JP7664170B2 - Catalyst-supporting porous substrate for water electrolysis, electrode for water electrolysis, gas diffusion layer, stack cell for water electrolysis, and cell module for water electrolysis - Google Patents

Description

The present invention relates to a catalyst-supported porous substrate for water electrolysis and an electrode for water electrolysis. More specifically, the present invention relates to a catalyst-supported porous substrate for water electrolysis, an electrode for water electrolysis, a gas diffusion layer, a stack cell for water electrolysis, and a cell module for water electrolysis, which are used when performing water electrolysis using a polymer electrolyte membrane (PEM) and are characterized by the catalyst support form, and the ionomer in contact with the catalyst and its presence form (filling form).

A membrane electrode assembly (MEA) used in a fuel cell or a "water electrolysis cell for a hydrogen generation device" has an electrode on which a catalyst layer is formed, and a polymer electrolyte membrane (PEM membrane) sandwiched between an anode and a cathode.
In a conventional membrane electrode assembly (MEA), the catalyst is present in the form of a layer, i.e., as a catalyst layer, in the membrane electrode assembly (MEA), as shown in Fig. 1(b), Fig. 2(b), and Fig. 2(c). A gas diffusion layer is in contact with the catalyst layer, so that the generated gas can be taken out.

The invention described in Patent Document 1 relates to a manufacturing method of a membrane electrode assembly (MEA) in which an MEA is determined to be a pass product when the amount of sulfate ions contained in an electrode catalyst layer is equal to or less than a specified value. Patent Document 1 discloses a membrane electrode assembly in which an electrode catalyst layer is formed on the surface of a polymer electrolyte membrane, and a manufacturing method thereof.
The manufacturing steps in this manufacturing method are described as follows: an electrolyte membrane is prepared, an electrode catalyst layer is prepared, a catalyst-coated membrane is fabricated using the prepared electrolyte membrane and electrode catalyst layer, a gas diffusion layer is prepared, and a membrane electrode assembly (MEA) is fabricated using the prepared catalyst-coated membrane and the prepared gas diffusion layer.

However, the membrane electrode assembly (MEA) in Patent Document 1 is for use in fuel cells, and the electrode catalyst layer of the membrane electrode assembly (MEA) is produced by continuously applying and drying an ink containing an ionomer and a catalyst onto the surface of a substrate or an electrolyte membrane (thereby reducing the amount of sulfate ions).

Patent Document 2 discloses a method for producing a durable membrane electrode assembly in which the "diffusion medium on which a catalyst layer is deposited" is not hot-pressed to a membrane. In this patent document 2, a catalyst layer is deposited on a diffusion medium layer, and then an ionomer layer is provided on the surface of the catalyst layer to produce a membrane electrode assembly (MEA) for a fuel cell.

However, the membrane electrode assembly (MEA) in Patent Document 2 is for a fuel cell, and the catalyst is formed on a diffusion medium in the form of a catalyst layer, and the ionomer is also sprayed onto the entire surface of the catalyst layer. In addition, the catalyst layer of the membrane electrode assembly (MEA) is formed by continuously rolling or applying a slurry onto a diffusion medium layer, and the catalyst is first coated on a transfer substrate and then transferred to a membrane by a hot press.

In recent years, the demand for hydrogen has increased, creating a demand for superior water electrolysis technology. However, conventional technology has not been sufficient, and there has been a demand for superior technology in terms of the performance and durability of water electrolysis cells.

Patent No. 6128099 Patent No. 4738350

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a catalyst-supporting porous substrate which has a new configuration (form) different from conventional ones and which provides excellent water electrolysis performance and durability for water electrolysis cells (including single cells for water electrolysis, stack cells for water electrolysis, etc.).
In other words, an object of the present invention is to provide a water electrolysis electrode having excellent water electrolysis performance and durability, and a catalyst-supporting porous substrate for use in the water electrolysis electrode.

As a result of intensive research aimed at solving the above problems, the present inventors have found that water electrolysis performance and durability are excellent when the concept of a "catalyst layer" is deviated from and an embodiment or configuration in which a catalyst layer is not provided on a (porous) substrate is not used.
The inventors have also found that by supporting a catalyst in the pores constituting the porous substrate or in the fibers forming the porous substrate, excellent water electrolysis performance and durability can be achieved.

In addition, it was discovered that by supporting a catalyst as described above and sandwiching a polymer electrolyte membrane (PEM) between the resulting catalyst-supported porous substrate, catalyst detachment due to gas lift is less likely to occur, and performance degradation is prevented.

Furthermore, they discovered that by filling the electrode surface in contact with the polymer electrolyte membrane, i.e., from the surface to the inside of the catalyst-supported porous substrate, with ionomer in a specific manner (state), it is possible to reduce the cell voltage (electrolysis voltage) and obtain a catalyst-supported porous substrate for water electrolysis that has excellent water electrolysis performance and durability, which led to the completion of the present invention.

That is, the present invention provides a catalyst-supporting porous substrate in a single cell for water electrolysis, the substrate sandwiching a polymer electrolyte membrane (PEM), forming a cathode or an anode in contact with the polymer electrolyte membrane (PEM), and having a structure that also functions as a gas diffusion layer,
the catalyst is supported on the side surfaces of the pores of the porous substrate or on the side surfaces of the fibers forming the porous substrate, and is present from the surface to the inside of the porous substrate itself;
The present invention also provides a catalyst-supporting porous substrate, characterized in that the ionomer is in contact with the catalyst and is filled from the surface of the porous substrate toward the inside thereof while having a concentration gradient in the thickness direction of the porous substrate.

The present invention also provides the above-mentioned catalyst-supported porous substrate, which is obtained by a process of adhering a metal catalyst or a metal catalyst precursor to the side surfaces of the pores of the porous substrate before firing, or to the side surfaces of the fibers forming the porous substrate before firing, and then firing the substrate.

The present invention also provides the above-mentioned catalyst-supporting porous substrate, wherein the thickness or particle size of the catalyst is smaller than the average diameter of the "voids consisting of the pores and/or the gaps between the fibers."
In other words, the catalyst-supporting porous substrate is provided such that, when the catalyst is supported by a membrane, the membrane thickness, or, when the catalyst is supported by particles, the particle diameter, is smaller than the size of the "voids which are the 'holes' and/or 'gaps between the fibers.'"

The present invention also provides the above catalyst-supporting porous substrate, wherein the material of the porous substrate is titanium (Ti) or a titanium (Ti) alloy, or carbon (C).

The present invention also provides an electrode for water electrolysis, which is characterized by being a porous substrate carrying the above-mentioned catalyst.

The present invention also provides a gas diffusion layer characterized by being a catalyst-supporting porous substrate as described above.

The present invention also provides a single cell for water electrolysis, characterized in that it has a structure in which a polymer electrolyte membrane (PEM) is sandwiched between the above-mentioned catalyst-supported porous substrate.

The present invention also provides a stack cell for water electrolysis, characterized in that when a single water electrolysis cell has a structure in which a polymer electrolyte membrane (PEM) is sandwiched between catalyst-supported porous substrates, two or more of the above-mentioned single water electrolysis cells are stacked together.

The present invention also provides a cell module for water electrolysis, characterized in that the above-mentioned stack cells for water electrolysis are arranged in a two-dimensional or three-dimensional manner.

The present invention solves the above problems and issues, and provides an electrode for water electrolysis that has excellent water electrolysis performance, such as a stable cell voltage at a constant (low) value, and also has excellent durability due to strong catalyst support and good contact. It also provides a single cell for water electrolysis and a stack cell for water electrolysis that use the electrode for water electrolysis.

Specifically, by supporting a catalyst on a porous substrate in a specific mode or form, the catalyst and the porous substrate can be integrated. Specifically, by supporting the catalyst on the "surface of the holes or fibers that make up the porous carbon substrate" as catalyst particles or a catalyst film, it is possible to provide superior water electrolysis performance and durability compared to a mode in which the catalyst is deposited on the upper (or lower) surface of a (porous) substrate in the form of a catalyst layer.

