JP2008077913A - Inter-connector for fuel cell and cell stack - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inter-connector evading deterioration in cell performance due to chrome poisoning in a solid electrolyte type fuel cell. <P>SOLUTION: As a base material of the inter-connector, a pure Ni or Ni-base alloy not containing Cr or Ni metal composed of Fe-Ni alloy is used. A coating layer composed of spinel type composite oxide containing at least one type from Co, Cu, and Mn as well as Ni as metallic element is formed on the surface of the matrix. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a fuel cell interconnector used for a fuel cell, in particular, a solid oxide fuel cell, and a cell stack for a fuel cell using the same.

  One typical fuel cell is a solid oxide fuel cell (SOFC). In this fuel cell, normally, a fuel electrode layer is formed on one surface of a thin and brittle solid electrolyte layer made of a sintered body such as yttria stabilized zirconia (YSZ), and an air electrode layer is formed on the other surface. A laminated body having a structure is used as a single battery cell. Ni and YSZ cermet are used as the fuel electrode, and lanthanum strontium manganite (LSM) is used as the air electrode. Both are porous sintered bodies.

  Since this single battery cell has an electromotive force as low as 1 V or less, it is usually used by stacking a plurality of sheets in the thickness direction and connecting them in series. More specifically, the cell stack is configured by stacking interconnectors in the thickness direction while sandwiching the unit cells so that reaction spaces are formed on both sides of the unit cells. The cell stack is housed in a furnace and operated at a high temperature of 800-1000 ° C.

  The interconnector is a conductive plate that electrically connects battery cells in series and is a separator plate that separates fuel gas and oxidizing gas, and is required to have excellent electrical conductivity and heat resistance. From these viewpoints, Fe-Cr alloy, especially ferritic stainless steel with high heat resistance and comparatively low thermal expansion coefficient, is often used as the material of the interconnector, and both surfaces are adjacent to adjacent battery cells. A gas distribution groove for forming a reaction space is formed.

  In the operation of the fuel cell, after preheating the inside of the furnace to the operating temperature, a hydrogen-rich fuel gas and an oxidizing gas such as air are supplied to the center of the cell stack in the furnace. The fuel gas supplied into the cell stack flows from the central part to the outer peripheral part in the reaction space on the fuel electrode side formed on one surface side of the unit cell. Moreover, oxidizing gas distribute | circulates from the center part to the outer peripheral part in the reaction space by the side of the air electrode formed in the other surface side of the single battery cell. In this way, both gases circulate from the central part to the outer peripheral part across each single battery cell, whereby electric power is generated in each single battery cell. The operating temperature is 800 to 1000 ° C. as described above.

  The conventional fuel cell having such a configuration has the following problems due to the interconnector of the cell stack.

When the interconnector of the cell stack is an Fe-Cr alloy such as ferritic stainless steel, high-temperature humidified air flows through the reaction space on the air electrode side of the battery cell, so that the surface of the interconnector on the air electrode side is oxidized. A scale [mainly chromia (Cr ₂ O ₃ )] is formed. This oxide scale has excellent heat resistance, protects the interconnector from high temperatures, has a relatively high electrical conductivity, and does not hinder the electrical connection of the battery cells. The high electrical conductivity of the oxide scale is one of the major reasons why Fe—Cr alloys are used for interconnectors.

However, chromium vapor composed of chromium compounds such as CrO ₃ and CrO ₂ (OH) ₂ is generated from chromia (Cr ₂ O ₃ ), which is the main component of the oxide scale. These chromium vapors [CrO ₃ (g), CrO ₂ (OH) ₂ (g)] are deposited on the air electrode layer side as high-resistance SrCrO ₄ or the like, and gradually deteriorate the battery characteristics. The decrease in battery performance due to chromium vapor is called chromium poisoning, and is one of the problems that must be solved when an Fe—Cr alloy is used for an interconnector.

  In order to solve this problem, coating various compounds on the surface of an interconnector made of an Fe—Cr alloy such as ferritic stainless steel is widely performed. If a dense coating layer is formed on the surface of the Fe—Cr alloy base material, generation of chromium vapor from the base material can be effectively suppressed.

