Gas diffusion layer for an electrolysis cell and methods for producing a gas diffusion layer

Abstract

The invention relates to a gas diffusion layer (1) for an electrolysis cell (3), comprising a first gas diffusion layer (5) and a second gas diffusion layer (7). The first gas diffusion layer (5) has a fine-pored layer with fine pores, and the second gas diffusion layer (7) has a coarse structure with coarse pores. The second gas diffusion layer (7) is applied to the first gas diffusion layer (5) and is bonded to it in such a way that a bonding surface (21) is formed with an inner active area (23A) and an outer edge area (23B) surrounding the inner active area (21A). The outer edge area (23B) features the bonded connection. The invention further relates to a method for producing a gas diffusion layer (1), an electrolysis cell (3) with a gas diffusion layer (1) and an electrolyzer.

Description

[0001] The present invention relates to a gas diffusion layer for an electrolysis cell and a manufacturing process for a gas diffusion layer.

[0002] The splitting of water by electric current to produce hydrogen and oxygen gas using an electrolysis cell is well known. A distinction is mainly made between two technical systems: alkaline electrolysis and PEM (proton exchange membrane electrolysis).

[0003] An electrolysis cell comprises two half-cells, an anodic half-cell and a cathodic half-cell, with both half-cells connected by a membrane. Each half-cell has a bipolar plate that contacts a gas diffusion layer. A gas diffusion layer can be composed of several gas diffusion layers, where, for example, a gas diffusion layer contacts the electrode on the membrane or is itself the electrode. The electrode is the component located on the catalyst where the electrochemical reaction takes place. The arrangement of the electrodes and the application of an electrolysis voltage during electrolysis create an electrical potential and a current flow, which sets ions in motion. This ion movement occurs in a conductive electrolyte.The cell consists of a liquid electrolyte in each half-cell and a potentially solid electrolyte (membrane) that separates the half-cells. The flow of ions through the electrolytes results in an ion current from one half-cell to the other, generating hydrogen or oxygen at the respective electrodes. The cell reactions for hydrogen and oxygen production in an alkaline environment are: Anode: 4OH - → 2H2O + O2 + 4e - , E = +0.40 V Kathode: 2H2O + 2e - → 2OH - + H2, E° = -0.83 V

[0004] The cell reactions for hydrogen and oxygen formation during PEM electrolysis are: Anode: 2H2O → 4H + + O2 + 4e - , E = +1.23 V Kathode: 4H + + 4e - → 2H2 E° = 0.00 V

[0005] Spatial separation of the cell reactions is enabled by the aforementioned membrane, which allows ionic transport through the electrolysis cell. In the case of anion exchange membrane electrolysis (AEM water electrolysis), this is achieved through the use of a hydroxide ion-conducting membrane. A central cell component is the membrane electrode assembly (MEA). The MEA consists of the respective catalyst-electrode assembly and a solid-state polymerization membrane (SPA) on both sides.

[0006] In PEM electrolysis, the proton-conducting polymer membrane is a semipermeable membrane made of ionomers. Ionomers belong to the group of thermoplastics, but have the advantage over thermoplastics that ionic bonds are effective within them, and unlike most plastics, they can be used as electrolytes. Due to its gas impermeability, the PEM prevents the products hydrogen and oxygen from mixing. In this process, the hydrogen has a higher purity than in other electrolysis methods. The PEM electrolysis cell is supplied with demineralized water on the O₂ side, which is converted at the anode into oxygen gas and protons (H₂). +The protons migrate through the proton-conducting membrane and recombine at the cathode (H₂ side) to form hydrogen gas. The gas diffusion layer, which is in contact with the electrodes and typically consists of several layers, ensures optimal water distribution, thus guaranteeing membrane wetting and the removal of product gases. Therefore, the gas diffusion layer must be electrically conductive and porous, providing good, long-lasting contact with the electrode. An additional requirement is that any component tolerances arising in the electrolyzer must be compensated for to ensure the most uniform possible contact of the MEA under all tolerance conditions.

[0007] Several electrolysis cells are then typically connected in series in so-called stacks. Each electrolyzer has one or more stacks.

[0008] An electrolysis cell with its electrodes constitutes, in particular, an electrical assembly, whereby electrical resistances or impedances also occur at the individual components of the assembly. These electrical resistances ultimately determine the operating voltage of the electrolysis cell.

[0009] Two resistances are of particular importance for industrial operation: the activation resistance of the electrolysis reaction and the ohmic resistance of the electrolysis cell. The activation resistance is primarily determined by the electrodes, or the catalyst of the electrolysis reaction, and its surroundings. The ohmic resistance is influenced by all conductive components. In particular, membrane resistances, electrolyte resistances, and contact resistances play a major role in the case of ohmic resistance.

[0010] The contact resistance between a gas diffusion layer and a bipolar plate, for example, is a function of the contact pressure applied by tensioning elements such as tie rods, tension springs or tension straps.

[0011] According to the current state of the art, gas diffusion layers are implemented as expanded metal, wire mesh, and rigid, embossed bipolar plates, onto which a metal- or carbon-based fiber fleece may be applied. The stiffness of these assemblies is exceptionally high, while their compressibility is relatively low. Consequently, if the planes are not sufficiently parallel, the bipolar plates and the gas diffusion layers are pressed together inhomogeneously, resulting in contact areas of varying sizes. Deviations in local contact pressure ultimately lead to locally differing contact resistances and thus to an undesirable current density distribution, which, for example, promotes melting, welding, or corrosion of the components. These effects are accompanied by degradation of the electrolysis cell and electrolyzer over their service life.

[0012] To keep contact resistance low, individual metal layers or wire fibers are currently welded, rigidly woven, or sintered together. However, this leads to increased stiffness and potential scale formation. Gas diffusion layers composed of expanded metal exhibit irreversible settling behavior, or rather, such gas diffusion layers are plastically deformed under the applied compression.

[0013] In PEM water electrolyzers and many other galvanic cells (e.g., AEM electrolyzers, fuel cells), the electrodes are contacted via a so-called porous transport layer (PTL) or gas diffusion layer (GDL). This layer ensures the electrical connection of the electrodes and simultaneously enables sufficiently efficient mass transport through the pores. Reactant molecules must reach the active sites of the electrode, and product molecules must be transported away to guarantee efficient, safe, and long-lasting cell operation. Due to these requirements for local electrode contact and mass transport, the gas diffusion layer plays a crucial role as a functional layer in an electrolysis cell. This is also important for minimizing the need for expensive catalyst materials such as iridium and platinum while simultaneously ensuring good transport properties.

[0014] Starting from the known state of the art, it is an object of the present invention to provide a gas diffusion layer with improved electrical and transport properties. A further object is to specify a manufacturing process for a corresponding gas diffusion electrode.

[0015] The object is achieved according to the invention by a gas diffusion layer for an electrolysis cell, comprising a first gas diffusion layer and a second gas diffusion layer, wherein the first gas diffusion layer has a fine-porous layer with fine pores and the second gas diffusion layer has a coarse structure with coarse pores, wherein the second gas diffusion layer is applied to the first gas diffusion layer and is materially bonded to it in such a way that a connecting surface with an inner active area and with an outer edge area surrounding the inner active area is formed, wherein the outer edge area has the materially bonded connection.