Specifically, for example, in a conventional method in which a catalyst is deposited in the form of a layer on a substrate, there is a risk that the catalyst layer may potentially peel off from the substrate due to gas lift. However, according to the present invention, it is possible to provide a catalyst-supported porous substrate for water electrolysis and an electrode for water electrolysis in which the catalyst is prevented from peeling off from the porous substrate and from being detached from the material constituting the porous substrate.
Furthermore, it is possible to provide a single water electrolysis cell or a stack cell for water electrolysis using the catalyst-supporting porous substrate (electrode for water electrolysis).

Furthermore, by having an ionomer be present (or filled) in the catalyst-supporting porous substrate while having a concentration gradient in the thickness direction, a stable low cell voltage (applied voltage) can be achieved, and an electrode for water electrolysis in which peeling or detachment of the catalyst due to gas lift is more unlikely to occur can be provided.
Such "catalyst (membrane or particle) detachment" is effectively prevented or suppressed, in particular, by the synergistic action of the above-mentioned "support of the catalyst on the porous substrate in a specific form" and the above-mentioned "presence of the ionomer layer in a specific form".

A single water electrolysis cell using the catalyst-supported porous substrate of the present invention as an electrode for water electrolysis and having a water electrolysis cathode, a polymer electrolyte membrane (PEM), and a water electrolysis anode in that order is capable of water electrolysis at a low electrolysis voltage and is extremely durable because there is no peeling or detachment of the catalyst membrane or catalyst particles.

In addition, because the single cell operates even at a low cell voltage (applied voltage), a stack cell for water electrolysis, which is made by stacking two or more "single cells for water electrolysis using the catalyst-supported porous substrate," also operates at a low applied voltage. In other words, when two or more single cells for water electrolysis are stacked, the effect of such "low voltage operation" is more pronounced, and more hydrogen (and oxygen) can be obtained with less power.

1 is a schematic exploded perspective view of a water electrolysis cell having a single cell for water electrolysis, (a) is a schematic exploded perspective view of a water electrolysis cell using a catalyst-supporting porous substrate (electrodes for water electrolysis, gas diffusion layer) of the present invention, (b) is a schematic exploded perspective view of a conventional water electrolysis cell, 1 is a schematic cross-sectional view of a single cell for water electrolysis showing the form of catalyst present; (a) is a schematic cross-sectional view showing "the form of catalyst supported on a porous substrate" in the catalyst-supported porous substrate of the present invention; (b) and (c) are schematic cross-sectional views showing the form of catalyst present in a conventional method; 1 is a schematic enlarged cross-sectional view of a catalyst-supporting porous substrate of the present invention. 3A is a schematic enlarged cross-sectional view showing that an ionomer is filled in the catalyst-supporting porous substrate of the present invention with a concentration gradient from the surface to the inside of the porous substrate while being in contact with the catalyst. (a) is a schematic enlarged cross-sectional view corresponding to FIG. 3. (b) is an SEM photograph showing a cross-section of an actual catalyst-supporting porous substrate. FIG. 1 is a schematic enlarged cross-sectional view of a single cell for water electrolysis of the present invention, in which a polymer electrolyte membrane (PEM) is sandwiched between the catalyst-supporting porous substrate of the present invention (which serves as both a water electrolysis electrode and a gas diffusion layer). Schematic enlarged cross-sectional views showing examples of an embodiment in which an ionomer is filled from the surface of a water electrolysis electrode (catalyst-supporting porous substrate) toward the inside with a concentration gradient in the thickness direction: (a) An embodiment in which an ionomer is filled from the surface on the side in contact with a polymer electrolyte membrane (PEM) and a power feeder (i.e., from both sides) toward the inside with a concentration gradient, (b) An embodiment in which an ionomer is filled from the surface on the side in contact with a polymer electrolyte membrane (PEM) toward the inside with a concentration gradient FIG. 1 is a schematic exploded perspective view of a stack cell for water electrolysis according to the present invention, which is formed by stacking two unit cells for water electrolysis according to the present invention. 1 is a schematic diagram of a stack cell for water electrolysis according to the present invention; (a) a perspective view of the stack cell to which wiring and piping are connected; (b) a schematic cross-sectional view showing a "hydrogen outlet" on the cathode side and a "water inlet" and a "water outlet/oxygen outlet" on the anode side; and (c) a perspective view of the stack cell from which wiring and piping are omitted. FIG. 1 is a schematic perspective view of a water electrolysis cell module in which stack cells for water electrolysis according to the present invention are three-dimensionally arranged.

The present invention is described below, but is not limited to the specific forms below and can be modified as desired within the scope of the technical concept.

<Catalyst-supporting porous substrate>
The catalyst-supporting porous substrate of the present invention is a catalyst-supporting porous substrate that is present in a single cell for water electrolysis with a polymer electrolyte membrane (PEM) sandwiched therebetween, constitutes a cathode or an anode in contact with the polymer electrolyte membrane (PEM), and has a structure that also functions as a gas diffusion layer,
the catalyst is supported on the side surfaces of the pores of the porous substrate or on the side surfaces of the fibers forming the porous substrate, and is present from the surface to the inside of the porous substrate itself;
The ionomer is filled in the porous substrate from the surface toward the inside thereof while being in contact with the catalyst, with a concentration gradient in the thickness direction of the porous substrate.

The catalyst-supporting porous substrate of the present invention is used in a "single cell for water electrolysis" or a "stack cell for water electrolysis comprising two or more of the single cells for water electrolysis stacked together" (hereinafter, both may be collectively referred to as "cell for water electrolysis").
As shown in Figs. 1(a), 2(a), 5 and the like, the catalyst-supporting porous substrate 3 of the present invention is disposed across a polymer electrolyte membrane (PEM) 5 in a single cell 1 for water electrolysis, and constitutes an electrode 2 (cathode or anode) for water electrolysis in contact with the polymer electrolyte membrane (PEM) 5.

In the present invention, it is essential that the cathode or anode of the single water electrolysis cell 1 is composed of the catalyst-supported porous substrate 3 of the present invention. It is preferable that both the cathode and the anode are composed of the catalyst-supported porous substrate 3 of the present invention.
However, the types and shapes of the actual catalyst 3c, porous substrate 3p, etc. in the cathode and anode, as well as the manufacturing methods thereof, may be different for the cathode and the anode in accordance with their respective polarities.

The catalyst-supporting porous substrate 3 of the present invention functions as the water electrolysis electrode 2, and also has a structure that functions as a gas diffusion layer. Therefore, the catalyst-supporting porous substrate 3 of the present invention also functions as a gas diffusion layer.
As shown in the cross-sectional views of Figure 3, Figure 4(a)(b) and Figure 5, in the catalyst-supporting porous substrate 3 of the present invention, a catalyst is supported on a porous substrate through which gas can diffuse. That is, the catalyst-supporting porous substrate itself also functions as a gas diffusion layer, so that gas generated near the catalyst in the catalyst-supporting porous substrate 3 can be transported (diffused) and taken out to the outside.

<<Porous substrate>>
In the present invention, "porous" refers to a structure (or a property having such structure) that a catalyst can be supported even inside, but is not limited to a form in which holes are simply opened on the flat surface (upper surface), and refers to all of the following: a state roughened by chemical or physical etching, sputtering, etc.; a porous state; a state in which there is space; an assembly of fibrous materials; a knitted, woven, or nonwoven fabric state; etc. The above-mentioned "holes" and "voids formed by an assembly of fibrous materials" may form irregular voids in the thickness direction. In addition, "porous" refers to a state or property having air permeability.

Figure 3 is a schematic cross-sectional view before the ionomer 4 is applied, with the left figure showing before the catalyst is applied and the right figure showing after the catalyst is applied. The porous substrate 3p in the present invention only needs to have enough space to support the catalyst not only on the outside but also inside, and may have so-called through holes (however, this does not exclude the presence of independent holes), may be made of woven or knitted fibers, or may be a nonwoven fabric, etc. For example, an aggregate of fibers such as a nonwoven fabric also has holes in the gaps between the fibers, so in the present invention, it is referred to as a "porous substrate" in a broad sense.

