WO2022077064A1 - Membrane for hydrogen generation and method of forming same - Google Patents

Abstract

A membrane for water splitting, wherein the membrane is in the form of a hydrogel membrane. The hydrogel membrane may be formed from a hydrophilic hydrogel. A method of forming a hydrogel membrane, including the step of copolymerising acrylate with acrylamide. The method may further include the step of using Ν,Ν'- Methylenebisacrylamide as a cross-linking agent to produce a cross-linked polymeric network.

Description

MEMBRANE FOR HYDROGEN GENERATION AND METHOD OF FORMING SAME

FIELD OF THE INVENTION

The invention relates to a membrane for hydrogen generation and to a method of forming a membrane for hydrogen generation. The membrane may be used in an electrolysis cell. More particularly, but not exclusively, the invention relates to a method and apparatus for the generation of hydrogen and/or oxygen from water with improved conversion efficiency.

BACKGROUND TO THE INVENTION

The previously published documents referred to in this patent specification are incorporated in their entirety herein by reference.

Broadly, the invention relates to the field of electrochemistry, particularly, electrolysis of water (and/or other chemicals such as ammonia) for hydrogen generation. Specifically, the invention relates to hydrogel membranes, their methods of manufacture and to an apparatus for generating hydrogen. Examples of the present invention may be suitable for use in water splitting.

It will be convenient to hereinafter describe a non-limiting aspect of the invention in relation to splitting to produce hydrogen and oxygen. However, it should be appreciated that the broadest scope of the present invention may not be limited to that use and may be used for production of other compounds such as, but not limited to, ammonia with chemical formula NH3, which can be produced from atmospheric nitrogen N2 and hydrogen H2 contained in water or dissociation of ammonia into nitrogen and hydrogen. Examples of the invention may be used with catalysts for methanol and ethanol (and other carbon-based value-add chemicals) generation from carbon dioxide and hydrogen.

Typically, existing water electrolysis (water splitting) processes rely on an electrochemical phenomena where two electrodes are immersed in water, often with the addition of salts to increase electrical conductivity, and are separated by a membrane. Upon supply of electric charge, electrodes (an anode and a cathode) attract oxygen and hydrogen atoms in the water, effectively breaking bonds and allowing for separation of these two gases.

The overall reaction of water splitting, 2H2O — 2H2 + O2 produces oxygen and hydrogen gases as end products. These gases must be kept separate for later individual use and to avoid production of a flammable and potentially explosive gas mixture.

The electrolyser used for water splitting through the means of electrolysis has previously been proposed and typically comprises two electrodes separated by a membrane and immersed in water. Salts may be added to the water to increase electrical conductivity. Passage of an electric current across the electrodes causes water to be reduced by the supply of electrons from the cathode and hydrogen gas is formed. A corresponding oxidation reaction occurs at the anode to generate gaseous oxygen.

A number of electrolyser types have been manufactured and used commercially to date. However, those devices typically rely on the concept of using an aqueous medium as a source of hydrogen and oxygen. Two main types of electrolysers have been broadly used in the past: alkaline electrolysers which typically include an electrolyte in the form of an aqueous alkaline solution and proton exchange membrane (PEM) electrolysers. In recent years, the idea of using solid oxide electrolysers where the membrane is based on oxide (ceramic) materials has been explored. Solid oxide electrolysers allow for high-temperature operation increasing the reaction kinetics and reaction rate. However, all three concepts utilize water in liquid form as a source of hydrogen and oxygen. Membrane electrode assemblies (MEAs) typically have a multi-layered structure comprising the PEM, a current collecting electrode and an electro-catalyst layer on each side.

In traditional electrolysers (both alkaline and PEM), the reaction occurs at a so-called three-phase boundary (sometimes also referred to as a “triple phase boundary” or “TPB” as an alternative) where catalyst, electrolyte and generated gas meet, forming solid - liquid - gas phase. In the preferred example of the present invention, the reaction occurs at a two- phase boundary because the electrolyte is in a solid state (hydrogel) forming a solid - solid - gas boundary. More recently, new types of gas-to-gas reactors have been developed, an example of which is described in International patent publication WO2016/148637 entitled “Electrolysis System”. The concept relies on evaporating water and subsequent separation of hydrogen and oxygen. This concept overcomes a number of limitations associated with traditional water-based electrolysers. In particular, the concept has the advantage that no gas bubbles are formed on the electrode, a phenomenon which over time reduces the active surface of the electrode and effectively decreases the conversion efficiency.

It has been previously proposed by the National Aeronautics and Space Administration to provide a water-vapour electrolysis cell which uses a phosphoric acid electrolyte such that when the cell is operated on recirculated cabin air, dehumidification and generation of oxygen occur simultaneously so as to be suitable for integration in a cabin air-conditioning system. However, the focus of this cell was on the suitability for an air- conditioning system and the applicant has identified that such a cell would not be suitable for the efficient generation of hydrogen.

The applicant has determined that it would be advantageous for there to be provided an improved membrane and apparatus for electrolysis which at least alleviate one or more disadvantages of existing electrolysis systems.

SUMMARY OF INVENTION

In accordance with one aspect of the present invention, there is provided a membrane for water splitting, wherein the membrane is in the form of a hydrogel membrane.

Preferably, the hydrogel membrane is formed from a hydrogel. The hydrogel may be in the form of a porous polymer hydrogel.

In a preferred form, the hydrogel membrane is formed from a hydrogel being a copolymer of acrylate copolymerized with acrylamide. More preferably, the hydrogel is formed with N,N' -Methylenebisacrylamide (molecular formula is C7H10N2O2) used as a cross-linking agent to produce a cross-linked polymeric network. As will be appreciated by those skilled in the art, acrylates are the salts, esters, and conjugate bases of acrylic acid and its derivatives. Altematively, the hydrogel may be formed with N,N,N',N'- Tetramethylethylenediamine used as a cross-linking agent to produce a cross-linked polymeric network.

In another form, the hydrogel membrane is formed from a hydrogel being a copolymer of polyacrylate copolymerized with polyacrylamide.

Preferably, the hydrogel is in the form of sodium polyacrylate or potassium polyacrylate.

In a preferred form, the hydrogel membrane is formed from poly(potassium acrylate- co-acrylamide), a hydrogel being a copolymer of polymerisation of acrylate with acrylamide.

Preferably, the hydrogel membrane is formed from a hydrogel being a hydrophilic material containing a network of polymer chains. The hydrogel may also contain a water dispersion medium.

It is preferred that the hydrogel membrane is formed from a hydrogel being a poly(potassium acrylate-co-acrylamide) and/or poly(sodium acrylate-co-acrylamide) hydrogel.

Preferably, the hydrogel membrane is adapted to serve as both a gas separation membrane and an electrolyte in a hydrogen electrolyser.

In a preferred form, the hydrogel membrane is adapted to have an optimal ratio of acrylate to acrylamide, and an optimal ratio of sodium and/or potassium ions responsible for conductivity.

Preferably, the hydrogel membrane is optimised for mechanical properties of the membrane, water content and/or gas permeation within the membrane.

It is preferred that the membrane is adapted to be a source of water for hydrogen and oxygen production in water splitting. Preferably, the hydrogel membrane is formed from a hydrophilic hydrogel. More preferably, the hydrogel membrane is formed from a hydrophilic hydrogel having water content by weight above 50%. Even more preferably, the hydrogel membrane is formed from a hydrophilic hydrogel having water content by weight above 80%. In an example, the hydrogel membrane is formed from a hydrophilic hydrogel having water content by weight above 90%. In one particular form, the hydrogel membrane is formed from a hydrophilic hydrogel having water content by weight above 95%.

In accordance with another aspect of the present invention, there is provided a method of forming a hydrogel membrane, including the step of copolymerising acrylate with acrylamide. Alternatively, there is provided a method of forming a hydrogel membrane, including the step of copolymerising polyacrylate with polyacrylamide.

