Electrolyzer, use, operation and a method of production thereof

Abstract

An electrolyzer stack (17) wherein each of the electrode compartments (5, 6) is delimited by the ion-transporting separator (15) and a bipolar plate (14) as well as a gasket (25) that is surrounding the electrode compartments (5, 6). An electrolyte manifold (27) extends along the stack (17) and through an opening (20) in each of the bipolar plates (14). The electrolyte manifold (27) is formed by multiple serially connected flow restrictors (28), of which one flow restrictor (28) is provided per cell (1) and inserted between neighbouring gaskets (25) and comprises a flow canal (30) having a canal inlet (30 A) and a canal outlet (30B), wherein the canal inlet (30 A) is provided in the throughput opening and fluid-flow connected to the electrolyte manifold (27) and the canal outlet (30B) is fluid-flow connected to one of the cells (1) for supply of electrolyte from the electrolyte manifold (27) to the respective cell (1). The canal is narrow and long for reducing shunt currents and for providing a pressure drop larger, for example at least 10 times higher, than a pressure drop along the electrolyte manifold (27).

Description

[0001] Electrolyzer, use, operation and a method of production thereof

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to an electrolyzer, especially alkaline electrolyzer, and its use for production of hydrogen gas. The electrolyzer comprises a stack of electrolyzer cells, each having two electrode compartments separated by an ion-transporting separator, wherein each of the electrode compartments is delimited by the ion-transporting separator and a bipolar plate, as well as a gasket that is surrounding the electrode compartments. In particular, the invention relates to control of flow into the electrode compartment. The invention also related to use, operation and a method of production of the electrolyzer.

[0004] BACKGROUND OF THE INVENTION

[0005] An efficient method for production of hydrogen gas is electrolysis. In an electrolyzer, an ion conducting membrane is sandwiched between two electrodes, and a voltage is applied over the electrodes. The voltage results in water from the aqueous electrolyte being split into hydrogen and oxygen with a final separation of hydrogen gas and oxygen gas on opposite sides of the membrane.

[0006] Traditional alkaline electrolysis is based on a stacked series of electrolytic cells. In each cell, two electrode plates are separated by a certain distance. The gap between the electrodes is filled with a liquid alkaline electrolyte. When sufficient voltage is applied, hydrogen is released on the cathode surface, and oxygen is released on the anode surface. An ion conducting diaphragm between the electrodes act as separator and prevents mixing of the gases. The electrolyte is circulated to remove the heat generated by the electrolytic process. The gap needs to be of a sufficient width to allow the escape of hydrogen and oxygen bubbles without blocking through the electrolyte and to allow electrolyte circulation without excessive pressure loss.

[0007] An electrolyzer stack comprises a series of electrolytic cells. In some series of cells, each electrode acts as an anode on its one side and as a cathode on its opposite side. In such application, the electrodes are designated as bipolar electrodes because they have different polarity on the two sides. In later years, the configuration of traditional alkaline electrolyzers has been replaced by a so-called zero-gap configuration. In the zero-gap configuration, the cell design works by pressing two porous electrodes onto either side of a hydroxide ion conducting membrane. This achieves a gap between the two electrodes equal to the thickness of the membrane, typically 0.5 mm or even less, rather than the 2-5 mm required for the traditional gap configuration. The smaller gap reduces the ohmic resistance contribution to the losses in the electrolytic cells.

[0008] In zero gap configuration, the electrodes need to have pores in order to allow the escape of hydrogen and oxygen bubbles to the side of the electrode not facing the membrane. In a bipolar electrode, this arrangement would lead to mixing of hydrogen and oxygen in the chamber established between the cathode and the anode, which is not wanted. Therefore, a separator plate is inserted between the anode of one cell and the cathode of the neighboring cell, which prevents such mixing of the gases that are created by the electrolytic process. Hence, a bipolar electrode for a zero-gap electrolysis stack is typically composed of three metallic plates - a porous anode plate and a porous cathode plate which are sandwiching a single-sheet or double-sheet bipolar plate, the bipolar plate functioning as a solid, liquid-tight separator plate between neighboring cells. The distance from the anode to the bipolar plate and from the bipolar plate to the cathode must be of sufficient width in order to allow electrolyte circulation without excessive pressure loss, and most importantly, in order to allow the escape of hydrogen and oxygen bubbles without excessive blocking, which otherwise would cause backpressure.

[0009] Examples of electrolyzer arrangements are illustrated in US patent applications US2021 / 0234237 and US2021 / 0202963, where opposite corrugated separator sheets are welded to each other to form a double-sheet bipolar plate.

[0010] US2021 / 0234237 discloses bipolar plates made of two combined corrugated plates so that the corrugation form cooling channels in between the two metal sheets and gas transport channels on their outer sides towards the electrodes. Such bipolar plates are stacked and arranged on both sides of membrane electrode assemblies, MEA, typically sandwiched between gas diffusion layers, for example nonwovens. On its outer side, the corrugations are in contact with the gas diffusion layer. Generally, gas diffusion layers between the membrane and the electrodes are often used for achieving proper flow and diffusion of the gas away from the membrane.

[0011] However, the more layers the electrolyzer cell comprises, the higher is the risk that components are moving relatively to each other and cause reduced efficiency or even malfunctioning of the electrolyzer. Accordingly, there is an interest of providing electrolyzer systems with high rigidity and sturdiness.

[0012] Among electrolyzer systems, there is a great variety. In some systems, membranes are provided as part of membrane electrode assemblies, MEA, in some cases flexible membrane electrode assemblies, others have metal meshes or grids pressing on the membrane or are provided with flexible gas diffusion layers. Some have a single-sheet bipolar plate between electrolyzer modules in a stack, others have double walled bipolar plates. Each principle is representing an attempt to optimise hydrogen production. No conclusion has yet been found on the most efficient configuration, and for a skilled person, there is no specific starting point for an optimised system and no direction for how to optimize in the best way. Often, improvements are found by multiple trial and error attempts, where various features are put together in the hope to find further optimised systems.

