JP2007042432A - Separator for fuel cell, fuel cell stack, fuel cell vehicle, and manufacturing method of separator for fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a separator for a fuel cell with low cost, having low contact resistance between electrodes and itself, excellent in corrosion resistance, and to provide a fuel cell stack and a fuel cell vehicle loading the same. <P>SOLUTION: A base layer 13 made of a base material containing transition metal or an alloy of transition metal and a nitrided layer 14 obtained by nitriding a base material under a nitrogen atmosphere are provided, of which, the nitrided layer 14 has a cleavage 15. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fuel cell separator, a fuel cell stack, a fuel cell vehicle, and a method for manufacturing a fuel cell separator, and more particularly to a solid polymer electrolyte fuel cell separator formed using stainless steel.

  From the viewpoint of protecting the global environment, it has been studied to use a fuel cell as a power source for a motor that operates in place of an internal combustion engine of an automobile, and to drive the automobile with this motor. Since this fuel cell does not require the use of fossil fuels that have a problem of resource depletion, it does not generate exhaust gas or the like. In addition, the fuel cell has excellent characteristics such that it hardly generates noise, and further, the energy recovery efficiency can be increased as compared with other energy engines.

  Fuel cells include a solid polymer electrolyte type, a phosphoric acid type, a molten carbonate type, and a solid oxide type, depending on the type of electrolyte used. One of them, the polymer electrolyte fuel cell (PEFC), uses a polymer electrolyte membrane having a proton exchange group in the molecule as an electrolyte, and saturates the polymer electrolyte membrane with water. In other words, the battery functions as a proton conductive electrolyte. A solid polymer electrolyte fuel cell operates at a relatively low temperature and has high power generation efficiency. Furthermore, since the solid polymer electrolyte fuel cell is small and lightweight together with other ancillary facilities, various uses including those for mounting on electric vehicles are expected.

  The solid polymer electrolyte fuel cell has a fuel cell stack. The fuel cell stack is configured integrally by stacking a plurality of single cells serving as a basic unit for generating power by an electrochemical reaction, sandwiching both end portions with end flanges, and pressurizing and holding with fastening bolts. The single cell is composed of a polymer electrolyte membrane, an anode (hydrogen electrode) and a cathode (oxygen electrode) bonded to both sides thereof.

  FIG. 14 is a cross-sectional view showing the configuration of a single cell forming the fuel cell stack. As shown in FIG. 14, the single cell 90 has a membrane electrode assembly in which an oxygen electrode 92 and a hydrogen electrode 93 are joined and integrated on both sides of a solid polymer electrolyte membrane 91. The oxygen electrode 92 and the hydrogen electrode 93 have a two-layer structure including a reaction film 94 and a gas diffusion layer 95 (GDL: gas diffusion layer), and the reaction film 94 is in contact with the solid polymer electrolyte film 91. On both sides of the oxygen electrode 92 and the hydrogen electrode 93, an oxygen electrode side separator 96 and a hydrogen electrode side separator 97 are respectively installed for lamination. The oxygen electrode side separator 96 and the hydrogen electrode side separator 97 form an oxygen gas channel, a hydrogen gas channel, and a cooling water channel.

  In the unit cell 90 having the above-described configuration, the oxygen electrode 92 and the hydrogen electrode 93 are arranged on both sides of the solid polymer electrolyte membrane 91, and are usually joined together by a hot press method to form a membrane electrode assembly. The separators 96 and 97 are disposed on both sides of the membrane electrode assembly. In the fuel cell composed of the single cell 90, when a mixed gas of hydrogen, carbon dioxide, nitrogen and water vapor is supplied to the hydrogen electrode 93 side and air and water vapor are supplied to the oxygen electrode 92 side, An electrochemical reaction occurs at the contact surface between the molecular electrolyte membrane 91 and the reaction membrane 94. Hereinafter, a more specific reaction will be described.

  In the single cell 90 configured as described above, when oxygen gas and hydrogen gas are respectively supplied to the oxygen gas flow channel and the hydrogen gas flow channel, the oxygen gas and hydrogen gas are supplied to the reaction film 94 side through the respective gas diffusion layers 95. Then, the following reactions occur in each reaction film 94.

Hydrogen electrode side: H ₂ → 2H ⁺ + 2e ⁻ Expression (1)
Oxygen electrode side: (1/2) O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O (2)
When hydrogen gas is supplied to the hydrogen electrode 93 side, the reaction of the formula (1) proceeds and H ⁺ and e ⁻ are generated. H ⁺ moves through the solid polymer electrolyte membrane 91 in a hydrated state and flows to the oxygen electrode 92 side, and e ⁻ flows from the hydrogen electrode 93 to the oxygen electrode 92 through the load 98. On the oxygen electrode 92 side, the reaction of Formula (2) proceeds by H ⁺ , e ^−, and the supplied oxygen gas, and electric power is generated.

As described above, since the fuel cell separator has a function of electrically connecting each single cell, it is required to have good conductivity and low contact resistance with a constituent material such as a gas diffusion layer. The solid polymer electrolyte membrane is formed of a polymer having a large number of sulfonic acid groups, and has proton conductivity because the sulfonic acid groups are used for proton exchange in a wet state. Since the polymer electrolyte membrane is strongly acidic, the fuel cell separator is required to have corrosion resistance against sulfuric acid acidity of about pH 2-3. Further, the temperature of each gas supplied to the fuel cell is as high as 80 to 90 [° C.], and not only H ⁺ is generated at the hydrogen electrode, but the oxygen electrode through which oxygen, air, etc. pass is standard hydrogen. It is in an oxidizing environment in which a potential of about 0.6 to 1 [Vvs SHE] is loaded with respect to the extreme potential. For this reason, like the oxygen electrode and the hydrogen electrode, the fuel cell separator is required to have chemically stable corrosion resistance in a strongly acidic atmosphere.

  Therefore, attempts have been made to use a titanium material such as stainless steel or industrial pure titanium, which has good conductivity and excellent corrosion resistance, as the fuel cell separator. Stainless steel has a dense passive film formed on its surface, such as an oxide, hydroxide or hydrate of which chromium is the main metal element. Similarly, a dense passive film such as titanium oxide, titanium hydroxide or a hydrate thereof is formed on the surface of titanium. For this reason, stainless steel and titanium have good corrosion resistance.

  However, the above-mentioned passive film produces contact resistance with carbon paper that is usually used as a gas diffusion layer. The overvoltage caused by resistance polarization in the fuel cell can recover the exhaust heat by cogeneration or the like in the stationary application, thereby improving the total thermal efficiency. On the other hand, in automobile applications, heat loss due to contact resistance can only be thrown out of the radiator through the cooling water, so that if the contact resistance increases, the power generation efficiency decreases. In addition, a decrease in power generation efficiency is equivalent to an increase in heat generation, and it becomes necessary to equip a larger cooling system. Therefore, an increase in contact resistance is an important issue to be solved.

In a fuel cell, the theoretical voltage per unit cell is 1.23 [V], but the voltage that can actually be taken out decreases due to reaction polarization, gas diffusion polarization, and resistance polarization, and the voltage drops as the extracted current increases. Further, since it is desired to increase the output density per unit volume / weight in automobile applications, it is used at a higher current density side than stationary use, for example, at a current density of 1 [A / cm ² ]. When the current density is 1 [A / cm ² ], it is considered that the power generation efficiency is reduced by the contact resistance if the contact resistance between the separator and the carbon paper is 40 [mΩ · cm ² ] or less.

