Bipolar plate, method of production thereof, plate assembly and electrochemical system

Abstract

The invention relates to a bipolar plate (1) for use in an electrochemical system (10), formed from a metal sheet having an anode side (1a) and a cathode side (1b), wherein an edge region (6) of the bipolar plate (1) defines a reference plane (EE) and, in the edge region (6), there are ports (7, 8) for input and output of media to or from an active field (9), in the region of which electrochemical reactions take place in the electrochemical system (10), and wherein, in the active field (9), structuring (12) of the bipolar plate (1) is designed such that an active field base plane (EA) formed by two-dimensional material of the bipolar plate (1) is offset in a parallel spaced-apart manner from the reference plane (EE), namely in the direction toward the cathode side (1b), and individual embossed elements (13, 22) present in the active field (9) all project from the active field base plane (EA) in the direction of the anode side (1a). The invention further relates to a method of producing the bipolar plate (1), to a plate assembly (11) and to an electrochemical system (10).

Description

[0001] Bipolar plate, method for its manufacture, plate arrangement and electrochemical system

[0002] The invention relates to a bipolar plate intended for use in an electrochemical system, in particular an electrolyzer, and a method for its manufacture. The invention further relates to a plate arrangement comprising at least one bipolar plate and an electrolyzer, in particular for the electrolysis of water.

[0003] A possible design for a bipolar plate and an electrochemical cell is described, for example, in DE 10 2022 122 717 B3. The bipolar plate according to DE 10 2022 122 717 B3 consists of two half-sheets that are firmly joined together, with fluid inlet and outlet openings formed by the bipolar plate. Furthermore, distribution fields and an active field are formed. Seals are located on both sides of the bipolar plate, arranged congruently one above the other and reinforced by embossed structures. The electrochemical cell according to DE 10 2022 122 717 B3 can, in particular, be an electrolysis cell for the electrolysis of water.

[0004] DE 10 2021 206 090 A1 discloses a fuel cell system comprising several distributor plates. Flow channels for a reactant are formed through the first side of a distributor plate, while flow channels for a coolant are formed through the second side of the distributor plate.

[0005] Another bipolar plate designed for use in a fuel cell stack is described in DE 10 2006 059 857 A1. In this case, flow fields in the form of a positive and a negative structure are provided by a component generally referred to as a disk section, with channel-shaped structures in the inflow and outflow regions potentially exhibiting a meandering structure. WO 2015 / 126 746 A1 describes a separator plate and a frame element for an electrochemical cell. The separator plate comprises a plurality of protrusions that define a flow field, as well as a pair of end features arranged at opposite ends of the separator plate and extending along the length of the flow field. A circumferential region has first openings and second openings. The protrusions and the end features extend from a plane in which this circumferential region is located in only one direction perpendicular to that plane.A hydroforming process is also used.

[0006] The invention is based on the objective of further developing stacked electrochemical systems compared to the aforementioned prior art, whereby a high power density in relation to the stack height is sought in a manufacturing-friendly, robust design.

[0007] This problem is solved according to the invention by a bipolar plate designed as a component of an electrochemical system, having the features of claim 1. The bipolar plate is suitable for use in a plate arrangement according to claim 8. The plate arrangement, in turn, is suitable for use in an electrochemical system, in particular in an electrolyzer, according to claim 11. The problem is further solved by a method for manufacturing the bipolar plate according to claim 12.

[0008] The bipolar plate according to the invention, for use in an electrochemical system, is formed from a metal sheet with an anode side and a cathode side. A reference plane is defined by an edge region of the bipolar plate. Ports for supplying media to an active field of the electrochemical system or for discharging media from the active field are located in this edge region. Electrochemical reactions take place in the active field region of an electrochemical system. Within the active field, the bipolar plate is structured such that an active field base plane, formed by planar material, in particular undeformed material, of the bipolar plate, is spaced parallel to the reference plane. The active field base plane is offset towards the cathode side. Simultaneously, individual embossed elements exist, all of which project from the active field base plane towards the anode side.

[0009] The metal sheet, in particular made of titanium or steel, especially stainless steel, preferably has a thickness of 0.5 to 1.5 mm in the undeformed edge region. The stainless steel used is selected particularly with regard to good corrosion resistance. The metal sheet may also have at least one coating, in particular a coating formed by a PVD or CVD process or an electroplated coating.

