JP7310759B2 - Ionoma-coated catalyst and manufacturing method thereof, protective material-coated electrode catalyst and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to an ionomer-coated catalyst and its production method, and a protective material-coated electrode catalyst and its production method. , a protective material-coated electrode catalyst in which the surfaces of catalyst particles are coated with a protective material, and a method for producing the same.

A polymer electrolyte fuel cell is based on a membrane electrode assembly (MEA) in which electrodes are joined to both sides of a solid polymer electrolyte membrane. Moreover, in polymer electrolyte fuel cells, electrodes generally have a two-layer structure of a diffusion layer and a catalyst layer. The diffusion layer is for supplying reaction gas and electrons to the catalyst layer, and is made of carbon paper, carbon cloth, or the like. The catalyst layer is a reaction field for electrode reaction, and is generally composed of a composite of carbon supporting an electrode catalyst such as platinum and a solid polymer electrolyte (catalyst layer ionomer).

A catalyst layer used in a polymer electrolyte fuel cell is generally
(a) preparing a catalyst ink containing an electrocatalyst and an ionomer and having a solids concentration of about 10%;
(b) forming a catalyst layer on the substrate surface by applying a catalyst ink to the substrate surface and volatilizing the solvent in the coating film using various methods;
(c) It is manufactured by transferring the catalyst layer on the substrate surface to the electrolyte membrane.
There is also a method of applying the catalyst ink directly to the solid polymer electrolyte membrane instead of the base material.

Known methods for applying the catalyst ink to the substrate surface include, for example, a spray method, a blade coating method using a doctor blade or an applicator, a die coating method, a reverse roll coater method, and an intermittent die coating method. Whichever method is used, the properties of the catalyst ink affect the soundness and productivity of the catalyst layer. Therefore, various proposals have been conventionally made regarding methods for manufacturing a catalyst layer, with the aim of improving the soundness and productivity of the catalyst layer.

For example, in Patent Document 1,
(a) preparing a dispersion containing a highly oxygen-permeable ionomer and platinum-supported carbon, and evaporating the solvent from the dispersion to obtain a dry powder;
(b) A fuel cell electrode is disclosed that is obtained by preparing a dispersion containing a dry powder and a Nafion® solution and applying the dispersion onto a sheet.
The document describes that by making the catalyst layer ionomer a two-layer structure, oxygen permeability is improved even if the amount of high oxygen permeability material used is small, and the amount of catalyst used can be reduced. .

In Patent Document 2,
(a) platinum-supported carbon and a highly oxygen-permeable ionomer are dispersed in a solvent having a water ratio of 0.8 or more to form a dispersion;
(b) drying the dispersion to a powder;
(c) An ionomer-coated catalyst obtained by heat-treating a powder at a heat-treatment temperature of 130° C. or more and 200° C. or less is disclosed.
The document describes that the use of such an ionomer-coated catalyst makes it possible to produce a catalyst ink having a low viscosity and a high solid content concentration.

In Patent Document 3,
(a) preparing a catalyst in which a catalytic metal is supported inside mesoporous carbon;
(b) mixing materials including a catalyst, a solvent, and an ionomer to prepare a catalyst ink;
(c) disclosed is an electrode catalyst layer obtained by applying a catalyst ink onto a substrate.
This document describes that poisoning of the catalyst metal by the ionomer can be suppressed by supporting the catalyst metal inside the mesoporous carbon.

Patent Document 4 discloses a cathode-side catalyst layer containing a supported catalyst in which platinum is supported on a hollow carbon carrier.
In the same document,
(A) Since the anode side has a low potential, catalyst poisoning by the ionomer is not a big problem, whereas the cathode side has a high potential, so catalyst poisoning by the ionomer becomes a problem;
(B) It is described that poisoning of platinum by an ionomer can be reduced by using a supported catalyst in which platinum is supported on a hollow carbon carrier as the cathode-side catalyst.

In Patent Document 5,
(a) dispersing a carbon support having pores in water and degassing to prepare a dispersion in which the carbon support having water intruded into the pores is dispersed;
(b) adding a catalyst metal salt to the dispersion to support the catalyst metal salt on the carbon support;
(c) A fuel cell electrode catalyst obtained by reducing a salt of a catalytic metal is disclosed.
The document describes that an electrode catalyst in which a catalytic metal is supported in the pores of a carbon support can be obtained by such a method.

Patent Document 6 discloses a noble metal-supported catalyst in which a noble metal is supported on mesoporous carbon having a pore diameter of 2 to 5 nm and a pore volume of 2.1 to 2.4 mL/g.
This document describes that the use of mesoporous carbon that satisfies predetermined conditions improves gas diffusibility, thereby improving voltage performance.

Patent Document 7 discloses a catalyst in which platinum alloy fine particles are supported in the mesopores of a carrier having a mesopore radius of 1 to 10 nm and a mesopore mode radius of 2.5 to 10 nm. .
The document describes that platinum alloy fine particles can be supported inside the carrier by controlling the size of the mesopores.

In Patent Document 8,
(a) heat-treating a carrier made of mesoporous carbon having a crystallite diameter Lc of 002 plane of 1.5 nm or less at 1700° C. or more and less than 2300° C.;
(b) supporting catalyst particles on at least the interior of the heat-treated support;
(c) A method for producing a fuel cell electrode is disclosed in which a carrier on which catalyst particles are supported is coated with an ionomer.

In the same document,
(A) When carbon black is used as a catalyst carrier, if heat treatment is applied to the carbon black in order to improve the durability of the carbon black, the pores in the carbon black are crushed, and the catalyst particles are included in the pores. It becomes difficult to let
(B) When mesoporous carbon that satisfies predetermined conditions is heat-treated, the pores of the mesoporous carbon are not crushed and the durability of the mesoporous carbon is improved;
(C) It is described that when the catalyst is supported in the pores of such a carrier, the ionomer hardly enters the pores, so that poisoning of the catalyst by the ionomer can be suppressed.

Although Patent Document 9 does not aim to suppress catalyst poisoning by ionomer,
(a) a method of immersing a copper plate in an acid aqueous solution containing copper ions and platinum-supported carbon dispersed therein and stirring the acid aqueous solution;
(b) a step of immersing a Pt wire (working electrode) in the acid aqueous solution while stirring the acid aqueous solution, and measuring the time required for the potential of the Pt wire to reach the standard oxidation-reduction potential of copper; and removing the copper layer deposited on the surface of the Pt wire after the measurement is completed.

In the same document,
(A) When platinum-supported carbon dispersed in an acid aqueous solution contacts a copper plate, underpotential deposition (UPD) occurs almost instantaneously, and a one-atom-layer copper layer is formed on the surface of the platinum particles. ,
(B) Whether or not the surface of the platinum particles contained in the platinum-carrying carbon is completely covered with a copper layer by measuring the change over time in the time required for the potential of the Pt wire to reach the standard oxidation-reduction potential of copper. can be determined, and
(C) Galvanic replacement of the copper monoatomic layer deposited on the surface of the platinum particles with gold can provide a catalyst in which the surface of the platinum particles is modified with gold, thereby suppressing oxidation and elution of the platinum particles. It mentions what you can do.

Furthermore, Non-Patent Document 1 discloses the composition of a plating bath for performing electroless copper plating for metallizing the surface of non-conductors such as plastics, although it is not intended for the production of electrode catalysts for fuel cells. ing.

