Membrane electrode assembly, electrolysis cell, and use of a core-shell catalyst

Abstract

The invention relates to a membrane electrode assembly (1) comprising a cathode (2), an anode (3), and an ion-conductive membrane (4) arranged between the cathode (2) and the anode (3), wherein the membrane electrode assembly (1) comprises a gas-recombination catalyst (6), wherein the gas-recombination catalyst (6) is a core-shell catalyst, wherein a shell (8) of the core-shell catalyst surrounds at least one core (10), wherein the at least one core (10) is formed from at least one catalytically active component (11) that comprises at least one catalytically active substance (7), and wherein the shell (8) is formed from a material having high gas permeability and low proton conductivity.

Description

[0001] HOEFER & PARTNER

[0002] GNT260301PCT 03 / 10 / 2026 Applicant:

[0003] Greenerity GmbH

[0004] Industrial Area South E11

[0005] 63755 Alzenau

[0006] Membrane electrode arrangement, electrolysis cell and use of a core-shell catalyst

[0007] Description

[0008] The invention relates to a membrane electrode arrangement and its use, as well as an electrolysis cell with increased lifetime and high performance over its entire lifetime, and the use of a core-shell catalyst.

[0009] Membrane electrode arrangements, including catalyst-coated membranes with an ion exchange membrane coated on one side with an anode and on the opposite side with a cathode, are known in the art under the term COM (catalyst-coated membrane). When a COM is used in water electrolysis and a proton-conducting membrane is employed, the term PEM-WE (proton-exchange membrane water electrolysis) is also commonly used. In the case of anion-conducting membranes, the term AEM-WE is commonly used.

[0010] Proton exchange membranes for use in PEM-WE are typically extruded perfluorosulfonic acid (PFSA) polymer membranes. The most established examples of PEM-WE are Nation® N115 and Nation® N117 from Chemours. Current literature also utilizes thinner, cast membranes, i.e., solvent-printed membranes such as Nation® NR212. Furthermore, hydrocarbon-based ionomers can also be used for sustainability reasons. Compared to perfluorosulfonic acid membranes (PFSA membranes), hydrocarbon membranes offer several advantages. For example, they exhibit lower gas permeability through the membrane, allowing for higher cell current yields, even when using very thin membranes.which result in very low ion resistance (and thus very good performance). Furthermore, they can be operated for extended periods at high temperatures, e.g., >100 °C, with limited degradation, due to their low gas permeability (even at high temperatures) and the high glass transition temperature typical of hydrocarbon-based polymers. The hydrocarbon membrane itself is not limited. Particularly stable PEM carbon membranes are selected from sulfonated polyaryl ethers (SPAE), sulfonated polyaryl ether nitriles (SPAEEN), sulfonated polyaryl ether ketones (SPAEK), sulfonated polyaryl ether nitriles (SPAEN), sulfonated polyaryl ether sulfones (SPAES), sulfonated polyaryl ether sulfone ketones (SPAESK), sulfonated polyether ether ketones (SPEEK), sulfonated polyether ketones (SPEK), sulfonated polyether sulfones (SPES), sulfonated polyimides, sulfonated polyketone ketones (SPKK), and sulfonated polyphosphazenes (SPPh).sulfonated polyphenylenesulfones (SPPSf), sulfonated polyphenylene sulfide sulfones (SPPSSf), sulfonated polyphenylene sulfide sulfonenitriles (SPPSSfN), sulfonated polystyrenes (SPS), sulfonated polysulfones (SPSf), sulfonated polyphenylenes (sPP), sulfonated phenylated polyphenylenes (sPPP) and mixtures thereof, and are in particular selected from sulfonated polyetherketones, sulfonated polyetheretherketones, sulfonated polyketoneketones, sulfonated polyphenylenes, sulfonated phenylated polyphenylenes and mixtures thereof.

[0011] Furthermore, membranes can incorporate one or more reinforcing structures, such as expanded PTFE (ePTFE) or woven or non-woven polymer structures. For example, a reinforcing structure can be introduced during the manufacturing process of the ionomer membrane from an ionomer dispersion or solution. In this case, a pre-formed reinforcing structure, such as ceramic materials or polymeric materials like (bi-)axially stretched PTFE (ePTFE, also known as expanded PTFE), or woven structures like fabrics made of polyketone (PK) fibers, polyetherketone (PEK) fibers, polyetheretherketone (PEEK) fibers, perfluoroalkoxyalkane (PFA) fibers, or polyphenylene sulfide (PPS) fibers, is impregnated with a suitable ionomer dispersion and then dried so that the pores of the reinforcing structure are filled with ionomer.

[0012] Well-known examples of AEM membranes include Fumasep® FAA-3, Tokuyama A201 and A901, Sustainion® X37-50, PiperlON®, Aemion™, and Orion TM1. Membranes of this polymer class can also feature reinforcements or reinforcing layers.

[0013] In the anode electrode layer (anode for short), a catalyst is used for the oxidation of water (water splitting). This catalyst is often referred to as an OER catalyst (oxygen evolution reaction). OER catalysts are usually based on noble metals and comprise noble metal oxides that exhibit high catalytic activity for water splitting. These catalysts are typically based on iridium and / or ruthenium. Furthermore, an ion-conducting polymer, a so-called proton-conducting ionomer of the PFSA type in the case of PEM-WE or an anion-conducting ionomer in the case of AEM-WE, is usually used as a binder in which the OER catalyst is dispersed. In AEM-WE, non-noble metal-based catalysts can also be used, including, for example, Co3O4 and Cu. x Co 3.x O4, or Ni x Fe 3.xO4 (including Ni0.75Fe2.25O4), which facilitates the oxidation of hydroxide ions to water. A catalyst for the reduction of protons to hydrogen (hydrogen evolution reaction = HER catalyst) is used in the cathode electrode layer (cathode). These catalysts are usually platinum and / or palladium-based, with the platinum and / or palladium preferably being finely dispersed on carbon powders. The cathode also typically includes a PFSA-based ionomer as a binder. In AEM-WE, non-precious metal catalysts, such as metallic nickel, possibly alloyed with other metals such as Co, Fe, Mo, Ce, Zn, and Cu, can alternatively be used to facilitate the reduction of water to hydrogen.

[0014] The membrane is coated with electrode materials using established methods as described in the prior art. Examples include direct printing of an electrode dispersion onto the membrane or a decal process, in which an electrode dispersion is first coated and dried onto a coating substrate and then hot-pressed onto the membrane. Alternatively, electrodes can be applied to the porous transport layer (PTL) and then brought into contact with the membrane to create a catalyst-coated membrane. For this, the cathode is applied to a gas diffusion layer and the anode to a porous transport layer. This can be achieved, for example, using solvent-based coating processes or chemical or physical vapor deposition.For example, a dispersion of a carbon-supported platinum catalyst, an ionomer, and solvents can be coated onto a gas diffusion layer to create a gas diffusion electrode (GDE), or an iridium compound can be deposited onto a porous transport layer made of a titanium sintered material by vapor deposition to create an anode electrode PTL. Both methods—applying the electrode to the membrane or to a porous transport layer—can also be combined. For instance, the cathode can be applied to the membrane using direct printing or decal techniques, and the anode can first be applied to the PTL and then brought into contact with the membrane.To prevent oxygen from the anode from reacting with hydrogen passing through the membrane from the cathode to form explosive mixtures (a phenomenon known as oxyhydrogen reaction), membrane electrode assemblies for water electrolysis cells typically include a gas recombination catalyst (GRC), which may consist of platinum particles. The GRC catalyzes the reaction of hydrogen passing from the cathode to the anode with oxygen from the anode side, thus preventing the formation of explosive mixtures on the anode side of the cell. The GRC is either finely dispersed and homogeneously distributed throughout the entire volume of the membrane or, embedded in an ionomer, forms a sublayer of the membrane electrode assembly. Such a gas recombination layer can be considered part of the membrane.

