DE102021113591A1 - METHOD FOR FORMING A HYDROPHILIC SURFACE ON A GRAPHITE-CONTAINING MATERIAL AND METHOD FOR MANUFACTURING A BIPOLAR PLATE AND BIPOLAR PLATE AND FUEL CELL RESPECTIVELY. FLOW BATTERY WITH THE SAME - Google Patents

Abstract

A method for forming a hydrophilic surface (3) on a graphite-containing material (7) is described, the surface (3) to be formed hydrophilic being irradiated with a pulsed laser (11) with a power density of at least 0.5 MW/mm2. Using the method, for example, a bipolar plate (1) can be made hydrophilic in partial areas (9) of its surface (3) in a simple manner and with little outlay on equipment.

Description

FIELD OF THE INVENTION

The present invention relates to a method for forming a hydrophilic surface on a graphite-containing material. Furthermore, the invention relates to a method for manufacturing a bipolar plate of a fuel cell or a flow battery. The invention also relates to a bipolar plate and an energy storage arrangement, for example in the form of a fuel cell or a flow battery.

BACKGROUND OF THE INVENTION

Materials containing graphite can be used in a wide variety of applications, for example to form different components. In this case, physical properties of the graphite contained therein, such as its high electrical conductivity, mechanical properties, thermal stability and/or chemical stability, can be advantageously used.

In some applications, it can be advantageous here if a surface of a component produced with a graphite-containing material has certain physical properties when it comes into contact with other materials. In particular, it can be advantageous for some applications if this surface is hydrophilic, i.e. interacts strongly with water and can therefore be easily wetted by water.

Embodiments of a method for forming a hydrophilic surface on a graphite-containing material and physical properties and/or advantages that can be achieved therewith are detailed below with reference to forming a hydrophilic surface on a bipolar plate formed with graphite-containing material, such as is used in a fuel cell or a flow battery can come, described. However, it is pointed out that embodiments of the method described herein can also be used in the production of other components in which at least partial surfaces are to be made hydrophilic.

Bipolar plates are intended to fulfill several different tasks for fuel cells which, when stacked, form a core of a fuel cell system. On the one hand, they should connect neighboring fuel cells with one another, i.e. physically and electrically conductively connect an anode of a cell to a cathode of a neighboring cell. On the other hand, it should be possible for gas to be distributed over a surface of the bipolar plate to reaction spaces within the fuel cells, ie the bipolar plate should conduct reaction gases into reaction zones. For this purpose, the bipolar plate typically has flow profiles (so-called flow fields) on both sides, which can be primary formed and/or formed, i.e. milled or pressed in, for example, and through which hydrogen flows on one side and air is supplied on the other side. In this case, the bipolar plate generally also regulates the removal of water vapor or the release of thermal and electrical energy. In addition, the bipolar plate should also ensure gas separation between adjacent cells, a seal to the outside and, if necessary, cooling.

In order to be able to meet the requirements placed on them, at least parts of the surface of the bipolar plate should be able to be wetted by water as well as possible, i.e. they should be highly hydrophilic. For example, the bipolar plate should enable uniform humidification of an electrode and a membrane in the fuel cell on the anode side, i.e. on the fuel gas side. On the cathode side, i.e. on the oxygen side, water that is produced during a reaction within the fuel cell should be able to be removed effectively, since this can otherwise block a pore system in the electrode and/or air channels in the bipolar plate. By making associated areas of the surface of the bipolar plate hydrophilic and thus having very good wetting properties, it can be achieved that water is no longer present in the form of drops but forms a surface film and can then be easily removed, for example, with a gas stream.

Various approaches are known for making the surfaces of materials hydrophilic.

For example, a surface can be coated with a hydrophilic material. Polar polymers, for example, are suitable as coating materials.

Another way to form a hydrophilic surface is to treat it with a siloxane-containing plasma, which produces a very thin layer of pyrogenic silica, similar to a coating or surface modification.

Another option that is frequently used is the direct oxidation of the surface to be made hydrophilic. Various methods can be used for this, such as wet-chemical oxidation with strongly oxidizing acids or hydrogen peroxide, dry oxidation in the gas phase, for example with fluorine or sulfur trioxide, and atmospheric plasma treatment, a low-pressure plasma treatment or a corona treatment.

