Electrolysis of water

Abstract

A method and associated apparatus 50 for the production of gas via electrolysis of water. The method comprises: performing electrolysis of water within one or more electrolysis cells (figure 1,2), to produce a mixture comprising a liquid and at least one of hydrogen and oxygen. The gas(es) and liquid are separated, where the separator 53 operates at a higher pressure than the pressure at which the one or more electrolysis cells operate. An additional pressurising step 55 can be performed on the gaseous mixture before separation. The gas output from the separator may be supplied to a compressor. A energy harvesting device may be provided as a part of a depressuring system 56.

Description

The present invention relates to a method of producing one or more of hydrogen and oxygen from water using an electrolyser. The invention further extends to a corresponding electrolysis system. The use of hydrogen as a fuel can reduce the amount of emissions released from burning fossil fuels and, unlike fossil fuels, offers an almost endless supply of fuel. Combusting hydrogen produces water as opposed to the CO2 produced by combusting fossil fuels, and hence a reduction in the greenhouse emissions from energy production can be obtained by using hydrogen fuel. Capitalising on the environmental benefits of hydrogen fuel cells also requires improving the energy efficiency and environmental impact of the process with which hydrogen is produced. So-called ‘blue’ hydrogen is hydrogen obtained from natural gas. Methods of producing blue hydrogen include methane reformation, which uses methane, high temperature steam and a catalyst to produce hydrogen and carbon dioxide (CO2). It therefore requires carbon capture methods to prevent the CO2 produced from entering the atmosphere as emissions. Green hydrogen is hydrogen produced in a carbon-neutral way; that is, green hydrogen is produced without emitting CO2. Hydrogen can be obtained from water by utilising an electrolysis reaction which splits the water into constituent hydrogen and oxygen. The Gibbs free energy of 2H2O is lower than the combined Gibbs free energy of 2H2 and O2, hence oxygen and hydrogen tend to combine via combustion to form water. Consequently, energy input is required to separate H2O to form hydrogen. If this energy is obtained from fossil fuels, carbon emissions would be generated in the production of hydrogen and the environmental advantages of using hydrogen as a fuel would be reduced. Thus, in order to produce green hydrogen from water via electrolysis, the electrolysis process must be powered by carbon-neutral energies such as nuclear power and renewable energies, including wind, solar, hydro, tidal and geothermal power. The inventors have realised that further process and environmental advantages can be achieved from the use of hydrogen as a fuel by improving the electrolysis process used to produce it. Since hydrogen or oxygen output from an electrolyser is usually mixed with a liquid (water or an electrolyte solution), a separator is required to obtain a pure gas. The operation of the separator has an impact on the efficiency of the electrolysis process for producing hydrogen. In conventional systems, the separator is directly connected to the electrolyser, meaning that the electrolyser and the separator operate at the same, or substantially the same, pressure. Operating a separator at an increased pressure would increase the efficiency of that separator. However, in the conventional arrangement, raising the pressure of the separator will also raise the pressure of the electrolyser which (as will be explained further below) introduces operational challenges and increases the occurrence of gas crossover which causes a reduction in the safety, reliability and efficiency of the electrolysis system. According to an aspect of the invention, there is provided a method of producing a gas via electrolysis of water, the method comprises performing electrolysis of water within one or more electrolysis cells to produce a mixture comprising a liquid and one of hydrogen and oxygen; and separating, within a separator, the mixture into a gas and a liquid, wherein the separator operates at a higher pressure than the pressure at which the one or more electrolysis cells operate. By means of the present invention, increased efficiency of separating gas and liquid from the mixture can be realised without causing detrimental effects relating to the electrolyser and the electrolysis of water. The inventors have realised that employing a pressure differential between the electrolyser and separator, and thus removing the direct connection between the two, allows both the separator and electrolyser to work at pressures beneficial to the efficiency of their operation. Employing a pressure differential between the electrolyser and separator is particularly advantageous because the electrolysis can be carried out at a low pressure, and therefore the occurrence of gas crossover is reduced, whilst the separation can be performed at a high pressure and therefore efficiency of the gas and liquid separation is increased. The method may thus comprise separating the gas of the mixture produced from the one or more electrolysis cells from the liquid of the mixture produced from the one or more electrolysis cells. The method is particularly useful for separating hydrogen gas from water or electrolyte solution and thus the method may be used for producing hydrogen via electrolysis of water. The method may alternatively comprise separating oxygen gas from water or electrolyte solution and thus the method may be for producing oxygen via the electrolysis of water. The invention is preferably used to obtain both hydrogen and oxygen and accordingly, performing electrolysis of water within one or more electrolysis cells may produce a first output of an oxygen mixture, and a second output of a hydrogen mixture, and the separator may receive either the first output or the second output; a further separator may then receive the other