Green paper for producing a gas diffusion layer for a fuel cell

Abstract

The invention relates to a method for producing a green paper for producing a gas diffusion layer (GDL) for a fuel cell, the surface of said GDL having structures for distributing gases. The method according to the invention has the following steps: a. a pulp (1), which is mixed with a powder and / or fibers made of metal or carbon, is produced from water and paper fibers and / or regenerated cellulose and / or polymer fibers, b. a mold (2) which has a surface with structures that are shaped negatively relative to the structures for distributing gases on the surface of the GDL and which has perforations is then introduced into the pulp (1), c. a negative pressure is then applied to the pulp (1) so that water is suctioned through the perforations of the mold (2), and the powder and / or the fibers and the paper fibers accumulate on the surface of the mold (2) in the form of a fibrous material mat (3), d. the mold (2) is then guided out of the pulp (1), e. the fibrous material mat (3) is then dried, and f. the edges of the fibrous material mat (3) are then cut to size or punched out such that the green paper is produced from the fiber material mat (3).

Description

[0001] Green paper for the production of a gas diffusion layer for a fuel cell

[0002] The invention relates to a method for producing a green paper for producing a gas diffusion layer (GDL) for a fuel cell, wherein the GDL has structures for distributing gases on its surface.

[0003] In the conventional design of the Proton Exchange Membrane Fuel Cell (PEMFC), gas distribution is achieved via a bipolar plate (BPP) and the gas diffusion layer (GDL) to the catalytic platinum-coated membrane (also called the catalyst layer, CL). The entire structure between the two bipolar plates is also called the membrane-electrode assembly (MEA).

[0004] A generic method for producing a green paper for producing a gas diffusion layer (GDL) for a fuel cell is known, for example, from DE 102020005480 A1. In this process, at least one first paper web, preferably mixed with metal powder and / or metal fibers, is produced, wherein at least one watermark is introduced into the paper web to form a structure for the flow field of the gas diffusion layer (GDL) produced from the green paper.

[0005] DE 102020005481 A1 discloses a method for producing a green paper for producing a gas diffusion layer (GDL) for a fuel cell, in which a first paper web is formed and a second paper web is formed, which is combined with the first paper web while still wet and firmly bonded. The first paper web and the second paper web together form the green paper. DE 102020005477 A1 discloses a method for producing a green paper for producing a gas diffusion layer (GDL) for a fuel cell, in which a first paper web, preferably mixed with metal powder and / or metal fibers, is produced, and a microporous layer (MPL) in the form of at least one coating is applied to the paper web.

[0006] For the automotive sector, a GDL made from a fiber material, such as carbon fiber, and a coated BPP made from steel have now become established. The fiber material can be a textile woven / knitted fabric or a fiber mat produced using paper technology, as is known, for example, from DE 102008 042415 B3. However, this current technology has the disadvantage of being relatively expensive, and the resulting fiber composite is pressure-sensitive, which can lead to fiber breakage, which can then potentially damage the CL / PEM, or partially close or constrict the gas distribution channels of the bipolar plate due to the excessive flexibility of the GDL.

[0007] Some of these disadvantages of previous GDL technologies have been overcome by the processes known from DE 102020005480 Al, DE 102020005481 Al, or DE 102020005477 Al. However, even with these processes, the thickness of the GDL is limited to 100 pm to a maximum of 300 pm due to the nature of the process. Furthermore, the resolution, fineness, and edge steepness of the producible structures are limited by the dynamic cylinder mold process and tend to round. Sharp-edged channels are not possible.

[0008] The invention is therefore based on the object of developing a generic method in such a way that the disadvantages of the prior art are eliminated. This object is achieved by the features of the independent claims. Further developments of the invention are the subject of the dependent claims.

[0009] According to the invention, the method for producing a green paper for producing a gas diffusion layer (GDL) for a fuel cell, wherein the GDL has structures for distributing gases (so-called flow field channels) on its surface, comprises the following method steps a to f. In method step a, a pulp is first produced from water and paper fibers and / or regenerated cellulose, for example viscose or lyocell, and / or polymer fibers, to which a powder and / or fibers made of metal or carbon have been added. The concentration of the powder and / or the fibers made of metal or carbon in the pulp is preferably 60% to 90% (here and in all following details in each case % by weight), particularly preferably 80% to 90%. For the regenerated cellulose as an additive for debinding, the concentration in the pulp is preferably in the range up to 10%.Subsequently, in process step b, a mold with perforations on its surface is introduced into the pulp. The perforations are designed, for example, as a multitude of through-holes drilled or lasered into the mold. However, the mold can also be designed, for example, as a sieve or net-like structure, with the spaces or holes forming the perforations.

