JP2009032686A - Fuel cell separator, manufacturing method thereof, fuel cell stack, and fuel cell vehicle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell separator suppressed in change and variation in contact resistance which is generated accompanied by aging. <P>SOLUTION: A nitride layer 11 is composed of a first nitride layer 11b formed on a base layer and a second nitride layer 11c which is formed on the first nitride layer and comprises the surface of the nitride layer. The atomic percentage of Cr and N in the second nitride layer is set higher than that of Cr and N in the first nitride layer. On the surface 11a of the nitride layer 11, nitrogen is driven into the surface 10a of a base material 10 by plasma nitriding to form the nitride layer. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

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

  2. Description of the Related Art Conventionally, attempts have been made to use titanium materials such as stainless steel or industrial pure titanium that have good electrical conductivity and excellent corrosion resistance as fuel cell separators. Stainless steel has a dense passive film formed on its surface, such as an oxide, hydroxide or hydrate of which chromium is the main metal element. Similarly, a dense passive film such as titanium oxide, titanium hydroxide or a hydrate thereof is formed on the surface of titanium. For this reason, stainless steel and titanium have good corrosion resistance.

  However, the passive film 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 electrode is 40 [mΩ · cm ² ] or less, the efficiency reduction due to the contact resistance can be suppressed. Therefore, a fuel cell separator has been proposed in which after forming stainless steel and processing it into the shape of a fuel cell separator, the nitride film is formed by removing the passive film on the surface that generates contact resistance by contact with the electrode. (See Patent Document 1).
Japanese Patent Laid-Open No. 2006-134855

  However, only by forming the nitride layer on the surface of the fuel cell separator, the contact resistance changes and varies with time.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a fuel cell separator in which changes and variations in contact resistance with time change are small.

  Another object of the present invention is to provide a method for producing a separator for a fuel cell in which changes and variations in contact resistance with time change are small.

  Another object of the present invention is to provide a fuel cell stack including a fuel cell separator in which changes and variations in contact resistance with time change are small.

  Another object of the present invention is to provide a fuel cell vehicle including as a power source a fuel cell stack having a fuel cell separator with little change and variation in contact resistance with time.

  The present invention comprises a first nitride layer formed on a base layer, and a second nitride layer formed on the first nitride layer and including the surface portion of the nitride layer, to form a nitride layer, The gist is to make the atomic concentrations of Cr and N in the nitride layer higher than the atomic concentrations of Cr and N in the first nitride layer.

  According to the present invention, it is possible to provide a fuel cell separator, a fuel cell separator, a fuel cell stack, and a fuel cell vehicle in which changes and variations in contact resistance with time change are small.

  Hereinafter, a fuel cell, a fuel cell manufacturing method, and a fuel cell vehicle 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 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 formed with a gas diffusion layer having an oxygen electrode and a gas diffusion layer having a hydrogen electrode on both surfaces of a solid polymer electrolyte membrane to form a membrane electrode assembly, and fuel on both sides of the membrane electrode assembly. The battery 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. 3 (a) is a schematic perspective view of the fuel cell separator 3, FIG. 3 (b) is a sectional view taken along the line IIIb-IIIb in which the main part of the fuel cell separator 3 is emphasized in an easily understandable manner, and FIG. 3 (c). These are the IIIc-IIIc sectional view taken on the line which emphasized the principal part of the separator 3 for fuel cells clearly. As shown in FIG. 3, the fuel cell separator 3 is composed of a base material 10 made of stainless steel containing any element selected from Fe, Cr, and Ni, and by nitriding the surface 10 a of the base material 10. The resultant is composed of a nitrided layer 11 formed in the depth direction of the surface 10a of the substrate 10 and a base layer 12 that is an unnitrided non-nitrided layer.

  The fuel cell separator 3 is formed with a groove-like flow path 13 of fuel or oxidant having a rectangular cross section by press molding. A flat plate portion 14 defined by the flow channel 13 and the flow channel 13 is provided between the flow channel 13 and the flow channel 13, and the nitride layer 11 extends along the outer surfaces of the flow channel 13 and the flat plate portion 14. To do. The flat plate portion 14 contacts the gas diffusion layer on the adjacent solid polymer membrane when the fuel cell separators 3 and the single cells 2 are alternately laminated.

  As shown in FIG. 3C, the nitride layer 11 is formed on the first nitride layer (first layer) 11b formed on the base layer 12 and the first nitride layer 11b. And a second nitride layer 11 c including the surface portion 11 a of the layer 11. The surface portion 11a of the nitride layer 11 is nitrogen struck by the surface 10a of the substrate 10 by plasma nitriding. The atomic concentration of Cr and N in the second nitride layer 11c is higher than the atomic concentration of Cr and N in the first nitride layer 11b.

