JPH10208759A - Separator for molten carbonate type fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for a molten carbonate type fuel cell having a corrosion resistant film with high corrosion resistance. SOLUTION: This separator is for a molten carbonate type fuel cell, which is produced by layering unit cells constituted of an anode, a cathode, and an electrolyte plate sandwiched between these electrodes, to support the electrolyte plate while being set adjacently to the plate, and the separator has an Al/Fe alloy layer formed by heating treatment of an Al/Fe elemental component layer containing Al and Fe given to a seal face in the seal face side of the separator on the opposite to the electrolyte plate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a separator for use in a molten carbonate fuel cell, and more particularly, to a separator in which a seal portion in contact with an electrolyte has improved corrosion resistance.

[0002]

2. Description of the Related Art In recent years, fuel cells that generate electricity by reacting hydrogen and air have attracted attention as new power generation devices. As one of such fuel cells, there is a molten carbonate fuel cell (hereinafter, also referred to as MCFC). MCFC
As shown in FIG. 11, both surfaces of an electrolyte plate 10 formed by impregnating a molten carbonate of an alkali metal as an electrolyte into a porous material such as lithium aluminate are connected to a fuel electrode (anode) 12 and an air electrode (cathode). 14 to form a single cell. Further, in order to obtain practical power, the single cell is laminated in multiple layers with a separator 16 interposed therebetween.
H ₂ and CO _{2 serving as} fuel are supplied to a passage space 18 formed between the fuel cell 6 and the fuel electrode 12, and air and C are supplied to a passage space 20 formed between the separator 16 and the air electrode 14.
The basic configuration is that O ₂ is supplied.

[0003] In order for such an MCFC to be put into practical use, a life of 40,000 hours or more is required.
One of the factors limiting the life of the FC is corrosion of the wet seal portion (contact portion with the molten carbonate) 22 of the separator 16 by the molten carbonate. That is, the separator 16
As shown in FIG. 12, the electrolyte plate 10 normally supports the fuel electrode 12 and the air electrode 14 and also supports the electrolyte plate 10 impregnated with a highly corrosive carbonate. For this reason,
The material of the separator 16 is, for example, SUS316L or SUS31 in order to secure corrosion resistance against molten carbonate.
Generally, stainless steel such as 0S is used. However, in particular, since the wet seal portion 22 is directly in contact with and supports the electrolyte, oxygen, Fe eluted from stainless steel, and Li ₂ CO ₃ in the molten carbonate are simultaneously present, so that LiFeO ₂ is present on the surface layer. Corrosion progresses as _two layers are formed. Corrosion causes an increase in internal resistance and a problem of disappearance of molten carbonate due to corrosion. As a result, the life of the battery becomes shorter than the target value.

[0004]

In order to solve such a problem of corrosion of the wet seal portion 22, several methods have been studied. As a typical method,
Al / F obtained by diffusing the Al film formed on the sealing surface
A method of coating with an e-alloy layer is used (see FIG. 13). It is known that the corrosion resistance effect of this Al / Fe alloy layer contributes to the improvement of corrosion resistance by forming a LiAlO ₂ main layer (including LiFeO ₂ ) by reaction with molten carbonate in the early stage of operation. However, it has been found that in this corrosion resistant film, LiAlO ₂ is in the form of fine powder and easily peels off. This is not preferable because a new corrosion-resistant film is further formed in the peeled portion, and as a result, corrosion proceeds. In addition, there is a problem that elution of the Al / Fe alloy layer and corrosion of the separator material are inevitable in the early stage of operation when the formation of the corrosion resistant film is not sufficient. Therefore, an object of the present invention is to provide a molten carbonate fuel cell separator having a corrosion resistant film having excellent corrosion resistance.

[0005]

Means for Solving the Problems In order to solve the above-mentioned technical problems, the present inventors have proposed a method of forming a separator by using an Al / Fe alloy layer formed by applying Fe on a sealing surface in addition to Al. It has been found that an excellent corrosion resistant film can be obtained by coating the sealing surface, and the present invention has been completed. That is, the invention described in claim 1 is in contact with the electrolyte plate of a molten carbonate fuel cell formed by stacking single cells each having an anode electrode, a cathode electrode, and an electrolyte plate interposed therebetween. An Al / Fe single-component layer containing an Al component and an Fe component applied to the sealing surface is formed on the sealing surface of the supporting separator, the sealing surface facing the electrolyte plate, by heat treatment. A separator for a molten carbonate fuel cell, comprising an Al / Fe alloy layer. In this specification, an Al / Fe single-component layer means Al and Fe by heat treatment.
And a layer containing an Al component and an Fe component capable of forming an alloy of According to the present invention, an Al / Fe single-component layer containing an Al component and an Fe component is formed on the sealing surface of the separator, and then heat-treated. By this heat treatment, these are alloyed and an Al / Fe alloy layer is formed. This Al / Fe alloy layer contacts the molten carbonate,
Upon reaction, a good corrosion-resistant film is formed on the surface side of the alloy layer.

