JP2024082451A - Electrode, film electrode joint body, fuel cell, and manufacturing method for electrode - Google Patents

Abstract

To provide an electrode for performing efficient power generation in a fuel cell using formic acid or formate as a fuel.SOLUTION: An electrode 10 according to the present invention is used for a fuel cell 100 using at least one selected from the group consisting of formic acids and formates as a fuel. The electrode 10 includes a catalyst layer 1 containing a metal catalyst. The content of the metal atoms in the catalyst layer 1 is 20 at% or more. A film electrode joint body 50 according to the present invention includes an electrode 10 and an electrolyte film 20. The fuel cell 100 according to the present invention includes the film electrode joint body 50.SELECTED DRAWING: Figure 1

Description

The present invention relates to an electrode, a membrane electrode assembly, a fuel cell, and a method for manufacturing an electrode. In the present invention, the electrode or the membrane electrode assembly is used in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts.

Polymer electrolyte fuel cells (PEFCs) are fuel cells that can operate at low temperatures and have advantages such as high power density, and are expected to become more widespread in the future. PEFCs have a membrane between the anode and cathode electrodes, and this membrane is made of a polymer electrolyte membrane with ion conductivity.

The use of formic acid and formate salts, which are easier to handle than hydrogen or methanol and have a relatively high energy density, as fuel for fuel cells is being considered. For example, Patent Document 1 discloses a fuel cell that uses formic acid as fuel.

JP 2012-195131 A

For fuel cells that use formic acid or formate salts as fuel, there is a demand for electrodes that can generate electricity efficiently.

In the field of fuel cells that use formic acid or formate salts as fuel, there is a tendency to fabricate the catalyst layer of the electrode by a coating method (e.g., Patent Document 1). In the coating method, for example, a coating liquid containing a metal catalyst is applied to a diffusion layer or the like, and the resulting coating film is dried to fabricate the catalyst layer. However, according to the inventors' investigations, the coating method requires the addition of a relatively large amount of organic binder and support particles (e.g., carbon fine particles such as carbon black) to support the metal catalyst to the coating liquid, making it difficult to adjust the metal atom content in the catalyst layer to 20 at% or more.

The present invention relates to
An electrode for use in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts,
A catalyst layer including a metal catalyst is provided.
The electrode has a metal atom content of 20 at % or more in the catalyst layer.

Further, the present invention relates to
An electrode for use in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts,
A catalyst layer including a metal catalyst is provided.
The catalyst layer provides an electrode having a vapor-deposited layer including the metal catalyst.

Further, the present invention relates to
A membrane electrode assembly for use in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts,
The above electrode,
An electrolyte membrane;
The present invention provides a membrane electrode assembly comprising:

Further, the present invention relates to
A fuel cell using a fuel containing at least one selected from the group consisting of formic acid and formate salts,
A fuel cell is provided, comprising the above membrane electrode assembly.

Further, the present invention relates to
A method for producing an electrode for use in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts, comprising the steps of:
A method of manufacturing is provided that includes forming a catalyst layer comprising a metal catalyst by a vapor deposition process.

The present invention provides an electrode for efficient power generation in a fuel cell that uses formic acid or formate salts as fuel.

FIG. 2 is a cross-sectional view illustrating an example of an electrode according to the present embodiment. 1 is a scanning transmission electron microscope (STEM) image of the surface of a catalyst layer having an atomic ratio of Pd/C=100/0. 2B is an energy dispersive X-ray (EDX) image of the surface of the catalyst layer of FIG. 2A. 1 is an STEM image of the surface of a catalyst layer having an atomic ratio of Pd/C=72/28. 3B is an EDX image of the surface of the catalyst layer of FIG. 3A. 1 is an STEM image of the surface of a catalyst layer having an atomic ratio of Pd/C=39/61. 4B is an EDX image of the surface of the catalyst layer of FIG. 4A. FIG. 1 is a cross-sectional view illustrating an example of a membrane electrode assembly according to an embodiment of the present invention. 1 is a cross-sectional view showing a schematic diagram of an example of a fuel cell according to an embodiment of the present invention. FIG. 1 is an exploded perspective view showing a schematic diagram of an example of a fuel cell according to an embodiment of the present invention.

The electrode according to the first aspect of the present invention comprises:
An electrode for use in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts,
A catalyst layer including a metal catalyst is provided.
The catalyst layer has a metal atom content of 20 at % or more.

In the second aspect of the present invention, for example, in the electrode according to the first aspect, the content is 90 at% or less.

The electrode according to the third aspect of the present invention comprises:
An electrode for use in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts,
A catalyst layer including a metal catalyst is provided.
The catalyst layer comprises a vapor-deposited layer containing the metal catalyst.

In a fourth aspect of the present invention, for example, in an electrode according to any one of the first to third aspects, the metal catalyst contains a precious metal.

In a fifth aspect of the present invention, for example, in an electrode according to any one of the first to fourth aspects, the metal catalyst contains palladium.

In a sixth aspect of the present invention, for example, in an electrode according to any one of the first to fifth aspects, the catalyst layer further contains carbon.

In the seventh aspect of the present invention, for example, in the electrode according to the sixth aspect, the catalyst layer has a first phase containing the metal catalyst and a second phase containing the carbon, and the first phase and the second phase form a phase-separated structure.

In the eighth aspect of the present invention, for example, in an electrode according to any one of the first to seventh aspects, the thickness of the catalyst layer is 1 nm to 2000 nm.

In the ninth aspect of the present invention, for example, in the electrode according to the eighth aspect, the thickness of the catalyst layer is 300 nm or less.

In a tenth aspect of the present invention, for example, the electrode according to any one of the first to ninth aspects further comprises a diffusion layer.

