Green Paper for Manufacturing Gas Diffusion Layers for Fuel Cells

Abstract

The present invention relates to a green paper for producing a gas diffusion layer (GDL) for a fuel cell. The present invention also relates to a method for producing a green paper for producing a gas diffusion layer (GDL) for a fuel cell. In the present invention, the green paper comprises at least one first watermarked paper web. This watermark forms a pattern for the flow field or gas distribution structure of the gas diffusion layer (GDL) produced from the green paper. The first paper web is very preferably mixed with metal powder and / or metal fibers. The final GDL is formed after binder removal, sintering, coating, atomic layer deposition (ALD - atomic layer deposition) and optionally further processing steps.

Description

[Technical Field]

[0001] The present invention relates to a green paper for manufacturing a gas diffusion layer (GDL) for a fuel cell. The present invention further relates to a method for manufacturing a green paper for manufacturing a gas diffusion layer (GDL) for a fuel cell. [Background technology]

[0002] In the case of proton exchange membrane fuel cells (PEMFC), also known as polymer electrolyte fuel cells, bipolar plates (BPP) and gas diffusion layers (GDL) distribute gases to a catalytic platinum-coated membrane (also called a CL or catalyst layer). 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, GDLs made of fibrous materials, such as carbon fibers, and BPPs made of coated steel are currently established. Here, the fibrous materials can be realized as woven / knitted fabrics or as fiber mats produced by papermaking techniques. Such mats are known, for example, from US Pat. No. 5,699,493. The material 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 and thereby further processed to GDL.

[0006] A particular disadvantage 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 bend or swell, penetrating the channels of the BPP and thereby reducing the transient flow of gas and water, impairing the efficiency of the fuel cell. Furthermore, the ability to adjust the porosity of the GDL is limited. 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 offer the possibility of patterning. Therefore, the flow field must be entirely formed by the BPP. For this purpose, embossing of the BPP is required to achieve the patterning for the gas distribution structure or the flow field, or processing of the green paper is required. This is generally a separate, expensive, and inconvenient procedure. [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] The object underlying the present invention is therefore to develop a universal green paper for manufacturing gas diffusion layers (GDLs) for fuel cells and a universal method for manufacturing green paper for manufacturing gas diffusion layers (GDLs) for fuel cells in a manner that avoids the disadvantages 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.

[0011] In the present invention, the green paper has at least one first paper web with at least one watermark formed thereon. This watermark forms a pattern for the flow field or gas distribution structure of the gas diffusion layer (GDL) produced from the green paper. Particularly preferably, the first paper web is mixed with metal powder and / or metal fibers. The final GDL is formed after binder removal, sintering, coating, atomic layer deposition (ALD - atomic layer deposition), and optionally further processing steps.

[0012] The present invention further relates to a method for producing green paper for producing gas diffusion layers (GDLs), in which at least one first paper web, preferably mixed with metal powder and / or metal fibers, is produced and at least one watermark is created therein, which is subsequently processed by binder removal, sintering, coating, atomic layer deposition by thermal ALD process (ALD - atomic layer deposition) and optionally further manipulation steps to form the final GDL.

[0013] After sintering, all organic components of the green paper are no longer present in the GDL due to thermal decomposition, which consists almost exclusively of a metal framework, the porosity of which, according to current knowledge, 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.

[0014] Surprisingly, it has now been found that green paper produced by papermaking can be patterned by introducing a watermark into the paper web of green paper in a way that eliminates, or at least makes easier to achieve, costly and inconvenient embossing of the BPP or post-processing of the green paper or GDL produced from the green paper.

[0015] In the present invention, the flow field is subsequently incorporated into the green paper without a separate processing operation by incorporating a corresponding watermark into the watermark ply on the cylindrical wire of the paper machine. In this case, any desired shape and gradation of the flow field channels can be realized by patterning depending on the design of the watermark screen, without any special cost or complexity, with the associated thickness adjustment of the paper. To increase the resolution of the patterning, it is also possible to use high-resolution or multi-level watermarks, as known, for example, from EP 1 432 868 A1 or WO 2014 / 040706 A1.

[0016] A watermark in the sense of the present invention is Paper Web The thickness of the Paper Web This is a true watermark where 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 wire is the same in all areas. On the other hand, for example, by introducing recesses or protrusions into the cylindrical wire, the density of the wire from the pulp can be increased. Paper Web This type of watermark can be created in the paper web during papermaking by increasing or decreasing the accumulation of paper fibers in these depressions or protrusions during the creation of the watermark. Alternatively, it can be created mechanically, for example by milling or laser processing. Paper Web By removing a portion of the watermark, a watermark can later be created in the paper web.

