JP2948453B2 - Solid oxide fuel cell - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid oxide fuel cell, and more particularly to an improvement in a heat-resistant alloy separator.

[0002]

2. Description of the Related Art Since a fuel cell directly converts chemical energy of supplied gas into electric energy, high power generation efficiency can be expected. In particular, solid oxide fuel cells (SO
Since the FC operates at a high temperature of about 1000 ° C., the power generation efficiency including the use of waste heat is reduced by the phosphoric acid fuel cell (PA).
FC) and a molten carbonate fuel cell (MCFC). Therefore, P
It is attracting attention as a third-generation fuel cell after AFC and MCFC, and is being studied in various fields. The development of such SOFCs has been preceded by the cylindrical type, but now the flat type SOFC is expected to increase the power generation efficiency per volume.
The development is in the spotlight.

FIG. 5 is an exploded perspective view showing a basic structure of a conventional flat type SOFC, and includes a cell 14 having an air electrode 12 and a fuel electrode 13 arranged on both sides via a solid electrolyte plate 11,
This is a structure in which a plurality of separators 15 are alternately stacked.
Here, the separator 15 has a role of electrically connecting the cells 14 and separating the reaction gas between the air electrode 12 side and the fuel electrode 13 side. Properties required for this type of separator include (1) gas impermeability (density),
(2) good electrical conductivity, (3) good thermal conductivity, and (4) accurate molding (thickness uniformity). In recent years, heat-resistant alloy separators such as Ni-Cr-Fe-based alloys have been widely used in place of the ceramic separators because the conditions (1) to (4) are satisfied and the processing is relatively easy. Have been. The chromium contained in the heat-resistant alloy forms a layer of chromium oxide on the surface of the heat-resistant alloy separator when the battery is operated at a high temperature of about 1000 ° C., and thus plays a role in preventing oxidation and corrosion of the separator at a high temperature. .

[0004]

However, in the above-mentioned conventional battery, chromium contained in the alloy separator and chromium oxide formed on the surface of the alloy move to the air electrode side due to thermal diffusion or the like during the operation of the battery. Since chromium and chromium oxide are mixed in the air electrode, the electrode activity of the air electrode decreases.
Therefore, there is a problem that the battery life is shortened because the battery reaction resistance and the ohmic resistance increase.

[0005] In view of the above problems, it is an object of the present invention to provide a solid oxide fuel cell that suppresses the incorporation of chromium or chromium oxide into the air electrode and has an improved battery life.

[0006]

According to the present invention, in order to solve the above problems, a cell in which a fuel electrode and an air electrode face each other via a solid electrolyte and a heat-resistant alloy separator containing at least chromium are alternately laminated. In the solid oxide fuel cell, the amount of chromium in the heat-resistant alloy separator interposed between the fuel electrode and the air electrode is smaller in the alloy surface portion in contact with the electrode than in the alloy central portion before the temperature rise of the battery. Is characterized by a low

[0007]

Generally, it is assumed that chromium oxide formed on the surface of an alloy separator reacts with oxygen gas supplied through an air electrode to form chromium gas, which diffuses and mixes into the air electrode. In the above configuration, since the chromium content on the surface of the heat-resistant alloy separator (that is, the side in contact with the electrode) is small, the amount of chromium mixed into the air electrode is also reduced as compared with the conventional case. Therefore, the electrode reaction resistance and the ohmic resistance are reduced, and the battery life is improved.

In addition, it was also confirmed that in a battery using the above alloy separator, the incorporation of chromium from the separator into the air electrode was suppressed even after operation for thousands of hours. Although the reason for this is not clear, if the chromium content on the alloy surface is low before the battery temperature rises, even if the battery is operated for a long time after that, even if the battery is operated for a long time, other alloys ( For example, F
This is probably because an oxide of e or Ni) is mainly formed, and a chromium oxide layer is formed therein.

[0009]

【Example】

[Embodiment] FIG. 1 is a sectional view of a main part of a solid oxide fuel cell (10-cell stack) according to an embodiment of the present invention.
ol% yttria-added solid electrolyte plate 1 made of partially stabilized zirconia (size 10 cm × 10 cm, thickness 0.2
mm) via La _0.9 Sr _0.1 MnO ₃ -YS
And a heat-resistant alloy separator 5 for separating a gas. The cells 4 each having an air electrode 2 made of Z and a fuel electrode 3 made of NiO-YSZ are alternately stacked. A sealing material 6 made of a non-conductive high-viscosity melt such as Pyrex glass was interposed between the electrode non-applied surface and the separator 5. A platinum net was disposed between the air electrode 2 and the separator 5 as a current collector 7, and a nickel felt was disposed between the fuel electrode 3 and the separator 5 as a current collector 8.

Hereinafter, the fabrication of the battery will be specifically described. First, the heat-resistant alloy separator 5 was formed as follows. Cr: Fe: Ni = 16: 8: 76 (w
(t%), a heat-resistant alloy separator was produced by cutting, polishing, or the like using the heat-resistant alloy having the composition of (t%). Next, this separator was heat-treated in air at 1050 ° C. for 48 hours to form an oxide film (Cr ₂ O ₃ ) on the surface. Subsequently, the surface of the alloy is polished to form an oxide film (Cr ₂ O) formed on the surface.
₃ ) was removed.

