JP7511245B2 - Electrode material, electrode, membrane electrode assembly, and polymer electrolyte fuel cell - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applies. 1. January 8, 2021, 6th NEXT-FC Basic Research Reporting Session Handout "Notice for the 6th NEXT-FC Basic Research Reporting Session" Announced in the presentation materials (relevant part)

The present invention relates to an electrode material suitable for use as an electrode in a polymer electrolyte fuel cell or a polymer electrolyte water electrolysis device, and to an electrode and a membrane electrode assembly using the electrode material.

A polymer electrolyte fuel cell (PEFC) is composed of a basic unit, a cell having a structure in which a power generation module consisting of a membrane electrode assembly (MEA) including an electrolyte membrane and electrodes (anode and cathode) laminated on both sides of the electrolyte membrane, and gas diffusion layers (GDLs) laminated on both sides of the membrane electrode assembly is sandwiched between two separators. Fuel cell electrodes (particularly PEFC electrodes) generally consist of an electrode catalyst layer made of an electrode material having electrocatalytic activity and a polymer electrolyte, and a gas diffusion layer that combines gas permeability and electronic conductivity.

The electrode catalyst layers (anode and cathode) of first-generation PEFCs are made by mixing an electrode material with fine platinum-based particles supported on the surface of a carbon-based support with an electrolyte material. Under the operating conditions of a PEFC, electrode catalyst layers using such carbon-based supports have the problem that the platinum-based particles on the carbon-based support tend to aggregate, and the platinum-based particles tend to fall off due to oxidation of the carbon-based support, resulting in degradation of cell performance.

As an electrode catalyst capable of providing superior electrode performance, a fuel cell electrode material has been developed that uses an "electronically conductive oxide support" such as tin oxide ( _SnO2 ) instead of a carbon-based support (see Patent Document 1). In this electrode material, the oxide support is thermodynamically stable under PEFC operating conditions (strong acidity, high potential), and therefore is stable for a long period of time without being oxidized and corroded.
As an electrode material that provides an electrode catalyst layer with better electrode performance, an electrode catalyst material for fuel cells has been developed in which an electronically conductive oxide support such as tin oxide ( _SnO2 ) or titanium oxide ( _TiO2 ) is supported on the surface of a fibrous carbon material such as carbon nanotubes (CNT), and platinum (Pt) particles are further highly dispersed on the electronically conductive oxide support (see Patent Documents 2 and 3). This electrode catalyst material for fuel cells reduces the electrical resistance caused by the electronically conductive oxide support by supporting the electronically conductive oxide support as fine particles in a highly dispersed manner on the surface of the fibrous carbon material, and improves durability and electrochemical catalytic activity, resulting in an electrode catalyst material that can withstand 60,000 potential cycles, which corresponds to the lifespan of a fuel cell vehicle.

Japanese Patent No. 5322110 Patent No. 6598159 Patent No. 6624711

The most important components that affect the power generation performance of a fuel cell are the electrode catalyst and the electrode material, which consists of a carrier that supports the electrode catalyst. Usually, the electrode is composed of a mixture of the electrode catalyst and an electrolyte material (ionomer material).
In general, a material is prepared in which fine platinum-based particles are supported as an electrode catalyst on the surface of a conductive carbon material (carbon black, etc.) as a carrier, and then an ionomer is mixed into this to be used as a catalyst layer.Currently, issues with this catalyst layer material include the detachment of platinum particles due to oxidation of the carbon carrier, the elution and aggregation of platinum, and the balance between the amount of ionomer and the gas supply and water discharge.
The electrode reaction of a PEFC proceeds at a so-called three-phase interface where an electrode catalyst such as Pt, an electrolyte, and a reactant (oxygen or hydrogen) coexist. In order to increase the three-phase interface, which is the electrode reaction field, the electrode catalyst layer (anode and cathode) contains an ionomer, which is an electrolyte material, in addition to the electrode catalyst material. It is desirable for the ionomer to have excellent proton conductivity and not hinder the supply of oxygen or hydrogen gas to the three-phase interface.

However, while improvements have been made to the electrode catalyst and catalyst carrier, which are the materials that make up the electrode catalyst layer, little attention has been paid to the electrolyte material contained in the electrode catalyst layer, and the reality is that no fundamental solutions have been made. In conventional electrode catalyst layer manufacturing, the electrolyte material is usually mixed with the electrode material using fluorine-based polymer ionomers such as Nafion and Aquivion, which have been used for many years, and is then physically coated and adsorbed onto the surface of the electrode catalyst layer. However, this method causes an excess of electrolyte material to be contained in the electrode, which can hinder the supply of gas to the electrode catalyst.

In this situation, the present invention aims to provide an electrode material that can provide an electrode capable of generating electricity without including an electrolyte material in the electrode. It also aims to provide an electrode and a membrane electrode assembly that include the electrode material.

As a result of extensive research into solving the above problems, the inventors discovered that the following invention meets the above objectives, and thus arrived at the present invention.

