JP2017179408A - Electrode material for water electrolysis and method for producing the same, and electrode for water electrolysis - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode material for water electrolysis that can reduce the amount of use of Ir, is stable even under potential in water electrolysis, and can show sufficient catalytic activity.SOLUTION: An electrode material for water electrolysis comprises a carbon-based conductive aid, an electron conductive oxide carrier carried on the carbon-based conductive aid, and iridium oxide particles that are dispersedly carried on the electron conductive oxide carrier and has an average particle size of 10 nm or less.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an electrode material for water electrolysis suitably used for a polymer electrolyte water electrolysis cell, a method for producing the same, and an electrode for water electrolysis.

A polymer electrolyte fuel cell (PEFC) has already been marketed for home use, and a fuel cell vehicle has also started to be marketed. The most important member for determining the performance in a fuel cell is an electrode material containing an electrode catalyst. The first generation fuel cell vehicle uses a material carrying fine platinum-based fine particles on the surface of carbon black. However, the carbon support is oxidized, and the supported platinum particles are detached, resulting in deterioration of battery performance. It became a problem.
So far, the present inventors have developed an electrode material in which noble metal particles are dispersed in a tin oxide support instead of a carbon-based material (Patent Document 1). Since the electrode material is thermodynamically stable under the operating conditions of the PEFC cathode, the cathode manufactured using the electrode material is not subject to oxidative corrosion. Can withstand potential cycles. Furthermore, in the method for producing a fuel cell electrode material disclosed in Patent Document 2, the present inventors use a carbon nanotube-based material including a vapor-grown carbon fiber as a conductive auxiliary material as a highly conductive path. It has succeeded in improving the overall conductivity of the electrode by improving the defects of the conductive oxide carrier that is less likely to pass electrons compared to carbon-based carrier materials, and obtaining excellent performance as a PEFC electrode (patent) Reference 2).

  On the other hand, a solid polymer type water electrolysis cell using a solid polymer membrane as in the case of PEFC (hereinafter sometimes simply referred to as “water electrolysis cell”) is known. As shown in FIG. 1, in the water electrolysis cell, oxygen is generated on the anode side and hydrogen is generated on the cathode side by electrolysis of water. The water electrolysis reaction theoretically requires a voltage of 1.23 V or higher under standard conditions (25 ° C., 1 atm).

  The water electrolysis cell is used under a higher potential than the fuel cell (about 0.6 V to 1.0 V). When the water electrolysis cell is used particularly for the purpose of storing renewable energy, it is used in a situation where the potential fluctuation is high at a high potential (about 1.5 V to 2.0 V). High durability at a high potential is required compared with the electrode material for PEFC. For example, platinum-based electrocatalysts used in PEFC electrode materials become Pt oxides with low catalytic activity under high potential conditions (1.5 V or higher) in water electrolysis cells, or dissolve and deposit on electrolyte membranes. Therefore, even if diverted as it is to the electrode material for water electrolysis cells, it cannot be used for a long time.

Therefore, iridium (Ir), which is more expensive than Pt, is often used as an oxide as an electrode catalyst used for an electrode material for water electrolysis. Iridium oxide (IrO ₂ ) is stable even at a high potential of 1.5 V to 2.0 V, and has a high catalytic activity for the anode reaction in water electrolysis. For practical use of electrode materials for water electrolysis, it is necessary to reduce the amount of expensive iridium noble metal materials used as much as possible, but there are currently several electrocatalysts for water electrolysis that are generally commercially available and used. In many cases, iridium oxide (IrO ₂ ) powder having a micron diameter is used as it is.

On the other hand, the reduction of the amount of iridium used and the development of a catalyst that can replace iridium are being carried out. For example, Patent Document 3 discloses an electrode material for water electrolysis in which iridium oxide and an inorganic oxide are combined, and an inorganic oxide such as TiO ₂ , SiO ₂ , and Al ₂ O _{3 is} used as the catalyst. It is disclosed that, by combining less than 20% by mass with respect to mass, higher catalytic activity is exhibited than in the case of iridium oxide alone. In this electrode material, in order to ensure electronic conductivity, the amount of the inorganic oxide must be suppressed to less than 20% by mass, and the effect of reducing the amount of iridium oxide used by combining with the inorganic oxide is limited. is there.
Patent Document 4 discloses an electrode material for water electrolysis in which a metal oxide catalyst provided with oxygen defects is supported on a carrier containing an oxide having electron conductivity from Sn, Sb, Nb, Ta and Ti. ing. In the electrode material, since a metal oxide of a non-noble metal is used as an electrode catalyst, no noble metal such as iridium is used, but there is room for improvement in terms of catalytic activity.

Japanese Patent No. 5322110 International Publication No. 2015/141595 Pamphlet International Publication No. 2006/019128 Pamphlet Japanese Patent Laying-Open No. 2015-129347

Thus, there is a demand for the development of an electrode material for water electrolysis that minimizes the amount of noble metal such as iridium used, is stable even at a high potential (1.5 V or more) in water electrolysis, and exhibits sufficient catalytic activity.
Under such circumstances, an object of the present invention is to provide an electrode material for water electrolysis that can reduce the amount of Ir used, is stable even under potential in water electrolysis, and can exhibit sufficient catalytic activity, and a method for producing the same. It is to be.

  As a result of intensive studies to solve the above problems, the present inventor has found that the following inventions meet the above object, and have reached the present invention.

That is, the present invention relates to the following inventions.
<1> A carbon-based conductive auxiliary material, an electron conductive oxide supported on the carbon-based conductive auxiliary material, and iridium oxide particles dispersed and supported on the electron conductive oxide and having an average particle diameter of 10 nm or less. Contains electrode material for water electrolysis.
<2> The electrode material for water electrolysis according to <1>, wherein the carbon-based conductive auxiliary material is a conductive auxiliary material made of fibrous carbon having a graphite structure on the surface.
<3> The electrode material for water electrolysis according to <1> or <2>, wherein the electron conductive oxide carrier is composed of an electron conductive oxide mainly composed of tin oxide.
<4> A method for producing an electrode material for water electrolysis having the following steps.
(1) A step of supporting an electron conductive oxide on a conductive auxiliary material made of fibrous carbon having a graphite structure on the surface. (2) The conductive auxiliary material supporting an electron conductive oxide carrier is converted into an iridium oxide precursor. (3) The step of supporting the iridium oxide precursor on the surface of the electron conductive oxide carrier is carried out in an oxidizing atmosphere under a oxidative atmosphere. A process of heat-treating at a temperature of from ℃ to 500 ℃ and converting to iridium oxide <5> The method for producing an electrode material for water electrolysis according to <4>, wherein the supporting in the step (2) is performed by evaporation to dryness.
<6> The method for producing an electrode material for water electrolysis according to <4> or <5>, wherein the electron conductive oxide carrier is composed of an electron conductive oxide mainly composed of tin oxide.
<7> An electrode for water electrolysis comprising the electrode material for water electrolysis according to any one of <1> to <3> and a proton conductive electrolyte material.

<1a> A membrane / electrode assembly including 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. A membrane electrode assembly, wherein the anode is the electrode for water electrolysis according to <7>.
<2a> A polymer electrolyte water electrolysis cell comprising the membrane electrode assembly according to <1a>.

