Method for forming hydrophilic surface of graphite-containing material, method for manufacturing bipolar plate, bipolar plate, and fuel or flow cell comprising bipolar plate

Abstract

A method of forming a hydrophilic surface (3) on a graphite-containing material (7) is described, in which the surface (3) to be made hydrophilic is irradiated with a pulsed laser (11) having a power density of at least 0.5 MW / mm 2 Using the above-described method, it is possible, for example, to simply render the surface (3) of a bipolar plate (1) hydrophilic in a partial region (9) thereof using low-cost equipment.

Description

Technical Field

[0001] This invention relates to a method for forming a hydrophilic surface on a graphite-containing material. It also relates to a method for manufacturing a bipolar plate for a fuel cell or flow battery. Furthermore, it relates to a bipolar plate and an energy storage device, such as a fuel cell or flow battery. Background Technology

[0002] Graphite-containing materials can be widely used in a wide variety of applications, such as forming different components. The physical properties of the contained graphite, such as its high electrical conductivity, mechanical properties, thermal stability, and / or chemical stability, can be advantageously utilized.

[0003] In some applications, it may be advantageous for the surface of components made from graphite-containing materials to have specific physical properties when in contact with other materials. In particular, it may be advantageous for some applications for the surface to be hydrophilic, that is, the surface interacts strongly with water and is thus easily wetted with water.

[0004] The following details embodiments of methods for forming hydrophilic surfaces on graphite-containing materials, and the physical properties and / or advantages that may be achieved therefrom, in conjunction with the formation of hydrophilic surfaces on bipolar plates made of graphite-containing materials, applicable to fuel cells or flow batteries. However, it should be noted that the embodiments of the methods described herein can also be used to manufacture other components where at least a portion of the surface should be hydrophilic.

[0005] When fuel cells are stacked, they form the core of the fuel cell system, with bipolar plates designed to perform several different functions. On one hand, bipolar plates connect adjacent fuel cells, physically and electrically connecting the anode of one cell to the cathode of an adjacent cell. On the other hand, the surface of the bipolar plate allows gas to be distributed into the reaction space within the fuel cell; that is, the bipolar plate guides the reactant gases into the reaction region. For this purpose, bipolar plates typically have flow patterns (so-called flow fields) on both sides, which can be pre-formed and / or reshaped, for example, milled or pressed, providing water flow on one side and air flow on the other. Bipolar plates also generally control the dissipation of water vapor or the dissipation of heat and electrical energy. Additionally, bipolar plates should provide gas separation between adjacent cells, external sealing, and, where appropriate, cooling.

[0006] To meet the fabrication requirements of bipolar plates, at least a portion of their surface should be thoroughly wettable with water; that is, they should be highly hydrophilic. For example, on the anode side, i.e., the fuel gas side, the bipolar plate should allow for uniform wetting of the electrodes and membranes within the fuel cell. On the cathode side, i.e., the oxygen side, it should be able to effectively remove water generated inside the fuel cell during the reaction; otherwise, the generated water may clog the pore system in the electrodes and / or the air passages in the bipolar plate. If relevant areas of the bipolar plate surface are made hydrophilic and thus possess excellent wettability, water can no longer be in droplet form but rather form a film on the surface, allowing for easy removal, for example, by airflow.

[0007] Currently, there are various known methods for creating hydrophilic properties on material surfaces.

[0008] For example, a hydrophilic material can be used to coat the surface. Suitable coating materials include, for example, polar polymers.

[0009] Another possible approach to forming a hydrophilic surface is to treat it with siloxane-containing plasma, thereby producing an extremely thin layer of pyrolytic silica, similar to a coating or surface modification.

[0010] Another common approach is to directly oxidize the hydrophilic surface to be formed. This can be achieved using various methods, such as wet chemical oxidation with strong oxidizing acids or hydrogen peroxide, dry gas-phase oxidation with fluorine or sulfur trioxide, atmospheric pressure plasma treatment, low-pressure plasma treatment, or corona treatment.

