An ammonia electrolysis cell

Abstract

The present disclosure relates broadly to ammonia electrochemical cells. The ammonia electrolysis cell may comprise: a chamber for containing an electrolyte; two electrodes disposed within the chamber; and an anion exchange membrane disposed between the electrodes, wherein each electrode comprises a bifunctional catalyst having ammonia oxidation reaction activity and hydrogen evolution reaction activity, and wherein each electrode is capable of alternating in polarity when subjected to an alternating potential. There is also disclosed herein a method of operating an ammonia electrolysis cell as well as the use of an ammonia electrolysis cell to produce hydrogen from ammonia.

Description

[0001] An Ammonia Electrolysis Cell

[0002] References to Related Application

[0003] This application claims priority to Singapore Application No. 10202401270R filed with the Intellectual Property Office of Singapore on 30 April 2024, the contents of which are incorporated herein by reference.

[0004] Technical Field

[0005] The present invention generally relates to an ammonia electrolysis cell. The present invention also relates to a method of operating an ammonia electrolysis cell and use of the ammonia electrolysis cell.

[0006] Background Art

[0007] To reduce reliance on fossil fuels and move toward carbon-free energy economy, increasing developments have been made towards the conversion and storage of renewable energy. Molecular hydrogen (H2) is often seen as an ultimate energy carrier. However, there arc technical challenges involved with safely transporting and storing liquid H2. Accordingly, there is a need to develop hydrogen carriers which may mitigate these issues. Ammonia (NH2) has been proposed as an ideal carbon-free hydrogen carrier with excellent gravimetric and volumetric hydrogen capacitance.

[0008] The electrolysis of ammonia (with Eo = 0.06 VRHE) can couple with a cathodic hydrogen evolution reaction (HER) in an electrochemical cell to produce H2 and 2 at a relatively low cell voltage (~0.6 V). This electrochemical NH2 cracking (“ammonia e-cracking”) reaction can occur at a low temperature compared to thermal NH2 cracking. Decentralised cell-based production of H2 from NH2 provides an advantage in that it avoids the necessity for H2 transportation and storage. To date, the successful implementation of NH2 electrolysis has not yet been realised as one of tire major issues is anode durability owing to tire effects of the ammonia oxidation reaction (AOR). While Pt and Pt-based electrocatalysts promote the AOR effectively, the anode performance decays significantly due to catalyst poisoning.

[0009] The poisoning mechanisms of Pt-based catalysts include (a) deactivation caused by dehydrogenated nitrogen atoms irreversibly covering the platinum surface (via the Gcrischcr- Mauerer mechanism), and (b) blocking of active sites by *NOXadsorbates, which are produced at high overpotential.

[0010] Examples of methods used to mitigate catalyst poisoning include the introduction of iridium into platinum-based catalyst, for example, Pt?Irz, which was reported to be more resistant to poisoning; however, the cost of iridium makes this less commercially viable. Another approach involved in-situ cleaning of the adsorbates by applying desorption potentials to the Pt-based catalyst during AOR to recover catalyst activity. However, the catalyst recovery rate dropped substantially over time wherein only 30% catalyst performance could be maintained after 2 hours of cell operation. There is thus a need to provide an ammonia electrolysis cell and methods of operating such ammonia electrolysis cells that overcomes, or at least ameliorates, one or more of the disadvantages of the prior art.

[0011] Accordingly, there is a need to provide an ammonia electrolysis cell or methods of operating such ammonia electrolysis cells that ameliorates the effect of catalyst poisoning and is able to maintain performance over extended periods while remaining relatively cost-effective.

[0012] Summary

[0013] According to a first aspect, there is provided an ammonia electrolysis cell comprising: a chamber for containing an electrolyte; two electrodes disposed within the chamber; and an anion exchange membrane disposed between the electrodes, wherein each electrode comprises a bifunctional catalyst having ammonia oxidation reaction activity and hydrogen evolution reaction activity, and wherein each electrode is capable of alternating in polarity when subjected to an alternating potential.

[0014] Advantageously, the use of bifunctional catalysts may allow for both the ammonia oxidation reaction (AOR) and the hydrogen evolution reaction (HER) to occur at each electrode, wherein the HER may help to recover catalyst active sites and thereby reduce the catalyst poisoning effected by the AOR when alternating, i.e. reversing, the potential. Further advantageously, the ammonia electrolysis cell may have improved durability.