However, the known approaches are usually accompanied by disadvantages or problems that are difficult to overcome. For example, a layer applied to the surface of a bipolar plate can increase electrical contact resistance at the surface of the bipolar plate. In addition, a uniform coating of sometimes very filigree structures on the surface of the bipolar plate can be difficult. Permanent and reliable adhesion of the coating to the surface of the bipolar plate can also be difficult to achieve. Oxidation with highly toxic gases such as fluorine or sulfur trioxide can place high demands on the equipment to be used, since such treatment should generally only be carried out in hermetically sealed systems. In some of the methods mentioned above, such as plasma treatment or corona treatment, often satisfactory hydrophilicity is recognized immediately after the treatment, but it has often been observed that this hydrophilicity is not stable over time and, for example, only lasts for a few hours or days . In addition, it is difficult with many of the known methods to make small-area sub-areas of a surface specifically hydrophilic locally, for example in that other sub-areas are masked in a complex manner.

In the EP 2 615 675 A1 and the EP 2 960 973 A1 describes methods for producing fuel cell separators in which a surface is first roughened in a targeted manner and then irradiated with a laser. The processes described generally require several process steps. The processes are therefore complicated to carry out or require significant outlay in terms of equipment.

SUMMARY OF THE INVENTION AND ADVANTAGEOUS EMBODIMENTS

There may be a need for a method for forming a hydrophilic surface on a graphite-containing material, with the aid of which at least some of the disadvantages or problems of conventionally known approaches mentioned at the outset can be avoided or reduced. In particular, there may be a need for such a method in which a component made from the graphite-containing material has a low electrical contact resistance on its surface, in which even filigree surface structures can be made hydrophilic, in which no highly toxic substances or catalyst poisons need to be used, in which hydrophilicity can be generated with long-term stability and/or which can be carried out easily, with little outlay on equipment and/or inexpensively. Furthermore, there may be a need for a method for manufacturing a bipolar plate of a fuel cell or a flow battery, in which partial areas of the bipolar plate are made hydrophilic by using the method described herein. Furthermore, there may be a need for a correspondingly manufactured bipolar plate and an energy storage arrangement, for example in the form of a fuel cell or a flow battery.

Such a need can be met with the subject matter of the independent claims. Advantageous embodiments are defined in the dependent claims and described in the following description and illustrated in the figures.

A first aspect of the invention relates to a method for forming a hydrophilic surface on a graphite-containing material, the surface to be formed hydrophilic being treated with a pulsed laser with a power density of at least 0.5 MW/mm ² , preferably at least 1 MW/mm ² or at least 2 MW/mm ² .

A second aspect of the invention relates to a method for manufacturing a bipolar plate of a fuel cell or a flow battery, the method comprising: providing a plate-like substrate which consists of a graphite-containing material at least adjacent to an exposed surface of the substrate, and forming at least partial areas of the exposed Surface as hydrophilic surface by the method according to an embodiment of the first aspect of the invention.

A third aspect of the invention relates to a bipolar plate of a fuel cell or a flow battery, which is manufactured using a method according to an embodiment of the second aspect of the invention.

A fourth aspect of the invention relates to an energy storage arrangement, in particular in the form of a fuel cell or a flow battery, with a bipolar plate according to an embodiment of the third aspect of the invention.

Without limiting the scope of the invention in any way, ideas and possible features relating to embodiments of the invention can be considered, inter alia, to be based on the thoughts and findings described below.

Briefly and roughly summarized, a basic idea for the idea described here can be seen in the fact that it was surprisingly observed that the surface of a graphite-containing material develops hydrophilic properties when it is irradiated with a pulsed laser with a relatively high power density. Although materials containing graphite have also previously been processed with the aid of lasers, lasers were used here whose power density was significantly lower than that of the lasers that are to be used for the invention described herein. When treated with such low-power lasers, the surfaces of the graphite-containing material, which are generally hydrophobic in any case, developed increased hydrophobicity as a rule. It was therefore not to be expected that the surface of a graphite-containing material irradiated with a laser would not become more hydrophobic but even hydrophilic due to a suitable choice of properties of the laser used for this purpose.

Possible details on configurations of the methods and products proposed herein are explained below.

As a carbonaceous material, graphite offers advantageous properties for many applications. For use in bipolar plates, for example, graphite offers very high electrical conductivity together with high thermal resilience and sufficiently high mechanical strength.

Materials containing graphite, in which graphite particles are embedded in a polymer matrix, are used to form components such as bipolar plates, for example. The graphite particles give the material desired electrical and/or thermal properties. The polymer matrix serves, among other things, to hold the graphite particles together mechanically and to transfer the load in the component. The polymer matrix can contain an epoxy resin, for example. The graphite particles thus act as a filler and the polymer matrix as a kind of binder. In addition to graphite particles and polymers, the mixture of materials can also contain other components, for example in the form of carbon black, other binders or the like.

The graphite-containing material can advantageously have a graphite content of at least 60%, preferably at least 70% or even at least 80%. The percentages here can relate to the volume. Due to the high proportion of graphite, the material can, among other things, offer very good electrical conductivity, which is particularly advantageous when used to form bipolar plates.

