JPH0554904A - Manufacture of solid electrolyte for fuel cell - Google Patents

Abstract

PURPOSE:To improve the electric conductivity and heat resistance. CONSTITUTION:Lace powders and SrF2 powders are weighed to make a material including LaF3 95mol% and SrF2 5mol%, which is then ground and dry mixed in an agate mortar for about one hour. Obtained ground powders are weighed by 2g, and pressed under a pressure of 1ton/cm<2> to obtain a molded body. This molded body is heated in an argon atmosphere furnace at a speed of 200 deg.C/h, and it is baked at 1180 C for one hour to obtain a baked body. One of obtained baked bodies is then crushed, and an X-ray refraction is taken.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a solid electrolyte for a fuel cell.

[0002]

2. Description of the Related Art Generally, a solid oxide fuel cell using stabilized zirconia (YSZ) is known. However, the operation of the fuel cell using this YSZ is about 1
Due to the high temperature of 000 ° C., a heat resistant material must be used as the material used. Therefore, in order to lower the operating temperature of the cell, the development of a fuel cell that operates at 700 to 800 ° C. using another solid electrolyte such as CeO ₂ or Bi ₂ O ₃ is under way. However, these materials have a drawback that they are reduced in a reducing atmosphere such as a hydrogen atmosphere.

Therefore, the inventor has found that the low temperature (300.degree.
In order to increase the electric conductivity of lanthanum fluoride (LaF ₃ ) or lanthanum fluoride as a solid electrolyte capable of operating at 0 ° C., divalent strontium Sr, barium Ba, etc. are doped to form La _1-x M _x F _{3 -x} (M = Sr, Ba, C
a) Research has been carried out using a material having a composition such as (x = 0.05 to 0.10).

[0004]

[Problems to be Solved by the Invention] Sr, B in LaF ₃
La _1-x M _x F _3-x (x = 0.0) doped with a, Ca
The electric conductivity of 5 to 0.10, M = Sr, Ba, Ca) is much higher than that of LaF ₃ and is promising as a solid electrolyte for fuel cells. However, LaF ₃ , La _1-x M
material _x F _3-x, because there is no heat resistance, as raising the operating temperature, or by reaction of LaF ₃ crystal structure and O ₂ of the air electrode side, there is a disadvantage that changes in LaOF crystal structure.
Further, in order to reduce the electrode interface resistance, an oxygen electrode side electrode (for example, La ₀ .6 Sr ₀ .4 C ₀ O ₃ or La ₀ .6 Sr ₀ .4 Co) is used.
LaF may be increased by increasing the firing temperature during the formation of a perovskite electrode such as _0.98 Ni _0.02 O ₃ ) on the solid electrolyte.
There is a problem that ₃ or La _1-x M _x F _3-x s reacts with oxygen in the oxygen electrode side electrode to change to a LaOF crystal structure.

[0005] Density of LaOF is 6.02 g / cm ^3, since larger 5.96 g / cm ³ of LaF _3, La
When F ₃ changes to LaOF, there are problems that volume contraction occurs, cracks occur in the solid electrolyte, and power generation as a fuel cell cannot be performed. On the other hand, LaO
The electric conductivity of F. It has been measured by Pelloux et al. (Reference CR Acad. Sci, Ser.
C, 276, 241 (1973)), for example, 500 ° C.
However, the electric conductivity is too low to be used as a solid electrolyte for fuel cells, which is as low as about 1.5 × 10 ⁻³ Scm ⁻¹ .

The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for producing a solid electrolyte for a fuel cell, which is intended to improve electric conductivity and heat resistance.

[0007]

In order to achieve the above object, the present invention provides La _1-x M _x F _1-x (x = 0.05 to
0.20, M = Sr, Ba, Ca), the substance represented by the composition formula is fired in a zirconia container at 1200 ° C. for several hours, and then La _1-x M _x OF _1-x (x = 0.05). ~ 0.20,
M = Sr, Ba, Ca) is a composition represented by a composition formula.

[0008]

Since [action] which fired body prior to firing in the air is _{La 0 · 95 Sr 0 · 05} F 2 · 95 becomes crystallographically The LaOF crystal structure after sintering, the sintered body is La 0 _{· 95} Sr _{0 · 05} F _{2 · 35}
Fluorine in the inside is replaced with oxygen O, and La _{0 .95} Sr _{0 .05} O ₁
The composition is ± _y F _{2 · 95} (y is a number close to 0). For example, assuming y = 0, La _{0 · 98} Sr _{0 · 05} OF
_This is 0.95, which means that F defects have been introduced into the crystal.

