SE2130036A1

SE2130036A1 - An electrolyzer comprising a catalyst supported on a nanostructure

Info

Publication number: SE2130036A1
Application number: SE2130036A
Authority: SE
Inventors: Fabian Wenger; Vincent Desmaris
Original assignee: Smoltek Ab
Priority date: 2021-02-05
Filing date: 2021-02-05
Publication date: 2022-08-06
Also published as: SE545366C2

Abstract

An electrolyzer (100) comprising a first (110) and a second (120) electrode and an ion exchange membrane (13) arranged in-between the first and the second electrode is disclosed. Each electrode comprises a conductive element (111, 121). At least one of the electrodes comprises a catalyst structure (140), the catalyst structure comprising a plurality of elongated nanostructures (141) arranged to connect the conductive element (111,121) to a corresponding plurality of catalyst particles (142). Each catalyst particle (142) is localized at an end of a respective elongated nanostructure (141) opposite from the conductive element (111,121).

Description

TITLE An electrolyzer comprising a Catalyst supported on a nanostructure TECHNICAL FIELD The present disclosure relates to devices used in electrolysis, particularly forthe electrolysis of water.

BACKGROUND The production of hydrogen gas through the electrolysis of water is a promisingtechnology both for replacing the production of hydrogen gas from fossil fuelsand as a means of converting excess electrical energy from intermittent energysources such as solar and wind power to chemical energy for storage.However, existing water electrolyzers suffer from problems related to thecorrosive conditions within the electrolysis cell and the use of expensivecatalysts. For electrolysis cells comprising ion exchange membranes it may benecessary to use catalysts comprising e.g., platinum or iridium, which entailsa significant cost. Additionally, current electrolysis cells are limited in terms ofthe ion current per area through the cell. An improvement in this regard wouldresult in increased production capability.

WO2018185617 discloses a water electrolyzer comprising platinum or platinum oxide as a catalyst for the electrolysis reactions.

Still, there is a need for improved water electrolyzers.

SUMMARYlt is an object of the present disclosure to provide improved electrolyzers.

This object is at least in part obtained by an electrolyzer comprising a first anda second electrode and an ion exchange membrane arranged in-between thefirst and second electrode. Each electrode comprises a conductive element and at least one of the electrodes comprises a catalyst structure. The catalyst structure comprises a plurality of elongated nanostructures arranged toconnect the conductive element to a corresponding plurality of cata|ystparticles. Each cata|yst particle is localized at an end of a respective elongated nanostructure opposite from the conductive element.

Advantageously, each cata|yst particle being localized at an end of arespective elongated nanostructure provides an improved control over theposition of the cata|yst particles relative to the ion exchange membrane andother components of the electrolyzer, which makes it possible to achieve amore efficient operation. As an example, the cata|yst particles need to be inclose proximity to the ion exchange membrane and present a large surfacearea in order to efficiently promote the chemical reactions comprised in theelectrolysis process. With improved control over the position of the cata|ystparticles, both the distance to the ion exchange membrane and the exposedsurface area can be adjusted to improve performance.

Advantageously, since a larger fraction of the cata|yst particles can be reliablypositioned close to the ion exchange membrane, the total number of cata|ystparticles used can be reduced compared to electrolyzers where the cata|yststructure does not afford the same degree of control over the position ofcata|yst particles. ln other words, in the electrolyzer disclosed here a smalleramount of cata|yst particles may be used to achieve the same efficiency as in previously known electrolyzers.

According to aspects, the conductive elements may be conductive plates.Conductive plates are commonly used with electrolyzers, it is an advantagethat the techniques disclosed herein may be implemented with the same form factor.

According to aspects, the elongated nanostructures may comprise carbonnanostructures. Advantageously, carbon nanostructures have good electricalconductivity and structural stability, thereby providing both control over theposition of the cata|yst particle and a good electrical connection between theconductive elements and the cata|yst particles, which is beneficial for efficient electrolyzer operation. The elongated carbon nanostructures may compriseany of carbon nanofibers, carbon nanotubes, and/or carbon nanowires.

Carbon nanofibers, as well as carbon nanotubes and nanowires, have the advantage of being easy to grow on a wide range of substrates. ln particular, for carbon nanofibers, the shape and surface structure can bea|tered by adjusting the conditions under which the nanofibers are grown. Thisprovides the possibility of creating a p|ura|ity of nanofibers that are arrangedin a particularly suitable configuration. They are also structurally and chemically robust.

The process of growing carbon nanostructures may entail the use of a growth catalyst to promote the formation of the carbon nanostructure.Advantageously, the growth catalyst used to grow carbon nanofibers maycomprise the same materials as the catalyst particles used to promote thechemical reactions comprised in the electrolysis process. lf the same materialscan be used as a growth catalyst and as electrolysis catalyst particles, fabrication of the electrolyzer may be made simpler and more efficient.

According to aspects, the catalyst particles are positioned less than 10 nmfrom the ion exchange membrane, or preferably less than 5 nm from the ionexchange membrane. According to other aspects, the catalyst particles arepositioned in contact with the ion exchange membrane, or within 0.1 -1 nm fromthe ion exchange membrane. Advantageously, positioning the catalystparticles in close proximity to the ion exchange membrane, i.e., less than 10nm, and preferably closer, allows more efficient use of the catalyst particles,as the hydrogen or hydroxide ions generated during the reactions can moreeasily enter the ion exchange membrane. Also, hydrogen or hydroxide ionsexiting the ion exchange membrane can more easily adsorb to a catalystparticle, which is important for the electrolysis reactions.

