JP2010113841A - Fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell for suppressing discharging of toxic substance. <P>SOLUTION: The fuel cell includes a negative electrode for oxidizing fuel, a positive electrode for reducing oxygen, and an electrolyte film arranged between the negative electrode and the positive electrode. The positive electrode includes a catalyst layer contacting to the electrolyte film, and a gas diffusion layer arranged on the side opposite to the electrolyte of the catalyst layer. An addition layer arranged in the gas diffusion layer or on the side opposite to the catalyst layer of the gas diffusion layer contains oxidation catalyst which promotes combustion of a substance in the fuel that has not been combusted yet or an incomplete combustion product. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a fuel cell capable of suppressing emission of harmful substances.

  In recent years, with the increase in functionality of portable electronic devices such as notebook computers, mobile phones, and PDAs, the power consumption of such devices is increasing. At present, the power source of such portable electronic devices is the lithium ion secondary battery, but the energy density cannot follow the increase in power consumption in recent years, which hinders the enhancement of the functionality of portable electronic devices. It has become.

  As a power source for portable electronic devices replacing lithium ion secondary batteries, polymer electrolyte fuel cells have attracted attention. A polymer electrolyte fuel cell has gas diffusive catalyst layers on both sides of a polymer electrolyte membrane, and the negative electrode side is exposed to fuel gas (hydrogen, etc.) and the positive electrode side is exposed to oxidant gas (air, etc.). The basic principle is to synthesize water by a chemical reaction through a polymer electrolyte membrane and to electrically extract the reaction energy generated thereby. Above all, the liquid fuel supply type fuel cell, which can directly oxidize the liquid fuel at room temperature without being reformed to hydrogen and take out the electrical energy, does not need a reformer, so the power supply is downsized. In particular, it is most expected as a power source for portable electronic devices.

  By the way, in a fuel cell that directly supplies liquid fuel such as methanol, by performing discharge, the fuel is oxidized on the negative electrode side to generate carbon dioxide, and the oxidant such as oxygen is reduced on the positive electrode side. A reaction for generating water occurs, and these products are discharged to the outside from the discharge port of the fuel cell. However, in some cases, not all of the fuel is completely oxidized. In this case, unburned fuel (unreacted fuel) and incompletely burned products (intermediate products) are discharged to the outside from the exhaust port. . This incomplete combustion product may contain harmful formaldehyde, formic acid, methyl formate, carbon monoxide, etc., and it is necessary to prevent the emission of these harmful substances together with unreacted fuel.

  As a method to prevent such harmful substances from being discharged to the outside, a method of arranging an adsorbent material such as activated carbon or zeolite that physically adsorbs the intermediate product at the outlet of the fuel cell, There is known a method of arranging a catalyst such as a precious metal catalyst, a photochemical catalyst, or a metal oxide that promotes oxidation of a product (Patent Documents 1 and 2). Moreover, in order to suppress the discharge | release of the intermediate product to the exterior, the structure (patent document 3) which covers the harmful substance discharge port of a fuel cell with the filter containing the nitrogen-containing compound which absorbs an intermediate product, and produced | generated in the positive electrode In order to suppress discharge of an intermediate product dissolved in water to the outside, a configuration (Patent Document 4) is known in which an absorption part including a nitrogen-containing compound and a water-absorbing material is provided.

JP 2003-223920 A JP 2003-346862 A JP 2006-261053 A JP 2007-213982 A

However, the above-mentioned conventional method does not have sufficient adsorption / absorption rate and oxidation efficiency with respect to the amount of harmful substances produced, impedes the air flow necessary for the operation of the fuel cell, and further reduces the size of the fuel cell. There is a problem of preventing.
Accordingly, an object of the present invention is to provide a fuel cell capable of suppressing the discharge of harmful substances while reducing the above-mentioned problems.

