Method for manufacturing a bipolar plate for an electrochemical cell and bipolar plate

Abstract

The present invention relates to a method (10) for manufacturing a bipolar plate (1) for an electrochemical cell (2), in which a fluid-impermeable carrier (6) is provided and a fluid-impermeable coating (7) is applied onto at least one sub-region of the surface of the carrier (6), the coating (7) being applied by cold gas spraying, plating (in particular roll cladding) or high velocity flame spraying (in particular using air or oxygen as combustion gas). The present invention also relates to a bipolar plate (1) for an electrochemical cell (2).

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a bipolar plate for an electrochemical cell, in which a fluid-impermeable carrier is provided and a fluid-impermeable coating is applied onto at least one subregion of the surface of the carrier. The present invention also relates to a bipolar plate for an electrochemical cell.

[0002] Electrochemical cells known from the prior art are generally based on the arrangement of two electrodes electrically connected to each other by an ionic conductor. Important examples of such cells are electrolysis cells or fuel cell systems, and accumulators for storing electrical energy. A common configuration of an electrolysis cell or fuel cell system is a polymer electrolyte membrane cell, in which the ionic conductor is formed by a proton-permeable polymer membrane (PEM, "proton exchange membrane" or "polymer electrolyte membrane"), and hydrogen ions generated at the anode migrate to the cathode to form hydrogen molecules (electrolysis cells) or react with reduced oxygen at the cathode to produce water (fuel cell systems). The electrochemically active core of a PEM cell is a membrane electrode assembly (MEA), which consists of a solid polymer membrane coated on both sides with electrode materials. The membrane electrode assembly is part of a sandwich structure, in which two electrodes can each be placed on a current collector, on which a bipolar plate is placed. The bipolar plate has a flow structure (flow field) on the surface facing the respective current collector, through which base substances (e.g., water or hydrogen and oxygen gases) are supplied to the cell. By connecting several cells of MEAs, current collectors, and bipolar plates, a cell stack can be formed, which can be used to correspondingly multiply the performance of the system. Bipolar plates are also used in rechargeable energy storage cells. An example of this is a metal-air battery, where a metal anode is oxidized by atmospheric oxygen during discharge and a corresponding reduction reaction occurs during charging. Similar to electrolysis cells and fuel cell systems, bipolar plates can have flow structures that can supply or remove oxygen.

[0003] Of the main components mentioned, the interconnects (bipolar plates and current collectors) constitute a major part of the manufacturing costs of such cells. A common material here is titanium, which is characterized by particularly good corrosion resistance, but at the same time is expensive to manufacture. Another possibility is to improve the surface properties of the bipolar plates with a suitably selected coating. Plasma-based methods for coating bipolar plates are known, for example, from DE 10 2014 109 321 A1 and the publications "Bipolar plates materials for polymer electrolyte membrane electrolysis" (by M. Langemann, Forschungszentrum Julich 2016) and "Development and integration of novel components for polymer electrolyte membrane (PEM) electrolyzers" (by P. Lettenmeyer, Universitat Stuttgart 2018).

[0004] A high-pressure plasma beam coating method for coating electrode surfaces is known from DE 10 2006 031 791 A1.

[0005] DE 10 2013 213 015 A1 describes a method for producing bipolar plates, in which a layer is applied to a substrate by plasma spraying.

[0006] Against this background, the object is to provide a method for manufacturing bipolar plates that meet the special material requirements for electrochemical applications and can be produced in a cost-effective manner.

[0007] The object is achieved by a method for manufacturing a bipolar plate for an electrochemical cell, in which a fluid-impermeable carrier is provided and a fluid-impermeable coating, in particular a metallic or ceramic coating, is applied onto at least one sub-region of the surface of the carrier, the coating being applied by cold gas spraying, plating or high-velocity flame spraying.

[0008] The coating consists, for example, of a ductile material and is applied by cold gas spraying ("cold gas dynamic spraying" or simply "cold spray, CGDS, CS"), particularly preferably using nitrogen and / or helium as process gas, by high velocity flame spraying, particularly preferably using air or oxygen (HVAF (high velocity air fuel) or HVOF (high velocity oxygen fuel)) as combustion gas, or by plating, preferably by rolling onto a metal layer (e.g. roll weld cladding), welding, ion plating, electroplating, immersion or explosion plating.

[0009] In high velocity flame spraying, the coating material is preferably melted as a powdered spray additive and applied to the surface of a carrier by a carrier gas to form a dense coating with high adhesive strength and low porosity. For example, nitrogen is a suitable carrier gas, while thermal energy is generated by burning propane, propylene, or hydrogen with the addition of oxygen (HVOF) or air (HVAF).

