Method for manufacturing a graphite bipolar plate by adhesive bonding of a monopolar plate, and bipolar plate and fuel cell or redox flow battery with a monopolar plate

Abstract

A method for manufacturing a graphite bipolar plate (1) for a fuel cell (21) or flow battery is described. A pair of graphite monopolar plates (3) are provided. The monopolar plates (3) are bonded together along opposing contact surfaces (7) of the monopolar plates (3). The bonding is performed using a 1K-epoxy resin (9). The properties and / or process parameters of the 1K-epoxy resin (9) can be specifically adapted to the application method.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing graphite bipolar plates for use in, for example, fuel cells or flow batteries. The present invention further relates to bipolar plates and energy storage assemblies comprising such bipolar plates. [Background technology]

[0002] The bipolar plates are intended to perform several different tasks for the fuel cells and are stacked to form the core of the fuel cell system: on the one hand, they are intended to interconnect adjacent fuel cells, i.e., to physically and electrically connect the anode of one cell with the cathode of the adjacent cell; on the other hand, they should be able to distribute gases across their surfaces towards the reaction spaces within the fuel cells, i.e., they should transport the reactant gases to the reaction compartments.

[0003] For this purpose, bipolar plates generally have flow profiles (so-called flow fields) on both sides, which can be cast and / or formed, i.e., for example, milled or engraved, on one side of which hydrogen flows and on the other side air or oxygen is supplied. The bipolar plate also generally controls the removal of water vapor or the release of thermal and electrical energy. Furthermore, the bipolar plate should also separate the gases between adjacent cells, seal against the outside, and cool them, if necessary.

[0004] To meet these requirements, bipolar plates typically have an internal structure of channels through which reactant gases can be supplied or exhausted. To enable the formation of such internal channel structures in a manufacturing environment, bipolar plates are typically constructed from two separate monopolar plates. The desired channel structure can be easily formed, for example, in the form of elongated recesses, by casting or molding, on one or both sides of the monopolar plates that, when assembled, face each other to form the bipolar plate. The two monopolar plates can then be mechanically joined together such that the desired internal channels are formed between them by the channel structures formed in the individual monopolar plates.

[0005] To meet further requirements, it has been recognized that it may be advantageous to fabricate the bipolar plates from a graphite material. For example, such a graphite material may consist of many small graphite particles embedded and / or pressed together in a matrix, such as a polymer material. Graphite materials have advantageous properties for bipolar plates, such as high electrical conductivity, high thermal conductivity, good chemical resistance to chemicals typically used in fuel cell cells, and sufficiently high mechanical load-bearing capacity, among others.

[0006] However, it has been recognized that mechanically joining two graphite monopolar plates to form a bipolar plate is not a simple task, especially considering the demands placed on the bipolar plate when used in a fuel cell. The connection between the two monopolar plates should, for example, have sufficient mechanical strength to withstand the thermal and chemical conditions within the fuel cell, on the one hand, connect the two monopolar plates to each other sufficiently tightly so that, for example, process gases cannot leak from undesired locations, and on the other hand, have sufficient gas permeability, for example, to oxygen, so that, for example, chemical processes within the fuel cell are not disturbed. Furthermore, it should be possible to mechanically connect the two monopolar plates to produce a bipolar plate by a simple, reliable, and / or cost-effective process.

[0007] Conventionally, graphite monopolar plates are often glued together to mechanically bond them into bipolar plates, using adhesives based on 2K-epoxy resins (i.e., two-component epoxy resins), cyanoacrylates, or silicone polymers.

[0008] Although conventionally bonded bipolar plates meet most of the requirements placed on fuel cells in use, there are recognized shortcomings, particularly in the manufacturing process. Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, there may be a need for improved methods of manufacturing graphite bipolar plates. In particular, there may be a need for methods of manufacturing bipolar plates that, on the one hand, meet the demands placed on fuel cells or flow batteries in use, and, on the other hand, can be realized by simple, reliable, and / or cost-effective processes. There may also be a need for suitably manufacturable bipolar plates, and energy storage assemblies comprising bipolar plates, in the form of fuel cells or flow batteries.

[0010] Such a need may be met by the subject matter of the independent claims. Advantageous embodiments are defined in the dependent claims, explained in the following description and shown in the drawings. [Means for solving the problem]

[0011] A first aspect of the present invention relates to a method for manufacturing graphite bipolar plates for a fuel cell or flow battery, comprising the process steps of: (i) providing a pair of graphite monopolar plates; and (ii) bonding the monopolar plates together along their opposing contact surfaces, the bonding being carried out using 1K-epoxy resin. The 1K-epoxy resin is adapted to have an activation temperature of at least 50°C. do.

[0012] A second aspect of the present invention comprises a pair of graphite monopolar plates bonded together with 1K-epoxy resin along their opposing contact surfaces. The 1K-epoxy resin is adapted to have an activation temperature of at least 50°C. It also relates to bipolar plates for fuel cells or flow batteries.