That is, the present invention relates to the following inventions.
<1> An electrode material comprising: an oxide support made of an electronically conductive oxide; a monomolecular layer made of ionomer molecules having an ion-conductive functional group fixed to a surface of the oxide support; and electrode catalyst particles supported on the surface of the oxide support.
<2> The electrode material according to <1>, wherein the monomolecular layer is formed by fixing the ionomer molecule having the ion-conducting functional group to the surface of the oxide support by a covalent bond.
<3> The electrode material according to <1> or <2>, wherein the ionomer molecule having an ion-conducting functional group has a sulfonic acid group.
<4> The electrode material according to <3>, wherein the ionomer molecule having an ion-conductive functional group has a hydrocarbon chain or a fluorocarbon chain having 2 to 50 carbon atoms which may be branched.
<5> The electrode material according to any one of <1> to <4>, wherein the electronically conductive oxide is tin oxide or an electronically conductive oxide mainly composed of tin oxide.
<6> The electrode material according to <5>, wherein the electronic conductive oxide is niobium-doped tin oxide.
<7> The electrode material according to any one of <1> to <6>, wherein the electronically conductive oxide is fixed to a conductive auxiliary material.
<8> An electrode comprising the electrode material according to any one of <1> to <7>.
<9> The electrode according to <8>, which is for a polymer electrolyte fuel cell or a polymer electrolyte water electrolysis device.

<10> A membrane electrode assembly having a solid polymer electrolyte membrane, a cathode bonded to one side of the solid polymer electrolyte membrane, and an anode bonded to the other side of the solid polymer electrolyte membrane, wherein at least one of the cathode and the anode is the electrode according to <8> or <9>.
<11> A polymer electrolyte fuel cell comprising the membrane electrode assembly according to <10>.
<12> A polymer electrolyte water electrolysis device comprising the membrane electrode assembly according to <10>.

The present invention provides an electrode material that can generate electricity without including an electrolyte material in the electrode.

FIG. 1 is a schematic diagram of an electrode material of the present invention. FIG. 2 is an explanatory diagram showing the state of ionomer molecules in a conventional electrode material and the electrode material of the present invention. FIG. 2 is an explanatory diagram of a reaction for forming an ionomer molecule monolayer on the surface of an oxide support of an electrode material. 1 shows microstructural evaluation of the electrode material (ionomer molecule modified) of Example 1, (a) FE-SEM image, (b) EDX. FIG. 2 is an explanatory diagram of a start-stop simulated potential cycle. 1 shows an evaluation of IV characteristics of a single cell using the MEA of Example 1.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to the examples below and can be modified as desired without departing from the spirit of the present invention. In this specification, the word "~" is used as an expression that includes the numerical values or physical quantities before and after it.

The present invention relates to an electrode material (hereinafter referred to as the "electrode material of the present invention") that has an oxide support made of an electronically conductive oxide, a monolayer made of ionomer molecules having ion-conductive functional groups fixed to the surface of the oxide support, and electrode catalyst particles supported on the surface of the oxide support.

FIG. 1 is a schematic diagram of the electrode material of the present invention, and FIG. 2 is an explanatory diagram showing the ionomer state of a conventional electrode material and the electrode material of the present invention.
The electrode material of the present invention has a configuration in which electrode catalyst particles are directly supported on an oxide support made of an electronically conductive oxide having higher electrochemical stability than a carbon support, and the surface of the oxide support (parts on which the electrode catalyst particles are not supported) is modified with ionomer molecules having ion-conductive functional groups in the form of a thin film (monolayer). As a result, since the electrode material contains ionomer molecules having ion-conductive functional groups, ions can be supplied to the electrode reaction field (three-phase interface) without including an electrolyte material in the electrode when manufacturing the electrode. Since low-molecular-weight ionomer molecules having ion-conductive functional groups (typically sulfonic acid groups) are arranged in a monolayer (thickness of 3 nm or less) on the entire surface of the oxide support (excluding the part supported by the electrode catalyst particles), there is an advantage that even a small amount (less than 1 wt%) of ionomer molecules in the electrode can carry out ion conduction as an electrode.

As shown in the left diagram of FIG. 2, in PEFC electrodes in which a conventional electrolyte material (ionomer) is mixed, in order to develop sufficient ion conductivity (proton conductivity), the electrolyte material (high molecular weight ionomer) is physically coated and adsorbed onto the catalyst layer surface. Therefore, in the case of an oxide carrier made of an electronically conductive oxide, about 10 wt % of the material is required as the electrolyte material (ionomer molecules). As shown in the left diagram of FIG. 2, in the conventional electrode material, the electrode catalyst particles are covered with the electrolyte material (ionomer), which inhibits the supply of reactive gases such as oxygen to the electrode reaction field. On the other hand, as shown in the right diagram of FIG. 2, in the electrode material of the present invention, only the surface of the oxide carrier is modified with ionomer molecules in a thin film (monolayer), so that the electrode catalyst particles are not covered with ionomer molecules and are exposed, which prevents the gas supply to the electrode reaction field (so-called three-phase interface) from being inhibited. Therefore, it is possible to smoothly supply gases such as oxygen, which serves as fuel, without reducing the catalytic activity of the electrode catalyst particles.

The electrolyte material of the present invention can be suitably used as a cathode electrode material or an anode electrode material for PEFCs. It is also suitable as an electrode material for solid polymer water electrolysis devices.

The electronically conductive oxide, ionomer molecules, and electrode catalyst particles in the electrode material of the present invention are described in more detail below.

In this specification, "PEFC cathode conditions" refers to the conditions at the cathode during normal operation of the PEFC, where the temperature is between room temperature and 150°C and an oxygen-containing gas such as air is supplied (oxidizing atmosphere), and "PEFC anode conditions" refers to the conditions at the anode during normal operation of the PEFC, where the temperature is between room temperature and 150°C and an oxygen-containing fuel gas is supplied (reducing atmosphere). Hereinafter, "at least one of the PEFC cathode conditions and anode conditions" may be referred to as "fuel cell use conditions."

(Electron-conducting oxide)
The electronically conductive oxide in the electrode material of the present invention is preferably an electronically conductive oxide that is stable under at least one of the cathode conditions and the anode conditions of a PEFC.

