Bipolar plate for an electrochemical system

Abstract

Bipolar plate (2) for an electrochemical system (1), comprising a first separator plate (2a) and a second separator plate (2a) arranged one above the other, wherein - the first separator plate (2a) has embossed ribs (32) and channels (30) on one side facing away from the second separator plate (2b) for guiding a first fluid; - the second separator plate (2b) has embossed ribs (32) and channels (30) on the side facing away from the first separator plate (2a) for guiding a second fluid; - the first and second separator plates (2a, 2b) each have webs (42) and channels (46) on their facing sides for guiding a cooling medium along an inner side of the bipolar plate (2); wherein at least one of the separator plates (2a, 2b) has only partially coated surface areas on the side facing the other separator plate (2b, 2a), which are provided with a coating (80) that increases electrical conductivity.

Description

[0001] The disclosure relates to a bipolar plate for an electrochemical system and to an electrochemical system comprising at least one such bipolar plate.

[0002] Common electrochemical systems, such as fuel cell systems, electrochemical compressor systems, redox flow batteries, and electrolyzers, typically comprise a multitude of bipolar plates arranged in a stack, such that each pair of adjacent bipolar plates encloses an electrochemical cell. The bipolar plates usually each include two separator plates connected along their back surfaces facing away from the electrochemical cells. The bipolar plates can serve, for example, to electrically contact the electrodes of individual electrochemical cells (e.g., fuel cells) and / or to electrically connect adjacent cells (series connection of the cells).

[0003] The separator plates of the bipolar plates can have channel structures for supplying the cells with one or more media and / or for removing media. In the case of fuel cells, these media can be, for example, fuels (e.g., hydrogen or methanol), reaction gases (e.g., air or oxygen), or a coolant as supplied media, and reaction products and heated coolant as discharged media. Furthermore, the bipolar plates can serve to dissipate the waste heat generated in the electrochemical cell, such as that produced during the conversion of electrical or chemical energy in a fuel cell, and can also be designed to seal the various media and / or cooling channels from each other and / or from the outside. In fuel cells, the reaction media, i.e., the reaction gases, are typically located on the surfaces of the individual plates facing away from each other.Fuel and reaction gases are guided through the bipolar plates, while the coolant is guided between them. The bipolar plates typically each have at least one or more through-holes. Through these through-holes, the media and / or reaction products can be directed to the electrochemical cells bounded by adjacent bipolar plates of the stack, or into the cavity formed by the separator plates of the bipolar plate, or discharged from the cells or the cavity. The electrochemical cells, particularly those of a fuel cell, can each comprise, for example, a membrane electrode assembly (MEA) with a polymer electrolyte membrane (PEM), electrodes, and catalyst layers.Adjacent to the MEA, the MEA system can also include one or more gas diffusion layers (GDLs), which are typically oriented towards the separator plates, particularly the bipolar plates of fuel cell systems, and are, for example, made of carbon fleece. In a typical fuel cell, hydrogen supplied on one anode side is converted to water by oxygen supplied on the other. Protons produced during the oxidation of the hydrogen pass from the anode side through the MEA to the cathode, where they react with the oxygen and electrons to form water. The fuel cell membrane is electrically insulating and prevents an electrical short circuit between the anode and cathode. The electrons are separately supplied from the anode to the cathode, thus making the electrical current usable.

[0004] Nowadays, most fuel cell systems are operated with an excess of oxidant. It has been observed that the reaction occurs more intensely in areas with a particularly high concentration of oxidant than in other parts of the electrochemically active region. This can be the case, for example, in the immediate vicinity of oxidant entry points into the electrochemically active region. The increased conversion of reactants leads to a particularly high current density in areas with a greater excess of oxidant. This results in an uneven distribution of current density across the active area. High reactant conversion places a high load on the membrane, while the membrane experiences only low load in areas of low conversion. Overloaded areas negatively impact the lifespan of the MEA (Mechanical Energy Exchange) or the cell itself.their membrane. Low-load areas have the disadvantage of high area-related costs compared to performance, especially since the MEA is a comparatively expensive component. Similar phenomena can occur in PEM electrolyzers, since water is the only medium supplied and, as it travels across the bipolar plate, an increasing proportion of oxygen is carried along with the water, while the actual water content decreases. Here, it can happen, in particular, that low-load areas develop towards the end of the water path on a bipolar plate.

[0005] In other electrochemical systems, especially other polymer membrane-based electrochemical systems, other factors can lead to unequal stresses on the surface of the polymer membrane.

[0006] The object of the invention is therefore to propose an improved bipolar plate and / or an improved electrochemical system, in particular with respect to increasing the lifetime of the bipolar plate and / or the lifetime of an MEA or polymer membrane arranged between bipolar plates in a bipolar plate stack. Additionally or alternatively, it may be an object to reduce the costs depending on the performance of an electrochemical system.

[0007] These problems are solved according to the invention by the bipolar plate according to claim 1 or by an electrochemical system according to the dependent claim. Further developments are the subject of the dependent claims and the following description.

[0008] The proposed bipolar plate for an electrochemical system has a first separator plate and a second separator plate arranged one on top of the other.

[0009] The first separator plate has embossed ridges and channels on one side facing away from the second separator plate for guiding a first fluid.

[0010] The second separator plate has embossed ridges and channels on one side facing away from the first separator plate for guiding a second fluid.

[0011] The first and second separator plates each have ribs and channels on their facing sides for guiding a cooling medium along an inner surface of the bipolar plate. These facing sides can be referred to as the coolant sides in the following.

[0012] At least one of the separator plates has surface areas on the side facing the other separator plate, i.e., its coolant side, that are only partially coated and are provided with a coating that increases electrical conductivity.

[0013] Only partially coated surface areas can have the technical effect of increasing the electrical conductivity of the bipolar plate in these areas. This allows for a more homogeneous current density.

