EP4649184A1

EP4649184A1 - Electrolyser and method for its operation

Info

Publication number: EP4649184A1
Application number: EP24740325.6A
Authority: EP
Inventors: Henrik Stiesdal; Niels Vinter VOIGT; Thomas Tøttrup Jensen
Original assignee: Stiesdahl Hydrogen AS
Current assignee: Stiesdahl Hydrogen AS
Priority date: 2023-01-13
Filing date: 2024-01-12
Publication date: 2025-11-19
Also published as: DK181744B1; WO2024149432A1; DK202300028A1

Abstract

Electrolyser and method for its operation In an electrolyser (1) stack for production of hydrogen gas, multiple bipolar electri- cally conducting metal separator plates (21, 25) sandwich membranes. Each separator plate has raised surface portions (50) towards the membrane (23), forming minor gas channels (40) between the separator plate (21, 25) and the membrane (23) for transport of produced gas along the separator plate (21, 25). Each structured area (30A, 30B) with the minor channels (40) is surrounded by a combination of an upper major channel (41) above and a lower major channel (47) below the first structured 10 area (30A), as well as a first major channel (42) and second major channel (49) connecting the lower major channel (47) with the upper major channel (41) on a first and second side. Gas flow through the channels leads to circulation of electrolyte through and around the structured areas (30A, 30B).

Description

Electrolyser and method for its operation

FIELD OF THE INVENTION

The present invention relates to an electrolyser, especially alkaline electrolyser, and its operation, for production of hydrogen gas. The electrolyser comprises a stack of bipolar electrodes, each two of which are sandwiching an ion-transporting membrane.

Each bipolar electrode forms an anode chamber on one side and a cathode chamber on the other side. In particular, the invention relates to an electrolyser and a method of its operation according to the preamble of the independent claims.

BACKGROUND OF THE INVENTION

Electrolysis is an efficient method for production of hydrogen gas from electricity. In an electrolyser, an ion conducting membrane is sandwiched between two electrodes, and a voltage is applied over the electrodes. The voltage results in water from the aqueous electrolyte being split into hydrogen and oxygen with a final separation of hydrogen gas and oxygen gas on opposite sides of the membrane.

The primary objective of the electrolyser is for production of hydrogen gas. Hydrogen is collected for later use, for example in fuel cells or industrial applications. However, due to the splitting of water in the electrolyte when applying electrical power, oxygen is also produced. The oxygen may also be collected for later use.

Traditional alkaline electrolysis is based on a series of electrolytic cells. In each cell, two electrode plates are separated by a certain distance. The gap between the electrodes is filled with a liquid alkaline electrolyte. When sufficient voltage is applied, hydrogen is released on the cathode surface and oxygen is released on the anode surface. An ion conducting diaphragm between the electrodes prevents mixing of the gases. The electrolyte is circulated to remove the heat generated by the electrolytic process. The gap needs to be of sufficient width to allow the escape of hydrogen and oxygen bubbles without excessive blocking of the conductive path through the electrolyte from the anode to the cathode, and to allow electrolyte circulation without excessive pressure loss.

In later years, the configuration of traditional alkaline electrolysers has been replaced by a so-called zero-gap configuration. In the zero-gap configuration the cell design works by pressing two porous electrodes onto either side of a hydroxide ion conducting membrane. This achieves a gap between the two electrodes equal to the thickness of the membrane, typically 0.5 mm or even less, rather than the 2-5 mm required for the traditional gap configuration. The smaller gap reduces the ohmic resistance contribution to the losses in the electrolytic cells.

In a conventional zero gap electrolyser configuration, the electrodes need to have pores in order to allow the escape of hydrogen and oxygen bubbles to the side of the electrode not facing the membrane. In a basic setup this arrangement would lead to mixing of hydrogen and oxygen in the chamber established between the cathode and the anode, which is not wanted. Therefore, a separator plate is inserted between the anode of one cell and the cathode of the neighbouring cell, which prevents such mixing of the gases that are created by the electrolytic process. Hence, a bipolar electrode for a zero-gap electrolysis stack is typically composed of three metallic plates, namely a porous anode, a solid separator plate, and a porous cathode.

The distance from the anode to the separator plate and from the separator plate to the cathode must be of sufficient width in order to allow electrolyte circulation without excessive pressure loss, and most importantly, in order to allow the escape of hydrogen and oxygen bubbles without excessive blocking, which otherwise would cause backpressure on the bubbles.

An alkaline zero-gap electrolyser stack comprises a significant number of electrolytic cells, each cell comprising the electrode chamber and the anode from a first bipolar electrode, the hydroxide ion conducting membrane separating the first bipolar electrode from a second bipolar electrode, and the cathode and the electrode chamber from the second bipolar electrode. Electrolyte is fed to the electrode chambers through electrolyte conduits passing through the bipolar metal plates and/or the gaskets providing the combines sealing and electrical insulation between the adjacent bipolar metal plates. A mixture of electrolyte and gases produced is removed from the electrode chambers through gas removal conduits passing through the bipolar metal plates and/or through the gaskets providing the combines sealing and electrical insulation between the adjacent bipolar metal plates.

Voltage is applied to the electrodes at either end of the electrolyser stack. The voltage required for the desired hydrogen production is the product of the voltage required for the desired rate of hydrogen formation at each electrolytic cell multiplied with the number of electrolytic cells. A minimum equilibrium voltage of 1.47 V is required to initiate hydrogen formation, and a typical electrolytic cell voltage is in the order of 1.75 V. Since the electrolyser stack often comprises a hundred or more electrolytic cells, the total applied voltage may be in the order of several hundred volts. The power delivered to the electrolyser stack is the product of the total applied voltage and the current through the stack.

The electrolytic cell efficiency is the ratio of the equilibrium voltage divided by the actual voltage applied to the electrolytic cell. At an applied voltage of 1.75 V, the cell efficiency is 1.47 V / 1.75 V = 84%. This means that 84% of the electric power delivered to the electrolytic cell is used for the decomposition of water into hydrogen and oxygen. The remaining 16% of the electric power is converted into waste heat.

The waste heat needs to be removed from the electrolytic cells. Such removal of waste heat is normally implemented by circulating the electrolyte, removing electrolyte together with the gases through the gas removal conduits, separating the gases from the electrolyte, cooling the electrolyte in an external cooler, and feeding it back again into the electrolytic cells through the electrolyte feed conduits.

Inside the electrode chambers that are forming the electrolytic cells, random circulation of the electrolyte is often not sufficient to avoid electrolyte concentration gradients and uneven temperature distribution, and, therefore, the electrolyte needs to be circulated by significant pumping force of the electrolyte that is delivered through the electrolyte feed conduits. In addition, the inlet temperature of the electrolyte needs to be maintained at a level fairly close to the outlet temperature of the electrolyte in order to maintain a desired average temperature of the electrolyte throughout the electrode chambers.

The amount of heat removed with the circulating electrolyte is proportional to the flow rate multiplied with the difference between the inlet and outlet temperature of the electrolyte. Consequently, due to the combined effects of the circulation needed in the electrolytic cells and the small difference between the inlet and outlet temperatures of the electrolyte, conventional electrolysers require a high flow rate of the electrolyte.