Specific examples of electronically conductive oxides include those that are primarily made of one selected from tin oxide, molybdenum oxide, niobium oxide, tantalum oxide, titanium oxide, and tungsten oxide. In this specification, "electronically conductive oxide primarily made of" refers to (A) a composite oxide that is made of only a parent oxide, and (B) an oxide that is doped with another element and contains 50 mol % or more of the parent oxide.

Specific examples of doped elements include Sn, Ti, Sb, Nb, Ta, W, In, V, Cr, Mn, and Mo (but these elements are different from the parent oxide). The doped element is an element with a higher valence than the parent oxide. For example, when the parent oxide is tin oxide, an element other than Sn (e.g., Nb) is selected from the above doping element types.

A suitable example of the electronically conductive oxide when the electrode material of the present invention is used for the cathode of a PEFC is an oxide mainly composed of tin oxide. Here, the term "oxide mainly composed of tin oxide" refers to a composite oxide containing 50 mol % or more of the target tin oxide.
As an element, tin (Sn) has an oxide, _SnO2 , which is thermodynamically stable under the cathode conditions of a PEFC and does not undergo oxidative decomposition. In addition, tin oxide has sufficient electronic conductivity and can serve as a support capable of supporting electrode catalyst particles (especially precious metal particles) in a highly dispersed state.
When used for the anode, an oxide mainly composed of tin oxide is not preferable because it is reduced to metallic Sn under the anode conditions of the PEFC.

Among oxides mainly composed of tin oxide, niobium-doped tin oxide doped with 0.1 to 20 mol% niobium (Nb) is particularly preferred because it can form fuel cell electrodes with superior electrode performance.

Another suitable example of the electronically conductive oxide of the electrode material of the present invention is an oxide mainly made of titanium oxide. Titanium (Ti) as an element is thermodynamically stable as an oxide _TiO2 under the anode conditions of PEFC, and reduction does not occur. Furthermore, an oxide mainly made of titanium oxide can be suitably used as a cathode, since the oxide _TiO2 is thermodynamically stable not only under the anode conditions of PEFC, but also under the cathode conditions.

Another preferable example of the electronically conductive oxide of the electrode material of the present invention is an embodiment in which the skin layer is made of a Ti-rich oxide and the core particle is made of a Ti-containing composite oxide.
A Ti-rich oxide layer is preferred as a skin layer because it is stable under both the cathode and anode conditions of a PEFC and has some electronic conductivity.

Examples of the Ti-containing composite oxide of the core particle include Ti-containing perovskite-type oxides and spinel-type oxides, etc. Since both the skin layer and the core particle surface contain Ti, there is good matching of the crystal lattices.
In particular, the Ti-containing composite oxide is preferably a perovskite-type oxide having a composition formula of _ABO3 , in which the A site is at least one element selected from the group consisting of Ca, Sr, Ba, and La, and the B site is Ti.
A particularly suitable Ti-containing perovskite oxide is _SrTiO3 , in which the A site is Sr and the B site is Ti. Note that a part of the Sr in the A site may be substituted with other atoms.

In addition, in the Ti-containing perovskite oxide, a portion of the Ti in the B site may be replaced with at least one element selected from the group consisting of Sb, Nb, Ta, W, Co, V, Cr, Mn, and Mo.

In the electrode material of the present invention, the electronically conductive oxide serves as a support that supports the electrode catalyst particles. In the following description, the electronically conductive oxide may be referred to as the "oxide support of the present invention."

The shape of the electronically conductive oxide is not particularly limited, but is typically in the form of particles or a thin film.
In the case of a particulate electronically conductive oxide, the average particle size is preferably 3 to 200 nm. The "average particle size of the particulate electronically conductive oxide" can be obtained by averaging the particle sizes of any particles (20 particles) examined from an electron microscope image.
In the case of a thin film of an electronically conductive oxide, the average thickness is preferably 2 to 50 nm. The "average thickness of a thin film of an electronically conductive oxide" can be obtained by averaging thicknesses (five points) at arbitrary positions obtained by examining a cross-sectional electron microscope image of the thin film of an electronically conductive oxide in the thickness direction.

In order to increase the amount of the electrode catalyst supported, the electronic conductive oxide preferably has a large surface area within a range where mechanical strength can be maintained. Specifically, the BET specific surface area is preferably 70 ^m2 /g or more, more preferably 100 ^m2 /g or more, and even more preferably 150 ^m2 /g or more.

The amount of electronically conductive oxide in the electrode material of the present invention is appropriately determined within a range that can support a sufficient amount of electrode catalyst particles, since the optimal value varies depending on the particle size (film thickness in the case of a thin film) and physical properties of the electronically conductive oxide, such as surface area, and the manufacturing method of the electronically conductive oxide. If the amount of electronically conductive oxide is too small, a sufficient amount of electrode catalyst particles cannot be supported as an electrode material. If the amount of electronically conductive oxide is too large, the particle size (film thickness in the case of a thin film) of the electronically conductive oxide may become too large, resulting in high electrical resistance of the electrode material.

The electronically conductive oxide can be used as it is as a support for the electrode catalyst, or it can be used in combination with a conductive auxiliary material. In this specification, the term "conductive auxiliary material" refers to a material that is included in the electrode material and has the role of improving the electronic conductivity when the electrode is formed.

