JP2002329501A - Gas diffusion electrode and polyelectrolyte fuel cell using this - Google Patents

Abstract

PROBLEM TO BE SOLVED: To resolve a problem of an extensive drop of output voltage due to reduction of a humidifying amount in a polyelectrolyte fuel cell needing to maintain a polyelectrolyte in a wet state and commonly operating the cell while humidifying by using some kind of humidifying means. SOLUTION: Water repellent conductive particles mainly composed of water repellent material such as fluororesin and conductive particles such as carbon are arranged throughout the whole area of a conductive porous electrode base material by impregnation or press filling or the like. The gas diffusion electrode is used in at least one of an anode or cathode of the polyelectrolyte fuel cell.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel cell useful as a cogeneration system for a consumer or a generator for an automobile or a moving body, and more particularly to a polymer electrolyte fuel cell using a polymer electrolyte.

[0002]

2. Description of the Related Art In a polymer electrolyte fuel cell, a fuel gas such as hydrogen and an oxidizing gas such as air are electrochemically reacted by a gas diffusion electrode having a catalyst layer on which a catalyst such as platinum is disposed, so as to generate electric heat and heat. At the same time. FIG. 1 shows a general configuration of such a polymer electrolyte fuel cell. In FIG. 1, a catalyst layer 2 mainly composed of a carbon powder carrying a platinum group metal catalyst is disposed in close contact with both surfaces of a polymer electrolyte membrane 1 for selectively transporting hydrogen ions. On the outer surface of the catalyst layer 2, a pair of diffusion layers 3 having both gas permeability and conductivity are disposed in close contact with the catalyst layer. The electrode 4 is constituted by the diffusion layer 3 and the catalyst layer 2. Outside the electrode 4, an electrolyte membrane-electrode assembly (MEA) 5 formed by the electrode 4 and the polymer electrolyte membrane 1 is mechanically fixed, and adjacent MEAs are electrically connected to each other in series. The conductive separator plate 6 is disposed. The separator plate 6 has a gas flow path 7 on a surface in contact with the electrode 4 for supplying a reaction gas to the electrode and carrying away water and excess gas generated by the reaction. Although this gas flow path can be provided separately from the separator plate, a method in which a groove is provided on the surface of the separator plate to form a gas flow path is generally used. In the illustrated example, a separator plate 6a having a gas passage 7a for supplying a fuel gas to the anode and a separator plate 6b having a gas passage 7b for supplying an oxidizing gas to the cathode are combined to form a cooling water passage. 8 are formed. Reference numeral 9 denotes a gasket arranged on the periphery of the electrode.

[0003] Many fuel cells have a stacked structure in which a number of unit cells having the above structure are stacked. In the polymer electrolyte fuel cell stack as described above, in order to reduce the electrical contact resistance of the components such as the separator plate, and to maintain the sealing property of the fuel gas and the oxidizing gas, the entire cell is constantly maintained. Need to be tightened. For this purpose, end plates are arranged at both ends of a large number of cells stacked in one direction, and both end plates are fixed to each other with a fastening member, and a fastening pressure is applied.

The diffusion layer 3 needs to have gas permeability and electron conductivity, and carbon paper, carbon cloth, and the like are generally used. These gas diffusion layers are often subjected to a water-repellent treatment using a dispersion of a fluororesin in order to increase the water-repellency. Also,
In order to provide further water repellency, the diffusion layer 13 is formed by mixing a carbon layer containing carbon powder as a main component or a water repellent carbon layer obtained by mixing a fluorine-based water repellent material such as polytetrafluoroethylene (PTFE) with carbon powder. Is also provided on the catalyst layer side. By these, the moisturizing of the polymer electrolyte and the safe and prompt removal of excess water due to the battery reaction are effectively performed. At the time of battery operation, at the cathode, oxygen or air as a reactant diffuses from the gas flow path through the diffusion layer to the catalyst layer, and excess oxygen generated by the reaction and penetrating from the catalyst layer to the diffusion layer. The moisture is removed to the outside of the battery together with the excess gas.

