Separator plate having a periodic surface structure in the range from nanometers to micrometers

Abstract

To provide a separator plate improved with regard to the corrosion resistance, electrical contact resistance, production costs and / or service life thereof, and a method for producing the same.SOLUTION: A separator plate has, at least in some regions, periodic surface structures 40 with a mean spatial period of less than 10 μm. A method for producing the separator plate includes the steps of: providing the separator plate; irradiating the separator plate by means of a pulsed laser, a pulse duration of the pulsed laser being less than 1 ns, preferably less than 100 ps; and creating the periodic surface structures 40 on the separator plate by way of the laser irradiation.SELECTED DRAWING: Figure 6A

Description

Technical Field

[0001] The present invention relates to a separator plate for an electrochemical system and a method for manufacturing such a separator plate. The electrochemical system may in particular be a fuel cell system, an electrochemical compressor, an electrolytic cell, or a redox flow battery.

Background Art

[0002] Separator plates can have different functions depending on the application field. On the one hand, separator plates are used to ensure a conductive connection to an adjacent layer (for example, a gas diffusion layer). On the other hand, separator plates are commonly used to supply and / or remove reactants and / or reaction products, in which case a channel structure is usually provided for such purposes. In addition, the reaction heat may be removed by the separator plate, for example, using a coolant. In particular, in the case of mobile applications such as automobiles, it is desirable for the separator plate to be made of metal (because the mechanical stability of the metal makes the design of the fuel cell and the fuel cell stack more compact compared to other materials).

Summary of the Invention

Problems to be Solved by the Invention

[0003] However, although metal is appropriate from an economic point of view and has corrosion resistance that can sufficiently withstand the harsh conditions that usually occur in fuel cells, it has the problem of being easily passivated. For example, stainless steel forms a passivation layer of chromium oxide (which has corrosion resistance initially). However, the passivation layer significantly increases the electrical contact resistance at the contact surface, and as a result, it adversely affects the function of the separator plate for establishing a low-loss electrical connection beyond a reasonable level. The above-mentioned harsh reaction conditions also have a negative impact on the service life of the separator plate.

[0004] Therefore, there is always a need to improve separator plates in terms of corrosion resistance, electrical contact resistance, manufacturing cost, and / or service life.

Means for Solving the Problem

[0005] Accordingly, one object of the present invention is to specify a separator plate that solves at least one of the above problems. Another object of the present invention is to specify a method capable of manufacturing such a separator plate.

[0006] Such an object is achieved by the separator plate according to the main claim and the manufacturing method of the separator plate according to the further independent claims. Advantageous embodiments of the present invention will become apparent from the features of the independent claims and the following description.

[0007] Accordingly, a separator plate for an electrochemical system is provided. The separator plate has a periodic surface structure with an average spatial period of less than 10 μm in at least some regions. Preferably, the average spatial period is at most 2 μm, particularly at most 1.5 μm.

[0008] The periodic surface structures are usually periodically arranged with respect to each other in at least one spatial direction. The periodic surface structures may be periodically arranged with respect to each other in two spatial directions. In some embodiments, the periodic surface structures are arranged adjacent to and parallel to each other, and / or parallel to each other in the front and back directions, in at least some sections. The aligned state may span a relatively large area or a relatively small area. The periodic surface structures extend parallel to each other within a region surrounded at least by grain boundaries. Individual regions having parallel periodic surface structures and having an orientation different from that of another region may be adjacent to each other, for example, by grain boundaries.

[0009] Therefore, the shape of the surface structure is repeated in at least one spatial direction. The spatial period generally means the maximum distance between two adjacent periodic surface structures having the same or similar shapes. Due to manufacturing-related reasons, usually, the periodic surface structures are not completely identical to each other. Rather, they may vary along the surface. Therefore, the average spatial period is specified to be less than 10 μm. The spatial period of the periodic surface structures may be less than 10 μm in any case.

[0010] Such periodic surface structures are generally created by laser irradiation from an ultrashort pulse laser (see below) and are also known in the literature such as "Laser-induced Periodic Surface Structure" (LIPSS). Further explanations, detailed descriptions, and definitions regarding LIPSS are incorporated herein by reference in their entirety from the following literature, namely, "Dynamics of creation and mechanisms of formation of periodic surface structures in the nanometre range (LIPSS) by irradiation of solids with femtosecond laser pulses (Dynamik der Erzeugung und Mechanismen der Entstehung von periodischen Oberflaechenstrukturen im Nanometerbereich (LIPSS) durch die Bestrahlung von Festkoerpern mit Femtosekunden-Laserpulsen)" (a paper by Sandra Hoehm in Berlin in 2014, hereinafter referred to as Hoehm2014). Therefore, the separator plate is generally surface-treated by a laser in the region of the periodic surface structures. The spatial period of the periodic surface structures depends directly, in particular, on the wavelength of the laser light used and is usually of the same order of magnitude as the wavelength of the laser light used.

[0011] The inventors of the present disclosure have found that the periodic surface structure is particularly suitable for use as a separator plate in an electrochemical system. This is because the surface properties of the separator plate can be adjusted in a targeted manner by the periodic surface structure. For example, the periodic surface structure can affect or improve the chemical, electrical, and / or mechanical properties of the surface.

[0012] The periodic surface structure may, for example, extend in a wavy or linear manner along the longitudinal direction. In one embodiment, the periodic surface structure may have depressions and / or elevations. The depressions may extend between the elevations and are typically bounded and / or formed by the latter. In at least some sections, the depressions and / or elevations may extend substantially parallel to each other (e.g., adjacent to each other in parallel or parallel in the front and back). The periodic surface structure often forms a trench structure having a plurality of depressions that are at least locally oriented substantially parallel to each other and extend long. The number of periodic surface structures, depressions, and / or elevations may vary depending on the requirements. For example, the number of depressions may depend on the size of the surface that becomes the surface periodic structure. In regions having similar or identical periodic surface structures, in at least some sections, there are at least 10 or at least 20 trench structures (e.g., depressions) extending parallel to each other. It is also possible to provide different numbers of periods over a specific length in different regions in at least one spatial direction of the surface. When the periodic surface structure is provided in the region of the channel structure of the separator plate, a different number of periods than the number of periods in the recessed region of the periodic surface structure, i.e., particularly in the bottom of the channel and in the section of the channel side wall, may be provided in a specific distance in one spatial direction in the raised region of the channel structure (e.g., the web or the section of the channel side wall).

[0013] The size of the depression, in particular the period, usually depends at least on the wavelength of the laser irradiation used. For example, the depression has a depth of at least 8 nm, preferably at least 20 nm, for example at least 50 nm and / or at most 3 μm, preferably at most 1 μm, in particular at most 500 nm, preferably at most 300 nm, usually at most 250 nm. The depth is usually measured with respect to the normal to the surface formed by the ridge or to the surface of the separator plate without the periodic surface structure. Furthermore, the depression may have a width of at least 0.1 μm and / or at most 2 μm. The width is usually measured at half-height at a position perpendicular to the local longitudinal direction of the depression. In addition, the depression may have a period of at least 100 nm, usually at least 0.3 μm and / or at most 3 μm, preferably at most 1.5 μm, in particular at most 1.2 μm, in particular at most 1000 nm, typically at most 700 μm in one spatial direction. Thus, the periodic surface structure often has a nanostructure with a depth, width, and / or period of less than 1 μm in each case, or in particular a nanostructure with a period slightly exceeding 1 μm.

