JP5052314B2 - Electrode for polymer electrolyte fuel cell - Google Patents

Description

  The present invention relates to an electrode for a polymer electrolyte fuel cell.

  In various electrochemical devices such as a polymer electrolyte fuel cell and a water electrolysis apparatus, the polymer electrolyte is used in the form of a membrane electrode assembly (MEA) in which a membrane is formed and electrodes are bonded to both sides thereof. . In the polymer electrolyte fuel cell, the electrode generally has a two-layer structure of a diffusion layer and a catalyst layer. The diffusion layer is for supplying reaction gas and electrons to the catalyst layer, and carbon fiber, carbon paper, or the like is used. The catalyst layer is a part that becomes a reaction field for electrode reaction, and is generally composed of a composite of an electrode catalyst and a solid polymer electrolyte.

  Electrocatalysts used in such various electrochemical devices conventionally include fine particles of noble metals such as Pt (such as Pt black), those in which noble metal fine particles such as Pt are supported on a carbonaceous carrier such as carbon black, electrolytes, etc. A noble metal thin film formed on the surface of the film by a method such as plating or sputtering is used.

  However, noble metals such as Pt exhibit high catalytic activity and high catalytic activity stability, but are expensive and limited in terms of resources. For this reason, the electrode catalyst contributes to increase the cost of various electrochemical devices. In particular, since a fuel cell is used in a state where a large number of MEAs are stacked in order to obtain a predetermined output, the amount of electrode catalyst used per fuel cell is also large, which hinders the spread of fuel cells. .

In order to solve this problem, various proposals have heretofore been made.
For example, in Non-Patent Document 1,
(1) Using a thermal decomposition method, vertically aligned carbon nanotubes (CNT) having a density of 0.18 g / cm ² , a diameter of 20 nm, and a length of 40 μm are grown on an Si substrate supporting an Fe catalyst. ,
(2) By repeating injection / H ₂ reduction method using Pt (NO ₂ ) ₂ (NH ₃ ) ₂ solution, 0.09 mg / cm ² (anode) or 0.26-0.52 mg / to support Pt of cm ² (cathode),
(3) After depositing Pt, the surface of Pt / CNT is coated with Nafion (registered trademark) to form a CNT electrode.
(4) The CNT electrode is transferred to the electrolyte membrane by hot pressing.
A method for manufacturing an MEA is disclosed.
In the same document,
(A) MEA (CNT-MEAs) using a Pt / CNT electrode is excellent in gas diffusivity, so that a higher limit current density can be obtained compared to a conventional MEA using a Pt / C electrode.
(B) The proton conduction resistance of the catalyst layer of CNT-MEAs is significantly smaller than that of conventional MEA, and
(C) The total ohmic resistance of the catalyst layer of CNT-MEAs is small under both wet and dry conditions,
Points are listed.

ECS Transactions, 3 (1) 277-284 (2006)

The CNT has a three-dimensional structure in which a graphene sheet corresponding to one layer of graphite (a sheet in which carbon atoms are arranged in a hexagonal network) is rolled into a cylindrical shape. The CNT includes a single-walled CNT made of one cylindrical graphene sheet and a multi-walled CNT in which a plurality of cylindrical graphene sheets are concentrically overlapped. In addition, the tip of the synthesized unprocessed CNT usually has a structure closed with a hemispherical graphite layer called “cap”.
CNTs have a diameter on the order of nm and a length on the order of μm to cm, have an extremely large aspect ratio, and have a feature that the radius of curvature of the tip is as small as several nm to several tens of nm. CNT is mechanically tough, has excellent chemical and thermal stability, and is characterized by being both a metal and a semiconductor depending on the helical structure of the cylindrical portion.

The catalyst layer obtained by vertically aligning such CNTs with respect to the substrate and transferring it to the electrolyte membrane has higher gas diffusivity, proton conductivity and electron conductivity than the catalyst layer produced by the conventional method. Show. Moreover, since the gas diffusibility is high, high performance is exhibited in a high current region.
However, in CNT-MEAs, there is a problem that the performance in a low current region cannot be secured unless the amount of Pt used as a catalyst is increased rather than the conventional electrode. This is considered to be due to the fact that the surface area of CNT, which is a carrier for supporting the catalyst, is small. On the other hand, in order to increase the surface area of the carrier, it is necessary to increase the length (L) of the CNTs or increase the number density (A) of the CNTs. However, there is no example in which these parameters are optimized.

An object of the present invention is to provide an electrode for a polymer electrolyte fuel cell that exhibits high performance not only in a high current region but also in a low current region.
Another problem to be solved by the present invention is to provide a polymer electrolyte fuel cell electrode capable of reducing the amount of Pt used.