Examples and possible properties of graphite-containing materials are inter alia in the earlier patent application PCT/EP2020/078489 described by the applicant. The graphite-containing materials described there can be processed in embodiments of the methods described herein and made hydrophilic on their surface. The content of the earlier patent application is incorporated herein in its entirety by reference.

It is known that pure graphite generally has no polar groups, so surfaces of graphite usually have hydrophobic properties.

Up to now it was assumed that the surfaces of a material containing graphite can be processed using a laser, but that in many cases their hydrophobic properties are at least not reduced but rather increased. In particular, it was assumed or observed that microscopic surface textures, such as are typically formed when treating a surface with a laser, cause the treated surface to develop even more hydrophobic properties, since such microscopic surface structures typically inhibit wetting with water due to the lotus effect.

In the EP 2 615 675 A1 it is described that a fuel cell separator made of a composition of graphite powder, epoxy resin, phenolic resin and other components is to be treated with a powerful laser in order to influence, among other things, hydrophilic properties of its surface. However, in the EP 2 615 675 A1 only information regarding the power and the pulse duration of the laser to be used is described. Among other things, it is stated that pulse durations that are too short, for example less than 30 ns, should be avoided, since otherwise there is a fear that warping of the substrate may occur. Information about a pulse repetition frequency and/or a cross-sectional area of a laser pulse beam is given in EP 2 615 675 A1 not made, so that a statement about the power densities caused by the pulsed laser cannot be derived from this publication.

Contrary to such expectations or previous observations, the inventors of the present invention surprisingly observed that a surface of a graphite-containing material actually after the Irradiation with a pulsed laser that meets certain requirements is more hydrophilic than before this irradiation. In other words, a contact angle that water forms with a surface treated in this way is smaller than before the irradiation. It was recognized that it seems to be decisive for the improvement in hydrophilicity that the pulsed laser irradiates the surface with a power density that is above a limit value. 0.5 MW/mm ² is assumed to be such a limit value. A very good hydrophilicity of the treated surface was observed, for example, when irradiated with pulsed laser light with a power density of at least 1 MW/mm ² or in particular at least 1.5 MW/mm ² . A short-pulse laser briefly illuminates a generally very small area of significantly less than 1 mm ² with a very high light output during a very short pulse duration.

• Es wird angenommen, dass die Bestrahlung des graphithaltigen Werkstoffs mit einer sehr hohen Leistungsdichte dazu führt, dass in dem Graphit Störstellen, insbesondere Fehlstellen, generiert werden. Solche Störstellen in einem Kristallgitter könnten an sich bereits die hydrophilen Eigenschaften an der Oberfläche des Werkstoffs beeinflussen. Ergänzend wird angenommen, dass an solchen Störstellen nachfolgend Hydroxylgruppen ankoppeln können. Diese Hydroxylgruppen können aufgrund ihrer Polarität die hydrophilen Eigenschaften der Werkstoffoberfläche vermutlich signifikant verstärken. Auch ein Ankoppeln von Sauerstoff kann ähnliche Effekte bewirken.

An attempt was made to understand the surprising observation of improved hydrophilicity due to high power density irradiation. The model presented below was developed and assumptions were made regarding the effects caused by the laser irradiation. However, it is expressly pointed out that the microscopic, ie in particular atomic or molecular interactions on which the observation is based have not yet been fully understood. Therefore, the following description of the model and assumptions is not intended in any way to limit the scope of the invention:

• It is assumed that the irradiation of the graphite-containing material with a very high power density leads to imperfections, in particular defects, being generated in the graphite. Such imperfections in a crystal lattice could in themselves influence the hydrophilic properties on the surface of the material. In addition, it is assumed that hydroxyl groups can subsequently couple to such imperfections. Due to their polarity, these hydroxyl groups can presumably significantly increase the hydrophilic properties of the material surface. A coupling of oxygen can also cause similar effects.

In particular, it is believed that the hydrophilic properties of the treated surface can be enhanced if the pulsed laser pulses these surfaces with an area-based pulse energy of at least 0.1 J/mm ² , preferably at least 0.2 J/mm ² or even at least 0.3 J/mm ² , irradiated.

In other words, the pulsed laser used to treat the surface of the graphite-containing material should emit light pulses in which each individual light pulse radiates a relatively large amount of energy onto a small area, resulting in a pulse energy that has a lower limit of at least 0.1 J/mm exceeds ² . The irradiated pulse energy can vary locally within the irradiated area and the limit value mentioned can be an average value. A single light pulse can have an energy of more than 1 mJ, for example. An area irradiated by the light pulse can be approximately round or rectangular or have another shape and, for example, have a diameter or lateral dimensions of 0.1 mm or less.

