WO2025228586A1

WO2025228586A1 - A porous transport layer with a substantially flat surface and method for producing the same

Info

Publication number: WO2025228586A1
Application number: PCT/EP2025/058130
Authority: WO
Inventors: Xin Wen; Bastien PENNINCKX; Muhammad Amin Saleem
Original assignee: Smoltek AB
Current assignee: Smoltek AB
Priority date: 2024-04-29
Filing date: 2025-03-25
Publication date: 2025-11-06
Anticipated expiration: 2026-10-29

Abstract

A porous transport layer, PTL, (200) for a water electrolyzer (100). The porous transport layer comprises a porous layer (210), where the porous layer (210) is a porous structure comprising irregular pores (212) and solid sections (213). At least a first surface (211) of the porous layer (210) is formed by a first plurality of solid sections (213). At least some of the solid sections (213) in the first plurality have at least one surface that is substantially flat and arranged facing outwards from the porous layer such that it forms part of the first surface (211).

Description

A POROUS TRANSPORT LAYER WITH A SUBSTANTIALLY FLAT SURFACE AND

METHOD FOR PRODUCING THE SAME

TECHNICAL FIELD

The present disclosure relates to electrodes for devices used in electrolysis, particularly for the electrolysis of water.

BACKGROUND

The production of hydrogen gas through the electrolysis of water is a promising technology both for replacing the production of hydrogen gas from fossil fuels and as a means of converting excess electrical energy from intermittent energy sources such as solar and wind power to chemical energy for storage. Among the existing electrolyzer types, proton exchange membrane water electrolyzers or PEMWE stand out due to their flexibility, rapid start-up, and high current density. However, PEMWE require the use of platinum-group metals as catalysts both on the anode and cathode side. On the anode side in a PEMWE the acidic conditions and high electrostatic potential also lead to a need for highly chemically stable materials.

PEMWE generally contain a porous transport layer, PTL, positioned between the electrocatalyst and the current collector or bipolar plate. This layer should facilitate mass transport of reactants and products as well as electrical contact with the electrocatalyst. Porous metal materials such as titanium felt, and sintered titanium powder, are often used for the anode PTL.

It is increasingly common to deposit the electrocatalyst onto the PTL, forming a porous transport electrode (PTE). With the electrocatalyst positioned on the PTL the morphology of the underlying porous material becomes more important for the contact between the electrocatalyst and the proton exchange membrane.

Attempts have been made to devise anode-side PTL or PTE that improve contact with the membrane, such as the one disclosed in US 2023/0047374 A1.

However, there is still a need for improved porous transport layers and porous transport electrodes. SUMMARY

It is an objective of the present disclosure to provide improved porous transport layers and porous transport electrodes, which, i.a. , offer improved contact between the PTL I PTE and the membrane of the electrolyzer cell. This objective is at least in part obtained by a porous transport layer, PTL, for a water electrolyzer. The porous transport layer comprises a porous layer, where the porous layer is a porous structure comprising irregular pores and solid sections. At least a first surface of the porous layer is formed by a first plurality of solid sections, where at least some of the solid sections in the first plurality of solid sections has at least one surface that is substantially flat. The substantially flat surface is arranged facing outwards from the porous layer such that it forms part of the first surface. That is, the first surface is arranged so that at least some of the solid sections forming the first surface are substantially flat.

In an electrolyzer using an ion exchange membrane, a PTL such as the one described here would be arranged with the first surface facing towards the membrane. Compared to regular PTL, which have highly uneven surfaces with feature sizes of at least tens of microns, the PTL described above will improve the contact between the PTL and the membrane. This is due to the flat surfaces of the solid sections forming the first surface, which enable the membrane to be in close contact with the PTL over a comparatively large area.

The size of a substantially flat surface forming part of the first surface will generally depend on the size of the solid sections. According to some aspects, the substantially flat surfaces may be between 1 and 100 microns in size. According to other aspects, at least some of the flat surfaces may be 5 microns in size, 10 microns in size, 20 microns in size, or more. Here, the size is defined as the length of the longest straight line that can be drawn between two points on the periphery of the flat surface.

PTLs, especially PTL:s for use on the anode side of proton exchange membrane (PEM) water electrolyzers, commonly comprise porous metallic material. The porous layer mentioned above may be such a regular PTL that is treated so as to make the solid sections forming the first surface flat. Thus, the porous layer may be a felt comprising metal fibers. The porous layer may also be formed from a plurality of sintered metal grains. According to some examples, the porous layer may comprise titanium. Advantageously, titanium is known to be durable and to be able to withstand the conditions at the anode of a PEM electrolyzer.

According to some aspects, the PTL may also comprise a plurality of elongated nanostructures arranged on the first surface. The presence of elongated nanostructures increases the surface area of the first surface while still maintaining the advantages of a relatively flat surface mentioned above. This is an advantage as it increases the area of contact between the PTL and the membrane further.

The plurality of elongated nanostructures may comprise elongated carbon nanostructures. Advantageously, carbon nanostructures have good electrical conductivity and are mechanically stable. They can also be grown directly on the porous layer with relative ease.

According to one example, the elongated nanostructures are arranged in a first plurality of regions on the first surface, wherein the first plurality of regions does not cover the entire first surface. Ideally, the regions in the first plurality of regions are between 50 nm and 10 microns in size and are interspersed with surface regions where no nanostructures are present. When the PTL is arranged in an electrolyzer with the first surface, and therefore the plurality of elongated nanostructures, facing the membrane, products and reactants such as water and oxygen gas will need to be transported between the nanostructures to the pores of the porous layer. Having some surface regions that are occupied by nanostructures and some that are not can facilitate this mass transport.