[0011] However, most known methods are accompanied by drawbacks or at least intractable problems. For example, the layer applied to the surface of the bipolar plate may increase the contact resistance of the bipolar plate surface. Additionally, it can sometimes be difficult to uniformly coat an extremely fine structure on the bipolar plate surface. It may also be difficult to achieve a durable and reliable adhesion of the coating to the bipolar plate surface. Oxidation using highly toxic gases such as fluorine or sulfur trioxide can place high demands on the equipment used, as this treatment can generally only be performed in a closed system. In some of the aforementioned methods, such as plasma treatment or corona treatment, although generally satisfactory hydrophilicity is randomly observed after treatment, this hydrophilicity is often observed to become unstable over time, for example, lasting only a few hours or days. Furthermore, many known methods struggle to selectively and locally create hydrophilicity in small areas of the surface, for example, by extensively masking other areas.

[0012] Patent documents EP 2 615 675 A1 and EP 2 960 973 A1 describe methods for producing fuel cell separator membranes, in which the surface is first selectively roughened and then irradiated with a laser. These methods generally require multiple steps. Therefore, such methods are costly to implement and require significant expenditure on equipment. Summary of the Invention

[0013] In view of this, there is a need for a method for forming a hydrophilic surface on a graphite-containing material, which can avoid or reduce at least some of the disadvantages or problems of the aforementioned conventional and known methods. In particular, it may be necessary to have a method in which components produced from graphite-containing materials have low contact resistance on their surfaces, exhibiting hydrophilicity even with fine surface structures, without the use of any highly toxic substances or catalytic poisons, and can stably generate hydrophilicity over a long period, and / or the method can be easily implemented, reducing equipment costs and / or being cost-effective. Furthermore, it may be necessary to have a method for manufacturing bipolar plates for fuel cells or flow batteries, which can provide hydrophilicity to certain regions of the bipolar plate using the method described herein. Additionally, it may be necessary to have a bipolar plate to be manufactured accordingly, and an energy storage device, for example, in the form of a fuel cell or flow battery.

[0014] To achieve the above objectives, the present invention provides the subject matter of the independent claims. Advantageous embodiments can be found in the independent claims, as well as in the following summary and drawings.

[0015] The first aspect of the present invention relates to a method for forming a hydrophilic surface on a graphite-containing material, wherein the surface to be formed as hydrophilic is irradiated with a pulsed laser having a power density of at least 0.5 MW / mm². 2 Preferably, it should be at least 1MW / mm 2 Or at least 2MW / mm 2 .

[0016] A second aspect of the present invention relates to a method for manufacturing a bipolar plate for a fuel cell or flow battery, the method comprising: providing a plate-shaped substrate, the plate-shaped substrate being composed of a graphite-containing material at least adjacent to an exposed surface of the substrate; and forming at least a portion of the exposed surface as a hydrophilic surface by means of a method according to an embodiment of the first aspect of the present invention.

[0017] The third aspect of the present invention relates to a bipolar plate for a fuel cell or flow battery, which is manufactured by a method according to an embodiment of the second aspect of the present invention.

[0018] The fourth aspect of the present invention relates to an energy storage device, particularly an energy storage device in the form of a fuel cell or a flow battery, the energy storage device having bipolar plates according to an embodiment of the third aspect of the present invention.

[0019] Without limiting the scope of the invention in any way, the concept and possible features of the embodiments of the invention can be regarded as being based on the following considerations and understandings.

[0020] In summary, the basic concept of this invention is that, unexpectedly, irradiating the surface of a graphite-containing material with a pulsed laser of relatively high power density results in the formation of hydrophilicity on the surface. While laser irradiation of graphite-containing materials is known, the power density of the laser used in this invention is significantly lower than that of the laser employed in this invention. When treated with such a low-power laser, the surface of the graphite-containing material (which is generally quite hydrophobic) typically develops stronger hydrophobicity. Therefore, it is unexpected that, by appropriately selecting the characteristics of the laser used, the hydrophobicity of the laser-irradiated graphite-containing material surface will not be enhanced, and may even result in hydrophilicity.

[0021] The following section will describe possible details of the methods and product implementations presented in this paper.

[0022] Graphite, as a carbon-containing material, offers advantageous properties for many applications. For example, when used in bipolar plates, graphite provides extremely high electrical conductivity, high thermal load capacity, and sufficiently high mechanical strength.

[0023] For forming components, such as bipolar plates, graphite-containing materials with graphite particles embedded in a polymer matrix are particularly used. The graphite particles provide the material with the required electrical and / or thermal properties. The polymer matrix is ​​used, in particular, to agglomerate the graphite particles by mechanical force and to transfer loads in the component. For example, the polymer matrix may contain epoxy resin. Thus, the graphite particles act as a filler, and the polymer matrix acts as a binder. In addition to graphite particles and polymers, the material mixture may also contain other components, such as carbon black, other binders, etc.

