JP2007073370A - Intermediate plate for fuel cell, fuel cell stack, and manufacturing method of intermediate plate for fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an intermediate plate for a fuel cell having reduced contact resistance between an anode separator and a cathode separator, enhanced corrosion resistance, and low cost. <P>SOLUTION: The intermediate plate for the fuel cell interposed between the anode separator and the cathode separator constituting a fuel cell separator is made of a substrate of transition metal or alloy of the transition metal, and has a nitrided layer 103 having hexagonal system crystal structure, on the surface layer. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an intermediate plate for a fuel cell, a fuel cell stack, and a method for manufacturing the intermediate plate for a fuel cell, and more particularly to a solid polymer electrolyte type fuel cell intermediate plate 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. 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 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 order to prevent the deformation of each separator by improving the rigidity of the oxygen electrode side separator and the hydrogen electrode side separator, and to absorb the dimensional change of each part of the cell due to thermal expansion, etc., between these separators. A fuel cell stack in which an intermediate plate (not shown) is sandwiched has been developed (see, for example, Patent Document 1).

  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.

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 2-3. Further, the temperature of each gas supplied to the fuel cell is as high as 80 to 90 [° C.], and not only H ⁺ is generated at the hydrogen electrode, but the oxygen electrode through which oxygen, air, etc. pass is standard hydrogen. It is in an oxidizing environment in which a potential of about 0.6 to 1 [Vvs SHE] is loaded with respect to the extreme potential. For this reason, like the oxygen electrode and the hydrogen electrode, the fuel cell separator is required to have 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.

  Therefore, an attempt has been made to use a titanium material such as stainless steel or industrial pure titanium having excellent 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 passive film described above generates contact resistance with carbon paper that is normally used as a gas diffusion layer. The overvoltage caused by resistance polarization in the fuel cell can recover the exhaust heat by cogeneration or the like in the stationary application, thereby improving the total thermal efficiency. On the other hand, in automobile applications, heat loss due to contact resistance can only be thrown out of the radiator through the cooling water, so that if the contact resistance increases, the power generation efficiency decreases. In addition, a decrease in power generation efficiency is equivalent to an increase in heat generation, and it becomes necessary to equip a larger cooling system. Therefore, an increase in contact resistance is an important issue to be solved.

In a fuel cell, the theoretical voltage per unit cell is 1.23 [V], but the voltage that can 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 ], it is considered that if the contact resistance between the separator and the carbon paper is 40 [mΩ · cm ² ] or less, the efficiency reduction due to the contact resistance can be suppressed.

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 (for example, see Patent Document 2). 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, for example, Patent Document 3).
JP 2002-367665 A Japanese Patent Laid-Open No. 10-228914 (2nd page, FIG. 2) JP 2001-6713 A (page 2)

  However, when precious metal is plated or coated on the surface of a separator for fuel cells or an intermediate plate, there is a problem that not only labor is required at the time of production but also material costs are increased.

  Therefore, an object of the present invention is to provide a fuel cell intermediate plate, a fuel cell stack, and a method for manufacturing a fuel cell intermediate plate that can be manufactured at low cost without labor.

  An intermediate plate for a fuel cell according to the present invention is made in order to solve the above-mentioned problem, and is nitrided having a cubic crystal structure sandwiched between an anode separator and a cathode separator constituting the fuel cell separator. The gist of the invention is that it is composed of a base material made of a transition metal or an alloy of transition metals and having a layer formed on the surface layer.

  The gist of the fuel cell stack according to the present invention is that the intermediate plate is provided.

Furthermore, the method for producing an intermediate plate for a fuel cell according to the present invention comprises subjecting a base material made of a transition metal or a transition metal alloy to a plasma nitriding treatment at a temperature of 500 [° C.] or less, thereby providing a surface layer of the base material. A method of manufacturing an intermediate plate for a fuel cell in which a nitride layer is formed on the substrate, wherein the nitride layer is formed of metal atoms of at least one transition metal element selected from Fe, Cr, Ni, and Mo The gist is that it has an M ₄ N type crystal structure in which nitrogen atoms are arranged in an octahedral void at the center of a unit cell of a face-centered cubic lattice.

  According to the fuel cell intermediate plate of the present invention, a fuel cell intermediate plate having low contact resistance generated between the anode separator and the cathode separator, excellent corrosion resistance, and low cost can be obtained.