output. The pressure within the separator while the gas is being separated from the liquid therein is higher than the pressure within the one or more electrolyser cells while water is being electrolysed therein. In some embodiments where the electrolyser employs a liquid electrolyte, the mixture may comprise the electrolyte, and the liquid output by the separator may also comprise the electrolyte - typically a solution thereof. Although any significant pressure differential will provide the advantage described above, the separator preferably operates at a pressure at least one bar greater than the pressure at which the one or more electrolysis cells operate. Accordingly, the pressure within the separator while the gas is being separated from the electrolyte therein may be more than one bar greater than the pressure within the one or more electrolyser cells while water is being electrolysed therein. However, larger pressure differentials are more advantageous and the pressure within the separator while the gas is being separated from the liquid therein may be between 1 and 60 bar greater than the pressure within the one or more electrolysis cells while water is being electrolysed therein. Thus, the separator may operate at an increased efficiency due to the higher pressure during operation compared to the prior art system, without the drawback that the electrolysis cells experience an increased occurrence of gas cross over that would otherwise occur as a result of increased pressure in the electrolysis cells. In particular, the pressure within the separator while the gas is being separated from the liquid therein may be, or may be more than, 5 barg (6 bar), 10 barg (11 bar), 20 barg (21 bar), 30 barg (31 bar), 40 barg (41 bar) or 50 barg (51 bar) greater than the pressure within the one or more electrolyser cells while water is being electrolysed therein. (For the avoidance of doubt, Barg refers to gauge pressure in bar, so one barg is approximately 2 bar absolute pressure (2 bar), and 0 barg is approximately atmospheric pressure (1 bar).) The pressure within the one or more electrolysis cells while water is being electrolysed therein may be 0 barg or 1 barg (1 bar or 2 bar) and the pressure within the separator while the gas is being separated from the liquid therein may be, or may be more than, 5 barg (6 bar), 10 barg (11 bar), 20 barg (21 bar), 30 barg (31 bar), 40 barg (41 bar) or 50 barg (51 bar). Although only a small difference in pressure is required to benefit from the increased efficiency according to the discussion above, a larger difference in pressure between the separator and electrolyser cells will result in a greater increase in efficiency. The method may comprise providing the mixture produced from the one or more electrolysis cells to a suitable pressurising means, typically a multiphase pump; increasing the pressure of the mixture; and providing the pressurised mixture to the separator. Thus the pressure of the mixture is increased after the mixture has exited the one or more electrolyser cells, and before the mixture enters the separator. The gas and liquid in the separator is thus at a higher pressure than the gas and liquid in the one or more electrolyser cells. The gas exiting the separator may be further compressed as required for storage and / or transport. However, in preferred embodiments, the gas exiting the separator may be used, or stored for subsequent use, without an additional pressurising step. Thus, the separator may operate at a pressure such that the gas produced therefrom is at a pressure suitable for further use. For example, hydrogen gas pipelines operate at around 50 to 60 bar, or higher and so the separator may operate at around 50 to 60 bar or higher. The separator may operate at a pressure such that the gas produced therefrom is at a pressure suitable for pressurised storage. For example, hydrogen gas can be stored at around 50 to 60 bar and the separator may operate at around 50 to 60 bar. Where no additional pressurizing step is required, the complexity of the method and system for providing hydrogen or oxygen for subsequent use or storage can be reduced. This is at least because fewer method steps and component parts will be required. Although the invention may be employed with a single electrolysis cell, in most practical applications, the electrolysis will be performed in a plurality of electrolysis cells and the plurality of electrolysis cells may be arranged to form an electrolysis stack. As such any significant pressure differential between the electrolyser stack and the separator will provide the advantage described above; however, the separator preferably operates at a pressure at least one bar greater than the pressure at which the one or more electrolysis cells of the electrolyser stack operate. As such, the mixture comprising liquid and one of hydrogen and oxygen produced from each of the plurality of electrolyte cells may be provided to the same separator. Thus, the volume in which electrolysis of water takes place is increased without the number of separators also increasing. This in turn allows the rate of hydrogen or oxygen production to be increased without increasing the number of separators. Liquid output from the separator may be returned to the one or more electrolysis cells via a de-pressurising means. Liquid, i.e. water (and, where present, electrolyte) is removed from the one or more electrolysis cells in the mixture, the liquid within the one or more electrolysis cells may be replenished via the liquid obtained from the separator. In this case, the pressure within the one or more electrolyser cells remains lower than that of the separator because the high-pressure liquid exiting the separator is depressurised before re-entering the one or more electrolysis cells. Thus, the liquid exiting the separator can be recycled within the method of producing a gas via electrolysis of water for increased efficiency. The de-pressurising means may act to control the flow rate of the gas and the liquid around the circuit provided by