Method and system of operating a polymer electrolyte membrane electrolyser with oxygen recirculation

Abstract

The present invention relates to a method and system of producing hydrogen gas using a water electrolyser cell that comprises both a catalytic reactor and polymer electrolyte membrane (PEM) electrolyser stack of cells. Oxygen generated by the PEM electrolyser stack may be recycled. The system and method include controlling the concentration of hydrogen in the anode compartment of the PEM electrolyser stack by varying the amount of recycled oxygen.

Description

[0001] Method and System of Operating a Polymer Electrolyte Membrane Electrolyser with Oxygen Recirculation

[0002] PRIORITY

[0003] This application claims priority under 35 U. S. C. § 119(e) to U. S. Provisional Application No. 63 / 737,837, filed December 23, 2024, the contents of which are incorporated herein by reference in their entirety.

[0004] TECHNICAL FIELD

[0005] The invention relates to a system and method of producing hydrogen in a polymer electrolyte membrane (PEM) water electrolyser system. In some embodiments consistent with the disclosed invention, oxygen generated by a PEM electrolyser stack is recycled.

[0006] Disclosed systems and methods may control the concentration of hydrogen in the anode compartment of the PEM electrolyser stack by varying the amount of recycled oxygen.

[0007] Purified oxygen may be produced as a second product.

[0008] BACKGROUND

[0009] A water electrolysis cell is an electrochemical device that dissociates water to produce hydrogen and oxygen gases. Electrolysis cells typically include a cathode, an anode and an electrolyte. The electrolyte is positioned between the cathode and the anode and transports ions between the electrodes while preventing the transport of electrons. One electrolyte alternative is a polymer electrolyte membrane (PEM), also called a proton exchange membrane. During operation of an electrolyser cell, water is oxidized to oxygen gas, protons and electrons at the anode. The protons migrate from the anode to the cathode due to the applied electric field across the polymer electrolyte membrane. At the cathode, the protons combine with electrons transferred through an external circuit to produce hydrogen gas. The pressure at the cathode is, in most cases, above ambient atmospheric pressure (~0 barg) and, typically, above 15 barg or even 30 barg. The anode pressure, on the other hand, is usuallyslightly above atmospheric pressure and, typically, significantly lower than the cathode pressure. FIG. 2 shows a schematic diagram of a conventional membrane electrode assembly (MEA) of a PEM water electrolyser cell, as well as the main transport phenomena and reactions occurring.

[0010] Conventional PEM electrolysis cells consume water at the anode side, and this water must continuously be supplied to the anode. The water can either be supplied directly to the anode (as shown in FIG. 2) or be supplied to the cathode and transported through the polymer electrolyte membrane to the anode. The rate of consumption of water, and thus, the rate of hydrogen and oxygen generation, is governed by Faraday’s law, in that an increase of the current passed through the cell will result in a corresponding increase in the generation of gas and consumption of water.

[0011] In addition to water transport, oxygen (O2 (diff)) and hydrogen (H2 (diff) are transported through the membrane through a diffusion / convection mechanism due to the partial pressure gradient of the gases across the membrane. This gas flux across the membrane — as well as the consequent mixing of hydrogen in oxygen on the anode and oxygen in hydrogen on the cathode — is a design and operational constraint of conventional PEM water electrolysers. Even small amounts of hydrogen in oxygen in the anode can form flammable and / or explosive gas mixtures that can ignite or explode and damage electrolyser equipment or cause injuries or death to persons in close proximity.

[0012] The hydrogen transported through the membrane may exceed safe levels if the oxygen production on the anode is too low (e.g., low current densities at startup or standby) or the transport of hydrogen is too high (e.g., due to a thin membrane and / or high permeability). An unwanted hydrogen leak from the cathode to the anode may also occur because of a leak in a gasket or seal between the anode and cathode compartments or through one or moremembrane defects, such as a pinhole or tear. This hydrogen leak can be small and gradually increase over time or, alternatively, can be an abrupt immediate large leak.

