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

Abstract

The purpose of the present invention is to provide an electrode catalyst layer, a membrane electrode assembly, and a solid polymer fuel cell, which are capable of improving material transport properties and proton conductivity in the electrode catalyst layer, and which are capable of exhibiting high power generation performance over the long term and have good durability. An electrode catalyst layer (10) is an electrode catalyst layer used in a solid polymer fuel cell, and includes: a catalyst material (12); a conductive carrier (13) that supports the catalyst material (12); a polymer electrolyte (14); and a fibrous material (15), wherein the fibrous material (15) contains a substance having a nitrogen atom, and is included in an amount of 1-12 wt% (exclusive of 12) in the electrode catalyst layer (10).

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 for 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 with different particle sizes has been proposed (Patent Documents 1 and 2).

[0007] In Patent Document 1, dense packing of carbon particles in the electrode catalyst layer is prevented by combining carbon particles with moderately different particle sizes. When large particles with large particle sizes are mixed with small particles with small particle sizes, the small particles may enter the gaps between the large particles, resulting in a dense packing. Furthermore, when the carbon material is only particles, cracks are easily induced in the catalyst layer, which can lead to problems with reduced durability.

[0008] In Patent Document 2, carbon fibers having different fiber lengths are included and their ratio is set within a certain range, so that suitable pores occupy a large proportion of the electrode catalyst layer. When carbon fibers are used, dense packing can be prevented, but the ratio of electron conductors in the catalyst layer increases and the ratio of proton conductors decreases, resulting in increased proton transfer resistance and a decrease in power generation performance.

[0009] The power generation performance of a fuel cell varies greatly depending on the material transport properties, electronic conductivity, and proton conductivity. Therefore, ultimately, there is a limit to how much power generation performance can be improved by using a combination of carbon particles or carbon fibers that only increases electronic conductivity. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Patent No. 3617237 [Patent Document 2] Patent No. 5537178 Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention has been made in light of the above-mentioned points, and aims to provide an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell that have improved substance transport properties and proton conductivity in an electrode catalyst layer, and that are capable of exhibiting high power generation performance over the long term and have good durability. [Means for solving the problem]

[0012] In order to solve 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, and is characterized in that it contains a catalyst material, a conductive support that supports the catalyst material, a polymer electrolyte, and a fibrous material, the fibrous material containing a substance having a nitrogen atom, and the content of the fibrous material in the electrode catalyst layer is 1% by weight or more and less than 12% by weight. Furthermore, an electrode catalyst layer according to another aspect of the present invention is an electrode catalyst layer used in a polymer electrolyte fuel cell, and is characterized in that it comprises a catalytic substance, a conductive support that supports the catalytic substance, a polymer electrolyte, and a fibrous substance, the fibrous substance containing a Lewis base having an unshared electron pair, and the content of the fibrous substance in the electrode catalyst layer is 1% by weight or more but less than 12% by weight.

[0013] Furthermore, a membrane electrode assembly according to yet another aspect of the present invention is characterized in that the electrode catalyst layer according to the above aspect of the present invention is provided on at least one surface of a polymer electrolyte membrane.

[0014] A polymer electrolyte fuel cell according to yet another aspect of the present invention is characterized by including the membrane electrode assembly according to the above-described other aspect of the present invention. [Effects of the Invention]

[0015] According to one aspect of the present invention, it is possible to provide an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell that have improved substance transport properties and proton conductivity in an electrode catalyst layer, and that are capable of exhibiting high power generation performance over the long term and have good durability. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a cross-sectional view schematically showing an example of the cross-sectional structure of an electrode catalyst layer according to a first embodiment of the present invention. [Figure 2] 1 is an exploded perspective view schematically illustrating the configuration of a polymer electrolyte fuel cell according to a first embodiment of the present invention. [Figure 3] FIG. 4 is a cross-sectional view schematically showing an example of the cross-sectional structure of an electrode catalyst layer according to a second embodiment of the present invention. [Figure 4] FIG. 2 is an exploded perspective view schematically showing the configuration of a polymer electrolyte fuel cell according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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.

[0018] 1. First embodiment 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. The inventors then used a fibrous material containing nitrogen atoms in the electrode catalyst layer, which formed wide voids, improving gas diffusivity and reducing proton conduction resistance. As a result, the inventors succeeded in suppressing the decline in cell output and the deterioration of the electrode catalyst layer, thereby obtaining a polymer electrolyte fuel cell that exhibits high power generation performance over the long term and has excellent durability.

[0019] (Configuration of electrode catalyst layer) An electrode catalyst layer according to a first embodiment of the present invention (hereinafter referred to as "this embodiment") will be described below with reference to Fig. 1. Fig. 1 is a cross-sectional view that schematically shows an example of the cross-sectional structure of an electrode catalyst layer according to this embodiment.

[0020] 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 supporting the catalyst material 12, a polymer electrolyte 14, and a fibrous material 15. The portions where none of the above components are present form voids 4.

[0021] The fibrous material 15 contained in the electrode catalyst layer 10 according to this embodiment contains a substance having nitrogen atoms and is formed of one or more types of fibrous material.

[0022] The content of the fibrous material 15 in the electrode catalyst layer 10 may be in the range of 0.5 wt % to 15 wt %, and more preferably in the range of 1.0 wt % to less than 12 wt %. When the content of the fibrous material 15 having nitrogen atoms in the electrode catalyst layer 10 is within this range, an interaction between the unshared electron pairs of the nitrogen atoms and the protons of the polymer electrolyte can occur. This improves the proton conductivity in the electrode catalyst layer 10, thereby improving the output characteristics. Note that if the content of the fibrous material 15 in the electrode catalyst layer 10 is 12 wt % or more, the power generation performance tends to decrease.

[0023] The weight ratio of the fibrous material 15 contained in the electrode catalyst layer 10 can be determined by the ratio of the weight after removing other substances by chemical and electrochemical methods to the weight before removal. For example, the catalyst material can be dissolved by an acid such as aqua regia, which contains a strong oxidizing agent, and the conductive support can be burned off by applying a high potential. Furthermore, the polymer electrolyte and polymer electrolyte membrane can be decomposed by hydrogen peroxide or the like.

[0024] If the content of the nitrogen-containing fibrous material 15 in the electrode catalyst layer 10 is less than the above range, the interaction with the sulfonic acid groups in the polymer electrolyte 14 may be weakened, resulting in a lack of proton conduction paths and increased resistance. If the content of the nitrogen-containing fibrous material 15 in the electrode catalyst layer 10 is greater than the above range, the fibers may become entangled or aggregated, blocking voids and making it impossible to ensure sufficient drainage and gas diffusion.