[0013] A third aspect of the invention relates to an energy storage assembly, in particular comprising at least one fuel cell or flow battery, comprising a bipolar plate according to an embodiment of the second aspect of the invention.

[0014] Without limiting the scope of the invention in any way, the ideas and envisaged features of the embodiments of the invention may be considered to be based, inter alia, on the ideas and findings set out below.

[0015] Briefly and broadly summarized, the rationale for the invention described herein lies in the surprising finding that, contrary to previous preconceptions, graphite bipolar plates can be advantageously bonded in a manner suitable for application in fuel cells or flow batteries using adhesives based on 1K-epoxy resins (i.e., one-component epoxy resins). It has been surprisingly recognized that, if the 1K-epoxy resin is appropriately adapted to the conditions present, on the one hand, during the manufacture of the bipolar plates and, on the other hand, during the subsequent use of the bipolar plates, bonding of monopolar plates with such specially adapted 1K-epoxy resins may be superior in many respects to bonding with adhesives traditionally used for this purpose. The 1K-epoxy resin can be adapted both to bond two monopolar plates together with sufficient mechanical strength to form a bipolar plate and to act as a sealant for adequately sealing areas within the bipolar plate or to seal the bipolar plate against the environment. For this purpose, the properties of the 1K-epoxy resin should be adapted to be sufficiently chemically resistant, hermetically sealed, and mechanically resilient, particularly for use inside a fuel cell, for example.

[0016] The following provides a detailed description of the methods and product embodiments proposed herein.

[0017] As already mentioned above, bipolar plates made of graphite material can offer great advantages when used in fuel cells or flow batteries.

[0018] As a carbon-containing material, graphite offers advantageous properties for many applications. For example, when used in bipolar plates, graphite offers very high electrical conductivity, along with high thermoelasticity and conductivity and sufficiently high mechanical strength. To form bipolar plates, graphite-containing materials are used in which graphite particles are embedded in a polymer matrix. The graphite particles impart desired electrical, thermal, and / or mechanical properties to the material. The polymer matrix, among other things, mechanically holds the graphite particles together and transfers loads within the component. The polymer matrix may contain, for example, an epoxy resin. In this way, the graphite particles act as a filler, and the polymer matrix acts as a type of binder. In addition to the graphite particles and polymer, the material mixture may also contain other components, such as carbon black, other binders, or other components in similar forms. Advantageously, the graphite-containing material may have a graphite content of at least 60%, preferably at least 70%, or even at least 80%. Percentages may refer to volume. Due to their high graphite content, such materials may provide, among other things, very good electrical conductivity, which is particularly advantageous when used to form bipolar plates. Examples and possible properties of graphite-containing materials are described, inter alia, in the applicant's earlier patent application PCT / EP2020 / 078489. The graphite-containing materials described therein may be used in embodiments of the methods described herein for monopolar plates. The entire contents of the earlier patent application are incorporated herein by reference.

[0019] To mechanically bond two monopolar plates together by assembling them into a bipolar plate, adhesives have been used whose known properties make them ideal for such a task. Among these, adhesives with sufficient mechanical strength based on 2K epoxy resins, cyanoacrylates, or silicone polymers have been used. Such adhesive systems can be activated, for example, thermally, hygroscopically, or by UV irradiation.

[0020] 2K epoxy resins consist of two components that are mixed together immediately before processing. The two components then chemically react with each other, hardening the adhesive. 2K epoxy resins are known to be adhesives that can be processed in a defined manner and achieve very high strength after curing. Furthermore, due to their chemical properties, 2K epoxy resins appear to be suitable as adhesives for bonding monopolar plates. The same applies to adhesives based on cyanoacrylates or silicone polymers.

[0021] However, it has been recognized that the aforementioned conventional adhesives often suffer from drawbacks related to their workability. For example, such adhesives often have a short pot life, meaning they can only be stored for a short time and must be processed quickly. Furthermore, such adhesives may require a relatively long time to cure. When processing conventional adhesives, bonding typically requires a narrow process window, which can limit workability. In particular, the chemical reaction between the two components of the adhesive releases a significant amount of heat energy, which can significantly heat the adhesive and, for example, can cause overheating or even a fire hazard if excessive amounts of adhesive are accidentally mixed together. Furthermore, the aforementioned adhesives often have low viscosity and therefore low stability. Furthermore, such adhesives can be difficult to apply. Some of the aforementioned adhesives even contain catalyst poisons, for example, as fillers, which can damage catalysts used in, for example, fuel cell cells. Some of the aforementioned adhesives also have low final strength, particularly poor cohesion and / or adhesion. Thus, the use of previously utilized adhesives can lead to high processing costs and / or require elaborate processing infrastructure, including, for example, maintaining strict storage and / or processing temperatures, avoiding sunlight, etc.

[0022] In principle, adhesives based on 1K epoxy resins have been known for a long time. In 1K epoxy resins, all components are already mixed before application and placed in a single container. At room temperature, this mixture is chemically inactive or crosslinks only very slowly (latently). Therefore, activation is not achieved by mixing with another component. Rather, activation is achieved by introducing activation energy. This is generally introduced thermally, i.e., by applying heat. Depending on its exact chemical composition, 1K epoxy resins generally must be heated above their activation temperature. Above this activation temperature, a generally exothermic chemical reaction begins, which, on the one hand, hardens the adhesive and, on the other hand, releases additional heat energy.

