Technical field

The present invention relates to an electrochemical flow reactor such as a flow cell for electrochemical ammonia synthesis.

Background

Ammonia is one of the most important necessities for modern society, and is currently the second most produced industrial chemical. It is primarily used as a fertilizer, enabling the explosive growth of the global population during the past century, as well as a reactant in the chemical industry. Recently, ammonia is also being considered as an energy carrier for renewable energy sources, also referred to as power-to-X. The main advantage as an energy carrier lies in its ease of transportation, as ammonia can be liquefied and stored at comparatively milder conditions than hydrogen.

The production of ammonia currently relies on the Haber-Bosch process, which requires high temperatures of 400-500 °C, high pressures above 100-150 bar, and a hydrogen source. Consequently, the Haber-Bosch process is highly energy demanding, resulting in ca. 1 % of the global energy consumption, and since the hydrogen is typically supplied from steam-reformed natural gas, the process gives rise to significant CO2 emissions. Additionally, the high-pressure reaction conditions require large centralized facilities, with a high cost of installation and cost for transportation to the point of use of the produced ammonia.

Alternatively, ammonia may be produced electrochemically by reduction of nitrogen (N2) to ammonia (NH3) at a cathode, as shown by equation 1 , where the electrical energy can be provided from renewable sources like wind or solar power:

(Eq. 1) N ₂ + 6 H ⁺ + 6 e- ^ 2 NH ₃

The electrochemical ammonia synthesis may be carried out under mild conditions, i.e. below 100 °C and at near atmospheric pressure. However, the process selectivity towards ammonia, and hence the faradaic efficiency of the process, will depend on the process parameters, including temperature, pressure, current supply and potential, the types and supply of reactants, and presence of reaction catalysts or mediators. For example, the electrochemical ammonia synthesis may be lithium mediated, as observed experimentally and illustrated in Figure 1. The Li mediated process typically involves an aprotic solvent, a proton source, and a lithium salt, in addition to a nitrogen supply. When applying a potential of -3 V vs. reversible hydrogen electrode (RHE) and a current load, the Li ions in solution undergo reduction on the surface of the cathode, forming Li metal (shown in Figure 1 , to the left, as the lithium reduction). This potential is also referred to as the lithium reduction potential. The formed Li metal is extremely reactive, and is therefore able to split the strong triple bond and disassociate N2, forming intermediate compounds, such as for example lithium nitride U3N , in a nonelectrochemical reaction at room temperature (shown in Figure 1, second image to the left). The proton source subsequently hydrogenates the intermediate compounds, e.g. lithium nitride, whereby ammonia may be formed and Li ions released to the solution (shown in Figure 1, two images to the right). The exact mechanism is however not yet fully elucidated, but the process is known to reliably form ammonia from N2 and a proton source at ambient conditions with Faradaic efficiencies of around 10-20%.

The reaction line for converting Li ions (Li ⁺ ) to metallic lithium (Li°), further to lithium nitride U3N as intermediate compound, and further into ammonia (NH3) is also illustrated in the middle part of Figure 2 (not balanced equations).

Simultaneously with the ammonia synthesis at the cathode, hydrogen evolution occurs at the cathode by reaction of metallic lithium (Li°) and the proton source (HA), as illustrated by equation 2 below.

The hydrogen reaction competes with the ammonia synthesis, and thus affects the ammonia selectivity and faradaic efficiency. Initial faradaic efficiencies of 18.5 % (at ambient pressure, and a current density of 8 mA/cm ² ) and 30 % (at 10 bar, and a current density of 2 mA/cm ² ) may be obtained via the lithium mediated nitrogen reduction to ammonia.

The electrochemical ammonia synthesis is however generally limited to low efficiencies below 30 %, and further suffers from poor stability due to degradation mechanisms at the cathode. The cathode degradation is speculated to be related to the intermediate lithium compounds, such as lithium nitride, which remains deposited, and decreases the efficiency. Accordingly, the cathode resistance and selectivity degrades rapidly, and the ammonia synthesis is effectively terminated within hours.

The applicant’s applications WO 2021/176041 and WO 2022/175548 describe methods and apparatuses for improving the efficiency and stability of electrochemical ammonia synthesis. The applications are hereby incorporated by reference.

US 2021/301411 discloses use of gas diffusion electrodes (GDE) for reducing the resistance of an electrochemical reaction between e.g. gaseous nitrogen and a liquid electrolyte. The electrode comprises a metal support with a metal catalyst, such as a silver, gold, platinum, nickel, lithium, zinc, or titanium catalyst.

Summary

The present disclosure relates to an electrochemical flow reactor or flow cell for continuous ammonia synthesis when supplied with a flow of reactants. The flow cell provides ammonia synthesis with a surprisingly high efficiency, and facilitates continuous ammonia synthesis with a surprisingly high stability.

The high efficiency and stability may be obtained due to the selected material composition of the electrodes, e.g. the cathode and/or anode, providing higher reaction selectivity and reduced degradation, as well as resulting in reduced resistance of the flow cell components. For example, the anode may advantageously comprise a bimetallic platinum (Pt) catalyst providing high selectivity for the hydrogen oxidation reaction (HOR) thereby making the presence of membranes for improving the selectivity redundant.

The high efficiency and stability may additionally or alternatively be obtained due to an electrolyte chamber with a selected dimensional configuration and/or flow circulation guides facilitating a variable electrolyte composition during operation and a lower resistance. Advantageously, the flow circulation guides further form a grid, or comprises one or more separators dividing the electrolyte chamber into two or more subchambers, which may mechanically support the electrodes, thereby facilitating flexible and robust upscaling of the reactor size. Further advantageously, the electrolyte chamber comprises chamber openings dimensional configurated for improving the electrolyte flow circulation and the mechanical stability.

A first aspect of the disclosure relates to a flow cell for electrochemical ammonia synthesis, comprising a cathode, an anode, and an electrolyte chamber, wherein the anode comprises a HOR catalyst comprising bimetallic Pt, and/or wherein the electrolyte chamber comprises one or more spacers having a height defining a distance between the anode and cathode.

In a preferred embodiment of the first aspect, the spacers are configured as flow circulation guides.

A second aspect of the disclosure relates to a flow cell system comprising a plurality of flow cells according to the first aspect.

Description of Drawings

The invention will in the following be described in greater detail with reference to the accompanying drawings.

Figure 1 shows an embodiment of lithium mediated electrochemical nitrogen reduction to ammonia, according to the present disclosure.

Figure 2 shows an embodiment of possible cathode reactions during lithium mediated electrochemical ammonia synthesis, according to the present disclosure.

Figure 3 shows an embodiment of an electrolysis flow cell according to the present disclosure in 3D exploded view.

Figure 4 shows an embodiment of a flow cell according to the present disclosure in 3D exploded view.

Figure 5 shows an embodiment of a half flow cell according to the present disclosure in 3D exploded view (A), and in assembled perspective view (B).

Figure 6 shows an embodiment of a flow cell with a membrane (A), and without a membrane (B) in cross sectional view.

Figure 7 shows embodiments of electrolyte chambers according to the present disclosure in perspective view and cross sectional view.

Figure 8 shows embodiments of electrolyte chambers according to the present disclosure. Figure 9 shows representative SEM images of an embodiment of a cathode according to the present disclosure, as further described in Example 3.

Figure 10 shows representative SEM images of an embodiment of an anode according to the present disclosure, as further described in Example 4.

Figure 11 shows stability curves of the anode potential as a function of time when tested in an electrolyte without ammonia (A-B), and with ammonia (C-D), as further described in Example 2.

Figure 12 shows mass spectrometer data of the gas flow exiting the flow cell, as further described in Example 1.

Figure 13 shows mass spectrometer data of the gas flow exiting the flow cell, as described in Example 1.

Figure 14 shows electrochemical results (the electrode potentials and the Faradaic efficiency) as a function of time for a flow cell during operation, as further described in Example 1.

Figure 15 shows an embodiment of an electrolyte chamber according to the present disclosure, where it is indicated how a chamber opening 4.7 may be formed by cutting, e.g. cutting using a saw wheel or a wheel shaped drill.

Figure 16 shows an embodiment of an electrolyte chamber according to the present disclosure, where it is indicated how multiple chamber openings 4.7 may be formed by drilling and/or cutting, and where the chamber comprises a separator spacer or flow circulation guide 4.4 such that the electrolyte chamber is divided into two subchambers. Figure 17 shows a close up of the stippled area in Figure 15, showing two chamber openings 4.7 in perspective view.

Figure 18 shows an embodiment of the flow velocity field in (m/s) for an embodiment of the electrolyte chamber according to the present disclose, where the velocity field is based on streamline multislices, where a slice eguals sgrt(u2+v2+w2).

Figure 19 shows embodiments of electrolyte chambers according to the present disclosure imaged in perspective view, where the chamber comprises multiple chamber openings 4.7 and a separator spacer or flow circulation guide 4.4.

Detailed description

The invention is described below with the help of the accompanying figures. It would be appreciated by the people skilled in the art that the same feature or component of the device are referred with the same reference numeral in different figures. A list of the reference numbers can be found at the end of the detailed description section.

Flow cell

Electrochemical ammonia synthesis may be carried out in any type of electrolysis cell, such as a single compartment cell or a flow cell, where the electrochemical reduction of nitrogen (N2) to ammonia (NH3) occurs at the cathode, as shown by equation 1. The cathode is thus also referred to as the working electrode (WE), and the consumed electrons referred to as the cathodic current load. At the anode, oxidation takes place, and the corresponding amount of electrons are released e.g. by oxidation of hydrogen. Thus, the anode is also referred to as the counter electrode (CE), and the produced electrons or current may be referred to as an anode current load.

The electrochemical ammonia synthesis is advantageously carried out in an electrochemical flow reactor, also referred to as a flow cell. In contrast to a single compartment cell where the ammonia is synthesized in batches, a flow cell renders continuous ammonia synthesis possible, in principle for as long as the flow cell is supplied with a flow of reactants and electrical energy.

Figure 3 shows an embodiment of a flow cell 1 for electrochemical ammonia synthesis in 3D exploded view, where the reactants supplied as continuous gas flows. At the cathode side, nitrogen is supplied as a continuous gas flow to the cathode 2 and electrolyte 4. At the anode side, hydrogen is supplied as a continuous gas flow to the anode 3 and electrolyte 4. The gas may optionally be supplied to the flow cell via gas manifolds 5 (also referred to as gas flow field elements), and the electrodes may be electrically connected via current collectors 6, as illustrated in Figure 3. The gas manifolds and current collectors may reduce the gas diffusion resistance and contact resistance of the cell, and accordingly improve the reaction kinetics and reduce the resistance or impedance of the assembled flow cell. It is found that gas manifolds comprising an aperture having a meandering shape as shown in Figures 4-5 may significantly reduce the gas diffusion resistance. The gas manifold facilitates that gas is delivered as a flow field by the gas channels and distributed essentially uniformly to the surface of the electrodes. To further improve the reaction kinetics, the electrodes 2, 3 may comprise or be gas diffusion electrodes (GDE), as also indicated in Figure 3. In an embodiment of the disclosure, the anode and cathode comprise gas diffusion electrodes. In a further embodiment, the cathode and/or anode is fluidly connected to a gas manifold. In a further embodiment, the gas manifold(s) comprise an aperture having a meandering shape.

The nitrogen reduction at the cathode 2 implies the presence of protons (H ⁺ ) and lithium (Li) ions (Li ⁺ ), as sketched in Figures 1-2. Since the Li cations are not consumed and are regenerated during the ammonia synthesis, the cations may be stored at a sufficient concentration within an electrolyte chamber 4, e.g. as part of a liquid electrolyte. For example, the electrolyte may be a liquid solution of LiCICL or UBF4 in tetra hydrofuran (THF), as illustrated in Figure 3. Accordingly, the electrolyte may act as a cation source with a fixed Li ion concentration, having a concentration being sufficient to facilitate the mediation, and at the same time not impeding the availability of other reactants at the reaction sites.

The protons may also be supplied to the cathode 2 via the electrolyte 4. For example, the oxidation of hydrogen gas at the anode 3 results in protons, which are dissolved into the electrolyte 4 and thus provided to the reaction sites at the cathode.