[0025] Although the example in which the fibrous material 15 contains nitrogen atoms has been given, the present invention is not limited to this. The fibrous material 15 may contain a Lewis base having an unshared electron pair, which can cause an interaction with the protons of the polymer electrolyte. More specifically, the Lewis base (a Lewis base having an unshared electron pair) contained in the fibrous material 15 is preferably a soft or intermediate Lewis base according to the HSAB principle, from the viewpoint of compatibility with the polymer electrolyte 14. A hard Lewis base according to the HSAB principle (for example, a carbonyl group) strongly interacts with the sulfonic acid group in the polymer electrolyte 14, causing an increase in viscosity and gelling of the ink, which can make coating difficult.

[0026] Examples of fibrous materials having nitrogen atoms that satisfy the conditions of a Lewis base with an unshared electron pair include polybenzimidazole, polybenzoxazole, etc. Furthermore, aromatic polymer materials having phosphine or phosphine oxide groups can also be used as fibrous materials that do not contain nitrogen atoms for the fibrous material 15.

[0027] The average fiber diameter of the fibrous material 15 contained in the electrode catalyst layer 10 according to this embodiment is preferably 50 nm or more and 400 nm or less. By setting the fiber diameter within this range, the voids 4 in the electrode catalyst layer 10 can be increased and a decrease in proton conductivity can be suppressed, enabling higher output. If the average fiber diameter of the fibrous material 15 is smaller than the above range, the fibrous material may clog the voids, making it impossible to ensure sufficient drainage and gas diffusion. Furthermore, if the average fiber diameter of the fibrous material 15 is larger than the above range, the conduction of electrons and protons by the conductive support 13 and the polymer electrolyte 14 may be inhibited, resulting in increased resistance.

[0028] The fiber length of the fibrous material 15 is preferably 1 μm to 100 μm, and more preferably 5 μm to 50 μm. By setting the fiber length in the range of 1 μm to 100 μm, the strength of the electrode catalyst layer 10 can be increased, and ultimately, the occurrence of cracks in the electrode catalyst layer 10 during formation of the electrode catalyst layer 10 can be suppressed.

[0029] The fiber diameter of the fibrous material 15 can be obtained, for example, by observing the cross section of the electrode catalyst layer 10 using a scanning electron microscope (SEM) and measuring the diameter of the fibrous material 15 exposed at the cross section. When the fibrous material 15 is cut obliquely, the shape of the exposed cross section may be elliptical. In this case, the fiber diameter of the fibrous material 15 can be obtained by measuring the diameter of a perfect circle fitted along the minor axis. The average fiber diameter can be obtained by measuring the fiber diameters of the fibrous material 15 at multiple locations, for example, 20 locations, and calculating the arithmetic average.

[0030] Known methods such as ion milling and ultramicrotome can be used to expose the cross section of the electrode catalyst layer 10. When processing to expose the cross section, it is particularly preferable to use cryo-ion milling, which performs processing while cooling the electrode catalyst layer 10, in order to reduce damage to the polymer electrolyte membrane 11 and the polymer electrolyte 14 that constitutes the electrode catalyst layer 10.

[0031] The thickness of the electrode catalyst layer 10 is preferably 5 μm or more and 30 μm or less. If the thickness is greater than 30 μm, cracks are likely to occur, and when used in a fuel cell, the diffusibility and conductivity of gas and generated water may decrease, resulting in a decrease in output. Furthermore, 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 may 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.

[0032] The thickness of the electrode catalyst layer 10 can be measured, for example, by observing the cross section of the electrode catalyst layer 10 using a scanning electron microscope (SEM). Methods for exposing the cross section of the electrode catalyst layer 10 include known methods such as ion milling and ultramicrotome. When processing to expose the cross section, it is particularly preferable to use cryo-ion milling, in which processing is performed while cooling the electrode catalyst layer 10, in order to reduce damage to the polymer electrolyte membrane 11 and the polymer electrolyte 14 that constitutes the electrode catalyst layer 10.

[0033] For example, platinum group metals, metals other than platinum group metals, and alloys, oxides, double oxides, and carbides of these metals can be used for the catalytic material 12. Platinum group metals include platinum, palladium, ruthenium, iridium, rhodium, and osmium. Non-platinum group metals include iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum.

[0034] The conductive support 13 can be a support that is conductive and can support the catalytic material 12 without being eroded by the catalytic material 12. Carbon particles can be used for 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 approximately 10 nm to 1000 nm, and more preferably approximately 10 nm to 100 nm. A particle size of 10 nm or more prevents the carbon particles from packing too densely in the electrode catalyst layer 10, thereby preventing a decrease in the gas diffusion properties of the electrode catalyst layer 10. A particle size of 1000 nm or less prevents cracks from occurring in the electrode catalyst layer 10. The particle size of the carbon particles is the volume average diameter measured by a laser diffraction / scattering method.

[0035] The polymer electrolyte 14 contained in the polymer electrolyte membrane 11 and the electrode catalyst layer 10 can be a proton-conductive electrolyte. The polymer electrolyte can be, for example, a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte. The fluorine-based polymer electrolyte can be a polymer electrolyte having a tetrafluoroethylene skeleton. An example of the polymer electrolyte having a tetrafluoroethylene skeleton is Nafion (registered trademark) manufactured by DuPont. The hydrocarbon-based polymer electrolyte can be, for example, sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, or sulfonated polyphenylene.

[0036] The polymer electrolyte contained in the polymer electrolyte membrane 11 and the polymer electrolyte 14 contained in the electrode catalyst layer 10 may be the same electrolyte or different electrolytes. However, taking into consideration the interfacial resistance at the interface between the polymer electrolyte membrane 11 and the electrode catalyst layer 10 and the rate of dimensional change in the polymer electrolyte membrane 11 and the electrode catalyst layer 10 when humidity changes, it is preferable that the polymer electrolyte contained in the polymer electrolyte membrane 11 and the polymer electrolyte 14 contained in the electrode catalyst layer 10 be the same electrolyte or polymer electrolytes with similar thermal expansion coefficients.

[0037] (Configuration of membrane electrode assembly) Next, the configuration of the membrane electrode assembly will be described with reference to Fig. 2. Fig. 2 is an exploded perspective view schematically showing the configuration of a polymer electrolyte fuel cell 3 equipped with a membrane electrode assembly 1 having an electrode catalyst layer 10 according to this embodiment.

