JP2005251490A - Current collector and solid oxide fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a current collector which is chemically stable in an oxidative and reducing high temperature atmosphere, which can be used for all of an air pole and a fuel electrode when applied to a fuel cell, which has further a low contact resistance and a small current collection loss, and which can improve the output performance of a battery; and to provide a solid oxide fuel cell using such a current collector. <P>SOLUTION: A metal carbide film F, such as, for example, a chromium carbide (CrC) and a vanadium carbide (VC) is formed on the front surface of a plate-like substrate B which has an unevenness on the contact surface with a fibrous or at least an electrode made of a metal material or a carbon material containing, for example, a carbon. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a current collector that is applied to various types of batteries such as fuel cells and storage batteries, and in particular, is excellent in stability at high temperatures and has an operating temperature in both oxidizing and reducing atmospheres. The present invention relates to a current collector particularly preferably used for a high solid oxide fuel cell (SOFC).

In a fuel cell stack formed by stacking a plurality of single cells, in order to obtain a desired output voltage, a current collector is used to connect the single cells in series or in parallel, or to improve current collection efficiency. Is used, and the power generation output is taken out to an external circuit through the current collector.
In the solid oxide fuel cell, a heat-resistant metal mesh or felt such as Ag or Ni is used as a current collector in consideration of heat resistance and gas permeability at an operating temperature.

  For example, Patent Document 1 discloses that silver is used as a main material of a current collector, and silver felt is applied to silver felt, silver mesh, or metal fiber having a higher temperature strength than silver. Is used as a current collector for an air electrode.

Further, in Patent Document 2, when nickel felt is used as a fuel electrode side conductor of a solid oxide fuel cell, the gap between the nickel fibers is prevented from being reduced due to the sintering, thereby ensuring gas permeability. It is described that an electrolyte material is adhered to the surface of nickel felt fibers.
JP 2002-280026 A Japanese Patent Application Laid-Open No. 07-045297

However, in the current collector described in the above cited reference 1 using silver as a main material, not only the current collector is limited to use for an air electrode but also the electrode performance due to silver migration. There is a problem that deterioration occurs or costs increase. Further, when the metal fiber is used after being plated with silver, there is a problem in the adhesion of the plating layer.
On the other hand, the current collector described in the cited document 2 is not limited to use as a fuel electrode. In addition, the resistivity of the electrolyte supported on the surface of the nickel fiber is not so low. There has been a problem that it hardly contributes to the reduction of the above.

  The present invention has been made paying attention to the above-mentioned problems in conventional current collectors using Ag as a main material or using nickel felt having an electrolyte material attached to the fiber surface. However, it is chemically stable even in oxidizing and reducing high-temperature atmospheres. For example, when applied to a fuel cell, it can be used for both an air electrode and a fuel electrode, and has low contact resistance. An object of the present invention is to provide a current collector capable of reducing the current collection loss and improving the output performance of the battery, and a solid oxide fuel cell (SOFC) using such a current collector.

  In order to achieve the above object, the present inventors have conducted extensive studies on various materials that can be used as current collectors and surface treatment methods thereof, and as a result, focused on the conductivity and chemical stability of metal carbides. It has been found that the above object can be achieved by forming a metal carbide film on the surface of a suitable substrate, and the present invention has been completed.

  The present invention is based on the above knowledge, and the current collector of the present invention is formed by forming a metal carbide film on the surface of a base material, for example, a metal material containing carbon For example, a metal carbide film such as chromium carbide (CrC) or vanadium carbide (VC) is formed on the surface of a fibrous substrate made of carbon or a carbon material, or at least the surface of a plate-like substrate having irregularities on the contact surface with the electrode. Can be.

  The solid oxide fuel cell of the present invention is an application of the current collector, and the current collector of the present invention is provided on one or both of two electrode layers sandwiching an electrolyte made of a solid oxide. Is in contact.

  In general, metal carbonaceous materials have electrical conductivity, strong interatomic bonds, and stable characteristics even at high temperatures. Therefore, by forming a film of such a metal carbonaceous material on the surface of an appropriate substrate, Oxidation and reduction of the substrate can be prevented, and it can be used regardless of the surrounding environment and atmosphere, and a low-cost current collector can be obtained. Therefore, for example, when applied to a solid oxide fuel cell, it can be used for both an air electrode and a fuel electrode, and can exhibit excellent current collecting performance over a long period of time.

