Patent Application
20080076001
This application claims priority from Japanese Patent Application Serial Nos. 2006-222989, filed Aug. 18, 2006, and 2007-141014, filed May 28, 2007, each of which is incorporated herein in its entirety by reference.
The invention relates generally to a transition metal nitride, a fuel cell separator, a method for producing the transition metal nitride, a method for producing the fuel cell separator, a fuel cell stack and a fuel cell vehicle.
From the viewpoint of global environmental protection, it has been studied to use a fuel cell substituted in place of an internal combustion engine for vehicle, as a power supply for a motor, and to drive the vehicle by means of the motor. Fuel cells do not require a fossil fuel that bears a depletion problem, and therefore do not produce exhaust gas and the like. Additionally, the fuel cells have such excellent characteristics such as to hardly make noise and to allow an energy recovery efficiency to improve as compared with other energy engines.
Types of fuel cells include those of a solid polymer electrolyte type, a phosphoric acid type, a molten carbonate type and a solid oxide type. A solid polymer electrolyte fuel cell or polymer electrolyte fuel cell (PEFC), one of the above, is such a cell as to be used in the presence of a polymer electrolyte membrane having a proton exchange group as an electrolyte in its molecules. The polymer electrolyte membrane is applied thereto since it functions as a proton-conducting electrolyte when saturated with water. The solid polymer electrolyte fuel cell is activated at relatively low temperatures, and is high in electricity generation efficiency. Further, not only the solid polymer electrolyte fuel cell but also other accessory facilities are compact and lightweight, so that the solid polymer electrolyte fuel cell for use in an electric vehicle or for other various uses is expected.
The solid polymer electrolyte fuel cell includes a fuel cell stack. The fuel cell stack is assembled by stacking a plurality of unit cells (each of which serves as a base unit in an electricity generation due to electrochemical reactions), sandwiching both end portions of the unit cells with end flanges, and then pressingly holding it by a fastening bolt. Thus, the fuel cell stack is formed into a single-piece. A unit cell is comprised of a polymer electrolyte membrane, an anode (or hydrogen electrode) and a cathode (or oxygen electrode), which are respectively bonded to both sides of the membrane, and separators respectively disposed outside the hydrogen and oxygen electrodes.
A fuel cell separator performs a function of electrically connecting the unit cells with each other, and therefore is required to be excellent in electrical conductivity and to be low in contact resistance against components such as a gas diffusion layer.
Further, a solid polymer electrolyte membrane is formed of a polymer having a number of sulfonic acid groups and has a proton conductivity since it uses the wet state sulfonic acid groups as a proton exchange group. Further, as the solid polymer electrolyte membrane has strong acidity, the fuel cell separator is required to have a corrosion resistance against an acidity of sulfuric acid of about pH 2 to 3.
Moreover, a temperature of each gas supplied to the fuel cell is as high as 80 to 90° C. Additionally, H+ is generated in the hydrogen electrode. Further, the oxygen electrode, through which oxygen, air and the like pass, is in an oxidative environment where potential of about 0.6 to 1 V vs SHE is applied. Therefore, similar to the oxygen and hydrogen electrodes, the fuel cell separator is required to have sufficient corrosion resistance to endure a strong acid atmosphere.
There have been attempts to use stainless steel or a titanium material such as industrial pure titanium as the fuel cell separator since they have good electrical conductivity and high corrosion resistance. Under a normal condition, stainless steel has on its surface a closely-packed passive state film of oxide, hydroxide, hydrate thereof or the like containing chromium as its main metallic element. Similarly, titanium has on its surface a closely-packed passive state film of titanium oxide, titanium hydroxide, hydrate thereof or the like. Therefore, stainless steel and titanium have good corrosion resistance.
However, the above-mentioned passive state film causes contact resistance with a carbon paper that is normally used as a gas diffusion layer. With regard to excessive voltage due to resistance polarization within a stationary type fuel cell, exhaust heat can be recovered by cogeneration or the like, so that heat efficiency is improved as a whole. However, as for a fuel cell for use in a vehicle, a heat loss based on the contact resistance has to be exhausted to outside by a radiator through cooling water, which results in reduction of electricity generation efficiency when contact resistance is increased. Moreover, electric efficiency reduction is an equivalent to an increase in heat, and therefore there will be a need for providing a larger cooling system. Accordingly, an increase in contact resistance is a grave issue to be resolved.
In the fuel cell, a theoretical voltage per unit cell is 1.23 V. However, an actual voltage extracted is reduced due to reaction polarization, gas diffusion polarization and resistance polarization, and further is reduced as a current to be extracted is increased. Moreover, since higher power density per unit volume and weight are demanded in the fuel cell for vehicle use, the fuel cell is used at a higher current density, for example, a current density of 1 A/cm2, than the stationary type fuel cell. It is thus considered that, when a current density is 1 A/cm2, an efficiency decrease due to contact resistance between the separator and the carbon paper can be suppressed if the contact resistance is not larger than 40 mΩcm2.
For this purpose, a fuel cell separator is proposed in Japanese Patent Provisional Publication No. 10-2289214, in which the separator is formed by carrying out a press forming on a stainless steel and then directly covering its surface to be contacted with an electrode with a gold-plated layer (see pg. 2 and
An aspect of the invention includes, for example, transition metal nitrides. One transition metal taught herein includes, for example, a first layer formed of a nitride of stainless steel containing at least Fe and Cr and a second layer formed on the first layer and having an exposed surface. In this example, the second layer is formed of another nitride having contents of components that differ from those in the first layer. In the transition metal nitride, the first and second layers have a composition distribution in which a Cr concentration is continuously changed from the first layer to the second layer in a thickness direction of these layers. Additionally, the second layer has at the exposed surface thereof a nitride deposition protruding from a base portion of the exposed surface.
Another aspect of the invention resides in fuel cell separators. A fuel cell separator includes, for example, a base layer formed of a stainless steel containing at least Fe and Cr and a nitride layer formed of a transition metal nitride. The nitride layer is formed on the base layer, and the first layer of the transition metal nitride is directly connected to the base layer. Also, a crystal lattice of the first layer is continuously connected to that of the base layer, a crystal orientation of the first layer is the same as that of the base layer, and a crystal grain of the first layer is continuously connected to that of the base layer.
