JP2001338653A - Separator for fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for a fuel cell which has a function of inhibiting absorption of carbon monoxide existing in a fuel gas supplied to the fuel cell, onto an electrode surface. SOLUTION: The separator for fuel cells has a contact surface to an electrode or to an electricity collector and a reaction gas aeration slot, and on the surface of the reaction gas aeration slot, an oxidation catalyst or a reduction catalyst is equipped.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel cell separator, and more particularly to a separator that can be used in a vehicle-mounted fuel cell for powering an automobile.

[0002]

2. Description of the Related Art A fuel cell has been attracting attention as a next-generation power generator because it has a high energy conversion efficiency from fuel to electricity and does not emit harmful substances. In particular, polymer ion exchange membrane fuel cells that operate in a temperature range of 150 ° C. or less have been actively studied, and are expected to be put into practical use several years later.
This fuel cell can be operated at a relatively low temperature, has a high power generation output density, and can be downsized, so that it is suitable as a home or vehicle fuel cell.

In general, a polymer ion exchange membrane fuel cell is
A fuel cell and an oxygen electrode (air electrode) are fixed on both sides of the solid electrolyte membrane to form a unit cell (cell), and these are laminated via a plate-like separator provided with a ventilation groove for supplying fuel gas and air. It is constituted by. As the solid electrolyte membrane, a fluororesin-based ion exchange membrane having a sulfonic acid group or the like is used, and the electrode is formed of carbon black in which a water-repellent material PTFE and a noble metal fine particle catalyst are dispersed. When the hydrogen-oxygen fuel cell operates, protons generated by oxidizing hydrogen gas enter the electrolyte and move to the positive electrode side. On the positive electrode side, oxygen introduced from the ventilation groove obtains electrons generated by the oxidation reaction of hydrogen and combines with protons in the electrolyte to form water. By continuing these reaction processes, electric energy can be continuously taken out.

From the viewpoint of fuel storage density and supply cost,
Rather than supplying hydrogen gas directly to the fuel cell, it is preferable to generate and supply hydrogen gas from hydrocarbons such as methanol and gasoline. The fuel gas thus generated usually contains about 1% of carbon monoxide, and even if a carbon monoxide removing device is used, about tens to 100 ppm of carbon monoxide remains. If a very small amount (generally, 10 ppm or more) of carbon monoxide is contained in the fuel gas, the carbon monoxide is adsorbed on the surface of the noble metal catalyst of the electrode, and the electrode reaction of hydrogen is inhibited, and the performance of the battery is greatly reduced.

As a method for suppressing the adsorption of carbon monoxide to the electrode surface, for example, a method of filtering a fuel gas with a palladium thin film is known. However, the mechanical strength of the palladium thin film is low, and the permeation speed also poses a problem. Further, a method has been proposed in which only hydrogen is selectively permeated by an electrochemical hydrogen pump using a proton electrolyte membrane and supplied to a battery.
Since the pump is driven by consuming a part of the electric power, the power generation efficiency of the fuel cell is reduced. Therefore, at present, a method is employed in which a separate catalyst device is provided in front of the battery cell to remove carbon monoxide in the fuel gas. Recently, a carbon monoxide removal system using zeolite was proposed (M.
Watanabe, et al., Chem. Lett., 21 (1995)), and there remains the need for a dedicated carbon monoxide removal device. Therefore, development of a separator that can effectively suppress the adsorption of carbon monoxide to the electrode without using such a removing device is desired.

