WO2024106185A1

WO2024106185A1 - Anode catalyst layer, water electrolytic cell, and water electrolytic cell stack

Info

Publication number: WO2024106185A1
Application number: PCT/JP2023/038997
Authority: WO
Inventors: 篤宇根本
Original assignee: 東京瓦斯株式会社
Priority date: 2022-11-15
Filing date: 2023-10-27
Publication date: 2024-05-23
Also published as: JP2024072199A

Abstract

Provided is an anode catalyst layer comprising: an oxide catalyst having a perovskite structure, wherein the A site ion includes an alkaline earth metal ion and the B site ion includes at least one kind selected from the group consisting of an iridium ion and a ruthenium ion and a metal ion; and an ionomer.

Description

Anode catalyst layer, water electrolysis cell, and water electrolysis cell stack

This disclosure relates to an anode catalyst layer, a water electrolysis cell, and a water electrolysis cell stack.

Water electrolysis (hereinafter sometimes referred to as "water electrolysis") is a method of producing hydrogen and oxygen from water by electrolysis. For example, among technologies that use hydrogen as an energy source, water electrolysis is a promising technology for sustainable hydrogen production.

The water electrolysis cell used for water electrolysis is equipped with an anode separator, an anode gas diffusion layer, an anode catalyst, an electrolyte membrane, a cathode catalyst, a cathode gas diffusion layer, a cathode separator, etc. Regarding the anode catalyst and cathode catalyst, catalysts suitable for water electrolysis are being considered.

For example, JP 2018-519420 A (Patent Document 1) discloses an oxygen evolution reaction electrode containing nanostructured whiskers, which has an oxygen evolution reaction electrocatalyst containing at least one layer on nanostructured whiskers, and each layer of the oxygen evolution reaction electrocatalyst contains at least 95 atomic percent Ir and 5 atomic percent or less Pt in total, based on the total content of cations and elemental metals in each layer.

Anode catalysts containing elements with anode activity, such as iridium and ruthenium, have high catalytic activity. However, because the anode catalyst is exposed to a strong oxidizing atmosphere during water electrolysis, elements such as ruthenium and iridium are easily dissolved, and anode catalysts containing these elements are easily destabilized.

For example, in a catalyst containing iridium such as that described in Patent Document 1, the iridium is easily dissolved, and there is a risk that the catalyst may become unstable.

The objective of this disclosure is to provide an anode catalyst layer that exhibits high catalytic activity and can maintain stable catalytic activity, as well as a water electrolysis cell and a water electrolysis cell stack that include this anode catalyst layer.

The present disclosure includes the following aspects.
＜1＞
an oxide catalyst having a perovskite structure, the A-site ions of which include alkaline earth metal ions, and the B-site ions of which include at least one selected from the group consisting of iridium ions and ruthenium ions and metal ions (excluding iridium ions and ruthenium ions);
Ionomer and
an anode catalyst layer comprising:
＜2＞
The anode catalyst layer according to <1>, wherein a logarithm of a mass ratio (log(X/Y)) of a content (X) of the ionomer to a total content (Y) of the iridium ions and the ruthenium ions contained in the B site ions is −0.860 to 0.060.
＜3＞
The anode catalyst layer according to <2>, wherein a logarithm of a mass ratio (log(X/Y)) of a content (X) of the ionomer to a total content (Y) of the iridium ions and the ruthenium ions contained in the B site ions is −0.620 to −0.090.
＜4＞
The anode catalyst layer according to <3>, wherein a logarithm of a mass ratio (log(X/Y)) of a content (X) of the ionomer to a total content (Y) of the iridium ions and the ruthenium ions contained in the B site ions is −0.500 to −0.180.
＜5＞
The alkaline earth metal ion includes at least one selected from the group consisting of a calcium ion, a strontium ion, and a barium ion,
The metal ion includes at least one selected from the group consisting of a titanium ion, a zirconium ion, and a tin ion;
The anode catalyst layer according to any one of <1> to <4>, wherein a total molar concentration of the iridium ions and the ruthenium ions in the B site ions is 5 mol % or more and 67 mol % or less.
＜6＞
The anode catalyst layer according to any one of <1> to <5>, wherein the alkaline earth metal ions include strontium ions, and the metal ions include titanium ions.
＜７＞
The anode catalyst layer according to any one of <1> to <5>, wherein the alkaline earth metal ions include strontium ions, and the metal ions include zirconium ions.
＜8＞
<8> The anode catalyst layer according to any one of <1> to <7>, wherein the ionomer contains a perfluorosulfonic acid group.
＜9＞
A water electrolysis cell comprising: an anode gas diffusion layer; the anode catalyst layer according to any one of <1> to <8>; an electrolyte membrane; a cathode catalyst layer; a cathode gas diffusion layer; and a separator.
＜10＞
A water electrolysis cell stack in which the water electrolysis cells according to <9> are stacked.

The present disclosure provides an anode catalyst layer that exhibits high catalytic activity and can maintain stable catalytic activity, as well as a water electrolysis cell and a water electrolysis cell stack that include this anode catalyst layer.

FIG. 1 is a schematic cross-sectional view of a water electrolysis cell. 1 is a graph showing the relationship between log(X/Y) and log{current density per iridium mass at 1.8 V (A/mg-Ir)}.

Below, an embodiment of the present disclosure will be described. The present disclosure is not limited to the following embodiment, and can be implemented with appropriate modifications within the scope of the purpose of the present disclosure. The dimensional ratios in the drawings do not necessarily represent the actual dimensional ratios.

In this disclosure, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as the lower and upper limits.

