JPH04301372A - Fuel cell - Google Patents

Abstract

PURPOSE:To provide an excellent fuel cell such as possible to maintain stable high performance relating to operation over a long period of containing from start to stop by enabling electrolyte distribution to adequately hold its balance in the unit cell. CONSTITUTION:In a fuel cell constituted by forming a unit cell 1 by interposing a matrix layer 4, mainly composed of silicon carbide for holding an electrolyte, between a pair of gas diffusion electrodes formed by arranging an anode 2 and a cathode 3 opposed, a mixed material of silicon carbide fine grains of different size is arranged in a side 4a in contact with the anode 2 of the matrix layer 4, and a mixed material, obtained by mixing silicon carbide whisker in addition to the silicon carbide fine grains of different size, is arranged in a side 4b in contact with the cathode 3 of the matrix layer 4.

Description

[Detailed description of the invention]

[Object of the invention]

0002

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to fuel cells, and more particularly to improvements in the electrolyte retention matrix layer thereof.

0003

2. Description of the Related Art A fuel cell is a battery that can directly convert the chemical energy of fuel into electrical energy, and generally has the following configuration.

That is, a unit cell constituting a fuel cell has a pair of gas diffusion electrodes arranged opposite to each other as an anode and a cathode, and a matrix layer for retaining an electrolyte interposed between the electrodes. Among these, gas diffusion electrodes are made by coating a catalyst layer on one side of a conductive porous substrate whose main component is carbon, and grooves for gas flow channels are formed on the back side. . A pair of gas diffusion electrodes having such a configuration are arranged with their catalyst surfaces facing each other, and a matrix layer for retaining an electrolyte is interposed between them to form a unit cell.

[0005] Furthermore, during operation of a fuel cell, a fuel gas containing hydrogen as a main component is passed through one pole of such a unit cell, and air is passed as an oxidant gas through the other pole. As a result, an electromotive force is generated between the two electrodes due to the reverse reaction of the water electrolysis reaction, and electric power can be extracted through an external load.

Among these fuel cells, in fuel cells that use concentrated phosphoric acid as an electrolyte, the catalyst layer is made of fine particles of noble metals such as platinum or alloys thereof dispersed and supported on carbon powder as an electrode catalyst. The catalyst material is kneaded with polytetrafluoroethylene (PTFE), which is a binder and a water repellent, and then applied onto a substrate. In addition, in such fuel cells, the matrix layer for retaining the electrolyte consists of fine particles of silicon carbide and a small amount of polytetrafluoroethylene (PTFE).
It can be formed by kneading a binder such as PEG and a thickener such as PEG with phosphoric acid and applying it to the contact surface with the electrode catalyst layer, or it can be formed by pre-carbonizing the contact surface with the electrode catalyst layer. It is formed by sintering a thin layer of silicon and impregnating it with phosphoric acid.

[0007]

By the way, it is known that when a fuel cell having the conventional structure as described above is operated for a long period of time, the cell voltage gradually decreases as the operating time passes.

The causes of this voltage drop are mainly divided into two factors: deterioration (sintering) of the electrode catalyst and progress of electrode leakage.

[0009] Of these, the former voltage drop due to catalyst deterioration becomes noticeable over a longer period of time from the perspective of changes over time (in the case of alloy catalysts, There are reports that base metal elements elute in just a few hundred hours, but it is said that the activity after element elution does not decrease due to the effect of surface roughness. It is believed that the decrease is mainly due to the progress of electrode wetting.

It is presumed that the progress of wetting of the electrode is due to the movement of electrolyte between the electrode and the matrix. Conventionally, various improvements have been made to electrodes and matrix layers in order to suppress such deterioration in battery performance caused by wetting of electrodes.

[0011] For example, it is generally possible to adjust the water repellency by changing the amount of water repellent such as polytetrafluoroethylene (PTFE) in the catalyst layer between the cathode and the anode, or by changing the firing temperature of the electrode. I've been exposed to it. In addition to these general methods, in recent years, carbon black has been coated with a large number of water-repellent particles with a diameter equal to or smaller than carbon black, making it possible to maintain high gas supply performance over a long period of time with only a relatively small amount of water-repellent particles. A method for manufacturing such a high-performance electrode has been disclosed (Japanese Patent Application Laid-open No. 312989/1989).