[0009]

Embodiments of the present invention will be described below. As a starting material, -100 mesh manufactured by Optron Co., LaF ₃ powder having a purity of 99% and -100 mesh, SrF ₂ powder having a purity of 99.9% were used, and LaF ₃ 95 mol% and SrF ₂ were used.
The mixture was weighed so as to be 5 mol%, ground in an agate mortar for about 1 hour, and dry-mixed. 2 g of this mixed ground powder was weighed and pressed at a pressure of 1 ton / cm ² using a mold having a diameter of 18φ to obtain a molded body. The compact was heated in an argon atmosphere furnace at a rate of 200 ° C./h and heated at 1180 ° C. for 1 hour.
Burned for hours. When one of the fired bodies was crushed and the X-ray diffraction was measured by an X-ray diffractometer, only the X-ray diffraction peak of LaF ₃ was detected and the X-ray diffraction peak of SrF ₂ was not detected.

Further, when a quantitative analysis of the fired body was carried out,
Compared with La, it contained 5.04 at% and was almost La.
₀ fired bodies _{· 95 Sr 0 · 05 F 2} · 95 were obtained. Then, the fired body was transferred to a zirconia container, and the temperature was 1200 ° C. 6
Burned for hours. After firing, one of the fired bodies was crushed, and X-ray diffraction was measured by an X-ray diffractometer, and it was found that the crystal structure was pure LaOF. La _{0 · 95} Sr before firing in the air is
_{0 · 05} has become a composition of F 2 _{· 95,} since the crystal structural after firing in air became LaOF crystal structure, the sintered body is La 0 _{· 95} Sr 0 _{· 05} F 2 _{· 29} Fluorine F is replaced with oxygen O, and La _{0 · 95} Sr _{0 · 05} O ₁ ± _y F _{0 · 95} 2y
(Y is a number close to 0). For example, assuming that y = 0, La _{0 .95} Sr _{0 .05} OF ₀ .95 is obtained, which means that F defects are introduced into the crystal. In addition,
Calculated filling rate of fired pellets 84-90%
It was.

Platinum pt is sputtered on both end faces of the pellet to form electrodes, and the electric conductivity is measured at 400 ° to 800 ° C. by the complex impedance method. The results are shown in the following table.

[0012]

[Table 1]

In the table, La _1-X Sr _x O ₁ ± _y F _1-x ± _2y (x = 0.0) manufactured by the same manufacturing method as described above.
3 to 0.20, y = 0) and the measurement results of the electric conductivity of LaOF are also shown. Since y = 0, La _1-X Sr _x O ₁ ± _y F _1-x ± _2y is La _1-X Sr _x OF _1-x.
Is written.

As is clear from the above table, La _1-X S
The electrical conductivity of the r _x OF _1-x (x = 0.03 to 0.20) pellets is clearly higher than that of pure LaOF. When x = 0.03 or less, the effect of doping Sr does not appear so much. This is for the following reason. When Sr is doped into LaOF, Sr ²⁺ in the crystal lattice is replaced with La ³⁺ , but La ion is 3
Since the valence and the Sr ion are divalent, the three F ⁻ ions that corresponded to La ³⁺ correspond to Sr ²⁺ , so that the number of originally three becomes two, and the vacancy point is 1 Introduced individually. However, if x = 0.03 or less, the number of vacancies introduced is small, so that it does not contribute so much to the improvement of electric conductivity. X = to improve conductivity
0.05 or more is desirable.

On the other hand, it was confirmed that the conductivity was lowered when x = 0.20 or more was doped. This is for the following reasons. When trying to dope SrF ₂ into LaF ₃ , the solid solubility limit of SrF ₂ is 20 mo even at 1400 ° C.
It is about 1% or less (Ceramics Association Journal 93 [3] 1985)
Although the solid solubility limit of Sr in LaOF has not been investigated,
_This is because if it is considered to be about the same as _3- SrF ₂ , Sr cannot be replaced with La in the LaOF crystal even if 20 mol% or more of Sr is doped, and vacancies cannot be formed. In order to improve the conductivity, x = 0.15 or less is desirable. Therefore, La
If Sr is doped in the OF at x = 0.03 to 0.20, preferably x = 0.05 to 0.15, and La _1-X S is doped.
When r _x OF _1-x (x = 0.05 to 0.15), the electric conductivity was improved, and it was found that the solid electrolyte can be sufficiently used as a solid electrolyte for fuel cells.

Next, La _1-X Sr _x OF _1-x2 powder (x = 0.
03 to 0.20) in (a) oxygen atmosphere humidified at 30 ° C. (H ₂ O + O ₂ atmosphere) and (b) hydrogen atmosphere humidified at 30 ° C. (H ₂ O + H ₂ atmosphere). Were exposed to 975 ° C. for 30 hours and 900 ° C. for 500 hours, and then the presence or absence of the La ₂ O ₃ peak was examined by X-ray diffraction. As a result, La ₂ O ₃ was not detected in any of the samples. The time A until La ₂ O ₃ is formed is represented by the following equation.