In order to solve the above problems, an electrode for a polymer electrolyte fuel cell according to the present invention is:
A fibrous conductive carrier;
A catalyst supported on the surface of the fibrous conductive support;
A solid polymer electrolyte covering the catalyst surface;
The gist is to satisfy the following expressions (1) to (4).
R> 1 nm (1)
L <20000 nm (2)
1-AπR ² > 0.5 (3)
2πRLA> 200 (4)
Where R (nm) is the fiber radius of the fibrous conductive carrier, A (lines / nm ² ) is the number density of fibers per unit electrode area of the fibrous conductive carrier, and L (nm) is the fibrous conductive carrier. It is the fiber length of the sex carrier.

In a polymer electrolyte fuel cell electrode using a fibrous conductive support, the fiber radius (R), the fiber length (L), the porosity of the catalyst layer (1-AπR ² ), and the fibrous conductive support When the surface area (2πRLA) is optimized, high performance is exhibited not only in the high current region but also in the low current region. Further, when the length of the fibrous conductive carrier is set to a certain value or less, mass transfer in the electrode is promoted, so that the amount of Pt used can be reduced.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Electrode for polymer electrolyte fuel cell]
The electrode for a polymer electrolyte fuel cell according to the present invention comprises a fibrous conductive carrier, a catalyst, and a solid polymer electrolyte, and satisfies the following formulas (1) to (4).
R> 1 nm (1)
L <20000 nm (2)
1-AπR ² > 0.5 (3)
2πRLA> 200 (4)
Where R (nm) is the fiber radius of the fibrous conductive carrier, A (lines / nm ² ) is the fiber density per unit electrode area of the fibrous conductive carrier, and L (nm) is the fibrous conductive carrier. The fiber length.

[1.1. Fibrous conductive support]
[1.1.1. Vertical alignment structure]
“Fibrous conductive support” refers to a support made of carbonaceous fibers (carbon nanotubes (CNT), carbon nanofibers (CNF), etc.) and having electron conductivity. In particular, vapor-grown CNT is suitable as a fibrous conductive support because an electrode excellent in gas diffusibility, proton conductivity, and electron conductivity can be obtained.

As will be described later, a fibrous conductive carrier such as CNT supplies a carbon source to the surface of a substrate on which catalyst particles having a predetermined composition and a predetermined particle size are supported at a predetermined density. Obtained by thermal decomposition. When such a vapor phase growth method is used, the fibrous conductive carrier can be grown in a state of being substantially perpendicularly oriented with respect to the substrate surface.
The fibrous conductive support oriented substantially perpendicular to the substrate surface carries a catalyst, is combined with an electrolyte (electrolyte in the catalyst layer), and is then transferred to the electrolyte membrane. When the length of the fibrous conductive carrier is equal to or shorter than the predetermined length, the vertical alignment structure is maintained even after the transfer. However, if the length of the fibrous conductive carrier exceeds the predetermined length, the fibers are transferred during transfer. The conductive support may be compressed in the vertical direction, and the vertical alignment structure may be broken.

[1.1.2. Fiber radius]
The fiber radius R (nm) of the fibrous conductive support preferably satisfies the following formula (1).
R> 1 nm (1)
When the fiber radius R is too small, not only is it difficult to support the catalyst particles, but the electronic conductivity of the fibrous conductive support is lowered. Accordingly, the fiber radius R (nm) is preferably larger than 1 nm.

[1.1.3. Fiber length]
The fiber length L (nm) of the fibrous conductive carrier preferably satisfies the following formula (2).
L <20000 nm (2)
The longer the fiber length L, the longer the distance that the substance moves during the electrode reaction. Therefore, in a high current region where the mass transfer is rate limiting, the current density decreases as the fiber length L increases. On the other hand, in the low current region, a higher current density is obtained as the fiber length becomes longer. However, when the fiber length exceeds a certain value, the effect is saturated. Accordingly, the fiber length L (nm) is preferably smaller than 2 × 10 ⁴ nm.
The fiber length L (nm) particularly preferably satisfies the following formula (2 ′).
L <10000 nm (2 ′)

[1.1.4. Porosity]
The porosity of the fibrous conductive support immediately after being vertically aligned on the substrate surface (that is, before being combined with the electrolyte (electrolyte in the catalyst layer) and before being joined to the electrolyte membrane) is the fiber radius R (nm). ) And the number density A of fibers per unit electrode area (lines / nm ² ) can be expressed as 1-AπR ² . The porosity of the fibrous conductive carrier preferably satisfies the following formula (3).
1-AπR ² > 0.5 (3)
When the porosity of the catalyst layer becomes too small, the diffusion coefficient of mass transfer becomes small, and the characteristics are likely to deteriorate particularly in a high current region where the mass transfer is rate-limiting. Therefore, the porosity is preferably larger than 0.5.
In particular, the porosity preferably satisfies the following formula (3 ′).
1-AπR ² > 0.7 (3 ′)