According to aspects, the elongated nanostructures may extend generallyalong respective axes, where the axes are oriented in parallel to each otherand extended substantially perpendicularly to the conductive element. Thismeans that the catalyst particles localized to the end of each elongated nanostructure form a layer of Catalyst particles that can all simultaneously bepositioned in close proximity to the ion exchange membrane. A larger fractionof the catalyst particles will then be in close proximity to the ion exchangemembrane compared to a configuration where the axes are not oriented inparallel, leading to an increase in efficiency without the need to use morecatalyst particles. Alternately, a smaller amount of catalyst particles can beused without a reduction in efficiency. lt is appreciated that the elongated nanostructures are not perfectly straight.The elongated nanostructure extending along an axis should be understood tomean that the height axis of the nanostructure extends in the general directionof the axis.

According to aspects, the catalyst structure comprises a porous carbonmaterial. According to other aspects, at least one of the elongatednanostructures may be a branched nanostructure comprising a trunk and atleast two branches, where a catalyst particle is localized at the end of eachbranch. Advantageously, branched nanostructures make it possible toincrease the number of catalyst particles per unit area of the ion exchangemembrane, thereby increasing the surface area of catalyst where theelectrolysis reactions may take place.

According to aspects, at least one section of an elongated nanostructure maybe covered by a protective coating arranged to increase a resistance tocorrosion. The chemical environment at the electrodes of an electrolyzer iscorrosive, especially at the anode side of the ion exchange membrane.Applying a protective coating to the elongated nanostructures is thus anadvantage as it reduces wear and allows the catalyst structure to be used forlonger. According to aspects, the protective coating may comprise any of platinum, iridium, or titanium, or a combination thereof.

The object is also obtained at least in part by a method of producing a catalyststructure for an electrolyzer. The electrolyzer comprises a first and a secondelectrode and an ion exchange membrane arranged in-between the first andsecond electrode. Each electrode comprises a conductive element. The method comprises generating a plurality of elongated nanostructures, theelongated nanostructures being connected to one of the conductive elements.The method also comprises attaching a plurality of Catalyst particles to theplurality of elongated nanostructures such that each catalyst particle islocalized at an end of a respective elongated nanostructure opposite from the conductive element.

Advantageously, the elongated nanostructures being connected to theconductive element ensures good electrical contact between the conductiveelement and the catalyst particles, which is important for efficient electrolyzeroperation. Each catalyst particle being localized at an end of a respectiveelongated nanostructure provides control over the position of the catalystparticle, e.g., in relation to the ion exchange membrane.

According to aspects, generating a plurality of elongated nanostructures maycomprise growing the elongated nanostructures on a substrate. Growing theelongated nanostructures on a substrate presents the advantage that theproperties and shape of the nanostructures can be tailored by tuning theconditions under which the nanostructures are grown, in order to improve thefunctionality of the resulting catalyst structure. As an example, the thickness ofthe elongated nanostructures could be tuned to improve structural stability. Asanother example, the surface of the nanostructures could be altered to includestructure such as ridges and grooves, which increases the total surface areaand may provide more possible sites at which catalyst particles may beaüached.

According to aspects, growing the elongated nanostructures on a substratemay comprise depositing a growth catalyst layer on a surface of the substrateand growing the elongated nanostructures on the growth catalyst layer. Thegrowth catalyst layer promotes growth of the elongated nanostructures. Byaltering the properties of the growth catalyst layer, the properties of the grownelongated nanostructures can be tuned in order to improve the functionality ofthe resulting catalyst structure.

According to further aspects, depositing a growth catalyst layer may comprisedepositing a uniform growth catalyst layer and introducing a pattern onto thedeposited uniform growth catalyst layer. An advantage of introducing a patternonto the deposited uniform growth catalyst layer is that it makes it possible tocontrol the density of nanostructures per surface area on the substrate. Thedensity of nanostructures per surface area may affect the flow of fluids suchas water, oxygen gas, and hydrogen gas, to and from the catalyst particles. ltmay also affect the density of catalyst particles per surface area of themembrane. Both the flow and the catalyst particle density impact the efficiencyof the electrolyzer. As such, controlling the density of nanostructures persurface area of the substrate makes it possible to improve the efficiency of theelectrolyzer.

According to aspects, growing the elongated nanostructures on a substratemay comprise depositing a conducting layer on a surface of the substrate.Advantageously, depositing a conducting layer on the surface of the substratecan produce the effect of electrically grounding the substrate. Electricallygrounding the substrate may be advantageous for certain methods of growingnanostructures. lf a conductive layer is deposited on a surface of the substrateand a growth catalyst layer is deposited on top of the conductive layer, theconductive layer may also hinder diffusion of atoms and/or molecules between the catalyst layer and the substrate.

The object is also obtained at least in part by a method of producing a catalyststructure for an electrolyzer, where the electrolyzer comprises a first and asecond electrode, as well as an ion exchange membrane arranged in-betweenthe first and the second electrode. Each electrode comprises a conductiveelement. The method comprises configuring a substrate having a surface andselecting a growth catalyst for the growth of elongated nanostructures on thesubstrate. The growth catalyst is selected such that it can also be used as anelectrolysis catalyst in the electrolyzer. The method also comprises depositinga growth catalyst layer comprising the selected growth catalyst on the surfaceof the substrate and generating elongated nanostructures with a catalystparticle suitable for use in an electrolyzer localized at an end of each elongated nanostructure by growing e|ongated nanostructures on the growth Catalyst layer. ln addition to the advantages described above associated with growinge|ongated nanostructures on a substrate using a catalyst layer, this methodpresents the further advantage of using a growth catalyst that can also be usedas an electrolysis catalyst in an electrolyzer. This makes it possible to producethe catalyst structure in a simpler and more efficient way.