Preferably, the method further includes the step of using N,N'- Methylenebisacrylamide as a cross-linking agent to produce a cross-linked polymeric network. More preferably, the method includes the step of subsequently functionalising the hydrogel membrane with sodium hydroxide or potassium hydroxide to increase conductivity of the membrane. Even more preferably, the step of functionalising the hydrogel membrane is achieved by immersing the hydrogel membrane in an aqueous solution of sodium hydroxide or potassium hydroxide.

Alternatively, the method includes the step of using N,N,N',N'- Tetramethylethylenediamine as a cross-linking agent to produce a cross-linked polymeric network.

In a preferred form, the method includes the step of adjusting the ratio between acrylate and acrylamide to vary properties of the hydrogel membrane.

In accordance with another aspect of the present invention, there is provided a method of forming a hydrogel membrane, including the steps of forming an aqueous solution of monomers and cross-linker, and removing gas bubbles from the aqueous solution to achieve uniformity in the hydrogel membrane. In accordance with another aspect of the present invention, there is provided a method of forming a hydrogel membrane for electrolysis, including the step of forming the membrane with a thickness of between 1 mm and 2.5 mm.

Preferably, the method includes the step of forming the membrane with a thickness of between 1.4 mm and 1.8 mm. More preferably, the method includes the step of forming the membrane with a thickness of 1.55 mm.

In accordance with another aspect of the present invention, there is provided an apparatus for water splitting, including an anode, a cathode and hydrogel, wherein the hydrogel is arranged to transport hydroxide ions (OH") from one electrode to the other, to separate product gases and/or to isolate the cathode and/or anode. The hydrogel may be arranged to isolate the cathode and/or anode electrically and/or ionically. At the same time, the hydrogel may serve as an electrolyte to facilitate the reaction.

Preferably, the hydrogel is arranged to act as a source of water for hydrogen and oxygen production. More preferably, the hydrogel is in the form of a hydrogel membrane. Even more preferably, the hydrogel is a hydrophilic hydrogel. In one example, the hydrogel is a highly hydrophilic hydrogel.

In a preferred form, water content in the hydrogel is maintained by hydrating the membrane. Preferably, the apparatus includes a water management system arranged to maintain the water content in the hydrogel.

In accordance with yet another aspect of the present invention, there is provided a membrane for electrolysis of a chemical for hydrogen generation, wherein the membrane is in the form of a hydrogel membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described, by way of non-limiting example, with reference to the accompanying drawings in which:

Figure 1 shows the cell and stack assembly having a hydrogel membrane in accordance with an example of the present invention; Figure 2 shows an example hydrogel gas separation membrane;

Figure 3 shows the stack of electrolytic cells with flow explanation;

Figure 4 shows a graph depicting experimental measurement results of output power consumption versus percentage of rated power capacity;

Figure 5 shows swelling behaviour of poly(acrylamide-co-acrylic acid) hydrogels with 5% crosslinking agent in alkaline medium (pH = 10);

Figure 6 shows swelling behaviour of poly(acrylamide-co-acrylic acid) copolymerised in 70:30 ratio in alkaline medium (pH = 10);

Figure 7 shows swelling behaviour of poly(potassium acrylamide-co-acrylate) and poly(sodium acrylamide-co-acrylate) copolymerised in 70:30 ratio in alkaline medium (pH = 10);

Figure 8 shows absorbency of poly(potassium acrylamide-co-acrylate) and poly(sodium acrylamide-co-acrylate) copolymerised in 70:30 ratio under different temperature conditions in water after (a) 1 hour, (b) 2 hours and (c) 4 hours;

Figure 9 shows absorbency of poly(potassium acrylamide-co-acrylate) and poly(sodium acrylamide-co-acrylate) copolymerised in 70:30 ratio under different pH in water at 20 °C after 2 hours; and

Figure 10 shows average elongation of 20 mm long poly(potassium acrylamide-co- acrylate) copolymerised in 70:30 ratio sample - error bars are standard deviation of five samples.

DETAILED DESCRIPTION

Existing Hydrogels

A hydrogel is a hydrophilic material that contains a network of polymer chains and serves as water dispersion medium. While some hydrogels can contain over 98% of water by weight (Yuchao Wu, Darshil U. Shah, Chenyan Liu, Ziyi Yu, Ji Liu, Xiaohe Ren, Matthew J. Rowland, Chris Abell, Michael H. Ramage, and Oren A. Scherman (2017) Bioinspired supramolecular fibers drawn from a multiphase self-assembled hydrogel. PNAS 114 (31) 8163-8168, https://doi.org/10.1073/pnas.1705380114), due to their properties, including structural integrity, hydrogels are considered solid (sometimes semi-solid) materials. Polymer chains in the hydrogel are cross-linked, meaning that one polymer chain is chemically bonded with other polymer chains.

Like common polymer materials, certain hydrogels can be obtained synthetically, i.e., via various polymerisation routes or from living organisms, i.e., alginic acid, a hydrophilic polysaccharide material extracted from brown algae cells that form a hydrogel when hydrated.

Hydrogels find applications in sensors and biosensors, heat exchange materials, and novel adhesives because of their unique hydrophilic properties, high water content, and high structural integrity. However, most hydrogel applications rely on biodegradability and biocompatibility of certain polymeric networks that form hydrogels to be applied in novel drug delivery systems where specific water-soluble drugs can be captured within hydrogel and slowly released as biodegradable hydrogel degrades in the patient’s body. Due to the biocompatibility of many hydrogels, they find applications in cell culture growth and tissue engineering. Some hydrogels are also used in breast implants, contact lenses, agriculture, where hydrogel is used in arid areas to capture and maintain soil moisture.

Turning to an example of the present invention, the applicant presents poly(potassium acrylate-co-acrylamide) and poly(sodium acrylate-co-acrylamide) hydrogels used in energy applications, specifically water electrolysis. In this example of the invention, poly(acrylamide-co-acrylate) functionalized with sodium and potassium ions are formed into thin membranes and used in the electrolyser where they serve as both a gas separation membrane and electrolyte. The use of hydrogel membranes has various advantages, including a water source for hydrogen and oxygen production, preventing gas bubble formation during electrolysis process, and providing a unique interface (two-phase boundary) with solid catalyst and solid electrolyte and gaseous product meet. This is different from the traditional three-phase boundary systems where electrolyte is in a liquid phase. The materials have been tuned to identify the optimal ratio of acrylate to acrylamide, optimal membrane thickness, and optimal ratio of sodium and/or potassium ions responsible for conductivity. Further membrane optimization focused on membrane’s mechanical properties, water content, and gas permeation within the membrane.

Previous Electrolysis and Membranes

Traditionally, water electrolysis was performed in alkaline electrolyser which utilises an aqueous alkaline electrolyte solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH), and diaphragm membrane to separate gases produced at the cathode and anode while allowing for hydroxide ions (OH“) passage from one electrode to the other. Water electrolysis requires pure water feedstock to obtain high purity output hydrogen and oxygen gases. The presence of potassium and sodium ions dissolved in the electrolyte enables water to conduct electricity while at the same time enabling the electrochemical reaction of water splitting to occur.

PEM electrolysis is a newer concept where a polymer electrolyte membrane is applied. The membrane is responsible for the conduction of protons, electrical insulation of the electrodes and barrier for produced hydrogen and oxygen gases. PEM electrolysers use solid polymeric membrane as electrolyte with water saturated in it. However, the water and membrane in this case are in two different states, solid and liquid (Maria Bass, Amir Berman, Amarjeet Singh, Oleg Konovalov, and Viatcheslav Freger (2010) Surface Structure of Nafion in Vapor and Liquid. J. Phys. Chem. B 2010, 114, 11, 3784- -3790. https://doi.org/10.1021/jp9113128; Qiongjuan Duan, Huaping Wang, and Jay Benziger (2012) Transport of liquid water through Nafion membranes. Journal of Membrane Science, Vol. 392-393, 88-94, https://doi.Org/10.1016/j.memsci.2011.12.004 The most commonly used polymer membrane in PEM electrolysers is available under the brand name Nafion™.