  According to the present invention, there is provided an electrode material for water electrolysis in which iridium oxide fine particles are highly dispersed and supported on an electron conductive oxide, are stable even under potential in water electrolysis, and can exhibit sufficient catalytic activity. Provided.

It is a conceptual diagram which shows the typical structure of a solid polymer type water electrolysis cell. It is a schematic diagram of the electrode material for water electrolysis of the present invention. It is a cross-sectional schematic diagram of the membrane electrode assembly of this invention. 2 is an XRD profile of an electrode material of Example 1. FIG. 3 is an XRD profile of an electrode material of Reference Example 1. It is a FE-SEM image of an electrode material, (a) is Example 1 and (b) is the electrode material of Reference Example 1. It is a STEM image and EDS mapping of the electrode material of Example 1. 2 is a STEM image (high magnification) of the electrode material of Example 1. It is a STEM image and EDS mapping of the electrode material of the reference example 1. It is an Ir (4f) spectrum of the electrode material by XPS, (a) is Example 1 and (b) is the electrode material of Reference Example 1. 3 is an FE-SEM image of an electrode of Comparative Example 1. It is an evaluation result by the chronoamperometry (CA) of the electrode for water electrolysis of Example 1 and Comparative Example 1.

  Hereinafter, the present invention will be described in detail with reference to examples and the like, but the present invention is not limited to the following examples and the like, and can be arbitrarily modified and implemented without departing from the gist of the present invention. In the present specification, “to” is used as an expression including numerical values or physical quantities before and after.

<1. Electrode material for water electrolysis>
The present invention includes a carbon-based conductive auxiliary material, an electron conductive oxide supported on the carbon-based conductive auxiliary material, and iridium oxide particles having an average particle diameter of 10 nm or less dispersed and supported on the electron conductive oxide. The present invention relates to an electrode material for water electrolysis (hereinafter sometimes referred to as “electrode material of the present invention”).

In the electrode material of the present invention, the electrode catalyst particles (iridium oxide fine particles) supported on the electron conductive oxide hardly contact the conductive auxiliary material which is a carbon-based material, so that the conventional carbon-based support supports the electrode catalyst particles. It is possible to avoid a decrease in electrode performance due to the corrosion of the carbon-based support due to the electrochemical oxidation that occurs at the time. Since the conductive auxiliary material constituting the electrode material of the present invention is a carbon-based conductive auxiliary material having good mutual contact and excellent electronic conductivity, the electrode for the fuel cell is constituted using the electrode material. At this time, the conductive auxiliary materials come into contact with each other to form a low-resistance conductive path, and an electrode having excellent electron conductivity is obtained.
Thus, the electrode material of the present invention has both excellent durability to electrochemical oxidation caused by the electron conductive oxide and excellent electron conductivity caused by the carbon-based conductive auxiliary material. Therefore, the electrode for water electrolysis formed of the electrode material exhibits excellent electrode performance, has high durability, and can continue the water electrolysis reaction for a long period of time.

  Moreover, in the electrode material for water electrolysis of the present invention, since the carbon-based conductive auxiliary material plays a role as the skeleton of the electrode, the particle size of the electron conductive oxide carrier on which the electrode catalyst particles (iridium oxide fine particles) are supported. (Thickness in the case of a thin film) can be reduced. Therefore, in the electrode for water electrolysis formed using the electrode material for water electrolysis of the present invention, the electric resistance due to the electron conductive oxide can be reduced.

  In addition, since the electrical resistance due to the electron conductive oxide can be reduced, the electron conductive oxide (for example, having a high durability but poor electron conductivity and difficult to be practically used in the conventional electrode material for water electrolysis (for example, Titanium oxide etc. can also be used as the electrode material of the present invention.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 2 is a schematic diagram showing a typical configuration of the electrode material of the present invention. As shown in FIG. 1, the electrode material 1 for water electrolysis according to the present invention includes a (carbon-based) conductive auxiliary material 2, particulate electron conductive oxide 3 a supported on the conductive auxiliary material 2, and electron conduction. The electrode catalyst particles 3b are dispersed and supported on the conductive oxide 3a.

  The conductive auxiliary material 2 is a carbon-based conductive auxiliary material preferably made of fibrous carbon whose surface has a graphite structure. In the present specification, the “conductive auxiliary material” means a material that is included in the electrode material for water electrolysis and has a role of improving electron conductivity when the electrode for water electrolysis is formed. The conductive auxiliary material 2 has excellent electron conductivity derived from the carbon-based material, and can carry the electron conductive oxide 3a. Since the water electrolysis electrode material 1 uses such a conductive auxiliary material 2, when the water electrolysis electrode is formed, the adjacent conductive auxiliary material 2 can be continuously contacted, and the water electrolysis electrode inside A space capable of smoothly diffusing gas such as hydrogen and oxygen and discharging water (steam) can be formed.

  As a carbon-based conductive auxiliary material, fibrous carbon whose surface has a graphite structure has good mutual contact and excellent electronic conductivity. Therefore, when a water electrolysis electrode is formed using the water electrolysis electrode material 1, a conductive path is formed. Formed

  Fibrous carbon is a hollow or fibrous carbon material, and specifically includes carbon nanotubes (CNT) and carbon nanofibers. In the present invention, “carbon nanotube” includes single-walled CNT, double-walled CNT, multi-walled CNT, and a mixture thereof.

Here, in order to achieve both electrical conductivity and gas diffusibility in the electrode when the electrode for water electrolysis is formed, the fibrous carbon preferably has a diameter of 2 nm to 10 μm and a total length of 0.03 to 500 μm. is there.
Of the hollow or fibrous carbon materials, as represented by carbon nanotubes, the diameter is 100 nm or less, or the diameter is 100 such as vapor grown carbon fiber (VGCF). The diameter of the carbon material is about 1 to 20 μm, and there is no clear definition about the length and the name of these carbon materials. This is called fibrous carbon.

  In addition, if the surface is fibrous carbon having a graphite structure, it is chemically stable and the surface area is small, so that the electrode catalyst particles are difficult to be supported, and most of the electrode catalyst particles are selectively transferred to the electron conductive oxide. Supported. Therefore, the ratio of the electrode catalyst particles that are in direct contact with the fibrous carbon out of all the electrode catalyst particles is reduced, and the electrochemical oxidation corrosion of the fibrous carbon is suppressed when used as an electrode for water electrolysis. Is done. Examples of the fibrous carbon having a graphite structure on the surface include carbon nanotubes (including both single-walled CNTs, double-walled CNTs, and multi-walled CNTs) and vapor grown carbon fibers (VGCF). High crystallinity and high purity Are preferred.

(Electron conductive oxide)
As the electron conductive oxide constituting the electron conductive oxide 3a, any electron conductive oxide may be used as long as it has sufficient durability and electron conductivity under the anode conditions of the solid polymer water splitting cell.
The anode conditions are the conditions in the anode during normal operation of the solid polymer water splitting cell, the temperature is room temperature to 150 ° C., and the fuel gas containing hydrogen is supplied (reducing atmosphere), The voltage between the anode and the cathode means 1.5V or more and 2.0V or less.

