SE2130154A1 - An electrolyzer with a nanostructured catalyst support - Google Patents

Abstract

An electrolyzer comprising a first and a second electrode and an ion exchange membrane arranged in-between the first and the second electrode. Each electrode comprises a conductive element. At least one electrode comprising a catalyst structure, the catalyst structure comprising a plurality of elongated nanostructures and a plurality of electrocatalyst particles. The electrocatalyst particles are affixed to the plurality of elongated nanostructures and the elongated nanostructures extend generally along respective axes, where the axes are oriented in parallel to each other and substantially perpendicularly to the conductive element..

Description

TITLE An electrolyzer with a nanostructured catalyst support TECHNICAL FIELD The present disclosure relates to devices used in electrolysis, particularly for the electrolysis of water.

BACKGROUND The production of hydrogen gas through the electrolysis of water is a promising technology both for replacing the production of hydrogen gas from fossil fuels and as a means of converting excess electrical energy from intermittent energy sources such as solar and wind power to chemical energy for storage. However, existing water electrolyzers suffer from problems related to the corrosive conditions within the electrolysis cell and the use of expensive catalysts. For electrolysis cells comprising ion exchange membranes it may be necessary to use catalysts comprising e.g., platinum or iridium, which entails a significant cost. Additionally, current electrolysis cells are limited in terms of the ion current per area through the cell. An improvement in this regard would result 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.

SUMMARY lt is an object of the present disclosure to provide improved water electrolyzers, which, i.a., offer increased catalyst efficiency. This object is at least in part obtained by an electrolyzer comprising a first and a second electrode and an ion exchange membrane arranged in-between the first and the second electrode. Each electrode comprises a conductive element and at least one electrode comprises a catalyst structure. The catalyst structure comprises a plurality of elongated nanostructures and a plurality of electrocatalyst particles, where the electrocatalyst particles are affixed to the plurality of elongated nanostructures. The elongated nanostructures extend generally along respective axes, where the axes are oriented in parallel to each other and substantially perpendicularly to the conductive element.

The elongated nanostructures, extending along axes parallel to each other and substantially perpendicular to the conductive element, form a catalyst support structure that facilitates transport of the reactants and products of the chemical reaction and allows for optimal use of the available electrocatalyst particles, which is an advantage.

At least one of the elongated nanostructures may be arranged to extend at least partially into the ion exchange membrane.

Advantageously, allowing the elongated nanostructures to extend into the ion exchange membrane improves the contact between the ion exchange membrane and the electrocatalyst particles. The transport of ions to and from the surface of the electrocatalyst particles is thus improved, which makes the catalyst structure and the electrolyzer more efficient.

According to aspects, at least one electrocatalyst particle may be affixed to a first section of at least one elongated nanostructure, and at least this first section of the elongated nanostructure may then extend into the ion exchange membrane. Preferably, the first section is located at an end of the elongated nanostructure opposite from the conductive element, ln other words, electrocatalyst particles can be attached to a section near the tip of an elongated nanostructure, and this section is embedded in the ion exchange membrane. This allows for efficient use of the available electrocatalyst particles, as they are placed in good contact with the ion exchange membrane, while the part of the elongated nanostructures that is not embedded may facilitate transport of water and/ or gases to and from the electrocatalyst particles. lt may be that the elongated nanostructures are not all the same length, or they may be of substantially the same length but grown on an uneven substrate such as a porous metal or carbon material, leading to the tips of the elongated nanostructures being at different distances relative to the conductive element. Thus, a distance from the conductive element to an end of an elongated nanostructure opposite from the conductive element may vary among the elongated nanostructures. Elongated nanostructures for which the distance is larger may extend further into the ion exchange membrane than elongated nanostructures for which the distance is smaller, such that all elongated nanostructures extend at least partially into the ion exchange membrane. This ensures that electrocatalyst particles on all elongated nanostructures may be placed in close proximity to the membrane, which is an advantage.

According to aspects, the first section may comprise at least 90 % of the at least one elongated nanostructure.

According to other aspects, the first section may comprise less than half of the at least one elongated nanostructure.

At least one electrode may comprise a diffusion layer arranged between the conductive element and the catalyst structure. The diffusion layer may comprise a porous material. A diffusion layer may improve the transport of water and / or gases to and from the catalyst structure, which is an advantage.

According to aspects, the elongated nanostructures may comprise any of carbon nanofibers, carbon nanowires and / or carbon nanotubes. Carbon nanomaterials have good electrical conductivity and chemical stability, which is an advantage.

To prevent degradation of the elongated nanostructures, at least one of the elongated nanostructures may be covered at least in part by a protective coating arranged to increase a resistance to corrosion.

By coating the elongated nanostructures with a corrosion-resistant material, it is possible to prevent or slow degradation of the catalyst structure and increase the lifetime of the electrolyzer, which is an advantage. The protective coating may for example comprise any of platinum, iridium, titanium, or titanium nitride, or a combination thereof.