[0015] EP 3 559 314 A1 teaches a membrane with a laminate structure, wherein an intermediate layer comprises a recombination catalyst consisting of platinum or palladium supported on high-surface-area supports such as carbon, silica, titanium oxide, or zirconium oxide. Conventional PFSA membranes are described as the membranes.

[0016] A disadvantage of known GRC layers is that the catalyst particles, e.g., platinum particles, are electrically isolated within an ionomer matrix, causing them to degrade or be released from the gas recombination layer during repeated reduction-oxidation cycles, e.g., due to varying hydrogen and oxygen concentrations. Without being bound by theory, the inventors of the present invention postulate that the underlying mechanism for this phenomenon is the repeated dissolution and deposition of the platinum particles, leading to their migration. This migration ultimately results in the permanent loss of functionality of the gas recombination layer.

[0017] It is therefore an object of the present invention to provide a membrane electrode arrangement for an electrolysis cell, and in particular a water electrolysis cell, which includes a gas recombination catalyst and is characterized by a very long service life and very good performance, resulting in a permanently high gas purity (low hydrogen content in the anode), which in turn enables permanently reliable operation of the membrane electrode arrangement. Furthermore, it is also an object of the present invention to provide a permanently reliable electrolysis cell. In addition, it is also an object of the present invention to specify the use of a specifically designed core-shell catalyst.

[0018] These problems are solved by the features of the independent claims. The dependent claims contain advantageous further developments and embodiments of the invention.

[0019] Accordingly, the problem is solved by a membrane electrode arrangement (hereinafter referred to as MEA) comprising an anode, a cathode, and an ion-conducting membrane arranged between the anode and the cathode. In the simplest case, the MEA according to the invention is a three-layer arrangement. However, the MEA can also comprise multiple layers, i.e., multiple electrode layers and / or membrane layers, or even additional layers. The membrane can advantageously be a proton exchange membrane or an anion exchange membrane.

[0020] A key aspect of the invention is that the MEA comprises a gas recombination catalyst. The gas recombination catalyst (hereinafter referred to as GRC) is designed as a core-shell catalyst. In the core-shell catalyst, a shell surrounds at least one core. This is to be understood as meaning that a shell can also surround, i.e., enclose or enclose, several cores. The shell is arranged or formed around at least one core and preferably surrounds it completely.

[0021] The core of the core-shell catalyst provides the functionality as a catalyst and is formed from at least one catalytically active component, which comprises at least one catalytically active substance. According to the invention, a catalytically active substance is understood to be a substance that catalyzes the reaction of hydrogen passing through the membrane with oxygen to form water, thus preventing the formation of explosive mixtures. A catalytically active component comprises at least one catalytically active substance and can therefore also comprise two or more identical or different catalytically active substances. The catalytically active component can consist of one or more catalytically active substances and may also include further auxiliary materials, such as carriers for the catalytically active substance.

[0022] According to the invention, a shell of the core-shell catalyst is understood to be a layer formed from a material with high gas permeability and low ionic conductivity for protons, i.e., low proton conductivity. It surrounds at least one core, with the shell completely enclosing the core(s). The shell can be in complete direct contact with the core. Alternatively, cavities can be formed between the core and the shell. However, a characteristic feature of the invention is that the shell is closed and completely surrounds the core. The gas permeability refers in particular to the gases H₂, O₂, and water vapor.High gas permeability is important to ensure that hydrogen and oxygen are directed to the catalytically active substance as quickly and efficiently as possible and react to form water, and also to ensure that the resulting water is quickly removed so that excess product water does not impede the transport of hydrogen and oxygen to the catalytically active substance and the reaction of hydrogen and oxygen.

[0023] For reduced migration of the ions of the catalytically active substance, it is also essential that the shell exhibits low proton conductivity. Therefore, a proton-conducting ionomer, such as is typically used for the membrane of MEAs, is unsuitable as the shell material. The low proton conductivity is crucial for several reasons: firstly, it minimizes local acidity within the core-shell catalyst, so that essentially no acidic conditions prevail within the core-shell catalyst that would promote the release of the catalytically active substance(s). Secondly, the low proton conductivity also prevents the removal of cations from the catalytically active substance, which can be generated by multiple oxidation and reduction cycles.It was found that low proton conductivity correlates very well with low cation conductivity of the catalytically active substance(s). Proton conductivity is a measurable parameter and can be determined by electrochemical impedance spectroscopy; it is preferably in the range of up to 0.05 S / cm.

[0024] Due to the specific combination of a shell and at least one core surrounded by the shell, the GRC according to the invention is characterized by high efficiency in preventing explosive mixtures of hydrogen and oxygen and by exceptional longevity, without migrating from its intended location, thus increasing the lifetime of the MEA according to the invention. It was surprisingly found that the GRC exhibits high efficiency despite the shell, even though this type of catalyst does not demonstrate sufficient efficiency in applications such as fuel cell electrodes. While not bound by theory, this is presumably due to the fact that the GRC application involves much lower gas flows and therefore a smaller number of reactions taking place on the catalyst surface—on the order of 1000 times fewer reactions per unit time compared to applications in electrodes.

[0025] Due to its very high catalytic activity, the catalytically active substance is preferably selected from the group consisting of platinum, palladium, gold, silver, molybdenum disulfide, iridium, ruthenium, nickel, and alloys thereof—that is, alloys of these elements and compounds. As described above, the core of the core-shell GRC can contain a single catalytically active substance, or any combination of the aforementioned catalytically active substances can be surrounded by the shell. It is also possible for different catalytically active substances or mixtures thereof to be surrounded by separate shells. This allows for particularly precise control of the GRC's catalytic activity. Furthermore, the aforementioned catalytically active substance can be present in the form of an alloy with other elements of the periodic table.Among these alloys, alloys of platinum and a non-precious metal, as well as alloys of palladium and a non-precious metal, are particularly preferred, wherein the non-precious metal is particularly selected from cobalt and nickel. In principle, any element or combination of elements that catalyzes the reaction of hydrogen with oxygen to form water can be advantageously used as a catalytically active substance according to the present invention.

[0026] To increase the catalytically active surface area of ​​the catalytically active substance, the catalytically active component advantageously further comprises a support on which the catalytically active substance is supported. The specific support is not limited and is selected, in particular, from carbon or ceramic materials, including, in particular, silica, titanium oxide, niobium oxide, tungsten oxide, tantalum oxide, tin oxide, fluoride-doped tin oxide (FTO), and zirconium oxide. Different support materials can also be combined and mixed, especially mixtures of carbon and ceramic materials.

[0027] To further stabilize the GRC, according to an advantageous embodiment, the GRC itself can also be supported on a substrate, wherein the substrate is in particular selected from carbon or ceramic materials, including in particular silica, titanium oxide, niobium oxide, tungsten oxide, tantalum oxide, and zirconium oxide. A ceramic substrate for the GRC can also improve the bond strength, especially to hydrocarbon-based membranes.

[0028] To improve the efficiency of the GRC, the shell material advantageously has a gas permeability of more than 1*10 -17 mol x cm / (sx cm 2 x kPa), preferably of more than 1*10' 13 mol x cm / (sx cm 2 x kPa), preferably of more than 1*10 -11 mol x cm / (sx cm 2x kPa). The gas permeability corresponds to the intrinsic gas permeability of the shell material and is measured on thin films of the material. For this purpose, hydrogen or oxygen is supplied at a partial pressure of 2 bar at one side of the thin film at 25 °C and 100% RH (relative humidity). Gas diffusing through the thin film to the other side is collected with a nitrogen stream and directed to a reference cell. The reference cell comprises a catalyst-coated membrane, at which the amount of hydrogen or oxygen is determined electrochemically via the generated current (A). Hydrogen is determined by so-called "hydrogen pumping," in which hydrogen in the reference cell is first oxidized to protons. The protons migrate through the membrane of the catalyst-coated reference cell and are reduced back to hydrogen on the other side.The external current flow associated with this process corresponds to the amount of hydrogen that diffuses through the thin film of the shell material. To determine the oxygen, hydrogen is supplied to the other side of the catalyst-coated membrane of the reference cell and operated as a fuel cell. The external current flow associated with this process corresponds to the amount of oxygen that diffuses through the thin film of the shell material. To further improve the lifetime of the GRC, the shell material preferably has a proton conductivity of less than 0.05 S / cm, more preferably less than 0.01 S / cm, and more preferably less than 0.001 S / cm. To determine the proton conductivity, the shell material of the GRC according to the invention is pressed into the form of a pellet, and the surfaces are provided with electrodes.The proton conductivity of this sample is measured by impedance measurement with 20 mV oscillation at 10 Hz to 10 kHz and 80 °C under wet conditions of 100 % rF.

