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. The present invention further relates to the use of the gas diffusion layers (GDLs) produced thereby in fuel cells. According to the present invention, a first paper web, preferably loaded 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. The paper web is then subjected to a binder removal process, a sintering process, a coating process, atomic layer deposition (ALD) using a thermal ALD method, and optionally further process steps to obtain the final GDL. After the sintering process, all organic components of the green paper are pyrolyzed and therefore no longer contained in the GDL, and the GDL consists essentially of only a metal framework.

Description

[Technical Field]

[0001] The present invention relates to a method for producing green paper for producing gas diffusion layers (GDLs) for fuel cells. The present invention further relates to the use of the correspondingly produced gas diffusion layers (GDLs) in 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.

[0007] Finally, the GDLs known from the prior art do not have any means of structuring, so the flow fields must be formed entirely by the BPP. Therefore, to achieve the structuring for the gas distribution structure or the flow fields, the BPP must be embossed or the green paper must be embossed, which is generally a separate and complicated operation. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] German Patent No. 10 2008 042 415 B3 Summary of the Invention [Problem to be solved by the invention]

[0009] 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]

[0010] This object is achieved by the features of the independent claims. Developments of the invention are the subject of the dependent claims. DETAILED DESCRIPTION OF THE INVENTION

[0011] According to the present invention, a first paper web is produced, preferably loaded with metal powder and / or metal fibers, and a microporous layer (MPL) in the form of at least one coating is applied to the paper web, which is then processed by binder removal, sintering, coating, atomic layer deposition (ALD) by thermal ALD methods, and optionally further process steps to give the final MPL.

[0012] After sintering, all organic components of the green paper are no longer present in the GDL due to pyrolysis, which essentially consists of only a metal framework. It is currently believed that the porosity of the metal framework depends, inter alia, on the fiber density of the paper web, the size of the metal powder and / or metal fibers, and the added additives.

[0013] In a preferred embodiment, the coating is applied to the paper web by paper coating methods known from the prior art, screen printing methods or plasma coating.

[0014] If the coating is applied to the paper web before the green paper is sintered, the coating preferably comprises powders / fibers with particle / fiber diameters preferably between 0.4 μm and 5 μm, present in an organic binder matrix in a paper coating or screen printing process. After sintering, the resulting pore sizes in the GDL or MPL are on the order of 0.1 μm to 3 μm.

[0015] If a coating is applied to the paper web after the green paper has been sintered, this coating preferably has a composition known from the prior art, for example from DE 10 2008 042 415 B3.

[0016] The applicator used for paper coating can be either an LTDA (long dwell applicator) roll applicator or an STDA (short dwell applicator) nozzle applicator. After application, leveling is achieved by a suitable squeegee method (roll squeegee / vent / stiff blade, air blade). To texture the coating on the surface, the coating can be applied by a curtain coater or a film press (size press, gravure coater). For blade coating, a maximum of 30 g / m2 is possible. 2 On the other hand, film presses can achieve coatings of up to 7 g / m 2 It is possible to achieve a coating amount of

[0017] If the coating is applied to the paper web by screen printing before sintering, the screen printing is preferably carried out using a cylindrical screen on a roll-to-roll machine or lamination system. If the coating is applied to the paper web by screen printing after sintering, the screen printing is preferably carried out in a sheet-fed machine. Here, it is particularly preferred to use a cylinder machine, such as a NotaScreen or a flat screen machine. Here, it is particularly preferred to carry out screen printing in the printing press together with further processes, such as stamping of GDL and any further embossing.

[0018] When coating is performed by plasma coating, it is preferable to feed metal powder to a plasma head and spray it onto the paper web. The plasma energy causes the metal particles to "pack" together. The particle size of the metal powder should be selected to obtain the desired pore size. Very fine-grained powders with a particle size of 0.4 μm to 5 μm can produce pores with sizes of 0.1 μm to 3 μm. Plasma coating can be performed before or after sintering, preferably after sintering in a sheet-fed process.

[0019] In the case of plasma coating, particularly preferred is the use of atmospheric pressure high temperature plasma, which melts the metal particles, thereby bonding them to the paper fibers or substrate. In the case of MPL, it should be ensured that this melting process only involves the surface of the metal particles, so that the shape of the metal particles is essentially maintained, thereby forming the desired pore structure.

[0020] In a preferred embodiment, a watermark is created in at least one paper web. Surprisingly, it has been found that the green paper produced by papermaking can be structured by creating a watermark in the paper web of the green paper, such that complex embossing of the BPP or post-processing of the green paper or GDL produced from the green paper is not necessary or is at least simplified.

[0021] Here, the latter flow field is incorporated into the green paper without any additional work, in that the corresponding watermark is incorporated into the watermark ply in the cylindrical screen of the paper machine. Here, the desired shape and gradation of the flow field channel can be achieved by adjusting the thickness of the paper without any particular complexity, due to the structuring depending on the design of the watermark screen. To increase the resolution of the structuring, high-resolution or multi-level watermarks, as known from, for example, EP 1 432 868 A1 or WO 2014 / 040706 A1, can also be used.