[0013] EP0159138 discloses an electrolysis system for production of chlorine where a single corrugated bipolar electrode plate is sandwiched between membranes. The corrugation on either side comprises horizontal minor channels between major vertical channels, with a flow of electrolyte from the bottom, upwards through one major channel, then through minor channels to the adjacent major channel and then upwards through the adjacent channel and out of the chamber at the top. It is mentioned that this leads to rapid removal of the gases.

[0014] US5114547 takes offset in this system in EP0159138 and discloses an electrolysis system for production of chlorine where embossed corrugations in monopolar or bipolar metal electrode plates are formed in a herringbone pattern, where the minor channels extend inclined from the vertical major channels. This is explained as leading to improved flow and circulation of the electrolyte and further rapid removal of the formed gases. Optionally, the vertical major channels are provided with openings for electrolyte circulation.

[0015] Whereas some systems regard circulation of electrolyte out of the top together with the gas as an advantage, with a subsequent separation of the gas from the electrolyte, others prevent such transport of electrolyte together with the gas in order to reduce the risk for cross over currents.

[0016] WO2022 / 156869 discloses an electrolysis system with single-sheet or double-sheet bipolar plates between hydrogen-producing electrolysis chambers, each chamber comprising a membrane sandwiched between perforated electrode plates which abut the membrane. The perforations allow gas transport from the membrane through the perforations and into the respective anode chamber and cathode chamber on opposite sides of the membrane. When two bipolar plates are used, coolant is introduced in the volume between the two metal-sheets of the bipolar plate. The chambers are not fully filled with electrolyte, but a space for the gas accumulation and separation from the liquid is provided at the top, where the gas leaves the chamber through an opening above the liquid level. The separation of the gas from the electrolyte inside the chambers implies that only gas is transported out of the electrolyzer stack, thereby reducing the risk of shunt currents through the conductive electrolyte.

[0017] WO2023 / 104266Aldiscloses an electrolyzer comprising modules that sandwich iontransporting membranes in between adjacent modules. The modules are formed as four-layer structures of four metal sheets, typically steel plates, including two metal sheets combined into a bipolar plate as well as an anode plate and a cathode plate respectively on opposite sides of the bipolar plate. The four sheets are welded together to form a rigid module with three separate compartments, which are one anode compartment, one cathode compartment, and a coolant compartment inside the doublesheet bipolar plate. Around the welding region of the electrolyte inlet and outlet openings in the modules, the metal sheets of the bipolar plate, the anode, and the cathode have circular depressions bending inwards towards the local welding region for mutual common contact. Flow openings are provided in the depressions around the openings towards the central plane. US6878244, US2023 / 028509A1, and US4371433 disclose stacks of electrolyzer cells where liquid is provided through a series of cylindrical non-conducting bush inserts, one for each cell, which in serial cooperation provide a pressurized fluid-tight passage for the liquid along the stack and where the liquid is distributed to the cells through passages in the bush inserts.

[0018] In an electrolyzer stack, it is important to control both the uniformity of the electrolyte flow in the individual electrolytic cells and minimizing the shunt losses in the electrolyte feed channels, potentially also in the electrolyte discharge channels. Such control is difficult to achieve, particularly, when specific areas in the flow path have varying cross sections. Accordingly, it is desirable to improve control of uniform electrolyte flow into the individual anode compartments and cathode compartments and minimization of shunt currents.

[0019] Especially, with respect to uniform or non-uniform electrolyte supply, the following should be pointed out. When electrolyte flows in the electrolyte manifolds of the electrolyzer stack, the flow will be associated with a certain pressure drop through the liquid conduits. If this pressure drop is comparable to the pressure drop related to the flow of electrolyte through the electrolytic chambers, the rate of flow of electrolyte through a specific electrolytic chamber will depend on the relative position along the liquid manifolds, extending along the stack. An electrolytic chamber positioned close to the inlet end of the liquid manifold, for example at an end of the stack, will experience a higher inlet pressure from the electrolyte in the manifold than will be the case of an electrolytic chamber positioned at a greater distance from the inlet end of the liquid manifold, for example at the opposite end of the stack. This difference in inlet pressure will cause uneven flow of electrolyte into the electrolytic chambers, with the electrolytic chambers positioned closer to the inlet end of the liquid manifold experiencing a higher electrolyte flow rate than electrolytic chambers positioned further from inlet end of the liquid manifold. Such uneven flow of the electrolyte can lead to undesirable differences in electrochemical efficiency and / or cooling efficiency. For this reason, it is customary to dimension the liquid manifolds with a relatively large cross-sectional area so as to keep the pressure drop from one end of the liquid manifold to the other, for example from one end of the stack to the opposite end, at a low level, minimizing the uneven flow distribution in the electrolytic chambers. While such relatively large cross-sectional area of the liquid manifolds has the advantage of minimizing the pressure drop through the liquid manifolds, thereby minimizing the uneven flow distribution in the electrolytic chambers, it has the associated disadvantage of increasing the so-called shunt losses through the liquid manifolds.

[0020] Since the electrolyte needs to be electrically conductive, electric currents will flow from one end of the electrode stack to the other in the liquid manifolds. The electric currents through the liquid manifolds are typically designated shunt currents. This term stems from the manifolds appearing as shunts in an electrical diagram of the electrolyzer stack. Shunt currents are undesirable because the shunt currents do not contribute to hydrogen generation but are delivering waste heat only. Since the shunt currents increase proportionally with the cross-sectional area of the liquid manifolds, and since the liquid manifolds need to have a relatively large cross-sectional area so as to keep the pressure drop from one end of the liquid manifold to the other to a reasonable level in order to minimize the uneven flow distribution in the electrolytic chambers, it follows that it is difficult to minimize the shunt currents. For this reason, the dimensions of inflow channels to the electrolyzer cells has been the focus of considerable work. Reduction of shunt currents is discussed for various electrochemical cells in WO2022 / 269602, WO2015 / 082614, WO2013 / 049933, and WO2014 / 040165.