According to some aspects, the elongated nanostructures may be grown on a substrate comprising a component of the electrolyzer. Advantageously, this provides good contact between the nanostructures and the substrate.

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 rough surface such as a patterned flow plate or a porous diffusion layer, which improves electrical contact between components of the electrolyzer. A surface of the ion exchange membrane may be arranged to follow a contour of the structured surface in order to ensure close contact between the catalyst structure and the ion exchange membrane.

According to other aspects, the elongated nanostructures may be grown on a first substrate and transferred to a second substrate comprising a component of the electrolyzer. The elongated nanostructures may thus be grown on a substrate arranged for nanostructure growth and subsequently transferred to a substrate that is arranged for use in the electrolyzer. This means that the second substrate does not have to be suitable for nanostructure growth, which is an advantage.

According to aspects, the electrocatalyst particles may be attached to the elongated nanostructures using a deposition method that is an indirect or direct physicochemical and/ or physical method. Exampies of such methods include eiectrochemicai deposition, electroiess deposition, sputtering, spray-coating, dip- coating, and/ or soivent casting.

BRIEF DESCRIPTION OF THE DRAWINGS The present disclosure will now be described in more detail with reference to the appended drawings, where: Figure 1A, 1B, 1C schematically illustrate water electrolyzers; Figure 2 schematically illustrates a catalyst structure; Figure 3 schematically illustrates an apparatus for producing nanostructures; Figure 4 schematically illustrates a growth process for elongated nanostructures; Figure 5 is an image showing carbon nanofibers; and Figure 6 schematically illustrates a catalyst structure.

DETAILED DESCRIPTION By using carbon nanofibers (CNF) grown by chemical vapor deposition (CVD) as a catalyst support, it is possible to increase the active catalyst surface area in membrane electrolyzers and decrease the required catalyst load. Smoltek, as a technology provider known for nanostructure fabrication technology, is using previous experience and know-how regarding CNF production in this area of application.

The demand for improved water electrolyzer technology can be expected to increase in the near future. Producing hydrogen through electrolysis has been put fon/vard as a key method for storing surplus electricity from intermittent, renewable sources in the form of chemical energy. Simultaneously, the demand for sustainably produced hydrogen is increasing in e.g., the steel industry. ln view of this rising demand, there is an urgent need to solve issues related to existing electrolyzer technologies.

Low-temperature membrane electrolyzers, especially those using proton exchange membrane (PEl\/I) electrolytes, are of particular interest as they avoid both the use of a liquid electrolyte as used in conventional alkaline electrolyzers and the high operation temperatures seen in solid oxide electrolyzers. However, PEl\/l water electrolyzers require the use of platinum group metals as catalysts, usually platinum on the cathode side and iridium, in the form of iridium oxide, on the anode side. Ensuring that these expensive catalysts are used efficiently and reducing the necessary catalyst load is a challenge for PEM electrolyzer development.

A key component in this context is the catalyst support, which serves as a scaffolding on which nanoparticle catalysts can be dispersed as well as to electrically connect the catalyst material to the diffusion layer and separator plates.

Carbon materials are already used for this purpose on the cathode side of PEM water electrolyzers, but CNF and especially CVD-grown CNF have a number of additional advantages.

The CVD fabrication process enables the production of vertically aligned CNF with a well-defined average spacing, width and height, which gives a high degree of control over the structure of the resulting catalyst support. By selectively coating parts of the fibers with catalyst particles, this also results in improved control of the position of the catalyst particles, which makes it possible to optimize the active catalyst surface area and reduce catalyst load. The regular structure of CVD-CNF is expected to improve mass transport through the catalyst layer and facilitate the application of corrosion-resistant coatings such as titanium, which render the nanofibers more useful also on the anode side of a PEl\/l water electrolyzer.

As countries work to reduce their dependence on fossil fuels and lower greenhouse gas emissions, more renewable energy sources such as solar, water, and wind power are being introduced. The intermittent nature of these energy sources creates a demand for energy storage solutions, so that an excess of electricity produced under windy or sunny conditions can be stored and used at a later time. A recent report published by the European Commission mentions storage methods such as batteries and pumped-hydro storage. However, it also proposes that storing the excess as chemical energy through electrolysis of water, producing hydrogen, is important for reaching long-term goals [1]. Fuel cells can then provide an efficient method for converting the stored chemical energy back into electricity.

Production of hydrogen through electrolysis of water, rather than from fossil fuels, is also important for reducing the carbon footprint of industries such as steel production. Replacing current production methods with direct reduction of iron ore using hydrogen gas could reduce greenhouse gas emissions by as much as 90 % [2], but requires significant amounts of sustainably produced hydrogen. Companies around the world are dedicating research and development efforts to implementing this method [3], [4]. For example, in Sweden the steel production company SSAB, the mining company LKAB and state-owned energy company Vattenfall collaborate on technology development in the HYBRIT project [5], [6].