[0014] The coating that increases electrical conductivity can be referred to as a coating here and in the following.

[0015] In one embodiment, a proportion of coated surface areas is inhomogeneously distributed across the entire surface.

[0016] In one embodiment, the coated surface area can be located on the side of the first separator plate facing the second separator plate, and / or the coated surface area can be located on the side of the second separator plate facing the first separator plate. The coated surface areas on the coolant side of the first separator plate can be similar and / or uniform compared to the coated areas on the coolant side of the second separator plate. Alternatively, the coated surface areas on the coolant side of the first separator plate can have different shapes and / or be of different types compared to the coated areas on the coolant side of the second separator plate.

[0017] At least one of the separator plates can therefore be coated with the electrical conductivity-enhancing coating in specific sections on the coolant side. In particular, the separator plate with the electrical conductivity-enhancing coating in specific sections is not coated with the electrical conductivity-enhancing coating across its entire surface. The other separator plate of the bipolar plate, however, can optionally be coated with the electrical conductivity-enhancing coating across its entire surface on the coolant side, or partially coated with the electrical conductivity-enhancing coating, or not coated with the electrical conductivity-enhancing coating at all.

[0018] The first separator plate can be coated partially or completely on one side facing away from the second separator plate, subsequently the gas side.

[0019] The second separator plate can be coated partially or completely on the side facing away from the first separator plate, hereinafter referred to as the gas side.

[0020] In particular, the first separator plate may have coated web areas on its gas side, at least partially or completely, and / or the second separator plate may have coated web areas on its gas side, at least partially or completely.

[0021] The first separator plate can have a first electrochemically active area, and the second separator plate can have a second electrochemically active area. The first and second electrochemically active areas can overlap and form an active area of ​​the bipolar plate in the overlapping region. The coated surface areas can be located within the active area on at least one side of at least one of the separator plates.

[0022] The coated surface areas can be fully coated or partially coated, particularly in the area of ​​the web surfaces of one or both separator plates.

[0023] The first and second separator plates can form at least one contact surface with each other. The coated surface areas can be located particularly in the contact surfaces. In many bipolar plates, the two separator plates only touch in the rib areas within the active region, and the contact surfaces are formed accordingly on these rib areas. Therefore, preferably at least sections of the contacting rib surfaces are coated. On the other hand, there are bipolar plates in which the flow fields of the active regions of both separator plates are interlocked. In these bipolar plates, contact between the separator plates can occur particularly via the flanks. Therefore, preferably at least sections of the contacting flanks are coated.

[0024] The coated surface areas can alternatively or additionally be formed in sections in the areas between the web surfaces and / or in the areas between the contact surfaces of the first separator plate and / or the second separator plate. For example, an additional conductor can also be applied along the coated surface of a separator plate via such a coating area that is not directly connected to another separator plate.

[0025] The first separator plate can have a first region, a second region, and optionally a third region in the electrochemically active area of ​​at least one of its sides. The coated surface regions can be distributed such that the proportion of A11 coated surface regions in the first region is smaller than the proportion of A12 coated surface regions in the second region, and / or that the proportion of A13 coated surface regions in the third region is smaller than the proportion of A12 and / or A11.

[0026] The second separator plate can have a first region, a second region, and a third region in the electrochemically active area of ​​at least one of its sides. The coated surface regions can be distributed such that the proportion of A21-coated surface regions in the first region is smaller than the proportion of A22-coated surface regions in the second region, and / or that the proportion of A23-coated surface regions in the third region is smaller than the proportion of A22 and / or A21.

[0027] A ratio, A11 / A12 of the first and / or a ratio A21 / A22 of the second separator plate between the proportion A11 or A21 coated surface areas in the first area and the proportion A12 or A22 coated surface areas in the second area can be less than 0.9, preferably less than 0.7, preferably less than 0.5.

[0028] Additionally or alternatively, the ratio A13 / A12 of the first and / or the ratio A23 / A22 of the second separator plate, between the proportion of A13 or A23 coated surface areas in the third area and the proportion of A12 or A22 coated surface areas in the second area, can be less than 0.9, preferably less than 0.7, preferably less than 0.5.

[0029] At least the side of the first separator plate facing the second separator plate can be designed such that the proportion of coated surface areas on the total surface of the first and / or second and / or third area of ​​the first electrochemically active area differs from each other by more than 5%, preferably by more than 10%, particularly preferably by more than 20%, and especially by more than 30%.

[0030] At least the side of the second separator plate facing the first separator plate can be designed such that the proportion of coated surface areas on the total surface of the first and / or second and / or third area of ​​the second electrochemically active area differs from each other by more than 5%, preferably by more than 10%, and particularly preferably by more than 20%, and especially by more than 30%.

[0031] In one embodiment, the respective separator plate can be designed such that only one of the areas, in particular the second area, has the coated surface areas.

[0032] The bipolar plate can be configured to operate at a specific operating point of an electrochemical system. This operating point may encompass a specific concentration gradient of the first fluid along the first electrochemically active region. The coating can be adapted to the concentration gradient of the first fluid such that a relatively large proportion of coated surface areas exists in sections of the active region with comparatively low concentrations of the first fluid, and a relatively small proportion exists in sections of the active region with comparatively high concentrations of the first fluid. This is particularly relevant when oxygen or air is considered the first fluid in a system that otherwise operates with an excess of oxygen.

[0033] The first separator plate can have a first inlet opening for the inlet of the first fluid and a first outlet opening for the outlet of the first fluid. The first inlet opening can be fluidically connected to the first electrochemically active area via a first inlet region, and the first outlet opening can be fluidically connected to the first electrochemically active area via a first outlet region, such that the first fluid can pass sequentially through the first inlet region, the first electrochemically active area, and the first outlet region.