Many conventional fuel cell separators are obtained by machining a graphite plate. Graphite separators have low electrical resistance and high corrosion resistance, but have low mechanical strength and high processing costs. On the other hand, metal separators have low bulk electrical resistance, high airtightness and mechanical strength, and are easy to reduce processing costs. In addition, since the thickness of the separator can be reduced, miniaturization is easy. Further, when a low specific gravity metal material such as aluminum is used, the weight of the fuel cell can be further reduced. However, in aluminum separators, the problem is that the corrosion resistance of the aluminum base material itself is low. JP-A-11-162478 discloses a method of improving the corrosion resistance of a separator by plating a noble metal on the entire surface of a metal separator to form a film having low contact electric resistance and corrosion resistance. Although this method has no problem with respect to the separator performance, it is not practical because it increases the cost. In order to reduce costs, it is necessary to make the noble metal plating layer thinner.However, if the layer thickness is reduced during wet plating, fine pinholes will occur and cause corrosion,
Further, the dry plating (evaporation, sputtering, etc.) has poor production efficiency and deteriorates the uniformity of the coating. Therefore, an aluminum separator which is excellent in corrosion resistance and capable of suppressing adsorption of carbon monoxide to the electrode as described above is particularly desired.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a fuel cell separator having a function of suppressing adsorption of carbon monoxide present in a fuel gas supplied to a fuel cell to an electrode surface, and in particular, to such a fuel cell separator. An object of the present invention is to provide an aluminum fuel cell separator having both functions and excellent corrosion resistance.

[0008]

Means for Solving the Problems In view of the above problems, as a result of intensive studies, the present inventors have found that a fuel cell separator in which a catalyst for adsorbing carbon monoxide is fixed in a reactant gas vent groove is formed of a fuel gas separator. It has been found that the adsorption of carbon monoxide existing on the surface of the electrode can be suppressed, and the present invention has been reached.

That is, the fuel cell separator of the present invention has a contact surface with an electrode or a current collector and a reaction gas ventilation groove, and an oxidation catalyst or a reduction catalyst is supported on the surface of the reaction gas ventilation groove. It is characterized by being.

In the fuel cell separator of the present invention, it is preferable to use an aluminum metal plate as the base material from the viewpoint of improving mechanical strength and reducing processing cost. In this case, in order to improve the corrosion resistance of the separator, it is preferable that an alumite coating is formed on the surface of the above-mentioned reaction gas ventilation groove, and the oxidation catalyst or the reduction catalyst is supported on the alumite coating.

The alumite coating is preferably a porous alumite coating having a porosity of 10% or more. It is also preferable that the alumite coating is composed of a dense alumite coating having a porosity of 5% or less and a porous alumite coating having a porosity of 10% or more formed thereon. In these cases, the oxidation catalyst or the reduction catalyst is preferably supported inside the pores of the porous alumite coating or on the surface of the fine projections. The pore diameter of the pores of the porous alumite coating is preferably 10 to 1000 nm.

The oxidation catalyst or reduction catalyst is Pt, Au, Pd, Ru,
It is preferably made of at least one metal selected from the group consisting of Rh, Ir, Ag, Cs, Ni and Cu, or an alloy or oxide thereof. The supporting amount is preferably 0.01 to 2.0 mg / cm ² with respect to the unit area of the reaction gas vent groove.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The fuel cell separator of the present invention has a contact surface with an electrode or a current collector and a reaction gas ventilation groove, and an oxidation catalyst or a reduction catalyst is provided on the surface of the reaction gas ventilation groove. It is carried. INDUSTRIAL APPLICABILITY The separator of the present invention can be used for various fuel cells, and can be particularly suitably used for an in-vehicle fuel cell for driving an automobile.

The substrate used for the separator of the present invention is not particularly limited, and a metal plate such as aluminum or stainless steel, a graphite plate or the like can be used. From the viewpoint of reducing the weight of the separator, improving the electrical conductivity and strength, and reducing the processing cost, it is preferable to use a metal plate, and it is particularly preferable to use an aluminum metal plate. When a metal plate is used, it is preferable to appropriately treat the surface of the reaction gas ventilation groove in order to improve corrosion resistance. The thickness of the base material is not particularly limited, but is preferably 0.5 to 3 mm when used for a vehicle fuel cell.

Regardless of the type of substrate used, the shape and formation method of the separator, the type of catalyst used, and the supporting method used in the present invention are basically the same. The fuel cell separator according to the present invention when used will be described in detail with reference to the drawings. However, the present invention is not limited thereto, and various changes can be made without changing the gist of the present invention.

