Gas diffusion substrate made of a porous metal substrate and a gas-permeable polymer layer, method for producing same, gas diffusion electrode, and electrochemical cell comprising a gas diffusion substrate

Abstract

The invention relates to a gas diffusion substrate for use in an electrochemical cell, comprising at least one first flat porous metal substrate with a first and a second main surface and at least one first flat gas-permeable polymer layer arranged thereon. The metal substrate is provided with a catalyst layer for electric contacting purposes. The first flat polymer layer, which is made of a hydrophobic polymer, is arranged on the first main surface of the metal substrate In the process, a first surface of the gas diffusion substrate is formed substantially from the first flat polymer layer and the first main surface of the first flat metal substrate. At least sub-regions of said surface are provided with either a plurality of closed areas of the first metal substrate, which are surrounded by a continuous region of the first polymer layer, or a plurality of closed areas of the polymer layer, which are surrounded by a continuous region of the first main surface of the first flat metal substrate. The invention additionally relates to a gas diffusion electrode, to an electrochemical cell comprising such a gas diffusion substrate, and to a method for producing same.

Description

[0001] Patent application:

[0002] Gas diffusion substrate comprising a porous metal substrate and a gas-permeable polymer layer, method for its production, and gas diffusion electrode and electrochemical cell comprising a gas diffusion substrate

[0003] Applicant:

[0004] Fraunhofer Society for the Promotion of Applied Research

[0005] The application relates to a gas diffusion substrate and a gas diffusion electrode for use in electrochemical cells, as well as a method for producing the gas diffusion substrate. The gas diffusion electrode and the gas diffusion substrate comprise an electrocatalyst layer contacted by a metal substrate, as well as a gas-permeable polymer layer.

[0006] Particularly in the electrochemical conversion of gases in electrochemical cells, the porous gas diffusion substrate (also called diffusion layer) used with the catalyst layer arranged on it plays a central role. The diffusion layer serves to form a three-phase boundary between the electrolyte used for the electrochemical reaction, the electrocatalyst and the gas or liquid to be converted (i.e. the reactant). This three-phase boundary can improve mass transport in the electrochemical process, thus enabling a higher conversion of the reactant. Important factors here are, on the one hand, efficient transport of the reactant to the catalyst layer, which can usually be achieved through a hydrophobic pore structure of the gas diffusion substrate, and, on the other hand, good electrical contact with the catalyst layer arranged on the diffusion layer.The catalyst layer should also be in contact with the electrolyte of the electrochemical cell to ensure the transport of the ions required for the electrochemical conversion (e.g., protons or hydroxide ions). As a result, the three-phase boundary forms within the catalyst layer or at the interface between the catalyst layer and the gas diffusion substrate. According to the state of the art, such diffusion layers can be produced using various processes.

[0007] First, electrically conductive filler materials can be sintered (and optionally compacted) in the form of powder mixtures with hydrophobic polymer binders and, optionally, pore-forming agents. Two- or three-dimensional porous structures or lattice structures typically serve as supports for the resulting materials. This results in a three-dimensional porous composite layer that exhibits electrical conductivity perpendicular to the surface and hydrophobic channels within the pore structure. The disadvantage of this is the consistent electrical conductivity in the diffusion layer, which tends to irreversible electrowetting upon application of electrical potentials, which can lead to flooding of the pore structure with electrolyte. If a salt is formed through the electrochemical reaction, this effect is further enhanced.Ultimately, flooding leads to limited mass transfer of the reactant to the catalytically active layer, which is associated with a significant loss of performance. A multilayer structure with varying proportions of the hydrophobic binder can reduce this effect, but cannot completely eliminate it. A disadvantage, however, is that multilayer electrodes typically require high surface loadings of the catalyst.

[0008] Alternatively, three-dimensional porous electrically conductive materials such as carbon fiber fabrics or metal foams can be impregnated with a hydrophobic binder to produce diffusion layers. Such diffusion layers have established themselves as state-of-the-art in fuel cell applications. However, the disadvantage here is that flooding of the pore structure due to potential-induced electrowetting can be observed due to the continuous conductivity.

[0009] To prevent flooding of a diffusion electrode, the use of hydrophobic polymer films and polymer structures as barrier layers has been described. Examples include polymer foams or porous polymer layers (obtained from polymer binders and pore-forming agents). Due to their lack of electrical conductivity, these are resistant to electrowetting-induced flooding and thus prevent mass transport limitations. However, since these polymer materials are integrated by lamination onto porous, catalytically active electrodes or by applying catalytically active materials directly to the polymer film, the electrodes are insulated on one side, making them unsuitable for bipolar arrangements, which are common in large-scale applications. The electrical contact of the diffusion electrode is then made on the electrolyte side and is conducted outward in the electrode plane.An improvement can be achieved by using metal meshes as current collectors. However, this approach generally limits the scaling of the electrode size, as this is dependent on the electrical conductivity in the electrode plane. Furthermore, the current density is distributed inhomogeneously across the electrode surface, which leads to a reduction in service life due to agglomeration of the catalytically active sites.

[0010] The application is therefore based on the object of specifying a gas diffusion substrate, a method for its production, a gas diffusion electrode and an electrochemical cell with such a gas diffusion substrate or such a gas diffusion electrode, with which the disadvantages of the prior art can be at least partially overcome and which are in particular flood-resistant and provide suitable three-phase boundaries.

[0011] At least one of these objects is achieved by the gas diffusion substrate, the method for its production, the electrode, and the electrochemical cell according to the independent claims. Subclaims, the description, and the accompanying figures teach advantageous further developments.

[0012] A gas diffusion substrate according to the application comprises at least a first planar porous metal substrate with a first main surface and a second main surface opposite the first main surface, and at least one planar gas-permeable polymer layer arranged on the first main surface. The metal substrate is intended, among other things, for electrically contacting a catalyst layer. A plurality of three-phase boundaries for electrochemical processes are formed by the metal substrate, the polymer layer, and the catalyst layer. The gas diffusion substrate now has a first surface, which is essentially formed by both the first main surface of the porous metal substrate and the planar polymer layer.This first main surface of the gas diffusion substrate thus has regions formed by the first main surface of the metal substrate as well as regions formed by the planar polymer layer. At least partial regions of this first main surface, in particular those partial regions intended for contacting the catalyst layer, are designed such that either a plurality of closed regions of the polymer layer are present that are completely enclosed by a continuous region of the first main surface of the metal substrate, or a plurality of closed regions of the first main surface of the metal substrate are present that are completely enclosed by a continuous region of the polymer layer. In simplified terms, one can speak of a plurality of window-like regions of one material that are completely enclosed by a region of the second material.

[0013] In order to provide efficient three-phase boundaries, the first flat polymer layer is made of a hydrophobic polymer.

[0014] The fact that the first surface of the gas diffusion substrate is "essentially" formed by the first main surface of the porous metal substrate and the planar polymer layer means here and below that, in gas diffusion substrates with more than one polymer layer, and in particular in gas diffusion layers with more than one metal substrate, smaller portions of the second polymer layer or the second metal substrate can also be part of this surface. Particularly in embodiments with two metal substrates, embodiments are often realized in which, depending on the manufacturing process, both metal substrates can equally trigger a perforation of the polymer layer.In embodiments with two polymer layers, this will rarely be observed because, on the one hand - as explained below - a material bond can be realized between the two polymer layers so that this can no longer be detected and, on the other hand, typically only small portions of the second polymer layer are exposed in the area of ​​the first polymer layer.

[0015] The metal substrate primarily serves to provide electrical conductivity, allowing electrical contact with the catalyst coating or catalyst layer. Furthermore, the metal substrate typically provides mechanical stabilization, which is usually further enhanced by the contacting of the catalyst layer.

[0016] According to the application, a polymer layer is understood to mean a polymer membrane with a substantially uniform thickness of less than 500 μm. Therefore, commercially available polymer films are used as the polymer layer (also referred to as a polymer membrane in the application), from which the gas diffusion substrate according to the application is then constructed. The use of such films allows for the greatest possible flexibility with regard to the required properties in relation to pore size, hydrophobicity, and layer thickness, while at the same time, the aforementioned properties are uniform across the entire surface of the polymer film used, apart from the area in which compression of the metal substrate and polymer layer has taken place to create the perforations using the method according to the application (although, in principle, a gradient in pore size is also conceivable).According to the application, the uniform thickness can be achieved in particular by ensuring that, during the manufacturing process according to the application and the associated pressing of the polymer layer(s) and metal substrate(s), no significant deformation of the polymer membrane occurs (apart from the perforation) (provided the thickness of the polymer layer is not greater than that of the metal substrate). Essentially, with regard to the uniform thickness, this means that the thickness in the area of ​​the perforations, i.e., where compression of the metal substrate and polymer layer has occurred and therefore deformation has occurred, is not taken into account, and that outside these areas, the deviation from the stated thickness is a maximum of 10%, in particular a maximum of 5%.