  Specific examples of the electron conductive oxide include an electron conductive oxide mainly composed of one selected from tin oxide, molybdenum oxide, niobium oxide, tantalum oxide, titanium oxide, and tungsten oxide. Here, in the present invention, the “mainly electron-conducting oxide” means (A) an oxide composed only of a base oxide and (B) an oxide doped with other elements, wherein the base oxide is It means that contained at 50 mol% or more.

  Specific examples of the element to be doped include Sn, Ti, Sb, Nb, Ta, W, Co, V, Cr, Mn, and Mo (however, they are elements different from the base oxide). The element to be doped is an element having a higher valence than the base oxide. For example, when the base oxide is titanium oxide, an element other than Ti (for example, Nb) is selected from the above doped seed elements. Is done.

  Among these, the electron conductive oxide 3a is preferably an oxide mainly composed of tin oxide. Here, the “main oxide” refers to an oxide containing 50 mol% or more of the target oxide.

Further, as the electron conductive oxide 3a, an electron conductive oxide having high conductivity such as SrTiO _{3 is} used as a core, and an oxide having excellent durability under the anode conditions of the water electrolysis cell (TiO ₂ or the like) is formed on the surface thereof. It is also possible to use a carrier having a core-shell structure covered with a skin layer (layer thickness: several atomic layers to several tens of atomic layers).

In the electrode material 1 for water electrolysis of the present invention, since the conductive auxiliary material 2 plays a role as a skeleton of the electrode for water electrolysis, the electron conductive oxide 3a having a smaller electron conductivity than the carbon-based material is A smaller particle size is preferable as long as the electrode catalyst particles 3b can be dispersedly supported. The electron conductive oxide 3a may be either primary particles or secondary particles. However, the electron conductive oxide 3a is preferably primary particles. This is because when the electron conductive oxide 3a is a secondary particle, the electric resistance is increased due to the grain boundary resistance between the primary particles constituting the secondary particle.
The electron conductive oxide 3a is preferably a particulate electron conductive oxide having an average particle diameter of 1 to 200 nm, and more preferably a particulate electron conduction having an average particle diameter of 1 to 40 nm which is substantially a primary particle. Oxide. From the viewpoint of conductivity of the electrode material 1 for water electrolysis, part of the conductive auxiliary material 2 is exposed without the particulate electron conductive oxide 3a being densely packed, and the conductive auxiliary material 2 and other conductive materials are exposed. It is preferable that the electron conductive oxide 3a is dispersed and supported to such an extent that direct contact with the auxiliary material 2 is not hindered.

That is, one of the preferred embodiments of the electron conductive oxide in the electrode material for water electrolysis of the present invention is that the particulate electron conductive oxide is such that a part of the surface of the conductive auxiliary material 2 is exposed. In this embodiment, the conductive auxiliary material 2 is supported. The exposed portion of the conductive auxiliary material only needs to be in such a degree that the exposed portions can contact each other. The average particle size of the particulate electron conductive oxide is preferably 1 to 200 nm, and more preferably 1 to 40 nm.
The “average particle diameter of the particulate electron conductive oxide” can be obtained from the average value of the particle diameters of any particulate electron conductive oxide (20 particles) examined from an electron microscope image.

In FIG. 2, the electron conductive oxide 3 a is a particulate electron conductive oxide dispersedly supported on the conductive auxiliary material 2, but is not limited thereto, and the electron conductive oxide 3 a is the conductive auxiliary material 2. As long as it is supported on the surface. For example, the conductive auxiliary material 2 may be covered with a thin-film electron conductive oxide. The thin film electron conductive oxide can be formed, for example, by coating the conductive auxiliary material with the electron conductive oxide by a dry method such as vapor deposition.
From the viewpoint of the conductivity of the electrode material 1 for water electrolysis, the film thickness of the thin-film electron conductive oxide is preferably as thin as possible within the range that can be formed. That is, one of the preferred embodiments of the electron conductive oxide in the electrode material for water electrolysis of the present invention is a thin film electron conductive oxide having an average film thickness of 1 to 50 nm. In this embodiment, a part or all of the thin-film electron conductive oxide is supported so as to cover the conductive auxiliary material. If the electron conductive oxide has an average film thickness of 1 to 50 nm, the electrical resistance due to the electron conductive oxide is not substantially a problem, and therefore, the exposed portions of the conductive auxiliary material do not need to contact each other. The “average film thickness of the thin-film electron conductive oxide” can be obtained from the average value of the thicknesses (5 points) at arbitrary positions examined from the cross-sectional electron microscope image in the thickness direction of the thin-film electron conductive oxide. it can.

  The electron conductive oxide preferably has a large surface area as long as the mechanical strength can be maintained in order to increase the amount of the electrode catalyst supported.

In addition, the amount of the electron-conductive oxide supported varies depending on the physical properties of the electron-conductive oxide such as the particle size (film thickness in the case of a thin film) and surface area, and the manufacturing method of the electron-conductive oxide. It is determined as appropriate as long as a sufficient amount of electrode catalyst particles can be supported.
In the case of tin oxide, it is usually 5 to 95% by weight, preferably 45 to 95% by weight when the total of the conductive auxiliary material and the electron conductive oxide is 100% by weight. If the amount of the electron-conductive oxide supported is too small, a sufficient amount of electrode catalyst particles as an electrode material for water electrolysis cannot be supported. If the amount of the electron conductive oxide supported is too large, the particle size (or film thickness in the case of a thin film) of the electron conductive oxide becomes too large, and the electric resistance of the electrode material for water electrolysis may be increased.

Here, when the electron conductive oxide is an oxide mainly composed of tin oxide, the electrode for water electrolysis of the present invention is preferably used as the anode.
As an element, tin (Sn) is an oxide of SnO _{2 which} is thermodynamically stable and does not undergo oxidative decomposition under the anode conditions of the water electrolysis cell. In addition, tin oxide has sufficient electron conductivity and becomes a carrier capable of supporting electrode catalyst particles (IrO ₂ fine particles) with high dispersion.

  Among oxides mainly composed of tin oxide, niobium-doped tin oxide doped with 0.1 to 20 mol% of niobium (Nb) is particularly preferable in that an electrode for water electrolysis having better electrode performance can be formed.

(Electrocatalyst particles)
The electrode catalyst particles 3b are made of iridium oxide and are dispersed and supported on the electron conductive oxide 3a. The electrode catalyst particles 3b are preferably selectively dispersed and supported on the electron conductive oxide. Here, “selectively dispersed and supported on the electron conductive oxide” means 80% or more, preferably 90% or more, more preferably 95% or more (100%) of all the electrode catalyst particles (number). Means that it is supported on an electron conductive oxide. The ratio of the electrocatalyst particles supported on the electron conductive oxide was determined by selecting any electrocatalyst particles (100 or more) obtained by observing the electrode material for water electrolysis to be evaluated with an electromagnetic microscope. Evaluation can be made by counting the number carried on the oxide and the number carried on the carbon-based conductive auxiliary material.

