JP2000048836A - Electrolyte and electrolyte holding material for fused carbonate fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolyte holding material to prevent internal short circuit due to elution and deposition of metal of an air electrode material. SOLUTION: An electrolyte holding material of a fused carbonate fuel cell is made so as to contain one or more than two kinds of metallic oxides, specifically transition metals of the third to eleventh groups of the element periodic table, i.e., zinc, lead, and rare-earth metals.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolyte layer of a molten carbonate fuel cell (MCFC), and more particularly to an electrolyte and an electrolyte holding material for preventing a short circuit due to deposition of an air electrode material in the electrolyte layer.

[0002]

2. Description of the Related Art An MCFC comprises a fuel electrode (negative electrode), an air electrode (positive electrode), and an electrolyte layer (electrolyte layer) interposed between the fuel electrode and the air electrode and held by an electrolyte holding material. And one cell is formed, and the cells are stacked with a separator interposed therebetween. Hydrogen (H
₂ ) Fuel gas and carbon dioxide gas mainly
The structure is such that a mixed gas of air and carbon dioxide gas is supplied. Usually, a porous sintered body of a transition metal such as nickel (Ni) is used as a fuel electrode, an oxide porous body such as nickel oxide (NiO) is used as an air electrode, and lithium aluminate (LiAlO ₂ ) is used as an electrolyte holding material. ) Is used by impregnating a molten carbonate (lithium carbonate / potassium carbonate or lithium carbonate / sodium carbonate) into the porous body. Such MCFC
However, in order to be put to practical use, a service life of 40,000 hours or more is required.

[0003]

However, for example, when NiO is used as the air electrode material, the generation of Ni ²⁺ ions by the elution of Ni into the electrolyte during battery operation (formula (1)) and the fuel Ni ^{2+ with} extreme hydrogen (H ₂ )
The reduction of the ions causes the formation of metallic nickel (N
The precipitation of i) (formula (2)) occurs. NiO + CO ₂ → Ni ²⁺ + CO ₃ ² -... Formula (1) Ni ²⁺ + H ₂ + CO ₃ ^2- → Ni + H ₂ O + CO ₂ formula (2)

That is, Ni ²⁺ ions eluted from the air electrode by the long-time battery operation diffuse in the electrolyte toward the fuel electrode. Hydrogen (H ₂ ) supplied as fuel gas from the fuel electrode is dissolved in the molten carbonate and supplied to the electrolyte. As a result, dissolved hydrogen (H ₂ ) acts as a reducing agent for Ni ²⁺ ions in the electrolyte,
The precipitation of metallic nickel (Ni) according to equation (2) occurs. Further, the reaction of the formulas (1) and (2) continues due to the continuation of the battery operation, and the metal nickel grows in a chain or net shape. Connected by metallic nickel. That is, an internal short circuit occurs in the cell, and operation as a battery becomes impossible. According to equation (2), the internal short circuit of the cell due to the deposition of nickel metal mainly depends on the concentration of Ni ²⁺ ions and dissolved hydrogen (H ₂ : reducing agent) in the electrolyte.

[0005] From the above, as means for solving such a problem of the internal short circuit, (1) suppression of elution of the air electrode material into the electrolyte (2) concentration of reducing agent such as dissolved hydrogen in the electrolyte (3) Physically preventing the deposition of chain-like or net-like metal between the fuel electrode and the air electrode. As the means (1), a method of adding a component that suppresses elution of Ni ²⁺ ions into an air electrode material, an electrolyte material, or an electrolyte holding plate has been attempted. As means (2), hydrogen (H
₂ ) A method of lowering the concentration of dissolved hydrogen in the electrolyte by lowering the concentration has been disclosed (JP-A-7-10).
No. 5968). As means (3), a metal mesh layer that electrochemically captures hydrogen (H ₂ ) diffusing from the fuel electrode side is interposed in the electrolyte layer, so that metal is deposited only on the fuel electrode side. Attempts have been made to limit this.

However, in any of the above methods,
Also, the reducing agent (hydrogen (H _Two))
It is disclosed that the active component is present in the electrolyte layer.
Not.