Hydrogel membranes have been previously used in fuel cell applications, where hydrogen and oxygen are combined in electrochemical process producing electricity and water (Nurul A. Choudhury, Jia Ma, Yogeshwar Sahai (2012) High performance and eco-friendly chitosan hydrogel membrane electrolytes for direct borohydride fuel cells. Journal of Power Sources 210, 358- 365; Mehrdad Mashkour, Mostafa Rahimnejad, Mahdi Mashkour, Gholamreza Bakeri, Sang-Eun Oh and Rafael Luquee (2017) Application of Wet Nano-Structured Bacterial Cellulose as a Novel Hydrogel Bio- Anode for Microbial Fuel Cells. ChemElectroChem 10.1002/celc.201600868; N.A. Choudhury, S.K Prashant, S. Pitchumani, P. Sridhar and A.K. Shukla (2009) Poly (vinyl alcohol) hydrogel membrane as electrolyte for direct borohydride fuel cells. J. Chem. Sci., 121 (5), 647- -654; Shuangshuang Yuan, Qunwei Tang, and Benlin He (2014) Three-dimensional hydrogel frameworks for high-temperature proton exchange membrane fuel cells. J Mater Sci 49:5481- -5491 , DOI 10.1007/sl0853-014-8261-9). However, no reports showed application of hydrogels in reverse to fuel cell application, namely water electrolysis. Previous reports show use of hydrogels in other energy generation applications, mainly thermogalvanic low-grade heat energy harvesting devices (Shirui Pu, Yutian Liao, Kyle Chen, Jia Fu, Songlin Zhang, Lurong Ge, Giorgio Conta, Sofia Bouzarif Ting Cheng, Xuejiao Hu, Kang Liu, and Jun Chen (2020) Thermogalvanic Hydrogel for Synchronous Evaporative Cooling and Low-Grade Heat Energy Harvesting. Nano Lett. 20, 5, 3791-3797, https://doi.org/10.1021/acs.nanolett.0c00800).

Attempts to apply various hydrogel membranes in the water purification and treatment industry have also been made, although mainly as proof of concept studies. The mode of operation differs depending on the type of impurities. In some instances, hydrophilic hydrogel allows for the extraction of water, leaving impurities away and subsequent water extraction by hydrogel dehydration. In other cases, water, together with impurities, is absorbed by hydrogel, upon dehydration, extracted water is purified, leaving impurities in the hydrogel.

Publication US20200240028A1 ("Kobayashi") discloses a water splitting device, however it uses a polymeric membrane which does not serve as both a gas separation membrane and a source of water for water splitting. The polymeric membrane described in Kobayashi serves as a gas separation membrane, while a membrane electrode assembly is immersed in aqueous solution “S” which provides a source of water. In other words, the device described in Kobayashi still uses aqueous electrolyte. The hydration of the membrane described in Kobayashi appears to assist in improving the electrical and ionic conductivity of the system.

Drawings

The current invention relates to the synthesis and application of porous polymer hydrogel s-based on a copolymer of acrylate copolymerized with acrylamide (PAAm), with N,N' -Methylenebisacrylamide used as a cross-linking agent to produce a cross-linked polymeric network.

Produced hydrogel membranes are subsequently functionalized with sodium hydroxide (NaOH) or potassium hydroxide (KOH) to increase the membrane’s conductivity. Functionalization is performed by membrane immersion in an aqueous solution of NaOH or KOH.

With reference to Figures 1 to 10, there is shown an example of an improved electrolytic cell apparatus having a special gas separation membrane in the form of a hydrogel membrane. Advantageously, the applicant has determined that the provision of the hydrogel membrane may result in an improved efficiency and lower system maintenance. More specifically, Figure 1 shows on the left-hand side an expanded view depicting each of the components which together form an electrolytic cell 10. To the right-hand- si de of the expanded view there is shown a single, fully assembled electrolytic cell 10 for generating hydrogen and/or oxygen from water in a solid/semi-solid state, using a hydrogel membrane 12. The single assembled electrolytic cell 10 has holes 14 which are used for the delivery of water vapour. Below the fully assembled electrolytic cell 10 is shown a stack of electrolytic cells, expanded to show each of the components in perspective view. At the far right-hand side of Figure 10 is shown in perspective view an example of a multiple (10) stacked cell assembly.

Accordingly, the cell 10 is suitable for use in a method of generating hydrogen and/or oxygen, the method including the steps of evaporating water to produce a water vapour, and subsequently separating hydrogen and oxygen from the evaporated water.

As noted above, Figure 1 also shows the cell 10 in an exploded view depicting the separate components of the cell 10. In particular, the cell includes a gasket 16, a cathode 18 which may be in the form of a nickel mesh electrode (catalyst), the polymer membrane in the form of hydrogel membrane 12, an anode 20 which may be in the form of a nickel mesh electrode (catalyst), a frame, an aluminium corrugated plate 24, and a stainless steel corrugated mesh 26. The frame 22 provides support, while water enters the cell 10 from the left (in the direction that is towards the gasket 16 side of the cell 10) and hydrogen exits the cell 10 to the right (in the direction that is away from the stainless steel corrugated mesh 26). Accordingly, in this way, the cell 10 includes a pair of electrodes 18, 20 separated by the hydrogel membrane 12. Nickel mesh can also be replaced with nickel foam, which provides a more expensive solution but results in higher active surface area and improved performance. In addition, the catalyst (nickel nanoparticles, or nickel-iron nanoparticles, or other nanoparticles) can be deposited on the membrane surface with a prior conductive layer deposited on the membrane for an electrical conductivity purpose. In certain cases, density of sputtered catalyst nanoparticles and their conductivity allows omission of the requirement for an additional conductive layer.

The nickel mesh electrode (catalyst) 18 / polymer membrane 12 / nickel mesh electrode (catalyst) 20 / aluminium corrugated plate 24 / stainless steel corrugated mesh 26 form a separate electrically conductive assembly cell following the sequence as described. The stainless steel corrugated mesh 26 in one electrochemical cell is in close contact with the nickel mesh electrode (catalyst) 18 in the subsequent cell allowing for electric charge to pass through the system. All electrochemical cells are connected in series. The electric charge passes from one copper current collector though all subsequent cells in the system to the copper current collector at the other end of the stack at the same time facilitating the electrolysis reaction. Importantly, this novel construction with the hydrogel membrane leads to increased efficiency and results in superior performance when using a nickel catalyst, as compared to the performance of an electrolyser using platinum (considered the best water splitting catalyst).

Where water vapour is used instead of liquid water to replenish water in the hydrogel used to generate hydrogen and oxygen, an evaporator is used, which is also developed by the applicant. Because of the three-dimensional modular design with multiple vertical cells, each supplying large amounts of water vapour, it is possible to achieve nearly ten times more water vapour than what would normally be achieved from the same area. Advantageously, an integrated evaporation system enables water purification. As such, even sea water can be used directly (although this would require higher maintenance and salt removal). Most electrolysers require very pure water otherwise impurities will reduce the lifetime of the system. Impurities can also block pores in the membranes and reduce the active electrode surface. Further, impurities in feedwater can get into hydrogen/ oxygen output streams. PEM electrolysers require ultrapure deionized water, whereas alkaline electrolysers can work with lower purity water grade.

