Membrane catalyst layer structure for water electrolysis, membrane electrode assembly for water electrolysis, water electrolysis cell, water electrolysis device, and water electrolysis method

Abstract

The present invention addresses the problem of providing a membrane catalyst layer structure for water electrolysis, the membrane catalyst layer structure being capable of maintaining good electrolysis performance over a long period of time by improving adhesion between a cathode catalyst layer and a diaphragm that includes a polymer electrolyte membrane. In order to solve the problem, the present invention provides a membrane catalyst layer structure for water electrolysis, the membrane catalyst layer structure comprising at least: a diaphragm that includes a polymer electrolyte; and an anode catalyst layer and a cathode catalyst layer which are disposed so as to face each other with the diaphragm being interposed therebetween. The anode catalyst layer contains elemental iridium, the cathode catalyst layer contains elemental platinum, the cathode catalyst layer additionally contains carbon black and a polymer electrolyte, the ratio (I / C) of the mass (I) of the polymer electrolyte to the mass (C) of the carbon black in the cathode catalyst layer is not less than 0.40 but less than 1.00, and the carbon black contained in the cathode catalyst layer has a volatile content of less than 1.4 mass%.

Description

Membrane / Catalyst Layer Assembly for Water Electrolysis, Membrane / Electrode Assembly for Water Electrolysis, Water Electrolysis Cell, Water Electrolysis Device, and Water Electrolysis Method

[0001] The present invention relates to a membrane / catalyst layer assembly for water electrolysis, a membrane / electrode assembly for water electrolysis, a water electrolysis cell, a water electrolysis device, and a water electrolysis method.

[0002] In recent years, due to environmental problems such as global warming, hydrogen has attracted attention as a clean energy source to replace fossil fuels. Hydrogen is expected to be a clean energy because it basically emits only water even when burned and does not emit carbon dioxide, which causes global warming. The production of hydrogen is mainly carried out by electrolysis of water.

[0003] As hydrogen production methods by electrolysis of water, alkaline water electrolysis and polymer electrolyte membrane (PEM) type water electrolysis are known. Among them, the PEM type water electrolysis method has the merit that it can be operated at a high current density and can flexibly cope with the output fluctuations of renewable energy.

[0004] The PEM type water electrolysis method is a method in which water is supplied to a water electrolysis cell in which an anode and a cathode are arranged opposite to each other with a diaphragm including a polymer electrolyte membrane interposed therebetween, and oxygen is generated at the anode and hydrogen is generated at the cathode. At this time, it is known to use a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte as the polymer electrolyte constituting the polymer electrolyte membrane, an iridium catalyst as the anode catalyst, and a platinum catalyst supported on carbon black as the cathode catalyst (see, for example, Patent Documents 1 and 2).

[0005] JP-A-2023-112816 JP-A-2019-141792

[0006] In order to promote the spread of hydrogen energy, reduction of the production cost of hydrogen is required. To reduce the production cost of hydrogen, it is effective to use a water electrolysis cell that can maintain relatively high electrolysis performance for a long time in the same cell without replacing the water electrolysis cell. However, the technologies described in the above patent documents are still insufficient.

[0007] To address the above-mentioned problems, the inventors focused on the adhesion between the diaphragm containing the polymer electrolyte membrane and the catalyst layer. They found that by improving the adhesion between the diaphragm containing the polymer electrolyte membrane and the cathode catalyst layer in water electrolysis, good electrolysis performance can be maintained for a long period of time.

[0008] In other words, in view of the above problems, the object of the present invention is to provide a membrane / catalyst layer structure for water electrolysis that can maintain good electrolytic performance for a long time by improving the adhesion between a diaphragm containing a polymer electrolyte membrane and a cathode catalyst layer.

[0009] The above object of the present invention can be achieved by the following invention: [1] A membrane / catalyst layer structure for water electrolysis comprising at least a diaphragm containing a polymer electrolyte membrane, and an anode catalyst layer and a cathode catalyst layer arranged opposite each other with respect to the diaphragm, wherein the anode catalyst layer contains iridium, the cathode catalyst layer contains platinum, the cathode catalyst layer further contains carbon black and a polymer electrolyte, the ratio (I / C) of the mass (C) of the carbon black to the mass (I) of the polymer electrolyte in the cathode catalyst layer is 0.40 or more and less than 1.00, and all or part of the carbon black contained in the cathode catalyst layer is carbon black (carbon black A) with a volatile content of less than 1.4% by mass. [2] The BET specific surface area of ​​the carbon black A contained in the cathode catalyst layer is 850 m². ２[1] The water electrolysis membrane / catalyst layer structure described above, wherein the ion exchange capacity of the polymer electrolyte contained in the cathode catalyst layer is less than 1.50 meq / g. [3] The water electrolysis membrane / catalyst layer structure described above, wherein the diaphragm contains carbon black. [4] The water electrolysis membrane / catalyst layer structure described above, wherein all or part of the carbon black contained in the diaphragm is carbon black A. [5] The water electrolysis membrane / catalyst layer structure described above, wherein the ion exchange capacity of the polymer electrolyte contained in the cathode catalyst layer is less than 1.50 meq / g. [6] The water electrolysis membrane / catalyst layer structure described above, wherein the cathode catalyst layer contains platinum-supported carbon particles, and all or part of the platinum element is the platinum element in the platinum-supported carbon particles. [7] The water electrolysis membrane / catalyst layer structure described above, wherein all or part of the carbon particles of the platinum-supported carbon particles is carbon black A. [8] The water electrolysis membrane / catalyst layer structure according to any one of [1] to [7], wherein all or part of the iridium elements contained in the anode catalyst layer are iridium elements in iridium oxide. [9] The water electrolysis membrane / catalyst layer structure according to any one of [1] to [8], wherein when the mass of all metal elements contained in the anode catalyst layer is 100% by mass, the mass of iridium elements in the anode catalyst layer is 50% by mass or more.

[10] The water electrolysis membrane / catalyst layer structure according to any one of [1] to [9], wherein the anode catalyst layer contains iridium elements and a polymer electrolyte, and when the mass of iridium elements contained in the anode catalyst layer is (Wir) and the mass of the polymer electrolyte is (WPE), the ratio (WPE / Wir) is 0.05 or more and less than 0.5.

[11] The water electrolysis membrane / catalyst layer structure according to any one of [1] to

[10] , wherein the ion exchange capacity of the polymer electrolyte contained in the polymer electrolyte membrane is 1.50 meq / g or more.

[12] The ion exchange capacity (IEC) of the polymer electrolyte contained in the cathode catalyst layer. ＣＡ ) and the ion exchange capacity (IEC) of the polymer electrolyte contained in the polymer electrolyte membrane. ＰＥ ) ratio (IEC ＣＡ / IEC ＰＥ[1] A water electrolysis membrane / catalyst layer structure according to any one of [1] to

[11] , wherein the ratio is 0.90 or less.

[13] A water electrolysis membrane / electrode assembly, wherein an electrode substrate is bonded to the anode catalyst layer and the cathode catalyst layer of the water electrolysis membrane / catalyst layer structure according to any one of [1] to

[12] .

[14] A water electrolysis cell, comprising the water electrolysis membrane / electrode assembly according to

[13] .

[15] A water electrolysis apparatus, comprising the water electrolysis cell according to

[14] .

[16] A water electrolysis method comprising supplying water to a water electrolysis cell, whose interior is divided into an anode and a cathode by a diaphragm containing a polymer electrolyte membrane, and electrolyzing the water to produce oxygen in the anode and hydrogen in the cathode, wherein the anode catalyst layer constituting the anode contains iridium, the cathode catalyst layer constituting the cathode contains platinum, the cathode catalyst layer further contains carbon black and a polymer electrolyte, the ratio (I / C) of the mass of the carbon black (C) to the mass of the polymer electrolyte (I) in the cathode catalyst layer is 0.40 or more and less than 1.00, and all or part of the carbon black contained in the cathode catalyst layer is carbon black with a volatile content of less than 1.4% by mass.

[0010] According to the present invention, the adhesion between the diaphragm containing the polymer electrolyte membrane and the cathode catalyst layer is improved, and a water electrolysis membrane / catalyst layer structure that can maintain good electrolysis performance over a long period of time can be provided. Furthermore, a water electrolysis apparatus using the water electrolysis membrane / catalyst layer structure of the present invention can maintain good electrolysis performance over a long period of time.

[0011] A schematic cross-sectional view showing an example of a water electrolysis cell that can be used in the present invention.

[0012] In this invention, carbon black A is carbon black having a volatile content of less than 1.4% by mass.

[0013] The embodiments of the present invention will be described in detail below, but the present invention is not limited to the embodiments described below and can be implemented with various modifications depending on the purpose and application.

[0014] A membrane / catalyst layer structure for water electrolysis according to an embodiment of the present invention (hereinafter sometimes simply referred to as "membrane / catalyst layer structure") comprises a diaphragm containing at least a polymer electrolyte membrane, and an anode catalyst layer and a cathode catalyst layer arranged opposite each other across the diaphragm. In such a membrane / catalyst layer structure, good electrolysis performance can be obtained by the anode catalyst layer containing iridium and the cathode catalyst layer containing platinum. Furthermore, the cathode catalyst layer further contains carbon black and a polymer electrolyte, and the ratio of the mass of the carbon black (C) to the mass of the polymer electrolyte (I) (I / C) (hereinafter sometimes simply referred to as "(I / C)") is in the range of 0.40 or more and less than 1.00, and all or part of the carbon black is carbon black A, thereby enabling the maintenance of good electrolysis performance over a long period of time. Note that the units of the mass of the carbon black (C) and the mass of the polymer electrolyte (I) are the same, for example, milligrams.

[0015] Hereinafter, the ability to maintain electrolytic performance over a long period of time will be referred to as "durability." In other words, the membrane / catalyst layer structure according to the embodiment of the present invention can achieve good electrolytic performance and good durability. Note that "maintaining electrolytic performance" means maintaining a constant current density (for example, 1.2 A / cm²) during long-term operation of the water electrolysis apparatus. ２ This refers to the small increase in the applied voltage required to maintain the specified state.

[0016] Here, the volatile content of carbon black is an indicator used to evaluate the amount of surface functional groups present in the carbon black. Examples of surface functional groups include oxygen-containing functional groups such as phenolic hydroxyl groups, ether groups, carboxyl groups, carbonyl groups, and lactone groups. In other words, carbon black A can be said to be a carbon black with relatively few surface functional groups. The volatile content of carbon black in this invention is a value calculated by the method described in the Examples section below.

[0017] From the viewpoint of improving durability, the volatile content of carbon black A used in the water electrolysis membrane / catalyst layer structure of the present invention is preferably less than 1.2% by mass, more preferably less than 1.0% by mass, even more preferably less than 0.7% by mass, and particularly preferably less than 0.5% by mass. The lower limit is 0.0% by mass.

[0018] Carbon black A can be appropriately selected and used from commercially available products. Examples of commercially available products include Mitsubishi Chemical Corporation's 3050B (volatile content 0.5% by mass), 3400B (volatile content 1.0% by mass), Tokai Carbon Co., Ltd.'s Toka Black #3845 (volatile content 0.38% by mass), #3855 (volatile content 0.12% by mass), #3885 (volatile content 0.28% by mass), #4300 (volatile content 0.7% by mass), #4500 (volatile content 0.6% by mass), #7270SB (volatile content 1.0% by mass), and Asahi Carbon Co., Ltd. Examples include F200 (0.7 mass volatile content) from ) Ltd., Ketjenblack EC300JD (0.5 mass volatile content) and EC-600JD (0.7 mass volatile content) from Lion Corporation, Denka Black (0.16 mass volatile content) from Denka Corporation, Printex 300 (0.6 mass volatile content) from Orion Engineered Carbons Co., Ltd., and Ensaco 250G (0.2 mass volatile content) from Imerys GC Japan Co., Ltd. The mechanism by which the inclusion of carbon black A in the cathode catalyst layer improves durability in water electrolysis is not clear, but it is presumed to be as follows: In a water electrolysis device, it is thought that oxygen generated at the anode by water electrolysis moves to the cathode and reacts with hydrogen generated at the cathode to produce hydrogen peroxide and hydrogen peroxide radicals (hereinafter, hydrogen peroxide and hydrogen peroxide radicals are collectively referred to as "hydrogen peroxides") as by-products. Furthermore, these hydrogen peroxides are known to migrate to the diaphragm and degrade the polymer electrolyte membrane.