[0017] The polymer layer serves - as already explained - to transport the reactants of the electrochemical reaction to the catalytically active centers. The polymer layer is gas-permeable, thus allowing unhindered access of gases to the reaction centers. Typically, the pore structure or porosity of the polymer layer is designed such that even low-molecular-weight organic compounds can pass through the polymer layer (low-molecular-weight compounds are understood to mean, in particular, compounds with a molecular weight of up to 600 g / mol; for a tailored pore structure, however, polymer layers can also be used that can also be passed through by compounds with a molecular weight of up to 1000 g / mol). Typically, polymer membranes with pore sizes of 0.05 pm to 10 pm are used for this purpose. Pore sizes of 0.05 pm to 5 pm, for example, 0.1 pm to 1 pm, are particularly suitable.According to the application, the pore size specification consistently refers to a measurement using capillary flow porometry based on DIN 66140, which can be used to specify the maximum pore size. When using the method according to the application, these pore sizes are found in the gas diffusion layers in the non-perforated areas, i.e., outside the area where compression of the metal substrate and polymer layer has occurred. The pore sizes of the typically used polymer membranes are thus at least a factor of 10 smaller than the perforations described in more detail below. The pores in the specified size range (like the thin polymer layers) serve to ensure particularly effective mass transport of the reactant.If porosity gradients are to be established, instead of a polymer layer consisting of just one membrane, a polymer layer consisting of two (or possibly more than two) membranes arranged on top of one another (which then together have the thickness disclosed for the polymer layer) can be formed, with membranes each having different pore sizes or porosities being used. By means of a gradient, a more uniform surface distribution of the catalyst can be achieved, particularly on the side facing the catalyst with the smaller pores, and on the side facing away from the catalyst with larger pores, easier gas transport and, at the same time, easier drainage of possible condensate droplets can be enabled.

[0018] With regard to the porosity of the polymer layer (via the density of the polymer layer), the ratio of pore volume to the total volume of the membrane can also be determined. The porosity calculated in this way is typically between 40 and 90%, particularly between 50 and 80%, and frequently between 60 and 75%. Below 40%, there is a risk of impeding mass transport; above 80%, and especially above 90%, mechanical stability typically suffers, so there is a risk of significant damage to the pores and pore structure during the production of the gas diffusion substrate (for example, by compressing the pores).In general, it can be stated that the gas diffusion substrates according to the application, particularly in at least three-layer structures consisting of two metal substrates and one polymer layer, or of two polymer layers and one metal substrate, already exhibit high mechanical stability without the presence of a catalyst layer and are therefore free-standing. This enables a broad spectrum for the application of catalyst layers. In addition to the application of knife-coated, pressed, and sintered catalyst layers, processes for the application of thin layers smaller than 20 μm can also be carried out using galvanic, wet-chemical (application of ink formulations), or gas-phase processes (physical or chemical processes). Furthermore, other catalytically active materials, in particular metallic ones, can also be applied to the existing catalyst layer or catalyst coating using galvanic, wet-chemical, or gas-phase processes.The properties of the catalyst layer can be adjusted and its thickness can be varied from very thin (< 1 pm) to thick (500 pm).

[0019] The gas diffusion substrate according to the application is outstandingly suitable for achieving efficient electrochemical conversion of gases or organic compounds. By combining it with a suitable catalytically active coating or layer, this gas diffusion substrate enables spatial decoupling of the material diffusion properties from the electrical conductivity provided by the metal substrate. In particular, the gas diffusion substrate according to the application makes it possible to provide a flood-resistant system with which electrical conductivity is realized within the plane formed by the gas diffusion substrate, on the one hand, and perpendicular to this plane, on the other. This multidimensional conductivity has the significant advantage that a majority of the gas diffusion substrates according to the application can be integrated as layers into a bipolar cell stack.Because the main surface of the gas diffusion substrate is formed simultaneously by a metal substrate and a polymer substrate, a multitude of electrical contacting options are provided. At the same time, a multitude of hydrophobic regions are also provided, resulting in a network of phase boundaries between the electrically conductive metal on the one hand and the hydrophobic polymer on the other. If, on the other hand, the catalyst layer is also arranged on the metal, a "network" of three-phase boundaries forms on the surface of the gas diffusion substrate.The flooding resistance of the system is therefore based in particular on the fact that - unlike the systems of the prior art - the metal / polymer / catalyst system according to the application does not allow any potential-related electrowetting with respect to the electrolytes present (for example aqueous solutions) through the said network (or at least only allows it to a reduced extent), so that the existing pore structures do not block (or flood).

[0020] Irrespective of the above explanations, the gas diffusion substrate according to the application can, as an alternative to the nature of the first surface of the gas diffusion substrate explained above in two-dimensional terms, also be explained by any three-dimensional structural features of the metal substrate contained therein.

[0021] A gas diffusion substrate according to the application has a metal substrate with a large number of elevations that are formed at least on the first main surface of the metal substrate. These are often formed both on the first main surface of the metal substrate and on the second main surface of the metal substrate opposite this. The gas diffusion substrate further comprises a first flat, gas-permeable polymer layer that has a large number of perforations. The perforations are arranged such that elevations of the metal substrate at least partially protrude into them, in particular a plurality of elevations, often the larger parts of the elevations, for example all elevations on the first main surface. A gas diffusion substrate designed in this way can then be used to make electrical contact with a catalyst layer ora catalyst coating. Alternatively, another electrical connection (for example, to another layer of the gas diffusion substrate) can be made via these elevations; the catalyst layer or catalyst coating can then optionally be arranged on the side of the metal substrate facing away from the polymer layer (in a two-layer system consisting of a metal substrate and a polymer layer). With such a gas diffusion substrate, in addition to the aspects of two-dimensional conductivity and flooding resistance already described above in the explanation of the two-dimensional nature, the network of three-phase boundaries and the spacing of the three-phase boundaries can be predefined by the choice of the metal substrate or the elevations provided thereon.

[0022] According to one embodiment, at least some of the maxima of the elevations on the first and optionally also the second main surface of the metal substrate protrude through the perforations of the first planar polymer layer and / or are located at least in the side of the first planar polymer layer (facing away from the first main surface of the planar metal substrate). Such an arrangement allows for a particularly space-saving geometry, particularly when used in stacks.

[0023] The diffusion substrates according to the application also generally allow for space-efficient installation. Due to the flexible polymer layers (with a thickness of up to 0.5 mm) and the thin metal substrates (for example, with a thickness of less than 1 mm), very thin diffusion layers are obtained that are conductive perpendicular to the plane. Since numerous cells are stacked on top of one another in bipolar electrochemical converters, this thinner design can save on expensive material for current collectors and frame materials, and a more compact structure can be realized. Typically, the polymer layers, which—as already explained—can also be referred to as polymer membranes due to their essentially uniform thickness of less than 500 μm across the entire surface, have a thickness of 10 to 250 μm (so that, in particular, the potentially required passage of low-molecular-weight compounds can be easily realized).In general, thinner membranes are usually more suitable, provided they can be purchased with sufficient mechanical stability. Polymer membranes made of fluorinated or partially fluorinated polymers with thicknesses of 10 to 125 pm are particularly suitable. Membrane thicknesses of 20 to 50 pm, and especially 30 to 35 pm, deliver particularly good results, especially when using diatomic or triatomic gases (e.g., CO2). If other polymers are used, the upper and lower limits must be slightly offset; in this case, thicknesses of 60 to 200 pm are typically particularly suitable (especially for PES, PP, PE, and PAN).Layer thicknesses of 500 pm or more, and typically even more than 250 pm, typically lead to a loss of mass transport effectiveness and to a significantly reduced conductivity across the diffusion layer; layer thicknesses of less than 10 pm are feasible in principle, but according to the application, have significantly less favorable mechanical stability. For the manufacturing process described in the application, membrane thicknesses of at least 20 pm, in particular at least 30 pm, are particularly suitable in terms of mechanical stability. The metal substrates typically have thicknesses of 50 to 4000 pm, in particular 100 to 1000 pm. Metal substrates with thicknesses of 150 to 500 pm produce particularly good results. The thickness of the polymer layers or polymer membranes is always smaller than that of the metal substrates and usually at most half as large.For metal substrates with thicknesses of more than 250 pm, the diekene ratio (at least for membranes made of fluorinated polymers) can be even more different, for example greater than 4: 1 or greater than 6: 1 (the same applies to the embodiments described below with two metal substrates or with two polymer layers).

[0024] According to a further embodiment, the gas diffusion substrate, in addition to the first planar, gas-permeable polymer layer, also comprises at least one second planar, gas-permeable polymer layer (where the first and second planar polymer layers are often formed from the same material, in particular, the same film is used as the starting material; however, two polymer layers with different porosities can also be used, for example, to adjust porosity gradients). The second polymer layer is then arranged on the second main surface of the metal substrate. With such an arrangement, a second surface of the gas diffusion substrate can also be realized, which is essentially formed from regions of a planar polymer layer (here, the second planar polymer layer) and regions of the first planar metal substrate (here, the second main surface of the first planar metal substrate).Here, too, the arrangement can be configured such that a plurality of closed regions of the second polymer layer are surrounded by a continuous region of the second main surface of the metal substrate. If a precisely three-layer structure consisting of two polymer layers and the (first) metal substrate is present, then typically not only an arrangement is realized in which, on the second surface of the gas diffusion substrate, a plurality of closed regions of the second main surface of the metal substrate are surrounded by a continuous region of the second polymer layer, but usually also, on the first surface of the gas diffusion substrate, a plurality of closed regions of the first main surface of the metal substrate are surrounded by a continuous region of the first polymer layer.The thickness of such a precisely three-layer gas diffusion substrate is then typically determined essentially by the thickness of the metal substrate and is typically 50 to 4000 pm, in particular 100 to 1000 pm, for example 150 to 500 pm. The thickness of each individual polymer membrane is always smaller than that of the metal substrate and often at most half as large.