It is known that when the surface of a platinum catalyst is coated with an ionomer, platinum is poisoned by the anions (sulfonic acid group, imidic acid group, etc.) in the ionomer, and the oxygen reduction activity decreases (for example, non-patent literature 2, 3). Therefore, in order to construct a highly efficient fuel cell, it is necessary to reduce the poisoning of the catalyst by the ionomer.
In order to solve this problem, Patent Documents 3 to 8 propose using a porous carrier and supporting catalyst particles in the pores of the carrier. However, porous carriers are generally more expensive than solid carriers such as carbon black.

In addition, since no ionomer is present inside the porous carbon support, protons are supplied to the inside of the porous support by adsorbed water of the carbon. However, since the amount of adsorbed water decreases under low humidity conditions, it becomes difficult for protons to reach the catalyst, and there is a problem that the platinum utilization rate inside decreases.
In order to reduce catalyst poisoning by the ionomer and improve the catalyst utilization rate under low humidity conditions, the ionomer is separated from the catalyst particles to the extent that the catalyst particles are not poisoned, and protons can be supplied. It is desirable for the ionomer to be close to the catalyst particles.
However, there is no example in which an electrode catalyst that satisfies such conditions has been proposed. In particular, in the case of an electrode catalyst using a low-cost solid carrier, there has been no example of a proposed method for reducing ionomer poisoning.

JP 2014-216157 A JP 2018-152333 A JP 2018-181838 A JP 2018-163843 A JP 2018-098198 A JP 2018-098196 A WO2014/175106 WO2014/185498 Japanese Patent Application Laid-Open No. 2012-240002

Tomoyuki Fujinami, "Electroless Copper Plating", Surface Technology, Vol.50, No.2 (1999), p129-134 Ram Sbbaraman et al., "Oxygen Reduction Reaction at Three-Phase Interface," ChemPhysChem 2010, 11, 2825-2833 Kazuma Shinozaki et al., "Suppression of oxygen reduction reaction activity on Pt-based electrocatalysts from ionomer incorporation," Journal of Power Sources, 2016, 325, 745-751

The problem to be solved by the present invention is to provide an ionomer-coated catalyst that is less poisoned by ionomer and has a high catalyst utilization rate under low humidity conditions, and a method for producing the same.
Another problem to be solved by the present invention is to provide a protective material-coated electrode catalyst used for producing such an ionomer-coated catalyst and a method for producing the same.

In order to solve the above problems, the method for producing an ionomer-coated catalyst according to the present invention comprises:
A first step of preparing an electrode catalyst in which catalyst particles are supported on a carrier surface;
a second step of coating the surface of the catalyst particles with a protective material to obtain a protective material-coated electrode catalyst;
a third step of further coating the surface of the protective material-coated electrode catalyst with an ionomer to obtain a precursor;
and a fourth step of extracting the protective material from the precursor.

The ionomer-coated catalyst according to the invention is obtained by the method according to the invention.

The method for producing a protective material-coated electrode catalyst according to the present invention comprises:
A first step of preparing an electrode catalyst in which catalyst particles are supported on a carrier surface;
and a second step of coating the surface of the catalyst particles with a protective material to obtain a protective material-coated electrode catalyst.

Furthermore, the protective material-coated electrode catalyst according to the present invention is
an electrode catalyst in which catalyst particles are supported on the surface of a carrier;
and a protective material that coats the surface of the catalyst particles.

After the catalyst particles are supported on the carrier surface, the surface of the catalyst particles is coated with a protective material, and the surface of the protective material is further coated with an ionomer, the protective material is extracted. Also, it is possible to obtain an ionomer-coated catalyst that is less poisoned by the ionomer and has a high catalyst utilization rate under low-humidity conditions.
This is because the ionomer is arranged close to the catalyst particles by coating and extracting the protective material, and at the same time, a space of an appropriate size is formed between the ionomer and the catalyst particles. Conceivable. In other words, it is believed that the ionomer and the catalyst particles are separated enough to prevent the catalyst particles from being poisoned, and are close enough to allow the supply of protons even in a low-humidity environment. be done.

1 is a schematic diagram of a method for producing an ionomer-coated catalyst according to the present invention; FIG. 1 shows oxygen reduction activity retention rates of ionomer-coated catalysts obtained in Example 1 and Comparative Examples 1 and 2. FIG. 3 shows element distribution mapping (FIG. 3(A): copper, FIG. 3(B): platinum) and a transmission electron microscope image (FIG. 3(C)) of the protective material-coated electrode catalyst obtained in Example 2. FIG. The relationship between the logarithm of the specific activity (SA) and the potential of Pt / C (Comparative Example 3), Naf / Pt / C (Comparative Example 4), and Naf / (Cu removal) / Pt / C (Example 2) FIG. 10 shows. It is the oxygen reduction activity retention rate at 0.9 V of Naf/Pt/C (Comparative Example 4) and Naf/(Cu removal)/Pt/C (Example 2). FIG. 4 is a diagram showing the I/C dependence of the oxygen reduction activity retention rate at 0.9V. FIG. 4 is a diagram showing the ionomer coating thickness dependence of the oxygen reduction activity retention rate at 0.9V.

An embodiment of the present invention will be described in detail below.
[1. Method for producing ionomer-coated catalyst]
The method for producing an ionomer-coated catalyst according to the present invention comprises:
A first step of preparing an electrode catalyst in which catalyst particles are supported on a carrier surface;
a second step of coating the surface of the catalyst particles with a protective material to obtain a protective material-coated electrode catalyst;
a third step of further coating the surface of the protective material-coated electrode catalyst with an ionomer to obtain a precursor;
and a fourth step of extracting the protective material from the precursor.

[1.1. First step]
First, an electrode catalyst in which catalyst particles are supported on a carrier surface is prepared (first step).

[1.1.1. carrier]
The carrier is for supporting the catalyst particles. In the present invention, the carrier is not particularly limited as long as it can support the catalyst particles and supply the electrons necessary for the electrode reaction to the catalyst particles.
The carrier is
(a) a porous carrier having pores capable of supporting catalyst particles therein, or
(b) any solid support that does not have such pores;

When a porous carrier is used as the carrier, the catalyst particles carried in the pores are less likely to be poisoned by the ionomer. However, porous carriers with pores are generally expensive. On the other hand, solid carriers are less expensive than porous carriers. However, when a solid carrier is used as the carrier, the catalyst particles are generally susceptible to ionomer poisoning.
On the other hand, when the present invention is applied to a solid carrier, an electrode catalyst having performance equal to or higher than that obtained by supporting catalyst particles on a porous carrier using a conventional method can be obtained.

Porous carriers include, for example, mesoporous carbon such as Escarbon (registered trademark), nanodendrite (MCND), and Knobel (registered trademark).
Examples of solid carriers include acetylene blacks such as Denka Black (registered trademark), Vulcan (registered trademark), VGCF (registered trademark), and carbon nanotubes.
The average particle size of the carrier is not particularly limited, and an optimum average particle size can be selected depending on the purpose. Further, the material of the carrier is not limited to carbon, and conductive metal oxides (TiO _x , SnO ₂ ) can also be used.

