Porous transport layer

Abstract

The present invention provides a porous transport layer for an electrolytic cell or fuel cell, comprising: a first metal fiber nonwoven fabric layer provided for contact with a proton exchange membrane, comprising a first metal fiber nonwoven fabric layer comprising metal fibers of a first equivalent diameter and having a first surface roughness and a first porosity; and a second metal fiber nonwoven fabric layer comprising metal fibers of a second equivalent diameter, wherein the second metal fiber nonwoven fabric layer has a second surface roughness and a second porosity, wherein the first surface roughness is less than 10 μm, the first equivalent diameter is smaller than the second equivalent diameter, the first surface roughness is at least 20% smaller than the second surface roughness, for example in the range of 20% to 120%, and the first porosity is at least 10% smaller than the second porosity, for example in the range of 10% to 50%, and the first nonwoven fabric layer is metallurgically bonded to the second nonwoven fabric layer.

Description

[Technical Field]

[0001] The present invention relates to the field of gas diffusion layers, such as those used in electrolytic cells and fuel cells. [Background technology]

[0002] Proton exchange membrane (PEM) electrolytic cells can be used to convert water into separate hydrogen and oxygen streams. Such a PEM electrolytic cell contains a polymer electrolyte located between the anode and cathode electrodes. The anode-side porous transport layer and the cathode-side gas diffusion layer are located adjacent to the anode and cathode electrodes, respectively. Porous metals, such as titanium or titanium alloys, can be used as the anode-side porous transport layer.

[0003] International Publication No. 03 / 059556A2 discloses a stack for use as a porous transport layer in a fuel cell or electrolytic cell. The stack comprises an impermeable metal structure, a first metal fiber layer, and a second metal fiber layer. The impermeable metal structure is sintered on one side of the first metal fiber layer, and the second metal fiber layer is sintered on the other side of the first metal fiber layer. The second metal fiber layer is provided as a contact layer to the PEM in the fuel cell or electrolytic cell. The planar air permeability of the stack is greater than 0.02 l / min*cm.

[0004] A further modified porous transport layer is published in International Publication No. 2018 / 189005. The porous transport layer comprises a first nonwoven layer of metal fibers, a second nonwoven layer of metal fibers, and a third porous metal layer, all provided for contact with a proton exchange membrane. The first nonwoven layer contains metal fibers of a first equivalent diameter. The second nonwoven layer contains metal fibers of a second equivalent diameter, the second equivalent diameter being greater than the first equivalent diameter. The third porous metal layer contains open pores, the open pores of the third porous metal layer being larger than the open pores of the second nonwoven layer of metal fibers. The second nonwoven layer is provided between the first nonwoven layer and the third porous metal layer and is in contact with both the first and third porous metal layers. The second nonwoven layer is metallurgically bonded to the first and third porous metal layers.

[0005] The efficiency of fuel cell systems or electrolytic cell systems is greatly influenced by the properties of the porous transport layer. International Publication No. 2020 / 151997 discloses a porous transport layer based on multiple micro and nano-sintered porous layers. The sintered porous layer is permeable to gaseous and liquid substances in electrochemical cells, and the multilayer porous transport layer is suitable for integration between the bipolar plate and catalyst layer of an electrochemical cell. Multiple micro and nano-sintered porous layers are fabricated from irregularly shaped particles of conductive material. Micro-sintered porous layers are fabricated from particles having a larger diameter than the particles in nano-sintered porous layers. The multilayer structure is achieved by coaxial pressing of multiple irregularly sputtered powders, where the average particle diameter decreases from layer to layer. The mechanical integrity and specific bulk properties of the porous transport layer are obtained by the sintering process.