[0025] Such a three-layer gas diffusion substrate is typically more open-pored than a gas diffusion substrate consisting of two metal substrates and one polymer layer, because the openings of the first metal substrate are not partially "covered" by a second metal substrate. This results in a comparatively larger number of gas diffusion paths. Furthermore, this embodiment has a smoother surface, which can offer advantages for contacting in an electrochemical cell and exerts less mechanical stress on the polymer layers in a zero-gap structure.

[0026] Such a structure with at least two polymer layers can be used, in particular, to produce a gas diffusion substrate in which the two hydrophobic polymer layers adhere to one another; frequently, the first and second polymer layers form a cohesive bond at a plurality of contact points. Such a cohesive bond can be achieved during the production process of the gas diffusion substrate, for example, by selecting the pressure / temperature ratios appropriately during pressing in a pressing process. In a gas diffusion substrate in which at least two interconnected or adherent polymer layers and a metal substrate are present, thinner polymer layers can be used than if only one polymer layer is present, while at the same time high mechanical stability can be achieved.Furthermore, there is no risk of the individual layers shifting relative to each other during installation in an electrochemical cell; such shifting would have the adverse effect of impairing the electrical contact. Finally, such gas diffusion substrates are also more stable over the long term because the risk of delamination is significantly reduced.

[0027] According to a further embodiment, the gas diffusion substrate according to the application comprises, in addition to the first planar porous metal substrate, at least one second planar porous metal substrate. The second planar porous metal substrate is then arranged on the surface of the polymer substrate opposite the side facing the first planar porous metal substrate. With such an arrangement, in particular, a second surface of the gas diffusion substrate can be realized, which is essentially formed from regions of a planar polymer layer (here, the first planar polymer layer) and regions of the second planar metal substrate (here, the main surface of the second planar metal substrate facing away from the first planar polymer substrate).The first and second metal substrates can be made of the same material, but two different metal substrates, in particular two different mesh types and sizes, can also be used, allowing porosity gradients to be adjusted and better control over the perforations to be achieved. According to the above embodiment, the arrangement can again be designed such that a plurality of closed regions of the second metal substrate are surrounded by a continuous region of a polymer layer.If a precisely three-layer structure consisting of two metal substrates and one polymer layer is present, then typically not only is an arrangement realized in which a plurality of closed regions of the polymer substrate are surrounded by a continuous region of a main surface of the first metal substrate, but in which a continuous region of a main surface of the second metal substrate usually also encloses a plurality of further closed regions of the polymer substrate (which are opposite the polymer surface adjacent to the first metal substrate). The thickness of such a precisely three-layer gas diffusion substrate is then typically determined essentially by the thickness of the two metal substrates (it does not have to correspond to the combined thickness of the two metal substrates and can therefore also be smaller) and is typically 100 to 6000 pm, in particular 200 to 1500 pm, for example 250 to 800 pm.The thickness of the polymer membrane is always smaller than that of each of the metal substrates (which can also have different thicknesses) and is often at most half as large as that of each of the metal substrates.

[0028] Such a three-layer gas diffusion substrate is typically more mechanically stable than a gas diffusion substrate consisting of two polymer layers and one metal substrate, since the two metal substrates have contact points for electrical contact, thus limiting the degrees of freedom or mobility of the individual metal substrate. Furthermore, the metal-to-metal contact points in this embodiment provide improved electrical contact.

[0029] With such a structure, in particular, a gas diffusion substrate can be realized in which the two metal substrates are non-positively connected to one another via the intermediate polymer layer. Typically, there are then a large number of metal-metal contacts between the first and the second metal substrate, thus enabling through-plane conductivity. Here, too, the metal-polymer-metal arrangement achieves a mechanically significantly more stable structure and, in particular, enables improved long-term stability, since the risk of delamination is significantly reduced. Finally, such an arrangement enables a greater range of variation in polymer materials compared to a polymer-metal-polymer structure, since adhesion or delamination is avoided.a bonding of the polymer layers plays no role here and accordingly, in particular for the polymer layer, a thermoplastic polymer material which would otherwise be advantageously used for the other embodiment does not have to be used.

[0030] According to a further embodiment, the first metal substrate and / or the second metal substrate, but typically all metal substrates used, are formed from a lattice structure (in particular a two-dimensional lattice structure). Metallic wire mesh, expanded metal, and perforated sheets are particularly suitable. In addition, the first metal substrate and / or the second metal substrate, but typically all metal substrates used, can be formed at least partially, but usually entirely, from metal fibers or comprise a porous metal layer with metal fibers arranged directly thereon. Materials such as a metal fleece (with random metal fibers) or a metal woven fabric, metal knitted fabric, or metal knitted fabric (each with ordered metal fibers) are particularly suitable as metal substrates formed from metal fibers.

[0031] With such metal substrates, an arrangement can be easily realized in which elevations protrude from the first or second main surface of the metal substrate, but typically from both main surfaces.

[0032] According to the application, "made of metal fibers" means, in particular, that the fibers consist essentially or entirely of metal. Only in exceptional cases is it conceivable that non-metallic materials with a metal coating could be used for the metal fibers. However, non-metallic materials with a coating are generally less suitable for reasons of mechanical stability alone.

[0033] The geometries of the metal meshes are not limited to regular geometries (where, for example, square openings are created between the parallel fibers); geometries with other "weaving patterns" can also be used, for example a 5-shaft satin weave (as used in metal filters), as this allows greater flexibility with regard to the openings in the mesh, particularly since these are rectangular and have different edge lengths, and also allow a wide variation in the edge length ratio. In addition, when using two metal substrates, different surfaces of the metal substrates can be combined in different ways (for example, the 5-shaft satin weave has a smooth and a rough surface).This allows for the creation of very small mesh openings in the metal substrate, which, in addition to the pores in the polymer membrane, can influence mass transport. Furthermore, the surface texture can be varied widely, allowing the mechanical stability of the diffusion substrate to be improved—if necessary—by selecting the appropriate metal mesh.

[0034] According to a further embodiment, the at least one first metal substrate (typically all metal substrates) is selected such that (before combining the various layers to form the gas diffusion substrate) the two main surfaces of the metal substrate have a plurality of openings or the metal substrate itself has continuous openings. In particular, these openings should be formed in the respective main surface of the metal substrate such that these openings occupy an area of ​​25 to 80% of the main surface, in particular 30 to 60%, for example 35 to 40%. Typically, these openings have an average area of ​​0.01 to 0.1 mm 2 The measurement can be performed using light microscopy.

[0035] With such a metal substrate, it is possible, on the one hand, to create sufficiently large recesses or areas in the surface of the gas diffusion substrate that are occupied by the polymer layer (and, in a polymer-metal-polymer structure, allow the two polymer layers to be bonded or adhered to one another). Furthermore, such a structure with openings also allows the use of metal meshes, metal knits, and the like, and finally also a geometry that can be adapted to the parameters of a pressing process used, for example, to produce the gas diffusion substrate. Typically, the mesh size of such metal meshes and metal knits corresponds to approximately 0.5 to 5 times the diameter of the metal fibers or wires from which the metal meshes and metal knits are constructed.

[0036] As already mentioned, in a polymer layer-metal substrate-polymer layer structure, the metal substrate is at least twice as thick as one of the polymer layers. In a metal substrate-polymer layer-metal substrate embodiment, the polymer layer should have a maximum thickness equal to that of one of the metal substrates. For arrangements with four or more layers, i.e., at least two metal substrates and at least two polymer layers, these two key values ​​are typically used as a guide. When selecting the layer thickness, the aforementioned parameters have proven particularly useful with regard to manufacturing processes and mechanical stability.

[0037] According to a further embodiment, the perforations of the first and / or second polymer layer (typically in all polymer layers contained in the gas diffusion substrate) have an average area (determined by means of a light microscope) of 0.01 to 1 mm 2 , especially from 0.05 to 0.2 mm2 According to a further embodiment, the perforations, at least in partial areas of the first and / or second polymer layer (typically for all polymer layers contained), occupy a surface area that lies between 10 and 90% of the total surface area of ​​the polymer layer. The total surface area is understood to be the area composed of the actual polymer layer itself and the surface area of ​​the perforations (which, strictly speaking, no longer contain any polymer). As a rule, the surface area of ​​the perforations will occupy 30 to 70% and frequently correspond to between 40 and 60% of the total surface area of ​​the polymer layer. Typically - if only from the point of view of producing the gas diffusion substrate - the aforementioned surface areas apply not only to partial areas of the first and / or second polymer layer or partial areas of all polymer layers, but to the entire polymer layer.

[0038] In principle, larger perforation ratios lead to an improvement in electrical conductivity perpendicular to the gas diffusion substrate plane; however, excessively large perforation ratios have negative effects on gas diffusion.

