JP2008091290A - Separator for solid electrolyte fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a separator for a solid electrolyte fuel cell in which firing can be done at a relatively low temperature and a mechanical strength can be enhance. <P>SOLUTION: The separator 1 for a solid electrolyte fuel cell is provided with a main body 10 made of an insulating oxide and an electric conductive electron passage 20 formed on the main body 10, and the main body 10 contains ceria stabilized zirconia as a principal composition. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention generally relates to a separator for a solid oxide fuel cell, and specifically includes a main body portion made of an electrically insulating oxide and an electrically conductive electron passage portion formed in the main body portion. The present invention relates to a solid oxide fuel cell separator.

  In general, a flat solid electrolyte fuel cell (also referred to as a solid oxide fuel cell (SOFC)) includes a plurality of flat plate-shaped power generation elements each composed of an anode (negative electrode), a solid electrolyte, and a cathode (positive electrode). And a separator (also referred to as an interconnector) disposed between a plurality of cells. The separator is a fuel gas (for example, hydrogen) specifically supplied to the anode in order to electrically connect the plurality of cells to each other in series and to separate the gas supplied to each of the plurality of cells. And an oxidant gas (for example, air) supplied to the cathode is disposed between the plurality of cells.

Conventionally, the separator is formed from a heat-resistant metal material or a conductive ceramic material such as lanthanum chromite (LaCrO ₃ ). When a separator is formed using such a conductive material, a member that performs the functions of electrical connection and gas separation can be formed using a single material.

  On the other hand, the separator is joined to the three-layer members constituting the cell, that is, the three-layer members constituting the anode (fuel electrode), the electrolyte and the cathode (air electrode), and leakage of fuel gas and oxidant gas. In order to prevent this, the peripheral portions of the separator and the three-layer member are hermetically sealed. For this reason, it is necessary to prevent the separator or the three-layer member from being cracked or deformed when being joined to the three-layer member. In order to solve this problem, it is necessary to select a material having a thermal expansion coefficient approximate to that of a three-layer member as a material for the separator.

  For example, Japanese Patent Laid-Open No. 2000-36310 (Patent Document 1) discloses a configuration of a separator that can expand the selection range of the material of the separator so as to satisfy such a requirement.

  FIG. 4 is a diagram showing a configuration of a conventional separator disclosed in the above publication.

As shown in FIG. 4, the separator includes a separator body 102 made of an electrically insulating oxide and an electrically conductive electron flow path member 105 provided so as to penetrate the separator body 102. This separator is composed of magnesia (MgO) and spinel (MgAl ₂ O ₄ ), and a through hole is formed in the separator body 102 obtained by firing, and an electrically conductive oxide such as lanthanum is formed in the through hole. The electronic channel material 105 made of chromite (LaCrO ₃ ) is fitted or joined. In this case, since the separator body 102 is formed from an electrically insulating material, the material may be selected so as to have an appropriate thermal expansion coefficient. The laminating frame 103 and the gas distribution frame 104 are joined to two opposing sides of the flat separator body 102.

Japanese Patent Laying-Open No. 2003-132914 (Patent Document 2) discloses that a gas separator as a separator is composed of one or more layers of a dense ceramic material including a plurality of conductive vias. Yes. In this case, the gas separator is made of yttria-stabilized zirconia (YSZ), for example, 3 mol% yttria-stabilized zirconia, or a ceramic material such as a ceria-based electrolyte or ceria-doped LaGaO ₃ electrolyte.
JP 2000-36310 A JP 2003-132914 A

  However, in the separator disclosed in the above Japanese Patent Laid-Open No. 2000-36310 (Patent Document 1), since the main body contains spinel, the firing temperature is high, and thus the manufacturing cost is high.

  Moreover, when the firing temperature of the separator main body is high, the selection range of the electrically conductive material constituting the electron channel material is narrowed. If a silver (Ag) -palladium (Pd) based alloy is used as an electrically conductive material, it is necessary to use a material with a palladium content close to 100% when the firing temperature of the separator body is 1500 ° C. Material costs increase. On the contrary, as the firing temperature of the separator body becomes lower, a silver-palladium alloy having a higher silver content can be used as the electrically conductive material, and the material cost can be reduced.

