Electrode catalyst layer and membrane electrode assembly

Abstract

To provide an electrode catalyst layer and a membrane electrode assembly capable of achieving high mechanical strength and excellent electrochemical performance.SOLUTION: The electrode catalyst layer comprises catalyst-supported particles, a polymer electrolyte, and exfoliated graphite having a coating layer. The coating layer of the exfoliated graphite contains a polymer including a Lewis basic functional group.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to an electrode catalyst layer and a membrane electrode assembly suitable for an electrolysis cell for water electrolysis and organic hydride synthesis. [Background technology]

[0002] In recent years, there has been an accelerating movement to use hydrogen as a primary energy source, as a CO2-free energy source that can be produced from a variety of resources, in order to achieve carbon neutrality. Water electrolysis using renewable energy is seen as a promising method for producing such hydrogen. Alkaline water electrolysis and polymer electrolyte membrane (PEM) water electrolysis are commonly known as water electrolysis methods. While alkaline water electrolysis is easy to scale up, it is less adaptable to fluctuations. PEM water electrolysis, on the other hand, is more adaptable to fluctuations and has a high affinity with renewable energy, and has therefore attracted attention in recent years.

[0003] An electrolytic cell generally includes a pair of main electrodes and a membrane electrode assembly provided between the pair of main electrodes. In the case of water electrolysis, for example, the membrane electrode assembly includes a solid polymer electrolyte membrane that conducts protons and hydroxide ions, an anode-side electrode catalyst layer provided on one side of the electrolyte membrane, and an anode-side electrode catalyst layer provided on the opposite side of the electrolyte membrane.

[0004] When external electric power is applied to the anode-side electrode catalyst layer and the cathode-side electrode catalyst layer, the electrolysis reaction of water proceeds on the catalyst in each electrode catalyst layer, producing oxygen at the anode and hydrogen at the cathode.

[0005] The membrane electrode assembly is obtained by forming an electrode catalyst layer on one surface of a solid polymer electrolyte membrane, for example, by a coating method (see, for example, Patent Document 1 below). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2017 / 159820 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the membrane electrode assembly described in Patent Document 1 may suffer from cracks in the electrode catalyst layer, and there is room for improvement in terms of preventing cracks from occurring. The occurrence of cracks may inhibit the transfer of hydroxide ions and electrons to the catalyst, resulting in a decrease in water electrolysis performance.

[0008] The present disclosure has been made in view of the above-mentioned problems, and aims to provide an electrode catalyst layer and a membrane electrode assembly that can achieve high mechanical strength and high electrolysis performance. [Means for solving the problem]

[0009] [1] An electrode catalyst layer comprising catalyst-supporting particles, a polymer electrolyte, and exfoliated graphite having a coating layer, the coating layer containing a polymer having a Lewis basic functional group. [2] The electrode catalyst layer according to [1], wherein the polymer of the coating layer has an imide structure or an azole structure. [3] The electrode catalyst layer according to [1], wherein the polymer of the coating layer has a benzazole structure. [4] The electrode catalyst layer according to [1], wherein the polymer compound of the coating layer is polybenzimidazole. [5] The electrode catalyst layer according to any one of [1] to [4], wherein the arithmetic mean value of the aspect ratio L / D (where L is the major axis of the exfoliated graphite and D is the minor axis of the exfoliated graphite) of the exfoliated graphite is 10 to 200. [6] The electrode catalyst layer according to [5], wherein the arithmetic mean value of the major axis L is within a range of 1 μm or more and 20 μm or less. [7] The electrode catalyst layer according to any one of [1] to [6], which is for an electrolysis cell. [8] The electrode catalyst layer according to any one of [1] to [6], which is for use as a cathode in an electrolysis cell. [9] A membrane electrode assembly comprising a polymer electrolyte membrane, and an anode-side electrode catalyst layer and a cathode-side electrode catalyst layer sandwiching the polymer electrolyte membrane, wherein the cathode-side electrode catalyst layer is the electrode catalyst layer according to any one of [1] to [8]. [Effects of the Invention]

[0010] The electrode catalyst layer and membrane electrode assembly according to one embodiment of the present invention can achieve both high mechanical strength and high electrolysis performance. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is an exploded perspective view schematically showing a membrane electrode assembly having an electrode catalyst layer for an electrolysis cell according to one embodiment of the present invention. [Figure 2] FIG. 1 is an exploded perspective view schematically illustrating an electrolysis cell according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] <Embodiment> Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. The drawings are schematic, and the relationship between thickness and planar dimensions, the thickness ratio of each layer, etc., differ from the actual ones. Furthermore, the embodiments shown below are merely examples of configurations for embodying the technical idea of ​​the present invention, and the technical idea of ​​the present invention is not limited to the materials, shapes, structures, etc. of the components described below. The technical idea of ​​the present invention can be modified in various ways within the technical scope defined by the claims.

