Method for producing a gas diffusion electrode for an electrochemical cell, gas diffusion electrode, and electrolytic cell having a gas diffusion electrode

Abstract

The invention relates to a method for producing a gas diffusion electrode (35) for an electrochemical cell (25). In the method, a pulverous organic binder polymer (1) is first provided in a step (S1). In a further step (S2), the organic binder polymer (1) is dispersed in a solvent (3) to form a plastisol (5). Then, in a step (S3), a catalyst material (7) and a pore-forming material (37) are added to the plastisol (5). In a step (S4), this compound is intimately mixed and processed to form a highly viscous catalyst paste (9). In a further step (S5), the catalyst paste (9) produced in this way is applied to an electrode substrate (39), and finally, in a step (S6), the solvent (3) is removed. The invention also relates to a gas diffusion electrode (35) produced according to the method.

Description

[0001] Description

[0002] Method for producing a gas diffusion electrode for an electrochemical cell, gas diffusion electrode and electrolysis cell with a gas diffusion electrode

[0003] The present invention relates to a method for producing a gas diffusion electrode for an electrochemical cell, in particular for a PEM electrolysis cell. Furthermore, the invention relates to a gas diffusion electrode and an electrolysis cell with a gas diffusion electrode.

[0004] The so-called PEM electrolysis (PEM stands for "polymer electrolyte membrane" or "proton exchange membrane") is gaining increasing importance due to its significant potential for producing cost-effective green hydrogen, both for industrial applications and as a storage medium or component in energy storage systems. In the context of climate change, hydrogen and / or the possibility of producing H₂ from renewable energy sources via PEM or water electrolysis has long since proven to be a key factor for the energy sector and related industries. Even though most hydrogen is still produced today through steam reforming of methane, aggressive investments, regulations, and subsidies will undoubtedly lead to a trend toward renewable hydrogen production in the foreseeable future.

[0005] A particularly promising method for producing hydrogen (H2) is the electrolysis of water, especially using renewable electrical energy. Hydrogen can serve as an energy storage medium, for example, by being used as a fuel to stabilize the electrical energy supply, particularly from renewable sources such as wind power, photovoltaics, or similar technologies. Hydrogen can also be used in other processes that require a fuel or a reducing agent. The hydrogen produced by electrolysis can thus be used industrially, or electrical energy can be generated again electrochemically using fuel cells.

[0006] The separation of water into its chemical components, hydrogen (H₂) and oxygen (O₂), can therefore be carried out using suitable electrolysis cells. A particularly important method is the described PEM electrolysis, which—compared to alkaline electrolysis approaches—proves to be more load-dynamic and better suited for coupling fluctuating power sources due to its less complex peripherals. In particular, high current densities and power outputs can be achieved with PEM electrolysis even under higher load gradients, while the high quality or purity of the hydrogen product is advantageously maintained even under partial or overload operation.

[0007] Hydrogen is already used in countless industrial and technological applications. Its potential to produce large quantities of H2 in a climate-neutral way and / or to store or transport it "carbon-free" via hydrogen carriers such as ammonia opens up entirely new avenues for various industrial sectors, such as transportation, chemicals, and steel, to supply entire sectors with green energy or operate them in a climate-friendly manner. Furthermore, hydrogen is already highly interesting as a fuel or fuel additive, and will continue to be so in the future, due to its potential to produce no or fewer emissions.

[0008] In a PEM electrolysis cell, a membrane is provided which has a catalyst layer on each of its opposite surfaces (CCM, English: Catalyst-Coated Membrane or 3-Layer Membrane Electrode Assembly (MEA), English: Membrane Electrode Assembly). Adjacent to the catalyst layers are usually gas diffusion layers, which in turn are connected to electrically conductive contact plates, called bipolar plates, which, among other things, serve for electrical contact. Preferably, these gas diffusion layers are also designed to enable the necessary substance transport during the intended operation of the electrolysis cell. The gas diffusion layer provides the necessary electrical conductivity to couple the contact plates and the catalyst layers electrically.This allows the desired electrochemical reaction to be achieved in the area of ​​the catalyst layers.

[0009] Hydrogen is produced electrolytically from water as a reactant. This is an electrochemical process in which water is separated into its chemical components, oxygen and hydrogen. The electrochemical cell reactions can be described and differentiated as follows:

[0010] In polymer electrolyte membrane electrolysis, the two partial reactions are spatially separated by an ion-conducting membrane, which must expediently be equipped with electrodes, in particular a cathodic catalyst and an anodic catalyst (CCM). Besides material improvements, significant cost reductions can be achieved, among other things, through improvements in manufacturing processes.

[0011] Since the production of PEM hydrogen electrolyzers (PEMWE) will increase significantly in terms of throughput and scale, and indeed must increase sharply to achieve agreed climate targets, there is an urgent need for technologies that make it possible to improve the throughput and manufacturing capacity of so-called corresponding CCMs.

[0012] A CCM comprises a membrane with a catalyst material applied directly to each of its two opposing surfaces. In many applications, particularly PEM water electrolysis, very expensive and therefore very rare precious metals are used as catalyst materials. A common method for manufacturing or providing a CCM structure is to apply and fix the electrode containing the catalyst material directly to the membrane using a fluorine-containing ionomer binder, such as Nafion® containing perfluorosulfonic acid (PFSA).

[0013] Today's common methods utilize a catalyst paste to create the catalyst layers or electrodes for the anode and cathode. The catalyst paste typically consists of the catalyst powder itself, an ionomer, optionally a polymeric binder, and a solvent. After application, the solvent is usually thermally removed. If the catalyst layer is deposited on a thermally stable support film, it must be transferred to the membrane in a further process step under pressure and temperature ("decaling process"). This ensures that the electrode is permanently fixed to the membrane and that good ionic contact is maintained between the catalyst materials and the membrane. As an alternative to the decaling process, the catalyst paste can also be applied directly to the membrane ("direct membrane coating").

[0014] An industrially established technology for hydrogen production is polymer electrolyte membrane water electrolysis (PEMWE), whose name derives from the electrolyte, the polymer membrane. According to current technology, both the membrane and components of the electrodes consist of polyfluorosulfonic acid (PFSA) ionomers. However, fluoropolymers are suspected of being hazardous to health because they do not degrade in the environment or in the human body due to their exceptional chemical stability. In response to these findings, the European Chemicals Agency (ECHA) recently published a proposal for a Europe-wide ban on polyfluoroalkyl substances (PFAS), which include PFSA polymers.The impending ban, as well as the announcement by major polymer manufacturers that they will phase out the production of all fluorinated substances in the near future, underscores the urgent need to develop long-term stable, marketable fluorine-free alternatives. This is crucial to ensure the continued production of hydrogen via PEMWE electrolysis using an environmentally friendly alternative. In addition to new approaches for developing and supplying suitable fluorine-free materials in accordance with future legal requirements, fundamental approaches for manufacturing corresponding fluorine-free electrode structures and other functional components for use in an electrochemical cell must also be developed to achieve the desired electrical, chemical, and mechanical properties.

[0015] The invention is therefore based on the objective of providing a method for producing a gas diffusion electrode for an electrochemical cell that is preferable from an environmental perspective. A gas diffusion electrode produced accordingly should exhibit both good mechanical and chemical stability and show a low tendency to degrade when used in an electrochemical cell.

[0016] The problem is solved according to the invention by a method for producing a gas diffusion electrode for an electrochemical cell, comprising the steps:

[0017] - Providing a powdered organic binder polymer,

[0018] - Dispersing the organic binder polymer in a solvent to form a plastisol ,

[0019] - Addition of a catalyst material and a pore-forming material to the plastisol ,

[0020] - Mixing the mass to form a catalyst paste ,

[0021] - Applying the catalyst paste to an electrode substrate,

[0022] - Removal of the solvent. The invention is based on the understanding that the formulation of the catalyst paste is of crucial importance in the production of a gas diffusion layer. It has a significant influence on the formation and long-term stable function and bonding of the catalyst layer to the electrode substrate. It has been shown that approaches based on known formulations and application methods for catalyst pastes from PFSA technology are only transferable to the development and bonding of fluorine-free polymers in coating processes to a very limited extent. This applies in particular to known methods for the production of membrane electrode assemblies (MEAs), in which the membrane consists of perfluorinated ionomers and the directly applied electrode layer with the catalyst material is fixed to the PFSA membrane using Nafion® as a polymer binder.

