KR101027222B1 - OAE/Co Coating for Planar Solid Oxide Fuel Cell Interconnects - Google Patents

Abstract

The present invention is characterized in that the metal separator for a plate-type solid oxide fuel cell is coated with an OAE (Oxygen Active Elements) element on a lower layer of an existing ferritic stainless steel sheet, and then coated with cobalt on an upper layer. Accordingly, the present invention by coating a two-layer coating of OAE / cobalt (Co) on a ferritic stainless steel sheet is excellent in thermal stress resistance in a high temperature oxidizing atmosphere, excellent conductivity on the surface even after prolonged exposure, preventing chromium poisoning There is an effect of forming an oxide.

Flat solid oxide fuel cell, separator, ferritic stainless steel sheet, OAE element, cobalt

Description

OAE / Co Coating for Planar Solid Oxide Fuel Cell Interconnects}

The present invention relates to a metal separator for a planar solid oxide fuel cell (P / SOFC), and more specifically, to an OAE (Oxygen Active Elements) coated on the surface of a ferritic stainless steel sheet, By sequentially coating two layers of cobalt (Co) metal thereon, a separator having excellent electrical conductivity is prepared.

Separators used in solid oxide fuel cells should have excellent conductivity at high temperatures for a long time and have low toxicity due to chromium (Cr) components. This is because the separator should not only have the function of electrically connecting the anode of one cell and the cathode of the neighboring cell, but also reduce the toxicity caused by the gaseous chromium compound component generated from the base stainless steel sheet.

Various alloys used as solid oxide fuel cell separators up to now include Cr-based alloys based on Cr, Fe-Cr alloys based on Fe, and Ni-based superalloys based on Ni.

First, since Cr-based alloys form Cr ₂ O ₃ oxides that are stable at high temperatures, they have been developed as separators for flat SOFCs. In addition, the Cr-based alloy has a thermal expansion coefficient similar to that of other ceramic materials constituting SOFC, and has excellent mechanical properties at high temperatures. In general, research and development on Cr-based alloys have been progressed to develop alloys that can increase the adhesion of Cr ₂ O ₃ oxides and lower the growth rate. In order to satisfy this goal, most Cr-based alloys include elements such as Y, La, Ce, and Zr in the form of ODS. Representative Cr-based alloys include Plan-5's Cr-5Fe-1Y ₂ O ₃ and Cr-0.4La ₂ O ₃ , which were developed based on this background.

However, Cr-5Fe-1Y ₂ O ₃ is rarely used as a separator for SOFCs below 800 ° C. This is because, in SOFCs operating in the low temperature range of 650 to 800 ° C., Fe or Ni alloys can be applied in place of Cr-based alloys, which are poor in workability and expensive.

Among them, Ni-based alloys, which are already used commercially, have been studied as a separator for SOFC. Ni-based superalloys Haynes 230, Haynes 242, Inconel 625, Inconel 718, and the like are used. Haynes 242 is known to exhibit the highest electrical conductivity. However, Haynes 242 alloys are also expensive because they are more expensive than Fe-based alloys and do not sufficiently satisfy the SOFC requirements.

Fe-Cr alloys include ZMG232 developed by Hitachi Metals and Crofer22 developed by ThyssenKrupp. ZMG232 is a Ferritic Fe-22Cr alloy containing 22% Cr and added 0.04% La and 0.22% Zr. ZMG232 has a thermal expansion coefficient of 12.8 × 10 ^-6 / ℃ and is reported to show better oxidation resistance and electrical conductivity than the existing STS430 in the temperature range of 700 ~ 1000 ℃. This is known to be related to the characteristics of the oxide formed on the surface in the oxidizing atmosphere. That is, in the case of ZMG232 alloy, the structure of the oxide is dense, has high adhesion, and has excellent electrical conductivity.

Crofer22, developed by ThyssenKrupp, is a Ferritic Fe-Cr alloy originally developed for automotive APU (Auxiliary Power Unit). Crofer22 contains a trace amount of La of 0.08% to minimize evaporation of Cr and lower the coefficient of thermal expansion. Mn and Ti are added thereto to form an oxide structure of (Mn, Cr) ₂ O ₄ in the upper layer and Cr ₂ O _{3 in} the lower layer in a high temperature oxidation atmosphere. The (Mn, Cr) ₂ O ₄ oxide of the spinel structure also has a function of preventing the evaporation of Cr.