[0006] Another improved porous transport layer is a single-layer titanium sheet, as disclosed in U.S. Patent Application No. 2022 / 0023946. The porous titanium layer used as the anode-side porous transport layer is formed by a powder process, e.g., by tape casting, or by powder metallurgy techniques in which titanium powder is pressed into a porous titanium sheet using a compression process. The cast or compressed sheet is then sintered to produce a porous transport layer with established metallurgical joints. The porous titanium sheet may have a porosity of 40–60 percent. [Overview of the Initiative]

[0007] A first aspect of the present invention is a porous transport layer for an electrolytic cell or fuel cell, A nonwoven fabric layer of first metal fibers provided for contact with a proton exchange membrane, comprising metal fibers of first equivalent diameter, A nonwoven fabric layer of first metal fibers having a first surface roughness and a first porosity, A second metal fiber nonwoven fabric layer comprising a second metal fiber nonwoven fabric layer having a second equivalent diameter, a second surface roughness, and a second porosity, The first surface roughness is less than 10 μm. The first equivalent diameter is smaller than the second equivalent diameter, and the first surface roughness is at least 20% smaller than the second surface roughness, for example, in the range of 20% to 120%. The first porosity is at least 10% smaller than the second porosity, for example, in the range of 10% to 50%. The first nonwoven fabric layer is a porous transport layer that is metallurgically bonded to the second nonwoven fabric layer.

[0008] The first and second surface roughnesses are mean surface roughness (Ra) and are measured according to the standard ASME B46.1.

[0009] The first nonwoven layer is provided to act as a contact layer toward the anode electrode. It is beneficial that the porosity of the first layer is at least 10% lower than that of the second layer. The side of the porous transport layer with higher porosity, i.e., the second metal fiber nonwoven layer, will be oriented toward the flow plate side of the electrolytic cell or fuel cell. Therefore, higher porosity allows more water to enter the pores. On the other hand, the side of the porous transport layer with lower porosity, i.e., the first metal fiber nonwoven layer, will be oriented toward the anode electrode side of the electrolytic cell or fuel cell. Therefore, lower porosity provides improved electrical contact with the anode electrode. According to the present invention, the porous transport layer comprises two layers having different porosities. Preferably, the two layers also have different pore sizes. Importantly, the first nonwoven layer is metallurgically bonded to the second nonwoven layer. Therefore, the porous transport layer made from the first and second nonwoven layers behaves like an assembly. Therefore, the porosity of each side and the overall porosity of the porous transport layer can be well controlled by the assembly of two separate layers. This is advantageous compared to a single-layer porous transport layer, such as the one described in U.S. Patent Application No. 2022 / 0023946, in which the first main surface of the porous titanium sheet has at least 10 percent higher porosity than the second surface opposite the second main surface. This is also advantageous compared to International Publications No. 2018 / 189005 and 2020 / 151997, which disclose porous transport layers based on multiple sintered porous layers, because the porosity of each layer in those multiple sintered porous layers is similar as a result of a single sintering process.

[0010] Furthermore, the first surface roughness is at least 20% smaller than the second surface roughness, for example, in the range of 20% to 120%, and the first surface has an average roughness of less than 10 μm, preferably less than 8 μm, and more preferably less than 5 μm, as measured by standard ASME B46.1. PEM electrolytic cells typically have a catalyst coating film. The catalyst coating the film is platinum (Pt) on the cathode side and iridium oxide (IrOx) on the anode side. The deficiency of Ir is very relevant to the development of PEM. The global Ir production rate is about 7 tons / year. Current catalyst packing rate (2 mg / cm³) 2 Under these conditions, the annual installation of PEM electrolytic cells is limited to 2 GW. Unfortunately, due to its high stability and efficiency, there are no alternative catalysts for the oxygen evolution reaction (anode side) that can compete with Ir. As there are no competing alternatives, research is focused on significantly reducing the amount of Ir catalyst packed while maintaining the same performance. To maintain acceptable performance, the catalyst needs to be in close contact with the porous transport layer (e pathway) and the ionomer (H+ pathway). A lower amount of Ir packing means a thinner catalyst layer, reducing contact with the porous transport layer. Furthermore, Ir particles tend to aggregate, which also negatively affects contact with the porous transport layer, and therefore reduces performance due to poor catalyst utilization.