the one or more electrolyser cells, the separator and back to the one or more electrolyser cells. An energy harvesting device may be combined with the de-pressurising means such that energy may be harvested from the depressurisation of the liquid. Thus, the energy efficiency of the method of producing a gas via electrolysis of water may be increased. The energy harvesting device may be a turbine ora reciprocating engine, such as a motor. The energy harvesting device may be part of a pressure-reducing valve system. The energy stored within the liquid from the separator by virtue of its high pressure may therefore be captured as the liquid de-pressurises. The harvested energy may be reused for other applications. The harvested energy may be used within the method of producing hydrogen and oxygen via electrolysis of water such as to provide energy to the electrolysis process or to provide energy for increasing the pressure of the separator. The gas output from the separator may be provided to a compressor. In this case, the pressure within the separator during operation need not be as high as the pressure required for the further use or storage of the gas obtained from the separator. The pressure in the separator can be optimized independently of the pressure required for further use or storage of the gas obtained from the separator. Thus the separator may operate at a pressure such that the gas produced therefrom is not at a pressure suitable for further use and an additional pressurisation stage may be required. The separator may operate at a pressure such that the gas produced therefrom is not at a pressure suitable for pressurised storage and an additional pressurisation stage may be required. For example, hydrogen gas can be stored in large scale pressurised cylinders (particularly for the purpose of transportation) at around 200 bar. In this case the separator may operate at a pressure lower than 200 bar and the gas output from the separator undergoes additional pressurisation to reach a pressure required within the pressurised cylinders. According to another aspect of the invention, there is provided a processing plant for electrolysis of water, the processing plant comprising; one or more electrolysis cells, each of the electrolysis cells comprising a hydrogen outlet and an oxygen outlet; and a gas-liquid separator connected to one of the hydrogen outlet and the oxygen outlet of the one or more electrolysis cells, wherein the separator is configured to operate at a higher pressure than the one or more electrolysis cells. By means of the present invention, increased efficiency of the separator can be realised without causing detrimental effects relating to the electrolyser. The inventors have realised that providing a pressure differential between the electrolyser and separator, and thus removing the direct connection between the two, allows both the separator and electrolyser to work at pressures beneficial to the efficiency of their operation. Employing a pressure differential between the electrolyser and separator is particularly advantageous because the electrolyser can operate at a low pressure, and therefore the occurrence of gas crossover is reduced, whilst the separator can work at a high pressure and therefore efficiency of the gas separation is increased. The gas-liquid separator may be connected to the hydrogen outlet and the separator may be configured to separate hydrogen from water. The separator may be connected to the oxygen outlet and the separator may be configured to separate oxygen from water. The separator may be configured to operate at a pressure at least one bar greater than the pressure at which the one or more electrolysis cells operate. The separator may be configured to operate between 1 and 60 bar greater than the pressure at which the one or more electrolysis cells are configured to operate. The separator may be configured to operate at, or at more than, 5 barg (6 bar), 10 barg (11 bar), 20 barg (21 bar), 30 barg (31 bar), 40 barg (41 bar) or 50 barg (51 bar) greater than the pressure at which the one or more electrolyser cells are configured to operate at. The one or more electrolysis cells may be configured to operate at 0 barg (1 bar) or 1 barg (2 bar) and the separator may be configured to operate at, or at more than, 5 barg (6 bar), 10 barg (11 bar), 20 barg (21 bar), 30 barg (31 bar), 40 barg (41 bar) or 50 barg (51 bar). In some embodiments where the electrolyser employs a liquid electrolyte, the mixture may comprise the electrolyte, and the liquid output by the separator may also comprise the electrolyte (typically a solution thereof). The processing plant may comprise a pressurizing means connected between the separator and one of the hydrogen outlet or the oxygen outlet of the one or more electrolysis cells. The pressurising means may be configured to receive a hydrogen mixture comprising hydrogen and water (and, where present, a liquid electrolyte) from each of the one or more electrolysis cells and may be configured to provide the hydrogen mixture at an elevated pressure to the separator. The pressurising means may be configured to receive an oxygen mixture comprising oxygen and water (and, where present, a liquid electrolyte) from each of the one or more electrolysis cells and may be configured to provide the oxygen mixture at an elevated pressure to the separator. The pressurizing means may be a liquid pump, a multiphase pump or multiphase compressor. The one or more electrolysis cells may comprise a plurality of electrolysis cells and the plurality of electrolysis cells may be arranged in an electrolysis stack. The separator may be configured to receive a hydrogen mixture comprising hydrogen and water (and where present, a liquid electrolyte) from the electrolysis stack. The separator may be configured to receive an oxygen mixture comprising oxygen and water (and, where present, a liquid electrolyte) from the electrolysis stack. Thus, the volume in which electrolysis of water takes place is increased without the number of separators