  That is, since the intermediate plate flows the current generated by the cell in contact with the anode separator and the cathode separator, if the contact resistance is large, a loss occurs and the cell voltage decreases. However, since the nitride layer is formed on the intermediate plate according to the present invention, the contact resistance is reduced and the cell voltage can be kept high.

  Moreover, according to the fuel cell stack according to the present invention, a fuel cell stack having a high-performance fuel cell intermediate plate can be obtained.

  Furthermore, according to the method for manufacturing an intermediate plate for a fuel cell according to the present invention, an intermediate plate for a fuel cell having low contact resistance generated between the anode separator and the cathode separator, excellent corrosion resistance, and low cost is provided. Can be manufactured.

  Hereinafter, an intermediate plate for a fuel cell, a fuel cell stack, and a method for producing the intermediate plate 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.

(Separator with fuel cell intermediate plate and fuel cell stack)
FIG. 1 is a perspective view showing an external appearance of a fuel cell stack provided with an intermediate plate for a fuel cell according to an embodiment of the present invention, and FIG. 2 is an exploded perspective view of the fuel cell stack shown in FIG.

  As shown in FIG. 2, the fuel cell stack 1 is configured by alternately laminating a plurality of membrane electrode assemblies 6 and fuel cell separators 3 serving as a basic unit for generating power by an electrochemical reaction. The single cell 2 includes these membrane electrode assemblies 6 and a fuel cell separator 3. Each single cell 2 forms a membrane electrode assembly 6 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. 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 laminating the membrane electrode assembly 6 and the fuel cell separator 3, the end flange 4 is disposed at both ends, and the outer peripheral portion is fastened by the fastening bolt 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 conceptual cross-sectional view of a fuel cell separator and a membrane electrode assembly according to an embodiment of the present invention.

  As shown in FIG. 3, the membrane electrode assembly 6 includes a solid polymer electrolyte membrane 7, a gas diffusion layer 8 having an oxidant electrode formed on the upper surface of the solid polymer electrolyte membrane 7, A gas diffusion layer 9 having a fuel electrode formed on the lower surface of the molecular electrolyte membrane 7 is formed.

  The fuel cell separator 3 includes an anode plate 90 disposed on the upper side in FIG. 3, a cathode plate 91 disposed on the lower side, and an intermediate plate 92 sandwiched between the anode plate 90 and the cathode plate 91. It is composed of The fuel cell separator 3 is disposed in contact with the membrane electrode assembly 6.

  The anode plate 90 and the cathode plate 91 have recesses having a substantially hat-shaped cross section, and the passage defined by the cathode plate 91 and the gas diffusion layer 8 is set as an oxidant passage 99. A passage defined by the anode plate 90 and the gas diffusion layer 9 (not shown) is set as a fuel passage 100. The passage defined by the anode plate 90 and the cathode plate 91 is a cooling water passage 101, and an intermediate plate 92 is disposed at the center in the height direction.

  Then, both surfaces of the peripheral portion of the solid polymer film 7 are sandwiched by the resin plate 93, and the peripheral portion of the fuel cell separator 3 is sandwiched by the frame member 94. The frame member 94 includes a frame main body 95 that holds the end portions 90a and 91a of the anode plate 90 and the cathode plate 91, and a rubber gasket 96 having a substantially triangular cross section that is fixed to the frame main body 95 and protrudes in the vertical direction. , 97 and a liquid sealant 98 sandwiched between the frame bodies 95, 95.

  4 is a perspective view of the fuel cell intermediate plate shown in FIG. 3, and FIG. 5 is an enlarged cross-sectional view taken along line AA of FIG.

  As shown in FIG. 4, the intermediate plate 92 is a plate-like member having a rectangular shape in plan view, and is sandwiched between the anode plate 90 and the cathode plate 91 to improve the rigidity of the plates 90 and 91. It is arranged for the purpose of preventing deformation and absorbing the dimensional change of each part of the cell due to thermal expansion or the like.

  The intermediate plate 92 is formed with a nitride layer on the surface layers of all of the upper surface 92a, the lower surface 92b, and the side surface 92c exposed to the outside.

  That is, as shown in FIG. 5, the intermediate plate 92 uses a transition metal or a transition metal alloy as a base material, and is provided on a base layer 102 provided on the inner side and a surface layer portion of the base layer 102. The nitride layer 103 is formed. Accordingly, the nitride layer 103 is formed on all of the surface layer of the upper surface 92a, the surface layer of the lower surface 92b, and the surface layer of the side surface 92c in the intermediate plate 92. The nitride layer 103 is formed in the depth direction from the outer surface of the intermediate plate 92 and has a cubic crystal structure.