[0010] In cold gas spraying, particles of the coating material are sprayed onto a surface in an unmolten state by a carrier gas such as nitrogen and / or helium, creating a very dense, substantially oxide-free layer with good adhesion.

[0011] In plating, the surface is preferably first cleaned and prepared by brushing or polishing, and then the coating material is substantially integrally bonded to the carrier by rolling under high pressure.

[0012] These coating methods advantageously allow the creation of low oxide and low porosity layers that meet the special requirements for use in electrochemical cells, and material costs are accordingly reduced by replacing solid materials such as titanium, titanium alloys, etc. with low-cost carrier materials.

[0013] Preferably, the coating comprises at least one of the following materials: titanium (Ti), niobium (Nb), tantalum (Ta), molybdenum (Mo), tin (Sn), silver (Ag), copper (Cu), gold (Au), platinum (Pt), vanadium (V), aluminum (Al), ruthenium (Ru), nickel (Ni), silicon (Si), tungsten (W), or oxides or carbides thereof. In particular, the coating can be formed by a carbide layer of, for example, silicon carbide (SiC) or tungsten carbide, in particular tungsten monocarbide (WC), or a ceramic made therefrom. Various oxide or oxide ceramic coatings are also possible, such as substoichiometric titanium oxides, doped oxides, or mixed oxides.

[0014] According to an advantageous embodiment of the invention, the coating comprises the following materials: titanium or a titanium alloy, which comprises at least one of niobium, tantalum, molybdenum, tin, silver, copper, gold, platinum, vanadium, aluminum, ruthenium, nickel, silicon, or oxides or carbides thereof. In this way, a corrosion-resistant surface with good electrical conductivity can be achieved, and the material costs are advantageously relatively low compared to plates made of the corresponding solid materials.

[0015] The fluid-impermeable carrier is preferably made of an electrically conductive material. Preferably, the carrier is made of metal, in particular stainless steel, or a conductive polymer material. Austenitic stainless steel, nickel-based stainless steel, copper, aluminum or graphite, composite materials, conductive thermoplastics or thermosetting resins are particularly suitable as base materials for the carrier.

[0016] In a preferred embodiment of the present invention, the composition of the coating material applied to the surface and / or one or more process parameters and / or spray additives are changed during the application of the coating. Preferably, a gradual change in the components or chemical composition of the layer occurs with increasing layer thickness, thereby enabling targeted improvement of the layer's properties. Similar improvements can also be achieved by changing one or more process parameters or spray additives. The change can be made gradually over the layer thickness or by multiple layer applications.

[0017] To supply the cells with the base substances required for the electrochemical reaction or to remove the corresponding reaction products, the bipolar plate preferably has a profile configured as a flow structure (flow field). This flow structure is preferably formed by channel-shaped depressions in the surface, which can, for example, run straight or serpentine (parallel serpentine flow field). Preferably, the flow structure has a plurality of separate channels, which particularly preferably run parallel to one another. The manufacturing method according to the invention allows several variations in the formation of flow channels of this type. In particular, the channels can be created before, after, and during the coating process.

[0018] According to an advantageous embodiment of the invention, flow channels are formed in the surface of the fluid-impermeable carrier before applying the coating. The flow channels can be produced, for example, by tension-compression forming, in particular by hydroforming. In this process, a plate or sheet is inserted between an upper tool and a lower tool, the upper tool having the desired profile to which the workpiece conforms under the action of a high-pressure fluid. Alternatively, forming can be carried out by purely mechanical forming processes, such as deep drawing, punching, or extrusion. It is also conceivable to produce the channels by ablation methods, such as machining, in particular milling.

[0019] Preferably, the coating is applied to the elevated portions formed between the flow channels, while the depressions formed by the flow channels remain uncoated. Once the flow channels have been formed on the surface of the carrier, it is advantageously possible during subsequent coating to coat only the ridges (elevated portions) of the structures, leaving the depressions of the individual structures uncoated. When using a spraying method, localized coating can be achieved, in particular, by appropriately targeted movement of the nozzle used to spray the material onto the carrier. When using a plating method, the coating material is first placed on a sub-region of the surface to be coated and then bonded thereto in a shape-fitting manner, for example by rolling.