[0029] To measure proton conductivity, a potentiostat (e.g., from Zahner or BioLogic) is used. An alternating current (e.g., with an amplitude of 20 mV) is applied in a frequency range of 10 Hz to 10 kHz, and the AC resistance is determined. The proton resistance corresponds to the high-frequency resistance, since protons, unlike electrons, react very slowly to the frequency of the alternating current.

[0030] To further prevent the migration of ions of the catalytically active substance, the shell material advantageously has a diffusion coefficient for ions of the catalytically active substance of less than 10. -10 m 2 / s, preferably of less than 10 -11 m2 / s, preferably less than 10 -12 m 2 / s, preferably of less than 10 -13 m 2 / s. To determine the diffusion coefficient, films of the shell material in different thicknesses, e.g., 1, 3, 5, and 10 pm, are deposited onto a silicon wafer sputtered with the catalytically active substance, and the dissolution behavior is tested in a scanning flow cell (SFC) setup at 50 °C. The amount of diffused material is measured using inductively coupled plasma mass spectrometry (ICP-MS), and the diffusion coefficient is calculated from this data.

[0031] Not only because of its high permeability to oxygen, hydrogen, and water vapor, and its low proton conductivity, but also because of its good stability under operating conditions, thus enabling extremely long service life, the shell material is preferably selected from ceramic materials, carbon-containing materials, and polymers, in particular amorphous elastomeric and amorphous glassy poly- and perfluoropolymers and functionalized perfluoropolyethers, and mixtures of the aforementioned materials. Glassy polymers are understood to be non-equilibrium materials with excess free volume due to kinetic restrictions on the movement of polymer segments, which prevent such materials from fully reaching equilibrium properties (e.g., specific volume) once they fall below their glass transition temperature.

[0032] Examples of glassy polymers include the following polymer structures: FC — CF — C Fg -CFg

[0033] °\ / F 2

[0034]

[0035] < CF2

[0036] Examples of brand names are Hyflon AD from Syensqo, Teflon AF from Chemours and Cytop from AGC.

[0037] Other glassy polymers are available under the trade name CyclAFIor™ and represent a PBVE-PDD copolymer with the following structure:

[0038]

[0039] The manufacturer of these glass-like polymers is Chromis Technologies.

[0040] Furthermore, perfluoropolyethers (PFPE) can be used as a polymer for the shell. The basic structure is formed as follows:

[0041] X1(CF2) m O[CF(CF3)CF2O] n (C2F4O) p (CF2O) q (CF2CF2CF2O) r(CF2)sX2X1 and X2 are independently selected from -CF3, -CF2H, -CFH2, -CH3, -COF, -COOH, -COOR (where the residue R is an aliphatic chain), -CONH2, -CH2OH, alkylamides, ethoxylated alcohols, urethane methacrylate and ethoxysilanes.

[0042] If p, q, r = 0 and n = 0, the PFPE is called K-type PFPE or branched PFPE. If p, r = 0 and n, q = 0, the PFPE is called Y-type PFPE or branched PFPE. If n, r = 0 and p, q = 0, the PFPE is called Z-type PFPE or linear PFPE. If n, p, q = 0 and r = 0, the PFPE is called D-type PFPE or linear PFPE. The molecular weight of PFPE polymers ranges from 500 to 100,000 Daltons.

[0043] If both X1 and X2 are -CF3, the PFPE polymer is called non-functional. If only one of X1 and X2 is -CF3, the PFPE polymer is called monofunctional. If neither X1 nor X2 is -CF3, the PFPE polymer is called bifunctional. Bifunctional PFPE polymers are preferred, especially if the end groups are polar, because these exhibit stronger bonding to the metallic surface of the active GRC catalyst, e.g., platinum.

[0044] Exemplary brand names for functional PFPE polymers are Fluorolink® and certain Fomblin® products from Syensqo and Demnum from Daikin. Specific examples of suitable PFPE polymers are Fluorolink® A10-P, Fluorolink® E 10 / H, Fluorolink® P56, Fluorolink® AD 1700, Fluorolink® MD 700, Fluorolink® PEG 45, and Demnum SA.

[0045] Regarding non-functional PFPE polymers, exemplary brand names are Galden and Fomblin from Syensqo, Demnum from Daikin and Krytox from Chemours.

[0046] If non-functional PFPE polymers are used, those with a higher molecular weight and higher boiling point are preferred so that the PFPE polymers are not lost over time due to the high volatility of low molecular weight types. Preferably, the boiling point is above 300 °C and the molecular weight is more than 10,000 Daltons.

[0047] Both functional and non-functional PFPE polymers can be applied to the surface of the catalytically active component by coating. For this purpose, the PFPE polymer can be dissolved in a fluorinated solvent with a significantly lower boiling point, e.g., a boiling point of 50 °C to 100 °C. The catalytically active component is then added to the PFPE solution and well dispersed. The solvent is removed by evaporation, leaving the PFPE polymer on the surface of the catalytically active component. In this way, a GRC with a PFPE polymer shell is obtained.If the casing material is a ceramic material, for reasons of stability, particularly under acidic conditions, it is preferably selected from at least one of the following elements: silicon, tantalum, niobium, tin, titanium, zirconium, cerium, tungsten, antimony, and mixtures thereof. The ceramic material is specifically selected from silicon oxide, tantalum oxide, niobium oxide, tungsten oxide, and zirconium oxide. These five oxides have proven to be particularly stable under MEA operating conditions, while simultaneously exhibiting good compatibility with common catalytically active substances, high oxygen, hydrogen, and water vapor permeability, and low proton conductivity.

[0048] If the shell material is selected from carbon-containing materials, the production can be realized by two methods: polymer coating-based approaches and precursor ligand-induced formation, e.g., Platinum(II) bis(acetylacetonate) (Pt(acac)2), wherein the carbon-containing materials are carbonized in an inert atmosphere, e.g., argon or nitrogen, at a temperature of 500-3000 °C, preferably at a temperature of 600-1000 °C.

[0049] Preferably, the catalytically active substance of the gas recombination catalyst is present in the form of particles within the nucleus(s). The shape of the particles is not limited and can be, for example, round, polygonal, elongated, or whisker-shaped. A round particle shape is preferred for reasons of stability. The particles preferably have an average diameter of 1 to 20 nm, more preferably 1.5 to 15 nm, and most preferably 2 to 10 nm. In the case of particles exhibiting any aspect ratio or different dimensions in different directions, the diameter specified here refers to the greatest distance between two points within the particle.

[0050] GRC also preferably exists in particle form, the three-dimensional shape of which is essentially unlimited. The particle shape offers advantages in that the particles can be incorporated very well and uniformly into a layer while still possessing a large surface area, resulting in particularly high catalytic activity. Suitable particle shapes for GRC include spheres, rods, stars, fibers, cubes, and the like.

[0051] In particular, the characteristic dimensions of the particles are from 2 to 1000 nm, preferably from 3 to 500 nm, and most preferably from 5 to 250 nm. The term "characteristic dimensions" refers to the diameter or the maximum distance between two parallel tangents applied to the particle surface. It thus correlates with the Feret diameter, which is usually understood as the measurement result of a caliper (caliper principle) used to measure the distance between two parallel tangents of a particle at any angle. Due to the relatively small size of the GRC according to the invention, the characteristic dimensions are determined analogously using SEM (scanning electron microscopy) or TEM (transmission electron microscopy).

