Method of electrolysis and system for generating hydrogen

Abstract

The invention is directed to methods of electrolysis and cells used for the same. The method comprising generating and delivering a humidified gas stream or liquid water to an electrolysis cell comprising an anode side, a cathode side and an ion permeable membrane located between them wherein the anode side has a first catalytic layer and the cathode side has a second electrolytic layer, contacting the humidified gas stream or liquid water with the first catalytic layer and contacting a portion of the ion- permeable membrane on the cathode side with liquid water, applying a voltage such that oxygen gas is generated at the anode and hydrogen gas is generated at the cathode. The invention is also related to an electrolytic cell for performing the methods and a kit that allows for retrofitting existing cells to perform the methods.

Description

[0001] METHOD OF ELECTROLYSIS AND SYSTEM FOR GENERATING HYDROGEN

[0002] TECHNICAL FIELD

[0003] This disclosure relates to the field of hydrogen generation through water electrolysis.

[0004] BACKGROUND ART

[0005] Hydrogen is considered an attractive alternative to replace fossil fuels as an inexhaustible energy carrier. Various techniques have been developed for producing hydrogen. However, to date, the commonly used route for hydrogen production involves the use of steam reforming / cracking of fossil fuels. This is not considered a ‘green’ technique for hydrogen production. Water electrolysis is considered promising in terms of its sustainability. Abundant water (available for example from oceans) can be split using green electricity generated from renewable sources such as wind, solar, hydropower etc to provide a renewable supply of hydrogen. However, water electrolysis presents many challenges. For example, bubble formation in water during conventional electrolysis can affect mass transport thereby affecting reaction kinetics. Similarly, water electrolysis requires the use of catalysts, which can be poisoned by contaminants in the water. Vapor electrolysis is another technique that is being developed as an alternative method to address some of these issues. In this method, water vapor (instead of liquid water) is fed to an electrolysis cell / stack and undergoes decomposition to produce hydrogen gas and oxygen gas. Because water is in vapor form, issues associated with mass transport to and from electrodes due to bubble formation (i.e. hydrogen / oxygen bubbles in liquid water) are avoided. Similarly, the risk of catalysts being poisoned due to contact with contaminants in water can be avoided. Vapor electrolysis has its own share of challenges as well, however. For example, vapor electrolysis can require high overpotentials to drive the water splitting process. These overpotentials arise due to numerous factors such as slower kinetics of the Oxygen Evolution Reaction (OER), water transport through the membranes etc. Furthermore, the effect of overpotentials can be exacerbated at high current densities. Thus, it is desirable to provide an alternative technique of electrolysis or an electrolysis cell that can alleviate some of the challenges of conventional electrolysis techniques or at least provides a useful alternative to these.

[0006] It is to be understood that, if any prior art is referred to herein, such reference does not constitute an admission that the prior art forms a part of the common general knowledge in the art, in Australia or any other country.

[0007] SUMMARY

[0008] Disclosed in one aspect is an electrolysis method comprising: generating a humidified gas stream; delivering the humidified gas stream into an electrolysis cell, the cell comprising an anode side, a cathode side and an ion permeable membrane located between the anode side and the cathode side for conducting hydrogen ions generated at the anode side to the cathode side, wherein the anode side has a first catalytic layer that facilitates formation of oxygen gas and hydrogen ions, the cathode side has a second catalytic layer that facilitates generation of hydrogen gas from the hydrogen ions, and the ion permeable membrane contacts both the first and second catalytic layers; contacting the humidified gas stream with the first catalytic layer and contacting a portion of the ion-permeable membrane adjacent the cathode side with liquid water; and applying a voltage across the anode and the cathode such that oxygen gas is generated at the anode side and hydrogen gas in generated at the cathode side. The method allows the application of the principles of vapor electrolysis while attempting to alleviate at least some of the challenges associated with vapor electrolysis.

[0009] Disclosed in one aspect is an electrolysis method comprising: generating a humidified gas stream; delivering the humidified gas stream and liquid water into an electrolysis cell, the cell comprising an anode having a first catalytic layer that facilitates formation of oxygen gas and hydrogen ions from the humidified gas stream; a cathode having a second catalytic layer that facilitates generation of hydrogen gas from the hydrogen ions; and an ion-permeable membrane sandwiched between the anode and the cathode, and being in contact with both the first and second catalytic layers for conducting the hydrogen ions generated at the anode side to the cathode side, wherein the humidified gas stream contacts the first catalytic layer, and the liquid water contacts the second catalytic layer; and applying a voltage across the anode and the cathode to generate oxygen gas at the anode and generate hydrogen gas at the cathode.

[0010] In a conventional, liquid water-based electrolysis cell, water is supplied to both the anode and cathode sides and contacts the catalyst layers on both sides. In some cases, the catalyst on the anode side that comes into contact with liquid water may undergo undesirable reactions leading to the dissolution and loss of catalyst particles from the catalytic layer, resulting in degraded performance and ultimately failure of the cell. Consequently, the lifetime of the catalyst can thus be significantly reduced.

[0011] Providing water in a vapor form to the anode side alleviates this issue since water molecules in the vapor form do not result in the dissolution of catalysts (if at all there is dissolution, it is kept to minimal levels that do not affect the catalyst function) and / or poisoning of catalysts (due to contaminants present in liquid water) to the same extent as liquid water. In other words, the method can address dissolution / poisoning of the catalyst material at the anode side, at least because of the use of water vapor to contact the catalyst (as opposed to liquid water, which may accelerate degradation of the catalyst). This in turn allows for the use of catalytic materials that may be considered less than ideal for liquid-water electrolysis (e.g. Ruthenium oxide) as catalysts. In other words, this method may be used to prolong the life of catalysts that are prone to dissolution in liquid water. The energy efficiency of the cell may be increased through the use of ruthenium- based catalysts. Ruthenium is more catalytically active than some other catalysts, which may allow for a given hydrogen gas production rate to be achieved at lower voltages.

[0012] Providing water in the vapor form to the anode side also removes the need for water treatment (which would be required for supplying liquid water in the case of a conventional electrolysis cell). This may result in associated energy savings for the system as a whole.

[0013] Similarly, the use of water vapor on one side of the electrolysis cell, while maintaining liquid water on the other side, may assist in maintaining the membrane in a hydrated condition and thus reducing efficiency losses (e.g. ohmic losses) arising from a lack of hydration of the membrane. The ohmic resistance of the electrolyzer, which includes the ionic resistance of the membrane, the electronic resistance of the catalyst layers, as well as the contact resistances of the components, determines the efficiency of high current electrolysis of water. Amongst these, the ionic resistance is considered to be a main contributor to the ohmic resistance of the electrolyzer. A hydrated membrane can ensure that the ionic resistance is maintained at low levels thus facilitating quick and efficient transfer of hydrogen ions across the membrane and in turn facilitating the hydrogen gas production process at the cathode side. This hydration of the membrane may also assist in minimizing temperature inhomogeneities across the membrane.

[0014] Use of water vapor in the electrolysis cell removes the need for the vaporization of liquid water to occur within the cell itself, with the vaporization process able to be carried out separately, away from the electrolysis cell. Because water is converted from a liquid phase to a gaseous phase outside the cell, only a single -phase (i.e. gas phase) reaction is taking place inside the cell at the anode side. This minimizes the simultaneous presence of both liquid water and water vapor inside the cell at the anode side. This has the benefit of improved mass transport behaviour as well as enhanced utilization of catalytic sites available at the anode side. Therefore, for a given amount of energy input to the system, the hydrogen gas production rate is improved.

[0015] In some embodiments, generating the humidified gas stream can comprise heating water obtained from a water source to generate water vapour and mixing said water vapour with a carrier gas.

[0016] In some embodiments, generating the humidified gas stream can comprise passing the carrier gas through a microceramic diffuser prior to mixing with water that is obtained from the water source and is being heated. This allows for creation of an extended gas-liquid interface that enhances vaporization (bubbling-enhanced vaporization) of the water. In one embodiment, the microceramic diffuser may comprise a microdiffusion stone.

[0017] In some embodiments, the water source can be an impure water source. The ability to use an impure water source can minimize or eliminate costs associated with treating the water prior to use.

[0018] In some embodiments, compressed air can be used as the carrier gas.

[0019] In some embodiments, generating the humidified gas stream comprises utilizing, at least partly, waste heat generated from operation of the electrolysis cell. It is thus possible to use a low-grade waste heat source to generate the humidified gas stream.

[0020] Disclosed in another aspect is an electrolysis method comprising: delivering liquid water into an electrolysis cell, the cell comprising an anode side having a first catalytic layer that facilitates formation of oxygen gas and hydrogen ions from the liquid water; a cathode side having a second catalytic layer that facilitates generation of hydrogen gas from the hydrogen ions; and an ion- permeable membrane sandwiched between the anode side and the cathode side, and being in contact with both the first and second catalytic layers for conducting the hydrogen ions generated at the anode side to the cathode side, wherein the cell further comprises an anode gasket assembly configured to seal the first catalytic layer on the anode side; a cathode gasket assembly configured to seal the second catalytic layer on the cathode side, and the liquid water stream contacts both the first catalytic layer and the second catalytic layer; and applying a voltage across the anode side and the cathode side to generate oxygen gas at the anode side and generate hydrogen gas at the cathode side.

[0021] Disclosed in another aspect is an electrolysis cell for performing the previously described electrolysis methods.

[0022] In this respect there is provided an electrolysis cell for performing water electrolysis, the electrolysis cell comprises an anode side having a first catalytic layer that facilitates formation of oxygen gas and hydrogen ions from the humidified gas stream; a cathode side having a second catalytic layer that facilitates generation of hydrogen gas from the hydrogen ions; and an ion-permeable membrane sandwiched between the anode side and the cathode side, and being in contact with both the first and second catalytic layers for conducting the hydrogen ions generated at the anode side to the cathode side, wherein the cell further comprises an anode gasket assembly configured to seal the first catalytic layer on the anode side, and a cathode gasket assembly configured to seal the second catalytic layer on the cathode side.

[0023] The electrolysis cell may be configured to receive a liquid water or humidified gas stream to the anode side and a liquid water stream to the cathode side.

[0024] There is also provided an electrolysis cell for performing water electrolysis, the electrolysis cell comprises an anode side having a first catalytic layer that facilitates formation of oxygen gas and hydrogen ions from the humidified gas stream; a cathode side having a second catalytic layer that facilitates generation of hydrogen gas from the hydrogen ions; and an ion-permeable membrane sandwiched between the anode side and the cathode side, and being in contact with both the first and second catalytic layers for conducting the hydrogen ions generated at the anode side to the cathode side; the electrolysis cell being configured to be connectable to a humidified gas stream delivery system and a liquid water delivery system to supply a humidified gas stream and liquid water into the electrolysis cell, respectively, wherein the first catalytic layer is arranged to contact the humidified gas stream to generate oxygen gas and the second catalytic layer is arranged to contact the liquid water.

