JP2008004453A - Membrane electrode assembly for fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a membrane electrode assembly capable of preventing oxidation corrosion at the time of fuel deficiency of a carbon material which is a catalyst carrier, while holding power generation performance at normal power generation. <P>SOLUTION: The membrane electrode assembly has a fuel electrode on one face side of an electrolyte membrane and an oxidizer electrode on the other face side. In the membrane electrode assembly for a fuel cell, the fuel electrode contains a carbon nanotube containing a water electrolysis catalyst having catalyst activity against water electrolysis, and an electrode catalyst. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a membrane electrode assembly for a fuel cell.

  A fuel cell directly converts chemical energy into electrical energy by supplying fuel and an oxidant to two electrically connected electrodes and causing the fuel to be oxidized electrochemically. Unlike thermal power generation, fuel cells are not subject to the Carnot cycle and thus exhibit high energy conversion efficiency. A fuel cell is usually formed by laminating a plurality of single cells having a basic structure of a membrane / electrode assembly in which an electrolyte membrane is sandwiched between a pair of electrodes. Among them, a solid polymer electrolyte fuel cell using a solid polymer electrolyte membrane as an electrolyte membrane has advantages such as being easy to downsize and operating at a low temperature. It is attracting attention as a power source for the body.

In a solid polymer electrolyte fuel cell using hydrogen as a fuel and air (oxygen) as an oxidant, the reaction of formula (1) proceeds at the fuel electrode (anode) during normal power generation.
2H ₂ → 4H ⁺ + 4e ⁻ (1)
The electrons generated by the equation (1) reach the oxidant electrode (cathode) after working with an external load via an external circuit. The proton generated in the formula (1) moves from the fuel electrode to the oxidant electrode side in the solid polymer electrolyte membrane in a hydrated state with water.

On the other hand, the reaction of the formula (2) proceeds at the oxidant electrode.
4H ⁺ + O ₂ + 4e ⁻ → 2H ₂ O (2)
That is, as a whole battery,
2H ₂ + O ₂ → 2H ₂ O (3)
The reaction proceeds to generate electricity.

However, it is known that if the fuel electrode is deficient in fuel such as hydrogen (hereinafter referred to as fuel shortage) due to some cause, for example, gas channel blockage or flooding, the battery characteristics will deteriorate. Yes. In a single cell in which an abnormality occurs in the fuel supply state and fuel shortage occurs, protons and electrons generated by fuel oxidation are insufficient. In order to replenish these deficient protons and electrons, in the single cell, the water present in the fuel electrode or the water electrolysis of the electrolyte membrane and the oxidant electrode (H ₂ O → 2H ⁺ + 2e ⁻ + 1 / 2O ₂ ) proceeds. At this time, the potential of the fuel electrode of the single cell rises to the electrolysis potential of water, and as a result, the potential of the fuel electrode (anode) and the oxidant electrode (cathode) is reversed.

While the electrolysis of water in the fuel electrode can sufficiently supply protons and electrons, the potential of the fuel electrode is stable, but the electrolysis of water alone does not secure enough protons and electrons. In this case, the potential of the fuel electrode further increases, and the oxidation reaction (C + 2H ₂ O → CO ₂ + 4H ⁺ + 4e) of the carbon material constituting the electrode (for example, carbon particles supporting the electrode catalyst). ^- ) Progresses and replenishes electrons and protons. In this oxidative corrosion of the carbon material, the surface of the carbon material is covered with an oxide film, and as a result, the carbon material is decomposed and lost.

  Surface oxidation or disappearance of the carbon material, which is a conductive material, lowers the conductivity of the cell and causes an increase in resistance due to poor contact. In addition, the surface oxidation or disappearance of the carbon material, which is a water repellent material, reduces the water repellency of the cell and easily causes flooding that hinders the supply of the reaction gas. In addition, due to decomposition or disappearance of the carbon material carrying the metal catalyst particles, the metal catalyst particles are lost or moved, and the effective catalyst surface area is reduced. Thus, due to the oxidative corrosion of the carbon material, the power generation performance of the fuel cell is greatly reduced, and stable power generation performance cannot be expressed.

  Since the oxidation of the carbon material at the fuel electrode as described above is an irreversible reaction, the performance of the fuel cell does not return to the state before the fuel shortage even if the fuel shortage is eliminated. Examples of a technique for solving the problem caused by such a fuel shortage state include those described in Patent Document 1 and Patent Document 2.

Patent Document 1 discloses an anode used in a fuel cell having improved resistance to voltage reversal, wherein the anode is a first catalyst composition for electrochemically oxidizing fuel for the anode. And an anode comprising a second catalyst composition for generating oxygen from water is disclosed.
Patent Document 2 discloses that a fuel electrode of a solid polymer electrolyte fuel cell that promotes a fuel cell reaction that oxidizes fuel introduced through a diffusion layer is in contact with a solid polymer electrolyte membrane and performs the fuel cell reaction. A fuel for a solid polymer electrolyte fuel cell, comprising: at least one reaction layer to be advanced; and at least one water decomposition layer that is in contact with the diffusion layer and electrolyzes water in the fuel electrode The pole is disclosed.

Special table 2003-508877 gazette JP 2004-22503 A JP 2005-141966 A

  However, since the water electrocatalyst having catalytic activity for the electrolysis of water has high affinity with water, when the water electrocatalyst is simply added and mixed in the electrode as in Patent Document 1 and Patent Document 2, the electrode Water stays inside and flooding is likely to occur. As a result, the power generation performance of the fuel cell during normal power generation is reduced.