[0024] Advantageously, the graphite content of the graphite-containing material can be at least 60%, preferably at least 70%, or even at least 80%. These percentages are by volume. Due to the high graphite content, the material can provide particularly excellent electrical conductivity, which is especially advantageous for use in forming bipolar plates.

[0025] The polymer content (i.e., polymer matrix content) in the graphite-containing material is preferably at least 20 vol.%, preferably in the range of 20 vol.% to 40 vol.%, and more preferably in the range of 25 vol.% to 35 vol.%. In other words, during the laser treatment of the graphite-containing material, the material contains a substantial proportion of polymer, which can act as a binder for the graphite particles and / or be an element required for the mechanical stability and / or hermeticity of the bipolar plate. That is, the graphite-containing material is preferably neither carbonized nor calcined before laser treatment. In most cases, carbonization or calcination of graphite-containing material components leads to significant shrinkage and / or mechanical stress in the components, potentially resulting in distortion, increased dimensional tolerances, and / or fracture. Therefore, it is generally not possible to use carbonized or calcined graphite-containing materials to manufacture large components, especially large-area bipolar plates. However, in the methods described herein, carbonization or calcination is preferably omitted, thereby enabling the manufacture of large and / or extremely thin bipolar plates (e.g., length greater than 300 mm or 400 mm, width greater than 100 mm or 130 mm, and / or thickness from 0.3 mm to 2 mm, for example 0.6 ± 0.2 mm). Preferably, the laser treatment is performed such that the polymer binder contained in the graphite-containing material is not damaged or excessively damaged by the introduction of high energy input, but rather the polymer binder is simply removed from the surface of the bipolar plate.

[0026] In particular, examples and possible properties of graphite-containing materials are described in the applicant's earlier patent application PCT / EP2020 / 078489. In embodiments of the methods described herein, the graphite-containing materials described in the aforementioned documents can be treated to form a hydrophilic surface. The entire contents of the aforementioned applications are incorporated herein by reference.

[0027] As is well known, pure graphite usually does not contain any polar groups, so the surface of graphite is generally hydrophobic.

[0028] It is now believed that, although lasers can be used to treat surfaces containing graphite materials, in many cases the surface hydrophobicity is not at least not weakened, but rather enhanced. In particular, it is assumed or found that the microscopic surface textures typically formed when lasers are used to treat surfaces result in the treated surfaces developing even stronger hydrophobicity, because such microscopic surface structures usually suppress wetting due to the lotus leaf effect.

[0029] Patent document EP 2 615 675 A1 describes a method for treating a fuel cell separator membrane comprising a composition of graphite powder, epoxy resin, phenolic resin, and other components using a high-power laser, specifically to affect its surface hydrophilicity. However, EP 2 615 675 A1 only describes details regarding the power and pulse duration of the laser used. In particular, the document indicates that excessively short pulse durations, such as less than 30 ns, should be avoided, otherwise substrate warping may occur. EP 2 615 675 A1 does not provide details regarding the pulse repetition frequency and / or cross-sectional area of ​​the laser pulse beam, making it impossible to obtain information about the power density achieved by the pulsed laser from this publication.

[0030] Contrary to expectations or previous observations, the inventors of the invention described herein unexpectedly discovered that irradiating graphite-containing materials with a pulsed laser meeting certain requirements actually makes the materials more hydrophilic than before such irradiation. In other words, the contact angle between water and the treated surface is smaller compared to before irradiation. It is recognized here that irradiating the surface with a pulsed laser at a power density exceeding a certain limit appears to be crucial for improving hydrophilicity. This limit is assumed to be 0.5 MW / mm². 2 Observations showed that the treated surface exhibited excellent hydrophilicity; for example, when irradiated with a pulsed laser, the power density of the pulsed laser was at least 1 MW / mm². 2 Or especially at least 1.5MW / mm 2 At this point, within a very short pulse duration, the short-pulse laser irradiates a much smaller area (less than 1 mm) with extremely high optical power for a short period. 2 It is a very small area.

[0031] We sought to understand the unexpected phenomenon that high-power-density irradiation improves hydrophilicity. This led to the development of the model outlined below, which envisions the effects of laser irradiation. However, it should be clearly stated that the microscopic interactions observed (i.e., particularly the interactions between atoms or molecules) are not fully understood. Therefore, the models and assumptions described below are not intended to limit the scope of this invention in any way.