A further aspect of the invention resides in methods for producing a transition metal nitride and methods for producing a fuel cell separator. One exemplary method includes carrying out a plasma nitriding on a surface of a base material formed of a stainless steel containing at least Fe and Cr while holding the surface at a temperature not less than 425° C., thereby forming a first layer and a second layer. The first layer has at least a M4N type crystal structure where a nitrogen atom is located in an octahedral gap at a center of an unit cell of a face-centered cubic lattice formed of at least one transition metal atom selected from the group consisting of Fe, Cr, Ni and Mo, which are contained as stainless steel components. The second layer has a deposition that protrudes from the surface of the base material and is continuously connected to the first layer.
A still further aspect of the invention resides in a fuel cell stack including a fuel cell separator according to the teachings herein.
A still further aspect of the invention resides in a fuel cell vehicle that includes a fuel cell stack according to the teachings herein and serving as a power source.
The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein:
Plating or coating the surface of the fuel cell separator with noble metal as is known not only requires effort during manufacturing but also results in high material costs. Moreover, the fuel cell is still required to have low contact resistance against the electrode and high corrosion resistance, so that it is desired to further improve these characteristics. The corrosion resistance as discussed above means a durability where the fuel cell separator maintains an electrical conductive performance even in an oxidative environment at strong acidity. In other words, it is necessary to obtain corrosion resistance in an environment in which a cation is dissolved into humidifying water or generated water by chemical reaction so as to bond to or occupy sulfonic acid groups, which is designed to serve as pathways for protons, thereby lowering an electricity generation characteristic of the electrolyte membrane.
According to the disclosure herein, a transition metal nitride low in contact resistance and excellent in corrosion resistance can be obtained. In addition, a fuel cell separator low in contact resistance that occurs between the separator and the electrode and excellent in corrosion resistance can be obtained. Thus, a fuel cell separator high in performance can be produced with ease. Additionally, downsizing and cost reduction are allowed.
When a fuel cell stack that achieves downsizing thereof and cost reduction is mounted, flexibility in styling can be ensured while increasing mileage.
A transition metal nitride, a fuel cell separator, a fuel cell stack, a method for producing the fuel cell separator and a fuel cell vehicle according to embodiments of the invention are next discussed by using an example where they are applied to a solid polymer fuel cell and another example where the solid polymer fuel cell is used.
First discussed are a transition metal nitride, fuel cell and fuel cell stack.
As shown in
After stacking the membrane electrode assembly 2 and the fuel cell separators 3, end flanges 5 are put at both ends, and then the circumferences thereof are fastened with fastening bolts 6, thereby constructing the fuel cell stack 1. Additionally, the fuel cell stack 1 is provided with hydrogen supply lines HL for supplying fuel gas containing hydrogen to each membrane electrode assembly 2. The fuel gas may be a hydrogen gas H2. The fuel cell stack 1 is further provided with air supply lines AL for supplying air as an oxidant and with cooling water supply lines WL for supplying cooling water.
The unit cell 4 having the above construction is manufactured as follows. The oxygen electrode 202 and the hydrogen electrode 203 are disposed on opposing sides of the solid polymer electrode membrane 201. The oxygen electrode 202, hydrogen electrode 203 and membrane 201 are joined together usually by hot pressing, thereby forming the membrane electrode assembly 2. Then, the separators 301 and 302 are disposed on opposing sides of the membrane electrode assembly 2.
Mixed gas of hydrogen, carbon dioxide, nitrogen and moisture vapor is supplied to the side of the hydrogen electrode 203 of a fuel cell configured by the above unit cells 4, and air and moisture vapor are supplied to the side of the oxygen electrode 202 of the same. Then, electrochemical reactions occur mainly on the contact surfaces between the polymer electrolyte membrane 201 and the reaction membranes 204. This reaction is described more specifically below.
Once oxygen gas and hydrogen gas are supplied respectively to the oxygen gas flow passage 401 and the hydrogen gas flow passage 402 in the unit cell 4, the oxygen gas and hydrogen gas are supplied to the reaction membranes 204 through each gas diffusion layer 205. The following reactions occur in each reaction membrane 204:
Hydrogen electrode side: H2→2H++2e−; and (1)
Oxygen electrode side: (½)O2+2H++2e−→H2O. (2)
As shown in
A fuel cell separator 10 that can be used as the fuel cell separator 3 shown in
As shown in
As shown in
As shown in the schematic cross-sectional view of
As shown in the photograph of
The stainless steel used as the base material and serving as the base layer 12 of the fuel cell separator 10 is a stainless steel containing at least Fe and Cr. Particularly, austenitic stainless steel containing at least 8 wt % or more of Ni is preferable. Examples of austenitic stainless steel containing at least 8 wt % or more of Ni includes SUS304, SUS310S, SUS316L and SUS317L. The reason for selecting austenitic stainless steel is its excellent press-formability. In a case where stainless steel is used as the base material of the fuel cell separator 10, projections and depressions in the gas flow passage and the cooling water flow passage are press-formed. When the base material structure is austenite, as single-phase in the above case, it is excellent in ductility, drawability and press-formability. Additionally, in a case where plasma nitriding is made on the base material or austenite as single-phase, the amount of nitrogen solid solution on a surface of the base material is so increased that transition metal nitride containing high concentration nitrogen is easily formed on the surface of the base material by plasma nitriding. In a case where the base material is ferritic or martensitic stainless steel having a Ni content of less than 8 wt % or having no Ni, ductility, drawability and press-formability are lowered.
In a case where the base material contains Mo, Mo has an effect of thinly forming the passive state film over the whole surface of the nitride layer, so as to suppress metal ions from eluting while providing good electrical conductivity.
One of characteristics of a transition metal nitride and fuel cell separator according to certain embodiments of the invention is that the second nitride layer 112 has depositions that irregularly protrude from the base portion of the surface portion 11a of the nitride layer 11 such as depositions 112a, 112c and 112e. The depositions have at least one crystal structure of MN type, M2-3N type and M4N type.