[0023] A long-standing preconception has been that adhesives based on 1K epoxy resins, while offering certain advantages, for example in terms of their workability, are only suitable for certain applications due to certain other properties. For example, such adhesives are used in vehicle assembly to bond components together across significant gap widths, for example, over 1 mm. During such bonding processes, the temperatures prevailing must usually be kept relatively low due to the materials of the components being bonded, which can be ensured by using conventional 1K epoxy resins due to their generally slow exothermic reaction. It is generally accepted that such adhesives require a relatively long curing time.

[0024] In particular, it has been previously believed that 1K-epoxy resin-based adhesives do not have suitable or even advantageous properties for bonding monopolar plates to form bipolar plates, and instead the generally slow curing of such adhesives has been considered a major drawback for the production of bipolar plates.

[0025] Contrary to such preconceptions, it is now recognized that adhesives based on 1K-epoxy resins may be perfectly suitable and even advantageous for use in bonding monopolar plates, provided that their chemical composition, and therefore their physical properties, are appropriately adapted and / or the process parameters during the processing of the adhesive are appropriately adapted. In particular, it has been recognized that such adhesives may be very suitable for bonding monopolar plates based on graphite-containing materials to one another, since they have physical properties that allow them to be used advantageously in such bonding processes.

[0026] In particular, it has been recognized that the chemical composition of 1K epoxy resins can be adapted to withstand relatively high temperatures. At such high temperatures, 1K epoxy resins can react relatively quickly and thus cure. However, it was previously thought that 1K epoxy resins should not be overheated, because if they were, a subsequent, more violent exothermic reaction would typically result in a very rapid further temperature rise within the resin, resulting in temperatures of several hundred degrees Celsius that would normally cause thermal damage to the resin when applied using conventional methods.

[0027] However, as will be explained in more detail below, the application methods described herein for bonding monopolar plates together take advantage of the physical properties, particularly the thermal properties, of these monopolar plates, which can quickly dissipate heat energy to prevent excessive heating of the resin.

[0028] According to one embodiment, the 1K-epoxy resin is adapted to withstand temperatures up to a limit temperature of at least 200° C. without damage. Furthermore, the 1K-epoxy resin is heated to at least 200° C. during the bonding process.

[0029] In fact, adhesives based on 1K epoxy resins have generally not been designed to withstand high temperatures. Instead, the compositions of conventional 1K epoxy resins are usually optimized with respect to other adhesive properties, resulting in 1K epoxy resins often beginning to degrade due to heat damage at temperatures below 180°C, or even below 160°C. However, this may be of secondary importance in many of the previous application areas of 1K epoxy resins, since higher temperatures must be avoided for other reasons by then.

[0030] However, when using 1K-epoxy resin to bond the monopolar plates as described herein, higher temperatures may be tolerated, especially since graphite monopolar plates can withstand higher temperatures without problems.

[0031] Therefore, it has been found advantageous to improve the composition of 1K epoxy resins so that they can withstand higher temperatures without significant thermal decomposition. In particular, the resin composition should be adapted so that it can withstand temperatures up to a limiting temperature without damage. This limiting temperature should preferably be at least 200°C, more preferably at least 210°C, or even at least 220°C. The heat resistance of epoxy resins can be influenced in various ways. For example, various amine curing agents with aromatic or aliphatic structures along the chain may be used to influence the temperature behavior of the epoxy resin. Aromatic amines are preferred here because they act as solid curing agents, melt, and are highly latent. The use of catalysts such as tertiary amines, Lewis acids and Lewis bases, dicyandiamine, polyols (e.g., phenols), or dimethylaminomethylphenol, and / or accelerators such as urea and imidazole can also influence the temperature behavior of the epoxy resin.

[0032] During bonding, the 1K epoxy resin may be heated to a temperature of at least 200°C, preferably at least 210°C, or even at least 220°C. For this purpose, for example, one or preferably both monopolar plates may be appropriately heated, resulting in indirect heating of the resin between them. This heating may raise the resin above its activation temperature, triggering chemical reactions that result in curing. Since these chemical reactions are significantly faster at higher temperatures than at lower temperatures, the described increase in process temperature may result in a significantly faster curing process than is typically the case when conventional 1K epoxy resins are cured at lower temperatures.

[0033] However, it is important to avoid excessively high temperatures during curing due to the exothermic reactions that occur within the 1K epoxy resin, especially temperatures that exceed the critical temperature at which irreversible damage may occur.

[0034] For this purpose, according to a more particular embodiment, the monopolar plate may be tempered during bonding to a temperature below the limit temperature of the 1K-epoxy resin.