[0023] In contrast to the production of a catalyst-coated membrane (CCM), the process described here aims to produce a gas diffusion layer. In this process, so-called binder plastisols are used as the organic polymer, provided in a fine-granular form and added to the mass. The organic polymer binder is an organic polymer. This is dispersed in a solvent to form the plastisol, and then catalyst material and a pore-forming agent are added, which specifically induces pore formation. The pore-forming agent serves to create and maintain a desired very high porosity in the paste for the gas diffusion electrode. As a result, a gas diffusion electrode produced in this way enables very good substance transport when used in an electrochemical cell, such as an electrolysis cell or a fuel cell.A very high porosity in the range of 75% to 95% is targeted, typically 80% is preferred. The mixture is thoroughly blended to produce a highly viscous catalyst paste that is easy to process and can be applied over a large area to a substrate. The catalyst paste is applied evenly in a single layer to a thin, flat, or rollable porous electrode substrate. The electrode substrate material can be electrically conductive or insulating. Finally, the solvent is extracted from the compound, for example, by drying or vacuum extraction, resulting in a gas diffusion electrode with an electrically conductive, catalytically active, and open-pored structure for media transport. [The last sentence appears to be incomplete and requires context to be translated accurately.]In a known CCM catalyst layer, a gas diffusion electrode produced according to the process exhibits an extended layer thickness of up to 500 pm at high porosity. It has been shown that the process of the invention is particularly well-suited and applicable to the addition and processing of fluorine-free polymers as binder material in order to produce a fluorine-free gas diffusion electrode. To date, no fluorine-free alternatives for the production of gas diffusion electrodes have been described. Instead, previous approaches for gas diffusion electrodes are based on fluorine-based materials, in particular PFSA as binders in carbon fleeces or PFSA ionomers as binders and proton conductors in the electrodes of membrane electrode assemblies.

[0024] The invention's method overcomes numerous disadvantages of previous MEA concepts proposed in the context of the discussed ban on fluoropolymers in the European Union. High manufacturing and energy costs, as well as technical challenges such as mechanical, thermal, and chemical degradation, necessitate the development of fluorine-free membrane electrode assemblies (MEAs). These approaches are based on functionalized polymers obtained by sulfonation of, for example, PEEK, PPX, or PI. Problems here include low chemical stability (PI) under electrolysis conditions and significant swelling, even leading to water solubility, due to the high ionic conductivity required for sufficient ion transport.Known solutions involve ionic cross-linking of the molecular chains, which, however, unlike the more stable covalent bonds, provide points of attack for chemical degradation, i.e., they are insufficient for industrial application in an electrolyzer. Nowadays, electrochemical conversion processes are used for various purposes, for example, the electrochemical conversion of water into hydrogen and oxygen. The conversion takes place in an electrolysis cell with appropriate electrodes, which are separated from each other by a separator consisting of an ion-conducting membrane. In the case of PEM water electrolysis, the electrodes are usually applied directly to the membrane and fixed in place by an ionomer binder, e.g., Nafion™ (PFSA).

[0025] A suitable manufacturing or preparation process for a fluorine-free gas diffusion electrode with both good mechanical properties and high chemical stability has not yet been proposed. A particular advantage is that a gas diffusion electrode produced according to this method exhibits a low tendency to degradation when used in an electrochemical cell and meets the other requirements for industrial manufacturing.

[0026] In a particularly preferred embodiment of the process, sulfonic acid R-HSO3 is added, so that sulfonic acid R-HSO3 is introduced into the binder polymer as a side group and the binder polymer is thereby functionalized.

[0027] Thus, a targeted increase in the degree of functionalization is undertaken. In chemistry, functionality describes the presence of functional groups in a molecule. In organic chemistry, the functionality of a molecule has a decisive influence on its reactivity. In polymer chemistry, the functionality of a monomer refers to the number of its polymerizable groups; it affects the formation and degree of cross-linking of polymers. According to IUPAC, the functionality of a monomer is defined as the number of bonds that a monomer, or its repeating unit, forms with other monomers in a polymer. With a functionality of f = 2, polymerization results in a linear polymer (a thermoplastic). Monomers with a functionality f > 3 lead to a branching point, which can result in cross-linked polymers (thermosets).Monofunctional monomers therefore do not exist, since such molecules lead to chain termination.

[0028] In a particularly preferred embodiment of the process, a polymer from the group of polysulfones is selected as the binder polymer, which contains at least polyphenylene sulfone (PPSU), polysulfone (PSU) and polyphenylene sulfide (PPS) or mixtures thereof.

[0029] Initial investigations have shown that polysulfones are particularly well-suited as organic binder polymers for the production of fluorine-free gas diffusion electrodes and represent a preferred and environmentally friendly alternative to fluoropolymers. This allows for the use of a sulfonated polymer binder material or mixture in the manufacturing process, which is selective and highly effective in achieving the required properties of a gas diffusion electrode for use in an electrochemical cell, while simultaneously being fluorine-free. Polysulfones are a class of high-performance thermoplastic polymers. They are known for their toughness and stability at high temperatures. Technically used polysulfones therefore contain an aryl-SCp-aryl subunit. Due to high material and processing costs, polysulfones are used only in specialized applications, often as a superior alternative to polycarbonates.Three polysulfones have technical applications: polysulfone (PSU), polyethersulfone (PES), and polyphenylenesulfone (PPSU). They can be used in temperatures ranging from -100 °C to +200 °C and are employed in electrical devices, automotive engineering, and medical technology. They are composed of para-linked aromatics, sulfone and ether groups, and sometimes also alkyl groups. Polysulfones exhibit outstanding heat and oxidation resistance, hydrolysis resistance in aqueous and alkaline media, and good electrical properties. Polyphenylene sulfide (PPS) can also be used as a binder polymer. PPS, also known as polythio-p-phenylene, is a high-temperature-resistant thermoplastic with the general formula (SCeth). n Technically, it can be produced by polycondensation of 1,4-dichlorobenzene with sodium sulfide in aprotic solvents such as N-methylpyrrolidone.

[0030] In a further preferred embodiment of the process, a polymer from the group of compounds polyacrylonitrile, polyethylene oxide, polymethyl methacrylate or a copolymer thereof is selected as the binder polymer.

[0031] A significant advantage is that specific fluorine-free alternatives for the binder polymer in the manufacturing process have been selected and proposed, possessing the required properties for a gas diffusion electrode. The selected polymers exhibit hydrophilic and hydrophobic regions, high heat resistance, oxidation / reduction resistance, and long-term stability during electrolysis. It has been shown that, from the polysulfone group, PPSU exhibits the highest chemical stability under electrolysis conditions. The rigid molecular chain structure of aromatic polymers results in high thermal, mechanical, and chemical stability. However, due to the molecular structure, the ionic conductivity is somewhat lower than that of PFSA, as hardly any proton-conducting channels are formed.The targeted increase of the degree of functionalization by electrophilic aromatic substitution, here the ion exchange capacity IEC, of ​​the aromatic polymer tends to lead to weaker mechanical and chemical properties of the polymer.

[0032] Therefore, the advantageous preparation steps of the invention allow for a good compromise between increasing the ionic conductivity and the mechanical and chemical properties of a fluorine-free gas diffusion electrode. The manufacturing process of the invention thus provides a good starting point and basis for the expected further development of hydrocarbon polymers without fluorinated units.

[0033] In principle, polyvinylidene fluoride (PVDF), Nafion®, and perfluorosulfonic acid (PFSA), e.g., available under the trade name Aquivion®, could still be used as organic binder polymers for the production of a gas diffusion electrode for the time being. However, all of the aforementioned polymers contain fluorine, which is why their use is expected to be limited to a transitional period due to the anticipated ban. The invention already proposes, in a preferred embodiment, the use of suitable fluorine-free substitutes.

[0034] In a particularly preferred embodiment of the process, a crosslinking agent is added to the binder polymer, in particular triallyl cyanurate (TAIC), so that crosslinking of the binder polymer is brought about.

[0035] This process also selectively induces cross-linking of the binder polymer. This can be achieved by supplying energy, i.e., by introducing heat into the dispersed mass of solvent and binder polymer. Cross-linking can also be induced or promoted by irradiation with ultraviolet light or electron beams. Generally, the cross-linking of existing polymers is also referred to as cross-linking. In macromolecular chemistry, cross-linking refers to reactions in which a large number of individual macromolecules are linked to form a three-dimensional network. Linking can be achieved either directly during the synthesis of the macromolecules or through reactions on existing polymers. The cross-linking process alters the properties of the cross-linked substances. Generally, an increase in hardness, toughness, and melting point, and a decrease in solubility are observed.The change increases with the degree of crosslinking, the proportion of crosslinked sites relative to the total amount of polymer. A crosslinking agent is characterized by at least two reactive groups. Crosslinkers with two identical reactive groups are called homobi-functional crosslinkers, while those with two different groups are called heterobi-functional crosslinkers.