[0005] An outline of a general manufacturing process of an MEA for a polymer electrolyte fuel cell is described below. First, a catalyst layer is formed on both surfaces of a polymer electrolyte membrane by a transfer method, a direct printing method, or the like. On the other hand, the aforementioned water-repellent carbon layer is formed on a porous diffusion layer such as carbon paper or carbon cloth. The diffusion layer with a water-repellent carbon layer and the electrolyte membrane with a catalyst layer can be stacked as individual members when a stack is formed. However, it is advantageous in terms of handling and the like that the diffusion layer with the water-repellent carbon layer and the electrolyte membrane with the catalyst layer are previously integrated as an MEA by thermocompression bonding or the like. A gasket for gas sealing is often arranged at the periphery of the MEA at the same time.

[0006]

The polymer electrolyte membrane of the polymer electrolyte fuel cell has such properties that the ionic conductivity increases as the water content increases and the gas permeability decreases. For this reason, it is necessary to keep the polymer electrolyte in the polymer electrolyte membrane and the catalyst layer in a wet state. However, when the wet state is excessive, the pores of the diffusion layer and the gas flow path of the separator plate are blocked by the condensed water, and a flooding state occurs in which the cell performance is extremely deteriorated due to inhibition of gas diffusion. In order to provide a high-performance polymer electrolyte fuel cell, while maintaining the polymer electrolyte in an appropriate wet state,
It is essential to remove excess water safely and promptly.

[0007] Since it is usually difficult to maintain an appropriate wet state only by generated water generated during battery operation, it is common to supply a water component together with a reaction gas to the MEA by using some humidifying method to humidify the MEA. When the humidification amount is reduced, there is a problem that the output voltage is significantly reduced. However, as the humidification method is simpler or the humidification amount is smaller, more stable and safe humidification and battery operation are possible, and the operation efficiency of the fuel cell system can be improved. SUMMARY OF THE INVENTION An object of the present invention is to provide a polymer electrolyte fuel cell whose performance is not deteriorated even by a small amount of humidification.

[0008]

A gas diffusion electrode according to the present invention comprises an electrode substrate made of a conductive porous material and a catalyst layer on a main surface of the electrode substrate in contact with the electrolyte membrane. The electrode is characterized in that the water-repellent conductive particles are distributed over substantially the entire area of the electrode substrate. The conductive particles are preferably carbon particles containing a fluororesin. The porous electrode substrate is
What consists of carbon materials, such as carbon paper, carbon felt, and carbon cloth, is preferred.

[0009]

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming conductive particles such as carbon and water-repellent conductive particles mainly composed of a water-repellent material such as fluororesin on a catalyst for a porous electrode substrate such as carbon paper. By coating on the surface in contact with the layer, it is not distributed unevenly near one surface of the electrode substrate, but is distributed over the entire area of the porous electrode substrate by impregnation or press-fitting. On a porous electrode substrate in which such water-repellent conductive particles are arranged over almost the entire area, the main surface on the side in contact with the catalyst layer is further coated with water-repellent conductive particles or the like. It is also effective. Conventionally, gas diffusion electrodes made by applying water-repellent conductive particles to the porous electrode substrate in the vicinity of the surface of the electrode substrate by blade coating or spray coating are polymer electrolytes. When the humidification amount was reduced during the operation of the battery while humidifying the outside of the cell, the decrease in the cell voltage was remarkable.

In order to solve such problems, the present invention provides a porous electrode substrate, that is, a gas diffusion layer over substantially the entire area of a porous electrode substrate, ie, a gas diffusion layer, in order to realize a more appropriate wet state of a polymer electrolyte. It is distributed over. This makes it possible to provide a gas diffusion electrode having a sufficient thickness, excellent performance for moisturizing the polymer electrolyte, and having conductivity and water repellency. For the purpose of low humidification, it is also common to apply a water-repellent treatment to the porous electrode substrate in advance using a fluororesin dispersion or the like. However, as a result of the comparative study, it was not only revealed that the gas diffusion electrode of the present invention had a higher moisturizing ability, but also the manufacturing process of the gas diffusion electrode of the present invention was simpler.