  As described above, the intermediate plate 92 is composed of a base material made of a transition metal or an alloy of transition metals, in which the nitride layer 103 having a cubic crystal structure is formed on the surface layer. A fuel cell intermediate plate 92 having a low contact resistance, excellent corrosion resistance and low cost can be obtained.

  That is, since the intermediate plate flows the current generated by the cell in contact with the anode separator and the cathode separator, if the contact resistance is large, a loss occurs and the cell voltage decreases. However, since the nitride layer is formed on the intermediate plate according to the present invention, the contact resistance is reduced and the cell voltage can be kept high. Furthermore, since the intermediate plate 92 is disposed in the cooling water passage 101 as described with reference to FIG. 3, the intermediate plate 92 is in contact with the cooling water. Accordingly, ions are easily eluted from the surface of the intermediate plate 92. When this elution occurs, the electrical conductivity of the cooling water increases and the insulation resistance to the outside of the fuel cell stack decreases. However, since the nitride layer 103 is formed on the intermediate plate 92 according to the present invention, the insulation resistance to the outside of the fuel cell stack can be kept high.

  In addition, since the nitride layer 103 is formed on the outer surface of the base material exposed to the outside among the portions of the intermediate plate 92, the anode plate 90 and the cathode plate 91 are in contact with any portion of the outer surface of the intermediate plate 92. Low contact resistance and excellent corrosion resistance.

  Further, since the nitride layer 103 is formed in the depth direction from the outer surface of the intermediate plate 92 and has a cubic crystal structure, the transition metal atom in the nitride layer is rich in affinity with the nitrogen atom. In addition to forming a bond, since a metal bond is formed between metal atoms, a fuel cell intermediate plate 92 having excellent electrical conductivity can be obtained.

  Further, the nitride layer 103 having a cubic crystal structure is excellent in corrosion resistance because it is chemically stable even in a strongly acidic atmosphere having a pH of 2 to 3 that is usually used as a fuel cell. Therefore, it is possible to obtain the fuel cell intermediate plate 92 that keeps the contact resistance between the anode plate 90 and the cathode plate 91 low and continuously exhibits good electrical conductivity even in a strongly acidic atmosphere. In addition, unlike the conventional case, the contact resistance can be suppressed without providing a gold plating layer directly on the surface in contact with the electrode, so that the cost can be reduced.

  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.

FIG. 6 shows an M ₄ N type crystal structure.

As shown in FIG. 6, 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. Thus, nitrogen atoms are arranged in octahedral voids in the center of the unit cell of the face-centered cubic lattice in which the nitride layer is formed of at least one or more transition metal atoms selected from the group of Fe, Cr, Ni, and Mo. In the case of having an M ₄ N type crystal structure, the corrosion resistance in a strongly acidic atmosphere having a pH of 2 to 3 can be further improved.

  In addition, it is preferable that the thickness of the nitride layer is formed in the range of 0.5-5 [micrometers] in the base material surface. In this case, the corrosion resistance in a strong acid atmosphere is excellent. If the thickness of the nitride layer is less than 0.5 [μm], cracks may occur between the nitride layer and the substrate, or the adhesion strength between the nitride layer and the substrate may be insufficient. When used for a long time, the nitrided layer easily peels 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 exceeds 5 [μm], as the thickness of the nitride layer increases, the stress in the nitride layer becomes excessive and cracks occur in the nitride layer, resulting in holes in the fuel cell separator. Corrosion tends to occur, and it becomes difficult to contribute to improvement of corrosion resistance.

  Furthermore, it is preferable that the nitrogen amount on the extreme surface of the nitrided layer is 10 [at%] or more and the oxygen amount is 35 [at%] or less. Here, the extreme surface of the nitride layer refers to an atomic layer having a depth of 3 to 4 nm from the outermost surface of the nitride layer, that is, a depth of several tens of layers. Further, the outermost surface refers to the outermost atomic layer of the nitride layer. When the coverage of oxygen molecules adsorbed on the 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 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 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 It is possible to suppress the contact resistance between the two, and it is possible to obtain a fuel cell separator excellent in corrosion resistance in a strongly acidic atmosphere.