[0032] It is assumed that irradiating graphite-containing materials with extremely high power densities will induce defects, particularly flaws, within the graphite. These defects in the crystal lattice may themselves affect the hydrophilicity of the material surface. Furthermore, it is assumed that hydroxyl groups may subsequently couple with such defects. These hydroxyl groups, due to their polarity, may significantly enhance the hydrophilicity of the material surface. Coupling with oxygen will also cause a similar effect.

[0033] In particular, it is believed that if a pulsed laser is used to irradiate a surface with a pulse, the pulse energy per unit area is at least 0.1 J / mm². 2 Preferably, it is at least 0.2 J / mm2 Or even at least 0.3 J / mm 2 This can enhance the hydrophilicity of the treated surface.

[0034] In other words, a pulsed laser used to process graphite-containing surfaces should emit light pulses, each of which radiates a relatively large amount of energy onto a small area, thereby achieving a result exceeding at least 0.1 J / mm². 2 The lower limit of the pulse energy. The pulse energy of the radiation can vary locally within the irradiated area, and the above limit can be an average value. A single light pulse can have, for example, an energy greater than 1 mJ. The area irradiated by the light pulse can be approximately circular, rectangular, or other shapes, for example, it can have a diameter or side dimension of 0.1 mm or less.

[0035] It is assumed that, in addition to power density, the pulse energy per unit area of ​​the laser emitted light pulse also has a significant impact on the formation of hydrophilicity, and preferably both parameters should exceed certain limits.

[0036] Given the scope of the above model, it can be assumed that as the pulse energy per unit area increases, the defect density may increase, for example, defects may be generated due to laser irradiation.

[0037] However, it is believed that during the formation of hydrophilicity, the surface irradiated with a pulsed laser should have a pulse energy of less than 1 J / mm² per unit area. 2 Preferably less than 0.8 J / mm 2 or even less than 0.7 J / mm 2 Irradiation with pulses.

[0038] In other words, a pulsed laser used to treat surfaces containing graphite should emit a light pulse, the energy of which should not exceed 1 J / mm² based on the irradiated area. 2 The upper limit.

[0039] It is assumed that when the pulse energy per unit area exceeds this limit, a thermal effect with negative consequences may occur. In other words, it is assumed that under such circumstances, a laser pulse with a very high pulse energy per irradiated area may result in localized, brief, and intense heating of the graphite-containing material, potentially causing thermal damage to the material.

[0040] It is advantageous to assume that a pulsed laser irradiates the surface to be made hydrophilic with a pulse duration of less than 1 μs, preferably less than 100 ns, or even less than 20 ns or less than 10 ns.

[0041] In other words, it is considered advantageous to irradiate the surface to be hydrophilic using a nanosecond short-pulse laser. It is believed here that the desired high power density needs to be irradiated only for a very short period of time in order to achieve the desired hydrophilicity while avoiding the negative effects of laser radiation.

[0042] In particular, on the one hand, it is believed that even extremely brief, highly concentrated irradiation can generate impurities within the scope of the aforementioned model. On the other hand, it is assumed that excessively long irradiation times can lead to localized overheating of the material, potentially resulting in associated negative thermal effects. Specifically, it has been found that surface damage and / or increased contact resistance can occur on laser-irradiated surfaces when pulse durations exceed 20 ns or even 50 ns, depending on the selected pulse energy, laser spot size, pulse frequency, and / or scanning speed.

[0043] Preferably, the surface is irradiated with pulses of duration greater than 1 ns, and more preferably greater than 5 ns. Although it has been found that hydrophilicity can be improved with irradiation using shorter pulse durations, special ultrashort pulse lasers are usually required to generate pulse durations in the picosecond or even femtosecond range, which can be expensive and / or cumbersome to maintain.

[0044] It is believed that within the scope of the method proposed in this paper, irradiating the surface to be made hydrophilic with a laser pulse from a sufficiently powerful nanosecond laser is sufficient. Such nanosecond lasers are relatively inexpensive and / or require less maintenance. For example, a nanosecond fiber laser designed for large-scale use can be employed, capable of generating short laser pulses with high power density.

[0045] Preferably, the pulsed laser can irradiate the surface to be made hydrophilic with a pulse frequency of less than 100 kHz, preferably less than 50 kHz or even less than 30 kHz.

