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 lamina (5) and a second gas diffusion lamina (7), the first gas diffusion lamina (5) comprising a microporous layer with fine pores that is formed by a sintered composite structure composed of a conductive nonwoven material (9) and a sinter material (11), and the second gas diffusion lamina (7) comprising a coarse structure with coarse pores, the second gas diffusion lamina (7) being applied on and joined to the first gas diffusion lamina (5). The invention additionally relates to a method for producing a gas diffusion layer (1).

Description

[0001] Description

[0002] Gas diffusion layer for an electrolysis cell

[0003] The present invention relates to a gas diffusion layer for an electrolysis cell and a method for producing a gas diffusion layer.

[0004] The splitting of water by electrical 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).

[0005] An electrolysis cell comprises two half-cells, an anodic half-cell and a cathodic half-cell, with both half-cells being connected to a membrane. Each half-cell has a so-called bipolar plate that contacts a gas diffusion layer. A gas diffusion layer can be composed of several gas diffusion layers, with a gas diffusion layer, for example, contacting the electrode on the membrane or representing the electrode itself. The electrode describes the component that is arranged on the catalyst and at which the electrochemical reaction takes place. The arrangement of the electrodes and the application of an electrolysis voltage during electrolysis operation results in an electrical potential and a current flow, which sets ions in motion. The movement of the ions takes place in a conductive electrolyte, i.e.the liquid electrolyte in each half-cell, as well as a potentially solid electrolyte (membrane) that separates the half-cells. The ion flow through the electrolytes ultimately causes an ion current to flow from one half-cell to the other, producing hydrogen and oxygen at the respective electrodes. The cellular reactions for hydrogen and oxygen formation in an alkaline environment are: anode 40H- 2H2O + O2 + 4e~, E = +0.40 V.

[0006] Cathode 2H2O + 2e~ 2OH- + H2E° = -0.83 V

[0007] The cell reactions for hydrogen and oxygen formation in PEM electrolysis are:

[0008] Anode 2H2O 4H+ + O2+ 4e~, E = +1.23 V

[0009] Cathode 4H+ + 4e~ 2H2E° = 0.00 V

[0010] Spatial separation of the cell reactions is made possible 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 for short). The MEA consists of the respective catalyst-electrode assembly and a solid polymembrane (SPA) on both sides.

[0011] In the case of 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 active in them and, unlike most plastics, they can be used as electrolytes. The PEM's gas impermeability ensures that the hydrogen and oxygen products do not mix. In this process, the hydrogen is of a higher purity than in other electrolysis processes. The PEM electrolysis cell is fed with demineralized water on the O2 side, which is broken down into oxygen gas and protons (H+) at the anode. The protons migrate through the proton-conducting membrane and recombine at the cathode (H2 side) to form hydrogen gas.The gas diffusion layer adjacent to the electrodes, typically composed of several gas diffusion layers, ensures not only electrode contact but also optimal water distribution, thus wetting the membrane, and the removal of product gases. Therefore, an electrically conductive, porous element with good, permanent contact with the electrode is required as the gas diffusion layer. As an additional requirement, any component tolerances that may arise in the electrolyzer must be compensated to ensure the most uniform contact with the MEA under all tolerance conditions.

[0012] Several electrolysis cells are then typically connected in series in so-called stacks. Each electrolyzer has one or more stacks.

[0013] An electrolysis cell with its electrodes represents, in particular, an electrical assembly, with electrical resistances or impedances also occurring at the individual components of the assembly. These electrical resistances ultimately determine the operating voltage of the electrolysis cell.

[0014] 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 largely determined by the electrodes, or rather, the catalyst of the electrolysis reaction, and its environment. The ohmic resistance is influenced by all conductive components. In the case of the ohmic resistance, the membrane resistances, or rather, the electrolyte resistances and contact resistances, play a particularly important role.

[0015] The contact resistance between a gas diffusion layer and a bipolar plate is a function of the contact pressure, which is applied by tensioning elements such as tension rods, tension springs or tension belts.

