JP2009293092A - Nitride of transition metal, separator for use in fuel cell, fuel cell stack, fuel cell vehicle, method for producing nitride of transition metal, and method for manufacturing separator for use in fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitride of a transition metal, which has low contact resistance which is generated in between a separator and an electrode, is excellent in corrosion resistance and is produced at a low cost; a separator for use in a fuel cell; a fuel cell stack; and a fuel cell vehicle. <P>SOLUTION: The nitride of the transition metal is obtained by nitriding a substrate including an austenitic stainless steel, and includes a first layer (first nitrided layer) formed continuously on a base layer and a second layer (second nitrided layer) 13A formed continuously on the first layer. The second layer 13A has precipitates projecting from the surface thereof. The precipitate projecting from the surface of the second layer 13A includes an aggregate in which a plurality of nitrides 13C having an M<SB>4</SB>N type crystal structure comprising cubic crystals mainly containing Fe and a plurality of nitrides 13B having a CrN type crystal structure comprising hexagonal crystals mainly containing Cr are gathered, and has a Cr oxide film 13D formed on the surface. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a transition metal nitride, a fuel cell separator, a fuel cell stack, a fuel cell vehicle, a method for producing a transition metal nitride, and a method for producing a fuel cell separator.

  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 200 has a membrane electrode assembly in which an oxygen electrode 202 and a hydrogen electrode 203 are integrated on both sides of a solid polymer electrolyte membrane 201. The oxygen electrode 202 and the hydrogen electrode 203 have a two-layer structure including a reaction film 204 and a gas diffusion layer 205 (GDL: gas diffusion layer), and the reaction film 204 is in contact with the solid polymer electrolyte film 201. On both sides of the oxygen electrode 202 and the hydrogen electrode 203, an oxygen electrode side separator 206 and a hydrogen electrode side separator 207 are respectively installed for lamination. The oxygen electrode side separator 206 and the hydrogen electrode side separator 207 form an oxygen gas channel, a hydrogen gas channel, and a cooling water channel.

  In the unit cell 200 having the above-described configuration, the oxygen electrode 202 and the hydrogen electrode 203 are arranged on both sides of the solid polymer electrolyte membrane 201, and are usually joined together by a hot press method to form a membrane electrode assembly. The separators 206 and 207 are disposed on both sides of the membrane electrode assembly. In the fuel cell including the single cell 200, when a mixed gas of hydrogen, carbon dioxide, nitrogen, and water vapor is supplied to the hydrogen electrode 203 side, and air and water vapor are supplied to the oxygen electrode 202 side, An electrochemical reaction occurs at the contact surface between the molecular electrolyte membrane 201 and the reaction membrane 204. Hereinafter, a more specific reaction will be described.

  In the single cell 200 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 204 side through the gas diffusion layers 205. Then, the following reactions occur in each reaction film 204.

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 203 side, the reaction of the formula (1) proceeds, and H ⁺ and e ⁻ are generated. H ⁺ moves through the solid polymer electrolyte membrane 201 in a hydrated state and flows to the oxygen electrode 202 side, and e ⁻ flows from the hydrogen electrode 203 to the oxygen electrode 202 through the load 208. On the oxygen electrode 202 side, the reaction of Formula (2) proceeds by H ⁺ , e ^−, and the supplied oxygen gas to generate electric power.

As described above, since the fuel cell separator has a function of electrically connecting each single cell, it is required to have good electrical 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 2-4. 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 1 [Vvs SHE] is loaded from the natural potential to the extreme potential. For this reason, like the oxygen electrode and the hydrogen electrode, the fuel cell separator is required to have corrosion resistance that can withstand a strongly acidic atmosphere. The corrosion resistance required here means the durability with which the fuel cell separator can maintain its electric conductivity even in a strongly acidic oxidizing environment. In other words, the cation dissolves in the humidified water or the water generated by the reaction of the formula (2), so that the cation is bonded to the sulfonic acid group that should originally be a path for protons to occupy the sulfonic acid group, and the power generation of the electrolyte membrane Corrosion resistance needs to be measured in an environment that degrades properties.

In a fuel cell, the theoretical voltage per unit cell is 1.23 [V], but the voltage that can be actually taken out decreases due to reaction polarization, gas diffusion polarization, and resistance polarization. Descent. Further, in automotive applications, since it is desired to increase the output density per unit volume / weight, 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 if the contact resistance between the separator and the electrode is 40 [mΩ · cm ² ] or less, the efficiency reduction due to the contact resistance can be suppressed.

  Therefore, attempts have been made to use a titanium material such as stainless steel or industrial pure titanium, which has good electrical conductivity and excellent corrosion resistance, as a 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.

Therefore, a fuel cell separator in which a gold plating layer is directly formed on a contact surface with an electrode after press forming stainless steel has been proposed (see Patent Document 1). In addition, a fuel cell separator in which a noble metal or a noble metal alloy is adhered by removing stainless steel from a passive film formed on a surface that generates contact resistance by contact with an electrode after being formed into a fuel cell separator shape. Has been proposed (see Patent Document 2).
Japanese Patent Laid-Open No. 10-228914 (2nd page, FIG. 2) JP 2001-6713 A (page 2)

  However, when stainless steel is used for the separator base material, since conductivity and contact resistance are contradictory properties, it is difficult to achieve both corrosion resistance and conductivity. In addition, when the surface of stainless steel is coated with a noble metal or the like, not only is it time-consuming for manufacturing, but the cost may increase.

The present invention has been made to solve the above problems, the transition metal nitride according to the present invention is a transition metal nitride obtained by nitriding a substrate containing austenitic stainless steel, The transition metal nitride is continuously formed on a base layer formed by a base material, and has a cubic M ₄ N type crystal structure and a hexagonal M _2-3 N type crystal structure. A first layer having a nano-level stacked crystal structure including a nitride having a crystal structure, and a hexagonal Cr ₂ N, CrN, and M _2-3 N type formed continuously on the first layer. And a nitride having at least one crystal structure of a cubic M ₄ N type crystal structure, and is continuous in the depth direction from the surface of the substrate as a surface nitriding portion of the substrate And the second layer is formed of the second layer. Having precipitates projecting from face portion, precipitates projecting from the surface portion of the second layer, a CrN hexagonal nitride and Cr mainly with M ₄ N type cubic crystal structure of Fe principal And a precipitate projecting from the surface portion of the second layer having a Cr oxide film on the surface thereof.

  A fuel cell separator according to the present invention includes a base layer formed of a base material containing austenitic stainless steel, and a transition metal nitride nitride layer according to the present invention formed directly on the base layer. And

A method for producing a transition metal nitride according to the present invention is a method for producing a transition metal nitride formed by nitriding a base material containing austenitic stainless steel, and a cubic M ₄ N is formed on the surface of the base material. Forming a first layer having a nano-level stacked crystal structure including a nitride having a crystal structure of a type and a nitride having a hexagonal M _2-3 N-type crystal structure, and over the first layer And a nitride having at least one of a hexagonal Cr ₂ N, CrN and M _2-3 N type crystal structure and a cubic M ₄ N type crystal structure And forming a second layer formed continuously from the surface of the base material in the depth direction as a surface nitriding portion of the base material, and after forming the second layer, the surface of the second layer is It is characterized by oxidation treatment with an acid.

The method for producing a separator for a fuel cell according to the present invention includes press forming a base material containing austenitic stainless steel to form a fuel or oxidant passage, nitriding the press-formed base material, The substrate surface has a nano-level stacked crystal structure including a nitride having a cubic M ₄ N type crystal structure and a nitride having a hexagonal M _2-3 N type crystal structure. Formed in succession on this first layer, hexagonal Cr ₂ N, CrN and M _2-3 N type crystal structures, and cubic M ₄ N type crystal structures Including a nitride having at least one crystal structure, and forming a second layer formed continuously from the surface of the substrate in the depth direction as a surface nitriding portion of the substrate; The surface of the second layer is oxidized with an acid after forming To.

  The fuel cell stack according to the present invention is characterized by using the fuel cell separator according to the present invention.

  A fuel cell vehicle according to the present invention includes the fuel cell stack according to the present invention as a power source.

  ADVANTAGE OF THE INVENTION According to this invention, the transition metal nitride which has the chemical stability and corrosion resistance which maintain an electroconductive function also in a strong acid atmosphere, and low cost and favorable productivity can be provided.

  ADVANTAGE OF THE INVENTION According to this invention, the contact resistance generate | occur | produced between a separator and an electrode is low also in the sulfuric acid environment under a fuel cell environment, it is excellent in corrosion resistance, and the separator for fuel cells with low manufacturing cost can be provided. .

  According to the present invention, it is possible to form a transition metal nitride nitride layer having both opposite conductivity and corrosion resistance on the stainless steel surface.

  According to the present invention, it is possible to manufacture a fuel cell separator having excellent conductivity and corrosion resistance.

  According to the present invention, it is possible to provide a high-performance fuel cell stack that is downsized and reduced in cost.

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

  Hereinafter, the transition metal nitride, the separator for fuel cell, the fuel cell stack, the fuel cell vehicle, the method for producing the transition metal nitride, and the method for producing the separator for the fuel cell according to the present invention are applied to the polymer electrolyte fuel cell. An example will be described.

(Transition metal nitrides, fuel cell separators and fuel cell stacks)
FIG. 1 is a perspective view showing an external appearance of a fuel cell stack 1 constituted by using a fuel cell separator 3 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 stacking a plurality of unit cells 2 and fuel cell separators 3 as basic units for generating power by electrochemical reaction. 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 flow path and a fuel gas flow path are respectively defined inside the fuel cell separator 3. As the 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.

  Details of the fuel cell separator 3 shown in FIG. 2 will be described. 3 (a) is a schematic perspective view of the fuel cell separator 3, FIG. 3 (b) is a sectional view taken along the line IIIb-IIIb in which the main part of the fuel cell separator 3 is emphasized in an easily understandable manner, and FIG. 3 (c). These are the IIIc-IIIc sectional view taken on the line which emphasized the principal part of the separator 3 for fuel cells clearly. FIG. 4A is an enlarged schematic view of the main part of FIG. 3C, and FIG. 4B is a schematic cross-sectional view of the second nitride layer 11c of the fuel cell separator 3. FIG. As shown in FIG. 3, the fuel cell separator 3 is obtained by nitriding a surface portion 10a as a surface nitriding portion of the base material 10 using a base material 10 containing austenitic stainless steel as a raw material. 10 includes a nitride layer 11 formed in the depth direction of the surface portion 10a and a base layer 12 which is an unnitrided layer that is not nitrided. The fuel cell separator 3 is formed with a groove-like flow path 13 of fuel or oxidant having a rectangular cross section by press molding. A flat plate portion 14 defined by the flow channel 13 and the flow channel 13 is provided between the flow channel 13 and the flow channel 13, and the nitride layer 11 extends along the outer surfaces of the flow channel 13 and the flat plate portion 14. To do. The flat plate portion 14 contacts the gas diffusion layer on the adjacent solid polymer membrane when the fuel cell separators 3 and the single cells 2 are alternately laminated.