A nitride compound of MN type, M2-3N type and M4N type, forming these depositions, is a nitride compound in which nitrogen intrudes into a fcc or fcp crystal structure and is excellent in electrical conductivity since it has a metallic bond and a covalent bond, as will be discussed specifically in the below description of the first layer 111, particularly with regard to those of M2-3N type and M4N type. Therefore, transition metal nitride and the fuel cell separator according to the present invention achieve a low contact resistance required in fuel cell separators. Particularly when the surface portion 11a of the nitride layer 11, which serves as an outermost surface 10a of the fuel cell separator 10, has a structure including such projections and depressions as to have a maximum height difference (Dmax) or the height of the depositions 112a, 112c and 112e from the surface portion 11a of the nitride layer 11 of no more than 400 nm in a state where the fuel cell separator 10 is installed to the fuel cell stack 1, a contact surface between the fuel cell separator 10 and the gas diffusion layer is to become into an infinitesimal point contact in the oxidative environment. This facilitates acceptance of electrons since an oxide film forms on the contact surface with difficulty. As a result of this, the transition metal nitride and the fuel cell separator according to these embodiments are excellent in electrical conductivity and maintain a low contact resistance value. Additionally, these transition metal nitrides and fuel cell separators maintain a low contact resistance without forming a novel metal plating layer such as gold plating, thereby achieving cost reduction.
In certain embodiments, the above-discussed maximum height difference in the depositions of the second layer 112 is within a range of 10 to 400 nm. In a case where the maximum height difference is less than 10 nm, the contact surface formed between the fuel cell separator and the gas diffusion layer becomes a surface and not a infinitesimal point, so that the contact area is increased, thereby facilitating formation of the oxide film on the contact surface formed between the fuel cell separator and the gas diffusion layer. When the oxide film is formed, acceptance of electrons becomes difficult to be continued. Further, when the maximum height difference of the depositions 112a, 112 and 112e is larger than 400 nm, the depositions are to be Cr-based nitride such as CrN, and not nitride such as of N type that contains Fe and the like. Specifically, Cr is concentrated in the protruding depositions 112a, 112c and 112e formed on the outermost surface so that the second nitride layer 112 has Cr-based nitride of NaCl type as a main component. As a result of this, Cr-based oxide film is easily and stably formed on the protruding depositions 112a, 112c and 112e formed on the surface portion 11a, thereby making acceptance of electrons difficult and increasing the contact resistance value.
The first nitride layer 111 of the nitride layer 11 of the transition metal nitride and the fuel cell separator can have an M4N-type crystal structure where a nitrogen atom is located in the octahedral gap at the center of the unit cell of a face-centered cubic lattice formed of at least one transition metal atom selected from the group consisting of Fe, Cr, Ni and Mo, which are contained as stainless steel components. The M4N-type crystal structure is shown in
As shown in
The M4N-type crystal structure realizes strong covalent bonds between transition metal atoms 21 and the nitrogen atom 22 while maintaining metallic bond among the transition metal atoms 21, thereby lowering reactivity against oxidation of each transition metal atom. Therefore, the first nitride layer 111 having the M4N-type crystal structure 20 is chemically stable even in an oxidative environment within the fuel cell while having an electrical conductivity that the fuel cell separator 10 utilizes, a chemical stability that maintains the function of electrical conductivity in the presence of the separator 10 and corrosion resistance. Further, the fuel cell separator 10 in which the first nitride layer 111 having such a crystal structure is directly formed on the base layer 12 can maintain low contact resistance against the gas diffusion layer used as a common fuel cell, even if placed in an oxidative environment. Additionally, contact resistance can be limited without directly forming a novel metal plating layer such as gold plating on the surface in contact with the electrode including the gas diffusion layer, thereby achieving cost reduction over conventional techniques. Moreover, the nitride layer 111, including the M4N-type crystal structure, has chemical stability so as to maintain a low value of contact resistance between the separator and the electrode in an oxidative environment while achieving cost reduction in the fuel cell separator 10.
In certain advantageous embodiments, the transition metal atoms 21 are mainly Fe but may include an alloy obtained by partially substituting atoms of other transitional metals such as Cr, Ni or Mo for Fe. Also, the transition metal atoms 21 constructing the M4N-type crystal structure can be in irregular arrangement. With an irregular arrangement, partial molar free energy of each transition metal atom is reduced, thus reducing an activity of each transition metal atom. With this, reactivity within the nitride layer 14 to oxidation of each transition metal atom is also reduced, and the first nitride layer 111 thus stays chemically stable even in an oxidative environment within the fuel cell. Furthermore, since contact resistance between the separator 10 and the electrode including the gas diffusion layer is low, durability is thereby improved. Additionally, a low contact resistance is maintained without forming a noble metal plating layer on the separator 3 serving as the contact surface against the electrode, thereby achieving cost reduction. In these embodiments, the transition metal atoms 21 can be increased in mixing entropy with the irregular arrangement, or an activity of each transition metal atom can be lower than a value estimated based upon Raoult's law.
In the M4N-type crystal structure 20, in a case where the atom ratio of Cr to Fe is high, nitrogen contained in the nitride layer is bonded to Cr in the nitride layer 111, and a Cr-based nitride such as CrN (that is, the NaCl-type nitride compound) becomes a main component, thereby lowering corrosion resistance of the first nitride layer 111. Accordingly, the transition metal atoms 21 are mainly Fe in these embodiments. This type of crystal structure is considered to be nitride having the fcc or fct structure with high-density transition and twin crystal, high hardness of not less than 1000 HV, and supersaturated nitrogen solid solution (Yasumaru and Kamachi, Journal of Japan Institute of Metals, 50, pp. 362-368, 1986). The closer to the surface, the higher the concentration of nitrogen becomes. Additionally, CrN does not become a main component, so that Cr, which is effective for corrosion resistance, is not reduced while corrosion resistance is thus maintained even after nitriding. Where the first nitride layer 111 has the M4N-type crystal structure 20 such that an N atom is located in the octahedral gap at the center of the unit cell of a face-centered cubic lattice formed of at least one of metal atoms selected from Fe, Cr, Ni and Mo, corrosion resistance between the separator and the electrode can be even in a strong acid atmosphere of pH 2 to 3.