[0019] Generally, carbon black has the function of decomposing or capturing hydrogen peroxides. However, it is presumed that the surface functional groups of carbon black react with hydrogen peroxides, changing the surface properties of carbon black such as wettability, and reducing the adhesion between the diaphragm and the cathode catalyst layer during long-term operation. This reduction in adhesion reduces the durability of the film and catalyst layer structure. In contrast, carbon black A has relatively few surface functional groups, so the above problems do not occur, and the impact on the adhesion between the diaphragm and the cathode catalyst layer is suppressed, resulting in improved durability.

[0020] The mechanism by which an (I / C) ratio of 0.40 or higher and less than 1.00 improves durability is not clear, but it is presumed to be as follows: To suppress the by-production of hydrogen peroxides in the cathode, it is effective for the hydrogen generated in the cathode to diffuse rapidly from the cathode catalyst layer and be released. It is presumed that an (I / C) ratio of less than 1.00 effectively enhances the diffusivity of hydrogen in the cathode catalyst layer. Furthermore, an (I / C) ratio of less than 1.00 is thought to be advantageous for the adhesion between the cathode catalyst layer and the diaphragm, which also contributes to durability.

[0021] On the other hand, ensuring the physical strength of the cathode catalyst layer is considered effective in improving durability. The polymer electrolyte contained in the cathode catalyst layer has the function of a binder resin in addition to its proton conduction function, and contributes to the physical strength of the cathode catalyst layer. In other words, it is thought that the physical strength of the cathode catalyst layer is ensured and durability is improved when (I / C) is 0.40 or higher. Furthermore, (I / C) of 0.40 or higher is also advantageous in terms of electrolytic performance.

[0022] [Anode Catalyst Layer] The anode catalyst layer contains iridium. The iridium contained in the anode catalyst layer functions as a catalyst. In water electrolysis, the anode electrolyzes water to produce oxygen and protons, and therefore the anode catalyst is sometimes called an oxygen evolution catalyst. Iridium is useful as an oxygen evolution catalyst. In other words, the electrolysis performance of the anode catalyst layer is enhanced by containing iridium, which acts as a catalyst.

[0023] Examples of the catalyst containing an iridium element include zero-valent iridium (iridium black) and iridium compounds. As the iridium compounds, iridium oxide, iridium carbide, iridium nitride, etc. can be used. From the viewpoint of maintaining good electrolytic performance for a longer time, it is preferable to use iridium oxide as the catalyst for the anode catalyst layer. The catalyst containing an iridium element is preferably in the form of particles.

[0024] The catalyst containing an iridium element can be supported on a carrier composed of metal oxides such as titanium oxide, tin oxide, tantalum oxide, niobium oxide, zirconium oxide, tungsten oxide, etc. and used. In that case, as such a carrier, since the anode in water electrolysis is in a high-potential environment, those having relatively high electrochemically oxidation resistance are preferable. In that regard, using the above metal oxides is preferable because of their relatively high electrochemically oxidation resistance.

[0025] On the other hand, as the carrier of the catalyst-supported particles, carbon particles such as carbon black are generally known. However, carbon particles generally have low electrochemically oxidation resistance, and if contained too much, it may affect the durability of the catalyst layer. Therefore, in the anode catalyst layer in water electrolysis, it is preferable to reduce the content of carbon particles. The mass per unit area of carbon particles in the anode catalyst layer is preferably less than 0.1 mg / cm ２ , more preferably less than 0.05 mg / cm ２ and even more preferably less than 0.02 mg / cm ２ , and particularly preferably not contained at all.

[0026] The mass of the iridium element contained in the anode catalyst layer is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, and particularly preferably 80% by mass or more when the mass of all metal elements contained in the anode catalyst layer is 100% by mass. The upper limit is preferably 100% by mass or less.

[0027] The anode catalyst layer may further contain at least one platinum group element other than iridium as a catalyst, such as platinum, ruthenium, rhodium, palladium, and osmium. In water electrolysis, there is a risk of explosion due to the mixing of hydrogen into the oxygen produced at the anode by back diffusion of hydrogen produced at the cathode. However, the platinum group elements mentioned above function as catalysts that produce water from hydrogen and oxygen, thus avoiding the risk of explosion. From this viewpoint, platinum and palladium are preferred as platinum group elements, with platinum being particularly preferred. Examples of catalysts containing platinum and palladium include zero-valent elements, oxides, carbides, and nitrides, respectively. Among these, zero-valent elements are preferred.

[0028] When the anode catalyst layer contains platinum group elements, the content of platinum group elements is preferably in the range of 1 to 90 parts by mass, more preferably in the range of 5 to 80 parts by mass, and particularly preferably in the range of 10 to 50 parts by mass, per 100 parts by mass of iridium element.

[0029] In the anode catalyst layer, the basis weight of iridium element per unit area is 0.2 to 2.0 mg / cm². ２ A range of 0.4 to 1.5 mg / cm² is preferred. ２ A range of 0.6 to 1.3 mg / cm² is more preferable. ２ The range is particularly preferable.

[0030] The anode catalyst layer preferably further contains a polymer electrolyte. Hydrocarbon-based polymer electrolytes and fluorine-based polymer electrolytes can be used as the polymer electrolyte. Since the anode is in a high-potential environment in a water electrolysis device, it is preferable to use a fluorine-based polymer electrolyte, which has relatively good resistance to electrochemical oxidation, and perfluorocarbon sulfonic acid polymers are even more preferable. These polymer electrolytes can be the same as those described and exemplified in the section on polymer electrolyte membranes, which will be described later.

[0031] When the anode catalyst layer contains a polymer electrolyte, the ratio of the mass of the polymer electrolyte (WPE) in the anode catalyst layer to the mass of the element iridium (Wir), i.e., "WPE / Wir", is preferably 0.05 or higher, more preferably 0.07 or higher, and particularly preferably 0.1 or higher, from the viewpoint of electrolytic performance. Furthermore, from the viewpoint of the diffusibility of oxygen gas generated at the anode, the above ratio is preferably less than 0.5, more preferably less than 0.4, even more preferably less than 0.3, and particularly preferably less than 0.25. Note that the units of the mass of the element iridium (Wir) and the mass of the polymer electrolyte (WPE) are the same, for example, milligrams.

[0032] From the viewpoint of electrolytic performance, the thickness of the anode catalyst layer is preferably 0.5 μm or more, more preferably 1 μm or more, and particularly preferably 3 μm or more. Furthermore, from the viewpoint of the diffusibility of oxygen gas generated in the anode and physical stability (such as suppressing cracking during operation), the above thickness is preferably 25 μm or less, more preferably 20 μm or less, even more preferably 15 μm or less, and particularly preferably 10 μm or less.

[0033] [Cathode Catalyst Layer] The cathode catalyst layer contains platinum. The platinum contained in the cathode catalyst layer functions as a catalyst. In water electrolysis, the cathode reduces protons generated at the anode to produce hydrogen, so the cathode catalyst is sometimes called a proton reduction catalyst (or hydrogen generation catalyst). Platinum is useful as a proton reduction catalyst.

[0034] Examples of catalysts containing the element platinum include zero-valent platinum and platinum compounds (e.g., platinum oxide, platinum halides, platinum hydroxide, platinum cyanide, platinum carbonyl, platinum ammonium, etc.). Among these, zero-valent platinum is preferred.

[0035] As a catalyst containing platinum, particles in which zero-valent platinum is supported on carbon particles (hereinafter referred to as "platinum-supported carbon particles") are preferred from the viewpoint of electrolytic performance. Furthermore, the cathode catalyst layer preferably contains both platinum and ruthenium as catalysts. This further improves durability. As the catalyst containing ruthenium, zero-valent ruthenium is preferably used.

[0036] When the cathode catalyst layer contains platinum and ruthenium as catalysts, the mass ratio of platinum (WPt) to ruthenium (WRu) (WRu / WPt) is preferably 0.1 or higher, more preferably 0.2 or higher, even more preferably 0.3 or higher, and particularly preferably 0.4 or higher. Furthermore, the above mass ratio is preferably 2.0 or lower, more preferably 1.7 or lower, even more preferably 1.5 or lower, and particularly preferably 1.3 or lower.

[0037] Catalysts containing platinum and catalysts containing ruthenium may be used individually, as a platinum-ruthenium alloy, or supported on carbon particles. Hereinafter, carbon particles with ruthenium supported will be referred to as "ruthenium-supported carbon particles," carbon particles with platinum and ruthenium supported will be referred to as "platinum-ruthenium-supported carbon particles," and carbon particles with a platinum-ruthenium alloy supported will be referred to as "platinum-ruthenium alloy-supported carbon particles." These, along with the aforementioned "platinum-supported carbon particles," are collectively referred to as "platinum-and-other catalyst-supported carbon particles." Carbon black is preferably used as the carbon particles.

[0038] In this invention, carbon black is used as the carbon particles in the platinum catalyst-supported carbon particles, and it is more preferable from the viewpoint of durability that all or part of this carbon black is carbon black A. When the total amount of carbon particles used as the catalyst support in the cathode catalyst layer is 100% by mass, the proportion of carbon black A is preferably 30% by mass or more, more preferably 50% by mass or more, even more preferably 70% by mass or more, and particularly preferably 90% by mass or more. The upper limit is 100% by mass.

[0039] When the cathode catalyst layer contains platinum and ruthenium elements as catalysts, it is preferable that the carbon particles supported by a platinum-ruthenium alloy are used from the viewpoint of durability.

[0040] The loading rate of the catalyst element in the platinum- or other catalyst-supported carbon particles (the ratio of the mass of the catalyst element to the mass of the platinum- or other catalyst-supported carbon particles) is preferably in the range of 20 to 70% by mass, more preferably in the range of 30 to 65% by mass, and particularly preferably in the range of 35 to 60% by mass.

[0041] In the cathode catalyst layer, the basis weight of platinum element per unit area is 0.05 mg / cm², considering the electrolytic performance. ２ The above is preferable, and 0.1 mg / cm³ ２ The above is more preferable, 0.2 mg / cm³ ２ The above is particularly preferable. Furthermore, the mass of the platinum element is 1.0 mg / cm³ from the viewpoint of cost. ２ Preferably less than 0.7 mg / cm³. ２ Less than 0.5 mg / cm³ is more preferable, and 0.5 mg / cm³ is preferable. ２ Less than is particularly preferable.

[0042] In the present invention, the cathode catalyst layer contains, in addition to the platinum element, carbon black and a polymer electrolyte, with a ratio (I / C) of the mass of carbon black (C) to the mass of polymer electrolyte (I) in the range of 0.40 or more and less than 1.00. From the viewpoint of durability, (I / C) is preferably 0.95 or less, more preferably 0.90 or less, even more preferably 0.85 or less, and particularly preferably 0.83 or less. On the other hand, from the viewpoint of the physical strength of the cathode catalyst layer, (I / C) is preferably 0.45 or more, more preferably 0.50 or more, even more preferably 0.55 or more, and particularly preferably 0.60 or more. In this case, it is preferable that all or part of the carbon black is carbon black A, and the content of carbon black A in the cathode catalyst layer is preferably 30% by mass or more, more preferably 50% by mass or more, even more preferably 70% by mass or more, and particularly preferably 90% by mass or more, based on 100% by mass of the total amount of carbon black contained in the cathode catalyst layer. The upper limit is 100% by mass. There are no particular restrictions on the type of carbon black other than carbon black A.

[0043] In the present invention, the form of carbon black A is not particularly limited. For example, carbon black A may be included in its original form, as described above, as carbon particles (supports) in platinum or other catalyst-supported carbon particles, or in combination of these forms.

