Electrode catalyst layer, membrane electrode assembly and polymer electrolyte fuel cell

Abstract

Provided are an electrode catalyst layer, a membrane electrode assembly, and a solid polymer fuel cell with which it is possible to improve material transportability and proton conductivity in the electrode catalyst layer, and to exhibit high electricity generation performance for an extended period of time. An electrode catalyst layer (10) according to an embodiment of the present invention is employed in a solid polymer fuel cell (3), and includes, joined to a polyelectrolyte membrane (11), a catalyst material (12), an electrically conductive carrier (13) supporting the catalyst material (12), a polyelectrolyte (14), and one or more types of fibrous material (15) including at least polymer fibers, wherein, among the fibrous materials (15), the number of fibrous materials (15) of which an angle θ of the axis with respect to the joining surface between the polyelectrolyte membrane (11) and the electrode catalyst layer (10) is 0°≤θ<45° is greater than 50% of the overall number of contained fibrous materials (15).

Description

[Technical Field]

[0001] The present invention relates to an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell. [Background technology]

[0002] Fuel cells are power generation systems that generate electricity from the chemical reaction between hydrogen and oxygen. Compared to conventional power generation methods, fuel cells are characterized by high efficiency, low environmental impact, and low noise, and are attracting attention as a clean energy source of the future. In particular, solid polymer fuel cells, which can be used at around room temperature, are seen as promising for use in automotive power sources and stationary power sources for homes, and in recent years, various research and development efforts have been conducted on solid polymer fuel cells. Challenges for their practical application include improving cell performance such as power generation characteristics and durability, establishing infrastructure, and reducing manufacturing costs.

[0003] A polymer electrolyte fuel cell is generally constructed by stacking a large number of unit cells. A unit cell has a structure in which a membrane electrode assembly, in which a fuel electrode (anode) that supplies fuel gas and an oxygen electrode (cathode) that supplies oxidant are bonded to both sides of a polymer electrolyte membrane, is sandwiched between separators having gas flow paths and cooling water flow paths. The fuel electrode (anode) and oxygen electrode (cathode) are mainly composed of an electrode catalyst layer containing at least a catalytic material such as a platinum-based noble metal, a conductive support, and a polymer electrolyte, and a gas diffusion layer that is both gas permeable and electrically conductive.

[0004] In a polymer electrolyte fuel cell, electricity can be generated through the following electrochemical reaction. First, in the fuel electrode-side electrode catalyst layer, hydrogen contained in the fuel gas is oxidized by a catalytic substance to produce protons and electrons. The generated protons pass through the polymer electrolyte in the electrode catalyst layer and the polymer electrolyte membrane in contact with the electrode catalyst layer, and reach the oxygen electrode-side electrode catalyst layer. At the same time, the generated electrons pass through the conductive support in the fuel electrode-side electrode catalyst layer, the gas diffusion layer in contact with the fuel electrode-side electrode catalyst layer, the separator, and an external circuit, and reach the oxygen electrode-side electrode catalyst layer. Finally, in the oxygen electrode-side electrode catalyst layer, the protons and electrons react with oxygen contained in an oxidant gas, such as air, to produce water. In this series of reactions, the proton conduction resistance is greater than the electron conduction resistance. Therefore, efficient proton conduction is important for improving reactivity and fuel cell performance.

[0005] The gas diffusion layer diffuses the gas supplied from the separator and supplies it to the electrode catalyst layer. The pores in the electrode catalyst layer are located beyond the separator through the gas diffusion layer and act as pathways for transporting multiple substances. The pores in the fuel electrode are required to smoothly supply the hydrogen contained in the fuel gas to the three-phase interface, which is the oxidation-reduction reaction site. The pores in the oxygen electrode are also required to smoothly supply the oxygen contained in the oxidant gas. Furthermore, the pores in the oxygen electrode are also required to smoothly discharge the water produced by the reaction. Here, to smoothly supply the gas and smoothly discharge the produced water, it is important that the electrode catalyst layer has sufficient gaps to smoothly discharge the produced water, and that it does not have a dense structure.

[0006] As a means of controlling the structure of the electrode catalyst layer so that it does not become dense and improving power generation performance, for example, an electrode catalyst layer containing carbon or carbon fibers of different particle sizes has been proposed (see Patent Documents 1 and 2, etc.).

[0007] In Patent Document 1, carbon particles with moderately different particle sizes are combined to prevent dense packing of carbon particles in the electrode catalyst layer. Patent Document 2, meanwhile, uses carbon fibers with different fiber lengths, adjusting their ratio within a certain range to ensure that the appropriate pores occupy a large proportion of the electrode catalyst layer. On the other hand, mixing large particles with small particles can result in small particles filling the gaps between the large particles, resulting in dense packing. Furthermore, while using carbon fibers can prevent dense packing, the proportion of electron conductors in the catalyst layer increases and the proportion of proton conductors decreases, resulting in increased proton transfer resistance and reduced power generation performance. While these methods make it difficult to densely pack the pores in the electrode catalyst layer, they do not take into account the shape of the pores in the electrode catalyst layer. Because fuel cell power generation performance is significantly affected by mass transport, electronic conductivity, and proton conductivity, ultimately, methods that only increase electronic conductivity, such as using a combination of carbon particles or carbon fibers, have limited ability to improve power generation performance. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 3617237 [Patent Document 2] Patent No. 5537178 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention has been made in light of the above-mentioned points, and an object of the present invention is to provide an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell that are capable of improving the substance transport properties and proton conductivity in an electrode catalyst layer and thereby exhibiting high power generation performance over the long term. [Means for solving the problem]

[0010] As a means for solving the above-mentioned problems, an electrode catalyst layer according to one embodiment of the present invention is an electrode catalyst layer used in a polymer electrolyte fuel cell, which is bonded to a polymer electrolyte membrane and includes a catalyst material, a conductive support that supports the catalyst material, a polymer electrolyte, and one or more types of fibrous material including at least polymer fibers, and is characterized in that the number of fibrous materials whose axes have an inclination θ of 0°≦θ<45° with respect to the bonding surface between the polymer electrolyte membrane and the electrode catalyst layer is more than 50% of the total number of fibrous materials contained.

[0011] Furthermore, the electrode catalyst layer according to one embodiment of the present invention is characterized in that the number of fibrous materials having an axis inclination θ of 45°≦θ<90° with respect to the joining surface of the polymer electrolyte membrane and the electrode catalyst layer is more than 5% of the total number of fibrous materials contained. [Effects of the Invention]

[0012] The electrode catalyst layer of the present invention makes it possible to provide an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell that are capable of improving the substance transport properties and proton conductivity in the electrode catalyst layer and demonstrating high power generation performance over the long term. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 2 is a diagram illustrating voids formed in a fibrous material. [Figure 2] FIG. 2 is a schematic cross-sectional view showing an example of the configuration of an electrode catalyst layer according to the present embodiment. [Figure 3] FIG. 2 is an explanatory diagram of the inclination θ of the axis of a fibrous material. [Figure 4] 4A and 4B show a configuration example of a membrane electrode assembly according to the present embodiment, in which FIG. 4A is a plan view of the membrane electrode assembly as viewed from the oxygen electrode side of the electrode catalyst layer, and FIG. 4B is a cross-sectional view taken along line XX′ in FIG. 4A. [Figure 5] 1 is an exploded perspective view showing an example of the configuration of a polymer electrolyte fuel cell according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the embodiments described below, and modifications such as design changes can be made based on the knowledge of those skilled in the art. Such modified embodiments are also included in the scope of the present invention. In addition, the drawings are appropriately exaggerated to facilitate understanding.