[0013] [Membrane electrode assembly] As shown in FIG. 1, the membrane electrode assembly 11 of this embodiment includes a polymer electrolyte membrane 1, an anode-side electrode catalyst layer 2 (shown on the upper side in FIG. 1) that sandwiches the polymer electrolyte membrane 1 from above and below, and a cathode-side electrode catalyst layer 3 (shown on the lower side in FIG. 1).

[0014] (Polymer electrolyte membrane 1) The polymer electrolyte membrane 1 may be any membrane as long as it has proton conductivity. Examples of the proton-conductive polymer electrolyte membrane that can be used include a fluorine-based polymer electrolyte membrane and a hydrocarbon-based polymer electrolyte membrane. Examples of the fluorine-based polymer electrolyte membrane 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 Gore Select (registered trademark) manufactured by Gore. Examples of the hydrocarbon-based polymer electrolyte membrane that can be used include electrolyte membranes made of sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene. In particular, it is preferable to use a Nafion (registered trademark)-based material manufactured by DuPont as the proton-conductive polymer electrolyte membrane 1.

[0015] (electrode catalyst layer) The anode-side electrode catalyst layer 2 and the cathode-side electrode catalyst layer 3 each contain catalyst-supporting particles and a polymer electrolyte.

[0016] (Catalyst-supported particles) The catalyst-supporting particles include a catalyst and a conductive carrier on which the catalyst is supported.

[0017] As the catalyst, platinum group elements, metals, alloys, oxides, double oxides, etc. of these metals can be used. Examples of platinum group elements include platinum, palladium, ruthenium, iridium, rhodium, and osmium, and examples of metals include iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum. Note that the double oxide referred to here refers to an oxide made of two types of metals.

[0018] When the catalyst is one or more metals selected from platinum, gold, palladium, rhodium, ruthenium, and iridium, the electrode reactivity is excellent and the electrode reaction can be carried out efficiently and stably.When the catalyst is one or more metals selected from platinum, gold, palladium, rhodium, ruthenium, and iridium, the polymer electrolyte fuel cell 12 equipped with the anode-side electrode catalyst layer 2 and the cathode-side electrode catalyst layer 3 preferably exhibits high power generation performance.

[0019] An example of an electron-conductive support for supporting a catalyst is carbon particles. The type of carbon particles is not limited as long as they are fine particles, conductive, and not affected by the catalyst. Examples of carbon particles that can be used include carbon black, graphite, activated carbon, carbon fiber, carbon nanotubes, and fullerenes.

[0020] The arithmetic mean particle diameter of the carbon particles is preferably in the range of 10 nm to 1000 nm, more preferably in the range of 10 nm to 100 nm. Here, the arithmetic mean particle diameter is the arithmetic mean particle diameter determined from an SEM image. When the arithmetic mean particle diameter of the carbon particles is in the range of 10 nm to 1000 nm, the activity and stability of the catalyst particles are improved, which is preferable. Furthermore, when the arithmetic mean particle diameter of the carbon particles is in the range of 10 nm to 1000 nm, an electron conduction path is easily formed, and the gas diffusion properties and catalyst utilization rate of the anode-side electrode catalyst layer 2 and the cathode-side electrode catalyst layer 3 are improved, which is preferable.

[0021] (Polymer electrolyte) The polymer electrolyte may be any material as long as it has proton conductivity. The same materials as those used for the polymer electrolyte membrane 1 can be used for the polymer electrolyte. For example, a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte can be used as the proton-conductive polymer electrolyte. For example, a Nafion®-based material manufactured by DuPont can be used as the fluorine-based polymer electrolyte. Furthermore, for example, electrolytes such as sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene can be used as the hydrocarbon-based polymer electrolyte. In particular, it is preferable to use a Nafion®-based material manufactured by DuPont as the proton-conductive fluorine-based polymer electrolyte.