[1.1.5. Surface area]
The surface area of the fibrous conductive support per unit electrode area is determined by using the fiber radius R (nm), the fiber length L (nm), and the fiber density A (units / nm ² ) per unit electrode area. It can be expressed as 2πRLA. The surface area of the fibrous conductive carrier preferably satisfies the following formula (4).
2πRLA> 200 (4)
The characteristics of the fuel cell in the low current region are correlated with the number of reaction fields (that is, the number of catalysts), and the higher the number of reaction fields, the higher the characteristics are obtained. If the surface area of the fibrous conductive support becomes too small, the number of catalyst particles that can be supported decreases, and the characteristics tend to deteriorate particularly in a low current region. Therefore, the surface area of the fibrous conductive support is preferably larger than 200 (nm ² / nm ² ).
In particular, the surface area preferably satisfies the following formula (4 ′).
2πRLA> 400 (4 ′)

[1.2. catalyst]
The catalyst is supported on the surface of the fibrous conductive support. In the present invention, the composition of the catalyst is not particularly limited, and any material that contributes to the electrode reaction can be used as the catalyst. Specifically, as a catalyst,
(1) noble metals such as platinum, palladium, rhodium, ruthenium, iridium, gold, silver, or alloys containing any two or more thereof,
(2) An alloy of one or more kinds of noble metals and one or more kinds of transition metal elements made of cobalt, nickel, iron, chromium, etc.
and so on.

The particle size of the catalyst particles affects the fuel cell performance. In general, when the particle size of the catalyst particles becomes too small, the catalytic activity is reduced. On the other hand, when the particle size of the catalyst particles becomes too large, the specific surface area of the catalyst decreases, and the utilization factor of the catalyst decreases. Accordingly, it is preferable to select an optimum particle size of the catalyst particles according to the composition of the catalyst particles, required characteristics, and the like.
In addition, the amount of catalyst supported on the surface of the fibrous conductive support also affects the fuel cell performance. In general, if the loading density of catalyst particles is too low, a high output cannot be obtained. In order to obtain a high output, the loading density of the catalyst particles is preferably as large as possible.

[1.3. Solid polymer electrolyte (electrolyte in catalyst layer)]
The solid polymer electrolyte (electrolyte in the catalyst layer) is for exchanging protons between the electrolyte membrane and the catalyst, and is formed so as to cover the catalyst surface. In the present invention, the material of the electrolyte in the catalyst layer is not particularly limited, and various materials can be used. The catalyst layer electrolyte is generally made of the same material as the electrolyte membrane, but may be made of a different material.

That is, the material of the solid polymer electrolyte is a hydrocarbon electrolyte containing a C—H bond in the polymer chain and no C—F bond, and a fluorine electrolyte containing a C—F bond in the polymer chain. Either may be sufficient. Further, the fluorine-based electrolyte may be a partial fluorine-based electrolyte containing both C—H bonds and C—F bonds in the polymer chain, or contains C—F bonds in the polymer chain, and A perfluorinated electrolyte containing no C—H bond may be used.
In addition to the fluorocarbon structure (—CF ₂ —, —CFCl—), the fluorine-based electrolyte includes a chlorocarbon structure (—CCl ₂ —) and other structures (eg, —O—, —S—, —C ( ═O) —, —N (R) —, etc. provided that “R” may be an alkyl group). The molecular structure of the polymer constituting the solid polymer electrolyte membrane is not particularly limited, and may be either linear or branched, or may have a cyclic structure.

  Also, the type of acid group provided in the solid polymer electrolyte is not particularly limited. Examples of the acid group include a sulfonic acid group, a carboxylic acid group, a phosphonic acid group, and a sulfonimide group. The solid polymer electrolyte may contain only one kind of these electrolyte groups, or may contain two or more kinds. Furthermore, these acid groups may be directly bonded to the linear solid polymer compound, or may be bonded to either the main chain or the side chain of the branched solid polymer compound.

Specifically, as the hydrocarbon electrolyte,
(1) Polyamide, polyacetal, polyethylene, polypropylene, acrylic resin, polyester, polysulfone, polyether, etc. in which an acid group such as a sulfonic acid group is introduced into any of the polymer chains, and derivatives thereof (aliphatic hydrocarbons) System electrolyte),
(2) Polystyrene having an acid group such as a sulfonic acid group introduced into any of the polymer chains, polyamide having an aromatic ring, polyamideimide, polyimide, polyester, polysulfone, polyetherimide, polyethersulfone, polycarbonate, and the like, and These derivatives (partial aromatic hydrocarbon electrolytes),
(3) Polyetheretherketone, polyetherketone, polysulfone, polyethersulfone, polyimide, polyetherimide, polyphenylene, polyphenylene ether, polycarbonate, in which an acid group such as a sulfonic acid group is introduced into any of the polymer chains, Polyamide, polyamideimide, polyester, polyphenylene sulfide, etc., and derivatives thereof (fully aromatic hydrocarbon electrolytes),
and so on.