FIG. 1 is a partial schematic view showing an example of a fuel cell including an aluminum fuel cell separator according to one embodiment of the present invention. The fuel cell shown in FIG. 1 is configured by stacking unit cells 1 each comprising a solid electrolyte 2 and anodes 3 and cathodes 4 provided on both sides thereof with a separator 5 interposed therebetween. Both ends of the stack are connected to an external circuit (not shown).

(A) Reactive Gas Vent Groove As shown in FIG. 1, the separator 5 of the present invention has reactive gas vent grooves 8 and 9. Reaction gas vent groove 9 and anode 3
Is supplied with fuel gas, and an oxidant gas is supplied into a passage formed by the reaction gas vent groove 8 and the cathode 4. The reactive gas ventilation groove may be formed in a predetermined pattern by a method such as machining, pressing, precision casting, chemical polishing (etching), and electrolytic polishing. Although the shape of the reaction gas ventilation groove is U-shaped in FIG. 1, the shape is not particularly limited as long as a reaction gas passage can be formed in a portion in contact with the electrode, and the reaction gas ventilation resistance is small and the power generation efficiency is high. It is preferable to set so that Usually, the depth of each reaction gas vent groove is preferably 0.2 to 2 mm, and the width is 0.5
Preferably, it is set to ５5 mm.

When an aluminum substrate is used, a chemically and physically stable alumite film is formed on the surface of the reaction gas ventilation groove not in contact with the electrodes, etc., in order to ensure the corrosion resistance of the separator. Preferably, it is fixed on the coating. FIG. 2 shows an example of the aluminum separator according to the present invention on which the alumite film 6 supporting the catalyst 12 is formed.

The alumite film can be formed by a usual anodic oxidation method. For example, the γ-alumina coating may be formed on the surface of the substrate by performing electrolysis using an aqueous solution of oxalic acid, sulfuric acid, chromic acid, or the like as an electrolytic solution. The thickness of the alumite coating is preferably from 5 to 50 μm, more preferably from 10 to 30 μm. Further, by performing the treatment with boiling water or steam after performing the anodizing treatment, it is possible to close the fine pores peculiar to the alumite film and form a dense film called boehmite.

The porosity of the alumite coating can be adjusted by selecting the electrolysis conditions. Alumite coating 6 having columnar pores as shown in FIG. 3 (porous alumite coating 10)
It is also possible to form Alumite coating is porous
Preferably, it is a porous alumite coating of 10% or more.
Since a large number of micropores are present on the surface of such a porous alumite coating, the surface area per unit area (specific surface area) increases, and the catalyst is more highly dispersed. In addition, the bonding force of the catalyst fine particles is increased, and the fixation is strengthened, and the activity of the catalyst is improved. That is, by making the alumite coating porous, the function of suppressing adsorption of carbon monoxide to the electrode can be further improved.

As shown in FIG. 4, it is also preferable to form a dense alumite coating 11 and a porous alumite coating 10 formed thereon by changing the electrolytic conditions. The lower dense alumite coating 11 improves the corrosion resistance of the aluminum substrate, and the upper porous alumite coating 10 can efficiently support the catalyst as described above. In this case, the porosity of the dense alumite coating is 5%
It is preferably at most 2%, more preferably at most 2%. The porosity of the porous alumite coating is preferably at least 10%, more preferably at least 20%. The thickness of the dense alumite coating is preferably 2 to 30 μm, and the thickness of the porous alumite coating is preferably 5 to 50 μm.

When a porous alumite film is formed, it is preferable that the oxidation catalyst or the reduction catalyst is fixed inside the pores of the film or on the surface of the fine projections. In order to fix the catalyst fine particles inside the pores of the coating, the pore diameter of the pores of the coating is 10 to 1000 nm.
It is preferred that

(B) Oxidation catalyst or reduction catalyst In the separator of the present invention, an oxidation catalyst or a reduction catalyst is carried on the surface of the above-mentioned reaction gas ventilation groove to have a catalytic function. When an alumite film or the like is formed on the surface of the reaction gas ventilation groove, the film is fixed on the film. The catalyst may be in a state of being dispersed on the separator substrate or in a state of covering the separator substrate. When a hydrogen fuel gas containing carbon monoxide is fed into the fuel cell, carbon monoxide in the gas that has passed through the reaction gas vent groove is adsorbed by the catalyst, and the concentration of carbon monoxide in the fuel gas reaching the electrode is increased. Can be reduced. Although the adsorbed carbon monoxide may be left as it is, it is preferable to oxidize and remove the adsorbed carbon monoxide by adding a small amount of air or oxygen at regular time intervals. By performing such a treatment, the adsorption ability of the catalyst is regenerated.