According to aspects, depositing a growth catalyst layer may comprisedepositing a uniform growth catalyst layer and introducing a pattern onto thedeposited uniform growth catalyst layer. An advantage of introducing a patternonto the deposited uniform growth catalyst layer is that it makes it possible tocontrol the density of nanostructures per surface area on the substrate. Thedensity of nanostructures per surface area may affect the flow of fluids suchas water, oxygen gas, and hydrogen gas, to and from the catalyst particles. ltmay also affect the density of catalyst particles per surface area of themembrane. Both the flow and the catalyst particle density impact the efficiencyof the electrolyzer. As such, controlling the density of nanostructures persurface area of the substrate makes it possible to improve the efficiency of theelectrolyzer.

According to aspects, the method may comprise depositing a conducting layeron a surface of the substrate. Advantageously, depositing a conducting layeron the surface of the substrate can produce the effect of electrically groundingthe substrate. Electrically grounding the substrate may be advantageous forcertain methods of growing nanostructures. lf a conductive layer is depositedon a surface of the substrate and a growth catalyst layer is deposited on top ofthe conductive layer, the conductive layer may also hinder diffusion of atomsand/or molecules between the catalyst layer and the substrate.

Generally, all terms used in the claims are to be interpreted according to theirordinary meaning in the technical field, unless explicitly defined otherwiseherein. 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 statedotherwise. The steps of any method disclosed herein do not have to beperformed in the exact order disclosed, unless explicitly stated. Furtherfeatures of, and advantages with, the present invention will become apparentwhen studying the appended claims and the following description. The skilledperson realizes that different features of the present invention may becombined 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 tothe appended drawings, where: Figure 1 schematically illustrates an electrolyzer,Figures 2A-B show catalyst structures,Figure 3 schematically illustrates an energy harvesting system, and Figures 4A-B are flow charts illustrating methods.

DETAILED DESCRIPTION Aspects of the present disclosure will now be described more fully withreference to the accompanying drawings. The different devices and methodsdisclosed herein can, however, be realized in many different forms and shouldnot 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 onlyand 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.

Although the following description is focused on electrolyzers suitable for the electrolysis of water, a person skilled in the art will realize that the devices and methods herein described can also be used for electrolysis of other liquids orgases, provided that the reduction and oxidation reactions comprised in theelectrolysis process take place on the surface of a catalyst and that the electrodes are separated by a solid electrolyte.

An electrolyzer comprises two electrodes, of which one is the positivelycharged anode and one is the negatively charged cathode, and a mediumwhich allows for transport of ions, known as an electrolyte. The electrodes areconnected to a power supply which provides electrical energy, driving the electrolysis reaction. ln some electrolyzers, a solid electrolyte or ion exchange membrane is usedas the ion transport medium. An ion exchange membrane is a solid materialthat can be traversed by ions. Since this material conducts ions it can also beknown as an ionic conductor. Use of ion exchange membranes allows for acompact electrolyzer design, as well as good separation of oxygen and hydrogen gas, which is an advantage.

The ion exchange membranes used in electrolyzers can be categorizedaccording to the ionic species moving through the membrane. Anion exchangemembranes, AEl\/l, conduct the negative anion, in this case the hydroxide ion,from the cathode to the anode. Proton exchange membranes, PElVl, conductthe positive hydrogen ion or proton from the anode to the cathode. Both anionand proton exchange membranes may be permeable to water but minimizethe amount of hydrogen and / or oxygen gas that travels between the electrodes. ln electrolyzers comprising ion exchange membranes, each electrodecomprises an electrically conductive element connected to the power sourceand an electrolysis catalyst that facilitates the chemical reactions comprised inthe electrolysis process. The electrolysis catalyst may be in the form of discreteparticles, henceforth referred to in this description as catalyst particles, inwhich case the electrode may also comprise a support structure that connectsthe conductive element and the electrolysis catalyst while still being permeable to water and gases.

Herein, a conductive element is an element that has a high electricconductivity. A high electric conductivity could be an electric conductivitynormally associated with metallic or semiconducting materials, or an electricconductivity of more than 100 (Qm)'1.

A catalyst is a material or chemical compound that facilitates a chemicalreaction, e.g., by lowering the amount of energy needed to start the chemicalreaction. An electrolysis catalyst facilitates chemical reactions comprised in theelectrolysis process, such as the reaction taking place at the anode side or thereaction taking place at the cathode side. The anode- and cathode-side electrolysis catalysts will frequently comprise different materials. ln general, the support structure needs to be able to conduct electricity inaddition to being chemically stable under the conditions present in theelectrolyzer. The support structure could for example have an electricconductivity comparable to that of a metal or a semiconductor. l\/letallicmaterials such as a porous titanium mesh may be used. lt is also common to use porous carbon, carbon paper, or materials comprising carbon fibers.

During electrolysis, water enters the electrolyzer on the side of the ionexchange membrane where the anode is located. For a PEM electrolyzer,water molecules that come into contact with the electrolysis catalyst on the cathode side undergo the reaction:ZHZO _) 4H+ + 02 + 46-.

