WO2003050902A2 - Method for the production of a plate and plate - Google Patents

Abstract

The invention relates to a method for the production of a plate, comprising a thermally-hardening and/or thermoplastic plastic with carbon filler, with a filler proportion of preferably 70 wt. % to 95 wt. %, whereby a starting mixture containing the plastic and the carbon filler is filled in a press mould and moulded to give the plate by means of a pressing tool. According to the invention, the plate may have the desired properties, whereby the starting mixture is fed to the press mould in portions in layers or regions and the supplied portions are serially subjected to the desired pressing parameters.

Description

description

Process for producing a plate and plate

The invention relates to a method for producing a plate, consisting of at least two layers of different composition and / or different chemical and / or physical properties, in particular intended for a fuel cell, wherein at least one starting mixture for forming a layer is a thermosetting and / or thermoplastic Plastic with carbon filler with a filler content of preferably 70% by weight to 95% by weight is used, and the initial openings are introduced into a press mold and at least partially, preferably completely, heat-cured under pressure. Furthermore, the invention relates to a plate, in particular a bipolar plate intended for a fuel cell or separator plate or electrode plate, consisting of several layers of different chemical and / or physical properties, the layers consisting of the same or different starting mixtures, at least one layer consisting of a thermosetting and / or thermoplastic and a carbon filler with a filler content of preferably 70 wt% to 95 wt%.

In fuel cells, chemical energy can be converted directly into electrical current with high efficiency. The basic principle is the spatial separation of reactants such as hydrogen or methanol on the one hand and oxygen or air on the other hand by an ion-conductive electrolyte such as a polymer electrolyte membrane realized, which is in contact on both sides with porous electrodes - the anode and the cathode. In this way, a chemical reaction between hydrogen and oxygen cannot take place explosively as an oxyhydrogen gas reaction, but can be carried out in a controlled manner in such a way that an electron exchange between the reaction partners takes place via an external circuit and thus supplies the electrical energy.

The electrodes frequently consist of a soot layer provided with a catalyst, which is applied to the membrane, platinum, but also other suitable noble metals such as palladium, being used as catalysts.

The reactants can be supplied to the electrodes via bipolar plates which consist of a thermosetting plastic with carbon filler with a filler content of in particular 70% to 95% by weight. Channels along which the reactants flow are embedded in the surfaces of the bipolar plates facing the electrodes. If hydrogen is fed to the anode of the fuel cell as the reaction gas, cations are formed in the catalyst layer of the artode and electrons are simultaneously released to the electron-conducting anode. Oxygen or air is supplied to the cathode side of the cell as the oxidizing agent. By absorbing the hydrogen ions (protons) diffused through the ion-conductive membrane and the electrons flowing through the external circuit from the anode to the cathode, the reaction gas oxygen is reduced. This reaction takes place in the catalyst layer of the cathode which is in contact with the membrane. Water is the reaction product. The reaction enthalpy is released in the form of electrical energy and heat.

A diffusion layer is arranged between the respective electrode and the bipolar plate so that the reactants come into contact with the respective electrode or the catalyst present therein to a sufficient extent. Thus, the electrode is not covered by the webs or walls between the channels of the bipolar plate, as a result of which the efficiency could otherwise be impaired. Crucial parameters for achieving a high degree of efficiency are the quick and complete supply and removal of reaction partners and products at the active centers of the electrodes and an optimal water balance of the membrane.

Drying out of the membrane drastically reduces its proton conductivity; In contrast, excess water leads to the blocking of the pores in the gas diffusion layer and finally to the blockage of the gas guide channels.

Usually one tries to solve the problem of an optimal water content over the entire active area by the design and the arrangement of the gas guiding channels on the bipolar plate. At the same time, either external or internal humidification systems are used.

From DE 195 42 721 AI it is known to form plastic-filler mixtures by extrusion into sheets which can be intended for electrical and electrochemical purposes. Due to the extrusion, the mixtures fed to the extruder must maintain certain temperatures in order to ensure a mass requirement.