More specifically, Figure 1 illustrates on the left-hand-side the electrolytic cell 10 expanded to show each of the components. This view shows a printed and cured gasket 16 approximately 0.1 mm thick. The polymer membrane 12 is located between two mesh electrodes 18, 20 comprising a nickel catalyst. A polymer, preferably polypropylene, frame 22 separates the electrodes 18, 20 from a stamped aluminium corrugated plate 24 that facilitates oxygen and water vapour flow in the electrolytic cell 10. The plate 24 is adjacent a stamped stainless steel corrugated mesh 26 that also facilitates oxygen and water vapour flow. The frame 22 serves as a support and provides a means for assembly. The small holes/channels 14 on both sides of the frame 22 allow for water vapour to enter and for excess water vapour plus oxygen generated on the anode (open anode cell) side to leave the system. Both aluminium plate 24 and stainless steel mesh 26 also close the entire system electrically and allow for the charge to flow through from one copper collector to the other, at the same time facilitating the reaction. The applicant has also investigated the effect of using stainless steel instead of aluminium for the corrugated plate and what effect it will have on the system and its performance. This creates the possibility of using cheaper stainless steel for the stamped corrugated plate.

Figure 1 shows a stack 28 of 10 electrolytic cells. The stack 28 is further explained in Figure 3 which illustrates the concept of an open anode cell. Normally, the water vapour inlet side of the cell is at the bottom. By putting one electrolyser stack on top of another, the excess water vapour leaving one stack enters the other stack and is used there. The arrow 30 depicts the water vapour inlet from an evaporation unit, the arrow 32 depicts the flow of excess water vapour mixed with generated oxygen gas. The arrows 34 depict inlet and outlet holes on both sides of each cell 10 (open anode cell concept).

Figure 1 also illustrates the stack 28 of electrolytic cells expanded to show each of the components in perspective view. Fasteners 36, in the non-limiting example as M10 screws, pass through the edges of all elements at their edges to ensure proper sealing of the stack 28. Two thick stainless steel end plates 38, 40 are located at both ends of the stack 28 and may be used to collect gas - particularly hydrogen gas - generated during the electrolytic process. The end plates 38, 40 are present to compress the cells and the entire stack. The end plates 38, 40 must be thick to distribute the pressure - generated by tightened screws - evenly, otherwise there could be potentially a gas (hydrogen) leak. The fasteners 36 are tightened so as to ensure that pressure evenly distributes across the stack 28. Individual electrolytic cells 10 are located between the end plates 38, 40. Plastic pipes 42 are also located at the edges of the electrolytic cells 10 to insulate the fasteners 36 from the electrodes and other metal components of the stack 28.

The small holes 14 on the sides of the stack 28 are used to deliver water or water vapour to the membranes 12 (that is, the membrane 12 of each cell 10) to keep them hydrated. The water may, for example, be pumped or supplied in a passive manner from a reservoir or by any other method known in the art. The holes 14 may also be used for removal of gas, and, in particular, oxygen generated during the electrolytic process. This functionality is important to the operation of the system. It also allows the system to be much simpler and cheaper since it obviates the need for additional channels to remove oxygen in each of the cells. These channels are free-flowing and no additional pump or other means are required to force the oxygen flow and remove it from the system.

Figure 2 illustrates the hydrogel membrane 12 assembled in the electrolysis cells shown in Figure 1.

The membrane 12 is in the form of a polymeric membrane, specifically a hydrogel membrane, with high water-content. The membrane 12 may be hydrated with water such that the membrane forms a source of hydrogen and oxygen to the electrolytic process. The membrane 12 may be hydrated to maintain water content in the membrane 12 during electrolysis. In particular, this may be achieved with water influx into the membrane equivalent to an amount of water used to generate hydrogen and oxygen. In one example, the hydration of the membrane 12 is achieved by calculating the amount of water used based on one or more parameters including temperature, input voltage, input current, and/or hydrogen flow output, and by circulating liquid water through the cell 10 accordingly. Alternatively, the hydration may be achieved by introducing water to the membrane 12 in the gaseous phase. The hydrogel membrane 12 may form the electrolyte for the electrolysis process. More specifically, the membrane 12 may act as an electrolyte, the membrane 12 including a hydrophilic polymer, and water absorbed by the polymer being electrolysed when an electric current passing through the electric circuit. In this way, an example of the invention provides a method of electrolysing water including a first step of locating a hydrophilic polymer membrane between an anode and a cathode of an electric circuit, a second step of absorbing water into the hydrophilic polymer membrane, and a third step of passing an electric current through the electric circuit such that the water in the hydrophilic polymer membrane is reduced to hydrogen at the cathode and oxygen at the anode.

Owing to the nature of the hydrogel membrane, in the example of the invention as described above, this provides a solid/semi-solid membrane electrolyser and a method of electrolysis for gas synthesis. Examples of the invention may seek to improve the efficiency of electrolytic reactions used for synthesis.

In one example of the invention, the gas produced at the electrodes diffuses out of the cell 10 via the polymer membrane 12, separating the gas from the reaction at the electrode. Ideally, the gas is separated without gas bubble formation on the electrode. Avoiding bubble formation permits reactions such as water splitting to be achieved with a low over-potential, thereby contributing to the efficiency of the electrolytic cell 10.

Advantageously, hydrophilic hydrogel is used as a solid/semi-solid membrane and electrolyte in one. It allows separation of produced gases and, at the same time, being a source of water to generate hydrogen and oxygen. Water in the hydrogel is replenished either by supplying liquid water through the electrolyser channels or by supplying water vapour through special inlet holes on the side of the electrolyser cells 10. The opposite side of each of the cells have similar holes to remove the excess water vapour as well as produced oxygen. In the applicant’s arrangement, the anode side where oxygen is produced is referred to as the “open cell” or “open anode cell” which allows oxygen to be easily removed. This simplifies the entire system as well as all the processes.

Although there has been some discussion previously of water vapour electrolysis, the applicant has identified that it would be beneficial to actually have water vapour as a means of water delivery to a membrane, with the reaction occurring at a two-phase boundary (solid- solid-gas) instead of a three-phase boundary (solid-gas-liquid) as in traditional electrolysers.

Turning to Figure 4, there is shown experimental results for five measurements of how power consumption changes according to percentage of electrolyser’s rated power capacity. A 12-kW system (100-cell stack) has a power consumption of 3.5 to 4.4 kWh/Nm³ H2 depending on the operation mode, and applied power, as well as energy consumption of auxiliary systems, processes, i.e., control system pumps, etc.

Figures 5 to 7 show Swelling versus Time graphs for different materials and conditions (as labelled), Figures 8 and 9 show Absorbency versus Temperature and pH, and Figure 10 shows Elongation versus Force. More specifically, Figure 5 shows swelling behaviour of poly(acrylamide-co-acrylic acid) hydrogels with 5% crosslinking agent in alkaline medium (pH = 10); Figure 6 shows swelling behaviour of poly(acrylamide-co- acrylic acid) copolymerised in 70/30 ratio in alkaline medium (pH = 10); Figure 7 shows swelling behaviour of poly(potassium acrylamide-co-acrylic acid) and poly(sodium acrylamide-co-acrylic acid) copolymerised in 70/30 ratio in alkaline medium (pH = 10); Figure 8 shows absorbency of poly(potassium acrylamide-co-acrylic acid) and poly(sodium acrylamide-co-acrylic acid) under different temperature conditions in water after (a) 1 hour, (b) 2 hours and (c) 4 hours; Figure 9 shows absorbency of poly(potassium acrylamide-co- acrylic acid) and poly(sodium acrylamide-co-acrylic acid) under different pH in water at 20 °C after 2 hours; and Figure 10 shows average elongation of 20 mm long poly(potassium acrylamide-co-acrylic acid) sample - error bars are standard deviation of five samples.