The iridium oxide constituting the electrode catalyst particles 3b has an excellent electrochemical catalytic activity for water electrolysis (H ₂ O → 2H ⁺ + 1 / 2O ₂ + 2e ⁻ ). The electrode catalyst particles 3b may contain a metal element other than Ir as long as the performance is not impaired. Alternatively, the surface of the metal Ir particle may be oxidized and the surface layer may be an iridium oxide layer.

  The shape of the electrode catalyst particles 3b is not particularly limited, and those having the same shape as known electrode catalyst particles can be used. Specific examples include a spherical shape, an elliptical shape, a polyhedron, and a core-shell structure. The structure of the electrode catalyst particles 3b is not limited to crystals, and may be amorphous or a mixture of crystals and amorphous.

As the size of the electrode catalyst particle 3b is smaller, the effective surface area through which the electrochemical reaction proceeds increases, and therefore the electrochemical catalyst activity tends to increase. However, if the size is too small, the electrochemical reaction activity decreases. Therefore, the size of the electrode catalyst particles 3b is 10 nm or less, preferably 0.5 nm to 5 nm, as an average particle diameter.
The “average particle diameter of the electrode catalyst particles” in the present invention can be obtained from the average value of the particle diameters of the electrode catalyst particles (20 particles) examined from an electron microscope image. When calculating the average particle diameter based on the electron microscope image, if the shape of the fine particles is other than a spherical shape, the length in the direction indicating the maximum length of the particles is taken as the particle diameter.

  The amount of the electrode catalyst particles supported is appropriately determined in consideration of conditions such as the type of catalyst and the size (thickness) of the electron conductive oxide as a carrier. If the amount of the catalyst supported is too small, the electrode performance becomes insufficient, and if it is too large, the electrode catalyst particles may aggregate to deteriorate the performance.

The supported amount of the electrode catalyst particles is preferably 1 to 60% by mass, more preferably 10 to 50% by mass with respect to the total weight of the electrode material for water electrolysis, and the supported amount is excellent per unit mass. It is possible to obtain a desired electrode reaction activity according to the above.
By increasing the amount of electrode catalyst particles supported per unit weight of the electrode material, the thickness of the electrode catalyst layer in the electrode can be reduced. Therefore, gas diffusibility such as hydrogen and oxygen as a whole electrode and water (steam) Improves diffusibility.

  The supported amount of the electrode catalyst particles is preferably 10 to 50% by mass with respect to the electron conductive oxide. If it is such a range, it will be excellent in the catalyst activity per unit mass, and can obtain the desired electrochemical catalyst activity according to the load. When the supported amount is too small, the electrode reaction activity is insufficient, and when it is too large, the electrode catalyst particles are likely to aggregate and the effective surface area is reduced. Note that the amount of electrode catalyst particles supported can be examined by, for example, inductively coupled plasma emission spectrometry (ICP).

<2. Manufacturing method of electrode material for water electrolysis>
The electrode material for water electrolysis of the present invention described above can be suitably manufactured by the manufacturing method described below (hereinafter referred to as “the manufacturing method of the present invention”).
That is, the manufacturing method of the electrode material for water electrolysis of the present invention includes the following steps.
(1) A step of supporting an electron conductive oxide on a conductive auxiliary material made of fibrous carbon having a graphite structure on the surface. (2) The conductive auxiliary material supporting an electron conductive oxide carrier is converted into an iridium oxide precursor. (3) The step of supporting the iridium oxide precursor on the surface of the electron conductive oxide carrier is carried out in an oxidizing atmosphere under a oxidative atmosphere. Heat treatment at a temperature of from ℃ to 500 ℃ to convert to iridium oxide

  Hereafter, each process in the manufacturing method of this invention is demonstrated in detail.

"Process (1)"
Step (1) is a step of supporting an electron conductive oxide on a conductive auxiliary material made of fibrous carbon having a graphite structure on the surface.
Conductive aids and electron conductive oxides are <1. As described above in the description of the fuel cell electrode material of the present invention, detailed description is omitted here. Step (1) of the production method of the present invention is a method suitable for supporting an electron-conductive oxide mainly composed of tin oxide on fibrous carbon having a graphite structure on the surface as an electron-conductive oxide. It is. Since the electron conductive oxide mainly composed of tin oxide is as described above, the description thereof is omitted.

The conductive auxiliary material is preferably fibrous carbon having a graphite structure on the surface. The conductive auxiliary material may oxidize a part of the surface by surface modification. By doing in this way, the supporting property of an electron conductive oxide may improve. The method for modifying the surface of the conductive auxiliary material is not particularly limited, but a method of treating with an inert gas (for example, N ₂ ) containing 0.5 to 30% water (water vapor) at a temperature of 200 to 400 ° C. is exemplified. It is done.

In the production method of the present invention, after supporting an electron conductive oxide on a conductive auxiliary material made of fibrous carbon having a graphite structure, an iridium oxide precursor (IrO ₂ precursor) is supported.
That is, fibrous carbon having a graphite structure on the surface can carry an electron conductive oxide, but has a property that it is difficult to carry an IrO ₂ precursor.
After carrying an electron conductive oxide to conductive auxiliary material, when immersed in a solution containing the electrode catalyst precursor (IrO ₂ precursor), the electrode catalyst precursor (IrO ₂ precursor) is selectively electronically conducting oxide It is converted into IrO ₂ fine particles by heat-treating it under predetermined conditions. Therefore, according to the production method of the present invention, an electrode material for water electrolysis in which most of the electrode catalyst particles (IrO ₂ fine particles) are selectively dispersed and supported on the electron conductive oxide can be obtained.

As a method for supporting the electron conductive oxide, any method can be adopted as long as it can support the electron conductive oxide on the conductive auxiliary material. Among them, the “ammonia precipitation method” in which ammonia is directly reacted with an electron conductive oxide precursor described below and the “homogeneous precipitation method” in which ammonia generated by decomposing an ammonia generating compound such as urea is reacted are suitable. It is.
As will be described in detail later, the homogeneous precipitation method is a kind of ammonia precipitation method in that it uses ammonia as a decomposition product of the ammonia generating compound, but in the present specification, a method of directly using ammonia itself. Only the “ammonia precipitation method” is referred to as “ammonia precipitation method”, and a method of decomposing an ammonia generating compound to produce ammonia is excluded, and the two are distinguished.

The ammonia precipitation method is a method in which an electron conductive oxide generated by directly reacting an electron conductive oxide precursor and ammonia in a solvent is supported on a conductive auxiliary material.
An advantage of this method is that the reaction rate can be controlled by sequentially reacting while dropping the ammonia solution and changing the concentration and dropping speed of the ammonia solution. The precursor of the electron conductive oxide in the ammonia precipitation method is not particularly limited, and sulfate, oxynitrate, oxysulfate, acetate of constituent metal elements (for example, tin) of the electron conductive oxide, Chlorides, ammonium complexes, phosphates, carboxylates and the like can be used. A suitable precursor in the case where the electron conductive oxide is tin oxide is tin chloride (including hydrate).

The solvent may be any solvent that can dissolve the precursor of the electron conductive oxide, and examples thereof include water, lower alcohols such as methanol and ethanol.
Although the detailed reason is not completely clear at present, it is preferable to use anhydrous ethanol as the solvent because the amount of the electron conductive oxide supported on the conductive auxiliary material increases.