[0021] These examples illustrate only a few attempts in different directions for improvements electrolysis. However, for optimization, there is still room for improvements, especially with respect to minimization of shunt currents. In addition, it would be desirable to provide improved hydrogen-producing alkaline electrolyzers that have well-defined flow sharing to the electrolytic cells, and where the electrolyte flow is uniformly distributed to all the cells in the electrolyzer stack.

[0022] DESCRIPTION / SUMMARY OF THE INVENTION Accordingly, it is an objective to provide an improvement in the art. In particular, it is an objective to provide an electrolyzer with a high degree of operational reliability that allows higher voltage levels across the electrolyzer stack without undesirable increase in the level of shunt currents. Furthermore, it is an objective to provide an electrolyzer with uniform or nearly uniform flow of electrolyte in the individual cells of the electrolyzer stack. These objectives and further advantages are achieved with an electrolyzer in which an electrolyte manifold extends along at least a portion of the electrolyzer and is connected to multiple electrode compartments through corresponding electrolyte inlets and where flow restrictors are positioned as part of the electrolyte manifold for restricting the flow from the electrolyte manifold into each electrolytic cell in the electrolyzer stack. The flow restrictor creates a pressure drop between the manifold and the connected electrode chamber, where the pressure drop is multiple times, for example at least 5 times or at least 10 times, higher than a pressure drop along the electrolyte manifold from which electrolyte is replenished. The back pressure is useful for providing approximately equal electrolyte flow into the various electrolyzer cells.

[0023] In particular, an electrolyzer stack is provided wherein each of the electrode compartments is delimited by the ion-transporting separator and a bipolar plate as well as a gasket that is surrounding the electrode compartments. An electrolyte manifold extends along the stack and through an opening in each of the bipolar plates. The electrolyte manifold is formed by multiple serially connected flow restrictors, of which one or more flow restrictors is / are provided per cell and comprise(s) a flow canal having a flow canal inlet and a flow canal outlet. The flow canal inlet is provided in the throughput opening and fluid-flow connected to the electrolyte manifold, and the flow canal outlet is fluid-flow connected to one of the cells for supply of electrolyte from the electrolyte manifold to the respective cell. The canal is narrow and long for reducing shunt currents and for providing a pressure drop higher than a pressure drop along the electrolyte manifold, for example at least 5 or 10 times higher. Further details and further advantages are described in the following.

[0024] In more detail, the electrolyzer comprising a stack of multiple electrolyzer cells, each of the electrolyzer cells comprising two electrode compartments, which are a cathode compartment and an anode compartment, separated by an ion-transporting separator. Each of the electrode compartments is delimited by the ion-transporting separator and a bipolar plate as well as a gasket that is surrounding the electrode compartments in order to seal against leakage of electrolyte and gas. The stack comprises at least two gas manifolds, extending along the stack and, typically provided at the top, one of which being connected to the anode compartments and the other connected to the cathode compartments for drain of corresponding gases produced therein. For example, the gas manifolds are used only for transport of gas. Alternatively, the gas manifolds are also used as return conduits for electrolyte, in which case, the gases are separated from the electrolyte in a gas separator, typically remote from the stack.

[0025] The stack comprises at least one electrolyte manifold, but typically two electrolyte manifolds, for supply of water and / or liquid electrolyte to the anode compartments and the cathode compartment in the stack. Such electrolyte manifold extends along the stack, typically along the entire stack, although, it is also possible that it only extends along a fraction of the length of the stack for providing electrolyte to a subset of the cells, while further electrolyte manifolds are provided for other subsets of the total number of electrolyzer cells in the stack.

[0026] An opening for the electrolyte manifold is provided in each of the bipolar plates. For this purpose, the openings are aligned along a central axis. Multiple flow restrictors are provided, each flow restrictor comprising a throughput opening having a throughput width. The flow restrictors are connected with each other so as to form a continuous or almost continuous tubular inner wall of the electrolyte manifold along the central axis. Each of the flow restrictors forms a portion of the inner wall of the electrolyte manifold. At least one flow restrictor is provided per cell, optionally inserted between neighbouring gaskets in a fluid-sealing configuration. Each flow restrictor comprises a flow canal having a flow canal inlet and a flow canal outlet, wherein the flow canal inlet is provided in the throughput opening, and thus in the electrolyte manifold, and is fluid-flow connected to the electrolyte manifold. The flow canal outlet is fluidflow connected to one of the cells, for example only the anode compartment or only the cathode compartment, or alternatively to both, for supply of electrolyte from the electrolyte manifold to the respective cell.

[0027] By a useful dimensioning of the canal relatively to the electrolyte manifold, an equal or approximately equal flow into the various cells is established. At the same time the shunt currents in the manifold are controlled. During production of the electrolyser or during upgrade of an existing electrolyzer, flow restrictors are selected according to predetermined criteria and inserted into the electrolyzer stack. For example, the following production process of such an electrolyzer is useful where flow and pressure of electrolyte through the electrolyte manifold from a first end to a second end of the electrolyte manifold are predetermined. For the predetermined flow of electrolyte through the electrolyzer cells in the stack, a first pressure drop through the electrolyte manifold from the first end to a second end of the electrolyte manifold is determined, for example measured or estimated. According to the predetermined criteria in the selection process, flow restrictors are selected with flow canals having a width and a length so as to provide a second pressure drop through the flow canals, which is higher, for example at least 5 times or at least 10 times higher, than the first pressure drop through the electrolyte manifold, while providing a predetermined flow of electrolyte through the cells. As the pressure drop is high through the flow canals, the flow canals largely determine the flow into the cells and not the pressure drop along the electrolyte manifold, seeing that it is much lower and, accordingly, has much smaller influence.

[0028] The selection of flow restrictors also has great influence in the operation, where the pressure of the electrolyte in the electrolyte manifold is adjusted and the electrolyte pumped through the electrolyte manifold and into the electrolyzer cells. In this case, similarly, a first pressure drop through the electrolyte manifold from a first end to a second end of the electrolyte manifold is determined, for example measured or estimated, and a second pressure drop through the flow canals from the flow canal inlet to the flow canal outlet is estimated. The width and length of the flow canals are dimensioned so as to provide the second pressure drop through the flow canals, larger, for example at least 5 times or at least 10 times larger, than the first pressure drop through the electrolyte manifold.