[0016] According to the state of the art, the gas diffusion layers are made of expanded metal, wire mesh, and rigid, embossed bipolar plates, onto which a metal- or carbon-based fiber fleece is optionally placed. The rigidity of these assemblies is extremely high, while their compressibility is relatively low. This means that, if the plane parallelism is insufficient, the bipolar plates and the gas diffusion layers are pressed against one another inhomogeneously, resulting in contact surfaces of different sizes. Deviations in the local contact pressure ultimately lead to locally different contact resistances and thus to an undesirable current density distribution, which can, for example, promote melting or welding together, or corrosion of the components. These effects are accompanied by degradation of the electrolysis cell and the electrolyzer over the operating period, which is detrimental.

[0017] To keep contact resistance low, the individual metal layers or wire fibers are currently welded or rigidly woven together. However, this leads to increased rigidity and potential scaling.

[0018] Gas diffusion layers composed of expanded metals show irreversible settling behavior, or such gas diffusion layers are plastically deformed under the applied compression.

[0019] Starting from the known prior art, it is an object of the present invention to provide a gas diffusion layer which is improved with regard to the tendency to degradation, and to provide a corresponding manufacturing method.

[0020] 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, the first gas diffusion layer having a microporous layer with fine pores which is formed by a sintered composite structure made of a conductive nonwoven material and a sintered material, and the second gas diffusion layer having a coarse structure with coarse pores, the second gas diffusion layer being applied to the first gas diffusion layer and being connected to it. The invention is based on the knowledge that the configuration of a gas diffusion layer known from the prior art is disadvantageous. Here, the components of the coarsely structured part of a gas diffusion layer and the finely structured gas diffusion layer are laid loosely on top of one another in the electrolysis cell.However, this loose stacking brings with it numerous disadvantages in electrical contact, handling during assembly, and potential corrosion attack points. In addition, even the mesh size of so-called micromeshes is too coarse for future product developments, which is why the typically rolled expanded metal mesh is to be expanded with a significantly finer PTL ("porous transport layer") or even MPL ("microporous transport layer").

[0021] The invention has recognized that the associated technical challenges require a comprehensive consideration and consideration of both the mechanical and the electrical integration or combination of a very fine-pored first gas diffusion layer with a coarse-structured, coarse-pored second gas diffusion layer.

[0022] The degree of the finest porosity is significantly increased by the use of a sintered structure in the first gas diffusion layer. The sintered composite structure is formed from the conductive nonwoven material and the sintered material, whereby the conductive nonwoven material is evenly embedded in the sintered material and intimately bonded 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 very uniform electrical contact across the surface and 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 assembly.At the same time, a very efficient and uniform media transport of the reactants and products of electrolysis is achieved through the sintered composite structure of the first gas diffusion layer. The fine-pored sintered composite structure can be used both for contacting the anode electrode with applied catalyst and for 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, with the conductive fleece material embedded in and enclosed in the sintered material creating an intrinsic support structure in the fine-pored first gas diffusion layer.

[0023] The first gas diffusion layer can be connected to the second, coarse-pored 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-fitting, form-fitting or material-fitting manner, so that there is a strong mechanical connection between the two gas diffusion layers and, at the same time, good electrical contact. For this purpose, the material of the second gas diffusion layer has high electrical conductivity and a coarse-pored structure while at the same time being corrosion-resistant for use in an electrolysis cell. The second gas diffusion layer can, for example, be designed as an open-pored metallic fleece or an expanded metal mesh. In this way, a two-layer system is provided with a fine-pored and sintered composite layer and a layer with larger pores or coarse pores for media transport and power supply.

[0024] In a particularly preferred embodiment of the gas diffusion layer, the conductive nonwoven material in the first gas diffusion layer is formed from a metallic fiber material.

[0025] The first gas diffusion layer therefore has a fine structure which provides the most homogeneous current distribution and media transport possible. The first layer is preferably made from a fleece of conductive fibers which, however, does not have the necessary mechanical stability on its own 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 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.

[0026] In a further preferred embodiment of the gas diffusion layer, the fine pores of the microporous layer in the first gas diffusion layer are formed by a fibrous structure, wherein the fiber thickness is between 10pm and 50pm.