The oxidation catalyst or reduction catalyst is Pt, Au, Pd, Ru,
It is preferably made of at least one metal selected from the group consisting of Rh, Ir, Ag, Cs, Ni and Cu, or an alloy or oxide thereof.

The oxidation catalyst or reduction catalyst (carbon monoxide adsorption catalyst) includes, for example, salts such as Pt, Pd, and Au, noble metal acids, complexes,
After other inorganic compounds, organic noble metal compounds, etc. are adhered to the surface of the base material and dried sufficiently, they are reduced by a method such as a thermal decomposition method or a chemical reduction method to have a catalytic function, and are supported in the reaction gas ventilation groove. I just need. The noble metal compound or the like may be attached using a solution of water or an organic solvent by a method such as a dipping method, a coating method, and a spraying method. Loading amount of the catalyst is preferably a 0.01 to 2.0 mg / cm ² with respect to the unit area of the reaction gas ventilation grooves, and more preferably, 0.05~1.0mg / cm ^2.

(C) Contact Surface with Electrode or Current Collector The separator of the present invention has a contact surface with the electrode or current collector. The shape of the contact surface may be any shape as long as it is suitable for contacting the electrode of the fuel cell or the carbon paper or carbon cloth of the primary current collector, and is not limited by the drawings. It is preferable to secure the flatness by polishing the contact surface by lapping or the like. When producing an aluminum separator having an alumite coating, such polishing removes the alumite coating that has also been formed on the electrode contact surface, thereby enabling electrical conduction.

In the present invention, in order to reduce the contact electric resistance between the separator and the electrode or the primary current collector, such as carbon paper or carbon cloth, as shown in FIG. It is preferable to form a conductive film 7 on the electric conductive surface). That is, when aluminum is used as the substrate, it is preferable to cover the entire surface of the substrate with a non-conductive alumite coating and a conductive coating.

The conductive film is preferably formed using a material having good electric conductivity and corrosion resistance, and Pt, Au, Pd,
More preferably, it is formed of a metal selected from the group consisting of Ru, Rh, Ir, and Ag or an alloy thereof, carbon, or a conductive carbide. Noble metal-based coatings such as Au, Ag, Pt, and Pd have low contact resistance and extremely good corrosion resistance. As the carbon, a graphite film by CVD, a DLC film (diamond-like carbon film) and the like are preferable. Further, graphite powder to which a water repellent is added may be applied. When the electrode is made of carbon black to which a small amount of Pt is added, the use of a carbon coating provides good contact familiarity. As the conductive carbide, silicon carbide, niobium carbide, tungsten carbide and the like are preferable. The carbide coating not only has low contact resistance, but also has good corrosion resistance and oxidation resistance, and thus acts as a protective film for the separator.

The conductive film can be formed by a method such as sputtering, electroplating, electroless plating, sputtering, wet plating, and CVD. The thickness of the conductive film is 0.01 to
It is preferably 5 μm. If the film thickness is smaller than 0.01 μm, the film strength is weak and unstable, and if it is larger than 5 μm, the cost increases, which is not preferable.

[0030]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the present invention is limited thereto.