The plurality of elongated nanostructures and/or the first surface may be at least partially covered with a coating arranged to reduce corrosion. Advantageously, this enables the use of nanostructures that would not normally be chemically stable under the conditions at the anode side of a PEM electrolyzer, such as carbon nanostructures.

According to some examples, a catalytically active material is deposited on the first surface. The catalytically active material is preferably an electrocatalyst arranged to promote the oxygen evolution reaction at the PEM water electrolyzer anode. If elongated nanostructures are present on the first surface, the catalytically active material is preferably deposited on the elongated nanostructures. If the elongated nanostructures are covered with a coating arranged to reduce corrosion as mentioned above, the catalytically active material is advantageously deposited on top of the coating so as to be in direct contact with the membrane when the PTL is arranged in an electrolyzer.

The catalytically active material may for example comprise, alone or in combination, any of iridium, ruthenium, iridium oxide, hydrous iridium oxide, and ruthenium oxide. The catalytically active material may also be an alloy of iridium and another metal.

It may be noted that the combination of elongated nanostructures, which provide a large surface area, and a catalytically active material that is arranged on said elongated nanostructures provides a particularly advantageous variant of the present invention. The large surface area provided by the nanostructures forms a catalyst support on which the catalytically active material can be deposited, while the direct contact between the catalytically active material and the membrane serves to promote the chemical reaction at the electrode, e.g. the oxygen evolution reaction. It also makes it possible to reduce the amount of catalytically active material that is used while maintaining the same efficiency.

There is also herein disclosed an electrolyzer comprising a first electrode and a second electrode and a membrane arranged in-between the first and second electrode. Each electrode comprises a catalyst layer, a porous transport layer, and a conductive element. At least one of the porous transport layers is a porous transport layer as described above, arranged so that the first surface faces the membrane. The porous transport layer as described above will contribute to better contact between the PTL and the membrane, which in turn leads to more efficient operation of the electrolyzer.

When assembling an electrolyzer cell using a typical PTL on the anode side, a protonconducting ionomer is frequently mixed in the electrocatalyst layer to ensure sufficient contact with the membrane and to provide a path for protons from the anode electrocatalyst and into the membrane. With a PTL comprising nanostructures as described above, the amount of ionomer required can be reduced as the elongated nanostructures improve the contact between the PTL and the membrane. This is especially the case if the elongated nanostructures are arranged at least partially embedded in the membrane.

There is furthermore herein described a method for producing a porous transport layer, PTL, for a water electrolyzer. The method comprises arranging a porous layer, where the porous layer is a 3D porous structure comprising irregular pores and solid sections. The method further comprises applying a bulk filler onto the porous layer such that it covers a first surface of the porous layer and fills at least some of the pores, and machining the coated first surface such that the solid sections forming the first surface become substantially flat. Also, the method comprises removing the bulk filler.

This method makes it possible to produce a PTL as described above from a standard PTL comprising e.g. sintered titanium grains or titanium fibers, which is an advantage.

The bulk filler provides the possibility to selectively deposit a material, such as a growth catalyst or an electrocatalyst, on only the substantially flat parts of the solid sections forming the first surface. This can also be referred to as the flat areas of the PTL. Depending on the type of bulk filler used, method may comprise performing a solidification treatment on the filler. This has the advantage of making it possible to apply the bulk filler in liquid form and then hardening or solidifying it, which facilitates the application step.

According to an example, the method further comprises depositing a growth catalyst on the first surface and growing elongated nanostructures on the first surface. This creates a PTL with elongated nanostructures on an otherwise substantially flat surface, which has several advantages as described above.

Furthermore, it should be noted that the step of removing the bulk filler may be performed between the steps of depositing a growth catalyst on the first surface and growing elongated nanostructures on the first surface. Since the growth catalyst would be deposited on both the bulk filler and the PTL, and the bulk filler would subsequently be removed, only the growth catalyst on the flat areas of the PTL remains. The elongated nanostructures would thus be grown only on the flat areas of the PTL, which is an advantage particularly in comparison with if the growth catalyst was deposited without the bulk filler. In this case, it would also be deposited inside some of the pores that are open to the first surface. The nanostructures would then be grown also inside the pores, rather than selectively on just the flat areas. Thus, the method disclosed herein has the advantage of reducing parasitic growth of elongated nanostructures on parts of the PTL where they do not contribute to improved contact with the membrane, as well as providing greater predictability with regard to the surface area enhancement provided by the elongated nanostructures.

In addition, it is possible to restrict the nanostructures to only some regions on the flat areas, which is advantageous for mass transport as described above. This can be done according to different methods. According to a first alternative, depositing a growth catalyst on the first surface comprises applying a second layer of filler onto the first surface, and performing selective removal of at least a part of the second layer of filler. The method may further comprise depositing a layer of growth catalyst on the second layer of filler, and removing the remaining parts of the second layer of filler.

According to a second alternative, depositing a growth catalyst on the first surface comprises depositing a layer of growth catalyst on the first surface, applying a second layer of a filler on top of the layer of growth catalyst, and performing selective removal of at least a part of the second layer of filler. The method may further comprise etching the layer of growth catalyst in regions where it is not covered the second layer of filler and removing the remaining parts of the second layer of filler.

According to a third alternative, depositing a growth catalyst on the first surface comprises depositing polymer droplets on the first surface and drying the polymer droplets. The method may further comprise depositing a layer of growth catalyst on the first surface and removing the polymer droplets.

