the catalyst layer 20 at the portion that penetrates the uneven portion 12.

[0041] Furthermore, from the viewpoint of reducing the proton transport resistance, it is effective to increase the height dimension HL of the concave-convex portion 12. However, if the height dimension HL of the concave-convex portion 12 is made too large, there is a risk that the convex portions of the concave-convex portion 12 will collapse when the catalyst ink or the like that forms the catalyst layer 20 is placed on the surface of the proton conductor 10. If the convex portions of the concave-convex portion 12 collapse, the catalyst bonding surface 11 will decrease, thereby preventing a reduction in the proton transport resistance.

[0042] In contrast, according to the study by the present inventors, it has been found that it is desirable to limit the ratio of the width dimension WL of the base portion of the concave-convex portion 12 to the height dimension HL of the concave-convex portion 12 (=HL / WL) to a range of 1.8 or less. Furthermore, according to further study by the present inventors, it has been found that "0.3≦HL / WL" is preferable from the viewpoint of reducing proton transport resistance.

[0043] In consideration of these, the uneven portion 12 is set so that the ratio of the height dimension HL of the uneven portion 12 to the width dimension WL of the base portion of the uneven portion 12 is in the range of 0.3 or more and 1.8 or less. In the uneven portion 12 of this embodiment, the height dimension HL of the uneven portion 12 is set to about 5 μm so that the ratio of the height dimension HL of the uneven portion 12 to the width dimension WL of the base portion of the uneven portion 12 is in the range of 0.3 or more and 1.8 or less.

[0044] Next, an example of a method for manufacturing a membrane electrode assembly MEA will be described with reference to Fig. 8. As shown in Fig. 8, the membrane electrode assembly MEA of this embodiment is manufactured through a preparation step S10, a catalyst ink preparation step S20, a proton conductor 10 formation step S30, a catalyst ink application step S40, a hot pressing step S50, etc.

[0045] The preparation step S10 is a step of preparing various materials and devices required for manufacturing the membrane electrode assembly MEA, such as materials for forming the proton conductor 10 and constituent materials of the catalyst ink for forming the catalyst layer 20.

[0046] The catalyst ink preparation step S20 is a step of preparing catalyst ink for forming the catalyst layer 20. The catalyst ink can be prepared, for example, by mixing Nafion (registered trademark), a carbon-based powder carrier carrying precious metals including platinum, into an ionomer solution, and stirring and dispersing the mixture.

[0047] The step S30 of forming the proton conductor 10 is a step of forming the proton conductor 10 having the uneven portion 12 on its surface. The proton conductor 10 can be formed, for example, by placing a material such as an electrolyte membrane that forms the proton conductor 10 in a mold having an uneven structure corresponding to the uneven portion 12, applying heat and pressure, and then cooling and releasing the mold. The uneven portion 12 may be formed by removal processing using cutting, a laser, or an electron beam, plastic processing in which a mold is pressed against the electrolyte membrane or a polymer membrane material is poured into a mold, or lamination processing using spraying, inkjet, or coating.

[0048] The catalyst ink application step S40 is a step of applying the catalyst ink to the surface of the proton conductor 10. The application of the catalyst ink can be achieved by, for example, die coating, inkjet printing, spraying, or the like. Die coating is a method of coating the surface of the proton conductor 10 while extruding the liquid catalyst ink from a slit die, as shown in (A) of the balloon in FIG. 8 . Inkjet printing is a method of spraying the liquid catalyst ink from a minute nozzle onto the surface of the proton conductor 10, as shown in (B) of the balloon in FIG. 8 . Spraying is a method of spraying the liquid catalyst ink as mist, foam, or the like onto the surface of the proton conductor 10 using mechanical motion such as electrostatic force, high-pressure air, or ultrasound.