[1.1.2. catalyst particles]
The material of the catalyst particles is not particularly limited as long as it is a material exhibiting oxygen reduction reaction activity or hydrogen oxidation reaction activity.
Materials for the catalyst particles include, for example,
(a) noble metals (Pt, Au, Ag, Pd, Rh, Ir, Ru, Os),
(b) an alloy containing two or more precious metal elements;
(c) alloys containing one or more noble metal elements and one or more base metal elements (e.g., Fe, Co, Ni, Cr, V, Ti, etc.);
(d) a metal oxynitride;
(e) Carbon alloys and the like.

[1.1.3. Preparation of electrode catalyst]
A commercially available electrode catalyst may be used as it is, or an electrode catalyst in which catalyst particles are supported on the surface of a carrier may be prepared. Electrocatalyst preparation is usually
(a) dispersing the carrier in a solution in which the catalyst particle precursor is dissolved, and adsorbing the catalyst particle precursor onto the surface of the carrier in the solvent; forming a precursor-supported electrocatalyst precursor,
(b) It is carried out by reducing the catalyst particle precursor.

[1.2. Second step]
Next, the surfaces of the catalyst particles are coated with a protective material. (Second step). As a result, an electrode catalyst in which at least the surfaces of the catalyst particles are coated with the protective material (hereinafter also referred to as "protective material-coated electrode catalyst") is obtained.

[1.2.1. Protective layer]
The “protective material” is a space of an appropriate size between the catalyst particles and the ionomer (which can reduce catalyst poisoning by the ionomer and supply protons from the ionomer to the catalyst particles even in a low-humidity environment). It is used to temporarily cover the surface of the catalyst particles in order to form a space of a size that can be The protective material may cover the surface of the carrier in addition to the surface of the catalyst particles.

To that end, the protective material
(a) capable of coating at least the surface of the catalyst particles, and
(b) It must be possible to selectively extract only the protective material after the surface of the electrode catalyst containing the catalyst particles coated with the protective material is further coated with an ionomer.
The protective material is not particularly limited as long as it has such a function.

As a protective material, for example,
(a) of the main metal elements contained in the catalyst particles, a metal that is less noble than the most noble metal element,
(b) a phosphonic acid group (--PO ₃ ^2- ), a sulfonic acid group (--SO ₃ ^- ), a sulfamyl group (--SH), a carboxylic acid group (--COO ^- ), an ammonium group (--NR ₃ ⁺ ), a pyridinium group (--N ⁺ C ₅ H ₅ ), and an imidazolium group (--N ₂ R ₂ ⁺ C ₃ R ₂ ) (where R is an alkyl group or H ) an organic compound to which one or more functional groups are bonded,
(c) a polymer soluble in a solvent in which the ionomer is not soluble, or
(d) There are acid- or alkali-soluble inorganic compounds in which catalyst particles and ionomers are insoluble.

[A. metal]
A first specific example of the protective material consists of a metal that is less noble than the most noble metal among the main metal elements contained in the catalyst particles.
Here, the term "major metal element" refers to a metal element whose content in the catalyst particles is 1 mass % or more. To facilitate selective extraction of the protective agent, the protective agent is
(a) a metal element that is more base than the most noble metal element among the main metal elements and is more base than the metal element that has the largest content in the catalyst particles, or
(b) Metals less noble than all major metallic elements contained in the catalyst particles are preferred.

When the protective material is made of metal, the optimum material for the protective material is selected according to the composition of the catalyst particles. For example, when the catalyst particles are made of platinum or a platinum alloy, the protective material is preferably Cu, Ag, Sb, Bi, Pd, Zn, Cr, Fe, Cd, Co, Ni, Sn, or Pb. The protective material may be a pure metal containing any one of these metal elements, or may be an alloy or mixture containing two or more metal elements.

[B. Organic compound having an adsorptive functional group]
A second specific example of the protective material consists of an organic compound having an adsorptive functional group.
Here, the "adsorptive functional group" is a functional group having a property of being easily adsorbed on the surface of catalyst particles, and includes phosphonic acid group (--PO ₃ ^2- ), sulfonic acid group (--SO ₃ ^- ), a sulfamyl group (--SH), a carboxylic acid group (--COO ^- ), an ammonium group (--NR ₃ ⁺ ), a pyridinium group (--N ⁺ C ₅ H ₅ ), or an imidazolium group (--N ₂ R ₂ ⁺ C ₃ R ₂ ) (provided that R is an alkyl group or H).
The term “organic compound having an adsorptive functional group” refers to an organic compound in which one or two or more adsorptive functional groups are bound to a hydrocarbon chain having 5 or more carbon atoms.

An organic compound having an adsorptive functional group is used as a dispersing agent for fine particles because of its strong adsorptive power to the surface of fine particles. Such a dispersant functions as a protective material for temporary adsorption on the surface of the catalyst particles.
The lower the number of carbon atoms in the hydrocarbon chain, the less space is left after the protectant is extracted. As a result, the distance between the ionomer and the catalyst particles becomes excessively short, making the catalyst particles more likely to be poisoned by the ionomer. Therefore, the number of carbon atoms in the hydrocarbon chain is preferably 5 or more. The number of carbon atoms is preferably 10 or more, more preferably 20 or more.

Tables 1 to 3 show examples of organic compounds (dispersants) that can be used as protective materials (cited from Reference 1). The protective material may consist of any one of these, or may be used in combination of two or more.
[Reference 1] J. Jpn. Soc. Color Mater., 78(3), 141-148(2005)

[C. polymer]
A third specific example of the protective material consists of a polymer soluble in a solvent in which the ionomer is not soluble.
When the surfaces of catalyst particles are coated with a polymer and an ionomer and then immersed in a solvent that dissolves only the polymer, the polymer can be preferentially dissolved.
Such polymers include, for example, polybutylene terephthalate (PBT), polyacrylonitrile (PAN), and the like. The protective material may consist of any one of these, or may be used in combination of two or more.

[D. Inorganic compound]
A fourth specific example of the protective material consists of an acid- or alkali-soluble inorganic compound in which the catalyst particles and the ionomer are insoluble.
After coating the surfaces of the catalyst particles with an inorganic compound and an ionomer, the inorganic compounds can be preferentially dissolved by immersing the particles in an acid or alkali that dissolves only the inorganic compounds.
Examples of such inorganic compounds include silica, titanium oxide, niobium oxide, and tantalum oxide. The protective material may consist of any one of these, or may be used in combination of two or more.

[1.2.2. Coat method]
As for the coating method of the protective material, an optimum method is selected according to the type of the protective material.
For example, when the protective material is metal, coating methods include electrodeposition (electroplating), electroless plating, vapor deposition, sputtering, and the like.

When the protective material is an organic compound having an adsorptive functional group, the coating method includes, for example, dispersing the electrode catalyst in a solution in which the organic compound is dissolved or dispersed, and volatilizing the solvent.
When the protective material is an organic compound having an adsorptive functional group, the coating of the protective material (second step) and the ionomer coating (third step) may be performed separately or simultaneously. can be When the electrode catalyst is dispersed in a solution containing both the protective material and the ionomer, the surface of the catalyst particles can be coated with an organic compound due to equilibrium reactions in the solution.