In the numerical ranges described in stages in this disclosure, the upper limit value described in a certain numerical range may be replaced with the upper limit value of another numerical range described in stages, and the lower limit value described in a certain numerical range may be replaced with the lower limit value of another numerical range described in stages. In the numerical ranges described in stages in this disclosure, the upper limit value or lower limit value described in a certain numerical range may be replaced with a value shown in the examples.

In this disclosure, a combination of two or more preferred aspects is a more preferred aspect.

<Anode catalyst layer>
The anode catalyst layer of the present disclosure includes an oxide catalyst having a perovskite structure, the A-site ions of which include alkaline earth metal ions, and the B-site ions of which include at least one selected from the group consisting of iridium ions and ruthenium ions and metal ions (excluding iridium ions and ruthenium ions). The anode catalyst layer of the present disclosure also includes an ionomer.

According to the present disclosure, it is possible to obtain an anode catalyst layer that exhibits high catalytic activity and is capable of maintaining stable catalytic activity. Although the reason why this effect is achieved is not necessarily clear, it is presumed as follows.
The anode catalyst layer of the present disclosure can maintain stable catalytic activity by including an oxide catalyst having a perovskite structure. Furthermore, the anode catalyst layer includes at least one of iridium ions and ruthenium ions as B-site ions of the perovskite structure, and the A-site ions and B-site ions are the above ions, so that the current density of a water electrolysis cell including the anode catalyst layer can be increased, and high catalytic activity is exhibited. Furthermore, the anode catalyst layer includes an ionomer, which provides excellent ionic conductivity and high catalytic activity.
From the above, it is presumed that the catalyst exhibits high catalytic activity and can maintain stable catalytic activity.

(Oxide catalyst)
First, an oxide catalyst having a perovskite structure will be described, which contains an alkaline earth metal ion as an A-site ion and a metal ion and at least one selected from the group consisting of an iridium ion and a ruthenium ion as a B-site ion.
The oxide catalyst can be produced by a conventionally known method. For example, the oxide catalyst having the perovskite structure can be produced by a conventionally known solid-phase method, liquid-phase method, etc. The solid-phase method includes a method of direct reaction of solid raw materials, and the liquid-phase method includes the Pezzini method, complex polymerization method, hydrothermal synthesis method, etc.

An oxide catalyst having a perovskite structure is generally represented by the chemical formula _ABO3 . Some oxide catalysts having a perovskite structure have oxygen instability. The amount of oxygen may be deficient or excessive compared to 3. In addition, the A-site ions and B-site ions may be partially substituted with different elements.

-Alkaline earth metal ions (A site ions)
The A-site ions of the perovskite structure include alkaline earth metal ions. Examples of the alkaline earth metal ions include calcium ions, strontium ions, barium ions, and radium ions. The alkaline earth metal ions preferably include at least one selected from the group consisting of calcium ions, strontium ions, and barium ions, and more preferably include strontium ions. The A-site ions may be one type of alkaline earth metal ion or two or more types of alkaline earth metal ions.

- Iridium ions and ruthenium ions (B site ions)
The B-site ions of the perovskite structure include at least one selected from the group consisting of iridium ions and ruthenium ions. The B-site ions may include either one of iridium ions or ruthenium ions, or may include both of them. It is preferable that the B-site ions include iridium ions.

From the viewpoint of maintaining stable catalytic activity, the logarithm of the mass ratio (log(X/Y)) of the ionomer content (X) to the total content (Y) of iridium ions and ruthenium ions contained in the B site ions is preferably −0.860 to 0.060, more preferably −0.620 to −0.090, and even more preferably −0.500 to −0.180.
Here, from the viewpoint of reducing the internal resistance of the cell and increasing the efficiency of the water electrolysis reaction, it is preferable to highly disperse the catalyst particles in the anode catalyst layer to increase the reaction area, while adjusting the amount of the ionomer responsible for ion conduction to an appropriate amount. By setting the ratio of the ionomer to the iridium ions and the ruthenium ions in a range that is not too small, sufficient ion conduction paths are obtained, the internal resistance of the cell is reduced, and the efficiency of the water electrolysis reaction is improved. On the other hand, by setting the ratio of the ionomer to the iridium ions and the ruthenium ions in a range that is not too large, the ionomer is prevented from covering the catalyst surface and reducing the reaction area, the internal resistance of the cell is reduced, and the efficiency of the water electrolysis reaction is improved.

The total molar concentration of iridium ions and ruthenium ions as B site ions in the anode catalyst layer of the present disclosure is preferably 5 mol% or more and 67 mol% or less, more preferably 7 mol% or more and 60 mol% or less, and even more preferably 10 mol% or more and 50 mol% or less, from the viewpoint of maintaining stable catalytic activity.

In the anode catalyst layer of the present disclosure, the total coating amount of the iridium ions and ruthenium ions (iridium + ruthenium coating amount) is preferably 0.081 mg / cm ² or more, more preferably 0.093 mg / cm ² or more, and even more preferably 0.102 mg / cm ² or more. When the iridium + ruthenium coating amount is 0.081 mg / cm ² or more, high catalytic activity can be obtained. The upper limit of the iridium + ruthenium coating amount is not particularly limited, but is preferably 0.300 mg / cm ² or less, more preferably 0.200 mg / cm ² or less, and even more preferably 0.193 mg / cm ² or less.

・Metal ions (B site ions)
The B site ions of the perovskite structure include metal ions (excluding iridium ions and ruthenium ions). Examples of metal ions include ions of metals such as titanium (Ti), zirconium (Zr), tin (Sn), scandium (Sc), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), gallium (Ga), indium (In), and antimony (Sb). Among them, the metal ions are stable elements, and from the viewpoint of being able to maintain stable catalytic activity of the anode catalyst layer, it is preferable to include at least one selected from the group consisting of titanium ions, zirconium ions, and tin ions, and it is more preferable to include at least one selected from the group consisting of titanium ions and zirconium ions. The metal ions in the B site ions may be only one type of metal ion, or may be two or more types of metal ions.