[0017] Alternatively, false watermarks are also possible, in which the paper web is embossed by an embossing procedure while still wet, for example after it has been removed from the cylindrical wire. This type of watermark is also called a dandy roll watermark. Paper Web The thickness of the Paper WebThe density of the paper increases. Thus, a densification or compaction of the paper fibers occurs. The advantage of this densification is that it prevents excessive gas diffusion through the GDL in the direction of the catalyst layer (CL), even in the main area of ​​the channel, thus ensuring a more uniform gas distribution.

[0018] Particularly preferably, true and pseudo-watermarks can be combined with one another, for example by forming one part of the watermark with a true watermark and another part with a pseudo-watermark.

[0019] According to a further preferred embodiment, the green paper consists of a first paper web and at least one second paper web. The green paper is formed from the first paper web and at least one second paper web. The second paper web, while still wet, is combined with the first paper web and firmly bonded thereto. In this case, the second and / or any further paper webs may also have a watermark.

[0020] 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 stock is applied to a cylindrical wire via a nozzle. These manufacturing methods are known from WO 2006 / 099971 A2 for the production of security documents or securities or identity documents such as banknotes, and are also preferred methods according to the present invention for producing GDL from at least one paper web.

[0021] Thus, in one operation, a green paper highly loaded with metal powder and / or metal fibers is formed and processed according to DE 10 2008 042 415 B3 using at least two different formulations to result in 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 may also differ between the two paper webs.

[0022] Furthermore, when the green paper is made up of two paper webs, each of which has a watermark, it is particularly advantageous if the watermark patterns of the first and second paper webs are not identical but instead are exact mirror images in area and thickness. In other words, the watermark pattern 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 combined with their watermark-patterned surfaces, the convex portions of the first paper web overlap with the concave portions of the second paper web. One particular advantage of this embodiment is 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. As a result, the second paper web has little resistance to the gas and only functions as a spacer for the bipolar plate. In this way, optimal gas distribution can be combined with optimal stackability and optimally uniform distribution of mechanical pressure across the PEM membrane. Particularly advantageously, a microporous layer (MPL) is arranged between the first paper web and the membrane. This layer has a fine surface with less roughness and smaller pores than the first and second paper webs.

[0023] According to one preferred embodiment, the first paper web has a higher density than the second paper web. The first paper web has 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 Here, it is particularly preferred that the first paper web is formed with a finer paper fiber slurry than the second paper web, resulting in correspondingly finer pores in this small area of ​​the sintered paper.

[0024] 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 50 μm to 400 μm, more preferably 80 μm to 200 μm.

[0025] According to a further preferred embodiment, the watermark is configured as a recess in the form of at least one channel. This channel serves to transport gas, i.e., fuel or oxygen. This channel is preferably realized in a serpentine form over the area of ​​the paper web. Another possibility includes a plurality of channels in a lattice form or in a radial form with arcuate connecting channels.

[0026] There may also be additional channels created for water transport in one or more of the paper plies by one of the methods described above. These channels ensure a balanced water transport and have the particular advantage that the PEM cell does not become flooded or dry out, since both of these have a negative effect on the efficiency of the cell. On the other hand, the water channels can also be used to maintain the cooling of the cell.

[0027] According to a further preferred embodiment, in addition to the watermark, a pattern is created on the surface of the green paper or sintered green paper by laser machining. The advantage of this is that the laser beam allows the creation of, for example, deeper patterns or patterns with steeper sidewalls, or an existing pattern can be deepened or provided with steeper sidewalls. Furthermore, laser machining can also be performed on one or more former plies to introduce patterning or channels in the intermediate layer between the watermark and the former ply, thereby further improving gas distribution.

[0028] According to a further preferred embodiment, the gas is coupled into the GDL in the center of the bipolar plate (based on the plan view of the bipolar plate) and then distributed outwards or towards the outer edge of the GDL by various openwork patterns and / or channels in the GDL. These openwork patterns and / or channels can, for example, start from the center of the GDL and lead outwards in a radial or spiral fashion. These can be complemented by concentric annular openwork patterns and / or channels.