[0015] The inventors of the present invention conducted extensive research into the initial and durable power generation performance of polymer electrolyte fuel cells and found that these performances are significantly affected by the gas diffusivity and proton conductivity in the electrode catalyst layer. They then found that by using a fibrous material containing at least polymer fibers to form wide pores in the in-plane direction perpendicular to the film thickness direction of the electrode catalyst layer, the proton conduction resistance is reduced and the gas diffusivity in the in-plane direction is improved, thereby suppressing output reduction and deterioration of the electrode catalyst layer and successfully obtaining a polymer electrolyte fuel cell that exhibits high power generation performance over the long term.

[0016] FIG. 1 is a diagram illustrating voids formed in a fibrous material. The overlapping of multiple fibrous materials 15 creates voids between the fibrous materials 15. Conventionally, the inclination of the fibrous materials 15 has not been considered. However, the inventors of the present invention conducted research and found that, when considering two adjacent fibrous materials 15 as shown in FIG. 1, if the inclination of one axis of the two fibrous materials 15 is less than 45 degrees, especially when both axes are less than 45 degrees, voids are more likely to form between the fibrous materials 15 in an in-plane direction perpendicular to the film thickness direction, contributing to improved gas diffusion and uniform in-plane power generation. Furthermore, they found that if the number of fibrous materials 15 whose axes are inclined at an angle greater than 45 degrees but less than 90 degrees is greater than 5% of the total number of fibrous materials contained, favorable voids are formed in the in-plane direction perpendicular to the film thickness direction, and proton transfer resistance in the film thickness direction is reduced.

[0017] [Configuration of electrode catalyst layer] The specific configuration of the electrode catalyst layer according to this embodiment will be described below with reference to Figs. 2 and 3. Fig. 2 is a schematic cross-sectional view showing an example of the configuration of the electrode catalyst layer according to this embodiment. Fig. 3 is an explanatory diagram of the tilt θ of the axis of the fibrous material. As shown in the schematic diagram of Fig. 2, the electrode catalyst layer 10 according to this embodiment is bonded to the surface of a polymer electrolyte membrane 11, and is composed of a catalyst material 12, a conductive support 13, a polymer electrolyte 14, and a fibrous material 15. The portions where none of the above components are present are voids.

[0018] The electrode catalyst layer 10 according to this embodiment is configured so that the number of fibrous materials 15 having an axial inclination θ of 0°≦θ<45° relative to the joining surface that joins with the surface of the polymer electrolyte membrane 11 is greater than 50% of the total number of fibrous materials contained in the electrode catalyst layer 10. Preferably, the number of fibrous materials 15 having an axial inclination θ of 0°≦θ<30° is greater than 50% of the total number of fibrous materials contained, and more preferably, the number of fibrous materials 15 having an axial inclination θ of 0°≦θ<20° is greater than 50% of the total number of fibrous materials contained. If the number of fibrous materials 15 having an axial inclination θ of 0°≦θ<45° is less than 50% of the total number of fibrous materials contained in the electrode catalyst layer 10, the voids extending in the in-plane direction perpendicular to the membrane thickness direction will be narrow, and sufficient drainage and gas diffusion properties may not be ensured.

[0019] Furthermore, the number of fibrous materials 15 whose axial inclination θ is 45°≦θ<90° is preferably greater than 5% of the total number of fibrous materials contained, and more preferably 15% or less of the total number of fibrous materials contained in the electrode catalyst layer 10. If the number of fibrous materials 15 whose axial inclination θ is 45°≦θ<90° is less than 5% of the total number of fibrous materials contained, the proton transfer resistance in the membrane thickness direction may not be sufficiently suppressed. In this case, it may not be possible to contribute to improving gas diffusibility or uniform power generation within the plane.

[0020] The fibrous material 15 for determining the tilt θ of this axis is, for example, one in which the fiber length exposed in the cross section of the electrode catalyst layer 10 is 1 μm or more and the fiber diameter is 100 nm or more when the cross section is observed using a scanning electron microscope (SEM). This makes it easier for voids with sufficient drainage and gas diffusion properties to be formed in the electrode catalyst layer 10. Furthermore, the bonding surface between the polymer electrolyte membrane 11 and the electrode catalyst layer 10, which serves as the basis for determining the tilt θ of the axis of the fibrous material 15, can be seen as the boundary line between the polymer electrolyte membrane 11 and the electrode catalyst layer 10 when the cross section of the electrode catalyst layer 10 is observed using a scanning electron microscope (SEM). If the boundary line has large irregularities, the line that approximates the most average straight line serves as the basis.

[0021] 4 and 5, the reference plane, which is the bonding surface between the polymer electrolyte membrane 11 and the electrode catalyst layer 10, is the surface of the polymer electrolyte membrane 11 in contact with the electrode catalyst layer 10, in other words, a plane parallel to the surfaces 10Aa and 10Ca of the electrode catalyst layer 10 that are bonded to the surface of the polymer electrolyte membrane 11. Furthermore, if the thickness of the electrode catalyst layer 10 is uniform, the reference plane is a plane parallel to the surface of the electrode catalyst layer 10 that is in contact with the gas diffusion layer 17. Therefore, the tilt θ of the axis of the fibrous material 15 can be found using any one of the surface of the polymer electrolyte membrane 11, the surfaces 10Aa and 10Ca of the electrode catalyst layer 10 that are bonded to the surface of the polymer electrolyte membrane 11, or the surface of the electrode catalyst layer 10 that is in contact with the gas diffusion layer 17 as the reference plane.

[0022] Here, the inclination θ of the axis of the fibrous material 15 with respect to the reference plane will be described. FIG. 3 is an explanatory diagram schematically illustrating the inclination θ of the axis of the fibrous material 15 with respect to the surface of the polymer electrolyte membrane 11. The dashed line in FIG. 3 indicates the axis of the fibrous material 15, and the angle formed between the axis of the fibrous material 15 and the surface of the polymer electrolyte membrane 11 is the inclination θ of the axis of the fibrous material 15 with respect to the reference plane. As shown in FIGS. 3(a) and 3(b), regardless of the direction of inclination of the fibrous material 15, the inclination θ is an acute angle and is within the range of 0°≦θ≦90°. Note that the fibrous materials 15 are not oriented in the same direction. In other words, the inclination θ of the axes of the fibrous materials 15 contained simultaneously in the electrode catalyst layer is not all the same, but can take on various values ​​within the range of 0°≦θ≦90°.

[0023] 3(a) and 3(b) illustrate an example in which the fibrous material 15 is linear and unbranched, but in reality, the axis of the fibrous material 15 may be bent in multiple steps, branched, or curved. When the axis is bent in multiple steps or branched, the same effect can be achieved by making the longest linear extension satisfy the above range, as shown in FIG. 3(c). When the axis is curved, the surface with which the fibrous material 15 is most in contact is taken as the standard plane, and the inclination θ between the reference plane and the standard plane of the fibrous material 15 satisfies the above range, thereby achieving the same effect.

