Hybrid structured porous transport layer

Abstract

The invention is provided with a porous transport layer for an electrolyser or for a fuel cell, comprising a first layer and a second layer. The first layer is made from conductive fibers having an average equivalent diameter of less than 100 µm and an aspect ratio of discrete length to diameter of at least 5, wherein said conductive fibers have a standard deviation between fibers of the equivalent fiber diameter of less than 30% of the equivalent fiber diameter. The second layer is made from irregularly shaped conductive particles, wherein said second layer has an average pore size smaller than the average pore size of the first layer, and wherein the porosity within the thickness of said second layer has a variation less than 10% from the nominal value. The first layer is metallurgically bonded to the second layer.

Description

Hybrid Structured Porous Transport LayerDescriptionTechnical Field

[0001] The invention relates to the field of Gas Diffusion Layers (GDL) or Porous Transport Layer (PTL) as are, e.g. used in electrolysers and fuel cells.Background Art

[0002] Proton exchange membrane (PEM) or Anion Exchange Membrane (AEM) electrolysers may be used to convert water into separate hydrogen and oxygen streams. Such PEM or AEM electrolysers include a polymer electrolyte located between an anode electrode and a cathode electrode. Anode side porous transport layer and cathode side gas diffusion layer are located adjacent to the respective anode and cathode electrodes. A porous metallic, e.g., titanium or titanium alloy, may be used as the anode side porous transport layer for PEM; nickel or nickel alloy, may be used as the anode side porous transport layer for AEM.

[0003] WO03 / 059556A2 discloses a stack for use as porous transport layer in a fuel cell or in an electrolyser. The stack comprises an impermeable metal structure, a first metal fiber layer and a second metal fiber layer. The impermeable metal structure is sintered to one side of the first metal fibers layer and the second metal fibers layer is sintered to the other side of the first metal fibers layer. The second metal fiber layer is provided as contact layer to a PEM in a fuel cell or in an electrolyser. The planar air permeability of the stack is more than 0.02 l / min*cm.

[0004] A further modified porous transport layer is published in WO2018 / 189005.The porous transport layer comprises a first nonwoven layer of metal fibers provided for contacting a proton exchange membrane, a second nonwoven layer of metal fibers, and a third porous metal layer. The first nonwoven layer comprises metal fibers of a first equivalent diameter. The second nonwoven layer comprises metal fibers of a second equivalent diameter. The second equivalent diameter is larger than the first equivalent diameter. The third porous metal layer comprises open pores. The open pores of the third porous metal layer are larger than the open pores of the secondnonwoven layer of metal fibers. The second nonwoven layer is provided in between and contacting the first nonwoven layer and the third porous metal layer. The second nonwoven layer is metallurgically bonded to the first nonwoven layer and to the third porous metal layer.

[0005] The efficiency of a fuel cell system or an electrolyser system is highly affected by the properties of the porous transport layers. WO2020 / 151997 discloses a porous transport layer based on multiple micro and nano sintered porous layers. The sintered porous layers have a permeability for gaseous and liquid substances in an electrochemical cell, and the multilayer porous transport layer is suited to be assembled between a bipolar plate and a catalyst layer of the electrochemical cell. The multiple micro and nano sintered porous layers are made from irregularly shaped particles of a conductive material. The micro sintered porous layer is made by particles having bigger diameter than the particles in the nano sintered porous layer. The multiple layer structure is achieved by coaxial pressing of multiple irregularly spattered powders where mean particle diameter decreases from layer to layer. The mechanical integrity and specific bulk properties of the porous transport layers are obtained by sintering process.

[0006] Recently publication EP3958360 disclosed the use of a hybrid, multilayer designed porous transport electrode, comprised of a plurality of porous layers based on different particle geometries and a deposited porous catalyst layer provides simultaneously economic and technical improvement by unification of single layers to one component and offers optimization of technical and electrochemical properties. Also reduced catalyst layer thickness can be obtained due to extended, larger surface area when employing a second intermediate layer between a porous support layer and the catalyst layer. Thinner catalyst layers translate into reduced loss by mass, electrical and ionic transport and boost cell efficiency. This approach of unifying several components in a single component design results in a simultaneous decrease of operational expenditure as well as capital expenditure due to improved efficiency as well as materials cost savings.

[0007] While the use of multilayer designed porous transport electrode provide significant benefit, special attention is needed to avoid delaminationbetween multilayers. Also, there is a demand for a robust multilayer porous transport layer with desired mechanical and electrical properties.Disclosure of Invention

[0008] The objective of the present invention is to provide a robust multilayer designed porous transport layer.

[0009] Another objective of the invention is to provide a multilayer porous transport layer having optimised property as a whole and as well for each of layers.