DE 26 35 636 C2 describes a fuel cell and a method for producing one. Sheets of electrode material and sheets of nonwoven material are connected, the plates being punched out of these openings in order to achieve a desired geometry, in order to provide desired chambers, channels and passages in a finished fuel cell.

A graphitized composite material is known from WO 96/33520, which is used for the production of plates of fuel cells.

DE 198 29 142 AI relates to a gas-tight composite of bipolar plate and membrane-electrode assembly of polymer electrolyte membrane fuel cells. Bipolar plates with cooling channels are also known, which consist of two plate-shaped sections lying one on top of the other, the surfaces lying one on top of the other having channel-shaped depressions which form one another and form the cooling channels. The plate-shaped sections themselves are connected by adhesive, which means that losses in the conductivity of the bipolar plate in the area of the connection points have to be accepted.

An electrode material for fuel cells according to DE 35 12 326 AI has an integrated press-formed five-layer structure. The starting materials for the electrode material are then put into a press mold one after the other in order to then obtain a finished structure together by means of printing forms, post-curing and calcining. If necessary, corresponding structures produced as a unit can be re-entered into a printing form in order to connect them to one another by interposing a graphite sheet.

The problem underlying the present invention is to further develop a method for producing a plate and such itself in such a way that layers of desired properties can be achieved with simple measures, wherein an individual design of the plate should be possible. At the same time, a good water balance for a membrane should be ensured when using the plate for a fuel cell. A humidification system should not be necessary or only in a simplified form.

According to the invention, the problem is essentially solved by a method of the type mentioned in the introduction by the following method steps

Introducing a first starting mixture into the printing form and at least partially thermosetting the starting mixture under pressure to form a first layer,

Introducing a second starting mixture onto the first layer into the printing form and at least partially heat-curing the second starting mixture under pressure to form the second layer and in accordance with further layers to be formed, in each case introduction of a starting mixture onto a layer previously at least partially thermoset in the printing form and at least partial hardening of the respectively introduced starting mixture under the action of pressure.

According to the invention, a uniform plate is produced which, through the targeted supply of starting mixtures and the subsequent formation of the individual layers, obtain the desired chemical or physical properties in their final properties. Due to the individual design of the individual layers, these can consist of the same starting mixtures, but their physical properties, in particular porosity, can be set differently due to differing pressure effects or temperatures. For example, one flat side of a plate can be porous and the other flat side can be made liquid-tight without the need for different starting mixtures. Rather, the porosity can be adjusted solely by different pressure effects.

In other words, the different material properties can be achieved in that a starting mixture of the same composition is added to the mold in portions and then the pressing process is carried out after the respective portion has been supplied, wherein different portions can be exposed to different pressing parameters.

In particular, it is provided that at least one cover layer of the plate is made hydrophilic by, for example, oxidizable aftertreatment such as wet chemical or by corona or plasmas treatment.

A plate of the type mentioned at the outset is distinguished by the fact that it consists of successive layers which are at least partially heat-hardened when exposed to pressure, which layers are in turn connected to one another by the action of heat and pressure when the respective layer is formed. The plate can be structured in at least one of its flat sides by, for example, molded-in channels, the flat sides being of different pores. Of course, each flat side of the plate can also be structured, wherein when used as a bipolar plate, one flat side, specifically the porous flat side of the cathode and the denser flat side of the anode, can face a membrane electrode assembly (MEA).

According to a further development of the invention, the porous cathode side is made hydrophilic or aftertreated using conventional additives or methods. If the gas routing channels are arranged so that the inlet and outlet are closely adjacent, water can penetrate from the water-rich outlet side through the pore system to the dry inlet side and humidify the incoming air. Excess water will wet the plate surface well and be discharged as a film; Closing pores and channels by drop-shaped water is avoided.

Depending on the membrane thickness and the operating conditions, the cathode-side moistening of the membrane is sufficient to ensure the optimal water content; however, the anode-side fuel gas may need to be slightly moistened before entering the fuel cell.