[0029] For example, the flow canal has a length in the range of 0.05 to 2 m, a width in the range of 0.2 mm to 3 mm, and a length-to- width ratio in the range of 10-2000. However, final dimensioning depends on the size of the stack and the desired electrolyte flow through the cells, which in turn depends on the hydrogen gas production capacity and production target. The flow restrictors can be provided with various shapes. Some shapes that have been found useful are described in the following.

[0030] For example, each of the flow restrictors is ring-shaped. It has an outer diameter of the outer periphery and an inner diameter determined by the width of the throughput opening.

[0031] In some embodiments, the flow canal from the flow canal inlet to the flow canal outlet is following a curve extending more than 180 degrees about the central axis. The length of the curve is longer than a cross sectional width of the throughput opening and longer that a cross sectional distance from the throughput opening to an outer periphery of the flow restrictor. The winding of the flow canal around the electrolyte manifold is advantageous for providing the flow canal narrow and long, reducing shunt currents and increasing pressure loss.

[0032] In some embodiments, multiple serially connected flow restrictors have throughput openings with different throughput area and are arranged such that the throughput areas progressively decrease along the electrolyte manifold in a downstream direction. It is particular useful if the areas of the throughput openings of the serially connected flow restrictors are adjusted for constant flow speed of electrolyte along the electrolyte manifold despite flow of portions of electrolyte from the electrolyte manifold into the cells.

[0033] This is useful because of the following. Normally, for an electrolyte manifold having constant diameter, flow becomes smaller towards the end of the electrolyte manifold due to each cell along the stack receiving a portion of the electrolyte flowing through the electrolyte manifold and less electrolyte being left towards the end of the electrolyte manifold. This implies that the flow speed of the electrolyte towards the end of the electrolyte manifold becomes slower. Taking this into account, selecting flow restrictors with varying throughput area and arranging these such that the throughput areas getting progressively smaller along the electrolytic stack towards the downstream end, the flow through the electrolyte manifold can be adjusted such that the flow speed is approximately constant along the electrolyte manifold. Not only does the smaller throughput area reduce shunt currents as compared to a constant, large throughput area, but it also assist in equalising the pressure along the stack. Accordingly, multiple advantages are obtained by a simple adjustment if only the flow restrictors are produced with varying throughput areas and properly selected and arranged in accordance with predetermined criteria to pressure, flow and reduction of shunt currents.

[0034] Controlling the throughput areas of the relatively small and low cost flow restrictors is simple and allows the use and upgrade of standardized electrolyzer stacks with the associated benefits of mass production.

[0035] In some embodiments, each flow restrictor comprises two parts attached to each other with the flow canal located in between the two parts and provided as a recess in one or the other part or in both. For example, the two part are formed as discs attached to each other face-to-face with the flow canal in between formed by a recess

[0036] In other embodiments, each flow restrictor has a male portion and an opposite directed female portion, and the multiple restrictors are serially connected by uptake of their corresponding male portion in a female portion of a neighbouring flow restrictor. For example, each flow canal is provided as a recess in the male portion or the female portion and located between the connected male and female portion of the corresponding neighbouring flow restrictors.

[0037] In some useful concrete embodiments, the stack comprises a stack of modules, sandwiching one of the ion-transporting separators between each two of the modules, wherein each module comprises an assembly, for example welded assembly, of a first and second metal sheet, which are perforated cathode and anode sheets, respectively, and a bipolar plate, BPP, by the welding sandwiched between the first and second metal sheets. One of the gaskets is provided sealingly between each two neighbouring modules. In this case, not only the bipolar plate but also each of the metal sheets has an opening through which at least one of the flow restrictors extends for forming a portion of the inner wall of the electrolyte manifold. Optionally, the bipolar plate is made from two metal sheets welded, glued or gasketed together face-to-face. In such embodiments, the stack comprises a stack of four-sheet modules, sandwiching one of the ion-transporting separators between each two of the modules, wherein each module comprises an assembly of a first and second metal sheet, which are perforated cathode and anode sheets, respectively, and a third and fourth metal sheet, which are non-perforated and in combination form a two-layer bipolar plate, BPP, sandwiched between the first and second perforated metal sheets. A coolant compartment is delimited by the third and fourth metal sheets and fluid-flow separated from the anode and cathode compartments in order to prevent mixing of the coolant with the electrolyte. Also, in this case, one of the gaskets is provided sealingly between each two neighbouring modules. Each of the metal sheets has an opening through which at least one of the flow restrictors extends for forming a portion of the inner wall of the electrolyte manifold.

[0038] For example, the first metal sheet or the second metal sheet or both are perforated and abutting the ion-transporting separator as electrode in a zero-gap configuration.

[0039] Alternatively, there is provided a porous electrically conducting sheet between the ion-transporting separator and the first metal sheet and / or between the ion-transporting separator and the second metal sheet. The porous electrically conducting sheet and the first or second metal sheet in combination form a composite cathode or anode, respectively, as all the metal sheets are electrically conducting.

[0040] The electrolyzer presented herein is useful for producing hydrogen gas in an alkaline electrolyte. Typically, the electrolyzer is operated with an alkaline electrolyte based on NaOH or KOH, for example having a temperature in the range of in the range of 50-90°C.

[0041] Optionally, each of the cathode compartments has an electrolyte and hydrogen gas outlet connecting the electrolytic cell to a gas manifold that functions as a cathode electrolyte return conduit that receives electrolyte and hydrogen gas from all or a subset of the cathode compartments in the electrolyzer stack and delivers the electrolyte and gas mixture to a separator tank. Optionally, each of the anode compartments has an electrolyte and oxygen gas outlet connecting the electrolytic cell to an anode electrolyte return conduit that receives electrolyte and oxygen gas from all or a subset of the anode compartments in the electrolyzer stack and delivers the electrolyte and gas mixture to a separator tank.

