Methods and devices for forming catalytically active membranes or membrane-electrode assemblies

Abstract

The present invention relates to a method for manufacturing a catalytically active membrane-electrode assembly (20) comprising one or more, in particular two, electrodes, said method comprising at least the following steps: i) depositing a heterogeneous layer (3) on a substrate (5), the heterogeneous layer (3) comprising a base metal (1) and a precious metal (2) that are heterogeneously distributed in the heterogeneous layer (3), ii) removing the base metal (1) from the heterogeneous layer (3) such that a first self-supporting nanoporous catalyst layer (4) comprising the precious metal (2) is formed on the substrate (5). (1); iii) adding at least one proton conducting ionomer (40) and / or at least one hydrophobic particle (41) and / or an ionic liquid (42) to the first self-supporting nanoporous catalyst layer (4); v) forming a catalytically active membrane-electrode assembly (20) by attaching the self-supporting nanoporous catalyst layer (4) to a first side of the membrane (10) to form a catalytically active membrane-electrode assembly (20) having one electrode.

Description

[Technical field]

[0001] The present invention relates to a method for forming a catalytically active membrane or a membrane-electrode assembly The present invention further relates to a membrane-electrode assembly. [Background technology]

[0002] The present invention teaches a method for manufacturing catalytically active membranes, electrodes, or membrane-electrode assemblies for proton exchange membrane (PEM) fuel cells and electrolyzers (also referred to in the art as electrolytic cells). These types of fuel cells and electrolyzers have the advantage of being able to achieve high current densities and are very flexible to fluctuating demand or supply. In particular, the core of fuel cells and electrolyzers is the membrane-electrode assembly. The membrane-electrode assembly consists of a catalytically active membrane and a gas-permeable electrode or a catalytically active electrode and membrane. In particular, in PEM electrolyzers, a membrane-electrode assembly is commonly used, which consists of a membrane sandwiched between two electrodes, an anode and a cathode, where the electrodes and / or membranes have catalytic activity. The anode of a PEM electrolyzer usually requires an oxide containing a noble metal, such as iridium oxide, ruthenium oxide, etc. Due to the demanding reaction conditions, platinum is the material of choice for the cathode of a PEM electrolyzer. However, other catalysts such as MoS or other non-noble metal catalysts can also be used for the cathode. In PEM fuel cells, the hydrogen oxidation reaction (HOR) at the anode is then relatively active, and therefore platinum, the most common catalyst used at the anode, can be used at low loadings. Conversely, the oxygen reduction reaction (ORR) that occurs at the cathode also requires high loadings of platinum to maximize the surface area for reaction and achieve maximum activity.

[0003] To increase the efficiency of a catalyst, a high surface-to-volume ratio needs to be achieved, which is usually achieved by using nanoparticles of the catalyst, which are applied to the electrode or membrane in the form of a suspension using a spraying process. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Xinrong Zhang, Wei Zhang, Weijing Yang, Wen Liu, Fanqi Min, Samuel S. Mao, and Jingying Xie, “Catalyst-coated proton exchange membrane for hydrogen production with high pressure water electrolysis”, Appl. Phys. Lett. 119, 123903 (2021) https: / / doi.org / 10.1063 / 5.0060150 [Non-Patent Document 2] Maximilian Bernt et al., 2018 J.Electrochem.Soc.165 F305 DOI 10.1149 / 2.0641805jes, “Analysis of Voltage Losses in PEM Water Electrolyzers with Low Platinum Group Metal Loadings” Summary of the Invention [Problem to be solved by the invention]

[0005] On this basis, the subject of the present invention is to provide a method for forming a catalytically active membrane-electrode assembly with one or more, in particular two, electrodes, which is improved over the prior art, in particular with regard to an increased surface-to-volume ratio of the catalyst. [Means for solving the problem]

[0006] This object is solved by a membrane-electrode assembly according to claim 15 and by a method having the features of claim 1.

[0007] Advantageous embodiments of the invention are set out in the dependent claims and are explained below.

[0008] According to the present invention and a first aspect thereof, a method for manufacturing a catalytically active membrane-electrode assembly comprising one or more, in particular two, electrodes comprises at least the following steps: i) depositing a heterogeneous layer on a substrate, the heterogeneous layer comprising a base metal and a noble metal that are non-uniformly distributed in the heterogeneous layer; ii) leaching the base metal from the heterogeneous layer such that a first self-supporting nanoporous catalyst layer comprising the precious metal is formed on the substrate; iii) adding at least one proton conducting ionomer and / or at least one hydrophobic particle and / or an ionic liquid to the first self-supporting nanoporous catalyst layer; iv) forming a catalytically active membrane-electrode assembly by attaching a self-supporting nanoporous catalyst layer to a first side of the membrane to form a catalytically active membrane-electrode assembly with one electrode.

[0009] Thus, the method according to the invention describes a process for producing a nanoporous and self-supporting catalyst layer, rather than nanoparticles. A self-supporting layer has a very large surface area compared to its volume. Moreover, because this layer is self-supporting, unlike nanoparticles, it does not require any support structure or scaffolding, which results in a higher resistance to the corrosive environment found in reactions in acidic media in which the membrane-electrode assembly may be placed.

[0010] The pores of the self-supporting nanoporous catalyst layer are preferably in the nanometer scale range, for example having diameters in the range of 1 nm to 200 nm, more particularly in the range of 2 nm to 50 nm.