[0044] It is preferable that the BET specific surface area of ​​carbon black A used in the cathode catalyst layer be relatively small. Carbon black tends to have a higher bulk density as its BET specific surface area decreases. In other words, as the BET specific surface area of ​​carbon black decreases, the volume per unit mass of carbon black in the cathode catalyst layer tends to decrease.

[0045] In other words, by using carbon black A, which has a relatively small BET specific surface area, the physical strength of the cathode catalyst layer can be ensured even if the aforementioned (I / C) becomes relatively small. Therefore, by using carbon black A, which has a relatively small BET specific surface area, the (I / C) can be made relatively small while ensuring the physical strength of the cathode catalyst layer, and as a result, improved durability can be expected. As mentioned above, making the (I / C) relatively small is effective in increasing the diffusivity of hydrogen in the cathode catalyst layer, thereby suppressing the by-production of hydrogen peroxides, and as a result, it is presumed that the adhesion between the diaphragm and the catalyst layer is ensured, and durability is improved.

[0046] From the above perspective, the BET specific surface area of ​​carbon black A is 850 m². ２ Preferably less than / g, and 750m ２ More preferably less than / g, and 500m ２ It is even more preferable to be less than or equal to 300m ２ It is even more preferable to be less than or equal to 200m ２ A value of less than or equal to / g is particularly preferred. That is, a BET specific surface area of ​​850 m² is preferred. ２ By using carbon black A with a concentration of less than / g, further improvements in durability can be expected. On the other hand, from the standpoint of water electrolysis performance, the BET specific surface area of ​​carbon black A is 20 m². ２ Preferably 40 m ２More preferably 60 m ２ More preferably 70 m ２ A value of 1 / g or more is particularly preferred.

[0047] The polymer electrolyte contained in the cathode catalyst layer is not particularly limited, and known polymer electrolytes can be used. Among these, it is preferable to use a polymer electrolyte with an ion exchange capacity (IEC) greater than 0 meq / g and less than 1.50 meq / g. The IEC of the polymer electrolyte contained in the cathode catalyst layer is more preferably less than 1.40 meq / g, and particularly preferably less than 1.30 meq / g. The lower limit is preferably 0.40 meq / g or more. If the cathode catalyst layer contains multiple polymer electrolytes with different IECs, the IEC should be determined by a weighted average.

[0048] Furthermore, from the standpoint of electrolytic performance, it is preferable that the IEC of the polymer electrolyte contained in the cathode catalyst layer is smaller than that of the polymer electrolyte contained in the polymer electrolyte membrane constituting the diaphragm. Details will be described later.

[0049] Here, IEC is expressed as the chemical equivalent of ionic groups introduced per gram of dry weight of the polymer electrolyte, and a larger value indicates a greater amount of ionic groups introduced. In this invention, IEC is defined as the value obtained by neutralization titration.

[0050] As the polymer electrolyte, one having the above-mentioned IEC is preferably used. For example, hydrocarbon-based polymer electrolytes and fluorine-based polymer electrolytes can be used. Among these, fluorine-based polymer electrolytes are preferred, and perfluorocarbon sulfonic acid polymers are even more preferred. As these polymer electrolytes, those similar to those described and exemplified in the section on polymer electrolyte membranes described later can be used.

[0051] From the viewpoint of electrolytic performance, the thickness of the cathode catalyst layer is preferably 0.5 μm or more, more preferably 1 μm or more, and particularly preferably 3 μm or more. Furthermore, from the viewpoint of the diffusibility of hydrogen gas generated in the cathode, the thickness is preferably 25 μm or less, more preferably 20 μm or less, even more preferably 15 μm or less, and particularly preferably 10 μm or less.

[0052] [Diaphragm] The diaphragm used in the water electrolysis membrane / catalyst layer structure of the present invention includes at least a polymer electrolyte membrane. Here, "the diaphragm includes at least a polymer electrolyte membrane" means that, in addition to the polymer electrolyte membrane, it may include a layer that contains no polymer electrolyte at all or only a small amount of polymer electrolyte. Hereinafter, the layer other than the polymer electrolyte membrane will be referred to as "other layers". That is, the diaphragm may consist only of a polymer electrolyte membrane, or it may be a laminated structure of a polymer electrolyte membrane and other layers. Here, a polymer electrolyte membrane is defined as a membrane containing 50% by mass or more of polymer electrolyte per 100% by mass of the solid content of the membrane, and other layers are defined as layers in which the polymer electrolyte content is less than 50% by mass per 100% by mass of the solid content of the layer.

[0053] One possible lamination configuration of the polymer electrolyte membrane with other layers is one in which the other layers are arranged on one or both sides of the polymer electrolyte membrane. In the above lamination configuration, it is preferable that the other layers are arranged on at least the cathode catalyst layer side of the polymer electrolyte membrane.

[0054] The polymer electrolyte membrane that constitutes the diaphragm may be composed of multiple layers. Details of the polymer electrolyte membrane and other layers will be described later.

[0055] The diaphragm preferably contains carbon black. The inclusion of carbon black in the diaphragm further improves its durability. Furthermore, it is preferable that all or part of the carbon black contained in the diaphragm is carbon black A. The use of carbon black A further improves its durability.

[0056] When the diaphragm contains carbon black A, the BET specific surface area of ​​the carbon black A is 850 m². ２ Preferably less than / g, and 750m ２ It is more preferable that it be less than or equal to 500m ２ It is even more preferable to be less than or equal to 300m ２ It is even more preferable to be less than or equal to 200m ２ A value of less than or equal to / g is particularly preferred. That is, the diaphragm has a BET specific surface area of ​​750 m². ２Further improvements in durability can be expected by including carbon black A at a concentration of 0 / g or less. On the other hand, from the viewpoint of water electrolysis performance, when this diaphragm contains carbon black A, the BET specific surface area of ​​said carbon black A is 20 m². ２ It is preferable that it be 40m or more / g. ２ More preferably 60 m ２ More preferably 70 m ２ A value of 1 / g or more is particularly preferred.

[0057] When the diaphragm takes a laminated form, carbon black can be included in one or more layers among the layers constituting the diaphragm (including when the polymer electrolyte membrane is a laminate of multiple layers). From the viewpoint of improving the adhesion between the diaphragm and the cathode catalyst layer and thereby improving durability, it is preferable to include carbon black in the layer of the diaphragm that is located on the cathode catalyst layer side. Specific embodiments include the following embodiments (I) and (II). <Embodiment (I)> An embodiment in which another layer containing carbon black is placed on the cathode catalyst layer side of the polymer electrolyte membrane. <Embodiment (II)> An embodiment in which the polymer electrolyte membrane is composed of multiple layers, and the layer located on the cathode catalyst layer side contains carbon black.

[0058] In the above configuration (I), for example, when the diaphragm has a laminated structure of "(polymer electrolyte membrane) / (other layer)", the membrane / catalyst layer structure is "(anode catalyst layer) / (polymer electrolyte membrane) / (other layer) / (cathode catalyst layer)", and carbon black is contained in the above (other layer).

[0059] In embodiment (I), it is preferable that the polymer electrolyte membrane does not contain carbon black. Other layers containing carbon black may or may not be adjacent to the cathode catalyst layer, but it is preferable that they are adjacent. In embodiment (I), the polymer electrolyte membrane may also be configured as a stack of multiple layers.

[0060] In the above configuration (II), for example, when the diaphragm has a two-layer structure of "(polymer electrolyte layer 1) / (polymer electrolyte layer 2)", the membrane / catalyst layer structure is "(anode catalyst layer) / (polymer electrolyte layer 1) / (polymer electrolyte layer 2) / (cathode catalyst layer)", and carbon black is contained in the above (polymer electrolyte layer 2).

[0061] In embodiment (II), it is preferable that the polymer electrolyte layer 1 does not contain carbon black. That is, in embodiment (II), it is preferable that carbon black is contained only in the polymer electrolyte layer closest to the cathode catalyst layer among the multiple polymer electrolyte layers. The polymer electrolyte layer containing carbon black may or may not be adjacent to the cathode catalyst layer, but it is preferable that it is adjacent.

[0062] Furthermore, in form (II), the diaphragm may consist solely of a polymer electrolyte membrane, or it may be a laminated configuration of a polymer electrolyte membrane and other layers. An example of such a laminated configuration is one in which the other layers are placed on the anode catalyst layer side of the polymer electrolyte membrane.

[0063] In forms (I) and (II), it is most preferable that the layer containing carbon black is in contact with the cathode catalyst layer, but an intermediate layer that does not hinder adhesion can be placed between the two layers. For example, if the cathode catalyst layer contains a fluorine-based polymer electrolyte as a polymer electrolyte, an intermediate layer containing a fluorine-based polymer can be placed. Examples of such fluorine-based polymers include polytetrafluoroethylene, poly(vinylidene fluoride), copolymer of vinylidene fluoride and hexafluoropropylene, copolymer of vinylidene fluoride and trifluoroethylene, copolymer of vinylidene fluoride and tetrafluoroethylene, poly(vinylidene fluoride), and perfluorocarbon sulfonic acid polymers. The thickness of the intermediate layer is preferably less than 5 μm, more preferably less than 4 μm, and particularly preferably less than 3 μm. The lower limit of the thickness is preferably 0.1 μm or more.

[0064] From the viewpoint of durability, the amount of carbon black contained in the diaphragm is preferably 1 part by mass or more, more preferably 2 parts by mass or more, even more preferably 3 parts by mass or more, and particularly preferably 4 parts by mass or more, when the total mass of the polymer electrolyte contained in the diaphragm is 100 parts by mass. Furthermore, the above content is preferably 25 parts by mass or less, more preferably 20 parts by mass or less, even more preferably 15 parts by mass or less, and particularly preferably 10 parts by mass or less.

[0065] [Polymer Electrolyte Membrane] The polymer electrolyte membrane contains a polymer electrolyte. In the present invention, the polymer electrolyte content in the polymer electrolyte membrane is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more, when the mass of the solid content of the polymer electrolyte membrane is taken as 100% by mass. The upper limit is 100% by mass.

[0066] Known polymer electrolytes such as fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes can be used as the polymer electrolytes contained in the polymer electrolyte membrane. In the present invention, it is preferable that all or part of the polymer electrolytes contained in the polymer electrolyte membrane are hydrocarbon-based polymer electrolytes.

[0067] Examples of fluorinated polymer electrolytes include fluorinated polymers having ionic groups. A fluorinated polymer is defined as a polymer in which most or all of the hydrogen atoms in the alkyl and / or alkylene groups in the molecule are replaced with fluorine atoms.

[0068] Examples of fluorine-based polymer electrolytes include perfluorocarbon sulfonic acid polymers, perfluorocarbon phosphonic acid polymers, trifluorostyrene sulfonic acid polymers, trifluorostyrene phosphonic acid polymers, ethylene tetrafluoroethylene-g-styrene sulfonic acid polymers, ethylene-tetrafluoroethylene sulfonic acid polymers, and polyvinylidene fluoride-perfluorocarbon sulfonic acid polymers.

[0069] Among these, perfluorocarbon sulfonic acid polymers are preferred from the viewpoint of heat resistance and chemical stability, and examples of such polymers include commercially available products such as "Nafion" (registered trademark) (manufactured by Chemours), "Flemion" (registered trademark) (manufactured by AGC Inc.), and "Aciplex" (registered trademark) (manufactured by Asahi Kasei Corporation).

[0070] Examples of hydrocarbon-based polymer electrolytes include hydrocarbon polymers having ionic groups. A hydrocarbon polymer is defined as a polymer having a main chain whose main constituent units are hydrocarbons. Among the above hydrocarbon polymers, aromatic hydrocarbon polymers having aromatic rings in the main chain are preferred. In other words, aromatic hydrocarbon polymer electrolytes are preferred among hydrocarbon-based polymer electrolytes.

[0071] Specific examples of aromatic hydrocarbon polymers include polymers having a structure selected from polysulfone, polyethersulfone, polyphenylene oxide, polyarylene ether, polyphenylene sulfide, polyphenylene sulfide sulfone, polyparaphenylene, polyarylene polymers, polyarylene ketone, polyether ketone, polyarylene phosphine oxide, and polyether phosphine oxide, along with an aromatic ring, in their main chain.