  Patent Document 3 discloses a catalyst-carrying electrode including a catalyst-carrying conductive member in which a catalyst metal is carried on a conductive carrier and an electrode catalyst layer containing an electrolyte polymer. A catalyst-supporting electrode characterized by containing a conductive carrier and / or an electrolyte polymer containing a water material is disclosed. The technique of Patent Document 3 aims to provide a catalyst-carrying electrode that exhibits high catalytic activity over a long period of time by suppressing corrosion of the catalyst carrier, and is contained in a conductive carrier and / or electrolyte polymer. By releasing the water repellent material over time, it is possible to prevent deterioration of the catalyst layer, which is likely to occur due to retention of water in the catalyst layer. However, corrosion of the carbon material that is a catalyst carrier cannot be sufficiently suppressed only by preventing water retention by the water repellent material.

  The present invention has been accomplished in view of the above circumstances, and is a membrane electrode assembly capable of preventing oxidative corrosion at the time of fuel shortage of a carbon material as a catalyst support while maintaining power generation performance during normal power generation. The purpose is to provide.

A membrane electrode assembly for a fuel cell according to the present invention is a membrane electrode assembly for a fuel cell comprising a fuel electrode on one side of an electrolyte membrane and an oxidant electrode on the other side, wherein the fuel electrode is electrolyzed water. It contains a carbon nanotube encapsulating a water electrocatalyst having catalytic activity and an electrode catalyst.
The membrane electrode assembly for a fuel cell according to the present invention contains water electrocatalysts in carbon nanotubes and suppresses exposure of the highly hydrophilic water electrocatalyst to the fuel electrode, so that water stays in the fuel electrode. Occurrence of so-called flooding can be suppressed. Also, when carbon nanotubes are oxidatively corroded and the water electrocatalyst is released, the electrolysis of water proceeds ahead of the oxidative corrosion reaction of the carbon material due to the catalytic action of the water electrocatalyst. It is possible to suppress oxidative corrosion of the carbon material in the pole, particularly the carbon material that is a support for the electrode catalyst.

The carbon nanotube encapsulating the water electrocatalyst is not particularly limited as long as it is contained in the fuel electrode, and the fuel is contained in the catalyst layer, in the gas diffusion layer, at the interface between the catalyst layer and the gas diffusion layer. Although it may be the entire pole region, it is preferably disposed at the interface with the electrolyte membrane so that the carbon nanotubes are preferentially oxidized and corroded over the conductive particles supporting the electrode catalyst. Specifically, it is preferable that the fuel electrode has a structure including a carbon nanotube layer containing a carbon nanotube containing the water electrocatalyst at a boundary surface with the electrolyte membrane.
In order to more reliably suppress oxidative corrosion of the conductive particles supporting the electrode catalyst, it is preferable to use conductive particles supporting the electrode catalyst that have higher oxidation resistance than the carbon nanotubes.

  The membrane electrode assembly for a fuel cell of the present invention can suppress the occurrence of flooding that tends to occur due to the hydrophilicity of the water electrocatalyst by encapsulating the water electrocatalyst in the carbon nanotube. Therefore, the membrane electrode assembly for a fuel cell of the present invention is less likely to cause a decrease in power generation performance due to the hydrophilicity of the water electrocatalyst. Also, the water electrocatalyst encapsulated in the carbon nanotubes is released to the outside of the carbon nanotubes over time due to the oxidative corrosion of the carbon nanotubes, promotes water electrolysis, and suppresses the oxidative corrosion of the carbon material in the fuel electrode. it can. Therefore, according to the present invention, it is possible to prevent the carbon material that is the catalyst carrier of the fuel electrode from being oxidatively corroded when the fuel is insufficient, while maintaining the power generation performance during normal power generation.

  A membrane electrode assembly for a fuel cell according to the present invention is a membrane electrode assembly for a fuel cell comprising a fuel electrode on one side of an electrolyte membrane and an oxidant electrode on the other side, wherein the fuel electrode is electrolyzed water. It contains a carbon nanotube encapsulating a water electrocatalyst having catalytic activity and an electrode catalyst.

  Hereinafter, the membrane electrode assembly for a fuel cell according to the present invention will be described with reference to FIGS. FIG. 1 is a view showing an embodiment of a fuel cell membrane electrode assembly according to the present invention, and is a cross-sectional view of a single cell comprising the fuel cell membrane electrode assembly according to the present invention. FIG. It is a partial enlarged view.

In FIG. 1, a single cell 100 includes a membrane / electrode assembly 5 in which a fuel electrode (anode) 4a is provided on one surface of an electrolyte membrane 1 and an oxidant electrode (cathode) 4b is provided on the other surface. In the present embodiment, the fuel electrode 4a has a structure in which the carbon nanotube layer 8, the fuel electrode side catalyst layer 2a, and the fuel electrode side gas diffusion layer 3a are laminated in order from the electrolyte membrane side. On the other hand, the oxidant electrode 4b has a structure in which an oxidant electrode side catalyst layer 2b and an oxidant electrode side gas diffusion layer 3b are laminated in order from the electrolyte membrane side.
The catalyst layer 2 of each electrode (fuel electrode, oxidant electrode) contains an electrode catalyst (not shown) having catalytic activity for the electrode reaction, and serves as a field for the electrode reaction. The gas diffusion layer 3 is for increasing the diffusibility of the reaction gas to the catalyst layer 2. The carbon nanotube layer 8 provided only in the fuel electrode is a layer containing carbon nanotubes in which a water electrocatalyst is included.