[0010] The distance between the reference plane and the active field ground plane, hereinafter also referred to as the embossing depth, is preferably in the range of 0.3 to 2 mm.

[0011] In particular, all the embossing elements, starting from the active field base plane, touch the reference plane or extend beyond the reference plane by a maximum of 0.2 mm.

[0012] Within the active field of the bipolar plate, flow-guiding structures are formed by the embossed elements, with this structuring also significantly contributing to the mechanical stability of the bipolar plate. If the offset of the active field's base plane relative to the reference plane is considered a deformation in a first direction, then the numerous embossed elements protruding from the active field's base plane are present as elements deformed in the opposite direction. This makes no statement about simultaneous or sequential forming steps. In particular, it is possible to produce the structuring in a single forming process.

[0013] Accordingly, the embossing elements extending from the active field base plane and pointing towards the reference plane reach 100% of their height on the anode side. On the cathode side, only depressions are visible in the active field area; no protrusions extend beyond the active field base plane. An intermediate region of the bipolar plate can be formed between the edge region and the structuring present in the active field. In this intermediate region, a plate-level intermediate plane formed by planar material, particularly undeformed material, is arranged parallel between the reference plane and the active field base plane. Supporting embossing elements project from this plate-level intermediate plane towards the reference plane on the anode side, while maintaining unobstructed flow cross-sections.The term "supporting embossing elements" is intended to clarify that these elements, in addition to a fluid-technical function (which will be discussed in more detail later), also have a supporting function. This does not mean that other embossing elements formed by the bipolar plate do not have such a mechanical function. Rather, the term "supporting embossing elements" was chosen to distinguish them linguistically from the embossing elements in the active field.

[0014] One possible further development involves arranging several embossing elements formed in the active field parallel to each other, each with an elongated shape. Cross-flow paths are formed by recesses created within the embossing elements. These recesses are arranged such that a series of recesses formed by a plurality of embossing elements is oriented perpendicular to the longitudinal direction of the embossing elements.

[0015] Similarly, variations of the bipolar plate are possible in which a herringbone pattern is formed by its structuring. Such a herringbone pattern is composed of numerous embossed elements with an elongated rectangular base shape. Furthermore, embossed elements with a different shape, particularly a triangular base shape, can be located at at least one edge of the herringbone pattern.

[0016] With various designs of the embossed structure, the free flow cross-sections formed by the structuring on the anode side can be larger than those formed on the cathode side. In this case, at least a subset of the embossed elements can be knob-shaped, allowing flow around them on the anode side. Viewed from the cathode side, however, these same embossed elements represent blind-hole-like depressions, i.e., spaces unsuitable for the straight passage of a fluid.

[0017] The plate arrangement according to the invention comprises, in addition to at least one bipolar plate configured according to claim 1, frames between which a bipolar plate is inserted. Furthermore, the plate arrangement comprises various open-porous transport layers, namely an anode-side transport layer and a cathode-side transport layer, between which the bipolar plate is located. The anode-side open-porous transport layer and the cathode-side open-porous transport layer each border, on their sides facing away from the bipolar plate, a proton- or anion-permeable, catalytically coated membrane. Open-porous transport layers are generally flow-permeable and electrically conductive materials. The open-porous transport layers of the plate arrangement are also capable of absorbing forces acting in the active field that act in the stacking direction of the electrochemical system.

[0018] One possible configuration of the plate arrangement involves the active field being framed on the cathode side by an active surface seal, upon which the membrane, adjacent to the cathode-side open-porous transport layer, rests. The anode-side open-porous transport layer rests on the membrane on the side facing away from the active surface seal and has a larger surface area than the cathode-side open-porous transport layer. The active surface seal and the cathode-side open-porous transport layer together form a common, flat contact surface for the membrane.

[0019] In particular, the thickness of the anode-side open-porous transport layer is chosen to be greater than the thickness of the cathode-side open-porous transport layer.

[0020] The previously mentioned support embossing elements can be combined with this design by forming media channels between the anode-side open-porous transport layer and one of the ports. These channels connect to the flow cross-sections maintained between the support embossing elements, facing the active field. The media channels can be formed, for example, by a frame or by an insert part integrated into a frame. The frame, as well as the optional insert part, can be made of plastic – even in other configurations of the plate arrangement.

