JP4561239B2 - Fuel cell separator and fuel cell using the same - Google Patents

Description

  The present invention relates to a fuel cell separator constituting a polymer electrolyte fuel cell and a fuel cell using the same.

  In general, a fuel cell using a polymer electrolyte generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. It is. In the structure, first, a catalytic reaction layer composed mainly of carbon powder carrying a platinum-based metal catalyst is formed on both surfaces of a polymer electrolyte membrane that selectively transports hydrogen ions. Next, a diffusion layer is formed of, for example, carbon paper or carbon cloth having both air permeability of fuel gas and electronic conductivity on the outer surface of the catalytic reaction layer, and the diffusion layer and the catalytic reaction layer are combined to form an electrode. To do.

  Next, in order to prevent the supplied fuel gas and oxidant gas from leaking out and mixing the fuel gas and oxidant gas with each other, a gas seal material or Place the gasket. This sealing material or gasket may be integrated with the electrode and the polymer electrolyte membrane and assembled in advance, and this may be referred to as MEA (membrane electrode assembly). On the outside of the MEA, a conductive separator plate for mechanically fixing the MEA and electrically connecting adjacent MEAs to each other in series is disposed. In the portion of the separator plate that contacts the MEA, a reaction gas is supplied to the electrode surface, and a gas flow path for carrying away the generated gas and surplus gas is formed. The gas flow path can be provided separately from the separator plate, but a system in which a groove is provided on the surface of the separator to form a gas flow path is common.

  In order to supply the fuel gas to the groove, a pipe jig for branching the pipe for supplying the fuel gas to the number of separators to be used and directly connecting the branch destination to the separator-like groove is required. This jig is called a manifold, and the type connected directly from the fuel gas supply pipe as described above is called an external manifold. There is a type of this manifold called an internal manifold with a simplified structure. The internal manifold is a separator plate in which a gas flow path is formed with a through-hole, through the gas flow path to the hole, and fuel gas is directly supplied from the hole.

  Since the fuel cell generates heat during operation, it is necessary to cool it with cooling water or the like in order to maintain the battery at a good temperature. Usually, a cooling unit that allows cooling water to flow every 1 to 3 cells is inserted between the separator and the separator. However, a cooling water channel is often provided on the back surface of the separator to form a cooling unit. These MEAs, separators, and cooling units are alternately stacked, and after stacking 10 to 400 cells, they are sandwiched between end plates via current collector plates and insulating plates, and fixed from both ends with fastening bolts. This is a structure of a laminated battery.

  The separator used in such a polymer electrolyte fuel cell has high conductivity, high gas tightness with respect to the fuel gas, and high corrosion resistance against the reaction during oxidation / reduction of hydrogen / oxygen. Must have acid resistance. For this reason, conventional separators have a gas channel formed by cutting on the surface of a glassy carbon plate or a resin-impregnated graphite plate, or expanded graphite together with a binder in a press die having a gas channel groove. It was produced by putting a powder, pressing it, and then heat-treating it.

  In recent years, attempts have been made to use metal plates such as stainless steel instead of conventionally used carbon materials. In the separator using the metal plate, the metal plate is exposed to an oxidizing atmosphere at a high temperature. Therefore, when used for a long time, the metal plate may be corroded or dissolved. When the metal plate is corroded, the electric resistance of the corroded portion increases and the output of the battery decreases. Further, when the metal plate is dissolved, the dissolved metal ions are diffused into the polymer electrolyte and trapped at the ion exchange site of the polymer electrolyte, resulting in a decrease in the ionic conductivity of the polymer electrolyte itself. In order to avoid such deterioration, it is customary to apply gold plating having a certain thickness on the surface of the metal plate.

  In polymer electrolyte fuel cells, in order to facilitate the movement of ionized hydrogen in the polymer electrolyte membrane, water vapor is mixed with gas containing hydrogen as the fuel gas or oxygen gas as the oxidant. It is common to do. On the other hand, since water (water vapor) is generated by the combustion reaction during power generation, water vapor mixed with fuel and oxidant and water (water vapor) generated by power generation pass through the channel groove formed in the separator. The separator surface is generally controlled at a constant temperature so that the generated water does not condense more than necessary, but the amount of heat generated within the fuel cell changes due to changes in the amount of power generated and the fuel supply. However, the internal temperature fluctuates and the amount of generated water fluctuates.