[0038] The membrane electrode assembly 1 includes a polymer electrolyte membrane 11 and electrode catalyst layers 10C and 10A bonded to the front and back surfaces of the polymer electrolyte membrane 11, respectively. In this embodiment, the electrode catalyst layer 10C formed on the upper surface (front surface) of the polymer electrolyte membrane 11 is a cathode-side electrode catalyst layer constituting an oxygen electrode, and the electrode catalyst layer 10A formed on the lower surface (back surface) of the polymer electrolyte membrane 11 is an anode-side electrode catalyst layer constituting a fuel electrode. Hereinafter, the pair of electrode catalyst layers 10C and 10A may be abbreviated as "electrode catalyst layers 10" when there is no need to distinguish between them. In the membrane electrode assembly 1 according to this embodiment, the electrode catalyst layer 10 may be provided on at least one surface of the polymer electrolyte membrane 11. In addition, to prevent gas leakage from the outer periphery of the polymer electrolyte membrane 11 to which the electrode catalyst layer 10 is not bonded, a gasket 16C on the oxygen electrode side and a gasket 16A on the fuel electrode side are disposed in the membrane electrode assembly 1.

[0039] (Method for manufacturing membrane electrode assembly) A method for producing the membrane electrode assembly 1 will be described.

[0040] First, the catalyst ink is prepared by mixing the catalyst material 12, conductive support 13, polymer electrolyte 14, and fibrous material 15 in a dispersion medium, and then subjecting the mixture to a dispersion treatment. The dispersion treatment can be carried out using, for example, a planetary ball mill, a bead mill, or an ultrasonic homogenizer.

[0041] The dispersion medium of the catalyst ink can be a solvent that does not corrode the catalyst material 12, conductive support 13, polymer electrolyte 14, and fibrous material 15, and that can dissolve the polymer electrolyte 14 while maintaining high fluidity, or that can disperse the polymer electrolyte 14 as a fine gel. The dispersion medium may contain water. The catalyst ink preferably contains a volatile liquid organic solvent. If the solvent is a lower alcohol, there is a risk of fire, so it is preferable to mix water with such a solvent. Water can be mixed with the solvent to the extent that separation of the polymer electrolyte 14 does not cause the catalyst ink to become cloudy or gel.

[0042] The prepared catalyst ink is applied to a substrate and then dried, thereby removing the solvent from the catalyst ink coating. This forms an electrode catalyst layer 10 on the substrate. The substrate can be a polymer electrolyte membrane 11 or a transfer substrate. When a polymer electrolyte membrane 11 is used as the substrate, for example, the electrode catalyst layer 10 can be formed by applying the catalyst ink directly to the surface of the polymer electrolyte membrane 11 and then removing the solvent from the catalyst ink coating.

[0043] When a transfer substrate is used, a catalyst ink is applied to the transfer substrate and then dried to produce a substrate with a catalyst layer. Thereafter, for example, the surface of the electrode catalyst layer 10 in the catalyst layer-attached substrate 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, thereby producing a membrane electrode assembly 1.

[0044] 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.

[0045] When forming the electrode catalyst layer 10 on a transfer substrate, 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 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.

[0046] The transfer substrate may 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. A gas diffusion layer may also be used as the transfer substrate.

[0047] Here, by adjusting the blending ratio of the fibrous material 15, the blending 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 electrode catalyst layer 10 can be made to have sufficient gas diffusivity and proton conductivity.

[0048] 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 weight of the conductive support 13. The blending ratio of the fibrous material 15 is preferably about the same as or less than the weight of the conductive support 13. The solids ratio of the catalyst ink is preferably as high as possible, provided that it can be applied to form a thin film.

[0049] (Structure of polymer electrolyte fuel cells) 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. 2. Note that Fig. 2 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.

[0050] The polymer electrolyte fuel cell 3 includes a membrane electrode assembly 1, a gas diffusion layer 17C on the oxygen electrode side, and a gas diffusion layer 17A on the fuel electrode side. 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.

[0051] The polymer electrolyte fuel cell 3 further 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.

[0052] 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.

[0053] By adopting the membrane electrode assembly 1 according to this embodiment, the polymer electrolyte fuel cell 3 according to this embodiment has sufficient drainage and gas diffusibility, and is able to exhibit high power generation performance and high durability over the long term.

[0054] That is, according to this embodiment, it is possible to provide an electrode catalyst layer 10, a membrane electrode assembly 1, and a polymer electrolyte fuel cell 3 that have sufficient gas diffusivity and proton conductivity during operation of the polymer electrolyte fuel cell 3, and that are capable of exhibiting high power generation performance and high durability over the long term. Therefore, the present invention can be suitably used in stationary cogeneration systems, fuel cell vehicles, etc. that use polymer electrolyte fuel cells, and has great industrial utility value. First Example

[0055] Hereinafter, membrane electrode assemblies according to examples based on this embodiment will be described.

[0056] [Example 1-1] In Example 1, a platinum-supported carbon catalyst (TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.), water, 1-propanol, a polymer electrolyte (20% Nafion® dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and a fibrous material containing nitrogen atoms (diameter 250-300 nm) were mixed. This mixture was dispersed using a planetary ball mill at 300 rpm for 60 minutes. During this process, zirconia balls with a diameter of 5 mm were added to approximately one-third of the zirconia container. The weight of the polymer electrolyte was 100 wt% relative to the weight of the carbon support in the platinum-supported carbon catalyst, the weight of the fibrous material containing nitrogen atoms was 10 wt% relative to the weight of the carbon support in the platinum-supported carbon catalyst, the proportion of water in the dispersion medium was 50 wt%, and the solids concentration was 10 wt%. This produced a catalyst ink.

[0057] The catalyst ink was applied to one side of a polymer electrolyte membrane (Nafion (registered trademark) 211, manufactured by DuPont) using a slit die coater to form a coating film with a thickness of 200 μm. The polymer electrolyte membrane with the coating film formed thereon 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. The catalyst ink was then applied to the opposite side of the polymer electrolyte membrane using a slit die coater to form a coating film with a thickness of 50 μm. The polymer electrolyte membrane with the coating film formed thereon 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. This resulted in the production of the membrane / electrode assembly of Example 1-1. The proportion of the fibrous material in the electrode catalyst layer was 3 wt%.

[0058] [Example 1-2] A membrane electrode assembly of Example 1-2 was obtained in the same manner as in Example 1, except that when preparing the catalyst ink, the amount of the fibrous material having nitrogen atoms was double that of Example 1 (20 wt % relative to the weight of the carbon support). The proportion of the fibrous material in the electrode catalyst layer was 6% by weight.