[0011] Preferably, the heterogeneous layer formed by the base metal (as its first component) and the precious metal (as its second component) is highly heterogeneous, does not exhibit inter-alloy characteristics, and comprises a plurality of base metal particles and precious metal particles arranged in a highly heterogeneous manner. The heterogeneity of the base metal and the precious metal may be such that, when the base metal is leached from the layer, the base metal forms regions in the layer that essentially correspond to the pore size.

[0012] Preferably, steps i), ii), iii) and iv) are carried out chronologically, i.e. one after the other in the order given above.

[0013] Carrying out steps i) and ii) alone results in the formation of a catalytically active electrode. Applying step iv) allows the catalytically active electrode to be pressed with the membrane to form a catalytically active membrane or a catalytically active membrane assembly. Intermediate step iii) is particularly advantageous for establishing a strong contact between the catalytically active electrode and the membrane, resulting in a high surface-to-volume ratio of the catalyst, i.e. the precious metal, and thus substantially increasing the efficiency of the catalyst. Intermediate step iii) advantageously contributes to maximizing the three-phase boundary of the catalyst, which is in contact with the gas, liquid and solid phases when the membrane-electrode assembly is in operation. Thus, to transfer the protons to the catalyst, a proton conductor is required, which can be realized by a proton-conducting ionomer or an ionic liquid. To remove the water, a hydrophobic material is required, for example using an ionic liquid or hydrophobic particles. Thus, the liquid can be both hydrophobic and ionically conductive.

[0014] In particular, the term "base metal" may also include base metal compounds or alloys that are based on at least one base metal.

[0015] In particular, the term "noble metal" may also include a noble metal compound or alloy that is based on at least one noble metal.

[0016] For use in a fuel cell, for example in an electrolyser, it is necessary to electrically insulate the two (or more) catalytically active electrodes from one another. For this purpose, the two (or more) catalytically active electrodes are preferably separated and electrically insulated from one another by a membrane. Thus, according to one embodiment of the present invention, a second self-supporting nanoporous catalyst layer on a substrate can be formed according to steps ii) and iii), for example either simultaneously with the first self-supporting nanoporous catalyst layer, or sequentially, or by separating a part of the first self-supporting nanoporous catalyst layer forming the second self-supporting nanoporous catalyst layer. In step iv) or a subsequent step, the second self-supporting nanoporous catalyst layer can be attached to a second side of the membrane so as to form a catalytically active membrane-electrode assembly comprising two electrodes.

[0017] According to one embodiment of the invention, attachment of the first and / or second self-supporting nanoporous catalyst layers to the membrane is performed by pressing and / or decal transferring the first and / or second self-supporting nanoporous catalyst layers from a substrate to the membrane. When referring to decal transfer, preferably the substrate forms a decal and the first and / or second self-supporting nanoporous catalyst layers form a pattern that is transferred to the membrane by decal transfer.

[0018] To achieve the best contact between the electrode and the membrane, the pressing temperature should be close to the glass transition temperature of the membrane in order to achieve adhesion with the electrode without damaging the membrane structure. For this purpose, pressing and / or decal transfer can be preferably carried out at a pressure between 1 bar and 75 bar and / or at a substrate temperature between 115°C and 145°C, in particular for a time between 15 s and 600 s.

[0019] According to one embodiment of the present invention, at least one proton conducting ionomer and / or at least one hydrophobic particle and / or ionic liquid may be added to the first and / or second self-supporting nanoporous catalyst layer by at least one of spraying, ultrasonic spraying, decal transfer, dipping, drop casting, filtering, which is particularly advantageous for achieving robust contact of the self-supporting nanoporous catalyst layer in three-phase contact with the reactants, proton transport medium, and products.

[0020] According to one embodiment of the present invention, one type of hydrophobic particles may be or may include at least one of carbon, Nafion, titanium oxide, and / or Teflon.

[0021] The term "Nafion" refers to the brand name of a sulfonated tetrafluoroethylene-based fluoropolymer copolymer having the CAS number 31175-20-9.

[0022] The term "Teflon" refers to a synthetic fluoropolymer of tetrafluoroethylene having the CAS number 9002-84-0.

[0023] According to one embodiment of the present invention, the substrate may include or be a transfer substrate and / or a gas permeable electrode configured for decal transfer of the first and / or second self-supporting nanoporous catalyst layer onto the membrane. For example, the gas permeable electrode may include titanium, tantalum or carbon, and the transfer substrate may include poly(4,4'-oxydiphenylene-pyromellitimide), polytetrafluoroethylene (CAS:9002-84-0) or carbon. In particular, the first and / or second self-supporting nanoporous catalyst layer may be in contact with the gas permeable electrode or transfer substrate via at least one of the two sides of the membrane. Optionally, the transfer substrate may be removed after pressing and / or decal transfer of the first and / or second self-supporting nanoporous catalyst layer from the substrate to the membrane. The gas permeable electrode may be pressed together with the membrane.

[0024] According to one embodiment of the invention, the base metal may, for example, include or be at least one of the following: Co, Cu, Fe, Ni, Zn, Al, Mg, Cr, Mo, Gd, Ta, Ti, W, Nb, or Mn.

[0025] The noble metal may, for example, include or be at least one of the following: Pt, Ru, Ir, Au, iridium oxide, ruthenium oxide.

[0026] According to one embodiment of the present invention, the base metal and / or the precious metal may be deposited by a method of physical vapor deposition (PVD).