also increasing. This in turn allows the rate of hydrogen or oxygen production to be increased without increasing the number of separators. The processing plant may comprise a pressurising means configured to receive the hydrogen mixture from the electrolysis stack. The processing plant may comprise a pressurising means configured to receive the oxygen mixture from the electrolysis stack. Thus, the volume in which electrolysis of water takes place is increased without the number of pressurizing means also increasing. This in turn allows the rate of hydrogen or oxygen production to be increased without increasing the number of pressurizing means. The processing plant may comprise a de-pressurising means connected between a liquid outlet of the separator and a liquid inlet of the one or more electrolysis cells. The separator may comprise liquid outlet and a gas outlet, and the processing plant may be configured to provide the liquid from the liquid outlet to the one or more electrolysis cells. The processing plant may be configured with a closed loop circuit for the liquid in order to return the liquid from the separator to the one or more electrolysis cells for use in further electrolysis. A de-pressurising means may be provided between the separator and the one or more electrolysis cells so that the liquid returned to the one or more electrolysis cells does not raise the pressure within the one or more electrolysis cells. The de-pressurising means may be a choke valve. Thus, the liquid exiting the separator can be recycled within the processing plant for increased efficiency. The de-pressurising means may comprise an energy harvesting device. The energy harvesting device may be a turbine ora reciprocating engine, such as a motor. The energy harvesting device may be part of a pressure-reducing valve system. The energy harvesting device may be configured to harvest energy from the de-pressurisation of the liquid. Thus, the energy efficiency of using the processing plant may be increased. The processing plant may comprise a first separator connected to the oxygen outlet of the one or more electrolysis cells and a second separator connected to the hydrogen outlet of the one or more electrolysis cells, and the first and second separators may be configured to operate a higher pressure than the one or more electrolysis cells. Each of the one or more electrolysis cells may be configured to provide an oxygen mixture comprising oxygen and water (and, where present, a liquid electrolyte) to the first separator and provide a hydrogen mixture comprising hydrogen and water (and, where present, a liquid electrolyte) to the second separator. Thus, electrolysis of water may be carried out at a low pressure and the separation of both oxygen and hydrogen may be carried out at a higher pressure and the advantageous effects discussed above can be realized in relation to the production of both hydrogen and oxygen gas. Certain embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a conventional alkaline water hydrolysis cell; Figure 2 is a schematic diagram of a conventional PEM water hydrolysis cell; Figure 3 is a schematic diagram of a prior art hydrogen production system having a plurality of cells as shown in Figure 1 or Figure 2; and Figure 4 is a schematic diagram of a hydrogen production system having a plurality of cells as shown in Figure 1 or Figure 2 according to an embodiment of the invention. Hydrogen may be obtained from alkaline water electrolysis. The electrolysis reaction takes place within an electrolyser cell 10, such as that shown in Figure 1. Comprising a voltage source (V) connected to a negatively charged cathode 11 and a positively charged anode 12 located in a liquid alkaline electrolyte solution 13. The liquid alkaline electrolyte solution may comprise water and potassium hydroxide (KOH) or sodium hydroxide (NaOH), or a mixture thereof. The anode and cathode are separated by a porous diaphragm 14. At the cathode 11, water combines with electrons and is split into hydrogen and hydroxide anions according to equation 1. 4H2O + 4e“ -> 2H2 + 4OH~ (1) At the anode 12, hydroxide anions combine to form oxygen, water and electrons according to equation 2. WH' ^2H2O + O2 + 4e“ (2) It will be appreciated that the alkaline liquid electrolyte solution allows ions to pass from the anode to the cathode, and from the cathode to the anode. The porous diaphragm 14 provides a semi-permeable membrane that allows hydroxide anions to pass from the cathode to the anode whilst preventing oxygen produced at the anode from passing through to the cathode, or through to the hydrogen produced therefrom. Similarly, the hydrogen produced at the cathode cannot pass through the porous diaphragm to the anode, or to the oxygen produced therefrom. The porous diaphragm thus prevents recombination of the hydrogen and oxygen gasses (i.e. the porous diaphragm prevents gas cross-over) which can otherwise be hazardous. An anode separator plate (not shown) comprises an oxygen channel in fluid communication with the anode 12 for directing oxygen evolved at the anode to an oxygen and electrolyte mixture outlet 16 of the anode separator plate. The liquid alkaline electrolyte solution (i.e. water and hydroxide), as well as oxygen gas may exit the anode separator plate through the oxygen and electrolyte mixture outlet and this gas-liquid mixture is then typically separated using a gas-liquid separator, as discussed below. In addition, the cathode separator plate (not shown) comprises a water channel for supplying water to the cathode, and a hydrogen channel for directing hydrogen evolved at the cathode (combined with the liquid alkaline electrolyte solution) to a hydrogen and electrolyte mixture outlet 15 of the cathode separator plate. The liquid alkaline electrolyte solution, as well as hydrogen gas, may exit the cathode separator plate through the hydrogen and electrolyte mixture outlet and the gas-liquid mixture is then typically separated using a gas-liquid separator. As will be discussed further below, a large number