The electrons will enter the circuit connecting the two electrodes, the oxygengas leaves the electrolyzer, and the protons will diffuse across the PEl\/I to thecathode side and the cathode electrolysis catalyst, where they undergo the reaction: 4H+ + 4e" -> ZHZ. ln an AEl\/I electrolyzer, the water molecules will instead first diffuse throughthe AEl\/I to the cathode side, where the following reaction takes place at the cathode electrolysis catalyst: 11 ZHZO + 28- _) + H2.

The hydroxide ions will diffuse through the AEl\/I and undergo the followingreaction at the anode electrolysis catalyst: 4014- _ 48- _) 02 + For optimal operation of an ion exchange membrane electrolyzer, theelectrolysis catalyst needs to be in close proximity to the ion exchangemembrane so that protons or hydroxide ions can easily enter the ion exchangemembrane. As electron transport to and from the electrolysis catalyst isessential for the reactions, the electrolysis catalyst also needs to be inelectrical contact with the conductive elements. Additionally, the electrolysiscatalyst must have enough exposed surface area that water molecules caneasily adsorb to it and gas molecules can desorb. lf the electrolysis catalyst comprises particles supported by a support structure,any particle that is too far from the ion exchange membrane or has poor electriccontact with the conductive element will contribute little or not at all to theelectrolysis reactions, thereby lowering the efficiency of the electrolyzer. lt isthus an advantage to be able to control the position of the catalyst particles.

Figure 1 shows an electrolyzer 100 comprising a first 110 and a second 120electrode, and an ion exchange membrane 130 arranged in-between the firstand the second electrode. Each electrode comprises a conductive element111, 121 and at least one of the electrodes comprises a catalyst structure 140.The catalyst structure comprises a plurality of elongated nanostructures 141arranged to connect the conductive element 111, 121 to a correspondingplurality of catalyst particles 142, where each catalyst particle 142 is localizedat an end of a respective elongated nanostructure 141 opposite from theconductive element 111, 121. Both conductive elements are electrically connected to a power source 150. 12 Here, the conductive elements 111, 121 comprise conductive materials thatcan withstand the chemical environment in the electrolyzer. The conductiveelements 111, 121 may for example comprise materials such as titanium,tungsten, and /or zirconium. Optionally, a conductive element may be a steelelement coated with one or a combination of titanium, tungsten, and zirconium.

A conductive element may also comprise a carbon composite material.

As an example, the conductive elements 111, 121 may be conductive plates.A plate is taken to mean an object that is extended in two dimensions andcomparatively thin in the third dimension. The conductive elements 111, 121may also be in the shape of a sheet, or other structure suitable for electrolysis.

The ion exchange membrane comprises ionic conductors, i.e., materialsthrough which ions can travel. As an example, the ionic conductor may be apolymer such as sulfonated tetrafluoroethylene, also known as Nafion, orpolymers based on polysulfone or polyphenole oxide. However, the ionexchange membrane may also comprise other types of ionic conductors, forexample metal oxides such as doped barium zirconate, doped barium cerate, doped lanthanum gallate, or stabilized zirconia.

The catalyst particles comprise materials that are catalytically active andpromote the reactions taking place at the cathode and anode duringelectrolysis. As an example, the catalyst particles may comprise platinum,ruthenium, palladium, or iridium. The catalyst particles may also comprisemetal oxides such as platinum oxide or iridium oxide. As another example, thecatalyst particles may comprise cobalt or nickel. Optionally the catalystparticles may be nanoparticles, i.e., have a size that is substantially smallerthan one micrometer and mostly between 1 and 100 nm. Preferably, thecatalyst particles may be between 3 and 10 nm in size.

For the electrolyzer to operate efficiently, ions must be able to enter the ionexchange membrane from the catalyst surface where the chemical reactionstake place, which requires the catalyst particles to be in close proximity to theion exchange membrane 130. For example, the catalyst particles 141 may be positioned less than one micrometer from the ion exchange membrane 130. 13 Alternatively, the Catalyst particles 142 may be positioned less than 10 nm fromthe ion exchange membrane 130, or preferably less than 5 nm from the ionexchange membrane 130. Alternatively, the catalyst particles 142 may be positioned in contact with the ion exchange membrane 130.

According to yet other aspects, the catalyst particles 142 may be distributedsuch that the number of catalyst particles per unit volume is highest less thanone micrometer from the ion exchange membrane and decreases as the distance from the ion exchange membrane increases. lt should be noted that the surface of the ion exchange membrane may exhibitsurface structure, such as pores, grooves, and ridges, on a scale that is largerthan the size of the catalyst particles. ln this case, the distance between acatalyst particle and the ion exchange membrane surface is taken to be the shortest distance to the ion exchange membrane surface in any direction.

A nanostructure is a structure having a size that is substantially smaller thanone micrometer, and preferably between 1 and 100 nm, in at least onedimension. Herein, an elongated nanostructure is a nanostructure that issubstantially larger in at least one dimension, such as height, compared toanother dimension such as width or depth. As an example, consider asubstantially cylindrical nanostructure characterized by a height and a radius.The nanostructure may be considered elongated if the height is significantlylarger than the radius, e.g., if the height is more than twice as large as theradius. Similar reasoning may be applied to nanostructures that aresubstantially conical, rectangular, or of arbitrary shape.

The elongated nanostructures 141 may for example be straight, spiraling,branched, wavy or tilted. Optionally, they may be classifiable as nanowires,nano-horns, nanotubes, nano-walls, crystalline nanostructures, or amorphous nanostructu res.