  The base layer 12 is formed from an austenitic stainless steel containing at least one element selected from the group consisting of Fe, Cr, Ni, and Mo. Austenitic stainless steel has a face-centered cubic lattice crystal structure.

The nitride layer 11 includes a transition metal nitride obtained by nitriding a base material including austenitic stainless steel, and includes crystal structures of MN type, M ₄ N type, and M _2-3 N type. M represents a transition metal element selected from the group consisting of Cr, Fe, Ni and Mo. As shown in FIGS. 3C and 4, the nitride layer 11 is continuously formed on the first nitride layer (first layer) 11 b formed on the base layer 12 and the first nitride layer 11 b. And a second nitride layer (second layer) 11c which is the outermost surface of the nitride layer 11 including the surface portion 11a of the nitride layer 11. The surface portion 11a of the nitrided layer 11 is a solution in which nitrogen is dissolved in the surface portion 10a of the base material 10 by nitriding.

The first nitride layer 11b is continuously formed on the base layer 12 formed by the base material 10, and has a cubic M ₄ N type crystal structure and a hexagonal M _2-3 N type. And a nano-level stacked crystal structure 11b ₁ including a nitride having a crystal structure of The nano-level stacked crystal structure 11b ₁ includes a structure in which M ₄ N-type and M _2-3 N-type crystal structures are stacked.

The second nitride layer 11c is continuously formed on the first nitride layer 11b, and has a hexagonal Cr ₂ N, CrN and M _2-3 N type crystal structure, and a cubic M ₄ N type. The nitride has a crystal structure of at least one of the crystal structures, and is formed continuously from the surface portion 10a of the base material 10 as the surface nitriding portion of the base material 10 in the depth direction. The surface portion 11a of the nitride layer 11 has a structure in which Cr and nitrogen are concentrated, and includes MN-type, M ₄ N-type, and M _2-3 N-type crystal structures.

As shown in FIG. 4B, the surface portion 11a of the nitride layer 11 has a plurality of minute precipitates 11a _i (i = 1 to n) protruding irregularly from the surface portion 11a. For example, the precipitate 11a ₁ protrudes from the surface portion 11a of the nitride layer 11 by a height h _{1 at the} maximum. The precipitate 11a ₂ protrudes from the surface portion 11a of the nitride layer 11 by a height h _{2 at the} maximum. The precipitate 11a ₃ protrudes from the surface portion 11a of the nitride layer 11 by a height h _{3 at the} maximum. The maximum heights h ₁ , h ₂ , and h ₃ of the precipitates 11a ₁ , 11a ₂ , and 11a ₃ with respect to the surface portion 11a of the nitride layer 11 are obtained. These precipitates 11a _i include an aggregate in which a plurality of nitrides having a Fe-based cubic M ₄ N type crystal structure and Cr-based hexagonal CrN are gathered, and a Cr oxide film 11o is formed on the surface. Have Cr oxide film 11o of the surface of the deposit 11a _i, as described below, in which after the formation of the nitride layer 11 is obtained by oxidizing the surface portion 11a of the nitride layer 11 formed by the acid.

  By simply providing a nitride layer on the separator surface, for example, those with low contact resistance and excellent conductivity tend to have a high iron ion elution amount and a tendency to deteriorate the corrosion resistance. Conversely, the iron ion elution amount is small and the corrosion resistance is low. Those having excellent resistance tend to have high contact resistance and poor conductivity. For this reason, it is necessary to improve the corrosion resistance and conductivity of the fuel cell separator in order to cope with the drastic reduction in size and weight of the fuel cell stack. Improvement of the nitride layer is an important issue to be solved in order to make the film excellent.

  In the fuel cell separator 3 according to the embodiment of the present invention, since the nitride layer 11 has the structure described above, the nitride layer 11 can be used even in a strongly acidic oxidizing environment such as a hydrogen electrode of a fuel cell. The transition metal nitride that constitutes the material suppresses elution of metal ions from the fuel cell separator 3 and is excellent in corrosion resistance. In this case, the fuel cell separator 3 is excellent in durability for maintaining the electric conduction performance, the corrosion resistance and the conductivity are compatible, and the manufacturing cost can be kept low. In addition to this, the transition metal nitride makes the fuel cell separator 3 excellent in durability to maintain electric conductivity even in a strongly acidic atmosphere such as an oxygen electrode for introducing air into the fuel cell. The fuel cell separator 3 is excellent in conductivity. In addition, the elution of metal ions from the fuel cell separator 3 is suppressed to a low level even in a strongly acidic oxidizing environment, and both corrosion resistance and conductivity are compatible, and the manufacturing cost is suppressed to a low level. In this way, it combines the necessary electrical conductivity as a separator for fuel cells and the chemical stability and corrosion resistance to maintain the conductivity properties in the separator use environment, and it is low in cost and has good productivity, and adjacent gas diffusion. It is possible to provide a fuel cell separator having a low contact resistance with a constituent material such as an electrode and having good power generation performance of the fuel cell.

Moreover, since it has a surface portion 11a are a plurality of fine precipitates 11a _i of the second nitride layer 11c, in contact with the carbon paper made of carbon fiber for the separator 3 and the sub-micron unit fuel cell, the carbon fiber gap Precipitates 11a _i enter and the actual contact area (points) increases. For this reason, the contact resistance between the fuel cell separator 3 and the carbon paper is kept low, and the conductivity becomes excellent.

  Such a nitrided layer having a high nitrogen concentration and a high Cr concentration and having a plurality of fine precipitates on the surface can be easily obtained by plasma nitriding a base material including austenitic stainless steel having a particularly high Cr concentration. is there. The Cr oxide film on the precipitate surface can be easily obtained by oxidizing this nitride layer with an acid.

Next, details of each crystal structure are shown. FIG. 5A is a schematic diagram showing an M ₄ N-type crystal structure, and FIG. 5B is an M _2-3 N-type hexagonal crystal structure. Austenitic stainless steel has a face-centered cubic lattice crystal structure. When the surface portion 10a of the base material 10 having the crystal structure of the face-centered cubic lattice is nitrided, an M ₄ N type crystal structure shown in FIG. 5A is obtained. That is, the nitrogen atoms 22 that enter the face-centered cubic lattice by nitriding are arranged in the octahedral space at the center of the unit cell of the face-centered cubic lattice formed from the transition metal atoms (M) 24 (or 21). ₄ N-type crystal structure 20 is obtained. Here, M in the M ₄ N type represents a transition metal atom 24 (or 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 24 (or 21) selected from Fe, Cr, Ni, and Mo is mainly composed of Fe, but Fe is composed of Cr, Ni, Mo, or the like. An alloy partially substituted with another transition metal atom may be used.

This M ₄ N-type crystal structure is considered to be a fcc or fct structure nitride with high density of dislocations and twins, a hardness as high as 1000 [Hv] or higher, and nitrogen in a supersaturated solid solution. (Yasuma, Tsugaike; Journal of the Japan Institute of Metals, 50, pp 362-368, 1986). Further, the closer to the surface, the higher the nitrogen concentration, and CrN is not a main component, so that Cr effective for corrosion resistance does not decrease and corrosion resistance is maintained even after nitriding. Further, this M ₄ N-type crystal structure 20 includes metal bonds 23 between transition metal atoms M (for example, between Fe atoms 24 and adjacent Fe, Cr, or Ni atoms in FIG. 5A). Each of the transition metal atoms M is connected to the nitrogen atom N22 by a strong covalent bond 25 while maintaining the above, thereby reducing the reactivity of each transition metal atom M24 (or 21) to oxidation. For this reason, the nitride layer 11 having the M ₄ N-type crystal structure 20 is excellent in corrosion resistance even in a strongly acidic atmosphere of pH 2-4. In addition, conductivity is maintained by the metal bond 23 between the transition metal atoms M24 (or 21). In addition, the M ₄ N-type crystal structure 20 has a face-centered cubic lattice crystal structure similar to the base layer 12. For this reason, the base layer 12 and the nitride layer 11 formed on the base layer 12 have good consistency, and the movement of electrons between the base layer 12 and the nitride layer 11 is facilitated. A fuel cell having the nitride layer 11 The separator 3 for use is excellent in conductivity. In FIG. 5A, the bond indicated by reference numeral 25a is also a covalent bond between the transition metal atom M24 (or 21) and the nitrogen atom N22.

The transition metal atoms M selected from Fe, Cr, Ni and Mo forming the M ₄ N-type crystal structure 20 are preferably mixed irregularly. The atoms M of each transition metal are dispersed at irregular positions, the partial molar free energy of the component of the transition metal is lowered, and the activity of the transition metal atom M is kept low. Accordingly, the reactivity of each transition metal atom M24 (or 21) in the nitride layer 11 to oxidation can be kept low. The nitride layer 11 has chemical stability even in an oxidizing environment within the fuel cell. For this reason, the contact resistance between the fuel cell separator 3 and the corresponding electrode such as carbon paper can be kept low, and the durability of the fuel cell separator 3 can be enhanced. In addition, since the contact resistance can be kept low without forming a noble metal plating layer by gold plating or the like on the separator 3 serving as a contact surface with the electrode, cost reduction can be realized.

  In addition, transition metal atoms M24 (or 21) selected from Fe, Cr, Ni and Mo forming a face-centered cubic lattice are mixed irregularly to increase the mixing entropy and increase the component of each transition metal. It is preferred to reduce the partial molar free energy or to lower the activity of each transition metal atom M24 (or 21) than estimated by Raoul's law. Thereby, the reactivity with respect to the oxidation of each metal element M24 (or 21) can be further reduced, and the chemical stability is improved.

Next, the M _2-3 N type hexagonal crystal structure 26 shown in FIG. 5B will be described. This crystal structure is a crystal obtained when particles as hard spheres (that is, transition metal atoms M27 and nitrogen atoms N28 in FIG. 5B) are packed most closely together with the face-centered cubic (fcc) lattice. A lattice, called a close-packed hexagonal or hexagonal close-packed (hcp) lattice. The difference between a face-centered cubic (fcc) lattice and a close-packed hexagonal (hcp) lattice is that if hexagonal symmetric dense atomic planes are stacked in the order of ABAB, it becomes an hcp lattice, and ABCABC only becomes an fcc lattice. Here, A, B, and C are symbols indicating the positional relationship of atoms. As described above, the fcc lattice and the hcp lattice have a high matching property in terms of crystal structure, and thus have a deep relationship with each other, such as smooth electron movement and contribution to conductivity.