[0042] In the electrolyzer stack, the inlet temperature of the electrolyte needs to be maintained at a level close to the outlet temperature of the electrolyte in order to maintain a desired average temperature of the electrolyte throughout the electrode chambers. Therefore, a high flow rate of the electrolyte is required. A high electrolyte flow rate requires not only substantial pumping but also necessitates large cross-sectional areas of the electrolyte feed conduits and the gas removal conduits. As described above, there is a risk for undesirable shunt currents flowing from one end of the electrode stack to the other in the electrolyte feed conduits, and shunt currents may also flow from one end of the electrode stack to the other in the gas removal conduits, depending on the actual configuration of the gas removal conduits. However, as explained above, this problem has been overcome by the flow restrictors implemented in an electrolyser described herein.

[0043] Even at a fairly large cross-sectional areas of the electrolyte manifold, a pressure loss occurs over the length of the electrolyte manifold. This pressure loss causes the electrolyte flow to each electrolytic cell to vary as a function of the position of the electrolytic cell along the stack. The varying flow causes the concentration, mixing, and / or temperature of the electrolyte inside the electrolytic cell to differ from cell to cell, leading to variations in the gas production performance and efficiency of the individual electrolytic cells. As a consequence, not all cells will have optimal performance and efficiency. However, as explained above, this problem has been overcome by the flow restrictors implemented in the electrolyser described herein.

[0044] As explained above, it is useful if a flow restrictor is located at the electrolyte inlet of each cathode compartment, and a flow restrictor is located at the electrolyte inlet of each anode compartment. Advantageously, the flow restrictor of both the cathode compartments and the anode compartments are identical. Additionally as an option but not necessarily, a flow restrictor is located at the electrolyte and hydrogen gas outlet of each cathode compartment, and a flow restrictor is located at the electrolyte and oxygen gas outlet of each anode compartment. Advantageously, the flow restrictors for outlet flow control of both the cathode compartments and the anode compartments are identical.

[0045] As discussed above, one useful dimensioning of the flow restrictors concerns the throughput area of the electrolyte manifold and, where applied on the electrolyte and gas manifolds, which function as electrolyte return conduits, also the area of the electrolyte and gas manifolds. Thereby, it exerts a key influence on the electrical resistance and the pressure drop of the electrolyte and, where applied on the electrolyte and gas return conduits, also on the electrical resistance and the pressure drop of the electrolyte and gas in the corresponding gas manifolds.

[0046] As also discussed above, useful dimensioning of the flow restrictors defines the area and length of the flow canals for the electrolyte inlet into the electrolytic cell. Thereby, key influence is taken on the electrical resistance and the pressure drop of the electrolyte inlet from the electrolyte manifold to the electrolytic cell and, where applied on the electrolyte and gas manifolds, which function as electrolyte return conduits, also on the electrical resistance and the pressure drop of the electrolyte and gas outlet from the electrolytic cell to the manifolds functioning as electrolyte return conduits.

[0047] However, it is pointed out that the gas manifolds for receiving gas from the electrolysis and the manifolds functioning as electrolyte return conduits can be different manifolds, in which case, no specific device for gas separation from the electrolyte is necessary outside the stack.

[0048] SHORT DESCRIPTION OF THE DRAWINGS

[0049] FIG 1 A is a sketch of an electrolytic cell in a gap configuration;

[0050] FIG. IB is a sketch of an electrolytic cell in a zero-gap configuration, comprising a porous cathode, a porous anode;

[0051] FIG. 1C is a sketch of an electrolyzer stack;

[0052] FIG. 2 illustrates a principle sketch of an electrolyzer stack with modules; FIG. 3 illustrates an example of an assembly og sheets to form a module;

[0053] FIG. 4 illustrates an example of a perforated electrode sheet;

[0054] FIG. 5 A illustrates the area around openings in an example of four-sheet modules without gasket between the modules;

[0055] FIG. 5B illustrates the area around openings in an example of four-sheet modules with gasket inserted between the modules;

[0056] FIG. 6A illustrates an example in which multiple flow restrictors are inserted into the openings in between the gaskets;

[0057] FIG. 6B is a partial cross section view of FIG. 6A;

[0058] FIG. 7A shows one of two parts of the flow restrictor of FIG. 6A and 6B;

[0059] FIG. 7B shows another of two parts of the flow restrictor of FIG. 6 A and 6B;

[0060] FIG. 7C is a partial cross-sectional view of an assembled flow restrictor of FIG. 6A and 6B and FIG. 7A and 7B;

[0061] FIG. 8A illustrates a partial cross sectional illustration of multiple optional alternative flow restrictors inserted into the opening in between the gaskets;

[0062] FIG. 8B illustrate an alternative flow restrictor of FIG. 8 A in perspective view;

[0063] FIG. 8C illustrate the flow restrictor of FIG. 8B in a different perspective view;

[0064] FIG. 9A illustrates a partial cross sectional illustration of a further alternative flow restrictor;

[0065] FIG. 9B illustrate the further flow restrictor of FIG. 9A in perspective view;

[0066] FIG. 9C illustrate the flow restrictor of FIG. 9B in a different perspective view.

[0067] DETAILED DESCRIPTION / PREFERRED EMBODIMENT

[0068] FIG 1 A, B, and C show principle sketches of general variants of alkaline electrolytic cells and an electrolyzer stack.