[0036] Cross-linking can occur either via functionalities already present in the polymer through a clever choice of reaction conditions (self-crosslinking polymers), or by adding multifunctional, low-molecular-weight substances called cross-linking agents. Depending on the degree of cross-linking, cross-linking polymers first produce elastomers and, with increasing cross-linking, thermosets.

[0037] It has been shown that the addition of triallyl cyanurate as a crosslinking agent is particularly advantageous in achieving good crosslinking of the binder polymer during the manufacturing process. Triallyl cyanurate can generally be used as a trifunctional crosslinking component for the production of thermoplastics.

[0038] In a further preferred embodiment of the process, N-butyl-2-pyrrolidone, 2-pyrrolidone, N-octyl-2-pyrrolidone, N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide (DMF), formamide, dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAC), acetonitrile or a mixture thereof is used as the solvent.

[0039] In the manufacturing process for a gas diffusion electrode described here, the organic polymers or the functionalized, sulfonated polymers (SPPSU, SPPS, SPSU) are optionally dissolved in one of the aforementioned solvents. A catalyst paste is then produced based on the resulting plastisol. Specifically, a polar, high-boiling solvent is used and dispersed with the organic binder polymer to form the plastisol. Using a mixture of solvents allows for further customization in the production of the catalyst paste, ensuring, depending on the coating system of the catalyst layer, a sufficiently high degree of dissolution of the material on the electrode substrate surface and a close bond between the materials.

[0040] In the process, platinum (Pt), ruthenium (Ru) or a binary alloy of these metals is preferably added as a catalyst material.

[0041] The aforementioned metals or metal alloys can be used particularly advantageously as cathode catalysts, i.e., for the production of a catalytically active cathodic gas diffusion electrode. The catalyst material is provided in powder form and added to the dispersion, for example, in the form of powdered or fine-granular "platinum black" or "Pt-Black." Other cathode catalysts such as Pt-Black, Pt@C, PtRu@C, Ru@C, or multi-component powder mixtures made from these catalyst materials are advantageously used in the preparation of the gas diffusion electrode.

[0042] In a further preferred embodiment, carbon f C can be added as a particulate support material, so that the formation of a catalytic activity is effected in such a way that particles of the catalyst material are deposited on the high-surface carbon f C as support material.

[0043] Thus, the formation of a structure with clusters of highly surface-level carbon material with catalyst material adsorbed on the surface of the carbon material is achieved. Simultaneously, directed percolation is promoted, which can be specifically adjusted by the carbon support; that is, the formation of conductive paths or fibers in the gas diffusion electrode through the structure in the direction of current flow. Directed percolation can be intuitively explained using a coffee machine (coffee percolator) or porous rock.

[0044] When water is poured onto a porous medium, the question arises whether the medium can be permeated, i.e., whether there is a channel from the top to the bottom of the medium, or whether the water is absorbed by the medium. The probability that the water encounters an open channel is given by p, as in isotropic percolation. However, unlike isotropic percolation, there is a preferred direction: water in porous rock, and also in the coffee machine, moves in the direction determined by gravity. The percolation threshold is higher in directional percolation than in isotropic percolation.

[0045] In a further embodiment of the process, iridium (Ir), ruthenium (Ru), titanium (Ti) or an alloy and / or an oxide or a bimetallic mixed oxide made of these metals is preferably added as the catalyst material.

[0046] The addition of these catalyst materials is advantageously used in the preparation and provision of an anode catalyst for the fabrication of an anodic gas diffusion electrode. Powder mixtures of these metal alloys, optionally together with mixed oxides, can also be used and added to the plastisol as catalyst material. The following anode catalysts can be advantageously used: Ir, IrÜ2, IrOx, Ir@RuO, IrRu, IrO2@TiO2. The process is therefore equally applicable for the preparation of a gas diffusion electrode as either a cathode or an anode GDE.

[0047] In a particularly preferred embodiment of the process, a pore-forming agent made of a pore-forming material is added to the plastisol, wherein the pore-forming material is selected from sodium chloride NaCl, ammonium carbonate (NH4)2CO3, ammonium hydrogen carbonate NH4HCO3, ammonium chloride NH4C1, potassium carbonate K2CO3 or a polymer pore-forming agent and / or a metallic pore-forming agent, in particular zinc oxide ZnO or polyvinylpyrrolidone (PVP).

[0048] The pore-forming agent allows for the desired porosity to be set during the production of the gas diffusion electrode, thus ensuring the necessary transport properties for liquid and gaseous components.

[0049] Preferably, in the process, when mixing the mass to form a catalyst paste, the mass is heated to a temperature of 20°C to 80°C, in particular from 40°C to 60°C.

[0050] By selecting and setting a specific temperature during mixing, the overall porosity of the gas diffusion electrode can be modified according to the requirements for its use and specifically adapted and adjusted to the operating environment within certain limits. It has been shown that, in addition to the choice of pore-forming material, a temperature dependency regarding the mean pore size distribution during paste formulation and subsequent preparation of the gas diffusion electrode can be advantageously utilized.

[0051] Preferably, in the process a catalyst paste with a viscosity of 500 - 2000 Pa - s is provided at a temperature of 25 °C and applied to the electrode substrate.

[0052] In a preferred embodiment of the process, the catalyst paste is intimately mixed to form a paste mixture, wherein a high solids content of catalyst material and binder polymer of 35 wt% to 60 wt%, in particular of 45 wt% to 55 wt%, is established in the paste mixture.

[0053] In general, higher solids content has proven advantageous compared to PFSA-based systems for producing a fluorine-free gas diffusion electrode with the required properties, for example, for use in a PEM electrolysis cell. The solids content is adjustable and adaptable to the specific application-related paste formulation, which comprises the solvent, the functionalized organic binder polymer, and the catalyst material system. Therefore, when mixing the catalyst paste, a solvent-to-solids ratio (LMA) of, for example, 40 / 60, 45 / 55, or 50 / 50 by weight may be preferable, depending on the requirements.

[0054] The paste formulation or paste production process is divided into the following steps:

[0055] The polymer dispersion is produced and subsequently mixed with catalyst material in particle form. Particular attention must be paid to the interactions between the components, as these significantly determine the material properties of the catalyst paste. The rheological properties play a crucial role in the coating step following paste formulation, influencing, for example, the electrode structure, thickness, and weight of the gas diffusion electrode. A balance is preferably sought between the disadvantages of low-viscosity pastes (thickness variation, segregation) and high-viscosity pastes (clogging of the coating tool, problems with the mixing process).In general, for applying the catalyst paste to the electrode substrate in the production of a fluorine-free gas diffusion electrode, higher viscosity catalyst pastes are preferred in order to give preference to a good electrode structure of the GDE and a particularly intimate, uniform and at the same time long-term stable bonding of catalyst material to the electrode substrate, forming a stable interface.

[0056] In a particularly preferred embodiment of the process, a flexible, bendable fabric or nonwoven is used as the electrode substrate, onto which the catalyst paste is applied.

[0057] The electrode substrate can advantageously be formed into a rollable, continuous web of substrate material, enabling a continuous coating process as preferred for industrial applications. In this process, a carbon fabric, carbon fiber nonwoven, carbon paper, metal nonwoven, metal wire fabric, metal wire mesh, or a plastic fabric is preferably used as the electrode substrate.

[0058] When using synthetic fabrics or meshes based on polyphenylsulfone (PPSU), polysulfone (PSU), polyphenylene sulfide (PPS), polyetherketone (PEEK), or polybenzimidazole (PBI), these are first carbonized to achieve the necessary electrical conductivity for the electrode substrate. Conductive carbon fabrics, carbon paper, carbon fleeces, or metal fabrics are thin, already electrically conductive, and flexible. In particular, these materials can be prepared as continuous tapes or rolls for the electrode substrate and made available for coating.

[0059] In a particularly preferred embodiment of the process, the solvent is removed by extraction or drying or combinations thereof.

[0060] An extraction solvent is used, preferably a non-solvent such as water or isopropanol. The solvent extraction can be carried out in an immersion bath or, alternatively or additionally, by applying a vacuum to the substrate coated with the paste, preferably in a vacuum chamber in a continuous process for industrial manufacturing. Thermal removal of the solvent is also advantageously possible.

[0061] Preferably, the pore-forming material is removed in the process, in particular simultaneously with the removal of the solvent, whereby a high porosity is achieved.

[0062] Preferably, a mean pore diameter of 20 pm to 40 pm and a porosity of 75% to 95% are achieved. A pore diameter of 35 pm and a porosity of 80% are particularly preferred. The intended process, combined with the preparation method of the invention, achieves a high overall porosity with large pore diameters, enabling the necessary media transport in the gas diffusion electrode. Simultaneously, good electrical conductivity is achieved through percolation, and hydrophilic properties are attained through the cross-linked polymer structure.