The M _2-3 N-type hexagonal crystal structure 26 is formed as follows. M ₄ when dissolved nitrogen nitrides N-type crystal structure 20 is supersaturated, nitrides having a higher nitrogen of M _{2 to 3 N-type} crystal structure than M ₄ N type, M ₄ N-type crystal It will precipitate on the stacking faults of the structure 20 nitride. That is, with the M ₄ N-type crystal structure 20 as a matrix, stacking faults are generated at intervals of about several [nm] in the same crystal plane, and M _2-3 N nitriding with an interlayer distance of about several nm is formed on the stacking faults. A thing precipitates in layers.

Further, using the M ₄ N-type crystal structure 20 as a matrix, the amount of nitride nitrogen in which M _2-3 N nitride having an interlayer distance of about [nm] is deposited on the stacking fault, ie, the nitride layer 11 As the amount of nitrogen at a depth of 100 [nm] from the surface portion 11a increases, the chemical stability becomes stable and the reactivity of transition metal atoms decreases. As a result, the contact resistance is kept low even in a sulfuric acid environment. Sex is maintained.

The M _2-3 N-type hexagonal crystal structure 26 is preferably mainly composed of Cr. Cr is one of the transition metal atoms M24 (or 21) and 27 constituting the M ₄ N type crystal structure 20 and the M _2-3 N type hexagonal crystal structure 26, and is a transition metal element Fe. Among the elements Cr, Ni and Mo, these elements have the highest affinity for nitrogen atoms. For this reason, by nitriding a high austenitic stainless steel containing Fe, Cr, Ni, and Mo, Cr is concentrated on the surface portion 10a by lattice vibration. When Cr is concentrated on the surface portion 10a of the base material 10, the nitrogen concentration of the surface portion 10a is increased. In the embodiment of the present invention, the nitride layer 11 is obtained by plasma nitriding as will be described later. Due to the sputtering action of plasma nitriding, Fe atoms contained most in the surface portion 10a of the substrate 10 are once ejected from the surface portion 10a, and the concentration of Cr is promoted. As described above, CrN and Cr ₂ N Cr nitrides having a close-packed hexagonal lattice crystal structure are formed on the surface portion 10a of the base material 10 by the combination of Cr concentration and nitrogen by nitriding treatment. Become.

  On the other hand, Cr is an element having a high affinity with oxygen, and it becomes easy to form a passive film having conductivity of several tens [nm] or less on the surface of Cr nitride. This passive film has the effect of improving the corrosion resistance in a sulfuric acid environment.

  The first nitride layer 11b has a composition in which M is in the range of 66.6 to 80.0 [at%], N is in the range of 20.0 to 33.3 [at%], and the second nitride layer 11c The composition is preferably such that M is in the range of 50.0-75.0 [at%] and N is in the range of 25.0-50.0 [at%]. In this case, the coordination number of the metal atom and the nitrogen atom is bonded without excess or deficiency, that is, a stable bond state and a nitride with high Cr and high N concentration are obtained. Excellent in both performance and corrosion resistance. On the other hand, when an excess or deficiency occurs in the coordination number between the metal atom and the nitrogen atom, both the conductivity and the corrosion resistance are deteriorated due to chemical instability.

In the first nitride layer 11b, the nitride having a cubic M ₄ N type crystal structure has a composition of M of 80 [at%] and N of 20.0 [at%], which is a hexagonal crystal. The nitride having the M _2-3 N-type crystal structure of M is in the range of 66.6 to 75.0 [at%] for M and 25.0 to 33.3 [at%] for N Is preferred.

The second nitride layer 11c has a composition in the range of 10 to 30 [at%] of Cr and N of 20 to 40 [at%] at a depth of 10 [nm] or less from the surface portion 11a. It is preferable to have a portion where N is concentrated. In this case, the activity is low because it has a nano-level stacked crystal structure including a nitride having an M ₄ N type crystal structure and a nitride having a hexagonal M _2-3 N type crystal structure. It becomes an organization and becomes chemically stable and excellent in both conductivity and corrosion resistance. On the other hand, when it deviates from this range, it becomes a structure with high activity and becomes chemically unstable.

As shown in FIG. 4B, the second nitride layer 11c has a hexagonal Cr ₂ N, CrN and M _2-3 N type crystal structure, and a cubic M ₄ N type crystal structure. wherein, the surface portion 11a of the nitride layer 11, if a structure having a plurality of precipitates 11a _n that irregularly protrude from the surface portion 11a, the amount of the deposit 11a _n may contact with GDL the more Excellent electrical conductivity due to increased area. Therefore, the amount of precipitates 11a _n is preferably an area ratio 5 is% or more, more preferably 10% or more, and more preferably still 20% or more. By setting it as such an area ratio, contact resistance can be restrained low.

  Further, the protrusion-like precipitate protruding from the surface portion of the second nitride layer has a higher conductivity as the protrusion-like precipitate protruding from the surface obtained by image analysis has a larger equivalent circular diameter. The minimum equivalent circular diameter is 40 [nm] or more.

The surface portion 11a of the second nitride layer 11c and image analysis, the results obtained by this analysis, the conductive effect due to contact between the deposit 11a _n and GDL may also be determined. This analysis is to quantify the size distribution of the surface portion 11a image analysis to precipitate 11a _n electron microscope image by the technique of the image analysis of the second nitride layer 11c. From this analysis, precipitates 11a _n projecting from the surface portion 11a of the second nitride layer 11c is excellent in conductivity equivalent circular diameter as large, is preferably at 40 [nm] or more equivalent circular diameter. Further, it is preferable that precipitates having an equivalent circular diameter of 40 [nm] or more exist uniformly on the surface portion, and an equivalent circular diameter per area of 100 [μm ² ] is 40 [nm] or more. It is preferable that it is 800 [pieces] or more. In this case, in contact with the carbon paper made of carbon fibers of the second nitride layer 11c and the sub-micron (GDL), it contains the precipitates 11a _n the gap of the carbon fibers, the second nitride layer 11c and the carbon The contact area (point) with the paper (GDL) increases. By increasing the contact area in this way, the contact resistance is kept low and the conductivity is excellent.

  In this case, in contact with carbon paper made of submicron carbon fibers, precipitates enter the gaps between the carbon fibers, and the actual contact area (points) increases, so that the contact resistance is kept low and the conductivity is excellent. become.

As described above, it precipitates 11a _n is preferably be present a number on the surface portion 11a of the second nitride layer 11c are preferably, and uniformly distributed on the surface portion 11a. This is to make the contact with the GDL uniform. Thus, a number, and nitride layer 11 including the precipitates 11a _n are uniformly distributed is obtained with Mo concentration of substrates, including austenitic stainless steel nitrided high. Specifically, it is preferable that the Mo concentration of the base material including the austenitic stainless steel having the Cr concentration to be nitrided of 25 [%] or more is 0.5 [%] or more and 3.0 [%] or less. Thus when nitriding the substrates include precipitates 11a _n without segregated at grain boundaries, grain boundaries, also uniformly deposited in the grains. This is because by adjusting the amount of Mo, in plasma nitriding described later, Fe and Cr atoms are separated from the crystal lattice by a sputtering action and reattached to the surface of the base material, so that grain growth is facilitated.

Incidentally, the precipitate 11a _n is for forming an assembly gathered more and the CrN hexagonal nitride and Cr mainly with M ₄ N type cubic crystal structure of Fe principal as described above In addition, since the Cr-based nitride has a portion exposed on the surface layer, a Cr oxide film is likely to be formed on the surface of the precipitate by an oxidation treatment with an acid under atmospheric release described later.

  Next, an oxide layer formed by oxidizing the surface of the nitride layer with an acid will be described.

The nitrided layer obtained by plasma nitriding and nitriding stainless steel is immersed in a strong acidic aqueous solution having a pH of 2 to 4 under the atmosphere, and the surface of the nitrided layer is oxidized with an acid. As a result of performing element mapping analysis on this surface with a field emission Auger electron spectrometer, the surface of the nitride layer before oxidation treatment has a composition mainly composed of Fe, whereas the surface after oxidation treatment has protrusions protruding from the surface. It was found that only the portion of the precipitate was changed to a Cr-based composition. Further, when the surface of the nitride layer is observed by a cross-sectional observation with a field emission transmission electron microscope (FE-TEM), as shown in FIG. While the surface layer is an M ₄ N type crystal structure mainly composed of Fe and the inside thereof is a double structure of CrN, the protruding deposits on the surface of the nitrided layer after the oxidation treatment under the open atmosphere are It was found that the M ₄ N type crystal structure mainly composed of Fe in the surface layer was a structure of only CrN removed by oxidation treatment.

When the elemental composition of the nitrided layer 11 after the oxidation treatment is performed by XPS analysis (X-ray photoelectron spectroscopic analysis), the composition in the depth region of 2 to 3 [nm] from the surface portion 11a by the XPS analysis method is Cr for Fe. The atomic ratio (molar ratio) is preferably 1.5 or more. It means that the larger the Cr ratio to Fe, the greater the Cr ratio in the nitride layer. As the Cr ratio to Fe increases, a Cr oxide film having a thickness of 10 nm or less is formed on the surface portion 11a. Thus, when the atomic ratio of Cr to Fe is 1.5 or more, a passive film having conductivity is formed even in a strong acid acidic environment. In the oxidation treatment, Fe and Cr on the outermost surface of the nitride layer are simultaneously dissolved in a strongly acidic aqueous solution, and Fe ²⁺ is eluted in the solution, while Cr ³⁺ forms a complex film to form the surface of the nitride layer. This is because a Cr oxide film is formed.

  When the Cr ratio to Fe is less than 1.5, the Fe ratio in the nitride layer is increased, and not the Cr oxide film but the Fe oxide film is formed on the surface portion 11a. This Fe oxide film grows thicker than when a Cr oxide film is formed in the oxidation treatment. For this reason, when it uses as a separator as a result, resistance increases.

  When the elemental composition of the nitrided layer 11 after the oxidation treatment is performed by XPS analysis, the composition in the depth region of 2 to 3 [nm] from the surface portion 11a by the XPS analysis method has an O content of 30 [mol%]. The following is preferable. In this case, the nitride layer is conductive because it has a thickness that allows electrons to move through the oxide film. Moreover, it is preferable that it is more than 5.0 [mol%] nitrogen (N) in the depth of 2-3 [nm] from the surface part 11a. In this case, since the nitride is occupied mostly by the outermost surface portion 11a of the nitride layer 11, the nitride layer 11 has low resistance and conductivity.

Note that the protruding precipitate contains at least hexagonal CrN, and when this CrN is partially exposed on the surface of the protruding precipitate, a Cr oxide film is easily formed by the oxidation treatment. Structure of the protruding precipitates, or even one formed from only the crystal structure of the M ₄ N type cubic Fe mainly nitride having a crystal structure of the M ₄ N type cubic Fe Principals and Cr If CrN is not exposed on the surface even if a plurality of main hexagonal CrN aggregates to form an aggregate, the oxide film on the surface of the protrusion-like precipitate becomes Fe oxide. When the oxide film is Fe oxide, it is easily broken in a strong acid atmosphere, and the Fe-based cubic M ₄ N type crystal structure is dissolved. For this reason, the elution amount of Fe ions increases, and the oxide film is formed thick simultaneously with the elution of Fe ions, so that the resistance increases.