[0010] The first aspect of the invention is a porous transport layer for an electrolyser or for a fuel cell, comprising- a first layer made from conductive fibers, said conductive fibers having an average equivalent diameter of less than 100 pm and an aspect ratio of discrete length to diameter of at least 5, preferably at least 10, more preferably at least 50, e.g. between 100 and 1000, wherein said conductive fibers have a standard deviation between fibers of the equivalent fiber diameter of less than 30% of the equivalent fiber diameter.- a second layer made from conductive particles, wherein said second layer has an average pore size smaller than the average pore size of the first layer, wherein the porosity within the thickness of said second layer has a variation less than 10% from the nominal value,- wherein the ratio of plastic to elastic deformation of said porous transport layer is in a range from 10% to 28% at an applied load from 2 to 8 MPa, - wherein the first layer is metallurgically bonded to the second layer.

[0011] The multilayer or hybrid structured porous transport layer has a first layer made from conductive fibers. The first layer based on fiber materials provides high mechanical integrity and enables the manufacturing of thin and compact porous transport electrode designs in contrast to employment of powder particles as material for the porous transport layer bulk structure because of mechanical stability requirements and fabrication restrictions for elevated pressure applications. Also, the higher elastic modulus of fiberbased support layers is of interest when employing high porosities for improved water and gas transport distribution in the thin support structures.

[0012] According to the present invention, said conductive fibers have a standard deviation between fibers of the equivalent fiber diameter of less than 30%of the equivalent fiber diameter. With equivalent diameter of a fiber is meant the diameter of a circle having the same surface area as the cross section of a fiber which does not necessarily have a circular cross section. This fiber diameter distribution guarantees a well-controlled pore size distribution. This is important as a gas diffusion layer to reduce mass transport losses in the layer. In addition, the surface of the first layer formed with the conductive fibers having the limited deviation, is rather homogeneous and can efficiently induce high mechanical stress in the porous transport electrode due to highly heterogeneous contact pressure distribution. The homogeneity of the surface properties of the first layer also helps to reduce the delamination of a contacting layer.

[0013] Moreover, the first conductive layer can have a non-woven structure and the conductive fibers can be oriented within the first layer. The average orientation angle between the length direction of said conductive fibers and the surface of the porous transport layer is less than 30°. The length direction of said conductive fibers refers to the direction along the length of straight fibers or the direction defined by connecting the two ends (A, B) of curved fibers as shown in Figure 1. The elongated fibers can be laid with their length direction aligned within and along the plan parallel to surface of the porous transport layer and average orientation angle (angle BAC, i.e. angle a in Figure 1) can be less than 30°, preferably less than 20°. The average orientation angle is an average angle over all the measured orientated fibers, which can be measured and analyzed by tomography or microscopy. The well orientated fibers can further improve the mechanical properties and result in a desired plastic / elastic deformation ratio.

[0014] The first conductive layer of the porous transport layer is to be oriented towards the flow plate side of an electrolyser or a fuel cell. Thus, the high pore size permits more water to enter the pores. While the small pore size side of i.e., second conductive layer of the porous transport layer is to be oriented towards anode electrode side of an electrolyser or a fuel cell.

[0015] According to the current invention, the second layer is made from conductive particles, e.g. metallic powders. The particles can have irregular shape. They can have a near plate-like shape, near spherical shape or spherical shape. The equivalent particle size of the particles in the secondlayer maybe smaller than or equal to the equivalent diameter of the conductive fibers in the first layer. The second layer has an average pore size smaller than the average pore size of the first layer. The pore size of the nonwoven layers can be observed in several ways. A cross section through the thickness of the porous transport layer can be made, and the cross section can be analyzed under a microscope, in which the pores - and their sizes - become visible. A more advanced method is X-ray tomography of the porous transport layer. Alternatively, the pore size can be measured by mercury porosimetry or capillary flow porometry.

[0016] According to the invention, the porosity within said second layer has a variation less than 10%, preferably 5%, and more preferably 3% from the nominal value. The nominal value herein refers to a target or desired value in the design / manufacture of the porous layer. An improved mechanical property of the invention porous transport layer can be observed as an advantage of uniform pore size distribution within the individual first and the second layer. The ratio of plastic to elastic deformation of the invention porous transport layer can thus be in a desired range from 10% to 28% at an applied load from 2 to 8 MPa. The plastic to elastic deformation is also defined as energy ratio (presented as dominating dimensionless number in literatures, whose physical meaning denotes the plastic to elastic share characterizing the combined effects of structure, material, and load.