[0034] The second area can be located closer to the outlet opening than the first area and the optional third area. For example, the second area is located in a section of the electrochemically active area where a significant proportion of oxygen has already undergone electrochemical conversion.

[0035] The bipolar plate can be configured such that the flow direction of the first fluid is opposite to the flow direction of the second fluid. The proportion of coated surface areas, at least on the side of the first separator plate facing the second separator plate and / or on the side of the second separator plate facing the first separator plate, can increase, at least section by section, with the flow direction of the first fluid. Preferably, this proportion initially increases and then either decreases again or remains essentially constant.

[0036] In principle, it is possible to use the coating distribution to fine-tune the conductivity distribution, especially in the case of fixed embossing and stamping structures of a bipolar plate or its separator plates.

[0037] The aforementioned area proportions, i.e., proportions of coated surface areas to the total surface of an area, can be determined with reference to a projection perpendicular to the plate plane or with reference to a development of the embossed areas.

[0038] In one embodiment, at least one of the separator plates of the bipolar plate may be made of or contain steel, in particular stainless steel, or a titanium alloy, in particular a titanium alloy of group 1.

[0039] The coating on the side of the bipolar plate facing at least one of the other separator plates can contain one or more of the following substances or consist of one or more of these substances or their alloys: electrically conductive oxides, carbon, electrically conductive carbon layers, precious metals, in particular Au, Ag or Pt, metals, in particular Ti, Zr, Nb, Ta or Cr, metal nitrides, in particular TiN, TiON (with varying oxygen / nitrogen ratios), CrN, Cr₂N, metal carbides, metal borides, metal silicides and / or silicon carbide. If coatings are present on the facing sides of both separator plates, these coatings can be identical, different but containing the same substances, or fundamentally different.A coating that may be present on at least one of the gas sides may also contain one or more of the aforementioned substances or consist of one or more of these substances or their alloys, whereby the coating(s) may differ from the first-mentioned coating.

[0040] The coating can be essentially the same across the entire coated surface of one side of the separator plate. Alternatively, the coating can be of a different type in certain areas across the entire coated surface.

[0041] The coating can be single-layered or multi-layered. The coating thickness depends on the coating material used. In particular, the coating thickness should be greater than a minimum thickness, which in turn also depends on the material.

[0042] Inhomogeneous within the meaning of this disclosure shall preferably not mean that all bridge surfaces are fully coated and all channels are free of coating.

[0043] Furthermore, the invention comprises an electrochemical system with a plurality of bipolar plates as described above.

[0044] Examples of bipolar plate designs are shown in the figures and are explained in more detail below. They show: Fig. 1. Schematic representation in a perspective view of an electrochemical system with a plurality of separator plates or bipolar plates arranged in a stack; Fig. 2 schematically in a perspective view two bipolar plates of the system according to Fig. 1 with a membrane electrode assembly (MEA) arranged between the bipolar plates, using a fuel cell system as an example; Fig. 3 schematically a section BB through a stack of plates of a system according to the type of system according to Fig. 1, wherein the cooling sides of the separator plates are fully coated and the gas sides are coated on the ribbed roofs, Fig. 4A schematically shows a section BB through a stack of plates of a system according to the type of system according to Fig. 1; wherein one cooling side of one of the separator plates is fully coated and one cooling side of the other separator plate is coated only in the area that is further away from a transition area, Fig. 4B schematically shows a section BB through a stack of plates of a system according to the type of system according to Fig. 1; wherein one cooling side of one of the separator plates is completely uncoated and one cooling side of the other separator plate is coated only in the area that is closer to a transition area; Fig. 5 schematically a section AA through a stack of plates of a system according to the type of system according to Fig. 1, wherein the separator plates are nested within each other in the section shown; Fig. 6A schematically a top view of a cooling side of a section of an exemplary separator plate; Fig. 6B schematically a top view of a cooling side of a section of an exemplary separator plate; Fig. 6C schematically a top view of a cooling side of a section of an exemplary separator plate; Fig. 6D schematically a top view of a cooling side of a section of an exemplary separator plate; Fig. 7 schematically a top view of a cooling side of an exemplary separator plate with an unevenly distributed proportion of coated surface areas on the entire cooling side surface of the electrochemically active area; Fig. 8 schematically a top view of a cooling side of an exemplary separator plate with an unevenly distributed proportion of coated surface areas on the entire cooling side surface of the electrochemically active area; Fig. 9 schematically a top view of a cooling side of an exemplary separator plate with an unevenly distributed proportion of coated surface areas on the entire cooling side surface of the electrochemically active area; Fig. 10 schematically a top view of a cooling side of an exemplary separator plate with an unevenly distributed proportion of coated surface areas on the entire cooling side surface of the electrochemically active area; Fig. 11 schematically a top view of a cooling side of an exemplary separator plate with an unevenly distributed proportion of coated surface areas on the entire cooling side surface of the electrochemically active area; and Fig. 12 schematically a top view of a cooling side of an exemplary separator plate with an unevenly distributed proportion of coated surface areas on the entire cooling side surface of the electrochemically active area.

[0045] Here and in the following, recurring features in various figures are designated with the same or similar reference symbols. Some elements of the figures are shown schematically and in a fragmented form; therefore, they may depict distances—possibly only in sections—that do not actually exist.

[0046] Fig. Figure 1 shows an electrochemical system 1 with a plurality of identical metallic bipolar plates 2 arranged in a stack 6 and stacked along a z-direction 7. The bipolar plates 2 of the stack 6 are clamped between two end plates 3, 4. The z-direction 7 is also called the stacking direction. In this example, the system 1 is a fuel cell stack. Each pair of adjacent bipolar plates 2 of the stack encloses an electrochemical cell, which serves, for example, to convert chemical energy into electrical energy. To form the electrochemical cells of the system 1, a membrane electrode assembly (MEA) is arranged between each pair of adjacent bipolar plates 2 of the stack (see, for example, Figure 1). Fig. 3A). The MEAs typically each contain at least one membrane, e.g., an electrolyte membrane. Furthermore, a gas diffusion layer (GDL) can be arranged on one or both surfaces of the MEA.