[0059] [Examples 1-3] The membrane electrode assemblies of Examples 1-3 were obtained in the same manner as in Example 1, except that when preparing the catalyst ink, the amount of the fibrous material having nitrogen atoms was half that of Example 1 (5 wt % relative to the weight of the carbon support). The proportion of the fibrous material in the electrode catalyst layer was 1 wt %.

[0060] [Examples 1-4] A catalyst ink was prepared in the same manner as in Example 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. 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.

[0061] A transfer substrate with a cathode-side electrode catalyst layer and a transfer substrate with an anode-side electrode catalyst layer were placed on the front and back surfaces of a polymer electrolyte membrane (Nafion (registered trademark) 211, manufactured by DuPont) so that they faced each other, forming a laminate. Next, the laminate was hot-pressed at 120°C and 1 MPa to bond the electrode catalyst layers to the front and back surfaces of the polymer electrolyte membrane. Finally, the PTFE film was peeled off from each electrode catalyst layer to obtain a membrane electrode assembly of Example 1-4. The proportion of the fibrous material in the electrode catalyst layer was 3 wt%.

[0062] [Comparative Example 1-1] A membrane / electrode assembly of Comparative Example 1-1 was obtained in the same manner as in Example 1, except that no fibrous material having nitrogen atoms was added when preparing the catalyst ink. The proportion of the fibrous material in the electrode catalyst layer was 0 wt %.

[0063] [Comparative Example 1-2] A membrane electrode assembly of Comparative Example 1-2 was obtained in the same manner as in Example 1-1, except that the amount of fibrous material having nitrogen atoms was four times that of Example 1-1 (80 wt % relative to the weight of the carbon support) when preparing the catalyst ink. The proportion of the fibrous material in the electrode catalyst layer was 12 wt %.

[0064] [Comparative Example 1-3] A membrane / electrode assembly of Comparative Example 1-3 was obtained in the same manner as in Example 1, except that when preparing the catalyst ink, carbon nanofibers having no nitrogen atoms (VGCF (registered trademark)-H, manufactured by Showa Denko K.K.)) were used as the fibrous material instead of the fibrous material having nitrogen atoms. The proportion of the fibrous material having nitrogen atoms in the electrode catalyst layer was 0 wt %, and the proportion of the fibrous material having no nitrogen atoms was 3 wt %.

[0065] [evaluation] The power generation performance and durability of each of the polymer electrolyte fuel cells provided with the membrane electrode assemblies of Examples 1-1 to 1-4 and the membrane electrode assemblies of Comparative Examples 1-1 to 1-3 were evaluated.

[0066] (Evaluation of power generation performance) Measurements to evaluate power generation performance were performed using a method in accordance with the "Cell Evaluation and Analysis Protocol," a publication by the New Energy and Industrial Technology Development Organization (NEDO). A JARI standard cell was used as the evaluation unit cell, consisting of a gas diffusion layer, gasket, and separator on both sides of the membrane electrode assembly, and clamped to a specified surface pressure. An IV measurement was then performed as described in the "Cell Evaluation and Analysis Protocol." The conditions were set as standard conditions. The IV measurement was also performed with the anode and cathode relative humidity at 100% RH. The conditions were set as high humidity conditions.

[0067] If the current was 25 A or more when the voltage was 0.6 V under standard conditions and 30 A or more when the voltage was 0.6 V under high humidity conditions, the result was marked as "○", and if the current did not reach the above values, the result was marked as "×".

[0068] (Durability evaluation) The same evaluation cell as the evaluation cell used in the measurements to evaluate the power generation performance was used as the evaluation cell, and the humidity cycle test described in the above-mentioned "Cell Evaluation and Analysis Protocol" was carried out to evaluate durability.

[0069] A case where the hydrogen cross leakage current after 8000 cycles was less than 10 times the initial value was marked as "◯", and a case where it was 10 times or more the initial value was marked as "X".

[0070] As a result, the weight ratio of the fibrous material in the cathode-side electrode catalyst layer of fuel cells equipped with the membrane electrode assemblies of Examples 1-1 to 1-4 and the membrane electrode assemblies of Comparative Examples 1-1 to 1-3, as well as the power generation performance evaluation and durability evaluation, are shown in Table 1. In Table 1, "fibrous material content in electrode catalyst layer" indicates the total content of the fibrous material in the electrode catalyst layer.

[0071] [Table 1]

[0072] As shown in Table 1, the power generation performance and durability of Examples 1-1 to 1-4 were all rated "Good." It was confirmed that when the content of the fibrous material having nitrogen atoms in the electrode catalyst layer was 1.0 wt % or more and less than 12 wt %, a fuel cell having a membrane electrode assembly using this electrode catalyst layer had excellent power generation performance and durability.

[0073] In Comparative Examples 1-1 to 1-3, at least one of the power generation performance and durability was evaluated as "X." It was confirmed that at least one of the power generation performance and durability was reduced when the content of the fibrous material having nitrogen atoms in the electrode catalyst layer was not 1.0 wt % or more and less than 12 wt %.

[0074] 2. Second embodiment 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. By using both an electronically conductive fibrous material and proton-conductive fibers in the electrode catalyst layer, wide voids are formed, improving gas diffusivity and reducing proton conduction resistance. As a result, the inventors have succeeded in suppressing a decrease in cell output and deterioration of the electrode catalyst layer, thereby obtaining a polymer electrolyte fuel cell that exhibits high power generation performance over the long term and has excellent durability.

[0075] [Configuration of electrode catalyst layer] The specific configuration of an electrode catalyst layer according to a second embodiment of the present invention (hereinafter referred to as "this embodiment") will be described below with reference to the drawings. The electrode catalyst layer according to this embodiment differs from the electrode catalyst layer according to the first embodiment in that it contains at least one of electron conductive fibers or proton conductive fibers as a fibrous material. In the electrode catalyst layer 101, components equivalent to those in the electrode catalyst layer 10 according to the first embodiment are designated by the same reference numerals, and descriptions thereof will be omitted where appropriate.

[0076] 3, the electrode catalyst layer 101 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 supporting the catalyst material 12, a polymer electrolyte 14, a fibrous material 15 (first fibrous material), and a fibrous material 25 (second fibrous material). The portions where none of the above components are present form voids 4.

