An outlet duct of an electrolysis cell frame

Abstract

The invention relates to an electrolysis cell frame comprising: a compartment having an upper compartment surface; a compartment inlet connected to said compartment via an inlet duct; a compartment outlet fluidly connected said compartment via an outlet duct, and wherein said outlet duct is defined as a recess in said electrolysis cell frame wherein said recess is at least partly defined by a first recess par, wherein said first recess part is characterized in that it shapes said outlet duct in a direction increasing a first distance to said upper compartment surface and subsequently in a direction decreasing said first direction to said compartment.

Description

AN OUTLET DUCT OF AN ELECTROLYSIS CELL FRAMEField of the invention

[0001] The invention relates to an electrolysis cell frame comprising an outlet duct between a compartment and a compartment outlet, an electrolysis cell comprising two electrolysis cell frames and a method of controlling a volume of electrolyte in the outlet duct.Background of the invention

[0002] Electrolyser efficiency is continuously improved and one parameter to improve is the current efficiency i.e. to increase the percentage of current consumed by the electrolyser that is used for the actual production of hydrogen.

[0003] The design of the electrolysis cell may influence this. In the art various designs of an electrolysis cell is found having different electrolyte inlet and outlet designs. Such electrolysis cells can be found e.g. in EPl 119880 where a releasable outlet insert is disclosed. CN202308168, EP2895644, EP2532043 and US3717505 all disclose an electrolysis cell with an inlet and outlet duct fluidly connected to a production compartment of the electrolysis cell. WO2022156869 and EP4130340 specifically mentions aspects relevant for the design of the outlet of the electrolysis cell.Summary of the invention

[0004] The inventors have identified the above-mentioned problems and challenges related to current efficiency and electrolysis cell design and solved these problems by the present invention as described below.

[0005] In an aspect, the invention relates to an electrolysis cell frame comprising: a compartment having an upper compartment surface; a compartment inlet fluidly connected to said compartment via an inlet duct; a compartment outlet fluidly connected to said compartment via an outlet duct, and wherein said outlet duct is defined as a recess in said electrolysis cell frame wherein said recess is at least partlydefined by a first recess part, wherein said first recess part is characterized in that it shapes said outlet duct in a direction increasing a first distance to said upper compartment surface and subsequently in a direction decreasing said first direction to said compartment.

[0006] Forming the first recess part of the outlet duct in a hill -like structure with a first distance increasing and subsequently decreasing with respect to the compartment (such as the upper surface of the compartment) is advantageous in that it has the effect that electrolyte can be drained from said outlet duct thereby establishing a gas pocket leading to high ohmic resistance between electrolyte comprised by said compartment and electrolyte comprised by said compartment outlet. The ohmic resistance established by a gas pocket is higher than the ohmic resistance established by a length of outlet duct. Hence, a gas duct taking up a given length such as e.g. 10 cm of the outlet duct establishes a higher ohmic resistance than the same given length such as e.g. 10cm outlet duct with no gas pocket. In other words, it is possible to let the electrolyte escape from the outlet duct only under the impact of gravity either towards the compartment outlet or towards the compartment. Thereby discontinuing the flow of electrolyte between compartment and compartment outlet thereby establishing the high ohmic resistance.

[0007] An outlet duct with a hill-like structure should be understood an can be implemented as an outlet duct having / with a ridge. By forming such ridge the outlet duct will have an inlet establishing a flow path guiding gas and or electrolyte through one or more curves. In theory one such curve is sufficient to establish an outlet duct with a ridge. However, to establish efficient ohmic resistance, the outlet duct can comprise two or three such curves. By designing an outlet duct with e.g. three curves, a U-bend / water trap can be established. Thus, in such embodiment the outlet duct may comprise both a ridge and a valley. As explained or at least indicated elsewhere in this document an outlet duct with a U-bend facilitates a higher ohmic resistance than non-curved ducts. However as mentioned, the inventors have found that by establish the curves of the outlet duct so that the flow path at least partly surrounds the compartment outlet increases the ohmic resistance even more. One way ofimplementing such design is by establishing the flow path so that the compartment outlet is positioned between the outlet duct and the compartment i.e. so that the flow path extends over at least part of the compartment. This can be pictured by a compartment outlet that has an outlet duct around at least half (180 degrees) of the compartment outlet duct such as around at least 3 / 4 (270 degrees) of the compartment outlet duct. Thereby, the flow path is guided above the compartment outlet when the frame is in an upright position before the electrolyte is guided into the compartment outlet.

[0008] A high ohmic resistance in the outlet duct is advantageous in that it leads to a very low shunt current and increases the current efficiency of the electrolyser in which the electrolysis cell frame is used. Hence, the higher resistance, the more of the current applied to the electrolyser is conducted through the stack of cells and thereby used for production of hydrogen and oxygen.

[0009] Further, a high resistance in the outlet duct and the consequently stopping of current flow via the electrolyte through the cell is advantageous in that it has the effect, that during shutdown of the electrolysis process, stop of production of hydrogen in all cells is ensured.

[0010] Hence, by controlling of a pump forcing the electrolyte into the compartment inlet and thereby into the compartment, the level of electrolyte in the outlet duct can be controlled. Thereby, the ohmic resistance can be change between very high such above 20kohm (when no electrolyte is in the outlet duct which is advantageous during shutdown) and normal such as between 5ohm and lOOohm (when electrolyte is in the outlet duct which is advantageous during periods of gas production / when the electrolysis is not shutdown).

[0011] Accordingly, the present invention implementing a hill-like design of the outlet duct is advantageous in that it enables manipulation of the ohmic resistance between the electrolysis cells of a stack of an electrolyser and the outlet compartment.

[0012] An electrolysis cell is formed by two electrolysis cell frames connected with a diaphragm therebetween. The compartment of the electrolysis cell frame is the partof the electrolysis cell frame that is designed to hold the bipolar plate i.e. the cathode and anode and through which the flow of electrolyte is provided. Typically, the electrolysis cell frame is positioned upright such as substantially perpendicular to the gravity force. In such upright position, the compartment outlet is on at the top of the electrolysis cell frame above the compartment and with the compartment inlet at the bottom of the electrolysis cell frame i.e. below the compartment. Accordingly, when stating that the first recess part of the outlet duct is going upwards (in a direction increasing the distance to the compartment) or downwards (in a distance decreasing the distance to the compartment) with respect to the compartment, in the natural upright position of the electrolysis frame cell, this means that the outlet duct is going upwards and downwards with respect to the direction of the gravitational force. Similarly, an increasing and a decreasing distance between compartment and the first recess part is measure at a first half of the outlet duct and compared to a measurement at the second half of the outlet duct (in the direction of flow of the electrolyte).

[0013] The electrolysis cell frame should be understood as a structural element made of e.g. a metal. In this frame ducts (also referred to as recesses) between compartment inlet and compartment and between compartment outlet and compartment are provided. Such ducts are provided as recesses in the electrolysis cell frame material forming closed ducts having a cross-sectional area when two cell frames are connected. Further, the compartment inlet and outlet are provided as holes in the electrolysis cell frame. Together, the ducts, inlet and outlet form a fluid flow path in and out of the electrolysis cell frame.

[0014] Accordingly, a flow path for electrolyte is provided from outside the electrolysis cell frame via the compartment inlet and the inlet duct to the compartment. In the same way, a flow path for electrolyte, gas and / or a mixture electrolyte and gas is provided from the compartment via the outlet duct and out of the electrolysis cell frame via the compartment outlet.

[0015] According to an exemplary embodiment of the invention, said outlet duct is positioned in said electrolysis cell frame on an anode side fluidly connecting an anode compartment to an anode compartment outlet.

[0016] According to an exemplary embodiment of the invention said outlet duct is positioned in said electrolysis cell frame on a cathode side connecting a cathode compartment to a cathode compartment outlet.

[0017] As will be described below the cell frame and its bipolar plate separate a cathode compartment from an anode compartment which each have an output. Since it may be relevant to control the volume of electrolyte in the outlet ducts from both the anode and cathode compartments an outlet duct according to the present invention is preferably provided in / on both sides of the frame.

[0018] According to an exemplary embodiment of the invention a second recess part shapes said outlet duct in a direction increasing a second distance to said upper compartment surface and subsequently in a direction decreasing said second distance to said upper compartment surface.

[0019] These two recess parts shape the outlet duct. The distance that is increased / decreased, between the first and second recess parts of the outlet duct and the compartment, may be measured between an upper surface of the compartment i.e., the surface which is opened and thereby forming an inlet or beginning of the outlet duct, and a point of the outlet duct e.g. , measured at one of the first or second recess parts. So, following the flow path of the outlet duct from compartment to compartment outlet this distance will increase until it reaches its maximum after which is will decrease. The increase / decrease may be continuous or in discrete steps thereby shaping the flow path of the outlet duct.

[0020] According to an exemplary embodiment of the invention said first distance is longer than a third distance, wherein said third distance is measured between an upper surface of said compartment and an upper surface of said compartment outlet.

[0021] This is advantageous in that it has the effect, that thereby is ensured that the outlet duct can be drained from electrolyte.

[0022] According to an exemplary embodiment of the invention said outlet duct is completely positions above said compartment.

[0023] Above should be understood as when the cell frame is in an upright vertical position the outlet duct is higher from the ground on which the cell frame is positioned than the compartment. This is advantageous in that it has the effect, that the part of the cross-sectional area of the cell frame used for gas production is as large as possible.

[0024] According to an exemplary embodiment of the invention said outlet duct is surrounding at least half of the circumference of said compartment outlet, preferably at least three quarters of the circumference of said compartment outlet before ending in said compartment outlet.