The first nitride layer 111 preferably has a complex structure in which the M2-3N-type crystal structure and the M4N-type crystal structure are laminated. In a case of including the M2-3N-type crystal structure (s phase), transition metal atoms contained in the first nitride layer 111 realizes a strong covalent bond between the transition metal atoms and nitrogen atoms while maintaining metallic bond among the transition metal atoms. Therefore, there is conformity between the first nitride layer 111 and the second nitride layer 112 formed on the first nitride layer 111 and serving as an outermost surface layer of the nitride layer 11. A crystal orientation is the same as that of the second nitride layer 112, so that crystal grains are in a row while any defect is made between the first nitride layer 111 and the second nitride layer 112. With this, acceptance of electrons is facilitated to improve electrical conductivity.
The first nitride layer 111 is a complex structure including a matrix of the M4N-type crystal structure 20 and the crystal layers of the M2-3N-type hexagonal crystal structure 23 formed within the matrix. The crystal layers can have an interlayer distance within a range of several tens to several hundreds nm.
A fuel cell separator described herein comprises the base layer 12 consisting of stainless steel containing at least Fe and Cr and the nitride layer 11 formed of transition metal nitride and formed on the base layer 12 as previously described. The first layer 111 of the nitride layer 11 formed of transition metal nitride is directly connected to the base layer 12. Additionally, the crystal lattice of the first layer 111 is continuously connected to that of the base layer 12. The crystal orientations are the same, and the crystal grains are in a row. Such a structure is obtained by plasma nitriding the surface of the base material formed of stainless steel. Between the first layer 111 of the nitride layer 11 and the base layer 12, the crystal lattice is in a row, the crystal orientation is the same, and the crystal grains are in a row, Accordingly, the first layer 111 of the nitride layer 11 and the base layer 12 are in conformity to make strong covalent bond. With this, any defect is made between the nitride layer 11 and the base layer 12, thereby facilitating acceptance of electrons and improving electrical conductivity.
Thus, this arrangement is applied to transition metal nitrides and fuel cell separators taught herein, thereby lowering contact resistance that occurs between the separator and the electrode and achieving a low cost fuel cell separator. Moreover, the fuel cell stack according to certain embodiments includes fuel cell separators taught herein so as to maintain high electricity generation efficiency without any loss of electricity generation performance while achieving downsizing and cost reduction.
Referring now to
The fuel cell separator 10A as shown in
Though
Next, a method for producing an embodiment of a transition metal nitride and a method for producing the fuel cell separator are discussed. In a method for producing a transition metal nitride and a method for producing a fuel cell separator, plasma nitriding is carried out on a surface of a base material formed of a stainless steel containing at least Fe and Cr while holding the surface at a temperature of not less than 425° C., thereby forming a first nitride layer (or a first layer) and a second nitride layer (or a second layer). The first nitride layer can have at least a M4N type crystal structure where a nitrogen atom is located in an octahedral gap at a center of an unit cell of a face-centered cubic lattice formed of at least one transition metal atom selected from the group consisting of Fe, Cr, Ni and Mo, which are contained as stainless steel components. The second nitride layer can have a deposition that protrudes from the surface of the base material and is continuously connected to the first layer. M is at least one transition metal atom selected from the group consisting of Fe, Cr, Ni and Mo. With this method, a transition metal nitride including the first and second layers can be easily obtained. Additionally, there can be obtained a fuel cell separator including a base layer formed of stainless steel containing at least one transition metal atom selected from the group consisting of Fe, Cr, Ni and Mo and a nitride layer formed on the base layer. In the thus obtained fuel cell separator, the nitride layer includes a first nitride layer formed on the base layer and the second nitride layer formed connected to the first nitride layer and having a surface portion of the nitride layer. The second nitride layer has depositions protruding from the base portion of the surface of the nitride layer, the depositions having a height that can be within a range of 10 to 400 nm, measured from the base portion of the nitride layer.
In plasma nitriding, an object to be nitrided is set as a cathode, the object being a stainless steel foil in certain embodiments of the invention. A part of gas component is ionized by glow discharge, i.e., low-temperature non-equilibrium plasma produced by application of a direct current voltage. Then, the ionized gas component within non-equilibrium plasma collides at a very fast rate with the surface of the object to be nitrided. Thus the object is nitrided.
A nitriding apparatus 30 as shown in
The vacuum nitriding container 31a has an insulator 35 at its bottom 31c for holding the plasma electrodes 33a and 33b at high potential. The plasma electrodes 33a and 33b include thereon supporters 36 formed of stainless steel. The supporters 36 support a base material 100 formed of stainless steel foil. The base material 100 is provided by press-forming with a channel-like flow passage portion through which fuel or oxidant pass and with a flat portion, and the base material 100 is machined to be shaped into a separator.
The gas supply apparatus 32 has a gas chamber 38 and a gas supply path 39, and the gas chamber 38 is provided with openings (not shown). The openings respectively communicate with a hydrogen gas supply line, a nitrogen gas supply line and an argon gas supply line, each of which is provided with a gas supply valve (not shown). The gas supply apparatus 32 further has a gas supply opening 32a communicating with one end 39a of the gas supply path 39, the opening 32a being provided with a gas supply valve (not shown). The gas supply path 29 extends into the vacuum nitriding container 31a in such a manner as to hermetically pass through a bottom 31d of the outer container 31b of the nitriding furnace 31 and the bottom 31c of the vacuum nitriding container 31a, thereby reaching a standing portion 39 that stands vertically. The standing portion 39 has a plurality of openings 39c for ejecting gas into the vacuum nitriding container 31a.
A pressure of gas within the vacuum nitriding container 31a is detected by a gas pressure sensor (not shown) provided at the bottom 31c of the vacuum nitriding container 31a. The vacuum nitriding container 31a is heated by a lead wire 44a of a heater 44 of induction-type or resistance-type, the lead wire 44a being wound around the periphery of the vacuum nitriding container 31a. The vacuum nitriding container 31a and the outer container 31b define air flow passage 40 therebetween. The outer container 31b includes at its side wall 31e an air blower 41 for blowing air flown into the air flow passage 40 from an opening 31f of the side wall 31e of the outer container 31b. The air flow passage 40 has an opening 40a through which air flows out thereof.