[0014] The oxygen content of the surface material may be higher in the region of the periodic surface structure than outside the periodic surface structure. In the case of a surface with a higher chromium content than iron content, an increased oxygen content results in a thicker passivation layer, which is particularly advantageous. Such a thick passivation layer increases the corrosion resistance. Thus, the periodic surface structure may have an increased oxygen content compared to the material before laser irradiation or compared to the material in the region without the periodic surface structure. For example, this may be due to the increase in temperature during irradiation.

[0015] Despite the potential increase in oxygen content, the periodic surface structure can result in a decrease in electrical contact resistance and / or an increase in electrical conductivity. Thus, the separator plate has a higher electrical conductivity and / or a lower electrical resistance in the region of the periodic surface structure than outside the periodic surface structure.

[0016] The separator plate may preferably have a coating material different from the material of the separator plate. The coating may be provided, for example, over some regions or the entire surface. For example, in the regions where at least a periodic surface structure is provided, the coating is provided in at least some sections or completely (especially using a coating material that improves conductivity). The coating may include one or more of the following substances: conductive oxides, carbon, preferably a conductive carbon layer, noble metals such as gold, silver, platinum, metals such as titanium, chromium, metal nitrides, especially TiN, CrN, Cr2N, metal carbides, metal borides, metal silicides, and / or silicon carbide, and may also consist of one or more of the above substances or their alloys. The conductivity is not based only on the separator plate, but rather is generally compared based on an overall system consisting of two separator plates connected to each other and gas diffusion layers or gas diffusion media disposed on both sides of the composite. For test purposes, it is also possible to measure a single metal layer used for the separator plate in combination with the gas diffusion media usually applied to both of its sides.

[0017] The separator plate usually has two planes located on opposite sides. In certain embodiments, the periodic surface structure may be provided on only one of the planes or on both sides of the plane. In one embodiment, a periodic surface structure is provided over the entire surface of at least one of the planes. In other embodiments, only one region of the separator plate has a periodic surface structure, or several regions have it. Thus, there may be at least one region without a periodic surface structure. The two planes of the separator plate may have a reactant side or gas side and a coolant side. The periodic surface structure may be disposed on the reactant side or gas side of the separator plate. Additionally or alternatively, the periodic surface structure may be disposed on the coolant side of the separator plate.

[0018] The separator plate is preferably configured as a metal separator plate, in which case the separator plate is preferably manufactured from a metal sheet such as a stainless steel sheet. The separator plate usually has a flow field for guiding the reaction medium along the plane side of the separator plate. In one embodiment, the periodic surface structure is provided in the region of the flow field. According to one example, the periodic surface structure is provided only in the region of the flow field, preferably only in the electrochemically active region of the flow field. According to one preferred embodiment, the separator plate has a plurality of webs and channels formed between the webs. The webs and channels may in particular form a flow field for guiding the reaction medium along one of the plane sides of the separator plate. The webs generally form a contact surface for compressing the diffusion layer in particular. It is preferred that at least the webs have a periodic surface structure. The periodic surface structure may be present on the outside of the web, for example in the region of the channel. Alternatively, only the web may have a periodic surface structure, particularly in the region of the contact surface. Surprisingly, it has been found that the electrical resistance in the region of the contact surface is reduced by the use of the periodic surface structure. In other words, the electrical resistance in the region of the contact surface between the web and the diffusion layer is high if the periodic surface structure is not formed on the web.

[0019] Furthermore, on the opposite surface of the separator plate, a web may be formed on the rear side of the channel (hereinafter, such a web is referred to as a back web). In the assembled state, the back web is arranged opposite an equivalent back web. Also in this case, it has been found that the electrical resistance of the contact surface can be reduced by the use of the periodic surface structure.

[0020] Partial surface laser treatment that only treats the web and / or the back web may be applied to substantially the entire surface of each web or back web. However, it is also possible to use a laser to provide it only to the sections of the web or back web that have a periodic surface structure.

[0021] A bipolar plate for an electrochemical system is also provided by the present invention. The bipolar plate has two of the above-described separator plates. The two separator plates are joined to each other and are preferably firmly bonded to the surrounding environment (e.g., hermetically welded). In each case, the webs of the two separator plates form a contact surface for compressing the gas diffusion layer. The back web is formed on the surface facing the rear side of the channel, i.e., the inner side of the separator plate, and the two separator plates are generally in contact with each other in the region of the back web. Both the contact surface between the web and the gas diffusion layer and the contact surface of the adjacent back web have a low electrical resistance in the region of the periodically structured surface when laser fabricated.

[0022] The separator plate may be configured to guide a coolant. For such a purpose, the separator plate may preferably have a plurality of cooling channels on the planar side of the separator plate located on the opposite side of the flow field of the reaction medium, i.e., inside the bipolar plate. In this case, the two separator plates are not only connected to each other in a liquid-tight manner at a position slightly away from the outer edge, but also have a substantial bonding connection, particularly a welded joint, in the electrochemically active region of the flow field, particularly in the contact zone formed by the web, in order to improve conductivity. It has been found that the resistance is also reduced at such an interface by the periodically structured surface. Therefore, the periodically structured surface is preferably present over the entire surface or in some sections, particularly in the contact surface region between the separator plates, i.e., particularly in the aforementioned back web, on the coolant side of the separator plate. Therefore, the periodically structured surface may be present on one or both surfaces of the separator plate, i.e., on the coolant side and / or the reactant side.

[0023] As described above, the periodic surface structure significantly reduces the electrical contact resistance and / or significantly increases the conductivity. It has been found that due to such an improvement in conductivity, a substantial bonding connection, such as a welded joint in the contact zone as described above, can be omitted. Thus, in one embodiment, the bipolar plate does not need to use a substantial bonding connection, such as a welded joint, in the flow field or in the electrochemical active region on the coolant side, and in the contact zone between the two separator plates where the periodic surface structure is arranged. In this case, usually, the contact zone is formed by the back web of the separator plate. In particular, it is not necessary to apply a coating for increasing conductivity to the surface on the cooling side.

[0024] An electrochemical system having a plurality of such separator plates or bipolar plates is also proposed by the present invention. The electrochemical system may in particular be a fuel cell system, an electrochemical compressor, an electrolyzer, or a redox flow battery.

[0025] Known electrochemical systems of the above type usually have a number of electrochemical cells separated from each other by bipolar plates. Such bipolar plates may have, for example, the function of electrically contacting the electrodes of individual electrochemical cells (e.g., fuel cells) and / or the function of electrically connecting adjacent cells (connecting the cells in series). The bipolar plate is generally formed by two joined individual plates (also called separator plates within the scope of this specification). The individual plates may be joined by a substantial joining method, such as one or more welded joints, in particular one or more laser welded joints.

[0026] The bipolar plate or separator plate may each have, for example, a structure configured to supply one or more media to an electrochemical cell disposed between adjacent bipolar plates, and / or a structure configured to remove reaction products, or may be formed as such. The media may be a fuel (e.g., hydrogen or methanol), a reaction gas (e.g., air or oxygen), or a coolant. Further, the bipolar plate may be configured to conduct forward waste heat generated during the conversion of electrical energy and / or chemical energy in the electrochemical cell, and may be configured to seal various media channels and / or cooling channels to each other and / or to the outside.

[0027] Furthermore, the bipolar plate usually has at least one opening each, through which media and / or reaction products can be led to or excluded from an electrochemical cell disposed between adjacent bipolar plates in the stack. The electrochemical cell generally has one or more membrane electrode assemblies (MEAs) each. In addition, a gas diffusion layer may be provided, in which case the gas diffusion layer is usually disposed between the MEA and the bipolar plate in each case and is configured, for example, as a non-woven metallic fabric or carbon fabric.