[0027] In particular, braided, fine-mesh network structures made of metal fibers are incorporated into the sintered metal-ceramic composite structure. This allows for very good transverse conductivity, i.e., conductivity within the layer, and thus a very good current distribution. The current, which flows from the contact points to the electrodes of an electrolytic cell during installation, is distributed evenly with such a structure.

[0028] The conductive nonwoven material preferably comprises or is made from grade 1 technical purity titanium. Wires, braids, knitted fabrics and nonwovens based on titanium of this highest purity level are particularly suitable and advantageous for the intended use. For example, a particularly fine-pored or microporous structure with good electrical conductivity can be achieved using a titanium fiber nonwoven in a sintered structure. The sintering process increases the porosity compared to conventional micrononwovens, particularly due to sintering shrinkage. The target range for nonwoven material, for example, is 50-60% and for sintered powder at least 30-40%. The porosity can be easily measured by means of a volume-weight measurement of the gas diffusion layer and the corresponding ratio formation.

[0029] Preferably, the fine pores in the gas diffusion layer in the microporous layer of the first gas diffusion layer have a porosity of typically 30%-50%, in particular 30%-40%. It may even be particularly preferred to provide a slightly higher porosity of more than 50%, in particular approximately 55%-56%, in the first gas diffusion layer, which is expected to result in even better properties for electrode contacting in an electrolysis cell.

[0030] In a particularly preferred embodiment of the gas diffusion layer, the first gas diffusion layer has a layer thickness between 0.2 mm and 1 mm. This allows for good further processing and manufacturing manageability of the metal-ceramic sintered composite of the first gas diffusion layer, as well as easy application and bonding of the second gas diffusion layer to the first gas diffusion layer. Due to its low layer thickness, the first gas diffusion layer can, for example, be cut into the desired shape to suit the installation situation. Furthermore, the material used for this finely porous individual layer as a functional layer is reduced, resulting in even better results for electrical contact and media transport.

[0031] In a further preferred embodiment of the gas diffusion layer, the second gas diffusion layer comprises a metallic expanded metal mesh with a mesh size of 1.5 mm x 1 mm to 2.5 mm x 2 mm.

[0032] The metal expanded metal mesh provides a coarsely porous yet highly electrically conductive structure. At the same time, a certain degree of elasticity and resilience are achieved. In addition to expanded metal meshes or layers, wire mesh, or folded sheets, combinations can also be used. Structures that allow a certain degree of mechanical compression are advantageous, thus preventing material stresses and thus inhomogeneous pressure peaks, which, in the worst case, could lead to mechanical distortion or undesirable wedging during installation in an electrolysis cell.

[0033] In a particularly preferred embodiment of the gas diffusion layer, the coarse pores in the second gas diffusion layer have a diameter that corresponds to 100 times to 1000 times the diameter of the fine pores.

[0034] Due to the orders of magnitude larger pore diameter in the second gas diffusion layer compared to the first gas diffusion layer, a larger volume flow of fluid can be rapidly conducted through the second gas diffusion layer with moderate or low pressure losses. In particular, the product stream from 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 metal mesh, provides a mechanical support structure for the first gas diffusion layer and electrical conductivity for the current conduction and power supply of the first gas diffusion layer.

[0035] In a particularly advantageous embodiment of the gas diffusion layer, the first gas diffusion layer and the second gas diffusion layer are sintered together, in particular diffusion-sintered.

[0036] In this way, a particularly advantageous material-to-material bond between the layers is achieved, wherein the sintered material for sintering the first gas diffusion layer with the second gas diffusion layer is adapted; 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 greatly promoted by the material bond. The type of bond can then be designed in particular such that a structure of the first gas diffusion layer is initially provided from a titanium fiber fleece with the sintered material and an expanded metal mesh with a mesh size of 1.5 mm x 1 mm to 2.5 mm x 2 mm. The bond with the second coarse-pore gas diffusion layer is realized by a sintering process.In this process, fine-grained ceramic or metallic materials are heated as sintering material – often under elevated pressure – while remaining below the melting point of the main components, so that the shape of the workpiece is retained. Shrinkage typically occurs during this process because the particles of the starting material compact and pore spaces are at least partially filled. This creates a sintered two-layer composite in which the second gas diffusion layer is sintered to the first gas diffusion layer in a material-to-material bond.