Together, the demand for energy storage solutions and the need for hydrogen gas in industry point towards an increasing demand for water electrolyzer technology in the near future. However, existing electrolyzer technology is associated with issues relating to corrosive electrolytes and the need to reduce the amount of expensive noble-metal catalysts. Due to these issues, innovations in electrolyzer technology are very likely to be of high interest.

Among existing electrolyzer types, proton exchange membrane (PEM) electrolyzers typically show the highest production rates per unit area with current densities of 1-2 A/cm2, and current densities between 6 and 10 A/cm2 have been demonstrated in experimental cells even with low catalyst load [7], [8].

As a technology provider known for cutting-edge nanostructure fabrication technology, Smoltek has previously developed devices for the semiconductor and electronics industries [9]. Now, we reuse the same technology with the aim of modernizing hydrogen gas production and using carbon nanostructures to improve electrolyzer productivity. Carbon nanostructures enable a higher degree of control over the position of the catalyst particles and allows for optimization of the active surface area, leading to increased productivity and/or reduced need for expensive catalysts.

On a basic level, all water electrolyzer cells include two electrodes, a positively charged anode and a negatively charged cathode, as well as an electrolyte. The electrodes are connected to a DC power source that maintains an electrical current through the cell, thereby supplying the energy that drives the reaction. The overall electrochemical reaction taking place in a water electrolyzer is 1 H20 _) H2 + E02, with hydrogen gas being generated at the cathode and oxygen gas at the anode [10]. The cathode reaction is also frequently referred to as the hydrogen evolution reaction, or HER, while the anode reaction is referred to as the oxygen evolution reaction, OER.

Water electrolyzers can be divided into traditional alkaline electrolyzers and ion exchange membrane electrolyzers. The electrolyte and the materials used in the electrodes differ depending on the type of electrolyzer and at what pH and temperature the electrolysis is performed. ln most electrolyzers, electrocatalysts are used to lower the energy barrier for the electrochemical reactions.

Alkaline electrolysis has historically been the most common water electrolysis method [10]. Alkaline electrolysis uses a liquid electroiyte, typically a solution of water and potassium hydroxide or sodium hydroxide, in which the electrodes are submerged as i||ustrated in Figure 1A. The electrochemical reactions take place at the electrode surfaces, with the cathode reaction being 2H20 + ze- -> H2 + 20Hy and the anode reaction being 1 20H- -> E02 + H20 + zei ln alkaline electrolyzers it is possible to use cheap and abundant materials such as nickel and stainless steel in the electrodes, sometimes coated with additional electrocatalysts that facilitate the chemical reactions taking place at the electrode surface. The catalysts are usually also based on non-noble metals such as nickel, iron or zinc, often in the form of alloys or oxides [11] [12]. The electrolyzer cell is divided into an anode side and a cathode side by a diaphragm that separates the hydrogen and oxygen gases produced in the reaction, while still allowing passage of hydroxide ions from the cathode to the anode. The diaphragm often consists of a porous ceramic or polymer material [11].

Alkaline electrolysis is a mature technology, but it also has some known drawbacks. For example, the liquid electrolyte is associated with losses connected to its specific conductivity. Lately, this problem has been mitigated by using so-called zero-gap designs, where the electrodes are situated in contact with either side of the diaphragm [12]. Alkaline electrolysis also requires the handling of substantial amounts of strongly alkaline electrolyte.

Table 1: Summary of electrolyzer types. Data from [13], [14], [15].

Electrolyzer Current Operating Electrolyte Catalyst type density Temperature materials A/cm^2 (°C) Alkaline 0.2 - 0.7 60 - 80 Water-KOH Non-noble solution metal alloys / oxides, e.g., Fe, Ni, Co High 0.2 - 1.0 600-700 Yttrium- Ni-YSZ alloys, temperature stabilized perovskite zirconia oxides PEM 1.0 - 2.2 50 - 84 Polymer Platinum membranes, (cathode), Nafion iridium oxide (anode) AEM 0.1 - 0.5 50-70 Polymer Non-noble membranes metal alloys / oxides, e.g., Fe, Ni, Co ln ion exchange membrane electrolyzers, the liquid electrolyte is replaced by an ionically conducting solid electrolyte or membrane. Rather than merely being a porous material, like the diaphragm in an alkaline electrolyzer, the membrane selectively conducts one type of ion, which allows for a more efficient separation of the two sides of the electrolyzer and of the product gases [10]. Additionally, although in some ion exchange membrane electrolyzers the reactions still occur at a high pH, the required volume of alkaline solution is reduced and the electrolyzer design can be made more compact. lon exchange membrane electrolyzers can be categorized according to their operation temperature and according to which ionic species is conducted by the membrane. Operation temperatures in existing electrolyzers fall either in the high- temperature range above 600 °C, where oxides such as yttrium-stabilized zirconia can be used as ionic conductors, or below 100 °C where hydrated polymer membranes such as Nafion® can be used.