  Further, as proposed in the above-mentioned Japanese Patent Application Laid-Open No. 2003-132914 (Patent Document 2), in order to eliminate the need to form an airtight seal, the three-layer constituent material constituting the cell and the separator It is conceivable to manufacture the constituent materials by firing them at the same time. In this case, when the firing temperature of the separator main body is higher than 1300 to 1400 ° C., which is a general firing temperature of the three-layer constituent material, it becomes difficult to perform the simultaneous firing.

By the way, in said Unexamined-Japanese-Patent No. 2000-36310 (patent document 1), the mixture containing a spinel is baked at 1400-1800 degreeC, the separator baked body which consists of porous oxides is produced, and the surface of this separator baked body A separator body in which a dense oxide film is coated on the surface of a porous oxide plate is manufactured by applying a mixed slurry containing SiO ₂ , CaO and Al ₂ O ₃ and heat-treating at 1200 to 1400 ° C. A method is disclosed.

  However, in such a separator body, there is a problem that the dense oxide film is peeled off under the heat cycle of the use environment. Further, even if a dense oxide film is provided on the surface, there is a problem that the mechanical strength of the separator body is low because the inside of the separator body is made of a porous oxide. Furthermore, since the inside of the separator body contains spinel, the mechanical strength of the spinel itself is low, and therefore the separator body may be broken depending on the use environment.

In JP-A-2003-132914 (Patent Document 2), the gas separator as a separator is mainly composed of yttria-stabilized zirconia (YSZ), for example, 3 mol% yttria-stabilized zirconia, or ceria. And an electrolyte composed of ceria-doped LaGaO ₃ electrolyte. However, since these materials have oxide ion conductivity, they generate a reverse electromotive force for the three-layer members constituting the cell and cause a loss in the power generation efficiency of the battery. Has the problem of adversely affecting

  Accordingly, an object of the present invention is to provide a configuration of a solid oxide fuel cell separator that can be fired at a relatively low temperature and can increase mechanical strength.

  A separator for a solid oxide fuel cell according to the present invention is for a solid oxide fuel cell comprising a main body made of an electrically insulating oxide and an electrically conductive electron passage formed in the main body. It is a separator, Comprising: A main-body part contains a ceria stabilized zirconia as a main component.

  In the solid oxide fuel cell separator of the present invention, the main body portion contains ceria-stabilized zirconia as a main component, so that the firing temperature can be lowered.

  Ceria-stabilized zirconia, which is a kind of partially-stabilized zirconia, is less susceptible to mechanical stress than the conventional spinel that constitutes the separator body, and can therefore increase the mechanical strength of the separator. Ceria-stabilized zirconia can be fired at a lower temperature than the spinel that constitutes a conventional separator body. Therefore, in the case of co-sintering with an electrically conductive material that constitutes an electron passage, The selection range of the material can be expanded.

  Furthermore, ceria-stabilized zirconia is a kind of fluorite-type zirconia solid solution similar to yttria-stabilized zirconia (YSZ) and scandia-stabilized zirconia (ScSZ) constituting the solid electrolyte, and the thermal expansion of these solid electrolyte materials. It has a coefficient of thermal expansion close to that of the coefficient and a coefficient of thermal expansion that is slightly larger than the coefficient of thermal expansion of the solid electrolyte material. For this reason, when the separator of the present invention is joined to the solid electrolyte constituting the cell, the thermal stress acting on the solid electrolyte is small, and the compressive stress acts on the solid electrolyte, thereby suppressing the destruction of the solid electrolyte due to the thermal stress. Can do. Ceria-stabilized zirconia can be fired at almost the same temperature as the above-mentioned solid electrolyte material, and the main components are almost the same. And the three-layer members constituting the cell can be fired simultaneously.

  In addition, unlike other zirconia stabilizing materials such as yttrium and calcium, cerium does not form oxygen deficiency because ceria stabilized zirconia is stable in the same tetravalence as zirconia. Thereby, ceria-stabilized zirconia does not have oxide ion conductivity. Therefore, even if the separator main body is made of ceria-stabilized zirconia, a reverse electromotive force is generated with respect to the three-layer members constituting the cell, and no loss occurs with respect to the power generation efficiency of the battery. Therefore, the battery function is not adversely affected.

  In the solid oxide fuel cell separator of the present invention, it is preferable that the main body portion and the electron passage portion are formed by co-sintering.