When the protective material is a polymer, the coating method is as follows.
(a) a method of dispersing an electrode catalyst in a solution in which a polymer is dissolved or dispersed and volatilizing the solvent;
(b) A method of dispersing the electrode catalyst in a solvent in which the polymer is difficult to dissolve, pouring and dispersing the solution in which the polymer is dissolved or dispersed therein so that the polymer is adsorbed on the catalyst particles, and filtering;
and so on.

Furthermore, when the protective material is an inorganic material, as a coating method,
(a) a method of adding and mixing an electrode catalyst to a solution containing a metal alkoxide such as tetraalkoxysilane to hydrolyze the metal alkoxide;
(b) a sputtering method;
and so on.

[1.2.3. Thickness of protective material]
When the surface of the catalyst particles is coated with the protective material, the thickness of the protective material is not particularly limited, and an optimum thickness is selected according to the purpose. In general, if the thickness of the protective material is too thin, the ionomer will come too close to the catalyst particles after the protective material is extracted, and the catalyst particles will be easily poisoned by the ionomer. Therefore, the thickness of the protective material is preferably 1 nm or more. The thickness is preferably 3 nm or more, more preferably 5 nm or more.

On the other hand, if the protective material is too thick, it becomes difficult for protons to be supplied from the ionomer to the catalyst. Therefore, the thickness of the protective material is preferably 20 nm or less. The thickness is preferably 10 nm or less, more preferably 8 nm or less.

[1.3. Third step]
Next, the surface of the electrode catalyst, in which the surfaces of the catalyst particles are coated with the protective material, is further coated with an ionomer (third step). As a result, an electrode catalyst (hereinafter also referred to as "precursor") containing catalyst particles coated with a protective material and ionomer is obtained.

[1.3.1. Ionoma]
The ionomer is for supplying protons to the catalyst particle surface. The ionomer may coat only the surface of the catalyst particles coated with the protective material, or may additionally coat the surface of the carrier.
The ionomer is not particularly limited as long as it exhibits such functions. Ionomers include, for example, perfluorocarbon sulfonic acid polymers and highly oxygen-permeable ionomers. The ionomer may consist of any one of these, or may be used in combination of two or more.

A "perfluorocarbon sulfonic acid polymer" refers to a fluorine-containing ion exchange resin containing repeating units based on a sulfonyl fluoride vinyl ether monomer. Examples of perfluorocarbon sulfonic acid polymers include Nafion (registered trademark), Flemion (registered trademark), Aquivion (registered trademark), and Aciplex (registered trademark).

A "highly oxygen permeable ionomer" refers to a polymer compound containing an acid group and a cyclic structure in its molecular structure. A high oxygen permeability ionomer has a high oxygen permeability coefficient because it contains a ring structure in its molecular structure. Therefore, when this is used as an ionomer, the oxygen migration resistance at the interface with the catalyst becomes relatively small.
In other words, "high oxygen permeability ionomer" refers to an ionomer having a higher oxygen permeability coefficient than perfluorocarbon sulfonic acid polymers represented by Nafion (registered trademark).

In general, fuel cell performance is rate-determined by the diffusion of oxygen to the catalyst surface. On the other hand, when the surface of the electrode catalyst is coated with a highly oxygen-permeable ionomer, the oxygen permeability of the catalyst layer is improved, and the performance of the fuel cell is improved.
The molecular structure of the highly oxygen-permeable ionomer is not particularly limited as long as it exhibits relatively low resistance to oxygen transfer. In particular, an ionomer containing a cyclic structure (aliphatic ring structure) in its molecular structure has lower oxygen transfer resistance than an ionomer not containing a cyclic structure, and is therefore suitable as an ionomer for coating the surface of the electrode catalyst.

Examples of highly oxygen permeable ionomers include:
(a) an electrolyte polymer containing a perfluorocarbon unit having an alicyclic structure and an acid group unit having perfluorosulfonic acid as a side chain;
(b) an electrolyte polymer containing a perfluorocarbon unit having an alicyclic structure and an acid group unit having perfluoroimide as a side chain;
(c) an electrolyte polymer containing a unit in which a perfluorosulfonic acid is directly bound to a perfluorocarbon having an alicyclic structure;
etc. (see References 2-5).
[Reference 2] JP 2003-036856 [Reference 3] International Publication No. 2012/088166 [Reference 4] JP 2013-216811 [Reference 5] JP 2006-152249

[1.3.2. Coat method]
The ionomer can be coated by dispersing the electrode catalyst in a solution in which the ionomer is dissolved or dispersed, and volatilizing the solvent.
The solvent for dissolving or dispersing the ionomer is not particularly limited as long as it can dissolve or disperse only the ionomer without dissolving the protective material. In order to prevent dissolution of the protective material, it is preferable to adjust the atmosphere (for example, nitrogen atmosphere) and the pH of the solution as necessary.

[1.3.3. ionomer/carrier ratio]
When the surface of the electrode catalyst is coated with an ionomer, the ionomer/support ratio (mass ratio) affects the performance of the ionomer-coated catalyst. If the ionomer/support ratio becomes too small, it becomes difficult to supply the protons necessary for the oxygen reduction reaction. Therefore, the ionomer/carrier ratio is preferably 0.1 or more. The ionomer/carrier ratio is preferably 0.2 or higher, more preferably 0.4 or higher.

On the other hand, if the ionomer/support ratio is too large, the ionomer will reduce the porosity in the catalyst layer, impeding gas diffusion. Therefore, the ionomer/carrier ratio is preferably 1.5 or less. The ionomer/carrier ratio is preferably 1.2 or less, more preferably 1.0 or less.

[1.3.4. Ionomer coating thickness]
"Ionomer coating thickness" refers to a value obtained by dividing the volume of the dry ionomer contained in the ionomer-coated catalyst by the surface area of the electrode catalyst.
"Surface area of the electrode catalyst" is the sum of the surface area of the catalyst particles having oxygen reduction activity and the surface area of the support, and is the value measured by the BET method.

The ionomer coating thickness is strongly influenced by the specific surface area of the electrode catalyst (in particular, the specific surface area of the carrier). That is, even if the amount of ionomer is the same, the ionomer coating thickness is thinner when using an electrode catalyst with a high specific surface area than when using a catalyst with a low specific surface area. In general, the thinner the ionomer coating thickness, the lower the probability that the ionomer will adsorb to active species such as platinum after the protective material is removed. Therefore, in the third step, it is preferable to further coat the surface of the protective material-coated electrode catalyst with the ionomer so that the ionomer coating thickness becomes a predetermined value.

In order to suppress the decrease in oxygen reduction activity due to ionomer poisoning, the ionomer coating thickness is preferably 2.5 nm or less. The ionomer coating thickness is preferably 2.3 nm or less, more preferably 2.0 nm or less.
On the other hand, if the ionomer coating thickness is too thin, it becomes difficult to supply the protons necessary for the oxygen reduction reaction. Therefore, the ionomer coating thickness is preferably 0.2 nm or more. The ionomer coating thickness is more preferably 0.8 nm or more.

[1.4. Fourth step]
Next, the protective material is extracted from the precursor (fourth step). As a result, the ionomer-coated catalyst according to the present invention is obtained.

An optimum method for extracting the protective material is selected according to the type of the protective material.
For example, when the protective material is made of metal, the protective material is extracted by coating the surface of the catalyst particles with the protective material and ionomer, and then immersing the precursor in a solution capable of dissolving the protective material (e.g., diluted nitric acid aqueous solution). by At this time, when the protective material is a base metal compared to the catalyst particles, the protective material can be preferentially dissolved.