The membrane electrode assembly according to an eleventh aspect of the present invention comprises:
A membrane electrode assembly for use in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts,
An electrode according to any one of the first to tenth aspects;
An electrolyte membrane;
Equipped with.

A fuel cell according to a twelfth aspect of the present invention comprises:
A fuel cell using a fuel containing at least one selected from the group consisting of formic acid and formate salts,
The present invention includes a membrane electrode assembly according to an eleventh aspect.

A method for producing an electrode according to a thirteenth aspect of the present invention includes the steps of:
A method for producing an electrode for use in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts, comprising the steps of:
The method includes forming a catalyst layer including a metal catalyst by a vapor deposition method.

In the 14th aspect of the present invention, for example, in the manufacturing method according to the 13th aspect, the vapor deposition method is a physical vapor deposition method.

The present invention will be described in detail below, but the following description is not intended to limit the present invention to a specific embodiment.

<Electrode embodiments>
1 is a cross-sectional view showing a schematic example of an electrode 10 according to the present embodiment. The electrode 10 is used in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts. The electrode 10 includes a catalyst layer 1 and further includes, for example, a diffusion layer 5. In the electrode 10, the catalyst layer 1 is in direct contact with, for example, the diffusion layer 5.

The catalyst layer 1 contains a metal catalyst and contains metal atoms derived from the metal catalyst. The content of metal atoms (particularly, metal atoms derived from the metal catalyst) in the catalyst layer 1 is 20 at% or more. The content of metal atoms is preferably 30 at% or more, 40 at% or more, 50 at% or more, 60 at% or more, or even 70 at% or more. The content of metal atoms may be 90 at% or more, or 95 at% or more, depending on the case. The catalyst layer 1 may be substantially composed of metal atoms only. The catalyst layer 1 composed of only metal atoms (particularly, a single type of metal atom) is easy to fabricate by a deposition method described later. An electrode 10 having a high content of metal atoms tends to efficiently generate electricity in a fuel cell. In detail, this electrode 10 tends to greatly reduce the weight of metal atoms per output value of the fuel cell. The upper limit of the content of metal atoms is, for example, 100 at% or less, and may be 99 at% or less, 95 at% or less, 90 at% or less, or even 80 at% or less.

The content (at %) of metal atoms in the catalyst layer 1 can be calculated, for example, from the volume ratio or weight ratio of each component contained in the catalyst layer 1. The content of metal atoms may be specified by the following method. First, X-ray photoelectron spectroscopy (XPS) analysis and ion beam sputtering are repeated on the surface of the catalyst layer 1 (specifically, the surface of the catalyst layer 1 opposite to the diffusion layer 5) to measure the element distribution in the depth direction of the catalyst layer 1. The XPS analysis can be performed, for example, under the following conditions using a commercially available X-ray photoelectron spectroscopy device (for example, Quantera SXM, manufactured by ULVAC-PHI, Inc.).
Measurement conditions X-ray source: Monochrome Al Kα
X-ray setting: 100 μmφ [15 kV, 25 W]
Photoelectron take-off angle: 45° with respect to the surface of the catalyst layer 1
Neutralization conditions: Neutralization gun and Ar ion gun (neutralization mode) used in combination. Bond energy correction: The peak due to the C-C bond in the C1s spectrum was corrected to 285.0 eV.

Next, based on the results of the XPS analysis, the metal atom content (at %) is determined at a depth of 50% of the thickness of the catalyst layer 1 from the surface of the catalyst layer 1. The determined value can be regarded as the metal atom content (at %) in the catalyst layer 1.

[Catalyst layer]
The catalyst layer 1 has, for example, a vapor-deposited layer containing a metal catalyst, and is typically the vapor-deposited layer itself. The vapor-deposited layer makes it easy to adjust the content of metal atoms in the catalyst layer 1 to a high value.

From another aspect, the present invention provides
An electrode 10 for use in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts,
A catalyst layer 1 containing a metal catalyst is provided.
The catalyst layer 1 provides an electrode 10 having a vapor-deposited layer containing a metal catalyst.

In the catalyst layer 1, the metal catalyst functions as a catalyst for the electrochemical reaction of the fuel cell. The metal catalyst may contain a simple metal or an alloy. The metal catalyst may contain a metal oxide or a composite oxide. Examples of metals contained in the metal catalyst include platinum (Pt), palladium (Pd), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), and zinc (Zn). The metal catalyst preferably contains a precious metal,
It is particularly preferable that the metal catalyst contains palladium. The metal catalyst may be used alone or in combination of two or more kinds.

The weight of the metal catalyst in the catalyst layer 1 is not particularly limited and is, for example, 0.001 mg/cm ² to 15 mg/cm ^2. The weight of the metal catalyst may be 1 mg/cm ² or less, or 0.5 mg/cm ² or less.

The catalyst layer 1 may further contain other components in addition to the metal catalyst. Examples of the other components include carbon (C) and silica. These components can function as binders. It is preferable that the catalyst layer 1 further contains carbon as another component. The catalyst layer 1 may or may not contain a resin such as an ionomer.

The content of other components (especially carbon) in the catalyst layer 1 is not particularly limited, and may be, for example, 5 at% or more, 10 at% or more, 20 at% or more, 30 at% or more, or even 40 at% or more. A catalyst layer 1 with a high carbon content is less likely to fall off the electrode 10 during power generation in the fuel cell, and is suitable for suppressing deterioration of catalyst performance. The upper limit of the content of other components is, for example, 80 at% or less, and may be 50 at% or less.

The catalyst layer 1 has, for example, a first phase containing a metal catalyst. As shown in Figures 2A and 2B, the catalyst layer 1 may be composed of only the first phase 1A. Note that Figures 2A and 2B show the surface of a vapor deposition layer (catalyst layer 1) composed of only palladium. Figure 2B shows palladium detected by energy dispersive X-ray analysis (EDX).