[0021] In various configurations of the plate arrangement, the three-dimensional structuring of the bipolar plate, which separates a half-cell of a first electrochemical cell from a half-cell of a second electrochemical cell, can be designed such that one of the two half-cells, namely the anode-side half-cell, has a greater height in the stacking direction of the layered system compared to the second half-cell, i.e., the cathode-side half-cell. This design allows, in particular, the provision of a flow cross-section on the anode side that is larger than that on the cathode side.

[0022] If one considers only the flow cross-sections at the level of the bipolar plate, a defined flow cross-section on the anode side of the bipolar plate can even be contrasted with a non-existent flow cross-section on the cathode side. This applies, for example, to cases in which the embossed elements of the bipolar plate appear on the cathode side exclusively as island-shaped, approximately point-like depressions. In such cases, the individual embossed elements present as depressions on the cathode side may primarily serve to absorb the tensile forces of an electrochemical system, particularly an electrolysis system, that is generally stacked. The same embossed elements that appear as depressions on the cathode side appear on the anode side, assuming an at least approximately circular shape, as raised, knob-like embossed elements.These embossing elements are designed to allow flow around them on the anode side and can be combined with other, for example elongated raised embossing elements.

[0023] An electrochemical system in the form of an electrolyzer, particularly for the electrolysis of water, comprising at least one plate arrangement according to the invention, has proven effective. If the electrolyzer is configured for the electrolysis of water and the production of hydrogen, process water and the oxygen produced therefrom flow on the anode side of the bipolar plate in the open-porous transport layer on the anode side. The process water, whose hydrogen atoms, after donating an electron as protons, penetrate a proton-permeable, catalytically coated membrane of the electrochemical cell, forms gaseous hydrogen together with electrons on the cathode side of another bipolar plate. Alternatively, an anion-permeable, catalytically coated membrane can be used instead of a proton-permeable membrane.

[0024] The hydrogen, and also water (and optionally process water), flows in the open-porous transport layer on the cathode side. To dissipate the heat generated during electrolysis, the water used on the anode side, which is separate from the process water, is preferably used. Alternatively or additionally, the process water can also be used for cooling. Using process water is significantly more expensive because it is highly purified water.

[0025] The problem is solved for the method of manufacturing the bipolar plate according to the invention, wherein a flat metal sheet is deep-drawn, having a flat top surface and a flat bottom surface parallel to it.

[0026] - a first area of ​​the metal sheet is deformed according to a circumferential area of ​​the active field starting from the top side, wherein the reference plane is formed by the areas of the top side that remain undeformed, and the active field base plane is formed by a second area on the underside that is shifted parallel to the top side within the first area, and

[0027] - by deep drawing from the underside and the active field base plane, the embossed elements are formed in the direction of the reference plane.

[0028] This does not include any statement regarding simultaneous or sequential forming steps within the active field. In particular, it is possible to create the structure of the flat metal plate in a single forming process. Furthermore, no statement is made about the spatial arrangement of the flat metal plate, especially the arrangement of the top and bottom surfaces. For example, during forming, the top surface can be positioned next to the bottom surface, or the bottom surface can be positioned above the top surface.

[0029] The top side of the flat metal sheet is preferably used to form the cathode side and the bottom side of the flat metal sheet to form the anode side, or vice versa.

[0030] Each embossing element is specifically designed with an area of ​​maximum indentation or an embossing roof, which is tangent to the reference plane or extends beyond it by a maximum of 0.2 mm.

[0031] Several embodiments of the invention are explained in more detail below with reference to a drawing. These drawings show, in some cases simplified form:

[0032] Fig. 1 shows a section of an electrochemical system, namely an electrolyzer for the production of hydrogen,

[0033] Fig. 2 shows a section of the bipolar plate of the electrolyzer according to Fig. 1 in top view,

[0034] Fig. 3 shows several elongated embossing elements in the active field of the bipolar plate according to Fig. 2 in perspective view.

[0035] Figs. 4 and 5 show one of the embossing elements according to Fig. 3 in side view and in cutaway view, respectively.

[0036] Fig. 6 shows differently shaped embossing elements of a partially depicted bipolar plate in a three-dimensional representation; Figs. 7 and 8 show flow possibilities for operating and cooling media of an electrochemical system i given by three-dimensional structuring of bipolar plates according to Figure 2 or 6.