[0025] In some embodiments, a side of the ion-permeable membrane facing the anode side is provided with the first catalytic layer and a side of the ion-permeable membrane facing the cathode side is provided with the second catalytic layer.

[0026] In some embodiments, the first catalytic layer and / or the second catalytic layer are respectively provided on an anode diffusion layer and a cathode diffusion layer.

[0027] In some embodiments, the first catalytic layer comprises ruthenium oxide and an ionomer.

[0028] In some embodiments, the percentage weight of ionomer to ruthenium oxide is approximately 8% to 16%, suitably 8 - 12%.

[0029] In some embodiments, the second catalytic layer comprises a platinum - carbon black mixture and an ionomer.

[0030] In some embodiments, the percentage weight of the ionomer to carbon black is approximately 60%.

[0031] In some embodiments, the electrolysis cell further comprises: an anode gasket assembly configured to seal the first catalytic layer on the anode side; and a cathode gasket assembly configured to seal the second catalytic layer on the cathode side. The anode gasket assembly and the cathode gasket assembly may comprise different materials.

[0032] In one form, the cathode gasket assembly may comprise a combination of fluorine- ethylene-propylene (FEP) and polytetrafluoroethylene (PTFE) polymers while the anode gasket assembly may be formed from silicone rubber or fluoro rubber gasket.

[0033] The use of different materials for the anode gasket assembly and cathode gasket assembly offers benefits in terms of optimizing the physical contact between components. For example, in a cell using carbon paper for the cathode diffusion layer and porous titanium for an anode diffusion layer, there exists a difference in hardness / compressibility between the carbon paper and titanium that can result in uneven compression of the softer material (carbon paper in this case). Such uneven compression can have undesirable consequences. For example, higher compression of the carbon layer may result in closing of the pores that facilitate transport of reactants and / or products through the cathode diffusion layer thereby affecting performance. To mitigate such effects, the gasket material for the cathode diffusion layer can be chosen so as to allow the cathode gasket to soak up the forces experienced from uneven compression thereby preventing the cathode diffusion layer being compressed to an extent that affects its performance. In this manner, optimal contact may be ensured between the different components of the cell by using different materials for the anode gasket assembly and cathode gasket assembly. Furthermore, it will be appreciated that the use of different materials for the anode and cathode gasket assemblies can be adapted to other types of water electrolysis cells as well. For example, conventional water electrolysis cells that utilize liquid water as the feed instead of humidified gas stream or vapor - vapor electrolysis cells which utilize water vapor supply to both anode and cathode can also benefit from utilizing different materials for the anode and cathode gasket assemblies. In some embodiments, the anode gasket assembly is configured to contact the ion- permeable membrane on the anode side and seal the first catalytic layer on the anode side.

[0034] In some embodiments, the cathode gasket assembly configured to contact the ion- permeable membrane on the cathode side and seal the second catalytic layer on the cathode side.

[0035] In one embodiment, the ion-permeable membrane is selected to be larger than either or both first and second catalytic layers. For example, a cathode / another catalytic layer measuring 2x2 cm2is placed in the middle of an ion-permeable membrane measuring 2.5x2.5 cm2. This allows the gasket to press against the edges of the ion- permeable membrane to form a seal around the cathode / another catalytic layer.

[0036] In some embodiments, the humidified gas stream delivery system may be located external to the electrolysis cell.

[0037] In some embodiments, the electrolysis cell is configured to minimize pressure differential between the anode side and the cathode side. This will assist in minimizing hydrogen transport from the cathode side to the anode side which may lead to safety issues. For example, when the pressure on the cathode side is higher, hydrogen gas may be transferred across the ion-permeable membrane to the anode side where it may combine with the generated oxygen gas. If the concentration of the resulting mixture is beyond the explosive limit, then an explosion can occur. As such, systems where pressure differentials exist would require a way of diluting the hydrogen content on the anode side to a level below the explosive limit.

[0038] Suitably, the electrolysis cell is configured to prevent a pressure differential between the anode side and the cathode side from forming. This will avoid the need to dilute the hydrogen content on the anode side.

[0039] There is further provided an electrolysis cell for performing water electrolysis, the electrolysis cell comprises an anode side having a first catalytic layer that facilitates formation of oxygen gas and hydrogen ions from a liquid water stream; a cathode side having a second catalytic layer that facilitates generation of hydrogen gas from the hydrogen ions; and an ion-permeable membrane sandwiched between the anode side and the cathode side, and being in contact with both the first and second catalytic layers for conducting the hydrogen ions generated at the anode side to the cathode side; the electrolysis cell being configured to be connectable to a liquid water delivery system to supply the liquid water into the electrolysis cell, wherein both the first catalytic layer and second catalytic layer are arranged to contact the liquid water to generate oxygen gas hydrogen gas respectively. The cell further comprises an anode gasket assembly configured to seal the first catalytic layer on the anode side; and a cathode gasket assembly configured to seal the second catalytic layer on the cathode side. The anode gasket assembly and the cathode gasket assembly may comprise different materials.

[0040] In one form, the cathode gasket assembly may comprise a combination of fluorine- ethylene-propylene (FEP) and polytetrafluoroethylene (PTFE) polymers while the anode gasket assembly may be formed from silicone rubber or fluoro rubber gasket.

[0041] In some embodiments, the liquid water delivery system is configured to deliver liquid water at least to the cathode side of the electrolysis cell.

[0042] Disclosed in another aspect is a system for performing water electrolysis, the system comprising: an electrolysis cell described hereinabove; an electrical sub-system configured to drive the electrolysis cell; a humidified gas stream delivery system configured to supply a humidified gas stream into the electrolysis cell; a liquid water delivery system configured to supply liquid water into the electrolysis cell.

[0043] In some embodiments, liquid water permeates through the second catalytic layer into the ion permeable membrane. In one embodiment, water may travel through a cathode flow plate, a cathode diffusion layer and the second catalytic layer into the ion permeable membrane. It will be understood that the liquid water helps to hydrate the ion-permeable membrane. In some embodiments, the system includes one or more storage reservoirs configured to receive and store products of the water electrolysis. In some embodiments, the system includes a system controller configured to monitor and control one or more process parameters associated with the water electrolysis.

[0044] In some embodiments, the humidified gas stream delivery system is configured to produce water vapour from a water source and mix said water vapour with a carrier gas.

[0045] In some embodiments, the humidified gas stream delivery system comprises a microceramic diffuser through which the carrier gas is passed prior to mixing with the water that is obtained from the water source and heated.

[0046] In some embodiments, the carrier gas comprises, at least in part, oxygen gas that is generated at the anode side during operation of the cell.

[0047] In some embodiments, compressed air may be used as the carrier gas.

[0048] In some embodiments, the water source comprises an impure water source.

[0049] In some embodiments, the system can comprise a waste heat recovery device, configured to recover at least a portion of heat generated from operation of the electrolysis cell and use said heat at least in part to generate the humidified gas stream.

[0050] In some embodiments, the waste heat recovery device comprises a water recirculation system and a heat exchanger through which water is circulated, the heat exchanger acting as a heat sink for trapping heat generated from operation of the electrolysis cell.

[0051] In some embodiments, the heat exchanger is configured to function with low-grade waste heat.

[0052] Disclosed in another aspect is a kit for retrofitting an electrolysis cell configured to use a liquid water feed for electrolysis, the electrolysis cell comprising an anode side, a cathode side and an ion permeable membrane located between the anode side and the cathode side for conducting hydrogen ions generated at the anode side to the cathode side, wherein the anode side has an anode diffusion layer and a first catalytic layer in contact with the anode diffusion layer, the first catalytic layer facilitates formation of oxygen gas and hydrogen ions, the cathode side has a second catalytic layer that facilitates generation of hydrogen gas from the hydrogen ions, and the ion permeable membrane arranged to contact both the first and second catalytic layers, the kit comprising: a humidified gas stream generation device configured to generate and supply a humidified gas stream into the electrolysis cell.

[0053] In some embodiments, the kit further comprises a replacement anode flow plate configured to be located in fluid communication with the anode diffusion layer, the replacement anode flow plate comprising flow channels having a serpentine configuration.

[0054] In some embodiments, the kit comprises a replacement ion-permeable membrane.

[0055] In some embodiments, the replacement ion-permeable membrane comprises a replacement first catalytic layer having ruthenium oxide and an ionomer, wherein the percentage weight of ionomer to ruthenium oxide is approximately 8% to 16%, suitably in the range of 8 to 12%.

[0056] In some embodiments, the kit comprises anode and cathode gasket assembly materials that are different.

[0057] Disclosed in another aspect is a formulation for a catalytic layer of an anode of an electrolysis cell, the formulation comprising: a catalyst configured to facilitate water electrolysis; and a proton transfer material that facilitates transfer of protons generated in use during water electrolysis and configured to provide structural integrity to the catalytic layer. Such a formulation may reduce or minimize dissolution of the catalyst particles.

[0058] In some embodiments, the proton transfer material is hydrophilic. In some embodiments, the formulation comprises a binder and / or additional hydrophilic material.

[0059] In some embodiments, the percentage weight of catalyst to proton transfer material is approximately 8% to 16%, suitably 8 - 12%.

[0060] Disclosed in a further aspect is a method of electrolysis as defined hereinabove, wherein the first catalytic layer comprises the formulation defined hereinabove.

[0061] Disclosed in a further aspect is a system as defined hereinabove, wherein the first catalytic layer comprises the formulation defined hereinabove.

[0062] BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Embodiments will now be described by way of example only, with reference to the accompanying drawings in which:

[0064] Figure 1 shows a schematic of an electrolysis cell according to an embodiment of the invention.

[0065] Figure 2 shows a schematic of an electrode assembly according to an embodiment of the invention.

[0066] Figures 3A-3C show performance of an electrolysis cell according to an embodiment of the invention.

[0067] Figures 4A - 4C show the effect of ruthenium loading on the electrochemical performance of an electrolysis cell according to an embodiment of the invention.

[0068] Figure 5 shows the effect of iridium loading on the electrochemical performance of an electrolysis cell according to an embodiment of the invention.

[0069] Figures 6A-6B show the effect of different sources of water on the electrochemical performance of an electrolysis cell according to an embodiment of the invention. Figure 7 A - 7B shows the long term performance of an electrolysis cell according to an embodiment of the present invention.

[0070] Figure 8 shows a block diagram of a system for water electrolysis according to an embodiment of the invention.