[0063] In a particularly preferred embodiment of the process, the application of the catalyst paste to the electrode substrate is carried out by means of a roll-to-roll application process, wherein in particular an oven is used for heat treatment during the paste application, whereby the solvent is thermally driven off.

[0064] According to this design, the manufacturing process itself is subject to virtually no limitations, for example regarding the coating width, and can thus be scaled up to ever larger throughputs or production batches, thereby significantly increasing overall production capacities for electrolysis cells. In particular, these advantages mean that one is no longer bound by the procedural limitations of the prior art.

[0065] In a preferred embodiment of the method, the thickness of the gas diffusion electrode is set to between 90pm and 500pm, in particular between 250pm and 300pm.

[0066] Preferably, the process is carried out in such a way that the polymer content of the gas diffusion electrode is 5 wt% to 20 wt%, in particular 9 wt% to 13 wt%.

[0067] In a particularly preferred embodiment of the process, a polymer content of 6 wt% to 15 wt%, in particular 9 wt% to 12.5 wt%, is achieved in a gas diffusion electrode. A gas diffusion electrode produced according to the process can be designed as an anode GDE or as a cathode GDE and used accordingly in an electrochemical cell.

[0068] The desired polymer content can be adjusted in the paste formulation by varying the weight fraction of the solid component of the added powdered polymer in the catalyst paste, i.e., in the dispersion or pasty mixture of the solvent, the catalyst material, and the organic polymer, so that a correspondingly preferred polymer content results in the gas diffusion electrode after the drying step. It has been shown that polymer content with the aforementioned preferred weight fraction is particularly advantageous for the function of the fluorine-free gas diffusion layer and for providing the required transport properties. A sufficiently high porosity is necessary for the transport of liquid and gaseous media—e.g., water and product gas. This is typically well above 60%, with a high porosity of typically 75% to 95% being preferred for the gas diffusion electrode.Poor gas transport of product gas, e.g., from electrolysis, leads to increased gas pressure, which can cause gas to cross the membrane. Gas crossover is therefore related to the ionomer content. A favorable operating point for the ionomer content for the formulation of the catalyst paste and the dried gas diffusion electrode is thus a compromise between small pore volume and mass transport losses at a comparatively high ionomer content and poor proton conductivity and ionic ohmic voltage losses at a rather low ionomer content.

[0069] The ionomer content depends on the components and their respective use in the anode or cathode, and can, under certain operating conditions, assume higher values ​​up to 30 wt% compared to the generally preferred, somewhat lower ionomer contents. Finally, the final microstructure is influenced by the type of coating and drying process. Another aspect of the present invention relates to a gas diffusion electrode, which is produced or can be produced by the described process.

[0070] The gas diffusion electrode produced using the aforementioned preparation technique advantageously does not require a pre-formed conductive solid support as its core or electrode substrate. Rather, the gas diffusion electrode of the invention is not limited by the dimensions of a pre-formed conductive solid support and, despite the absence of such a support, remains physically stable, durable, economical, and functional. The self-supporting, electrically conductive integral network formed within the structure or layer of the gas diffusion electrode of this technique imparts physical integrity, mechanical strength, electrical conductivity, and catalytic properties to the gas diffusion electrode.

[0071] A further aspect of the present invention relates to an electrochemical half-cell which has such a gas diffusion electrode. The half-cell can be a half-cell of an electrolysis cell or a fuel cell and may have a correspondingly manufactured cathodic gas diffusion layer and / or cathodic gas diffusion layer.

[0072] Another aspect of the invention preferably relates to an electrolysis cell, in particular a PEM electrolysis cell, which has such a half-cell. This can be an electrolysis cell which uses a variant of the previously described gas diffusion layers, for example with a gas diffusion electrode inserted on one or both sides. Preferably, the application of a gas diffusion electrode can be arranged in the cathode compartment of an electrolysis cell.

[0073] Another advantageous aspect of the invention is a plurality of such electrolysis cells stacked and electrically connected in series. These form a cell stack or an electrolysis stack that is scalable for high electrolysis capacities.

[0074] Therefore, in a particularly preferred embodiment of the invention, an electrolysis plant is proposed which has such a cell stack.

[0075] The merits of the present invention are therefore not only evident in the small or minimal product unit, such as a gas diffusion electrode, but also significantly, through the industrial scale effect, in the electrolysis cell and a corresponding cell stack, electrolyzer, or electrolysis system comprising the electrolysis cell, which relates to further aspects of the present invention. This enables a new manufacturing approach for future fluorine-free gas diffusion electrodes for water electrolysis and their industrial-scale applications from an environmental perspective.

[0076] In particular, the present invention relates to PEM electrolyzers and furthermore to entire electrolysis or power-to-X power plants which have an electrolysis system as described herein.

[0077] The features, characteristics and / or advantages relating to the process described here also apply to the gas diffusion electrode produced accordingly, as well as to the half-cell, electrolysis cell, cell stack, electrolysis plant and / or an entire power-to-X power plant, and vice versa.

[0078] The expression "and / or" or "or" used here, when used in a series of two or more elements, means that each of the listed elements can be used alone, or any combination of two or more of the listed elements can be used. Exemplary embodiments of the invention are explained in more detail with reference to a drawing. In this drawing, the following are shown schematically and in a highly simplified manner:

[0079] FIG 1 shows the operation of an electrolysis cell for water electrolysis, in particular a PEM electrolysis cell, with a gas diffusion electrode used therein;

[0080] FIG 2 shows, based on a schematic flowchart, the process steps for manufacturing the gas diffusion electrode for an electrochemical cell;

[0081] FIG 3 shows a section of a membrane electrode arrangement with a cathodic gas diffusion electrode;

[0082] FIG 3A shows a detail enlargement from FIG 3 showing a number of catalyst particles deposited on carbon f as a support material;

[0083] FIG 3B shows a further enlarged section from FIG 3A, a catalyst particle with characteristic functional elements of the gas diffusion electrode;

[0084] FIG 4 shows, in comparison to a membrane electrode arrangement, a section of a catalyst-coated membrane of a cathodic half-cell;

[0085] FIG 4A shows a detail enlargement from FIG 4 showing a number of catalyst particles deposited on a carbon substrate as a support material;

[0086] FIG 4B shows a further enlarged detail from FIG 4A, a catalyst particle with characteristic functional elements of the electrode;

[0087] FIG 5 shows a characterization of a gas diffusion electrode prepared according to the procedure based on a flow measurement; FIG 6 shows a characterization of a gas diffusion electrode prepared according to the procedure, wherein the contributions of the pore size are determined;

[0088] FIG 7 shows a characterization of a gas diffusion electrode prepared according to the process using a current-voltage characteristic curve;

[0089] FIG 8 shows an exemplary embodiment of a fluorine-free gas diffusion layer in top view (photographic image);

[0090] FIG 8A a light microscopic image in cross-sectional view of the interface of the gas diffusion electrode shown in FIG 8;

[0091] FIG 8B shows a further magnified light microscopic image of the interface of the gas diffusion electrode shown in FIG 8, and

[0092] FIG 9 shows a section of a gas diffusion electrode photographed with a scanning electron microscope.

[0093] In the exemplary illustrations and figures, identical or equivalent elements may each be provided with the same reference symbols. The depicted elements and their relative sizes are generally not to be considered to scale; rather, individual elements may be exaggeratedly thick or large for better clarity and / or understanding.

[0094] Figure 1 shows an electrochemical cell 25 in the left-hand section, using an electrolysis cell 25, in particular a PEM electrolysis cell for water electrolysis, as an example. An important functional element of such a polymer electrolyte membrane electrolysis cell 25 is formed by the membrane 11, which consists of a polymer material. The membrane 11 separates the oxygen-containing anode compartment from the hydrogen-containing cathode compartment of the electrolysis cell 25, thus forming a cathodic half-cell 25A and an anodic half-cell 25B with a characteristic structure. Furthermore, the membrane 11 is proton-conducting. The membrane 11 is thinly coated on the anode side with an anode-specific catalyst material 7, so that an anode electrode is directly applied to and firmly connected with the membrane 11.In the anodic half-cell 25B, the anode electrode is designed as a catalyst-coated membrane, or CCM for short. The catalyst material 7 in half-cell 25B is selected and adapted for catalyzing the anode reaction. On the cathode side, however, in the cathodic half-cell 25A, the cathode electrode is designed as a gas diffusion electrode 35 and is arranged on the membrane 11, or rather, laid flat on the membrane 11 and brought into contact with it. The gas diffusion electrode 35 is designed as a porous and electrically conductive component and also incorporates a catalyst material 7. The cathodic catalyst material 7 is specifically adapted for catalyzing the cathode reaction and is introduced, at least in the side of the porous gas diffusion electrode 35 facing the membrane 11, in the form of supported catalyst particles.Thus, in the electrolysis cell 25, a combination of a gas diffusion electrode 35 and a catalyst-coated membrane 11, a CCM, is realized by the appropriately selected specific design of the half-cells 25A, 25B. This combination of the electrodes with the membrane 11 is referred to as a membrane-electrode arrangement 23.