[0007]

According to the present invention, there is provided an electrolyte holding material for a molten carbonate fuel cell containing a metal oxide capable of reducing the concentration of a reducing agent in an electrolyte.

Further, in the present invention, the third to eleventh groups of the periodic table are used.
The present invention also provides an electrolyte holding material for a molten carbonate fuel cell, which contains an oxide of one or more metals selected from group III transition metals, zinc, lead and rare earth metals.

The present invention provides an electrolyte for a molten carbonate fuel cell containing a metal oxide that can reduce the concentration of a reducing agent in the electrolyte. In the present invention, the third to eleventh groups of the periodic table are used.
Also provided is an electrolyte retention for a molten carbonate fuel cell containing an oxide of one or more metals selected from Group III transition metals, zinc, lead and rare earth metals.

Further, the present invention provides a molten carbonate fuel cell having a metal oxide capable of reducing the concentration of a reducing agent such as dissolved hydrogen in an electrolyte in an electrolyte layer.

Each of the above-mentioned inventions has the following effects. By containing the metal oxide in the electrolyte holding material or the electrolyte constituting the electrolyte layer, the concentration of the reducing agent such as dissolved hydrogen in the electrolyte is reduced, and the Ni ²⁺ ions eluted from the air electrode to the metal Ni Reduction is suppressed, and a precipitation reaction is prevented.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, an electrolyte layer is formed.
The concentration of the reducing agent in the electrolyte is added to the electrolyte holding material or electrolyte.
Metal oxides that can reduce
Also called thing. ). here,
The reducing agent that may be present in the electrolyte includes dissolved hydrogen.
(H_Two) And carbon monoxide (CO). MCFC smell
Hydrogen (H _Two)
It dissolves in the decomposition and exists as a reducing agent (dissolved hydrogen).
Therefore, the reducing agent in the electrolyte in the present invention is
Dissolved hydrogen (H_Two) Is preferable.

The case where the metal oxide can reduce the concentration of a reducing agent such as hydrogen in the electrolyte is, for example, a case where the metal oxide itself is a source of an oxidizing agent that oxidizes the reducing agent such as hydrogen in the electrolyte. There may be. In addition, when the metal oxide has a catalytic action to promote the reaction of the oxidizing agent such as oxygen oxidizing the reducing agent such as hydrogen in the electrolyte, or when the metal oxide has an action to increase the concentration of the oxidizing agent such as oxygen in the electrolyte. There is. When the concentration of the reducing agent in the electrolyte is reduced, the reduction of metal ions such as Ni ²⁺ ions eluted from the air electrode is suppressed, and the deposition of metal is suppressed. Further, when the concentration of the reducing agent in the electrolyte is reduced, the oxidation-reduction potential is increased in at least one portion in the range from the fuel electrode to the air electrode. If the metal ions eluted from the air electrode are not reduced at the increased oxidation-reduction potential, this also means that the reduction of the metal ions eluted from the air electrode is suppressed.

The case where the metal oxide promotes the reaction of oxidizing dissolved hydrogen in the electrolyte by an oxidizing agent such as oxygen means, for example, when the reducing agent in the electrolyte is dissolved hydrogen, the following formula (3) This is the case where the reaction of (2) is catalyzed. H ₂ +1/2 O ₂ → H ₂ O Formula (3) The case where the metal oxide serves as a supply source of the oxidizing agent for oxidizing the reducing agent in the electrolyte is, for example, the reduction in the electrolyte. When the agent is dissolved hydrogen, the reaction of the following formula (4) proceeds. H ₂ + MO _X → H ₂ O + MO _X-1 Formula (4)

Examples of the present metal oxide include oxides of one or more metals selected from transition metals of Groups 3 to 11, zinc, lead and rare earth metals. Preferably, a metal oxide of a transition metal belonging to Groups 3 to 9 is used. Also, individually and preferably mentioned metal oxides are cobalt, nickel, iron, titanium, vanadium, manganese, tungsten, copper, ruthenium, osmium, zinc, lead,
Cerium oxide. The present metal oxide may be an oxide of one kind of metal or a composite oxide of two kinds of metals. Further, the present metal oxide can be used alone or in combination of two or more. The present metal oxide can reduce the concentration of the reducing agent such as dissolved hydrogen in the electrolyte because the selective catalytic action on the reducing agent and the valence of the metal constituting the metal oxide are reduced.