[0015] According to a second aspect, there is provided a method of operating an ammonia electrolysis cell, wherein the cell comprises: two electrodes, wherein each electrode comprises a bifunctional catalyst having ammonia oxidation reaction activity and hydrogen evolution reaction activity'; and an electrolyte contacting the electrodes, wherein the electrolyte comprises ammonia, the method comprising the steps of: a) applying a potential to the electrodes for a predetermined time wherein the first electrode functions as an anode and wherein the second electrode functions as a cathode, b) alternating the potential for a predetermined time such that the first electrode functions as a cathode and the second electrode functions as an anode, and c) alternating the potential to repeat steps a) and b.

[0016] According to a third aspect, there is provided use of the ammonia electrolysis cell as described herein or the method as described herein to produce hydrogen from ammonia.

[0017] Advantageously, ongoing alternating potential may' allow for continuous cycles of catalyst poisoning and recovery at both electrodes, thereby substantially maintaining overall effective cell operation and continuous hydrogen production with improved durability. Definitions

[0018] The following words and terms used herein shall have the meaning indicated:

[0019] As used herein, the singular forms “a”, ‘"an”, and ‘"the” designate both the singular and the plural, unless expressly stated to designate the singular only.

[0020] The term “anode” refers to the electrode where the oxidation reaction occurs.

[0021] The term “cathode” refers to the electrode where the reduction reaction occurs.

[0022] The term “alternating potential” is used to refer to an electric potential difference or cell voltage applied across a set of electrodes that is intermittently reversed such that the electrodes alternatingly function as anodes and cathodes. This potential may be alternated by electronic or mechanical means from an external electrical energy source suitable for the intended use. A direct current is employed when a potential difference is applied across the electrodes. This direct current is reversed when alternating tire potential across the electrodes.

[0023] The “step time” refers to a predetermined amount of time that a potential is applied to the electrodes before alternating, i.e. reversing, the potential.

[0024] “Cyclic voltammetry” (or “CV”) is an electrochemical technique employed to analyse the current response of an electrode as a function of an applied potential. The potential of a working electrode is systematically varied within a predefined range, and the resulting current is measured. CV is generally used to investigate the redox behavior of chemical species to provide data regarding the kinetics, reaction mechanisms and other electrochemical characteristics. The current -potential curve is referred to as a cyclic voltammogram.

[0025] “Chronopotentiometry” is an electrochemical technique in which a constant current is applied to an electrode, and the potential response is monitored over time. It is generally used to study the electrochemical behavior of systems by observing the change in potential as a function of the applied current to provide information regarding the kinetics of electrochemical reactions, the diffusion coefficients of reactants, and other time -dependent behaviours.

[0026] “Chronoamperometry” (or “CA”) is an electrochemical technique in which a constant potential is applied to an electrode and the resulting current is measured as a function of time. CA is generally used to investigate the electrochemical processes occurring at the electrode surface by monitoring the time-dependent current response, providing information about the kinetics of electrochemical reactions, diffusion rates of reactants, and other properties such as capacitance or charge transfer characteristics.

[0027] “Faradaic Efficiency” (or “FE”) is a measure of the efficiency with which charge is utilised in an electrochemical reaction, specifically the proportion of the total current that contributes to the desired electrochemical transformation. FE is expressed as the ratio of the charge involved in the desired Faradaic (electron transfer) process to the total charge passed during the electrochemical experiment. FE is generally an important parameter in evaluating the effectiveness of electrochemical reactions. The “Reversible Hydrogen Electrode” (or “RHE”) is a reference electrode commonly used in electrochemical measurements as a standard reference potential. The RHE consists of a platinum electrode in contact with an aqueous solution of H3O+(1 M) under hydrogen gas (1 atm).

[0028] The “electrolyte” is a medium which facilitates ion movement between the electrodes. The electrolyte may be a liquid, a solid, a gel, or any other substance which is suitable for use in electrochemical cells.