Further, at a depth of 10 [nm] from the outermost surface of the nitride layer, 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%]. More preferably, the oxygen content is 22 [at%] or less. In this case, the corrosion resistance in a strong acid atmosphere is excellent. In addition, when it deviates from this range, the contact resistance generated between the separator and the electrode becomes high, the contact resistance value per single cell constituting the fuel cell stack exceeds 40 [mΩ · cm ² ], and power generation There is a problem that performance deteriorates.

  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. In this case, if the chemical potential of the nitrogen atom in the nitride layer is increased and the transition metal atom forms a compound with the nitrogen atom in a state where the activity of the transition metal atom is further reduced, the free energy of the transition metal atom , The reactivity of the transition metal atom with respect to 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 fuel cell intermediate plate 92 according to the embodiment of the present invention is excellent in corrosion resistance. In addition, it is possible to obtain a fuel cell intermediate plate 92 that is low in cost, has good productivity, has low contact resistance with a constituent material such as an adjacent gas diffusion electrode, and has good power generation performance of the fuel cell. Further, 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 intermediate plate 92 according to the embodiment of the present invention. And cost reduction can be realized.

(Manufacturing method of intermediate plate for fuel cell)
Next, a method for manufacturing the fuel cell intermediate plate 92 according to the embodiment of the present invention will be described. The method of manufacturing the fuel cell intermediate plate 92 includes a nitriding treatment in which a base material made of a transition metal or a transition metal alloy in which a fuel or oxidant passage is formed is subjected to nitriding treatment at a temperature of 500 [° C.] or less. Nitride layer having an M ₄ N type crystal structure in which nitrogen atoms are arranged in an octahedral void at the center of a unit cell of a face-centered cubic lattice formed by transition metal atoms selected from Fe, Cr, Ni, and Mo It has the process of forming.

  When the surface of the stainless steel is subjected to nitriding treatment at a high temperature, nitrogen is combined with Cr in the base material, and mainly nitride such as CrN having a NaCl-type crystal structure is deposited, so that the corrosion resistance of the fuel cell intermediate plate 92 is reduced. descend. 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 among the nitrided layers, the corrosion resistance of the fuel cell intermediate plate 92 is improved by performing nitriding at a low temperature of 500 [° C.] or less. In addition, the contact resistance between the plates 90 and 91 and the intermediate plate 92 can be kept low, the power generation efficiency of the fuel cell can be maintained, and the fuel cell intermediate plate 92 having excellent durability and reliability can be obtained at low cost. Can do.

  Note that when the nitriding temperature is lower than 350 [° C.], it takes a long time to obtain a nitrided layer having this crystal structure, and the productivity deteriorates. 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. Since the gas soft nitriding method has a high oxygen partial pressure during nitriding, the amount of oxygen in the nitrided layer 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 configuration of the nitriding apparatus used in the method for manufacturing the fuel cell intermediate plate according to the embodiment of the present invention will be described with reference to FIG. FIG. 7 is a schematic side view showing the configuration of the nitriding apparatus used in the method for manufacturing the fuel cell intermediate plate of the present embodiment.

As shown in FIG. 7, the nitriding apparatus 30 includes a batch type nitriding furnace 31, a gas supply apparatus 32 that supplies an 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 an intermediate plate for a fuel cell 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 with the carbon paper normally used as the gas diffusion layer of the fuel cell 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 with the carbon paper 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 is not formed when the processing time is less than 1 [minute], and conversely, the manufacturing cost increases when the processing time exceeds 60 [minute]. It is. Furthermore, the reason why the gas mixture ratio is defined within this range is that a nitride layer cannot be formed when the nitrogen ratio in the gas decreases, and conversely, hydrogen that acts as a reducing agent when the nitrogen ratio increases. This is because the amount is reduced and the surface of the base material is oxidized. By performing plasma nitriding under such processing conditions, a nitride layer having an M ₄ N type crystal structure can be formed on the substrate surface.

  As described above, according to the method for manufacturing an intermediate plate for a fuel cell according to the embodiment of the present invention, a low resistance value in an oxidizing environment is maintained, corrosion resistance is excellent, and the cost is low by a simple operation. Thus, it becomes possible to manufacture the intermediate plate 92 for a fuel cell that has been realized.

(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.