Therefore, among fuel cell separators formed by press-molding stainless steel, a fuel cell separator in which a gold plating layer is directly formed on a surface in contact with an electrode has been proposed (see Patent Document 1). In addition, a fuel cell separator is proposed in which after the stainless steel is processed into a fuel cell separator, the passive film on the surface that comes into contact with other members to generate contact resistance is removed and a noble metal or noble metal alloy is adhered. (See Patent Document 2).
Japanese Patent Laid-Open No. 10-228914 (2nd page, FIG. 2) JP 2001-6713 A (page 2)

  However, if the surface of the fuel cell separator is coated with a noble metal, the material cost increases, leading to an increase in cost.

  The present invention has been made to solve the above-mentioned problems. A fuel cell separator according to a first aspect of the present invention includes a base layer comprising a base material containing a transition metal or an alloy of transition metals, and the base material made of nitrogen. A nitride layer obtained by nitriding under an atmosphere, and the nitride layer has a cleavage.

  Moreover, the manufacturing method of the separator for fuel cells which is 2nd invention is an annealing process which anneals the plate-shaped base material containing a transition metal or a transition metal alloy, and a nitriding process continuously after the annealing process. A nitriding step for forming a nitride layer on the surface of the base material, a passage for fuel or oxidant by press forming the base material after the nitriding step, and a press forming step for forming a cleavage in the nitride layer. This is the gist.

  Furthermore, the fuel cell stack according to the third aspect of the invention is characterized by using the fuel cell separator according to the first aspect of the invention.

  The gist of the fuel cell vehicle according to the fourth invention is that the fuel cell stack according to the third invention is mounted and used as a power source.

  According to the first aspect of the invention, there is provided a fuel cell separator that is excellent in conductivity by suppressing a contact resistance generated between a separator and an electrode, and that is chemically stable and excellent in corrosion resistance even in a strongly acidic atmosphere. Can be provided.

  According to the second invention, annealing and nitriding can be continuously performed in-line, so that productivity is excellent and manufacturing cost can be reduced. In addition, by performing nitriding continuously after annealing, crystal defects are reduced and a nitride layer with few defects can be obtained, so that it is possible to manufacture a fuel cell separator with excellent conductivity. Become.

  According to the third aspect of the present invention, it is possible to provide a fuel cell stack that has high performance and is reduced in size and cost.

  According to the fourth invention, by mounting the fuel cell stack that is downsized and reduced in cost, it is possible to realize a long travel distance and to secure a degree of freedom in styling.

  Hereinafter, details of a method for manufacturing a fuel cell separator, a fuel cell stack, a fuel cell vehicle, and a fuel cell separator according to the present invention will be described based on embodiments.

(Fuel cell separator and fuel cell stack)
Embodiments of a fuel cell separator and a fuel cell stack according to an embodiment of the present invention will be described. A fuel cell separator according to an embodiment of the present invention includes a base layer made of a base material containing a transition metal or an alloy of transition metals, and a nitride layer obtained by nitriding the base material in a nitrogen atmosphere, This nitride layer is characterized by having a cleavage. In addition, the fuel cell stack according to the embodiment of the present invention is characterized by using the fuel cell separator according to the embodiment of the present invention, and a single cell serving as a basic unit for generating power by an electrochemical reaction. A fuel cell stack is formed by alternately laminating a plurality of fuel cell separators.

  FIG. 1 is a perspective view showing an external appearance of a fuel cell stack configured by using a fuel cell separator according to an embodiment of the present invention. FIG. 2 is a development view of the fuel cell stack 1 schematically showing a detailed configuration of the fuel cell stack 1 shown in FIG.

  As shown in FIG. 2, the fuel cell stack 1 is configured by alternately laminating a plurality of unit cells 2 serving as a basic unit for generating power by an electrochemical reaction and separators 3 for fuel cells as plate-like members. The Each single cell 2 forms a membrane electrode assembly by forming a gas diffusion layer having an oxidizer electrode and a gas diffusion layer having a fuel electrode on both sides of a solid polymer electrolyte membrane, and is formed on both sides of the membrane electrode assembly. The fuel cell separator 3 is disposed, and an oxidant gas channel and a fuel gas channel are formed inside the fuel cell separator 3. As the solid polymer electrolyte membrane, a perfluorocarbon polymer membrane having a sulfonic acid group (Nafion 1128 (registered trademark), DuPont) or the like can be used. After laminating the single cell 2 and the fuel cell separator 3, end flanges 4 are arranged at both ends, and the outer peripheral portion is fastened by fastening bolts 5 to constitute the fuel cell stack 1. The fuel cell stack 1 includes a hydrogen supply line for supplying a fuel gas containing hydrogen such as hydrogen gas to each single cell 2, an air supply line for supplying air as an oxidant gas, and cooling water. A cooling water supply line is provided.

  FIG. 3 shows a schematic diagram of the fuel cell separator 3 shown in FIG. 3A is a schematic perspective view of the fuel cell separator 3, FIG. 3B is a cross-sectional view taken along the line IIIb-IIIb of the fuel cell separator 3, and FIG. FIG. 3 is a sectional view taken along line IIIc-IIIc. FIG. 4 shows an SEM photograph of the fuel cell separator. FIG. 4A is a surface SEM photograph of the fuel cell separator, and FIG. 4B is a cross-sectional SEM photograph of the fuel cell separator. As shown in FIG. 3A, a fuel or oxidant passage 12 having a rectangular cross section, for example, is formed on the upper surface 11 of the fuel cell separator 3 by press molding. The fuel cell separator 3 includes a base layer 13 formed of a transition metal or an alloy of transition metals and a nitride layer 14 extending along the outer surface of the passage 12 and formed on the base layer. As shown in FIG. 3C and FIGS. 4A and 4B, the nitride layer 14 has a plurality of cleavages 15. The interface between the nitride layer 14 and the base layer 13 is the bottom 16 of the split 15, and the island 15 is formed in the nitride layer 14 by the split 15.

  The fuel cell separator according to the present embodiment includes a base layer made of a base material containing a transition metal or an alloy of transition metals, and a nitride layer obtained by nitriding the base material in a nitrogen atmosphere. Since the layer has a structure with cleavage, transition metal atoms in the nitride layer form a highly covalent bond with the nitrogen atom, and metal bonds are formed between the transition metal atoms. Therefore, a fuel cell separator having excellent conductivity can be obtained. Further, since this nitride layer is chemically stable even in a strongly acidic atmosphere having a pH of 2 to 3 that is usually used as a fuel cell, it has excellent corrosion resistance. For this reason, the contact resistance between the separator for fuel cells and the carbon paper is kept low, and a separator for fuel cells showing continuous good conductivity even in a strongly acidic atmosphere can be obtained. In addition, unlike the conventional case, the contact resistance can be suppressed without providing a gold plating layer directly on the surface in contact with the electrode, so that the cost can be reduced. In general, carbon paper and electrolyte membranes are easily deformed when they come into contact with the separator, so that it is said that the contact area with the separator is increased and the conductivity is increased. Therefore, when the nitride layer, which is the surface layer of the separator, has a plurality of cleavages, the surface in contact with the carbon paper is also easily deformed, so that the contact area is further increased and the conductivity is increased as compared with the conventional case.

