Method for manufacturing green paper for manufacturing gas diffusion layer for 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. According to the invention, a first paper web is formed and a second paper web is formed, which is combined with the first paper web and firmly connected in a still moist state. The first paper web and the second paper web are preferably admixed with metal powder and / or metal fibers and together with further constituents and / or coatings, if necessary, form a green paper. The final GDL is formed after the process steps of debinding, sintering, coating, (thermal) atomic layer deposition (ALD) and, if necessary, further process steps.

Description

[0001] The invention relates to a method for producing green paper for producing a gas diffusion layer (GDL) for a fuel cell.

[0002] In fuel cells of the type of the proton exchange membrane fuel cell (PEMFC), also known as polymer electrolyte fuel cell, the gas distribution onto the membrane coated with catalytic platinum (also known as CL or catalyst layer) is achieved by means of so-called bipolar plates (BPP) and gas diffusion layers (GDL). The entire structure between two bipolar plates is also known as membrane electrode assembly (MEA).

[0003] Under catalytic oxidation of hydrogen and oxygen, the fuel cell generates an electric current, water vapor and heat.

[0004] For the automotive sector, a GDL has become established which is produced from a fiber material composed of, for example, carbon fibers and a coated BPP composed of steel. The fiber material can be implemented here as a woven / knitted fabric or as a fiber mat produced by papermaking technology, for example known from DE 10 2008 042 415 B3. The GDL can also be composed of two layers, namely a fine layer which adjoins the CL, and a coarser layer which adjoins the BPP and the flow field.

[0005] The fiber mat produced by papermaking technology is known as green paper or sinter paper, which is debonded and / or sintered in one of the subsequent work steps and is thus further processed into a GDL.

[0006] It is particularly disadvantageous when producing a GDL on the basis of carbon fibers that the costs of carbon fibers and their further processing are relatively high. Furthermore, carbon fibers are sensitive to pressure, which can lead to fiber breakage and, in turn, can damage the CL / PEM. Furthermore, carbon fibers can buckle or swell and, in doing so, enter the channels of the BPP, thereby reducing the flow of gas and water and impairing the efficiency of the fuel cell. Furthermore, the porosity of the GDL can only be adjusted to a limited extent and requires at least two additional work steps for a double-layer GDL with a combination of coarse and fine porosity.

[0007] The technical problem addressed by the invention is therefore to improve the method for producing green paper for producing a gas diffusion layer (GDL) for a fuel cell of the type described at the outset in order to eliminate the disadvantages of the prior art.

[0008] The technical problem is solved by the features of the independent claim. The embodiments of the invention are the technical solutions of the dependent claims.

[0009] According to the application, a first paper web is formed and a second paper web is formed, which is combined with the first paper web and firmly connected in a still moist state. The first paper web and the second paper web are preferably admixed with metal powder and / or metal fibers and together with further components and / or coatings if necessary form a green paper. After the process steps of debinding, sintering, coating, (thermal) atomic layer deposition (ALD) and further process steps if necessary, the final GDL is formed. After sintering, all organic components of the green paper are pyrolyzed and thus no longer contained in the GDL, which consists almost exclusively of a metal framework. According to current views, the porosity of the metal framework depends, inter alia, on the fiber density of the paper web, the (grain) size of the metal powder and / or metal fibers and the additives added.

[0010] All metal powders and metal fibers of nanometer size, such as titanium, copper, zinc or rust-free fine steel known from DE 10 2008 042 415 B3, can be used as filler material for the sintered paper. It is important here that different mixtures are used for the forming layer and the ring screen layer in order to achieve different porosities of the paper layers. The forming layer is designed here finer than the ring screen. Nanopowders can also be used in the forming layer.

[0011] The first paper web and / or the second paper web can be produced here in a ring screen paper machine. Alternatively, the first paper web and / or the second paper web can also be produced in a short former, in which the paper stock is sprayed onto a ring screen. These manufacturing methods are known from WO 2006 / 099971 A2 for the manufacture of security documents or value documents, such as banknotes or identity cards, and are also preferred according to the application for the manufacture of a GDL from at least one paper web.

[0012] A highly filled green paper filled with metal powder and / or metal fibers is thus formed in the working process, which is processed according to DE 10 2008 042 415 B3 into a green paper with a combination of different properties by at least two different approaches. This is, for example, a thinner layer with fine pores and a thicker layer with coarse pores in the case of a fuel cell. The porosities between the two paper webs can also be different.

[0013] According to one preferred embodiment, it is provided that the first paper web has a higher density than the second paper web. The density of the first paper web is, for example, 3 g / cm 3 to 10 g / cm 3 and the density of the second paper web is 1 g / cm 3 to 5 g / cm 3The first paper web is particularly preferably formed here from a pulp of finer paper fibers than the second paper web, which accordingly results in a finer porosity in this partial region of the sintered paper.