  In the membrane / electrode assembly of the present invention, the structure of the fuel electrode is not limited to that shown in FIG. 1, and is a structure composed of two layers of a catalyst layer 2a and a gas diffusion layer 3a as shown in FIG. Alternatively, a structure (not shown) including only the catalyst layer may be used, or a structure (not shown) including layers other than the carbon nanotube layer, the catalyst layer, and the gas diffusion layer may be used. Moreover, the structure which consists only of a catalyst layer may be sufficient as an oxidizing agent electrode, and the structure with which layers other than a catalyst layer and a gas diffusion layer are equipped may be sufficient.

The membrane / electrode assembly 5 is sandwiched between the fuel electrode side separator 6 a and the oxidant electrode side separator 6 b to constitute a single cell 100. The separator 6 defines a flow path 7 (7a, 7b) for supplying a reaction gas (fuel gas, oxidant gas) to each electrode 4 and gas-seals between each single cell, and also functions as a current collector. To do. The fuel electrode 4a is supplied with fuel gas (a gas containing hydrogen or generating hydrogen, usually hydrogen gas) from the flow path 7a, and the oxidant electrode 4b is supplied with an oxidant gas (including oxygen or oxygen) from the flow path 7b. (Usually air).
A plurality of single cells 100 are usually stacked and assembled into a fuel cell as a stack.

  The membrane electrode assembly for a fuel cell of the present invention contains, in the fuel electrode, a water electrocatalyst that promotes electrolysis of water in addition to an electrode catalyst having catalytic activity for an electrode reaction. The electrocatalyst is characterized in that it is contained in a carbon nanotube.

  In the fuel shortage state, in the fuel electrode, in order to compensate for the shortage of protons and electrons, the electrolysis of water and the oxidation reaction of the carbon material proceed. When the oxidation reaction of the carbon particles as the catalyst carrier proceeds, the conductivity and water repellency of the electrode are lowered, and further, the catalytic activity is lowered due to peeling or movement of the electrode catalyst. Such a decrease in power generation performance due to the oxidation reaction of the carbon particles will not be restored even after the fuel shortage state is resolved. Therefore, it is important to suppress the oxidative corrosion of the catalyst carrier carbon particles in such a fuel shortage state in order to improve the durability of the fuel cell.

Therefore, in order to prevent oxidative corrosion of the carbon particles of the catalyst carrier when the fuel is absent, a water electrocatalyst has been conventionally used together with the electrode catalyst. The presence of a water electrocatalyst in the fuel electrode promotes the electrolysis of water, and the electrolysis of water replenishes more protons and electrons. This is because the progress can be suppressed.
However, since the water electrocatalyst has a high affinity with water, there is a problem that water tends to stay in the electrode, that is, flooding is likely to occur. Therefore, even if the water electrocatalyst is simply contained in the electrode, the oxidative corrosion of the carbon material at the time of fuel shortage can be suppressed, but the power generation performance is reduced due to flooding during normal power generation.

  On the other hand, the fuel cell of the present invention includes a water electrocatalyst having a high affinity for water by encapsulating the water electrocatalyst in a carbon nanotube having a lower hydrophilicity than the water electrocatalyst (see FIG. 3). Exposure can be suppressed and flooding can be prevented. The water electrocatalyst encapsulated in the carbon nanotubes is released into the fuel electrode when the oxidative corrosion of the carbon nanotubes proceeds, that is, in a fuel shortage state (see 2A → 2B in FIG. 2). Promotes water electrolysis. As a result, the progress of oxidative corrosion of carbon materials such as carbon particles carrying the electrode catalyst is prevented.

  As described above, in the fuel cell membrane electrode assembly of the present invention, when it is not necessary to promote the electrolysis of water, the water electrocatalyst is included in the carbon nanotube (FIG. 2A), and the electrolysis of water is performed. When it needs to be promoted, it is released from the carbon nanotubes (FIG. 2B). Therefore, in the fuel electrode of the membrane electrode assembly of the present invention, although it contains a water electrocatalyst, it is difficult for the power generation performance to be reduced by flooding during normal power generation. Oxidative corrosion of the carbon material can be suppressed. Moreover, it is also possible to reduce the usage-amount of a water electrolysis catalyst compared with the electrode which contained the water electrocatalyst conventionally in the electrode.

The fuel cell membrane electrode assembly of the present invention will be described in detail below.
The electrode catalyst is not particularly limited as long as it has catalytic activity for the electrode reaction, and those generally used as an electrode catalyst for a fuel electrode can be used. Usually, metals such as platinum, ruthenium, iridium, rhodium, palladium, osnium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum, or alloys thereof can be used. Preferred are platinum alloys such as platinum and platinum-ruthenium alloys. The amount of the electrode catalyst per unit area 0.02 to 10 mg / cm ² of electrode, particularly preferably a 0.05 to 1 mg / cm ^2.

