Membrane electrode assembly, method for manufacturing same, and use thereof

Abstract

The invention relates to a membrane electrode assembly (1) comprising a polymer electrolyte membrane (2) and at least one electrode (3) arranged on the polymer electrolyte membrane (2), the membrane electrode assembly (1) comprising silver and / or a silver-containing compound (4). The invention also relates to: an electrolysis cell; a cell stack; an electrolysis system; a method (100) for manufacturing a membrane electrode assembly (1); and uses of the membrane electrode assembly (1).

Description

[0001] Description

[0002] Membrane electrode assembly, process for its manufacture and its use

[0003] The invention relates to a membrane electrode assembly with a polymer electrolyte membrane (PEM) and at least one electrode arranged on the polymer electrolyte membrane, an electrolysis cell, a cell stack, an electrolysis system, a method for producing a membrane electrode assembly and uses of a membrane electrode assembly.

[0004] Hydrogen can be produced by electrolysis of water, using various technologies. One technology that is already industrially established is polymer electrolyte membrane water electrolysis (PEMWE). The core element here is a membrane electrode assembly with a polymer electrolyte membrane. The polymer electrolyte membrane usually functions as a proton exchange membrane, i.e., protons are transported through the polymer electrolyte membrane. Alternatively, the polymer electrolyte membrane can be designed as an anion-conducting polymer electrolyte membrane, allowing hydroxide anions to be transported through the polymer electrolyte membrane.

[0005] The membrane divides the electrolysis cell into two half-cells, the anode side and the cathode side. The electrodes are typically applied to the membrane as catalyst layers, with platinum on carbon typically used on the cathode side, while metallic iridium, which is at least partially converted to iridium oxide during electrolysis, or iridium oxide is used on the anode side.

[0006] Membrane electrode assemblies are typically manufactured using a so-called decal process, in which the catalyst layers are applied to a transfer film and the membrane is sandwiched between them. The catalyst layers are then typically bonded to the membrane by continuous hot pressing. As a result, the catalyst is deposited directly on the membrane, which can therefore also be referred to as a catalyst-coated membrane.

[0007] The electrodes are electrically contacted via a porous transport layer or gas diffusion layer. The porous layers serve to supply the electrolyte and / or reactants to the electrodes. The proton exchange membrane is operated with deionized water. The anion exchange membrane will, in the future, be operated with deionized water. Currently, dilute alkalis of around 1 mol / l are used to support the ionic contact. Materials for the gas diffusion layer are usually graphite fleece or carbon paper on the cathode side and sintered titanium plates or titanium metal-reinforced titanium fleece on the anode side.

[0008] In proton exchange membrane electrolysis, water is oxidized to molecular oxygen on the anode side, releasing protons and electrons. In anion exchange membrane electrolysis, however, hydroxide ions are oxidized to oxygen on the anode side. The membrane acts as an electrical insulator between the catalyst layers and is only able to conduct protons towards the cathodic catalyst layer or anions to the anode. On the cathode side, in proton exchange membrane electrolysis, the protons are reduced to molecular hydrogen. In anion exchange membrane electrolysis, hydrogen is formed from water at the cathode. The hydrogen and oxygen gases produced are removed from the electrolysis cell via the gas diffusion layers.However, the membrane, theoretically impermeable to the resulting gases, in practice allows a certain amount of undesirable gas exchange between the anode and cathode sides, known as gas crossover. Gas crossover reduces the overall efficiency of the electrolysis and can also pose a safety risk and lead to degradation effects.

[0009] For example, an oxygen crossover can lead to the formation of an explosive oxygen-hydrogen mixture as soon as at least 4 vol. -% oxygen is contained in the hydrogen or vice versa. Oxygen can also lead to the oxidation of components in the electrolysis cell, particularly components containing iron. Rust particles that form can settle in the electrolysis cell and lead to so-called microfouling. The cations released during such oxidation can also react with the electrode or membrane material and lead to an increase in membrane resistance, which in turn can have a negative effect on the efficiency of the electrolysis. The membrane can also be damaged by radicals formed by oxygen.