[0072] Furthermore, the term "polysulfone" refers to a general term for structures having sulfone bonds in their molecular chains, "polyethersulfone" refers to a general term for structures having both ether and sulfone bonds in their molecular chains, and "polyetherketone" refers to a general term for structures having both ether and ketone bonds in their molecular chains. Aromatic hydrocarbon polymers may have multiple of these structures.

[0073] Polyetherketone polymers are particularly preferred as aromatic hydrocarbon polymers. Examples of polyetherketone polymers include polyetherketone, polyetherketoneketone, polyetheretherketone, polyetheretherketoneketone, and polyetherketoneetherketoneketone.

[0074] Furthermore, the hydrocarbon polymer having ionic groups used as a hydrocarbon polymer electrolyte is preferably a block copolymer of an aromatic hydrocarbon polymer without ionic groups and an aromatic hydrocarbon polymer having ionic groups.

[0075] The ionic groups contained in the polymer electrolyte can be any ionic group having either cation exchange ability or anion exchange ability, but in the present invention, it is preferable to use proton-exchangeable ionic groups. Examples of such functional groups include sulfonic acid groups, sulfonimide groups, sulfate groups, phosphonic acid groups, phosphoric acid groups, carboxylic acid groups, ammonium groups, phosphonium groups, and amino groups. Two or more types of ionic groups can be contained in the polymer. Among these, sulfonic acid groups, sulfonimide groups, and sulfate groups are preferred because they have excellent water electrolysis performance, and sulfonic acid groups are more preferred from the viewpoint of raw material cost.

[0076] As described above, it is preferable to use a block copolymer of an aromatic hydrocarbon polymer without ionic groups and an aromatic hydrocarbon polymer having ionic groups as the polyelectrolyte, and it is even more preferable that the aromatic hydrocarbon polymer forming the main backbone of the block copolymer be a polyether ketone. As such a block copolymer, it is particularly preferable that it contains a segment containing a structural unit (S1) containing ionic groups as shown in the following formula and a segment containing a structural unit (S2) that does not contain ionic groups.

[0077]

[0078] In general formula (S1), Ar １１ ~Ar １４ This represents a divalent arylene group which may be the same or different and may have substituents, however Ar １１ and Ar １２ Either or both of these contain an ionic group. Also, Ar １３ and Ar １４ The compound may or may not contain an ionic group. * represents a bonding site with the general formula (S1) or other constituent units.

[0079]

[0080] In general formula (S2), Ar １５ ~Ar １８ This represents a divalent arylene group that may be the same or different, and may have substituents other than ionic groups (i.e., Ar １５ ~Ar １８ (This does not contain an ionic group.) * indicates a bonding site with the general formula (S2) or other constituent units.

[0081] Here, Ar １１ ~Ar １８ Preferred divalent arylene groups that serve as the core of the divalent arylene group include, but are not limited to, hydrocarbon arylene groups such as phenylene groups, naphthylene groups, biphenylene groups, and fluoroorangeyl groups, and heteroarylene groups such as pyridinediyl, quinoxalinediyl, and thiophenediyl. Here, "phenylene group" can be of three types depending on the location of the bond site between the benzene ring and other structural units: o-phenylene group, m-phenylene group, and p-phenylene group. In this specification, unless otherwise specified, these are used as a general term. The same applies to other divalent arylene groups such as "naphthylene group" and "biphenylene group". １１ ~Ar １４ The preferred core group for the divalent arylene group is a phenylene group, most preferably a p-phenylene group. １５ ~Ar １８ It is more preferable that the group is an unsubstituted divalent arylene group in terms of proton conductivity, chemical stability, and physical durability.

[0082] The ion exchange capacity (IEC) of the polymer electrolyte used in the polymer electrolyte membrane is preferably 1.50 meq / g or more, more preferably 1.70 meq / g or more, even more preferably 1.80 meq / g or more, and particularly preferably 1.90 meq / g or more, from the viewpoint of electrolytic performance. On the other hand, from the viewpoint of durability, it is preferably 2.70 meq / g or less, more preferably 2.50 meq / g or less, even more preferably 2.40 meq / g or less, and particularly preferably 2.30 meq / g or less. If the polymer electrolyte membrane contains multiple types of polymer electrolytes with different IECs, a weighted average should be used.

[0083] As mentioned above, the ion exchange capacity (hereinafter referred to as "IEC") of the polymer electrolyte contained in the cathode catalyst layer is ＣＡ The ion exchange capacity (hereinafter referred to as "IEC") of the polymer electrolyte used in the polymer electrolyte membrane that constitutes the diaphragm is the ion exchange capacity of the polymer electrolyte (hereinafter referred to as "IEC"). ＰＥ It is preferable that it be smaller than ( ). This is expected to improve water electrolysis performance. Specifically, IEC ＣＡ and IEC ＰＥ The ratio (IEC ＣＡ / IEC ＰＥ The ratio (IEC) is preferably 0.90 or less, more preferably 0.80 or less, even more preferably 0.70 or less, and particularly preferably 0.65 or less. ＣＡ / IEC ＰＥ The ratio is preferably 0.20 or higher, more preferably 0.30 or higher, even more preferably 0.35 or higher, and particularly preferably 0.40 or higher.

[0084] As for polymer electrolytes, the above IEC ＰＥ Materials having the above characteristics are preferably used, and among them, hydrocarbon polymer electrolytes are preferred because they have relatively high electrolytic performance. Among hydrocarbon polymer electrolytes, aromatic hydrocarbon polymer electrolytes are preferred, and polyetherketone block copolymers are particularly preferred. The content of hydrocarbon polymer electrolytes is preferably 60% by mass or more, more preferably 75% by mass or more, even more preferably 90% by mass or more, and particularly preferably 100% by mass, when the total mass of polymer electrolytes contained in the polymer electrolyte membrane is taken as 100% by mass.

[0085] A polymer electrolyte membrane may be composed of multiple layers. When a polymer electrolyte membrane is composed of multiple layers, the polymer electrolytes contained in each layer may have the same or different structures. Examples of the multiple layers include a laminated structure of a layer containing a hydrocarbon-based polymer electrolyte and a layer containing a fluorine-based polymer electrolyte (heterogeneous polymer electrolyte membrane), and a structure in which a layer impregnated with a polymer electrolyte in a porous substrate (composite layer) is laminated with a layer containing a polymer electrolyte on one or both sides and a layer not containing a porous substrate (non-composite layer) (composite polymer electrolyte membrane). Examples of porous substrates used in the composite layer include woven fabrics, nonwoven fabrics, porous films, and mesh fabrics.

[0086] In a composite polymer electrolyte membrane, it is preferable to use a hydrocarbon-based polymer electrolyte as the polymer electrolyte used in the composite layer and the non-composite layer. Furthermore, it is preferable to use a mesh fabric, which can be expected to have a high reinforcing effect, as the porous substrate used in the composite layer. A mesh fabric is a mesh-like fabric composed of warp and weft threads made of polymer fibers. As the material of the fibers constituting the mesh fabric, polymers having aromatic rings in the main chain are preferred. Mesh fabrics made of polymer fibers having aromatic rings in the main chain have good affinity with hydrocarbon-based polymer electrolytes, which has the advantage of suppressing the generation of voids in the electrolyte membrane. Examples of polymers having aromatic rings in the main chain include liquid crystal polyester, polyphenylene sulfide, polyether ketone, polyether ether ketone, and polyether ketone ketone. Among these, liquid crystal polyester and polyphenylene sulfide are preferred, and liquid crystal polyester is particularly preferred due to its high strength.

[0087] The average fiber diameter of the warp and / or weft threads constituting the mesh fabric is preferably 10 μm or more and 30 μm or less. The thickness of the mesh fabric (gauze thickness) is preferably 15 μm or more and 40 μm or less. Here, the thickness of the mesh fabric (gauze thickness) refers to the thickness at the intersection of the warp and weft threads.

[0088] In a composite polymer electrolyte membrane, the thickness ratio of the composite layer is preferably 10 to 90%, more preferably 20 to 80%, and particularly preferably 30 to 70%, with the thickness of the polymer electrolyte membrane being 100%. Here, the thickness of the composite layer can be said to be substantially equal to the thickness of the porous substrate. Specifically, the thickness of the composite layer is preferably in the range of 22 to 47 μm, more preferably in the range of 25 to 45 μm, and particularly preferably in the range of 30 to 43 μm. Furthermore, the thickness per layer of the non-composite layer is preferably 3 μm or more, more preferably 5 μm or more, even more preferably 7 μm or more, and particularly preferably 10 μm or more. Furthermore, the thickness per layer of the non-composite layer is preferably 45 μm or less, more preferably 40 μm or less, more preferably 35 μm or less, even more preferably 20 μm or less, and particularly preferably 15 μm or less.

[0089] A composite polymer electrolyte membrane can be manufactured, for example, as follows: A solution containing a polymer electrolyte (referred to as "solution (a)" in this section for convenience) is applied to a substrate such as a PET (polyethylene terephthalate) film; a porous substrate is bonded on top of the solution (a) to impregnate the porous substrate; and then a solution containing a polymer electrolyte (referred to as "solution (b)" in this section for convenience) is applied to the porous substrate and dried. Solution (a) and solution (b) may have the same composition or may have different compositions.

[0090] In the above manufacturing method, a composite layer is formed by impregnating a porous substrate with solution (a), and a non-composite layer is formed by applying solution (b) onto the porous substrate. By making the amount of solution (a) applied to the PET film exceed the amount impregnated into the porous substrate, a non-composite layer is formed between the PET film and the composite layer. Hereinafter, the non-composite layer formed between the PET film and the composite layer will be referred to as "non-composite layer 1," and the non-composite layer formed on the porous substrate will be referred to as "non-composite layer 2."

[0091] In the above manufacturing method, the porous substrate may be impregnated with solution (a), dried, and then the porous substrate may be coated with solution (b) and dried.

[0092] As described above, when the polymer electrolyte membrane is composed of multiple layers, each layer contains 50% by mass or more of polymer electrolyte relative to 100% by mass of the total solid content of the layer.

[0093] The polymer electrolyte membrane may contain various additives, such as antioxidants, surfactants, radical scavengers, hydrogen peroxide decomposers, non-electrolyte polymers, elastomers, fillers, etc., as long as they do not impair the effects of the present invention.

[0094] From the viewpoint of maintaining good electrolytic performance over a long period of time, the thickness of the polymer electrolyte membrane is preferably 40 μm or more, more preferably 50 μm or more, even more preferably 60 μm or more, and particularly preferably 70 μm or more. On the other hand, the thickness of the polymer electrolyte membrane is preferably 250 μm or less, more preferably 200 μm or less, even more preferably 180 μm or less, and particularly preferably 150 μm or less.

[0095] The polymer electrolyte membrane preferably contains platinum group elements, i.e., elements selected from ruthenium, rhodium, palladium, osmium, iridium, and platinum. As mentioned above, platinum group elements function as catalysts for producing water from hydrogen and oxygen, thus suppressing the back diffusion of hydrogen produced at the cathode to the anode.

[0096] Among these, platinum, palladium, and ruthenium are preferred, platinum and palladium are more preferred, and platinum is particularly preferred. The form of the platinum group elements contained in the polymer electrolyte membrane is preferably at least one selected from zero-valent metal particles, metal oxide particles, and sparingly soluble metal salt particles. Among these, zero-valent metal particles are more preferred. In other words, zero-valent platinum particles (platinum black) are particularly preferred.

[0097] Platinum group elements can be used in the form of being supported on carbon particles or metal oxide particles. Examples of carbon particles include carbon black, activated carbon, carbon fibers, carbon nanotubes, and graphene. Examples of metal oxide particles include tin oxide, titanium oxide, niobium oxide, molybdenum oxide, and tungsten oxide.