[0077] The fibrous material 25 contained in the electrode catalyst layer 101 according to this embodiment is formed of one or more types of fibers including either or both of electron conductive fibers and proton conductive fibers. That is, in the electrode catalyst layer 101, the fibrous material 25 may be formed of one or more types of fibers including at least either electron conductive fibers or proton conductive fibers. Thus, in the electrode catalyst layer 101 according to this embodiment, the fibrous material is configured to include at least one of electron conductive fibers or proton conductive fibers in addition to the fibrous material 15, which is the first fibrous material.

[0078] The electrode catalyst layer 101 is configured so that the content of the fibrous material 25 in the electrode catalyst layer 101 is 1% by weight or more and 15% by weight or less. If the content of the fibrous material 25 is less than the above range, the voids 4 may become narrower, making it difficult to ensure sufficient drainage and gas diffusion. Furthermore, cracks may occur in the electrode catalyst layer 101, resulting in a decrease in durability. If the content of the fibrous material 25 is greater than the above range, the proton conduction path through the polymer electrolyte 14 may be blocked, increasing resistance.

[0079] Furthermore, the fibrous material 25 preferably contains at least proton-conductive fibers, and the content of the proton-conductive fibers in the electrode catalyst layer 101 is more preferably 1% by weight or more and 10% by weight or less. If the content of the proton-conductive fibers is less than the above range, the proton-conductive fibers may not provide sufficient proton conduction paths, resulting in increased resistance. If the content of the proton-conductive fibers is greater than the above range, the fibers may become entangled or aggregated, blocking voids and making it impossible to ensure sufficient drainage and gas diffusion.

[0080] The weight ratio of the fibrous material 25 contained in the electrode catalyst layer 101 can be determined by the ratio of the weight after removing other substances by chemical and electrochemical methods to the weight before removal. For example, the catalyst material can be dissolved by an acid such as aqua regia, which contains a strong oxidizing agent, and the conductive support can be burned off by applying a high potential. Furthermore, the polymer electrolyte and polymer electrolyte membrane can be decomposed by hydrogen peroxide or the like.

[0081] An example of a proton-conducting fiber is a nanofiber (electrolyte nanofiber) made by processing a proton-conducting polymer electrolyte into a fibrous form. Because such proton-conducting fibers are flexible, excessive proton content can cause the above-mentioned problems. Materials for forming the proton-conductive fibers include fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes. Examples of fluorine-based polymer electrolytes that can be used 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 Aquivion (registered trademark) manufactured by Solvay. Examples of hydrocarbon-based polymer electrolytes that can be used include sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, sulfonated polyphenylene, sulfonated polyimide, and acid-doped polybenzazoles.

[0082] Examples of the electron conductive fiber include carbon fiber, carbon nanotube, carbon nanohorn, conductive polymer nanofiber, etc. In terms of conductivity and dispersibility, in this embodiment, it is preferable to use rigid carbon nanofiber as the electron conductive fiber contained in the fibrous material 25.

[0083] Only one type of the above-mentioned fibers may be used, or two or more types may be used as the fibrous material 25. An electron conductive fiber and a proton conductive fiber may be used in combination as the fibrous material 25. Among the above-mentioned fibrous materials 25, the fibrous material 25 preferably includes at least one selected from the group consisting of carbon nanofibers, carbon nanotubes, and electrolyte nanofibers.

[0084] Furthermore, in the electrode catalyst layer 101 according to this embodiment, the fibrous material 25 may contain a substance having a nitrogen atom. That is, the fibrous material 25 may be formed of one or more types of fibers containing either or both of an electron conductive fiber containing a substance having a nitrogen atom and a proton conductive fiber containing a substance having a nitrogen atom. Furthermore, when the fibrous material 25 is formed to contain both an electron conductive fiber and a proton conductive fiber, either the electron conductive fiber or the proton conductive fiber may contain a substance having a nitrogen atom.

[0085] The average fiber diameter of the fibrous material 25 contained in the electrode catalyst layer 101 according to this embodiment is preferably 50 nm or more and 400 nm or less. By setting the fiber diameter within this range, the voids 4 in the electrode catalyst layer 101 can be increased and a decrease in proton conductivity can be suppressed, enabling higher output. If the average fiber diameter of the fibrous material 25 is smaller than the above range, the fibrous material may clog the voids, making it impossible to ensure sufficient drainage and gas diffusion. Furthermore, if the average fiber diameter of the fibrous material 25 is larger than the above range, the conduction of electrons and protons by the conductive carrier 13 and the polymer electrolyte 14 may be inhibited, resulting in increased resistance. The fiber length of the fibrous material 25 is preferably 1 μm to 50 μm, and more preferably 5 μm to 20 μm. By setting the fiber length in the range of 1 μm to 50 μm, the strength of the electrode catalyst layer 101 can be increased, and ultimately, the occurrence of cracks in the electrode catalyst layer 101 during formation can be suppressed.

[0086] The fiber diameter of the fibrous material 25 can be obtained, for example, by observing the cross section of the electrode catalyst layer 101 using a scanning electron microscope (SEM) and measuring the diameter of the fibrous material 25 exposed at the cross section. When the fibrous material 25 is cut obliquely, the shape of the exposed cross section may be elliptical. In this case, the fiber diameter of the fibrous material 25 can be obtained by measuring the diameter of a perfect circle fitted along the minor axis. The average fiber diameter can be obtained by measuring the fiber diameters of the fibrous material 25 at multiple locations, for example, 20 locations, and calculating the arithmetic average.

[0087] The method for exposing the cross section of the electrode catalyst layer 101 is the same as the method for exposing the cross section of the electrode catalyst layer 10 in the first embodiment, and therefore a description thereof will be omitted.

[0088] The thickness of the electrode catalyst layer 101 is preferably 5 μm or more and 30 μm or less, similar to the electrode catalyst layer 10 according to the first embodiment. If the thickness is greater than 30 μm, cracks are likely to occur, and when used in a fuel cell, the diffusibility and conductivity of gas and generated water may decrease, resulting in a decrease in output. Furthermore, 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 may become non-uniform. Cracks on the surface of the electrode catalyst layer 101 and non-uniform thickness are likely to adversely affect the durability of the fuel cell when operated over a long period of time, and are therefore undesirable.

[0089] The thickness of the electrode catalyst layer 101 can be measured, for example, by observing the cross section of the electrode catalyst layer 101 using a scanning electron microscope (SEM). The method for exposing the cross section of the electrode catalyst layer 101 is the same as the method for exposing the cross section of the electrode catalyst layer 10 in the first embodiment, and therefore a description thereof will be omitted.