[0095] Such a membrane electrode arrangement 23 is illustrated in more detail – in a close-up – to the right of the electrochemical cell 25 in FIG. 1. Platinum, ruthenium, or a binary alloy of these metals is used as the catalyst material 7 in the gas fusion electrode 35. Platinum-Black, Pt@C, PtRu@C, and Ru@C are used as cathode catalysts. Carbon f C is additionally provided as a particulate support material 19, so that the formation of catalytic activity is effected by particles of the catalyst material 7 being deposited on the high-surface-area carbon as support material 19. This mechanism of action is illustrated and explained in more detail below with reference to FIG. 3.

[0096] On the anode side, the membrane electrode arrangement 23 has a membrane 11 coated with a catalyst material 7, made of a membrane substrate 11A. The catalyst material 7 applied to the membrane 11 on the anode side comprises iridium, ruthenium, titanium, or an alloy and / or an oxide or a bimetallic mixed oxide of these materials. The anode catalyst can be, for example, Ir, IrÜ2, or IrO. xIr@RuO, IrRu, IrO2@TiO2 can be used as catalyst material 7. Typically, the membrane substrate 11A, in known embodiments, is coated as a catalyst-coated membrane, or CCM for short, with a layer of the respective catalyst material 7 on both the anode and cathode sides, on two opposing, separately facing surfaces. In principle, it is also possible, in a membrane-electrode arrangement 23, to introduce a gas diffusion electrode 35 into the anodic half-cell 25B, to design it accordingly, and to place it on the membrane 11.

[0097] The respective cell reaction of electrolysis takes place in the area of ​​the functional layer formed by the respective catalyst material 7. During normal operation, electrons are conducted via the respective catalyst material 7 and a support or channel structure, which may be formed by or provide a gas diffusion layer 27, to the contact or bipolar plates 29 (compare the electrolysis cell 25 in the illustration on the left). In the present case, however, at least in half-cell 25A, the cathode electrode is designed as a gas diffusion electrode 23.

[0098] It is further evident that fully demineralized reactant water H2O is typically supplied on the anode side and fed to the PEM electrolysis cell 25. The reactant water H2O is decomposed into oxygen O2 and hydrogen H2 via the electrolysis process. Oxygen O2 is formed as the electrolysis product at the anode and hydrogen H2 at the cathode, and these are separately extracted and removed from the PEM cell.

[0099] Electrolysis cell 25 is routed out.

[0100] Membrane 11 is designed as a polymer membrane and comprises a membrane substrate made of a polymer material 11A. In the production of a catalyst-coated membrane 11 for the anodic half-cell 25B, the membrane 11 is directly coated with a pasty, highly viscous or pasty mass containing the catalyst material as a coating material.

[0101] Therefore, the membrane 11 is formed from a fluorine-free polymer material 11A and is thus designed as a fluorine-free membrane 11. The membrane material 11A contains a sulfonated hydrocarbon polymer, a so-called hydrocarbon, as an ionomer for proton conduction. Materials from the sulfonated polyaromatics are selected as the fluorine-free polymer material 11A for the membrane substrate. For example, as illustrated by way of example in FIG. 1, so-called sPEEK or sPPX can be used for the membrane substrate 11, or the membrane substrate 11A can be formed from them. The largest group are the poly(arylene ether)-based hydrocarbons, which include sulfonated polyetheretherketones, sPEEK. Another group consists of sulfonated polyphenylenes (sPPX), which exhibit similar properties. Their major advantage is their high stability against chemical degradation.Ionic ionomers are polymers with ionic groups that give them the characteristic property of proton conductivity. In the case of PEM water electrolysis, the ionic groups are sulfonic acid groups (-SO3H). These can be randomly distributed or fixed in place. In PEM water electrolysis, ionomers are used primarily because of their proton conductivity and simultaneous electrical insulating properties as membrane material in the membrane substrate 11 and in the electrode mounted on the membrane substrate 11, i.e., in the layer containing the catalyst material 7. Thus, a material adaptation has already been specifically implemented here with regard to a fluorine-free membrane 11 in the membrane-electrode assembly 23.

[0102] Of particular importance for the proposed electrochemical cell 25 is the fact that, at least in the cathodic half-cell 25A, the cathode electrode is designed as a fluorine-free gas diffusion electrode 35, which is in contact with the membrane 11. Such a fluorine-free gas diffusion electrode 35 requires carefully defined preparation, material selection, and design in a multi-step manufacturing process, which is described in more detail below with reference to FIG. 2. Previous structures are based on polytetrafluoroethylene (PTFE)-bonded fibers or are subsequently hydrophobized using PTFE. The production of fluorine-free variants is not yet known. A gas diffusion electrode 35 is a key component in various types of fuel cells and electrolyzers.The gas diffusion electrode 35 must, in addition to electrical contact, handle the transport of media, namely water (H₂O) and hydrogen (H₂) as the product gas of the cathodic half-cell 25A, as well as heat transport. Besides its chemical and physical properties, the gas diffusion electrode 35 must also withstand mechanical forces. The present gas diffusion electrode 35 is designed to enable the formation of a solid, liquid, and gaseous interface during cell operation, thus facilitating a catalytically supported cell reaction in a three-phase system.

[0103] The provision of the coating material is explained in more detail with reference to FIG. 2. FIG. 2 shows, by means of a schematic flowchart, process steps for the production of a gas diffusion electrode 35 for an electrochemical cell, which are significant for the preparation of a fluorine-free gas diffusion electrode 35. FIG. 2 merely indicates, by way of example, process steps according to the invention with reference to the flowchart shown, in order to illustrate in particular the paste formulation for the catalyst paste 9. The process according to the invention is thus designed and set up as a process for the production of a gas diffusion electrode 35 for an electrochemical cell 25, in this case an electrolysis cell for water electrolysis.

[0104] The process begins in step S1 by providing an organic binder polymer 1 as a starting material in solid form; that is, this starting material is preferably not in solution, but in the form of a fine-grained powder. A powdered sulfonated polymer 1 from the group of polysulfones is selected as the binder polymer 1, in this case, for example, a polyphenylenesulfone.

[0105] (PPSU), polysulfone (PSU), polyphenylene sulfide (PPS), or fine-granular mixtures thereof. Sulfonic acid R-HSO3 is added to this, so that sulfonic acid R-HSO3 is introduced as a side group into the binder polymer 1, thereby functionalizing the binder polymer 1. A polymer from the group consisting of polyacrylonitrile, polyethylene oxide, polymethyl methacrylate, or a copolymer thereof can also be selected as the binder polymer 1.

[0106] For the provision of the powdered fluorine-free binder polymer 1 in step S1, commercially available ready-made dispersions, which already contain the respective fluorine-free polymer to be used, can be employed. These can first be converted into the desired powder by a spray-drying step to obtain or separate the powdered fluorine-free binder polymer 1 from the dispersion. Such spray or atomization drying is a method from process engineering for drying solutions or, for example, suspensions. In this process, the material to be dried is introduced into or atomized in a hot gas stream, which dries it into a fine powder in a short time.

[0107] The drying process can be carried out, for example, with the following parameters or specifications: injection temperature of 120 °C, injection pressure of 10 bar, nozzle dimension of 0.5 mm, jacket or wall temperature of 250 °C, a gas temperature of 380 °C, and / or a gas flow rate of approximately 50 l / min. Alternatively, vacuum spray drying with a single- or dual-fluid nozzle can be carried out, for example, under the following parameters: a pressure of, for example, 100 mbar, a nozzle orifice of approximately 0.3 mm, an injection temperature of 130 °C, a jacket temperature of 200 °C, and a nozzle pressure of, for example, 10 bar.

[0108] The process further comprises, in step S2, the dispersion, introduction, or processing of the fluorine-free organic binder polymer 1 in a solvent 3, so that a plastisol 5 is produced. The term "plastisol" is here generally intended to denote a dispersion or a heterogeneous mixture.