High temperature electrolyzers have the advantage of being less dependent on efficient catalysts, as the increased operation temperature reduces the activation potential for the electrolysis reactions. However, the high temperature leads to degradation of the component materials and longer startup times, making this type of electrolyzer less well suited for applications that makes use of intermittent power sources [10], [16]. Here, the focus will therefore be on low temperature electrolyzers. However, it should be noted that water electrolysis in the intermediate temperature range, between 100 and 600 °C is a topic of ongoing research [10], [17]. So far, no electrolyzers in this temperature range appear to have been commercialized.

Proton exchange membranes (PEl\/l) are well established as electrolyte materials for low-temperature fuel cells. The most well-known is sulfonated tetrafluoroethylene, also known as Nafion® [10], although alternative polymer membranes are commercially available and also a topic of research (see, e.g., [18]). Generally, PEl\/l become proton-conducting upon absorbing liquid water, which limits the operation temperature to below 100 °C. ln a PEl\/l electrolyzer, illustrated in Figure 1B, the electrolysis reaction takes place under acidic rather than alkaline conditions. Under these conditions, the anode reaction becomes: 2H2o -> 4H+ + oz + Lie-_ The protons traverse the membrane and undergo the cathode reaction: 4H+ + 4eT -> ZHZ.

Electrolysis under acidic condition is associated with a higher potential barrier compared to electrolysis under alkaline conditions. Thus, for the reaction rate to be high enough, more efficient catalysts need to be used. The catalysts also need to be chemically stable in the acidic environment. Currently, the most common anode-side catalyst is iridium oxide, lrO2, while platinum is typically used on the cathode side. PEl\/l electrolyzers can achieve higher current densities than alkaline or anion exchange membrane electrolyzers (see summary in Table 1), but platinum and iridium 11 are also more expensive than the catalyst materials used in other electrolyzer types. Thus, a significant amount of research and development effort in both academia and industry is directed to reducing the necessary catalyst load and/or finding alternative catalysts [19], [20].

Anion exchange membrane (AEM) electrolyzers, illustrated in Figure 1C, can also be considered as alkaline electrolyzers with a zero-gap design, using a membrane that selectively conducts hydroxide ions instead of a conventional diaphragm. The AEM is typically a polymer membrane, consisting of e.g., polysulphones or polyphylene oxides, with cation groups attached to the polymer chain backbone. As is the case for PEM, the AEM needs to absorb liquid water in order to act as ionic conductors [15].

AEM electrolyzers retain many of the advantages associated with alkaline electrolysis, such as the possibility of using alloys or oxides of non-noble metals as catalysts, with the additional advantage of improved separation of the product gases that comes with using an ionically conducting membrane in the place of a liquid electrolyte [10]. However, existing AEM materials are not sufficiently chemically stable for long-term use, limiting the lifetime of electrolyzer cells to only around 1000 h [15]. Compared to proton exchange membrane electrolyzers, which operate at similar temperatures, AEM also show lower current densities (see Table 1).

The use of a membrane rather than a liquid electrolyte requires changes to the design of the electrolyzer cell. Examples of this can be seen in Figure 1B and Figure 1C, which show schematic depictions of PEl\/l and AEM electrolyzers, respectively. ln both cases, the membrane is in close contact with catalyst layers on both the anode and the cathode side. Further away from the membrane there is a material known as the current collector, porous transport layer, or gas (and/or liquid) diffusion layer. This is followed by the conducting plates, which are also sometimes referred to as separator plates or flow plates. ln systems where multiple electrolyzers are connected in series the conducting plate serves as part of the cathode for one electrolyzer cell and part of the anode for an adjacent electrolyzer cell, in which case it is often referred to as a bipolar plate [15], [21].

The electrochemical reactions in the cell take place at the catalyst surface and optimizing the reaction rate therefore places a number of requirements on the catalyst layer and its connection to other parts of the cell. Firstly, water must be able to reach the anode-side catalyst and gases (hydrogen or oxygen depending on the electrode) 12 must be able to diffuse away from the catalysts. Secondly, as the electrochemical reactions either generate or consume ions and electrons, the catalyst must be electrically connected to the power source, which among other things requires the contact resistance between catalyst layer and the diffusion layer and between the diffusion layer and the separator plate to be low.

The electrochemical reactions take place at the catalyst surface but depending on the structure of the catalyst layer only part of the catalyst surface may contribute. For the reactions to proceed efficiently the catalyst surface must be in contact with both the membrane, allowing ion transport to or from the catalyst, and the surrounding water and gas that allows for transport of reactants and products. This is sometimes described as the reaction occurring at the three-phase boundary between membrane, water, and catalyst. The surface area where these conditions are met is the active catalyst surface area, and the catalyst layer needs to be structured to make it as large as possible. Like all components of the electrolyzer cell, the catalyst layer must also be chemically stable [15], [19], [21]. ln practice, catalyst layers often contain nanoparticles of the catalytically active material, such as platinum or iridium oxide, either unsupported or dispersed on a porous, electrically conducting catalyst support such as carbon black or a titanium mesh [19], [21]. nanostructured supports may also be used [20]. The catalyst is often mixed with an Thin films of catalytically active material sputtered onto ionically conducting polymer in order to improve the transport of ions to and from the catalyst particles. Usually, the membrane and the catalyst layers are fabricated together in what is known as a membrane-electrode assembly (MEA).