Electron-conducting oxides supported on the conductive auxiliary material by the ammonia precipitation method include those that are in an amorphous state. Therefore, by drying and firing this, an electron-conducting oxide with high crystallinity can be obtained. Can do.
The drying method is not particularly limited, and the above-described solvent such as water or ethanol may be evaporated by a method such as heating, decompression, or natural drying. Also, the atmosphere during drying is not particularly limited, and the atmospheric conditions such as an oxidizing atmosphere containing oxygen or an air atmosphere, an inert atmosphere containing nitrogen or argon, a reducing atmosphere containing hydrogen, etc. Although it can be chosen arbitrarily, it is usually an atmospheric atmosphere.

  The electron conductive oxide formed and supported on the conductive auxiliary material by the ammonia precipitation method is 300 to 800 ° C., preferably 400 to 700, in an oxygen-containing oxidizing atmosphere (for example, in an air atmosphere). An electron conductive oxide having high crystallinity and high electron conductivity can be obtained by heat treatment at ° C., particularly preferably 450 to 650 ° C. When the heat treatment temperature is too low, the crystallinity is low and sufficient electron conductivity may not be obtained. When the temperature exceeds 800 ° C., the electron conductive oxide aggregates and the surface area becomes too small. In some cases, the electron conductive oxide may be detached from the conductive auxiliary material.

  The carbon material may oxidize (combust) when it exceeds a high temperature (for example, 500 ° C.) in an oxidizing atmosphere, but the fibrous carbon used as a conductive auxiliary material in the present invention has a graphite structure on the surface. High temperature durability. For this reason, the heat treatment temperature may be determined within the above temperature range so that it does not substantially burn and the electron conductive oxide does not desorb.

The uniform precipitation method is a method in which an electron conductive oxide produced by reacting a precursor of an electron conductive oxide with ammonia generated by decomposing an ammonia generating compound is supported on a conductive auxiliary material. In the uniform precipitation method, ammonia is generated from an ammonia generating compound at a molecular level in the solution, and the reaction occurs uniformly. Therefore, a homogeneous electron conductive oxide precipitate is generated, which becomes a conductive auxiliary material. Supported.
The ammonia generating compound may be any compound that decomposes at a temperature at which uniform precipitation is performed and generates ammonia. When the solvent is water, urea or urea derivatives that decompose at 100 ° C. or lower are used.

  In addition, since the precursor and solvent of the electron conductive oxide in the uniform precipitation method are the same as those in the ammonia precipitation method, description thereof is omitted. In addition, tin chloride (including hydrate) can be cited as a suitable precursor when the electron conductive oxide is tin oxide.

  As a method of decomposing the ammonia generating compound in the uniform precipitation method, a method of directly heating a solution containing the ammonia generating compound using heat conduction (hereinafter sometimes referred to as a uniform precipitation method (heating type)) The microwave heating uniform precipitation method described below is preferable in that a precipitate (electron conductive oxide) having a larger specific surface area can be supported on the conductive auxiliary material.

  The microwave heating uniform precipitation method is a uniform precipitation method in which an electron conductive oxide produced by the reaction of a precursor of an electron conductive oxide and ammonia generated by decomposing an ammonia generating compound is supported on a conductive auxiliary material. In this method, the ammonia generating compound in the solution is decomposed by heating by microwave irradiation. When the electron conductive oxide is mainly composed of tin oxide, a granular electron conductive oxide having a particle size of 10 nm or less (particularly 5 nm or less) can be produced by the microwave heating uniform precipitation method. Depending on the manufacturing conditions, the generated electron conductive oxide can be formed into a thin film, and a structure in which the thin film of the electronic conductive oxide is supported so as to cover a part or all of the conductive auxiliary material is manufactured. can do.

  The microwave heating uniform precipitation method has one advantage in that an electron conductive oxide having a larger surface area and a smaller particle size (film thickness in the case of a thin film) can be obtained than the ammonia precipitation method. Therefore, since the surface area is large, a large amount of electrode catalyst particles can be supported, and since the particle size (film thickness in the case of a thin film) is small, the electrical resistance due to the electron conductive oxide can be reduced.

  Also in the microwave heating uniform precipitation method, heat treatment can be performed after supporting the electron conductive oxide on the conductive auxiliary material. The conditions for the heat treatment are the same as in the ammonia precipitation method.

  The microwave heating uniform precipitation method is also preferable in that the dispersibility of the conductive auxiliary material in the solution can be improved by microwave irradiation.

  The solvent may be any solvent that can dissolve the precursor of the electron conductive oxide, and examples thereof include water, lower alcohols such as methanol and ethanol.

An example of the microwave heating uniform precipitation method will be specifically described in the case where the electron conductive oxide is tin oxide. For example, a tin oxide precursor such as tin chloride (SnCl ₄ .5H ₂ O) and ammonia generation A conductive auxiliary material is added to a solution containing urea, which is a compound, heated to a target temperature in the state of microwave irradiation, and then held for a predetermined time, so that the urea is decomposed in the solution and oxidized by ammonia generated. Tin oxide particles are generated from the tin precursor and are uniformly dispersed and supported on the conductive auxiliary material.
The intensity of the microwave irradiation is appropriately determined in consideration of the amount of the solution, the decomposability of the ammonia generating compound, the dispersibility of the conductive auxiliary material, and the like. The reaction temperature is determined in consideration of various conditions such as the type of the electron conductive oxide and the decomposability of the ammonia generating compound, but it is 90 to 100 ° C. in that a uniform quality electron conductive oxide is formed. Is preferred.

"Process (2)"
In the step (2), the conductive auxiliary material supporting the electron conductive oxide carrier is immersed in a solution containing the iridium oxide precursor, and the iridium oxide precursor is supported on the surface of the electron conductive oxide carrier. It is a process.

It is technically difficult to carry highly dispersed iridium oxide particles while maintaining high activity, so far it has not been possible to carry fine particles of 10 nm or less (particularly 5 nm or less) effectively.
One of the features of the production method of the present invention is that in step (2), an iridium oxide precursor is selected for an electron conductive oxide carrier supported on a conductive auxiliary material made of fibrous carbon having a graphite structure on the surface. In order to carry it, it has been found that a so-called evaporation-drying method in which the solvent is distilled off by immersing in a solution containing an iridium oxide precursor is suitable.

  In addition, the present inventors reported a method for supporting a Pt-based noble metal catalyst precursor, which is reported in Patent Document 2, a method for supporting an iridium oxide precursor on the surface of an electron conductive oxide carrier. Can be

Hereinafter, the case of the evaporation to dryness method will be described.
As the iridium oxide precursor, an inorganic compound or an organic compound containing Ir can be selected. A suitable example is chloroiridate (H ₂ IrCl ₆ .nH ₂ O).

Exemplifying the iridium oxide precursor by the evaporation to dryness method in the step (2), first, the solution containing the iridium oxide precursor is adjusted to a concentration at which the target amount of iridium oxide is supported after the evaporation of the solvent.
Next, the conductive auxiliary material carrying the predetermined amount of the electron conductive oxide obtained in the step (1) is stirred into this solution and the solvent is distilled off, whereby an electrode catalyst precursor (IrO ₂ precursor) is obtained. Is carried.
According to this method, the iridium oxide precursor can be supported on the electron conductive oxide with high dispersion without complicated chemical reaction. In addition, since a strong oxidizing agent or reducing agent is not used in the solution, there is an advantage that deterioration of the electron conductive oxide and the carbon-based conductive auxiliary material can be avoided.

