Method for recovering catalyst material from membrane electrode assembly for water electrolysis

Abstract

The invention relates to a method for recovering a catalyst material from a membrane electrode assembly (10) for water electrolysis, comprising the following steps: a step (1) of providing a membrane electrode assembly (10) comprising a membrane (24) coated with a metal catalyst material (20, 22); a step (2) of fragmenting the membrane electrode assembly (10); a step (3) of pyrolysis decomposition of the fragmented membrane electrode assembly (10), in which a solid pyrolysis product is obtained as a residue; (4) dissolving the solid pyrolysis product in a mixture containing concentrated hydrochloric acid and concentrated nitric acid; a step (5) of removing nitrate by heating the solution to 100 DEG C to 110 DEG C; a step (6) of filtering insoluble residues; a step (7) of drying the insoluble residue at a drying temperature (TD) wherein the metal catalyst material (20) is recovered. The method is preferably used for recovering a membrane electrode assembly (10) for PEM water electrolysis, in which iridium is recovered as a metal catalyst material (20).

Description

Technical Field

[0001] The present invention relates to a method for recovering catalyst material from a membrane electrode assembly for water electrolysis. Here, a used membrane electrode assembly is provided, which contains a membrane coated with a metal catalyst material. Background Art

[0002] Hydrogen is an important substance that has numerous applications in industry and technology. Hydrogen usually exists only in a bound state on Earth. One substance that contains bound hydrogen is water. In addition, hydrogen can be used as an energy store, in particular to store electrical energy generated by renewable energy production methods for later use.

[0003] An important method for obtaining hydrogen is the electrolysis of water, in particular when using electrical energy. In this case, hydrogen can be used as an energy storage unit in particular in the following way, that is, it is used, for example, as a fuel in order to stabilize the supply of electrical energy, in particular from renewable energy sources such as wind energy, photovoltaics, etc. However, hydrogen can also be used in other processes that require fuels or reducing agents. Thus, the hydrogen obtained in the electrolysis can be used, for example, in industry or can be used to generate electrical energy again electrochemically when using fuel cells.

[0004] Suitable electrolyzers can be used to decompose water into its chemical components hydrogen and oxygen. For this purpose, the electrolyzer can be designed as a so-called polymer-electrolyte-membrane-electrolyzer. In such an electrolyzer, a membrane is usually provided, which has a corresponding catalyst layer on the surface facing away from each other. Adjacent to the catalyst layer is usually a corresponding gas diffusion layer, and adjacent to the gas diffusion layer itself is a corresponding conductive contact plate, sometimes also referred to as a bipolar plate, which is particularly used for electrical contact. Preferably, the contact plate or bipolar plate is also designed at the same time so that they can achieve the required material transport in the prescribed operation during the electrolysis in the electrolyzer. For this purpose, corresponding channels can be provided to introduce corresponding suitable electrolytes and to draw out the reaction products of the electrolysis, i.e. hydrogen and oxygen. The gas diffusion layer usually provides electrical conductivity so that the contact plate and the catalyst layer are electrically coupled to each other. Therefore, the desired electrochemical reaction can be achieved in the region of the catalyst layer.

[0005] If the electrolysis reaction is a reaction in the alkaline region, an anion exchange membrane (AEM) is provided as the membrane. On the other hand, if the electrolysis reaction occurs in the acidic region, a proton exchange membrane (PEM) is provided instead.

[0006] The production of hydrogen from water is carried out by electrolysis. This is an electrochemical process in which water is decomposed into its chemical components oxygen and hydrogen. Depending on the mode of operation, the electrochemical cell reactions can be described and differentiated as follows:

[0007] Alkaline electrolysis:

[0008] Anode electrode <![CDATA[40H - →2H2O+O2+4e - ]]> (1) Cathode electrode <![CDATA[2H2O+2e - →2OH - +H2]]> (2)

[0009] Acid electrolysis:

[0010] Anode electrode <![CDATA[2H2O→4H + +O2+4e - ]]> (1) Cathode electrode <![CDATA[4H + +4e - →H2]]> (2)

[0011] In polymer-electrolyte-membrane-electrolysis, the two corresponding sub-reactions are spatially separated by an ion-conducting membrane. For electrolysis in the alkaline region, an anion exchange membrane (AEM) is provided, while for electrolysis in an acidic environment, a proton exchange membrane (PEM) is provided. However, in both cases, the structural design of the membrane electrode assembly (MEA) can be designed essentially similarly.