[0052] The characteristic extent of the GRC particles specified above according to this advantageous embodiment thus includes both a GRC in which a single 1 nm particle of a catalytically active substance is surrounded by a shell and a GRC in which a single approximately 20 nm particle of a catalytically active substance or several particles of any size with the same or different particle sizes are surrounded by a shell and thus form a GRC particle.

[0053] The shell of the core-shell GRC advantageously has a layer thickness in the range of 0.5 to 1000 nm, preferably 0.5 to 500 nm, more preferably 0.5 to 250 nm, even more preferably 0.5 to 20 nm, particularly preferably 1 to 10 nm, and most preferably 2 to 5 nm. The thinner the layer thickness, the better the accessibility of the catalytically active substance to H₂ and O₂, and thus the more effectively it can function as a GRC. Conversely, the dissolution stability and therefore the migration tendency of the catalytically active substance increase with the thickness of the shell. Layer thicknesses of 0.5 to 250 nm or even 1 to 10 nm are therefore particularly preferred in light of a balance between the efficiency of the gas recombination reaction and the dissolution stability of the catalytically active substance.The ideal layer thickness depends on the permeability of the shell material to hydrogen and oxygen, the diffusion coefficient of the catalytically active substance (e.g., platinum ions), and the proton conductivity of the shell material. For example, a material with higher permeability exhibits sufficiently high permeation even at greater thicknesses. The permeation is calculated as the permeability divided by the layer thickness. A material with a lower diffusion coefficient for the catalytically active substance or low proton conductivity suppresses the dissolution of the catalytically active substance even at thinner shells. A permeation greater than ICH is particularly advantageous. 1 mol / (sx cm 2 x kPa), preferably more than 5x10 -11 mol / (sx cm 2 x kPa), further preferably more than 2x10 -10 mol / (sx cm 2 x kPa).

[0054] According to a further advantageous refinement, the shell of the GRC is amorphous. This improves the diffusion of H₂ and O₂. The term "amorphous" here refers to the absence of long-range order, so that the shell material shows no reflections in the XRD, while the catalytically active substance will. If a polymer is used for the shell material, differential scanning calorimetry (DSC) is a common method for determining whether a polymer is amorphous. In this case, no melting point can be detected. In other words, a DSC of an amorphous polymer shows no endothermic peak attributable to melting.

[0055] A further advantageous development involves making the GRC essentially free of ionomers. This has the advantage of better preventing migration and thus the dissolution of the catalytically active substance.

[0056] Alternatively, the catalytically active substance of the GRC can be coated with an ionomer layer less than 4 nm thick. The ionomer layer surrounds the catalytically active substance and is then enclosed by the shell. Here, the ionomer layer acts as an adhesion promoter between the GRC and the shell. The thickness of the ionomer layer is determined at its thickest point. It is crucial that the ionomer layer around the core does not come into contact with the ionomer of the membrane or the gas recombination layer, but is completely enclosed by the shell to prevent migration and thus degradation of the catalytically active substance.

[0057] Furthermore, the gas recombination catalyst can include a radical scavenger component. Radicals can be formed by side reactions of the hydrogen-oxygen reaction, in which hydrogen peroxide (H₂O₂) is initially formed, which can be degraded to hydroxyl radicals (OH) in a subsequent reaction step. Hydroxyl radicals are highly reactive and thus lead to the degradation of even otherwise stable perfluoropolymers. For the purposes of the present invention, a radical scavenger component is understood to be any substance that neutralizes radicals, i.e., reacts them and thus renders them harmless with regard to their radical reactivity. In many cases, they can also be referred to as antioxidants. The radical scavenger component is selected in particular from

[0058] inorganic compounds including cerium, manganese, titanium, zirconium, aluminum, chromium, cobalt, nickel, tin and tungsten;

[0059] Heteropolyacids;

[0060] organic compounds including hydroxyphenyl-containing compounds, quinones, benzoquinones, hydroquinone, quercetin, caffeic acid, terephthalic acid, α-tocopherol, compounds including nitrogenous heterocycles, such as hindered amines and bipyridines;

[0061] Phosphorus-containing compounds, comprising additives as described in US 2017 / 0033387 A1, optionally in combination with nitrogenous aromatic compounds also described in US 2017 / 0033387 A1 and in US 2017 / 0125832 A1;

[0062] Compounds comprising transition metals, wherein the transition metal is selected from cobalt, nickel, ruthenium, rhodium, palladium, silver and gold, as described in US 2017 / 0037207 A1, optionally in combination with an azole-containing compound, as also described in US 2017 / 0037207 A1.

[0063] Among the inorganic compounds, those comprising cerium and manganese are preferred. These can be present as oxides or hydroxides, or as organic or inorganic salts such as acetates, oxalates, carbonates, fluorides, iodates, phosphates, nitrates, sulfides, arsenates, or selenates. Cerium oxide (CeO2) is preferably combined with Zr 4+ Cerium and manganese compounds can be doped to improve radical scavenging activity. Furthermore, CeO₂ can be nitrogen-doped by sintering in a nitrogen-rich atmosphere to achieve high radical scavenging activity. Additional metallic elements can be added to cerium and manganese compounds in variable proportions, including, in particular, Zn, Nb, Ta, Si, Ga, Sn, W, In, Zr, Ti, Mo, La, V, and Y, but other elements are also possible.

[0064] Heteropolyacids are inorganic compounds with acidic properties that include (a) a metal such as tungsten, molybdenum, or vanadium, (b) oxygen, (c) an element from the p-block (groups 13 to 17 of the periodic table), such as silicon, phosphorus, or arsenic—these are called heteroatoms—and (d) acidic hydrogen atoms. Examples of heteropolyacids proven to be suitable radical scavengers include H₆P₂W₆. 21 O 71 , H4SiW 12 O 40 , Cs x H 3.x PW 12 O 40 , H3PW 12 O 40 , H4PW 11 04O, HsSiWnO^.

[0065] Any mixture of the aforementioned substances is also conceivable.

[0066] According to the invention, the position of the radical scavenging component in the gas recombination catalyst is not restricted. For example, the radical scavenging component can be contained within the catalytically active component, particularly in the support. For instance, the pores of a porous carbon support can be impregnated with a cerium salt or an organic radical scavenging component. Alternatively, the radical scavenging component itself can form the main component or sole component of the support. For example, the catalytically active component can be formed by platinum nanoparticles supported on CeO₂ or MnCO₃. In an alternative embodiment, the radical scavenging component can be arranged between the core and the shell and can be deposited or adsorbed on the surface of the core. Furthermore, the radical scavenging component can be embedded in the shell.It is essential that the radical scavenger component is positioned in close proximity to the catalytically active substance to effectively neutralize radicals. The casing restricts the mobility of the radical scavenger component and binds it spatially, thus preventing its leaching and ensuring consistently stable performance of the membrane electrode assembly. This property is extremely important in water electrolysis, as radical scavenger components are known to be leached out in the presence of hot liquid water and rapidly lose their effectiveness over time, for example, after a few thousand operating hours (compared to a target operating life of approximately 100,000 hours).The fact that the radical scavenging component is embedded in the shell of the GRC therefore has several positive effects: The radicals are degraded as soon as they are formed, since the radical scavenging component is located nearby and held in place by the shell. Thus, the radicals cannot leave the GRC, and an attack on the ionomer surrounding the GRC is prevented. In this way, the known harmful polymer degradation that occurs when, for example, Pt particles are located near the ionomer, such as within the membrane, is suppressed. This polymer degradation is manifested by an increased fluoride release rate in many literature studies. The stabilization of the radical scavenging component according to the invention is permanent, since the radical scavenging component is immobilized by the encapsulation.The shell also prevents radical scavengers with pronounced acid solubility from dissolving in the ionomer and subsequently being lost, thereby increasing their lifetime and making more compounds suitable. For example, it is known that CeO₂ or MnO₂ exhibit some solubility in acid ionomers and dissolve and migrate over time. When used according to the present invention, such compounds do not come into contact with the ionomer, do not dissolve, and remain permanently stable.