[0039] According to a further embodiment, the metal substrate of the gas diffusion substrate is formed from a metal or has a coating with a metal which has a conductivity of at least 1 * 10 6 S / m (always measured at 25°C according to the application). Typically, the conductivity is selected to be at least equal to that of stainless steel (1.4* 10 6S / m) or higher. However, according to the application, the conductivity of the metal used plays a less important role than the accessibility of the porous metal substrates for the reactants. For cost / benefit considerations, the specific geometric design will therefore usually play a more central role in the selection of the metal substrate than the metal used. Nevertheless, the aspect of the material should be discussed again at this point: the fact that the metal substrate is formed from a metal means, in the context of this application, that it consists of it or contains it predominantly (> 90 wt%). In addition, a coating of the metal substrate with another metal is also possible. Typically, this will be a coating of a metal (which has an electrical conductivity that is lower than that stated above).In principle, however, a coated non-metal can also be present - as already explained above, this is often less suitable for stability reasons.

[0040] With regard to the polymer layer(s) contained in the gas diffusion substrate, it is essentially only important that they are hydrophobic, so that the three-phase boundaries can form and that the reactants can be transported to the catalytically active centers. The fact that the first planar polymer layer and all subsequent planar polymer layers are formed from a hydrophobic polymer means, in particular, that they consist of the hydrophobic polymer or contain it, in particular as a main component, for example, as a component with a content greater than 80 wt.%. Polysulfones and fluorinated polymers have proven to be particularly suitable hydrophobic polymers.In particular, suitable hydrophobic polymers are those polymers selected from the group consisting of polyethersulfone, fluorinated ethylene propylene (FEP), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), perfluoroalkoxy polymer (PFA), polyethylene tetrafluoroethylene (ETFE), polyethylene chlorotrifluoroethylene (ECTFE), perfluorinated elastomer [perfluoroelastomer] (FFPM / FFKM), fluorocarbon [chlorotrifluoroethylene vinylidene fluoride] (FPM / FKM), perfluoropolyether (PFPE), perfluorosulfonic acid (PFSA) and perfluoropolyoxethane as well as mixtures of the polymers mentioned. In principle, in addition to the polysulfones and fluorinated polymers mentioned, all other porous polymers are also suitable, in particular polyolefins such as polypropylene (PP) and polyethylene (PE), polyurethanes; PET, polyacrylates, cellulosic polymers, polyvinyl chloride, polycarbonates (such as PCTE), polyamides (e.g.Nylon), polyacrylonitrile (PAN), and polyester. Glass fibers are also conceivable.

[0041] The transport of reactants to the catalytically active centers is essential for polymer layers contained in gas diffusion substrates. Gas permeability, or the pore structure and pore properties of the polymer material, also play a role, particularly with gaseous reactants. The pore structure and pore properties must allow sufficient mass transport while still ensuring a separation effect between the compartments. Therefore, during the manufacturing process of the gas diffusion substrates, care must be taken to preserve the chemical composition, shape, and size of the pores as much as possible. This can be achieved using the process described below, provided the layer thickness ratios of the polymer layer to the metal substrate are observed. Using excessively thick polymer layers would lead to undesirable compaction of the polymer membrane, even outside the perforation areas.

[0042] The object of the application is also achieved by a gas diffusion electrode with a gas diffusion substrate as described above. An electrocatalyst layer or a coating comprising an electrocatalyst is arranged on a first planar metal substrate and at least one first planar polymer layer arranged thereon.

[0043] The fact that one layer is arranged on top of another means, here and generally within the scope of this application, that the two superimposed layers are in direct contact with each other and that no further layers are arranged between them. In exceptional cases, however, an adhesive or bonding layer may be arranged between two superimposed layers (it is essential that this adhesive or bonding layer does not impair the conductivity perpendicular to the plane, or at most only slightly).

[0044] As already explained above, the flat polymer layer is arranged on the first main surface of the first flat metal substrate and, together with it, forms the essential portion of a surface of the gas diffusion substrate. The electrocatalyst layer or the coating with an electrocatalyst can extend over the entire surface of the gas diffusion substrate thus formed, but it can also be present only in partial areas. Typically, it will be arranged in particular in the areas where direct contact with the first flat metal substrate is possible. As explained above, a larger portion of the surface area of ​​the perforations can be used to improve catalyst adhesion—particularly in embodiments with perforated polymer layers. This also results in improved through-plane conductivity.Typically, good adhesion of the catalyst layer or catalyst coating can be achieved with the metal substrate as a hydrophilic domain. If necessary, the adhesion of the catalyst layer can also be improved by an organic binder contained in the catalyst layer or catalyst coating. This binder can then be selected to match the material of the polymer layer. Binders that only develop their adhesive effect during the production of the gas diffusion substrate, for example, through thermal or mechanical treatment, are particularly suitable. Additives (e.g., conductive additives or pore formers) can also be incorporated into the catalyst layer.

[0045] The object according to the application is further achieved by an electrochemical cell with at least one gas diffusion substrate or at least one gas diffusion electrode, as described above. In other words, the electrochemical cell can be a structure containing exactly one gas diffusion electrode according to the application. To increase the conversion, however, a stack of several gas diffusion substrates or electrodes can also be used. In a stack, the solution according to the application can particularly well exploit its advantages over the prior art, since the electrical conductivity perpendicular to the plane of the gas diffusion substrate enables a particularly simple structure of the electrochemical cell. The electrochemical cell can in particular also be a zero-gap cell, i.e. a cell in which the distance between the electrodes is minimized as much as possible (e.g.by contacting the electrodes with an ion-conducting membrane or an electrolyte-impregnated porous separator) and in which, in particular, there is no gap for a liquid electrolyte in the cell.

[0046] An electrochemical cell according to the application has at least two chambers, with the gas diffusion electrode forming the interface between the liquid or gaseous reactant and the liquid or solid electrolyte. The gas diffusion substrate with the catalyst layer or catalyst coating is infiltrated, at least partially, by the electrolyte. Thus, a three-phase boundary is formed between the substrate, electrolyte, and catalyst. For the described applications, the gas diffusion electrode typically functions as a cathode for the reduction of CO2, CO, N2, or O2, or for the electroreduction of organic molecules (e.g., hydrogenation). Depending on the application, however, it can also be used as an anode for oxidation reactions (H2, N2, or CO oxidation, oxidation of organic compounds). In addition to the gas diffusion electrode, the electrochemical cell also comprises another electrode at which suitable oxidation or reduction processes can take place.In this case, both a diffusion electrode designed for a reductive process and a diffusion electrode designed for an oxidative process can be used simultaneously.

[0047] The object of the application is finally also achieved by a method for producing gas diffusion substrates. However, it should first be noted that the manufacturability of gas diffusion substrates is not limited to the methods described in more detail below, since, for example, even the most complex geometries and layer sequences can be realized within the framework of additive manufacturing, in particular the geometrically complex surfaces of the gas diffusion substrate, which are composed of polymer layer(s) and metal substrate(s). Rolling processes (e.g., roll-to-roll processes or calendering) are also conceivable.

[0048] The gas diffusion substrates according to the application can be produced by arranging the polymer layers and metal substrate layers required for the substrate to be produced between two pressing jaws of a press and then – depending on the materials used – adjusting the pressing pressure, pressing temperature, and pressing time so that a material-to-material or form-fitting bond between the layers is formed. The elevations on the main surface(s) of the metal substrate perforate the polymer layer during pressing, thus achieving the desired surface structure of the gas diffusion substrate. For example, the layer system to be pressed can be placed between two silicone plates as pressing jaws of a hydraulic hot press and subjected to 10 minutes at 260 °C and a pressure of, for example, 2.5 to 12 kN / cm 2By maintaining the die ratios of the polymer membrane and metal substrate as described above, the properties of the polymer membrane are essentially retained in the areas where no perforation is created.

[0049] The catalyst layer arranged on the gas diffusion electrode can be pressed together with the polymer layer(s) and metal substrate layer(s); however, a catalyst layer or catalyst coating can also be applied subsequently (for example, a catalyst ink can be sprayed on or applied in another way). Which method is appropriate is known to those skilled in the art and depends in particular on the pressure and temperature stability of the catalyst.

[0050] Alternatively or additionally, the surface of the pressing jaws can be designed so that it has raised portions at the locations where perforations are to be formed in the polymer layer. During the pressing process, these raised portions perforate the polymer layer, allowing the underlying metal substrate to form part of the surface of the gas diffusion substrate.

[0051] Typically, the temperature during the pressing process is selected so that it is below the glass transition temperature of the polymer contained in the polymer layer. Higher temperatures would result in the pore structure contained in the polymer layer suffering from the heat treatment and, in the worst case, complete caking of the pores. Nevertheless, a temperature is usually selected that is not too far below the glass transition temperature - especially in embodiments that contain at least two polymer layers, as this makes it easier for two polymer layers to adhere to one another or to create a cohesive bond. Temperatures between 180 and 260 °C, in particular 200 to 240 °C, have proven suitable, especially for layers made of fluorinated polymers. Typically, a contact pressure of at least 2 kN is required, independently of this or simultaneously. 2, for example from 2.25 to 8 kN rrr 2 suitable.