  The current collector of the present invention is excellent in high temperature characteristics as described above. For example, when applied to a solid oxide fuel cell operating at a high temperature of 600 ° C. or higher, the current characteristics are effectively exhibited. However, the use of the current collector is not limited to such a solid oxide fuel cell, and is applicable not only to other fuel cells but also to normal primary batteries and secondary batteries. It goes without saying that you can do it.

  Hereinafter, the current collector of the present invention will be described in more detail.

  As described above, the metal carbon material has electrical conductivity and exhibits chemical stability even at high temperatures, and the current collector of the present invention has such a metal carbon on the surface of an appropriate substrate. Since it forms a film of a product, it can withstand a long-term high temperature environment regardless of the oxidizing or reducing atmosphere.

As the base material in the current collector of the present invention, for example, a metal material can be used, and it is desirable to select an optimum material depending on the use temperature, atmosphere, and workability.
That is, as a material applicable from normal temperature to about 150 ° C., for example, a general steel material or a SUS304 material (austenite stainless steel) can be easily obtained, but is not limited thereto. Further, examples of alloys that can be used at higher temperatures include SUS430 (ferritic stainless steel), Fe—Cr alloys such as FeCrSi and FeCrAl, and SUS316 (Mo-added austenitic stainless steel) and Inconel and other Ni— A Cr-based alloy is mentioned.

As described later, these metal materials preferably contain carbon since a stable metal carbide film having high adhesion can be formed by salt bath treatment.
In the case of an alloy material that does not contain carbon or has a low carbon content, for example, heat treatment in a mixed atmosphere of carbon dioxide gas, hydrogen, etc. Will be able to handle.

  Moreover, as a base material other than the metal material, a carbon material can be preferably used. The carbon material is excellent in heat resistance. For example, as will be described later, the carbon material can be made elastic even if it is formed into a nonwoven fabric or a woven fabric using carbon fibers, and is optimal as a current collector for SOFC. Become. Although the carbon material itself is naturally easily oxidized, in the case of the current collector of the present invention, since the surface of the current collector is provided with a metal carbonaceous film, it is suitable for either the fuel electrode or the air electrode. Can be used without any problem.

  Thus, by using a metal material or carbon material having excellent heat resistance as a base material, a current collector that can be used even in a high temperature region can be obtained.

When such a current collector is used as a current collector for a fuel cell, hydrogen or hydrocarbon fuel gas introduced into the fuel electrode and oxidizing gas such as air or oxygen introduced into the air electrode Since it is necessary to supply efficiently to each electrode, it is an essential condition that the current collector has air permeability.
Therefore, as the base material shape of the current collector, it is desirable that when it is in the form of a fiber or a plate, at least the contact surface with the electrode is provided with irregularities.

That is, as shown in FIG. 1 (a), when the shape of the base material B of the current collector 1 is plate-like, the contact surface with the electrode 2 or 3 is made uneven so that FIG. ) And (c), a gap can be formed between the air electrode 2 or the fuel electrode 3, and the electrical connection to the electrodes 2 and 3 can be maintained while the electrical connection to the electrodes 2 and 3 is maintained. Air permeability can be secured.
In this case, the shape and size of these irregularities depend on the gas flow rate, gas flow rate, electrode resistivity, electrode porosity, plate substrate strength, thickness, etc., and the contact area with the electrode is extremely large. It is desirable to optimize so that it does not become small. In these figures, reference numeral 4 indicates an electrolyte layer.

  Moreover, as shown to Fig.2 (a), it can be set as the electrical power collector 1 with air permeability by making the base material B into a fibrous thing and forming the film | membrane F which consists of metal carbide on this surface. In addition, as shown in FIGS. 2B and 2C, air permeability can be ensured together with electrical conduction between the air electrode 2 and the fuel electrode 3.

At this time, as the fibrous metal, in addition to the fine wire by rolling, after growing the metal by using electrodeposition called electroforming, the cladding material from which the core wire was removed, or the wire diameter by the molten metal extraction method A metal fiber of about 40 μm can be used.
As the fibrous carbon material, for example, carbon fiber having a wire diameter of about 7 μm obtained by carbonizing acrylic fiber while drawing can be used.

  Further, such a fibrous metal or carbon fiber can be used to form a nonwoven fabric or a woven fabric, which makes it possible to form a current collector having both good gas permeability and a large contact area. Furthermore, since it is possible to ensure elasticity by using these fiber materials, the contact state with the electrode can be improved.