Catalyst supports used today, such as carbon black, typically have an irregular porous structure (see Figure 2). However, more regular nanostructured supports made from nanowires, nanowhiskers, nanofibers, and nanotubes have been considered [19], [20], [22]. A catalyst support with a more regular structure can enable better control over the placement of the catalyst, leading to better contact between the catalyst and the membrane.

The layer between the conductive plate and the catalyst layer, known as the diffusion layer, porous transport layer, or the current collector, faces a similar set of requirements, except for the need for ionic conductivity. The diffusion layer serves to connect the catalyst layer to the conductive plate in order to allow electric current to 13 flow between the power source and the catalyst, while also allowing mass transport of the reactants and products of the electrolysis reaction, i.e., water, oxygen, and hydrogen. lt can also serve as a heat conductor and as a mechanical support for the l\/IEA. Carbon paper or carbon felt are often used on the cathode side, while porous metal structures are more often used on the anode side due to the harsher chemical conditions. ln PEM cells this often means a titanium mesh, while in AEM cells the anode-side diffusion layer may be a nickel foam [15], [21].

As previously mentioned, the contact resistance between the diffusion layer, the conductive plate and the catalyst should be kept low in order to reduce Ohmic losses in the cell. This can particularly be a problem on the anode side of PEl\/l electrolyzers, as the titanium used in the diffusion layer forms an electrically insulating oxide layer on the surface. The oxide layer protects the remaining metal from the harsh environment but also increases the contact resistance. Attempts to mitigate this problem include both altering the structure of the diffusion layer and introducing new surface treatments for the porous titanium structure [23], [24], [25].

For PEM and AEM electrolyzers, the mass transport problem is compounded by the fact that the incoming water is a liquid, while the produced hydrogen and oxygen are gaseous. Thus, gas bubbles may form in the catalyst and/or diffusion layers on the anode side, and conversely water droplets may form on the cathode side. Gas bubbles in particular can block mass transport and, if they remain on the catalyst surface, reduce the amount of available catalyst surface area in the cell [26], [27].

The void fraction and pore size of the catalyst support and gas diffusion layer strongly influence the formation and movements of bubbles. lt has also been observed that the pore shape is important for bubble transport, leading to a tradeoff between maximizing the surface area in the catalyst support and ensuring pore size and shape do not interfere with bubble transport [28]. Recently, catalyst layers containing 1D nanostructures such as nanowires, nanorods or nanofibers have been found to offer more efficient mass transport than conventional structures, particularly when it comes to gas bubbles [29], [30]. Periodic catalyst layers formed from 1D nanostructures can be designed with more efficient channels for gas transport compared to conventional porous materials.

PEl\/l and AEM electrolyzers are promising alternatives in the search for a way to convert excess electrical energy into a more easily stored form, such as hydrogen. 14 However, there is room for improvement in today's systems. For AEM, the main obstacle appears to lie in the chemical stability of ion exchange membranes, while for PEM the main challenge is tackling the need for expensive catalyst materials either by reducing the necessary catalyst load or by finding alternatives. ln both PEl\/l and AEM electrolyzers, it is also important to develop catalyst and diffusion layers that allow for improved mass transport while maintaining good electrical conductivity and low contact resistance between components.

Developing better catalyst supports is crucial both to reducing the necessary catalyst load in PEl\/l cells and for improving mass transport. Nanostructured catalyst supports with a regular structure could both improve control over the position of the catalyst relative to the membrane and the gas and water transport properties of the catalyst layer.

As seen in the previous section carbon materials such as carbon black and carbon paper are used in catalyst and diffusion layers, especially on the cathode side of electrolyzer cells, due to their electrical conductivity and relatively low price. To solve the remaining problems related to electrolyzer electrodes, however, a different type of carbon material may be needed.

Carbon nanofibers (CNF) are elongated carbon nanostructures with diameters between 1 and 100 nm and lengths from 0.1 to 100 um [31]. Although CNF can be produced in multiple different ways, such as through electrospinning [32] or from bio- carbon, the focus here will be on vertically aligned CNF grown through chemical vapor deposition (CVD). Smoltek has previously developed and patented cutting-edge CVD- based CNF production methods [9].