[0027] PVD may for example include or be sputtering, in particular alternating magnetron sputtering.

[0028] According to one embodiment of the present invention, PVD is directed to the substrate from a sputtering target with a rectangular target area, at a pressure between 2 Pa and 10 Pa, for example using a radio frequency (RF) or direct current (DC) plasma source.

[0029] According to one embodiment of the present invention, the substrate may be moved back and forth along at least one spatial direction during PVD, which movement may correspond to vibration.

[0030] According to one embodiment of the present invention, a substrate having a lateral width L moves with respect to a stationary point by an offset of at least less than L / 4 in order to move back and forth during PVD along at least one direction.

[0031] According to one embodiment of the present invention, during step i), the deposition of base metal and precious metal, in particular by PVD, can be alternated at least three times, which advantageously contributes to obtaining a highly inhomogeneous layer comprising base metal and precious metal, this alternation being understood as meaning that the deposition of base metal and precious metal does not occur simultaneously, but sequentially.

[0032] According to another embodiment of the invention, the base metal forms a region in the layer constituted by the precious metal, in particular said region consists exclusively of the base metal.

[0033] According to another embodiment of the invention, the size of said domains of base metal is in the range of 1 nm to 200 nm, in particular the size may vary from domain to domain.

[0034] According to another embodiment of the invention, the precious metal forms a scaffold in a layer comprising a base metal.

[0035] According to one embodiment of the present invention, the weight ratio of deposited base metal to deposited precious metal can be 0.5:1 to 20:1, which provides the best catalytic performance.

[0036] According to one embodiment of the present invention, step ii) can be carried out in an electrolyte arranged between a first and a second leaching electrode, where the heterogeneous layer and the first leaching electrode are electrically connected and a leaching current is applied between the first and the second leaching electrodes, so that the base metal is leached out of the heterogeneous layer. For this purpose, the heterogeneous layer on the substrate, in particular on the transfer substrate, can in particular be arranged in the electrolyte or immersed in the electrolyte.

[0037] According to one embodiment of the present invention, the potential of the first leaching electrode can be controlled by a third leaching electrode, preferably in close proximity to the first electrode. The third leaching electrode can be a platinum electrode in a hydrogen atmosphere, where the third leaching electrode functions as a reference electrode configured to control the potential of the first leaching electrode and to allow measurement of the voltage between the first leaching electrode and the third leaching electrode. At the same time, the leaching process can be carried out by applying an electrical leaching current through the electrolyte between the first leaching electrode and the second leaching electrode, where the first leaching electrode is preferably connected to the heterogeneous layer. The leaching current between the first leaching electrode and the second leaching electrode can be controlled so that the measured voltage between the first leaching electrode and the third leaching electrode is a predetermined value, for example a voltage between -0.2V and 1.5V.

[0038] According to one embodiment of the present invention, the electrolyte may include an acid or a base, where the acid may include, for example, at least one of HClO4, H2SO4, HNO3, and the base may include, for example, at least one of KOH or NaOH.

[0039] According to one embodiment of the present invention, the concentration of the acid or base in the electrolyte may be in the range of 0.1 mol / l to 8 mol / l.

[0040] According to one embodiment of the invention, the potential of the first leaching electrode relative to the potential of the third leaching electrode may correspond to a voltage in the range of -0.2 V to +1.5 V. In particular, the voltage flowing between the first and second leaching electrodes and the resulting leaching current may be constant in time, but may also vary alternately in time, in particular periodically in time.

[0041] According to an embodiment of the invention, during step ii) the electrolyte (solution) may be at least partially exchanged with electrolyte (solution) not used in step ii) and / or the electrolyte may be subjected to a flow such that the electrolyte is constantly, in particular continuously, exchanged. This is particularly advantageous to avoid cross-contamination of the membrane with leached cations, for example by exchanging protons in the membrane with cations, which is known to reduce the performance of the catalyst and to deactivate the membrane.

[0042] According to one embodiment of the present invention, especially during step ii), the temperature of the electrolyte can be adjusted within the range of 10°C to 95°C.

[0043] According to one embodiment of the present invention, step ii) can be carried out in a leaching chamber comprising at least a first leaching electrode, said first leaching electrode being electrically connected, in particular a short-circuit connected, with the heterogeneous layer, in particular a heterogeneous layer deposited on a substrate; a second leaching electrode; and an electrolyte arranged between the first and second leaching electrodes, said heterogeneous layer being arranged between the first and second leaching electrodes. The leaching chamber can optionally further comprise a third leaching electrode.

[0044] According to one embodiment of the present invention, at least during step ii), a flow of at least one non-reactive (inert) gas, in particular N2 and / or Ar, may be carried out through the leaching chamber.

[0045] According to an embodiment of the invention, at least during step ii), a flow of at least one reactive gas, in particular H2 or O2, may be carried out through the leaching chamber, in particular above the first leaching electrode opposite the electrolyte.

[0046] According to one embodiment of the present invention, after the first and / or second catalytic layer have been attached to the membrane, the transfer substrate may be peeled off from the catalytically active membrane-electrode assembly.

[0047] According to a second aspect of the present invention, a method for producing a cellular radioisotope comprising the steps of: - a first gas-permeable electrode layer; - a first self-supporting nanoporous catalyst layer comprising a precious metal; - membrane, A membrane-electrode assembly for a proton exchange membrane (PEM) fuel cell or electrolyzer is disclosed, comprising: The first self-supporting nanoporous catalyst layer extends between the first gas permeable electrode layer and the membrane, and is characterized in that the first self-supporting nanoporous catalyst layer is formed by a plurality of grains of a first catalytic compound, where gaps are formed between the grains to form an increased surface area of ​​the first self-supporting nanoporous catalyst layer to enhance the catalytic reaction.

