JP2008243781A - Solid polymer electrolyte fuel cell and membrane electrode assembly thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance output density of a cell by improving fuel, diffusibility, reactivity or the like of an oxidant, especially, at an anode and cathode electrodes of a membrane electrolyte assembly (MEA), in order to provide one solution for improving the cell performance of a solid polymer electrolyte fuel cell. <P>SOLUTION: The MEA has a polymer electrolyte membrane 1, and the anode electrode 2 and the cathode electrode 3 formed on the surface of the polymer electrolyte membrane 1, and the anode electrode 2 and the cathode electrode 3 alike are constituted with a two-layer structure of dense inner catalyst layers 2A and 3A, and rough outer catalyst layers 2b and 3B. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a membrane / electrode assembly (hereinafter sometimes referred to as MEA) of a solid polymer electrolyte fuel cell, and the catalyst layer of an anode and a cathode electrode has a multilayer structure and is located on the polymer electrolyte membrane side. The electrode catalyst layer is made dense, and each layer is gradually roughened toward the diffusion layer side to improve cell performance.

FIG. 3 shows an example of an MEA that is a reaction part of a solid polymer electrolyte fuel cell. In FIG. 3, the code | symbol 1 shows a polymer electrolyte membrane.
A membrane-like anode electrode 2 having a thickness of about 10 μm is joined to one surface of the polymer electrolyte membrane 1 and a membrane-like cathode electrode 3 having a thickness of about 10 μm is joined and integrated on the other surface. A diffusion layer made of carbon paper or the like (not shown) is provided on the surfaces of the anode electrode 2 and the cathode electrode 3 to form an MEA.

As the polymer electrolyte membrane 1, a film made of a perfluorosulfonic acid polymer having a thickness of about 30 to 70 μm is used.
The anode electrode 2 and the cathode electrode 3 are made of polymer electrolytes made of ionomer, water, isopropanol, etc., with supported catalyst particles in which platinum particles having a diameter of about 2 to 5 nm are supported on carbon particles having a diameter of several tens of nm. Dispersed in a solution, this dispersion is applied onto carbon paper or the like that becomes a diffusion layer, and dried.

The anode electrode 2 and the cathode electrode 3 are joined to the polymer electrolyte membrane 1 by arranging the diffusion layers of the anode electrode 2 and the cathode electrode 3 to be outside and joining them by thermocompression bonding or the like.
The amount of catalyst in the anode and cathode electrodes is about 1 to 3 mg / cm ^{2 because} of the relationship between the power density and cost. There is also a proposal that it can be reduced to about cm ² .

  Further, when reformed hydrogen, methanol, dimethyl ether (DME) or the like is used as the fuel, there is a risk of poisoning of the catalyst by carbon monoxide contained in these fuels. Platinum / ruthenium alloy catalysts are used.

The principle of operation of this solid polymer electrolyte fuel cell is as follows. Hydrogen supplied to the anode electrode 2 is converted into hydrogen ions by the catalytic reaction here, moves through the polymer electrolyte membrane 1, reaches the cathode electrode 3, and is supplied here by the catalytic reaction at the cathode electrode 3. It reacts with oxygen to become water. Electrons generated in the anode electrode 2 flow to an external circuit via a separator (not shown) and move to the cathode electrode 3.
When methanol, dimethyl ether, or the like is used as the fuel, methanol or dimethyl ether is directly oxidized by a catalytic reaction at the anode electrode 2 to generate carbon dioxide, hydrogen ions, and electrons, and the hydrogen ions are converted into a polymer electrolyte membrane. It will move in 1.

Research and development for improving the cell performance such as the output density of such a solid polymer electrolyte fuel cell has been actively promoted, and many patent applications have been filed.
Japanese translation of PCT publication No. 2002-532833 Japanese translation of PCT publication No. 2003-502827 JP-A-9-27326 JP 2006-140134 A JP 2005-197195 A JP 2005-56583 A JP 2005-174620 A

  An object of the present invention is to provide one solution for improving the cell performance of a solid polymer electrolyte fuel cell, and in particular, fuel, oxidant diffusibility, reaction at the anode and cathode electrodes of MEA. The purpose is to improve the power density of the cell by improving the properties.

