Fuel cell separator and fuel cell stack including same

Abstract

The present invention relates to a separator for a fuel cell and a fuel cell stack, in which volatilization of chromium components and resultant corrosion of unit cells are minimized under operating conditions and durability is improved, by means of a fuel cell stack structure in which a Co-Ni-based first coating layer is formed on a surface of a stainless steel base material, and which includes an interfacial Mn-enriched layer and a Fe-Co-Ni-based second coating layer formed by Fe diffusion during SOFC operation.

Description

Separator for a fuel cell and fuel cell stack including the same

[0001] The present invention relates to a separator for a fuel cell and a fuel cell stack including the same.

[0002] More specifically, the present invention relates to a separator for a solid oxide fuel cell and a solid oxide fuel cell stack including the same.

[0003] As ammonia emerges as an alternative for green hydrogen transportation, fuel cell technology that generates electricity using ammonia as fuel is garnering attention. In particular, ammonia-fueled high-temperature solid oxide fuel cell (SOFC) technology is receiving particular interest for simultaneously possessing the convenience of ammonia storage and transportation with the high efficiency characteristics of SOFCs. However, since these fuel cells are exposed to various reaction gases in high-temperature environments, repeated use can lead to the degradation of internal components and a decline in performance. Therefore, there is a need for technological development to improve the durability of fuel cells.

[0004] (Patent Document 1) Republic of Korea Registered Patent Publication No. 10-2254196

[0005] The problem that the technical concept of the present invention aims to solve is to provide a separator capable of improving the durability of a fuel cell stack.

[0006] In addition, the problem that the technical concept of the present invention aims to solve is to provide a fuel cell stack with improved durability.

[0007] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall details of the specification.

[0008] According to exemplary embodiments for solving the problem of the present invention, a separator for a fuel cell is provided. The separator for a fuel cell comprises a first base material made of stainless steel; and a first coating layer located on the surface of the first base material, wherein the first coating layer comprises, in weight percent, cobalt (Co): 65 to 95%, the remainder being nickel (Ni) and unavoidable impurities.

[0009] The above first coating layer can satisfy the following relationship 1.

[0010] [Relationship 1]

[0011] 2≤[Co] / [Ni]≤19

[0012] In the above equation 1, [Co] and [Ni] represent the average content of Co and Ni in the first coating layer, respectively.

[0013]

[0014] According to other exemplary embodiments, a fuel cell stack may be provided. The fuel cell stack comprises a unit cell in which a fuel electrode; an electrolyte; and an air electrode are sequentially stacked; and a fuel cell separator plate in contact with the surface of the unit cell. The fuel cell separator plate comprises a second base material made of stainless steel and a second coating layer located on the surface of the second base material. The second coating layer may comprise, in weight percent, cobalt (Co): 10 to 30%, nickel (Ni): 9 to 20%, the remainder being Fe, and unavoidable impurities.

[0015] The above second coating layer can satisfy the following relationship 2.

[0016] [Relationship 2]

[0017] 1 < [Ni]top / [Ni]bottom ≤ 2

[0018] In the above relationship 2, [Ni]top refers to the average Ni content at the top of the second coating layer based on the center of the second coating layer, and [Ni]bottom refers to the average Ni content at the bottom of the second coating layer based on the center of the second coating layer.

[0019] The second base material may include a Mn enrichment layer having a depth range of 5 to 20 μm from the interface between the second base material and the second coating layer.

[0020] The above Mn enrichment layer may contain, in weight percent, manganese (Mn): 1 to 3%, the remainder being Fe and unavoidable impurities.

[0021] The ratio (T1 / T2) between the thickness (T1) of the Mn enrichment layer and the thickness (T2) of the second coating layer may be 1 to 10.

[0022] The above fuel cell stack can be configured to use ammonia directly as fuel.

[0023] According to exemplary embodiments of the present invention, a separator capable of improving the durability of a fuel cell stack can be provided.

[0024] In addition, according to exemplary embodiments of the present invention, a fuel cell stack with improved durability can be provided.