When the protective material consists of an organic compound, the method for extracting the protective material is as follows.
(a) a method of immersing the precursor in hot water;
(b) a method of oxidatively decomposing the protective material by controlling the potential to a high level after making a fuel cell using the precursor;
(c) a method of washing away the protective material with water generated by power generation after forming a fuel cell using the precursor;
and so on.

When the protective material is made of a polymer, the protective material is extracted by immersing the precursor in a solvent that dissolves the polymer but does not dissolve the ionomer. For example, when the protective material is PBT, PBT does not dissolve in the mixed solvent of water/alcohol, which is the solvent of the Nafion (registered trademark) solution, but dissolves in octochlorophenol. On the other hand, Nafion (registered trademark) does not dissolve in octochlorophenol. Therefore, when the precursor is immersed in octochlorophenol, only PBT can be selectively dissolved.

Furthermore, when the protective material consists of an inorganic compound, the protective material is extracted by immersing the precursor in a solvent in which the ionomer does not dissolve and the protective material can be dissolved. For example, when the protective material is silica, the silica can be removed with hydrofluoric acid, strongly basic aqueous solution (KOH, NaOH, etc.), or the like.

[2. Ionoma-coated catalyst]
FIG. 1 shows a schematic diagram of the method for producing an ionomer-coated catalyst according to the present invention. FIG. 1 shows an example in which the carrier is made of solid carbon particles, the catalyst particles are made of Pt, and the protective material is made of Cu. materials are not limited to these.

First, an electrode catalyst (for example, platinum-supported carbon) in which catalyst particles are supported on the surface of a carrier is prepared. Next, the surface of the catalyst particles is coated with a protective material (left figure in FIG. 1). When Cu is deposited on the surface of platinum-supported carbon using an electrolytic plating method, it is considered that Cu is also deposited on the surface of the carbon support. However, since the deposition rate on the Pt surface is faster than the deposition rate on the carbon support surface, Cu can be deposited mainly on the Pt surface by optimizing the electrolytic plating conditions.

Next, the surface of the electrode catalyst is further coated with an ionomer (middle diagram in FIG. 1). The ionomer may coat the surface of the carrier in addition to the surface of the protective material.
Furthermore, when the ionomer-coated electrode catalyst with a protective material (that is, the precursor) is immersed in a solution capable of dissolving only Cu (for example, a diluted nitric acid aqueous solution), only Cu is extracted (the right figure in FIG. 1). ). As a result, the ionomer-coated catalyst according to the present invention is obtained.

Since the ionomer-coated catalyst according to the present invention is obtained by the method described above, as shown in FIG. It is thought that a space of appropriate size is formed between them. As a result, proton supply becomes possible even in a low-humidity environment, and catalyst poisoning by the ionomer is suppressed.

Such a positional relationship between the catalyst and the ionomer can be observed by a method combining three-dimensional TEM and elemental analysis. However, since ionomers are easily damaged by electron beams, it is difficult to quantitatively analyze their positional relationship. That is, at the time of filing the present application, there is no means for directly observing the presence or absence of such a space and its size. However, by comparing the performance of the ionomer-coated catalyst obtained by the method of the present invention (for example, the oxygen reduction activity retention rate described later) with that of the catalyst of the same composition produced by the conventional method, , the presence or absence of an appropriately sized space around the catalyst particles can be indirectly confirmed.

Specifically, when the carrier is a solid carrier, optimizing the production conditions yields an ionomer-coated catalyst that satisfies the following formulas (1) and (2).
0.1≤x≤1.5 (1)
7.5758x ² -28.03x+100≦y≦100 (2)
however,
x is the ionomer/support ratio (mass ratio) of the ionomer-coated catalyst;
y is the oxygen reduction activity retention rate (%) of the ionomer-coated catalyst.

When the surface of the electrode catalyst is directly coated with an ionomer, the oxygen reduction activity retention rate drops sharply in the ionomer/support ratio range of 0 to 0.5. In addition, in the region where the ionomer/carrier ratio exceeds 0.5, the oxygen reduction activity retention rate takes a substantially constant value, or the oxygen reduction activity retention rate gradually decreases as the ionomer/carrier ratio increases. .
On the other hand, when an appropriate space is formed between the electrode catalyst and the ionomer, the oxygen reduction activity retention rate does not drop sharply in the range of the ionomer/support ratio of 0 to 0.5, and reaches a value close to 100%. to maintain In addition, the oxygen reduction activity retention rate varies depending on the ionomer/carrier ratio within the range of 0.5 to 2.0. Specifically, the smaller the ionomer/carrier ratio, the higher the oxygen reduction activity retention rate. By optimizing the production conditions, an ionomer-coated catalyst having an oxygen reduction activity retention rate of 90% or more can be obtained in the ionomer/support ratio range of 0.5 to 1.0.

Further, by optimizing the production conditions, an ionomer-coated catalyst having an ionomer coating thickness of 0.2 nm or more and 2.5 nm or less can be obtained. Since the details of the ionomer coating thickness are as described above, the description is omitted.

[3. Method for producing a protective material-coated electrode catalyst]
The method for producing a protective material-coated electrode catalyst according to the present invention comprises:
A first step of preparing an electrode catalyst in which catalyst particles are supported on a carrier surface;
and a second step of coating the surface of the catalyst particles with a protective material to obtain a protective material-coated electrode catalyst.

[3.1. First step]
First, an electrode catalyst in which catalyst particles are supported on a carrier surface is prepared (first step). Since the details of the first step are as described above, the description is omitted.

[3.2. Second step]
Next, the surfaces of the catalyst particles are coated with a protective material (second step). Thereby, the protective material-coated electrode catalyst according to the present invention is obtained. In the second step, the surfaces of the catalyst particles are preferably coated with the protective material so that the protective material has a thickness of 1 nm or more and 20 nm or less. Other points regarding the second step are the same as described above, so description thereof will be omitted.

[4. Protective material-coated electrode catalyst]
an electrode catalyst in which catalyst particles are supported on the surface of a carrier;
and a protective material that coats the surface of the catalyst particles.

[4.1. carrier]
The carrier is for supporting the catalyst particles. Since the details of the carrier are as described above, the description is omitted.

[4.2. catalyst particles]
The material of the catalyst particles is not particularly limited as long as it is a material exhibiting oxygen reduction reaction activity or hydrogen oxidation reaction activity. Since the details of the catalyst particles are as described above, the description is omitted.

[4.3. Protective layer]
The “protective material” is a space of an appropriate size between the catalyst particles and the ionomer (which can reduce catalyst poisoning by the ionomer and supply protons from the ionomer to the catalyst particles even in a low-humidity environment). It is used to temporarily cover the surface of the catalyst particles in order to form a space of a size that can be The protective material may cover the surface of the carrier in addition to the surface of the catalyst particles.

The thickness of the protective material is preferably 1 nm or more and 20 nm or less. Moreover, the protective material is preferably a metal such as Cu or Ni.
Since other points regarding the protective material are as described above, the description is omitted.

[5. action]
Poisoning of the catalyst by the ionomer can be reduced by using a porous carrier as the carrier and supporting the catalyst particles in the pores. However, porous carriers are generally expensive. In addition, since there is no ionomer inside the porous carrier, protons are less likely to be supplied to the catalyst particles carried in the pores under low humidity conditions. Therefore, catalyst utilization decreases under low humidity conditions.