[0022] In the context of the present invention, one of the watermarks is Paper Web The thickness of the Paper Web In this case, the density of the watermark does not change. Paper Web has areas that are thicker and / or thinner than adjacent areas, Paper Web The density of the watermark is the same in all areas. Such watermarks can be introduced into the paper web during any papermaking process. In this case, for example, Paper WebThe cylindrical screen contains depressions or protrusions that result in greater or less accumulation of paper fibers during the formation of the paper web. However, they can also be introduced into the paper web later. Paper Web A portion of the surface is mechanically removed, for example by machining or laser machining.

[0023] Alternatively, artificial watermarks are possible, in which the paper web is embossed in an embossing operation while still wet after it has been removed from, for example, a cylindrical screen. Such watermarks are also called dandy roller watermarks. Paper Web The thickness of the Paper Web The density of the paper increases. Thus, a densification or compaction of the paper fibers occurs. This densification has the advantage that excessive amounts of gas are prevented from diffusing directly through the GDL into the front region of the channel towards the catalyst layer (CL), thus ensuring a more uniform gas distribution.

[0024] 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.

[0025] In a further preferred embodiment, the green paper consists of a first paper web and at least one second paper web. In this case, the green paper is formed from the first paper web and at least one second paper web. The second paper web is combined with the first paper web while still wet and firmly bonded thereto. In this case, the second and / or any further paper webs can also have a watermark.

[0026] Here, the first and / or at least one second paper web can be produced in a cylinder paper machine. Alternatively, the first and / or at least one second paper web 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 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.

[0027] For example, a green paper with a large amount of metal powder and / or metal fiber filler is formed in one operation. This is processed using at least two different formulations according to DE 10 2008 042 415 B3 to produce composite green 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.

[0028] Furthermore, it is particularly advantageous if the green paper is composed of two paper webs, each with a watermark. The watermark structures of the first and second paper webs are not identical, but have exact mirror symmetry in the plane and thickness direction 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.

[0029] 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, in this case, 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.

[0030] 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.

[0031] In a further preferred embodiment, the watermark is configured as a recess in the form of at least one channel, which serves for the passage of gases, i.e., fuel or oxygen. This channel preferably meanders over the area of ​​the paper web. Another possibility includes a plurality of channels in a lattice or radial form, with arcuate connecting channels.

[0032] One or more of the paper plies can contain additional channels for water transport by one of the methods mentioned above. These channels ensure balanced water transport. PEM cells have the particular advantage that the water cannot flood or dry out, as both of these have a negative effect on the efficiency of the cell. Furthermore, the water channels can also be used to maintain cooling of the cell.

[0033] In a further preferred embodiment, in addition to the watermark, laser structuring is incorporated into the surface of the green paper or sintered green paper. This has the advantage that, for example, deeper structures or structures with steeper sides can be introduced using the laser beam, or existing structures can be deepened or made steeper sided. Furthermore, laser processing can be introduced into one or more former plies to introduce structuring or channels in the intermediate layer between the watermark and the former ply, thereby further improving gas distribution.

[0034] In a further preferred embodiment, the gas in the center of the bipolar plate (based on a top view of the bipolar plate) is connected into the GDL and then distributed towards the outside or towards the outer edge of the GDL by various openwork structures and / or channels in the GDL. These openwork structures and / or channels can, for example, lead outwards in a radial or spiral manner starting from the center of the GDL. They can be supplemented by concentrically arranged annular openwork structures and / or channels.

[0035] GDLs are typically 300cm long depending on their system and function. 2 ~350cm 2 The BPP typically has an area of ​​100 μm to 300 μm and a thickness of 100 μm to 300 μm. If flow field functions are integrated into the GDL, the GDL may be thicker. The channel depth is up to 350 μm. The GDL must have some compressibility while also carrying current between individual cells. Therefore, the GDL, including the original ply and cylindrical screen ply, has a thickness of 100 μm to 400 μm. The BPP should be in the form of a flat sheet with a thickness of 75 μm or less. The BPP also typically performs the cooling function of the fuel cell. Therefore, the BPP can also be in the form of a composite sandwich structure with porous or channel-like flow paths for the coolant. Alternatively, cooling channels can be integrated into the GDL or MEA.

[0036] In a further preferred embodiment, the BPP has a simple flow field structure, where a partial flow field is also formed in the GDL, where the original ply is thinned to avoid taking up too much space.

[0037] For a 120 kW fuel cell for automotive applications, approximately 400 cells are stacked on top of each other. Therefore, the cell spacing is preferably 0.8 mm to 1 mm. The fine original ply preferably has a thickness of 5 μm to 50 μm. The original ply preferably accounts for 2% to 40% of the total GDL.