The electrodes may be separated by a membrane 7, e.g. a proton exchange membrane, as sketched in Figure 3. The membrane allows the dissolved protons to migrate from the anode 3 to the cathode 2, simultaneously with preventing the produced ammonia (NH3) being oxidized at the anode. Ammonia is prone to be readily oxidized, and the resulting loss in ammonia product and the loss in current for the hydrogen oxidation reaction, reduces the efficiency of the ammonia synthesis. Accordingly, the produced ammonia is advantageously maintained at the cathode side of the flow cell, and may be recovered via an outlet of the electrolyte chamber 4, as indicated in Figure 3. Figure 6A shows another embodiment of a flow cell with a membrane 7. The membrane separates the electrolyte chamber into two chamber parts: a cathode chamber in contact with the cathode where ammonia may be present in the electrolyte, also referred to as the catholyte, and an anode chamber in contact with the anode where ammonia is not present in the electrolyte, also referred to as the anolyte. It follows that both the cathode chamber and anode chamber may be open and form fluid communications to allow the flow of catholyte and anolyte. In an embodiment of the disclosure, the cathode comprises a cathode chamber with a catholyte, and/or wherein the anode comprises an anode chamber with an anolyte.

It follows from Figure 3 that the chemical reactants and products of the flow cell are fluids, which may be stored outside the cell and fed by pumps into the cell to store electricity, e.g. by producing ammonia. The storage capacity and ammonia production capacity may depend on the size of the storage tanks or containers, as well as the size and performance of the flow cell. To increase the ammonia synthesis capacity and/or rate, the flow cell system may include a plurality of flow cells, such a cell stack of flow cells connected in series.

An embodiment of the disclosure relates to a flow cell system comprising a plurality of flow cells.

The chemical reactants may be continuously supplied from an external source to the cell, and the products (e.g. ammonia) may be extracted to a storage outside the system. The reactants and products are charge-neutral species, such as hydrogen, nitrogen and ammonia. The storage tanks can also be open for continuous flow to an external source or storage or grid, i.e. corresponding to a flow battery with infinite capacity. The voluminous tanks or containers to store reactants and/or products, and the flow controlling means ensuring the flow of fluid and/or gaseous reactants and products to and from the cell, influences the energy density and energy efficiency of the system. The flow controlling means, also known as balance-of-system components, may include a number of compressors, expanders, condensers, and pumps.

The external gas sources facilitate a more energy sustainable process, since the gas sources may be selected from green sources. For example, the hydrogen may be green hydrogen produced from water electrolysis powered by wind, solar, or wave electricity. This is in contrast to a process where the protons for the reaction are supplied from a chemically produced electrolyte solvent. The external gas sources further facilitate that isotope labelled ammonia may be produced.

In an embodiment of the disclosure, the cathode is fluidly connected to a nitrogen gas source, and/or the anode is fluidly connected to a hydrogen gas source. In a further embodiment, the nitrogen gas source comprises ¹⁵ N2 and/or the hydrogen gas source comprises deuterium, whereby the flow cell is configured to synthesize isotope labelled ammonia, such as ¹⁵ N and/or ² H labelled ammonia.

Example 1 further describes examples of the operation of flow cells according to the present disclosure. The flow cells are operated for ammonia synthesis by combined lithium mediated electrochemical nitrogen reduction and hydrogen oxidation. In one of the examples, isotope labelled ammonia is produced by using isotope labelled deuterium (D) as hydrogen source. Hence, ² H labelled ammonia was produced as also illustrated in Figures 12-13. This confirms that the gaseous hydrogen source provides the protons for the ammonia synthesis, since the ammonia containing deuterium can only be derived from deuterium oxidation at the anode.

To improve the performance and stability of the flow cell, the source of protons may be supplied to the cathode 2 via the electrolyte 4, both as protons derived from oxidation of hydrogen gas at the anode 3, and/or protons present in the electrolyte solvent. In addition, or alternatively, the composition of the electrolyte advantageously includes additives configured as proton shuttle or proton transport mediators, facilitating transport of the HOR derived protons to the reaction sites at the cathode. Accordingly, the additives may not necessarily act as proton donors.

An example of an electrolyte additive is ethanol, as further described in Example 1. The ethanol may act as proton donor and/or proton shuttle.

A surprisingly high Faradaic efficiency and/or stability was seen for flow cells with electrolytes comprising one or more proton shuttling additives, as further described in Example 6. Specifically, the presence of proton shuttling additives which are alcohols, such as ethanol or phenol, and optionally not ethanol, was seen to be advantageous for the performance and/or providing enhanced chemical stability. For example enhanced chemical stability of an electrolyte based on THF and Li salts may be obtained. An electrolyte based on THF and Li salts requires the additive to be soluble in the solution, as well as chemical and electrochemical stable. Improved performance as well as improved stability may be obtained for additives comprising an aromatic organic compound, i.e. including a benzene ring in the structure, as described in Example 6. In an embodiment of the disclosure, the electrolyte chamber contains an electrolyte comprising one or more proton shuttling additives. In a further embodiment, the additive is an alcohol, which is not ethanol. In a further embodiment, the additive comprises an aromatic organic compound, preferably selected from the group of: alcohols, phenol, Li-phenol oxide, hydroquinone, phloroglucinol, 1-naphthol, pyridine, and any combinations thereof.

Since protons are consumed during the synthesis, a sufficient concentration of protons may be ensured and controlled both by the HOR and the electrolyte composition. For example, the HOR may be controlled by the hydrogen gas supply, and the electrolyte composition may be controlled by circulation and replacement during operation. It follows that the proton concentration advantageously is controlled by the hydrogen gas supply, such that regeneration of the electrolyte composition is not needed.

Electrolyte chamber

As described above, the electrolyte chamber functions as a source of reactants, mediators, and products for the electrochemical reactions occurring at the anode and cathode. The reactants, mediators, and products may be present in a liquid electrolyte contained in the electrolyte chamber 4.

The electrolyte chamber 4 comprises a cavity 4.1 having two opposite ends or end planes defined by the adjacent cathode and anode, and side walls defined by an inner perimeter 4.3 of a chamber frame 4.2. Accordingly, the end planes and chamber frame defines a chamber plane, and the surface of the inner perimeter defines the chamber wall. Figure 4 shows an embodiment of an electrolyte chamber 4 in a flow cell according to the present disclosure in 3D exploded view. The cathode and anode may be planar layers, as shown in Figure 4, and the cavity 4.1 thus forms a plane shaped cuboid. Figure 5 shows an embodiment of a half flow cell according to the present disclosure in 3D exploded view (A), and in assembled perspective view (B). It follows that the chamber frame 4.2 may be considered as frame plane or plate with a cuboid cavity.

In an embodiment of the disclosure, the electrolyte chamber comprises a cavity having two opposite ends defined by the anode and cathode, and side walls defined by an inner perimeter of a chamber frame. The electrical resistance of the electrolyte will depend on the dimensions of the cavity 4.1 , and specifically the height or thickness of the cavity, corresponding to the distance between the cathode and anode. The bigger the distance, the higher the electrolyte resistance.

The electrode layers may be abutting each side plane of the chamber frame 4.2, and in this case the distance between the cathode and anode are determined by the thickness of the chamber frame. To reduce the electrical resistance, and thus improve the flow cell performance, the chamber frame advantageously has a thickness between 50 pm - 20 mm, more preferably between 500 pm - 10 mm, and most preferably between 1 mm - 6 mm , such as 2 or 4 mm.

Alternatively or additionally, the distance between the anode and cathode is defined by one or more spacers 4.4 or fixation elements within the electrolyte chamber 4, as illustrated in Figure 4. The spacers may be positioned within the chamber cavity 4.1 such that the height or thickness of the spacers define a distance between the anode and cathode. Additionally, the spacers or fixation elements may ensure that the electrodes are fixed or held in place, and does not easily move or deform, e.g. bulge, during assembly and cell operation. For example, the spacers may be one or more stubs essentially having a height corresponding to the thickness of the chamber frame. Alternatively, the height of the spacers may be smaller than the thickness of the chamber frame, such that the distance between the electrodes may be easily adjusted and controlled by the spacers.

In an embodiment of the disclosure, the spacers have a height of between 50 pm - 20 mm, more preferably between 500 pm - 10 mm, and most preferably between 1 mm - 6 mm, such as 2 or 4 mm.

To ensure a well-defined and/or fixed distance between the electrodes across the entire electrode area, the spacers are advantageously placed at a predetermined distance to each other, e.g. in a symmetrical pattern, and/or elongated in one or more directions, which may further simplify the assembly of the flow cell. For example, the spacers may be a plurality of elongated bars oriented in a first direction, such as three parallel bars at a fixed distance oriented horizontally, as shown in Figure 4. Alternatively, the parallel bars may be oriented vertically, as shown in Figure 3. The orientation of the first direction may be relative to the gas flow field of the manifolds and/or relative to an electrolyte flow, as indicated in Figure 4. Thus in Figure 4, the horizontally bars are parallel to the electrolyte inlet 4.5 and outlet 4.6 flow into the chamber, whereas in Figure 3, the vertically bars are perpendicular to the electrolyte inlet and outlet flow into the chamber.

In an aspect of the disclosure, the flow cell for electrochemical ammonia synthesis, comprises a cathode, an anode, and an electrolyte chamber, wherein the electrolyte chamber comprises one or more spacers having a height defining a distance between the anode and cathode.

In an embodiment of the disclosure, two or more spacers are placed at predetermined distance to each other. In a further embodiment, the spacers are elongated in one or more directions, and optionally shaped as bars. In a further embodiment, the spacers are horizontally oriented, and/or vertically oriented.

In a further example, the spacers may be a plurality of elongated bars intersecting each other at an angle to form a grid, in a similar manner as the grilles of a window. For example two or more elongated bars may cross each other at a perpendicular angle to form a chequered grid pattern. The grid extends in two directions and may accordingly form a plane having a first surface side facing the cathode, and a second surface side facing the anode. When the cathode layer and anode layer are abutting the grid surface sides, the multiple contact points ensures a well-defined and stable distance as the grid is sandwiched between a cathode plane and an anode plane.

In an embodiment of the disclosure, the spacers are intersecting each other at an angle to form a grid, optionally intersecting at a perpendicular angle. In a further embodiment, the grid is sandwiched between a cathode plane and an anode plane.

To simplify the assembly of the flow cell, the spacers may be detachably attached to the chamber frame, as illustrated in Figure 4.

In an embodiment of the disclosure, the spacers are detachably attached to the chamber frame. Example 1 further describes examples of the operation of flow cells according to the present disclosure, where the flow cell comprises an electrolyte chamber as shown in Figure 4.

The efficiency of the flow cells was seen to depend on the number of spacers.

Example 5 further describes example of the operation of flow cells according to the present disclosure, where the electrolyte chamber comprises a different number of spacers, such as 0, 1 , 3, or 4 spacers. For the embodiment absent spacers, a very low ammonia production was obtained. The presence of at least one spacer was seen to improve the ammonia synthesis, and/or possibly the ammonia recovery. For compact and cost-efficiency, an electrolyte chamber comprising fewer spacers, such as 1 spacer, may be advantageous.

In addition or alternatively, the spacers 4.4 may be configured as separators dividing the electrolyte chamber into two or more subchambers. Figures 16 and 18B show an embodiment of an electrolyte chamber according to the present disclosure, where the chamber comprises a separator 4.4 defining two subchambers within the chamber frame.

The configuration of the spacers and may affect the efficiency of the flow cell, and may further facilitate the recovery degree of the produced ammonia. Accordingly, spacers configured as separators dividing the electrolyte chamber into multiple subchambers may be advantageous.

In an embodiment of the disclosure, the flow cell comprises one or more spacers configured as separators diving the electrolyte chamber into two or more subchambers.

Electrolyte replacement and/or circulation

Flow cells are specifically exposed to electrolyte degradation due to the continuous operation. The electrolyte degradation affects the long-term stability of the ammonia synthesis, and may result in sudden termination of process.

It was surprisingly found that replacement and/or circulation of a liquid electrolyte may improve the continuous ammonia synthesis, and provide a surprisingly high long-term stability of the ammonia production. For example, circulation may regenerate the electrolyte e.g. by redissolving deposits, and the replacement and/or circulation may be used to replace a decomposed electrolyte, e.g. by inflow of fresh electrolyte with the same formulation, or by inflow of fresh electrolyte with a different formulation, e.g. a less toxic composition, and/or inflow of additives beneficial for e.g. cell start up procedures or electrode cleaning. Examples of additives include lithium salt stabilizers, and lithium deposition improver.