[0035] In other words, the monopolar plate may be used not only to initially heat the epoxy resin to its activation temperature or above, but also to dissipate excess heat energy from the epoxy resin. Graphite monopolar plates generally have very good thermal conductivity, and are therefore particularly suitable for adhesive layers with thicknesses of less than 200 μm, preferably less than 150 μm, and adhesive amounts applied of 0.4 g / m 2 When the temperature of the monopolar plate is less than the critical temperature, it may be used to quickly and efficiently dissipate heat energy from the adjacent 1K-epoxy resin. In particular, the monopolar plate may be tempered or cooled so that its temperature and the temperature of the adjacent epoxy resin are maintained below the critical temperature, thereby preventing significant thermal damage to the epoxy resin.

[0036] According to one embodiment, the 1K-epoxy resin may be adapted to cure at a temperature of at least 200° C. in less than 1 minute, preferably less than 30 seconds.

[0037] In other words, the composition of the 1K epoxy resin may be optimized to withstand relatively high process temperatures on the one hand, and to cure very quickly at such high process temperatures on the other hand. For example, it has been found that high-temperature 1K epoxy resins can be fully or largely cured within 30 seconds, or even as little as 20 seconds, at a process temperature of 220°C.

[0038] Such rapid curing of the 1K-epoxy resin used as the adhesive can be very advantageous in the industrial production of graphite bipolar plates, as the adhesive can not only be applied quickly but can then cure very quickly, which can allow, for example, very fast cycle times when manufacturing bipolar plates.

[0039] 1 The K-epoxy resin is adapted to have an activation temperature of at least 50°C, preferably at least 70°C, or even at least 90°C. can .

[0040] In other words, the chemical composition of the 1K epoxy resin may be selected so that its activation temperature far exceeds the typical ambient temperature during bipolar plate manufacturing. The activation temperature may be influenced, for example, by the type of curing agent or curing agents used. For example, aromatic amines with high melting points generally have high reaction onset temperatures. Latent catalysts may also be used. For imidazoles, for example, a temperature difference of 86°C to 250°C is expected. Generally, catalysts / curing agents are only chemically active when present in the molten phase.

[0041] This means that the 1K epoxy resin can be stored at ambient temperature for a very long time without any significant curing reaction occurring. In other words, the pot life of the 1K epoxy resin can be very long. For example, the 1K epoxy resin can be advantageously kept usable for many hours, days, or even weeks. Contamination or even damage to tools used to apply the resin, for example, caused by residues of the curable resin, can be minimized.

[0042] According to one embodiment, the 1K-epoxy resin is adapted to have a glass transition temperature of at least 100°C, preferably at least 110°C, or even at least 125°C.

[0043] The glass transition temperature, sometimes called the glass transformation temperature, refers to the temperature at which a solid glass or solid polymer changes from a rubbery to a viscous state. By tailoring the 1K-epoxy resin used in bipolar plates to have a relatively high glass transition temperature, it is possible to ensure that the adhesive holding the monopolar plates together remains sufficiently strong and avoids the rubbery or viscous state that often occurs with bipolar plates in operating fuel cells, typically at temperatures up to 90°C. This ensures that the bonded bipolar plates always maintain sufficient strength at normal operating temperatures. The glass transition temperature is generally determined to a large extent by the bond density and the strength of the individual bonds in the cured network. Short and small molecules with many active bonds that result in high bond strength are generally preferred here. Therefore, aromatic compounds with several functional groups usually have a high glass transition temperature (Tg).

[0044] According to one embodiment, the 1K-epoxy resin is adapted to have a pot life of at least 5 hours and / or a latency between 0°C and 50°C.

[0045] Latency here means that the reaction between the resin and the hardener is negligibly slow. Generally, latency is essentially determined by temperature. It can be influenced, among other things, by the choice of hardener and catalyst. Roughly speaking, the higher the melting point of the hardener / catalyst, the more latent the system. However, the strength of the bonds between the individual components also plays a role here. If the reactive bonds open easily, the molecules are more chemically active and react faster.

[0046] According to one embodiment, the 1K-epoxy resin is applied locally to the surface of the monopolar plate that serves as the contact surface and is adapted to remain self-supporting and stable on this surface.

[0047] In other words, the 1K epoxy resin is adapted to have a flowable or viscous property, so that it can be applied locally to one bonding surface of a monopolar plate and then not flow across the monopolar plate. Instead, the applied resin should not change shape, i.e., remain free-standing and stable on the surface, unless a force other than gravity is applied. Thus, the monopolar plate may be moved along with the locally applied resin without the resin flowing out during the movement. In particular, the monopolar plate may be moved, for example, from an apparatus where the resin is locally applied to an apparatus where the monopolar plate is pressed together with another monopolar plate. The rheology and / or free-standing stability of the 1K epoxy resin can be influenced by various factors. For example, the viscosity can be significantly affected by fillers (thixotropes) or diluents. The diluents may be reactive or chemically inert, and typically have aliphatic and therefore highly mobile chains. In this respect, the diluent is comparable to a lubricant, which promotes the sliding of aromatic bonds.

[0048] In particular, according to one embodiment, the 1K-epoxy resin may be adapted to be structurally viscous.

