Electroformed bipolar plate and electrolysis cell comprising an electroformed bipolar plate

Abstract

The invention relates to a system comprising an electroformed bipolar plate and a sealing member, electrolyser cells and electrolyser cell stacks comprising electroformed bipolar plates.

Description

[0001] Electroformed Bipolar Plate and Electrolysis Cell comprising an Electroformed Bipolar Plate

[0002] The invention relates to a system comprising an electroformed bipolar plate and a sealing member, electrolyser cells and electrolyser cell stacks comprising electroformed bipolar plates.

[0003] An efficient hydrogen production is for the de-carbonization of industry processes and the overall transformation of the energy landscape of utmost importance. Several different methods of hydrogen generation from different starting materials are known and established since several years, nevertheless, the provision of green hydrogen produced in a carbon-neutral way seems to be a very promising goal, because the substitution of fossil-based energy by green hydrogen leads to the highest reduction in carbon-dioxide emissions. One established way of hydrogen generation is the electrolysis of water performed in single electrolyser cells or, on an industrial scale, in electrolyser cell stacks, wherein the latter comprise a multitude of electrolyser cells connected in series. The components of the electrolyser cell compartment are determined by the underlying electrochemistry and, in short, a cell at least includes electrodes in the form of an anode and a cathode, a separator immersed in an aqueous electrolyte in between the anode and the cathode and a cell body defining the space of the overall cell compartment. In case of electrolyser cell stacks the cell body is formed by bipolar plates, wherein on each side of an bipolar plate an anode and a cathode is present. Besides encompassing the other components of the electrolysis cell, the bipolar plates may provide further functionalities, like the electrolyte and gas connection to and from the cell compartment. The basic design of the different electrolyser cells available on the market includes these components, nevertheless, there are pros and cons for every type and the overall electrical efficiency of a specific cell type is a complex function of the used materials and the detailed set-up of the functional component. Based on the vast amounts of hydrogen necessary for changing the energy landscape, the optimization of each cell component is crucial, in order to achieve the highest possible efficiency. This statement is valid for all cell components but especially true for the bipolar plates, because the bipolar plates are the crucial factor enabling the chemical and mechanical cell integrity during electrolysis and is responsible for the overall cell lifespan, the service intervals, the stack dimensions and the overall costs of the set-up.

[0004] Different strategies for an efficient bipolar plate design and bipolar plate production methods can also be found in the patent literature.

[0005] EP 1 103 636 Al, for instance, describes an electrolysis plate consisting of an outer, non-con- ductive frame, in particular consisting of a fiber-reinforced cresol resin, an electrically conductive, bipolar graphite plate mounted therein, preferably slotted on both sides, which has plastic aprons in the region of the electrolyte supply for the forced guidance of the electrolyte solutions.

[0006] Another patent document, WO 2021 080 064 Al, describes a bipolar plate structure having an electrical shielding means for preventing electrical corrosion due to a positive electrode and a negative electrode, and provides an electrolysis bipolar plate structure comprising: a bipolar plate; a negative electrode part including a negative electrode which is a gas diffusion electrode (GDE); a positive electrode part including a positive electrode; and an electrical shielding means disposed between the bipolar plate and each of the negative electrode part and the positive electrode part to prevent electrical corrosion due to the positive electrode and the negative electrode, the negative electrode part and the positive electrode part being coupled to one side and the other side of the bipolar plate, respectively, with the bipolar plate interposed therebetween, wherein: the bipolar plate has concave portions formed at a predetermined depth on one side and the other side thereof to allow the negative electrode and the positive electrode to be installed therein, respectively, and includes a flowing-water inlet port and a flowing-water outlet port formed through the upper and the lower corresponding ends of both sides of the bipolar plate, respectively, to allow the flow of water in the negative electrode part and the positive electrode part, the upper and the lower corresponding ends extending outward from the concave portions; the negative electrode part includes a negative electrode, a negative electrode shielding plate, and a gasket for covering the entire negative electrode part, which are coupled to one another; and the positive electrode part includes a positive electrode, a positive electrode shielding plate, and a gasket for covering the entire positive electrode part, which are coupled to one another.

[0007] In addition, EP 4 128 399 Al describes a bipolar plate assembly for forming an electrolytic or fuel cell stack comprising - a metallic separator unit adapted to create a fluid-tight seal between the anode side and the cathode side and provided with fluid supply channels and fluid discharge channels on both the anode and cathode sides respectively, - two metallic flow distributor units which are arranged adjacent to the separator unit on the anode and cathode sides, each flow distributor unit being designed to distribute a fluid supplied to it via the separator unit between the fluid supply channels and the fluid discharge channels, and - metallic frame elements which are connected to the separator unit in a fluid-tight manner and which each surround one of the flow distributor units circumferentially in a fluid-tight manner, the frame elements having through-openings which are designed to supply a fluid to the fluid supply channels and through- openings which are designed to discharge a fluid discharged via the fluid discharge ducts, wherein all the through-holes of one frame member are positioned in alignment with the through-holes of the other frame member, and in that the separator unit is provided with through-holes positioned in alignment with the through-holes of the frame members and connecting them to the fluid supply channels and fluid discharge channels of the separator unit.