[0020] In a further advantageous embodiment of the invention, after applying the coating, the flow channels are formed on the surface of the coated carrier by an ablation or forming method. In this manufacturing variant, the surface of the fluid-impermeable carrier is first partially or completely coated, and then the flow channels on the surface are created by forming or material removal. Forming can be carried out in particular by tension-compression forming, preferably by hydroforming, stamping or pressing. Alternatively, the channels can be formed by an ablation method, in particular a machining method, in which in particular only the sub-areas that were not covered when the coating was applied are ablated.

[0021] In a further advantageous embodiment of the invention, flow channels are formed on the surface of the fluid-impermeable carrier during the application of the coating. In particular, the material is applied by spraying, so that flow fields can be created on the carrier surface by targeted guidance of the spray nozzle. This allows for an almost freely selectable geometric configuration of the flow structures, which advantageously allows for a reduction in process steps in the production of bipolar plates.

[0022] Preferably, the coating is applied on a first sub-region of the surface to form elevated portions and is not applied on a second sub-region of the surface to form flow channels configured as depressions in the surface of the fluid-impermeable carrier. In this manufacturing variant, the ridges of the flow fields are formed by a layer applied by spraying material or by plating, so that the uncoated areas between the ridges form channel-shaped depressions in the surface of the bipolar plate.

[0023] In a preferred embodiment of the present invention, a first layer is applied to the surface of a fluid-impermeable carrier, followed by at least one additional layer applied over subregions of the first layer, so that flow channels are formed on the carrier surface between the elevated areas created by the additional layer. In other words, the spatial contours of the surface are created by the locally applied amounts of material. In particular, relatively freely selectable height profiles can be created layer by layer using a spray method with appropriate control of the nozzle, and these depressions form the flow channels of the bipolar plate. In this way, the carrier surface can be protected and the surface profile can be created at the same time.

[0024] According to an advantageous embodiment, particles are applied to the surface of the fluid-impermeable carrier or the coated fluid-impermeable carrier after and / or during the application of the coating, the particles comprising a conductive material, in particular reducing the contact resistance at the surface of the coated carrier. In particular, the particles are deposited as a spray additive on the surface or in an intermediate layer of the fluid-impermeable carrier, and the particles are preferably applied in a manner that does not cover the area. Particularly preferably, the particles are distributed sporadically on the surface after application. Partial coverage of the surface with the conductive material may already be sufficient to significantly reduce the contact resistance of the bipolar plate. For example, the conductive particles may comprise a metal such as silver or a silver alloy, or may be made of carbon or a carbon modification such as graphite. A binder may also be applied to the conductive particles to bind the particles to the surface.

[0025] A further object of the invention is a bipolar plate manufactured by an embodiment of the method according to the invention, with which the same technical effects and advantages as those explained in connection with the method according to the invention can be achieved.

[0026] A further aspect of the present invention relates to a polymer electrolyte membrane cell having two bipolar plates, a membrane electrode assembly, and two current collectors respectively arranged between the bipolar plates and the membrane electrode assembly, at least one of the two bipolar plates being manufactured by an embodiment of the method according to the present invention. The bipolar plates according to the present invention can be arranged on only one side of the membrane electrode assembly or on both sides, especially on the anode side, where very high corrosion resistance is required due to the high ion concentration at the anode.

[0027] PEM cells can be configured as both PEM electrolysis cells and PEM fuel cells.

[0028] Yet a further aspect of the invention relates to a metal-air cell, in particular a lithium-air storage battery, in which a bipolar plate according to the invention is arranged on a metallic anode.

[0029] A further aspect of the invention relates to an electrolyzer or fuel cell system assembly having at least one cell stack formed from a plurality of cells, each of which is an embodiment of a polymer electrolyte membrane cell according to the invention. Preferably, the cell stack has two end plates, which hold the stack under mechanical compressive stress to ensure intimate contact of the components. A further aspect of the invention relates to a metal-air energy storage unit, in particular a lithium-air energy storage unit, having at least one cell stack formed from a plurality of metal-air cells, each of which is an embodiment of a metal-air cell according to the invention.