The combination of a porous surface layer with finely structured channels can make the use of a gas diffusion layer unnecessary, in particular on the anode side. The fuel gas can reach the entire surface of the catalyst layer on the membrane via the porous structure of the webs between the channels. By dispensing with a gas diffusion layer, lower contact resistances, i. H. lower ohmic losses and thus better efficiency achievable.

There is also the possibility that the top layer of the plate contains at least one hydrophilic additive such as silica gel. Further details, advantages and features of the invention result not only from the claims, the features to be extracted from them - individually and / or in combination - but also from the following description of exemplary embodiments.

example 1

To produce a plastic filling mixture intended for a bipolar plate, 850 g of graphite from a fraction of 0 to 200 μm and 150 g of phenolic resin were mixed as mixture components in a mixer at a temperature of 80 ° over a period of 10 minutes. After the mixture had been discharged from the mixer and solidified, the mass was fed to a mill in order to obtain regrind from a fraction of less than 1 mm. A previously weighed and volumetrically metered mass of the mixture is then poured into a press mold (die) of a hot press tool, in order to then close the press by moving the upper punch axially, ie vertically. After the upper punch has been placed on the mass, it begins to soften, compact and harden as a result of the molding compound touching the hot punch walls. A first weighed and volumetrically dosed mass is exposed to a pressure of 50 MPa. As a result, the mass is compressed almost pore-free. After the mass has hardened, the press stone is withdrawn in order to fill a second weighed and volumetrically metered mass into the die. Then a pressing process is carried out again, but the pressure has been changed to 5 MPa. As a result, the layer formed by the second weighed volumetrically metered mass has a pore volume of 25%. Due to the hydrophilic nature of phenolic resin, the plate is well wetted by water.

Example 2

To produce a plastic filling mixture intended for a bipolar plate, 950 g of graphite from a fraction of 5 to 100 μm and 50 g of phenolic resin were mixed as mixture components in a mixer at a temperature of 80 ° over a period of 10 minutes. After discharging the mixture from the mixer and solidifying it, the mass was se fed to a mill in order to obtain regrind of a fraction smaller than 1 mm. A previously weighed and volumetrically metered mass of the mixture is then poured into a press mold (die) of a hot press tool, in order to then close the press by moving the upper punch axially, ie vertically. After the upper punch has been placed on the mass, it begins to soften, compact and harden as a result of the molding compound touching the hot punch walls. A first weighed and volumetrically dosed mass is exposed to a pressure of 50 MPa. As a result, the mass is compressed almost pore-free. After the mass has hardened, the press ram is withdrawn in order to fill a second weighed and volumetrically metered mass into the die. A pressing operation is then carried out again, but the pressure has been changed to 25 MPa. As a result, the cover layer formed by the second weighed, volumetrically metered mass has a pore volume of 15-20%.

Due to the low binder content, a relatively high pore volume remains even with the higher pressing pressure. The better contacting of the graphite particles in the porous layer leads to a lower electrical resistance of the multilayer plate.

Example 3

To produce a plastic filling mixture intended for a bipolar plate, 850 g of graphite from a fraction of 0 to 200 μm and 150 g of epoxy resin were mixed as mixture components in a mixer at a temperature of 80 ° over a period of 10 minutes. After the mixture had been discharged from the mixer and solidified, the mass was fed to a mill in order to obtain regrind from a fraction of less than 1 mm. A previously weighed and volumetrically metered mass of the mixture is then filled into a press mold (die) of a hot press tool in order to close the press by moving the upper punch axially, ie vertically. After the upper punch has been placed on the mass, it begins to soften, compact and harden as a result of the molding compound touching the hot punch walls. A first weighed and volumetrically dosed mass is exposed to a pressure of 50 MPa. hereby the mass is compressed almost pore-free. After the mass has hardened, the press ram is withdrawn in order to fill a second weighed and volumetrically metered mass into the die. Then a pressing operation is carried out again, but the pressure has been changed to 5 MPa. As a result, the layer formed by the second weighed volumetrically metered mass has a pore volume of 25%.