When the electronically conductive oxide is used as it is as a carrier for an electrode catalyst, it is sufficient that the size and shape are such that continuous contact is possible and sufficient space is secured to allow smooth diffusion of gases such as hydrogen and oxygen and discharge of water (vapor) in the fuel cell electrode. Specifically, examples include secondary particles with an average particle size of 0.1 to 5 μm (preferably 0.3 to 1 μm) formed by agglomeration of primary particles with an average particle size of 1 to 500 nm (preferably 1 to 100 nm).

It may also be a fused body with a chain-like or bunch-like structure formed by fusing primary particles (average primary particle size of 5 nm to 100 nm) of an electronically conductive oxide.

The electrode material of the present invention may be fixed to a conductive auxiliary material. There are no limitations on the manner in which the electrode material of the present invention and the conductive auxiliary material are fixed, but for example, a preferred embodiment is one in which the electrode material is dispersed and supported on the surface of the conductive auxiliary material. In this case, the average particle size of the electrode material is preferably 1 to 200 nm, and more preferably 1 to 40 nm. The conductive auxiliary material may also be coated with a thin film of electronically conductive oxide. The thin film of electronically conductive oxide can be formed, for example, by coating the conductive auxiliary material with electronically conductive oxide using a dry method such as vapor deposition.

There are no particular limitations on the conductive auxiliary material, but highly conductive carbon-based conductive auxiliary materials or metal-based conductive auxiliary materials can be used.

The carbon-based conductive auxiliary material may be a carbon material of any shape and size, such as granular or fibrous.
For example, fibrous carbon has good mutual contact and excellent electronic conductivity, and forms a conductive path when an electrode is formed, and therefore is one of the preferred carbon-based materials.
The fibrous carbon is a hollow or fibrous carbon material, and specific examples thereof include carbon nanotubes (CNTs) and carbon nanofibers. In the present invention, the term "carbon nanotubes" includes single-walled CNTs, double-walled CNTs, multi-walled CNTs, and mixtures thereof.

Here, in order to achieve both electrical conductivity and gas diffusion within the electrode when it is formed, it is preferable that the fibrous carbon has a diameter of 2 nm to 10 μm and a total length of 0.03 to 500 μm. Note that, among hollow or fibrous carbon materials, the term often refers to those with a diameter of 100 nm or less, such as carbon nanotubes, those with a diameter of about 100 to 1000 nm, such as vapor grown carbon fiber (VGCF), and those with a diameter of 1 μm to 20 μm, such as activated carbon fiber, but since there are no clear regulations regarding the length and names of these carbon materials, they are collectively referred to as fibrous carbon in this specification.

As the fibrous carbon, one having a graphite structure on the surface can be preferably used. Examples of fibrous carbon having a graphite structure on the surface include carbon nanotubes (including single-walled CNT, double-walled CNT, and multi-walled CNT) and vapor-grown carbon fibers (VGCF), and those with high crystallinity and high purity are preferred.

Metal-based conductive auxiliary materials include conductive auxiliary materials made of titanium or titanium alloys, and fibrous materials are particularly preferred. Note that "titanium alloy" means "an alloy containing 40 mol % or more of Ti." There are no particular limitations on the metal to be alloyed with Ti, as long as it does not impair the objective of the present invention.

(Ionomer molecules having ion-conducting functional groups)
The monolayer is formed by introducing an ionomer molecule having an ion-conductive functional group onto the surface of the oxide support. The ionomer molecule has a structure in which an ion-conductive functional group is introduced into a hydrocarbon-based organic molecule. The part having the functional group may be any part, such as the end of the organic molecule, the main chain of the organic molecule, or a branch. The amount of the ion-conductive functional group immobilized can be controlled by controlling the amount and type of the functional group in consideration of the type and surface area of the oxide support, etc., of the ionomer molecule.

There are no particular limitations on the length or number of branches of the fluorocarbon or hydrocarbon monomer in the ionomer molecule as long as the object of the present invention is not impaired, and the selection is made taking into consideration the particle size of the electrode catalyst particles, etc., but typically, a hydrocarbon chain or fluorocarbon chain with 2 to 50 carbon atoms (particularly 2 to 18 carbon atoms) that may have branches is selected.

The ion-conducting functional group to be introduced onto the surface of the oxide support is preferably a proton-conducting functional group, and for example, an ionomer molecule (organic molecule) having at least one functional group selected from the group consisting of a sulfonic acid group, a phosphoric acid group, and a carboxylic acid group is preferably used.
Among these, ionomer molecules having sulfonic acid groups, which are excellent in improving proton conductivity, are preferred.

The portion having the functional group may be any portion, such as the end or branch, of the ionomer molecule (hydrocarbon chain or fluorocarbon chain), as long as it does not impair the objective of the present invention.

A suitable method for forming a monolayer of ionomer molecules on the surface of an oxide support is, for example, a method using a coupling agent having an ion-conductive functional group. This method can form a monolayer in which the coupling agent having an ion-conductive functional group is covalently bonded (siloxane bonded) to the oxide support.

Examples of coupling agents include coupling agents selected from the group consisting of silane coupling agents, aluminate coupling agents, titanate coupling agents, and zirconate coupling agents.

A coupling agent is a molecule that contains functional groups and hydrolyzable groups that are expected to react or interact with organic substances. For example, in the case of a silane coupling agent, it is hydrolyzed by water to become active silanols, which react with hydroxyl groups on the surface of the oxide support and undergo a dehydration condensation reaction to form a strong covalent bond.

When introducing functional groups, for example, if a silane coupling agent having a sulfonic acid group is used, the sulfonic acid group has high acidity and promotes the hydrolysis reaction of the silane coupling agent. For this reason, silane coupling agents having sulfonic acid groups cannot be synthesized in a relatively stable manner. Therefore, a functional group is first introduced onto the surface of the oxide carrier using a silane coupling agent having another functional group that generates a sulfonic acid group, and then the functional group is converted into a sulfonic acid group by a chemical reaction, thereby introducing sulfonic acid groups onto the surface of the oxide carrier.