For efficient and flexible electrolyte replacement and/or circulation, the electrolyte chamber 4 advantageously comprises at least one fluid inlet 4.5 and at least one fluid outlet 4.6, as illustrated in Figure 4. For efficient and safe inflow and outflow, e.g. reducing the risk of material waste and leaks, the inlet and outlet are located on the chamber frame including the inner perimeter 4.3 of the chamber frame, as indicated in Figures 4-5. Hence, the inner perimeter comprises openings into the chamber providing the fluid communication into and out of the chamber from the fluid inlet and outlet.

In an embodiment of the disclosure, the electrolyte chamber comprises at least one fluid inlet and at least one fluid outlet such that the electrolyte chamber is configured for fluid replacement and/or circulation. In a further embodiment, the fluid inlet and fluid outlet are located on the chamber frame, optionally located on the inner perimeter of the chamber frame.

To obtain a compact and efficient flow cell, the fluid inlet(s) and fluid outlet(s) may be provided by one or more lumens or pipes within the chamber frame. Hence, the lumens or pipes of the frame are configured for fluid communication between the electrolyte chamber and one or more external electrolyte sources. The same lumen may be used for fluid inlet and fluid outlet at different points in the cell operation, or one or more first lumens may be used for fluid inlet 4.5, and one or more second lumens may be used for fluid outlet 4.6 during the cell operation.

In an embodiment of the disclosure, the fluid inlet is provided by one or more first lumens within the chamber frame, and/or wherein the fluid outlet is provided by one or more second lumens within the chamber frame. The position of the fluid inlets and outlets, as well as the orientation of the associated lumens, affect the efficiency and distribution of the electrolyte, when it is replaced and/or circulated. For example, the lumens may be parallel to the frame plane, such that the fluid transfer occurs mainly in-plane of the cavity 4.1 of the cuboid electrolyte chamber 4, as sketched in Figures 4-5. The lumens may further be parallel oriented to each other, such as both horizontally oriented as shown in Figures 4-5.

In an embodiment of the disclosure, the first and second lumens are parallel to a frame plane. In a further embodiment, the first and second lumens are parallel oriented.

The relative position of the fluid inlet(s) to the position of the fluid outlet(s) also affects the distribution of the electrolyte during transfer, and hence the efficiency and completeness of the electrolyte replacement and/or circulation. For example, the rectangular frame of Figures 4-5 has four side walls or frame sections, where two of the frame sections are positioned opposite each other. The fluid inlet 4.5 and outlet 4.6 may be located at the same frame section as seen in Figures 4-5 and 7E-F, such that the fluid flow in and out of the electrolyte chamber may induce a circular flow pattern across the chamber plane. Alternatively, the fluid inlet 4.5 and outlet 4.6 may be located at opposite frame sections as seen in Figure 3 and 7A-B, G-H, such that the fluid flow in and out of the electrolyte chamber may induce a linear flow pattern across the chamber plane. The inlet and outlet may further be symmetrically opposed as in Figures 3 and 7C-D, l-J, or mirror symmetrically opposed as in Figure 7A-B, G-H.

In an embodiment of the disclosure, the frame comprises at least two frame sections placed opposite each other. In a further embodiment, at least one fluid inlet and at least one fluid outlet is located at a first frame section. In an alternative embodiment, at least one fluid inlet is located at a first frame section, and at least one fluid outlet is located at an opposite frame section. In a further embodiment, the at least one fluid inlet and the at least one fluid outlet are symmetrically opposed or mirror symmetrically opposed.

For compact design of the system, the fluid inlet and/or outlet may comprise lumen sections that are perpendicular to each other, or where the lumens are supplied with a fluid flow that is perpendicular to the lumen or one or more of the lumen sections. For example, a part of the fluid inlet and outlet may occur perpendicular, and/or a part of the lumen may be perpendicular to the frame plane or the electrolyte chamber, as shown in Figures 7K-L.

Alternatively or additionally, the fluid inlet 4.5 and outlet 4.6 may be provided by first and second lumens comprising multiple lumen segments in-plane of the frame plate. For example as shown in Figure 15, the first lumen for the fluid inlet 4.5 may comprise a first segment extending in parallel and along the left inner perimeter of the chamber frame, and a second segment at an angle to the left inner perimeter such that the second segment forms a chamber opening 4.7 into the electrolyte chamber 4. Similarly, the second lumen for the fluid outlet 4.6 may comprise a first and second segment, which are advantageously arranged symmetrically, such as mirror symmetrically or rotational symmetrically, to the fluid inlet. For example, the second lumen may be 180 degrees rotational symmetrically arranged as shown in Figure 15.

In an embodiment of the disclosure, the fluid inlet and/or the fluid outlet comprise multiple lumen segments. In a further embodiment, the fluid inlet and/or the fluid outlet comprise one or more first lumen segments extending in parallel and along an inner perimeter of a chamber frame, and one or more second lumen segments at an angle to the inner perimeter, such that the second segment forms a chamber opening into the electrolyte chamber.

To improve the electrolyte circulation and stability, as well as the cell efficiency, the fluid inlet and outlet may advantageously comprise multiple second lumen segments at a predefined distance to each other along the first lumen segment. The predefined distance may for example be defined by the radius of two abutting circles with a diameter of 30 mm, as indicated in Figures 15-16. To further improve the circulation efficiency and stability, the second lumen segments of a corresponding fluid inlet and fluid outlet may be arranged oppositely, thereby defining a pair of oppositely located chamber openings. Figures 16-19 show embodiments, where the fluid inlet comprises two second lumen segments, and two oppositely located second lumen segments for the fluid outlet, such that two pairs of oppositely located chamber openings at a predefined distance are obtained.

In an embodiment of the disclosure, the fluid inlet and fluid outlet comprise oppositely located chamber openings. In a further embodiment, the cell comprises multiple pairs of oppositely located chamber openings, wherein the pairs are located at a predefined distance from each other along the first lumen segment.

To further improve the cell efficiency and electrolyte circulation, the electrolyte chamber may comprise one or more separators dividing the electrolyte chamber into two or more subchambers, where the subchambers are supplied with fluid flow from different lumen segments and chamber openings. For example, as shown in Figures 16 and 18, the electrolyte chamber may advantageously comprise a separator spacer 4.4, where the upper subchamber is supplied from a first second lumen segment and chamber opening 4.7, and the lower subchamber is supplied from a different second lumen segment and chamber opening 4.7. Depending on the size of the cells, multiple separators and correspondingly multiple subchambers and chamber openings may be preferred. For example for a cell size of 400 cm ² , the electrolyte chamber may comprise between 1-40 separators, more preferably between 2-20 or 5-15 separators, and most preferably between 6-10 separators, which may correspond to between 2-41 pairs of oppositely located chamber openings.

In an embodiment of the disclosure, the flow cell comprises between 1-40 separators, more preferably between 2-20 or 5-15 separators, and most preferably between 6-10 separators.

Further advantageously, the geometry and dimensions of the second lumen segments and the chamber openings are configured to improve the electrolyte flow circulation and the mechanical stability of the cell. Hence, advantageously, the chamber openings are longitudinally and may extend in parallel with the chamber plane or frame plane. For example, the chamber openings may be a slit extending in parallel with the chamber plane or the chamber frame plane. Further advantageously, the angle of the second lumen segment to the surface of the inner perimeter may have a diverging geometry towards the surface of the inner perimeter of the chamber/subchambers, such that the second lumen segments may obtain a trapezoid like geometry. For example, advantageously, the chamber openings 4.7 may be planar slits at the surface of the inner perimeter of the frame, as shown in Figures 17 and 19C, and the second lumen segments may further be planar slits with a diverging geometry towards the inner perimeter, such as a trapezoid shape, optionally where the legs of the trapezoid are curved, such as circular curved, as shown in Figures 15, 16, 19. Accordingly, the fluid transfer may occur efficiently in-plane of the chamber cavity. An example of an electrolyte flow velocity field is illustrated in Figure 18, where an essentially uniformly high and laminar fluid flow pattern is obtained.

In an embodiment of the disclosure, the chamber opening is a slit in parallel with a chamber plane. In a further embodiment, the angle of the second lumen segment to the surface of the inner perimeter is diverging towards the inner perimeter of the chamber, optionally wherein the second lumen segment has a shape selected from the group of: trapezoid, and/or trapezoid with curved legs, such as circular curved legs.

The multiple lumen segments in-plane the frame plate may be advantageously be formed by different shaping techniques, such as injection molding or formed by cutting and/or drilling, e.g. cutting using a saw wheel or a wheel shaped drill. For example, the first lumen segments of the fluid inlet and fluid outlet may be simply drilled from above and below the chamber frame 4.2. The dimension of the first lumen may be controlled by the drill bit size, and may be varied along the first lumen. For example, three different drill bits with decreasing size may be applied, as sketched with stippled lines in Figure 16.

The second lumens may be cut using a thin saw wheel on the side or the inner perimeter, as sketched by the circles in Figures 15 and 16. For example the thin saw wheel may have a diameter of 030.00 mm, and a thickness which is below the thickness of the frame or the height of the spacer, such that a slit shaped lumen is formed at the inner perimeter. Thus, the saw wheel is cutting into the inner perimeter from the side and will hit the first lumen segment to form a fluid connection. For example, a circular saw wheel may be placed at a distance of 6 mm from the inner perimeter, and accordingly cut and/or drill a slit shaped second lumen, as indicated in Figure 15. The resulting second lumen will have a trapezoid shape with diverging geometry towards the inner perimeter as seen from above, as indicated with dark stippled lines in Figure 15. Multiple slit shaped second lumens may be formed in a similar manner by translating the saw wheel, as indicated in Figure 16. The thickness of the saw wheel may advantageously be between 5-75% of the thickness of the frame or the height of the inner perimeter, more preferably between 10-65% or 15-60%, such as 20-50%, as shown in Figure 19. In an embodiment of the disclosure, the lumen segments are obtained by cutting and/or drilling.

Flow circulation guides

The electrolyte chamber 4 may comprise one or more flow circulation guides to further provide time efficient and flexible electrolyte replacement and/or circulation. For a compact and simple flow cell, the spacers may further be configured as flow circulation guides. For example, the spacers may be separators dividing the electrolyte chamber into two or more subchambers, such that the separators are also configured as flow circulation guides.

In an embodiment of the disclosure, the electrolyte chamber comprises flow circulation guides. In a further embodiment, the spacers are configured as flow circulation guides.

The flow circulation guides are configured such that when the electrolyte is forced between the fluid inlet 4.5 and fluid outlet 4.6, a certain electrolyte flow pattern is obtained within the cavity 4.1 of the electrolyte chamber 4. Figures 7A-B show a first embodiment of an electrolyte chamber 4 according to the present disclosure, as seen in perspective view (A) and cross sectional view (B). In this case the flow circulation guides are configured to generate a meandering fluid flow between the fluid inlet and fluid outlet, as indicated by the arrow in Figure 7B. This may be obtained by the flow circulation guides being a plurality of elongated bars, e.g. four bars as in Figure 7A-B, mounted at opposite side walls of the chamber frame. The bars are advantageously parallel oriented, and may further be oriented in parallel to the lumens of the fluid inlet and outlet for improved efficiency.

In an embodiment of the disclosure, the flow circulation guides are configured to generate a meandering fluid flow between the fluid inlet and fluid outlet. In a further embodiment, the flow circulation guides comprises a plurality of elongated bars within the electrolyte chamber, oriented in parallel to the fluid inlet and fluid outlet.

The bars may be configured as spacers and hence have a predetermined height or thickness, which defines the distance between the anode and cathode. The cross sectional shape of the bars will also affect the flow path, and may be varied to improve the flow circulation. Figure 8 shows embodiments of electrolyte chambers according to the present disclosure, where the cross sectional shape of the bars are smoothed rectangular (A), rectangular (B), or triangular (C).

In an embodiment of the disclosure, the elongated bars are rectangular or triangular shaped in cross sectional view.

Figures 7C-D show another embodiment of an electrolyte chamber 4 according to the present disclosure, as seen in perspective view (C) and cross sectional view (D). In this case the flow circulation guides are located within the frame 4.2 of the electrolyte chamber 4, and configured to generate a laminar fluid flow in the cavity 4.1 between the fluid inlet and fluid outlet, as indicated by the arrow in Figure 7D. This may be obtained by the flow circulation guides being a plurality of symmetrically opposed fluid inlets 4.5 and outlets 4.6, e.g. by the fluid inlet forming a branched fluid inlet and the fluid outlet forming a branched fluid outlet. It follows that the branching may be any symmetrical patterns, such as fractal trees.