  As shown in FIG. 3C and FIGS. 4A and 4B, the cleavage 15 preferably extends through the nitride layer 14 to the base layer 13. In this case, the nitride layer 14 on the separator surface in contact with the carbon paper is further easily deformed. Further, the base layer 13 is exposed by the cleavage 15 passing through the nitride layer 14 and reaching the base layer 13. In this case, a potential difference is generated between the base layer 13 and the nitride layer 14, and the passivation of the nitride layer is promoted by the anode anticorrosive effect. Therefore, a fuel cell separator excellent in conductivity and corrosion resistance can be obtained in strong acidity. . Moreover, it is preferable to provide the oxide film formed in the surface of the base layer which faces cleavage. In the atmosphere, as shown in FIG. 3 (c), when the bottom 16 of the cleavage 15 formed in the nitride layer 14 reaches the base layer 13, an oxide film (not shown) is formed on the exposed portion of the base layer 13. . Since this oxide film is excellent in corrosion resistance in a strongly acidic atmosphere, a separator further excellent in conductivity and corrosion resistance can be obtained. In addition, when this oxide film is an oxide film formed by passivation treatment, this oxide film is more stable in a strongly acidic atmosphere than a passive film naturally formed on the substrate surface in the air. Therefore, the corrosion resistance is further improved.

As the base material, it is preferable to use stainless steel containing at least one transition metal element selected from the group consisting of Fe, Cr, Ni and Mo. Examples of stainless steels containing such elements include austenitic, austenitic / ferrite, and precipitation hardening stainless steels. Among these, the base layer 13 is preferably formed from austenitic stainless steel. Examples of the austenitic stainless steel include SUS304, SUS310S, SUS316L, SUS317J1, SUS317J2, SUS321, SUS329J1, and SUS836. Stainless steel has good conductivity because the constituent elements are composed of transition metal elements. Further, the stainless steel surface, with dense passivation film, i.e., an excellent corrosion resistance for _{CrO · OH · nH 2 O,} Cr 2 O 3 · xH 2 Cr oxides O or the like is formed. As described above, this passive film generates contact resistance with the carbon paper used as the gas diffusion layer. However, in the fuel cell separator according to the present embodiment, since the nitride layer is provided on the surface of stainless steel made of a transition metal having excellent conductivity, the contact resistance with the carbon paper can be kept low. It becomes possible. Further, since this nitride layer is chemically stable, it has excellent corrosion resistance. For this reason, the contact resistance between the separator for fuel cells and the carbon paper is kept low, and a separator for fuel cells showing continuous good conductivity even in a strongly acidic atmosphere can be obtained.

  Further, a Ni-based alloy may be used as the base material. As the Ni-based alloy, for example, Inconel (registered trademark), Incoloy (registered trademark), Hastelloy (registered trademark), or the like can be used. These Ni-based alloys are industrial materials with excellent electrical conductivity and Ni, which is a transition metal, and a nitride layer that is easily chemically stable to Ni-based alloys by nitriding the surface. Can be formed. And this nitride layer is excellent in electroconductivity, Since it is chemically stable, the separator for fuel cells which is excellent also in corrosion resistance is obtained. For this reason, the contact resistance between the separator for fuel cells and the carbon paper is kept low, and a separator for fuel cells showing continuous good conductivity even in a strongly acidic atmosphere can be obtained.

More specifically, the crystal structure of the nitrided layer obtained by nitriding the stainless steel or Ni-based alloy substrate is formed by transition metal atoms selected from the group of Fe, Cr, Ni, and Mo. It is considered to be an M ₄ N type crystal structure in which nitrogen atoms are arranged in an octahedral void at the center of a unit cell of a face-centered cubic lattice. The M ₄ N-type crystal structure is shown in FIG. As shown in FIG. 5, the M ₄ N-type crystal structure 20 has a unit cell of a face-centered cubic lattice formed by transition metal atoms 21 mainly composed of Fe and selected from Fe, Cr, Ni, and Mo. In this structure, nitrogen atoms 22 are arranged in the central octahedral space. In the M ₄ N-type crystal structure 20, M represents a transition metal atom 21 selected from Fe, Cr, Ni, and Mo, and N represents a nitrogen atom 22. The nitrogen atoms 22 occupy 1/4 of the octahedral voids at the center of the unit cell of the M ₄ N type crystal structure 20. That is, the M ₄ N-type crystal structure 20 is an interstitial solid solution in which nitrogen atoms 22 invade into octahedral voids at the center of a unit cell of a face-centered cubic lattice of transition metal atoms 21, and is represented by a cubic lattice of lattices. Nitrogen atoms 22 are located at lattice coordinates (1/2, 1/2, 1/2) of each unit cell. In the M ₄ N-type crystal structure 20, the transition metal atom M 21 is mainly composed of Fe, but includes alloys in which Fe is partially substituted with other transition metal atoms such as Cr, Ni, and Mo.

This M ₄ N-type crystal structure exhibits a strong covalent bond between the transition metal atom and the nitrogen atom while maintaining the metal bond between the transition metal atoms, thereby reacting to the oxidation of each metal atom in the nitride. Decreases. For this reason, transition metal nitrides are chemically stable even in the oxidizing environment in the fuel cell as described above, and maintain the conductivity necessary for the separator for the fuel cell and the conductivity function in the environment where the separator is used. Thus, it is possible to provide a transition metal nitride having both chemical stability and corrosion resistance. Further, a fuel cell separator in which a nitride layer having such a crystal structure is formed directly on a base layer is normally used as a fuel cell even when placed in an oxidizing environment such as in a fuel cell stack. The contact resistance with the carbon paper can be kept low. Further, unlike the conventional case, the contact resistance can be suppressed without providing a gold plating layer directly on the surface in contact with the electrode, so that the cost can be reduced. Furthermore, since the nitride layer having the M ₄ N-type crystal structure has chemical stability, the contact resistance between the separator and the electrode in an oxidizing environment is maintained at a low value, excellent in corrosion resistance, and low in cost. A fuel cell separator that achieves the above can be provided.

  The nitride layer preferably has an atomic ratio of Cr (chromium) to Fe (iron) contained in the nitride layer smaller than an atomic ratio of Cr to Fe contained in the substrate. When the Cr atomic ratio with respect to Fe in the nitrided layer is higher than the Cr atomic ratio with respect to Fe in the base material, a Cr-based nitride such as CrN having a crystal structure of NaCl type is formed by combining nitrogen with the base material Cr. As a result of precipitation, a Cr-deficient layer is formed on the substrate, and the corrosion resistance is reduced. On the other hand, when the atomic ratio of Cr to Fe contained in the nitride layer is lower than the atomic ratio of Cr to Fe of the base material, Cr-based nitrides do not precipitate and are therefore included in the base material. The effective corrosion resistance of the fuel cell separator is maintained even after nitriding without reducing Cr effective for corrosion resistance, and the corrosion resistance in a strongly acidic atmosphere is further improved.

  You may use industrial pure titanium (JIS1 type) as a base material. Typical characteristics of titanium include good workability and the presence of an oxide film on the oxidized surface against corrosion in an oxidizing atmosphere, so that it has corrosion resistance superior to that of stainless steel. Thus, titanium is also excellent in corrosion resistance in a strongly acidic atmosphere having a pH of 2 to 3. Among these, industrial pure titanium (JIS class 1) has high purity and is excellent in terms of workability and corrosion resistance. However, since titanium has an oxide film on the surface, if it is used as it is without surface modification, the contact resistance value of the contact surface with the carbon paper becomes large, and as such, it can be used as a separator. difficult. On the other hand, when a nitride layer is formed on the surface of titanium, which is a transition metal element, the nitride layer contributes to corrosion resistance in an acidic atmosphere and exhibits high corrosion resistance. In addition, the nitride layer makes it possible to keep the contact resistance with the carbon paper normally used as a gas diffusion layer low. For this reason, the contact resistance value with the carbon paper which is a gas diffusion layer is low, and it becomes possible to obtain the separator which maintains high electroconductivity. Furthermore, when this nitrided layer is formed into a separator shape after forming the nitrided layer, the nitrided layer is cleaved, and when the base layer is exposed, the substrate titanium has a strongly acidic atmosphere with a pH of 2-3. Since the corrosion resistance is excellent, it maintains excellent corrosion resistance and conductivity. Thus, by using industrial pure titanium (JIS type 1) as the base material, there is also an effect that the application condition range at the time of separator molding is relaxed. In addition, since the density of industrial pure titanium (JIS type 1) is about 60% of stainless steel, the plate thickness can be reduced. For this reason, the unit price of industrial pure titanium (JIS class 1) is 1.3 times that of stainless steel, but by reducing the plate thickness, the cost can be kept lower than when stainless steel is used. Become. In addition, it is possible to provide a lightweight fuel cell separator and a lightweight fuel cell stack.