[0017] The fine structure of the second layer provides high mechanical, high thermal and electrical conductivity. Also intrinsically gives high surface areas while provides an extended surface for a desired deposition of the top layer. Importantly, the surface roughness of the second layer is low compared to the (first) fiber layer. The second layer is intended to contact the membrane of the electrolyser or fuel cell.

[0018] PEM electrolysers typically have a catalyst coated membrane. The catalysts coated on the membrane are Platinum (Pt) at the cathode side and Iridium oxide (IrOx) at the anode side. The scarcity of lr is very concerning for the development of PEM. Global lr production rate is about 7 ton / year. At current catalyst loadings (2mg / cm2), the annual installation of PEM electrolysers would be limited to 2 GW. Unfortunately, there are no alternative catalyst for the oxygen evolution reaction (anode side) that cancompete with Ir, due to its high stability and efficiency. Without any competitive alternative, research is focused on reducing Ir catalyst loading significantly while keeping the same performance. To maintain acceptable performances, the catalyst needs to be in close contact with the porous transport layer (e- path) and the ionomer (H+ path). With low loading of Ir, implying a thinner layer of catalyst, the contact with the porous transport layer will be reduced. In addition, Ir particles tend to agglomerate, which should also negatively affect the contact with the porous transport layer, and therefore lead to poor performances due to a bad catalyst utilization.

[0019] In addition to reduce the catalyst loading, the trend in PEM / AEM electrolyser is to decrease the thickness of the membrane, to reduce the overpotential and enhance the overall performances. This trend brings some challenges regarding the mechanical properties at the interface of porous transport layer / anode / membrane. Indeed, the membrane is a fragile component of the electrolyser cell, and a bad contact with the porous transport layer / anode can lead to the puncture of the membrane, with a strong risk of short circuit within the cell.

[0020] To bring a solution to these issues, the inventive porous transport layer having a second layer with a low roughness can be used to enhance the contact with the conductive particles of the catalyst layer and prevent the puncture of the membrane.

[0021] The microstructure of this second layer can have same porosity of the first layer. Preferably the second layer is finer and denser than the first layer, with a small inter-particle distance, to ensure a close contact with the catalyst layer. This also provides a more homogeneous repartition of the forces applied to the membrane and prevent its mechanical degradation.

[0022] The first layer is metallurgically bonded to the second layer. Metallurgical bonding can e.g., be performed by means of sintering or by means of welding (e.g., by means of capacitive discharge welding, CDW). Preferably, the metal fibers in the first nonwoven layer are metallurgically bonded to each other. Preferably, the conductive particles in the second nonwoven layer are metallurgically bonded to each other.

[0023] The presence of the second layer of conductive particles may negatively affect the inflow and outflow of molecules through the plane and mayconsequently negatively affect the functionality of the electrolyser or fuel cell as the reduced flow increase the required overvoltage of the electrolyser. The metallurgical bonds between the layers are important, as such bonds provide for a low electrical resistance between the layers. Providing a reliable metallurgical bonding between the first nonwoven layer and the second nonwoven layer is desirable. The benefit is a reduced ohmic resistance of the porous transport layer, reducing the overvoltage of the electrolyser, and an improved mechanical stability of the porous transport layer. There is inevitably a certain amount of hairiness on the surface of both nonwovens. Consequently, fibers from the first layer will penetrate to a certain extent in the second layer. The penetrations create enhanced metallic contacts and metallurgical bonding, both beneficial for the reduced ohmic resistance of the porous transport layer, reducing the overvoltage of the electrolyser, and an improved mechanical stability of the porous transport layer. The uniform porosity of porous transport layer provides good gas diffusion capability in comparison with gradient porosity. The uniform porosity and limited gradient region between two layers of the present invention provide good conductivity close to the membrane of electrolyser.

[0024] The conductive fibers in the first layer may have an equivalent diameter in the range of 10 to 50 pm and an average discrete length in the range of 50 pm to 200 mm. The equivalent diameter of the conductive fibers can be less than 30 pm, e.g., 22 pm, 14 pm, or 7pm. The equivalent diameter of fibers in the first conductive layer can be bigger than the equivalent diameter of particles in the second layer. Preferably, the thickness of the second layer is less than 100 pm. Such embodiments provide a particularly beneficial porous transport layer, as the second layer that provides the contact layer with the PEM / AEM is thin, such that in the available space for the provision of the porous transport layer in the electrolyser, a larger thickness of the first layer can be provided. The first layer provides for the in-plane inflow and outflow of water and reaction products. More preferably, the thickness of the first layer is at least double the thickness of the second layer. The planar flow is thus further improved thanks to the thickness of the first layer, thereby providing a large cross section for planar mass flow.