[0037] In a particularly advantageous embodiment of the gas diffusion layer, the second gas diffusion layer is designed in multiple layers, with a plurality of individual layers stacked one on top of the other and having coarse pores, wherein adjacent individual layers stacked one on top of the other are welded to one another.

[0038] The multi-layer design of the second gas diffusion layer comprising several individual layers is very advantageous because it enables layer-specific adaptation of the properties for the fluid transport of reactants and products and at the same time a uniform current supply. This means that the porosity can be adjusted across the individual layers. It has been shown that the design as a welded joint is particularly advantageous for connecting the individual layers of the second gas diffusion layer. Since the transport of larger volume flows of fluid predominates for the second gas diffusion layer during operation in an electrolysis cell, a finely porous sintered structure - as intended for the first gas diffusion layer - within the layer composite of the second gas diffusion layer would be disadvantageous.In contrast, the combination of a sintered connection between the first and second gas diffusion layers with the welded individual layer structure within the second gas diffusion layer has proven to be very advantageous. This results in considerable advantages both in terms of production technology and with regard to the electrical properties and the transport properties of the gas diffusion layer as a porous transport layer. The multi-layer design of the second gas diffusion layer can, for example, be made from a stack or composite of at least two or more expanded metals. The expanded metals preferably have a rectangular shape and an aspect ratio of at least 1.2. Expanded metals with an aspect ratio of at least greater than 2.0 are advantageous.Such a stack or multi-layer composite of expanded metal mesh can be placed in a gantry welding machine and welded together there with the help of so-called stamps, which form the welding electrodes.

[0039] 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 in a direction perpendicular to the layer normal from the first gas diffusion layer.

[0040] In this case, a mean value, i.e., a mean pore diameter of a distribution, can be considered as a porosity measure of an individual layer, via which a desired graded structure can be adjusted and achieved. This allows a porosity gradient to be adjusted and the fluid conductivity, particularly the volume flow rate, to be adjusted and specifically increased for the required media transport. In this case, the fluid conductivity decreases perpendicular to the layer normal, and the flow resistance decreases accordingly.

[0041] The above object is further 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 so that a microporous layer with fine pores is formed in the sintered composite structure, in which a second gas diffusion layer with a coarse structure of coarse pores is further 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 is effected.

[0042] Advantageous further developments of the method emerge from the subclaims as well as the present description and the figures.

[0043] Accordingly, a manufacturing method of a gas diffusion layer is proposed, wherein the first gas diffusion layer and the second gas diffusion layer are sintered together, in particular diffusion sintered.

[0044] During sintering, the first gas diffusion layer and the second gas diffusion layer are placed on top of each other and then baked together using a sintering material. The sintering material can be a metallic or ceramic sintering material, or even a metal-ceramic sintering material mixture. The first gas diffusion layer and the second gas diffusion layer are not welded to each other, but are bonded together.

[0045] This prevents the formation of mixed phases in the metal structure of the gas diffusion layer. Such mixed phases can be triggered, for example, by high local heat input when welding the expanded metal layers together. Such mixed phases can prevent the formation of passivation layers and, due to the lack of passivation, promote corrosion. Accordingly, sintering the first gas diffusion layer and the second gas diffusion layer enables the formation of a particularly uniform passivation layer, which can protect the gas diffusion layer against corrosion and degradation.

[0046] Furthermore, the first gas diffusion layer and the second gas diffusion layer form a materially bonded single component and an already functional unit for use in an electrolysis cell.

[0047] The first gas diffusion layer and the second gas diffusion layer can be sintered in a protective gas atmosphere.

[0048] Sintering in a protective gas atmosphere results in the first layer and the second layer bonding together via diffusion, but due to the lack of oxygen and the rather mild temperatures, no scale spots or oxide deposits are formed.