  In addition, it is preferable that the nitrided layer obtained by performing nitriding treatment on industrial pure titanium (JIS type 1) material contains TiN crystals. By having this crystal, it is possible to further improve the corrosion resistance in a strongly acidic atmosphere of pH 2 to 3, and to keep the contact resistance with the carbon paper low.

  The ratio t1 / t of the nitride layer thickness t1 to the plate thickness t of the plate-like member is preferably 0.02 to 0.2. FIG. 6 shows a schematic cross-sectional view of a fuel cell separator having nitride layers on both sides of the base layer. When the nitride layer 14 having the thickness t1 is formed on both surfaces of the plate-like member having the thickness t, and the t1 / t is in the range of 0.02 to 0.2, the nitride layer is due to metal bonding. Thus, the fuel cell separator is excellent in conductivity and corrosion resistance in a strongly acidic atmosphere. When t1 / t is smaller than 0.02, the nitride layer is thin and the conductivity is deteriorated. When t1 / t is larger than 0.2, the nitride layer is thick and the productivity is deteriorated.

  The sum y = Σyi of the width yi (i = 1, 2, 3...) Of the cleavage 15 at the interface 14 a of the nitride layer 14 with the base layer 13, that is, the portion where the base layer 13 is exposed by the cleavage 15. Sum x = Σxi of bottom 16 and sum x = Σxi of width xi (i = 1, 2, 3...) Of island 17 of nitride layer 14 formed by cleavage 15, that is, ridge line of nitride layer 14 The ratio y / x to the sum x = Σxi is preferably 0.052 to 19. When it exists in this numerical range, there exists an anode anticorrosion effect by the nitride layer and the base material having contact | connected. When y / x is smaller than 0.052 and when y / x is larger than 19, the area of either the nitride layer or the substrate is reduced, so that the anode anticorrosive effect cannot be obtained.

  It is preferable that the nitrogen amount on the extreme surface of the nitrided layer is 5 [at%] or more and the oxygen amount is 50 [at%] or less. Here, the extreme surface of the nitride layer refers to an atomic layer having a depth of 3 to 4 nm from the outermost surface of the nitride layer, that is, a depth of several tens of layers. Further, the outermost surface refers to the outermost atomic layer of the nitride layer. When the coverage of oxygen molecules adsorbed on the metal surface is increased, a clear bond is formed between the metal atom and the oxygen atom. This is the oxidation of metal atoms. Such oxidation of the metal surface first occurs when the outermost first atomic layer is oxidized. After the oxidation of the first atomic layer, oxygen adsorbed on the first atomic layer receives free electrons in the metal by the tunnel effect, and oxygen becomes negative ions. Then, due to the strong local electric field due to the negative ions, metal ions are pulled out from the inside of the metal onto the surface, and the pulled metal ions are combined with oxygen atoms. That is, a second oxide film is formed. Such a reaction occurs from one to the next, and the oxide film becomes thicker. Thus, when the amount of oxygen in the nitride layer is more than 50 [at%], an insulating oxide film is easily formed. On the other hand, if the metal atom forms a compound with nitrogen while increasing the chemical potential of the nitrogen atom in the nitride layer and the activity of the metal atom is further reduced, the free energy of the metal atom decreases, and the metal atom The reactivity with respect to the oxidation of can be lowered, and the metal atom is chemically stabilized. Since the oxygen atoms do not receive the free electrons and the metal atoms are not oxidized, the growth of the oxide film can be suppressed. Thus, when the nitrogen amount on the extreme surface of the nitrided layer is 5 [at%] or more and the oxygen amount is 50 [at%] or less, the growth of the oxide film can be suppressed and the contact with the carbon paper can be suppressed. It is possible to obtain a fuel cell separator that can keep resistance low and is excellent in corrosion resistance in a strongly acidic atmosphere. More preferably, the nitrogen amount is 7.5 [at%] or more and the oxygen amount is 30 [at%] or less.

  Further, the ratio O / N of the oxygen amount to the nitrogen amount on the extreme surface of the nitride layer is more preferably 10 or less. In this case, the condition that the amount of nitrogen is 5 [at%] or more and the amount of oxygen is 50 [at%] or less is satisfied, the corrosion resistance in the strongly acidic atmosphere is excellent, and the contact resistance with the carbon paper Can be kept low continuously. When the O / N exceeds 10, the surface is covered with an insulating film mainly composed of oxide, so that the contact resistance between the carbon paper and the strongly acidic atmosphere usually used as a fuel cell is low. High and poor electrical conductivity. More preferably, the ratio O / N of the nitrogen amount to the oxygen amount on the extreme surface of the nitride layer is 5 or less.

It is preferable that the nitrogen content is 10 [at%] or more and the oxygen content is 30 [at%] or less at a depth of 10 [nm] from the outermost surface of the nitride layer. In this case, the corrosion resistance in a strongly acidic atmosphere is excellent, and the contact resistance with the carbon paper can be continuously reduced. In addition, when it deviates from this range, the contact resistance generated between the separator and the electrode becomes high, the contact resistance value per single cell constituting the fuel cell stack exceeds 40 [mΩ · cm ² ], and power generation There is a problem that performance deteriorates. More preferably, the nitrogen amount is 17 [at%] or more and the oxygen amount is 18 [at%] or less.

  Furthermore, it is preferable that the nitrogen amount is 15 [at%] or more and the oxygen amount is 20 [at%] or less at a depth of 100 [nm] from the outermost surface of the nitride layer. In this case, the contact resistance can be further reduced. If it is out of this range, the surface of the base material is covered with an insulating film mainly composed of oxide, so that the contact resistance with the carbon paper is high in a strongly acidic atmosphere usually used as a fuel cell. , No gas conductivity. More preferably, the nitrogen amount is 23 [at%] or more and the oxygen amount is 10 [at%] or less.

  As described above, by adopting the above-described configuration, the fuel cell separator according to the embodiment of the present invention is a chemical that maintains the conductivity necessary for the separator for the fuel cell and the conductivity function in the environment where the separator is used. Combines stability and corrosion resistance. In addition, it is possible to obtain a fuel cell separator that is low in cost, has good productivity, has low contact resistance with a constituent material such as an adjacent gas diffusion electrode, and has good power generation performance of the fuel cell. In addition, the fuel cell stack according to the embodiment of the present invention can maintain high power generation efficiency without impairing the power generation performance by using the fuel cell separator according to the embodiment of the present invention. Cost reduction can be realized.