The electrode catalyst is usually supported on conductive particles so that electrons can be smoothly exchanged in the electrode reaction in the electrode catalyst, and in order to ensure the dispersibility of the electrode catalyst in the electrode. Examples of the conductive particles include carbon particles such as carbon black, metal particles such as titanium oxide (TiO ₂ ), other metal oxides, and metal carbides. The conductive particles are not limited to a spherical shape, and include particles having a relatively large aspect ratio such as a fibrous shape.

From the viewpoint of more reliably suppressing the oxidative corrosion of the conductive particles as the catalyst carrier when the fuel is absent, the conductive particles preferably have higher oxidation resistance than the carbon nanotubes. Specifically, carbon particles excellent in oxidation resistance, such as graphitized carbon particles subjected to high-temperature heat treatment (about 400 to 2000 ° C.) and carbon particles whose surfaces are fluorinated, titanium oxide, oxidation of other metals Metal particles excellent in oxidation resistance such as materials are preferred.
By using conductive particles with higher oxidation resistance compared to carbon nanotubes encapsulating a water electrocatalyst, the carbon nanotubes are sacrificed by oxidative corrosion, and oxidative corrosion of conductive particles carrying an electrode catalyst. Can be prevented. Since the carbon nanotubes are preferentially oxidized and corroded, the water electrocatalyst is released and the electrolysis of water is promoted, and further, the oxidative corrosion of the conductive particles is prevented.

  Here, the water electrocatalyst has high catalytic activity for electrolysis of water as compared with the electrode catalyst contained in the fuel electrode together with the water electrocatalyst. The degree of catalytic activity for water electrolysis can be defined by the water electrolysis current value in an aqueous solution, and the water electrocatalyst has a larger water electrolysis current value of electrode potential in the aqueous solution than the electrode catalyst. Is.

As described above, since platinum and platinum alloys are preferably used as the electrode catalyst, water electrocatalysts usually have higher catalytic activity for water electrolysis than platinum or platinum alloys. Used. Examples of such a water electrolysis catalyst include RuO ₂ , IrO ₂ , Pt, TiO ₂ , and solid solutions and alloys containing two or more of these. Among these, from the viewpoint of water splitting activity and electrochemical stability, a solid solution of RuO ₂ , IrO ₂ , RuO ₂ —IrO ₂ and a Ru—Ir alloy are preferably used.
The water electrocatalyst preferably has a primary particle size of about 0.1 to 10 nm from the viewpoint of reaction surface area and stability.

Moreover, the amount of the water electrocatalyst encapsulated in the carbon nanotube is not particularly limited, but it is preferably about 1 to 50%, particularly about 20 to 50% by weight ratio with respect to the carbon nanotube. The amount of carbon nanotubes, the fuel electrode per unit area 0.01 to 10 mg / cm ^2, particularly preferably a 0.2~5mg / cm ^2. That is, the amount of water electrolysis catalyst, the fuel electrode per unit area 0.0001~5mg / cm ^2, particularly preferably a 0.04~2.5mg / cm ^2.

The carbon nanotubes encapsulating the water electrocatalyst are not particularly limited in tube diameter, tube length, etc., but the tube inner diameter is 5 to 300 nm in terms of the particle diameter of the encapsulated water electrocatalyst and the amount of encapsulated water electrocatalyst. In particular, it is preferably 5 to 100 nm, more preferably 10 to 50 nm, and the tube length is preferably 0.01 to 1 μm, particularly preferably 0.01 to 0.5 μm. The specific surface area of the carbon nanotubes is preferably ²⁰ to 1000 cm ² / g, and it is desirable that the specific surface area be larger than the carbon particles supporting the electrode catalyst.
The structure of the carbon nanotube may be a single-walled carbon nanotube obtained by rolling a single graphene sheet, or may be a multi-walled carbon nanotube in which a plurality of graphene sheets are stacked in a nested manner.

  Examples of the method of encapsulating the water electrocatalyst in the hollow of the carbon nanotube include a method of heat treating the water electrocatalyst and the carbon nanotube at 400 to 500 ° C. for 1 to 48 hours under vacuum.

The carbon nanotube encapsulating the water electrocatalyst (hereinafter sometimes referred to as the water electrocatalyst encapsulating carbon nanotube) is not particularly limited as long as it is within the fuel electrode, and may be contained within the catalyst layer or the gas diffusion layer. The carbon nanotube may be disposed at the interface with the electrolyte membrane so that the carbon nanotube is preferentially oxidized and corroded over the conductive particles supporting the electrode catalyst, although it may be at the interface between the catalyst layer and the gas diffusion layer or the entire fuel electrode. It is preferable. When the carbon nanotube is disposed at a position close to the electrolyte membrane, the distance between the carbon nanotube and the oxidizer electrode is reduced. As a result, protons (H ⁺ ) generated by the decomposition of the carbon nanotubes easily move to the oxidant electrode side, so that the decomposition reaction of the carbon nanotubes easily proceeds and the carbon nanotubes are preferentially oxidized and corroded.
As a form of disposing the water electrocatalyst-encapsulated carbon nanotubes at the interface between the electrolyte membrane and the fuel electrode, a form in which the water electrocatalyst-encapsulated carbon nanotubes are mixed in the catalyst layer in contact with the electrolyte membrane may be used. Since the catalyst-encapsulated carbon nanotubes can be arranged in a concentrated manner, it is preferable to provide a carbon nanotube layer containing carbon nanotubes enclosing a water electrocatalyst at the interface between the electrolyte membrane and the fuel electrode, as shown in FIGS.