Specifically, for example, a method may be adopted in which a silane coupling agent having a sulfonate ester group is synthesized, the oxide carrier is surface-treated, and then the sulfonate ester group is converted to a sulfonic acid group by thermal decomposition. A silane coupling agent having a thiol group or a sulfide group may be synthesized, the surface treated, and then exposed to oxidizing conditions to convert the thiol group or the sulfide group to a sulfonic acid group.

The silane coupling agent having a sulfonate ester group is not particularly limited, but an alkoxysilane compound having a propyl sulfonate group or an isopropyl sulfonate group is preferred because the sulfonate ester group has high thermal decomposition properties. Examples of the silane coupling agent include 3-mercaptopropylmethyldimethoxysilane, 3-mercaptopropyltrimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, p-styryltrimethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane, 3-acryloxypropyl Examples of such compounds include trimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-phenyl-3-aminopropyltrimethoxysilane, N-phenyl-3-aminopropyltrimethoxysilane, N-(vinylbenzyl)-2-aminoethyl-3-aminopropyltrimethoxysilane hydrochloride, 3-ureidopropyltriethoxysilane, 3-chloropropyltrimethoxysilane, bis(triethoxysilylpropyl)tetrasulfide, and 3-isocyanatopropyltriethoxysilane.

Either (1) the wet method or (2) the dry method can be used as the surface treatment method with a silane coupling agent. However, (2) the dry method has the disadvantage that it is difficult to achieve a uniform treatment, so (1) the wet method is preferable.

As a method for introducing sulfonic acid groups, it is possible to use a known method for introducing sulfonic acid groups by treating with, for example, sulfuric acid gas, fuming sulfuric acid, sulfuric acid, etc.

(Electrode catalyst particles)
The electrode catalyst particles may be either a precious metal catalyst or a non-precious metal catalyst as long as they have electrochemical catalytic activity for the reduction of oxygen (and the oxidation of hydrogen), but are preferably selected from precious metals such as Pt, Ru, Ir, Pd, Rh, Os, Au, Ag, and alloys containing these precious metals. The term "alloy containing a precious metal" includes "alloys consisting of only the above-mentioned precious metals" and "alloys consisting of the above-mentioned precious metals and other metals and containing 10 mass% or more of the above-mentioned precious metals". The "other metals" to be alloyed with the precious metal are not particularly limited, but suitable examples include Co, Ni, Ti, W, Ta, Nb, and Sn, and one or more of these may be used. In addition, two or more of the above-mentioned precious metals and alloys containing the precious metals in a phase-separated state may be used. The above-mentioned precious metals and alloys containing these precious metals may be referred to as "electrode catalyst metals" hereinafter.

Non-precious metal catalysts include, for example, oxides (TaOx, ZrOx, TiOx, WOx), nitrides (TaNx, ZrNx, TiNx), and oxynitrides (TaOxNy, ZrOxNy, TiOxNy) of Ta, Zr, Ti, and W (where x and y are any numbers).

Among the electrode catalyst metals, when the electrode material of the present invention is used as an electrode material for a polymer electrolyte fuel cell (PEFC), Pt and alloys containing Pt are particularly suitable for use because they have high electrochemical catalytic activity for the reduction of oxygen (and the oxidation of hydrogen) in the temperature range around 80°C, which is the operating temperature of the PEFC.

When the electrode material of the present invention is used as an electrode material for a solid polymer water electrolysis device, Ir, Ir alloys, Pt, and Pt alloys, which have excellent water electrolysis activity, are suitable as electrode catalyst metals. In this case, Ir, Ir alloys, Pt, and Pt alloys may be used as Ir oxides or Pt oxides by heat treatment or the like.

The shape of the electrode catalyst particles is not particularly limited, and any shape similar to that of known electrode catalyst particles can be used. Specific shapes include spherical, elliptical, polyhedral, and core-shell structures. In addition, the structure of the electrode catalyst particles is not limited to crystalline, and may be amorphous, or may be a mixture of crystalline and amorphous.

The smaller the size of the electrode catalyst particles, the greater the effective surface area for the electrochemical reaction to proceed, and therefore the higher the electrochemical catalytic activity tends to be. However, if the size is too small, the electrochemical reaction activity decreases. Therefore, the size of the electrode catalyst particles is 0.3 to 30 nm, more preferably 1 to 10 nm, in terms of average particle size.
In the present invention, the "average particle size of electrode catalyst particles" can be obtained by averaging the particle sizes of electrode catalyst particles (20 particles) examined from an electron microscope image. When calculating the average particle size from an electron microscope image, if the shape of the fine particles is other than spherical, the length in the direction showing the maximum length of the particle is taken as the particle size. In other words, one suitable embodiment of the electrode catalyst particles in the electrode material of the present invention is electrode catalyst particles made of Pt and alloys containing Pt, with an average particle size of 0.3 to 30 nm.

The ionomer molecule is selected so that the thickness of the ionomer molecule monolayer fixed to the oxide support is thinner than the particle size of the electrode catalyst particles supported on the oxide support. The thickness of the ionomer molecule monolayer is usually 10 nm or less (particularly 3 nm or less, 1 nm or less), and the electrode catalyst particles are not covered with the ionomer molecule monolayer.