[0029] An “electrolysis cell” is an electrochemical cell that uses external electrical energy to drive a non-spontaneous chemical reaction. In an electrolysis cell, an electric potential is applied between the electrodes, forcing electrons produced from the oxidation reaction at the anode to move to the cathode where the electrons are consumed in the reduction reaction.

[0030] A “membrane” is a layer that be used to separate the electrodes while allowing for ion transfer between them. An “anion exchange membrane” (or “AEM”) is a semipermeable membrane intended to allow anion transfer but prevents the movement of other substances, such as gases.

[0031] A “membrane electrode assembly” (or “MEA”) is an arrangement within a cell comprising a membrane (such as an AEM) sandwiched between the electrodes. An MEA may facilitate the cell’s electrochemical reactions by allowing ion exchange between the electrodes while electrically insulating them.

[0032] A “bifunctional catalyst” is a catalyst that is able to increase the reaction rate of two different reactions. As used herein, a bifunctional catalyst refers to a catalyst that increases both tire rates of an oxidation reaction and a reduction reaction. Accordingly, a bifunctional catalyst at an electrode will be able to increase both the rate of oxidation (functioning as an anode) and increase the rate of reduction (functioning as a cathode) depending on the species that is present and the potential that is applied at the electrode.

[0033] The “hydrogen evolution reaction” (or “HER”) is a chemical reaction that produces hydrogen gas from water. In an electrochemical cell, the HER occurs at the surface of the cathode and may be characterised by the following half-reaction under basic conditions:

[0034] 2H2O + 2e- — 112+ 2OI I

[0035] The “ammonia oxidation reaction” (or “AOR”) is a chemical reaction that produces nitrogen gas from ammonia. In an electrochemical cell, the AOR occurs at the surface of the anode and may be characterised by the following half -reaction under basic conditions:

[0036] 2NH3+ 6OH- 6H2O + N2+ 6e"

[0037] Thc term “poisoning” as used herein refers to the inactivation of catalyst active sites, particularly at the catalyst surface, thereby reducing catalyst efficacy. This inactivation may be reversible or irreversible and may be caused by the ammonia oxidation reaction (AOR) at the anode. It may be caused by active site deactivation from the binding of dehydrogenated nitrogen atoms (*N) and / or the blocking of active sites by *NOXadsorbates, which may be produced at high overpotentials. The term “recovery” as used herein refers to the reactivation of poisoned catalyst active sites, thereby allowing for catalyst efficacy to be partially, substantially or fully recovered. The reactivation may be effected by the hydrogen evolution reaction (HER) at the cathode.

[0038] The word “substantially” does not exclude “completely” e.g. a composition which is “substantially free” from Y may be completely free from Y. Where necessary, the word “substantially” may be omitted from the definition of the invention.

[0039] Unless specified otherwise, the terms “comprising" and “comprise”, and grammatical variants thereof, are intended to represent “open” or “inclusive” language such that they include recited elements but also permit inclusion of additional, unrecited elements.

[0040] As used herein, the term “about”, in the context of concentrations of components of the formulations, typically means + / - 5% of the stated value, more typically + / - 4% of the stated value, more typically + / - 3% of the stated value, more typically, + / - 2% of the stated value, even more typically + / - 1% of the stated value, and even more typically + / - 0.5% of the stated value. Moreover, “about” may be understood by persons of ordinary skill in the art to allow for small or non-substantial variations reflecting the appropriate level of precision according to the context in which it is used.

[0041] Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

[0042] As used herein, the term “about” and the use of ranges in general, whether or not qualified by the term about, means that the number comprehended is not limited to the exact number set forth herein, and is intended to refer to values substantially within the quoted range while not departing from the scope of the invention.

[0043] Certain embodiments may also be described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the disclosure. This includes the generic description of the embodiments with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0044] Detailed Disclosure of Embodiments

[0045] Exemplary, non-limiting embodiments of an ammonia electrolysis cell will now be disclosed.

[0046] The ammonia electrolysis cell comprises: a chamber for containing an electrolyte; two electrodes disposed within the chamber; and an anion exchange membrane disposed between tire electrodes, wherein each electrode comprises a bifunctional catalyst having ammonia oxidation reaction activity (AOR) and hydrogen evolution reaction (HER) activity, and wherein each electrode is capable of alternating in polarity when subjected to an alternating potential.

