JP2007073429A - Method of manufacturing fuel cell stack and conductive porous member for fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a fuel cell stack provided with a gas diffusion layer having high corrosion resistant property, and a conductive porous member for a fuel cell serving as the gas diffusion layer. <P>SOLUTION: The fuel cell stack is composed of a film-electrode assembly 90 having the conductive porous members 93 formed on both faces of an electrolyte film 91 through a catalyst layer 92, and a fuel cell separator 3 arranged so as to contact the film-electrode assembly 90. The conductive porous members 93 is prepared by forming a nitride film on both surfaces of a substrate made of metal. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fuel cell stack and a method for producing a conductive porous member for a fuel cell, and more particularly to a conductive porous member serving as a solid polymer electrolyte type gas diffusion layer formed using stainless steel.

  From the viewpoint of protecting the global environment, it has been studied to use a fuel cell as a power source for a motor that operates in place of an internal combustion engine of an automobile, and to drive the automobile with this motor. Since this fuel cell does not require the use of fossil fuels that have a resource depletion problem, 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. 16 is a cross-sectional view showing a configuration of a single cell forming the fuel cell stack. As shown in FIG. 16, the single cell 80 has a membrane electrode assembly in which an oxygen electrode 82 and a hydrogen electrode 83 are joined and integrated on both sides of a solid polymer electrolyte membrane 81. The oxygen electrode 82 and the hydrogen electrode 83 have a two-layer structure including a reaction film 84 and a gas diffusion layer 85 (GDL: gas diffusion layer). The reaction film 84 is in contact with the solid polymer electrolyte film 81. On both sides of the oxygen electrode 82 and the hydrogen electrode 83, an oxygen electrode side separator 86 and a hydrogen electrode side separator 87 are provided for lamination. The oxygen electrode side separator 86 and the hydrogen electrode side separator 87 form an oxygen gas channel, a hydrogen gas channel, and a cooling water channel.

  In the unit cell 80 having the above-described configuration, the oxygen electrode 82 and the hydrogen electrode 83 are disposed on both sides of the solid polymer electrolyte membrane 81, and are usually joined together by a hot press method to form a membrane electrode assembly. The separators 86 and 87 are disposed on both sides of the membrane electrode assembly. In the fuel cell composed of the single cell 80, when a mixed gas of hydrogen, carbon dioxide, nitrogen and water vapor is supplied to the hydrogen electrode 83 side and air and water vapor are supplied to the oxygen electrode 82 side, An electrochemical reaction occurs at the contact surface between the molecular electrolyte membrane 81 and the reaction membrane 84. Hereinafter, a more specific reaction will be described.

  In the single cell 80 configured as described above, when oxygen gas and hydrogen gas are respectively supplied to the oxygen gas channel and hydrogen gas channel, the oxygen gas and hydrogen gas are supplied to the reaction film 84 side through the gas diffusion layers 85. Then, the following reactions occur in each reaction film 84.

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 83 side, the reaction of the formula (1) proceeds to generate H ⁺ and e ⁻ . H ⁺ moves through the solid polymer electrolyte membrane 81 in a hydrated state and flows to the oxygen electrode 82 side, and e ⁻ flows from the hydrogen electrode 83 to the oxygen electrode 82 through the load 88. On the oxygen electrode 82 side, the reaction of Formula (2) proceeds by H ⁺ , e ^−, and the supplied oxygen gas, and electric power is generated.

Here, since the gas diffusion layer has a function of electrically connecting the single cells, the gas diffusion layer is required to have good electrical conductivity and low contact resistance with a constituent material such as a separator. Further, the temperature of each gas supplied to the fuel cell is as high as 80 to 90 [° C.], and not only H ⁺ is generated at the hydrogen electrode, but the oxygen electrode through which oxygen, air, etc. pass is standard hydrogen. It is in an oxidizing environment in which a potential of about 0.6 to 1 [Vvs SHE] is loaded with respect to the extreme potential. For this reason, like the oxygen electrode and the hydrogen electrode, the gas diffusion layer is required to have corrosion resistance that can withstand in a strongly acidic atmosphere. In addition, the corrosion resistance required here means the durability which can maintain an electrical-conduction performance also in the oxidation environment where a gas diffusion layer is a strong acid. 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.

Here, a technique for forming a conventional gas diffusion layer using a porous metal, a punching metal, or the like is known (see, for example, Patent Document 1).
JP 2004-310443 A

  However, since the conventional gas diffusion layer uses a porous metal, a punching metal, or the like, there is a possibility that the corrosion resistance that can be endured in a strongly acidic atmosphere may not be sufficient.

  Then, an object of this invention is to provide the manufacturing method of the fuel cell stack which has a gas diffusion layer which has high corrosion resistance, and the conductive porous member for fuel cells used as a gas diffusion layer.

  The fuel cell stack according to the present invention is made to solve the above-mentioned problems, and is a membrane electrode joint in which a conductive porous member serving as a gas diffusion layer is provided on both surfaces of an electrolyte membrane via a catalyst layer. And a fuel cell separator disposed in contact with the membrane electrode assembly, wherein the conductive porous member has a porous interior at a certain depth from at least an outer surface of a metal substrate. It is characterized in that a nitride layer is formed as far as possible.

The method for producing a conductive porous member for a fuel cell according to the present invention includes nitriding by subjecting a porous base material made of a transition metal or a transition metal alloy to a plasma nitriding treatment at a temperature of 500 ° C. or lower. A method for producing a conductive porous member for a fuel cell comprising a nitride layer forming step of forming a layer from at least an outer surface of a base material into a porous interior of a certain depth, wherein the nitride layer comprises Fe, Cr, It has an M ₄ N type crystal structure in which nitrogen atoms are arranged in octahedral voids at the center of a unit cell of a face-centered cubic lattice formed by transition metal atoms selected from Ni and Mo.

  According to the fuel cell stack of the present invention, a fuel having a gas diffusion layer (conductive porous member) having low contact resistance between the separator and the catalyst layer, excellent corrosion resistance, and low cost. A battery stack is obtained.