[0047] In alternative embodiments, system 1 can also be configured as an electrolyzer, an electrochemical compressor, or a redox flow battery. Bipolar plates can also be used in these electrochemical systems. The structure of these bipolar plates can then correspond to the structure of the bipolar plates 2 described in more detail here, even though the media guided on or through the bipolar plate in an electrolyzer, an electrochemical compressor, or a redox flow battery may differ from the media used in a fuel cell system.

[0048] The z-axis 7, together with the x-axis 8 and y-axis 9, defines a right-handed Cartesian coordinate system. The bipolar plates 2 each define a plate plane, with the plate planes of the separator plates 2a and 2b being aligned parallel to the xy-plane and thus perpendicular to the stacking direction and the z-axis 7, respectively. The end plate 4 has a multitude of media connections 5 through which media can be supplied to and discharged from system 1. These media can include, for example, fuels such as molecular hydrogen or methanol, reaction gases such as air or oxygen, reaction products such as water vapor or depleted fuels, or coolants such as water and / or glycol.

[0049] Fig. Figure 2 shows, in perspective, two adjacent bipolar plates 2 of an electrochemical system of the type of system 1. Fig. 1 and a membrane electrode assembly (MEA) 10 arranged between these adjacent bipolar plates 2, known from the prior art, wherein the MEA 10 is in Fig. 2 is mostly obscured by the bipolar plate 2 facing the viewer. The bipolar plate 2 is formed from two separator plates 2a, 2b bonded together (see e.g. Fig. 3), of which in Fig. 2. Only the first separator plate 2a, facing the viewer, is visible, concealing the second separator plate 2b. The separator plates 2a and 2b can each be made of a single sheet of metal, e.g., stainless steel. The separator plates 2a and 2b can be welded together, e.g., by laser welding. The separator plates 2a and 2b can be welded tightly along sealing seams, particularly outside and / or inside the perimeter groove 12d, at a distance from their outer edges or at least along the inner edge of a portion of the through-openings. Further weld seams are possible.

[0050] The separator plates 2a, 2b have aligned through-holes that form through-holes 11a-c of the bipolar plate 2. When a plurality of bipolar plates of the type of bipolar plate 2 are stacked, the through-holes 11a-c form conductors that extend through the stack 6 in the stacking direction 7 (see Fig. 1) Typically, each of the conduits formed by the through-openings 11a-c is in fluid connection with one of the ports or media connections 5 in the end plate 4 of system 1. For example, coolant can be introduced into or discharged from the stack via the conduits formed by the through-openings 11a. The conduits formed by the through-openings 11b and 11c, on the other hand, can be configured to supply the electrochemical cells of the fuel cell stack 6 of system 1 with fuel and reaction gas, as well as to discharge the reaction products from the stack. The media-carrying through-openings 11a-11c are essentially parallel to the plane of the plate.

[0051] To seal the through-openings 11a-c from the interior of the stack 6 and from the environment, the first separator plates 2a each have sealing arrangements in the form of sealing beads 12a-c, which are arranged around the through-openings 11a-c and which completely enclose the through-openings 11a-c. The second separator plates 2b have, on the side visible to the observer, Fig. On the opposite side of the separator plates, two corresponding sealing beads are provided for sealing the through-openings 11a-c (not shown). Alternatively, elastomeric seals can also be used.

[0052] In an electrochemically active area 18, the first separator plates 2a exhibit, to the observer, the Fig. On the outer side facing 2, a flow field 17 with structures for guiding a reaction medium along the outer side of the separator plate 2a is located. These structures are in Fig. 2 is given by a multitude of footbridges 32, 42 and channels 30, 40 running between the footbridges 32, 42 and bounded by the footbridges 32, 42. At the viewer's Fig. On the outer surface of the bipolar plates 2, the first separator plates 2a each have a distribution and a collection area 20. Distribution or collection areas 20 each comprise structures designed to distribute a medium introduced into the distribution area 20 from one of the first of the two through-openings 11b across the active area 18, or to collect or concentrate a medium flowing from the active area 18 towards the second of the through-openings 11b. The fluid guide structures 29 of both distribution or collection areas 20 are arranged in Fig. Two channels are also provided, running through and between the channels and bounded by the channels. For simplicity, only the distribution area 20 is discussed below; the corresponding statements can equally apply to a collection area 20. A transition area 21 with a lower channel height on the gas sides compared to both adjacent areas can be arranged between the distribution and collection areas and the active area 20.

[0053] The sealing beads 12a-12c have feedthroughs 13a-13c which allow passage of medium between the associated through-openings 12a-12c and the distribution area 20 on a gas side or in the interior of the bipolar plate 2.

[0054] The first separator plates 2a each have a further sealing arrangement in the form of a perimeter bead 12d, which surrounds the flow field 17 of the active area 18, the distribution and collection areas 20, and the through-openings 11b, 11c, and seals them against the through-opening 11a, i.e., against the coolant circuit, and against the environment of the system 1. The second separator plates 2b each include corresponding perimeter beads. The structures of the active area 18, the distribution structures of the distribution and collection areas 20, and the sealing beads 12a-d are each formed integrally with the separator plates 2a and molded into the separator plates 2a or 2b, e.g., in an embossing or deep-drawing process or by means of hydroforming.