[0011] In addition to reducing the amount of catalyst packed, the trend in PEM electrolytic cells is to reduce membrane thickness, decrease overvoltage, and improve overall performance. This trend presents several challenges regarding the mechanical properties at the porous transport layer / anode / membrane interface. In fact, the membrane is a fragile component of PEM batteries, and poor contact with the porous transport layer / anode can lead to membrane rupture, increasing the risk of short circuits within the battery.

[0012] To solve these problems, the porous transport layer of the present invention, which has a first nonwoven fabric layer having a roughness of less than 10 μm, can be used to enhance contact with Ir particles and prevent film rupture.

[0013] The microstructure of this first nonwoven fabric layer should be finer and denser than that of the second nonwoven fabric layer, with smaller interparticle distances, to ensure close contact with the catalyst layer. This also provides a more uniform redistribution of the forces applied to the film, preventing its mechanical degradation. The surface roughness of the first layer is at least 20% less than that of the second layer, for example, in the range of 20% to 120%. The reason for the lower surface roughness of the first layer is to avoid bursting.

[0014] The second nonwoven fabric layer is metallurgically bonded to the first nonwoven fabric layer. The metallurgical bonding can be carried out, for example, by sintering or by welding (for example, by capacitive discharge welding, CDW). Preferably, the metal fibers in the first nonwoven fabric layer are metallurgically bonded to each other. Preferably, the metal fibers in the second nonwoven fabric layer are metallurgically bonded to each other.

[0015] The presence of a first nonwoven layer of metal fibers can adversely affect the inflow and outflow of molecules through the plane, and as a result, the reduced flow can increase the required overvoltage of the electrolytic cell, thus adversely affecting the functionality of the electrolytic cell or fuel cell. Interlayer metallurgical bonding is important because such bonding provides low electrical resistance between layers. It is desirable to provide a reliable metallurgical bonding between the first and second nonwoven layers. The advantages are that the ohmic resistance of the porous transport layer is reduced, the overvoltage of the electrolytic cell is reduced, and the mechanical stability of the porous transport layer is improved. Both nonwoven surfaces inevitably have some degree of fuzziness. As a result, fibers from the first nonwoven fabric penetrate to some extent into the second nonwoven fabric, and metal fibers from the second nonwoven fabric penetrate to some extent into the first nonwoven fabric. Penetration strengthens metal-to-metal contact and metallurgical bonding, both of which are beneficial in reducing the ohmic resistance of the porous transport layer, reducing the overvoltage of the electrolytic cell, and improving the mechanical stability of the porous transport layer.

[0016] The equivalent diameter of a fiber refers to the diameter of a circle that has the same surface area as the cross-section of a fiber, even if that fiber does not necessarily have a circular cross-section.

[0017] The pore size of the nonwoven fabric layer can be observed in several ways. A cross-section can be prepared through the thickness of the porous transport layer, and the cross-section can be analyzed under a microscope where the pores and their size are visible. A more advanced method is X-ray tomography of the porous transport layer. Alternatively, the pore size can be measured by mercury porosimetry.

[0018] According to the present invention, the first equivalent diameter may be less than 20 μm, preferably less than 15 μm. The use of fine fibers is beneficial because it provides a large contact area with the PEM for electrochemical reactions, and the fine pores present due to the use of fine fibers in the first nonwoven layer enable capillary action for efficient transport of mass to and from the reaction site in the PEM. The second equivalent diameter is preferably less than 50 μm, more preferably less than 30 μm. The second equivalent diameter is preferably larger than the first equivalent diameter. As an example, the first equivalent diameter is 14 μm and the second equivalent diameter is 22 μm. The second nonwoven layer of metal fibers has a larger diameter, and therefore the pores in the second nonwoven layer are larger than the pores in the first nonwoven layer. As an example, the first nonwoven layer has a porosity in the range of 30-50%, preferably 40-50%, and the second nonwoven layer has a porosity in the range of 51-90%, with the overall porosity of the porous transport layer being in the range of 50-80%. As a result, efficient planar mass inflow and outflow are obtained. Preferably, the thickness of the first nonwoven layer is less than 100 μm. Such embodiments provide a particularly beneficial porous transport layer because the first nonwoven layer providing the contact layer with the PEM is thin, and as a result, a second nonwoven layer of greater thickness can be provided in the space available to provide a porous transport layer in the electrolytic cell, and the second nonwoven layer provides planar inflow and outflow of water and reaction products.