On the other hand, since the separator forms an oxide on the surface in the solid oxide fuel cell operating environment, the resistance is increased to reduce the electrical conductivity. In other words, the electrical conductivity of the separator depends on the electrical conductivity of the oxide formed on the surface rather than the electrical conductivity of the metal itself. Therefore, it is important to form an oxide having excellent conductivity on the surface. This is why most separators for solid oxide fuel cells are designed based on Cr ₂ O ₃ -formers.

However, the major problem of Cr ₂ O ₃ -former type separator is that in the operating environment of solid oxide fuel cell, volatile Cr (VI) compound moves to the electrode in contact with the separator and reacts or moves to the electrolyte. . The Cr component acts as a factor to reduce the battery performance by disturbing the normal electrochemical reaction of the battery.

Therefore, in the development of a separator for a solid oxide fuel cell, along with the development of the alloy itself, a surface treatment technique for controlling surface characteristics, that is, a technique of coating on the surface in order to enhance the surface characteristics is being combined.

In the case of alloys, elements having a specific function can be sufficiently supplied from the base material, but in the case of coating, elements that exist only on the surface may have limitations in maintaining these functions.

Nevertheless, the advantage of the coating technology is that it is easily applicable. In addition, in view of the electrical conductivity of the metal material, it is possible to modify such surface properties through coating because control of the oxidation scale formed on the surface is important.

Therefore, there is a need for a technique for increasing the electrical conductivity by coating a suitable component on a conventional ferritic stainless steel sheet, and to reduce the toxicity by the chromium component.

The present invention has been completed to improve the performance of the conventional solid oxide fuel cell separator plate, by forming an oxide having excellent electrical conductivity by sequentially coating the OAE element and cobalt on the surface of the existing ferritic stainless steel sheet used as a solid oxide fuel cell. The present invention provides a separator for a solid oxide fuel cell having high temperature oxidation resistance and electrical conductivity and a method of manufacturing the same.

In addition, the chromium component from the ferritic stainless steel sheet in the fuel cell operating environment is prevented from being in a gas state or a solid state to prevent battery performance deterioration.

Therefore, the present invention

In a first embodiment, a separator plate for a solid oxide fuel cell in which an OAE coating layer in which OAE (Oxygen Active Elements) elements are vacuum deposited on a surface of a ferritic stainless steel sheet, which is a substrate, and a cobalt coating layer in which cobalt (Co) is vacuum deposited on the OAE coating layer are formed. ,

As a second embodiment, the OAE element is Y or La separation plate for a solid oxide fuel cell,

As a third embodiment, the thickness of the OAE coating layer is 50 to 500nm, the thickness of the cobalt coating layer provides a separator for a solid oxide fuel cell.

Furthermore, as a fourth embodiment, a ferritic stainless steel sheet is placed in a chamber, an OAE element is charged into a Cu crucible, and the OAE element is evaporated by electron beam deposition to coat the OAE element on the surface of the steel sheet; and Cobalt was coated on the OAE metal-coated steel sheet using a plating solution containing CoSO ₄ · 7H ₂ O, Na ₂ PO ₂ · 2H ₂ O, C ₄ H ₄ Na ₂ O ₆ · 2H ₂ O, H ₂ BO ₃ (Co) a method for producing a separator for a solid oxide fuel cell, comprising plating a metal by an electroless plating method, and

As a fifth embodiment, in the coating of the OAE element, the temperature of the steel sheet is raised to 300 ° C., and a vacuum is maintained at 5 × 10 ⁻⁵ torr or less, and a method of manufacturing a separator for a solid oxide fuel cell is provided. .

According to the present invention, since the OAE element and the cobalt (Co) are sequentially coated in a general ferritic stainless steel in two layers, the oxide having excellent conductivity is formed on the surface of the stainless steel sheet in a high temperature oxidation atmosphere. Can be used as a separator.

In addition, it is possible to maintain the performance of the fuel cell by preventing the emission of chromium that is evaporated from the stainless steel sheet during fuel cell operation to prevent the performance of the fuel cell due to poisoning by the chromium of the fuel cell.

The present invention relates to a separator plate for a solid oxide fuel cell, comprising: a flat plate type solid oxide comprising an OAE metal element coating layer as a lower layer on a surface of a ferritic stainless steel plate serving as a substrate, and a cobalt metal coating layer formed as an upper layer thereon. The present invention relates to a separator for a fuel cell.