In the vacuum nitriding container 31a, discharging is achieved with a discharge pipe 45 communicating with an opening 31h in the bottom 31c of the vacuum nitriding container 31a.
A temperature detector 37 passes through the vacuum nitriding container 31a, the bottoms 31c and 31d of the outer container 31b and the plasma electrodes 33a and 33b, thereby being connected to a temperature sensor 37b (such as a thermocouple) through a signal path 37a.
The microwave pulse plasma power 33 is turned on and off by receiving control signals from a process control unit 42. A potential difference between each base material 100 and a ground side object (e.g. an inner wall 31i of the vacuum nitriding container 31a) equates to a voltage supplied by the microwave pulse plasma power 33. The process control unit 42 can be, for example, a microcomputer including a central processing unit (CPU), input and output ports (I/O), random access memory (RAM), keep alive memory (KAM), a common data bus and read-only memory (ROM) as an electronic storage medium for executable programs and certain stored values.
The gas supply apparatus 32, the vacuum pump 34, the temperature detector 37 and the gas pressure sensor are also controlled by the process control unit 42. The process control unit 42 can be controlled by a personal computer 43.
Plasma nitriding employed in this embodiment is now discussed more specifically. First of all, the base material 100 serving as an object to be treated is disposed within the vacuum nitriding container 31a. Then, the furnace is vacuated to be less than 1 Torr (=133 Pa). Mixed gas of hydrogen and argon is introduced into the vacuum nitriding container 31a, and then voltage is applied thereto at the degree of vacuum of 665 to 2128 Pa in a state where the base material 100 is a cathode while the inner wall 31i of the vacuum nitriding container 31a is an anode. In this case, a glow discharge occurs on the base material 100. With the glow discharge, the base material 100 is heated and nitrided.
As a first operation of the method for producing the fuel cell separator or the transition metal nitride according to this embodiment, a spatter cleaning is carried out in order to remove a passive state film made on the surface of the base material 100 formed of stainless steel foil. During the spatter cleaning, hydrogen ions, argon ions and the like ionized by the introduced gas collide with the surface of the base material 100, thereby removing oxide film made on the base material 100. The oxide film is mainly Cr.
In a second operation carried out after the spatter cleaning, mixed gas of hydrogen and nitrogen is introduced into the nitriding furnace 31. Then voltage is applied thereto, thereby making the glow discharge on the base material 100, which is serving as the cathode. At this time, ionized nitrogen collides with and breaks into the surface of the base material 100 and then diffuses into the base material 100. This forms a continued nitride layer including a second layer having depositions protruding from the surface of the base material 100 and a first layer having at least the M4N-type crystal structure. Concurrently with formation of the nitride layer, the oxide film formed on the surface of the base material 100 is removed with a reduction reaction in which ionized hydrogen reacts with oxygen present on the surface of the base material 100.
With plasma nitriding, the surface of the base material 100 is strongly and locally heated at infinitesimal area as a result that N ions collide with acceleration by voltage applied thereto against the surface of the base material 100, while metal atoms such as Fe, Cr and Mo present on the surface of the base material 100 are spattered (or vaporized). The metal atoms separated by the action of spattering from the surface of the base material 100 are bonded to nitrogen highly activated within plasma in the vicinity of the surface of the base material 100. Then, absorption is carried out to irregularly form nitride depositions on the surface of the base material 100.
When the base material has a relatively high temperature of 425 to 450° C. during plasma nitriding, a collision energy of N ions against the surface of the base material becomes high so as to improve a spattering effect on the outer surface. Additionally, an atomic weight of Fe separated from the surface of the specimen is increased so as to increase an amount of iron nitride absorbed to the surface. With this, nitride containing nitrogen in high concentration and having the M2-3N-type hexagonal crystal structure and/or the M4N-type cubic crystal structure is deposited.
In plasma nitriding, the reaction that occurs on the surface of the base material 100 is not an equilibrium reaction but a non-equilibrium reaction, which allows rapid formation of the transition metal nitride having an M4N-type cubic crystal structure containing a high concentration of nitrogen according to certain embodiments of the invention. The nitride is formed from the surface of the base material 100 in a depth direction and has sufficient electrical conductivity and corrosion resistance.
Meanwhile, in a case where a nitriding method such as gas nitriding in which nitriding progresses under atmospheric pressure and under the equilibrium reaction is employed, it becomes difficult to remove the passive state film formed on the surface of the base material. Additionally, because of the equilibrium reaction it takes a long time to produce the M4N-type cubic crystal structure on the surface of the base material, and it becomes difficult to obtain a desired nitrogen concentration. This lowers electrical conductivity due to the presence of the oxide film on the surface of the base material, and chemical stability, so that in the nitride and nitride layer obtained in the nitriding method it is difficult to maintain electrical conductivity in a strong acidic environment.
As described herein, a microwave pulse plasma power is employed as a power supply. A power supply commonly used for plasma nitriding is a direct-current power supply, and not the microwave pulse plasma power. In the direct-current power supply, direct voltage is applied, and then a discharged waveform current is detected by a current detector and controlled by a thyristor to be a certain current. In this common power supply, glow discharge is continued, and the temperature of the base material is changed within a range of plus or minus 30° C. when measured by a radiation thermometer. Meanwhile, the microwave pulse plasma power is comprised of a high-frequency interrupting circuit in the presence of direct current and the thyristor. With this circuit, the waveform of direct current becomes a pulse waveform in which glow discharge repeats on and off. In this case, when plasma nitriding in the use of the microwave pulse plasma power that repeats discharging and interrupting is carried out in such a manner as to set times for discharging plasma and for interrupting plasma to 1 to 1000 μsec, temperature variation of the base material is within a range of plus or minus about 50° C. In order to obtain such a transition metal nitride as to have a high nitrogen concentration, accurate temperature control is required. Therefore, according to certain embodiments, a microwave pulse plasma power having little temperature variation of the base material is used. The power can repeat the plasma discharge and interruption in a cycle of 1 to 1000 μsec.