It is assumed that, in addition to the power density, the surface-related pulse energy of the light pulse emitted by the laser also has a significant influence on the formation of hydrophilic properties and that both parameters should preferably exceed certain limit values.

Within the framework of the model presented above, it can be assumed that with increasing surface-related pulse energy, a density of defects, such as those that can be generated by irradiation with the laser light, can be increased.

However, it is additionally assumed that when developing the hydrophilic properties, the surface irradiated with the pulsed laser with pulses with an area-related pulse energy of less than 1 J/mm ² , preferably less than 0.8 J/mm ² or even less than 0. 7 J/mm ² should be irradiated.

In other words, the pulsed laser used to treat the surface of the graphite-containing material should emit light pulses whose amount of energy, based on the irradiated area, should not exceed an upper limit value of 1 J/mm ² .

It is suspected that negative thermal effects can occur with area-related pulse energies above such a limit value. In other words, it is assumed that in the event that laser pulses with very high pulse energy per illuminated area can lead to the graphite-containing material heating up very quickly locally, which can lead to thermal damage to the material.

It is assumed to be advantageous if the pulsed laser cuts the surface to be hydrophilic with pulses 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.

In other words, the use of a nanosecond short-pulse laser is considered to be advantageous for irradiating the surface to be made hydrophilic. It is assumed that the desired high power density only needs to be irradiated over a very short period of time and should be irradiated in order on the one hand to develop the desired hydrophilic properties and on the other hand to avoid negative effects from the laser irradiation.

In particular, it is assumed in the context of the model presented above that imperfections can already be generated with very short-term, high-intensity illumination. On the other hand, it is assumed that excessively long illumination can lead to excessive local heating of the material and, as a result, to negative thermal effects.

The aim is preferably to illuminate the surface with pulses having a pulse duration of more than 1 ns, preferably more than 5 ns. It has been observed that the hydrophilic properties can also be improved with irradiation with even shorter pulse durations. However, in order to generate such even shorter pulse durations in the picosecond or even femtosecond range, special ultrashort pulse lasers are generally required, which can be expensive and/or maintenance-intensive.

It is assumed that within the scope of the method proposed here it is sufficient to irradiate the surface to be made hydrophilic with laser light pulses from a sufficiently powerful nanosecond laser. Such nanosecond lasers are available relatively inexpensively and/or can be operated with little maintenance. For example, nanosecond fiber lasers can be used, which are designed for large-scale use and can generate short laser pulses with high power density.

The pulsed laser can preferably 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.

The pulse frequency is understood to be the frequency with which the laser pulses are periodically repeated. It is assumed that it has a positive effect on the procedure to be performed if the pulse rate remains below an upper limit. A surface to be processed can be scanned particularly quickly with a very high pulse frequency by scanning along the surface. However, it is assumed that when irradiating with a very high power density, material can be detached from the irradiated surface and a kind of dust cloud can form. At very high pulse frequencies, i.e. pulses that follow each other very quickly, this cloud of dust could lead to partial absorption of the incident light and thus to a reduced effectiveness of the laser pulse.

The pulsed laser can preferably irradiate the surface to be made hydrophilic with wavelengths in a range from 800 nm to 1500 nm, preferably from 1000 nm to 1200 nm.

On the one hand, irradiated laser light of such wavelengths, i.e. in the near infrared range, can typically be well absorbed by material containing graphite, in particular preferably absorbed close to the surface. On the other hand, infrared lasers are generally available at low cost and are easy to use.

As a supplementary measure, the surface to be made hydrophilic can be exposed to a reactive gas atmosphere during the irradiation.

The reactive gas atmosphere can favor formation of hydroxyl groups or other polar chemical compounds on the surface irradiated by the laser. The reactive gas atmosphere can have radicals, in particular nitrogen radicals and/or oxygen radicals, for example. The reactive gaseous atmosphere may be at ambient pressure, i.e. a pressure of typically 1013 ± 50 hPa Alternatively, the gaseous atmosphere may be at reduced or elevated pressure. In addition, the gaseous atmosphere may be at ambient temperature, i.e. a temperature of typically 25 ± 15°C. Alternatively, the gas atmosphere can have a lower or higher temperature.

The pulsed laser, which is used to make the material surface more hydrophilic, is preferably also used to remove a superficial polymer layer and/or surface layer, which consists of a material other than the graphite-containing material, from the graphite-containing material.