According to the present invention,
(1) A fuel cell comprising a negative electrode for oxidizing fuel, a positive electrode for reducing oxygen, and an electrolyte membrane disposed between the negative electrode and the positive electrode, the positive electrode being in contact with the electrolyte membrane A catalyst layer and a gas diffusion layer disposed on the opposite side of the catalyst layer from the electrolyte, the gas diffusion layer or an additional layer disposed on the opposite side of the gas diffusion layer from the catalyst layer; A fuel cell comprising an oxidation catalyst that promotes combustion of unburned or incompletely burned products of the fuel;
(2) The fuel cell according to (1), wherein the additional layer containing the oxidation catalyst is formed by impregnating or coating the gas diffusion layer with the ink or paste containing the oxidation catalyst and a binder;
(3) The oxidation catalyst is contained in the gas diffusion layer by impregnating the entire gas diffusion layer with an ink or paste containing the oxidation catalyst and a nonionic conductive binder. Fuel cell;
(4) The fuel cell according to (1), wherein the oxidation catalyst is adsorbed inside the gas diffusion layer, so that the oxidation catalyst is contained in the gas diffusion layer;
(5) The oxidation catalyst includes at least one noble metal selected from the group consisting of gold, silver, platinum, palladium, rhodium, iridium, ruthenium and osmium or an alloy containing the noble metal. ) The fuel cell according to any one of
(6) The fuel cell according to any one of (1) to (5), wherein the fuel is at least one liquid fuel selected from the group consisting of methanol, ethanol, dimethyl ether and formic acid;
(7) The fuel cell according to any one of (1) to (6), wherein the incomplete combustion product is a compound selected from the group consisting of formaldehyde, formic acid, methyl formate, and carbon monoxide;
(8) The fuel cell according to any one of (1) to (7), comprising a microporous layer between the catalyst layer and the gas diffusion layer;
(9) The fuel cell according to any one of (1) to (8), wherein the electrolyte membrane is a polymer electrolyte membrane;
(10) A gas diffusion layer used for a positive electrode of a fuel cell, wherein the gas diffusion layer is impregnated with an ink or paste containing an oxidation catalyst and a binder that promotes combustion of unburned or incompletely burned products of the fuel. A gas diffusion layer carrying an additional layer formed by coating;
(11) A gas diffusion layer used for a positive electrode of a fuel cell, which gas diffuses an ink or paste containing an oxidation catalyst that promotes combustion of unburned or incompletely burned products of the fuel and a nonionic conductive binder A gas diffusion layer obtained by impregnating or coating the entire layer; and (12) a gas diffusion layer used for the positive electrode of a fuel cell, which promotes combustion of unburned or incompletely burned products of the fuel. A gas diffusion layer obtained by adsorbing an oxidizing catalyst to be adsorbed inside the gas diffusion layer is provided.

  In the fuel cell according to the present invention, the gas diffusion layer, or the additional layer disposed outside the gas diffusion layer, contains an oxidation catalyst that promotes combustion of the unburned product or incomplete combustion product of the fuel. Emission can be suppressed by efficiently burning. Further, the fuel cell according to the present invention does not hinder the flow of air necessary for its operation, and further does not require an external device, so that miniaturization is not hindered.

  A fuel cell according to the present invention is a fuel cell comprising a negative electrode that oxidizes fuel, a positive electrode that reduces oxygen, and an electrolyte membrane disposed between the negative electrode and the positive electrode. A catalyst layer in contact with the electrolyte membrane; and a gas diffusion layer disposed on the opposite side of the catalyst layer from the electrolyte, the gas diffusion layer or the gas diffusion layer disposed on the opposite side of the catalyst layer. The additional layer contains an oxidation catalyst that promotes combustion of unburned or incompletely burned products of the fuel.

  FIG. 1 is a schematic exploded perspective view of a five-layer structure type fuel cell which is an embodiment of a conventional fuel cell. In a five-layer structure type fuel cell, a negative electrode including a catalyst layer and a gas diffusion layer, a positive electrode including another catalyst layer and a gas diffusion layer, and a negative electrode and a positive electrode are disposed between two separators. An electrolyte membrane is disposed. Here, in the case of a fuel cell of a type that directly supplies liquid fuel such as methanol, by performing discharge, ideally, the fuel is oxidized on the negative electrode side to generate carbon dioxide, and oxygen or the like is generated on the positive electrode side. A reaction occurs in which the oxidant is reduced to produce water. These products pass through the flow path of the separator and are discharged to the outside from a discharge port (not shown) of the fuel cell. In practice, however, all of the fuel may not be completely oxidized. Specifically, a crossover phenomenon may occur in which a part of fuel such as methanol or hydrogen passes through the electrolyte membrane and reaches the positive electrode from the negative electrode side. In this case, at least a part of the permeated fuel reacts with oxygen to form incomplete combustion products (intermediate products) such as formaldehyde, formic acid, methyl formate, carbon monoxide, etc. Together with the reaction fuel), it is discharged from the outlet of the fuel cell through the flow path of the separator. Since such intermediate products and unreacted fuel are harmful, it is necessary to prevent their emission to the outside.