[0010] The cross-sectional area of ​​the perforations is preferably so small that the powder and / or the fibers or their flocculation products cannot pass through. Subsequently, in process step c, a vacuum is applied to the pulp so that the water is sucked out through the perforations of the mold and the powder and / or the fibers and the paper fibers are deposited on the surface of the mold in the form of a fiber mat. Subsequently, in process step d, the mold is removed from the pulp. Process steps b to d preferably last between 2 seconds and 5 seconds, particularly preferably about 3 seconds. Subsequently, in process step e, the fiber mat is dried. Subsequently, in process step f, the edges of the fiber mat are cut or punched out so that the green paper is produced from the fiber mat. Process step f can also take place after debinding and sintering of the GDL, orIn addition to step f, a fine cut or a fine punching can take place.

[0011] The drying of the fibrous mat in process step e can take place between two pressing tools structured as a matrix and a patrix, with the aid of which the flow field channels are embossed into the fibrous mat, and / or by the action of temperature, preferably in the range from 100°C to 250°C, particularly preferably in the range from 190°C to 220°C and most preferably at 200°C, and / or by the action of mechanical pressure, preferably in the range of 20 N / cm 2 up to 50 N / cm 2 , particularly preferably in the range of 25 N / cm 2 up to 35 N / cm 2 and most preferably at 30 N / cm 2onto the fiber mat. This process step preferably takes between 1 and 5 minutes, particularly preferably approximately 2 minutes. The structures of the mold and any fine structures on the surface of the two pressing tools are reproduced very precisely; otherwise, they would result in the surface of the fiber mat being smoothed.

[0012] According to a further advantageous embodiment, it is provided that the pulp in process step a is mixed with further additives such as sizing agents (for example synthetic sizing agents such as alkyl ketene dimer AKD and / or resin size dispersions with a concentration in the pulp in the range up to 2%) and / or wet strength agents (for example polyamide-polyamine-epichlorohydrin PAAE with a concentration in the pulp in the range up to 2%) and / or cationic flocculants with high charge density (with a concentration in the pulp in the range up to 0.2%) and / or an organic binder (for example styrene-butadiene latex with a concentration in the pulp in the range up to 2%).

[0013] According to a further advantageous embodiment, it is provided that in process step e or f the fiber web is removed from the mold and preferably placed on a base, for example on a drying belt.

[0014] The final GDL is created after debinding, sintering and coating, e.g. by atomic layer deposition (ALD) and, if necessary, further process steps.

[0015] After sintering, all organic components of the green paper are pyrolyzed and thus no longer contained in the GDL, which then consists almost exclusively of a metal framework. According to current knowledge, the porosity of the metal framework depends in particular on the fiber density of the paper webs, the grain size of the metal powders and / or metal fibers, and the fiber refining.

[0016] The invention also relates to the use of a gas diffusion layer (GDL) produced from a green paper according to one of the processes described above in a proton exchange membrane fuel cell (PEMFC), in a proton exchange membrane electrolyzer cell (PEMEC), in electrolyzer cells or another Power to X technology that requires a correspondingly porous, conductive material for gas / current / reactant distribution. Particular advantages of the process according to the invention are that a thickness of the GDL of over 300 μm can be easily produced to represent the gas distribution channels, that the shape details of the dewatering screen are reproduced more accurately due to the less dynamic processes compared to the rim screen paper machine, that the process volumes such asthe volume of the pulp is significantly lower than in the process of the cylinder mold paper machine, so that the process-related adjustments can be carried out more quickly and the wet chemistry can be better controlled. Furthermore, the method according to the invention has the advantage that even finer, more precise structuring is possible through a thermoforming process downstream of paper formation. The surface quality can also be significantly improved, thus producing an end result with a smoother surface and that precise outer contours are possible through fine punching. A particular advantage of the invention is that the resolution, fineness and flank steepness of the structures that can be produced are not limited and they do not tend to warp. Sharp-edged channels are therefore possible.

[0017] The method according to the invention is a timed, non-continuous process with a limited area per fiber, a fabric mat or green paper. In conventional machine formats, a fiber mat or green paper in the format of approximately 800 mm x 800 mm x 200 µm can be produced, whereby the thickness can vary between 100 µm and 1000 µm. The thickness of the fiber mat is preferably between 200 µm and 600 µm. For a fuel cell intended for use in the automotive sector, approximately four to eight GDLs can be produced in a single operation. One operation takes a total of approximately 30 to 300 seconds. The limiting factor here is the necessary drying time for the formed and pressed parts. This requires the parallel use of several machines to produce a large number of green papers simultaneously.An alternative to the parallel use of several machines could be a drying line with rotating drying forms or a suitably designed rotary machine with drying forms.