The combination of alkaline earth metal ions in the A site ions and metal ions in the B site ions will be described. The combination of alkaline earth metal ions and metal ions is not particularly limited, but from the viewpoint of maintaining stable catalytic activity of the anode catalyst layer, it is preferable to include strontium ions as the alkaline earth metal ions and titanium ions as the metal ions. Also, from the viewpoint of maintaining stable catalytic activity of the anode catalyst layer, it is preferable to include strontium ions as the alkaline earth metal ions and zirconium ions as the metal ions.

The molar concentration of metal ions contained in the A site ions, the molar concentration of metal ions contained in the B site ions, and the molar concentrations of iridium ions and ruthenium ions contained in the B site ions can be determined by analyzing the anode catalyst using inductively coupled plasma (ICP).

The content of the oxide catalyst (oxide catalyst having a perovskite structure) in the anode catalyst layer of the present disclosure is not particularly limited, and is preferably 70 mass% or more, more preferably 90 mass% or more, even more preferably 95 mass% or more, and most preferably 100 mass% relative to the total amount of the anode catalyst layer.

In the anode catalyst layer of the present disclosure, the coating amount (catalyst coating amount) of the oxide catalyst (oxide catalyst having a perovskite structure) is preferably 0.28 mg/cm ² or more, more preferably 0.34 mg/cm ² or more, and even more preferably 0.37 mg/cm ² or more. When the catalyst coating amount is 0.28 mg/cm ² or more, high catalytic activity can be obtained. The upper limit of the catalyst coating amount is not particularly limited, but is preferably 1.00 mg/cm ² or less, more preferably 0.70 mg/cm ² or less, and even more preferably 0.68 mg/cm ² or less.

(Ionomer)
Next, the ionomer will be described.
An ionomer is, for example, a resin having an ionically crosslinked ethylene skeleton as a basic structure. From the viewpoint of obtaining excellent ion conduction, the ionomer preferably contains a perfluorosulfonic acid group in the skeleton.

A representative example of an ionomer containing perfluorosulfonic acid groups is the ionomer shown in formula (1) below.

(In formula (1), m is 0 to 10, n is 1 to 10, x is 1 to 20, and y is 100 or more.)

For example, Nafion (registered trademark) 117 has the structure (m≧1, n=2, x=5-13.5, y=1000) in formula (1). The chemical structure of the ionomer can be evaluated by peeling the anode catalyst layer from the catalyst and using a combination of solid-state NMR (nuclear magnetic resonance) measurement and CHN analysis (carbon C, hydrogen H, nitrogen N atomic analysis). Furthermore, the ratio of catalyst to ionomer can be evaluated by ICP (inductively coupled plasma) analysis.

In the anode catalyst layer of the present disclosure, the coating amount of the ionomer (ionomer coating amount) is preferably 0.0056 mg/cm ² or more, more preferably 0.0068 mg/cm ² or more, and even more preferably 0.0370 mg/cm ² or more. When the ionomer coating amount is 0.0056 mg/cm ² or more, high catalytic activity can be obtained. The upper limit of the catalyst coating amount is not particularly limited, but is preferably 0.5000 mg/cm ² or less, more preferably 0.3500 mg/cm ² or less, and even more preferably 0.3050 mg/cm ² or less.

In the anode catalyst layer of the present disclosure, the mass ratio of the ionomer to the oxide catalyst (ionomer/catalyst mass ratio) is preferably 0.01 or more, more preferably 0.02 or more, and even more preferably 0.1 or more. By setting the ionomer/catalyst mass ratio to 0.01 or more, high catalytic activity can be obtained. The upper limit of the ionomer/catalyst mass ratio is not particularly limited, but is preferably 0.8 or less, more preferably 0.6 or less, and even more preferably 0.5 or less.

The anode catalyst layer of the present disclosure may contain components other than an oxide catalyst having a perovskite structure, which contains an alkaline earth metal ion as an A site ion and at least one selected from the group consisting of an iridium ion and a ruthenium ion and a metal ion (excluding iridium ion and ruthenium ion) as a B site ion, and an ionomer. Examples of such components include components having catalytic activity other than an oxide catalyst having a perovskite structure, unreacted components of the raw materials used to generate the oxide catalyst having a perovskite structure, side reaction components, and carriers. Examples of carriers include titanium oxide and tin oxide, which are stable under water electrolysis conditions.

<Water electrolysis cell>
The water electrolysis cell according to the present disclosure includes an anode gas diffusion layer, the anode catalyst layer according to the present disclosure, an electrolyte membrane, a cathode catalyst layer, a cathode gas diffusion layer, and a separator. Water to be used for water electrolysis is injected into the water electrolysis cell according to the present disclosure.

The anode gas diffusion layer, electrolyte membrane, cathode catalyst layer, cathode gas diffusion layer and separator may be made of materials used in conventional water electrolysis cells.

The water electrolysis cell may further include other components. The other components may be selected from known components of water electrolysis cells. Examples of the other components include gaskets, sealing materials, etc.

For example, the anode gas diffusion layer and the cathode gas diffusion layer can each independently be made of a material that allows fluid to flow through the layer, such as a porous material, a powder sintered body, a fiber sintered body, a metal mesh, or felt.

The anode gas diffusion layer may be coated with a corrosion-resistant conductive material to prevent high resistance due to oxidation. Examples of coating materials include platinum, gold, silver, titanium nitride, titanium carbide, and titanium carbonitride.