As already described above, materials in which highly electrically conductive graphite powder is embedded in a matrix made of a polymer material can be used in particular to form bipolar plates. Typically, when fabricating a bipolar plate, a Such a material has a thin polymer layer on the surface, similar to a surface skin. Additionally or alternatively, a surface layer can form on the surface of the graphite-containing material, which consists of a material other than the graphite-containing material. Such a surface layer can, for example, contain or consist of additives or external mold release agents. The polymer layer or the surface layer can lead to disadvantageous electrical properties, for example by increasing a contact resistance between the surface of the graphitic material and adjacent material such as an electrolyte or a reactant fluid. In order to expose the pressed graphite underneath and thus, among other things, to reduce an electrical contact resistance to an adjacent medium such as an electrolyte, this superficial polymer layer and/or surface layer must generally be removed at least in regions. It has been recognized that the same laser used to effect the more hydrophilic surface properties described herein can advantageously be used for this purpose.

In particular, it was observed that the polymer layer and/or surface layer can be removed with the same laser parameters as the formation of the hydrophilic surface.

In other words, it was recognized that the superficial polymer layer on the substrate can be removed with the same laser and the same laser parameters that can lead to the effecting of the more hydrophilic surface properties. Thus, preferably in a single, common process step, both the superficial polymer layer and/or disadvantageous surface layer can be removed from the substrate and the underlying portion of the graphite-containing material can be modified in such a way that it develops more hydrophilic properties.

In summary, with the method presented here, a surface of a component produced with graphite-containing material can be made hydrophilic in areas or over the entire surface with relatively little equipment and/or a disadvantageous superficial polymer layer or surface layer can be removed at the same time. Advantageously, a pulsed laser can be used for this purpose, which must meet certain conditions for this task, in particular with regard to a minimum power density to be achieved, but which can nevertheless be provided cost-effectively and industrially tested. In particular, a pulsed laser can be used for this purpose, which can also be used for other purposes, for example when processing the graphite-containing material or the production of bipolar plates, for example for removing a superficial polymer layer on the graphite-containing material. At best, sufficient hydrophilic properties of the surface can be brought about primarily or solely by the described irradiation with the powerful pulsed laser. In other words, supplementary measures can preferably be dispensed with.

For example, previous processing steps, such as roughening the surface to be made hydrophilic, can be dispensed with. Furthermore, subsequent processing steps such as further roughening, coating, oxidation, plasma treatment or corona treatment can be dispensed with.

Embodiments of the method presented here can be advantageously used, inter alia, to provide at least partial regions of the bipolar plate with a hydrophilic surface when manufacturing a bipolar plate. A plate-like substrate can consist entirely of graphite-containing material or have graphite-containing material at least on one of its surfaces. An exposed surface of this material can then be modified in the manner described by irradiation with laser pulses in such a way that it has hydrophilic properties. A bipolar plate manufactured in this way can advantageously be used in a fuel cell or a flow battery, which serves as an energy storage arrangement.

It is pointed out that here possible features and advantages of embodiments of the invention partly with reference generally to a method for forming a hydrophilic surface on a graphite-containing material and partly with reference to a method for manufacturing a bipolar plate and a bipolar plate manufactured according to the method and a so equipped energy storage arrangement are described. A person skilled in the art will recognize that the features described for individual embodiments can be transferred, adapted and/or exchanged in an analogous and suitable manner to other embodiments in order to arrive at further embodiments of the invention and possibly to achieve synergy effects.

character list

• 1 zeigt eine Bipolarplatte während eines Verfahrens gemäß einer Ausführungsform der vorliegenden Erfindung.
• 2 zeigt eine Energiespeicheranordnung mit einer Brennstoffzelle gemäß einer Ausführungsform der vorliegenden Erfindung.

Advantageous embodiments of the invention are explained in more detail below with reference to the accompanying drawings, in which: neither the drawings nor the explanations are to be construed as limiting the invention in any way.

• 1 12 shows a bipolar plate during a procedure according to an embodiment of the present invention.
• 2 12 shows an energy storage arrangement with a fuel cell according to an embodiment of the present invention.

The figures are merely schematic and not true to scale. In the various drawings, the same reference symbols denote the same features or features that have the same effect.

DESCRIPTION OF ADVANTAGEOUS EMBODIMENTS

1 shows a highly schematized representation of a bipolar plate 1 while it is being treated with a method according to the invention in order to form a surface 3 hydrophilic.

The bipolar plate 1 has a plate-like substrate 5. At least adjacent to the exposed surface 3, the substrate 5 is made of a graphite-containing material 7. The graphite-containing material 7 contains graphite particles. The graphite particles are typically held together by a matrix of polymeric material. On one surface, the substrate 5 can have a superficial polymer layer (not illustrated in the figure for reasons of clarity), which can have been formed, for example, when the substrate 5 was produced. The bipolar plate 1 can be designed structurally and/or functionally in the same or a similar way as conventional bipolar plates.