As shown in Figures 3A to 4B, the catalyst layer 1 may further have a second phase 1B containing carbon in addition to the first phase 1A, and the first phase 1A and the second phase 1B may form a phase-separated structure. The phase-separated structure may be a co-continuous structure (Figures 3A and 3B) in which the first phase 1A is continuously formed in a three-dimensional shape, or a structure in which the first phase 1A is particulate (Figures 4A and 4B). In particular, in the catalyst layer 1 in which a co-continuous structure is formed, a sufficient amount of metal catalyst is present on the surface of the catalyst layer 1, and the conductivity in the thickness direction tends to be high. An electrode 10 including this catalyst layer 1 allows the fuel cell to generate electricity more efficiently.

Figures 3A and 3B show the surface of a vapor deposition layer (catalyst layer 1) with an atomic ratio of Pd/C = 72/28. Figure 3B shows palladium detected by EDX. In the catalyst layer 1 of Figures 3A and 3B, the first phase 1A exists in islands on the surface. Figures 4A and 4B show the surface of a vapor deposition layer (catalyst layer 1) with an atomic ratio of Pd/C = 39/61. Figure 4B shows palladium detected by EDX.

The thickness of the catalyst layer 1 is not particularly limited, and is, for example, 1 nm to 2000 nm. The thickness of the catalyst layer 1 is preferably 1000 nm or less, and may be 800 nm or less, 500 nm or less, or even 300 nm or less. The thickness of the catalyst layer 1 may be 100 nm or less, 50 nm or less, or even 25 nm or less, depending on the case. In this embodiment, even if the thickness of the catalyst layer 1 is small, the basis weight of the metal catalyst in the catalyst layer 1 is easily adjusted to a sufficient value. When the thickness of the catalyst layer 1 is small, the diffusion of substances in the catalyst layer 1 is high, and the power generation performance of the fuel cell tends not to decrease. In addition, in the conventional coating method, when the thickness of the catalyst layer is adjusted to a small value, the basis weight of the metal catalyst is greatly reduced, and as a result, the power generation performance of the fuel cell tends to decrease significantly. Therefore, the thickness of the catalyst layer formed by the conventional coating method is usually adjusted to 10,000 nm or more from the viewpoint of ensuring the basis weight of the metal catalyst.

[Diffusion layer]
The diffusion layer 5 is a layer that diffuses fuel and the like, and is preferably conductive. The diffusion layer 5 also functions as a support that supports the catalyst layer 1. As the diffusion layer 5, a known material can be used, and examples of the material include carbon supports such as carbon cloth and carbon paper. The carbon support is preferably porous.

The carbon support may be subjected to a hydrophilization treatment. Examples of hydrophilization treatments include baking in an air atmosphere or an inert gas atmosphere; coating with a hydrophilic substance; surface treatment by introducing hydrophilic groups; and surface treatment by plasma treatment, excimer laser irradiation, corona discharge treatment, etc.

In coating with a hydrophilic substance, a low molecular weight or high molecular weight substance is used as the hydrophilic substance. It is also possible to polymerize the low molecular weight substance by carrying out a polymerization reaction. Coating with a high molecular weight can be carried out, for example, by the following method. First, a polymer such as polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone, polyacrylic acid, or hydroxypropyl cellulose is prepared. This polymer is dissolved in a solvent, and the carbon support is coated or impregnated with the resulting solution. Next, the solution is dried to perform coating. A crosslinking agent may be added to the above solution, and crosslinking reactions may be carried out between the polymers or between the polymer and the carbon support during drying.

Coating with small molecules can be performed, for example, by the following method. First, prepare polyhydric alcohols such as sucrose fatty acid esters, sorbitol, and glycerin; surfactants such as sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, and sodium lauryl sulfate; and small molecules such as sodium lactate and glycerin. The small molecules are dissolved in a solvent, and the resulting solution is applied to or impregnated into a carbon support. Next, the solution is dried to perform coating.

Coating with a hydrophilic substance may be performed by using a hydrophilic substance that contains a functional group that is reactive with the carbon support and reacting the hydrophilic substance with the carbon support.

The diffusion layer 5 may further include a microporous layer (MPL) formed on the carbon support. The microporous layer contains, for example, a binder such as Teflon and carbon particles.

[Electrode manufacturing method]
The manufacturing method of the electrode 10 of this embodiment includes forming a catalyst layer 1 containing a metal catalyst by, for example, a vapor deposition method. In detail, the electrode 10 can be manufactured by the following method. First, a diffusion layer 5 is prepared. Next, a vapor deposition method is performed so that the catalyst layer 1 is formed on the diffusion layer 5. In detail, a vapor deposition layer containing a metal catalyst is formed on the diffusion layer 5 by the vapor deposition method. In this way, the electrode 10 provided with the catalyst layer 1 (vapor deposition layer) can be obtained.

In this specification, deposition means, for example, vapor phase deposition. The deposition method is typically a physical deposition method such as a sputtering method. In the sputtering method, for example, a target containing a metal for forming a metal catalyst is used. In the manufacturing method of this embodiment, a co-sputtering method may be performed using a target containing carbon together with a target containing a metal. The co-sputtering method tends to make it easy to produce the catalyst layer 1 having the above-mentioned first phase 1A and second phase 1B. The conditions of the deposition method can be appropriately set depending on the composition and thickness of the desired catalyst layer 1.