[0067] The optional use of radical scavenger components within the gas recombination catalyst according to the invention is particularly advantageous when hydrocarbon ionomers are used in the GRC layer and thus in the immediate vicinity of the GRC. Hydrocarbon ionomers are particularly susceptible to radical attack due to the weaker carbon-hydrogen bond compared to perfluorinated compounds with their stronger carbon-fluorine bond, and contact with radicals must be avoided. In prior art gas recombination technology, recombination metals come into direct contact with the ionomer, and the use of hydrocarbon ionomers would not be possible because radicals do not react quickly enough, i.e., before they come into contact with the ionomer.According to the advantageous further development of the present invention described here, radicals generated on the catalytically active component are destroyed by the radical scavenger before they leave the GRC, and the ionomer itself in the immediate vicinity is not exposed to any chemical attack.

[0068] To improve the efficiency of the gas recombination reaction and thus increase the longevity of the MEA, it is further advantageously provided that the areal weight of the GRC, based on the catalytically active substance, is 0.01 to 0.5 mg / cm². 2 and in particular 0.01 to 0.15 mg / cm² 2 amounts.

[0069] The gas recombination catalyst can be homogeneously distributed over the entire thickness of the membrane, or with a concentration gradient over the entire thickness of the membrane, or in one or more sublayers of the membrane, or homogeneously or with a gradient or in sublayers of the anode within the anode.

[0070] Preferably, the majority (by mass%) of the gas recombination catalyst is located closer to the anode than to the cathode. In this case, hydrogen recombination is particularly efficient because the oxygen concentration of the oxygen diffusing from the anode is greater, and thus the rate of hydrogen oxidation on the gas recombination catalyst is higher.

[0071] Preferably, the gas recombination catalyst is arranged in an intermediate layer between the membrane and the anode, since it can carry out the gas recombination reaction particularly efficiently here due to the high oxygen concentration.

[0072] In an alternative embodiment, an additional ionomer layer with a small thickness is arranged between the anode and the gas recombination catalyst or between the anode and the membrane containing the gas recombination catalyst. The thickness of the additional ionomer layer is preferably 0.5 to 5 pm.

[0073] To improve the efficiency of the GRC, the GRC is preferably embedded in a gas recombination layer. This gas recombination layer can include other materials, such as ionomers as proton-conducting binders or ceramic particles to increase layer thickness or viscosity for improved processability or adhesion to hydrocarbon-based membranes. Preferably, the gas recombination layer includes ionomers to ensure good proton conductivity of the MEA across its entire thickness.

[0074] Furthermore, the invention also describes the use of the membrane electrode arrangement described above in a water electrolysis cell. This use is advantageous because it results in a particularly long service life for the water electrolysis cell. In addition, the invention also describes an electrolysis cell comprising the membrane electrode arrangement disclosed above. Due to the use of the MEA according to the invention in the electrolysis cell according to the invention, the electrolysis cell is characterized by a particularly long service life.

[0075] Furthermore, according to the invention, the use of a catalyst in the form of a core-shell catalyst, wherein the core is formed from at least one catalytically active component comprising at least one catalytically active substance, and wherein the shell is formed from a material with high gas permeability and low proton conductivity, is described as a gas recombination catalyst in a water electrolysis cell. The use of this catalyst increases the service life of the electrolysis cell because the formation of explosive mixtures of oxygen and hydrogen passing through the membrane of the electrolysis cell is permanently prevented.

[0076] The advantages and advantageous developments described for the respective aspects of the present invention are mutually applicable.

[0077] Examples

[0078] Example 1

[0079] An exemplary synthesis of a GRC according to the present invention comprised the following steps:

[0080] Polymers used as the shell material were dissolved in suitable solvents, and the active recombination catalyst (e.g., Pt-Black, Pt / C) was added to the solution. The solvent was then evaporated to obtain a coated (core-shell) gas recombination catalyst (GRC).

[0081] Example 2

[0082] A GRC with an amorphous fluoropolymer shell was obtained using the following procedure:

[0083] First, an amorphous fluoropolymer, such as Teflon AF from Chemours or Hyflon AD from Syensqo, was dissolved in a fluorinated solvent. The solvent was selected, for example, from the group of 3M's Fluorinert FC-72 (perfluorohexane), FC-75 (perfluoro-n-butyltetrahydrofuran), or FC-40 (perfluoroalkylamine), perfluoromethylcyclohexane, perfluorobenzene, perfluorodimethylcyclohexane, perfluorooctane, perfluorodecalin, perfluoro-1-methyldecalin, perfluorodimethyldecalin, or Galden perfluoropolyether from Syensqo. Solvents with a boiling point between 100 °C and 200 °C were preferred, taking into account changes in concentration due to unintentional evaporation after preparation and the need for rapid and complete drying after coating the catalyst with the fluoropolymer. Particularly advantageous solvents were Fluorinert FC-75 (boiling point = 103 °C), Fluorinert FC-40 (boiling point = 155 °C), Galden HT110 (boiling point = 103 °C).= 110 °C) and Galden HT135 (boiling point = 135 °C). Solvents with lower boiling points, such as perfluorobenzene (boiling point = 82 °C) and Galden HT55 (boiling point = 55 °C), could also be used if unwanted evaporation was carefully controlled. In fact, solvents with lower boiling points at room temperature tended to be better solvents for Teflon AF and Hyflon AD. Dissolution of the fluoropolymer could be improved by heating, by mixing the polymer with the fluorinated solvent and stirring at 10–20 °C below the boiling point of the solvent in a round-bottom flask with a reflux condenser. Polymer solutions with a concentration of 0.5 to 15 wt% could be obtained using this method.

[0084] After dissolving the polymer, the catalytically active component was added to the polymer solution, and a dispersion was obtained by mixing. For example, a platinum black could be used as the catalytically active component, such as Platinum Black TC HSTDP from Heraeus, Pt Black 32 from Umicore, or HyPer FC 100 from Ames Goldsmith.Furthermore, carbon-supported platinum catalysts (Pt / C) or carbon-supported platinum alloy catalysts, especially alloys with cobalt (PtCo / C), could be used, such as Elyst Pt500380, Elyst Pt50 0550, Elyst Pt200390 or Elyst Pt50 0690 (PtCo) from Umicore or H2FC-40Pt-C240, H2FC-50Pt-C700 or H2FC-50Pt-C700M from Heraeus or TEC10E50E, TEC10E60TPM, TEC10E70TPM, TEC10V40E, TEC10V50E, TEC36E32 (PtCo) or TEC36E52 (PtCo) from Tanaka or HyPerFC 140. HyPerFC 150, HyPerFC 150X or HyPer FC 170X from Ames Goldsmith or HiSPEC 3000 or HiSPEC 4000 from Johnson Matthey, and comparable carbon-supported platinum catalysts.

[0085] In an exemplary embodiment, 1 g of a Pt / C catalyst with a specific surface area of ​​approximately 100 m² was used. 2 / g, determined by nitrogen adsorption as the BET surface area, was dispersed in 90 g of a solution of Teflon AF 2400 containing 1 wt% Fluorinert FC-75 in a round-bottom flask and stirred for several hours. The dispersion was subjected to short vacuum cycles to facilitate complete wetting of the catalyst. The dispersion was then transferred to a beaker, and the solvent was removed in a vented oven at 80 °C overnight, specifically for a period of about 12 hours, to obtain the gas recombination catalyst.

[0086] Example 3: In an alternative embodiment, 1 g of Pt Black 32 (Umicore) with a specific (BET) surface area of ​​approximately 32 m² was used. 2 / g was dispersed in 25 g of a solution of Hyflon AD 80 at a wt% concentration in Galden HT110 in a round-bottom flask, stirred for 30 min, and then exposed to ultrasound for 1 min. The dispersion was then stirred for several hours. The dispersion was subsequently transferred to a beaker, and the solvent was removed in a vented oven at 90 °C overnight to obtain the gas recombination catalyst.