[0025] The various and beneficial advantages and effects of the present invention are not limited to those described above and will be more easily understood in the process of explaining specific embodiments of the present invention.

[0026] FIG. 1 is a drawing for illustrating a separator for a fuel cell according to exemplary embodiments.

[0027] FIG. 2 is a drawing for illustrating a fuel cell stack according to exemplary embodiments.

[0028] Figure 3 is an enlarged view of area A of Figure 2.

[0029] Figure 4 is a graph showing the change in sheet resistance of each fuel cell separator.

[0030] Figure 5 shows the EPMA analysis results of Example 1 before ammonia exposure.

[0031] Figure 6 shows the EPMA analysis results of Example 1 after ammonia exposure.

[0032] Figure 7 shows the EPMA analysis results of Comparative Example 1 before ammonia exposure.

[0033] Figure 8 shows the EPMA analysis results of Comparative Example 1 after ammonia exposure.

[0034] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Prior to this, terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings. Instead, based on the principle that the inventor can appropriately define the concepts of terms to best describe his invention, they should be interpreted in a meaning and concept consistent with the technical spirit of the present invention.

[0035] In the following descriptions with reference to the drawings, identical or corresponding components are assigned the same reference numerals, and redundant descriptions thereof will be omitted.

[0036] In the following embodiments, the terms first, second, etc. are used not in a limiting sense, but for the purpose of distinguishing one component from another component.

[0037] In the following embodiments, the singular expression includes the plural expression unless the context clearly indicates otherwise.

[0038] In the following embodiments, terms such as "include" or "have" mean that the features or components described in the specification are present, and do not preclude the possibility that one or more other features or components may be added.

[0039] In the drawings, the size of components may be exaggerated or reduced for convenience of explanation. For example, the size and thickness of each component shown in the drawings are depicted arbitrarily for convenience of explanation, so the present invention is not necessarily limited to what is illustrated.

[0040] Where an embodiment can be implemented differently, a specific process sequence may be performed differently from the order described. For example, two processes described consecutively may be performed substantially simultaneously or proceed in the reverse order of the description.

[0041] In addition, in describing the present invention, if it is determined that a detailed description of related known components or functions may obscure the essence of the invention, such detailed description is omitted.

[0042] The present invention will be described in detail below through each embodiment. It should be noted that each embodiment described in this specification is not limited to a single embodiment but may also be combined with other embodiments. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0043] The present invention will be described in detail below through examples. However, it should be noted that the following examples are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0044] [Fuel cell separator]

[0045] FIG. 1 is a drawing for illustrating a separator plate (100) for a fuel cell according to exemplary embodiments.

[0046] Referring to FIG. 1, a fuel cell separator (100) comprises a first base material (110) and a first coating layer (120). For convenience of explanation and illustration, the flow path shape of the fuel cell separator (100) is not specifically illustrated in FIG. 1.

[0047] A separator (100) for a fuel cell is a component that provides a flow path for gases supplied to the fuel cell. As a result, the separator (100) for the fuel cell is exposed to high-temperature reactive gases. Therefore, the separator (100) for the fuel cell requires high thermal stability and chemical resistance.

[0048] According to exemplary embodiments, the first base material (110) may be a stainless steel material. In this way, by using a material with excellent thermal stability as the base material, the thermal stability of the fuel cell separator (100) can be increased, and furthermore, the durability of the fuel cell stack can be improved.

[0049] The first base material (110) may be any one of austenitic stainless steel, martensitic stainless steel, ferritic stainless steel, and a combination thereof. The first base material (110) may be any one of Fe-Cr alloy steel, Fe-Cr-Ni alloy steel, Fe-Cr-Mn alloy steel, and a combination thereof.