According to aspects, the elongated nanostructures 141 may comprise carbonnanostructures. For example, the elongated nanostructures 141 may compriseany of, carbon nanofibers, carbon nanotubes, and/or carbon nanowires. The elongated nanostructures 141 may also comprise a combination of two or 14 more of carbon nanofibers, carbon nanotubes, and/ or carbon nanowires. Asan example, an elongated carbon nanostructure could comprise carbonnanotubes attached to a carbon nanofiber. An elongated carbon nanostructure could also be a graphene wall.

For carbon nanofibers, the shape and surface structure can be tuned byadjusting the conditions under which the nanofibers are grown in order toimprove the functionality of the resulting catalyst structure 140. As an example,the thickness of the nanofibers could be tuned to improve structural stability. ltis also possible to grow nanofibers that are arranged in a configuration that isparticularly suitable for the catalyst structure 140, e.g., with regard to thedensity of nanofibers per surface area, or the orientation of the nanofibers. lt is also possible to tune the available surface area or the number of carbonatoms per surface area. As an example, carbon nanofibers may be partlyformed by amorphous carbon, resulting in a higher number of carbon atomsper surface area. This may result in a larger number of possible sites wherecatalyst particles 142 can be attached. A similar effect may be achieved if thecarbon nanofibers have a corrugated surface structure. Herein, a corrugatedsurface structure is taken to mean that a surface has a series of grooves andridges of similar or different sizes.

Furthermore, the process of growing carbon nanofibers may involve the use ofa growth catalyst that promotes formation of carbon nanofibers. The growthcatalyst may comprise materials that are also comprised in the catalystparticles used to promote the chemical reactions comprised in the electrolysisprocess. lf the same material can be used as a growth catalyst and as anelectrolysis catalyst forming catalyst particles, fabrication of the electrolyzermay be made simpler and more efficient.

According to other aspects, the elongated nanostructures 141 may comprisecopper, aluminum, silver, gallium arsenide, zinc oxide, indium phosphate,gallium nitride, indium gallium nitride, indium gallium arsenide, silicon, or other materials.

Since the nanostructures are elongated nanostructures 141, they are larger inone dimension than in other dimensions. Consider an axis along thisdimension as the height axis of the nanostructure. lf this height axis extendsperpendicularly or nearly perpendicularly to the conductive element 111, 121,the elongated nanostructures can be considered as extending along an axis210 that is perpendicular to the conductive element 111, 121, as shown inFigure 2A.

This should not be taken to mean that the nanostructures are completelystraight or completely perpendicular to the conductive element 111, 121, asthey can for example have a moderate tilt relative to the axis 210, or they maycurve back and forth to form a spiraling or wavy shape. Rather, thenanostructures extend in the general direction of the axis 210. ln this context,a moderate tilt mean that the angle between the height axis and the axis 210is less than 45 degrees, and preferably may be less than 30 degrees.

For the elongated nanostructures 141 to effectively connect the conductiveelement 111, 121 to the catalyst particles 142, it is advantageous to have themextend from the conductive element 111, 121 in a uniform direction. Thus, theelongated nanostructures 141 may extend generally along respective axes210, where the axes are oriented in parallel to each other and extendedsubstantially perpendicularly to the conductive element 111, 121.

The catalyst structure 140 must also allow a sufficient flow of water and ofoxygen and hydrogen gas to and from the catalyst particles and the ionexchange membrane. To improve the flow of water and gases it may beadvantageous to use other structures in addition to the elongatednanostructures. As an example, the catalyst structure 140 may comprise aporous carbon material. A porous carbon material could for example be carbonmicrofiber cloth or carbon paper. This porous carbon material may be placedadjacent to the conductive element 111, 121, with elongated nanostructures141 extending from the porous carbon material towards the ion exchangemembrane 130. 16 When considering the flow of water and gases through the Catalyst structure,it should be noted that closely spaced elongated nanostructures may impedethe water flow, but an increase in the number of catalyst particles per unit areaof the ion exchange membrane 130 may be beneficial as it would make a largeramount of electrolysis catalyst available for the electrolysis reactions.Therefore, it is advantageous to use a branched nanostructure such as theone shown schematically in Figure 2B. Each branch of the nanostructure hasone catalyst particle localized at one end, in proximity to the ion exchangemembrane. As each nanostructure has multiple branches, this means that thenumber of catalyst particles per unit area of the ion exchange membrane 130can be increased without placing the elongated nanostructures more closelytogether.

Thus, according to aspects, at least one of the elongated nanostructures 141may be a branched nanostructure comprising a trunk 201 and at least twobranches 202, where a catalyst particle is localized at the end of each branch202.

The chemical environment in an electrolyzer may be corrosive, especially onthe anode side due to the high electrical potential. To prevent degradation ofthe catalyst structure 140, the elongated nanostructures 141 may be shieldedfrom the surrounding chemical environment. For example, at least one sectionof an elongated nanostructure 141 may be covered by a protective coatingarranged to increase a resistance to corrosion. As an example, the protectivecoating may comprise any of platinum, iridium, or titanium, or a combinationthereof. As another example, the protective coating may comprise ceramicmaterials or metal oxides such as aluminum oxide, cerium oxide and zirconium oxide.

According to aspects, the surface of the elongated nanostructures 141 mayalso be chemically altered to achieve hydrophobic or hydrophilic surfaceproperties. This may be achieved through a number of methods such ascoating, etching, or chemical functionalization. 17 Figure 3 shows the electrolyzer 100 incorporated into an energy harvestingsystem 300 comprising, in addition to the electrolyzer 100, an energy source310 and a hydrogen storage device 320. Although the example given in Figure3 is an energy source 310 comprising solar cells, other energy sources suchas hydropower or wind power may also be used. The energy harvestingsystem 300 may be particularly useful for storing excess energy fromintermittent energy sources such as solar cells and wind power in the form of hydrogen gas.