[0069] FIG. 1 A is a sketch of an electrolytic cell 1 in a gap configuration, comprising a cathode 2, an anode 3, as well as an ion-transporting separator 4 arranged therein between at a distance (gap). Hydrogen gas 8 is produced at the side of the cathode 2 facing the ion-transporting separator 4 in the cathode compartment 5, and oxygen gas 9 is produced at the side of the anode 3 facing the separator 4 in the anode compartment 6. A power supply 7 drives the electrolytic process. FIG. IB is a sketch of an electrolytic cell 1 in a zero-gap configuration, comprising a perforated cathode 12 and a perforated anode 13, both abutting an ion-transporting separator 4. The cathode compartment 5 and the anode compartment 6 are provided between metal separator plates 14. Electrical connections are provided by conductive elements 15 connecting the respective electrodes 12, 13 with the metal separator plates 14. Hydrogen is produced at the side of the perforated cathode 12 facing the ion-transporting separator 4 and is conveyed to the cathode compartment 5 through holes or pores 16 in the cathode 12. Oxygen is produced at the side of the perforated anode 13 facing the separator 4 and is conveyed to the anode compartment 6 through holes or pores 16 in the anode 13.

[0070] FIG. 1 C is a sketch of an electrolyzer stack 17, comprising a series of electrolytic cells 1 in zero-gap configuration of the type as illustrated in FIG. IB. Advantageously, a set of perforated electrodes 12, 13 and a bipolar plate 14 are assembled into a module 18, for example by welding or gluing the corresponding metal sheets together. Each pair of neighbouring modules 18 are sandwiching an ion-transporting separator 4

[0071] FIG. 2 illustrates numerous of such modules 18 in a stack 17, each neighbouring two of such modules 18 sandwiching an ion-transporting separator 4. Gaskets (not shown) between adjacent modules 18 and around the ion-transporting separator 4 seal the corresponding electrode compartments 5, 6.

[0072] FIG. 3 is an example of such module 18 of FIG. 2 in an exploded view. The module 18 is made by welded assembly with four metal sheets 13, 14A, 14B, 12, similar to the module disclosed in W02023 / 050257. The four-sheet module 18 comprises two perforated electrodes 12, 13 and a double-sheet bipolar plate 14 with two metal sheets 14 A, 14B forming the bipolar plate 14, the two sheets 14 A, 14B confining a coolant compartment 22 in between the two metal sheets 14A, 14B, which by welding is provided liquid tight and sealed against the electrolyte compartments 5, 6. In contrast thereto, the two metal sheets for the electrodes 12, 13 are perforated with perforations 23, which appears in more detail from FIG. 4, showing a portion of such metal sheet of the cathode 12. To provide stiffness and stability and in order to provide a sturdy module, the four metal plates are interconnected by local welding regions 24 distributed over most of the area of the module 18, as indicated in FIG. 4 and also visible in FIG. 3. Around the welding region 24, the metal sheets have circular deformations bending inwards towards the local welding region 24 for mutual common contact and where the welding is performed for fastening the four plates 12, 134, 14A, 14B to each other. Further details are described in W02023 / 050257A1.

[0073] With reference to FIG. 3, a number of openings 20 are provided in the sheets 12, 13, 14 A, 14B. These openings 20 are part of the fluid manifolds along the stack 17 for transport of coolant and electrolyte in and out of the stack as well as transport of gases out of the electrode compartments 5, 6. Each of the openings 20 is connecting the respective manifold of the stack only with the corresponding compartment.

[0074] FIG. 5A and FIG. 5B illustrate the area around one series of such openings 20 in a four-sheet module 18 with metal sheets for the electrodes 12, 13 and metal sheets 14 A, 14B for the bipolar plate 14 with the coolant compartment 22 in between the two metal sheets 14A, 14B of the double-walled bipolar plate 14. The openings 20 form part of the manifold 27 with flow of liquid, illustrated by arrow 26, through the such manifold 27.

[0075] For example, the module 18 is provided as explained above in connection with FIG. 2 and 3. As illustrated in FIG. 5 A and B, the sheets 12, 13, 14 A, 14B are corrugated and combined, similar to the structure described in relation to FIG. 11 of W02023 / 050257A1. However, the overall configuration can deviate therefrom. Focus in the following is on the openings 20 of the sheets 12, 13, 14A, 14B and not on details of the remaining portion of the sheets, where FIG. 2 and 3 were only exemplary embodiments for illustration.

[0076] FIG 5 A illustrates a portion of the stack with modules 18 arranged side by side without gaskets between the four-sheet modules 18, and FIG. 5B illustrates the same arrangement with gaskets 25. FIG. 6A illustrates a flow restrictor 28 inserted in the opening 20 in between the gaskets 25. The flow restrictor 28 is made of two flow-restrictor parts 28 A, 28B and shown in detail in FIG. 7A, 7B, and 7C. FIG. 6B is a partial cross-sectional view.

[0077] The flow restrictor 28 has a substantially smaller cross section in its throughput opening 29 than the opening 20 in the metal sheets for the electrodes 12, 13. In this connection, it is pointed out that the flow in the conduit of the electrolyte manifold 27 becomes smaller towards the downstream end of the manifold 27 due to each electrolyte compartment 5, 6 along the stack 17 receiving a portion of the electrolyte flowing through the manifold 27. This implies that the flow speed of the electrolyte towards the downstream end of the manifold 27 becomes lower. Taking this into account, the throughput openings 29 of the flow restrictors 28 can be made smaller along the stack 17, so that the area of the throughput opening 29 of the flow restrictors 28 is largest at the inlet to the electrolyte manifold 27 and gets progressively smaller along the stack 18 towards the downstream end of the electrolyte manifold 27. In particular, the areas of the throughput openings 29 of the flow restrictors 28 may be adjusted such that the flow speed is approximately constant along the electrolyte manifold 27. This leads to reduction of shunt currents due to the smaller area. A smaller conduit area of the throughput opening 29 of the flow restrictors 28 is good for shunt current reduction, but should also not restrict the flow too much, which is why a progressive reduction of the areas of the throughput openings 29 of the flow restrictors 28 is advantageous.