[0025] An outlet duct surrounding at least half of the circumference of the compartment outlet is advantageous in that it has the effect, that the length is increased and thereby the electric resistance is increased.

[0026] According to an exemplary embodiment of the invention said outlet duct is at least partly surrounding said compartment outlet thereby establishing a substantially circular flow path around said compartment outlet.

[0027] According to an exemplary embodiment of the invention said outlet duct is at least partly surrounding said compartment outlet thereby establishing a substantially triangular flow path FP around said compartment outlet.

[0028] According to an exemplary embodiment of the invention said outlet duct is at least partly surrounding said compartment outlet thereby establishing a substantially square-like flow path around said compartment outlet.

[0029] Various ways of designing the outlet duct can be made no matter the degree of it surrounding the compartment outlet. A long outlet duct is advantageous in that it increases the ohmic resistance and thereby reduce shunt currents through the compartment outlet. Hence, the outlet duct may be designed circular, triangular of like a square just to mention a few possible designs with which it is possible to increase the ohmic resistance.

[0030] If the geometry or design of the outlet duct include a change in direction from going upwards to going horizontally or downwards, the risk of electrolyte present in the duct creating a current path through the outlet duct is reduced.

[0031] Further, such changing direction may lead to a turbulent flow of fluid (i.e. gas mixed with electrolyte) in the duct. Such turbulent flow may be established as the gas flow up towards the second wall of the outlet duct and if the outlet duct change direction more than twice, the flow path may change 180 degrees leading to gas will flow up towards the fist wall of the outlet duct.

[0032] According to an exemplary embodiment of the invention said first recess part comprises an acute angle.

[0033] An acute angle in the first wall of the outlet duct is advantageous in that it has the effect, that it forces the fluid to change direction and thereby ensuring that there can be no continuous current conducting layer of electrolyte in the outlet duct when the pump pumping the electrolyte has stopped.

[0034] According to an exemplary embodiment of the invention the distance between said first recess part and said second recess part change along the length of said outlet duct.

[0035] A varying width of the outlet duct and thus a non-uniform width of said outlet duct is advantageous in that it may change the speed of the flow fluid in the outlet duct and / or create a swirling effect in said fluid following the flow path between the first and second recess parts of the outlet duct.

[0036] An example could be that this distance is measured horizontal in the beginning part of the outlet duct and subsequently vertical in the middle part of the outlet duct, two vertical measurements in the middle part of the outlet duct, or the like.

[0037] According to an exemplary embodiment of the invention said first recess part extends above a compartment outlet center line before ending in said compartment outlet.

[0038] Hence, the distance between at least one point of the first recess part and the upper surface of the compartment is longer than the distance between the compartment center line and the upper surface of the compartment. In this way draining of the outlet duct is possible and thus control of the ohmic resistance is possible.

[0039] According to an exemplary embodiment of the invention the length of said outlet duct along said first recess part is between 2cm and 50cm, preferably between 7cm and 40cm most preferably between 10 and 35cm.

[0040] The ducts may be shaped in various geometries depending. The geometry may be determined e.g. by a desired pressure drop in the duct, controllability of level of electrolyte in the duct, flow speed in the duct, etc.

[0041] According to an exemplary embodiment of the invention said outlet duct ends and enters said compartment outlet below said compartment outlet center line.

[0042] According to an exemplary embodiment of the invention said outlet duct, towards said compartment, has a funnel-like geometry with a first width which is larger than a second width and wherein said second width is measured further away from said compartment than said first width.

[0043] A wide outlet duct beginning narrowing into the outlet duct i.e. having a funnel-like shaped geometry is advantageous in that it has the effect, that gas produce in the chamber easier find its way to the outlet duct. Further, such shape is efficient to evacuate gas from the compartment and thereby reduce number / size of gas pockets in the compartment. When there are no gas pockets in the compartment, the active areas of the compartment available for gas production is increased.

[0044] According to an exemplary embodiment of the invention said outlet duct comprising a first outlet duct part and a second outlet duct part, wherein said first outlet duct part extends substantially perpendicular to an upper surface of said compartment and wherein said second outlet duct part extends substantially parallel to said upper compartment surface.

[0045] According to an exemplary embodiment of the invention said first and second outlet duct parts have different lengths.

[0046] According to an exemplary embodiment of the invention the length of the first outlet duct part is between 1cm and 20cm, preferably between 5cm and 15cm, most preferably between 7cm and 13cm.

[0047] According to an exemplary embodiment of the invention the length of the second outlet duct is between 1cm and 20cm, preferably between 5cm and 15cm, most preferably between 7cm and 13cm.

[0048] According to an exemplary embodiment of the invention said outlet duct further comprises a third outlet duct part substantially parallel to said first outlet duct part.

[0049] According to an exemplary embodiment of the invention said outlet duct further comprises a fourth duct part substantially parallel to said second outlet duct part.

[0050] Having two, three or four outlet duct part is advantageous in that the total length of the outlet duct can be increased in different directions which may increase the Ohmic resistance in the outlet duct as a whole. This is because the risk of an electric path from compartment to outlet duct in the form of electrolyte is reduced.

[0051] Further, it should be noted that a reference to substantially parallel and perpendicular related to the extend of the duct parts should be understood with respect to the direction of flow in the outlet duct parts. More specific, parallel and perpendicular should be understood as a predetermined flow path through the outlet duct, not necessarily the actual flow path traveled by the gas / liquid in the outlet duct.

[0052] According to an exemplary embodiment of the invention, said outlet duct further comprises a fifth duct part and a sixth duct part, wherein said sixth duct part is configured for establishing a fluid flow path in a direction which is opposite the direction of the fluid flow part in the fifth duct part.

[0053] Hence in the outlet duct between the compartment and the compartment outlet, flow of electrolyte is first established in one direction by the fifth outlet duct part. In the sixth outlet duct part, at least part of the flow, preferably all of the flow of electrolyte is turned 180 degrees to a flow in a second direction opposite the first direction. The second direction may as the first direction in embodiments be substantially perpendicular to the upper compartment surface.

[0054] According to an exemplary embodiment of the invention a flow separator is positioned in said outlet duct having one end aligned with said upper compartment surface.

[0055] A flow separator may be implemented as holes through which the gas produced in the chamber need to pass through following the flow path from compartment to compartment outlet. The flow separator is advantageous in that it provides structural support to adjacent cell frame at the beginning part of the outlet duct and in that a predetermined flow path in the outlet duct can be established.

[0056] According to an exemplary embodiment of the invention said flow separator is releasably mounted.

[0057] A releasably mounted flow separator is advantageous in that manufacturing of the cell frame is made easier in that the flow separator does not need to by a monolithic part of the cell frame.

[0058] According to an exemplary embodiment of the invention a first and a second of said electrolysis cell frame together forms an electrolysis cell and wherein the outlet duct has a non-uniform cross-sectional area.

[0059] The cross-sectional area should be understood as the plane of the outlet duct in a view perpendicular to the intended direction of flow from compartment to compartment outlet. Measuring such cross-sectional area two different locations along the outlet duct would result in two different results i.e. in a non-uniform cross-sectional area of the outlet duct.

[0060] An outlet duct having a non-uniform cross-sectional defined as described above is advantageous in that it has the effect, that it may facilitate a turbulent flow, a higher flow speed, etc.

[0061] In an aspect, the invention relates to an electrolysis cell comprising a first electrolysis cell frame and a second electrolysis cell frame assembled with a diaphragm therebetween thereby establishing an anode compartment and a cathode compartment, wherein said anode compartment comprise an anode and wherein said cathode compartment comprise a cathode, wherein said first and second electrolysis cell frames both comprises: a compartment having an upper compartment surface; a compartment inlet fluidly connected to said compartment via an inlet duct; a compartment outlet fluidly connected to said compartment via an outlet duct, and wherein said outlet duct is defined in said first and second electrolysis cell frames, as a recess, wherein said recess is characterized in that has a hill-like shape when said first and second electrolysis cell frames are in an upright substantially vertical position.

[0062] According to an exemplary embodiment of the invention said first and second electrolysis cell frames are identical.

[0063] In an aspect, the invention relates to a method of controlling a volume of electrolyte in a cathode outlet duct of an electrolysis cell of an electrolyser, said electrolysis cell comprises a first electrolysis cell frame and a second electrolysis cell frame which together are forming an anode compartment and a cathode compartment separated by a diaphragm, wherein each of said first electrolysis cell frame and said second electrolysis cell frame comprises at least part of a hydrogen manifold fluidly connected to said cathode compartment via said cathode outlet duct and at least part of an oxygen manifold fluidly connected to said anode compartment via an anode outlet duct, wherein said cathode outlet duct is provided as a recess in a side of said first electrolysis cell frames and wherein said anode outlet duct is provided as a recess in a side of said second electrolysis cell frame, wherein side of said first electrolysis cell frame is facing said side of said second electrolysis cell frame, wherein each of said first electrolysis cell frame and said second electrolysis cell frame comprises acathode compartment inlet fluidly connected to said cathode compartment via a cathode inlet duct thereby establishing a cathode fluid flow path from said cathode compartment inlet to said hydrogen manifold via said cathode compartment, wherein a first recess part of at least one of said cathode outlet duct and said anode outlet duct comprises a convex shape having a vertex, and wherein a controller is controlling a flow of electrolyte in said cathode fluid flow path and thereby said volume of said electrolyte in said cathode outlet duct according to a desired level of electrolyte relative to said vertex of said cathode outlet duct.