In order to make at least partial areas 9 of the exposed surface 3 hydrophilic, these partial areas 9 are irradiated with the aid of a pulsed laser 11 . The pulsed laser 11 can be a nanosecond fiber laser 13, for example. The pulsed laser 11 can direct a pulsed laser beam 15 onto an exposure area 17 on the surface 3 of the substrate 5 . The laser beam 15 and the exposure area 17 can be displaced successively along the surface 3 in order to illuminate the partial areas 9 to be made hydrophilic. The subregions 9 can be elongate, for example, and form the bottom of a channel structure, for example, in which water is to be conducted along the surface of the bipolar plate 1 during operation of a fuel cell or a flow battery.

The laser beam 15 strikes the exposure area 17 with a power density of 0.5 MW/mm ² or more. Each individual pulse preferably has an area-related pulse energy of more than 0.1 J/mm ² but less than 1 J/mm ² The pulse can have a pulse duration of between 5 ns and 20 ns, for example. For example, pulses can be repeated with a pulse frequency of between 20 kHz and 40 kHz. The laser beam 15 can be emitted with a wavelength of 1064 nm, for example. During the irradiation, the substrate 5 can be exposed to a reactive gas atmosphere. In the same work step or, optionally, in a separate work step, the laser 11 can also be used to remove a superficial polymer layer from the substrate 5 . In this case, the laser 11 can preferably be operated with the same laser parameters as are used to achieve the increasingly hydrophilic surface properties.

The bipolar plate 1 can be used, for example, in a fuel cell 19, as is shown schematically in 2 is shown. In a cell stack 21 of a fuel cell system 23 serving as an energy storage arrangement 25, adjacent fuel cells 19 can be separated from one another with the aid of bipolar plates 1, electrically connected to one another and supplied with fuel.

In the following, backgrounds as well as possible configurations and/or advantages of embodiments of the invention presented herein are described again with a partly different choice of words, whereby this description is to be interpreted merely as further explanation, but in no way restrictive.

The invention relates, inter alia, to a method for producing a bipolar plate for a flow battery, fuel cell or the like, and a bipolar plate produced with the method. The invention further relates to a fuel cell, in particular a fuel cell bundle (stack), or a flow battery, in particular redox flow battery, with a bipolar plate according to the invention. The invention also relates to the use of a laser, in particular an ultra-short pulse laser, to produce a bipolar plate.

Purpose of the invention:

Process for producing hydrophilic surfaces on graphitic materials, in particular on bipolar plates made from graphite-filled polymers for use in fuel cells. The performance and reliability of fuel cells depend to a large extent on water management in the cell. Uniform humidification of the electrode and the membrane must be ensured on the anode side, ie on the combustion gas side, and on the cathode side, ie on the oxygen side On the other hand, the water produced during the reaction must be effectively drained away, otherwise excess water can block the pore system in the electrode and the air channels in the bipolar plate.

Typically this requires bipolar plates with hydrophilic surfaces. Very good wetting properties mean that water is no longer in the form of droplets, but forms a surface film and can then be easily removed with the gas flow.

The surface of graphite-filled polymers is usually only poorly wetted by water, the contact angles are typically > 60°. The cause lies in a combination of hydrophobic properties of the graphitic fillers, but also of the polymeric binders and release agents that can accumulate on the surface.

For reasons of better electrical contact, it may be necessary to remove the surface skin rich in polymer and release agent from the functional surfaces. Common procedures include abrasive brushing, fine blasting, grinding and, in particular, cleaning with an infrared laser. These processes usually lead to a slight roughening of the surface. Given the hydrophobic properties of the graphite, this can even lead to superhydrophobic surfaces (lotus effect) with contact angles > 90°, from which water droplets roll off very easily.

• - Als Beschichtungsstoffe können polare Polymere (z. B. Phenolharze, vernetzter Polyvinylalkohol) eingesetzt werden, denen auch noch feinteilige polare Füllstoffe (z. B. pyrogene Kieselsäure, oxidierte Ruße) zugesetzt werden können.
• - Im Grenzbereich zwischen Beschichtung und Oberflächenmodifikation steht die Behandlung mit einem Siloxan-haltigen Plasma, was zur Abscheidung einer sehr dünnen Schicht von pyrogener Kieselsäure führt.
• - Überwiegend wird jedoch die direkte Oxidation der Graphitoberflächen angestrebt, wobei verschiedene Verfahren zum Einsatz kommen:
  • - Nasschemische Oxidation mit starken oxidierenden Säuren oder Wasserstoffperoxid
  • - Trockene Oxidation in der Gasphase mit Fluor oder Schwefeltrioxid
  • - Atmosphärische Plasmabehandlung
  • - Niederdruck-Plasmabehandlung
  • - Corona-Behandlung