As shown in Figures 3 to 5, the present invention is characterized in that the catalyst (catalyst particles or catalyst film) is present from the surface to the interior of the porous substrate 3p itself, and further characterized in that the ionomer 4 is present from the surface to the interior of the porous substrate 3p itself. Therefore, it is necessary that the porous substrate 3p in the present invention has sufficient voids to allow the catalyst or ionomer 4 to penetrate (be applied) into its interior.

The material of the porous substrate 3p in the present invention is not particularly limited as long as it is electrically conductive, but it is preferable that the material has properties such as electronic conductivity, resistance to corrosion (oxidation, etc.), resistance to various chemical reactions and electrochemical reactions, and high strength. Specifically, it is preferable that the material be a titanium group metal, a titanium group metal alloy, a titanium group metal compound, or carbon (C).

Here, "titanium group metal" refers to titanium, zirconium, or hafnium, i.e., titanium group substrates include titanium substrates, zirconium substrates, hafnium substrates, titanium alloy substrates, zirconium alloy substrates, and hafnium alloy substrates.
Examples of the "compound of a titanium group metal" include titanium nitride (titanium nitride (TiN)), titanium carbide (titanium carbide (TiC)), titanium boride (titanium diboride (TiB ₂ )), and the like.

As the titanium group metal or alloy, titanium or a titanium alloy is more preferable, and titanium is particularly preferable, since the cell voltage is low and the catalyst particles are suitably prevented from being detached from the electrode substrate.
Moreover, the carbon (C) preferably has a graphite structure (graphene structure).
The porous substrate 3p in the present invention is particularly preferably an aggregate of titanium fibers or titanium alloy fibers, or an aggregate of carbon fibers.

<<Catalyst>>
In the present invention, as described above, the catalyst is present from the surface to the inside of the porous substrate 3p itself, and more specifically, the catalyst is supported on the side surfaces of the pores of the porous substrate 3p or on the side surfaces of the fibers 3q forming the porous substrate 3p, and is present from the surface to the inside of the porous substrate 3p itself (Figures 3 to 5).
FIG. 5 is a schematic cross-sectional view of a catalyst-supporting porous substrate 3 of the present invention, in which fibers 3q such as carbon fibers, titanium fibers, etc. constitute a porous substrate 3p, and catalyst particles are supported not only on the surfaces of the fibers 3q on the surface of the porous substrate 3p itself, but also on the surfaces of the fibers 3q inside the porous substrate 3p itself.

It should be noted that the above-mentioned "surface of the porous substrate itself" is a term that is opposed to "the inside of the porous substrate itself", and therefore the above-mentioned "surface of the porous substrate itself" does not mean the side surfaces of the holes in the porous substrate 3p or the surface of the material (fibers, etc.) that constitutes the porous substrate 3p.
In the present invention, it is preferable that the catalyst is not deposited as a catalyst layer only on the surface of the porous substrate 3p itself, but is supported on the side surfaces of the holes in the porous substrate 3p or on the side surfaces of the fibers 3q forming the porous substrate 3p, and is present from the surface to the inside of the porous substrate 3p itself (see Figures 3 to 5).

If the catalyst is in the form of a catalyst layer, in other words, if the catalyst is in the form of a layer, and is present (formed or deposited) only on the surface of the polymer electrolyte membrane (PEM) 5 as in FIG. 2(b), or only on the surface of the porous substrate 3p itself as in FIG. 2(c), the contact area between the catalyst and the porous substrate 3p will be small, and the contact area between the catalyst and the polymer electrolyte membrane (PEM) 5 will also be small, and therefore the excellent effects of the present invention described above will not be obtained.

The catalyst-supporting porous substrate 3 of the present invention preferably has a configuration that can be obtained by a process of adhering a metal catalyst or a metal catalyst precursor to the side surfaces of the pores of the porous substrate 3p before firing, or to the side surfaces of the fibers 3q that form the porous substrate 3p before firing, and then firing the resulting product.
In the above-mentioned preferred embodiment of the present invention, it is impossible or almost impractical to directly specify or specify by parameters, etc., what form and composition the catalyst is formed in on the porous substrate 3p, what form the catalyst particles are supported in, etc. Therefore, the above-mentioned preferred configuration (embodiment) can only be specified by its manufacturing method.

Specifically, the catalyst of the present invention is preferably one obtained by applying a coating liquid in which a "metal catalyst or metal catalyst precursor" is dissolved or finely dispersed onto a porous substrate 3p, and then calcining the coating liquid to form a catalyst.
In other words, it is particularly preferable that the catalyst in the present invention is one which is obtained by applying a coating liquid, drying it, and then adhering a "metal catalyst or metal catalyst precursor" to the side surfaces of the pores of the porous substrate 3p or the side surfaces of the fibers 3q, and then calcining the same.
Here, the coating method is not particularly limited, and examples thereof include spray coating, dip coating, brush coating, screen printing, etc. After coating, if necessary, the coating solvent is removed by drying according to a conventional method.

"Baking" performed after deposition refers to a common heat treatment. The baking temperature depends on the type of catalyst (precursor) and is not particularly limited, but is preferably 160°C to 800°C, more preferably 230°C to 750°C, and particularly preferably 300°C to 700°C.

The calcination time (heat treatment time) is not particularly limited as long as the catalytic effect is exhibited, but is preferably from 10 minutes to 8 hours, more preferably from 30 minutes to 5 hours, and particularly preferably from 1 hour to 3 hours.
When the calcination time is not less than the above lower limit, the same effect as when the calcination temperature is not less than the above lower limit is obtained, and when the calcination time is not more than the above upper limit, the same effect as when the calcination temperature is not more than the above upper limit is obtained, and the catalyst can be preferably supported.

2(b), when the catalyst is present on the polymer electrolyte membrane (PEM) 5, if the catalyst is to be supported by baking, the polymer electrolyte membrane (PEM) 5 is weak against heat and melts or changes in quality at the above-mentioned baking temperature. Therefore, in the case of the form shown in FIG. 2(b), the catalyst cannot be produced by baking.
According to the present invention, a metal catalyst or metal catalyst precursor can be attached to a porous substrate 3p (which generally tends to be heat-resistant) and then fired to obtain a catalyst (supported thereon). Compared to the case where the catalyst is simply applied and dried to be attached, this improves adhesion to the porous substrate 3p, makes it possible to use catalysts with excellent forms and compositions, and also widens the range of catalysts that can be used.

In the present invention, firing may change the volume of the pores of the porous substrate 3p, enlarge the pores, or reduce the thickness of the fibers 3q that make up the porous substrate 3p. Rather, the occurrence of such phenomena makes it easier for the catalyst to be supported on "the side of the pores of the porous substrate or the side of the fibers 3q that form the porous substrate", and in the case of particles of the catalyst, there is a margin in size, making it easier for the catalyst to be present (supported) gradually (uniformly) from the surface to the inside of the porous substrate 3p itself.
Furthermore, an ionomer 4, which will be described later, may be easily filled from the surface of the porous substrate 3p toward the inside while having a concentration gradient in the thickness direction of the porous substrate 3p.

The catalyst-supported porous substrate 3 in the present invention is preferably one obtained by attaching a metal catalyst or a metal catalyst precursor to a porous substrate 3p and firing the metal catalyst or metal catalyst precursor, but the metal in the metal catalyst or metal catalyst precursor is not particularly limited as long as the catalyst particles or catalyst film obtained by firing the metal catalyst or metal catalyst precursor act as a catalyst. Among them, a metal selected from the group consisting of platinum (Pt), ruthenium (Ru), iridium (Ir), palladium (Pd), tantalum (Ta), and nickel (Ni) is preferable from the viewpoint of high catalytic effect. The above metals may be one type, or two or more types may be used in combination. In addition, they may be used in combination with metals other than those mentioned above.
In other words, the catalyst in the present invention is preferably one or more metals or metal-containing compounds selected from the group consisting of platinum (Pt), ruthenium (Ru), iridium (Ir), palladium (Pd), tantalum (Ta), and nickel (Ni).

