JP2007073432A - Separator for fuel cell, fuel cell stack, fuel cell vehicle, and method of manufacturing separator for fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a separator for a fuel cell having low contact resistance between a separator and an electrode, high corrosion resistance, and low cost, and to provide a fuel cell stack and a fuel cell vehicle equipped with the fuel cell stack. <P>SOLUTION: The separator 3 for the fuel cell obtained by nitriding the surface of a substrate 10 comprising alloy containing a transition metal element selected from Fe, Cr, Ni, and Mo has a nitrided layer 11 having hexagonal system crystal structure in the depth direction from the surface 10a of the substrate 10, and the substrate 10 contains nitrogen in an interface region 10b between the substrate 10 and the nitrided layer 11. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

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

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

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

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

  FIG. 11 is a cross-sectional view showing the configuration of a single cell forming the fuel cell stack. As shown in FIG. 11, the unit 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 arranged 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 ⁻ (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 the 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 elutes in the humidified water or the water generated by the reaction of the formula (2), so that the cation binds 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 above-mentioned passive film produces contact resistance with carbon paper that is usually used as a gas diffusion layer. The overvoltage caused by resistance polarization in the fuel cell can recover the exhaust heat by cogeneration or the like in the stationary application, thereby improving the total thermal efficiency. On the other hand, in automobile applications, heat loss due to contact resistance can only be thrown out of the radiator through the cooling water, so that if the contact resistance increases, the power generation efficiency decreases. In addition, a decrease in power generation efficiency is equivalent to an increase in heat generation, and it becomes necessary to equip a larger cooling system. Therefore, an increase in contact resistance is an important issue to be solved.

In a fuel cell, the theoretical voltage per unit cell is 1.23 [V], but the voltage that can 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 (see Patent Document 1). In addition, a fuel cell separator in which a noble metal or a noble metal alloy is adhered by removing stainless steel from a passive film formed on a surface that generates contact resistance by contact with an electrode after being formed into a fuel cell separator shape. Has been proposed (see Patent Document 2).
Japanese Patent Laid-Open No. 10-228914 (2nd page, FIG. 2) JP 2001-6713 A (page 2)

  However, if the surface of the fuel cell separator is plated or coated with a noble metal, not only labor is required during production but also material costs are incurred.

  The present invention has been made to solve the above problems, and the fuel cell separator according to the first invention is made of an alloy containing a transition metal element selected from Fe, Cr, Ni and Mo. A separator for a fuel cell obtained by nitriding the surface of a base material, comprising a nitride layer having a cubic crystal structure in the depth direction from the surface of the base material, wherein the base material is an interface region with the nitride layer The main point is to contain nitrogen.

  According to a second aspect of the present invention, there is provided a method for producing a fuel cell separator comprising an alloy containing a transition metal element selected from Fe, Cr, Ni and Mo and nitrogen having a concentration of 0.1% or more. The gist is to have a nitriding treatment step of nitriding the surface of the substrate.

  The gist of a fuel cell stack according to a third aspect of the invention is that it includes the fuel cell separator according to the first aspect of the invention.

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

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

  According to the second invention, it becomes easy to manufacture a high-performance fuel cell separator.

  According to the third invention, a high-performance fuel cell stack can be obtained, and miniaturization and cost reduction can be achieved.

  According to the fourth aspect of the invention, by mounting the fuel cell stack that is reduced in size and cost, it is possible to increase the travel distance and to ensure the freedom of styling.

  Hereinafter, a fuel cell separator, a fuel cell stack, a fuel cell vehicle, and a method for manufacturing a fuel cell separator according to an embodiment of the present invention will be described with reference to an example applied to a polymer electrolyte fuel cell.