A high electrolyte flow rate requires fairly large cross-sectional areas of the electrolyte feed conduits and the gas removal conduits.

Since the electrolyte needs to be electrically conductive, currents will flow from one end of the electrode stack to the other in the electrolyte feed conduits, and currents may also flow from one end of the electrode stack to the other in the gas removal conduits, depending on the actual configuration of the gas removal conduits. The currents flowing through the conduits are typically designated shunt currents. This term stems from the conduits appearing as shunts in an electrical diagram of the electrolyser stack.

Shunt currents are undesirable because the current flowing through the shunts does not contribute to hydrogen generation but is delivering waste heat only.

Shunt currents can be reduced by lowering the voltage across the stack or by reducing the cross-sectional area of the conduits forming the electrical conductors. However, in order to avoid too high pressure drops in the conduits, the flow rates will typically define a certain minimum cross-sectional area. Consequently, the need to maintain shunt currents at an acceptable level sets an upper limit for the voltage applied to the electrolyser stack. Electrolyser stacks are typically fed with voltages in the range of SOO- SOO V.

Electrolysers are fed with DC current. Since electrolysers are normally connected to an AC system, a rectifier is required to convert the AC current into DC current. To minimize electronic noise on the grid and to allow full control over the current that is supplied, the rectifier is, typically, constructed as an active rectifier using insulated- gate bipolar transistors (IGBTs). IGBTs are expensive, and they are dimensioned by the maximum current. IGBTs can typically handle up to 1800 V DC. In order to optimize utilization of such expensive components, it would be advantageous to feed elec- trolyser stacks with higher voltages than the typical 300-500 V of conventional alkaline electrolysers, as this would be more cost-effective in that they would be able to convert more power without increasing the current rating of the rectifier. Unfortunately, as already discussed above, it can be expected that such higher voltage levels would cause undesirable levels of shunt currents.

Among electrolyser systems, there is a great variety. In some systems, membranes are provided as part of membrane electrode assemblies, in some cases flexible membrane electrode assemblies, others have metal meshes or grids pressing on the membrane or are provided with flexible gas diffusion layers. Some have a single bipolar plate between electrolyser modules in a stack, for example a corrugated bipolar plate, as disclosed in US4208451, others have double walled electrolysers, examples of which are given below. Each principle represents an attempt to optimize hydrogen production. No conclusion has yet been found on the most efficient configuration, and for a skilled person, there are no specific starting points for an optimized system and no direction for how to optimize in the best way. Often, improvements are found by multiple trial and error attempts, where various features are put together in the hope of finding further optimized systems.

Examples of the electrolyser arrangements are illustrated in US patent applications US2021/0234237 and US2021/0202963, where opposite separator plates are welded to each other.

US2021/0234237 discusses separator plates for electrochemical systems and discloses a bipolar separator made of two combined corrugated plates so that the corrugation form cooling channels in between the two metal plates and gas transport channels on their outer sides. Bipolar plates are stacked and arranged on both sides of membrane electrode assemblies, MEA, typically sandwiched between gas diffusion layers, for example nonwovens. On its outer side, the corrugations are in contact with the gas diffusion layer. Generally, gas diffusion layers, between the membrane and the electrodes are often used for proper flow and diffusion of the gas away from the membrane.

However, the more layers the electrolyser cell comprises, the higher is the risk that components are moving relative to each other and cause reduced efficiency or even malfunctioning of the electrolyser. Accordingly, there is an interest in providing electrolyser systems with high rigidity and sturdiness.

US2004/0038102 discloses a fuel cell stack with alternately arranged membrane-electrode units and separator plates for the introduction and removal of the reactant and oxidative fluid. The separator plate comprises a first structured area on its side comprising multiple raised surface portions towards the membrane and forming walls of multiple minor gas channels extending between the separator plate and the membrane for transport of produced gas in the minor gas channels along the separator plate.

US4608144 discloses an electrolysis system for production of chlorine where a single corrugated bipolar electrode plate is sandwiched between membranes. The corrugation on either side comprises horizontal minor channels between major vertical channels, with a flow of electrolyte from the bottom, upwards through one major channel, then through minor channels to the adjacent major channel and then upwards through the adjacent channel and out of the chamber at the top. It is mentioned that this leads to rapid removal of the gases.

US5114547 takes offset in this system in US4608144 and discloses an electrolysis system for production of chlorine where embossed corrugations in monopolar or bipolar metal electrode plates are formed in a herringbone pattern, where the minor channels extend inclined from the vertical major channels. This is explained as leading to improved flow and circulation of the electrolyte and further rapid removal of the formed gases. Optionally, the vertical major channels are provided with openings for electrolyte circulation. US5114547 does not, however, teach any means whereby the electrolyte may be caused to recirculate inside the electrode chamber. These examples illustrate only a few attempts in different directions for improvements electrolysis, going. However, for optimization, there is still room for improvement. In particular, it would be desirable to provide improved hydrogen-producing electrolys- ers that are simple in construction but robust, reliable, and efficient, and that would allow for higher voltage levels across the electrolyser stack without undesirable increase in the level of shunt currents.

DESCRIPTION / SUMMARY OF THE INVENTION

It is therefore an objective of the invention to provide an improvement in the art. In particular, it is an objective to provide an electrolyser with a high degree of operational reliability that allows for higher voltage levels across the electrolyser stack without undesirable increase in the level of shunt currents. Furthermore, it is an objective to provide an electrolyser with a simple construction of a plurality of separa- tor/electrode modules that sandwich membranes in between, where the modules are rigid and suitable for mass production at relatively low cost, and which allow for good temperature control of the electrolyser. These objectives and further advantages are achieved with an electrolyser, in particular an alkaline electrolyser, for production of hydrogen gas as described below and in the claims.

In short, in an electrolyser stack for production of hydrogen gas, multiple bipolar electrically conducting metal separator plates sandwich membranes. Each separator plate has raised surface portions towards the membrane, forming minor gas channels between the separator plate and the membrane for transport of produced gas along the separator plate. Each structured area with such minor channels is surrounded by a combination of an upper major channel above and a lower major channel below the first structured area, as well as a first major channel and second major channel connecting the lower major channel with the upper major channel on a first and second side. Gas flow through the channels leads to circulation of electrolyte through and around the structured areas.

Details and further advantages are explained in the following. In the electrolyser according to the invention, a stack of electrolyser cells is provided, filled with liquid electrolyte. An ion conducting membrane separates adjacent electrolyser cells. On either side of the membrane, there is provided a bipolar electrically conducting metal separator plate. Each two of the bipolar separator plates are sandwiching an ion transporting membrane, either by abutment or in a near-zero gap or in combination with supplemental electrodes layers between the separator and the membrane.

Each metal separator plate in combination with the respective membrane on opposite sides of the separator plate is delimiting an anode chamber on an anode side and a cathode chamber on an opposite, cathode side of the separator plate, the separator metal plate separating the anode side from the cathode side of the bipolar electrode.

