JP2009187866A - Membrane-electrode assembly - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a membrane-electrode assembly for restraining deterioration in an electrolyte membrane caused by platinum separated in the electrolyte membrane and having excellent durability. <P>SOLUTION: In the membrane-electrode assembly having the electrolyte membrane 1 and a catalyst layer arranged on the surface of the electrolyte membrane and containing at least platinum fine particles, a platinum atom 10 is created at random on the electrolyte membrane by making the electrolyte membrane contain gold fine particles 9, and an exposed surface area of the platinum is lessened in a growing process of a platinum particle in comparison with the case that the platinum particle 11 grows. Generation of hydrogen peroxide caused by a catalyst action is restrained as its results, and deterioration in the polymer electrolyte membrane and the other formation members are prevented. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a membrane / electrode assembly.

  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 easy miniaturization and operation at a low temperature. It is attracting attention as a power source for the body.

In the solid polymer electrolyte fuel cell, the reaction of the following formula (1) proceeds at the fuel electrode (anode).
H ₂ → 2H ⁺ + 2e ⁻ (1)
The electrons generated by the equation (1) reach the oxidant electrode (cathode) after working with an external load via an external circuit. Then, the proton generated in the formula (1) moves in the solid polymer electrolyte membrane from the fuel electrode side to the oxidant electrode side by electroosmosis while being hydrated with water.
On the other hand, the reaction of the following formula (2) proceeds at the oxidant electrode.
4H ⁺ + O ₂ + 4e ⁻ → 2H ₂ O (2)

The cathode potential may rise temporarily under the operating environment of the fuel cell, such as when the fuel gas is insufficient or when the fuel cell is idle. Due to such a change in cathode potential, platinum contained as an electrode catalyst in the catalyst layer of the oxidizer electrode is dissolved (Pt ²⁺ ). The generated platinum ions diffuse in the electrolyte membrane having proton conductivity and are reduced by the hydrogen gas diffused from the fuel electrode to become platinum atoms (Pt ²⁺ + H ₂ → Pt + 2H ⁺ ). It is known to precipitate within. The platinum atoms thus deposited serve as nuclei for platinum particle growth, and platinum particles are formed or aggregated in the film, and it is considered that platinum particles grow using the aggregates as nuclei.

In recent years, it has been found that the coexistence state of electrolyte, oxygen, hydrogen and platinum is a cause of deterioration of the fuel cell. That is, as described above, precipitation of platinum particles in the electrolyte membrane is one of the major factors that cause deterioration of the electrolyte membrane and other components of the fuel cell. It is considered that hydrogen gas is generated by the catalytic action of platinum when hydrogen gas diffuses from the fuel electrode side and oxygen gas diffuses from the oxidant electrode side while platinum is deposited in the electrolyte membrane. . Hydrogen peroxide has a strong oxidizing power and is a source of radicals such as a hydroxy radical having a strong oxidizing power, and is one of the causative substances that cause deterioration of fuel cell constituent materials such as electrolytes.
It has been reported that the coexistence state of electrolyte, oxygen, hydrogen, and platinum particles is formed in several tens of hours after the start of operation of the fuel cell depending on the type of the electrolyte membrane. Therefore, avoiding this coexistence has become a major issue.

  In Patent Document 1, by changing the position of a so-called platinum band formed by aggregation of platinum particles precipitated in an electrolyte membrane, local degradation of the electrolyte membrane is prevented, and the electrolyte membrane and the fuel cell stack A fuel cell operating method for improving the durability of the battery is described. Specifically, by changing the partial pressure ratio between the hydrogen gas and the oxygen gas, changing the differential pressure between the fuel electrode and the oxidant electrode, or changing the humidity of the oxygen gas and the hydrogen gas at a specified timing, An operation method for changing the position of a platinum band formed by aggregation of platinum particles in a solid polymer electrolyte membrane is described.

  On the other hand, a fuel cell system that cleans impurities contained in the reaction gas, ion components derived therefrom, and dirt that adversely affects the operation of the fuel cell has also been proposed. For example, Patent Document 2 discloses a fuel cell configured by stacking a plurality of single cells (single cells), humidifying means capable of humidifying the inside of the fuel cell for each of a plurality of regions, and a part of the plurality of regions. Describes a fuel cell system comprising a humidification control means for controlling the humidification means so as to temporarily reach an excessive humidity state. The technique described in Patent Document 2 generates condensed water by setting a part of the plurality of regions in the fuel cell to an excessively humidified state, that is, a state exceeding the saturation relative humidity. Is used to clean impurities in the fuel cell.