[0028] A method for manufacturing a separator plate for an electrochemical system is also provided by the present invention. The method includes the following steps, namely, providing a separator plate, irradiating the separator plate with a pulsed laser, wherein the pulse duration of the laser pulse is less than 1 ns, preferably less than 100 ps, and creating a periodic surface structure on the separator plate by the laser irradiation.

[0029] In this case, the plurality of periodic surface structures described above are generally produced by irradiating a separator plate with spatially adjacent laser pulses. A plurality of periodic surface structures can be produced for each laser pulse. The production of each periodic surface structure by each laser pulse is completed before the next laser pulse is irradiated. At least 5, at least 10, or at least 20 periodic surface structures, i.e., particularly trench structures, can be produced for each laser pulse. Therefore, the periodic surface structures are those produced by each laser pulse within adjacent surfaces irradiated by each laser pulse, and are not, for example, those produced by spatially periodically scanning a separator plate or those irradiated using a spatially periodic non-adjacent light pattern, such as a diffraction pattern or an interference pattern.

[0030] Importantly, the laser pulse has a pulse duration of less than 1 ns, preferably 100 ps, particularly less than 10 ps, or for example less than 1 ps. The laser pulse preferably has a pulse frequency of less than 1 MHz. In particular, it is advantageous if the ratio of the pulse frequency to the pulse duration is at least 1000. Such a short pulse duration and relatively low pulse frequency can achieve the extremely high intensities required to perform surface ablation and / or to rearrange surface materials. On the other hand, since the pulse duration is short relative to a fairly long dead time, the surface material can be processed without substantial heat diffusion, and thus periodic surface structures can be produced.

[0031] In a preferred embodiment, the pulse duration is less than 100 ps, less than 50 ps, less than 20 ps, less than 10 ps, and further less than 1 ps. In some embodiments, a pulse duration in the fs range, for example, greater than 30 fs and / or less than 1000 fs and / or less than 500 fs, preferably greater than 50 fs and / or greater than 100 fs is used. Therefore, in the method of the present invention, in particular, picosecond or femtosecond lasers (collectively referred to as ultrashort pulse lasers) can be used.

[0032] The periodic surface structure usually has the shape of a periodic trench structure, and its shape depends on process parameters. Possible process parameters are disclosed in Hoehm2014.

[0033] In particular, the periodic surface structure may be produced by the interaction between the incident laser light and the irradiated surface. Due to the above interaction, an inward coupling of spatially adjusted energy occurs between the material, and as a result of ablation, a periodic surface structure is produced. The periodic surface structure is generally caused by the optical interference between the incident laser light and the electromagnetic surface wave in the material of the separator plate generated by the laser pulse. The fluence of the laser irradiation is in the range of the order of the ablation threshold of the material used for the separator plate. The fluence of the laser light should preferably be at least large enough to enable ablation of the material. The fluence may be selected, for example, such that the difference from the ablation threshold of the material used for the separator plate is at most 20%. Fluence is the unit of the energy density of the laser pulse and is generally expressed in J / cm 2 For example, the fluence is at least 0.1 J / cm 2 and / or at most 10.0 J / cm 2 The repetition rate of the laser may be, for example, at least 10 Hz, preferably at least 1 kHz and / or at most 1000 kHz, preferably at most 20 kHz. A low repetition rate results in a rather long dead time, thus limiting the total energy input and only adjusting the surface layer.

[0034] In one preferred embodiment, the laser irradiation is linearly polarized. The periodic surface structures are generally oriented perpendicular to the polarization direction of the incident laser light. This applies in particular to the core region of the irradiation area and especially to the regions extending within the grain boundaries of the untreated sheet. In regions that are adjacent to each other but separated from each other by grain boundaries, the periodic surface structures may, in contrast, have different orientations. The average spatial period of the periodic surface structures may be at least 2%, preferably 5%, in particular at least 20% and / or at most 200%, preferably at most 120% of the laser wavelength used. Thus, the separator plate may have various regions each having a periodic structure but with different orientations in different regions. In that case, the above regions may be adjacent to each other.

[0035] The most common LIPSS are referred to in the above literature as "Low Spatial Frequency LIPSS" (LSFL) (see Hoehm 2014). LSFL has an orientation perpendicular to the polarization of the laser beam and a period in the wavelength region of the laser used. Depending on the angle of incidence of the light on the surface, there are both the possibility that the period varies with each irradiation and the possibility that the orientation of the LIPSS rotates at an angle of up to 90 degrees. The above literature reports that the period P of LSFL is in the order of magnitude of the wavelength λ of the incident laser light, i.e., for example, P≒λ. The second type of LIPSS, so-called HSFL ("High Spacial Frequency LIPSS"), has a period P HSFL that is significantly smaller compared to the wavelength of the laser (P HSFL <<λ). Its directionality is linked to the polarization of the radiation in a material-dependent manner and is usually parallel or perpendicular to it.

[0036] Due to such variability, the periodic surface structures are produced by a single laser beam. The surface of the separator plate can be continuously scanned by the laser beam.

[0037] The laser beam or laser pulse irradiated onto the separator plate may have a beam diameter or minimum lateral size of at least 20 μm, preferably at least 40 μm. In other words, the adjacent irradiation of the laser beam onto the separator plate (also called a laser spot) may have the above beam diameter or the above minimum lateral size of at least 20 μm, preferably at least 40 μm. When using a line-type laser, the laser line has a width (minimum lateral size) of at least 20 μm, particularly at least 40 μm.

[0038] In another embodiment, at least two laser beams, particularly two linearly polarized laser beams, are superimposed. By superimposing them, an interference pattern for producing a periodic surface structure can be formed. A diffraction pattern may be used to produce the periodic surface structure. By using the interference pattern or the diffraction pattern, it is no longer necessary to scan the surface of the separator plate with only one laser beam. Therefore, the method can be implemented much more quickly overall. For such a purpose, it is preferable to divide the laser beam of the laser into two partial beams. The interference pattern or the diffraction pattern usually has a plurality of spatially non-adjacent light spots. It should be noted that the spatial period of the periodic surface structure is established by the interaction between the short laser pulse and the separator plate, rather than the spatial period of the interference pattern or the diffraction pattern of the laser irradiation (see also the above). Therefore, the spatial period of the periodic surface structure is different from the spatial period of the interference pattern or the diffraction pattern of the laser irradiation, and is usually significantly smaller (for example, 10 times smaller) than that.

[0039] Comparative measurements between an untreated single sheet and a single sheet surface-treated on both sides with a laser, i.e., a single sheet without a typed channel structure, showed a decrease in volume resistance in each case when compressed at the same pressure against gas diffusion layers from the same batch on both sides. However, the decrease in volume resistance in the sheet with the LSFL structure (showing an average decrease of 87%) is much more significant than that in the sheet with the HSFL structure (showing an average decrease of only 76%). All the single plates used in this comparison are from the same batch. Also in this case and in the following, the change in surface contact resistance is derived from the change in volume resistance.

[0040] In addition, the separator plate may be provided with a coating material. The coating of the separator plate is preferably provided after the periodic surface structure is formed. In particular, in one embodiment of the method, at least the region provided with the periodic surface structure may be coated, at least in some sections or completely, with the coating material. In particular, the separator plate may be coated with a coating material that improves conductivity. Conductivity is generally compared not only based on the separator plate alone but based on a bipolar plate consisting of two separator plates (connected to each other with a gas diffusion layer or a gas diffusion medium arranged on both sides). For test purposes, it is also possible to measure a single metal layer used for the separator plate in combination with the gas diffusion medium usually applied on both sides thereof.

