Gas diffusion layer for an electrolysis cell

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), wherein the first gas diffusion layer (5) has a microporous layer with fine pores, which is formed by a sintered composite structure made of a conductive nonwoven material (9) and a sintered material (11), and wherein 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 connected to it. The invention further relates to a method for producing a gas diffusion layer (1).

Description

The present invention relates to a gas diffusion layer for an electrolytic cell and a method for manufacturing a gas diffusion layer.The splitting of water by electric current to produce hydrogen and oxygen gas by means of an electrolysis cell is well known. A distinction is made here mainly between two industrial systems, alkaline electrolysis and PEM (proton exchange membrane electrolysis.An electrolytic cell comprises two half-cells, an anodic half-cell and a cathodic half-cell, wherein both half-cells are located together on a membrane. Each half-cell has a so-called bipolar plate which contacts a gas diffusion layer, wherein a gas diffusion layer can be composed of a plurality of gas diffusion layers, wherein a gas diffusion layer contacts the electrode on the membrane or itself represents the electrode, for example. The electrode describes the component which is arranged on the catalyst and on which the electrochemical reaction takes place. The arrangement of the electrodes and the application of an electrolysis voltage in electrolysis operation result in an electrical potential and a current flow through which ions are set in motion. The ions are moved in a conductive electrolyte, i.e. the liquid electrolyte in each half-cell, and a potentially solid electrolyte (membrane) separating the half-cells from one another. As a result of the ion flow through the electrolytes, an ion current finally flows from one half cell to the other half cell, wherein hydrogen or oxygen is generated at the respective electrodes. The cellular reactions of hydrogen and oxygen formation in an alkaline medium are:Anode40H -->2H 2 O+O 2 + 4 e -,E = +0.40 VCathode Cathode2H 2 O+2e->2OH-+H 2,E° = -0.83 VThe cell reactions of hydrogen and oxygen formation in a PEM electrolysis are:Anode2H 2 O→4H++ O 2 + 4 e -,E = +1.23 VCathode Cathode4H + + 4e - → 2H 2E° = 0.00 VSpatial separation of the cell reactions is made possible by the abovementioned membrane, which allows ionic transport through the electrolysis cell. In the case of anion exchange membrane electrolysis (AEM water electrolysis), this is effected by the use of a hydroxide ion-conducting membrane. A central cell component is the membrane electrode unit (MEA for short). The MEA consists on both sides of the respective catalyst-electrode composite and a solid-state membrane (SPA).In the case of PEM electrolysis, the proton-conducting polymer membrane is a semi-permeable membrane of ionomers. Ionomers belong to the group of thermoplastics, but have the advantage over the thermoplastics that ionic bonds become effective in them and, in contrast to most plastics, they can be used as electrolytes. The gas impermeability of the PEM ensures that the products hydrogen and oxygen do not mix. In this process, the hydrogen has a higher purity than in other electrolysis processes. The PEM electrolysis cell is fed on the O 2- side with demineralized water, which is decomposed at the anode into oxygen gas and protons (H +). The protons migrate through the proton-conducting membrane and recombine at the cathode (H 2- side) to form hydrogen gas. The gas diffusion layer which bears against the electrodes and is typically composed of a plurality of gas diffusion layers ensures, in addition to the electrode contacting, the optimum water distribution and thus the wetting of the membrane and the removal of the product gases. What is therefore required as gas diffusion layer is an electrically conductive, porous element with good permanent contacting of the electrode. As an additional requirement, component tolerances which may occur in the electrolyser should be compensated for in order to enable the most uniform possible contacting of the MEA in each tolerance case.A plurality of electrolysis cells are then typically connected in series in so-called stacks. Each electrolyser has a single or a plurality of stacks.An electrolysis cell with the electrodes represents in particular an electrical assembly, wherein electrical resistances or impedances likewise occur at the individual components of the assembly. These electrical resistances ultimately dictate the operating voltage of the electrolysis cell.Two resistors are of particular importance for industrial operation, namely the activation resistance of the electrolysis reaction and the ohmic resistance of the electrolysis cell. The activation resistance is decisively predefined by the electrodes, or the catalyst of the electrolysis reaction and its environment. The ohmic resistance is influenced by all conductive components. In particular, in the case of ohmic resistance, membrane resistances or electrolyte resistances and contact resistances play a higher-order role.The contact resistance between a gas diffusion layer and a bipolar plate is a function of the contact pressure which is applied by clamping elements such as tie rods, clamping springs or clamping straps.According to the prior art, the gas diffusion layers are realized as expanded metal, wire braids and rigid embossed bipolar plates, on which a metal- or carbon-based nonwoven fabric is optionally laid. The rigidity of these assemblies is extremely high and the compressibility is relatively low. This has the consequence that, in the event of inadequate plane parallelism, the bipolar plates and the gas diffusion layers are pressed against one another inhomogeneously and contact