METHOD FOR MANUFACTURING GREEN PAPER FOR MANUFACTURING GAS DIFFUSION LAYERS FOR FUEL CELLS - Patent application

Abstract

The present invention relates to a method for producing green paper for producing gas diffusion layers (GDLs) for fuel cells. According to the present invention, 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 spliced ​​thereto. The first and second paper webs are preferably mixed with metal powder and / or metal fibers and together form green paper, optionally with additional components and / or coatings. The final GDL is provided after a debinding process, a sintering process, a coating process, a (thermal) deposition process (atomic layer deposition, ALD), and optionally additional process steps.

Description

[Technical Field]

[0001] The present invention relates to a method for producing green paper for producing gas diffusion layers (GDLs) for fuel cells. [Background technology]

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

[0003] Through the catalytic oxidation of hydrogen and oxygen, fuel cells produce electricity, water vapor and heat.

[0004] In the automotive sector, currently established GDLs are those made from fibrous materials, such as carbon fibres and coated steel BPPs. The fibrous material can take the form of a woven / knitted fabric or a fibre mat, which can be produced by papermaking techniques known, for example, from US Pat. No. 5,629,493. It can also consist of two plies: a fine ply adjacent to the CL and a coarser ply adjacent to the BPP and flow field.

[0005] The fiber mat produced by papermaking techniques is called green paper or sintered paper, which in one of the subsequent work steps is debindered and / or sintered, thereby further processed to give GDL.

[0006] One particular drawback of producing a carbon fiber-based GDL is the relatively high cost of the carbon fibers and their further processing. Furthermore, carbon fibers are susceptible to pressure, which can cause the fibers to break. This can potentially damage the CL / PEM. Furthermore, the carbon fibers can swell or expand and penetrate the channels of the BPP, thereby reducing the flow of gas and water and reducing the efficiency of the fuel cell. Furthermore, the porosity of the GDL can only be adjusted to a limited extent. For a two-layer GDL with a combination of coarse and fine porosity, at least two additional work steps are required. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] German Patent No. 10 2008 042 415 B3 Summary of the Invention [Problem to be solved by the invention]

[0008] It is therefore an object of the present invention to develop a versatile method for producing green paper for producing gas diffusion layers (GDLs) for fuel cells to address the shortcomings of the prior art. [Means for solving the problem]

[0009] This object is achieved by the features of the independent claims. Developments of the invention are the subject of the dependent claims.

[0010] According to the present invention, a first paper web is formed, and a second paper web is formed, and the second paper web is combined with the first paper web while still wet and firmly bonded thereto. The first and second paper webs preferably contain added metal powder and / or metal fibers, and together with any additional components and / or coatings, form a green paper. The final GDL is obtained as a result of binder removal, sintering, coating, atomic layer (thermal) deposition (ALD - atomic layer deposition), and any further process steps. After sintering, all organic components of the green paper are no longer present in the GDL due to pyrolysis. This GDL consists essentially of only a metal framework. At present, it appears that the porosity of the metal framework depends, inter alia, on the fiber density of the paper web, the metal powder and / or metal fibers, and the particle size of the added additives.

[0011] The filler material used in the sintered paper can be any microscale metal powder and metal fiber, such as titanium, copper, zinc, or anti-corrosion stainless steel, as known from German Patent No. 10 2008 042 415 B3. It is important to note that different mixtures are used for the former ply and the cylindrical screen ply to achieve different porosities of the paper ply. The former ply should be made finer than the cylindrical screen ply. Nano-sized powders can also be used in the former ply.

[0012] Here, the first and / or second paper webs can be produced in a cylinder paper machine. Alternatively, the first and / or second paper webs can also be produced in a short former, in which the papermaking raw material is jetted onto a cylindrical screen. These production methods are known, for example, from WO 2006 / 099971 A2 for the production of confidential documents or securities such as banknotes or ID cards, and are also preferred methods according to the invention for producing GDL from at least one paper web.

[0013] For example, green paper with a large amount of metal powder and / or metal fiber fillers is formed in one operation. This is then processed using at least two different formulations according to DE 10 2008 042 415 B3 to produce composite sintered paper with different properties. In the case of fuel cells, these are, for example, thin plies with fine pores and thicker plies with coarser pores. The porosity can also differ between the two paper webs.

[0014] In a preferred embodiment, the first paper web has a higher density than the second paper web. The first paper web may have a density of, for example, 3 g / cm 3 ~10g / cm 3 The second paper web has a density of 1 g / cm 3 ~5g / cm 3 More preferably, the first paper web is formed from a finer paper fiber slurry than the second paper web, resulting in correspondingly finer pores in this small area of ​​the sintered paper.