[0037] Fig. 9 shows a section of an active field providing longitudinal and transverse flow paths of a three-dimensionally structured bipolar plate in a three-dimensional representation,

[0038] Fig. 10 shows another design of a three-dimensionally structured bipolar plate for an electrochemical system,

[0039] Fig. 11 shows a partial herringbone pattern of the bipolar plate according to Fig. 10.

[0040] Unless otherwise stated, the following explanations apply to all embodiments. Corresponding or essentially equivalent parts are marked with the same reference numerals in all figures.

[0041] Bipolar plates 1 belong to an electrochemical system 10, which in the present case is an electrolyzer for producing hydrogen from water. The parallel bipolar plates 1 are part of a plate arrangement 11, also referred to as a stack, which constitutes the core component of the electrochemical system 10. Other components, such as containers for feedstocks, end products, and coolant, pumps, as well as measuring and control devices, are not shown. Regarding the basic structure and function of the electrolyzer 10, reference is made to the prior art cited above.

[0042] The stack 11 comprises a plurality of electrochemical cells 2, that is, electrolysis cells. Each electrolysis cell 2 comprises two half-cells 3, 4, namely an anode-side half-cell 3 and a cathode-side half-cell 4, which are separated from each other by a catalytically coated, proton- or anion-permeable membrane 5. The bipolar plate 1 is arranged between a half-cell 3 of a first electrochemical cell 2 and a half-cell 4 of another electrochemical cell 2.

[0043] In a peripheral region 6 of the bipolar plate 1, various ports 7, 8 are located for the passage of operating and cooling media. An active field, in which the desired electrochemical reactions take place, is designated 9. In all embodiments, a structuring 12 of the bipolar plate 1 is provided in the active field 9. In each case, the structuring 12 comprises rib-shaped embossed elements 13. Optional knob-shaped embossed elements are designated 22. Viewed from a cathode side 1b of the bipolar plate 1, the embossed elements 13, 22 represent elongated or point-like depressions, respectively. This is equivalent to the fact that the embossed elements 13, 22 project from an active field ground plane EA in the direction of an anode side 1a of the bipolar plate 1.

[0044] The active field base plane EA is parallel to and spaced apart from a reference plane EE, which is defined by the position of the boundary region 6. The distance between the reference plane EE and the active field base plane EA is denoted by dEA. In the cases shown in Figures 1 to 9, the distance dEA is identical to an embossing depth PT, which is also generally referred to as the height of the embossing elements 13, 22 or as the forming height. In these cases, the area of ​​maximum indentation, designated 14 and also referred to as the embossing roof, lies on or is tangent to the reference plane EE.

[0045] The bipolar plate 1 is arranged in a sandwich-like manner between an anode-side open-porous transport layer 15 and a cathode-side open-porous transport layer 16. As can be seen from Figure 1, the area of ​​the anode-side open-porous transport layer 15 is larger than the area of ​​the cathode-side open-porous transport layer 16. Furthermore, the thickness of the anode-side open-porous transport layer 15, denoted by dPa, is greater than the thickness of the cathode-side open-porous transport layer 16, denoted by dPk. Support embossing elements 19 are located on the anode-side open-porous transport layer 15, insofar as it extends beyond the active field 9 and the cathode-side open-porous transport layer 16. These support elements project towards the anode side from an area 18 of the bipolar plate 1. The area 18 lies in an intermediate plate plane PZ, which is arranged parallel between the reference plane EE and the active field base plane EA.The support embossing elements 19, as well as the embossing elements 13 and 22, are tangent to the reference plane EE. A plastic frame 23 rests on the side of the planar intermediate area 18 facing away from the support embossing elements 19. An active surface seal 17 rests on an inner strip 24 of the frame 23, which borders the active field 9 and is located at the level of the cathode-side open-porous transport layer 16.

[0046] Where geometric terms such as "resting" or "at the height of" are used in this text, these terms refer only to the arrangements shown in Figures 1 to 9 and do not imply any statement about the actual orientation of the components of the electrochemical system 10 in space. In particular, the bipolar plates 1, as well as the frames 23 and the membranes 5, can be vertically oriented.

[0047] Seals arranged in the edge region 6, located, among other places, between the ports 7 and the flat intermediate region 18, are generally designated as 20. Unlike the active surface seal 17, the seals 20 contact not only the frame 23 but also the bipolar plate 1. On its side facing away from the seal 20, the frame 23 contacts an insert 21, which is optionally designed as an integral part of the frame 23, laterally adjacent to the active surface seal 17. The insert 21 forms media channels MK, which extend from the ports 7 towards the active field 9. Free flow areas connecting the media channels MK to the active field 9 are kept open between the support embossed elements 19.