[0041] The coating may be provided by a plasma-based method, in particular PVD (physical vapor deposition), CVD (chemical vapor deposition), or PA-CVD (plasma-assisted CVD). Other methods for applying the coating to the separator plate include, for example, spraying, pen plating, screen printing, roller printing, stencil printing, metering processes, etc.

[0042] The method may have an additional step, i.e., a step of removing a section of the coating by further laser treatment. Such a coating may include the coating described above or another coating. For such a purpose, removal by the same laser is also possible. Alternatively, another laser may be used to remove the coating. The coating may be removed before the periodic surface structure is produced, but particularly when the material that coats the entire surface is used as the starting material, it may be preferable to remove it in some sections.

[0043] The separator plate may have a plurality of webs and channels formed between the webs. At least the webs may be irradiated with a laser. As a result, a periodic surface structure can be produced on the webs. Further characteristics of the separator plate are as described above.

[0044] It should be noted that according to the above method, the step of forming a flow field and the step of irradiating the separator plate with a pulsed laser may be interchanged. Therefore, the periodic surface structure of the separator plate may be performed before or after forming the flow field in the sheet. The flow field is generally formed by deep drawing or stamping of the separator plate.

[0045] The method can be used in particular to manufacture the separator plate described above. Features described only in the context of the separator plate, bipolar plate, or electrochemical system are also applicable in the method, and vice versa.

Brief Description of the Drawings

[0046] Exemplary embodiments of separator plates, bipolar plates, and electrochemical systems are shown in the drawings and are further described in the following description.

[0047]

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DETAILED DESCRIPTION OF THE INVENTION

[0048] In the following, features repeated in various drawings are denoted by the same or similar reference numerals.

[0049] Figure 1 shows an electrochemical system 1 of the type proposed in the present application, which has a plurality of structurally identical metal bipolar plates 2 arranged in a stacked manner along the z-direction 7. The bipolar plates 2 in the stack are compressed between two end plates 3, 4. The z-direction 7 is also called the stacking direction. The bipolar plate 2 usually has two metal separator plates 2a, 2b each, and the separator plates are connected to each other (see, for example, Figures 2 and 3). In the present example, the system 1 is a fuel cell stack. Thus, two adjacent bipolar plates 2 in the stack each have an electrochemical cell therebetween that has, for example, the function of converting chemical energy into electrical energy. The electrochemical cells usually each have a membrane electrode assembly (MEA) 10 (see, for example, Figures 2 and 3). The MEA generally has at least one membrane, for example an electrolyte membrane, in each example. Further, a gas diffusion layer (GDL) may be disposed on one or both sides of the MEA.

[0050] In an alternative embodiment, system 1 may be configured as an electrolytic cell, an electrochemical compressor, or a redox flow battery. The bipolar plate may be used in such an electrochemical system. In the electrolytic cell, the electrochemical compressor, or the redox flow battery, the medium conducted through the bipolar plate is different from the medium used in the fuel cell system, but the structure of such a bipolar plate may correspond to the structure of bipolar plate 2 described in detail herein. The same applies to the separator plate.

[0051] The z-axis 7 follows a right-handed Cartesian coordinate system together with the x-axis 8 and the y-axis 9. The bipolar plate 2 and the separator plates 2a, 2b each define a plate surface, and the plate surfaces of the separator plates 2a, 2b are each parallel to the x-y plane and thus perpendicular to the stacking direction, i.e., the z-axis 7. The end plate 4 has a plurality of medium ports 5 through which a medium can be supplied to or discharged from the system 1. Such a medium that can be supplied to or discharged from the system 1 may include, for example, fuels such as hydrogen molecules and methanol, reaction gases such as air and oxygen, reaction products such as water vapor and spent fuel, or coolants such as water and glycol.

[0052] Figure 2 shows in perspective two bipolar plates 2 well-known in the prior art and which can be used also in an electrochemical system of the type shown in FIG. 1 for example. FIG. 2 shows a membrane electrode assembly (MEA) 10 disposed between said adjacent bipolar plates 2, although the MEA 10 of FIG. 2 is in a substantially hidden state by the bipolar plate 2 facing the reader side. The bipolar plate 2 is formed by two single plates, namely separator plates 2a, 2b, which are connected in a substantially joined state (see for example FIG. 3), and in each case only the first single plate (facing the reader side and hiding the second single plate) can be seen in FIG. 2. Said single plates are each formed by a shaped metal sheet, for example a stamped or deep-drawn stainless steel sheet. Such a metal sheet has a thickness of at most 150 μm, preferably at most 100 μm, preferably 90 μm, and particularly preferably at most 80 μm. The single plates may be welded to each other, for example by laser weld joints.

[0053] The single plates usually have openings which are aligned with each other and form the openings 11a-c of the bipolar plate 2. When a plurality of bipolar plates 2 are stacked, the openings 11a-c, 11'a-c form media channels extending in the stacking direction 7 through the stack of the system 1 (see FIG. 1). Each of the media channels formed by the openings 11a-c, 11'a-c is generally fluidly connected to one of the ports 5 of the end plate 4 of the system 1. For example, a coolant may be introduced into the stack through the media channel formed by the opening 11a and discharged from the stack through the opening 11'a. In contrast, the lines formed by the openings 11b, 11c may be configured to supply fuel and reaction gas to the electrochemical cells of the fuel cell stack of the system 1, and the lines formed by the openings 11'b, 11'c may be configured to discharge reaction products from the stack.

[0054] The first separator plate, which faces the reader side of FIG. 2 and is located on the front side, has a flow field 17 with a structure for guiding the reaction medium along the front side of the separator plate. Such a structure of the electrochemical active region 18 is shown in FIG. 2 by a plurality of webs 15 and channels 16 (the channels extend between the webs 15 and are bounded by the webs 15). On the rear side of the channel, i.e., the opposite surface of the separator plate, a back web 19 is formed in the region where the separator plates 2a and 2b are compressed against each other. On the front side of the bipolar plate 2 facing the reader side of FIG. 2, the first separator plate 2a further has a distribution and collection region 20 for the reaction medium. On the opposite side, i.e., the rear side of the separator plate 2a, which is the inner side of the bipolar plate 2, there is a distribution and collection region (outside the visible region) for the coolant. The distribution region 20 has a structure configured to distribute the medium introduced from the first of the openings 11a, 11b, 11c, 11b into the distribution region 20 throughout the active region 18. The collection region 20 has a structure configured to collect or pool the medium flowing from the active region 18 to the first of the openings 11'a, 11'b, 11'c, 11'b. For such a purpose, the distribution and collection region 20 has a guiding structure defined by a web 35 and a channel 36 formed between the web 35 in FIG. 2. The channel 16 is preferably fluidly connected to one of the openings 11b, 11'b through the channel 36. Accordingly, the electrochemical active region 18 is fluidly connected to the openings 11b, 11'b through the distribution and collection region 20.

[0055] The structure of the active area 18 and the guiding structures of the distribution area 20 and the collection area 20 are each formed singly together with the separator plates 2a and are integrally formed within the separator plates 2a, 2b, for example, by stamping, hydroforming, or deep drawing processes. Similar content is usually also applicable to the corresponding guiding structures of the second separator plate 2b. The guiding structure of the flow field 17 may have a plurality of webs 15 and any channels 16 formed between the webs 15 in the electrochemical active area 18, and may also have a plurality of webs 35 and any channels 36 formed between the webs 35 in the distribution and collection areas 20.