[0013] Conventional PEM water electrolysers attempt to limit hydrogen crossover by using a thick membrane (usually above 125 pm), preferably made of perfluorosulfonic acid (PSFA) polymers, such as Nafion® or Aquvion®, to reduce the hydrogen diffusion through the membrane. A hydrogen / oxygen recombination catalyst — such as platinum or palladium — may also be introduced into the membrane to provide reaction sites for local recombination of oxygen and hydrogen to water. This, in turn, prevents diffusing gases from reaching the other electrode compartment. To have the necessary amount of recombination catalyst and time for the recombination reaction to take place, it is necessary to have a significant membrane thickness. Thus, conventional water electrolysers often use polymer electrolyte membranes with thicknesses of 125 microns (Nafion® 115 or equivalent) or higher.

[0014] The use of such thick membranes introduces a significant ohmic resistance and consequently a lower efficiency of the electrolyser, especially at current densities above 1 Acm-2.

[0015] Conventional water electrolysers operate with a stack efficiency around 65-70% (higher heating value HHV) which results in a demand of about 55 kWh of electricity for 1 kg H2. Of the 55 kWh, about 50 kWh is used by the electrolysis process and 5 kWh by the balance of plant (circulation and feed water pump, heat exchanger, ion exchanger, gas / water separators, valves and sensors). In most water electrolyser systems, the cost of electricity can amount to up to 80% of the cost of the produced hydrogen and an increase in the efficiency of the water electrolyser stack will improve both the overall primary electrical energy consumption and the total cost of hydrogen.

[0016] Current PEM electrolysers are mostly limited in efficiency by:

[0017] 1. Tire overpotential on the anode2. The ohmic resistance in the polymer membrane.

[0018] A novel method to mitigate the potential for flammable gas mixtures due to hydrogen crossover through the membrane was presented in U. S. Patent No. 11,408,081 (“the ’081 patent”), which is hereby incorporated by reference in its entirety. In the ’081 patent, air was added to the anode to dilute crossover hydrogen to below 1 vol% enabling the use of a thinner membrane. This resulted in a higher efficiency electrolyser. Such an approach, however, introduces nitrogen and carbon dioxide into any produced oxygen, making the utilization of this produced oxygen less desirable. Adding air can also introduce trace impurities of nitrogen and carbon dioxide in the produced hydrogen if these elements diffuse across the membrane from the anode to the cathode. This can, in some cases, lead to additional purification needs. The ‘081 patent also discloses addition of humidified air, which may result in increased water consumption.

[0019] Recent studies have highlighted that hydrogen may act as an indirect climate gas, contributing to global warming by prolonging the lifetime of atmospheric methane. Limiting fugitive hydrogen emissions is, therefore, becoming increasingly important, and solutions to remove trace amounts of hydrogen in the anode gas stream provide a direct environmental benefit by reducing unintended hydrogen release to the atmosphere.

[0020] It is an object of the present invention to provide an improved method and system for the production of hydrogen and oxygen from a PEM electrolyser. It is further an aim to reduce the need for anode gas humidification via higher pressure at the anode and, additionally, to reduce impurities in hydrogen products.

[0021] SHORT SUMMARY

[0022] In a first aspect of the disclosure, a method for operating a polymer electrolyte membrane (PEM) water electrolyser cell may comprise applying a direct electric current to aPEM electrolyser stack of the cell. The method may additionally comprise oxidizing water molecules at an anode catalyst layer of the PEM electrolyser stack into protons, oxygen and electrons. The method may further comprise supplying liquid water to a cathode compartment of the PEM electrolyser stack. The method may additionally comprise allowing the protons to migrate through the PEM electrolyser stack into the cathode compartment of the PEM electrolyser stack. The method further comprise reducing the protons at a cathode catalyst layer of the PEM electrolyser stack to produce hydrogen. And the method may additionally comprise recycling a portion of the oxygen by recirculating the portion to the anode compartment of the PEM electrolyser stack.

