JP2013118177A - Solid oxide fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an SOFC that can effectively prevent Cr scattering and offers high durability under long-term use.SOLUTION: A solid oxide fuel cell includes an alloy containing Cr, and an air electrode joined together. The alloy includes a substrate composed of SUS with a Mn content higher than or equal to 0.3% and lower than or equal to 1.0%, and a protective coat mainly composed of a spinel oxide is formed on the surface of the substrate.

Description

  The present invention relates to a solid oxide fuel cell (SOFC) formed by joining an alloy containing Cr and an air electrode.

  The solid oxide fuel cell (SOFC) targeted by the present invention is one in which electrodes having functions of oxidizing and reducing fuel gas and oxygen in the air are attached to both sides of a solid electrolyte that conducts oxygen ions. . In general, yttria-doped zirconia is used as an electrolyte material, and hydrogen, carbon monoxide, hydrocarbons in fuel gas and oxygen in oxidant gas are electrically connected at a high temperature of 700 to 1000 ° C. Electricity is generated by chemical reaction. SOFC has been developed as a promising power generation technology because it can generate power with particularly high power generation efficiency compared to other fuel cell systems and gas engines.

With the progress of development in recent years, the operating temperature of SOFC is decreasing.
The conventional operating temperature is about 1000 ° C., and metal oxides typified by lanthanum chromite (LaCrO ₃ ) have been used from the viewpoint of heat resistance, but recently the operating temperature has dropped to 700 ° C. to 800 ° C. Alloys have become available. The use of alloys can be expected to reduce costs and improve robustness.

In such SOFC, an air electrode is joined to one surface side of an electrolyte membrane, and a single cell formed by joining a fuel electrode to the other surface side of the electrolyte membrane is used to transfer electrons to the air electrode or the fuel electrode. It has a structure sandwiched between a pair of electron conductive alloys.
And in such SOFC, it operate | moves at the operating temperature of about 700-1000 degreeC, for example, and it moves between a pair of electrodes with the movement of the oxide ion through the electrolyte membrane from the air electrode side to a fuel electrode side. An electromotive force is generated, and the electromotive force can be taken out and used. SOFC has been developed as a promising power generation technology because it can generate power with particularly high power generation efficiency compared to other fuel cell systems and gas engines.

As the alloy used in such SOFC, ferritic stainless steel is often used because of its consistency with the coefficient of thermal expansion of the metal oxide to be joined, but austenitic stainless steel with superior heat resistance. Fe-Cr-Ni alloy or Ni-Cr alloy that is nickel-based alloy may be used. Further, the heat resistance of such an alloy is derived from a dense coating (oxide film) of chromia (Cr ₂ O ₃ ) formed on the surface of the alloy.

These alloys and the like almost always contain Cr, and an oxide film of Cr ₂ O ₃ or MnCr ₂ O ₄ is formed on the surface in a high-temperature air atmosphere that is an operating environment. This oxide film is known to increase in thickness over time, evaporate as a hexavalent chromium compound in a high-temperature atmospheric atmosphere as an operating environment, and poison the air electrode to cause deterioration (Cr Called poisoning).
That is, in an SOFC formed by joining an alloy containing Cr and an air electrode, when the alloy or the like is exposed to a high temperature during operation or the like, Cr contained in the alloy or the like scatters to the air electrode side and air There is a problem that extreme Cr poisoning occurs.
Such Cr poisoning of the air electrode inhibits the oxygen reduction reaction for the generation of oxide ions in the air electrode, increases the electrical resistance of the air electrode, and further decreases the Cr concentration of alloys and the like. This may cause problems such as a decrease in the heat resistance of the alloy itself, resulting in a decrease in SOFC performance.
Therefore, an attempt has been made to suppress deterioration by providing a protective film made of a metal oxide material having excellent heat resistance on the surface of the alloy (oxide film surface) (see, for example, Patent Document 1).