[0025] The first conductive layer may comprise Titanium (Ti), Nickel (Ni), Stainless- Steel (SS), Niobium (Nb), Tantalum (Ta), Zirconium (Zr), Platinum (Pt), Gold (Au), or Iridium (lr).

[0026] The second conductive layer may comprise Titanium (Ti), Nickel (Ni), Stainless-Steel (SS), Niobium (Nb), Tantalum (Ta), Zirconium (Zr), Platinum (Pt), Gold (Au), or Indium (lr). Preferably, the second conductive layer is made from conductive particles or powders from any one of Ti, Ni, SS, Nb, Ta, Zr, Pt, Au, or lr.

[0027] The conductive fibers have a discrete length and the cross section of the conductive fibers of the invention may have two neighboring straight-lined sides with an included angle of less than 90 degrees and one or more irregularly shaped curved sides. Such fibers can be made as described in WO2014 / 048738A1. Another technology for producing such fibers is described in US4,640,156.

[0028] Alternatively, the conductive fibers may have a quadrangular, and preferably a near rectangular cross section. A technology for manufacturing such fibers is disclosed in US4,930,199. This provides a benefit: the metal fibers of the first layer have a more compact cross section, which does not create obstructions for the planar inflow and outflow of gases in the electrolyser or fuel cell in which the porous transport layer is used.

[0029] In the present invention, the first layer may have a porosity in a range of 40 to 80%, the second layer has a porosity in a range of 10 to 50% and the overall porosity of the porous transport layer is in a range of 30 to 70%.

[0030] Moreover, the porous transport layer according to the present invention may further comprise a third layer deposited between the first layer and the second layer, and the third layer has an average pore size smaller than the average pore size of the first layer and bigger than the average pore size of the second layer.

[0031] Moreover, the porous transport layer may further comprise one or more expanded metal sheets or woven wire meshes next to the first layer; wherein the expanded metal sheets or woven wire meshes are metallurgically bonded to each other, e.g., by means of sintering or by means of welding, e.g., capacitive discharge welding (CDW). It is a benefit that expanded metal sheets have a higher stiffness than sintered nonwovens. A multilayerporous transport layer including one or more expanded metal sheets allows a high surface area at the PEM / AEM side, while the stiffness prevents sagging when compressed against a profiled surface provided by a bipolar plate in which flow channels have been machined.

[0032] The second aspect of the invention is a method of making a porous transport layer for an electrolyser or for a fuel cell, comprising the steps of:(a) making a first layer of non-sintered conductive fibers, preferably by wet webbing process, wherein said conductive fibers having an average equivalent diameter of less than 100 pm and an aspect ratio of discrete length to diameter of at least 5, preferably at least 10, more preferably at least 50, e.g. between 100 and 1000, and wherein said conductive fibers have a standard deviation between fibers of the equivalent fiber diameter of less than 30% of the equivalent fiber diameter, (b) optionally compressing the first layer of conductive fibers to have a compressed non-sintered first layer of conductive fibers,(c) making a second layer of non-sintered conductive particles onto the surface of the first layer of conductive fibers to provide a combined conductive layer made from the first layer and second layer, wherein the average pore size of the second layer is smaller than the average pore size of the first layer, and wherein the porosity within the thickness of said second layer has a variation less than 10% from the nominal value,(d) sintering the combined conductive layer,(f) compressing the sintered combined conductive layer to a determined thickness.

[0033] Preferably, the conducive fibers are oriented within the first layer, and the average orientation angle between the length direction of said conductive fibers and the surface of the porous transport layer is less than 30°.

[0034] The combined conductive layer can be sintered in an oxygen-free or low oxygen atmosphere at 900 to 1250 degrees Celsius for 1 to 2 hours, preferably under load of 70 kg / m2to 200 kg / m2The first layer of conductive fibers is optionally compressed under a load force of 30 to 80 kN / mm2, e.g.60 kN / mm2, to have a compressed non-sintered first layer of conductive fibers.

[0035] The second layer can be first made by tape casting or screen printing, and subsequently transferred onto the surface of the first layer of conductive fibers. The porosity within the second layer has a variation less than 10%, preferably 5%, and more preferably 3% from the nominal value.

[0036] The final compression step to a determined thickness, alone or in combination with the first optional compression step on the first layer of conductive fibers contribute to the alignment of the elongated fibers within and along the plan parallel to the surface of the porous transport layer, so that their average orientation angle (angle BAC, i.e. angle a in Figure 1 ) can be less than 30°, preferably less than 20°. The optional compression step and / or the final compression step can therefore be adapted to improve the mechanical properties and result in the desired plastic / elastic deformation ratio.