According to a second aspect of the present invention, there is provided the separator according to the first aspect, wherein the separator portion including the sealing surface contains Fe. is there. Conventionally, when the separator portion including the sealing surface contains Fe (for example, in the case of stainless steel), heat treatment of the Al single-component layer formed on the sealing surface causes Fe to move to the single-component layer side by thermal diffusion. . Also, with the progress of corrosion, Al
Fe moved to the diffusion layer side, and the Fe concentration on the surface layer side of the sealing surface was reduced. As a result, the integrity of the formed Al diffusion layer and the sealing surface was reduced. According to the present invention, Al / Fe containing not only Al but also Fe
Since the single-component layer is formed, the diffusion of Fe from the sealing surface side is suppressed in the heat treatment of the Al / Fe single-component layer, as compared with the conventional heat treatment with only the Al layer formed. . Furthermore, since Fe is previously added to the generated Al / Fe alloy layer, the movement of Fe from the separator side to the alloy layer side due to the progress of corrosion is suppressed,
The integrity of the Al / Fe alloy layer and the sealing surface is maintained.

According to a third aspect of the present invention, in the separator according to the first aspect, the separator portion including the sealing surface contains Fe, and Al and F provided on the sealing surface.
e is a separator for a molten carbonate fuel cell, having a weight ratio of 95: 5 to 90:10. The sealing surface is formed from a separator portion containing Fe,
When the weight ratio of Al and Fe provided on the sealing surface is within the above range, the obtained Al / Fe alloy layer reacts with the molten carbonate to form a good corrosion resistant film.

According to a fourth aspect of the present invention, there is provided the separator according to any one of the first to third aspects, wherein
A separator for a molten carbonate fuel cell, wherein a LiFeO ₂ / LiAlO ₂ layer containing LiFeO ₂ as a main component is formed on the surface side of the 1 / Fe alloy layer. According to the present invention, the main layer of LiFeO ₂ is Al / Fe
By being formed on the surface side of the alloy layer, good corrosion resistance is exhibited. The LiFeO _{2 main} layer is composed of the Al /
By incorporating the Fe alloy layer into a molten carbonate fuel cell and contacting and reacting with the molten carbonate, or
Before being incorporated in the molten carbonate fuel cell, the molten carbonate is formed by applying the molten carbonate or immersing the molten carbonate in the molten carbonate to react with the molten carbonate.

[0009]

Embodiments of the present invention will be described below in detail. According to the present invention, an Al / Fe alloy layer obtained by heat-treating an Al / Fe single-component layer formed on a sealing surface by thermal spraying or the like is formed on a sealing surface of an MCFC separator facing an electrolyte plate. Features. According to this alloy layer, by contact with molten carbonate,
When the Fe in the alloy layer is in the molten carbonate, Li ₂ CO ₃
React with, easily generates a LiFeO _2, contains primarily LiFeO ₂ / Li a LiFeO ₂ on the surface side of the alloy layer
AlO ₂ layer (hereinafter, referred to as LiFeO ₂ main layer.) Are formed. Since this LiFeO ₂ main layer is formed on the surface side of the Al / Fe alloy layer and is integrated with the Al / Fe alloy layer, it is difficult to peel off (FIG. 1).
reference). As a result, in the early stage of operation of the molten carbonate,
Corrosion-resistant LiFeO ₂ is easily generated to suppress corrosion, and in long-time operation, the main layer of LiFeO ₂ is hard to peel off, so that the progress of corrosion is suppressed. Thus, in the present invention, Al / Fe alloy layer is formed after adding Fe in addition to Al, and Al / Fe obtained by applying and diffusing Al by the conventional method is obtained.
It is possible to form a corrosion resistant film having a higher corrosion inhibiting effect than the alloy layer.

In the present invention, the Al / Fe alloy layer comprises:
It is formed of an Al / Fe single-component layer formed by applying a material containing Al and Fe on the sealing surface of the separator.
In this case, it does not matter whether the separator portion including the sealing surface contains Fe. As shown in FIG. 1, such an Al / Fe alloy layer is formed by applying a mixed material of Al and Fe onto a sealing surface by thermal spraying, sputtering, plating, or the like, and then diffusing it by heat treatment. Can be used. In addition, Al / F
The thickness of the e-alloy layer is not particularly limited.