[0048] It is noted that the definitions, features and embodiments relating to this method apply in the same or similar manner to the second aspect of the invention, namely the membrane-electrode assembly, and vice versa.

[0049] In particular, the electrode and / or catalyst compositions disclosed in connection with the present methods are optionally applicable to membrane-electrode assemblies.

[0050] In particular, the first gas permeable electrode can be formed as a layer, and likewise, the first self-supporting nanoporous catalyst can be formed as a layer as well.

[0051] It should be noted that the term "layer" may include the concept of a non-continuous layer, including recesses and gaps, and not necessarily completely interconnected. Thus, the thickness of a layer may vary locally, but one skilled in the art can relate the thickness of a layer, for example, by determining the envelope of the layer.

[0052] The term "increased surface area" particularly relates to an area that is greater than the surface area of ​​a layer that essentially consists of two smooth surfaces of a continuous manifold. The interstices and grain structure allow for such an increase in surface area.

[0053] Due to the increased surface area, the invention according to the second aspect allows for the use of less catalyst compound.

[0054] According to another embodiment of the present invention, the membrane-electrode assembly further comprises a second self-supporting nanoporous catalyst layer disposed on the second gas permeable electrode layer on the opposite side of the membrane, i.e., the side facing substantially opposite to the first self-supporting nanoporous catalyst layer, such that the second self-supporting nanoporous catalyst layer extends between the second gas permeable electrode layer and the membrane, and the second self-supporting nanoporous catalyst layer is formed by a plurality of grains comprising a second catalytic compound, with gaps formed between the grains to form an increased surface area of ​​the second self-supporting nanoporous catalyst layer for enhancing the catalytic reaction.

[0055] In particular, the second gas permeable electrode can be formed as a layer, and similarly, the second self-supporting nanoporous catalyst can be formed as a layer as well.

[0056] According to another embodiment of the present invention, the median equivalent spherical diameter of the plurality of grains of the first and / or second self-supporting nanoporous catalyst layers is in the range of 0.1 μm to 1.0 μm.

[0057] Equivalent sphere diameter is a measure of particle size known to those skilled in the art.

[0058] Although the grains may be at least partially interconnected, the skilled person is able to estimate entities / grains separated from the interconnected grains for the purpose of estimating the grain size distribution, e.g. by segmentation methods, and thus to determine the equivalent spherical diameter for a number of grains.

[0059] Since grains can also be considered as particles, the spherical equivalent diameter can also be applied to characterize the particle size distribution.

[0060] An advantage of this embodiment is that the particle size distribution allows for increased surface area, especially when interstices form a comparable sized volume.

[0061] According to another embodiment of the invention, the plurality of grains of the first and / or second self-supporting nanoporous catalyst layer have an equivalent spherical diameter in the range of 50 nm to 1,500 nm. This embodiment allows for an alternative or complementary measure of the grain size distribution that allows optimizing the surface area of ​​the grains.

[0062] The term "nanoporous" specifically refers to a grain structure that is also porous such that the surface area of ​​each grain is increased compared to a corresponding (smooth-surfaced) sphere of equivalent diameter.

[0063] The nanoporous grains and the interstices between the grains form the nanoporous electrode layer.

[0064] According to another embodiment of the invention, the ratio of the volume comprising voids to the volume comprising grains of the first and / or second self-supporting nanoporous catalyst layer is in the range of 0.3 to 5, in particular in the range of 0.5 to 3, more in particular in the range of 0.8 to 1.2.

[0065] This scale further details the structural features of the catalyst layer(s) that allow for increased surface area of ​​the grain for catalytic activity.

[0066] In this respect, the first and / or second catalyst layer can be formed discontinuously, which allows for a granular structure of the catalyst layer that increases the surface area compared to a continuous catalyst layer that may only represent a surface facing the membrane and a surface facing the electrode layer, where the surface area of ​​these surfaces will be smaller than the surface area of ​​the disclosed granular structure.

[0067] According to another embodiment of the present invention, the first catalyst compound comprises or is iridium.

[0068] According to another embodiment of the invention, the first gas permeable electrode layer comprises or is titanium.

[0069] According to another embodiment of the invention, the second gas permeable electrode layer comprises or is carbon.

[0070] According to another embodiment of the invention, the membrane is a proton permeable membrane configured to retain at least hydrogen, oxygen.

[0071] According to another embodiment of the invention, an electrode-membrane assembly is formed using the method according to the first aspect.