  By configuring in this way, it is possible to join between the main body part and the electron path part by mutual diffusion during co-sintering, so that the generation of voids between the main body part and the electron path part is suppressed. As a result, gas leakage can be prevented from occurring. Moreover, a separator can be manufactured at low cost.

In the solid oxide fuel cell separator of the present invention, it is preferable that the main body further contains zircon (ZrSiO ₄ ) or alumina (Al ₂ O ₃ ).

  The ceria-stabilized zirconia that constitutes the main body of the separator has a thermal expansion coefficient slightly larger than that of the solid electrolyte material. For this reason, the thermal expansion coefficient of the separator body can be reduced by mixing ceramic with high atmospheric stability such as zircon or alumina with ceria-stabilized zirconia. The thermal expansion coefficient of the electrolyte material can be approximately the same as or close to the thermal expansion coefficient of the solid electrolyte material. As a result, the thermal stress acting on the solid electrolyte can be controlled and finely adjusted when the separator of the present invention is joined to the solid electrolyte constituting the cell, and the destruction of the solid electrolyte due to the thermal stress can be suppressed more effectively. can do.

  Furthermore, in the solid oxide fuel cell separator of the present invention, the ceria-stabilized zirconia preferably contains 10 mol% or more and 20 mol% or less of ceria.

  If the ceria content is less than 10 mol%, volume fluctuations due to phase transition occur in the temperature raising process and temperature lowering process, so that it is large when the separator of the present invention is joined to a cell composed of a three-layer member containing an electrolyte. It causes thermal stress. On the other hand, when the ceria content exceeds 20 mol%, the mechanical strength decreases. Therefore, in order to prevent the occurrence of large thermal stress that may cause breakage such as cracking during joining with the cell, and to prevent the mechanical strength of the separator body from being lowered, The ceria content in the ceria-stabilized zirconia constituting the main body is preferably controlled within the above range.

  As described above, according to the present invention, a separator for a solid oxide fuel cell including a main body portion made of an electrically insulating oxide and an electrically conductive electron passage portion formed in the main body portion. As well as the firing temperature for producing the separator, the mechanical strength of the separator can be increased.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic plan view showing the configuration of a solid oxide fuel cell separator as one embodiment of the present invention.

  As shown in FIG. 1, the separator 1 includes a main body portion 10 made of an electrically insulating oxide and a plurality of electrically conductive electron passage portions 20 formed in the main body portion 10. The main body 10 contains ceria-stabilized zirconia as a main component.

  FIG. 2 is a cross-sectional view showing a cross section of the separator of one embodiment taken along line II-II in FIG.

  As shown in FIG. 2, the main body portion 10 is composed of two layers of main body portions 11 and 12. The main body portion 11 is formed with an electronic passage portion 21, and the main body portion 12 is formed with an electronic passage portion 22. Yes. An electrically conductive thin film made of the same material as that of the electron path portion is formed at the interface between the main body portions 11 and 12 so that the adjacent electron path portions 21 and 22 are electrically connected. The separator configured as described above is formed by co-sintering the constituent materials of the two main body portions 11 and 12 constituting the main body portion 10 and the constituent materials of the electron passage portions 21 and 22.

  FIG. 3 is a cross-sectional view showing a cross section of another embodiment of the separator taken along line III-III in FIG.

  As shown in FIG. 3, the main body 13 having a plurality of through holes formed for the electron passage is formed by firing. A separator is manufactured by press-fitting a rod 23 made of an electrically conductive material forming the electron passage portion into the plurality of through holes. Thus, after forming a main-body part by baking, you may produce the separator of this invention by joining the rod which comprises an electronic channel part to a main-body part.

  Examples of the present invention will be described below.

(Evaluation of separator body material)
First, each sample of the main body part of the separators of Examples 1 to 7 and Comparative Examples 1 to 3 shown in Table 1 was prepared as follows, and the material was evaluated.

As shown in Table 1, in each sample of Examples 1 to 7 and Comparative Examples 1 to 3, various oxide powders as the main material and additive material constituting the main body of the separator, and polyvinyl butyral system After mixing a binder and a mixture of ethanol and toluene as an organic solvent (weight ratio is 1: 4), a green sheet having a thickness of 300 μm was formed. After this green sheet was fired, it was cut into a sheet having a size of 5 cm × 5 cm. After the two obtained sheets were laminated, they were pressure-bonded at a pressure of 1000 kgf / cm ² and a temperature of 80 ° C. for 2 minutes. After degreasing the pressure-bonded body at a temperature in the range of 400 to 500 ° C., a fired body was produced for each firing temperature by holding at 50 ° C. intervals for 2 hours within a temperature range of 1000 to 1500 ° C. .