[0010] In addition to these possible harmful effects of oxygen, the gas crossover leads to contamination of the product streams with the other gas, so that depending on the intended use, purification of the product gas stream may be necessary.

[0011] As a result, gas crossover should be prevented if possible. It is already known that platinum, which can be present as electrode material, for example, also catalyzes the recombination reaction of oxygen and hydrogen to form water and can therefore contribute to a reduction in gas crossover. Due to the high costs, however, there is a need to keep the required quantities of platinum as low as possible and to prevent gas crossover in other ways. The use of a silver-containing oxygen-consuming electrode, which is used under strongly alkaline conditions, is known from chlor-alkali electrolysis (DE 10 2010 042 729 Al). Here, the silver acts as a catalyst in the formation of hydroxide ions from the supplied oxygen and water. Autoprotolysis of the water and thus the formation of hydrogen can be avoided.

[0012] Against this background, it is the object of the present invention to provide possibilities with which the gas crossover through a membrane of a membrane electrode assembly can be reduced cost-effectively.

[0013] This problem is solved by the subject matter of the independent claims. The dependent claims relate to embodiments.

[0014] A first aspect of the invention relates to a membrane electrode unit with a polymer electrolyte membrane and at least one electrode arranged on the polymer electrolyte membrane, wherein the membrane electrode unit comprises silver and / or a silver-containing compound.

[0015] The polymer electrolyte membrane can, for example, be designed as a proton exchange membrane and comprise a perfluorinated copolymer with ionic groups, e.g., sulfonic acid groups, or consist of such a polymer. The use of fluorine-free polymers is also possible. Alternatively, the polymer electrolyte membrane can be designed as an anion exchange membrane. The invention is not limited to a specific material for the polymer electrolyte membrane. The electrode can be designed as a catalyst layer.

[0016] One or two electrodes are arranged on one or both sides of the polymer electrolyte membrane, which simultaneously serve as electrodes and catalyze the electrolysis reaction. The electrode or electrodes can preferably be in direct contact with the polymer electrolyte membrane or can be arranged directly on the polymer electrolyte membrane and, for example, as described in the introduction, can be connected to the membrane to form the membrane-electrode unit using a decal process. In particular, there is no gap between the membrane and the electrodes, as is the case in chlor-alkali electrolysis, where there is a gap filled with, for example, potassium hydroxide or sodium hydroxide solution between the membrane and the electrodes.

[0017] The membrane electrode assembly can be used to conduct electrolysis. The membrane electrode assembly can preferably be used for the electrolysis of water, e.g., for so-called PEM water electrolysis (PEMWE). The membrane electrode assembly can be used, for example, as a proton exchange membrane under non-alkaline conditions, particularly under neutral conditions, or as an anion exchange membrane under alkaline or neutral conditions.

[0018] It is intended that the membrane electrode assembly comprises silver and / or a silver-containing compound. This means that the membrane electrode assembly comprises elemental silver and / or one or more silver-containing compounds.

[0019] The inventors of the present invention suspect that the silver or the silver ions that can be formed from silver and the incoming oxygen primarily serve as an oxygen reduction catalyst, i.e. as a catalyst for the reaction 0.5 O2 + H2O + 2 e" 2 OH". Silver or .

[0020] Silver ions catalyze this reaction significantly better than platinum. Through the catalyzed conversion of oxygen, the negative effects of oxygen in the hydrogen product gas described above can be reduced or even completely avoided. Therefore, the membrane electrode assembly preferably contains such amounts of silver or silver-containing compounds that enable a sufficient catalytic effect. The silver mass fraction in the membrane electrode assembly can, for example, be between 1% and 50%, preferably between 10% and 30%.

[0021] By introducing silver or silver ions into the membrane-electrode assembly, particularly between the polymer electrolyte membrane and the electrode, oxygen that has undesirably permeated the membrane can be reduced. Contamination of the actual electrolysis product gas can thus be avoided, so that product gas purification is either unnecessary or only required to a reduced extent.

[0022] The problems initially caused by gas crossover can also be reduced. This can prevent the formation of an explosive gas-hydrogen mixture by reducing the oxygen content in the hydrogen, particularly to below 4 vol.%, preferably below 2 vol.%. Oxygen-based corrosion, microfouling, and membrane damage can also be reduced. The use of silver also represents a cost-effective way to improve product gas quality.