According to the manufacturing method of this embodiment, the content of metal atoms in the catalyst layer 1 can be easily adjusted to a high value, and the above-mentioned electrode 10 can be easily manufactured. Furthermore, unlike the conventional coating method, the manufacturing method of this embodiment does not require the use of a solvent such as an organic solvent. Therefore, by adopting the manufacturing method of this embodiment, it is possible to reduce carbon dioxide emitted when preparing the solvent or drying the coating liquid. Since no organic solvent is used, there is also the advantage that there is no risk of fire due to the organic solvent. According to the deposition method, not only can the thickness of the catalyst layer 1 be easily reduced, but there is also a tendency to suppress the variation in thickness, compared to the conventional coating method. The deposition method is suitable for producing the catalyst layer 1 over a large area, and can be said to have high productivity.

The method for manufacturing the electrode 10 is not limited to the above. For example, the catalyst layer 1 may be produced by depositing metal particles (e.g., palladium black) on the diffusion layer 5 by a spray method or the like. However, from the viewpoint of ensuring a sufficient surface area that contributes to activity, it is preferable to produce the catalyst layer 1 by a vapor deposition method. The catalyst layer 1 produced by the vapor deposition method tends to have high conductivity and is suitable for improving the power generation performance of the fuel cell.

<Embodiments of Membrane Electrode Assembly>
5 is a cross-sectional view showing a schematic example of a membrane electrode assembly (MEA) 50 according to the present embodiment. The membrane electrode assembly 50 is used in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts. The membrane electrode assembly 50 includes the above-mentioned electrode 10 and electrolyte membrane 20.

The membrane electrode assembly 50 may include two electrodes 10 (10A and 10B). The two electrodes 10A and 10B may differ from each other in the composition or thickness of the catalyst layer 1. In the membrane electrode assembly 50, for example, an electrolyte membrane 20 is disposed between the two electrodes 10A and 10B, which are joined together to form an integrated structure. The membrane electrode assembly 50 does not have to include two electrodes 10A and 10B, and may be a combination of one electrode 10 and a conventional electrode. In this case, the electrode 10 may be used as an anode electrode or a cathode electrode.

[Electrolyte membrane]
The electrolyte membrane 20 functions, for example, as an electrolyte for conducting protons, and also functions as a separator for preventing direct mixing of fuel and oxygen. The electrolyte membrane 20 is, for example, a cation exchange membrane.

There are no particular limitations on the cation exchange membrane, so long as it is a membrane made of a material that has ion conductivity, can transport cationic components (e.g., hydrogen ions (H ⁺ ) and sodium ions (Na ⁺ )), and does not exhibit electronic conductivity, and any known cation exchange membrane can be used. The cation exchange membrane is preferably a membrane made of a polymer, and may be made of a cation exchange resin. The cation exchange resin makes it easy to recycle carbon when the fuel cell is disposed of.

There are no particular limitations on the cation exchange resin as long as it has cation exchange properties. The cation exchange resin may be a solid polymer electrolyte (ionomer) having cationic groups, in which a polymer having a cationic conductive functional group (cationic group) is introduced into a polymer substrate. Specific examples of cation exchange resins include perfluoroalkylsulfonic acid resins such as Nafion (registered trademark, DuPont), Flemion (registered trademark, AGC), and Aciplex (registered trademark, Asahi Kasei). Specific examples of cation exchange membranes include Nafion NR-211 and NR-212.

The thickness of the electrolyte membrane 20 is preferably 5 μm to 130 μm, and more preferably 10 μm to 70 μm. An electrolyte membrane 20 having a thickness of 5 μm or more has sufficient membrane strength for practical use, and is less likely to have defects such as breakage or pinholes. This electrolyte membrane 20 tends to be able to suppress an increase in the amount of fuel permeated and the amount of water permeated to the cathode side. An electrolyte membrane 20 having a thickness of 130 μm or less tends not to have too large membrane resistance. This electrolyte membrane 20 can ensure a practically sufficient amount of water permeation, and therefore it is easy to suppress a decrease in power generation efficiency due to a shortage of water required for the reaction at the cathode.

[Membrane Electrode Assembly Manufacturing Method]
The membrane electrode assembly 50 can be produced, for example, by a method (a catalyst coated on substrate (CCS) method) in which an electrode 10 obtained by forming a catalyst layer 1 on a diffusion layer 5 is bonded to an electrolyte membrane 20. The electrode 10 and the electrolyte membrane 20 can be bonded by a known method such as hot pressing.

The manufacturing method of the membrane electrode assembly 50 is not limited to the above-mentioned CCS method. The membrane electrode assembly 50 may be manufactured by a method in which a catalyst layer 1 is formed on the electrolyte membrane 20 and then bonded to the diffusion layer 5 (CCM method: Catalyst Coated on Membrane). The electrolyte membrane 20 on which the catalyst layer 1 is formed and the diffusion layer 5 can be bonded by a known method such as hot pressing.

<Fuel Cell Embodiment>
6 is a cross-sectional view showing an example of a fuel cell 100 according to the present embodiment. The fuel cell 100 uses a fuel containing at least one selected from the group consisting of formic acid and formate salts. The fuel cell 100 includes the membrane electrode assembly 50 described above.

As an example, in the fuel cell 100, the electrode 10A functions as an anode electrode, and the electrode 10B functions as a cathode electrode. The fuel cell 100 further includes an anode-side current collector 60A in contact with the electrode 10A, and a cathode-side current collector 60B in contact with the electrode 10B. The membrane electrode assembly 50 is located between the current collectors 60A and 60B. The electromotive force generated in the fuel cell 100 is taken out from the terminals provided on each of the current collectors 60A and 60B.

[Current collector]
The current collectors 60A and 60B may be conventionally known ones generally used for fuel cells. The current collectors 60A and 60B are made of a conductive material such as a metal, and examples of such materials include gold-plated stainless steel plates.

[Other configurations]
The fuel cell 100 may further include known components constituting a fuel cell. Examples of such components include a gasket, an oxidant supply plate, a fuel supply plate, an oxidant flow path, a fuel flow path, an external circuit, and various sensors for detecting the power generation status. Examples of the sensors include a temperature sensor, an oxygen sensor, a flow meter, and a humidity sensor. The fuel cell 100 may be connected to a fuel supply device, an oxidant supply device, a humidifier device, and the like.