[0049] The hot pressing step S50 is a step in which the surface of the proton conductor 10 onto which the catalyst ink has been applied is heated, compressed, and dried in a hot press. Through these steps, a membrane electrode assembly MEA is obtained in which part of the catalyst layer 20 has entered the uneven portion 12 of the proton conductor 10.

[0050] The membrane electrode assembly MEA described above includes a proton conductor 10 and a catalyst layer 20 that is disposed across the proton conductor 10 and bonded to the proton conductor 10. The proton conductor 10 has an uneven portion 12 that allows a portion of the catalyst layer 20 to penetrate into a catalyst bonding surface 11 to which the catalyst layer 20 is bonded. The dimensions of the uneven portion 12 are set so that the proportion of the low-resistance region LA in the entire catalyst layer 20 is larger than when the catalyst bonding surface 11 is a flat surface without any unevenness. This ensures a larger proportion of the low-resistance region in the catalyst layer 20 than when the catalyst bonding surface 11 is a flat surface without any unevenness. Therefore, the membrane electrode assembly MEA of this embodiment can reduce proton transport resistance and improve the reaction efficiency of the catalyst.

[0051] The membrane electrode assembly MEA of this embodiment also has the following features. (1) In the membrane electrode assembly MEA, the dimensions of the concave-convex portion 12 and the dimensions of the catalyst layer 20 are set so that the size of the low-resistance region LA in the catalyst layer 20 is equal to or larger than the size of the high-resistance region HA. By ensuring a sufficient proportion of the low-resistance region LA in the catalyst layer 20 in this way, proton transport resistance can be reduced, improving the reaction efficiency of the catalyst. (2) In the membrane electrode assembly MEA, the dimensions of the concave-convex portion 12 and the dimensions of the catalyst layer 20 are set so that a first range in the thickness direction of the catalyst layer 20, where the low-resistance region LA exists, is equal to or larger than a second range in which the low-resistance region LA does not exist. This ensures a sufficient proportion of the catalyst layer 20 occupied by the low-resistance region LA. Therefore, proton transport resistance can be reduced, improving the reaction efficiency of the catalyst. Here, if the uneven portion 12 is provided on the surface of the proton conductor 10 without taking into consideration the thickness of the catalyst layer 20, the proton transport resistance of the entire catalyst layer 20 will be greater in a catalyst layer 20 with a larger thickness than in a catalyst layer 20 with a smaller thickness.

[0052] In contrast to this, in the present invention, the dimensions of the concave-convex portion 12 and the dimensions of the catalyst layer 20 are set so that the first range in which the low-resistance region LA exists is equal to or larger than the second range in which the low-resistance region LA does not exist. This makes it possible to reduce the proton transport resistance throughout the catalyst layer 20, regardless of whether the thickness of the catalyst layer 20 is increased or decreased. This is particularly effective in improving the reaction efficiency of the catalyst.

[0053] (3) The dimensions of the proton conductor 10 and the catalyst layer 20 are set so that the size of the low-resistance region LA in the first range is equal to or larger than the size of the high-resistance region HA in the first range. This allows the low-resistance region LA in the catalyst layer 20 to be sufficiently enlarged, thereby suppressing proton transport resistance and improving efficiency.

[0054] (4) The uneven portion 12 of this embodiment is provided on the catalyst bonding surface 11 with regularity so that the intervals between the projections constituting the uneven portion 12 are uniform. In this way, if the intervals between the projections constituting the uneven portion 12 are uniform, it is possible to prevent the low-resistance region LA from being concentrated in one area in the proton conductor 10, and therefore it is possible to expect that a uniform electrochemical reaction will occur throughout the entire proton conductor 10.

[0055] (5) The protrusions constituting the uneven portion 12 include an upper end portion 121 forming a peak, a pair of side wall portions 122, 123 that are continuous with the upper end portion 121 and slope toward each other toward the upper end portion 121, and a bottom portion 124 that is continuous with the side wall portions 122, 123. This configuration can increase the contact area between the proton conductor 10 and the catalyst layer 20. In addition, since the catalyst ink that forms the catalyst layer 20 can be easily introduced into the uneven portion 12 during the manufacture of the membrane electrode assembly MEA, the generation of cavities in the catalyst layer 20 at the portion where it has penetrated into the uneven portion 12 can be suppressed.