[0024] The tilt θ of the axis of the fibrous material 15 with respect to a reference plane can be measured, for example, by observing a cross section of a membrane electrode assembly 1 in which an electrode catalyst layer 10 is bonded to the surface of a polymer electrolyte membrane 11 using a scanning electron microscope (SEM). For example, first, the orientation of the sample is adjusted so that the bonding surface between the polymer electrolyte membrane 11 and the electrode catalyst layer 10 is horizontal within a field of view at a magnification of 1000 times. Next, the tilt of the fibrous material with a fiber length of 1 μm or more and a fiber diameter of 100 nm or more exposed in the observed cross section within a field of view at a magnification of 10,000 times is measured, and this can be determined as the tilt θ of the axis of the fibrous material 15. To determine the tilt of the axis of the fibrous material 15 without bias within the catalyst layer, it is preferable to perform similar measurements at at least 20 observation points.

[0025] Known methods such as ion milling and ultramicrotome can be used to expose the cross section of the membrane electrode assembly 1. When processing to expose the cross section, it is preferable to perform the processing while cooling the membrane electrode assembly 1 in order to reduce damage to the polymer electrolyte membrane 11 and the polymer electrolyte 14 that constitutes the electrode catalyst layer 10. In particular, it is preferable to use cryo-ion milling, which reduces damage to the polymer electrolyte membrane 11 and the polymer electrolyte 14 that constitutes the electrode catalyst layer 10 and allows for the production of a clear cross section.

[0026] The thickness of the electrode catalyst layer 10 is preferably 5 μm or more and 20 μm or less. If the thickness is greater than 20 μm, cracks are likely to occur, and when used in a fuel cell, the diffusibility and conductivity of gas and generated water are reduced, resulting in reduced output. If the thickness is less than 5 μm, the layer thickness is likely to vary, and the catalyst material 12 and polymer electrolyte 14 inside are likely to become non-uniform. Cracks on the surface of the electrode catalyst layer 10 and non-uniform thickness are likely to adversely affect the durability of the fuel cell when used and operated over a long period of time, and are therefore undesirable.

[0027] The thickness of the electrode catalyst layer 10 can be measured, for example, by observing the cross section of the membrane electrode assembly 1 using a scanning electron microscope (SEM). For example, it can be measured by measuring the thickness of the electrode catalyst layer within a field of view that includes the entire electrode catalyst layer at an observation magnification of approximately 1000 to 10000 times. To grasp the thickness uniformly within the catalyst layer, it is preferable to perform similar measurements at at least 20 or more observation points. Methods for exposing the cross section of the membrane electrode assembly 1 include known methods such as ion milling and ultramicrotome.

[0028] Examples of the catalyst material 12 according to this embodiment include platinum group elements, metals, and their alloys, oxides, double oxides, and carbides. Examples of platinum group elements include platinum, palladium, ruthenium, iridium, rhodium, and osmium. Examples of metals include iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum.

[0029] Any material may be used as the conductive support 13 as long as it is conductive and can support the catalytic material 12 without being corroded by the catalytic material 12 and the polymer electrolyte 14, but carbon particles are generally used as the conductive support 13. Examples of carbon particles that can be used include carbon black, graphite, activated carbon, carbon nanotubes, carbon nanofibers, and fullerenes. The particle size of the carbon particles is preferably about 10 to 1000 nm, and more preferably about 10 to 100 nm. If the particle size is too small, they may be packed too densely in the electrode catalyst layer 10, reducing the gas diffusion properties of the electrode catalyst layer 10, while if the particle size is too large, they may cause cracks in the electrode catalyst layer 10 or reduce the catalyst utilization rate, which are undesirable. The particle size of the carbon particles is the volume average diameter measured by a laser diffraction / scattering method.

[0030] The polymer electrolyte 14 contained in the polymer electrolyte membrane 11 and the electrode catalyst layer 10 may be any material that has proton conductivity, and may include fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes. The fluorine-based polymer electrolyte may be a polymer electrolyte having a tetrafluoroethylene skeleton, such as DuPont's "Nafion®." The hydrocarbon-based polymer electrolyte may be sulfonated polyetherketone, sulfonated polyethersulfone, sulfonated polyetherethersulfone, sulfonated polysulfide, or sulfonated polyphenylene. The polymer electrolyte 14 contained in the polymer electrolyte membrane 11 and the polymer electrolyte 14 contained in the electrode catalyst layer 10 may be the same or different. However, considering the interfacial resistance between the polymer electrolyte membrane 11 and the electrode catalyst layer 10 and dimensional changes between the polymer electrolyte membrane 11 and the electrode catalyst layer 10 due to humidity changes, it is preferable that the polymer electrolyte 14 contained in the polymer electrolyte membrane 11 and the polymer electrolyte 14 contained in the electrode catalyst layer 10 be the same or have similar components.

[0031] The fibrous material 15 may be any material that can maintain its fibrous shape without being affected by the catalyst material 12 and the polymer electrolyte 14, and examples thereof include nanofibers made by processing polymer compounds, carbon, and conductive oxides into fibers. The fibrous material 15 may be made of one type of fiber listed below, or two or more types may be used in combination. In order to simultaneously improve the material transportability and proton conductivity in the electrode catalyst layer 10, it is preferable that the fibrous material 15 contains at least a polymer fiber having proton conductivity or basicity.

[0032] Examples of the polymer fibers according to this embodiment include nanofibers obtained by processing a proton-conductive polymer electrolyte into a fibrous form. For example, a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte can be used. Examples of fluorine-based polymer electrolytes include Nafion (registered trademark) manufactured by DuPont, Flemion (registered trademark) manufactured by Asahi Glass Co., Ltd., Aciplex (registered trademark) manufactured by Asahi Kasei Corporation, and Gore Select (registered trademark) manufactured by Gore. Examples of hydrocarbon-based polymer electrolytes include electrolytes such as sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene. Acid-doped polybenzazoles, which exhibit proton conductivity by being doped with an acid, can also be suitably used.

[0033] Furthermore, examples of polymer fibers according to this embodiment include nanofibers made by processing basic polymer compounds into fibers. Examples include polymer fibers having an azole structure such as imidazole, thiazole, or oxazole, or a pyrrole ring or pyridine ring. The basic functional groups contained in the polymer fibers bond with acidic proton-conducting sites such as sulfonyl groups contained in the polymer electrolyte 14 via an acid-base bond, allowing the polymer fibers to be coated with a polymer electrolyte film. In particular, the presence of a basic functional group containing a nitrogen (N) atom in its molecular structure allows the polymer electrolyte 14 to uniformly coat the polymer fibers, thereby simultaneously improving the mass transport properties and proton conductivity in the electrode catalyst layer 10.