The heat-resistant alloy separator thus produced is hereinafter referred to as (a) separator. Next, the cell 4
Was produced as follows. 8 mol% yttria-added stabilized zirconia (YSZ) powder having an average particle diameter of 0.5 μm and nickel oxide powder having an average particle diameter of 1 μm were prepared as raw materials for the fuel electrode. And slurried using a terpineol solvent and PVB to obtain a slurry for a fuel electrode.

On the other hand, the raw material of the air electrode has an average particle size of 1 μm
La _0.9 Sr _0.1 MnO ₃ powder and an average particle size of 1 μm
m YSZ powders were prepared, mixed at a weight ratio of 8: 2, and slurried in the same manner as the fuel electrode 3 to obtain a slurry for an air electrode. The fuel electrode slurry is applied to one surface of the solid electrolyte plate so as to have a thickness of 50 μm, dried, and then dried in air at 1250 ° C. for 2 hours.
Bake for hours. Subsequently, the slurry for the air electrode was applied to the other surface of the solid electrolyte plate so as to have a thickness of 50 μm, and dried.
After firing for a time, the above cell was prepared.

Finally, (a) ten cells were stacked with the separator interposed therebetween to assemble a battery. The battery fabricated in this manner is hereinafter referred to as (A) battery. Comparative Example 1 Cr: Fe: Ni = 16: 8: 76 (wt
%) Of a heat-resistant alloy separator having a composition of 10% in air.
Heat treated at 50 ° C. for 48 hours to form an oxide film (Cr ₂ O) on the surface.
After forming ₃ ), a heat-resistant alloy separator was produced in the same manner as in Example 1 except that the oxide film was not removed.

The heat-resistant alloy separator thus manufactured is hereinafter referred to as (x ₁ ) separator. Next, this (x
₁ ) A battery was assembled in the same manner as in Example 1 using the separator. The battery fabricated in this manner is hereinafter referred to as (X ₁ ) battery. Comparative Example 2 Cr: Fe: Ni = 16: 8: 76 (wt
%), Except that neither heat treatment nor polishing treatment was performed on the heat-resistant alloy separator having the composition of (%).

The heat-resistant alloy separator thus manufactured is hereinafter referred to as (x ₂ ) separator. Next, this (x
₂ ) A battery was assembled in the same manner as in Example 1 using the separator. The battery fabricated in this manner is hereinafter referred to as (X ₂ ) battery. [Experiment 1] The (a) separator of the present invention and the (x ₁ ) ·
(X ₂ ) Using a separator, the distribution of chromium on the alloy surface (air electrode side surface) was examined from inside the alloy, and the results are shown in FIG. Also, (x ₁ ) of the comparative example
The distribution of chromium , iron, and nickel in the separator was measured, and the results are shown in FIG. 2 and 6
Represents the result of the EPMA line analysis.

As is apparent from FIG. 2, in the (x ₂ ) separator of the comparative example in which neither heat treatment nor polishing treatment was performed, chromium was distributed almost uniformly from the inside of the alloy to the surface of the alloy. Furthermore, only heat treatment is performed,
In the (x ₁ ) separator of the comparative example in which the polishing treatment was not performed thereafter, the distribution of chromium in the vicinity of the air electrode side surface was slightly reduced, but a large amount of the Cr ₂ O ₃ layer was present on the alloy surface. You can see that. In addition, it can be seen from the EPMA line in FIG. 6 that a large amount of the Cr ₂ O ₃ layer exists on the alloy surface.

On the other hand, in the separator (a) of the present invention, since the oxide film formed on the surface by the heat treatment is removed by the polishing treatment, the distribution of chromium on the air electrode side surface is reduced. I understand. Therefore, it is considered that the amount of chromium mixed into the air electrode from the separator during the operation of the battery is reduced. Even when the separator (a) of the present invention is used, chromium is slightly mixed into the air electrode from the separator side.
1000 ° C. by using _1-X Sr _X MnO ₃
In _{La 1-X} Sr _X MnO ₃ and the chromium reacts readily _{La 1-X Sr X Mn 1} -y Cr y O 3 and (Mn
It changes to _{1-Z Cr Z) 3 O} 4. This La _1-X Sr
_{X Mn 1-y Cr y O} 3 is The less substitution amount y of the chromium electrode activity high as a _{La 1-X Sr X MnO 3} , Supin'neru type oxide _{(Mn 1-Z Cr Z)} 3 O 4 Even if a small amount is mixed, the electrode activity is not significantly affected, and thus, the life characteristics of the battery are greatly improved by applying the above surface treatment. [Experiment 2] A continuous discharge test was performed using the battery (A) of the present invention and the batteries (X ₁ ) and (X ₂ ) of the comparative example (all of which are 10-cell laminates). 3 is shown. In the experiment, Air was used as an oxidizing gas, and H ₂ gas humidified at room temperature was used as a fuel gas.
The condition is that the current density is 300 mA / cm ² at 00 ° C.