surfaces of different sizes are thus formed. Deviations in the local contact pressure thus ultimately lead to locally different contact resistances and thus to an undesired current density distribution, which promotes, for example, the melting or welding together or the corrosion of the components. These effects are associated with a degradation of the electrolysis cell and of the electrolyser which is disadvantageous over the operating time.In order to keep the contact resistance low, for example, the individual metal layers or wire fibers have hitherto been welded or rigidly woven together. However, this leads to increased rigidity and to possible scale points. Gas diffusion layers composed of expanded metals exhibit irreversible settling behavior, or such gas diffusion layers are plastically deformed under the compression applied.Proceeding from the known prior art, it is an object of the present invention to specify a gas diffusion layer which is improved with respect to the degradation tendency, and to provide a corresponding production method.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 microporous layer with fine pores which is formed by a sintered composite structure made of a conductive nonwoven material and made of a sintered material, and wherein 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 connected thereto.The invention is already based on the finding that the configuration of a gas diffusion layer known from the prior art is disadvantageous. Here, the components of the coarse structured part of a gas diffusion layer and the fine structured gas diffusion layer are loosely put one upon another in the electrolytic cell. However, this loose stacking entails a wide variety of disadvantages in electrical contacting, handling during assembly and possible points of attack by corrosion. In addition, for future product developments, even the mesh width of so-called micromeshes is too coarse-mesh, for which reason the typically rolled expanded lattice is to be expanded by a significantly finer PTL "porous transport layer" or even MPL "microporous transport layer".The invention has recognized that the associated technical challenges require 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.The degree of finest porosity is significantly increased by the use of a sintered structure in the first gas diffusion layer. The sintered composite structure is formed by the conductive nonwoven material and the sintered material, wherein the conductive nonwoven material is uniformly embedded in the sintered material and is intimately connected by the sintering process. This provides a very uniform and homogeneous microporous first gas diffusion layer with fine pores. At the same time, the sintered composite structure creates a very uniform electrical contact via the surface and a homogeneous current conduction, in particular with a very homogeneous local surface current density, through the first gas diffusion layer. A first gas diffusion layer configured in this way is particularly suitable for electrically contacting an electrode of an electrolysis cell, for example a membrane electrode unit. At the same time, a very efficient and uniform media transport of the educts and products of the electrolysis is achieved by the sintered composite structure of the first gas diffusion layer. The finely porous sintered composite structure can be used here both for contacting the anode electrode with applied catalyst and a catalytically coated cathode electrode of an electrolysis cell. The first gas diffusion layer thus acts as a sintered functional layer for contacting and media transport, wherein an intrinsic support structure is realized in the fine-porous first gas diffusion layer by the conductive nonwoven material embedded and enclosed in the sintered material.The first gas diffusion layer can be connected to the second coarse-porous gas diffusion layer in various ways, for example by the second gas diffusion layer being connected to the first gas diffusion layer in a force-fit, form-fit or firmly bonded manner, so that there is a firm mechanical connection of the two gas diffusion layers with simultaneously good electrical contact. For this purpose, the material of the second gas diffusion layer has a high electrical conductivity and a roughly porous structure with simultaneous corrosion resistance for use in an electrolysis cell. The second gas diffusion layer can be designed, for example, as an open-pored metallic nonwoven or an expanded lattice. In this way, a two-layer system is provided with a fine-porous and sintered composite layer and a layer with larger pores or coarse pores for the media transport and the power supply.In a particularly preferred embodiment of the gas diffusion layer, the conductive nonwoven material is formed from a metallic fiber material in the first gas diffusion layer.The first gas diffusion layer therefore has a fine structure which provides as homogeneous a current distribution as possible and a transport of media. The first layer is preferably provided from a nonwoven of conductive fibers, which, however, alone does not have the necessary mechanical stability and is therefore embedded in the sintered material in a sintered composite structure and is thereby mechanically supported. The coarse structure of the second gas diffusion layer comprises in particular coarse pores, while the fine structure has fine pores. A pore is a recess in the surface of a respective layer and within a layer, such that flow channels are formed for effective fluid transport through the gas diffusion layer perpendicular to the normal of the gas diffusion layer. A coarse pore is only a significantly larger spatial cutout than a fine pore or micropore.In a further preferred configuration of the gas diffusion layer, in the first gas diffusion layer, the fine pores