[0029] The GDL is typically 300 cm, depending on the system and function. 2 ~350cm 2 The BPP has an area of ​​100 μm to 300 μm thick. If the flow field function is also integrated into the GDL, the GDL can be even thicker. The channel depth is up to 350 μm. The GDL also needs to have a certain compressibility and simultaneously conduct power between individual cells. Therefore, a GDL consisting of original plies and cylindrical wire plies has a thickness of 100 μm to 400 μm. The BPP should be realized as a flat plate with a thickness of 75 μm or less. Typically, the BPP also performs the cooling function of the fuel cell. Therefore, the BPP in this case can also be realized as a composite sandwich structure with porous or channel-like flow paths for the coolant. Alternatively, the cooling channels can be integrated into the GDL or MEA.

[0030] According to a further preferred embodiment, the BPP has a simple flow field pattern. Furthermore, a partial flow field is generated in the GDL. The original ply in this case has a thin embodiment so as not to take up too much space.

[0031] 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.

[0032] According to a further preferred embodiment, high resolution or multi-level watermarks are used to generate registration marks, alignment aids, centering aids and passage starting points, which advantageously simplifies further processing of the GDL to form the fuel cell stack, since precise alignment of the GDL with respect to other components such as the BPP or CL is possible, for example, by a transmitted / reflected light imaging system.

[0033] According to a further preferred embodiment, the patterns of the GDLs on the anode and cathode sides are not identical, but instead are exactly mirror-symmetric in area and material thickness. In other words, the pattern of the GDL on the anode side is 180° out of phase with the pattern of the GDL on the cathode side. This means that when the anode GDL is placed near the flow field side of the cathode GDL, the convex portions of one GDL exactly overlap the concave portions of the other GDL. Therefore, when placed together, the combination of two anode / cathode GDLs with 3D mirror symmetry results in a strictly planar piece of green paper. This embodiment has the advantage that the green paper can be densified with any mechanical pressure without losing its channel pattern. This is because the convex and concave portions in the green paper, created by the watermark and forming the flow field channels, are not damaged, pushed back, or flattened by subsequent pressing and other mechanical loads, allowing the channels to remain effective. This embodiment has the additional advantage that the anode GDL and cathode GDL can have different porosities. Instead of an alternating structure of anode GDLs and cathode GDLs, it is also possible to provide 3D mirror-symmetric GDLs for every other anode / cathode pair in the stack or for every other stack.

[0034] Particularly preferably, the fuel cell is a proton exchange membrane fuel cell (PEMFC). According to 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 for gas distribution, such as proton exchange membrane electrolyzer cells (PEMEC), electrolyzer cells or other power conversion technologies.

[0035] The components of the paper web preferably comprise paper obtained from cellulose or cotton fibers, as used for example for banknotes, or from other natural or synthetic fibers or a mixture of natural and synthetic fibers. More preferably, the paper web consists of a combination of at least two different substrates arranged and connected to one another in a hybrid. Data on the weight of the paper webs used are reported, for example, in DE 102 43 653 A9, the relevant remarks of which are incorporated in their entirety into the present patent application. The metal-filled green paper has a weight of 100 g / m 2 ~1200g / m 2 The sheet may have a basis weight of

[0036] The filler material used in the sintered paper can be any microscale metal powder or metal fiber. Examples are titanium, copper, zinc, or anti-corrosion stainless steel, as known from DE 10 2008 042 415 B3. It is important to use different mixtures for the original ply and the cylindrical wire ply to achieve different porosities in the paper ply. The original ply should be made finer than the cylindrical wire ply. It is also possible to use nanopowder in the original ply.

[0037] To protect the metal contained in the smallest pores from corrosion and to provide the generally desired hydrophobicity, preferably on the side facing the catalyst, according to a further preferred embodiment, a (thermal) ALD coating or another coating method is used in one of the subsequent operating steps, preferably after binder removal and sintering and before stamping and conversion of the GDL, where the cut is either outside the area at risk of corrosion or the cut is provided with additional sealing in a further operating step of the finished cell. Alternatively, there is also the possibility of coating the GDL after stamping and conversion using ALD, etc.

[0038] It will be recognized that, as far as is covered by 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.