[0078] The flow restrictor 28 in the illustration of FIG. 6 and 7 is made of two separate ringshaped parts 28A, 28B with identical inner and outer circular periphery and which are combined face-to-face for use. The first part 28 A has a flow canal 30 with a flow canal inlet 30A at the throughput openings 29 of the flow restrictors 28 and a flow canal outlet 30B, which is connected to the respective compartment, for example anode compartment 5 or cathode compartment 6. or Such flow restrictors can also be used for supply of coolant to the coolant compartments 22. The flow through the flow canal 30 is illustrated by arrow 31 in FIG. 7C. The flow canal 30 connects the flow canal inlet 30A with the flow canal outlet 30B. The flow canal 30 extends along a curve 30C around the throughput openings 29 in order to be longer than its cross sectional width 36 from the throughput openings 29 to its periphery 37, which reduces shunt currents. With respect to the flow canals 30, the following discussion is important. As already mentioned, the flow canal 30 in the flow restrictor 28 is narrow and multiple times longer than its cross section and it is multiple times narrower than the manifold 27, which results in a pressure drop through the flow canal 30 from the electrolyte manifold 27 to the respective electrode compartment 5, 6. This pressure drop is larger, in particular, at least 10 times larger, than the pressure drop throughout the electrolyte manifold 27 along the stack 18 and is used to overcome the problem of decreasing electrolyte pressure at the entrances of subsequent electrode compartments 5, 6 along the stack 17.

[0079] As discussed above, in prior art electrolyzer systems, electrolyte manifold with even relatively large cross-sectional areas have a pressure loss over the length of the electrolyte manifold, which causes the flow into each electrolytic cell to vary as a function of the position of the electrolytic cell along the stack. Such flow of different sizes causes the concentration, mixing and / or temperature of the electrolyte inside the electrolytic cell to differ from cell to cell, leading to unequal gas production performance and efficiency in the individual electrolytic cells. As a consequence, not all cells will have optimal performance and efficiency.

[0080] However, by using flow restrictors 28 as described herein, in which the pressure drop of the electrolyte through the flow restrictor 28 is multiple times higher, for example at least 10 times higher, than the pressure drop along the corresponding electrolyte manifold 27, the effect of the pressure drop along the electrolyte manifold 27 is prevented and the electrolyte flow into the various electrode compartment 5, 6 largely constant along the stack 17.

[0081] For example, the flow restrictor 28 as shown in FIG. 6 and 7 is used in one orientation for transport of electrolyte from a first electrolyte manifold 27 into multiple cathode compartments 6 along the stack 17 and in an opposite orientation for transport of electrolyte, optionally from a second electrolyte manifold, into multiple anode compartments 6 along the stack 17. Whereas, the flow restrictor of FIG. 6 and 7 comprised two parts 28 A, 28B between each gasket 25, other embodiments require only a single-part flow restrictor 28 between the gaskets.

[0082] FIG. 8A, B, and C illustrate an alternative embodiment of such single-part flow restrictor 28. The flow restrictor 28 has a conical outer portion 33 in extension of a ring portion 32, and the flow canal 30 is also made long and narrow and is winding around the throughput openings 29 of the flow restrictors 28 inside the conical outer portion 33 of the flow restrictor 28. The restrictor 28 has a conical inner side 34 as a female portion for matching the conical outer portion 33 as a male portion when being stacked during assembly, as illustrated in FIG. 8A.

[0083] In some embodiments, and in agreement with FIG. 3, 5 and 6, one electrolyte manifold 27 is used for supplying electrolyte and replenishing water to the anode compartments 6 and a second electrolyte manifold 27 is used for supplying electrolyte and replenishing water in the cathode compartments 5. However, in principle, it is also possible to feed electrolyte into the cathode compartments 5 as well as anode compartments 6 from a single electrolyte manifold 27.

[0084] FIG. 9A, B, and C illustrate a further alternative embodiment of a single-part flow restrictor 28. The flow restrictor 28 has a conical outer portion 33 in extension of a ring portion 32, and the flow canal 30 is also made long and narrow by winding around the throughput openings 29 of the flow restrictors 28 inside the conical outer portion 33 of the flow restrictor 28. The flow restrictor 28 has a conical groove 35 for accommodating the conical outer portion 33 when being stacked, as illustrated in FIG. 8 A.

[0085] The flow restrictors 28 need to be non-conductive in order not to conduct stray currents or cause additional shunt losses. Advantageously, the flow restrictors 28 are made of hard plastics such as poly ether ether ketone (PEEK) or polyphenylene sulphide (PPS), or more elastic materials like synthetic rubbers, such as ethylene propylene diene monomer (EPDM) or synthetic fluoropolymer elastomers such as Viton.

[0086] The material for the flow restrictors has to be resistant against the aggressive nature of alkaline electrolyte, such as KOH or NaOH, at elevated temperature.

Claims

CLAIMS1. An electrolyzer comprising- a stack (17) of multiple electrolyzer cells (1), each of the electrolyzer cells (1) comprising two electrode compartments, which are a cathode compartment (5) and an anode compartment (6), separated by an ion-transporting separator (4), wherein each of the electrode compartments (5, 6) is delimited by the ion-transporting separator (15) and a bipolar plate (14) as well as a gasket (25) that is sealingly surrounding the electrode compartments (5, 6); wherein the stack (17) comprises at least two gas manifolds, extending along the stack (17), at least one of which being connected to the anode compartments (6) and at least one other connected to the cathode compartments (5) for drain of corresponding gases produced therein; wherein the stack (17) comprises at least one electrolyte manifold (27) for supply of water and / or liquid electrolyte to the electrode compartments (5, 6) in the stack (17), wherein the electrolyte manifold (27) extends along the stack (17) and through an opening (20) in each of the bipolar plates (14), the openings (20) of the bipolar plates (14) in the stack being aligned along a central axis (X);- multiple non-conducting flow restrictors (28), each flow restrictor (28) comprising a throughput opening (29) having a throughput width (29A), wherein the flow restrictors (28) extend thorough the openings (20) and are serially connected with each other and in combination form a tubular inner wall of the electrolyte manifold (27) along the central axis (X), each of the flow restrictors (28) forming a portion of the inner wall of the electrolyte manifold (27), wherein at least one flow restrictor (28) is provided per cell (1), wherein each flow restrictor (28) comprises a flow canal (30) having a flow canal inlet (30 A) and a flow canal outlet (30B), wherein the flow canal inlet (30A) is provided in the throughput opening (29) and fluid-flow connected to the electrolyte manifold (27) and the flow canal outlet (30B) is fluid-flow connected to one of the cells (1) for supply of electrolyte from the electrolyte manifold (27) to the respective cell (1).