  It is desirable that the atomic concentration of O (oxygen) at a position 5 nm deep from the surface of the second nitride layer 11c is in the range of 5% to 20%. When the atomic concentration of oxygen exceeds 20%, the contact resistance is increased by an oxide having low conductivity, and the structure in which nitrides are connected to each other is easily broken. Conversely, when the atomic concentration of oxygen is less than 5%, the oxide film that suppresses the elution of metal ions is not densely formed, so that the metal ions are eluted and the performance variation (electric conductivity and elution property) is large. Become.

  The thickness of the oxide film formed on the surface of the second nitride layer 11c is desirably 200 nm or less. When the thickness of the oxide film is 200 nm or more, the contact resistance is increased by the oxide having low conductivity, and the low contact resistance performance required for the fuel cell separator cannot be realized. The oxide film is desirably formed by pre-oxidation treatment in which stainless steel is heat-treated in an air (oxygen) -containing atmosphere before nitriding treatment, or pickling after nitriding treatment. Unlike the method of forming an oxide film by adding Si or Mn, since there is no precipitate such as MnS that deteriorates the corrosion resistance, elution of metal ions can be reduced. Further, since it is not necessary to add Si or Mn, the cost is reduced.

  The thickness of the second nitride layer 11c is preferably 50 [nm] or less. In this case, the corrosion resistance in an oxidizing environment is excellent. When the second nitrided layer 11c is not formed, the outermost surface is the first nitrided layer, so that the ratio of Fe in the passive film is high and Cr concentration is difficult to occur, so that the corrosion resistance is high. Getting worse. On the other hand, when the thickness of the second nitride layer 11c exceeds 50 [nm], the thickness of the second nitride layer 11c increases and the Cr concentration of the first nitride layer 11b and the base layer 12 decreases. Since a Cr-deficient layer is formed, the corrosion resistance is inferior due to this Cr-deficient layer.

  The second nitride layer 11c is preferably formed with a single layer of a nitride compound having an MN type crystal structure. Here, M is a transition metal element selected from the group consisting of Cr, Fe, Ni and Mo. When the second nitride layer 11c is formed with a single phase of a nitride compound having an MN type crystal structure, Cr concentration occurs in the passive film. For this reason, the passive potential shifts to the noble side, and the effect of improving the corrosion resistance is obtained.

  In the second nitride layer 11c, it is preferable that Cr is distributed throughout and there is no Cr-deficient layer. That is, it is more preferable that a single phase of a nitride compound having CrN is formed in the second nitride layer 11c. CrN itself is known to exhibit high corrosion resistance. However, when CrN is deposited by nitriding or the like, the Cr concentration of the base layer 12 decreases and a Cr-deficient layer is generated, so this Cr-deficient layer deteriorates corrosion resistance. Let Further, when the CrN layer is coated on the base layer 12 by coating or the like, the adhesion strength is not sufficient, and defects are likely to occur in the film, which deteriorates the corrosion resistance.

  On the other hand, by forming a very thin CrN layer of 1 to 30 [nm] on the outermost surface of the nitrided layer 11 by nitriding, particularly plasma nitriding, the Cr concentration of the base layer 12 is lowered and a Cr-deficient layer is formed. Without this, the first nitride layer 11b and the second nitride layer 11c, and the first nitride layer 11b and the base layer 12 have high consistency. For this reason, there are no defects such as slip lines at the boundary with the nitride layer 11, the metal bond is not broken, and there is an advantage that the adhesion strength between the respective layers is ensured.

  The second nitride layer 11c preferably has a Cr to Fe ratio of 1.0 or more. In this case, since a Cr-based oxide film is formed on the outermost surface of the second nitride layer 11c, the standard electrode potential is shifted to the noble side, and the corrosion resistance in a strongly acidic atmosphere of pH 2 to 3 is further improved. It becomes a thing.

  The stainless steel used as the substrate 10 is preferably an austenitic stainless steel containing at least 8 wt% Ni. Examples of the austenitic stainless steel containing 8 wt% or more of Ni include SUS304L, SUS316L, SUS310S, and the like. The reason for selecting austenitic stainless steel is that when austenitic stainless steel is used as the base material 10 of the fuel cell separator 3, it is necessary to press-mold irregularities such as gas flow paths and cooling water flow paths.

  In this case, when the base material structure to be press-molded is an austenite single phase, it is excellent in elongation and drawability and excellent in press-formability. In addition, when plasma nitriding is performed on the base material 10, the amount of nitrogen solid solution with respect to the surface 10a of the base material 10 is increased, and a transition metal nitride containing a high concentration of nitrogen by plasma nitriding is formed on the base material surface 10a. It is because it becomes easy to form. When the Ni content is less than 8 wt% or when ferritic or martensitic stainless steel not containing Ni is used as the base material, the elongation and drawability are inferior, and the press formability becomes inferior. .