The methods disclosed herein are associated with the same advantages as discussed above in connection to the different apparatuses. There are also disclosed herein computer programs, computer program products, and control units associated with the above-mentioned advantages.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realizes that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will now be described in more detail with reference to the appended drawings, where: Figure 1 schematically illustrates a PEM water electrolyzer;

Figure 2 schematically illustrates a porous transport layer;

Figure 3 schematically illustrates a porous transport layer comprising elongated nanostructures;

Figure 4 is a SEM image of a porous transport layer,

Figures 5A, 5B, and 5C are SEM images of porous transport layers;

Figure 6 is a flow chart illustrating methods; and

Figures 7 A-C are schematic illustrations of a PTL at different steps of a production method.

DETAILED DESCRIPTION

Aspects of the present disclosure will now be described more fully with reference to the accompanying drawings. The different devices and methods disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the aspects set forth herein. Like numbers in the drawings refer to like elements throughout.

The terminology used herein is for describing aspects of the disclosure only and is not intended to limit the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

The following description relates to electrodes for electrochemical cells, with a focus on water electrolyzers using proton exchange membranes. However, a person skilled in the art realizes that the methods and devices described herein are also applicable to other types of water electrolyzers, such as alkaline electrolyzers or electrolyzers comprising anion exchange membranes. The methods and devices are also applicable to electrolyzers producing other compounds, for example ammonia, and to other electrochemical cells such as fuel cells.

Figure 1 schematically illustrates a PEM water electrolyzer 100. The electrolyzer comprises a first electrode 110 and a second electrode 120, with a proton exchange membrane (PEM) 130 positioned in-between the electrodes. Each electrode comprises an electrocatalyst layer 111 , 121 positioned next to the PEM. Each electrode also has a porous transport layer (PTL) 112, 122 arranged on the side of the respective electrocatalyst layer that is facing away from the PEM. Both the first and the second electrode also comprise a respective conductive element 113, 123 which are connected to a power source 140.

During operation, the electrolyzer 100 uses electrical energy to split water into oxygen gas and hydrogen gas. The chemical reactions take place at the electrodes 110, 120. One electrode is functioning as the anode, with a positive charge, and the other functions as the cathode and is negatively charged. In Figure 1 , the first electrode 110 is shown as the cathode and the second electrode 120 as the anode. At the anode electrocatalyst layer, water molecules undergo the oxygen evolution reaction:

2H₂O -> 4H⁺ + O₂ + 4e“.

The electrons produced in the reaction will pass via the porous layer to the conductive elements. The positive hydrogen ions (protons) will diffuse through the PEM 130 and reach the cathode electrocatalyst layer, where they undergo the hydrogen evolution reaction:

4H⁺ + 4e“ 2H₂.

Each electrocatalyst layer 111 , 121 comprises a catalytically active material, i.e. a material or chemical compound that facilitates a chemical reaction by lowering the amount of energy needed to drive the reaction.

Here, the PEM 130 acts as an electrolyte, i.e. as a medium that allows transport of ions, in this case specifically protons. It is also an electrical insulator, which assists in keeping the anode and cathode from coming into electrical contact, as well as minimizing leakage of the produced oxygen and hydrogen gas. PEM:s commonly comprise polymer materials with a polytetrafluoroethylene backbone and side chains comprising any of ether groups, sulfonic acids, and sulfonyl fluoride vinyl ether. Such membranes can for example be found under brand names such as Nation and Aquivion. Membranes may also comprise polymers based on polysulfone or polyphenole oxide.

It may be noted that other types of electrolyzers use other electrolytes. For example, anion exchange membrane electrolyzers use membranes that conduct negative anions, in this case hydroxide ion, rather than protons.

The electrocatalysts comprised in the anode and cathode electrocatalyst layers are selected to facilitate the oxygen reduction reaction and hydrogen evolution reaction respectively. The electrocatalysts should also be chemically stable under the conditions at the respective electrode. On the cathode side of PEM water electrolyzers, platinum-group metals such as platinum or palladium are frequently used due to their high catalytic activity with regard to the hydrogen evolution reaction. On the anode side, it is common to use oxides of platinum-group metals, e.g. iridium oxide, ruthenium oxide or platinum oxide. The chemical conditions at the anode side are more corrosive than at the cathode side due to a combination of an acidic environment and a high electrical potential that is necessary to drive the electrolysis reaction. In addition to high catalytic activity with regard to the oxygen evolution reaction, the anode-side electrocatalyst must therefore also be chemically stable under such conditions.

The electrocatalyst layer may also comprise catalyst supports on which the electrocatalyst material is deposited. As an example, cathode-side electrocatalysts in PEMWE often comprise carbon nanostructures or carbon black, on which an electrocatalyst such as platinum is deposited in the form of nanoparticles. In order for the electrolysis reaction to proceed the electrocatalysts must be in electrical contact with the porous layer and the conductive element, so it is advantageous if the catalyst support is electrically conductive. The catalyst support can then provide an electrical connection between the electrocatalyst and other components such as the porous layer.

Herein, a nanoparticle is a particle with a size smaller than 100 nm. The size may be a diameter, a length, or any other relevant size parameter.

On the anode side, catalyst supports can comprise metals such as titanium or metal oxides such as titanium oxide. Metal oxides may have insufficient electrical conductivity to provide an electrical connection between the electrocatalyst and the porous layer, in which case the electrocatalyst itself is often used to form the electrical connection. This requires a larger amount of electrocatalyst per unit area of the electrolyzer and leads to higher costs. The amount of electrocatalyst per unit area of the electrolyzer membrane is referred to as the catalyst load. The anode-side electrocatalyst can be used without a catalyst support, but this also requires a high catalyst load.

The conductive elements 113, 123 of each electrode 110, 120 can also be known as separator elements, separator plates, or flow plates. If the electrolyzer 100 is part of an electrolyzer stack, that is of several electrolyzer cells arranged in series, a conductive element 113, 123 may serve as the anode-side conductive element for one electrolyzer cell and as the cathode-side conductive element for a neighboring cell. In this case it may be referred to as a bipolar plate.