In a strongly acidic aqueous solution having a pH of 2 to 4 in a deaeration environment close to the anode electrode in an actual fuel cell, a nitride having a Fe-based cubic M ₄ N type crystal structure and a Cr-based hexagonal crystal In the precipitate protruding from the surface portion of the second layer in which a plurality of CrN aggregates to form an aggregate, the Fe-based nitride containing the M ₄ N type crystal structure is more than the Cr-based nitride such as CrN. However, it is inferior in oxidation resistance and is easily eluted. And when it elutes, it becomes easy to form an oxide film on the nitride surface, and conductivity deteriorates. On the other hand, a Cr-based nitride such as CrN is superior in oxidation resistance to the Fe-based M ₄ N type, so that it is difficult to elute even in a strongly acidic aqueous solution of pH 2 to 4 in a degassing environment. In the contact between the fuel cell separator and the carbon paper made of sub-micron unit carbon fibers, Fe-based nitride containing an M ₄ N-type crystal structure is left as a precipitate protruding from the surface of the layer. The actual contact area (projection height) is reduced by the amount of dissolution compared to before immersion. For this reason, the electrical conductivity between the fuel cell separator and the carbon paper is deteriorated.

In the case where the precipitate protruding from the surface portion of the second layer is composed only of a cubic M ₄ N type crystal structure mainly composed of Fe, the oxide film to be formed becomes Fe oxide, and the degassing environment is in a strongly acidic atmosphere. It is easily broken and the cubic M ₄ N type crystal structure mainly composed of Fe is dissolved. For this reason, the elution amount of Fe ions increases, and the oxide film is formed thick simultaneously with the elution of Fe ions, so that the resistance increases.

On the other hand, in strongly acidic aqueous solution having a pH of 2 to ₄ in an open-air environment close to the cathode in an actual fuel cell, a Fe-based nitride containing an M ₄ N-type crystal structure and CrN or other Cr Both the main nitrides are difficult to elute and the protruding precipitates contain at least hexagonal CrN. When this CrN is partially exposed on the surface of the protruding precipitates, a Cr oxide film is formed by oxidation treatment. Therefore, in the contact between the fuel cell separator and the carbon paper made of submicron carbon fibers, precipitates enter the gaps between the carbon fibers, and the actual contact area (point) increases. For this reason, since the contact resistance between the separator for fuel cells and the carbon paper is kept low, the conductivity is excellent.

That is, as an oxidation treatment of a nitrided layer formed by plasma nitriding a substrate with an acid, a nitride layer formed by nitriding in a strong acidic aqueous solution having a pH of 2 to 4 under the atmosphere is immersed, so that a cubic structure mainly composed of Fe is obtained. Without dissolving the precipitate protruding from the surface of the second layer, which is an aggregate of a plurality of nitrides having an M ₄ N type crystal structure and Cr-based hexagonal CrN, Then, a chemically stable Cr oxide film is formed on the surface of the second layer. In other words, the precipitate protruding from the surface portion of the second nitride layer after the oxidation treatment of the transition metal nitride with the acid is a hexagonal structure composed of a Fe-based cubic M ₄ N type crystal structure and a Cr-based hexagon. In the aggregate of a plurality of crystal CrN, the nitride mainly composed of Cr is exposed on at least a part of the precipitate surface layer protruding from the surface portion of the second layer, so that the surface of the second nitride layer is chemically exposed. By forming a highly stable Cr oxide film, it becomes difficult for the nitrided layer to elute even in an environment close to both the anode and cathode electrodes in an actual fuel cell. In addition, the surface portion of the second layer, which is excellent in oxidation resistance, is an aggregate of a plurality of nitrides having a cubic M ₄ N type crystal structure mainly composed of Fe and Cr-based hexagonal CrN. Since the protruding deposits do not dissolve, the deposits enter the gaps between the carbon fibers in the contact between the fuel cell separator and the carbon paper made of submicron carbon fibers, and the actual contact area (points) increases. . For this reason, since the contact resistance between the separator for fuel cells and the carbon paper is kept low, the conductivity is excellent. In this way, the surface of the nitrided layer becomes stable in both strong acid atmospheres in open air and in a deaerated environment.

  Note that the larger the amount of the precipitate protruding from the surface portion of the second nitride layer after the oxidation treatment in the open atmosphere, the more the contact area with the GDL increases, so that the conductivity is excellent. For this reason, the amount of precipitates is at least an area ratio and requires a precipitation area of 5 [%] or more, preferably 10 [%] or more, more preferably 20 [%] or more. It is.

In addition, the projection-like precipitate protruding from the surface portion of the second nitride layer after the oxidation treatment in the open atmosphere has a small equivalent circular diameter of the projection-like precipitate protruding from the surface obtained by image analysis. Larger ones are more excellent in conductivity, and at least 40 [nm] or more in equivalent circular diameter is required. The precipitates protruding from the surface having an equivalent circular diameter of 40 [nm] or more are preferably present uniformly dispersed in the surface area, and 800 [pieces per 100 [μm ² ] in the measurement visual field. ] Because of the above, the contact area with the GDL is increased, so that the conductivity is excellent.

  As described above, by adopting the above-described configuration, the transition nitride according to the embodiment of the present invention has the opposite conductivity and corrosion resistance at the same time. In addition, the fuel cell separator according to the embodiment of the present invention combines the necessary conductivity as a fuel cell separator, the chemical stability and the corrosion resistance to maintain the conductivity function in the separator use environment, and is produced at a low cost. In addition, it is possible to obtain a fuel cell separator that has good properties and low contact resistance with a constituent material such as an adjacent gas diffusion electrode and that 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.

(Transition metal nitride manufacturing method and fuel cell separator manufacturing method)
Next, the transition metal nitride manufacturing method and the fuel cell separator manufacturing method according to the embodiment of the present invention will be described.

A method for producing a transition metal nitride according to an embodiment of the present invention is a method for producing a transition metal nitride formed by nitriding a base material containing austenitic stainless steel. A first nitride layer having a nano-level stacked crystal structure (first layer) including a nitride having an M ₄ N-type crystal structure and a nitride having a hexagonal M _2-3 N-type crystal structure Of the hexagonal Cr ₂ N, CrN and M _2-3 N type crystal structure, and the cubic M ₄ N type crystal structure. A second nitride layer (second layer) comprising a nitride having at least one crystal structure, and continuously formed in the depth direction from the surface of the substrate as a surface nitriding portion of the substrate; After forming the second nitride layer, the surface of the second nitride layer is oxidized with an acid. And wherein the door. According to this method, a transition metal nitride formed continuously over the entire surface of the substrate and in the depth direction from the surface can be obtained. And this nitride has Cr oxide on the surface, and has both conductivity and corrosion resistance.

A method for manufacturing a separator for a fuel cell according to an embodiment of the present invention is a method of press-forming a base material containing austenitic stainless steel to form a fuel or oxidant passage, and forming the press-formed base material. Nitrided to have a nano-level stacked crystal structure including a nitride having a cubic M ₄ N type crystal structure and a nitride having a hexagonal M _2-3 N type crystal structure on the surface of the substrate Forming a first nitride layer (first layer), continuously formed on the first nitride layer, a hexagonal Cr ₂ N, CrN and M _2-3 N type crystal structure, and cubic A second nitride layer including a nitride having at least one crystal structure of M ₄ N type crystal structure and continuously formed in the depth direction from the surface of the substrate as a surface nitriding portion of the substrate; After forming the second nitride layer, the second nitride layer is formed, and then the second nitride layer is formed. Wherein the oxidation treatment with an acid the surface of the nitride layer. According to the present invention, a transition metal nitride nitride layer having a Cr oxide film on the surface and having both conductivity and corrosion resistance can be formed on the stainless steel surface. When nitriding is performed after press molding, defects such as cracks do not occur in the nitride layer. For this reason, the separator for fuel cells rich in electroconductivity and corrosion resistance is obtained.

In addition, it is preferable that Mo density | concentration of the base material containing the austenitic stainless steel whose Cr density | concentration to nitride is 25 [%] or more is 0.5 [%] or more and 3.0 [%] or less. When such a base material is nitrided, as shown in FIG. 4 (b), the second nitride layer 11c is mainly composed of hexagonal Cr ₂ N, CrN and M _2-3 N type crystal structures, In addition, a nitride layer including a cubic M ₄ N type crystal structure and having a structure in which the precipitates 11 a _i protruding from the surface portion 11 a protrude irregularly is obtained on the surface portion 11 a of the nitride layer 11.

  Next, with reference to FIG. 6, the manufacturing method of the transition metal nitride and the separator for fuel cells is demonstrated. FIG. 6 is a schematic side view of the nitriding apparatus 30 used in the method for manufacturing the transition metal nitride and the fuel cell separator according to the embodiment of the present invention.

  Plasma nitriding uses a workpiece (here, stainless steel) as a cathode and applies a DC voltage to generate glow discharge, that is, low-temperature non-equilibrium plasma, ionizes a part of gas components, and in the non-equilibrium plasma. In this method, the ionized gas component is nitrided by colliding with the surface of the workpiece at high speed.

  The nitriding device 30 is a batch type nitriding furnace 31, a vacuum pump 34 for evacuating a vacuum nitriding vessel 31a installed in the nitriding furnace 31, and supplying an atmospheric gas to the vacuum nitriding vessel 31a. Gas supply device 32, and plasma electrodes 33a and 33b charged to a high voltage to generate plasma in the vacuum nitriding vessel 31a, and these electrodes 33a and 33b are pulsed to a high frequency of 45 [kHz]. A pulse plasma power source 33 for supplying a direct current voltage and a temperature detector 37 for detecting the temperature in the vacuum nitriding vessel 31a. The nitriding furnace 31 includes an outer container 31b made of a heat insulating insulating material that accommodates the vacuum nitriding container 31a, and includes a plasma observation window 31g. The vacuum nitriding vessel 31a includes an insulator 35 at the bottom 31c for holding the plasma electrodes 33a and 33b at a high potential. The plasma electrodes 33a and 33b are provided with a stainless steel support 36 thereon. The support 36 is formed with a flow path of fuel or oxidant by press molding, and supports a stainless steel 44 (hereinafter also referred to as a base material) processed into a separator shape. The gas supply device 32 includes a gas chamber 38 and a gas supply pipe 39, and the gas chamber 38 has a predetermined number of gas introduction openings (not shown), each of which has a gas supply valve (not shown). ) Communicated with a hydrogen gas supply line (not shown), a nitrogen gas supply line (not shown), and an argon gas supply line (not shown). The gas supply device 32 further has a gas supply opening 32a communicating with one end 39a of the gas supply line 39, and a gas supply valve (not shown) is provided in the opening 32a. The gas supply line 39 extends through the bottom 31d of the outer vessel 31b of the nitriding furnace 31 and the bottom 31c of the vacuum nitriding vessel 31a into the vacuum nitriding vessel 31a and rises vertically. It reaches part 39b. The rising portion 39b is provided with a plurality of openings 39c for ejecting gas into the vacuum nitriding vessel 31a. The gas pressure in the vacuum nitriding vessel 31a is detected by a gas pressure sensor (not shown) provided at the bottom 31c of the vacuum nitriding vessel 31a. The conductive wire 31k of the resistance heating type or induction heating type heater 31j is wound around the outer periphery of the vacuum nitriding vessel 31a, and is heated by this. An air flow path 40 is defined between the vacuum nitriding vessel 31a and the outer vessel 31b. The side wall 31e of the outer container 31b is provided with a blower 41 that sends air flowing into the air flow path 40 from an opening 31f provided in the side wall 31e of the outer container 31b. The air flow path 40 includes an opening 40a through which air flows out. The vacuum pump 34 evacuates through an exhaust line 45 communicating with the opening 31h provided in the bottom 31c of the vacuum nitriding vessel 31a. The temperature detector 37 passes through the bottoms 31c and 31d of the vacuum nitriding vessel 31a and the outer vessel 31b and the plasma electrodes 33a and 33b and is connected to a temperature sensor 37b (for example, a thermocouple) through a signal line 37a. .