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

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

  FIG. 3 shows a schematic diagram of the fuel cell separator 3 shown in FIG. 3A is a schematic perspective view of the fuel cell separator 3, FIG. 3B is a cross-sectional view taken along the line IIIb-IIIb of the fuel cell separator 3, and FIG. FIG. 3D is a sectional view showing another example of the fuel cell separator 3. As shown in FIG. 3, the fuel cell separator 3 includes a base material 10 made of an alloy containing a transition metal element selected from Fe, Cr, Ni, and Mo. The surface of the base material 10 is nitrided. The nitride layer 11 having a cubic crystal structure formed in the depth direction of the surface 10a of the substrate 10 and the base layer 12 which is an unnitrided layer that is not nitrided. The base material 10 contains nitrogen in the interface region 10 b between the nitride layer 11 and the base layer 12. The fuel cell separator 3 is formed with a groove-like passage 13 of fuel or oxidant having a rectangular cross section by press molding. Between the passage 13 and the passage 13, a flat plate portion 14 defined by the passage 13 and the passage 13 is provided, and a cubic nitride layer 11 extends along the outer surface of the passage 13 and the flat plate portion 14. . The flat plate portion 14 contacts the gas diffusion layer on the adjacent solid polymer membrane when the fuel cell separators 3 and the single cells 2 are alternately laminated. In FIG. 3, for convenience of explanation, the surface 10 a of the base material 10, that is, the surface of the nitride layer 11 is described by a straight line, but the surface of the nitride layer 11 has a plurality of cleavages.

  In the fuel cell separator according to the present embodiment, an alloy containing a transition metal element selected from Fe (iron), Cr (chromium), Ni (nickel) and Mo (molybdenum) is used as a base material. In order to provide a nitride layer having a cubic crystal structure in the depth direction from the surface of the substrate, the transition metal atom in the nitride layer forms a bond with a high affinity with the nitrogen atom. In addition, since a metal bond is formed between the metal atoms, a fuel cell separator having excellent electrical conductivity can be obtained. Further, the nitrided layer having a cubic crystal structure is chemically stable even in a strongly acidic atmosphere of pH 2 to 3 that is usually used as a fuel cell, and therefore has excellent corrosion resistance. Therefore, it is possible to obtain a fuel cell separator that keeps the contact resistance between the fuel cell separator and the carbon paper 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.

  Moreover, since the base material contains nitrogen in the interface region with the nitride layer, the substrate is further excellent in corrosion resistance, and elution of metal ions is reduced due to the cleavage formed in the nitride layer. In order for the base material to contain nitrogen in the interface region with the nitrided layer, the base material 10 itself contains nitrogen as shown in FIG. 3C, that is, the entire base layer 12 that is an unnitrided layer contains nitrogen. In addition, as shown in FIG. 3D, the base material 10 has a layer 12a containing nitrogen and a layer 12b not containing nitrogen, and the nitride layer 11 is a layer 12a containing nitrogen. It may be formed on the side.

  Since the nitride layer is hard and brittle, as shown in FIGS. 4A and 4B, the nitride layer is cleaved as a defect in the nitride layer. FIG. 4A shows an example in which the base material 10 contains nitrogen in the interface region 10 b with the nitride layer 11. As shown in FIG. 4A, the nitride layer 11 has a nitrogen concentration n1% (here, 0.2%) contained in the base layer 12 and a nitrogen concentration n2% permeating the base material 10 by nitriding treatment. N1 + n2% nitrogen. A curve indicating the nitrogen concentration is composed of l 1, l 2, l 3, and l 4, and a region A represents an interface region 10 b with the nitride layer 11 of the substrate 10. Region A has a nitrogen concentration of 0.2%. In the nitride layer 11, for example, cracks are formed as nitride layer defects such as fine cracks 15a and 15b and fine pits 15c. In addition to the cracks that are split to the middle of the nitride layer 11 such as the cracks 15 a, there are cracks that pass through the nitride layer 11 and extend to the base layer 12, such as cracks 15 b and pits 15 c. When the cleavage extends through the nitrided layer 11 to the base layer 12, the non-nitrided base layer 12 is exposed. In this case, since nitrogen is contained in the interface region 10b of the base material 10 with the nitride layer 11, elution of metal ions can be suppressed even if the base layer 12 is exposed. This is considered to be because ammonia is generated in the defect portion of the nitride layer and the effect of neutralizing the acidic atmosphere in the defect acts, and as a result, ion elution from the exposed portion of the base layer is suppressed. By this action, elution of metal ion elution is suppressed, and deterioration of the solid polymer electrolyte membrane can be suppressed.

