Electrolyzer with nanostructure-supported catalyst

Abstract

The electrolytic cell (100, 400) comprises first and second electrodes and an ion exchange membrane (130, 430) disposed between the first and second electrodes. Each electrode comprises a conductive element (113, 123, 413, 423) and a catalyst layer (111, 121), at least one of the catalyst layers comprising a catalyst structure (200). The catalyst structure comprises a plurality of elongated nanostructures (221) and a plurality of electrocatalyst particles (222) attached to the plurality of elongated nanostructures (221), the plurality of elongated nanostructures (221) being arranged to control the position of the plurality of electrocatalyst particles (222) relative to the ion exchange membrane (130, 430).

Description

[Technical field]

[0001] The present disclosure relates to devices used in electrolysis, particularly for the electrolysis of water. [Background technology]

[0002] The production of hydrogen gas by electrolysis of water is a promising technology, both as an alternative to 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 into chemical energy for storage. However, existing water electrolysers suffer from problems related to corrosive conditions in the electrolysis cell and the use of expensive catalysts. In the case of electrolysis cells with ion exchange membranes, it may be necessary to use catalysts containing, for example, platinum or iridium, which entails significant costs. In addition, current electrolysis cells have limitations regarding the ion current per unit area flowing through the cell. Improvements in this regard would result in increased production capacity.

[0003] Patent Document 1 discloses a water electrolysis cell that includes platinum or platinum oxide as a catalyst for the electrolysis reaction.

[0004] There remains a need for improved water electrolysis cells. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2018 / 185617 Summary of the Invention [Problem to be solved by the invention]

[0006] It is an object of the present disclosure to provide an improved water electrolyser, inter alia, with improved electrocatalyst utilization efficiency. [Means for solving the problem]

[0007] This object is achieved, at least in part, by an electrolytic cell comprising first and second electrodes and an ion exchange membrane disposed between the first and second electrodes, each electrode comprising a conductive element and a catalyst layer, at least one of the catalyst layers comprising a catalyst structure comprising a plurality of elongated nanostructures and a plurality of electrocatalyst particles attached to the plurality of elongated nanostructures, the plurality of elongated nanostructures positioned to control the position of the plurality of electrocatalyst particles relative to the ion exchange membrane.

[0008] For efficient operation of an electrolyzer cell with an ion exchange membrane, it is important to ensure good contact between the electrocatalyst particles and the ion exchange membrane, since ions such as hydrogen or hydroxide ions must be able to move between the catalyst particles and the membrane. Controlling the position of the electrocatalyst particles allows for improved contact between the electrocatalyst particles and the ion exchange membrane, which is an advantage.

[0009] The elongated nanostructures may extend approximately along respective axes that are oriented parallel to one another and extend perpendicular to the conductive elements. One advantage of elongated nanostructures extending along axes that are parallel to one another and substantially perpendicular to the conductive elements is that they form a catalyst support structure that facilitates transport of reactants and products of a chemical reaction and allows for efficient use of available electrocatalyst particles.

[0010] This means that the electrocatalyst particles attached to the elongated nanostructures form a layer of catalyst particles that can all be positioned adjacent to the ion exchange membrane at the same time. In this case, a larger percentage of catalyst particles will be adjacent to the ion exchange membrane at the same time compared to configurations with non-parallel oriented axes, which leads to improved efficiency without the need to use more catalyst particles. Alternatively, a smaller amount of catalyst particles can be used without a loss in efficiency.

[0011] The elongated nanostructures may include carbon nanostructures. Advantageously, carbon nanostructures have good electrical conductivity and structural stability, thus providing both control over the position of the catalyst particles and good electrical connection between the electrocatalyst particles and other components of the electrolytic cell, such as porous transport layers or conductive elements. This is beneficial for efficient electrolytic cell operation. By way of example, the elongated carbon nanostructures may include any of carbon nanofibers, carbon nanotubes, carbon nanosheets, and / or carbon nanowires.

[0012] Carbon nanofibers, and also carbon nanotubes and nanowires, have the advantage of being easy to grow on a wide variety of substrates.

[0013] In particular, for carbon nanofibers, the shape and surface structure can be altered by adjusting the conditions under which the nanofibers are grown. This allows the creation of multiple nanofibers arranged in a particularly suitable configuration. Carbon nanofibers are also structurally and chemically robust.

[0014] The process of growing carbon nanostructures may involve the use of a growth catalyst to promote the formation of carbon nanostructures. Advantageously, the growth catalyst used to grow the carbon nanofibers may comprise the same material as the catalyst particles used to promote the chemical reactions involved in the electrolysis process. The ability to use the same material as the growth catalyst and the electrolysis catalyst particles may make the manufacture of the electrolysis cell simpler and more efficient.

[0015] To prevent degradation of the elongated nanostructures, at least one section of the elongated nanostructures may be covered with a protective coating arranged to increase resistance to corrosion. The chemical environment at the electrodes of the electrolytic cell is corrosive, especially on the anode side of the ion exchange membrane. Coating the elongated nanostructures with a corrosion-resistant material makes it possible to prevent or slow degradation of the catalytic structure and to extend the life of the electrolytic cell, which is an advantage. The protective coating may include any of platinum, iridium, titanium, and titanium nitride, or a combination thereof.

[0016] According to embodiments, elongated nanostructures may be grown on a first substrate and transferred to a second substrate equipped with components of an electrolytic cell, such as conductive elements or a porous transport layer. Thus, elongated nanostructures may be grown on a substrate arranged for nanostructure growth and then transferred to a substrate arranged for use in an electrolytic cell. This means that the second substrate does not have to be suitable for nanostructure growth, which is an advantage.

[0017] According to another aspect, the elongated nanostructures may be grown on a substrate comprising components of the electrolytic cell such as conductive elements or porous transport layers, advantageously resulting in good adhesion between the nanostructures and said electrolytic cell components.

[0018] The substrate may comprise a structured surface and the elongated nanostructures may be grown on the structured surface. That is, the nanostructures may be grown on a patterned or roughened surface, such as a patterned flow plate or a porous diffusion layer, to improve electrical contact between components of the electrolytic cell. The surface of the ion exchange membrane may be arranged to follow the contours of the structured surface to ensure intimate contact between the catalytic structures and the ion exchange membrane.

[0019] According to an embodiment, at least one of the elongated nanostructures may be positioned to extend at least partially into the ion exchange membrane.

[0020] Advantageously, having the elongated nanostructures extend into the ion exchange membrane improves contact between the ion exchange membrane and the electrocatalyst particles, thus improving ion transport to and from the surface of the electrocatalyst particles, making the catalyst structure and electrolytic cell more efficient.

[0021] The at least one electrocatalyst particle may be fixed to a first section of the at least one elongated nanostructure, where the first section of the at least one elongated nanostructure may extend at least into the ion exchange membrane, and preferably the first section of the at least one elongated nanostructure is located at an end of the at least one elongated nanostructure opposite the conductive element.

[0022] That is, electrocatalyst particles may be attached to the apical sections of the elongated nanostructures, which are then embedded in the ion exchange membrane, allowing for efficient use of the available electrocatalyst particles as they are placed in good contact with the ion exchange membrane, while the portions of the elongated nanostructures that are not embedded may facilitate water and / or gas transport to and from the electrocatalyst particles.

[0023] The elongated nanostructures may not all be the same length, or they may be substantially the same length but grown on an uneven substrate such as a porous metal or carbon material, which leads to the tips of the elongated nanostructures being at different distances from the conductive element. Thus, the distance from the conductive element to the end of the elongated nanostructure opposite the conductive element may vary between the elongated nanostructures. Elongated nanostructures with a larger distance may extend further into the ion exchange membrane than elongated nanostructures with a smaller distance, such that all elongated nanostructures extend at least partially into the ion exchange membrane. This ensures that the electrocatalyst particles on all elongated nanostructures may be located close to the membrane, which is an advantage.

[0024] According to one example, the first section may comprise more than 90% of the at least one longitudinal nanostructure. According to another example, the first section may comprise less than half of the at least one longitudinal nanostructure. According to a third example, the first section may comprise between 50% and 90% of the at least one longitudinal nanostructure.

[0025] The electrocatalyst particles can be attached to the elongated nanostructures using deposition techniques that are indirect or direct physicochemical and / or physical methods, examples of which include electrolytic plating, electroless plating, sputtering, spray coating, dip coating, and / or solvent casting.

[0026] At least one electrode may include a porous transport layer disposed between the conductive element and the catalytic structure. The porous transport layer includes a porous material. The porous transport layer may improve water and / or gas transport to and from the catalytic structure, which is an advantage.