[0022] (Improved electrode catalyst layer) At least one of the pair of anode-side electrode catalyst layer 2 and cathode-side electrode catalyst layer 3 further contains exfoliated graphite having a coating layer in addition to catalyst-supporting particles and a polymer electrolyte. The coating layer is provided on the surface of the exfoliated graphite. Hereinafter, exfoliated graphite having a coating layer will also be referred to as "coated exfoliated graphite." Hereinafter, an electrode catalyst layer containing coated exfoliated graphite will also be referred to as "improved electrode catalyst layer." It is preferable that both of the pair of anode-side electrode catalyst layer 2 and cathode-side electrode catalyst layer 3 are improved electrode catalyst layers.

[0023] (exfoliated graphite) Exfoliated graphite refers to a sheet-like laminate in which 1 to 2,000 graphene layers are stacked. The number of stacked layers is preferably 2 or more, more preferably 5 or more, and preferably 1,000 or less, more preferably 500 or less. Because exfoliated graphite is a sheet-like material, it has high shape anisotropy and can have an extremely large specific surface area. Exfoliated graphite is commercially available.

[0024] Because exfoliated graphite is in sheet form, when an electrode catalyst layer is formed using it, the exfoliated graphite tends to be oriented so that the widest surface of the exfoliated graphite is nearly parallel to the main surface of the bridge catalyst layer. When a cross section of such a catalyst layer along the thickness direction is observed with an SEM, the exfoliated graphite is observed as elongated regions.

[0025] Although there are no particular limitations on the arithmetic mean value of the aspect ratio L / D of exfoliated graphite, the arithmetic mean value of the aspect ratio L / D may be 4 or more and 300 or less. The arithmetic mean value of the aspect ratio L / D is preferably within the range of 10 or more and 200 or less. The arithmetic mean value of the aspect ratio may be 15 or more and 150 or less.

[0026] In this specification, the "aspect ratio" of exfoliated graphite refers to the ratio (=L / D) of the long diameter L to the short diameter D of exfoliated graphite appearing in a cross section along the thickness direction of the electrode catalyst layer. Methods for exposing the cross section of the electrode catalyst layer include known methods such as ion milling and ultramicrotome. Methods for observing cross-sectional images include SEM and TEM.

[0027] The "major axis L" refers to the length of the straight line (longest straight line) connecting the two most distant points on the outline of exfoliated graphite in the cross-sectional image. The "minor diameter D" refers to the length of the straight line (shortest line) connecting the two closest points on the outline of exfoliated graphite in a cross-sectional image. The "arithmetic mean value of the aspect ratio" refers to the arithmetic mean value of the aspect ratio (= L / D) measured for 20 or more exfoliated graphite particles appearing in the cross section of the electrode catalyst layer.

[0028] When the aspect ratio is such, the occurrence of cracks in the electrode catalyst layer is more likely to be suppressed. Furthermore, the adhesion between the polymer electrolyte membrane and the electrode catalyst layer can be further improved. Therefore, the occurrence of voids due to peeling between the polymer electrolyte membrane and the electrode catalyst layer can be suppressed, and an increase in the resistance of the membrane electrode assembly due to these voids can be further suppressed. It is estimated that when the aspect ratio of exfoliated graphite is small, it may be difficult to improve the mechanical properties. Furthermore, when the aspect ratio of exfoliated graphite is too large, it may not be able to be dispersed as an ink.

[0029] The arithmetic mean value of the major axis L of the exfoliated graphite is not particularly limited, but is preferably 1 to 20 μm, and more preferably 5 to 15 μm. In this case, pores of an appropriate size can be formed in the electrode catalyst layer, and the mechanical properties of the electrode catalyst layer can be improved. The arithmetic mean value of the minor axis D of the exfoliated graphite is not particularly limited, but is preferably 80 to 500 nm, and more preferably 100 to 350 nm.

[0030] The arithmetic mean values ​​of the major axis L and minor axis D of the exfoliated graphite refer to the arithmetic mean values ​​of the major axis and minor axis of at least 20 pieces of exfoliated graphite in the above cross section.

[0031] (covering layer) The coating layer contains a polymer having Lewis basic functional groups in its molecular structure. Because the polymer electrolyte is adsorbed onto the polymer (coating layer) having Lewis basic functional groups, the occurrence of cracks in the electrode catalyst layer is suppressed, and the mechanical properties of the electrode catalyst layer are improved, resulting in improved durability. In addition to the improved mechanical properties, the adsorption of the polymer electrolyte onto the coated exfoliated graphite forms a proton conduction path, resulting in high electrolysis performance. In other words, rather than directly adsorbing the polymer electrolyte onto the exfoliated graphite, the polymer electrolyte can be more strongly adsorbed onto the exfoliated graphite via the coating layer containing the polymer having Lewis basic functional groups. In particular, when the exfoliated graphite in the cathode electrode catalyst layer has a coating layer containing a polymer having a Lewis basic functional group, the mechanical properties of the electrode catalyst layer are improved and deterioration due to hydrogen generation can be suppressed.