[0034] Examples of the electron-conductive fiber according to this embodiment include carbon fibers, carbon nanotubes, carbon nanohorns, and conductive polymer nanofibers. Carbon nanofibers are particularly preferred in terms of conductivity and dispersibility. Furthermore, the use of electron-conductive fibers with catalytic activity is more preferable because it reduces the amount of precious metal catalyst used. When used as an air electrode for a polymer electrolyte fuel cell, for example, a carbon alloy catalyst made from carbon nanofibers is an example. Furthermore, an electrode active material for an oxygen reduction electrode processed into a fibrous form may also be used, and for example, a material containing at least one transition metal element selected from Ta, Nb, Ti, and Zr may be used. Examples include partial oxides of carbonitrides of these transition metal elements, or conductive oxides or conductive oxynitrides of these transition metal elements.

[0035] The average fiber diameter of the fibrous material 15 is preferably 10 to 500 nm, more preferably 100 to 400 nm. By setting the average fiber diameter within this range, it is possible to increase the voids in the electrode catalyst layer 10 and suppress a decrease in proton conductivity, thereby enabling higher output. The average fiber length of the fibrous material 15 is preferably 1 to 100 μm, more preferably 5 to 50 μm. By setting the average fiber length within this range, the strength of the electrode catalyst layer 10 can be increased, and thus the occurrence of cracks during the formation of the electrode catalyst layer 10 can be suppressed. In addition, the voids within the electrode catalyst layer 10 can be increased, enabling higher output.

[0036] [Configuration of membrane electrode assembly] Next, a specific configuration of a membrane electrode assembly 1 including an electrode catalyst layer 10 according to this embodiment will be described with reference to Fig. 4. Fig. 4 shows a configuration example of a membrane electrode assembly according to this embodiment, in which (a) is a plan view of the membrane electrode assembly as seen from the oxygen electrode side of the electrode catalyst layer 10, and (b) is a cross-sectional view taken along line XX' in (a).

[0037] As shown in FIG. 4, the membrane electrode assembly 1 includes a polymer electrolyte membrane 11 and electrode catalyst layers 10C and 10A bonded to either side of the polymer electrolyte membrane 11. The surface of the electrode catalyst layer 10C bonded to the surface of the polymer electrolyte membrane 11 is designated 10Ca, and the surface of the electrode catalyst layer 10A bonded to the surface of the polymer electrolyte membrane 11 is designated 10Aa. In this embodiment, the electrode catalyst layer 10C formed on the upper surface of the polymer electrolyte membrane 11 is a cathode-side electrode catalyst layer constituting the oxygen electrode, and the electrode catalyst layer 10A formed on the lower surface of the polymer electrolyte membrane 11 is an anode-side electrode catalyst layer constituting the fuel electrode. Hereinafter, the pair of electrode catalyst layers 10C and 10A may be abbreviated as "electrode catalyst layer 10" when there is no need to distinguish between them. The outer periphery of the electrode catalyst layer 10 may be sealed with a gasket or the like (not shown).

[0038] [Method for manufacturing electrode catalyst layer and membrane electrode assembly] A method for manufacturing the electrode catalyst layer 10 and membrane electrode assembly 1 described above will be described below. First, a catalyst ink is prepared. At least the catalyst material 12, conductive support 13, polymer electrolyte 14, and fibrous material 15 described above are mixed in a dispersion medium, and then the mixture is subjected to a dispersion treatment to prepare the catalyst ink. The dispersion treatment can be performed using, for example, a planetary ball mill, a bead mill, or an ultrasonic homogenizer.

[0039] The solvent used as the dispersion medium for the catalyst ink may be any solvent that does not corrode the catalyst material 12, conductive support 13, polymer electrolyte 14, and fibrous material 15, and that can dissolve or disperse the polymer electrolyte 14 as a fine gel in a highly fluid state. The solvent may also contain water. It is desirable for the catalyst ink to contain at least a volatile liquid organic solvent, but since solvents using lower alcohols have a high risk of fire, it is preferable to use a mixture of such solvents with water. There are no particular restrictions on the amount of water added, as long as it is sufficient to prevent the polymer electrolyte 14 from separating and becoming cloudy or gelling.

[0040] The prepared catalyst ink is applied to a substrate and then dried, thereby removing the solvent component from the catalyst ink coating, and forming an electrode catalyst layer 10 on the substrate. The substrate can be a polymer electrolyte membrane 11, a transfer substrate, a gas diffusion layer 17, or the like. When the polymer electrolyte membrane 11 is used as the substrate, for example, the catalyst ink is applied directly to the surface of the polymer electrolyte membrane 11, and the solvent is then removed from the catalyst ink coating to form the electrode catalyst layer 10C. Thereafter, the catalyst ink is applied directly to the surface opposite the polymer electrolyte membrane 11 so as to face the electrode catalyst layer 10A across the polymer electrolyte membrane 11, and the solvent is then removed from the catalyst ink coating to form the electrode catalyst layer 10A, thereby obtaining the membrane electrode assembly 1.

[0041] When a transfer substrate is used, for example, a catalyst ink is applied to the transfer substrate and then dried to produce a transfer substrate with a catalyst layer. Thereafter, the surface of the electrode catalyst layer 10 on the transfer substrate with the catalyst layer is brought into contact with the polymer electrolyte membrane 11, and the electrode catalyst layer 10 and the polymer electrolyte membrane 11 are bonded together by applying heat and pressure. The electrode catalyst layers 10 are bonded to both sides of the polymer electrolyte membrane 11, and the transfer substrate is then removed, thereby producing a membrane / electrode assembly 1.

[0042] When a gas diffusion layer 17 is used as the substrate, for example, a catalyst ink is applied to the surface of the gas diffusion layer 17 and then dried to produce the catalyst layer-equipped gas diffusion layer 17. Thereafter, the surface of the electrode catalyst layer 10 in the catalyst layer-equipped gas diffusion layer 17 is brought into contact with the polymer electrolyte membrane 11, and the electrode catalyst layer 10 and the polymer electrolyte membrane 11 are bonded together by applying heat and pressure. The membrane electrode assembly 1 can be produced by bonding the electrode catalyst layers 10 to both sides of the polymer electrolyte membrane 11.

[0043] Various coating methods can be used to apply the catalyst ink to the substrate. Examples of coating methods include die coating, roll coating, curtain coating, spray coating, and squeegee coating. Die coating is preferably used as the coating method. Die coating is preferred because it stabilizes the film thickness during the coating period and allows for intermittent coating. Methods for drying the catalyst ink coating include drying using a hot air oven, IR (far infrared) drying, drying using a hot plate, and reduced pressure drying. The drying temperature is 40°C or higher and 200°C or lower, preferably approximately 40°C or higher and 120°C or lower. The drying time is 0.5 minutes to 1 hour, preferably approximately 1 minute to 30 minutes.

[0044] When the electrode catalyst layer 10 is formed on the transfer substrate or gas diffusion layer 17, the pressure and temperature applied to the electrode catalyst layer 10 during transfer of the electrode catalyst layer 10 affect the power generation performance of the membrane electrode assembly 1. To obtain a membrane electrode assembly 1 with high power generation performance, the pressure applied to the electrode catalyst layer 10 is preferably 0.1 MPa or more and 20 MPa or less. A pressure of 20 MPa or less prevents the electrode catalyst layer 10 from being excessively compressed. A pressure of 0.1 MPa or more prevents a decrease in power generation performance due to a decrease in the bonding strength between the electrode catalyst layer 10 and the polymer electrolyte membrane 11. Considering the improvement of the bonding strength at the interface between the polymer electrolyte membrane 11 and the electrode catalyst layer 10 and the suppression of interfacial resistance, the temperature during bonding is preferably near the glass transition point of the polymer electrolyte membrane 11 or the polymer electrolyte 14 contained in the electrode catalyst layer 10.