[0050] These stainless steels contain a high content of chromium. This chromium can form an oxide film (Cr2O3) with excellent high-temperature stability on the surface of the base material, thereby increasing the thermal stability of the first base material. However, if continuously exposed to high-temperature reactive gases, Cr may volatilize and contaminate the catalyst of the fuel cell, potentially causing performance degradation. In particular, when ammonia is used as fuel, a nitridation reaction of the iron component of the base material occurs on the surface of the separator (100) for the fuel cell, and the nitrided leaching may occur on the outer surface of the base material, causing pressure loss in the flow path. Additionally, there is a risk that pores may form on the surface and increase the resistance of the separator, thereby causing performance degradation of the fuel cell. Furthermore, nitrogen may penetrate into the base material and induce a phase transition within the base material. As a result, the coefficient of thermal expansion within the base material becomes non-uniform, which may cause cracks to occur within the base material. To prevent this, according to exemplary embodiments, a fuel cell separator (100) includes a first coating layer (120) located on the surface of a first base material (110).

[0051] The first coating layer (120) contains, in weight percent, cobalt (Co): 65–95%, the remainder being nickel (Ni) and unavoidable impurities. In this way, direct contact between the reaction gas and the first base material (110) can be blocked, and the volatilization of chromium can be suppressed. Additionally, it can prevent nitrogen from penetrating into the interior of the base material. At this time, the content of cobalt and nickel refers to the average content in the first coating layer (120).

[0052] Cobalt (Co) can form a spinel-structured oxide on the surface of the first coating layer (120) in a high-temperature ammonia environment. This effectively suppresses the volatilization of chromium and contributes to preventing the penetration of nitrogen. To achieve this effect, the cobalt content is set to 65% or more. A higher cobalt content is desirable. However, if the cobalt content becomes excessively high, an uneven oxide layer may be formed on the surface of the first coating layer (120). In this case, it may cause a pressure loss of the fuel gas / air gas supplied to the fuel cell (unit cell). Therefore, the cobalt content may be 95% or less.

[0053] Nickel (Ni) can form a spinel-structured oxide with cobalt or alone in a high-temperature ammonia environment. This prevents nitrogen from penetrating into the interior of the first base material (110) and contributes to minimizing the volatilization of chromium. Therefore, the remainder of the first coating layer (120) is nickel.

[0054] According to exemplary embodiments, the first coating layer (120) can satisfy the following relationship 1.

[0055] [Relationship 1]

[0056] 2≤[Co] / [Ni]≤19

[0057] In the above equation 1, [Co] and [Ni] each represent the average content of Co and Ni in the first coating layer (120).

[0058] The composition of the first coating layer (120) can be analyzed using EPMA (Electron Probe MicroAnalyzer).

[0059] In this way, by optimizing the content of cobalt and nickel in the first coating layer (120), nitrogen diffusion into the first base material (110) and chromium loss in the first base material (110) can be minimized.

[0060] [Fuel cell stack]

[0061] FIG. 2 is a drawing for illustrating a fuel cell stack (10) according to exemplary embodiments.

[0062] Figure 3 is an enlarged view of area A of Figure 2.

[0063] Referring to FIGS. 2 and 3, the fuel cell stack (10) includes a fuel cell separator (101) and a unit cell (200). The operating temperature of the fuel cell stack (10) may be 600 to 950°C. In this temperature range, the fuel cell stack (10) can decompose ammonia to produce hydrogen and energy.

[0064] A fuel cell separator (101) may be interposed between a plurality of unit cells (200). Unit cells (200) may be arranged on both sides of the fuel cell separator (101). The fuel cell separator (101) may provide a flow path (P) for fuel gas or air gas supplied to the unit cells (200). At this time, the direction of the fuel gas flow path (P1) and the direction of the air gas flow path (P2) may be substantially perpendicular to each other, but are not necessarily limited thereto.

[0065] The fuel cell separator (101) is derived from the fuel cell separator (100) described above. When high-temperature ammonia is continuously supplied, the iron component of the first base material (110) can diffuse into the first coating layer (120) of the fuel cell separator (100). Additionally, the cobalt and nickel components of the first coating layer (120) can also diffuse into the first base material (110). Due to this mutual diffusion of alloy elements, the thermal stability and chemical resistance of the fuel cell separator (101) can be improved. As a result, the durability of the fuel cell stack (10) can be improved.