  For example, when the temperature is lowered, the separator surface is likely to condense, and it is impossible to completely eliminate such a phenomenon. When dew condensation occurs, water drops block the flow path, resulting in a shortage of fuel supply to the electrodes and catalyst after the plugged place, so the voltage gradually drops and the water drops are blocked when the water drops are discharged. As a result, the fuel supply is restored and voltage instability (flooding) occurs.

  In order to solve such problems, a method has been proposed in which the hydrophilicity of the separator surface is increased and the water drainage is improved by spreading condensed water into a thin film. For example, as proposed in Patent Document 1, a separator made of a conductive resin prepared by adding a hydrophilic resin or a metal oxide for hydrophilization has been studied. In order not to cause voltage instability phenomenon (flooding), it is necessary to make the hydrophilicity of the separator surface super-hydrophilic with a water contact angle of 10 ° or less. For this purpose, it is necessary to increase the amount of hydrophilic resin or metal oxide added.

  However, when the addition amount is increased, there is a problem that the electrical resistance of the separator material is increased, the voltage drop due to the resistance loss is increased, and the battery performance is lowered. Further, instead of adding a hydrophilic material, plasma treatment is performed on a separator surface obtained by mixing conductive carbon and a binder and compression-molding to form a hydrophilic group such as C═O or C—OH on the carbon element C. There was a way. The compression-molded carbon surface has many chemically stable basal planes (crystal basal planes) of the graphite crystals that make up the carbon particles exposed on the separator surface, so it is chemically stable even after plasma treatment. Therefore, there has been a problem that a sufficient amount of hydrophilic groups cannot be formed on the surface.

  On the other hand, the prism surface (crystal side surface) of graphite crystal is chemically unstable, so it is desirable to expose a large amount of hydrophilic groups on the surface. However, in a compression molded body in which carbon particles and a binder are kneaded, In the surface layer having a thickness of 1 μm, the graphite crystal is often present in a layered manner with the basal surface exposed, and the prism surface of the graphite crystal is exposed if it is deleted from the surface layer to the inner layer by 1 μm or more.

  Further, although depending on the degree of mixing of the binder and the carbon material, the optimum value of the ratio of the total carbon material to satisfy the conductive properties required for the separator and maintain the strength is generally 70% to It is 85%, and the area ratio of the exposed area of the basal surface before removing the surface layer to the entire surface is 70% to 90%. If the surface layer is deleted, the area ratio of the exposed area of the basal surface to the entire surface becomes about 10% to 70%. That is, when the area ratio of the surface of the prism surface to the entire surface is greater than 30%, a sufficient amount of hydrophilic functional groups are formed to impart surface hydrophilicity.

The schematic diagram is shown in FIG. The carbon particles 1 are aggregates 9 in which graphite crystals 8 are bonded in a plane direction or a layer direction. When the carbon particles 1 and the binder 2 are compression-molded, the carbon particles 1 in the vicinity of the surface in contact with the mold are arranged in parallel to the surface, so that the basal surface 7 of the graphite crystal 8 constituting the carbon particle 1 is also Similarly, they are arranged parallel to the surface. The aggregate 9 of the graphite crystals 8 constituting the carbon particles 1 has a scaly shape and has a thickness of 1 to 5 μm and a diameter of 50 to 100 μm. It is highly established that the carbon particles are aligned and formed up to a depth of 1 μm from the molding surface, but in the portion deeper than 1 μm, the orientation of the carbon particles 1 is random and the particles are not aligned. The existing establishment is high. In other words, if 1 μm or more of the surface layer is removed, many chemically unstable prism surfaces appear.
JP 2003-217608 A

  Even when water vapor mixed with fuel gas or oxidant gas or moisture generated by power generation (water vapor) condenses on the separator surface, it does not form water droplets, spreads thinly as a water film on the flow surface of fuel gas and oxidant gas, and opens the flow channel A separator that does not close is realized. Furthermore, the present invention provides a fuel cell having a stable power generation performance that does not cause a voltage drop or increase even when a load connected to the fuel cell fluctuates or a fuel supply amount fluctuates.