To solve this problem,
The invention according to claim 1 is a membrane electrode assembly comprising a polymer electrolyte membrane, an anode and a cathode formed on the surface of the polymer electrolyte membrane, and a diffusion layer provided on the surfaces of the anode and the cathode electrode Because
Either one or both of the anode and the cathode electrode have a multilayer structure, and among these layers, the catalyst layer of the electrode on the polymer electrolyte membrane side is made dense, and each of the anode and cathode electrodes becomes more individual toward the diffusion layer side. A membrane electrode assembly of a solid polymer electrolyte fuel cell, characterized in that the layer is gradually roughened.

  The invention according to claim 2 is a solid polymer electrolyte fuel cell comprising the membrane electrode assembly according to claim 1.

  Originally, the anode and cathode electrodes of MEA are dense and have no voids or voids, so that the catalyst-supported particles are connected, the electric resistance is low, the electrical conductivity is good, and the polymer electrolyte is connected. The proton conductivity is improved, and the proton conductivity is preferably improved. However, if it is too dense, there are disadvantages in terms of fuel supply at the anode and cathode electrodes, oxidant supply, exhaust gas discharge, moisture supply, and generated water discharge.

On the other hand, if voids exist in the anode and cathode electrodes, the substantial surface area increases, and the utilization rate of the catalyst present in the voids increases. Further, it is advantageous in terms of fuel supply, oxidant supply, exhaust gas discharge, moisture supply, and generated water discharge, but electrical conductivity and proton conductivity are reduced.
As in the present invention, in the structure in which the layer on the polymer electrolyte membrane side is dense and the individual layers are gradually roughened toward the diffusion layer side, the advantages of both can be utilized well. As a result, cell characteristics can be improved.

FIG. 1 shows an example of the MEA of the present invention. In the MEA of this example, the catalyst for the anode electrode 2 and the cathode electrode 3 has a two-layer structure.
That is, the anode 2 is composed of an inner layer 2A on the polymer electrolyte membrane 1 side, an outer layer 2B outside the inner layer 2A, and a diffusion layer (not shown) on the surface of the outer layer 2B, and the cathode electrode 3 on the polymer electrolyte membrane 1 side. The inner layer 3A includes an outer layer 3B outside the inner layer 3A and a diffusion layer (not shown) on the surface of the outer layer 3B.

The catalyst inner layer 2A of the anode electrode 2 is dense, the electrode catalyst of the outer layer 2B is coarse, and similarly, the inner layer 3A of the cathode electrode 3 is dense and the outer layer 3B is coarse. This is a feature of the present invention.
The denseness in the inner layers 2A and 3A refers to a dense state in which the entire layer is dense and free from cracks, voids, voids, bubbles, etc., and the supported catalyst particles and the polymer electrolyte are uniformly dispersed.
The roughness in the outer layers 2B and 3B is a state in which many fine cracks and voids are formed in the entire layer and minute openings are formed on the surface. The supported catalyst particles and the polymer electrolyte are aggregated, and the aggregation It is also a state where voids exist between the particles.

The thickness ratio between the inner layers 2A and 3A and the outer layers 2B and 3B is preferably such that the inner layers 2A and 3A occupy 10 to 50% of the whole, and the outer layers 2B and 3B occupy the remaining 50 to 90%. . The inner layers 2A and 3A need to have a thickness of 10 μm or less because the gas permeability decreases as the thickness increases.
Note that the types of catalysts contained in the inner layers 2A and 3A and the outer layers 2B and 3B may be the same, or different types of catalysts may be used. In the anode electrode 2, a platinum catalyst is used for the inner layer 2A, and a platinum / ruthenium alloy catalyst is used for the outer layer 2B, so that catalytic performance due to catalyst poisoning by carbon monoxide when methanol, dimethyl ether, or the like is used as a fuel. Deterioration can be prevented.