Further, as the partial fluorine-based electrolyte, specifically, a polystyrene-graft-ethylenetetrafluoroethylene copolymer (hereinafter referred to as “PS”) in which an acid group such as a sulfonic acid group is introduced into any of the polymer chains. -G-ETFE "), polystyrene-graft-polytetrafluoroethylene, and derivatives thereof.
Further, as the perfluorinated electrolyte, specifically, Nafion (registered trademark) manufactured by DuPont, Aciplex (registered trademark) manufactured by Asahi Kasei Co., Ltd., Flemion (registered trademark) manufactured by Asahi Glass Co., Ltd., etc. There are derivatives.
Among these, fluorinated electrolytes, particularly perfluorinated electrolytes, have a C—F bond in the polymer chain and are excellent in oxidation resistance. Therefore, if the present invention is applied thereto, A polymer electrolyte fuel cell electrode excellent in oxidation resistance and durability can be obtained.

[1.4. Diffusion layer]
The electrode constituting the MEA usually has a two-layer structure of a catalyst layer and a diffusion layer, but may be constituted only by the catalyst layer. When the polymer electrolyte fuel cell electrode according to the present invention is applied to an electrode having such a two-layer structure, a diffusion layer is further joined on the electrode (catalyst layer) according to the present invention.

  The diffusion layer is for supplying and receiving a reaction gas to the catalyst layer at the same time as transferring electrons to and from the catalyst layer. Generally, carbon paper, carbon cloth, or the like is used for the diffusion layer. In order to increase water repellency, a surface of carbon paper or the like coated with a mixture of water repellent polymer powder such as polytetrafluoroethylene and carbon powder (water repellent layer) is used as a diffusion layer. Also good.

[2. Method for producing electrode for polymer electrolyte fuel cell]
The polymer electrolyte fuel cell electrode according to the present invention can be produced by the following method.

[2.1. Fibrous conductive carrier forming step]
First, a substantially vertically oriented fibrous conductive carrier is formed on a substrate surface made of a suitable material (fibrous conductive carrier forming step). The vertically oriented fibrous conductive carrier can be produced by the following method.
That is, first, a catalyst for synthesizing a fibrous conductive carrier is supported on the substrate surface.
As a catalyst for synthesis,
(1) Fe, Co, Ni, or alloys thereof,
(2) an alloy containing Fe, Co and / or Ni and one or more elements selected from group 4A elements (Ti, Zr, Hf) and group 5A elements (V, Nb, Ta),
and so on.

The substrate is for supporting the fibrous conductive carrier. The fibrous conductive carrier carrying the catalyst particles is transferred to the electrolyte membrane surface as will be described later. Accordingly, the substrate is not particularly limited as long as it can form a fibrous conductive carrier on its surface and can be easily transferred.
Specific examples of the substrate include Si, Si with thermal oxide film, sapphire, magnesia, Si substrate on which various metals, oxides, and nitrides are deposited, mesoporous material, and the like. In particular, the mesoporous material is suitable as a substrate for forming a vertically oriented fibrous conductive support.

  Catalysts for synthesis include organic complexes containing the above-described elements (eg, acetylacetonate, trifluoroacetylacetonate, diisopropoxide bistetramethylheptanedionate) and organic / inorganic acid salts (eg, acetate, Acid salt, sulfate, etc.) are dissolved in an organic solvent, and an alcohol as a reducing agent and an organic acid or organic amine as a protective layer are further added thereto and heated at a predetermined temperature. The heating temperature is preferably 300 to 500 ° C. When the obtained synthesis catalyst is dispersed in an appropriate solvent, applied to the substrate surface, and the solvent is volatilized, the synthesis catalyst can be supported on the substrate surface. At this time, if a mesoporous material is used as the substrate, the synthesis catalyst can be supported on the substrate surface in a highly dispersed manner.

The substrate carrying the synthesis catalyst is placed in a reaction vessel, and the pressure is reduced to a predetermined pressure (for example, 10 ⁻⁵ Torr (1.3 × 10 ⁻³ Pa) or less). Next, the substrate is heated up to the synthesis temperature using a heating device. The synthesis temperature is preferably 500 to 900 ° C. When the substrate reaches the synthesis temperature, the carrier gas and the carbon-containing compound gas (for example, hydrocarbon gas such as methane, ethane, propane, etc.) are mixed at a predetermined flow ratio for several minutes to several hours using the reaction gas supply device. When flowing while adjusting the pressure, the fibrous conductive support can be grown substantially perpendicular to the substrate surface.

