fuel cells

Abstract

To enhance durability of a fuel battery.SOLUTION: A solid polymer type fuel battery (10) onto which a catalyst layer (21) is laminated onto both sides of an electrolyte film (1), comprises a microporous layer (22) onto the catalyst layer (21). The microporous layer (22) contains a conductive metal component.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a fuel cell. [Background technology]

[0002] Fuel cells generally have a laminated structure with an anode and a cathode on either side of an electrolyte membrane, each containing a catalyst layer made of platinum catalyst supported on support particles such as carbon particles.

[0003] It is known that the material of the catalyst layer deteriorates depending on the operating conditions of the fuel cell. For example, at the cathode, which is at a high potential when the fuel cell is started or stopped, an oxidation reaction between carbon and water progresses, which makes it easy for the carbon in the catalyst layer to corrode. To improve the durability of the cathode, the use of support particles made of metal oxide instead of carbon has been proposed (see, for example, Patent Document 1).

[0004] At the anode, if the supply of hydrogen becomes insufficient due to a sudden output fluctuation or the like, the oxidation reaction described above may occur, causing corrosion of the carbon. Therefore, the use of water electrolysis catalyst particles such as iridium oxide as a catalyst has been proposed (see, for example, Patent Document 2). In the presence of water electrolysis catalyst particles, water is easily electrolyzed and is less likely to react with carbon, thereby suppressing carbon corrosion. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-157353 [Patent Document 2] Japanese Patent Publication No. 2020-47432 Summary of the Invention [Problem to be solved by the invention]

[0006] However, carbon is often used not only in the catalyst layer but also in the microporous layers or gas diffusion layers on both sides of the catalyst layer. Therefore, suppressing corrosion of the carbon in the catalyst layer does not suppress corrosion of the carbon in the microporous layers or gas diffusion layers. In addition, materials such as iridium are expensive, which increases costs.

[0007] An object of the present invention is to improve the durability of a fuel cell. [Means for solving the problem]

[0008] One embodiment of the present invention is a polymer electrolyte fuel cell (10) having catalyst layers (21) laminated on both sides of an electrolyte membrane (1). The fuel cell (10) includes a microporous layer (22) on the catalyst layer (21). The microporous layer (22) contains a conductive metal compound. [Effects of the Invention]

[0009] According to the present invention, the durability of the fuel cell can be improved. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view showing the cell structure of the fuel cell of this embodiment. [Figure 2] FIG. 2 is a cross-sectional view showing a gas diffusion layer with a microporous layer. [Figure 3] FIG. 3 is a cross-sectional view showing a microporous layer provided alone in place of a gas diffusion layer. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of a fuel cell according to the present invention will be described with reference to the drawings. The configuration described below is one example (typical example) of the present invention, and the present invention is not limited to this configuration.

[0012] (fuel cell) FIG. 1 shows the cell structure of a fuel cell 10 of this embodiment. The fuel cell 10 may be a single cell having such a structure, or may be a stack of multiple cells. The fuel cell 10 of this embodiment is a polymer electrolyte fuel cell. The fuel cell 10 is mounted on a mobile object such as a vehicle, and supplies driving power to the mobile object by generating electricity through a chemical reaction of fuel gas. However, the present invention is not limited to mobile objects and can also be applied to fuel cells in stationary power generation systems and the like.

[0013] 1, a fuel cell 10 includes a membrane electrode assembly (MEA) 3, a pair of separators 4 arranged on either side of the MEA 3, and a subgasket 5 surrounding the outer periphery of the MEA 3. The MEA 3 is a laminate in which electrodes 2 are arranged on either side of an electrolyte membrane 1.

[0014] (electrolyte membrane) The electrolyte membrane 1 is an ion-conductive polymer electrolyte membrane. Examples of polymer electrolytes that can be used for the electrolyte membrane 1 include perfluorosulfonic acid polymers such as Nafion (registered trademark) and Aquivion (registered trademark), aromatic polymers such as sulfonated polyether ether ketone (SPEEK) and sulfonated polyimide, and aliphatic polymers such as polyvinyl sulfonic acid and polyvinyl phosphoric acid.