of such cells may be combined for form an electrolyser stack. When an electrolyser stack is used, the oxygen and electrolyte mixture outlets 16 of each of the plurality of anode separator plates combine at an electrolyser stack oxygen outlet. Likewise, the hydrogen and electrolyte mixture outlets 15 of the plurality of cathode separator plates combine at an electrolyser stack hydrogen outlet. Alternatively, hydrogen may be obtained from proton exchange membrane water electrolysis using electrolyser cells such as those shown in Figure 2. The electrolysis reaction takes place within an electrolyser cell 20 comprising a positively charged anode 21 and a negatively charged cathode 22. The anode and cathode are each provided with a catalyst layer and separated by a solid polymer proton exchange membrane (PEM) 23. At the anode 21, water is split into oxygen, electrons and hydrogen ions according to equation 3. 2H2O -> O2 + 4e +4H+ (3) At the cathode 22 hydrogen ions and electrons combine to form hydrogen according to equation 4. 2H + + 2e -> H2 (4) These reactions take place at the catalyst layer coated on each electrode 21,22. A diffusion layer 25, 27 on each side helps in efficient current distribution and also connects the PEM to distribution or separator plates 28, 29. The proton exchange membrane (23) allows protons to pass from the anode to the cathode whilst preventing oxygen produced at the anode from passing through to the cathode or through to the hydrogen produced therefrom. Similarly, the hydrogen produced at the cathode cannot pass through to the anode or to the oxygen produced therefrom. The PEM thus prevents recombination of the hydrogen and oxygen gasses (i.e. the PEM prevents gas cross-over) which can otherwise be hazardous. The anode separator plate 28 comprises a water channel 30 for supplying water to the anode, and an oxygen channel in fluid communication with the anode for directing oxygen evolved at the anode to an oxygen and water mixture outlet 31 of the anode separator plate. Water, as well as oxygen gas, may exit the separator plate through the oxygen outlet and the gas-liquid mixture is then separated using a gas-liquid separator. The cathode separator plate comprises a hydrogen channel for directing hydrogen evolved at the cathode to a hydrogen outlet of the cathode separator plate. Water may in some arrangements be supplied via water inlet 32and water as well as hydrogen gas, may exit the separator plate through the hydrogen outlet 33. The gas-liquid mixture is then separated using a gas-liquid separator. When an electrolyser stack is used, the hydrogen outlets of each of the plurality of anode separator plates combine at an electrolyser stack hydrogen outlet. Likewise, the oxygen outlets of the plurality of cathode separator plates combine at an electrolyser stack oxygen outlet. As with the alkaline hydrolysis system, a number of cells may be combined in an electrolyser stack. Such an electrolyser stack comprises a plurality of electrolysis cells electrically connected in series. Figure 3 shows a conventional hydrogen production system 40 comprising an electrolyser stack 42, a liquid / gas separator 43, and a final compressor 44. The electrolyser stack 42 comprises a plurality of electrolyser cells, which may be alkaline or PEM hydrolysis cells as described above with reference to Figures 1 and 2. In either case, the cells are operated at relatively low pressure to prevent crossover of hydrogen via the diaphragm or PEM. Water is supplied to the electrolyser stack via inlet conduit 41. The electrolyser stack 42 has a first multiphase fluid outlet 45 which supplies a mixture of hydrogen and liquid (water or electrolyte solution) from each of the cells forming the stack. The fluid outlet is directly connected to the inlet of the separator 43 means of a first conduit 46 such that the pressure within the electrolyser stack 42 is the same, or close to the same, as the pressure within the separator 43. Within the separator 43, the hydrogen is separated from the liquid. The liquid obtained from the outlet of the separator 43 is returned to the electrolyser stack 42 via second conduit 47 for use in further electrolysis. The separated hydrogen from the outlet of the separator 43 is directed via third conduit 48 to compressor 44, where the pressure of the hydrogen gas is increased to be suitable for further use such as storage, transportation or utilisation as a hydrogen gas supply. The above discussion concerns the treatment of the hydrogen / liquid mixture. In the illustrated arrangement, oxygen is simply vented via oxygen outlet 49. However, if it is desired to provide separated oxygen as an output, similar arrangements to those described above may be provided for treating the oxygen / liquid mixture produced by the stack. The electrolyser stack 42 and separator 43 operate at close to atmospheric pressure (0 barg or 1 bar). However, high volumes of low-pressure hydrogen will be output from the separator 43 as a result. As well as requiring large apparatus to handle such large volumes, the process of compressing the low-pressure hydrogen gas to a high enough pressure for storage and / or further use has a high energetic cost. Due to the inefficiency of the hydrogen production process in this case, operating the electrolyser stack 42 at close to atmospheric pressure may not be desirable. The electrolyser stack 42 and separator 43 may alternatively operate at elevated pressure, for example around 30 barg (31 bar), thus less energy is required to compress the hydrogen gas output from the separator 43. The higher pressure within the electrolyser and the separator allows the electrolyser 42, separator 43 and compressor 44 to handle lower volumes of reactants and products per mass of hydrogen produced and thus each component may have smaller dimensions. For example, the bubble size of the gas produced at the anodes and cathodes within the electrolyser cells will be smaller the higher the pressure is within the electrolyser cells, and the volume per mass of hydrogen produced