7 is an exploded perspective view showing a more specific configuration of the fuel cell 100. As shown in FIG. 7, the fuel cell 100 further includes a fuel supply plate 61A, an oxidizer supply plate 61B, and gaskets 62A and 62B in addition to the membrane electrode assembly 50 and the current collectors 60A and 60B. The fuel supply plate 61A is located between the current collector 60A and the membrane electrode assembly 50 and is configured to supply fuel to the electrode 10A. The oxidizer supply plate 61B is located between the current collector 60B and the membrane electrode assembly 50 and is configured to supply oxidizer to the electrode 10B. The gasket 62A is configured to surround, for example, the outer edge of the electrode 10A so as to prevent fuel from leaking to the outside. The gasket 62B is configured to surround, for example, the outer edge of the electrode 10B so as to prevent oxidizer from leaking to the outside.

[Fuel cell power generation method]
In the fuel cell 100, fuel is supplied to the anode electrode (electrode 10A) and an oxidant is supplied to the cathode electrode (electrode 10B), and an electrochemical reaction is caused to proceed in the anode electrode and the cathode electrode, thereby generating electricity.

The fuel supplied to the anode electrode contains at least one selected from the group consisting of formic acid and formate salts. In particular, formate salts are usually solid at room temperature (25°C) and are safer for the human body than formic acid, and therefore have the advantage of being easily transported and stored. As described below, formate salts can be synthesized from hydrogen carbonate contained in the waste liquid generated by power generation in the fuel cell 100, and are therefore suitable for realizing carbon circulation. Examples of formate salts include calcium formate, cesium formate, sodium formate, and potassium formate, with sodium formate and potassium formate being preferred.

The fuel supplied to the anode electrode is, for example, a solution containing a formate. The solvent contained in the solution is water and/or alcohol. Examples of the alcohol include lower alcohols such as methanol, ethanol, propanol, and isopropanol. The fuel is preferably an aqueous solution of a formate.

In the above solution, the concentration of the formate can be adjusted as appropriate depending on the type of formate. From the viewpoint of the output of the fuel cell 100, the concentration of the formate in the solution is preferably 0.1 mol/L or more, more preferably 0.5 mol/L or more, and even more preferably 1 mol/L or more. From the viewpoint of the solubility of the formate, the concentration of the formate is preferably 10 mol/L or less, more preferably 9 mol/L or less, and even more preferably 8 mol/L or less. The concentration of the formate is preferably 0.1 to 10 mol/L.

The solution containing the formate may further contain an additive. Examples of additives include aqueous solutions of acids such as sulfuric acid and hydrochloric acid (acidic aqueous solutions), and aqueous solutions of bases such as sodium hydroxide and potassium hydroxide (alkaline aqueous solutions). The amount of additive added is not particularly limited and is set appropriately depending on the application of the fuel cell 100.

The oxidant supplied to the cathode electrode may be a liquid, but is preferably a gas. Examples of the oxidant include air and oxygen, and oxygen is preferred. The oxidant is preferably humidified, but does not have to be humidified. The humidity of the oxidant can be appropriately adjusted within a range that sufficiently moistens the electrolyte membrane 20 and does not impede the diffusion of the oxidant in the cathode electrode.

The humidity of the oxidant can be controlled, for example, by a humidification tube installed in the oxidant flow path. When the humidification tube is immersed in a water bath, the humidity of the oxidant can be increased by increasing the temperature of the water bath. The temperature of the water bath can be controlled, for example, in the range from room temperature (25°C) to 100°C, and is preferably 45°C to 85°C.

When an aqueous solution of formate is used as the fuel, an electrochemical reaction at the anode electrode produces, for example, bicarbonate, hydrogen ions, and electrons (e ⁻ ). The produced hydrogen ions are supplied to the cathode electrode via the electrolyte membrane 20. When oxygen is used as the oxidizing agent, the electrons (e ⁻ ) moving from the anode electrode via an external circuit, the hydrogen ions moving from the anode electrode via the electrolyte membrane 20, and oxygen react at the cathode electrode to produce water. The produced water can be reused at the anode electrode.

As an example, when an aqueous solution of sodium formate is used as the fuel and oxygen is used as the oxidant, the anode reaction at the anode electrode, the cathode reaction at the cathode electrode, and the overall reaction are expressed by the following reaction formulas.
Anode reaction: HCOONa + _H2O → _NaHCO3 + 2H ⁺ + ^2e-
・Cathode reaction 1/ _2O2 + 2H ⁺ + ^2e- → _H2O
Overall reaction: HCOONa + 1/ _2O2 → _NaHCO3

The operating conditions of the fuel cell 100 are not particularly limited. As an example, the temperature of the fuel cell 100 during operation is, for example, 30°C or higher, preferably 60°C or higher, and more preferably 65°C or higher. The temperature of the fuel cell 100 is, for example, 100°C or lower, and preferably 90°C or lower.

When an aqueous solution of formate is used as fuel, waste liquid containing bicarbonate is obtained when the fuel cell 100 generates electricity. The bicarbonate contained in the waste liquid can be reacted with hydrogen to synthesize formate. Carbon circulation can be achieved by using the formate synthesized from the bicarbonate contained in the waste liquid again as fuel.

Formate salts can be synthesized by reacting hydrogen carbonate with hydrogen in the presence of a solvent using a catalyst, for example, by using the method described in Nature Chemistry, 2012, Vol. 4, p. 383-388. In the synthesis of formate salts, the reactivity with hydrogen may be improved by further supplying carbon dioxide as a carbon source.