In a method of forming an alloy layer by heat treatment after applying a mixed material of an Al component and a Fe component on a sealing surface, if the separator portion including the sealing surface contains Fe, Fe contained inside the plane contributes to the formation of the alloy layer by diffusion during the heat treatment. However, the present invention is significantly different from an Al / Fe alloy layer formed by forming an Al single-component layer on the sealing surface of a separator containing Fe such as stainless steel and then thermally diffusing the same. When the Al / Fe alloy layer reacts with the molten carbonate in such a case, Li containing LiAlO ₂ as a main component is used.
This is because an AlO ₂ / LiFeO ₂ corrosion resistant layer is formed. The surface of such a corrosion-resistant layer is made of LiAl in the form of fine particles.
O ₂ is mainly generated, and LiAlO ₂ is in a state of being very easily desorbed. As a result, corrosion progresses further inside.

The Al / Fe alloy layer is one of the separators.
In particular, it is formed on the surface facing the electrolyte plate. This is because the corrosion by molten carbonate is most severe in this part. However, it may be formed at a portion that comes into contact with the molten carbonate via this electrode. In addition, Al /
The Fe alloy layer is often formed on the sealing surface of the separator that is interposed between the single cells and simultaneously supports the electrolyte plate. In the case where the separator is formed by combining a portion forming a gas passage with a single cell and a seal portion supporting the electrolyte plate, the Al / Fe alloy layer forms a seal on the electrolyte plate side of the seal portion. Even if it is formed on the surface, it can be said that it is formed on the sealing surface of the separator interposed between the single cells.

The weight ratio between Al and Fe in the Al / Fe alloy layer is appropriately set such that a reaction with molten carbonate forms a corrosion resistant film having good corrosion resistance on the surface side. The corrosion resistance can be confirmed by an immersion test or the like in molten carbonate simulating the state of use in a molten carbonate fuel cell. In addition, LiFeO ₂ is formed on the surface of the corrosion resistant film.
You may set so that a main layer may be formed. LiFeO ₂
The formation of the main layer can be confirmed by various surface qualitative and quantitative analysis means such as X-ray diffraction and electron microscope observation.

In particular, an alloy whose separator base material contains Fe,
Alternatively, in the case of stainless steel, Al and Fe
Is preferably 98: 2 to 85:15. More preferably, it is 95: 5 to 90:10. Within this range, when Fe is contained on the separator side, a better corrosion resistant film is formed on the surface side of the Al / Fe alloy layer formed by the thermal diffusion treatment as compared with the conventional Al diffusion layer.

The Al / Fe alloy layer further reacts with the carbonate during or before the operation of the MCFC to form a corrosion-resistant film on the surface side.
As the form of this corrosion resistant film, it is particularly preferable that LiFeO ₂ is mainly formed on the surface layer side as a good corrosion resistant film. When the corrosion resistant film is formed prior to the operation of the MCFC, the corrosion resistant film can be formed simultaneously with the Al / Fe alloy layer. In this case, the heat treatment after applying Al and Fe to the sealing surface is performed in a state where Li ions exist on the surface side of the Al / Fe mixed material. in this case,
Prior to the start of operation of the battery, a good corrosion-resistant film is formed in advance, so that further prevention of corrosion can be achieved in the early stage of operation.

In order to perform the heat treatment in a state where Li ions are applied to the surface side of Al and Fe applied to the sealing surface, a lithium carbonate is applied to a separator provided with an Al / Fe material on the sealing surface ( Carbonate coating method), a method in which the separator is immersed in a molten lithium salt and a heat treatment (a molten carbonate immersion method), or a method in which the lithium atmosphere gas is Al /
There is a method of performing heat treatment while spraying on the surface of the Fe material (Li atmosphere method).

As the carbonate coating method, for example, (Li
_0.62 K _0.38 ) ₂ CO ₃ is applied to the surface of the Al / Fe mixed material and heat-treated at 750 ° C. to 850 ° C. for 2 hours in 5% H ₂ —Ar. As the molten carbonate immersion method, 5% H ₂ -Ar is used in a state where the separator is immersed in a similar molten carbonate.
And heat-treat at 750 ° C. to 850 ° C. for 2 hours. As the Li atmosphere method, for example, an aqueous solution of Li acetate is heated to 50 ° C. and heat-treated at 750 ° C. for 2 hours in an Ar atmosphere.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments. Using stainless steel SUS316L having a plate thickness of 2 mm and 10 mm square as a separator base material, and blasting one surface of the separator base material, Examples 1 and 2, a comparative example, and a control example were respectively subjected to Ar plasma spray spraying. Prepared. In the control example, Fe was added to the lower layer side.
After forming the thermal spray layer, an Al layer is formed by overlapping.