[0056] (6) The ratio of the height of the uneven portion 12 to the width of the base of the uneven portion 12 is set to a range of 0.3 or more and 1.8 or less.

[0057] In order to reduce the proton transport resistance, it is effective to increase the height dimension HL of the uneven portion 12. However, if the height dimension HL of the uneven portion 12 is made too large, there is a risk that the convex portions of the uneven portion 12 will collapse when the catalyst ink or the like that forms the catalyst layer 20 is placed on the surface of the proton conductor 10.

[0058] In contrast, if the ratio of the height dimension HL of the concave-convex portion 12 to the width dimension WL of the base portion of the concave-convex portion 12 is set in the range of 0.3 to 1.8, it is possible to reduce the proton transport resistance while preventing the convex portions of the concave-convex portion 12 from collapsing. That is, according to the present invention, it is possible to make the convex portions of the concave-convex portion 12 self-supporting and ensure a low-resistance region LA in the vicinity of the concave-convex portion 12.

[0059] (7) In this embodiment, the membrane electrode assembly MEA of the present disclosure is applied to a fuel cell FC that outputs electrical energy through an electrochemical reaction between an oxidant gas and a reducing gas, which are reactant gases. This makes it possible to expand the low resistance region LA in the catalyst layer 20 of the fuel cell FC, thereby reducing proton transport resistance and improving the power generation efficiency of the fuel cell FC.

[0060] (Modifications) In the above embodiment, the uneven portion 12 of the proton conductor 10 has been described in detail, but the shape, dimensions, etc. of the uneven portion 12 may be different from those described in the above embodiment. For example, the uneven portion 12 may be curved such that at least the portion of the side wall portions 122, 123 that is continuous with the upper end portion 121 becomes increasingly inclined toward the upper end portion 121 relative to the catalyst bonding surface 11. Specifically, the uneven portion 12 may be configured, for example, as in the following first to third modifications.

[0061] (First Modification) The uneven portion 12 may be curved toward the upper end portion 121 so that the inclination angle θα2 of the corner portion connecting to the upper end portion 121 of the side wall portions 122, 123 is larger than the inclination angle θα1 of the other portions, as shown in Figure 9, for example.

[0062] (Second Modification) Furthermore, as shown in FIG. 10 , the uneven portion 12 may be curved such that the portions of the side wall portions 122, 123 that connect to the upper end portion 121 are partially arc-shaped, and the inclination angle θα relative to the catalyst bonding surface 11 continuously increases toward the upper end portion 121.

[0063] (Third Modification) Furthermore, as shown in FIG. 11 , the uneven portion 12 may have a rounded shape at the peak of the convexity, and may be curved so that the angle of inclination relative to the catalyst bonding surface 11 increases continuously toward the upper end 121.

[0064] As in the first to third modified examples, if the sidewalls 122, 123 are curved so that their inclination relative to the catalyst bonding surface 11 increases toward the upper end 121, the catalyst ink will be more likely to spread to the bottom 124 of the uneven portion 12. As a result, it is possible to further prevent cavities from forming in the catalyst layer 20 at the portion that extends into the uneven portion 12.

[0065] 12, the uneven portion 12 may be curved toward the bottom 124 so that the inclination angle θβ of the corner portions of the side wall portions 122, 123 that are continuous with the bottom 124 is larger than the inclination angle θα of the other portions. Although not shown, the portions of the side wall portions 122, 123 that are continuous with the bottom 124 may be formed in an arc shape.