In an embodiment of the disclosure, the flow circulation guides are configured to generate a laminar flow between a fluid inlet and a fluid outlet. In a further embodiment, the flow circulation guides comprise a plurality of symmetrically opposed fluid inlets and fluid outlets. In a further embodiment, the symmetrically opposed fluid inlets and fluid outlets comprise a branched fluid inlet and a branched fluid outlet.

The flow circulation guides may be combined with different configurations of the fluid inlet 4.5 and outlet 4.6, as further illustrated in Figure 7E-L, where the fluid flow direction is indicated by arrow. A meandering fluid flow between the inlet and outlet may include inlet and outlet on the same frame section side, as seen in Figure 7E-F, or opposite frame section sides, as seen in Figure 7G-H. A laminar fluid flow between the inlet and outlet may include inlet and outlet symmetrically opposed, i.e. in-plane symmetric, as seen in Figures l-J, and/or cross-plane symmetric, as seen in Figures K- L.

Example 5 further describes examples of the operation of flow cells according to the present disclosure, where the electrolyte is replaced and/or circulated during start-up and/or operation, and an effect of the electrolyte chambers with different flow circulation guides may be seen.

Electrodes

The flow cell is operated on gaseous reactants, such as nitrogen gas and hydrogen gas, and the electrodes are therefore advantageously gas diffusion electrodes (GDE) such that the reaction kinetics of the cell are improved. Gas diffusion electrodes are electrode microstructures which improves the transport of gaseous species into a liquid electrolyte. It is found that certain gas diffusion electrodes are particularly efficient for transporting nonpolar gaseous reactants such as nitrogen and hydrogen into nonaqueous electrolytes, e.g. organic based electrolytes such as THF based electrolytes. Advantageously, the electrodes are gas diffusion electrodes obtained by hydrogen bubbling templating (HBT) the electrode material on a substrate. Improved microstructures may specifically be obtained by hydrogen bubbling templating on substrates of stainless steel (SS) and/or carbon. For example the electrode materials may be deposited on a stainless steel mesh or cloth with the HBT method.

Example 3 further describes an example of the synthesis of a cathode for a flow cell according to the present disclosure. The Cu/stainless steel cathode is made by the HBT method, and Figure 9 shows representative SEM images of an embodiment of the cathode.

Example 4 further describes an example of the synthesis of an anode for a flow cell according to the present disclosure. The PtAu/stainless steel anode is made by the HBT method, and Figure 10 shows representative SEM images of an embodiment of the anode. Alternatively, the anode may be Pt/stainless steel made by the HBT method.

The GDE may also be referred to as a high surface area (HSA) electrode or substrate. General properties of the GDE is further described in the section “Cathode substrate” below, where the properties and manufacture are exemplified based on a cathode. Further specification on the cathode structure is described in the application WO 2022/175548 which describes methods and apparatuses for improving the efficiency and stability of electrochemical ammonia synthesis. The application is hereby incorporated by reference. The gas diffusion electrode manufacturing may be readily scaled to different flow cell sizes. Advantageously, the electrodes are configured to have an area of between 2- 200 cm ² , such as 25 or 100 cm ² .

Cathode substrate

To further improve the performance and FE, as well as the electrochemical and mechanical stability of the anode and cathode, then for example the cathode advantageously comprises a high surface area (HSA) electrode or substrate, By the term “high surface electrode” is meant an electrode with high porosity and fine pore sizes, such that the specific surface area or the electrochemical active surface area (ECSA) is high, compared to the geometrical surface area as measurable on the bulk electrode.

For example, a high surface cathode may have a geometrical surface area of 1 cm _geo ² , corresponding to the electrode having a length and width of 1 cm, whereas the specific surface area or the ECSA including the surface roughness and tortuosity due to the porosity, is much higher.

A minor degree of surface roughness, e.g. a roughness factor due to scratches, may result in an ECSA of ca. 1.5-2.0 cm ² EcsA/cm _geo ² , as measured via capacitive cycling as described in Examples 11-12. In contrast, high surface electrodes may have a roughness factor above 5, more preferably above 10. Particularly for the present disclosure, surprisingly improved performance and FE may be obtained for a cathode configured to have a surface roughness factor between 10-100 or 30-80 such as 50, 60, or 70, as measured via capacitive cycling as described in Examples 11-12.

High surface area electrodes may be obtained by any suitable synthesis routes, which may provide porosity between 25-55%, such as 30-50%, and/or average pore diameters of between 100 nm - 50 pm, such as 500 nm - 1 pm, and/or specific surface areas or ECSA of between 1-100 cm ² /g. For example, a high surface electrode may be synthesized by hydrogen bubbling templating (HBT) on a substrate, which results in an alveolate, highly and finely porous dendritic structure. The electrode porosity and surface area characteristics are typically measured by gas adsorption techniques, such as the BET method. In an embodiment, a high surface area electrode may be a high surface area Cu electrodes synthesized through hydrogen bubbling templating (HBT) on a transition metal substrate, preferably a porous transition metal substrate, such as Ni foam or stainless steel mesh substrates. The porous transition metal substrate is advantageously a highly porous substrate, having macropores and a porosity between 50-95%, more preferably 75-90%, and pore, such as a metal foam or mesh.

The resulting Cu electrode may be referred to as HBTCu. The improved performance is particularly observed for HBTCu structures comprising deposited Cu forming an alveolate, highly porous, secondary dendritic structure on the surface of the primary, pristine Ni foam.

In an embodiment of the disclosure, the cathode comprises a high surface area metal electrode, preferably a high surface area electrode comprising a metal selected from the group of: Cr, Fe, Ni, Cu, Zn, and combinations thereof. In a further embodiment, the cathode comprises a Cu electrode made by hydrogen bubbling templating on a transition metal substrate, preferably a porous transition metal substrate, such as Ni foam or stainless steel mesh substrate.

In an embodiment of the disclosure, the cathode comprises a porosity of between 25- 55%, such as 30-50%. In a further embodiment, the cathode comprises pores having an average pore diameter of between 100 nm - 50 pm, such as 500 nm - 1 pm, In a further embodiment, the cathode is configured to have a specific surface area between 1-100 cm ² /g, such as 2-50 cm ² /g, 3-25 cm ² /g, or 2-10 cm ² /g, as measured by BET. In a further embodiment, the cathode is configured to have a surface roughness factor of above 5, more preferably above 10, and most preferably between 10-100 or 30-80 such as 50, 60, or 70, as measured by capacitive cycling.

Anode

A flow cell with a surprisingly high efficiency and continuous ammonia synthesis with a surprisingly high stability may be obtained for selected anode materials. It was surprisingly found that anodes comprising a bimetallic platinum catalyst provides both high and stable faradaic efficiency for ammonia synthesis. This may be due to one or more synergistic properties of the bimetallic platinum, e.g. the material is stable in organic based electrolytes and resistant to carbon poisoning, the material is not active towards ammonia oxidation, the material is resistant to ammonia poisoning, and the HOR overpotential is low.

An aspect of the disclosure relates to a flow cell for electrochemical ammonia synthesis, comprising a cathode, an anode, and an electrolyte chamber, wherein the anode comprises a HOR catalyst comprising bimetallic Pt.

Accordingly, the high efficiency and stability may be due to the selected material composition of the anode providing higher reaction selectivity and reduced degradation.

It is further found that bimetallic platinum in combination with a metal selected from the groups 8-12, 14, 15 may provide further improved efficiency and stability. Further, combinations thereof, including combinations comprising a third or fourth metal, may provide improved efficiency and stability.

In an embodiment of the disclosure, the bimetallic Pt is in combination with a metal selected from the groups 8-12, 14, 15, and combinations thereof, and optionally comprises a third or fourth metal. In a further embodiment, the bimetallic Pt is in combination with a metal selected from the group of: Au, Ru, Rh, Pb, Bi, Sn, Sb, and combinations thereof, and preferably in combination with Au, such as Pt-Au in atomic ratio 1:1 , 1 :2, 1 :3, 1:4, 1:5, 2:3, 2:5, 2:1, 3:1 , 4:1 , 5:1 , 3:2, 5:2.

Due to the selectivity of the anode material towards HOR including the resistance of the material to ammonia poisoning results in the ion conducting membrane 7 being redundant. For example, an anode comprising a bimetallic platinum (Pt) catalyst may provide sufficient high selectivity for the hydrogen oxidation reaction (HOR), and thereby make the presence of membranes for improving the selectivity redundant.

Figure 6B shows an embodiment of a flow cell without a membrane in cross sectional view. The operation without a membrane implicitly means that the cell resistance is decreased due to the fewer components and contact interfaces. This further has the advantage that the heating of the electrolyte during operation is reduced, and the decomposition rate of the electrolyte is reduced, such that the system stability and energy efficiency are further improved.

In an embodiment of the disclosure, the electrolyte chamber is absent an ion conducting membrane.

The synthesized ammonia may be separated and extracted from the electrolyte, and/or separated from the gaseous nitrogen out flow from the cell.

Example 1 further describes examples of the operation of flow cells according to the present disclosure, where the anode is bimetallic platinum. Specifically, an anode comprising bimetallic platinum-gold was tested. An example of the electrochemical results are shown in Figure 14, showing the electrode potentials (left y-axis) and the Faradaic efficiency (right y-axis) as a function of time during flow cell operation. A stable Faradaic efficiency of around 60 % is seen.

Example 2 further describes examples of the stability of the bimetallic anode compared to a conventional Pt. The performance in terms of overpotential and long term stability is shown for PtAu and Pt in Figure 11.

Figures 11 A-B show the anode performances without ammonia present in the electrolyte, corresponding to a flow cell shown in Figure 6A. Figure 11A is a close-up of the first 50 minutes time interval.

Figures 11C-D show the anode performances with ammonia present in the electrolyte, corresponding to a flow cell shown in Figure 6B. Figure 11 C is a close-up of the first 50 minutes time interval.

Figure 11 shows that the cathode working electrode potential (EWE) is almost identical with and without ammonia present. The PtAu anode show a better performance than Pt. A lower over-potential at the counter electrode (ECE) is seen for PtAu compared to Pt, both without ammonia present and with ammonia present. Further, an improved stability of PtAu may be seen, even in the presence of ammonia. Hence, the PtAu activity towards HOR is not affected by the presence of ammonia. Similar poor performances of different single metal anodes, e.g. either Pt or Au anodes, are seen. The higher performance and stability is surprisingly seen for a bimetallic catalyst, such as PtAu.

Cell operation configurations

The operation and further configurations of the flow cell for operation is further described below. The applicant’s applications WO 2021/176041 and WO 2022/175548 describe methods and configurations for improving the efficiency and stability of electrochemical ammonia synthesis, e.g. for a flow cell. The applications are hereby incorporated by reference.

Electrolysis cell

Ammonia may be produced electrochemically by reduction of nitrogen (N2) to ammonia (NH3). In addition to nitrogen as reactant, protons and electrons are required as indicated by equation (1). The electrochemical reaction may further be mediated by the presence of additional substances. For example, the selectivity of the electrochemical production of ammonia may be promoted by the presence of cations, e.g. lithium cations, as well as specific solvents and solvent additives, into which the cations may be dissolved.

The reactants and substances taking part in the electrochemical ammonia synthesis are either continuously supplied from externally to the reaction site in the cell, or present and stored within the cell. For example, an ammonia electrolysis cell may be operated by external sources supplying power, nitrogen, oxygen, cations, and protons, e.g. supplied as hydrogen. The substances which are not directly consumed reactants, e.g. the cations, may be supplied or stored within the cell, e.g. in the form of an electrolyte comprising a solvent with dissolved cations and additives.

In an embodiment of the disclosure, the electrolysis cell is connectable to at least one power source, at least one nitrogen source, and at least one oxygen source.

Preferably, the cell is further fluidly connectable to at least one proton source, and/or cation source. For example, the electrolysis cell have an electrolyte comprising a proton source and/or cation source. Hence, electrochemical ammonia synthesis is carried out in an electrolysis cell, i.e. a device where an external voltage and/or current load, may be applied to drive the synthesis reaction. For example, when Li ions in a solution are subjected to a potential of -3 V vs. reversible hydrogen electrode (RHE), the so-called lithium reduction potential, including a current supply at the cathode, the Li ions are reduced to Li metal on the surface of the cathode by electrolysis.

The electrical potential is applied across the electrodes of the electrolysis cell, i.e. the anode and cathode, where the electrodes are separated by the electrolyte comprising the solution of Li ions. However, to precisely control the potential of the cathode, the cathode potential is measured by use of a reference electrode (RE). Hence, the reference electrode only controls, or more specifically only measures, the cathode potential and passes no current.