[0019] More preferably, the thickness of the second nonwoven fabric layer is at least twice the thickness of the first nonwoven fabric layer. Thus, thanks to the thickness of the second nonwoven fabric layer, the planar flow is further improved, thereby providing a large cross-section for planar mass flow.

[0020] Preferably, the first non-woven fabric layer and the second non-woven fabric layer contain titanium fibers, and more preferably consist of titanium fibers. Alternatively, the first and second non-woven fabric layers can be made of titanium alloy fibers, nickel fibers, nickel alloy fibers, or stainless steel fibers.

[0021] In a preferred embodiment, the metal fibers of the first non-woven fabric layer and / or the second non-woven fabric layer have a discontinuous length and a cross-section, and the cross-section has two adjacent straight sides with an included angle of less than 90 degrees and one or more irregularly shaped curved sides. The first non-woven fabric layer of metal fibers has a large surface area due to the irregular shape of its cross-section. As a result, when the surface of the first non-woven fabric layer is coated with a catalyst, or when the proton exchange membrane is coated with a catalyst, the surface area in contact with the proton exchange membrane where an electrochemical reaction can occur is increased. Such fibers can be produced as described in WO 2014 / 048738 A1. Another technique for producing such fibers is described in US Patent No. 4,640,156.

[0022] Alternatively, the second non-woven fabric layer of metal fibers has a square, preferably rectangular cross-section. The technique for manufacturing such fibers is disclosed in US Patent No. 4,930,199. This provides the advantage that the second non-woven fabric layer of metal fibers has a more compact cross-section, thereby not causing an obstacle to the planar inflow and outflow of gas in an electrolytic cell or fuel cell where a porous transport layer is used.

[0023] As an example, the second non-woven fabric layer may further include one or more expanded metal sheets or wire meshes, and the expanded metal sheets or wire meshes are metallurgically joined to each other, for example, by sintering or by welding, such as capacitor discharge welding (CDW). It is beneficial that the expanded metal sheet has higher rigidity than the sintered non-woven fabric. The porous transport layer of the present invention may further include a third porous metal layer having a larger pore diameter and / or porosity, adjacent to the second non-woven fabric layer of metal fibers. The multi-layer porous transport layer including one or more expanded metal sheets enables a large surface area on the PEM side, while the rigidity prevents sagging when compressed against the profiled surface provided by the machined bipolar plate of the flow path.

[0024] A second aspect of the present invention is another method of producing the porous transport layer for an electrolytic cell or a fuel cell according to any one of the preceding claims, preferably in the following order: (a) a step of producing a first non-woven fabric layer of metal fibers; (b) sintering the first non-woven fabric layer of metal fibers at 1000 to 1200 °C for 1 to 2 hours, preferably under a load of 70 kg / m 2 ~200 kg / m 2 in an oxygen-free or low-oxygen atmosphere to obtain a sintered first non-woven fabric layer of metal fibers; (c) compressing the sintered first non-woven fabric layer of metal fibers to obtain a compressed and sintered first non-woven fabric layer of metal fibers; (d) a step of producing a second non-woven fabric layer of metal fibers; (e) stacking the compressed and sintered first non-woven fabric layer of metal fibers with the second non-woven fabric layer of metal fibers to form a double-layer structure; (f) sintering the double-layer structure at 1000 to 1200 °C for 1 to 2 hours, preferably under a load of 70 kg / m 2 ~200 kg / m 2 in an oxygen-free or low-oxygen atmosphere; (g) optionally, a step of compressing the sintered double-layer structure to a predetermined thickness, which is a method.

[0025] The first nonwoven layer of metal fibers is compressed after sintering. Preferably, the compression is 30-80 kN / mm². 2 For example, 60kN / mm 2 The load is applied under the following conditions. The compression step (c) applied before the second sintering of the first nonwoven layer, i.e., the sintering of the double layer (f), provides significant advantages in terms of reducing surface roughness and controlling the porosity of the first nonwoven layer. The double-layer porous transport layer formed by the present invention has a top surface with low porosity and roughness, which is desirable for its application.