[0071] Figure 9 shows the back-diffusion rate of water from cathode to anode side under different temperature and humidity conditions.

[0072] Figure 10 shows the performance of an asymmetric electrode assembly used in electrolysis cells operating in different modes.

[0073] DETAILED DESCRIPTION

[0074] In the following detailed description, reference is made to accompanying drawings which form a part of the detailed description. The illustrative embodiments described in the detailed description, depicted in the drawings and defined in the claims, are not intended to be limiting. Other embodiments may be utilised and other changes may be made without departing from the spirit or scope of the subject matter presented. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings can be arranged, substituted, combined, separated and designed in a wide variety of different configurations, all of which are contemplated in this disclosure.

[0075] Electrolysis cell - general description

[0076] Referring to Figure 1, disclosed is a schematic of an electrolysis cell 10 configured to perform water electrolysis according to the method of the present invention. The cell 10 comprises an anode side 12 having a first catalytic layer 12a and a cathode side 14 having a second catalytic layer 14a. The anode side 12 and cathode side 14 are separated by an ion permeable membrane 16 having two sides 16a and 16b that respectively contact the first and second catalytic layers 12a, 14a. The anode side 12 is configured to receive a humidified gas stream 18 comprising water vapour, while the cathode side 14 is configured to receive liquid water, such that the side 16b of the permeable membrane 16 is in contact with liquid water. The water vapor present in the humidified gas stream 18 contacts the first catalytic layer 12a and undergoes electrolysis to produce a stream of oxygen gas 20 and hydrogen ions (protons). Oxygen gas 20 produced from the electrolysis process exits the cell 10 from the anode side 12. The hydrogen ions are transferred across the membrane 16 to the second catalytic layer 14a, where they combine to form a stream of hydrogen gas 22 that exits the cell 10 from the cathode side 14.

[0077] The anode side 12 and the cathode side 14 further comprise electrodes assemblies 26, 34 (best shown in Figure 2 and discussed further below) that are in contact with the catalytically active layers 12a and 14a and are connected to an external circuit. The electrode assemblies facilitate distribution of the reactants (i.e. vapor and liquid water) to the catalytically active layers, withdrawal of products of the electrolysis reaction (oxygen gas and hydrogen gas) as well as transfer of electrons generated during the electrolysis reaction from the anode side 12, to the cathode side 14.

[0078] In some embodiments, the anode side may be configured to receive liquid water instead of humidified gas stream 18 to produce the stream of oxygen gas 20 and hydrogen ions (protons).

[0079] Ion permeable membrane 16

[0080] As stated above, the ion permeable membrane 16 allows passage of hydrogen ions generated at the anode side 12 to the cathode side 14, thereby completing the electrical circuit within the cell. The ion permeable membrane 16 also functions to prevent the electrode assembly 26 located on the anode side 12 from coming into direct contact with electrode assembly 34 located at the cathode side 14, thereby preventing a short circuit from occurring. The thickness of the ion permeable membrane 16 may be chosen based on the performance requirements desired. Generally speaking, a thinner member can be advantageous. However, any membrane thickness that offers the desired functionality may be suitable for the present invention. For example, commercial membranes having thickness in the range of 150 - 180 pm may be utilized. Similarly, membranes of 50 pm thickness may be utilized, with membranes of 8 - 10 pm thickness also potentially suitable for use with the present invention. The membrane 16 also needs to withstand the operating temperatures encountered during the electrolysis process.

[0081] In the preferred embodiment, the ion permeable membrane is chosen from various commercially available products such as Nafion NR212 (50 pm). It will be appreciated that other membranes that satisfy the functional requirements described above can be employed. For example, perfluorosulphonic acid (PFSA) based membranes such as Nafion 117, Nafion 115, AQUIVION E98-05S, Aquivion® E87-05S may also be suitable choices for the ion permeable membrane.

[0082] First and second catalytic layers

[0083] The first and second catalytic layers 12a, 14a enable the decomposition of water by facilitating the reactions for oxygen gas and hydrogen gas formation respectively. These layers need to maintain structural integrity while ensuring the required functionality (of facilitating the desired reactions). Accordingly, these layers may comprise more than one component to satisfy these requirements.

[0084] A catalytic layer typically comprises the catalyst which is the active component on which the reactions are facilitated, a binder which provides structural integrity to the catalytic layer and one or more additives that may impart a specific functionality. The term structural integrity in the context of the present invention refers to the ability to physically hold together the various components that form part of the catalytic layer, such that the required performance levels are obtained for a desired amount of time (e.g. for the designed life of the catalyst layer). One or more additives may improve the transport of a desired ion (e.g. protons) from the catalytic layer to the membrane 16. Similarly, the additive may also include a hydrophilic material that improves the ability of the catalytic layer to absorb and retain moisture which can facilitate the transport of ions.

[0085] It has been discovered that it is possible to select materials with more than one desirable characteristic for the catalytic layer. For example, it is possible to select a material that has a) desired proton conduction properties, b) desired binding ability and c) desired hydrophilicity. Such material selection would simplify the design and fabrication of the catalytic layer by reducing the number of components to be combined and processed. Such material selection can also offer operational benefits. For example, choosing a hydrophilic material as part of the first catalytic layer would allow the layer to operate more efficiently under conditions of low humidity (e.g. with water vapor on the anode side).

[0086] In this regard, the first catalytic layer 12a may comprise a catalyst and a proton conducting material that is able to act as a binder, thus also providing structural integrity to the catalytic layer. The rationale for choosing materials with more than one desirable characteristic can be understood from the following description. Electrochemical reactions (such as electrolysis of water) typically occur at a triplephase boundary (i.e. meeting points) formed between a catalyst, an ionic conductor (in this case the proton conducting material) and a reactant (in this case water). The catalyst provides active sites for the desired reaction, thereby facilitating the reaction, as well as providing a path for the effective conduction of electrons formed in the reaction. The ionic conductor on the other hand provides a path for the transport of protons formed in the reaction, to the cathode side. An effective triplephase boundary may be built with suitable proportions of these three components (i.e. catalyst, ionic conductor and water), enabling the reaction to occur at a sufficient interface, while simultaneously ensuring effective conduction of ions and electrons.

[0087] While not wishing to be bound by a particular theory, it is believed that by selecting the proton conducting material (e.g. PFSA ionomer) and catalyst combination in suitably balanced proportions, so as to provide a continuous conduction network for electrons and ions, the efficiency of the electrolysis cell may be maximised. The rate of the electrolysis reaction can depend on at least the following parameters: a) the amount of triple-phase boundary (i.e. meeting points) available, b) the amount and continuity of transport paths available to move the electrons formed, c) the amount and continuity of transport paths available for the protons formed and d) the ability of the catalyst in facilitating the oxygen evolution reaction. Using a separate binder (i.e. one that provides suitable structural integrity, but does not provide an ion-conduction ability) results in a reduction of at least the amount of triple-phase boundary and / or the amount / continuity of transport paths available for both electron and proton transport. By using a proton conducting material that has adequate binding (structural enhancing) ability, the need for an additional binder (having potentially less desirable electrical and / or ionic properties) is avoided. In other words, the binding material itself can act as a pathway for moving the protons generated from the reactions occurring at the anode side. The elimination of additional binder material in the catalytic layer may allow for a greater access to catalytic sites (i.e. increased number of meeting points) for a given amount of the catalyst present in the layer. This in turn may have the effect of allowing more reactions to be catalysed in a given volume of the catalytic layer. This may assist in improving the yield / output of the cell (e.g. in terms of the H2 output). Accordingly, by choosing an appropriate amount of binder which can also act as a proton conductor, and a catalyst that facilitates water catalysis as well as electron transport, the amount and continuity of transport paths available for both electron and proton transport can be optimized. Similarly, choosing a catalyst material that has the best ability in facilitating the oxygen evolution reaction would ensure that the oxygen evolution reaction occurs at the desired potential once the reactants arrive at the catalyst site.

[0088] Without being bound by a particular theory, the catalyst for the first catalytic layer 12a may be chosen from a group of materials having an overpotential in the range of 200 - 450 millivolts (mV). This range of overpotential is considered to provide the best oxygen evolution performance for a catalyst material. Generally speaking, platinum group metal oxides and compounds containing Ru, Ir, Pt, Rh, Pd, Ag, Cu, Os, Re, Au, and Hg display an overpotential in this range. Of these, Ru and Ir based oxides display overpotentials in the range of 200 - 250 mV. However, other materials may also be suitable. For example, oxides containing Co, and Mn, including AB2O4 spinels and AMmOs manganites (where A and B are rare earth metal or transition metal elements) have overpotentials in the range of 200 - 550 mV.

[0089] The material chosen can further be in the form of oxide nanoparticles such as RuCh, IrCh, core-shell nanoparticles such as Ir@IrC>2, Ru@RuC>2 and composite nanoparticles such as RuCL-SiCh, RUO2-WO3 and RuCh-SnCK

[0090] Without being bound by any particular theory, polymers with a backbone that provides a binding ability and a side chain that provides the desired proton conduction ability may be chosen as the proton conducting material. For example, a PFSA ionomer that has a poly(tetrafluoroethylene) backbone with perfluoroether pendant side chains terminated by sulfonic acid groups can be chosen. The pendant sulfonic acid groups in the polymer have the ability to conduct protons, while the hydrophobic poly(tetrafluoroethylene) backbone provides the ability to connect the catalyst particles, acting as a binder. In this manner, a continuous proton conducting network that holds together catalyst particles may be formed in the catalytic layer. In this regard, a PFSA ionomer with lower Equivalent Weight (EW) have been found beneficial to yield higher proton conductivity and water uptake. One reason for this is that a lower equivalent weight ionomer has a higher ion exchange capacity and allows protons to be transported more efficiently during the reaction. Long sidechain ionomers with EW in the range of 900-1100, and short side-chain ionomers with EW in the range of 700-1000 may be suitable. Suitable proton conducting materials may be chosen from a group consisting of Nafion D520CS, Nafion D521CS, Nafion D2020CS, Nafion D2021CS, AQUIVION D79-25BS, Aquivion® D72-25BS, Aquivion® D83-24B, Aquivion® D98-25BS and Nafion 1100EW.

[0091] It is also believed that catalysts with high specific surface area may require higher proportions of proton conducting material (e.g. PFSA ionomers) to build effective ion conduction networks.

[0092] It should be understood that the anode and cathode can have different proton conducting material / catalyst compositions. For example, Pt / C cathodes having high specific areas (due to carbon) may require ionomer to carbon percentage of 60 % by weight.

[0093] In a preferred embodiment, the first catalytic layer 12a comprises a catalyst in the form of the rutile phase of ruthenium oxide (RuO2) and the proton conducting material comprises an ionomer in the form of a perfluorinated resin containing Nafion 1100W. The combination of RuO2and ionomer provides unique advantages discussed above.