The conductive elements comprise conductive materials that can withstand the chemical environment in the electrolyzer. For example, a conductive element may be a steel element coated with one or a combination of titanium, tungsten, and zirconium. A conductive element may also comprise a carbon composite material.

Herein, a conductive material, element, or component is a material, element, or component with high electrical conductivity. A high electrical conductivity could be an electrical conductivity normally associated with metallic or semiconducting materials, or an electrical conductivity of more than 100 (Qm)’¹.

The proton exchange membrane 130 and electrodes 110, 120 are most commonly in the form of planar elements, plates, or sheets. The same is true for the component layers of each electrode. Here, a planar element, a layer, a plate, or a sheet is an element that is comparatively large in two spatial dimensions and comparatively thin in the third spatial dimension, with two large surfaces substantially perpendicular to this third spatial dimension. The two large surfaces will generally be referred to as the sides or surfaces of the layer, while the smaller surfaces of the element can be referred to as the edges. The distance between the two large surfaces will be referred to as the thickness.

The proton exchange membrane 130, electrocatalyst layers 111 , 121 , etc. are stacked together in the direction of the smallest dimension of each layer with the large surfaces facing the adjacent layers. Thus, the first electrocatalyst layer 111 is arranged with a first large surface of the electrocatalyst layer facing a first large surface of the proton exchange membrane 130, while the second electrocatalyst layer 121 is arranged with a first large surface facing a second large surface of the proton exchange membrane opposite to the first electrocatalyst layer 111. The porous layers 112, 122 are then arranged with large surfaces facing the second large surface of the respective electrocatalyst layer. The electrolyzer cell is thus formed from a stack of component layers. Examples of this type of electrolyzer cell are well known in the art.

The porous transport layer (PTL) may also be referred to as a mass transport layer, gas diffusion layer (GDL), liquid/gas diffusion layer (LGDL), or diffusion layer. Its function is to allow transport of reactants and products, i.e. water, oxygen gas, and hydrogen gas, to and from the electrocatalyst layers while simultaneously maintaining electrical contact between the electrocatalyst layer and the conductive element that is connected to the power source. This leads to a requirement that the PTL must have both enough conductive material positioned to maintain a galvanic connection throughout the layer, and enough pore space to allow for sufficiently rapid transport of reactants and products. Furthermore, the PTL must be mechanically rigid enough to withstand the pressure in the cell during both operation and production. As electrolyzer cells are frequently manufactured by hot pressing the components together, the pressure during production can amount to 10 MPa or more. The PTL must also be chemically stable in the environment of the electrolyzer.

Particular challenges are associated with PTL:s for the anode side of PEM electrolyzers. As mentioned above with regard to electrocatalysts, the chemical conditions at the anode are harsh and require materials that are resistant to corrosion and oxidation. Also, mass transport of both a liquid and a gas are required at the anode due to the influx of water and production of oxygen gas during the oxygen evolution reaction.

Known electrolyzer cells frequently comprise carbon felt or carbon paper as a PTL on the cathode side, while the anode PTL is more commonly made up of a titanium mesh or porous sintered titanium.

Figure 2 shows a schematic of a porous transport layer, PTL, 200 for a water electrolyzer 100. The porous transport layer comprises a porous layer 210, where the porous layer 210 is a porous structure comprising irregular pores 212 and solid sections 213. At least a first surface 211 of the porous layer 210 is formed by a first plurality of solid sections 213 where at least some of the solid sections 213 in the first plurality of solid sections has at least one surface that is substantially flat. The substantially flat surface is arranged facing outwards from the porous layer such that it forms part of the first surface 211. That is, the first surface 211 is arranged so that at least some of the solid sections 213, forming the first surface 211 are substantially flat.

Optionally, all of the solid sections 213 in the first plurality of solid sections 213 have at least one surface that is substantially flat and arranged as described above. The solid sections 213 forming the first surface 211 are herein considered to be the solid sections 213 that are part of an outer layer of solid sections 213 of the porous layer 210. This does not include solid sections that form the walls of pores open to the first surface 211 but that are not part of the outermost layer of solid sections 213. Put another way, the solid sections 213 forming the first surface are the ones that would be in direct contact with the membrane 130 when the PTL is arranged in an electrolyzer cell.

The porous layer 210 is generally a three-dimensional (3D) porous structure. Herein, a 3D porous structure with irregular pores 212 and solid sections 213 refers to a porous structure wherein both pores and solid sections extend in three dimensions and are not generally arranged at regular intervals. A 3D porous structure can for example be a porous felt material such as carbon fiber felt or metal fiber felt. Preferably, the porous layer 210 is formed from a plurality of sintered metal grains, so that the grains are the solid sections 213. Porous layers formed from sintered metal grains are frequently used as PTLs in PEM water electrolyzers. The porosity of the porous layer, i.e. the ratio of the volume of the pores 212 to the total volume, may be between 30% and 50 %.

The PTL 200 is a planar element, i.e., it has two approximately parallel large surfaces and is narrow in the direction perpendicular to at least one of the large surfaces. Here, the first surface 211 is one of the abovementioned large surfaces of the PTL. In Figure 2, the PTL is shown with the narrow edge facing the viewer, i.e., the PTL is viewed along the first surface 211. This is herein referred to as a side view of the PTL.

In a typical PTL used for a PEM water electrolyzer, the size of solid sections or grains, as well as the size of pores, will generally be on the order of several tens of microns up to around 100 microns. A surface of such a PTL will thus be uneven on the scale of tens of microns. Figure 4 shows a scanning electron microscope (SEM) image of such a PTL, with one of the large surfaces facing the viewer. This type of image is herein occasionally referred to as a top view of the PTL.