The carbon nanotube layer may be composed of only carbon nanotubes enclosing a water electrocatalyst, or water repellent such as polytetrafluoroethylene (PTFE) as necessary in addition to carbon nanotubes encapsulating a water electrocatalyst. It may contain a material, an electrolyte resin, or the like.
In general, the carbon nanotube layer preferably has a thickness of about 0.1 to 1 μm from the viewpoint of securing the power generation characteristics of the fuel cell.

  The method for forming the carbon nanotube layer is not particularly limited. Examples of a method for forming a carbon nanotube layer composed only of carbon nanotubes enclosing a water electrocatalyst include, for example, an ink in which carbon nanotubes encapsulating a water electrocatalyst in alcohols are dissolved or dispersed in an ink jet method, a spray method, a doctor The method of apply | coating to an electrolyte membrane by the blade method etc. is mentioned.

  In addition, as a method of forming a carbon nanotube layer containing a carbon nanotube encapsulating a water electrocatalyst and a water repellent material and / or an electrolyte resin, for example, the water electrocatalyst is placed in a suitable solvent such as alcohols. Examples thereof include a method in which an ink in which the encapsulated carbon nanotubes and the water repellent material and / or the electrolyte resin are dissolved or dispersed is applied to the electrolyte membrane by an inkjet method, a spray method, a doctor blade method, or the like.

When carbon nanotubes containing water electrocatalysts are contained in the catalyst layer, the water electrocatalyst encapsulating carbon nanotubes are mixed in the catalyst ink containing the conductive particles carrying the electrode catalyst, and the resulting water electrocatalyst containing A catalyst layer may be formed using a catalyst ink according to a general method.
In addition to the conductive particles carrying the electrode catalyst, the catalyst layer is usually formed using a catalyst ink that contains an electrolyte resin and in which the conductive particles carrying the electrode catalyst and the electrolyte resin are dissolved and dispersed in a solvent. The The electrolyte resin is not particularly limited, and those commonly used in polymer electrolyte fuel cells can be used. For example, hydrocarbons such as polyethersulfone, polyimide, polyetherketone, polyetheretherketone, polyphenylene as well as fluorine-based electrolyte resins such as perfluorocarbonsulfonic acid resin represented by Nafion (trade name, manufactured by DuPont) A hydrocarbon electrolyte resin into which an ion exchange group such as a sulfonic acid group, a boronic acid group, a phosphonic acid group, a hydroxyl group, or a carboxylic acid group is introduced can be used.

  Catalyst ink containing water electrocatalyst-encapsulated carbon nanotubes (water electrocatalyst-containing catalyst ink) is obtained by mixing and stirring conductive particles carrying an electrode catalyst, an electrolyte resin, and water electrocatalyst-encapsulated carbon nanotubes in a solvent. Obtainable. Examples of the solvent include alcohols such as methanol, ethanol, propanol, propylene glycol, N, N-dimethylformamide, N, N-diethylformamide, N, N-dimethylacetamide, N, N-diethylacetamide, and the like. These mixtures and mixtures with water can be used, but are not limited thereto. Among these, ethanol and propylene glycol are preferably used from the viewpoint of dispersibility of the carbon nanotubes.

  The water-electrocatalyst-containing catalyst ink is an electrolyte membrane, a gas diffusion layer sheet for a gas diffusion layer, a base film for a transfer film, etc. The catalyst layer can be formed by coating and drying the support. That is, the catalyst ink is applied directly to the electrolyte membrane and dried, or directly applied to the gas diffusion layer sheet and dried, or is applied onto the substrate film to form a transfer film, and the electrolyte membrane Alternatively, the catalyst layer can be formed by thermal transfer to a gas diffusion layer sheet. Examples of the base film for the transfer film include polytetrafluoroethylene (PTFE) and polypropylene.

  When forming a catalyst layer by applying and drying a catalyst ink (catalyst ink containing a water electrolysis catalyst) on the electrolyte membrane, the catalyst layer is sandwiched between the electrolyte membrane on which the catalyst layer is formed and the gas diffusion layer sheet. The membrane / electrode assembly can be formed by superimposing and bonding by hot pressing or the like. When a catalyst ink is applied to a gas diffusion layer sheet and dried to form a catalyst layer, the gas diffusion layer sheet on which the catalyst layer is formed and the electrolyte membrane are stacked so as to sandwich the catalyst layer, and hot A membrane / electrode assembly can be formed by bonding with a press or the like. In addition, when a transfer film is formed using a catalyst ink, the catalyst layer is transferred to the electrolyte membrane or the gas diffusion layer sheet, and the gas diffusion layer and the electrolyte membrane are overlapped and bonded so as to sandwich the catalyst layer. By this, a membrane / electrode assembly can be formed.

The application method of the catalyst ink (catalyst ink containing water electrocatalyst) is not particularly limited, and general methods such as an ink jet method, a spray method, a doctor blade method, a gravure printing method, and a die coating method can be used. From the viewpoint of the thickness and structure of the resulting catalyst layer, the degree of freedom of the coating amount, and the like, the spray method is suitable. The drying method of the catalyst ink is not particularly limited, and examples thereof include reduced pressure drying, heat drying, combined use of reduced pressure drying and heat drying. Usually, it is preferable to dry under the conditions of 120 ° C., vacuum and 24 hours.
Although the film thickness of a catalyst layer is not specifically limited, What is necessary is just to be about 1-100 micrometers in a dry state. The catalyst layer may contain other materials such as a water-repellent polymer and a binder as necessary in addition to the conductive particles supporting the catalyst metal, the polymer electrolyte, and the water-electrocatalyst-encapsulating carbon nanotubes. Good.