The separator of the present invention uses a ferritic stainless steel sheet as a substrate, specifically, STS444 alloy. Such STS444 contains 18-20% of chromium (Cr) component in the base metal and does not include rare earth metals such as yttrium (Y) and lanthanum (La). Such ferritic stainless steel sheets have been commercialized as substrates for separation plates.

The separator for a solid oxide fuel cell of the present invention is coated on the surface of a ferritic stainless steel sheet with two layers of an OAE metal element coating layer on a lower layer and a cobalt metal coating layer on an upper layer.

First, the lower OAE metal element coating layer formed on the substrate is to increase the high temperature oxidation resistance of the conventional commercialized ferritic stainless steel sheet. It serves to lower the growth rate of the oxide formed on the surface of the ferritic stainless steel sheet as a substrate and to reduce the thickness of the scale formed on the surface.

Oxygen Active Elements (OAE) metal elements forming the lower coating layer include elements such as yttrium (Y) and lanthanum (La). These OAE elements serve to lower the growth rate of the oxide formed on the surface of the substrate and to increase adhesion.

The coating thickness of the OAE coating layer is preferably 50nm to 500nm. If the coating thickness is less than 50nm, the effect of increasing the thermal stress resistance and the oxidation resistance does not show much. If the coating thickness exceeds 500nm, since the peeling of the coating layer occurs due to the formation of an internal oxide, it is not preferable.

On the other hand, only the OAE element cannot sufficiently prevent chromium poisoning by the gaseous chromium compound component. Therefore, instead of forming chromium oxide, it is desirable to form another oxide having excellent conductivity.

In the present invention, by coating cobalt (Co) to the existing ferritic stainless steel sheet used for the solid oxide fuel cell to prevent the formation of chromium oxide on the surface while preventing the external diffusion of chromium from the ferritic stainless steel sheet as a substrate will be.

Cobalt (Co) is easy to form a spinel-type oxide such as Co ₃ O ₄ on the surface, the conductivity of the Co ₃ O ₄ oxide is about 6.7 S / cm at 800 ℃ compared to Cr ₂ O ₃ oxide, so the electrical conductivity The effect of increasing the amount can also be great.

The coating thickness of the cobalt coating layer of the upper layer is preferably formed in the range of 2㎛ 10㎛. Basically, if the coating thickness of cobalt (Co) is thinner than 2 μm, it is highly likely to form Cr ₂ O ₃ oxide by chromium components diffused from the base material. In addition, when the coating thickness of cobalt (Co) is too large to exceed the coating thickness required to perform the basic function, there is no economical efficiency, and as the thickness of the coating layer increases, there is a problem that the thermal stress resistance of the coating layer decreases. It is preferable to form 10 micrometers or less.

In the present invention, it is preferable to form a coating layer of cobalt metal after forming an OAE element coating on the surface of the ferritic stainless steel sheet serving as the substrate. Unlike the present invention, when the Co coating layer / OAE element coating layer is formed on the substrate, the oxidation resistance of the Co coating layer may be increased by the OAE element coating layer on the surface layer in a high temperature oxidizing atmosphere. Therefore, the OAE element is exposed to the surface to control the growth of the cobalt oxide, but does not contribute to the oxidation resistance of the substrate itself. However, by forming a Co coating layer on the surface layer as in the present invention, by coating the OAE element between the substrate and the Co coating layer it can increase the oxidation resistance of the substrate. In the high temperature oxidation atmosphere, the Co coating layer forms cobalt oxides such as Co ₃ O ₄ through the oxidation process, and Cr and Fe components of the substrate participate in the oxidation process through the oxidation process, but the OAE element is coated on the interface. Therefore, the oxidation rate of these Cr oxides or Fe oxides can be reduced.

Next, the method of forming the coating layer of this invention is demonstrated.

First, the OAE element is coated on the surface of a ferritic stainless steel sheet, specifically, STS444, which is a substrate. EB evaporation may be used for coating OAE elements.

The ferritic stainless steel sheet is seated in a chamber for vacuum deposition, an OAE element such as yttrium (Y) or lanthanum (La) is charged in a water-cooled Cu crucible inside the chamber, and the temperature is raised to a predetermined temperature. Below the degree of vacuum, the metal is evaporated using the electron beam to coat the substrate.