[0016] The fine pores of the fine-porous layer have a significantly smaller pore diameter than the coarse pores of the coarse structure. It is possible that micropores are incorporated into the first gas diffusion layer, thus forming a microporous layer. Therefore, micropores can also be introduced into the fine-porous layer. Furthermore, the second gas diffusion layer, in particular, can be multilayered, meaning it can comprise several layers that have or form a base structure with coarse pores.

[0017] The invention is based on the understanding that, for example, in PEM water electrolyzers, the fine-pored contacting of the electrode, particularly the anode electrode, is becoming increasingly important. Very expensive iridium is typically used as a catalyst material for the production of anode-side electrodes. Due to its limited availability and high cost, reducing the amount of catalyst material used is a key objective. However, with a lower iridium content and the associated reduced transverse conductivity of the electrode, the distance between two electrical contacts, i.e., the pore diameter, must be reduced to achieve the same efficiency of the electrolysis cell. The same considerations apply accordingly to saving platinum and other catalyst materials in a cathode electrode.

[0018] The different layers of a gas diffusion layer are usually bonded together using a material-bonded process, e.g., welding or sintering. The reason for this is improved conductivity, i.e., no or very low contact resistance, as well as easier handling during assembly of the electrolysis cell.

[0019] When joining these gas diffusion layers, the use of very fine-pored structures can lead to manufacturing problems. Welding, sintering, or other joining processes can melt the fine gas diffusion layers to such an extent that the corresponding pores are locally blocked. This locally impairs the transport properties, which can lead to a reduction in efficiency and damage to a membrane electrode assembly.

[0020] The invention proposes not to provide a full-surface connection across the entire contact area between the first and second gas diffusion layers. Instead, the connection and simultaneous electrical contacting of the porous transport layers provided by the first and second gas diffusion layers is to occur only in a local and selective area. For this purpose, a material-bonded connection is proposed that is limited to the outer edge region of the contact area, whereas no such close material bond is provided in the inner active area. In the inner active area, the first and second gas diffusion layers lie against each other, thus providing electrical contact.Thus, a functional separation of the connection surface between the diffusion layers is achieved, enabling both mechanical fastening and connection on the one hand, and low-resistance and uniform electrical contact between the fine-pored gas diffusion layers on the other. At the same time, the fluid transport through the active area with its fine pores is advantageously not impaired. Pore clogging or loss in the active area, with its associated damaging effects on the function of the gas diffusion layer, can be reliably avoided through the locally bonded joining process.

[0021] This allows for the use of a very fine-pored and microporous first gas diffusion layer without the risk of pore clogging or loss of electrochemical and transport properties in the active area. Previously known gas diffusion layers have employed the unsatisfactory workaround of simply not reducing the pore size in the gas diffusion layers below a minimum value. With sufficiently large pores, the problem then does not occur, or at most only to a very small extent, when two gas diffusion layers are contacted at their interface. Only the recently increased additional requirements for reducing the precious metal content and increasing efficiency necessitate the densest possible contact between the gas diffusion layers. This, in turn, leads to losses due to fused or clogged pores resulting from the joining process in the active area.

[0022] A further alternative option being pursued involves implementing additional contact structures on the electrode. Sintered or nonwoven structures are used that are not metallurgically bonded to the rest of the second gas diffusion layer, which serves as the substructure. Here, too, the problem of pore closure or pore loss does not occur. However, this conventional, prior art approach requires very complex handling during assembly and can therefore lead to efficiency losses due to increased contact resistance caused by the assembly process. If the additional contact structures are welded or sintered, the same problems arise, namely a loss of pore volume because the metal is melted during welding or sintering.

[0023] These disadvantages are overcome in the invention by the functional separation and design of the contact surface with an inner active area and an outer edge area. This enables reliable and very uniform electrical contact without loss of porosity in the active area, while simultaneously reducing the use of catalyst material.

[0024] The problem of blocked transport pores due to welding or similar processes is solved in the present invention by a novel concept for the gas diffusion layer and GDL fabrication, which is adapted to the overall cell design. The connection of the individual layer structures, in this case the first gas diffusion layer with the second gas diffusion layer, is specifically limited to the electrochemically inactive area. The second gas diffusion layer functions as a substructure or support structure and as a contact structure to the electrode – anode or cathode – when installed in an electrolysis cell.

[0025] Depending on the cell design, the edge region of the gas diffusion layer may primarily serve to seal the two cell halves and is therefore not in direct contact with the electrode. Since no product gases from electrolysis are formed in such a region, clogged pores have less of an impact on the performance and efficiency of the electrolysis cell. Therefore, the material bond between the finest layer(s) forming the first gas diffusion layer (contact structure) and the coarse-pored substructure of the second gas diffusion layer should occur almost exclusively in this edge region.

[0026] The invention incorporates the additional insight that the prior art configuration of a gas diffusion layer is disadvantageous. Consequently, the components of the coarsely structured part (substructure) of a gas diffusion layer and the finely structured gas diffusion layer (contact layer) in the electrolysis cell were sometimes simply placed loosely on top of each other without any further connection or fixation. This loose stacking, however, entails numerous disadvantages in terms of electrical contact, handling during assembly, and potential points of corrosion. Furthermore, for the development of more efficient electrolyzers, even the mesh size of fine expanded metal is too coarse. Therefore, the typically rolled expanded metal is to be augmented by a significantly finer PTL (porous transport layer) or even MPL (microporous transport layer).

[0027] In contrast, the invention recognizes that the associated technical challenges require a comprehensive consideration and consideration of both the mechanical and electrical integration or combination of a very fine-pored first gas diffusion layer with a coarsely structured, coarse-pored second gas diffusion layer.

[0028] The material of the second gas diffusion layer exhibits high electrical conductivity and a coarse-pored structure, combined with corrosion resistance, for use in an electrolysis cell. This second gas diffusion layer can be, for example, an open-pore metallic nonwoven, a coarse-pored powder sinter, a metal woven fabric, a metal mesh, a wire mesh, or an expanded metal sheet. This creates a two-layer system with a fine-pored, sintered composite layer and a layer with larger or coarse pores for media transport and current supply.

[0029] In a particularly preferred embodiment of the gas diffusion layer, the first gas diffusion layer and the second gas diffusion layer are welded together in the edge region.

[0030] To ensure that only the electrochemically inactive edge region is permanently bonded, a locally and precisely controlled joining process is used, preferably a welding process. Capacitor pulse welding is particularly suitable among other welding methods, as the punch size and shape can be freely selected. This is then adapted to the size of the edge region, enabling highly targeted welding. The location, dimensions, and density of the weld points within the edge region can be tailored to the specific requirements by adjusting the welding punch design.

[0031] To ensure that the microporous first gas diffusion layer lies flat, it can be weighted down with a non-conductive material during welding. Conductive and corrosion-resistant materials, such as titanium and stainless steel, are preferred for the gas diffusion layers.