[0064] According to an exemplary embodiment of the invention each of said first electrolysis cell frame and said second electrolysis cell frame comprises an anode compartment inlet fluidly connected to said anode compartment via an anode inlet duct thereby establishing an anode fluid flow path from said anode compartment inlet to said oxygen manifold via said anode compartment, wherein said controller is controlling a flow of electrolyte in said anode fluid flow path and thereby said volume of said electrolyte in said anode outlet duct according to a desired level of electrolyte relative to said vertex of said anode outlet duct.

[0065] This is advantageous in that it has the effect that the temperature of the electrolysis cell can be regulated by increasing the flow of electrolyte into the cathode / anode fluid flow path and thus through the anode / cathode outlet ducts (together referred to as outlet ducts) where the electrolyte is having a temperature below the temperature of the electrolysis cell.

[0066] This is furthermore advantageous in that it has the effect the shunt currents can be controlled such as eliminated by controlling the volume of electrolyte in the outlet ducts. More specifically, this is obtained by controlling the level of electrolyte above or below the vertex of the convex shaped first recess part of the output ducts. By flooding the convex shaped first recess part, a low ohmic resistance is obtained and by ensuring that no electrolyte is covering the vertex of the convex shaped first recess part, a high ohmic resistance is obtained.

[0067] This is furthermore advantageous in that it has the effect that it is possible to control the flow regime in the outlet duct i.e. the gas versus electrolyte fraction.

[0068] Accordingly, the desired level / volume of electrolyte in the cathode and / or anode outlet duct is determining for e.g. the heat exchange between electrolyte and cell frame and ohmic resistance in the outlet ducts.

[0069] According to an exemplary embodiment of the invention said volume of said electrolyte in said outlet duct is predetermined.

[0070] This is advantageous in that it has the effect, that the control of flow of electrolyte can be simplified at least in the situation where no volume of electrolyte is required or where a volume covering the convex shaped first recess part is required. Between these outer extremes, flow control parameters can be determined and thus, the controller need no feedback from sensors to control the flow (such as by controlling a pump).

[0071] According to an exemplary embodiment of the invention said volume is controlled during operation of said electrolyser.

[0072] According to an exemplary embodiment of the invention said flow of electrolyte is controlled based on feedback from a temperature sensor.

[0073] Such temperature sensors may be located outside the stack of cells in the pipe conducting the electrolyte back to the compartment inlets. Preferably, such sensor is located as close as possible to the stack.

[0074] According to an exemplary embodiment of the invention said flow of electrolyte is stopped during shut-down of said electrolyser.

[0075] This is advantageous in that it has the effect, that no electrolyte will be forced into the outlet duct and thus, due to the convex shaped first recess part, the outlet duct will, due to gravity, at least at the vertex of the convex shape, no electric path via electrolyte is created between compartment and outlet. This is leading to areduction, preferably elimination of shunt currents during shutdown and thus no secondary electrolysis in the compartment / outlet takes place.

[0076] Note that compartment outlets from a plurality of stacked electrolysis cells are forming what is sometimes referred to as an outlet manifold (oxygen / hydrogen manifold). Accordingly, secondary electrolysis can be prevented in this / these manifolds.

[0077] The invention relates to a method according to any of the below method claims, implemented in an electrolysis cell according to any of the blow electrolysis cell claims, wherein said electrolysis cell comprising two electrolysis cell frames according to any of the below electrolysis cell frame claims.The drawings

[0078] For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. The drawings illustrate embodiment of the invention and elements of different drawings can be combined within the scope of the invention:Fig. 1 illustrate elements of an electrolysis cell in a perspective view according to an embodiment of the invention,Fig. 2 illustrates an electrolyser comprising an electrolysis cell frame according to an embodiment of the invention,Fig. 3a illustrates a front view of an electrolysis cell frame according to an embodiment of the invention,Fig 3b illustrates an enlarge view of an outlet duct and compartment outlet,Fig 3c illustrates a cross-sectional view fig. 3b a section AA,Fig. 4a-4d illustrates enlarged views of various outlet duct designs according to embodiments of the invention,Fig. 5 illustrates a flow separator and various compartment outlet designs,Fig. 6 illustrates part of a stack with electrical equivalents, andFig. 7 illustrates a specific design of an outlet duct.Detailed description

[0079] The present invention is described in view of exemplary embodiments only intended to illustrate the principles and implementation of the present invention. The skilled person will be able to provide several embodiments within the scope of the claims.

[0080] Fig. 1 illustrates an example of an electrolysis cell CE of an electrolyser ELY according to an embodiment of the invention. Such cell CE is typically one in a plurality of similar adjacent cells that are forced together e.g. by hydraulics, bolts and nuts or similar arrangements to form what is referred to as an electrolyser stack STA (see fig. 2). The elements of the cell is schematically illustrated and thus may be implemented different from how it is illustrated e.g. in terms of geometry and dimensions.

[0081] The illustrated cells CE comprise two cell frames CF each of the cell frames comprises openings referred to as compartment inlet CI, compartment CMP (also referred to as cell volume) and compartment outlet CO. The inlets CI are located towards the bottom part of the cell frame CF and is supplying the compartment with electrolyte (also referred to as lye). The compartment CMP is where the electrolysis process happens i.e. the production of the gaseous Hydrogen or Oxygen. The produced Hydrogen and Oxygen are leaving the compartment CMP via the outlets CO located towards the top part of the cell frame CF. These references are generic and can in this document be used both in relation an anode and to a cathode side / compartment of the cell frame CF.

[0082] The opening referred to as compartment CMP is in this embodiment divided by a bipolar plate BP. The bipolar plate BP may be a metal or plastic plate that is forming a gas and liquid tight barrier separating flow of gas and electrolyte on the twosides of the cell frame CF. Thus, the bipolar plate fills out the entire opening referred to as compartment CMP. Accordingly, one side of the bipolar plate BP could be said to form one end wall of one compartment CMP of the cell CE. The other end wall of such compartment CMP is the diaphragm DI. In fig. 1, the bipolar plated is illustrated as a plurality of crosses which is intended to illustrate the part of the bipolar plate that is electrically connected to the electrodes. As will be described below, the cell CE will comprise two such compartments one could be referred to as an anode compartment ACO and the other as cathode compartment CCO referring to the location of an anode AN and a cathode CA located in these compartments respectively.

[0083] The generic references to openings mentioned above, may be associated with the term’s anode and cathode when a specific reference is made to the side of a cell frame forming part of the anode and cathode compartments respectively. Hence, on fig. 1, in addition to the generic references, the anode / cathode specific references are used whereas on the rest of the figures only the generic references are used. More specifically, the anode specific references are used at the first cell frame 1CF and the cathode specific reference are used at the second cell frame 2CF knowing that it is relevant for all “anode sides” and “cathode sides” in a stack STA. As the cathode side is facing away in the figure, the lines illustrating hydrogen H and the part of the hydrogen fluid flow path HFFP which is on the other side of the cell frame are stipulated. All this is to try to make the figure as detailed as possible and in the same way as readable as possible.

[0084] The anode compartment outlet CO from the anode compartment ACO may be referred to as anode or oxygen outlet OO which together with anode outlets of other cell frames are forming an anode / oxygen manifold OM. In the same way, the cathode compartment outlet CO from the cathode compartment CCO may be referred to as cathode or hydrogen outlet HO which together with cathode outlets of other cell frames are forming a cathode / hydrogen manifold HM.

[0085] The anode and cathode may together be referred to as electrodes. The anode AN of one cell CE is electrically connected to one side of the bipolar plate of a first cell frame 1CF and the cathode CA of the same cell CE is electrically connected toone side of the bipolar plat of a second cell frame 2CF. The electrical connection between electrodes and bipolar plates BP may be established by welding the electrodes to each side of the bipolar plate BP.

[0086] As the bipolar plated BP may physically be supported by or attached to the cell frame CF, the cell frame CF is preferably electrically insulated. Such insulation may be provided e.g. by manufacturing the cell frame CF, which may be a self- supporting structure, in a thermoplastic material. Alternatively, the cell frame CF may be made in a metal. A metal cell frame CF may coated with a thermoplastic material or in an elastomer to provide electric insulation, sealing, fastening of the bipolar plate, etc.

[0087] As a non-limiting example, the diameter of the cell frame CF may be between 50cm and 200cm in 5cm intervals therebetween. Hence, examples of outer diameter of a cell frame CF may be 75cm, 80cm, 85cm, ...190cm, 195cm, 200cm. With this said, depending on application and location of installation the diameter of a cell frame may be both above and below the above-mentioned range.

[0088] The width of one cell frame is typically between 1cm and 3cm such as 1,2cm, 1,4cm, 1,6cm, 1,8cm, 2,0cm, etc Accordingly, as one cell CE includes two cell frames the width of a cell is typically between 2cm and 6cm. A stack STA may comprise from two cells CE to several hundred cells such as from up to 100, 150, 200, 230, . . .500 or even more cells. Hence, the length of a stack STA may be from 4cm to e.g. lOmeters. Several stacks may be connected in series or in parallel. The design of a stack may be determined by voltage and thus requirements to the power supply of the electrolyser. Further stack design parameters may be depending on available footprint, required capacity, etc.

[0089] Accordingly, a bipolar plate BP of one cell frame CF is on one side electrically connected to an anode AN and on the other side electrically connected to the cathode, thus the bipolar plate BP facilitates an electric connection between an anode AN of a first cell and a cathode CA of a second cell. As illustrated in fig. 1 the bipolar plate of the left most cell frame CF is on its left side connected to an anode ANforming part of an adjacent cell and on its right side connected to the cathode CA forming part of the illustrated cell CE. Similarly, the bipolar plate of the right most cell frame on fig. 1 is on its left side connected to the anode AN forming part of the illustrated cell CE and on its right side connected to a cathode forming part of an adjacent cell.