[0008] Such solutions, known from the prior art, may offer further potential for improvement, especially regarding the ease of electrolyser stack assembly, the operational safety and long service life of electrolyser cells even under harsh process conditions including high pressures and temperatures. It is therefore the task of the present invention to at least partially overcome the disadvantages known from the prior art. In particular, it is the task of the present invention to disclose a bipolar plate, comprising an improved mechanical and electrochemical performance even in the context of large electrolyser cells as typically found in alkaline electrolysis, only. Furthermore, it is the task of the present invention to provide an inventive system of a bipolar plate and a sealing member, and an electrolyser cell or an electrolyser cell stack comprising the inventive system. Especially the cells and the stacks are easily up-scalable, simple and material effective to manufacture and maintain, less prone to leakages and comprising a good corrosion resistance resulting in long service life periods.

[0009] The task is solved by the features of the independent claims, directed to the system comprising the bipolar plate and a sealing member and electrolyser cells and electrolyser cell stacks according to the invention. Preferred embodiments of the invention are described in the dependent claims or in the description, whereby further features described or shown in the dependent claims or in the description may individually or in any combination constitute an object of the invention if the context does not clearly indicate the contrary.

[0010] A system is within the scope of the invention, wherein the system comprises a bipolar plate for an electrolyser cell and a sealing member, wherein the bipolar plate is an electroformed bipolar plate comprising a thickness of larger than or equal to 0.1 mm and smaller than or equal to 0.8 mm and a surface area of larger than or equal to 0.5 m2and smaller than or equal to 6.0 m2, wherein the bipolar plate is in physical contact with the sealing member, and wherein the sealing member comprises an elastic modulus, measured according to DIN EN ISO 527-1 :2019, of larger than or equal to 0.05 GPa and smaller than or equal to 2.0 GPa. In the case of electrolys- ers, and especially in the case of alkaline water electrolyser comprising large surface areas, it has been found favourable to combine electroformed plates and rather elastic gaskets. Without being bound by the theory it is assumed that especially the tailorable surface characteristics of electroformed plates is suitable to be combined with gasket materials comprising an elasticity in above defined range. This mechanical sealing results in electrolysis cells and electrolysis cell stacks comprising a superior pressure resistance and a lower failure rate. In addition, the service life of the cells and stacks can be prolonged, resulting in a reduction of processing costs. Such advantages can for instance apply in the case of electroformed bipolar nickel -plates. In general, gaskets for electroformed nickel sheets may not need to accommodate as many surface irregularities as those for standard hot-rolled sheets. The mechanical properties of a gasket, including its hardness and elasticity, play a crucial role in its ability to create a leak-tight seal. For a smoother surface, like that of an electroformed nickel sheet, a harder gasket material may be used because the smooth surface allows for better contact between the gasket and the sheet. This can create a tighter seal with less potential for leaks. However, for a rougher surface, like that of a hot-rolled nickel sheet, a softer, more flexible gasket material may be needed. This is because the gasket needs to conform to the surface irregularities of the sheet to create a tight seal. A harder gasket material may not be able to conform as well to these irregularities, which could result in leaks. Consequently, the harder seal material claimed above comprises a real benefit compared to state-of-the-art systems. Preferably, the elastic modulus can larger than or equal to 0.1 GPa and smaller than or equal to 1.5 GPa and further preferred larger than or equal to 0.2 GPa and smaller than or equal to 1.0 GPa.

[0011] The electrolysis cell for the system can, for instance, be a water electrolysis cell or a chloralkali (CA) cell. A water electrolysis cell is able to generate oxygen and hydrogen from water, and a CA cell is able to generate chlorine from aqueous halide salt solutions by the application of a DC electrical power. The water electrolyser can be of the PEM (Polymer Electrolyte Membrane), AEM (Anion Exchange Membrane) or the alkaline-type. PEM and AEM electrolysers are able to process pure water as the electrolyte. The electrodes for the PEM-type are made from noble metals, and the separator is a proton-conducting membrane usually made from a perfluorinated sulfonic acid polymer. The electrodes for the AEM type are made from nonnoble metals, and the separator is a hydroxide-conducting membrane. Preferably the water electrolysis cell is a cell for alkaline electrolysers or AWEs. AWEs preferably use aqueous KOH solutions. The chemistry of the reaction allows the use of cheap non-noble electrodes, e.g. based on nickel. The Chlor-alkaline electrolyser cell (CA) can be of the NaCl- or KCl-type, using respective salt solutions. It can also be of the HCl-type, using aqueous hydrochloric acid as electrolyte. Preferably, the electrolysis cell is an AWE or CA cell.