As one of the coating techniques, for example, Patent Document 1 discloses that a dense layer of a perovskite complex oxide is formed on the surface of an Fe—Cr alloy base material. In addition, an Fe—Cr alloy (such as SUS430) that forms a coating layer of a spinel type complex oxide such as (Mn, Cr) ₃ O ₄ on the surface in an operating environment has also been developed. Furthermore, a countermeasure is also known in which the surface of the Fe—Cr alloy base material is coated with a spinel type complex oxide not containing Cr such as (Mn, Co) ₃ O ₄ .

JP 2004-281105 A

When these conventional measures are compared, the spinel composite oxide coating layer can be easily formed by heating the base material in an oxidizing atmosphere or heating in an oxidizing atmosphere after metal coating. It also has good electrical conductivity, even higher than chromia (Cr ₂ O ₃ ). However, as long as the base material contains chromium, chromia (Cr ₂ O ₃ ) is generated at the interface between the base material and the coating layer in the course of use, and a decrease in battery performance due to chromium poisoning is inevitable.

On the other hand, perovskite-type composite oxides have many gaps in the crystal structure compared to spinel-type composite oxides, and ions are easily diffused, so that electric conductivity is good, and they are widely used as electrode materials in solid oxide fuel cells. Has been. However, in order to form a dense coating layer on the surface of the interconnector base material, a special film forming operation such as electron beam vapor deposition or plasma spraying is required, which increases the cost of forming the coating layer. Further, it is difficult to increase the film thickness, and there is a problem that the coating layer is peeled off during the use process and chromium poisoning occurs. Even if the coating layer does not peel off, as long as the base material contains chromium, there is a risk that chromia (Cr ₂ O ₃ ) is generated at the interface between the base material and the coating layer in the course of use. The problem of chromium poisoning remains. The problem of chromium poisoning is more conspicuous because the crystal density is lower than that of the spinel complex oxide and there are many gaps.

  An object of the present invention is to provide a fuel cell interconnector capable of stably suppressing chromium poisoning in a solid oxide fuel cell for a long period of time, and a fuel cell capable of exhibiting stable cell performance for a long period of time by using the interconnector. It is to provide a cell stack.

  In order to achieve the above object, the present inventor has paid attention to pure Ni, a Ni-based alloy, and a Fe—Ni alloy as a material for an interconnector in a solid oxide fuel cell. These Ni-based metallic materials are considered as interconnector materials in part because of their excellent heat resistance. However, it is not actually used for an interconnector. This is because the oxide scale formed on the surface of the Ni base material has low electrical conductivity and lacks basic performance as an interconnector (cell voltage does not reach the rating). This oxide scale is mainly nickel oxide (NiO).

  However, it is a fact that Ni-based metals can exhibit excellent heat resistance even without containing Cr. Further, by eliminating Cr, the problem of chromium poisoning can be solved fundamentally. Emphasizing this possibility, the present inventor made pure Ni, Ni-based alloy, and Fe-Ni alloy containing no Cr as the base material of the interconnector, and the electrical conductivity of the oxide scale, which is a problem with the base material. We planned to solve this lowness with coating technology, and conducted experiments and examinations from various perspectives. As a result, the present inventor has found that it is effective to form a specific spinel-type composite oxide as a coating layer on the surface of a base material made of a Ni-based metal not containing Cr.

  The fuel cell interconnector according to the present invention has been completed on the basis of such knowledge, and is an interconnector used for a solid oxide fuel cell, which does not contain pure Ni or Cr, or an Fe-based alloy or Fe- Ni alloy as a base material, at least one of Co, Cu, Mn as a metal element on the surface of the base material, preferably the face facing the fuel electrode layer of the unit cell and the face facing the air electrode layer, and It has a coating layer made of a spinel complex oxide containing Ni.

  The cell stack for a fuel cell according to the present invention includes the interconnector while sandwiching a single battery cell in which a fuel electrode layer is formed on one surface of a solid electrolyte layer and an air electrode layer is formed on the other surface. Are laminated in the thickness direction.