[0072] In particular, exemplary embodiments are described below in conjunction with figures, which are appended to the claims and are accompanied by texts explaining individual features of the illustrated embodiments and aspects of the invention. Individual features shown in the figures and / or mentioned in the text of the figures can be incorporated (even in separate manner) in claims relating to the method according to the invention. [Brief description of the drawings]

[0073] [Figure 1] FIG. 1 shows a membrane-electrode assembly formed by a method according to the present invention. [Diagram 2] FIG. 2 illustrates one embodiment of a method according to the present invention for depositing base and precious metals on a substrate to form a heterogeneous layer that can be processed into a catalytically active electrode. [Diagram 3] FIG. 3 illustrates one embodiment of a method according to the present invention, in which the base metal is leached from the heterogeneous layer to form a catalytically active electrode. [Figure 4] FIG. 4 shows an embodiment of the method according to the invention, in which the base metal is leached from a heterogeneous layer in a leaching chamber comprising an electrolyte and three leaching electrodes. [Diagram 5] FIG. 5 is a schematic cross-sectional view of a membrane-electrode assembly according to the present invention. [Figure 6] FIG. 6 shows an SEM recording of a membrane-electrode assembly according to the invention. [Figure 7] FIG. 7 shows a histogram of particle size distribution on the catalyst layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0074] 1 is an exploded view of a membrane 10 sandwiched between two self-supporting nanoporous catalyst layers 4 comprising ionomer 40 and two gas permeable electrodes 50, illustrating a membrane-electrode assembly 20 that may be formed using the method according to the present invention. The membrane-electrode assembly 20 is particularly applicable to PEM fuel cells and electrolysers.

[0075] According to the method of the present invention, a self-supporting nanoporous catalyst layer 4 can be formed in a first step by depositing a heterogeneous layer 3 (not shown in FIG. 1 ) on a substrate 5, the heterogeneous layer 3 preferably comprising a base metal 1 and a precious metal 2.

[0076] The base metal 1 may include or be, for example, at least one of Co, Cu, Fe, Ni, Zn, Al, Mg, Cr, Mo, Gd, Ta, Ti, W, Nb, or Mn.

[0077] Noble metal 2 may include or be, for example, at least one of Pt, Ir, IrO2, Ru, RuO2, and Au.

[0078] Substrate 5 may comprise or be a gas permeable electrode 50, for example comprising titanium, tantalum, or carbon. Substrate 5 may also comprise or be a transfer substrate 51, for example comprising Kapton, Teflon, or carbon foil.

[0079] In a second step, the base metal 1 is preferably leached from the heterogeneous layer such that a self-supporting nanoporous catalyst layer 4 comprising the precious metal 2 is formed on the substrate 5 .

[0080] Leaching can be carried out chemically or electrochemically, for example in a leaching chamber 30 comprising an electrolyte 35 as further described in FIG.

[0081] In a third step, at least one proton conducting ionomer 40 and / or at least one hydrophobic particle 41 and / or an ionic liquid 42 are added to the preferably self-supporting nanoporous catalyst layer 4 .

[0082] For example, the at least one hydrophobic particle 41 may include or be one of carbon, sulfonated tetrafluoroethylene-based fluoropolymer copolymer (CAS number: 31175-20-9), titanium oxide, or poly(1,1,2,2-tetrafluoroethylene) (CAS number: 9002-84-0).

[0083] According to one embodiment of the present invention, the at least one proton conducting ionomer 40 and / or the at least one hydrophobic particle 41 and / or the ionic liquid 42 can be added, for example, by spraying, ultrasonic spraying, decal transfer, dipping, drop casting, and / or filtering.

[0084] Finally, in a fourth step of the method according to the invention, a catalytically active membrane or membrane-electrode assembly 20 can be formed by pressing and / or decal transferring a self-supporting nanoporous catalyst layer 4 on a substrate 5 with the membrane 10, or by pressing and / or decal transferring two self-supporting nanoporous catalyst layers 4 on respective substrates 5 with the membrane 10, the membrane 10 preferably being disposed between two self-supporting nanoporous catalyst layers 4.

[0085] According to one embodiment of the present invention, pressing and / or decal transfer can be carried out at a pressure between 1 bar and 75 bar and / or at a substrate temperature in the range of 115°C to 145°C, in particular for a time between 15 seconds and 600 seconds.

[0086] Optionally, if a transfer substrate 51 is used, the transfer substrate 51 can be peeled off from the catalytically active membrane 10 or membrane-electrode assembly 20 after pressing and / or decal transfer.

[0087] FIG. 2 relates to step i) of the method according to the invention, in which a heterogeneous layer 3 is formed on a substrate 5 , the heterogeneous layer 3 comprising a base metal 1 and a precious metal 2 .

[0088] For this purpose, the substrate 5 can be arranged on a substrate holder 6 .

[0089] According to an embodiment of the invention, the deposition of the base metal 1 and / or the precious metal 2 can be performed by PVD, which can in particular involve sputtering, in particular (alternating) magnetron sputtering 7.

[0090] As shown in FIG. 2, PVD can be carried out at pressures between 2 Pa and 10 Pa from a sputtering target 8 with a rectangular target area towards a substrate 5, in particular using an RF or DC plasma source.

[0091] In order to cover a large area of ​​the substrate 5, the substrate 5 can be moved back and forth with respect to a stationary point periodically in time, as indicated in particular by the arrows. For example, a substrate 5 having a lateral dimension L, in particular a width or length, can be moved back and forth with an offset of at least L / 4 with respect to the stationary point.

[0092] Moving the substrate 5 in this manner advantageously minimizes the target material required, especially in combination with a rectangular target area, and is scalable to a variety of sputtering sizes.

[0093] According to one embodiment of the present invention, the deposition of base metal 1 and precious metal 2 can be alternated at least three times. That is, the deposition of base metal 1 and precious metal 2 is performed sequentially in a series of cycles to achieve a desired loading at a specific layer thickness. Preferably, the weight ratio between the deposited base metal 1 and the deposited precious metal 2 can be between 0.5:1 and 20:1. These parameters advantageously result in a highly heterogeneous layer that can be further processed into a self-supporting nanoporous catalyst layer 4.