In the gas diffusion electrode of the present invention, the effect of low humidification can be recognized in each case even if the gas diffusion electrode is provided in only one of the anode and the cathode. Shows the greatest effect of low humidification. As described above, the present invention provides a polymer electrolyte fuel cell in which it is important to keep the electrolyte in an appropriate wet state, even in a state where humidification from outside the cell is small, a gas diffusion electrode that exhibits a high output voltage. provide.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below in detail. Example 1 First, a method for forming a catalyst layer will be described. A carbon powder having a particle diameter of several microns or less was immersed in an aqueous chloroplatinic acid solution, and a platinum catalyst was supported on the surface of the carbon powder by a reduction treatment. The weight ratio of carbon and supported platinum was 1: 1. Next, this carbon powder carrying platinum is dispersed in an alcohol solution of a polymer electrolyte,
A slurry containing the slurried catalyst-supporting carbon powder was applied to a film substrate and dried to form a catalyst layer. This catalyst layer was transferred to a polymer electrolyte membrane to produce a polymer electrolyte membrane with a catalyst layer. The size of the catalyst layer was made smaller than the carbon paper to be joined later, and the outer peripheral portion of the carbon paper was set to protrude several mm from that of the catalyst layer.

Next, the water-repellent carbon layer will be described. After dispersing acetylene black granule powder in water to which a surfactant was added, an aqueous dispersion of polytetrafluoroethylene (PTFE) was added and mixed well to prepare a slurry of water-repellent carbon. A 360-μm-thick carbon paper was immersed in the water-repellent carbon slurry, and the container was kept under reduced pressure in each container to impregnate the entire area of the carbon paper with the water-repellent carbon slurry. This was pulled up and dried, and then fired at 350 ° C. for 2 hours. The carbon paper impregnated with the water-repellent carbon was cut into a predetermined size. On both sides of the above-mentioned polymer electrolyte membrane with a catalyst layer, carbon paper is overlapped so that the catalyst layer is entirely covered, and a gasket is positioned on the periphery of the carbon paper.
Press for minutes to make MEA.

After aging operation of the polymer electrolyte fuel cell A manufactured as described above for 12 hours, current-voltage characteristics were evaluated under two conditions having different dew points on the cathode side. Hydrogen humidified so as to have a dew point of 70 ° C. was supplied to the anode at a flow rate of 70%, and humidified air was supplied to the cathode at a flow rate of 40%. First, the current-voltage characteristics were measured under the condition that the dew point (Tdc) of the air supplied to the cathode was 60 ° C. Thereafter, the dew point of the air supplied to the cathode was changed to 45 ° C., and after the aging operation for 8 hours, the current-voltage characteristics were measured again, and this was defined as the current-voltage characteristics at the air dew point of 45 ° C. The battery temperature was kept at 75 ° C.

Comparative Example 1 An example using a conventional battery B, that is, a gas diffusion electrode coated with a water-repellent carbon layer so as to be unevenly distributed on the surface of the porous electrode substrate in contact with the catalyst layer by blade coating. The same fuel cell B as in Example 1 was constructed.

FIG. 2 shows the current-voltage characteristics of the fuel cell A of Example 1 and the fuel cell B of Comparative Example 1. From these results, it is understood that the polymer electrolyte fuel cell using the gas diffusion electrode according to the present invention exhibits a high output voltage even when the humidification amount is small.

<< Embodiment 2 >> The characteristics of the battery a1 in which the gas diffusion electrode of Embodiment 1 is provided only on the anode and the battery a2 in which only the cathode is provided are shown in FIG. 3 in comparison with the battery A provided with both electrodes. . From these results, it can be seen that the output voltage in a state where the humidification amount is small can be improved in both cases where the gas diffusion electrode of the present invention is provided only on the anode and when the gas diffusion electrode is provided only on the cathode. And when the gas diffusion electrode of the present invention is arranged on both the anode and the cathode, the greatest effect of reducing humidification is exhibited.