[0047] The two electrodes may comprise the same bifunctional catalyst. The bifunctional catalysts may allow for any catalyst poisoning at the anode caused by the AOR to be substantially reversed by the HER, when that electrode functions as a cathode as a result of alternating the potential. The continuous catalyst recovery at both electrodes may maintain effective overall electrolysis operation and thereby improved performance and durability in hydrogen production.

[0048] The electrodes may be capable of alternating in polarity when subjected to an external alternating potential. The potential may be alternated by mechanical means. The potential may be alternated by electronic means. The potential may be provided from an external energy source.

[0049] The bifunctional catalyst may have AOR activity and HER activity. The bifunctional catalyst may enable both the AOR and HER to alternatingly occur at the same electrode. The bifunctional catalyst may increase the reaction rate of the AOR, HER, or both.

[0050] The bifunctional catalyst may be one or more of a nickel-based catalyst, a copper-based catalyst, a cobalt-based catalyst, an iron-based catalyst, an iridium-based catalyst, a titanium- based catalyst, a ruthenium-based catalyst and a platinum -based catalyst. The bifunctional catalyst may be a platinum or a platinum-based catalyst. The platinum catalyst may be Pt / C. The Pt / C catalyst may be selected from, but not limited to, about 1% to about 70% w / w Pt / C, or about 5% to about 60% w / w Pt / C, or about 10% to about 50% w / w Pt / C, or about 20% to about 40% w / w Pt / C, based on the weight of the catalyst. The Pt / C catalyst may be about 20% or about 40% w / w Pt / C.

[0051] The anion exchange membrane (AEM) disposed between the electrodes may help to prevent any short-circuiting between the electrodes. Furthermore, the AEM may prevent bubble formation between the electrodes or on the electrode surface, which could reduce the active surface area and ultimately decrease electrode reactivity.

[0052] The membrane and electrodes may be arranged in a membrane electrode assembly (MEA) within the cell, wherein the MEA comprises the membrane, such as an AEM, sandwiched between the electrodes.

[0053] The ammonia electrolysis cell may comprise a chamber suitable for containing an electrolyte. The electrolyte may comprise ammonia. The electrolyte may comprise one or more of a liquid, a solid, a polymer or a gel. The electrolyte may be a liquid. The electrolyte may be an aqueous electrolyte. The electrolyte may be alkaline. The electrolyte may comprise an aqueous hydroxide base, such as LiOH, NaOH or KOH. The electrolyte may comprise aqueous KOH.

[0054] The electrolyte may comprise an aqueous hydroxide base at a concentration that is selected from, but not limited to, between about 0.1 M to about 10 M, or about 0.3 M to about 5 M, or about 0.5 M to about 2 M, or about 1 M to about 1.5 M. The aqueous hydroxide base concentration may be about 1.0 M. The electrolyte may comprise aqueous ammonia at a concentration that is selected from, but not limited to, between about 0.1 M to about 10 M, or about 0.3 M to about 5 M, or about 0.5 M to about 2 M, or about 1 M to about 1.5 M. The ammonia concentration may be about 1.1 M or about 1.5 M.

[0055] The electrolyte may be degassed or aerated with an inert gas, such as nitrogen or argon.

[0056] Exemplary; non-limiting embodiments of a method of operating an ammonia electrolysis cell will now be disclosed.

[0057] The method of operating an ammonia electrolysis cell, wherein the cell comprises: two electrodes, wherein each electrode comprises a bifunctional catalyst having ammonia oxidation reaction activity7and hydrogen evolution reaction activity; and an electrolyte contacting the electrodes, wherein the electrolyte comprises ammonia, the method comprising the steps of: a) applying a potential to the electrodes for a predetermined time wherein the first electrode functions as an anode and wherein the second electrode functions as a cathode, b) alternating the potential for a predetermined time such that the first electrode functions as a cathode and the second electrode functions as an anode, and c) alternating the potential to repeat steps a) and b.