[0026] A third aspect of the present invention is a stack for an electrolytic cell or fuel cell, comprising a porous transport layer and a bipolar plate according to the first aspect of the present invention. The bipolar plate is in contact with a second nonwoven fabric layer. Preferably, the bipolar plate is metallurgically bonded to the second nonwoven fabric layer, for example, by sintering or welding. Preferably, the bipolar plate is flat across the entire surface in contact with the second nonwoven fabric layer, meaning that no flow field is provided on the bipolar plate.

[0027] A fourth aspect of the present invention is an assembly of a porous transport layer and a proton exchange membrane according to the first aspect of the present invention. The first nonwoven layer is in contact with the proton exchange membrane. Preferably, the catalyst is provided on the first nonwoven layer on the side in contact with the proton exchange membrane, or the catalyst is provided on the proton exchange membrane on the side in contact with the first nonwoven layer.

[0028] A fifth aspect of the present invention is an assembly of a stack according to the third aspect of the present invention and a proton exchange membrane. A first nonwoven layer is in contact with the proton exchange membrane. Preferably, the catalyst is provided on the first nonwoven layer on the side in contact with the proton exchange membrane, or the catalyst is provided on the proton exchange membrane on the side in contact with the first nonwoven layer. [Brief explanation of the drawing]

[0029] [Figure 1(a)] A schematic cross-section of the porous transport layer according to the present invention is shown. [Figure 1(b)] This shows a magnified photograph of a cross-section of the porous transport layer according to the present invention. [Figure 2] This shows a comparison of the pore size of the porous transport layer and the pore size of the single-layer porous transport layer according to the present invention. [Figure 3] This shows a comparison of the average surface roughness of the porous transport layer and the average surface roughness of the single-layer porous transport layer according to the present invention. [Figure 4] The electrical resistance under compression across the thickness of the porous transport layer according to the present invention is compared with that of a single-layer porous transport layer. [Modes for carrying out the invention]

[0030] An exemplary porous transport layer 10 according to the present invention is schematically shown in Figure 1(a). A magnified photograph of a cross-section of this exemplary porous transport layer 40 is shown in Figure 1(b). The exemplary porous transport layers 10 and 40 consist of a first titanium fiber nonwoven fabric layer 12 and 42 and a second titanium fiber nonwoven fabric layer 22 and 52. The first titanium fiber nonwoven fabric layers 12 and 42 are provided to be in contact with the proton exchange membrane in the electrolytic cell. The first titanium fiber nonwoven fabric layer contains titanium fibers with an equivalent diameter of 14 μm. The titanium fibers of the first nonwoven fabric layer have a length of 10 mm and a cross-section, the cross-section having two adjacent straight sides with an angle of less than 90 degrees and one or more irregularly shaped curved sides. The first nonwoven fabric layer has a thickness di of about 40 μm. The first nonwoven fabric layer has a porosity of about 43%. The second titanium fiber nonwoven fabric layer 22, 52 has an equivalent diameter of 22 μm and weighs 400 g / m². 2 It consists of titanium fibers. The thickness d2 of the second nonwoven fabric layer is approximately 200 μm. The second nonwoven fabric layer has a porosity of approximately 78%. The overall porosity of the porous transport layer, consisting of the first and second nonwoven fabric layers, is approximately 56%. Because the equivalent diameter of the fibers in the second nonwoven fabric layer is larger than that of the first nonwoven fabric layer, the number of open pores is larger in the second nonwoven fabric layer than in the first nonwoven fabric layer.