  A schematic exploded perspective view of a fuel cell according to an embodiment of the present invention is shown in FIG. The difference from the conventional fuel cell of the five-layer structure type shown in FIG. 1 is that an additional layer is disposed between the positive electrode separator and the gas diffusion layer. This additional layer includes an oxidation catalyst that promotes combustion of unburned or incompletely burned products of the fuel. Therefore, unreacted fuel and incomplete combustion products resulting from the crossover phenomenon pass through the additional layer before entering the separator flow path, and at that time, are oxidized by the action of the oxidation catalyst to become water and carbon dioxide. Therefore, discharge of harmful substances from the discharge port of the fuel cell is suppressed.

  The additional layer according to the present invention can be formed by impregnating or applying an ink or paste containing an oxidation catalyst in a binder to one side of the gas diffusion layer. As the binder, an ion exchange resin represented by Nafion (manufactured by Nafion (registered trademark) DuPont) can be used. The thickness of the additional layer is generally 1 to 300 μm, preferably 10 to 200 μm, more preferably 30 to 100 μm. In FIG. 2, the gas diffusion layer and the additional layer are shown separately for easy understanding of the concept. However, in practice, the impregnated or applied ink or paste partially penetrates into the gas diffusion layer. When there is a portion of the ink or paste deposited on the surface of the gas diffusion layer, the entire oxidation catalyst-containing region including the portion is regarded as an additional layer.

  A schematic exploded perspective view of a fuel cell according to another embodiment of the present invention is shown in FIG. The difference from the five-layer structure type conventional fuel cell shown in FIG. 1 is that the gas diffusion layer on the positive electrode side contains an oxidation catalyst that promotes combustion of unburned or incompletely burned products of fuel. It is. Therefore, unreacted fuel and incomplete combustion products resulting from the crossover phenomenon pass through the gas diffusion layer before entering the separator flow path, and at that time, they are oxidized by the action of the oxidation catalyst to be converted into water and carbon dioxide. Therefore, discharge of harmful substances from the outlet of the fuel cell is suppressed.

  The gas diffusion layer containing the oxidation catalyst according to the present invention can be formed by impregnating the entire gas diffusion layer with an ink or paste containing the oxidation catalyst in a nonionic conductive binder. Thus, when an oxidation catalyst is arranged in the entire gas diffusion layer, it is necessary to use a non-ion conductive binder in order to break the ion conductivity with the catalyst layer of the positive electrode. As such a nonionic conductive binder, polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), polyvinylidene fluoride (PVDF), or the like can be used.

  Alternatively, the gas diffusion layer containing the oxidation catalyst according to the present invention can also be formed by adsorbing the oxidation catalyst inside the gas diffusion layer. For example, platinum can be deposited on the inner surface of the gas diffusion layer by impregnating the porous gas diffusion layer with a platinum complex such as chloroplatinic acid and then performing a reduction treatment. In this case, the oxidation catalyst may be present in the entire gas diffusion layer or may be partially present in the thickness direction. In the case where the oxidation catalyst is partially present in the gas diffusion layer, defining the oxidation catalyst-containing region as the above-described additional layer is an expressional difference and is possible. Moreover, the combination aspect which has the additional layer by this invention and an oxidation catalyst exists in the whole gas diffusion layer is also included by the scope of the present invention.

As the oxidation catalyst, any catalyst capable of promoting the combustion of unburned or incompletely burned products of fuel can be used. In the present invention, in particular, at least one noble metal selected from the group consisting of gold, silver, platinum, palladium, rhodium, iridium, ruthenium, and osmium or an alloy containing the noble metal can be preferably used. A person skilled in the art can select a suitable oxidation catalyst according to the type of harmful substance to be combusted. The content of the oxidation catalyst is generally in the range of 0.01 to 5 mg / cm ² , preferably in the range of 0.3 to 1.5 mg / cm ² , regardless of whether in the additional layer or in the gas diffusion layer. .