Processes for coating or for modifying the surface are known from the literature. The overall goal is to create a sufficient number of polar functional groups, which are a prerequisite for good wetting properties:

• - Polar polymers (e.g. phenolic resins, crosslinked polyvinyl alcohol) can be used as coating materials, to which finely divided polar fillers (e.g. pyrogenic silica, oxidized carbon blacks) can also be added.
• - In the border area between coating and surface modification is the treatment with a siloxane-containing plasma, which leads to the deposition of a very thin layer of pyrogenic silica.
• - However, the direct oxidation of the graphite surfaces is predominantly aimed at, whereby various processes are used:
  • - Wet chemical oxidation with strong oxidizing acids or hydrogen peroxide
  • - Dry gas phase oxidation with fluorine or sulfur trioxide
  • - Atmospheric plasma treatment
  • - Low pressure plasma treatment
  • - Corona treatment

• - Beschichtungen mit polymeren Bindemitteln können die graphitischen Füllstoffe teilweise wieder bedecken und damit den elektrischen Kontaktwiderstand erhöhen. Weiterhin gestaltet sich eine gleichmäßige Beschichtung von filigranen Kanalstrukturen, vor allem bei füllstoffhaltigen Systemen sehr schwierig. Die Einhaltung von sehr engen Toleranzen der Kanalgeometrien, was wiederum eine notwendige Voraussetzung für eine gleichmäßige Strömung und einen homogenen Stoffaustausch ist, ist damit nicht mehr möglich.
• - Eine ausreichende Schichthaftung, die auch einem längeren Betrieb der Brennstoffzelle Stand hält, ist bei Beschichtungen mit geringem Bindemittelgehalt nicht zu gewährleisten. Dies gilt umso mehr bei der bindemittelfreien Abscheidung von pyrogener Kieselsäure über eine Plasmabehandlung.
• - Die Oxidation mit Fluor oder Schwefeltrioxid in der Gasphase kann zu einer dauerhaft hydrophilen Oberflächenmodifikation führen, aufgrund der Toxizität der Gase kann die Behandlung aber nur in hermetisch abgeschlossenen Systemen durchgeführt werden, was für eine Großserienfertigung ein gravierender Nachteil ist. Weiterhin ist eine Behandlung von Teilflächen nicht oder nur mit einer sehr aufwändigen Maskierung der Platten möglich.
• - Die Plasmabehandlung, insbesondere bei Normaldruck, und das technologisch verwandte Corona-Verfahren sind dagegen sehr gut in einen Fertigungsprozess zu integrieren und führen auch kurzfristig zu sehr gut benetzenden Oberflächen mit Kontaktwinkeln < 15°. Es zeigt sich jedoch, dass dieser Zustand nur über wenige Stunden bzw. Tage anhält. Nach längerer Liegezeit stellt sich ein weitgehend stationärer Zustand mit einer mittleren Benetzbarkeit und Kontaktwinkeln im Bereich von ca. 25 - 50° ein.

Disadvantages of the previously known solutions:

• - Coatings with polymeric binders can partly cover the graphitic fillers again and thus increase the electrical contact resistance. Furthermore, a uniform coating of filigree channel structures is very difficult, especially in systems containing filler. It is therefore no longer possible to comply with very narrow tolerances of the channel geometries, which in turn is a necessary prerequisite for a uniform flow and a homogeneous mass transfer.
• - Adequate layer adhesion, which also withstands prolonged operation of the fuel cell, cannot be guaranteed with coatings with a low binder content. This applies all the more to the binder-free deposition of pyrogenic silica via plasma treatment.
• - The oxidation with fluorine or sulfur trioxide in the gas phase can lead to a permanently hydrophilic surface modification, but due to the toxicity of the gases, the treatment can only be carried out in hermetically sealed systems, which is a serious disadvantage for large-scale production. Furthermore, a treatment of partial areas is not possible or only possible with a very complex masking of the plates.
• - The plasma treatment, especially at normal pressure, and the technologically related corona process, on the other hand, can be easily integrated into a manufacturing process and also lead to very well-wetted surfaces with contact angles < 15° for a short time. It turns out, however, that this condition only lasts for a few hours or days. After a long period of lying there, a largely stationary state sets in with average wettability and contact angles in the range of approx. 25 - 50°.

Object of the invention:

The goal is permanently hydrophilic surfaces with contact angles of water droplets < 25° through the use of an efficient large-scale production process.