  FIG. 4B is an example in which the base material 17 does not have a layer containing nitrogen in the interface region 17 b with the nitride layer 16. As shown in FIG. 4B, the nitride layer 16 has a nitrogen concentration of N% that penetrates the base material 17 by nitriding treatment. A curve indicating the nitrogen concentration is composed of l5, l6, l7, and l8, and a region B represents an interface region 17b of the base material 17 with the nitride layer 16. Similarly to FIG. 4A, the nitride layer 16 is formed with cracks such as fine cracks 19a and 19b and fine pits 19c. The cleavage extends through the nitride layer 16 to the base layer 18 to expose the non-nitrided base layer 18. In this case, as indicated by an arrow in FIG. 4B, ions of the transition metal element constituting the substrate 17 are eluted from the exposed portion of the base layer 18, and the solid polymer electrolyte membrane is deteriorated. Thus, when it does not have the layer containing nitrogen in the interface area | region with the nitride layer of a base material, it is chemically unstable in a strong acidic atmosphere of pH 2-3, and corrosion resistance is inferior.

  The interface region with the nitride layer preferably has a nitrogen concentration of 0.1 [%] or more from the viewpoint of pitting corrosion resistance. Furthermore, the nitrogen concentration is preferably 0.2% or more because the pitting corrosion resistance is further improved.

  The base material is preferably stainless steel containing at least one metal element selected from the group consisting of Fe, Cr, Ni and Mo, and further containing nitrogen. Examples of stainless steels containing such elements include austenitic, austenitic / ferrite, and precipitation hardening stainless steels. Among these, the base material is preferably made of austenitic stainless steel. Examples of the austenitic stainless steel include SUS304LN, SUS316LN, SUS304N1, SUS304N2, and SUS316N.

More specifically, the cubic crystal structure is such that 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 Fe, Cr, Ni, and Mo. preferably made of M _{4 N} type crystal structure which is. The crystal structure of the M ₄ N type is shown in FIG. As shown in FIG. 5, 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 this M ₄ N type crystal structure 20, the transition metal atom M 21 is preferably mainly composed of Fe, but even if Fe is an alloy partially substituted with other transition metal atoms such as Cr, Ni, Mo, etc. good.

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, the M ₄ N type in which the nitrogen layer is arranged in the octahedral void at the center of the unit cell of the face-centered cubic lattice formed by the transition metal atom selected from Fe, Cr, Ni, and Mo in the nitride layer. In the case of this crystal structure, the corrosion resistance in a strongly acidic atmosphere of pH 2 to 3 is further improved, and the contact resistance with the carbon paper can be kept low.

  The nitride layer is preferably formed on the substrate surface in a thickness range of 0.5 to 5 [μm]. In this case, the corrosion resistance in a strong acid atmosphere is excellent, and the contact resistance with the carbon paper can be kept low. In addition, when the thickness of the nitride layer is less than 0.5 [μm], it is used for a long time due to the occurrence of cracks between the nitride layer and the base layer or insufficient adhesion strength between the nitride layer and the base layer. As a result, the nitrided layer easily peels off from the interface with the base layer, 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.

  Thus, by adopting the above-described configuration, the fuel cell separator according to the embodiment of the present invention is excellent in corrosion resistance. In addition, it is possible to obtain a fuel cell separator that is low in cost, has good productivity, has low contact resistance with a constituent material such as an adjacent gas diffusion electrode, and has good power generation performance of the fuel cell. In addition, the fuel cell stack according to the embodiment of the present invention includes the fuel cell separator according to the embodiment of the present invention, so that high power generation efficiency can be maintained without impairing power generation performance, and the size and size of the fuel cell stack can be reduced. Cost reduction can be realized.