[0023] In a second aspect of the disclosure, an exemplary water electrolyser cell system may comprise a cathode compartment of a polymer electrolyte membrane (PEM) electrolyser stack comprising a cathode catalyst layer. The water electrolyser cell system may additionally comprise an anode compartment of the PEM electrolyser stack comprising an anode catalyst layer. The anode compartment may be configured to generate oxygen gas. The water electrolyser cell system may further comprise a catalytic reactor configured to remove hydrogen from the oxygen gas generated at the anode compartment. And the water electrolyser cell system may additionally comprise a controllable valve configured to recirculate a portion of the oxygen gas released by the system to the anode compartment of the PEM electrolyser stack.

[0024] In a third aspect of the disclosure, a method for controlling hydrogen levels in an oxygen stream of a polymer electrolyte membrane (PEM) electrolyser stack in a water electrolyser cell may comprise supplying liquid water to a cathode compartment of the PEM electrolyser stack. The method may additionally comprise applying a direct electric current to the PEM electrolyser stack of the cell. The system may be configured to recycle a portion ofthe oxygen stream formed by the PEM electrolyser stack to an anode compartment of the PEM electrolyser stack.

[0025] The concentration of hydrogen in the oxygen-containing anode gas can be monitored, for example, with a sensor or monitoring device before and / or after the catalytic reactor and before the controllable recirculation valve. The temperature of the catalytic reactor and / or the temperature of the anode gas before and / or after the catalytic reactor may also be monitored. Tire temperature change of the anode gas can be used to determine the amount of hydrogen removed by the catalytic reactor, which is directly related to the amount of hydrogen in the anode gas (calculated from the reaction enthalpy, specific heat capacity of the gas, and the gas flow rate). The temperature change can be used as an additional safety or control measure to ensure that the concentration of hydrogen is maintained below the Lower Flammability Limit in the system.

[0026] FIGURES FIG. 1 is a schematic diagram of an electrolyser cell constructed to be operated with supply of oxygen on the anode and liquid water on the cathode.

[0027] FIG. 2 is a schematic diagram of a membrane electrode assembly, MEA, according to the state of the art.

[0028] FIG. 3 is a schematic diagram of a PEM water electrolyser system according to the invention.

[0029] FIG. 4 is a schematic representation of the experimental setup for Example 1.

[0030] FIG. 5 depicts hydrogen concentration in an anode outlet, anode flow, and applied current when a catalytic reactor was bypassed during operation of the experiments performed in Example 1.FIG. 6 depicts hydrogen concentration in an anode outlet, anode flow, and applied current when anode flow streamed through a catalytic reactor during operation of the experiments performed in Example 1.

[0031] DETAILED DESCRIPTION

[0032] The objects and features of embodiments consistent with the invention can be better understood with reference to the drawings described below.

[0033] FIG. 1 is a schematic diagram of an electrolyser cell constructed to be operated with supply of oxygen to the anode and liquid water to the cathode.

[0034] The electrolyser cell comprises an anode compartment having an anode bi-polar plate (la), an anode porous transport layer (2a), and an anode catalyst layer (3) coated on top of a thin polymer electrolyte membrane (4). The cathode compartment comprises a cathode catalyst layer (5) coated on top of the polymer electrolyte membrane (4), a cathode porous transport layer (2b) and a cathode metallic bi-polar plate (1b). The anode bi-polar plate (la) is made of a metallic material with high corrosion resistance and high electrical conductivity. In addition, the anode bi-polar plate (la) is designed with a flow field pattern (6) and corresponding inlet (7) and outlet (8) flow distribution manifolds for optimal gas and water distribution along the active area of the electrolyser. Both the anode bi-polar plate (la) and the anode porous transport layer (2a) are optimised to minimise electrical contact resistances in the electrolyser. The anode porous transport layer (2a) is made of a highly corrosion resistant and highly electronic conductive porous material that enables the transport of humidified or non-humidified air or oxygen from or to the anode catalyst layer (3). The anode catalysts layer (3) comprises a catalyst that is highly efficient for the oxygen evolution reaction and a proton conductive polymer that allows for the migration of protons out and water into the anode catalyst layer (3). The cathode metallic bi-polar plate (1b) is also madeof a metallic material with high corrosion resistance and high electrical conductivity. The cathode bi-polar plate (1b) is designed with a flow field pattern (9), but not necessarily the same as the flow field pattern (6) of the anode bi-polar plate (la), a corresponding inlet (10) and outlet (11) flow distribution manifolds for optimal water and gas distribution along the active area of the electrolyser device, but not necessarily the same as (7) and (8) on the anode side. The cathode porous transport layer (2b) is made of a highly corrosion resistant and highly electronic conductive porous material that enables the transport of water and hydrogen in and out of the cathode catalyst layer (5). The cathode catalyst layer (5) comprises a catalyst that is highly efficient for the hydrogen evolution reaction and a proton conductive polymer that allows for the migration of protons in and water out of the cathode catalyst layer (5).