However, while the above-mentioned conventional method can suppress the increase in wetting of the electrode catalyst layer over time, it has the following drawbacks. In other words, in the phenomenon of electrode wetting progressing, the main factors that cause actual battery performance deterioration are:
It is estimated that this is due to an imbalance in the electrolyte distribution at the electrode/matrix layer interface within the unit cell. On the other hand, the conventional method described above is merely an improvement of the electrode alone, and from the perspective of the unit battery as a whole, the balance of electrolyte distribution such as phosphoric acid at the electrode/matrix layer interface is not considered at all. do not have. Therefore, in the conventional method described above, it is not possible to properly maintain the balance of electrolyte distribution, which is the main cause of actual deterioration of battery performance, and as a result, it is difficult to sufficiently suppress the deterioration of battery performance. Ta.

The present invention has been made to solve the problems of the prior art as described above, and its purpose is to maintain an appropriate balance of electrolyte distribution in a unit battery, so that it can be easily maintained from startup to shutdown. The object of the present invention is to provide an excellent fuel cell that can maintain stable and high performance during long-term operation including.

[0014]

[Means for Solving the Problems] The present invention solves the problem of unbalanced electrolyte distribution, which is the main cause of deterioration of battery performance, for long-term operation including from startup to shutdown. Assuming that this is due to migration from the matrix layer to the cathode, we attempted to maintain an appropriate balance in the distribution of electrolytes such as phosphoric acid at the electrode/matrix interface by making improvements to the electrolyte retention matrix layer. It is something.

That is, in the fuel cell of the present invention, a unit cell is formed by interposing a matrix layer mainly composed of silicon carbide for retaining an electrolyte between a pair of gas diffusion electrodes arranged oppositely as an anode and a cathode. In the fuel cell, a mixed material of silicon carbide fine particles with different particle sizes is placed on the side of the matrix layer that contacts the anode, and a mixed material of silicon carbide fine particles with different particle sizes is placed on the side of the matrix layer that contacts the cathode. It is characterized by disposing a mixed material made of a mixture of silicon carbide whiskers.

[0016]

[Operation] According to the present invention having the above structure, the electrolyte holding power of the anode-side matrix layer can be made relatively strong, and the electrolyte can be sufficiently held on the anode. Therefore, it is possible to prevent the electrolyte from moving from the matrix layer to the cathode, and to maintain an appropriate balance in the distribution of electrolytes such as phosphoric acid at the electrode/matrix interface, ensuring long-term operation from startup to shutdown. However, stable and high performance can be maintained.

[0017]

[Embodiment] An embodiment in which the fuel cell according to the present invention is applied to a phosphoric acid fuel cell using phosphoric acid as an electrolyte will be described below.

First, FIG. 1 is a schematic side view showing an embodiment of a fuel cell according to the present invention. As shown in FIG. 1, the unit cell 1 consists of a pair of gas diffusion electrodes, an anode 2 and a cathode 3, and a matrix layer 4 for retaining phosphoric acid interposed between the two. That is, an anode catalyst layer 5 and a cathode catalyst layer 6 are formed on one side of the anode 2 and cathode 3, respectively, and the anode 2 and cathode 3 are arranged with these catalyst layers 5 and 6 facing each other. There is. Then, on the back of anode 2,
A fuel gas containing hydrogen as a main component is passed through, and air is passed as an oxidizing gas behind the cathode 3, and electrical energy is extracted by utilizing the electrochemical reaction that occurs at this time. Further, the output voltage of such a unit battery 1 is 0.7 to 0.8 under normal operating conditions.
In reality, several hundred unit cells 1 are stacked with separators 7 in between to form a fuel cell body called a cell stack, so that a predetermined output voltage can be obtained. It has become.

In the fuel cell shown in FIG. 1 having the above structure, each element of the unit cell 1 has a fixed composition according to the present invention. This point will be explained below. That is, first, as the anode 2 and cathode 3 of the unit cell 1, carbon-supported platinum catalyst (10W) was prepared on a conductive porous substrate by a known method.
t% Pt content) and polytetrafluoroethylene (PTF
Form a catalyst layer consisting of the mixture of E) and heat at 350°C in air.
Electrodes that are fired for 30 minutes are used.