(Manufacturing method of fuel cell separator)
Next, an embodiment of a method for manufacturing a fuel cell separator according to an embodiment of the present invention will be described. The method of manufacturing a fuel cell separator includes a surface of a substrate made of an alloy containing a transition metal element selected from Fe, Cr, Ni and Mo and nitrogen having a concentration of 0.1% or more. And a nitriding treatment step for nitriding the substrate. This method makes it easy to obtain a high-performance fuel cell separator that includes a nitride layer having a cubic crystal structure in the depth direction from the surface of the substrate, and the substrate contains nitrogen in the interface region with the nitride layer. It is done.

  After the nitriding treatment step, it is preferable to have a step of forming a fuel or oxidant passage by pressing. By press-molding after nitriding, the warpage and undulation of the separator due to distortion during heat treatment (nitriding) can be reduced.

The nitriding process includes a plasma nitriding process at a temperature of 500 [° C.] or less, and the face-centered cubic lattice formed by transition metal atoms selected from Fe, Cr, Ni, and Mo by the plasma nitriding process. It is preferable to form a nitride layer having an M ₄ N type crystal structure in which nitrogen atoms are arranged in the octahedral voids at the center of the unit cell. When the surface of stainless steel is subjected to nitriding treatment at a high temperature, nitrogen is combined with Cr in the base material, and nitrides such as CrN having a NaCl-type crystal structure are mainly precipitated, so that the corrosion resistance of the fuel cell separator is lowered. . On the other hand, when plasma nitriding is performed at a temperature of 500 [° C.] or lower, an M ₄ N type crystal structure is formed on the surface of the substrate. Since this crystal structure is particularly rich in corrosion resistance among the nitrided layers, the corrosion resistance of the fuel cell separator is improved by performing plasma nitriding at a low temperature of 500 [° C.] or lower. Moreover, the contact resistance between the separator and the constituent material such as the adjacent gas diffusion electrode can be kept low, the power generation efficiency of the fuel cell can be maintained, and a fuel cell separator having excellent durability and reliability can be obtained at low cost. Can do. Note that when the nitriding temperature is lower than 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.].

  Details of plasma nitriding are shown below. FIG. 6 is a schematic side view of the nitriding apparatus 30 used in the method for manufacturing a fuel cell separator according to the embodiment of the present invention, and FIG. 7 is a system diagram of the nitriding apparatus 30.

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, plasma electrodes 33a and 33b that generate plasma in the nitriding furnace 31, and these electrodes 33a and 33b. A DC power source 33 that supplies a DC voltage to the nitriding furnace 31, a pump 34 that discharges the gas in the nitriding furnace 31, and a temperature sensor 37 that detects the temperature in the nitriding furnace 31 are included. 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 fuel cell separator is provided on the ceiling 31c of the inner wall 31a. The gas supply device 32 has a gas chamber 38 and a gas supply conduit 39, and the gas chamber 38 is provided with openings 32a, 32b, 32c and 32d. The openings 32a, 32b, and 32c communicate with an H ₂ gas supply line 32e, an N ₂ gas supply line 32f, and an Ar gas supply line 32g each having a gas supply valve V1, a gas supply valve V2, and a gas supply valve V3. The gas supply device 32 has an opening 32 d that communicates with one end of the gas supply line 39. 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 V4. 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 V5 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 35e in the range of 0 [V] to a bias voltage, and the obtained voltage is passed through the bias circuit 35a. Supplied to each stainless steel foil 44. The DC power supply 33 is turned on / off by a control signal from the control panel 43. The potentiometer 45 is supplied with a bias control signal from the control panel 33 via the bias control circuit 35b, and the movable contact 35e slides in response to the control signal. Accordingly, each stainless steel foil 44 has a voltage difference between the terminal voltage of the DC power supply 33 and the bias voltage supplied via the movable contact 35e with respect to the inner wall 31a. 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 1 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 the separator for a fuel cell according to the embodiment of the present invention, the low resistance value in the oxidizing environment is maintained, the corrosion resistance is excellent, and the cost is reduced by a simple operation. It is possible to easily manufacture a high-performance fuel cell separator that realizes the above.