Metal compounds in the metal catalysts and metal catalyst precursors that are the raw materials for the catalyst include, but are not limited to, platinum-containing compounds such as chloroplatinic (IV) acid n-hydrate, ammonium chloroplatinic (IV) acid, dinitrodiammine platinum (II), platinous chloride (II), platinic chloride (IV), tetraammineplatinum (II) dichloride n-hydrate, tetraammineplatinum (II) hydroxide, and hexahydroxyplatinic (IV) acid; ruthenium-containing compounds such as ruthenium chloride (III) hydrate, ruthenium nitrate (III), and ruthenium oxide (IV) hydrate; iridium chloride (IV) acid n-hydrate, iridium chloride Examples of iridium-containing compounds include iridium(III) n-hydrate, iridium(III) chloride anhydrate, iridium(IV) nitrate, ammonium chloride iridate(IV), and hexaammineiridium(III) hydroxide; palladium-containing compounds include palladium(II) chloride, palladium(II) nitrate, dinitrodiamminepalladium(II), palladium(II) acetate, and tetraamminepalladium(II) dichloride; nickel-containing compounds include nickel(II) chloride anhydrate, nickel(II) chloride hexahydrate, and nickel(II) nitrate hexahydrate; tantalum pentachloride, tantalum alkoxides, and the like.

The metal catalyst precursor is not limited, but specific examples include those in which an alcohol is coordinated to the above metal compound. It is particularly preferable to prepare a catalyst-supported porous substrate 3 by dissolving the above metal compound in an alcohol solvent or the like to prepare a metal catalyst precursor, applying a coating liquid containing the metal catalyst precursor to a porous substrate 3p, drying the coating liquid, and then calcining the porous substrate 3p to convert the metal catalyst precursor into a catalyst and support the catalyst.

When the metal catalyst or metal catalyst precursor is attached to the porous substrate 3p by coating, preferred examples of the "solvent (dispersion medium) of the coating liquid used in the coating" and/or the "alcohol to be coordinated to the metal compound" include methanol, ethanol, propyl alcohol, isopropyl alcohol, butanol, pentanol, hexanol, cyclohexanol, etc.

When adhesion to a porous carbon substrate is performed by coating, there are no limitations on the method for applying the coating liquid, but preferred methods include brush coating, spraying, spray coating, and immersion.

In addition, the catalyst may be supported on the side of the hole or the side of the fiber 3q in the form of a membrane, for example, as shown in Figures 3 and 4, or may be supported on the side of the hole or the side of the fiber 3q in the form of particles, for example, as shown in Figure 5.
Typically, iridium (Ir), tantalum (Ta), etc. are supported in the form of a film on the pores or the sides of the fibers 3q, and platinum (Pt), etc. are supported in the form of particles on the pores or the sides of the fibers 3q.

The average particle diameter of the catalyst particles is not particularly limited depending on the type of catalyst, but the number average particle diameter is preferably 300 μm or less, more preferably 200 μm or less, and particularly preferably 100 μm or less. The number average particle diameter in the present invention can be obtained by observing the cross section of the catalyst-supported porous material obtained after firing with a scanning electron microscope (SEM), randomly selecting 20 catalyst particles, and taking the arithmetic mean of the diameters, and is defined as the arithmetic mean value of the diameters thus obtained.

If the average particle size of the catalyst particles is too large, the catalytic effect decreases, the cell voltage increases, and the catalyst particles may be detached from the electrode substrate.
In addition, in the case of the present invention, as described later, it may be difficult for catalyst particles to penetrate into the interior of the porous substrate 3p and to be supported therein.
On the other hand, if the particle size is too small, it may penetrate into the microscopic depths from the side of the pores of the porous substrate 3p or the side of the fibers 3q, etc. However, as long as the catalyst surface area can be increased by adjacently bonding multiple microparticles, the particle size is not necessarily limited to the above particle size range.
The "cell voltage" refers to the electrolysis voltage applied between the cathode and anode of a water electrolysis cell for the electrolysis of water.

When the catalyst is in the form of a membrane and is supported on the side of the holes of the porous substrate 3p or on the side of the fibers 3q, the average thickness of the membrane is not particularly limited depending on the type of catalyst, but is preferably 50 μm or less, more preferably 40 μm or less, and particularly preferably 30 μm or less. The average thickness can be obtained by observing the cross section of the catalyst-supported porous material obtained after firing with a scanning electron microscope (SEM), and is defined as the thickness obtained in this manner.

<<Ionomer>>
In the catalyst-supporting porous substrate 3 of the present invention, the ionomer 4 is preferably present at least on the side adjacent to the polymer electrolyte membrane (PEM) 5 .
Here, the term "ionomer" is also called a cation exchange polymer, a polymer having a strong acid group in the side chain, a proton conductive polymer, an ion conductive polymer, etc., and the term "ionomer" refers to a polymer having the above-mentioned chemical structure and physical properties.

For example, the upper surface of the porous substrate 3p, which is an aggregate of carbon fibers, titanium fibers, etc., is not flat to begin with (FIG. 2(a), FIGS. 3 to 5), and therefore the contact with the polymer electrolyte membrane (PEM) 5 may be insufficient, and therefore the contact between the catalyst and the polymer electrolyte membrane (PEM) may also be insufficient.
Furthermore, in the present invention, since the catalyst is also present inside the porous substrate 3p, and moreover, since much of the catalyst is also present inside the porous substrate 3p, without the ionomer 4, the contact between the catalyst and the polymer electrolyte membrane (PEM) becomes even less sufficient.

According to the present invention, the ionomer 4 is present in the manner shown in Figure 2 (a) and Figures 3 to 6, so that the voids present inside the porous substrate 3p itself and between the porous substrate 3p and the polymer electrolyte membrane (PEM) are filled, improving the contact between them, as well as the contact between the ionomer 4 and the catalyst, and further between the polymer electrolyte membrane (PEM) 5 and the catalyst.

In the catalyst-supporting porous body of the present invention, the ionomer 4 is in contact with the supported catalyst and is filled from the surface of the porous substrate 3p toward the inside, with a concentration gradient in the thickness direction of the porous substrate 3p (see Figure 2(a) and Figures 4 to 6).
The direction in which the ionomer concentration is high is the side in contact with the polymer electrolyte membrane (PEM) 5 in the catalyst-supporting porous body (Figure 6(b)), but in addition, it is also preferable that the concentration is high on the opposite side, i.e., the side in contact with the current collector 6, and that the ionomer is filled with a concentration gradient from the surface toward the inside in the thickness direction of the porous substrate 3p (Figure 6(a)).

In the present invention, it is important from the viewpoint of the necessity of contact to surround the catalyst film or catalyst particles with the ionomer 4. Furthermore, the ionomer 4 is filled with a concentration gradient in the thickness direction of the porous substrate 3p.
As shown in FIG. 6( b ), there may be no ionomer 4 or only a small amount of ionomer 4 in the vicinity of the catalyst supported on the deep portion of the porous substrate 3 p (i.e., the side opposite to the side in contact with the polymer electrolyte membrane (PEM) 5); furthermore, from the viewpoint of permeability, it is preferable that there are portions where there is no ionomer 4 or where there is only a small amount of ionomer 4.
Furthermore, as shown in FIG. 6( a), there may be no ionomer 4 or only a small amount of ionomer 4 near the catalyst supported on the deep portion of the porous substrate 3p (i.e., the portion away from the contact surface with the PEM 5 and the current collector 6); furthermore, from the standpoint of permeability, it is preferable that there are portions where there is no ionomer 4 or where there is only a small amount of ionomer 4.