As apparent from FIG. 3, in the (X ₂ ) battery of the comparative example using the (x ₂ ) separator without any treatment on the alloy separator, the cell voltage peaks at 830 mV after several tens of hours. Began to decrease and after 400 hours 35
It can be seen that the voltage has dropped to 0 mV. Further, in the (X ₁ ) battery of the comparative example using the (x ₁ ) separator which was only subjected to the heat treatment but was not subjected to the subsequent polishing treatment, the Cr ₂ O ₃ layer formed on the alloy surface by the heat treatment contained a large amount of air. It can be seen that the cell voltage is significantly lower than that of the (X ₂ ) battery of the comparative example due to the movement to the center.

On the other hand, in the battery (A) of the present invention using the separator (a) subjected to both the heat treatment and the polishing removal, the decrease in cell voltage is small, and the life characteristics are greatly improved. You can see that there is. This is because the oxide film formed on the surface by the heat treatment is removed by the polishing treatment, so that the distribution of chromium on the air electrode side surface is reduced. [Experiment 3] Using the battery (A) of the present invention and the batteries (X ₁ ) and (X ₂ ) of the comparative example, a continuous discharge was performed for 300 hours. Since the EPMA surface analysis was performed, the results are shown in FIGS.
And FIG.

As is apparent from FIGS. 7 and 8, the (X ₁ ) battery of the comparative example using the separator (x ₁ ) without the surface treatment after the heat treatment, and neither treatment (x ₂ ) In the (X ₂ ) battery of the comparative example using the separator, it can be seen that a large amount of chromium (white image) was mixed into the air electrode from the alloy separator side. In FIGS. 7 and 8, the black mass reflected on the air electrode is considered to be a zirconia mass.

On the other hand, in the battery (A) of the present invention using the separator (a) subjected to the polishing treatment after the heat treatment,
As is clear from FIG. 4, the distribution of chromium on the alloy separator surface is small, and the chromium diffused from the separator side into the air electrode is small. [Others] The heat treatment temperature of the heat-resistant alloy separator is preferably 1000 ° C. or higher, which is the battery operating temperature.
When applied to a low-temperature operating battery of less than 00 ° C., heat treatment at a lower temperature has the same effect.

[0022]

According to the present invention, the amount of chromium on the alloy surface of the heat-resistant alloy separator (that is, the side in contact with the electrode) is smaller than that inside the alloy. The amount of chromium moving in is also reduced.
Therefore, chromium is prevented from being mixed into the air electrode as compared with the related art, so that the electrode reaction resistance and the ohmic resistance are reduced, and the battery life is improved.

[Brief description of the drawings]

FIG. 1 is a sectional view of a main part of a solid oxide fuel cell (10-cell stack) according to an embodiment of the present invention.

FIG. 2 is a graph showing the distribution of chromium from the inside of the alloy to the air electrode side surface in the (a) separator of the present invention and the (x ₁ ) · (x ₂ ) separator of the comparative example.

FIG. 3 shows (A) the battery of the present invention and (X ₁ ) · of the comparative example.
(X ₂₎ in the case of performing continuous discharge test using a battery, is a graph showing the change over time in the average cell voltage.

FIG. 4 shows EPMA of chromium in the air electrode cross section after performing continuous discharge for 300 hours using the battery (A) of the present invention.
It is an X-ray photograph which shows a surface analysis result.

FIG. 5 is an exploded perspective view showing the basic configuration of a conventional flat solid oxide fuel cell.

FIG. 6 shows chromium in a (x ₁ ) separator of a comparative example ,
It is a graph which shows distribution of iron and nickel.

FIG. 7: EPM of chromium in the air electrode cross section after performing continuous discharge for 300 hours using the (X ₁ ) battery of the comparative example
It is an X-ray photograph showing an A-plane analysis result.

FIG. 8: EPM of chromium in the air electrode cross section after performing continuous discharge for 300 hours using the (X ₂ ) battery of the comparative example
It is an X-ray photograph showing an A-plane analysis result.

[Explanation of symbols]

 Reference Signs List 1 solid electrolyte plate 2 air electrode 3 fuel electrode 4 cell 5 separator

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yukinori Akiyama 2--18 Keihanhondori, Moriguchi City Sanyo Electric Co., Ltd. (72) Inventor Toshihiko Saito 2-18-18 Keihanhondori, Moriguchi City Sanyo Electric Co., Ltd. 56) References JP-A-3-119663 (JP, A) JP-A-4-137465 (JP, A) JP-A-5-121060 (JP, A) JP-A-6-188003 (JP, A) (58) ) Surveyed field (Int.Cl. ⁶ , DB name) H01M 8/02 H01M 8/12

Claims (1)

(57) [Claims] 1. A solid oxide fuel cell comprising a cell in which a fuel electrode and an air electrode face each other via a solid electrolyte and a heat-resistant alloy separator containing at least chromium are alternately stacked, A solid oxide fuel cell characterized in that the amount of chromium in the heat-resistant alloy separator interposed between the air electrodes is smaller at the surface of the alloy in contact with the electrode than at the center of the alloy before the battery is heated.