[0058] The predetermined time or “step time” may be selected to ensure sufficient HER reactivity at the cathode. The predetermined time may enable sufficient hydrogen to be produced and provide enough time for said hydrogen to diffuse away from the cathode, while allowing for any catalyst poisoning from the AOR at the anode to be substantially' recovered by the subsequent HER when the cell potential is alternated. The predetermined time may be sufficient for the HER at the cathode to substantially reverse or recover the catalyst poisoning effected by the AOR at the anode. The predetermined time may be selected to substantially avoid unrecoverable or irreversible catalyst poisoning.

[0059] The predetermined time may be selected from, but not limited to, about 0.5 seconds to about 10 minutes, or about 1 seconds to about 8 minutes, or about 5 seconds to about 5 minutes, or about 10 seconds to about 3 minutes, or about 20 seconds to about 120 seconds, or about 30 seconds to about 100 seconds, or about 40 seconds to about 80 seconds, or about 50 seconds to about 70 seconds. The predetermined time may be about 60 seconds.

[0060] The predetermined time of step a) may be the same as tire predetermined time of step b). The predetermined time of step a) may' be different from the predetermined time of step b). The predetermined time of step a) and the predetermined time of step b) may be independently selected or determined. The predetermined time of step a) may be considered a first predetermined time and the predetermined time of step b) may' be considered a second predetermined time.

[0061] The cell potential may be alternated between ranges that are selected from, but not limited to, the range of about -0.5V to about -1.0V to the range of about +0.5V to about +1.0V, or the range of about -0.6V to about -0.9V to the range of about +0.6V to about +0.9V, or about the range of -0.7V to about -0.85V to the range of about +0.7V to about +0.85V. The cell potential may' alternate between about -0.75V to about +0.75V, about -0.8V to about +0.8V, or about -0.85V to about +0.85V. The cell potential may alternate between about -0.8V to about +0.8V.

[0062] The ammonia elecholysis cell may be operated at ambient temperatures or at elevated temperatures. The ammonia electrolysis cell may be operated at an elevated temperature. The temperature may be selected from, but not limited to between about 0 °C to about 100 °C. The temperature may be between about 20 °C to about 100 °C, or about 40 °C to about 100 °C, or about 60 °C to about 95 °C, or about 70 °C to about 90 °C, or about 75 °C to about 85 °C. The operating cell temperature may be about 80 °C.

[0063] The electrolyte may be pumped through the ammonia electrolysis cell. The electrolyte may be continuously or periodically circulated through the ammonia electrolysis cell. The flow rate of the electrolyte through the ammonia electrolysis cell may be selected from, but not limited to, about 0.1 mL / minute to about 100 mL / minute, or about 0.3 mL / minute to about 30 mL / minute, or about 1 mL / minute to about 10 mL / minute, or about 1.5 mL / minute to about 5 mL / minute. The flow rate of the electrolyte may be about 2.0 mL / minute.

[0064] Exemplary; non-limiting embodiments of use of the ammonia electrolysis cell as defined herein or method as defined herein will now be disclosed.

[0065] The use of the ammonia electrolysis cell as defined herein or the method as defined herein is to produce hydrogen from ammonia. The ammonia electrolysis cell or method of operating thereof may utilise ammonia derived from industrial and biological byproducts or waste products. The ammonia electrolysis cell or method of operating thereof may be configured for decentralised or portable hydrogen production. The hydrogen produced may be used immediately or stored.

[0066] Brief Description of Drawings

[0067] The accompanying drawings illustrate a disclosed embodiment and serve to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

[0068] Figure 1 (a)

[0069] Figure 1 (a) is a graph showing cyclic voltammetry7(CV) performance of Pt / C electrode in N2- saturated KOH (1 M) + NH3 (1 M) within four potential ranges in order of (T) -0.2 to 0.9 V, @ 0 to 0.9 V, @ 0.2 to 0.9 V and (4) 0.4 to 0.9 V.

[0070] Figure 1 (b)

[0071] Figure 1 (b) is a graph showing chronopotcntiomctry measurements of two-clcctrodc NHi electrolysis cell with (Alternating Anode-Cathode) and without (Fixed Anode-Cathode) alternating the potential between the Pt / C electrodes. The current density of the cell was kept at 1 mA cm2.