As mentioned above, if the fibers 3q constituting the porous substrate 3p become thinner as a result of calcination to prepare the catalyst, contact becomes more difficult. However, for example, as shown in Figure 5, if the ionomer 4 is in contact with the catalyst 3c and exists with a concentration gradient from the surface to the inside of the porous substrate 3p, contact between the catalyst-supported porous substrate (catalyst 3c) and the polymer electrolyte membrane (PEM) 5 becomes extremely good.

In other words, the ionomer 4 has the function of conducting "hydrogen ions (protons) to the cathode side", and by having it present on the side of the catalyst-supporting porous substrate 3 adjacent to the polymer electrolyte membrane (PEM) 5, it is possible to significantly reduce the resistance when conducting protons from the polymer electrolyte membrane (PEM) 5 to the catalyst surface.
Therefore, by filling the ionomer 4 toward the inside of the catalyst-supporting porous substrate 3, protons generated in the "catalyst on the power feeder and/or resin tank side, which cannot come into direct contact with the polymer electrolyte membrane (PEM)" can be conducted to the cathode side, thereby improving the utilization efficiency of the catalyst.

As a result, by filling the ionomer 4 in the above-mentioned embodiment of the present invention, when the water electrolysis cell is operated under constant current conditions, the cell voltage (electrolysis voltage) can be operated even if the cell voltage (electrolysis voltage) is low, and detachment of catalyst particles due to the generated gas can be suppressed.

The amount of ionomer 4 present varies depending on the thickness and porosity of the catalyst-supported porous substrate 3, as well as the operating conditions of the water electrolysis cell, for example, the amount of hydrogen generated per unit time, and is therefore not necessarily limited; however, because gas is generated during water electrolysis and this gas needs to be removed from the cell, it is desirable from a microscopic perspective that the entire catalyst surface is not thickly covered with ionomer 4, but that the catalyst surface is microscopically and partially exposed; and from a macroscopic perspective, it is desirable that not all of the voids in the catalyst-supported porous substrate 3 are filled with ionomer 4, but that a concentration gradient is present in the thickness direction of the catalyst-supported porous substrate 3, allowing the generated gas to escape.

Since the generated gas needs to be extracted from the power supply 6 and/or resin tank 7 side, it is preferable to provide a gradient in the amount of ionomer 4 present between the "polymer electrolyte membrane (PEM)" and the "power supply 6 and/or resin tank 7" so that the gas can be extracted more efficiently.
Therefore, although not necessarily limited, the proportion of the volume of the voids filled by the ionomer 4 in the void volume of the catalyst-supporting porous substrate 3 (hereinafter, sometimes referred to as the "filling rate") is preferably 10 vol. % or more and 90 vol. % or less, more preferably 20 vol. % or more and 80 vol. % or less, and particularly preferably 30 vol. % or more and 70 vol. % or less.

In other words, the present invention also relates to the above-mentioned catalyst-supporting porous substrate 3, in which the ionomer 4 penetrates into the voids of the catalyst-supporting porous substrate 3, and the proportion of the voids filled with the ionomer 4 of the void volume of the catalyst-supporting porous substrate 3 is 10 volume % or more and 90 volume % or less.

The above-mentioned effect of the ionomer 4 is effectively achieved for both electrodes, whether they are cathodes for water electrolysis or anodes for water electrolysis (see Figure 5).

The ionomer 4 can be filled, for example, by applying an ionomer dispersion or an ionomer solution to the porous substrate 3p carrying the catalyst.
The present invention also relates to the catalyst-supporting porous substrate 3, in which the ionomer 4 is obtained by supporting the catalyst on the porous substrate 3p, applying a solution of the ionomer 4 and drying it, and filling the porous substrate 3p with a concentration gradient from the surface toward the inside.
The catalyst may be applied from the side that will come into contact with the polymer electrolyte membrane (PEM) 5 to obtain a catalyst-supported porous substrate 3 having a concentration gradient as shown in FIG. 6(b) or, after doing so, the catalyst may be applied from the side that will come into contact with the current collector 6 to obtain a catalyst-supported porous substrate 3 having a concentration gradient on both sides as shown in FIG. 6(a).

In the preferred embodiment of the present invention, it is impossible or almost impractical to directly specify or specify by parameters, etc., the mass and form in which the ionomer 4 is packed in the porous substrate 3p, and the manner in which it is in contact with the catalyst. Therefore, the preferred embodiment (configuration) described above can only be specified by its manufacturing method.

Examples of the application method include a brush application method, a spray application method, and a spray application method.
After applying the ionomer dispersion (ionomer solution), it is preferable to volatilize the solvent (dispersion medium) at about 60° C., and then perform heat treatment at preferably 120° C. or higher and 250° C. or lower, particularly preferably 140° C. or higher and 200° C. or lower, preferably for 1 minute or higher and 1 hour or lower, particularly preferably for 3 minutes or higher and 30 minutes or lower.

<<<<Chemical structure of ionomer>>>
The ionomer 4 is not particularly limited as long as it has ion conductivity, particularly proton conductivity, and can be used in the water electrolysis unit cell 1. That is, the "ionomer" in the present invention refers to a proton-conducting polymer.
The ionomer 4 is not particularly limited, and may be a fluorine-based ionomer 4 having a fluorine atom in the molecule, or a non-fluorine-based ionomer 4 having no fluorine atom in the molecule.

Although not limited thereto, fluorine-based ionomers are preferred, and ion-conducting polymers (proton-conducting polymers) having a polyfluoroethylene skeleton as the main chain and a perfluoroethylene ether skeleton having a sulfonic acid group at the end as the side chain are more preferred.
Although not particularly limited, examples of the ionomer 4 more preferably used in the present invention are shown below.

［式（１）中、ｍは自然数］

[In formula (1), m is a natural number]

［式（２）中、ｐは自然数］

[In formula (2), p is a natural number]

［式（３）中、ｎは自然数］

[In formula (3), n is a natural number]

The ionomer represented by formula (1) is a polymer consisting of a polytetrafluoroethylene (PTFE) backbone as the main chain and perfluoroether pendant side chains having sulfonic acid groups at their ends. Because the side chains are relatively long, it is called a long-side-chain (LSC) ionomer.
The equivalent weight (EW) of the ionomer represented by formula (1) (the mass of the polymer required to supply 1 mole of protons) is particularly preferably, but not limited to, 900 g/mol to 1200 g/mol. The preferred range of m in formula (1) is the range that can be calculated from the equivalent weight.

As the long side chain (LSC) ionomer represented by formula (1), a commercially available product is preferably used, but is not limited thereto, and examples thereof include Nafion (registered trademark) manufactured by DuPont.
As the long side chain (LSC) ionomer represented by formula (1), Nafion (registered trademark) (EW=1100 g/mol, m=6.6 in formula (1)) is preferred.

The ionomers represented by formula (2) or (3) are polymers consisting of a polytetrafluoroethylene (PTFE) skeleton as the main chain and perfluoro pendant side chains having sulfonic acid groups at their ends. Because the side chains are relatively short, they are called short-side-chain (SSC) ionomers.
The equivalent weight (EW) of the ionomer represented by formula (2) or formula (3) is particularly preferably, but not limited to, 700 g/mol to 950 g/mol. The preferred range of p in formula (2) and the preferred range of n in formula (3) are ranges that can be calculated from the equivalent weight.

Short side chain (SSC) ionomers represented by formula (2) or formula (3) may be commercially available, but are not limited to these, such as 3M ionomers manufactured by 3M Corporation.

<Relationship between "film thickness or particle size of catalyst" and "voids in porous substrate">
In the catalyst-supporting porous substrate 3 of the present invention, the thickness or particle size of the catalyst supported on the porous substrate 3p is preferably smaller than the average diameter length of the "voids consisting of gaps between the pores and/or fibers" of the porous substrate 3p. If it is larger, it may be difficult for the catalyst to be suitably supported on the material constituting the porous substrate 3p.
The voids referred to above are voids in the porous substrate 3p after the catalyst is supported by calcination, etc., but the voids in the porous substrate 3p before the catalyst is supported by calcination, etc. are preferably larger than the size (film thickness or particle size) of the catalyst obtained by supporting. If they are smaller, the "metal catalyst or metal catalyst precursor" dissolved or finely dispersed in the catalyst coating liquid may have difficulty penetrating into the pores, and as a result, the catalyst may not be present from the surface to the inside of the porous substrate 3p itself.