[0012] Electroformed bipolar plates in electrolysers comprise several advantages compared to state-of the art formed (and coated) bipolar plates. This finding is surprising, because the here considered bipolar plates do not just form the margins of electrolyser cells but have to fulfil certain side-conditions, which are only found in electrolyser cells. Suitable formed bipolar plates, e.g. for alkaline electrolysis, have to comprise in contrast to PEM electrolysis and fuel cells a relatively large surface area, and cell surface areas larger than 1 - 2 m2can easily be processed by electroforming, e.g. by forming pure and homogeneous bipolar plates, e.g. made from nickel. The bipolar plates can be provided as one piece, irrespective of the size, eliminating the need for welding different nickel plates together or relying on large steel plate which need to be coated in a subsequent step, as performed in the state of the art. The electroformed bipolar plates comprise homogeneous material characteristics throughout the entire plate and elastic and thin plates can be provided, which are able to withstand the harsh chemical surrounding in electrolysers, especially in the case of alkaline water electrolyser (AWE). Here the process surrounding is very demanding, putting more and different constraints on the mechanical properties in comparison, for instance, to fuel cells. A further advantage is, that a homogeneous hardness throughout large surface areas of the bipolar plate can be achieved. State of the art electrolyser bipolar plates comprise inhomogeneous elasticity profiles, because the common bipolar plate structure is achieved by mechanical forming steps, wherein the structure of the formed areas is significantly altered compared to the unprocessed surface areas. This effect is known as work hardening. In consequence, state-of-the art bipolar plates for large area alkaline cells are typically rather thick and made from hot-rolled nickel sheets or prepared from thick steel sheets subsequently coated with nickel. Very thin bipolar plates can be formed by electroforming instead, and the bipolar plates can ensure safe operating pressures of 50 kPa and above. Besides the macroscopic mechanical properties also the surface features of the bipolar plates can be tailored by electroforming. Very smooth and homogeneous surfaces can be achieved, even for very large areas, wherein the surfaces are free of surface defects. Surface defects are important for bipolar plates, because surface defects generally degrade the corrosion resistance of the plates. This is especially true for full-Ni-plates, wherein corrosion resistance is especially important during transport and storage periods when plates may be subjected to chloride containing environments.

[0013] The bipolar plate is a bipolar plate for an electrolyser cell stack. The bipolar plate is mechanically supporting the electrodes in an electrolytic cell, is in physical contact to one or two electrodes and defines the inner cell volume by surrounding the electrolytic cell or separating different electrolyser cells in an electrolyser cell stack. In an electrolyser cell a liquid or a solid dissolved in a liquid is electrochemically transformed to the desired product or products. Preferably the electrolysis cell is especially adapted to generate gases from an aqueous electrolyte. In case of a single electrolyser cell or single element electrolyser cell the bipolar plate just contacts one electrode. In this case the bipolar plate can also be considered an electrode support, also including the function of an electrolysis cell wall. In case of a cell stack, comprising more than one electrolyser cell, bipolar plates are contacting two electrodes, one on each side and in this case the bipolar plate comprises a different electrical polarity at each side. Besides having the function of forming the electrolyser cell volume a bipolar plate can also fulfil further tasks. It is, for instance, possible that the bipolar plate comprises means for guiding electrolyte into the cell volume. In addition, the bipolar plates can be used for removal of the process gases, i.e. oxygen and hydrogen, out of the electrolyser cell, by incorporating internal inlet and / or outlet manifolds.

[0014] The bipolar plate is an electroformed bipolar plate. An electroformed bipolar plate is a bipolar plate obtained by electrodeposition upon a mandrel or mould that is subsequently separated from the deposit. Electroforming per se is a method of fabricating parts that are usually free standing once separated from the mandrel. It is further possible that a piece of the mandrel may be intentionally bonded to or encapsulated by the electrodeposit and become an integral part of the finished product. The electroformed bipolar plate is a metal bipolar plate. Preferably the bipolar plate can be a pure nickel bipolar plate. In nickel electroforming, electrolytes like nickel sulphamate solutions or Watts solutions can be used for forming the surface or the complete bipolar plate body. In the latter, nickel sulphate and nickel chloride provide the nickel units. Plate properties such as hardness, ductility, strength and internal stress can be varied significantly by changing electrolyte composition and operating conditions, by the use of inorganic or organic additives or by co-deposition of alloying metals such as cobalt. Preferably, bipolar plates are electroformed comprising high Ni -contents, e.g. higher than 95 weight-%, preferably higher than 99 weight-% and even more preferred higher than 99.9 weight-%.

[0015] The bipolar plate comprises a thickness of larger than or equal to 0.1 mm and smaller than or equal to 0.8 mm and a surface area of larger than or equal to 0.5 m2and smaller than or equal to 6.0 m2. It has surprisingly been found that via electroforming the specific side-conditions for bipolar plater in electrolysis, especially water electrolysis and preferably alkaline water electrolysis can be met. Large and thin bipolar plates can be provided using electroforming, wherein it is surprising that the produced plates comprise the right mechanical strength and elasticity range even in case when the surface area of the plates is near the upper range. In addition, the electroformed bipolar plates also maintain their favourable mechanical features in case that additional functional features are integrated during electroforming. These additional functional features might include manifolds for electrolyte distribution into the cell or removal of generated reaction gases like oxygen or hydrogen. Preferably, the bipolar plate thickness is larger than or equal to 0.15 mm and smaller than or equal to 0.5 mm and the surface area is larger than or equal to 1.0 m2and smaller than or equal to 2.9 m2, and further preferred, the thickness is larger than or equal to 0.15 mm and smaller than or equal to 0.3 mm and the surface area is larger than or equal to 1.5 m2and smaller than or equal to 2.8 m2. In case that the bipolar plate is intended to comprise a varying thickness, the area weighted average plate thickness is taken into account.