[0046] Pulse frequency refers to the frequency of a periodically repeating laser pulse. It is believed that keeping the pulse frequency below an upper limit has a positive impact on the execution method. Although extremely high pulse frequencies can be used to scan the surface to be treated very quickly by scanning along the surface, it is assumed that under high power density irradiation, the material can separate from the irradiated surface, forming a dust cloud. At extremely high pulse frequencies, i.e., when successive pulses are time-sequential, this dust cloud may cause partial absorption of the irradiated light, thus reducing the effectiveness of the laser pulse.

[0047] Preferably, the pulsed laser can irradiate the hydrophilic surface to be formed with a wavelength in the range of 800nm ​​to 1500nm, preferably 1000nm to 1200nm.

[0048] On the one hand, this wavelength of radiant laser light (i.e., in the near-infrared range) is typically readily absorbed by graphite-containing materials, particularly preferably near the surface. On the other hand, infrared lasers are generally inexpensive and easy to use.

[0049] As an additional measure, the surface to be made hydrophilic can be exposed to the reaction atmosphere during the irradiation process.

[0050] The reactive atmosphere can promote the formation of hydroxyl groups or other polar chemical components on the laser-irradiated surface. The reactive atmosphere can, for example, include free radicals, particularly nitrogen and / or oxygen radicals. The reactive atmosphere can have ambient pressure, typically 10¹³ ± 50 hPa. Alternatively, the atmosphere can have a lower or higher pressure. Furthermore, the atmosphere can be at ambient temperature, typically 25 ± 15 °C. Alternatively, the atmosphere can have a lower or higher temperature.

[0051] Preferably, the pulsed laser used to make the material surface more hydrophilic can also be used to remove the surface polymer layer and / or surface layer composed of materials other than graphite-containing materials from graphite-containing materials.

[0052] As described above, materials in which highly conductive graphite powder is embedded in a polymer matrix are particularly suitable for forming bipolar plates. Typically, when a bipolar plate is made from such a material, a thin polymer layer, similar to a skin, is formed on the surface. Alternatively or supplementarily, a surface layer composed of materials other than the graphite-containing material can be formed on the surface of the graphite-containing material. This surface layer may, for example, contain or consist of additives or external release agents. The polymer layer or surface layer can lead to undesirable electrical properties, for example, by increasing the contact resistance between the surface of the graphite-containing material and adjacent materials (e.g., electrolytes or reaction companion fluids). Generally, at least locally, this surface polymer layer and / or surface layer must be removed to expose the compressed graphite beneath, thereby reducing the contact resistance, especially relative to adjacent materials (e.g., electrolytes). It has been recognized that the same laser, which also possesses the function of enhancing surface hydrophilicity as described herein, can be advantageously employed for this purpose.

[0053] It has been discovered that the same laser parameters used to form hydrophilic surfaces can be used to remove polymer layers and / or surface layers.

[0054] In other words, it has been recognized that the same laser and the same laser parameters, which can also be used to enhance surface hydrophilicity, can be used to remove the surface polymer layer on the substrate. Thus, it is preferable that the surface polymer layer and / or undesirable surface layer can be removed from the substrate in a single combined process step, and the subsurface portion of the graphite-containing material is modified in a manner that gradually increases surface hydrophilicity.

[0055] In summary, using the method of the present invention and relatively low-cost equipment, the surface of components made of graphite-containing materials can be designed to be partially or entirely hydrophilic, and / or simultaneously remove undesirable surface polymer layers. For this purpose, a pulsed laser can advantageously be used, which, while meeting certain conditions, particularly regarding the minimum power density to be provided, is both cost-effective and industrially proven. Specifically, a pulsed laser that can also be used in other applications, such as processing graphite-containing materials or manufacturing bipolar plates, can be used to remove surface polymer layers from graphite-containing materials. Ideally, sufficient surface hydrophilicity can be achieved primarily or solely through irradiation with a high-power pulsed laser, as described above. In other words, other measures are preferably omitted.

[0056] For example, pretreatment steps, such as roughening the surface to be hydrophilic, may no longer be required. Furthermore, subsequent treatment steps, such as further roughening, coating, oxidation, plasma treatment, or corona treatment, may no longer be necessary.

[0057] Embodiments of the method of the present invention can be particularly advantageously used in the manufacture of bipolar plates, providing a hydrophilic surface for at least a portion of the bipolar plate. Here, the plate-like substrate can be composed entirely of a graphite-containing material, or at least one surface thereof has a graphite-containing material. Thus, the exposed surface of the material can be modified to be hydrophilic by laser pulse irradiation in the manner described above. The bipolar plates thus produced can be advantageously used in fuel cells or flow batteries as energy storage devices.

