JP2007073371A - FUEL CELL STACK AND METHOD FOR PRODUCING FUEL CELL FRAME MEMBER - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell stack having a frame member for a fuel cell excellent in rigidity and corrosion resistant property, and to provide a method of manufacturing the frame member for the fuel cell. <P>SOLUTION: The fuel cell stack is composed of film-electrode assemblies 96 having a gas diffusion layer 104 at nearly middle part of an electrolyte film 97 through a catalyst layer 103, and a frame member 90 fitted to the film-electrode assemblies 96 in a state of contacting peripheral part 105 thereof. The frame member 90 is constituted by a base material made of transition metal or an alloy thereof, on outer surface of which a nitride layer having crystal structure of cubic system is formed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fuel cell stack and a method of manufacturing a fuel cell frame member, and more particularly to a solid polymer electrolyte type fuel cell frame member 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 solid polymer electrolyte membrane having a proton exchange group in the molecule as an electrolyte to saturate the polymer electrolyte membrane. It is a battery that utilizes the function as a proton conductive electrolyte when it is hydrated.

  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 light with other ancillary facilities, various uses including for mounting on an electric vehicle are expected.

  The solid polymer electrolyte fuel cell has a fuel cell stack. This 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 ends between 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. 15 is a cross-sectional view showing the configuration of a single cell forming the fuel cell stack. As shown in FIG. 15, 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), and 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 ⁻ 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 solid polymer electrolyte membrane is strongly acidic, the components contacting the solid polymer electrolyte membrane are required to have corrosion resistance against sulfuric acid acidity of about pH 2 to 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 components of the fuel cell stack are required to have corrosion resistance that can withstand in a strongly acidic atmosphere. In addition, the corrosion resistance required here means the durability which can maintain an electrical-conducting performance also in the oxidizing environment where a component is a strong acid. In other words, the cation dissolves in the humidified water or the water generated by the reaction of the formula (2), so that the cation is bonded to the sulfonic acid group that should originally be a path for protons to occupy the sulfonic acid group, and the power generation of the electrolyte membrane Corrosion resistance needs to be measured in an environment that degrades properties.

In addition, since the membrane electrode assembly has a relatively low rigidity, the membrane electrode assembly is bent when the fuel cell stack is assembled, and the assembly workability may be reduced. Therefore, a technique for improving the rigidity of the membrane electrode assembly by forming an opening by cutting out the central portion of the resin frame member into a rectangular shape and disposing the membrane electrode assembly in the opening. Is known (see, for example, Patent Document 1).
JP 2004-165125 A

  However, when the resin frame member is used, there is a risk of deformation due to a stacking load when the single cell is assembled. In addition, due to this deformation, the reaction force of the gasket fixed to the resin frame is reduced, and the sealing performance may be deteriorated.

  Accordingly, an object of the present invention is to provide a fuel cell stack having a fuel cell frame member having high rigidity and excellent corrosion resistance, and a method of manufacturing the fuel cell frame member.

  The fuel cell stack according to the present invention includes a membrane electrode assembly in which a gas diffusion layer is provided at a substantially central portion of an electrolyte membrane via a catalyst layer, and the membrane so as to surround the catalyst layer and the gas diffusion layer. And a frame member attached in contact with the peripheral edge of the electrode assembly, wherein the frame member is formed with a nitride layer having a cubic crystal structure formed on the outer surface. It is characterized by comprising a base material made of a metal or an alloy of a transition metal.

The method for producing a fuel cell frame member according to the present invention includes 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 an outer surface of the base material. A method of manufacturing a fuel cell frame member, wherein the nitride layer is formed of metal atoms of at least one transition metal element selected from Fe, Cr, Ni, and Mo. 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 stack of the present invention, since the fuel cell frame member using the base material made of a transition metal or a transition metal alloy is provided, the rigidity becomes high, and deformation during stacking is suppressed. Can do. Further, since the nitride layer having a cubic crystal structure is formed on the outer surface, the corrosion resistance of the portion in contact with the membrane electrode assembly can be improved.