[0045] The transfer substrate can be, for example, a polymer film or a sheet formed from a fluorine-based resin. Fluorine-based resins have excellent transferability. Examples of fluorine-based resins include ethylene tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroperfluoroalkyl vinyl ether copolymer (PFA), and polytetrafluoroethylene (PTFE). Examples of polymers that form the polymer film include polyimide, polyethylene terephthalate, polyamide (Nylon (registered trademark)), polysulfone, polyethersulfone, polyphenylene sulfide, polyether ether ketone, polyetherimide, polyarylate, and polyethylene naphthalate.

[0046] Here, by adjusting the composition ratio of the fibrous material 15, the composition ratio of the polymer electrolyte 14, the solvent composition of the catalyst ink, the dispersion strength when preparing the catalyst ink, the heating temperature and heating rate of the applied catalyst ink, etc., the axis θ of the fibrous material 15 contained in the electrode catalyst layer 10 can be within the above-mentioned range, and the electrode catalyst layer 10 can have sufficient gas diffusivity and proton conductivity. For example, the blending ratio of the polymer electrolyte 14 in the electrode catalyst layer 10 is preferably about the same as or about half the mass of the conductive support 13. The blending ratio of the fibrous material 15 in the electrode catalyst layer 10 is preferably about 1% by mass or more and 10% by mass or less. If the blending ratio of the fibrous material 15 in the electrode catalyst layer 10 is less than 1% by mass, the effects of reducing proton conduction resistance and improving gas diffusibility are not sufficiently achieved, and cracks may occur during the formation of the electrode catalyst layer 10, reducing durability during long-term operation. On the other hand, if the blending ratio of the fibrous material 15 in the electrode catalyst layer 10 is more than 10% by mass, the catalytic reaction may be inhibited, potentially reducing battery performance. The solids content of the catalyst ink is preferably as high as possible, provided that it can be applied to form a thin film.

[0047] [Structure of polymer electrolyte fuel cells] Next, a specific example of the configuration of a polymer electrolyte fuel cell 3 including the membrane electrode assembly 1 according to this embodiment will be described with reference to Fig. 5. Fig. 5 is an exploded perspective view showing an example of the configuration of a polymer electrolyte fuel cell 3 equipped with the membrane electrode assembly 1. Note that Fig. 5 shows an example of the configuration of a single cell, and the polymer electrolyte fuel cell 3 is not limited to this configuration and may have a configuration in which a plurality of single cells are stacked.

[0048] As shown in FIG. 5, the polymer electrolyte fuel cell 3 includes a membrane electrode assembly 1, a gas diffusion layer 17C, and a gas diffusion layer 17A. The gas diffusion layer 17C is disposed opposite an electrode catalyst layer 10C, which is a cathode-side electrode catalyst layer on the oxygen electrode side of the membrane electrode assembly 1. The gas diffusion layer 17A is disposed opposite an electrode catalyst layer 10A, which is an anode-side electrode catalyst layer on the fuel electrode side of the membrane electrode assembly 1. The electrode catalyst layer 10C and the gas diffusion layer 17C form an oxygen electrode 2C, and the electrode catalyst layer 10A and the gas diffusion layer 17A form a fuel electrode 2A. In addition, a gasket 16C on the oxygen electrode side and a gasket 16A on the fuel electrode side are disposed to prevent gas leakage from the outer peripheral portion of the polymer electrolyte membrane 11 to which the electrode catalyst layer 10 is not joined.

[0049] Furthermore, the polymer electrolyte fuel cell 3 includes a separator 18C disposed opposite the oxygen electrode 2C and a separator 18A disposed opposite the fuel electrode 2A. The separator 18C includes gas channels 19C for flowing reactant gases formed on the surface facing the gas diffusion layer 17C, and cooling water channels 20C for flowing cooling water formed on the surface opposite to the surface on which the gas channels 19C are formed. The separator 18A has a similar configuration to the separator 18C, and includes gas channels 19A formed on the surface facing the gas diffusion layer 17A, and cooling water channels 20A formed on the surface opposite to the surface on which the gas channels 19A are formed. The separators 18C and 18A are made of a conductive and gas-impermeable material. In the polymer electrolyte fuel cell 3, an oxidant such as air or oxygen is supplied to the oxygen electrode 2C through the gas flow path 19C of the separator 18C, and a fuel gas containing hydrogen or an organic fuel is supplied to the fuel electrode 2A through the gas flow path 19A of the separator 18A, thereby generating electricity.

[0050] By employing the membrane electrode assembly 1 according to this embodiment, it is possible to obtain sufficient drainage, gas diffusibility, and proton conductivity, and to exhibit high power generation performance over a long period of time. That is, according to this embodiment, by specifying the inclination of the axis of the fibrous material 15, it is possible to provide an electrode catalyst layer 10, a membrane electrode assembly, and a polymer electrolyte fuel cell that have sufficient drainage, gas diffusibility, and proton conductivity during operation of the polymer electrolyte fuel cell and are capable of exhibiting high power generation performance over the long term. Therefore, the present invention can be suitably used in stationary cogeneration systems, fuel cell vehicles, and the like that use polymer electrolyte fuel cells, and has great industrial utility value.

[0051] Hereinafter, a membrane electrode assembly 1 according to each embodiment (first embodiment and second embodiment) of the present invention will be described. (First Example) [Example 1-1] In Example 1-1, a platinum-supported carbon catalyst (TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.), water, 1-propanol, a polymer electrolyte 14 (Nafion® dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and electrolyte nanofibers (diameter 200-400 nm, fiber length approximately 30 μm) were mixed. This mixture was dispersed at 500 rpm for 60 minutes using a planetary ball mill (P-7, manufactured by Fritsch). Approximately one-third of the zirconia container was filled with 5 mm diameter zirconia balls. The catalyst ink was prepared by adjusting the mass of the polymer electrolyte 14 to 100% by mass relative to the mass of the carbon particles, the mass of the electrolyte nanofibers to 10% by mass relative to the mass of the carbon particles, the proportion of water in the dispersion medium to 50% by mass, and the solids concentration to 10% by mass.

[0052] The catalyst ink was applied to one side of a polymer electrolyte membrane 11 (Nafion (registered trademark) 211, manufactured by DuPont) using a slit die coater to form a coating film having a thickness of 200 μm. The polymer electrolyte membrane 11 on which the coating film was formed was then dried in a hot air oven at 80°C until the coating film became tacky, thereby forming a cathode-side electrode catalyst layer 10C. The catalyst ink was then applied to the other side of the polymer electrolyte membrane 11 using a slit die coater to form a coating film having a thickness of 50 μm. The polymer electrolyte membrane 11 on which the coating film was formed was then dried in a hot air oven at 80°C until the coating film became tacky, thereby forming an anode-side electrode catalyst layer 10A. This resulted in the production of a membrane electrode assembly 1 of Example 1-1.