[0090] Furthermore, the catalytic material 12, the conductive support 13, and the polymer electrolyte 14 in the electrode catalyst layer 101 according to this embodiment are equivalent to the catalytic material 12, the conductive support 13, and the polymer electrolyte 14 in the electrode catalyst layer 10 according to the first embodiment described above, and therefore the same reference numerals are used and their description will be omitted.

[0091] The polymer electrolyte contained in the polymer electrolyte membrane 11 and the polymer electrolyte 14 contained in the electrode catalyst layer 101 may be the same electrolyte or different electrolytes. However, taking into consideration the interfacial resistance at the interface between the polymer electrolyte membrane 11 and the electrode catalyst layer 101 and the rate of dimensional change in the polymer electrolyte membrane 11 and the electrode catalyst layer 101 when humidity changes, it is preferable that the polymer electrolyte contained in the polymer electrolyte membrane 11 and the polymer electrolyte 14 contained in the electrode catalyst layer 101 be the same electrolyte or polymer electrolytes with similar thermal expansion coefficients.

[0092] [Configuration of membrane electrode assembly] Next, the configuration of the membrane electrode assembly will be described with reference to Fig. 4. Fig. 4 is an exploded perspective view showing an example of the configuration of a polymer electrolyte fuel cell 30 equipped with a membrane electrode assembly 100 having an electrode catalyst layer 101 according to this embodiment. The membrane electrode assembly 100 includes a polymer electrolyte membrane 11 and electrode catalyst layers 101C and 101A bonded to the front and back surfaces of the polymer electrolyte membrane 11, respectively. In this embodiment, the electrode catalyst layer 101C formed on the upper surface (front surface) of the polymer electrolyte membrane 11 is a cathode-side electrode catalyst layer constituting an oxygen electrode, and the electrode catalyst layer 101A formed on the lower surface (back surface) of the polymer electrolyte membrane 11 is an anode-side electrode catalyst layer constituting a fuel electrode. Hereinafter, the pair of electrode catalyst layers 101C and 101A may be abbreviated as "electrode catalyst layer 101" when there is no need to distinguish between them. In the membrane electrode assembly 100 according to this embodiment, the electrode catalyst layer 101 may be provided on at least one surface of the polymer electrolyte membrane 11. Furthermore, in order to prevent gas leakage from the outer peripheral portion of the polymer electrolyte membrane 11 to which the electrode catalyst layer 101 is not bonded, the membrane electrode assembly 100 is provided with a gasket 16C on the oxygen electrode side and a gasket 16A on the fuel electrode side, similar to the membrane electrode assembly 1 according to the first embodiment.

[0093] [Method for manufacturing membrane electrode assembly] A method for manufacturing the above-mentioned membrane electrode assembly 100 will now be described. First, a catalyst ink is prepared. The method for producing a membrane electrode assembly 100 according to this embodiment differs from the method for producing the membrane electrode assembly 1 according to the first embodiment in that the catalyst material 12, the conductive support 13, the polymer electrolyte 14, the fibrous material 15, and the fibrous material 25 are mixed in a dispersion medium. Otherwise, the membrane electrode assembly 100 according to this embodiment can be produced in the same manner as the method for producing the membrane electrode assembly 1 according to the first embodiment.

[0094] Furthermore, when the electrode catalyst layer 101 is formed on a transfer substrate, the pressure and temperature applied to the electrode catalyst layer 101 during transfer of the electrode catalyst layer 101 affect the power generation performance of the membrane electrode assembly 100. To obtain a membrane electrode assembly with high power generation performance, the pressure applied to the electrode catalyst layer 101 is preferably 0.1 MPa or more and 20 MPa or less, similar to the electrode catalyst layer 10 according to the first embodiment. A pressure of 20 MPa or less prevents the electrode catalyst layer 101 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 101 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 101 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 101.

[0095] As in the first embodiment, the transfer substrate may be, for example, a polymer film or a sheet formed of 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. A gas diffusion layer may also be used as the transfer substrate.

[0096] As described above, the fibrous material 25 in the electrode catalyst layer 101 is formed of one or more types of fibers including at least either electron conductive fibers or proton conductive fibers. The catalytically active electron conductive fiber is more preferable because it can reduce the amount of catalyst formed from a precious metal used. When the electrode catalyst layer 101 is used as the electrode catalyst layer 101C that constitutes the oxygen electrode, the catalytically active electron conductive fiber can be a carbon alloy catalyst made from carbon nanofibers.

[0097] The catalytically active electron conductive fiber may be a fiber obtained by processing an electrode active material into a fibrous form. The electrode active material may be a material containing at least one transition metal element selected from the group consisting of Ta, Nb, Ti, and Zr. Examples of the transition metal element-containing material contained in the electron conductive fiber include partial oxides of carbonitrides of transition metal elements, conductive oxides of transition metal elements, and conductive oxynitrides of transition metal elements. The electron conductive fiber in the electrode catalyst layer 101 according to this embodiment may contain at least one of these three types of materials as the transition metal element-containing material. In other words, the electron conductive fiber may be a fiber containing one or more materials selected from these three types of materials.

[0098] Here, by adjusting the blending ratio of the fibrous material 25, the blending 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 electrode catalyst layer 101 can be made to have sufficient gas diffusivity and proton conductivity. For example, the blending ratio of the polymer electrolyte 14 in the electrode catalyst layer 101 is preferably about the same as or about half the weight of the conductive support 13. The blending ratio of the fibrous material 25 is preferably about the same as or less than the weight of the conductive support 13. The solids ratio of the catalyst ink is preferably as high as possible within the range that allows it to be applied to a thin film.

[0099] [Structure of polymer electrolyte fuel cells] Next, a specific example of the configuration of a polymer electrolyte fuel cell 30 including the membrane electrode assembly 100 according to this embodiment will be described with reference to Fig. 4. Note that Fig. 4 shows an example of the configuration of a single cell, and the polymer electrolyte fuel cell 30 is not limited to this configuration, and may have a configuration in which a plurality of single cells are stacked.

[0100] 4, a polymer electrolyte fuel cell 30 includes a membrane electrode assembly 100, a gas diffusion layer 17C on the oxygen electrode side, and a gas diffusion layer 17A on the fuel electrode side. The gas diffusion layer 17C is disposed opposite an electrode catalyst layer 101C, which is a cathode-side electrode catalyst layer on the oxygen electrode side of the membrane electrode assembly 100. The gas diffusion layer 17A is disposed opposite an electrode catalyst layer 101A, which is an anode-side electrode catalyst layer on the fuel electrode side of the membrane electrode assembly 100. The electrode catalyst layer 101C and the gas diffusion layer 17C form an oxygen electrode 200C, and the electrode catalyst layer 101A and the gas diffusion layer 17A form a fuel electrode 200A.