At the cathode, reduction can take place, and electrons are consumed to e.g. reduce Li ions to Li metal. Thus, the cathode is also referred to as the working electrode (WE), and the consumed electrons referred to as the cathodic current load. At the anode, oxidation takes place, and the corresponding amount of electrons are released e.g. by oxidation of hydrogen. Thus, the anode is also referred to as the counter electrode (CE), and the produced electrons or current may be referred to as an anode current load.

According to the present disclosure, the cathode potential is advantageously varied. For example, it may be changed between the lithium reduction potential, i.e. -3 V, and a less negative cathode potential, such as the cell voltage corresponding to the open circuit voltage. The open circuit voltage (OCV), also referred to as the open circuit potential (OCP), is the potential when no external load is connected to the cell, corresponding to the cathode potential, where the cathode current load is zero. Hence, at the lithium reduction potential the cathode potential is negative, and includes a cathodic current load, and at the less negative cathode potential, e.g. cell OCP, no cathodic current load is present.

A change in the cathode potential from e.g. the lithium reduction potential and to cell OCP may be referred to as one cycle. Advantageously, the cathode potential, and the associated cathodic current load, is operated cyclic, i.e. the cycle is repeated multiple times, and preferably repeated in a periodic manner without interruption of the operation cell. This operation may also be referred to as a continuously pulsed operation, comprising pulses of a first cathode potential, including a first cathodic current load, and pulses of a second cathode potential, including a second cathodic current load.

The electrolysis selectivity towards ammonia, and hence the faradaic efficiency of the process, will depend on the process parameters, including the voltage/current supply pattern, as well as the operational temperature, pressure, and the types of reactants. The energy efficiency will further depend on the electrolysis configuration and cell type, e.g. whether it is a single compartment cell or a flow cell.

In the present disclosure, the electrochemical ammonia synthesis is exemplified as being mediated by lithium ions. However, the skilled person will know that the synthesis may be similarly mediated by other cations, and/or additional cations, and their corresponding metal, having similar properties to lithium. Metals in the vicinity of lithium in the periodic table of elements may have similar solubility, reactivity, and/or reduction potentials as lithium. Thus, advantageously, the synthesis may be mediated by one or more metal cations selected from the groups 1-13 of the periodic table of elements. This means that the synthesis is mediated by one or more metals and their corresponding cations. Advantageously, the synthesis is mediated by one or more metal cations selected from the groups consisting of: alkali metals, alkaline earth metals, and/or transition metals. Advantageously, the synthesis is mediated by cations which are reduced to metal at a similar cation reduction potential as lithium, and/or which have similar reactivity towards nitridation and protonation, such as sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), barium (Ba), yttrium (Y), and combinations thereof.

It also follows that the associated apparatus for the electrochemical ammonia synthesis may be adapted for different types of electrolysis cells, and preferably the apparatus is adapted for electrolysis cells, which comprise a source of cations. Preferably, the apparatus comprises electrolysis cells comprising a source of cations, e.g. an electrolyte comprising dissolved cations, which preferably are lithium cations. In an embodiment of the disclosure, the cations are one or more metal cations, where the metal is selected from groups 1-13 of the periodic table and combinations thereof, more preferably the metal is selected from the group consisting of: alkali metals, alkaline earth metals, and/or transition metals, more preferably the metal is selected from groups 1 , 2, 3 of the periodic table and combinations thereof, and most preferably the metal is selected from the group consisting of: lithium (Li), sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), barium (Ba), yttrium (Y), and combinations thereof.

The Faradaic efficiency (FE) of an electrochemical ammonia synthesis is calculated based on the concentration, CNH3, of synthesized ammonia in the electrolyte, which is measured via either a colorimetric or isotope sensitive method, along with the total electrolyte volume, V, after each measurement. This is compared with the total charged passed, Q, as shown in Equation 3, where F is Faraday’s constant, and 3 is the number of electrons transferred during the reaction for each mole of NH3.

(Eq. 3)

3 F C _NH3 V

FE _NH3

Q

The energy efficiency may be defined in various ways and taking different factors into account. The energy efficiency for the electrochemical synthesis of ammonia may for example take into account for the energy efficiency of the synthesis of hydrogen, which is expected to come from the electrolysis of water.

However, generally the energy efficiency, , of an electrochemical ammonia synthesis may be based on the total amount of energy put into the system via the potentiostat, Em, and compared that to the energy contained in the total amount of ammonia produced during the experiment, E _0U f., as shown in Equation 4.

(Eq. 4)

Eout

^{T1 =} ~E t~n Eout is defined by the free energy of reaction of ammonia oxidation to N2 and water times the amount of ammonia produced, while Em is given by the total cell voltage between the counter electrode (CE) and working electrode (WE), multiplied by the current to get the instantaneous power, and integrated over time, as shown in Equations 5 and 6.

(Eq. 5)

(Eq. 6)

Continuous deposition without oxygen

In an embodiment of the disclosure, electrochemical ammonia synthesis experiments were carried out where a steady cathodic current load of e.g. -2 mA/cm ² was applied. The steady cathodic current load implies continuous Li ion reduction and continuous Li metal deposition at the cathode, and the operation condition of the cell is therefore also denoted as the deposition potential.

The resulting electrode potentials as a function of time for lithium mediated ammonia synthesis under the constant cathodic current load of -2 mA/cm ² show that the working electrode (WE) potential, or the cathode potential, is not stable and degrades rapidly over time from 0 V vs Li ⁺ /Li to around -12 V vs Li ⁺ /Li. The decrease and degradation of the WE potential corresponds to an increase in the system energy input to sustain the desired current density of -2 mA/cm ² . After less than 1 hour of operation at -2 mA/cm ² the system is overloaded.

The cathode degradation mechanism is speculated to be related to the lithium salt reduction, where not all of the metallic lithium undergoes further reactions, e.g. nitridation, as illustrated by the possible reaction mechanism (not balanced) in Figure 2. In addition, or in alternative, to nitridation into LisN , the metallic lithium may form Li- amides or hydrides, as illustrated by the lower and upper reaction paths in Figure 2 (not balanced). However, the deposited metallic lithium which do not undergo further reactions, forms fresh lithium deposits that do not promote formation of ammonia and which are not released as lithium ions back to the solution, as illustrated in Figure 1. The deposits therefore decrease the overall efficiency of the system, as well as decrease the ionic conductivity of the solution as the lithium ions are depleted from solution, thereby increasing the overall resistance in the cell. The continuous deposition of lithium limits the up-scalability of the process, as a continued supply of lithium salt would be required to sustain synthesizing ammonia. This also leads to an accumulation of lithium species on the electrode surface, which slowly increases the needed potential to run the reaction.

Pulsed operation without oxygen

In an embodiment of the disclosure, electrochemical ammonia synthesis experiments were carried out using cyclic or pulsed cathode potential and current load. For example, the cathode current load may be pulsed between -2 mA/cm ² and 0 A, corresponding to cathode potential pulses between the lithium reduction potential and OCV. The pulsed operation implies alternating periods of Li deposition and no deposition. Alternatively, the cathode current load may be pulsed between -6 mA/cm ² and 0 A, as for the flow cell tested in Example 1.

The resulting electrode potentials as a function of time for lithium mediated ammonia synthesis under the pulsed cathodic current load shows a cathode potential, or working electrode (WE) potential which is more stable compared to the operation without pulses.

The regeneration of the degraded cathode during the periods of cell OCV, is speculated to be due to removal of the build-up lithium species on the surface of the electrode. The resting time between the deposition pulses may allow the lithium to react fully with nitrogen in solution significantly prevented the WE potential from drifting cathodic over time. Hence, the cycling procedure stabilizes the WE potential because it “resets” the surface by removing the deposited material, and replenishes the lithium in the solution, and produces ammonia.

The Faradaic efficiency also increases with the continuous cycling method, as charge is not wasted on forming unreactive lithium deposits. Furthermore, the overall energy efficiency is improved, due to the decrease in needed potential to sustain the same current, i.e. the average WE potential is lower. Moreover, by cycling the potential from a very negative lithium reducing potential, to a less negative potential at which lithium is not reduced, while potentially still synthesizing ammonia, the Faradaic and energy efficiency is further increased, since ammonia may be formed at potentials less negative than -3 V vs RHE.

The improvement in Faradaic efficiency and energy efficiency, as well as the efficiency of the cathode regeneration, will depend on the cyclic or pulsed operation patterns. Further, for operational simplicity, the pulsed operation is regular and periodical, i.e. similar pulse sizes and durations are applied. Advantageously, the cathode potential, including the cathodic current load, is changed between two configurations, such that the cathode potential is pulsed between a first cathode potential, including a first cathodic current load, and a second cathode potential, including a second cathodic current load. Further advantageously, the cathode potential may be pulsed between the lithium reduction potential, and a less negative cathode potential, such as the potential corresponding to the cell OCV.

In an embodiment of the disclosure, the cathode potential is pulsed between a first cathode potential, including a first cathodic current load, and a second cathode potential, including a second cathodic current load. In a further embodiment, the cathode potential is pulsed between the cation reduction potential and a less negative cathode potential. In a further embodiment, the cathode potential is pulsed between the lithium reduction potential and a less negative cathode potential. In a further embodiment, the cathode potential is pulsed between the lithium reduction potential and the cell OCP.

It was surprisingly found that by increasing/decreasing the current load of the pulses and the duration of the pulses, the Faradaic efficiency, energy efficiency, and cathode regeneration, may be further improved. For example, advantageously, the duration of the pulses at the second cathode current load may be longer than the duration of the pulses at the first cathode current load. However, for electrochemical ammonia synthesis including oxygen as described below, the duration of the pulses at the second cathode current load may advantageous be the same or shorter than the duration of the pulses at the first current load. For example, the duration of both the first and second pulses may be 1 min. In an embodiment of the disclosure, the duration of the pulses at the first cathode potential is between 0.5-60 min, more preferably between 0.7-30 min, and most preferably between 0.8-10 min, such as 1 or 2 min. In a further embodiment, the duration of the pulses at the second cathode potential is between 1-120 min, such as 1 or 2 min, more preferably between 2-60 min, and most preferably between 3-30 min, such as 3-5 or 10 min.

In an embodiment of the disclosure, the pulses of at the first cathodic current load has a duration of between 0.5-60 min, more preferably between 0.7-30 min, and most preferably between 0.8-10 min, such as 1 or 2 min. In a further embodiment, the pulses at the second cathodic current load has a duration of between 1-120 min or 5-120 min, such as 1 or 2 min, more preferably between 2-60 min or 6-60 min, and most preferably between 3-30 min or 7-30 min, such as 8 or 10 min.

In a further embodiment of the disclosure, the duration of the pulses is preferably 1 min, such as alternatingly 1 min deposition and 1 min rest, as e.g. described for the flow cell tested in Example 1.

It was further found that by increasing/decreasing the current load of the pulses, as well as the relative current load between the pulses, the Faradaic efficiency, energy efficiency, and cathode regeneration, may be further improved. For example, advantageously, the first cathodic current load is below (i.e. more negative than) -1 mA/cm ² , preferably around -100 mA/cm ² , and the second cathodic current load is -0.5 mA/cm ² , preferably 0 mA/cm ² or even positive, where the current load is based on the geometrical area of the electrode, referred to in the units by cm _geo ² . When the second cathodic current is negative or zero, the pulsed operation may be referred to as pulsating DC. When the second cathodic current is positive, the pulsed operation may be referred to as pulsating AC. Advantageously, high current load pulses are obtainable for cathodes comprising high surface area electrodes, such as gas diffusion electrodes.

In an embodiment of the disclosure, the pulsed cathodic current load is pulsating DC and/or pulsating AC. In a further embodiment, the pulses at the first cathodic current load has a current density below (i.e. more negative than) -1 mA/cm _geo ² , such as -2, -5, or -10 mA/cm _geo ² , more preferably above -50 mA/cm _geo ² , such as -60, -70, -80, -90, - 100, -200, -400, -600, -800, or -1000 mA/cm _geo ² . In a further embodiment, the pulses at the second cathodic current load has a current density above (i.e. more positive or more anodic than) -0.5 mA/cm _geo ² , such as 0 mA/cm _geo ² or 0.1 mA/cm _geo ² .

Additives, reactants and conditions

The faradaic efficiency of the process and the energy efficiency, will depend on other process parameters than the voltage/current pattern. For example, it was found that surprisingly high efficiencies may be obtained at mild temperature and pressure conditions, such as temperatures between 10-150 °C, and/or a pressure equal to or below 20 bar.