[0048] Details of the three-dimensional structuring 12 in the area of ​​the active field 9, as shown in the embodiment according to Figure 1, are illustrated in Figures 3 to 5. According to these figures, flow regions Sin are formed by the strip-shaped, parallel embossing elements 13 on the cathode side 1b of the bipolar plate 1. These flow regions are located in the embossing elements 13, which are designed as recesses. These are to be distinguished from the outer flow regions Sout on the anode side 1a of the bipolar plate 1. The outer flow regions Sout are located between the outer surfaces of the strip-shaped embossing elements 13. Overall, the flow cross-section on the anode side 1a is larger than on the cathode side 1b.

[0049] Figures 6 to 8 illustrate an embodiment of a bipolar plate 1, which, in addition to strip-shaped embossed elements 13, has several stud-shaped embossed elements 22. On the anode side 1a, the stud-shaped embossed elements 22 are open to flow. In contrast, the stud-shaped embossed elements 22 on the cathode side 1b have the form of blind holes, which do not allow flow through them. In this case, the static function of the stud-shaped embossed elements 22 is paramount.

[0050] The embossing elements 13, 22 can project beyond the reference plane EE by a maximum of 0.2 mm, in particular by 0.05 mm, starting from the active field ground plane EA.

[0051] The partial active field 9 of a bipolar plate shown in Figure 9 differs from the embodiment shown in Figures 1 to 5 in that each of the strip-shaped embossed elements 13 contains several recesses 25, in this case two per embossed element 13. In the recesses 25, the material of the bipolar plate 1 is located at least approximately at the level of the plate intermediate plane PZ, that is, between the active field base plane EA and the reference plane EE. The recesses 25 form transverse flow paths Squ on the anode side 1a of the bipolar plate 1, oriented orthogonally to the embossed elements 13. In the embodiment shown in Figures 10 and 11, the structuring 12 is a herringbone pattern 26. This pattern 26 comprises a plurality of embossed elements 27 with a rectangular base shape. In addition, there are 26 individual embossed elements 28 with a triangular base shape at the edge of the herringbone pattern.

[0052] List of reference signs

[0053] Bipolar plate a anode side b cathode side electrochemical cell, electrolysis cell

[0054] Half-cell, anode side

[0055] Half-cell, cathode-side

[0056] Catalytically coated membrane

[0057] Edge area

[0058] port

[0059] port

[0060] Active field 0 electrochemical system, electrolyzer 1 plate arrangement, stack 2 structuring 3 embossing element, strip-shaped 4 area of ​​maximum imprint, embossing roof 5 open-porous transport layer, anode-side 6 open-porous transport layer, cathode-side 7 active surface seal 8 planar intermediate area 9 support embossing element 0 seal 1 insert 2 embossing element, stud-shaped 3 frame 4 inner strip 5 retraction 6 herringbone pattern 7 embossing element with rectangular base shape 8 embossing element with triangular base shape dPa thickness of the anode-side open-porous transport layer dPk thickness of the cathode-side open-porous transport layer dEA distance between the reference plane and the active field base plane

[0061] EA Active Field Basic Level

[0062] EE Reference Level

[0063] MK Media Channel

[0064] PT Embossing depth, height of the embossing elements, forming height

[0065] PZ plate intermediate level

[0066] Sin flow area in the embossing element

[0067] South flow area between embossing elements

[0068] Squ crossflow path

Claims

Patent claims 1. Bipolar plate (1 ) for use in an electrochemical system (10), which is formed from a metal sheet with an anode side (1a) and a cathode side (1b), wherein a reference plane (EE) is defined by an edge region (6) of the bipolar plate (1 ), and in the edge region (6) ports (7, 8) for supplying and removing media to or from an active field (9) are located, in the region of which electrochemical reactions take place in the electrochemical system (10), and wherein in the active field (9) a structuring (12) of the bipolar plate (1 ) is formed such that an active field base plane (EA) formed by planar material of the bipolar plate (1 ) is spaced parallel to the reference plane (EE), namely offset in the direction towards the cathode side (1b), and all individual embossing elements (13, 22) present in the active field (9) are offset from the active field base plane. (EA) protrude in the direction of the anode side (1a).