[0032] On the other hand, if a coating layer containing a polymer with a Lewis basic functional group is not used, the mechanical properties of the electrode catalyst layer are insufficient, making it difficult to suppress cracking, and it is presumed that durability cannot be improved. Furthermore, since the polymer electrolyte is dispersed, the formation of proton conduction paths is inhibited, and electrolysis performance cannot be improved.

[0033] Examples of Lewis basic functional groups are imide structures and azole structures. An example of a polymer having an imide structure is a polyimide such as an aromatic polyimide. An example of an azole structure is a benzazole structure, and examples of the benzazole structure are a benzimidazole structure, a benzoxazole structure, and a benzothiazole structure. Examples of these polymers include polybenzimidazole, polybenzoxazole, and polybenzothiazole.

[0034] The coating layer preferably contains a polymer containing a Lewis basic functional group as a main component. The main component means that the polymer accounts for 50% by mass or more. The mass proportion of the Lewis basic functional group in the coating layer may be 70%, 80%, or 90% by mass.

[0035] In particular, it is preferable that the coating layer contains polybenzimidazole, which is a polymer that has affinity for both the polymer electrolyte and exfoliated graphite.

[0036] The polybenzimidazole is not particularly limited as long as it contains a benzimidazole structure in the repeating unit.

[0037] There is no particular limitation on the thickness of the coating layer, but it may be, for example, about 1 to 5 nm.

[0038] The material of the coating layer may contain a polymer containing a Lewis acidic functional group in its molecular structure in addition to a polymer containing a Lewis basic functional group. This may make it easier for the polymer electrolyte to be present in the vicinity of the exfoliated graphite via the coating layer. Examples of the coating layer containing a Lewis acidic functional group include polymers containing a hydroxyl group, a carbonyl group, a sulfonic acid group, and a phosphorous group.

[0039] (Composition ratio of electrode catalyst layer) There are no particular limitations on the composition ratio of the anode-side electrode catalyst layer 2 and the cathode-side electrode catalyst layer 3, but the mass of the conductive support such as the carbon support in the catalyst-supporting particles can be 1, the mass of the polymer electrolyte (solid content) can be 0.2 to 0.8, and the mass of the coated exfoliated graphite can be 0.05 to 0.3.

[0040] (Action and effect) The inventors of the present application have confirmed that the electrode catalyst layer having the above-described structure has high mechanical properties and also exhibits high electrolysis performance. The detailed mechanism is speculated as follows, but the present invention is not limited to the mechanism described below.

[0041] In coated exfoliated graphite, the polymer electrolyte is adsorbed to the polymer having a Lewis basic functional group in the coating layer, thereby improving the mechanical properties of the electrode catalyst layer and improving durability, such as by suppressing the occurrence of cracks in the electrode catalyst layer. In addition to the improved mechanical properties, the adsorption of the polymer electrolyte onto the coated exfoliated graphite forms a proton conduction path, resulting in high electrolysis performance.

[0042] On the other hand, if a polymer having a Lewis basic functional group is not used in the coating layer, the mechanical properties of the electrode catalyst layer are insufficient, making it difficult to suppress the occurrence of cracks and presumably preventing improvement in durability. Furthermore, the dispersion of the polymer electrolyte inhibits the formation of proton conduction paths, presumably preventing improvement in electrolysis performance.

[0043] When both the anode-side electrode catalyst layer 2 and the cathode-side electrode catalyst layer 3 are modified electrode catalyst layers, the adhesion between the coated exfoliated graphite and the polymer electrolyte is improved by adsorption, and proton conduction paths are formed, which suppress the occurrence of cracks in the electrode catalyst layer that cause a decrease in durability, and thus improve durability due to the high mechanical properties.