Furthermore, the fiber diameter of the fibrous metal and carbon used as the base material is preferably in the range of 1 to 100 μm.
As described above, the fiber diameter is generally about 40 μm for metal fibers and about 7 μm for carbon fibers, but if it exceeds 100 μm, flexibility is lost and the contact area with the electrode surface is reduced. Contact resistance increases, and the meaning of adopting a fibrous base material is lost. In addition, it is possible to reduce the contact resistance by using a fiber body having a small wire diameter. However, it is difficult to produce a fiber having a diameter of less than 1 μm. This is because it is not so low.

In the present invention, it is desirable that the metal carbide formed on the surface of the substrate is CrC, VC or TiC. In addition, it can also be used in the state which mixed 2 or more types of these carbide | carbonized_materials.
Such a metal carbide is used as a surface film of a metal mold for press or casting, has excellent strength and heat resistance, and has high conductivity, and can reduce contact resistance.

  In particular, CrC has high electrical conductivity even in a high-temperature oxidizing atmosphere, and is optimal as a film for a current collector used in a high-temperature environment. Moreover, since VC and TiC have the same conductivity, they can be used as a film.

Such a carbide film can be formed by, for example, a vapor phase film formation method such as a PVD method or a CVD method, or a salt bath treatment.
However, in a vapor deposition method such as PVD method or CVD method, a carbide film is formed only on the surface, and it is difficult to cover the inside, so that it can be applied to a plate-like substrate. It is unsuitable for forming a carbide film on a fibrous base material after being formed into a nonwoven fabric or a woven fabric.

On the other hand, the salt bath treatment is to form a metal carbide film on the surface of the substrate by immersing the substrate in a high-temperature molten metal salt and reacting with carbon contained in the substrate. A film can also be formed on the surface of the fibrous base material after being formed into a woven fabric or the like.
Here, the salt bath treatment is to immerse the base material in the molten salt to which the metal powder is added, thereby causing the carbon contained in the base material to react with the metal in the molten salt, and the surface of the base material is made of metal carbide. It is a technique for forming a film, and since a strong film can be formed with good adhesion, it is often used as a coating method for press or casting dies.

For example, in the case of chromium carbide (CrC), chromium (Cr) is mixed in a molten salt of about 1000 ° C. made of KCl, BaCl ₂ or the like, and the base material is immersed therein. In the case of this method, a chromium carbide film is formed by a reaction between carbon contained in the base material and chromium in the molten salt, so that the base material contains carbon.
At this time, since the molten salt has higher fluidity as the temperature is higher, it is desirable that the temperature be as high as possible when forming a film on the inside of a fibrous material such as a nonwoven fabric or a woven fabric or on a complicated structure. In addition, by using a deep salt bath and sinking the substrate to a deep position, the pressure acting on the substrate is increased, so that the bath agent gets into the interior of the fiber structure and the details of the complex structure. Such a film can be processed. In addition, as a method of covering the inside of such a fiber assembly, there is chemical vapor diffusion (CVI) or the like. However, these manufacturing methods have a very slow film formation rate and have a difficulty in productivity.

Thus, a metal carbide layer having high adhesion can be formed by coating the base material by the salt bath method.
In particular, when a fuel cell is mounted on a vehicle, vibration may be transmitted to the stack and the electrode and current collector may rub, and it is necessary to withstand heat shock due to frequent starting and stopping. Thus, a film having such a high adhesion is desirable. Further, since this method is a wet method, it is possible to cover the inside even with a nonwoven fabric or a woven fabric, and it is also possible to use a material that is vulnerable to oxidation such as carbon fiber.

  The application of the current collector of the present invention described above is not particularly limited and can be widely applied to various power generation devices, batteries, storage batteries, etc., but fuel cells, among others, operating temperature is particularly high, and material restrictions It can also be suitably used as a current collector for a solid oxide fuel cell under severe conditions.

That is, as shown in FIG. 1 (a) or 2 (a), for example, a current collector 1 formed by forming a metal carbide film F on the surface of a plate-like or fibrous substrate B having irregularities is formed. 1 (b) and 1 (c) or 2 (b) and 2 (c), the air electrode 2 in the battery element in which the solid electrolyte 4 is sandwiched between the air electrode 2 and the fuel electrode 3 is used. Or it can be set as the solid oxide fuel cell of this invention by making it contact | abut to the fuel electrode 3 or both.
In the solid oxide fuel cell, examples of the solid electrolyte 4 include YSZ (yttria stabilized zirconia), SSZ (scandium stabilized zirconia), SDC (samarium-doped ceria), and the air electrode 2. It is, Pt, or a metal material such as _{Ag, LSM (La 1-X} Sr X MnO 3), LCM (La 1-X Ca X MnO 3), LSC (La 1-X Sr X CoO 3), SSC (Sm _1-X Sr _X CoO ₃ ) and the like, and the fuel electrode 3 includes metal materials such as Pt, Ni, and Cu, and cermets such as Ni—SDC, Ni—YSZ, and Cu—CeO ₂ (ceria). A material or a mixture of these materials can be used.