[0094] The RUO2facilitates the water splitting reaction on the anode side (i.e. splitting of water into protons and oxygen ions, and formation of oxygen gas). The ionomer functions to complement the RuO2(e.g. by providing structural integrity) and provides a pathway that transfers the protons formed at reaction sites in the first catalytic layer 12a to the membrane 16. Such movement can result in improved kinetics of the decomposition reaction. The ionomer also facilitates ‘binding’ of the catalyst particles to provide structural integrity of the first catalytic layer 12a. Additionally, the ionomer is also hydrophilic in nature, allowing moisture to be retained in the first catalytic layer 12a. A hydrated catalytic layer 12a allows for efficient generation and transfer of the generated protons. It should be noted that the moisture that is trapped by the hydrophilic ionomer may not cause dissolution of the Rut as the moisture is in the form of ‘bound’ water (i.e. water molecules that are bound to the ionomer and not free to dissolve the RuO2).

[0095] The percentage weight of the ionomer to the RuO2(i.e. (weight of ionomer / weight of RUO2) x 100) in the first catalytic layer may be controlled as desired. Generally speaking, the percentage weight is chosen such that there is an optimum amount of coverage of the catalyst by the ionomer. For example, a lower amount of ionomer (e.g. <8% by weight) may result in the formation of a discontinuous network of the ionomer and / or be insufficient to obtain the desired level of proton conduction. Similarly, too high an amount of ionomer may result in the catalyst particles being covered completely by the ionomer and thus hinder electron conduction between catalyst particles. In a preferred embodiment, the weight of the ionomer ranges from 8 - 16% of the RUO2. In other embodiments, the weight of the ionomer may be as high as 20%. In some embodiments with precious metal free oxide catalysts, the ionomer and catalyst can be present in equal amounts (i.e. the mixture of ionomer and catalyst has 50% by weight of ionomer and 50% by weight of catalyst). Furthermore, catalyst loading in the fabricated catalytic layer may be controlled to obtain the desired level of catalyst in the catalytic layer. For example, in some embodiments, RuO2loading may range from 0.5 mg / cm2to 4 mg / cm2.

[0096] The RUO2particles can have a particle size in the range of 200 - 600 pm, for example for RuO2particles produced by thermal methods. In other embodiments, RUO2particles in the size range 2 - 20 pm may also be suitable for use, such as those produced by wet chemical methods. In general, the preferred size ranges may be in the micrometre / nanometre range. However, it should be noted that there are no specific requirements for particle size as such, in order for the catalyst to be able to perform its intended function.

[0097] In some forms, additional binder may be added to the mixture of RuO2and ionomer. This additional binder can further improve the structural integrity of the first catalytic layer 12a. Similarly, additional hydrophilic materials may be added to the mixture of RuO2and ionomer, to improve the moisture uptake of the first catalytic layer 12a.

[0098] It will be understood by the person skilled in the art that any suitable material may be employed in the first catalytic layer, as long as the performance requirements are met.

[0099] In the preferred embodiment, the second catalytic layer 14a comprises a catalyst in the form of Platinum / carbon black composite. However, it will be understood by the person skilled in the art that any suitable catalyst may be employed in the second catalytic layer, as long as the functional requirement of facilitating the hydrogen generation reaction is met.

[0100] Location of first and second catalytic layers The first and second catalytic layers 12a, 14a comprising a number of components are typically provided on one of the components in the cell 10, e.g. as a layer applied thereto. This is because these layers are typically formed from particles / powders that require a substrate component to provide structural integrity.

[0101] In preferred embodiments, the first catalytic layer 12a and second catalytic layer 14a are deposited on opposite sides of the ion permeable membrane 16 to form a ‘sandwich’ type structure. Such a structure can provide benefits including higher catalytic site utilisation and lower ohmic resistance. Because the first catalytic layer 12a and ion permeable membrane 16 are located in close proximity, the protons that are produced at the anode side 12 can be transferred more efficiently through the membrane 16 to the cathode side. Accordingly, there is no need to use higher catalyst loading to generate more protons (i.e. in other words, existing catalyst sites are able to perform efficiently as protons generated there are moved through the ion permeable membrane more easily). The protons then move through the membrane 16 to the second catalytic layer 14a where they combine with electrons to form hydrogen gas (2H++ 2e" H2). Enhanced catalytic site utilisation may allow for lower catalyst loadings to be used (without a corresponding decrease in cell performance), which can provide cost advantages and decrease manufacturing complexity.

[0102] It should be noted that the term ‘sandwich’ refers to an arrangement in which the membrane 16 is located in between the first and second catalytic layers 12a, 14a, and is not intended to be limited to embodiments in which these layers are deposited directly onto the membrane 16. For example, a sandwich construction can also include an arrangement wherein the first and second catalytic layers 12a, 14a are deposited onto other components of the cell (see discussion on anode and cathode diffusion layers below) and brought into contact with the membrane 16 from opposing sides. Such a configuration may also satisfy the functional requirement of the first and second catalytic layers 12a, 14a being in contact with the membrane 16. Electrode assemblies 26, 34

[0103] The first and second catalytic layers 12a, 14a are respectively in contact (i.e. electrical contact) with electrode assemblies 26 and 34, that are located respectively at the anode side 12 and cathode side 14 of the cell 10. The electrode assemblies perform multiple functions. Firstly, they assist with delivering the reactant (humidified gas stream 18, shown in Fig. 1 or liquid water in some embodiments) to the first catalytic layer 12a. The electrode assemblies also provide support to the membrane 16 (which is typically a mechanically flexible membrane). The electrode assembly can also assist with transport of reactants / products in / out of the cell (e.g. water, oxygen gas etc). The electrode assemblies can also act as sealing members, ensuring that the humidified gas stream 18 (or liquid water) only flows to the desired region of the cell (i.e. the first catalytic layer 12a) and likewise the products of electrolysis (e.g. oxygen gas) do not leak to other parts of the cell (e.g. the cathode side where hydrogen gas is generated).

[0104] In a preferred embodiment, electrode assembly 26 (Anode Electrode assembly - AES), can comprise an anode flow plate 28, a gasket assembly 30 and an anode diffusion layer 32, that is in contact with the first catalytic layer 12a. The anode flow plate 28 is made from titanium (coated with Pt / Au) and has an inlet and an outlet. The inlet receives the humidified gas stream 18 (or liquid water) while the outlet facilitates removal of the product stream comprising residual water vapor, oxygen gas. The titanium flow plate 28 is in contact with the anode diffusion layer 32 and comprises flow channels designed to distribute the humidified gas stream 18 across the surface of the anode diffusion layer 32. This ensures that the anode diffusion layer 32 receives the humidified gas stream 18 (or liquid water) at different locations along its surface. The gasket assembly 30 may be selected based on general principles so as to ensure optimal compression of the anode diffusion layer 32 and ensure close contact between the anode diffusion layer 32 and first catalytic layer 12a, as well as the anode diffusion layer 32 and current collector (titanium flow plate 28). The gasket assembly 30 may comprise a single gasket or more than one gasket

[0105] When used for a single electrolysis cell (i.e. comprising just one anode and cathode side separated by a membrane), the anode flow plate 28 has flow channels only on one side (i.e. it is a monopolar flow plate). When used in stacks (i.e. multiple cells stacked together), the anode flow plate 28 may have channels on both sides (to allow humidified gas stream to be supplied to two anode diffusion layers that may be located on either side).

[0106] The configuration of the channels employed in the anode flow plate 28 may be varied. A conventional, parallel flow configuration that is used in a water-based electrolysis cells may be utilised to supply the humidified gas stream 18 (or liquid water), for example. In a preferred embodiment, the anode flow plate 28 comprises serpentine flow channels. A serpentine flow channel arrangement may provide an enhanced pressure drop between the inlet and outlet of the flow channel, thereby enhancing vapour flow. In some embodiments, a 3-dimensional patterned flow field may be utilized as channels.

[0107] The dimensions of the anode flow plate 28 may be chosen according to the functional requirements of the cell. For example, the anode flow plate 28 may have a channel of depth 1.5 mm, groove width 1.5 mm, ridge width 1.5 mm and be made from a plate 5 mm thick. Furthermore, internal circulating water cooling channels may also be employed in the anode flow plate 28, to regulate the temperature to the desired levels. It will be appreciated that the cooling water used to cool the anode flow plate 28 may be directed to a heat recovery device to enable waste heat recovery as discussed above.

[0108] The anode diffusion layer 32 is in contact with the first catalytic layer 12a. Its function is to distribute the humidified gas stream 18 (or liquid water) received from the flow plate 28, in a uniform manner to the first catalytic layer 12a. The anode diffusion layer 32 also serves to collect oxygen gas 20 generated during electrolysis and pass it to the anode flow plate 28. The gathered oxygen gas flows out through an outlet of the anode flow plate 28. In a preferred embodiment, the anode diffusion layer 32 is made from a titanium fibre mesh. In one form, the anode diffusion layer may comprise a 0.27mm platinized porous titanium as the gas diffusion layer. In other embodiments, other suitable materials may also be chosen.

[0109] In the preferred embodiment, the gasket assembly 30 of AES 26 is made from a single material comprising soft fluorinated rubber (278 - 300 pm). In use, this enables the gasket assembly 30 to deform against the anode diffusion layer 32 (titanium being relatively resistant to deformation) to form a tight seal and provide application of uniform in-plane pressure. This may result in optimal contact of the anode diffusion layer 32 with both the flow plate 28 as well as the first catalytic layer 12a thereby facilitating optimal electron conduction. It is essential that the thickness of the gasket assembly 30 be close to, but not less than that of the anode diffusion layer 32. This will ensure effective sealing given the high hardness and near incompressibility of the anode diffusion layer 32 made from Titanium fiber felt.

[0110] Similar to AES 26, the cathode side 14 comprises a cathode electrode assembly (CES) 34 comprising a cathode flow plate 36, a gasket assembly 38 and a cathode diffusion plate 40. In a preferred embodiment, the cathode flow plate 36 is made from stainless steel. Like the anode flow plate 28, the cathode flow plate 36 includes an inlet and an outlet. The inlet is configured to receive water while the outlet is configured to facilitate removal of a product stream comprising water and hydrogen gas 22 generated from the electrolysis process. The cathode flow plate 36 comprises flow channels designed to distribute the water at different locations across the surface of the cathode diffusion layer 40.