[0109] In the dispersion preparation, one of the following solvents is used: N-butyl-2-pyrrolidone, 2-pyrrolidone, N-octyl-2-pyrrolidone, N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide (DMF), formamide, dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAC), acetonitrile, or a mixture thereof. High-boiling, anhydrous solvents are selected. The ionomer content in the dispersed stock solution is typically adjusted to a range of 5–20 wt%, depending on the solvent chosen, for example, 7–18 wt% for DMF, 7–16 wt% for DMSO, or 7–20% for NMP. The viscosity of the dispersion is adjusted between 100 and 500 mPa·s. These specified ranges for the ionomer fractions and viscosities of the dispersion are generally preferred windows for the formulation and can be adapted to the specific requirement.Mixed solutions of two of the aforementioned solvents 3 have proven effective, for example, binary mixtures of N-methyl-2-pyrrolidone and a selected proportion of N-ethyl-2-pyrrolidone. Depending on the application and material composition of the catalyst paste 9 for preparation, viscosities of up to 500 mPa·s, for example up to 1000 mPa·s, are possible for the respective gas diffusion electrode 25. It has proven particularly advantageous to add a crosslinking agent 41 to the organic binder polymer 1 when preparing the dispersion in process step S2 in order to prepare the plastisol 5. Triallyl cyanurate (TAIC) has proven suitable as a crosslinking agent 41, thus ensuring good crosslinking of the organic binder polymer 1.

[0110] The process further comprises, in process step S3, the addition of a catalyst material 7 to the dispersion and the intimate and homogeneous mixing of the catalyst material 7 with the plastisol 5. For the preparation of a cathodic gas diffusion electrode 35, a catalyst material 7 supporting the cathode reaction is introduced. Platinum (Pt), ruthenium (Ru), or a binary alloy of these metals is added as a fine-granular powder as the cathodic catalyst material 7. During the preparation of the cathode catalyst, carbon (C) is additionally used as a particulate support material 19. This results in the formation of catalytic activity such that particles of the catalyst material 7 are deposited on the high-surface-area carbon (C) support material 19. This configuration of the gas diffusion electrode 25 is described in more detail below in FIG. 3.In the case of the preparation of an anodic gas diffusion electrode 35, iridium Ir, ruthenium Ru, titanium Ti or an alloy and / or an oxide or a bimetallic mixed oxide of these metals is optionally added to the paste mixture as catalyst material 7.

[0111] In addition to the catalyst material 7, a pore-forming material 37, i.e., a pore former, is added. The pore-forming material 37 is selected from sodium chloride NaCl, ammonium carbonate (NH4)2CO3, ammonium hydrogen carbonate NH4HCO3, ammonium chloride NH4CI, potassium carbonate K2CO3, or a polymer pore former and / or a metallic pore former, in particular zinc oxide ZnO or polyvinylpyrrolidone (PVP).

[0112] In process step S4, the mixture obtained from process step S3 is intimately blended to form a catalyst paste 9 using a stirring tool. The paste is produced with a high-speed mixing tool, preferably a vacuum disperser, forced mixer, or rotor-stator system. Alternatively, a planetary ball mill or planetary mixer can also be used. The paste is degassed before application. This process step S4 thus yields a pasty, generally viscous or highly viscous coating material that forms the catalyst paste 9 for the gas diffusion electrode 35. During the blending of the mass to form a catalyst paste 9, the mass is heated to a temperature of 20°C to 80°C, particularly 40°C to 60°C. The catalyst paste 9 is provided at a viscosity of 500 - 2000 Pa -s, preferably 1000 Pa -s, at a temperature of 25°C and is applied to an electrode substrate 39 in process step S5.It is possible to add a small amount of flow improver or stabilizers (surfactants), approximately 0.1–0.5 wt% based on the catalyst weight, to the catalyst paste 9. The catalyst paste 9 obtained from process step S4 has a high solids content of catalyst material 7 and binder polymer 1 of 35 wt% to 60 wt%, particularly 45 wt% to 55 wt%, in the paste mixture. A flexible, bendable, elastic fabric or nonwoven is used as the electrode substrate 39, onto which the catalyst paste 9 is applied. The electrode substrate 39 can be a carbon fabric, a carbon fiber nonwoven, carbon paper, a metal nonwoven, a metal wire mesh, a metal wire braid, or a plastic fabric. For example, when using a tissue that is inserted into the gas diffusion electrode 25, a tissue made of metal wires can be used.Square meshes with mesh sizes of 0.025 mm to 0.10 mm and wire thicknesses ranging from 0.16 mm to 0.41 mm are preferred. This allows for a relative open area of ​​the metal wire mesh of 25% to 41%, which has proven advantageous. The application of the catalyst paste 9 to the electrode substrate 39 in process step S5 is carried out using a roll-to-roll application process. In particular, an oven can be used for heat treatment during the paste application.

[0113] Finally, in process step S6, the solvent 3 is removed from the prepared gas fusion electrode 35 by extraction, drying, or a combination thereof. If an oven is used, the solvent 3 can be thermally driven off either concurrently with or in combination with process step S5. The pore-forming material 37 is also removed or driven out of the structure, resulting in the desired high porosity for the gas fusion electrode. This can be done simultaneously with, or in a single process step with, the removal of the solvent 3. The fabrication of the gas fusion electrode 35 structure is based on a phase inversion process from the homogeneous casting solution of the catalyst paste 9. The liquid-liquid extraction leads to a separation into a polymer-rich phase and a polymer-poor phase within the structure.

[0114] The pores are formed by nucleation and growth of the solid phase. A sponge-like structure is observed for delayed demixing and at slow extraction rates. A finger-like structure is observed for rapid demixing. During the extraction of solvent 3, a high porosity is achieved for the gas diffusion electrode 35. The mean pore diameter is 20 pm to 40 pm, typically 35 pm. The porosity of the gas diffusion electrode 35 produced in this way is very high, ranging from 75% to 95%. A porosity of approximately 80% is typically targeted. The thickness D of the gas diffusion electrode 35 prepared in this way is 90 pm to 500 pm. A thickness D of 250 pm to 300 pm is preferred. Furthermore, a gas diffusion electrode 35 produced in this manner is characterized by a high polymer content of 5 wt% to 20 wt% .A polymer content of 9 wt% to 13 wt% is typically used, resulting in a hydrophobic effect. This facilitates the transport of the fluids involved.

[0115] Figure 3 shows a section of the basic structure of a membrane electrode assembly 23 with a cathodic gas diffusion electrode 35. The gas diffusion electrode 35 is prepared according to the preparation method described above and is designed as a fluorine-free gas diffusion electrode 35. The gas diffusion electrode 35 is placed in a cathodic half-cell 25A and rests on the membrane 11, which consists of a fluorine-free polymer 11A, such as a fluorine-free sulfonated polymer 11A. The sulfonated fluorine-free polymer 11A is provided by a fluorine-free hydrocarbon compound, in particular an aromatic hydrocarbon compound having a cation-conducting group. The sulfonated fluorine-free polymer 11A can be a polyaromatic ionomer having a sulfonated side group as its cation-conducting group.The polycyclic ionomer is selected from the group of sulfonated polyetherketones (sPEEK), sulfonated polyphenylenes (sPPX), and sulfonated polyimides (sPI). The gas diffusion electrode 35 rests on the membrane 11 constructed in this manner. The gas diffusion electrode 35 has a highly porous structure with a multitude of pores 13. The porosity of the gas diffusion electrode 35 increases with distance from the surface of the membrane 11. The gas diffusion electrode 35 has a total thickness D of approximately 100 pm to 350 pm. A fiber material, such as a carbon fabric or carbon fiber fleece, is used as the electrode substrate 39. The cathode electrode has a cathodic catalyst material 7, which is applied or deposited on a highly surface-level carbon C as a particulate support material 19 or substrate.The catalyst material 7 comprises, for example, platinum (Pt) or ruthenium (Ru), which adheres uniformly to the surface of a carbon (C) support particle in the form of a plurality of catalyst particles, thus forming a cluster. The catalytically active zone or layer with catalyst material 7 facing the membrane 11 of the cathodic gas diffusion electrode 35 shown here has a thickness D. E from 5 pm to 15 pm. The pore size of a pore 13 is 20 pm to 40 pm. On the side of the membrane 11 opposite the gas diffusion electrode 35, the anode is arranged and designed as a catalyst-coated membrane 11 (CCM), i.e., in the anodic half-cell 25B, the surface of the membrane 11 is coated with catalyst material 7 and designed as a catalytically active thin electrode layer and applied to the membrane 11. On the anode side, iridium is preferably considered as the catalyst material 7.