of the microporous layer are formed by a fibrous structure, the fiber thickness being between 10 μm and 50 μm.In this case, in particular braided fine-mesh mesh mesh structures made of metal fibers are introduced into the sintered metal-ceramic composite structure. As a result, a very good transverse conductivity, i.e. a conductivity within the layer and thus a very good distribution of the current, can be achieved. The current flowing from the contact points to the electrodes of an electrolysis cell in the installation situation is distributed homogeneously with such a structure.Preferably, the conductive nonwoven material comprises or is formed from grade 1 technical grade titanium. Wires, braids, knits and titanium-based webs of this highest purity grade are particularly suitable and advantageous for use. Thus, for example, a titanium fiber fleece in a sintered structure can achieve a particularly fine-porous or microporous structure with good electrical conductivity. In this case, the porosity is increased by the sintering process, in particular by the sintering shrinkage, compared to conventional micro fleeces. The target area in the case of the nonwoven material is, for example, 50-60% and, in the case of sintered powder, at least 30-40%. The porosity can be measured in a simple manner by means of a volume-weight measurement of the gas diffusion layer and corresponding ratio formation.Preferably, in the gas diffusion layer in the microporous layer of the first gas diffusion layer, the fine pores have a porosity of typically 30%-50%, in particular of 30%-40%. It may even be particularly preferred to provide a somewhat greater porosity of more than 50% in the first gas diffusion layer, in particular about 55%-56%, which suggests even better properties for electrode contacting in an electrolysis cell.In a particularly preferred embodiment of the gas diffusion layer, the first gas diffusion layer has a layer thickness of between 0.2 mm and 1 mm. This allows good further processing and handling by production of the metal-ceramic sintered composite of the first gas diffusion layer and easy application and connection of the second gas diffusion layer to the first gas diffusion layer. The first gas diffusion layer can be cut into the desired shape, for example, to fit the installation situation, because of the small layer thickness. The material use for this fine-porous individual layer as a functional layer is also reduced with an even better result for the electrical contacting and the media transport.In a further preferred configuration of the gas diffusion layer, the second gas diffusion layer comprises a metallic expanded lattice having a mesh width of 1.5 mm×1 mm to 2.5 mm×2 mm.The metallic expanded lattice provides a roughly porous structure which at the same time has good electrical conductivity. At the same time, a certain elasticity and spring property are achieved. In addition to expanded metal grids or layers, wire mesh fabrics or folded sheets, combinations can also be used. Structures are advantageous which allow a certain mechanical compression and thus do not lead to material stresses and thus to inhomogeneous pressure peaks which in the worst case lead to mechanical stresses or undesired wedging in the installation situation in an electrolysis cell.In a particularly preferred embodiment of the gas diffusion layer, the coarse pores in the second gas diffusion layer have a diameter which corresponds to 100 times to 1000 times the diameter of the fine pores.Due to the pore diameter in the second gas diffusion layer set larger by orders of magnitude in comparison to the first gas diffusion layer, a larger volume flow of fluid can be quickly discharged through the second gas diffusion layer with moderate or low pressure losses. Above all, the product stream from a water electrolysis, which contains water and gas bubbles of hydrogen or oxygen, is characterized by a large volume flow. These fluid transport properties are ensured by the coarse pores. At the same time, the metallic and large-pore design of the second gas diffusion layer, in particular a metallic expanded lattice, realizes a mechanical supporting structure of the first gas diffusion layer and also electrical conductivity for the current conduction and power supply of the first gas diffusion layer.In a particularly advantageous embodiment of the gas diffusion layer, the first gas diffusion layer and the second gas diffusion layer are sintered to one another, in particular diffusion sintered.In this way, a particularly advantageous cohesive connection of the layers is achieved, wherein the sintered material is adapted for sintering the first gas diffusion layer to the second gas diffusion layer; in particular, the same sintered material can be used as for the first gas diffusion layer. The interfacial adhesion between the first gas diffusion layer and the second gas diffusion layer is very promoted by the material bond. The type of connection can then be configured in particular in such a way that firstly a construction of the first gas diffusion layer from a titanium fiber fleece with the sintered material and expanded lattice connected thereto with a mesh width of 1.5 mm x 1 mm to 2.5 mm x 2 mm is provided. The connection to the second coarse-porous gas diffusion layer is realized by a sintering process. In this process, fine-grained ceramic or metallic substances are heated as the sintered material-often under elevated pressure-but the temperatures remain below the melting temperature of the main components so that the shape (shape) of the workpiece is maintained. This usually results in shrinkage because the particles of the starting material compress and pore spaces are at least partially filled. This realizes a sintered two-layer composite in which the second gas diffusion layer is integrally sintered