The electrolyte membrane in the present invention is not particularly limited as long as it has high proton (H ⁺ ) conductivity, electronic insulation, and gas impermeability, and is a known polymer electrolyte membrane. I just need it. A typical example is a resin having a fluorine-containing polymer as a skeleton and a group such as a sulfonic acid group, a carboxyl group, a phosphoric acid group, or a phosphonic group. Since the thickness of the polymer electrolyte membrane has a great influence on the resistance, a thinner one is required as long as the electronic insulation and gas impermeability are not impaired. Specifically, the thickness is 1 to 200 μm, preferably 5 to 5 μm. It is set within the range of 140 μm. The material of the polymer electrolyte membrane in the present invention is not limited to a perfluorinated polymer compound, but a mixture of a hydrocarbon polymer compound or an inorganic polymer compound, or a C—H bond and C in a polymer chain. It may be a partially fluorinated polymer compound containing both -F bonds. Specific examples of hydrocarbon polymer electrolytes include polyamides, polyacetals, polyethylenes, polypropylenes, acrylic resins, polyesters, polysulfones, polyethers, etc., and derivatives thereof (aliphatic carbonization) into which electrolyte groups such as sulfonic acid groups have been introduced. Hydrogen-based polymer electrolytes), polystyrene having electrolyte groups such as sulfonic acid groups, polyamides having aromatic rings, polyamideimides, polyimides, polyesters, polysulfones, polyetherimides, polyethersulfones, polycarbonates, and derivatives thereof (Partial aromatic hydrocarbon polymer electrolyte), polyetheretherketone, polyetherketone, polyethersulfone, polycarbonate, polyamide, polyamideimide, polyester into which electrolyte groups such as sulfonic acid groups are introduced Polyphenylene sulfide and the like, and derivatives thereof (fully aromatic hydrocarbon-based polymer electrolyte), and the like. Specific examples of the partially fluorinated polymer electrolyte include polystyrene-graft-ethylenetetrafluoroethylene copolymer, polystyrene-graft-polytetrafluoroethylene, etc., into which an electrolyte group such as a sulfonic acid group has been introduced, and derivatives thereof. Can be mentioned. Specific examples of the perfluorinated polymer electrolyte membrane include Nafion (registered trademark) membrane (manufactured by DuPont) and Aciplex (registered trademark) membrane (manufactured by Asahi Kasei), which are perfluoropolymers having a sulfonic acid group in the side chain. And Flemion (registered trademark) membrane (manufactured by Asahi Glass Co., Ltd.). As the inorganic polymer compound, a siloxane-based or silane-based, particularly alkylsiloxane-based organosilicon polymer compound is suitable, and specific examples include polydimethylsiloxane, γ-glycidoxypropyltrimethoxysilane, and the like. It is done.

The negative electrode and the positive electrode according to the present invention generally include a gas diffusion layer and a catalyst layer in contact with the electrolyte membrane. The catalyst layer is not particularly limited as long as it contains catalyst particles and an ion exchange resin, and conventionally known ones can be used. The catalyst is usually made of a conductive material carrying catalyst particles. The catalyst particles may be any catalyst particles that have a catalytic action in the oxidation reaction of hydrogen or the reduction reaction of oxygen. In addition to platinum (Pt) and other noble metals, iron, chromium, nickel, and alloys thereof may be used. it can. As the conductive material, carbon-based particles such as carbon black, activated carbon, graphite and the like are suitable, and fine powder particles are particularly preferably used. Typically, carbon black particles having a surface area of 20 m ² / g or more carry noble metal particles such as Pt particles or alloy particles of Pt and other metals. In particular, for negative electrode catalysts, Pt is vulnerable to carbon monoxide (CO) poisoning. Therefore, when using a fuel that produces CO by side reaction, such as methanol, or a gas that is reformed from methane or the like. It is preferable to use alloy particles of Pt and ruthenium (Ru). The ion exchange resin in the catalyst layer is a material that supports the catalyst and serves as a binder for forming the catalyst layer, and has a role of forming a passage for ions and the like generated by the catalyst to move. As such an ion exchange resin, the thing similar to what was previously demonstrated in relation to the polymer electrolyte membrane can be used. The catalyst layer is preferably porous so that vapor of liquid fuel such as methanol can be brought into contact with the catalyst as much as possible on the negative electrode side and oxygen gas such as oxygen and air can be brought into contact with the catalyst as much as possible on the positive electrode side. Further, the amount of catalyst contained in the catalyst layer is generally sufficient if the range of 0.01 to 10 mg / cm ^2, but in particular in the range of 1.0~7mg / cm ² for anode, and for the positive electrode Is preferably in the range of 0.3-5 mg / cm ² . The thickness of the catalyst layer may generally be in the range of 1 to 500 μm, but it is particularly preferable that the thickness of the catalyst layer is in the range of 50 to 300 μm, and that of the positive electrode is in the range of 10 to 150 μm.