[0017]

[Equation 1]

However, A ₀ is a constant, k is Boltzmann's constant, T is absolute temperature, and Q is activation energy. The above experimental value corresponds to a life of 40,000 hours required for a fuel cell, and this time is achieved even at 800 ° C. In the above embodiment, LaF ₃ was doped with Sr to obtain La _1-x
Although the solid electrolyte made of Sr _x OF _1-x has been described as an example, even if Sr is changed to Ba or Ca, similar electric conductivity characteristics and heat resistance characteristics are shown.

[0019]

As described above, according to the present invention,
There is an advantage that the electric conductivity can be improved and the heat resistance can be improved.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] October 7, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0004

[Correction method] Change

[Correction content]

[0004]

[Problems to be Solved by the Invention] Sr, B in LaF ₃
a, _{La 1-x} was doped with _{Ca M x F 3-x (} x =
The electric conductivity of 0.05 to 0.10, M = Sr, Ba, Ca) is much higher than that of LaF ₃ and is promising as a solid electrolyte for fuel cells. However, LaF ₃ , L
For _{a 1-x M x F 3} -x is a material that is not heat resistant, as raising the operating temperature, or by reaction of LaF ₃ crystal structure and _{O 2} of the air electrode side, changes in LaOF crystal structure There is a drawback that ends up. Further, in order to reduce the electrode interface resistance, an electrode on the oxygen electrode side (for example, La _0.6 Sr _0.4 C ₀ O ₃ or La _0.6 Sr _0.4 Co _0.98 Ni _0.02 O
_If the firing temperature at the time of forming a perovskite electrode such as No. ₃ ) on the solid electrolyte is increased, LaF ₃ or La
_1-x M _x F _3-x reacts with oxygen in the oxygen electrode side electrode,
There is a problem that it changes to the LaOF crystal structure.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0008

[Correction method] Change

[Correction content]

[0008]

[Operation] La _0.95 Sr _0.05 before firing in air
The crystal structure of the fired product of F _2.95 is L after firing.
Since it has an aOF crystal structure, the fired body is La
_0.95 Sr _0.05 F _{2.95 Then,} it becomes _{La 0.95} Sr _0.05 OF _0.95 , which means that F defects are introduced into the crystal.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0010

[Correction method] Change

[Correction content]

Further, when a quantitative analysis of the fired body was carried out,
Sr is included at 5.04 at%, which is almost La _0.95.
A fired product of Sr _0.05 F _2.95 was obtained. afterwards,
The fired body is transferred to a zirconia container, and 1200
Calcination was performed for 6 hours. After firing, crush one of the fired bodies and
When X-ray diffraction was measured with a line diffractometer, pure LaO
It had an F crystal structure. La before firing in air
It has a composition of _0.95 Sr _0.05 F _2.95 , and has a LaOF crystal structure in terms of crystal structure after firing in the air. Therefore, the fired body is La _0.95 Sr.
Fluorine F in _0.05 F _2.95 is replaced by oxygen O,
La _0.95 S If y = 0, then La _0.95 Sr _0.05 OF
_{It was 0.95} , which means that F defects were introduced into the crystal. The filling rate of the fired pellets was calculated to be 84 to 90%.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0013

[Correction method] Change

[Correction content]

[0013] _2y is described as La _1-x Sr _x OF _1-x .

[Procedure Amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0016

[Correction method] Change

[Correction content]

Next, La _1-x Sr _x OF _1-x powder (x
= 0.03 to 0.20) in (a) oxygen atmosphere humidified at 30 ° C (H ₂ O + O ₂ atmosphere) and (b) hydrogen atmosphere humidified at 30 ° C (H ₂ O + H ₂ atmosphere). In two atmospheres, 975 ℃ -30 hours and 900 ℃ -50
After exposure for 0 hour, the presence or absence of a La ₂ O ₃ peak was examined by X-ray diffraction. As a result, L
a ₂ O ₃ were detected. The time A until La ₂ O ₃ is formed is represented by the following equation.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasuhiro Yoshioka 2-1-1 Osaki, Shinagawa-ku, Tokyo Incorporated company Meidensha

Claims (1)

[Claims] 1. La _1-x M _x F _1-x (x = 0.05-0.
20, M = Sr. Ba. After calcining the substance represented by the composition formula of Ca) at 1200 ° C. for several hours in a zirconia container, La _1-x M _x OF _1-x (x = 0.05 to 0.20, M =
A method for producing a solid electrolyte for a fuel cell, characterized in that it is composed of a substance represented by a composition formula of Sr, Ba, Ca).