  8A and 8B are views showing the appearance of a fuel cell electric vehicle equipped with a fuel cell stack. FIG. 8A is a side view of the fuel cell electric vehicle, and FIG. 8B is a plan view of the fuel cell electric vehicle. is there.

  As shown in FIG. 8B, 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 for connecting the left and right hood ridges including the front side member to each other is combined and welded. A joined engine compartment 52 is formed. In the fuel cell electric vehicle 50 shown in FIGS. 8 (a) and 8 (b), the fuel cell stack 1 is mounted in the engine compartment 52.

  By mounting the fuel cell stack 1 having high power generation efficiency to which the fuel cell intermediate plate 92 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 50 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 intermediate plate 92 for a fuel cell according to the embodiment of the present invention will be described. In these examples, the effectiveness of the intermediate plate 92 for a fuel cell according to the present invention was examined. 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 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 nitride layer thickness>
The thickness of the nitride layer 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 nitrided layer was obtained by measuring the Fe concentration and Cr concentration of the nitrided layer by X-ray electron spectroscopy (XPS). Further, the amount of nitrogen and the amount of oxygen on the surface of the nitrided layer 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>
Measurements of nitrogen and oxygen at depths of 10 nm and 100 nm from the outermost surface of the nitride layer were performed using a scanning Auger electron spectrometer. The apparatus used was MODEL4300 manufactured by PHI. The measurement is performed under the conditions of an electron beam acceleration voltage of 5 [kV], a measurement area of 20 [μm] × 16 [μm], an ion gun acceleration voltage of 3 [kV], and a sputtering rate of 10 [nm / min] (SiO ₂ conversion value). went.

<Measurement of contact resistance value>
The 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.

9A, a carbon paper 63 is interposed between the electrode 61 and the sample 62, and as shown in FIG. 9B, 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. 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. 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, the thickness of the nitride layer, the amount of nitrogen and oxygen on the extreme surface of the nitride layer, the ratio of the amount of nitrogen to the amount of oxygen O / N, 10 [nm] and 100 [nm] from the outermost surface of the nitride layer Table 2 shows the amounts of nitrogen and oxygen at the depth, contact resistance values, and ion elution amounts.

  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 was observed from the cross-sectional structure shown in FIG. 10A, a nitride layer of about 5 [μm] was formed on the surface in Example 1. As described above, although the surface is covered with the nitride layer having the M ₄ N type crystal structure, the X-ray diffraction peak is also derived from the austenite base material. 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 no nitrided layer was formed as in Comparative Example 1, only the peak derived from the austenite 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. 11A, and a cross-sectional structure photograph of the sample obtained in Comparative Example 1 is shown in FIG. In FIG. 11A, nitride layers 72 are formed on both surfaces of the base material 71, but in FIG. 11B, a modified layer such as a nitride layer is not formed on the surface of the base material 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 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 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, the ion elution amount is low in each of Examples 1 to 6 in which the thickness of the nitrided layer is 0.5 to 5 [μm], and the corrosion resistance is excellent. I understood.

  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 mainly includes CrN having a NaCl-type crystal structure, Comparative Examples 1 to 3 in which the nitriding treatment is not performed. Compared with, the contact resistance is low, but the corrosion resistance is inferior.

Further, from Table 2, the nitrogen amount on the surface of the nitrided layer measured by XPS, the amount of oxygen, and the ratio O / N of the amount of nitrogen to the amount of oxygen, the nitrogen amount on the surface of the nitrided layer is 10 [at%]. In Examples 1 to 6 in which the oxygen amount was 35 [at%] or less and the 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. 12 shows an element profile in the depth direction by scanning Auger electron spectroscopy analysis of the sample obtained in Example 1.

As shown in FIG. 12, an oxide film exists on the outermost surface of the nitride layer because there is a slight partial pressure of oxygen during the nitriding process, and electrons can freely move back and forth through the thickness of the oxide film. 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 having a contact resistance value of 40 [mΩ · cm ² ] or less, the nitrogen amount is 18 [at%] or more at a depth of 10 nm from the outermost surface of the nitride layer. 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, the nitrogen amount is 17 [at%] or more and the oxygen amount is 18 [at%] or less. It was. 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 are deviated 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 of 40 is obtained by forming a nitride layer having an M ₄ N type crystal structure in which nitrogen atoms are arranged in the octahedral voids in the center of a unit cell of a face-centered cubic lattice formed of at least one transition metal atom. It exhibits low contact resistance of [mΩ · cm ² ] or less, and also has a low ion elution amount and excellent corrosion resistance, so it has both low contact resistance and corrosion resistance at the same time.