  The gas diffusion layer in which an additional layer is arranged according to the invention and / or contains an oxidation catalyst is generally a sheet material having electrical conductivity and breathability. Representative examples include those obtained by subjecting a breathable conductive base material such as carbon paper, carbon woven fabric, carbon nonwoven fabric, carbon felt or the like to a water repellent treatment. A porous sheet obtained from carbon-based particles and fluorine-based resin can also be used. For example, a porous sheet obtained by forming carbon black into a sheet using polytetrafluoroethylene as a binder can be used. The thickness of the gas diffusion layer is generally 50 to 500 μm, preferably 100 to 350 μm.

  The gas diffusion layer before the additional layer according to the present invention is arranged and / or containing the oxidation catalyst, or the gas diffusion layer after the additional layer according to the present invention is arranged and / or contains the oxidation catalyst, and the catalyst layer And a membrane electrode assembly can be produced by joining the polymer electrolyte membrane. As a joining method, any conventionally known method can be adopted as long as a precise joining with a low contact resistance is achieved without impairing the polymer electrolyte membrane. In joining, after first forming a negative electrode or a positive electrode by combining the catalyst layer and the gas diffusion layer, these can be joined to the polymer electrolyte membrane. For example, a negative electrode or a positive electrode is formed by preparing a coating solution for forming a catalyst layer containing catalyst particles and an ion exchange resin using an appropriate solvent, and applying it to a sheet material for a gas diffusion layer. The film can be joined by hot pressing. Further, after the catalyst layer is combined with the polymer electrolyte membrane, the gas diffusion layer may be combined on the catalyst layer side. When combining the catalyst layer and the polymer electrolyte membrane, a conventionally known method such as a screen printing method, a spray coating method, or a decal method may be employed.

  The fuel cell according to the present invention reduces the contact resistance with the catalyst layer and promotes drainage of water generated during operation of the fuel cell. Therefore, in the negative electrode and / or the positive electrode, the fuel cell is interposed between the catalyst layer and the gas diffusion layer. A microporous layer can also be disposed. The thickness of the microporous layer is generally in the range of 10 to 100 μm, preferably 15 to 50 μm.

  Generally, in a fuel cell, a current collector for taking out generated electricity is further combined. As the current collector, any conventionally known material can be employed. Specific examples of the current collector include a titanium (Ti) porous plate, a SUS porous plate, and a carbon porous plate. The thickness of the current collector is generally in the range of 10 μm to 5 mm.

  The fuel cell according to the present invention can supply liquid fuel to the negative electrode side. Examples of the liquid fuel include methanol aqueous solution, ethanol aqueous solution, 1-propanol aqueous solution, 2-propanol aqueous solution, dimethyl ether aqueous solution, sodium borohydride aqueous solution, potassium borohydride aqueous solution, lithium borohydride aqueous solution, formic acid aqueous solution and the like. The solute concentration in these aqueous solutions is generally in the range of 2 to 90 mass%, preferably 20 to 80 mass%, more preferably 50 to 70 mass%. If the solute concentration is less than 2% by mass, the energy density of the fuel cell is lowered. Conversely, if the solute concentration exceeds 90% by mass, the electrolyte membrane may be dissolved.

  When used as a fuel cell, a membrane electrode assembly is solidified by laminating 10 to 100 cells alternately with a separator plate and a cooling unit so that the negative electrode side and the positive electrode side are on a predetermined side according to a conventionally known method. A polymer fuel cell stack can be assembled. Alternatively, a planar series fuel cell in which membrane electrode assemblies are juxtaposed in the same plane so that the positive electrode and the negative electrode are on the same side and the positive electrode and the negative electrode are connected in series can be assembled. .