[0037] The third aspect of the invention is a method of making a porous transport layer for an electrolyser or for a fuel cell, comprising the steps of:(a) making a first layer of non-sintered conductive fibers, preferably by wet webbing process, wherein said conductive fibers having an average equivalent diameter of less than 100 pm and an aspect ratio of discrete length to diameter of at least 5, preferably at least 10, more preferably at least 50, e.g. between 100 and 1000, and wherein said conductive fibers have a standard deviation between fibers of the equivalent fiber diameter of less than 30% of the equivalent fiber diameter,(b) optionally compressing the first layer of non-sintered conductive fibers to have a compressed non-sintered first layer of conductive fibers,(c) making a second layer of non-sintered conductive particles, preferably by tape casting, wherein the average pore size of the second layer is smaller than the average pore size of the first layer and wherein the porosity within the thickness of said second layer has a variation less than 10% from the nominal value,(d) making a third layer of non-sintered conductive fibers or particles, e.g. by wet webbing or tape casting, wherein the average pore size of the third layer is smaller than the average pore size of the first layer and is bigger that the average pore size of the second layer,(f) depositing the third layer of non-sintered conductive fibers or particles onto the surface of the first layer of conductive fibers to provide an intermediate conductive layer made from the first layer and third layer, wherein the third layer is optionally compressed,(g) depositing the second layer of non-sintered conductive fibers or particles onto the surface of the third layer of the intermediate conductive layer to provide a combined conductive layer made from the first layer, third layer and second layer, wherein the third layer is deposited between the first layer and the second layer,(h) sintering the combined conductive layer,(i) compressing the sintered combined conductive layer to a determined thickness.

[0038] The conductive fibers before sintering step can have an equivalent diameter in the range of 10 to 25 pm and an average discrete length in the range of 50 pm to 200 mm.

[0039] In order to have a durable compressed nonwoven layer, the optional compression can be applied under a load force of 30 to 80 kN / mm2, e.g., 60kN / mm2. Such applied forces can result in a nonwoven fiber layer having a porosity of 30% to 50%. The applied force is selected depending on a desired porosity. This applied force according to the present invention is significantly bigger than the normal force applied for calendaring of fiber web. The fibers are better entangled in the nonwoven layer after the compression.

[0040] As in the previous embodiment, the final compression step to a determined thickness, contributes further to the alignment of the elongated, non-woven fibers within and along the plan parallel to the surface of the porous transport layer, so that their average orientation angle (angle BAC, i.e. angle a in Figure 1) can be less than 30°, preferably less than 20°. The optional compression step and / or the final compression step can therefore be adapted to improve the mechanical properties and result in the desired plastic / elastic deformation ratio.

[0041] The inventive porous transport layer according to the present invention is made by one sintering process. In comparison with a dual-layer porous transport layer that is made by two sintering steps, the inventive poroustransport layer is less expensive since one sintering step is skipped. Surprisingly, the inventive porous transport layer presents a smoother surface and better performance than a similar dual-layer porous transport layer made by two sintering steps.

[0042] A fourth aspect of the invention is a stack for an electrolyser or a fuel cell, comprising a porous transport layer as in the first aspect of the invention, and a bipolar plate and / or a mesh. The bipolar plate or the mesh contacts the first conductive layer. Preferably, the bipolar plate or the mesh is metallurgically bonded to the first conductive layer, e.g., by means of sintering or welding. Preferably, the bipolar plate is flat over its entire surface that is contacting the second nonwoven layer; meaning that no flow fields are provided in the bipolar plate.

[0043] A fifth aspect of the invention is an assembly of a porous transport layer as in the first aspect of the invention and an exchange membrane. The second layer contacts the exchange membrane. Preferably a catalyst is provided on the second layer at the side where the second conductive layer contacts the exchange membrane, or a catalyst is provided on the exchange membrane at the side in contact with the second conductive layer.

[0044] A sixth aspect of the invention is an assembly of a stack as in the fourth aspect of the invention and an exchange membrane. The second conductive layer contacts the exchange membrane. Preferably a catalyst is provided on the second conductive layer at the side where the second conductive layer contacts the exchange membrane, or a catalyst is provided on the exchange membrane at the side in contact with the second conductive layer.Brief Description of the Figures

[0045] Figure 1 shows schematically how an orientation of individual fiber is defined in a cross section of a PTL according to the invention.Figure 2 shows a comparison of porosity distribution of different PTLs along depth according to the invention.Figure 3 shows a pore size distribution of an invention PTL.Figure 4 shows the resistance of an invention PTL compared with a fiber PTL at different pressures.Figure 5 compares compression rate of an invention PTL compared with a fiber PTL and a powder PTL at different pressures.Figure 6 shows a comparison of surface roughness of an invention PTL with a fiber PTL.Figure 7 shows the energy ratio ( ) of invention PTLs compared with a power PTL at different loads.Mode(s) for Carrying Out the Invention