In an embodiment of the disclosure, the temperature is between 10-150 °C such as 20 °C, more preferably between 20-130 °C, and most preferably between 25-120 °C, such as 50 or 100 °C. In a further embodiment, the pressure is equal to or below 20 bar, such as 15, 10, 5, 1 bar or ambient pressure.

The faradaic efficiency of the process and the energy efficiency, will also depend on the reactant type and concentrations, as well as their accessibility and costs. For example, certain reactants were found advantageous as sources of Li ions, nitrogen, and protons. Furthermore, to ensure sufficient concentration of the reactants, the reactants may be supplied via a filter, e.g. protons may be supplied to the cathode via a proton exchange membrane.

Since the cations are not consumed and regenerated during the ammonia synthesis, the source of cations is advantageously comprised within the electrolysis cell, e.g. as part of a liquid electrolyte. Hence, the cation source is stored within the cell from which it may be supplied to the reaction sites. The liquid may be a molten salt or a solution comprising the cations, such as lithium cations. To improve the mediation and reaction kinetics and selectivity for the ammonia synthesis, a cation concentration which is sufficient for facilitating the mediation, and which at the same time do not impede the availability of other reactants at the reaction sites, is further advantageous. For example, for a solvent electrolyte, the lithium concentration is preferably between 0.1 - 3 M. In an embodiment of the disclosure, the source of Li ions is selected from the group consisting of: molten Li salt, Li solutions, and combinations thereof, such as LiCIC>4, LiPF6, LiBF4, LiAsF6, Lithium tri(pentaflouroethyl)trifluorophosphate, lithium trifluoromethanesulfonate, lithium bis(fluorosulfonyl)imide, lithium cyclo- difluoromethane-1 ,1-bis(sulfonyl)imide, lithium cyclo-hexafluoropropane-1,1- bis(sulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide), lithium bis(perfluoroethanesulfonyl)imide, lithium bis(oxalate)borate, lithium difluoro(oxalate)borate, lithium bis(fluoromalnato)borate solutions. In a further embodiment, the solutions has a Li concentration below 3 M or 1 M, such as 0.1 , 0.2, 0.5, or 2 M.

The source of nitrogen is advantageously continuously supplied from externally to the cell, such that the consumed nitrogen is continuously replaced and the synthesis may be carried out continuously. Nitrogen is easily accessible as air, which comprises ca. 78 vol% N2, or correspondingly a mole fraction of 78%. However, the Faradaic efficiency will depend on the nitrogen concentration. Advantageously the nitrogen source is oxygen separated or purified nitrogen. To easily provide the nitrogen at the electrochemical reaction sites, the gaseous nitrogen may be supplied as gas to the liquid electrolyte, where it dissolved into the liquid.

In an embodiment of the disclosure, the source of nitrogen is selected from the group consisting of: gaseous N2, liquidly dissolved N2, and combinations thereof.

The source of protons may also be continuously supplied from externally to the cell, such that the consumed protons are continuously replaced and the synthesis may be carried out continuously. For example, gaseous hydrogen may be supplied to an anode of the electrolysis cell, where the hydrogen is oxidized to protons that are dissolved in the liquid electrolyte. Alternatively, the source of protons may be supplied or stored within the cell, e.g. as part of an electrolyte which acts as a proton source or comprises dissolved protons. To further improve the reaction kinetics and selectivity for the ammonia synthesis, a sufficient proton concentration is desired. This may for example be obtained by the dissolved protons being transferred to the reaction sites at the cathode via a proton exchange membrane. In an embodiment of the disclosure, the source of protons is selected from the group consisting of: gaseous H2, liquidly dissolved H2, ethanol (EtOH), water (H2O), alkyl alcohols, especially tert-butanol, perfluorinated alcohols, polyethyleneglycols, ethanethiol, alkyl thiols, alkyl ketones, alkyl esters, and combinations thereof. In a further embodiment, the concentration of the protons within the proton source is between 0.01-100 vol%, more preferably between 0.01 - 5 vol%, and most preferably between 0.05-3 or 0.1-2 vol%. In a further embodiment, the source of protons is combined with a proton exchange membrane.

The reaction kinetics and the selectivity of the ammonia synthesis at the cathode, also depends on the simultaneous electrochemical reactions occurring, e.g. the competing hydrogen evolution which may occur at the cathode, as described in equation (2). To improve the ammonia selectivity, the method or the electrolysis cell advantageously comprises a liquid electrolyte comprising an essentially aprotic solvent, such as tetrahydrofuran (THF) or propylene carbonate or any organic carbonates, which can be diethyl carbonate, ethyl methyl carbonate, ethylene carbonate and variations of these.

In an embodiment of the disclosure, the method or electrolysis cell comprises an essentially aprotic solvent, selected from the group of: tetrahydrofuran (THF), oxane, diethyl ether, dipropyl ether, diglyme, dimethoxyethane, triglyme, tetraglyme, polyethyleneglycol alkyl ethers, dioxane, organic carbonates, e.g. dimethyl carbonate, ethylene carbonate, diethyl carbonate, ethyl methyl carbonate, propylene carbonate, dialkyl carbonates, butyrolactone, cyclopentanone, cyclohexanone, sulfolane, ethylene sulfate (DTD), trimethylglycerol, and mixtures thereof, and preferably is selected from the group of: tetrahydrofuran, organic carbonates, propylene carbonate, and mixtures thereof.

By the term essentially aprotic is meant that the electrolyte may comprise a mixture of the aprotic solvent and the proton source, whereby the electrolyte solvent is essentially or near aprotic. For example, the electrolyte may comprise a mixture of THF with 1 vol% ethanol as proton source.

In an embodiment of the disclosure, the aprotic solvent is selected from the group consisting of: tetrahydrofuran (THF), ethanol (EtOH), and combinations thereof, such as THF-1 vol% EtOH or THF with 1 vol% EtOH. In addition to specific solvents, the selectivity and stability of the electrochemical production of ammonia may be further promoted by the presence of solvent additives. For example, additives which may prevent solvent degradation under the operational potential and current loads, are preferably included. Such additives are preferably included in a suitable concentration, which is typically below 5 vol% of the solvent.

In an embodiment of the disclosure, the essentially aprotic solvent comprises one or more additives selected from the group of: perfluorinated hydrocarbons, perfluorinated ethers, highly fluorinated organic tetrkisalkyl phosphonium perfluorinated phosphates, tetrakisalkyl phosphonium perfluoroalkyl sulfonates, tetrakisalkyl phosphonium perfluoroalkyl carboxylates, crown ethers, and mixtures thereof, wherein preferably the concentration of the additives is between 0-100 vol%, more preferably between 0.01-5 vol%, and most preferably is between 0.05-3 or 0.1-2 vol%.

Oxygen source

As described above, the selectivity and stability of the electrochemical ammonia synthesis may be mediated and/or promoted by the presence of specific cations, solvents, and solvent additives. It was further surprisingly seen that the selectivity and stability of the electrochemical ammonia synthesis may be mediated by the presence of low concentrations of oxygen at the cathode. Hence, the electrochemical ammonia synthesis is advantageously carried out in the presence of oxygen, meaning that oxygen must be present in a defined amount. Specifically this is obtained when the oxygen is present in a predefined or specified concentration supplied by a source of oxygen, thereby providing a predefined oxygen concentration. Particularly high efficiencies, selectivity, and/or stability may be obtained with a source of oxygen providing a predefined oxygen concentration below 20 vol% or correspondingly a mole fraction below 20%. For example, the oxygen concentration is advantageously below 20 mol%, such as between 0.1-10 mol%, more preferably between 0.2-5 mol%. The partial pressure of a gas, e.g. oxygen (pCh), is generally directly proportional to the gas mole fraction, e.g. the oxygen mole fraction, and the temperature. Hence, a specific oxygen mole fraction range of ca. 1.4% may correspond to an oxygen partial pressure of ca. 0.14 bar at 10 bar. Correspondingly, an oxygen mole fraction of ca. 0.7% may correspond to an oxygen partial pressure of ca. 0.14 bar at 20 bar. Further advantageously, the predefined oxygen concentration corresponds to the source of oxygen comprising an oxygen partial pressure of between 0.02 - 2.5 bar, such as 0.01

- 0.5 bar or 0.02 - 0.4 bar, more preferably between 0.05 - 0.4 bar, and most preferably between 0.06 - 0.3 bar, such as 0.07, 0.1 , 0.15, or 0.2 bar.

Thus, the preferred partial pressure of oxygen is directly related to the amount of oxygen present irrespective of the pressure. Specifically it was seen that oxygen concentrations below 2 vol%, more preferably oxygen concentrations below 1 vol%, such as between 0.2 - 0.8 vol%, resulted in surprisingly high Faradaic efficiencies for the ammonia synthesis. It follows from the above that though oxygen may be present as an impurity or trace component in various systems and gasses, then such impurity or trace amounts cannot be present in a defined amount which is sufficient to obtain the improved performance and efficiency. For example, oxygen impurities may be highly variable during the operation of a system, and e.g. be absent at some points, and typically amount to very small amounts such as less than 10 ppm.

The surprising effect of small amount of oxygen particularly improves the costefficiency of the method and related apparatus and systems. Since highly pure nitrogen gas (>99.999%), where the O2 is removed from air down to ppm levels via cryogenic separation in large facilities, is not needed. Thus, the method is particularly suitable for decentralized systems. The positive effect of the O2 content on the Faradaic efficiency is surprising, because previously established experiments using synthetic air was shown to be detrimental to the system, and significantly reducing the FE to <4%, In an embodiment of the disclosure, the cathode is contacted with a source of oxygen, wherein the oxygen concentration is below 2%, while subjecting the cell to a potential and current load, whereby ammonia is synthesized.

In an embodiment of the disclosure, the cathode is contacted with a source of oxygen providing a predefined oxygen concentration. In a further embodiment of the disclosure, the oxygen concentration is below 20%, such as between 0.1 -10 %, such as 0.2 - 5%, 0.2 - 2% or 0.2 - 1.5%, more preferably between 0.3 - 1%, and most preferably between 0.4 - 0.8%. In an alternative or further embodiment, the source of oxygen comprises an oxygen partial pressure of between 0.02 - 2.5 bar, such as 0.01 - 0.5 bar or 0.02 - 0.4 bar, more preferably between 0.02 - 0.3 bar or between 0.05 - 0.4 bar, and most preferably between 0.05 - 0.2 bar or between 0.06 - 0.3 bar, such as 0.07, 0.1 , 0.15, or 0.2 bar. Air is a convenient and accessible source of both nitrogen and oxygen. Hence, advantageously the air is continuously supplied from externally to the cell in combination with the nitrogen. For example the nitrogen and oxygen source may be oxygen separated or purified nitrogen, which is supplied as gas to the electrolysis cell, e.g. to the liquid electrolyte, where it liquidly dissolved. Other sources of oxygen which may be utilized and may show an equally beneficial behaviour include, but is not limited to, gasses such as CO2, CO, NO _X , or H2O, and alcohols, aldehydes, peroxides, superoxides, and organic acids which contain oxygen, and oxygen from transition metal electrodes in the form of oxides and carbonates. The sources of oxygen may be continuously supplied from externally to the cell, e.g. as gas to the cell, and/or be supplied or stored within the cell, e.g. as part of an electrolyte.

In an embodiment of the disclosure, the sources of nitrogen and/or oxygen are supplied to the electrolysis cells, and more preferably the source is a combined nitrogen and oxygen source.

Oxygen mediation of the electrochemical ammonia synthesis is particularly surprising because the presence of oxygen conventionally is expected to decrease the Faradaic efficiency, because oxygen reduction together with hydrogen evolution as mentioned in Equation (2), will be competing reactions to the ammonia synthesis.

However, despite this prejudice, a surprising peak in Faradaic efficiency may be observed for oxygen concentrations below 2 vol%, and particularly 1 vol%. The FE may be seen to increase with the relative diffusion rates of nitrogen and protons, which corresponds to the concentrations of nitrogen and protons at the reaction sites. The FE may also be seen to increase significantly, when a system without is exposed to the presence of 1 vol% oxygen.

The steady cathodic current load may imply continuous cation reduction and deposition, e.g. continuous Li metal deposition, at the cathode. Alternatively, or additionally, to a steady cathodic current, the ammonia synthesis may advantageously be operated by using cyclic or pulsed cathode potential and current load, where the pulsed operation implies alternating periods of cation/Li deposition and no deposition. For example, the cathode current load may be pulsed between -2 mA/cm ² and 0 A, corresponding to cathode potential pulses between the lithium reduction potential and OCV.