[0044] [Method for producing improved electrode catalyst layer] Next, an example of a method for producing the improved electrode catalyst layer having the above structure will be described. The improved electrode catalyst layer can be produced by a method including the following first to third steps. The first step is to form a coating layer containing a polymer having a Lewis basic functional group, such as polybenzimidazole, on the surface of exfoliated graphite. The second step is a step of producing a catalyst ink containing catalyst-supported particles, a polymer electrolyte, the coated exfoliated graphite obtained in the first step, and a solvent. The third step is a step of forming an improved electrode catalyst layer by applying the catalyst ink obtained in the second step onto a substrate and drying the solvent. An electrode catalyst layer that is not an improved electrode catalyst layer may also be produced by the same process. Then, the pair of prepared anode-side electrode catalyst layer 2 and cathode-side electrode catalyst layer 3 are attached to the top and bottom surfaces of the polymer electrolyte membrane 1, thereby obtaining a membrane electrode assembly 11.

[0045] [Detailed explanation] (first step) First, a polymer containing a Lewis basic functional group, such as polybenzimidazole, is dissolved or dispersed in a solvent to prepare a polymer dispersion / solution. An example of the solvent is dimethylacetamide. Exfoliated graphite is added to this polymer dispersion / solution, and ultrasonic dispersion treatment is performed as necessary. The solid content is then recovered by filtration and dried to obtain coated exfoliated graphite.

[0046] (Second process) The solvent constituting the catalyst ink does not corrode the catalyst-supported particles, polymer electrolyte, and coated exfoliated graphite, and dissolves the polymer electrolyte or disperses it as a fine gel. Examples of the solvent constituting the catalyst ink include alcohols, ketone-based solvents, ether-based solvents, polar solvents, etc. Examples of alcohols include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, tert-butyl alcohol, pentanol, etc. Examples of ketone-based solvents include acetone, methyl ethyl ketone, pentanone, methyl isobutyl ketone, heptanone, cyclohexanone, methylcyclohexanone, acetonylacetone, and diisobutyl ketone.

[0047] Examples of the ether solvent include tetrahydrofuran, dioxane, diethylene glycol dimethyl ether, anisole, methoxytoluene, dibutyl ether, etc. Examples of the polar solvent include dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, diethylene glycol, diacetone alcohol, 1-methoxy-2-propanol, etc. The solvent may also be a mixed solvent of two or more of the above-mentioned materials.

[0048] Furthermore, since a dispersion medium using a lower alcohol has a high risk of ignition, when a lower alcohol is used as the dispersion medium, it is preferable to use a mixed solvent of a lower alcohol and water. Furthermore, the dispersion medium may contain water that is compatible with the polymer electrolyte, i.e., water that has a high affinity for the polymer electrolyte. The amount of water added to the dispersion medium is not particularly limited as long as it is an amount that does not cause separation of the polymer electrolyte, resulting in cloudiness, or gelation. The catalyst ink may contain a dispersant to disperse the catalyst-supported carbon particles in the catalyst ink.

[0049] The catalyst ink may also be subjected to a dispersion treatment as needed. The viscosity of the catalyst ink and the size of the particles contained in the catalyst ink can be controlled by the conditions of the dispersion treatment of the catalyst ink. The dispersion treatment can be performed using various devices. The method of the dispersion treatment is not particularly limited. Examples of dispersion treatment include treatment using a ball mill or roll mill, treatment using a shear mill, treatment using a wet mill, and ultrasonic dispersion treatment. The dispersion treatment may also be performed using a homogenizer that stirs using centrifugal force. As the dispersion time of the catalyst ink during the dispersion treatment increases, the aggregates of the catalyst-supported particles are broken down, resulting in a smaller pore volume in the electrode catalyst layer formed using the catalyst ink.

[0050] If the solids content of the catalyst ink is too high, the viscosity of the catalyst ink increases, making it more likely for cracks to form on the surfaces of the anode-side electrode catalyst layer 2 and the cathode-side electrode catalyst layer 3. On the other hand, if the solids content of the catalyst ink is too low, the film-forming rate becomes very slow, reducing productivity. Therefore, the solids content of the catalyst ink is preferably 1% by mass (wt%) or more and 50% by mass or less. That is, by ensuring that the solids content of the catalyst ink is 1% by mass or more, it is possible to prevent the film-forming rate from becoming excessively slow, thereby preventing a decrease in productivity. By ensuring that the solids content of the catalyst ink is 50% by mass or less, it is possible to prevent the viscosity of the catalyst ink from becoming excessively high, thereby making it more likely that cracks will form on the surfaces of the anode-side electrode catalyst layer 2 and the cathode-side electrode catalyst layer 3.

[0051] The catalyst ink can be applied to the substrate by a doctor blade method, a dipping method, a screen printing method, a roll coating method, or the like.