  In addition, according to the method for manufacturing a conductive porous member for a fuel cell according to the present invention, a high-performance gas diffusion layer (conductive porous member) can be easily manufactured.

  Hereinafter, a fuel cell stack and a method for producing a conductive porous member for a fuel cell according to an embodiment of the present invention will be described with reference to an example applied to a polymer electrolyte fuel cell.

(Conductive porous member that serves as gas diffusion layer and fuel cell stack)
FIG. 1 is a perspective view showing an external appearance of a fuel cell stack configured using a conductive porous member according to an embodiment of the present invention. FIG. 2 is an exploded perspective view of FIG.

  As shown in FIG. 2, the fuel cell stack 1 is configured by alternately laminating a plurality of membrane electrode assemblies 90 and fuel cell separators 3 serving as basic units for generating power by electrochemical reaction. Each single cell 2 forms a membrane electrode assembly 90 by forming a gas diffusion layer having an oxidant electrode and a gas diffusion layer having a fuel electrode on both surfaces of a solid polymer electrolyte membrane, respectively. The fuel cell separator 3 is disposed on both sides, and an oxidant gas channel and a fuel gas channel are formed inside the fuel cell separator 3.

  As the polymer electrolyte membrane, for example, a fluorine resin membrane having a sulfonic acid group can be used. After the membrane electrode assembly 90 and the fuel cell separator 3 are stacked, the end flange 4 is disposed at both ends, and the outer peripheral portion is fastened by the fastening bolts 5 to constitute the fuel cell stack 1. The fuel cell stack 1 includes a hydrogen supply line for supplying a fuel gas containing hydrogen such as hydrogen gas to each single cell 2, an air supply line for supplying air as an oxidant gas, and cooling water. A cooling water supply line is provided.

  FIG. 3 is a cross-sectional view showing a membrane electrode assembly and a separator according to an embodiment of the present invention.

  The membrane electrode assembly 90 includes an electrolyte membrane 91 and a conductive porous member 93 serving as a gas diffusion layer disposed on both surfaces of the electrolyte membrane 91 via a catalyst layer 92. The separators 3 are disposed on both surfaces of the membrane electrode assembly 90 in contact with the conductive porous member 93.

  4 is a perspective view showing a conductive porous member according to an embodiment of the present invention, and FIG. 5 is an enlarged sectional view taken along line AA of FIG.

  The conductive porous member 93 to be the gas diffusion layer is a rectangular metal plate material, and as shown in FIG. 5A, the entire conductive porous member, that is, the surface layer in the conductive porous member. Nitride layer 95 is formed on the inner surfaces of the inner pores. Usually, since the porous member is a high-porosity material having a thickness of about 100 to 300 μm, nitriding of the entire porous substrate is preferable in view of the corrosion resistance inside the porous member. However, when nitriding to the inside is difficult due to the specifications of the material of the substrate itself, the porosity and the pore diameter, partial nitriding treatment may be used. That is, in the example shown in FIG. 5B, the conductive porous member 93 has a nitride layer 95 formed on the front surface and the back surface of the base layer 96. These nitride layers 95 are formed on both surfaces in the example shown in FIG. 5B, but may be provided only on the front surface, only the back surface, or both the front and back surfaces. Further, the nitride layer 95 can be formed by, for example, a plasma process described later.

  FIG. 6 is an enlarged cross-sectional view of a conductive porous member according to another embodiment of the present invention.

  Further, as shown in FIG. 6, it is preferable to form a noble metal layer 97 on at least one of the front surface and the back surface of the nitride layer 95 of the conductive porous member 93. As this noble metal, it is preferable to use at least one of Au, Ag, Pt, Rd, Rd, Pd, Os, and Ir, and Au is most preferable. The noble metal layer 97 can be formed by electroplating or cladding.

  Further, as the conductive porous member 93, it is preferable to use a metal base material formed by compression molding a stainless steel or a fibrous metal material.

  In the present embodiment, since the nitride layer 95 is formed on the conductive porous member 93 serving as the gas diffusion layer, the corrosion resistance can be improved while maintaining the conductivity. Moreover, in this embodiment, since the metal base material is used, mechanical strength and electroconductivity can be improved compared with the conventional carbon diffusion layer.

  Further, since the noble metal layer 97 is formed on the conductive porous member 93 according to the present embodiment, the contact resistance with the catalyst layer 92 and the separator 3 can be reduced while maintaining the corrosion resistance.

  Furthermore, by using a metal base material obtained by compression-molding a fibrous metal material as the conductive porous member 93, the surface of the porous base material can be uniformly nitrided.

  FIG. 7 is an enlarged cross-sectional view of a conductive porous member 93 according to still another embodiment of the present invention.

  The conductive porous member 93 is made of a sintered metal that is a porous body, has a high porosity, and has a nitride layer 95 formed on the front and back surfaces. That is, a base layer 98 and a nitride layer 95 made of sintered metal are provided. The nitride layer 95 can also be formed, for example, by plasma processing described later. Further, a noble metal layer 97 is preferably provided on the nitride layer 95.

  Since the conductive porous member 93 is made of sintered metal, the required porosity can be ensured while suppressing deformation of the stack against compression. Therefore, since there are many pores, it is not necessary to form a flow path in the separator 2. In this case, the cell pitch can be set short and the volumetric efficiency can be kept high.

  Further, the overall corrosion resistance of the conductive porous member 93 can be improved by nitriding the sintered metal body itself.

  Furthermore, in the conductive porous member 93 according to the present embodiment, a transition metal or an alloy of transition metal is used as a base material, and a nitride layer 95 having a cubic crystal structure is provided in the depth direction from the base material surface. Since the transition metal atom in the nitride layer 95 forms a highly covalent bond with the nitrogen atom in addition to the formation of a metal bond between the metal atoms, The conductive porous member 93 having excellent conductivity can be obtained. Further, the nitride layer 95 having a cubic crystal structure is chemically stable even in a strongly acidic atmosphere of pH 2 to 3 that is normally used as a fuel cell, and therefore has excellent corrosion resistance. Therefore, it is possible to obtain the conductive porous member 93 that keeps the contact resistance with the fuel cell separator 3 low and continuously exhibits good electrical conductivity even in a strongly acidic atmosphere.