  The pulsed plasma power source 33 receives a control signal from the process control device 42 and is turned on / off. Each stainless steel 44 has a potential difference corresponding to the voltage supplied from the pulse plasma power supply 33 with respect to the ground side (for example, the inner wall 31i of the vacuum nitriding vessel 31a). The gas supply device 32, the vacuum pump 34, the temperature detector 37 and the gas pressure sensor are also controlled by the process control device 42, and the process control device 42 is operated by the personal computer 43.

  The plasma nitriding used in the embodiment of the present invention will be described in more detail. First, stainless steel 44, which is an object to be processed, is placed in the nitriding furnace 31, and the inside of the furnace is evacuated to a vacuum of 1 [Torr] (= 133 [Pa]) or less. Next, after introducing a mixed gas of hydrogen gas and argon gas into the nitriding furnace 31, the stainless steel 44 has a vacuum degree of several [Torr] to several tens [Torr] (665 [Pa] to 2128 [Pa]). Is applied as a cathode, and the inner wall 31i of the vacuum nitriding vessel 31a as an anode. In this case, glow discharge is generated on the stainless steel 44 as the cathode, and the stainless steel 44 is heated and nitrided by the glow discharge.

  As a manufacturing method according to an embodiment of the present invention, as a first step, sputter cleaning is performed to remove the nonconductive film on the surface of the base material 44 made of stainless steel. During this sputtering cleaning, hydrogen ions, argon ions, etc., ionized by the introduced gas collide with the sample surface, so that the oxide film mainly composed of Cr on the surface of the stainless steel 44 can be removed.

As a second step, after sputtering cleaning, a mixed gas of hydrogen gas and nitrogen gas is introduced into the nitriding furnace 31, and a voltage is applied to generate glow discharge on the substrate 44 which is a cathode. At this time, the ionized nitrogen collides, penetrates and diffuses on the surface of the base material 44, whereby a continuous nitride layer having an M ₄ N type crystal structure is formed on the surface of the base material 44. Simultaneously with the formation of the nitride layer, the oxide film formed on the substrate surface is removed by a reduction reaction in which ionized hydrogen reacts with oxygen on the substrate surface.

  In the first step and the second step, extremely high-temperature ions such as hydrogen, argon, and nitrogen collide and invade the surface of the base material, and local micro regions on the base material surface are strongly heated. Then, metal atoms of alloy elements such as Fe, Cr, and Mo contained in the base material are separated by the sputtering action and evaporated. Metal atoms of alloy elements such as Fe, Cr, and Mo separated and evaporated from the stainless steel surface are combined with highly activated nitrogen present in the plasma near the substrate surface, and then precipitate as nitride on the substrate surface. .

  When the treatment temperature in the first step and the second step is relatively low, that is, 400 [° C.] or less, the collision energy when ions such as hydrogen, argon, nitrogen collide with the substrate surface is low. Therefore, there is little separation and evaporation by the sputtering action of metal atoms of alloy elements such as Fe, Cr, and Mo contained in the base material. For this reason, the surface of the nitrided layer formed by nitriding is smooth, and the amount of precipitates deposited as protruding nitrides protruding from the surface is small or hardly precipitated. On the other hand, when the treatment temperature is 400 [° C.] or higher, the collision energy of ions such as hydrogen, argon, nitrogen and the like on the substrate surface increases, so an alloy such as Fe, Cr, Mo contained in the substrate Separation and evaporation due to the sputtering effect of elemental metal atoms increase. For this reason, the amount of the nitride of the projecting nitride deposited on the substrate surface increases. When the easiness of evaporation of metal atoms of Fe, Cr, and Mo is compared with the melting point, Fe is 1539 [° C.] and Cr is 1900 [° C.], whereas Mo is as high as 2622 [° C.] and evaporates. It turns out to be difficult.

It is preferable that the Mo concentration of the base material containing the austenitic stainless steel having a Cr concentration of 25 [%] or more to be nitrided is 0.5 [%] or more and 3.0 [%] or less. By increasing the Mo content, the second nitride layer comprises mainly hexagonal Cr ₂ N, CrN and M _2-3 N type crystal structures, and cubic M ₄ N type crystal structures, A nitride layer having a structure in which precipitates protruding from the surface portion irregularly protrude is obtained on the surface portion of the nitride layer.

  If the Mo content is increased to some extent, the Mo itself is difficult to evaporate, and the lattice strain is increased in a substitution type, so that an fcc crystal lattice is formed. In addition, the addition of Mo has an effect of easily separating Fe and Cr from the lattice, so that the number of granular nitrides increases. Furthermore, since the addition of Mo has the effect of reducing grain boundary segregation such as Cr, desired projection-like precipitates are uniformly dispersed within the crystal grain boundaries and grains. The effect of addition of Mo is more likely to appear from 0.5 [%] or more, and when it exceeds 3.0 [%], the σ phase is precipitated at the grain boundaries, so the corrosion resistance is deteriorated. There is a fear.

In this plasma nitriding, the reaction on the surface of the base material 44 is not an equilibrium reaction but a non-equilibrium reaction. In addition, the surface of the base material 44 is treated at a temperature of 400 [° C.] to 500 [° C.]. Thus, a transition metal nitride containing an M ₄ N-type crystal structure having a high nitrogen concentration in the depth direction can be rapidly obtained, and this nitride is rich in conductivity and corrosion resistance.

On the other hand, when using a nitriding method in which nitriding proceeds at atmospheric pressure and by an equilibrium reaction, such as a gas nitriding method, it is difficult to remove the non-conductive film on the surface of the substrate, and because of the equilibrium reaction, It takes a long time to obtain the M ₄ N-type crystal structure on the surface of the base material, and it becomes difficult to obtain a desired nitrogen concentration. For this reason, the conductivity deteriorates due to the presence of an oxide film on the surface of the substrate, and the chemical stability is lacking. Therefore, it is difficult to maintain the conductivity in a strongly acidic atmosphere with the nitride and nitride layer obtained by this nitriding method. It becomes.

  In the embodiment of the present invention, it is preferable to use a pulsed plasma power source as the power source. As a power source used for the plasma nitriding method, it is common to use a DC power source having a DC waveform that is applied with a DC voltage, this discharge current is detected by a current detector, and is controlled by a thyristor so as to be a predetermined current. . In this case, glow discharge is continuously continued, and when the substrate temperature is measured with a radiation thermometer, the substrate temperature changes in a range of about ± 30 [° C.]. On the other hand, the pulse plasma power source is composed of a high-frequency cutoff circuit using a DC voltage and a thyristor, and this circuit makes the DC power source waveform a pulse waveform in which glow discharge is repeatedly turned on and off. In this case, when the substrate temperature is measured by a radiation thermometer by performing plasma nitridation using a pulsed plasma power source that repeatedly discharges and shuts off the plasma discharge time and plasma cut-off time from 1 to 1000 [μsec]. The change in substrate temperature is in the range of about ± 5 [° C.]. In order to obtain a transition metal nitride having a high nitrogen concentration, precise temperature control of the substrate temperature is required. Therefore, a pulsed plasma power source with a small change in the substrate temperature is used. It is preferable that the discharge and interruption of the plasma can be repeated with a period of μsec].

Further, the nitriding treatment is preferably performed at a base material temperature of 400 [° C.] or more and 500 [° C.] or less. 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 Cr nitride mainly precipitates as a sub- [μm] thick layer or lump exceeding several [nm] level. The corrosion resistance of the fuel cell separator is reduced due to the Cr-deficient layer formed in a part of the nitride layer. In contrast, when nitriding is performed at a temperature of 400 [° C.] to 500 [° C.], a cubic M ₄ N type crystal structure is formed continuously on the base layer formed by the base material. A first nitride layer having a nano-level stacked crystal structure including a nitride having a hexagonal M _2-3 N-type crystal structure and a first nitride layer continuously formed on the first nitride layer A nitride having at least one of a hexagonal Cr ₂ N, CrN and M _2-3 N type crystal structure, and a cubic M ₄ N type crystal structure, As the surface nitriding portion, a transition metal nitride provided with a second nitride layer continuously formed in the depth direction from the surface of the substrate is obtained.

  Note that when the nitriding temperature is lower than 400 [° C.], a long time treatment is required to obtain the first nitride layer, and thus the productivity is deteriorated. Therefore, nitriding is preferably performed in the range of 400 [° C.] to 500 [° C.].

  Further, in order to obtain a nitride enriched with Cr and N, the stainless steel used as the base material preferably has a high Cr concentration, and in particular, the Cr concentration is preferably 25 [%] or more. Since Cr has a high affinity with nitrogen, the use of stainless steel with a high Cr concentration results in the formation of nitrides at other locations where plasma is difficult, such as groove portions of the substrate, during plasma nitriding. The

  Next, an oxidation process using an acid after plasma nitriding will be described.

The surface of the nitrided layer (the surface of the second nitrided layer) is immersed in a strong acidic aqueous solution having a pH of 2 to 4 in an open air environment by plasma nitriding a base material containing stainless steel and nitriding. Oxidation with acid. When immersed in an open atmosphere with a high oxygen partial pressure, Fe-based transition metal nitrides such as M ₄ N type and Cr-based transition metal nitrides such as CrN hardly dissolve even in strongly acidic aqueous solutions. Even in an open atmosphere, when immersed in a strongly acidic aqueous solution having a pH value lower than pH 2 (particularly about pH 1), the transition metal nitride mainly composed of Fe such as M ₄ N type transition metal nitride composed mainly of Cr such as CrN. It elutes more preferentially and dissolves part of the granular nitride, reducing the surface area of the granular nitride. For this reason, the contact area between the nitride layer and the GDL is reduced, and the conductivity is deteriorated. On the other hand, when immersed in a solution having a pH value higher than pH 4 even in an open atmosphere, a natural oxide film that is not dense is formed on the granular nitriding surface. A thick oxide film is formed. For this reason, electroconductivity deteriorates.