The present metal oxide may be present in the MCFC electrolyte layer. That is, the present metal oxide may be present together with either or both of the electrolyte and the electrolyte holding material. Here, the electrolyte in the present invention is M
It is an electrolyte in CFCs and is a carbonate that melts at operating temperatures. Usually, a eutectic salt of lithium carbonate and potassium carbonate or a eutectic salt of lithium carbonate and sodium carbonate is used. In addition to these electrolyte materials, for example,
A component for preventing the elution of nickel oxide, which is an air electrode material, may be added in a range that does not impair the function of the present metal oxide. Further, the electrolyte holding material is an electrolyte holding material in MCFC, and usually, γ-LiAlO ₂ is preferably used. In order to obtain an electrolyte layer having the present metal oxide, the present metal oxide is added to a layer constituting material (a material of an electrolyte or an electrolyte holding material) in any one of various conventionally known methods for forming an electrolyte layer. Then, an electrolyte layer to which the present metal oxide is finally applied may be formed. After applying the present metal oxide to the electrolyte, the electrolyte may be applied to the electrolyte holding material by impregnation or the like to form an electrolyte layer, or after applying the present metal oxide to the electrolyte holding material, the electrolyte To form an electrolyte layer. Further, an electrolyte layer may be obtained by mixing an electrolyte, an electrolyte holding material, and the present metal oxide.

For example, to obtain an electrolyte holding material provided with the present metal oxide, an aqueous solution or an alcohol solution of various metal salt compounds such as acetate, nitrate and sulfate of the metal element constituting the present metal oxide can be obtained. There is a method in which the electrolyte holding material is impregnated, the impregnated electrolyte holding material is dried, and then thermally decomposed. Also,
There is also a method in which the electrolyte holding material and various metal salt compounds are uniformly mixed and then thermally decomposed. Further, there is a method of mechanically coating the surface of the electrolyte holding material with various kinds of the present metal oxide. Thereafter, the electrolyte holding material may be formed before applying the electrolyte. When the electrolyte holding material or the electrolyte layer provided with the present metal oxide is obtained as described above, the content of the present metal oxide with respect to the electrolyte holding material is preferably 0.05 mol% or more. If the content is less than 0.05 mol%, the effect of suppressing the precipitation of the metal component of the air electrode in the electrolyte is small. In addition,
The present metal oxide is preferably provided in a range of 1 mol% or less. This is because, even if it is added in excess of 1 mol%, the effect of suppressing precipitation no longer significantly improves.

In order to obtain an electrolyte provided with the present metal oxide, a molten carbonate used in MCFC, for example, a eutectic salt of lithium carbonate and potassium carbonate (molar ratio of 62:38), a eutectic of lithium carbonate and sodium carbonate Salt (52: 4 molar ratio)
8) and the like, and can be obtained by mixing the present metal oxide, melting, cooling, and then pulverizing. When the electrolyte layer is formed by mixing such an electrolyte and an electrolyte holding material, the content of the metal oxide with respect to the electrolyte holding material is preferably adjusted to 0.05 mol% or more and 1 mol% or less.

Further, the metal oxide raw material, the electrolyte raw material, and the electrolyte holding material raw material are mixed in a ball mill in an organic solvent such as toluene, ethanol, etc., then dried and crushed. Alternatively, a metal oxide raw material and an electrolyte holding material raw material are added to a molten electrolyte, mixed, cooled, solidified and then pulverized to obtain an electrolyte layer powder.

The electrolyte holding material, the electrolyte and the electrolyte layer of the present invention can be applied to both internally modified and externally modified MCFCs. Further, the air electrode of the MCFC to which the electrolyte holding material, the electrolyte, and the electrolyte layer of the present invention are applied includes:
It is preferable to use nickel oxide (NiO). Further, it is preferably a solid solution of nickel oxide and magnesium oxide. More preferably, it is a solid solution having a molar ratio of nickel to magnesium of 95: 5 to 75:25. By using such an air electrode material, elution of Ni into the electrolyte under pressure is suppressed, so that precipitation of metallic Ni in the electrolyte is further suppressed.