  The base material is preferably stainless steel containing at least one metal element selected from the group consisting of Fe, Cr, Ni and Mo. Examples of stainless steels containing such elements include austenitic, austenitic / ferrite, and precipitation hardening stainless steels. Among these, the base material is preferably formed from austenitic stainless steel. Examples of the austenitic stainless steel include SUS304, SUS310S, SUS316L, SUS317J1, SUS317J2, SUS321, SUS329J1, and SUS836.

More specifically, the crystal structure of the cubic crystal is a face-centered cubic formed by at least one metal atom selected from the group consisting of Fe (iron), Cr (chromium), Ni (nickel), and Mo (molybdenum). It is preferably an M ₄ N type crystal structure in which nitrogen atoms are arranged in octahedral voids at the center of the unit cell of the lattice.

The crystal structure of the M ₄ N type is shown in FIG.

As shown in FIG. 8, the M ₄ N-type crystal structure 20 has an octahedral void at the center of a unit cell of a face-centered cubic lattice formed by transition metal atoms 21 selected from Fe, Cr, Ni, and Mo. In which nitrogen atoms 22 are arranged. In the M ₄ N type crystal structure 20, M represents a transition metal atom 21 selected from Fe, Cr, Ni, and Mo, and N represents a nitrogen atom 22. The nitrogen atom 22 occupies a quarter of the octahedral void 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 the unit cell of the face-centered cubic lattice of transition metal atoms 21, and is represented by a cubic crystal lattice. The nitrogen atoms 22 are located at the lattice coordinates (1/2, 1/2, 1/2) of each unit cell. By adopting the M ₄ N type crystal structure, strong covalent bonding is exhibited between the transition metal atom 21 and the nitrogen atom 22 while maintaining the metal bond between the transition metal atoms 21.

In the M ₄ N-type crystal structure 20, the transition metal atom M 21 is preferably mainly composed of Fe, but Fe is an alloy partially substituted with other transition metal atoms such as Cr, Ni, and Mo. May be.

The M ₄ N type crystal structure is considered to be a fcc or fct structure nitride in which high hardness is 1000 [HV] or more accompanied by high density dislocations and twins, and nitrogen is supersaturated in a solid solution. (Yasumaru, 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. In this way, nitrogen atoms are arranged in the octahedral voids in the center of the unit cell of the face-centered cubic lattice in which the nitride layer 95 is formed of at least one or more transition metal atoms selected from the group of Fe, Cr, Ni, and Mo. In addition, in the case of having an M ₄ N type crystal structure, it becomes possible to further improve the corrosion resistance in a strongly acidic atmosphere having a pH of 2 to 3.

  The thickness of the nitride layer 95 is preferably formed in the range of 0.5 to 5 [μm] on the surface of the base material. In this case, the corrosion resistance in a strong acid atmosphere is excellent. When the thickness of the nitride layer 95 is less than 0.5 [μm], a crack occurs between the nitride layer 95 and the base material, or the adhesion strength between the nitride layer 95 and the base material is insufficient. Accordingly, when used for a long time, the nitrided layer 95 is easily peeled off from the interface with the base material, so that it is difficult to obtain sufficient corrosion resistance when used for a long time. Further, when the thickness of the nitride layer 95 exceeds 5 [μm], the stress in the nitride layer 95 becomes excessive as the thickness of the nitride layer 95 increases, and the nitride layer 95 cracks, and the fuel cell It becomes easy for pitting corrosion to occur in the separator 3 for use, and it becomes difficult to contribute to improvement of corrosion resistance.

  Furthermore, it is preferable that the amount of nitrogen on the pole surface of the nitride layer 95 is 10 [at%] or more and the amount of oxygen is 35 [at%] or less. Here, the extreme surface of the nitride layer 95 refers to an atomic layer having a depth of 3 to 4 nm from the outermost surface of the nitride layer 95, that is, a depth of several tens of atoms. The outermost surface refers to the outermost atomic layer of the nitride layer 95. When the coverage of oxygen molecules adsorbed on the surface of the transition metal is increased, a clear bond is formed between the transition metal atom and the oxygen atom. This is the oxidation of transition metal atoms. Such oxidation of the transition metal surface first occurs when the outermost first atomic layer is oxidized. When the oxidation of the first atomic layer is completed, next, oxygen adsorbed on the first atomic layer receives free electrons in the transition metal by the tunnel effect, and oxygen becomes negative ions. Then, due to the strong local electric field due to the negative ions, transition metal ions are pulled out from the inside of the transition metal onto the surface, and the pulled transition metal ions are combined with oxygen atoms. That is, a second oxide film is formed. Such a reaction occurs from one to the next, and the oxide film becomes thicker. Thus, when the amount of oxygen in the nitride layer 95 is greater than 35 [at%], an insulating oxide film is likely to be formed. On the other hand, when the nitrogen amount on the extreme surface of the nitride layer 95 is 10 [at%] or more and the oxygen amount is 35 [at%] or less, the growth of the oxide film can be suppressed, and the carbon paper can be suppressed. It is possible to reduce the contact resistance between the conductive porous member 93 and the conductive porous member 93 having excellent corrosion resistance in a strongly acidic atmosphere.

Further, at a depth of 10 [nm] from the outermost surface of the nitride layer 95, the nitrogen amount is preferably 15 [at%] or more and the oxygen amount is 26 [at%] or less, and further the nitrogen amount is 18 [at]. %] Or more and the oxygen amount is more preferably 22 [at%] or less. In this case, the corrosion resistance in a strong acid atmosphere is excellent. In addition, when it remove | deviates from this range, the contact resistance generate | occur | produced between the separators 3 becomes high, and the contact resistance value per single cell which comprises a fuel cell stack exceeds 40 [mohm * cm < ² >], There is a problem that power generation performance deteriorates.