[0027] The object is also achieved, at least in part, by an electrolytic cell comprising first and second electrodes and an ion exchange membrane disposed between the first and second electrodes, each electrode comprising a conductive element and at least one of the electrodes comprising a catalytic structure comprising a plurality of elongated nanostructures arranged to connect the conductive element with a plurality of electrocatalyst particles, each electrocatalyst particle being localized at an end of a respective elongated nanostructure opposite the conductive element.

[0028] Advantageously, each catalyst particle localized at the end of its respective elongated nanostructure allows for improved control over the position of the catalyst particle relative to the ion exchange membrane and other components of the electrolytic cell, thereby enabling more efficient operation. By way of example, the catalyst particles need to be in close proximity to the ion exchange membrane and to present a large surface area in order to efficiently promote the chemical reactions involved in the electrolysis process. With improved control over the position of the catalyst particles, both the distance to the ion exchange membrane and the exposed surface area can be adjusted for improved performance.

[0029] Advantageously, by ensuring that a greater proportion of the catalyst particles are located closer to the ion exchange membrane, the total number of catalyst particles can be reduced compared to electrolysers in which the catalyst structure does not provide the same degree of control over the location of the catalyst particles, i.e., the electrolysers disclosed herein can use a smaller amount of catalyst particles to achieve the same efficiency as previously known electrolysers.

[0030] According to one example, the electrocatalyst particles may be located less than 10 nm from the ion exchange membrane, preferably less than 5 nm from the ion exchange membrane. According to another example, the electrocatalyst particles may be located in contact with the ion exchange membrane or within 0.1-1 nm from the ion exchange membrane. Advantageously, by locating the catalyst particles in close proximity to the ion exchange membrane, i.e. less than 10 nm, preferably closer than this, hydrogen or hydroxide ions generated during the reaction can more easily enter the ion exchange membrane, allowing for more efficient use of the catalyst particles. Also, hydrogen or hydroxide ions exiting the ion exchange membrane can more easily be adsorbed on the catalyst particles, which is important for the electrolysis reaction.

[0031] At least one of the elongated nanostructures may be a branched nanostructure with a trunk and at least two branches, with the electrocatalyst particles being localized at the ends of each branch. Advantageously, the branched nanostructure allows for an increased number of catalyst particles per unit area of ​​the ion exchange membrane, thereby increasing the catalytic surface area on which the electrolytic reaction can take place. The catalyst structure may also comprise a porous carbon material.

[0032] The object is further achieved, at least in part, by a method of manufacturing a catalytic structure for an electrolytic cell, the electrolytic cell comprising first and second electrodes and an ion exchange membrane disposed between the first and second electrodes, each electrode comprising a conductive element, the method comprising: creating a plurality of elongated nanostructures connected to the conductive elements of the first or second electrode, and attaching a plurality of electrocatalyst particles to the plurality of elongated nanostructures such that each electrocatalyst particle is located at an end of the respective elongated nanostructure opposite the conductive element of the first or second electrode.

[0033] Advantageously, the elongated nanostructures connected to the conductive elements ensure good electrical contact between the conductive elements and the catalyst particles, which is important for efficient electrolyzer operation. The localization of each catalyst particle at the end of a respective elongated nanostructure allows for control of the position of the catalyst particle, for example relative to the ion exchange membrane.

[0034] Creating a plurality of elongated nanostructures may include growing the elongated nanostructures on a substrate, such as one of the conductive elements of the first or second electrode, a porous transport layer, or some other substrate. Growing elongated nanostructures on a substrate provides the advantage that the properties and shape of the nanostructures can be tailored by adjusting the conditions under which the nanostructures are grown to improve the performance of the resulting catalytic structure. As an example, tailoring the thickness of the elongated nanostructures can improve the structural stability. As another example, the surface of the nanostructures can be modified to include structures such as ridges and grooves, which can increase the total surface area and provide more sites to which catalytic particles can be attached.

[0035] Growing elongated nanostructures on a substrate may include depositing a growth catalyst layer on a surface of the substrate and growing the elongated nanostructures on the growth catalyst layer. The growth catalyst layer promotes the growth of the elongated nanostructures. The properties of the grown elongated nanostructures can be tuned by varying the properties of the growth catalyst layer to improve the performance of the resulting catalyst structures.

[0036] According to an embodiment, depositing the growth catalyst layer may include depositing a uniform growth catalyst layer and introducing a pattern onto the deposited uniform growth catalyst layer. An advantage of introducing a pattern onto the deposited uniform growth catalyst layer is that it allows for control of the nanostructure density per unit surface area on the substrate. The nanostructure density per unit surface area may affect the flow of fluids, such as water, oxygen gas, and hydrogen gas, to and from the catalyst particles. It may also affect the catalyst particle density per unit surface area of ​​the membrane. Both the flow and the catalyst particle density strongly affect the efficiency of the electrolytic cell. Thus, controlling the nanostructure density per unit surface area of ​​the substrate allows for improved efficiency of the electrolytic cell.

[0037] Growing elongated nanostructures on a substrate may also involve depositing a conductive layer on the surface of the substrate. Advantageously, depositing a conductive layer on the surface of the substrate has the effect of electrically grounding the substrate. Electrically grounding the substrate may be advantageous in certain methods of growing nanostructures. When a conductive layer is deposited on the surface of the substrate and a growth catalyst layer is deposited on the conductive layer, the conductive layer may also inhibit the diffusion of atoms and / or molecules between the growth catalyst layer and the substrate.

[0038] Also disclosed herein is a method for producing a catalyst structure for an electrolytic cell. The electrolytic cell comprises a first and a second electrode and an ion exchange membrane disposed between the first and the second electrodes. Each electrode comprises an electrically conductive element. The method includes constructing a substrate having a surface. The substrate may be one of the electrically conductive elements of the first or second electrode, a porous transport layer, or any other substrate. The method further includes selecting a growth catalyst for growing elongated nanostructures on the substrate such that the growth catalyst can also be used as an electrocatalyst in the electrolytic cell, and depositing a growth catalyst layer including the selected growth catalyst on the surface of the substrate. The method also includes growing elongated nanostructures on the growth catalyst layer to create elongated nanostructures having catalyst particles localized at an end of each elongated nanostructure suitable for use in an electrolytic cell.

[0039] In addition to the above-mentioned advantages associated with growing elongated nanostructures on a substrate using a catalyst layer, this method offers the additional advantage of using a growth catalyst that can also be used as an electrocatalyst in an electrolytic cell, allowing for a simpler and more efficient method of producing the catalytic structures.

[0040] According to an embodiment, depositing the growth catalyst layer may include depositing a uniform growth catalyst layer and introducing a pattern onto the deposited uniform growth catalyst layer. An advantage of introducing a pattern onto the deposited uniform growth catalyst layer is that it allows for control of the nanostructure density per unit surface area on the substrate. The nanostructure density per unit surface area may affect the flow of fluids, such as water, oxygen gas, and hydrogen gas, to and from the catalyst particles. It may also affect the catalyst particle density per unit surface area of ​​the membrane. Both the flow and the catalyst particle density strongly affect the efficiency of the electrolytic cell. Thus, controlling the nanostructure density per unit surface area of ​​the substrate allows for improved efficiency of the electrolytic cell.

[0041] According to another embodiment, depositing a growth catalyst layer on the face of the substrate may include depositing an electrically conductive layer on the face of the substrate. Advantageously, depositing a conductive layer on the face of the substrate can have the effect of electrically grounding the substrate. Electrically grounding the substrate can be advantageous in certain methods of growing nanostructures. When a conductive layer is deposited on the face of the substrate and a growth catalyst layer is deposited on the conductive layer, the conductive layer can also inhibit the diffusion of atoms and / or molecules between the catalyst layer and the substrate.

[0042] Further disclosed herein is a method of making a catalytic structure for an electrolytic cell, the electrolytic cell comprising first and second electrodes and an ion exchange membrane disposed between the first and second electrodes, each electrode comprising a conductive element, the method including generating a plurality of elongated nanostructures, attaching a plurality of catalytic particles to the plurality of elongated nanostructures, and positioning the plurality of elongated nanostructures to control the position of the plurality of electrocatalyst particles relative to the ion exchange membrane.

[0043] The method disclosed herein entails the same advantages as discussed above in relation to different devices. In general, all terms used in the claims should be interpreted according to their ordinary meaning in the art, unless expressly stated otherwise herein. All references to "a / an / the element, apparatus, component, means, step, etc." should be interpreted in an open-ended manner to mean at least one example of the element, apparatus, component, means, step, etc., unless expressly stated otherwise. The steps of any method disclosed herein need not be performed in the exact order disclosed, unless expressly stated otherwise. Further features of the present invention and advantages thereof will become apparent upon examination of the appended claims and the following description. It will be appreciated by those skilled in the art that different features of the present invention can be combined to create embodiments other than those described below, without departing from the scope of the present invention.