Example 1 A reaction gas ventilation groove having a depth of 1.0 mm and a width of 3.0 mm was formed in an aluminum metal plate (1 mm x 150 mm x 150 mm) by press working, and used as a separator substrate. This substrate was anodized in an oxalic acid aqueous solution, then immersed in boiling water for 30 minutes, and dried to form an alumite film having a porosity of about 25% and a film thickness of about 30 μm on the surface of the substrate. Next, the electrode contact surface was lapped and polished to improve the flatness. The alumite film formed on the electrode contact surface by this step was removed. Subsequently, a Pd-Au alloy was sputtered on the electrode contact surface in a 5 mTorr pure argon gas atmosphere at a substrate temperature of 200 ° C. to form a conductive film. The thickness of the conductive film was about 1 μm. The obtained separator is sprayed with a 5% aqueous solution of potassium chloroplatinate and the like, dried, and then sprayed with an aqueous solution of sodium tetrahydroborate (1%) to fix the catalyst and to carry the catalyst of the present invention shown in Table 1. Were prepared respectively. The loading amount of the catalyst was about 0.5 mg / cm ² with respect to the unit area of the above-mentioned reaction gas vent groove. In addition, a comparative separator not supporting a catalyst was similarly manufactured.

The separator of the present invention prepared as described above was incorporated into a solid polymer fuel cell (cell) to prepare fuel cells 1a to 1r, respectively. In addition, a fuel cell 1s using the above-described comparative separator was similarly manufactured. Simulated reformed fuel gas (75% H ₂ , 25% CO ₂ , about 30 ppm CO) was supplied to each fuel cell, and the CO concentration in the outlet gas was analyzed. Table 1 also shows the catalyst carried by the separator used in each fuel cell, the CO concentration in the simulated reformed fuel gas at the inlet, and the CO concentration in the outlet gas.

[0033]

[Table 1]

From Table 1, it was confirmed that the fuel cell separator of the present invention had a function of greatly reducing the CO concentration in the fuel gas.

Example 2 Pt-Ru was used as a catalyst using the separator base material shown in Table 2.
(50 wt%), except that the above-mentioned Example 1 was used.
Fuel cells 2a to 2j were respectively manufactured. The fuel cell 2a
The catalyst was fixed in the same manner to the graphite separator, stainless steel separator, and aluminum separator having no alumite coating film used in Nos. 2c.
In the fuel cells 2d to 2j, the porosity was changed as shown in Table 2 by changing the conditions for forming the alumite coating. Using each of these fuel cells, Example 1
The same analysis was performed. Table 2 shows the separator substrate used for each fuel cell, the porosity of the alumite coating, the CO concentration in the simulated reformed fuel gas at the inlet, and the CO concentration in the outlet gas.
Shown in FIG. 5 shows the relationship between the CO removal rate and the porosity derived from these results.

[0036]

[Table 2]

According to Table 2, when any of the substrates was used,
It can be seen that the function of reducing the CO concentration can be obtained by fixing the catalyst. Furthermore, from Table 2 and FIG. 5, when the porosity of the alumite coating is about 10% or more, the catalytic effect increases,
In particular, it can be seen that the effect becomes remarkable when the porosity is 18.7% or more.

Example 3 The porosity of the film was changed from the middle by changing the electrolysis conditions during the formation of the alumite film, and a dense alumite film (thickness: about 5 to 10 μm) having the porosity shown in Table 3 was obtained. Porous alumite coating (porosity: about 20-35%, film thickness: about 15-
20 μm) in the same manner as in Example 1 except that
Fuel cells 3a to 3j were manufactured, respectively. However, in this example, Pt-Ru (50 wt%) was used as a catalyst. The same analysis as in Example 1 was performed using each of these fuel cells. The corrosion resistance was also evaluated by immersing the separator used in each fuel cell in an acetic acid solution (pH: about 4) for one week and confirming the state of occurrence of corrosion. Table 3 also shows the porosity, corrosion resistance, and CO removal rate of the dense alumite coating of each fuel cell.

[0039]

[Table 3]

From Table 3, when the alumite coating of the separator of the present invention is composed of a dense alumite coating and a porous alumite coating, the porosity of the dense alumite coating is preferably 5.06% or less from the viewpoint of corrosion resistance. 2.
It is understood that the content is more preferably 01% or less.