  Furthermore, it is preferable that the nitrogen amount is 16 [at%] or more and the oxygen amount is 21 [at%] or less at a depth of 100 [nm] to 200 [nm] from the outermost surface of the nitride layer 95. In this case, if the transition metal atom forms a compound with the nitrogen atom in a state where the chemical potential of the nitrogen atom in the nitride layer 95 is increased and the activity of the transition metal atom is further reduced, the freedom of the transition metal atom is reduced. The energy is lowered, the reactivity of the transition metal atom to the oxidation can be lowered, and the transition metal atom is chemically stabilized. For this reason, oxygen atoms receive no free electrons and the transition metal atoms are not oxidized, so that the growth of the oxide film can be suppressed, and the contact resistance after the corrosion resistance test can be suppressed low.

  Thus, by adopting the above-described configuration, the conductive porous member 93 according to the embodiment of the present invention is excellent in corrosion resistance. In addition, it is possible to obtain the conductive porous member 93 that has low cost and good productivity, low contact resistance with adjacent constituent materials, and 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 conductive porous member 93 according to the embodiment of the present invention. And cost reduction can be realized.

(Method for producing conductive porous member)
Next, a method for manufacturing a conductive porous member according to an embodiment of the present invention will be described. The conductive porous member 93 is manufactured from Fe, Cr, Ni, and Mo by a nitriding step in which a base material made of a transition metal or a transition metal alloy is subjected to nitriding at a temperature of 500 [° C.] or less. Forming a nitride layer 95 having an M ₄ N type crystal structure in which nitrogen atoms are arranged in octahedral voids at the center of a unit cell of a face-centered cubic lattice formed by selected transition metal atoms; A noble metal layer forming step of forming a noble metal layer 97 on at least one of the front surface side and the back surface side of the nitride layer 95 of the porous member 93.

  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 nitride such as CrN having a crystal structure of NaCl type is mainly precipitated, so that the corrosion resistance of the fuel cell separator 3 is lowered. To do. On the other hand, when nitriding is performed at a temperature of 500 [° C.] or lower, the surface of the substrate is not mainly a nitride compound such as CrN having a NaCl type crystal structure, but selected from the group of Fe, Cr, Ni, and Mo A crystal structure is formed in which nitrogen atoms are arranged in octahedral voids at the center of a unit cell of a face-centered cubic lattice or a face-centered tetragonal lattice formed of at least one metal atom. Since this crystal structure is particularly rich in corrosion resistance in the nitride layer 95, the corrosion resistance of the fuel cell separator is improved by performing nitriding at a low temperature of 500 [° C.] or lower. Further, the contact resistance between the separator 3 and the adjacent constituent material such as the gas diffusion electrode can be kept low, the power generation efficiency of the fuel cell can be maintained, and the fuel cell separator 3 having excellent durability and reliability can be obtained at low cost. Obtainable.

  Note that when the nitriding temperature is lower than 350 [° C.], it takes a long time to obtain the nitrided layer 95 having this crystal structure, so that the productivity is deteriorated. Therefore, the nitriding treatment is preferably performed in the range of 350 to 500 [° C.], more preferably 350 to 500 [° C.].

  The nitriding treatment is preferably a plasma nitriding method. For the nitriding treatment, a gas nitriding method, a gas soft nitriding method, a salt bath method, a plasma nitriding method, or the like can be used. In the gas soft nitriding method, the oxygen partial pressure during nitriding is high, so that the amount of oxygen in the nitride layer 95 is high. In contrast, plasma nitriding is a nitriding process that uses a workpiece as a cathode, ionizes nitrogen gas by glow discharge generated by applying a DC voltage, and the ionized nitrogen accelerates rapidly to the surface of the workpiece. This is a method of nitriding by collision. For this reason, the plasma nitriding method is a nitriding method suitable for stainless steel because nitriding is performed while easily removing the passive film on the surface of the stainless steel, which is the object to be processed, by the sputtering action by ion bombardment, and by non-equilibrium reaction. Since the nitrogen ions are infiltrated into the substrate, the crystal structure can be easily obtained in a short time, and the corrosion resistance is improved.

  Next, the structure of the nitriding apparatus used for the manufacturing method of the electroconductive porous member which concerns on embodiment of this invention is demonstrated based on FIG. FIG. 9 is a schematic side view showing the configuration of the nitriding apparatus used in the method for manufacturing the conductive porous member of the present embodiment.

As shown in FIG. 9, the nitriding apparatus 30 includes a batch-type nitriding furnace 31, a gas supply apparatus 32 that supplies atmospheric gas to the nitriding furnace 31, and plasma electrodes 33 a and 33 b that generate plasma in the nitriding furnace 31. And a DC power source 33 for supplying a DC voltage to the electrodes 33a and 33b, a pump 34 for discharging the gas in the nitriding furnace 31, and a temperature sensor 37 for detecting the temperature in the nitriding furnace 31. The nitriding furnace 31 has an inner wall 31a and an outer wall 31b, and a stainless hanger 36 for suspending a stainless steel foil 44 processed into the shape of a conductive porous member is provided on the ceiling 31c of the inner wall 31a. The gas supply device 32 includes a gas chamber 38 and a gas supply conduit 39. The gas chamber 38 is provided with an opening, and a gas supply valve is provided in each of the openings, and H ₂ gas, N ₂ Gas and Ar gas are supplied. The ceiling portion 31 c of the nitriding furnace 31 has an opening 31 d that communicates with the other end of the gas supply conduit 39. The gas supply line 39 is provided with a gas supply valve. The gas pressure in the nitriding furnace 31 is detected by a gas pressure sensor 40 provided at the bottom 31 e of the nitriding furnace 31. The nitriding furnace 31 is provided with a cooling water flow path (not shown), and the cooling water flows into the cooling water flow path from the opening 31f provided in the outer wall 31b of the nitriding furnace 31, and flows out from the opening 31g. A cooling water supply valve is provided in the opening 31f to adjust the flow rate of the cooling water. The pump 34 is connected to a discharge pipe 41 communicating with an opening 31h provided in the bottom 31e. The temperature sensor 37 is installed in an installation port 31 i provided in the outer wall 31 b of the nitriding furnace 31.