[0021]

The present invention will be described below with reference to specific examples. The present invention is not limited to the following specific examples. Example 1 A predetermined amount of cobalt nitrate was dissolved in a mixture of 95% propanol and 5% ethanol to prepare an alcohol solution of cobalt. Γ-L as electrolyte holding material
Using iAlO ₂ , the cobalt alcohol solution was added and mixed in γ-LiAlO ₂ such that the mole percent of cobalt was 0.05, 0.5, ₁ , and then dried in air. Pyrolyzed at 800 ° C, each 0.05
Γ-LiAlO ₂ to which mol%, 0.5 mol%, and 1 mol% of cobalt oxide were added was obtained.

These various electrolyte retaining materials and a eutectic salt of lithium carbonate: potassium carbonate (62 mol%: 38 mol%) were mixed at a weight ratio of 40:60, and samples 1 to 3 ( Cobalt oxide content with respect to electrolyte holding material: 0.
(Corresponding to 0.5 mol%, 0.5 mol%, and 1 mol%, respectively) of the molten carbonate type fuel cell.

Γ-L without cobalt
An MCFC electrolyte material of a comparative example was obtained in the same manner as in the example sample, except that iAlO ₂ was used as an electrolyte holding material. Furthermore, only the eutectic salt of the carbonate, which was not retained in γ-LiAlO _2, was used as a control.

Samples 1 to 3 of the examples, a comparative example, and a control example were each placed in an alumina crucible, melted at 650 ° C., and passed through an alumina tube in a molten electrolyte.
200 ppm hydrogen (H ₂ ), 100 ppm oxygen (O
Argon gas containing ₂ ) was bubbled at a flow rate of 10 ml / min to capture gas coming out of the electrolyte, and the hydrogen concentration in this gas was measured by gas chromatography. Table 1 shows the results.

[0025]

Table 1 Hydrogen concentration (ppm) at the electrolyte material outlet Sample 1 0.05 mol% cobalt oxide-containing γ-LiAlO ₂ + carbonate 160 Sample 2 0.5 mol% cobalt oxide-containing γ-LiAlO ₂ + carbonate 130 Sample 3 1 mol% oxidation Cobalt-containing γ-LiAlO ₂ + carbonate 125 Comparative Example γ-LiAlO ₂ + carbonate 180 Control Example Carbonate only 180

As shown in Table 1, in the comparative example and the control example, the hydrogen concentration was almost the same and there was almost no change with respect to the inlet concentration. The outlet hydrogen concentration was reduced as compared with the example and the control example. That is, in samples 1 to 3, it was found that the concentration of hydrogen in the electrolyte was reduced by the applied cobalt oxide.

Example 2 A mixed powder of the γ-LiAlO ₂ containing 0.5 mol% cobalt oxide obtained in Example 1 and an electrolyte material (sample 2) was molded (50 MPa, 200 M).
(Pa pressure for 20 minutes) to form three 1 mm-thick plate-like electrolyte plates, and these were stacked to form an electrolyte layer. An MCFC-like cell shown in FIG. 1 was constructed using this electrolyte layer. In order to support the electrolyte layer, a Ni porous plate was arranged, and then a gold electrode was arranged on the anode side (the lower side in FIG. 1). On the cathode side (upper side in FIG. 1), a gold (Au) electrode was arranged as a reference electrode. Further, in order to measure the oxidation-reduction potential inside the electrolyte layer, a gold (Au) wire is inserted between the electrolyte plates constituting the electrolyte layer, so that 1 / the distance from the anode side to the cathode side inside the electrolyte layer is measured.
A gold (Au) electrode as a working electrode was arranged at the position 3 and the position 2/3. On the anode (fuel electrode) side of this cell,
A mixed gas of CO ₂ and H ₂ (CO ₂ / H
₂ ; 3/7), and a mixed gas of CO ₂ and air (CO ₂ / air; 3/7) is supplied to the cathode (reference electrode) side as an air electrode gas. It has become so.