As can be seen in the exploded view of Figure 1, each cell 10 of the example depicted includes a printed and cured gasket 16 (0.1 mm in thickness in the non-limiting example depicted/described), a cathode in the form of a nickel mesh electrode (catalyst) 18, a special gas separation membrane manufactured in-house 12, an anode in the form of a nickel mesh electrode (catalyst) 20, a polypropylene frame 22, a second printed and cured gasket 44 (thickness 0.1 mm), a stamped aluminium corrugated plate to facilitate oxygen and water vapour flow 24, and a stamped stainless steel corrugated mesh to facilitate oxygen and water vapour flow 26. These components are pressed together as a single cell assembly as shown by reference numeral 10 in Figure 1. In turn, the single cell assemblies 10 are combined and stacked together into a stack 28 as shown in exploded view in Figure 1 and as an assembly on the far right-hand- si de of Figure 1. The components of the stack 28 of 10 cells includes two tube fittings/hydrogen outlets 46, a thick stainless steel end plate 38 to ensure equal pressure distribution by screws and to collect produced hydrogen gas, two rubber insulators 48 separating end plates from current collectors, two copper current collectors 50 at each end, single cell assemblies 10 (as shown above), plastic pipes 42 to separate fasteners (screws) from electrodes and other metal components of the stack 28, (20 x M10 screws in the non-limiting example depicted/described) 36 to ensure proper sealing of the stack 28 (screws can be larger if needed), and a second stainless steel end plate 40.

Further information on the membrane and on hydrogels is provided below.

Hydrophilic Polymer Membrane

The hydrophilic polymer membrane can absorb a large proportion of water rapidly and has a robust physical structure. Preferably, the hydrophilic polymer membrane is a hydrogel, a macromolecular polymer gel which can hydrogen-bond molecules of water within interstices located throughout a network of cross-linked polymer chains. The water content in the polymer membrane can be as high as 98 wt% of the polymer, but typically the water content is at least 90 wt%. The electrolytic splitting is effectively carried out in a hydrogel environment.

Hydrophilic polymers typically include charged functional groups. In a preferred embodiment, the hydrophilic polymer of the present invention is chosen from the group comprising acrylic acid, acrylamide, maleic anhydride, polyacrylic acid, polyacrylamide, polyvinyl alcohol polymers and copolymers thereof. The membrane may comprise one or more polymers. The main composition of the membrane is a copolymer of poly(potassium acrylate-co-acrylamide) with some other ingredients to reinforce the mechanical strength or tune the hydrophilicity.

During electrolysis, water in the polymer membrane is converted into hydrogen and oxygen and it will be necessary to compensate for the water loss. The polymer membrane must be adequately hydrated at all times during electrolysis because dehydration can adversely affect the chemical and mechanical properties of the membrane, potentially causing the membrane to break or wear off. As such, water influx into the membrane preferably balances the amount of water converted into hydrogen and oxygen.

A number of water management strategies can be used to ensure constant hydration of the polymer membrane at a desired level. In a preferred embodiment, the desired level of water can be calculated by a simple mathematical model based on parameters such as temperature, voltage and current of the electric circuit, or hydrogen flow output.

In a preferred embodiment, the membrane is hydrated by circulating liquid water through the cell or by introducing water vapour to the polymer membrane. Other suitable hydration methods may also be used. For example, patent publication EP2463407A entitled "Electrolysis method and electrolysis cells" in the name of Astrium GmbH describes pumping water into microchannels in a hydrophobic membrane. Patent publication US2014224668A1 entitled "Method for operating an electrolytic cell" in the name of Astrium GmbH describes a hydrophobic membrane for electrolytic water splitting, the membrane being supplied with liquid water in a passive manner from a reservoir without using a pump. Water from the reservoir may pass by capillary effect via at least one cavity structure in the membrane.

As will be apparent to a person skilled in the art, an electrolytic system may comprise a plurality of connected electrolytic cells, each cell being in accordance with the present invention. Each electrolytic cell may have separate or common water feeds. In particular, single electrolytic cells of the present invention can be stacked in a manner well known in the prior art to increase the power capacity or hydrogen generation.

The polymer membrane can be made by any convenient means known in the art for constructing membranes, such as polymer moulding or phase inversion technique. Polymer moulding is appropriate for small scale and prototype applications, whereas the phase inversion technique may be used with a support layer so that the entire process can be automated. In a preferred embodiment the polymer precursor in liquid/semi-liquid form is injected into customized stainless steel or ceramic moulds. The polymer is then subjected to a predetermined process at desired temperatures, pressures and cycle times. The electrolytic cell of the present invention may additionally comprise a model catalyst associated with the hydrophilic polymer membrane. The catalyst may be deposited upon the porous membrane.

Generally, precious metals such as platinum, gold or palladium are used for water splitting. However, the electrolytic cell and method of the present invention may be used in association with less expensive and non-precious catalysts, such as nickel and manganese- based catalysts. The present invention may include incorporation of different catalysts and different chemistries, and may be used for production of compounds other than hydrogen and oxygen. This could include, for example, ammonia (from water and atmospheric nitrogen) or methane, methanol or ethanol from water and carbon dioxide. The applicant foresees that it may also be possible to utilise substances having longer carbon chains.

Significant Improvements in the Membrane

Improvements relate to the use of highly hydrophilic hydrogel membranes for water splitting. The use of hydrogel as a membrane but also the source of water for hydrogen and oxygen production allows for innovative and efficient water splitting. Traditional electrolysis is performed in so-called three-phase (gas - liquid - solid) boundary (TPB) where catalyst, electrolyte, and produced gas meet and react. A hydrogel is a solid/semi-solid material with very high-water contents by weight, sometimes higher than 95%. In the current invention, electrolysis is performed in a two-phase (solid - solid/semi-solid - gas) boundary, where solid-state catalyst, solid/semi-solid electrolyte, and gas meet. This allows for more efficient operation and overcomes issues of producing gas bubbles, which over time, significantly reduce the active electrode surface. The water content in the membrane is maintained by hydrating the membrane via the water management system.

Various hydrogel membranes with different hydrophilicity and water content by weight have been developed and tested and used. The best performing membrane identified to date is a copolymer of poly(potassium acrylate-co-acrylamide), however, other hydrogel membrane have also been tested and proven suitable. Hydrogels

It appears there is no prior application of hydrophilic hydrogel membranes for water splitting reaction. There are, however, some reports where hydrogel was used in batteries and fuel cells but for electrolytes rather than as membranes.

In contrast, the applicant’s system achieves solid/semi-solid state water splitting, since hydrogels can be considered to be solid or semi-solid materials due to their solid rather than liquid behaviour.

The applicant has determined that hydrogel materials are of benefit for electrolysis because their structures consist of a cross-linked network of polymer chains having interstitial spaces filled with solvent water. Hydrogels are thus both wet and soft, making them ideal candidates for electrolyte materials in flexible energy storage devices, and rechargeable batteries.

Hydrogel is a gel in which the swelling agent is water. The network component of a hydrogel is usually a cross-linked polymer network. More specifically, a hydrogel is a network of polymer chains that are hydrophilic, sometimes found as a colloidal gel in which water is the dispersion medium. A three-dimensional solid results from the hydrophilic polymer chains being held together by cross-links. The structural integrity of the hydrogel network does not dissolve from the high concentration of water because of the inherent cross-links. Hydrogels are highly absorbent (they can contain over 90% - or even 98% - water) natural or synthetic polymeric networks. Hydrogels also possess a degree of flexibility very similar to natural tissue, due to their significant water content. As responsive "smart materials," hydrogels can encapsulate chemical systems which upon stimulation by external factors such as a change of pH may cause specific compounds such as glucose to be liberated to the environment, in most cases by a gel-sol transition to the liquid state. Chemomechanical polymers are mostly also hydrogels, which upon stimulation change their volume and can serve as actuators or sensors.