[0044] The present disclosure will now be described in more detail with reference to the accompanying drawings. [Brief description of the drawings]

[0045] [Figure 1] FIG. 1 is a schematic diagram of a water electrolysis cell. [Figure 2A] FIG. 2A is a schematic diagram of elongated nanostructures on a substrate. [Figure 2B] FIG. 2B is a schematic diagram of elongated nanostructures on a substrate. [Figure 2C] FIG. 2C is a schematic diagram of elongated nanostructures on a substrate. [Diagram 3] FIG. 3 is a schematic diagram of a catalyst structure. [Figure 4] FIG. 4 is a schematic diagram of a water electrolysis cell. [Figure 5A] FIG. 5A is a flow chart illustrating the method. [Figure 5B] FIG. 5B is a flow chart illustrating the method. [Figure 5C]FIG. 5C is a flow chart illustrating the method. [Figure 6A] FIG. 6A is an electron microscope image of carbon nanofibers. [Figure 6B] FIG. 6B is an electron microscope image of the carbon nanofibers. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] Aspects of the present disclosure will now be described more fully with reference to the accompanying drawings. However, the different devices and methods disclosed herein may be embodied in many different forms and should not be construed as being limited to the aspects shown herein. Like reference numerals in the drawings refer to like elements throughout.

[0047] The terminology used herein is merely for the purpose of describing aspects of the present disclosure and is not intended to limit the present invention. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms unless the context clearly indicates otherwise.

[0048] Although the following description focuses on electrolytic cells suitable for the electrolysis of water, those skilled in the art will recognize that the devices and methods described herein can also be used for the electrolysis of other liquids or gases, so long as the reduction and oxidation reactions involved in the electrolysis process occur on the surface of a catalyst and the electrodes are separated by a solid electrolyte.

[0049] An electrolytic cell contains two electrodes, one positively charged anode and one negatively charged cathode, and a medium allowing the movement of ions, known as the electrolyte. The electrodes are connected to a power source that provides the electrical energy that drives the electrolytic reaction.

[0050] Some electrolyzers use a solid electrolyte or ion exchange membrane as the ion transport medium. An ion exchange membrane is a solid material through which ions can pass. This material is also known as an ion conductor because it conducts ions. The use of ion exchange membranes allows for a compact electrolyzer design as well as good separation of oxygen and hydrogen gases, which is an advantage.

[0051] Ion exchange membranes used in electrolyzers can be classified according to the ionic species that pass through the membrane. Anion exchange membranes, AEMs, conduct negative ions, in this case hydroxide ions, from the cathode to the anode. Proton exchange membranes, PEMs, conduct hydrogen cations or protons from the anode to the cathode. Both anion and proton exchange membranes are permeable to water, but may minimize the amount of hydrogen and / or oxygen gas that moves between the electrodes.

[0052] In electrolyzers with ion exchange membranes, each electrode comprises an electrically conductive element connected to a power source, and a catalyst layer containing an electrocatalyst that facilitates the chemical reactions involved in the electrolysis process. Most electrolyzer electrodes also comprise a porous transport layer disposed between the electrically conductive element and the catalyst layer, which facilitates the transport of products and reactants to and from the catalyst.

[0053] As used herein, a conductive element is an element that has a high electrical conductivity. High electrical conductivity is defined as the electrical conductivity typically associated with metallic or semiconducting materials, or greater than 100 (Ωm). -1 The electrical conductivity of the material may be as high as 1000 .mu.m.

[0054] A catalyst is a material or chemical compound that facilitates a chemical reaction, for example by lowering the amount of energy required to initiate the chemical reaction. An electrocatalyst facilitates the chemical reactions involved in an electrolysis process, such as the reactions occurring on the anode side or the reactions occurring on the cathode side. The anode and cathode side electrocatalysts often comprise different materials.

[0055] During the electrolysis process, water can be introduced into the electrolyser through the side of the ion exchange membrane where the anode is located. In the case of a PEM electrolyser, water molecules that come into contact with the electrocatalyst on the anode side undergo an oxygen evolution reaction. 2H 2 O→4H + +O 2 +4e -

[0056] The electrons enter a circuit connecting the two electrodes, oxygen gas is pumped out of the electrolyzer, and the protons diffuse through the PEM to the cathode side and to the cathode electrocatalyst where they undergo the hydrogen evolution reaction. 4H + +4e - →2H 2

[0057] In an AEM electrolyser, water molecules may instead diffuse through the AEM first to the cathode side, where the following reaction occurs at the cathode electrocatalyst: 2H 2 O+2e - →2OH - +H 2

[0058] The hydroxide ions diffuse through the AEM and undergo the following reaction at the anode electrocatalyst: 4OH - -4e - →O 2 +2H 2 O

[0059] Since the electrochemical reactions in the cell occur on the surface of the electrocatalyst, several requirements are placed on the catalyst layer and its connection with the rest of the cell to achieve high reaction rates. First, water must be able to reach the catalyst on the anode side, and gas (hydrogen or oxygen, depending on the electrode) must be able to diffuse out of the catalyst. Second, since the electrochemical reactions either produce or consume ions and electrons, the catalyst must be in electrical connection with a power source, which requires, among other things, low contact resistance between the catalyst layer and the porous transport layer, as well as between the porous transport layer and the conductive element.

[0060] The electrochemical reaction occurs at the catalyst surface, but depending on the structure of the catalyst layer, only a portion of the catalyst surface may participate. For the reaction to proceed efficiently, the catalyst surface needs to be in contact both with a membrane that allows the transport of ions to and from the catalyst, and with the surrounding water and gas that allows the transport of reactants and products. This is sometimes expressed as the reaction occurring at the three-phase interface between the membrane, the water or gas, and the catalyst. The surface area where these conditions are met is the active catalyst surface area, and the catalyst layer needs to be constructed so that it is as large as possible. Like all components of the electrolyzer cell, the catalyst layer must also be chemically stable.

[0061] In practice, the catalyst layer often contains nanoparticles of catalytically active material such as platinum or iridium oxide. Such particles are hereafter referred to as catalyst particles or electrocatalyst particles. The electrocatalyst particles may be unsupported or may be dispersed on a porous catalyst support such as carbon black, titanium mesh, or metal oxide. Thin films of catalytically active material sputtered onto nanostructured supports may also be used. In general, the catalyst support structure needs to be able to conduct electricity as well as be chemically stable under the conditions present in the electrolytic cell. The catalyst layer often also contains an ion-conducting polymer to improve the transfer of ions to and from the catalyst particles. Usually, the membrane and catalyst layer are manufactured together into what is known as a membrane electrode assembly (MEA).

[0062] Currently used catalyst supports, such as carbon black, usually have an irregular porous structure. However, more regular nanostructured supports made from nanowires, nanowhiskers, nanofibers, and nanotubes can also be used. Catalyst supports with a more regular structure can allow better control over the placement of the catalyst, which leads to better contact between the catalyst and the membrane, as well as improved transport of water and oxygen and hydrogen gases.

[0063] The layers between the conductive plate and the catalyst layer, known as the diffusion layer, porous transport layer, or current collector, have a similar set of requirements, apart from the need to be ionically conductive. The diffusion layer serves to connect the catalyst layer and the conductive plate, allowing the current to flow between the power source and the catalyst, and also the transport of the reactants and products of the electrolysis reaction, namely water, oxygen, and hydrogen. It can also serve as a thermal conductor and as a mechanical support for the MEA. On the cathode side, carbon paper or carbon felt is often used, while on the anode side, due to the harsher chemical conditions, porous metal structures are more often used. In the case of PEM cells, this is often a titanium mesh, while in AEM cells, the diffusion layer on the anode side may be nickel foam.

[0064] For optimal operation of an ion exchange membrane electrolyzer, the electrocatalyst must be in close proximity to the ion exchange membrane so that protons or hydroxide ions can easily enter the ion exchange membrane. Because the transfer of electrons to and from the electrocatalyst is essential for the reaction, the electrocatalyst must also be in electrical contact with the conductive element. In addition, the electrocatalyst must have sufficient exposed surface area so that water molecules or ions can easily adsorb onto it and gas molecules can desorb.

[0065] When the electrocatalyst comprises catalyst particles supported on a support structure, any catalyst particles that are too far from the ion exchange membrane or that have poor electrical contact with the conductive element will make little or no contribution to the electrolysis reaction, thereby reducing the efficiency of the electrolyzer. Therefore, being able to control the position of the electrocatalyst particles is an advantage.

[0066] 1 shows an electrolytic cell 100 comprising first and second electrodes and an ion exchange membrane 130 disposed between the first and second electrodes. Each electrode comprises an electrically conductive element 113, 123 and a catalyst layer 111, 121, the catalyst layer being disposed in intimate contact with the ion exchange membrane 130. Both of the electrically conductive elements 113, 123 are connected to a power source 140.