with the first gas diffusion layer with the sintering material.In a particularly advantageous embodiment of the gas diffusion layer, the second gas diffusion layer is designed in multiple layers, having a plurality of individual layers layered one above the other and having coarse pores, wherein adjacent individual layers layered one above the other are welded to one another.The multilayer embodiment of the second gas diffusion layer comprising a plurality of individual layers is very advantageous since it enables layer-specific adaptation of the properties for the fluid transport of starting materials and products and at the same time enables uniform current supply. The porosity can thus be adjusted via the individual layers. It has been found that the configuration as a welded connection for the connection of the individual layers of the second gas diffusion layer is particularly advantageous. Since during operation in an electrolysis cell for the second gas diffusion layer the transport of relatively large volume flows of fluid predominates, a finely porous sintered structure-as provided for the first gas diffusion layer-would be disadvantageous within the layer composite of the second gas diffusion layer. In contrast, it has proven very advantageous to combine a sintered connection between the first and the second gas diffusion layer with the welded single-layer structure within the second gas diffusion layer. Considerable advantages arise here, both from the manufacturing standpoint and with regard to the electrical properties and the transport properties of the gas diffusion layer as a porous transport layer. The multilayer construction of the second gas diffusion layer can be effected, for example, from a stack or composite of at least two or more expanded grids. The expanded gratings have a preferably rectangular shape and an aspect ratio of at least 1.2. expanded gratings having an aspect ratio of in particular at least greater than 2.0 are advantageous. Such a stack or multilayer composite of expanded grids can be introduced into a portal welding machine and welded together there with the aid of so-called punches, which form the welding electrodes.In an advantageous embodiment of the gas diffusion layer, the diameter of the coarse pores increases layer-specifically with the distance of a layered individual layer from the first gas diffusion layer in a direction perpendicular to the layer normal.In this case, a mean value, i.e. a mean pore diameter of a distribution, can be considered to be a porosity measure of an individual layer, by means of which a desired graded structure can be adjusted and achieved. A porosity gradient can thus be set and an adaptation and specific increase of the fluid conductivity, in particular of the volume flow line, for the required media transport. In this case, the fluid conductivity perpendicular to the layer normal increases, or the flow resistance decreases accordingly.The object set above is furthermore achieved by a method for producing a gas diffusion layer, in which a first gas diffusion layer is provided, in which a conductive nonwoven material and a sintered material are sintered together, such that a microporous layer having fine pores is formed in the sintered composite structure, in which a second gas diffusion layer having a coarse structure of coarse pores is furthermore provided, wherein the second gas diffusion layer is connected to the first gas diffusion layer (3) in such a way that both a mechanical connection and an electrical contacting of the gas diffusion layers are effected.Advantageous refinements of the method are evident from the dependent claims and from the present description and the figures.Accordingly, a method for producing a gas diffusion layer is proposed, wherein the first gas diffusion layer and the second gas diffusion layer are sintered to one another, in particular diffusion sintered.In sintering, the first gas diffusion layer and the second gas diffusion layer are laid one on top of the other and then baked together using a sintered material. The sintered material can be a metallic or ceramic sintered material or else metal-ceramic sintered material mixtures. The first gas diffusion layer and the second gas diffusion layer are in particular not welded to one another, but are connected to one another in a materially bonded manner.As a result, the formation of mixed phases in the metal structure can be avoided in the gas diffusion layer. Such mixed phases can be triggered, for example, when the expanded metal layers are welded together by a high local introduction of heat. Mixed phases of this type can prevent the formation of passivation layers and promote corrosion as a result of the lack of passivation. Accordingly, by sintering the first gas diffusion layer and the second gas diffusion layer, the formation of a particularly uniform passivation layer is made possible, as a result of which the gas diffusion layer can be protected against corrosion and degradation.Furthermore, the first gas diffusion layer and the second gas diffusion layer form an integrally bonded individual component and an already functional unit for use in an electrolysis cell.The first gas diffusion layer and the second gas diffusion layer may be sintered in a protective gas atmosphere.Sintering in a protective gas atmosphere has the consequence that the first layer and the second layer connect to one another via diffusion, but no scale sites or oxidic deposits form on account of the absence of oxygen and the rather mild temperatures.The bond during sintering is always materially bonded and therefore offers optimum and uniform contact with a minimum contact resistance between the components, or the boundary surface between the first and the second gas diffusion layer. The surfaces of the first layer and the second layer may be