  An object of the present invention is to realize highly stable operation of a fuel cell. In addition, separator surface characteristics can be measured in advance by contact angle, etc., but voltage instability phenomena are manifested by interactions between separator characteristics and electrode characteristics after stacking fuel cells and starting power generation operation. Since the voltage is unstable after power generation, work such as exchanging the separator may be performed, and the object is to reduce such work loss.

First, a conductive separator obtained by pressurizing and thermoforming a mixed material obtained by kneading a conductive carbon material and a binder, wherein at least a part of a surface of a gas flow path formed in the separator has the surface More protruding carbon particles and concave carbon particles having a depressed crystal structure are exposed, the surfaces of both the carbon particles are composed of graphite crystals , and the entire surface of the basal surface (crystal bottom surface) exposed portion of the graphite crystals. The area ratio is 10% or more and 70% or less.

In particular, it is preferable to form a hydrophilic group on the surface.

At this time, the hydrophilic group is more preferably formed by plasma treatment containing at least oxygen.

The hydrophilic group may be formed by UV ozone treatment.

It is preferable that a polymer electrolyte fuel cell is formed by using the polymer electrolyte fuel cell separator described above.

  According to the present invention, the separator surface constituting the fuel cell can be hydrophilized. Therefore, the generated water generated by the power generation adheres to the separator surface and forms a thin water film. And by uniformly forming a water film in the flow path, the contact angle between the generated water and the flow path surface becomes equal to zero, and the generated water becomes water droplets and discharges the water without clogging the flow path. It becomes possible. Since the fuel gas is stably supplied to the flow path, stable power generation is possible.

  First, a method for producing an electrode having a catalyst layer will be described. An electrode catalyst comprising 25% by weight of platinum particles having an average particle diameter of about 30% supported on acetylene black powder was used. A dispersion solution in which perfluorocarbonsulfonic acid powder was dispersed in ethyl alcohol was mixed with a solution in which the catalyst powder was dispersed in isopropanol to form a catalyst paste.

On the other hand, the carbon paper which becomes a support body of an electrode was water-repellent treated. After impregnating a carbon nonwoven fabric (made by Toray, TGP-H-120) having an outer size of 14 cm × 14 cm and a thickness of 360 μm into a fluororesin-containing aqueous dispersion (manufactured by Daikin Industries, Neoflon ND1), this is dried. Water repellency was imparted by heating at 400 ° C. for 30 minutes. A catalyst layer was formed on one surface of the carbon nonwoven fabric by applying the catalyst paste using a screen printing method. At this time, a part of the catalyst layer is embedded in the carbon nonwoven fabric. The catalyst layer thus prepared and the carbon nonwoven fabric were combined to form an electrode. Amount of platinum contained in the reaction electrode after forming the 0.6 mg / cm ^2, the amount of perfluorocarbon sulfonic acid was adjusted to be 1.2 mg / cm ^2.

  Next, a pair of electrodes are joined by hot pressing on both sides of the proton conductive polymer electrolyte membrane having an outer dimension of 15 cm × 15 cm so that the catalyst layer is in contact with the electrolyte membrane, and this is joined to the electrode electrolyte membrane assembly. (MEA). Here, as the proton conductive polymer electrolyte, a perfluorocarbon sulfonic acid thinned to a thickness of 30 μm was used.