With reference to Figure 4A and Figure 1, there is also disclosed a method ofproducing a catalyst structure 140 for an electrolyzer 100. The electrolyzer 100comprises a first 110 and a second 120 electrode and an ion exchangemembrane 130 arranged in-between the first and the second electrode. Eachelectrode comprises a conductive element 111, 121. The method comprisesgenerating SA1 a plurality of elongated nanostructures 141, the elongatednanostructures 141 being connected to a conductive element 111, 121. Themethod further comprises attaching SA2 a plurality of catalyst particles 142 tothe plurality of elongated nanostructures 141 such that each catalyst particle142 is localized at an end of a respective elongated nanostructure 141opposite from the conductive element 111, 121.

Attaching SA2 catalyst particles 142 to elongated nanostructures 141 may beaccomplished through methods such as sputtering, chemical vapor deposition,or other methods.

Elongated nanostructures 141 may be generated through lithographicmethods such as colloidal lithography or nanosphere lithography, focused ionbeam machining and laser machining, among other methods. For nanofiberscomprising carbon or organic compounds, methods such as electrospinning orchlorination of carbides such as titanium carbide or metalloorganic compounds such as ferrocene may also be used.

According to aspects, generating SA1 a plurality of elongated nanostructures141 may comprise growing SA11 the elongated nanostructures 141 on asubstrate. Growing SA11 elongated nanostructures 141 on a substrate allows 18 extensive tailoring of the properties of the nanostructures. For instance, thegrowth conditions may be selected to increase the surface area of eachnanostructure. According to aspects, the elongated nanostructures may be grown in a plasma.

The substrate may comprise materials such as silicon, glass, stainless steel,ceramics, silicon carbide, or any other suitable substrate material. Thesubstrate may also comprise high temperature polymers such as polymide.Optionally, the substrate may be a component of the electrolyzer 100, such as a conductive plate 111, 121 or the ion exchange membrane 130.

Growing SA11 the elongated nanostructures 141 on a substrate may comprisedepositing a growth catalyst layer on one surface of the substrate and growingSA11 the elongated nanostructures 141 on the growth catalyst layer.

Herein, a growth catalyst is a substance that is catalytically active andpromotes the chemical reactions comprised in the formation of nanostructures.

The growth catalyst may comprise materials such as nickel, iron, platinum,palladium, nickel-silicide, cobalt, molybdenum, gold, or alloys thereof. As anexample, the growth catalyst layer may be between 1 and 100 nm thick. Asanother example, the growth catalyst layer may comprise a plurality of particlesof growth catalyst.

Growing SA11 the elongated nanostructures 141 on the growth catalyst layermay comprise heating the growth catalyst layer to a temperature wherenanostructures can form and providing a gas comprising a reactant in such away that the reactant comes into contact with the growth catalyst layer. Here,the reactant is a chemical compound or mix of chemical compounds thatcomprises the chemical elements used to form the nanostructure. For a carbonnanostructure, the reactant may comprise a hydrocarbon such as methane or acetylene, or it may comprise carbon monoxide.

According to aspects, the growth catalyst materials and the parameters of thegrowth process may be selected to achieve so-called tip growth of thenanostructures. During tip growth, a nanostructure will grow beneath a section 19 of the growth Catalyst, resulting in an elongated nanostructure with a remainingparticle of growth catalyst at the tip of the elongated nanostructure.

Attaching SA2 catalyst particles 142 to the elongated nanostructures 141 maybe accomplished using the remaining particle of growth catalyst. As anexample, chemical elements present in the remaining particle of growthcatalyst may be replaced with other chemical elements through methods suchas galvanic replacement. As another example, the remaining particle of growthcatalyst may be selectively coated with an electrolysis catalyst material suitable for use in the electrolyzer 100.

According to aspects, depositing a growth catalyst layer may comprise spincoating the surface of the substrate with particles of growth catalyst. Accordingto other aspects, depositing a growth catalyst layer comprises depositing auniform growth catalyst layer and introducing a pattern onto the depositeduniform growth catalyst layer. lntroducing a pattern onto the deposited uniformgrowth catalyst layer could comprise altering the thickness of the growthcatalyst layer according to a pattern, or selectively removing the growthcatalyst layer in some places. lntroducing a pattern onto the growth catalystlayer may for example be accomplished through lithographic methods such ascolloidal or nanosphere lithography. The patterning of the growth catalyst layermakes it possible to control the density of nanostructures per surface area onthe substrate.

According to aspects, growing SA11 the elongated nanostructures on asubstrate comprises depositing a conducting layer on a surface of thesubstrate. The growth catalyst layer may then be deposited on top of theconducting layer. After growing the elongated nanostructures, parts of theconductive layer that extend between or around the elongated nanostructuresmay be selectively removed. This removal may for example be accomplishedthrough etching, e.g., plasma etching, pyrolysis etching or electrochemicaletching.

The conducting layer electrically grounds the substrate, which is an advantagefor certain methods of nanostructure growth such as growth in a plasma. lt may also prevent the diffusion of atoms between the growth Catalyst layer andthe substrate.

According to aspects, the conducting layer may be between 1 and 100 micronsthick.