At this time, the temperature and vacuum degree of the steel sheet can be appropriately selected by a person skilled in the art, but in order to improve the deposition efficiency by the electron beam deposition method, the temperature of the substrate is raised to 300 ℃, the vacuum degree is 5 × 10 It is desirable to keep it below ^-5 torr.

Cobalt (Co) metal plating is performed on the surface of an OAE metal-coated ferritic stainless steel sheet using an electroless plating method. For electroless plating, the plating solution is prepared by using a solution composed of CoSO ₄ · 7H ₂ O, Na ₂ PO ₂ · 2H ₂ O, C ₄ H ₄ Na ₂ O ₆ · 2H ₂ O, H ₂ BO _3, etc. Can be.

In this manner, a separator for a solid oxide fuel cell in which a coating layer of an OAE metal element / cobalt metal layer is sequentially formed can be obtained, and the separator thus obtained has an oxide having excellent conductivity on the surface of a stainless steel sheet in a high temperature oxidation atmosphere. It is possible to form, and to prevent the emission of chromium evaporated from the stainless steel sheet during the fuel cell operation, it is possible to keep the performance of the fuel cell constant.

Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not limited by these Examples.

[Example]

Assessment Methods

High temperature conductivity and chromium poisoning were evaluated to evaluate the quality characteristics of the two-layer coated specimens of the present invention.

(1) high temperature conductivity

After exposing the test piece in an air atmosphere at 800 ° C. for 100 hours, the electrical conductivity of the test piece was measured. In order to measure the electrical conductivity of the oxidized test piece, ASR (Area specific resistance) was measured. The ASR measurement was performed as follows.

The uncoated STS444 stainless steel sheet and the coated specimens were all cut to a size of 3 × 3 cm and then oxidized for 100 hours in an 800 ° C. air atmosphere to simulate the SOFC Cathode atmosphere. For ASR measurements, first, Pt paste (Engelhead® # 6082) was first applied to both sides of the oxidized specimens by sintering at 800 ° C for 1 hour. After that, the Pt paste was secondarily applied in the same manner, and then the Pt mesh and the Pt line were connected. Subsequently, after sintering again at 800 ° C. for 1 hour, the specimen was mounted in a furnace and maintained for 200 hours under pressure at 0.5 bar. Finally, a current of 100 mA / cm 2 was applied to the specimen through the Pt line, and the ASR value of the specimen was measured according to Ohm's law.

Evaluation criteria were (circle) when ASR value was less than 10 mPacm <2>, (circle) when 10 mPacm <2> or more and less than 20 mPacm <2>, (triangle | delta), 20m <2> cm <2> or more and less than 50mPacm <2> made it into x.

(2) chromium poisoning

After exposing the test piece at 800 ° C. for 24 hours in an air atmosphere containing 3% H ₂ O, the amount of evaporated chromium component was measured.

(Circle), when the density | concentration of the measured chromium component was less than 10 ng / cm <^2> / h, if it was 10 ng / cm <^2> / h or more and less than 100 ng / cm <^2> / h, it was made into (triangle | delta) and 100 ng / cm <^2> / h or more.

(3) Thermal stress resistance

The test piece of 3 x 3 cm ² was kept at a high temperature of 800 ° C. for 24 hours, and then subjected to 10 cycles of exposure to room temperature for one cycle, and then the peeling of the coating layer was evaluated as ○ or ×. When the peeling area was 5% or more in the whole test piece area, it judged as x.

Example  1 ~ Example  3

On the STS444 alloy substrate which is a ferritic stainless steel sheet, Y metal was coated with a coating thickness of 200 nm by using an electron beam evaporation method. Then, Co was changed to 2, 5, and 10 μm on the Y metal-coated substrate by electroless plating.

The coating method is specifically as follows.

The ferritic stainless steel sheet was placed in a chamber for vacuum deposition, and yttrium (Y) was charged into a water-cooled Cu crucible in the chamber, and then the temperature of the substrate was raised to 300 ° C., and the vacuum degree was 5 × 10 ^−5. The Y metal was evaporated and coated on the substrate by using an electron beam while maintaining below torr.

Then, a plating solution consisting of CoSO ₄ · 7H ₂ O, Na ₂ PO ₂ · 2H ₂ O, C ₄ H ₄ Na ₂ O ₆ · 2H ₂ O and H ₂ BO ₃ was prepared. Using the prepared plating solution, the surface of the ferritic stainless steel sheet coated with Y metal was subjected to cobalt metal plating by an electroless plating method. In the electroless plating, the plating solution had a pH of 9 and a temperature of 90 ° C.