[0039] The advantages of the present invention will be described with reference to the following exemplary embodiments and auxiliary figures. The exemplary embodiments constitute preferred embodiments, but are not intended to limit the present invention thereto. Furthermore, for easier understanding, the representations in the figures are highly schematic and do not reflect actual situations. In particular, the proportions shown in the figures do not correspond to conditions that actually exist, but merely serve to make things clearer. Furthermore, the embodiments described in the following exemplary embodiments are reduced to essential information for easier understanding. In actual implementation, substantially more complex designs or images may be used. [Brief explanation of the drawings]

[0040] [Figure 1] 1 shows a double cylinder paper machine for producing the green paper of the present invention. [Figure 2] The diagram shows a cylinder paper machine and a short former. [Figure 3] A two-ply GDL with serpentine channels formed by openwork is shown in plan view on the left and in cross section along section AB on the right. [Figure 4]4 shows the two-ply GDL of FIG. 3 further having registration marks, alignment aids, and centering aids. [Figure 5] Two 3D mirror-symmetric anode and cathode GDL combinations are shown in plan view on the left and in cross section along section AB on the right in each case. [Figure 6a] 1 shows a GDL with channels formed by watermarks, which run from the center of the GDL outwards and have radial channels. [Figure 6b] 1 shows a GDL with channels formed by watermarks, which run from the center of the GDL outwards, with radial and concentric channels. [Figure 6c] 1 shows a GDL with a channel formed by a watermark, which runs from the center of the GDL to the outside and has a spiral-shaped channel. DETAILED DESCRIPTION OF THE INVENTION

[0041] 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.

[0042] 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 joined to the first paper web 20 in the area of ​​pinch rollers 36. The joined paper webs 38 form a GDL and are passed through further processing stations.

[0043] As shown in Figure 2, the second paper web 30 can also be produced using a short former 40 in which the papermaking stock is applied to the surface of a cylindrical wire 44 by a headbox nozzle 42. This type of short former can be used to produce particularly thin paper plies, for example having a basis weight of 15 to 25 g / m2.

[0044] It will be appreciated that the paper machines 12, 14, 40 shown may be used to produce and join three or more paper webs as well.

[0045] FIG. 3 shows a two-ply GDL 1 with a serpentine channel 2 formed by openwork, shown diagrammatically in plan view on the left and in cross section along section AB on the right.

[0046] In cross section AB, the black area 3 represents the cylindrical wire ply with the patterned openwork as channel 2. The shaded area 4 represents the original ply with its microporous structure. Depending on the configuration, the individual plies 3 and 4 may have different base thicknesses. Evident along the meandering pattern of channel 2 in the example above is the cross-sectional profile shaped by the openwork. This is evident in the drawing as a thickness variation due to the cylindrical wire ply with a semicircular shape. In principle, any conceivable profile shape is possible here, as long as it does not have undercuts and the wall angle is less than 80°. The large arrows indicate the gas inlet / outlet. Therefore, a gasket needs to be designed around the GDL.

[0047] FIG. 4 shows a schematic representation of the two-ply GDL of FIG. 3, further including registration marks, alignment aids, and centering aids.

[0048] The highlight watermark can incorporate registration marks, alignment aids, centering aids, and path start points to simplify further processing of the GDL to form a fuel cell stack, ensuring that accurate alignment of the GDL with respect to another component, such as a BPP or CL, is possible using, for example, a transmitted / reflected light imaging system.

[0049] The lines 5 are intended to represent cutting marks of the GDL, for example realized as highlight watermarks. The circles 6 are intended to represent centering / alignment aids. Naturally, these can be made in any desired form. An HD watermark laser screen can also be used.

[0050] Figure 5 shows a schematic diagram of the combination of an anode GDL7.1 and a cathode GDL7.2 formed in 3D mirror symmetry with respect to the anode. The diagram shows a plan view of the surface of anode GDL7.1 in the upper left, a plan view of the surface of cathode GDL7.2 in the lower left, and a cross-section along section AB in each case on the right.

[0051] The protrusions and recesses in the green paper and finished GDL that are created by the watermark and that form the flow field channels 8.1 and 8.2 may be damaged again, pushed back or even flattened by the press and other mechanical loads, which means that the channels 8.1 and 8.2 may no longer be fully effective.

[0052] This problem can be solved by creating patterns created by the watermarks in the anode and cathode GDLs that are not identical, but are instead precisely mirror-symmetrical not only in area but also through the thickness of the material. This means that when the anode GDL 7.1 is placed near the flow-field side of the cathode GDL 7.2, the convex and concave portions of the parallel-arranged channels 8.1 and 8.2 exactly cancel each other out. The combination of the two anode / cathode GDLs, with their 3D mirror symmetry, results in a planar piece of sintered paper that can be densified at any pressure without losing its channel pattern.