  The coating layer made of a spinel complex oxide containing at least one of Co, Cu, and Mn and Ni is first electroplated with, for example, a metal made of at least one of Co, Cu, and Mn on the surface of the base material. It can be easily formed by coating with electroless plating or the like and heat-treating in an oxidizing atmosphere. Secondly, according to electrolytic plating or electroless plating of metal, the layer thickness can be easily increased, and durability can be enhanced.

  Third, the spinel complex oxide exhibits the characteristics of a p-type semiconductor, and in particular, it becomes a spinel complex oxide of a transition metal having a plurality of valences. Therefore, the spinel complex oxide exhibits high electrical conductivity by a hopping conduction mechanism, and perovskite. Although it is difficult for ions to diffuse as compared with the type complex oxide, it exhibits much higher electrical conductivity than NiO. Fourth, since the spinel type complex oxide has a close-packed structure of oxygen and an extremely dense crystal structure in terms of density, the oxide ion (oxygen ion) compared to the perovskite type complex oxide. Ni ions and Co ions are difficult to diffuse. For this reason, the reactivity with a base material is low, and the production | generation of the Ni-type oxide scale in an interface is suppressed. In addition, since the oxide scale formed at this interface is a spinel oxide showing the characteristics of a p-type semiconductor, the electrical conductivity is better than that of NiO. For these reasons, the spinel complex oxide is applied to the coating layer of the interconnector and does not hinder the electrical connection of the single battery cell.

Fifth, the heat resistance of the coating layer is excellent, and the base material is protected from the harsh operating environment of the fuel cell. Sixth, since the base material does not contain Cr, chromia (Cr ₂ O ₃ ) is not generated at the interface between the base material and the coating layer on the air electrode side of the single battery cell, resulting in chromium poisoning. There is no danger.

  That is, when a spinel-type composite oxide coating layer is formed on the surface of the Fe—Cr alloy, the oxide scale formed at the interface causes problems such as chromium poisoning. On the other hand, when a spinel-type composite oxide coating layer is formed on the surface of pure Ni or Cr-free Ni-based alloy or Fe-Ni alloy, problems due to oxide scale formed at the interface (chromium poisoning, electrical No decrease in conductivity occurs.

  The material of the base material in the interconnector of the present invention is a Ni-based alloy or Fe—Ni alloy that does not contain pure Ni or Cr. The Ni content in the Ni-based alloy is preferably 60% by weight or more. This is to ensure the heat resistance and corrosion resistance necessary for the fuel cell. Suitable elements other than Ni include Mo, Fe, Co, and Cu. A typical Ni-based alloy is permalloy. It should be noted that these base metals can contain a trace amount of Cr that does not cause chromium poisoning. Specifically, it is 1% by weight or less.

  The spinel type complex oxide formed on the surface of the base material is represented by, for example, Chemical Formula 1. As described above, the spinel-type composite oxide contains at least one of Co, Cu, and Mn as a metal element other than Ni, and has excellent electrical conductivity and chemical stability.

  Examples of metal elements that can be contained in addition to Co, Cu, and Mn include Fe.

Such a spinel-type composite oxide coating layer can be formed on the surface of the base material as follows. First, the surface of the base metal is coated with each metal or alloy of Co, Cu, and Mn. Alternatively, an oxide such as MO (M = Co, Cu, Mn) or M ₃ O ₄ (M = Co, Cu, Mn) is coated. Alternatively, a hydroxide such as M (OH) ₂ (M = Co, Cu, Mn) is coated.

  Examples of the coating method include electroless plating method, electrolytic plating method, PVD method, spin coating method, spray coating method, and screen printing method in the case of metal. In the case of oxides and hydroxides, electroplating, spin coating, spray coating, screen printing, and the like can be exemplified.

  The base material that has been coated with metal or the like is heated in an oxidizing atmosphere. As a result, the coating is fired or co-fired, and a spinel complex oxide coating layer is formed on the surface of the base material. Heating here can be combined with preheating prior to operation after assembling the interconnector into the cell stack, but in order to improve film characteristics, dedicated heating is performed at the stage of the interconnector alone before assembling into the cell stack. Is preferred.

  As heating conditions and heating atmosphere, 800 ° C. or higher and 48 hours or longer in an oxidizing atmosphere are appropriate. This is because a temperature and a time for interdiffusion between the coating layer formed on the surface of the base material and the base material are required.