In this way, by using a base material with high heat resistance, having a conductivity, and forming a metal carbide film with high heat resistance and chemical stability, it is used for both the air electrode 2 and the fuel electrode 3. Thus, good contact with the electrode can be obtained even in a high temperature environment, and the contact resistance can be reduced. In particular, when a fiber material is used, good air permeability can be ensured, so that reaction resistance can be reduced.
In a solid oxide fuel cell, Cr poisoning of the air electrode may be a problem. However, since CrC has a very strong bond with carbon, a current-carrying member made of CrC as a metal carbide film is formed. Even if it is applied to the battery, it is considered that the possibility that the carbides dissociate and the electrode performance deteriorates is extremely low.

  Hereinafter, the present invention will be specifically described based on examples. In addition, this invention is not limited only to these Examples.

(Example 1)
First, a 40 × 4 × 3 mm prismatic test piece made of austenitic stainless steel SUS304 material is prepared, and in the mixed gas atmosphere containing CO ₂ , CO, H ₂ , and H ₂ O with respect to the test piece, Carburizing treatment was performed at 1000 ° C. for 2 hours, and then a solution treatment was performed by rapidly cooling from a temperature of 750 to 950 ° C.
Next, a salt bath containing KCl and BaCl ₂ is melted at 1050 ° C. in a furnace having a depth of 1 m, and after adding the metal chromium powder, the test piece is immersed in the bottom of the furnace to obtain a surface of the test piece. A CrC film having a thickness of 3 μm was formed.

  And after removing salt bath agent from the test piece after salt bath treatment by high-pressure hot water and ultrasonic cleaning, the resistivity at each temperature is measured by 4-terminal measurement with a Pt probe, and the temperature dependence of the chromium carbide film resistivity In addition, the temperature dependence of the resistivity of these oxides is the same by using a sample in which ITO (indium-tin oxide) and Ga-added ZnO are formed to a thickness of 3 μm by sputtering on the same test piece. Investigated. These results are shown in FIG.

  As is apparent from the results of FIG. 3, in these oxides (ITO and Ga-doped ZnO), the resistivity rapidly increases after exceeding 200 to 400 ° C., whereas in CrC, the resistance near room temperature is increased. It was confirmed that the rate was lower than that of the above oxide, and the value hardly changed up to 800 ° C.

(Example 2)
A metal fiber nonwoven fabric (average fiber diameter: 40 μm) made of an Fe—Cr—Si heat-resistant alloy with C added is immersed in the same salt bath as in Example 1 to form a CrC film having a thickness of 3 μm on the fiber surface. did.

Then, using the thus obtained non-woven fabric after forming the CrC film as a current collector, an oxide of a perovskite structure containing a lanthanum, strontium, and cobalt of a solid oxide fuel cell (30 mm diameter): 1 .2 mm diameter) and power generation evaluation was performed by pressing a Pt mesh on the fuel electrode side.
As a result, it was confirmed that the battery output was improved by about 20% compared to the case where Pt mesh was pressed against both electrodes. In addition, defects such as peeling of the CrC film did not occur in the nonwoven fabric after power generation evaluation.

(Example 3)
Using the non-woven fabric after CrC film formation obtained in Example 2 as a current collector, the fuel cell is pressed against the fuel electrode (nickel oxide and stabilized zirconia cermet) side, and the Pt mesh is pressed against the air electrode side Thus, the same power generation evaluation was performed.
As a result, it was confirmed that the battery output was improved by about 10% compared to the case where Pt mesh was pressed against both electrodes.

Example 4
A groove having a width of the convex portion: 0.5 mm, a width of the concave portion: 0.5 mm, and a depth: 0.5 mm was formed in the gas flow direction on a plate material made of ferritic stainless steel SUS430 and having a thickness of 1 mm. And after giving the carburizing process to the said board | plate material, it immersed in the salt bath similar to the said Example 1, and formed the CrC film | membrane with a thickness of 3 micrometers on the board | plate surface.