[0052] Finally, the gas diffusion substrates according to the application can also be produced alternatively or in addition to the pressing process by bonding a metal substrate and a polymer layer together at least in a form-fitting manner and then grinding the side of the polymer layer facing away from the metal substrate until perforations form in the polymer layer, enabling electrical contact with the metal substrate, or until the proportion of perforations has reached an area that ensures sufficient conductivity.

[0053] The gas diffusion electrode and the electrochemical cell according to the application are particularly suitable for the reduction of gases, including CO, CO2, N2, and NO. X (nitrogen oxides), SO X(sulfur oxides) or oxygen. In addition, it is also suitable for the reduction or oxidation of organic substances, for example, the hydrogenation of multiple bonds, carboxylation, or partial oxidation. However, it is important to ensure that the material, especially the polymer layer material, is compatible with the organic compounds and the resulting products. It goes without saying that the polymer layer must also be chemically stable with respect to the electrolyte.

[0054] In summary, it can be stated once again that the gas diffusion substrate according to the application provides systems for electrochemical processes with the formation of three-phase boundaries consisting of an electrolyte, catalyst, and reactants. These systems are conductive perpendicular to the plane and yet do not exhibit significant flooding behavior with electrolyte due to electrowetting. Furthermore, due to the perforated porous polymer layers or polymer films, they are no longer electrically insulating and therefore allow for surface scaling and bipolar assembly in electrochemical stacks. These properties and the design as free-standing gas diffusion substrates enable a flexible platform for the application of catalytically active layers and thus for properties tailored to the respective processes. The adaptation of the catalyst layer properties is thus decoupled from the diffusion layer.Properties such as permeability, porosity, electrical conductivity or mechanical stability can be controlled by selecting the material types and morphologies.

[0055] Without limiting the generality, the diffusion layer according to the application, the electrode according to the application and the electrochemical cell as well as the method for producing the diffusion layer are described in more detail below with reference to examples and figures.

[0056] Figures 1 and 2 show a schematic view of a section of the diffusion substrate according to the application, each in a three-layer embodiment. The thicknesses of the polymer layer(s) are shown larger for clarity. In fact, as explained above, the thickness of the polymer layers is generally significantly smaller than that of the metal substrates.

[0057] Figure 1 shows a first polymer layer 31 and an underlying second polymer layer 32, between which a first flat metal substrate 21 in the form of a metal grid is arranged. The largest part of the metal substrate 21 is not part of the first surface 5 of the gas diffusion substrate; however, the course of the metal grid between the polymer layers 31, 32 is shown with dashed lines (the grid has very large grid windows for clarity). The surface 5 of the gas diffusion substrate is formed by the side of the polymer layer 31 facing away from the metal substrate 21 (the continuous region of the first polymer layer is clearly visible) and ten closed regions 23 of the first metal substrate 21 (three of which are located at the edge of the illustrated section).At the upper edge of the cutout, a region 23 extending from the main surface 5 is shown in the form of a protruding elevation 24 of the first metal substrate 21 (this is thus part of the first main surface of the first flat metal substrate 21). At the location of this elevation 24, a perforation 34 has formed in the polymer layer 31. A perforation 34 can also be seen at the right edge of the cutout. Finally, at the lower edge of the cutout, an area 37 can be seen in which, due to the manufacturing process, a material bond is formed between the two polymer layers 31, 32 (schematically shown by a partially broken line).

[0058] Figure 2 shows a first flat metal substrate 21 and a second flat metal substrate 22 located underneath, between which a first polymer layer 31 is arranged. As in Figure 1, both the first metal substrate 21 and the second metal substrate 22 are shown as metal grids (again with very large grid windows for the sake of clarity). The largest part of the underlying metal substrate 22 is not part of the first surface 5 of the gas diffusion substrate; however, the course of the metal grid is shown in partial areas with dashed lines (the side of this metal substrate 22 facing away from the polymer layer forms part of the second surface of the gas diffusion substrate). The first surface 5 of the gas diffusion substrate is formed here by the side of the polymer layer 31 facing the metal substrate 21 and approximatelyeighteen closed regions 33 of the first polymer layer 31 and by the metal grid of the first metal substrate 21 in the form of a continuous area. In addition, some elevations 25 of the second metal substrate 22 can be seen here, which have perforated the polymer layer 31 due to the manufacturing process and are thus also part of the first surface 5. For the sake of better clarity, in this schematic figure, elevations 24 pointing upwards (in the direction of the viewer) are formed only at every second intersection point of the first metal grid 21 (these are indicated by hatched lines). The same applies to the second metal substrate 22. At the remaining intersection points, downward-facing elevations 26 are formed (indicated by dashed hatching), so that perforations (not shown here) are formed in the polymer layer 31 there.On the right-hand edge of the cutout, a perforation 34 can be seen in the area of ​​such a downwardly directed elevation. The upwardly or downwardly directed elevations can, for example, be part of a fabric made of individual metal fibers (with elevations inevitably occurring at the intersection points of the longitudinally and transversely running fibers); however, they can also, for example, be specifically created at individual points of a completely flat metal grid using additive manufacturing (this also applies to the grid in Figure 1).

[0059] Figures 3a and 3b show examples of the structure of electrochemical cells with the diffusion substrates and diffusion electrodes according to the application.

[0060] Figure 3a shows a cell for CO2 electrolysis in zero-gap with anion exchange membrane as electrolyte with gas diffusion electrodes with a layer structure polymer layer / metal substrate / polymer layer:

[0061] A (bi-)polar plate with flow guide 103 is arranged on each of the two end plates 101, with an insulator layer 102 arranged thereon. The actual diffusion electrode 104 (a three-layer structure of polymer layer / metal substrate / polymer layer is shown in Figure 3a) is arranged on one of the bipolar plates 103 (the electrocatalyst layer is not shown here for clarity). The gas diffusion electrode 104 is followed by an anion exchange membrane 106 and an anode 107 for O2 formation. The spaces between the gas diffusion electrode 104 and the anion exchange membrane 106 on the one hand, and the anion exchange membrane 106 and the anode 107 on the other, are sealed to the outside with a gasket 106.

[0062] Figure 3b shows a cell for O2 or CO2 reduction in the flow cell with liquid catholyte with gas diffusion electrodes with a layer structure polymer layer / metal substrate / polymer layer):

[0063] A (bi-)polar plate with flow guide 203 is arranged on each of the two end plates 201, each with an insulator layer 202 arranged thereon. The actual diffusion electrode 204 (a three-layer structure of polymer layer / metal substrate / polymer layer is shown in Figure 3b) is arranged on one of the bipolar plates 203 (the electrocatalyst layer is not shown here for clarity). The gas diffusion electrode 104 is followed by an electrolyte compartment 206, an anion exchange membrane 207, another electrolyte compartment 206, and an anode 208 for O2 formation. Sealing is also achieved here by means of several seals 206.

[0064] Example 1 - Fabrication of an electrode with a mesh-reinforced PTFE diffusion substrate with a layer structure of polymer layer / metal substrate / polymer layer

[0065] By means of hot pressing, a galvanically silver-plated stainless steel grid (Haver & Boecker, 0.2 mm x 0.16 mm) is pressed 4 x 2 min at 200 °C and 3.6 kN cm 2 between two ePTFE polymer layers (Fluortex Multiflon ePTFE membrane with a thickness of 30 μm and a pore size of 0.45 μm) as polymer layers. For the catalyst coating, a mixture of carbon black (Imerys Graphite & Carbon), PTFE (3M), and silver nanoparticles (Alfa Aesar) is homogenized in a knife mill and knife-coated onto the resulting diffusion substrate in a layer thickness of 3 mm. Using the same press, the gas diffusion layer is heated for 10 minutes at 260 °C and 3.6 kN cm. 2pressed onto the catalyst coating. The electrodes were functionalized with an anion exchange (ionomer) layer (Dioxide Materials) by drop-casting and conditioned in 1 M KOH. Figures 4a to 4c and 5a to 5c show scanning electron microscope (SEM) images of a diffusion substrate with a polymer layer / metal substrate / polymer layer layer structure at different magnifications.

[0066] Figures 4a to 4c show images of a stainless steel grid or fabric with an ePTFE polymer layer (Fluortex MultiFlon membrane with a thickness of 30 μm and a pore size of 0.45 μm) consisting of a polymer layer / metal substrate / polymer layer structure. A stainless steel grid with a mesh size of 0.16 mm according to the manufacturer's specifications was used as the metal substrate. Clearly visible in Figures 4b and especially 4a are the maxima or elevations of a fabric-like metal substrate with a fiber diameter of approximately 0.16 mm protruding through the perforations of the polymer layer. The perforations of the polymer layer have dimensions of approximately 0.35 to 0.55 mm in length and 0.13 to 0.17 mm in width, and occupy an average area of ​​approximately 0.05 to 0.06 mm. 2so that the perforations in the polymer layer take up approximately 40-45% of the total area of ​​the polymer layer. The surface of the diffusion substrate is thus formed by the continuous area of ​​the perforated polymer layer and the enclosed areas of the metal substrate visible in the perforations of the polymer layer, which are completely enclosed by the polymer layer (see Figure 4b). It can also be seen that the production process (hot pressing) pressed the elevations on the main surface of the metal substrate into the polymer layer. Figure 4b shows an approximately diamond-shaped area between these elevations in the center of the figure, i.e. the area in which no compression of the polymer layer has occurred, so that the layer thickness and the pore structure of the polymer film used to produce the gas diffusion layer have essentially been retained.In Figure 4c, this open-pore structure of the polymer layer can be clearly seen, which in particular enables gas transport.