[0058] It should be noted that this document generally describes possible features and advantages of embodiments of the invention in part with a method for forming a hydrophilic surface on a graphite-containing material, part with a method for manufacturing a bipolar plate and a bipolar plate manufactured according to the method, and an energy storage device equipped with the bipolar plate. Those skilled in the art will recognize that features described for a single embodiment can be appropriately transferred, modified, and / or interchanged in other embodiments to obtain other embodiments of the invention and possible synergistic effects. Attached Figure Description

[0059] Advantageous embodiments of the present invention are further described below with reference to the accompanying drawings, which should not be construed as limiting the present invention in any way.

[0060] Figure 1 A bipolar plate is shown in a method according to an embodiment of the present invention.

[0061] Figure 2 An energy storage device with a fuel cell according to an embodiment of the present invention is shown.

[0062] The accompanying drawings are for illustrative purposes only and are not drawn to scale. The same reference numerals in different drawings represent the same or equivalent features. Detailed Implementation

[0063] Figure 1 A schematic diagram of the height of bipolar plate 1 is shown, in which the bipolar plate 1 is treated according to the method of the present invention to make the surface 3 hydrophilic.

[0064] In this figure, the bipolar plate 1 has a plate-shaped substrate 5. At least adjacent to the exposed surface 3, the substrate 5 includes a graphite-containing material 7. The graphite-containing material 7 comprises graphite particles. The graphite particles are typically agglomerated from a polymer material matrix. The substrate 5 may have a surface polymer layer (not shown in the figure for clarity), which may be formed, for example, during the manufacturing process of the substrate 5. The bipolar plate 1 may be designed structurally and / or functionally in the same or similar manner as a conventional bipolar plate.

[0065] To make at least a portion of the exposed surface 3 9 hydrophilic, these portions 9 are irradiated by a pulsed laser 11. The pulsed laser 11 can be, for example, a nanosecond fiber laser 13. The pulsed laser 11 can guide a pulsed laser beam 15 onto the exposure area 17 on the surface 3 of the substrate 5. The laser beam 15 and the exposure area 17 can be gradually shifted along the surface 3 to irradiate the portion 9 to be made hydrophilic. The portion 9 can be, for example, elongated and can form, for example, the bottom of a channel structure in which water will be guided along the surface of the bipolar plate 1 during the operation of a fuel cell or flow battery.

[0066] Laser beam 15 at 0.5 MW / mm 2 The above power density is applied to the exposure area 17. Each pulse preferably has a power density greater than 0.1 J / mm². 2 But less than 1J / mm 2 The pulse energy per unit area. The pulse duration can be, for example, 5 ns to 20 ns. The pulse can be repeated, for example, at a pulse frequency of 20 kHz to 40 kHz. The laser beam 15 can be emitted, for example, at a wavelength of 1064 nm. During irradiation, the substrate 5 can be exposed to a reactive atmosphere. The surface polymer layer 5 can also be removed from the substrate 5 using the laser 11 in the same working step or optionally in a separate working step. Here, the laser 11 can preferably be operated with the same laser parameters as those used to achieve enhanced surface hydrophilicity.

[0067] like Figure 2 As illustrated, the bipolar plate 1 can be used, for example, in a fuel cell 19. In the fuel cell stack 21 of the fuel cell system 23, which serves as an energy storage device 25, adjacent fuel cells 19 can be separated from each other, electrically connected to each other, and supplied with fuel by means of the bipolar plate 1.

[0068] The background of the embodiments of the present invention and possible configurations and / or advantages will be described again below, with different wording used in some cases. Such descriptions should be understood as further explanation only and not as limitation in any way.

[0069] This invention relates in particular to a method for producing bipolar plates for flow batteries, fuel cells, etc., and to a bipolar plate produced by this method. The invention also relates to a fuel cell, particularly a fuel cell stack, or a flow battery, particularly a redox flow battery, having a bipolar plate according to the invention. Furthermore, the invention relates to the use of a laser, particularly an ultrashort pulse laser, in the production of bipolar plates.