[0032] In a particularly preferred embodiment of the gas diffusion layer, a large number of weld points are placed over the entire edge surface, with a density of 1 to 100 weld points per cm². 2 amounts.

[0033] The close and uniform arrangement of the weld points across the entire edge surface at high density ensures a secure local bond. The gas diffusion layers are uniformly and stably bonded across the edge area via a frame connection. The inner active area can function without welding and its function is not impaired, as the locally applied frame connection prevents pore closure in the active area. It has been shown that when using the capacitor pulse welding process, 4-100 weld points per cm² are required. 2 are suitable. These are optionally placed in a planar square grid, a planar rectangular grid, or a planar hexagonal grid, or in combinations of these grid variants. For highly stressed structures, such as in pressure electrolysis, at least 36 weld points per cm² are required. 2They can be attached for local fastening. The weld point density in the edge area can be flexibly adapted to the cell design and the operating conditions in an electrochemical cell.

[0034] In a particularly preferred embodiment of the gas diffusion layer, the first gas diffusion layer is bent over outside the edge region, with the side surface of the second gas diffusion layer being at least partially enclosed, so that the first gas diffusion layer accommodates the second gas diffusion layer.

[0035] Thus, the geometric dimensions of the first gas diffusion layer exceed the dimensions of the second gas diffusion layer, so that during manufacturing it is possible to bend or fold the first gas diffusion layer around the outer edges of the second gas diffusion layer to enclose or frame it.

[0036] This advantageously creates the possibility of attaching the contact layer not only to the edge area but also to the side surfaces or even the top surface of the substructure. For this purpose, the first gas diffusion layer (contact layer) is bent or folded during manufacturing, for example, so that it wraps around the edges and, if necessary, corners of the substructure. Subsequently, the first gas diffusion layer can be optionally attached to the second gas diffusion layer forming the substructure, either by a positive locking mechanism, such as a snap-fit ​​or interlocking system, or by a material-bonded connection, such as welding.

[0037] In the case of the gas diffusion layer, this is preferably achieved by bending the first gas diffusion layer in such a way that the end surface of the second gas diffusion layer opposite the bonding surface is at least partially enclosed. The variant of joining both layers by bending one layer around the other is particularly advantageous when the inactive edge region, in which both layers can be welded without blocking important transport pores, is very small, or when the entire surface is electrochemically active.

[0038] In a further preferred embodiment of the gas diffusion layer, a positive-locking connection between the first gas diffusion layer and the second gas diffusion layer is provided outside the edge area.

[0039] In a particularly preferred embodiment of the gas diffusion layer, a material-bonded connection between the first gas diffusion layer and the second gas diffusion layer is provided outside the edge region.

[0040] By selecting the same, or at most slightly modified, material-bonding joining process both within and outside the edge region, the complexity and costs of joining the gas diffusion layers are reduced, since the same material-bonding joining process can be used. Here, it is advantageous to employ a capacitor discharge welding process, which can be performed on the same gantry welding machine.

[0041] In a particularly preferred embodiment of the gas diffusion layer, a connecting surface is formed with an inner active area and with an outer edge area surrounding the inner active area, wherein in the inner active area the first gas diffusion layer is isolated and point-fixed to the second gas diffusion layer.

[0042] This creates the possibility, particularly in the case of a very extensive active area, of only very lightly attaching the first gas diffusion layer as a contact structure to the second gas diffusion layer as a substructure. This can be achieved, for example, by selecting locally different welding parameters in the active area. This prevents melting in the electrochemically active area of ​​the gas diffusion layer. The isolated and localized, material-bonded application ensures sufficient adhesion in the active area, if required, without impairing the mass transport through the pores of the first gas diffusion layer.

[0043] Therefore, in a particularly preferred embodiment of the gas diffusion layer in the active area, isolated welding points are placed, which are designed in such a way that point fixings are achieved without melting of material in the active area.

[0044] Thus, at least a slight material bond without melting is provided in the active area.

[0045] The design of the gas diffusion layer is preferably such that the density of the weld points placed in the active area is lower than the density of the weld points placed in the boundary area.

[0046] This results in the fastening and contacting of the connection surface occurring predominantly or exclusively via the edge region. The edge region features a large number of weld points with a high weld point density, whereas in the inner active region, only tack welding or fixing of the two gas diffusion layers is intended, with a significantly smaller number of fixing points per unit area. This combination of different and adapted weld point densities in the edge region and the active region proves particularly advantageous for the function of the gas diffusion layer, as a smaller pore diameter is now possible in the active region, thus achieving increased efficiency.A lower loading with expensive catalyst material - such as iridium on the anode side or platinum on the cathode side - is possible without impairing the electrochemical activity and conversion performance of the electrochemical cell.

[0047] Therefore, in a particularly advantageous embodiment of the gas diffusion layer, it is provided that the density of the weld points in the active area is limited to a maximum of 9 per cm. 2 , especially up to a maximum of 4 per cm 2 , amounts.

[0048] In a manner analogous to the edge area, a regular, in particular square, rectangular or hexagonal, planar point grid is present as a fastening structure over the surface of the active area, formed by the welding points.

[0049] In a preferred embodiment of the gas diffusion layer, the finely porous layer has a porosity of 30% - 85%, in particular of 50% - 80%.

[0050] The finely porous or, if applicable, even microporous layer is formed entirely by the first gas diffusion layer, or the first gas diffusion layer may itself have a finely porous or, if applicable, even microporous layer or a single layer. Particularly in an installation or operating situation within an electrolysis cell, the first gas diffusion layer at the contact surface with an electrode of a membrane-electrode assembly has a porous layer with fine pores. It may even be particularly preferred to provide a somewhat higher porosity of more than 50% in the first gas diffusion layer, especially approximately 50% to 85%, which promises even better properties for electrode contacting in an electrolysis cell.

[0051] Preferably, the first gas diffusion layer in the gas diffusion layer has a layer thickness of 0.1 mm to 2.0 mm, in particular of 0.2 mm to 1.0 mm.

[0052] This allows for good further processing and manufacturing handling of the first gas diffusion layer, as well as simple application and material-bonded local bonding of the second gas diffusion layer to the first. Due to its thinness, the first gas diffusion layer can be cut to the desired shape to suit the installation situation. The material required for this finely porous single layer as a functional layer is also reduced, while even improving electrical contact and media transport. The first gas diffusion layer is a thin functional layer that can also be multi-layered. The first gas diffusion layer thus has a fine structure that, when used in an electrochemical cell, such as a PEM electrolysis cell, ensures the most homogeneous current distribution possible and simultaneously facilitates media transport.

[0053] Preferably, the first gas diffusion layer in the gas diffusion layer may comprise or be formed by a fine-pored expanded metal. It is also preferred that the first gas diffusion layer comprises a metal fleece or a powder sintered fleece.

[0054] In particular, when used in expanded metal applications, braided, fine-mesh structures made of metal fibers can also be incorporated. This allows for very good transverse conductivity, meaning conductivity within the layer, and thus very good current distribution. With such a structure, the current flowing from the contact points to the electrodes of an electrolysis cell is distributed homogeneously.