[0038] In a further preferred embodiment, high resolution or multi-level watermarks are used to form pass marks, alignment aids, centering aids and passageway access points, which advantageously allow for precise positioning of the GDL relative to other components such as the BPP or CL, for example using a transmitted / incident light imaging system, thus simplifying further processing of the GDL to obtain a fuel cell stack.

[0039] In a further preferred embodiment, the structures of the anode and cathode GDLs are not identical but are precisely mirror-symmetric in the plane and through the thickness of the material. In other words, the structure of the anode GDL is 180° phase-shifted relative to the structure of the cathode GDL. This means that when the anode GDL is positioned near the flow field side of the cathode GDL, the convex portions of one GDL overlap exactly with the concave portions of the other GDL. Therefore, the combination of two 3D mirror-symmetric anode / cathode GDLs results in a precisely planar green paper part when stacked on top of each other. This embodiment has the advantage that the green paper can be densified using any mechanical pressure without damaging its channel structure. This is because the convex and concave portions of the green paper, formed by the watermarks and forming the flow field channels, are not damaged, collapsed, or flattened by subsequent pressure and other mechanical stresses, allowing the channels to remain effective. This embodiment also has the additional advantage that the anode GDL and cathode GDL can have different porosities. As an alternative to the alternating anode and cathode GDLs, every second anode / cathode pair in the stack or every second stack can be provided with a 3D mirror symmetric GDL.

[0040] The fuel cell is more preferably a proton exchange membrane fuel cell (PEMFC). 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 the 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 that require a porous conductive layer to distribute gases, such as proton exchange membrane electrolyzer cells (PEMEC), electrolyzer cells, or other power conversion technologies.

[0041] 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. The paper web also preferably 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 document 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 / m². 2 ~1200g / m 2 The weight in grams of

[0042] 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. In this case, it is important to use different mixtures for the original ply and the cylindrical screen ply to achieve different porosity in the paper ply. In this case, the original ply should be made finer than the cylindrical screen ply. Nano-sized powders can also be used in the original ply.

[0043] 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.

[0044] It will be recognized that, when falling within the scope of protection of the claims, the above features can be used not only in the combinations specified but also in other combinations without departing from the scope of the invention.

Claims

1. 1. A method for producing a green paper for producing a gas diffusion layer (GDL) for a fuel cell, comprising: producing a first paper web; and applying a microporous layer (MPL) in the form of at least one coating to the paper web; the coating is applied to the paper web by a paper coating method, a screen printing method or a plasma coating method; and the coating is applied to the paper web before sintering of the green paper and comprises a powder having a particle size of 0.4 μm to 5 μm, the powder being present in an organic binder matrix in a paper coating process or a screen printing process; A method, characterized in that the watermark is created in at least one paper web.

2. 2. The method of claim 1, wherein the first paper web is manufactured with the addition of metal powder and / or metal fibers.

3. 3. The method according to claim 1 or 2, characterized in that the coating takes the form of multiple plies, the multiple plies having different grain sizes and / or different layer thicknesses and / or being applied in a partial and / or structured manner to the side of the green paper facing the catalyst layer (CL) or bipolar plate (BPP) in the fuel cell.

4. 4. The method according to claim 1, wherein prior to the application of the coating, at least one second paper web is formed, said at least one second paper web being combined with the first paper web while still wet and firmly bonded thereto.

5. 5. The method according to claim 4, characterized in that the first paper web and / or each second paper web is produced in a cylinder paper machine and / or in a short former in which the papermaking raw material is jetted onto a cylindrical screen.

6. 6. The method of claim 4 or 5, wherein the first paper web has a higher density than the second paper web.

7. The first paper web has a density of 3 g / cm ３ ~10g / cm ３ and said second paper web has a density of 1 g / cm ３ ~5g / cm ３ 7. The method of claim 6, wherein the granules have a density of

8. 8. The method of claim 6 or 7, wherein the first paper web is formed from a finer paper fiber slurry than the second paper web.

9. 9. The method according to claim 4, wherein the first paper web in the gas diffusion layer (GDL) made from the green paper forms a diffusion layer for a membrane (CL) coated with a catalytic metal, and the second paper web forms a distribution layer with flow fields in the gas diffusion layer (GDL) made from the green paper.

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

11. A method according to any one of claims 1 to 10, characterized in that the watermark is a true watermark, in which the thickness of the paper web changes but the density of the paper web does not change, and / or the watermark is a pseudo-watermark, in which the thickness of the paper web decreases but at the same time the density of the paper web increases.

12. 12. The method according to any one of claims 11 to 11, wherein the first and second paper webs each have a watermark, and the watermark structures of the first and second paper webs are not identical but have exact mirror symmetry in the plane and in the thickness direction of the material.

13. 13. Use of a gas diffusion layer (GDL) made from the green paper of any one of claims 1 to 12 in a proton exchange membrane fuel cell (PEMFC) which requires a correspondingly porous conductive material for gas / power / reactant distribution.