  The nitriding device 30 is provided with a bias potentiometer 35 in addition to the DC power source 33 controlled from the operation panel 43 for glow discharge. The DC power supply 33 has a positive (+) electrode 33 a connected to the inner wall 31 a of the nitriding furnace 31 and a negative (−) electrode 33 b grounded. The potentiometer 35 divides the potential difference between the bias DC power supply terminal 35c and the ground circuit 35d by a movable contact in the range of 0 [V] to the bias voltage, and the obtained voltage is passed through the bias circuit 35a. Supply to each stainless steel foil 44. The DC power source 33 is turned on / off by a control signal from the operation panel 43. The potentiometer 35 is supplied with a bias control signal from the operation panel 43 via the bias control circuit 35b, and the movable contact slides in accordance with the control signal. Accordingly, each stainless steel foil 44 has a voltage difference with respect to the inner wall 31a, which is obtained by adding the voltage between the terminals of the DC power supply 33 and the bias voltage supplied via the movable contact. The gas supply device 32 and the gas pressure sensor 40 are also controlled by the operation panel 43.

  Nitrogen gas and hydrogen gas are used for plasma nitriding, and a negative bias voltage is applied to the stainless steel material in a low temperature non-equilibrium plasma in which the nitrogen gas and hydrogen gas are discharged. Nitriding is preferably performed at a temperature. In the plasma nitriding treatment, the passive film on the surface of the metal material can be easily removed by the sputtering action by ion bombardment. On the other hand, when nitriding is performed using gas nitriding or salt bath nitriding, which is commonly used, oxidation occurs on the outermost layer of the order of several to several tens [nm] of the nitrided layer to form an insulating oxide. Therefore, the contact resistance increases. On the other hand, in the nitriding treatment using the plasma nitriding method as in the present invention, the nitriding reaction can proceed while removing oxygen on the surface of the metal material. It becomes possible to keep it low enough. Furthermore, the contact resistance can be maintained at a low value so as to be suitable as a fuel cell.

Further, the processing conditions for plasma nitriding are as follows: temperature 400 to 500 [° C.], processing time 10 to 60 [min], gas mixing ratio N ₂ : H ₂ = 3: 7 to 7: 3, processing pressure 3 -7 [Torr] (= 399 to 931 [Pa]) is preferable. The reason why the nitriding conditions are defined in this range is that the nitrided layer 95 is not formed when the processing time is less than 1 [min], and conversely, the manufacturing cost increases when the processing time exceeds 60 [min]. Because. Furthermore, the reason why the gas mixture ratio is defined within this range is that the nitride layer 95 cannot be formed if the nitrogen ratio in the gas decreases, and conversely, if the nitrogen ratio increases, it acts as a reducing agent. This is because the amount of hydrogen decreases and the substrate surface is oxidized. By performing plasma nitriding under such processing conditions, a nitride layer 95 having an M ₄ N type crystal structure can be formed on the substrate surface.

  Thus, according to the method for manufacturing the conductive porous member 93 according to the embodiment of the present invention, a low resistance value in an oxidizing environment is maintained, corrosion resistance is excellent, and low by a simple operation. It becomes possible to manufacture the conductive porous member 93 that realizes cost reduction.

(Fuel cell vehicle)
As an example of the fuel cell vehicle according to the embodiment of the present invention, a fuel cell electric vehicle using the fuel cell stack according to the embodiment of the present invention as a power source will be described.

  FIG. 10 is a diagram showing the appearance of a fuel cell electric vehicle equipped with a fuel cell stack. FIG. 10A is a side view of the fuel cell electric vehicle 50, and FIG. 7B is a plan view of the fuel cell electric vehicle 50.

  As shown in FIG. 10 (b), in front of the vehicle body 51, 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. The engine compartment portion 52 is formed. In the fuel cell electric vehicle 70 shown in FIGS. 10A and 10B, the fuel cell stack 1 is mounted in the engine compartment portion 52.

  By mounting the fuel cell stack 1 having high power generation efficiency to which the conductive porous member 93 according to the embodiment of the present invention is applied to a mobile vehicle such as an automobile, the fuel efficiency of the fuel cell electric vehicle can be improved. . In addition, by mounting the miniaturized lightweight fuel cell stack 1 on the vehicle, the vehicle weight can be reduced to save fuel, and the travel distance can be increased. Furthermore, by mounting a miniaturized fuel cell on a mobile vehicle or the like, the vehicle interior space can be used more widely, and the degree of freedom in styling can be increased.

  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 apply also to the aircraft and other engines in which electric energy is requested | required. .

  Hereinafter, Examples 1 to 6 and Comparative Examples 1 to 5 of the conductive porous member 93 according to the embodiment of the present invention will be described. In these examples, the effectiveness of the conductive porous member 93 according to the present invention was examined, and each sample was prepared by treating different raw materials under different conditions. It is not limited to.

<Preparation of sample>
In each of Examples and Comparative Examples, as shown in Table 1, the brightness of each JIS standard austenitic stainless steel (SUS304, SUS316, SUS316L, SUS310) having a thickness of 0.1 [mm] as a base material is shown. An annealed (BA) material was used. After these substrates were degreased and cleaned, plasma nitriding treatment (plasma nitriding treatment by direct current glow discharge) was performed on both surfaces. As shown in Table 1, the plasma nitriding conditions are as follows: the processing temperature is 350 to 550 [° C.], the processing time is 10 to 60 [min], and the gas mixing ratio is N ₂ : H ₂ = 3: 7 in Examples 1 to 6. To 7: 3, and a processing pressure of 3 to 7 [Torr] (= 399 to 931 [Pa]).

  Here, each sample was evaluated by the following method.

<Identification of crystal structure of nitride layer>
The crystal structure of the nitride layer 95 of the sample obtained by the above method was identified by performing X-ray diffraction measurement on the base material surface modified by nitriding treatment. The apparatus used was an X-ray diffractometer (XRD) manufactured by Mac Science. The measurement was performed under the conditions of a CuKα ray as a radiation source, a diffraction angle of 20 to 100 [°], and a scanning speed of 2 [° / min].

