JPS6314815B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to a fuel cell and a method for manufacturing the same, and particularly to a molten carbonate fuel cell and a method for manufacturing the same. The bipolar plate of a fuel cell is also called a separator, a gas separation plate, or a current collector. The bipolar plate is generally made of a rust-resistant material such as austenitic stainless steel or nickel. Although the bipolar plate is made of a rust-resistant material, an electrically insulating oxide film forms on the entire surface of the cathode bipolar plate on the oxidant passage side during use in molten carbonate fuel cells. I found out that it would happen. This is because during operation of the battery, the battery is oxidized by the bipolar plate oxidizer, forming an oxide film on the surface, and this film progresses to the contact surface with the cathode. It has been found that when such an oxide film is formed, the contact resistance increases and the output of the battery decreases. An object of the present invention is to provide a fuel cell in which the contact surface of the cathode-side bipolar plate with the cathode has good conductivity, and a method for manufacturing the same. In the present invention, the contact surface of the cathode-side bipolar plate with the cathode is made of a conductive member containing an alkali metal element. The present invention is based on the discovery that a conductive member containing an alkali metal element has conductivity even when heated to a high temperature (500 to 800°C) in an oxidizing atmosphere. The cathode side bipolar plate may have a structure in which the conductive member and another member are laminated together,
Preferably, it should be made entirely of the electrically conductive material. This is easier to manufacture and is also stronger in terms of mechanical strength. In the cathode side bipolar plate, the electrically insulating film is a problem at the contact surface with the cathode, so it is desirable to have the highest concentration of alkali metal elements on this surface to prevent loss of conductivity. . In the present invention, the electrodynamic member is not only made of austenitic stainless steel or nickel, but also pure iron, carbon steel, silicon steel sheets, molded products of metal powder, such as sintered products, molded products of conductive metal oxide powder, etc. It can be made with. The ability to use a wide variety of conductive members in this way is another advantage brought about by the present invention.
This has two major effects. Considering the corrosion resistance or oxidation resistance of the molten salt electrolyte, the conductive member is preferably made of austenitic stainless steel or pure nickel. The cathode-side bipolar plate in the present invention can be made, for example, by placing a conductive member in an alkali metal salt and heat-treating it to contain an alkali metal element. This method is extremely preferable because it allows the concentration of the alkali metal element to be highest at the surface. A specific method for carrying out such a method is to immerse the conductive member in an alkali metal salt that has been heated to 400 to 1200° C. and is in a molten state. A conductive member may be placed in solid alkali metal salt powder and heated. Of the above two methods, the former method using an alkali metal salt allows the alkali metal element to enter the conductive member more easily. When the heat treatment is performed in an oxidizing atmosphere, the increase in contact resistance during use is extremely small. Therefore, in the present invention, the most desirable method is to immerse the conductive member in a melt of an alkali metal salt in an oxidizing atmosphere. It is desirable that the alkali metal salt contains at least a lithium salt. When a material containing lithium salt is used, a layer of a solid solution containing lithium is formed on the surface of the conductive member. This layer has excellent electrical conductivity at the operating temperatures of the molten carbonate fuel cell. Example 1 A conductive member made of SUS316 austenitic stainless steel was immersed in a carbonate melt made by mixing lithium carbonate and potassium carbonate at a weight ratio of 1:1, and was heated at 700°C in air for 3 hours. I heated it up and made a bipolar plate. Example 2 A conductive member made of pure nickel was immersed in a molten carbonate mixture of lithium carbonate and potassium carbonate at a weight ratio of 1:1, and heated in air at 750°C for 5 hours to form a bipolar plate. I made it. Comparative Example 1 A bipolar plate was made from SUS316. Comparative Example 2 A bipolar plate was made from pure nickel. Experimental Example Using the bipolar plates obtained in Examples 1 and 2 and Comparative Examples 1 and 2, contact resistance was measured by the method shown in the figure. Two bipolar plates 2 are prepared, and a disc-shaped electrode plate 1 with a diameter of 30 mmφ and a thickness of 1.5 mm is placed between them.
was installed. Oxygen and carbon dioxide gas are added to gas chambers 3 and 4.
Gases mixed at a ratio of 1:1 were fed at a flow rate of 1 l/min. Press pressure was applied to the bipolar plate 2 from both sides in the direction indicated by the arrow. In the above condition, several ~
A direct current of 10 mA was applied while being adjusted with a variable resistor 6. The current value was read with an ammeter 7, and the voltage drop at that time was measured with a voltmeter 8 to determine the contact resistance.
The total contact area between the electrode plate and the bipolar plate 2 is 8 cm ² on both sides.

【table】

[Table] Comparative Example 1 was used for measurement after the oxide film on the surface of the bipolar plate was removed by polishing with a grinder. As is clear from the table, the contact resistance of Examples 1 and 2 at 500 to 700° C., which is the operating temperature of a molten carbonate fuel cell, is smaller than that of Comparative Examples 1 and 2.
Further, as is clear from Example 2, the contact resistance increases little with the passage of time in the case of the present invention. Therefore, it is clear that if such a bipolar plate is used as the cathode side bipolar plate, the decrease in battery output caused by this can be suppressed.

[Brief explanation of the drawing]

The figure is an explanatory diagram showing a method for measuring contact resistance. 2...Bipolar plate.

Claims (1)

[Claims] 1. The contact surface of the cathode-side bipolar plate with the cathode is made of a conductive member containing an alkali metal element, and the conductive member is made of at least one of austenitic stainless steel and nickel. Characteristics of molten carbonate fuel cells. 2. The molten carbonate fuel cell according to claim 1, wherein the entire bipolar plate is made of the conductive member. 3 In claim 1 or 2,
A molten carbonate fuel cell characterized in that a surface of the bipolar plate in contact with the cathode has the highest alkali metal element concentration. 4. In a molten carbonate fuel cell in which the contact surface of the cathode-side bipolar plate with the cathode is made of a conductive member containing an alkali metal element, and the conductive member is made of at least one of austenitic stainless steel and nickel, A method for producing a molten carbonate fuel cell, which comprises immersing the conductive member in a melt of an alkali metal salt and subjecting the conductive member to a heat treatment to cause the conductive member to contain an alkali metal element.