  Furthermore, according to the method for manufacturing a fuel cell frame member according to the present invention, a fuel cell frame member having high rigidity and excellent corrosion resistance can be manufactured.

  Hereinafter, a fuel cell stack and a method for manufacturing a fuel cell frame member 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 frame member and fuel cell stack)
FIG. 1 is a perspective view showing an external appearance of a fuel cell stack provided with a fuel cell frame member 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 96 and fuel cell separators 3 serving as basic units for generating power by electrochemical reaction. As will be described later, each single cell 2 includes a membrane electrode assembly 96 in which a gas diffusion layer having an oxidant electrode and a gas diffusion layer having a fuel electrode are formed on both surfaces of a solid polymer electrolyte membrane, and the membrane. A frame member 90 bonded to the peripheral edge of the electrode assembly 96 and the fuel cell separator 3 are provided. Further, the fuel cell separator 3 is disposed on both sides of the membrane electrode assembly 96, and an oxidant gas flow path and a fuel gas flow path are formed inside the fuel cell separator 3, respectively.

  As the polymer electrolyte membrane, for example, a fluorine resin membrane having a sulfonic acid group 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.

  As shown in FIG. 1, the fuel cell stack 1 has a hydrogen supply line for supplying a fuel gas containing hydrogen such as hydrogen gas to each single cell 2, and air for supplying air as an oxidant gas. A supply line and a cooling water supply line for supplying cooling water are provided.

  3 is a perspective view of the frame member according to the embodiment of the present invention, FIG. 4 is a plan view of the membrane electrode assembly according to the embodiment of the present invention, and FIG. 5 is a membrane electrode junction according to the embodiment of the present invention. It is a conceptual sectional view of a body and a frame member.

  The frame member 90 is a metal plate formed in a substantially frame shape in plan view. That is, the opening 91 is provided by cutting out the central portion of a rectangular metal plate member elongated in the left-right direction in FIG. 3 into a rectangular shape. Therefore, the outer surface of the frame member 90 includes a contact surface 92 to the membrane electrode assembly 96 that is the upper surface in FIG. 3, a back surface 93 opposite to the contact surface 92, an inner surface (side surface of the opening 91) 94, and It consists of an outer surface 95.

  As shown in FIGS. 4 and 5, the membrane electrode assembly 96 is provided with a gas diffusion layer 104 at a substantially central portion of the electrolyte membrane 97 through the catalyst layer 103, and these catalyst layer 103 and gas diffusion layer are provided. As shown in FIG. 5, the frame member 90 is bonded to both surfaces of the peripheral edge portion 105 of the 104. Thus, the shape of the frame member 90 is formed substantially the same as the shape of the peripheral edge portion 105 of the membrane electrode assembly 96.

  As shown in FIG. 5, the single cell 106 includes a membrane electrode assembly 96 and a frame member 90.

  The membrane electrode assembly 96 includes a solid polymer electrolyte membrane 97, a catalyst layer 103 formed on both surfaces of the solid polymer electrolyte membrane 97, and a gas diffusion layer 104 formed on both surfaces of the catalyst layer 103. It is configured.

  FIG. 6 is an enlarged cross-sectional view taken along line AA in FIG.

  As shown in FIG. 6, a nitride layer 102 is formed on the outer surface of the frame member 90.

  The frame member 90 uses a transition metal or a transition metal alloy as a base material, and is formed of a base layer 101 provided on the inner side and a nitride layer 102 provided on a surface layer portion of the base layer 101. Yes. The nitride layer 102 is formed in the depth direction from the outer surface and has a cubic crystal structure.

  Specifically, the nitride layer 102 is formed on a contact surface 92 to the membrane electrode assembly 96, a back surface 93 opposite to the contact surface 92, and an inner side surface 94 (side surface of the opening 91). In the present embodiment, the outer surface 95 is not formed. However, the nitride layer 102 may be provided on the outer surface 95.