<Measurement of the thickness of the nitride layer 95>
The thickness of the nitride layer 95 was measured by observing a cross section using an optical microscope or a scanning electron microscope.

<Measurement of Cr atomic ratio to Fe and measurement of nitrogen amount and oxygen amount on the surface>
The measurement of the Cr atomic ratio with respect to Fe of the nitride layer 95 was obtained by measuring the Fe concentration and the Cr concentration of the nitride layer 95 by X-ray electron spectroscopy (XPS). Further, the amount of nitrogen and the amount of oxygen on the surface of the nitride layer 95 were measured using XPS, and the ratio O / N of the amount of nitrogen to the amount of oxygen was determined from the measurement results. The apparatus used was a photoelectron spectrometer Quantum-2000 manufactured by PHI. Measurement is performed using a Monochromated-Al-kα ray (voltage 1486.6 [eV], 20.0 [W]) as a radiation source, a photoelectron extraction angle of 45 [°], a measurement depth of about 4 [nm], and a measurement area φ200 [ The sample was irradiated with X-rays at [μm].

<Measurement of Nitrogen Content and Oxygen Content at 10 [nm] and 100 [nm] Depth from the Top Surface of Nitride Layer>
The nitrogen amount and the oxygen amount at the depth of 10 nm and 100 nm from the outermost surface of the nitride layer 95 were measured by a scanning Auger electron spectrometer. The apparatus used was MODEL4300 manufactured by PHI. The measurement is performed under the conditions of an electron beam acceleration voltage of 5 [kV], a measurement area of 20 [μm] × 16 [μm], an ion gun acceleration voltage of 3 [kV], and a sputtering rate of 10 [nm / min] (SiO ₂ conversion value). went.

<Measurement of contact resistance value>
The obtained sample was cut into a size of 30 [mm] × 30 [mm], and the contact resistance was measured. The apparatus used was TRS-2000SS, a pressure load contact electrical resistance measuring device manufactured by ULVAC-RIKO. 11A, a carbon paper 63 is interposed between the electrode 61 and the sample 62, and as shown in FIG. 11B, 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 an average value of each electrical resistance was obtained to obtain a contact resistance value. 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. This contact resistance measurement was performed twice before and after an electrolytic test described later.

<Evaluation of corrosion resistance>
In the fuel cell, a potential of about 1 [Vvs SHE] is applied to the oxygen electrode side in comparison with the hydrogen electrode side. The solid polymer electrolyte membrane 91 uses proton conductivity by saturating the polymer electrolyte membrane 91 having a proton exchange group such as a sulfonic acid group in the molecule, and exhibits strong acidity. For this reason, the corrosion resistance is evaluated using a constant potential electrolysis test that is an electrochemical technique, and the amount of metal ions that dissolve in the solution after being held for a certain period of time with a predetermined constant potential applied by fluorescent X-ray analysis. The degree of reduction in corrosion resistance was evaluated from the value of the ion elution amount.

  Specifically, a sample obtained by cutting out the central portion of each sample into a size of 30 [mm] × 30 [mm] in a sulfuric acid aqueous solution of pH 2 at a temperature of 80 [° C.] and a potential of 1 [V vs SHE] for 100 [hours] Retained. Thereafter, the elution amounts of Fe, Cr, and Ni dissolved in the sulfuric acid aqueous solution were measured by fluorescent X-ray analysis.

The crystal structure of the nitride layer 95, the thickness of the nitride layer 95, the amount of nitrogen and the amount of oxygen on the extreme surface of the nitride layer 95, the ratio O / N of the amount of nitrogen to the amount of oxygen, 10 nm from the outermost surface of the nitride layer 95, and Table 2 shows the amounts of nitrogen and oxygen, contact resistance values, and ion elution amounts at a depth of 100 nm.

  Furthermore, the X-ray analysis pattern of the sample obtained by the said Example 1 and the comparative example 1 is shown in FIG.

In Comparative Example 1, only the peak derived from the austenite which is the base material was clearly observed, whereas in Example 1, in addition to the peak derived from the austenite which is the base material indicated by γ in the figure, the above M ₄ S1-S5 peaks showing an N-type crystal structure were observed. Here, M is mainly composed of Fe, and includes an alloy of Cr, Ni, and Mo in addition to Fe. When the thickness of the nitride layer 95 was observed from the cross-sectional structure shown in FIG. 10A, the nitride layer 95 of about 5 [μm] was formed on the surface in Example 1. Thus, although the surface is covered with the nitride layer 95 having the M ₄ N type crystal structure, the peak derived from the austenite which is the base material is also observed as the X-ray diffraction peak. This was determined that the base material was detected because the incident depth of X-rays on the base material under this measurement condition was about 10 [μm]. In Examples 2 to 6, the peak of the M ₄ N type crystal structure was observed in addition to the peak derived from the austenite as the base material, as in Example 1.

In Comparative Example 2 and Comparative Example 3, since the nitrided layer 95 was not formed as in Comparative Example 1, only the peak derived from the austenite that was the base material was observed. In Comparative Example 4, peaks indicating CrN and γ ′ phases were observed. In the γ ′ phase, Cr combines with N to form a Cr-based nitride compound such as CrN, so that Fe atoms form a face-centered cubic lattice by decreasing the Cr concentration of the base material. The γ ′ phase is a crystal structure in which nitrogen atoms have entered the octahedral voids at the center of the unit cell of this lattice, that is, a Fe ₄ N type crystal structure in which nitrogen atoms are arranged in 1/4 of the octahedral voids, It contains no Cr or Ni alloy in addition to Fe. Thus, it has been found that when the processing temperature exceeds 500 [° C.], a Cr-based nitride compound such as CrN having a NaCl-type crystal structure is formed.

  Next, a cross-sectional structure photograph of the sample obtained in Example 1 at a magnification of 400 is shown in FIG. 13 (a), and a cross-sectional structure photograph of the sample obtained in Comparative Example 1 is shown in FIG. 13 (b).