In contrast, the surface of the solid sections 213 forming the first surface 211 in the PTL 200 described herein are substantially flat. Substantially flat should not be taken to mean that the surfaces of the solid sections 213 are completely flat, but rather that any remaining surface roughness is at least an order of magnitude smaller than the surface roughness of a typical PTL such as is shown in Figure 4. Preferably, the average roughness of the surfaces of the solid sections 213 is less than one micron. The average roughness of a surface may for example be defined as the average height deviation from a mean line, where the mean line describes the average height of the surface. Definitions of surface roughness can be found in the art. It should be noted that when arranged in an electrolyzer such as the one shown in Figure 1 , the PTL 200 is placed with the first surface facing the membrane 130. The substantially flat solid sections 213 will therefore be in contact with the membrane 130, and the low surface roughness will contribute to better contact between the PTL and the membrane.

In Figure 2, the surfaces of the solid sections 213 forming the first surface 211 are shown as flat areas designated with the reference number 211a. The first surface 211 will also comprise holes between these flat areas, see reference number 211b in Figure 2, where the surface 211 intersects with pores 212. Alternatively, the first surface 211 of the porous layer 210 may be described as a flat surface made up of the top surfaces of a number of solid sections 213, where the flat surface has holes that open towards the pores 212 that intersect with the surface.

Yet another way to describe the first surface 211 is to consider that any surface of a porous layer 210 is made from solid sections 213 arranged in a non-periodic manner. That the solid sections 213 forming the first surface 211 are substantially flat can thus be taken to mean that each solid section has at least one substantially flat surface, and that substantially flat surface is facing outwards from the porous layer 210 in such a way that it forms part of the first surface 211 . Between these solid sections 213 are holes that lead to pores 212 that are open to the first surface 211.

The size of the substantially flat surfaces of the solid sections 213 depend on the sizes of the solid sections themselves. As an example, if the solid sections are sintered metal grains, the size of a flat surface will be the size of a cross-section of one of the grains taken at some point along the grain. Thus, if the grain size is between 10 and 100 microns as mentioned above and seen in Figure 4 and Figure 5A, the flat surfaces of the solid sections may be up to 100 microns in size. Preferably, at least some of the flat surfaces are above 5 microns in size, between 10 and 100 microns in size, or between 20 and 50 microns in size. At least some of the flat surfaces may have a size of above 20 microns.

According to some aspects, a second surface of the porous layer formed by a second plurality of solid sections 213 may also be arranged so that at least some of the solid sections 213 in the second plurality have a flat surface facing outwards from the porous layer such that they form part of the second surface. That is, the second surface may be similar to the first surface 211 in terms of shape, geometry, and surface characteristics such as surface roughness. The second surface will generally be positioned opposite of the first surface. Referring back to the discussion of planar elements above, the first and second surface are the large, opposed surfaces of the porous layer.

The porous layer 210 may comprise titanium. For example, the porous layer 210 may be a titanium felt or a sintered porous titanium plate comprising sintered titanium grains. The porous layer 210 may also comprise a titanium mesh.

The PTL 200 may also comprise a plurality of elongated nanostructures 310 arranged on the first surface 211. In particular, the elongated nanostructures 310 are arranged on the flat areas 211a. Here, a nanostructure refers to a structure that has a size substantially smaller than one micrometer, and preferably between 1 and 100 nm, in at least one dimension. An elongated nanostructure is a nanostructure that is substantially larger in one dimension, such as length or height, in comparison to another dimension such as the width. For example, the length or height may be at least twice as large as the width.

As an example, consider a substantially cylindrical nanostructure characterized by a length and a diameter. This nanostructure may be considered elongated if the length is twice as large as the diameter or more. Similar reasoning may be applied to nanostructures that are substantially conical, frustoconical, rectangular, or of arbitrary shape.

The elongated nanostructures 310 may be of any suitable shape, such as conical, frustoconical, or irregularly shaped. They can also for example be straight, spiraling, branched, wavy, or tilted. Optionally, they can be classified as nanowires, nano-horns, nanotubes, nanowalls, crystalline nanostructures, or amorphous nanostructures. According to some examples, they can also be classifiable as aggregates of smaller nanostructures, such as a bundle of nanotubes.

Preferably, the elongated nanostructures 310 are attached to the first surface 211 , and more specifically to the flat areas 211a. An elongated nanostructure 310 is considered attached to the surface if it is in contact with a point on the surface and remains in contact with that point during operation of the electrolyzer. For example, there may be a chemical bond between the nanostructure 310 and the surface 211. The chemical bond may be a covalent bond.

It is also an advantage to have the elongated nanostructures 310 extend along a direction perpendicular to the first surface 211. That is, to have the elongated nanostructures 310 extend along a normal to the flat area 211a to which they are attached. That the elongated nanostructures 310 extend along the normal vector of a flat area 211a should not be taken to mean that they are completely parallel to the normal of the area. Rather, the elongated nanostructures 310 extend at an angle of 45° or less to the normal of the surface, or preferably at an angle of 30° or less to the normal, or even more preferably at an angle of 10° or less to the normal.

According to some aspects, the elongated nanostructures 310 may be between 1 and 10 pm in height, or preferably around 3 pm. The width of the nanostructures at the base, i.e. at the end closest to the first surface 211 , may be between 30 and 200 nm, or preferably between 50 and 100 nm.