[0055] In a particularly preferred embodiment of the gas diffusion layer, the second gas diffusion layer comprises a metallic expanded metal mesh with a mesh size of 0.5 mm × 0.5 mm to 20.0 mm × 20.0 mm.

[0056] In particular, it may be preferred that the second gas diffusion layer comprises a metallic expanded metal mesh with a mesh size of 1.5 mm × 1 mm to 2.5 mm × 2 mm.

[0057] Several expanded metal grids can also be inserted into the second gas diffusion layer and stacked. The expanded metal grid provides a coarsely porous yet electrically conductive structure.

[0058] This achieves a certain degree of elasticity and springiness. In addition to expanded metal meshes or layers, wire fabrics, or folded sheets, combinations of these materials can also be used. Structures that allow for a certain degree of mechanical compression are advantageous, as they do not lead to material stresses and thus to inhomogeneous pressure peaks, which in the worst case can result in mechanical stresses or undesirable wedging during installation in an electrolysis cell.

[0059] By means of expanded metal mesh adapted to the installation situation and function, a coarse structure with large pores is provided in the second gas diffusion layer in a particularly advantageous manner. The coarse structure of the second gas diffusion layer comprises, in particular, large pores, while the fine structure of the first gas diffusion layer features fine pores or micropores. A pore is a recess in the surface of a given layer, and within a layer, such that flow channels for effective fluid transport through the gas diffusion layer are formed perpendicular to the normal of the gas diffusion layer. A coarse pore is simply a significantly larger spatial recess than a fine pore or micropore.

[0060] In a further preferred embodiment of the gas diffusion layer, the coarse pores in the second gas diffusion layer have a diameter that corresponds to 100 to 1000 times the diameter of the fine pores in the microporous layer of the first gas diffusion layer.

[0061] Due to the significantly larger pore diameter in the second gas diffusion layer compared to the first, a larger fluid flow rate can be rapidly dissipated through the second layer with moderate or low pressure losses. The product stream from water electrolysis, in particular, which contains water and gas bubbles of hydrogen or oxygen, is characterized by a high flow rate. These fluid transport properties are ensured by the large pores. Simultaneously, the metallic and large-pore design of the second gas diffusion layer, especially a metallic expanded metal mesh, provides mechanical support for the first layer and electrical conductivity for conducting and supplying current to the first layer.

[0062] In a particularly preferred embodiment of the gas diffusion layer, the second gas diffusion layer is multilayered with a plurality of superimposed individual layers having large pores, wherein superimposed adjacent individual layers are welded together.

[0063] The multi-layered design of the second gas diffusion layer, comprising several individual layers, is highly advantageous because it allows for layer-specific adjustment of the properties for fluid transport of reactants and products, while simultaneously ensuring a uniform current supply. Thus, the porosity can be adjusted across the individual layers. It has been shown that a welded joint is particularly beneficial for connecting the individual layers of the second gas diffusion layer. Since the transport of larger fluid volumes predominates in the operation of the second gas diffusion layer within an electrolysis cell, a finely porous sintered structure—as used for the first gas diffusion layer—would be disadvantageous within the layered structure of the second gas diffusion layer.In contrast, the combination of a material-bonded, particularly welded, connection in the edge region of the bonding surface between the first and second gas diffusion layers with the welded single-layer structure within the second gas diffusion layer has proven to be very advantageous. This offers significant advantages both in terms of manufacturing technology and with regard to the electrical and transport properties of the gas diffusion layer as a porous transport layer. The multi-layered design of the second gas diffusion layer can, for example, consist of a stack or composite of at least two or more expanded metal sheets. The expanded metal sheets preferably have a rectangular shape and an aspect ratio of at least 1.2. Expanded metal sheets with an aspect ratio of at least greater than 2.0 are particularly advantageous.Such a stack or multi-layered composite of expanded metal mesh can be placed in a portal welding machine and welded together there using so-called punches, which form the welding electrodes. It is possible for adjacent individual layers within the multi-layered structure of the second gas diffusion layer to be fully welded together, or for the bond to be localized only in the edge areas of adjacent individual layers.

[0064] In an advantageous embodiment of the gas diffusion layer, the diameter of the macropores increases layer-specifically with the distance of a layered single layer in a direction perpendicular to the layer normal from the first gas diffusion layer.

[0065] In this context, a mean value, i.e., the average pore diameter of a distribution, can be conveniently considered a porosity measure for a single layer, allowing for the adjustment and achievement of a desired graded structure. Thus, a porosity gradient can be set, enabling the adaptation and targeted increase of the fluid conductivity, particularly the volumetric flow rate, for the required media transport. The fluid conductivity increases perpendicular to the layer normal, and the flow resistance decreases accordingly.

[0066] The above-mentioned task is further addressed by a method for producing a gas diffusion layer, in which - a first gas diffusion layer is provided, which has a finely porous layer with fine pores; - a second gas diffusion layer is provided, which has a coarse structure of large pores; - the second gas diffusion layer is connected to the first gas diffusion layer in such a way that a connecting surface with an inner active area and with an outer edge area surrounding the active area is formed, and wherein - a material-bonded connection is established locally in the outer edge area between the first gas diffusion layer and the second gas diffusion layer.

[0067] Advantageous further developments of the method result from the dependent claims as well as the present description and the figures.

[0068] Accordingly, a manufacturing process for a gas diffusion layer is proposed, wherein the first and second gas diffusion layers are locally bonded and contacted in the outer edge region, for example, by welding them together in the edge region via a multitude of targeted and exclusively local weld points placed in the edge region. In this local bonding process, the first and second gas diffusion layers are placed on top of each other and then bonded together in the edge region. For this bonding in the edge region, a welding process is preferred over a sintering process due to the local application. The first and second gas diffusion layers are sintered together, in particular using a sintering material, but are bonded together using a welding process.

[0069] The local application of the welding process reliably prevents the formation of mixed phases within the metal structure or pore closure in the active zone of the interface between the first and second gas diffusion layers. Such mixed phases can be triggered, for example, by high local heat input during the welding of expanded metal layers. These mixed phases can prevent the formation of passivation layers and, due to the lack of passivation, promote corrosion. Accordingly, the local application of a welding process exclusively or predominantly within the active zone of the first and second gas diffusion layers ensures the maintenance of a particularly fine-pored, uniform, and electrically conductive contact layer.

[0070] Furthermore, the first gas diffusion layer and the second gas diffusion layer can form a single component bonded together and constitute a functional building unit for use in an electrolysis cell, for example in the anode compartment of a PEM electrolysis cell with iridium as catalyst material.

[0071] The local joining of the first and second gas diffusion layers by welding can always be achieved with a metallurgical bond, thus providing optimal and uniform contact with minimal contact resistance between the components, or rather, at the interface between the first and second gas diffusion layers. The surfaces of the first and second layers can be degreased and pickled beforehand to facilitate the sintering of the metallic expanded metal layers, wires, fibers, etc.