[0116] Figure 3A shows a close-up of Figure 3, revealing a number of catalyst particles deposited on carbon (C) as a support material 19, forming a cluster. The approximately spherical structure of the carbon particle as support is visible, with a plurality of catalyst material 7 particles uniformly adhering to its surface. The polymeric binder material 1, present as an ionomer, is also visible, partially enclosing the carbon (C) cluster with the adhering catalyst material 7 particles. Figure 3B, a further close-up of Figure 3A, schematically depicts a single particle of cathodic catalyst material 7, e.g., platinum, with characteristic functional elements of the gas diffusion electrode 35. High-surface carbon f C as carrier material 19 and the binder polymer 1 or ionomer 1 are shown.Furthermore, a pore 13 is visible for fluid transport.

[0117] With the gas diffusion electrode 35 produced in this manner, a structure with clusters of high-surface-area carbon material C with catalyst material 7 adsorbed on the surface of the carbon material is formed. At the same time, directed percolation is promoted and can be specifically adjusted by the carbon C support, i.e., the formation of conductive paths or fibers in the gas diffusion electrode 35 through the structure in the current direction. Furthermore, by choosing the binder polymer 1 in the form of an ionomer structure embedded in the gas diffusion electrode 35, good hydrophobicity and high porosity for fluid transport are achieved.In particular, the gas diffusion electrode 35 of the present technique is not limited by the dimensions of the preformed conductive solid support and is physically stable, particularly durable, economical, and functional despite the absence of the preformed conductive solid support. The self-supporting, electrically conductive network formed in the layer of the gas diffusion electrode 35 of the present preparation technique confers physical integrity, mechanical strength, electrical conductivity, and good catalytic properties on the gas diffusion electrode 35. Furthermore, a fluorine-free alternative for the fabrication of a functional gas diffusion electrode 35 is enabled for the first time.

[0118] To further illustrate the differences between the gas diffusion electrode 35 (GDE approach) and a catalyst-coated membrane 11 (CCM approach), FIGS. 4, 4A, 4B show, in comparison to a GDE-based membrane electrode arrangement 35 described in FIGS. 3, 3A, 3B, a section of a catalyst-coated membrane 11 (CCM) of a cathodic half-cell 25A, into which a gas diffusion layer 19 is additionally introduced.

[0119] As can be seen from FIG. 4, the electrode microstructure comprises a three-dimensional network of ionomer 1, catalyst material 7, and pores 15, 17. Furthermore, it can be seen that primary pores 15 and secondary pores 17 are formed in the electrode microstructure. The thickness D E The thickness of the electrode layer is approximately 5 pm to 15 pm. A gas diffusion layer 27, a separate functional element or component of the half-cell 25A, lies on the electrode layer; its thickness D LThe gas diffusion layer 27 is realized as a fibrous porous structure, e.g., using metal wires or metal grids. The intersection of the three components is called the triple phase boundary (TPB) and is shown in FIG. 4B. There, in a further enlarged detail from FIG. 4A, a platinum particle is shown as the cathodic catalyst material 7 with characteristic functional elements of the electrode. The electrochemical reaction takes place here, since, in addition to the availability of the catalyst material 7, the provision of the reactants is also crucial for the reaction. The platinum catalyst Pt can be supported on carbon C, as shown here. The carbon C forms agglomerates, which are partially enclosed by the ionomer 1.FIG. 4A shows a close-up of FIG. 4 of a number of particles of catalyst material 7 deposited on a carbon substrate C as a support material 19. A further distinction can be made between primary pores 15 and secondary pores 17. Primary pores 15 are located within an agglomerate, and secondary pores 17 are located between agglomerates. They also differ in that the ionomer 1 is mainly present in the secondary pores 17, and thus an increased ionic resistance exists in the primary pores 15.

[0120] Figures 5, 6, and 7 below illustrate the results of characterizing a gas diffusion electrode 35 produced according to the process. Standardized measurement methods were used for this characterization. The basis for the analysis was samples of the gas diffusion electrode 35 prepared according to the process, which were introduced, for example, into an electrolysis cell 25 as a measuring cell or into a specific measuring device. The gas diffusion electrode 35 typically exhibits the following specific characteristics:

[0121] Mean pore diameter range: 2 Opm-4 Opum, typically 35pm; Total porosity: 75-95%, preferably 80%; Polymer content: 5-20 wt%, preferably 9-13 wt%; Binder polymer preferably PPSU or SPPSU;

[0122] Catalyst cathode: Pt@C, Ru@C, PtRu@C, Pt, Ru; preferably Pt@C;

[0123] Catalyst anode: Ir, IrO, IrOx; preferably Ir-black. Specific water permeability (according to Darcy). Electrical resistance 4-wire: 0.050 hm - 50 ohms, preferably 0.01-3 ohms.

[0124] Electrode thickness: 90–500 µm; preferably 250–300 µm. Reinforcement metal mesh cathode: Preferably mesh size 0.09 mm, wire thickness 0.05 mm.

[0125] Reinforcement Metal Mesh Anode: Titanium Grade 1, Grade 2. Figure 5 shows a flow measurement—a porometer measurement with an average pore diameter of 21 pm—through such a gas diffusion electrode 35. Porometry is used to determine the pore size distribution of a continuous pore system. Before the measurement, the sample is immersed in a wetting agent so that all existing pores are filled with this liquid. The wetting agent is then forced out through the gas. The pore size distribution can be determined using the required gas pressure. In the measurement diagram shown in Figure 5, the flow rate of a test liquid, for example, water (H₂O), in 1 / min is plotted against an increasing measurement pressure of a test gas for a fully saturated gas diffusion electrode 35 (“wet curve”). Here, the porous structure of the gas diffusion electrode 35 is completely saturated and filled with water.Initially, at low pressures up to 0.035 bar, the flow rate is practically zero or very low. However, at a specific minimum pressure of approximately 0.03 bar, the flow rate of the sample increases very steeply and in steps up to a certain flow rate. At this point, a breakthrough occurs; that is, the flow resistance of the porous structure is overcome, and the gas diffusion electrode 35 switches on, forcing the liquid out.

[0126] The subsequent pressure-dependent increase in flow rate is approximately linear, but a slightly flattened, monotonically rising curve. In addition to the measurement curve ("Wet Curve"), interpolations were performed for a dry gas diffusion electrode 35, i.e., one not saturated with water. For illustration and better comparison, a first interpolation ("Interpolated Dry Curve") and a second interpolation ("Half Interpolated Dry Curve") are presented side by side. Here, a test gas is passed through the dry, porous structure of the gas diffusion electrode 35 under an increasing test pressure.

[0127] Figure 6 shows a further characterization of the flow properties of a gas diffusion electrode 35 prepared according to the process. It shows the pore distribution of the gas diffusion electrode 23. Here, the contributions in percent of a specific pore size or a specific pore diameter to a total flow rate achieved at a test pressure are determined and plotted. A differentiated picture emerges, namely that a specific (singular) pore size of approximately 21 pm contributes a significant proportion, more than 70%, of the total flow rate. The contributions of many other pore sizes are each considerably smaller, at less than 5% each, and total only about 30% of the total flow rate.

[0128] Figure 7 shows a current-voltage characteristic curve of a gas fusion electrode 35 prepared according to the process described above. Electrochemical parameters of a gas fusion electrode 35 produced according to the aforementioned process are shown. The gas fusion electrode 23 was tested in combination with a half-coated membrane 11. This is compared to a membrane coated on both sides with a catalyst ("full-MEA"). The GDE sample achieved significantly better efficiency values ​​in the test run. A characteristic curve for a gas fusion electrode 35 ("GDE") as a test sample is shown, and in comparison, the characteristic curve of a corresponding membrane electrode arrangement 23 ("MEA") in an electrolysis measuring cell, into which a gas fusion electrode 35 prepared according to the manufacturing process is inserted, is shown.It is evident that the characteristic curve for the membrane electrode arrangement 23 ("full-MEA") requires significantly higher cell voltages at the same current density. On the other hand, the two characteristic curves are very similar, and the electrical properties and contributions for the membrane electrode arrangement 23 are largely determined by the gas diffusion electrode 23, which, however, is considerably more efficient and requires lower cell voltages. The samples used for the measurement have an active area of ​​25 cm². 2 The characteristic curve was measured at a sample temperature of 60 °C. Significantly lower voltages are observed at current densities in the ohmic region of the characteristic curve, at current densities above approximately 0.2 A / cm². 2 , which were achieved with the new GDE approach proposed here.