[0090] As mentioned, and as illustrated in fig. 1, a cell CE comprises two similar cell frames CF. Hence, when these two cell frames CF are physically connected, e.g. via threaded rods RD and bolts (see fig. 2), with a diaphragm DI therebetween, two compartments CO (two reactive zones) are provided in the cell CE. The diaphragm DI is impermeable to gases such as Hydrogen and Oxygen (non-polar components) but allow passage of water and polar components (ions). Accordingly, the cell frame CF is facilitating structural support for the compartments CMP. The compartments CMP being defined by the bipolar plate BP and the diaphragm DI and establishes a fluid flow path FFP from the compartment inlet CI, through the compartment CMP to the compartment outlet CO.

[0091] The inlet to the compartment should be designed to establish an even distribution of electrolyte in the compartment to ensure optimal gas production and optimal cooling of the compartment. The outlet from the compartment should be designed to effectively evacuated gas from the compartment and thereby ensure that no gas pocket occurs in the compartment. This is not desired because it will reduce efficiency of the cell and there will be a risk of gas migrating or passing the diaphragm.

[0092] As mentioned, Hydrogen and Oxygen are produced in the anode compartment ACO and cathode compartment CCO respectively. More specific the compartments CMP comprises the anode AN are producing Oxygen and the compartments CMP comprises the cathode CA are producing Hydrogen. The anode compartment is on one side of the cell frame and thus the compartment outlet and outlet duct fluidly connected to the anode compartment may be referred to as anode compartment outlet / Oxygen outlet and anode outlet duct. Similarly, the cathode compartment is on the other side of the cell frame and thus the compartment outlet and outlet duct fluidly connected tothe cathode compartment may be referred to as cathode compartment outlet / hydrogen outlet and cathode outlet duct.

[0093] The gas production starts when a current is allowed to flow through the cells of the stack. More specific, when a negative potential of a direct current power supply is applied to a cathode CA in one end of a stack of cells CE and a positive potential of the direct current power supply is applied to an anode AN in the other end of the stack STA.

[0094] In an embodiment, the electrolyte is an alkaline electrolyte established by adding a strong base to demineralized water such as potassium hydroxide (KOH) producing K+ cations and OH- anions when the DC power is applied. The alkaline electrolyte facilitates transport of negatively charged ions OH- through the diaphragm DI and thus Hydrogen and Oxygen is produced at the electrodes according to the general principle of electrolysis (see eql and eq2 below).

[0095] Water molecules are dissociated at the cathode into H+ and OH-. Further, due to the presence of electrons e- from the direct current power source, a recombination of H into gaseous H2 is facilitated according to eql (1 = liquid, g = gas, ag = aqueous).EQI : 2 H2O (1) + 2 e- H2 (g) + 2 OH- (aq)

[0096] The gaseous hydrogen H2, travels upwards in the cathode compartment CCO and leaves, typically together with some electrolyte, the cathode compartment CCO via the outlet CO.

[0097] The OH- ions in the electrolyte are attracted towards the anode AN by the electric field and is allowed to pass through the diaphragm DI. As mentioned, the diaphragm DI ensures that no Hydrogen or Oxygen can pass, only OH- ions can travel from the cathode compartment CCO to the anode compartment ACO. At the anode, OH- anions are oxidised and producing oxygen while releasing electrons e- (seen eq2).EQ2: 2 OH- (aq)102 (g) + H2O (I) + 2 e-

[0098] The gaseous oxygen 02 travels upwards in the anode compartment ACO and leaves, typically together with some electrolyte, the anode compartment ACO via the outlet CO.

[0099] It should be mentioned that the cell CE and components thereof illustrated in fig. 1 is only one example of a cell CE / cell frame CF of the present invention. Other geometric shapes may be suitable. Alternative locations of the bipolar plates BP, electrodes AN, CA and diaphragm DI may be relevant. Further it should be mentioned that a cell frame CF according to the invention may be used for both finite-gap and zero-gap electrolysers.

[0100] The bipolar plate BP may e.g. be located in the opening of the cell frame such as its main plate structure does not protrude outside the plane of the sides of the cell frame CF. The bipolar plate BP may on each side comprise a plurality of electric connections / attachment points for the electrodes and which may serve to distribute the lye in the compartment.

[0101] The cell frames CF may, when joint form two top / upper manifolds UM which may be referred to as hydrogen manifold HM and oxygen manifold OM respectively. Further two bottom manifolds BM may be provided (illustrated by stipulated lines on fig. 1). Hence, the bottom manifolds BM are formed by the compartment inlets CI of a plurality of cell frames CF and the upper manifolds UM are formed by the compartment outlets CO of a plurality of cell frames CF. The electrolyte supplied to the compartments CMP via the compartment inlet / bottom manifolds may be the same i.e. origin from the said fluid system FLS. Via ducts in one side of the cell frame CF one bottom manifold BM supplies electrolyte to the anode compartment ACO and via ducts in the other side of the cell frame CF another bottom manifold BM supplies electrolyte to the cathode compartment CCO. The upper manifolds UM may be formed by compartment outlets CO on each side of the cell frame CF and serves to conduct Hydrogen and Oxygen to storage vessels SV located e.g. at the end of the stack STA. Together with the gasses, electrolyte also leaves the compartments to be recirculated in cells CE via the bottom manifolds BM. Beforerecirculated, demineralized water may be added, and the electrolyte may be cooled thereby also functioning as a cooling fluid inside the stack ST.

[0102] The diaphragm DI is as mentioned impermeable to gases but must be selectively permeable to ions. Therefore, it may be of a hydrophilic material that attracts water or other polar components, like OH-, but repels non-polar components like H2 or 02. As a non-limiting example, the diaphragm may be made of an open mesh polyphenylene sulfide fabric, which is coated with a mixture of poly sulfone and of zirconium oxide. The diaphragm may have a thickness between 0.2mm and 0.9mm such as 0.5 mm, a porosity of 55%, the same density as water and an operating temperature between 75 °C and 110°C such as 80°C.

[0103] The electrode may be made of a corrosion-resistant metal. The oxygenproducing anode may be made of e.g. nickel, a nickel coating on a steel core or of a nickel-iron alloy coated on a simple steel or nickel plate, etc. Similar to the cathode, it can also be a coating on the anode side (the coating on anode-nickel may be selfforming). The hydrogen-producing cathode may generally be made of a a steel core with some catalytic coating. Such coatings may include plain unactivated nickel to activated nickel alloys (NiMo, NiSn, NiS) or even platinum group metals. The electrodes may be either solid or use thin porous or perforated plates. For example, a thin 0.3mm pure nickel plate with 50% perforation can be used for the anode, while a similarly thin 0.3mm perforated nickel plate coated with NiS can be used for the cathode.

[0104] An electrolyser ELY according to an embodiment of the invention is illustrated as a circular tube comprising a plurality cells CE. The cells are pushed together by endplates which are squeezed together via a rod RD and nut system.

[0105] As mentioned, the gas leaves the stack STA via the upper manifolds UM. The fluid system FLS is connected to the manifolds and the gas from each manifold is separated from the electrolyte which is subsequently recirculated. As the gas separation is not essential for the invention, this is not described in further details.

[0106] The fluid system FLS is illustrated with a vessel denoted K0H+H20. This vessel serves to illustrate that the base (in case of an alkaline electrolysis) such as KOH is mixed with water (demineralized water). Further, the vessel serves to illustrate that additional water (H2O) is added to the electrolyte separated from the gas before the electrolyte is recirculated. It should be mentioned that the fluid system FLS may also comprise a pump used for controlling the flow speed of the electrolyte, a heat exchanger for temperature regulation of the electrolyte thereby allowing increasing or decreasing temperature in the cells CE of the stack STA.

[0107] Temperature regulation may be relevant e.g. during commissioning where water may be circulated (i.e. prior to addition of e.g. KOH). An increase or decrease of the water temperature during commissioning may be relevant e.g. to ensure optimal sealing, check for leakages, etc. After commissioning, the temperature regulation may be used as a control parameter in the gas production. Further, the temperature regulation may be used to circulated water or electrolyte that are either warmer or colder than the ambient temperature whenever relevant.

[0108] The flow speed or gas vs electrolyte fraction is advantageous to control in that in this way it is possible to control the volume of electrolyte in the outlet duct OD and thus, if there is a liquid connection between electrolyte in the compartment CMP and electrolyte in the upper manifold UM. The gas / electrolyte fraction can be controlled in various ways requiring a process system (not illustrated) which comprises a controller, pumps, sensors, valves, meters, etc.

[0109] Non-limiting examples of control of gas vs electrolyte fraction can be by controlling the power supply. This is because there is a relationship between gas production and power consumption. Alternatively, or in addition, the gas vs electrolyte fraction can be controlled by controlling the pressure in the stack STA or the flow of quantity of electrolyte per unit time into the bottom manifold.

[0110] An electrolyser is typically designed to operate at a given pressure and temperature based on a trade off between OPEX and CAPEX. Hence, as a general rule the higher operation pressure, the more expensive electrolyser. At the same time, athigh pressure gas is compressed in the stack STA and thus higher gas production can be expected. With this said, it is possible to brake away from these design parameters e.g. if energy prices are increasing or decreasing. As an example, if energy prices are low and hydrogen prices are high, gas production can be increased by an increase in operation pressure and the following power consumption and vice versa.