[0049] The bond during sintering is always cohesive, thus providing optimal and uniform contact with minimal contact resistance between the components, or rather, 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 wires, fibers, and nonwovens.

[0050] The sintering temperature can typically be between 700°C and 1200°C, with a sintering temperature of 900°C being preferably set. Alternatively or additionally, when applying the temperature, the first gas diffusion layer and the second gas diffusion layer can be sintered at a pressure between 10MPa and 100MPa, preferably 40MPa. 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 yet locally limited material bond at the interface, so that the coarse-porous second gas diffusion layer is not impaired in its transport function for fluids. At the same time, the material bond creates a conductive connection in which a large number of electrical contact points are formed and sintered with the first gas diffusion layer.

[0051] The second gas diffusion layer can be coated with Ni and / or NiP and / or NiTi and / or NiFe if required.

[0052] This allows the current carrying capacity and the ohmic resistance of the gas diffusion layer to be reduced, while the nickel-based coating simultaneously has wear-inhibiting properties, such as abrasion and corrosion protection.

[0053] The coating with a corresponding coating material can be deposited electrolessly, galvanically or by gas phase processes, in particular sputtering, arc evaporation or electron beam evaporation.

[0054] In an alternative embodiment of the method, it is possible for the first gas diffusion layer and the second gas diffusion layer to be welded together, in particular using a capacitor discharge welding process.

[0055] For example, the type of connection on the anode side of an electrolysis cell can be configured as follows: A structure consisting of a titanium fiber fleece in a composite structure with sintered material as the first gas diffusion layer in a finely porous sintered composite structure. A second gas diffusion layer is applied to this, comprising an open-pore expanded mesh with a mesh size of 1.5 mm x 1 mm to 2.5 mm x 2 mm. This second gas diffusion layer is applied to the first gas diffusion layer by capacitor pulse welding, firmly bonding the two layers together.

[0056] Both a structure with a sintered composite structure and a welded connection between the first and second gas diffusion layers can be advantageous. Both basic structures can be easily processed as a basic composite, for example by capacitor pulse welding, and successively expanded to form a more complex multi-layer gas diffusion layer. In particular, the second gas diffusion layer can advantageously be designed in multiple layers comprising several individual layers. This creates GDL structures that offer a correspondingly greater current carrying capacity than the conductive fleece with the fine pores alone. The lattice-shaped metallic structure creates defined contact points, on the one hand to the sintered first gas diffusion layer and, on the other hand, to further individually structured individual layers of the second gas diffusion layer.

[0057] Therefore, in a particularly advantageous embodiment of the manufacturing method, it is provided that a second gas diffusion layer is provided by providing a plurality of individual layers stacked one above the other and having coarse pores, wherein adjacent individual layers stacked one above the other are each welded to one another, wherein in particular a capacitor discharge welding process is used.

[0058] The method can be advantageously configured such that, in the second gas diffusion layer, the porosity of the coarse pores in an individual layer is adjusted such that the layered individual layers form a graded stratification, with the porosity decreasing with the distance of a layered individual layer perpendicular to the layer normal from the first gas diffusion layer. In this way, a graded porous structure is achieved with an adapted fluid conductivity in the direction of the normal; thus, a desired porosity gradient can be considered and adjusted during production.

[0059] The finer or more fine-pored the contact between the gas diffusion layer (GDL) and the electrode is in the installation situation 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). In contrast to the so-called "Micromesh" expanded metal used previously, a fleece or sintered metal enables much closer contact points and thus a more extensive contact of the electrode. A very dense and at the same time very fine-pored design of the first gas diffusion layer as the key contact layer also leads to significantly improved support of the MEA as such. The processing of a solid sintered structure in downstream welding processes is also considerably easier.A major advantage is that this allows for savings in the use of very expensive catalyst material, such as iridium, at the MEA electrode. Furthermore, it results in improved and, in particular, homogeneous current distribution. Easier handling and further processing, as well as subsequent joining and expansion to form a multilayer gas diffusion layer with a plurality of individual layers, e.g., by capacitor pulse welding, are very easily possible. This also makes the gas diffusion layer easier to handle during assembly and quality control.