  The thickness of the spinel complex oxide coating layer is preferably 50 μm or less. This is because a thicker layer is preferable in consideration of a decrease in the coating layer due to mutual diffusion with the base material, but cracking or peeling of the coating layer due to a difference in thermal expansion from the base material occurs when the thickness is too large. About a minimum, 0.5 micrometer or more is preferable from a viewpoint of the reduction | decrease of the coating layer by the mutual diffusion with the base material mentioned above. The particularly preferred layer thickness is 20 μm or less for the upper limit and 1 μm or more for the lower limit.

  The fuel cell interconnector of the present invention has a coating layer made of a spinel complex oxide containing Ni, at least one of Co, Cu, and Mn as metal elements, on the surface of a Ni-based metal base material that does not contain Cr. By being formed, it exhibits the high heat resistance required for gas separators, electric conductivity that does not hinder the electrical connection of single battery cells, and deterioration in battery performance due to chromium poisoning over a long period of time. It can be stably prevented.

  In addition, the fuel cell stack of the present invention employs a structure in which the interconnectors of the present invention are laminated in the plate thickness direction with a single battery cell sandwiched therebetween, thereby providing excellent durability in terms of mechanical strength and chromium. Stable performance without deterioration of battery performance due to poisoning can be continued for a long period of time.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic longitudinal sectional view of a fuel cell stack showing one embodiment of the present invention.

  The interconnector 10 of the present invention is used for a cell stack of a solid oxide fuel cell. The cell stack is configured by laminating the disk-shaped interconnectors 10 in the thickness direction while sandwiching the disk-shaped single battery cells 20 therebetween.

The unit cell 20 is a thin disk having a three-layer structure in which a fuel electrode layer 22 is laminated on one surface of a solid electrolyte layer 21 and an air electrode layer 23 is laminated on the other surface. The solid electrolyte layer 21 is made of, for example, YSZ and has a thickness of 5 to 30 μm. The thermal expansion coefficient of YSZ is about 10 × 10 ⁻⁶ K ⁻¹ smaller than cermet described later. The fuel electrode layer 22 is made of, for example, Ni and YSZ cermet and has a thickness of 0.5 to 2 mm. The thermal expansion coefficient of cermet is about 12 × 10 ⁻⁶ K ⁻¹ which is the same as that of ferritic stainless steel. Usually, a co-sintering method in which the solid electrolyte and the fuel electrode are simultaneously sintered is employed, and the sintering temperature is around 1300 ° C. The air electrode layer 23 is made of, for example, LSM and has a thickness of 20 to 50 μm. The firing temperature of LSM is about 900 ° C., and the thermal expansion coefficient is about 10 × 10 ⁻⁶ K ⁻¹ equivalent to YSZ.

  Two types of through holes are provided in the central portion of the unit cell 20 in order to allow the fuel gas and the oxidizing gas to pass therethrough.

  The interconnector 10 is a metal disk having the same outer diameter as that of the single battery cell 20 and is made of Ni-based alloy or Fe—Ni alloy not containing pure Ni and Cr. At the center of the interconnector 10, fuel gas and oxidant gas through holes corresponding to the through holes of the unit cells 20 are provided. The thickness of the interconnector 10 is preferably 0.5 to 2.5 mm, particularly preferably 1.0 to 2.0 mm. If it is too thin, a temperature gradient is generated due to a decrease in thermal conductivity in the radial direction, and cell cracking due to thermal distortion or thermal stress of the interconnector occurs. On the other hand, if it is too thick, the material cost increases and the stack capacity increases.

  The interconnector 10 is used to electrically connect a plurality of stacked unit battery cells 20 in series and to form reaction spaces on both sides of the battery cell 20. It is the same structure except for things. The interconnector 10 excluding those at both ends, that is, the interconnector 10 sandwiched between the two unit cells 20, 20 penetrates the fuel gas through the center of the unit cell 20 on the surface facing the fuel electrode side surface. It has a gas distribution groove 11 for fuel gas that circulates while being diffused from the hole to the outer periphery. Further, a gas distribution groove 12 for oxidizing gas is provided on the surface of the single battery cell 10 facing the air electrode side surface to allow the oxidizing gas such as air to flow while diffusing from the through hole in the central portion to the outer peripheral portion. ing.