The separator metal plate comprises a first structured area on at least one of its sides, which is fitted or embossed with a structural surface pattern comprising multiple raised surface portions forming walls of multiple minor gas channels extending between the separator plate and the membrane for transport of produced gas in the minor gas channels along the separator plate.

The raised surface portions form ridges, and the lower surface portions in between the raised portions form valleys between the ridges. The lower surface portions are distal to the respective membrane as compared to the raised surface portions, which are close to the membrane, so as to form multiple minor gas channels between the separator plate and the membrane for transport of produced gas in the lower surface portions between the raised surface portions.

In particular, the first structured area with the minor channels is surrounded by a combination of major channels. This includes an upper major channel, for example largely horizontally oriented upper major channel, above the first structured area and a lower major channel, for example largely horizontally oriented lower major channel, below the first structured area. It also includes a first major channel, for example vertical or largely vertical first major channel, connecting the lower major channel with the upper major channel on a first side of the first structured area and a second major channel, for example vertical or largely vertical second major channel, connecting the upper major channel with the lower major channel on an opposite second side of the first structured area.

Each minor channel has an inlet end and an outlet end. When the separator is oriented for operation of the electrolyser, the outlet end is arranged higher than the inlet end, relatively to a horizontal plane. The outlet ends of the minor channels in the first structured area are connected to the first major channel for flow of gases by buoyancy through the minor channels towards the outlet end and into the first major channel and up to the upper major channel for release of the gas through a respective outlet. Furthermore, the inlet end of each minor channel is connected to the second major channel in order for the flow of gas through the minor channels and the first major channel forcing circulation of electrolyte through the minor channels, upwards through the first major channel, sideways through the upper major channel, and downwards in the second major channel. This circulation inside the electrolyte chambers is highly useful as will be explained in more detail in the following with some examples and specific embodiments.

For example, a fraction of the raised and lower surface portions in the first structures area of the separator plate combine into a corrugated pattern.

For the embodiments, where the raised and lower surface portions are embossed, the embossed structures have a function on both sides of the separator plate. Accordingly, a fraction of the lower surface portions forms ridges on the opposite side. In particular, the lower surface portions on the cathode side with minor channels for transport of hydrogen gas form raised surface portions on the anode side, and the raised surface portions on the cathode side form lower surface portions with minor channels for transport of oxygen gas on the anode side.

In some concrete embodiments, the raised surface portions of the metal separator plate are pressed against the ion-conducting membrane or against an electrode abutting the ion-conducting membrane. The two opposite sides of the metal separator plate facing the respective one of two ion-transporting membranes forms an anode chamber and a cathode chamber with the respective membranes.

The electrode chambers contain electrolyte, in particular alkaline electrolyte, for example a NaOH or KOH based electrolyte.

The metal separator plate may serve as the electrolytically active bipolar electrode. Such an arrangement can be said to represent a “mixed gap” configuration, comprising a zero-gap arrangement where the raised surface portions of the metal separator plate touch the membrane, and a gap arrangement where the lower surface portions of the metal separator plate extends as a valley between the ridges to a certain distance from the membrane in order to form the gas-transport channels. Typically, the channel has a depth D, where D is in the range of 1 to 10 mm when measured as a distance from the ridges to the lower most portion of the channel.

As an alternative to the metal separator plate serving as the electrolytically active bipolar electrode, a porous electrode may be placed as a layer on top of one or both the two opposite bipolar electrode surfaces, so that the porous electrode is resting on the raised surface portions of the metal separator plate, partly or completely abutting the respective membranes, and thereby, correspondingly, partly or completely serving as the electrolytically active parts of the electrolytic cell. In this embodiment, a zero-gap arrangement may be established across the entire surface of the membrane, as the gas is able to flow from the membrane, through the pores of the porous electrode, and into the channels that are formed by the lower surface portions.

Advantageously, the separator plate comprises at least two structured areas with the minor channels but typically more than two structured areas.

In some embodiments, a second structured area is provided beside the first structure on the opposite side of the first major channel, and the minor channels of the first structured area and the minor channels of the second structured area in combination with the first major channel form a herringbone configuration with minor channels inclined upwards towards the first major channel. For example, the minor channels, arranged in a herringbone pattern, are connecting two major channels and are fluidflow communicating with the major channels, which, in turn, are oriented in a vertical or largely vertical direction. Relative to vertical, the angle of the minor channel herringbone pattern is, typically, larger than the angle of the major channel connecting to the outlet end of the minor channel.

One or more pairs of major vertical channels are connecting to a respective set of minor channels. At the top and bottom of the herringbone pattern that is created by the minor channels, the vertical major channels are connected to horizontal or largely horizontal major channels. The effect of such arrangement is that a series of minor channels, arranged in a herringbone pattern, is surrounded by two vertical channels and two horizontal channels.

The metal separator plate may be embossed with several sets of minor channels, for example each set provided in a herringbone pattern, each set delimited by major channels.

In more detail, when the electrolyser is operational, oxygen and hydrogen gases created by electrolysis at the active surfaces on or adjacent to a minor channel are transported through the inclined minor channel towards the outlet end of the minor channel and into a first major channel, for example vertically or largely vertically oriented major channel. The transport is caused or facilitated by gas bubble buoyancy, and when the gas bubbles move through the minor channels, they drag along electrolyte, which is highly useful for circulation of the electrolyte in the electrolyte chamber. Once, the bubbles and the associated electrolyte enter the first major channel, the flow continues upwards through the first major channel until it reaches a first, upper major channel, for example upper horizontal or approximately horizontal channel. Here, the gas and part of the associated electrolyte will leave the electrode chamber through corresponding gas outlets into gas-removal conduits for release of the gases. After having released all or part of the gas bubbles contained in the electrolyte, the electrolyte recirculates, flowing in a second major channel, for example vertically or largely vertically oriented second major channel. Here, the electrolyte flows downwards through one or more of the second major channels. On passing the inlet end of a minor channel, connecting to the second vertical major channel, part of the electrolyte flows into this minor channel, replenishing the electrolyte that was dragged along by the gas bubbles created by electrolysis. At the bottom of the second major channel, a remaining flow of electrolyte enters a lower major channel, for example horizontal or approximately horizontal lower major channel, where the electrolyte flows largely horizontally. On passing an inlet end of a minor channel connected to the lower horizontal channel, part of the electrolyte flows upwards into such minor channel, replenishing the electrolyte that was dragged along by the gas bubbles created by electrolysis. After passing the inlet ends of the minor channels that connect to the lower horizontal channel, remaining electrolyte flows upwards into and through one of the first major channels.

This arrangement of minor and major channels creates a simple path for bubble-driven electrolyte circulation in the electrode chamber, from the lower part upwards through the first major channel to the upper major channel in the upper part of the electrolysis cell above the minor channels and then, to the second major channel prior to flowing downwards through the second major channel back to the lower part. Typically, as already outlined above, there are plural major channels of the first type with upwards flow and plural major channels of the second type with the downwards flow.

By suitable dimensioning of the width and depth of the minor and major channels, this bubble-driven circulation through the electrode chamber will cause rapid, self-driven circulation of the electrolyte.