Example  4 ~ Example  5

A Y / Co bilayer coating layer was formed in the same manner as in Example 1 except that the thickness of the lower Y metal coating layer was changed to 50 and 500 nm and the coating thickness of the upper layer Co was fixed at 5 μm.

Example  6

A La / Co bilayer coating layer was formed on the surface of STS444 alloy, which is a ferritic stainless steel sheet, using the same method as Example 1 except that La was used as the OAE metal. The La metal coating layer was coated with a thickness of 200 nm, and the Co coating layer was coated with a thickness of 5 μm.

Comparative example  1 ~ Comparative example  3

The thickness of the Y coating layer was coated at 200 nm, and the thickness of the Co coating layer was changed to 0, 0.5, and 15 μm, respectively, except that the coating layer was formed.

Comparative example  4 ~ Comparative example  5

A Y / Co bilayer coating layer was formed in the same manner as in Example 1 except that the thickness of the lower Y metal coating layer was changed to 20 and 1000 nm and the coating thickness of the upper layer Co was fixed at 5 μm.

Comparative example  6

The same process as in Example 1 was performed except that only the Co coating layer was formed to a thickness of 5 μm without forming an OAE layer on the surface of the STS444 alloy, which is a ferritic stainless steel sheet.

Comparative example  7

STS444 alloy, a ferritic stainless steel sheet having no coating, was used as a separator for a solid oxide fuel cell.

division Coating thickness Electrical conductivity Chromium Peptide Toxicity Thermal stress resistance OAE (nm) Co (μm) Example One 200 (Y) 2 ◎ ○ ○ 2 200 (Y) 5 ◎ ○ ○ 3 200 (Y) 10 ◎ ○ ○ 4 50 (Y) 5 ◎ ○ ○ 5 500 (Y) 5 ◎ ○ ○ 6 200 (La) 5 ◎ ○ ○ Comparative example One 200 (Y) 0 △ × ○ 2 200 (Y) 0.5 ○ △ ○ 3 200 (Y) 15 △ ○ × 4 20 (Y) 5 ○ ○ × 5 1000 (Y) 5 △ ○ × 6 0 5 ○ ○ × 7 0 0 × × ○

As can be seen from the above table, the separator for a solid oxide fuel cell in which the AEO / cobalt two-layer coating layer according to the present invention is formed has all excellent electrical conductivity, chromium resistance and thermal stress resistance, but does not include a cobalt coating layer. In Comparative Example 1, the chromium resistance was poor, and when the OAE coating layer was not included (Comparative Example 6), the thermal stress resistance was poor, and in the case where no coating layer was included (Comparative Example 7), the electrical conductivity and It can be seen that the thermal stress resistance is not good. Furthermore, when the thickness of the coating layer is out of the range of the present invention (Comparative Example 2-5), the result is a lack of chrome resistance, poor thermal stress resistance, and poor electrical conductivity.

Claims (5)

In the flat plate type solid oxide fuel cell separator, an OAE coating layer in which an OAE (Oxygen Active Elements) metal element is vacuum deposited on a surface of a ferritic stainless steel sheet as a substrate, and a cobalt coating layer coated with cobalt (Co) is formed on the OAE coating layer. Separation plate for a solid oxide fuel cell characterized in that. The separator plate for solid oxide fuel cells according to claim 1, wherein the OAE element is Y or La. The separator of claim 1 or 2, wherein the thickness of the OAE coating layer is 50 to 500 nm and the thickness of the cobalt coating layer is 2 to 10 µm. Seating a ferritic stainless steel sheet inside the chamber, charging the OAE element into a Cu crucible, and then evaporating the OAE element by electron beam evaporation to coat the OAE element on the surface of the steel sheet; And Cobalt was coated on the OAE metal-coated steel sheet using a plating solution containing CoSO ₄ · 7H ₂ O, Na ₂ PO ₂ · 2H ₂ O, C ₄ H ₄ Na ₂ O ₆ · 2H ₂ O, H ₂ BO ₃ Plating (Co) metal by electroless plating Separation plate manufacturing method for a solid oxide fuel cell comprising a. 5. The method of claim 4, wherein in the coating of the OAE element, the temperature of the steel sheet is raised to 300 ° C., and the vacuum is maintained at 5 × 10 ⁻⁵ torr or less.