The amount of electrode catalyst particles supported is determined appropriately taking into consideration the type of catalyst, the shape and size of the oxide support (electronically conductive oxide), and other conditions. If the amount of catalyst supported is too small, the electrode performance will be insufficient, and if it is too large, the electrode catalyst particles may aggregate, resulting in reduced performance.

When the amount of electrode catalyst particles supported is preferably 1 to 60 mass %, more preferably 10 to 50 mass %, of the total weight of the electrode material, excellent catalytic activity per unit mass can be achieved, and the desired electrode reaction activity can be obtained according to the amount supported.

The amount of the electrode catalyst particles supported is usually 10 to 50 mass % relative to the oxide support (electronically conductive oxide). Within this range, excellent catalytic activity per unit mass can be obtained, and a desired electrochemical catalytic activity according to the supported amount can be obtained.
If the amount of the electrode catalyst particles is too small, the electrode reaction activity is insufficient, and if the amount is too large, the electrode catalyst particles are likely to aggregate, resulting in a problem of a decrease in the effective surface area for the electrochemical reaction of oxygen and hydrogen. The amount of the electrode catalyst particles supported can be examined, for example, by inductively coupled plasma emission spectrometry (ICP).

As a method for supporting the electrode catalyst precursor or electrode catalyst particles on the oxide support (electronically conductive oxide), a known metal supporting method can be adopted.
The electrode catalyst precursor may be any that supports the above-described electrode catalyst precursor and is then reduced to become a zero-valent electrode catalyst metal.
Specific examples of the electrode catalyst precursor when the electrode catalyst particles are platinum (Pt) include platinum halides such as platinum chloride, platinum bromide, platinum iodide, etc.; inorganic acid salts of platinum such as chloroplatinic acid, ammonium tetrachloroplatinate, potassium hexachloroplatinate, etc.; organic acid salts of platinum such as platinum acetylacetonate, platinum hexafluoroacetylacetonate, dichloro(1,5-cyclooctadiene)platinum, platinum cyanide, etc. When the electrode catalyst particles are a metal other than Pt, a corresponding precursor may be used.

As a support method, suitable methods that can obtain highly dispersed and small particle size electrode catalyst particles include the noble metal acetylacetonate method, which uses noble metal acetylacetonate, and the colloid method, which uses noble metal colloid.

The method of using a precious metal acetylacetonate as an electrode catalyst precursor (the precious metal acetylacetonate method) involves supporting the electrode catalyst precursor, a precious metal acetylacetonate, on a carrier, and then directly converting the electrode catalyst precursor into electrode catalyst particles. With this method, the precious metal precursor does not contain residual impurities, so improved catalytic activity is expected.

Examples of precious metal acetylacetonates include acetylacetonates of precious metals such as Pt, Ru, Ir, Pd, Rh, Os, Au, and Ag, and one or more of these can be used. The solvent may be any organic solvent capable of dispersing the precious metal acetylacetonate, such as acetone or dichloromethane.

The colloidal method involves dispersing a support in a solution containing colloids of electrode catalyst precursors (especially precious metal colloids), reducing the colloids of the electrode catalyst precursors, and supporting them on the support as electrode catalyst particles. The colloidal method can produce electrode metal particles with a uniform nano-sized particle size distribution without using surfactants or organic solvents.

The electrode catalyst particles or electrode catalyst precursor supported on an oxide support (electronically conductive oxide) may contain non-stoichiometric metal oxides and have low activity as is. Therefore, the electrochemical catalytic action of the metal that becomes the electrode catalyst is activated by heat treatment in an inert atmosphere such as nitrogen or argon, or in a reducing atmosphere containing hydrogen.

The heat treatment conditions are appropriately selected depending on the composition of the electronically conductive oxide and the type of metal or precursor that serves as the electrode catalyst.

We will now explain electrodes formed using the electrode material of the present invention. Electrodes containing the electrode material of the present invention have excellent electrochemical catalytic activity for the reduction of oxygen and the oxidation of hydrogen, and therefore can be used as cathodes and anodes.

The electrode of the present invention may be composed only of the above-mentioned electrode materials, but may also contain components other than the above-mentioned electrode materials as long as the purpose of the present invention is not impaired. Examples of such components include conductive auxiliary materials, electrolyte materials, binders, etc.

An electrode containing the electrode material of the present invention can be suitably used as an electrode for a PEFC. It can also be suitably used as an electrode for a water electrolysis device that uses a polymer electrolyte membrane similar to that of a PEFC.

The membrane electrode assembly of the present invention is a membrane electrode assembly having a solid polymer electrolyte membrane, a cathode bonded to one side of the solid polymer electrolyte membrane, and an anode bonded to the other side of the solid polymer electrolyte membrane, characterized in that at least one of the cathode and anode is the electrode of the present invention.

The membrane electrode assembly (MEA) of the present invention can be suitably used in applications such as polymer electrolyte fuel cells and polymer electrolyte water electrolysis devices. In the membrane electrode assembly of the present invention, the components other than the electrodes of the present invention are similar to those of known membrane electrode assemblies for polymer electrolyte fuel cells or polymer electrolyte water electrolysis devices (see, for example, Japanese Patent No. 6,598,159 and Japanese Patent No. 6,779,470), and therefore detailed description thereof will be omitted.

In the polymer electrolyte fuel cell of the present invention, the components other than the membrane electrode assembly of the present invention are similar to those of known polymer electrolyte fuel cells, and detailed description will be omitted. Typically, a fuel cell stack is formed in which the polymer electrolyte fuel cells (single cells) of the present invention are stacked in a number according to the power generation performance, and the fuel cell is used by assembling other associated devices such as a gas supply device and a cooling device.