[0066] 13, the uneven portion 12 may be curved so that the inclination angle θα2 of the corner portions of the side wall portions 122, 123 that connect to the upper end portion 121 and the inclination angle θβ of the corner portions that connect to the bottom portion 124 are larger than the inclination angle θα2 of the other portions. Although not shown, the portions of the side wall portions 122, 123 that connect to the upper end portion 121 and the portions of the side wall portions 122, 123 that connect to the bottom portion 124 may be formed in an arc shape.

[0067] Other Embodiments Although typical embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments and can be modified in various ways, for example, as follows.

[0068] As in the above-described embodiment, in the membrane electrode assembly MEA, the dimensions of the concave-convex portion 12 and the dimensions of the catalyst layer 20 are desirably set so that the size of the low-resistance region LA in the catalyst layer 20 is equal to or larger than the size of the high-resistance region HA, but this is not a limitation. In the membrane electrode assembly MEA, the dimensions of the concave-convex portion 12 and the dimensions of the catalyst layer 20 may be set so that the size of the low-resistance region LA in the catalyst layer 20 is smaller than the size of the high-resistance region HA. Furthermore, in the catalyst layer 20, the dimensions of the concave-convex portion 12 and the dimensions of the catalyst layer 20 are desirably set so that a first range in the thickness direction of the catalyst layer 20 in which the low-resistance region LA exists is equal to or larger than a second range in which the low-resistance region LA does not exist, but this is not a limitation. As long as the size of the low-resistance region LA in the first range is equal to or larger than the size of the high-resistance region HA in the first range, in the catalyst layer 20, the first range in the thickness direction of the catalyst layer 20 in which the low-resistance region LA exists may be smaller than a second range in which the low-resistance region LA does not exist.

[0069] As in the above-described embodiment, it is desirable that the size of the low-resistance region LA in the first range be equal to or larger than the size of the high-resistance region HA in the first range in the catalyst layer 20. However, this is not limitative. As long as the first range in which the low-resistance region LA exists in the thickness direction of the catalyst layer 20 is equal to or larger than the second range in which the low-resistance region LA does not exist, the size of the low-resistance region LA in the first range may be smaller than the size of the high-resistance region HA in the first range.

[0070] As in the above-described embodiment, it is desirable that the uneven portion 12 be provided on the catalyst bonding surface 11 with regularity so that the intervals between the convex portions constituting the uneven portion 12 are uniform. However, this is not limitative, and for example, the uneven portion may be provided on the catalyst bonding surface 11 so that the intervals between the convex portions are partially different. In a fuel cell FC, the power generation performance, water drainage, gas diffusion properties, etc. may differ within the cell surface. In such cases, it is possible to adjust the power generation performance, water drainage, and gas diffusion properties within the cell surface by partially changing the intervals between the convex portions.

[0071] As in the above-described embodiment, the convexities constituting the uneven portion 12 are preferably inclined so that the pair of side wall portions 122, 123 approach each other toward the upper end portion 121, but are not limited to this. For example, the convexities constituting the uneven portion 12 may be such that the pair of side wall portions 122, 123 extend perpendicular to the bottom portion 124 or the like.

[0072] As in the above-described embodiment, it is desirable that the ratio of the height dimension of the uneven portion 12 to the width dimension of the base portion of the uneven portion 12 is set in a range of 0.3 or more and 1.8 or less, but this is not necessarily the case.

[0073] In the above-described embodiment, an example has been described in which the membrane electrode assembly MEA of the present disclosure is applied to a fuel cell FC, but the application of the membrane electrode assembly MEA is not limited to a fuel cell FC. The membrane electrode assembly MEA of the present disclosure can be applied, for example, to an electrolysis cell EC that outputs fuel by an electrochemical reaction between an oxidant gas and a reducing gas, which are reactant gases.

[0074] In the above-described embodiments, it goes without saying that the elements constituting the embodiments are not necessarily essential unless they are specifically stated as essential or are clearly considered essential in principle.