[0018] It is understood that the features mentioned above and those to be explained below can be used not only in the combinations specified, but also in other combinations without departing from the scope of the present invention, insofar as this is covered by the scope of protection of the claims.

[0019] The advantages of the invention are explained using the following exemplary embodiment and the supplementary figures. The exemplary embodiment represents a preferred embodiment, to which, however, the invention is in no way limited. Furthermore, the representations in the figures are highly schematic for the sake of better understanding and do not reflect the actual conditions. In particular, the proportions shown in the figures do not correspond to the actual conditions and serve solely to improve clarity. Furthermore, the embodiment described in the following exemplary embodiment has been reduced to the essential core information for the sake of better comprehensibility. In practical implementation, considerably more complex patterns or images can be used.

[0020] In detail, the single Figure 1 schematically shows a method according to the invention for producing a green paper for producing a gas diffusion layer (GDL) for a fuel cell. According to Fig. 1a, a pulp 1 mixed with a powder and / or fibers made of metal or carbon is produced from water and paper fibers. According to Fig. 1b, a mold 2 is introduced into this pulp 1. The mold 2 has structures on its surface that are negatively shaped to the structures of the surface of the GDL and has perforations. Subsequently, a vacuum is applied to the underside of the pulp 1 so that the water is sucked off through the perforations of the mold 2 and the powder and / or the fibers and the paper fibers are deposited in the form of a fiber mat 3 on the upper side of the surface of the mold 2. According to Fig. 1c, the mold 2 is then removed from the pulp 1 and the fiber mat 3 is taken over by a transfer tool 4.The transfer tool 4 places the fiber mat 3 on a base 5, for example, a drying belt, as shown in Fig. 1d. At this point, it may be useful to place a corresponding embossing tool on the underside, opposite the transfer tool, so that the desired shape and thus the design of the gas distribution channels are reproduced even more precisely and smoothly. Subsequently, the transfer tool 4 is removed as shown in Fig. 1e, and the fiber mat 3 is dried. Finally, the edges of the fiber mat 3 are trimmed or punched out to create the green paper.

Claims

P a t e n t a n s p r ü c h e 1. A method for producing a green paper for producing a gas diffusion layer (GDL) for a fuel cell, the GDL having structures for distributing gases on its surface, characterized by the following method steps: a. a pulp (1) made of water and paper fibers and / or regenerated cellulose and / or polymer fibers, mixed with a powder and / or fibers made of metal or carbon, is produced, b. a mold (2) having structures on its surface that are negatively shaped to the structures for distributing gases on the surface of the GDL and having perforations is then introduced into the pulp (1), c. a vacuum is then applied to the pulp (1) so that the water is sucked off through the perforations of the mold (2) and the powder and / or the fibers and the paper fibers are deposited on the surface of the mold (2) in the form of a fiber mat (3), d.then the form (2) is removed from the pulp (1), e. then the fiber mat (3) is dried, f. then the edges of the fiber mat (3) are cut or punched out, so that the green paper is created from the fiber mat (3).

2. Method according to claim 1, characterized in that the drying of the fibrous mat (3) in method step e takes place between two structured pressing tools, in which the shape of the structures for distributing gases is embossed as a matrix or patrix, and / or by the action of temperature which is 100°C to 250°C, preferably 190°C to 220°C and particularly preferably 200°C, and / or by the action of mechanical pressure which is 20 N / cm 2 until 50 N / cm 2 , preferably 25 N / cm 2 up to 35 N / cm 2 and particularly preferably 30 N / cm 2 amounts.

3. Process according to one of the preceding claims, characterized in that the pulp is mixed in process step a with further additives such as sizing agents and / or wet strength agents and / or cationic flocculants with high charge density and / or an organic binder.

4. Method according to one of the preceding claims, characterized in that in method step e or f the fiber web is removed from the mold and preferably laid on a base, for example on a drying belt.

5. Use of a gas diffusion layer (GDL) made from a green paper according to any one of the preceding claims in a proton exchange membrane fuel cell (PEMFC), in a proton exchange membrane electrolyzer cell (PEMEC), in electrolyzer cells or another power to X technology which requires correspondingly porous, conductive material for gas / current / reaction flow distribution.

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