In addition, the catalyst-supporting porous substrate 3 of the present invention preferably has a porosity, excluding the ionomer 4, represented by the following formula (1) of 3 volume % or more and 80 volume % or less.
Porosity (volume%)
= 100 × [pore volume of catalyst-supporting porous substrate] / [volume of catalyst-supporting porous substrate] (1)

The term "catalyst-supporting porous substrate" refers to a substrate on which a catalyst is already supported. Therefore, when the catalyst is supported by firing, the "voids" in the above "porosity" refer to the voids in the porous substrate 3p after firing.
Since the volume of the catalyst particles (catalyst film) is sufficiently small compared with the volume of the above-mentioned voids, the "voids in the porous substrate" are approximately equal to the "voids in the catalyst-supporting porous substrate".

The numerator of the above definition formula (1) can be calculated by measuring the weight and volume of the catalyst-supporting porous substrate 3 and using the true specific gravity of the material (Ti, C, etc.), and the denominator can also be easily measured, so the above porosity is determined in this way and is defined as the value thus determined.
The porosity is preferably measured before the ionomer 4 is applied (filled).

The porosity is more preferably from 5 vol. % to 70 vol. %, and particularly preferably from 10 vol. % to 60 vol. %.
If the porosity is too small, it is difficult to support the catalyst, and the catalyst may not be supported inside the porous substrate 3p. In addition, the ionomer solution/dispersion may not penetrate into the porous substrate 3p, and the ionomer 4 may not be filled into the porous substrate 3p, or may not be filled with a concentration gradient in the thickness direction of the porous substrate 3p. In addition, gas diffusibility may be deteriorated, and the function as a gas diffusion layer may be deteriorated.
On the other hand, if the porosity is too large, the strength of the catalyst-supporting porous substrate 3 may decrease, and the electrical conductivity may decrease.

In the catalyst-supported porous substrate 3 of the present invention, it is preferable that the sum of the "micro-area of the catalyst present from the surface to the inside of the porous substrate itself in contact with the ionomer" is at least twice the "macro-area of the surface of the catalyst-supported porous substrate 3 in contact with the polymer electrolyte membrane (PEM) 5." This ratio is called the "contact area ratio."

The above-mentioned micro area can be calculated by observing the cross section of the catalyst-supporting porous substrate 3 after filling with the ionomer with a scanning electron microscope (SEM) and calculating the surface area of the catalyst from the cross section of the catalyst therein. The surface area of the catalyst inside the catalyst-supporting porous substrate 3 that is not filled with the ionomer 4 is excluded from the micro area.
For example, when the catalyst-supporting porous substrate 3 is rectangular, the macro area is obtained by multiplying its length and width.
The "contact area magnification" is determined by dividing the micro area by the macro area, and is defined as the value thus determined.

The contact area magnification is more preferably 3 times or more, further preferably 4 times or more and 300 times or less, and particularly preferably 5 times or more and 100 times or less.
If the contact area ratio is too small, the various "effects of (being filled with) the ionomer 4" described above may not be obtained, whereas if it is too large, it may be difficult to manufacture the catalyst-supporting porous substrate 3.

<Water electrolysis electrodes>
As described above, the present invention also relates to the water electrolysis electrode 2 which is the catalyst-supporting porous substrate 3 of the present invention.
As described above, the present invention also relates to a gas diffusion layer which is characterized by being the above-mentioned catalyst-supporting porous substrate 3 of the present invention.
That is, as described above, because of the shape and function of the catalyst-supporting porous substrate 3 of the present invention, the catalyst-supporting porous substrate 3 of the present invention also functions and is used as the water electrolysis electrode 2, and also functions and is used as a gas diffusion layer.

<Single cell for water electrolysis>
The present invention also relates to a single cell 1 for water electrolysis, characterized in that it has a structure in which a polymer electrolyte membrane (PEM) 5 is sandwiched between the catalyst-supporting porous substrate 3 of the present invention.
The water electrolysis unit cell 1 of the present invention, an example of which is shown in Fig. 1(a), has a structure in which a polymer electrolyte membrane (PEM) 5 is sandwiched between catalyst-supported porous substrates 3 of the present invention, which are further sandwiched between, specifically, a gasket, a power feeder 6, a resin tank body 7, etc. The water electrolysis unit cell 1 of the present invention has a catalyst-supported porous substrate 3 (electrodes for water electrolysis) as shown in Fig. 5 sandwiched between the above-mentioned structures.
The structures such as the power feeder 6 and the resin tank 7 are not particularly limited, and known structures may be used.

FIG. 7 shows a stack cell 9 for water electrolysis in which two unit cells for water electrolysis of the present invention are connected together, with unit cells 1 for water electrolysis on the right and left sides, respectively.
The water electrolysis unit cell 1 of the present invention does not exclude the use (combination) of other layers, other members, structures, and the like in addition to those described above and illustrated in the drawings.

<Stack cell for water electrolysis>
The present invention also relates to a stack cell 9 for water electrolysis, characterized in that when one unit cell 1 for water electrolysis has a structure in which a polymer electrolyte membrane (PEM) 5 is sandwiched between catalyst-supporting porous substrates 3, two or more of the above-described unit cells for water electrolysis are stacked.
A schematic diagram of a stack cell 9 for water electrolysis of the present invention is shown in Fig. 7. As shown in Fig. 7, the stack cell 9 for water electrolysis of the present invention is configured by sandwiching a bipolar plate 8 between the unit cells 1 for water electrolysis of the present invention.

The stack cell 9 for water electrolysis of the present invention is formed by stacking two or more, preferably 3 to 10, and particularly preferably 4 to 6, unit cells for water electrolysis of the present invention.
If the number of layers is too small, the efficiency of water electrolysis (hydrogen generation efficiency) may decrease, whereas if the number of layers is too large, the voltage required for water electrolysis may increase.

As described above, the single water electrolysis cell 1 using the catalyst-supported porous substrate 3 of the present invention is characterized by its small cell voltage, so that even if multiple single water electrolysis cells 1 are connected in series, the electrolysis voltage between the cathode and anode of the water electrolysis stack cell 9 can be kept low.

The stack cell 9 for water electrolysis of the present invention does not exclude the use (combination) of other layers, other members, structures, and the like in addition to those described above and illustrated in the drawings.
8( a) and 8(b) are conceptual diagrams showing a stack cell 9 for water electrolysis of the present invention further provided with cathode wiring, anode wiring, a water inlet, a water outlet, a hydrogen discharge pipe (hydrogen outlet), an oxygen discharge pipe (oxygen outlet), etc. The water outlet and the oxygen discharge pipe (oxygen outlet) may be the same (shared).

<Water electrolysis cell module>
The present invention also relates to a water electrolysis cell module 10 comprising the above-mentioned water electrolysis stack cells 9 arranged two-dimensionally or three-dimensionally.
FIG. 9 is a schematic perspective view of a water electrolysis cell module 10 in which the stack cells 9 for water electrolysis of the present invention are arranged three-dimensionally.

By adopting the water electrolysis cell module 10, it is possible to efficiently electrolyze a large amount of water and obtain a large amount of hydrogen and oxygen.
In addition, as shown in FIG. 9 , an installation board can be provided to allow only a portion of the water electrolysis stack cells 9 to be removed for replacement, maintenance, or to stop the flow of electricity (to provide a power-off period), thereby improving the efficiency of operation.

In addition, the invention of the water electrolysis cell module 10 of the present invention does not exclude the use (combination) of other components, etc. in addition to those described above and illustrated in the figures.

The present invention will be explained in more detail below by giving examples and comparative examples, but the present invention is not limited to these examples as long as it does not depart from the gist of the present invention.
Hereinafter, unless otherwise specified, values relating to ratios and percentages are mass ratios and mass %.