In contrast, when catalyst particles were supported on the carrier surface, the surface of the catalyst particles was coated with a protective material, and the surface of the electrode catalyst was further coated with an ionomer, the protective material was extracted, and a solid carrier was used as the carrier. Even in this case, it is possible to obtain an ionomer-coated catalyst that is less poisoned by the ionomer and has a high catalyst utilization rate under low-humidity conditions.
This is because the ionomer is arranged close to the catalyst particles by coating and extracting the protective material, and at the same time, a space of an appropriate size is formed between the ionomer and the catalyst particles. Conceivable. In other words, it is believed that the ionomer and the catalyst particles are separated enough to prevent the catalyst particles from being poisoned, and are close enough to allow the supply of protons even in a low-humidity environment. be done.

(Example 1, Comparative Examples 1 and 2)
[1. Preparation of sample]
[1.1. Reference electrode]
29 mass% Pt/C (TEC10V30E manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., carbon type: Vulcan (registered trademark) XC-72) was used as the electrode catalyst. Electrocatalyst: 16.4 mg, water: 1 g, and 1-propanol: 7.23 g were mixed and dispersed using an ultrasonic homogenizer. 5 μL of the above dispersion was dropped on a polished glassy carbon (GC) rod of 5.5 mmφ and dried at 80° C. to form a thin catalyst layer. Hereinafter, the catalyst consisting only of Pt/C will also be referred to as "reference catalyst", and the GC rod supporting only the reference catalyst will also be referred to as "reference electrode".

[1.2. Example 1]
A plating solution was placed in a three-electrode cell, and a reference electrode was immersed in the plating solution. A 0.1 mol/L sulfuric acid aqueous solution containing 0.01 mol/L copper sulfate was used as the plating solution. Next, using the reference electrode as a cathode, the sample was held at 0.3 V vs. RHE for 5 minutes in an inert atmosphere to deposit Cu on the Pt surface to obtain a Cu-coated electrode. After pulling up the Cu-coated electrode from the sulfuric acid aqueous solution, it was immediately rinsed with ultrapure water and air-dried.

Next, a 0.15 mass% Nafion (registered trademark) solution (corresponding to I/C = 1.0) was dropped on the surface of the Cu-coated electrode and dried at 80°C to obtain a Cu-ionomer-coated electrode.
Further, an electrolytic solution was put into the three-electrode cell, and the Cu/ionomer coated electrode was immersed in the electrolytic solution. 0.1 mol/L perchloric acid was used as the electrolyte. Next, the potential was swept from 0.3 V to 1.0 V (vs. RHE) using a Cu/ionomer-coated electrode as a working electrode to elute only Cu to obtain an ionomer-coated catalyst.

[1.3. Comparative Example 1]
A 0.15 mass% Nafion (registered trademark) solution (corresponding to I/C = 1.0) was dropped directly on the surface of the reference electrode and dried at 80°C.

[1.4. Comparative Example 2]
29 mass% Pt/C (TEC10V30E manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., carbon type: Vulcan (registered trademark) XC-72) was used as the electrode catalyst. Nafion (registered trademark) solution (D2020) was used as the ionomer solution.
Electrocatalyst: 16.4 mg, water: 0.98 g, 1-propanol: 7.23 g, and ionomer solution: 50 mg were mixed and dispersed using an ultrasonic homogenizer. 5 μL of the above dispersion was dropped on a ground glassy carbon rod of 5.5 mmφ and dried at 80° C. to form a thin catalyst layer.

[2. Test method]
[2.1. ECSA]
A voltammogram was measured in an inert atmosphere in a 0.1 mol/L perchloric acid aqueous solution. The electrochemical effective surface area (ECSA) of platinum was calculated from the hydrogen adsorption/desorption waves.

[2.2. Oxygen reduction activity retention rate]
Using a rotating electrode method (400 rpm), an oxygen reduction current was measured at 30° C. and 0.9 V (vs. RHE) in an oxygen (1 atm) atmosphere. A 0.1 mol/L perchloric acid aqueous solution was used as the electrolytic solution. The oxygen reduction activity @ 0.9 V (vs. RHE) (ie, the specific activity (SA) of the oxygen reduction reaction) was calculated by normalizing the oxygen reduction current at 0.9 V with ECSA.

Furthermore, using the obtained oxygen reduction activity @ 0.9 V (vs. RHE), the oxygen reduction activity retention rate was calculated according to the following formula (3).
Oxygen reduction activity retention rate (%)=ORR _x ×100/ORR ₀ (3)
however,
ORR _x is the oxygen reduction activity of the ionomacoated catalyst @ 0.9 V vs. RHE;
ORR ₀ is the oxygen reduction activity @ 0.9 V vs. RHE of the reference catalyst (a catalyst having the same composition as the ionomer-coated catalyst but not coated with ionomer).

[3. result]
FIG. 2 shows the oxygen reduction activity retention rate of the ionomer-coated catalysts obtained in Example 1 and Comparative Examples 1 and 2. As shown in FIG. 2 that Example 1 has a higher oxygen reduction activity retention rate than Comparative Examples 1 and 2. FIG. It is believed that this is because a suitable space was formed between the Pt and the ionomer due to the steps of deposition of Cu on the Pt surface, coating with the ionomer, and extraction of Cu.

(Example 2, Comparative Examples 3-4)
[1. Preparation of sample]
[1.1. Example 2]
[1.1.1. Preparation of protective material-coated electrode catalyst]
480 g of ultrapure water is weighed into a 1 L beaker, and (a) 3.59 g of copper sulfate pentahydrate as a copper source,
(b) 7.66 g of Rossel salt (potassium sodium tartrate tetrahydrate; KOOCCH(OH)CH(OH)COONa.4H ₂ O) as a complexing agent; and (c) 13% aqueous formaldehyde solution as a reducing agent. .0g
was added and mixed to obtain a solution.

1 g of Pt/C (TEC10V30E manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., carbon type: Valcan (registered trademark) XC-72) was added to this solution and suspended. Further, while stirring this suspension with a magnetic stirrer, 7.85 g of sodium hydroxide was gradually added. Concentrations determined from the input amount of each reagent were CuSO ₄ .5H ₂ O: 0.03 mol/L, Rossel's salt: 16 g/L, and formaldehyde: 0.25 mol/L. Also, the reaction was carried out at room temperature.

After precipitating copper, filtration under reduced pressure was performed, and the product was washed with water several times to obtain platinum-supported carbon on which copper was precipitated. Hereinafter, this is described as "Cu/Pt/C".
Assuming that all the copper put into the solution was supported on Pt/C, the composition of Cu/Pt/C was estimated to be 47.7 mass% copper and 15.0 mass% platinum. In addition, assuming that all the injected copper uniformly covers the surfaces of the Pt particles, the thickness of the copper layer (thickness of the protective material) was estimated to be about 10 nm.