[0049] Structurally viscous, also known as shear thinning, is a fluid property that exhibits a decrease in viscosity under high shear forces. With respect to the epoxy resins described herein, structurally viscous means that the resin is highly viscous and therefore very stable unless subjected to significant shear forces. Thus, after being locally applied to a monopolar plate, the resin may be maintained in the absence of significant force and moved with the monopolar plate, after which a strong shear force is applied to the resin by pressing the monopolar plate with another monopolar plate. The strong shear force then temporarily thins the resin, allowing it to flow across the surface of the monopolar plate and spread over a larger area between the two monopolar plates.

[0050] According to one embodiment, 1K-epoxy resin may be locally applied as a bead by a dispenser to one of the contact surfaces of the monopolar plates, and the monopolar plates may then be pressed together at their opposing contact surfaces.

[0051] In other words, the resin may be locally applied by a dispenser to a region of the surface of one monopolar plate that is coupled to a second monopolar plate. The dispenser may function similarly to a spraying device, and the applied resin is relatively fluid during spraying due to its preferably structurally viscous properties and remains stable on the surface in the absence of applied force. However, the term "dispenser" should be interpreted broadly here and is not limited to spraying devices, but may include any device, in particular a printing device and / or application device, with the help of which a viscous fluid may be applied locally to a surface, i.e., for example, to a point, a line, or a region.

[0052] After applying the resin to the contact surface of one or both monopolar plates, they may be pressed together, and the resin may spread along the contact surface and adhere strongly to the contact surface.

[0053] According to more specific embodiments, monopolar plates having opposing contact surfaces may be formed and pressed together to obtain a gap width between the contact surfaces of less than 200 μm, preferably less than 150 μm, or less than 100 μm, or even less than 60 μm.

[0054] In other words, the two monopolar plates may be configured to complement each other at least at their contact surfaces, so that when the monopolar plates are pressed together, the contact surfaces are very close to each other. The perpendicular distance between the contact surfaces corresponds to the gap width and should be as small as possible. A gap width of less than 200 μm means, among other things, that there is only a small amount of epoxy resin between the two monopolar plates, and that this resin is in contact with the monopolar plates over a large area. For example, the mass of epoxy resin per contact area is 0.4 g / m 2 Therefore, the heat generated in the resin during curing can be transferred to the monopolar plate over a wide area and therefore efficiently dissipated through the monopolar plate, thereby avoiding excessive heat generation.

[0055] According to one embodiment, the 1K-epoxy resin may be free of fillers with a diameter greater than 50 μm.

[0056] Conventional 1K epoxy resins often contain fillers in the form of small particles or hollow bodies that can be used to affect the flow behavior, viscosity, and / or thermal behavior of the resin. However, in the application method described herein for bonding monopolar plates, in contrast to typical conventional application methods for 1K epoxy resins, a very narrow gap width is targeted, so it is preferable to use a resin that does not contain fillers or at most contains small-sized fillers. In particular, when two monopolar plates are bonded and force is applied in the process, a deformable filler may be used in some cases.

[0057] According to one embodiment, 1K-epoxy resins may be specifically tailored with respect to specific structural and / or functional properties, taking into account the conditions or requirements to be considered when used in, for example, a bipolar plate for a fuel cell or flow battery. In particular, the 1K-epoxy resin may be substantially free of catalyst poisons that affect the catalyst used in the fuel cell. Alternatively or additionally, the 1K-epoxy resin may be substantially free of substances that promote corrosion of materials used in the fuel cell or substances that promote membrane degradation. Alternatively or additionally, the 1K-epoxy resin may be substantially free of substances that reduce proton conductivity in the fuel cell. Alternatively or additionally, the 1K-epoxy resin may have an oxygen permeability of at least 0.003 mol / s at a diffusion distance of greater than 1.5 mm and at a temperature ranging from 25°C to 90°C. Alternatively or additionally, the 1K-epoxy resin may be resistant to 0.5 mM sulfuric acid, glycantin, and deionized water at 95°C for 1000 hours.

[0058] Catalysts are typically used in fuel cells or flow batteries to support the chemical reactions occurring therein. However, the effectiveness of such catalysts can be inhibited by certain other substances called catalyst poisons. Therefore, it can be important that significant amounts of catalyst poisons never circulate anywhere in the fuel cell or flow battery, including the bipolar plates. "Substantially free of catalyst poisons" or "significant amounts of catalyst poisons" herein can be understood to mean that any amount of catalyst poison in the 1K-epoxy resin is at most such that the poison does not significantly adversely affect the functionality of the fuel cell or flow battery. Organic materials, such as free siloxanes, phthalates, amides, or similar substances, can act as catalyst poisons in fuel cells or flow batteries. The respective concentrations in the 1K-epoxy resin should be less than 50 ppm, preferably less than 30 ppm, if possible. The total concentration of semivolatile organic materials should preferably be less than 300 ppm, more preferably less than 100 ppm. The concentration of volatile and / or flammable organic materials, such as alcohol, should preferably be less than 10 ppm, more preferably less than 5 ppm. Such concentrations can be measured, for example, by methods such as dichloromethane (DCM) / methanol extraction (MSQS), EPA 8270D, or gas chromatography / mass spectrometry (GC / MS) using TBC. Anions, such as chloride, can also act as catalyst poisons. Their content should preferably be less than 10 ppm, more preferably less than 5 ppm, as measured, for example, by methods such as APHA 4110B Ion Chromatography with Chemical Suppression of Eluent Conductivity, NIOSH 6011, or APHA 4110B.