(Manufacturing method of fuel cell separator)
Next, an embodiment of a method for manufacturing a fuel cell separator according to an embodiment of the present invention will be described. This method for producing a separator for a fuel cell includes an annealing process in which a plate-like base material containing a transition metal or an alloy of transition metals is annealed, and a nitriding process is continuously performed after the annealing process, so It includes a nitriding step for forming a nitrided layer, a fuel or oxidant passage by press-forming the base material after the nitriding step, and a press-forming step for forming a tear in the nitrided layer. By subjecting a base material made of a transition metal or a transition metal alloy to nitriding after annealing, crystal defects such as spiral transition, edge transition, and vacancies are reduced, and a nitride layer with few crystal defects can be obtained. For this reason, the separator excellent in electroconductivity is obtained. Moreover, since the nitriding treatment can be continuously performed in the cooling process after the annealing treatment, the separator can be manufactured in-line. For this reason, it is excellent in productivity and can reduce manufacturing cost. Furthermore, the passage of fuel or oxidant can be formed thereafter by press molding, and at the same time, the cleavage can be formed in the nitride layer on the substrate surface.

  It is preferable to perform a passivation treatment after the press molding step. Examples of the passivation treatment include pickling. By pickling, an oxide film having a thickness of 10 [nm] or less having conductivity is formed on the surface of the nitride layer or the substrate surface exposed by cleavage of the nitride layer. When a strongly acidic solution adheres to the separator surface, the presence of this oxide film suppresses the formation of local batteries on the separator surface and the occurrence of corrosion on the separator. For this reason, the separator excellent in corrosion resistance is obtained.

  It is preferable to apply a tensile tension to the base material during the nitriding treatment. In this case, the flatness can be maintained even when the base material is thermally deformed, and a predetermined shape can be obtained by subsequent blanking, pressing or the like. Note that if the nitriding or annealing treatment is performed without applying a tensile tension, the base material is thermally deformed, and it becomes difficult to maintain the flatness.

  The nitriding treatment is preferably a plasma nitriding method or a plasma CVD method. Nitriding includes gas nitriding, gas soft nitriding, salt bath, plasma nitriding, and plasma CVD. However, since the gas soft nitriding method has a high oxygen partial pressure during nitriding, the amount of oxygen in the nitrided layer becomes high. On the other hand, among the nitriding treatments, the plasma nitriding method uses the object to be processed as a cathode, converts nitrogen gas into plasma by glow discharge generated by applying a DC voltage, and the converted nitrogen is rapidly transferred to the surface of the object to be processed. It is a method of nitriding by accelerated collision. The plasma nitriding method is a nitriding method suitable for a substrate having a passive film on the surface because nitriding is performed while easily removing the passive film on the surface of the substrate, which is an object to be processed, by sputtering action due to plasma impact, and Since the nitrogen ions are infiltrated into the substrate by non-equilibrium reaction, the crystal structure can be easily obtained in a short time. In the plasma CVD method, a nitride layer is formed by adsorbing nitrogen plasma generated by exciting a gas onto a substrate, and a nitride layer with improved crystallinity can be obtained. By using the plasma nitriding method or the plasma CVD method, the oxygen content of the nitrided layer is low, the conductivity is excellent, the oxidation resistance in the strong acid atmosphere is excellent, and the carbon paper is used in the strong acid atmosphere usually used in fuel cells. Thus, a fuel cell separator capable of continuously reducing the contact resistance between the two is obtained.

Further, the annealing treatment when stainless steel is used as the base material is preferably performed at a temperature of 1100 [° C.] or more and 1200 [° C.] or less, and the nitriding treatment is performed at a temperature of 300 [° C.] or more and 500 [° C.] or less. It is preferable. When the surface of stainless steel is subjected to nitriding treatment at a high temperature, nitrogen is combined with Cr in the base material, and nitrides such as CrN having a NaCl-type crystal structure are mainly precipitated, so that the corrosion resistance of the fuel cell separator is lowered. . On the other hand, when nitriding is performed at a temperature of 300 [° C.] or more and 500 [° C.] or less, the substrate surface is not a nitride compound such as CrN having a NaCl type crystal structure, but mainly Fe, A nitride layer having an M ₄ N-type crystal structure in which nitrogen atoms are arranged in an octahedral void at the center of a unit cell of a face-centered cubic lattice formed by transition metal atoms selected from the group of Cr, Ni, and Mo is formed. Therefore, a fuel cell separator with improved corrosion resistance is obtained. Moreover, the contact resistance between the separator and the constituent material such as the adjacent gas diffusion electrode can be kept low, the power generation efficiency of the fuel cell can be maintained, and a fuel cell separator having excellent durability and reliability can be obtained at low cost. Can do. Note that when the nitriding temperature is lower than 300 [° C.], it takes a long time to obtain a nitrided layer having this crystal structure, and the productivity is deteriorated. Therefore, the nitriding treatment is preferably performed in the range of 300 [° C.] to 500 [° C.].

  Next, a method for manufacturing a fuel cell separator will be described with reference to FIGS. Fig.7 (a) is explanatory drawing which shows the manufacturing method of the separator for fuel cells which concerns on embodiment of this invention, FIG.7 (b) is the time chart.

  As shown in FIG. 7 (a), in order to manufacture a fuel cell separator, for example, a stainless steel sheet steel strip 30, a vacuum furnace 31 for performing annealing and nitriding, annealing and nitriding are performed. A take-up reel 32 for winding the thin plate, dies 33a and 33b for blanking and press forming the thin plate, cutting teeth 34a and 34b for cutting the thin plate after press molding, and pickling the thin plate after cutting A pickling device 35 is used. The thin steel strip 30 and the take-up reel 32 enable the annealing treatment and pickling treatment to be performed continuously at a constant speed by applying tension on the thin plate during annealing and nitriding. Furthermore, it is possible to continuously wind the thin strip steel subjected to the annealing treatment and the pickling treatment at a constant speed.

  In order to manufacture the fuel cell separator, first, the thin plate 30a drawn from the thin strip steel 30 is introduced into the vacuum furnace 31 from time t0 to t1. During the time t1 to t2, the temperature in the vacuum furnace 31 is raised to the annealing heating temperature T1. And between time t2 and t3, the temperature of the vacuum furnace 31 is hold | maintained at T1, and the thin plate 30b which passes the vacuum furnace 31 is annealed at temperature T1. Next, the temperature in the vacuum furnace 31 is cooled to T2 from time t3 to t4. Then, from time t4 when the temperature drops to T2, the annealed thin plate is subjected to nitriding treatment at temperature T2. Then, the temperature in the vacuum furnace 31 is lowered to room temperature between time t5 and time t6, and the thin plate is taken up by the take-up reel 32. Thereafter, the thin plate 30c drawn from the take-up reel 32 is guided between the molds 33a and 33b. Then, the thin plate 30d is disposed between the molds 33a and 33b, and blanking and press molding are performed on the thin plate 30d. Next, the thin plate 30e after press molding is disposed between the cutting teeth 34a and 34b, and the thin plate 30e is cut by the cutting teeth 34a and 34b. And the thin plate 30f after blanking and press molding is fixed in the pickling apparatus 35, and pickling is performed.

  Next, a reference example of a method for manufacturing a fuel cell separator is shown. FIG. 8A is an explanatory view showing a method for manufacturing a fuel cell separator of a reference example, and FIG. 8B is a time chart thereof. As shown in FIG. 8 (a), in order to manufacture a separator for a fuel cell, for example, a stainless steel sheet steel strip 40, a vacuum furnace 41 for performing an annealing process, and a take-up reel for winding the annealed sheet 42, cutting teeth 43a and 43b for cutting the thin plate, dies 44a and 44b for blanking and press forming the cut thin plate, and a nitriding furnace 45 for nitriding the formed thin plate. The thin steel strip 40 and the take-up reel 42 enable the annealing process to be continuously performed at a constant speed by applying a tension on the thin sheet when the annealing process is performed. Further, the annealed thin sheet The steel strip can be continuously wound at a constant speed.