[0023] According to various embodiments, the silver and / or the silver-containing compound can be present at least partially in the form of nanoparticles.

[0024] The term "nanoparticles" refers to particles that are smaller than 100 nm in at least one dimension. For example, the nanoparticles can have a substantially spherical shape, with an average particle diameter of less than 100 nm, as determined, for example, by measuring microscopic images. By using nanoparticles, the active surface area of ​​the silver can be increased while maintaining the same mass, resulting in higher catalytic activity with the smallest possible amount of silver.

[0025] According to further embodiments, the silver-containing compound can be selected from a group comprising silver oxides, e.g. silver (I) oxide, silver complex compounds and silver halides, e.g. silver chloride.

[0026] The silver compounds mentioned offer a cost-effective way to introduce silver or silver ions into the membrane-electrode assembly. During electrolysis, at least partial reduction of the silver in the silver compound can occur, allowing elemental silver to be formed as the active species. The catalytic activity is therefore not necessarily dependent on whether silver or a silver-containing compound is used, nor on the type of silver-containing compound introduced.

[0027] The use of silver-containing compounds, especially silver oxides, also enables the creation of porous structures due to the reduction described during electrolysis and thus a higher catalytic effect for the same mass due to the increased surface area.

[0028] According to further embodiments, an electrode usable as a cathode can comprise silver and / or the silver-containing compound.

[0029] For example, the silver and / or the silver-containing compound can be applied to an electrode that can be used as a cathode. In this context, "usable as a cathode" means that the corresponding electrode serves as a cathode during electrolysis, so that, for example, during water electrolysis, hydrogen is formed at the electrode provided with the silver or the silver-containing compound. In this case, the silver or the silver ions can bring about a reduction in the oxygen that has undesirably passed through the membrane and is produced at the anode, thus reducing or preventing contamination of the hydrogen product stream with oxygen.

[0030] According to further embodiments, the silver and / or the silver-containing compound can be arranged between the polymer electrolyte membrane and the electrode.

[0031] In other words, a sequence of polymer electrolyte membrane - silver or silver-containing compound - electrode can result.

[0032] This arrangement advantageously ensures that the gas that undesirably passes through the membrane is converted immediately afterward, i.e., before reaching the electrode. This allows the gas crossover to be reduced particularly effectively.

[0033] In particular, the silver or the silver-containing compound can be applied as a silver coating to the electrode or catalyst layer, which is arranged in the membrane-electrode unit on the side of the electrode facing the membrane.

[0034] According to further embodiments, the electrode may comprise at least one precious metal catalyst.

[0035] The noble metal can be selected from a group comprising platinum, iridium, palladium, rhodium, gold, and ruthenium. The noble metal catalyst can comprise the noble metal(s) in metallic form or in a suitably chemically bonded form, e.g., as an alloy. A platinum-containing electrode can, in particular, be arranged on the cathode side, so that, if silver is present, silver and platinum can preferably interact on the cathode side.

[0036] In particular, an electrode containing iridium and / or iridium oxide can be arranged on the anode side, so that on the anode side, when silver is present, silver and iridium can preferably interact.

[0037] A further aspect of the invention relates to an electrolysis cell with a membrane electrode unit as described above.

[0038] A further aspect of the invention relates to a cell stack with a plurality of such electrolysis cells, i.e., comprising a plurality of such electrolysis cells stacked and electrically connected in series. These form a cell stack or electrolysis stack that is scalable for large electrolysis capacities.

[0039] A further aspect of the invention relates to an electrolysis plant with such a cell stack.

[0040] Embodiments, features and / or advantages which in the present case relate to the membrane electrode assembly also relate to the electrolysis cell, the cell stack, the electrolysis system and / or an entire power-to-X power plant, and vice versa.

[0041] A further aspect of the invention relates to a method for producing a membrane electrode assembly with a polymer electrolyte membrane and at least one electrode arranged on the polymer electrolyte membrane as an electrode, wherein the method comprises introducing silver and / or a silver-containing compound into the membrane electrode assembly. The membrane electrode assembly can in particular be designed as described above. The description of the membrane electrode assembly therefore equally serves to explain the proposed method. The advantages of the membrane electrode assembly are correspondingly associated with the method.