degreased and mordanted in advance to facilitate sintering of the wires and fibers, and nonwoven fabrics.The sintering temperature can typically be between 700° C. and 1200° C., wherein a sintering temperature of 900° C. is preferably set. Alternatively or additionally, in the temperature application, the first gas diffusion layer and the second gas diffusion layer can be sintered at a pressure between 10 MPa and 100 MPa, preferably 40 MPa. Alternatively or additionally, a heating rate of 50° C. / min can be set.Preferred parameters for sintering are a sintering temperature of 900° C. at a pressure of 40 MPa and heating rates of 50° C. / min. This achieves a particularly good and at the same time, however, locally limited material bond at the boundary surface, with the result that the coarsely porous second gas diffusion layer is not impaired in its transport function for fluids. At the same time, a conductive connection is achieved by the material bond, in which a plurality of electrical contact points are formed and sintered with the first gas diffusion layer.The second gas diffusion layer can be coated with Ni and / or NiP and / or NiTi and / or NiFe as required.As a result, the current-carrying capacity and the ohmic resistance of the gas diffusion layer can be reduced, while the nickel base coating simultaneously has wear-inhibiting properties, such as abrasion and corrosion protection, for example.The coating with a corresponding coating material can be deposited electrolessly, galvanically or by gas phase methods, in particular sputtering, arc evaporation or electron beam evaporation.In an alternative configuration of the method, it is possible for the first gas diffusion layer and the second gas diffusion layer to be welded to one another, wherein in particular a capacitor discharge welding process is used.The type of connection can be configured, for example, for an electrolysis cell on the anode side as follows: A structure made of a titanium fiber fleece in a composite structure with sintered material as the first gas diffusion layer in a fine-porous sintered composite structure. A second gas diffusion layer is applied thereon, which has an open-pore expanded lattice of mesh width 1.5 mm×1 mm to 2.5 mm×2 mm, which is applied to the first gas diffusion layer by capacitor pulse welding, whereby the two layers are firmly connected to one another.Both the structure with a sintered composite structure and a welded joint of the first and second gas diffusion layers may optionally be advantageous. Both basic structures can be processed further as a basic composite in a simple manner, for example by capacitor pulse welding, and successively extended to form a more complex multilayer gas diffusion layer. In particular, the second gas diffusion layer can advantageously be embodied in multiple layers comprising a plurality of individual layers. GDL structures are hereby created, since a correspondingly greater current-carrying capacity is provided than the conductive nonwoven with the fine pores alone. The grid-shaped metallic structure creates defined contact points on the one hand with the sintered first gas diffusion layer and on the other hand with further individually structured individual layers of the second gas diffusion layer.Therefore, in a particularly advantageous configuration of the production method, it is provided that a second gas diffusion layer is provided by providing a plurality of individual layers which are layered one above the other and have coarse pores, wherein adjacent individual layers which are layered one above the other are each welded to one another, wherein in particular a capacitor discharge welding process is used.The method can be advantageously configured to the effect that, in the second gas diffusion layer, the porosity of the coarse pores in a single layer is adjusted to the effect that a graded lamination is formed by the layered single layers, wherein the porosity decreases with the distance of a layered single layer perpendicular to the layer normal from the first gas diffusion layer.In this way, a graded porous structure is achieved with an adjusted fluidic conductivity in the direction of the normal, and thus a desired porosity gradient can be taken into account and adjusted during production.The finer or more fine-pored the contacting of the gas diffusion layer (GDL) toward the electrode is configured in the installation situation of an electrolysis cell, the better is the catalyst utilization and the associated transverse and normal conductivity and the mechanical support of a membrane-electrode unit (MEA). In contrast to the so-called "Micromesh" expanded mesh used hitherto, a nonwoven or sintered metal enables contact points which are situated significantly more densely and thus makes it possible to make contact with the electrode over a more extensive area. A very dense and at the same time very fine-pored configuration of the first gas diffusion layer as a decisive contacting layer moreover leads to a significantly improved support of the MEA as such. Also, processing a solid sintered structure in downstream welding processes is significantly easier to accomplish. It is very advantageous that this results in savings of the very expensive catalyst material, such as iridium, for example, at the electrode of the MEA. Furthermore, an improved and in particular homogeneous current distribution is effected. Easier handling and further processing and subsequent connection and expansion to form a multilayer gas diffusion layer having a plurality of individual layers, for example by capacitor pulse welding, is very easily possible. This also involves easier handling of the gas diffusion layer during assembly and quality control.A further aspect of the invention relates to an electrolysis cell having a gas diffusion layer according to