  The first nitride layer 11b is an M4N 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 the group of Fe, Cr, Ni, and Mo. It preferably has a type crystal structure. The M4N type crystal structure is shown in FIG. As shown in FIG. 4, the M4N type crystal structure 20 has a unit cell center of a face-centered cubic lattice formed by transition metal atoms 21 mainly composed of Fe and selected from Fe, Cr, Ni, and Mo. In this structure, nitrogen atoms 22 are arranged in the octahedral space.

  In this M4N 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 atoms 22 occupy 1/4 of the octahedral voids at the center of the unit cell of the M4N type crystal structure 20. That is, the M4N-type crystal structure 20 is an interstitial solid solution in which nitrogen atoms 22 enter the octahedral voids at the center of the unit cell of the face-centered cubic lattice of the transition metal atoms 21. 22 is located at the lattice coordinates (1/2, 1/2, 1/2) of each unit cell.

  In this M4N type crystal structure 20, the transition metal atom M21 is mainly composed of Fe, but includes an alloy in which Fe is partially substituted with other transition metal atoms such as Cr, Ni, and Mo. In this M4N type crystal structure 20, a strong covalent bond is exhibited between the transition metal atom 21 and the nitrogen atom 22 while maintaining the metal bond between the transition metal atoms 21. Sexuality decreases. Therefore, the first nitride layer 11b having the M4N type crystal structure 20 can provide the fuel cell separator 3 that is excellent in corrosion resistance even in an oxidizing environment in the fuel cell and realizes cost reduction. .

  In this M4N type crystal structure 20, the transition metal atom 21 is preferably mainly composed of Fe, but may be an alloy in which Fe is partially substituted with other transition metal atoms such as Cr, Ni, and Mo. Moreover, it is preferable that the transition metal atom 21 constituting the M4N type crystal structure 20 does not have regularity. In this case, the partial molar free energy of each transition metal atom is reduced, and the activity of each transition metal atom can be kept low. Accordingly, the reactivity of each transition metal atom in the first nitride layer 11b with respect to oxidation becomes low, and the first nitride layer 11b has chemical stability even in an oxidizing environment in the fuel cell. And since it becomes excellent in the corrosion resistance of the separator 3, durability can be improved. Moreover, since corrosion resistance can be maintained without forming a noble metal plating layer on the separator 3 used as a contact surface with an electrode, cost reduction is realizable. Moreover, the transition metal atom 21 has a mixed entropy increased due to the absence of regularity and the partial molar free energy of each transition metal atom has decreased, or the activity of each transition metal atom is less than the rule of Raoul. It is preferably lower than the estimated value.

  In this M4N-type crystal structure 20, when the Cr atomic ratio to Fe is high, nitrogen contained in the nitride layer is combined with Cr in the first nitride layer 11b to form a Cr-based nitride such as CrN, that is, NaCl. The type nitride compound is the main component, and the corrosion resistance of the first nitride layer 11b is lowered. For this reason, the transition metal atom 21 is preferably composed mainly of Fe. This crystal structure is considered to be a fcc or fct structure nitride in which nitrogen is supersaturated with high density of dislocations and twins and high hardness of 1000 [HV] (Yasumaru, Tsugaike) ; Journal of the Japan Institute of Metals, 50, pp 362-368, 1986).

  The closer to the surface, the higher the nitrogen concentration, and CrN is not the 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 the octahedral space at the unit cell center of the face-centered cubic lattice in which the first nitride layer 11b is formed by the transition metal atoms 21 selected from Fe, Cr, Ni, and Mo. When the crystal structure 20 of the M4N type is used, the corrosion resistance in a strongly acidic atmosphere having a pH of 2 to 3 can be further improved, and the contact resistance with the electrode can be reduced.

  The first nitride layer 11b is a composite structure including a matrix of an M4N type crystal structure 20 and a crystal layer of an ε-M2-3N type crystal structure called an ε phase formed in the matrix. It is preferable that the interlayer distance is in the range of several tens to several hundreds [nm]. By using a composite structure including the ε-M2-3N crystal structure in the M4N crystal structure matrix, the chemical stability of the first nitride layer 11b is ensured. In addition, when the first nitride layer 11b has a crystal layer with an interlayer distance in the range of several tens to several hundreds [nm], free energy is reduced due to the two-phase equilibrium of the nano-level fine layered structure. Therefore, the activity is kept low, the reactivity to oxidation is lowered, and the chemical stability is obtained. For this reason, especially in a strongly acidic atmosphere, the oxidation is suppressed and the corrosion resistance is improved.