[0067] Figures 5a to 5c show images of a stainless steel grid or fabric with a polyethersulfone (PES) polymer layer consisting of a polymer layer / metal substrate / polymer layer structure. A stainless steel grid with a mesh size of 0.2 mm according to the manufacturer's specifications was used as the metal substrate; the polymer layer had a thickness of 140 μm and a pore diameter of 0.45 μm. Also clearly visible in Figures 5b and especially 5a are the maxima or elevations of the fabric-like metal substrate protruding through the perforations of the polymer layer TI, with a fiber diameter of approximately 0.16 mm. The perforations of the polymer layer have dimensions of approximately 0.35 to 0.45 mm in length and 0.16 to 0.2 mm in width, and occupy an average area of ​​approximately 0.055 to 0.065 mm. 2so that the perforations in the polymer layer take up approximately 45-50% of the total area of ​​the polymer layer. Here, too, the surface of the diffusion substrate is formed by the continuous area of ​​the perforated polymer layer and the enclosed regions of the metal substrate visible in the perforations of the polymer layer, which are completely enclosed by the polymer layer (see Figure 5b). It can be seen again that the production process (hot pressing) pressed the elevations on the main surface of the metal substrate into the polymer layer. Figure 5c clearly shows the open-pore structure of the polymer layer, which in particular enables gas transport. Here, too, the layer thickness and the pore structure of the polymer film used to produce the gas diffusion layer have essentially been retained.

[0068] Figures 6a to 6c show scanning electron microscope (SEM) images of a diffusion substrate with a polymer layer / metal substrate / polymer layer layer structure with a catalyst coating applied thereon, at various magnifications. A silver-plated copper grid or fabric with a mesh size of 0.5 mm according to the manufacturer's specifications and an ePTFE polymer layer (Fluortex MultiFlon membrane with a thickness of 30 μm and a pore size of 0.45 μm) were used. Clearly visible in Figures 6b and especially 6a are the maxima or elevations of the fabric-like metal substrate protruding through the perforations of the polymer layer (these are the regular structures of perpendicular ovals or perforations) with a fiber diameter of approximately 0.16 to 0.25 mm. The perforations of the polymer layer have approximately dimensions of 0.25 to 0.35 mm in length and 0.13 to 0.17 mm in width and occupy an average area of ​​about 0.035 mm2 so that the perforations in the polymer layer occupy approximately 30% of the total surface area of ​​the polymer layer. Figure 6b shows that the closed regions of the metal substrate visible in the perforations of the polymer layer are completely enclosed by the polymer layer. It can also be seen that the open pore structure of the polymer layer (cf. Figure 6c) is retained and enables gas transport. Due to the pressing of the catalyst in the immediate vicinity of the metal fibers penetrating the polymer layer and thus also the three-phase boundaries, a higher pore concentration of the polymer layer can be seen than in the more distant regions.

[0069] Figure 7 shows, in contrast to all other figures, a scanning electron microscope (SEM) image of a diffusion substrate with a layered structure of polymer layer / metal substrate / polymer layer with metal fibers irregularly distributed on an expanded metal mesh. It can be seen that even diffusion substrates with such asymmetrical structures are functional, i.e., electrically conductive and gas-permeable. However, the pressing process for their production described in the application results in an uneven distribution of pressure across different sections of the polymer layers, which can lead to gas-impermeable domains in certain areas. The desired porosity of the polymer layer is maintained particularly in areas with a higher metal fiber content, while pore closures can occur in areas with fewer fibers. Therefore, when using inhomogeneous structures or loose metal fibers, care must be taken to ensure the most even distribution possible.

[0070] Example 2 - CO2 electrolysis with an electrode according to Example 1

[0071] CO2 electrolysis is carried out in a 3-electrode setup (RE: mini-RHE, CE: platinum-plated titanium wire) with 0.1 M K2SCh / 1.5 M KHCO3 as electrolyte at room temperature. The CO2 flow is 22.5 ml min -1 , mixed with 2.5 ml min -1 Ar is used as an internal standard, and a pressure of 40 mbar is applied. The concentrations of the product gases (H2, CO) generated during electrolysis are determined using online GC / MS.

[0072] Table 1 summarizes the electrolysis results for different catalyst layer compositions. The influence of the catalyst layer composition on the Faradaic efficiency for CO after 20 and 120 min, respectively, and the cathode potential against the reversible hydrogen electrode after 120 min were investigated. The current density was 50 mA cm for all measurements. -2and the anion exchanger loading 5 mg cm -2 Although higher silver loadings lead to lower electrolysis voltages, they also reduce the Faradaic efficiency for CO. A PTFE content of 15 wt.% in the catalyst coating represents an optimum for the performance of the electrodes with regard to CO2 reduction. Based on these data, a catalyst layer composition of 80 wt.% carbon black (CB), 15 wt.% PTFE, and 5 wt.% silver nanoparticles was selected for the following examples 3 and 4.

[0073] Table 1 :

[0074] Example 3 - Comparison of the electrodes according to Example 1 with commercially available electrodes

[0075] To compare the electrodes from Example 1 with those from Example 2, the electrolysis was also carried out using commercially available electrodes based on hydrophobic carbon fiber fabric with a microporous coating (CeTech). The current density was (as in Example 2) 50 mA cm-2 and the anion exchanger loading 5 mg cm -2Both the CO Faraday efficiency and the voltage are comparable for both electrode types. On average, the standardized CO Faraday efficiency for the electrode according to the application is between 88 ± 4 and 85 ± 9%, compared to 95 ± 1 to 82 ± 5% for the electrodes based on carbon fiber fabric. However, the electrode according to the application exhibits a much higher stability of the Faraday efficiency over the test period, while the electrode with reference material already records a loss of 13 percentage points. Of particular note is the lower scatter of the potential values ​​when using an electrode according to the application, which demonstrates high reproducibility of stable electrolysis operation for electrodes with a grid-reinforced gas diffusion layer. The high mechanical integrity of the electrodes according to the application, despite the commonly occurring carbonate formation, is particularly noteworthy here.Commercially available carbon fiber fabric gas diffusion layers, however, can break due to carbonate formation.

[0076] Figure 8a shows the time course of the normalized Faraday efficiency for CO and Figure 8b of the voltage (right) during a two-hour CO2 electrolysis for the electrode according to the application (squares) and the commercially available electrode (circles).

[0077] To test the mechanical stability, the tensile strength was used as a parameter for both electrodes. In tensile tests based on DIN 527-3 Type 4 with a test speed of 5 mm min -1 At a test temperature of 23 °C, the diffusion substrates according to the application exhibit a tensile strength of 105 ± 9 MPa and a tensile elongation of 27 ± 6%. In correspondingly cut test specimens made of commercially available carbon fiber fabric, a tensile strength 6.5 times lower (16 ± 3 MPa) and a lower tensile elongation of 13 ± 1% were observed.

[0078] Using the same electrode as claimed in the application, corresponding tests were also carried out at higher current densities and, in deviation from Example 2, with an anion exchanger loading of 2.5 mg cm -2 Figure 9a shows the Faraday efficiency for CO and Figure 9b the cathode potential against the reversible hydrogen electrode above for 50 mA cm -2 and below for 200 mA cm -2 Even at 200 mA cm -2 A standardized CO Faraday efficiency in the range of 40 to 10% is achieved over the entire test period.

[0079] Example 4 - O2 reduction with an electrode according to Example 1

[0080] The electrochemical O2 reduction is carried out in a three-electrode setup (Gaskatel FlexCell, RE: Mini-RHE, CE: Pt coil) with 1 M KOH as the electrolyte at room temperature. The O2 flow is 20 ml min. -1and an overpressure of 2 to 3 mbar is applied across a water column. The gas diffusion electrode according to the application was cycled 100 times between +1.2 V and +0.3 V. Figure 10a shows cyclic voltammometric investigations of O2 reduction. For selected cycles, the current density curve is shown as a function of the applied voltage. O2 reduction begins between 0.7 and 0.8 V and can be recognized by the increase in the current density. Figure 10b shows that the current density increases with increasing cycle numbers at a voltage of 0.3 V, approaching a value between -40 and -50 mA cm. -2 This behavior can be attributed to the increasing wetting of the hydrophobic, carbon-containing catalytic layer by the electrolyte. However, flooding and the associated decrease in O2 reduction activity do not occur due to the high barrier effect of the diffusion layer.

[0081] Example 5 - Fabrication of an electrode with a mesh-reinforced PTFE diffusion substrate with a layer structure of metal substrate / polymer layer / metal substrate

[0082] An ePTFE polymer layer (Fluortex; membrane with 30 pm thickness and 0.45 pm pore size) is heated for 2 min at 180 °C and 2.5 kN cm -2between two electroplated silver-plated Cu grids (Haver & Boecker, 0.5 mm x 0.25 mm). The design of the diffusion substrate in the form of a porous ePTFE polymer layer between two conductive grids prevents detachment of the diffusion layer from the conductive component and thus enables easier further processing. For the coating, a mixture of carbon black (Imerys Graphite & Carbon), nanometer-sized PTFE (Nanoshel), and silver nanoparticles (Alfa Aesar) is suspended in 2-propanol as a catalyst ink and homogenized for 30 minutes using an ultrasonic bath. The catalyst ink is applied to the grid-reinforced diffusion substrate with a spray gun (Iwata) at 90 °C until an Ag loading of 1 mg cm³ is achieved. -2 The electrodes are functionalized with an anion exchange (ionomer) layer (Dioxide Materials) by drop casting and conditioned in 1 M KOH.