  Thus, by oxidizing the surface of the nitride layer with a strong acidic aqueous solution having a pH of 2 to 4 in an open atmosphere, the nitride layer surface after nitriding is chemically stabilized. And since it becomes possible to maintain the initial state after nitriding, it will be excellent in corrosion resistance. Further, the contact area with the GDL is ensured without reducing the surface area of the nitride layer, in particular, the protruding nitride, and thus a fuel cell separator having further excellent conductivity and corrosion resistance can be obtained.

  As described above, according to the method for manufacturing the transition metal nitride and the fuel cell separator according to the embodiment of the present invention, the nitride layer including the transition metal nitride excellent in corrosion resistance and conductivity is formed. A separator is obtained. In addition, it is possible to manufacture a low-cost fuel cell separator by a simple operation.

(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. 7 is a diagram showing the external appearance of an electric vehicle equipped with a fuel cell stack. FIG. 7A is a side view of the electric vehicle, and FIG. 7B is a top view of the electric vehicle. As shown in FIG. 7B, in addition to the left and right front side members and the hood ridge, a dash lower member that connects the left and right hood ridges including the front side member to each other is combined and welded to the front of the vehicle body 51. 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 7 will be described. Each example is an examination of the effectiveness of the separator for a fuel cell according to the present invention. Each sample was prepared by treating the raw material under different conditions, and is limited to the illustrated example. It is not a thing.

<Preparation of sample>
In each Example and Comparative Example, a vacuum annealed material having a thickness of 0.1 [mm] and □ 100 × 100 [mm] adjusted with the composition shown in Table 1 was used as a base material. This base material was press-molded to form a fuel or oxidant passage to obtain a separator. The separator obtained by press molding was pickled and then subjected to plasma nitriding by micropulse direct current glow discharge on both sides. The conditions of plasma nitriding are as follows: nitriding temperature is 300 to 550 [° C.], nitriding time is 60 [min], gas mixing ratio during nitriding N ₂ : H ₂ = 7: 3, processing pressure 3 [Torr] (= 399 [Pa] ]). The sample of Comparative Example 1 was not plasma-nitrided. In Comparative Example 2, gas nitriding was performed instead of plasma nitriding. In Comparative Example 3, an austenitic stainless steel having a Cr concentration of 18% was used. In Comparative Example 4, nitriding was performed at 300 [° C.]. In Comparative Example 5, nitriding was performed at 550 [° C.]. In Comparative Example 6, a dissolution treatment with an acid in a degassed environment of pH 4 was performed instead of the oxidation treatment with an acid. In Comparative Example 7, neither an oxidation treatment with an acid nor an acid dissolution treatment with an acid was performed, and the plasma nitridation was used as it was.

Table 1 shows the amounts of Cr, Ni, and Mo of the stainless steel used as the base material, the presence or absence of plasma nitriding, the plasma power source used, and the control temperature during nitriding.

  Each obtained sample was evaluated using the following method.

<Identification of crystal structure of base layer>
The crystal structure of the base layer was identified by performing X-ray diffraction measurement on the substrate surface modified by nitriding. 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].

<Observation of nitride layer / Crystal structure identification of top surface nitride, morphology observation, nitride layer thickness, maximum height measurement of protruding nitride>
The nitride layer of the sample obtained by the above method was observed. In the observation method, a thin film sample near the surface for TEM observation was prepared using a FIB-μ sampling method using a focused ion beam apparatus (FIB) FB2000A manufactured by Hitachi, and a field emission transmission electron microscope (Hitachi, Ltd.). HF-2000) manufactured at 200 [kV].

<Projected nitride element mapping protruding from the nitride layer surface>
The surface of the sample was sputtered with an electron beam current value of 10 [nA] and Ar ion sputtering of 1 [kV] using a field emission Auger electron spectroscopy analyzer (FE-AES (PHI SAM-700)), and a sputtering time of 1 [ The surface of the sample after [minute] was observed.
<Measurement of amount of nitrogen and oxygen in nitride layer>
Nitrogen and oxygen content of the nitride layer, that is, measurement of nitrogen content and oxygen content in the outermost layer of the nitride layer by depth profile measurement of Auger electron spectroscopy in the range from the surface of the nitride layer to a depth of 200 [nm] Went. Fe, Ni at the outermost surface, that is, 0 [nm] depth, 2 [nm] depth, 5 [nm] depth, 10 [nm] depth, 50 [nm] depth, 100 [nm] depth, The amounts of Cr, N and O are shown in Table 2. For the measurement, a scanning Auger electron spectrometer (MODEL4300 manufactured by PHI) was used. Electron beam acceleration voltage 5 [kV], measurement area 20 [μm] × 16 [μm], ion gun acceleration voltage 3 [kV], was carried out under the conditions of sputtering rate 10 [nm / min] (SiO 2 converted value).

<Measurement of contact resistance>
The samples obtained in Examples 1 to 6 and Comparative Examples 1 to 7 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. 8A, a carbon paper 63 is interposed between the electrode 61 and the sample 62, and as shown in FIG. 8B, the electrode 61a / carbon paper 63a / sample 62 / carbon. The configuration is paper 63b / electrode 61b. Then, the electrical resistance when a current of 1 [A / cm ² ] was passed at a measurement surface pressure of 1.0 [MPa] was measured twice, and the average value of each electrical resistance was obtained to obtain the contact resistance. The contact resistance value is measured twice before and after a corrosion resistance test, which will be described later, and the contact resistance after the corrosion resistance test evaluates the corrosion resistance by simulating the environment where the fuel cell separator is exposed in the fuel cell stack. It is a thing. 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.

<Oxidation treatment with acid>
The sample was immersed in an aqueous sulfuric acid solution having a pH of 4 and 80 [° C.] for 100 [hours], then the surface was washed and dried.

<Image analysis>
The sample surface was observed with an electron microscope. For the observation, a field emission scanning electron microscope (Hitachi, Ltd. S-4000 type) was used. The sample was cut to a size of about 5 [mm] square, and the surface was washed with ethanol and observed. The magnification of observation was 10,000 times. A scanning electron microscope image was obtained as digital data of 2080 pixels × 1650 pixels. This visual field corresponds to a range of 12 [μm] × 7.5 [μm] in actual dimensions.

  An attempt was made to quantify the distribution of the size of the protrusion-like precipitates by using an image analysis technique on the electron microscope image. For the analysis, image analysis software “A image-kun” (Asahi Kasei Engineering Co., Ltd.) was used.

  First, the image size was once reduced to a half of 1040 pixels × 825 pixels. Further, a range of 1039 pixels × 764 pixels was appropriately cut out from the reduced image. This is an operation for removing characters such as scales and notes included in the original electron microscope image from the quantitative range.

  The “equivalent circle diameter” and “area” of the extracted region were output by the “particle analysis” operation of “A image” for an image of 1039 pixels × 764 pixels. The parameter settings in the analysis are as follows. The equivalent circle diameter, as the name implies, is an amount defined for a graphic having a certain area, and means the diameter of a circle having the same area.

<Parameters for analysis>
Particle brightness Brightness Method of binarization Automatic Range specification None Outer edge correction None Fill in hole None Small figure removal 500 nm ² (Particles less than this area are not counted)
Correction method Shrinkage Shrinkage separation Number of times 100 Small figure 10 Contact degree 1000
Noise reduction filter Yes Shading Yes Shading size 20
Result display nm
An image obtained by binarizing the measurement region was obtained by image analysis. The obtained data was statistically processed with spreadsheet software, and the number of protruding precipitates and the total area of the protruding precipitates in each segment were determined for each appropriate circle equivalent diameter section.

<Evaluation of corrosion resistance (corrosion resistance test after oxidation treatment)>
In the fuel cell, a potential of about 1 [Vvs SHE] is applied to the oxygen electrode side from the natural potential at the maximum compared to the hydrogen electrode side. In addition, the solid polymer electrolyte membrane is a polymer electrolyte membrane having a proton exchange group such as a sulfonic acid group in the molecule, saturated with water and utilizing proton conductivity, and exhibits strong acidity. Therefore, the corrosion resistance is evaluated by measuring the amount of metal ions dissolved in the solution after holding for a certain period of time using an immersion test, which is an electrochemical technique, and measuring the corrosion resistance from the value of the metal ion elution amount. The degree of decrease was evaluated.

Specifically, first, a sample obtained by cutting out the central portion of each sample after the oxidation treatment into a size of 30 [mm] × 30 [mm] is prepared, and the prepared sample is heated in a sulfuric acid aqueous solution of pH 4 at a temperature of 80 [ [° C.] and 100 [hour]. The atmosphere at that time was N ₂ gas degassing by simulating the anode electrode environment, and the atmosphere was opened to the atmosphere by simulating the cathode electrode environment. Thereafter, the elution amounts of Fe, Cr and Ni dissolved in the sulfuric acid aqueous solution were measured by fluorescent X-ray analysis.

Crystal structure, structure, form, layer thickness of layered nitride, maximum height of protruding nitride protruding from surface of nitride layer formed by nitriding in Examples 1 to 6 and Comparative Examples 1 to 7 , Protrusion structure, area ratio per unit area of protrusion nitride protruding from the surface by image analysis, and unit area of protrusion nitride protruding from the surface having an equivalent circular diameter of 40 [nm] or more 100 [μm ² ], Fe, Cr, O, N amount [mol%] up to 4 [nm] depth by X-ray photoelectron spectroscopy (XPS), contact resistance before and after corrosion test after oxidation, contact before and after corrosion test The measurement results of the resistance difference and the elution amounts of Fe, Ni and Cr ions in the test solution are shown in Tables 2 to 4, respectively.

  In the sample of Comparative Example 1, a nitrided layer is not formed, and a thick passive film is formed on the surface. For this reason, in the corrosion resistance test under the anode condition and the cathode condition, the contact resistance before and after the test showed a high value. Regarding ion elution, since a thick passive film was formed on the substrate surface, Fe, Ni, and Cr ions were hardly eluted under both anode conditions and cathode conditions.