[0035] FIG. 3 shows a schematic diagram of an embodiment of a PEM water electrolyser system according to the present disclosure. An electrolyser stack (14) generates oxygen gas and hydrogen gas from liquid deionized water, each of which travel to different components of the electrolyser system. The electrolyser stack (14) is configured to supply liquid water to the cathode compartment of each electrolyser cell. This combination secures the necessary water needed for the oxygen evolution reaction on the anode and to ensure a high water content in the membrane to retain a high proton conductivity. Ion exchanged water is supplied from a water purification device (19) to the cathode compartment of the electrolyser stack (14). The water purification device (19) is an inline ion exchange resin column that removes any ionic impurities in water that circulates through the system. Hydrogen produced exits the cathode compartment of the electrolyser stack (14) together with deionized water, and travels to a hydrogen / water separator (15). Hydrogen and water are separated in the hydrogen / water separator (15). The hydrogen flows through a deoxidizer / dryer (16). The separated water is recycled to the water purification unit (19) and into the PEM water electrolyser stack (14). A circulation pump (17) and a heat exchanger (18) may be included inthe circulation line. A catalytic reactor (12) is configured to remove traces of hydrogen in the oxygen generated at the electrolyser stack (14) before it is released from the system. In some embodiments, a condenser may be added on the media transfer line between the electrolyser stack (14) and the catalytic reactor (12) to remove excess gas humidity and / or add one or more heat exchangers to heat the fluid flow, thereby ensuring a proper reaction temperature in the catalytic reactor (12). The hydrogen-contaminated oxygen typically results from crossover of hydrogen molecules from the cathode of the electrolyser stack (14) to the anode, where oxygen molecules are produced. A controllable valve (20) facilitates recirculation of a portion of oxygen gas to the anode compartment of the electrolyser stack (14). The controllable valve (20) may be positioned such that it facilitates release of a portion of oxygen gas from the system. The remaining oxygen is circulated to the anode compartment of electrolyser stack (14) through a blower (13). The blower (13) provides the necessary energy and pressure to enable flow of oxygen through the electrolyser stack (14) and catalytic reactor (12). The blower (13) is configured to blow ambient air through the system both at startup and shutdown when, for example, there is insufficient oxygen generated by the system. In certain embodiments, in a startup phase of the system, it may be necessary to introduce ambient air through the system to ensure there is a sufficient volume of gas to properly dilute any crossover hydrogen. The controllable valve (20) may be controlled by such factors as pressure, temperature, the amount of oxygen needed by the system, and the amount of purification needed for oxygen generated by the electrolyser stack (14).

[0036] FIG. 3 improves upon electrolyser systems defined by the ’081 patent by eliminating the need for anode gas humidification and the use of ambient air during normal operation, thus reducing or eliminating the introduction of air impurities to the system and thereby enabling the production of high purity oxygen. Humidification of air introduced to the anode increases water consumption of the overall system as the water used for humidification isvented to the atmosphere. The use of ambient air as a dilution gas on the anode may introduce atmospheric nitrogen and carbon dioxide into generated oxygen, making the oxygen less desirable and more difficult to use as a downstream product from the electrolyser. Small amounts of impurities (e.g, nitrogen (N2), carbon dioxide (CO2) can also diffuse from the anode to the cathode and contaminate any generated hydrogen. The system illustrated in FIG.