[0066] A separator plate (101) for a fuel cell may include a second base material (111) made of stainless steel and a second coating layer (121) located on the surface of the second base material (111).

[0067] According to exemplary embodiments, the second base material may comprise, in weight percent, carbon (C): 0.001–0.10%, manganese (Mn): 0.1–2.0%, silicon (Si): 0.05–1.0%, chromium (Cr): 15.0–30.0%, nitrogen (N): 0.001–0.20%, nickel (N): 0.01–25.0%, cobalt (Co): 0.5–10%, the remainder being iron (Fe) and unavoidable impurities. In this case, the alloy content in the second base material refers to the average content within the second base material.

[0068] Carbon (C) is an element that increases the strength and hardness of the base material. However, if the carbon content is excessively high, it can form carbides and deplete chromium within the base material. In this case, it can lower the corrosion resistance of the base material and reduce its chemical resistance. Therefore, the carbon content may be 0.001 to 0.1%.

[0069] Manganese (Mn) is an element that has a deoxidizing effect. However, if manganese is added in excess, it can reduce oxidation resistance and cause hardening of the base material, which can reduce processability. Therefore, the manganese content may be 0.1 to 2.0%.

[0070] Silicon (Si) is an element that can improve oxidation resistance. However, if the silicon content is excessively high, silicon oxide (SiO2) may form on the surface of the substrate, which may reduce the electrical conductivity of the substrate. In addition, it may reduce the adhesion to the coating layer during subsequent coating processes. Therefore, the silicon content may be 0.05 to 1.0%.

[0071] Chromium (Cr) is an element that forms a Cr2O3 film with excellent high-temperature stability on the surface of the base material and has the effect of ensuring electrical conductivity. However, if the chromium content is excessively high, volatilization of chromium-based oxides is promoted, which may lead to the deterioration of the durability of the separator (100) or other components of the fuel cell in contact with the separator (100). Therefore, the chromium content may be 15.0 to 30.0%.

[0072] Nitrogen (N) forms nitrides and can consume chromium in the base material, so it is desirable for its content to be as low as possible. Therefore, the nitrogen content may be 0.001 to 0.20%.

[0073] Nickel (Ni) is an austenite phase stabilizing element and can contribute to improving corrosion resistance. Nickel may be derived from the first coating layer (120) or may be added during the manufacturing process of the first base material (110). However, if the nickel content is excessive, the processability of the base material may be reduced. Therefore, the nickel content may be 0.01 to 25.00%.

[0074] Cobalt (Co) can have the effect of increasing the hardenability of the base material and improving corrosion resistance. Cobalt may be derived from the first coating layer (120) or may be added during the manufacturing process of the first base material (110). If the cobalt content is excessive, it may lower the elongation of the base material and degrade its formability. Therefore, the cobalt content may be 0.5 to 10%.

[0075] It may contain iron and unavoidable impurities as a remainder. Unavoidable impurities refer to all elements that may inevitably be contained within the base material during the ironmaking process, and a specific explanation thereof is omitted.

[0076] In addition, the second base material (111) may further include one or more of niobium (Nb), molybdenum (Mo), titanium (Ti), copper (Cu), and selenium (Se), depending on the type of the first base material (110).

[0077] The composition of the second base material (111) can be analyzed using EPMA (Electron Probe MicroAnalyzer), SEM-EDS (Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy), ICP (Inductively Coupled Plasma), TEM (Transmission Electron Microscopy), and these means in combination.

[0078] According to exemplary embodiments, the second base material (111) may include a Mn enrichment layer having a depth range of 5 to 20 μm from the interface between the second base material (111) and the second coating layer (121). In this way, by forming a Mn enrichment layer on the surface of the second base material (111), it can contribute to preventing nitrogen diffusion into the interior of the second base material (111) and minimizing excessive volatilization of chromium.