[0053] [Example 1-2] A membrane / electrode assembly 1 of Example 1-2 was obtained in the same manner as in Example 1-1, except that when preparing the catalyst ink, carbon nanofibers (VGCF (registered trademark)-H, manufactured by Showa Denko K.K.) were further added in an amount of 10 mass % relative to the mass of the carbon particles. [Examples 1-3] A membrane / electrode assembly 1 of Example 1-3 was obtained in the same manner as in Example 1-1, except that the amount of electrolyte nanofibers used in preparing the catalyst ink was double that of Example 1-1.

[0054] [Examples 1-4] A membrane / electrode assembly 1 of Example 1-4 was obtained in the same manner as in Example 1-1, except that the dispersion treatment time when preparing the catalyst ink was 120 minutes instead of 60 minutes. [Examples 1-5] A membrane electrode assembly 1 of Example 1-5 was obtained in the same manner as in Example 1-1, except that the amount of polymer electrolyte 14 used in preparing the catalyst ink was two-thirds that of Example 1-1. [Examples 1-6] A membrane / electrode assembly 1 of Example 1-6 was obtained in the same manner as in Example 1-1, except that when forming the cathode-side electrode catalyst layer 10C, the catalyst ink was applied to a thickness of 100 μm.

[0055] [Examples 1-7] A catalyst ink was prepared in the same manner as in Example 1-1. The catalyst ink was applied to the surface of a PTFE film using a slit die coater to a thickness of 200 μm to form a coating film. The PTFE film on which the coating film was formed was then dried in a hot air oven at 80°C until the coating film became tacky, thereby obtaining a transfer substrate with a cathode-side electrode catalyst layer 10C. Next, the catalyst ink was applied to the surface of another PTFE film using a slit die coater to a thickness of 50 μm to form a coating film. The PTFE film on which the coating film was formed was then dried in a hot air oven at 80°C until the coating film became tacky, thereby obtaining a transfer substrate with an anode-side electrode catalyst layer 10A.

[0056] A transfer substrate with a cathode-side electrode catalyst layer 10C and a transfer substrate with an anode-side electrode catalyst layer 10A were arranged facing each other on the front and back surfaces of a polymer electrolyte membrane 11 (Nafion (registered trademark) 211, manufactured by DuPont), to form a laminate. Next, the laminate was hot-pressed at 120°C and 1 MPa to bond electrode catalyst layers 10 to the front and back surfaces of the polymer electrolyte membrane 11, respectively. Finally, the PTFE film was peeled off from each electrode catalyst layer 10 to obtain a membrane electrode assembly 1 of Example 1-7.

[0057] [Comparative Example 1-1] A membrane / electrode assembly 1 of Comparative Example 1-1 was obtained in the same manner as in Example 1-1, except that the electrolyte nanofibers were not added when the catalyst ink was prepared. [Comparative Example 1-2] A membrane / electrode assembly 1 of Comparative Example 1-2 was obtained in the same manner as in Example 1-1, except that the amount of electrolyte nanofibers used in preparing the catalyst ink was 10 times that of Example 1-1. [Comparative Example 1-3] A membrane / electrode assembly 1 of Comparative Example 1-3 was obtained in the same manner as in Example 1-1, except that the dispersion treatment time when preparing the catalyst ink was 5 minutes instead of 60 minutes. [Comparative Example 1-4] A membrane / electrode assembly 1 of Comparative Example 1-4 was obtained in the same manner as in Example 1-2, except that the electrolyte nanofibers were not added when the catalyst ink was prepared.

[0058] Below, we will explain the results of comparing the inclination θ of the axis of the fibrous material 15 relative to the reference plane and the power generation performance of each of the membrane electrode assemblies 1 of Examples 1-1 to 1-7 and the solid polymer fuel cells 3 equipped with the membrane electrode assemblies 1 of Comparative Examples 1-1 to 1-4.

[0059] [Measurement of the axial tilt θ of fibrous materials] The tilt θ of the axis of the fibrous material 15 relative to a reference plane was measured by observing the cross section of the membrane electrode assembly 1 using a scanning electron microscope (SEM). Specifically, a small piece of the membrane electrode assembly 1 was first bonded to a metal plate, and a cross section of the electrode catalyst layer 10 was exposed using a cross section specimen preparation device IB-19520CCP manufactured by JEOL Ltd. Next, the exposed cross section was observed using an FE-SEM S-4800 manufactured by Hitachi High-Technologies Corporation, and θ was measured. First, the orientation of the sample was adjusted so that the bonding surface between the polymer electrolyte membrane 11 and the cathode-side electrode catalyst layer 10C was horizontal within a field of view at 1000x magnification. Next, the observation area was moved parallel to the cathode-side electrode catalyst layer 10C so that the center of the field of view was the cathode-side electrode catalyst layer 10C. The tilt θ of the axis of the fibrous material 15 was measured using an angle measurement function within a field of view at 10000x magnification. This was carried out at 30 observation points evenly within the catalyst layer. The ratio of the number of fibrous materials 15 whose axis tilt θ was 0°≦θ<45° and the ratio of the number of fibrous materials 15 whose axis tilt θ was 45°≦θ<90° to the total number of fibrous materials 15 measured at the 30 observation points were calculated.

[0060] [Measurement of power generation performance] To measure the power generation performance, a JARI standard cell was used as the evaluation unit cell, in accordance with the "Cell Evaluation and Analysis Protocol" published by the New Energy and Industrial Technology Development Organization (NEDO). The gas diffusion layer 17, gasket 16, and separator 18 were placed on both sides of the membrane electrode assembly 1, and the assembly was clamped to a specified surface pressure. Then, IV measurements were performed under the "Cell Evaluation and Analysis Protocol" (standard conditions) and with the anode and cathode relative humidities both set at 100% RH (high humidity conditions).

[0061] [Comparison results] Table 1 shows the tilt θ of the axis of the fibrous material 15 relative to the reference plane and the power generation performance of each of the polymer electrolyte fuel cells 3 including the membrane electrode assemblies 1 of Examples 1-1 to 1-7 and the membrane electrode assemblies 1 of Comparative Examples 1-1 to 1-4. The tilt θ is shown as the percentage of the number of fibrous materials 15 whose axis tilt θ is 0°≦θ<45° and the percentage of the number of fibrous materials 15 whose axis tilt θ is 45°≦θ<90°, relative to the total number of fibrous materials 15 measured at 30 observation points. Regarding power generation performance, under "standard" conditions, a current of 25 A or more at a voltage of 0.6 V was marked with "○" and a current less than 25 A was marked with "×." Under "high humidity" conditions, a current of 30 A or more at a voltage of 0.6 V was marked with "○" and a current less than 30 A was marked with "×."

[0062] [Table 1]

[0063] As shown in Table 1, in all of Examples 1-1 to 1-7, the proportion of fibrous materials 15 in which the inclination θ of the axis relative to the surface of the catalyst layer was 0°≦θ<45° was greater than 50%. Furthermore, the power generation performance was evaluated as "good" under both the "standard" and "high humidity" conditions. In other words, in Examples 1-1 to 1-7, membrane electrode assemblies 1 capable of constituting fuel cells with excellent power generation performance were obtained.