The electrolyte membrane may be selected from known electrolyte membranes (which may be ion exchange membranes) used in water electrolysis. The electrolyte membrane preferably has a property of selectively permeating protons (H ⁺ ). Examples of the electrolyte membrane include polymer electrolyte membranes (PEM). Examples of the polymer electrolyte membrane include perfluorocarbon membranes having sulfonic acid groups. Examples of the perfluorocarbon membranes having sulfonic acid groups include Nafion membranes.

The electrolyte membrane is a polymer that has proton conductivity due to the presence of ionic groups, and may be, for example, either a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte.

Here, a fluoropolymer electrolyte refers to one in which most or all of the hydrogen atoms in the alkyl and/or alkylene groups in the polymer have been replaced with fluorine atoms. Representative examples of fluoropolymer electrolytes having ionic groups include commercially available products such as "Nafion" (registered trademark) (manufactured by Chemours), "Aquivion" (registered trademark) (manufactured by Solvay), "Flemion" (registered trademark) (manufactured by AGC), and "Aciplex" (registered trademark) (manufactured by Asahi Kasei).

As the hydrocarbon electrolyte, an aromatic hydrocarbon polymer having an aromatic ring in the main chain is preferable. Here, the aromatic ring may include not only a hydrocarbon aromatic ring consisting only of carbon atoms and hydrogen atoms, such as a benzene ring or a naphthalene skeleton, but also a heterocycle such as a pyridine ring, an imidazole ring, or a thiol ring. In addition, some aliphatic units may constitute the polymer together with the aromatic ring units.

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, polyarylene ketone, polyether ketone, polyarylene phosphine oxide, polyether phosphine oxide, polybenzoxazole, polybenzothiazole, polybenzimidazole, polyamide, polyimide, polyetherimide, and polyimide sulfone in the main chain together with an aromatic ring. Note that polysulfone, polyethersulfone, polyether ketone, etc. referred to here are general terms for structures having sulfone bonds, ether bonds, ketone bonds, etc. in the molecular chain, and include polyether ketone ketone, polyether ether ketone, polyether ether ketone ketone, polyether ketone ether ketone ketone, and polyether ketone sulfone. Aromatic hydrocarbon polymers may have a plurality of these structures. Among these, aromatic hydrocarbon polymers, particularly polymers having a polyetherketone skeleton, i.e., polyetherketone polymers, are preferred.

The electrolyte membrane may be combined with a reinforcing material. By using a reinforcing material, for example, gas leaks and short circuits within the electrodes caused by damage to the membrane when joining the electrolyte membrane and electrodes by hot pressing are less likely to occur.

Specific examples of reinforcing materials include homogeneous porous membranes made of fluorine-based polymers such as PTFE (polytetrafluoroethylene), PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer), PVDF (polyvinylidene fluoride), and FEP (tetrafluoroethylene-hexafluoropropylene copolymer), thermoplastic resins such as PE (polyethylene) and PP (polypropylene), and engineering plastics such as PI (polyimide), PSF (polysulfone), PES (polyethersulfone), PEEK (polyetheretherketone), PPSS (polyphenylene sulfide sulfone), PPO (polyphenylene oxide), PEK (polyetherketone), PBI (polybenzimidazole), PPS (polyphenylene sulfide), PPP (polyparaphenylene), PPQ (polyphenylquinoxaline), polybenzoxazole (PBO), polybenzothiazole (PBT), and polyparaphenylene terephthalamide (PPTA).

The cathode catalyst may be selected from known catalysts used in water electrolysis. Examples of catalyst components include platinum, gold, silver, palladium, iridium, rhodium, ruthenium, tin, iron, cobalt, nickel, molybdenum, tungsten, vanadium, and alloys and oxides thereof. The catalyst may be in the form of particles. The cathode catalyst may include a catalyst supported on a carrier. Examples of carriers include carbon black.

The water electrolysis cell includes an anode catalyst layer according to the present disclosure, which includes an oxide catalyst (anode catalyst, preferably oxide catalyst particles) and an ionomer. The cell may also include a cathode catalyst layer, which includes a cathode catalyst (preferably cathode catalyst particles) and an ionomer. This increases the contact area between the catalyst and the ionomer in the catalyst layer, which tends to promote the reaction.

The primary particle diameter of the oxide catalyst (anode catalyst) is preferably 1 nm to 10 μm, more preferably 2 nm to 1 μm, and even more preferably 5 nm to 100 nm. When the primary particle diameter of the oxide catalyst (anode catalyst) is 1 nm or more, the mixing ratio of the ionomer required to increase the contact area is not too large, and many electron conduction paths can be secured inside the anode catalyst layer, so that the resistance tends to be difficult to increase. When the primary particle diameter of the anode catalyst is 10 μm or less, the decrease in the contact area with the ionomer is suppressed, so that the resistance tends to be difficult to increase. The particle diameter of the catalyst can be evaluated using a scanning electron microscope or a transmission electron microscope.

The separator may be an anode separator arranged on the anode gas diffusion layer side, or a cathode separator arranged on the cathode gas diffusion layer side. Examples of separator materials include titanium, stainless steel, and carbon. From the viewpoint of suppressing oxidation due to oxygen generated on the anode side, it is preferable that the anode separator contains titanium.

The anode separator may be coated with a corrosion-resistant conductive material to prevent high resistance due to oxidation. Examples of coating materials include platinum, gold, silver, titanium nitride, titanium carbide, and titanium carbonitride.

The arrangement of each component in the water electrolysis cell may be determined with reference to known water electrolysis cells. In the water electrolysis cell, the electrolyte membrane is preferably located between the anode catalyst layer and the cathode catalyst layer. In the water electrolysis cell, the electrolyte membrane, the anode catalyst layer, and the cathode catalyst layer are preferably located between the anode gas diffusion layer and the cathode gas diffusion layer. In the water electrolysis cell, the electrolyte membrane, the anode catalyst layer, the cathode catalyst layer, and the anode gas diffusion layer and the cathode gas diffusion layer are preferably located between two separators.