[0060] A further aspect of the invention relates to an electrolysis cell with a gas diffusion layer according to the invention.

[0061] A further aspect of the invention relates to an electrolyzer with an electrolysis cell according to the invention. Further advantages, features and details of the invention emerge 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 the features and combinations of features mentioned below in the description of the figures and / or shown alone in the only figures can be used not only in the respective combination specified, but also in other combinations or on their own, without departing from the scope of the invention.

[0062] Examples of the invention are explained in more detail with reference to the drawings. They show schematically and in a highly simplified manner:

[0063] FIG 1 shows the basic structure of an electrochemical cell, which is exemplified as a PEM electrolysis cell, with a gas diffusion layer;

[0064] FIG 2 shows a simplified representation of the structure and connection of a multi-layer gas diffusion layer;

[0065] FIG 3 shows the structure of a multi-layer gas diffusion layer with a connection modified from FIG 2.

[0066] 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 electrolyzer (not shown in detail here) for splitting water H2O using direct electrical current to produce hydrogen and oxygen.

[0067] The electrochemical cell 3 comprises an electrolyte made of a proton-conducting membrane 13 (proton exchange membrane, PEM), on which the electrodes 15a, 15b are located on both sides. The unit consisting of membrane 13 and electrodes 15a, 15b is usually referred to as a membrane electrode assembly (MEA). The electrode 15a is referred to as a cathode, and the electrode 15b as an anode. The electrodes 15a, 15b comprise a catalyst material which is coated in a thin catalytically active layer of approximately 1-1.5 mg / cm 2 applied to the membrane. The catalyst material on the anode side, for example, is iridium, which catalytically accelerates the conversion reaction and is very expensive. Therefore, many efforts are being made to reduce the coating of catalyst material on the electrodes 15a, 15b.

[0068] A gas diffusion layer 1, an anodic gas diffusion layer 1 and a cathodic gas diffusion layer 1, is each applied to the electrodes 15a, 15b constructed in this way. The gas diffusion layers 1 are contacted by so-called bipolar plates 17. In the assembled state of an electrolysis stack consisting of a plurality of individual electrolysis cells 3 connected electrically in series, these plates spatially separate the electrolysis cells 3 from one another.

[0069] The electrolysis cell 3 is fed with demineralized water (H2O) as the reactant, which is decomposed at the anode 15b into oxygen gas (O2) and protons (H+). The protons (H+) migrate through the electrolyte membrane 13 toward the cathode 15a. On the cathode side, they recombine to form hydrogen gas (H2). Thus, hydrogen (H2) and oxygen (O2) are obtained as products.

[0070] In another 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 designed in this way are to be modified analogously to the electrolysis cell 3 shown in FIG. 1. Therefore, without restricting generality, reference is made below by way of example to an electrochemical cell 3 designed as an electrolysis cell 3.

[0071] The gas diffusion layer 1 is a flat component and a functional layer important 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 serve to supply reactants to the respective electrodes. A key aspect here is that the gas diffusion layer 1 is always sufficiently permeable to the gaseous products or reactants to allow their removal. For this purpose, porosity must be provided to enable and promote this transport.

[0072] The gas diffusion layer 1 also serves, particularly in an electrolysis cell 3, as a current distributor. For these reasons, the gas diffusion layer 1 is formed from an electrically conductive, porous material. It is important that a uniform, i.e. homogeneous, current distribution is achieved so that the current density at electrodes 15a, 15b is as homogeneous as possible over the catalytically active surface during operation, and indeed for a long operating time. This can reduce or prevent degradation of the electrolysis cell 3. The most uniform and multiple contacting possible of the electrodes 15a, 15b also has a favorable effect on the material required for the catalyst coating. Likewise, a structure with the finest possible porosity on the contact surface of the gas diffusion layer 1 with the respective electrode 15a, 15b.