[0075] In the above-described embodiments, when numerical values ​​such as the number, value, amount, and range of components of the embodiments are mentioned, they are not limited to the specific numbers, except when they are specifically stated as essential or when they are clearly limited to a specific number in principle. In the above-described embodiments, specific dimensions of the uneven portion 12 are exemplified, but are not limited to the above-described dimensions. The inventors assume that the specific dimensions of the uneven portion 12 are a height dimension HL ranging from 3 μm to 13 μm, a width dimension WU of the upper end portion 121 ranging from 0.5 μm to 3 μm, and a width dimension WL of the base portion ranging from 3 μm to 15 μm. The dimensions of the uneven portion 12 are not limited to the above-described numerical ranges assumed by the inventors, and may be outside these numerical ranges.

[0076] In the above-described embodiments, when referring to the shapes, positional relationships, etc. of components, etc., the shapes, positional relationships, etc. are not limited to those unless otherwise specified or when they are limited in principle to specific shapes, positional relationships, etc.

[0077] [Aspects of the Present Disclosure] [First Aspect] A membrane electrode assembly comprising: a proton conductor (10); and catalyst layers (20) disposed on either side of the proton conductor and joined to the proton conductor, wherein the proton conductor has an uneven portion (12) into which a part of the catalyst layer enters a catalyst joining surface (11) to which the catalyst layer is joined, and wherein, when a region of the catalyst layer in which proton transport resistance increases proportionally with increasing distance from the catalyst joining surface is defined as a high resistance region (HA), and a region in which the proton transport resistance is smaller than that of the high resistance region and in which the rate of increase in the proton transport resistance with increasing distance from the catalyst joining surface is smaller than that of the high resistance region, the catalyst layer has a dimension set such that a proportion of the low resistance region in the entire catalyst layer is larger than when the catalyst joining surface is a flat surface without unevenness. [Second Aspect] A membrane electrode assembly comprising: a proton conductor (10); and catalyst layers (20) disposed on either side of the proton conductor and joined to the proton conductor, wherein the proton conductor has an uneven portion (12) into which a part of the catalyst layer enters a catalyst joining surface (11) to which the catalyst layer is joined, and wherein a region of the catalyst layer in which proton transport resistance increases proportionally with increasing distance from the catalyst joining surface is defined as a high resistance region (HA), and a region in which the proton transport resistance is smaller than that of the high resistance region and in which the rate of increase in the proton transport resistance with increasing distance from the catalyst joining surface is smaller than that of the high resistance region is defined as a low resistance region (LA), wherein dimensions of the uneven portion and dimensions of the catalyst layer are set so that a size of the low resistance region in the catalyst layer is equal to or larger than a size of the high resistance region.[Third Aspect] A membrane electrode assembly comprising: a proton conductor (10); and catalyst layers (20) disposed on either side of the proton conductor and joined to the proton conductor, wherein the proton conductor has an uneven portion (12) into which a part of the catalyst layer enters a catalyst joining surface (11) to which the catalyst layer is joined, wherein a region of the catalyst layer in which proton transport resistance increases proportionally with increasing distance from the catalyst joining surface is defined as a high resistance region (HA), and a region in which the proton transport resistance is smaller than that of the high resistance region and in which the rate of increase in the proton transport resistance with increasing distance from the catalyst joining surface is smaller than that of the high resistance region, and further wherein dimensions of the uneven portion and dimensions of the catalyst layer are set so that, when the stacking direction of the catalyst layer with respect to the proton conductor is defined as a thickness direction of the catalyst layer, a first range in the thickness direction of the catalyst layer in which the low resistance region exists is equal to or larger than a second range in which the low resistance region does not exist. [Fourth Aspect] The membrane electrode assembly according to the third aspect, wherein the dimensions of the proton conductor and the catalyst layer are set so that the size of the low resistance region in the first range is equal to or larger than the size of the high resistance region in the first range.[Fifth Aspect] A membrane electrode assembly comprising: a proton conductor (10); and catalyst layers (20) disposed on either side of the proton conductor and joined to the proton conductor, wherein the proton conductor has an uneven portion (12) through which a part of the catalyst layer enters a catalyst joining surface (11) to which the catalyst layer is joined, wherein a region of the catalyst layer in which proton transport resistance increases proportionally with increasing distance from the catalyst joining surface is defined as a high resistance region (HA), and a region in which the proton transport resistance is smaller than that of the high resistance region and in which the rate of increase in the proton transport resistance with increasing distance from the catalyst joining surface is smaller than that of the high resistance region, and further wherein dimensions of the uneven portion and dimensions of the catalyst layer are set so that, when the stacking direction of the catalyst layer with respect to the proton conductor is defined as a thickness direction of the catalyst layer, a size of the low resistance region in a low resistance range in which the low resistance region exists in the thickness direction of the catalyst layer is equal to or larger than a size of the high resistance region in the low resistance range. [Sixth Aspect] The membrane electrode assembly according to any one of the first to fifth aspects, wherein the uneven portion is provided on the catalyst bonding surface with regularity such that the spacing between the convexities constituting the uneven portion is at least partially uniform. [Seventh Aspect] The membrane electrode assembly according to any one of the first to sixth aspects, wherein the convexities constituting the uneven portion include an upper end (121) forming a peak, a pair of side wall portions (122, 123) continuing from the upper end and sloping toward each other toward the upper end, and a bottom portion (124) continuing from the side wall portions. [Eighth Aspect] The membrane electrode assembly according to the seventh aspect, wherein at least a portion of the side wall portion continuing from the upper end is curved so that the inclination with respect to the catalyst bonding surface increases toward the upper end. [Ninth Aspect] The membrane electrode assembly according to the seventh or eighth aspect, wherein at least a portion of the side wall portion continuing from the bottom is curved so that the inclination with respect to the catalyst bonding surface increases toward the bottom.[Tenth Aspect] A fuel cell that outputs electric energy by an electrochemical reaction between an oxidant gas and a reducing gas, which are reactant gases, comprising: a membrane electrode assembly (MEA) according to any one of the first to ninth aspects; a pair of diffusion layers (DL1, DL2) disposed on either side of the membrane electrode assembly and diffusing the reactant gases; and a pair of separators (SP) disposed on either side of the pair of diffusion layers and having gas flow paths for the reactant gases.