Solution:

Surprisingly, it has been shown that permanently hydrophilic surfaces can be produced with laser treatment, which can also be used to remove the polymer-rich surface skin. Contrary to the previous assumption that the laser treatment tends to increase the hydrophobic properties, the treatment with a sufficiently high pulse energy not only leads to the removal of the polymer skin, but also leads to a structural change in the graphite surface. Short, intense lasers apparently induce defects in the layer planes of the graphite particles close to the surface and, in extreme cases, extensive destruction of the graphite crystals. The defects then spontaneously saturate themselves with oxygen and hydroxyl groups, which are a prerequisite for good wetting behavior.
The effect could first be demonstrated when using an ultra-short pulse laser with a pulse duration of approx. 15 ps and a pulse energy of approx. 0.4 mJ. However, due to the limited power and the comparatively high costs, the use of ultrashort pulse lasers in the large-scale production of bipolar plates is usually ruled out. Surprisingly, however, it was then shown that comparable surface properties can also be set with pulsed nanosecond fiber lasers if the pulse energy is at a comparable level, although the individual pulse powers of approx. 30 MW or 5 kW differ by more than three orders of magnitude. Apparently, the pulse energy per unit area should exceed a critical value. In the cases described, this value was around 0.3 - 0.7 J/mm ² ; the lower limit has not yet been precisely determined. Nanosecond fiber lasers are state-of-the-art in different power classes and can be easily scaled for large-scale use.

Permanently hydrophilic properties might also be possible through the combination of fillers with a low degree of graphitization and a plasma or corona treatment after the laser treatment with lower pulse energy. The fillers would then already have a sufficient number of defects that are needed to form the polar surface groups during the oxidizing treatment. As a result, however, the entire material then also has low electrical and thermal conductivity and an additional production step is required again.

Finally, it should be noted that terms such as "comprising," "comprising," etc. do not exclude other elements or steps, and terms such as "a" or "an" do not exclude a plurality. Furthermore, it should be pointed out that features or steps that have been described with reference to one of the above exemplary embodiments can also be used in combination with other features or steps of other exemplary embodiments described above. Any reference signs in the claims should not be construed as limiting.

Reference List

1

bipolar plate

3

surface to be hydrophilic

5

substrate

7

material containing graphite

9

subarea

11

pulsed laser

13

Nanosecond fiber laser

15

laser beam

17

exposure range

19

fuel cell

21

cell stack

23

fuel cell system

25

energy storage arrangement

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• EP 2615675 A1 [0012, 0028]
• EP 2960973 A1 [0012]
• EP 2020/078489 PCT [0025]

Claims (14)

Process for forming a hydrophilic surface (3) on a graphite-containing material (7), wherein the surface (3) to be formed hydrophilic is irradiated with a pulsed laser (11) with a power density of at least 0.5 MW/mm ² . procedure after claim 1 , wherein the pulsed laser (11) irradiates the surface (3) to be hydrophilic with pulses having an area-related pulse energy of at least 0.1 J/mm ² . Method according to one of the preceding claims, in which the pulsed laser (11) irradiates the surface (3) to be made hydrophilic with pulses having a pulse energy per unit area of less than 1 J/mm ² . Method according to one of the preceding claims, in which the pulsed laser (11) irradiates the surface (3) to be made hydrophilic with pulses having a pulse duration of less than 1 µs. Method according to one of the preceding claims, in which the pulsed laser (11) irradiates the surface (3) to be made hydrophilic with a pulse frequency of less than 100 kHz. Method according to one of the preceding claims, wherein the pulsed laser (11) irradiates the surface (3) to be made hydrophilic with wavelengths of between 800 nm and 1500 nm. Method according to one of the preceding claims, in which the surface (3) to be made hydrophilic is exposed to a reactive gas atmosphere during the irradiation. Method according to one of the preceding claims, in which the graphite-containing material (7) has graphite particles which are embedded in a polymer matrix. Method according to one of the preceding claims, wherein the pulsed laser is additionally used to remove a superficial polymer layer and/or surface layer, which consists of a material other than the graphite-containing material, from the graphite-containing material (7). procedure after claim 9 , wherein the removal of the polymer layer and/or surface layer takes place with the same laser parameters as the formation of the hydrophilic surface. Method according to one of the preceding claims, in which the graphite-containing material (7) has a graphite content of at least 60%. Method for manufacturing a bipolar plate (1) of a fuel cell (19) or a flow battery, the method comprising: providing a plate-like substrate (5) which consists of a graphite-containing material at least adjacent to an exposed surface (3) of the substrate (5). , and forming at least partial areas (9) of the exposed surface (3) as a hydrophilic surface (3) by means of the method according to one of Claims 1 until 11 . Bipolar plate (1) of a fuel cell (19) or a flow battery, which according to the method claim 12 is made. Energy storage arrangement (25), in particular with at least one fuel cell (19) or flow battery, with a bipolar plate (1) according to Claim 13 .

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