The hydrogen used in the synthesis of formate can be either gaseous hydrogen from a gas cylinder or liquid hydrogen. Hydrogen sources that can be used include, for example, hydrogen generated during the iron smelting process and hydrogen generated during the soda manufacturing process. Hydrogen generated from the electrolysis of water can also be used.

When carbon dioxide is used to synthesize the formate, the carbon dioxide may be pure carbon dioxide gas or may be mixed with other components other than carbon dioxide. A mixed gas with other components may be prepared by introducing carbon dioxide gas and other gases separately, or may be prepared in advance before the introduction. Examples of components other than carbon dioxide include inert gases such as nitrogen and argon, water vapor, and any other components contained in exhaust gases. Examples of carbon dioxide that can be used include gaseous carbon dioxide from a gas cylinder, liquid carbon dioxide, supercritical carbon dioxide, and dry ice.

Hydrogen gas and carbon dioxide gas may be introduced into the reaction system either individually or as a mixed gas. The method for introducing the carbon dioxide, hydrogen, catalyst, solvent, etc. used in the synthesis of the formate salt into the reaction vessel is not particularly limited, but all the raw materials may be introduced at once, some or all of the raw materials may be introduced stepwise, or some or all of the raw materials may be introduced continuously. Also, a combination of these introduction methods may be used.

The bicarbonate contained in the waste liquid comes from the formate used as fuel. Specific examples of bicarbonate include sodium bicarbonate and potassium bicarbonate.

[Fuel cell applications]
Applications of the fuel cell 100 include, for example, a power source for drive motors in automobiles, ships, aircraft, etc.; stationary power sources in factories, offices, homes, etc.; and a power source for communication terminals and electronic devices such as mobile phones.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited thereto.

Example 1
First, a target composed of Pd and a target composed of carbon (C) were set in the target installation part of the sputtering device. Next, a sufficiently polished glassy carbon (GC) electrode (L type, GC-3L, EC Frontier Co., Ltd.) was set in the sample introduction part of the sputtering device. Next, a co-sputtering method using the above two targets was performed, and a catalyst layer composed only of Pd and C was prepared on the GC electrode. In the co-sputtering method, the power output was controlled so that the content of Pd in the catalyst layer was 49 at %. The thickness of the catalyst layer was adjusted so that the basis weight of Pd in the catalyst layer was 0.015 mg/cm ^2. In this way, the electrode of Example 1 was prepared. In this electrode, Pd can function as a metal catalyst.

(Examples 2 to 4)
The electrodes of Examples 2 to 4 were produced in the same manner as in Example 1, except that the content of Pd in the catalyst layer was changed to the value shown in Table 1. In Example 4, a target made of carbon was not used.

(Comparative Example 1)
First, 5 mg of a mixed powder of Pd and C (Pd/C, Pd content 40 wt%, Ishifuku Metals Co., Ltd.), 695 μL of ultrapure water, 1623 μL of 2-propanol (IPA, Fujifilm Wako Pure Chemical Industries Co., Ltd.), and 510 μL of a solution containing Nafion at a concentration of 5 wt% (dispersion solution DE521 CS type, Chemours Co., Ltd.) were mixed and thoroughly mixed for 30 minutes using an ultrasonic cleaner (WK-113A, Honda Electronics Co., Ltd.) to prepare a catalyst ink. Next, 1.5 μL of the catalyst ink was dropped onto a sufficiently polished GC electrode (L type, GC-3L, EC Frontier Co., Ltd.) and dried for 60 minutes to prepare an electrode of Comparative Example 1.

[Surface observation]
For the electrodes of Examples 3 and 4, the surface of the catalyst layer was observed with a scanning transmission electron microscope (FE-TEM, JEM-2800, JEOL), and further, energy dispersive X-ray analysis (EDX) was performed. In EDX, Pd was detected using NORAN System7 (Thermo Fisher Scientific). Note that Figures 2A and 2B show the results of the above analysis for the electrode of Example 4. Figures 3A and 3B show the results of the above analysis for the electrode of Example 3. As can be seen from Figures 3A and 3B, in the electrode of Example 3, the catalyst layer had a bicontinuous structure in which the first phase composed of Pd was continuously formed in a three-dimensional shape.

[CV evaluation]
(Weight Activity)
Using the electrodes prepared in the examples and comparative examples, CV (cyclic voltammetry) measurements were performed by the following method. First, 80 mL of an aqueous solution containing sodium formate (FUJIFILM Wako Pure Chemical Industries, Ltd.) at a concentration of 1 mol/L was added to the beaker cell. Next, the working electrode, counter electrode, and reference electrode were set in the beaker cell. The above-prepared electrode was used as the working electrode. A platinum electrode (CE-200, EC Frontier Co., Ltd.) was used as the counter electrode. A silver/silver chloride electrode (RE-1A, EC Frontier Co., Ltd.) was used as the reference electrode. Next, the aqueous solution in the beaker cell was thoroughly degassed by bubbling nitrogen gas at a flow rate of 30 mL/min for 30 minutes.

Next, CV measurements were performed using a potentio/galvanostat (HZ-5000, Hokuto Denko Corporation) to obtain a voltammogram. The current value (mA) due to the oxidation of sodium formate was identified, and the current value (weight activity) per mg of Pd was calculated.