[0052] The substrate used in producing the electrode catalyst layers 2 and 3 may be a transfer sheet. The transfer sheet used as the substrate may be made of any material that has good transferability, such as fluorine-based resins, such as ethylene tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroperfluoroalkyl vinyl ether copolymer (PFA), and polytetrafluoroethylene (PTFE).

[0053] The transfer sheet can also be a polymer sheet or polymer film, such as polyimide, polyethylene terephthalate, polyamide (Nylon (registered trademark)), polysulfone, polyethersulfone, polyphenylene sulfide, polyether ether ketone, polyetherimide, polyarylate, or polyethylene naphthalate.

[0054] Furthermore, when a transfer sheet is used as the substrate, an electrode membrane, which is a coating film after solvent removal, is bonded to the polymer electrolyte membrane 1, and then the transfer sheet is peeled off, thereby making it possible to obtain a membrane electrode assembly 11 having an anode-side electrode catalyst layer 2 and a cathode-side electrode catalyst layer 3 on both sides of the polymer electrolyte membrane 1.

[0055] <Electrolytic cell> An embodiment of an electrolysis cell of the present disclosure, such as a water electrolysis cell and an organic hydride synthesis cell, will now be described with reference to Figure 2. Figure 2 is a cross-sectional view of an embodiment of an electrolysis cell 300 of the present disclosure.

[0056] As shown in FIG. 2 , the electrolysis cell 300 of this embodiment includes a membrane electrode assembly 11, an anode-side current collector (main electrode) 310 and a cathode-side current collector (main electrode) 320 that are disposed so as to sandwich the membrane electrode assembly 11, and a DC power supply (not shown) that is electrically connected to the anode-side current collector 310 and the cathode-side current collector 320.

[0057] The anode-side current collector 310 is connected to a DC power supply to function as the anode, and the anode-side current collector 310 is joined to the anode-side electrode catalyst layer 2 of the membrane electrode assembly 11. The cathode-side current collector 320 is connected to a DC power supply to function as the cathode, and the cathode-side current collector 320 is joined to the cathode-side electrode catalyst layer 3 of the membrane electrode assembly 11.

[0058] The current collector may be any conductive material. Specific examples include carbon paper, carbon nonwoven fabric, and oxide and metal plates. Examples of metal plates include titanium sintered bodies. The carbon paper may be water-repellent, and the oxide and metal plates may be plated with a precious metal. The current collector may be porous or may have flow paths for supplying or discharging gases or liquids. The current collector may function as a separator that retains liquids and gases supplied to the cathode and anode sides or generated and discharged.

[0059] In the case of a water electrolysis cell, when water and voltage are supplied, in the case of a proton exchange membrane type, oxygen and protons are generated from water at the anode-side electrode catalyst layer 2, and the generated protons are converted to hydrogen at the cathode-side electrode catalyst layer 3. In the case of an anion exchange membrane type, hydrogen and hydroxide ions are generated from water at the cathode-side electrode catalyst layer 3, and the generated hydroxide ions are converted to oxygen and water at the anode-side electrode catalyst layer 2. Ultrapure water or other water is used as the water.

[0060] In the case of an organic hydride synthesis cell, when water, an organic substance such as toluene, and a voltage are supplied, oxygen and protons are generated from the water in the anode-side electrode catalyst layer 2, and the generated protons hydrogenate the organic substance in the cathode-side electrode catalyst layer 3, converting it into an organic hydride such as methylcyclohexane.

[0061] <Other effects> According to this embodiment, it is possible to manufacture, without using any complicated steps, an electrode catalyst layer having high mechanical properties and excellent durability, and an electrode catalyst layer and a membrane electrode assembly that can achieve high electrolysis performance.

[0062] Although the embodiments of the present invention have been described in detail above, in reality, the present invention is not limited to the above-described embodiments, and even if modifications are made within the scope of the gist of the present invention, they are included in the present invention.