[0031] Such porous transport layers can preferably be manufactured according to the following sequence of steps. (a) A step of producing a first nonwoven fabric layer of titanium fibers, (b) sinter the non-woven fabric layer of the first titanium fiber at 1000-1200°C for 1-2 hours, preferably under a load of 70 kg / m 2 ~200 kg / m 2 under a load in an oxygen-free or low-oxygen atmosphere to obtain a sintered non-woven fabric layer of the first titanium fiber; (c) compress the sintered non-woven fabric layer of the first titanium fiber to obtain a compressed and sintered non-woven fabric layer of the first titanium fiber; (d) produce a non-woven fabric layer of the second titanium fiber; (e) stack the compressed and sintered non-woven fabric layer of the first titanium fiber on the non-woven fabric layer of the second titanium fiber to form a double-layer structure; (e) sinter the double-layer structure at 1000-1200°C for 1-2 hours, preferably under a load of 70 kg / m 2 ~200 kg / m 2 under a load in an oxygen-free or low-oxygen atmosphere; (g) optionally, compress the sintered double-layer structure to a predetermined thickness.

[0032] An exemplary porous transport layer according to the present invention was compared with a reference standard single-layer porous transport layer. The reference porous transport layer having the same thickness as the porous transport layer of the present invention was made from titanium fibers with an equivalent diameter of 22 μm and had a porosity of 56%.

[0033] The pore sizes of the porous transport layer of the present invention and the reference porous transport layer were measured and analyzed. The pore size distribution was analyzed based on the minimum pore (I), average pore (II), and maximum pore (III). As shown in FIG. 2, in all ranges, that is, for the minimum pore, average pore, and maximum pore, the size of the porous transport layer of the present invention (A in FIG. 2) is significantly reduced compared to the reference porous transport layer (B in FIG. 2) due to the densified first non-woven fabric layer. A smaller pore size distribution can be converted into a smaller interparticle distance, which should result in better contact with the catalyst layer.

[0034] In addition, the average surface roughness of the porous transport layer and the reference porous transport layer of the present invention is measured. Two sides (S1, S2) of the porous transport layer of the present invention (A in Figure 3) and the reference porous transport layer (B in Figure 3) are evaluated and compared in Figure 3. Side S1 of the porous transport layer of the present invention is the surface of a first nonwoven fabric layer made from titanium fibers with an equivalent diameter of 14 μm (12 in Figure 1), while S2 of the porous transport layer of the present invention is the surface of a second nonwoven fabric layer made from titanium fibers with an equivalent diameter of 22 μm (32 in Figure 1). The average surface roughness (Ra) measured according to standard ASME B46.1 is shown on the Y axis (μm). As shown in Figure 3, the roughness of one side (S2) of the porous transport layer of the present invention and the reference porous transport layer are considerably equivalent. The roughness (5.0 μm) of the other side (S1) of the porous transport layer of the present invention is significantly reduced compared to the roughness (10.0 μm) of the reference porous transport layer. The side of the porous transport layer of the present invention with lower roughness (S1) is intended to contact the catalyst, while the other side (S2) is intended to contact the bipolar plate. Using the porous transport layer of the present invention should be beneficial in preventing membrane rupture.

[0035] Electrical resistance was measured as a function of the compressive force on the porous transport layer, through its thickness. The porous transport layer operates under compressive forces, e.g., 4 MPa, or even higher compressive forces, in electrolytic cells and fuel cells. Figure 4 shows the compressed electrical resistance measured through the thickness of the porous transport layer of the present invention (curve A) and the reference porous transport layer (curve B) on the Y axis (mOhm), as a function of the compressive force (compressive force or pressure P, expressed in MPa on the X axis) on the porous transport layer of the present invention or the reference porous transport layer. The measured samples of the porous transport layer of the present invention and the reference porous transport layer had the same dimensions and were measured under the same conditions. The results show that the electrical resistance through the porous transport layer of the present invention under compressive forces such as those used in electrolytic cells or fuel cells (curve A) is slightly reduced compared to the reference porous transport layer (curve B). The porous transport layer of the present invention results in a reduction of ohmic overpotential at the interface between the porous transport layer and the catalyst. Furthermore, it is clearly shown that the deformation of the porous transport layer made from fibers is mainly elastic, which is considered clearly advantageous for applications compared to the plastic deformation of powder-based porous transport layers.