  Then, the thickness of the nitride layer 102 on the contact surface 92 of the frame member 90 that contacts the peripheral edge 105 of the membrane electrode assembly 96 is thicker than the nitride layer 102 provided on the back surface 93 of the contact surface 92. Forming. Therefore, the thickness of the nitride layer 102 on the side in contact with the strongly acidic electrolyte membrane 97 can be increased, and the corrosion resistance of the frame member 90 can be improved more efficiently.

  7 is a conceptual cross-sectional view of a membrane electrode assembly and a frame member according to a comparative example. FIG. 8A is a conceptual cross-sectional view of a membrane electrode assembly and a frame member according to another comparative example. Is a cross-sectional view of the frame member in a state before assembly, and (c) is a cross-sectional view of the frame member after assembly.

  As shown in FIG. 7, a gasket 98 having a substantially triangular cross section is fixed to both surfaces of the periphery of the electrolyte membrane 97. This gasket 98 is in contact with the separator and seals the gas. However, if the gasket 98 is fixed to the thin electrolyte membrane 97 having a thickness of 10 to 50 μm, it will bend when the electrolyte membrane 97 is assembled, and the electrolyte membrane 97 will be wrinkled. May decrease.

  On the other hand, since the frame member 99 shown in FIG. 8 is made of resin, a stacking load is applied when assembling the fuel cell stack at a temperature of 60 to 90 ° C., for example, as shown in FIG. The frame member 99 may be deformed. This deformation is not preferable because the reaction force of the gasket 100 decreases and the sealing performance may decrease.

  Further, the nitride layer 102 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. As described above, since the nitride layer 102 is formed on the outer surface of the base material exposed to the outside of the part of the frame member 90, the frame member 90 is corroded even if the electrolyte membrane 97 and the gas diffusion layer 104 come into contact with each other. There is no fear.

  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. 9 shows an M ₄ N type crystal structure.

As shown in FIG. 9, 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 thickness 0.5-5 [micrometers] on the surface of a base material. 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. In contrast, when 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 as described above, the fuel cell excellent in corrosion resistance in a strongly acidic atmosphere is used. A separator can be obtained.

  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.

  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.

  Thus, by adopting the above-described configuration, the fuel cell frame member 90 according to the embodiment of the present invention is excellent in corrosion resistance.

(Manufacturing method of fuel cell frame member)
Next, a method for manufacturing a fuel cell frame member according to an embodiment of the present invention will be described. The manufacturing method of the fuel cell frame member 90 is selected from Fe, Cr, Ni, and Mo by nitriding a base material made of a transition metal or a transition metal alloy at a temperature of 500 ° C. or lower. Forming a nitride layer having an M ₄ N type crystal structure in which nitrogen atoms are arranged in octahedral voids at the center of a unit cell of a face-centered cubic lattice formed by transition metal atoms formed .

  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 crystal structure of NaCl type is precipitated, so that the corrosion resistance of the fuel cell frame member 90 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 frame member 90 is improved by performing nitriding at a low temperature of 500 [° C.] or less.

  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 structure of the nitriding apparatus used in the method for manufacturing the fuel cell frame member according to the embodiment of the present invention will be described with reference to FIG. FIG. 10 is a schematic side view showing the configuration of the nitriding apparatus used in the method for manufacturing the fuel cell frame member of the present embodiment.