According to aspects, additional layers may be present in addition to thesubstrate, the growth catalyst layer, and the conducting layer. The materialscomprised in the additional layers may be selected to tune properties of thegrown nanostructures, facilitate vertically oriented growth, or otherwiseimprove the result of the growth process. The additional layers may alsocomprise a conducting element 111, 121 forming part of an electrode 110, 120for an electrolyzer 100.

According to aspects, depositing any layer including the conducting layer andthe growth catalyst layer may be carried out by methods such as evaporating,plating, sputtering, molecular beam epitaxy, pulsed laser depositing, chemicalvapor deposition, spin-coating, spray-coating, or other suitable methods.

As an example, producing a catalyst structure 140 may comprise generatingSA1 elongated nanostructures by depositing a conducting layer on an uppersurface of a substrate; depositing a layer of growth catalyst on the conductinglayer; growing the elongated nanostructures 141 on the layer of growthcatalyst; and selectively removing the conducting layer between and aroundthe elongated nanostructures. lt may also comprise attaching SA2 catalystparticles 142 to the elongated nanostructures following the growth process.

According to aspects, the elongated nanostructures may be grown on asubstrate comprising a component of the electrolyzer 100, such as one of theconductive elements 111, 121 or the ion exchange membrane 130. Accordingto other aspects, the elongated nanostructures may be grown on some othersubstrate and subsequently transferred onto for example one of the conductiveelements 111, 121 or the ion exchange membrane 130.

Figure 4B shows a flowchart of a second method of producing a catalyststructure 140 for an electrolyzer 100, where the electrolyzer 100 comprises afirst 110 and a second 120 electrode, and an ion exchange membrane 130 21 arranged in-between the first and the second electrode. Each electrodecomprises a conductive element 111, 121. The method comprises configuringSBO a substrate having a surface and selecting SB1 a growth catalyst for thegrowth of elongated nanostructures on the substrate. The growth catalyst isselected such that it can also be used as an electrolysis catalyst in theelectrolyzer 100. The method also comprises depositing SB2 a growth catalystlayer comprising the selected growth catalyst on the surface of the substrateand generating SB3 elongated nanostructures 141 with a catalyst particle 142suitable for use in an electrolyzer 100 localized at an end of each elongatednanostructure 141 by growing SB31 elongated nanostructures 141 on thegrowth catalyst layer. ln addition to the previously described advantages of growing elongatednanostructures on a substrate, such as being able to tailor the properties ofthe nanostructures, this method has the advantage of simplifying theproduction of the catalyst structure 140 by using the same material in thegrowth catalyst as in the electrolysis catalyst particles 142. Attaching catalyst particles to the elongated nanostructures is thus not done in a separate step.

The substrate may comprise materials such as silicon, glass, stainless steel,ceramics, silicon carbide, or any other suitable substrate material. The substrate may also comprise high temperature polymers such as polymide.

To select a growth catalyst that is also suitable for use as an electrolysiscatalyst in an electrolyzer 100, it is preferred to find materials that cansuccessfully act as catalysts for both the growth process and the chemicalreactions comprised in the electrolysis process. Examples of suitable materialsmay be platinum, palladium, and nickel, which are used both in growth catalysts for growing nanostructures and in electrolysis catalysts. 22 Growing SB31 the elongated nanostructures 141 on the growth Catalyst layermay comprise heating the growth catalyst layer to a temperature wherenanostructures can form and providing a gas comprising a reactant in such away that the reactant comes into contact with the growth catalyst layer. Here,the reactant is a chemical compound or mix of chemical compounds thatcomprises the chemical elements used to form the nanostructure. For a carbonnanostructure, the reactant may comprise a hydrocarbon such as methane or acetylene, or it may comprise carbon monoxide.

According to aspects, the elongated nanostructures may be grown in a plasma.

According to aspects, the growth catalyst materials and the parameters of thegrowth process may be selected to achieve so-called tip growth of thenanostructures. During tip growth, a nanostructure will grow beneath a sectionof the growth catalyst, resulting in an elongated nanostructure with a remainingparticle of growth catalyst at the tip of the elongated nanostructure.

As an example, a substrate may be configured SBO to comprise one of theconductive elements 111, 121 of an electrolyzer electrode 110, 120 and agrowth catalyst may be selected SB1 such that it can also be used as anelectrolysis catalyst. lf, after depositing SB2 the growth catalyst layer, theparameters of the growth process are tuned to achieve tip growth, growingSB31 elongated nanostructures on the growth catalyst layer will result in aplurality of elongated nanostructures attached to a conductive element with acatalyst particle localized at one end of each elongated nanostructure,opposite from the conductive element.

Depositing a growth catalyst layer SB2 may also comprise depositing auniform growth catalyst layer and introducing SB22 a pattern onto thedeposited uniform growth catalyst layer. As previously mentioned, introducinga pattern onto the deposited uniform growth catalyst layer could comprisealtering the thickness of the growth layer according to a pattern, or selectivelyremoving the growth catalyst layer in some places. lntroducing a pattern ontothe growth catalyst layer may for example be accomplished through 23 Iithographic methods such as colloidal or nanosphere lithography. Thepatterning of the growth Catalyst layer makes it possible to control the density of nanostructures per surface area on the substrate The method may also comprise depositing SB21 a conducting layer on asurface of the substrate. The growth catalyst layer may be deposited on top ofthe conducting layer. After growing the elongated nanostructures, parts of theconductive layer that extend between or around the elongated nanostructuresmay be selectively removed. This removal may for example be accomplishedthrough etching, e.g., plasma etching, pyrolysis etching or electrochemicaletching. According to aspects, the conducting layer may be between 1 and 100 microns thick.