  In FIG. 13A, nitride layers 72 are formed on both surfaces of the substrate 71, but in FIG. 13B, a modified layer such as a nitride layer is not formed on the surface of the substrate 73. I understand that.

Further, as shown in Table 2, in Examples 1 to 6 in which a nitride layer having an M ₄ N type crystal structure was formed, the contact resistance value was 10.5 [mΩ · cm ² ] or less. there were. On the other hand, in Comparative Examples 1 to 3 and Comparative Example 5 in which no nitride layer was formed, the contact resistance value was remarkably high. 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, and the voltage drops as the extracted current increases. . 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 ], the contact resistance between the separator 3 and the carbon paper is 20 [mΩ · cm ² ], that is, the measured value by the measuring method shown in FIG. If it is less than mΩ · cm ² ], it is considered that the efficiency reduction due to contact resistance can be suppressed. In each of Examples 1 to 6, since the contact resistance value is 40 [mΩ · cm ² ] or less, it is possible to form a fuel cell stack with high electromotive force per unit cell and high electromotive force. It becomes.

  Next, as a result of measuring the ion elution amount, all of Examples 1 to 6 in which the thickness of the nitrided layer 95 is 0.5 to 5 [μm] have a low ion elution amount and excellent corrosion resistance. I understood it.

  Compared to the examples, when the surface of the austenitic stainless steel is not nitrided as in Comparative Examples 1 to 3, the contact resistance is high because a passive film is formed on the surface of the substrate. Further, since there is a passive film, austenitic stainless steel is generally excellent in corrosion resistance, but it was found to be inferior compared with Examples 1 to 6.

  Further, even if the nitriding treatment is performed as in Comparative Examples 4 to 5, when the nitrided layer 95 mainly includes CrN having a NaCl-type crystal structure, the nitriding treatment is not performed. Compared with 3, the contact resistance is low, but the corrosion resistance is inferior.

Further, from Table 2, when the nitrogen amount on the surface of the nitrided layer 95 measured by XPS, the amount of oxygen, and the ratio of the amount of nitrogen to the amount of oxygen O / N are examined, the amount of nitrogen on the surface of the nitrided layer 95 is 10 [at %] And the amount of oxygen was 35 [at%] or less, and in each of Examples 1 to 6 where O / N was 3.5 or less, the contact resistance value was 40 [mΩ · cm ² ] or less. . On the other hand, when the surface of the austenitic stainless steel is not nitrided as in Comparative Examples 1 to 3, since there is a passive film on the surface of the substrate, the amount of oxygen on the extreme surface is large, and O / N Was also very high. Further, in Comparative Example 4, Cr-based nitride such as CrN mainly precipitates, so that the contact resistance is low and the O / N is also low, but the ion elution amount is large and the corrosion resistance is deteriorated.

  Next, FIG. 14 shows an element profile in the depth direction of the sample obtained in Example 1 by scanning Auger electron spectroscopy analysis.

As shown in FIG. 14, an oxide film exists on the outermost surface of the nitride layer because there is a slight partial pressure of oxygen during the nitriding process. The highest. However, since the range in which electrons can freely travel is 3 to 4 [nm] from the outermost surface, the amount of oxygen gradually decreased and the amount of nitrogen increased. Further, at a depth of 10 nm from the outermost surface of the nitride layer, the nitrogen content was 18 [at%] and the oxygen content was 22 [at%]. The nitrogen content was 17 [at%] and the oxygen content was 17 [at%] at a depth of 100 [nm] from the outermost surface of the nitride layer. Note that the ratio of Fe, which is a component of the base material, increased from around the sputtering depth of 50 [nm]. The contact resistance value at this time was 3 [mΩ · cm ² ], and the ion elution amount was also low. Thus, in Example 1 in which the nitride layer having the M ₄ N type crystal structure was formed, the contact resistance and the corrosion resistance were satisfied.

Similarly, in any of Examples 1 to 6 in which the contact resistance value is 40 [mΩ · cm ² ] or less, the nitrogen amount is 18 [at%] at a depth of 10 [nm] from the outermost surface of the nitride layer 95. In addition, the oxygen amount is 25 [at%] or less, and further, at a depth of 100 [nm] from the outermost surface of the nitride layer 95, the nitrogen amount is 17 [at%] or more and the oxygen amount is 18 [at%] or less. Met. On the other hand, in Comparative Examples 1 to 3 in which the contact resistance value is 245 or more, the nitrogen amount and the oxygen amount at a depth of 10 [nm] from the outermost surface of the nitride layer 95 deviate from the above values. The amounts of nitrogen and oxygen at a depth of 100 [nm] from the outermost surface of the nitride layer also deviated from the above values. In Comparative Examples 4 to 5 in which no passive film was formed, the above values were satisfied.

From the above measurement results, Examples 1 to 6 are selected from the group of Fe, Cr, Ni, and Mo, which are the crystal structures of the cubic crystals in any of the Examples compared to Comparative Examples 1 to 5. A contact resistance value is obtained by forming the nitride layer 95 having an M ₄ N type crystal structure in which nitrogen atoms are arranged in the octahedral voids at the center of a unit cell of a face-centered cubic lattice formed of at least one transition metal atom. The low contact resistance is 40 [mΩ · cm ² ] or less, and the ion elution amount is small and the corrosion resistance is excellent. Therefore, both low contact resistance and corrosion resistance are simultaneously provided.

  In this example, austenitic stainless steel was used as the base material, but the present invention is not limited to this. Even if ferritic or martensitic stainless steel is used, plasma nitriding treatment is also used as nitriding treatment. However, the same effect can be obtained by gas nitriding.