In addition, in the solid polymer water electrolysis device of the present invention, the components other than the membrane electrode assembly of the present invention are similar to those of known solid polymer water electrolysis devices, so detailed description will be omitted.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these.

Example 1
The electrode material (ionomer molecule unmodified) of Example 1 described below was prepared according to the method disclosed in Japanese Patent No. 6598159 and Japanese Patent No. 6779470. The calculation and evaluation methods of the amounts of electrode catalyst particles and oxide carriers supported were also similar.
In addition, carbon nanotubes (CNTs) (manufactured by Meijo Nano Carbon Co., Ltd., MW-1, nanotube diameter: 15 nm), which are fibrous carbon having the following physical properties, were used as the conductive auxiliary material.

First, a predetermined amount of the fibrous carbon was added with ultrapure water and stirred with an ultrasonic homogenizer to obtain a dispersion of fibrous carbon. Tin chloride hydrate ( _SnCl2.2H2O ) was added to this dispersion, and _niobium chloride ( _NbCl5 ) was added in a ratio of Sn:Nb=98:2 (molar ratio). The mixture was kept at 50°C with a hot stirrer, and ammonia water was added dropwise with a burette while stirring. After the addition of ammonia water, stirring was continued for 1 hour, and then the dispersion was filtered, washed, and dried, and then heat-treated at 600°C for 2 hours in an air atmosphere to obtain fibrous carbon carrying niobium-doped tin oxide particles.
The fibrous carbon carrying the obtained tin oxide particles was loaded with Pt catalyst particles, which are electrode catalyst particles, by the platinum _{acetylacetonate} method. The amount of Pt precursor (Pt( _C5H7O2 ) ₂ ) was set so that Pt was 17.3 _wt %. The fibrous carbon carrying the tin oxide particles and the Pt precursor were added to an eggplant flask, and acetone was added to dissolve them. Next, while cooling the eggplant flask with ice, the mixture was stirred with an ultrasonic stirrer until all the solvent was evaporated. The obtained powder was reduced in an _N2 atmosphere at 210°C for 3 hours and at 240°C for 3 hours to obtain an electrode material (Pt/Nb- _SnO2 /CNT, ionomer molecule unmodified).

The electrode materials (unmodified with ionomer molecules) consisted of 17.3 wt % electrode catalyst particles (Pt), 57.9 wt % oxide support (Nb- _SnO2 ), and 24.8 wt % fibrous carbon, with the oxide support:fibrous carbon ratio being 70:30 (weight ratio).
Observation of the microstructure of the electrode material (unmodified with ionomer molecules) revealed that tin oxide particles of a particle size were supported on the surface of the fibrous carbon, and that Pt catalyst particles were not present on the surface of the fibrous carbon but were selectively dispersed and supported on the tin oxide particles. Most of the tin oxide particles had a particle size of 40 nm or less, and many particles with a particle size of 20 nm or less were also observed. The particle size of all of the observed Pt catalyst particles was 5 nm or less.

The electrode material (unmodified with ionomer molecules) was stirred at room temperature for 24 hours in a mixed solution of ethanol:water = 1:1 (vol) with diethoxysilane having a propyl chloro group as an ionomer molecule precursor, thereby condensing the hydroxyl groups on the surface of the oxide support (Nb-SnO ₂ ) with the ethoxy groups of the diethoxysilane having a propyl chloro group through hydrolysis, forming a monolayer of the ionomer molecule precursor immobilized by covalent bonds (siloxane bonds) on the surface of the oxide support (see the top diagram in Figure 3).
The obtained product was placed in a dialysis membrane with a molecular weight cutoff of 1000, and low molecular weight impurities were removed using ultrapure water. After dialysis, the product was freeze-dried and heat-treated at 120° C. for 24 hours.
This sample was dispersed in a solution of ethanol:water = 1:3 (vol), sodium sulfate _Na2SO3 was added, and the _mixture was refluxed for 48 hours to convert the chlorine (Cl) at the end of the ionomer molecule precursor into a sulfonic acid group (see the lower diagram in Figure 3). The resulting product was added to a dialysis membrane with a molecular weight cutoff of 1000, and low molecular weight impurities were removed using ultrapure water. After that, concentrated hydrochloric acid was added to the dialysis membrane to protonate the sulfonic acid group, and dialysis was further performed using ultrapure water. The sample solution was dialyzed until it became neutral, and the sample was recovered by freeze-drying to obtain the electrode material (ionomer molecule modified) of Example 1.

The amount of organic functional groups introduced into the electrode material (modified with ionomer molecules) of Example 1 was evaluated by measuring with a TG (thermobalance) and it was confirmed that 0.34 wt% of ionomer molecules were introduced into the electrode material (Pt/Nb- _SnO2 /CNT, unmodified with ionomer molecules). The distribution of the ionomer molecules was evaluated by SEM-EDX and it was confirmed that sulfur elements were distributed on the surface of the electrode material (see Figure 4), confirming that sulfonic acid groups were fixed on the surface.
From the above, it was determined that the electrode material of Example 1 (modified with ionomer molecules) had ionomer molecules having sulfonic acid groups immobilized on the surface of the electrode material (oxide support).

<Electrochemical evaluation (single cell, power generation, durability evaluation)>
An MEA having a cathode formed from the electrode material of Example 1 was used to carry out electrochemical evaluation.