2. The electrolyzer according to claim 1, wherein each of the flow restrictors (28) is ring-shaped with an outer diameter (38) of an outer periphery (37) and an inner diameter determined by the width (29A) of the throughput opening (29).

3. The electrolyzer according to claim 1 or 2, wherein the flow canal (30) from the flow canal inlet (30 A) to the flow canal outlet (30B) is following a curve extending more than 180 degrees about the central axis (X), a length of the curve being (30C) longer than a cross sectional width (29A) of the throughput opening (29) and longer that a cross sectional distance (36) from the throughput opening (29) to an outer periphery (37) of the flow restrictor (28)4. The electrolyzer according to any preceding claim, wherein the flow canal (30) has a length in the range of 0.05 to 2 m, a width in the range of 0.2 mm to 3 mm, and a length-to-width ratio in the range of 10-5000.

5. The electrolyzer according to any preceding claim, wherein the multiple serially connected flow restrictors (28) have throughput openings (29) with different throughput areas and are arranged such that the throughput areas progressively decrease along the electrolyte manifold (27) in a downstream direction.

6. The electrolyzer according to claim 5, wherein the throughput areas of the throughput openings (29) of the serially connected flow restrictors (28) are adjusted for constant flow speed of electrolyte along the electrolyte manifold (27) despite flow of portions of electrolyte from the electrolyte manifold (27) into the cells (1).

7. The electrolyzer according to any one of the claims 1-6, wherein each flow restrictor (28) comprises two parts (28A, 28B) attached to each other with the canal (30) located in between the two parts (28A, 28B) and provided as a recess in one or the other part (28A, 28B) or in both.

8. The electrolyzer according to claim 7, wherein the two parts (28 A, 28B) are formed as discs attached to each other face-to-face with the canal (30) in between formed by a recess.

9. The electrolyzer according to any one of the claims 1-6, wherein each flow restrictor (28) has a male portion (33) and an opposite directed female portion (34), and the multiple restrictors (28) are serially connected by uptake of their corresponding male portion (33) in a female portion (34) of a neighbouring flow restrictor (28), andwherein each canal (30) is provided as a recess in the male portion (33) or the female portion (34) and located between the connected male and female portion (33, 34) of neighbouring flow restrictors (28).

10. The electrolyzer according to any one of the claims 1-9, wherein the stack (17) comprises a stack of modules (18), sandwiching one of the ion-transporting separators (4) between each two of the modules (18), wherein each module (18) comprises a welded assembly of a first and second metal sheet (12, 13), which are perforated cathode and anode sheets, respectively, and a bipolar plate, BPP, (14) sandwiched between the first and second metal sheets (12, 13); wherein one of the gaskets (25) is provided sealingly between each two neighbouring modules (18); wherein each of the metal sheets (12, 13) and the bipolar plate (14) has an opening (20) through which at least one of the flow restrictors (28) extends for forming a portion of the inner wall of the electrolyte manifold (27).

11. The electrolyzer according to any one of the claims 1-9, wherein the stack (17) comprises a stack of modules (18), sandwiching one of the ion-transporting separators (4) between each two of the modules (18), wherein each module (18) comprises a four-sheet assembly of a first and second metal sheet (12, 13), which are perforated cathode and anode sheets, respectively, and a third and fourth metal sheet (14A, 14B), which are non-perforated and in combination form a two-layer bipolar plate, BPP, (14) sandwiched between the first and second metal sheets (12, 13) and having a coolant compartment (22) delimited by the third and fourth metal sheets (14A, 14B), the coolant compartment (22) being separated from the anode and cathode compartments (5, 6); wherein one of the gaskets (25) is provided sealingly between each two neighbouring modules (18); wherein each of the four metal sheets (12, 13, 14A, 14B) has an opening (20) through which at least one of the flow restrictors (28) extends for forming a portion of the inner wall of the electrolyte manifold (27).

12. Electrolyzer according to claim 10 or 11, wherein the first metal sheet (12) or the second metal sheet (13) or both are abutting the ion-transporting separator (4) as electrode in a zero-gap configuration; or wherein there is provided a porous electrically conducting sheet between the ion-transporting separator (4) and the first metal sheet (12) or between the ion-transporting separator (4) and the second metal sheet (13), orboth, the porous electrically conducting sheet and the first or second metal sheet in combination being a composite cathode or anode, respectively.

13. Use of an electrolyzer according to any preceding claim for producing hydrogen gas in an alkaline electrolyte.

14. A method of operating an electrolyzer according to anyone of the claims 1-12, the method comprising adjusting the pressure of the electrolyte in the electrolyte manifold (27) and pumping electrolyte through the electrolyte manifold (27) and into the electrolyzer cells (1), determining a first pressure drop through the electrolyte manifold from a first end to a second end of the electrolyte manifold (27) and a second pressure drop through the flow canals (30) from the flow canal inlet (30A) to the flow canal outlet (30B), and wherein width and length of the flow canals (30) are dimensioned so as to provide the second pressure drop through the flow canals (30) at least 10 times higher than the first pressure drop through the electrolyte manifold (27).

15. A method of production of an electrolyzer according to anyone of the claims 1-12, the method comprising- in a selection step, selecting a set of flow restrictors (28) and inserting the flow restrictors in the electrolyzer stack (27),- determining a flow and a pressure of electrolyte through the electrolyte manifold (27) from a first end to a second end of the electrolyte manifold (27),- for a predetermined flow of electrolyte through the electrolyzer cells (1) in the stack (17), determining a first pressure drop through the electrolyte manifold (27) from the first end to a second end of the electrolyte manifold (27), wherein the selection step comprises selecting flow restrictors with flow canals (30) having a width and a length so as to provide a second pressure drop through the flow canals (30) at least 10 times higher than the first pressure drop through the electrolyte manifold (27).

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