[0059] Furthermore, the performance of fuel cells or flow batteries should be prevented from deteriorating over time due to corrosion or decomposition of the membranes therein. Therefore, care should be taken with 1K-epoxy resins to ensure that they do not contain any substances that would have a negative effect in this regard. For example, so-called Fenton metals should be avoided as much as possible. Such metals include iron, copper, chromium, nickel, and titanium. Their concentration in the resin should be kept below 30 ppm, preferably below 10 ppm, as measured by, for example, inductively coupled plasma / atomic emission spectroscopy (ICP / AES) or EPA 3050B.

[0060] Proton conductivity in fuel cells should also be maintained at a high level. Therefore, 1K-epoxy resins should contain as few proton conductivity-reducing substances as possible, in the smallest possible amounts. Such substances include, but are not limited to, aluminum, arsenic, barium, bismuth, cadmium, cobalt, lithium, magnesium, manganese, molybdenum, lead, antimony, selenium, tin, vanadium, zinc, zirconium, calcium, sodium, and sulfur. With the exception of the last three substances mentioned, each of these substances should be present, if possible, in a concentration not exceeding 30 ppm, preferably not exceeding 10 ppm, in the resin, and the total concentration of all these substances should be kept below 300 ppm, preferably below 100 ppm, as measured, for example, by ICP / AES, EPA 3050B. Each of the last three substances mentioned should preferably be present at a concentration kept below 300 ppm, more preferably below 100 ppm, as measured, for example, by PRC 7100207, EPA 3050B.

[0061] Since oxygen is necessary for various chemical reactions inside the fuel cell or flow battery and must also circulate through the bipolar plates, the oxygen permeability of the 1K-epoxy resin must also be sufficiently low so as not to impede such circulation. In particular, oxygen should not be able to pass through the joints formed by the epoxy resin, which act as seals, or pass through them only to a negligible extent at most. Therefore, the properties of the 1K-epoxy resin should be adapted to be nearly gas-tight, especially with regard to the passage of oxygen. For this purpose, the oxygen permeability through the epoxy resin should be less than 0.006 mol / s, preferably less than 0.003 mol / s, over a distance of, for example, more than 1.5 mm. This should apply within the temperature range in which fuel cells or flow batteries generally operate, i.e., between 25°C and 90°C.

[0062] Furthermore, in a fuel cell or flow battery, the 1K-epoxy resin may come into contact with chemicals typically present therein, such as sulfuric acid, Glysantin, and deionized water, and should therefore be resistant to these chemicals for a sufficiently long period of time, such as at least 500 hours or even at least 1000 hours, at typical operating temperatures.

[0063] Embodiments of bipolar plates according to the second aspect of the invention may be manufactured using embodiments of the manufacturing method described herein, and the 1K-epoxy resin adhesive used for bonding between those monopolar plates may therefore have the properties described above in the method description.

[0064] An energy storage assembly according to the third aspect of the present invention may comprise one or more bipolar plates as described herein. The bipolar plates may be layered to form a stack. In addition to the bipolar plates, further components such as electrodes, membranes, and / or further materials such as electrolytes may be contained in the energy storage assembly.

[0065] It should be noted that the anticipated features and advantages of embodiments of the present invention are described herein in part with general reference to a method for manufacturing bipolar plates and in part with reference to the bipolar plates produced according to this method and the 1K-epoxy resin used in this method. Those skilled in the art will recognize that features described for individual embodiments may be transferred, adapted, and / or exchanged in a suitable manner similar to other embodiments to arrive at further embodiments of the present invention, and in some cases, synergistic effects.

[0066] Advantageous embodiments of the present invention are further described below with reference to the accompanying drawings, in which neither the drawings nor the description should be construed as limiting the invention in any way. [Brief explanation of the drawings]

[0067] [Figure 1] 1A and 1B are plan views of a monopolar plate in a method according to an embodiment of the present invention; [Figure 2] 1A-1C are cross-sectional views of a monopolar plate in a method according to an embodiment of the present invention. [Figure 3] 10A-10C are another cross-sectional views of a monopolar plate in a method according to an embodiment of the present invention. [Figure 4] FIG. 1 illustrates an energy storage assembly including a fuel cell according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0068] The drawings are only schematic and are not to scale. The same reference symbols in different drawings indicate features that act the same or similarly.