[1.1.2. Preparation of evaluation electrode]
Cu/Pt/C: Weighed out 127 mg. Water: 6.29 g and 2-propanol: 25.15 g were added in this order and mixed to obtain a catalyst dispersion.
Separately, Nafion (registered trademark) solution (manufactured by Chemours Co., Ltd., D2020) was diluted with 2-propanol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for precision analysis), and the concentration was 5.5 mass%. An ionomer solution was obtained. Ionomer solution: 0.34 g was added to the catalyst dispersion, and dispersed for 6 minutes with a horn-type ultrasonic homogenizer (manufactured by SMT Co., Ltd., MODELUH-600) to obtain an ionomer/catalyst dispersion.

Next, the prepared ionomer/catalyst dispersion was applied onto the electrode using an ultrasonic spray nozzle (WS130K50ST, manufactured by Sonia). A GC electrode of 5.5 mmφ whose surface was polished with alumina particles (1 μm, 0.3 μm, and 0.05 μm) was used as the electrode. The spray conditions were ultrasonic output: 70%, liquid feed rate: 100 μL/min, nitrogen gas flow rate: 6 L/min, distance between nozzle and electrode: 10 cm, application time: 5 minutes.
After coating, the GC electrode was placed in a constant temperature bath at 80° C. and dried for 10 minutes to obtain an ionomer-coated Cu/Pt/C-carrying GC electrode. Cu/Pt/C coated with ionomer is hereinafter referred to as "Naf/Cu/Pt/C". The mass ratio (I/C) of ionomer to carbon was set to 0.4.

[1.1.3. Removal of Cu]
A GC electrode supporting Naf/Cu/Pt/C was set on a rotating electrode holder. In this state, the GC electrode was immersed in a 0.5 mol/L nitric acid aqueous solution for 5 minutes to elute copper. Hereinafter, a catalyst obtained by removing Cu from Naf/Cu/Pt/C is referred to as "Naf/(Cu removal)/Pt/C".
[1.2. Comparative Example 3]
Pt/C not coated with ionomer was deposited on the GC electrode. That is, Pt/C: 67 mg was weighed out. Water: 6.29 g and 2-propanol: 25.15 g were added in this order and mixed to obtain a catalyst dispersion. The prepared catalyst dispersion was applied onto the GC electrode. The coating method and conditions were the same as in Example 2.

[1.3. Comparative Example 4]
An ionomer-coated platinum-supported carbon (Naf/Pt/C) was supported on a GC electrode without depositing copper. That is, Pt/C: 66 mg was weighed out. Water: 6.29 g and 2-propanol: 25.15 g were added in this order and mixed to obtain a catalyst dispersion. Separately, an ionomer solution with a concentration of 5.5 mass % was prepared in the same manner as in Example 2. Ionomer solution: 0.34 g was added to the catalyst dispersion liquid and subjected to dispersion treatment to obtain an ionomer/catalyst dispersion liquid. Further, the prepared ionomer/catalyst dispersion was applied onto the GC electrode. The method and conditions of dispersion and application were the same as in Example 2.

[2. Test method]
[2.1. Evaluation of protective material-coated electrode catalyst]
For Example 2, the concentration of copper ions contained in the first filtrate obtained by filtration under reduced pressure after precipitating copper was measured. The copper ion concentration was measured by colorimetric analysis using Packtest (registered trademark) (manufactured by Kyoritsu Rikagaku Kenkyusho Co., Ltd., for copper ion analysis, WAK-Cu).
In addition, the protective material-coated electrode catalyst was observed with a transmission electron microscope.

[2.2. Evaluation of oxygen reduction activity]
The obtained electrode for evaluation was inserted into a rotating electrode holder and used as a working electrode. Hereinafter, in the same manner as in Example 1, the electrochemical effective surface area (ECSA), specific activity (SA), and oxygen reduction activity retention rate of platinum were determined.

[3. result]
[3.1. Evaluation of protective material-coated electrode catalyst]
Pt/C was added to and mixed with a mixed solution of copper sulfate, Rossel's salt and formaldehyde, and sodium hydroxide was gradually added while stirring to generate air bubbles. This is probably because the reduction of copper ions represented by the following formula (4) and the oxidation of formaldehyde caused the deposition of copper and the generation of hydrogen.
Cu ²⁺ +2HCHO+4OH ⁻ →Cu+2HCOO ⁻ +H ₂ +2H ₂ O (4)

In addition, the filtrate obtained by filtering and washing after the completion of the reaction was colorless and transparent. The concentration of copper ions in the filtrate was simply measured by PACKTEST (registered trademark) and found to be 0.1 mg/L. Since the concentration of copper ions before the reduction reaction calculated from the amount of reagent input was 1.9 g/L, it is considered that almost all copper ions were reduced and copper was deposited on Pt/C.
From the knowledge of electroless plating technology, it is known that this reduction reaction of copper ions progresses due to the catalytic action of the surface of the noble metal or the surface of the deposited copper. Therefore, it is considered that the place where copper is deposited is not on the carbon support but on the platinum as shown in FIG.

FIG. 3 shows element distribution mapping of the protective material-coated electrode catalyst obtained in Example 2 (FIG. 3(A): copper, FIG. 3(B): platinum), and a transmission electron microscope image (FIG. 3(C)). indicate. From FIG. 3, it can be confirmed that copper and platinum are present at the same place, and that there is little copper on the carbon.

[2.2. Evaluation of oxygen reduction activity]
FIG. 4 shows the logarithm of the specific activity (SA) and the potential of Pt/C (Comparative Example 3), Naf/Pt/C (Comparative Example 4), and Naf/(Cu removal)/Pt/C (Example 2). (Tafel plot). From FIG. 4, it can be seen that the SA of Naf/(Cu removal)/Pt/C is almost the same as the SA of Pt/C. On the other hand, it can be seen that the SA of Naf/Pt/C is lower than that of Pt/C.

Using this SA value at 0.9 V, the oxygen reduction activity retention rate was obtained from the equation (3). FIG. 5 shows the oxygen reduction activity retention rate at 0.9 V for Naf/Pt/C (Comparative Example 4) and Naf/(Cu removal)/Pt/C (Example 2). When Pt/C was coated with an ionomer, its oxygen reduction activity retention rate was halved to 51%. On the other hand, the oxygen reduction activity retention rate of Naf/(Cu removal)/Pt/C was 93%. It can be seen from FIG. 5 that ionomer poisoning is reduced by depositing and removing copper.

(Example 4, Comparative Example 5)
[1. Preparation of sample]
[1.1. Example 4]
[1.1.1. Preparation of reference electrode]
29 mass% Pt/C (TEC10V30E manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., carbon type: Vulcan (registered trademark) XC-72) was used as the electrode catalyst. Pt/C: 66.2 mg was weighed out. To this, 6.29 g of ultrapure water and 25.15 g of 2-propanol were added in this order, mixed, and dispersed with a horn-type ultrasonic homogenizer for 6 minutes to obtain a catalyst dispersion. The prepared catalyst dispersion was applied onto a polished 5.5 mmφ GC electrode using an ultrasonic spray nozzle (WS130K50ST, manufactured by Sonia).

[1.1.2. Preparation of evaluation electrode]
The ionomer solution concentration (0.1 to 0.15 mass%) and dropping amount (4 to 9 μL) were adjusted so that the I/C was 0.5, 1.0, 1.5, or 2.0. Except for this, Cu electrodeposition, ionomer coating, and Cu removal were performed in the same manner as in Example 1, and an evaluation electrode supporting an ionomer coating catalyst (Naf/(Cu removal)/Pt/C) was prepared. Obtained.