[0063] The electrode catalyst layer and membrane electrode assembly for an electrolysis cell according to this embodiment will be described below with reference to specific examples and comparative examples, but this embodiment is not limited to the following examples and comparative examples. [Example]

[0064] Example 1 [Production of polybenzimidazole-coated exfoliated graphite] Polybenzimidazole of the following formula was dissolved in dimethylacetamide to prepare a polybenzimidazole dispersion. Next, exfoliated graphite having an arithmetic mean aspect ratio L / D of 13 and an arithmetic mean major axis L of 4.3 μm was added to this polybenzimidazole dispersion, and ultrasonic dispersion treatment was performed. After that, filtration and drying were performed to obtain polybenzimidazole-coated exfoliated graphite. The thickness of the polybenzimidazole coating layer was as thin as a few nm, and the arithmetic mean aspect ratio L / D and arithmetic mean major axis L of the obtained polybenzimidazole-coated exfoliated graphite were substantially the same as those of the exfoliated graphite before coating. [ka]

[0065] [Production of cathode-side catalyst ink] The catalyst-supported particles, polybenzimidazole-coated exfoliated graphite, and polymer electrolyte shown below were mixed in a solvent and dispersed for 30 minutes using a planetary ball mill to prepare a catalyst ink. The solvent for the catalyst ink was a mixed solvent of ultrapure water and 1-propanol. The volume ratio of ultrapure water to 1-propanol was 1:1. The catalyst ink was adjusted so that the solid content in the catalyst ink was 10% by mass. Catalyst support: carbon particles ·Catalyst particles: PtRu Polymer electrolyte: Fluorine-based polymer electrolyte (Nafion® dispersion) Blending ratio: The catalyst ink had a catalyst particle mass of 1, a polymer electrolyte (solid content) mass of 0.4, and a polybenzimidazole-coated exfoliated graphite mass of 0.1.

[0066] [Production of anode-side catalyst ink] The catalyst-supported particles, polymer electrolyte, and polybenzimidazole-coated exfoliated graphite shown below were mixed in a solvent and dispersed for 30 minutes using a planetary ball mill to prepare a catalyst ink. The solvent used for the catalyst ink was a mixed solvent of ultrapure water and 1-propanol. The volume ratio of ultrapure water to 1-propanol was 1:1. The catalyst ink was adjusted so that the solid content in the catalyst ink was 10% by mass. Catalytic particles: Iridium oxide Polymer electrolyte: Fluorine-based polymer electrolyte (Nafion® dispersion) Blending ratio: The catalyst ink had a catalyst particle mass of 1, a polymer electrolyte (solid content) mass of 0.2, and a polybenzimidazole-coated exfoliated graphite mass of 0.05.

[0067] [Fabrication of Membrane Electrode Assembly] The prepared cathode catalyst ink was applied to one side of a proton-conductive polymer electrolyte membrane (fluorinated polymer electrolyte membrane) using a die coater to form a rectangular coating film measuring 10 mm long x 10 mm wide. The amount of the cathode catalyst ink applied was 0.5 mg / cm. 2 Then, a drying treatment was carried out using an oven to volatilize the dispersion medium contained in the coating film, thereby forming a cathode-side electrode catalyst layer. Next, the prepared anode catalyst ink was applied to the surface of the polymer electrolyte membrane opposite to the surface on which the cathode electrode catalyst layer was formed, to form a rectangular coating film measuring 10 mm long x 10 mm wide. The amount of the anode catalyst ink applied was 0.5 mg / cm. 2Then, a drying treatment was carried out using an oven to volatilize the dispersion medium contained in the coating film, thereby forming an anode-side electrode catalyst layer, thereby obtaining a membrane electrode assembly. Both electrode catalyst layers in Example 1 were free from cracks and did not peel off from the polymer electrolyte membrane.

[0068] <Example 2> A membrane electrode assembly of Example 2 was obtained in the same manner as in Example 1, except that exfoliated graphite having an arithmetic mean value of aspect ratio L / D of 18 and an arithmetic mean value of major axis L of 5.9 μm was used as the exfoliated graphite in the polybenzimidazole-coated exfoliated graphite. Both electrode catalyst layers in Example 2 were free from cracks and did not peel off from the polymer electrolyte membrane.

[0069] <Comparative Example 1> A membrane electrode assembly of Comparative Example 1 was obtained in the same manner as in Example 1, except that exfoliated graphite having an arithmetic mean aspect ratio L / D of 13 and an arithmetic mean major axis L of 4.3 μm was used without being subjected to a polybenzimidazole coating treatment, instead of the polybenzimidazole-coated exfoliated graphite.

[0070] In Comparative Example 1, no cracks were observed during the formation of the cathode-side electrode catalyst layer, and no peeling from the polymer electrolyte membrane occurred. However, cracks occurred in the cathode-side electrode catalyst layer during the formation of the anode-side electrode catalyst layer, and partial peeling from the polymer electrolyte membrane was observed.