[0069] FIG. 1 shows a highly simplified plan view of the surface 5 of a monopolar plate 3 used to produce bipolar plates for fuel cells. In the finished bipolar plate, the surface 5 faces another monopolar plate of similar or identical geometry on the opposite side. The surface 5 has a plurality of regions 13. Within these regions 13, channel-like structures, e.g., in the form of depressions (not shown for clarity), are provided in the surface 5 and extend across the surface 5. A narrow linear contact surface 7 runs along the periphery of each region 13 or between adjacent regions 13. Each region 13 is completely surrounded by this area of ​​contact surface 7.

[0070] As shown in the cross section of FIG. 2 , in a method for producing a bipolar plate 1, a 1K-epoxy resin 9 is first applied to the surface 5 of a monopolar plate 3 along the contact area 7 using a dispenser 15. The dispenser 15 may be similar in design to a spraying device, and the 1K-epoxy resin 9 may be continuously applied in the form of a bead 11 along the contact area 7 by moving the dispenser 15 relative to the monopolar plate 3 in a direction parallel to the surface 5 of the monopolar plate 3. Because the 1K-epoxy resin 9 is relatively tough and preferably structurally viscous, the bead 11 may be applied with a limited width, for example, of a few millimeters, and a high aspect ratio (ratio of height to width), for example, greater than 0.5. The 1K-epoxy resin 9 is preferably sufficiently self-supporting and stable so that, after application, the bead 11 of the 1K-epoxy resin 9 maintains its shape, particularly its width, without flowing across the surface 5 of the monopolar plate 3.

[0071] Next, as shown in FIG. 3, the monopolar plate 3 provided with the epoxy resin 9 is stacked with another monopolar plate 3. The stack is then placed in a heat press 17, where pressure is applied to the stacked monopolar plates 3 from both sides to press them together. The applied force, particularly the resulting shear force, causes the structurally viscous 1K-epoxy resin to flow across the surfaces 5. This means that the area of ​​the contact surface 7 covered by the resin increases while the height of the resin layer decreases. The two monopolar plates 3 are preferably pressed toward each other to such an extent that the remaining gap width s is less than 150 μm, e.g., only 50 μm or less.

[0072] Simultaneously or subsequently, the two monopolar plates 3 are heated using a heat press 17 to a high temperature, e.g., 200°C or higher. At temperatures generally between 90°C and 130°C, the 1K-epoxy resin used in this case reaches its activation temperature, resulting in an exothermic curing reaction within the 1K-epoxy resin. The thermal energy released during this process supports the heating process. However, the heat press 17 is designed and operated, e.g., through its thermal properties and / or additional active temperature control, to ensure that the monopolar plates 3, and in particular the 1K-epoxy resin 9 between them, never become too hot, i.e., never exceed a critical temperature at which the 1K-epoxy resin may be damaged. Thermal properties of the heat press 17 include, for example, its thermal inertia, heat capacity, and thermal conductivity. In addition to the heating assembly, the heat press 17 may also optionally include a cooling assembly and / or sensors to actively control or adjust its thermal behavior. Since the graphite monopolar plate 3 generally has a very high thermal conductivity, the thermal energy released in the 1K-epoxy resin 9 during curing can be dissipated very efficiently, thus preventing overheating.

[0073] After hardening, the 1K-epoxy resin 9 ensures, on the one hand, a mechanically very elastic bond between the two monopolar panels 3. On the other hand, the 1K-epoxy resin 9 forms a kind of seal around each of the areas 13.

[0074] The properties of the 1K-epoxy resin 9 used to manufacture the graphite bipolar plate 1 described herein are specifically tailored for this purpose in various ways. For example, the 1K-epoxy resin 9 should be able to withstand high temperatures of at least 200°C without damage and cure very quickly at such high process temperatures, e.g., within 30 seconds. Such an accelerated curing process allows for very rapid manufacturing, i.e., fast cycle times, in the production of the bipolar plate 1. The 1K-epoxy resin 9 should also have a relatively high activation temperature, e.g., above 90°C, so that it can be stored and processed for very long periods of time, e.g., more than two weeks, at normal ambient temperatures. Furthermore, the 1K-epoxy resin 9 should have a relatively high glass transition temperature, e.g., above 120°C, so that it remains solid and stable at typical operating temperatures within a fuel cell.

[0075] It should be noted that means for influencing the properties of adhesives, especially 1K epoxy resins, are widely known, and experts are generally able to mix adhesives that are properly adapted to their applications, provided they know the limitations and target conditions of the application.The expert has extensive background knowledge in this field, as described, for example, in the textbook "Epoxy Adhesive Formulations" by Edward M. Petrie, published by McGraw Hill Professional, 2005, ISBN 0071589082, 9780071589086, and in the textbook "Handbook of Adhesives and Surface Preparation: Technology, Applications and Manufacturing" by Sina Ebnesajjad, published by Plastics Design Library, William Andrews, 2010, ISBN 1437744621, 9781437744620.