[0072] Figure 2

[0073] Figure 2 is a schematic showing an electrolyser cell design without (left) and with (right) alternating potential at the electrodes. Figure 3

[0074] Figure 3 is a schematic showing the arrangement of the anion exchange membrane (AEM), catalyst, and bifunctional / bipolar AOR / HER electrodes in the electrolyser cell as disclosed herein as well as the reactions occurring at each electrode.

[0075] Figure 4 (a)

[0076] Figure 4 (a) is a graph showing CV curves in KOH (I M) wilh / without NHj (1.5 M).

[0077] Figure 4 (b)

[0078] Figure 4 (b) is a graph showing chronoamperometry (CA) curves at 0.6 V and -0.6 V for 5, 10, 20 and 30 minutes each.

[0079] Figure 4 (c)

[0080] Figure 4 (c) is a graph showing corresponding Faradaic Efficiency (FE) towards hydrogen production with CA switching step time of 1, 2, 5, 10, 20 and 30 minutes.

[0081] Figure 4 (d)

[0082] Figure 4 (d) is a graph showing the stability curve obtained via repetitive CA tests at 0.6 V and - 0.6 V for 10 minutes each.

[0083] Figure 4 (e)

[0084] Figure 4 (e) is a graph showing CV curves obtained after every 24 h CA stability tests.

[0085] Figure 4 (f)

[0086] Figure 4 (f) is a graph showing the corresponding CV anodic and cathodic peaks to monitor the degradation process.

[0087] Examples

[0088] Non-limiting examples of the invention will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

[0089] Example 1

[0090] 20% Pt / C (obtained from Premetek of New Jersey, USA) was dispersed in 1:4 isopropanol / water (V / V) solvent, followed by the addition of Na+-exchanged Nafion (obtained from Sigma- Aldrich of Massachusetts, USA) as a binder to produce an ink with Pt / C concentration at 5 mg / mL. The electrocatalyst ink was dropped onto an electrode (obtained from Dioxide Materials of Florida, USA), maintaining an overall Pt loading of 0.05 mg / cm2. The Pt / C electrodes were dried under ambient conditions.

[0091] Cyclic voltammetry (CV) was performed on the resulting Pt / C electrode in an ^-saturated electrolyte comprising KOH (1 M) and NH rFEO (1 M) in a three-electrode configuration, with a platinum plate ( Iz2 cm2) as a counter electrode and Hg / HgO (KOH, 1 M) reference electrode. The measurements were conducted within four potential ranges sequentially and the corresponding steady CV profile is shown in Figure 1 (a). In the cyclic voltammogram, the notable oxidative peaks observed at a potential of -0.75 V (vs RHE) are attributed to the A OR current. The AOR current shows a notable decrease along with a narrowing of the scan range from (T) to (4). This is due to the Pt surface being substantially poisoned by the surface adsorbates and the recovery process occurring at a lower potential. When the Pt surface is not recovered, tire AOR activity decreases substantially, demonstrating the serious issues related to Pt-based electrocatalyst stability during the AOR.

[0092] Chronopotentiometry measurement was then carried out on a two-electrode electrolysis cell with two Pt / C electrodes as anode and cathode, respectively, using a Nz-saturated electrolyte comprising KOH (1 M) and NH3 H2O (1 M). One electrode functioned as the AOR electrode while the other functioned as the HER electrode. The measurements were performed with and without alternating potential applied to the two Pt / C electrodes at a current density of 1 mA cm2(Figure 1 (b)). Without alternating potential, the cell voltage was maintained at -0.7 V for a very limited time, and then dramatically increased to over 1.5 V (vs RHE), at which point, water hydrolysis was observed. Accordingly, the AOR could not be sustained for long on the Pt / C electrode without surface recovery. However, when an alternating potential was applied, the cell voltage was maintained at -0.6 V for more than 1.5 hours with negligible activity drop. This indicated that the poisoned Pt surface during the AOR was activated completely during the HER, which is likely due to a thorough removal of *N / *NOx adsorbates on the surface of the Pt catalyst. The electrochemical characterisations and analysis accordingly provided sufficient fundamental evidence for the presence of poisoning and recovery on the Pt-catalyst electrode.