An example of a water electrolysis cell is shown in Figure 1. Figure 1 is a schematic cross-sectional view of a water electrolysis cell. As shown in Figure 1, the water electrolysis cell 100 includes, from the top of Figure 1, an anode separator 60, an anode gas diffusion layer 20, an anode catalyst layer 12, an electrolyte membrane 11, a cathode catalyst layer 13, a cathode gas diffusion layer 30, and a cathode separator 70. In addition, a gasket 40 is disposed between the anode separator 60 and the electrolyte membrane 11, and a gasket 50 is disposed between the cathode separator 70 and the electrolyte membrane 11.

<Water electrolysis device>
The water electrolysis device according to the present disclosure may be a water electrolysis cell stack formed by stacking a plurality of the above-described water electrolysis cells according to the present disclosure, or may be a device including the water electrolysis cell stack or the water electrolysis cell according to the present disclosure and other components.

The other components may be selected from known components of a water electrolysis device. Examples of the other components include auxiliary equipment such as a power conditioner, a water pump, an ion exchange resin, a heat exchanger, and a dehumidifier.

Below, the present disclosure will be described in detail with reference to examples. However, the present disclosure is not limited to the following examples. The matters shown in the following examples may be modified as appropriate without departing from the spirit of the present disclosure.

Example 1
A catalyst powder containing an oxide having a perovskite structure in which iridium ions are incorporated into the B site of strontium titanate (SrTiO ₃ ) was obtained by the Pezzini method. The starting materials were strontium nitrate (Sr(NO ₃ ) ₂ ), titanium tetrabutoxide (C ₁₆ H ₃₆ O ₄ Ti), and potassium hexachloroiridate (K ₂ IrCl ₆ ). 30.2 g of Sr(NO ₃ ) ₂ , 5.7 g of K ₂ IrCl ₆ , and 20.1 g of citric acid monohydrate (C ₆ H ₈ O _7.H ₂ O) were weighed, put into 750 mL of pure water, mixed, and dissolved by stirring at room temperature for more than 1 hour. This mixed solution was named solution A. C ₁₆ H ₃₆ O ₄ Ti was weighed out at 8.1 g, mixed with 300 mL of ethylene glycol (C ₂ H ₆ O ₂ ), and stirred for 1 hour or more. This mixed solution was designated as solution B. After adding solution A to solution B, the mixture was stirred and mixed at 70° C. for 3 hours or more using a hot stirrer. The mixed solution was then transferred to a zirconia crucible and heat-treated at 180° C. for 12 hours, 200° C. for 6 hours, 300° C. for 6 hours, 500° C. for 3 hours, and 600° C. for 6 hours. The powder after the heat treatment was collected and put into a beaker together with 500 mL of a 1M aqueous hydrochloric acid solution and stirred for 6 hours or more to remove unreacted components. The obtained mixed solution was washed with water using a suction filter, dried in an oven at 60° C., and then a catalyst powder was obtained. When an X-ray diffraction measurement was performed on the synthesized catalyst powder, a diffraction pattern derived from a perovskite structure was obtained. The obtained powder was dissolved in aqua regia and analyzed by high-frequency inductively coupled plasma (ICP), which revealed that the molar concentration of iridium ions at the B site was 67 mol %.

A test anode catalyst layer for measuring current density was prepared using the synthesized catalyst powder. 10 g of the obtained catalyst powder and 5 mass % Nafion dispersion solution (Sigma-Aldrich, 70160) equivalent to the ionomer were weighed out so that log(X/Y) (logarithm of the mass ratio of the ionomer content (X) to the total content (Y) of iridium ions and ruthenium ions contained in the B site ions) was −0.662, transferred to a glass container together with 2-propanol and pure water, and mixed with a homogenizer for 30 minutes or more to obtain an anode slurry.
Commercially available Pt/C (platinum/carbon) was used as the cathode catalyst, and was mixed with an ionomer to obtain a cathode slurry.
The anode slurry and the cathode slurry were sprayed onto a polytetrafluoroethylene (Teflon (registered trademark)) sheet with a length and width of 5 cm, respectively, and transferred onto the electrolyte membrane using a hot press to produce an electrolyte membrane with a catalyst layer. The amount of anode catalyst was determined from the mass difference before and after application to the Teflon sheet, and the amount of iridium applied was calculated from the ratio of metal elements in the catalyst. When the mass of the Teflon sheet before and after the transfer was compared, the transfer rate of the anode catalyst layer was 100%.

The catalyst layer-attached electrolyte membrane thus prepared was cut into 2 cm squares, the anode catalyst was peeled off and scraped off with a spatula, and dissolved in aqua regia. The catalyst composition and coating amount were analyzed by ICP (Hitachi High-Tech Science Corporation, PS3520VDDII) analysis, and the results were consistent with the composition and coating amount of the catalyst. In addition, solid-state ¹⁹ F-NMR analysis (Bruker, AVANCE NEO400) was performed. The test was performed using a single pulse method, a spectrum width of 200 kHz, a pulse width of 2.4 μsec, and a sample rotation speed of 20 kHz to obtain a spectrum. Furthermore, fluorine and sulfur components were analyzed by ion chromatography, and the structure of the ionomer was estimated, which was consistent with the composition of the ionomer in the coating. From the estimated ionomer structure and the results of elemental analysis by ion chromatography, the mass of the ionomer contained in the catalyst layer was estimated, and the coating amount of the ionomer per unit area was calculated, which was consistent with the coating amount in the coating.