In the above description and in the drawings, a simple layout of the channel system is assumed, comprising vertical and horizontal major channels delimiting structured areas filled with minor channels in a herringbone pattern. Alternatively, layouts are provided comprising triangular, polygonal, or curved paths of the major channels, and with minor channels in different patterns, provided such alternative layouts make use of minor channels that are oriented in an inclined, and thus non-horizontal, direction and fluid-flow communicating with first major channels, thereby establishing a rapid, well-defined bubble-driven internal circulation in the electrode chamber.

This rapid bubble-driven circulation of the electrolyte in the electrode chamber leads to a number of advantages. Firstly, the circulation ensures fast replenishment of the electrolyte at all active areas of the electrodes, thereby eliminating the need for pump-driven circulation of the electrolyte inside the chambers as is otherwise generally required in conventional electro- lysers.

In contrast thereto, inside the electrode chambers forming the electrolytic cells of conventional electrolysers, random circulation of the electrolyte is not sufficient to avoid electrolyte concentration gradients and uneven temperature distribution. Therefore, the electrolyte needs to be circulated by significant pumping of the electrolyte flowing through the electrolyte feed conduits.

Furthermore, in conventional electrolysers, the inlet temperature of the electrolyte needs to be maintained at a level fairly close to the outlet temperature of the electrolyte in order to maintain a desired average temperature of the electrolyte throughout the electrode chambers. Due to the combined effects of the circulation needed for the replenishment of the electrolyte at all active areas of the electrodes and the need for maintenance of the inlet temperature of the electrolyte at a level fairly close to the outlet temperature of the electrolyte, conventional electrolysers require a high flow rate of the electrolyte. A high electrolyte flow rate requires not only substantial pumping but also necessitates fairly large cross-sectional areas of the electrolyte feed conduits and the gas removal conduits. As described above, undesirable shunt currents will flow from one end of the electrode stack to the other in the electrolyte feed conduits, and currents may also flow from one end of the electrode stack to the other in the gas removal conduits, depending on the actual configuration of the gas removal conduits. The need to maintain shunt currents at an acceptable level sets an upper limit for the voltage applied to the electrolyser stack of conventional electrolysers. Electrolyser stacks are typically fed with voltages in the range of 300-500 V. For a given power rating, this leads to a need for larger and costlier rectifiers than would otherwise be possible if a higher voltage could be applied.

In contrast to these traditional arrangements, the arrangement of the electrolyser stack according to the invention provides significant internal circulation of the electrolyte in the electrolytic cell through the bubble-driven circulation, as described above. Consequently, no pump-driven circulation of the electrolyte is necessary to ensure that the electrolyte is evenly distributed through the electrolyte chamber, avoiding differences in electrolyte temperature and concentration. The controlled circulation obtained with the embossed or otherwise structured bipolar metal plates ensures that no ’’dead areas” without circulation occur in the electrolyte chambers.

Furthermore, the significant internal circulation of the electrolyte in the electrode chambers through the bubble-driven circulation leads to rapid mixing of fresh electrolyte with the electrolyte already present in the electrode chambers. The consequence of this rapid mixing is that the fresh electrolyte can be fed to the electrolyte chambers at much lower temperature than allowable in conventional electrolysers.

The amount of heat removed with the circulating electrolyte is proportional to the product of the flow rate multiplied by the difference between the inlet and outlet temperature of the electrolyte. Consequently, since the need for maintenance of the inlet temperature of the electrolyte at a level close to the outlet temperature of the electrolyte, in order to maintain a desired average temperature of the electrolyte throughout the electrode chambers, can be significantly relaxed, a larger temperature difference can be accepted. This in turn leads to a significant reduction in the required flow rate.

Furthermore, as no pumping is required to force circulation in the electrolyte chambers, the electrolyte flow rates in the electrolyte feed conduits required for this purpose can also be drastically reduced.

The drastically reduced flow rates in the electrolyte feed conduits allow for much smaller cross-sectional areas of the electrolyte feed conduits. These smaller cross-sectional areas serve to reduce the shunt currents. As a consequence, the applied voltage across the electrolyser stack can be dramatically increased without increasing the shunt currents above normally acceptable levels, leading to much more cost-efficient rectifier designs.

In the following, the advantage is demonstrated by simple calculations.

A conventional electrolyser may be provided with a 400 V DC power supply capable of delivering 2500 A, delivering a total power of 1 MW. It may be constructed as a series of electrolytic cells having a thickness of each cell of 1 cm and a voltage drop across each cell of 1.84 V. At a total applied voltage of 400 V and a cell voltage of 1.84 V a total of 217 cells are required, and the stack will have a length of 2.17 m. The stack efficiency is 80%, so the waste heat power is P = 200 kW

The temperature of the electrolyte is kept close to constant by circulation of the electrolyte, maintaining a temperature difference of 5 K between electrolyte outlet and electrolyte inlet. The electrolyte has a heat capacity of 4 kJ/kg K and a density of 1200 kg/m³. The electrolyte further has an electrical conductance of 1 S/cm.

Applying these values, the electrolyte flow rate is 0.0083 m³/s.

Assuming that the electrolyte is fed through two conduits, also assuming conservatively that the flow rate is constant through the conduits and targeting a pressure drop of 0.1 bar through each conduit, each of the two conduits needs to have a diameter of 40 mm, leading to a cross-sectional area of 0.0013 m².

Based on this cross-sectional area and the conductance of the electrolyte, the two parallel conductors formed by the electrolyte contained in the electrolyte inlet conduits have a total resistance of 9 Ohm.

At an applied voltage of 400 V this resistance leads to a total shunt current of 46 A and a total shunt loss of 19 kW. This shunt loss corresponds to 1.9% of the total applied power to the electrolyser.

A shunt loss of 1.9% could be considered reasonable for a conventional electrolyser.

Likewise, an electrolyser according to the invention may also be provided with a 400 V DC power supply capable of delivering 2500 A, delivering a total power of 1 MW. It may be constructed as a series of electrolytic cells having a thickness of each cell of 1 cm and a voltage drop across each cell of 1.84 V. At a total applied voltage of 400 V and a cell voltage of 1.84 V a total of 217 cells are required, and the stack will have a length of 2.17 m. The stack efficiency is 80%, so the waste heat power is P = 200 kW Also, for the electrolyser according to the invention, the temperature of the electrolyte is kept close to constant by circulation of the electrolyte, but the bubble-driven internal circulation in the electrode chambers makes it possible to raise the temperature difference between the electrolyte outlet and the electrolyte inlet to at least 20 K, for example 30 K (Kelvin).

Applying these values, the electrolyte flow rate is 0.0014 m³/s. This is a factor of six lower than in the above prior art comparative example.

Assuming that the electrolyser according to the invention has the same layout as the conventional electrolyser and the same pressure drop of 0.1 bar through each conduit, each of the two conduits needs to have a diameter of 20 mm, leading to a cross-sectional area of 0.00031 m². This is a factor of four smaller than in the comparative example above.