[0042] The membrane electrode assembly can preferably be manufactured using a decal process by means of which the electrode or electrodes are connected to the membrane to form the membrane electrode assembly.

[0043] The introduction of the silver and / or the silver-containing compound may comprise applying the silver and / or the silver-containing compound to the electrode and / or the polymer electrolyte membrane.

[0044] In other words, the silver or the silver-containing compound can be applied to the membrane and / or the electrode and / or introduced into the electrode.

[0045] The application of the silver and / or the silver-containing compound can preferably be carried out by means of a method selected from a group comprising chemical vapor deposition, physical vapor deposition, spin coating, spray coating, thermal spraying, dip coating, slot die coating, blade coating, roller coating, powder coating and printing.

[0046] Particularly preferably, the silver and / or the silver-containing compound can be applied by spray coating. For example, silver nanoparticles dispersed in isopropanol or another dispersant can be applied to the electrode by spraying, e.g., airbrush spray coating. A further aspect of the invention relates to the use of a membrane electrode assembly as described above for proton exchange membrane electrolysis or anion exchange membrane electrolysis.

[0047] In other words, an electrolysis process is proposed in which the membrane electrode assembly is used for electrolysis under non-alkaline conditions or alkaline conditions.

[0048] A further aspect of the invention relates to the use of a membrane electrode assembly as described above for the electrolysis of water.

[0049] In other words, an electrolysis process is proposed in which the membrane electrode assembly is used for the electrolysis of water. The electrolysis can be carried out under non-alkaline conditions, preferably under neutral conditions. For example, deionized water can be used for the electrolysis. Alternatively, water enriched with hydroxide ions can be used, so that the electrolysis is carried out under alkaline conditions.

[0050] The proposed uses are associated with the advantages of the membrane electrode assembly. The electrolysis cell, cell stack, and electrolysis system can be used accordingly.

[0051] The term "and / or" as used herein, when used in a series of two or more elements, means that any one of the listed elements may be used alone, or any combination of two or more of the listed elements may be used.

[0052] The above-described properties, features and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more clearly understandable in connection with the following description of the embodiments, which are explained in more detail in connection with the figures, wherein the figures show the following:

[0053] Figure 1 is a schematic representation of an exemplary membrane electrode assembly;

[0054] Figure 2 is a schematic representation of another exemplary membrane electrode assembly;

[0055] Figure 3 shows a flow chart of an exemplary

[0056] Method for producing a membrane electrode assembly;

[0057] Figure 4 shows a representation of the temporal course of the

[0058] Oxygen-gas crossovers; and

[0059] Figure 5 shows a representation of the temporal

[0060] Voltage curve .

[0061] Figure 1 shows schematically an embodiment of a membrane electrode unit 1 which can be used for the electrolytic conversion of water into hydrogen and oxygen under neutral conditions.

[0062] The membrane electrode assembly 1 has a polymer electrolyte membrane 2, which in the exemplary embodiment is made of Nafion®. Furthermore, the membrane electrode assembly 1 has an electrode 3, which serves as the cathode during electrolysis and is made of platinum. The electrode 3 or cathode is arranged on a gas diffusion layer 5, which in the exemplary embodiment is designed as a graphite fleece. A bipolar plate 6, which can also be made of graphite or stainless steel, is optionally connected to the gas diffusion layer 3.

[0063] To prevent oxygen passing through the polymer electrolyte membrane 2 from contaminating the hydrogen formed at the cathode and leading to the additional negative effects described above, a layer of silver and / or a silver-containing compound 4 is arranged between the polyelectrolyte membrane 2 and the electrode 3. In the exemplary embodiment, silver nanoparticles are used for this purpose.

[0064] Alternatively, the silver or the silver-containing compound 4 can be arranged elsewhere in the membrane electrode assembly 1, for example, between the electrode 3 and the gas diffusion layer 5 or as a component of the gas diffusion layer 5. In addition to silver nanoparticles, silver can additionally or alternatively be used in non-nanoparticulate form or in a compound, such as silver oxide, which is reduced under operating conditions.