  In addition, in the production process of the SOFC, in order to reduce the contact resistance between the alloy and the like, the air electrode and the fuel electrode as much as possible, in a state in which they are laminated, the operating temperature is higher than 1000 ° C. to 1250 ° C. There is a case where heat treatment is performed at a firing temperature of a certain degree (see, for example, Patent Document 2). When heat treatment is performed in a state where an alloy or the like and an air electrode are joined at the time of manufacturing an SOFC, an oxide of Cr (VI) is generated and evaporated by being exposed to a temperature condition higher than the operating temperature. In some cases, it reacts with the air electrode to produce a Cr compound, and Cr poisoning of the air electrode may occur.

  Further, in this firing treatment, even when the occurrence of initial Cr poisoning during production is suppressed, during subsequent operation, the air supplied to the air electrode is exposed to a high temperature in an oxidizing atmosphere, whereby Cr Oxidation to (VI) proceeds, and Cr poisoning with time may occur.

  It has also been proposed to increase the oxidation resistance of the alloy itself by using SUS steel with a low Mn content as a steel material suitable as the alloy (see, for example, Patent Document 3).

International Publication WO2009 / 131180 Pamphlet JP 2004-259634 A JP 2006-057153 A

  When a protective film made of a metal oxide material is provided on the surface of the alloy, as described above, it has been clarified experimentally that Cr poisoning by the protective film can be suppressed. It has become clear that the suppression effect does not necessarily increase depending on the film thickness of the metal oxide material, and in some cases, a sufficient poisoning suppression effect is not always obtained.

  Therefore, an object of the present invention is to provide an SOFC that can more reliably expect an effect of suppressing Cr poisoning when an SOFC is manufactured using an alloy containing Cr.

  In addition to the Cr scattering prevention effect that the protective film itself chemically captures and reacts with Cr, the present inventors have the Cr poisoning suppression effect by the protective film, and at the interface between the base material and the protective film, The protective film forms a dense layer, and it is newly found that the Cr scattering prevention effect that physically blocks the scattering of Cr is effective. As a result of earnest research, a specific base material and a specific protective film The present invention has been completed by clarifying that the physical Cr scattering prevention effect can be enhanced in the above combination.

[Configuration 1]
In order to achieve the above object, the characteristic configuration of the SOFC of the present invention is as follows:
A solid oxide fuel cell formed by joining an alloy containing Cr and an air electrode,
The alloy is made of SUS having a Mn content of 0.3% or more and 1.0% or less as a base material, and a protective film mainly containing a spinel oxide is formed on the surface of the base material.
In addition, in this application, the content rate of a metal is displayed by the mass%.

[Function 1]
According to the research of the present inventors, when the protective film is formed on the base material, the protective film interacts with Mn contained in the base material to form the original protective film material. A coating layer (hereinafter referred to as a dense layer) that is much denser and has higher adhesion to the substrate is formed. Thereby, Cr which is going to evaporate from the base material surface is interrupted by the dense layer and becomes difficult to scatter. Therefore, the dense layer can be formed by using a combination of SUS steel having a high Mn content as the base material and using a spinel oxide as a main material as a protective film.

When the protective film is not present, manganese in the base material diffuses on the surface of the base oxide layer (chromia) to form Cr ₂ O ₃ or MnCr ₂ O ₄ spinel containing Cr on the outermost surface. Therefore, when Cr is easily scattered to the outside, when the protective film is present, the constituent element Mn diffuses in the protective film to form a dense layer that does not contain Cr. Thus, the protective film can be formed and the scattering of Cr can be suppressed. By making the alloy of the base material have a Mn content of 0.3% or more, scattering of Cr can be prevented by reliably diffusing Mn and forming a dense layer, but a base material having a high Mn content. Is preferably 1% or less because it is considered that the oxidation resistance is low.

[Configuration 2]
Moreover, it is preferable that the said alloy contains 16% or more and 30% or less of Cr.

[Operation effect 2]
When the Cr content of the base material is large, the Cr content diffusing from the base material also increases. Therefore, the Cr content is preferably 30% or less, and the alloy itself has high corrosion resistance, and against the protective film. In order to provide high adhesion, the content is preferably 16% or more.
In addition, according to the base material containing Cr and Mn at such a ratio, Mn diffuses preferentially compared with Cr. Then, when a material that forms a spinel oxide is used as the protective film, Mn diffuses in the protective film instead of Cr, thereby forming a dense layer that does not contain Cr. It is thought that the effect of suppressing occurs. A more preferable range is 16% or more and 25% or less, and a still more preferable range is 18% or more and 22% or less.