In the sample of Comparative Example 2, gas nitriding was used as the nitriding method, and a thick oxide film was formed on the surface of the nitrided layer due to the treatment at atmospheric pressure. Since this nitride layer contains a nitride having a high oxygen concentration and a low nitrogen concentration, the contact resistance before and after the test showed a high value in the corrosion resistance test under the anode condition and the cathode condition. Regarding ion elution, since a thick passive film was formed on the surface, Fe, Ni and Cr ions were hardly eluted under both anode conditions and cathode conditions.
In the sample of Comparative Example 3, SUS316L was used as the base material. In this case, since the Cr content of the substrate is as low as 18 [wt%], the composition of the second nitride layer including the protruding nitride is an M ₄ N type single phase mainly composed of Fe. The surface of the second nitride layer is an Fe oxide film, and the ratio of Cr to Fe is low. The oxygen (O) content in the oxide film is high and the nitrogen (N) content is low. Further, the area ratio of the protruding nitride is low. For this reason, under the anode conditions, the elution amount of metal ions of Fe, Ni and Cr slightly increased, and the contact resistance after the corrosion resistance test also slightly increased. In the cathode condition, the oxygen partial pressure in the solution was higher than that in the anode condition, so an oxide film was formed on the surface. For this reason, the metal ion elution amount of Fe, Ni, and Cr was slightly increased as compared with the anode condition, and the contact resistance after the corrosion resistance test was slightly increased.

  In the sample of Comparative Example 4, since the nitriding temperature was as low as 300 [° C.], the nitrided layer was not formed at the nitriding time of 60 [minutes], and the substrate surface remained as it was. Thus, in Comparative Example 4, a thick passive film was present on the surface of the sample. Further, since the collision energy of N ions on the substrate surface is low, the amount of N atoms implanted into the substrate surface portion is low, and the sputtering effect on the surface portion of the substrate is low. For this reason, it is thought that the amount of Fe atoms separated from the substrate surface was small and the amount of iron nitride adsorbed on the substrate surface was also small. Thus, the nitrogen concentration was low on the surface of the nitride layer, and the surface of the nitride layer was a smooth shape with almost no granular nitride. For this reason, the surface area of the nitride layer is small, the contact area with the GDL is reduced, and as a result, the contact resistance is increased. Further, in the corrosion resistance test under the anode condition and the cathode condition, the contact resistance before and after the test showed a high value. Since a thick passive film was formed on the surface, Fe, Ni, and Cr ions were hardly eluted under both anode conditions and cathode conditions.

  In the sample of Comparative Example 5, since the nitriding temperature was as high as 550 [° C.], large protruding CrN having an average particle diameter exceeding 150 [nm] was formed on the surface layer. For this reason, a Cr-deficient layer was formed in a part of the nitride layer, and Fe, Ni, and Cr ions were eluted during the corrosion resistance test under the anode condition, and the corrosion resistance was deteriorated. Further, the contact resistance after the test showed a high value due to the thick oxide film generated during the corrosion resistance test. Further, in the corrosion resistance test under the cathode condition, since the oxygen partial pressure in the solution was higher than that under the anode condition, a thick oxide film was formed particularly on the surface near the Cr-deficient layer. For this reason, although it was difficult to elute ions as compared with the anode conditions, the contact resistance increased.

On the other hand, in each sample of Examples 1 to 6, the contact resistance after the corrosion resistance test under the anode condition and the cathode condition is low, and the elution amounts of Fe, Ni, and Cr ions are small. This is because Cr having high affinity with N is concentrated on the surface by lattice vibration by oxidizing with acid after nitriding, and many N atoms are implanted or invaded at the surface portion of the substrate by plasma nitriding. This is thought to be because Cr and N atoms were bonded to form CrN and Cr ₂ N Cr nitrides having a close-packed hexagonal lattice crystal structure on the surface of the substrate. In addition, as a result of plasma nitriding, N ions on the surface of the base material are accelerated and collided by voltage, and as a result, they are strongly heated in a very small area. At the same time, metal atoms such as Fe, Cr and Mo of the base material are sputtered (evaporated). Therefore, the most abundant Fe element in the surface portion of the substrate is separated from the sample surface by the sputtering action, and the separated Fe atoms are combined with highly activated nitrogen in the plasma near the sample surface. Then, it is absorbed as iron nitride on the sample surface by absorption.

When the substrate temperature during plasma nitriding is 425 to 500 [° C.], which is a relatively high temperature, the collision effect of N ions on the substrate surface is high, so the sputtering effect on the substrate surface portion is high, and the sample surface The amount of Fe atoms separated from the iron is large and the amount of iron nitride adsorbed on the surface is also large. Therefore, a high nitrogen nitride having a crystal structure of any one of a projecting hexagonal M _2-3 N and cubic M ₄ N type is deposited on the surface of the substrate.

  In addition, Cr / Fe, which is the Cr ratio to Fe, is high, the O content is 30 or less, and the N content is high. Even in the environment, the movement of electrons is not hindered, conductivity is maintained, and ion elution is further suppressed.

Next, FIG. 9 shows the result of SEM image observation by a scanning electron microscope showing the state of the sample surface before and after the treatment with acid. FIG. 9A shows the surface of Comparative Example 7 in which only plasma nitriding was performed at 450 [° C.]. In FIG. 9B, after plasma nitriding, the sample was immersed in an aqueous sulfuric acid solution having a pH of 4 and 80 [° C.] for 100 [hour] in the open atmosphere, and then the surface was washed and then oxidized with a dried acid. It is the surface of Example 2. FIG. 9 (c) shows the surface of Comparative Example 6 which was immersed in a sulfuric acid aqueous solution having a pH of 4 and 80 [° C.] degassed with N ₂ for 100 [hours], washed the surface, and then subjected to an acid dissolution treatment with a dried acid. is there. The protruding nitride indicated by 9A in FIG. 9A is subjected to either oxidation treatment with acid or acid dissolution treatment with acid, as indicated by 9B in FIG. 9B and 9C in FIG. 9C. There are the same number as before.

  The distribution of the size of the protruding nitrides 9A, 9B, 9C was examined. FIG. 10 shows a graph arranged with respect to the area ratio of the equivalent circular diameter of the protruding nitrides 9A, 9B, and 9C. In FIG. 10, 10A indicates the distribution of the protruding nitride 9A, 10B indicates the distribution of the protruding nitride 9C, and 10C indicates the distribution of the protruding nitride 9B. As shown in FIG. 10, the size of the protruding nitride 9B obtained by the oxidation treatment with acid shows almost the same distribution as that before the treatment (9A) (10A, 10C). On the other hand, it has been found that the size of the protruding nitride 9C obtained by the acid dissolution treatment with an acid is high as shown in 10B, although the equivalent circular diameter is large.

  From this distribution, FIG. 11 (a) shows a comparison of the surface after oxidation with an acid after nitriding and the surface after acid dissolution with an acid by area ratio, and FIG. 11 (b) shows a comparison with the number of particles. . As shown in FIG. 11 (a), the area ratio of the protruding nitride is 11A after nitriding, whereas it becomes as high as 11B after acid dissolution treatment, and the area ratio 11C after oxidation treatment. Is substantially the same as the area ratio 11A after nitriding. Further, as shown in FIG. 11 (b), the number of particles of the protrusion-like nitride is 11D after nitriding, whereas it becomes as high as 11E after acid dissolution treatment, and the particles after oxidation treatment. The number 11F is substantially the same as the number 11D of particles after nitriding. As described above, the area ratio and the number of particles of the protruding nitride are almost the same as after the nitriding (that is, before the oxidation treatment) by the oxidation treatment with the acid, but after the dissolution treatment with the acid, compared with that before the treatment. The area ratio and the number of particles increased. This indicates that, as shown in FIG. 12, a part of the protruding nitride is melted, and the equivalent circular diameter is increased by increasing the thickness of the oxide film on the nitride surface. It was also found that the number of particles that can be observed increased because the equivalent circular diameter at the tip of the protrusion-like nitride was increased by the dissolution and oxidation of the acid.

Next, FIG. 12 is a diagram in which the surface of Example 2 obtained by further oxidizing the sample after nitriding and the surface of Comparative Example 6 obtained by acid dissolving treatment with acid are observed. Show. 12A and 12B show a thin film sample prepared by FIB-μ sampling method using a focused ion beam apparatus (FIB) for the obtained sample, and is obtained by a field emission transmission electron microscope (FE-TEM). This is a result of cross-sectional observation. Figure 12 (a), the surface of the nitrided layer surface after oxidation treatment with acid, protruding nitrides 12B ₁ of CrN principal to the surface of the M _{4 N} type crystal structure of Fe entities indicated by 12C protrudes It has been deposited Te, a structure in which the layer 12A of the M _{4 N} type crystal structure further Fe principal on the 12B ₁ is formed. On the other hand, as shown in FIG. 12B, the surface of the nitrided layer surface after the acid dissolution treatment with an acid is formed in a projective nitridation mainly composed of CrN on the surface of the M ₄ N type crystal structure mainly composed of Fe shown by 12C. This is a structure in which the product 12B ₂ protrudes and precipitates and the Fe-based M ₄ N type crystal structure layer 12A is not dissolved. Thus, the protruding nitride protruding from the surface of the nitride layer formed by plasma nitriding has a M ₄ N type crystal structure whose surface layer is mainly Fe, and has a double structure with CrN inside. After the acid dissolution treatment with acid, the M ₄ N type mainly composed of Fe on the surface layer is eluted, and only the inner CrN remains. Although not observable in FIGS. 12A and 12B, an oxide film is formed on the surface of each protruding nitride.

Thus, after the acid dissolution treatment with acid, the M ₄ N type crystals mainly composed of Fe on the surface of the protruding nitride elute, so that the height of the protruding nitride is reduced, and the fuel cell separator and In contact with carbon paper made of carbon fibers of submicron units, precipitates enter between the carbon fiber gaps, and the actual contact area is reduced. After the acid dissolution treatment with acid, the M ₄ N type crystal mainly composed of Fe on the surface of the protruding nitride is eluted to expose CrN, and a thin oxide film is formed on the surface layer of CrN. For this reason, the surface area of the projection-like nitride is increased by the amount of oxide film formed and grown on the surface. In addition, due to the acid dissolution treatment with acid, oxide growth occurs with a minute nitride that is not detected in the image analysis after nitriding as a nucleus, and as a result, the equivalent circular diameter per area of 100 [μm ² ] is about The number of protrusion-shaped nitrides of 40 [nm] or more also increases.

Next, FIG. 13 shows the structure of the protruding nitride. FIG. 13A shows a Cr-based nitride 13B mainly composed of Cr and a Fe-based nitride 13C mainly composed of Fe formed thereon on the surface of the nitrided layer (second nitrided layer) 13A. The nitride is composed of two layers, the surface of which is covered with a Cr oxide film 13D. FIG. 13B shows a Fe-based nitride 13C mainly composed of Fe and a Cr-based nitride 13B mainly composed of Cr formed thereon on the surface of the nitrided layer (second nitrided layer) 13A. The nitride is composed of two layers, the surface of which is covered with a Cr oxide film 13D. FIG. 13 (c) shows a Cr-based nitride 13B mainly composed of Cr on the surface of the nitrided layer (second nitrided layer) 13A and Fe-based nitriding mainly composed of Fe formed in contact therewith. The nitride is composed of two layers of the product 13C and the surface thereof is covered with a Cr oxide film 13D. FIG. 13D shows a nitride layer (second nitride layer) 13A having a surface made of Cr-based nitride 13B containing Cr as a main component and the surface covered with a Cr oxide film 13D. Is shown. In FIG. 13E, the nitride layer (second nitride layer) 13A is made of Fe-based nitride 13C mainly composed of Fe, and the surface is covered with an oxide film 13E of Fe. Show. The Cr-based nitride 13B has a hexagonal CrN crystal structure. The Fe-based nitride 13C has a cubic M ₄ N type crystal structure mainly composed of Fe.