by the separator will also be smaller the higher the pressure within the separator. However, as noted above, operating the electrolyser stack 42 at elevated pressures reduces the reliability and effectiveness of the semi-permeable barrier separating the anode and cathode and requires a higher DC voltage to be provided between the anode and cathode of each electrolyser cell, increasing the energy requirement per mass of hydrogen produced. Another consequence of operating the electrolyser stack 42 at high pressure is that the oxygen gas output from the electrolyser stack 42 will also be provided at the elevated pressure. This can cause problems for further processing of the oxygen. Oxygen disposal may be carried out by venting the high-pressure gas to the atmosphere. In this case, the needless pressurisation of oxygen is energy inefficient, plus the decompression of the oxygen during venting results in objectionable noise levels. Figure 4 shows a hydrogen production system 50 according to an embodiment of the invention. It comprises an electrolyser stack 52, a pressurising means 55, a separator 53 and a compressor 54. Water is supplied to the electrolyser stack, indicated by arrow A. Water is supplied in the form of deionised water in order to avoid introducing impurities into the system. In the case of alkaline water electrolysis, water is supplied via the water channel of the cathode separator plates. In the case of proton exchange membrane water electrolysis, water is supplied via the water channel of the anode separator plates. The electrolyser stack 52 operates at close to atmospheric pressure and may operate at a pressure of around 1 barg, i.e. around 1 bar above atmospheric pressure. It outputs a hydrogen / liquid mixture and an oxygen / liquid mixture as previous described. In electrolysis systems comprising a liquid electrolyte the hydrogen mixture will also comprise the electrolyte solution. The oxygen mixture is output from the electrolyser stack, as indicated by arrow B. As shown, the oxygen is released into the atmosphere. However, the oxygen mixture can be provided to a separator (not shown) and processed and / or stored for later use, particularly within the medical or industrial sectors. As discussed above in relation to the conventional system, the output of the hydrogen channel is provided to the separator 53. However, in the present embodiment, it flows via pressurizing means 55. Recovery of water (and, where present, electrolyte) from the hydrogen mixture and compression of hydrogen (or another gas such as oxygen) are both favourable at high pressure. It is therefore beneficial to carry out separation of the gas and liquid at elevated pressure. However, as discussed above, raising the pressure within the electrolysis stack reduces the reliability of the electrolysis cells. This is because the increase in pressure increases the rate of occurrence of gas crossover. Gas crossover is the phenomenon of hydrogen gas travelling from the cathode to the anode through the barrier separating the anode and cathode (i.e. the porous diaphragm in the case of alkaline water electrolysis and the solid polymer electrolyte membrane in the case of PEM electrolysis) or conversely, oxygen gas travelling from the anode to the cathode through the barrier separating the anode and cathode. When gas crossover occurs, dangerous operating conditions may arise since hydrogen and oxygen can combine to form water initiating a fire or explosion. High pressures in the electrolysis cells can force the hydrogen and / or oxygen through the barrier separating the anode and cathode. The presence of defects in the barrier separating the anode and cathode causes the electrolyser cell to be particularly at risk of gas crossover especially at elevated pressures. In particular, raising the pressure within an alkaline water electrolysis electrolyser cell causes particular susceptibility to gas crossover due to the porous nature of the diaphragm. Providing a pressurizing means 55 between the electrolysis stack 52 and the separator 53 in order to increase the pressure of the separator compared to the stack will reduce the energy consumption of the hydrogen production system 50, whilst avoiding in increase in the occurrence of gas crossover. The pressurizing means 55 raises the pressure of the hydrogen mixture. The pressure within the separator 53 is therefore significantly greater than the pressure within the electrolyser stack 52. The hydrogen mixture is therefore provided from the hydrogen outlet of the electrolyser stack 52 to the pressurising means 55, as indicated by arrow C. The pressure within the electrolyser stack 52 may be 0 barg (i.e. atmospheric pressure, 1 bar) or maybe around 1 barg, up to 5 barg, or up to 10 barg. The pressurising means 55 may raise the pressure of the electrolyte and hydrogen mixture by around 5 barg, around 10 barg, around 20 barg or around 30 barg or more. The pressure in the separator 13 may be around 20 barg, around 30 barg, or around 40 barg. A prior art hydrogen production system such as that shown in figure 3, must compromise between having a low pressure in the electrolyser stack 42 for safety and a high pressure in the separator 43 for efficiency. No such compromise is required in the hydrogen production system of figure 4, the pressure of the hydrogen mixture is increased (for improving the efficiency of the separation process) after the mixture leaves the electrolysers allowing for the pressure within the electrolyser stack 52 to remain low. Hence the occurrence of gas crossover can be reduced and the safety of the system improved. In examples where the hydrogen / liquid mixture comprises a moderate hydrogen gas content (e.g. a gas content of less than approximately 40 %), a conventional liquid pump may be suitable as the pressurising means 55. In examples where the hydrogen mixture comprises a high hydrogen gas content (e.g. a gas content of more than 40 %), a multiphase pump may be suitable and / or required as the pressuring means 5. The specification of the