[Configurations 3 to 5]
It is preferable that the protective film is mainly composed of a spinel oxide containing two or more metals selected from Mn, Co, Zn, Fe, Ni, Cr, Ti, V, Y, W, and lanthanoid. .
Specifically, the protective film is made of NiCo ₂ O ₄ , (Zn _x Co _1-x ) Co ₂ O ₄ (0.45 ≦ x ≦ 1.00), TiCo ₂ O ₄ , ZnFe ₂ O ₄ , FeCo _The main material is at least one spinel oxide selected from the group consisting of ₂ O ₄ , CoFe ₂ O ₄ , MgCo ₂ O ₄ , Co ₃ O ₄ and a mixture of two or more thereof. Preferably
More specifically, it is preferable that the protective film is composed mainly of a spinel oxide made of ZnCo ₂ O ₄ .

  In addition, in the present application, when “material X is a main material”, it means that the material X as a constituent material is one of the main raw materials, and an additive may be added as necessary. As long as the characteristics possessed by the material appear, the blending ratio is not particularly limited, and the material X alone does not necessarily need to be the most abundant material in the mixture, and preferably 50% or more is composed of the material X. Although it is preferable, it may be less.

[Effects 3-5]
In general, these spinel type oxides take in Mn and easily grow a spinel type oxide structure. Therefore, these spinel type oxides are preferable for capturing Mn diffused from a substrate and forming a dense layer. These spinel oxides are also suitable as a material for forming a protective film from the viewpoint that they can be tightly bonded to the air electrode.

As a protective film used in such a case, a spinel oxide containing two or more metals selected from Mn, Co, Zn, Fe, Ni, Cr, Ti, V, Y, W, and a lanthanoid is known. NiCo ₂ O ₄ , (Zn _x Co _1-x ) Co ₂ O ₄ (0.45 ≦ x ≦ 1.00), TiCo ₂ O ₄ , ZnFe ₂ O ₄ , FeCo ₂ O ₄ , CoFe ₂ O ₄ At least one spinel oxide selected from the group consisting of MgCo ₂ O ₄ , Co ₃ O ₄ and a mixture of two or more thereof is advantageously used, and the coefficient of thermal expansion with the substrate, the air electrode, etc. This is advantageous in that the discrepancy between the two is small.

In other words, according to the SOFC of this configuration, the protective film containing the spinel oxide composed of ZnCo ₂ O ₄ is most preferably used, and therefore, the thermal expansion coefficient mismatch between the base material and the air electrode is extremely high. It is small and can suppress the peeling of the joint part between the base material and the protective film for the connection member between cells and the protective film and the cell, and the formation of cracks caused by the mismatch of the thermal expansion coefficient, and the accompanying increase in electrical resistance, etc. .

In particular, the spinel oxide made of ZnCo ₂ O ₄ has a coefficient of thermal expansion of 10.0 × 10 ⁻⁶ / ° C. (30 to 800 ° C.), and ferritic stainless steel is 11.7 × 10 ⁻⁶ / ° C. (30 to 800 ° C), the protective film containing the spinel oxide of this structure is a protective film that is not easily peeled off from the alloy or the like even if the alloy or the air electrode is thermally expanded, and has excellent durability. It can be said that there is.

In the present application, “including two or more kinds of metals” means that they are contained as A and B as metal materials constituting the ABO ₄ type spinel structure, and spinel structure as another additional component. Exclude items that are not included.

[Configuration 6, 7]
The protective film preferably has a thickness of 1 μm or more and 50 μm or less,
The protective film preferably includes a dense layer having a thickness of 1 μm or more and 20 μm or less.