In Example 1 to Example 6, as shown in FIGS. 13A to 13C, the protrusion-like nitride is composed of a nitride having an Fe-based cubic M ₄ N type crystal structure and a Cr-based nitride. It was found that a plurality of hexagonal CrNs gather to form an aggregate. On the other hand, in the comparative example, it was the structure shown to FIG.13 (d), (e). In the embodiment, the structure of the projecting nitride formed on the surface of the second nitride layer includes a nitride having a Fe-based cubic M ₄ N type crystal structure and a Cr-based hexagonal CrN. Aggregates are formed to form Cr, and Cr is exposed on the surface of the nitride. Therefore, when Cr is exposed on the surface of the nitride, a Cr oxide film is easily formed on the surface of the nitride. It was thought that. Since this Cr oxide film was chemically stable, it was considered that the contact resistance before and after the corrosion resistance test was low, and the ion elution amount was suppressed.

As shown in Table 2, in each of the samples of Examples 1 to 6, the nitride layer adjacent to the base layer has M ₄ N type and M _{2 to 3} N precipitates at intervals of several 10 to 100 [nm]. A composite structure laminated on the M ₄ N matrix was formed on the substrate surface. By nitriding the surface of the stainless steel used as the base material, a nitrided layer was formed in the depth direction of the surface of the base material, and a base layer, which is an unnitrided layer that was not nitrided, was formed immediately below the nitrided layer. The nitride layer includes a surface portion of the nitride layer and a first nitride layer and a second nitride layer formed adjacent to the base layer, and the second nitride layer is a two-phase composite in which a layered structure is repeated A texture was observed, and it was revealed that the matrix was a matrix of M ₄ N type crystal structure and a layered M _2-3 N type crystal structure formed in this matrix.

Thus, the reason why the electrochemical stability in the strong acid environment and the corrosion resistance in Examples 1 to 6 were excellent and the corrosion resistance was good was that the nitrided layer was hexagonal M ₂ N, M _2-3 N, nitride having a nitride having any crystal structure of the M ₄ N type MN type or cubic, the layered crystal structure of a nano-level consisting of cubic M ₄ N type and M _2-3 N-type hexagonal This is considered to be because the electron transfer between the base layer and the nitride is easy and the conductivity is excellent by being continuously present with a gas layer having a cubic crystal structure. Further, the surface portion of the nitride layer has a crystal structure of any one of hexagonal M ₂ N, M _2-3 N, MN type and cubic M ₄ N type, and the Cr amount and N amount are Many thin and stable passive films are formed on the nitride surface, and the amount of oxygen is relatively large. Therefore, as in the fuel cell separator environment, a high temperature and strongly acidic environment of 80 to 90 [° C.] Even underneath, it is considered that the movement of electrons is not hindered, the conductivity is maintained, and the ion elution is improved. The nano-level stacked crystal structure consisting of cubic M ₄ N type and hexagonal M _2-3 N type maintains the metal bond between transition metal atoms and is strong between transition metal atom and nitrogen atom. This is considered to be due to showing the covalent bond. In addition, it is considered that the transition metal atoms constituting the face-centered cubic lattice are irregularly mixed, so that the partial molar free energy of each transition metal component is lowered and the activity can be kept low. . In addition, when the nano-level fine layered structure is in two-phase equilibrium, the free energy is reduced, the activity can be kept low, the reactivity to oxidation is lowered, and the chemical stability is obtained. For this reason, it is thought that it will become excellent in corrosion resistance by suppressing oxidation especially in a strongly acidic atmosphere. Further, since a thin oxide film of several tens of nanometer level is formed on the outermost layer, it is considered that the corrosion resistance is improved without deteriorating the conductivity.

In addition, the amount of precipitate protruding from the surface portion of the second nitride layer after the oxidation treatment in the open atmosphere is an area ratio obtained by image analysis, which is a precipitation area of 10% or more of the measurement field of view. Above, the equivalent circular diameter is 45 [nm] or more, and it is uniformly distributed over the surface area and more than 800 [pieces] per 100 [μm ² ] area in the measurement field of view. In contact with carbon paper made of carbon fiber, precipitates enter into the gaps between the carbon fibers, and the contact area with the GDL increases, which is considered to be excellent in conductivity.

In a fuel cell, the theoretical voltage per unit cell is 1.23 [V]. However, the voltage that can be actually extracted drops due to reaction polarization, gas diffusion polarization, and resistance polarization, and the voltage increases as the extracted current increases. Descent. Further, in automotive applications, since it is desired to increase the output density per unit volume / weight, it is used at a higher current density side than stationary use, for example, at a current density of 1 [A / cm ² ]. Therefore, when the current density is 1 [A / cm ² ], the contact resistance between the separator and the carbon paper is 20 [mΩ · cm ² ], that is, the measured value with the apparatus shown in FIG. If it is [mΩ · cm ² ] or less, it is considered that the efficiency reduction due to contact resistance can be suppressed. In all of Examples 1 to 6, since the contact resistance is 30 [mΩ · cm ² ] or less, the fuel cell has high electromotive force per unit cell, excellent power generation performance, downsizing and cost reduction. A stack can be formed.

  From the above measurement results, Examples 1 to 6 show low contact resistance as compared with the comparative example, and moreover, the amount of ion elution is small and the corrosion resistance is excellent. Therefore, both low contact resistance and corrosion resistance are obtained. At the same time, it was shown to be combined.

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 an expansion schematic diagram of the principal part of FIG.3 (c). (B) It is a typical sectional view of the 2nd nitride layer of the separator for fuel cells. (A) is a schematic diagram showing an _{M 4} N type crystal structure. (B) M _2-3 N type hexagonal crystal structure. It is a typical side view of the nitriding apparatus used for the manufacturing method of the separator for fuel cells which concerns on embodiment of this invention. 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) The cross-sectional structure | tissue photograph by TEM of the magnification of 100,000 times of the sample obtained by the comparative example 7. FIG. (B) It is the cross-sectional structure | tissue photograph by TEM of the magnification of 100,000 times of the sample obtained by Example 2. FIG. (C) It is a cross-sectional structure | tissue photograph by TEM of the magnification of 100,000 times of the sample obtained by the comparative example 6. It is a graph which shows the relationship between the equivalent circular diameter of protrusion-like nitride, and the area ratio of nitride. (A) It is the figure which compared the nitride layer surface after the nitridation, the oxidation process by an acid, and the acid dissolution by an acid by the area ratio. (B) It is the figure which compared the nitrided layer surface after the nitriding, the oxidation process by an acid, and the acid dissolution by an acid by the number of particles. (A) It is the result of having observed the cross section of the sample obtained in Example 2 by FE-TEM. (B) It is the result of having observed the cross section by FE-TEM after the dissolution process by the acid of the sample obtained in the comparative example 6. FIG. (A) It is a figure which shows the structure of protrusion-like nitride. (B) It is a figure which shows the structure of protrusion-like nitride. (C) It is a figure which shows the structure of protrusion-like nitride. (D) It is a figure which shows the structure of protrusion-like nitride. (E) It is a figure which shows the structure of protrusion-like nitride. 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 11 Nitride layer 12 Base layer 20 M ₄ N-type crystal structure 21 Transition metal atom 22 Nitrogen atom

Claims (9)

A transition metal nitride obtained by nitriding a base material containing austenitic stainless steel,
The transition metal nitride is continuously formed on a base layer formed by the base material, and has a cubic M ₄ N type crystal structure and a hexagonal M _2-3 N type crystal. A first layer having a nano-level stacked crystal structure including a nitride having a structure;
At least one of a hexagonal Cr ₂ N, CrN and M _2-3 N type crystal structure and a cubic M ₄ N type crystal structure formed continuously on the first layer. A second layer formed continuously from the surface of the base material in the depth direction as a surface nitriding portion of the base material, including a nitride having a crystal structure of
The second layer has a precipitate protruding from the surface portion of the second layer, and the precipitate protruding from the surface portion of the second layer is an Fe-based cubic M ₄ N type crystal. A transition metal nitride comprising an aggregate of a plurality of nitrides having a structure and Cr-based hexagonal CrN and having a Cr oxide film on a surface thereof. A base layer formed from a substrate comprising austenitic stainless steel;
A fuel cell separator comprising the transition metal nitride nitride layer according to claim 1 formed directly on the base layer. A method for producing a transition metal nitride formed by nitriding a base material containing austenitic stainless steel,
A first surface having a nano-level stacked crystal structure including a nitride having a cubic M ₄ N type crystal structure and a nitride having a hexagonal M _2-3 N type crystal structure on the substrate surface. Forming a layer of
At least one of a hexagonal Cr ₂ N, CrN and M _2-3 N type crystal structure and a cubic M ₄ N type crystal structure formed continuously on the first layer. A second layer formed continuously from the surface of the base material in the depth direction as a surface nitriding portion of the base material, including a nitride having a crystal structure of
A method for producing a transition metal nitride, comprising: oxidizing the surface of the second layer with an acid after forming the second layer. A base material containing austenitic stainless steel is press-molded to form fuel or oxidant passages, the press-molded base material is nitrided, and a cubic M ₄ N type is formed on the surface of the base material. Forming a first layer having a nano-level stacked crystal structure including a nitride having a crystal structure and a nitride having a hexagonal M _2-3 N-type crystal structure;
At least one of a hexagonal Cr ₂ N, CrN and M _2-3 N type crystal structure and a cubic M ₄ N type crystal structure formed continuously on the first layer. A second layer formed continuously from the surface of the base material in the depth direction as a surface nitriding portion of the base material, including a nitride having a crystal structure of
After forming the second layer, the surface of the second layer is oxidized with an acid.   The method for producing a fuel cell separator according to claim 4, wherein the nitriding is a plasma nitriding method.   6. The fuel cell separator according to claim 5, wherein the plasma nitriding method uses a pulsed plasma power source that repeatedly discharges and shuts off, with a plasma discharge time and a plasma cut-off time being 1-1000 [μsec]. Manufacturing method.   The method of manufacturing a fuel cell separator according to claim 5 or 6, wherein the plasma nitriding method is performed in a state where the temperature of the base material is 400 [° C] or more and 500 [° C] or less.   A fuel cell stack using the fuel cell separator according to claim 2.   A fuel cell vehicle comprising the fuel cell stack according to claim 8 as a power source.