[0056] FIG. 2 also shows two rectangular end regions 21 of the flow field 17, which are arranged at the ends of the electrochemical active area 18 facing the direction of the distribution and collection areas 20 and extend longitudinally across the entire width of the flow field 17 so as to cross the path of the channels 16 of the flow field 17. In the stack of the system 1, the above-mentioned end regions 21 each function to accommodate and compress the extended range of the membrane electrode assembly (MEA) 10. The end region 21 will hereafter be referred to as the transition region 21. As can be seen in FIG. 3, since the transition region 21 is recessed compared to the active area 18 (flow field 17) and the distribution or collection area 20, the separator plates 2a, 2b or the bipolar plate 2 as well as the MEA 10 have optimal compression within the stack. Further details are described in WO2018 / 114819A1, which is incorporated herein by reference.

[0057] FIG. 3 is a partial cross-sectional view of the stack of the electrochemical system 1 of FIG. 1. The stack has a plurality of structurally identical bipolar plates 2 of the same type as shown in FIG. 2. The cutting plane is positioned perpendicular to the flat surface of the bipolar plate 2 or the separator plates 2a, 2b and is by the cutting line A-A emphasized in FIG. 2. The active area 18, the transition region 21, and the distribution or collection area 20 of the separator plates 2a, 2b or the bipolar plate 2 are particularly emphasized.

[0058] In each case, the membrane composite 29 is disposed between two adjacent bipolar plates 2 within the stack. Each membrane composite has a membrane electrode 26 having an ionomer and at least one catalyst layer, gas diffusion layers 27 disposed on both sides of the membrane electrode 26, and reinforcing layers 28 disposed on both sides of the membrane electrode 26 in some sections. The membrane electrode 26 and the reinforcing layer 28 together form a membrane electrode assembly (MEA) 10. For clarity of explanation, FIG. 3 shows a stack in a state where the bipolar plate 2 and the membrane composite 29 are not fully compressed along the z direction 7 (i.e., perpendicular to the flat surfaces of the bipolar plate 2 or the separator plates 2a, 2b).

[0059] In the region of the flow field 17, i.e., the electrochemical active region 18 of the bipolar plate 2, the membrane composite 29 only has the membrane electrode 26 and the gas diffusion layers 27 disposed on both sides of the membrane electrode, respectively. The membrane electrode 26 has catalyst coatings on both sides. In the distribution or collection region 20, the membrane composite 29 only has two reinforcing layers 28 (usually interconnected) respectively. Each membrane composite 29 has a reinforcing region 30 only in the region corresponding to the transition region 21 of the separator plate 2. In each case, the reinforcing region has the membrane electrode 26, the reinforcing layers 28 disposed on both sides of the membrane electrode 26, and the gas diffusion layers 27 disposed on both sides of the membrane electrode 26 and both sides of the reinforcing layer 28. The reinforcing region 30 of the membrane composite 29 generally forms the end of the electrochemical active region of the membrane composite 29. Further, in the transition region 21, the separator plate has a web 45 and channels 46 formed between the webs 45.

[0060] The separator plates 2a, 2b are usually in contact with each other in a contact area formed by the rear sides of the depressions 16, 36, 46 and are compressed against each other at that part. The separator plates 2a, 2b are usually connected by a substantial bonding connection such as a laser welding joint in order to improve conductivity and / or to reduce the volume resistance in the active area 18 and optionally in the transition area 21. On the other hand, in the distribution area 20, it is preferable to use a laser welding joint in order to avoid local expansion of the channels, that is, to define the volume available for the fluid flow. Therefore, the laser welding joint 24 is provided, for example, in the flow field 17, particularly in the electrochemical active area 18, the transition area 21, and the distribution area 20, particularly in their contact zones or contact areas. In the x-direction (i.e., the flow direction of the cooling fluid), for example, the welding joints 24 may form continuous welds or stitch welds (each having a row of linear welding sections), particularly spaced apart from each other.

[0061] According to the present invention, the separator plates 2a, 2b have a periodic surface structure 40 with an average spatial period of less than 10 μm in at least some areas. That is, the periodic surface structures 40 are arranged at periodic intervals. The periodic surface structure 40 is formed on the surfaces of the separator plates 2a, 2b by irradiation with an ultra-short pulse laser. In particular, one adjacent area or several adjacent areas of the separator plates 2a, 2b may have the periodic surface structure 40. The periodic surface structure 40 will be further described below in connection with FIGS. 4A to 6H.

[0062] Figures 4A, 4B, and 5 show highly magnified images of a periodic surface structure 40 formed by laser surface treatment on the surface of a metal sheet such as a stainless steel sheet. The enlarged extracted images are extracted images from the surface of the separator plate or the metal sheet for the separator plate. The stainless steel sheet may be formed into the separator plates 2a, 2b by stamping, hydroforming, or deep drawing processes. Alternatively, the separator plates 2a, 2b may first be formed by stamping, hydroforming, or deep drawing processes, and then the periodic surface structure 40 may be provided.

[0063] The periodic surface structure 40 may be formed on one side and / or both sides of the separator plates 2a, 2b, that is, on the coolant side and / or the reactant / gas side.

[0064] As can be seen from FIGS. 4A to 6H, the periodic surface structure 40 (hereinafter, the surface structure 40) preferably has a plurality of depressions 42 and protrusions 44. The depressions 42 extend between the protrusions 44 and are delimited and / or formed by the latter. The surface structures 40 are periodically arranged relative to each other in at least one spatial direction x, y. For example, the surface structures 40 may be aligned with each other along the longitudinal direction. For example, the surface structures 40, that is, the depressions 42 and the protrusions 44, extend substantially parallel to each other. In particular, the surface structures 40 are arranged adjacent to each other in parallel and / or in front and behind. For example, as shown in FIGS. 6C and 6F, the surface structures 40 are arranged adjacent to each other in parallel, that is, perpendicular to the longitudinal direction of the surface structures. Further, FIG. 5 shows that the surface structures 40 are arranged parallel to each other (in the longitudinal direction) in front and behind and are also arranged adjacent to each other in parallel. Such surface structures 40 can also be seen in FIGS. 6B and 6H in the same manner.

[0065] The surface structure 40 may extend, for example, in a wavy or linear manner along the longitudinal direction. Examples of the wavy extension of the surface structure 40 are shown in FIGS. 6C and 6F.

[0066] Figure 6G shows the surface structure 40, particularly the depth t, width b, and period Px of the depression 42. The surface structure 40 may have a depth t of at least 8 nm, preferably at least 5 nm and / or at most 3 μm, preferably at most 1 μm, particularly at most 500 nm and / or at most 300 nm and / or at most 250 nm. In the current example, t = 0.4 μm or t = 100 nm. In one exemplary embodiment, the surface structure 40 has a width b of at least 0.1 μm and / or at most 2 μm. In the current example, b = 0.45 μm. Additionally, the surface structure 40 may have a period Px of at least 0.3 μm and / or at most 3 μm in one spatial direction. In the current example, it is 1 μm. In Figure 6G, the period Px means the lateral spacing between two adjacent ridges 44.

[0067] In Figure 6H, the surface structure 40 has a length l of 5 μm in one spatial direction y. The surface structures are arranged parallel to each other with a period Py of 5 μm.