Comparative Example 1 Production of Conventional Fuel Cell A conventional fuel cell as shown in FIG. 4 was produced as follows.
(Gas diffusion layer: GDL)
Carbon paper (manufactured by Toray, model number TGP-H-060) is immersed in a solution obtained by diluting PTFE dispersion (manufactured by Daikin, D1E, content of PTFE 60% by mass) with water 5 times and dried in an oven at 90 ° C. Then, baking was performed at 350 ° C. for 2 hours to produce a water-repellent gas diffusion layer.

(Micro porous layer: MPL)
To 100 g of water, 15 g of carbon black (manufactured by Denki Kagaku, Denka Black) and 12 g of PTFE dispersion (manufactured by Daikin, D1E, PTFE content 60 mass%) were added and mixed well to prepare a dispersion. The dispersion was coated on one side of the gas diffusion layer, dried in an oven at 90 ° C., and then fired at 350 ° C. for 40 minutes to form a microporous layer having a dry thickness of 20 μm.

(Negative electrode catalyst layer)
Carbon-supported platinum ruthenium catalyst (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., TEC61E54DM: platinum / ruthenium (PtRu) supported amount 50% by mass) 5 g of 17% by mass ethanol solution of perfluorosulfonic acid electrolyte (Nafion 117 (registered trademark); manufactured by DuPont) A mixed ink was prepared by mixing 17.6 g so that the mass ratio of the carbon content of the catalyst to the electrolyte was 1.2. The mixed ink was sprayed onto the surface of the microporous layer to deposit a catalyst layer, thereby forming a negative electrode catalyst layer having a catalyst noble metal amount of 3 mg / cm ² .

(Cathode layer)
5 g of a carbon-supported platinum catalyst (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E10: platinum-supported amount of 50 mass%) is mixed with the perfluorosulfonic acid electrolyte solution so that the mass ratio of the carbon amount of the catalyst to the electrolyte is 1.0. Thus, a mixed ink was prepared. The mixed ink was sprayed onto the surface of the microporous layer to deposit a catalyst layer, thereby forming a positive electrode catalyst layer having a catalyst noble metal amount of 1.3 mg / cm ² .

(Membrane electrode assembly: MEA)
An ion exchange membrane Nafion 117 (registered trademark) (manufactured by DuPont) having a size of 5 × 5 cm and a thickness of 175 μm is prepared as a polymer electrolyte membrane. The negative electrode catalyst layer is provided on one side, and the positive electrode catalyst layer is provided on the opposite side. The membrane electrode assembly was formed by laminating and laminating by applying hot pressure (130 ° C., 1 MPa, 5 minutes) with a hot press.

(Separator)
As the separator, a cut carbon separator (thickness 2 cm, size 8 × 8 cm) was used. By cutting, a flow path with a width of 2 mm and a depth of 1 mm is linearly formed at intervals of 2 mm from the lower part to the upper part of the carbon separator. Is formed.

Example 1 Fabrication of Fuel Cell According to the Present Invention As shown in FIG. 5, the present invention was performed in the same manner as in Comparative Example 1 except that an additional layer was disposed on the opposite side of the positive electrode gas diffusion layer from the microporous layer. A fuel cell according to the invention was made. 5 g of a carbon-supported platinum catalyst (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E10: platinum-supported amount of 50% by mass) in the perfluorosulfonic acid electrolyte solution so that the solid content ratio of the catalyst to the electrolyte is 1: 0.5. A paste was prepared by mixing. The paste was applied to the opposite side of the gas diffusion layer from the microporous layer by a doctor blade method so that the platinum loading was 1 mg / cm ² . The coating was then dried and further fired at about 130 ° C. for 1 hour to place an additional layer containing an oxidation catalyst. The thickness of the additional layer was 50 μm.

Example 2 Production of Fuel Cell of the Present Invention A fuel cell according to the present invention was produced in the same manner as in Comparative Example 1 except that an oxidation catalyst was included in the gas diffusion layer of the positive electrode as shown in FIG. 5 g of a carbon-supported platinum catalyst (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E10: platinum-supported amount of 50 mass%) is mixed with the perfluorosulfonic acid electrolyte solution so that the mass ratio of the carbon amount of the catalyst to the electrolyte is 1.0. Thus, an ink was prepared. The ink was impregnated from the side opposite to the microporous layer of the gas diffusion layer by a knife coating method so that the amount of platinum supported was 1 mg / cm ² .