[0111] Independent of which of these different parameters, the gas production is controlled it is important for an efficient gas production that it is possible to be able to control the electrolyte vs gas fraction and thereby ohmic resistance in the outlet duct. In fact, this may also be true for electrolysers that are in a stop of gas production mode of operation e.g. to ensure reduction of secondary electrolysis in the top manifolds, temperature regulation of the stack e.g. to maintain a desired operation start temperature, etc. Hence, the higher flow of electrolyte / volume of electrolyte in the outlet duct, the more cold electrolyte / less ohmic resistance is in the outlet duct. In the same way, if the flow is reduced the cooling of the cell by the electrolyte is reduced as the volume of electrolyte in the outlet duct is reduce. This will also lead to an increase of the ohmic resistance in the outlet duct.

[0112] In fig. 3a a cell frame CF is illustrated in a front view. As illustrated the cell frame comprises a compartment input CI fluidly connected to the compartment CMP via an inlet duct ID. The compartment CMP is fluidly connected to the compartment outlet CO via an outlet duct OD. Thus, a fluid flow path FFP is created from compartment inlet CI to compartment outlet CO. The compartment outlet CO has a center line COC dividing the compartment outlet CO into two halves. Illustrated in fig. 3 and 4A these halves are substantially the same cross-sectional area, however in other embodiments this is not always the case. This is illustrated in fig. 5.

[0113] In some embodiments, the point of connection between the compartment outlet CO and the outlet duct OD is at least partly preferably completely below the center line COC.

[0114] The outlet duct OD starts at an opening in the upper part of the compartment CMP which in fig. 3a is referred to as upper compartment surface UCS. It should bementioned, that preferably, the upper compartment surface UCS is inclining a bit towards the outlet duct OD to encourage gas produced in the compartment CMP to escape the compartment CMP via the outlet duct OD.

[0115] It should be mentioned that the inlet duct is preferably designed to increase ohmic resistance in order to prevent shut currents through the compartment inlet CI into the inlet manifold. Such design may include a long and narrow path opposite to what is illustrated.

[0116] The outlet and inlet at the right top and bottom parts of the cell frame CF are associated with the compartment on the opposite side of the of the cell frame CF. The other side of the cell frame CF including inlet and outlet ducts may be similar to what is described below.

[0117] A stipulated circle is surrounding the compartment outlet CO and outlet duct OD. This is to illustrate that these parts are illustrated in an enlarged view in fig. 4A and in various designs in the figures 4B-4E.

[0118] Fig. 3b illustrated an enlarge view of the outlet duct OD and compartment outlet CO of fig. 3a and fig. 3c illustrates a cross-sectional view of fig. 3b at section AA.

[0119] As illustrated in fig. 3c, the outlet duct OD is formed into the cell frame CF as a recess RE. This recess RE may have a hemispheric geometry i.e. a half pipe like geometry. With this said, the inner wall(s) of the outlet duct may also be formed by two or more straight lines e.g. in combination with one or more bending lines. Thus, as can be understood, the geometry of the outlet duct OD may be implemented in many different designs.

[0120] Common to each design is that a first recess part 1RP and a second recess part 2RP can be defined. These walls may ultimately be reduced to points if the outlet duct is of a hemispheric geometry as illustrated in fig. 3c. In this case, the first recess part 1RP is defined as a point on the line, closest to the compartment CMP, defining the start of the recess in the cell frame plane forming outlet duct. In the same way, thesecond recess part 2RP would then be defined as a point on the line, most distant to the compartment CMP, defining the start of the opposite side of the recess in the cell frame plan forming the outlet duct. Thus, in this document, a reference made to “walls” may in practice depending on outlet duct design be a reference to a point rather than a plane / wall.

[0121] Thus, independent of design of the recess RE, the outlet duct OD is forming a fluid flow path FFP in the cell frame CF between the first recess part of the outlet duct 1RP and the second recess part of the outlet duct 2RP.

[0122] As illustrated in fig. 3b, the outlet duct OD is formed by two lines referred to as first recess part 1RP and second recess part 2RP as described with reference to fig. 3c. A distance can be measured between the upper compartment surface UCS and both of the first and second recess parts 1RP, 2RP. This distance is varying along the fluid flow path FFP starting to increase from the upper compartment surface UCS towards a maximum distance MD and ending by decreasing towards the compartment outlet. This is illustrated by arrow 1DIS and 1DIS’. The arrow 1DIS represents the distance between the upper compartment surface UCS and a point on the first recess part 1RP between the upper compartment surface UCS and the maximum distance MD. The arrow 1DIS’ represents the distance between the upper compartment surface UCS and a point on the first recess part 1RP between the maximum distance MD and the compartment outlet CO.

[0123] As illustrated, in this embodiment, the distance 1DIS is increasing all the way to the maximum distance MD after which the distance is decreasing all the way to the compartment outlet. It should be noted, that obviously, the increase and decrease may be made in steps with parts having a constant distance. Hence, the increase and decrease should be understood as an average sufficient to establish a hill-like structure of the first recess part 1RP and thereby of the outlet duct OD.

[0124] As mentioned, such hill-like / swan neck structure is beneficial in that it allows control of volume of electrolyte in the outlet duct OD. Such control includes completely emptying the outlet duct and thereby ensure than no shunt current is presentin the outlet manifold formed by the compartment outlet of the plurality of cell frames in a stack STA. This is at least relevant when there is metal in the manifolds and when the electrolyser is shut off to avoid secondary electrolysis in the outlet manifold. Further, the current control is particularly relevant if the electrolyser is pressurized e.g. in relation to electrode corrosion during shut-down, choice of electrode coating and the possibility to use current protection. _Further, such control includes ensuring electrolyte above the hill-like structure and thereby flow in the electrolyte in the fluid flow path FFP and thereby a controlled cooling provided by the flow of electrolyte. This is at least relevant when the electrolyser is in operation producing hydrogen.

[0125] Fig. 4a illustrate an outlet duct OD according to an embodiment of the invention. In this embodiment, the outlet duct OD is having a varying width between the compartment and the compartment outlet. At the beginning of the outlet duct OD, the outlet duct is having a funnel-like geometry which serves to collect and direct gaseous Hydrogen or Oxygen, together with electrolyte into the outlet duct OD. This is illustrated by the widths denoted 1W and 2W. Further into the outlet duct, the width denoted 3W illustrates that the outlet duct narrows further. After the width 3W, the outlet duct is increased again towards the compartment outlet.

[0126] The design of the outlet duct with varying width is only one simple example to illustrated that the design of the outlet duct may have a changing design including a changing width.

[0127] It is however preferred, that the outlet duct includes at least one hill-like structure to be able to break the flow of electrolyte from compartment to outlet. Further, with one hill-like structure it is possible to completely draining the outlet duct. It should be mentioned, that with a design having more than one such hill-like structures, e.g. with one or more valleys therebetween, the same effects may be established. However, a complete drain of the outlet duct would then not be possible and thereby clocking of the outlet duct due to phase shift / crystallization of the electrolyte over time. Such clocking may lead to failure of the associated cell and even hazardous situations related to heating of such cell.

[0128] One advantage of the present invention is that the electrolyte e.g. in the form of a liquid film, foam or liquid can be broken by forming one or more gas pockets in the outlet duct. Such gas pocket may be established in the outlet duct between the compartment and the compartment outlet i.e. in the outlet ducts downstream the compartment outlet (so that part of the flow path exists between the gas pocket and the compartment outlet). Thereby, the current path between the compartment and compartment outlet is broken, leading to a possible increase of the ohmic resistance between the compartment and compartment outlet. Thus, provide optimized operation conditions for the electrolyser by facilitating control of the ohmic resistance indirectly by controlling gas, liquid or foam level in the outlet duct.

[0129] Fig. 4b illustrate an outlet duct OD divided in what could be referred to as a first outlet duct part 1ODP and a second outlet duct part 2ODP before it end in the right side of the compartment outlet. The two duct parts are substantially perpendicular leading to a flow of gas and electrolyte along the fluid flow path FFP that is changing direction from a direction substantially perpendicular to the upper compartment surface UCS to a direction substantially parallel to the upper compartment surface ucs.

[0130] In addition to the two duct parts 1ODP, 2ODP of fig. 4b, a third duct part 3OPD is illustrated in fig. 4c. In the illustrated embodiment, the outlet duct ends in the upper part of the compartment outlet CO. The third part 3OPD is substantially parallel with the first part 1OPD thereby shaping the outlet duct with a maximum height / distance along the second part 2OPD.

[0131] In addition to the embodiments illustrated in fig. 4b and 4c, the embodiment illustrated in fig. 4e, includes a fourth outlet duct part 4ODP. The fourth outlet duct part 4OPD is substantially parallel with the second outlet duct part 2ODP leading to an outlet duct OD that is encircling the compartment outlet CO. This also leads to an entrance into the compartment outlet CO from the left side of the compartment outlet CO.

[0132] In this embodiment, the flow of fluid (gas and liquid) is forces to change position as illustrated by the dotted (gas) and stipulated (liquid) lines. Thereby a turbulence in fluid flow is created which may lead to an increased cooling effect.

[0133] This design inevitably also is leading to the creation of an acute angle AA along first recess part 1RP around which gravity facilitates a change of position of gas and liquid in the outlet duct OD.

[0134] Further, this design also ensures that the distance 1DIS from the upper compartment surface UCS to a point on the first recess part 1RP is longer than a third distance 3DIS from the upper compartment surface UCS to the upper surface of compartment outlet UCO. Hence, in this design the outlet duct and thereby possible also the hill-like structure hereof is above the compartment outlet CO. Such as the compartment outlet CO is almost completely enclosed by a flow of fluid from the compartment CMP.