Apparatus

The electrolysis cells may be assembled into an apparatus connectable to one or more independent or decentralized power sources, which advantageously are renewable power sources such as wind power, hydropower, solar energy, geothermal energy, bioenergy, and mixtures thereof. Thus, the apparatus may be operated as an on-site ammonia production unit at a decentralised location, and the apparatus may further be adapted to be mobile, and to synthesize ammonia in amounts of 0.01 - 10 kg/day, more preferably 0.1 - 10 kg/day, and most preferably 0.1-5 kg/day, such as up to 1, 2, 3, or 4 kg/day, with a Faradaic efficiency above 50%, and operated at current loads equal to or above 100 mA/cm ² .

An on-site, decentralised ammonia production unit, further has the advantage that voluminous tanks or containers for storing the produced ammonia product may be avoided or reduced. Due to the controllable and restricted amount of power, and thus corresponding restricted amounts of synthesized ammonia per day, the ammonia may be extracted from the electrolysis cell and directly distributed to a site of demand and further matched to the demand. For example, the ammonia may be extracted from the electrolyte of the cell, and continuously supplied to an irrigation system of a greenhouse or farm, thereby providing fertilizer for the plants after demand. This way a more simple apparatus and system may be obtained without, or with a reduced, need for product storage.

The operational conditions of the electrolysis cells, including the potential and current load, may be controlled by a controller, such as a potentiostat. Further advantageously the controller is configured for both regulating the power source input to the cells, as well as the supply of reactants and additives into the cells, and particularly the supply of nitrogen and/or oxygen.

In an embodiment of the disclosure, the apparatus comprises at least one electrolysis cell and a potentiostat configured for carrying out the method according to the present disclosure. In another embodiment of the disclosure, the apparatus comprises one or more electrolysis cells connectable to one or more power sources and one or more nitrogen and/or oxygen sources, and at least one controller configured for regulating the power source input and the oxygen input to the electrolysis cells, such that the cells are operated according to the method according to the present disclosure.

In a further embodiment, the apparatus comprises one or more power sources, preferably renewable power sources, optionally selected from the group of: wind power, hydropower, solar energy, geothermal energy, bioenergy, and mixtures thereof. In a further embodiment, the apparatus is configured as a decentralized and/or mobile unit, adapted to synthesize ammonia in amounts of 0.01 - 10 kg/day, more preferably 0.1 - 10 kg/day, and most preferably 0.1 - 5 kg/day, such as up to 1, 2, 3, or 4 kg/day, preferably with a Faradaic efficiency above 50%, and operated at current loads equal to or above 100 mA/cm ² .

The nitrogen and oxygen source are advantageously a combined nitrogen and oxygen source, such as air. To ensure sufficient nitrogen and oxygen supply, the apparatus preferably further comprises an oxygen separator fluidly connectable to the oxygen and/or nitrogen source. For flexible and cost-efficient operation, the oxygen separator is preferably configured to provide separated air with an oxygen concentration above 0% and below 20%, preferably below 2, 5, or 10%, and most preferably between 0.8 - 1.5%, such as 0.3, 0.4, 0.5, 0.8, or 1%.

In an embodiment of the disclosure, the apparatus comprises an oxygen separator fluidly connectable to the oxygen source and/or nitrogen source.

Examples

The invention is further described by the examples provided below.

Example 1: Electrochemical flow cell for combined lithium mediated electrochemical nitrogen reduction and hydrogen oxidation

Flow cells according to the present disclosure were used for synthesizing ammonia. The synthesis is for example carried out in a flow cell as shown in Figure 4, where nitrogen is supplied to the electrolyte as a continuous gas flow, and hydrogen is supplied as a continuous gas flow. For example, tests were carried out as further described below.

Cell

Electrochemical ammonia synthesis was conducted in the flow cell. The dimension of the flow cell was 10.7x10.7x5.1 cm ³ . The effective area of the flow field was 25 cm ² .

The size of gas flow channels was 1 mm. Before every experiment, the flow-cell parts were boiled in ultra-pure water for 3 h, and dried overnight at 100 °C in air.

Gases

The N2 (5.0, Air Liquide) and H2 (5.0, Air Liquide) gas flow rates were controlled using a mass flow controller (Brooks Instrument) and set to 75 seem. In order to synthesize ² H and/or ¹⁵ N isotope labelled ammonia, bottles of ² H2 (purity 99.995 %, isotope purity 99.8 %) and/or ¹⁵ N2 (purity 99 %, isotope purity 98 %) were used. The N2 and H2 (and optionally ² H2 and ¹⁵ N2) used in the experiments were further cleaned by purifiers (NuPure), prior to entering the reactor, and all labile N-containing compounds can be down to parts per trillion by volume (ppt-v) level.

Electrodes

The anode and cathode according to Examples 3-4 may be applied. Specifically: Cu electrodeposited on stainless steel cloth (SSC: McMaster-Carr, 325 x 2300 mesh, pore size: 5 pm, thickness: 70 pm) was used as the working electrode (WE).

Pt or PtAu alloy were electrodeposited on stainless steel cloth (SSC: McMaster-Carr, 325 x 2300 mesh, pore size: 5 pm, thickness: 70 pm).

The Pt wire used as pseudo reference electrode was flame annealed before the measurement to clean its surface

Electrolyte formulation

The UBF4 (Sigma Aldrich, > 98 %, anhydrous) was dried at 120 °C for 48 h in a vacuum oven before use. The ethanol (Honeywell, anhydrous) was treated by the 3 A molecular sieves. Electrolyte solution consisted of 1.0 M IJBF4 in 99-100 vol. % tetrahydrofuran (THF, anhydrous, >99.9 %, inhibitor-free, Sigma Aldrich) and 0-1 vol. % ethanol and was prepared in an Ar glovebox.

Electrolyte water content The electrolyte water content was measured via Karl Fischer Titration (831 KF Coulometer and 728 Stirrer, Metrohm). Before the experiment the water content of the electrolyte was generally 20-40 ppm; after the experiment 100-150 ppm.

Electrolyte start-up and circulation

A syringe pump (World Instruments) was used to control the flow rate of the electrolytes at 1.0 mL/min. The syringe (Trajan Scientific and Medical, 100 mL) consisted of the borosilicate glass tube and the PTFE plunger (Gas tight).

Prior to injection of electrolyte into the liquid chamber, the purified N2 and H2 (75 seem) were introduced into the empty assembled flow cell for at least 30 min. Afterward, we injected electrolyte solution into the cell in N2 and H2 atmosphere.

Electrochemistry

The electrochemistry experiments were performed using a BioLogic Potentiostat (VMP2). The resistance between the WE and RE was measured using the potentiostatic electrochemical impedance spectroscopy (PEIS) and the current interrupt technique. The /R-corrected linear sweep voltammetry (LSV) was recorded from the open-circuit voltage (OCV) until lithium reduction is clearly seen, where iR was the voltage drop from the electrolyte resistance.

Subsequently, chronopotentiometry (CP) was measured with or without potential cycling. The potential cycling refers to that -6 mA/cm ² (the corresponding potential denoted deposition potential) was applied for 1 min and then 0 mA/cm ² (the corresponding potential denoted resting potential) for 1-3 min. The typical potential cycling is -6 mA/cm ² for 1 min and 0 mA/cm ² for 1 min (denoted 1 min deposition + 1 min resting). Unless otherwise specified, 700 C of charge was passed for each experiment to determine the ammonia (NH3) Faradaic efficiency (FE). Potential cycling is used because it leads to an almost double Faradaic efficiency.

Pressure gradient across the GDE

The pressure gradient (/.e., 15 mbar) between the gas inlet and gas outlet of the flow cell was modulated by 10.5 centimeters of water column (50 mL) above the gas.

Ammonia gas trap

The 10 mM of H2SO4 (Sigma Aldrich, 99.999%) or HCI (Sigma Aldrich, Suprapur) solution was used as NH3 trapped solution. Boundary conditions

All experiments were conducted at room temperature and 1 bar. Typically, flow cell experiments were performed in a fume hood. For analysis of the solid-electrolyte interface (SEI) on the working electrode, the flow cell was run in the Ar glovebox.

Produced ammonia

When the synthesis experiment was finished, the produced NH3 was distributed in three parts,

(1) gas-phase NH3 trapped in 10 mM of H2SO4 or 10 mM HCI;

(2) NH3 trapped in the electrolyte;

(3) NH3 trapped in the SEI layer. We typically used 120-130 mL of ultra-pure water to dissolve the SEI layer to release trapped NH3. It was noted that H2O can react with N-containing compounds {e.g., U3N) to produce NH3.

Results and conclusions

Figure 14 shows the resulting electrochemical results for the flow cell including a bimetallic platinum-gold anode. Figure 14 shows the electrode potentials (left y-axis) and the Faradaic efficiency (right y-axis) as a function of time during flow cell operation. A high performance and stability over the ca. 600 minutes of test is seen. Specifically, a surprisingly high and stable Faradaic efficiency of around 60 % is seen.

Figures 12-13 show mass spectrometer data of the gas flow exiting the flow cell, when isotope labelled ammonia is produced by using isotope labelled deuterium (D) as hydrogen source. The initial products are mainly H-containing as the cathode is surrounded by H-containing electrolyte. But over time, more and more D-containing products are observed/monitored, as H-atoms in the proton shuttle is replaced by D- atoms from the anodic oxidation of D2 gas. Both fully deuterated hydrogen gas (D2 curve) and fully deuterated ammonia (ND3 curve) is observed/monitored, while less H- containing products are seen (H curves).

Accordingly, ² H labelled ammonia was produced, confirming that the gaseous hydrogen source provides the protons for the ammonia synthesis, since the ammonia containing deuterium can only be derived from deuterium oxidation at the anode. Example 2: Stability test of anode in presence of ammonia

Electrochemical ammonia synthesis were carried out in a single-compartment cell with different anodes (Pt or PtAu), and different electrolyte compositions (with or without ammonia).

All tests were conducted in an Ar-filled glovebox to avoid moisture and oxygen. A single-compartment glass cell was used with: a 4 cm ² stainless steel working electrode (McMaster, 316 Stainless Steel Wire Cloth, 500 x 500 Mesh); a 4 cm ² stainless steel counter electrode (McMaster, 316 Stainless Steel Wire Cloth, 500 x 500 Mesh) on which either Pt or PtAu were deposited; a Pt wire was used as a pseudo reference electrode. The Pt wire was flame annealed before the measurement to clean the Pt surface.

The electrolyte consisted of 1M LiBF4 in THF for the experiments without ammonia. For the experiments with 40 mM ammonia, this was added from a 0.4 M ammonia in THF solution (Sigma, 718939).

At the beginning of the experiment the electrolyte was saturated with H2 for 20 min, then cyclic voltammograms were taken with H2 bubbling and without H2 bubbling at a scan rate of 20 mV s’ ¹ . Afterward, the H2 bubbling was turned to a very slow bubbling rate just to keep the solution saturated with minimal convection. A chronopotentiometry was then initiated at -2 mA cm ^-2 to observe the HOR activity.

Results and conclusions

Figures 11 A-B show the anode performances without ammonia present in the electrolyte, corresponding to a flow cell shown in Figure 6A. Figure 11A is a close-up of the first 50 minutes time interval.

Figures 11C-D show the anode performances with ammonia present in the electrolyte, corresponding to a flow cell shown in Figure 6B. Figure 11 C is a close-up of the first 50 minutes time interval.

Figure 11 shows that the cathode working electrode potential (EWE) is almost identical with and without ammonia present. The PtAu anode show a better performance than Pt. A lower over-potential at the counter electrode (ECE) is seen for PtAu compared to Pt, both without ammonia present and with ammonia present. Further, an improved stability of PtAu may be seen, even in the presence of ammonia. Hence, the PtAu activity towards HOR is not affected by the presence of ammonia.

Similar poor performances of different single metal anodes, e.g. either Pt or Au anodes, are seen. The higher performance and stability is surprisingly seen for a bimetallic catalyst, such as PtAu.

As seen from the chronopotentiometry data, both with the presence and absence of ammonia, the Pt electrodes are not able to consistently oxidize H2. In the first few minutes the potential increases to above 1 V vs. Pt _pse udo at which THF oxidation takes place. On the other hand, the potential of PtAu stays below 0.5 V, proving the stability of HOR in the given electrolyte, also in presence of ammonia.