[0030] Further details and advantages of the invention are explained below with reference to exemplary embodiments shown in the drawings. [Brief explanation of the drawings]

[0031] [Figure 1] 1 shows schematic diagrams of two exemplary embodiments of an electrochemical cell. [Figure 2] 1 shows in schematic diagrams four exemplary embodiments of a bipolar plate according to the invention. [Figure 3] 1 shows an exemplary embodiment of the method according to the invention in a schematic diagram;

[0032] FIG. 1 shows a schematic example of a typical structure of an electrochemical cell 2 configured as a polymer electrolyte membrane cell. A membrane electrode assembly (MEA) 5 is positioned in the center of the cell 2, which is flanked on each side by current collectors 3 and bipolar plates 1. Base materials for the electrochemical reaction are introduced into the cell 2 via flow structures 4 on the surface of the bipolar plates 1 and then flow through the porous current collectors 3 to the MEA 5, where they are converted into reaction products. The PEM cell 2 can be either an electrolysis cell or a fuel cell system. In electrolysis, the base material is water, which is split into hydrogen and oxygen by electrochemical splitting in the MEA 5. In fuel cell systems, the base materials, hydrogen and oxygen, are converted into water, releasing electrical energy.

[0033] For this purpose, the MEA 5 consists of a polymer-based proton-permeable membrane coated on both sides with electrode / catalyst materials. Hydrogen ions are formed at the anode by the catalyst and migrate through the membrane of the MEA 5 to the opposite cathode layer, where they form water in the case of a fuel cell system or molecular hydrogen gas in the case of an electrolysis cell. The current collector 3 not only provides a transport path for the base material flowing toward the MEA 5 and the outgoing reaction products, but also ensures the electrical contact of the MEA 5. Due to the high ion concentration, highly corrosive conditions occur near the catalyst / electrode layers of the MEA 5, placing special demands on the materials of the current collector 3 and the bipolar plates 1.

[0034] According to the invention, at least one of the bipolar plates 1 is formed by applying a coating 8 onto a fluid-impermeable carrier 6. Titanium or titanium alloys are particularly suitable here due to their excellent corrosion resistance. In the embodiment shown in FIG. 1a, flow channels 4 are formed on the surface of the bipolar plate 1, while the bipolar plate shown in FIG. 1b does not have any channels. Similarly, bipolar plates can also be used in other electrochemical cells, such as storage batteries.

[0035] FIG. 2 illustrates various options for structuring and / or coating a bipolar plate 1 according to the present invention. In the embodiment shown in FIG. 2a, a fluid-impermeable coating 7 is applied to a substantially flat surface of a fluid-impermeable carrier 6. In the embodiment shown in FIG. 2b, flow structures 4 are formed on the surface of the fluid-impermeable carrier 6 before coating, and the fluid-impermeable coating 7 is applied in a subsequent step over the entire structured surface of the carrier 6. Alternatively, it is possible to coat only the elevated areas formed between the flow channels 4, leaving the depressions uncoated. In a variant shown in FIG. 2c, only subregions 8′ of the fluid-impermeable carrier 6 are coated, with the intermediate uncoated subregions 8 forming the flow channels 4. In the embodiment shown in FIG. 2d, the entire surface of the fluid-impermeable carrier 6 is first coated with a first layer 9′, while a further layer 9 is applied only over certain subregions, thereby forming the flow structures 4 with different thicknesses of the fluid-impermeable coating 7. Such a layer system consisting of two or more layers 9, 9′ allows for the creation of relatively freely configurable height profiles on the surface of the fluid-impermeable carrier 6. According to the invention, the fluid-impermeable coating 7 is applied by cold gas spraying, plating (particularly roll cladding), or high velocity flame spraying (particularly using air or oxygen as combustion gas).

[0036] FIG. 3 shows various method steps 11, 12, and 13 of a possible embodiment of a method 10 according to the present invention for manufacturing a bipolar plate 1. In the first step 11, a fluid-impermeable carrier 6, made of, for example, stainless steel or a polymer material, is provided. In the second step 12, a fluid-impermeable coating 7 is deposited on the surface of the carrier 6. The coating 7 is made of a ductile material, such as titanium or a titanium alloy, and is applied by cold gas spraying, (rolled) cladding, or high-velocity oxygen flame spraying (HVOF or HVAF). The coating 7 is formed by a single-layer or multi-layer coating system, with or without flow structures 4 formed on the surface. The coating 7 can be applied over an existing flow field 4, but it is also possible to apply the structures for generating the flow field 4 directly onto the substrate surface without a coating covering the area. In the optional third method step 13, conductive particles (e.g., as a spray additive) are applied to the surface of the fluid-impermeable carrier 6 or the intermediate layer. The application preferably does not cover the area, so that the particles are distributed sporadically on the surface. Such a proportional coverage of the surface with conductive material can advantageously reduce the contact resistance of the bipolar plate 1 . [Explanation of symbols]