[0076] The following is but one of many possible compositions of 1K epoxy resins that may be used in the manufacturing methods described herein. An exemplary epoxy resin comprises 50-60% resin component and 40-50% filler. The resin component may be composed as follows:

[0077] - 10-30% epichlorohydrin resin with an average molecular weight distribution not exceeding 700 (i.e., average MW<=700)

[0078] -Oligomer reaction products using 10-30% formaldehyde, 1-chloro, 2,3-epoxypropane and phenol

[0079] -5-10% of 4,4'-isopropylidenediphenol, oligomeric reaction products with 1-chloro-2,3-epoxypropane

[0080] -1-5% of 4,4'-isopropylidenediphenol, oligomeric reaction products with 1-chloro-2,3-epoxypropane, reaction products with [(demethylamino)methyl]phenol and piperazine

[0081] The filler may be composed of 95-100% magnesium silicate and 0-5% other metal oxides.

[0082] Bipolar plates 1 manufactured in the manner described herein may be used, for example, in fuel cells 21 of an energy storage assembly 19, as shown schematically in Figure 4. In a cell stack of a fuel cell system 23 that serves as the energy storage assembly 19, the bipolar plates 1 may separate, electrically connect, and supply fuel to adjacent fuel cells 21 from one another.

[0083] Finally, it should be noted that terms such as "having" and "comprising" do not exclude other elements or steps, and terms such as "one" or "a" do not exclude a plurality. It should further be noted that features or steps described with reference to one of the above embodiments may be used in combination with other features or steps of other of the above embodiments. Reference signs in the claims should not be considered as limitations. [Explanation of symbols]

[0084] 1 bipolar plate 3 Monopolar Plates 5. Monopolar plate surface 7 Monopolar plate contact surface 9. 1K-Epoxy Resin 11 Bead 13 areas 15 Dispenser 17 Heat press equipment 19 Energy Storage Assembly 21 Fuel cell 23 Fuel Cell System s Gap width between monopolar plates

Claims

1. A method for manufacturing a graphite bipolar plate (1) for a fuel cell (21) or flow battery, comprising the steps of: Providing a pair of graphite monopolar plates (3); and joining the monopolar plates (3) together along the opposing contact surfaces (7) of the monopolar plates (3), 1K-epoxy resin (9) is used for bonding, 1. A process characterized in that the 1K-epoxy resin (9) is adapted to have an activation temperature of at least 50°C by applying, as a hardener, an aromatic amine acting as a solid hardener.

2. The 1K-epoxy resin (9) is adapted to withstand temperatures up to a limit temperature of at least 200°C without damage, The 1K-epoxy resin (9) is heated to at least 200°C during bonding. The method of claim 1.

3. the monopolar plate (3) is tempered during bonding to a temperature below the limit temperature of the 1K-epoxy resin (9); The method of claim 2.

4. The 1K-epoxy resin (9) is adapted to cure in less than 1 minute at a temperature of at least 200°C; The method of claim 2.

5. The 1K-epoxy resin (9) contains a curing agent and / or catalyst that becomes chemically active only in the melt phase upon heating above an activation temperature.

5. The method according to any one of claims 1 to 4.

6. The 1K-epoxy resin (9) is adapted to have a glass transition temperature of at least 100°C.

6. The method according to any one of claims 1 to 5.

7. The 1K-epoxy resin (9) is adapted to have a pot life of at least 5 hours and / or a latency between 0°C and 50°C.

7. The method according to any one of claims 1 to 6.

8. The 1K-epoxy resin (9) is applied locally to the surface (5) of the monopolar plate (3) that serves as the contact surface (7) and is adapted to remain self-supporting and stable on this surface (5).

8. The method according to any one of claims 1 to 7.

9. The 1K-epoxy resin (9) is adapted to be structurally viscous, 9. The method according to any one of claims 1 to 8.

10. The 1K-epoxy resin (9) is applied by a dispenser (15) as a bead (11) locally restricted to one of the contact surfaces (7) of the monopolar plates (3), and then the monopolar plates (3) are pressed together at their opposing contact surfaces (7).

10. The method according to any one of claims 1 to 9.

11. The monopolar plates (3) are formed with their opposing contact surfaces (7) and pressed together so as to obtain a gap width (s) between the contact surfaces (7) of less than 200 μm. The method of claim 10.

12. The 1K-epoxy resin (9) does not contain fillers with a diameter greater than 50 μm.

12. The method according to any one of claims 1 to 11.

13. the 1K-epoxy resin (9) is substantially free of catalyst poisons that affect the catalysts used in the fuel cell (21) or flow battery, and / or the 1K-epoxy resin (9) is substantially free of substances that promote corrosion of materials used in the fuel cell (21) or flow battery, or substances that promote membrane degradation, and / or the 1K-epoxy resin (9) is substantially free of substances that reduce proton conductivity in the fuel cell (21) or flow battery, and / or The 1K-epoxy resin (9) has an oxygen permeability of at least 0.003 mol / s at a diffusion distance of more than 1.5 mm and at a temperature ranging from 25°C to 90°C, and / or The 1K-epoxy resin (9) is resistant to 0.5 mM sulfuric acid, glycantin, and deionized water at 90°C for at least 500 hours.

13. The method of any one of claims 1 to 12.

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