[0093] Example 2

[0094] A prototype of anion exchange membrane (AEM) NH3 electrolyser cell was designed using the alternating potential approach. Figure 2 shows a clear comparison of the different approaches with and without the alternating potential in the AEM eletrolyser cell. Without alternating electrodes, tire two chambers of the MEA produce H2 and Nz, respectively, wherein the AOR electrode would experience significant catalyst poisoning. When alternating the potential between the electrodes in the two chambers, the two electrodes exchange roles as the AOR and HER electrodes repeatedly. Accordingly, tire poisoned AOR electrode can be recovered by the subsequent HER process and is then able to deliver recovered activity in the subsequent AOR process.

[0095] To assemble the AEM-based MEA electrolyser cell, two bipolar plates were used as gas flow channels and functioned as a current conductor to the gas diffusion layer (GDL), as shown in Figure 3.

[0096] Referring to Figure 3, there is an expanded schematic showing an ammonia electrolytic cell (1) comprising a chamber for containing an electrolyte; two electrodes (2) and (3) disposed within the chamber; an anion exchange membrane (4) disposed between the electrodes, wherein each electrode comprises a bifunctional catalyst (5) having AOR activity and HER activity. The electrodes (2) and (3) also function as a current conductor to the gas diffusion layer (GDL) (6). In the state of the cell (1) shown, the electrode (2) is functioning as the anode, showing the formation of nitrogen from ammonia at the location of the bifunctional catalyst (5) and GDL (6) at the surface of the electrode (2). Electrode (3) is functioning as the cathode, showing the formation of hydrogen from water at the location of the bifunctional catalyst (5) and GDL (6) at the surface of the electrode (3). A catalyst ink was prepared by mixing 40% Pt / C (100 mg) into a mixture (prepared from mixing 15 mL isopropanol, 5 mL deionised water and 0.24 mL 10% FAA-3 ionomer solution (obtained from FuMA-Tech of Bietigheim-Bissingen, Germany), then ultrasonicating the resulting mixture for 1 hour. The anode and cathode were both prepared by air-spraying the ink onto a nickel fiber paper with a Pt loading of 1 mg / cm2, and drying on a hot plate at 90 °C. A Sustainion X37-50 Grade T membrane (obtained from Dioxide Materials of Florida, USA) was used as the AEM and this was immersed in KOH solution (1 M) for 24 hours to substantially basify the AEM. The AEM was also used prior to any dehydration to avoid membrane cracking. Stainless steel 316L blocks with a single serpentine channel were used as bipolar plates. Viton gaskets with suitable thickness (no compression for nickel fiber paper) were also placed to prevent any liquid / gas leakage. The torque applied to assemble the cell was 8.5 Nm and the active area of the MEA was 4 cm2. The cell temperature was maintained at 80 °C and an Ar-saturated electrolyte comprising KOH (1 M) and NH3 (1.5 M) was preheated and pumped into the cell with a flow rate of 2 mL / minute.

[0097] The following tests shown in Figure 4 (a) to Figure 4 (f) were conducted in the AEM-based MEA electrolyser at 80 °C.

[0098] As shown in Figure 4 (a), a symmetric cyclic voltammetry (CV) curve was achieved when using the electrodes alternatingly as anode and cathode in the AEM-based MEA electrolyser cell. An anodic and a cathodic peak at ~±0.9 V indicated that the AOR occurred at each electrode. It was also noteworthy that the alternating frequency was also crucial for not only the ammonia cracking activity but also hydrogen generation efficiency. If the step time was too short, the hydrogen oxidation reaction after switching cathode to anode could consume a significant amount of the hydrogen just produced but not yet dissociated from the electrode. If the step time was too long, the Pt catalyst was significantly deactivated during AOR and became difficult to reactivate. Therefore, the optimal frequency of the alternating electrode was examined to optimise the Faradaic Efficiency (FE) towards NI L e-cracking. As shown in Figure 4 (b), the step time was varied from 5 to 30 minutes for chronoamperometry (CA) measurement for a total duration of 60 minutes. It was surprising found that the AOR activity could be repeatedly recovered.

[0099] The FE values for H2 production are summarised in Figure 4 (c). The highest FEHZ can be achieved when at a step time of 10 minutes. The alternating frequency was then fixed at a step time of 10 minutes for CA measurement using an alternating potential of ±0.6 V for 192 hours (8 days), as shown in Figure 4 (d), during which the CV measurement was recorded every 24 h.