JP 2006-302578 A JP 2003-163023 A

However, in the method of Patent Document 1, it is possible to change the position where platinum particles are formed from platinum atoms, but it is not possible to change the position where platinum ions are reduced by hydrogen gas and platinum atoms are deposited. . Further, according to the knowledge of the present inventor, the effect of suppressing the deterioration of the electrolyte membrane by changing the position where the platinum particles grow is low.
In addition, the cleaning method using condensed water, a chelating agent, electroosmosis and the like represented by Patent Document 2 can remove ions and water-soluble low molecules from the fuel cell. The Pt atoms and particles deposited inside cannot be washed away.

  The present invention has been accomplished in view of the above circumstances, and an object of the present invention is to provide a membrane / electrode assembly that can suppress deterioration of the electrolyte membrane due to platinum deposited in the electrolyte membrane and has excellent durability. To do.

  The membrane / electrode assembly of the present invention is a membrane / electrode assembly comprising an electrolyte membrane and a catalyst layer provided on the surface of the electrolyte membrane and containing at least platinum particles, wherein the electrolyte membrane is a gold It contains fine particles.

  In the membrane / electrode assembly of the present invention, the reduction reaction of platinum ions in the electrolyte membrane by the hydrogen gas diffusing from the fuel electrode side proceeds selectively on the surface of the gold fine particles contained in the electrolyte membrane. That is, platinum particles grow using the gold fine particles as nuclei. In this way, by containing gold fine particles that serve as particle growth nuclei in advance in the electrolyte membrane, platinum atoms are randomly generated in the electrolyte membrane, and they aggregate to form platinum fine particles. Compared with the case where platinum particles grow using fine particles as nuclei, the exposed surface area of platinum in the growth process of platinum particles becomes smaller. As a result, the generation of hydrogen peroxide due to the catalytic action of platinum is suppressed, and deterioration of the polymer electrolyte membrane and other components can be prevented.

From the viewpoint of reducing the amount of gold used and the stability of the gold fine particles when the gold fine particles are dispersed in the electrolyte membrane, the gold fine particles preferably have a particle size of 20 to 200 nm.
In addition, from the viewpoint of suppressing the exposed surface area of platinum in the growth process of platinum particles while sufficiently capturing platinum atoms, the electrolyte membrane contains a fluorine-based polymer electrolyte, and the gold fine particles are reduced to 0.5%. It is preferable to contain -5 particles / μm ³ , or a hydrocarbon polymer electrolyte and 5-50 gold particles / μm ³ .

  Since the membrane / electrode assembly of the present invention can reduce the exposed surface area of platinum deposited on the electrolyte membrane, it is possible to suppress the generation of hydrogen peroxide in the electrolyte membrane. Therefore, it is possible to prevent deterioration of the constituent members of the membrane-electrode assembly, particularly the electrolyte membrane, caused by hydrogen peroxide generated in the electrolyte membrane.

  The membrane / electrode assembly of the present invention is a membrane / electrode assembly comprising an electrolyte membrane and a catalyst layer provided on the surface of the electrolyte membrane and containing at least platinum particles, wherein the electrolyte membrane is a gold It contains fine particles.

The membrane / electrode assembly provided by the present invention will be described below with reference to FIGS. FIG. 1 is a schematic cross-sectional view showing an embodiment of a single cell provided with the membrane / electrode assembly of the present invention.
In FIG. 1, a single cell 100 is a membrane in which a fuel electrode (anode) 2 and an oxidant electrode (cathode) 3 are provided on one surface side of a polymer electrolyte membrane (hereinafter sometimes simply referred to as an electrolyte membrane) 1. An electrode assembly 6 is provided. The fuel electrode 2 has a structure in which a fuel electrode side catalyst layer 4a and a fuel electrode side gas diffusion layer 5a are laminated in order from the electrolyte membrane 1 side. Similarly, the oxidant electrode 3 has a configuration in which an oxidant electrode side catalyst layer 4b and an oxidant electrode side gas diffusion layer 5b are laminated in order from the electrolyte membrane 1 side.

  The membrane / electrode assembly 6 is sandwiched between two separators 7 a and 7 b to constitute a single cell 100. Each separator 7a, 7b is formed with a groove that forms a flow path for each reaction gas (fuel gas, oxidant gas), and a flow for supplying and discharging the reaction gas and the outermost surface of each electrode 2, 3. Paths 8a and 8b are defined. The flow path 8a on the fuel electrode side is a flow path for supplying fuel gas (a gas containing hydrogen or generating hydrogen) to the fuel electrode 2 and discharging unreacted fuel gas, and on the oxidant electrode side. The flow path 8b is a flow path for supplying an oxidant gas (a gas containing oxygen or generating oxygen) to the oxidant electrode 3 and discharging an unreacted portion of the oxidant gas.