  The above-mentioned dense inner layers 2A and 3A are prepared by, for example, using a catalyst-supported fine particle supporting a platinum catalyst or the like and an ink-like catalyst dispersion liquid dispersed in a polymer electrolyte solution made of ionomer, water, isopropanol, etc. When applying to a release paper by spraying, it is possible to increase the amount of spray sprayed to spray very fine droplets and to stack a number of thin layers. At this time, the viscosity of the catalyst dispersion is lowered so that small droplets are formed by spraying. The viscosity of the catalyst dispersion can be adjusted by adjusting the type and amount of solvent used.

  In addition, there are two types of methods for producing the rough outer layers 2B and 3B. The first method is to apply the catalyst dispersion liquid to carbon paper or release paper serving as a diffusion layer and dry the coating conditions. This is a method of forming a large number of cracks in the layer by controlling the drying conditions.

The catalyst dispersion is applied by spraying, doctor blade method, screen printing, etc .. At this time, the amount of coating is taken into account, and the catalyst dispersion is held in a liquid state immediately after application, forming a liquid surface for a certain period of time. To be.
If the coating amount is small, the catalyst dispersion is immediately dried and fixed at the position where the supported catalyst particles are applied, so that the supported catalyst particles are evenly arranged. It is difficult to crack.

Next, drying is performed. At this time, heat is applied only to the surface of the catalyst dispersion that is a liquid layer. For example, light from a lamp such as an incandescent lamp is irradiated on the liquid level of the catalyst dispersion. Although it is possible to heat the entire heating furnace or the like, it is preferable to apply heat only to the surface.
By heating the surface of the catalyst dispersion, water, isopropanol, etc., which are the solvent of the catalyst dispersion, volatilize. However, since the boiling point of isopropanol is low, it volatilizes quickly and is present on the surface side of the applied catalyst dispersion. The catalyst particles form an initial agglomeration.

The initially agglomerated supported catalyst particles are not yet completely dried, but water also volatilizes with time, so that the supported catalyst particles are dried and fixed on the surface side of the catalyst dispersion, and slightly fluid on the lower layer side. It becomes a state.
Since the drying proceeds from the surface layer to the inner layer, the cracks initially formed in the surface layer trigger the cracks on the inner layer side like wedges, and the entire catalyst layer obtained by drying has cracked. It becomes.

When the cross section of the catalyst layer thus formed is observed, a crack is generated in the thickness direction of the catalyst layer.
This number of cracks is controlled by changing the mixing ratio of two or more solvents having different boiling points contained in the catalyst dispersion, the solvent content, the heating temperature after coating, and the like. be able to.

  In the second method, a catalyst dispersion having a high viscosity with a reduced amount of solvent used is used. As the solvent, it is preferable to use isopropanol having a high volatilization rate to reduce water as much as possible. This highly viscous catalyst dispersion is sprayed in a state where the spray amount is small. Then, relatively large droplets are ejected from the spray nozzle and applied. Since these droplets have a small amount of solvent, they quickly solidify as they become large and become large particles, and without forming gaps between the particles, particles are formed one after another with large voids. It becomes the layer of.

In such an MEA, since both the anode electrode 2 and the cathode electrode 3 are composed of the rough outer layers 2A and 3A and the dense inner layers 2B and 3B, it is easy to supply fuel and oxidant at the anode and cathode electrodes. Thus, the generated gas and water can be easily discharged, the water can be smoothly supplied, and the surface area actually involved in the catalytic reaction in the outer layer is increased, resulting in a high utilization rate of the catalyst. In the inner layer, electrical conductivity and proton conductivity are increased.
For this reason, a solid polymer electrolyte fuel cell equipped with such an MEA has high cell characteristics such as power density.