the invention.A further aspect of the invention relates to an electrolyser having an electrolysis cell according to the invention.Further advantages, features and details of the invention are evident from the following description of preferred exemplary embodiments and with reference to the drawing. The features and combinations of features mentioned above in the description and the features and combinations of features mentioned below in the description of the figures and / or shown alone in the single figures can be used not only in the respectively specified combination but also in other combinations or alone without departing from the scope of the invention.Exemplary embodiments of the invention are explained in more detail with reference to the drawings. Here, diagrammatically and greatly simplified show: FIG. 1 shows the basic structure of an electrochemical cell, which is exemplarily designed as a PEM electrolysis cell, with a gas diffusion layer; FIG. 2 shows a simplified illustration of the structure and bonding of a multilayer gas diffusion layer; FIG. 3 shows the structure of a multilayer gas diffusion layer with a compound modified from FIG. 2.FIG. 1 schematically shows the structure of an electrochemical cell 3, which is designed, for example, as an electrolysis cell 3 for a PEM electrolysis cell. The electrochemical cell 3 is part of an electrolyser, not shown in detail here, for splitting water H 2 O by direct electrical current for producing hydrogen and oxygen.The electrochemical cell 3 comprises an electrolyte made of a proton-conducting membrane 13 (proton exchange membrane, PEM), on both sides of which the electrodes 15 a, 15 bare located. The unit of membrane 13 and electrodes 15 a, 15 bis usually referred to as a membrane-electrode unit (MEA). The electrode 15a is a cathode and the electrode 35b is an anode. The electrodes 15a, 15b comprise a catalyst material coated on the membrane in a thin catalytically active layer of about 1-1.5 mg / cm 2. The catalyst material is, for example, iridium on the anode side, which catalytically accelerates the conversion reaction, and which is very expensive. Therefore, there are many efforts at the electrodes 15a, 15b to cope with less catalyst loading.A gas diffusion layer 1, an anodic gas diffusion layer 1 and a cathodic gas diffusion layer 1 are each in contact with the electrodes 15 a, 15 b thus configured. In the assembled state of an electrolysis stack comprising a multiplicity of individual electrolysis cells 3 connected electrically in series, these separate the electrolysis cells 3 spatially from one another.The electrolysis cell 3 is fed with demineralized water H 2 O as reactant, which is decomposed at the anode 15 binto oxygen gas O 2 and protons H +. The protons H + migrate through the electrolyte membrane 13 in the direction of the cathode 15a. On the cathode side, they recombine to hydrogen gas H 2. Thus, hydrogen H 2 and oxygen O 2 are obtained as products.In another exemplary embodiment, the electrochemical cell 3 can be designed as a galvanic cell or fuel cell designed for power generation. According to the invention, the gas diffusion layers 1 of electrochemical cells 3 formed in this way are to be modified analogously to the electrolysis cell 3 shown in FIG. 1. Without limiting generality, therefore, reference is made below by way of example to an electrochemical cell 3 designed as an electrolysis cell 3.The gas diffusion layer 1 is a planar component and a functional layer which is important for the electrolysis cell 3 and has various tasks during operation of an electrolysis cell 3. In the case of a galvanic cell, the gas diffusion layers 1 serve in accordance with the supply of reactants to the respective electrodes. An essential aspect here is that the gas diffusion layer 1 is in any case sufficiently permeable to the gaseous products or educts so that their removal is possible. For this purpose, a porosity is to be provided in order to enable and promote this transport.The gas diffusion layer 1 additionally serves as a current distributor in particular at the same time in an electrolysis cell 3. For these reasons, the gas diffusion layer 1 is formed of an electrically conductive porous material. It is important that a uniform, i.e. homogeneous, current distribution is achieved, so that during operation the current density at electrodes 15 a, 15 bis as homogeneous as possible over the catalytically active surface, namely for a long operating time. Degradation of the electrolysis cell 3 can thereby be reduced or avoided. The most uniform and multiple contacting possible of the electrodes 15 a, 15 balso has a favourable effect on the necessary material use for the catalyst coating. Likewise, a structure with the smallest possible pores at the contact surface of the gas diffusion layer 1 with the respective electrode 15 a, 15 b.In the exemplary embodiment shown, component tolerances, in particular those of the adjacent bipolar plates 17, are compensated for by the gas diffusion layer 1. The gas diffusion layer 1 therefore contains a plurality of layers stacked one above the other, wherein an outer layer is formed as a spring component which can have, for example, 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 their composition. The gas diffusion layer 1 of electrolysis cells 3 comprising a plurality of layered diffusion layers must also meet particularly stringent requirements for the respective height or thickness profile and, owing to the predefined installation space of the cell frame, must be designed and manufactured with a very precise fit for use in an electrolysis cell 3. The dimensional accuracy and dimensional stability with respect to the permissible thickness are becoming increasingly important here, in particular in the case of large-format sheet-like gas diffusion layers 1, as they