  In order to incorporate the MEA produced in Comparative Example 1, Example 1 and Example 2 into a fuel cell (single cell) and analyze the exhaust gas during operation of the fuel cell, a system as shown in the experimental diagram of FIG. It was constructed. The upper surface of the separator on the positive electrode side of the fuel cell was sealed, an exhaust port was provided through an adapter, a pump was connected, and the exhaust amount was controlled with a flow meter (mass flow controller). A drain pod is provided on the outlet side to collect condensed water and exhausted by an air pump through a flow meter. FIG. 7A shows the flow of exhaust gas when measuring the power generation performance of the fuel cell (when measuring performance). When analyzing the exhaust gas (at the time of gas sampling), as shown in FIG. 7B, a three-way valve provided downstream of the flow meter is switched to transfer a part of the exhaust gas to a sampling bag (Tedlar bag). ).

  The fuel cell was connected to a load device (not shown). The cell temperature was set to 40 ° C. or 65 ° C., and the load was varied at each temperature to create a load curve (IV curve). The maximum output density was determined from this load curve, and the fuel cell was operated with the current value set at that time. As a reference (control), the adapter at the top of the separator was removed, and a load curve during natural convection in which air was not sucked with an air pump was created. The adapter was then attached and the air pump was varied to determine the air flow rate when the same load curve as the reference was obtained. The following analysis was performed according to the determined air flow rate.

  The fuel cells of Comparative Example 1, Example 1 and Example 2 were operated at the set current value at a cell temperature of 40 ° C. or 65 ° C. by supplying 1-10 mass% methanol aqueous solution to the negative electrode. As described above, a part of the exhaust gas of the positive electrode is collected in a sampling bag, and the concentration of methanol (unburned fuel) and formaldehyde (incomplete combustion product) in the exhaust gas is measured with a gas detector tube (not shown). It was measured.

The unburned methanol concentration in the exhaust gas of Comparative Example 1, Example 1 and Example 2 was as follows.
(Operating temperature 40 ℃)
Supply methanol concentration Comparative example 1 Example 1 Example 2
1% by mass 190ppm 10ppm 15ppm
3% by mass 185ppm 20ppm 15ppm
5% by mass 180 ppm 10 ppm 20 ppm
10% by mass 950ppm 300ppm 280ppm
(Operating temperature 65 ° C)
Supply methanol concentration Comparative example 1 Example 1 Example 2
1% by mass 200ppm 20ppm 20ppm
3% by mass 195ppm 20ppm 15ppm
5% by mass 240ppm 20ppm 20ppm
10% by mass 1200ppm 200ppm 180ppm

  FIG. 8 shows a graph in which the measured unburned methanol concentration is plotted against the supplied aqueous methanol solution concentration for the fuel cells of Comparative Example 1 and Example 1. In FIG. 8, “ACGIH recommended working environment standard value (200 ppm)” is an allowable methanol exposure set by the American Conference of Governmental Industrial Hygienist (ACGIH). Comparative Example 1 having no additional layer is at the environmental standard value level even when the concentration of the supplied aqueous methanol solution is a low concentration of 1% by mass. On the other hand, Example 1 having an additional layer is significantly lower than the environmental standard value, and it can be seen that emission of harmful substances from the fuel cell was significantly suppressed.

The formaldehyde concentrations in the exhaust gases of Comparative Example 1, Example 1 and Example 2 were as follows.
(Operating temperature 40 ℃)
Supply methanol concentration Comparative example 1 Example 1 Example 2
1% by mass 0.1 ppm 0.1 ppm 0.1 ppm
3% by mass 0.1 ppm 0.2 ppm 0.1 ppm
5% by mass 0.2 ppm 0.2 ppm 0.2 ppm
10% by mass 1.1 ppm 0.25 ppm 0.2 ppm
(Operating temperature 65 ° C)
Supply methanol concentration Comparative example 1 Example 1 Example 2
1% by mass 0.1 ppm 0.1 ppm 0.1 ppm
3% by mass 0.1 ppm 0.2 ppm 0.1 ppm
5% by mass 0.35 ppm 0.2 ppm 0.2 ppm
10% by mass 1.2 ppm 0.25 ppm 0.2 ppm

  FIG. 9 shows a graph in which the measured formaldehyde (incomplete combustion product) concentration is plotted against the supplied aqueous methanol solution concentration for the fuel cells of Comparative Example 1 and Example 1. In Comparative Example 1, the formaldehyde concentration increased from around the methanol concentration exceeding 5% by mass, but in Example 1 having an additional layer, the formaldehyde concentration hardly increased even when the methanol concentration exceeded 5% by mass. .