The conducting layer electrically grounds the substrate, which is an advantagefor certain methods of nanostructure growth such as growth in a plasma. ltmay also prevent the diffusion of atoms between the growth catalyst layer andthe substrate.

According to aspects, additional layers may be present in addition to thesubstrate, the growth catalyst layer, and the conducting layer. The materialscomprised in the additional layers may be selected to tune properties of thegrown nanostructures, facilitate vertically oriented growth, or otherwiseimprove the result of the growth process. The additional layers may alsocomprise a conducting element 111, 121 forming part of an electrode 110, 120 for an electrolyzer 100.

According to aspects, the elongated nanostructures may be grown on asubstrate comprising a component of the electrolyzer 100, such as one of theconductive elements 111, 121 or the ion exchange membrane 130. Accordingto other aspects, the elongated nanostructures may be grown on some other 24 substrate and subsequently transferred onto for example one of the conductiveelements 111, 121 or the ion exchange membrane 130.

Claims

1. An electrolyzer (100) comprising a first (110) and a second (120)electrode and an ion exchange membrane (130) arranged in-between the firstand the second electrode, each electrode comprising a conductive element(111, 121), at least one electrode comprising a catalyst structure (140), thecatalyst structure comprising a plurality of elongated nanostructures (141)arranged to connect the conductive element (111,121) to a correspondingplurality of catalyst particles (142), where each catalyst particle (142) islocalized at an end of a respective elongated nanostructure (141) oppositefrom the conductive element (111,121).

2. The electrolyzer according to claim 1, wherein the elongatednanostructures (141) comprise carbon nanostructures.

3. The electrolyzer (100) according to claim 2, where the elongatedcarbon nanostructures (141) comprise any of: carbon nanofibers, carbon nanotubes, and/or carbon nanowires.

4. The electrolyzer (100) according to any previous claim, whereinthe catalyst particles (142) are positioned less than 10 nm from the ionexchange membrane (130), and preferably less than 5 nm from the ion exchange membrane.

5. The electrolyzer (100) according to claim 4, wherein the catalystparticles (142) are positioned in contact with the ion exchange membrane(130).

6. The electrolyzer (100) according to any previous claim, whereinthe elongated nanostructures (141) extend generally along respective axes,where the axes are oriented in parallel to each other and extendedsubstantially perpendicularly to the conductive element (111,121).

7. The electrolyzer (100) according to any previous claim, whereinthe catalyst structure (140) comprises a porous carbon material.

8. The electrolyzer (100) according to any previous claim, wherein atleast one of the elongated nanostructures (141) is a branched nanostructurecomprising a trunk (201) and at least two branches (202), where a Catalyst particle is Iocalized at the end of each branch (202).

9. The electrolyzer according to any previous claim, wherein at leastone section of an elongated nanostructure (141) is covered by a protective coating arranged to increase a resistance to corrosion.

10. The electrolyzer according to claim 9, wherein the protectivecoating comprises any of platinum, iridium, or titanium, or a combination thereof.

11. The electrolyzer according to any previous claim, wherein the conductive elements (111, 121) are conductive plates.

12. A method of producing a catalyst structure (140) for anelectrolyzer (100), the electrolyzer (100) comprising a first (110) and a second(120) electrode and an ion exchange membrane (130) arranged in-betweenthe first and the second electrode, each electrode comprising a conductiveelement (111, 121), the method comprising: generating (SA1) a plurality of elongated nanostructures (141 ), theelongated nanostructures (141) being connected to a conductiveelement (111, 121); and attaching (SA2) a plurality of catalyst particles (142) to the pluralityof elongated nanostructures (141) such that each catalyst particle(142) isnanostructure (141) opposite from the conductive element(111,121). Iocalized at an end of a respective elongated

13. The method according to claim 12, wherein generating (SA1) aplurality of elongated nanostructures (141) comprises growing (SA11) theelongated nanostructures (141) on a substrate.

14. The method according to claim 13, where the growing (SA11) theelongated nanostructures (141) on a substrate comprises depositing a growthcatalyst layer on a surface of the substrate and growing the elongatednanostructures (141) on the growth catalyst layer.

15. The method according to claim 14, where depositing a growthCatalyst layer comprises depositing a uniform growth catalyst layer andintroducing a pattern onto the deposited uniform growth catalyst layer.

16. The method according to any of claims 13 to 15, comprisingdepositing a conducting layer on a surface of the substrate.

17. A method of producing a catalyst structure (140) for anelectrolyzer (100), the electrolyzer (100) comprising a first (110) and a second(120) electrode, and an ion exchange membrane (130) arranged in-betweenthe first and the second electrode, where each electrode comprises aconductive element (111, 121), the method comprising: configuring (SBO) a substrate having a surface; selecting (SB1) a growth catalyst for the growth of elongatednanostructures on the substrate, such that the growth catalyst can also beused as an electrolysis catalyst in the electrolyzer (100); depositing (SB2) a growth catalyst layer comprising the selectedgrowth catalyst on the surface of the substrate; and generating (SB3) elongated nanostructures (141) with a catalystparticle (142) suitable for use in an electrolyzer (100) localized at an end ofeach elongated nanostructure (141) by growing elongated nanostructures(141) on the growth catalyst layer.

18. The method according to claim 17, where depositing a growthcatalyst layer (SB2) comprises depositing a uniform growth catalyst layer andintroducing (SB22) a pattern onto the deposited uniform growth catalyst layer.

19. The method according to any of claims 17 or 18, comprising depositing (SB21) a conducting layer on a surface of the substrate.