[0037] 3 employs a catalytic reactor to remove hydrogen gas from the oxygen gas generated at the anode. A portion of oxygen gas is recirculated by a controllable valve into the anode of the electrolyser stack to lower levels of hydrogen gas, while the remaining oxygen gas is vented or delivered to downstream processes. In some embodiments, a blower provides the necessary energy and pressure to enable flow of oxygen through the system, In particular, through the electrolyser stack and catalytic reactor. The embodiment of the invention presented in FIG. 3 provides for a purer oxygen gas product and reduces hydrogen gas product impurities.

[0038] Methods consistent with the invention may include operating a polymer electrolyte membrane (PEM) water electrolyser cell like those described in this disclosure by supplying liquid water to a cathode compartment of a PEM electrolyser stack (14) and applying a direct electric current to the PEM electrolyser stack (14), wherein the PEM electrolyser stack (14) is configured to recycle a portion of oxygen to the anode compartment of the PEM electrolyser stack (14). Oxygen-containing gases from the PEM electrolyser stack (14) may be introduced into the catalytic reactor (12) to remove crossover hydrogen, resulting in a purified oxygen gas stream. This oxygen may then be recycled via controllable valve (20) or removed from the system. In some embodiments, the PEM may have a thickness below 50 pm. The method may further comprise operating the anode compartment of the PEM electrolyser stack (14) at a pressure that is greater than an ambient pressure. In some methods, the cathode compartmentof the PEM electrolyser stack (14) may be operated at a pressure between 0.5 bar to 35 bar greater than the pressure of the anode compartment of the PEM electrolyser stack (14).

[0039] Controllable valve (20) may be adjusted (by an operator, a control system, or other means) to control an amount of oxygen that is recycled. The amount of recycled oxygen may be selected to reduce or control the concentration of hydrogen present in the anode compartment of the PEM electrolyser stack (14). The concentration of hydrogen present in the gas stream entering the catalytic reactor (12) may also be controlled by adjusting the amount of oxygen recycled. This may, in turn, enhance the efficiency of the catalytic reactor (12). The controllable valve (20) may also be adjusted to control the pressure in the anode compartment of the PEM electrolyser stack (14). Blower (13), or (if present) compressor or pump, may be selectively activated and deactivated. These components may increase the pressure of the recycled oxygen stream.

[0040] A non-recycled portion of the oxygen exiting the catalytic reactor (12) may be released as a waste stream or commercialized as a product. By reducing the concentration of cross-over hydrogen in this oxygen stream, systems and methods consistent with the invention enhance safety and increase commercial value of this oxygen stream.

[0041] Certain embodiments may comprise a method and system for producing hydrogen, wherein a polymer electrolyte membrane (PEM) water electrolyser cell is provided. The method may comprise applying a direct electric current to the water electrolyser cell, allowing water molecules from a cathode compartment to diffuse through a PEM into an anode compartment, oxidizing water molecules at an anode catalyst layer into protons, oxygen and electrons, allowing the protons to migrate through a PEM into the cathode compartment, reducing the protons at a cathode catalyst layer to produce hydrogen, supplying water to the cathode compartment, and supplying oxygen or humidified oxygen to the anode compartment.In some embodiments, the oxygen supplied to the anode compartment may be humidified and may have a relative humidity between 20 and 100%. The humidified oxygen may also be, supersaturated. The oxygen may be supplied to the anode by use of a compressor, blower, or pump and distributed through flow distribution manifolds and via flow patterns on the anode bi-polar plate for optimal distribution along the active area of the anode.

[0042] During operation, the pressure on the cathode side of the electrolyser cell may be controlled to be higher than the pressure on the anode side. In an embodiment, the pressure on the cathode side may be between 0.5 bar to 35 bar higher than the pressure in the anode compartment. In some embodiments, the anode compartment may be operated at a pressure greater than ambient pressure. By way of example, the pressure on the anode compartment may be controlled to be between 0.2 bar to 20 bar above ambient pressure.