As is briefly discussed above, a typical PTL as shown in Figure 4 presents a microstructured surface towards the membrane of the electrolyzer, with surface features on the order of tens of microns. A PTL 200 as is disclosed herein instead presents a mostly flat surface 211 made up of the flat areas 211a of Figure 2, interspersed with gaps 211 b. A SEM image of such a PTL is shown in Figure 5A. The PTL in Figure 5A is shown with one of the large surfaces facing the viewer. This type of surface facilitates contact between the PTL 200 and the membrane of the electrolyzer, as the surface of the membrane can be arranged in close contact with the flat areas 211a.

With elongated nanostructures 310 arranged on the first surface 211 as shown in Figure 3, the PTL 200 instead presents a nanostructured surface towards the membrane. Due to the small size of the nanostructures 310 compared to the size of grains and pores in the porous layer 210, this preserves the advantage mentioned above of improving contact between the PTL and the membrane. However, it also introduces the further advantage of providing a larger surface area than that of the first surface 211 and flat areas 211a in themselves. That is, the presence of the elongated nanostructures 310 provide a larger surface area of the PTL 200. This is particularly important if the electrocatalyst layer is deposited on the PTL, that is, if it is used as a porous transport electrode (PTE). SEM images of a PTL with elongated nanostructures are shown in Figures 5B and 5C. Figure 5B is a top view, while Figure 5C shows the PTL from the side and in higher magnification.

Note that the elongated nanostructures are here arranged on the first surface 211 of the PTL that is facing outwards from the PTL, and that the elongated nanostructures 310 themselves extend out from the first surface. That is, if the PTL 200 was arranged in an electrolyzer the elongated nanostructures 310 would be extending from the first surface towards the membrane 130.

According to some aspects, elongated nanostructures may also be arranged on a second surface of the porous layer, particularly a second surface that is similar to the first surface in terms of shape and surface roughness as described above.

The plurality of elongated nanostructures 310 may comprise nanostructures of any material suitable for use in an electrolyzer, although the nanostructures should preferably be electrically conductive. The plurality of elongated nanostructures 310 may for example comprise nanostructures formed from metals, metal oxides, semiconductors, or any other suitable material.

According to one example, the plurality of elongated nanostructures 310 comprises elongated carbon nanostructures. Carbon nanostructures are electrically conductive, which is an advantage. The plurality of elongated nanostructures 310 may thus comprise carbon nanotubes, carbon nanofibers, carbon nanowalls, carbon nanohorns, or carbon nanowires. In some cases, an elongated carbon nanostructure can also comprise several smaller nanostructures, such as a bundle of carbon nanotubes or carbon nanofibers positioned closely together that can function as a single elongated nanostructure 310 for the purposes of this invention.

Optionally, the elongated nanostructures 310 are arranged in a first plurality of regions on the first surface 211 , wherein the first plurality of regions does not cover the entire first surface 211. More specifically, the nanostructures may cover only part of the available surface on the flat areas 211a. The regions on which the elongated nanostructures are arranged may be regularly or non-regularly distributed over the first surface 211. The regions on which the elongated nanostructures are arranged may be between 50 nm and 10 microns in size. The size can for example be measured as the length of the longest straight line that can be drawn between two points on the periphery of the region.

An effect of this is to leave gaps 320 among the elongated nanostructure 310, as can be seen in Figure 3. The presence of gaps 320 facilitates mass transport of e.g. water or oxygen gas around the elongated nanostructures 310. Optionally, the regions on which the elongated nanostructures are arranged may be distributed in a periodic pattern. According to an example, a gap 320 could be arranged to be largely rectangular or square in shape and extend from the interior of a flat area 211a to the edge of the flat area 211a. Largely rectangular should herein be taken to mean that the gap 320 has an approximately rectangular shape when seen from above, i.e. in a top view. The width of such a gap may for example be between 1 and 10 microns.

The plurality of elongated nanostructures 310 and/or the first surface 211 may be at least partially covered with a coating arranged to reduce corrosion. The coating may comprise any of platinum, manganese oxide, tantalum oxide, hafnium oxide, antimony oxide, titanium oxide, and niobium oxide.

According to some examples, a catalytically active material may be deposited on the first surface 211. Preferably, the catalytically active material is an electrocatalyst suitable for use in a water electrolyzer anode, i.e., an electrocatalyst that promotes the oxygen evolution reaction. The catalytically active material may for example comprise iridium oxide and/or ruthenium oxide.

For a PTL 200 with a plurality of elongated nanostructures 310 arranged on the first surface 210, the catalytically active material is preferably deposited on top of the elongated nanostructures 310. If a coating arranged to reduce corrosion is present as described above, the catalytically active material should be deposited on top of the coating so that when the PTL 200 arranged in an electrolyzer, the catalytically active material can be in direct contact with the membrane. Optionally, the elongated nanostructures 310 may be embedded in the membrane.

With reference again to Figure 1 , there is also herein disclosed an electrolyzer 100 comprising a first electrode 110 and a second electrode 120 and a membrane 130 arranged in-between the first 110 and second 120 electrode. Each electrode comprises a catalyst layer 111 , 121 , a porous transport layer 112, 122 and a conductive element 113, 123, wherein at least one porous transport layer 112, 122 is a porous transport layer as described above.

With reference also to Figure 2, in the electrolyzer, the porous transport layer is arranged so that the first surface 211 faces the membrane 130.

With reference to Figures 2 and 6, there is also herein disclosed a method for producing a porous transport layer, PTL, 200 for a water electrolyzer. The method comprises arranging S1 a porous layer, where the porous layer 210 is a 3D porous structure comprising irregular pores 212 and solid sections 213. The method further comprises applying S2 a bulk filler onto the porous layer 210 such that it covers a first surface 211 of the porous layer 210 and fills at least some of the pores 212, machining S4 the coated first surface 211 until the solid sections 213 forming the first surface 211 are substantially flat, and removing S6 the bulk filler.