pressurising means will be determined according to the appropriate gas-liquid composition / fractions of the given stream. The pressurised hydrogen mixture is provided from an outlet of the pressurising means 55 to a separator 53 (as indicated by arrow D) in order to separate hydrogen gas from the water (and, where present, the electrolyte). The separator 53 is a liquid-gas separator. In some examples, the separator 53 separates the hydrogen from the water (and, where present, the electrolyte) utilising gravity. In such examples, the separator comprises an inlet, a drum, a gas outlet and a liquid outlet. The residence time of the hydrogen mixture in the drum is such that the hydrogen gas bubbles are allowed to rise to the surface of the liquid and be released into the gas phase. The pressure of the gas phase will be at, or very close to, the pressure generated by the pressurising means 55. The inlet for the flow of the hydrogen mixture entering the separator 53 may comprise an inlet device to facilitate gas-liquid separation. The gas outlet may be equipped with a device to capture liquid droplets to avoid liquid carry-over, i.e. to prevent water (and, where present, electrolyte) exiting the separator 53 though the gas outlet. The gas leaving the separator may comprise water vapour, and in some instances may be saturated with water vapour. The amount of water dissolved in the gas will decrease with increasing pressure. Water transported out with the hydrogen gas may be condensed and returned to the process. The liquid obtained from the liquid output of the separator 53 is returned to the electrolyser stack 52 for use in further electrolysis. However, the liquid output from the separator 53 must be decompressed before entering the electrolyser stack 52 in order to prevent an increase in the pressure of the electrolyser stack 52. The liquid output from the separator 53 is therefore provided to a de-pressurising means 56, as indicated by arrow E, before being provided to the electrolyser stack 52, as indicated by arrow F. The de-pressurising means 56 may be a choke, an expander, a motor or a turbine. The de-pressurising means 56 reduces the pressure of the liquid from the pressure at which the separator 53 operates to the pressure at which the electrolyser stack 52 operates. The de-pressurising means 56 can be combined with an energy harvesting device, such as a turbine, in order to harvest the energy released during depressurisation of the liquid output from the separator 53. In some examples, the energy so harvested may be used to power the pressurising means 55 (or another component of the hydrogen production system 50) for increased energy efficiency of the hydrogen production system 50. The de-pressuring means 56 may also be used for process control, i.e. controlling the rate at which the liquid exits the separator. Furthermore, pressurising means 55 and de-pressurising means 56 may in combination be used for process control, i.e. controlling the rate at which the hydrogen mixture enters the separator and the rate at which the liquid exits the separator. The pressurising means 55 and the de-pressurising means 56 can be integrated in to a single unit or can be provided as individual components within the hydrogen production system 50. Likewise, the energy harvesting device and the de-pressurising means 56 can be integrated into a single unit (with or without the pressurising means 55 integrated therein also), or the energy harvesting device and the de-pressurising means 56 can be provided as two suitably linked individual components. The hydrogen obtained from the output from the separator 53 is provided from the gas outlet of the separator 53 to the compressor means 54, as indicated by arrow G. The compressor means 54 further increases the pressure of the hydrogen gas so that the hydrogen gas is suitable for further use such as storage, transportation or utilisation as a hydrogen gas supply. The pressure of the hydrogen may be raised by around 5 bar, around 10 bar, around 20 bar or more. The pressure of the hydrogen provided by the compressor means 54 may be around 40 barg (41 bar), around 50 barg (51 bar) or more. The higher pressure within the separator 53 facilitates the further compression of hydrogen so that the efficiency of the compressor means 54 is increased. In alternative examples, the pressurising means 55 increases the pressure of the hydrogen and electrolyte mixture to the pressure required for further use such as storage, transportation or utilisation as a hydrogen gas supply. In this case, the separator 53 operates at the final pressure required for the hydrogen and thus no compressor means 54 is required. The pressurising means 55 may therefore raise the pressure of the electrolyte and hydrogen mixture by around 40 bar, around 50 bar or more. The pressure in the separator may be around 40 barg (41 bar), around 50 barg (51 bar), or around 60 barg (61 bar). In some examples, a separator is provided for separating oxygen from the water (and, where present, the electrolyte) of the oxygen mixture output from the electrolysing stack. As discussed above, this separator may operate at a higher pressure than the electrolysing stack. A pressurising means (operating akin to the pressurising means 55) is provided so that the output from the oxygen channel of the anode separator pates is directed to a pressurising means, before being supplied to a separator. Thus, arrows C and D in figure 4 can indicate the flow of the oxygen mixture and arrow B indicates the flow of the hydrogen mixture. Arrow G in figure 4, in this case, indicates the flow of oxygen. The description above in relation to obtaining pressurised hydrogen gas and the operating considerations of the components thereby equally applies to obtaining pressurised oxygen gas. This would be advantageous for applications where a high-pressure oxygen source is required. The same process advantages, such as space savings and energy 5 efficiency savings, are achieved when applied obtaining pressurised oxygen gas in this way.