[0055] The bipolar plate 2 is formed from two separator plates joined together by material bonding, namely separator plates 2a, 2b (see e.g. Fig. 3 to 5), of which in Fig. 2. Only the first separator plate 2a, facing the viewer, is visible, concealing the second separator plate. The separator plates 2a and 2b can each be formed from a formed metal sheet, e.g., from an embossed or deep-drawn stainless steel sheet. This metal sheet can, for example, have a thickness of at most 150 µm, preferably at most 100 µm, preferably at most 90 µm, and particularly preferably at most 80 µm. The separator plates can be welded together, for example, by laser welding.

[0056] The Fig. Figure 3 schematically shows a section BB, drawn in Fig. 2 through a section of the plate stack 6 of system 1 from Fig. 1, wherein the cutting plane is aligned in the z-direction and thus perpendicular to the plate planes of the bipolar plates 2.

[0057] The first separator plate 2a has, on its side facing away from the second separator plate 2b, molded-in, parallel channels 30 for media guidance, which are separated from each other by first webs 32 formed between them. On one side of the first separator plate 2a facing the second separator plate 2b, the first channels 30 form first raised areas 34 and the first webs 32 form first grooves 36. The second separator plate 2b has, on its side facing away from the first separator plate 2a, molded-in, parallel second channels 40 for media guidance, which are separated from each other by second webs 42 formed between them. On one side of the second separator plate 2b facing the first separator plate 2a, the second channels 40 form second raised areas 44 and the second webs 42 form second grooves 46.

[0058] The identical bipolar plates 2 of the stack each comprise the previously described first metallic separator plate 2a and the previously described second metallic separator plate 2b. The media flow structures along the outer surfaces of the separator plates 2 (hereinafter also referred to as gas sides) are visible, particularly in the form of the webs 32 and 42 respectively, and the channels 30 and 40 bounded by the webs 32 and 42 respectively. Cooling channels in the cavity 19 between adjacent separator plates 2a and 2b are shown in particular. Adjacent to the sealing beads and between the cooling channels, both in the distribution / collection area 20 and in the active area 18, the two separator plates 2a and 2b rest against each other in a contact area 24 (see Figure 1). Fig. 2) Furthermore, the two separator plates 2a, 2b are tightly connected to each other along the sealing ribs, for example by means of laser welding seams.

[0059] Between adjacent bipolar plates 2 of the stack, a membrane electrode assembly (MEA) 10, known from the prior art, is arranged. The MEA 10 typically comprises a membrane 14, e.g., an electrolyte membrane including electrodes and catalyst layers (not shown separately here), and an edge section 15 connected to the membrane 14. For example, the edge section 15 can be metallurgically bonded to the membrane 14. Furthermore, gas diffusion layers 16 can be arranged in the active region 18. The gas diffusion layers 16 allow gas to flow across the membrane 14 over as large an area of ​​its surface as possible, thus improving proton transfer across the membrane 14. The gas diffusion layers 16 can be arranged, for example, on both sides of the membrane, at least in the active region 18 between the adjacent bipolar plates 2.be formed from or comprise a nonwoven fabric.

[0060] The term "electrochemically active area" is used here to refer to the area of ​​the separator plate 2a, 2b that faces an electrochemically active area of ​​the MEA 10 when the separator plate 2a, 2b is arranged in an electrochemical system. The membrane 14 of the MEA 10 extends at least over the active area 18 of the adjacent separator plates 2 and enables proton transfer across or through the membrane 14. The actual active area 18 is bounded by the edge section 15 of the MEA. Thus, the membrane does not extend into the distribution or collection area 20. The edge section 15 of the MEA 10 serves to position, secure, and seal the membrane between the adjacent separator plates 2.

[0061] In Fig. 3. The inner surfaces, i.e., the coolant surfaces of the first separator plate 2a and the second separator plate 2b, are fully coated with a coating 80 that increases electrical conductivity. This is known from the prior art. The outer surfaces, i.e., the gas surfaces of the first separator plate 2a and the second separator plate 2b, are coated with a coating 90 on the web roofs. The coating 90 on the gas surfaces can be the same coating as the coating 80 on the coolant surface(s), or a different coating that also increases electrical conductivity, or another coating, for example, for corrosion prevention.

[0062] The Fig. 4A schematically shows a section BB, drawn in Fig. 2 through a section of the plate stack 6 of system 1 from Fig. 1, wherein the section plane is aligned in the z-direction and thus perpendicular to the plate planes of the bipolar plates 2. The first separator plate 2a has a full-surface, electrically conductivity-enhancing coating 80 on the side facing the other separator plate 2b, i.e., its coolant side. The second separator plates 2b have only partially coated surface areas on the side facing the other separator plate 2a, i.e., its coolant side, which are provided with the electrically conductivity-enhancing coating 80. The coolant side of the separator plates 2b is coated only in the area that is further away from the transition zone, recognizable by the edge sections 15. The outer sides (gas sides) of the first and second separator plates 2a and 2b are coated with a coating 90 only on the web tops. For example, the transition zone on the inlet side for oxygen or gas can be described here.Air may be arranged and, in areas further away from this inlet, particularly in significantly distant regions, an oxygen depletion may have occurred on the oxygen-carrying gas side. Here, a more uniform conductivity can be achieved by means of the coating 80 on the opposite coolant side, thus ensuring a more even utilization of the MEA.

[0063] The Fig. 4B schematically shows a section BB, drawn in Fig. 2 through a section of the plate stack 6 of system 1 from Fig. 1, where the section plane is aligned in the z-direction and thus perpendicular to the plate planes of the bipolar plates 2. The first separator plate 2a is completely uncoated on the side facing the other separator plate 2b, i.e., its coolant side. The outer surfaces (gas sides) of the first separator plates 2a have a coating 90 that increases electrical conductivity across their entire surface. The second separator plates 2b have the coating 80 only in certain areas on the side facing the other separator plates 2a, specifically only in an area closer to the transition zone. The outer surfaces (gas sides) of the second separator plates 2b are coated with a coating 90 only on the web tops. For example, the transition zone on the outlet side for oxygen or...Air may be arranged and, in areas near this outlet, compared to areas further away, especially in significantly distant areas, a depletion of oxygen may occur on the oxygen-carrying gas side. Here, near the outlet, a more uniform conductivity can be achieved by means of the coating 80 on the opposite coolant side, thus resulting in a more even utilization of the MEA.