[0087] Example 4

[0088] In another alternative embodiment, 1 g of Pt Black 32 (Umicore) with a specific (BET) surface area of ​​approximately 32 m² was used. 21 g of the gas recombination catalyst was dispersed in 50 g of a 5 wt% solution of Hyflon AD 60 in Galden HT55 in a round-bottom flask, stirred for 30 min, and then subjected to ultrasound for 1 min. The dispersion was then stirred for several hours. It was subsequently transferred to a beaker, and the solvent was removed overnight in a ventilated oven at 50 °C. The temperature was then increased to 90 °C for 3 h. The resulting material was then milled to obtain the gas recombination catalyst as a fine powder. The GRC, prepared in one way or another, was then added to and dispersed in an ionomer dispersion, for example, by high-energy mixing, e.g., with an IKA ULTRA TURRAX disperser, or by ultrasonic treatment. This ionomer dispersion with GRC could then be processed into a layer, a gas recombination layer, using conventional methods.

[0089] Example 5

[0090] A gas recombination layer according to the invention could be obtained according to the procedure described below.

[0091] To 50 g of an ionomer dispersion D2020CS from Chemours (20 wt% ionomer in water and 1-propanol), 2-propanol was added to obtain a dispersion with 15 wt% ionomer. The dispersion was then heated to 50 °C for 4 h. 1 g of a powdered gas recombination catalyst according to the invention, comprising 50 wt% Pt / C based on the gas recombination catalyst (60 wt% Pt based on the catalytically active component of the Pt / C; the carbon is a Vulcan-type carbon) and a shell of glassy amorphous perfluoropolymer, was added to the ionomer dispersion, stirred, exposed to ultrasound for 1 min, and then stirred for a further hour. The dispersion thus obtained was coated onto a PTFE film by a doctor blade coating and dried at 80 °C in a ventilated oven.The resulting film, in which the gas recombination catalyst according to the invention was embedded in an ionomer matrix, was laminated onto the surface of a Nation N115 membrane using a decal process in a hot press at 160 °C to obtain a membrane with a gas recombination layer. The gas recombination layer had a basis weight of 0.05 mg(Pt) / cm² based on the platinum content of the gas recombination catalyst. 2 .

[0092] Example 6

[0093] Polymers used as shell material could also be dissolved in supercritical sc-CO2. In this case, the catalytically active component was mixed with the polymer and dispersed / dissolved in sc-CO2, which was then deaerated, and the resulting GRC could be further processed as a powder.

[0094] Example 7

[0095] In another exemplary process, a gas recombination catalyst according to the invention with a shell of amorphous fluoropolymer was produced using supercritical sc-CO2 solvent and dispersant as follows:

[0096] A 100 mL high-pressure vessel was charged with 100 mg of the catalyst TEC10V50E (Pt / C catalyst) and 100 mg of Teflon AF 2400. CO₂ was then introduced into the vessel via a pressure generator, building up a CO₂ pressure of 10 MPa. The high-pressure vessel was then tightly sealed and heated to 120 °C in a silicone oil bath while stirring with a magnetic stirrer. After 3 hours, the high-pressure vessel was removed from the silicone oil bath and cooled to room temperature, then cooled with water for 1 minute. The pressure was then released from the high-pressure vessel over 2-3 minutes, yielding the modified core-shell gas recombination catalyst.

[0097] Optional further training

[0098] To increase the compatibility of the catalytically active substance with an amorphous perfluoropolymer, one possible option is to first coat the catalytically active substance with a small amount (thin layer) of ionomer (or another fluoropolymer with ionic or polar functional groups) and then apply an amorphous perfluoropolymer to its surface. The sulfonic acid groups of the ionomer adhere to the surface of the catalytically active substance (high surface energy) and leave the fluorinated backbone unaffected, which is compatible with the nonpolar amorphous perfluoropolymer (low surface energy). This coating of a bilayer can be carried out in two successive steps: first, the catalytically active substance is dispersed in a polar (e.g., water-alcohol) dilute ionomer dispersion, dried, and the ionomer-coated catalyst is obtained.This powder is then dispersed in a solution of amorphous perfluoropolymer in a fluorinated solvent, dried, and the core-shell GRC is obtained. In this case, the ionomer essentially acts as a compatibilizer between the active catalyst and the shell to obtain a closed shell.

[0099] As an example, a GRC with an inorganic shell could be produced according to the following description (using SiO2 as an example).

[0100] To produce the platinum particles encapsulated by SiO₂ as GRC, a 50 mM solution of Pt(NH₃)₄Cl₂ in ethylene glycol (VWR Chemicals) was prepared in the first step. The salt was initially dissolved in ethylene glycol under stirring. The pH was adjusted to between 11 and 12 by adding aqueous NaOH. The suspension was then heated to 140–150 °C while stirring. The resulting dispersion contained Pt nanoparticles, which served as the core of the core-shell particles. An organic silicate source (e.g., tetraethyl orthosilicate) was then added and stirred for 1–24 hours, with the stirring time influencing the reaction process of hydrolysis of the silicate source and the growth of the silicate layer. Within this period, hydrolysis of the silicate source resulted in the formation of a silicate layer around the core.The layer thickness of the material could be controlled depending on the stirring speed, reaction time, and amount of silicon added. A Pt:Si ratio of 1:0.5 by mass was preferred, with a stirring time of 24 hours and a stirring speed of 400 RPM. After completion of the reaction, the precipitated material was separated by subsequent filtration. The isolated product was washed several times with water and ethanol and finally dried in an oven at 80 °C.

[0101] In an alternative embodiment, a silicate shell analogous to the above description was formed on a core / support of platinum particles on cerium oxide (CeO2) with 5 wt% Pt. First, a cerium source (in the example, 1.89 g Ce(NO3)3*6H2O) was dissolved in ethylene glycol, and the pH was adjusted to 10-11 using a NaOH solution. The solution was heated to 80 °C and stirred for 2 h until it became cloudy. Then, the solution was cooled to room temperature, and a 1.05 M H2PtCl6 solution of ethylene glycol was added. The pH of the solution was adjusted to 10-11 by adding NaOH, and the solution was heated to 160 °C for 4 h. After completion of the reaction, the solution was cooled to room temperature, and after the addition of 0.074 g of tetraethyl orthosilicate, it was stirred at room temperature for 24 h. This resulted in a mass ratio of Pt to CeO2:Si of 1:0.02.The gas recombination catalyst obtained in this way according to the invention, comprising an inorganic radical scavenger as a support for the catalytically active substance, was subsequently precipitated, filtered, washed with distilled water and dried in an oven.

[0102] In an alternative embodiment, a silicate shell analogous to the above description was formed on a core of platinum particles on cerium oxide (CeO2) (10 wt% platinum, 90 wt% CeO2). The preparation of a cerium oxide-supported platinum catalyst was not further restricted and could be obtained by wet impregnation, chemical deposition of platinum on cerium oxide, co-precipitation of Ce(NO3)3 and K2PtCl4 in an alkaline medium, microemulsion methods, or sol-gel-based methods. The average particle size of the Pt / CeO2 catalyst was 50 nm. 1 g of the platinum-cerium oxide catalyst was dispersed in 90 g of ethylene glycol. Subsequently, 0.74 g of tetraethyl orthosilicate was added, and the mixture was stirred for 12 hours. This resulted in a mass ratio of Pt@CeO2:Si of 1:0.1.The gas recombination catalyst thus obtained according to the invention, comprising an inorganic radical scavenger as a carrier of the catalytically active substance, was subsequently precipitated, filtered, washed with distilled water and dried in an oven.

