JPH0719606B2 - Molten carbonate fuel cell - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molten carbonate fuel cell which operates by using hydrogen or the like as a fuel and operates by using air, oxygen or the like as an oxidant, and particularly by improving the material of the anode, the cell performance is improved. And to improve the battery life.

2. Description of the Related Art A molten carbonate fuel cell generally has two electrodes, an anode (cathode) and a cathode (anode), an electrolyte tile in contact with both electrodes, and a current collector or a type that also serves as a current collector depending on the structure. It is composed of a bipolar plate, a battery housing for physically holding the battery structure, an active material gas distributor, an active material gas supply pipe, and the like. When the fuel cell is operating, the temperature is between about 500 and 700 ° C and the entire electrolyte tile, ie carbonate and inert carrier, forms a paste, which separates the anode and cathode. . This electrolyte is in direct contact with the electrode, has gas tightness between the electrolyte and the battery housing, and also serves to electrically insulate the anode side and the cathode side of the battery.

The anode of these battery components has conventionally been sintered or taped with nickel or nickel / chromium alloy powder in consideration of cost, hydrogen oxidation catalytic ability, electronic conductivity, stability in molten carbonate and the like. It has been made by molding by the casting method. On the other hand, cathodes have been made by molding lithium-doped nickel oxide powder in a similar manner or by oxidizing a nickel sintered plate similar to the anode.

Problems to be Solved by the Invention A major problem regarding the anode for maintaining the performance of the molten carbonate fuel cell for a long period of time is that, during operation of the cell at high temperature, oversintering of nickel, which is a main constituent material, with time, That is, the porosity and specific surface area of the porous anode are reduced. Since this anode is a gas diffusion electrode, the area of the part that contributes to the electrode reaction is closely related to the porosity and specific surface area of the electrode. Therefore, the over-sintering of the porous anode causes a decrease in the performance of the molten carbonate fuel cell, which eventually leads to a decrease in the life, which is a serious problem.

As one of the methods for solving the above-mentioned problems, there is a method of using a nickel-chromium alloy as an anode constituent material for preventing oversintering. However, in this method, if the particles are large to some extent, oversintering can be prevented, but if the particles are small, the rate of oversintering is high, so it is difficult to maintain a large porosity and a specific surface area for a long time.

There is also a method of covering the nickel particles with fine particles of alumina to form an anode to prevent oversintering, but if alumina is an insulator, adding a large amount will increase the resistance of the anode and the resistance between the anode and the current collector. Contact resistance increases, which adversely affects battery performance.

Therefore, there is a demand for measures to prevent oversintering of the anode that can maintain the high performance of the molten carbonate fuel cell at the beginning of operation.

[Means for Solving the Problems] Therefore, the present invention uses a metal for an anode, preferably nickel, mixed with a powder of a highly electron-conductive material having hydrogen permeability.

Action Hydrogen-permeable metals are themselves difficult to sinter,
It has been found that such a difficulty of sintering brings about an effect of effective over-sintering conversely when it is mixed with nickel which is an anode material or alloy powder mainly containing nickel containing chromium or aluminum. .

By the way, the hydrogen oxidation reaction on a normal nickel-based anode in a molten carbonate fuel cell is generally carried out in the solid phase (electrode).
It is thought to occur in a three-phase zone where the three phases of liquid phase (electrolyte) and gas phase (fuel) are in contact with each other. However, the portion where the above three phases are in contact should be strictly one-dimensional and never have an area. Actually, it is considered that the hydrogen oxidation reaction (electrochemical reaction) occurs near the three-phase zone, but in any case, the area of the part that contributes to this electrochemical reaction is limited. Therefore, in order to maintain the high performance of the molten carbonate fuel cell, it is considered necessary to keep the area of the portion contributing to the electrochemical reaction as large as possible, that is, the porosity and the specific surface area of the anode.

On the other hand, when the hydrogen-permeable metal powder is mixed and used in the anode, atomic hydrogen or molecular hydrogen diffuses in the hydrogen-permeable metal particles 1 constituting the anode as shown in FIG. As soon as it reaches the surface in contact with 2, it reacts electrochemically with carbonate ions. That is, in addition to the three-phase zone 3 which was conventionally thought to cause an electrochemical reaction, the solid phase and the gas phase are integrated, and the portion where this and the liquid phase (electrolyte) are in contact (that is, the portion like the two-phase zone) Since the electrochemical reaction also occurs in No. 4, the area of the active portion necessary for this reaction to occur is larger than that of the nickel anode having the same porosity and the same specific surface area, which contributes to the improvement of the battery performance. Therefore, unlike the conventional chromium and alumina, in the present invention, addition of a hydrogen permeable metal makes it possible to prevent oversintering of nickel and improve the performance. In FIG. 1, 4 indicates nickel and 5 indicates a vapor phase.