  In FIG. 1, each electrode (fuel electrode, oxidant electrode) has a structure in which a catalyst layer and a gas diffusion layer are laminated. A structure in which a functional layer is provided in addition to the layer and the gas diffusion layer may be used.

FIG. 2 is an enlarged schematic cross-sectional view of the electrolyte membrane in the membrane / electrode assembly of the present invention. The electrolyte membrane 1 contains gold fine particles 9.
When the cathode potential fluctuates due to an increase in the potential of the oxidant electrode or the like, the platinum particles that are the electrode catalyst for the oxidant electrode are dissolved, and the generated platinum ions (Pt ²⁺ ) move into the electrolyte membrane due to the concentration gradient. The platinum ions in the electrolyte membrane are reduced by the hydrogen gas that has been supplied to the fuel electrode and diffused in the electrolyte membrane, and precipitates as platinum atoms 10. At this time, in the membrane / electrode assembly of the present invention containing the gold fine particles 9 in the electrolyte membrane 1, the generation of platinum atoms 10 proceeds selectively on the surface of the gold fine particles 9, Platinum particles 11 grow as nuclei.

  On the other hand, in the case of a conventional electrolyte membrane that does not contain gold fine particles that serve as the nucleus of grain growth, the platinum atoms generated in the electrolyte membrane aggregate to form stable nuclei, and platinum ions are reduced on the surface of the nuclei. Platinum particles grow by progressing or by reducing platinum ions on the surface of platinum atoms. The formation of platinum particles with such platinum atoms or aggregates of platinum atoms as the nucleus is slow in the formation of the grains, so that many minute platinum atoms, aggregates thereof, and platinum particles exist in the electrolyte membrane. It becomes.

On the other hand, in the electrolyte membrane of the membrane / electrode assembly of the present invention, the growth of platinum particles with the gold fine particles as the nucleus is selected such that the gold fine particles become the nucleus and the formation of platinum atoms is selected on the surface of the gold fine particles. Therefore, platinum particles grow rapidly and platinum particles having a large particle diameter are formed.
That is, when the same amount of platinum is present in the electrolyte membrane, the electrolyte membrane according to the present invention containing gold fine particles has an exposed surface area (ratio of platinum) as compared with the conventional electrolyte membrane not containing gold fine particles. Surface area) is reduced. As the exposed surface area of platinum is reduced, the catalytic action of platinum is reduced. Therefore, according to the present invention, the generation of deterioration-causing substances due to the catalytic action of platinum, such as the generation of hydrogen peroxide and the generation of hydrogen peroxide radicals, is suppressed. can do.
As described above, according to the present invention, it is possible to suppress the generation of hydrogen peroxide and radicals, and to suppress the deterioration of the constituent materials of the membrane / electrode assembly, particularly the electrolyte membrane, caused by these deterioration-causing substances. Therefore, a membrane / electrode assembly excellent in durability can be provided.

  Furthermore, it is known that metal fine particles in which gold and platinum coexist have a strong tendency to be covered with gold (Applied Surface Science. Vol. 26, Issue 1, June-July 1986, Pages 27-41). . Accordingly, when platinum atoms are deposited on the surface of the gold fine particles as nuclei, there is a possibility that the gold atoms in the first layer of the gold fine particles and the platinum atoms present on the surface are exchanged. In this case, since the exposed surface area of platinum can be further reduced, the catalytic action of platinum can be further reduced, and an improvement in the effect of suppressing the generation of deterioration-causing substances can be expected.

  As a growth nucleus of platinum particles in the electrolyte membrane, it is desirable that the platinum particles do not dissolve in the potential range (0 V to 1.23 V) in the operating environment of the fuel cell, and since only such gold exists as such a metal, Uses gold fine particles.

The particle size of the gold fine particles is 3 to 200 nm, particularly 20 to 200 nm, from the viewpoint of reducing the amount of gold used (cost reduction) and the stability of the gold fine particles when the gold fine particles are dispersed in the electrolyte membrane. Is preferred. If the particle size of the gold fine particles exceeds 200 nm, the amount of gold used increases and the cost may increase.
The particle size of the gold fine particles can be measured by, for example, a light scattering method. The gold fine particles may have a primary particle size or a secondary particle size.