Based on this cross-sectional area and the conductance of the electrolyte, the two parallel conductors formed by the electrolyte contained in the electrolyte inlet conduits have a total resistance of 35 Ohm, which is four times higher than the prior art comparative example above.

At an applied voltage of 400 V this resistance leads to a total shunt current of 12 A and a total shunt loss of 5 kW. This shunt loss corresponds to 0.5% of the total applied power to the electrolyser.

Compared with the 1.9% shunt loss of the conventional electrolyser, the bubble-driven circulation in the electrode chambers leads to a relative reduction of the shunt loss by a factor of four.

The reduction in shunt loss may be used to increase the applied voltage without increasing the shunt losses above those of a conventional electrolyser.

An electrolyser according to the invention may be provided with an 800 V DC power supply capable of delivering 2500 A, yielding a total power of 2 MW, i.e., double the power rating of the conventional electrolyser. Since the current level is the same as the current level delivered to the conventional electrolyser, and given that at voltage levels below 1200 V or 1800 V, depending on the type of IGBTs used in the rectifier, the IGBTs and the cables are dimensioned exclusively by the current level, the doubling of the power rating does not lead to any need for changes of the rectifier. Applying electrolytic cells having the same thickness and voltage drop as for the conventional electrolyser, the stack length becomes 4.34 m. As for the conventional electrolyser, the stack efficiency is 80%, so the waste heat power is P = 400 kW

The temperature of the electrolyte is kept close to constant by the natural circulation of the electrolyte created by the gas bubble generation in the flow system according to the invention. Therefore, there is no need for maintaining a small temperature difference between the electrolyte outlet and the electrolyte inlet. A realistic temperature difference may be set to be in the range of 10 K to 40 K, for example in the order of 30 K.

Relative to the conventional electrolyser, the waste power has been doubled, while the temperature difference has been reduced by a factor of six. Consequently, the electrolyte flow rate is one third of the flow rate in the conventional electrolyser, 0.0028 m³/s.

Assuming again that the electrolyte is fed through two conduits, also assuming conservatively that the flow rate is constant through the conduits, and again targeting a pressure drop of 0.1 bar through each conduit, each of the two conduits needs to have a diameter of 30 mm, leading to a cross-sectional area of 0.00071 m².

Based on this cross-sectional area and the conductance of the electrolyte, the two parallel conductors formed by the electrolyte contained in the electrolyte inlet conduits have a total resistance of 31 Ohm.

At an applied voltage of 800 V this resistance leads to a total shunt current of 26 A and a total shunt loss of 21 kW. This shunt loss corresponds to 1.0% of the total applied power to the electrolyser. Consequently, since the rapid internal circulation in the electrode chambers created by the bubble-driven circulation provides the opportunity to significantly increase the temperature difference between the electrolyte outlet and the electrolyte inlet, it is possible to increase the power rating of the electrolyser by a factor of two without making any change to the rectifier, while at the same time reducing the shunt loss to slightly more than half of the shunt loss of a conventional electrolyser. This is a very important consequence of the invention.

The combination of a doubling of the power rating without any change to the power supply and a reduction of the shunt losses by almost half leads to significant improvement of the cost-effectiveness of an electrolyser according to the invention relative to a conventional electrolyser.

In practical embodiments, the gas outlets connect the anode chamber with an oxygen transport conduit and the cathode chamber with a hydrogen transport conduit. Optionally, the oxygen transport conduit and/or the hydrogen transport conduit extend along the stack through openings in the anode and cathode plates. For example, gaskets are provided between the metal separator plates so that the openings in the stack of bipolar plates, optionally in addition to corresponding openings in membrane-holding frames, form a longitudinal gas conduit through and along the stack.

This arrangement with the bipolar metal plate fitted or embossed with raised portions provides a structurally sound stack that has excellent stiffness. It is easily manufactured applying conventional pressing methods. And due to the arrangement of the flow channels, with minor channels in a herringbone pattern, feeding major channels, a well-defined internal flow of electrolyte is established in the electrode chamber, once, bubbles start forming as a consequence of gas creation by electrolysis.

Alternatively, to embossing the metal plate, the raised portions of the bipolar metal plate, which form the minor and major channels, may be made by fitting metal strips to the surface of the bipolar metal plate, forming gas flow channels in valleys between the metal strips. A cross-section of an embossed pattern that forms the channels may optionally be smoothly alternating, for example being approximately sinusoidal. An approximately sinusoidal cross-section reduces the contact area between the membrane and the anode and cathode surfaces. Despite deformation of the membrane and the embossed plate, the contact area is minimized to only a few percent of the membrane surface area.

Alternatively, the cross-section of the embossed pattern that forms the channels is polygonal. A polygonal cross-section can be made to have the same small contact area between the membrane and the anode and cathode surfaces, but it may also be made to have a significant part of the anode and cathode surfaces in close contact, dependent on the preference.

In order to permit unrestricted flow of gas bubbles, a certain minimum depth of the channels is required. Typically, at depth of less than 1 mm will lead to flow restrictions. At the other extreme, a depth much larger than 10 mm may lead to less- than-optimal internal circulation, since the ratio of bubbles to electrolyte may become too small for the bubbles to drag along the electrolyte in the desired manner. Consequently, the minor channels advantageously have a depth in the range of 1 mm to 10 mm with areas in between the minor channels abutting the membrane or, alternatively, abutting an electrode layer.

In some advantageous embodiments, the minor channels have a length in the order of 50 to 200 mm and a width in the order of 4 to 40 mm. For example, the length may be 10 to 50 times the width.

Typical dimensions are given in the following:

Thickness of bipolar plates: 0.3 mm to 1.0 mm Length/width of the bipolar plates: 0.3 m to 3 m Depth of minor channels in herringbone pattern: 1-10 mm

In some embodiments, supplementary electrodes may be placed on the anode and/or the cathode. Such supplementary electrodes may be plates with holes, meshes or other advantageous designs. The supplementary electrodes are optionally coated with a catalytic material. As mentioned above, a porous electrode may be placed on top of one or both surfaces of the bipolar separator plate, the electrodes resting on the raised surfaces of the bipolar separator plate, partly or completely abutting the respective membranes, and thereby partly or completely serving as the electrolytically active parts of the electrolytic cell.

It would be obvious to arrange the stacking of the bipolar separator plates, the porous electrodes and the membranes so that the porous electrodes and the membranes are flat. It is always necessary, however, that the porous electrodes have good electrical contact to the bipolar separator plates. Ensuring this could require quite narrow tolerances in the manufacturing of the constituent parts of the stack.

In order to reduce the tolerance requirements, a particularly advantageous arrangement can be made by arranging the stack system so that the raised surface portions of the bipolar separator plate project slightly beyond a level defined by the raised surfaces of the adjacent and opposite bipolar separator plate with the addition of the thickness of the one or more porous electrodes and the membrane. This arrangement causes the membrane, or alternatively the combination of the porous electrodes and the membrane, to deform elastically, forming a wave pattern with elevations at the contact areas with the raised surfaces of the bipolar separator plate and with depressions between the raised surfaces of the bipolar separator plate where the raised surfaces of the adjacent bipolar separator plate are in contact with the membrane, or alternatively with the combination of the porous electrodes and the membrane.