[0046] Two exemplary porous transport layers according to the invention have been tested. Both porous transport layers have a first layer made from conductive Ti fibers and a second layer made from Ti powders. The thickness of the second layer of sample 1 (S1) is about 35 pm while the thickness of the second layer of sample 2 (S2) is about 70 pm. The total thickness of both samples is about 220 pm. The porosity distribution of both samples along the depth of the PTL is illustrated in Figure 2. It can be derived that the porosity of first fiber layer is about 56% and the porosity of the second layer is about 22%. The porosity within the thickness of the second layer has a variation less than 5% from the nominal value (22%). At the interlayer of the first fiber layer and the second powder layer has a porosity drop. The Ti fibers was produced according to the method described in patent WO2014 / 048738A1. Representative sets of the fibers have been tested, the results can be found in Table I.Table I: Dimensions of exemplary masses of Ti fibersaverage equivalent fiber diameter (pm) 22 standard deviation between fibers of the equivalent 6.0 fiber diameter (pm)maximum bisector (pm) 28 minimum bisector (pm) 18 ratio maximum bisector over minimum bisector 1.56percentage of fibers (percentage by numbers of 87.7 fibers) with a ratio maximum bisector over minimum bisector

[0047] The first nonwoven layer of Ti fibers consists of 400 g / m2of Ti fibers with an equivalent diameter 22 pm. The Ti fibers have 10 mm length, and have a cross section, wherein the cross section has two neighbouring straight-lined sides with an included angle of less than 90 degrees and one or more irregularly shaped curved sides. The open pores are larger in the first layer than those in the second layer. The pore size distribution of first layer in sample 1 (S1L1) & sample 2 (S2L1) and the pore size distribution of the second layer in sample 1 (S1L2) & sample 2 (S2L2) are measured. As illustrated in Figure 3, volume percent (V.P. % in y-axis) is shown as a function of local void size (S in x-axis). Both in sample 1 and 2, the open pores of the first fiber layer are bigger and broadly distributed mainly between 15 to 35 pm, while the pores of the second powder layer is smaller and mainly distributed between 4 to 12 pm. The smaller pore size distribution can be translated to a smaller inter-particle distance, which should lead to a better contact with the catalyst layer.

[0048] Such a porous transport layer can be made according to the steps of, preferably in order:(a) making a first layer of non-sintered conductive fibers, preferably by wet webbing process, wherein said conductive fibers having an average equivalent diameter of less than 100 pm and an aspect ratio of discrete length to diameter of at least 5, preferably at least 10, more preferably at least 50, e.g. between 100 and 1000, and wherein said conductive fibers have a standard deviation between fibers of the equivalent fiber diameter of less than 30% of the equivalent fiber diameter,(b) optionally compressing the first layer of conductive fibers to have a compressed non-sintered first layer of conductive fibers,(c) making a second layer of non-sintered conductive particles by tape casting or screen printing, drying the second layer and transfer the dried second layer onto the surface of the first layer of conductive fibers to provide a combined conductive layer made from the first layer and second layer,wherein the average pore size of the second layer is smaller than the average pore size of the first layer, and wherein the porosity within the thickness of said second layer has a variation less than 10% from the nominal value,(d) sintering the combined conductive layer, e.g., in an oxygen-free or low oxygen atmosphere at 900 to 1250 degrees Celsius for 1 to 2 hours, preferably under a load of 70 kg / m2to 200 kg / m2,(f) compressing the sintered combined conductive layer to a determined thickness.

[0049] The exemplary Microporous Transport Layer (MPL) according to the invention are compared with a reference fiber PTL (F). The reference fiber PTL (F) having a similar thickness of the invention porous transport layer is made from titanium fibers with equivalent diameter 22 pm and has a porosity of 56%.

[0050] The resistance of invention MPL and the reference fiber PTL is compared in Figure 4. As illustrated in figure 4, in all the range of applied pressure, the resistance of the microporous transport layer (MPL in figure 4) of the invention is lower than the resistance of the fiber PTL (F in figure 4). The resistance of the invention MPL is significantly reduced compared with the reference fiber porous layer, thanks to the densified second powder top layer.