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

  FIG. 8 is a view showing an appearance of a fuel cell electric vehicle equipped with the fuel cell stack 1. FIG. 8A is a side view of the fuel cell electric vehicle 50, and FIG. 8B is a top view of the fuel cell electric vehicle 50. As shown in FIG. 8 (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 for connecting 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 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 separator according to the embodiment of the present invention is mounted on 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 3 and Comparative Examples 1 to 6 of the separator for a fuel cell according to the embodiment of the present invention will be described. In these examples, the effectiveness of the separator for a fuel cell according to the embodiment of the present invention was examined. Each sample was prepared by treating different raw materials under different conditions. The present invention is not limited to the examples.

<Preparation of sample>
In the examples, SUS304LN and SUS316LN were used as austenitic stainless steels having different JIS standard nitrogen contents, and SUS304, SUS316, and SUS316L were used as austenitic stainless steels not containing nitrogen in the comparative example. Table 1 shows the base material used and the composition of the base material. In each example, a bright annealed (BA) material having a thickness of 0.1 [mm] was used, and after degreasing and cleaning these substrates, plasma nitriding treatment by direct current glow discharge was performed on both surfaces. Table 2 shows nitriding methods and nitriding conditions. As shown in Table 2, the plasma nitriding conditions are as follows: the processing temperature is 400 to 550 [° C.], the processing time is 1 to 60 [min], and the gas mixing ratio is N ₂ : H ₂ = 1: 1 or 7: 3. The pressure was 3 to 7 [Torr] (= 399 to 931 [Pa]).

  Here, each sample was evaluated by the following method.

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

<Measurement of 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 the amount of nitrogen in the interface region>
Measurement of the amount of nitrogen in the interface region with the nitride layer of the base material (for convenience, a nitrogen-containing layer) was performed with 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. The detection limit is 0.05 [mass%].

<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 paper 63b. The electrode 61b is configured. 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.

Measurement results of crystal structure of nitride layer, thickness of nitride layer, nitrogen concentration of nitrogen-containing layer, contact resistance value before and after electrolytic test and ion elution amount of Examples 1 to 3 and Comparative Examples 1 to 6 Is shown in Table 3.

  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 Examples 1 and 2, 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, Peaks of S1 to S5 showing an M ₄ 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. In Examples 2 and 3, 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 3, peaks indicating CrN and γ ′ phase were observed. In the γ ′ phase, Cr is combined with nitrogen atoms to form a Cr-based nitride compound such as CrN, so that the 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 nitriding temperature exceeds 500 [° C.], a Cr-based nitride compound such as CrN having an NaCl type crystal structure is formed.

As shown in Table 3, in Examples 1 to 3 in which the nitride layer having the M ₄ N type crystal structure was formed, the contact resistance value was 40 [mΩ · cm ² ] or less. 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 3, 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. Further, in Examples 1 to 3, it was found that the ion elution amount is low, the corrosion resistance is excellent, and the higher the nitrogen content at the base-side interface, the more the ion elution tends to be suppressed. .

Compared to the examples, when the surface of the austenitic stainless steel was not nitrided as in Comparative Examples 1 and 2, the contact resistance was high because there was a passive film on the substrate surface. In general, austenitic stainless steel is excellent in corrosion resistance due to the presence of a passive film, but was found to be inferior to Examples 1 to 3. Even when the nitriding treatment is performed as in Comparative Example 3, when the nitride layer mainly contains CrN having a NaCl type crystal structure, the contact resistance is smaller than that of Comparative Example 1 and Comparative Example 2 where the nitriding treatment is not performed. Although lower, corrosion resistance is inferior. Further, even when a nitride layer having an M ₄ N type crystal structure is formed as in Comparative Examples 4 and 5, when nitrogen is not contained at the substrate interface, Examples 1 to 3 containing nitrogen and Although there was almost no difference in contact resistance, the amount of ion elution increased and the corrosion resistance was inferior.