  In order to manufacture the fuel cell separator, first, the thin plate 40a drawn from the thin strip steel 40 is introduced into the vacuum furnace 41 from time t0 to t1. During the time t1 to t2, the inside of the vacuum furnace 41 is raised to the temperature T1. And the temperature of the vacuum furnace 41 is hold | maintained to T1 between time t2 and t3, and the thin plate 40b which passes the vacuum furnace 41 is annealed at temperature T1. From time t3 to t4, the temperature in the vacuum furnace 41 is cooled to room temperature. After that, the stainless steel is taken up by the take-up reel 42, and the thin plate 40c drawn out from the take-up reel 42 is guided between the cutting teeth 43a and 43b. Next, the thin plate 40d is cut by the cutting teeth 43a and 43b. Then, the thin plate 40e cut | disconnected between metal mold | die 44a, 44b is arrange | positioned, and blanking and press molding are performed to the thin plate 40e. Next, the blank plate 40f after blanking and press forming is fixed in the nitriding furnace 45, and the temperature in the nitriding furnace 45 is raised to a temperature T2 between time t5 and time t6. When the inside of the nitriding furnace 45 rises to the temperature T2, the temperature is maintained at T2 from time t6 to t7, and the thin plate 40f is nitrided. Then, the temperature in the nitriding furnace 45 is lowered to room temperature between times t7 and t8. And the thin plate 40f after nitriding is fixed in the pickling apparatus 45, and pickling is performed.

  Compared to the reference example, when manufactured by the method for manufacturing a separator for a fuel cell according to an embodiment of the present invention, annealing and nitriding can be continuously performed in-line, resulting in excellent productivity and reduced manufacturing costs. It becomes possible to do. Also, by performing nitriding treatment continuously after annealing treatment, nitriding treatment can be carried out from the annealing temperature without passing through cold, so crystal defects are reduced and a nitride layer with few defects can be obtained. This makes it possible to produce a fuel cell separator that is excellent in performance. In addition, a fuel or oxidant passage can be formed by press molding, and at the same time, a cleavage can be formed in the nitride layer on the surface of the substrate, thereby reducing the number of manufacturing steps and obtaining a fuel cell separator with excellent conductivity. It is done. Then, by pickling the base material after the press molding step, a conductive oxide film having a thickness of 15 nm or less is formed on the surface of the nitride layer or the base material surface exposed by cleavage of the nitride layer. Therefore, a separator with more excellent corrosion resistance can be obtained. Further, during the nitriding treatment, the thin plate is tensioned by the thin strip steel 30 and the take-up reel 32, so that the flatness can be maintained even if the base material is thermally deformed, and blanking thereafter. A predetermined shape can be obtained by pressing or the like.

  Thus, according to the method for manufacturing a fuel cell separator in the embodiment of the present invention, a base layer comprising a base material containing a transition metal or an alloy of a transition metal and nitriding the base material in a nitrogen atmosphere are obtained. The fuel cell separator having the cleavage layer is obtained. In addition, since the annealing and nitriding treatment can be continuously performed in-line, the productivity is excellent and the manufacturing cost can be reduced. Furthermore, by performing nitriding continuously after annealing, crystal defects are reduced and a nitride layer with few defects can be obtained, so that it is possible to manufacture a fuel cell separator with excellent conductivity. Become.

(Fuel cell vehicle)
In this embodiment, a fuel cell electric vehicle using a fuel cell including a fuel cell stack manufactured by the above method as a power source will be described as an example of a fuel cell vehicle.

  FIG. 9 is a diagram showing the external appearance of an electric vehicle equipped with a fuel cell stack. FIG. 9A is a side view of the electric vehicle 50, and FIG. 9B is a top view of the electric vehicle 50. As shown in FIG. 9B, in addition to the left and right front side members and the hood ridge, a dash lower member for connecting the left and right hood ridges including the front side member to each other is welded and joined to the front of the vehicle body 51, respectively. An engine compartment portion 52 is formed. In the electric vehicle according to the embodiment of the present invention, the fuel cell stack 1 is mounted in the engine compartment portion 52.

  By mounting a fuel cell stack having a good power generation performance to which the fuel cell separator according to the embodiment of the present invention is mounted on a vehicle such as an automobile, the fuel efficiency of the fuel cell electric vehicle can be improved. In addition, according to the present embodiment, by mounting a small and lightweight fuel cell stack on a vehicle, the vehicle weight can be reduced and fuel consumption can be reduced, and the travel distance can be increased. it can. Furthermore, according to this embodiment, by mounting a small fuel cell on a mobile vehicle or the like, the vehicle interior space can be used more widely, and the degree of freedom of styling can be ensured.

  In addition, although the electric vehicle was mentioned as an example of a fuel cell vehicle, this invention is not limited to vehicles, such as an electric vehicle, It can be applied also to the aircraft and other engines in which electric energy is requested | required. .

  Hereinafter, Examples 1 to 6 and Comparative Examples 1 to 2 of the separator for a fuel cell according to the embodiment of the present invention will be described. These examples are examinations of the effectiveness of the fuel cell separator according to the present invention, and show examples of fuel cell separators produced by treating different raw materials under different conditions. It is.

<Preparation of sample>
In each example, SUS304, SUS316, SUS310, Ni-based alloy Inconel 600 (INCONEL600 (registered trademark)), industrial pure titanium (JIS type 1), which is austenitic stainless steel having a plate thickness of 0.1 [mm] as a base material. ), Which is a cold rolled steel sheet that has not been brightly annealed. These sheet steels were annealed and nitrided as shown in FIG. 7 or FIG. In Examples 1 to 6, nitriding was performed by plasma nitriding. The plasma nitriding was performed under the conditions of a processing temperature of 320 to 700 [° C.], a processing time of 30 to 60 [min], and a gas mixing ratio N ₂ : H ₂ = 5: 5. The processing pressure was 4 [Torr] (= 532 [Pa]). In Comparative Example 1, nitriding treatment is not performed. Table 1 shows the base materials and methods used, and Table 2 shows the annealing heating temperature, nitriding method, and nitriding conditions.

  Here, each sample was evaluated by the following method.

<Presence / absence of nitride layer>
The presence or absence of the nitrided layer is determined by corroding the cross section of the nitrided substrate with aqua regia after polishing and observing the cross section at 400 times using an optical microscope, or using a scanning electron microscope (S-4000 manufactured by Hitachi, Ltd.). The cross section was observed at 1000 to 10000 times.

<Measurement of nitride layer thickness t1, island width x, and cleavage width y>
The thickness of the nitride layer was measured by observing the cross section of each sample using an optical microscope or a scanning electron microscope. The width x of the island part and the width y of the cleavage were obtained by image processing.

<Identification of crystal structure of nitride layer>
The crystal structure of the nitride layer of the sample obtained by the above method was identified by performing X-ray diffraction measurement on the surface of the base material modified by nitriding treatment. The apparatus used was an X-ray diffractometer (XRD) manufactured by Mac Science. The measurement was performed under the conditions of a CuKα ray as a radiation source, a diffraction angle of 20 to 100 [°], and a scanning speed of 2 [° / min].

<Measurement of the amount of nitrogen and oxygen on the surface>
The measurement of the amount of nitrogen and the amount of oxygen on the pole surface was performed using X-ray electron spectroscopy (XPS), and the ratio O / N of the amount of nitrogen to the amount of oxygen was determined from the measurement results. The apparatus used was a photoelectron spectrometer Quantum-2000 manufactured by PHI. Measurement is performed using a Monochromated-Al-kα ray (voltage 1486.6 [eV], 20.0 [W]) as a radiation source, a photoelectron extraction angle of 45 [°], a measurement depth of about 4 [nm], and a measurement area φ200 [ The sample was irradiated with X-rays at [μm].