Example 3: Synthesis of cathode by HBT

Cu was electrodeposited on stainless steel cloth (SSC: McMaster-Carr, 325 x 2300 mesh, pore size: 5 pm, thickness: 70 pm) was used as the working electrode (WE). The SSC was cut into the desired size and spot-welded to a copper wire for electrical contact. To achieve the uniform electrodeposition, the two Pt meshes (Goodfellow, 1.5 cmx1 .5 cm, 99.9 %) were electrically connected and used as a split counter electrode, where the working electrode (SSC) was placed in the middle between counter electrodes during electrodeposition.

The 0.4 M CuSC>4 (Merck, 98 %) in 1.5 M H2SO4 (Sigma Aldrich, 99.999 %) solution was used as an electrolyte for electrodeposition Cu on SSC.

To deposit porous Pt or PtAu on the SSC, a constant current (e.g. a current density between -10 and -2.5 A cm ^-2 , such as -10 A cm ^-2 ) was applied for 5 min, during which rigorous hydrogen evolution and metal deposition take place at the same time, leading to high surface area structures. After the electrodeposition, the electrodes were cleaned in EtOH and ultra-pure water (18.2 MQ resistivity, Millipore, Synergy UV system) three times to remove all residual electrolytes.

Example 4: Synthesis of anode by HBT

Pt or PtAu alloy were electrodeposited on stainless steel cloth (SSC: McMaster-Carr, 325 x 2300 mesh, pore size: 5 pm, thickness: 70 pm). The SSC was cut into the desired size and spot-welded to a copper wire for electrical contact. To achieve uniform electrodeposition, the two Pt meshes (Goodfellow, 1.5 cmx1.5 cm, 99.9 %) were electrically connected and used as a split counter electrode, where the working electrode (SSC) was placed in the middle between counter electrodes during electrodeposition.

The 10 mM H2PtCle-6H2O (Sigma Aldrich, ACS reagent) or 10 mM H2PtCle-6H2O with 10 mM HAUCI ₄ '3H ₂ O (Sigma Aldrich, 99%) in 3 M H ₂ SO ₄ (Sigma Aldrich, 99.999%) solution was used as an electrolyte for electrodeposition Pt/SSC or PtAu/SSC, respectively.

To deposit porous Pt or PtAu on the SSC, a constant current (a current density of between -0.2 and -0.5 A cm ^-2 , such as -0.2 A cm ^-2 ) was applied for 2 min, during which rigorous hydrogen evolution and metal deposition take place at the same time, leading to high surface area structures. After the electrodeposition, the electrodes were cleaned in EtOH and ultra-pure water (18.2 MQ resistivity, Millipore, Synergy UV system) three times to remove all residual electrolytes

Example 5: Electrolyte chamber and electrolyte replacement and/or circulation

Flow cells with electrolyte chambers comprising different numbers of spacers were tested. The flow cell shown in Figure 4 having 4 elongated bar shaped spacers was tested, and similar chambers with respectively 0, 1 , or 3 spacers were also tested . The efficiency of the flow cells was seen to depend on the number of spacers.

For the embodiment absent spacers, a very low ammonia production was obtained. The presence of at least one spacer was seen to improve the ammonia synthesis, and/or possibly the ammonia recovery. For compact and cost-efficiency, an electrolyte chamber comprising fewer spacers, such as 1 spacer, may be advantageous.

Flow cells comprising electrolyte chambers with flow circulation guides are tested during start-up and/or operation of the cells.

The circulation efficiency and the replacement time efficiency and completeness is depending on the flow circulation guides position, numbers, and shapes.

6: Flow cells with different

Example 1 is repeated with different electrolyte compositions. Optionally, the electrolyte composition is replaced and/or circulated during operation, as described in Example 5, such that the electrolyte composition is moved/stirred and/or changed at some points during the operation time. Example 1 describes an electrolyte composition based on THF and Li-salts and ethanol as additive. Example 1 was repeated where the ethanol (EtOH) of the electrolyte was replaced with phenol (PhOH), and specifically Li-phenol oxide or lithium phenoxide (CeHsLiO). Similar high performance and/or stability may be seen, and specifically a surprisingly high Faradaic efficiency of around 75 % was seen, compared to the 60 % in Example 1.

Example 1 was repeated where the ethanol of the electrolyte was replaced with hydroquinone. Improved Faradaic efficiency and/or stability is seen.

Example 1 is repeated where the ethanol of the electrolyte is replaced with one or more of: phloroglucinol, 1-naphthol, or pyridine. Improved Faradaic efficiency and/or stability is seen.

Example 1 is further repeated, where the ethanol of the electrolyte is partly substituted by one or more of: phenol, Li-phenol oxide, hydroquinone, phloroglucinol, 1-naphthol, pyridine, and combinations thereof. Improved Faradaic efficiency and/or stability is seen.

Reference numbers

1 - Flow cell

2 - Cathode

3 - Anode

4 - Electrolyte chamber

4.1 - Cavity

4.2 - Frame

4.3 - Inner perimeter

4.4 - Spacer, separator and/or flow circulation guides

4.5 - Fluid inlet

4.6 - Fluid outlet

4.7 - Chamber opening

5 - Manifold

6 - Current collector

7 - Membrane Items

The presently disclosed may be described in further detail with reference to the following items.

1. A flow cell for electrochemical ammonia synthesis, comprising a cathode, an anode, and an electrolyte chamber, wherein the anode comprises a HOR catalyst comprising bimetallic Pt, and/or wherein the electrolyte chamber comprises one or more spacers having a height defining a distance between the anode and cathode.

2. The flow cell according to item 1, wherein the spacers are configured as separators diving the electrolyte chamber into two or more subchambers.

3. The flow cell according to any of the preceding items, wherein the spacers have a height of between 50 pm - 20 mm, more preferably between 500 pm - 10 mm, and most preferably between 1 mm - 6 mm, such as 2 or 4 mm.

4. The flow cell according to any of the preceding items, wherein two or more spacers are placed at predetermined distance to each other.

5. The flow cell according to any of the preceding items, wherein the spacers are elongated in one or more directions, and optionally shaped as bars.

6. The flow cell according to item 5, wherein the elongated spacers are horizontally oriented, and/or vertically oriented.

7. The flow cell according to any of items 5-6, wherein the elongated spacers are intersecting each other at an angle to form a grid, optionally intersecting at a perpendicular angle.

8. The flow cell according to item 7, wherein the grid is sandwiched between a cathode plane and an anode plane.

9. The flow cell according to any of the preceding items, wherein the electrolyte chamber comprises a cavity having two opposite ends defined by the anode and cathode, and side walls defined by an inner perimeter of a chamber frame. 10. The flow cell according to item 9, wherein the spacers are detachably attached to the chamber frame.

11. The flow cell according to any of the preceding items, wherein the electrolyte chamber comprises at least one fluid inlet and at least one fluid outlet such that the electrolyte chamber is configured for fluid replacement and/or circulation.

12. The flow cell according to item 11 , wherein the fluid inlet and fluid outlet are located on the chamber frame, optionally located on the inner perimeter of the chamber frame.

13. The flow cell according to item 12, wherein the fluid inlet is provided by one or more first lumens within the chamber frame, and/or wherein the fluid outlet is provided by one or more second lumens within the chamber frame.

14. The flow cell according to item 13, wherein the first and second lumens are parallel to a frame plane.

15. The flow cell according to any of claims 13-14, wherein the first and second lumens are parallel oriented.

16. The flow cell according to any of items 9-15, wherein the frame comprises at least two frame sections placed opposite each other.

17. The flow cell according to item 16, wherein at least one fluid inlet and at least one fluid outlet is located at a first frame section.

18. The flow cell according to item 16, wherein at least one fluid inlet is located at a first frame section, and at least one fluid outlet is located at an opposite frame section.

19. The flow cell according to item 18, wherein the at least one fluid inlet and the at least one fluid outlet are symmetrically opposed or mirror symmetrically opposed.

20. The flow cell according to any of items 13-19, wherein the fluid inlet and/or the fluid outlet comprise multiple lumen segments. 21. The flow cell according to item 20, wherein the fluid inlet and/or the fluid outlet comprise one or more first lumen segments extending in parallel and along an inner perimeter of a chamber frame, and one or more second lumen segments at an angle to the inner perimeter, such that the second lumen segment forms a chamber opening into the electrolyte chamber.

22. The flow cell according to item 21 , wherein the fluid inlet and fluid outlet comprise oppositely located chamber openings.

23. The flow cell according to item 22, comprising multiple pairs of oppositely located chamber openings, wherein the pairs are located at a predefined distance from each other along the first lumen segment.

24. The flow cell according to any of items 21-23, wherein the chamber opening is a slit in parallel with a chamber plane.

25. The flow cell according to any of items 21-24, wherein the angle of the second lumen segment to the surface of the inner perimeter is diverging towards the inner perimeter of the chamber, optionally wherein the second lumen segment has a shape selected from the group of: trapezoid, and/or trapezoid with curved legs, such as circular curved legs.

26. The flow cell according to any of items 20-25, wherein one or more lumen segments are obtained by cutting and/or drilling.

27. The flow cell according to any of items 2-26, comprising between 1-40 separators, more preferably between 2-20 or 5-15 separators, and most preferably between 6-10 separators.

28. The flow cell according to any of the preceding items, wherein the electrolyte chamber comprises flow circulation guides.

29. The flow cell according to item 28, wherein the spacers are configured as flow circulation guides.

30. The flow cell according to any of items 28-29, wherein the flow circulation guides are configured to generate a meandering fluid flow between the fluid inlet and fluid outlet. 31. The flow cell according to any of items 28-31 , wherein the flow circulation guides comprises a plurality of elongated bars within the electrolyte chamber, oriented in parallel to the fluid inlet and fluid outlet.

32. The flow cell according to any of items 28-31 , wherein the elongated bars are rectangular or triangular shaped in cross sectional view.

33. The flow cell according to any of items 28-32, wherein the flow circulation guides are configured to generate a laminar flow between a fluid inlet and a fluid outlet.

34. The flow cell according to item 33, wherein the flow circulation guides comprise a plurality of symmetrically opposed fluid inlets and fluid outlets.

35. The flow cell according to item 34, wherein the symmetrically opposed fluid inlets and fluid outlets comprise a branched fluid inlet and a branched fluid outlet.

36. The flow cell according to any of the preceding items, wherein the electrolyte chamber is absent an ion conducting membrane.

37. The flow cell according to any of the preceding items, wherein the bimetallic Pt is in combination with a metal selected from the groups 8-12, 14, 15, and combinations thereof, and optionally comprises a third or fourth metal.

38. The flow cell according to item 37, wherein the bimetallic Pt is in combination with a metal selected from the group of: Au, Ru, Rh, Pb, Bi, Sn, Sb, and combinations thereof, and preferably in combination with Au, such as Pt-Au in atomic ratio 1:1, 1:2, 1:3, 1 :4, 1 :5, 2:3, 2:5, 2:1 , 3:1 , 4:1, 5:1 , 3:2, 5:2.

39. The flow cell according to any of the preceding items, wherein the anode and cathode comprise gas diffusion electrodes.

40. The flow cell according to any of the preceding items, wherein the cathode comprises a cathode chamber with a catholyte, and/or wherein the anode comprises an anode chamber with an anolyte. 41 . The flow cell according to any of the preceding items, wherein the cathode is fluidly connected to a nitrogen gas source, and/or the anode is fluidly connected to a hydrogen gas source.

42. The flow cell according to item 41 , wherein the nitrogen gas source comprises 1 ⁵ N2 and/or the hydrogen gas source comprises deuterium, whereby the flow cell is configured to synthesize isotope labelled ammonia, such as ¹⁵ N and/or ² H labelled ammonia.

43. The flow cell according to any of the preceding items, wherein the cathode and/or anode is fluidly connected to a gas manifold.

44. The flow cell according to item 45, wherein the gas manifold(s) comprise an aperture having a meandering shape.

45. The flow cell according to any of the preceding items, wherein the electrolyte chamber contains an electrolyte comprising one or more proton shuttling additives.

46. The flow cell according to item 45, wherein the additive is an alcohol, which is not ethanol.

47. The flow cell according to any of items 45-46, wherein the additive comprises an aromatic organic compound, preferably selected from the group of: phenol, Li-phenol oxide, hydroquinone, phloroglucinol, 1 -naphthol, pyridine, and any combinations thereof.

48. A flow cell system comprising a plurality of flow cells according to any of items 1-47.