  Next, the plate material after such CrC film formation was used as a current collector and pressed against the air electrode of the solid oxide fuel cell, and the power generation evaluation was performed by pressing a Pt mesh on the fuel electrode side. . It was confirmed that a battery output substantially equivalent to that obtained when Pt mesh was pressed against both electrodes was obtained. In addition, since the manufacturing cost of the said plate-shaped collector is about 1/10 of Pt mesh, the cost of a collector can be reduced and it can contribute to the cost reduction of a fuel cell.

(Example 5)
The same power generation evaluation was performed by pressing the plate material after the CrC film formation obtained in Example 4 as a current collector to the fuel electrode side of the fuel cell and pressing the Pt mesh on the air electrode side. It was.
As a result, a battery output comparable to that obtained when Pt mesh was pressed against both electrodes was obtained, and it was confirmed that it has an advantage over Pt mesh from the viewpoint of cost.

(Example 6)
Using a carbon fiber having an average fiber diameter of 7 μm and an average length of 20 mm, a non-woven fabric having a thickness of 4 mm and a fiber density of 600 g / m ² is prepared, and the non-woven fabric is immersed in the same salt bath as in Example 1 to obtain carbon. A CrC film having a thickness of 3 μm was formed on the fiber surface. FIGS. 4A and 4B show enlarged cross-sectional photographs of the carbon fibers after the CrC coating obtained as described above, using a scanning electron microscope.

Then, the carbon fiber non-woven fabric after CrC film formation obtained in this way is used as a current collector and pressed against the air electrode of the solid oxide fuel cell, and power is generated by pressing a Pt mesh on the fuel electrode side. As a result of the evaluation, it was confirmed that the battery output was improved by about 15% compared to the case where Pt mesh was pressed against both electrodes.
Moreover, as a result of performing power generation evaluation by pressing the current collector made of the carbon fiber non-woven fabric against the fuel electrode and pressing the Pt mesh on the air electrode side, the battery output is compared with the case where the Pt mesh is pressed against both electrodes. Was confirmed to be improved by about 20%. In addition, peeling of the CrC film | membrane was not recognized by the nonwoven fabric after power generation evaluation.

(A) It is a schematic diagram which shows the structure of the electrical power collector which formed the metal carbide film in the plate-shaped base material provided with the unevenness | corrugation in the contact surface with an electrode. (B) It is a schematic sectional drawing which shows the cell structure which has arrange | positioned the electrical power collector shown to Fig.1 (a) to the air electrode side. (C) It is a schematic sectional drawing which shows the cell structure which has arrange | positioned the electrical power collector shown to Fig.1 (a) to the fuel electrode side. (A) It is a schematic diagram which shows the structure of the electrical power collector which formed the metal carbide film in the surface of the fibrous base material. (B) It is a schematic sectional drawing which shows the cell structure which has arrange | positioned the electrical power collector shown to Fig.2 (a) to the air electrode side. (C) It is a schematic sectional drawing which shows the cell structure which has arrange | positioned the electrical power collector shown to Fig.2 (a) to the fuel electrode side. It is a graph which shows the influence of the temperature which acts on the resistivity of chromium carbide compared with the case of an oxide. (A) It is a cross-sectional photograph which shows the state of the carbon fiber which formed the chromium carbide membrane | film | coat on the surface. (B) It is the expanded cross-section photograph which expanded FIG. 4 (a) further.

Explanation of symbols

B Base material F Metal carbide film 1 Current collector 2 Air electrode (electrode layer)
3 Fuel electrode (electrode layer)
4 electrolyte

Claims (10)

  A current collector formed by forming a metal carbide film on a substrate surface.   The current collector according to claim 1, wherein the substrate is made of a metal material containing carbon.   The current collector according to claim 1, wherein the substrate is made of a carbon material.   The current collector according to any one of claims 1 to 3, wherein the base material has a plate shape having at least a contact surface with an electrode.   The current collector according to any one of claims 1 to 3, wherein the base material has a fibrous shape.   The current collector according to claim 5, wherein the fibrous substrate has a diameter of 1 to 100 μm.   The current collector according to claim 1, wherein the metal carbide film contains at least one carbide selected from the group consisting of CrC, VC, and TiC.   The current collector according to any one of claims 1 to 7, wherein the metal carbide film is formed by a salt bath treatment.   A fuel cell, wherein the current collector according to any one of claims 1 to 8 is used. A solid oxide fuel cell in which an electrolyte composed of a solid oxide is sandwiched between two electrode layers having a catalytic function, wherein the current collector is one of the electrode layers. Or a solid oxide fuel cell characterized by contacting both surfaces.

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