<Measurement of Nitrogen Content and Oxygen Content at 10 [nm] and 100 [nm] Depth from the Top Surface of Nitride Layer>
Measurements of nitrogen and oxygen at depths of 10 nm and 100 nm from the outermost surface of the nitride layer were performed using a scanning Auger electron spectrometer. The apparatus used was MODEL4300 manufactured by PHI. The measurement is performed under the conditions of an electron beam acceleration voltage of 5 [kV], a measurement area of 20 [μm] × 16 [μm], an ion gun acceleration voltage of 3 [kV], and a sputtering rate of 10 [nm / min] (SiO ₂ conversion value). went.

<Measurement of contact resistance value>
The samples obtained in Examples 1 to 6 and Comparative Examples 1 to 2 were cut into a size of 30 [mm] × 30 [mm], and contact resistance was measured. The apparatus used was TRS-2000SS, a pressure load contact electrical resistance measuring device manufactured by ULVAC-RIKO. Then, as shown in FIG. 10 (a), a carbon paper 63 is interposed between the electrode 61 and the sample 62, and as shown in FIG. 10 (b), the electrode 61a / carbon paper 63a / sample 62 / carbon. The configuration is paper 63b / electrode 61b. The carbon paper is a carbon paper coated with a platinum catalyst supported by carbon black (carbon paper TGP-H-090 manufactured by Toray Industries, Inc., thickness 0.26 [mm], bulk density 0.49 [g / cm ³ ], Porosity 73 [%] and thickness direction volume resistivity 0.07 [Ω · cm ² ]) were used. As the electrode, a Cu electrode having a diameter of φ20 was used. And the penetration resistance at the time of flowing the electric current of 1 [A / cm < ² >] with the measurement surface pressure of 1.0 [MPa] was measured. The contact resistance value was measured twice before and after the constant potential electrolysis test, and indicated as an average value. The contact resistance value after the constant potential electrolysis test is an evaluation of the corrosion resistance in an oxidizing environment by simulating the environment where the fuel cell separator is exposed in the fuel cell stack. The conditions of the constant potential electrolysis test were a sulfuric acid aqueous solution with a temperature of 80 [° C.] and a pH of 2 and a natural potential measurement with a potential of 1 [V vs SHE] and 100 [hour] while degassing with hydrogen gas.

<Evaluation of corrosion resistance>
Corrosion resistance is evaluated by measuring the amount of Fe, Cr, Ni, and Ti ions eluted in the sulfuric acid aqueous solution after holding for a certain period of time with the prescribed constant potential applied in the constant potential electrolysis test by fluorescent X-ray analysis. The degree of decrease in corrosion resistance was evaluated from the value of metal ion elution amount.

The crystal structures of the nitride layers of Examples 1 to 6 and Comparative Examples 1 to 2, the thicknesses t1 and t1 / t of the nitride layers, and the ratio y / x of the cleavage width y to the width x of the island portion are shown. 3 shows the measurement results of the contact resistance value and the ion elution amount before and after the electrolytic test.

Example 1 to Example 6 and Comparative Example 1 to Comparative Example 1 to Comparative Example 1 to Comparative Example 1 of Nitrogen Layer, Nitrogen and Oxygen, Ratio of Oxygen to Nitrogen O / N, 10 nm Table 5 shows the amounts of nitrogen and oxygen at a depth of 100 [nm].

  11A shows a cross-sectional structure photograph of the sample obtained in Example 2 at a magnification of 400 times, and FIG. 11B shows a cross-sectional structure photograph of the sample obtained in Comparative Example 2 at a magnification of 400 times. FIG. 13 shows the X-ray diffraction pattern of the sample obtained in Example 2 and Comparative Example 2, and FIG. 13 shows the element profile in the depth direction by scanning Auger electron spectroscopy analysis of the sample obtained in Example 2.

  11A that the nitride layer 72 is formed on both surfaces of the base layer 71 in the sample obtained in Example 2. FIG. A plurality of cleaves 73 are formed in the nitride layer 72. The cleaves 73 pass through the nitride layer 72 and the bottom 74 extends to the base layer 71. Although not shown in FIG. 11A, when the bottom 74 of the cleavage 73 reaches the base layer 71, an oxide film is formed on the exposed portion of the base layer 72. The existence of this oxide film is clear from the result of the scanning Auger electron spectroscopy analysis shown in FIG. Since this oxide film is excellent in corrosion resistance in a strongly acidic atmosphere, the separator on which the oxide film is formed is further excellent in conductivity and corrosion resistance. In addition, when this oxide film is an oxide film formed by a passivating process such as pickling, it is more stable in a strongly acidic atmosphere than a passive film that is naturally formed on the substrate surface in the air. Therefore, the corrosion resistance is further improved. On the other hand, in FIG. 11B, continuous nitride layers 72 without cleavage are formed on both surfaces of the base layer 81.

From FIG. 12, in Comparative Example 2, only the peak derived from austenite, which is the substrate indicated by γ in the figure, was clearly observed, whereas in Example 2, M ₄ N indicated by M ₄ N in the figure. A peak indicating the type crystal structure was observed. In XRD, although the surface layer is a nitride layer having an M ₄ N type crystal structure, a peak derived from austenite as a base material is also observed. This is because the X-ray incident depth on the sample under this measurement condition is about 10 [μm], and it was determined that the base layer portion where the nitride layer was not formed was also detected. Similarly, in Examples 1 to 5, the peak of the M ₄ N type crystal structure was observed in addition to the peak derived from the austenite as the base material. In Example 6, a TiN peak was observed. Moreover, in Comparative Example 1, only the peak derived from the austenite which is the base material was observed. In Comparative Example 2, a peak indicating CrN was observed. In Comparative Example 2, since SUS316L stainless steel was nitrided at high temperature, it is considered that CrN having a NaCl-type crystal structure was mainly used.

Further, as shown in Table 3, in Examples 1 to 6 in which a nitride layer having an M ₄ N type or TiN crystal structure was formed, the contact resistance value measurement method shown in FIG. The contact resistance value was 40 [mΩ · cm ² ] or less. On the other hand, in Comparative Example 1 in which the nitride layer was not formed, a passive film was formed on the surface of the base material, and since this passive film showed insulating properties, the contact resistance was high before and after the electrolytic test. In Comparative Example 2, since the nitride layer is formed on the surface before the electrolysis test and exhibits good conductivity, the contact resistance before the electrolysis test is low, but the contact resistance value after the electrolysis test is the surface of the nitride layer Since a passive film was formed on the film and the film grew, the contact resistance value increased. Thus, in Examples 1 to 6, a nitrided layer was formed instead of a passive film, and since this nitrided layer was cleaved, a low contact resistance value was obtained. Further, in all of Examples 1 to 6, the ion elution amount was low, whereas in Comparative Examples 1 to 2, the ion elution amount was large and the corrosion resistance was inferior.

Further, from Table 5, when the nitrogen amount on the surface of the nitrided layer measured by XPS, the amount of oxygen, and the ratio of the amount of nitrogen to the amount of oxygen O / N are examined, the amount of nitrogen on the surface of the nitrided layer is 5 [at%]. In Examples 1 to 6 in which the oxygen amount was 50 [at%] or less and the O / N was 10 or less, the contact resistance value was 40 [mΩ · cm ² ] or less. On the other hand, in Comparative Example 2 having CrN in the nitrided layer, the amount of oxygen on the pole surface was large and O / N was also high because iron nitride having a reduced Cr concentration was preferentially dissolved.