[0071] <Comparative Example 2> A membrane electrode assembly of Comparative Example 2 was obtained in the same manner as in Example 2, except that exfoliated graphite having an arithmetic mean aspect ratio L / D of 18 and an arithmetic mean major axis L of 5.9 μm was used without being subjected to polybenzimidazole coating treatment instead of the polybenzimidazole-coated exfoliated graphite.

[0072] In Comparative Example 2, there were no cracks during formation of the cathode-side electrode catalyst layer, and no peeling from the polymer electrolyte membrane. However, during formation of the anode-side electrode catalyst layer, cracks occurred in the cathode-side electrode catalyst layer, and partial peeling from the polymer electrolyte membrane was observed.

[0073] <Comparative Example 3> A membrane / electrode assembly of Comparative Example 3 was obtained in the same manner as in Example 1, except that polybenzimidazole-coated exfoliated graphite was not used. In Comparative Example 3, significant cracks occurred during the formation of the cathode-side electrode catalyst layer, and significant peeling from the polymer electrolyte membrane was observed, making it impossible to apply the anode-side catalyst ink.

[0074] [Evaluation of initial electrolysis performance] For each of the membrane electrode assemblies obtained in the Examples and Comparative Examples, gas diffusion layers were attached to sandwich the membrane electrode assembly to prepare samples. Each sample was placed in an electrolysis cell, and electrolysis voltage was measured using a water electrolysis device. The cell temperature during measurement was set to 60°C, and water heated to 60°C was circulated between the anode and cathode.

[0075] Regarding the electrolysis performance of each membrane electrode assembly measured, the current density of 0.5 A / cm was used as an index of power generation performance at low current density. 2 If the voltage is 1.6 V or less at this time, it is designated as "A", and if the voltage is over 1.6 V, it is designated as "B". Similarly, as an index of power generation performance at high current densities, a current density of 2.0 A / cm 2 When the voltage at this time was 1.85 V or higher, it was rated as "A," and when the voltage exceeded 1.85 V, it was rated as "B." As a result, in Examples 1 and 2, the indices at both low and high current densities were "A," but in Comparative Examples 1 and 2, the indices at both low and high current densities were "B." Note that in Comparative Example 3, an anode-side electrode catalyst layer could not be formed, and therefore evaluation was not possible.

[0076] [Evaluation of durability performance] Current density 1.0A / cm 2The electrolysis evaluation was carried out for 8 hours a day, and repeated for 5 days. As an index of durability, a case where the voltage increase from the initial stage of electrolysis was 0.1 V or less was rated as "A", and a case where the voltage exceeded 0.1 V was rated as "B". As a result, Examples 1 and 2 were rated as "A", while Comparative Examples 1 and 2 were rated as "B". Note that in Comparative Example 3, an anode-side electrode catalyst layer could not be formed, and therefore evaluation was not possible.

[0077] [Table 1] [Explanation of symbols]

[0078] 1...polymer electrolyte membrane, 2...anode side electrode catalyst layer, 3...cathode side electrode catalyst layer, 11...membrane electrode assembly, 300...electrolysis cell

Claims

1. The catalyst-supporting particle includes a polymer electrolyte and exfoliated graphite having a coating layer, The coating layer comprises a polymer containing a Lewis basic functional group.

2. The electrode catalyst layer according to claim 1 , wherein the polymer of the coating layer has an imide structure or an azole structure.

3. The electrode catalyst layer according to claim 1 , wherein the polymer of the coating layer has a benzazole structure.

4. The electrode catalyst layer according to claim 1 , wherein the polymer of the coating layer is polybenzimidazole.

5. 5. The electrode catalyst layer according to claim 1, wherein the arithmetic mean value of the aspect ratio L / D of the exfoliated graphite (where L is the major axis of the exfoliated graphite and D is the minor axis of the exfoliated graphite) is 10 to 200.

6. 6. The electrode catalyst layer according to claim 5, wherein the arithmetic mean value of the major axis L is 1 to 20 μm.

7. The electrode catalyst layer according to claim 1 or 2, which is for an electrolysis cell.

8. 3. The electrode catalyst layer according to claim 1, which is for use as a cathode in an electrolysis cell.

9. a polymer electrolyte membrane; and an anode-side electrode catalyst layer and a cathode-side electrode catalyst layer that sandwich the polymer electrolyte membrane; A membrane electrode assembly, wherein the cathode-side electrode catalyst layer is the electrode catalyst layer according to claim 1 or 2.

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