Comparative Example 2 A gas diffusion electrode according to the present invention was compared with a gas diffusion electrode coated with a conductive water-repellent layer after the electrode substrate had been subjected to a water-repellent treatment in advance. A method for manufacturing a fuel cell as a comparative example will be described. Thickness 36 to be the electrode substrate
0 micron carbon paper was immersed in an aqueous dispersion of fluororesin and then dried. The slurry of the water-repellent carbon used in Example 1 was applied to one main surface of the porous electrode substrate provided with the water-repellency, and then fired at 350 ° C. for 2 hours. Next, the water-repellent treated carbon paper coated with the two water-repellent carbon layers cut to a predetermined size was applied to both surfaces of the polymer electrolyte membrane provided with the catalytic reaction layer in the same manner as in Example 1; The sheets were overlapped so as to be covered with carbon paper, and a gasket was further positioned on the periphery, and then pressed at 100 ° C. for 5 minutes to obtain an MEA.

FIG. 4 shows the characteristics of the battery C using the MEA under the same conditions as in the first embodiment. From these results, it can be seen that the polymer electrolyte fuel cell A using the gas diffusion electrode of the present invention and the polymer electrolyte fuel using the gas diffusion electrode coated with a conductive water repellent layer after the electrode substrate has been subjected to a water repellent treatment in advance. It can be seen that in the battery C, the former has a higher output voltage under the condition that the humidification amount is smaller. That is, the battery using the gas diffusion electrode of the present invention can suppress a decrease in performance due to a decrease in the amount of humidification.

[0020]

As described above, by dispersing the water-repellent conductive particles over the entire area of the conductive porous electrode substrate, a gas diffusion electrode excellent in moisturizing the polymer electrolyte can be obtained. By using this gas nucleic acid electrode,
It has become possible to provide a polymer electrolyte fuel cell in which a decrease in output in a state where the amount of humidification is small is suppressed.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a configuration of a general polymer electrolyte fuel cell.

FIG. 2 is a diagram showing current-voltage characteristics of a fuel cell of Example 1 and a fuel cell of Comparative Example 1.

FIG. 3 is a diagram showing current-voltage characteristics of the fuel cell of Example 2.

FIG. 4 is a diagram showing current-voltage characteristics of the fuel cell of Example 1 and the fuel cell of Comparative Example 2.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Polymer electrolyte membrane 2 Catalyst layer 3 Gas diffusion layer 4 Electrode 5 MEA 6, 6a, 6b Conductive separator plate 7, 7a, 7b Gas flow path 8 Cooling water flow path 9 Gasket

Continued on the front page (72) Inventor Masao Yamamoto 1006 Kadoma Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Inventor Teruhito Kanbara 1006 Ojidoma Kadoma, Kadoma City Osaka Pref. Reference) 5H018 AA06 AS01 BB01 BB03 BB05 BB06 BB08 BB11 BB12 BB17 CC06 DD08 EE03 EE05 EE18 5H026 AA06 CC03 CX05 EE05 EE19

Claims (3)

[Claims] 1. An electrode substrate comprising a conductive porous material,
And a gas diffusion electrode having a catalyst layer on the main surface of the electrode substrate in contact with the electrolyte membrane, wherein the conductive particles having water repellency are distributed over substantially the entire area of the electrode substrate. Characterized gas diffusion electrode. 2. The gas diffusion electrode according to claim 1, wherein the conductive particles are carbon particles containing a fluorine-based resin. 3. A hydrogen ion conductive polymer electrolyte membrane, a pair of gas diffusion electrodes sandwiching the polymer electrolyte, and a pair of gas diffusion electrodes sandwiching the gas diffusion electrode and forming a gas flow path on a surface in contact with the gas diffusion electrode. 3. A polymer electrolyte fuel cell comprising a conductive separator, wherein at least one of the gas diffusion electrodes is the gas diffusion electrode according to claim 1 or 2.