[0094] FIG. 3 shows an embodiment of the method according to the invention, and in particular relates to steps i) and ii) of the method according to the invention.

[0095] In this embodiment, the base metal 1 and the precious metal 2 can be deposited by magnetron sputtering 7 on a substrate 5 mounted on a substrate holder 6, where the substrate 5 mounted on the substrate holder 6 is moved relative to a sputtering target 8. The deposition of the base metal 1 and the precious metal 2 can be performed alternately by moving the substrate 5 mounted on the substrate holder 6 relative to the respective sputtering targets 8 containing the base metal 1 and the precious metal 2, and then starting the sputtering process.

[0096] The resulting heterogeneous layer 3 can be further processed into a self-supporting nanoporous catalyst layer 4 by leaching the base metal 1 from the heterogeneous layer 3, such that a self-supporting nanoporous catalyst layer 4 comprising the precious metal 2 is formed.

[0097] FIG. 4 shows an embodiment of the method according to the invention, referring to the leaching of base metals 1 from a heterogeneous layer 3, as proposed by step ii) of the method according to the invention.

[0098] Preferably, leaching is carried out in an electrolyte 35 arranged between a first leaching electrode 31 and a second leaching electrode 32, the heterogeneous layer 3 and the first leaching electrode 31 being electrically connected and a leaching current being applied between the first leaching electrode 31 and the second leaching electrode 32, such that the base metal 1 is leached from the heterogeneous layer 3. The heterogeneous layer 3 on the substrate 5, in particular on the transfer substrate 51, is preferably arranged in the electrolyte 35. The potential in the electrolyte 35 can be further controlled by an optional third leaching electrode 33.

[0099] The electrolyte 35 may include at least one acid, such as HClO4, H2SO4, or HNO3. The electrolyte 35 may also include at least one base, such as KOH or NaOH. The concentration of the acid or base in the electrolyte 35 may be in the range of, for example, 0.1 mol / l to 8 mol / l.

[0100] In the leaching step, the leaching current is preferably applied between the first leaching electrode 31 and the second leaching electrode. The third leaching electrode 33 can function as a reference electrode located closer to the first leaching electrode 31 than the second leaching electrode 32. In this way, the third leaching electrode 33 can be configured to control the potential of the first electrode 31 and to allow voltage measurement between the first electrode 31 and the third electrode 33. The third leaching electrode 33 can comprise a platinum wire in a hydrogen atmosphere. Preferably, for the leaching process, the leaching current between the first leaching electrode 31 and the second leaching electrode 32 can be controlled such that the potential difference between the first leaching electrode 31 and the third leaching electrode 33 corresponds to a predetermined voltage, preferably within the range of -0.2V to 1.5V.

[0101] Preferably, the first leaching electrode 31 connected to the heterogeneous layer 3 can be kept at a positive potential in order to avoid redeposition of the base metal 1 on the resulting self-supporting nanoporous catalyst layer 4. The voltage between the first and third reference electrodes 31 and 33 and the leaching current between the first and second electrodes 31 and 32 can be constant or can alternate in time, in particular can alternate periodically in time.

[0102] To avoid cross-contamination of the leached cations, the electrolyte 35 is preferably at least partially exchanged or constituted flow during step ii).

[0103] According to one embodiment of the present invention, leaching is carried out at a temperature of the electrolyte 35 between 10°C and 95°C.

[0104] Preferably, the first, second and third leaching electrodes 31, 32, 33 and the electrolyte 35 may be disposed at least partially within the leaching chamber 30, as shown in FIG.

[0105] The leaching chamber 30 may further comprise an inlet 34 and optionally an outlet (not shown) for conducting and controlling the flow of non-reactive (inert) gases, particularly N2 and / or Ar, or reactive gases, particularly H2 and / or O2, through the leaching chamber 30 to create desired electrochemical conditions within the leaching chamber 30.

[0106] The leaching chamber 30 may be configured such that a third leaching electrode 33, which may function as a reference electrode, is at least partially disposed in the electrolyte 35 so as to control the potential in the electrolyte 35. The first and second leaching electrodes 31, 32 may be disposed such that the electrolyte 35 is located between the first leaching electrode 31 and the second leaching electrode 32. In particular, the first leaching electrode 31, which is preferably connected to the heterogeneous layer 3, may be disposed in a floating state on top of the electrolyte 35, as shown in FIG. 4. The second leaching electrode 32 may be disposed, for example, at the bottom of the leaching chamber 30. The inlets 34 and outlets for any flow of non-reactive or reactive gases may be disposed on top of the electrolyte 35, in particular on top of the first leaching electrode 31, so as to establish the desired electrochemical conditions in the leaching chamber 30, such that the flow is directed over the electrolyte 35 and to the top of the first electrode 31 facing away from the electrolyte 35.

[0107] A schematic cross-section of one embodiment of a membrane-electrode assembly 20 according to the invention is shown in Figure 5. In this example, the membrane-electrode assembly 20 comprises two nanoporous catalyst layers 4-1, 4-2 disposed between a membrane 10 and a respective first or second gas-permeable electrode layer 5-1, 5-2.