[0135] The outlet duct illustrated in fig. 5 comprises a fluid separator FS the purpose of which is two folded. First, the fluid separator FS is a structural support which may be needed in that cell frame material is removed to provide the opening into and the first part of the outlet duct. The flow separator has the further effect that it guides the fluid from the compartment into the outlet duct in predetermined streams / directions.

[0136] Finally, it should be mentioned, that the flow separator may be removably insertable. In this way the stream of fluid into the outlet duct of a generic cell frame may easily be change.

[0137] The embodiment illustrated in fig. 5 also illustrated, by the stipulated lines, various geometries of the outlet duct OD and compartment outlet CO. This is only included to underline that the design of these two elements may, within the scope of the present invention, have different designs.

[0138] The current efficiency of an electrolyser stack STA is among others depending on what is referred to as shunt currents (sometimes also referred to as creep or bypass currents). The shunt current is almost always present to some extend inbipolar electrolyser stacks where a common inlet and outlet manifold system (outlet duct, compartment outlet, inlet duct and compartment inlet) is fluidly connected to the compartments CMP of each of the individual cells CE. The shunt current lead to a loss of production (production of heat, not hydrogen), secondary electrolysis (hydrogen production in outlet manifold), gas impurities (limiting low load) and corrosion of catalysts (i.e. of the electrodes referred to as cathodes and electrodes or coating hereof). Further, shunt currents may complicate applying a protection current during shut-down and standby.

[0139] The shunt current is a part of the main current that is shunted via the manifold system instead of passing through the bipolar plates from one electrode to the next electrode in the stack. The shunt current is mainly determined by the overall stack voltage and resistivity of the manifold system and therefore, the manifold system can be seen as a parallel ohmic resistance. Thus, the manifold design, especially the outlet manifold design, is important to be able to control the shunt currents from each cell.

[0140] During normal operation, current flow through the stack from plus to minus. During shut-down, due to an internal short-circuit of the cell, the current direction is reversed, and the current is higher in the middle cells compared to the current in the cells at the end of the stack.

[0141] An electrolyser stack may, from an electric point of view, be seen as a huge resistor. This is illustrated in Fig. 6 illustrating two cells in series of such stack including reference to electric equivalent components Rcl, Rc2, Rdl, Rd2, Rai, Ra2, Rduct, Rmanifold. In the illustrated example a voltage is applied supplying a current of 100 A. This starts the splitting of water to hydrogen (cathode) and oxygen (anode) via the mentioned electrodes that may electrically be seen as a cathode resistance Rc and an anode resistance Ra. As the current also travels through the diaphragm, the diaphragm may be referred to as a diaphragm resistance Rd. Each of the electrodes and diaphragms in the stack can be represented by these ohmic resistances and thus the sum hereof may be referred to as a main resistor. In parallel to such main resistor an outlet manifold resistor could be defined by the sum of resistance of the outlet ductsRduct (between compartments and manifold) and resistance of the outlet manifold Rmanifold.

[0142] It should be noted that an inlet manifold resistor may be defined in the same way. Further, note that Fig. 6 is simplified and thus other not mentioned contributions may be present to the main circuit (bipolar plate, diaphragm and electrodes) and the parallel circuit (outlet duct and manifold).

[0143] In the example of Fig. 6, a current of 1A is bypassing the main resistor (electrodes and diaphragm) via the manifold resistor (ducts and manifold), this current may be referred to as the shunt current. Accordingly, a current efficiency of the system illustrated in fig. 6 could be 100% at the cathode and anode at the end-plates and 99% at the middle cathode / anode. Thus, the overall stack efficiency of the illustrated example is 99,5%. It should be noted that when stacking a plurality of cells, the efficiency in terms of hydrogen production is lower in the middle cells compared to cells towards the end-plates. Hence, if the production of cells towards the end-plates of a stack of 25 cells are 100%, then the production of cell number 13 may only be 98% thereby forming a U-shaped production curve. Such, U-shaped curve may be established by what is referred to as the local Faradaic efficiency as function of the number of cells in a stack.

[0144] From the primitive and generalized example above, it is clear that by increasing the number of cells, the current efficiency of the stack is reduced and that the shunt-currents increases with increasing internal resistance of the cells. Further, it is clear that the design and dimensions of the manifold systems especially of the inlet / outlet duct and the manifolds is important to current efficiency of the stack. More specific, shunt currents decreases when the resistance of the inlet / outlet ducts increases. It should be noted, that increased pressure increases the shunt current issues as the gas volume decreases which again decreases the resistance in the outlet duct. Further, that increasing the main current (also referred to as stack current) reduces the impact on inlet / outlet duct resistance.

[0145] Regarding the ducts, the length and dimensions hereof can be changed which as indicated above, in combination with the load of the stack influences the current efficiency. The inventors have found that there is a non-linear relationship between length of inlet / outlet ducts and load of the stack that impacts the current efficiency. The longer duct, the higher current efficiency at reduced load of the stack. The difference of current efficiency at 100% load with duct lengths of e.g. 5cm and 155cm is within 10%. However, at 20% load with the same duct lengths, the difference in current efficiency may be close to 20%. The duct diameters are the same such as between 3mm and 20mm, such as between 4mm and 15mm such as 5mm.

[0146] The longer duct, the more current effective also at reduced load of the stack. In fact, there may be no significant reduction in current efficiency if the inlet duct is 155cm between a stack load of 20% and 100%. Whereas the current efficiency may drop e.g. 10%-20% if the inlet duct is 5cm and the stack load drops from 100% to 20%. Hence, for shorter duct length, the current efficiency drops fast at reduced stack load.

[0147] Current efficiency issues related to shunt currents is relevant to consider both ducting normal production but especially also during shut-down and intermitted production. Intermitted production may be a consequence of a varying power supply which is not unusual if the stack is power from a renewable power generator such as a wind turbine or a solar system.

[0148] During shut-down and reduced load of the stack, the shunt currents may cause what is referred to as deep discharge of larger stacks (such as stacks having 15 or more cells) and thereby cause corrosion of the electrodes (or coating hereof). The corrosion of the electrodes have a tendency to increase towards the cells at the center of the stack.

[0149] When cutting power to the stack, as mentioned above, a reverse current may be generated in the cells. The voltage generating this current comes from the charged electrodes being discharged and thus, the voltage (and current) will drop over time until system equilibrium is reached. The close to the center cell of the stack the faster the voltage drops. The electrodes / coating hereof may start to corrode if the voltage over the cell drops below a deep discharge limit specific for the electrode / coatingmaterial. Hence, if nothing is done, the end of lifetime of the electrodes in the center of the stack may be reached much earlier than electrodes towards the end-plates of the stack. Thus, the remaining coating of the electrode towards the end of lifetime may form a U-shaped curve form where cells at the ends of the stack have a longer lifetime compared to the cells towards the middle.

[0150] To ensure that the volage over the cells do not drop below the deep discharge limit where speed of corrosion increases a protective current is injected. Hence, a voltage is applied between the end-plates leading to a current between 5A and 30A, such as 10A and 25 A, such as between 10A and 15A through the middle cells of the stack. Note that the deep discharge limit may be IV, such as 0.9V, such as 0.8V depending on material of electrode / coating. Accordingly, it is preferred that there is always a voltage above e.g. IV across all cells when the electrolyser stack is not producing hydrogen. Hence, a separate rectifier (smaller than the main) takes over and supplies protective current. The minimum voltage depends from stack to stack but between IV and 3 V such as 1.5 V as minimum may be used in some stacks. If the electrolyser is to be shut off for a longer time, then cooling will reduce fast discharge and the protective current can be removed. In renewable energy-setting, short-time intermittency, the protection current will be idle and electrolyser is ready to kick back in operation.

[0151] On the other hand, it is important that the voltage and current is maintained in the middle of the stack to avoid production of Oxygen at the anode. The current required to start Oxygen production is lower than what is needed to start Hydrogen production. Thus, the current should be kept below the limit at which Oxygen production start. A production of Oxygen will result in the need of a flushing of the stack e.g. with Nitrogen which is expensive and time consuming and therefore not desired.

[0152] Another factor which may reduce corrosion of the electrodes / coating is temperature. Hence, if the temperature of the stack is reduced, the speed with which corrosion takes place is also reduced. The stack may be temperature regulated (up at startup of production or down during production and during shutdown) by theelectrolyte that is conducted through the manifolds / compartments by conducting it through an external heat exchanger.

[0153] Hence, as can be understood it is important to be able to control temperature and shunt current to prevent corrosion of the electrodes / coatings and to increase current efficiency. Both are important during intermittent operation (varying load of the stack) and during shutdown and especially the current efficiency is important during hydrogen production at full load and at partly full load such as between 20% and 100% load of the stack. In order to increase the current efficiency and reduce corrosion, this invention focuses especially on the shunt current which is reduced by the design of the outlet duct OD.

[0154] History has shown that the inlet duct must have a certain diameter to avoid clogging which may lead to hazardous situations. Such diameter should preferably not be below 4-5mm. Therefore, the freedom in terms of designing the inlet duct is reduced to its length. The length is limited by the design of the cell frame so an S-like inlet duct having a width as large as possible is implemented in the cell frame (not illustrated).