[0072] In a particularly preferred embodiment of the method, it is provided that the first gas diffusion layer and the second gas diffusion layer are welded together using a capacitor discharge welding process.

[0073] The type of connection for a gas diffusion layer of an electrolysis cell on the anode side can be designed as follows: A first gas diffusion layer comprises a porous expanded metal with fine pores. A second gas diffusion layer is applied to this, consisting of an open-pore expanded metal mesh with a mesh size of 0.5 mm × 0.5 mm to 20.0 mm × 20.0 mm, which is applied to the first gas diffusion layer by capacitor pulse welding, thereby firmly bonding the two layers locally in the outer edge region.

[0074] A structure with a welded connection between the first and second gas diffusion layers, executed only locally within the bonding surface, can be highly advantageous. This basic structure of a gas diffusion layer can be easily processed further, for example by capacitor discharge welding, and successively expanded into a more complex, multi-layered gas diffusion layer. In particular, the second gas diffusion layer can advantageously be multi-layered, comprising several individual layers. This creates GDL structures that offer a correspondingly higher current-carrying capacity than the first gas diffusion layer with its fine pores alone. The grid-like metallic structure creates defined, material-bonded contact points, both with the first gas diffusion layer and with further individually structured individual layers of the second gas diffusion layer.

[0075] Therefore, in a particularly advantageous embodiment of the manufacturing process, a second gas diffusion layer is provided by providing a plurality of superimposed individual layers having large pores, wherein superimposed adjacent individual layers are welded together, in particular by using a capacitor discharge welding process.

[0076] The process can be advantageously designed in such a way that in the second gas diffusion layer the porosity of the macropores in a single layer is adjusted such that a graded layering is formed by the layered single layers, whereby the pore size decreases with the distance of a layered single layer perpendicular to the layer normal from the first gas diffusion layer.

[0077] In this way, a graded porous structure is achieved with an adapted fluidal conductivity in the direction of the normals; thus, a desired porosity gradient can even be taken into account and set during production.

[0078] The finer or more finely porous the contact between the gas diffusion layer (GDL) and the electrode via the first gas diffusion layer in the installation of an electrolysis cell, the better the catalyst utilization and the associated transverse and normal conductivity, as well as the mechanical support of a membrane electrode assembly (MEA). A lower loading of catalyst material on the electrodes can be used without any loss of efficiency.

[0079] In the case of the fine-pored layer preferably used in the first gas diffusion layer, for example, a fiber structure or expanded metal, denser contact points and thus a more extensive contact of the electrode in the active area are possible. A very dense and simultaneously very fine-pored design of the first gas diffusion layer as the primary contact layer also leads to significantly improved support of the MEA itself. A major advantage is that this results in savings of the very expensive catalyst material, such as iridium, at the electrode of the MEA. Furthermore, an improved and, in particular, homogeneous current distribution is achieved. Easier handling and further processing, as well as subsequent joining and expansion to a multi-layer gas diffusion layer with multiple individual layers, e.g., by capacitor discharge welding, is very easily accomplished.This also includes easier handling of the gas diffusion layer during assembly and quality control. A particular advantage, however, is that the pores of the micro- or fine-porous contact structures in the active area are not affected. Improved media transport properties are also ensured without compromising efficiency, while requiring less material for the precious metal catalyst. Furthermore, assembly is easier compared to unconnected layers. The invention's method enables the use of even finer-pored expanded metals and other structures, offering a significant cost advantage over sintered or nonwoven structures.

[0080] Another aspect of the invention relates to an electrolysis cell with a gas diffusion layer according to the invention.

[0081] Another aspect of the invention relates to an electrolyzer with an electrolysis cell according to the invention.

[0082] Further advantages, features, and details of the invention will become apparent from the following description of preferred embodiments and from the drawings. The features and combinations of features mentioned above in the description, as well as those mentioned below in the figure description and / or shown individually in the figures, can be used not only in the combinations specified, but also in other combinations or individually, without departing from the scope of the invention.

[0083] Exemplary embodiments of the invention are explained in more detail with reference to the drawing. The drawing shows, schematically and in a highly simplified form: Fig. 1 the basic structure of an electrochemical cell, exemplified as a PEM electrolysis cell, with a gas diffusion layer; Fig. 2 Simplified representations of a multilayer gas diffusion layer according to a conventional structure; Fig. 3 corresponding views of a multilayer gas diffusion layer with one opposite Fig. 2 improved design; Fig. 4 a sectional view of a gas diffusion layer with fastening of two layers; Fig. 5 a sectional view of a gas diffusion layer with alternative fastening of two layers; Fig. Figure 6 shows a simplified representation of a multi-layered gas diffusion layer.

[0084] In Fig. Figure 1 schematically shows the structure of an electrochemical cell 3, which is configured as an example electrolysis cell 3 for a PEM electrolysis cell. The electrochemical cell 3 is part of an electrolyzer (not shown in detail here) for splitting water (H₂O) by direct current to produce hydrogen and oxygen.

[0085] The electrochemical cell 3 comprises an electrolyte consisting of a proton-conducting membrane 13 (proton exchange membrane, PEM), on which electrodes 15a and 15b are located on both sides. The unit consisting of membrane 13 and electrodes 15a and 15b is commonly referred to as a membrane electrode assembly (MEA). Electrode 15a is designated as the cathode, and electrode 15b as the anode. The electrodes 15a and 15b contain a catalyst material in a thin, catalytically active layer of approximately 1–1.5 mg / cm². 2The catalyst material is applied to the membrane. For example, on the anode side, it is iridium, which catalytically accelerates the conversion reaction, but is very expensive. Therefore, many efforts are being made to reduce the amount of catalyst material used on electrodes 15a and 15b.

[0086] Each of the electrodes 15a, 15b thus configured has a gas diffusion layer 1 present, an anodic gas diffusion layer 1 at the anode electrode 15b and a cathodic gas diffusion layer 1 at the cathode electrode 15a. The gas diffusion layers 1 are contacted by so-called bipolar plates 17. In the assembled state, these plates spatially separate an electrolysis stack consisting of a plurality of individual electrolysis cells 3 connected in series.

[0087] The electrolysis cell 3 is supplied with fully demineralized water H2O as reactant, which is converted at the anode 15b into oxygen gas O2 and protons H + is broken down. The protons H + They migrate through the electrolyte membrane 13 towards the cathode 15a. On the cathode side, they recombine to form hydrogen gas H2. Thus, hydrogen H2 and oxygen O2 are obtained as products.

[0088] In another embodiment, the electrochemical cell 3 can be designed as a galvanic cell or fuel cell for generating electricity. According to the invention, the gas diffusion layers 1 of such electrochemical cells 3 are analogous to those in Fig. The electrolysis cell 3 shown in Figure 1 is only to be modified accordingly with regard to this type of application. Therefore, without limiting the generality of the information, the following refers by way of example to an electrochemical cell 3 designed as an electrolysis cell 3.