The contact force between the raised surface portions of the bipolar separator plate and the one or more porous electrodes and the membrane will depend on the stiffness of the porous electrodes and membrane, the depth of the lower surface portions and the spacing between the raised surface portions of the bipolar separator plate. By varying these parameters, it is possible to adjust the contact force to establish sufficient electrical contact while at the same time providing allowance for increased manufacturing tolerances. Increased manufacturing tolerances will in turn lead to a more robust arrangement with reduced risk of scrap of out-of-tolerance components. As it appears from the above, the presented separator is a single-plate separator, which is in contrast to some prior art systems where double-plate separators are used with a coolant chamber in between the two plates, for example identical plates welded to each other. In particular, in the invention, no additional coolant is required, as the electrolyte also fulfils the coolant function. In the latter case, the electrolyte is the only liquid in the system, apart from the water that has to be supplemented to the system in order to substitute the water consumed for hydrogen production.

SHORT DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail with reference to the drawing, were FIG. 1A is a sketch of an electrolytic cell in a gap configuration;

FIG. IB is a sketch of an electrolytic cell in a zero-gap configuration;

FIG. 1C is a sketch of an electrolyser stack;

FIG. 2A illustrates an assembly of an electrolytic cell;

FIG. 2B illustrates an upper portion of a metal separator plate;

FIG. 2C illustrates an upper portion of a metal separator plate with a gasket, demonstrating the electrolyte circulation;

FIG. 3 illustrates the circulation of the electrolyte;

FIG. 4A illustrates a sinusoidal cross section of the metal separator plate;

FIG. 4B illustrates a sinusoidal cross section of the metal separator plate abutting the membrane;

FIG. 4C illustrates a triangular cross section of the metal separator plate;

FIG. 4D illustrates a triangular cross section of the metal separator plate with rounded edges;

FIG. 5A is a cross section of a metal separator plate in combination with a supplementary electrode layer in the form of a perforated sheet;

FIG. 5B is a cross section of a metal separator plate in combination with a supplementary electrode in the form of a metal mesh.

FIG. 6A is a pre-assembly cross section of a metal separator plate in combination with sandwich structure made of two secondary electrodes and the membrane;

FIG. 6B is similar to FIG. 6A but after assembly:

FIG. 6C illustrates the assembly of FIG. 6B with a slightly deformed membrane. DETAILED DESCRIPTION / PREFERRED EMBODIMENT

FIG 1 shows principle sketches of general variants of electrolytic cells and an electro- lyser stack.

FIG. 1 A is a sketch of an electrolytic cell 1 in a gap configuration, comprising a cathode 2, an anode 3, and an ion-transporting membrane 4. Hydrogen gas 8 is produced at the side of the cathode 2 facing the membrane 4 in the cathode chamber 5, and oxygen gas 9 is produced at the side of the anode 3 facing the membrane 4 in the anode chamber 6. A power supply 7 drives the electrolytic process.

FIG. IB is a sketch of an electrolytic cell 1 in a zero-gap configuration, comprising a porous cathode 12, a porous anode 13, and an ion-transporting membranes 4. The cathode chamber 5 and the anode chamber 6 are contained within metal separator plates 14. Electrical connections are provided between the electrodes 12, 13 and the metal separator plates 14 with conductive elements 15, not detailed in the sketch. Hydrogen is produced at the side of the porous cathode 12 facing the membrane 4 and is conveyed to the cathode electrode chamber 5 through holes or pores 16 in the cathode 12, and oxygen is produced at the side of the porous anode 13 facing the membrane 4 and is conveyed to the anode electrode chamber 6 through holes or pores 16 in the anode 13.

FIG. 1 C is a sketch of an electrolyser stack 17, comprising a series of electrolytic cells 1 in zero-gap configuration of the type as illustrated in FIG. IB.

FIG. 2 illustrates some details of a practical example of an electrolytic cell according to the invention.

FIG 2A is an exploded view of the electrolytic cell 20 according to the invention. The cell 20 comprises an ion-conducting membrane 23, a first metal separator plate 21 which also is acting as cathode towards the illustrated membrane 23, a first gasket 22 and a second gasket 24, which also function as holders, for example frames, for the membrane 23, and a second metal separator plate 25 acting as anode towards the membrane 23.

The first metal separator plate 21, which is acting as cathode, the first gasket 22, and the ion-conducting membrane 23 jointly delimit the cathode chamber 5, and the ionconducting membrane 23, the gasket 24 and the second metal separator plate 25, which is acting as anode, jointly delimit the anode chamber 6. Both electrode chambers 5, 6 contain the necessary electrolyte for the electrolytic reaction where water is split into oxygen and hydrogen, respectively.

Also illustrated is a further ion-conducting membrane 23A and a further gasket 24A on the opposite side of the first separator plate 21. Due to its bipolar nature, the singleplate first separator plate 21 acts, in this configuration, as cathode relatively to the membrane 23, but acts as an anode towards the further membrane 23 A. In the shown embodiment, the single plate bipolar separator plate 21 is directed towards membranes on both sides. A stack of this configuration would comprise altematingly a sequence of : further membrane 23 A - further gasket 24 A - separator 21 - gasket 22 - membrane 23 - gasket 24 - separator 25 - even further gasket (not illustrated) - even further membrane (not illustrated) - and so forth.

However, as will be explained and shown below, electrode layers may be placed between the bipolar separator 21 and the respective membranes 23, 23 A on opposite sides of the separator plate 21.

In order to replenish the water that is consumed during the electrolysis reaction, and in order to cool the electrolyser stack 17, electrolyte that has externally been refreshed with the required fresh water and that has also been cooled is continuously supplied and circulated through the two electrolyte feed conduit 31 formed by the relevant openings in the metal separator plates 21 and 25 and the gaskets 22 and 24. Optionally, one conduit is used for the feed of electrolyte to the anode and another for feed of electrolyte to the cathode. An outlet 32 for the electrolyte is optionally provided for cooling purposes and recirculation after cooling.

For removing the oxygen and hydrogen gases from the respective electrode chambers, similar gas outlet conduits 33 and 34 are established. The gas outlets conduits 33 and 34 are filled with a mixture of electrolyte and the respectively produced gases and lead into external gas separation vessels for separating the gases from the electrolyte.

Typically, the metal separator plates 21 and 25 are manufactured from stainless steel or nickel and may be provided with surface coatings.

Typically, the gaskets 22 and 24 are manufactured from an elastomeric material such as EPDM or Viton. Many other types of soft and hard gasket materials are also available, and selection of the preferred material will depend on a number of factors, such as pressure, oxygen saturation level, and temperature.

Typically, polymer membranes are used in alkaline electrolysers. For example, the membrane comprises an open mesh polyphenylene sulfide fabric which is symmetrically coated with a mixture of a polymer and zirconium oxide. The latter is advantageous for systems in which the electrolyte is used at elevated temperatures, for example in the range of 50°C-90°C.