[Method for producing separator for fuel cell]
Next, the manufacturing method of the separator for fuel cells which becomes embodiment of this invention is demonstrated. The method for producing a fuel cell separator according to an embodiment of the present invention maintains the surface of a base material made of stainless steel containing any element selected from Fe, Cr and Ni at a temperature lower than 450 [° C.]. M4N type crystal structure 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 the group of Fe, Cr, Ni and Mo by plasma nitriding A first step of forming a first nitride layer (first layer), and plasma nitriding while maintaining the temperature of the substrate surface at a temperature lower than 450 ° C. Forming a second nitride layer (second layer) in which the atomic concentration of Cr and N is higher than the atomic concentration of Cr and N in the first nitride layer. To do. Here, M is a transition metal element selected from the group consisting of Cr, Fe, Ni and Mo. Also, the Cr and N concentrations in the second nitride layer can be made higher than the atomic concentrations of Cr and N in the first nitride layer by controlling the energy and gas composition during the plasma nitriding process. .

  Plasma nitriding uses a workpiece (here, stainless steel foil) as a cathode, applies a direct current voltage to generate glow discharge, that is, generates a low-temperature non-equilibrium plasma, ionizes a part of the gas component, and produces a non-equilibrium plasma. This is a method in which the ionized gas component therein is collided with the surface of the object to be processed at a high speed for nitriding. FIG. 5 is a schematic side view of a nitriding apparatus 30 used in the method for manufacturing a fuel cell separator according to an embodiment of the present invention.

  The nitriding device 30 is a batch type nitriding furnace 31, a vacuum pump 34 for evacuating a vacuum nitriding vessel 31a installed in the nitriding furnace 31, and supplying an atmospheric gas to the vacuum nitriding vessel 31a. Gas supply device 32, and plasma electrodes 33a and 33b charged at a high voltage to generate plasma in the vacuum nitriding vessel 31a, and these electrodes 33a and 33b are pulsed to a high frequency of 45 [KHz]. A micro-pulse plasma power source 33 for supplying a direct current voltage and a temperature detector 37 for detecting the temperature in the vacuum nitriding vessel 31a.

  The nitriding furnace 31 includes an outer container 31b made of a heat-insulating insulating material that accommodates the vacuum nitriding container 31a, and includes an air inlet 31g with a valve valve. The vacuum nitriding vessel 31a includes an insulator 35 at the bottom 31c for holding the plasma electrodes 33a and 33b at a high potential. The plasma electrodes 33a and 33b are provided with a stainless steel support 36 thereon. This support 36 is formed with a flow path of fuel or oxidant by press molding, and supports a stainless steel foil 44 (hereinafter often referred to as a base material) processed into a separator shape. The gas supply device 32 includes a gas chamber 38 and a gas supply pipe 39, and the gas chamber 38 has a predetermined number of gas introduction openings (not shown), each of which has a gas supply valve (not shown). ) Communicated with a hydrogen gas supply line (not shown), a nitrogen gas supply line (not shown), and an argon gas supply line (not shown).

  The gas supply device 32 further has a gas supply opening 32a communicating with one end 39a of the gas supply line 39, and a gas supply valve (not shown) is provided in the opening 32a. The gas supply line 39 extends through the bottom 31d of the outer vessel 31b of the nitriding furnace 31 and the bottom 31c of the vacuum nitriding vessel 31a into the vacuum nitriding vessel 31a and rises vertically. It reaches part 39b.

  The rising portion 39b is provided with a plurality of openings 39c for ejecting gas into the vacuum nitriding vessel 31a. The gas pressure in the vacuum nitriding vessel 31a is detected by a gas pressure sensor (not shown) provided at the bottom 31c of the vacuum nitriding vessel 31a. The conductive wire 39a of the resistance heating type or induction heating type heater 39 is wound around the outer periphery of the vacuum nitriding vessel 31a, and is heated by this.

  An air flow path 40 is defined between the vacuum nitriding vessel 31a and the outer vessel 31b. The side wall 31e of the outer container 31b is provided with a blower 41 that sends air flowing into the air flow path 40 from an opening 31f provided in the side wall 31e of the outer container 31b. The air flow path 40 includes an opening 40a through which air flows out. The vacuum pump 34 evacuates through an exhaust line 45 communicating with the opening 31h provided in the bottom 31c of the vacuum nitriding vessel 31a. The temperature detector 37 passes through the bottoms 31c and 31d of the vacuum nitriding vessel 31a and the outer vessel 31b and the plasma electrodes 33a and 33b and is connected to a temperature sensor 37b (for example, a thermocouple) through a signal line 37a. .

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

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

  As a method for manufacturing a separator used in a fuel cell according to an embodiment of the present invention, as a first step, sputtering cleaning is performed to remove a non-conductive film on the surface of a base material 44 made of stainless steel foil. During this sputtering cleaning, hydrogen ions, argon ions, etc., ionized by the introduced gas collide with the surface of the base material 44, thereby removing the oxide film mainly composed of Cr on the surface of the base material 44. As a second step, after sputtering cleaning, a mixed gas of hydrogen gas and nitrogen gas is introduced into the nitriding furnace 31, and a voltage is applied to generate glow discharge on the substrate 44 which is a cathode.