[0068] Due to the surface structure 40, the surfaces of the separator plates 2a, 2b have different chemical, electrical, and / or mechanical properties from the regions of the separator plates 2a, 2b that do not have the surface structure 40. For example, the oxygen content of the surface material of the separator plates 2a, 2b may be higher in the region of the periodic surface structure 40 than outside the periodic surface structure 40 due to the surface structure 40.

[0069] Surprisingly, despite the increase in oxygen content, it was found that the conductivity of the separator plates 2a and 2b combined with the gas diffusion layer was higher in the region of the surface structure 40 than outside the surface structure 40. FIG. 9 shows the comparative measurement values in a composite formed by a stainless steel sheet and each gas diffusion layer (GDL) on both sides. The stainless steel sheet has a thickness of 75 μm, and each gas diffusion layer has a thickness of about 190 μm. The electrical volume resistance was measured in different portions 1 to 6 of the composite formed by the stainless steel sheet and the gas diffusion layer. In portions 1 to 4, the stainless steel sheet has the aforementioned surface structure 40 with different contents. In portions 5 to 6, the stainless steel sheet does not have the surface structure 40. Two measurements A and B were carried out for each portion. It can be seen from the measurement results shown in FIG. 9 that the volume resistance of portions 1 to 4 provided with the surface structure 40 is significantly lower than the volume resistance in portions 5 to 6 without the surface structure 40. That is, due to the low electrical resistance, portions 1 to 4 having the surface structure 40 have improved conductivity. Therefore, it is particularly preferable to provide the surface structure 40 in the contact region with the gas diffusion layer. In other words, in particular, the web 15 of the active region 18 may have the surface structure 40. The difference between the range of the decrease in electrical resistance in the surface structure of the present case provided with the LSFL structure and the decrease value of the aforementioned LSFL structure may be due to different sheet batches and different laser parameters.

[0070] The separator plates 2a and 2b may have one or more coatings (not shown). Such coatings may be provided to improve the conductivity and / or corrosion resistance of the separator plates 2a and 2b. The coating may be applied to the entire surface. Alternatively, the coating is provided in a specific region of the separator plates 2a and 2b. It is preferable that the coating is applied to at least the region where the periodic surface structure 40 is provided, and in particular, it is preferable to provide a coating material for improving conductivity. The coating may be provided only in the region of the web 15 and omitted in the region of the channel 16.

[0071] The coating may include, for example, one or more of the following substances, namely, conductive oxides, carbon, preferably a conductive carbon layer such as graphite, noble metals such as gold, silver, platinum, metals such as titanium, chromium, metal nitrides, especially TiN, CrN, Cr2N, metal carbides, metal borides, metal silicides, and / or silicon carbide, or may consist of one or more of the above substances or their alloys.

[0072] A method for manufacturing a separator plate for an electrochemical system is described below. The method is particularly suitable for manufacturing the above-described separator plates 2a, 2b.

[0073] The method is characterized by laser treatment using the laser 100 shown in FIGS. 7 and 8A. The laser treatment is shown in an extracted image from the separator plates 2a, 2b in which the channels 16 and the webs 15 are already provided. The treatment may be similarly applied to the region of the back web 19 and the coolant channels formed between the back webs, i.e., the coolant side of each separator plate 2a, 2b. FIG. 8B shows some preferred components of the laser 100, namely, the laser head 101, the first mirror 102, the second mirror 103, the λ / 2 plate 104, a polarizer, especially a linear polarizer 105, the beam splitter 106, the cap 107, and the lens 108. Of course, different configurations of the laser 100 are also possible.

[0074] In this method, a pulsed laser 100 is particularly used, and each pulse has a pulse duration of less than 1 ns, preferably less than 100 ps. Thus, the laser 100 can be, in particular, a picosecond laser (pulse duration less than 1 ns and greater than or equal to 1 ps) or a femtosecond laser (pulse less than 1 ps, in particular less than 500 fs and / or greater than or equal to 30 fs). The laser 100 preferably generates linearly polarized laser irradiation. The beam diameter or minimum lateral size of the laser parallel to the surfaces of the separator plates 2a, 2b can be, for example, at least 20 μm and / or at most 2 mm, and in the example shown, it is about 60 μm. The wavelength λ generated by the laser 100 is, for example, between 200 nm and 2000 nm, preferably between 400 nm and 1500 nm. Typical wavelengths are, for example, 700 - 1000 nm corresponding to a titanium sapphire laser system, 1064 nm (fundamental wavelength) or 532 nm, 355 nm, or 266 nm (frequency doubling) corresponding to a neodymium YAG laser system. The fluence of the laser should be selected as a function of the material of the separator plates 2a, 2b, and can be, for example, at least 0.1 and / or at most 10.0 J / cm 2 and may be. The repetition rate of the laser is, for example, at least 10 Hz, preferably at least 1 kHz and / or at most 1000 kHz, preferably at most 20 kHz.

[0075] Hoehm2104 describes in detail the interaction between the laser irradiation and the material for producing the periodic surface structure 40, and the advantageous combinations of laser parameters are also published in Hoehm2104. For this reason, no further explanation is necessary here.

[0076] The method has at least the following steps. · Step of providing separator plates 2a, 2b. · Step of irradiating the separator plates 2a, 2b with the pulsed laser 100. · Step of producing a periodic surface structure 40 on the separator plates 2a, 2b by laser irradiation.

[0077] For example, when only the area of the web 15 or the area of the back web 19 is processed, the laser beam may be directed parallel to the web direction, perpendicular to the web direction, or at an angle different from these. In particular, when performing laser surface treatment on the back web 19 of the two separator plates 2a, 2b connected to form the bipolar plate 2, the laser may be directed, for example, in the same direction, in the opposite direction, or perpendicularly in each case.

[0078] The production of the plurality of periodic surface structures 40 described above is already completed before the next laser pulse is irradiated onto the surfaces of the separator plates 2a, 2b. For example, for each laser pulse, at least 10 or at least 20 surface structures, particularly trench structures, may be produced. The surface structures 40 are generally arranged perpendicular to the linear polarization direction of the incident laser light. Therefore, the laser 100 may be directed onto the surfaces of the separator plates 2a, 2b in such a way that surface structures 40 with a desired directionality are produced. This is particularly the case in the core region of the irradiated area. When the laser pulse is irradiated onto the surfaces of the separator plates 2a, 2b, the incident laser light causes interference with the electromagnetic surface wave of the surface material of the separator plates 2a, 2b generated by the laser pulse. Such interaction forms the periodic surface structure 40.

[0079] The average spatial period Px of the surface structure 40 usually depends on the wavelength λ of the laser 100. In the case of a metal (metal sheet, stainless steel sheet), the period P is generally within the order of magnitude of the frequency λ. For example, the average spatial period Px of the surface structure 40 is at least 2%, preferably at least 5%, particularly at least 20% and / or at most 200%, preferably at most 120% of the laser wavelength used.

[0080] In principle, a single laser beam is sufficient to fabricate the surface structure 40. Such a laser beam can scan the surfaces of the separator plates 2a, 2b to be processed. In this case, the plurality of periodic surface structures are fabricated by individual laser pulses within the spatial irradiation of the laser light onto the separator plates. If an interference pattern or a diffraction pattern is formed by at least two laser beams, the method is accelerated, so that the surface structure can be fabricated by scanning the surface with the interference pattern. For such a purpose, the linearly polarized laser beam of the laser 100 is preferably split by a beam splitter 106. The two linearly polarized partial beams fabricated in such a manner are used to form an interference pattern. The interference pattern of the laser beam used only enlarges the surface area to be scanned and does not directly affect the periodicity of the adjacent surface structures 40. Therefore, the spatial period of the surface structure 40 is different from the spatial period of the interference pattern or the diffraction pattern and is usually significantly smaller (for example, 10 times smaller) than that. However, comparative measurements show that the volume resistance cannot be reduced to the same level as in the case of using a single laser beam in such a method for fabricating the surface structure applied by the acceleration method. In addition or as an alternative method, a line-type laser may be used, in which case the laser line preferably has a width of at least 20 μm.