[Effects 6, 7]
If the thickness of the protective film is too thin, the Cr scattering suppression effect is not expected, so it is preferably 1 μm or more. If it is too thick, cracks occur due to thermal stress, and the peel strength tends to decrease, so it is 50 μm or less. It is preferable to do. Practically, it is preferable that the dense layer formed on the protective film has a thickness of 1 μm or more and 20 μm or less depending on the manufacturing conditions of the protective film because a sufficient Cr scattering suppression effect can be obtained.

  Therefore, it was possible to provide a SOFC having a high Cr scattering suppression effect, and to provide a highly durable SOFC for long-term use.

Diagram showing detailed cross-sectional structure of SOFC cell The figure which shows the protective film formed in the interconnector SEM photograph of a protective film made of spinel oxide formed on the surfaces of test pieces (a) to (c) SEM photo of protective film after heat treatment The figure which shows the relationship between the Mn content of the base material and the thickness of the dense layer The figure which shows the Cr scattering suppression effect of a test piece (b) and (c) SEM photograph after heat treatment of a protective film made of spinel oxide formed on the surface of the test piece (d) The figure which shows the Cr scattering suppression effect of a test piece (d)

  The SOFC of the present invention will be described below. Preferred embodiments are described below, but these embodiments are described in order to more specifically illustrate the present invention, and various modifications can be made without departing from the spirit of the present invention. However, the present invention is not limited to the following description.

<Solid oxide fuel cell>
Embodiments of an SOFC interconnector and a method for manufacturing the same according to the present invention will be described with reference to the drawings.
The SOFC cell C shown in FIG. 1 has an air electrode 31 made of an oxide ion and an electroconductive porous body bonded to one surface side of an electrolyte membrane 30 made of a dense oxide oxide conductive solid oxide. In addition, a single cell 3 formed by joining a fuel electrode 32 made of an electron conductive porous body to the other surface side of the electrolyte membrane 30 is provided.
Further, the SOFC cell C includes an interconnector 1 made of an alloy in which the single cell 3 is exchanged with electrons to the air electrode 31 or the fuel electrode 32 and the groove 2 for supplying air and hydrogen is formed. Thus, the gas seal body is sandwiched in a state where the gas seal body is sandwiched appropriately at the outer peripheral edge portion. And the said groove | channel 2 by the side of the air electrode 31 functions as the air flow path 2a for supplying air to the air electrode 31 because the air electrode 31 and the interconnector 1 are closely_contact | adhered, on the other hand, a fuel electrode The groove 2 on the 32 side functions as a fuel flow path 2 b for supplying hydrogen to the fuel electrode 32 by arranging the fuel electrode 32 and the interconnector 1 in close contact with each other.

In addition, when a general material used in each element constituting the SOFC cell C is described, for example, the material of the air electrode 31 is LaMO ₃ (for example, M = Mn, Fe, Co). A perovskite oxide of (La, AE) MO ₃ in which a part of La of Al is substituted with an alkaline earth metal AE (AE = Sr, Ca) can be used. And yttria-stabilized zirconia (YSZ) can be used, and yttria-stabilized zirconia (YSZ) can be used as the material of the electrolyte membrane 30.

  Furthermore, in the SOFC cell C described so far, the interconnector 1 is made of a ferritic stainless steel Fe-Cr alloy, an austenitic stainless steel Fe-Cr-Ni alloy, or a nickel-based alloy. An alloy containing Cr of 16% to 30%, preferably 16% to 25%, more preferably 18% to 22%, such as Ni—Cr alloy is used. Among these, stainless steel ZMG232L (SUS-A material described later) is preferably used in the present invention.

In a state where a plurality of SOFC cells C are arranged in a stacked manner, a pressing force is applied in the stacking direction by a plurality of bolts and nuts to form a cell stack.
In this cell stack, the interconnector 1 disposed at both ends in the stacking direction may be any one in which only one of the fuel flow path 2b or the air flow path 2a is formed, and the other interconnector disposed in the middle. 1 may use a fuel channel 2b formed on one surface and an air channel 2a formed on the other surface. In the cell stack having such a laminated structure, the interconnector 1 may be called a separator.
An SOFC having such a cell stack structure is generally called a flat-plate SOFC. In the present embodiment, a flat SOFC will be described as an example. However, the present invention is applicable to SOFCs having other structures.