Claims

1. A membrane electrode assembly comprising: a proton conductor (10); and catalyst layers (20) disposed on either side of the proton conductor and joined to the proton conductor, wherein the proton conductor has an uneven portion (12) formed on a catalyst joining surface (11) to which the catalyst layer is joined, where a portion of the catalyst layer penetrates the catalyst joining surface; and wherein, when a region of the catalyst layer in which proton transport resistance increases proportionally with increasing distance from the catalyst joining surface is defined as a high resistance region (HA), and a region in which the proton transport resistance is smaller than that of the high resistance region and where the rate of increase in the proton transport resistance with increasing distance from the catalyst joining surface is smaller than that of the high resistance region is defined as a low resistance region (LA), at least the dimensions of the uneven portion are set so that the proportion of the low resistance region in the entire catalyst layer is larger than when the catalyst joining surface is a flat surface without any unevenness.

2. A membrane electrode assembly comprising: a proton conductor (10); and catalyst layers (20) disposed on either side of the proton conductor and joined to the proton conductor, wherein the proton conductor has an uneven portion (12) into which a portion of the catalyst layer penetrates a catalyst joining surface (11) to which the catalyst layer is joined, and wherein a region of the catalyst layer in which proton transport resistance increases proportionally with increasing distance from the catalyst joining surface is defined as a high resistance region (HA), and a region in which the proton transport resistance is smaller than that of the high resistance region and in which the rate of increase in the proton transport resistance with increasing distance from the catalyst joining surface is smaller than that of the high resistance region is defined as a low resistance region (LA), wherein the dimensions of the uneven portion and the dimensions of the catalyst layer are set so that the size of the low resistance region in the catalyst layer is equal to or larger than the size of the high resistance region.