[0098] The content of platinum group elements per unit area in polymer electrolyte membranes is 0.005 to 0.5 mg / cm². ２ Preferably, 0.01 to 0.3 mg / cm³ ２ More preferably, 0.02 to 0.2 mg / cm³ ２ More preferably, 0.03 to 0.1 mg / cm³ ２ That is particularly preferable.

[0099] When incorporating platinum group elements into a polymer electrolyte membrane, it is preferable to have a multi-layered polymer electrolyte membrane, with the platinum group elements contained in the layer closest to the anode catalyst layer. While platinum group elements may be contained in multiple layers, it is more preferable to contain them only in the layer closest to the anode catalyst layer. Here, the number of layers constituting the polymer electrolyte membrane is preferably 2 to 5, and more preferably 2 to 4.

[0100] In a polymer electrolyte membrane, in a layer containing platinum group elements (hereinafter referred to as the "platinum group element-containing layer"), the mass ratio of platinum group elements to the polymer electrolyte (mass of platinum group elements / mass of polymer electrolyte) is preferably 0.03 or higher, more preferably 0.05 or higher, even more preferably 0.10 or higher, and particularly preferably 0.20 or higher. Furthermore, the above mass ratio is preferably 0.95 or lower, more preferably 0.90 or lower, even more preferably 0.80 or lower, and particularly preferably 0.70 or lower. It is preferable that part or all of the polymer electrolyte contained in the platinum group element-containing layer is a hydrocarbon-based polymer electrolyte. Note that the units of mass of the platinum group elements and the mass of the polymer electrolyte are the same, for example, milligrams.

[0101] The thickness of the platinum group metal-containing layer is preferably 0.1 μm or more, more preferably 0.5 μm or more, and particularly preferably 1 μm or more. The thickness of the platinum group metal-containing layer is preferably 15 μm or less, more preferably 10 μm or less, and particularly preferably 8 μm or less.

[0102] The diaphragm used in the water electrolysis membrane / catalyst layer structure of the present invention preferably takes the form of (I) or (II) described above, even when the polymer electrolyte membrane contains platinum group elements. Since hydrogen adsorption can be expected by carbon black placed on the cathode catalyst layer side of form (I) or (II), a further suppression of back diffusion of hydrogen to the anode can be expected when combined with the platinum group-containing layer on the anode catalyst layer side.

[0103] The above-mentioned form (II) will now be explained in detail. As previously stated, form (II) consists of multiple layers of polymer electrolyte membrane, and the layer closest to the cathode catalyst layer contains carbon black. The number of layers constituting the polymer electrolyte membrane is preferably 2 to 5, and more preferably 2 to 4. Here, the layer closest to the cathode catalyst layer is called the "cathode-side electrolyte layer," and the layers other than the cathode-side electrolyte layer are collectively called the "main electrolyte layer." In other words, the polymer electrolyte membrane consists of the "main electrolyte layer" and the "cathode-side electrolyte layer." Here, the cathode-side electrolyte layer may or may not be adjacent to the cathode catalyst layer, but it is preferable that it is adjacent.

[0104] The main electrolyte layer preferably does not contain carbon black. That is, in form (II), it is preferable that carbon black is contained only in the cathode-side electrolyte layer.

[0105] The main electrolyte layer preferably contains a hydrocarbon-based polymer electrolyte as described above, from the viewpoint of electrolytic performance. The cathode-side electrolyte layer preferably contains a fluorine-based polymer electrolyte as described above, from the viewpoint of adhesion, and more preferably a perfluorocarbon sulfonic acid-based polymer.

[0106] The carbon black content in the cathode-side electrolyte layer is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and particularly preferably 40% by mass or more, based on 100% by mass of the total solid content of the cathode-side electrolyte layer. The upper limit is preferably less than 50% by mass.

[0107] The thickness of the cathode-side electrolyte layer is preferably 0.1 μm or more, more preferably 0.5 μm or more, even more preferably 1 μm or more, and particularly preferably 3 μm or more. The above thickness is preferably 30 μm or less, more preferably 20 μm or less, even more preferably 15 μm or less, and particularly preferably 12 μm or less.

[0108] The thickness of the main electrolyte layer is preferably 35 μm or more, more preferably 40 μm or more, even more preferably 50 μm or more, and particularly preferably 60 μm or more. The above thickness is preferably 200 μm or less, more preferably 180 μm or less, even more preferably 150 μm or less, and particularly preferably 120 μm or less.

[0109] The main electrolyte layer is preferably configured to have a non-composite layer on one or both sides of the composite layer, and more preferably to have a non-composite layer on both sides of the composite layer (non-composite layer 1 / composite layer / non-composite layer 2).

[0110] Furthermore, the thickness of the cathode-side electrolyte layer is preferably 1 to 30%, more preferably 2 to 20%, and particularly preferably 3 to 15%, relative to 100% of the thickness of the polymer electrolyte membrane (total thickness of the main electrolyte layer and the cathode-adjacent electrolyte layer).

[0111] In form (II), it is preferable to further arrange a platinum group metal-containing layer on the anode catalyst layer side. That is, it is preferable that the layers are composed of "platinum group metal-containing layer / main electrolyte layer / cathode-side electrolyte layer" in that order from the anode catalyst layer side. Furthermore, it is preferable that the main electrolyte layer has a non-composite layer on one or both sides of the composite layer, and it is more preferable that it has a non-composite layer on both sides of the composite layer (non-composite layer 1 / composite layer / non-composite layer 2).

[0112] [Other Layers] The diaphragm may contain layers other than the polymer electrolyte membrane. These other layers are not particularly limited and include various functional layers. For example, a protective layer (for protecting the polymer electrolyte membrane), an adhesion-enhancing layer (for strengthening the adhesion between the polymer electrolyte membrane and the catalyst layer), and a gas transport suppression layer (for decomposing or capturing gases such as hydrogen, oxygen, and hydrogen peroxide). The other layers may also be layers for containing carbon black in the diaphragm. The other layers may contain less than 50% by mass of the polymer electrolyte, as long as it does not impair their function.

[0113] The aforementioned form (I) will now be described in detail. Form (I) is a laminated structure in which the diaphragm consists of a polymer electrolyte membrane and another layer, and the other layer containing carbon black is positioned on the cathode catalyst layer side relative to the polymer electrolyte membrane. Here, the other layer containing carbon black may or may not be adjacent to the cathode catalyst layer, but it is preferable that it be adjacent.

[0114] The other layers containing carbon black preferably further contain a polymer electrolyte. As the polymer electrolyte, the aforementioned fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes can be used. Among these, fluorine-based polymer electrolytes are preferred, and perfluorocarbon sulfonic acid polymers are even more preferred.

[0115] The carbon black content in the other layers is preferably 20% by mass or more, more preferably 30% by mass or more, even more preferably 40% by mass or more, and particularly preferably 50% by mass or more, based on 100% by mass of the total solid content of the other layers. The above content is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less, and particularly preferably 65% ​​by mass or less.

[0116] When other layers contain carbon black and a polymer electrolyte, the ratio of the mass of the polymer electrolyte (PE) to the mass of the carbon black (CB) (PE / CB) is preferably 0.3 or higher, more preferably 0.4 or higher, and particularly preferably 0.5 or higher. Furthermore, the above ratio (PE / CB) is preferably less than 1.2, more preferably less than 1.1, even more preferably less than 1.0, and particularly preferably less than 0.9. Note that the units of the mass of the carbon black (CB) and the mass of the polymer electrolyte (PE) are the same, for example, in milligrams.

[0117] Furthermore, if other layers contain carbon black, it is preferable that the polymer electrolyte membrane does not contain carbon black.

[0118] The thickness of the other layers is preferably 0.1 μm or more, more preferably 0.5 μm or more, even more preferably 1 μm or more, and particularly preferably 3 μm or more. The thickness of the other layers is preferably 30 μm or less, more preferably 20 μm or less, even more preferably 15 μm or less, and particularly preferably 12 μm or less. Furthermore, the thickness of the other layers is preferably 1 to 30%, more preferably 2 to 20%, and particularly preferably 3 to 15%, when the thickness of the polymer electrolyte membrane is taken as 100%.

[0119] In the above embodiment (I), the polymer electrolyte membrane is preferably a composite polymer electrolyte membrane. That is, the polymer electrolyte membrane is preferably configured to have a non-composite layer on one or both sides of the composite layer, and more preferably to have a non-composite layer on both sides of the composite layer (non-composite layer 1 / composite layer / non-composite layer 2). The composite layer and non-composite layer can preferably be those described above.

[0120] In the above embodiment (I), the polymer electrolyte membrane preferably contains platinum group elements. When the polymer electrolyte membrane contains platinum group elements, the layer configuration described above can be preferably applied. That is, the polymer electrolyte membrane has a multi-layer configuration, and it is preferable to place the platinum group-containing layer in the layer closest to the anode catalyst layer. A preferred specific example is a configuration in the following order from the anode catalyst layer side: "platinum group-containing layer / non-composite layer 1 / composite layer / non-composite layer 2 / other layer (containing carbon black)". The platinum group-containing layer can preferably be the one described above.

[0121] [Membrane / Catalyst Layer Structure] The water electrolysis membrane / catalyst layer structure of the present invention has an anode catalyst layer arranged on one side of a diaphragm and a cathode catalyst layer arranged on the other side.

[0122] The membrane-catalyst layer structure according to an embodiment of the present invention can be obtained, for example, by stacking an anode catalyst layer and a cathode catalyst layer on a diaphragm. Alternatively, the membrane-catalyst layer structure of the present invention can be completed during the assembly process of the membrane-electrode assembly described later. The latter embodiment will be described in detail later.

[0123] From the viewpoint of adhesion between the diaphragm and the catalyst layer, it is preferable that both the anode catalyst layer and the cathode catalyst layer are laminated on the diaphragm. Hereinafter, the anode catalyst layer and the cathode catalyst layer may be collectively referred to as the "catalyst layer."

[0124] Methods for laminating the catalyst layer onto the diaphragm include, for example, a coating method, a transfer method, or a combination of the coating method and the transfer method. These methods are not particularly limited, and known methods can be employed.

[0125] The coating method involves applying a coating solution for the catalyst layer to the diaphragm using a known coating method. The transfer method involves stacking a catalyst layer transfer sheet, which has the catalyst layer laminated on a transfer substrate, with the diaphragm and then heating and pressing them together.

[0126] Furthermore, if the diaphragm includes a polymer electrolyte membrane and other layers, the other layers can be laminated on the catalyst layer of the catalyst layer transfer sheet, and the membrane / catalyst layer structure can be manufactured by stacking this catalyst layer transfer sheet and the polymer electrolyte membrane and then heating and pressing them together. Specifically, the polymer electrolyte membrane is sandwiched between the "other layer / cathode catalyst layer transfer sheet," which has other layers laminated on the cathode catalyst layer of the cathode catalyst layer transfer sheet, and the anode catalyst layer transfer sheet, and then heated and pressed together.

[0127] In the membrane / catalyst layer structure according to the embodiment of the present invention, the thicknesses of the diaphragm, anode catalyst layer, and cathode catalyst layer are as described above, but it is preferable to adjust the relationship between the above thicknesses from the viewpoint of maintaining good electrolytic performance for a longer period of time. For example, it is preferable that the thicknesses of the anode catalyst layer and cathode catalyst layer are 25% or less relative to 100% of the thickness of the diaphragm. More specifically, the thickness of the anode catalyst layer is preferably 25% or less, more preferably 20% or less, and particularly preferably 15% or less, relative to 100% of the thickness of the diaphragm. The lower limit is preferably 1% or more. The thickness of the cathode catalyst layer is preferably 25% or less, more preferably 20% or less, and particularly preferably 15% or less, relative to 100% of the thickness of the diaphragm. The lower limit is preferably 1% or more.

[0128] [Membrane-Electrode Assembly for Water Electrolysis (MEA)] The membrane-catalyst layer structure of the present invention becomes a membrane-electrode assembly for water electrolysis (hereinafter sometimes simply referred to as "membrane-electrode assembly") when electrode substrates are arranged on both sides thereof. That is, the membrane-electrode assembly has an anode catalyst layer and an anode electrode substrate on one side of the diaphragm, and a cathode catalyst layer and a cathode electrode substrate on the other side.