[0073] 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 on top of one another, with an outer layer being designed as a spring component which, for example, can have a progressive spring characteristic. The gas diffusion layer 1 comprises in particular a contacting component, a diffusion component and the spring component, which differ specifically from one another in terms of their structure and / or composition. The gas diffusion layer 1 of electrolysis cells 3 comprising a plurality of layered diffusion layers must also meet particularly high requirements for the respective height and thickness profile and, due to the specified installation space of the cell frame, be designed and manufactured to fit very precisely for use in an electrolysis cell 3.Dimensional accuracy and dimensional control with regard to the permissible thickness are becoming increasingly important, particularly for large-format, flat gas diffusion layers 1, such as those currently being developed and designed for use in high-performance electrolyzers. Therefore, for an industrial manufacturing process with high volumes and large effective functional areas, quality assurance in the thickness adjustment of the gas diffusion layer 1 is very important. Furthermore, the permissible thickness tolerances must be monitored during the axial stacking and mechanical bracing of a large number of electrolysis cells 3 to form a high-performance electrolyzer.

[0074] 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 with a second gas diffusion layer 7, as described in more detail below in FIG. 2 and in FIG. 3.

[0075] FIG. 2 shows a simplified representation of the successive construction and connection of a multi-layer gas diffusion layer 1. A first gas diffusion layer 5 is shown, which is composed of an electrically conductive nonwoven material 9 and a sintered material 9. With the sintered composite structure, a highly microporous layer with fine pores is formed for the first gas diffusion layer. The electrically conductive nonwoven material 9 comprises titanium of purity grade 1, for example with the material designation ASTM B265, in a fine-fiber-like structure. This nonwoven material 9 is sintered by a sintering process with a metallic, a ceramic or a metal-ceramic sintered material 9. This results in a fine-porous, solid first gas diffusion layer 5. This has a conductivity through the layer plane, which is characterized by a volume resistance of maximum 50 mΩ-cm 2At the same time, the sintered composite structure of the first gas diffusion layer 5 creates a finely porous structure, characterized by a high porosity of 56% ± 3% and a high density of the sintered structure with a layer thickness of only approximately 0.2 mm to 1 mm. Thus, a gas diffusion layer 5 configured in this way is specially designed and suitable for the requirements as an anodic contact layer on an anode-side electrode 15 of an electrolysis cell 3. The catalyst efficiency in the electrolysis cell 3 is thereby increased, and there is a more comprehensive utilization of the available catalyst surface for the electrochemical material conversion. This makes it possible for the catalyst coverage of a membrane electrode assembly to be less than 1.2 g / cm 2 can be reduced, typically coverage levels between 0.8 g / cm 2 and 1.1 g / cm 2achieved and adjusted without significantly reducing the catalytic activity.

[0076] A second gas diffusion layer 7, on the other hand, comprises a plurality of stacked individual layers 7a, 7b, 7c, 7d, each of which has a coarse structure with coarse pores and is firmly connected to one another to form the second gas diffusion layer 7. In FIG. 2, the second gas diffusion layer 7 is initially connected directly to the first gas diffusion layer via a provided individual layer 7a. In the exemplary embodiment, the individual layer 7a is first sintered or baked to the first gas diffusion layer 5, using a sintering material 11. This forms a sintered two-layer system (19) comprising the first diffusion layer 5 and the individual layer 7a in a cohesive structure and is prepared for the subsequent manufacturing steps of the gas diffusion layer 1.This sintered two-layer system 19 is therefore an intermediate product which is, however, easy to handle and process further, for example, it can be cut to a desired installation dimension. The material bond provides both a mechanically strong connection and also a very low-resistance electrical contact. In the subsequent production steps, further coarse-porous metallic individual layers 7b, 7c, 7d are successively joined and applied to the sintered two-layer system (19) by means of a capacitor discharge welding process. The adjacent individual layers 7b, 7c, 7d are layered one above the other on the sintered two-layer system (19) onto the individual layer 7a of the two-layer system (19) and are 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 coarse pores, 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 a large volume flow of fluid can be transported. The porosity across the individual layers 7a, 7b, 7c, 7d is advantageously adjusted such that the diameter of the coarse pores increases layer-specifically with the spacing of the layered individual layers 7a, 7b, 7c, 7d.