Chemical vapor deposition is a fabrication method where a precursor, typically a gas, is deposited on a substrate. On the substrate, it undergoes a reaction to form the fabricated material. One type of CVD that is particularly suitable for growing CNF is plasma-enhanced CVD or PECVD. ln PECVD, a carbon-containing gas such as methane or acetylene, known as the process gas, is introduced into the reactor along with an inert gas [33]. ln the reactor gases are converted into a plasma, e.g., using AC or DC discharge between two electrodes as shown schematically in Figure 3. From the plasma, carbon is deposited on the substrate (labeled as “wafer” in Figure 3). ln order for CNF to form, a catalyst must be present on the surface of the substrate. Common catalyst materials are nickel, iron, cobalt, and palladium. The catalyst can be deposited in the form of a uniform layer, or it can be patterned using lithographic techniques. The catalyst may also be deposited in the form of nanoparticles, e.g., through spin-coating. lf the catalyst is not already in the form of nanoparticles as it is deposited, it will form nanoparticles on the substrate through a process known as de- wetting (see Figure 4). During the CVD process, carbon will be deposited on the catalyst particles and diffuse across the surface, eventually forming nanofibers. Depending on parameters such as the size of the catalyst particle and the interaction between the catalyst and the substrate, the CNF will display either base growth or tip growth. During base growth, the CNF will grow upwards from the catalyst particle, which remains on the substrate. During tip growth the CNF will grow underneath the catalyst particle as illustrated in Figure 4 [33].

Using PECVD to grow CNF has several advantages. Unlike many other types of CVD, PECVD can be performed at temperatures down to around 350 °C, making it possible to grow CNF on substrates that cannot tolerate higher temperatures. PECVD can be used to produce vertically aligned CNF, which extend mostly perpendicularly from the substrate as seen in Figure 5. Also, by using a patterned catalyst it is possible to control the spacing between the nanofibers. Thus, it is possible to grow an array of nanofibers with relatively well-defined widths and heights and a desired spacing between the fibers. Other properties of CVD-grown CNF, such as mechanical properties and electrical resistivity, will vary depending on the exact growth conditions (see Table 2).

Table 2: Typical properties of PECVD-grown CNF, from [31].

Parameter Typical values Diameter 1-100 nm Length 0.1-100 um Fill factor (grown as films) 5-80 % Density < 2 g/cm2 Thermal expansion coefficient 106 -107 /K Young's modulus 80-800 GPa Poisson's ratio 0.2-0.25 Tensile strength 30 GPa Electrical resistivity 0.1 uQ-m - 2 mQ-m Thermal conductivity -3000 W/m-K Temperature tolerance >1000 °C without oxygen, > 400 °C with oxygen The rate of growth of CNF also depends on a number of growth parameters and on the catalyst that is used. CNF of around 30 um, like the ones seen in Figure 5, can be grown in around 20 to 30 minutes. Shorter CNF of around 3 um in length can be grown in as little as 5 minutes or less.

Comparing the properties of PECVD-grown carbon nanofibers with the previous discussion on membrane electrolyzers, it is clear that carbon nanofibers fulfil several of the requirements of good catalyst support and diffusion layer materials.

As can be seen in Table 2, PECVD-CNF have low electrical resistivity, as is required for both catalyst support and diffusion layer materials. Additionally, they may be useful for reducing the contact resistance between adjacent components in the electrolyzer 17 cell. Previous studies on separator plates in fuel cells, which face similar requirements, have shown that CVD-grown carbon nanofibers can reduce contact resistance [34]. PECVD-CNF grown on either a separator plate or the material of the diffusion layer may thus improve the electrical contact between the catalyst and the DOWGI' SOUFCG.

An existing application of PECVD-CNF is in supercapacitors, where they function as electrodes with high surface area as previously demonstrated by Smoltek [31], [33], [35]. High surface area is also relevant in the context of electrolyzer electrodes, particularly for the catalyst support. ln membrane electrolyzers, the catalyst particles and catalyst support are frequently mixed with ionically conducting material to improve ion transport as previously described [19], [21]. With PECVD-CNF as catalyst support, the tips of the CNF can be embedded in the ion exchange membrane, resulting in a large active surface area or three-phase boundary, where the catalyst is in good contact with the membrane (see Figure 6). This can aid in reducing the necessary catalyst load, particularly on the anode side of PEM water electrolyzers where high catalyst loads are common.

Using vertically aligned PECVD-CNF as catalyst supports also gives better control over the position of the electrocatalyst particles. lf the tips of the CNF are selectively coated with catalyst particles, or even if the catalyst used for growing the CNF is also useful as an electrolysis catalyst, positioning the tips of the CNF either 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 an efficient use of the available catalyst and enables the use of a lower catalyst loading (patent pending, see [36]).

The regular structure that can be achieved with a catalyst support made from vertically aligned PECVD-CNF may also improve mass transport. As previously mentioned, recent studies have shown improved mass transport in catalyst supports with regular structures made from nanowires or nanofibers [29], [30], particularly on the anode side where the problem of gas bubbles in water blocking the transport may occur. PECVD-CNF form such a regular structure, with the advantage that the nanofiber spacing can be precisely controlled during manufacture. This makes it possible to optimize both the void fraction and the size of the voids in such a catalyst support material. 18 One reason for carbon materials such as CNF being used in catalyst supports is their chemical stability even under harsh conditions [10]. PECVD-grown CNF can be expected to show adequate chemical stability under conditions where carbon c|oth or carbon black are used today, such as at the cathode side of an electrolyzer cell [19], [21].