[0016] A sealing member for sealing an electrolyte in the anode and the cathode compartment is present in the system. The sealing member can for instance be arranged between the electrically conducting anode support and the electrically conducting cathode support or the bipolar plates, respectively. The sealing member can be configured for sealing the electrolyte in the anode compartment and the electrolyte in the cathode compartment. The anode and the cathode compartment combined can form together with the separator an electrolysis cell volume. This cell volume is separated with respect to the cell surrounding by the sealing member, e.g. in the form of a gasket. The sealing member is electrically isolating the anode and the cathode in the cell. The sealing member can be a single- or a multi-part item. The sealing member may be formed by a gasket between the bipolar plates, by a frame and a gasket between the bipolar plates, or by a frame around at least one support or bipolar plate with gaskets in between. The seal or the gasket is a polymeric seal or gasket and the seal is in direct or indirect physical contact with the supports or in case of a stack the bipolar plates. The sealing member can be arranged in between the supports and prevents any leakage of electrolyte or gas from the cell volume to the surrounding. Preferably, the seal or gasket is arranged at the outer circumference of the support, e.g. 0,5 cm to 2 cm away from the outer circumference of the support or flush with the outer circumference or protruding beyond the outer circumference. This seal forms as such or as a part of a more sophisticated combination of a frame and a gasket the pressure bearing part or, e.g. further in combination with a pressure ring, e.g. a metallic hoop, part of the pressure bearing part of the cell in case that the inner cell volume and the outer surrounding of the cell comprise a pressure difference. Usually, AWEs for instance are performed at an operating pressure higher than standard pressure, i.e. the hydrogen and / or oxygen has a gas pressure higher than ambient. Therefore, the sealing member has at least to withstand this inner cell to surrounding pressure difference during operation. The sealing member may comprise a polymer blend of at least two different polymers A and B.

[0017] The polymer A can selected from the group consisting of polymers according to the following formula 1 wherein n = 10 - 20.000, m = 0, 1 or 2, each R1is independently selected from the group consisting of Cl-C3-Alkyl or a further polymer chain according to formula 1, X is independently selected from the group consisting of S, O, SO2, CH2, CO, or isomeric forms or mixtures thereof. The polymer can be a “simple” polymer just comprising sulfur-heteroatoms. Nevertheless, also polymers are possible comprising besides sulfur also sulfur oxides, alkylenes, oxygen, or CO-bridging groups in the polymeric chain. Mixtures of different X may be present, e.g. in the form of SO2- and -O- bridging units in the polymer. The side groups R1may be present at different positions at the aromatic ring. In case that more than one side group R1is present the R1can be the same but do not have to be the same. The side group R1can also be formed by covalently crosslinking another polymer to that unit. In the latter case a branching of the polymer is generated. Isomeric variants of the monomeric unit encompass the attachment of the bridging group at different positions of the aromatic ring structure. Mixtures of the polymer A may include polymeric chains according to the above depicted Markush formulae, but different chemical compositions.

[0018] The polymer B can be selected from the group consisting of synthetic elastomeric rubbers or mixtures thereof. The second component of the polymer blend is a synthetic elastomeric rubber. This means, that the rubber is made from monomers under controlled chemical conditions. The term elastomeric means, that the rubber as such comprises an elastic modulus of larger or equal to 1 MPa and smaller or equal to 50 MPa. The elastomeric rubber can, for instance, be selected from the group consisting of ethylene-propylene-diene monomer (EPDM) rubber, EPDM rubber derivatives, fluoro-elastomer rubbers (FKM a family of fluorocarbon-based fluoro-elas- tomer materials), NBR (Nitrile rubber, also known as nitrile butadiene rubber), Viton / FKM or SBR (styrene-butadiene rubber). Especially preferred are EPDM rubbers, wherein these rubbers are copolymers of three different monomers, i.e. a ethylene, a propylene and a non -conjugated diene monomer. The diene monomer enables vulcanization of the monomers into a polymeric rubber. The vulcanization can be performed by a cross-liking with sulphur, peroxides, phenolic resins or via radiation curing. Possible non-conjugated dienes may be selected from the group consisting of 2-ethylidene-5-norbomene (ENB), dicyclopentadiene (DCPD), 2-vinyl-5-nor- bomene (VNB), 1,4-hexadiene (HD) and 2-methylidene-5-norbornene (MNB) or mixtures thereof. Possible EPDM rubber derivatives are selected from the group consisting of partially fluorinated EPDM-rubbers epoxidized EPDM or EPDM functionalised by other functional groups such as carboxylic groups. Especially preferred are epoxidized EPDM rubbers, wherein the polymers of group B may or may not comprise epoxidation. All the other rubbers in the above defined list may also the further functional moieties as defined for the EPDM rubber.

[0019] In a preferred embodiment of the system the polymer A is polyphenylene sulfide (PPS) and polymer B is EPDM rubber. Highly resistant but elastic sealing members can be achieved in cases, wherein polymer A is a PPS. The PPS -polymer may also include the branched PPS- polymers and the oxidized PPS-polymers. Preferably, polymer B can be an EPDM-rubber or an EPDM-rubber derivative in this case. Such polymer combination results in sealing members comprising an improved mechanical interaction in the system including the electroformed bipolar plates according to the invention.