FIG. 5 is a schematic exploded perspective view showing a five-layer structure type fuel cell which is an embodiment of a conventional fuel cell. 1 is a schematic exploded perspective view showing a fuel cell according to an embodiment of the present invention. FIG. 5 is a schematic exploded perspective view showing a fuel cell according to another embodiment of the present invention. 6 is a schematic exploded perspective view showing a fuel cell manufactured in Comparative Example 1. FIG. 1 is a schematic exploded perspective view showing a fuel cell manufactured in Example 1. FIG. 6 is a schematic exploded perspective view showing a fuel cell manufactured in Example 2. FIG. It is a schematic diagram which shows a system for analyzing the exhaust gas at the time of fuel cell operation. It is the graph which plotted the unburned methanol density | concentration with respect to the supply methanol aqueous solution density | concentration about the fuel cell of the comparative example 1 and Example 1. FIG. It is the graph which plotted the formaldehyde density | concentration with respect to the supply methanol aqueous solution density | concentration about the fuel cell of the comparative example 1 and Example 1. FIG.

Claims (12)

  A fuel cell comprising a negative electrode for oxidizing fuel, a positive electrode for reducing oxygen, and an electrolyte membrane disposed between the negative electrode and the positive electrode, the positive electrode comprising a catalyst layer in contact with the electrolyte membrane, A gas diffusion layer disposed on the opposite side of the catalyst layer from the electrolyte, and the gas diffusion layer or an additional layer disposed on the gas diffusion layer opposite to the catalyst layer, A fuel cell comprising an oxidation catalyst that promotes combustion of unburned products or incomplete combustion products.   2. The fuel cell according to claim 1, wherein the additional layer containing the oxidation catalyst is formed by impregnating or coating the gas diffusion layer with an ink or paste containing the oxidation catalyst and a binder.   The fuel cell according to claim 1, wherein the gas diffusion layer is impregnated with the oxidation catalyst by impregnating the entire gas diffusion layer with an ink or paste containing the oxidation catalyst and a nonionic conductive binder.   The fuel cell according to claim 1, wherein the oxidation catalyst is adsorbed inside the gas diffusion layer, so that the oxidation catalyst is contained in the gas diffusion layer.   The oxidation catalyst includes at least one noble metal selected from the group consisting of gold, silver, platinum, palladium, rhodium, iridium, ruthenium and osmium or an alloy containing the noble metal. The fuel cell according to item.   The fuel cell according to any one of claims 1 to 5, wherein the fuel is at least one liquid fuel selected from the group consisting of methanol, ethanol, dimethyl ether and formic acid.   The fuel cell according to any one of claims 1 to 6, wherein the incomplete combustion product is a compound selected from the group consisting of formaldehyde, formic acid, methyl formate and carbon monoxide.   The fuel cell according to claim 1, comprising a microporous layer between the catalyst layer and the gas diffusion layer.   The fuel cell according to claim 1, wherein the electrolyte membrane is a polymer electrolyte membrane.   A gas diffusion layer used for a positive electrode of a fuel cell, wherein the gas diffusion layer is impregnated or coated with an ink or paste containing an oxidation catalyst and a binder that promotes combustion of unburned or incompletely burned products of the fuel. A gas diffusion layer carrying an additional layer formed by   A gas diffusion layer used for a positive electrode of a fuel cell, wherein an ink or paste containing an oxidation catalyst that promotes combustion of unburned or incompletely burned products of the fuel and a nonionic conductive binder is added to the entire gas diffusion layer. A gas diffusion layer obtained by impregnating or applying to a metal.   A gas diffusion layer used for a positive electrode of a fuel cell, which is obtained by adsorbing an oxidation catalyst that promotes combustion of unburned or incompletely burned products of the fuel inside the gas diffusion layer.

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