  When the catalyst layer does not contain water electrocatalyst-encapsulated carbon nanotubes, or the catalyst layer of the oxidizer electrode, instead of the water electrocatalyst-containing catalyst ink, a catalyst ink that does not contain water electrocatalyst encapsulated carbon nanotubes is used. The catalyst layer can be formed in the same manner as described above.

  The gas diffusion layer has gas diffusibility, conductivity, and strength required as a material constituting the gas diffusion layer, for example, carbon paper, carbon cloth, carbon, which can efficiently supply gas to the catalyst layer. Carbonaceous porous body such as felt, titanium, aluminum, copper, nickel, nickel-chromium alloy, copper and its alloys, silver, aluminum alloy, zinc alloy, lead alloy, titanium, niobium, tantalum, iron, stainless steel, gold Further, it can be formed using a gas diffusion layer sheet made of a conductive porous material such as a metal mesh composed of a metal such as platinum or a metal porous material. The thickness of the conductive porous body is preferably about 50 to 300 μm.

  The gas diffusion layer sheet may be composed of a single layer of the conductive porous body as described above, but a water repellent layer may be provided on the side facing the catalyst layer. The water-repellent layer usually has a porous structure containing conductive particles such as carbon particles and carbon fibers, water-repellent resin such as polytetrafluoroethylene (PTFE), and the like. The water-repellent layer is not necessarily required, but it can improve the drainage of the gas diffusion layer while maintaining an appropriate amount of water in the catalyst layer and the electrolyte membrane. There is an advantage that electrical contact can be improved.

  The method for forming the water repellent layer on the conductive porous body is not particularly limited. For example, water repellent obtained by mixing conductive particles such as carbon particles, water repellent resin, and other components as necessary with an organic solvent such as ethanol, propanol, propylene glycol, water or a mixture thereof. The layer ink may be applied to at least the side facing the catalyst layer of the conductive porous body, and then dried and / or fired. Examples of the method for applying the water repellent layer ink to the conductive porous body include a screen printing method, a spray method, a doctor blade method, a gravure printing method, and a die coating method.

  In addition, the conductive porous body is formed by impregnating and applying a water-repellent resin such as polytetrafluoroethylene to the side facing the catalyst layer with a bar coater or the like, so that the moisture in the catalyst layer is efficiently removed from the gas diffusion layer. You may process so that it may be discharged well.

  Water electrocatalyst-encapsulated carbon nanotube solution in which water electrocatalyst-encapsulated carbon nanotubes are dispersed in a solvent such as ethanol or propylene glycol is used as a method of incorporating water electrocatalyst-encapsulated carbon nanotubes in the gas diffusion layer. Examples thereof include a method of impregnating the inside and a method of applying a water electrocatalyst-encapsulated carbon nanotube solution to a conductive porous body. Moreover, the water-repellent layer ink may contain water electrocatalyst-encapsulated carbon nanotubes, or a method of impregnating and applying the water-electrocatalyst-encapsulated carbon nanotubes to the conductive porous body together with the water-repellent resin.

  As the electrolyte membrane, those used in general fuel cells can be used. For example, Nafion (trade name, manufactured by DuPont), Flemion (trade name, manufactured by Asahi Glass), Aciplex (trade name) (Manufactured by Asahi Kasei Co., Ltd.) and other fluorinated electrolyte resin membranes represented by perfluorocarbon sulfonic acid resins, and hydrocarbon resins such as polyethersulfone, polyimide, polyetherketone, polyetheretherketone and polyphenylene , Boronic acid groups, phosphonic acid groups, hydroxyl group, carboxylic acid group introduced hydrocarbon resin electrolyte membranes such as polymer electrolyte membranes, polybenzimidazole, polypyrimidine, polybenzoxazole, etc. Examples include a composite electrolyte membrane of a basic polymer in which a molecule is doped with a strong acid and a strong acid. Although the thickness of the electrolyte membrane is not particularly limited, it is usually about 10 to 100 μm.

  The membrane / electrode assembly provided with a pair of electrodes on the electrolyte membrane is further sandwiched between separators to form a single cell. As the separator, for example, a carbon separator containing a high concentration of carbon fiber and made of a composite material with a resin, a metal separator using a metal material, or the like can be used. Examples of the metal separator include those made of a metal material excellent in corrosion resistance, and those coated with a coating that enhances the corrosion resistance by coating the surface with carbon or a metal material excellent in corrosion resistance. .

[Reference Experimental Example 1]
(Production of single cell 1)
Platinum Pt / C catalyst supported on carbon black (Pt loading of 30 wt%. Electrocatalyst) and fluorine-based electrolyte resin (trade name Nafion, Aldrich Co.) and, IrO _2, which was supported IrO ₂ carbon black / C catalyst (IrO ₂ loading rate 45 wt%, water electrocatalyst), fluorine electrolyte resin / carbon black (Pt / C catalyst component) weight ratio is 1.0, fluorine electrolyte resin / carbon black (IrO ₂ / C catalyst component) Mixing was performed so that the weight ratio was 1.0, and ethanol and water were further added as solvents to prepare a catalyst ink containing a water electrocatalyst for an anode.
The water electrocatalyst-containing catalyst ink for anode was applied to the surface of carbon paper to be a gas diffusion layer and dried to produce a water electrocatalyst catalyst-containing catalyst layer / gas diffusion layer assembly (for anode).