A hydrogel usually consists of water-swollen polymers (long molecular chains of repeating units, also called “mers”) that are “cross-linked” / attached to each other forming a three-dimensional network. This network traps a large volume of water, resulting in a solid/semi-solid material consisting of mostly water. While many polymers that form hydrogels - especially those natural ones - can be water soluble, cross-linking makes the network more robust and prevents those networks from being water soluble.

Particular hydrogels tested for water splitting

Various hydrogels have been tested for their usefulness in the method of water splitting according to the present invention. Hydrogels that have been tested include crosslinked polymers based on acrylic acid, acrylamide, maleic anhydride, polyacrylic acid, polyacrylamide, polyvinyl alcohol polymers and copolymers thereof.

The best performing hydrogels were found to be based on cross-linked poly(potassium acrylate-co-acrylamide), followed by poly(sodium acrylate-co-acrylamide), both cross-linked using N,N' -Methylenebisacrylamide (a cross-linking agent used during the formation of polymers such as polyacrylamide).

Other tested hydrogels include biopolymer-based hydrogels such as alginate, carrageenan and xanthan gum, as well as their potassium and sodium salt compounds, i.e., potassium alginate, sodium carrageenan, etc. Both carrageenan and xanthan gum are natural polysaccharides produced by red and purple seaweeds, and bacterial fermentation, respectively. Glutaraldehyde, sodium sulfate, and potassium sulfate have been used as cross-linking agents.

Alginic acid is a polysaccharide distributed widely in the cell walls of brown algae that is hydrophilic and forms a viscous gum when hydrated. With metals such as sodium and calcium, its salts are known as alginates.

Hydrogels of sodium alginate and potassium alginate have been tested for hydrogen generation. The presence of metal ions enhances ionic conductivity of the hydrogel membrane.

Some other tested hydrogels include commercially available products like AmGel hydrogels (AG900 Series and AG2500 Series) by Axelgaard Manufacturing Co., Ltd. Other hydrogels might be suitable as well, however their practical applicability for this particular application (water splitting and hydrogen generation) still needs to be evaluated.

(i) Polyacrylates

Both potassium polyacrylate and sodium polyacrylate with a chemical formula of [- CH2- CH(CO2K)- ]_n and [-CH2-CH(CO2Na)-]_n, respectively, are known water absorbent polymers capable of absorbing hundreds times its original weight in purified water. Both polymers are based on respective salts of acrylate, a monomer used to produce polyacrylate. As explained above, alkaline electrolysers use an aqueous solution of KOH (and sometimes NaOH) as electrolytes to increase conductivity. In the current invention, gas separation membrane, electrolyte, and salt (used to increase conductivity) are all captured in one solid/semi-solid material. Polyacrylates are also classified as anionic polyelectrolytes because they form ionic bonds with monovalent and divalent metal cations, including bonds with monovalent Na⁺ and K⁺ cations.

The composition of the hydrogel may vary by adjusting the ratio between polyacrylate and polyacrylamide. Varying the composition changes the properties of the materials, including its mechanical strength, water uptake, hydrophilicity, flexibility, gas separation properties of the material, among other factors. Preferably, the electrolyser's membrane material should have good mechanical and thermal properties, high hydrophilicity and water uptake, high hydrogen, and oxygen gas barriers, and high conductivity. In reality, changing composition results in each of those properties becoming more or less favorable. As such, finding an optimal ratio of the polymer is necessary.

The application of sodium polyacrylate or potassium polyacrylate hydrogel membranes could be sufficient for the application in water electrolysis. However, copolymerisation with polyacrylamide gives more preferential properties.

(ii) Polyacrylamide

Polyacrylamide is a polymer with a chemical formula of [-CH2CHCONH2-]. The polymer can be polymerized into chains or cross-linked, typically using N,N'- Methylenebisacrylamide. Polyacrylamide is highly hydrophilic and water absorbent. When hydrated, polyacrylamide forms hydrogel used in electrophoresis, contact lenses, and many other applications. Polyacrylamide is often used to form or stabilize gels and as a thickening or clarifying agent. Polyacrylamide, just like polyacrylate, can form ionic bonds with cationic metals such as sodium and potassium.

Polymerisation of acrylamaide with N,N' -Methylenebisacrylamide as a cross-linking agent using the synthetic procedure described herein. The acrylamide monomer was dissolved in deionized water. The water content was fixed at 85 wt%. Ammonium persulphate as a photo-initiator and N,N' -Methylenebisacrylamide as the crosslinker for polyacrylamide were added into an aqueous solution of acrylamide. The resulting solution was degassed in a vacuum chamber. Degassing was followed by the addition of N,N,N',N'- Tetramethylethylenediamine, as the cross-linking accelerator for polyacrylamide. The resulting gel was cured in a transparent airtight container with ultraviolet light for one hour, after which gel was left for one day to finalize reactions. The concentration of cross-linking agent varied 5 to 20 mole%. However, 13 mole%, together with the addition of 1.5 mole% of N,N,N',N'-Tetram ethyl ethylenediamine was shown to provide the most optimal performance.

(iii) Poly(potassium acrylate-co-acrylamide) and poly(sodium acrylate-co- acrylamide)

Both polyacrylate and polyacrylamide contribute their unique properties to resulting poly(potassium acrylate-co-acrylamide) and poly(sodium acrylate-co-acrylamide) as well as unique properties arising from copolymerization. Generally, both polymers are hydrophilic and can absorb significant water amounts by weight. Polyacrylate can generally capture and contain a higher water percentage. In addition, both polymers can form ionic bonds with metal cations such as potassium or sodium, resulting in increased conductivity and other electrochemical properties. Besides, acrylamide serves as a stabilising agent and thickener, allowing for improved mechanical and thermal stability.

A study from 2012 published in Nature by Sun et al. (Jeong-Yun Sun, Xuanhe Zhao, Widusha R. K. Illeperuma, Ovijit Chaudhuri, Kyu Hwan Oh, David J. Mooney, Joost J. Vlassak, and Zhigang Suo (2012) Highly stretchable and tough hydrogels. Nature 489, 133-136, doi:10.1038/nature 11409) have shown that copolymerisation of two types of cross-linked polymers can result in new properties. In this study, authors copolymerised ionically crosslinked alginate, and covalently cross-linked polyacrylamide resulting in a hydrogel material with superior stretchability and mechanical strength.

Acrylic acid used as polyacrylate precursor was dissolved in distilled water. Subsequently, the acrylamide monomer was added to the solution so that the water content was fixed at 85 wt%. The acrylamide to acrylic acid ratio varied from 10: 1 to 0.2: 1. However, the optimal ratio in terms of mechanical, thermal, and water retention properties found to be 2.3: 1. Ammonium persulphate as a photo-initiator and N,N'- Methylenebisacrylamide as the cross-linker were added into an aqueous solution of acrylamide and acrylic acid. The resulting solution was degassed in a vacuum chamber. Degassing was followed by the addition of N,N,N',N'-Tetramethylethylenediamine, as the cross-linking accelerator for polyacrylamide. The resulting gel was cured in an airtight glass container with ultraviolet light for one hour, after which gel was left for one day to finalize reactions. The concentration of cross-linking agent varied 5 to 20 mole%. However, 15 mole%, together with the addition of 1.7 mole% of N,N,N',N'-Tetramethylethylenediamine showed to provide the most optimal synthesis performance. Although it is described above that N,N,N',N'-Tetram ethyl ethylenediamine may be used as an agent for cross-linking of polymers, the applicant has identified that other agents may be used as alternatives.