Example 1
<Production of catalyst-supporting porous substrate and water electrolysis electrode>
An alcohol-based coating solution containing chloroiridate (IV) hexahydrate as a metal catalyst (metal precursor) was spray-coated evenly onto a porous substrate 3p in which titanium fibers with a fiber diameter of 50 μm were aggregated into a flat plate.
At that time, it was observed how the coating liquid permeated from the surface to the inside of the porous substrate 3p itself.

Next, the porous substrate was dried by heating at 70°C for 30 minutes to volatilize the solvent of the coating liquid. After that, in order to improve the adhesion between the metal catalyst 3c and the surface of the material of the porous substrate (the side of the fiber), a heat treatment was performed for 2 hours at 600°C, which is a temperature higher than the temperature during the drying.

When the obtained "catalyst-supported porous substrate before ionomer filling" was observed with a scanning electron microscope (SEM), it was found that the iridium catalyst was supported on the side of the titanium fibers forming the porous substrate 3p, and existed as a coating covering the titanium fibers from the surface to the inside of the porous substrate 3p itself, as shown in Figures 2 to 4. The thickness of the catalyst film was 0.3 μm to 10 μm.

A dispersion of ionomer (Nafion (registered trademark) manufactured by DuPont) was spray-coated on the porous substrate 3p carrying the metal catalyst 3c from the side in contact with the PEM.
During this process, the state in which this dispersion liquid permeated from the surface of the catalyst-supporting porous substrate 3 to the inside was observed.

Next, the catalyst-supported porous substrate 3 was heated at 100°C for 30 minutes and dried to volatilize the dispersion medium of the ionomer dispersion.

When the obtained "catalyst-supported porous substrate after ionomer loading" was observed with a scanning electron microscope (SEM), it was confirmed that the ionomer 4 was in contact with the iridium catalyst and was loaded from the surface to the inside of the porous substrate 3p with a concentration gradient in the thickness direction, as shown in Figures 4 and 5 (see particularly Figures 4(b) and 6(b)).

The resulting "catalyst-supported porous substrate after ionomer filling" had a porosity of 40 volume %, a contact area ratio of 10 times, and an average diameter of the "voids consisting of gaps between the fibers constituting the porous substrate" of 25 μm to 50 μm.

Example 2
A catalyst-supporting porous substrate 3 (electrode 2 for water electrolysis) was obtained in the same manner as in Example 1, except that a porous substrate 3p made of carbon fibers was used instead of the porous substrate 3p made of titanium fibers in Example 1.
During the process, it was observed that the catalyst coating solution was seeping into the porous substrate 3p from its surface.
During the process, it was confirmed that the ionomer dispersion liquid was permeating from the surface of the catalyst-supporting porous substrate 3 to the inside.

When the obtained catalyst-supported porous substrate 3 was observed with a scanning electron microscope (SEM), it was found that the iridium catalyst was supported on the side surfaces of the carbon fibers forming the porous substrate 3p and was present from the surface to the interior of the porous substrate 3p itself.
The ionomer 4 was filled in the porous substrate 3p from the surface toward the inside while being in contact with the catalyst, with a concentration gradient in the thickness direction (see, in particular, FIG. 4(b) and FIG. 6(b)).

Example 3
A catalyst-supporting porous substrate 3 (electrode for water electrolysis 2) was obtained in the same manner as in Example 1, except that a platinum catalyst was used instead of the iridium catalyst in Example 1.
During the process, it was observed that the catalyst coating solution was seeping into the porous substrate 3p from its surface.
During the process, it was confirmed that the ionomer dispersion liquid was permeating from the surface of the catalyst-supporting porous substrate 3 to the inside.

When the obtained catalyst-supported porous substrate 3 was observed with a scanning electron microscope (SEM), it was found that the platinum catalyst was supported on the side of the titanium fibers forming the porous substrate 3p and was present from the surface to the interior of the porous substrate 3p itself (Figure 5).
Moreover, the ionomer 4 was filled from the surface of the porous substrate 3p toward the inside while being in contact with the catalyst, with a concentration gradient in the thickness direction (particularly, FIG. 5 and FIG. 6(b)).

Comparative Example 1
A water electrolysis electrode 2 (catalyst layer-formed substrate) was obtained in the same manner as in Example 1, except that a substrate made of titanium fibers and having a porosity of 1 volume % was used as the (porous) substrate.
The (porous) substrate used was made of titanium fibers with a porosity of 1% by volume, and since its pores (voids) were smaller than the metal catalyst particles, it was difficult for the catalyst coating liquid to penetrate from the surface of the porous substrate 3p itself into the interior.

When the obtained water electrolysis electrode 2 was observed with a scanning electron microscope (SEM), it was found that the catalyst was not present inside the porous substrate 3p itself, but rather that a catalyst layer had formed on the (porous) substrate. In other words, the layer structure was "porous substrate \ metal catalyst layer \ ionomer layer."

Comparative Example 2
A catalyst-supporting porous substrate 3 was obtained in the same manner as in Example 1, except that ionomer 4 was not filled, that is, the dispersion liquid of ionomer 4 was not applied.

When the obtained catalyst-supported porous substrate 3 was observed with a scanning electron microscope (SEM), it was found that the catalyst was present even inside the porous substrate 3p itself, but because the ionomer 4 was not filled, the catalyst was isolated.

Comparative Example 3
A conventional membrane electrode assembly (MEA) was used as an electrode for water electrolysis.
The membrane electrode assembly (MEA) used was made by coating one side of a PEM with carbon particles and "catalytic platinum particles" and the other side with "catalytic iridium oxide particles," which were then dried and fixed.

Evaluation Example 1
<Assembly of single cell for water electrolysis>
A water electrolysis unit cell 1 was assembled having the configuration shown in FIG.
Specifically, a polymer electrolyte membrane (PEM) was placed in the center, and on both outer sides thereof, the "catalyst-supported porous substrate 3 (electrodes 2 for water electrolysis) having a metal catalyst 3 c and an ionomer 4" obtained in the above-mentioned Examples or the electrodes for water electrolysis obtained in the Comparative Examples were placed.
Further, power feeders 6 were arranged on both outer sides of the above, and resin tank bodies 7 were arranged on both outer sides of the above, and then both ends were fastened with bolts to sandwich each component therebetween, thereby assembling the single cell 1 for water electrolysis.

<Initial performance evaluation by water electrolysis test>
Pure water at a temperature of 20° C. was circulated by a pump through the single cell for water electrolysis 1 obtained in the above Examples and Comparative Examples, and the pure water for electrolysis was supplied to the anode side of the water electrolysis cell, and the cell voltage value during water electrolysis at a predetermined current density was recorded using a DC power supply.

The initial cell voltages (V) at a current density of 100 A/ ^dm2 are shown in Table 1 below.

<Evaluation of durability of single water electrolysis cells through water electrolysis tests>
As in the above-described initial performance evaluation, pure water was circulated and supplied to the single cell 1 for water electrolysis, and a current value was set using a DC power supply so that the current density became 100 A/ ^dm2 , electrolysis was performed continuously, and the cell voltage was recorded.

The cell voltage [V] after 500 hours is shown in Table 2 below.

Evaluation Example 2
<Assembly of stack cells for water electrolysis>
The water electrolysis unit cells 1 were stacked in the configuration shown in FIG. 7 to fabricate a water electrolysis stack cell 9 .
Specifically, "catalyst-supported porous substrate (anode)/polymer electrolyte membrane (PEM)/catalyst-supported porous substrate (cathode)" constituted one water electrolysis unit cell 1, and five such unit cells were stacked with a bipolar plate 8 interposed therebetween to form one water electrolysis stack cell 9. Note that while two unit cells are stacked in Fig. 7, five unit cells were stacked in Evaluation Example 2.