On the other hand, a Pt / C catalyst in which platinum is supported on carbon black (Pt support rate: 30 wt%, electrode catalyst) and a fluorine-based electrolyte resin (trade name: Nafion, manufactured by Aldrich), fluorine-based electrolyte resin / carbon black weight The mixture was mixed so that the ratio was 1.0, and ethanol and water were further added as solvents to prepare a catalyst ink for cathode.
Cathode catalyst ink was applied to the surface of carbon paper to be a gas diffusion layer and dried to prepare a catalyst layer / gas diffusion layer assembly (for cathode).

  The water-based electrocatalyst-containing catalyst layer / gas diffusion layer assembly (for anode) and catalyst layer / gas diffusion layer assembly (for cathode) are sandwiched with a fluorine-based polymer electrolyte membrane, and thermocompression bonding (press pressure 3 MPa, The pressing temperature was 140 ° C.) to obtain a membrane / electrode assembly. Further, this cell-electrode assembly was sandwiched between two carbon separators to produce a single cell 1.

(Production of single cell 2)
Pt / C catalyst in which platinum is supported on carbon black (Pt supporting rate 30 wt%, electrode catalyst), fluorine-based electrolyte resin (trade name Nafion, manufactured by Aldrich), and carbon nanotubes (not supporting IrO ₂ ) , Fluorine electrolyte resin / carbon black (Pt / C catalyst content) Weight ratio is 1.0, Fluorine electrolyte resin / carbon nanotube weight ratio is 1.0, and ethanol and water are added as solvent Thus, a carbon nanotube-containing anode catalyst ink was prepared.
The carbon nanotube-containing anode catalyst ink was applied to the surface of carbon paper to be a gas diffusion layer and dried to prepare a carbon nanotube-containing catalyst layer / gas diffusion layer assembly (for anode).

  Using the obtained carbon nanotube-containing catalyst layer / gas diffusion layer assembly (for anode) and the catalyst layer / gas diffusion layer assembly (for cathode) produced in the same manner as in the single cell 1, a fluorine-based polymer electrolyte membrane was prepared. The membrane / electrode assembly was obtained by sandwiching and thermocompression bonding (pressing pressure 3 MPa, pressing temperature 140 ° C.). Further, this membrane / electrode assembly was sandwiched between two carbon separators to produce a single cell 2.

(Evaluation of power generation performance)
About the single cell 1 and the single cell 2, the IV characteristic was measured on condition of the following. The results are shown in FIG.

<IV characteristics measurement conditions>
・ Fuel electrode: hydrogen gas 500 cm ³ / min (1 atm, 25 ° C. constant condition, humidity 100% RH)
・ Oxidizer electrode: Air 1000 cm ³ / min (1 atm, 25 ° C. constant condition, humidity 100% RH)
-Cell temperature: 80 ° C

  As shown in FIG. 5, the single cell 2 provided with the anode catalyst layer containing carbon nanotubes has superior IV characteristics compared to the single cell 1 provided with the anode catalyst layer containing the water electrocatalyst. It was. This is probably because the water electrocatalyst has a higher affinity with water than the carbon nanotubes, so that water tends to stay in the anode catalyst layer containing the water electrocatalyst of the single cell 1 and the gas diffusibility is lowered. .

[Reference Experiment Example 2]
(Production of single cell 3)
Platinum Pt / C catalyst supported on carbon black (Pt loading of 30 wt%. Electrocatalyst), a fluorine-based electrolyte resin (trade name Nafion, Aldrich Co.), RuO ₂ which was supported RuO ₂ to the carbon black / C catalyst (RuO ₂ loading ratio 45 wt%, water electrocatalyst), fluorine electrolyte resin / carbon black (Pt / C catalyst component) weight ratio is 1.0, fluorine electrolyte resin / carbon black (RuO ₂ / C catalyst component) Mixing was performed so that the weight ratio was 1.0, and ethanol and water were further added as solvents to prepare a catalyst ink containing a water electrocatalyst for an anode.
The anode water electrolysis-containing catalyst ink was applied to a carbon paper surface to be a gas diffusion layer and dried to prepare a water electrolysis catalyst-containing catalyst layer / gas diffusion layer assembly (for anode).

On the other hand, a Pt / C catalyst in which platinum is supported on carbon black (Pt support rate: 30 wt%, electrode catalyst) and a fluorine-based electrolyte resin (trade name: Nafion, manufactured by Aldrich), fluorine-based electrolyte resin / carbon black weight The mixture was mixed so that the ratio was 1.0, and ethanol and water were further added as solvents to prepare a catalyst ink for cathode.
Cathode catalyst ink was applied to the surface of carbon paper to be a gas diffusion layer and dried to prepare a catalyst layer / gas diffusion layer assembly (for cathode).

  The water-based electrocatalyst-containing catalyst layer / gas diffusion layer assembly (for anode) and catalyst layer / gas diffusion layer assembly (for cathode) are sandwiched with a fluorine-based polymer electrolyte membrane, and thermocompression bonding (press pressure 3 MPa, The pressing temperature was 140 ° C.) to obtain a membrane / electrode assembly. Further, this cell-electrode assembly was sandwiched between two carbon separators to produce a single cell 3.