Next, the conductive separator which is the point of the present invention will be described. First, artificial graphite powder having an average particle size of about 50 μm is prepared, and 80% by weight of the artificial graphite powder is kneaded with an extruding kneader with 20% by weight of a thermosetting phenol resin. It put into the metal mold | die which gave the process for shape | molding the groove | channel for cooling water flow paths, and a manifold, and hot-pressed. The hot pressing conditions were a mold temperature of 150 ° C. and a pressure of 100 kg / cm ² for 10 minutes. The obtained separator had an outer size of 20 cm × 20 cm, a thickness of 3.0 mm, and a depth of the gas channel and the cooling water channel of 1.0 mm. Therefore, the thickness of the thinnest part of the separator plate is 1.0 mm. In this embodiment, artificial graphite is used for the conductive carbon material. However, for example, natural graphite, carbon black, ketjen black and the like can be applied.

  On the surface of the separator thus prepared, there is a carbon particle layer composed of graphite particles having a thickness of about 1 μm to 5 μm and a basal surface having a diameter of 50 to 100 μm parallel to the surface. This layer is sprayed with aluminum oxide powder having an average particle diameter of 20 μm from a nozzle having a diameter of 5 mm at a discharge rate of 1 kg / min at a plane moving speed of 0.5 m / min to remove a thickness of 10 μm.

  FIG. 1 shows the cross-sectional shape of the conductive separator according to the present example prepared as described above. The graphite crystal layer with the orientation of the basal surface exposed on the surface is removed, and a large number of protruding carbon particles 3 protruding from the surface and concave carbon particles 4 having a depressed crystal structure are exposed. An enlarged view of the concave carbon particles 4 is shown in FIG. The graphite particles 6a to 6c expose prism surfaces (crystal side surfaces) 5a to 5c on the surface. When further enlarged, the prism surfaces (crystal side surfaces) 5a to 5c are composed of crystals having many unbonded portions such as a graphite crystal 10 shown in FIG.

  Similarly, the surface of the protruding carbon particle 3 is composed of a crystal having a large number of unbonded portions such as the graphite crystal 10 in which covalent bonds are partially bonded. In other words, the chemically stable carbon surface is destroyed, and a lot of chemically unstable surfaces appear such that many such unbonded portions are exposed on the surface. In this way, the hydrophilicity of the surface is increased because water molecules and the like are easily adsorbed to the chemically unstable surface. Furthermore, since the irregular shape is formed when the surface layer is removed, there is an effect that the surface area is increased, and the hydrophilicity is further increased.

  Since the surface after removing the surface layer is a mixture of the carbon material and the resin as the binder, the resin portion, the basal surface of the graphite crystal, and the prism surface of the graphite crystal are mixed and exposed. When the cross section was observed with an electron scanning microscope (SEM), the ratio of the area of the prism surface to the entire surface area was about 30% even at a small portion, and about 90% at the large portion.

  Next, oxygen plasma treatment is performed for the purpose of reacting oxygen active radicals on the separator surface in such a state. The plasma processing apparatus used is a general RF plasma apparatus of a decompression parallel plate type, the RF power source has a frequency of 13.56 MHz, the output is 500 W, the oxygen supply amount is 500 sccm, the processing time is 5 minutes, and the pressure in the chamber is 0.5 Torr. It was. When the plasma treatment is performed in this manner, a part of the four covalent bonds around one carbon atom is in a chemically unstable unbonded state, so that it reacts with active oxygen radical species and becomes hydrophilic with oxygen. Form a functional group.

  When the surface analysis of this separator was performed by X-ray photoelectron spectroscopy (XPS), the oxide functional group shown in Chemical Formula 1 and Chemical Formula 2, that is, the hydrophilic functional group, was imparted to the carbon surface. Confirmed that it was.

  Further, when the contact angle to water was measured, as shown in FIG. 5, the contact angle immediately after compression molding was 80 ° and 100 ° after blasting, but it was 10 ° after plasma treatment. The contact angle described above is the contact angle of the water droplet when the separator surface is in a dry state, and when the surface is exposed to a humidified state of 100%, the contact angle is measured in the same manner. The contact angle immediately after compression molding was 70 °, 20 ° after blasting, and 0 ° after plasma treatment. When plasma treatment is performed for the purpose of attaching a hydrophilic functional group to a portion where many chemically unstable surfaces are exposed on the surface where many covalent bonds are not bonded, the bonded sites of C and O are Many of them are formed, and the bond strength between C and O is increased, so that it exhibits extremely high hydrophilicity and durability.