[0111] Like the anode flow plate 28, the cathode flow plate 36 can come in different configurations (e.g. monopolar / bipolar) for use with a single cell / stacks. The configuration and dimensions of the flow channels may be chosen so as to facilitate flow of water (e.g. a parallel flow configuration may be optimal for liquid waterbased systems). The cathode diffusion layer 40 is in contact with the second catalytic layer 14a. Its function is to distribute the liquid water received from the cathode flow plate 36, in a uniform manner to the second catalytic layer 14a. The cathode diffusion layer 40 also serves to collect hydrogen gas 22 generated during electrolysis and pass it to the cathode flow plate 36. The gathered hydrogen gas flows out through the outlet of the cathode flow plate 36.

[0112] In a preferred embodiment, the cathode diffusion layer 40 is made from a carbon paper (Toray-TGP-H-090, 280 pm). In one embodiment, the cathode diffusion layer can comprise a 0.28 pm, 5 wt% water-proofed carbon paper. In other embodiments, other suitable materials may also be chosen.

[0113] In a preferred embodiment, the gasket assembly 38 of CES 34 is made from a combination of polyethylene terephthalate (PET, 100 pm) and PTFE (150 pm). This combination is relatively harder / stiffer compared to the carbon paper. In use, this enables the cathode diffusion layer 40 to deform against the gasket assembly 38 (i.e. conform to the gasket assembly 38) to form a tight seal and apply uniform in-plane pressure. In some embodiments, the materials chosen for the gasket assembly 38 and the thickness may allow the gasket assembly to compress the cathode diffusion layer by 20 - 30%. In some other embodiments, the gasket assembly 38 can comprise a combination of 0.1mm thick fluorine-ethylene- propylene (FEP) and 0.05mm PTFE gasket.

[0114] In some forms, the electrode assemblies 26, 34 are asymmetric. Such an asymmetric assembly may prevent excessive compression of the cathode diffusion layer caused by differences in compressibility between the cathode and anode gas diffusion layers, thereby enhancing the overall structural stability and performance of the electrolysis cell. It will be appreciated by a person skilled in the art that such an asymmetric electrode assembly may be employed for other types of water electrolysis cells (e.g. conventional electrolysis cells with water being supplied both to the anode and cathode sides). In this regard, Figure 10 shows that the asymmetric electrode assembly may show similar performance across water electrolysis cells in different modes.

[0115] It will be appreciated that the components described above for the anode and cathode electrode assemblies may be fabricated from any suitable material, as long as the respective functional requirements are met.

[0116] As discussed above, in some embodiments, the first and second catalytic layers 12a, 14a may be applied on one of the sides of the anode and cathode diffusion layers 32, 40 respectively. The coated sides of the anode and cathode diffusion layers can then be brought into contact with the ion permeable membrane 16, to facilitate the function of transferring protons formed during the reactions from the anode side to the cathode side. In this regard, additional pressure may be applied to the anode and cathode diffusion layers 32, 40 to ensure optimal contact between the layers and the membrane 16. Such pressure may be applied for example by means of bolts fastened to the respective flow plates 28 and 36, that in turn compress the gaskets and the diffusion layers. In some embodiments, the pressure applied may in the range of 2.2 - 5.5 Nm.

[0117] Techniques for deposition of first and second catalytic layers in a sandwich construction

[0118] As discussed above, the first and second catalytic layers 12a, 14a may be deposited on either side of the ion permeable membrane 16. This deposition may be accomplished through any suitable means. In a preferred embodiment, the first catalytic layer 12a may be deposited on the membrane 16 followed by deposition of the second catalytic layer 14a. In other embodiments, the second catalytic layer 14a may be deposited first on the membrane.

[0119] In a preferred embodiment, the first catalytic layer 12a may be deposited on the membrane 16 using techniques such as spray coating. It will be appreciated that other suitable methods may also be employed. For example, bar coating, slot-die coating and decal transfer are some other methods that may also be employed. The mixture required for spray coating can be formed using solvents such as isopropyl alcohol (IPA), water, etc using conventional techniques such as sonication, dissolution etc. The concentration of the mixture can be adjusted to give the desired mass loading of the catalyst in the catalytic layers. Following spray coating, the layers may be subjected to heat treatment in several steps to remove the solvents used, and then subjected to further processing through techniques such as hot pressing, to improve the properties of the layer.

[0120] In other embodiments, the first catalytic layer 12a may be deposited on the anode diffusion layer 32 using spray coating. Similarly, the second catalytic layer 14a may be deposited on the cathode diffusion layer 40 using spray coating.

[0121] Humidified gas stream 18

[0122] The cell 10 requires water to be input as feed in order for it to be split. In some embodiments, a humidified gas stream 18 is supplied to the anode side 12. The humidified gas stream 18 is a gas stream comprising a desired amount of water vapor suitable for operation of the cell 10. The main function of the humidified gas stream 18 is to supply the water required for the electrolysis process, in the form of water vapor, to the anode side 12.

[0123] In the preferred embodiment, the humidified gas stream 18 can comprise a carrier gas (e.g. argon / helium / nitrogen etc.) that is combined with water vapor generated from a source of water.

[0124] Since water is required only in vapor form, any source of water may be utilized to generate the required water vapor. In some forms, this may include (in a nonlimiting manner) sources such as lakes, sea, rivers, tap or rain-water. Because the water does not need to be purified (i.e. the process of vaporization itself acts as a purifying mechanism), complex / expensive water pre-treatment systems (e.g. to remove contaminants, as for conventional liquid water electrolysis systems) may not be required for operation. This in turn may offer additional benefits in terms of reduced operational and maintenance costs. To determine the suitability of water sources, the quality of vapor produced from these sources may be tested and characteristics identified. For example, the ionic conductivity of the water vapor generated from the various water sources can be measured to determine suitability of the water sources.

[0125] It should be noted a dedicated system may not be required to generate the humidified gas stream 18. For example, the humidified gas stream 18 may be a byproduct of various processes (e.g. boilers / process steam / drying processes that generate water vapor etc). It is sufficient if the gas stream 18 obtained from these processes is suitable as an input feed to the cell 10. In some embodiments, the humidified gas stream 18 may be in the form of a waste gas stream from another process and / or may comprise waste streams from other parts of the electrolysis system.

[0126] Properties of humidified gas stream 18

[0127] Various characteristics of the humidified gas stream 18 such as temperature, flow rate of gas, vapor pressure, dew point, relative humidity, gas dispersion, liquid level and pipeline temperature can be controlled to enable supply of a saturated, humidified gas stream to the anode and obtain the desired performance from the cell 10.

[0128] Among other factors, the amount of water vapor supplied to the anode side 12 is an important parameter that is critical to the operation of the cell 10. In this regard, it is known that the water vapor supplied to the anode side 12 performs two functions - a) act as a source of water that undergoes an oxygen evolution reaction and b) facilitates electro-osmosis which is responsible for the diffusion of protons formed in the oxygen evolution reaction (thereby enabling hydrogen evolution at the cathode side). Accordingly, a minimum amount of vapor must be supplied to the anode side 12 to ensure that there is enough water to satisfy both functions a) and b). In this regard, the saturation level of the humidified gas stream 18 can be controlled to ensure that the desired amount of water vapor is being supplied to the anode side 12. The saturation level can in turn be controlled by controlling, among other things, the temperature of the humidified gas generation device 24 (discussed further below), the flow rate of the carrier gas and the pipeline temperature. In preferred embodiments, the temperature of the humidified gas produced by the generation device may be controlled in the range of 60 - 95 °C. At a carrier gas flow rate of 200 ml / min and temperature of 90 °C, this may result in a relative humidity level greater than 90%, which is sufficient to facilitate electrolysis.

[0129] In one embodiment, air may be used as the carrier gas (see further discussion below on carrier gas). In other embodiments, nitrogen may be used as carrier gas.

[0130] In this regard, some typical carrier gas flow rates (applicable to both air and nitrogen) and associated vapor flow rates at given current and temperatures are shown in the table below.

[0131] Table 1. Possible combinations of carrier gas flow rates and vapor flow rates when using air as the carrier gas. Ideally, the vapor supply is also able to maintain sufficient and stable vapor partial pressure at the surface of the anode to prevent any dehydration during the reaction. In this regard, supplying vapor at a higher temperature may be advantageous owing to the higher saturated vapor pressure that may be present. The high saturated vapor pressure may in turn enable the use of low carrier gas flow rates while ensuring sufficient vapor supply.

[0132] The relative humidity level of the humidified gas stream 18 may also be controlled to prevent back-diffusion of liquid water from the cathode side to the anode side. Back-diffusion is a process wherein differential water activity on the two sides of the membrane causes liquid water to be moved from a region of high activity (e.g. the cathode side) to a region of low activity (e.g. the anode side). When this occurs, the benefits associated with operation of the cell using water vapor supply may be lost. For example, back-diffusion may result in dissolution of the catalyst as liquid water contacts the first catalytic layer 12a on the anode side. A person skilled in the art will appreciate that such back-diffusion may also be affected by the operating temperatures and that a combination of higher operating temperatures and lower relative humidity may accelerate the back-diffusion processes. Carrier gas

[0133] As mentioned above, the humidified gas stream 18 can be generated by combining a carrier gas with water vapor generated from a source. The function of the carrier gas is to act as a ‘carrier’ for the water vapor. The carrier gas may be generally selected such that it does not undergo any chemical reactions that would interfere with the electrolysis process. In some forms, this gas can include inert gases such as argon, helium etc. In other forms, gases such as air, nitrogen may also be employed as the carrier gas.

[0134] In some other forms, oxygen gas 20 that is generated at the anode side 12 may be used as the carrier gas. In this regard, the generated oxygen gas 20 may be withdrawn from the cell 10 and directed to the humidified gas generation device (described below). By using a gas that is generated in-situ as the carrier gas, the need for external gas supply may be avoided, thereby reducing operational costs. In some forms, the oxygen gas 20 that is generated at the anode side 12 may be stored as a separate product, for later transport or treatment (e.g. to reduce moisture levels) to produce a source of oxygen gas.

[0135] Humidified gas generation device 24

[0136] In a preferred embodiment, the humidified gas stream 18 is generated using a humidified gas generation device / system 24. The device 24 may comprise a reservoir configured to hold water obtained from one or more of the water sources discussed above. The device / system 24 may be configured to allow a carrier gas (such as the oxygen gas 20 that is generated at the anode side 12 or alternatively compressed air supplied from suitable sources) to be bubbled through water contained in the reservoir. In one embodiment, half of the vessel may be filled with water from the desired source. The volume level may be controlled by a liquid level gauge or other similar devices. To facilitate generation of the humidified gas stream 18, the device / system 24 can be equipped with a heating means. In some forms, the heating means can comprise heating coils inserted into the reservoir, into which both water and the carrier gas are fed. It will be appreciated that other means of heating such as gas burners and the like may be used as well.