Using this cell, the internal potential of the electrolyte at 650 ° C. with respect to a standard oxygen electrode (CO ₂ / O ₂ ; 2/1) was measured. As a comparative example, the electrolyte material of Comparative Example in Example 1 (γ containing no cobalt oxide) was used.
-LiAlO ₂ + carbonate) to form an electrolyte layer in the same manner as in the case of forming the electrolyte layer of the present embodiment, and in the same manner as in the present embodiment, the electrolyte internal potential when applied to the cell shown in FIG. Was measured. The result is shown in FIG. Further, based on the internal potential, the concentrations of hydrogen and oxygen (mol /
L) is shown in FIG.

As shown in FIG. 2, when the electrolyte layer in Example 2 was used, the internal potential increased almost linearly from the anode side to the cathode side. It has been reported that the oxidation-reduction potential at which Ni precipitates is -600 to -700 mV (Journal of Power Source, Vol. 71 (19).
98) pp239-243). Considering this deposition potential, it was found that the use of the electrolyte layer in Example 2 was in a state where Ni deposition could occur only in the immediate vicinity of the anode. On the other hand, when the electrolyte layer of the comparative example was used, it was found that Ni deposition could occur in the entire electrolyte layer.

Further, from the table of oxygen (O ₂ ) concentration and hydrogen (H ₂ ) concentration shown in FIG. 3, when the electrolyte layer of Example 2 is used, the electrolyte layer on the cathode side is compared with the electrolyte layer of Comparative Example. It is clear that the closer it is, the lower the hydrogen concentration.
At the same time, it was clear that the oxygen concentration increased nearer the cathode side. Further, by using the electrolyte layer of Example 2 over the entire region from the anode side to the cathode side, the hydrogen concentration was lower and the oxygen concentration was higher than when the electrolyte layer of the comparative example was used.

As described above, according to the electrolyte layer of Example 2, since the concentration of the reducing agent such as hydrogen in the electrolyte can be reduced, the deposition of metallic nickel (Ni) only in the vicinity of the anode in the electrolyte layer. A possible area is formed,
In other regions, precipitation of Ni was suppressed. Therefore, it was found that formation of Ni from the anode side to the cathode side in the form of a net or a chain in the electrolyte was suppressed. It has been found that the use of such an electrolyte layer prevents the internal short circuit of the cell. From the above results, by using an electrolyte layer that reduces the concentration of a reducing agent such as hydrogen in the electrolyte or increases the concentration of an oxidizing agent, the oxidation-reduction potential in at least a part of the range from the anode to the cathode can be reduced to metallic nickel. It was found that the formation of the Ni deposition suppression region so as to exceed the deposition potential of (Ni) can prevent the internal short circuit of the cell.

[0032]

According to the present invention, it is possible to provide an MCFC capable of preventing an internal short circuit due to the deposition of a metal element of an air electrode material.

[Brief description of the drawings]

FIG. 1 is a diagram showing an MCFC-like cell used in Example 2.

FIG. 2 is a diagram showing a measurement result of an oxidation-reduction potential inside an electrolyte layer in Example 2.

FIG. 3 is a table showing a table of oxygen and hydrogen concentrations based on the oxidation-reduction potential obtained in Example 2.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Gen Okawa 2-4-1 Rokuno, Atsuta-ku, Nagoya-shi, Aichi F-term in the Fine Ceramics Center (reference) 5H026 AA05 EE12

Claims (5)

[Claims] 1. An electrolyte holding material for a molten carbonate fuel cell containing a metal oxide capable of reducing the concentration of a reducing agent in an electrolyte. 2. An electrolyte for a molten carbonate fuel cell containing an oxide of one or more metals selected from transition metals of Groups 3 to 11 of the periodic table, zinc, lead and rare earth metals. Holding material. 3. An electrolyte for a molten carbonate fuel cell containing a metal oxide capable of reducing the concentration of a reducing agent in the electrolyte. 4. An electrolyte for a molten carbonate fuel cell containing an oxide of one or more metals selected from transition metals of Groups 3 to 11 of the periodic table, zinc, lead and rare earth metals. . 5. A molten carbonate fuel cell having a metal oxide capable of reducing the concentration of a reducing agent in an electrolyte in an electrolyte layer.