[1.2. Comparative Example 5]
An electrode for evaluation carrying an electrode catalyst (Naf/Pt/C) on which no Cu was deposited was prepared in the same manner as in Example 4, except that the electrodeposition of Cu and the removal of Cu were not performed.

[2. Test method]
The obtained electrode for evaluation was inserted into a rotating electrode holder and used as a working electrode. Hereinafter, in the same manner as in Example 1, the electrochemical effective surface area (ECSA), specific activity (SA), and oxygen reduction activity retention rate of platinum were determined.

[3. result]
[3.1. I/C dependence of oxygen reduction activity retention rate]
FIG. 6 shows the I/C dependence of the oxygen reduction activity retention rate at 0.9V. With Naf/Pt/C, the oxygen reduction activity retention rate was almost constant at 0.75 regardless of the I/C. This is probably because Pt is poisoned by the ionomer in every I/C.
On the other hand, with Naf/(Cu removal)/Pt/C, the oxygen reduction activity retention rate was about 100% when the I/C was 0.5, and the oxygen reduction activity retention rate decreased as the I/C increased. Further, when I/C was 1.5 to 2.0, the oxygen reduction activity retention rate of Naf/(Cu removal)/Pt/C was approximately the same as that of Naf/Pt/C. From FIG. 6, it can be seen that I/C is preferably 1.0 or less in order to reduce catalyst poisoning by the ionomer.

[3.2. Ionomer coating thickness dependence of oxygen reduction activity retention rate]
It is believed that the degree of ionomer adsorption after removal of the protective material is affected not only by the mass of the coated ionomer, but also by the thickness of the ionomer coating. The ionomer coating thickness is strongly influenced by the specific surface area of the electrode catalyst (including active species and carrier). That is, even if the amount of ionomer is the same, the ionomer coating thickness is thinner when using an electrode catalyst with a high specific surface area than when using an electrode catalyst with a low specific surface area.
FIG. 7 shows the dependence of the oxygen reduction activity retention rate at 0.9V on the ionomer coating thickness. From FIG. 7, it can be seen that when the ionomer coating thickness is 2.5 nm or less, the oxygen reduction activity retention rate is 85% or more. This is probably because the thinner the ionomer coating thickness, the less likely the ionomer will move after removing the protective material, and the lower the probability that the ionomer will adsorb to active species such as platinum.

Although the embodiments of the present invention have been described in detail above, the present invention is by no means limited to the above-described embodiments, and various modifications are possible without departing from the gist of the present invention.

The ionomer-coated catalyst according to the present invention can be used as an electrode catalyst for the air electrode of polymer electrolyte fuel cells.

Claims (12)

A first step of preparing an electrode catalyst in which catalyst particles are supported on a carrier surface;
a second step of coating the surface of the catalyst particles with a protective material to obtain a protective material-coated electrode catalyst;
a third step of further coating the surface of the protective material-coated electrode catalyst with an ionomer to obtain a precursor;
and a fourth step of extracting the protective material from the precursor. 2. The method for producing an ionomer-coated catalyst according to claim 1, wherein the carrier is a solid carrier. The protective material is
(a) of the main metal elements contained in the catalyst particles, a metal that is less noble than the most noble metal element;
(b) a phosphonic acid group (--PO ₃ ^2- ), a sulfonic acid group (--SO ₃ ^- ), a sulfamyl group (--SH), a carboxylic acid group (--COO ^- ), an ammonium group (--NR ₃ ⁺ ), a pyridinium group (--N ⁺ C ₅ H ₅ ), and an imidazolium group (--N ₂ R ₂ ⁺ C ₃ R ₂ ) (where R is an alkyl group or H ) an organic compound to which one or more functional groups are bonded,
(c) a polymer soluble in a solvent in which the ionomer is not soluble, or
3. The method for producing an ionomer-coated catalyst according to claim 1, wherein (d) the catalyst particles and the ionomer are composed of an acid- or alkali-soluble inorganic compound in which the catalyst particles and the ionomer are insoluble. The catalyst particles are made of platinum or a platinum alloy,
The protective material is made of a metal or alloy containing at least one selected from the group consisting of Cu, Ag, Sb, Bi, Pd, Zn, Cr, Fe, Cd, Co, Ni, Sn, and Pb. Item 4. A method for producing an ionomer-coated catalyst according to any one of Items 1 to 3. 5. The method according to any one of claims 1 to 4, wherein the second step comprises coating the surface of the catalyst particles with the protective material so that the protective material has a thickness of 1 nm or more and 20 nm or less. and a method for producing an ionomer-coated catalyst. 6. The method for producing an ionomer-coated catalyst according to any one of claims 1 to 5, wherein the ionomer comprises a perfluorocarbon sulfonic acid polymer and/or a highly oxygen-permeable ionomer. 7. The method according to any one of claims 1 to 6, wherein the third step comprises further coating the surface of the electrode catalyst with the ionomer so that the ionomer/support ratio (mass ratio) is 0.1 or more and 1.5 or less. A method for producing an ionomer-coated catalyst according to any one of items 1 to 3. 8. Any one of claims 1 to 7, wherein the third step comprises further coating the surface of the protective material-coated electrode catalyst with the ionomer so that the ionomer coating thickness is 0.2 nm or more and 2.5 nm or less. 2. A method for producing an ionomer-coated catalyst according to item 1. An ionomer-coated catalyst having the following composition.
(1) The ionomer coat catalyst is
an electrode catalyst in which catalyst particles are supported on the surface of a carrier;
and an ionomer that coats the surface of the electrode catalyst,
There is a space between the catalyst particles and the ionomer.
(2) The ionomer coat catalyst is
the carrier comprises a solid carrier,
It satisfies the relationship of the following equations (1) and (2).
0.1≤x≤1.5 (1)
7.5758x ² -28.03x+100≦y≦100 (2)
however,
x is the ionomer/support ratio (mass ratio) of the ionomer-coated catalyst;
y is the oxygen reduction activity retention rate (%) of the ionomer-coated catalyst. 10. The ionomer-coated catalyst according to claim 9, wherein the ionomer coating thickness is 0.2 nm or more and 2.5 nm or less. A protective material-coated electrode catalyst having the following configuration.
(1) The protective material-coated electrode catalyst is
an electrode catalyst in which catalyst particles are supported on the surface of a carrier;
and a protective material that coats the surface of the catalyst particles,
The catalyst particles are
(a) noble metals (Pt, Au, Ag, Pd, Rh, Ir, Ru, Os),
(b) an alloy containing two or more precious metal elements, or
(c) including alloys containing one or more noble metal elements and one or more base metal elements;
(2) The protective material-coated electrode catalyst is
The surface of the protective material-coated electrode catalyst is further coated with an ionomer to obtain a precursor,
It is used to make an ionomer-coated catalyst by extracting the protective material from the precursor. A first step of preparing an electrode catalyst in which catalyst particles are supported on a carrier surface;
a second step of coating the surface of the catalyst particles with a protective material to obtain the protective material-coated electrode catalyst according to claim 11;
The catalyst particles are
(a) noble metals (Pt, Au, Ag, Pd, Rh, Ir, Ru, Os),
(b) an alloy containing two or more precious metal elements, or
(c) A method for producing a protective material-coated electrode catalyst containing an alloy containing one or more noble metal elements and one or more base metal elements.

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