[0012] The core of such a polymer electrolyte membrane electrolyzer is usually formed by a membrane electrode assembly (MEA). The membrane electrode assembly has at least a membrane, which is usually coated with a layer containing a corresponding catalyst material on two surfaces facing away from each other on the anode side and the cathode side. The corresponding cell reaction of the electrolysis takes place in the region of the layer formed by the corresponding catalyst material. During the prescribed operation, the electrons are conducted to the contact plate via the corresponding catalyst material and the support structure, which can be formed by a gas diffusion layer or can provide a gas diffusion layer. For this reason, a high electrical conductivity of the catalyst layer is desired.

[0013] When reacting in the alkaline region, in addition to electrons, hydroxide anions OH are generated. - , while in the acidic region, protons H + , which move through the respective membrane as charge carriers. In this context, therefore, good conductivity of the structure of the layer consisting of the respective catalyst material is desired in order to facilitate the respective ions to reach the membrane and from the membrane to the respective catalyst material. This poses the technical challenge of simultaneously providing good ionic bonding of the catalyst material to the membrane and good electrical conductivity of the catalyst material.

[0014] In the prior art, a membrane electrode assembly or unit is produced by providing a catalyst material in the form of a paste, which is applied directly to the corresponding surface of the membrane or to the corresponding surface of the substrate. The catalyst material in the form of a paste or slurry contains the catalyst material itself, an ionomer, a polymer binder and a solvent. After application, the solvent is usually removed or driven out thermally, for which purpose the layer containing the catalyst material is pressed with the membrane at high temperatures, which are usually above 100° C. This ensures ionic contact of the ionomer and the catalyst material with the membrane.

[0015] A membrane electrode assembly comprises a membrane which has a corresponding catalyst material on two surfaces facing away from each other. In many applications, especially in PEM water electrolysis, very expensive and therefore very rare noble metals are used as catalyst materials. Therefore, in addition to the ecological aspects that contribute to the circular economy, it is of great economic interest to process used membrane electrode assemblies in order to recover the valuable metal catalyst material and use it in new membrane electrode assemblies if necessary or to supply it to other applications. Summary of the invention

[0016] The object of the present invention is to specify a method for recovering catalyst material from a membrane electrode assembly which allows a particularly simple and cost-effective recovery of the catalyst material.

[0017] According to the invention, this object is achieved by a method for recovering catalyst material from a membrane electrode assembly for water electrolysis, the method comprising the following steps:

[0018] - providing a membrane electrode assembly comprising a membrane coated with a metal catalyst material,

[0019] - fragmentation of membrane electrode assemblies,

[0020] - pyrolytically decomposing the fragmented membrane electrode assembly, wherein a solid pyrolysis product is obtained as a residue,

[0021] - dissolving the solid pyrolysis product in a mixture containing concentrated hydrochloric acid and concentrated nitric acid,

[0022] - Removal of nitrates by heating the solution to 100°C to 110°C,

[0023] - Filter out the insoluble residue,

[0024] - Drying the insoluble residue at a drying temperature, wherein the metal catalyst material is recovered.

[0025] Here, the invention has been based on the general basic goal that recycling makes a significant contribution to protecting the environment. Recycling means sustainability, since raw material resources do not exist forever. Recycling means money, since metal catalyst materials are usually precious metals that are too valuable not to be recycled. On the other hand, in the technical large-scale use of electrolyzers for water electrolysis, it can be expected that the demand for membrane electrode assemblies will grow rapidly, so that the economy of the known recycling methods will also be increasingly required from a cost point of view and in terms of efficiency.

[0026] Although laboratory scale processing and separation methods have been well described in the scientific literature, they are too complex in terms of process control to be implemented economically and on a large scale and are therefore very costly.Therefore, there is a need for simple and cost-effective recovery of metal catalyst materials, which has been recognised by the present invention.