[0103] In another exemplary embodiment, 10 g of powdered terephthalic acid were dissolved at room temperature in 90 g of dimethyl sulfoxide (DMSO) in a flask to obtain a 10 wt% terephthalic acid solution. Terephthalic acid is a known radical scavenger. Furthermore, 1 g of a Pt / C catalyst with a specific surface area of ​​approximately 100 m² was added. 2A BET surface area of ​​Pt / C catalyst, determined via nitrogen adsorption, was added to the terephthalic acid solution. Short vacuum cycles were applied, and the resulting dispersion was then dispersed for 120 minutes. The vacuum cycles removed air from the pores of the carbon support, allowing the terephthalic acid solution to more effectively impregnate, or fill, the pores. To prepare the dispersion, the Pt / C catalyst was milled with the terephthalic acid solution in a ball mill (grinding medium: ZrO₂ balls). The milling time was, for example, 120 minutes and depended on the dispersibility of the catalyst. Thus, the milling time could generally be adjusted accordingly. Alternatively, ultrasonic grinding or various grinding media mills could also be used to prepare the dispersion. Grinding media mills include, for example, ball mills, stirred bead mills, stirred mills, attritors, and specific roller mills.The resulting catalyst with radical scavenger component was then separated from the solvent and, if applicable, from the grinding medium by filtration, transferred to a Petri dish and dried in an oven at 140 °C.

[0104] The resulting Pt / C catalyst, impregnated with terephthalic acid as a radical scavenger, was dispersed in 90 g of a solution of Teflon AF 2400 containing 1 wt% Fluorinert FC-75 in a round-bottom flask and stirred for several hours. The dispersion was then transferred to a beaker, and the solvent was removed in a vented oven at 80 °C for approximately 12 hours to obtain the radical scavenger-containing gas recombination catalyst.

[0105] In an alternative example, a solution containing 5 wt% alpha-tocopherol (α-TOH) in ethanol was prepared using a similar procedure to the one described above. α-TOH is also a known radical scavenger. After adding the Pt / C catalyst under a nitrogen atmosphere, the dispersion was subjected to short vacuum cycles and then dispersed for one hour. The resulting α-TOH-impregnated catalyst was filtered, transferred to a Petri dish, and dried in an oven at 70 °C. The resulting α-TOH-impregnated catalyst powder was processed in a Teflon AF 2400 solution as described above to obtain a gas recombination catalyst according to the invention, comprising a core of α-TOH-impregnated Pt / C catalyst and a shell of Teflon AF 2400.

[0106] The GRC produced in this or a similar way could be redispersed by adding it to an ionomer dispersion, for example using high-energy mixing, e.g. with an IKA ULTRA TURRAX disperser or an ultrasonic bath. The resulting ionomer / GRC dispersion could then be processed into a layer, a gas recombination layer, using conventional methods.

[0107] A gas recombination layer according to the invention can be obtained by the following procedure: 1-propanol and water were added to 30 g of an ionomer dispersion D2020CS from Chemours (21 wt% ionomer in water and 1-propanol) to obtain a dispersion with 20 wt% ionomer. The dispersion was then mixed with 0.29 g of a powdered gas recombination catalyst according to the invention, containing 99 wt% Pt and a coating consisting of 1 wt% SiO2, based on the gas recombination catalyst. The resulting suspension was dispersed for 2 h in a LAU disperser. The resulting dispersion was then coated onto a PTFE film using a doctor blade and dried at 80 °C in a ventilated oven.The resulting film, in which the gas recombination catalyst according to the invention was embedded in an ionomer matrix, was laminated onto the surface of a Nation N115 membrane using a decal process in a hot press at 160 °C to obtain a membrane with a gas recombination layer. The gas recombination layer had a basis weight of 0.1 mg(Pt) / cm² based on the platinum of the gas recombination catalyst. 2 Further details, advantages, and features of the present invention will become apparent from the following description of exemplary embodiments with reference to the drawing. Fig. 1 shows a section of an MEA according to a first embodiment.

[0108] Fig. 2 shows a sectional view of a gas recombination catalyst according to a second embodiment and

[0109] Fig. 3 shows a sectional view of a gas recombination catalyst according to a third embodiment.

[0110] The figures show only the essential aspects, components, and elements of the present invention. All other aspects have been omitted for the sake of clarity. Furthermore, identical reference numerals denote identical components.

[0111] In detail, Fig. 1 shows a cross-sectional view of an MEA 1 for an electrolysis cell, in particular a water electrolysis cell, comprising four layers: a cathode 2, an anode 3, and a membrane 4 arranged between the cathode 2 and the anode 3.

[0112] Membrane 4 can be designed in particular as a proton exchange membrane or as an anion exchange membrane.

[0113] Between the membrane 4 and the anode 3, a further layer, a gas recombination layer 5, is arranged. This layer contains a gas recombination catalyst (GRC) 6. The GRC 6 is designed as a core-shell catalyst and comprises a shell 8 (see Figures 2 and 3) that surrounds at least one core 10 (see Figures 2 and 3). Each core 10 is formed from at least one catalytically active component 11, wherein each of the catalytically active components 11 comprises at least one catalytically active substance 7.

[0114] Shell 8 is made of a material with high gas permeability and low proton conductivity, allowing H₂ and O₂ to easily reach the catalytically active substance 7, while the water vapor generated at the GRC by the catalytic reaction can be rapidly transported away. The low proton conductivity also prevents ions of the catalytically active substance, generated during the oxidation and reduction cycles of the catalytic reaction, from migrating out of shell 8 and also out of the gas recombination layer 5. Furthermore, the solubility stability of the catalytically active substance 7 is improved by effectively preventing protons, which create an acidic environment and thus increase the solubility of catalytically active substances and their ions, from diffusing into the GRC.

[0115] The MEA 1 shown in Figure 1 is characterized by a particularly long lifetime and high efficiency due to the specifically designed GRC 6 in the form of a core-shell catalyst, in which the core contains the catalytically active substance and the shell serves as protection for the catalytically active substance.

[0116] Figures 2 and 3 show exemplary further developments of a GRC 6.

[0117] Figure 2 shows a GRC 6 with a round shape, in which a shell 8 exemplarily surrounds a catalytically active component 11 as a core 10. The catalytically active component 11 is completely surrounded by the shell 8.

[0118] As an example, it is shown that four polygonal particles of a catalytically active substance 7 are supported on a support 12, wherein the support 12 is selected in particular from carbon or ceramic materials, including in particular silica, titanium oxide, niobium oxide, tungsten oxide, tantalum oxide, tin oxide, fluoride-doped tin oxide (FTO), and zirconium oxide. Thus, the catalytically active component 11 or components 11 are formed from one or more catalytically active substances 7 and the support 12. The catalytically active substance 7 is selected in particular from the group consisting of platinum, palladium, gold, silver, molybdenum disulfide, iridium, nickel, ruthenium, and alloys thereof, alloys of platinum and a non-precious metal, and alloys of palladium and a non-precious metal, wherein the non-precious metal is selected in particular from cobalt and nickel.

[0119] The material of the shell has a proton conductivity of less than 0.05 S / cm and a gas permeability of more than 1*10 -17 mol x cm / (sx cm 2 x kPa). The shell material is selected in particular from ceramic materials and polymers, especially from amorphous elastomeric and amorphous glassy poly- and perfluoropolymers and functionalized perfluoropolyethers, and carbon-containing materials.

[0120] The shell 8 has, in particular, a layer thickness S in a range of 0.5 to 1000 nm, measured at the widest point.

[0121] Figure 3 shows an elongated GRC 6 in which a shell 8 surrounds, by way of example, two catalytically active components 11. The two catalytically active components 11 are each surrounded by an ionomer film 9 and together form the cores 10. The catalytically active components 11 are completely enclosed by the shell 8.

[0122] The catalytically active substance 7 is in particular selected from the group consisting of platinum, palladium, gold, silver, molybdenum disulfide, iridium, nickel, ruthenium, and alloys thereof, alloys of platinum and a base metal, and alloys of palladium and a base metal, wherein the base metal is in particular selected from cobalt and nickel. The shell material has a proton conductivity of less than 0.05 S / cm and a gas permeability of more than 1 × 10 -17 mol x cm / (sx cm 2 x kPa). The shell material is selected in particular from ceramic materials and polymers, especially from amorphous elastomeric and amorphous glassy poly- and perfluoropolymers and functionalized perfluoropolyethers, and carbon-containing materials.