[0015] From the viewpoint of improving durability, the electrolyte membrane 1 can be a composite membrane in which a porous substrate 1a is impregnated with a polymer electrolyte. The porous substrate 1a is not particularly limited as long as it has pores capable of supporting the polymer electrolyte, and a membrane in a porous, woven, nonwoven, fibril, or other form can be used. The material of the porous substrate 1a is also not particularly limited, but from the viewpoint of improving ion conductivity, the above-mentioned polymer electrolytes can be used. Among them, fluorine-based polymers such as polytetrafluoroethylene, polytetrafluoroethylene-chlorotrifluoroethylene copolymer, and polychlorotrifluoroethylene have excellent strength and shape stability.

[0016] Of the pair of electrodes 2, one electrode 2 is the anode, also called the fuel electrode, and the other electrode 2 is the cathode, also called the air electrode. Hydrogen gas is supplied to the anode as the fuel gas, and air containing oxygen gas is supplied to the cathode.

[0017] At the anode, electrons (e - ) and protons (H + ) occurs. The electrons move to the cathode via an external circuit (not shown). This electron movement generates a current in the external circuit. The protons move to the cathode via the electrolyte membrane 1. (1) 2H2→4H + +4e -

[0018] At the cathode, electrons transferred from the external circuit convert oxygen gas (O2) into oxygen ions (O2 - ) is generated. The oxygen ions are transferred to the protons (2H + ) to form water (H2O). (2) O2+4H + +4e - →2H2O

[0019] When the fuel cell 10 is started or stopped, if the cathode reaches a high potential, an oxidation reaction shown in the following reaction formula (3) occurs, which may accelerate corrosion of the carbon used in the cathode. A similar oxidation reaction may also occur in the anode, causing corrosion of the carbon, if the supply of hydrogen becomes insufficient due to a sudden output fluctuation or the like. (3) C+2H2O→CO2+4H + +4e -

[0020] In this embodiment, the electrode 2 includes a catalyst layer 21, a microporous layer 22, and a gas diffusion layer 23. On both sides of the electrolyte membrane 1, the catalyst layer 21, the microporous layer 22, and the gas diffusion layer 23 are laminated in this order.

[0021] The catalyst layer 21 promotes the reaction between hydrogen gas and oxygen gas by the catalyst. The catalyst layer 21 includes a catalyst, a carrier that supports the catalyst, and an ionomer that coats the carrier and the carrier. The catalyst layer 21 may further contain water electrolysis catalyst particles, and it is particularly preferable that the anode-side catalyst layer 21 contain such catalyst particles.

[0022] Examples of catalysts include particles of metals such as platinum (Pt), ruthenium (Ru), iridium (Ir), rhodium (Rh), palladium (Pd), and tungsten (W), as well as mixtures and alloys of these metals. Among these, platinum and mixtures or alloys containing platinum are preferred from the viewpoints of catalytic activity, resistance to carbon monoxide poisoning, heat resistance, and the like.

[0023] Examples of the support include porous particles of conductive metal compounds such as mesoporous carbon and Pt black. Among these, mesoporous carbon is preferred because it has good dispersibility, a large surface area, and little particle growth at high temperatures even when a large amount of catalyst is supported. As the ionomer, an ion-conductive polymer electrolyte similar to that of the electrolyte membrane 1 can be used.

[0024] Examples of water electrolysis catalyst particles include particles of iridium, ruthenium, rhenium, palladium, rhodium, or oxides of these. These particles promote the electrolysis of water, thereby suppressing the oxidation reaction between carbon and water in the catalyst layer 21 when hydrogen is insufficient, thereby improving the durability of the catalyst layer 21.

[0025] The content of the water electrolytic catalyst particles in the catalyst layer is preferably 0.2% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1.0% by mass or more, from the viewpoint of suppressing carbon corrosion. From the viewpoint of avoiding excessive water electrolysis, the content is preferably 5.0% by mass or less, more preferably 3.0% by mass or less, and even more preferably 2.0% by mass or less.

[0026] (microporous layer) The microporous layer 22 is porous and can facilitate the flow of fuel gas (air containing hydrogen gas and oxygen) from the gas diffusion layer 23 to the catalyst layer 21, or the flow of wastewater from the catalyst layer 21 to the gas diffusion layer 23.