[0089] The gas diffusion layer 1 is a planar component and a crucial functional layer for the electrolysis cell 3, performing various tasks during its operation. The gas diffusion layer 1 ensures optimal water distribution and the removal of the product gases hydrogen (H₂) and oxygen (O₂). In the case of a galvanic cell, the gas diffusion layers 1 facilitate the supply of reactants to the respective electrodes. A key aspect here is that the gas diffusion layer 1 must be sufficiently permeable to the gaseous products or reactants to allow their removal. This requires a certain degree of porosity to enable and facilitate this transport.

[0090] The gas diffusion layer 1 also serves as a current distributor, particularly in an electrolysis cell 3. For this reason, the gas diffusion layer 1 is made of an electrically conductive, porous material. It is important to achieve a uniform, i.e., homogeneous, current distribution so that, during operation, the current density at electrodes 15a, 15b is as homogeneous as possible across the catalytically active area, and this persists over a longer operating time. This reduces or prevents degradation of the electrolysis cell 3. A uniform and multiple contact pattern at electrodes 15a, 15b also has a positive effect on the amount of material required for the catalyst.

[0091] Likewise, a structure that is as fine-pored or microporous as possible at the contact surface of the gas diffusion layer 1 with the respective electrode 15a, 15b.

[0092] In the illustrated embodiment, the gas diffusion layer 1 also compensates for component tolerances, in particular those of the adjacent bipolar plates 17. The gas diffusion layer 1 therefore contains a plurality of superimposed layers, with an outer layer, which rests against the bipolar plate, being designed as a spring component that may, for example, exhibit a progressive spring characteristic. The gas diffusion layer 1 comprises, in particular, a contacting component, a diffusion component, and the spring component, which differ from one another specifically with regard to their structure and / or composition. The gas diffusion layer 1 of electrolysis cells 3, comprising several layered diffusion layers, must also meet particularly high requirements with regard to the respective height and thickness profile and, due to the predetermined installation space of the cell frame, must be designed and manufactured with a very precise fit for use in an electrolysis cell 3.Dimensional accuracy and control of the permissible thickness are becoming increasingly important, particularly for large-format, planar gas diffusion layer 1, such as those currently being developed and designed for use in high-power electrolyzers. Therefore, for industrial manufacturing processes with high production volumes and large effective functional areas, quality assurance during the thickness adjustment of the gas diffusion layer 1 is crucial. Furthermore, the permissible thickness tolerances must be monitored during the axial stacking and mechanical clamping of numerous electrolysis cells 3 to form a high-performance electrolyzer.

[0093] For example, in PEM water electrolyzers, a fine-pored contact of the anode electrode 15b is becoming increasingly important, since the conventional production of the anode electrode 15b uses very expensive iridium, which is only available in small quantities. Due to the limited availability and high cost of iridium, efforts are being made to reduce its use. With a lower iridium content and the associated reduced transverse conductivity of the electrode, the distance between two electrical contacts, i.e., the pore diameter, must be reduced to achieve the same efficiency. The same applies to saving platinum and other catalyst materials on the cathode electrode 15a.

[0094] In the present case, the gas diffusion layer 1 is advantageously designed as an integrated multilayer structure, with a first gas diffusion layer 5 and a second gas diffusion layer 7, as shown in Fig. 2 and in Fig. 3 is described in more detail below.

[0095] A simplified representation of different views of a multilayer gas diffusion layer 1 is shown in Fig. Figure 2 shows the structure of the gas diffusion layer 1 according to a conventional design. The gas diffusion layer 1 is multi-layered and comprises a first gas diffusion layer 5 and a second gas diffusion layer 7, each made of a porous, electrically conductive material. The gas diffusion layers 5 and 7 are stacked tightly on top of each other and fully connected to one another via a connecting surface 21 by means of numerous weld points (not shown). Thus, the welded, metallurgical connection extends over and fills the entire connecting surface 21 between the gas diffusion layers 5 and 7. The gas diffusion layer 1 is installed in an electrolysis cell 3 accordingly. Fig. 1 The fine-pored or microporous gas diffusion layer 5, for example, lies on the anode electrode 15b coated with iridium as catalyst material. Contact is made over a surface area via a contact region 9. The second gas diffusion layer 7, on the other hand, has a coarse-pored structure and exhibits large pores. In the installation situation of the gas diffusion layer 1 in an electrolysis cell 3 – approximately corresponding to Fig. 1 - The coarse-pored gas diffusion layer 7 rests on the bipolar plate 17, which functions as a power and media distributor. In Fig. Figure 2 shows a sectional view in the left part of the figure and a top view of the connection surface 21 with the underlying second gas diffusion layer 7 in the right part. In this configuration of the gas diffusion layer 1, the weld extends over the entire connection surface 21 and, when installed in an electrolysis cell 3, also completely covers the contact area 9 with the anode electrode 15b. This entails numerous disadvantages and limits the pore size in the finely porous first gas diffusion layer 5, since the problem of blocked transport pores is particularly associated with the large-area weld or any other type of metallurgical bond. Due to the limitation of the pore size, the requirement for expensive catalyst material such as iridium or platinum is correspondingly higher.

[0096] However, a lower iridium loading per unit area of ​​the anode electrode 15b results in a reduced transverse conductivity of the anode electrode 15b. Therefore, the distance between two electrical contacts, i.e., the pore diameter in the first gas diffusion layer 5, must be reduced to achieve the same efficiency of the electrolysis cell 3.

[0097] In Fig. Figure 3 shows corresponding views of a multilayer gas diffusion layer 1 with a opposite Fig. 2 improved connection of the first gas diffusion layer 5 with the second gas diffusion layer 7 is shown, which avoids or reduces the aforementioned problems of pore closure, thus enabling the use of a fine-porous or microporous gas diffusion layer 5.

[0098] The gas diffusion layer 1 comprises a first gas diffusion layer 5 and a second gas diffusion layer 7, wherein the first gas diffusion layer 5 has a porous layer with fine pores and the second gas diffusion layer 7 has a coarse structure with coarse pores. The second gas diffusion layer 7 is applied to the first gas diffusion layer 5 and is metallurgically bonded to it by a weld joint that is only locally introduced within the connection surface 21. This results in an inner active area 23A and an outer edge area 23B that completely surrounds and is path-connected to the inner active area 21A within the connection surface 1. The intimate metallurgical bond in the form of a weld joint is limited to the edge area 23B. The connection between the first gas diffusion layer 5 and the second gas diffusion layer 7 is limited to the electrochemically inactive area of ​​the connection surface 21. In contrast to the one described in Fig. In the embodiment shown in Figure 2, the contact area 9 does not form an intimate, material-bonded connection in the form of a weld. A large number of weld points are placed over the entire edge area 21B, with a density of 4 to 100 weld points per cm². 2 This results in a firmly welded closed frame connection between the gas diffusion layers 5, 7 in the edge area 21B.