[0053] Furthermore, the anode GDL 7.1 and cathode GDL 7.2 may have different porosities. For example, the anode GDL 7.1 may have a porosity between 20% and 75%. The cathode GDL 7.2 may have a porosity between 30% and 90%. Therefore, the cathode GDL 7.2 has little gas resistance and instead acts only as a spacer for the bipolar plate.

[0054] Figure 6 shows, in plan views in Figures 6a, 6b and 6c, three embodiments in which gas is coupled into the GDL in the center of the bipolar plate (not shown) and then distributed outwards or towards the outer edge of the GDL by various openwork patterns and / or channels in the GDL.

[0055] According to Figure 6a, the channels of the openwork pattern have a radial design, starting from the center of the GDL. Gas is supplied through circular openings in the center of the GDL. The black areas constitute the openwork regions where the GDL is thinner and thicker than the white areas that form the channels.

[0056] Figure 6b shows an exemplary embodiment in which the radial channels of the openwork pattern are supplemented with concentric annular channels, thereby creating a spider's web-like pattern. Gas is supplied through a circular opening in the center of the GDL. The black areas constitute the openwork regions where the GDL is thinner and thicker than the white areas that form the channels.

[0057] Figure 6c shows an exemplary embodiment in which the openwork pattern channels have a spiral design starting from the center of the GDL. Gas is supplied through a circular opening in the center of the GDL. The black areas constitute the openwork regions where the GDL is thinner and thicker than the white areas that form the channels.

Claims

1. A green paper for manufacturing a gas diffusion layer (GDL) for a fuel cell, comprising at least one first paper web having at least one watermark formed thereon, the watermark forming a pattern for a flow field of the gas diffusion layer (GDL) manufactured from the green paper; the watermark patterns on the anode and cathode sides of the fuel cell are not identical, but instead are exact mirror images in area and material thickness; and The green paper is characterized in that the watermark is a true watermark in which the thickness of the first paper web changes but the density of the first paper web does not change, and / or the watermark is a pseudo watermark in which the thickness of the first paper web decreases but at the same time the density of the first paper web increases.

2. The green paper of claim 1 comprising a first paper web and at least one second paper web.

3. 3. Green paper according to claim 1 or 2, characterized in that the watermark is configured as an indentation in the form of at least one channel.

4. 4. The green paper of claim 1, wherein the watermark has little or substantially no patterning to create registration marks, alignment aids, centering aids or passageway starting points in the GDL.

5. 1. A method for manufacturing a green paper for manufacturing a gas diffusion layer (GDL) for a fuel cell, comprising: generating at least one first paper web to form a pattern for a flow field of the gas diffusion layer (GDL) manufactured from the green paper; and forming at least one watermark in the paper web; A first paper web (20) is formed, and a second paper web (30) is formed, the second paper web (30) in a wet state is brought together with the first paper web (20) and firmly bonded thereto, the first paper web (20) and the second paper web (30) together forming a green paper for the GDL; and The method, wherein the first paper web (20) has a higher density than the second paper web (30).

6. The method of claim 5 , wherein the at least one first paper web is mixed with metal powder and / or metal fibers.

7. 7. The method according to claim 5 or 6, characterized in that the watermark is formed by a true watermark, in which the thickness of the first paper web changes but the density of the first paper web does not change, and / or the watermark is formed by a pseudo watermark, in which the thickness of the first paper web decreases but at the same time the density of the first paper web increases.

8. 8. The method of claim 7, wherein the first paper web (20) and / or the second paper web (30) are produced in a cylinder paper machine (12, 14).

9. 9. The method according to claim 7 or 8, 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 applied to a cylindrical wire (44) through a nozzle.

10. 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 10. The method of claim 9, wherein the granules have a density of

11. 11. The method according to claim 9 or 10, characterized in that the first paper web (20) is formed from a finer paper fiber slurry than the second paper web (30).

12. 12. The method according to any one of claims 5 to 11, characterized in that the first paper web (20) forms a diffusion layer for a membrane (CL) coated with a catalytic metal in the gas diffusion layer (GDL) produced from the green paper, and the second paper web (30) forms a distribution layer with flow fields in the gas diffusion layer (GDL) produced from the green paper.

13. 13. The method of claim 12, wherein the catalytic metal is platinum.

14. 5. Use of a gas diffusion layer (GDL) made from the green paper of any one of claims 1 to 4 in a proton exchange membrane fuel cell (PEMFC) which requires a correspondingly porous conductive material for gas / power / reactant distribution.

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