[0027] The catalyst layer 21, the microporous layer 22, and the gas diffusion layer 23 are porous, but the pore size in each layer is usually smaller in the microporous layer 22 than in the gas diffusion layer 23, and smaller in the catalyst layer 21 than in the microporous layer 22. This gradient tends to facilitate the flow of gas or water.

[0028] (gas diffusion layer) The gas diffusion layer 23 can diffuse the fuel gas supplied to the fuel cell 10 uniformly over the entire surface of the catalyst layer 21 . The gas diffusion layer 23 includes a conductive porous substrate. Examples of the porous substrate include a fiber sheet such as carbon felt or a carbon fiber sheet, a foam sheet such as a foam metal sheet, or a mesh sheet such as an expanded metal sheet.

[0029] The subgasket 5 is a film or plate provided on the outer peripheral edge of the MEA 3. The subgasket 5 protects the edge of the electrolyte membrane 1 and functions as a support for the MEA 3. A resin with low electrical conductivity can be used as the material for the subgasket 5. There are no particular limitations on the resin material, and examples include polyphenylene sulfide (PPS), glass-filled polypropylene (PP-G), polystyrene (PS), silicone resin, and fluorine-based resin.

[0030] The separator 4 is also called a bipolar plate. The separator 4 is made of a conductive material such as carbon or stainless steel.

[0031] The separator 4 of this embodiment has a surface on which recesses 4a are formed. When the surface of the separator 4 on which the recesses 4a are formed faces the MEA 3, a flow path is formed between the separator 4 and the MEA 3. The flow path not only serves as a supply path for fuel gas but also as a discharge path for water produced by chemical reactions during power generation.

[0032] (Microporous layer material) In this embodiment, the microporous layer 22 is a layer of a composition containing a conductive metal compound. The use of such a non-carbon metal compound reduces the amount of carbon in the microporous layer 22 and reduces the carbon corrosion caused by the oxidation reaction under high potential or hydrogen gas deficiency. This suppresses deterioration of the microporous layer 22 and improves the durability of the electrode 2.

[0033] From the viewpoint of further reducing corrosion, it is preferable that the content of the conductive metal compound in the composition is high, and it is more preferable that the composition does not contain carbon but instead contains the conductive metal compound.

[0034] The microporous layer 22 can be formed by preparing a composition containing a conductive metal compound, forming a porous film of the composition, or coating the composition on a porous substrate and then drying it. The composition can contain a binder, a solvent, etc. in addition to the conductive metal compound.

[0035] (Conductive metal compounds) Examples of the conductive metal compound include conductive metal oxides, nitrides, and oxynitrides, etc. These may be used alone or in combination of two or more.

[0036] Among these, from the viewpoints of electrical conductivity or cost, metal compounds of transition metals are preferred, and among metal compounds, metal oxides are more preferred. As the transition metal, from the viewpoints of electrical conductivity or cost, Group 4 elements such as titanium or Group 14 elements such as tin are preferred, and titanium or tin is more preferred. Specific examples of metal compounds include titanium oxide, titanium nitride, tin oxide, palladium oxide, vanadium oxide, and tantalum nitride.

[0037] The metal compound may be doped with a metal element other than the transition metal. Doping generates holes or electrons, which tends to improve the conductivity of the metal compound. Metal elements that can be used as dopants include, for example, rare earth elements such as yttrium, Group 5 elements such as niobium or tantalum, Group 6 elements such as tungsten, and Group 15 elements such as antimony. From the viewpoint of conductivity or cost, the dopant is preferably niobium or tantalum. Specific examples of doped compounds include tantalum-doped tin oxide, which is tin oxide doped with tantalum.

[0038] From the viewpoint of durability, the content of the conductive metal compound in the composition is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The upper limit of the content may be 100% by mass, but is usually less than 100% by mass due to the inclusion of other components such as binders.

[0039] (binder) Although the binder is not particularly limited, an ionomer having high proton conductivity is preferable from the viewpoint of electrical conductivity. The ionomer may be the same compound as the ionomer in the catalyst layer 21 described above, but a water-repellent fluorine-based resin is preferable from the viewpoint of drainage.