[2.2. Hydrophilization treatment process]
Next, the fibrous conductive carrier formed on the substrate surface is subjected to a hydrophilic treatment (hydrophilization treatment step). The hydrophilization treatment step is not always necessary, but if the fibrous conductive support is hydrophilized in advance, there is an advantage that the catalyst can be supported very easily.
Any hydrophilic treatment may be used as long as the surface of the fibrous conductive carrier can be uniformly hydrophilized. Specific examples of the hydrophilic treatment method include UV ozone treatment, water plasma treatment, oxygen plasma treatment, and the like. In particular, the UV ozone treatment is convenient and can be hydrophilized without being affected by the water repellency of the fibrous conductive carrier, and thus is suitable as a hydrophilization treatment method.

[2.3. Immersion process]
Next, if necessary, the surface of the fibrous conductive carrier is hydrophilized, and the substrate on which the fibrous conductive carrier is formed is immersed in a solution in which catalytic metal ions are dissolved in a polar solvent. Thereby, a solution penetrate | invades in the space | gap of a fibrous conductive support | carrier.
Water, ethanol, methanol, 1-propanol, 2-propanol, butanol, ethylene glycol, propylene glycol, or the like can be used as the polar solvent for dissolving the catalytic metal ion. In particular, water is particularly suitable as a solvent because there is no risk of poisoning the catalyst metal.

The noble metal source and the transition metal source only need to be soluble in the polar solvent described above.
Specific examples of the noble metal source include the following. These may be used alone or in combination of two or more.
(1) Hexachloroplatinum (IV) acid hexahydrate, dinitrodiammineplatinum (II), hexaammineplatinum (IV) chloride, tetraammineplatinum (II) chloride, bis (acetylacetonato) platinum (II), etc. Pt compound.
(2) Pd compounds such as palladium sulfate, palladium chloride, palladium nitrate, dinitrodiammine palladium (II), bis (acetylacetonato) palladium (II), and trans-dichlorobis (triphenylphosphine) palladium (II).
(3) Rhodium chloride, rhodium sulfate, rhodium nitrate, rhodium acetate (II), tris (acetylacetonato) rhodium (III), chlorotris (triphenylphosphine) rhodium (I), acetylacetonatodicarbonyl rhodium (I), Rh compounds such as tetracarbonyldi-μ-chlorodirhodium (I).
(4) Ru compounds such as ruthenium chloride, ruthenium nitrate, dodecacarbonyltriruthenium (0), and tris (acetylacetonato) ruthenium (III).
(5) Ir compounds such as hexachloroiridium (IV) acid hexahydrate, dodecacarbonyltetriiridium (0), carbonylchlorobis (triphenylphosphine) iridium (I), tris (acetylacetonato) iridium (III) and the like.
(6) Au compounds such as hexachlorogold (IV) acid hexahydrate and gold gold cyanide.
(7) Ag compounds such as silver nitrate and silver cyanide.

Specific examples of the transition metal source include the following. These may be used alone or in combination of two or more.
(1) Nitrates such as Co (NO ₃ ) ₂ , Ni (NO ₃ ) ₂ , Fe (NO ₃ ) ₂ , and Cr (NO ₃ ) ₂ .
(2) Hydroxides such as Co (OH) ₂ .
(3) Sulfates such as CoSO ₄ , NiSO ₄ , FeSO ₄ , CrSO ₄ .
(4) Chlorides such as CoCl ₂ , NiCl ₂ , FeCl ₂ , and CrCl ₂ .
Among these, nitrates and sulfates are particularly suitable as transition metal sources because they are highly soluble in polar solvents, are not easily oxidized even under anaerobic conditions, and do not contain ions that cause catalyst poisoning.

  In general, if the concentration of the noble metal source and the transition metal source in the solution is too low, the catalyst particle deposition reaction becomes inefficient. On the other hand, if the concentrations of the noble metal source and the transition metal source are too high, it becomes difficult to deposit catalyst particles uniformly and finely. Therefore, it is preferable to select the optimum concentration of the noble metal source and the transition metal source dissolved in the solvent according to the kind of the noble metal source and the transition metal source, the composition of the catalyst particles, and the like. Further, the amount of the solution may be an amount sufficient to immerse the substrate.