Example 1
First, 46 wt% Pt/C (Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E50E) was dispersed in a predetermined organic solvent containing a Nafion solution on a Nafion membrane (thickness: 50 μm) as an electrolyte membrane to prepare a dispersion solution for forming an anode. The obtained dispersion solution was spray-printed on the Nafion membrane to prepare an anode (electrode catalyst layer) of a predetermined thickness on the Nafion membrane. On the anode (electrode catalyst layer), water-repellent carbon paper (manufactured by Toray Industries, Inc., model number: EC-TP1-060T) was placed as a gas diffusion layer. In addition, the amount of Pt was adjusted to 0.3 mg/cm ² in the formation of the anode.
Next, the electrode material (ionomer molecular modification) of Example 1 was dispersed in a predetermined organic solvent (not containing Nafion solution) to prepare a dispersion solution for forming a cathode. As a binder for fixing the electrode material to the Nafion membrane, Aedotron ^TM C3-NM, C12-PEG-block-PEDOT-block-PEG-C12, PEDOT:PEG (manufactured by Sigma-Aldrich) was added to 5 wt% of the solid component. The obtained dispersion was spray-printed on the opposite side of the Nafion membrane on which the anode was formed, to prepare a cathode (electrode catalyst layer) of a predetermined thickness on the Nafion membrane. Carbon paper was placed on each of the anode and cathode, and pressed under predetermined conditions (0.3 kN, 130 ° C.) to obtain the MEA of Example 1. The amount of Pt in the cathode of the MEA of Example 1 was 0.27 mg / cm ² .

(Reference Example 1)
As a reference example, a conventional MEA was produced that contains a cathode containing unimmobilized Nafion. The MEA of Reference Example 1 was produced in the same manner as in Example 1, except that, instead of the electrode material (ionomer molecule modified) of Example 1, an electrode material not containing immobilized ionomer molecules (Pt/Nb- _SnO2 /CNT, unmodified ionomer molecules) was used as a dispersion solution for forming the cathode, and the dispersion solution was prepared by dispersing the electrode material in a predetermined organic solvent containing a Nafion solution. (Cathode composition: electrode material 92 wt%, Nafion 8 wt%)

Using the MEAs of Example 1 and Reference Example 1, cycle durability tests were performed according to the method described in "III-3-3 Test name: Potential cycle test method 1/2" (start-stop simulated potential cycle) in the Fuel Cell Commercialization Promotion Council's "Proposal of goals, research and development issues and evaluation methods for polymer electrolyte fuel cells (revised in 2011)." Figure 5 shows an explanatory diagram of the start-stop simulated potential cycle.
First, the initial IV characteristics were measured using the MEAs of Example 1 and Reference Example 1. Next, as a start-stop simulated potential cycle, the potential was scanned from 1.0 V to 1.5 V at 0.5 V/sec to perform an accelerated test with a triangular wave, and after a predetermined number of cycles, the IV measurement was performed.
The evaluation devices used were a fuel cell evaluation device (manufactured by Toyo Corporation, model number: PE-8900K) and a potentio/galvanostat (manufactured by Solatron, model number: SI1287).
(Anode Conditions)
Electrode area: ^1cm2
Supply gas type: 100% _H2
Gas supply rate: 77 mL / min
(Cathode conditions)
Electrode area: ^1cm2
Supply gas type: _N2
Gas supply rate: 166 mL / min

Figure 6 shows the evaluation results of the current-voltage (IV) characteristics of a single cell using the MEA of Example 1. As shown in Figure 6, in the MEA of the example using the electrode material of Example 1 as the cathode, the IV characteristics showed improved performance at 2000 cycles compared to 0 cycles. Although the electrode material of Example 1 had an ionomer molecular weight of 0.34 wt% having sulfonic acid groups, the IV characteristics showed performance comparable to that of Reference Example 1 (Nafion 8 wt%). It was revealed that the material developed in this invention can generate electricity with only 0.34 wt% ionomer molecules without using Nafion. In addition, it showed high performance equivalent to that of 2000 cycles up to 5000 cycles.

The electrode material of the present invention is suitable for use as electrodes for fuel cells (particularly electrodes for PEFCs) that require long-term operation, and electrodes for water electrolysis devices.

Claims (10)

The present invention comprises an oxide support made of an electronically conductive oxide, a monolayer made of ionomer molecules having an ion-conductive functional group fixed to a surface of the oxide support, and electrode catalyst particles supported on the surface of the oxide support,
13. An electrode material, comprising: a monolayer formed by immobilizing an ionomer molecule having an ion-conducting functional group on a surface of the oxide support through a covalent bond; The electrode material according to claim 1 , wherein the ionomer molecule having an ion-conducting functional group has a sulfonic acid group. 3. The electrode material according to claim 2 , wherein the ionomer molecule having the ion-conducting functional group has a hydrocarbon chain or a fluorocarbon chain having 2 to 50 carbon atoms which may be branched. 4. The electrode material according to claim 1, wherein the electronically conductive oxide is tin oxide or an electronically conductive oxide mainly composed of tin oxide. 5. The electrode material according to claim 4 , wherein the electronically conductive oxide is niobium-doped tin oxide. 6. The electrode material according to claim 1, wherein the electronically conductive oxide is fixed to a conductive auxiliary material. An electrode comprising the electrode material according to any one of claims 1 to 6 . 8. The electrode according to claim 7 , which is for a polymer electrolyte fuel cell or a polymer electrolyte water electrolysis device. 9. A membrane electrode assembly comprising a solid polymer electrolyte membrane, a cathode bonded to one surface of the solid polymer electrolyte membrane, and an anode bonded to the other surface of the solid polymer electrolyte membrane, wherein at least one of the cathode and the anode is the electrode according to claim 7 or 8 . A polymer electrolyte fuel cell comprising the membrane electrode assembly according to claim 9.