[0027] By using the method of the invention, it is possible to very effectively recover especially metal catalyst materials, especially precious metals. The catalyst material on the membrane of the membrane electrode assembly can, for example, have one or more of the following substances on the anode side, namely nickel-aluminum, nickel-zinc, cobalt-aluminum, cobalt-iron, nickel-iron, nickel-iron-vanadium, nickel-cobalt, nickel-molybdenum, nickel-iron-double hydroxide, nickel-iron-cobalt, iridium, ruthenium oxide, nickel hydroxide, nickel oxide and nickel.

[0028] The catalyst material on the membrane may for example have one or more of the following on the cathode side, namely nickel, nickel-molybdenum on carbon black, nickel-molybdenum, nickel-platinum, platinum, nickel on carbon black, nickel phosphate, nickel-vanadium.

[0029] Here, the used membrane electrode assembly is first mechanically shredded to a predetermined degree of shredding. The economic and technical service life of ion-conducting membranes is limited by various influencing factors. For example, degradation effects on the membrane have been described, which damage the membrane material and impair its function. Due to the harsh electrochemical conditions that always occur in water electrolysis, small amounts of H2O2 or OH radicals can be formed in the electrolysis device, for example in the cell area. It is well known that such substances can chemically attack the membrane material of the PEM electrolysis device, wherein fluoride is released as a degradation product in this decomposition. This is because the membrane has a fluorine component and consists, for example, of PFSA "perfluorosulfonic acid". The membrane is a particularly important element for the function of the electrolysis cell in PEM electrolysis, so that its service life and its limitation due to degradation effects are of great concern, especially from an economic point of view. On the other hand, the metal catalyst material is hardly consumed and is not easily degraded, so it is very advantageous to handle and recycle it.

[0030] The invention is characterized in this context by a significantly simplified process which, as surprisingly has been shown, requires only a few separation steps for separating the noble metals from the high-quality catalyst material.

[0031] After the membrane electrode assembly has been shredded, pyrolysis first occurs. Pyrolysis or pyrolytic decomposition refers to various thermochemical conversion processes in which organic compounds are cracked at high temperatures and largely in the absence of oxygen. Some chemical bonds in the starting material are cracked at high temperatures, wherein the lack of oxygen prevents complete combustion. The resulting products are diverse. In the method according to the invention, a solid pyrolysis product, ash, is obtained as a residue, which is present in a suitable particle size or particle size and contains metal catalyst material. In this case, the ash contains a mixture of catalyst material used on the anode side and catalyst material used on the cathode side.

[0032] In the pyrolysis, a complex product mixture of solid, liquid and gaseous products is usually produced, wherein the exact ratio depends on the specific conditions and the starting material. Basically, it can be said that the higher the temperature and the longer the pyrolysis time, the more gaseous products are obtained, and the lower the temperature and the shorter the pyrolysis time, the more liquid products are obtained. If polymers are pyrolyzed, the corresponding monomers usually appear as products. Since these products have a high energy content, they can be used both as secondary energy carriers in terms of energy and further used as materials by obtaining various chemicals therefrom. In the present method, higher temperatures are preferred in order to pyrolyze and crack the fragmented membrane electrode units.

[0033] Preferably, the membrane is completely converted to the gaseous state during pyrolysis so that no carbon residues remain in the solid pyrolysis product, ash.

[0034] The solid pyrolysis product, ash, is now dissolved in aqua regia, thereby forming a solution. Aqua regia is a mixture of hydrochloric acid HCl and nitric acid HNO3 in a ratio of 3:1. Here, the process control according to the invention mainly distinguishes between catalyst materials that are insoluble in aqua regia and catalyst materials that are soluble in aqua regia, and separates them accordingly and continuously in only two subsequent steps. First, the solution is heated to 100° C. to 110° C., and the nitrates are thermally driven out of the solution here. Therefore, in this step, the noble metal nitrates are separated and the catalyst materials dissolved in the aqua regia are processed, which are preferably used on the cathode side of the membrane electrode assembly and applied to the intact membrane. These are noble metals that are soluble in aqua regia, for example, preferably platinum or binary platinum alloys (for example nickel-platinum), which are used as catalysts. Therefore, metal catalyst materials that are insoluble in aqua regia and are preferably used on the anode side of the membrane electrode assembly, such as iridium, are advantageously left in the residual solution after this process step. This solid component can now be separated very advantageously and easily in a filtration step and thus separated. An insoluble residue is thus present which consists mainly of the already high-value metallic catalyst material.