When the SOFC including the SOFC cell C is operated, air is passed through the air flow path 2a formed in the interconnector 1 adjacent to the air electrode 31, as shown in FIG. And hydrogen is supplied through the fuel flow path 2b formed in the interconnector 1 adjacent to the fuel electrode 32, and operates at an operating temperature of about 800 ° C., for example. Then, the air electrode 31 O ₂ electrons e ^- are reacting with O ^2- is generated, the O ^2- passes through the electrolyte membrane 30 to move to the fuel electrode 32, H ₂ supplied in the fuel electrode 32 Reacts with the O ²⁻ to generate H ₂ O and e ⁻ , so that an electromotive force E is generated between the pair of interconnectors 1, and the electromotive force E can be taken out and used outside. it can.

<Interconnector>
As shown in FIGS. 1 and 3, the interconnector 1 is configured by providing a protective film 12 on the surface of a substrate 11 for an interconnector. And it forms in the shape of a groove plate which can be connected, forming the air flow path 2a and the fuel flow path 2b between the cells 3.

The protective film 12 is preferably composed mainly of a spinel oxide containing two or more metals selected from Mn, Co, Zn, Fe, Ni, Cr, Ti, V, Y, W, and lanthanoid. In the embodiment described below, a spinel oxide made of Co _x Mn _3−x O ₄ (0 ≦ x ≦ 3, specifically x = 1.5) can be preferably used.

  The spinel oxide used here is considered to react with Mn and be converted from a porous coating to a dense coating. The dense protective film 12 formed in this way is considered to have a higher effect of preventing Cr transpiration because it has a higher function of blocking Cr transpiration than a porous coating.

  The protective film 12 is formed as a thick film by dip-coating the interconnector 1 with a film-forming material containing a conductive ceramic material.

  Next, embodiments of SOFC in which the protective film 12 of the present invention is formed on the interconnector 1 and comparative examples will be described in detail below.

(Embodiment)
A test piece in which a protective film 12 made of a spinel oxide film was provided on the surface of the interconnector base material 11 made of the following stainless steel material was subjected to a heat treatment by heating the test piece at 950 ° C. for 145 hours. The protective film 12 was slurry-coated under the condition that each test piece had a film thickness of about 6 μm, and an adhesive layer 15 was adhered to the surface of the protective film 12 to conduct a heat treatment test. For each test piece, the thickness of the dense layer 12a occupying the protective film 12 was measured by SEM, and changes in the dense layer 12a due to heat treatment were examined. 3 to 8, the symbols (a) to (d) corresponding to the test pieces of each SOFC interconnector 1 correspond to the stainless steel materials (a) to (d) constituting the interconnector 1 below. Made up of.

(A) SUS-A material Cr: 22%, Mn: 0.4%, C: 0.02%, Si: 0.08%
(B) SUS-B material Cr: 21%, Mn: 0.2%, C: 0.01%, Si: 0.1% (c) SUS-C material Cr: 18%, Mn: 1.0% , C: 0.01%, Si: 0.29%
(D) SUS-D material Cr: 22%, Mn: 0.3%, C: 0.01%, Si: 0.1%

  FIG. 3 shows the state of the protective film 12 before the heat treatment of the test pieces (a) to (c). From the figure, for any test piece, when a protective film 12 is provided on the surface of the interconnector 1, an oxide film (chromia) 11b of the substrate 11a is formed on the surface of the interconnector recording substrate 11; The protective film 12 forms a dense layer 12a on the contact surface side with the substrate 11a. Moreover, it turns out that it forms in the 2 layer form which formed the porous porous layer 12b in the surface on the opposite side to the base material 11a.