[0064] On the other hand, in the comparative examples, in all of Comparative Examples 1-1 to 1-3, the proportion of fibrous material 15 whose axis inclination θ relative to the surface of the catalyst layer was 0°≦θ<45° was less than 50%. Furthermore, the power generation performance was evaluated as "×" under both the "standard" condition and the "high humidity" condition. In other words, when the inclination of the fibrous material 15 in the electrode catalyst layer 10 was outside the predetermined range, the power generation performance decreased. Furthermore, a large number of cracks occurred in the electrode catalyst layer of Comparative Example 1-4, making measurement and evaluation impossible. In other words, when the configuration of the fibrous material 15 in the electrode catalyst layer 10 was outside the predetermined range, a membrane electrode assembly 1 capable of constituting a fuel cell could not be obtained.

[0065] (Second Example) [Example 2-1] In Example 2-1, a platinum-supported carbon catalyst (TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.), water, 1-propanol, a polymer electrolyte 14 (Nafion® dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and polyazole fibers (average fiber diameter: 200 nm, average fiber length: approximately 30 μm) were mixed. This mixture was dispersed at 500 rpm for 60 minutes using a planetary ball mill (P-7, manufactured by Fritsch). Approximately one-third of the zirconia container was filled with 5 mm diameter zirconia balls. The catalyst ink was prepared by adjusting the mass of the polymer electrolyte 14 to 100% by mass relative to the mass of the carbon particles, the mass of the polyazole fibers in the solid content to 1% by mass, the proportion of water in the dispersion medium to 50% by mass, and the solid content concentration to 10% by mass.

[0066] The catalyst ink was applied to one side of a polymer electrolyte membrane 11 (Nafion (registered trademark) 211, manufactured by DuPont) using a slit die coater to form a coating film having a thickness of 150 μm. The polymer electrolyte membrane 11 on which the coating film was formed was then dried in a hot air oven at 80°C until the coating film became tacky, thereby forming a cathode-side electrode catalyst layer 10C. The catalyst ink was then applied to the other side of the polymer electrolyte membrane 11 using a slit die coater to form a coating film having a thickness of 50 μm. The polymer electrolyte membrane 11 on which the coating film was formed was then dried in a hot air oven at 80°C until the coating film became tacky, thereby forming an anode-side electrode catalyst layer 10A. This resulted in the production of a membrane electrode assembly 1 of Example 2-1.

[0067] [Example 2-2] A membrane / electrode assembly 1 of Example 2-2 was obtained in the same manner as in Example 2-1, except that when preparing the catalyst ink, carbon nanofibers (VGCF (registered trademark)-H, manufactured by Showa Denko K.K., average fiber diameter 150 nm, average fiber length approximately 7 μm) were further added in an amount equal to the mass of the polyazole fibers. [Example 2-3] A membrane / electrode assembly 1 of Example 2-3 was obtained in the same manner as in Example 2-1, except that the amount of polyazole fiber used in preparing the catalyst ink was five times that of Example 2-1.

[0068] [Example 2-4] A membrane / electrode assembly 1 of Example 2-4 was obtained in the same manner as in Example 2-1, except that the dispersion treatment time when preparing the catalyst ink was 120 minutes instead of 60 minutes. [Example 2-5] A membrane electrode assembly 1 of Example 2-5 was obtained in the same manner as in Example 2-1, except that the amount of polymer electrolyte 14 used in preparing the catalyst ink was half that of Example 2-1. [Example 2-6] A membrane / electrode assembly 1 of Example 2-6 was obtained in the same manner as in Example 2-1, except that when forming the cathode-side electrode catalyst layer 10C, the catalyst ink was applied to a thickness of 100 μm.

[0069] [Example 2-7] A catalyst ink was prepared in the same manner as in Example 2-1. The catalyst ink was applied to the surface of a PTFE film using a slit die coater to a thickness of 150 μm to form a coating film. The PTFE film on which the coating film was formed was then dried in a hot air oven at 80°C until the coating film no longer had any tack, thereby obtaining a transfer substrate with a cathode-side electrode catalyst layer 10C. Next, the catalyst ink was applied to the surface of another PTFE film using a slit die coater to a thickness of 50 μm to form a coating film. The PTFE film on which the coating film was formed was then dried in a hot air oven at 80°C until the coating film no longer had any tack, thereby obtaining a transfer substrate with an anode-side electrode catalyst layer 10A.

[0070] A transfer substrate with a cathode-side electrode catalyst layer 10C and a transfer substrate with an anode-side electrode catalyst layer 10A were arranged facing each other on the front and back surfaces of a polymer electrolyte membrane 11 (Nafion (registered trademark) 211, manufactured by DuPont), to form a laminate. Next, the laminate was hot-pressed at 120°C and 1 MPa to bond electrode catalyst layers 10 to the front and back surfaces of the polymer electrolyte membrane 11, respectively. Finally, the PTFE film was peeled off from each electrode catalyst layer 10, to obtain a membrane electrode assembly 1 of Example 2-7.

[0071] [Example 2-8] A membrane electrode assembly 1 of Example 2-8 was obtained in the same manner as in Example 2-1, except that polyazole fibers (average fiber diameter 500 nm, average fiber length approximately 30 μm) were added instead of polyazole fibers (average fiber diameter 200 nm, average fiber length approximately 30 μm) when preparing the catalyst ink. [Example 2-9] A membrane / electrode assembly 1 of Example 2-9 was obtained in the same manner as in Example 2-1, except that when forming the cathode-side electrode catalyst layer 10C, the catalyst ink was applied to a thickness of 300 μm.

[0072] [Comparative Example 2-1] A membrane / electrode assembly 1 of Comparative Example 2-1 was obtained in the same manner as in Example 2-1, except that no polyazole fiber was added when preparing the catalyst ink. [Comparative Example 2-2] A membrane / electrode assembly 1 of Comparative Example 2-2 was obtained in the same manner as in Example 2-1, except that the amount of polyazole fiber used in preparing the catalyst ink was 15 times that of Example 2-1. [Comparative Example 2-3] A membrane / electrode assembly 1 of Comparative Example 2-3 was obtained in the same manner as in Example 2-1, except that the dispersion treatment time when preparing the catalyst ink was 5 minutes instead of 60 minutes. [Comparative Example 2-4] A membrane / electrode assembly 1 of Comparative Example 2-4 was obtained in the same manner as in Example 2-1, except that carbon nanofibers were added instead of polyazole fibers when preparing the catalyst ink.

[0073] Below, we will explain the results of comparing the inclination θ of the axis of the fibrous material 15 relative to the reference plane and the power generation performance of each of the membrane electrode assemblies 1 of Examples 2-1 to 2-9 and the solid polymer fuel cells 3 equipped with the membrane electrode assemblies 1 of Comparative Examples 2-1 to 2-4.