Here, the bulk filler is a material that can be easily applied to the porous layer 210 and that can fill the pores 212, but that can also be easily removed after machining. During application S2 of the bulk filler, it may be in the form of a liquid, gel, or an aqueous or non-aqueous solution. The bulk filler may for example be a wax, a resin, or a photoresist. If a material such as a wax or resin is used, the application step S2 may comprise controlling the temperature of the bulk filler to give it a desired viscosity.

Applying S2 the bulk filler to the porous layer 210 can be performed using any suitable method that allows the bulk filler to fill the pores 212 and cover the first surface. The method may advantageously be adapted to the type of bulk filler that is used.

According to one example, if the bulk filler is in liquid form or in an aqueous or nonaqueous solution, said solution can be drop cast on the first surface 211 and allowed to spread both over the surface and into the pores of the porous layer 210. This method can for example be used if the bulk filler is a photoresist.

According to another example, the porous layer 210 may be dipped into the bulk filler, either partially or completely. This is applicable especially to bulk fillers that are in the form of high-viscosity liquids during the step of applying S2 the bulk filler.

That the bulk filler fills at least some of the pores 212 should be taken to mean that it occupies most of the space in at least some of the pores 212. However, the bulk filler does not have to fill all pores 212. For example, it may fill only the pores 212 that are closer than a cutoff distance to the first surface 211. The cutoff distance may be approximately half of the thickness of the porous layer 210.

Machining S4 the coated first surface can be performed using any method suitable for producing a flat surface. For example, machining S4 the coated first surface may comprise grinding and polishing the surface, using chemical mechanical polishing or electrochemical grinding, or cutting the porous layer 210 and the bulk filler using methods such as laser cutting and water cutting. Advantageously, both chemical mechanical polishing and methods involving cutting the porous layer 210 can be scaled to large production volumes. Removing S6 the bulk filler may for example be accomplished using a solvent. The solvent should be selected for material compatibility with the porous layer 210, that is, it should be a solvent that will do minimal damage to the porous layer 210 while removing most or all of the bulk filler. If any other material is applied to the first surface 211 before removal of the bulk filler, as discussed below, the solvent must also be selected so as to be compatible with this other material.

According to some examples, removal S6 of the bulk filler may also be accomplished using a high-temperature treatment arranged to burn away the bulk filler.

The method may also comprise performing S3 a solidification treatment on the bulk filler. Here, a solidification treatment is intended to refer to a treatment that e.g. turns a liquid filler solid or semi-solid. The solidification treatment may comprise a heat treatment. For example, in the case where the bulk filler is applied in the form of an aqueous or non-aqueous solution, it may subsequently be heated in order to dry the bulk filler and remove the solvent. Rendering the bulk filler more solid facilitates the machining S4 of the coated first surface 211.

Figures 7 A-C show a porous layer 210 at different stages of the method described above. As is the case for Figures 2 and 3 discussed above, the porous layer 210 is shown with the narrowest dimension facing the viewer. In Figure 7A, the bulk filler 710 has been applied and fills the voids 212 as well as covering the first surface 211. Figure 7B shows the porous layer 210 after machining S3 of the first surface 211 , with the grains 213 that make up the first surface 211 being substantially flat. 7C shows the porous layer 210 1 PTL 200 after removal of the bulk filler 710.

According to an example, the method also comprises depositing S5 a growth catalyst on the first surface 211 and growing S7 elongated nanostructures 310 on the first surface 211.

A growth catalyst is a catalytically active substance that promotes one or more chemical reactions comprised in the formation of nanostructures. The growth catalyst may for example comprise materials such as nickel, iron, platinum, palladium, nickelsilicide, cobalt, molybdenum, gold, or alloys thereof.

Deposition of the growth catalyst can be carried out using methods such as evaporating, plating, sputtering, molecular beam epitaxy, pulsed laser deposition, chemical vapor deposition, spin-coating, spray-coating, or any other suitable method. According to some aspects, the growth catalyst can be deposited in the form of a plurality of particles, which may be nanoparticles. According to other aspects, the growth catalyst may be deposited in the form of a thin layer or film. The layer of growth catalyst may for example be between 1 and 100 nm thick. It is also possible to limit the growth catalyst to selected areas on the first surface, as will be discussed below.

Growing S7 elongated nanostructures 310 on the first surface 211 generally entails growing the elongated nanostructures with the aid of the growth catalyst. The growth process may for example involve chemical vapor deposition, particularly plasma- enhanced chemical vapor deposition. Methods for growing elongated nanostructures by chemical vapor deposition are known in the art.

Advantageously, if removal S6 of the bulk filler is performed between the steps of depositing S5 the growth catalyst layer and growing S7 elongated nanostructures on the first surface 211 , the nanostructures will only be grown on the substantially flat surfaces of the grains 213 that make up the first surface 211. That is, the elongated nanostructures will be grown on the flat areas denoted by 211a in Figure 2 and not on any of the grains 213 below the surface 211 that are exposed via the gaps 211b.

In addition to only growing elongated nanostructures on the flat areas 211a, it may be desirable to grow the elongated nanostructures only in some regions of the flat areas 211a, so that the elongated nanostructures do not cover the entire first surface 211. This can be accomplished either by selective depositing of the growth catalyst, or by selectively removing some of the growth catalyst layer after deposition.

One alternative is to have the step of depositing S5 a growth catalyst on the first surface 211 comprise applying S511 a second layer of filler onto the first surface 211 and performing S512 selective removal of at least a part of the second layer of filler. The method also includes depositing S513 a layer of growth catalyst on the second layer of filler and removing S514 the remaining parts of the second layer of filler.