[0040] As the solvent, water, ethanol, etc. can be appropriately used. The viscosity of the composition can be adjusted by the solvent, and the composition can be prepared in a solid, paste, ink, or other form.

[0041] (Microporous layer structure) 2, the microporous layer 22 in this embodiment is a layer of the above composition laminated on a part of the surface of the porous substrate of the gas diffusion layer 23. For example, the surface of the porous substrate may be coated by, for example, immersing a part of the porous substrate of the gas diffusion layer 23 in the above composition, and then drying the composition to laminate the layer of the above composition on the surface of the porous substrate.

[0042] (materials for other layers) From the viewpoint of further enhancing the durability of the electrode 2, it is preferable that the porous substrate of the gas diffusion layer 23 also be made of the above-mentioned composition. The porous substrate made of the above-mentioned composition can be produced by forming a fiber sheet, a foam sheet, a mesh sheet, or the like using the above-mentioned composition.

[0043] The preferred content of the conductive metal compound in the composition of the gas diffusion layer 23 is the same as that of the microporous layer 22. From the viewpoint of reducing carbon corrosion, it is preferable that the content of the conductive metal compound in each layer is high, and it is even more preferable that the carbon material in each layer is replaced with the conductive metal compound so that the electrode 2 does not contain carbon. The conductive metal compounds used in each layer may be the same or different.

[0044] As described above, according to this embodiment, a layer of a composition containing a conductive metal compound is provided as the microporous layer 22 on the catalyst layer 21. Because the metal compound is not carbon, it is possible to reduce carbon that corrodes under high potential or when hydrogen is deficient. This can suppress deterioration of the microporous layer 22 and improve the durability of the electrode 2.

[0045] By using the above-mentioned conductive metal compound not only in the microporous layer 22 but also in the gas diffusion layer 23, it is possible to improve the durability of the entire electrode 2. It is also possible to reduce the resistance that occurs at the interface between the microporous layer 22 and the gas diffusion layer 23 due to corrosion of the gas diffusion layer 23.

[0046] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments and various modifications and changes are possible.

[0047] For example, the microporous layer 22 may be a porous substrate made of a composition containing the above-mentioned conductive metal compound. In this case, the microporous layer 22 can be formed without the gas diffusion layer 23, so the gas diffusion layer 23 may be replaced with the microporous layer 22. Figure 3 shows an example of a microporous layer 22 disposed alone in place of the gas diffusion layer 23. In this case, the microporous layer 22 contacts the separator 4.

[0048] The microporous layer 22 disposed alone may be a porous substrate made of the above-mentioned composition, or may be a laminate in which a layer of the above-mentioned composition is laminated on the surface of a porous substrate made of another material such as carbon or metal. From the viewpoint of durability, the microporous layer 22 is preferably a porous substrate made of a composition of a conductive metal compound. [Explanation of symbols]

[0049] 10...Fuel cell, 1...Electrolyte membrane, 2...Electrode, 21...Catalyst layer, 22...Microporous layer, 23...Gas diffusion layer, 3...Membrane electrode assembly, 4...Separator, 5...Subgasket

Claims

1. In a solid polymer fuel cell (10) in which catalyst layers (21) are laminated on both sides of an electrolyte membrane (1), A microporous layer (22) is provided on the catalyst layer (21), The microporous layer (22) contains a conductive metal compound. Fuel cell (10).

2. A gas diffusion layer (23) is provided on the microporous layer (22), The microporous layer (22) is a layer of a composition containing the conductive metal compound laminated on the surface of the gas diffusion layer (23). The fuel cell (10) of claim 1.

3. The gas diffusion layer (23) comprises a porous substrate, The porous substrate is made of a composition containing a conductive metal compound. The fuel cell (10) of claim 2.

4. The microporous layer (22) is a porous substrate made of a composition containing the conductive metal compound, or a laminate in which a layer of the composition is laminated on the surface of a porous substrate. The fuel cell (10) of claim 1.

5. The metal compound is a transition metal compound. A fuel cell (10) according to any one of claims 1 to 4.

6. The metal compound is doped with a metal element different from the transition metal. The fuel cell (10) of claim 5.

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