[2.4. Reduction process]
Next, the catalytic metal ions injected into the fibrous conductive support are reduced to deposit catalyst particles on the surface of the fibrous conductive support (reduction step).
As a reduction method,
(1) A method of injecting a solution containing catalytic metal ions into a fibrous conductive support formed on a substrate surface, drying the solution, and then performing hydrogen reduction,
(2) A method in which a substrate on which a fibrous conductive support is grown is immersed in a solution containing catalytic metal ions, a reducing agent is added to the solution, and the catalytic metal ions are wet-reduced.
and so on. Any method may be used in the present invention.

  When the catalytic metal ion is reduced with hydrogen, the optimum reducing condition is selected according to the type of the catalytic metal ion. If the fibrous conductive carrier is not hydrophilized, a sufficient amount of catalyst may not be supported by one immersion / reduction. In such a case, it is preferable to repeat the immersion / reduction multiple times. Generally, the greater the number of repetitions of immersion / reduction, the greater the amount of catalyst supported.

On the other hand, when the catalytic metal ion is wet-reduced, an optimum reducing agent is selected according to the type of the catalytic metal ion. Specific examples of the reducing agent include ethanol, methanol, formic acid, sodium borohydride (NaBH ₄ ), hydrazine, ethylene glycol, and propylene glycol. These may be used alone or in combination of two or more.

When the reducing agent has a relatively strong reducing power, the reduction reaction proceeds only by adding the reducing agent to the solution. However, it is not preferable that the reducing agent has too strong reducing power because the probability that the catalyst particles precipitate in the liquid is higher than the surface of the fibrous conductive support.
On the other hand, when a reducing agent having a relatively low reducing power is used, the reduction reaction hardly proceeds only by adding the reducing agent to the solution. Therefore, in such a case, it is preferable to increase the reaction rate of the reduction reaction by increasing the temperature of the solution.
In both cases of using a reducing agent having a relatively strong reducing power and using a reducing agent having a relatively low reducing power, the reduction reaction is performed while convection of catalytic metal ions in the presence of the reducing agent. preferable. When the reduction reaction is performed while convection is performed, the noble metal source and the transition metal source are forcibly supplied toward the porous carbon film, so that the catalyst particles can be efficiently supported on the surface of the fibrous conductive support.
In particular, when a reducing agent having relatively weak reducing power is used and the temperature is raised while convection with a solution containing catalytic metal ions in the presence of the reducing agent, the catalyst particles are uniformly and finely formed on the surface of the fibrous conductive support. It can be supported. Suitable reducing agents for such methods include ethanol, methanol, ethylene glycol, propylene glycol and the like.

The temperature, reaction time, amount of reducing agent used during the reduction reaction, etc. should be optimized according to the type of noble metal source, transition metal source and reducing agent used, the concentration of noble metal source and transition metal source in the solution, etc. select. In general, the catalyst particles can be uniformly and finely supported on the surface of the fibrous conductive support when the reduction reaction proceeds under mild conditions.
In addition, the reduction reaction rate can be controlled by optimizing the amount and timing of use of the reducing agent. Generally, as the amount of the reducing agent used is relatively small, the reduction reaction proceeds more slowly, so that the catalyst particle diameter can be reduced. On the other hand, when the reduction reaction proceeds to some extent and the concentration of catalytic metal ions remaining in the solution becomes low, the reaction rate of the reduction reaction becomes extremely slow. Therefore, a relatively small amount of reducing agent (specifically, 1 to 20% of the total amount of reducing agent used) is added at the beginning of the reaction to promote nucleation of the catalyst particles, When a reducing agent is added, catalyst particles having a small particle size can be produced efficiently.

[2.5. Compounding process]
Next, the surface of the fibrous conductive support carrying the catalyst is covered with a solid polymer electrolyte (electrolyte in the catalyst layer) (compositing step). Compounding with the electrolyte in the catalyst layer is performed by dissolving the solid polymer electrolyte in an appropriate solvent, impregnating the solution in the voids of the fibrous conductive support, and volatilizing the solvent. As a result, the polymer electrolyte fuel cell electrode according to the present invention is obtained.
The electrode for the polymer electrolyte fuel cell thus obtained and the electrolyte membrane or the diffusion layer are superimposed and hot pressed, and when the substrate is peeled off, the electrode (catalyst layer) is transferred to the electrolyte membrane or the diffusion layer. Can do. Since the catalyst for synthesizing the fibrous conductive support remains on the substrate surface even after the electrode transfer, the catalyst for synthesizing hardly affects the catalyst layer of the fuel cell.