[0099] It is possible that a certain degree of fixation between the first gas diffusion layer 5 and the second gas diffusion layer 7 is also provided in the active area 23A. In this embodiment, the connecting surface 21, with an inner active area 23A and an outer edge area 23B surrounding the inner active area 23A, is designed such that the first gas diffusion layer 5 and the second gas diffusion layer 7 are only sporadically and point-fixed within the inner active area 23A. In this case, only a few weld points are placed within the active area 23A, designed to achieve point-fixed connections without melting any material in the active area 23A. The first gas diffusion layer 5 is then only very lightly attached to the second gas diffusion layer 7, a connection that can be adjusted via the local welding parameters.In this case, the area density of the weld or joint points placed in active area 23A is to be set significantly lower than the area density of the weld points placed in the boundary area 21B. Typically, the weld point density in active area 23A is set to a maximum of 9 per cm. 2 , especially up to a maximum of 4 per cm 2, selected. In the microporous layer of the first gas diffusion layer 5, the pores have a porosity of 30%–85%, particularly 50%–80%, wherein the first gas diffusion layer 5 can have a layer thickness typically between 0.2 mm and 1.1 mm and is formed by a particularly fine-pored expanded metal or another fine-pored structure that is both electrically conductive and fluid-conducting. The invention enables the use of even finer-pored expanded metals and other layer structures, which offer a cost advantage over sintered or nonwoven structures. The second gas diffusion layer 7 can be multi-layered and has a very open-pored or coarse-pored metallic expanded metal mesh with a mesh size in the range of 0.5 mm × 0.5 mm to 20.0 mm × 20.0 mm, for example from 1.5 mm × 1 mm to 2.5 mm × 2 mm.In the second gas diffusion layer 7, the coarse pores have a diameter that is 100 to 1000 times the diameter of the fine pores or micropores of the first gas diffusion layer 5.

[0100] In Fig. 4 a sectional view of a gas diffusion layer 1 accordingly Fig. Figure 3 shows, in which, however, the finely porous first gas diffusion layer 5 and the coarsely porous second gas diffusion layer 7 are connected to each other by an extended or extended attachment. Thus, it shows Fig. 4 For example, a gas diffusion layer 1 in which the first gas diffusion layer 5 is more extensive in area and correspondingly larger. The first gas diffusion layer 5 is bent over outside the edge region 23B, so that the side surface 25 of the second gas diffusion layer 7 is at least partially enclosed. The first gas diffusion layer 5 accommodates the second gas diffusion layer 7.

[0101] In Fig. Figure 5 shows a gas diffusion layer 1 in which the first gas diffusion layer 5 is even bent over in such a way that the end surface 27 of the second gas diffusion layer 7 opposite the connection surface 21 is at least partially enclosed. This creates a positive-locking connection between the first gas diffusion layer 5 and the second gas diffusion layer outside the spot-welded edge region 23B. It is possible that in both the embodiment of the Fig. 5 as well as in the embodiment of the Fig. 5. A material-bonded connection between the first gas diffusion layer 5 and the second gas diffusion layer 7 is also provided outside the edge area 23B. Accordingly, weld points can be locally introduced into selected side surfaces 25 and / or end surfaces 27 to provide an additional material bond. Both the first gas diffusion layer 5 and the second gas diffusion layer 7 can be multi-layered, meaning they can have finer graded porous substructures.

[0102] This shows Fig. 6 For example, a gas diffusion layer 1 in which the second gas diffusion layer 7 is multilayered with a plurality of superimposed individual layers 7a, 7b, 7c, 7d exhibiting macropores. Superimposed adjacent individual layers 7a, 7b, 7c, 7d are each locally welded to one another in the edge region 23B, so that the active region 23A does not exhibit a material bond. The diameter of the macropores in the second gas diffusion layer increases layer-specifically with the distance of a superimposed individual layer 7a, 7b, 7c, 7d in a direction perpendicular to the layer normal from the first gas diffusion layer 5.

[0103] Fig. Figure 6 shows a simplified representation of the structure and the material bond of a multilayer gas diffusion layer 1. The gas diffusion layer 1 is advantageously designed as an integrated multilayer structure, with a first gas diffusion layer 5 and a second gas diffusion layer 7. Except for the multilayer design, the basic structure essentially corresponds to that shown in Fig. 3 to Fig. 5 illustrated examples, where in Fig. 6 The successive construction and the material-bonded connection of a multilayer gas diffusion layer 1 is significant. A first gas diffusion layer 5 is shown, which has an electrically conductive microporous expanded metal structure 11. With this structure, a highly microporous layer with fine pores for fluid transport with high electrical conductivity is formed for the first gas diffusion layer 5. This layer exhibits a conductivity through the layer plane that is characterized by a volume resistance of at most 50 mΩ·cm. 2is characterized by. At the same time, the expanded metal structure 11 of the first gas diffusion layer 5 creates a finely porous structure characterized by a high porosity of, for example, 56% ± 3% with a layer thickness of only about 0.2 mm to 1.1 mm. Thus, a gas diffusion layer 5 designed in this way is specially configured and suitable for the requirements in an electrolysis cell 3, e.g., as an anodic contact layer on an anode electrode 15b of an electrolysis cell 3. Fig. 1. This increases the catalyst efficiency in electrolysis cell 3, or rather, it allows for more comprehensive utilization of the available catalyst surface for the electrochemical conversion. This makes it possible to reduce the catalyst coverage of a membrane electrode assembly to values ​​below 1.2 g / cm². 2 It can be reduced; typically, occupancy levels can be between 0.8 g / cm². 2 and 1.1 g / cm³ 2can be achieved and adjusted without significantly affecting the catalytic activity.

[0104] In contrast, a second gas diffusion layer 7 comprises a plurality of stacked individual layers 7a, 7b, 7c, 7d, each of which has a coarse structure with large pores and is firmly connected to form the second gas diffusion layer 7. Fig. 6 is the second gas diffusion layer 7, initially directly connected to the first gas diffusion layer 5 via a provided single layer 7a by means of a welded connection, specifically only locally in the edge region 23B of the connection surface 21. In contrast, the active area 23A does not have such a welded connection, so that pore closure is excluded and at the same time the loading with catalyst material, such as iridium, is correspondingly reduced.

[0105] In the exemplary embodiment, the single layer 7a is first welded to the first gas diffusion layer 5 in the manufacturing process. This forms a welded two-layer system 19 consisting of the first gas diffusion layer 5 and the single layer 7a in a structure that is locally bonded in the edge region 23B and is prepared for the subsequent manufacturing steps of the gas diffusion layer 1.