[0137] In other embodiments, the humidified gas generation device 24 may have separate systems for generating / supplying water vapor and carrier gas and the humidified gas stream may be formed by mixing the water vapor and carrier gas (i.e. water vapor may be generated from water obtained from a water source in a separate system and then mixed separately with the carrier gas).

[0138] The heating means heats the water stored in the reservoir to the desired temperature. In preferred embodiments (as discussed above), the temperature of the device 24 can be maintained in the range of 60 - 95 °C. In some forms, the device 24 may be covered with a heating jacket to prevent heat loss and ensure that no condensation of water occurs at the top of the vessel.

[0139] In some forms, the carrier gas may be conveyed through a micro-ceramic gas diffuser into the reservoir. This can increase the amount of gas-liquid interface, thereby resulting in an increased vaporization rate. In this manner, sufficient gas humidification may be achieved even under relatively low carrier-gas flow rates. In one form, the micro-ceramic gas diffuser comprises a microdiffusion stone that includes pores with size of 2 microns that is located within the water reservoir. The microdiffusion stone may advantageously be located at the bottom surface of the reservoir to maximise the distance travelled by the carrier gas through the water so that the bubbles become fully saturated with water vapor. It will be appreciated that the carrier gas flow rate can be controlled through a mass flow controller (i.e. the carrier gas source is connected to a mass flow controller which in turn is connected to the microdiffusion stone). Similarly, it will be appreciated that the vapor generation process can be controlled to avoid condensation of the vapor on top of the vessel. In some embodiments, microceramic diffuser forms bubbles, suitably bubbles in the micro-nano range.

[0140] Some embodiments may receive water already heated from an external water source, for example the water may be byproduct or output from a cooling system or industrial process. Thus, instead of heat energy stored in the water being treated as waste, for example and the water output to a cooling pond, the heated water may be input to the reservoir to thereby reducing additional energy input required to produce the humidified gas stream. In some embodiments, a waste / byproduct output from a process may be usable directly as an input humidified gas stream for the electrolysis system.

[0141] Once the humidified gas stream 18 is generated, it is delivered to the cell 10 for the electrolysis reaction. The delivery may be accomplished through suitable means. For example, insulated tubes / pipelines may be used to prevent or limit a drop in temperature of the gas stream 18 as it moves from the device / system 24 to the cell 10. Similarly, suitable pumps may be utilized to adjust the pressure and flow rate of the humidified gas stream 18. In some preferred embodiments, the temperature of the pipeline is maintained at 150 °C by wrapping heating tapes or other insulation along the length of the pipeline.

[0142] Waste Heat recovery

[0143] In some preferred embodiments, the device / system 24 may use, at least partly, waste heat recovered from operation of the electrolysis cell 10 to heat the water for the production of the gas stream 18. During operation of the electrolysis cell, heat may be generated due to irreversible processes such as irreversible ohmic loss and irreversible electrochemical reactions etc, that occur during the operation of the cell. At least part of this heat may be recovered and reused. For example, the water that is condensed in the cell 10 or exiting the cathode side may exit the cell 10 at a higher temperature than ambient. The heat trapped in this water can be recovered by using a heat exchanger / heat pumps.

[0144] Where a heat sink is used to conduct thermal energy away from the cell or other electrical components to maintain operating temperatures within tolerance levels, the heat sink may be arranged to exchange heat energy with water being input to the humidified gas stream generation. The device / system 24 may also include auxiliary systems such as pumps that circulate gases and water into the reservoir, various sensors to monitor the quality of the input and output streams etc. For example, water temperature sensors, dew point sensors, relative humidity sensors may be employed to monitor various parameters of the streams to ensure that the resulting humidified gas stream 18 has the desired properties before it is fed into the cell 10.

[0145] It will be apparent to a person skilled in the art that the humidification device 24 need not be limited to the above-described embodiment and other devices that are capable of producing a similar humidified gas stream 18 may also be utilized. For example, it is known that there exist devices that control relative humidity by adding water vapor in controlled amounts to a gas / air streams. Such devices may also be configured for the purpose of generating the humidified gas stream 18.

[0146] Operation of cell 10

[0147] The operation of the electrolysis cell 10 will be described now with reference to the embodiments described above that include delivering a humidified gas stream 18 to the anode side of the electrolysis cell.

[0148] To start up the electrolysis cell 10, a humidified gas stream 18 (generated as described above) is supplied to the anode side 12 while the cathode side 14 is supplied with liquid water. A voltage is applied across the cell to begin the electrolysis. The water supplied to the cathode side may be pure water / deionised (DI) water that is supplied from an external source. This water serves to keep the membrane 16 in a hydrated condition. As electrolysis proceeds, oxygen gas 20 and hydrogen gas 22 are generated and withdrawn from the anode and cathode sides, respectively. These gases are subjected to treatment to remove any residual moisture / other impurities. The separated gas streams may be stored / supplied as desired. In particular, the oxygen gas stream may be employed as feed in production of the humidified gas stream, as described above.

[0149] The voltage applied for operation of the cell may be chosen as desired. Advantageously, the voltage applied may be about 1.6 - 1.8V to ensure optimal hydrogen gas production rates. The voltage may be controlled to below 1.8V, in order to maintain a relatively high energy efficiency (e.g. greater than 70%). It will be apparent to a person skilled in the art that these ranges may be different depending on a number of factors such as cell construction, selection of materials, etc.

[0150] During electrolysis, water vapor from the humidified gas stream 18 that does not undergo decomposition at the first catalytic layer 12a, can saturate the ion permeable membrane 16 and move across the membrane 16 to the cathode side 14, where it may condense as liquid water. Over a period of time, this condensation can lead to an increase in the level of water on the cathode side 14. After a certain amount of time of electrolysis, the level of water at the cathode side 14 can reach a predetermined level, at which point the pure / DI water supplied from an external source may no longer be required because there is sufficient water being formed within the cell 10, from supply of the humidified gas stream 18. The water being formed within the cell 10 may be recirculated via a pump, in order to feed the cell 10.

[0151] In the event that the humidified gas stream 18 does not provide a sufficient amount of moisture to keep the membrane 16 hydrated, as well as maintain the water level on the cathode side 14, external water supply to the cathode side 14 may be restarted to compensate for the reduction in water levels at the cathode side 14.

[0152] Furthermore, any residual water that is obtained from optional treatment of the output oxygen gas and hydrogen gas streams, may also be separated and stored to resupply the cathode side 14, if required. This water may also be utilized to prepare potable water for other uses. Thus, the method may be used to generate hydrogen gas as well as oxygen gas and potable water, using water from untreated sources as input feed.

[0153] Stacks formed from individual cells

[0154] As noted briefly above, it is possible to use the cell 10 to form an electrolysis stack. It will be apparent to a person skilled in the art that any number of cells 10 can be combined together, using bipolar cathode and anode flow plates as required, to form an electrolysis stack to increase the hydrogen gas output.

[0155] Furthermore, it will be apparent that the heat generated from an electrolysis stack can be utilized in generation of the humidified gas stream 18. For example, the heat present in the oxygen gas 20, hydrogen gas 22 streams and water present in the cell, can be recovered by passing these streams through a heat exchanger before they are sent to storage containers or subsequent processing steps. By configuring the stack appropriately, it may be possible to obtain the bulk of the heat necessary for vaporization of water, from the waste heat alone. This may result in the minimization or elimination of any external heating required for vaporization.

[0156] System

[0157] The above-described components can be put together to form a system 100 (as shown in Figure 8) that may be used as part of an electrolysis plant. One or more such systems may be installed in the electrolysis plant to generate hydrogen gas on an industrial scale. The system 100 may comprise an electrolysis cell 10 / a stack of electrolysis cells 10 as described above. The system can further comprise a humidified gas generation and delivery system 124 configured to receive water from a water source 104 and a carrier gas from a carrier gas source 102 to generate and supply a humidified gas stream 118 to the cell 10. The humidified gas generation and delivery system 124 may in turn comprise one or more humidified gas generation devices 24 described hereinabove that may be coupled together to deliver the required amount of humidified gas 118 to be input into the cell 10 / stack of cells 10 to meet the operating requirements. The cell 10 / stack of cells 10 may be driven using an electrical subsystem with a DC power supply circuit 108 configured to apply a voltage across the cell 10 to facilitate electrolysis. Generally, the power source 108 will include electric circuitry to connect the cell or stack of cells 10 to a power supply and control DC power supply to the cell(s). In some embodiments the electrical subsystem can include a power supply or the electric subsystem can be connected to an external power supply. For example in an embodiment the electrical subsystem is a DC power converter (running on mains electricity obtained from the grid) configured to supply direct current to the cell 10 / stack of cells 10. An AC power generator coupled to an appropriate converter may also be employed. Depending on the voltage to be applied, the electrical subsystem 108 may also have provisions to adjust voltage. For example, the voltage may be stepped up or down before being applied to the cell 10 / stacks of cells 10. In some forms, the source of electricity supplied to the DC power circuit may also be a standalone power source such as a solar / wind device connected to a battery that is housed within the premises of the plant. In other forms, the power source may be a standalone generator. The system 100 may further comprise one or more heat sinks 110 that are configured to recover heat from the products of the cell (e.g. hydrogen / oxygen gas). Such heat may be supplied to the humidified gas generation and delivery system 124. Similarly, oxygen gas generated from the cell may be used as carrier gas as described above.

[0158] It will be apparent that a number of processes and parameters of such processes are critical to water electrolysis using the proposed system 100. For example, water and carrier gas must be supplied at a desired rate to the humidified gas generation and delivery system 124 in order to generate the desired amount of humidified gas stream 118. Similarly, the humidified gas generation may need to be heated to a desired temperature when it is supplied to the cell 10. Accordingly, a number of sensors may be implemented throughout the system 100 that monitors and controls various parameters across the system. For example, sensors to monitor flow rates of carrier gas, water supplied to the humidified gas generation device 24, flow rate of the humidified gas stream to the cell 10, voltage sensors to determine voltage applied to the cell, temperature sensors to monitor the temperature of the device 24, sensors to measure flow rate of oxygen gas and hydrogen gas generated from the cell 10 etc are some such sensors that may be employed as part of the system. All these sensors can generate input that is fed to a system controller 150 which is able to monitor and control processes parameters associated with water electrolysis. The system controller 150 may house one or more subsystems (150a - 150f). Each of these subsystems may receive the input, calculate whether the desired parameter / s are within the required operating ranges and are configured to trigger signals to correct any deviations from the required operating ranges. For example, one subsystem 150a may be configured to control the flow rate of humidified gas stream. If the flow rate of the humidified gas stream 118 to the cell 10 is lower than required, then the subsystem 150a may trigger a signal that opens up a flow control valve to increase the flow rate of humidified gas stream 118 to the cell. Similarly, another subsystem 150b may be tasked with monitoring and controlling the temperature of the humidified gas generation device 24. If the temperature value detected is below a certain threshold, subsystem 150b may trigger the heating function to increase the temperature. It will be apparent that the system controller 150 can house any number of subsystems as desired.