[0108] The first nanoporous catalyst layer 4-1 consists of a plurality of grains 100 (indicated by the black spaces in the fill pattern of the corresponding boxes) forming the nanoporous catalyst layer 4-1. Between the grains 100 there are gaps 101 (white spaces in the fill pattern) which allows a relatively large surface area of ​​each grain 100 accessible such that the catalytic compound provided by the grains 100 is highly exposed to the surroundings of the grain 100, as compared to, for example, a continuous or quasi-continuous catalyst layer.

[0109] The assembly 20 essentially forms a stack which can be used in an electrolyser or fuel cell.

[0110] In Fig. 6 a scanning electron microscope (SEM) image of one embodiment of the assembly 20 is shown. The image is taken at a cross-section of a layer of the assembly. The recording parameters are an electron acceleration energy of 15 kV and a magnification of 15.000x. The recording was taken at a working distance of 7.8 mm. The pixel size of the image corresponds to 7 nm and a scale bar showing a length of 2 μm is superimposed on the image.

[0111] The top panel A) shows the SEM recording, the bottom panel B) shows the same area recorded with false colours indicating the compounds detected.

[0112] As can be seen, the first nanoporous catalyst layer 4-1 is formed by a number of grains 100, which in this example are made of iridium. Each grain 100 can be recognized as a separate entity or particle of the catalyst layer 4-1, even if the grains are partially connected. An equivalent spherical diameter can be associated with the grains 100 so that differences in grain shape can be compared and measured. Gaps 101 extend and at least partially separate the grains 100 from one another, thereby exposing a relatively large surface area around the grains 100. The gaps may be filled with the compound of the membrane 10.

[0113] The relevant thickness of the catalyst layer is of the order of 2 μm, which is relatively thinner than the layers known from the prior art [1], [2].

[0114] Figure 7 shows a histogram of the relative occurrence of particle sizes based on the determined equivalent spherical diameters of the particles, demonstrating the heterogeneity of the particle size distribution and the granular structure of the catalyst layer. The median particle size from this histogram is in the range of 450 nm to 750 nm, while the particle size range is approximately 50 nm to 1,300 nm.

[0115] References: [1]Xinrong Zhang, Wei Zhang, Weijing Yang, Wen Liu, Fanqi Min, Samuel S. Mao, and Jingying Xie, “Catalyst-coated proton exchange membrane for hydrogen production with high pressure water electrolysis”, Appl. Phys. Lett. 119, 123903 (2021) https: / / doi.org / 10.1063 / 5.0060150 [2]Maximilian Bernt et al., 2018 J. Electrochem. Soc. 165 F305 DOI 10.1149 / 2.0641805jes, “Analysis of Voltage Losses in PEM Water Electrolyzers with Low Platinum Group Metal Loadings”

Claims

1. A method for manufacturing a catalytically active membrane-electrode assembly (20) with one or more electrodes, in particular two electrodes, comprising at least the following steps: i) depositing a heterogeneous layer (3) on a substrate (5), the heterogeneous layer (3) comprising a base metal (1) and a noble metal (2) distributed heterogeneously in the heterogeneous layer (3); ii) leaching the base metal (1) from the heterogeneous layer (3) so as to form a first self-supporting nanoporous catalyst layer (4) comprising the noble metal (2) on the substrate (5); iii) adding at least one proton-conducting ionomer (40) and / or at least one hydrophobic particle (41) and / or an ionic liquid (42) to the first self-supporting nanoporous catalyst layer (4); and iv) forming a catalytically active membrane-electrode assembly (20) by attaching a self-supporting nanoporous catalyst layer (4) to a first side of the membrane (10) to form a catalytically active membrane-electrode assembly (20) comprising one electrode. The method comprising:

2. 10. The method of claim 1, wherein the second self-supporting nanoporous catalyst layer (4) on the substrate (5) is formed by steps ii) and iii), for example either simultaneously or sequentially with the first self-supporting nanoporous catalyst layer (4), or by separating a portion of the first self-supporting nanoporous catalyst layer (4) to form the second self-supporting nanoporous catalyst layer (4), and wherein the second self-supporting nanoporous catalyst layer (4) is attached to the second side of the membrane (10) in step iv) or a subsequent step to form a catalytically active membrane-electrode assembly (20) comprising two electrodes.

3. 3. The method according to claim 1 or 2, wherein the attachment of the first and / or second self-supporting nanoporous catalyst layer (4) is carried out by pressing a substrate (5) carrying the first and / or second self-supporting nanoporous catalyst layer (4) onto the membrane (10) and / or by decal transfer of the first and / or second self-supporting nanoporous catalyst layer (4) from the substrate (5) to the membrane (10).

4. 4. The method according to claim 1, wherein the at least one proton-conducting ionomer (40) and / or the at least one hydrophobic particle (41) and / or the ionic liquid (42) are added to the first and / or second self-supporting nanoporous catalyst layer (4) by at least one of the following methods: spraying, ultrasonic spraying, decal transfer, dipping, drop casting, filtering.

5. 5. The method according to any one of claims 1 to 4, wherein the substrate (5) comprises or is an electrically non-conductive or conductive transfer substrate (51) configured for decal transfer of the first and / or second self-supporting nanoporous catalyst layer (4) onto the membrane (10), and / or a gas permeable electrode (50).

6. 6. The method according to any one of claims 1 to 5, wherein in step i) the base metal (1) and / or the noble metal (2) are deposited by physical vapor deposition (PVD).

7. 7. The method according to claim 6, characterized in that the PVD comprises or is a sputtering method, in particular an alternating magnetron sputtering method (7).