The content of the gold fine particles in the electrolyte membrane is such that if the density of the gold fine particles is too low, the gold fine particles do not act effectively as the growth nuclei of the platinum particles, and the platinum atoms cannot be sufficiently captured. If the density is too high, the exposed surface area of platinum in the growth process of platinum particles may not be sufficiently suppressed. From such a viewpoint, it is preferable that the electrolyte membrane has 0.5 to 100 gold particles / μm ³ , particularly 0.5 to 50 gold particles / μm ³ .
In particular, when containing a fluorine-based polymer electrolyte as the polymer electrolyte, 0.5 to 10 pieces of gold particles / [mu] m ^3, more preferably contains 0.5 to 5 pieces / [mu] m ^3, high when it contains a hydrocarbon-based polymer electrolyte as a polymer electrolyte is 5 to 100 fine gold particles / [mu] m ^3, more preferably it is ³ containing 5 to 50 / [mu] m. Gas permeability varies depending on the type of polymer electrolyte contained in the electrolyte membrane, and hydrocarbon polymer electrolytes tend to have higher gas barrier properties than fluorine polymer electrolytes. Hydrogen gas from the fuel electrode The diffusion rate is low. That is, an electrolyte membrane containing a hydrocarbon-based polymer electrolyte has a slower rate of platinum particles than an electrolyte membrane containing a fluorine-based polymer electrolyte, and the exposed surface area of platinum in the platinum particle growth process is high. Therefore, as described above, by adjusting the content of the gold fine particles according to the polymer electrolyte contained in the electrolyte membrane, the platinum exposed surface area in the growth process of the platinum particles while sufficiently capturing the platinum atoms. Can be suppressed.

  The gold fine particles may be uniformly dispersed throughout the film, or may be limited to a specific region. As a specific example of the arrangement of the gold fine particles in the specific region, for example, when the position in the film thickness direction of the electrolyte membrane is specified as 0 on the cathode side surface and 1 on the anode side surface, 0.1 to 0.6 The gold fine particles are unevenly distributed in the region corresponding to the position, or the precipitation of platinum particles in the electrolyte membrane is observed in advance using a fuel cell having an equivalent structure, and the gold fine particles are observed in the region where the precipitation of platinum particles is observed. The form which makes uneven distribution exist. Thus, the amount of gold fine particles used can be reduced by disposing gold fine particles in a limited area in which precipitation of platinum particles is likely to occur.

  The manufacturing method of the electrolyte membrane containing gold fine particles is not particularly limited. For example, a colloidal gold solution containing fine gold particles can be obtained by adding citric acid to a chloroplatinic acid solution by a technique known as the Turkevich method. At this time, the particle diameter of the gold fine particles can be controlled by adjusting the amount of citric acid. Then, the colloidal gold solution is mixed with the polymer electrolyte solution, and the resulting mixed solution is cast-coated and evaporated to dryness to obtain an electrolyte membrane containing gold fine particles.

  As a polymer electrolyte, what can be used as an electrolyte of a polymer electrolyte fuel cell can be used. For example, in addition to a fluorine-based polymer electrolyte such as a perfluorocarbon sulfonic acid resin film represented by Nafion (trade name), a sulfonic acid group, a hydrocarbon resin such as polyether ketone, polyether ether ketone, and polyether sulfone, Examples thereof include hydrocarbon polymer electrolytes into which ion exchange groups such as phosphate groups and carboxyl groups are introduced.

The catalyst layers 4 provided on both surfaces of the electrolyte membrane 1 are usually formed using a catalyst ink containing an electrode catalyst having catalytic activity for an electrode reaction in each electrode and an electrolyte material.
As the electrode catalyst, one in which a catalyst component is supported on conductive particles is usually used. As the catalyst component, any catalyst component may be used as long as it has catalytic activity for the oxidation reaction of the fuel at the fuel electrode or the reduction reaction of the oxidant at the oxidant electrode, but usually platinum particles are used. The platinum particles include those made of a platinum alloy in addition to pure platinum. Examples of the platinum alloy include alloys of platinum and a metal such as ruthenium, iron, nickel, manganese, cobalt, copper, iridium, and osmium. Catalyst components other than platinum particles may be used in combination with platinum particles.
As the conductive particles as the catalyst carrier, carbon particles such as carbon black, conductive carbon materials such as carbon fibers, and metal materials such as metal particles and metal fibers can also be used.
As the electrolyte material, those exemplified as the polymer electrolyte constituting the electrolyte membrane can be used.

  The catalyst ink can be obtained by dissolving or dispersing the electrode catalyst and the electrolyte material as described above in a solvent. The solvent of the catalyst ink may be appropriately selected. For example, alcohols such as methanol, ethanol and propanol, organic solvents such as N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO), or organic solvents such as these Mixtures and mixtures of these organic solvents and water can be used. In addition to the electrode catalyst and the electrolyte material, the catalyst ink may contain other components such as a binder and a water repellent resin as necessary.