[0159] Generally speaking, the various components of the system may be located in proximity to each other. For example, the carrier gas source and humidified gas generation and delivery system may be located in proximity to the cell 10 / stacks of cell 10. However, it will be understood that the various components may be located apart as well. For example, it is known that carrier gases such as nitrogen may be delivered from a centralized source of gas for the plant through pipelines. Similarly, water from a lake source may be pumped to a water tank that is located in a separate section of the plant. It will be appreciated that the system controller 150 may also be located away from the cell 10 and other components of the system 100 but still be connected to the various components. For example, the system controller may be wirelessly connected to the various components of the system 100. Alternatively, the system controller 150 may be located in a separate control room and connected through wiring to the various components of the system 100.

[0160] It will be further be appreciated that other components of the system that are not explicitly described herein will be apparent to a person skilled in the art who is familiar with the operation of proton-exchange membrane electrolyser setups.

[0161] Kit and its use A conventional electrolysis cell that utilizes liquid water on both the anode and cathode side may be retrofitted with a kit according to the present invention. This may make the cell suitable for electrolysis using a humidified gas stream. In this manner, the benefits associated with the method and / or system of the present invention may be realized in an existing set up, without the need for significant rework and downtime, resulting in minimal operational disruptions and capital expenditure. The conventional electrolysis cell may be broadly similar in configuration to the electrolysis cell of the present invention, in that it comprises an anode side having an anode diffusion layer and a first catalytic layer in contact with the anode diffusion layer, an ion-permeable membrane configured to be in contact with the first catalytic layer and separating the anode side from a cathode side of the cell which comprises a second catalytic layer. Furthermore, the conventional electrolysis cell may also comprise an asymmetric electrode assembly as discussed above. However, the conventional electrolysis cell will differ from the electrolysis cell of the present invention in that it is not configured to operate using water vapour on one side of the cell and does not comprise a humidified gas generation device configured to generate a humidified gas stream. This component can be provided by the kit. The kit may comprise a humidified gas generation device 24 described above. In some embodiments, the kit may also comprise a replacement anode flow plate (not shown in the Figures) that is configured to supply water vapor to the electrolysis cell and to be in fluid communication with an anode diffusion layer 32 of the conventional electrolysis cell. The replacement anode flow plate may have flow channels comprising a serpentine configuration as described previously.

[0162] In some embodiments, the kit may also comprise a replacement ion-permeable membrane that can be used to replace the existing ion-permeable membrane of the conventional electrolysis cell. The replacement ion-permeable membrane may advantageously comprise a replacement first catalytic layer, e.g. comprising a ruthenium oxide catalyst and an ionomer wherein the percentage weight of ionomer to ruthenium oxide is 8 to 16%. This may be especially advantageous in cases where an existing catalytic layer and / or the ion permeable membrane of the conventional electrolysis cell are nearing end of life / not functioning efficiently.

[0163] In use, a user may simply need to remove the water inlet and cooling water connections to the existing cell, dismantle the anode side of the cell and replace the anode flow plate of the cell with the anode flow plate provided with the kit. In cases where the user wishes to replace the existing first catalytic layer as well, In some cases, the existing membrane comprising the first catalytic layer can be replaced with a replacement membrane comprising the replacement catalytic layer of the present invention. It will be apparent to a person skilled in the art, from the above discussions that, if the first catalytic layer is coated on the anode diffusion layer (rather than the ion-permeable membrane), then the anode diffusion layer can be replaced with a new anode diffusion layer that comprises the desired first catalytic layer. Following this, the remaining components including gasket assembly 30 and flow plate 28 can be fixed in position. Finally, the humidification stream generation device 24 can be connected to the appropriate inlets on the anode flow plate 28. It will be apparent that the accessories required for performing this retrofitting are commonly available and well-known in the field.

[0164] The kit offers a cheap and attractive modification to an existing liquid water based electrolyzer, that can provide the advantages disclosed above.

[0165] It will also be appreciated that when a kit is employed to retrofit one or more cells 10 in a stack, suitable modifications required to the other components of the system may be made. For example, flow controllers that are configured to supply liquid water to the cell 10 may need to be replaced with flow controllers that are configured to supply humidified gas stream 118 to the cell 10. A person skilled in the art will be aware of such required modifications.

[0166] Examples

[0167] Examples of the present invention will now be described.

[0168] Example 1 - Fabrication of the catalyst-coated membrane 16 This example illustrates the fabrication of the ion-permeable membrane 16 comprising a first catalytic layer 12a and the second catalytic layer 14a.

[0169] The ion permeable membrane 16 was fabricated through the following steps:

[0170] Nafion™ NR212 (50 pm) membrane was purchased commercially from Chemours and used as received.

[0171] The first catalytic layer 12a is first coated onto the Nafion™ NR212 substrate. To prepare the mixture required for this layer, RuCT particles (commercially available, 200 - 600 pm in size) is mixed with ionomer (Perfluorinated resin solution containing 5% Nafion™ 1100W in Ethanol) at a percentage weight of 8-16% (i.e. the ionomer content is 8 - 16% by weight of the RuCL), solvent (IPA: H2O = 2:1 by volume) is added to form the catalyst ink at the concentration of 40 mg / mL, then the catalyst ink is dispersed under ultrasonic water bath for 30-60 min at a temperature of 25 °C, then the uniform catalyst ink as obtained is sprayed immediately onto the membrane to form a thin catalyst film. Spray coating was performed manually by a spray gun (TAIWAN, HD-131, 0.2 mm caliber). The spray rate and base temperature are controlled to allow immediate evaporation of the solvent to form uniform catalyst layers. The trimmed NR212 membrane was placed onto a hot plate (RCT basic IKAMAG®) preheated to 98 °C, flattened and secured with masking tape (Silicone adhesive Polyimide, 3M). An active area of 2 X 2 cm2or 5 X 5 cm2was demarcated to be sprayed with the as-formed anode catalyst ink. Spray rate was controlled to allow immediate evaporation of the solvent to form uniform catalyst layers. Then it is dried under 90-98 °C for 15-25 min to form the anode catalyst layer.

[0172] Following this, the second catalytic layer 14a is prepared. Pt / C (60 wt% Pt on carbon EC-300J (obtained from Fuel Cell Store)) is mixed with ionomer (Perfluorinated resin solution containing Nafion™ 1100W) at a percentage weight of 40-60%, solvent (EtOH: H2O = 1:1) is added to form the catalyst ink at the concentration of 20 mg / mL, then the catalyst ink is dispersed under ultrasonic water bath for 30-60 min, then the cathode catalyst film is formed by the same spray coating procedures.

[0173] The membrane 16 is then hot-pressed at 130 °C, 0.7 - 1.0 MPa for 30s.

[0174] To prepare an IrCh catalyst containing membrane, a similar procedure as discussed above for RuCh was employed except that the RuCh was replaced with Core / Shell Ir / IrOxcatalyst (Fuel Cell Store).

[0175] Example 2 - Set up of the hydrogen generation system (on a lab scale)

[0176] This example illustrates the set-up of the cell 10 of the present invention. The setup can comprise an ion-permeable membrane 16 fabricated according to Example 1.

[0177] The bulk of the electrolyzer was made of pure titanium (Ti) and was customized with the anode and cathode flow plates engraved with parallel flow field. Platinized titanium fiber felt (-250 / 265 pm, Fuel Cell Store) was used as anode gas diffusion layer (GDL) and equipped with silicone gasket for anode chamber sealing. Carbon paper (-280 pm, Toray-TGP-H-090) was used as cathode GDL, and for cathode sealing, polytetrafluoroethylene (PTFE) film and fluorinated ethylene propylene (FEP) film were combined also to achieve an appropriate carbon paper compression rate of 25 % in assembly. To make full contact of the components in electrolyzer, an assembly force of 2.5 Nm was applied to the 2 x 2 cm2electrolyzer and 3.5 Nm to the 5 x 5 cm2unit.

[0178] Once the cell 10 is set up, it is connected to a humidified gas generation device on the anode side through a pipeline. The pipeline is maintained at a temperature of around 150 °C by wrapping it with a heating tape. The humidified gas generation device is configured to supply saturated vapor generated at dew points of 60 - 95 °C. The carrier gas employed is nitrogen and this is supplied through a microceramic diffusion stone described above. The carrier gas is injected into the bottom of a water reservoir which is filled to half of its full capacity with water from a desirable source. The cathode side of the cell is connected to a deionized water supply system that provides water at a rate of 80 mL / min.

[0179] In operation of the cell, the water and gaseous products (oxygen / hydrogen) are separated at the respective outlets. Part of the separated water is passed through a heat exchanger to harvest waste heat from the water and is recirculated back to the electrolyser. Another part of the water is collected and stored in a storage tank.

[0180] Example 3 - Electrochemical performance with 11 O2 and Ruth catalysts

[0181] This example demonstrates the performance of an electrolysis cell 10, prepared according to Example 2 and comprising two different catalysts, IrCL and RuCh- Additionally, the catalyst loading for IrCh was set to 3.0 mg / cm2.

[0182] The carrier gas was delivered at a constant rate of 200 - 350 mL / min by a mass flow controller to a micro-ceramic gas diffuser before being bubbled into a tank of the humidified gas generation device. Similarly, the cathode side was supplied with deionized water flow at a rate of 60 - 80 mL / min to maintain the temperature of the electrolyzer.

[0183] Polarization curves were obtained by stepping the voltage from 1.2V to 1.8V with an increment of 10 mV for three times, and each voltage point was held for 30s to record the stabilized electrolysis current. Prior to recording polarization curves, activation of electrodes was performed by running stepwise at small current densities of 100 mA / cm2, 200 mA / cm2and 400 mA / cm2for 10 mins at each point.

[0184] Figure 3(A) reveals the performance of electrolysis cells employing a humidified gas stream 18 on the anode side 12 and using RuCh and IrCh as catalysts.

[0185] The results reveal that using a humidified gas stream allows a higher current density to be obtained at the same voltage for RuCh (due to the RuCh being intrinsically more active than I1O2 ). With saturated vapor supplied at 90 °C under a fixed carrier gas flow rate of 200 mL min-1(at Standard Temperature and Pressure STP conditions), the cell delivered a current density of 4.67 A cm2at 1.8 V with RuCh- based anode, and 3.65 A cm2at 1.8 V with IrCh-based anode. These currents are in the industrially relevant range of 1.00 - 4.00 A cm'2. The results indicate that using a humidified gas stream is more energy efficient for producing the same amount of hydrogen gas, as compared with a conventional liquid water electrolyzer.