[0070] Technical issues:

[0071] A method for producing hydrophilic surfaces on graphite materials, particularly graphite-filled polymer bipolar plates for fuel cells. The performance and reliability of fuel cells depend heavily on water management within the cell. The anode side (i.e., the fuel gas side) must ensure uniform wetting of the electrodes and membranes, while the cathode side (i.e., the oxygen side) must effectively remove water generated during the reaction; otherwise, residual water may clog the pore system in the electrodes and the air channels in the bipolar plates.

[0072] This typically requires bipolar plates with hydrophilic surfaces. Excellent wettability results in water forming a film instead of droplets, making it easy to remove with an airflow.

[0073] In most cases, the surface of graphite-filled polymers is not easily wetted, and the contact angle is usually greater than 60°. This is due to the hydrophobicity of the graphite filler and the combination of the polymer binder and the release agent that can accumulate on the surface.

[0074] For reasons of improving electrical contact, it may be necessary to remove the polymer- and release agent-rich skin from functional surfaces. Common methods include abrasive brushing, fine sandblasting, grinding, and especially infrared laser cleaning. Most of these methods result in a slightly rough surface. Given the hydrophobicity of graphite, this can even lead to a superhydrophobic surface (lotus effect) with a contact angle greater than 90°, from which water droplets can easily roll off.

[0075] Surface coating or modification methods can be found in the literature. The overall goal is to create a sufficient number of polar functional groups, which is a prerequisite for good wettability.

[0076] - Polar polymers (such as phenolic resins and cross-linked polyvinyl alcohol) can be used as coatings, and finely divided polar fillers (such as pyrolytic silica and carbon black) can be added.

[0077] - In the boundary region between the coating and the surface modification, the area is treated with plasma containing siloxane, resulting in the deposition of an extremely thin layer of pyrolytic silica.

[0078] -But the primary goal is to directly oxidize the graphite surface, which involves various methods:

[0079] - Wet chemical oxidation using strong oxidizing acids or hydrogen peroxide;

[0080] - Dry gas-phase oxidation using fluorine or sulfur trioxide;

[0081] -Ambient pressure plasma treatment;

[0082] -Low-pressure plasma treatment;

[0083] - Corona treatment.

[0084] Disadvantages of existing technical solutions:

[0085] Coatings containing polymer binders may partially cover the graphite filler, thus increasing contact resistance. Furthermore, especially in systems containing fillers, it is extremely difficult to uniformly coat fine channel structures. Therefore, maintaining extremely narrow tolerances in channel geometry becomes impossible, which is a prerequisite for uniform flow and homogeneous material exchange.

[0086] - In the case of coatings with low binder content, it is impossible to ensure sufficient coating adhesion even during long-term operation of the fuel cell. This is especially true in the case of pyrolytic silica deposition via plasma treatment without binder.

[0087] Oxidation using gaseous fluorine or sulfur trioxide can result in permanent hydrophilic surface modification, but due to the toxicity of the gases, this can only be done in a closed system, which is a serious disadvantage for large-scale production. Furthermore, some surfaces cannot be treated, or can only be treated by using very complex masking materials.

[0088] Plasma treatment (especially at atmospheric pressure) and the technically related corona treatment process can be easily integrated into the manufacturing process, resulting in a surface with excellent wettability and a contact angle of less than 15° in the short term. However, it has been shown that this state only lasts for a few hours or days. After a long period of inactivity, a basic steady state with moderate wettability and a contact angle in the range of approximately 25° to 50° is established.

[0089] Purpose of the invention:

[0090] The purpose of this invention is to obtain a permanently hydrophilic surface with a water droplet contact angle of less than 25° by applying a reasonable mass production manufacturing method.

[0091] Technical solution:

[0092] Surprisingly, it has been shown that laser treatment can produce permanently hydrophilic surfaces, and it can also be used to remove polymer-rich skins. Contrary to previous assumptions that laser treatment tends to increase hydrophobicity, treatment with sufficiently high pulse energies not only leads to the removal of polymer skins but also induces structural changes on the graphite surface. Short, intense laser pulses obviously cause defects at the near-surface graphite grain level, and in extreme cases, cause large-scale destruction of the graphite crystals. These defects then spontaneously saturate oxygen and hydroxyl groups, which are prerequisites for good wetting behavior.