[0155] Looking at the outlet duct several characteristics are described above and an additional design or variation of the embodiment illustrated in fig. 4D is illustrated in fig. 7. The embodiment of fi. 7 may benefit from some or all of these. It should be mentioned, that in both sides of the cell frame CF a similar outlet duct may be provided to obtain the advantages (reduction in shunt current) in relation to both the production of hydrogen and oxygen. As illustrated, the outlet duct OD is connecting the compartment CMP ending at the upper compartment surface UCS and the compartment outlet CO.

[0156] The outlet duct illustrated in fig. 7 is advantageous in that it combines at least two design features that together facilitates a preferred fluid flow path for flow of fluid (i.e. of the gas and electrolyte) in the outlet duct. These features are in fig. 7 referred to as fifth and sixth outlet duct part 5ODP, 6ODP which as illustrated do not cover the entire outlet duct OD.

[0157] During production of e.g. hydrogen, a mixture of liquid electrolyte and hydrogen gas leaves the compartment CMP into the outlet duct OD. After having passed the first turn of the outlet duct, the mixture of liquid and gas enters the first design feature in the form of a slightly declining fifth duct part 5ODP (declining towards the compartment). During this part of the outlet duct, the fluid mixture will separate in its liquid component (illustrated as waves) that will flow along the lower part of the duct 1RP and a gas component (illustrated as circles) that will flow along the upper part of the duct 2RP. This fifth part of the duct 5ODP is sufficiently long to allow this separation of fluids. Thus, the length of this part of the duct preferably is as long as possible within the physical space available as exemplified in fig. 7. Hence, the length depend of the physical layout of the frame and could e.g. be within the nonlimiting range between 4cm can 30cm.

[0158] The dimensions such as length and width (and dept into the cell frame) of fifth and sixth duct parts 5ODP, 6ODP of the duct illustrated in fig. 7 may vary. Hence, one can imagen the embodiment illustrated in fig. 4d and which is cut through at the arrows denoted 2ODP and 4ODP allowing straight duct parts to be inserted as illustrated in fig. 7.

[0159] The length of the fifth outlet duct part 5ODP may be longer than the length of the sixth outlet duct part 6ODP. The length of the fifth outlet duct part 5ODP should be sufficiently long to enable a stable flow of fluid where the gas separates from the liquid as illustrated. As the length required to facilitate this separation in the limited space of the frame the duct, in this embodiment, continues above the compartment outlet. Therefore, the duct needs the turn back to guide the gas and liquid into the compartment outlet CO. This is done via the sixth outlet duct part 6ODP.

[0160] It should be noted that the declining duct part 5ODP may not necessarily have a uniform gradient (i.e. tangent of the angle of the surface of the first recess part 1RP to the upper compartment surface USC (which typically is horizontal)). However, an average gradient below 45 degrees has proven to be optimal. Further, it has proven to be optimal if the individual gradient contributions are between 55 degrees and 35degrees as a trade-off between steady flow that allows for the separation and flow speed.

[0161] The beginning of the first part of the duct 5ODP may be defined when the first recess part 1RP i.e. the lower part of the ducts starts to decline towards the compartment. The end of the first part of the duct may be defined by the acute angle AA. If an acute angle is not easy to define, the fist part of the duct may end at the tangent to the curved first recess part 1RP which tangent is perpendicular to the upper compartment surface UCS at the opening of the outlet duct OD. Thus, looking in a cross-sectional view of the duct, the acute angle is measured at the vertex of the ridge formed by the parts of the ducts leading to this vertex (also referred to as apex in this document).

[0162] The second design feature is characterized by facilitating a change in flow direction of the gas and liquid in the outlet duct. Hence, from being guided up and over the compartment outlet CO, the gas and liquid is in the sixth duct part 6ODP then guided into the outlet compartment (also referred to as manifold) in a direction back towards the part of the outlet duct comprising the mixed fluid. The second design feature thus, forces the flow of liquid and gas to change so that in the sixth part into the compartment outlet the liquid flows along the second recess part 2RP and the gas separated from the liquid along the first recess part 1RP.

[0163] The beginning of the sixth part of the duct 6ODP may be defined as where the fifth part of the duct 5ODP ends. The end of the sixth part of the duct may be defined as where the compartment outlet starts.

[0164] Accordingly, in both the fifth and sixth part of the outlet duct illustrated in fig. 7 what may be referred to as a Stratified flow may be established. Between the two parts, i.e. at the acute angle AA part of the outlet duct, liquid and gas mixes as they change position in the duct. Thereby, the liquid flow (which is the main shunt current carrier) is broken, contributing to increase the ohmic resistance. Put in another way, the part of the outlet duct carrying a mixture of liquid and gas is reduced. Such mixturehas a tendency to appear in a foam-like state which when found in the entire outlet duct will create a current path from compartment to compartment outlet.

[0165] The effect of the two design features, i.e. the separation of liquid and gas, is that the ohmic resistance in the outlet duct is increased with factor 5-10. More specific, the ohmic resistance in the outlet duct is five to ten times higher with the outlet duct design illustrated in fig. 7 comprising one implementation of the two design features compared to known outlet duct designs.

[0166] Examples of the ohmic resistance that is possible to obtain with a two-phase flow i.e. flow of both liquid and gas is in the range of 40-1500hm, such as below lOOOhm. The numbers may depend on liquid (electrolyte) / gas (hydrogen / oxygen) flow. Hence, if the liquid flow is reduced such as below 0.5L / min a gas pocket is trapped in the outlet duct due to its design which will ensure ohmic resistance towards to higher end of the specified range. If the liquid flow speed is increased, the ohmic resistance will drop. Following that if the liquid flow is stopped, the gas pocket will cut off the liquid path to the compartment outlet and increase the ohmic resistance. A none limiting example of such ohmic resistance would be above 2000hm, such as above 4000hm such as above 6000hm or even higher.

[0167] From the above it is now clear that the invention relates to an electrolysis cell frame having an outlet duct OD that comprises a hill -like structure established by a first side of a recess (that is also referred to as outlet duct) that is changing distance to the upper surface of the compartment UCS. The effect hereof is that the volume or level of liquid electrolyte inside the compartment can be regulated to a determined level relative to the maximum distance (top point of the hill-like structure) between the first recess part 1RP and the upper compartment surface UCS.

[0168] By regulating the level of electrolyte, the ohmic resistance is regulated e.g. between very high such as above lOkOhm, 20kOhm, 30kOhm (no electrolyte above maximum distance MD level) and relatively low such as below lOOOhm, 500hm, 250hm, lOOhm, 50hm.

[0169] The ohmic resistance is regulated according to the present invention by control flow of electrolyte in the outlet duct having a design according the present invention. The duct design enables change of electrolyte flow between mixed two- phase flow, slug-flow and other all having minor impact on the resistance compared to when stratified flow is provided. The stratified flow provides a long channel and a small cross section area. Hence, to increase resistance, the length of the outlet duct can be increased and / or the cross-section area can be reduced.

[0170] Further, by regulating the level of electrolyte, the temperature of the cell frame / electrolyte comprised thereby can be regulated such as preferably decreased or maintained at a desired temperature. With this said, during commissioning it may be relevant to maintain a higher temperature during a period of time e.g. to test the stack STA of cells CE e.g. for leakages.

[0171] The invention has been exemplified above with the purpose of illustration rather than limitation with reference to specific embodiments. Details of specific embodiment have been provided in order to understand the aim of the invention and can be combined where appropriate. Please note, that detailed descriptions of well- known systems, devices, circuits, and methods have been omitted so as to not obscure the description of the invention with unnecessary details. Furthermore, note that the electrolysis cell claimed below may comprise a cell frame also claimed below and that the below claimed method of controlling the flow of electrolyte (and thereby volume of electrolyte in the outlet duct) may be implemented in a stack comprising one or more cell frames / electrolytic cell frames as claimed below.ListAA. Acute angleACO. Anode CompartmentAN. AnodeAOD. Anode outlet ductAID. Anode inlet ductBP. Bipolar PlateBM. Bottom ManifoldCA. CathodeCCI. Cathode compartment inletCCO. Cathode CompartmentCE. Electrolysis cellCF. Electrolysis cell frameCID. Cathode inlet ductCMP. CompartmentCI. Compartment inletCO. Compartment outletCOC. Compartment outlet center lineCOD. Cathode outlet ductDI. Diaphragm (also referred to as a membrane)ELY. ElectrolyserFLS. Fluid SystemFS. Flow separatorFFP. Fluid flow pathHO. Hydrogen outletHM. Hydrogen manifoldID. Inlet ductMD. Maximum distanceOD. Outlet duct00. Oxygen outletOM. Oxygen manifoldRD. RodSTA. StackSV. Storage VesselsUM. Upper / top ManifoldUCS. Upper compartment surfaceUCO. Upper surface of compartment outlet1RP. First recess part2RP. Second recess part1DIS. First distance2DIS. Second distance3DIS. Third distance10DP 6ODP. First - sixth outlet duct parts 1W. First width2W. Second width

Claims

Claims1. An electrolysis cell frame (CF) comprising:- a compartment (CMP) having an upper compartment surface (UCS);- a compartment inlet (CI) fluidly connected to said compartment (CMP) via an inlet duct (ID);- a compartment outlet (CO) fluidly connected to said compartment (CMP) via an outlet duct (OD), and wherein said outlet duct (OD) is defined as a recess in said electrolysis cell frame (CF) wherein said recess is at least partly defined by a first recess part (1RP), wherein said first recess part (1RP) is characterized in that it shapes said outlet duct (OD) in a direction increasing a first distance (1DIS) to said upper compartment surface (UCS) and subsequently in a direction decreasing said first direction (1DIS’) to said compartment (CMP).