[0064] The Fig. Figure 5 schematically shows a section AA, drawn in Fig. 2 through a section of the plate stack 6 of system 1 from Fig. 1, where the section plane is aligned in the z-direction and thus perpendicular to the plate planes of the bipolar plates 2. In the section shown, the separator plates 2a, 2b do not lie with their web surfaces against each other, but rather touch in the region of their flanks 35, 45. The first separator plate 2a is fully coated on the surface facing away from the second separator plate 2b. The second separator plate 2b is coated on the surface facing away from the first separator plate 2a in the region of its webs 42. Adjacent to the outer edge 18' of the active area, the second separator plate 2b has a coating 80 extending over the flanks 35 and the groove 36, which extends both into the contact surfaces and also enables conduction through the coating 80 in the groove 36.Moving further into the interior of the active area, only the flanks 35 are coated; a section without coating 80 is shown on the far right.

[0065] Fig. Figures 6A to 6D each show a top view of a coolant side of a separator plate 2a or 2b of an electrochemical system of type 1. Fig. 1. At least some of the lines separating the differently inclined or curved areas of the raised sections 34 and 44 and grooves 36 and 46 are also shown. The separator plate 2a and / or 2b has an unevenly distributed coating 80 that increases electrical conductivity on at least one coolant side of the separator plate 2a and / or 2b. The examples of Fig. Figures 6A to 6D are merely examples. Coated areas are shown in black, uncoated areas in white.

[0066] In Fig. 6A, the coating 80 is specifically intended for elevations 34 and 44. However, areas with coated elevations have different widths and extend partly into areas between the web crests, especially along the flanks.

[0067] In Fig. 6B, the coating 80 is applied to a portion of the elevations 34 and 44. Additionally, a coating 80 forming a transverse connection 80' is applied.

[0068] In Fig. 6C does not provide a coating 80 along the elevations 34 and 44. However, the coating 80 is applied forming a multitude of cross-connections 80' essentially perpendicular to the course of the elevations 34 and 44.

[0069] In Fig. 6D is formed by the coating 80 partly on elevations 34 or 44 and partly in the area in between, creating an arbitrary superimposition pattern.

[0070] Fig. Figures 7 to 12 each show top views of a separator plate 2a, 2b of a bipolar plate 2 of an electrochemical system. The representation can correspond to either a cooling side of a first separator plate 2a or a cooling side of a second separator plate 2b. It is possible that only one of the separator plates 2a, 2b has one of the depicted configurations, or both; examples from different figures of the Fig. 7 - 12 can be combined. The description refers to areas that are coated with a coating 80 that increases electrical conductivity.

[0071] The Fig. Figure 7 shows a top view of a separator plate 2a, 2b of an electrochemical system of the type of system 1. Fig. 1. In the image plane shown, the first fluid flows from left to right. The active region 18 is bounded by its outer edge 18'. Within the active region, there are sections 70 in which the raised areas 34 and 44 are uncoated. The sections in which the electrically conductivity-enhancing coating 80 is present are highlighted here with hatching and are distributed differently across the active region 18. In a first region I, only a short section 81 is present in the flow direction, in which raised areas with the electrically conductivity-enhancing coating 80 are present. In a centrally located third region III, the coating 80 is evenly distributed on the raised areas in two sections 82 and 83, which are again shown with hatching. In a second region II located to the right, the coating is evenly distributed on the raised areas in a hatched area 84.The first area I and the third area III have the same area as the second area II. The proportion of section 81 to the total raised area of ​​the first area I is less than the proportion of section 84 to the second area II. The proportion of sections 82 and 83 to the total raised area of ​​the third area III is less than the proportion of area 84 to the total raised area of ​​the second area II. The proportion of coated raised areas to the total raised area is inhomogeneously distributed across the entire raised area. The coated raised areas are spaced from an outer edge 18' of the active area 18; a circumferential section 71 is formed here, in which the raised areas are formed without the coating 80. In the present example, the raised flanks and bottoms of the grooves 36 and 46, respectively, are also formed without coating 80 in the hatched sections.

[0072] The Fig. Figure 8 shows a top view of a bipolar plate 2 of an electrochemical system of the type of system 1. Fig. 1 or Fig. 2. In the depicted image plane, the first fluid flows from left to right. The active region 18 is bounded by its outer edge 18', which borders a circumferential section 71 in which no coating 80 is applied. 70 denotes further sections in which no coating 80 is applied. In a hatched section 81, the coating 80 of the type described above is uniformly arranged on the raised areas. Section 81 extends from left to right, i.e., in the x-direction, in a <-shape. In areas where the coating 80 is applied, electrical conductivity is improved. Again, three regions I, II, and III are shown.The coating 80 is distributed such that the proportion of coated enhancement areas in the first region I is smaller than the proportion of coated enhancement areas in the second region II. Furthermore, the proportion of coated enhancement areas 80 in the third region III is also significantly smaller (less than 50%) than the proportion in the second region II and slightly smaller than the proportion in the first region I. This inhomogeneous distribution of the proportion of coated enhancement areas across the entire enhancement area of ​​the active region 18 allows for a more homogeneous current density. The inhomogeneous distribution of coated enhancement areas... Fig. 8 is particularly advantageous for use in bipolar plates where the hydrogen is directed in the opposite direction to the oxygen. The largest proportion of coated enhancement areas is located in a centrally arranged zone where a portion of both oxygen and hydrogen has already reacted.