[0067] At least one electrode may also include a porous transport layer 112, 122 disposed between the conductive element 113, 123 and the catalyst layer 111, 121 as shown in Figure 1. The porous transport layer comprises a porous material.

[0068] Here, the conductive elements 113, 123 comprise a conductive material capable of withstanding the chemical environment in the electrolytic bath. The conductive elements 113, 123 may comprise materials such as titanium, tungsten, and / or zirconium, for example. If desired, the conductive elements may be steel elements coated with one or a combination of titanium, tungsten, and zirconium. The conductive elements may comprise a carbon composite material.

[0069] The conductive element may also be known as a separator element, a separator plate, or a flow plate. If the electrolyzer cell is part of a stack of electrolyzer cells arranged in series, the conductive element may serve as an anode-side conductive element for one electrolyzer cell and as a cathode-side conductive element for an adjacent electrolyzer cell. In this case, it may be referred to as a bipolar plate.

[0070] By way of example, the conductive elements 113, 123 may be conductive plates. A plate is taken to mean a body that is extended in two dimensions and relatively thin in the third dimension. The conductive elements 113, 123 may also be in the form of a sheet or other structure suitable for electrolysis.

[0071] The ion exchange membrane 130 comprises an ion conductor, i.e., a material through which ions can pass. By way of example, the ion conductor may be a polymer, such as sulfonated tetrafluoroethylene, also known as Nafion, or a polymer based on polysulfone or polyphenole oxide. However, the ion exchange membrane may also comprise other types of ion conductors, for example metal oxides such as doped barium zirconate, doped barium cerate, doped lanthanum gallate, or stabilized zirconia.

[0072] The porous transport layers 112, 122 may comprise a porous conductive material such as titanium mesh or nickel foam, or may comprise a porous carbon material such as carbon paper or carbon felt. The porous transport layer may be a planar or plate-shaped layer with parallel conductive elements.

[0073] The catalyst particles include materials that are catalytically active and promote the reactions that occur at the cathode and anode during electrolysis. As examples, the catalyst particles may include platinum, ruthenium, palladium, or iridium. The catalyst particles may also include metal oxides, such as platinum oxide or iridium oxide. As another example, the catalyst particles may include cobalt or nickel. If desired, the catalyst particles may be nanoparticles, i.e., have a size substantially smaller than 1 micrometer, most often between 1 and 100 nm. Preferably, the catalyst particles may be between 3 and 10 nm in size.

[0074] In addition to catalyst particles, the catalyst layers 112, 122 may have a catalyst support that includes a conductive material. The catalyst support may include, for example, carbon black, carbon nanofibers, carbon nanotubes, or porous carbon materials. Metallic materials such as porous titanium mesh or nickel foam may also be used. The catalyst layers may also include an ionically conductive material such as sulfonated tetrafluoroethylene.

[0075] According to the present disclosure, at least one of the catalyst layers 111, 121 provided on the first and second electrodes of the electrolytic cell 100 comprises a catalyst structure comprising a plurality of longitudinal nanostructures, as shown diagrammatically in Figures 2A, 2B and 2C. The catalyst structure comprises a plurality of longitudinal nanostructures 221 and a plurality of electrocatalyst particles 222 attached to the plurality of longitudinal nanostructures 221. The plurality of longitudinal nanostructures 221 are arranged to control the position of the plurality of electrocatalyst particles 222 relative to the ion exchange membrane 130. Controlling the position of the electrocatalyst particles 222 relative to the ion exchange membrane 130 may mean, for example, positioning the electrocatalyst particles within a certain distance from the ion exchange membrane 130 or in contact with the ion exchange membrane 130.

[0076] Nanostructures are structures having a size in at least one dimension substantially smaller than 1 micrometer, preferably 1-100 nm. As used herein, elongated nanostructures are nanostructures in which at least one dimension, such as height, is substantially larger than another dimension, such as width or depth. As an example, consider a substantially cylindrical nanostructure characterized by height and radius. The nanostructure may be considered elongated if the height is significantly larger than the radius, for example, if the height is more than twice as large as the radius. Similar reasoning may be applied to nanostructures that are substantially conical, rectangular, or of any shape.

[0077] The elongated nanostructures 221 may be, for example, straight, helical, branched, wavy, or tilted. As desired, they may be classifiable as nanowires, nanohorns, nanotubes, nanowalls, crystalline nanostructures, or amorphous nanostructures.

[0078] The nanostructures included in the catalytic structure 200 are elongated nanostructures, and thus have one dimension greater than the other. The axis along this dimension is considered the height axis of the nanostructure. If this height axis extends perpendicular or nearly perpendicular to the conductive elements 113, 123 and / or the porous transport layers 112, 122, the elongated nanostructures 221 can be considered to extend approximately along their respective axes 230 that are oriented parallel to each other and perpendicular to the conductive elements 113, 123 or the porous transport layers 112, 122.

[0079] This is shown in Figures 2A, 2B, and 2C, which show schematic longitudinal nanostructures 221 extending along respective axes 230 that are parallel to each other and perpendicular to the substrate 210. The substrate may be the conductive element 113, 123, the porous transport layer 112, 122, or some other substrate. Figures 2A and 2C show longitudinal nanostructures with clusters of electrocatalyst particles 222 attached to their top ends, while Figure 2B shows a branched longitudinal nanostructure with electrocatalyst particles attached to the end of each branch.

[0080] This should not be taken to mean that the nanostructures are perfectly straight or perfectly perpendicular to the conductive elements 111, 121, because the nanostructures may, for example, have a moderate inclination with respect to the axis 230, or because the nanostructures may bend back and forth to form a helical or wavy shape. Instead, the nanostructures extend approximately in the direction of the axis 230. In this context, a moderate inclination means that the angle between the height axis and the axis 230 may be less than 45 degrees, preferably less than 30 degrees, and even more preferably less than 10 degrees.

[0081] According to an embodiment, the plurality of elongated nanostructures 221 may include carbon nanostructures. For example, the elongated carbon nanostructures may include any of carbon nanofibers, carbon nanotubes, carbon nanosheets, and / or carbon nanowires. The elongated nanostructures 221 may include a combination of two or more of carbon nanofibers, carbon nanotubes, and / or carbon nanowires. As an example, the elongated carbon nanostructures may include carbon nanotubes attached to carbon nanofibers. The elongated carbon nanostructures may be graphene walls or carbon nanosheets.

[0082] Carbon nanofibers (CNFs) are longitudinal carbon nanostructures with diameters of 1-100 nm and lengths of 0.1-100 μm. In carbon nanofibers, the shape and surface structure can be adjusted by adjusting the conditions for growing the nanofibers in order to improve the function of the resulting catalyst structure 200. For example, adjusting the thickness of the nanofibers can improve the structural stability. It is also possible to grow nanofibers arranged in a configuration that is particularly suitable for the catalyst structure 200, for example with respect to the density of nanofibers per unit surface area or the orientation of the nanofibers.

[0083] It is also possible to adjust the available surface area or the number of carbon atoms per unit surface area. As an example, the carbon nanofibers may be formed partly by amorphous carbon, which increases the number of carbon atoms per unit surface area. This may result in an increase in the number of possible sites to which the catalyst particles 222 can attach. A similar effect may be achieved if the carbon nanofibers have a corrugated surface structure, where a corrugated surface structure is taken to mean that the surface has a series of grooves and ridges of similar or different sizes.

[0084] Additionally, the process for growing carbon nanofibers may use a growth catalyst that promotes the formation of the carbon nanofibers. The growth catalyst may include materials that are also included in the catalyst particles used to promote the chemical reactions involved in the electrolysis process. If the same material can be used as the growth catalyst and as the electrolysis catalyst that forms the catalyst particles, the manufacture of the electrolysis cell may be simpler and more efficient.

[0085] According to other embodiments, the elongated nanostructures 221 may comprise copper, aluminum, silver, gallium arsenide, zinc oxide, indium phosphate, gallium nitride, indium gallium nitride, indium gallium arsenide, silicon, or other materials.

[0086] The chemical environment in the electrolytic cell may be corrosive, especially on the anode side, due to the high potential. To prevent degradation of the catalytic structure 200, the elongated nanostructures 221 may be shielded from the surrounding chemical environment. For example, at least one section of the elongated nanostructures 221 may be covered by a protective coating arranged to increase resistance to corrosion. By way of example, the protective coating may include any of platinum, iridium, titanium, niobium, and titanium nitride, or a combination thereof. As another example, the protective coating may include a ceramic material or a metal oxide, such as aluminum oxide, cerium oxide, and zirconium oxide.