[3. Action of electrode for polymer electrolyte fuel cell according to the present invention]
Next, the operation of the polymer electrolyte fuel cell electrode according to the present invention will be described.
In order to carry a predetermined amount of catalyst particles having a predetermined size, the fibrous conductive support needs to have a certain size and surface area. When CNT is used as the fibrous conductive carrier, there is a close relationship between the radius of CNT and the electron conductivity. Furthermore, in order to facilitate mass transfer during the electrode reaction, the shorter the length of the fibrous conductive support, the better the porosity of the electrode (catalyst layer).
Accordingly, in the polymer electrolyte fuel cell electrode using the fibrous conductive support, the fiber radius (R), the fiber length (L), the porosity of the catalyst layer (1-AπR ² ), and the fibrous conductivity When the surface area (2πRLA) of the conductive carrier is optimized, high performance is exhibited not only in the high current region but also in the low current region. Further, when the length of the fibrous conductive carrier is set to a certain value or less, mass transfer in the electrode is promoted, so that the amount of Pt used can be reduced.

(Example 1: Dependence of fiber diameter)
[1. Preparation of sample]
Commercially available CNTs having different radii were dispersed in ethanol, an appropriate amount of a commercially available dinitrodiammine nitric acid solution was added, and reduction was performed with an aqueous NaBH ₄ solution, whereby Pt was supported on the CNT surface.
[2. experimental method]
The CNT surface was observed by TEM.
[3. result]
As a result of TEM observation, Pt was hardly supported on CNTs having a radius of 1 nm. As literature information, it is known that CNTs become semiconductors when the radius is 1 nm or less because the graphene sheet is a single layer. From these facts, it was found that the fiber diameter is suitably larger than 1 nm.

(Example 2: Dependence on fiber length)
[1. Preparation of sample]
As a model experiment, a commercially available Pt / C, a Nafion (registered trademark) solution, water, and ethanol were mixed and spray-coated to produce an electrode having an arbitrary thickness. The obtained electrode was joined to both surfaces of the electrolyte membrane to obtain MEA.
[2. experimental method]
Using the obtained MEA, the current density at 0.8 V (low current characteristics) and the current density at 0.6 V (high current characteristics) were evaluated.
[3. result]
FIG. 1 shows the relationship between electrode thickness and fuel cell characteristics. In general, the low current characteristics increase in proportion to the number of reaction fields (that is, the amount of catalyst). Therefore, as the electrode thickness increases, the amount of catalyst that can be supported on the surface of the carrier increases and the low current characteristics are improved. However, when the number of reaction fields exceeds a certain amount, such an effect is saturated. FIG. 1 shows that the low current characteristics are saturated when the electrode thickness is 2 × 10 ⁴ nm or more.
On the other hand, the high current characteristics decrease as the distance traveled by the substance increases. From FIG. 1, it can be seen that when the electrode thickness is 1 × 10 ⁴ nm or more, the high current characteristics start to decrease, and when the electrode thickness is 2 × 10 ⁴ nm or more, the high current characteristics rapidly decrease.
Although the results obtained in FIG. 1 are model experiments using Pt / C, the electrode performance is determined by the porosity, the number of reaction sites, proton conductivity, and electron conductivity, and the shape of the carrier Does not depend on Therefore, the result of FIG. 1 can also be applied to an electrode using a fibrous conductive carrier.

(Example 3: Dependency of porosity)
[1. Preparation of sample]
As a model experiment, an electrode was prepared according to the same procedure as in Example 2. Subsequently, the carrier carbon was corroded by holding the electrode at a high potential of 1.3 V and changing the holding time. When the carrier carbon is corroded, the voids in the electrode contract and the porosity can be arbitrarily changed.
[2. experimental method]
Using the obtained MEA, the current density (high current characteristics) at 0.6 V was evaluated. The porosity was determined by image processing from SEM observation of the electrode cross section.
[3. result]
FIG. 2 shows the relationship between porosity and high current characteristics. As shown in FIG. 2, when the porosity is larger than 50%, high high current characteristics are exhibited. However, when the porosity is 40% or less, the high current characteristics start to decrease, and when the porosity is 30% or less, the high current characteristics are exhibited. It turns out that falls rapidly. This is because mass transfer becomes the rate-determining electrode reaction in the high current region.
The volume of the electrolyte in the catalyst layer contained in the electrode is generally about 20 vol%. Therefore, from FIG. 2, the porosity (1-AπR ² ) of the fibrous conductive carrier alone before being combined with the electrolyte in the catalyst layer is preferably greater than at least 50%, more preferably more than 60%. More preferably, it turns out that it is more than 70%.