[0073] The Fig. Figure 9 shows a top view of another example of a schematically represented bipolar plate 2 of an electrochemical system of the type of system 1. Fig. 1 or Fig. 2. Arrow A marks a concentration gradient of the fluid flowing from through hole 11c (in Fig. 9 (arranged top left) via the distribution area into the active area 18 and further to the through hole 11c (in Fig. 9 (arranged at the bottom right) is directed. The active area 18 is bounded by its outer edge 18'. In a hatched section 81, the coating 80 of the type described above is applied uniformly. Section 81 extends in Fig. 9 from the bottom left of the active area 18, the coating 80 extends in a stepped pattern on both sides to the top right of the active area 18. The coating 80 is spaced from the upper edge of the active area 18'; no coating 80 is provided in either the corresponding section 71 or the sections 70. Unlike the two preceding embodiments, here not only the webs are coated, but also the raised flanks and / or bottoms of the grooves 36 and 46 in the hatched section 81 are coated section by section according to the pattern shown. Fig. 6D coated with coating 80.

[0074] The Fig. Figure 10 shows a top view of another example of a schematically represented bipolar plate 2 of an electrochemical system of the type of system 1. Fig. 1 or Fig. 2. Arrow A marks a concentration gradient of the fluid flowing from through hole 11c (in Fig. 10 (arranged top left) via the distribution area into the active area 18 and further to the through hole 11c (in Fig. 10 (arranged at the bottom right) is conducted. The active area 18 is bounded by its outer edge 18'. In a hatched section 81, an electrical conductivity-enhancing coating 80 is uniformly applied to the protrusions and grooves. The section 81 extends in Fig. 10 from the bottom left of the active area 18, in a step-like manner almost to the right outer edge and to the right upper corner of the active area 18. The coating 80 is only slightly spaced from the edge of the active area 18'.

[0075] The Fig. Figure 11 shows a top view of another example of a schematically represented bipolar plate 2 of an electrochemical system of the type of system 1. Fig. 1 or Fig. 2. The active area 18 is bounded by its outer edge 18'. In sections 81 and 82, shown in black, the coating 80 of the type described above is uniformly arranged on the raised areas. In sections 83, also shown in black, however, the coating 80 extends in a manner comparable to Fig. 6c essentially perpendicular to the course of the webs, i.e., across raised sections, flanks, and grooves. The intervening sections 70, shown in white, do not have such a coating 80. Section 81 in Fig. 11 has the shape of an elongated semicircle and is in Fig. 11 on the left in the active area 18 arranged. Section 82 in Fig. 11 has the shape of a compressed semicircle and is in Fig. 11 is located on the right side of the active area 18. Strip-shaped sections 83, of which only some are designated with the reference numeral 83 for clarity, extend essentially parallel to the y-axis and are located between sections 81 and 82. The coating 80 is spaced from the upper and lower edges 18' of the active area. Areas I, II, and III are again marked, but the sequence from left to right is I, III, and II, with the smallest proportion of raised areas formed with the coating 80 in the third area, III. Here, the second area, II, is located closer to the outlet opening 11 than the first area, I, and the third area, III.

[0076] The Fig. Figure 12 shows a top view of another example of a schematically represented bipolar plate 2 of an electrochemical system of the type of system 1. Fig. 1 or Fig. 2. The active area 18 is bounded by its outer edge 18'. In sections 81 and 82, shown in black, the coating of the type described above is uniformly applied to the raised areas and flanks, and in section 82 also in the grooves. The raised areas of the intervening sections 70, shown in white, do not have such a coating 80. Section 81 in Fig.The active area 18 has the shape of a trapezoid, extending centrally from right to left over approximately three-quarters of the area. Strip-shaped sections 82, of which only a few are designated with the reference numeral 82 for clarity, extend essentially parallel to the y-axis and are arranged essentially over the entire active area. The coating 80 is further spaced from the upper and lower edges 18' of the active area 18. The left area, in which only the strip-shaped sections 82 are coated with the coating 80, can be considered the first area I, while the right area, in which both the strip-shaped sections 82 and the trapezoidal section 81 are coated with the coating 80, can be considered the second area II. Therefore, there do not always have to be three areas I, II, III, and the area of ​​these areas can differ.