PECVD-grown CNF can also be coated with layers of even more chemically stable materials such as titanium, for example through atomic layer deposition [35], [37]. This enables the use of PECVD-CNF structures also on the anode side of water electrolyzer cells, where the chemical conditions typically require using metals or metal oxides in catalyst supports and diffusion layers [19], [21].

Although electrolyzer technology is of high interest in the search for efficient energy storage solutions, there are improvements to be made to lower initial investment costs and render the technology more efficient in terms of produced hydrogen per cell area. This is especially the case for low-temperature membrane electrolyzers such as PEl\/l electrolyzers, which thanks to their low operating temperature are suitable for handling of excess electricity from intermittent power sources.

Such improvements to electrolyzer technology are likely to require innovation regarding the materials used. CNF are attractive for electrolyzer applications due among other things to their electrical conductivity and the high degree of control over their orientation and spacing provided by the PECVD production method. ln particular, PECVD-CNF show promise when used as a catalyst support, where they can provide improved control over catalyst placement and a large active catalyst surface area. Smoltek as a company possesses both technology and know-how regarding CNF production and experience from industrialization of nanofabrication technologies in the semiconductor and electronics industries and is therefore well placed to realize the promises of PECVD-CNF in electrolyzer technology.

For PEM electrolyzers, a main challenge is improving the efficiency of the anode-side catalyst. PECVD-CNF are promising in this regard, provided that they can be rendered sufficiently chemically stable. Thus, future efforts will include demonstrating the efficacy of titanium and other coatings in achieving this, as well as showing the hydrogen and oxygen production rate achievable with a PECVD-CNF catalyst suppon. 19 ln summary, what is disclosed herein is an electrolyzer comprising a first and a second electrode and an ion exchange membrane arranged in-between the first and the second electrode. Each eiectrode comprises a conductive element. At least one of the electrodes comprises a catalyst structure, which in turn comprises a plurality of elongated nanostructures and a plurality of electrocatalyst particles. The electrocatalyst particles are affixed to the plurality of elongated nanostructures and the elongated nanostructures extend generally along respective axes, where the axes are oriented in parallel to each other and substantially perpendicularly to the conductive element.

Here, the conductive elements are understood to be bipolar plates, conductive plates, or separator plates as described previously. The electrocatalyst particles are electrocatalysts as described in the preceding text.

An elongated nanostructure is a nanostructure that is substantially larger in at least one dimension, such as height, compared to another dimension such as width or depth. As an example, consider a substantially cylindrical nanostructure characterized by a height and a radius. The nanostructure may be considered elongated if the height is significantly larger than the radius, e.g., 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 arbitrary shape.

The elongated nanostructures 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 nanostructures.

Since the nanostructures are elongated nanostructures, they are larger in one dimension than in other dimensions. Consider an axis along this dimension as the height axis of the nanostructure. lf this height axis extends perpendicularly or nearly perpendicularly to the conductive element, the elongated nanostructures can be considered as extending along an axis that is perpendicular to the conductive element. This should not be taken to mean that the nanostructures are completely straight or completely perpendicular to the conductive element, as they can for example have a moderate tilt relative to the axis, or they may curve back and forth to form a spiraling or wavy shape. Rather, the nanostructures extend in the general direction of the axis. ln this context, a moderate tilt may mean that the angle between the nanostructure and the axis is less than 45 degrees, and preferably may be less than 30 degrees.

As an example, the elongated nanostructures may be grown on a substrate so as to be vertically oriented, as described above, and subsequently incorporated into the electrolyzer such that they extend from the conductive element or diffusion layer towards the ion exchange membrane.

The substrate may be a conductive element, a diffusion layer, or some other substrate.

The electrocatalyst particles may be nanoparticles, i.e., have a size that is substantially smaller than one micrometer and mostly between 1 and 100 nm. Preferably, the electrocatalyst particles may be between 3 and 10 nm in size.

At least one of the elongated nanostructures may be arranged to extend at least partially into the ion exchange membrane. For an elongated nanostructure to be extended at least partially into the ion exchange membrane may mean that the nanostructure extends into the membrane by at least 5 % of its length.

According to aspects, at least one electrocatalyst particle may be affixed to a first section of at least one of the elongated nanostructures. At least this first section of the elongated nanostructure may then extend into the ion exchange membrane, as shown in Figure 6. The first section may be located at an end of the at least one elongated nanostructure opposite from the conductive element.

According to aspects, the first section comprises at least 90 % of the at least one elongated nanostructure. According to other aspects, the first section comprises less than half of the at least one elongated nanostructure.