[0093] Firstly, the above-mentioned effects were confirmed using an ultrashort pulse laser with a pulse duration of approximately 15 ps and a pulse energy of approximately 0.4 mJ. However, due to limited power and relatively high cost, ultrashort pulse lasers are generally not considered for large-scale production of bipolar plates. Surprisingly, however, it was shown that even if individual pulse powers (approximately 30 MW or 5 kW) differ by more than three orders of magnitude, similar surface properties can be achieved using pulsed nanosecond fiber lasers if the pulse energy is at a similar level. The pulse energy per unit area should obviously exceed a critical value. In the above case, this value is approximately 0.3 J / mm². 2 Up to 0.7J / mm 2 The lower limit has not yet been precisely determined. Nanosecond fiber lasers are leading in different power levels and can be easily scaled up for large-scale use.

[0094] Permanent hydrophilicity can also be achieved by combining low-graphitization fillers with plasma treatment or corona treatment using low-pulse-energy lasers. The fillers already possess a sufficient number of defects to form surface polar groups as required by oxidation treatment. However, this reduces the overall electrical and thermal conductivity of the material, thus requiring additional manufacturing steps.

[0095] Finally, it should be noted that terms such as "having" or "comprising" do not exclude any other elements or steps, and the terms "an" or "a" do not exclude multiples. It should also be pointed out that features or steps described in conjunction with one of the exemplary embodiments described above may also be used in combination with other features or steps of the other exemplary embodiments described above. Reference numerals in the claims should not be considered limiting.

[0096] Figure Labels

[0097] 1. Bipolar plate

[0098] 3. To form a hydrophilic surface

[0099] 5 substrates

[0100] 7. Graphite-containing materials

[0101] 9 Partial Areas

[0102] 11-pulse laser

[0103] 13 nanosecond fiber laser

[0104] 15 laser beams

[0105] 17 Exposure Areas

[0106] 19 fuel cells

[0107] 21 battery stack

[0108] 23 Fuel Cell Systems

[0109] 25 energy storage devices

Claims

1. A method for forming a hydrophilic surface (3) on a graphite-containing material (7), wherein, With a power density of at least 0.5 MW / mm 2 The pulsed laser (11) irradiates the surface (3) to be made hydrophilic.

2. The method according to claim 1, wherein, The pulsed laser (11) uses a pulse energy of at least 0.1 J / mm² per unit area. 2 The surface to be made hydrophilic was irradiated with pulsed light (3).

3. The method according to any one of the preceding claims, wherein, The pulsed laser (11) uses a pulse energy of less than 0.1 J / mm² per unit area. 2 The surface to be made hydrophilic was irradiated with pulsed light (3).

4. The method according to any one of the preceding claims, wherein, The pulsed laser (11) irradiates the surface (3) to be made hydrophilic with pulses of less than 1 μs.

5. The method according to any one of the preceding claims, wherein, The pulsed laser (11) irradiates the surface (3) to be made hydrophilic with a pulse frequency of less than 100 kHz.

6. The method according to any one of the preceding claims, wherein, The pulsed laser (11) irradiates the surface (3) to be made hydrophilic with a wavelength of 800 nm to 1500 nm.

7. The method according to any one of the preceding claims, wherein, The surface (3) to be made hydrophilic is exposed to a reaction atmosphere during irradiation.

8. The method according to any one of the preceding claims, wherein, The graphite-containing material (7) has graphite particles embedded in a polymer matrix.

9. The method according to any one of the preceding claims, wherein, The pulsed laser (11) is also used to remove the surface polymer layer and / or the surface layer composed of materials other than the graphite-containing material (7) from the graphite-containing material.

10. The method according to claim 9, wherein, The removal of the polymer layer and / or surface layer is performed using the same laser parameters as those used to form the hydrophilic surface.

11. The method according to any one of the preceding claims, wherein, The graphite-containing material (7) has a graphite content of at least 60 vol.%.

12. The method according to any one of the preceding claims, wherein, The polymer content of the graphite-containing material (7) is at least 20 vol.%.

13. A method for manufacturing a bipolar plate (1) for a fuel cell (19) or a flow battery, the method comprising: A plate-shaped substrate (5) is provided, the substrate (5) being composed of a graphite-containing material at least in the area adjacent to the exposed surface (3) of the substrate (5); At least a portion (9) of the exposed surface (3) is formed into a hydrophilic surface (3) by means of any one of claims 1 to 12.

14. A bipolar plate (1) for a fuel cell (19) or flow cell manufactured by the method according to claim 13.

15. An energy storage device (25), particularly comprising at least one fuel cell (19) or flow battery having a bipolar plate (1) according to claim 14.

Images