[0037] 1 bipolar plate 2. Electrochemical Cell 3 Current collector 4 Flow Channel 5. Membrane Electrode Assembly 6. Fluid-impermeable carrier 7 Fluid-impermeable coating 8 Uncoated sub-areas 8' coated subregion 9 First Layer 9' More layers 10 Manufacturing method 11. Providing a career 12 Applying the coating 13 Applying conductive particles

Claims

1. A method (10) for manufacturing a bipolar plate (1) for an electrochemical cell (2), comprising providing a fluid-impermeable carrier (6) and applying a fluid-impermeable coating (7) onto at least one sub-region of the surface of said carrier (6), said coating (7) being applied by cold gas spraying or high-velocity flame spraying; the coating (7) comprises at least one of niobium, tantalum, tin, silver, gold, platinum, vanadium, aluminum, ruthenium, silicon, or an oxide or carbide thereof; flow channels (4) are formed in the surface of the carrier (6) before the application of the coating (7); A method (10) characterized in that the coating (7) is applied to the elevated areas formed between the flow channels (4), leaving the depressions formed by the flow channels (4) uncoated.

2. A method (10) for manufacturing a bipolar plate (1) for an electrochemical cell (2), comprising providing a fluid-impermeable carrier (6) and applying a fluid-impermeable coating (7) onto at least one sub-region of the surface of said carrier (6), said coating (7) being applied by cold gas spraying or high-velocity flame spraying; the coating (7) comprises at least one of niobium, tantalum, tin, silver, gold, platinum, vanadium, aluminum, ruthenium, silicon, or an oxide or carbide thereof; A method (10) characterized in that flow channels (4) are formed in the surface of the carrier (6) during the application of the coating (7).

3. A method (10) for manufacturing a bipolar plate (1) for an electrochemical cell (2), comprising providing a fluid-impermeable carrier (6) and applying a fluid-impermeable coating (7) onto at least one sub-region of the surface of said carrier (6), said coating (7) being applied by cold gas spraying or high-velocity flame spraying; the coating (7) comprises a titanium alloy, the titanium alloy comprising at least one of niobium, tantalum, molybdenum, tin, silver, copper, gold, platinum, vanadium, aluminum, ruthenium, nickel, silicon, or oxides or carbides thereof; flow channels (4) are formed in the surface of the carrier (6) before the application of the coating (7); A method (10) characterized in that the coating (7) is applied to the elevated areas formed between the flow channels (4), leaving the depressions formed by the flow channels (4) uncoated.

4. A method (10) for manufacturing a bipolar plate (1) for an electrochemical cell (2), comprising providing a fluid-impermeable carrier (6) and applying a fluid-impermeable coating (7) onto at least one sub-region of the surface of said carrier (6), said coating (7) being applied by cold gas spraying or high-velocity flame spraying; the coating (7) comprises a titanium alloy, the titanium alloy comprising at least one of niobium, tantalum, molybdenum, tin, silver, copper, gold, platinum, vanadium, aluminum, ruthenium, nickel, silicon, or oxides or carbides thereof; A method (10) characterized in that flow channels (4) are formed in the surface of the carrier (6) during the application of the coating (7).

5. The method (10) according to any one of claims 1 to 4, wherein air or oxygen is used as the combustion gas in the high velocity flame spray.

6. The method (10) according to any one of claims 1 to 5, characterized in that the carrier (6) is made from an electrically conductive material.

7. 7. The method (10) according to any one of claims 1 to 6, characterized in that during the application of the coating (7) the composition of the coating material applied onto the surface and / or one or more process parameters and / or spray additives are changed.

8. 8. The method (10) according to any one of claims 1 to 7, characterized in that after the application of the coating (7), flow channels (4) are formed in the surface of the coated carrier (6) by an ablation or forming method.

9. 5. The method (10) according to claim 2 or 4, characterized in that the coating (7) is applied on a first sub-region (8') of the surface to form elevated portions and is not applied on a second sub-region (8) of the surface to form flow channels (4) designed as depressions in the surface of the carrier (6).

10. 5. A method (10) according to claim 2 or 4, characterized in that a first layer (9) is applied onto the surface of the carrier (6), and subsequently at least one further layer (9') is applied onto sub-regions of the first layer (9) in such a way that flow channels (4) are formed on the surface of the carrier (6) between the high portions created by the further layer (9').

11. 11. The method (10) according to any one of claims 1 to 10, characterized in that particles are applied onto the surface of the carrier (6) or the coated carrier (6) after and / or during the application of the coating (7), the particles comprising an electrically conductive material, in particular reducing the contact resistance at the surface of the coated carrier (6).

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