  The plasma processing apparatus used here is a parallel plate type in which the electrodes face each other in parallel, but may be a barrel type in which the electrodes are arranged on the side surface of the cylindrical chamber. In addition, although the decompression process is performed this time, a method of irradiating plasma in an atmospheric pressure atmosphere may be used. Further, UV ozone treatment in which ultraviolet light is irradiated in an ozone atmosphere may be used.

Using the two separators thus prepared, the separator according to this example in which the oxidant gas flow path was formed on one surface of the MEA sheet, and the separator according to this example in which the fuel gas flow path was formed on the back surface Were stacked to form a single cell. After stacking two cells of this single cell, the two-cell stacked battery was sandwiched by a separator having a cooling channel groove, and this pattern was repeated to create a battery stack of 100 cells. At this time, the both ends of the battery stack were fixed with a current collector plate made of stainless steel, an insulating plate made of an electrically insulating material, and an end plate and a fastening rod. The fastening pressure at this time was 15 kg / cm ² per separator area.

  The polymer electrolyte fuel cell of this example produced in this way was held at 80 ° C., and hydrogen gas that had been humidified and heated to a dew point of 75 ° C. on one electrode side was placed on the other electrode side. Air that was humidified and heated to a dew point of 65 ° C. was supplied. As a result, a battery open voltage of 96 V was obtained at no load when no current was output to the outside. Further, when the internal resistance of the whole laminated battery at this time was measured, it was about 45 mΩ.

This battery was subjected to a continuous power generation test under the conditions of a fuel utilization rate of 85%, an oxygen utilization rate of 50%, and a current density of 0.7 A / cm ² , and the time change of the output characteristics was measured. As a result, it was confirmed that the battery of this example maintained a battery output of about 14 kW (62V-224A) for 8000 hours or more. In addition, the plasma treatment was not performed on the surface, only the blast treatment was performed, the battery was constructed as described above, and the continuous power generation test was performed. Similarly, the battery output of about 14 kW (62V-224A) was maintained for 8000 hours or more. Confirmed to do.

  As described above, according to the present invention, there has been provided a method capable of superhydrophilic treatment of the surface of a carbon molded body. In addition to application to a solid polymer fuel cell separator flow path, an oxide type, carbonated molten salt It is also applicable to a type fuel cell separator. Moreover, it is applicable also to the hydrophilic treatment of the joint surface for joining a carbon raw material.

The schematic diagram which shows the cross section of the separator surface vicinity in one Example of this invention. The schematic diagram which shows the cross section of the concave carbon particle of the separator surface vicinity in one Example of this invention. Schematic diagram showing a cross section near the separator surface in one conventional example Schematic diagram of graphite crystal The graph showing the contact angle of the surface of the separator in one Example of this invention

Explanation of symbols

1 Carbon particles
2 Binder
7 Basal surface
8 Graphite crystals

Claims (5)

A conductive separator obtained by pressurizing and heat-molding a mixed material obtained by kneading a conductive carbon material and a binder, wherein at least a part of a surface of a gas flow path formed in the separator protrudes from the surface. Protruding carbon particles and concave carbon particles having a depressed crystal structure are exposed, the surfaces of both carbon particles are composed of graphite crystals , and the area occupied by the entire surface of the basal surface (crystal bottom surface) exposed portion of the graphite crystals A polymer electrolyte fuel cell separator, wherein the ratio is 10% or more and 70% or less. Claim 1 Symbol placement polymer electrolyte fuel cell separator, characterized in that the formation of the hydrophilic group on the surface. The polymer electrolyte fuel cell separator according to claim 2, wherein the hydrophilic group is formed by plasma treatment containing at least oxygen. The polymer electrolyte fuel cell separator according to claim 2, wherein the hydrophilic group is formed by UV ozone treatment. A polymer electrolyte fuel cell comprising the separator according to any one of claims 1 to 4 .

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