[0020] Within a preferred embodiment of the bipolar plate, the bipolar plate comprises a surface area of larger than or equal to 75% comprising an arithmetic average surface roughness (Ra) of smaller than or equal to 1.0 pm. Even for very large surface areas of the bipolar plates a consistent and small surface roughness can be established. The surface roughness can be assessed according to DIN EN ISO 21920-2. The measurement length 1 can be set to 20 cm and an average of 10 measurements points can be taken. This level of surface roughness enables a high corrosion resistance and good electrochemical performance of the cells, including a low level of absorption of generated gases to the bipolar surface. Furthermore, the attachment of the electrodes and the mechanical sealing via gaskets is improved. It is further preferred that the bipolar plate comprises a surface area of larger than or equal to 75% comprising an arithmetic average surface roughness (Ra) of larger than or equal to 0.1 pm and smaller than or equal to 1.0 pm. Preferably, the surface roughness (Ra) can be larger than or equal to 0.2 pm and smaller than or equal to 0.75 pm, further preferred, the surface roughness (Ra) can be larger than or equal to 0.25 pm and smaller than or equal to 0. 5 pm.

[0021] In another preferred embodiment of the bipolar plate, the bipolar plate comprises a thickness uniformity of smaller than or equal to 0.1 mm. The thickness uniformity is defined as the difference between the maximum and minimum thickness, as measured by the method described in ASTM Al 073. The bipolar plate can, on the one hand, be very thin, reducing the costs of the bipolar plate and increasing the cell-number density in electrolyser stacks. On the other hand, the electroformed bipolar plates can also comprise a very uniform thickness over large surface areas. This feature may enhance the electrical cell performance and increase the service lifetime. The thickness uniformity of the plate can be accessed via a calliper or micrometre gauge measurement. At least 10 measurements may be performed in the area of interest. The thickness can be measured at representative locations at least 25 mm from an edge. In case that the bipolar plate is intended to provide more than one area of defined thickness, the thickness uniformity is assessed for each area. In case that certain parts of the bipolar plate comprise a coating or intended macroscopic surface structure these areas are not included in the assessment or in case of a coating the coating thickness is assessed separately. Preferably, the bipolar plate comprises a thickness uniformity of larger than or equal to 0.005 mm and smaller than or equal to 0.1 mm. Preferably, the bipolar plate comprises a thickness uniformity of larger than or equal to 0 mm and smaller than or equal to 0.1 mm, further preferred, the bipolar plate comprises a thickness uniformity of larger than or equal to 0.001 mm and smaller than or equal to 0.05 mm. In a further preferred aspect of the bipolar plate, at least a part of one or both bipolar plate surface comprises a brazing filler coating, wherein the average thickness of the brazing filler coating on one side is larger than or equal to 20 pm and smaller than or equal to 200 pm. In order to ease the attachment of the electrodes to the electroformed bipolar plate it has been found useful, to add a brazing filler on the surface of the bipolar plate. The used range of coating thickness is sufficient to assure a sufficient mechanical fixation of the electrode to the bipolar plate. In this respect, it is surprising that considering the relatively large sizes of the electrodes and the bipolar plate in electrolysis cells, especially in alkaline water electrolysis cells, such coating is sufficient for a mechanical fixation of the electrode. The brazing fillers can be Ni- based brazing fillers, for instance compositions comprising or consisting of B-Ni82CrSiBFe. Such coatings add an important functionality to the plate without deterioration of the plate resistance against harsh process conditions, for instance in the context of an alkaline water electrolysis. The thickness of the brazing filler coating can be assessed by microscopic measurements. Preferably, the average thickness of the brazing filler coating on one side can be larger than or equal to 25 pm and smaller than or equal to 100 pm, further preferred, the average thickness of the brazing filler coating on one side can be larger than or equal to 30 pm and smaller than or equal to 80 pm.

[0022] In another preferred characteristic of the bipolar plate, the brazing filler coating covers a surface area at least on one side of the bipolar plate of larger than or equal to 5 % and smaller than or equal to 50%. In order to assure a homogeneous and mechanical sufficient connection between the electrodes and the electroformed bipolar plate, even in case of large electrodes and large bipolar plates, above defined covered surface area ratio has been found useful. Smaller surface areas might be insufficient for a secure attachment of large electrodes and large cell volumes. Larger areas might deteriorate the other physical and electrochemical functions of the electroformed bipolar plate. Preferably, the brazing filler coating covers a surface area at least on one side of the bipolar plate of larger than or equal to 15 % and smaller than or equal to 45%. In another preferred embodiment of the bipolar plate, the bipolar plate comprises at least two different coherent areas comprising a difference in surface roughness, wherein in one area the arithmetic average surface roughness (Ra) is smaller than or equal to 1.0 pm and in the other surface area the arithmetic average surface roughness (Ra) is smaller than or equal to 0.5 pm. The surface roughness can be assessed according to DIN EN ISO 21920-2, as detailed earlier. For an electrolysis cell bipolar plate, it has been found useful to establish different surface areas comprising a different surface roughness. This means, that within a certain surface area of the bipolar plate a smaller surface roughness is present compared to another defined surface area. The area is coherent in a sense, that the areas of different surface roughness are not randomly scattered throughout the surface of the bipolar plate. A coherent surface area at least comprises an area of larger than 50 cm2or 20% of the overall bipolar plate surface area, whichever surface area is larger. The different regions comprising different surface roughness’s for instance include a different roughness at the mechanical sealing part of the bipolar plate or at the electrode attachment part. Such tailored roughness as a function of the surface task can increase the overall bipolar plate performance. Preferably, the bipolar plate comprises at least two different coherent areas comprising a difference in surface roughness, wherein in one area the arithmetic average surface roughness (Ra) is larger than or equal to 0.1 pm and smaller than or equal to 1.0 pm and in the other surface area the arithmetic average surface roughness (Ra) is larger than or equal to 0.1 pm and smaller than or equal to 0.5 pm. Further preferred, in one area the surface roughness (Ra) can be larger than or equal to 0.3 pm and smaller than or equal to 0.8 pm and in the other surface area the surface roughness (Ra) can be larger than or equal to 0.1 pm and smaller than or equal to 0.25 pm.