3. A membrane electrode assembly comprising: a proton conductor (10); and catalyst layers (20) disposed on either side of the proton conductor and joined to the proton conductor, wherein the proton conductor has an uneven portion (12) into which a portion of the catalyst layer enters a catalyst joining surface (11) to which the catalyst layer is joined; a region of the catalyst layer in which proton transport resistance increases proportionally with increasing distance from the catalyst joining surface is defined as a high resistance region (HA), and a region in which the proton transport resistance is smaller than that of the high resistance region and in which the rate of increase in the proton transport resistance with increasing distance from the catalyst joining surface is smaller than that of the high resistance region is defined as a low resistance region (LA); and further, when the stacking direction of the catalyst layer relative to the proton conductor is defined as the thickness direction of the catalyst layer, dimensions of the uneven portion and dimensions of the catalyst layer are set so that a first range in the thickness direction of the catalyst layer in which the low resistance region exists is equal to or larger than a second range in which the low resistance region does not exist.

4. A membrane electrode assembly as described in claim 3, wherein the dimensions of the uneven portion and the dimensions of the catalyst layer are set so that the size of the low resistance region in the first range is equal to or greater than the size of the high resistance region in the first range.

5. A membrane electrode assembly comprising: a proton conductor (10); and catalyst layers (20) disposed on either side of the proton conductor and joined to the proton conductor; the proton conductor has an uneven portion (12) formed on a catalyst joining surface (11) to which the catalyst layer is joined, where a portion of the catalyst layer penetrates; a high-resistance region (HA) is a region of the catalyst layer in which proton transport resistance increases proportionally with increasing distance from the catalyst joining surface; a low-resistance region (LA) is a region in which the proton transport resistance is smaller than that of the high-resistance region and in which the rate of increase in the proton transport resistance with increasing distance from the catalyst joining surface is smaller than that of the high-resistance region; and the dimensions of the uneven portion and the catalyst layer are set so that, when the stacking direction of the catalyst layer relative to the proton conductor is the thickness direction of the catalyst layer, the size of the low-resistance region in a low-resistance range in which the low-resistance region exists in the thickness direction of the catalyst layer is equal to or larger than the size of the high-resistance region in the low-resistance range.

6. A membrane electrode assembly according to any one of claims 1, 2, 3 and 5, wherein the uneven portion is provided on the catalyst bonding surface with regularity so that the intervals between the projections constituting the uneven portion are at least partially uniform.

7. A membrane electrode assembly as described in any one of claims 1, 2, 3, and 5, wherein the convexities constituting the uneven portion include an upper end portion (121) forming a peak portion, a pair of side wall portions (122, 123) continuing from the upper end portion and sloping toward each other toward the upper end portion, and a bottom portion (124) continuing from the side wall portions.

8. The membrane electrode assembly according to claim 7, wherein at least a portion of said side wall portion connected to said upper end portion is curved so that the inclination with respect to said catalyst bonding surface increases toward said upper end portion.

9. The membrane electrode assembly according to claim 7, wherein at least the portion of the side wall that is continuous with the bottom is curved so that the inclination with respect to the catalyst bonding surface increases toward the bottom.

10. A fuel cell that outputs electrical energy through an electrochemical reaction between an oxidant gas and a reducing gas, which are reactant gases, comprising: a membrane electrode assembly (MEA) according to any one of claims 1, 2, 3, and 5; a pair of diffusion layers (DL1, DL2) disposed on either side of the membrane electrode assembly for diffusing the reactant gas; and a pair of separators (SP) disposed on either side of the pair of diffusion layers and having gas flow paths for the reactant gas.

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