[0081] It is preferable to irradiate at least the webs 15 and / or the back webs 19 of the separator plates 2a, 2b with the laser 100, and as a result, the surface structure 40 is formed on the webs 15 and / or the back webs 19. In this case, the webs 15 and / or the back webs 19 and / or the electrochemically active regions 18 of the flow field 17 may be formed before or after the laser treatment. In other words, the steps (for example, deep drawing or die stamping) necessary to form the separator plates 2a, 2b by shaping a flat stainless steel sheet are performed before or after the irradiation with the laser 100.

[0082] To improve the conductivity of the separator plates 2a, 2b, for example, a coating may be applied to the separator plates 2a, 2b. The coating may have a thickness of at least 10 nm and / or at most 100 μm, preferably at least 50 nm and / or at most 50 μm. Possible materials for the coating are described above. It is particularly advantageous if the coating is applied to the surface structure 40. Depending on the type of coating, various coating processes can be envisaged. For example, the coating may be performed by PVD, CVD, or PA-CVD. Methods in which the technical conditions are not too strict for applying the coating to the separator plates 2a, 2b include, for example, spraying, pen plating, screen printing, roller printing, stencil printing, metering processes, etc. The coating material may include a thermoplastic or thermosetting binder for applying the coating material in a liquid state to the separator plate. When coating the entire surface, in some sections, the coating material may be removed in the next step. For such a purpose, a laser 100 or another laser configured to remove the coating material can be used.

[0083] Figure 10 shows further comparative measurements for various test specimens C1 - C3, D1 - D4, each test specimen comprising a composite forming two separator plates 2a, 2b having gas diffusion layers (GDLs) applied on both sides. The separator plates of test specimens D1 - D4 have a surface structure with LSFL laser surface treatment applied substantially over the entire surface in the active regions on both surfaces, while such surface treatment is not applied to test specimens C1 - C3. Except for the laser surface treatment, test specimens C1 - C3, D1 - D4 are identical. The separator plates 2a, 2b and the gas diffusion layers of test specimens C1 - C3, D1 - D4 are compressed at 1250 N. In addition, test specimens C1 - C3, D1 - D4 are stored at 100 degrees Celsius and volume resistance measurements have been performed at various time intervals. In the drawing, T indicates the number of days and ΩA is the volume resistance (mOhm cm 2) are the measured values. Time T = 0 indicates the manufacturing time of test pieces C1 to C3, D1 to D4 and the start time of storage at 100 degrees Celsius. Over the entire measurement time, it will be seen that the untreated test pieces C1 to C3 have a significantly higher volume resistance than the test pieces D1 to D4 that have been surface-treated with a laser to have a periodic surface structure 40. A sharp increase in volume resistance between T = 0 and T = 21 is also shown. Such a sharp increase may be due to the growth of a passivation layer induced by temperature (which increases the volume resistance). Thereafter, the test piece volume resistance value stabilizes, and a further slight increase in volume resistance is seen from T = 37 to T = 55.

[0084] Figures 11 to 13 each show a cross-sectional view of another electrochemical system along the cut line A-A shown in Figure 2. For clarity of explanation, the same names and reference numerals as in Figure 3 are also used in Figures 11 to 13.

[0085] The bipolar plates 2 of the electrochemical systems in Figures 11 to 13 differ in that the periodic surface structure 40 is provided at different locations or on different sides of the separator plates 2a, 2b.

[0086] For example, in the electrochemical system 1 of Figure 11, it can be seen that the periodic surface structure 40 is provided on the outer side 22 of the bipolar plate and faces the direction of the gas diffusion layer 27. The outer side 22 is also called the reactant side or the gas side. In particular, the periodic surface structure is provided only on the web 15 of the active region 18. For improving conductivity and / or reducing volume resistance, the separator plates 2a, 2b are connected by a substantial bonding connection such as a laser welding joint 24. The laser welding joint 24 is preferably arranged in the contact zone or contact region formed by the electrochemical active region 18 of the flow field 17, particularly the channel 16. For example, in the x direction (i.e., the flow direction of the cooling fluid), the welding joints 24 may particularly form stitch welds (each having a row of linear welding sections) at intervals from each other. The separator plates 2a, 2b are in contact with each other and are compressed against each other in the above contact zone.

[0087] The surface structure 40 considered in this specification provides, for example, a significant reduction in electrical contact resistance and / or a significant increase in conductivity on the contact surface of the outer side 22 of the bipolar plate 2, particularly in the web 15. The comparison between FIGS. 11 and FIGS. 12, 13 shows that the surface structure 40 can be used instead of the coating 25 for improving conductivity.

[0088] Due to the improvement in conductivity, if the surface structure 40 is present on the surfaces where the separator plates 2a, 2b are in contact with each other, i.e., on the back web 19, the laser welding joint 24 in the contact zone on the contact side of the separator plates 2a, 2b can be omitted. Thus, in one embodiment, the bipolar plate 2 may be free of a substantial bonding connection such as a laser welding joint in the flow field 17 or in the electrochemical active region 18, and in the contact zone of the two separator plates 2a, 2b on the coolant side where the surface structure 40 is disposed (see FIGS. 12 and 13).

[0089] In the electrochemical system 1 of FIGS. 12 and 13, the periodic surface structure 40 may be provided on the inner side 23 of the bipolar plate 2 (the inner side 23 is also referred to as the coolant side). In particular, the surface structure 40 is provided in the contact zone, i.e., in each case on the back web 19, i.e., in the region where the separator plates 2a, 2b are in contact with each other and compressed. The surface structures 40 of the two separator plates 2a, 2b face each other and are in contact. Both separator plates 2a, 2b have a region including the periodic surface structure 40. The periodic surface structure may be provided on only one of the two separator plates 2a, 2b. It can be seen in FIGS. 12 and 13 that the periodic surface structure 40 is present only on the inner side 23, and in the contact zone, and in the active region 18 and the transition region 21. In contrast, the distribution region 20 does not have a periodic surface structure, but in other embodiments, it may have the periodic surface structure 40.

[0090] As already described above, the bipolar plate 2 of FIGS. 12 and 13 has no substantial bonding connection, such as a welding joint, for connecting and electrically contacting the separator plates 2a, 2b having the periodic surface structure 40 disposed on the inner side 23.

[0091] It can also be seen in FIGS. 12 and 13 that the separator plates 2a, 2b of the bipolar plate 2 have a coating 25 on the outer side 22 for improving the electrical conductivity and / or corrosion resistance of each separator plate 2a, 2b. In the embodiment of FIG. 12, the coating 25 is provided only on the webs 15, 45, particularly on its back web 19, and is thus omitted in the regions of the channels 16, 46. Alternatively, the coating 25 may be provided in the regions of the channels 16, 46 or on the entire surface of the separator plates 2a, 2b of the bipolar plate 2 (see FIG. 13). The coating 25 may optionally be combined with the periodic surface structure 40 and may thus be applied to the periodic surface structure 40.