[0023] In another preferred embodiment of the bipolar plate, the bipolar plate comprises at least two coherent areas of different bipolar plate thickness, wherein one area comprises a thickness of smaller than or equal to 0.3 mm and another area comprises a thickness of smaller than or equal to 0.8 mm. The thickness can be measured by the method described in ASTM A1073, as discussed earlier. For a large area electrolyser cell, it has been found useful to establish two different plate segments, wherein on segment comprises a smaller thickness compared to the other plate segment. It is possible to tailor the electroformed plate in a way that these different regions are already achieved within the electroforming process, without the need for further mechanical processing of the plate. Mechanical processing significantly alters the material properties and results in unwanted mechanical and structural inhomogeneities. It might be favourable to electroform a plate comprising a larger thickness at the edges of the plate, wherein a smaller thickness might be used at the inner part of the plate. The overall location specific thickness can be assessed by a micrometre or calliper. The areas are at least larger than 50 cm2and intended surface structures like bumps or other functional structure on the plate surface are excluded from measurement. At least 10 different locations are assessed in each area and the thickness average is calculated. Preferably, one area comprises a thickness of larger than or equal to 0.1 mm and smaller than or equal to 0.3 mm and another area comprises a thickness of larger than or equal to 0.4 mm and smaller than or equal to 0.8 mm. Further preferred, one area comprises a thickness of larger than or equal to 0.15 mm and smaller than or equal to 0.25 mm and another area comprises a thickness of larger than or equal to 0.3 mm and smaller than or equal to 0.8 mm.

[0024] In another preferred characteristic of the bipolar plate, the bipolar plate comprises a constant Vickers hardness range according to the following ratio, wherein the Vickers hardness is assessed according to a micro- Vickers HV0.5 / 15 as described in DIN EN ISO 6507-1 1 :2023, and the ratio of the largest to the smallest Vickers hardness, determined by largest Vickers hardness value divided by smallest Vickers hardness value, is larger than or equal to 1.0 and smaller than or equal to 1.5. Especially for large scale electrolyser cells and electrolyser cell stacks it has been found suitable, that bipolar plates comprising a nearly constant hardness are provided. This ensures, that the forces during electrolysis are evenly absorbed by the bipolar plate and no weak spots occur. This may enhance the service life of the bipolar plate. In addition, state of the art bipolar plate forming processes usually require a mechanical treatment of the plates in order to achieve the right surface architecture by rolling and / or pressing steps. Such mechanical treatments result in a work-hardening of the treated parts and, consequently, result in an uneven Vickers hardness of treated vs. untreated parts. Such differences are significant and well above the described range for electroformed bipolar plates. For the assessment of the Vickers hardness at least 1 measurement is performed per 10 cm2throughout the plate surface.

[0025] In another preferred embodiment of the bipolar plate, a surface area of larger than or equal to 25% and smaller than or equal to 90 % comprises surface protrusions, wherein the surface protrusions comprise a height of larger than or equal to 10 pm and smaller than or equal to 50 pm. Large electroformed bipolar plates comprise the advantage of very low surface roughness. Nevertheless, it is also possible to generate special feature intentionally on the bipolar plate surface, adding further functionalities to the surface. In this case it is possible to generate an even and smooth basic surface, showing such additional features. Additional surface structures might alter the absorption characteristics of generated oxygen and hydrogen on the bipolar plate surface. Furthermore, intentionally generated surface structures might also be used to guide the electrolyte flow within the inner cell compartment. In electroforming it is also possible to generate large electrolyser bipolar plates for instance showing a riblet spacing of 15 to 100 pm. The riblet height might be in the range from 10 to 50 pm, the riblet width from 5 to 50 pm. The aspect ratio (h / s) of the riblet can be from 0.5 to 1.0 and a triangular riblet shape can be chosen.

[0026] Within a preferred embodiment of the bipolar plate, the bipolar plate comprises a rectangular shape, wherein a ratio between the length of the longest to the length of the shortest rectangle side, determined by the length of the longest side divided by the length of the shortest side, is larger than or equal to 1.1 and smaller than or equal to 5. For electrolysis cells and especially for AWE cells asymmetric bipolar plates obtained by electroforming has been found useful. Without being bound by the theory it appears that electroforming shows especial advantages in the case of asymmetric bipolar plate dimensions. In the state-of-the art processes such shapes are difficult to obtain without alteration the of the material properties. Electroforming provides such plates without deterioration of the surface and mechanical properties. Preferably, the ratio is larger than or equal to 1.2 and smaller than or equal to 4.0 and, further preferred, is larger than or equal to 1.5 and smaller than or equal to 2.5. In addition, it is further preferred that the asymmetric bipolar plates comprise a surface area of larger than 0.5 m2, further preferred larger than 1 m2and even further preferred larger than 2.0 m2.