[0087] According to embodiments, the surfaces of the elongated nanostructures 221 may be chemically altered to achieve hydrophobic or hydrophilic surface properties, which can be achieved through a number of methods, such as coating, etching, or chemical functionalization.

[0088] At least one of the elongated nanostructures 221 may be arranged to extend at least partially into the ion exchange membrane 130. By elongated nanostructures extending at least partially into the ion exchange membrane, it may be meant that the nanostructure extends through the plane of the membrane by at least 5% of its length. This results in a large active surface area or three-phase interface where the electrocatalyst is in good contact with the membrane and, in turn, with the reactants of the electrochemical reaction. This large active surface area may allow for a reduced catalyst loading requirement.

[0089] At least one electrocatalyst particle 222 may be fixed to a first section of at least one longitudinal nanostructure 221. This first section of the longitudinal nanostructure 221 may then be positioned to place the electrocatalyst particle 222 in contact with the ion exchange membrane 130, or as shown in Figure 3, this first section may extend into the ion exchange membrane 130. The first section of the at least one longitudinal nanostructure 221 may be located at an end of the at least one longitudinal nanostructure opposite the conductive element 113, 123 and the porous transport layer 112, 122.

[0090] According to an embodiment, the first section may comprise at least 90% of the at least one longitudinal nanostructure 221. According to other embodiments, the first section may comprise less than half of the at least one longitudinal nanostructure 221, or between 50% and 90% of the longitudinal nanostructure.

[0091] The electrocatalyst particles 222 may be attached to the elongated nanostructures 221 using a suitable deposition method. Examples of suitable deposition methods include indirect or direct physicochemical and / or physical methods, such as electrolytic plating, electroless plating, sputtering, spray coating, dip coating, and / or solvent casting.

[0092] One method of fabricating elongated nanostructures is to grow them on a substrate using a method such as chemical vapor deposition (CVD). CVD is a fabrication method in which precursors, typically gases, are deposited on a substrate where they undergo a reaction to form the fabrication material. Some types of CVD, such as plasma enhanced chemical vapor deposition (PECVD), can be used to grow vertically oriented nanostructures, i.e., nanostructures that extend approximately perpendicularly from the plane of the substrate on which they are grown.

[0093] According to an embodiment, the elongated nanostructures 221 may be grown on a substrate comprising components of the electrolytic cell 100, such as the conductive elements 113, 123 or the porous transport layers 112, 122. This results in an electrocatalytic structure 200 in which the elongated nanostructures 221 are attached to and in good electrical contact with the components of the electrolytic cell forming the substrate. By way of example, the elongated nanostructures may be grown on a substrate in a vertical orientation as described above and subsequently incorporated into the electrolytic cell such that the elongated nanostructures extend from the conductive elements or diffusion layers towards the ion exchange membrane.

[0094] According to another embodiment, the elongated nanostructures 221 may be grown on a first substrate and transferred onto a second substrate comprising components of the electrolytic cell 100 such as the conductive elements 113, 123 or the porous transport layers 112, 122. That is, the nanostructures may be grown on a substrate that is not part of the electrolytic cell itself but serves only as a growth substrate. This allows the use of a growth substrate optimized for the growth of nanostructures without the need for it to also be suitable for use in an electrolytic cell. Conversely, the second substrate may be optimized for use in an electrolytic cell without the need for it to be suitable for the growth of nanostructures.

[0095] The substrate on which the elongated nanostructures 221 are grown may comprise a structured surface on which the elongated nanostructures 221 are grown. A structured surface is a surface that is not flat but exhibits, for example, holes, ridges, or bumps. This may be expressed as surface roughness, non-planarity, or patterning. For example, the substrate may be a porous material that exhibits surface roughness due to pores that intersect the surface of the material.

[0096] By using an undulating substrate, the tips of the elongated nanostructures 221 can reach different heights, for example relative to the conductive elements 113, 123, even if the elongated nanostructures themselves are of similar length. Thus, the distance from the conductive elements 113, 123 to the end of the elongated nanostructure 221 opposite the conductive elements can differ between the elongated nanostructures. If the elongated nanostructures extend into the ion exchange membrane, the elongated nanostructures with a larger distance can extend further into the ion exchange membrane than the elongated nanostructures with a smaller distance, such that all of the elongated nanostructures extend at least partially into the ion exchange membrane. The surface of the ion exchange membrane 130 can be arranged to follow the contour of the structured surface.

[0097] As an example, carbon nanostructures, such as carbon nanofibers, may be grown using plasma enhanced chemical vapor deposition (PECVD). When growing carbon nanostructures by PECVD, a carbon-containing gas, known as a process gas, such as methane or acetylene, is introduced into a reactor along with an inert gas, such as nitrogen or argon, and a reducing gas, such as hydrogen or ammonia. In the reactor, the gas is converted into a plasma at a specific temperature, for example, using an AC or DC discharge between two electrodes. From the plasma reaction, carbon is deposited on the substrate.

[0098] To form carbon nanofibers, a growth catalyst may need to be present on the surface of the substrate. Common growth catalyst materials are nickel, iron, cobalt, and palladium. The growth catalyst may be deposited in the form of a uniform layer, or the growth catalyst may be patterned using lithographic techniques. The growth catalyst may also be deposited in the form of nanoparticles, for example via spin coating. If the growth catalyst is not already in the form of nanoparticles as deposited, it may form nanoparticles on the substrate via a process known as de-wetting. During the CVD process, carbon is deposited on the growth catalyst particles and diffuses across the surface, eventually forming nanostructures such as nanofibers or nanotubes. In the case of nanofibers and nanotubes, depending on parameters such as the size of the growth catalyst particles and the interaction between the growth catalyst and the substrate, the nanofibers may exhibit either base growth or tip growth. During base growth, the nanofibers and / or nanotubes grow upward from the growth catalyst particles that are maintained on the substrate. During tip growth, the growth catalyst particle is maintained at the tip of the nanofiber, and the nanofiber grows below the growth catalyst particle.

[0099] There are several advantages to using PECVD for growing carbon nanofibers. Unlike many other types of CVD, PECVD can be performed at temperatures as low as approximately 350° C., making it possible to grow carbon nanofibers on substrates that cannot withstand higher temperatures. Using PECVD, it is possible to produce carbon nanofibers that are aligned vertically and extend almost perpendicularly from the substrate. It is also possible to control the spacing between the nanofibers by using patterned catalysts. Thus, it is possible to grow arrays of nanofibers with well-defined widths and heights, as well as the desired spacing between the fibers. Other properties of CVD-grown carbon nanofibers, such as mechanical properties and electrical resistance, vary depending on the exact growth conditions.

[0100] According to an embodiment, the structure and morphology of a plurality of CVD grown carbon nanostructures, such as carbon nanotubes and carbon nanofibers, can be altered after the nanostructures are grown. This can be accomplished by methods such as liquid induced densification. During liquid induced densification, a liquid such as acetone, deionized water, or isopropyl alcohol is introduced onto the sample and subsequently evaporated, causing the carbon nanostructures to form bundles, leaving more space between the bundles. If desired, a second plurality of carbon nanostructures can be grown in the spaces between the bundles.

[0101] The use of vertically aligned PECVD grown carbon nanofibers as the elongated nanostructures 221 of the catalyst structure 200 allows for better control of the location of the electrocatalyst particles. If the tips of the carbon nanofibers are selectively coated with catalyst particles, or even if the growth catalyst used to grow the carbon nanofibers is also useful as an electrocatalyst, then locating the tips of the carbon nanofibers at the surface of the ion exchange membrane or embedded in the ion exchange membrane ensures good contact between the catalyst and the membrane. This leads to efficient use of the available catalyst, allowing a lower catalyst loading to be used.

[0102] The regular structure that can be achieved with catalyst supports made from longitudinally aligned carbon nanofibers can also improve mass transport, especially on the anode side where air bubbles in the water can be problematic for impeding transport. The nanofiber spacing can also be precisely controlled during fabrication, allowing for optimization of both the void fraction and void size in such catalyst support materials.

[0103] One reason carbon materials such as carbon nanofibers are used as catalyst supports is that they are chemically stable even under harsh conditions. PECVD-grown carbon nanofibers can be expected to exhibit adequate chemical stability under conditions where carbon cloth or carbon black are currently used, such as the cathode side of electrolyzer cells. Carbon nanofibers may also be coated, for example via atomic layer deposition, with layers of even more chemically stable materials such as titanium, titanium nitride, iridium, niobium, or platinum. This allows the use of carbon nanofiber-based structures even on the anode side of water electrolyzer cells, where chemical conditions typically require the use of metals or metal oxides for catalyst supports and porous transport layers.

[0104] Figures 6A and 6B show transmission electron microscopy (TEM) images of a carbon nanofiber 610. Figure 6A shows the entire nanofiber, while Figure 6B shows a portion of the nanofiber where iridium nanoparticles 620 are visible as black dots.