[0051] The compression rate (CR) of the invention MPL is also compared to the reference fiber PTL (F in Fig.5) and powder PTL (P in Fig.5) in Figure 5. As shown in Figure 5, at the tested pressures, the compression rate of the invention MPL is similar to the compression rate of the fiber PTL F and are significantly lower than the compression rate of the powder PTL. The compression rate of invention MPL is about 50-70% of the compression rate of a powder PTL at a test pressure range of 2-8 Mpa. The fiber layer is made from fibers having a standard deviation between fibers of the equivalent fiber diameter of less than 30% of the equivalent fiber diameter. The resulted fiber layer has a robust structure and excellent mass transport capability. It has been shown by means of an impedance study that increasing the compression of the PTL in a PEM fuel cell leads to an increase of the low frequency resistance. This corresponds to the results for PEM waterelectrolysis that a higher ionomer content in the cathode increases the low frequency resistance in the impedance spectra, which corresponds to a higher mass transport resistance. It shows that the compression of the PTL in PEM water electrolysis can also affect the mass transport resistance and thus the gas permeation through the membrane.

[0052] In addition, the average surface roughness of the invention MPL and reference fiber PTL (F) are measured. The surface roughness of the invention MPL herein refers to the surface roughness on the side of the second layer, i.e. the surface roughness of the second layer. The surface roughness measured according to standard ASME B46.1 is shown in Y-axis (in pm) of Figure 6. The average surface roughness (Ra) is the arithmetic average of the absolute values of the profile height deviations recorded within the evaluation length and measured from the mean line. The average maximum height (Rz) of the profile is the average of successive values of Rzi calculated over the evaluation length: Rz = ( / ?zl + Rz2 + Rz3 + Rz4 + RzSwherein Rzi is maximum height over the evaluation length and n is the number of sampling lengths. As shown in figure 6, the average surface roughness (Ra) of the of the invention MPL (1.9 pm) is significantly decreased compared with the roughness of the reference fiber PTL (10.0 pm). The root mean square roughness (not shown in Figure 6) of the invention MPL (2.2 pm) is also significantly decreased compared with the roughness of the reference fiber PTL (12.5 pm). The maximum height (not shown in Figure 6) of the invention MPL (13 pm) measured is significantly smaller than the maximum height of the reference fiber PTL (80 pm). The average maximum height (Rz) of the invention MPL (9.5 pm) measured is also significantly smaller than the maximum height of the reference fiber PTL (65 pm). This side with lower roughness of the invention MPL is intended to contact with catalyst, while the other side with fiber surface is intended to contact with bipolar plate. Using the invention MPL should be beneficial to prevent the puncture of the membrane.

[0053] The plastic to elastic deformation of invention PTLs is compared with a power PTL under step loading pressure in a range from 2 to 8 MPa as shown in Figure 7. The invention PTLs are represented by sample 1 and sample 2. The thickness of the second layer of sample 1 (S1) is about 35m while the thickness of the second layer of sample 2 (S2) is about 70 pm. The total thickness of both samples is about 220 pm, while the powder PTL (P in Fig.7) has a similar total thickness. According to the loading intensity, the ratios of plastic to elastic deformation ( ) of the PTLs are shown in Figure 7. The of both invention sample 1 (S1 ) and sample 2 (S2) are in the range of 10 to 28% which is lower than the of the powder PTL (P) in the range or 30 to 35%. Moreover, as shown in Figure 7, sample 1 having thicker first layer made from conductive fibers has lower value than sample 2 in all the tested pressure loads. Moreover, the value of invention PTLs decrease as the applied pressure loads increase. To the contrary, thevalue of the powder PTL slightly increase as the applied pressure loads increase. The invention PTLs show more elasticity due to the presence of fiber layer and are thus more robust in the applications.

Claims

Claims1. Porous transport layer for an electrolyser or for a fuel cell, comprising- a first layer made from non-woven conductive fibers, said conductive fibers having an average equivalent diameter of less than 100 pm and an aspect ratio of discrete length to diameter of at least 5, wherein said conductive fibers have a standard deviation between fibers of the equivalent fiber diameter of less than 30% of the equivalent fiber diameter,- a second layer made from conductive particles, wherein said second layer has an average pore size smaller than the average pore size of the first layer, wherein the porosity within the thickness of said second layer has a variation less than 10% from the nominal value,- wherein the first layer is metallurgically bonded to the second layer.

2. Porous transport layer as in claim 1, wherein the ratio of plastic to elastic deformation of said porous transport layer is in a range from 10% to 28% at an applied load from 2 to 8 MPa.

3. Porous transport layer as in claim 2, wherein said non-woven conductive fibers are oriented within the first layer, and wherein the average orientation angle between the length direction of said conductive fibers and the surface of the porous transport layer is less than 30°.

4. Porous transport layer as in any one of the preceding claims, wherein the first layer and the second layer comprise or are made from any one of titanium (Ti), nickel (Ni), stainless-steel (SS), Niobium (Nb), Tantalum (Ta), Zirconium (Zr), Platinum (Pt), Gold (Au), or Iridium (lr).