[0123] The shell 8 has a layer thickness S in the range of 0.5 to 1000 nm. The GRC 6 is in the form of an elongated particle and has a characteristic dimension A of 2 to 1000 nm. In the individual catalytically active components 11, which form the cores 10, the catalytically active substances 7 have a diameter D of 1 to 20 nm. For the corresponding dimensions, only the longest distances are used to determine the respective dimension.

[0124] In Fig. 3, the catalytically active substances 7 also have a coating, an ionomer layer 9 with a layer thickness of less than 4 nm. This improves the compatibility of the catalytically active components 11 of the GRC 6 with the material of the shell 8.

[0125] Both GRC 6 shown in Figures 2 and 3 improve the longevity and thus the long-term stability of an electrolysis cell (especially a water electrolysis cell) and can be used in the MEA 1 from Fig. 1.

[0126] In addition to the foregoing written description of the invention, explicit reference is hereby made to the graphic representation of the invention in the figures for its supplementary disclosure. List of reference symbols

[0127] 1 Membrane electrode arrangement

[0128] 2 Cathode

[0129] 3 Anode

[0130] 4 Membran

[0131] 5 Gas recombination layer

[0132] 6 Gas recombination catalyst

[0133] 7 catalytically active substances

[0134] 8 case

[0135] 9 ionomer layer

[0136] 10 cores

[0137] 11 catalytically active components

[0138] 12 carriers

[0139] A characteristic extent

[0140] D Diameter of the catalytically active substance S Layer thickness of the shell

Claims

Claims 1. Membrane electrode arrangement (1) comprising a cathode (2), an anode (3) and an ion-conducting membrane (4) arranged between the cathode (2) and the anode (3), wherein the membrane electrode arrangement (1) comprises a gas recombination catalyst (6), wherein the gas recombination catalyst (6) is a core-shell catalyst, wherein a shell (8) of the core-shell catalyst surrounds at least one core (10), wherein the at least one core (10) is formed from at least one catalytically active component (11) comprising at least one catalytically active substance (7), and wherein the shell (8) is formed from a material with high gas permeability and low proton conductivity.

2. Membrane electrode arrangement (1) according to claim 1, wherein the catalytically active substance (7) is selected from the group consisting of platinum, palladium, gold, silver, molybdenum disulfide, iridium, nickel, ruthenium, and alloys thereof, alloys of platinum and a non-precious metal and alloys of palladium and a non-precious metal, wherein the non-precious metal is in particular selected from cobalt and nickel.

3. Membrane electrode arrangement (1) according to one of the preceding claims, wherein the catalytically active component (11) further comprises a support (12) which supports the catalytically active substance (7), wherein the support (12) is in particular selected from carbon or ceramic materials, and including in particular silica, titanium oxide, niobium oxide, tungsten oxide, tantalum oxide, tin oxide, fluoride-doped tin oxide and zirconium oxide.

4. Membrane electrode arrangement (1) according to one of the preceding claims, wherein the gas recombination catalyst (6) is supported on a support, wherein the support is in particular selected from carbon or ceramic materials, including in particular silica, titanium oxide, niobium oxide, tungsten oxide, tantalum oxide and zirconium oxide.

5. Membrane electrode arrangement (1) according to one of the preceding claims, wherein the material of the shell (8) has a gas permeability of more than 1*10 -17 mol x cm / (sx cm 2 x kPa), preferably of more than 1*10' 13 mol x cm / (sx cm 2 x kPa), preferably of more than 1*10 -11 mol x cm / (sx cm 2 has x kPa).

6. Membrane electrode arrangement (1) according to any one of the preceding claims, wherein the material of the shell (8) has a proton conductivity of less than 0.05 S / cm, preferably less than 0.01 S / cm, more preferably less than 0.001 S / cm.

7. Membrane electrode arrangement (1) according to any one of the preceding claims, wherein the material of the shell (8) has a diffusion coefficient for ions of the catalytically active substance (7) of less than 10⁻⁴. 10 m 2 / s, preferably of less than 10 -11 m 2 / s, preferably of less than 10 -12 m 2 / s, preferably of less than 10 -13 m 2 / s has.

8. Membrane electrode arrangement (1) according to one of the preceding claims, wherein the material of the shell (8) is selected from ceramic materials and polymers, in particular from amorphous elastomeric and amorphous glassy poly- and perfluoropolymers and functionalized perfluoropolyethers, and carbon-containing materials.

9. Membrane electrode arrangement (1) according to claim 8, wherein the ceramic material is selected from at least one of oxides, nitrides, carbides, silicides, borides, fluorides and mixtures thereof, from at least one of silicon, tantalum, niobium, tin, titanium, zirconium, cerium, tungsten, antimony and mixtures thereof, wherein the ceramic material is in particular selected from silicon oxide, tantalum oxide, niobium oxide, tungsten oxide and zirconium oxide.

10. Membrane electrode arrangement (1) according to one of the preceding claims, wherein the catalytically active substance (7) of the gas recombination catalyst (6) is in the form of particles, wherein the particles have a diameter of 1 to 20 nm, preferably of 1.5 to 15 nm and particularly preferably of 2 to 10 nm.

11. Membrane electrode arrangement (1) according to one of the preceding claims, wherein the gas recombination catalyst (6) is in the form of particles, wherein the particles have a characteristic size (A) of 2 to 1000 nm, preferably of 3 to 500 nm and particularly preferably of 5 to 250 nm.

12. Membrane electrode arrangement (1) according to one of the preceding claims, wherein the shell (8) has a layer thickness (S) in a range of 0.5 to 1000 nm, preferably 0.5 to 500 nm, more preferably 0.5 to 250 nm, particularly preferably 0.5 to 20 nm, even more preferably 1 to 10 nm and particularly preferably 2 to 5 nm.

13. Membrane electrode arrangement (1) according to one of the preceding claims, wherein the shell (8) is amorphous.

14. Membrane electrode arrangement (1) according to any one of claims 1 to 13, wherein the gas recombination catalyst (6) is substantially free of ionomer.

15. Membrane electrode arrangement (1) according to any one of claims 1 to 13, wherein the catalytically active substance (7) of the gas recombination catalyst (6) has a coating with an ionomer layer (9) having a layer thickness of less than 4 nm.

16. Membrane electrode arrangement (1) according to one of the preceding claims, wherein the gas recombination catalyst (6) comprises a radical scavenger component, in particular selected from the group consisting of inorganic compounds comprising cerium and manganese and organic compounds comprising hydroxyphenyl-containing compounds, quinones, benzoquinones, hydroquinones, quercetin, caffeic acid, terephthalic acid, α-tocopherol, compounds comprising nitrogenous heterocycles such as hindered amines and bipyridines and phosphorus-containing compounds.

17. Membrane electrode arrangement (1) according to one of the preceding claims, wherein the basis weight of the gas recombination catalyst (6) based on the catalytically active substance (7) is 0.01 to 0.5 mg / cm² 2 , in particular 0.01 to 0.15 mg / cm² 218. Membrane electrode arrangement (1) according to one of the preceding claims, wherein the gas recombination catalyst (6) is arranged in an intermediate layer between the membrane (4) and the anode (3).

19. Membrane electrode arrangement (1) according to any one of the preceding claims, wherein the gas recombination catalyst (6) is located in a gas recombination layer (5).

20. Use of the membrane electrode arrangement (1) according to any one of the preceding claims in a water electrolysis cell.

21. Electrolysis cell comprising a membrane electrode arrangement (1) according to any one of claims 1 to 19.

22. Use of a catalyst formed in the form of a core-shell catalyst, wherein the core (10) is formed from at least one catalytically active component (11) comprising at least one catalytically active substance (7), and wherein the shell (8) is formed from a material with high gas permeability and low proton conductivity, as a gas recombination catalyst (6) in a water electrolysis cell.

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