Table 1 Conditions for forming sprayed film Separator material Stainless steel (SUS316L) 10 × 10 × 2 mmthick Sprayed material Al: powder with 99% purity Fe: 0.2 carbon steel (C: 0.2%, Mg: 0.5%, S : 0.04%, P: 0.04%, Fe: residual amount) Examples and Comparative Examples Al / Fe mixing ratio (weight ratio) Film thickness (μm) Example 1 95/5 50 Example 2 90/10 50 Comparative Example 100 / 0 50 Control example Lower layer side 0/100 10 Upper layer side 100/0 70-80

Thus, an embodiment in which an Al / Fe mixed film is formed on the substrate surface, a comparative example in which only an Al film is formed,
For each test piece of the control example in which a double film of an Al layer and an Fe layer was formed, the surface analysis of the sprayed film was performed by X-ray diffraction. The result is shown in FIG. According to the results, only Al was observed in the sprayed films of all the test pieces, and no remarkable difference was observed between the test pieces on the sprayed film surface.

Next, each test piece was subjected to a diffusion treatment at 750 ° C. for 2 hours in a 5% H ₂ -Ar atmosphere to form an alloy layer from the sprayed film. FIG. 3 shows the result of observing the surface of each test piece after the diffusion treatment by X-ray diffraction. According to this result, it was confirmed that Al ₁₃ Fe ₄ was generated on the surface.

Each test piece (Example and Comparative Example) thus heat-treated was subjected to the following corrosion test to evaluate the corrosion resistance. (1) Corrosion test in the early stage of MCFC operation In this test, the amount of change in corrosion weight due to molten carbonate in the early stage of MCFC operation (test time: about 60 hours) was measured using a thermobalance. The test was performed by applying Li / K carbonate to the entire surface of the test piece on which the alloy layer was formed, drying the test piece,
The temperature was raised to 0 ° C. and maintained for about 60 hours, and the change in corrosion weight at 650 ° C. was measured. Table 2 shows the measurement conditions. Table 2 Corrosion test conditions in the early stage of operation Coating carbonate Composition: Li / K = 62/38 mol% Coating amount 20 mg / cm ² Atmosphere 70% Air-30% CO ₂ Temperature 650 ° C Time 60 hours

FIG. 4 shows the test results. In this figure, the vertical axis indicates the amount of weight loss. The lower the bar, the less the material elutes from the surface during the early stages of MCFC operation, ie, the less corrosion. According to this result, the weight change was smallest in Example 1 (Al: Fe = 95: 5), indicating that a stable corrosion resistant film was formed. Then, Example 2 (Al:
Fe = 90: 10), the weight change is small, and
In each of the examples, since the change in weight was smaller than that of the comparative example (Al100), it is clear that the corrosion resistance of the corrosion resistant film according to the present invention is superior to that of the Al diffusion film according to the conventional method.

(2) Corrosion test during long-term MCFC operation In this test, Li / K carbonate was applied to the entire surface of the test piece (Example and Comparative Example), dried, and then heated to 650 ° C. And then held. The test time was 100 hours and 225 hours. In this test, the corrosion weight was measured by measuring the weight of the test piece before applying the carbonate, removing the carbonate attached to the test piece after the test, and then measuring the weight of the test piece. It was calculated by subtracting the weight of the test piece before the test from the weight of the test piece. Table 3 shows the test conditions. Table 3 Corrosion test conditions in the early stage of operation Coating carbonate Composition: Li / K = 62/38 mol% Coating amount 50 mg / cm ² Atmosphere 70% Air-30% CO ₂ Temperature 650 ° C Time 100, 225 hours

FIG. 5 shows the change in corrosion weight after 225 hours. In this figure, the vertical axis indicates the amount of weight increase, and the smaller the amount of weight increase, the smaller the amount of oxide generated by the reaction with the molten carbonate, that is, the smaller the amount of corrosion. . According to this result, in Examples 1 and 2, the increase in the corrosion weight was smaller than that in the comparative example. That is, it was confirmed that the corrosion resistance of the corrosion resistant film of the present invention was superior to that of the conventional method. In particular, in Example 1, the amount of weight increase was the smallest, and the corrosion resistance effect was the largest.