[0129] FIG. 8 illustrates an exemplary embodiment of a fluorine-free gas diffusion electrode 35 prepared according to the manufacturing process. It is a photographic top view of a sample with an active area of ​​25 cm². 2The gas diffusion electrode has a square geometry. It features an electrode substrate 39 made of a support material 19, on which a cathode layer containing a catalyst material 7 is applied. The support material 19 is carbon paper (carbon fleece). The cathode layer contains the catalyst material and the fluorine-free ionomer 1. The image shown in FIG. 8 was taken after drying following the curing step of the structure. FIG. 8A and FIG. 8B show corresponding light micrographs of the gas diffusion electrode 35 from the interface, in a sectional and side view of the structure, respectively. In FIG. 8A, an uncoated area 21A and a coated area 21B of the gas diffusion electrode 35, which appears dark in the image and adjoins the uncoated area 21A below it, can be seen in the section.Platinum (Pt) is incorporated as catalyst material 7 into the coated area 21B, along with a fluorine-free ionomer 1. The support material 19 consists of fibrous structures of carbon fleece, which, like a woven or knitted fabric, permeate both areas 21A and 21B and simultaneously provide mechanical stability to the gas diffusion electrode 35. FIG. 8B shows a further magnified view of the coated area 21B. Numerous pores 13, formed by the network of the support material 19, are visible. Furthermore, very fine particles of the dark-appearing catalyst material 7 – in this case, platinum (Pt) – are discernible, as well as somewhat lighter areas containing the fluorine-free ionomer 1.

[0130] FIG 9 shows a scanning electron microscope image of a section of a gas diffusion electrode 35. Here, Pt@C is used as the cathodic catalyst material 7, i.e., platinum Pt supported by a high-surface-area carbon C. Polyphenylenesulfone (PPSU) is incorporated into the gas diffusion electrode 35 as an organic, fluorine-free binder polymer 1. The extraction pattern and the resulting exposed porous structure with a multitude of pores 13 are visible. Further properties include a layer thickness of 250 pm and a resistance of 0.05 ohms. A polymer content of 9 wt% and a high porosity of 85% are also specified. The mean pore size is 21 pm.

[0131] Fluorine-free membrane electrode assemblies 35 offer a number of advantages compared to fluorine-containing ionomers. These include, firstly, technical advantages such as higher operating temperatures due to the superior thermal stability of hydrocarbons for PEM water electrolysis. Furthermore, an alternative technology is proposed should an EU ban on PFAS materials be imposed, which is widely expected. Thus, the method of the invention and the GDEs produced according to the method ensure that no environmentally harmful fluorine compounds need to be processed, which could otherwise be released into the environment along the process chain. Finally, a downstream recycling process is also significantly simplified, as the formation of hydrogen fluoride during thermal reprocessing in a combustion process of a GDE is not a concern.This eliminates the previously necessary and very complex gas scrubbing of the combustion products. Furthermore, fewer corrosion problems induced by fluorine ions are expected during the operation of an electrolysis plant, which can lead to detrimental degradation.

[0132] The formulation of the catalyst paste 9 allows for particularly precise adjustment of the ionomer / catalyst ratio and adaptation to the respective solvent 3. Furthermore, known or standardized manufacturing dimensions can be easily scaled up, for example, to significantly increase the production capacities of high-purity hydrogen. A fluorine-free gas diffusion electrode 35 produced according to this process is therefore particularly interesting for future applications in electrochemical cells 25, such as a fuel cell or an electrolysis cell 25. It is therefore possible for such a gas diffusion electrode 35 to be used in a cathodic half-cell 25A or in an anodic half-cell 25B. When used in an electrolysis cell 25, a PEM electrolysis cell based on a fluorine-free gas diffusion electrode 35 can be implemented.A cell stack or stack with a large number of serially stacked electrolysis cells 25 is possible, as is their use in an electrolysis plant that is composed of or has a large number of cell stacks or stacks.

Claims

Patent claims 1. Method for producing a gas diffusion electrode (35) for an electrochemical cell (25) comprising the steps: - Sl: Providing a powdered organic binder polymer ( 1 ) , - S2 : Dispersing the organic binder polymer (1) in a solvent (3) to form a plastisol (5) , - S3: Addition of a catalyst material (7) and a pore-forming material (37) to the plastisol (5) , - S4: Mixing the mass to form a catalyst paste (9) , - S5: Applying the catalyst paste (9) to an electrode substrate (39) , and - S6: Removal of the solvent (3) .

2. The method of claim 1, wherein sulfonic acid R-HSO3 is added, such that sulfonic acid R-HSO3 is introduced as a side group into the binder polymer (1) and the binder polymer (1) is thereby functionalized.

3. A method according to claim 1 or 2, wherein the binder polymer (1) is selected from the group of polysulfones which includes at least polyphenylene sulfone (PPSU), polysulfone (PSU) and polyphenylene sulfide (PPS) or mixtures thereof.

4. A method according to one of claims 1, 2 or 3, wherein the binder polymer (1) is selected from the group consisting of polyacrylonitrile, polyethylene oxide, polymethyl methacrylate or a copolymer thereof.

5. A method according to any of the preceding claims, wherein a crosslinking agent (YY) is added to the binder polymer (1), in particular triallyl cyanurate (TAIC) , such that crosslinking of the binder polymer (1) is effected.

6. Method according to any one of the preceding claims, wherein the solvent (3) is N-butyl-2-pyrrolidone, 2-pyrrolidone, N- Octyl-2-pyrrolidone, N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide (DMF), formamide, dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAC), acetonitrile or a mixture thereof is used.

7. Method according to any of the preceding claims wherein platinum (Pt), ruthenium (Ru) or a binary alloy of these metals is added as catalyst material (7).

8. Method according to claim 7, wherein carbon (C) is added as a particulate support material (19) such that the formation of a catalytic activity is effected by particles of the catalyst material (7) being deposited on the high-surface carbon (C) as support material (19).

9. Method according to any one of claims 1 to 6, wherein iridium (Ir) , ruthenium (Ru) , titanium (Ti) or an alloy and / or an oxide or a bimetallic mixed oxide of these metals is added as catalyst material (7).

10. Method according to one of the preceding claims, wherein a pore-forming agent from a pore-forming material (37) is added to the plastisol (5), wherein the pore-forming material (37) is selected from sodium chloride NaCl, ammonium carbonate (NH4)2CO3, ammonium hydrogen carbonate NH4HCO3, ammonium chloride NH4CI, potassium carbonate K2CO3 or a polymer pore-forming agent and / or a metallic pore-forming agent, in particular zinc oxide ZnO or polyvinylpyrrolidone (PVP) .

11. Method according to one of the preceding claims, wherein when mixing the mass to form a catalyst paste (9) the mass is heated to a temperature of 20°C to 80°C, in particular of 40°C to 60°C.

12. Method according to one of the preceding claims, wherein the catalyst paste (9) is provided with a viscosity of 500 - 2000 Pa -s at a temperature of 25°C and applied to the electrode substrate (39).

13. Method according to one of the preceding claims, wherein the catalyst paste (9) is intimately mixed to form a paste mixture, wherein a high solids content of catalyst material (7) and binder polymer (1) of 35 wt% to 60 wt%, in particular of 45 wt% to 55 wt%, is established in the paste mixture.

14. Method according to one of the preceding claims, wherein a flexible, bendable fabric or nonwoven is used as the electrode substrate (39), onto which the catalyst paste (9) is applied.

15. Method according to claim 14, wherein the electrode substrate (39) is a carbon fabric, a carbon fiber fleece, carbon paper, a metal fleece, a metal wire fabric, a metal wire mesh or a plastic fabric.

16. A method according to any of the preceding claims, wherein the solvent (3) is removed by extraction or drying or combinations thereof.

17. Method according to any of the preceding claims, wherein the pore-forming material (37) is removed, in particular simultaneously with the removal of the solvent (3), whereby a high porosity is achieved.

18. Method according to claim 16, wherein a mean pore diameter of 20pm to 40pm, in particular 35pm and a porosity of 75% to 95%, in particular 80%, is set.

19. Method according to one of the preceding claims, wherein the application of the catalyst paste (9) to the electrode substrate (39) in process step (S5) is carried out by means of a roll-to-roll application process, wherein in particular an oven (15) is used for heat treatment during the paste application, wherein the solvent (3) is thermally driven off in process step (S6).

20. Method according to one of the preceding claims, wherein a thickness (D) of the gas diffusion electrode (35) is set from 90pm to 500pm, in particular from 250pm to 300pm.

21. Method according to one of the preceding claims, wherein the polymer content of 5 wt% to 20 wt%, in particular 9 wt% to 13 wt%, is adjusted in the gas diffusion electrode (35).

22. Gas diffusion electrode (35) which is manufactured according to the method of any of the preceding claims.

23. Half-cell (25A, 25B) which has a gas diffusion electrode (35) according to claim 21.

24. Electrolysis cell (25) , in particular a PEM electrolysis cell comprising a half-cell (25A, 25B) according to claim 22.

25. Cell stack comprising a plurality of electrolysis cells (25) according to claim 23.

26. Electrolysis plant, with a cell stack according to claim

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