[0079] The Mn enrichment layer is a region where Mn is relatively dense compared to the surrounding region. According to exemplary embodiments, the Mn enrichment layer may contain, in weight percent, manganese (Mn): 1 to 3%, the remainder being Fe and unavoidable impurities. The manganese content of the Mn enrichment layer refers to the average manganese content in the surface layer of the second base material (111) (deep from the interface with the second coating layer to 5 to 20 μm), and it will be obvious to a person skilled in the art that the manganese content of the Mn enrichment layer may be higher than the overall average manganese weight of the second base material (111).

[0080] The Mn enrichment layer can be determined based on the Mn distribution in the surface area of ​​the second base material (111) through EPMA analysis.

[0081] According to exemplary embodiments, the ratio (T1 / T2) between the thickness (T1) of the Mn enrichment layer and the thickness (T2) of the second coating layer (121) may be 1 to 10. This may help prevent nitrogen diffusion into the second substrate (111) and minimize excessive volatilization of chromium. The thickness of the second coating layer may be 0.5 to 20 μm.

[0082] The second coating layer (121) can minimize the corrosion of the unit cell caused by the volatilization of the chromium component of the fuel cell separator (101) in the operating environment of the fuel cell stack (10). Additionally, it can minimize the penetration of nitrogen into the second base material (111) during the ammonia decomposition process.

[0083] The second coating layer (121) may contain, in weight percent, 10–30% cobalt (Co), 9–20% nickel (Ni), the remainder iron (Fe), and unavoidable impurities. Thus, due to the high-temperature fuel gas atmosphere, mutual diffusion of alloy components occurs between the first base material (110) and the first coating layer (120), so that the cobalt and nickel content of the second coating layer (121) may be relatively reduced compared to the first coating layer (120). Additionally, iron components diffused from the first base material (110) may be included. Oxides of cobalt and nickel are distributed on the surface of the second coating layer (121). This suppresses the penetration of nitrogen, thereby minimizing the nitriding of iron components partially diffused into the second coating layer (121). The content range of cobalt, nickel, and iron refers to the average content in the second coating layer (121).

[0084] The second coating layer (121) can satisfy the following relationship 2.

[0085] [Relationship 2]

[0086] 1 < [Ni]top / [Ni]bottom ≤ 2

[0087] In the above equation 2, [Ni]top refers to the average Ni content at the top of the coating layer relative to the center of the coating layer, and [Ni]bottom refers to the average Ni content at the bottom of the coating layer relative to the center of the coating layer.

[0088] In this way, the performance of the fuel cell stack (10) can be improved by forming a nickel-based oxide or a nickel-chromium-based oxide by densely concentrating nickel on the surface of the second coating layer (121).

[0089] The unit cell (200) may be formed by sequentially stacking a fuel electrode; an electrolyte; and an air electrode. The unit cell (200) may constitute a single unit cell.

[0090] The unit cell (200) may be a solid oxide type fuel cell in which the electrolyte contains a solid oxide.

[0091] According to exemplary embodiments, the fuel cell stack (10) may be configured to use ammonia directly as fuel. That is, it may be configured to produce hydrogen and energy by directly supplying ammonia to the fuel electrode of the unit cell (200).

[0092] The fuel electrode may include an ammonia decomposition catalyst. As an active metal, the ammonia decomposition catalyst may include any one of nickel (Ni), cobalt (Co), ruthenium (Ru), palladium (Pd), platinum (Pt), iron (Fe), molybdenum (Mo), and alloys thereof.

[0093] The air electrode may be an electrode that comes into contact with an air gas containing oxygen. According to exemplary embodiments, the air electrode may include any one of LSC (Lanthanum Strontium Cobalt Oxide), LSCF (Lanthanum Strontium Cobalt Iron Oxide), LSM (Lanthanum Strontium Manganite), and combinations thereof.

[0094] An electrolyte can be interposed between the fuel electrode and the air electrode. The electrolyte can provide a pathway for the movement of ions necessary for the reactions occurring within the fuel cell.