Example 4 The porosity of the dense alumite coating was set to 1.25%, the porosity of the porous alumite coating was set to about 30%, and Pt or Pd was used as a catalyst with respect to the unit area of the above-mentioned reaction gas ventilation groove. A fuel cell was produced in the same manner as in Example 3 except that the amount of the carrier was fixed at about 0 to ² mg / cm ² . The same analysis as in Example 1 was performed using each fuel cell. FIG. 6 is a graph showing the relationship between the amount of supported catalyst and the CO removal rate (solid circle: Pt
Catalyst, open circle: Pd catalyst). FIG. 6 shows that when a Pt catalyst or a Pd catalyst is used, the amount
It is preferable to be 0.01 to 2.0 mg / cm ² about, 0.05～1.0Mg
/ cm ² was found to be more preferable.

[0042]

As described in detail above, according to the present invention, the separator is provided with catalytic functionality, and the concentration of carbon monoxide in the fuel gas can be greatly reduced. Therefore, the fuel cell separator of the present invention can greatly contribute to improvement of electrode deterioration due to carbon monoxide.

[Brief description of the drawings]

FIG. 1 is a partial schematic view showing an example of a fuel cell including a fuel cell separator according to one embodiment of the present invention.

FIG. 2 is a schematic view showing a fuel cell separator according to another embodiment of the present invention, and a partially enlarged view showing the structure of an alumite film thereof.

FIG. 3 is a schematic view showing a fuel cell separator according to another embodiment of the present invention, and a partially enlarged view showing a structure of an alumite film thereof.

FIG. 4 is a schematic view showing a fuel cell separator according to another embodiment of the present invention, and a partially enlarged view showing a structure of an alumite film thereof.

FIG. 5 is a graph showing the relationship between the porosity of the alumite coating and the CO removal rate in the separator of the present invention.

FIG. 6 shows the amount of catalyst carried and CO in the separator of the present invention.
It is a graph which shows the relationship with a removal rate.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Single cell 2 ... Solid electrolyte 3 ... Anode 4 ... Cathode 5 ... Separator 6 ... Alumite coating 7 ... Conductive coating 8, 9 ... Reactive gas vent groove 10 ・ ・ ・ Porous alumite coating 11 ・ ・ ・ Dense alumite coating 12 ・ ・ ・ Catalyst

Claims (7)

[Claims] 1. A fuel cell separator having a contact surface with an electrode or a current collector and a reaction gas ventilation groove, wherein an oxidation catalyst or a reduction catalyst is supported on the surface of the reaction gas ventilation groove. For fuel cells. 2. The fuel cell separator according to claim 1, comprising an aluminum metal plate, wherein the oxidation catalyst or the reduction catalyst is supported on an alumite film formed on a surface of the reaction gas ventilation groove. A fuel cell separator comprising: 3. The fuel cell separator according to claim 2, wherein the alumite coating is a porous alumite coating having a porosity of 10% or more, and wherein the oxidation catalyst or the reduction catalyst is formed inside the pores of the porous alumite coating. Alternatively, a separator for a fuel cell, which is supported on a surface of a microprojection. 4. The fuel cell separator according to claim 2, wherein the alumite coating comprises a dense alumite coating having a porosity of 5% or less and a porous alumite coating having a porosity of 10% or more formed thereon. A fuel cell separator, wherein the oxidation catalyst or the reduction catalyst is supported inside the pores of the porous alumite coating or on the surface of the fine projections. 5. The fuel cell separator according to claim 3, wherein the pore diameter of the porous alumite coating is 10 to 1000 nm. 6. The fuel cell separator according to claim 1, wherein the oxidation catalyst or the reduction catalyst is
A fuel cell separator comprising at least one metal selected from the group consisting of Pt, Au, Pd, Ru, Rh, Ir, Ag, Cs, Ni and Cu, or an alloy or oxide thereof. 7. The fuel cell separator according to claim 1, wherein the supported amount of the oxidation catalyst or the reduction catalyst is 0.01 to 0.01 with respect to a unit area of the reaction gas ventilation groove.
A fuel cell separator characterized by being 2.0 mg / cm ² .