  In addition, one of the advantages is that an aqueous solvent can be used in the evaporation to dryness method. Since water is used as the solvent, the iridium oxide precursor is easily supported on the surface of the electron conductive oxide because the hydrophobic surface is not particularly supported on the graphite carbon material.

  The water solvent may contain other solvent (ethanol or the like) for the purpose of controlling the evaporation rate.

"Process (3)"
The step (3) is a step of converting the iridium oxide precursor supported on the electron conductive oxide carrier in the step (2) into iridium oxide by heat treatment at 300 ° C. or more and 500 ° C. or less in an oxidizing atmosphere. . In the step (2), since the iridium oxide precursor supported on the electron conductive oxide has low activity as it is, it is converted into iridium oxide and activated by heat treatment in an oxidizing atmosphere.
One of the features of the production method of the present invention is that, in the step (3), the heat treatment condition is defined as a temperature range of 300 ° C. or more and 500 ° C. or less in an oxidizing atmosphere, whereby a conductive auxiliary material or an electron conductive oxide carrier is provided. Therefore, it is possible to obtain iridium oxide fine particles having an excellent electrochemical catalytic activity inherent to iridium oxide without degrading the quality. By such a specific supporting method and manufacturing method through the activation method, the IrO ₂ fine particles become fine particles having an average particle diameter of 10 nm or less and have high catalytic activity, so that the supported amount is at least an excellent electrode catalyst.

The heat treatment conditions may vary depending on the type of the electron conductive oxide and the precursor, but highly active IrO ₂ fine particles are obtained, and the influence on the electron conductive oxide and the carbon-based conductive auxiliary material is affected. As a condition that does not exist, the temperature range is 300 ° C. or higher and 500 ° C. or lower (preferably 400 ° C. or higher and 450 ° C. or lower) in an oxidizing atmosphere. If the temperature is too low, highly active IrO ₂ fine particles cannot be obtained. If the temperature is too high, the IrO ₂ fine particles may aggregate, the effective reaction surface area may be small, and the carbon-based conductive additive may be oxidized (burned) and decomposed.
The oxidizing atmosphere is usually an atmospheric atmosphere, but only oxygen or a mixed gas of oxygen and inert gas may be used. Water vapor may be added to the atmosphere as necessary, and a mixed gas of an inert gas and water vapor becomes an oxidizing atmosphere.

<3. Electrode for water electrolysis>
The electrode for water electrolysis of the present invention includes the above-described electrode material for water electrolysis and a proton conductive electrolyte material. In the water electrolysis electrode, the carbon-based conductive auxiliary materials come into contact with each other to form a conductive path.

  With such a configuration, the conductive auxiliary material constituting the electrode material of the present invention described above is fibrous carbon having a long diameter and excellent electronic conductivity. Excellent. Furthermore, since the gap between the long-diameter conductive auxiliary materials can be formed at least to the extent that air permeability is exhibited, it has excellent diffusibility of gases involved in electrode reactions such as hydrogen, oxygen, and water vapor, and proton conductivity. The electrolyte material can be sufficiently retained. Therefore, the electrode for water electrolysis formed with the electrode material exhibits excellent electrode performance, has high durability, and can perform electrolysis of water for a long period of time.

  Below, the electrode for water electrolysis formed using the electrode material for water electrolysis of this invention is demonstrated.

  The water electrolysis electrode of the present invention may be composed only of the above-mentioned water electrolysis electrode material, but is usually a proton conductive electrolyte material (hereinafter referred to as “proton conductive electrolyte material” used for water electrolysis. Or simply “electrolyte material”). The electrolyte material contained in the electrode together with the electrode material for water electrolysis may be the same as or different from the electrolyte material used for the electrolyte membrane for water electrolysis. From the viewpoint of improving the adhesion between the electrode for water electrolysis and the electrolyte membrane, it is preferable to use the same one.

  Examples of the electrolyte material used for the water electrolysis electrode and the electrolyte membrane include a proton conductive electrolyte material. This proton conductive electrolyte material is roughly classified into a fluorine-based electrolyte material that contains fluorine atoms in all or part of the polymer skeleton, and a hydrocarbon-based electrolyte material that does not contain fluorine atoms in the polymer skeleton. Can be used.

  Specific examples of fluorine-based electrolyte materials include Nafion (registered trademark, manufactured by DuPont), Aciplex (registered trademark, manufactured by Asahi Kasei Co., Ltd.), Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.), and the like. Can be mentioned.

  Specific examples of the hydrocarbon electrolyte material include polymers such as polysulfonic acid, polystyrene sulfonic acid, polyaryl ether ketone sulfonic acid, polyphenyl sulfonic acid, polybenzimidazole sulfonic acid, polybenzimidazole phosphonic acid, and polyimide sulfonic acid. In addition, a polymer having a side chain such as an alkyl group can be mentioned as a suitable example.

  The mass ratio of the electrode material for water electrolysis and the electrolyte material mixed with the electrode material for water electrolysis gives good proton conductivity in the electrode formed using these materials, and gas diffusion in the electrode In addition, it may be determined as appropriate so that water vapor can be discharged smoothly. However, when the amount of the electrolyte material mixed with the electrode material for water electrolysis is too large, the proton conductivity is improved, but the gas diffusibility is lowered. Conversely, when the amount of the electrolyte material to be mixed is too small, the gas diffusibility is improved, but the proton conductivity is lowered. Therefore, 10-50 mass% is a suitable range for the mass ratio of the electrolyte material to the electrode material for water electrolysis. When this mass ratio is smaller than 10% by mass, the continuity of the material having proton conductivity is deteriorated, and sufficient proton conductivity as an electrode for water electrolysis cannot be ensured. On the other hand, when it is larger than 50% by mass, the continuity of the electrode material for water electrolysis is deteriorated, and it may not be possible to have sufficient electronic conductivity as an electrode for water electrolysis. Furthermore, the diffusibility of gas (oxygen, hydrogen, water vapor) or water inside the electrode may be reduced.

  The electrode for water electrolysis of the present invention may contain components other than the above-described electrode material for water electrolysis and proton conductive material as long as the object of the present invention is not impaired.

<4. Membrane electrode assembly (MEA)>
The membrane electrode assembly of the present invention comprises a solid polymer electrolyte membrane, a cathode joined to one surface of the solid polymer electrolyte membrane, and an anode joined to the other surface of the solid polymer electrolyte membrane. In the membrane electrode assembly, the anode is the electrode for water electrolysis according to the present invention.

As a preferred embodiment of the present invention, an electrode for water electrolysis including an electrode material using an oxide mainly composed of tin oxide as an electron conductive oxide is used as a membrane electrode joint using the electrode for water electrolysis of the present invention as an anode. Explain the body.
FIG. 3 schematically shows a cross-sectional structure of the membrane electrode assembly according to the embodiment of the present invention. As shown in FIG. 3, the membrane electrode assembly 10 has a structure in which the cathode 4 and the anode 5 are arranged facing the solid polymer electrolyte membrane 6.