  At this time, the ionized nitrogen collides, penetrates and diffuses on the surface of the base material 44, whereby the first nitride layer including the M4N type crystal structure is formed on the surface of the base material 44, and the MN type crystal structure is formed on the surface of the first nitride layer. A second nitride layer containing is formed. Simultaneously with the formation of the nitride layer, the oxide film formed on the surface of the base material 44 is removed by a reduction reaction in which ionized hydrogen and oxygen on the surface of the base material 44 react. In this plasma nitriding method, the reaction on the surface of the base material 44 is not an equilibrium reaction but a non-equilibrium reaction, so that the transition metal nitride containing an M4N type crystal structure with a high nitrogen concentration in the depth direction from the surface of the base material 44 Is quickly obtained, and this nitride is rich in corrosion resistance.

  In the embodiment of the present invention, a micro pulse plasma power source is used as a power source. As a power source used for the plasma nitriding method, it is common to use a DC power source having a DC waveform that is applied with a DC voltage, this discharge current is detected by a current detector, and is controlled by a thyristor so as to be a predetermined current. . In this case, glow discharge is continuously continued, and when the substrate temperature is measured with a radiation thermometer, the substrate temperature changes in a range of about ± 30 [° C.]. On the other hand, the micro-pulse plasma power source is composed of a high-frequency cutoff circuit using a DC voltage and a thyristor, and this circuit makes the DC power source waveform a pulse waveform in which glow discharge is repeatedly turned on and off.

  In this case, the substrate temperature is measured with a radiation thermometer by performing plasma nitridation using a micro-pulse plasma power source that repeatedly discharges and shuts off, with the plasma discharge time and the plasma cut-off time being 1-1000 [μsec]. Then, the change in substrate temperature is in the range of about ± 5 [° C.]. In order to obtain a transition metal nitride having a high nitrogen concentration, precise temperature control of the substrate temperature is required. Therefore, a micropulse plasma power source having a small change in the substrate temperature is used. It is preferable that the plasma can be repeatedly discharged and interrupted at a period of [μsec].

  The nitriding treatment is preferably performed while maintaining the temperature of the substrate surface at a temperature lower than 450 [° C.]. When the surface of stainless steel is nitrided at a high temperature, nitrogen is combined with Cr in the base material, and a nitride such as CrN having a NaCl-type crystal structure with a high Cr concentration is precipitated so that a Cr-deficient layer is formed in the base layer or nitrided layer. For this reason, the corrosion resistance of the separator is lowered. In contrast, when nitriding is performed at a temperature lower than 450 [° C.], the surface of the base material is not initially a nitride compound such as CrN having a NaCl type crystal structure, but mainly an M4N type crystal structure on the base material surface. A nitride layer is formed. At the same time, on the outermost surface of the base material, nitrogen ions are implanted from the surface in a state where Cr in the base layer is concentrated on the surface by plasma heating, thereby forming a nanometer order CrN layer on the outermost surface of the base material. The Thereby, a separator having improved corrosion resistance is obtained. Moreover, the power generation efficiency of the fuel cell can be maintained, and a fuel cell having excellent durability and reliability can be obtained at low cost.

  When the nitriding temperature is lower than 300 [° C.], Cr concentration is difficult to occur, and it is difficult to form a nanometer-order CrN layer on the outermost surface, and a nitride layer having an M4N type crystal structure is formed. In order to obtain it, since a long-time process is required, productivity deteriorates. For this reason, it is preferable to perform nitriding in the range of 300 [° C.] or higher and lower than 450 [° C.]. The first nitride layer has a composite structure including a matrix having an M4N type crystal structure and a crystal layer having an ε-M2-3N type crystal structure called an ε phase formed in the matrix. In which the interlayer distance is in the range of several tens to several hundreds [nm], the second nitride layer is formed with a single layer of a nitride compound having an MN type crystal structure, and the thickness is In order to set it to 50 [nm] or less, the nitriding treatment is preferably performed in the range of 380 [° C.] to 450 [° C.]. The method for producing a separator for a fuel cell according to an embodiment of the present invention includes a step of forming a passage for a fuel or an oxidant by subjecting a base material to press molding. This step may be performed at any stage before or after nitriding, but is preferably performed before nitriding. In this case, defects such as cracks do not occur in the nitride layer.

  The oxide film on the surface of the second nitride layer is preferably formed by pre-oxidation treatment in which stainless steel is heat-treated in an air (oxygen) -containing atmosphere before nitriding treatment, or pickling after nitriding treatment. Unlike the method of forming an oxide film by adding Si or Mn, since there is no precipitation part such as MnS that deteriorates the corrosion resistance, elution of metal ions can be reduced. Moreover, since it is not necessary to perform an addition process, cost becomes cheap.