[0129] The electrode substrate (which may also serve as a gas diffusion layer) is primarily intended for the application of electric current and is composed of a conductive material. As the electrode substrate, porous substrates such as metal or carbon can be used. Examples of metal porous substrates include metal nonwoven fabric, metal fiber sintered body, metal powder sintered body, and metal foam sintered body, while examples of carbon porous substrates include carbon felt, carbon paper, carbon cloth, and graphite particle sintered body.

[0130] As the anode electrode substrate, a porous metal substrate that exhibits excellent corrosion resistance in environments such as high potential, oxygen presence, and strong acidity is preferably used. From the above viewpoint, as the metal constituting the porous metal substrate, titanium, aluminum, nickel, stainless steel, and alloys mainly composed of at least one of these metals are preferred, and titanium and alloys mainly composed of titanium are particularly preferred.

[0131] As the cathode electrode substrate, a carbon porous substrate is preferred from the viewpoint of material cost and conductivity, and carbon paper is particularly preferred.

[0132] In the membrane-catalyst layer structure according to an embodiment of the present invention, the anode catalyst layer and the cathode catalyst layer can be laminated on an electrode substrate, either one or both. During the assembly process of the membrane-electrode assembly, the electrode substrate on which the catalyst layers are laminated and a diaphragm are placed, resulting in a configuration where the anode catalyst layer and the cathode catalyst layer are positioned opposite each other with the diaphragm in between. In other words, the membrane-catalyst layer structure is completed during the assembly process of the membrane-electrode assembly, and this form is also included in the present invention.

[0133] In the above configuration, the anode catalyst layer and the cathode catalyst layer may be laminated on the electrode substrate, but depending on the type of electrode substrate, sufficient adhesion to the catalyst layer may not be obtained, so it is preferable to select the appropriate type of electrode substrate. For example, since a carbon porous substrate has relatively good adhesion to the catalyst layer, the cathode catalyst layer may be laminated on a suitable cathode electrode substrate made of a carbon porous substrate, and the anode catalyst layer may be laminated on a diaphragm. As a method for laminating the catalyst layer onto the electrode substrate, the coating method or transfer method described above can be used.

[0134] [Water electrolysis cell and water electrolysis apparatus] The water electrolysis cell in the present invention includes a membrane-electrode assembly (MEA). The water electrolysis cell is divided internally by a diaphragm into an anode (composed of an anode catalyst layer and an anode electrode substrate) and a cathode (composed of a cathode catalyst layer and a cathode electrode substrate).

[0135] Figure 1 is a schematic cross-sectional view showing an example of a water electrolysis cell that can be used in the present invention. The water electrolysis cell 1 is divided into an anode 20 and a cathode 30 by a diaphragm 10. Here, the anode 20 is composed of an anode catalyst layer 21 and an anode electrode substrate 22, and the cathode 30 is composed of a cathode catalyst layer 31 and a cathode electrode substrate 32. These are sandwiched from both sides by separators 41 and 42.

[0136] The water electrolysis apparatus has multiple water electrolysis cells arranged together, and a power supply (not shown) is connected to the anode 20 and cathode 30 of each cell, to which a voltage is applied. The water electrolysis apparatus includes, as basic components, a water supply unit that supplies water to the water electrolysis cells, a power supply unit that supplies power to the water electrolysis cells, an oxygen discharge unit that discharges the generated oxygen, a hydrogen discharge unit that discharges the generated hydrogen, and a water discharge unit that discharges excess water after electrolysis.

[0137] The method of supplying water to the water electrolysis cell is not particularly limited, and known methods can be used. Specifically, methods such as supplying water to the anode, supplying water to the cathode, and supplying water to both the anode and the cathode can be used. In the above water supply methods, it is preferable to supply water from outside the water electrolysis cell using means such as a pump.

[0138] [Water Electrolysis Method] The water electrolysis method in the present invention is carried out using the water electrolysis cell and water electrolysis apparatus described above. That is, the water electrolysis method according to an embodiment of the present invention is a water electrolysis method in which water is supplied to a water electrolysis cell, whose interior is divided into an anode and a cathode by a diaphragm containing at least a polymer electrolyte membrane, and electrolyzed to produce oxygen at the anode and hydrogen at the cathode, wherein the anode catalyst layer constituting the anode contains iridium, the cathode catalyst layer constituting the cathode contains platinum, the cathode catalyst layer further contains carbon black and polymer electrolyte, the ratio of the mass of carbon black (C) to the mass of polymer electrolyte (I) (I / C) in the cathode catalyst layer is 0.40 or more and less than 1.00, and the carbon black contains less than 1.4% by mass of volatile matter (carbon black A). The polymer electrolyte membrane, the diaphragm containing it, the anode catalyst layer and the cathode catalyst layer in this method can preferably be those described above.

[0139] As mentioned above, water can be supplied to the anode only, the cathode only, or both the anode and the cathode in a water electrolysis cell. In particular, it is preferable to supply water to at least the anode, where oxygen and protons are produced by the oxidation reaction of water.

[0140] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The various measurement conditions are as follows.

[0141] (1) Molecular weight of the polymer The number-average molecular weight and weight-average molecular weight of the polymer were measured by gel permeation chromatography (GPC). A Tosoh Corporation HLC-8022GPC was used as the GPC instrument. Two Tosoh Corporation TSK gel SuperHM-H columns (inner diameter 6.0 mm, length 15 cm) were used as the GPC columns. Measurements were taken with N-methyl-2-pyrrolidone solvent (N-methyl-2-pyrrolidone solvent containing 10 mmol / L of lithium bromide) at a flow rate of 0.2 mL / min, and the number-average molecular weight and weight-average molecular weight were determined by converting to standard polystyrene.

[0142] (2) Ion exchange capacity (IEC) was measured by the neutralization titration method shown in 1) to 4) below. The measurement was performed three times and the arithmetic mean was taken. 1) After proton substitution and thorough washing with pure water, the moisture of the block copolymer was wiped off and the material was vacuum dried at 100°C for more than 12 hours, and the dry weight was determined. 2) 50 mL of 5 wt% sodium sulfate aqueous solution was added to the block copolymer and allowed to stand for 12 hours to perform ion exchange. 3) The resulting sulfuric acid was titrated using a 0.01 mol / L sodium hydroxide aqueous solution. A commercially available titration phenolphthalein solution of 0.1 w / v% was added as an indicator, and the endpoint was taken when the solution turned a pale reddish-purple. 4) The IEC was calculated using the following formula: IEC (meq / g) = [Concentration of sodium hydroxide aqueous solution (mol / ml) × Volume added (ml)] / Dry weight of sample (g).

[0143] (3) Measurement of the thickness of the diaphragm and catalyst layer Cross-sections of the diaphragm and each catalyst layer, which had undergone the following pretreatment according to the conditions below, were observed with a scanning electron microscope (SEM), and the thickness of the diaphragm, anode catalyst layer and cathode catalyst layer were measured from the obtained images. Apparatus: Field emission scanning electron microscope (FE-SEM) S-4800 (manufactured by Hitachi High-Technologies) Acceleration voltage: 2.0 kV Pretreatment: Platinum was coated onto the cross-sectional sample prepared by the BIB method described below and measured. BIB method: A cross-sectional sample preparation apparatus using an argon ion beam. A shielding plate was placed directly above the sample, and an argon broad ion beam was irradiated from above to perform etching, thereby creating an observation surface and an analysis surface (cross-section).

[0144] (4) Measurement of volatile components of carbon black First, the sample was heated at 105-110°C for 2 hours to dry it. Next, the dried sample was fired at 950°C for 10 minutes under a vacuum of 3 torr. Then, the volatile components were calculated from the mass (weight loss) before and after firing using the following formula: Volatile components (%) = [(Mass before firing) - (Mass after firing)] / (Mass before firing) × 100.

[0145] (5) Measurement of the BET specific surface area of ​​carbon black The measurement was performed in accordance with the method compliant with JIS K6217-2:2017. Specifically, a container containing degassed carbon black was immersed in liquid nitrogen, and the amount of nitrogen adsorbed on the carbon black surface at equilibrium was measured, and the specific surface area (m²) was calculated from this value. ２ The value per g was calculated.

[0146] (6) Evaluation of electrolytic performance In order to evaluate the electrolytic performance of the film-catalyst layer laminate, the film-electrode assembly was fabricated in the following manner.

[0147] [Fabrication of film-electrode assemblies] Film-electrode assemblies were fabricated by laminating commercially available carbon paper as the cathode electrode substrate on the cathode catalyst layer side of the film-catalyst layer laminates fabricated in the examples and comparative examples, and commercially available porous titanium sintered plate as the anode electrode substrate on the anode catalyst layer side.

[0148] [Evaluation of Electrolytic Performance] The film-electrode assembly prepared above was placed in the JARI standard cell "Ex-1" manufactured by Eiwa Co., Ltd. (electrode area 25 cm²). ２ The device was set to a specific configuration, and the cell temperature was set to 70°C. Deionized water with an electrical conductivity of 1 μS / cm or less was supplied to both the anode and cathode at atmospheric pressure at a flow rate of 0.2 L / min, resulting in a current density of 1.2 A / cm². ２ A voltage was applied to the water to achieve the desired result, and water electrolysis was carried out for 700 hours. The applied voltage was measured at 0 hours and 700 hours of electrolysis, and the voltage increase rate after 700 hours was calculated using the following formula 1. Voltage increase rate (%) = (V １ -V ０ ) / V ０ ×100 ...Formula 1 In the formula, V １ V is the applied voltage after 700 hours. ０ This represents the initial (0 hour) applied voltage.

[0149] A lower voltage (initial voltage) during the above 0-hour period indicates higher electrolytic performance, and a lower voltage rise rate indicates less degradation of electrolytic performance, i.e., better durability.

[0150] (7) Adhesion (1) The adhesion between the diaphragm and the cathode catalyst layer was evaluated in the following manner. The membrane-electrode assembly was removed from the cell used in "(6) Evaluation of Electrolytic Performance" above (the cell after 700 hours of operation). The carbon paper was peeled off from this membrane-electrode assembly. At this time, if the adhesion between the diaphragm and the cathode catalyst layer has decreased, part or all of the cathode catalyst layer will peel off together with the carbon paper, exposing the diaphragm. The exposed state of the diaphragm was observed visually and evaluated according to the following criteria. A: The diaphragm is not exposed at all. B: Only a part of the diaphragm is exposed. C: The entire diaphragm is exposed.

[0151] (8) Adhesion (2) In the same manner as in "(6) Evaluation of Electrolytic Performance" above, however, the film-electrode assembly was removed from the cell after 2,500 hours of operation. Carbon paper was peeled off this film-electrode assembly and evaluated in the same manner as in Adhesion (1) above. The same evaluation criteria as in Adhesion (1) were used.

[0152] (9) Measurement of hydrogen gas concentration in oxygen gas Under the above electrolysis performance evaluation conditions, the generated oxygen gas discharged from the anode was collected 50 hours after the start of operation, water was removed from this oxygen gas, and it was introduced into a gas chromatography analyzer. The hydrogen gas concentration (ppm, volume ratio) in the oxygen gas was measured according to the following conditions. Apparatus: Agilent Technologies, Inc., 490 microGC Column: Agilent Technologies, Inc., part number 493001360, type BF CHA 10m MS5A Unl, Facl Carrier gas: Ar Column temperature: 100℃ [Carbon black] The following carbon black (CB) was prepared. ・CB1: Tokai Carbon Co., Ltd., Toka Black #3855 (volatile content 0.12 mass%, BET specific surface area 95m²) ２ / g) ・CB2: Denka Black manufactured by Denka Co., Ltd. (Volatile content 0.16% by mass, BET specific surface area 69 m²) ２ / g) ・CB3: 3050B manufactured by Mitsubishi Chemical Corporation (volatile content 0.5% by mass, BET specific surface area 50 m²) ２ / g) ・CB4: Tokai Carbon Co., Ltd. Toka Black #4300 (Volatile content 0.7% by mass, BET specific surface area 33m²) ２ / g) ・CB5: 3400B manufactured by Mitsubishi Chemical Corporation (volatile content 1.0% by mass, BET specific surface area 165 m²) ２ / g) ・CB6: Ketjenblack EC300JD manufactured by Lion Corporation (volatile content 0.5% by mass, BET specific surface area 800m²) ２ / g) ・CB7: Cabot Vulcan XC-72 (Volatile content 1.6% by mass, BET specific surface area 250m²) ２ / g) [Preparation of platinum-supported carbon particles] <Preparation of Pt / CB1> Dinitrodiamine platinum nitric acid solution (platinum content: 30.8g) was diluted with pure water in a colloid mill to make a 4685 mL aqueous solution.