  One of the interconnectors 10 and 10 at both ends is in contact with the surface of the unit cell 20 on the fuel electrode side, and on the surface, the fuel gas is circulated while diffusing from the central through hole to the outer periphery. Gas distribution grooves 11 are provided. The other is in contact with the surface of the unit cell 20 on the air electrode side, and the surface is provided with a gas distribution groove 12 for oxidizing gas that circulates while diffusing the oxidizing gas from the central through hole to the outer periphery. Yes.

On both surfaces of each interconnector 10, a spinel-type composite oxide coating layer 13 represented by (Co, Ni) ₃ O ₄ , (Mn, Ni) ₃ O ₄ , (Cu, Ni) ₃ O _4, or the like is formed. Is formed. The coating layer 13 is formed by electrolytically or electrolessly plating Co, Mn, Cu, or the like on the surface of a Ni-based metal that is a base material of the interconnector 10, and then holding the base material in a high-temperature oxidizing atmosphere. Is formed. The thickness of the coating layer 13 is 50 μm or less.

  In the interconnector 10 disposed between two adjacent unit cells 20, 20, the coating layer 13 is necessary on both surfaces because it is necessary to electrically connect the unit cells 20, 20. In the interconnectors 10 and 10 at both ends where only one surface is opposed to the unit cell 20, it is only necessary to provide it on the opposing surfaces where the gas distribution grooves 11 and 12 are provided, and it is not always necessary to provide them on both surfaces.

  The cell stack is configured by stacking a predetermined number of interconnectors 10, 10,... With the single battery cells 20 sandwiched therebetween, and fixing them with a connecting rod (not shown).

  The characteristics of the cell cell stack assembled in this way are as follows.

  The cell stack is preheated to an operating temperature in a furnace (not shown), and hydrogen-rich fuel gas is circulated from the central part to the outer peripheral part between the fuel electrode 22 of the unit cell 20 and the interconnector 10 on the fuel electrode side. At the same time, an oxidizing gas such as air is circulated from the central portion to the outer peripheral portion between the air electrode 23 of the single battery cell 20 and the interconnector 10 on the air electrode side. As a result, power is generated in each single battery cell 20. Each single battery cell 20 is connected in series by an interconnector 10. The operating temperature is, for example, 800 ° C.

Here, the interconnector 10 is made of Ni-based alloy or Fe—Ni alloy not containing pure Ni and Cr. Since these metals are excellent in heat resistance and have high electrical conductivity, they function as gas separators and electrical connection members between adjacent unit cells 20 and 20. In particular, in each interconnector 10, (Co, Ni) ₃ O ₄ , (Mn, Ni) ₃ O ₄ , (Cu, Ni) ₃ are formed on the surface of the unit cell 20 on the fuel electrode side and on the air electrode side. A spinel-type complex oxide coating layer such as O ₄ is formed.

Without this coating layer, an oxide scale is formed on the surface of the interconnector 10 at the operating temperature of the fuel cell. This oxide scale has poor electrical conductivity and causes a decrease in battery performance due to a voltage drop. However, in the cell stack, a coating layer 13 of a spinel-type complex oxide indicated by (Co, Ni) ₃ O ₄ or the like is formed on the surface of the interconnector 10. The covering layer 13 has higher electrical conductivity than the oxide scale, and does not cause deterioration of battery performance due to voltage drop. Further, since Cr is excluded from both the base material of the interconnector 10 and the covering layer 13, chromium poisoning does not occur on the air electrode side of the single battery cell 20. Furthermore, since the coating layer 13 is dense and highly stable, ion diffusion hardly occurs at the interface between the base material and the coating layer 13. For this reason, generation | occurrence | production of the oxide scale in an interface is suppressed. In addition, since the chromium poisoning does not occur on the air electrode side of the unit cell 20 and the generation of high-resistance oxide scale at the interface between the fuel electrode side and the air electrode side is suppressed, A decrease is avoided, and stable battery performance is maintained for a long time.

  FIG. 2 is a graph showing the cell performance of a fuel cell using the interconnector of the present invention compared with the cell performance of a fuel cell using another interconnector.