[0027] In a preferred embodiment of the system, the sealing member comprises a sealing member main body and at least one sealing member protrusion extending from the sealing member main body, wherein the cross-sectional area of the sealing member main body is larger than or equal to 15 mm2and smaller than or equal to 500 mm2. Based on the elasticity and chemical resistance it has been found useful to establish a system comprising an electroformed bipolar plate and sealing member in the form of a three-dimensional body instead of a two-dimensional film. The three-dimensional body exhibits a sealing member main body comprising the major weight of the sealing member. The sealing member main body can be defined as the structural part of the sealing member comprising the smallest surface area with respect to the volume of that structural part. The sealing member main body can for instance be present in the form of a closed ring structure, wherein from the ring structure protrusions extend. In this case the main body is the ring structure, wherein the protrusions do not belong to the sealing member main body. Smaller cross-sectional areas of the sealing member main body can be disadvantageous, because the sealing member main body may be too small to achieve a sufficient mechanically stable seal. Larger sealing member main bodies may be disadvantageous, because the necessary mechanic forces to squeeze the sealing member in place may be too high. Examples of the definition of the sealing member main body are given below. In case that the cross-sectional area of the sealing member main body varies along the sealing member main body, an average cross-sectional area of the sealing member main body is calculated. Preferably, the cross-sectional area of the sealing member main body is larger than or equal to 30 mm2and smaller than or equal to 400 mm2, further preferred, the cross-sectional area of the sealing member main body is larger than or equal to 50 mm2and smaller than or equal to 300 mm2. In a preferred aspect of the system, the bipolar plate comprises a recess in the bipolar plate surface adapted for contacting at least a part of the sealing member. Besides the sealing of the electrolysis cell the mechanical properties of the sealing member enable the integration of further functions. One possibility is the mechanical fixing of the separator within the cell. The interaction with the separator is established by a recess within the sealing member main body, wherein the separator extends into the recess. This set-up enables an easy assembly of the electrolysis cell or cell stacks and prevents unintended movements of the separator in the cell, even in case of fluctuating pressure or temperature conditions. The mechanical interaction is established by the recess walls in the sealing member main body, wherein the wall is in close surface contact to the separator. The separator is clamped between the recess walls of the sealing member main body in case that a mechanical force is acting on the sealing member. The overall elasticity range of the sealing member enablers a tight clamping of the separator, without risking a mechanical damage of the separator. Such additional function is especially suitable in the context of the inventive system, because the use of electroformed bipolar plates comprising specific surface characteristics enables the use of higher mechanical forces for fixing the electrolysis cell stack. These pressures also interact on the recess and, therefore, a system of an electroformed bipolar plate and a hard sealing member including a recess might result in a better fixation of a separator.

[0028] An electrolyser cell is further within the scope of the invention, wherein the electrolyser cell comprises a system according to the invention, wherein the electrolyser cell comprises an anode compartment with an anode in contact with an anode support, a cathode compartment with a cathode in contact with a cathode support and a separator between the anode and the cathode compartment, wherein the sealing member seals an electrolyte in the anode and the cathode compartment, and wherein the anode and / or the cathode support is the electroformed bipolar plate. The electroformed bipolar plates show the above-mentioned advantages in the context of an electrolysis cell as well as in the case of electrolysis cell stacks obtained by combination of several electrolysis cells. The electrolysis cell comprises an anode compartment with an anode in contact with an anode support and a cathode compartment with a cathode in contact with a cathode support. In the anode compartment in the case of an AWE cell the oxidation of the hydroxide ions in the electrolyte to oxygen and water is performed. The reaction takes place at the anode electrode, which is either directly or indirectly connected to the DC power source. The electrode is in electrical contact to an electrically conducting support, wherein in case of a combination of several cells to a stack the support is called a bipolar plate, because electrodes are present on both sides of the support. At both ends of the stack the cells comprise an end plate. In the case of a single cell, only, one electrode, the anode, is present at the anode support and attached or fixed at one or more positions to the support. The anode electrode can for instance be a nickel-electrode. The anode compartment is filled during the reaction with the alkaline aqueous electrolyte. The reaction products e.g. electrolyte (or anolyte) and oxygen leave, at least in part, the anode compartment. In the cathode compartment of e.g. an AWE the reduction of the water in the electrolyte to hydrogen and hydroxide ions is performed. The reaction takes place at the cathode electrode, which is either directly or indirectly connected to the DC power source. The cathode is in electrical contact to an electrically conducting support, wherein in case of a combination of several cells to a stack the support is called a bipolar plate, because electrodes are present on both sides of the support. In the case of a single cell, only, one electrode, the cathode, is present at the cathode support and attached or fixed at one or more positions to the support. The cathode electrode can for instance be a nickel-electrode. The cathode compartment is filled during the reaction with the electrolyte. The reaction products hydrogen and electrolyte (catholyte) leave, at least in part, the cathode compartment. The overall benefits of using an electroformed bipolar plate in the context of an electrolysis cell can inter alia be seen in an enhanced mechanical stability, lower costs, and an easier assembly.

[0029] An electrolyser cell stack is further within the scope of the invention, wherein the electrolyser cell stack comprises two or more electrolyser cells in physical contact, wherein the electrolyser cell stack comprises a system according to the invention or at least one electrolyser cell according to the invention. The advantage of electroformed bipolar plates can be exploited to the full extent in cases, wherein more than one electrolysis cell is connected in series. The electroformed bipolar plates can provide higher mechanical stabilities and flexibilities at lower thicknesses. In consequence, compared to standard stacks the inventive stack may comprise more cells, resulting in a higher performance per meter. Alternatively, the costs of the cell stack may significantly be reduced. Further advantages are discussed above in the context of the inventive system including an electroformed bipolar plate and a sealing member and the inventive electrolyser cell.