[0015] The thickness of the first paper web is preferably 5 μm to 50 μm, more preferably 10 μm to 20 μm, and the thickness of the second paper web is preferably 50 μm to 400 μm, more preferably 80 μm to 200 μm.

[0016] In a further preferred embodiment, additional paper webs can be attached to the first and second paper webs, either in the same manner as the first and second paper webs in the wetted area of ​​the paper machine or by subsequent lamination. All paper webs can have different porosities or different channel structures, for example, different lengths or different diameters. More preferably, paper webs of different porosities can be combined to form a paper stack with a porosity gradient. This is particularly advantageous in that it allows for more uniform gas distribution within the fuel cell.

[0017] One or more of the paper webs can also incorporate additional channels in the form of watermarks for water transport. These ensure balanced water transport and have the particular advantage that the PEM cell cannot become flooded or dry out, both of which have a negative effect on the cell's efficiency. Furthermore, the water channels can also be used to maintain cooling of the cell.

[0018] Furthermore, it is particularly advantageous if the watermarks are created in the first and second paper webs, and the structures of the watermarks in the first and second paper webs are not identical, but have exact mirror symmetry in the plane and through the thickness of the material. In other words, the watermark structure of the first paper web is 180° out of phase with the watermark structure of the second paper web. This means that when the first and second paper webs are joined at their watermark-structured surfaces, the convex portions of the first paper web overlap the concave portions of the second paper web. This embodiment has the particular advantage that the first and second paper webs can have different porosities after sintering. For example, the first paper web facing the membrane can have a lower porosity of 20% to 75% after sintering, while the second paper web can have a higher porosity of 30% to 90% after sintering. Therefore, the second paper web hardly functions as a gas barrier, but merely as a spacer for the bipolar plates. In this way, optimal gas distribution can be combined with optimal lamination and optimally uniform distribution of mechanical pressure across the PEM membrane. Particularly advantageously, between the first paper web and the membrane there is a microporous layer (MPL) with a fine surface having less roughness and smaller pores than the first and second paper webs.

[0019] In the context of this invention, one type of watermark is a true watermark, where the thickness of the paper changes but the density of the paper does not. Here, the paper has areas that are thicker and / or thinner than adjacent areas. However, the density of the paper is the same in all areas. Such a watermark can be introduced into a paper web during any papermaking process. For example, in the formation of paper from pulp, depressions or protrusions where there is a greater or lesser accumulation of paper fibers are included in a cylindrical screen. However, it can also be introduced into the paper web later. Portions of the paper are mechanically removed, for example, by mechanical or laser processing.

[0020] Alternatively, artificial watermarks are also possible, for example, by embossing the wet paper web after it has been removed from the cylindrical screen. These watermarks are also called dandy roller watermarks. Embossing reduces the thickness of the paper but simultaneously increases its density. Thus, densification or compaction of the paper fibers occurs. This densification has the advantage of preventing excessive gas from diffusing directly through the GDL into the front region of the channel toward the catalyst layer (CL), thus ensuring a more uniform gas distribution.

[0021] More preferably, real watermarks and pseudo-watermarks can be combined with each other, for example, one part of the watermark is formed by a real watermark and another part is formed by a pseudo-watermark.

[0022] The fuel cell is more preferably a proton exchange membrane fuel cell (PEMFC) or a proton exchange membrane electrolyzer cell (PEMEC). In a preferred embodiment, the first paper web in this case forms a diffusion layer for a membrane (CL) coated with a catalytic metal, preferably platinum, in a gas diffusion layer made from the green paper. The second paper web forms a distribution layer with flow fields in a gas diffusion layer made from the green paper. However, GDLs made from the green paper of the present invention can also be used in other types of fuel cells or other power conversion technologies that require a porous conductive layer for gas / power / reactant distribution, such as electrolyzer cells.

[0023] The paper web consists in particular of paper made of cellulose or cotton fibers, such as those used for banknotes, or of natural or synthetic fibers or a mixture of natural and synthetic fibers. Preferably, the paper web also consists of a combination of at least two different substrates arranged and bonded to one another, known as a hybrid. Details of the weight of the paper web used are reported, for example, in DE 102 43 653 A9, the details of which are incorporated herein by reference in their entirety. The metal-filled green paper has a weight of 100 g / m 2 ~1200g / m 2 The weight in grams may be

[0024] To prevent corrosion of the metal down to the smallest pores and to provide the usually desired hydrophobicity preferentially on the side facing the catalyst, according to a further preferred embodiment, (thermal) ALD coating or another coating method is used in one of the subsequent process steps, preferably after binder removal and sintering and before stamping and finishing of the GDL, if discontinuities exist outside the area at risk of corrosion or if the discontinuities are to be sealed especially during further process steps to obtain the finished cell. Alternatively, the GDL can also be coated, such as by ALD, after stamping and finishing.