[0043] In certain embodiments, the PEM water electrolyser cell may comprise an anode compartment comprising an anode bi-polar plate, an anode porous transport layer, and an anode catalyst layer, a cathode compartment comprising a cathode bi-polar plate, a cathode porous transport layer, and a cathode catalyst layer. The anode catalyst layer and the cathode catalyst layer may be coated on either side of the PEM. The cathode compartment may be configured to be supplied with ion exchanged water through a first set of inlet and outlet flow distribution manifolds, and the cathode bi-polar plate is designed with a first flow field pattern. The anode compartment may be configured to be supplied with oxygen or humidified oxygen through a second set of inlet and outlet flow distribution manifolds, and the anode bipolar plate is designed with a second flow field pattern.

[0044] In some embodiments, the anode catalyst layer and the cathode catalyst layer may comprise catalysts in powder form.In some embodiments, the temperature of the supplied oxygen and the relative humidity values may typically be at nominal operating temperature of the electrolyser of 50 to 90° C.

[0045] In some embodiments, the PEM may have a thickness below 50 microns, preferably in the range of 5 to 49 microns, and most preferably from 10 to 35 microns.

[0046] In some embodiments, the PEM water electrolyser stack may comprise a plurality of PEM water electrolyser cells connected in series.

[0047] In certain embodiments, the PEM water electrolyser system may comprise the PEM electrolyser stack together with a water and oxygen management system, a hydrogen gas management system, a water input system, a mounting and packaging cabinetry subsystem, a ventilation system, power electronics and power supply, system controls and instrumentation, and a humidified air supply and humidification system.

[0048] hi certain embodiments, the PEM electrolyser system may include a stack of cells and a catalytic reactor. The system may comprise an anode gas recirculation loop where generated oxygen and crossover hydrogen are introduced into a catalytic reactor to remove hydrogen. After purification by the catalytic reactor, oxygen may be circulated by a blower or compressor into the anode of the electrolyser stack to maintain a low hydrogen concentration in the anode. Remaining oxygen may be delivered to downstream processes or vented. This aspect provides a purified oxygen product, reduces the need for anode gas humidification, provides a higher anode pressure operation, and reduces impurities in hydrogen products. In other aspects, other gases may be employed to purify oxygen and ventilate the system, including nitrogen and / or carbon dioxide. Alternatively, hydrogen may be emitted from the PEM system to purify oxygen. Such an approach has real-world applications in aviation and steel production.

[0049] Example 1: Measuring hydrogen concentration in an exemplary embodiment of the disclosed systemA catalyst coated membrane (CCM ) with a membrane thickness of 30 micrometers was mounted in a 20 cm2test cell using a titanium porous transport layer on the anode and a carbon gas diffusion layer on the cathode. The test cell was connected to a PEM electrolyser test station (Greenlight Technologies).

[0050] FIG. 4 depicts the experiments performed that involved anode oxygen circulation. A diaphragm pump (21) and catalytic reactor (22) were included in the anode fluid loop. This setup enabled circulation of gas through the catalytic reactor (22) and the cell. The net flow out of the anode fluid loop was controlled by a pressure control valve (23) connected downstream of the catalytic reactor (22). A sensor downstream of the pressure control valve (23) measured the amount of hydrogen (24) in oxygen gas from the anode (25).

[0051] The cell was operated at 60°C with a cathode water flow rate of 10 L / min, a cathode outlet pressure of 4barg. and an anode outlet pressure of 0.5 barg. At the beginning of the experiment, the anode gas bypassed the catalytic reactor (22), but still recirculated through the PEM cell via the diaphragm pump (21). During this period, the catalytic reactor (22) was heated to a temperature of 250°C to ensure a rapid startup and avoid any potential condensation of water in the catalytic reactor (22).

[0052] Over a period of 1.5 hours, the current was increased to 65 A, and the concentration of hydrogen in the anode gas was continuously monitored. The hydrogen concentration gradually increased in the anode gas loop until a stable concentration of 1.3% was reached. After this, the current was held constant for 1 hour with no changes in hydrogen concentration observed. FIG. 5 depicts hydrogen concentration, anode outlet flow, and current during this initial run, where the catalytic reactor (22) was bypassed.