This results in growth catalyst remaining on the parts of the first surface 211 where the second layer of filler was selectively removed S512. Elongated nanostructures can subsequently be grown on these parts of the surface only.

A second alternative is that depositing S5 a growth catalyst on the first surface 211 comprises depositing S521 a layer of growth catalyst on the first surface 211 , applying

5522 a second layer of a filler on top of the layer of growth catalyst, and performing

5523 selective removal of at least a part of the second layer of filler. The method also comprises etching S524 the layer of growth catalyst in regions where it is not covered the second layer of filler, and removing S525 the remaining parts of the second layer of filler.

According to this alternative, the growth catalyst will remain on the parts of the first surface 211 where the second layer of filler is not removed, the growth catalyst on these parts of the surface will not be etched away.

For both the abovementioned alternatives, the selective removal of the second layer of filler can be accomplished by methods such as photolithography. This, however, requires that the second layer of filler comprise a material that can act as a photoresist and be patterned by means of illumination e.g. with UV light.

According to a third alternative, depositing S5 a growth catalyst on the first surface 211 comprises depositing S531 polymer droplets on the first surface 211 and drying S532 the polymer droplets. The method further comprises depositing S533 a layer of growth catalyst on the first surface 211 , and removing S534 the polymer droplets.

Here, depositing S531 polymer droplets on the first surface 211 may for example be accomplished using electrospraying. The polymer droplets may for example comprise polyvinylpyrrolidone (PVP).

An example implementation of the methods described above is as follows. A porous layer 210 in the form of porous sintered titanium is provided S1 . A bulk filler is applied S2 in the form of a photoresist which is drop cast onto the surface. Subsequently, a solidification treatment is performed S3 by drying the photoresist at a temperature of 110 °C. Machining S4 the coated first surface 211 is performed by grinding the surface with sandpaper. A growth catalyst is subsequently deposited S5 on the surface.

The bulk filler is removed S6 by means of a suitable solvent. The PTL is subsequently washed with deionized water and dried. Elongated nanostructures such as carbon nanofibers can then be grown on the first surface 211.

Claims

1. A porous transport layer, PTL, (200) for a water electrolyzer (100), the porous transport layer comprising a porous layer (210), the porous layer (210) being a porous structure comprising irregular pores (212) and solid sections (213), wherein at least a first surface (211) of the porous layer (210) is formed by a first plurality of solid sections (213), at least some of the solid sections (213) in the first plurality having at least one surface that is substantially flat and facing outwards from the porous layer such that it forms part of the first surface (211), wherein the at least one substantially flat surface has a size of at least 5 microns, and where the PTL (200) comprises a plurality of elongated nanostructures (310) arranged on the first surface (211).

2. The PTL (200) according to claim 1 , wherein the porous layer (210) is formed from a plurality of sintered metal grains.

3. The PTL (200) according to claim 1 or 2, wherein the porous layer (210) comprises titanium.

4. The PTL (200) according to any previous claim, wherein the plurality of elongated nanostructures (310) comprises elongated carbon nanostructures.

5. The PTL (200) according to any previous claim, wherein the elongated nanostructures (310) are arranged in a first plurality of regions on the first surface (211), wherein the first plurality of regions does not cover the entire first surface (211).

6. The PTL (200) according to any previous claim, wherein the plurality of elongated nanostructures (310) and/or the first surface (211) are at least partially covered with a coating arranged to reduce corrosion.

7. The PTL (200) according to any previous claim, where a catalytically active material is deposited on the first surface (211).

8. An electrolyzer (100) comprising a first electrode (110) and a second electrode (120) and a membrane (130) arranged in-between the first (110) and second (120) electrode, each electrode comprising a catalyst layer (111 , 121), a porous transport layer (112, 122) and a conductive element (113, 123), wherein at least one porous transport layer (112, 122) is a porous transport layer according to any of claims 1 to 7.

9. A method for producing a porous transport layer, PTL, (200) for a water electrolyzer (100), the method comprising arranging (S1) a porous layer, the porous layer (210) being a 3D porous structure comprising irregular pores (212) and solid sections (213), applying (S2) a bulk filler onto the porous layer (210) such that it covers a first surface (211) of the porous layer (210) and fills at least some of the pores (212), machining (S4) the coated first surface (211) until the solid sections (213) forming the first surface (211) are substantially flat, with the flat surface of at least one solid section (213) having a size of at least 5 microns, depositing (S5) a growth catalyst on the first surface (211), removing (S6) the bulk filler, and growing (S7) elongated nanostructures (310) on the first surface (211).

10. The method according to claim 9, further comprising performing (S3) a solidification treatment on the filler.

11 . The method according to claim 9, wherein depositing (S5) a growth catalyst on the first surface (211) comprises applying (S511) a second layer of filler onto the first surface (211), performing (S512) selective removal of at least a part of the second layer of filler, depositing (S513) a layer of growth catalyst on the second layer of filler, and removing (S514) the remaining parts of the second layer of filler.

12. The method according to claim 9, wherein depositing (S5) a growth catalyst on the first surface (211) comprises: depositing (S521) a layer of growth catalyst on the first surface (211), applying (S522) a second layer of a filler on top of the layer of growth catalyst, performing (S523) selective removal of at least a part of the second layer of filler, etching (S524) the layer of growth catalyst in regions where it is not covered the second layer of filler, and removing (S525) the remaining parts of the second layer of filler.

13. The method according to claim 9, wherein depositing (S5) a growth catalyst on the first surface (211) comprises: depositing (S531) polymer droplets on the first surface (211), drying (S532) the polymer droplets, depositing (S533) a layer of growth catalyst on the first surface (211), and removing (S534) the polymer droplets.