(Active Surface Area)
Except for using a 0.5 mol/L aqueous sulfuric acid solution (FUJIFILM Wako Pure Chemical Industries, Ltd.) instead of the aqueous sodium formate solution, CV measurement was performed by the same method as the above-mentioned method for measuring gravimetric activity, and a voltammogram was obtained. Based on the amount of electricity Q _H (C) due to hydrogen adsorption and the basis weight I (mg/cm ² ) of Pd in the catalyst layer, the active surface area (ECSA: electrochemically active surface area) of the catalyst layer was calculated by the following formula. Details of the calculation method of the active surface area are described in H. Khuntia et al., Colloids and Surfaces A, 2021, Vol.615, 126237.
Active surface area (cm ² /mg-Pd)=Q _H (C)/(0.405×I (mg/cm ² ))

(Number of cycles until catalyst performance deteriorates)
Furthermore, the above-mentioned measurement of weight activity was repeated to identify the number of cycles at which the catalytic performance deteriorated. The number of cycles at which the catalytic performance deteriorated was specifically identified by the following method. First, the above-mentioned measurement of weight activity was repeated, and the weight activity (mA/mg-Pd) for each number of cycles of the CV measurement was recorded. The weight activity value usually increases as the number of cycles of the CV measurement increases, and as the number of cycles increases further, the increase in the value stabilizes and the value eventually decreases. The number of CV measurements at which a tendency for the weight activity value to decrease was confirmed was regarded as the "number of cycles at which the catalytic performance deteriorated."

As can be seen from Table 1, the electrodes of the examples, in which the content of metal atoms (Pd) in the catalyst layer is 20 at% or more, had larger weight activity and active surface area than the comparative examples. In particular, in Examples 2 and 3, in which the content of metal atoms (Pd) in the catalyst layer is 60 to 90 at%, the weight activity and active surface area increased significantly. From these results, it can be seen that the electrodes of the examples enable efficient power generation in fuel cells.

Furthermore, the electrodes of the Examples had a tendency to have high durability, with the number of cycles at which the catalytic performance was reduced being greater than that of the Comparative Examples. In particular, it is presumed that the electrodes of Examples 1 to 3 exhibited extremely high durability due to the carbon contained in the catalytic layer functioning as a binder. In the electrode of Example 4, the CV measurement was repeated five times, and then observation by STEM and EDX analysis were performed on the catalytic layer. As a result, peeling of the catalytic layer was confirmed in the electrode of Example 4. It is presumed that the catalytic performance of the electrode of Comparative Example 1 was reduced relatively quickly due to the aggregation of Pd particles contained in the catalytic layer by the CV measurement.

Example 5
First, carbon paper (Mitsubishi Chemical Corporation, MFK-A) on which a microporous layer (MPL) was formed was baked in a baking furnace. The baking was performed at 500°C for 3 hours under an inert gas atmosphere. As a result, the hydrophobic resin was removed, and a substrate (diffusion layer) with hydrophilized carbon paper was obtained. This substrate was set in the sample introduction section of the sputtering device. Furthermore, a target composed of Pd and a target composed of carbon (C) were set in the target installation section of the sputtering device. Next, a co-sputtering method using the above two targets was performed, and a catalyst layer composed only of Pd and C was prepared on the substrate. In the co-sputtering method, the power output was controlled so that the content of Pd in the catalyst layer was 60 at%. The thickness of the catalyst layer was adjusted so that the basis weight of Pd in the catalyst layer was 0.17 mg/ ^cm2 . As a result, an anode electrode was obtained.

Next, 88 mg of a mixed powder of Pd and C (Pd/C, Pd content 40 wt%, Ishifuku Metals Co., Ltd.), 700 μL of ultrapure water, 7000 μL of 2-propanol (IPA, Fujifilm Wako Pure Chemical Industries Co., Ltd.), and 1.76 g of a solution containing Nafion at a concentration of 5 wt% (dispersion solution DE521 CS type, Chemours Co., Ltd.) were mixed and mixed well for 30 minutes using an ultrasonic cleaner (WK-113A, Honda Electronics Co., Ltd.) to prepare a catalyst ink. Next, the catalyst ink was sprayed onto the substrate (diffusion layer) prepared by the method described above for the anode electrode using a spray coater to obtain a cathode electrode.

Next, a cation exchange membrane (Nafion NR-212, Sigma-Aldrich) was cut out to form an electrolyte membrane. The electrolyte membrane was placed between the above-mentioned anode electrode and cathode electrode, and these were thermocompressed to obtain a membrane electrode assembly. Thermocompression was performed at 140°C for 3 minutes using a heat press. The fabricated membrane electrode assembly was set in a fuel cell to obtain the fuel cell of Example 5.

Next, the power generation performance of the prepared fuel cell was measured by the following method. First, the fuel cell was heated to 80° C., and the fuel was supplied to the anode electrode and the oxidant was supplied to the cathode electrode. As the fuel, an aqueous solution containing sodium formate (FUJIFILM Wako Pure Chemical Industries, Ltd.) at a concentration of 1 mol/L was used and supplied at a flow rate of 1.0 mL/min. As the oxidant, humidified oxygen gas was used and supplied at a flow rate of 100 mL/min. Next, an IV measurement was performed using a potentio/galvanostat (HZ-5000, Hokuto Denko Corporation) to obtain an IV curve. Furthermore, the maximum output value (W) was calculated from the IV curve. Based on the calculated value, the maximum output value per area of the main surface of the catalyst layer in the anode electrode (maximum output density (mW/cm ² )), the weight of Pd contained in the catalyst layer of the anode electrode per output value (Pd weight (g/W)) were specified. Furthermore, based on the results of the power generation performance in Comparative Example 2 described later, the ratio of the Pd weight (g/W) per output value in Comparative Example 2 to the Pd weight (g/W) per output value in Example 5 (catalyst utilization efficiency (times)) was calculated. The results are shown in Table 2.

Example 6
In measuring the power generation performance, a fuel cell was produced and the power generation performance was measured in the same manner as in Example 5, except that an aqueous solution containing sodium formate at a concentration of 2 mol/L was used as the fuel.

(Example 7)
A fuel cell was produced and its power generation performance was measured in the same manner as in Example 5, except that the anode and cathode electrodes were produced using a base material obtained by baking carbon paper (Mitsubishi Chemical Corporation, MFK).