A water electrolysis cell was prepared that includes an electrolyte membrane with a catalyst layer, an anode gas diffusion layer, an anode-side separator, a cathode gas diffusion layer, a cathode-side separator, an anode-side end plate, an anode-side current collector, an insulating sheet disposed between the anode-side end plate and the current collector, an end plate on the cathode side, a current collector on the cathode side, and an insulating sheet disposed between the cathode-side end plate and the current collector. The anode-side separator was made of Pt-plated titanium, with the electrode installation section having a length and width of 5 cm, and 26 parallel flow paths with grooves and peaks of 1 mm each and a depth of 2 mm within a 5 cm side range. The cathode-side separator was made of carbon, with the electrode installation section having a length and width of 5 cm, and 26 parallel flow paths with grooves and peaks of 1 mm each and a depth of 2 mm within a 5 cm side range. The anode gas diffusion layer was made of Pt-plated titanium fiber sintered body. The cathode gas diffusion layer was made of carbon material (manufactured by SGL Carbon Japan Co., Ltd.) and cut to a length and width of 5 cm. The anode and cathode gaskets were made of polytetrafluoroethylene (Teflon (registered trademark)) sheets with the electrode installation portion cut out. The electrode layer was placed on the portion where the electrode installation portion of the gasket was cut out, and the anode side separator, the anode gas diffusion layer, the electrolyte membrane with catalyst layer, the cathode gas diffusion layer, and the cathode side separator were laminated so that the electrode installation portion and the anode and cathode flow path portions overlap. The anode side and cathode side separators were laminated in the order of the current collector plate, the insulating sheet, and the end plate, and fastened with bolts to prepare a water electrolysis cell. The anode side separator was provided with a water inlet and a pipe for discharging the generated oxygen and unreacted water, and the cathode side separator was provided with a pipe for discharging the generated hydrogen. The current collectors on the anode side and the cathode side were each connected to an external power supply (PWR1201L, manufactured by Kikusui Electronics Co., Ltd.).
After connecting an external power source to the water electrolysis cell, the temperature of the water electrolysis cell was raised to 60°C, and water was supplied to the water electrolysis cell at a flow rate of 100 ml/min. From 1.50 V to 1.90 V, the voltage was maintained at 0.1 V increments for 2 minutes each under constant voltage control, and after reaching 1.90 V, the voltage was maintained at 0.1 V increments for 2 minutes each up to 1.50 V. This constitutes one cycle, and a total of 10 cycles of conditioning were performed. Next, the voltage of the water electrolysis cell was maintained at 2.3 V under constant voltage control for 100 hours, and then the current value at a voltage of 1.8 V was measured to calculate log {current density per iridium mass at 1.8 V (A/mg-Ir)}, which was 0.891.

Example 2
A catalyst powder containing an oxide _having a perovskite structure _was prepared in the _same manner as in Example 1, except that the starting materials were 62.4 g of Sr( _NO3 ) ₂ , _11.9 g of _K2IrCl6 , _4.2 g of _C16H36O4Ti , 41.6 g of _C6H8O7.H2O , 1500 mL of pure water, and ₆₀₀ mL of _C2H6O2 . The _molar concentration of _iridium ions at the B site was 33 mol%.
Furthermore, a Nafion dispersion solution was mixed so that log(X/Y) was −0.438 to obtain an anode slurry. Thereafter, an electrolyte membrane with a catalyst layer was produced in the same manner as in Example 1. The log{current density per mass of iridium at 1.8 V (A/mg-Ir)} was 1.073.

Example 3
Except for adjusting log(X/Y) to be −0.137, an anode slurry was obtained in the same manner as in Example 2. Thereafter, an electrolyte membrane with a catalyst layer was obtained in the same manner as in Example 1. The log{current density per mass of iridium at 1.8 V (A/mg-Ir)} was 0.843.

Example 4
Except for adjusting log(X/Y) to 0.039, an anode slurry was obtained in the same manner as in Example 2. Thereafter, an electrolyte membrane with a catalyst layer was produced in the same manner as in Example 1. The log{current density per mass of iridium at 1.8 V (A/mg-Ir)} was 0.808.

Example 5
A catalyst _powder containing _an oxide having a perovskite structure was prepared in the _same _manner as in Example 1, except that _Ba ( _NO3 ) ₂ , _K2IrCl6 , _and _C16H36O4Ti _{were used as starting materials, Ba(NO3)2 was 58.7g, K2IrCl6 was 5.5g, C16H36O4Ti was 4.8g, C6H8O7.H2O} _was _19.3g _, _pure _water _was _700mL , and _C2H6O2 was _300mL . The molar concentration of _iridium ions at the B site was 45mol%.
Furthermore, a Nafion dispersion solution was mixed so that log(X/Y) was −0.463 to obtain an anode slurry. Thereafter, an electrolyte membrane with a catalyst layer was produced in the same manner as in Example 1. The log{current density per mass of iridium at 1.8 V (A/mg-Ir)} was 1.037.

Example 6
The starting materials were Sr(NO ₃ ) ₂ , K ₂ IrCl ₆ and zirconium oxychloride octahydrate (ZrOCl ₂ .8H ₂ O). 86.8 g of Sr(NO ₃ ) ₂ , 14.1 g of K ₂ IrCl ₆ , 14.4 g of ZrOCl ₂ .8H ₂ O, and 345 g of C ₆ H ₈ O ₇ .H ₂ O were weighed, and mixed with 600 mL of C ₂ H ₆ O ₂ in 1500 mL of pure water and stirred at room temperature for more than 1 hour. Then, the mixture was stirred and mixed at 75 ° C for more than 3 hours using a hot stirrer. The mixed solution was then transferred to a zirconia crucible and heat-treated for 12 hours at 180° C., 6 hours at 200° C., 6 hours at 300° C., 3 hours at 500° C., 6 hours at 600° C., and 6 hours at 700° C. The powder after heat treatment was collected, and the subsequent operations were carried out in the same manner as in Example 1 to produce a catalyst powder. The molar concentration of iridium ions at the B site was 40 mol%.
Furthermore, a Nafion dispersion solution was mixed so that log(X/Y) was −0.459 to obtain an anode slurry. Thereafter, an electrolyte membrane with a catalyst layer was produced in the same manner as in Example 1. The log{current density per mass of iridium at 1.8 V (A/mg-Ir)} was 1.049.