Example First, a titanium-manganese system was used as a hydrogen permeable metal. This is known as a hydrogen storage alloy near room temperature, but in the present invention, hydrogen is not stored and attention is paid to its permeability. Although this may be used directly, in this example, electroless plating of nickel is applied to this powder to improve corrosion resistance, and a mixture of this nickel powder and the nickel powder in a weight ratio of 1: 1 is used at 800 ° C. It was sintered in a hydrogen atmosphere furnace and used as an anode. The alloys known as hydrogen storage alloys at room temperature are known to be used as anodes of fuel cells as hydrogen storage and hydrogen ionization catalysts.
However, each of these hydrogen storage alloys has an appropriate temperature for storing hydrogen depending on its system, which is generally around room temperature. When this is used in a high temperature region, hydrogen is not occluded and hydrogen is only permeated in the form of atoms or molecules. Therefore, the use and purpose of the hydrogen storage or ionization catalyst are different from those of this embodiment. In fact, the operating temperature of this battery is 650
At ° C, the hydrogen permeable metal used for the anode is under the condition that hydrogen is not absorbed.

Further, porous lithium-doped nickel oxide was used for the cathode, and an electrolyte having a lithium carbonate: potassium carbonate molar ratio of 62:38 was prepared by tape casting together with lithium aluminate, which is an electrolyte holder, and used.
The fuel gas has a hydrogen: carbon dioxide gas: water vapor ratio of 80: 14: 6, and the oxidant has an air: carbon dioxide gas ratio of 70: 3.
Tests were conducted at a temperature of 650 ° C. and a current density of 150 mA / cm ² by applying the one having a ratio of 0.

In FIG. 2, the relationship between time and performance of the battery A in which the hydrogen permeable metal was mixed in the anode and the battery B using a normal nickel-based anode were compared under the same conditions. It can be seen from FIG. 2 that, when the hydrogen permeable metal is mixed, the high performance is maintained for a long time and the effect of preventing oversintering is great.

Next, the IV characteristics of this battery and the IV characteristics of a battery under the same conditions using a normal nickel porous body were compared and shown in FIG. Clearly an improvement in performance was seen when using a hydrogen permeable metal anode. Similarly, when the hydrogen permeable metal was coated with copper, aluminum, silver, platinum, palladium or the like, the performance of the molten carbonate fuel cell was improved as compared with the case where the nickel anode was used.

EFFECTS OF THE INVENTION As described above, according to the present invention, it is possible to prevent oversintering of the anode, which has been a big problem in the past, and to improve the performance of the battery.

[Brief description of drawings]

Figure 1 shows the three-phase zone for a typical nickel-based anode,
A schematic diagram showing a two-phase zone when a hydrogen permeable metal is used for the electrode, and FIG. 2 shows an ordinary nickel porous body and a titanium / manganese alloy which is a hydrogen permeable metal mixed and used as an anode material. FIG. 3 is a diagram showing the relationship between time and performance of a molten carbonate fuel cell, and FIG. 3 shows a case where an ordinary nickel porous body and a titanium / manganese alloy, which is a hydrogen-pervious metal, are used as an anode material. It is a figure showing IV characteristics of a molten carbonate fuel cell.

Claims (6)

[Claims] 1. A molten carbonate fuel cell which operates using a gas containing hydrogen as a fuel, wherein the anode metal is mixed with a hydrogen-permeable metal powder. 2. The molten carbonate fuel cell according to claim 1, wherein the metal for the anode is nickel or a nickel-based alloy. 3. The molten carbonate fuel cell according to claim 1, wherein the hydrogen-permeable metal powder is made of a metal that absorbs hydrogen at room temperature and does not absorb hydrogen at a battery operating temperature. 4. A molten carbonate fuel cell which operates using a gas containing hydrogen as a fuel, wherein the anode metal is coated with hydrogen-permeable metal powder of copper, aluminum, nickel, silver, platinum, or palladium. A molten carbonate fuel cell, characterized in that: 5. The molten carbonate fuel cell according to claim 4, wherein the metal for the anode is nickel or a nickel-based alloy. 6. The molten carbonate fuel cell according to claim 4, wherein the hydrogen-permeable metal powder is made of a metal that absorbs hydrogen at room temperature and does not absorb hydrogen at a battery operating temperature.