  The vertical axis in FIG. 2 is the unit cell voltage in the fuel cell, and the horizontal axis is the operating time. The relationship between the two is the fuel cell using the interconnector of the present invention, and the fuel using the interconnector made of Fe—Cr alloy. It shows a fuel cell using a battery and an uncoated interconnector made of pure Ni.

In the interconnector of the present invention, coating layers having a thickness of 5 μm made of (Co, Ni) ₃ O ₄ are formed on both surfaces of a disk-shaped base material made of pure Ni having a thickness of 1.5 mm and a diameter of 120 mm. It is. This coating layer is obtained by coating Co on both surfaces of a disk-shaped base material made of pure Ni by electroless plating and holding the disk-shaped base material in an oxidizing heating atmosphere at 800 ° C. for 48 hours. Formed. The interconnector made of Fe—Cr alloy is made of ferritic stainless steel (SUS430) that forms a coating layer of spinel type complex oxide made of (Mn, Cr) ₃ O ₄ on the surface in an operating environment. The uncoated interconnector made of pure Ni is the same as the base material in the interconnector of the present invention.

  The cell stack is configured by stacking 80 single battery cells, and is housed in a furnace (not shown) to form a fuel cell. The operating conditions were the same for each fuel cell.

As can be seen from FIG. 2, when using an interconnector made of pure Ni, the cell voltage is low from the beginning of operation and does not reach the rated voltage (0.7V) due to the high-resistance oxide scale formed on the surface. . In the case of using an interconnector formed of a Fe-Cr alloy (SUS430) that forms a coating layer of a spinel type complex oxide on the surface in an operating environment, the electrical conductivity of the spinel type complex oxide is good. Initially, a cell voltage exceeding the rating can be obtained. However, the cell voltage decreases as the operation continues. This is because chromia (Cr ₂ O ₃ ) is generated at the interface between the base material and the spinel-type complex oxide, and chromium vapor generated from the chromia is transferred to the air electrode of the unit cell, and high-resistance SrCrO ₄ is generated at the air electrode. This is because of the so-called chromium poisoning.

On the other hand, when the interconnector of the present invention in which the coating layer of the spinel type complex oxide made of (Co, Ni) ₃ O ₄ is formed on the surface of the base material made of pure Ni, the cell voltage at the start of operation is used. Is lower than when using an interconnector made of Fe-Cr alloy (SUS430). This is because the electrical conductivity of the Ni-based composite oxide is slightly lower than that of the Cr-based composite oxide. However, since Cr is excluded from both the base material and the coating layer, Cr-based oxide scale (chromia) that causes chromium poisoning on the air electrode side is not generated. For this reason, the cell voltage does not decrease with time due to chromium poisoning, and the rated cell voltage is maintained during long-time operation.

It is a model longitudinal cross-sectional view of the cell stack for fuel cells which shows one Embodiment of this invention. It is the graph which showed the battery performance of the fuel cell using the interconnector of this invention compared with the battery performance of the fuel cell using another interconnector.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Interconnector 11,12 Gas distribution groove 13 Coating layer 20 Single battery cell 21 Solid electrolyte layer 22 Fuel electrode layer 23 Air electrode layer

Claims (5)

  An interconnector used in a solid oxide fuel cell, which uses pure Ni or Cr-free Ni-base alloy or Fe-Ni alloy as a base material, and Co, Cu, Mn as metal elements on the surface of the base material A fuel cell interconnector having a coating layer made of a spinel complex oxide containing at least one of the above and Ni.   2. The fuel cell interconnector according to claim 1, wherein the surface of the base material on which the coating layer is formed is a surface facing the fuel electrode layer of the single battery cell and a surface facing the air electrode layer.   The fuel cell interconnector according to claim 1, wherein the coating layer is a spinel-type composite oxide containing no Cr.   The fuel cell interconnector according to claim 1, wherein the coating layer has a thickness of 50 μm or less.   3. The interconnector according to claim 1 or 2 is laminated in the thickness direction while sandwiching a single battery cell in which a fuel electrode layer is formed on one surface of the solid electrolyte layer and an air electrode layer is formed on the other surface. A cell stack for a fuel cell formed by