  The heating temperature and heating time of the pre-oxidation treatment are within the range of 300 to 1100 [° C.] and 0 to 10 [hour] in consideration of the crystal grain size and mechanical properties according to the components of the stainless steel of the base material. It is desirable to do. When the heating temperature is 300 [° C.] or less, the oxide film is not sufficiently formed, and the ion elution suppression effect is reduced. On the other hand, when the heating temperature is 1100 [° C.] or higher, oxygen enters too much or the crystal grain size becomes finer, so that the contact resistance increases.

  For the pickling treatment, it is desirable to use an acid solution of sulfuric acid, hydrofluoric acid, hydrochloric acid or the like having a pH of 2 to 4. Further, the treatment temperature is preferably from room temperature to 80 [° C.], the treatment time is from 1 minute to 100 hours, and it is desirable to carry out under conditions suitable for the components of stainless steel as the substrate. When the pH of the acid solution is 2 or less, even the nitride layer is dissolved. Further, when the pH of the acid solution is 4 or more, or when the treatment temperature is room temperature or less and the treatment time is 1 minute or less, there is no pickling effect. Further, when the processing temperature is 80 [° C.] or higher or when the processing time is 100 hours or longer, the oxide film becomes too thick and the resistance increases.

[Fuel cell vehicle]
As an example of the fuel cell vehicle according to the embodiment of the present invention, a fuel cell electric vehicle 50 using the fuel cell stack 1 according to the embodiment of the present invention as a power source will be described.

  FIG. 6 is a view showing an appearance of a fuel cell electric vehicle 50 on which the fuel cell stack 1 is mounted. 6A is a side view of the fuel cell electric vehicle 50, and FIG. 6B is a top view of the fuel cell electric vehicle 50. As shown in FIG. 6 (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. 6A and 6B, 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 50 can be improved. In addition, by mounting the miniaturized lightweight fuel cell stack 1 on the vehicle, the vehicle weight can be reduced to save fuel, and the travel distance can be increased. Furthermore, by mounting a miniaturized fuel cell on a mobile vehicle or the like as a power source, the vehicle interior space can be used more widely, and the degree of freedom of 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. .

[Example]
Hereinafter, examples and comparative examples of a separator for a fuel cell according to an embodiment of the present invention will be described.

[Sample preparation]
In Example 1 and Comparative Example 1, JIS standard SUS304L (18Cr-9Ni-low C), SUS316L (18Cr-12Ni-2Mo-low C), and SUS310S (25Cr-20Ni-low C) as raw materials are used as base materials. The vacuum annealed material having a thickness of 0.1 [mm] and □ 100 × 100 [mm] was press-formed into a separator shape and used. After the separator-shaped press-molded material was degreased and cleaned, plasma nitridation by glow discharge using a micro-pulse DC power source was performed on both sides of the separator-shaped press-formed material. The conditions of plasma nitriding are as follows: nitriding temperature is 380 [° C.] to 450 [° C.], nitriding time is 60 [min], gas mixing ratio during nitriding N ₂ : H ₂ = 7: 3, processing pressure 3 [Torr] (399 [Pa]). In the sample of Example 1, when the first nitride layer is formed so that the concentration of Cr and N in the second nitride layer is higher than the atomic concentration of Cr and N in the first nitride layer. The plasma nitridation conditions for forming the second nitride layer were made different from those for the plasma nitridation. On the other hand, in the sample of Comparative Example 1, the plasma nitridation conditions for forming the first nitride layer and the plasma nitridation conditions for forming the second nitride layer were the same.

[Evaluation]
The obtained samples of Example 1 and Comparative Example 1 were evaluated using the evaluation methods shown below.

[Atomic concentrations of Cr and N in the first and second nitride layers]
The atomic concentrations of Cr and N in the samples of Example 1 and Comparative Example 1 were measured using a scanning Auger electron spectrometer (MODEL 4300 manufactured by PHI). The measurement is performed under the conditions of an electron beam acceleration voltage of 5 [kV], a measurement region of 20 [μm] × 16 [μm], an ion gun acceleration voltage of 3 [kV], and a sputtering rate (SiO ₂ conversion value) of 10 [nm / min]. I went there. Then, the maximum value of the atomic concentration of Cr and N in the outermost surface to a depth of 50 nm and the maximum value of the atomic concentration of Cr and N in a depth of 50 nm to 5000 nm are respectively in the second nitride layer and the first nitride layer. As the atomic concentration of Cr and N. The measurement results are shown in Table 1 below.

[Contact resistance value]
The contact resistance values of the samples of Example 1 and Comparative Example 1 were measured twice using a pressure load contact electrical resistance measuring device (TRS-2000SS manufactured by ULVAC-RIKO) before and after the corrosion test described later. Specifically, after the samples of Example 1 and Comparative Example 1 were cut into a size of □ 30 [mm] × 30 [mm], carbon paper was interposed between the electrode and the sample, and the electrode / carbon paper / Sample / electrode configuration. And the electrical resistance at the time of flowing 1 [A / cm < ² >] with the measurement surface pressure of 1.0 [MPa] was measured twice, and the average value of each electrical resistance was computed as a contact resistance value. Carbon paper is carbon paper coated with platinum catalyst supporting carbon black (TGP-H-090 manufactured by Toray Industries, Inc., thickness 0.26 [mm], bulk density 0.49 [g / cm ³ ], voids Ratio 73 [%], thickness direction volume resistivity 0.07 [Ω · cm ² ]). An electrode made of Cu having a diameter of φ10 was used. The measurement results are shown in Table 1 below.