[0153] Then, 30.8 g of carbon black (CB1) was added to the above-mentioned dinitrodiammineplatinic acid aqueous solution while being crushed. After crushing for 1 hour, 318 mL of 98% ethanol (34.5 mol per 1 mol of platinum, 6.4 vol%) was added as a reducing agent and mixed. This mixed solution was refluxed at approximately 95°C for 6 hours to reduce the platinum. After that, it was filtered, dried and washed. Through the above process, Pt / CB1 in which platinum was supported on carbon black (CB1) was obtained. The platinum load of this platinum-supported CB1 was 48 mass%.

[0154] <Preparation of Pt / CB2 to Pt / CB7> The preparations were carried out in the same manner as in the preparation of Pt / CB1 above, except that CB2 to CB7 were used instead of carbon black (CB1).

[0155] [Preparation of Platinum-Ruthenium Alloy-Supported Carbon Particles] <Preparation of PtRu / CB1> 4500 g of dinitrodiamine platinum nitric acid solution containing 1.5% by mass of platinum was mixed with 100 g of carbon black (CB1) and stirred. Then, 550 ml of 98% ethanol was added as a reducing agent. This solution was stirred and mixed at its boiling point (approximately 95°C) for 6 hours to support the platinum on the carbon black (CB1).

[0156] Next, 35.96 g of a ruthenium chloride solution containing 8.232 mass% ruthenium (ruthenium: 2.96 g) was mixed with 710 ml of water, stirred, and then 9.5 g of the platinum-supported carbon black (platinum: 3.8 g) was immersed in it. 65 ml of 95% ethanol was then added, and the mixed solution was stirred and reacted at its boiling point (approximately 95°C) for 6 hours. After the reaction was complete, the mixture was filtered, washed, and dried at 60°C to obtain platinum and ruthenium-supported carbon black. Next, a heat treatment for alloying platinum and ruthenium was performed by holding the mixture at 900°C for 1 hour in a 50% hydrogen gas (nitrogen balance) environment. The molar ratio of platinum to ruthenium was 4:6. The loading rate of the platinum-ruthenium alloy was 62 mass%.

[0157] [Synthesis of Polyether Ketone Block Copolymers (PEK Blocks)] [Synthesis Example 1] (Synthesis of 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane (K-DHBP) represented by the following chemical formula (G1)) 49.5 g of 4,4'-dihydroxybenzophenone, 134 g of ethylene glycol, 96.9 g of trimethyl orthoformate, and 0.50 g of p-toluenesulfonic acid monohydrate were charged into a 500 mL flask equipped with a stirrer, thermometer, and distillation tube and dissolved. The mixture was then kept warm and stirred at 78-82°C for 2 hours. The internal temperature was then gradually increased to 120°C and heated until the distillation of methyl formate, methanol, and trimethyl orthoformate completely stopped. After the reaction mixture was cooled to room temperature, it was diluted with ethyl acetate, and the organic layer was washed with 100 mL of 5% potassium carbonate aqueous solution. After liquid-liquid extraction, the solvent was removed by distillation. 80 mL of dichloromethane was added to the residue to precipitate crystals, which were filtered and dried to obtain 52.0 g of 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane, represented by the following chemical formula (G1). GC analysis of these crystals revealed 99.9% 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane and 0.1% 4,4'-dihydroxybenzophenone. The purity was 99.9%.

[0158]

[0159] [Synthesis Example 2] (Synthesis of disodium-3,3'-disulfonate-4,4'-difluorobenzophenone represented by the chemical formula (G2) below) 109.1 g of 4,4'-difluorobenzophenone (reagent from Sigma-Aldrich Japan) was reacted in 150 mL of fuming sulfuric acid (50% SO3) (reagent from Fujifilm Wako Pure Chemical Industries, Ltd.) at 100°C for 10 hours. Then, it was gradually added to a large amount of water, neutralized with sodium hydroxide, and then 200 g of sodium chloride (NaCl) was added to precipitate the product. The obtained precipitate was filtered off and recrystallized in an aqueous ethanol solution to obtain disodium-3,3'-disulfonate-4,4'-difluorobenzophenone represented by the chemical formula (G2) below. The purity was 99.3%.

[0160]

[0161] [Synthesis Example 3] (Synthesis of nonionic oligomer a1 represented by the following general formula (G3)) In a 2,000 mL SUS polymerization apparatus equipped with a stirrer, nitrogen inlet tube, and Dean-Stark trap, 16.59 g of potassium carbonate (Aldrich reagent, 120 mmol), 25.83 g (100 mmol) of K-DHBP obtained in Synthesis Example 1, and 20.3 g of 4,4'-difluorobenzophenone (Aldrich reagent, 93 mmol) were added. After nitrogen purging, 300 mL of N-methylpyrrolidone (NMP) and 100 mL of toluene were added, dehydration was performed at 150°C, the temperature was raised to remove the toluene, and polymerization was carried out at 170°C for 3 hours. Reprecipitation and purification were performed with a large amount of methanol to obtain the terminal hydroxyl compound of nonionic oligomer a1. The number-average molecular weight of this terminal hydroxyl compound of nonionic oligomer a1 was 10,000.

[0162] In a 500 mL three-necked flask equipped with a stirrer, nitrogen inlet tube, and Dean-Stark trap, 1.1 g of potassium carbonate (Sigma-Aldrich Japan reagent, 8 mmol) and 20.0 g (2 mmol) of the terminal hydroxyl form of the nonionic oligomer a1 were added. After purging the apparatus with nitrogen, 100 mL of NMP and 30 mL of toluene were added, and the mixture was dehydrated at 100°C, followed by heating to remove the toluene. Furthermore, 2.2 g of hexafluorobenzene (Sigma-Aldrich Japan reagent, 12 mmol) was added, and the reaction was carried out at 105°C for 12 hours. Reprecipitation and purification with a large amount of isopropyl alcohol were performed to obtain nonionic oligomer a1 (terminal: fluoro group) represented by the following general formula (G3). The number average molecular weight was 11,000.

[0163]

[0164] [Synthesis Example 4] (Synthesis of ionic oligomer a2 represented by the following general formula (G4)) In a 2,000 mL SUS polymerization apparatus equipped with a stirrer, nitrogen inlet tube, and Dean-Stark trap, 27.6 g of potassium carbonate (Sigma-Aldrich Japan reagent, 200 mmol), 12.9 g (50 mmol) of K-DHBP obtained in Synthesis Example 1, 9.3 g of 4,4'-biphenol (Sigma-Aldrich Japan reagent, 50 mmol), 39.3 g (93 mmol) of disodium-3,3'-disulfonate-4,4'-difluorobenzophenone obtained in Synthesis Example 2, and 17.9 g of 18-crown-6 (Fujifilm Wako Pure Chemical Industries, Ltd., 82 mmol) were added. After nitrogen purging, 300 mL of NMP and 100 mL of toluene were added, dehydration was performed at 150°C, the temperature was raised to remove the toluene, and polymerization was carried out at 170°C for 6 hours. Reprecipitation purification with a large amount of isopropyl alcohol yielded ionic oligomer a2 (terminal: hydroxyl group) represented by the following general formula (G4). The number-average molecular weight was 16,000. In general formula (G4), M represents a hydrogen atom, Na, or K.

[0165]

[0166] (Synthesis of polyetherketone block copolymer (indicated as "PEK block" in Table 1)) 16 g of ionic oligomer a2 and 11 g of nonionic oligomer a1 were placed in a 2,000 mL SUS polymerization apparatus equipped with a stirrer, nitrogen inlet tube, and Dean-Stark trap. NMP was added so that the total amount of oligomers was 7 wt%, and the reaction was carried out at 105°C for 24 hours.

[0167] The precipitate was reprecipitated in a large amount of isopropyl alcohol / NMP mixture (mass ratio 2 / 1), and the resulting precipitate was recovered by filtration and washed with a large amount of isopropyl alcohol block to obtain block copolymer b1. The weight-average molecular weight of this polyetherketone-based block copolymer was 340,000, and the ion exchange capacity (IEC) was 2.10 meq / g.

[0168] [Fluorine-based polymer electrolyte] A commercially available "Nafion" (registered trademark) product, part number D2020, manufactured by Chemours Co., Ltd., was used as the fluorine-based polymer electrolyte. Its ion exchange capacity (IEC) was 0.91 meq / g.

[0169] [Example 1] [Preparation of Polymer Electrolyte Membrane] Toray Industries, Inc.'s PET film "Lumirror" (registered trademark) 125T60 was bonded and fixed to a SUS plate using "Kapton" (registered trademark) tape. 20 g of the polyetherketone-based block copolymer synthesized above was dissolved in NMP to obtain a transparent solution with a polymer concentration of 13% by mass. The obtained solution was pressure filtered using a 1 μm polypropylene filter, then cast onto the PET film and dried to obtain a film-like membrane. Furthermore, this membrane was immersed in a 10% by mass sulfuric acid aqueous solution at 80°C for 24 hours to undergo proton substitution and deprotection reactions, and then thoroughly washed by immersion in a large excess of pure water for 24 hours to obtain a polymer electrolyte membrane (thickness 85 μm). This polymer electrolyte membrane was used as a diaphragm.

[0170] [Fabrication of Membrane and Catalyst Layer Structure] A membrane and catalyst layer laminate was fabricated by laminating the following anode catalyst layer on one side of the polymer electrolyte membrane (diaphragm) prepared above, and the following cathode catalyst layer on the other side. The mass of iridium element in the anode catalyst layer was 0.9 mg / cm³. ２The mass of platinum element in the cathode catalyst layer is 0.3 mg / cm³. ２ The thickness of each component was adjusted to achieve this result.

[0171] <Anode catalyst layer> Total solid content: Catalyst particles (IrO2 manufactured by Umicore) ２ The catalyst contained 10 parts by mass of Elist Ir75 0480 (Ir content 75%) and 1.3 parts by mass of a fluorine-based polymer electrolyte ("Nafion" (registered trademark), product number D2020, manufactured by Chemours K.K.). The ratio of the mass of the polymer electrolyte to the mass of iridium element (WPE / Wir) was 0.17. The thickness of this anode catalyst layer was 9 μm.

[0172] <Cathode Catalyst Layer> The total solid content includes 10 parts by mass of catalyst particles (Pt / CB1) and 4 parts by mass of fluorine-based polymer electrolyte ("Nafion" (registered trademark), product number D2020, manufactured by Chemours Co., Ltd.). The ratio (I / C) of the mass of carbon black (CB1) (C) to the mass of fluorine-based polymer electrolyte (I) is 0.77. The thickness of this cathode catalyst layer was 8 μm.

[0173] [Examples 2-9 and Comparative Examples 1-3] In Example 1, the membrane / catalyst layer structure was prepared in the same manner as in Example 1, except that the type of catalyst particles (Pt / CB2 to Pt / CB7) in the cathode catalyst layer and the mass ratio (I / C) of carbon black to fluorine-based polymer electrolyte were changed as shown in Table 1. The (I / C) ratio was adjusted by increasing or decreasing the amount of fluorine-based polymer electrolyte added per 10 parts by mass of catalyst particles.

[0174] [Comparative Example 4] A film / catalyst layer structure was prepared in the same manner as in Example 6, except that the anode catalyst layer was changed as described below. The mass of platinum element in this anode catalyst layer was 0.3 mg / cm³. ２ The thickness was adjusted to achieve this result.