  The cathode 4 is composed of an electrode catalyst layer 4a and a gas diffusion layer 4b, and there is no particular limitation on the configuration thereof, and conventionally known electrode catalyst layers and gas diffusion layers can be used as cathodes for water electrolysis cells.

The anode 5 is composed of an electrode catalyst layer 5a and a gas diffusion layer 5b. As described above, the electrode catalyst layer 5a is an electrode for water electrolysis according to the present invention (electroconductive oxide: oxide mainly composed of tin oxide). Therefore, detailed description is omitted.
As the gas diffusion layer 5 b of the anode 5, a conventionally known gas diffusion layer of a water electrolysis cell can be used in the same manner as the gas diffusion layer 4 b described for the cathode 4.

  As the solid polymer electrolyte membrane 6, a known electrolyte membrane for a solid polymer type water electrolysis cell may be used as long as it has proton conductivity and has chemical stability and thermal stability. In FIG. 3, the thickness is shown with emphasis, but in order to reduce the electrical resistance, the thickness of the solid polymer electrolyte membrane 6 is preferably a thin film that does not cause damage.

As the electrolyte material constituting the solid polymer electrolyte membrane 6, a material that does not decompose under the operating conditions of the water electrolysis cell may be used, and conventionally known materials are used as the electrolyte material of the water electrolysis cell. , Fluorine-based electrolyte materials, and hydrocarbon-based electrolyte materials. In particular, an electrolyte membrane formed of a fluorine-based electrolyte material is preferable because of its excellent heat resistance, chemical stability, and the like. Specific examples include Nafion (registered trademark, manufactured by DuPont), Aciplex (registered trademark, manufactured by Asahi Kasei Co., Ltd.), Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.), and the like.
Examples of the hydrocarbon polymer electrolyte material include polymers such as polysulfonic acid, polystyrene sulfonic acid, polyaryl ether ketone sulfonic acid, polyphenyl sulfonic acid, polybenzimidazole sulfonic acid, polybenzimidazole phosphonic acid, and polyimide sulfonic acid. These include polymers having side chains such as alkyl groups. Further, as the electrolyte membrane, an electrolyte membrane made of an inorganic proton conductor such as phosphate or sulfate can also be used.

  The embodiments of the MEA of the present invention have been described above with reference to the drawings. However, these are examples of the MEA of the present invention, and various configurations other than the above are employed as long as the water electrolysis electrode of the present invention is employed as an anode. Can also be adopted.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.

The raw material compounds and conductive auxiliary materials used are as follows.
(Sn precursor)
Tin chloride hydrate (SnCl ₂ · 2H ₂ O) (Kishida Chemical Co., Ltd.)
(Nb precursor)
Niobium chloride (NbCl ₅ ) (Mitsuwa Chemical Co., Ltd.)
(Ir precursor)
Chlorinated iridium acid (H ₂ IrCl ₆ · nH ₂ O) (Wako Pure Chemical Industries, Ltd.)
(Conductive auxiliary material)
Fibrous carbon having the following physical properties (made by Showa Denko KK, vapor grown carbon fiber, VGCF-H (registered trademark)) was used.
Fiber diameter: 150 nm
True density: 2.1 g / cm ³
Specific surface area: 11.4m ² / g
Thermal conductivity: 1200W / (m · K)
Conductivity: 1 × 10 ⁻⁴ Ωcm

1. Production of Electrocatalyst Material <Example 1>
Process (1)
Step (1-1): Production of niobium-doped tin oxide-supported fibrous carbon In Example 1, fibrous carbon carrying niobium-doped tin oxide (Nb-SnO ₂ ) particles was produced by an ammonia precipitation method.
First, ultrapure water was added to the fibrous carbon (0.2519 g), and the mixture was stirred with an ultrasonic homogenizer to obtain a fibrous carbon dispersion. Tin chloride hydrate (SnCl ₂ .2H ₂ O) (0.7698 g) was added to this dispersion, and niobium chloride (NbCl ₅ ) was further added at a ratio of Sn: Nb = 98: 2 (mol ratio). While maintaining at 50 ° C. with a hot stirrer, ammonia water (NH ₃ 28 wt%) was added dropwise with a burette while stirring (5 cc / min). After dropping ammonia water, stirring was continued for 1 hour, and then the dispersion was filtered and washed, and dried at 100 ° C. for 10 hours. After drying, heat treatment was performed at 600 ° C. for 2 hours in an air atmosphere to obtain fibrous carbon carrying the tin oxide particles of Example 1.
Also, using a thermal analyzer (ThermoPlus TG8120, manufactured by Rigaku Corporation), the temperature of the fibrous carbon carrying tin oxide particles was raised to 800 ° C in an air atmosphere, and the mass difference before and after the temperature increase was reduced by weight. As the weight of the fibrous carbon burned, the supporting rate of the tin oxide particles was determined to be 75% by weight.

Step (1-2): Supporting IrO ₂ Precursor by Evaporation Drying Method Fibrous carbon carrying tin oxide particles obtained in step (1) (hereinafter sometimes referred to as “carrier powder”) Then, an IrO ₂ precursor as electrode catalyst particles was supported by an evaporation to dryness method. In the evaporation to dryness method, a predetermined amount of carrier powder, pure water, and IrO ₂ precursor (H ₂ IrCl ₆ · nH ₂ O) are added to an evaporating dish and heated until the liquid evaporates while stirring. An IrO ₂ precursor was supported on the carrier powder.

Process (2)
The electrode material of Example 1 (IrO) was obtained by subjecting the powder obtained in the step (1-2) to heat treatment in an air atmosphere at 440 ° C. for 1 hour (temperature increase rate: 2 ° C./min). ₂ / Nb-SnO ₂ / VGCF). Note that the amount of IrO ₂ supported by the electrode material of Example 1 was 23 wt%. The amount of IrO ₂ supported is a value calculated from the change in the weight of fibrous carbon before and after combustion determined from TG-DTA and the weight ratio (preparation) of niobium-doped tin oxide and fibrous carbon.

<Reference Example 1>
In the step (2), the electrode material (Ir / Nb-SnO) of Reference Example 1 was used in the same manner as in Example 1 except that the heat treatment condition was 150 ° C. for 1 hour in a 5% H ₂ —N ₂ atmosphere. ₂ / VGCF).

2. Evaluation 2-1: Evaluation by XRD The electrode catalyst materials of Example 1 and Reference Example 1 were evaluated by the X-ray diffraction method.
As shown in FIG. 4, in the electrocatalyst material of Example 1, Sn and SnO signals were not confirmed, but only SnO ₂ signals were confirmed. In addition, SnO ₂ in Example 1 was confirmed to be doped with Nb because the signal was shifted as compared with pure SnO ₂ . On the other hand, for IrO ₂ , no clear signal was confirmed by XRD.
On the other hand, as shown in FIG. 5, in the electrode catalyst material of Reference Example 1, a SnO signal was confirmed together with SnO ₂ . For Ir, only the Ir signal was confirmed.