(iv) Polymer functionalization using sodium hydroxide or potassium hydroxide

Functionalisation of polymers with potassium hydroxide and sodium hydroxide was performed by placing them for extended periods (3 hours to overnight) in alkaline solutions of potassium hydroxide or sodium hydroxide to enable ionic bond formation between the polymer film and metal cation. Molar concentration in aqueous solution varied from 0.1 to 1 M KOH or NaOH. However, higher molar concentrations proved to be more effective, in terms of time required and cation absorption into the hydrogel. The concentration of potassium or sodium in the resulting polymers varied between 10 to 30% of the final polymer weight. Obtaining a higher percentage is also doable. Generally, higher potassium or sodium concentrations showed to be preferable for described application as a high concentration of metal ions significantly increases materials conductivity. Polymers with potassium showed superior electrochemical performance over those with sodium ions.

(v) Membrane formation and membrane thickness

Achieving membrane uniformity in terms of composition and thickness is a crucial aim. In addition, gas bubbles can be suspended in a concentrated aqueous solution of monomers and cross-linker. Removal of those gas bubbles is essential in achieving uniformity of resulting membrane.

To date polymerisation has been performed in a sealed container in the absence of oxygen and using ultraviolet light curing, by which ultraviolet light initiates a photochemical reaction that generates a cross-linked network of polymers. Alternative synthesis routes can be explored. For example, inverse-suspension polymerization ammonium persulfate as initiator and N,N'-Methylenebisacrylamide. (K Mohana Raju, and M Padmanabha Raju (2001) Synthesis of novel superab sorbing copolymers for agricultural and horticultural applications. Polymer International, https://doi.org/! 0.1002/pi.721).

Measurements and optimisation found that for water electrolysis application, the most optimal membrane thickness range in terms of mechanical and electrochemical properties was between 1 to 2.5 mm, and preferably 1.4 and 1.8 mm, and further preferably 1.55 mm.

Examples and Variations

In a preferred example, the electrolytic cell 10 and method of the present invention is used for synthesis. In another preferred example, the cell 10 forms part of a fuel cell or a fuel cell system. For example, the electrolysis cell 10 may produce hydrogen through water splitting, then hydrogen is stored in a storage tank, and fuel cells use hydrogen to power a car or other appliances.

The electrolytic cell 10 and method of the present invention may be used for water splitting to synthesise hydrogen and oxygen, but it will be readily apparent to a person skilled in the art that other gases could be synthesised by adjusting the catalyst and/or chemistry of the process. For example, ammonia could be synthesised. The applicant has conducted initial work on methanol production using the same setup but with a different catalyst. Other reactions (such as using carbon dioxide to form formic acid, carbon monoxide or formaldehyde; or halide oxidation to halide gas; or hydrogen peroxide oxidation to oxygen gas; or nitrite reduction to nitrous oxide gas or ammonia gas) could possibly be achieved with the current setup and the same or similar hydrogel membrane 12, although the catalyst would most likely have to be changed for each of the reactions, as nickel may not be suitable or the most efficient catalyst to facilitate each of those reactions.

The applicant also foresees that to produce ammonia, it is more efficient and industrially more feasible to have two units (a two-step reaction process) where the first unit generates hydrogen from water and supplies this hydrogen to the second similar unit (with different catalyst) to combine it electrochemically with nitrogen. Using a two-step reaction process may be preferable and more efficient than using one system where water and nitrogen are supplied to generate ammonia in a single step. More specifically, performing a reaction of N2+3H2 -> 2NH3 may be less complex than performing a reaction of 2N2 + 6H2O -> 4NH3 + 302. However, generating ammonia (NH3) (and oxygen) from water and atmospheric nitrogen in a single step may be a state of the art and industrially preferred solution since it would be a single device (single step) rather than a two-step process. Catalysts that allow for taking water and nitrogen and converting them directly to ammonia (and oxygen) are still relatively inefficient.

In essence, examples of the present invention stem from the realization that the separation membrane of an electrolytic cell, in addition to separating gasses, may also be a source of reactant. For example, the membrane that separates oxygen and hydrogen formed during electrochemical splitting of water may also be the source of water for the electrochemical splitting.

Competitive advantages

The present invention shows superior performance over aqueous KOH electrolytes used in alkaline electrolysers and Nafion™ membranes used in proton exchange membrane electrolysers. Other advantages include: • Improved efficiency;

• Integration of a gas separation membrane and electrolyte / water source in one material (hydrogel membrane);

• Smart chemical route to capturing metal cations in a polymeric material, which are essential for high conductivity;

• High water intake up to 93% of water by weight (compared to polymer weight) of the membrane allowing for uninterrupted hydrogen and/or oxygen production;

• Formation of effective gas separation membrane which allows separating two gases;

• Lack of liquid phase preventing gas bubble formation during the electrolysis process;

• Simple manufacturing process and use of common polymeric materials;

• High mechanical and thermal properties of the membrane, same time high flexibility and stretchability;

• Cost competitive alternative to Nafion™ membranes used in PEM electrolysers;

• Novel two-phase boundary water splitting methodology;

• Continuous performance of the system without efficiency drop due to gas bubble formation in the electrode surface. Overcoming the bubble gas formation that is often present in water-based electrolysis. Where gas bubbles (hydrogen and oxygen) build up on the electrode, these effectively reduce the conversion efficiency;

• Possible high-temperature operation, no cooling requirement, the system uses waste heat to further reduce activation losses and increase the conversion efficiency;

• Low maintenance;

• Possibility of incorporating and capturing ions in the membrane which increase membrane’s ionic conductivity, i.e., sodium ions in sodium acrylate, or potassium ions in potassium acrylate; the process allows for superior power conversion (balance of plant power conversion), in some cases, as low as 3.5 kWh/Nm3 of hydrogen, which translates to the conversion efficiency of about 85.6%;

• Simplified structure and assembly processes, allowing for the use of inexpensive materials and manufacturing processes, effectively reducing the system cost;

• Allows for the use of inexpensive and non-precious catalysts whilst at the same time achieving superior efficiencies;

• Excellent hydrogen purity (up to 99.999%); • Possibility of using lower purity water feedstocks - in some instances, possible use of any water feedstock, including tap water, rainwater or even seawater;

• No electrolyte requirement, water is replenished in the membrane;

• Compact design - small footprint and scalability; and

• Relies on electrochemical principles, hence the ability for quick turn on and off, as well as quick ramp up and ramp down.

Accordingly, it will be understood from the foregoing that the invention provides a novel type of electrolyser / gas reactor, where input gas or gases are electrochemically converted into other gases (products). One example would be water vapour being converted into hydrogen and oxygen. In another instance, hydrogen and nitrogen are electrochemically converted into ammonia, whereas in the third instance, conversion of carbon dioxide and hydrogen is also possible. Other conversions are also possible, for example electrochemical reduction of carbon dioxide to methanol, ethanol and other carbon-based value-added chemicals. Catalysts and chemistries of this invention can be tailored to achieve specific outcomes or products. The developed gas reactor can work at room temperatures and temperatures of 100 °C or above. The system utilizes a novel approach whereby hydrogel with water content of up to 95 wt% is used as a gas separation membrane, and also, in the case of water splitting, to generate hydrogen, the hydrogel is used as a source of water feedstock.

Inventive elements include:

• the overall system design;

• solid/semi-solid water splitting principle with water supplied from a novel hydrogel membrane;

• the use of water vapour instead of liquid water;

• novel water treatment and water management system;

• unique hydrogel membrane with water contents of up to 95 wt% as a source of the feedstock for water splitting; and

• novel cell design (an open anode concept), which allows for water feedstock delivery and efficient removal of oxygen from the system. The concept relies on the evaporation of water and the consequent separation of hydrogen and oxygen. This solution allows overcoming a number of limitations that are present in traditional water-based electrolysers, namely no gas bubble formation is present on the electrodes which would otherwise over time reduce the active surface of the electrode and effectively decrease the conversion efficiency. The applicant determined that dealing with water vapour proved to be much easier than dealing with liquid water, especially when forcing the hydrogen-rich medium through very narrow channels present in the electrolyser cells.