[0095] According to exemplary embodiments, the electrolyte is oxygen ions (O 2- It can be a conductive electrolyte. In this case, oxygen is reduced at the air electrode to form oxygen ions (O 2- ) can be provided. Oxygen ions (O 2- Oxygen ions (O) diffused toward the fuel electrode through the electrolyte. 2- ) can generate water vapor and electrons by causing an electrochemical reaction with hydrogen at the triple phase boundary between the fuel electrode and the electrolyte. The generated electrons can move to the air electrode through an external circuit configured to electrically connect the fuel electrode and the air electrode.

[0096] The oxygen reduction reaction described above can follow the following reaction equation.

[0097] [Reaction Equation 2]

[0098] 0.5O2 + 2e- → O 2-

[0099] The above-described reaction for the generation of water vapor and electrons can follow the following reaction equation.

[0100] [Reaction Equation 3]

[0101] H2+ O 2- → H2O + 2e -

[0102] According to exemplary embodiments, the electrolyte may be any one of YSZ (Y2O3-stabilized ZrO2), SDC (Sm2O3-doped CeO2), GDC (Gd2O3-doped CeO2), ScCeSZ (Sc2O3 & CeO2-stabilized ZrO2), ESB (Er2O3-stabilized Bi2O3), LSGM (Sr and Mg-doped LaGaO3), and combinations thereof.

[0103] According to other exemplary embodiments, the electrolyte is hydrogen ions (H + It can be a conductive electrolyte. In this case, at the fuel electrode, hydrogen generated from ammonia is oxidized to form hydrogen ions (H + ) and electrons can be provided. The generated electrons can move to the air electrode through an external circuit configured to electrically connect the fuel electrode and the air electrode. Hydrogen ions (H + ) can diffuse and move through the electrolyte to the air electrode. Hydrogen ions (H) diffused toward the air electrode + ) can react with oxygen at the three-phase interface between the air electrode and the electrolyte to generate water vapor.

[0104] The hydrogen oxidation reaction described above can follow the following reaction equation.

[0105] [Reaction Equation 4]

[0106] H2→ 2H + + 2e -

[0107] The above-described water vapor generation reaction may follow the following reaction equation.

[0108] [Reaction Equation 5]

[0109] 0.5O2+ 2H + + 2e - → H2O

[0110] According to exemplary embodiments, the electrolyte is BCY(BaCe 1-x Y x O 3-δ ), BZY(BaZr 1-x Y x O 3-δ ), BCZY(BaCe 1-x-y Zr x Y y O 3-δ ), BCG(BaCe 1-x Gd x O 3-δIt may include any one of ), and combinations thereof. In this case, x is 0 < x ≤ 0.2, y is 0 < y ≤ 0.8, and δ is 0 < δ < 0.3.

[0111] [Test Example]

[0112] (Example 1)

[0113] A fuel cell separator was prepared by electroplating stainless steel 460FC to form a coating layer of 90 wt% cobalt and 10 wt% nickel.

[0114] (Comparative Example 1)

[0115] A separator for a fuel cell was prepared in the same manner as in Example 1, except that a coating layer was not formed.

[0116] (Comparative Example 2)

[0117] A fuel cell separator was prepared in the same manner as in Example 1, except that it was electroplated to form a coating layer of 50 wt% cobalt and 50 wt% nickel.

[0118] Subsequently, each fuel cell separator was exposed to ammonia at an atmosphere of approximately 700°C to measure the sheet resistance.

[0119] Figure 4 is a graph showing the change in sheet resistance of each fuel cell separator.

[0120] Referring to Fig. 4, the highest sheet resistance was observed in Comparative Example 1, in which no coating layer was formed. In addition, the sheet resistance was also measured to be high in Comparative Example 2, in which the composition of the coating layer fell outside the range presented in the present invention.

[0121] However, it was confirmed that the separator for the fuel cell according to Example 1 maintains a low sheet resistance.

[0122] Subsequently, EPMA analysis was performed at a depth of approximately 400 μm from the surface of the fuel cell separator according to Example 1 and Comparative Example 1 to observe the alloy component composition and distribution before and after exposure to ammonia of Example 1 and Comparative Example 1.