Example 4

To produce a plastic filling mixture intended for a bipolar plate, 850 g of graphite from a fraction of 0 to 200 μm and 150 g of epoxy resin were mixed as mixture components in a mixer at a temperature of 80 ° over a period of 10 minutes. After the mixture had been discharged from the mixer and solidified, the mass was fed to a mill in order to obtain regrind from a fraction of less than 1 mm. A previously weighed and volumetrically metered mass of the mixture is poured into a press mold (die) of a hot press tool, in order to then close the press by moving the upper punch axially, ie vertically. After the upper punch has been placed on the mass, it begins to soften, compact and harden as a result of the molding compound touching the hot punch walls. A first weighed and volumetrically dosed mass is exposed to a pressure of 50 MPa. As a result, the mass is compressed almost pore-free. After the mass has hardened, the press ram is withdrawn in order to fill a second weighed and volumetrically metered mass into the die. Then a pressing process is carried out again, but the pressure has been changed to 25 MPa. As a result, the layer formed by the second weighed, volumetrically metered mass has a pore volume of 15-20%.

Due to the low binder content, a relatively high pore volume remains even with the higher pressing pressure. The better contacting of the graphite particles in the porous layer leads to a lower electrical resistance of the multilayer plate. By using epoxy resin in Examples 3 and 4, the plates are hydrophobic after shaping. A hydrophilic adjustment is possible through oxidizing aftertreatment (wet chemical or through corona or plasma treatment).

The invention is also explained below with reference to exemplary embodiments to be taken from the drawing, from which further features which characterize the invention result.

Show it:

Fig. 1 is an exploded perspective view of a portion of a

fuel cell,

Fig. 2 is a schematic diagram of a press tool and

Fig. 3 is a schematic diagram of an embodiment of a bipolar plate.

In Fig. 1 purely in principle and in an exploded view, a cutout of a fuel cell is shown, a membrane-electrode arrangement (MEA) 10 being arranged between two bipolar plates 12, 14. In the exemplary embodiment, the bipola plate 12 has a homogeneous structure with the same properties in each area, whereas the bipola plate 14 consists of areas 16, 18 which have different properties. Region 16 may be more porous than region 18.

Regardless of this, the Bipolaφlatten 12, 14 consist of a thermosetting and / or thermoplastic plastic with carbon filler with a filler content of in particular 70% to 95% by weight. In this regard, however, reference is made to well-known techniques. The bipolar plates 12, 14 should also preferably have channels 92, at least on the surface 20 facing the membrane electrode assembly 10, which may be divided into sections, through which a reactant - hydrogen or methane on the anode side and air or oxygen on the cathode side - can flow. The membrane electrode assembly 10 comprises a membrane 24 which is permeable to cations, along the surfaces of which a soot layer with a noble metal catalyst such as platinum or palladium as anode 26 or cathode 28 is arranged. Anode 26 and cathode 28 are in turn covered by a gas diffusion layer 30, 32, which completely cover the channels 22 of the bipolar plates 12, 14 when the unit is composed of the bipolar plates 12, 14 and the membrane electrode assembly 10, but at the same time the possibility offer that reactants flowing in the channels 22 can be distributed over the entire electrode surfaces 26, 28, so that the desired chemical reaction can take place with high efficiency.

According to the teaching of the invention, the bipolar plates can consist of sections of different material compositions or chemical or physical properties that differ from one another.

In Fig. 3, a bipolar plate 34 is shown as an example, which consists of a cuboid carrier body 36 and on this existing layer 48 having a structure formed by channels 38, 40, 42, 44, 46, the layer 48 having a greater porosity than the base plate 36 has. The base plate 36 and the layer 48, however, are not separately produced sections of the bipolar plate 34, but are successively produced in a pressing tool 50 (FIG. 2), which can be a hot pressing tool.

The pressing tool 50 comprises a pressing mold or a die 52 with an inner space 54 which specifies the geometry of a plate to be produced, as well as a pressing die 56 which is axially adjustable therein.