[0030] It is further within the scope of the invention to disclose the use of the system according to the invention in an electrolyser, the use of such a system in an electrolyser cell stack, the use of such a system in an alkaline water electrolyser (AWE) cell or the use of such a system in an AWE cell stack. Electroformed bipolar plates provide exceptional improvements in comparison with state-of-the art (hot) rolled or welded bipolar plates.

[0031] It is further within the scope of the invention to disclose process of manufacturing an electrolyser cell or electrolyser cell stack, characterized in that, the process at least comprises the steps of: a) providing two or more electroformed bipolar plates, two or more sealing members and further components for assembly of an electrolyser cell or electrolyser cell stack; and b) assembling the electrolyser cell or the electrolyser cell stack.

[0032] Within this manufacturing process improved electrolysis cells or electrolysis cell stacks are achievable.

Claims

Claims1. System comprising a bipolar plate for an electrolyser cell and a sealing member, wherein the bipolar plate is an electroformed bipolar plate comprising a thickness of larger than or equal to 0.1 mm and smaller than or equal to 0.8 mm and a surface area of larger than or equal to 0.5 m2and smaller than or equal to 6.0 m2, wherein the bipolar plate is in physical contact with the sealing member, and wherein the sealing member comprises an elastic modulus, measured according to DIN EN ISO 527-1 :2019, of larger than or equal to 0.05 GPa and smaller than or equal to 2.0 GPa..

2. System according to claim 1, wherein the bipolar plate comprises a surface area of larger than or equal to 75% comprising an arithmetic average surface roughness (Ra) of smaller than or equal to 1.0 pm, and wherein the arithmetic average roughness (Ra) is described by ISO 21920.

3. System according to any one of the preceding claims, wherein the bipolar plate comprises a thickness uniformity of smaller than or equal to 0.1 mm and wherein the thickness uniformity is defined as the difference between the maximum and minimum thickness, as measured by the method described in ASTM A1073.

4. System according to any one of the preceding claims, wherein at least a part of one or both bipolar plate surfaces comprises a brazing filler coating, wherein the average thickness of the brazing filler coating on one side is larger than or equal to 20 pm and smaller than or equal to 200 pm.

5. System according to claim 4, wherein the brazing filler coating covers a surface area at least on one side of the bipolar plate of larger than or equal to 5 % and smaller than or equal to6. System according to any one of the preceding claims, wherein the bipolar plate comprises at least two different coherent areas comprising a difference in surface roughness, wherein, in one area, the arithmetic average surface roughness (Ra) is larger than or equal to 0.1 pm and smaller than or equal to 1.0 pm and, in the other surface area, the arithmetic average surface roughness (Ra) is larger than or equal to 0.1 pm and smaller than or equal to 0.5 pm, and wherein the arithmetic average roughness (Ra) is described by ISO 21920.

7. System according to any one of the preceding claims, wherein the bipolar plate comprises at least two coherent areas of different bipolar plate thickness, wherein one area comprises a thickness of smaller than or equal to 0.3 mm and another area comprises a thickness of smaller than or equal to 0.8 mm, as measured by the method described in ASTM Al 073.

8. System according to any one of the preceding claims, wherein the bipolar plate comprises a constant Vickers hardness according to the following ratio, wherein the Vickers hardness is assessed according to a micro- Vickers HV0.5 / 15 as described in DIN EN ISO 6507-1 1 :2023, and the ratio of the largest to the smallest Vickers hardness, determined by largest Vickers hardness value divided by smallest Vickers hardness value, is larger than or equal to 1.0 and smaller than or equal to 1.5.

9. System according to any one of the preceding claims, wherein a surface area of larger than or equal to 25% and smaller than or equal to 90 % of the bipolar plate comprises surface protrusions, wherein the surface protrusions comprise a height of larger than or equal to 10 pm and smaller than or equal to 50 pm.

10. System according to any one of the preceding claims, wherein the bipolar plate comprises a rectangular shape, wherein a ratio between the length of the longest to the length of theshortest rectangle side, determined by the length of the longest side divided by the length of the shortest side, is larger than or equal to 1.1 and smaller than or equal to 5.

11. System according to any one of the preceding claims, wherein the sealing member comprises a sealing member main body and at least one sealing member protrusion extending from the sealing member main body, wherein the cross-sectional area of the sealing member main body is larger than or equal to 15 mm2and smaller than or equal to 500 mm2.

12. System according to any one of the preceding claims, wherein the bipolar plate comprises a recess in the bipolar plate surface adapted for contacting at least a part of the sealing member.

13. Electrolyser cell comprising a system according to any one of the preceding claims, wherein the electrolyser cell comprises an anode compartment with an anode in contact with an anode support, a cathode compartment with a cathode in contact with a cathode support and a separator between the anode and the cathode compartment, wherein the sealing member seals an electrolyte in the anode and the cathode compartment, and wherein the anode and / or the cathode support is the electroformed bipolar plate.

14. Electrolyser cell stack comprising two or more electrolyser cells in physical contact, wherein the electrolyser cell stack comprises a system according to any one of claims 1 to 12 or at least one electrolyser cell according to claim 13.

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