[Corrosion test]
Samples of Example 1 and Comparative Example 1 were cut into a size of □ 30 [mm] × 30 [mm] and then immersed in an aqueous sulfuric acid solution having a pH of 4 and a temperature of 80 [° C.] for 100 hours.

[Oxygen concentration at 5 nm depth on the surface]
The oxygen concentration at a depth of 5 nm on the surface was measured using a scanning Auger electron spectrometer (MODEL4300 manufactured by PHI). The measurement is performed under the conditions of an electron beam acceleration voltage of 5 [kV], a measurement region of 20 [μm] × 16 [μm], an ion gun acceleration voltage of 3 [kV], and a sputtering rate (SiO ₂ conversion value) of 10 [nm / min]. I went there. The measurement results are shown in FIG.

[Discussion]
As shown in Table 1, for the sample of Comparative Example 1 in which the atomic concentration of Cr and N in the second nitride layer is lower than the atomic concentration of Cr and N in the first nitride layer, contact after the corrosion test The resistance value was in the range of 90 to 150 [mΩ · cm ² ]. On the other hand, for the sample of Example 1 in which the atomic concentration of Cr and N in the second nitride layer is higher than the atomic concentration of Cr and N in the first nitride layer, the contact resistance value after the corrosion test is 30 to 30. It was within the range of 45 [mΩ · cm ² ]. Therefore, by making the atomic concentration of Cr and N in the second nitride layer larger than the atomic concentration of Cr and N in the first nitride layer, the change and variation in contact resistance with time change can be reduced. It was found that it was possible. Further, as shown in FIG. 7, when the oxygen concentration at the surface depth of 5 nm is 20% or more, the contact resistance value after the corrosion test increases, whereas the oxygen concentration at the surface depth of 5 nm is 20% or less. In this case, the contact resistance value after the corrosion test becomes small. From this, it was found that by changing the oxygen concentration at a surface depth of 5 nm to 20% or less, changes and variations in contact resistance with time change can be reduced.

[Sample preparation]
In Examples 2 to 4 and Comparative Examples 2 and 3, a 0.1 [mm] flat plate made of JIS standard SUS310S (25Cr-20Ni-low C) as a base material was press-formed into a separator shape. Used. The separator-shaped flat plate was degreased and washed, and then pre-oxidation treatment and plasma nitriding were performed (Comparative Example 3 is only plasma nitriding). The pre-oxidation treatment was performed at a temperature of 300 to 400 [° C.], a treatment temperature of 10 to 60 [min], and atmospheric pressure. The plasma nitriding conditions were the same as those in Example 1 and Comparative Example 1 so that the effect of only the pre-oxidation treatment could be confirmed.

[Evaluation]
The obtained samples of Examples 2 to 4 and Comparative Examples 2 and 3 were evaluated using the following evaluation methods.

[Oxygen concentration at 5 nm depth on the surface]
The oxygen concentration at a depth of 5 nm on the surface was measured using a scanning Auger electron spectrometer (MODEL4300 manufactured by PHI). The measurement is performed under the conditions of an electron beam acceleration voltage of 5 [kV], a measurement region of 20 [μm] × 16 [μm], an ion gun acceleration voltage of 3 [kV], and a sputtering rate (SiO ₂ conversion value) of 10 [nm / min]. I went there. The measurement results are shown in Table 2 below.

[Contact resistance value]
The contact resistance values of the samples of Examples 2 to 4 and Comparative Examples 2 and 3 were measured twice using a pressure load contact electrical resistance measuring device (TRS-2000SS manufactured by ULVAC-RIKO) before and after the corrosion test described later. Specifically, after cutting the samples of Examples 2 to 4 and Comparative Examples 2 and 3 into a size of □ 30 [mm] × 30 [mm], carbon paper was interposed between the electrode and the sample, The configuration was electrode / carbon paper / sample / electrode. And the electrical resistance at the time of flowing 1 [A / cm < ² >] with the measurement surface pressure of 1.0 [MPa] was measured twice, and the average value of each electrical resistance was computed as a contact resistance value. Carbon paper is carbon paper coated with platinum catalyst supporting carbon black (TGP-H-090 manufactured by Toray Industries, Inc., thickness 0.26 [mm], bulk density 0.49 [g / cm ³ ], voids Ratio 73 [%], thickness direction volume resistivity 0.07 [Ω · cm ² ]). An electrode made of Cu having a diameter of φ10 was used. The measurement results are shown in Table 2 below.