FIG. 2B shows the top end of a metal separator plate 21 with the gas outlet conduits 33 and 34. The metal separator plate is embossed with minor channels 40 in herringbone patterns 35. Each upward directed herringbone configuration 35 is provided by a first structural area 30A comprising a first set of minor channels 40 inclined towards the first major channel 38 and a second structural area 30B with a second set of minor channels 40 inclined in an opposite way towards the same first major channel 38.

FIG 2C shows the top end of a metal separator plate 21 with the gas outlet conduits 33 and 34, and with a gasket 22 placed on the metal separator plate 21. Notice that the orientation of the metal separator plate 21 and the gasket 22 is from the opposite side as compared to FIG. 2A. The gasket 22 has gas outlet slits 37 to allow for the escape of gas to the gas outlet conduit 34. The metal separator plate 21 with the embossed herringbone pattern 35 of minor channels 40, the gasket 22, and the ion-conducting membrane 23, which will be placed on top of the gasket 22, form a chamber with the minor channels 40 and the major channels 38. Due to the inclination of the raised surface portions that form ridges and the lower surface portions that form valleys between the ridges in the herringbone pattern 35, gas bubbles and attached electrolyte will flow from the minor channels 40 into a first vertical major channel 38 and there continue in the upwards direction, indicated by arrow 42. On arrival in the upper horizontal major channel 41, the gas bubbles and electrolyte will partly separate, and a substantial portion and typically most of the gas bubbles and some of the electrolyte is escaping through the gasket slits 37 to the gas outlet conduit 34, and a small portion of the remaining gas bubbles and electrolyte is moving downwards through the second vertical major channels 39 on the opposite sides of the first structured area 30A and the second structured area 30B.

The arrows 42 and 43 show the upwards and downwards flow directions in the major channels 38 and 39, and the arrows 44 show how the flow is split in the horizontal major channel 41 at the top of the vertical major channel 38.

As the herringbone pattern 35 of the minor channels 40 is provided by embossing an alternating pattern into the metal separator plate 21, the inclined minor channels 40 are not only present on the side of the metal separator plate 21 shown in the figure, but are likewise present on the opposite side of the metal separator plate 21, and similar flow patterns will occur in both electrode chambers 12, 13.

FIG. 3 illustrates by the arrows 42, 43, 44 and 45 the principle of the flow of the electrolyte in the complete anode chamber 12. The flow of the gas bubbles in the minor channels 40 is upwards in an inclined direction, as indicated by the arrow 45, driven by the gas buoyancy. When the flow, which comprises a mixture of gas bubbles and electrolyte, reaches the end 40B of the minor channel 40 and enters the first vertical channel 38, the flow continues vertically upwards, as indicated by arrow 42, and is reinforced by the sum of all the contributing flows from the minor channels 40, each driven by the gas bubbles therein. The moving gas bubbles also drag the electrolyte 46 along, which causes a circulation of the electrolyte 46 upwards in the vertical first major channels 38 to the upper horizontal channel 41. After having released all or part of the gas bubbles contained in the electrolyte 46, the electrolyte recirculates, flowing downwards in the neighbouring vertical second major channels 39. On passing the inlet end 40A of a minor channel 40, connecting to one of the vertical second major channels 39, part of the electrolyte flows into the minor channel 40, replenishing the electrolyte that was dragged along by the gas bubbles created by electrolysis. At the bottom of the vertical second major channel 39, remaining flow of electrolyte enters the horizontal lower major channel 47 where the electrolyte flows horizontally. On passing the inlet end of a minor channel 49 that is connected to the horizontal lower major channel 47, part of the electrolyte flows into such minor channel 40, replenishing the electrolyte that was dragged along by the gas bubbles created by electrolysis. After passing all inlet ends 40A of those minor channels 40 that are connected to the horizontal lower major channel 47, any remaining electrolyte 46 flows upwards in one of the vertical first major channels 38.

The cross-section of the embossed pattern that forms the minor channels 40 and the major channels 38, 39, 41 and 47 may be smoothly alternating, for example being sinusoidal. Alternatively, the cross-section of the embossed pattern forming the minor channels 40 and the major channels 38, 39, 41 and 47 is polygonal. A polygonal crosssection optionally has a similar small contact area between the membrane 23 and the anode and cathode surfaces, but it may also be made to have a significant part of the anode surface and cathode surface in close contact, dependent on the preference.

FIG. 4 A, B, C, and D illustrate examples of possible embossing in a metal separator plate 21 in a cross section along a line perpendicular to the minor channels 40 formed by the herringbone pattern 35 along the membrane 23.

In FIG. 4A, the cross section follows a sinusoidal curve along a line transverse to the minor channels 40. The raised surface portions 50 forming ridges 50A of the curve are provided in close proximity of the membrane 23.

Alternatively, as illustrated in FIG. 4B, for which the same numerals are valid as for FIG. 4A, the raised surface portions 50 forming ridges 50A of the embossing are resting against the membrane 23. By a pressing force, the ridges 50A press slightly into the membrane 23 for good conductivity and optimized electric field strength, while the gases produced at the ridges 50A flow into the minor channels 40 and from there into the first major channels 38. 1

Lower surface portions 51 forming valleys 51A of the embossing, so that the gases are transported in these formed minor channels 40 upwards to the gas outlets 33 and 34 as explained in connection with FIG. 2.

Other cross-sectional alternating shapes are possible, for example a triangular alternating curve with sharp edges, as illustrated in FIG. 4C, or a triangular alternating curve with rounded edges, as illustrated in FIG. 4D.

FIG. 5A illustrates an arrangement in which the electrode 9, 10 comprises a corrugated metal separator plate 21 as described above, and a supplementary porous electrode, provided as a perforated electrode layer 60, for example a thin perforated conducting sheet, placed between the metal separator plate 21 and the membrane 23 and resting on the ridges 50A of the raised surface portions 50 of the embossed part of the metal separator plate 21.

FIG. 5B illustrates a similar arrangement where the supplementary porous electrode is implemented as a metal mesh with first wires 61 and second wires 62 that are crossing each other.

The porous electrode is not limited to the embodiments illustrated in FIG 5A and FIG. 5B but may be of other types of perforated or porous conducting material having catalytic properties in as far its fits the purpose.

FIG. 6A illustrates a pre-assembly cross section of a metal separator plate 21 in combination with two secondary porous electrodes 70, 71, namely an anode, a cathode, and the membrane 23 in between the porous electrodes 70, 71, where the surface 72 of the upper porous electrode 71 is at a certain level 73.

In this configuration the embossing of the metal separator plate 21 comprises raised surface portions 50 with separate ridges 50A and lower surface portions 51 as valleys 50A between the ridges 51A and forming minor channels 40 for transport of gas and electrolyte. As illustrated herein, with reference to a symmetry plane 59 of the separator plate 21, including reference to FIG. 6B and FIG. 6C in this discussion, the term raised portions 50 and lower portions 51 is determined relatively to such symmetry plane 59 and with respect to a direction towards the membrane 21. They are not understood as related to up and down when the electrolyser is in operation, in particular when having in mind that the orientation of the separator plate 21 can be different, for example vertical. As the separator plate 21 is facing membranes 23 on both sides of the separator plate 21, the raised portions 50 on one side of the separator plate 21 correspond to lower portions 51 on the opposite side, and vice versa. However, this is due to the raised and lower portions being made by embossing, which, however, need not be the case

FIG. 6B illustrates the same arrangement as FIG. 6A during assembly with the adjacent metal separator plate 25 placed on the surface 72 of the upper porous electrode layer 71 at the level 73.