[0065] The membrane electrode assembly 1 can also be constructed symmetrically, as shown in Figure 2. This means that electrodes 3a, 3b are arranged on both sides of the polymer electrolyte membrane 2, one of the electrodes 3a acting as the cathode and the other of the electrodes 3b acting as the anode. The anode can, for example, comprise iridium and / or iridium oxide or consist of iridium and / or iridium oxide. For further details, please refer to the explanation of Figure 1.

[0066] Alternatively or additionally, a layer with silver and / or a silver-containing compound 4 can also be arranged on the anode side, for example between the electrode 3b acting as the anode and the polymer electrolyte membrane 2, in order to catalyze a recombination of hydrogen that has passed through the polymer electrolyte membrane 2 with oxygen formed at the anode. On the side of the anode facing away from the polymer electrolyte membrane 2, a gas diffusion layer 5b, e.g. in the form of a titanium or steel fleece, can optionally also be arranged, analogous to the cathode side. The membrane electrode assembly 1 shown in Figure 2 can be produced, for example, by means of the method 100 explained below with reference to Figure 3. The method 100 can preferably be a so-called decal method.

[0067] In process step S1, an anode and a cathode are provided. For this purpose, a so-called decal substrate, e.g., a transfer film, can be coated with the catalyst material of the anode or cathode, so that the anode and cathode are provided as catalyst layers arranged on a decal substrate.

[0068] In process step S2, silver 4 is applied to the cathode in the form of silver nanoparticles. In the exemplary embodiment, this is done by spray coating with a dispersion of silver nanoparticles in isopropanol at a mass ratio of 1 to 40 (silver nanoparticles to isopropanol). The isopropanol is removed by drying after coating.

[0069] The cathode coated with silver nanoparticles is then punched to the required size, just like the anode and the polymer electrolyte membrane 2. Optionally, all three components of the membrane electrode assembly 1 can be preheated, e.g., to a temperature of 100 °C, to facilitate subsequent bonding.

[0070] In process step S3, the polymer electrolyte membrane 2 is arranged between the anode and the cathode, with the cathode being aligned such that the surface of the cathode coated with silver nanoparticles points in the direction of the polymer electrolyte membrane 2. All three components of the membrane-electrode assembly 1 are also arranged together between polyimide plates. In process step S4, the anode and the cathode are connected to the polymer electrolyte membrane 2 by applying pressure at an elevated temperature. This is done with the help of laminating rollers heated to approx. 160 °C, which transfer pressure via the polyimide plates. For this purpose, a pressure of 4 bar is applied to the laminating rollers themselves using a pneumatic cylinder. The rotation speed of the laminating rollers is approx. 0.02 m / s.

[0071] The decal substrates can then be removed and the membrane electrode assembly 1 can be further processed, e.g. by arranging gas diffusion layers.

[0072] The mass of the applied silver nanoparticles is between 10% and 50%, preferably between 10% and 30%, based on the mass of the cathode applied to the polymer membrane 2 during the production of the membrane electrode assembly 1.

[0073] Figure 4 shows the oxygen content (volume percent) in the hydrogen product stream, i.e. the oxygen gas crossover to the cathode side, as a function of time for four different membrane electrode assemblies 1, designated by the letters A, B, C, and D. The determination was carried out by gas chromatography.

[0074] Membrane electrode assemblies A and B are identically manufactured reference examples without silver or silver-containing compound. Membrane electrode assemblies 1 C and D, on the other hand, have silver nanoparticles. They were manufactured using the method 100 described with reference to Figure 3 under identical conditions and are designed as shown in Figure 2. To determine the gas crossover, membrane electrode assemblies 1 were used in an electrolytic cell and the composition of the product gas streams was analyzed. Figure 4 shows that the oxygen crossover for the silver-containing samples C and D is significantly lower, i.e. approx. 0.04, and more stable over time compared to reference samples A and B, for which an oxygen crossover between 0.06% and 0.09% with widely scattered values ​​can be observed.It can therefore be concluded that by introducing silver, particularly in the form of silver nanoparticles, the oxygen content in the hydrogen product stream can be effectively reduced, i.e. by up to 45%, and the problems associated with gas crossover can be correspondingly reduced or even avoided.