[Corrosion test]
Samples of Examples 2 to 4 and Comparative Examples 2 and 3 were cut into a size of □ 30 [mm] × 30 [mm] and then immersed in an aqueous sulfuric acid solution having a pH of 4 and a temperature of 80 [° C.] for 70 hours.

[Ion dissolution]
The ion elution properties of the samples of Examples 2 to 4 and Comparative Examples 2 and 3 were measured using an inductively coupled plasma mass spectrometer (SPQ9000 type ICP-MS manufactured by SII Nano Technology) for the amount of iron ions in the solution after the corrosion test. And measured. The measurement results are shown in Table 2 below.

[Discussion]
As shown in Table 1, in the samples of Examples 2 to 4 and Comparative Example 2 in which the oxygen concentration at the 5 nm depth of the nitride layer surface is 5 atomic% or more, the oxide film eluting the metal ion elution is densely formed. Therefore, the ion elution amount could be kept low. On the other hand, in the sample of Comparative Example 3 in which the oxygen concentration at a depth of 5 nm from the nitride layer surface was 5 atomic% or less, the amount of ion elution increased because of the small oxide film. Further, in the samples of Examples 2 to 4 in which the oxygen concentration at the 5 nm depth of the nitride layer surface was 20 atomic% or less, the initial contact resistance was low, and the resistance could be kept low even when the corrosion test was performed. On the other hand, in the sample of Comparative Example 2 in which the oxygen concentration at a depth of 5 nm from the nitride layer surface is 20 atomic% or more, the initial contact resistance is increased by the low conductivity oxide, and the nitrides are interconnected. The contact resistance after the corrosion test was remarkably increased due to the structure being easily broken. From this, it was found that according to the samples of Examples 2 to 4, the contact resistance can be kept low and ion elution can be kept low. Therefore, by applying the samples of Examples 2 to 4 to the fuel cell separator, the output of the fuel cell can be maintained in a high state and the durability can be improved.

  As mentioned above, although embodiment which applied the invention made by the present inventors was described, this invention is not limited by the description and drawing which make a part of indication of this invention by this embodiment. That is, it should be added that other embodiments, examples, operation techniques, and the like made by those skilled in the art based on this embodiment are all included in the scope of the present invention.

1 is a perspective view showing an external appearance of a fuel cell stack configured using a fuel cell separator according to an embodiment of the present invention. It is an expanded view of the fuel cell stack comprised using the separator for fuel cells used as 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. It is a schematic diagram which shows the M4N type | mold crystal structure contained in the transition metal nitride used as embodiment of this invention. It is a typical side view of the nitriding apparatus used for the manufacturing method of the separator for fuel cells used as the embodiment of the present invention. It is a figure which shows the external appearance of the electric vehicle carrying the fuel cell stack which becomes 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 relationship between the oxygen resistance in the surface 5nm depth, and the contact resistance after a corrosion test.

Explanation of symbols

1: Fuel cell stack 2: Single cell 3: Fuel cell separator 4: End flange 5: Fastening bolt 10: Base material 10a: Surface 11: Nitride layer 11a: Surface portion 11b: First nitride layer (first layer) )
11c: second nitride layer (second layer)
12: Base layer 13: Channel 14: Flat plate portion

Claims (7)

  A fuel cell separator having a base layer formed of a base material made of stainless steel and a nitride layer formed on the base layer, wherein the nitride layer is a first layer formed on the base layer A nitride layer; and a second nitride layer formed on the first nitride layer and including a surface portion of the nitride layer, wherein the atomic concentrations of Cr and N in the second nitride layer are 1. A fuel cell separator characterized by being higher in atomic concentration of Cr and N in one nitride layer.   2. The fuel cell separator according to claim 1, wherein the atomic concentration of O in a position 5 nm deep from the surface of the second nitride layer is in the range of 5% to 20%. Separator.   3. The fuel cell separator according to claim 2, wherein the thickness of the oxide film formed on the surface of the second nitride layer is 200 nm or less. Forming a first nitride layer on the surface of the base material by plasma nitriding while maintaining the temperature of the base material made of stainless steel at a temperature lower than 450 ° C .;
Plasma nitriding is performed while maintaining the surface temperature of the first nitride layer at a temperature lower than 450 [° C.], whereby the atomic concentration of Cr and N is increased on the surface of the first nitride layer. And a step of forming a second nitride layer having a higher atomic concentration of Cr and N in the layer.   5. The fuel cell separator manufacturing method according to claim 4, wherein the plasma nitridation is performed using a micro pulse plasma power source capable of repeatedly discharging and shutting off plasma at a cycle of 1 to 1000 [mu] sec. Manufacturing method of battery separator.   A fuel cell stack comprising the fuel cell separator according to any one of claims 1 to 3.   A fuel cell vehicle comprising the fuel cell stack according to claim 6 as a power source.

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