[0105] 4 shows an electrolytic cell 400 comprising first and second electrodes and an ion exchange membrane 430 disposed between the first and second electrodes. Each electrode comprises a conductive element 413, 423, at least one of the electrodes comprising a catalytic structure 200. The catalytic structure comprises a plurality of elongated nanostructures 221 arranged to connect the conductive elements 413, 423 with a corresponding plurality of catalytic particles 222, each catalytic particle 222 being located at an end of the respective elongated nanostructure 221 opposite the conductive elements 413, 423.

[0106] In order for the elongated nanostructures 221 to effectively connect the conductive elements 413, 423 with the catalyst particles 222, it is advantageous to have the elongated nanostructures extend in the same direction from the conductive elements 413, 423. Thus, the elongated nanostructures 221 may extend approximately along their respective axes 230, as shown in Figure 2, where the axes are oriented parallel to each other and extend substantially perpendicular to the substrate 210, in this case the conductive elements 413, 423.

[0107] For the electrolyser to operate efficiently, ions must be able to enter the ion exchange membrane from the catalytic surface where the chemical reaction occurs, which requires that the catalyst particles are in close proximity to the ion exchange membrane 430. For example, the catalyst particles 221 may be positioned less than 1 micrometer from the ion exchange membrane 430, or alternatively less than 10 nm from the ion exchange membrane 430, and preferably less than 5 nm from the ion exchange membrane. As yet another option, the electrocatalyst particles 222 may be positioned in contact with the ion exchange membrane 430.

[0108] According to yet other embodiments, the catalyst particles 222 may be distributed such that the number of catalyst particles per unit volume is greatest less than 1 micrometer from the ion exchange membrane and decreases as the distance from the ion exchange membrane increases.

[0109] It should be noted that the surface of the ion exchange membrane may exhibit surface structures such as pores, grooves, ridges, etc., on a scale larger than the size of the catalyst particles, in which case the distance between the catalyst particle and the surface of the ion exchange membrane is taken to be the shortest distance in any direction to the surface of the ion exchange membrane.

[0110] Considering the flow of water and gas through the catalytic structure, it is noted that closely spaced longitudinal nanostructures may hinder the flow of water, but an increase in the number of catalyst particles per unit area of ​​the ion exchange membrane 430 may be beneficial to make a larger amount of electrocatalyst available for the electrolysis reaction. Therefore, it is advantageous to use branched nanostructures such as those shown diagrammatically in FIG. 2B. Each branch of the nanostructure has one catalyst particle localized at one end proximate to the ion exchange membrane. Since each nanostructure has multiple branches, this means that the number of catalyst particles per unit area of ​​the ion exchange membrane 430 can be increased without placing the longitudinal nanostructures closer to each other. Thus, referring to FIG. 2B, at least one of the longitudinal nanostructures 221 may be a branched nanostructure with a trunk portion 241 and at least two branches 242, where the electrocatalyst particles 222 are localized at the ends of each branch 242.

[0111] The catalytic structure 200 should also allow sufficient flow of water, as well as oxygen and hydrogen gases, to and from the catalytic particles and the ion exchange membrane. It may be advantageous to use other structures in addition to the elongated nanostructures to improve the flow of water and gases. By way of example, the catalytic structure 200 may include a porous carbon material. The porous carbon material may be, for example, a carbon microfiber cloth or carbon paper. The porous carbon material may be disposed adjacent to the conductive elements 413, 423, with the elongated nanostructures 221 extending from the porous carbon material toward the ion exchange membrane 430. The porous carbon material may be a porous transport layer, as previously mentioned.

[0112] With reference to Fig. 5A and Figs. 1 and 4, a method for manufacturing a catalyst structure 200 for an electrolytic cell 100, 400 is also disclosed. The electrolytic cell 100, 400 comprises a first and a second electrode and an ion exchange membrane 130, 430 arranged between the first and the second electrode. Each electrode comprises a conductive element 113, 123, 413, 423. The method for manufacturing includes producing a plurality of elongated nanostructures 221, SA1, which are connected to the conductive element 113, 123, 413, 423 provided on the first or second electrode. The method for manufacturing also includes attaching a plurality of electrocatalyst particles 222 to the plurality of elongated nanostructures 221, SA2, such that each electrocatalyst particle 222 is located at an end of the respective elongated nanostructure 221 opposite the conductive element 113, 123, 413, 423 provided on the first or second electrode.

[0113] Deposition of catalyst particles 222 onto elongated nanostructures 221, SA2, may be accomplished via methods such as sputtering, spray coating, dip coating, atomic layer deposition, chemical vapor deposition, or other methods.

[0114] The elongated nanostructures 221 can be produced by methods such as lithographic methods such as colloidal or nanosphere lithography, focused ion beam processing, and laser processing, among others. In the case of nanofibers containing carbon or organic compounds, methods such as electrospinning or chlorination of carbides such as titanium carbide or organometallic compounds such as ferrocene can also be used.

[0115] According to an embodiment, generating the plurality of elongated nanostructures 221, SA1, may include growing the elongated nanostructures 221, SA11, on a substrate. The substrate may be one of the conductive elements 113, 123, 413, 423 of the first or second electrode, the porous transport layer 112, 122, or some other substrate.

[0116] Growing elongated nanostructures 221 on a substrate, SA11, allows for a wide range of tailoring of the nanostructure's properties. For example, growth conditions may be selected to enhance the surface area of ​​each nanostructure. According to an embodiment, the elongated nanostructures may be grown in a plasma.

[0117] The substrate may comprise materials such as silicon, glass, stainless steel, ceramic, silicon carbide, or any other suitable substrate material. The substrate may also comprise a high temperature polymer, such as polyimide. As desired, the substrate may be a component of the electrolytic cell 100, 400, such as the conductive plate 113, 123, 413, 423, the porous transport layer 112, 122, or the ion exchange membrane 130, 430.

[0118] Growing elongated nanostructures 221 on the substrate, SA11, may include depositing a growth catalyst layer on a surface of the substrate, and growing the elongated nanostructures 221 on the growth catalyst layer.

[0119] As used herein, a growth catalyst is a substance that has catalytic activity and promotes the chemical reactions that occur during the formation of nanostructures.

[0120] The growth catalyst may include a material such as nickel, iron, platinum, palladium, nickel silicide, cobalt, molybdenum, gold, or alloys thereof. As an example, the growth catalyst layer may be 1-100 nm thick. As another example, the growth catalyst layer may include a plurality of growth catalyst particles.

[0121] Growing elongated nanostructures 221 on the growth catalyst layer, SA11, may include heating the growth catalyst layer to a temperature capable of forming the nanostructures, and supplying a gas containing reactants such that the reactants contact the growth catalyst layer, where the reactants are chemical compounds or mixtures of chemical compounds containing the chemical elements used to form the nanostructures. In the case of carbon nanostructures, the reactants may include hydrocarbons such as methane or acetylene, or the reactants may include carbon monoxide.

[0122] According to an embodiment, the growth catalyst material and the parameters of the growth process may be selected to achieve so-called tip-growth of the nanostructures, during which the nanostructures grow below a section of the growth catalyst, resulting in elongated nanostructures with particles of the growth catalyst remaining at the tips of the elongated nanostructures.

[0123] Attachment of catalyst particles 222 to elongated nanostructures 221, SA2, can be accomplished using the remaining growth catalyst particles. As an example, chemical elements present in the remaining growth catalyst particles can be replaced with other chemical elements via methods such as galvanic replacement. As another example, the remaining growth catalyst particles can be selectively coated with an electrocatalytic material suitable for use in the electrolytic cells 100, 400.

[0124] According to an embodiment, depositing the growth catalyst layer may include spin-coating the surface of the substrate with particles of the growth catalyst. According to another embodiment, depositing the growth catalyst layer includes depositing a uniform growth catalyst layer and introducing a pattern onto the deposited uniform growth catalyst layer. Introducing a pattern onto the deposited uniform growth catalyst layer may include varying the thickness of the growth catalyst layer according to a pattern or selectively removing the growth catalyst layer in some places. Introducing a pattern onto the growth catalyst layer may be achieved by lithographic methods, such as, for example, colloidal lithography or nanosphere lithography. Patterning of the growth catalyst layer allows for controlling the density of nanostructures per unit surface area on the substrate.

[0125] Growing elongated nanostructures 221 on a substrate, SA11, may also include depositing a conductive layer on the surface of the substrate. A growth catalyst layer may then be deposited on the conductive layer. After growth of the elongated nanostructures, the portions of the conductive layer that extend between or around the elongated nanostructures may be selectively removed. This removal may be accomplished by etching, for example, plasma etching, pyrolysis etching, or electrochemical etching.