[0025] It will be recognized that, when falling within the scope of protection of the claims, the features mentioned above and those described below can be used not only in the combinations specified but also in other combinations without departing from the scope of the invention.

[0026] The advantages of the present invention will be described with reference to the following embodiments and auxiliary figures. These examples are preferred embodiments, but are not intended to limit the present invention thereto. Furthermore, the figures are highly schematic for better understanding and do not reflect the actual situation. In particular, the proportions shown in the figures do not correspond to the actual proportions, but only serve to make things clearer. Furthermore, the embodiments described in the following examples are reduced to essential information for easier understanding. In actual implementation, substantially more complex patterns or images can be used.

[0027] The figures show, in particular, in schematic form: [Brief explanation of the drawings]

[0028] [Figure 1] 1 is a schematic diagram of a double cylinder paper machine for producing the green paper of the present invention. [Figure 2] In a schematic view, a paper machine with a cylinder paper machine and a short former. DETAILED DESCRIPTION OF THE INVENTION

[0029] 1 shows a schematic diagram of a double cylinder paper machine 10 known from WO 2006 / 099971 A2 for the production of security paper, for example. The paper machine 10 comprises two cylinder paper machines 12 and 14, which are connected to each other by a transfer felt 16.

[0030] In the first paper machine 12, a paper web 20 is formed on a cylindrical wire 18. In parallel to this, in the second paper machine 14, a second uniform paper web 30 is produced, removed from a cylindrical wire 34 by a transfer felt 16, and passed through the first paper machine 12, where it is combined with the first paper web 20 in the area of ​​a contact roll 36. The combined paper webs 38 together form a GDL and are passed through further processing stations.

[0031] As shown in Figure 2, the second paper web 30 can also be produced using a short former 40 in which the papermaking material is sprayed onto the surface of a cylindrical screen 44 by a headbox nozzle 42. By using such a short former, particularly thin paper plies can be produced, for example having a gram weight of 15 to 25 g / m2.

[0032] It will be appreciated that the paper machines 12, 14, 40 shown may be used to produce and combine more than two paper webs as well.

Claims

1. A method for manufacturing a green paper for manufacturing a gas diffusion layer (GDL) for a fuel cell, comprising: forming a first paper web (20); forming a second paper web (30); and combining the second paper web (30) with the first paper web (20) while still wet and firmly bonding it thereto; the first paper web (20) and the second paper web (30) together forming the green paper; and The method, wherein the first paper web (20) has a higher density than the second paper web (30).

2. 2. The method of claim 1, wherein the first paper web (20) and / or the second paper web (30) are produced in a cylinder paper machine (12, 14).

3. 3. The method according to claim 1 or 2, characterized in that the first paper web (20) and / or the second paper web (30) are produced in a short former (40) in which papermaking raw material is jetted onto a cylindrical screen (44).

4. The first paper web (20) has a density of 3 g / cm ３ ~10g / cm ３ and said second paper web (30) has a density of 1 g / cm ３ ~5g / cm ３ 4. The method according to claim 1, wherein the granules have a density of 0.01 to 0.

01.

5. The method according to any one of claims 1 to 3, characterized in that the first paper web (20) is formed from a finer paper stock slurry than the second paper web (30).

6. The method according to any one of claims 1 to 5, characterized in that the first paper web (20) and / or the second paper web (30) contain added metal powder and / or metal fibers.

7. 7. The method according to any one of claims 1 to 6, characterized in that the fuel cell is a proton exchange membrane fuel cell (PEMFC), which requires a correspondingly porous conductive material for gas / vapor / reactant distribution.

8. 8. The method according to claim 1, wherein the first paper web (20) forms a diffusion layer for a membrane (CL) coated with a catalytic metal in the gas diffusion layer (GDL) made from the green paper, and the second paper web (30) forms a distribution layer with flow fields in the gas diffusion layer (GDL) made from the green paper.

9. 9. The method of claim 8, wherein the catalytic metal is platinum.

10. 10. The method according to any one of claims 1 to 9, characterized in that watermarks are made in the first paper web (20) and in the second paper web (30), the structures of the watermarks in the first paper web (20) and the watermarks in the second paper web (30) are not identical but have exact mirror symmetry in the plane and in the thickness direction of the material.

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