[0053] After the current was held constant for 1 hour, the anode gas stream was introduced through the catalytic reactor (22), and an immediate drop in hydrogen concentration to 0% was registered. The hydrogen concentration remained at this level until the catalytic reactor(22) was again bypassed after 20 minutes of operation, when the concentration again increased to 1.3%. These observations are depicted in FIG. 6.

[0054] While the disclosure has been described in connection with exemplary embodiments, it will be understood that it is capable of further modifications and this application covers any variations, uses, or adaptations following, in general, the principles of the disclosure and including such departures as come within known or customary practice within the art to which the disclosure pertains.

Claims

Claims1. A method of operating a polymer electrolyte membrane (PEM) water electrolyser cell, the method comprising:applying a direct electric current to a PEM electrolyser stack of the cell; oxidizing water molecules at an anode catalyst layer of the PEM electrolyser stack into protons, oxygen and electrons;supplying liquid water to a cathode compartment of the PEM electrolyser stack; allowing the protons to migrate through the PEM electrolyser stack into the cathode compartment of the PEM electrolyser stack;reducing the protons at a cathode catalyst layer of the PEM electrolyser stack to produce hydrogen; andrecycling a portion of the oxygen by recirculating the portion to the anode compartment of the PEM electrolyser stack.

2. The method of claim 1, wherein the recycled oxygen is introduced into a catalytic reactor to remove crossover hydrogen.

3. The method of claims 1 or 2, wherein the PEM has a thickness below 50 pm.

4. The method of any one of claims 1-3, wherein the anode compartment of the PEM electrolyser stack is operated at a pressure that is greater than an ambient pressure.

5. The method of any one of claims 1 -4, wherein the cathode compartment of the PEM electrolyser stack is operated at a pressure between 0.5 bar to 35 bar greater than the pressure of the anode compartment of the PEM electrolyser stack.

6. The method of any one of claims 1-5, wherein the amount of recycled oxygen is controlled by a controllable valve.

7. The method of any one of claims 2-6. wherein the recycled oxygen passes through a blower to ensure proper oxygen flow through the PEM electrolyser stack and the catalytic reactor.

8. The method of any one of claims 1-7, wherein the cathode compartment is configured to generate hydrogen gas.

9. A water electrolyser cell system comprising:a cathode compartment of a polymer electrolyte membrane (PEM) electrolyser stack comprising a cathode catalyst layer;an anode compartment of the PEM electrolyser stack comprising an anode catalyst layer, the anode compartment configured to generate oxygen gas;a catalytic reactor configured to remove hydrogen from the oxygen gas generated at the anode compartment; anda controllable valve configured to recirculate a portion of the oxygen gas released by the system to the anode compartment of the PEM electrolyser stack.

10. The system of claim 9, wherein the PEM has a thickness below 50 pm.

11. The system of claims 9 or 10, wherein the anode compartment of the PEM electrolyser stack is configured to operate at a pressure greater than an ambient pressure.

12. The system of any one of claims 9-11, wherein the cathode compartment of the PEM electrolyser stack is configured to operate at a pressure between 0.5 bar to 35 bar greater than the pressure of the anode compartment of the PEM electrolyser stack.

13. The system of any one of claims 9-12, wherein the system further comprises a controllable valve configured to control the amount of recirculated oxygen.

14. The system of any one of claims 9-13, wherein the portion of oxygen is circulated more than once through the catalytic reactor for further purification before being recirculated to the anode compartment of the PEM electrolyser stack.

15. The system of any one of claims 9-14, wherein the system further comprises a blower configured to ensure proper oxygen flow through the PEM electrolyser stack and the catalytic reactor.

16. The system of any one of claims 9-15, wherein the cathode compartment is configured to generate hydrogen gas.

17. A method of controlling hydrogen levels in an oxygen stream of a polymer electrolyte membrane (PEM) electrolyser stack in a water electrolyser cell, the method comprising:supplying liquid water to a cathode compartment of the PEM electrolyser stack; andapplying a direct electric current to the PEM electrolyser stack of the cell, wherein the PEM electrolyser stack is configured to recycle a portion of the oxygen stream formed by the PEM electrolyser stack to an anode compartment of the PEM electrolyser stack.

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