[0123] Figure 5 shows the EPMA analysis results of Example 1 before ammonia exposure.

[0124] Figure 6 shows the EPMA analysis results of Example 1 after ammonia exposure.

[0125] Figure 7 shows the EPMA analysis results of Comparative Example 1 before ammonia exposure.

[0126] Figure 8 shows the EPMA analysis results of Comparative Example 1 after ammonia exposure.

[0127] Referring to FIGS. 5 to 8, it can be seen that an Mn enrichment layer is formed on the surface of the second substrate (see FIG. 6). In addition, compared to the first coating layer (see FIG. 5), nickel and cobalt in the second coating layer (see FIG. 6) showed a more dense distribution on the surface of the coating layer. This is interpreted as being due to nickel and cobalt forming spinel-structured oxides on the surface of the coating layer.

[0128] In addition, in the case of Comparative Example 1, it was confirmed that nitrogen penetrated into the surface of the base material after ammonia exposure and accumulated in excessive amounts in the surface layer. In particular, as the iron component reacted excessively with nitrogen at the surface, iron was lost, and a large number of pores were observed in the surface layer (see arrow in Fig. 8). In addition, it was confirmed that chromium in the base material volatilized and was lost after ammonia exposure. However, in the case of Example 1, it was confirmed that nitrogen did not accumulate in any area even after ammonia exposure, and relatively fewer pores were formed.

[0129] Although the invention has been described with reference to the above embodiments, those skilled in the art will understand that various modifications and changes can be made to the invention without departing from the spirit and scope of the invention as described in the following claims.

[0130] (Explanation of symbols)

[0131] 10: Fuel cell stack

[0132] 100,101: Separator for fuel cell

[0133] 110: 1st base material

[0134] 111: Second base material

[0135] 120: First coating layer

[0136] 121: Second coating layer

Claims

1. First base material made of stainless steel; It includes a first coating layer located on the surface of the first base material, and The first coating layer comprises, in weight percent, cobalt (Co): 65-95%, the remainder being nickel (Ni) and unavoidable impurities, for a fuel cell separator.

2. In Paragraph 1, The above first coating layer is a fuel cell separator satisfying the following relationship 1. [Relationship 1] 2≤[Co] / [Ni]≤19 (In the above Equation 1, [Co] and [Ni] represent the average content of Co and Ni in the first coating layer, respectively.) 3. A unit cell in which a fuel electrode; an electrolyte; and an air electrode are sequentially stacked; and A fuel cell separator plate in contact with the surface of the above unit cell; comprising, The above fuel cell separator comprises a second base material made of stainless steel and a second coating layer located on the surface of the second base material. The above second coating layer comprises, in weight percent, cobalt (Co): 10~30%, nickel (Ni): 9~20%, the remainder Fe, and unavoidable impurities, in a fuel cell stack.

4. In Paragraph 3, The above second coating layer is a fuel cell stack satisfying the following relationship 2. [Relationship 2] 1 < [Ni]top / [Ni]bottom ≤ 2 (In the above Equation 2, [Ni]top refers to the average Ni content at the top of the second coating layer relative to the center of the second coating layer, and [Ni]bottom refers to the average Ni content at the bottom of the second coating layer relative to the center of the second coating layer.) 5. In Paragraph 3, The above second base material is, A fuel cell stack comprising a Mn enrichment layer having a depth range of 5 to 20 μm from the interface between the second base material and the second coating layer.

6. In Paragraph 5, The above Mn enrichment layer comprises, in weight percent, manganese (Mn): 1 to 3%, the remainder being Fe and unavoidable impurities, in a fuel cell stack.

7. In Paragraph 5 A fuel cell stack in which the ratio (T1 / T2) between the thickness (T1) of the Mn enrichment layer and the thickness (T2) of the second coating layer is 1 to 10.

8. In Paragraph 3, The above fuel cell stack is a fuel cell stack configured to use ammonia directly as fuel.

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