Claims

[1] Bipolar plate (2) for an electrochemical system (1), with a first separator plate (2a) and a second separator plate (2a) which are arranged one on top of the other, wherein - the first separator plate (2a) has embossed webs (32) and channels (30) for guiding a first fluid on a side facing away from the second separator plate (2b); - the second separator plate (2b) has embossed webs (32) and channels (30) for guiding a second fluid on a side facing away from the first separator plate (2a); - the first and second separator plates (2a, 2b) each have webs (42) and channels (46) on mutually facing sides for guiding a cooling medium along an inner side of the bipolar plate (2); wherein at least one of the separator plates (2a, 2b) has, on the side facing the other separator plate (2b, 2a), surface areas which are only partially coated and which are provided with a coating (80) which increases the electrical conductivity. [2] Bipolar plate according to the preceding claim, wherein a proportion of coated surface areas on the entire surface is distributed inhomogeneously over the entire surface. [3] Bipolar plate according to one of the preceding claims, wherein the proportion of coated surface areas is formed on the side of the first separator plate (2a) which faces the second separator plate (2b) and / or wherein the proportion of coated surface areas is formed on the side of the second separator plate (2b) which faces the first separator plate (2a). [4] Bipolar plate according to one of the preceding claims, wherein the first separator plate (2a) has a first electrochemically active region (18a) and the second separator plate (2b) has a second electrochemically active region (18b), wherein the first electrochemically active region (18a) and the second electrochemically active region (18b) overlap one another and form an active region (18) of the bipolar plate in an overlap region, wherein the coated surface regions in the active region (18) lie on at least one side of at least one of the separator plates (2a, 2b). [5] Bipolar plate according to one of the preceding claims, wherein the coated surface areas are formed in particular in the area of the web surfaces of one or both separator plates (2a, 2b). [6] Bipolar plate according to one of the preceding claims, wherein the first separator plate (2a) and / or the second separator plate (2b) form at least one contact surface with each other, wherein the coated surface regions are formed in particular in the region of the contact surfaces. [7] Bipolar plate according to one of the preceding claims, wherein the coated surface areas are formed in sections in the areas between the web surfaces and / or in the areas between the contact surfaces of the first separator plate (2a) and / or the second separator plate (2b). [8] Bipolar plate according to one of the preceding claims, wherein the first separator plate (2a) has in the electrochemically active region (18a) of at least one of its sides a first region (I), a second region (II) and optionally a third region (III), wherein the coated surface regions are distributed such that a proportion A11 of coated surface regions in the first region (I) of the entire surface of the first region (I) is smaller than a proportion A12 of coated surface regions in the second region (II) of the entire surface of the second region (II) and / or that a proportion A13 of coated surface regions in the third region (III) of the entire surface of the third region (III) is smaller than a proportion A12 and / or A11. [9] Bipolar plate according to one of the preceding claims, wherein the second separator plate (2b) has a first region (I), a second region (II) and a third region (III) in the electrochemically active region (18b) of at least one of its sides, wherein the coated surface regions are distributed such that a proportion A21 of coated surface regions in the first region (I) of the entire surface of the first region (I) is smaller than a proportion A22 of coated surface regions in the second region (II) of the entire surface of the second region (II) and / or that a proportion A23 of coated surface regions in the third region (III) of the entire surface of the third region (III) is smaller than a proportion A22 and / or A21. [10] Bipolar plate according to one of the two preceding claims, wherein a ratio A11 / A12 of the first and / or a ratio A21 / A22 of the second separator plate (2a, 2b), between the proportion A11 or A21 of coated surface areas in the first region (I) and the proportion A12 or A22 of coated surface areas in the second region (II) is less than 0.9, preferably less than 0.7, preferably less than 0.5, and / or wherein a ratio A13 / A12 of the first and / or a ratio A23 / A22 of the second separator plate (2a, 2b), between the proportion A13 or A23 of coated surface areas in the third region (III) and the proportion A12 or A22 of coated surface areas in the second region (II) is less than 0.9, preferably less than 0.7, preferably less than 0.

5. [11] Bipolar plate according to one of the preceding claims, wherein at least the side of the first separator plate (2a) facing the second separator plate (2b) is designed such that a proportion of coated surface areas on the entire surface of the first and / or second and / or third area (I, II, III) of the first electrochemically active area (18a) differ from one another by more than 5%, preferably differ from one another by more than 10%, particularly preferably differ from one another by more than 20%, in particular differ from one another by more than 30%. [12] Bipolar plate according to one of the preceding claims, wherein at least the side of the second separator plate (2b) facing the first separator plate (2a) is designed such that a proportion of coated surface areas on the entire surface of the first and / or second and / or third area (I, II, III) of the second electrochemically active area (18b) differ from one another by more than 5%, preferably differ from one another by more than 10%, particularly preferably differ from one another by more than 20%, in particular differ from one another by more than 30%. [13] Bipolar plate according to one of the two preceding claims, wherein only one of the regions, in particular the second region (II), has the coated surface regions. [14] Bipolar plate according to one of the preceding claims, wherein the bipolar plate is configured to be operated at a specific operating point of an electrochemical system (1), wherein the specific operating point comprises a specific concentration gradient of the first fluid along the first electrochemically active region (18a), wherein the coating is adapted to the concentration gradient of the first fluid, so that a proportion of coated surface areas in sections of the active region (18) with a comparatively low concentration of the first fluid is comparatively large and a proportion of coated surface areas in sections of the active region (18) with a comparatively high concentration of the first fluid is comparatively small. [15] Bipolar plate according to one of the preceding claims, wherein the first separator plate (2a) has a first inlet opening (11) for the inlet of the first fluid and a first outlet opening (11) for the outlet of the first fluid, wherein the first inlet opening (11) is fluidically connected to the first electrochemically active region (18a) via a first inlet region (20) and the first outlet opening (11) is fluidically connected to the first electrochemically active region (18a) via a first outlet region (20), so that the first fluid can be conducted successively through the first inlet region (20), the first electrochemically active region (18a) and the first outlet region (20). [16] Bipolar plate according to claim 15 as dependent on claim 10, wherein the second region (II) is arranged closer to the outlet opening (11) than the first region (I) and the optional third region (III). [17] Bipolar plate according to one of the preceding claims, designed such that a flow direction of the first fluid is opposite to a flow direction of the second fluid, wherein a proportion of coated surface areas of at least the side of the first separator plate (2a) facing the second separator plate and / or the side of the second separator plate (2b) facing the first separator plate increases at least in sections with the flow direction of the first fluid. [18] Bipolar plate according to one of the preceding claims, wherein at least one of the separator plates (2a, 2b) of the bipolar plate (2) consists of or contains a steel, in particular a stainless steel, or a titanium alloy, in particular a group 1 titanium alloy. [19] Bipolar plate according to one of the preceding claims, wherein the coating (90) contains one or more of the following substances or consists of one or more of these substances or their alloys: electrically conductive oxides, carbon, electrically conductive carbon layers, precious metals, in particular Au, Ag or Pt, metals, in particular Ti, Zr, Nb, Ta or Cr, metal nitrides, in particular TiN, TiON (with varying oxygen / nitrogen ratio), CrN, Cr2N, metal carbides, metal borides, metal silicides and / or silicon carbide. [20] Bipolar plate according to one of the preceding claims, wherein the coating is single-layer or multi-layer. [21] Electrochemical system (1) comprising a plurality of bipolar plates (2) according to one of the preceding claims.

Images