To form layers corresponding to those of FIG. 3 with different material properties and / or physical or chemical properties, layers 58, 60, 62 are poured into the die, that is to say the interior thereof, one after the other, with the press ram 56 after each layer has been introduced onto it acts to form. The press parameters of the press ram 56 can be set differently become so z. B. adjust the porosity of the individual layers 58, 60, 62 differently. Thus, with the same starting mixture, a plate can be produced that has different material properties such as porosity in layers. Of course, the individual layers can also show different material compositions.

For example, layer 58 has a composition of 95% graphite in a fraction of 50 to 100 μm and 50% phenolic resin, and layers 60 and 62 have a composition of 85% graphite in a fraction of 0 to 200 μm and 15% phenolic resin. After applying a respective layer, the press cylinder 56 acts at a pressure of 25 MPa or 50 MPa or 5 MPa. The layers 58, 60, 62 of a bipolar plate produced in this way have the following physical properties:

Bulk density porosity medium pore size

62 1.55 g / cm ³ 21.5% 2.0 μm

60 1.95 g / cm ³ 0.0% ./.

58 1.75 g / cm ³ 15.0% 1.5 µm.

The porous structure enables z. B. with a Bipolaφlatte a problem-free water transport, so that there is no risk that channels are blocked by water drops.

Claims

claims Process for producing a plate and plate 1. Method for producing a plate (12, 14, 34) consisting of at least two layers (36, 48, 58, 60, 62) of different composition and / or different chemical and / or physical properties, in particular intended for a fuel cell, wherein at least one starting mixture for forming a layer is a thermosetting and / or thermoplastic plastic with carbon filler with a filler content of preferably 70% by weight to 95% by weight, and the starting mixtures are introduced into a press mold and at least under pressure are partially, preferably completely heat-cured, characterized by the process steps Introducing a first starting mixture into the printing form and at least partially thermosetting the starting mixture under pressure to form a first layer, Introducing a second starting mixture onto the first layer in the printing form and at least partially thermosetting the second starting mixture under pressure to form the second layer and further layers to be formed accordingly, introducing a starting mixture onto the layer previously at least partially thermoset in the printing form and at least partially curing the respectively introduced one Starting mixture under pressure. A method according to claim 1, characterized in that the same starting mixtures are used for different layers. 3. The method according to claim 1 or 2, by means of the fact that the same and / or different starting mixtures are exposed to different pressure parameters and / or different temperatures. 4. The method according to at least one of the preceding claims, characterized in that at least one cover layer (48) of the plate is made hydrophilic by, for example, oxidizable aftertreatment such as wet chemical or by corona or plasma treatment. 5. The method according to at least one of the preceding claims, characterized by the fact that porosity of a layer is set by pressure or temperature acting on the corresponding starting mixture. 6. The method according to at least one of the preceding claims, d ad urch g ekennz ei chn et that in order to achieve different porosity of layers of the same starting mixture, these different pressures and / or temperatures are exposed. 7. plate (12, 14, 34), in particular Bipolaφlatte intended for a fuel cell or Separatoφlatte or electrode plate, consisting of several layers of different chemical and / or physical properties, wherein the layers consist of the same or different starting mixtures, at least one layer of thermosetting and / or thermoplastic resin and a carbon filler with a filler proportion of preferably 70 wt%> to 95% by _weight> consists, characterized in that the plate (12, 14, 34) consists of successive layers (12, 14, 34) which are at least partially heat-hardened when subjected to pressure, which in turn are connected to one another by the action of heat and pressure when the respective layer is formed. 8. Plate according to claim 7, characterized in that the plate (14, 34) is structured in at least one of its flat sides by, for example, molded channels (38, 40, 42, 44, 46) and that the flat sides are differently porous. 9. Plate according to claim 7 or 8, characterized in that the respective flat side of the plate (34) is structured. 10. Plate according to at least one of claims 7 to 9, characterized in that the cover layer (48) of the plate (34) contains at least one hydrophilic additive such as silica gel. 11. Plate according to at least one of claims 7 to 10, characterized in that at least one cover layer (48), preferably all cover layers of the plate (34) are made hydrophilic.