FIG. 6C illustrates the same arrangement as FIG. 6B but after assembly, with the adjacent metal separator plate 25 moved a certain distance below the level 73. This arrangement will cause the porous electrodes 70, 71 and the membrane 23 becoming deformed elastically, so as to form a wave pattern with deformations at the contact areas with the ridges 50A of the raised surface portions 50 of the metal separator plates 21, 25 and with lower surface areas between the ridges 50A. By proper selection of the level of depression, the contact force to establish sufficient electrical contact is adjusted, while at the same time providing increased manufacturing tolerances. Increased manufacturing tolerances, in turn, leads to a more robust arrangement with reduced risk of scrap of out-of-tolerance components.

Claims

1. An electrolyser (1) for production of hydrogen gas, the electrolyser (1) comprising a stack of electrolysis cells (20) with multiple bipolar electrically conducting metal separator plates (21, 25), on opposite sides of each of multiple ion-transporting membranes (23), wherein each separator plate (21, 25) in combination with the respective membrane (23) delimits an anode chamber (6) on an anode side of the separator plate (23) and a cathode chamber (5) on an opposite, cathode side of the separator plate (21, 25), wherein the separator plate (21, 25) comprises a first structured area (30A) on at least one of its sides, the first structured area (30A) comprising multiple raised surface portions (50) towards the membrane (23), the raised surface portions (50) forming walls of multiple minor gas channels (40) extending between the separator plate (21, 25) and the membrane (23) for transport of produced gas in the minor gas channels (40) along the separator plate (21, 25) , characterized in that the first structured area (30A) is surrounded by a combination of an upper major channel (41) above and a lower major channel (47) below the first structured area (30A), as well as a first major channel (38) and second major channel (39) connecting the lower major channel (47) with the upper major channel (41) on a first and second side, respectively, of the first structured area (30A); wherein each minor channel (40) has an inlet end (40A) and an outlet end (40B), the outlet end (40B) being connected to the first major channel (38) and arranged higher than the inlet end (40A) for flow of gases by buoyancy through the minor channel (40) towards the outlet end (40B) and into the first major channel (38) and up to the upper major channel (41) for release of the gas through a respective gas outlet conduits (33, 34), wherein the inlet end (40A) of each minor channel (40) is connected to the second major channel (39) in order for the flow of gas through the minor channels (40) and the first major channel (38) forcing circulation of electrolyte (46) through the minor channels (40), upwards through the first major channel (38), sideways through the upper major channel (41), and downwards in the second major channel (39).

2. The electrolyser according to claim 1, wherein the raised surface portions (50) are provided by embossing in the metal separator plate (21, 25), and wherein the raised surface portions (50) on the cathode side form minor gas channels (40) for transport of oxygen on the anode side.

3. The electrolyser according to claim 1 or 2, wherein the separator plate (21, 25) comprises a second structured area (30B) beside the first structured area (30A) on the opposite side of the first major channel (38), and wherein the minor channels (40) of the first structured area (30A) and the minor channels (40) of the second structured area (30B) in combination with the first major channel (38) form a herringbone configuration (35) with minor channels (40) inclined upwards towards the first major channel (38).

4. The electrolyser according to any preceding claims, wherein the electrolyser comprises a porous electrode (60, 61, 62, 70, 71) with a first side abutting the ion-conducting membrane (23) and a second side abutting the raised surface portions (50) of the metal separator plate (21, 25).

5. The electrolyser according to claim 4, wherein the combination of the membrane (23), a porous anode (70) and a porous cathode (71) form a sandwich structure in which the porous anode (70) and porous cathode (71) abut the membrane (23), wherein the separator plate (21, 25) on the anode side of the membrane (23) is abutting the porous anode (70) by first raised surface portions (50) of the separator plate (21, 25) and abutting the porous cathode (71) by second raised surface portions (51), wherein the first raised portions (50) are offset relatively to the second raised portions (51), and wherein the sandwich structure (70, 23, 71) is deformed into an undulating shape by force exerted laterally on the sandwich structure (70, 23, 71) by the first and second raised surface portions (50, 51) on the two opposite sides of the sandwich structure (70, 23, 71).

6. The electrolyser according to any one of the preceding claims, wherein the minor channels has a depth in the range of 1 - 10 mm, the separator plate has a material thickness in the range of 0.3 - 1.0 mm, the minor channels (40) have a length in the range of 50 - 200 mm and a width in the range of 4 - 40 mm, wherein the length is 10 - 50 times the width.

7. Method of operating an electrolyser for production of hydrogen gas, the electrolyser (1) comprising a stack of electrolysis cells (20) with multiple bipolar electrically conducting metal separators (21, 25) on opposite sides of each of multiple ion-transporting membranes (23), wherein each separator plate (21, 25) in combination with the respective membrane (23) delimits an anode chamber (6) on an anode side of the separator plate (23) and a cathode chamber (5) on an opposite, cathode side of the separator plate (21, 25), wherein the separator plate (21, 25) comprises a first structured area (30A) on at least one of its sides, the first structured area (30A) comprising multiple raised surface portions (50) towards the membrane (23), the raised surface portions (50) forming walls of multiple minor gas channels (40) extending between the separator plate (21, 25) and the membrane (23) for transport of produced gas in the minor gas channels (40) along the separator plate (21, 25) , characterized in that the first structured area (30A) is surrounded by a combination of an upper major channel (41) above and a lower major channel (47) below the first structured area (30A), as well as a first major channel (38) and second major channel (39) connecting the lower major channel (47) with the upper major channel (41) on a first and second side, respectively, of the first structured area (30A); wherein each minor channel (40) has an inlet end (40A) and an outlet end (40B), the outlet end (40B) being connected to the first major channel (38) and arranged higher than the inlet end (40A), the method comprising causing flow of gas by buoyancy through the minor channel (40) towards the outlet end (40B) and into the first major channel (38) and up to the upper major channel (41) and releasing the gas through a respective gas outlet conduits (33, 34), wherein the inlet end (40A) of each minor channel (40) is connected to the second major channel (39) and the method comprises causing flow of gas through the minor channels (40) and the first major channel (38) forcing circulation of electrolyte (46) through the minor channels (40), upwards through the first major channel (38), sideways through the upper major channel (41), and downwards in the second major channel (39).

8. The method according to claim 7, wherein the method comprises cooling the electrolysis cells (20) by extracting heated electrolyte from the electrode chambers (5, 6) through an electrolyte outlet (32) and replenishing cooled electrolyte to the electrode chambers through an electrolyte inlet (31).

9. The method according to claim 8, wherein the method comprises replenishing the electrolyte through the electrolyte inlet (31) at a temperature differing by no less than 10 Kelvin relatively to the temperature of the electrolyte at the electrolyte outlet (32).