  The method for forming the catalyst layer is not particularly limited. For example, the catalyst layer can be formed on the surface of the gas diffusion layer by applying and drying the catalyst ink on the surface of the gas diffusion layer sheet serving as the gas diffusion layer. Moreover, a catalyst layer can be formed on the electrolyte membrane surface by applying and drying the catalyst ink on the electrolyte membrane surface. Alternatively, a transfer sheet is prepared by applying and drying a catalyst ink on the surface of the transfer substrate, and the transfer sheet is bonded to the electrolyte film or the gas diffusion layer sheet by thermocompression bonding or the like. A catalyst layer can be formed on the surface of the gas diffusion layer.

  The electrolyte membrane with the catalyst layer formed on the surface is superposed on the gas diffusion layer sheet and thermocompression-bonded, and joined together to form a membrane / electrode assembly in which the gas diffusion layer-catalyst layer-electrolyte membrane is laminated. can get. Further, the gas diffusion layer sheet having a catalyst layer formed on the surface thereof is superposed on the electrolyte membrane so that the catalyst layer is sandwiched between the gas diffusion layer and the electrolyte membrane, and is bonded to each other by thermocompression bonding. A membrane / electrode assembly in which a gas diffusion layer-catalyst layer-electrolyte membrane is laminated is obtained. Specific conditions such as heating temperature and pressure in the thermocompression bonding may be set as appropriate.

The method for applying the catalyst ink, the drying method, and the like can be selected as appropriate. For example, examples of the coating method include a spray method, a screen printing method, a doctor blade method, a gravure printing method, and a die coating method. Examples of the drying method include reduced-pressure drying, heat drying, and reduced-pressure heat drying. There is no restriction | limiting in the specific conditions in reduced pressure drying and heat drying, What is necessary is just to set suitably.
The amount of catalyst ink applied varies depending on the composition of the catalyst ink and the catalyst performance of the catalyst metal used in the electrode catalyst, but the amount of catalyst component per unit area is about 0.1 to 2.0 mg / cm ^2. What should I do? The thickness of the catalyst layer is not particularly limited, but may be about 1 to 50 μm.

  A water-repellent layer can be provided between the gas diffusion layer and the catalyst layer as necessary. 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 always necessary, 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 between the gas diffusion layer and the catalyst layer 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 the side of the gas diffusion layer sheet facing the catalyst layer, and then dried and / or fired. The thickness of the water repellent layer may usually be about 1 to 50 μm. Examples of the method of applying the water repellent layer ink to the gas diffusion layer sheet include a screen printing method, a spray method, a doctor blade method, a gravure printing method, and a die coating method.

  The produced membrane / electrode assembly is further sandwiched between separators to form a single cell. The separator has conductivity and gas sealing properties, and can function as a current collector and gas sealing body, for example, a carbon separator containing a high concentration of carbon fiber and made of a composite material with resin, metal A metal separator using a material 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. .

It is a cross-sectional schematic diagram which shows one example of a single cell provided with the membrane electrode assembly of this invention. It is an expanded section schematic diagram of an electrolyte membrane which constitutes a membrane electrode assembly of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Electrolyte membrane 2 ... Fuel electrode (anode)
3 ... Oxidant electrode (cathode)
4a ... Fuel electrode side catalyst layer 4b ... Oxidant electrode side catalyst layer 5a ... Fuel electrode side gas diffusion layer 5b ... Oxidant electrode side gas diffusion layer 6 ... Membrane / electrode assembly 7a ... Fuel electrode side separator 7b ... Oxidant electrode Side separator 8a ... Fuel electrode side gas flow path 8b ... Oxidant electrode side gas flow path 9 ... Gold fine particle 10 ... Platinum atom 11 ... Platinum particle 100 ... Single cell

Claims (3)

A membrane-electrode assembly comprising an electrolyte membrane and a catalyst layer provided on the surface of the electrolyte membrane and containing at least platinum particles,
The membrane / electrode assembly, wherein the electrolyte membrane contains gold fine particles.   The membrane / electrode assembly according to claim 1, wherein the gold fine particles have a particle size of 20 to 200 nm. The electrolyte membrane contains a fluorine polymer electrolyte and contains 0.5 to 5 gold fine particles / μm ³ or contains a hydrocarbon polymer electrolyte and the gold fine particles. The membrane / electrode assembly according to claim 1, wherein 5 to 50 / μm ^{3 is} contained.

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