[0083] Figure 11a shows a light micrograph (100x magnification) and Figure 11b a scanning electron microscope (SEM) image of a diffusion substrate with a layered structure of metal substrate / polymer layer / metal substrate. Here, too, the fabric-like metal substrate is visible. The fiber diameter is approximately 0.15 mm, and the openings in the metal substrate have an area of ​​approximately 0.47 mm. 2, so that the openings cover an area of ​​more than 90% of the main surface of the metal substrate. The surface of the diffusion substrate is therefore also formed here by the continuous region of the first main surface of the fabric-shaped metal substrate and the closed regions of the polymer layer visible in the openings of the metal substrate, which are completely enclosed by the main surface of the metal substrate. Figure 11a shows that the manufacturing process (hot pressing) caused the metal substrate lying beneath the polymer layer to push through the polymer layer, creating perforations in the polymer layer. Figure 11b shows a side view. The thin polymer layer winds its way between the two metal grid / metal fabric layers.

[0084] Figure 12a shows a scanning electron microscope (SEM) image of a diffusion substrate with a layered structure of metal substrate / polymer layer / metal substrate. A silver-plated copper grid or fabric with a mesh size of 0.565 mm and an ePTFE polymer layer (Fluortex MultiFlon membrane with a thickness of 30 μm and a pore size of 0.45 μm) were used. The fiber diameter is approximately 0.15 mm. The surface of the diffusion substrate is formed by the continuous area of ​​the first main surface of the fabric-shaped metal substrate and the enclosed regions of the polymer layer visible in the openings of the metal substrate, which are completely enclosed by the main surface of the metal substrate. It can also be seen that the second metal substrate is arranged beneath the polymer membrane at an angle of approximately 45° relative to the first metal substrate. It can also be seen that openings resulting from the manufacturing process are arranged in the polymer membrane.It can also be seen that only in the area where the polymer layer was compressed by the lower (second) metal substrate, the pore structure of the polymer film used was lost (Fig. 12b).

[0085] Example 6 - CO2 electrolysis with an electrode according to Example 5

[0086] CO2 electrolysis was carried out in a 3-electrode setup (RE: Mini-RHE (Gaskatel), CE: Ni foam) with 1 M KHCO3 as electrolyte at room temperature. The CO2 flow was 22.5 ml min -1 , mixed with 2.5 ml min -1 Ar as an internal standard and 40 mbar overpressure was applied. The concentrations of the product gases (H2, CO) generated during the electrolysis were determined using online GC / MS. Table 2 shows the Faradaic efficiencies for CO of the gas diffusion electrodes according to Example 5 at a current density of 50 mA cm -2The CO selectivity varies depending on the catalyst layer composition. A Faradaic efficiency for CO of 78% is achieved at a half-cell voltage of -1.54 V vs. RHE with a catalyst layer composition of equal mass parts of Ag nanoparticles, carbon black (CB), and PTFE nanoparticles.

[0087] Table 2:

[0088] Example 7 - O2 reduction with an electrode according to Example 5

[0089] The electrochemical O2 reduction was carried out using an electrode according to Example 5 but otherwise analogous to Example 4. Figure 13a shows cyclic voltammometric investigations of the O2 reduction. For selected cycles, the current density curve is shown as a function of the applied voltage. Figure 13b shows the achieved current densities at 0.3 V. At the beginning of electrolysis, the electrodes used show comparable values ​​to Example 4 at approximately -50 mA cm. -2 down to -40 mA cm-2 During electrolysis, the current density decreases. This can be attributed to oxidation of the copper grid used, as can be seen in the cyclic voltammogram at approximately +0.8 V vs. RHE. Cu oxide species on the copper support grid of the diffusion layer increase the contact resistance between the catalyst layer and the diffusion layer, thus causing a gradual decrease in current density.

[0090] Example 8- CO2 electrolysis with an electrode according to Example 1 and solid electrolyte

[0091] The CO2 electrolysis is carried out galvanostatically at a current density of 100 mA cm -2 in a zero-gap setup with 0.1 M KHCO3 as the anode substrate at 40 °C cell temperature. A 40 pm thick Sustainion membrane is used as the anion exchange membrane. A 1 mm thick titanium felt from Bekaert with 1 mg cm -2 IrO2 serves as the anode. The CO2 flow is 50 ml min -1 , mixed with 5.5 ml min -1Ar is used as an internal standard, and a pressure of 40 mbar is applied. The concentrations of the product gases (H2, CO) generated during electrolysis are determined using online GC / MS. Figure 14a shows the time course of the normalized Faraday efficiency for CO, and Figure 14b shows the time course of the total cell voltage (left) during a 40-minute CO2 electrolysis. A constant CO Faraday efficiency of 13% is achieved at a cell voltage of < 4 V.

[0092] Example 9 - Hydrogenation of 2-methyl-3-butyn-2-ol using a diffusion substrate according to Example 1

[0093] A diffusion substrate according to Example 1 is used, but a different catalyst layer than in Example 1 is applied to produce the electrode. For the catalyst coating, a catalyst ink consisting of a Fe3Ni6S8 catalyst or a Pd catalyst and polyvinylidene fluoride (PVDF) in MeOH is applied to the grid-supported PTFE diffusion substrate up to a catalyst loading of 5 mg cm -2 sprayed using an Iwata SBS spray gun.

[0094] The electrochemical hydrogenation is carried out in a three-electrode setup (working electrode: diffusion electrode according to the application, reference: Hydroflex Mini-RHE, counter electrode: Ni foam). The diffusion electrode forms the boundary between a cathode compartment and a compartment for the organic substrate 2-methyl-3-butyn-2-ol. A 2 M potassium hydroxide solution serves as the electrolyte on both the anode and cathode sides. Both the substrate and the electrolytes are passed through the cell at 10 ml min-1 each. The electrolysis was carried out at 100 mA cm-1 -2 over a period of 60 min. The quantification of the hydrogenation products was carried out via 1 H-NMR analysis using potassium hydrogen phthalate as internal standard.

[0095] Table 3 shows the hydrogenation results for the Fe3Ni6S8 catalyst and the Pd catalyst, respectively. Hydrogenation to the alkene (2-methyl-3-buten-2-ol, MBE) was observed with a total Faradaic efficiency of 31% (Fe3Ni6S8) and 19% (Pd). Additionally, a total Faradaic efficiency of 12% was observed for the Fe3Ni6S8 catalyst. A product transfer of approximately 50% of the yield was observed in each case. However, this can be achieved by adjusting the polymeric material and thus the hydrophobicity / hydrophilicity of the diffusion layer. The cathode voltages were 4.5 (Fe3Ni6S8) and 1.5 V vs. RHE (Pd). Comparative experiments using a carbon cloth as a diffusion layer failed due to the high penetration of the substrate into the catholyte and demonstrate the advantage of the electrodes according to the application.

[0096] Table 3:

[0097] Example 10 - Measurement of electrical conductivity perpendicular to the electrode surface

[0098] To determine the surface resistance, a test specimen (40 mm diameter) is clamped between two gold-plated copper blocks (5 x 5 cm²) and pressed together with a pneumatic piston at different surface pressures. To improve the contact resistance, a carbon fleece is placed above and below the test specimen. The voltage drop across the test specimen is determined from the difference between the voltage drop of the test measurement and a blank measurement using two carbon fleeces. The measurements are carried out at current densities of 200 mA cm². -2carried out and the resistance is determined from a linear representation of the voltage loss as a function of the current density. In addition to the diffusion substrates from Examples 1 and 5 according to the application, the carbon fiber fabric from CeTech and a porous PTFE polymer layer (Fluortex ePTFE membrane with a thickness of 30 pm and a pore size of 0.45 pm) were measured as reference materials. Table 4 shows that the porous PTFE polymer layer cannot be measured due to the high electrical resistance in the measurement setup and the limited maximum voltage of the test bench (0-1 V) and therefore does not exhibit any significant electrical conductivity perpendicular to the electrode plane. The diffusion substrates according to the application, on the other hand, exhibit significant electrical conductivity. The electrical resistance only shows a significant dependence of the sheet resistance on the surface pressure for the diffusion substrate according to Example 1.This effect is not evident for the diffusion substrate according to Example 5, since the grids exhibit lower compressibility than the polymer layers. At a surface pressure of 13 N cm. -2 This results in surface resistances of 156 (Example 5) or 230 mO cm -2 (Example 1) measured. The carbon fiber fabric reference material exhibits a lower surface resistance at a similar surface pressure. At a low surface pressure of 3 N cm -2 comparable values ​​of 150 mfi cm -2 for the reference material and the diffusion substrate Example 5.