[0126] The conductive layer electrically grounds the substrate, which is an advantage in certain nanostructure growth methods, such as growth in a plasma, and may also prevent atomic diffusion between the growth catalyst layer and the substrate.

[0127] According to an embodiment, the conductive layer can be between 1 and 100 microns (1 to 100 micrometers) thick.

[0128] According to embodiments, in addition to the substrate, the growth catalyst layer, and the conductive layer, additional layers may be present. The materials included in the additional layers may be selected to tailor the properties of the nanostructures after growth, to promote vertically oriented growth, or to improve the outcome of the growth process. The additional layers may also constitute conductive elements 113, 123, 413, 423 or porous transport layers 112, 122 that form part of the electrodes of the electrolytic cell 100, 400.

[0129] According to embodiments, deposition of any of the layers, including the conductive layer and the growth catalyst layer, may be performed by methods such as evaporation, plating, sputtering, molecular beam epitaxy, pulsed laser deposition, chemical vapor deposition, spin coating, spray coating, or other suitable methods.

[0130] As an example, fabricating the catalytic structures 200 may include depositing a conductive layer on the top surface of a substrate, depositing a layer of growth catalyst on the conductive layer, growing elongated nanostructures 221 on the layer of growth catalyst, and selectively removing the conductive layer between and around the elongated nanostructures to produce the elongated nanostructures, SA1. It may also include attaching catalytic particles 222 to the elongated nanostructures after the growth process, SA2.

[0131] According to embodiments, the elongated nanostructures may be grown on a substrate that comprises a component of the electrolytic cell 100, 400, such as one of the conductive elements 113, 123, 413, 423, the porous transport layer 112, 122, or the ion exchange membrane 130, 430. According to other embodiments, the elongated nanostructures may be grown on some other substrate and then transferred, for example, to one of the conductive elements 113, 123, 413, 423, the porous transport layer 112, 122, or the ion exchange membrane 130, 430.

[0132] 5B shows a method for producing a catalyst structure 200 for an electrolytic cell 100, 400. The electrolytic cell 100, 400 comprises a first and a second electrode and an ion exchange membrane 130, 430 disposed between the first and second electrodes. Each electrode comprises a conductive element 113, 123, 413, 423. The method includes constructing a substrate having a surface, SB0. The substrate may be one of the conductive elements 113, 123, 413, 423 of the first or second electrode, a porous transport layer 112, 122, or any other substrate. The method further includes selecting a growth catalyst for the growth of elongated nanostructures 221 on the substrate, SB1, such that the growth catalyst can also be used as an electrocatalyst in the electrolytic cell 100, 400, and depositing a growth catalyst layer comprising the selected growth catalyst on the surface of the substrate, SB2. The method further includes growing elongated nanostructures 221 on the growth catalyst layer to produce elongated nanostructures 221 having electrode catalyst particles 222 localized at the ends of each elongated nanostructure 221 suitable for use in the electrolytic cell 100, 400, SB3.

[0133] In addition to the advantages already mentioned of growing elongated nanostructures on a substrate, such as the ability to tailor the properties of the nanostructures, this method has the advantage of simplifying the manufacture of the catalytic structures 200 by using the same material for the growth catalyst as for the electrocatalyst particles 222. Thus, attachment of the catalytic particles to the elongated nanostructures is not performed in a separate step.

[0134] The substrate may include materials such as silicon, glass, stainless steel, ceramic, silicon carbide, or any other suitable substrate material. The substrate may also include a high temperature polymer, such as a polyimide.

[0135] The growth catalyst may include a material such as nickel, iron, platinum, palladium, nickel silicide, cobalt, molybdenum, gold, or alloys thereof. By way of example, the growth catalyst layer may be 1-100 nm thick. As another example, the growth catalyst layer may include a plurality of growth catalyst particles.

[0136] In order to select a growth catalyst that is also suitable for use as an electrocatalyst in the electrolytic cells 100, 400, it is preferable to find a material that can act well as a catalyst for both the chemical reactions involved in the electrolysis process and the growth process. Examples of suitable materials can be platinum, palladium, and nickel, which are used both as growth catalysts and electrocatalysts for nanostructure growth.

[0137] Growing elongated nanostructures 221 on the growth catalyst layer, SB31, may include heating the growth catalyst layer to a temperature capable of forming the nanostructures, and supplying a gas containing a reactant such that the reactant contacts the growth catalyst layer, where the reactant is a chemical compound or mixture of chemical compounds containing the chemical elements used to form the nanostructures. In the case of carbon nanostructures, the reactant may include a hydrocarbon such as methane or acetylene, or the reactant may include carbon monoxide.

[0138] According to an embodiment, the elongated nanostructures may be grown in a plasma.

[0139] According to an embodiment, the growth catalyst material and the parameters of the growth process may be selected to achieve so-called tip growth of the nanostructures, during which the nanostructures grow below a section of the growth catalyst, resulting in elongated nanostructures with particles of the growth catalyst remaining at the tips of the elongated nanostructures.

[0140] By way of example, the substrate may be configured to comprise one of the conductive elements 113, 123, 413, 423 of the electrolytic cell electrodes (SB0) and the growth catalyst may be selected such that it can also be used as an electrocatalyst (SB1). After depositing a growth catalyst layer, SB2, and if the parameters of the growth process are adjusted to achieve tip growth, growing elongated nanostructures on the growth catalyst layer, SB31, results in a plurality of elongated nanostructures attached to the conductive element, with catalyst particles localized at the end of each elongated nanostructure opposite the conductive element.

[0141] Depositing a growth catalyst layer, SB2, may include depositing a uniform growth catalyst layer and introducing a pattern, SB22, onto the deposited uniform growth catalyst layer. As already mentioned, introducing a pattern onto the deposited uniform growth catalyst layer may include varying the thickness of the growth catalyst layer according to a pattern or selectively removing the growth catalyst layer in some places. Introducing a pattern onto the growth catalyst layer may be achieved, for example, by lithographic methods such as colloidal lithography or nanosphere lithography. Patterning of the growth catalyst layer allows to control the density of nanostructures per unit surface area on the substrate.

[0142] The method may also include depositing a conductive layer, SB21, on the surface of the substrate. A growth catalyst layer may be deposited on the conductive layer. After growth of the elongated nanostructures, portions of the conductive layer that extend between or around the elongated nanostructures may be selectively removed. This removal may be accomplished by etching, for example, plasma etching, pyrolytic etching, or electrochemical etching. According to an embodiment, the conductive layer may be between 1 and 100 microns (1 to 100 micrometers) thick.

[0143] The conductive layer electrically grounds the substrate, which is an advantage in certain nanostructure growth methods, such as growth in plasma, and may also prevent atomic diffusion between the growth catalyst layer and the substrate.

[0144] According to embodiments, in addition to the substrate, the growth catalyst layer, and the conductive layer, additional layers may be present. The materials included in the additional layers may be selected to tailor the properties of the nanostructures after growth, to promote vertically oriented growth, or to improve the outcome of the growth process. The additional layers may also include conductive elements 113, 123, 413, 423 that form part of the electrodes for the electrolytic cells 100, 400.

[0145] According to embodiments, deposition of any of the layers, including the conductive layer and the growth catalyst layer, may be performed by methods such as evaporation, plating, sputtering, molecular beam epitaxy, pulsed laser deposition, chemical vapor deposition, spin coating, spray coating, or other suitable methods.

[0146] According to embodiments, the elongated nanostructures may be grown on a substrate that comprises a component of the electrolytic cell 100, such as one of the conductive elements 113, 123, 413, 423, the porous transport layer 112, 122, or the ion exchange membrane 130, 430. According to other embodiments, the elongated nanostructures may be grown on some other substrate and then transferred, for example, to one of the conductive elements 113, 123, 413, 423, the porous transport layer 112, 122, or the ion exchange membrane 130, 430.

[0147] 5C, a method of manufacturing a catalytic structure 200 of an electrolytic cell 100, 400 is also disclosed herein. The electrolytic cell 100, 400 comprises a first and a second electrode and an ion exchange membrane 130, 430 disposed between the first and second electrodes. Each electrode also comprises a conductive element 113, 123, 413, 423 and a catalytic layer 111, 121. The method includes generating a plurality of elongated nanostructures 221, SC1, and attaching a plurality of catalytic particles 222 to the plurality of elongated nanostructures 221, SC2. The method also includes positioning the plurality of elongated nanostructures 221 to control the position of the plurality of electrocatalyst particles 222 relative to the ion exchange membrane 130, 430, SC3.