are currently being developed and projected for use in electrolysers of high performance. Therefore, for an industrial manufacturing process with high production volumes and large effective functional surfaces, quality assurance is very important in the thickness adjustment of the gas diffusion layer 1. In addition, the permissible thickness tolerances must be monitored during the axial stacking and mechanical bracing of a multiplicity of electrolysis cells 3 to form a high-performance electrolyser.In the present case, the gas diffusion layer 1 is advantageously configured as an integrated multilayer structure, having a first gas diffusion layer 5 and having a second gas diffusion layer 7, as is described in more detail below in FIG. 2 and in FIG. 3.In this case, FIG. 2 shows, in a simplified illustration, the successive structure and the connection of a multilayer gas diffusion layer 1. a first gas diffusion layer 5 is shown which is composed of an electrically conductive nonwoven material 9 and of a sintered material 9. With the sintered composite structure, a highly microporous layer having fine pores is formed for the first gas diffusion layer. The electrically conductive nonwoven material 9 comprises titanium of the grade 1, for example with the material designation ASTM B265 in a fine-fiber-like structure. This nonwoven material 9 is sintered with a metallic, ceramic or metal-ceramic sintered material 9 by a sintering process. This results in a finely porous solid first gas diffusion layer 5. This has a conductivity through the layer plane, which is characterized by a volume resistance of at most 50 mΩ·cm 2. At the same time, a fine-porous structure is realized with the sintered composite structure of the first gas diffusion layer 5, which is distinguished by a high porosity of 56%±3% with a simultaneously high density of the sintered structure, with a layer thickness of only approximately 0.2 mm to 1 mm. A gas diffusion layer 5 configured in this way is thus specially packaged and suitable for the requirements as an anodic contact layer on an anode-side electrode 15 of an electrolysis cell 3. It is thereby possible that the catalyst loading of a membrane electrode unit can be reduced to values of less than 1.2 g / cm 2 typically loading levels between 0.8 g / cm 2 and 1.1 g / cm 2 can be achieved and adjusted without significantly reducing the catalytic activity.A second gas diffusion layer 7, on the other hand, comprises a plurality of individual layers 7 a, 7 b, 7 c, 7 dstacked one on top of the other, each of which has a coarse structure with coarse pores and are firmly connected to one another overall to form the second gas diffusion layer 7. In FIG. 2, the second gas diffusion layer 7 is firstly directly connected to the first gas diffusion layer via a provided individual layer 7 a. In the exemplary embodiment, the individual layer 7 ais firstly sintered or bonded to the first gas diffusion layer 5 from the manufacturing standpoint, a sintered material 11 being used. As a result, a sintered two-layer system ( 19) is formed from the first diffusion layer 5 and the individual layer 7 ain a cohesive structure and is provided for the subsequent production steps of the gas diffusion layer 1. This sintered two-layer system 19 is therefore an intermediate product which, however, is easy to handle and further process, for example can be cut to a desired installation dimension. In this case, both a mechanically fixed connection and also already a very low-ohmic electrical contact-connection are effected via the material connection. In the subsequent production steps, further coarsely porous metallic individual layers 7 b, 7 c, 7 dare successively connected and applied to the sintered two-layer system ( 19) by means of a capacitor discharge welding process. The adjacent individual layers 7 b, 7 c, 7 dare in this case layered on top of one another on the sintered two-layer system ( 19) onto the individual layer 7 aof the two-layer system ( 19) and are welded together overall in a welding operation. In this case, for example, a portal welding machine can be used. Thus, the second gas diffusion layer 7 is provided with coarse pores as a multi-layer structure, wherein the diameter of the pores is increased by a factor of 100 to 1000 compared to the pore diameter in the first gas diffusion layer, so that the large volume flow of fluid can be transported. In this case, the porosity via the individual layers 7 a, 7 b, 7 c, 7 dis advantageously set such that the diameter of the coarse pores increases layer-specifically with the spacing of the layered individual layer 7 a, 7 b, 7 c, 7 d.FIG. 3 shows the structure of a multilayer gas diffusion layer 1 with a connection of the first gas diffusion layer 5 to the second gas diffusion layer 7 that is slightly modified compared to FIG. 2. Specifically, the connection of the single layer 7 awith the first gas diffusion layer 5 is modified. In the production, the first gas diffusion layer 1 and a single layer 7 aof the second gas diffusion layer 7 are first welded to each other, specifically using a capacitor discharge welding process. In further production steps, the further individual layers 7 b, 7 c, 7 dare successively layered one on top of the other and the respectively adjacent individual layers 7 a, 7 b, 7 c, 7 dare welded to one another. A capacitor discharge welding process is applied.Depending on the material selection, the gas diffusion layer 1 is suitable for polymer membrane electrolysis, in particular with acidic proton exchange membranes or alkaline anion exchange membranes. The use in alkaline electrolysis with diaphragms is also possible.Where applicable, all individual features illustrated in the exemplary embodiments can be combined with one another and / or interchanged without departing from the scope of the invention.