Claims

1. A method of producing a gas via electrolysis of water, the method comprising:performing electrolysis of water within one or more electrolysis cells to produce a mixture comprising a liquid and one of hydrogen and oxygen; and separating, within a separator, the mixture into a gas and a liquid, wherein the separator operates at a higher pressure than the pressure at which the one or more electrolysis cells operate.

2. The method as claimed in claim 1, wherein the separator operates at a pressure at least one bar greater than the pressure at which the one or more electrolysis cells operate.

3. The method as claimed in claim 1 or 2, comprising:providing the mixture produced from the one or more electrolysis cells to a pressurising means;increasing the pressure of the mixture; andproviding the pressurised mixture to the separator.

4. The method as claimed in claims 1, 2 or 3, wherein gas exiting the separator is used, or stored for subsequent use, without an additional pressurising step.

5. The method as claimed in any preceding claim, wherein the electrolysis is performed in a plurality of electrolysis cells and the plurality of electrolysis cells are arranged to form an electrolysis stack.

6. The method as claimed in any preceding claim, wherein the liquid output from the separator is returned to the one or more electrolysis cells via a depressurising means.

7. The method as claimed in claim 6, wherein an energy harvesting device is combined with the de-pressurising means such that energy is harvested from the depressurisation of the liquid.

8. The method as claimed in any preceding claim, wherein the gas output from the separator is provided to a compressor.

9. A processing plant for electrolysis of water, the processing plant comprising;one or more electrolysis cells, each of the electrolysis cells comprising a hydrogen outlet and an oxygen outlet; anda gas-liquid separator connected to one of the hydrogen outlet and the oxygen outlet of the one or more electrolysis cells,wherein the separator is configured to operate at a higher pressure than the one or more electrolysis cells.

10. The processing plant of claim 9, comprising a pressurizing means connected between the separator and one of the hydrogen outlet or the oxygen outlet of the one or more electrolysis cells.

11. The processing plant of claim 10, wherein the pressurizing means is a liquid pump, a multiphase pump, or multiphase compressor.

12. The processing plant of any of claims 9 to 11, wherein the one or more electrolysis cells comprise a plurality of electrolysis cells and the plurality of electrolysis cells are arranged to form an electrolysis stack.

13. The processing plant of any of claims 9 to 12, comprising a depressurising means connected between a liquid outlet of the separator and a liquid inlet of the one or more electrolysis cells.

14. The processing plant of any of claim 13, wherein the de-pressurising device comprises an energy harvesting device.

15. The processing plant of any of claims 9 to 14, comprising a first separator connected to the oxygen outlet and a second separator connected to the hydrogen outlet, the first and second separators configured to operate a higher pressure than the one or more electrolysis cells.

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