While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not by way of limitation. It will be apparent to a person skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the present invention should not be limited by any of the above described exemplary embodiments. The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A membrane for water splitting, wherein the membrane is in the form of a hydrogel membrane.

2. A membrane as claimed in claim 1, wherein the hydrogel membrane is formed from a hydrophilic hydrogel.

3. A membrane as claimed in claim 1, wherein the hydrogel membrane is formed from a hydrogel being a copolymer of acrylate copolymerized with acrylamide.

4. A membrane as claimed in claim 3, wherein the hydrogel is formed with N,N'- Methylenebisacrylamide used as a cross-linking agent to produce a cross-linked polymeric network.

5. A membrane as claimed in claim 3, wherein the hydrogel is formed with N,N,N',N'-Tetram ethyl ethylenediamine used as a cross-linking agent to produce a cross-linked polymeric network.

6. A membrane as claimed in claim 1, wherein the hydrogel is in the form of sodium polyacrylate or potassium polyacrylate.

7. A membrane as claimed in claim 1, wherein the hydrogel membrane is formed from a hydrogel being a copolymer of poly(sodium acrylate-co-acrylamide).

8. A membrane as claimed in claim 1, wherein the hydrogel membrane is formed from a hydrogel being a hydrophilic material containing a network of polymer chains.

9. A membrane as claimed in claim 1, wherein the hydrogel membrane is formed from a hydrogel being a copolymer of poly(potassium acrylate-co-acrylamide). A membrane as claimed in any one of claims 1 to 9, wherein the hydrogel membrane is adapted to serve as both a gas separation membrane and an electrolyte in a hydrogen electrolyser. A membrane as claimed in claim 1, wherein the hydrogel membrane is adapted to have an optimal ratio of acrylate to acrylamide, and an optimal ratio of sodium and/or potassium ions responsible for conductivity. A membrane as claimed in claim 1, wherein the hydrogel membrane is optimised for mechanical properties of the membrane, water content and/or gas permeation within the membrane. A membrane as claimed in any one of claims 1 to 12, wherein the membrane is adapted to be a source of water for hydrogen and oxygen production in water splitting. A membrane as claimed in claim 1, wherein the hydrogel membrane is formed from a hydrophilic hydrogel. A membrane as claimed in claim 14, wherein the hydrogel membrane is formed from a hydrophilic hydrogel having water content by weight above 50%. A membrane as claimed in claim 15 wherein the hydrogel membrane is formed from a hydrophilic hydrogel having water content by weight above 80%. A membrane as claimed in claim 16, wherein the hydrogel membrane is formed from a hydrophilic hydrogel having water content by weight above 90%. A membrane as claimed in claim 17, wherein the hydrogel membrane is formed from a hydrophilic hydrogel having water content by weight above 95%. A method of forming a hydrogel membrane, including the step of copolymerising acrylate with acrylamide to form poly(acrylamide-co-acrylic acid). A method of forming a hydrogel membrane as claimed in claim 19, further including the step of using N,N' -Methylenebisacrylamide as a cross-linking agent to produce a cross-linked polymeric network. A method of forming a hydrogel membrane as claimed in claim 19, further including the step of using N,N,N',N'-Tetramethylethylenediamine as a crosslinking agent to produce a cross-linked polymeric network. A method of forming a hydrogel membrane as claimed in claim 20 or 21, further including the step of subsequently functionalising the hydrogel membrane with sodium hydroxide or potassium hydroxide to increase conductivity of the membrane. A method of forming a hydrogel membrane as claimed in claim 22, wherein the step of functionalising the hydrogel membrane is achieved by immersing the hydrogel membrane in an aqueous solution of sodium hydroxide or potassium hydroxide. A method of forming a hydrogel membrane as claimed in any one of claims 19 to 23, including the step of adjusting the ratio between acrylate and acrylamide to vary properties of the hydrogel membrane. A method of forming a hydrogel membrane, including the steps of forming an aqueous solution of monomers and cross-linker, and removing gas bubbles from the aqueous solution to achieve uniformity in the hydrogel membrane. A method of forming a hydrogel membrane for electrolysis, including the step of forming the membrane with a thickness of between 1 mm and 2.5 mm. A method of forming a hydrogel membrane for electrolysis as claimed in claim 26, including the step of forming the membrane with a thickness of between 1.4 mm and 1.8 mm. A method of forming a hydrogel membrane for electrolysis as claimed in claim 27, including the step of forming the membrane with a thickness of 1.55 mm. An apparatus for water splitting, including an anode, a cathode and hydrogel, wherein the hydrogel is arranged to conduct protons, to separate product gases and/or to electrically isolate the cathode and/or anode. An apparatus for water splitting as claimed in claim 29, wherein the hydrogel is arranged to act as a source of water for hydrogen and oxygen production. An apparatus as claimed in claim 29 or claim 30, wherein the hydrogel is in the form of a hydrogel membrane. An apparatus as claimed in any one of claims 29 to 31, wherein the hydrogel is a hydrophilic hydrogel. An apparatus as claimed in claim 32, wherein the hydrogel is a highly hydrophilic hydrogel. An apparatus for water splitting as claimed in any one of claims 29 to 33, wherein water content in the hydrogel is maintained by hydrating the membrane. An apparatus for water splitting as claimed in claim 34, further including a water management system arranged to maintain the water content in the hydrogel. A membrane for electrolysis of a chemical for hydrogen generation, wherein the membrane is in the form of a hydrogel membrane. A membrane as claimed in claim 36, wherein the hydrogel membrane is formed from a hydrophilic hydrogel. A membrane as claimed in claim 36, wherein the hydrogel membrane is formed from a hydrogel being a copolymer of acrylate copolymerized with acrylamide. A membrane as claimed in claim 38, wherein the hydrogel is formed with N,N'- Methylenebisacrylamide used as a cross-linking agent to produce a cross-linked polymeric network. A membrane as claimed in claim 38, wherein the hydrogel is formed with N,N,N',N'-Tetram ethyl ethylenediamine used as a cross-linking agent to produce a cross-linked polymeric network. A membrane as claimed in claim 36, wherein the hydrogel is in the form of sodium polyacrylate or potassium polyacrylate. A membrane as claimed in claim 36, wherein the hydrogel membrane is formed from a hydrogel being a copolymer of poly(sodium acrylate-co-acrylamide). A membrane as claimed in claim 36, wherein the hydrogel membrane is formed from a hydrogel being a hydrophilic material containing a network of polymer chains. A membrane as claimed in claim 36, wherein the hydrogel membrane is formed from a hydrogel being a copolymer of poly(potassium acrylate-co-acrylamide). A membrane as claimed in any one of claims 36 to 44, wherein the hydrogel membrane is adapted to serve as both a gas separation membrane and an electrolyte in a hydrogen electrolyser. A membrane as claimed in claim 36, wherein the hydrogel membrane is adapted to have an optimal ratio of acrylate to acrylamide, and an optimal ratio of sodium and/or potassium ions responsible for conductivity. A membrane as claimed in claim 36, wherein the hydrogel membrane is optimised for mechanical properties of the membrane, water content and/or gas permeation within the membrane. A membrane as claimed in any one of claims 36 to 47, wherein the membrane is adapted to be a source of water for hydrogen and oxygen production in water splitting. A membrane as claimed in claim 37, wherein the hydrogel membrane is formed from a hydrophilic hydrogel having water content by weight above 50%. A membrane as claimed in claim 49 wherein the hydrogel membrane is formed from a hydrophilic hydrogel having water content by weight above 80%. A membrane as claimed in claim 50, wherein the hydrogel membrane is formed from a hydrophilic hydrogel having water content by weight above 90%. A membrane as claimed in claim 51, wherein the hydrogel membrane is formed from a hydrophilic hydrogel having water content by weight above 95%.