[0099] Table 4:

[0100] Example 1 1 - Use of metal substrates with two different surfaces in a layered structure metal substrate / polymer layer / metal substrate

[0101] An ePTFE polymer layer (Fluortex; membrane with 30 pm thickness and 0.45 pm pore size) is heated for 2 min at 200 °C and 3 kN cm-2 between two metal meshes with a 5-shaft satin weave (different meshes from GKD - Gebr. Kufferath AG). The surfaces of the metal substrate can be combined in different ways (depending on how the smooth and rough surfaces are aligned). By selecting such metal meshes, rectangular openings can be created in the metal substrates. It can be seen that the mechanical stability of the gas diffusion layer can suffer depending on the mesh size and geometry, as well as the alignment of the smooth or rough surface. Table 5 shows the results. Table 5:

[0102] Example 12 - Relevance of Pressing Parameters in the Production of Diffusion Substrates: Polymer Layer / Metal Substrate A woven metal substrate made of copper mesh (Haver & Boecker, 0.5 mm x 0.25 mm) was pressed onto a Bola PTFE polymer layer (0.2 mm thick; pore size 0.45 μm) for 90 s at different pressures and temperatures. The resulting sheet resistance was measured at a current density of 1 A cm. -2 for two different pressures (1 or 2 N*cnr 2). Furthermore, the generated substrates were roughened using sandpaper to remove layers of PTFE from the Cu grid. Table 6 shows that the gas diffusion substrates produced using the process according to the application initially exhibit significant conductivity perpendicular to the layer (as is evident from the comparison with the pure PTFE starting material / Table 6, line 2) and consistently exhibit similar or better conductivity than the reference material carbon cloth (Table 6, line 1) at both 180 °C and 200 °C during substrate production, with the conductivity increasing for higher contact pressures - see lines 4 to 19. With constant contact pressure and increasing contact temperature, it can be seen that the sheet resistance decreases with increasing contact temperatures (up to approximately 220 / 240 °C). Higher temperatures lead to deformation of the PTFE layer, which impedes the flow of current.In general, it can be stated that the sheet resistance decreases and thus the conductivity can be maximized without any destructive influence on the polymer layer if the contact pressure during the fabrication of the gas diffusion layers is increased and the temperatures are moderately increased. Roughening with sandpaper (Table 5, odd rows starting from row 5) exposes additional maxima of the metal substrate, and the sheet resistance further decreases to values ​​close to those of the Cu lattice used as the starting material (Table 6, row 3).

[0103] Table 6:

Claims

Patent claims 1. A gas diffusion substrate for use in an electrochemical cell, comprising at least a first planar porous metal substrate and at least one first planar gas-permeable polymer layer arranged thereon, wherein the first planar metal substrate has a first main surface and a second main surface, wherein electrical contact with a catalyst layer is provided via the metal substrate, and wherein the first planar polymer layer is arranged on the first main surface of the metal substrate and is formed from a hydrophobic polymer, wherein a first surface of the gas diffusion substrate is formed essentially from the first planar polymer layer and the first main surface of the first planar metal substrate, and wherein at least in partial areas of this surface of the gas diffusion substrate, in particular in partial areas provided for contact with the catalyst layer,either a plurality of closed regions of the first metal substrate surrounded by a continuous region of the first polymer layer, or a plurality of closed regions of the polymer layer surrounded by a continuous region of the first main surface of the first planar metal substrate.

2. Gas diffusion substrate according to the preceding claim, wherein at least the first main surface of the first metal substrate has a structure with a plurality of elevations protruding from the first main surface, and the first polymer layer has a plurality of perforations into which at least some of the elevations of the first main surface of the metal substrate protrude, so that the elevations can be directly electrically contacted with the catalyst layer.

3. Gas diffusion substrate according to one of the preceding claims, wherein the polymer layer has a substantially uniform layer thickness of 10 to 500 μm, in particular 10 to 250 μm, for example 10 to 100 μm or 20 to 50 μm. Gas diffusion substrate according to the preceding claim, wherein the thickness of the polymer layer is smaller than that of the metal substrate and in particular is at most half as thick as the thickness of the metal substrate. Gas diffusion substrate according to one of the preceding claims, wherein the polymer layer has pore sizes of 0.05 μm to 10 μm, in particular from 0.05 μm to 5 μm, for example 0.1 μm to 1 μm. Gas diffusion substrate according to the preceding claim, wherein at least some of the maxima of the elevations protrude through the perforations of the first polymer layer and / or lie in the surface formed by the side of the first polymer layer facing away from the first main surface of the metal substrate.Gas diffusion substrate according to one of the preceding claims, wherein a second planar, gas-permeable polymer layer is arranged on the second main surface of the metal substrate, and wherein a second surface of the gas diffusion substrate is formed substantially from the second polymer layer and from regions of the second main surface of the first metal substrate, and wherein at least in partial regions of this second surface there are a plurality of closed regions of the first metal substrate which are surrounded by a continuous region of the second polymer layer, and in particular the first planar metal substrate has a plurality of elevations protruding from the main surfaces on the first and on the second main surfaces, and the second polymer layer has a plurality of perforations into which at least some of the elevations of the second main surface of the first metal substrate protrude at least partially.. Gas diffusion substrate according to one of the preceding claims, wherein the first planar polymer layer is arranged between the first planar metal substrate and a second planar porous metal substrate, and wherein a second surface of the gas diffusion substrate is formed essentially from regions of the first polymer layer and from the main surface of the second metal substrate facing away from the polymer layer, and wherein at least in partial regions of this second surface, a plurality of closed regions of the first polymer layer are present, which are surrounded by a continuous region of the second metal substrate, and in particular, the first and second metal substrates have, on their main surfaces facing the polymer layer, a plurality of elevations protruding from the main surfaces, and the first polymer layer has a plurality of perforations,into which at least some of the elevations on these main surfaces of the first and second metal substrates protrude at least partially. . Gas diffusion substrate according to one of the preceding claims, wherein the first and the optionally present second metal substrate are formed at least partially or completely from metal fibers or comprise a porous metal layer with metal fibers arranged directly thereon.

0. Gas diffusion substrate according to the preceding claim, wherein the first and the optionally present second metal substrate are designed in the manner of a metal fleece with random metal fibers or a metal woven fabric, metal knitted fabric, or metal knitted fabric with ordered metal fibers.

1. Gas diffusion substrate according to one of the preceding claims, wherein the main surfaces of the first and the optionally present second metal substrate have a plurality of openings which are designed such thatthat at least in partial areas of the gas diffusion substrate, these openings occupy an area of ​​at least 25-80%, in particular 30-60%, for example 35-40% of the main surfaces of the metal substrate. . Gas diffusion substrate according to one of the preceding claims, wherein the main surfaces of the first and optionally present second metal substrate have a plurality of openings with an average area of ​​0.01 to 0.1 mm 2 . Gas diffusion substrate according to one of the preceding claims, wherein the perforations in the first and optionally present second polymer layer have an average area of ​​0.01 to 1 mm 2 , especially from 0.05 to 0.2 mm 2. Gas diffusion substrate according to one of the preceding claims, wherein the area of ​​the perforations of the first and optionally present second polymer layer, at least in partial areas, takes up between 10% and 90%, in particular between 30% and 70%, for example between 40 and 60% of the total area of ​​the polymer layer allocated to these partial areas.

5. Gas diffusion substrate according to one of the preceding claims, wherein the metal substrate is formed from a metal or comprises a coating with a metal having a conductivity of at least 1 * 10 6 S / m and in particular of at least 1 .4* 10 6S / m. Gas diffusion substrate according to one of the preceding claims, wherein the porous polymer substrate comprises or consists of a polysulfone, in particular polyethersulfone (PES) or a fluorinated polymer, wherein the fluorinated polymer is in particular selected from the group consisting of fluorinated ethylene propylene (FEP), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), perfluoroalkoxy polymer (PFA), polyethylene tetrafluoroethylene (ETFE), polyethylene chlorotrifluoroethylene (ECTFE), perforated elastomer [perfluoroelastomer] (FFPM / FFKM), fluorocarbon [C h lortrif luorethylenevinylidenefluoride] (FPM / FKM), perfluoropolyether (PFPE), perfluorosulfonic acid (PFSA) and perfluoropolyoxethane. A gas diffusion electrode comprising a gas diffusion substrate made of a first planar metal substrate and at least one first planar polymer layer according to one of the preceding claims arranged thereon, as well as an electrocatalyst layer, wherein the first planar metal substrate has a first main surface and a second main surface, wherein the first planar polymer layer is arranged on the first main surface, and wherein the electrocatalyst layer is arranged at least on a partial area of ​​the first and / or the second main surface of the metal substrate. An electrochemical cell comprising at least one gas diffusion substrate according to one of the preceding claims. A method for producing a gas diffusion substrate according to one of the preceding claims, wherein - a gas-permeable flat metal substrate and at least one first flat polymer layer are arranged one above the other between two pressing jaws of a press - Pressing the metal substrate and the at least one porous polymer layer at a temperature below the glass transition temperature of the polymer of the polymer layer, so that a composite of metal substrate and polymer layer is formed. Use of a gas diffusion electrode according to one of the preceding claims for the reduction of gases, in particular CO, CO2, N2, NO X , SO X or oxygen.

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