5. Porous transport layer as in any one of the preceding claims, wherein the cross section of the conductive fibers has two neighbouring straight-lined sides with an included angle of less than 90 degrees and one or more irregularly shaped curved sides.

6. Porous transport layer as in any one of the claims 1 to 4, wherein the cross section of the conductive fibers has near-rectangular shape.

7. Porous transport layer as in any one of the preceding claims, wherein the first layer has a porosity in a range of 40 to 80%, the second layer has a porosity in a range of 10 to 50% and the overall porosity of the porous transport layer is in a range of 30 to 70%.

8. Porous transport layer as in any one of the preceding claims, wherein said porous transport layer further comprise a third layer deposited between the first layer and the second layer, and the third layer has an average pore size smaller than the average pore size of the first layer and bigger than the average pore size of the second layer.

9. Porous transport layer as in any one of the preceding claims, wherein the conductive fibers in the first layer have an equivalent diameter in the range of 10 to 50 pm and an average discrete length in the range of 50 pm to 200 mm.

10. A method of making a porous transport layer for an electrolyser or for a fuel cell, comprising the steps of:(a) making a first layer of non-sintered non-woven conductive fibers, preferably by wet webbing process, wherein said conductive fibers having an average equivalent diameter of less than 100 pm and an aspect ratio of discrete length to diameter of at least 5, and wherein said conductive fibers have a standard deviation between fibers of the equivalent fiber diameter of less than 30% of the equivalent fiber diameter,(b) optionally compressing the first layer of non-woven conductive fibers to have a compressed non-sintered first layer of conductive fibers,(c) making a second layer of non-sintered conductive particles onto the surface of the first layer of conductive fibers to provide a combined conductive layer made from the first layer and second layer, wherein the average pore size of the second layer is smaller than the average pore size of the first layer, and wherein the porosity within the thickness of said second layer has a variation less than 10% from the nominal value,(d) sintering the combined conductive layer,(f) compressing the sintered combined conductive layer to a determined thickness.

11. A method of making a porous transport layer for an electrolyser or for a fuel cell as in claim 10, wherein said non-woven conductive fibers are oriented within the first layer, and the average orientation angle between the length direction of said conductive fibers and the surface of the porous transport layer is less than 30°.

12. A method of making a porous transport layer for an electrolyser or for a fuel cell as in claim 10 or 11 , wherein in step (c) the second layer is first made by tape casting or screen printing, and subsequently transferred onto the surface of the first layer of conductive fibers.

13. A method of making a porous transport layer for an electrolyser or for a fuel cell, comprising the steps of:(a) making a first layer of non-sintered non-woven conductive fibers, preferably by wet webbing process, wherein said conductive fibers having an average equivalent diameter of less than 100 pm and an aspect ratio of discrete length to diameter of at least 5, and wherein said conductive fibers have a standard deviation between fibers of the equivalent fiber diameter of less than 30% of the equivalent fiber diameter,(b) optionally compressing the first layer of non-sintered non-woven conductive fibers to have a compressed non-sintered first layer of conductive fibers, (c) making a second layer of non-sintered conductive particles, preferably by tape casting or screen printing, wherein the average pore size of the second layer is smaller than the average pore size of the first layer, and wherein the porosity within the thickness of said second layer has a variation less than 10% from the nominal value,(d) making a third layer of non-sintered conductive fibers or particles, e.g. by wet webbing or tape casting, wherein the average pore size of the third layer is smaller than the average pore size of the first layer and is bigger that the average pore size of the second layer, wherein the third layer is optionally compressed, (f) depositing the third layer of non-sintered conductive fibers or particles onto the surface of the first layer of conductive fibers to provide an intermediate conductive layer made from the first layer and third layer,(g) depositing the second layer of non-sintered conductive fibers or particles onto the surface of the third layer of the intermediate conductive layer to provide a combined conductive layer made from the first layer, third layer and second layer, wherein the third layer is deposited between the first layer and the second layer,(h) sintering the combined conductive layer,(i) compressing the sintered combined conductive layer to a determined thickness.

14. A method of making a porous transport layer as in any one of claims 10 to 13, wherein the non-woven conductive fibers before sintering step have an equivalent diameter in the range of 10 to 25 pm and an average discrete length in the range of 50 pm to 200 mm.

15. Stack for an electrolyser or a fuel cell, comprising- a porous transport layer as in any one of the preceding claims 1 to 9, and - a bipolar plate and / or a meshwherein the bipolar plate or the mesh contacts the first layer,preferably wherein the bipolar plate or the mesh is metallurgically bonded to the first layer.

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