2. Bipolar plate (1 ) according to claim 1 , characterized in that the embossing elements (13, 22) starting from the active field base plane (EA) all touch the reference plane (EE) or extend beyond the reference plane (EE) by a maximum of 0.2 mm.

3. Bipolar plate (1 ) according to claim 1 or 2, characterized in that an intermediate region (18) of the bipolar plate (1) is formed between the edge region (6) and the structuring (12) given in the active field (9), in which a plate intermediate plane (PZ) formed by planar material is arranged parallel between the reference plane (EE) and the active field base plane (EA), wherein support embossing elements (19) project from the plate intermediate plane (PZ) in the direction towards the anode side while keeping flow cross-sections clear.

4. Bipolar plate (1) according to claim 3, characterized in that the support embossing elements (19) are all limited by the plate intermediate plane (PZ) and the reference plane (EE) or the reference plane (EE) by a maximum of 0.2 mm tower over.

5. Bipolar plate (1 ) according to one of claims 1 to 4, characterized in that several embossing elements (13) formed in the active field (9) are arranged parallel to each other and each has an elongated shape, wherein transverse flow paths (Squ) are formed by recesses (25) formed within the embossing elements (13), which are arranged such that a series of recesses (25) formed by a plurality of embossing elements (13) is oriented perpendicular to the longitudinal direction of the embossing elements (13).

6. Bipolar plate (1 ) according to one of claims 1 to 5, characterized in that the free flow cross-sections formed on the anode side (1a) by the structuring (12) are larger than the free flow cross-sections formed on the cathode side (1b), wherein at least a subset of the embossed elements (22) are knob-shaped, so that the embossed elements (22) in question can be flowed around on the anode side.

7. Bipolar plate (1 ) according to one of claims 1 to 4, characterized in that a herringbone pattern (26) is formed by the structuring (12).

8. Plate arrangement (11 ) for an electrochemical system (10), comprising at least one bipolar plate (1) designed according to one of claims 1 to 7, and a frame (23) between which a bipolar plate (1 ) is inserted, wherein the bipolar plate (1 ) is located between an anode-side open-porous transport layer (15) and a cathode-side open-porous transport layer (16), and wherein the anode-side open-porous transport layer (15) and the cathode-side open-porous transport layer (16) each border a proton- or anion-permeable, catalytically coated membrane (5) on their sides facing away from the bipolar plate (1 ).

9. Plate arrangement (11) according to claim 8, characterized in that the active field (9) on the cathode side (1b) is framed by an active surface seal (17) on which the membrane (5) adjacent to the cathode-side open-porous transport layer (16) rests, wherein the anode-side open-porous transport layer (15) rests on the side of the membrane (5) facing away from the active surface seal (17) and has a larger area than the cathode-side open-porous transport layer (16), and wherein the active surface seal (17) forms a common flat bearing surface for the membrane (5) with the cathode-side open-porous transport layer (16).

10. Plate arrangement (11) according to claim 8 or 9 in conjunction with claim 5, characterized in that media channels (MK) are formed between the anode-side open-porous transport layer (15) and at least one of the ports (7), which connect to the flow cross-sections kept free between the support embossing elements (19) towards the active field (9).

11. Electrochemical system (10) in the form of an electrolyzer, in particular for the electrolysis of water, comprising at least one plate arrangement (11 ) according to one of claims 8 to 10.

12. Method for manufacturing the bipolar plate (1) according to any one of claims 1 to 7, wherein a flat metal sheet is deep-drawn having a flat top surface and a flat bottom surface parallel thereto. - a first area of ​​the metal sheet is deformed according to a circumferential area of ​​the active field (9) starting from the top side, wherein the reference plane (EE) is formed by the areas of the top side that remain undeformed and the active field base plane (EA) is formed by a second area on the underside that is shifted parallel to the top side within the first area, and - by deep drawing from the underside and the active field base plane (EA) the embossing elements (13,22) are formed in the direction of the reference plane (EE).

13. Method according to claim 12, wherein the top side of the flat metal sheet is used to form the cathode side (1 b) and the bottom side of the flat metal sheet is used to form the anode side (1a).

14. Method according to claim 12 or 13, wherein each embossing element (13, 22) is formed with a maximum embossing area (14) which is tangent to the reference plane (EE) or extends beyond it by a maximum of 0.2 mm.

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