Example 7
Except for adjusting log(X/Y) to -0.158, an anode slurry was obtained in the same manner as in Example 6. Thereafter, an electrolyte membrane with a catalyst layer was produced in the same manner as in Example 1. The log{current density per mass of iridium at 1.8 V (A/mg-Ir)} was 0.792.

Example 8
Except for adjusting log(X/Y) to 0.018, an anode slurry was obtained in the same manner as in Example 6. Thereafter, an electrolyte membrane with a catalyst layer was produced in the same manner as in Example 1. The log{current density per mass of iridium at 1.8 V (A/mg-Ir)} was 0.732.

<Example 9>
Ba( _NO3 ) ₂ , _K2IrCl6 and _ZrOCl2.8H2O were used as starting materials, _and 97.6g of Ba( _NO3 ) ₂ , _12.8g of _K2IrCl6 , _8.5g of _ZrOCl2.8H2O , _and _{315g of C6H8O7.H2O} _were _weighed out and added to 1400mL of pure water together with _550mL of _C2H6O2 to synthesize catalyst powder in the same manner as in Example 6. The molar concentration of iridium ions at the B site was _50mol _% .
Furthermore, a Nafion dispersion solution was mixed so that log(X/Y) was −0.468 to obtain an anode slurry. Thereafter, an electrolyte membrane with a catalyst layer was produced in the same manner as in Example 1. The log{current density per mass of iridium at 1.8 V (A/mg-Ir)} was 1.041.

Example 10
The starting materials were calcium carbonate (CaCO ₃ ), K ₂ IrCl ₆ and ZrOCl 2.8H ₂ O, and 52.0 g of CaCO ₃ , 16.0 g of K ₂ IrCl ₆ , 25.0 g of ZrOCl _2.8H ₂ O, and 400 g of C ₆ H ₈ O _7.H ₂ O were weighed out _and added to 1700 mL of pure water together with 70 mL of nitric acid and 700 mL of C ₂ H ₆ O ₂ , and the catalyst powder was synthesized in the same manner as in Example 6. The molar concentration of iridium ions at the B site was 50 mol%.
Furthermore, a Nafion dispersion solution was mixed so that log(X/Y) was −0.440 to obtain an anode slurry. Thereafter, an electrolyte membrane with a catalyst layer was produced in the same manner as in Example 1. The log{current density per mass of iridium at 1.8 V (A/mg-Ir)} was 1.009.

Example 11
Except for adjusting log(X/Y) to 0.261, an anode slurry was obtained in the same manner as in Example 2. Thereafter, an electrolyte membrane with a catalyst layer was produced in the same manner as in Example 1. The log{current density per mass of iridium at 1.8 V (A/mg-Ir)} was −0.398.

Example 12
Except for adjusting log(X/Y) to be −1.137, an anode slurry was obtained in the same manner as in Example 2. Thereafter, an electrolyte membrane with a catalyst layer was produced in the same manner as in Example 1. The log{current density per mass of iridium at 1.8 V (A/mg-Ir)} was 0.146.

Example 13
Except for adjusting log(X/Y) to be −1.158, an anode slurry was obtained in the same manner as in Example 6. Thereafter, an electrolyte membrane with a catalyst layer was produced in the same manner as in Example 1. The log{current density per mass of iridium at 1.8 V (A/mg-Ir)} was 0.079.

<Example 14>
Except for adjusting log(X/Y) to 0.240, an anode slurry was obtained in the same manner as in Example 6. Thereafter, an electrolyte membrane with a catalyst layer was produced in the same manner as in Example 1. The log{current density per mass of iridium at 1.8 V (A/mg-Ir)} was −0.523.

<Comparative Example 1>
Commercially available _RuO2 was used as the anode catalyst. Furthermore, a Nafion dispersion solution was mixed so that log(X/Y) was -1.000 to obtain an anode slurry. Thereafter, an electrolyte membrane with a catalyst layer was obtained in the same manner as in Example 1. The log{current density per ruthenium mass at 1.8 V (A/mg-Ru)} was -0.843.

<Comparative Example 2>
A catalyst powder containing an oxide (SrTi _0.67 Ir _0.33 O ₃ ) having a perovskite structure was prepared in the same manner as in Example 2. Next, polyvinylidene fluoride (PVdF) (Kureha Corporation, L#1120) dissolved in N-methylpyrrolidone (NMP) was used as a binder to obtain an anode slurry. The catalyst powder and PVdF were weighed out so that the mass ratio of the solid content was 95:5, and NMP was added to adjust the viscosity of the slurry to obtain an anode slurry. An electrolyte membrane with a catalyst layer was obtained by the method described in Example 1, except that the anode slurry was applied to a Teflon sheet with a bar coater so that the total mass of the catalyst powder and the binder was 1.0 mg/cm ² .
The log{current density per iridium mass (A/mg-Ir) at 1.8 V} was −0.907.