Claims

1. A porous transport layer for electrolytic cells or fuel cells, A nonwoven fabric layer of first metal fibers provided for contact with a proton exchange membrane, comprising metal fibers of first equivalent diameter, A nonwoven fabric layer of first metal fibers having a first surface roughness and a first porosity, A second metal fiber nonwoven fabric layer comprising a second metal fiber nonwoven fabric layer having a second equivalent diameter, a second surface roughness, and a second porosity, The first surface roughness is less than 10 μm. The first equivalent diameter is smaller than the second equivalent diameter, and the first surface roughness is at least 20% smaller than the second surface roughness, for example, in the range of 20% to 120%. The first porosity is at least 10% smaller than the second porosity, for example, in the range of 10% to 50%. The first nonwoven fabric layer is a porous transport layer that is metallurgically bonded to the second nonwoven fabric layer.

2. The porous transport layer according to claim 1, wherein the thickness of the second nonwoven fabric layer is at least twice the thickness of the first nonwoven fabric layer.

3. The porous transport layer according to claim 1 or 2, wherein the first equivalent diameter is less than 20 μm.

4. The porous transport layer according to any one of claims 1 to 3, wherein the second equivalent diameter is greater than the first equivalent diameter and less than 50 μm.

5. The porous transport layer according to any one of claims 1 to 4, wherein the first equivalent diameter is 14 μm and the second equivalent diameter is 22 μm.

6. The porous transport layer according to any one of claims 1 to 5, wherein the thickness of the first nonwoven fabric layer is less than 100 μm.

7. The porous transport layer according to any one of claims 1 to 6, wherein the first nonwoven fabric layer and the second nonwoven fabric layer contain, preferably consist of, one of titanium fibers, nickel fibers, or stainless steel fibers.

8. The porous transport layer according to any one of claims 1 to 7, wherein the first nonwoven metal fiber layer has a discontinuous length and a cross-section, the cross-section having two adjacent straight sides having an angle of less than 90 degrees and one or more irregularly shaped curved sides.

9. The porous transport layer according to any one of claims 1 to 8, wherein the first nonwoven fabric layer has a porosity in the range of 30 to 50%, the second nonwoven fabric layer has a porosity in the range of 51 to 90%, and the overall porosity of the porous transport layer is in the range of 50 to 80%.

10. The porous transport layer according to any one of claims 1 to 9, further comprising a third porous metal layer adjacent to the second nonwoven metal fiber layer, having a larger pore diameter and / or porosity.

11. A method for producing a porous transport layer for an electrolytic cell or fuel cell according to any one of claims 1 to 10, (a) A step of producing a first nonwoven fabric layer of metal fibers, (b) A step of sintering the first metal fiber nonwoven fabric layer to obtain a sintered first metal fiber nonwoven fabric layer, (c) A step of compressing the sintered first metal fiber nonwoven fabric layer to obtain a compressed and sintered first metal fiber nonwoven fabric layer, (d) A step of producing a second nonwoven fabric layer of metal fibers, (e) A step of stacking the compressed and sintered first metal fiber nonwoven fabric layer with the second metal fiber nonwoven fabric layer to form a double layer structure, (f) A step of sintering the double layer structure, (g) A method comprising the optional step of compressing the sintered double-layer structure to a predetermined thickness.

12. In process (c), the compression is 30-80 kN / mm ２ A method for producing a porous transport layer according to claim 11, applied under a load force.

13. A stack for an electrolytic cell or fuel cell, A porous transport layer according to any one of claims 1 to 10, Equipped with a bipolar plate, The bipolar plate is in contact with the second nonwoven fabric layer. Preferably, the bipolar plate is metallurgically bonded to the second nonwoven fabric layer, forming a stack.

14. An assembly of a porous transport layer and a proton exchange membrane according to any one of claims 1 to 10, or an assembly of a stack and a proton exchange membrane according to claim 13, wherein the first nonwoven fabric layer is in contact with the proton exchange membrane, and preferably, a catalyst is provided on the first nonwoven fabric layer on the side in contact with the proton exchange membrane.

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