[0092] The periodic surface structure 40 may be variously configured on the inner side 23 and the outer side 22 using different laser parameters in each case. Similarly, regions of the various surface structures 40 may also be present on one side 22, 23, again particularly due to different laser parameters.

[0093] Further comparative measurements were carried out to demonstrate the influence of the position of the periodic surface structure 40 on the bipolar plate 2 on the volume resistivity.

[0094] In two compressed separator plates 2a, 2b without surface treatment and without welding joints in the contact regions of the separator plates, a volume resistivity of 161 mOhm cm 2 was measured.

[0095] In two compressed separator plates 2a, 2b without surface treatment and having stitch-weld type welding joints 24 in the contact regions of the separator plates 2a, 2b, a volume resistivity of 121 mOhm cm 2The volume resistance was measured (see separator plates 2a and 2b in Fig. 3).

[0096] In two compressed separator plates 2a and 2b having a periodic surface structure 40 type of surface treatment on the outside 22 and having a stitch welding type weld joint 24 in the contact area of the separator plates, 5.3 mOhm cm 2 The volume resistance was measured (see separator plates 2a and 2b in Fig. 11).

[0097] In two compressed separator plates 2a and 2b having a periodic surface structure 40 type of surface treatment on both the outside 22 and the inside 23 and having no weld joint 24 in the contact area of the separator plates 2a and 2b, 4.8 mOhm cm 2 The volume resistance was measured (see separator plates 2a and 2b in Figs. 12 and 13 having an additional surface structure 40 on the outside 22).

[0098] For separator plates that are geometrically different and manufactured from different batches of stainless steel, are compressed against the gas diffusion layer, and are paired, the following volume resistances were measured. A pair of separator plates with an LSFL surface structure provided on the entire surface of both sides of the active area: 6.1 mOhm cm 2 A pair of separator plates connected by stitch welding and having an LSFL surface structure provided on the entire surface only outside the active area: 6.6 mOhm cm 2 The volume resistance of a pair of separator plates having neither a laser surface structure nor stitch welding was 74 mOhm cm under otherwise the same conditions. 2 It was.

[0099] From such facts, it is presumed that the inner laser treatment effect can achieve a resistance value similar to that of the stitch welding 24, or in some cases, a more excellent resistance value.

[0100] The separator plates 2a, 2b and the bipolar plate 2 shown in FIGS. 11 to 13 are the subject of the claims alone, i.e., without the other features of the electrochemical system 1. However, the features of FIGS. 11 to 13 can be combined, for example, the surface structure 40 on the gas side of the separator plates 2a, 2b in FIG. 11 can be combined with the surface structure 40 on the coolant side of the separator plates 2a, 2b as shown in FIGS. 12 and 13. In that case, the stitch welding 24 can be omitted.

Explanation of Signs

[0101] 1: Electrochemical system 2: Bipolar plate 2a: Separator plate 2b: Separator plate 3: End plate 4: End plate 5: Media port 7: z-axis 8: x-axis 9: y-axis 10: Membrane electrode assembly (MEA) 11a: Opening 11b: Opening 11c: Opening 15: Web 16: Channel 17: Flow field 18: Active area 20: Distribution area or collection area 21: Transition area 22: Outside of the bipolar plate 23: Inside of the bipolar plate 24: Laser welding joint 25: Coating 26: Membrane electrode 27: Gas diffusion layer 28: Reinforcement layer 29: Membrane composite 30: Reinforcement area 35: Web of the distribution area or collection area 36: Channel of the distribution area or collection area 40: Surface structure 42: Depression 44: Protrusion 45: Web of the transition region 46: Channel of the transition region 100: Laser system 101: Laser head 102: First mirror 103: Second mirror 104: λ / 2 plate 105: Polarizer 106: Beam splitter 107: Cap 108: Lens b: Width t: Depth l: Length Px: Period in the x direction Py: Period in the y direction

Claims

1. A separator plate for an electrochemical system, wherein the separator plate has a periodic surface structure with an average spatial period of less than 10 μm in at least some regions, and the separator plate has a plurality of webs and channels formed between the webs, a back web is formed on the surface of the separator plate on the opposite side of the bottom of the channel, and the back web has the periodic surface structure in at least some sections. Separator plate.

2. The periodic surface structure has a plurality of depressions that are at least partially arranged in a uniaxial direction and extend along a direction perpendicular to the uniaxial direction. The separator plate according to claim 1.

3. The plurality of depressions have a depth of at least 8 nm, a width of at least 0.1 μm and at most 2 μm, and a period in one spatial direction of at least 0.3 μm and at most 3 μm. The separator plate according to claim 2.

4. The separator plate is surface-treated with a laser in the region of the periodic surface structure. The separator plate according to any one of claims 1 to 3.

5. The periodic surface structures are periodically arranged with respect to each other in at least one spatial direction. The separator plate according to any one of claims 1 to 4.

6. The oxygen content of the surface material is higher in the region of the periodic surface structure than outside the periodic surface structure. The separator plate according to any one of claims 1 to 5.

7. At least in the region where the periodic surface structure is provided, a coating is provided in at least some sections, and in particular, a coating for improving conductivity is provided. The separator plate according to any one of claims 1 to 6.

8. The coating contains one or more of conductive oxides, carbon, noble metals such as gold, silver, and platinum, metals such as titanium and chromium, metal nitrides, metal carbides, metal borides, metal silicides, and silicon carbide, or consists of one or more of these substances or their alloys. The separator plate according to claim 7.

9. The web has the periodic surface structure in at least some sections. The separator plate according to any one of claims 1 to 8.

10. A bipolar plate having the separator plate according to any one of claims 1 to 9, wherein the web forms a contact surface that compresses the diffusion layer, or the back web forms contact surfaces that compress against each other, bipolar plate.

11. The electrical resistance of the region of the contact surface in at least one of between the web and the gas diffusion layer and between the back webs of the separator plate is reduced by the periodic surface structure. The bipolar plate according to claim 10.

12. A method for manufacturing a separator plate for an electrochemical system, comprising the steps of providing a separator plate, irradiating the separator plate with a pulsed laser, wherein the pulse duration of the pulsed laser is less than 1 ns, and creating a periodic surface structure on the separator plate by laser irradiation, wherein the separator plate has a plurality of webs and channels formed between the webs, a back web is formed on the surface of the separator plate on the opposite side of the bottom of the channel, and the back web has the periodic surface structure in at least some sections. method.

13. The method according to claim 12, wherein the laser irradiation is linearly polarized.

14. The method according to claim 12 or 13, wherein the periodic surface structure is oriented perpendicular to the polarization direction of the incident laser light.

15. The average spatial period of the periodic surface structure is at least 20% of the wavelength of the pulsed laser used. The method according to any one of claims 12 to 14.

16. The beam diameter or the minimum lateral size of the pulsed laser is at least 20 μm. The method according to any one of claims 12 to 15.

17. Additionally, at least the region where the periodic surface structure is provided is coated with a coating material in at least some sections, particularly with a coating material for improving conductivity. The method according to any one of claims 12 to 16.

18. The back web formed as the back side of the bottom of the channel is irradiated with the pulsed laser over the entire surface or in some sections, and the periodic surface structure is created on the back web. The method according to any one of claims 12 to 17.

19. The plurality of the periodic surface structures are formed on the separator plate within spatially adjacent laser irradiations. The method according to any one of claims 12 to 18. **Claim 20** The periodic surface structure is generated by optical interference between incident laser light and an electromagnetic surface wave in the material of the separator plate generated by the pulsed laser. The method according to claim 19.

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