Nitriding can be carried out while maintaining the surface of the base material at a temperature of 425° C. In this case, the height of depositions from the surface of the nitride layer is within a range of from 10 to 400 nm while obtaining a nitride layer in which the depositions have a crystal structure selected from MN type, M2-3N type and M4N type.
The method for producing the fuel cell separator according to certain embodiments can include making press-forming on the base material to form a flow passage portion and a flat portion adjacent thereto, the flow passage portion serving as a path of fuel or oxidant. Though the formation of the flow passage portion and the flat portion may be carried out after or before nitriding, it is more preferably carried out before nitriding in certain embodiments. In this case, any defect such as cracks is not made on the nitride layer.
According to a method for producing transition metal nitrides of the present embodiment, a transition metal nitride high in corrosion resistance and low in contact resistance is easily obtained. Further, methods for producing a fuel cell separator according to embodiments of the invention produce a fuel cell separator excellent in durability. Electricity generation performance is easily obtained by plasma nitriding. This facilitates production of a high-performance fuel cell separator, thereby lowering the production cost.
In forming the nitride layer on the surface of the base material 100, the nitride layer may be selectively formed on the flat portion of the base material by selectively conducting masking on the flow passage portion of the base material 100. By carrying out masking, a selective formation of the nitride layer on the flat portion is easily achieved.
As an example of a fuel cell vehicle according to an embodiment of the invention, there is next discussed a fuel cell electric vehicle that has as its power source the fuel cell stack 1 according to the above-mentioned embodiments.
By mounting on a mobile vehicle such as an automotive vehicle the fuel cell stack 1 according to the teachings herein that has a good power generation efficiency, an improvement of fuel efficiency of the fuel cell electric vehicle 50 can be achieved. Moreover, by mounting the small-sized and light-weighted fuel cell stack 1 on the vehicle, the vehicle weight can be reduced to save fuel and to deliver more mileage. Furthermore, by mounting the small-sized fuel cell on the mobile vehicle or the like as a power source, the usable interior space of the vehicle becomes wider, thereby securing design freedom.
Though the electric vehicle is described as an example of the fuel cell vehicle, the invention is not limited to such vehicle as an electric vehicle and can be applied to engines of an aircraft and the like that require electric energy.
Hereinafter are discussed Examples 1 to 5 and Comparative Examples 1 to 3 of a transition metal nitride and a fuel cell separator according to certain embodiments of the invention. Each example is discussed for examining effectiveness of the transition metal nitrides and the fuel cell separators taught herein. These were prepared by treating each specimen under a condition different from a row material, and therefore the invention is not limited to these examples.
First, preparation of specimens is described. In each of the Examples and Comparative Examples, vacuum annealing materials thickness 0.1 mm and width 100×100 mm of Japan Industrial Standards (JIS)-accredited SUS304L (18Cr-9Ni-low C), SUS316L (18CCr-12Ni-2Mo-low C), SUS310S (25Cr-20Ni-low C) were used as a base material upon being subjected to press-forming to be shaped into a separator. After degreasing the vacuum annealing materials press-formed to be shaped into the separator, plasma nitriding was carried out by glow discharge, using microwave pulse direct-current, on both sides of the vacuum annealing materials. As for the plasma nitriding conditions, nitriding temperature was 350 to 500° C., nitriding time was 60 minutes, gas mixing ratios in nitriding was N2:H2=7:3, and a processing pressure was 3 Torr (or 399 Pa). The temperature was varied. Note that plasma nitriding was not performed in Comparative Example 1. In Comparative Examples 2 and 3, the vacuum annealing materials press-formed to be shaped into the separator were subjected to plasma nitriding by using direct-current glow discharge. Table 1 shows types of steel used as the base materials, plasma power supplies used and base material temperatures used during nitriding.
Each of the thus obtained specimens was evaluated in the following method.
First discussed is the observation of the nitride layer and measurements of the height of protruding portions.
As a specimen to be observed with a transmission electron microscope, a thin layer specimen was prepared in the vicinity of a surface of a nitride layer obtained in each of Examples 1 to 5 and Comparative Examples 1 to 3. A focused ion beam (FIB) available from Hitachi, Ltd. under the trade name of FB2000A was used as an apparatus in the preparation. The specimen was prepared by employing an FIB-μ sampling method. This specimen was observed by using a field emission transmission electron microscope (available from Hitachi, Ltd. under the trade name of HF-2000) at 200 kV.
Next discussed is the identification of the crystal structure of the nitride layer.
Identification of the crystal structure of the nitride layer was carried out by using the field emission transmission electron microscope (available from Hitachi, Ltd. under the trade name of HF-2000), an EDS analyzer at an acceleration voltage of 200 kV and a μ-diffraction electron diffraction. In identification of the crystal structure, as a first step, measurement of spacing between crystal faces was carried out by using a photograph showing electron diffraction. More specifically, a distance between lattice planes d was determined by formula (3) obtained by combining formula (1) and Bragg's formula (2) as below:
R=L tan 2θ; (1)
λ=2d sin θ; and (2)
d=nλL(1/R); wherein (3)
R is the center of a diffraction pattern; and
L is the length of the camera.
Then, crystal structure was identified by the distance between lattice planes d.
Next, contact resistance of the separator obtained in each of Examples 1 to 5 and Comparative Examples 1 to 3 was measured before and after the corrosion resistance test by using a pressure-loading contact electrical resistance measuring device made by ULVAC-RIKO, Inc. under the trade name of TRS-2000SS. As shown in
Next discussed is evaluation of the corrosion resistance. In a fuel cell, an electric potential of about 1 V versus SHE at the maximum is applied to an oxygen electrode side in comparison with a hydrogen electrode side. In addition, a solid polymer electrolyte membrane exerts proton conductivity by saturating a polymer electrolyte membrane having a proton exchange group such as a sulfonic acid group within a molecule, and exhibits strong acidity. Therefore, corrosion resistance was evaluated by constant-potential electrolysis testing, that is, an electrochemical method where the specimen is measured in terms of contact resistance upon being held for a certain period of time while applying predetermined constant electric potential thereto. More specifically, a specimen was prepared by cutting the center of each specimen to have a size of 30 mm×30 mm. The thus prepared specimen was then held for 100 hours in a sulfuric acid aqueous solution of pH 2 at temperature of 80° C., and at electric potential of 1 V vs. SHE.