[0101] The polymer electrolyte fuel cell 30 further includes a separator 18C disposed opposite the oxygen electrode 200C and a separator 18A disposed opposite the fuel electrode 200A. 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 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 the surface on which the gas channels 19A are formed. The separators 18C and 18A are made of a conductive and gas-impermeable material.

[0102] In the polymer electrolyte fuel cell 30, an oxidant such as air or oxygen is supplied to the oxygen electrode 200C 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 200A through the gas flow path 19A of the separator 18A, thereby generating electricity.

[0103] By adopting the membrane electrode assembly 1 according to this embodiment, the polymer electrolyte fuel cell 30 according to this embodiment has sufficient drainage and gas diffusibility, and is able to exhibit high power generation performance and high durability over the long term. That is, according to this embodiment, it is possible to provide an electrode catalyst layer 101, a membrane electrode assembly 100, and a polymer electrolyte fuel cell 30 that have sufficient gas diffusivity and proton conductivity during operation of the polymer electrolyte fuel cell 30, and that are capable of exhibiting high power generation performance and high durability over the long term. Therefore, the present invention can be suitably used in stationary cogeneration systems, fuel cell vehicles, and the like that utilize polymer electrolyte fuel cells, and has great industrial utility value. Second Example

[0104] Hereinafter, membrane electrode assemblies according to examples based on this embodiment will be described. [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 (20% Nafion® dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and carbon nanofibers (VGCF®-H, manufactured by Showa Denko K.K.) as a second fibrous material were mixed. This mixture was dispersed using a planetary ball mill at 300 rpm for 60 minutes. During this process, zirconia balls with a diameter of 5 mm were added to approximately one-third of the zirconia container. The weight of the polymer electrolyte was 100 wt % relative to the weight of the carbon support in the platinum-supported carbon catalyst, the weight of the carbon nanofibers was 20 wt % relative to the weight of the carbon support in the platinum-supported carbon catalyst, the proportion of water in the dispersion medium was 50 wt %, and the solids concentration was adjusted to 10 wt %.

[0105] The catalyst ink was applied to one side of a polymer electrolyte membrane (Nafion (registered trademark) 211, manufactured by DuPont) using a slit die coater to form a coating film with a thickness of 200 μm. The polymer electrolyte membrane with the coating film formed thereon 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. Next, the catalyst ink was applied to the opposite side of the polymer electrolyte membrane using a slit die coater to form a coating film with a thickness of 50 μm. The polymer electrolyte membrane with the coating film formed thereon 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. This resulted in the production of the membrane electrode assembly of Example 2-1. The proportion of the second fibrous material in the catalyst layer was 6 wt%.

[0106] [Example 2-2] A membrane electrode assembly of Example 2-2 was obtained in the same manner as in Example 2-1, except that when preparing the catalyst ink, electrolyte nanofibers (sulfonated polyphenylene, diameter 250-300 nm) were used as the second fibrous material instead of carbon nanofibers (VGCF (registered trademark)-H, manufactured by Showa Denko K.K.). The proportion of the second fibrous material in the catalyst layer was 6 wt%.

[0107] [Example 2-3] The membrane electrode assembly of Example 2-3 was obtained in the same manner as in Example 2-1, except that the amount of carbon nanofibers as the second fibrous material was double that of Example 2-1 (40 wt % relative to the weight of the carbon support) when preparing the catalyst ink. The proportion of the second fibrous material in the catalyst layer was 12 wt %.

[0108] [Example 2-4] In Example 2-4, a platinum-supported carbon catalyst (TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.), water, 1-propanol, a polymer electrolyte (Nafion® dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), carbon nanofibers (VGCF®-H, manufactured by Showa Denko K.K.), and electrolyte nanofibers (sulfonated polyphenylene, diameter 250-300 nm) were mixed. In other words, the second fibrous material contained carbon nanofibers and electrolyte nanofibers. This mixture was dispersed using a planetary ball mill at 300 rpm for 60 minutes. During this process, zirconia balls with a diameter of 5 mm were added to approximately one-third of the zirconia container. The weight of the polymer electrolyte was 100 wt% of the weight of the carbon particles, and the weights of the carbon nanofibers and electrolyte nanofibers (weight of the second fibrous material) were each 10 wt% of the weight of the carbon particles, for a total of 20 wt%. A catalyst ink was prepared by adjusting the proportion of water in the dispersion medium to 50 wt % and the solid content to 10 wt %. A membrane electrode assembly of Example 2-4 was obtained in the same manner as in Example 2-1, except that this catalyst ink was used to form the cathode-side electrode catalyst layer. The proportion of the second fibrous material in the catalyst layer was 6 wt %.

[0109] [Example 2-5] The membrane electrode assembly of Example 2-5 was obtained in the same manner as in Example 2-2, except that the amount of electrolyte nanofibers as the second fibrous material was half that of Example 2-1 (10 wt % relative to the weight of the carbon support) when preparing the catalyst ink. The proportion of the second fibrous material in the catalyst layer was 3 wt %.

[0110] [Example 2-6] The membrane electrode assembly of Example 2-6 was obtained in the same manner as in Example 2-4, except that when preparing the catalyst ink, the weights of the carbon nanofibers and the electrolyte nanofibers (weight of the second fibrous material) were each 2 wt % relative to the weight of the carbon particles, for a total of 4 wt %. The proportion of the second fibrous material in the catalyst layer was 1 wt %.

[0111] [Example 2-7] A catalyst ink was prepared in the same manner as in Example 2-4. 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 no longer had any tack, thereby obtaining a transfer substrate with a cathode-side electrode catalyst layer. 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.

[0112] A transfer substrate with a cathode-side electrode catalyst layer and a transfer substrate with an anode-side electrode catalyst layer were placed on the front and back surfaces of a polymer electrolyte membrane (Nafion (registered trademark) 211, manufactured by DuPont) so that they faced each other, forming a laminate. Next, the laminate was hot-pressed at 120°C and 1 MPa to bond the electrode catalyst layers to the front and back surfaces of the polymer electrolyte membrane. Finally, the PTFE film was peeled off from each electrode catalyst layer to obtain a membrane electrode assembly of Example 2-7. The proportion of the second fibrous material in the catalyst layer was 6 wt%.

[0113] [Comparative Example 2-1] A membrane electrode assembly of Comparative Example 2-1 was obtained in the same manner as in Example 2-1, except that carbon nanofibers were not added as the second fibrous material when preparing the catalyst ink. The proportion of the second fibrous material in the catalyst layer was 0 wt %.