8. 8. Method according to claim 6 or 7, characterized in that during PVD the substrate (5) is moved back and forth along at least one direction.

9. 9. The method according to claim 1, characterized in that step ii) is carried out in an electrolyte (35) arranged between a first leaching electrode (31) and a second leaching electrode (32), wherein the heterogeneous layer (3) and the first leaching electrode (31) are electrically connected and a leaching current is applied between the first (31) and the second (32) leaching electrodes so that the base metal (1) is leached from the heterogeneous layer (3), and wherein the heterogeneous layer (3), in particular on a substrate (5), in particular on a transfer substrate (51), is arranged in the electrolyte (35).

10. 10. The method of claim 9, wherein the potential of the first leaching electrode (31) is controlled by a third leaching electrode (33) immersed in the electrolyte adjacent to the first leaching electrode (31), wherein the voltage between the first leaching electrode (31) and the third leaching electrode (33) is determined.

11. 11. A method according to claim 10, characterized in that during step ii) the leaching current between the first (31) and second (32) leaching electrodes is controlled so that the voltage between the first (31) and third (33) leaching electrodes reaches a selectable value.

12. Step ii) is carried out in a leaching chamber (30), which comprises: a first leaching electrode (31) electrically connected to the heterogeneous layer (3), in particular to the heterogeneous layer (3) deposited on the substrate (5); a second leaching electrode (32), and an electrolyte (35) placed between the first leaching electrode (31) and the second leaching electrode (32); Equipped with A method according to any one of claims 9 to 11, characterized in that the heterogeneous layer (3) is arranged between a first leaching electrode (31) and a second leaching electrode (32).

13. At least during step ii), at least one non-reactive gas, in particular N ２ 13. The method according to claim 12, characterized in that a flow of HCl and / or Ar is carried out through the leaching chamber (30).

14. At least during step ii), at least one reactive gas, in particular H ２ Or O ２ 14. The method according to claim 12 or 13, characterized in that the flow of the sulphur dioxide through the leaching chamber (30), in particular above the first leaching electrode (31).

15. A membrane-electrode assembly (20) for a proton exchange membrane (PEM) fuel cell or electrolyzer, comprising: a first gas-permeable electrode layer (5, 5-1), a first self-supporting nanoporous catalyst layer (4, 4-1) comprising a noble metal (2), - membrane (10) and Equipped with The membrane-electrode assembly (20) comprises a first self-supporting nanoporous catalyst layer (4, 4-1) extending between a first gas-permeable electrode layer (5, 5-1) and a membrane (10), the first self-supporting nanoporous catalyst layer (4, 4-1) being characterized by being formed by a plurality of grains (100) of a first catalyst compound, wherein gaps (101) are formed between the grains (100) to increase the surface area of ​​the first self-supporting nanoporous catalyst layer (4, 4-1) to enhance catalytic reaction, and wherein the median equivalent spherical diameter of the grains is in the range of 450 nm to 750 nm, while the range of equivalent spherical diameter of the grains is in the range of 200 nm to 1300 nm.

16. 16. The membrane-electrode assembly (20) of claim 15, wherein the assembly further comprises a second self-supporting nanoporous catalyst layer (4, 4-2) disposed on the second gas permeable electrode layer (5, 5-2) on a side of the membrane (10) opposite the first self-supporting nanoporous catalyst layer (4, 4-1), such that the second self-supporting nanoporous catalyst layer (4, 4-2) extends between the second gas permeable electrode layer (5, 5-1) and the membrane (10), and wherein the second self-supporting nanoporous catalyst layer (4, 4-2) is formed by a plurality of grains comprising a second catalytic compound, with gaps formed between the grains to increase the surface area of ​​the second self-supporting nanoporous catalyst layer (4, 4-1) for enhancing catalytic reaction.

17. A membrane-electrode assembly (20) as described in claim 15 or 16, wherein the median equivalent spherical diameter of the multiple particles (100) of the second, or the first and second self-supporting nanoporous catalyst layers (4-1, 4-2) is in the range of 450 nm to 750 nm.

18. A membrane-electrode assembly (20) described in any one of claims 15 to 17, wherein the spherical equivalent diameter of the multiple particles of the second, or the first and second self-supporting nanoporous catalyst layers, is in the range of 200 nm to 1300 nm.

19. 19. The membrane-electrode assembly (20) according to any one of claims 15 to 18, wherein the ratio of the volume comprising voids (101) to the volume comprising grains (100) of the first and / or second self-supporting nanoporous catalyst layers (4, 4-1, 4-2) is in the range of 0.3 to 5.

20. The membrane-electrode assembly (20) of any one of claims 15 to 19, wherein the first catalytic compound comprises or is iridium.

21. A membrane-electrode assembly (20) according to any one of claims 15 to 20, wherein the first gas-permeable electrode layer (5-1) comprises or is titanium (5-1).

22. A membrane-electrode assembly (20) according to any one of claims 16 to 21, wherein the second catalytic compound comprises or is platinum.

23. A membrane-electrode assembly (20) according to any one of claims 16 to 22, wherein the second gas-permeable electrode layer (5-2) comprises or is carbon (5-1).

24. The membrane-electrode assembly (20) according to any one of claims 15 to 23, wherein the membrane (10) is a proton-permeable membrane configured to retain at least hydrogen and oxygen.

25. The membrane-electrode assembly (20) according to any one of claims 15 to 24, wherein the assembly (20) is formed using a method according to any one of claims 1 to 14.

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