Claims

A gas diffusion layer (1) for an electrolytic 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 microporous layer with fine pores formed by a sintered composite structure of a conductive non-woven material (9) and a sintered material (11), and wherein the second gas diffusion layer (7) has a coarse structure with coarse pores, wherein the second gas diffusion layer (7) is applied to and bonded to the first gas diffusion layer (5).The gas diffusion layer (1) according to claim 1, wherein in the first gas diffusion layer (5), the conductive non-woven material (9) is formed of a metallic fiber material.The gas diffusion layer (1) according to claim 1 or 2, wherein in the first gas diffusion layer (5), the fine pores of the microporous layer are formed by a fibrous structure, wherein the fiber thickness is between 10 μm and 50 μm.The gas diffusion layer (1) according to claim 1, 2 or 3, wherein the conductive non-woven material (9) comprises technical grade 1 titanium.Gas diffusion layer (1) according to one of the preceding claims, in which, in the microporous layer, the fine pores have a porosity of 30%-50%, in particular of 30%-40%.Gas diffusion layer (1) according to one of the preceding claims, in which the first gas diffusion layer (5) has a layer thickness between 0.2 mm and 1 mm.The gas diffusion layer (1) according to any one of the preceding claims, wherein the second gas diffusion layer (7) comprises a metallic expanded mesh having a mesh width of 1.5 mm×1 mm to 2.5 mm×2 mm.The gas diffusion layer (1) according to any 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.Gas diffusion layer (1) according to one of the preceding claims, in which the first gas diffusion layer (5) and the second gas diffusion layer (7) are sintered to one another, in particular are diffusion sintered.Gas diffusion layer (1) according to one of the preceding claims, in which the second gas diffusion layer (7) is designed in multiple layers with a plurality of individual layers (7a, 7b, 7c, 7d) layered one above the other and having coarse pores, wherein adjacent individual layers (7a, 7b, 7c, 7d) layered one above the other are welded to one another.Gas diffusion layer (1) according to Claim 9, in which the diameter of the coarse pores increases layer-specifically with the distance of a layered single layer (7a, 7b, 7c, 7d) from the first gas diffusion layer (5) in a direction perpendicular to the layer normal.Method for producing a gas diffusion layer (1) according to one of the preceding claims, wherein - a first gas diffusion layer (5) is provided by sintering a conductive nonwoven material (9) and a sintered material (11) to one another, so that a microporous layer with fine pores is formed in the sintered composite structure, - a second gas diffusion layer (7) with a coarse structure of coarse pores is provided, - the second gas diffusion layer (7) is connected to the first gas diffusion layer (3) in such a way that both a mechanical connection and an electrical contacting of the gas diffusion layers (5, 7) are effected.Method according to Claim 12, in which the first gas diffusion layer (5) and the second gas diffusion layer (7) are sintered to one another, in particular diffusion-sintered.Method according to claim 13, wherein a sintering temperature is set between 700°C and 1200°C, preferably 900°C, and / or - the first gas diffusion layer (5) and the second gas diffusion layer (7) are sintered in a protective gas atmosphere and / or - the first gas diffusion layer (5) and the second gas diffusion layer (5) are sintered with a pressure between 10 MPa and 100 MPa, preferably 40 MPa and / or - the heating rate is 50°C / min.The method according to claim 12, wherein the first gas diffusion layer (5) and the second gas diffusion layer (7) are welded together, in particular using a capacitor discharge welding process.The method according to any one of claims 12 to 15, wherein a second gas diffusion layer (7) is provided, wherein a plurality of single layers (7a, 7b, 7c, 7d) stacked one on the other and having coarse pores are provided, wherein adjacent single layers (7a, 7b, 7c, 7d) stacked one on the other are welded to each other, respectively, wherein a capacitor discharge welding process is particularly applied.Method according to Claim 16, in which, in the second gas diffusion layer (7), the porosity of the coarse pores in a single layer (7a, 7b, 7c, 7d) is adjusted, such that a graded lamination is formed by the layered single layers (7a, 7b, 7c, 7d), the porosity increasing with the distance of a layered single layer (7a, 7b, 7c, 7d) perpendicular to the layer normal from the first gas diffusion layer (5).An electrolytic cell (3) having a gas diffusion layer (1) according to any one of claims 1 to 11.Electrolyser having an electrolysis cell (3) according to Claim 18.

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