2. An electrolysis cell frame (CF) according to claim 1, wherein said outlet duct (OD) is positioned in said electrolysis cell frame (CF) on an anode side fluidly connecting an anode compartment (ACO) to an anode compartment outlet.

3. An electrolysis cell frame (CF) according to claim 1 or 2, wherein said outlet duct (OD) is positioned in said electrolysis cell frame (CF) on a cathode side connecting a cathode compartment (CCO) to a cathode compartment outlet.

4. An electrolysis cell frame according to any of the preceding claims, wherein a second recess part (2RP) shapes said outlet duct (OD) in a direction increasing a second distance (2DIS) to said upper compartment surface (UCS) and subsequently in a direction decreasing said second distance (2DIS’) to said upper compartment surface (UCS).

5. An electrolysis cell frame according to any of the preceding claims, wherein said first distance (1DIS) is longer than a third distance (3DIS), wherein said third distance(3DIS) is measured between an upper surface of said compartment (USC) and an upper surface of said compartment outlet (UCO).

6. An electrolysis cell frame according to any of the preceding claims, wherein said outlet duct (OD) is completely positions above said compartment (CMP).

7. An electrolysis cell frame according to any of the preceding claims, wherein said outlet duct (OD) is surrounding at least half of the circumference of said compartment outlet (CO), preferably at least three quarters of the circumference of said compartment outlet (CO) before ending in said compartment outlet (CO).

8. An electrolysis cell frame according to any of the preceding claims, wherein said outlet duct (OD) is at least partly surrounding said compartment outlet (CO) thereby establishing a substantially circular flow path (FP) around said compartment outlet (CO).

9. An electrolysis cell frame according to any of the preceding claims, wherein said outlet duct (OD) is at least partly surrounding said compartment outlet (CO) thereby establishing a substantially triangular flow path (FP) around said compartment outlet (CO).

10. An electrolysis cell frame according to any of the preceding claims, wherein said outlet duct (OD) is at least partly surrounding said compartment outlet (CO) thereby establishing a substantially square-like flow path (FP) around said compartment outlet (CO).

11. An electrolysis cell frame according to any of the preceding claims, wherein said first recess part (1RP) comprises an acute angle (AA).

12. An electrolysis cell frame according to any of the preceding claims, wherein the distance between said first recess part (1RP) and said second recess part (2RP) change along the length of said outlet duct (OD).

13. An electrolysis cell frame according to any of the preceding claims, wherein said first recess part (1RP) extends above a compartment outlet center line (COC) before ending in said compartment outlet (CO).

14. An electrolysis cell frame according to any of the preceding claims, wherein the length of said outlet duct (OD) along said first recess part (1RP) is between 2cm and 50cm, preferably between 7cm and 40cm most preferably between 10 and 35cm.

15. An electrolysis cell frame according to any of the preceding claims, wherein said outlet duct (OD) ends and enters said compartment outlet (CO) below said compartment outlet center line (COC).

16. An electrolysis cell frame according to any of the preceding claims, wherein said outlet duct OD, towards said compartment (CMP), has a funnel-like geometry with a first width 1 W which is larger than a second width 2W and wherein said second width 2W is measured further away from said compartment (CMP) than said first width 1 W.

17. An electrolysis cell frame according to any of the preceding claims, wherein said outlet duct (OD) comprising a first outlet duct part (1ODP) and a second outlet duct part (21ODP), wherein said first outlet duct part (1ODP) extends substantially perpendicular to an upper surface (USC) of said compartment (CMP) and wherein said second outlet duct part (21ODP) extends substantially parallel to said upper compartment surface (UCS).

18. An electrolysis cell frame according to any of the preceding claims, wherein said first and second outlet duct parts (1ODP, 2ODP) have different lengths.

19. An electrolysis cell frame according to any of the preceding claims, wherein the length of the first outlet duct part (1ODP) is between 1cm and 20cm, preferably between 5cm and 15cm, most preferably between 7cm and 13cm.

20. An electrolysis cell frame according to any of the preceding claims, wherein the length of the second outlet duct (21ODP) is between 1cm and 20cm, preferably between 5cm and 15cm, most preferably between 7cm and 13cm.

21. An electrolysis cell frame according to any of the preceding claims, wherein said outlet duct (OD) further comprises a third outlet duct part (310DP) substantially parallel to said first outlet duct part (10DP).

22. An electrolysis cell frame according to any of the preceding claims, wherein said outlet duct (OD) further comprises a fourth duct part (40DP) substantially parallel to said second outlet duct part (210DP).

23. An electrolysis cell frame according to any of the preceding claims, wherein said outlet duct OD further comprises a fifth duct part 5(0D)P and a sixth duct part 60DP, wherein said sixth duct part (60DP) is configured for establishing a fluid flow path (FFP) in a direction which is opposite the direction of the fluid flow part (FFP) in the fifth duct part (50DP).

24. An electrolysis cell frame according to any of the preceding claims, wherein a flow separator (FS) is positioned in said outlet duct having one end aligned with said upper compartment surface (UCS).

25. An electrolysis cell frame according to claim 24, wherein said flow separator FS is releasably mounted.

26. An electrolysis cell frame according to any of the preceding claims, wherein a first and a second of said electrolysis cell frame (CF) together forms an electrolysis cell (CE) and wherein the outlet duct (OD) has a non-uniform cross-sectional area.

27. An electrolysis cell (CE) comprising a first electrolysis cell frame (1CF) and a second electrolysis cell frame (2CF) assembled with a diaphragm (DI) therebetween thereby establishing an anode compartment (ACO) and a cathode compartment (CCO), wherein said anode compartment (ACO) comprise an anode (AN) and wherein said cathode compartment comprise a cathode (CA), wherein said first and second electrolysis cell frames (1CF, 2CF) both comprises: a compartment (CMP) having an upper compartment surface (UCS);a compartment inlet (CI) fluidly connected to said compartment (CMP) via an inlet duct (ID);- a compartment outlet (CO) fluidly connected to said compartment (CMP) via an outlet duct (OD), and wherein said outlet duct (OD) is defined in said first and second electrolysis cell frames (1CF, 2CF) as a recess (RE), wherein said recess (RE) is characterized in that has a hill-like shape when said first and second electrolysis cell frames (1CF, 2CF) are in an upright substantially vertical position.

28. An electrolysis cell (CE) according to claim 27, wherein said first and second electrolysis cell frames (1CF, 2CF) are identical.

29. A method of controlling a volume of electrolyte in a cathode outlet duct (COD) of an electrolysis cell (CE) of an electrolyser (ELY), said electrolysis cell (CE) comprises a first electrolysis cell frame (1CF) and a second electrolysis cell frame (2CF) which together are forming an anode compartment (ACO) and a cathode compartment (CCO) separated by a diaphragm (DI), wherein each of said first electrolysis cell frame (1CF) and said second electrolysis cell frame (2CF) comprises at least part of a hydrogen manifold (HM) fluidly connected to said cathode compartment (CCO) via said cathode outlet duct (COD) and at least part of an oxygen manifold (OM) fluidly connected to said anode compartment (ACO) via an anode outlet duct (AOD), wherein said cathode outlet duct (COD) is provided as a recess (RE) in a side of said first electrolysis cell frames (1CF) and wherein said anode outlet duct (AOD) is provided as a recess (RE) in a side of said second electrolysis cell frame (2CF), wherein side of said first electrolysis cell frame (1CF) is facing said side of said second electrolysis cell frame (2CF), wherein each of said first electrolysis cell frame (1CF) and said second electrolysis cell frame (2CF) comprises a cathode compartment inlet (CCI) fluidly connected tosaid cathode compartment (CCO) via a cathode inlet duct (CID) thereby establishing a cathode fluid flow path (CFFP) from said cathode compartment inlet (CCI) to said hydrogen manifold (HM) via said cathode compartment (CCO), wherein a first recess part (1RP) of at least one of said cathode outlet duct (COD) and said anode outlet duct (AOD) comprises a convex shape having a vertex, and wherein a controller (CTRL) is controlling a flow of electrolyte in said cathode fluid flow path (CFFP) and thereby said volume of said electrolyte in said cathode outlet duct (COD) according to a desired level of electrolyte relative to said vertex of said cathode outlet duct (COD),30. A method according to claim 29, wherein each of said first electrolysis cell frame (1CF) and said second electrolysis cell frame (2CF) comprises an anode compartment inlet (ACI) fluidly connected to said anode compartment (ACO) via an anode inlet duct (AID) thereby establishing an anode fluid flow path (AFFP) from said anode compartment inlet (ACI) to said oxygen manifold (OM) via said anode compartment (ACO), wherein said controller (CTRL) is controlling a flow of electrolyte in said anode fluid flow path (AFFP) and thereby said volume of said electrolyte in said anode outlet duct (AOD) according to a desired level of electrolyte relative to said vertex of said anode outlet duct (AOD).

31. A method according to claim 27 or 28, wherein said volume of said electrolyte in said outlet duct (OD) is predetermined.

32. A method according to any of the preceding claims 29-31, wherein said volume is controlled during operation of said electrolyser (ELY).

33. A method according to any of the preceding claims 2932, wherein said flow of electrolyte is controlled based on feedback from a temperature sensor.

34. A method according to any of the preceding claims 29-33, wherein said flow of electrolyte is stopped during shut-down of said electrolyser.

35. A method according to any of the preceding claims 29-34, implemented in an electrolysis cell (CE) according to any of the claims 27 and 28, wherein said electrolysis cell (CE) comprising two electrolysis cell frames (CF) according to any of the claims 1-26.

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