The elongated nanostructures may be grown on a substrate comprising a component of the electrolyzer. For example, the substrate may be the conductive element, or a porous diffusion layer arranged between the conductive element and the catalyst structure. l\/lethods for growing elongated nanostructures are described earlier in the text. ln particular, a substrate may comprise a structured surface, and the elongated nanostructures are grown on the structured surface. Here, a structured surface is a surface that is not flat but displays e.g., holes, ridges, or bumps. This could be described as surface roughness, unevenness, or patterning. For example, the substrate may be a porous material which displays surface roughness due to pores intersecting with the material surface. 21 An uneven substrate may lead to the tips of the elongated nanostructures reaching different heights relative e.g., to the conductive element even if the elongated nanostructures themselves are of a similar length. Thus, a distance from the conductive element to an end of an elongated nanostructure opposite from the conductive element may vary between elongated nanostructures. Elongated nanostructures for which the distance is larger may extend further into the ion exchange membrane than elongated nanostructures for which the distance is smaller, such that all elongated nanostructures extend at least partially into the ion exchange membrane. A surface of the ion exchange membrane may be arranged to follow a contour of the structured surface in order to ensure contact between the ion exchange membrane and the catalyst structure.

According to aspects, the elongated nanostructures may be grown on a first substrate and transferred to a second substrate comprising a component of the electrolyzer. That is, the nanostructures can be grown on a substrate that is never itself made part of the electrolyzer cell, but only serves as a growth substrate. This enables the use of a growth substrate optimized for nanostructure growth, without also needing to be suitable for use in the electrolyzer. Conversely, the second substrate may be optimized for use in the electrolyzer without needing to be suitable for nanostructure growth.

According to aspects, the electrocatalyst particles may be attached to the elongated nanostructures using a deposition method that is an indirect or direct physicochemical and/ or physical method. Examples of such methods include electrochemical deposition, electroless deposition, sputtering, spray-coating, dip- coating, and/ or solvent casting.

At least one electrode may comprise a diffusion layer arranged between the conductive element and the catalyst structure. The diffusion layer may comprise a porous material such as carbon paper or a metal foam, or a metallic mesh.

The elongated nanostructures may comprise any of carbon nanofibers, carbon nanowires and /or carbon nanotubes.

At least one in the plurality of elongated nanostructures may be covered at least in part by a protective coating arranged to increase a resistance to corrosion. For the elongated nanostructure to be covered at least in part may mean that at least 10 % of the surface of the elongated nanostructure is covered, or preferably that at least 90 22 % of the surface is covered, or even more preferably that 100 % of the surface is covered.

The protective coating may comprise any of platinum, iridium, titanium, or titanium nitride, or a combination thereof.

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Claims (15)

1. An electrolyzer comprising a first and a second electrode and an ion exchange membrane arranged in-between the first and the second electrode, each electrode comprising a conductive element, at least one electrode comprising a catalyst structure, the catalyst structure comprising a plurality of elongated nanostructures and a plurality of electrocatalyst particles, wherein the electrocatalyst particles are affixed to the plurality of elongated nanostructures, the elongated nanostructures extending generally along respective axes, where the axes are oriented in parallel to each other and extend substantially perpendicularly to the conductive element.

2. The electrolyzer according to claim 1, wherein at least one of the elongated nanostructures is arranged to extend at least partially into the ion exchange membrane.

3. The electrolyzer according to claim 2, wherein at least one electrocatalyst particle is affixed to a first section of the at least one elongated nanostructure, and wherein at least the first section of the at least one elongated nanostructure extends into the ion exchange membrane.

4. The electrolyzer according to claim 3, wherein the first section of the at least one elongated nanostructure is located at an end of the at least one elongated nanostructure opposite from the conductive element.

5. The electrolyzer according to any of claims 2 to 4, wherein the first section comprises more than 90 % of the at least one elongated nanostructure.

6. The electrolyzer according to any of claims 2 to 4, wherein the first section comprises less than half of the at least one elongated nanostructure.

7. The electrolyzer according to any previous claim, wherein at least one in the plurality of elongated nanostructures is covered at least in part by a protective coating arranged to increase a resistance to corrosion.

8. The electrolyzer according to claim 7, wherein the protective coating comprises any of platinum, iridium, titanium, or titanium nitride, or a combination thereof.

9. The electrolyzer according to any previous claim, wherein the elongated nanostructures are grown on a substrate comprising a component of the electrolyzer.

10. The electrolyzer according to claim 9, wherein the substrate comprises a structured surface, and the elongated nanostructures are grown on the structured surface.

11. The electrolyzer according to claim 10, wherein a surface of the ion exchange membrane is arranged to follow a contour of the structured surface.

12. The electrolyzer according to any of claims 1 to 8, wherein the elongated nanostructures are grown on a first substrate and transferred to a second substrate comprising a component of the electrolyzer.

13. The electrolyzer according to any previous claim, wherein the electrocatalyst particles are attached to the elongated nanostructures using a deposition method, the deposition method being an indirect or direct physicochemical and /or physical method, such as electrochemical deposition, electroless deposition, sputtering, spray-coating, dip-coating, and/ or solvent casting.

14. The electrolyzer according to any previous claim, wherein at least one electrode comprises a diffusion layer arranged between the conductive element and the catalyst structure, and the diffusion layer comprises a porous material.

15. The electrolyzer according to any previous claim, where the elongated nanostructures comprise any of carbon nanofibers, carbon nanowires, and/ or carbon nanotubes.