[0106] This welded two-layer system 19 is therefore an intermediate product, which, however, is easy to handle and further process, for example, it can be cut to a desired installation dimension. The material bond provides both a mechanically strong connection and a very low-resistance electrical contact. In the subsequent manufacturing steps, further coarse-pored metallic single layers 7b, 7c, 7d are successively bonded and applied to the welded two-layer system 19 by means of a capacitor discharge welding process. The adjacent single layers 7b, 7c, 7d are stacked on top of the single layer 7a of the two-layer system 19 on the two-layer system 19 – which is preferably also produced by a capacitor discharge welding process – and welded together in a single welding operation. A gantry welding machine, for example, can be used for this purpose.Thus, the second gas diffusion layer 7 is provided as a multilayer structure with large pores, the diameter of which is increased by a factor of 100 to 1000 compared to the pore diameter in the first gas diffusion layer, enabling the transport of a large volume flow of fluid. Advantageously, the porosity across the individual layers 7a, 7b, 7c, 7d is adjusted such that the diameter of the large pores increases layer-specifically with the spacing between the layered individual layers 7a, 7b, 7c, 7d. It is also possible for the first gas diffusion layer 5 to be designed not as a monolayer but as a multilayer fine-pored expanded metal structure 11, with the porosity of the expanded metal structure 11 being set to its maximum at the contact area 9 towards the electrode. This allows the loading with catalyst material to be reduced compared to known designs, while the weld-free or...At most, in the low-welding active area 23A, pore blockage is not to be feared.

[0107] Gas diffusion layer 1 is very well suited for polymer membrane electrolysis, particularly with acidic proton exchange membranes (PEM) or alkaline anion exchange membranes (AEM), depending on the material selection. Its use in alkaline electrolysis with diaphragms is also possible in principle.

[0108] Where applicable, all individual features shown in the exemplary embodiments can be combined and / or exchanged without leaving the scope of the invention.

Claims

[1] Gas diffusion layer (1) for an electrolysis cell (3), comprising a first gas diffusion layer (5) and a second gas diffusion layer (7), wherein the first gas diffusion layer (5) has a fine-porous layer with fine pores and the second gas diffusion layer (7) has a coarse structure with coarse pores, wherein the second gas diffusion layer (7) is applied to the first gas diffusion layer (5) and is metallurgically bonded to it in such a way that a connecting surface (21) with an inner active area (23A) and with an outer edge area (23B) surrounding the inner active area (21A) is formed, wherein the outer edge area (23B) has the metallurgical bond. [2] Gas diffusion layer (1) according to claim 1, wherein the first gas diffusion layer (5) and the second gas diffusion layer (7) are welded together in the edge region (25B). [3] Gas diffusion layer (1) according to claim 2, wherein a plurality of weld spots are placed over the entire edge surface (21B), the density of the weld spots being 1 to 100 per cm 2 amounts. [4] Gas diffusion layer (1) according to one of the preceding claims, wherein the first gas diffusion layer (5) is bent over outside the edge region (23B), the side surface (25) of the second gas diffusion layer (7) is at least partially enclosed, so that the first gas diffusion layer (5) accommodates the second gas diffusion layer (7). [5] Gas diffusion layer (1) according to claim 4, wherein the first gas diffusion layer (5) is bent such that the end surface (27) opposite the connecting surface (21) of the second gas diffusion layer (7) is at least partially enclosed. [6] Gas diffusion layer (1) according to claim 4 or 5, wherein a positive-locking connection is provided outside the edge region (23B) between the first gas diffusion layer (5) and the second gas diffusion layer (7). [7] Gas diffusion layer (1) according to claim 4, 5 or 6, wherein a material-bonded connection is provided between the first gas diffusion layer (5) and the second gas diffusion layer (7) outside the edge region (23B). [8] Gas diffusion layer (1) according to one of the preceding claims, in which a connecting surface (21) is formed with an inner active area (23A) and an outer edge area (23B) surrounding the inner active area (23A), wherein in the inner active area (23A) the first gas diffusion layer (5) is attached to the second gas diffusion layer (7) at isolated points. [9] Gas diffusion layer (1) according to claim 8, wherein in the active area (23A) isolated welding points are placed which are designed such that point fixings are effected without melting of material in the active area (21A). [10] Gas diffusion layer (1) according to claim 9, wherein the density of the weld spots placed in the active area (23A) is lower than the density of the weld spots placed in the boundary area (21B). [11] Gas diffusion layer (1) according to claim 9, wherein in the active area (23A) the density of the weld spots is up to a maximum of 9 per cm 2 , especially up to a maximum of 4 per cm 2 , amounts. [12] Gas diffusion layer (1) according to one of the preceding claims, wherein the finely porous layer has a porosity of 30% - 85%, in particular of 50% - 80%. [13] Gas diffusion layer (1) according to one of the preceding claims, wherein the first gas diffusion layer (5) has a layer thickness of 0.1 mm to 2.0 mm, in particular 0.2 mm and 1.1 mm. [14] Gas diffusion layer (1) according to one of the preceding claims, wherein the first gas diffusion layer (5) comprises a fine-pored plug-in metal, a metal fleece or a powder sintered fleece. [15] Gas diffusion layer (1) according to one of the preceding claims, wherein the second gas diffusion layer (7) is a metallic expanded metal mesh with a mesh size of 0.5 mm × 0.5 mm to 20.0 mm × 20.0 mm. [16] Gas diffusion layer (1) according to one of the preceding claims, wherein in the second gas diffusion layer (7) the coarse pores have a diameter corresponding to 100 times to 1000 times the diameter of the fine pores. [17] Gas diffusion layer (1) according to one of the preceding claims, wherein the second gas diffusion layer (7) is multilayered with a plurality of superimposed individual layers (7a, 7b, 7c, 7d) having large pores, wherein superimposed adjacent individual layers (7a, 7b, 7c, 7d) are welded together. [18] Gas diffusion layer (1) according to claim 16, wherein the diameter of the macropores increases layer-specifically with the distance of a layered single layer (7a, 7b, 7c, 7d) in a direction perpendicular to the layer normal from the first gas diffusion layer (5). [19] Method for producing a gas diffusion layer (1) according to any one of the preceding claims, wherein - a first gas diffusion layer (5) is provided which has a porous layer with fine pores; - a second gas diffusion layer (7) is provided which has a coarse structure of large pores; - the second gas diffusion layer (7) is connected to the first gas diffusion layer (3) in such a way that a connecting surface (21) is formed with an inner active area (23A) and with an outer edge area (23B) enclosing the active area (23A), and wherein - locally in the outer edge area (23B) a material-bonded connection is established between the first gas diffusion layer (5) and the second gas diffusion layer (7). [20] Method according to claim 19, wherein the first gas diffusion layer (5) and the second gas diffusion layer (7) are welded together in the edge region (23B) using a capacitor discharge welding process. [21] Method according to one of claims 19 or 20, wherein a second gas diffusion layer (7) is provided, wherein a plurality of superimposed and coarse-pored single layers (7a, 7b, 7c, 7d) are provided, wherein superimposed adjacent single layers (7a, 7b, 7c, 7d) are each welded together, wherein a capacitor discharge welding process is applied. [22] Method according to claim 21, wherein in the second gas diffusion layer (7) the porosity of the macropores in a single layer (7a, 7b, 7c, 7d) is adjusted, such that a graded layering is formed by the layered single layers (7a, 7b, 7c, 7d), wherein the porosity increases with the distance of a layered single layer (7a, 7b, 7c, 7d) perpendicular to the layer normal from the first gas diffusion layer (5). [23] Electrolysis cell (3) with a gas diffusion layer (1) according to any one of claims 1 to 17. [24] Electrolyzer with an electrolysis cell (3) according to claim 23.

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