[0077] FIG. 3 shows the structure of a multi-layer gas diffusion layer 1 with a connection between the first gas diffusion layer 5 and the second gas diffusion layer 7 that is slightly modified compared to FIG. 2. In particular, the connection between the individual layer 7a and the first gas diffusion layer 5 is modified. During production, the first gas diffusion layer 1 and an individual layer 7a of the second gas diffusion layer 7 are first welded to one another, in particular using a capacitor discharge welding process. In further production steps, the further individual layers 7b, 7c, 7d are successively layered on top of one another, and the adjacent individual layers 7a, 7b, 7c, 7d are welded to one another. A capacitor discharge welding process is used here.

[0078] Depending on the material selection, the gas diffusion layer 1 is suitable for polymer membrane electrolysis, particularly with acidic proton exchange membranes or alkaline anion exchange membranes. Application in alkaline electrolysis with diaphragms is also possible. Where applicable, all individual features presented in the exemplary embodiments can be combined and / or exchanged without departing from the scope of the invention.

Claims

Patent 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 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.

2. Gas diffusion layer (1) according to claim 1, wherein in the first gas diffusion layer (5) the conductive nonwoven material (9) is formed from a metallic fiber material.

3. 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, the fiber thickness being between 10pm and 50pm.

4. Gas diffusion layer (1) according to claim 1, 2 or 3, wherein the conductive nonwoven material (9) comprises titanium of grade 1 technical purity.

5. Gas diffusion layer (1) according to one of the preceding claims, wherein in the microporous layer the fine pores have a porosity of 30%-50%, in particular of 30%-40%.

6. Gas diffusion layer (1) according to one of the preceding claims, wherein the first gas diffusion layer (5) has a layer thickness between 0.2 mm and 1 mm.

7. Gas diffusion layer (1) according to one of the preceding claims, wherein the second gas diffusion layer (7) comprises a metallic expanded metal mesh with a mesh size of 1.5 mm x 1 mm to 2.5 mm x 2 mm.

8. 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 which corresponds to 100 times to 1000 times the diameter of the fine pores.

9. Gas diffusion layer (1) according to one of the preceding claims, wherein the first gas diffusion layer (5) and the second gas diffusion layer (7) are sintered together, in particular diffusion-sintered.

10. 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) stacked one above the other and having coarse pores, wherein adjacent individual layers (7a, 7b, 7c, 7d) stacked one above the other are welded to one another.

11. Gas diffusion layer (1) according to claim 9, wherein the diameter of the coarse pores increases layer-specifically with the distance of a layered individual layer (7a, 7b, 7c, 7d) in a direction perpendicular to the layer normal from the first gas diffusion layer (5).

12. A 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) together, 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) is effected.

13. The method according to claim 12, wherein the first gas diffusion layer (5) and the second gas diffusion layer (7) are sintered together, in particular diffusion sintered.

14. The 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 at a pressure between 10MPa and 100MPa, preferably 40MPa and / or - the heating rate is 50°C / min.

15. 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.

16. Method according to one of claims 12 to 15, in which a second gas diffusion layer (7) is provided, wherein a plurality of individual layers (7a, 7b, 7c, 7d) stacked one above the other and having coarse pores are provided, wherein adjacent individual layers (7a, 7b, 7c, 7d) stacked one above the other are each welded to one another, wherein in particular a capacitor discharge welding process is used.

17. The method according to claim 16, wherein in the second gas diffusion layer (7) the porosity of the coarse pores in an individual layer (7a, 7b, 7c, 7d) is adjusted so that a graded layering is formed by the layered individual layers (7a, 7b, 7c, 7d), wherein the porosity increases with the distance of a layered individual layer (7a, 7b, 7c, 7d) perpendicular to the layer normal from the first gas diffusion layer (5).

18. Electrolysis cell (3) with a gas diffusion layer (1) according to one of claims 1 to 11.

19. Electrolyzer with an electrolysis cell (3) according to claim 18.

Images