From these results, when the corrosion reaction rate for the molten carbonate was calculated, the corrosion reaction rate of the comparative example was 3.9 × 10 ⁻⁶ mg / cm ² · h, and in Example 1, it was 2.8.
× 10 ^-6 mg / cm ² · h, and in Example 1, the reaction rate was suppressed by about 30% compared to the comparative example. From this, it is clear that the long-term stability is improved.

Further, FIG. 6 shows the results of X-ray diffraction of the test piece after the corrosion test on the corrosion resistant film forming side for the example, comparative example and control example. According to this result, LiAlO ₂ , LiFeO ₂ , Al ₁₃ F
e ₄ was observed. In addition, the results of observing the surfaces of these corrosion resistant films with a scanning electron microscope are shown in FIGS.
Shown in According to this result, FIG. 8A and FIG.
In the corrosion-resistant film of the comparative example of (a), fine particulate L
Although iAlO ₂ was observed, Examples 1 (see FIGS. 7A and 9A) and Example 2 (see FIGS. 7B and 9)
In (b), a state in which LiFeO ₂ crystals were densely formed was observed, and LiAlO _{2 in the} form of fine particles was hardly observed. Also, in the corrosion-resistant films of the control examples (see FIGS. 8B and 10B), Examples 1 and 2 were used.
The surface state as observed in the above was not observed, but mainly LiAlO _{2 in the} form of fine particles. From these results, L
It is clear that the formation of the iFeO ₂ main layer contributes to the improvement of the corrosion resistance of the corrosion resistant film.

[0028]

According to the first, second, third and fourth aspects of the present invention, an Al / Fe alloy layer containing Al and Fe provided on the sealing surface is formed on the sealing surface of the separator. Therefore, the corrosion-resistant film formed on the surface side of this alloy layer has good corrosion resistance.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of the present invention.

FIGS. 2A to 2D show the results of X-ray diffraction of a thermal sprayed film formed in an example.

FIG. 3 shows X of a film after diffusion processing formed in an example.
It is a figure (a)-(d) which shows a line diffraction result.

FIG. 4 is a diagram showing the results of a corrosion test in an early stage of operation.

FIG. 5 is a diagram showing the results of a corrosion test during long-term operation.

6A to 6D show X-ray diffraction results of the corrosion resistant film after the corrosion test.

FIGS. 7A and 7B are scanning electron micrographs of the surface of the corrosion resistant film after the initial corrosion test of Examples 1 and 2 (magnification: 10,000 times).

FIGS. 8A and 8B are scanning electron micrographs (magnification: 10,000 times) of the surfaces of the corrosion resistant films after the initial corrosion test of the comparative example and the control example.

FIGS. 9A and 9B are scanning electron micrographs of the surface of the corrosion-resistant film after the long-term corrosion test of Examples 1 and 2 (a magnification of 10000).

FIGS. 10A and 10B are scanning electron micrographs of the surface of the corrosion-resistant film of the comparative example and the control example after the long-term corrosion test (magnification: 10,000 times).

FIG. 11 is a diagram showing a schematic structure of an MCFC.

FIG. 12 is a view showing a wet seal portion of a separator.

FIG. 13 is a view showing a process of forming a corrosion-resistant film using a conventional Al diffusion film.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takehisa Fukui 2-4-1 Rokuno, Atsuta-ku, Nagoya-shi, Aichi Prefecture Inside the Fine Ceramics Center (72) Inventor Gen Okawa Ryuno, Atsuta-ku, Nagoya-shi, Aichi Prefecture Chome 4-1 Fine Ceramics Center

Claims (4)

[Claims] 1. A separator which is in contact with and supports the electrolyte plate of a molten carbonate fuel cell formed by stacking single cells each comprising an anode electrode, a cathode electrode, and an electrolyte plate interposed therebetween. A sealing surface of the separator facing the electrolyte plate includes A containing Fe and Al given to the sealing surface.
Al / Fe formed by heat treatment of 1 / Fe single component layer
A separator for a molten carbonate fuel cell, comprising an alloy layer. 2. The separator for a molten carbonate fuel cell according to claim 1, wherein the separator portion including the sealing surface contains Fe. 3. The separator according to claim 1, wherein the separator portion including the sealing surface contains Fe, and the weight ratio of Al to Fe provided on the sealing surface is 95: 5 to 5: 5.
A separator for a molten carbonate fuel cell, which has a ratio of 90:10. 4. The separator according to claim 1, wherein a surface of the Al / Fe alloy layer is provided with LiFeO ₂ / LiA containing LiFeO ₂ as a main component.
A separator for a molten carbonate fuel cell, wherein a 10 ₂ layer is formed.