(Example 4: Dependence of surface area)
[1. Preparation of sample]
As a model experiment, electrodes having different thicknesses were produced according to the same procedure as in Example 2.
[2. experimental method]
Using the obtained MEA, the current density (low current characteristics) at 0.8 V was evaluated. The surface area of the catalyst was determined from the charge amount of the hydrogen desorption wave of the electrode.
[3. result]
FIG. 3 shows the relationship between the catalyst surface area and the low current characteristics. From FIG. 3, it can be seen that the low current characteristics increase almost in proportion to the catalyst surface area. This is because there is a correlation between the low current characteristics and the catalyst surface area (the number of reaction fields).
In general, the catalyst is supported on a part of the support surface, and the catalyst does not completely cover the support surface. In this case, from the geometrical arrangement, the support surface area> the catalyst surface area. Therefore, from FIG. 3, in order to obtain a low current characteristic equal to or higher than that of the conventional product, the surface area (2πRLA) of the fibrous conductive support is preferably at least larger than 200, more preferably more than 400. I understand.

FIG. 4 shows a preferable range of the fiber radius R and the fiber number density A when the fiber length L is 1 × 10 ⁴ nm. As shown in FIG. 4, when the fiber radius R and the fiber density A are optimized so as to enter the region sandwiched between ^two solid lines (1-AπR ² > 0.5, 2πRLA> 200), a low current is obtained. An electrode having excellent characteristics and high current characteristics and a relatively small amount of catalyst can be obtained. In particular, when the fiber radius R and the fiber number density A are optimized so as to enter a region (hatched region) sandwiched between ^two broken lines (1-AπR ² > 0.7, 2πRLA> 400), high characteristics and A low-cost electrode can be obtained.

(Example 5, Comparative Example 1)
[1. Preparation of sample]
Using known vertical alignment CNT technology, fibrous conductivity with fiber radius R = 10 nm, fiber length L = 1 × 10 ⁴ nm, fiber number density per unit area A = about 0.001 / nm ² A carrier was prepared. Pt was supported on the obtained carrier and combined with Nafion (registered trademark), and then transferred to the electrolyte membrane to obtain MEA (Example 5).
For comparison, an MEA was produced using the method described in Non-Patent Document 1 (Comparative Example 1).
[2. experimental method]
High current characteristics and low current characteristics were evaluated in the same manner as in Example 1.
[3. result]
FIG. 5 shows the relationship between electrode thickness, low current characteristics, and high current characteristics. FIG. 5 also shows the results of the conventional electrode using Pt / C. From FIG. 5, when the fiber radius R = 1 nm and the fiber length L = 1 × 10 ⁴ nm, when the porosity (1-AπR ² ) and surface area (2πRLA) are optimized, the low current characteristics are Although it is substantially equivalent, it can be seen that the high current characteristics are greatly improved as compared with the conventional electrode and the electrode described in Non-Patent Document 1.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention.

  The polymer electrolyte fuel cell electrode according to the present invention is particularly suitable as an electrode used in a polymer electrolyte fuel cell, but the application of the present invention is not limited to this, and a gas diffusion electrode is used. Various electrochemical devices used and other electrochemical devices (for example, solar cells, lithium batteries, water electrolysis devices, hydrohalic acid electrolysis devices, salt electrolysis devices, oxygen and / or hydrogen concentrators, humidity sensors, gas sensors, It can be used for an anode and / or a cathode of a photocatalyst and the like.

It is a figure which shows the relationship between an electrode thickness, a low current characteristic, and a high current characteristic. It is a figure which shows the relationship between a porosity and a high current characteristic. It is a figure which shows the relationship between a catalyst surface area and a low electric current characteristic. It is a figure which shows the suitable range of the fiber radius R and fiber number density A in case the fiber length L is 1 * 10 < ⁴ > nm. It is a figure which shows the low current characteristic and high current characteristic of MEA obtained in Example 5 and Comparative Example 1, and the conventional MEA using Pt / C.

Claims (5)

A fibrous conductive carrier;
A catalyst supported on the surface of the fibrous conductive support;
A solid polymer electrolyte covering the catalyst surface;
An electrode for a polymer electrolyte fuel cell satisfying the following formulas (1) to (4):
R> 1 nm (1)
L <20000 nm (2)
1-AπR ² > 0.5 (3)
2πRLA> 200 (4)
Where R (nm) is the fiber radius of the fibrous conductive carrier, A (lines / nm ² ) is the number density of fibers per unit electrode area of the fibrous conductive carrier, and L (nm) is the fibrous conductive carrier. It is the fiber length of the sex carrier. The electrode for a polymer electrolyte fuel cell according to claim 1, which satisfies the following expression (2 ').
L <10000 nm (2 ′) The electrode for a polymer electrolyte fuel cell according to claim 1 or 2, satisfying the following expression (3 ').
1-AπR ² > 0.7 (3 ′) The electrode for a polymer electrolyte fuel cell according to any one of claims 1 to 3, which satisfies the following expression (4 ').
2πRLA> 400 (4 ′)   The electrode for a solid polymer fuel cell according to any one of claims 1 to 4, wherein the fibrous conductive carrier is made of vapor-grown CNT.

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