[0114] [Comparative Example 2-2] A membrane electrode assembly of Comparative Example 2-2 was obtained in the same manner as in Example 2-1, except that the amount of carbon nanofibers as the second fibrous material was three times that of Example 2-1 (60 wt % relative to the weight of the carbon particles) when preparing the catalyst ink. The proportion of the second fibrous material in the catalyst layer was 17 wt %.

[0115] [Comparative Example 2-3] A membrane electrode assembly of Comparative Example 2-3 was obtained in the same manner as in Example 2-4, except that when preparing the catalyst ink, the weights of the carbon nanofibers and electrolyte nanofibers (weight of the second fibrous material) were each 40 wt % relative to the weight of the carbon particles, for a total of 80 wt %. The proportion of the second fibrous material in the catalyst layer was 21 wt %.

[0116] [evaluation] The power generation performance and durability of each of the polymer electrolyte fuel cells provided with the membrane electrode assemblies of Examples 2-1 to 2-7 and the membrane electrode assemblies of Comparative Examples 2-1 to 2-3 were evaluated.

[0117] (Evaluation of power generation performance) Measurements to evaluate power generation performance were performed using a method that conforms to the "Cell Evaluation and Analysis Protocol," a publication by the New Energy and Industrial Technology Development Organization (NEDO). Specifically, a JARI standard cell was used as the evaluation unit cell, in which gas diffusion layers, gaskets, and separators were placed on both sides of the membrane electrode assembly and clamped to a specified surface pressure. IV measurements were then performed as described in the "Cell Evaluation and Analysis Protocol." The conditions used were set as standard conditions. In addition, the IV measurement was carried out with the relative humidity of the anode and the relative humidity of the cathode set to RH 100%, which was set as a high humidity condition. Regarding the evaluation of power generation performance, a rating of "○" was given if the current was 25 A or more when the voltage was 0.6 V under standard conditions, and 30 A or more when the voltage was 0.6 V under high humidity conditions, and an rating of "×" was given if the current values ​​were less than the above.

[0118] (Durability evaluation) The durability was evaluated by using the same evaluation cell as that used in the measurement for evaluating the power generation performance, and the humidity cycle test described in the above-mentioned "Cell Evaluation and Analysis Protocol" was carried out. In the evaluation of durability, a hydrogen cross leakage current after 8000 cycles that was less than 10 times the initial value was marked as "good", and a hydrogen cross leakage current that was 10 times or more the initial value was marked as "poor".

[0119] [Evaluation results] Table 2 shows the weight ratio of the fibrous material in the cathode-side electrode catalyst layer of fuel cells equipped with the membrane electrode assemblies of Examples 2-1 to 2-7 and the membrane electrode assemblies of Comparative Examples 1 to 3, as well as the evaluation results of power generation performance and durability. In Table 2, "fibrous material content" refers to "the content of the second fibrous material in the catalyst layer."

[0120] [Table 2]

[0121] As shown in Table 2, in all of Examples 2-1 to 2-7, the content of the second fibrous material in the electrode catalyst layer was 1% by weight or more and 15% by weight or less. The results for power generation performance and durability were all "Good." That is, in Examples 2-1 to 2-7, membrane electrode assemblies capable of constituting fuel cells with excellent power generation performance and durability were obtained.

[0122] On the other hand, in the comparative examples 2-1 to 2-3, the content of the second fibrous material in the electrode catalyst layer was outside the range of 1% by weight or more and 15% by weight or less. At least one of the power generation performance and durability was evaluated as "X." That is, when the content of the second fibrous material in the electrode catalyst layer was outside the above range, at least one of the power generation performance and durability was reduced.

[0123] The content of the proton-conductive fiber containing the electrolyte was 6 wt % in Example 2-2, which had good power generation performance and durability, 3 wt % in Examples 2-4, 2-5, and 2-7, and 1 wt % in Example 2-6. On the other hand, the content of the proton-conductive fiber in Comparative Example 2-3, which did not have good power generation performance, was 11%. Therefore, it was found that when the content of the proton-conductive fiber is 1 wt % or more and 10 wt % or less, a membrane electrode assembly capable of constituting a fuel cell with even better power generation performance can be obtained. [Explanation of symbols]

[0124] 1, 100...Membrane electrode assembly 2C, 200C...Oxygen electrode 2A, 200A…Fuel electrode 3, 30...Polymer fuel cell 4...Void 10, 10C, 10A, 101, 101C, 101A...electrode catalyst layer 11...Polymer electrolyte membrane 12...catalyst material 13...Conductive carrier 14...polymer electrolyte 15, 25...fibrous materials 16C, 16A...Gasket 17C, 17A...Gas diffusion layer 18C, 18A...Separator 19C, 19A...Gas flow path 20C, 20A...Cooling water flow path

Claims

1. An electrode catalyst layer used in a polymer electrolyte fuel cell, The catalyst includes a catalytic material, a conductive support that supports the catalytic material, a polymer electrolyte, and a fibrous material, the fibrous material contains a Lewis base having an unshared electron pair, The content of the fibrous material in the electrode catalyst layer is 1% by weight or more and less than 12% by weight. An electrode catalyst layer characterized by:

2. The average fiber diameter of the fibrous material is 50 nm or more and 400 nm or less. The electrode catalyst layer according to claim 1 .

3. The average fiber length of the fibrous material is 1 μm or more and 100 μm or less.

3. The electrode catalyst layer according to claim 1 or 2.

4. The fibrous material has nitrogen atoms. The electrode catalyst layer according to any one of claims 1 to 3.

5. The fibrous material includes at least one of an electron conductive fiber and a proton conductive fiber.

5. The electrode catalyst layer according to claim 1, wherein the electrode catalyst layer is a metal oxide.

6. The fibrous material includes the proton-conducting fiber. The electrode catalyst layer according to claim 5 .

7. The content of the proton conductive fiber in the electrode catalyst layer is 1% by weight or more and 10% by weight or less. The electrode catalyst layer according to claim 6 .

8. The electron conductive fiber is a carbon nanofiber. The electrode catalyst layer according to any one of claims 5 to 7,

9. The electron conductive fiber contains at least one 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. The electrode catalyst layer according to any one of claims 5 to 8,

10. The electrode catalyst layer according to any one of claims 1 to 9 is provided on at least one surface of a polymer electrolyte membrane. A membrane electrode assembly characterized by:

11. A device comprising the membrane electrode assembly according to claim 10. A polymer electrolyte fuel cell characterized by:

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