In all of Examples 1 to 14, the value of log {current density per iridium mass at 1.8 V (A/mg-Ir)} was larger than the value of log {current density per ruthenium mass at 1.8 V (A/mg-Ru)} in Comparative Example 1. It is known that RuO ₂ dissolution proceeds at high potential. This indicates that the catalyst was dissolved and deactivated during constant voltage operation at 2.3 V. After the test, the water electrolysis cell was disassembled, and microstructure and elemental analysis were performed on the cross section of the electrolyte membrane with the catalyst layer using a scanning electron microscope and energy dispersive X-ray spectroscopy in combination, confirming the presence of oxide precipitates containing Ru in the electrolyte near the anode catalyst layer. In contrast, in Example 2, which had the largest log {current density per iridium mass at 1.8 V (A/mg-Ir)} among Examples 1 to 14, dissolution of the catalyst constituent elements into the electrolyte membrane as seen in Comparative Example 1 was not observed. This indicates that the catalyst having a perovskite structure is stable even at a high potential.

All of Examples 1 to 14 had a higher log {current density per mass of iridium at 1.8 V (A/mg-Ir)} than Comparative Example 2. In Examples 1 to 14, the catalyst layer contains an ionomer that allows protons to move and is necessary for forming active sites for the water electrolysis reaction, whereas Comparative Example 2 contains PVdF as a binder in the catalyst layer, which does not contribute to the movement of protons or the formation of active sites for the water electrolysis reaction. For this reason, it is believed that in Comparative Example 2, no proton paths or active sites for the reaction are formed inside the anode catalyst layer, which is why the performance of the catalyst could not be brought out.

In Examples 1 to 14, when log(X/Y) was in the range smaller than -0.3, there was a tendency for log{current density per mass of iridium at 1.8 V (A/mg-Ir)} to increase as log(X/Y) increased. This is thought to be because by increasing the mass of ionomer per mass of iridium, a proton path was efficiently formed inside the anode catalyst layer, improving performance. Furthermore, when log(X/Y) was in the range larger than -0.3, there was a tendency for log{current density per mass of iridium at 1.8 V (A/mg-Ir)} to increase as log(X/Y) decreased. It is thought that when the amount of ionomer per mass of iridium increases, the catalyst surface is covered with the ionomer, resulting in a decrease in the amount of active sites for the water electrolysis reaction; however, within this range, as log(X/Y) decreases, the amount of excess ionomer decreases, and the active sites for the water electrolysis reaction are efficiently introduced into the anode catalyst layer, improving performance.

Furthermore, as shown in FIG. 2 , in a water electrolysis cell including an anode catalyst layer containing an anode catalyst having a perovskite structure and an ionomer, when log(X/Y) is P and log{current density per mass of iridium at 1.8 V (A/mg-Ir)} is Q, the relationship between P and Q does not depend on the constituent elements of the anode catalyst or the concentration of iridium ions contained in the B site, and
Q = -2.1232P ³ -5.2892P ² -2.7969P + 0.6662
It was found that the cell can be expressed by the formula: From this, it was found that when P is -0.860 or more and 0.060 or less, Q is 0.5 or more, which is a high-performance water electrolysis cell. It was found that when P is -0.620 or more and -0.090 or less, Q is 0.86 or more, which is an even higher-performance water electrolysis cell. Furthermore, it was found that when P is -0.500 or more and -0.180 or less, Q is 1 or more, which is an even higher-performance water electrolysis cell.

The disclosure of Japanese Application No. 2022-182922 is incorporated herein by reference in its entirety.
All publications, patent applications, and standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or standard was specifically and individually indicated to be incorporated by reference.

11: Electrolyte membrane 12: Anode catalyst layer 13: Cathode catalyst layer 20: Anode gas diffusion layer 30: Cathode gas diffusion layer 40: Gasket 50: Gasket 60: Anode separator 70: Cathode separator 100: Water electrolysis cell

Claims

an oxide catalyst having a perovskite structure, the A-site ions of which include alkaline earth metal ions, and the B-site ions of which include at least one selected from the group consisting of iridium ions and ruthenium ions and metal ions (excluding iridium ions and ruthenium ions);
Ionomer and
an anode catalyst layer comprising:
The anode catalyst layer according to claim 1, wherein the logarithm (log(X/Y)) of the mass ratio between the content (X) of the ionomer and the total content (Y) of the iridium ions and ruthenium ions contained in the B site ions is -0.860 to 0.060.
The anode catalyst layer according to claim 2, wherein the logarithm (log(X/Y)) of the mass ratio of the ionomer content (X) to the total content (Y) of the iridium ions and ruthenium ions contained in the B site ions is -0.620 to -0.090.
The anode catalyst layer according to claim 3, wherein the logarithm (log(X/Y)) of the mass ratio of the ionomer content (X) to the total content (Y) of the iridium ions and ruthenium ions contained in the B site ions is -0.500 to -0.180.
The alkaline earth metal ion includes at least one selected from the group consisting of a calcium ion, a strontium ion, and a barium ion,
The metal ion includes at least one selected from the group consisting of a titanium ion, a zirconium ion, and a tin ion;
2. The anode catalyst layer according to claim 1, wherein a total molar concentration of the iridium ions and the ruthenium ions in the B site ions is 5 mol % or more and 67 mol % or less.
The anode catalyst layer according to claim 1, wherein the alkaline earth metal ions include strontium ions, and the metal ions include titanium ions.
The anode catalyst layer according to claim 1, comprising strontium ions as the alkaline earth metal ions and zirconium ions as the metal ions.
The anode catalyst layer according to claim 1, wherein the ionomer contains perfluorosulfonic acid groups.
A water electrolysis cell comprising an anode gas diffusion layer, the anode catalyst layer according to claim 1, an electrolyte membrane, a cathode catalyst layer, a cathode gas diffusion layer, and a separator.
A water electrolysis cell stack in which the water electrolysis cells according to claim 9 are stacked.