From the above measurement results, Examples 1 to 3 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. Since it has a nitride layer 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, it exhibits low contact resistance, Furthermore, it has been shown that since it has a nitrogen-containing layer at the substrate-side nitrided layer interface, the amount of ion elution from the substrate is small and the corrosion resistance is excellent, so that it has both low contact resistance and corrosion resistance at the same time.

It is a perspective view which shows the external appearance of the fuel cell stack comprised using the separator for fuel cells which concerns on embodiment of this invention. It is an expanded view of the fuel cell stack comprised using the separator for fuel cells which concerns on embodiment of this invention. (A) It is a typical perspective view of the separator for fuel cells. (B) It is the IIIb-IIIb sectional view taken on the line of the separator for fuel cells. (C) It is the IIIc-IIIc sectional view taken on the line of the separator for fuel cells. (D) It is sectional drawing which shows the other example of the separator for fuel cells. (A) This is an example in which the base material contains nitrogen in the interface region with the nitride layer. (B) This is an example in which the base material does not contain nitrogen in the interface region with the nitride layer. 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 separator for fuel cells which concerns on embodiment of this invention. It is a system diagram of a nitriding apparatus. 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. It is sectional drawing which shows the structure of the single cell which forms a fuel cell stack.

Explanation of symbols

1 the fuel cell stack 2 single cells 3 fuel cell separator 4 End flange 5 fastening bolts 10 substrate 10a surface 10b interface region 11 nitride layer 12 the base layer 13 passage 14 flat plate part 20 M ₄ N type crystal structure 21 transition metal atom 22 nitrogen atoms

Claims (11)

A fuel cell separator obtained by nitriding the surface of a substrate made of an alloy containing a transition metal element selected from Fe, Cr, Ni and Mo,
A fuel cell separator comprising a nitride layer having a cubic crystal structure in a depth direction from a surface of the substrate, wherein the substrate contains nitrogen in an interface region with the nitride layer.   The fuel cell separator according to claim 1, wherein the nitride layer has a cleavage.   3. The fuel cell separator according to claim 1, wherein the interface region with the nitride layer has a nitrogen concentration of 0.1 [%] or more. 4.   The fuel cell separator according to claim 3, wherein the interface region with the nitride layer has a nitrogen concentration of 0.2% or more. The cubic crystal structure is M ₄ N in which nitrogen atoms are arranged in octahedral voids in the center of a unit cell of a face-centered cubic lattice formed by transition metal atoms selected from Fe, Cr, Ni, and Mo. The fuel cell separator according to any one of claims 1 to 4, wherein the fuel cell separator has a crystal structure of a type.   6. The fuel cell separator according to claim 1, wherein the nitrided layer has a thickness in a range of 0.5 to 5 μm.   A nitriding treatment step of nitriding a surface of a base material made of an alloy containing a transition metal element selected from Fe, Cr, Ni, and Mo and nitrogen having a concentration of 0.1% or more. A method for producing a fuel cell separator.   8. The method of manufacturing a fuel cell separator according to claim 7, further comprising a step of forming a fuel or oxidant passage by pressing after the nitriding step. The nitriding step includes a plasma nitriding step at a temperature of 500 [° C.] or less, and a face center formed by transition metal atoms selected from Fe, Cr, Ni, and Mo by the plasma nitriding step. The fuel cell separator according to claim 7 or 8, wherein a nitride layer having an M ₄ N type crystal structure in which nitrogen atoms are arranged in an octahedral void at the center of a unit cell of a cubic lattice is formed. Manufacturing method.   A fuel cell stack comprising the fuel cell separator according to any one of claims 1 to 6.   A fuel cell vehicle comprising the fuel cell stack according to claim 10 as a power source.

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