As shown in FIG. 10, the nitriding apparatus 30 includes a batch-type nitriding furnace 31, a gas supply device 32 that supplies atmospheric gas to the nitriding furnace 31, and plasma electrodes 33 a and 33 b that generate plasma in the nitriding furnace 31. And a DC power source 33 for supplying a DC voltage to the electrodes 33a and 33b, a pump 34 for discharging the gas in the nitriding furnace 31, and a temperature sensor 37 for detecting the temperature in the nitriding furnace 31. The nitriding furnace 31 has an inner wall 31a and an outer wall 31b, and a stainless hanger 36 for suspending a stainless steel foil 44 processed into the shape of a fuel cell frame member is provided on the ceiling 31c of the inner wall 31a. The gas supply device 32 includes a gas chamber 38 and a gas supply conduit 39. The gas chamber 38 is provided with an opening, and a gas supply valve is provided in each of the openings, and H ₂ gas, N ₂ Gas and Ar gas are supplied. The ceiling portion 31 c of the nitriding furnace 31 has an opening 31 d that communicates with the other end of the gas supply conduit 39. The gas supply line 39 is provided with a gas supply valve. The gas pressure in the nitriding furnace 31 is detected by a gas pressure sensor 40 provided at the bottom 31 e of the nitriding furnace 31. The nitriding furnace 31 is provided with a cooling water flow path (not shown), and the cooling water flows into the cooling water flow path from the opening 31f provided in the outer wall 31b of the nitriding furnace 31, and flows out from the opening 31g. A cooling water supply valve is provided in the opening 31f to adjust the flow rate of the cooling water. The pump 34 is connected to a discharge pipe 41 communicating with an opening 31h provided in the bottom 31e. The temperature sensor 37 is installed in an installation port 31 i provided in the outer wall 31 b of the nitriding furnace 31.

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

  Nitrogen gas and hydrogen gas are used for plasma nitriding, and a negative bias voltage is applied to the stainless steel material in a low temperature non-equilibrium plasma in which the nitrogen gas and hydrogen gas are discharged. Nitriding is preferably performed at a temperature. In the plasma nitriding treatment, the passive film on the surface of the metal material can be easily removed by the sputtering action by ion bombardment. On the other hand, when nitriding is performed using gas nitriding or salt bath nitriding, which is commonly used, oxidation occurs on the outermost layer of the order of several to several tens [nm] of the nitrided layer to form an insulating oxide. Therefore, the contact resistance 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 the fuel cell frame member 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 low by a simple operation. Thus, it becomes possible to manufacture the fuel cell frame member 90 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.

  FIG. 11 is a diagram showing the appearance of a fuel cell electric vehicle equipped with a fuel cell stack, in which (a) is a side view of the fuel cell electric vehicle and (b) is a plan view of the fuel cell electric vehicle. is there.

  As shown in FIG. 11 (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. A joined engine compartment 52 is formed. In the fuel cell electric vehicle 50 shown in FIGS. 11 (a) and 11 (b), the fuel cell stack 1 is mounted in the engine compartment 52.

  By mounting the fuel cell stack 1 with high power generation efficiency to which the fuel cell frame member 90 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. it can. 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. .

  Examples 1 to 6 and Comparative Examples 1 to 5 of the fuel cell frame member 90 according to the embodiment of the present invention will be described below. In these examples, the effectiveness of the fuel cell frame member 90 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.

<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 electrolyte membrane 97 for solid polymer uses proton conductivity by saturating the polymer electrolyte membrane 97 having proton exchange groups such as sulfonic acid groups in the molecule, and exhibits strong acidity. For this reason, the corrosion resistance is evaluated using a constant potential electrolysis test that is an electrochemical technique, and the amount of metal ions that dissolve in the solution after being held for a certain period of time with a predetermined constant potential applied by fluorescent X-ray analysis. The degree of reduction in corrosion resistance was evaluated from the value of the ion elution amount.

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

The crystal structure of the nitride layer, 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. 13 (a), and a cross-sectional structure photograph of the sample obtained in Comparative Example 1 is shown in FIG. 13 (b). In FIG. 13A, nitride layers 72 are formed on both surfaces of the substrate 71, but in FIG. 13B, a modified layer such as a nitride layer is not formed on the surface of the substrate 73. I understand that.