<Initial performance evaluation of stack cells for water electrolysis>
Pure water at a temperature of 20° C. was circulated by a pump through a stack cell 9 for water electrolysis in which five "single cells for water electrolysis obtained in the above Examples and Comparative Examples" were stacked as described above, and the pure water for electrolysis was supplied to the anode side of the water electrolysis cells (see FIG. 8 ). The stack cell voltage value during water electrolysis at a predetermined current density using a DC power supply was recorded.

The initial stack cell voltage at a current density of 100 A/ ^dm2 is shown in Table 3.

<Durability evaluation of stack cells for water electrolysis>
Using a stack cell 9 for water electrolysis obtained by stacking the catalyst-supporting porous substrates 3 (electrodes for water electrolysis 2) obtained in Example 1 as in Evaluation Example 2, water electrolysis was continuously performed at a constant current value, and the change in the stack cell voltage was recorded.

After 500 hours, the stack cell voltage increased slightly compared to the initial stack cell voltage, but this was at a level corresponding to Table 2 for a single cell and was not a problem.

Initial performance evaluation of single cell for water electrolysis (initial cell voltage)

Durability evaluation of single cell for water electrolysis (cell voltage after 500 hours)

Initial performance evaluation of stack cell 9 for water electrolysis (initial stack cell voltage)

As can be seen from Tables 1 to 3, the water electrolysis electrode 2, the single water electrolysis cell 1, and the stack cell 9 for water electrolysis each using the catalyst-supporting porous substrate 3 of the present invention had a stable low cell voltage (required applied voltage) at the start of water electrolysis.
Moreover, this value was maintained even after the durability test.
On the other hand, the water electrolysis electrode 2, the water electrolysis unit cell 1, and the water electrolysis stack cell 9 of the comparative examples had a high cell voltage (required applied voltage) at the start of water electrolysis, and maintained the high value even after the durability test.

When the catalyst-supporting porous substrate 3 obtained in Examples 2 and 3 was used, the cell voltage was stable and low, as in Example 1. That is, the initial cell voltage, the cell voltage after 500 hours, the initial stack cell voltage, and the stack cell voltage after 500 hours were almost the same as the results in Tables 1 to 3 of Example 1, and the voltages were low.

The catalyst-supported porous substrate 3 of the present invention is excellent as an electrode or gas diffusion layer for water electrolysis, and a single water electrolysis cell using it and a stack cell 9 for water splitting in which the single cells are stacked have a low cell voltage, are prevented from detaching the catalyst from the electrode substrate, and are easy to manufacture and durable, and therefore are widely used in all fields requiring hydrogen and oxygen.

1 Single cell for water electrolysis 2 Electrode for water electrolysis 3 Catalyst-supporting porous substrate 3p Porous substrate 3q Fiber forming the porous substrate 3c Metal catalyst or metal oxide catalyst 4 Ionomer 5 Polymer electrolyte membrane (PEM)
6 Power supply body 7 Resin tank body 8 Bipolar plate 9 Water electrolysis stack cell 10 Water electrolysis cell module

Claims (18)

A catalyst-supporting porous substrate is disposed across a polymer electrolyte membrane (PEM) in a single cell for water electrolysis, constitutes a cathode or an anode in contact with the polymer electrolyte membrane (PEM), and has a structure that also functions as a gas diffusion layer,
the catalyst is supported on the side surfaces of the pores of the porous substrate or on the side surfaces of the fibers forming the porous substrate, and is present from the surface to the inside of the porous substrate itself;
and an ionomer is filled in the porous substrate from the surface toward the inside thereof while being in contact with the catalyst, with a concentration gradient in the thickness direction of the porous substrate;
The catalyst-supported porous substrate is characterized in that the ionomer is obtained by supporting the catalyst on the porous substrate, applying a solution or dispersion of the ionomer and drying it, and filling the porous substrate with a concentration gradient from the surface to the inside. The catalyst-supporting porous substrate according to claim 1, which is obtained by a process comprising adhering a metal catalyst or a metal catalyst precursor to the side surfaces of the pores of the porous substrate before firing, or to the side surfaces of the fibers forming the porous substrate before firing, and then firing the substrate. 3. The catalyst-supporting porous substrate according to claim 1, wherein the thickness or particle size of said catalyst is smaller than the average diameter of "voids consisting of gaps between said pores and/or said fibers." A catalyst-supported porous substrate as described in any one of claims 1 to 3, wherein the catalyst is not deposited as a catalyst layer only on the surface of the porous substrate itself, but is supported on the sides of the holes in the porous substrate or on the sides of the fibers forming the porous substrate, and is present from the surface to the inside of the porous substrate itself. 5. A catalyst-supporting porous substrate according to claim 1, wherein the porosity of the catalyst-supporting porous substrate excluding the ionomer, as represented by the following formula (1), is 3 volume % or more and 80 volume % or less.
Porosity (volume%)
= 100 × [pore volume of catalyst-supporting porous substrate] / [volume of catalyst-supporting porous substrate] (1) A catalyst-supporting porous substrate as described in any one of claims 1 to 5, wherein the ionomer is filled into the voids of the catalyst-supporting porous substrate, and the ratio of the volume of the voids filled with the ionomer to the total volume of the voids of the catalyst-supporting porous substrate is 10 volume % or more and 90 volume % or less. A catalyst-supported porous substrate as described in any one of claims 1 to 6, wherein the sum of the micro-area of "the catalyst present from the surface to the interior of the porous substrate itself" in contact with the ionomer is at least twice the macro-area of the surface of the catalyst-supported porous substrate in contact with the polymer electrolyte membrane (PEM). The catalyst-supporting porous substrate according to any one of claims 1 to 7, wherein the catalyst is one or more metals or metal-containing compounds selected from the group consisting of platinum (Pt), ruthenium (Ru), iridium (Ir), palladium (Pd ) , tantalum (Ta), and nickel (Ni). 9. The catalyst-supporting porous substrate according to claim 1 , wherein the material of the porous substrate is titanium (Ti), a titanium (Ti) alloy, or carbon (C). 10. The catalyst-supporting porous substrate according to claim 1 , wherein the porous substrate is an aggregate of titanium fibers, titanium alloy fibers, or carbon fibers. 11. The catalyst-supporting porous substrate according to claim 1 , wherein the ionomer is a proton-conducting polymer. 12. An electrode for water electrolysis, comprising the catalyst-supporting porous substrate according to claim 1 . A gas diffusion layer comprising the catalyst-supporting porous substrate according to any one of claims 1 to 11 . 12. A single cell for water electrolysis, comprising a polymer electrolyte membrane (PEM) sandwiched between the catalyst-supporting porous substrates according to any one of claims 1 to 11 . 15. A water electrolysis stack cell comprising two or more unit cells for water electrolysis according to claim 14 stacked together, the unit cells being each configured as a single unit cell for water electrolysis having a structure in which a polymer electrolyte membrane (PEM) is sandwiched between catalyst-supporting porous substrates. A water electrolysis cell module comprising the stack cells for water electrolysis according to claim 15 arranged two-dimensionally or three-dimensionally. A method for producing a catalyst-supported porous substrate in a single cell for water electrolysis, the porous substrate sandwiching a polymer electrolyte membrane (PEM), forming a cathode or an anode in contact with the polymer electrolyte membrane (PEM), and also functioning as a gas diffusion layer, comprising the steps of:
the catalyst is supported on the side surfaces of the pores of the porous substrate or on the side surfaces of the fibers forming the porous substrate, and is present from the surface to the inside of the porous substrate itself;
A method for producing a catalyst-supporting porous substrate, comprising the steps of: supporting a catalyst on a porous substrate ; applying a solution or dispersion of the ionomer and drying the same to bring the ionomer into contact with the catalyst and filling the porous substrate with a concentration gradient from the surface to the inside. A method for producing a catalyst-supporting porous substrate according to claim 17, comprising a step of adhering a metal catalyst or a metal catalyst precursor to the side surfaces of the pores of the porous substrate before firing, or to the side surfaces of the fibers forming the porous substrate before firing , and then firing the substrate.