[0014] The thickness of the first paper web is preferably 5 μm to 50 μm, particularly preferably 10 μm to 20 μm, and the thickness of the second paper web is 50 μm to 400 μm, particularly preferably 80 μm to 200 μm.

[0015] According to a further preferred embodiment, further paper webs can be applied to the first paper web and the second paper web. This is either achieved in the wet zone of the paper machine as with the first and second paper web or post-hoc by lamination. Here, all paper webs can have a different porosity or a different channel-like structure, for example with different lengths or different diameters. Particularly preferably, paper webs with different porosities can be combined to form a paper stack with a porosity gradient. This enables particularly advantageous realization of a more uniform gas distribution within the fuel cell.

[0016] In one or more paper webs, additional channels for water transport in the form of watermarks can also be introduced. The additional channels serve for the balanced transport of water and have the particular advantage that the PEM cell is neither flooded nor dried, since both would have a negative effect on the efficiency of the cell. Furthermore, the water channels can also serve for the continuous cooling of the cell.

[0017] Furthermore, it is particularly advantageous to introduce watermarks in the first paper web and the second paper web, respectively, wherein the structure of the watermarks of the first paper web and the structure of the watermarks of the second paper web are not identical, but are mirror-symmetrical exactly in the plane and in the direction of the material thickness. In other words, the watermark structure of the first paper web is phase-shifted by 180° relative to the watermark structure of the second paper web. This means that, when the first paper web and the second paper web are joined by their sides structured by the watermarks, the protrusions of the first paper web coincide with the recesses of the second paper web. This embodiment has the particular advantage that, after sintering, the first paper web and the second paper web can have different porosities. For example, the first paper web towards the membrane has a lower porosity of 20% to 75% after sintering and the second paper web has a higher porosity of 30% to 90% after sintering, so that the second paper web hardly serves as a resistance to the gas, but only as a spacer relative to the bipolar plate. In this way, the best gas distribution can be combined with the best stackability and the best uniform distribution of the mechanical pressure over the PEM membrane. It is particularly advantageous if there is a microporous layer (MPL) between the first paper web and the membrane, which has a fine surface with a smaller roughness and smaller pores than the first paper web and the second paper web.

[0018] The watermark in the context of the present application is a true watermark, wherein the thickness of the paper is changed, whereas the density of the paper remains unchanged. The paper here has regions of greater and / or lesser thickness relative to adjacent regions, wherein the density of the paper is the same in all regions. Such a watermark can be introduced into the paper web during the paper production process, for example by introducing recesses or protrusions into the ring screen, with more or less paper fibers being gathered at the recesses or protrusions when the paper is produced from the paper pulp. However, the watermark can also be introduced into the paper web ex post, for example mechanically by milling or by means of a laser.

[0019] Alternatively, a false watermark is also possible, wherein the still moist paper web is embossed after it has been removed from the ring screen, for example, by means of an embossing process. Such a watermark is also referred to as an Egoutteur-Wasserzeichen. By means of the embossing, the thickness of the paper is reduced, whereas the density of the paper is increased at the same time. The paper fibers are thus compacted or compressed. The advantage of this compaction is that excessive gas is prevented from already diffusing in the front region of the channel in the direction of the catalyst layer (CL) by means of the GDL and it thus ensures a more uniform gas distribution.

[0020] It is particularly preferred that the true watermark and the false watermark can be combined with one another, for example such that the watermark is composed of a portion by means of the true watermark and of another portion by means of the false watermark.

[0021] The fuel cell is particularly preferably a proton exchange membrane fuel cell (PEMFC) or a proton exchange membrane electrolyser (PEMEC) fuel cell. According to one preferred embodiment, the first paper web here forms a diffusion layer in a gas diffusion layer produced from green paper, which diffusion layer is used for a membrane (CL) coated with a catalytic metal, preferably platinum, and the second paper web forms a distribution layer with a flow field in a gas diffusion layer produced from green paper. However, the GDL produced from the green paper according to the application can also be used in other types of fuel cell or other power-to-X technologies, for example electrolyser, which require a porous, electrically conductive layer in order to achieve a gas distribution / current distribution / reactant distribution.

[0022] The paper web is preferably composed of paper formed from cellulose fibers or cotton fibers, for example for banknotes, or of other natural fibers or artificial fibers or of a mixture of natural and artificial fibers. Furthermore, the paper web is preferably composed of a combination, i.e. a mixture, of at least two different substrates, which are arranged on top of one another and are connected to one another. Data on the weight of the paper web used are given, for example, in the patent document DE 102 43 653 A9, the design concept of which is included in its entirety in the present application in relation thereto. The metal-filled green paper can have a grammage of 100 g / m 2 to 1200 g / m 2 .