[0075] Figure 5 shows the voltage curve as a function of time for the reference sample B and the silver-containing samples C and D at a constant current density of 2 A / cm 2 .

[0076] Similar behavior can be observed for all samples. It can therefore be concluded that the introduction of silver nanoparticles does not have any significant negative effects on the overpotential.

[0077] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited to the disclosed examples and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

[0078] The invention relates to a membrane electrode assembly 1 with a polymer electrolyte membrane 2 and at least one electrode 3 arranged on the polymer electrolyte membrane 2, wherein the membrane electrode assembly 1 comprises silver and / or a silver-containing compound 4. The invention further relates to an electrolysis cell, a cell stack, an electrolysis system, a method 100 for producing a membrane electrode assembly 1, and uses of the membrane electrode assembly 1. List of reference symbols

[0079] 1 membrane electrode assembly

[0080] 2 polymer electrolyte membrane 3, 3a, 3b electrode

[0081] 4 Silver / silver-containing compound

[0082] 5, 5a, 5b gas diffusion layer

[0083] 6 bipolar plate 100 procedures

[0084] 51 Providing an anode and a cathode

[0085] 52 Applying silver to the cathode

[0086] 53 Placing a polymer electrolyte membrane between the anode and the cathode

[0087] 54 Connecting the anode and the cathode to the

[0088] Polymer electrolyte membrane by pressing

Claims

Patent claims 1. Membrane electrode assembly (1) with a polymer electrolyte membrane (2) and at least one electrode (3, 3a, 3b) arranged on the polymer electrolyte membrane (2), wherein the membrane electrode assembly (1) comprises silver and / or a silver-containing compound (4).

2. Membrane electrode assembly (1) according to claim 1, wherein the silver and / or the silver-containing compound (4) is at least partially in the form of nanoparticles.

3. Membrane electrode assembly (1) according to one of the preceding claims, wherein the silver-containing compound (4) is selected from a group comprising silver oxides, silver complex compounds and silver halides.

4. Membrane electrode assembly (1), wherein an electrode (3, 3a) usable as a cathode comprises the silver and / or the silver-containing compound (4).

5. Membrane electrode assembly (1) according to one of the preceding claims, wherein the silver and / or the silver-containing compound (4) are arranged between the polymer electrolyte membrane (2) and the electrode (3).

6. Membrane electrode assembly (1) according to one of the preceding claims, wherein the electrode (3, 3a, 3b) comprises at least one noble metal catalyst.

7. Membrane electrode assembly (1) according to claim 6, wherein the noble metal is selected from a group comprising platinum, iridium, palladium, rhodium, gold and ruthenium.

8. Electrolysis cell comprising a membrane electrode assembly (1) according to one of claims 1 to 7.

9. A cell stack comprising a plurality of electrolysis cells according to claim 8.

10. Electrolysis plant with a cell stack according to claim 9.

11. Method (100) for producing a membrane electrode assembly (1) with a polymer electrolyte membrane (2) and at least one electrode (3, 3a, 3b) arranged on the polymer electrolyte membrane (2), wherein the method (100) comprises introducing silver and / or a silver-containing compound (4) into the membrane electrode assembly (1).

12. The method (100) according to claim 11, wherein the introduction of the silver and / or the silver-containing compound (4) comprises applying the silver and / or the silver-containing compound (4) on the electrode (3, 3a, 3b) and / or the polymer electrolyte membrane (2).

13. The method (100) according to claim 12, wherein the application of the silver and / or the silver-containing compound (4) is carried out by means of a method selected from a group comprising chemical vapor deposition, physical vapor deposition, spin coating, spray coating, thermal spraying, dip coating, slot die coating, blade coating, roller coating, powder coating and printing.

14. Use of a membrane electrode assembly (1) according to one of claims 1 to 7 for proton exchange membrane electrolysis or anion exchange membrane electrolysis.

15. Use of a membrane electrode assembly (1) according to one of claims 1 to 7 for the electrolysis of water.

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