  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. 13A and 13B show the relationship between the amount of nitrogen and oxygen and the contact resistance value.

  FIG. 13 (a) shows the relationship between the amount of nitrogen and oxygen on the pole surface and the contact resistance value. FIG. 13B 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. 13 (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 is large, an insulating oxide film is formed on the surface of the substrate, so that the contact resistance value is high, and the nitride layer is formed on the surface of the substrate. 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. 13B, it was revealed that the contact resistance value is lower as the ratio O / N of the nitrogen amount to the oxygen amount on the pole surface is lower.

From the above measurement results, Examples 1 to 6 are different from Comparative Examples 1 to 5 in any example. The intermediate plate 92 according to this embodiment is a base material made of a transition metal or an alloy of transition metals. And having a cubic crystal structure formed directly on the base layer, the contact resistance value before the electrolysis test is 10 [mΩ · cm ² ] or less and exhibits a low contact resistance. It was also found that the contact resistance value after the test was kept low, and both the low contact resistance and the corrosion resistance were simultaneously provided.

It is a perspective view which shows the external appearance of the fuel cell stack comprised using the intermediate plate for fuel cells which concerns on embodiment of this invention. It is a disassembled perspective view of the fuel cell stack comprised using the intermediate board for fuel cells which concerns on embodiment of this invention. 1 is a conceptual cross-sectional view of a fuel cell separator and a membrane electrode assembly according to an embodiment of the present invention. 1 is a perspective view showing an intermediate plate for a fuel cell according to an embodiment of the present invention. It is an expanded sectional view by the AA line of FIG. The M _{4 N} type crystal structure of the nitride layer of the intermediate plate according to an embodiment of the present invention is a schematic diagram showing. It is sectional drawing which shows notionally the nitriding apparatus used for the manufacturing method of the intermediate plate for fuel cells which concerns on embodiment of this invention. It is a perspective view 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 14 Nitride layer 20 M ₄ N type crystal structure 21 Transition metal atom 22 Nitrogen atom 90 Anode plate (anode separator)
91 Cathode plate (cathode separator)
92 Intermediate plate 103 Nitrogen layer

Claims (10)

  It was composed of a base material made of transition metal or transition metal alloy, which was sandwiched between an anode separator and a cathode separator constituting a fuel cell separator and provided with a nitride layer having a cubic crystal structure on the surface layer. An intermediate plate for a fuel cell.   2. The fuel cell intermediate plate according to claim 1, wherein the nitrided layer is formed on an outer surface of a substrate exposed to the outside. 3. The nitride layer is obtained by nitriding the 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,
3. The fuel cell intermediate plate according to claim 1, wherein a nitride layer having a cubic crystal structure is formed in a depth direction from the surface of the base material. 4. The cubic crystal structure of the nitride layer is an octahedral void at the center of a unit cell of a face-centered cubic lattice formed by metal atoms of at least one transition metal element selected from the group of Fe, Cr, Ni, and Mo. intermediate plate for a fuel cell according to claim 1, wherein the nitrogen atom is the crystal structure of the deployed M _{4 N} type.   The thickness of the said nitrided layer is 0.5-5 [micrometer], The intermediate plate for fuel cells of any one of Claims 1-4 characterized by the above-mentioned.   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. An intermediate plate for a fuel cell according to 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. 7. The fuel cell intermediate plate according to claim 6.   A fuel cell stack comprising the intermediate plate according to any one of claims 1 to 7. A method for producing an intermediate plate for a fuel cell, in which a nitride layer is formed on a surface layer of a base material by subjecting the base material made of a transition metal or a transition metal alloy to a plasma nitriding treatment at a temperature of 500 [° C.] or less. And
In the nitride layer, nitrogen atoms are arranged in octahedral voids in the center of a unit cell of a face-centered cubic lattice formed of metal atoms of at least one transition metal element selected from Fe, Cr, Ni, and Mo. A method for producing an intermediate plate for a fuel cell, characterized by having an M ₄ N type crystal structure. The intermediate for a fuel cell according to claim 9, wherein the plasma nitriding treatment is performed by applying a negative bias voltage to the substrate in a low-temperature non-equilibrium plasma in which nitrogen gas and hydrogen gas are discharged. A manufacturing method of a board.