  The firing density of the fired body for each firing temperature thus obtained was measured by the Archimedes method, and the firing temperature of the fired body having a relative density of 95% or more was measured as in Examples 1-7 and Comparative Examples 1-3. The minimum firing temperature [° C.] for each sample was used.

Moreover, the thermal expansion coefficient in the temperature rising process from room temperature to 1000 degreeC was measured for the thermal expansion coefficient [* 10 < ^-6 >] of the sample baked at the minimum baking temperature by the thermomechanical analysis method (Thermomechanical Analysis). The thermal expansion behavior in the temperature lowering process from a temperature of 1000 ° C. to 100 ° C. was also measured, and it was verified whether there was a change between the temperature rising process and the temperature lowering process, that is, no volume fluctuation. This volume variation is caused by a phase transition. In zirconia that lacks stabilization, the monoclinic phase is a stable phase at a low temperature around room temperature, and the tetragonal phase is a stable phase at a high temperature around 1000 ° C. When a phase transition occurs between these two phases, Causes a volume fluctuation of about 6%. If volume fluctuation occurs, it may cause cracking or generate a large stress during joining.

  Furthermore, the obtained fired body was examined for the presence or absence of oxide ion conductivity at a temperature of 800 ° C. The bending strength [MPa] of the obtained fired body was also measured.

As a result of the above measurement, thermal expansion coefficient [× 10 ⁻⁶ ] (average thermal expansion coefficient from room temperature to 1000 ° C.), minimum firing temperature [° C.], bending strength [MPa], presence / absence of ionic conductivity, volume fluctuation The presence or absence of is shown in Table 1.

(Evaluation of separator)
As shown in Table 1, in Examples 1 to 7 and Comparative Examples 1 to 3, separators were produced by two types of production methods, co-sintering and bonding.

  When the separator is manufactured by co-sintering, as shown in FIG. 2, the formation positions of the corresponding through holes do not overlap at the positions where the electron passage portions 21 and 22 are formed in the main body portions 11 and 12, respectively. In addition, through holes having different arrangements were formed in the two green sheets obtained above. Each through-hole was filled with an alloy paste of 50% by weight of silver (Ag) -50% by weight of palladium (Pd). A paste having the same composition as described above was printed on one surface of two green sheets so as to connect the through holes forming the adjacent electron passage portions 21 and 22. Thereafter, as shown in FIG. 2, two green sheets were laminated and pressure-bonded under the same conditions as described above. The pressure-bonded body was cut into a size of 5 cm × 5 cm, and then fired at a temperature equal to or higher than the minimum firing temperature.

  When the separator is manufactured by joining, the two green sheets obtained above are arranged so that the corresponding through-hole formation positions overlap at the positions where the electron passage portions 21 are formed in the main body portion 11 shown in FIG. Through holes with the same arrangement were formed in the sheet. Thereafter, two green sheets were laminated and pressure-bonded under the same conditions as described above. The laminate was cut into a size of 5 cm × 5 cm and then fired at a temperature equal to or higher than the above minimum firing temperature. A cylindrical lanthanum chromite (LaCrO3) sintered body or silver (Ag) -palladium (Pd) sintered body separately produced by sintering as a conductive material for forming an electron passage portion in the through hole of the obtained fired body I pressed my body.

Each sample is sandwiched between 3 cm × 3 cm transmission cells so as to include the electron passage portion of each sample of the separator obtained as described above, and each sample is filled with nitrogen gas, and then each sample is filled. One side of the was decompressed to 1 Pa. The pressure fluctuation on the decompressed side was recorded, and the gas permeability coefficient [cm ³ / m ² · 24 h · atm] was measured. As a preferred embodiment of the separator of the present invention, the nitrogen gas permeability coefficient measured in accordance with JIS K7126 method B is 30 [cm ³ / m ² · 24 h · atm] or less, more preferably. Is 15 [cm ³ / m ² · 24 h · atm] or less. If the gas permeability is too large, fuel gas leaks when the separator is incorporated into the fuel cell, and the effects of the present invention cannot be achieved.