(Production of single cell 4)
A platinum-supported Pt / C catalyst (Pt support ratio 30 wt%, electrode catalyst) and a fluorine-based electrolyte resin (trade name: Nafion, manufactured by Aldrich) have a weight ratio of fluorine-based electrolyte resin / carbon black. The mixture was mixed to 1.0, and ethanol and water were further added as solvents to prepare an anode catalyst ink.
On the other hand, a fluorine-based electrolyte resin (trade name Nafion, Aldrich Co.) and an _RuO 2 / C catalyst was supported RuO ₂ carbon black (RuO ₂ supported ratio 45 wt%. Water electrolysis catalyst), a fluorine-based electrolyte resin / Carbon black was mixed so that the weight ratio was 1.0, and ethanol and water were further added as solvents to prepare a water electrocatalyst ink for anode.
The anode catalyst ink was applied to the surface of carbon paper to be a gas diffusion layer and dried to form a catalyst layer. Further, a water electrocatalyst ink for anode was applied on this catalyst layer and dried to prepare a joined body of water electrocatalyst layer / catalyst layer / gas diffusion layer (for anode).

  Next, a fluorine-based polymer comprising a water electrocatalyst layer / catalyst layer / gas diffusion layer assembly (for anode) and a catalyst layer / gas diffusion layer assembly (for cathode) prepared in the same manner as single cell 3 The electrolyte membrane was sandwiched and thermocompression bonded (pressing pressure 3 MPa, pressing temperature 140 ° C.) to obtain a membrane / electrode assembly. Further, this membrane / electrode assembly was sandwiched between two carbon separators to produce a single cell 4.

(Evaluation of power generation performance)
About the single cell 3 and the single cell 4, it drive | operated in the following fuel shortage conditions, and the time change of the cell voltage was measured. The results are shown in FIG.

<Fuel outage>
Fuel electrode: Nitrogen gas [272 sccm (25 ° C. standard), humidification temperature 80 ° C., gas pressure 0.2 MPa]
・ Oxidizer electrode: Air [866 sccm (25 ° C. standard), humidification temperature 80 ° C., gas pressure 0.2 MPa]
-Cell temperature: 80 ° C
・ Current density: 0.1 A / cm ²

  As shown in FIG. 6, compared with the single cell 3 having an anode side catalyst layer containing a water electrocatalyst, a water electrocatalyst layer containing a water electrocatalyst is provided at the interface between the anode side catalyst layer and the electrolyte membrane. The single cell 4 had a high cell voltage during fuel shortage operation.

  From the results of Reference Experimental Example 1 and Reference Experimental Example 2, during normal operation, the water electrocatalyst was contained in the carbon nanotubes rather than being contained in the anode side catalyst with the water electrocatalyst exposed. It is considered that the power generation characteristics are less deteriorated when contained in the anode side catalyst. In addition, the water electrocatalyst released by sacrificial corrosion of the carbon nanotubes during the fuel-deficient (hydrogen-deficient) operation is more at the interface between the electrolyte membrane and the anode-side catalyst layer than in the anode-side catalyst layer. It is considered that a higher cell voltage can be obtained by arranging them. That is, the water electrocatalyst layer is preferably contained in the anode electrode in a state of being encapsulated in carbon nanotubes, and particularly preferably disposed at the interface between the electrolyte membrane and the anode side electrode (anode side catalyst layer). It is guessed.

It is a figure which shows one example of a cell provided with the membrane electrode assembly for fuel cells of this invention. It is the elements on larger scale of FIG. It is a schematic diagram of the carbon nanotube which encloses a water electrolysis catalyst. It is a figure which shows one example of a cell provided with the membrane electrode assembly for fuel cells of this invention. 6 is a graph showing the results of Reference Experimental Example 1. 10 is a graph showing the results of Reference Experimental Example 2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Electrolyte membrane 2 ... Catalyst layer (2a: Fuel electrode side catalyst layer, 2b: Oxidant electrode side catalyst layer)
3. Gas diffusion layer (3a: fuel electrode side gas diffusion layer, 3b: oxidant electrode side gas diffusion layer)
4 ... Electrode (4a: fuel electrode, 4b: oxidant electrode)
5 ... Membrane / electrode assembly 6 ... Separator (6a: fuel electrode side separator, 6b: oxidant electrode side separator)
7 (7a, 7b) ... gas flow path 8 ... carbon nanotube layer 9 ... carbon nanotube 10 ... water electrocatalyst 11 ... electrode catalyst 12 ... conductive particle 100 ... single cell

Claims (3)

  A fuel cell membrane electrode assembly comprising a fuel electrode on one side of an electrolyte membrane and an oxidant electrode on the other side, wherein the fuel electrode contains a water electrolysis catalyst having catalytic activity for electrolysis of water. A membrane electrode assembly for a fuel cell, comprising the carbon nanotube thus produced and an electrode catalyst.   2. The membrane electrode assembly for a fuel cell according to claim 1, wherein the fuel electrode includes a carbon nanotube layer containing a carbon nanotube including the water electrolysis catalyst at a boundary surface with the electrolyte membrane.   The membrane electrode assembly for a fuel cell according to claim 1 or 2, wherein the electrode catalyst is supported on conductive particles having higher oxidation resistance than the carbon nanotube.

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