[0023] In order to protect the metal from corrosion even to the smallest pores and to produce the generally desired hydrophobic properties on the side facing the catalyst, according to a further preferred embodiment a (thermal) ALD coating or other coating method is used in one of the subsequent process steps. If the cutouts are outside the area at risk of corrosion or if the cutouts are additionally shielded in other process steps for the production of the battery cell, the (thermal) ALD coating or other coating method is preferably used after the GDL debinding and sintering and before the punching and cutting. There is also the possibility of coating the GDL by ALD or the like after the punching and cutting.

[0024] It goes without saying that the features mentioned above and still to be elucidated below can be used not only in the combinations indicated, but also in other combinations, without departing from the scope of the application, provided that this is not excluded by the claims.

[0025] The advantages of the application are elucidated in accordance with the following examples and the supplementary drawings. The examples embody preferred embodiments, however the application should not be limited to these examples. Furthermore, the views in the drawings are highly diagrammatic and do not reflect the actual situation for the sake of better understanding. In particular, the proportions shown in the drawings do not correspond to the actual proportions and are used only to improve the clarity. Furthermore, the embodiments described in the following examples are reduced to the essential core information for the sake of better understanding. In actual implementation, significantly more complex patterns or figures can be used.

[0026] In the drawings, shown in detail and diagrammatically are:

[0027] Figure 1 A double-cylinder screen paper machine for producing green paper according to the application is shown in a diagram,

[0028] Figure 2 A paper machine with a cylinder screen paper machine and a short former is shown in a diagram.

[0029] Figure 1 A double-cylinder screen paper machine 10 is shown in a diagram, which is known for example from WO 2006 / 099971 A2 for producing security paper. The paper machine 10 comprises two cylinder screen paper machines 12 and 14 which are connected to one another by means of a transfer felt 16.

[0030] In the first paper machine 12, a paper web 20 is formed on a cylinder screen 18. In the second paper machine 14, a uniform second paper web 30 is produced parallel to the paper web, which is removed from a cylinder screen 34 by means of the transfer felt 16 and is guided to the first paper machine 12, where it is connected to the first paper web 20 in the region of a press roll 36. The paper webs 38 connected to one another together form a GDL and are fed to further processing stations.

[0031] As Figure 2 shown, a second web 30 can also be produced by a short former 40, in which the pulp is sprayed through a headbox nozzle 42 onto the surface of a ring screen 44. With such a short former it is possible to produce particularly thin paper layers, for example with a grammage of 15 to 25 g / m 2 .

[0032] It goes without saying that with the shown paper machines 12, 14, 40, three or more webs can also be produced analogously and joined together.

Claims

1. A method of manufacturing green paper for manufacturing a gas diffusion layer for a fuel cell, characterized by, A first paper web (20) is formed and a second paper web (30) is formed, the formed second paper web is combined and firmly connected with the formed first paper web (20) in a still moist state, wherein the first paper web (20) and the second paper web (30) together form the green paper, wherein the first and second paper web are each paper formed from cellulose fibers and do not contain carbon fibers, the first paper web (20) has a higher density than the second paper web (30), wherein the first paper web (20) forms a diffusion layer for a membrane coated with catalytic metal in a gas diffusion layer manufactured from green paper, and the second paper web (30) forms a distribution layer with a flow field in a gas diffusion layer manufactured from the green paper, wherein the first paper web (20) and the second paper web (30) are admixed with metal fibers, and after sintering all organic components of the green paper are pyrolized and thus no longer contained in the gas diffusion layer, the gas diffusion layer consists only of a metal framework.

2. The method of claim 1, wherein, The first paper web (20) and / or the second paper web (30) is manufactured in a ring screen paper machine (12, 14).

3. The method of claim 1, wherein, The first paper web (20) and / or the second paper web (30) is produced in a short former (40), wherein the paper stock is sprayed onto a ring screen (44).

4. The method of claim 1, wherein, The first web has a density of 3 g / cm 3 to 10 g / cm3, and the second web has a density of 1 g / cm3to 5 g / cm3.

5. The method of claim 4, wherein, The first paper web (20) is formed from a finer pulp than the second paper web (30).

6. The method of claim 1, wherein, The fuel cell is a proton exchange membrane fuel cell or other type of fuel cell that requires a porous, electrically conductive material to achieve gas distribution / current distribution / reactant material distribution.

7. The method of claim 1, wherein, The catalytic metal is platinum.

8. The method of claim 1, wherein, A watermark is introduced in the first paper web (20) and in the second paper web (30), respectively, wherein the structure of the watermark of the first paper web (20) is not identical to the structure of the watermark of the second paper web (30), but is mirror-symmetrical in the plane and in the direction of the material thickness.

9. The method of claim 1, wherein, The cellulose fibers are cotton fibers.

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