  The gas permeability coefficient measured as described above is also shown in Table 1.

  In the column of “main material” in Table 1, for example, “12CeSZ” is zirconia partially stabilized with ceria having an addition amount of 12 mol%, and “3YSZ” is zirconia partially stabilized with yttria having an addition amount of 3 mol%. Indicates that

  From the results shown in Table 1, Examples 1 to 7 and Comparative Example 3 in which the main material is partially stabilized zirconia can be fired at a lower temperature than Comparative Examples 1 and 2 in which the main material is spinel. It can be seen that the bending strength is high. Comparative Example 3 in which the main material is yttria-stabilized zirconia showed ionic conductivity, but Examples 1 to 7 in which the main material is ceria-stabilized zirconia and Comparative Examples 1 and 2 in which the main material is spinel are It did not show ionic conductivity. When the amount of ceria added was small as in Example 6, a phase transition from a tetragonal phase to a monoclinic phase was observed, and a volume fluctuation that was detectable by thermomechanical analysis was observed. When the amount of ceria added is large as in Example 7, the cubic phase is increased, and the contribution by the martensitic phase transformation of the tetragonal phase that ensures the high strength of the partially stabilized zirconia is reduced, so the bending strength is reduced. I understand that

  More specifically, the comparison between Examples 1 to 7 and Comparative Examples 1 and 2 shows that when ceria-stabilized zirconia is used as the main material of the main body of the separator, it can be fired at a low temperature. . Further, from comparison between Examples 1 to 7 and Comparative Example 3, it can be seen that when ceria-stabilized zirconia is used as the main material of the main body of the separator, ionic conductivity is not exhibited.

  From the comparison between Examples 1 to 4, Examples 6 to 7, Comparative Example 3, Example 5, and Comparative Examples 1 to 2, the separator was produced by a co-sintering method with a smaller gas permeability coefficient and airtightness. It can be seen that a high-performance separator can be obtained. This is because, after sintering, the co-sintering method can be closely bonded to the main body portion and the electron path portion by diffusion rather than the method of bonding the conductive material of the electron path portion.

From the comparison between Examples 1-2 and Examples 3-5, when zircon or alumina is added to produce the main body of the separator, the thermal expansion coefficient of the main body of the separator can be reduced, and the solid electrolyte is configured. It can be seen that the thermal expansion coefficient of yttria-stabilized zirconia is 10.8 × 10 ⁻⁶ and that of scandia-stabilized zirconia is 10.7 × 10 ⁻⁶ . In this case, the added alumina or zircon does not contain an element that easily causes valence fluctuation such as a transition metal, so that no problem occurs even when exposed to a reducing atmosphere.

  From the comparison between Examples 1 to 5 and Examples 6 to 7, when the amount of ceria added was less than 10 mol%, volume fluctuation caused by phase transition due to temperature change occurred, and the amount of ceria added was 20 mol%. When it exceeds, it turns out that bending strength falls.

  It should be considered that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is shown not by the above embodiments and examples but by the claims, and is intended to include all modifications and variations within the meaning and scope equivalent to the claims. .

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic plan view showing a configuration of a solid oxide fuel cell separator as one embodiment of the present invention. It is sectional drawing which shows the cross section of the separator of one Embodiment in the II-II line | wire of FIG. It is sectional drawing which shows the cross section of the separator of another embodiment in the III-III line of FIG. It is a figure which shows the structure of the conventional separator.

Explanation of symbols

  1: Separator, 10: Main body, 20: Electron passage.

Claims (4)

A solid oxide fuel cell separator comprising a main body made of an electrically insulating oxide and an electrically conductive electron passage formed in the main body,
The separator for a solid oxide fuel cell, wherein the main body portion contains ceria-stabilized zirconia as a main component.   The separator for a solid oxide fuel cell according to claim 1, wherein the main body portion and the electron passage portion are formed by co-sintering.   The separator for a solid oxide fuel cell according to claim 1 or 2, wherein the main body further contains zircon or alumina.   The solid oxide fuel cell separator according to any one of claims 1 to 3, wherein the ceria-stabilized zirconia contains 10 mol% or more and 20 mol% or less of ceria.

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