Next, from the element profile shown in FIG. 13, an oxide film exists on the outermost surface of the nitride layer because there is a slight partial pressure of oxygen during the nitriding process, and electrons can freely travel through the thickness of the oxide film. Therefore, the amount of oxygen is the highest. However, since the range in which electrons can freely travel is a depth of 3 to 4 [nm] from the outermost surface, the amount of oxygen gradually decreases and the amount of nitrogen increases, resulting in a depth of 10 [nm] from the outermost surface of the nitride layer. The amount of nitrogen was 10 [at%] and the amount of oxygen was 30 [at%] or less. The nitrogen content was 15 [at%] and the oxygen content was 20 [at%] at a depth of 100 [nm] from the outermost surface of the nitride layer. Thus, in Example 5 in which the nitride layer having the M ₄ N type crystal structure was formed, the contact resistance and the corrosion resistance were satisfied.

Similarly, in any of Examples 1 to 6 where the contact resistance value is 40 [mΩ · cm ² ] or less, the nitrogen amount is 10 [at%] at a depth of 10 [nm] from the outermost surface of the nitride layer. ] And the oxygen content is 30 [at%] or less, and further, at a depth of 100 [nm] from the outermost surface of the nitride layer, the nitrogen content is 15 [at%] or more and the oxygen content is 20 [at%] or less. Met. On the other hand, in Comparative Example 2 in which the contact resistance value after the electrolytic test is larger than 40, the amount of nitrogen and the amount of oxygen at a depth of 10 nm from the outermost surface of the nitrided layer deviate from the above values. The amounts of nitrogen and oxygen at a depth of 100 [nm] from the outermost surface of the layer also deviated from the above values.

From the above measurement results, Examples 1 to 6 have a low contact resistance value of 40 [mΩ · cm ² ] or less even after the electrolytic test with respect to any of Comparative Examples 1 and ² . It has been shown that it has both low contact resistance and corrosion resistance at the same time.

It is a perspective view which shows the external appearance of the fuel cell stack comprised using the separator for fuel cells which concerns on embodiment of this invention. It is an expanded view of the fuel cell stack comprised using the separator for fuel cells which concerns on embodiment of this invention. (A) It is a typical perspective view of the separator for fuel cells. (B) It is the IIIb-IIIb sectional view taken on the line of the separator for fuel cells. (C) It is the IIIc-IIIc sectional view taken on the line of the separator for fuel cells. (A) It is the surface SEM photograph of the separator for fuel cells. (B) It is a cross-sectional SEM photograph of the separator for fuel cells. The M _{4 N} type crystal structure of the transition metal nitride according to an embodiment of the present invention is a schematic diagram showing. It is typical sectional drawing of the separator for fuel cells. (A) It is explanatory drawing which shows the manufacturing method of the separator for fuel cells which concerns on embodiment of this invention, (b) is the time chart. (A) It is explanatory drawing which shows the manufacturing method of the separator for fuel cells of a reference example, (b) is the time chart. It is a figure which shows the external appearance of the electric vehicle carrying the fuel cell stack which concerns on embodiment of this invention, (a) is a side view of an electric vehicle, (b) is a top view of an electric vehicle. (A) It is a schematic diagram explaining the measuring method of the contact resistance of the sample obtained in each Example. (B) It is a schematic diagram explaining the apparatus used for the measurement of contact resistance. (A) It is a cross-sectional structure | tissue photograph of the sample obtained by Example 2. FIG. (B) It is a cross-sectional structure | tissue photograph of the sample obtained by the comparative example 2. FIG. It is a figure which shows the X-ray-diffraction pattern of the sample obtained by Example 2 and Comparative Example 2. It is a figure which shows the element profile of the depth direction by the scanning Auger electron spectroscopy analysis of the sample obtained by Example 2. FIG. It is sectional drawing which shows the structure of the single cell which forms a fuel cell stack.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fuel cell stack 2 Single cell 3 Fuel cell separator 4 End flange 5 Fastening bolt 13 Base layer 14 Nitride layer 15 Cleavage 20 M ₄ N-type crystal structure 21 Transition metal atom 22 Nitrogen atom

Claims (19)

A base layer comprising a base material containing a transition metal or an alloy of the transition metal, and a nitrided layer obtained by nitriding the base material in a nitrogen atmosphere,
The fuel cell separator, wherein the nitrided layer has a cleavage.   2. The fuel cell separator according to claim 1, wherein the cleavage extends vertically through the nitride layer to the base layer. 3.   The fuel cell separator according to claim 2, further comprising an oxide film formed on a surface of the base layer facing the cleavage.   The said base material consists of stainless steel containing at least 1 type or more of transition metal elements chosen from the group of Fe, Cr, Ni, and Mo, The Claim 1 thru | or 3 characterized by the above-mentioned. Fuel cell separator.   The fuel cell separator according to any one of claims 1 to 3, wherein the base material includes a Ni-based alloy. The nitride layer has an M ₄ N-type crystal structure in which nitrogen atoms are arranged in an octahedral cavity at the center of a unit cell of a face-centered cubic lattice formed by transition metal atoms selected from the group of Fe, Cr, Ni, and Mo. The fuel cell separator according to claim 4 or 5, wherein   4. The fuel cell separator according to claim 1, wherein the base material is made of pure industrial titanium, and the nitride layer contains TiN crystals. 5.   The ratio t1 / t of the thickness t1 of the nitride layer to the plate thickness t of the plate-like member is 0.02 to 0.2, according to any one of claims 1 to 7. The fuel cell separator as described.   The ratio y / x between the sum y of the width of the cleavage at the interface between the nitride layer and the base layer and the sum x of the width of the island portion of the nitride layer formed by the cleavage is 0.052 to 19 The fuel cell separator according to claim 8, wherein the fuel cell separator is provided.   10. The fuel cell according to claim 1, wherein an amount of nitrogen on the pole surface of the nitride layer is not less than 5 [at%] and an amount of oxygen is not more than 50 [at%]. Separator for use.   11. The fuel cell separator according to claim 1, wherein a ratio O / N of an oxygen amount to a nitrogen amount on the pole surface of the nitride layer is 10 or less.   12. The nitrogen content is 10 [at%] or more and the oxygen content is 30 [at%] or less at a depth of 10 [nm] from the outermost surface of the nitride layer. The fuel cell separator according to claim 1.   13. The nitrogen content is 15 [at%] or more and the oxygen content is 20 [at%] or less at a depth of 100 [nm] from the outermost surface of the nitride layer. The fuel cell separator according to claim 1. An annealing step for performing an annealing treatment on a plate-like base material containing a transition metal or an alloy of the transition metal, and
A nitriding step of performing a nitriding treatment continuously after the annealing step to form a nitride layer on the substrate surface;
A method for producing a fuel cell separator, comprising: pressing a base material after the nitriding step to form a fuel or oxidant passage; and forming a cleavage in the nitrided layer.   The method for producing a fuel cell separator according to claim 14, wherein a passivation treatment is performed after the press molding step.   The method for producing a fuel cell separator according to claim 14 or 15, wherein a tensile tension is applied to the base material during the nitriding treatment.   The method for producing a fuel cell separator according to any one of claims 14 to 16, wherein the nitriding treatment is a plasma nitriding method or a plasma CVD method.   A fuel cell stack using the fuel cell separator according to any one of claims 1 to 13.   A fuel cell vehicle comprising the fuel cell stack according to claim 18 and using the fuel cell stack as a power source.