Next, as the matrix layer 4, a material made by kneading silicon carbide particles with average particle diameters of 5 μm and 1 μm with polytetrafluoroethylene (PTFE) is used. In this matrix layer 4, a material having a large ratio of small particle diameters to large particle diameters is used for a portion 4a in contact with the anode catalyst layer 5, and a material having a large ratio of small particle diameters to large particle diameters is used for a portion 4b in contact with the cathode catalyst layer 6. In this method, a material is used in which the ratio of large particle size to small particle size is increased, and in addition, silicon carbide whiskers are mixed therein. Then, using these two types of composition materials, a fixed amount of a binder such as polytetrafluoroethylene (PTFE) and a thickener such as PEG are mixed by a known method to form the matrix layer 4. ing. Further, a portion 4b of the matrix layer 4 on the cathode catalyst layer 6 side is of a sintered type,
It is formed by later impregnating it with phosphoric acid. Further, in the portion 4a of the matrix layer 4 on the anode catalyst layer 5 side, a paste type matrix is used which is made by kneading phosphoric acid as an electrolyte and a mixed material having the above composition.

The operation of this embodiment having the above configuration will be explained below with reference to FIGS. 2 to 4. In this case, FIG. 2 is a graph showing a comparison of changes over time in the phosphoric acid space factor of the cathode catalyst layer in a fuel cell according to the prior art and a fuel cell according to the present invention, and FIG. It is a graph showing a comparison of changes over time in the phosphoric acid space factor of the anode catalyst layer in the battery of the invention. In addition, FIG.
The battery characteristics of the conventional battery and the battery of the present invention are constant load current (
220 mA/cm2) is a graph showing a comparison of changes in cell voltage over time. The operating conditions are normal pressure (
Anode: 30% CO2 +70% H2, Cathode:
Air).

First, referring to FIG. 2, the phosphoric acid space factor in the pores of the cathode catalyst layer in the conventional battery is approximately 100
In contrast, the phosphoric acid occupancy factor of the pores in the cathode catalyst layer in the battery of the present invention did not show a significant increase and remained stable and constant over a long period of time. It can be seen that the product moment is maintained. Next, referring to FIG. 3, the phosphoric acid space factor of the pores in the anode catalyst layer in the conventional battery gradually decreases after about 1000 hours, whereas the anode It can be seen that the phosphoric acid space factor in the pores of the catalyst layer is higher from the beginning of operation than in conventional batteries, and that value remains stable over a long period of time.

Reflecting the above-mentioned difference in stability of the phosphoric acid space factor in the catalyst layer, the battery characteristics also change as shown in FIG. 4. In other words, while the characteristics of the conventional battery rapidly deteriorate after approximately 4000 hours, the battery of the present invention shows no such rapid deterioration in characteristics at all, and even continues for a long period of time. It can be seen that stable characteristics are maintained over a period of time.

It should be noted that the present invention is not limited to the above embodiments, and for example, the binder and thickener used in the matrix layer can be selected as appropriate. Further, since the present invention relates to improvement of the matrix layer, the configurations other than the matrix layer, ie, the configurations of the gas diffusion electrode and the catalyst layer thereof, can be appropriately selected.

[0025]

As described above, according to the present invention, by changing the composition of the matrix layer between the anode side and the cathode side, the electrolyte holding power on the anode side is increased and the electrolyte distribution balance in the unit battery is improved. Since it can be maintained appropriately, it is possible to provide an excellent fuel cell that can maintain stable and high performance during long-term operation, including from startup to shutdown.

[Brief explanation of drawings]

FIG. 1 is a schematic side view showing one embodiment of a fuel cell according to the present invention.

FIG. 2 is a graph comparatively showing changes over time in the phosphoric acid space factor of the cathode catalyst layer in a fuel cell according to the prior art and a fuel cell according to the present invention.

FIG. 3 is a graph comparatively showing changes over time in the phosphoric acid space factor of the anode catalyst layer in a fuel cell according to the prior art and a fuel cell according to the present invention.

FIG. 4 shows a constant load current (220 m
2 is a graph comparatively showing changes in cell voltage over time in A/cm2).

[Explanation of symbols]

1 Unit battery 2 Anode 3 Cathode 4 Matrix layer 5 Anode catalyst layer 6 Cathode catalyst layer 7 Separator

Claims (1)

[Claims] 1. A fuel cell in which a unit cell is formed by interposing a matrix layer mainly composed of silicon carbide for retaining an electrolyte between a pair of gas diffusion electrodes arranged oppositely as an anode and a cathode, A mixed material of silicon carbide fine particles with different particle sizes is arranged on the side of the matrix layer that contacts the anode, and silicon carbide whiskers in addition to the silicon carbide fine particles with different particle sizes are placed on the side of the matrix layer that contacts the cathode. A fuel cell characterized by disposing mixed materials formed by mixing.