Claims

1. 1. An electrolytic cell (100, 400) comprising a first electrode and a second electrode and an ion exchange membrane (130, 430) disposed between the first and second electrodes, each electrode comprising a conductive element (113, 123, 413, 423) and a catalyst layer (111, 121), at least one catalyst layer comprising a catalyst structure (200, 200), the catalyst structure comprising a plurality of elongated nanostructures (221) and a plurality of electrocatalyst particles (222) attached to the plurality of elongated nanostructures (221), the plurality of elongated nanostructures (221) being arranged to control the position of the plurality of electrocatalyst particles (222) relative to the ion exchange membrane (130, 430).

2. 2. The electrolytic cell (100, 400) of claim 1, wherein the elongated nanostructures (221) extend approximately along respective axes (230) that are oriented parallel to one another and perpendicular to the conductive elements (113, 123, 413, 423).

3. 3. The electrolytic cell (100, 400) of claim 1 or 2, wherein the elongated nanostructures (221) comprise carbon nanostructures.

4. Electrolyzer (100, 400) according to any one of claims 1 to 3, wherein said elongated nanostructures (221) comprise any of carbon nanofibers, carbon nanotubes and / or carbon nanowires.

5. Electrolytic cell (100, 400) according to any one of claims 1 to 4, wherein at least one section of the elongated nanostructures (221) is covered by a protective coating arranged to increase its resistance to corrosion.

6. 6. The electrolytic cell (100, 400) of claim 5, wherein the protective coating comprises any one or combination of platinum, iridium, titanium, and titanium nitride.

7. 7. The electrolytic cell (100, 400) of any one of claims 1 to 6, wherein the elongated nanostructures (221) are grown on a first substrate and transferred onto a second substrate comprising components of the electrolytic cell (100, 400) such as conductive elements (113, 123, 413, 423) or porous transport layers (112, 122).

8. 8. The electrolytic cell (100, 400) of any one of claims 1 to 7, wherein the elongated nanostructures (221) are grown on a substrate comprising components of the electrolytic cell (100, 400) such as conductive elements (113, 123, 413, 423) or porous transport layers (112, 122).

9. 9. The electrolytic cell (100, 400) of claim 8, wherein the substrate comprises a structured surface and the elongated nanostructures (221) are grown on the structured surface.

10. 10. The electrolytic cell (100, 400) of claim 9, wherein a surface of the ion exchange membrane (130, 430) is oriented to conform to the contour of the structured surface.

11. An electrolytic cell (100, 400) as described in any one of claims 1 to 10, wherein at least one elongated nanostructure (221) is arranged to extend at least partially into the ion exchange membrane (130, 430).

12. 12. The electrolytic cell (100, 400) of claim 11, wherein at least one electrocatalyst particle (222) is fixed to a first section of the at least one elongated nanostructure (221), and at least the first section of the at least one elongated nanostructure (221) extends into the ion exchange membrane (130, 430).

13. 13. The electrolytic cell (100, 400) of claim 12, wherein the first section of the at least one elongated nanostructure (221) is located at an end of the at least one elongated nanostructure opposite the conductive element (113, 123, 413, 423).

14. 14. The electrolytic cell (100, 400) of claim 12 or 13, wherein the first section constitutes more than 90% of the at least one elongated nanostructure (221).

15. 14. The electrolytic cell (100, 400) of claim 12 or 13, wherein the first section constitutes less than half of the at least one elongated nanostructure (221).

16. Electrolytic cell (100, 400) according to any one of claims 1 to 15, wherein the electrocatalyst particles (222) are attached to the elongated nanostructures (221) using a deposition method, the deposition method being an indirect or direct physico-chemical and / or physical method such as electroplating, electroless plating, sputtering, spray coating, dip coating and / or solvent casting.

17. An electrolytic cell (100, 400) as described in any one of claims 1 to 16, wherein at least one electrode comprises a porous transport layer (112, 122) arranged between the conductive element (113, 123, 413, 423) and the catalyst layer (111, 121), the porous transport layer comprising a porous material.

18. 9. The electrolytic cell (100, 400) according to any one of claims 1 to 8, wherein the plurality of elongated nanostructures (221) are arranged to connect the conductive element (113, 123, 413, 423) with the plurality of electrocatalyst particles (222), each electrocatalyst particle (222) being localised at an end of a respective elongated nanostructure (221) opposite the conductive element (113, 123, 413, 423).

19. 20. The electrolytic cell (100, 400) of claim 18, wherein the electrocatalyst particles (222) are positioned less than 10 nm from the ion exchange membrane (130, 430), preferably less than 5 nm from the ion exchange membrane.

20. 20. The electrolytic cell (100, 400) of claim 19, wherein the electrocatalyst particles (222) are positioned in contact with the ion exchange membrane (130, 430).

21. 21. The electrolytic cell (100, 400) according to any one of claims 18 to 20, wherein at least one of said elongated nanostructures (221) is a branched nanostructure comprising a trunk portion (241) and at least two branches (242), and an electrocatalyst particle (222) is localized at the end of each branch (242).

22. 22. The electrolytic cell (100, 400) of any one of claims 18 to 21, wherein the catalytic structure (200) comprises a porous carbon material.

23. A method for manufacturing a catalytic structure (200) for an electrolytic cell (100, 400), said electrolytic cell (100, 400) comprising a first electrode and a second electrode and an ion exchange membrane (130, 430) arranged between said first electrode and said second electrode, each electrode comprising an electrically conductive element (113, 123, 413, 423), said method comprising the steps of: Producing (SA1) a plurality of longitudinal nanostructures (221), the plurality of longitudinal nanostructures (221) being connected to the conductive elements (113, 123, 413, 423) provided on the first electrode or the second electrode; and Attaching (SA2) a plurality of electrocatalyst particles (222) to the plurality of elongated nanostructures (221) such that each electrocatalyst particle (222) is localized at an end of the respective elongated nanostructure (221) opposite the conductive element (113, 123, 413, 423) provided on the first electrode or the second electrode; A manufacturing method comprising:

24. The method of claim 23, wherein generating (SA1) the plurality of elongated nanostructures (221) comprises growing (SA11) the elongated nanostructures (221) on a substrate, such as one of the conductive elements (113, 123, 413, 423) provided on the first electrode or the second electrode, a porous transport layer (112, 122), or some other substrate.

25. The method of claim 24, wherein growing (SA11) the elongated nanostructures (221) on a substrate comprises depositing a growth catalyst layer on a surface of the substrate and growing the elongated nanostructures (221) on the growth catalyst layer.

26. 26. The method of claim 25, wherein depositing a growth catalyst layer comprises depositing a uniform growth catalyst layer and introducing a pattern onto the deposited uniform growth catalyst layer.

27. The method of any one of claims 24 to 26, wherein growing (SA11) the elongated nanostructures (221) on a substrate comprises depositing a conductive layer on a surface of the substrate.

28. A method for manufacturing a catalytic structure (200) for an electrolytic cell (100, 400), said electrolytic cell (100, 400) comprising a first electrode and a second electrode and an ion exchange membrane (130, 430) arranged between said first electrode and said second electrode, each electrode comprising an electrically conductive element (113, 123, 413, 423), said method comprising the steps of: configuring (SB0) a substrate having a surface, such as one of the conductive elements (113, 123, 413, 423) provided on the first electrode or the second electrode, a porous transport layer (112, 122), or any other substrate; Selecting (SB1) a growth catalyst for the growth of elongated nanostructures (221) on said substrate such that said growth catalyst can also be used as an electrocatalyst in said electrolytic bath (100, 400); depositing a growth catalyst layer on the surface of the substrate comprising the selected growth catalyst (SB2); and growing elongated nanostructures (221) on said growth catalyst layer to produce elongated nanostructures (221) having electrocatalyst particles (222) localized at the ends of each elongated nanostructure (221) suitable for use in an electrolytic cell (100, 400) (SB3); A manufacturing method comprising:

29. 30. The method of claim 28, wherein depositing a growth catalyst layer (SB2) comprises depositing a uniform growth catalyst layer and introducing a pattern onto the deposited uniform growth catalyst layer (SB22).

30. 30. The method of claim 28 or 29, wherein depositing (SB2) a growth catalyst layer on the face of the substrate comprises depositing (SB21) a conductive layer on the face of the substrate.

31. A method for manufacturing a catalytic structure (200) for an electrolytic cell (100, 400), said electrolytic cell (100, 400) comprising a first electrode and a second electrode and an ion exchange membrane (130, 430) arranged between said first electrode and said second electrode, each electrode comprising an electrically conductive element (113, 123, 413, 423), said method comprising the steps of: Producing (SC1) a plurality of elongated nanostructures (221); attaching (SC2) a plurality of electrocatalyst particles (222) to the plurality of longitudinal nanostructures (221); and Arranging the plurality of elongated nanostructures (221) to control the position of the plurality of electrocatalyst particles (222) relative to the ion exchange membrane (130, 430) (SC3); A manufacturing method comprising:

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