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

  As shown in FIG. 14, an oxide film exists on the outermost surface of the nitride layer because there is a slight partial pressure of oxygen during the nitridation process, and the amount of oxygen is reduced because 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].

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. By using 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 the unit cell of the face-centered cubic lattice formed of at least one transition metal atom, the ion elution amount is also reduced. Less corrosion resistance.

  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.

  From the above measurement results, Examples 1 to 6 are comparative examples 1 to 5 in any example. The frame member 90 according to this embodiment is a base material made of a transition metal or an alloy of transition metals. It was found that the nitride layer 102 having a cubic crystal structure formed directly on the base layer 101 is provided with high corrosion resistance.

It is a perspective view which shows the external appearance of the fuel cell stack comprised using the frame member which concerns on embodiment of this invention. It is a disassembled perspective view of the fuel cell stack comprised using the frame member which concerns on embodiment of this invention. It is a perspective view of the frame member concerning an embodiment of the invention. It is a top view of the membrane electrode assembly which concerns on embodiment of this invention. It is a conceptual sectional view of a membrane electrode assembly and a frame member concerning an embodiment of the invention. It is an expanded sectional view by the AA line of FIG. It is a conceptual sectional view of a membrane electrode assembly and a frame member concerning a comparative example. Among these drawings, (a) is a conceptual cross-sectional view of a membrane electrode assembly and a frame member according to another comparative example, (b) is a cross-sectional view of the frame member in a state before assembly, and (c) is a set. It is sectional drawing of the frame member after attachment. 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 frame member 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. It is a figure which shows the X-ray-diffraction pattern of the sample obtained by Example 1 and Comparative Example 1. In this figure, (a) is a cross-sectional structure photograph of the sample obtained in Example 1, and (b) is a cross-sectional structure photograph of the sample obtained in Comparative Example 1. 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. 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 20 M ₄ N type crystal structure 21 transition metal atom 22 nitrogen atoms 90 frame member 92 abutment surfaces 96 membrane electrode assembly 97 electrolyte membrane 102 nitride layer 103 catalyst layer 104 the gas diffusion layer 105 periphery 107 Substrate

Claims (9)

A membrane electrode assembly in which a gas diffusion layer is provided at a substantially central portion of the electrolyte membrane via a catalyst layer, and a peripheral portion of the membrane electrode assembly so as to surround the catalyst layer and the gas diffusion layer A fuel cell stack provided with a frame member attached in the state,
The fuel cell stack according to claim 1, wherein the frame member is made of a base material made of a transition metal or an alloy of a transition metal and having a nitride layer having a cubic crystal structure formed on the outer surface.   The nitride layer on the contact surface of the frame member that contacts the peripheral edge of the membrane electrode assembly is formed thicker than the nitride layer provided on the back surface of the contact surface. The fuel cell stack described in 1. The nitride layer is obtained by nitriding an outer 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 stack according to claim 1, wherein a nitride layer having a cubic crystal structure is formed in a depth direction from an outer surface of the base material. 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. the fuel cell stack according to claim 1, wherein the nitrogen atom is the crystal structure of the deployed M _{4 N} type.   5. The fuel cell stack according to claim 1, wherein the nitride layer has a thickness of 0.5 to 5 μm.   The nitrogen content is 15 [at%] or more and the oxygen content is 26 [at%] or less at a depth of 10 [nm] from the outermost surface of the nitride layer. The fuel cell stack according to 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. The fuel cell stack according to any one of 6. A method for producing a frame member for a fuel cell, in which a nitride layer is formed on the outer surface 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 lower. 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 manufacturing a fuel cell frame member, characterized by having an M ₄ N type crystal structure. 9. The fuel cell frame according to claim 8, wherein the plasma nitriding treatment is performed by applying a negative bias voltage to the base material in a low-temperature non-equilibrium plasma in which nitrogen gas and hydrogen gas are discharged. Manufacturing method of member.