2-2: Evaluation with an electron microscope (1) Evaluation with a scanning electron microscope (FE-SEM) The microstructure of the electrode catalyst material of Example 1 and Reference Example 1 was observed. FIG. 6A shows an FE-SEM image of the electrode catalyst material of Example 1, and FIG. 6B shows an FE-SEM image of the electrode catalyst material of Reference Example 1. The FE-SEM image of the electrode catalyst material of Example 1 shown in FIG. 6 (a), it is confirmed that SnO ₂ is supported on the surface of the fibrous carbon is a conductive auxiliary material, IrO ₂ particles It was not confirmed alone. On the other hand, from FIG. 6B, it was confirmed that SnO ₂ (SnO) was supported on the surface of the fibrous carbon in the electrode catalyst material of Reference Example 1, and Ir particles were also confirmed alone.

(2) Evaluation by Scanning Transmission Electron Microscope (STEM) and EDS Mapping Evaluation of the microstructure in more detail using the scanning transmission electron microscope (STEM) and EDS mapping of the electrode catalyst material of Example 1 and Reference Example 1 went. 7 shows the STEM image and EDS mapping, FIG. 8 shows the STEM image (high magnification), and FIG. 8 shows the STEM image and EDS mapping of the electrode catalyst material of Reference Example 1, respectively.
The electrode catalyst material of Example 1 from 7, the distribution of Ir atoms in the EDS mapping is spread SnO ₂ portions of support powder (fibrous carrying SnO ₂ carbon), STEM images (high magnification shown in Figure 8 ), It was confirmed that IrO ₂ having a diameter of 10 nm or less was supported on the surface of SnO ₂ particles having a diameter of about 10 to several tens of nm. Therefore, it is considered that IrO ₂ exists in a state where the surface is supported on SnO ₂ .
Further, as shown in FIG. 9, it was confirmed that Ir particles having a diameter of 10 nm or less were present on the SnO ₂ (SnO) surface in the electrode catalyst material of Reference Example 1.

2-3: Evaluation by XPS In order to confirm the component composition of the sample surface, evaluation by XPS was performed. FIG. 10 (a) shows the Ir (4f) spectrum of the electrode catalyst material of Example 1, and FIG. 10 (b) shows the Ir (4f) spectrum of the electrode catalyst material of Reference Example 1, respectively.
When comparing FIG. 10A and FIG. 10B, the ratio of the peak near 62 eV and the peak near 65 eV is different, and it can be said that it is IrO ₂ from the surface component in the electrode catalyst material of Example 1. That is, it was confirmed by XPS that IrO ₂ fine particles were present on the surface of the electrode catalyst material of Example 1.

2-4. Electrochemical evaluation (half-cell)
Electrodes for evaluation were prepared using the following electrode catalyst materials, and their performances were compared by a potential step method (chronoamperometry (CA)).
Electrocatalyst material of Example 1 (IrO ₂ / Nb-SnO ₂ / VGCF)
Electrocatalyst material of Comparative Example 1 (commercially available IrO ₂ powder, manufactured by Tokuri Honten)

As an electrode for evaluation, an electrode catalyst material, 2-propanol, and 5% Nafion dispersion were placed on a GC (Glassy Carbon, Hokuto Denko Co., Ltd., HR2-D1-GC5) having a diameter of 5 mm. Applying the mixture in proportion to the target / research and development issues and evaluation method for molecular fuel cells (issued in January 2011) so that the amount of IrO ₂ supported is 17.3 μg / cm ² An electrode was used.
FIG. 11 shows an FE-SEM image of an electrode using the electrode catalyst material of Comparative Example 1 (μm order IrO ₂ powder).

The measurement conditions for CA are as follows.
Measurement: Three-electrode cell (working electrode: electrode catalyst material / GC, counter electrode: Pt, reference electrode: Ag / AgCl)
Electrolyte: 0.1M HClO ₄ (pH: about 1)
Applied voltage: 1.6V
Rotation speed: 1600rpm
Voltage holding time: 30 min
The measurement was performed by changing the voltage stepwise from 0 V to 1.6 V at the start of measurement (at 0 min) and then holding for 30 min.

FIG. 12 shows the CA results of Example 1 and Comparative Example 1. In FIG. 12, the vertical axis represents the current density, and the larger the value, the more water electrolysis reaction proceeds.
As can be seen from FIG. 12, the current density at a constant voltage value (1.6 V) is higher in the electrode of Example 1 on which IrO ₂ fine particles are supported than in the electrode of Comparative Example 1 using IrO ₂ powder. It was confirmed to be high. This result suggests that the electrode made of the electrocatalyst material of the present invention can provide a high-performance electrode even with a small amount of IrO ₂ .

  According to the present invention, it is possible to produce an electrode material for water electrolysis that can reduce the amount of Ir used, is stable even under potential in water electrolysis, and can exhibit sufficient catalytic activity, at a low cost. The surplus is promising industrially because it can be a breakthrough that can fundamentally solve the high-cost problem of the water electrolysis system, which is the core of the system that stores electric power as hydrogen.

DESCRIPTION OF SYMBOLS 1 Electrolytic material for water splitting 2 Conductive auxiliary material 3a (Particulate) electron conductive oxide 3b Electrocatalyst particles 4 Electrode for water splitting cell (cathode)
4a Cathode electrode layer 4b Gas diffusion layer 5 Water splitting cell electrode (anode)
5a Cathode electrode layer 5b Gas diffusion layer 6 Solid polymer electrolyte membrane 10 Membrane electrode assembly (MEA)

Claims (7)

  A carbon-based conductive auxiliary material, an electron conductive oxide carrier supported on the carbon-based conductive auxiliary material, and iridium oxide particles having an average particle diameter of 10 nm or less dispersed and supported on the electron conductive oxide carrier. The electrode material for water electrolysis characterized by the above-mentioned.   The electrode material for water electrolysis according to claim 1, wherein the carbon-based conductive auxiliary material is a conductive auxiliary material made of fibrous carbon having a graphite structure on the surface.   The electrode material for water electrolysis according to claim 1 or 2, wherein the electron conductive oxide support is composed of an electron conductive oxide mainly composed of tin oxide. The manufacturing method of the electrode material for water electrolysis characterized by having the following processes.
(1) A step of supporting an electron conductive oxide carrier on a conductive auxiliary material made of fibrous carbon having a graphite structure on the surface. (2) The conductive auxiliary material supporting the electron conductive oxide carrier is converted into an iridium oxide precursor. (3) iridium oxide precursor supported on the electron conductive oxide support is immersed in an oxidizing atmosphere in an oxidizing atmosphere. Heat treatment at a temperature of from ℃ to 500 ℃ to convert to iridium oxide   The method for producing an electrode material for water electrolysis according to claim 4, wherein the supporting in the step (2) is performed by an evaporation to dryness method.   The method for producing an electrode material for water electrolysis according to claim 4 or 5, wherein the electron conductive oxide carrier is composed of an electron conductive oxide mainly composed of tin oxide.   An electrode for water electrolysis comprising the electrode material for water electrolysis according to any one of claims 1 to 3 and a proton conductive electrolyte material, wherein the conductive auxiliary material is in contact with each other to form a conductive path.