  Here, when the protective film 12 is observed, the film thickness of the oxide coating 11b is not greatly different regardless of the Mn content of the stainless steel to be the base material 11a, but the base material 11a having a high Mn content is obtained. It was found that the denser layer 12a occupying the protective film 12 was thicker as the protective film 12 was formed on the provided current collecting member. (Initial film thickness (c)> (a)> (b))

  Next, when the test pieces (a) to (c) were heat-treated, the state of the protective film 12 was as shown in FIG. As for any test piece, it turns out that the thickness of the dense layer 12a is increasing after the test. That is, it is expected that the protective film 12 becomes dense with the use of SOFC and the Cr scattering suppression effect is enhanced. However, the effect of suppressing Cr scattering particularly at the beginning of use is expected to be greatly affected by the thickness of the initial dense layer 12a on which the protective film 12 is formed. It is considered that Cr scattering can be effectively suppressed by forming the protective film 12 made of a spinel oxide on the base material 11a having a high Mn content, which is formed thick. Further, for the test piece (d), when a protective film was formed and heat treatment was performed in the same manner, it was as shown in FIG. It is thought that it can be suppressed. This Cr scattering suppression effect will be described later. (Film thickness after heat treatment (c)> (a)> (d)> (b))

  When the thickness of the dense layer 12a in the protective film 12 is plotted with respect to the Mn content of the base material 11a, it is as shown in FIG. 5, and the interconnector 1 (c) including the base material 11a having a high Mn content is shown. When (a) and (d) are used, compared with the interconnector 1 (b) provided with the substrate 11a having a low Mn content, the protection provided with the thick dense layer 12a even in the initial stage of forming the protective film 12 A film 12 can be formed.

  When the influence on the Cr scattering suppression effect by the difference in the thickness of the dense layer 12a was examined, the results were as shown in FIGS. From the drawing, in the thin protective film 12 (interconnector 1 (b) (FIG. 6)) of the dense layer 12a, a portion where the scattering of Cr is not sufficiently suppressed is observed. In the thick protective film 12 (interconnector 1 (c) (FIG. 6), (d) (FIG. 8)) of the layer 12a, it can be seen that the scattering of Cr is extremely suppressed. (Cr scattering suppression effect (c)> (a)> (d)> (b)) Therefore, when the protective film 12 made of a spinel oxide is provided, the Mn content of the substrate is 0.3% or more. It was found that the spinel type oxide containing Mn is preferably 1% or less from the point that the oxidation resistance is considered to be low.

  According to the present invention, since the Cr scattering from the alloy base material can be more reliably suppressed, the SOFC capable of effectively suppressing Cr poisoning to the air electrode and exhibiting stable performance over a long period of time. Can be used to provide.

1: Interconnector 2: Groove 2 a: Air flow path 2 b: Fuel flow path 3: Single cell 11: Interconnector base material 11 a: Base material 11 b: Oxide coating 12: Protective film 12 a: Dense layer 12 b: Porous layer 15 : Adhesive layer 30: Electrolyte membrane 31: Air electrode 32: Fuel electrode

Claims (7)

A solid oxide fuel cell formed by joining an alloy containing Cr and an air electrode,
Solid oxide fuel in which the alloy is made of SUS having a Mn content of 0.3% or more and 1.0% or less, and a protective film mainly composed of a spinel oxide is formed on the surface of the substrate. battery.   The solid oxide fuel cell according to claim 1, wherein the alloy contains 16% to 30% of Cr.   The main material of the protective film is a spinel oxide containing two or more metals selected from Mn, Co, Zn, Fe, Ni, Cr, Ti, V, Y, W, and a lanthanoid. 2. A solid oxide fuel cell according to 1. The protective film is made of NiCo ₂ O ₄ , (Zn _x Co _1-x ) Co ₂ O ₄ (0.45 ≦ x ≦ 1.00), TiCo ₂ O ₄ , ZnFe ₂ O ₄ , FeCo ₂ O ₄ , CoFe _The main material is at least one spinel oxide selected from the group consisting of ₂ O ₄ , MgCo ₂ O ₄ , Co ₃ O ₄ and a mixture of two or more thereof. Solid oxide fuel cell. The solid oxide fuel cell according to claim ₄ , wherein the protective film is mainly composed of a spinel oxide made of ZnCo ₂ O ₄ .   The solid oxide fuel cell according to claim 1, wherein the protective film has a thickness of 1 μm or more and 50 μm or less.   The solid oxide fuel cell according to claim 1, wherein the protective film includes a dense layer having a thickness of 1 μm to 20 μm.