[0175] <Anode Catalyst Layer> The catalyst layer contains 10 parts by mass of catalyst particles (platinum-supported carbon particles TEC10E50E manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. (platinum support rate 50% by mass)) and 4 parts by mass of fluorine-based polymer electrolyte ("Nafion" (registered trademark) product number D2020 manufactured by Chemours K.K.) as total solids. The ratio of the mass of the polymer electrolyte to the mass of the platinum element is 0.80. The thickness of this anode catalyst layer was 8 μm.

[0176] [Example 10] A film / catalyst layer structure was prepared in the same manner as in Example 1, except that the catalyst particles in the cathode catalyst layer were changed to PtRu / CB1.

[0177] [Example 11] A membrane / catalyst layer structure was prepared in the same manner as in Example 1, except that the polyetherketone block copolymer was replaced with a fluorine-based polymer electrolyte ("Nafion" (registered trademark) product number D2020, manufactured by Chemours K.K.).

[0178] <Example of form (I) in which the diaphragm contains carbon black> [Example 12] <Preparation of the diaphragm> Carbon black was incorporated into the diaphragm. Specifically, the coating solution of the "other layer" described below was applied to one side of the polymer electrolyte membrane of Example 1 with a solid content coating amount of 12 g / m². ２ The material was coated in this manner, dried, and then the "other layer" was laminated. The thickness of the "other layer" was 9 μm. The carbon black content per 100 parts by mass of the total mass of the polymer electrolyte contained in the diaphragm was 5.4 parts by mass.

[0179] <Coating solution for "other layers"> A coating solution with a solid content of 10% by mass was prepared by dispersing 110 parts by mass of carbon black (CB) and 8 parts by mass (on a solid content basis) of fluorinated polymer electrolyte ("Nafion" (registered trademark), product number D2020, manufactured by Chemours Co., Ltd.) in a solvent (a mixed solvent of water and 1-propyl alcohol in a mass ratio of 4:6). The ratio of the mass of polymer electrolyte (PE) to the mass of carbon black (CB) in this coating solution (PE / CB) is 0.8.

[0180] <Fabrication of the membrane and catalyst layer structure> The membrane and catalyst layer structure was fabricated in the same manner as in Example 1, except that the cathode catalyst layer was laminated on the "other layer" side of the diaphragm and the anode catalyst layer was laminated on the polymer electrolyte membrane side.

[0181] [Examples 13, 14] The film and catalyst layer structures were prepared in the same manner as in Example 12, except that the carbon black used in the other layers was changed to CB3 and CB5, respectively.

[0182] [Example 15] A film-catalyst layer structure was prepared in the same manner as in Example 12, except that the cathode catalyst layer was changed to the cathode catalyst layer of Example 6, and the carbon black used in the other layers was changed to CB7.

[0183] <Example of a form (II) in which the diaphragm contains carbon black> [Example 16] A membrane / catalyst layer structure was prepared in the same manner as in Example 12, except that the "other layer" was changed to the "cathode-side electrolyte layer" described below.

[0184] <Cathode-side electrolyte layer> 110 parts by mass of carbon black (CB) and 10.5 parts by mass (on a solid content basis) of fluorine-based polymer electrolyte ("Nafion" (registered trademark) product number D2020, manufactured by Chemours Co., Ltd.) were dispersed in a solvent (a mixed solvent of water and 1-propyl alcohol in a mass ratio of 4:6) to prepare a coating solution with a solid content concentration of 10% by mass.

[0185] [Evaluation] The electrolytic performance and adhesion of the film and catalyst layer structures prepared in the above examples and comparative examples were evaluated. The results are shown in Table 1. Comparative Example 4, which used platinum as the anode catalyst, became unable to perform electrolysis after 700 hours.

[0186]

[0187] The following symbols in the table have the meanings of their respective meanings:

[0188] *1: Volatile components of carbon black.

[0189] *2: "Other layers" or "cathode-side electrolyte layer". A diagonal line indicates that neither other layers nor the cathode-side electrolyte layer are used.

[0190] [Example 17] <Preparation of a polymer electrolyte membrane> A polymer electrolyte membrane consisting of a "platinum group metal-containing layer / non-composite layer 1 / composite layer / non-composite layer 2" was prepared according to the following procedure.

[0191] <Preparation of a laminate consisting of "non-composite layer 1 / composite layer / non-composite layer 2"> Using "Kapton" (registered trademark) tape, Toray Industries, Inc.'s PET film "Lumirror" (registered trademark) 125T60 was bonded and fixed to a SUS plate. 20 g of the polyetherketone-based block copolymer synthesized above was dissolved in NMP to obtain a transparent solution (coating solution 1) with a polymer concentration of 13% by mass. The obtained coating solution 1 was pressure filtered using a 1 μm polypropylene filter, and then cast onto the PET film. The mesh fabric described below was then bonded on top and impregnated with coating solution 1. Coating solution 1 was further applied to the mesh fabric and dried to obtain a laminate consisting of "non-composite layer 1 / composite layer / non-composite layer 2". Here, non-composite layer 1 refers to the non-composite layer formed on the side of the PET film as viewed from the mesh fabric, and non-composite layer 2 is the non-composite layer formed on the side opposite to the side of the PET film as viewed from the mesh fabric.

[0192] <Mesh Fabric> A mesh fabric made of liquid crystal polyester fibers, manufactured according to Manufacturing Example 1 of International Publication No. 2019 / 188960, was used.

[0193] <Lamination of Platinum Group Metal-Containing Layers> The following coating solution (coating solution 2) was applied to the non-composite layer 2 of the above laminate, dried to laminate the platinum group metal-containing layer, and then immersed in a 10% by mass sulfuric acid aqueous solution at 80°C for 24 hours to perform proton substitution and deprotection reactions. After that, it was thoroughly washed by immersing in a large excess amount of pure water for 24 hours to obtain a polymer electrolyte membrane (thickness 60 μm).

[0194] <Coating Solution 2> Ten parts by mass of the polyether ketone block copolymer synthesized above and five parts by mass of platinum particles (platinum nanoparticles manufactured by Sigma-Aldrich Japan, average particle size 200 nm) were dissolved and dispersed in NMP, and the resulting solution was dispersed in a bead mill to prepare a coating solution for a platinum group metal-containing layer (Coating Solution 2).

[0195] The composition and thickness of the polymer electrolyte membrane obtained above were "non-composite layer 1 (10 μm) / composite layer (35 μm) / non-composite layer 2 (10 μm) / platinum group metal-containing layer (5 μm)". The basis weight of the platinum particles was 0.05 mg / cm². ２ That was the case.

[0196] <Preparation of the diaphragm> A diaphragm was obtained by laminating another layer similar to the one used in Example 12 onto the non-composite layer 1 of the polymer electrolyte membrane obtained above.

[0197] <Preparation of the film and catalyst layer structure> The film and catalyst layer structure was prepared in the same manner as in Example 1, except that the cathode catalyst layer was laminated on the "other layer" side of the diaphragm and the anode catalyst layer was laminated on the platinum group metal-containing layer side.

[0198] [Evaluation] In evaluating the film / catalyst layer structure of Example 17, the film / catalyst layer structure of Example 12 was used as a blank. As a result, the electrolytic performance and adhesion of the film / catalyst layer structure of Example 17 were both about the same as those of Example 12. On the other hand, the hydrogen gas concentration in the oxygen gas was 4,000 ppm in Example 12 and 800 ppm in Example 17. From these results, it can be seen that the hydrogen gas concentration is significantly reduced by placing a platinum group metal-containing layer on the anode catalyst layer side.

[0199] 1 Water electrolysis cell 10 Diaphragm 20 Anode 21 Anode catalyst layer 22 Anode electrode substrate 30 Cathode 31 Cathode catalyst layer 32 Cathode electrode substrate 41, 42 Separator

Claims

1. A membrane / catalyst layer structure for water electrolysis comprising at least a diaphragm containing a polymer electrolyte membrane, and an anode catalyst layer and a cathode catalyst layer arranged opposite each other across the diaphragm, wherein the anode catalyst layer contains iridium, the cathode catalyst layer contains platinum, the cathode catalyst layer further contains carbon black and a polymer electrolyte, the ratio (I / C) of the mass of the carbon black (C) to the mass of the polymer electrolyte (I) in the cathode catalyst layer is 0.40 or more and less than 1.00, and all or part of the carbon black contained in the cathode catalyst layer is carbon black with a volatile content of less than 1.4% by mass (hereinafter, carbon black with a volatile content of less than 1.4% by mass is referred to as "carbon black A").

2. The BET specific surface area of ​​carbon black A contained in the cathode catalyst layer is 850 m². ２ The water electrolysis membrane / catalyst layer structure according to claim 1, wherein the amount is less than or equal to / g.

3. The membrane / catalyst layer structure for water electrolysis according to claim 1, wherein the diaphragm contains carbon black.

4. The water electrolysis membrane / catalyst layer structure according to claim 3, wherein all or part of the carbon black contained in the diaphragm is carbon black A.

5. The water electrolysis membrane / catalyst layer structure according to claim 1, wherein the ion exchange capacity of the polymer electrolyte contained in the cathode catalyst layer is less than 1.50 meq / g.

6. The water electrolysis membrane / catalyst layer structure according to claim 1, wherein the cathode catalyst layer contains platinum-supported carbon particles, and all or part of the platinum element is the platinum element in the platinum-supported carbon particles.

7. The water electrolysis membrane / catalyst layer structure according to claim 6, wherein all or part of the carbon particles of the platinum-supported carbon particles are carbon black A.

8. The water electrolysis membrane / catalyst layer structure according to claim 1, wherein all or part of the iridium elements contained in the anode catalyst layer are iridium elements in iridium oxide.

9. The water electrolysis membrane / catalyst layer structure according to claim 1, wherein when the total mass of all metal elements contained in the anode catalyst layer is taken as 100% by mass, the mass of iridium elements in the anode catalyst layer is 50% by mass or more.

10. The water electrolysis membrane / catalyst layer structure according to claim 1, wherein the anode catalyst layer contains an iridium element and a polymer electrolyte, and when the mass of the iridium element contained in the anode catalyst layer is (Wir) and the mass of the polymer electrolyte is (WPE), the ratio (WPE / Wir) is 0.05 or more and less than 0.

5.

11. The water electrolysis membrane / catalyst layer structure according to claim 1, wherein the ion exchange capacity of the polymer electrolyte contained in the polymer electrolyte membrane is 1.50 meq / g or more.

12. Ion exchange capacity (IEC) of the polymer electrolyte contained in the cathode catalyst layer. ＣＡ ) and the ion exchange capacity (IEC) of the polymer electrolyte contained in the polymer electrolyte membrane. ＰＥ ) ratio (IEC ＣＡ / IEC ＰＥ The water electrolysis membrane / catalyst layer structure according to claim 1, wherein the ratio is 0.90 or less.

13. A water electrolysis membrane / electrode assembly comprising an electrode substrate bonded to the anode catalyst layer and the cathode catalyst layer of the water electrolysis membrane / catalyst layer structure according to any one of claims 1 to 12.

14. A water electrolysis cell comprising the water electrolysis membrane / electrode assembly described in claim 13.

15. A water electrolysis apparatus comprising the water electrolysis cell described in claim 14.

16. A water electrolysis method comprising supplying water to a water electrolysis cell, whose interior is divided into an anode and a cathode by a diaphragm containing a polymer electrolyte membrane, and electrolyzing it to produce oxygen at the anode and hydrogen at the cathode, wherein the anode catalyst layer constituting the anode contains iridium, the cathode catalyst layer constituting the cathode contains platinum, the cathode catalyst layer further contains carbon black and a polymer electrolyte, the ratio (I / C) of the mass (C) of the carbon black to the mass (I) of the polymer electrolyte in the cathode catalyst layer is 0.40 or more and less than 1.00, and all or part of the carbon black contained in the cathode catalyst layer is carbon black with a volatile content of less than 1.4% by mass.

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