(Example 8)
In measuring the power generation performance, a fuel cell was produced and the power generation performance was measured in the same manner as in Example 7, except that an aqueous solution containing sodium formate at a concentration of 2 mol/L was used as the fuel.

Example 9
A fuel cell was produced and its power generation performance was measured in the same manner as in Example 5, except that the anode electrode and the cathode electrode were produced using a base material obtained by baking carbon cloth (Carbon Cloth Plain (Etek Cloth A), Fuel Cell Earth).

Example 10
In measuring the power generation performance, a fuel cell was produced and the power generation performance was measured in the same manner as in Example 9, except that an aqueous solution containing sodium formate at a concentration of 2 mol/L was used as the fuel.

(Comparative Example 2)
First, 88 mg of a mixed powder of Pd and C (Pd/C, Pd content 40 wt%, Ishifuku Metals Co., Ltd.), 700 μL of ultrapure water, 7000 μL of 2-propanol (IPA, Fujifilm Wako Pure Chemical Industries Co., Ltd.), and 0.88 g of a solution containing Nafion at a concentration of 5 wt% (dispersion solution DE521 CS type, Chemours Co., Ltd.) were mixed and thoroughly mixed for 30 minutes using an ultrasonic cleaner (WK-113A, Honda Electronics Co., Ltd.) to prepare a catalyst ink.

Next, the carbon cloth (Carbon Cloth Plain (Etek Cloth A), Fuel Cell Earth) was fired in a firing furnace. The firing was carried out at 500°C for 3 hours in an inert gas atmosphere. As a result, the hydrophobic resin was removed, and a substrate (diffusion layer) made of hydrophilized carbon cloth was obtained. A catalyst layer was produced by spraying catalyst ink onto this substrate using a spray coater. The calculated Pd content in the catalyst layer was less than 7 at%. As a result, an anode electrode was obtained.

Next, 88 mg of a mixed powder of Pd and C (Pd/C, Pd content 40 wt%, Ishifuku Metals Co., Ltd.), 700 μL of ultrapure water, 7000 μL of 2-propanol (IPA, Fujifilm Wako Pure Chemical Industries Co., Ltd.), and 1.76 g of a solution containing Nafion at a concentration of 5 wt% (dispersion solution DE521 CS type, Chemours Co., Ltd.) were mixed and mixed well for 30 minutes using an ultrasonic cleaner (WK-113A, Honda Electronics Co., Ltd.) to prepare a catalyst ink. Next, the catalyst ink was sprayed onto the substrate (diffusion layer) prepared by the method described above for the anode electrode using a spray coater to obtain a cathode electrode.

Next, a cation exchange membrane (Nafion NR-212, Sigma-Aldrich) was cut out to form an electrolyte membrane. The electrolyte membrane was placed between the above-mentioned anode electrode and cathode electrode, and these were thermocompressed to obtain a membrane electrode assembly. Thermocompression was performed at 140°C for 3 minutes using a heat press. The fabricated membrane electrode assembly was set in a fuel cell to obtain a fuel cell of Comparative Example 2. Furthermore, the power generation performance of the fabricated fuel cell was measured using the same method as in Example 5.

As can be seen from Table 2, in the fuel cells of Examples 5 to 10, in which an electrode having a metal atom (Pd) content of 20 at % or more in the catalyst layer was used as the anode electrode, the weight of Pd contained in the catalyst layer of the anode electrode per output value (Pd weight per output value) was smaller than that of the comparative example, and the ratio (catalyst utilization efficiency) was 6 times or more. From this result, it can be seen that the fuel cells of the examples were able to efficiently generate power. Note that in the fuel cell of Comparative Example 2, the maximum output value (maximum output density) per area of the main surface of the catalyst layer of the anode electrode was relatively large due to the large weight of Pd in the catalyst layer of the anode electrode.

The electrode of this embodiment is suitable for a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts.

1 Catalyst layer 1A First phase 1B Second phase 5 Diffusion layer 10 Electrode 20 Electrolyte membrane 50 Membrane electrode assembly 100 Fuel cell

Claims (14)

An electrode for use in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts,
A catalyst layer including a metal catalyst is provided.
An electrode, wherein the content of metal atoms in the catalyst layer is 20 at % or more. The electrode according to claim 1, wherein the content is 90 at% or less. An electrode for use in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts,
A catalyst layer including a metal catalyst is provided.
The catalyst layer comprises a vapor-deposited layer including the metal catalyst. The electrode according to claim 1 or 3, wherein the metal catalyst comprises a precious metal. The electrode according to claim 1 or 3, wherein the metal catalyst comprises palladium. The electrode according to claim 1 or 3, wherein the catalyst layer further contains carbon. the catalyst layer has a first phase including the metal catalyst and a second phase including the carbon,
The electrode of claim 6 , wherein the first phase and the second phase form a phase-separated structure. The electrode according to claim 1 or 3, wherein the thickness of the catalyst layer is 1 nm to 2000 nm. The electrode of claim 8, wherein the thickness of the catalyst layer is 300 nm or less. The electrode according to claim 1 or 3, further comprising a diffusion layer. A membrane electrode assembly for use in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts,
The electrode according to claim 1 or 3,
An electrolyte membrane;
A membrane electrode assembly comprising: A fuel cell using a fuel containing at least one selected from the group consisting of formic acid and formate salts,
A fuel cell comprising the membrane electrode assembly according to claim 11. A method for producing an electrode for use in a fuel cell that uses a fuel containing at least one selected from the group consisting of formic acid and formate salts, comprising the steps of:
A method of manufacturing comprising forming a catalyst layer comprising a metal catalyst by a vapor deposition method. The method of claim 13, wherein the vapor deposition method is a physical vapor deposition method.

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