[0186] Techno-economic analysis based on the results of this experiment indicate (see Figure 3C) that the electrolyzer using the humidified gas stream and RuCh catalyst consumes approximately 43.6 kWh of energy to produce one kilogram of hydrogen gas. This is relatively lower compared to an electrolyzer using humidified gas stream and IrCh catalyst as well as some commercially available electrolyzers.

[0187] Example 4 - Effect of different catalyst loading in first catalytic layer 12a

[0188] This example illustrates the effect of varying the mass of the catalytically active material (RuCh) in the first catalytic layer 12a on the performance of an electrolysis cell 10 prepared according to Example 2.

[0189] The results (illustrated in Figure 4) demonstrate that there is an optimal loading of approximately 1.0 mg. cm'2of RuCh in the first catalytic layer 12a. Beyond this loading, the increase in current is not significantly different, suggesting that a mass loading of even 1.0 mg. cm'2is sufficient to obtain industrially relevant performance.

[0190] Figure 5 shows the effect of varying the mass of IrCh catalyst in the first catalytic layer 12a. The results demonstrate that increasing catalyst content increases the performance up to a certain level and that the optimum loading for IrCh is around 3.0 mg / cm2offers the best performance.

[0191] Example 5 - Performance of Electrolysis with untreated water sources

[0192] This example illustrates the effect of the various untreated sources of water on the performance of an electrolysis cell 10 prepared according to Example 2. The performance is compared with water from these sources, used in the liquid form on the anode side 12. The results (illustrated in Figures 6a and 6b) show that the use of water vapor from untreated sources still permits operation of the cell 10, whereas using liquid water from impure water sources does not permit operation of the cell 10 (i.e. the required voltage shoots up to very high levels).

[0193] Example 6 - Long term performance and comparison between liquid water electrolysis and vapor-based electrolysis

[0194] This example illustrates the long term performance of a cell prepared according to Example 2 and containing a RuCh catalyst.

[0195] Water vapor was supplied at a temperature of 90 °C to the anode side 12 and liquid water at 90 °C was supplied to the cathode side. The current density was set at 200 mA / cm2. The results (shown in Figure 7A) demonstrate that the cell is stable for up to 2000 hours (at a current density of 0.2 A.cm'2). Furthermore, as can be seen from the inset of Figure 6, the level of ruthenium present in the vapor condensate obtained from anode side outlet, indicates that the dissolution of ruthenium from the catalyst layer is almost non-existent or significantly reduced.

[0196] Figure 7B demonstrates clearly the difference in long term stability offered by the use of water vapor on the anode side as opposed to using liquid water, when tested at industrially relevant current ranges of 1A. cm'2. The cell employing liquid water (WLE) showed a significant decrease in performance after 50 hours of operation (i.e. significantly increased voltage requirements) while the cell using water vapor on the anode side (SVE) demonstrated a performance decay of less than 1 % after 500 hours.

[0197] Example 7 - Operating conditions to avoid back-diffusion

[0198] This example illustrates conditions that may assist in avoiding back-diffusion of water from the cathode side to the anode side in a semi-vapor electrolysis (SVE) set up.

[0199] One way back-diffusion rates were measured in a cell according to Example 2 under static SVE conditions. Static SVE condition refers to a condition in which humidified gas stream is supplied to the anode side and water is supplied to the cathode side, and the cell / electrolyzer is maintained at the operational temperature without any applied potential to drive the reactions. In this regard, the anode is maintained with humidified gas atmosphere, and the relative humidity is controlled by controlling the partial vapor pressure from vapor generation (as shown below in Table 2). The vapor inlet as well as the electrolyzer temperature are controlled by an external water bath. The cathode is circulated with pure water at certain temperatures. The back diffusion rate is obtained by measuring the water decrease at the cathode side over a period of time. Table 2 The vapor generation temperature and anode inlet temperature controlled to obtain specific relative humidity for static SVEs conditions.

[0200] In Figure 9, the electrolyzer is operated at controlled temperatures of 60°C, 70°C, 80°C, and 90°C and the relative humidity of the anode supply is maintained at 0.50, 0.75, and 0.90, respectively. The results (shown in Figure 9) demonstrate that the back-diffusion remains at acceptable levels under static SVE conditions with a measured water migration rate of less than 27.0 mF min-1[STP], which is below the typical value of >55.7 mL min1[STP] observed due to electro-osmosis occurring at practical current densities of over 1.0 A cm'2.

[0201] Variations and modifications may be made to the parts previously described without departing from the spirit or ambit of the disclosure. In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

CLAIMS1. An electrolysis method comprising: generating a humidified gas stream; delivering the humidified gas stream into an electrolysis cell, the cell comprising an anode side, a cathode side and an ion-permeable membrane located between the anode side and the cathode side for conducting hydrogen ions generated at the anode side to the cathode side, wherein the anode side has a first catalytic layer that facilitates formation of oxygen gas and hydrogen ions, the cathode side has a second catalytic layer that facilitates generation of hydrogen gas from the hydrogen ions, and the ion-permeable membrane contacts both the first and second catalytic layers; contacting the humidified gas stream with the first catalytic layer and contacting a portion of the ion-permeable membrane adjacent the cathode side with liquid water; and applying a voltage across the anode and the cathode such that oxygen gas is generated at the anode side and hydrogen gas is generated at the cathode side.

2. A method according to claim 1, further comprising generating the humidified gas stream by heating water obtained from a water source to generate water vapour and mixing said water vapour with a carrier gas.

3. A method according to claim 2, further comprising generating the humidified gas stream by passing the carrier gas through a microceramic diffuser prior to mixing with water that is obtained from the water source and is being heated.

4. A method according to claims 2 or 3, wherein the water source is an impure water source.

5. A method according to any one of claims 2 to 4 wherein the carrier gas is compressed air.

6. A method according to any one of claims 2 to 5, wherein generating the humidified gas stream comprises utilizing, at least partly, waste heat generated from operation of the electrolysis cell.

7. An electrolysis cell for performing water electrolysis, the cell comprising: an anode side having a first catalytic layer that facilitates formation of oxygen gas and hydrogen ions from a humidified gas stream; a cathode side having a second catalytic layer that facilitates generation of hydrogen gas from the hydrogen ions; and an ion-permeable membrane sandwiched between the anode side and the cathode side and being in contact with both the first and second catalytic layers for conducting the hydrogen ions generated at the anode side to the cathode side; the electrolysis cell being configured to be connectable to a humidified gas stream delivery system and a liquid water delivery system to supply a humidified gas stream and liquid water into the electrolysis cell, respectively, wherein the first catalytic layer is arranged to contact the humidified gas stream to generate oxygen gas and the second catalytic layer is arranged to contact the liquid water.

8. An electrolysis cell according to claim 7, wherein a side of the ion- permeable membrane facing the anode side is provided with the first catalytic layer and a side of the ion-permeable membrane facing the cathode side is provided with the second catalytic layer.

9. An electrolysis cell according to claims 7 or 8, wherein the first catalytic layer and / or the second catalytic layer are respectively provided on an anode diffusion layer and a cathode diffusion layer.

10. An electrolysis cell according to any one of claims 7 to 9, further comprising: an anode gasket assembly configured to seal the first catalytic layer on the anode side; and a cathode gasket assembly configured to seal the second catalytic layer on the cathode side; wherein the anode gasket assembly and the cathode gasket assembly comprise different materials.

11. An electrolysis cell according to any one of claims 7 to 10 wherein, the first catalytic layer comprises ruthenium oxide and an ionomer.

12. An electrolysis cell according to claim 11, wherein the percentage weight of ionomer to ruthenium oxide is approximately 8% to 16%.

13. An electrolysis cell according to any one of claims 8 to 12, wherein the second catalytic layer comprises a platinum - carbon black mixture and an ionomer.

14. An electrolysis cell according to claim 13, wherein the percentage weight of the ionomer to carbon black is approximately 60%.

15. A kit for retrofitting an electrolysis cell configured to use a liquid water feed for electrolysis, the electrolysis cell comprising an anode side, a cathode side and an ion permeable membrane located between the anode side and the cathode side for conducting hydrogen ions generated at the anode side to the cathode side, wherein the anode side has an anode diffusion layer and a first catalytic layer in contact with the anode diffusion layer, the first catalytic layer facilitates formation of oxygen gas and hydrogen ions, the cathode side has a second catalytic layer that facilitates generation of hydrogen gas from the hydrogen ions, and the ion permeable membrane arranged to contact both the first and second catalytic layers the kit comprising: a humidified gas stream generation device configured to generate and supply a humidified gas stream into the electrolysis cell.

16. A kit according to claim 15, further comprising a replacement anode flow plate configured to be located in fluid communication with the anode diffusion layer, the replacement anode flow plate comprising flow channels having a serpentine configuration.

17. A kit according to any one of claims 15 or 16, comprising a replacement ion-permeable membrane.

18. A kit according to claim 17, wherein the replacement ion-permeable membrane comprises a replacement first catalytic layer having ruthenium oxide and an ionomer, wherein the percentage weight of ionomer to ruthenium oxide is approximately 8% to 16%.

19. An electrolysis method comprising: delivering liquid water into an electrolysis cell, the cell comprising an anode side having a first catalytic layer that facilitates formation of oxygen gas andhydrogen ions from the liquid water; a cathode side having a second catalytic layer that facilitates generation of hydrogen gas from the hydrogen ions; and an ion-permeable membrane sandwiched between the anode side and the cathode side, and being in contact with both the first and second catalytic layers for conducting the hydrogen ions generated at the anode side to the cathode side, wherein the cell further comprises an anode gasket assembly configured to seal the first catalytic layer on the anode side; a cathode gasket assembly configured to seal the second catalytic layer on the cathode side, and the liquid water stream contacts both the first catalytic layer and the second catalytic layer; and applying a voltage across the anode side and the cathode side to generate oxygen gas at the anode and generate hydrogen gas at the cathode.

20. An electrolysis cell for performing water electrolysis, the electrolysis cell comprises an anode side having a first catalytic layer that facilitates formation of oxygen gas and hydrogen ions from the humidified gas stream; a cathode side having a second catalytic layer that facilitates generation of hydrogen gas from the hydrogen ions; and an ion-permeable membrane sandwiched between the anode side and the cathode side, and being in contact with both the first and second catalytic layers for conducting the hydrogen ions generated at the anode side to the cathode side, wherein the cell further comprises an anode gasket assembly configured to seal the first catalytic layer on the anode side, and a cathode gasket assembly configured to seal the second catalytic layer on the cathode side.

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