[0074] [Measurement of the axial tilt θ of fibrous materials] The tilt θ of the axis of the fibrous material 15 relative to a reference plane was measured by observing the cross section of the membrane electrode assembly 1 using a scanning electron microscope (SEM). Specifically, a small piece of the membrane electrode assembly 1 was first bonded to a metal plate, and a cross section of the electrode catalyst layer 10 was exposed using a cross section specimen preparation device IB-19520CCP manufactured by JEOL Ltd. Next, the exposed cross section was observed using an FE-SEM S-4800 manufactured by Hitachi High-Technologies Corporation, and θ was measured. First, the orientation of the sample was adjusted so that the bonding surface between the polymer electrolyte membrane 11 and the cathode-side electrode catalyst layer 10C was horizontal within a field of view at 1000x magnification. Next, the observation area was moved parallel to the cathode-side electrode catalyst layer 10C so that the center of the field of view was the cathode-side electrode catalyst layer 10C. The tilt θ of the axis of the fibrous material 15 was measured using an angle measurement function within a field of view at 10000x magnification. This was carried out at 30 observation points evenly within the catalyst layer. The ratio of the number of fibrous materials 15 whose axis tilt θ was 0°≦θ<45° and the ratio of the number of fibrous materials 15 whose axis tilt θ was 45°≦θ<90° to the total number of fibrous materials 15 measured at the 30 observation points were calculated. That is, the tilt θ of the axis of the fibrous material was measured in the same manner as in the first example.

[0075] [Measurement of power generation performance] To measure the power generation performance, a JARI standard cell was used as the evaluation unit cell, in accordance with the "Cell Evaluation and Analysis Protocol" published by the New Energy and Industrial Technology Development Organization (NEDO). The gas diffusion layer 17, gasket 16, and separator 18 were placed on both sides of the membrane electrode assembly 1, and the assembly was clamped to a specified surface pressure. Then, IV measurements were performed under the "Cell Evaluation and Analysis Protocol" (standard conditions) and with the anode and cathode relative humidities both set at 100% RH (high humidity conditions). That is, the power generation performance was measured in the same manner as in the first example.

[0076] [Comparison results] Table 2 shows the tilt θ of the axis of the fibrous material 15 relative to the reference plane and the power generation performance of each of the polymer electrolyte fuel cells 3 including the membrane electrode assemblies 1 of Examples 2-1 to 2-9 and the membrane electrode assemblies 1 of Comparative Examples 2-1 to 2-4. The tilt θ is shown as the percentage of the number of fibrous materials 15 whose axis tilt θ is 0°≦θ<45° and the percentage of the number of fibrous materials 15 whose axis tilt θ is 45°≦θ<90°, relative to the total number of fibrous materials 15 measured at 30 observation points. Regarding power generation performance, under "standard" conditions, a current of 25 A or more at a voltage of 0.6 V was marked with "○" and a current of less than 25 A was marked with "×." Under "high humidity" conditions, a current of 30 A or more at a voltage of 0.6 V was marked with "○" and a current of less than 30 A was marked with "×." In other words, the same evaluation criteria as in the first embodiment were used.

[0077] [Table 2]

[0078] As shown in Table 2, in all of Examples 2-1 to 2-9, the proportion of fibrous materials 15 in which the inclination θ of the axis relative to the surface of the catalyst layer was 0°≦θ<45° was greater than 50%. Furthermore, the power generation performance was evaluated as "good" under both the "standard" and "high humidity" conditions. That is, in Examples 2-1 to 2-9, membrane electrode assemblies 1 capable of constituting fuel cells with excellent power generation performance were obtained. Furthermore, in Examples 2-1 to 2-7, the thickness of the electrode catalyst layer and the fiber diameter of the fibrous material were within suitable ranges. Therefore, in Examples 2-1 to 2-7, membrane electrode assemblies 1 capable of constituting fuel cells with particularly excellent power generation performance were obtained.

[0079] On the other hand, in the comparative examples, in all of Comparative Examples 2-1 to 2-3, the proportion of fibrous material 15 whose axis inclination θ relative to the surface of the catalyst layer was 0°≦θ<45° was less than 50%. Furthermore, the power generation performance was evaluated as "×" under both the "standard" condition and the "high humidity" condition. In other words, when the inclination of the fibrous material 15 in the electrode catalyst layer 10 was outside the predetermined range, the power generation performance decreased. Furthermore, a large number of cracks occurred in the electrode catalyst layer of Comparative Example 2-4, making measurement and evaluation impossible. In other words, when the configuration of the fibrous material 15 in the electrode catalyst layer 10 was outside the predetermined range, a membrane electrode assembly 1 capable of constituting a fuel cell could not be obtained. [Explanation of symbols]

[0080] 1...Membrane electrode assembly 2C: oxygen electrode 2A…Fuel electrode 3… Polymer electrolyte fuel cell 4...Void 10, 10C, 10A...electrode catalyst layer 11...Polymer electrolyte membrane 12...catalyst material 13...Conductive carrier 14...polymer electrolyte 15...Fibrous materials 16, 16C, 16A...Gasket 17, 17C, 17A...Gas diffusion layer 18, 18C, 18A...Separator 19, 19C, 19A...Gas flow path 20, 20C, 20A…Cooling water flow path

Claims

1. An electrode catalyst layer for use in a polymer electrolyte fuel cell, the electrode catalyst layer being bonded to a polymer electrolyte membrane, The catalyst includes a catalytic substance, a conductive support that supports the catalytic substance, a polymer electrolyte, and one or more types of fibrous materials that include at least polymer fibers, the number of the fibrous materials having an axis with an inclination θ of 0°≦θ<45° with respect to the joint surface between the polymer electrolyte membrane and the electrode catalyst layer is more than 50% of the total number of the fibrous materials contained, An electrode catalyst layer characterized in that the polymer fiber contains an azole structure.

2. An electrode catalyst layer for use in a polymer electrolyte fuel cell, the electrode catalyst layer being bonded to a polymer electrolyte membrane, The catalyst includes a catalytic substance, a conductive support that supports the catalytic substance, a polymer electrolyte, and one or more types of fibrous materials that include at least polymer fibers, the number of the fibrous materials having an axis with an inclination θ of 0°≦θ<45° with respect to the joint surface between the polymer electrolyte membrane and the electrode catalyst layer is more than 50% of the total number of the fibrous materials contained, An electrode catalyst layer characterized in that the polymer fibers have proton conductivity.

3. 3. The electrode catalyst layer according to claim 1, wherein the number of the fibrous materials having an axis with an inclination θ of 45°≦θ<90° with respect to the joining surface of the polymer electrolyte membrane and the electrode catalyst layer is more than 5% of the total number of the fibrous materials contained.

4. the fibrous material comprises electronically conductive fibers; 4. The electrode catalyst layer according to claim 1, wherein the electron conductive fibers are fibers containing one or more selected from the group consisting of a partial oxide of a carbonitride of a transition metal element, a conductive oxide of a transition metal element, and a conductive oxynitride of a transition metal element.

5. the fibrous material comprises electronically conductive fibers; 4. The electrode catalyst layer according to claim 1, wherein the electron conductive fibers are carbon nanofibers.

6. 6. The electrode catalyst layer according to claim 1, wherein the polymer fibers have an average fiber diameter of 100 nm or more and 400 nm or less.

7. 7. The electrode catalyst layer according to claim 1, wherein the thickness of the electrode catalyst layer is 5 μm or more and 20 μm or less.

8. A membrane electrode assembly, comprising the electrode catalyst layer according to claim 1 provided on at least one surface of the polymer electrolyte membrane.

9. A polymer electrolyte fuel cell comprising the membrane electrode assembly according to claim 8.

Images