[0100] As shown in the oxidation peak for the AOR CV (Figure 4 (e)), the catalyst activity shows only a very limited drop in performance over a long period. Figure 4 (f) shows the peak current density in the AOR CV versus the time. It was remarkably found that the Pt catalyst could maintain effective electrochemical performance through use of an alternating potential, with catalytic activities being maintained at >93% of initial performance throughout the 8- day test. Industrial Applicability

[0101] The ammonia electrolysis cell as defined herein may be used in hydrogen generation industries to produce hydrogen from ammonia or ammonia-containing sources.

[0102] It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

Claims

Claims1. An ammonia electrolysis cell comprising: a chamber for containing an electrolyte; two electrodes disposed within the chamber; and an anion exchange membrane disposed between the electrodes, wherein each electrode comprises a bifunctional catalyst having ammonia oxidation reaction activity and hydrogen evolution reaction activity, and wherein each electrode is capable of alternating in polarity when subjected to an alternating potential.

2. The ammonia electrolysis cell of claim 1, wherein the bifunctional catalyst is one ormore of a nickel -based catalyst, a copper-based catalyst, a cobalt-based catalyst, an iron-based catalyst, an iridium-based catalyst, a titanium-based catalyst, a ruthenium-based catalyst and a platinum-based catalyst.

3. The ammonia electrolysis cell of claim 1 or claim 2, wherein the bifunctional catalyst is a platinum-based catalyst.

4. The ammonia electrolysis cell of any one of claims 1 to 3, wherein the electrolyte comprises ammonia.

5. The ammonia electrolysis cell of any one of claims 1 to 4, wherein the electrolyte is alkaline.

6. The ammonia electrolysis cell of any one of claims 1 to 5, wherein tire electrolyte is an aqueous electrolyte.

7. A method of operating an ammonia electrolysis cell, wherein the ammonia electrolysis cell comprises: two electrodes, wherein each electrode comprises a bifimctional catalyst having ammonia oxidation reaction activity and hydrogen evolution reaction activity'; and an electrolyte contacting the electrodes, wherein the electrolyte comprises ammonia, tire method comprising the steps of: a) applying a potential to the electrodes for a predetermined time whereinthe first electrode functions as an anode and wherein the second electrode functions as a cathode, b) alternating the potential for a predetermined time such that the first electrode functions as a cathode and the second electrode functions as an anode, and c) alternating the potential to repeat steps a) and b.

8. The method of claim 7, wherein the predetermined time is sufficient for the hydrogen evolution reaction at the cathode to substantially reverse the catalyst poisoning effected by the ammonia oxidation reaction at the anode.

9. The method of claim 7 or claim 8, wherein the predetermined time is from about 0.5 seconds to about 10 minutes.

10. The method of any one of claims 7 to 9, wherein the predetermined time is from about 20 seconds minutes to about 2 minutes.

11. The method of any one of claims 7 to 10, wherein the cell comprises an anion exchange membrane disposed between the electrodes.

12. The method of any one of claims 7 to 11, wherein the bifiinctional catalyst is one or more of a nickel-based catalyst, a copper-based catalyst, a cobalt-based catalyst, an iron-based catalyst, an iridium-based catalyst, a titanium-based catalyst, a ruthenium-based catalyst and a platinum-based catalyst.

13. The method of any one of claims 7 to 12, wherein the bifunctional catalyst is a platinumbased catalyst.

14. The method of any one of claims 7 to 13, wherein the electrolyte is a liquid electrolyte.

15. The method of any one of claims 7 to 14, wherein the electrolyte is alkaline.

16. The method of any one of claims 7 to 15, wherein the electrolyte is an aqueous electrolyte.

17. The method of any one of claims 7 to 16, wherein the cell potential alternates between the range of about -0.5V to about -1 OV to the range of about +0.5V to about +1 ,0V.

18. The method of any one of claims 7 to 17, wherein the cell potential alternates between about -0.8V to about +0.8V.

19. Use of the ammonia electrolysis cell of any one of claims 1 to 6 or the method of any one of claims 7 to 18 to produce hydrogen from ammonia.

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