Table 2 shows a surface pattern, the height of protruding portion and crystal structures of the nitride layer (the first nitriding layer) adjacent to the outermost surface (the second nitriding layer), and of the base material. Table 3 shows the contact resistance values obtained before and after the corrosion resistance test.
The specimen obtained in Comparative Example 1 has a surface including a γ-type crystal structure in which a surface of the separator was coated with a stable passive state film and was not coated with the nitride layer. Accordingly, the contact resistance values obtained before and after the corrosion resistance test were high. Additionally, specimens obtained in Comparative Examples 2 and 3 were formed with the nitride layer adjacent to the base layer. However, the pattern shown in the outermost surface had no protrusion and therefore was flat. In a case where protruding depositions are not formed on the outermost surface, a stable passive state film can be formed easily. Therefore, the contact resistance value obtained after the corrosion resistance test took a high value exceeding 100 mΩcm2, and the nitride layer did not exhibit a sufficient electrochemical stability in an oxidative environment.
Meanwhile, each specimen obtained in Examples 1 to 5 was formed with the nitride layer that includes depositions containing crystal structure of the MN type, M2-3N type or M4N type and having a protruding portion height of 12 to 400 nm. This is considered to allow an excellent electrical conductivity and maintaining a low contact resistance value.
The highest of the protruding portions of the depositions had a height of 21 nm. In
Each specimen of Examples 1 to 5 had a contact resistance value not larger than 30 mΩcm2 both before and after the corrosion test. Contact resistance values before and after the corrosion test varied little. Without being bound to any theory, a reason why each specimen of Examples 1 to 5 is thus excellent in electrochemical stability in an oxidative environment is that an outermost layer of the nitride layer is covered at its surface with the protruding depositions, and that the depositions are protrusions whose protruding portion has a height of within a range of from 10 to 400 nm. Moreover, the complex structure in which the matrix having the M4N-type crystal structure and the crystal layer having the M2-3N-type crystal structure are laminated is stably formed adjacent to the base layer without lowering the Cr concentration in the base layer due to the depositions, so that the M4N-type crystal structure realizes strong covalent bonds between the transitional metal atoms and the nitrogen atom while maintaining metallic bond among the transition metal atoms.
With this, conformity among the protruding deposition, the M4N-type crystal structure and the base layer is greatly maintained to obtain chemical stability, electrical conductivity used by the fuel cell separator and chemical stability for maintaining the function of electrical conductivity in the use of the fuel cell. Further, the transition metal atoms constructing a face-centered cubic lattice are irregularly mixed so as to lower an activity to reduce partial molar free energy of each transition metal composition. This decreases reactivity within the protruding depositions and the first nitride layer to oxidation of each transition metal atom so that the protruding depositions and the first nitride layer are considered to have chemical stability. Accordingly, each specimen of Examples 1 to 5 had a low contact resistance value both before and after the corrosion test and had good corrosion resistance.
In a fuel cell, a theoretical voltage per unit cell is 1.23 V, but a voltage which can be actually extracted is reduced due to reaction polarization, gas diffusion polarization and resistance polarization. The larger a current to be extracted is, the lower the voltage becomes. Moreover, as higher power density per unit volume and weight is demanded, a fuel cell for a vehicle use is used at a high current density, for example, a current density of 1 A/cm2, in comparison with a stationary-type fuel cell. It is considered that, when the current density is 1 A/cm2, an efficiency decrease due to contact resistance between a separator and a carbon paper can be suppressed if the contact resistance is 20 mΩcm2, in other words, if a measurement value obtained by the apparatus as shown in
Hereinafter, an embodiment of a fuel cell separator differs from Examples 1 to 5 in terms of height of the protruding depositions formed on the surface of the nitride layer.
In each of the Examples and Comparative Examples, vacuum annealing materials thickness 0.1 mm and width 100×100 mm of Japan Industrial Standards (JIS)-accredited SUS304L and SUS316L were used as a base material subjected to press-forming to be shaped into a separator. After degreasing the vacuum annealing materials press-formed to be shaped into the separator, plasma nitriding was carried out on both sides of the vacuum annealing materials by direct-current glow discharge. As for the plasma nitriding conditions, nitriding temperature was 500° C., nitriding time was 60 minutes, gas mixing ratios in nitriding was N2:H2=7:3 and a processing pressure was 3 Torr (or 399 Pa). Table 4 shows the type of stainless steel used as the base material, the used plasma power supply and a base material temperature during nitriding.
Each of the thus obtained specimens was evaluated in the same method as the above-discussed Examples 1 to 5.
The specimens of Examples 6 and 7 had the nitride layer formed adjacent to the base layer, and the protruding depositions were formed on the outermost surface so as to be excellent in electrical conductivity and to maintain a low contact resistance. However, the protruding portion of the deposition formed on the outermost surface of each of Examples 6 and 7 had a height of larger than 400 nm. In this case, distribution of components such as Fe and Cr tends to occur locally among the protruding depositions. In other words, the protruding depositions formed on the outermost surface are to have a part in which Cr is extremely concentrated or a Cr concentration is extremely low. As a result of this, it is considered that the contact resistance value after the corrosion test was increased in comparison with Examples 1 to 5 since the protruding depositions low in Cr concentration tends to be oxidized in a strong acid atmosphere of pH 2 to 3.
There will be discussed hereinafter an embodiment of the fuel cell separator in which the nitride layer is selectively formed on the flat portion of the fuel cell separator. First the preparation of specimens is described.
In Examples 8 to 10 and Comparative Example 4, a vacuum annealing material thickness 0.1 mm and width 100×100 mm of Japan Industrial Standards (JIS)-accredited SUS310S was used as the base material. This base material was formed with a path of fuel or oxidant by being subjected to press-forming, so as to be shaped into a separator.