  Next, FIGS. 15A and 15B show the relationship between the amount of nitrogen and oxygen and the contact resistance value. FIG. 15A shows the relationship between the amount of nitrogen and oxygen on the pole surface and the contact resistance value. FIG. 15B shows the relationship between the ratio O / N of the nitrogen amount to the oxygen amount on the pole surface and the contact resistance value. As shown in FIG. 15 (a), it is clear that the contact resistance value is lower as the nitrogen amount is higher (the nitrogen concentration is higher) on the pole surface, and the contact resistance value is lower as the oxygen amount is lower (the oxygen concentration is lower). became. This is because, as described above, when the amount of oxygen in the nitride layer 95 is large, an insulating oxide film is formed on the substrate surface, so that the contact resistance value is high, and the nitride layer 95 is formed on the substrate surface. In this case, it is considered that the contact resistance value is lowered because the growth of the oxide film is suppressed. Similarly, as shown in FIG. 15 (b), it was revealed that the contact resistance value was lower as the ratio of the nitrogen amount to the oxygen amount on the pole surface was lower.

From the above measurement results, Examples 1 to 6 were compared to Comparative Examples 1 to 5 in any example, and the base layer formed from a base material made of a transition metal or an alloy of transition metals, and this base layer And a nitride layer 95 having a cubic crystal structure formed directly on the surface of the substrate, the contact resistance value before the electrolysis test is as low as 10 [mΩ · cm ² ] or less, and after the electrolysis test The conductive porous member 93 having both low contact resistance and corrosion resistance at the same time was obtained.

It is a perspective view which shows the external appearance of the fuel cell stack comprised using the electroconductive porous member which concerns on embodiment of this invention. It is a disassembled perspective view of the fuel cell stack comprised using the electroconductive porous member which concerns on embodiment of this invention. It is sectional drawing which shows the membrane electrode assembly and separator which concern on embodiment of this invention. It is a perspective view which shows the electroconductive porous member which concerns on embodiment of this invention. It is an expanded sectional view by the AA line of FIG. 4, (a) is sectional drawing which formed the nitrided layer in the whole conductive porous member among this figure, (b) is the surface of a conductive porous member It is sectional drawing in which the nitride layer was formed in the layer. It is an expanded sectional view of the conductive porous member concerning another embodiment of the present invention. It is an expanded sectional view of the conductive porous member concerning another embodiment of the present invention. The M _{4 N} type crystal structure of the transition metal nitride according to an embodiment of the present invention is a schematic diagram showing. It is a side surface schematic diagram of the nitriding apparatus used for the manufacturing method of the electroconductive porous member 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. It is a figure which shows the X-ray-diffraction pattern of the sample obtained by Example 1 and Comparative Example 1. (A) A cross-sectional structure photograph of the sample obtained in Example 1. (B) It is a cross-sectional structure | tissue photograph of the sample obtained by the comparative example 1. FIG. It is a figure which shows the element profile of the depth direction by the scanning Auger electron spectroscopy analysis of the sample obtained by Example 1. FIG. (A) It is a figure which shows the relationship between the amount of nitrogen and oxygen of a pole surface, and a contact resistance value. (B) It is a figure which shows the relationship between ratio O / N of the nitrogen amount with respect to the oxygen amount of a pole surface, and a contact resistance value. 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 3 Fuel cell separator 20 M ₄ N type crystal structure 21 Transition metal atom 22 Nitrogen atom 90 Membrane electrode assembly 91 Electrolyte membrane 92 Catalyst layer 93 Conductive porous member 95 Nitride layer 97 Noble metal layer

Claims (12)

A membrane electrode assembly in which a conductive porous member is provided on both sides of the electrolyte membrane via a catalyst layer, and a fuel cell separator disposed in contact with the membrane electrode assembly,
A stack for a fuel cell, wherein the conductive porous member is formed by forming a nitride layer from at least an outer surface of a base material made of metal to a porous interior at a certain depth.   2. The fuel cell stack according to claim 1, wherein a noble metal layer is formed on at least one of a front surface side and a back surface side of the nitride layer of the conductive porous member.   The fuel cell stack according to claim 1, wherein the conductive porous member is a metal substrate obtained by compression molding a fibrous metal material.   The fuel cell stack according to claim 1, wherein the conductive porous member is a sintered metal body.   The conductive porous member is obtained by nitriding a surface of a base material made of an alloy containing at least one transition metal element selected from the group consisting of Fe, Cr, Ni and Mo. 5. The fuel cell stack according to claim 1, wherein a nitride layer having a cubic crystal structure in a depth direction is formed. In the cubic crystal structure, nitrogen atoms are arranged in octahedral voids at the center of a unit cell of a face-centered cubic lattice formed of at least one transition metal atom selected from the group consisting of Fe, Cr, Ni, and Mo. 6. The fuel cell stack according to claim 5, wherein the fuel cell stack has an M ₄ N type crystal structure.   The fuel cell stack according to claim 1, wherein a thickness of the nitride layer is 0.5 to 5 μm.   The nitrogen content is 15 [at%] or more and the oxygen content is 26 [at%] or less at a depth of 10 [nm] from the outermost surface of the nitride layer. The fuel cell stack according to the item.   The nitrogen content is 16 [at%] or more and the oxygen content is 21 [at%] or less at a depth of 100 [nm] to 200 [nm] from the outermost surface of the nitride layer. 9. The fuel cell stack according to any one of 8 above. A nitrided layer is formed from at least the outer surface of the substrate to the porous interior at a certain depth by subjecting the porous substrate made of transition metal or transition metal alloy to plasma nitriding treatment at a temperature of 500 [° C.] or lower. A method for producing a conductive porous member for a fuel cell comprising a nitride layer forming step comprising:
The nitride layer is an M ₄ N-type crystal in which nitrogen atoms are arranged in an octahedral void at the center of a unit cell of a face-centered cubic lattice formed by transition metal atoms selected from Fe, Cr, Ni, and Mo. A method for producing a conductive porous member for a fuel cell, characterized by having a structure.   The noble metal layer forming step of forming a noble metal layer on at least one of the front surface side and the back surface side of the nitride layer of the conductive porous member after the nitride layer forming step. The manufacturing method of the electroconductive porous member for fuel cells as described in any one of. 12. The fuel cell according to claim 10, wherein the plasma nitriding treatment is performed by applying a negative bias voltage to the base material in a low-temperature non-equilibrium plasma in which nitrogen gas and hydrogen gas are discharged. For producing a conductive porous member for use.