  In the example described above, an example in which the anode and the cathode electrode have a two-layer structure is shown. However, a multilayer structure having three or more layers may be used. In this case, the degree of roughness gradually increases toward the diffusion layer. The layer in contact with the polymer electrolyte membrane 1 must be a dense layer.

The solid polymer electrolyte fuel cell of the present invention comprises the above-mentioned MEA. That is, it has a cell in which a plate-like separator (bipolar plate) made of the MEA and carbon or the like and having a flow path through which fuel or oxidant flows is formed through a gasket.
Therefore, this solid polymer electrolyte fuel cell has high performance.

Specific examples are shown below.
Example 1
Supported catalyst particles were used in which 50% by mass of platinum particles having an average particle size of 3 nm was supported on ketjen black having an average primary particle size of 30 nm. After dispersing the supported catalyst particles in a solvent of isopropanol and water (4: 1 by weight), 5% by weight of a polymer electrolyte Nafion solution (manufactured by DuPont, trade name) is mixed to prepare a catalyst dispersion. did.

The catalyst dispersion was applied onto carbon paper with a spray device. The catalyst dispersion was applied in a relatively large amount to such an extent that it could exist as a thin liquid on the carbon paper. Thereafter, using an incandescent light (200 W), incandescent light was irradiated from the coated surface for 1 to 3 minutes to completely dry the catalyst dispersion and volatilize the solvent.
When the coated surface after drying was observed, many cracks were generated. This crack was one in which the width of the crack became smaller toward the carbon paper side.

On this coating layer (a), the same catalyst dispersion was further coated and dried to form another coating layer (b). The coating layer B was prepared by spraying onto the coating layer a by reducing the amount of coating once, coating the coating layer in five times, and drying in a heating furnace. The coating layer B had a smooth surface and no cracks were observed.
The coating layer A and the coating layer B having a two-layer structure were used as an anode and a cathode electrode, and joined to the polymer electrolyte membrane by hot pressing to form an MEA.

The MEA produced in this way was sandwiched between carbon separators to form a cell. The separator has a structure in which a flow path through which fuel and oxidant flow is engraved.
Hydrogen was supplied as fuel to the cell, air was supplied as the oxidant, the cell temperature was 80 ° C., the catalyst amount was 1 mg / cm ² , and the output density was measured.
The result is shown by curve A in the graph of FIG.

As a comparison, when spraying on carbon paper using the same catalyst dispersion, the amount of coating is reduced once, the coating is divided into 10 times, and dried in a heating furnace. An anode and a cathode were produced, and an MEA was produced using the anode and the cathode. The surface of the anode and cathode electrode of this MEA was smooth and no cracks were observed. A cell was prepared in the same manner as described above, and the output density was measured in the same manner.
The result is shown by curve B in the graph of FIG.
From the graph of FIG. 2, it is understood that the cell performance is improved in the case of using the MEA having the dense structure.

It is a schematic sectional drawing which shows an example of MEA of this invention. It is a graph which shows the result of an Example. It is a schematic sectional drawing which shows the conventional MEA.

Explanation of symbols

1 ·· Polymer electrolyte membrane, 2 ·· Anode electrode, 2A ·· Inner catalyst layer, 2B ·· Outer catalyst layer, 3 ·· Cathode electrode, inner layer 3A, outer layer 3B

Claims (2)

A membrane electrode assembly comprising a polymer electrolyte membrane, an anode and a cathode electrode formed on the surface of the polymer electrolyte membrane, and a diffusion layer provided on the surface of the anode and cathode electrode,
Either one or both of the anode and the cathode electrode have a multilayer structure, and among these layers, the catalyst layer of the electrode on the polymer electrolyte membrane side is made dense, and each of the anode and cathode electrodes becomes more individual toward the diffusion layer side. A membrane electrode assembly for a solid polymer electrolyte fuel cell, wherein the layer is gradually roughened.   A solid polymer electrolyte fuel cell comprising the membrane electrode assembly according to claim 1.

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