After rinsing the separator formed by press-forming with acid, plasma nitriding was carried out by microwave pulse direct-current glow discharge on both sides of the vacuum annealing material. At the occasion of the plasma nitriding, the nitriding in Examples 8 and 9 was carried out selectively on the flat portion of the base material by using either the mask M1 or M2 shown in
Table 7 shows amounts of Cr, Ni and Mo contained in the used stainless steel base material, whether or not the plasma nitriding was carried out, the used plasma power supply, a controlling temperature during nitriding, and whether or not masking was carried out.
Each of the thus obtained specimens was evaluated according to the following method, starting with the identification of the crystal structure of the nitride layer.
Crystal structure of the nitride layer of the specimen obtained in the above method was identified by making X-ray diffraction measurement on the surface of the base material modified by nitriding. X-ray diffraction device (XRD) made by Mac Science Co., Ltd. was used as a device. Measurements were performed under conditions where a radiation source was CuKα ray, a diffraction angle was between 20 and 100 degrees and a scan speed was 2 degrees/min.
Observation of the nitride layer and identification of the crystal structure was performed. A thin layer specimen was prepared from the specimen using a focused ion beam device (FIB) available from Hitachi, Ltd. under the trade name of FB2000A in a FIB-μ sampling method, and then was observed by using a field emission transmission electron microscope (available from Hitachi, Ltd. under the trade name of HF-2000) at 200 kV.
Nitrogen content and oxygen content in the nitride layer, in other words, nitrogen content and oxygen content in the outermost layer of the nitride layer, were measured from the surface to the depth of 200 nm of the nitride layer by depth profile in Auger electron spectroscopic analysis, concerning Fe, Ni, Cr, N and O at depths of 0 nm (i.e., the outermost surface), 2 nm, 5 nm, 10 nm, 50 nm and 100 nm. The measurement was conducted by using a scanning Auger electron spectrometry equipment available from PHI, Inc. under the trade name of MODEL4300, under the following conditions: 1) an electron beam acceleration voltage of 5 kV; 2) a measuring region of 20 μm×16 μm; 3) an ion gun accelerating voltage of 3 kV; and 4) a sputtering rate of 10 nm/min (SiO2 converted value).
The specimen obtained in the above-mentioned Examples 1 to 6 and Comparative Examples 1 to 4 was cut to have a size of 30 mm×30 mm, and then its contact resistance was measured. Device and conditions used in this contact resistance measurement were the same as those of the above-discussed contact resistance value measurements conducted on Examples 1 to 6.
Evaluation of corrosion resistance was conducted by constant-potential electrolysis testing, that is, an electrochemical method, upon being held for a certain period of time in a state where predetermined constant electric potential was applied thereto. Thereafter, amounts of metallic ion elution within a solution were measured by X-ray fluorescence analysis. From values of amounts of metallic ion elution, the degree of deterioration of corrosion resistance was evaluated.
More specifically, a specimen having a size of 30 mm×30 mm was prepared in such a manner as to be taken from the center of each specimen. The thus prepared specimen was immersed for 100 hours in a sulfuric acid aqueous solution of pH 2 at a temperature of 80° C. At this time, a surrounding was made in such a manner as to purge N2 gas, simulating the anode environment, and so as to be brought into a state open to the atmospheric air, simulating the cathode environment. Thereafter, amounts of Fe, Cr and Ni ion elution within the sulfuric acid aqueous solution were measured by X-ray fluorescence analysis.
Tables 8 and 9 show measurement results in Examples 8 to 10 and Comparative Example 4, the measurement being conducted concerning the crystal structure of the nitride compound, amounts of Fe, Ni, Cr, N and O at a depth of 5 nm, which were obtained by depth profile in Auger electron spectroscopic analysis, contact resistance values obtained before and after an immersion test and amounts of Fe, Cr and Ni ion elution within a test solution. Additionally, regarding Examples 9 and 10 and Comparative Example 4,
As shown in Tables 8 and 9 and
Meanwhile, the specimens of Examples 8 and 9 in which the nitride layer is selectively formed on the flat portion have a low contact resistance value after the immersion test made under the anode conditions and the cathode conditions while having a low Fe, Ni and Cr ion elution. Without being bound to any theory, the low contact resistance results in that the flat portion in contact with GDL is covered at its surface with the nitride layer to facilitate electron's movement or to improve electrical conductivity between the base layer and the nitride layer. Further, the surface of the flow passage portion is covered with a passive state film that is thick and stable and formed of stainless steel. With this, it is difficult for electron movement to be interrupted even at a high temperature of 80 to 90° C. and in a strong acidic atmosphere, in other words, even in a fuel cell separator environment, so that it is considered to improve an ion elution ability and corrosion resistance.
Regarding each specimen of Examples 8 to 10, the first layer of the nitride layer had at the portion brought into contact with GDL, a complex structure in which M4N-type or M2-3N-type depositions were laminated on an M4N matrix and on the surface of the base material at several tens to several hundreds intervals, as shown in Table 8.
Moreover, the specimen of Example 10 was formed with the nitride layer by using the microwave pulse plasma power; plasma nitriding applied thereto was conducted without masking. Therefore, the GDL contact portion is formed with the nitride layer. Contact resistance value after the immersion test made under the anode conditions and the cathode conditions did not increase so as to be excellent in electrical conductivity. However, the passive state film as shown in Examples 8 and 9 was not formed at the gas flow passage, so that amounts of Fe, Cr and Ni ion elution in the immersion test made under the anode conditions and the cathode conditions were increased as compared with those of Examples 8 and 9.
Based on the results of the scanning Auger electron spectroscopic analysis conducted on the flat portion, a peak is confirmed in which both Cr and N are concentrated as compared with the base layer at a depth of 5 nm from the surface of the flat portion. Therefore, the peak is found to denote a nitride having Cr2N-type, CrN-type, M2-3N-type and/or M4N-type crystal structure where Cr and N are concentrated. It is also found that the nitride layer has such a composition distribution that a Cr concentration is continuously changed from the first layer to the second layer in the thickness direction of them.
Based on the result of analysis as shown in
The above-described embodiments have been described in order to allow easy understanding of the invention and do not limit the invention. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structure as is permitted under the law.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-222989 | Aug 2006 | JP | national |
| 2007-141014 | May 2007 | JP | national |