[0035] Finally, as a further step, the residue is dried at a drying temperature for a drying time, so that all liquid and possibly still gaseous components in the residue are removed or thermally driven out. The solid residue is now advantageously recovered, which contains the insoluble metal catalyst material or already consists essentially of it. The metal catalyst material can be directly processed (purified) and supplied for reuse, for example as anode-side catalyst material for application to new membranes.

[0036] The method described here makes it particularly advantageous to directly recover valuable anode precious metal catalysts (OER catalysts) from metal electrode assemblies of PEM water electrolysis without dissolving, melting or reprocessing them. In the method described herein, the number of process steps is significantly reduced, and loss paths in a large number of separation steps are avoided. From an economic point of view, the other complex reprocessing costs in separation (separation process) and the production costs for precipitation of new catalyst materials are eliminated. Here, the cathode catalyst material can also be advantageously reused without large expenditures. Here, the process control only distinguishes between catalyst materials that are insoluble in aqua regia and catalyst materials that are soluble in aqua regia, and effectively separates these materials in separate process steps.

[0037] Compared with the technical methods described so far, the process control of the present invention is technically and economically superior. These previous methods are, for example, based on the pyrolysis of the membrane, the dissolution of the catalyst or by completely dissolving the catalyst components in the solvent. Here, if an insoluble metal catalyst material is used, it is not necessary to dissolve it in aqua regia, because the insoluble anode noble metal catalyst material is insoluble in aqua regia. Therefore, the subsequent separation process required at this time for the separation of precious metals will include a complex and lossy process of Na2O2 oxidation melting, etc., so as to convert the metal catalyst material, for example, into a soluble oxide of the metal catalyst material. The present invention specifically avoids the lossy approach of treating the catalyst material by the conversion process using a soluble intermediate product. From the perspective of process technology, due to the numerous process steps when recovering the purest possible metal catalyst material, a higher process cost is thus caused. In addition, due to the corrosiveness of the chemicals used, expensive reactor linings are usually required. The present invention avoids this situation.

[0038] It has been advantageously shown that the pyrolysis step does not lead to any significant coarsening of the particle size compared to the new catalyst. Furthermore, it has surprisingly been shown that the metal catalyst material has sufficient purity and good quality after the drying step so that it can be used directly for the production of new membrane electrode assemblies. The method proposed here thus enables a significant reduction in the number of established process steps, thereby achieving significant cost and time savings in process management. Thus, due to the proposed process, in particular the recovery of precious metals can be increased.

[0039] In a preferred embodiment of the method, the insoluble residue is ground such that an average particle size of 10 μm to 80 μm, in particular 20 μm to 50 μm, is achieved.

[0040] After drying the insoluble residue at the drying temperature, the metal catalyst material is already in the form of solid particles. This residue can advantageously be subjected to a grinding process until the desired particle size distribution, in particular the average particle size, of the metal catalyst material is achieved. By means of the grinding process, a more favorable surface / volume ratio of the catalyst particles can be set, which facilitates the subsequent coating process of a new membrane with the recovered catalyst material while minimizing the amount of material used in the membrane electrode unit and facilitates a high catalytic efficiency.

[0041] The pyrolysis step is preferably carried out at a pyrolysis temperature of 600 to 1000° C., in particular 700 to 900° C. The high-temperature pyrolysis is thus carried out so that the components of the film are converted into gaseous products by the pyrolysis and do not remain in the solid pyrolysis product or residue but are driven off.

[0042] In a particularly preferred embodiment of the method, the solid pyrolysis product is dissolved in aqua regia at a temperature of 70° C. to 90° C., in particular at a temperature of 80° C., wherein this temperature is maintained during the dissolution for 3 to 5 hours, in particular for 4 hours. It has been shown that a pyrolysis temperature of 80° C. is particularly advantageous with an exposure time of 4 hours, so that good dissolution results of the soluble components in the solid pyrolysis product are achieved. However, this may deviate within certain limits depending on the batch, wherein a lower temperature requires a longer exposure time and vice versa.

[0043] Preferably, the metal components dissolved in the mixture of hydrochloric acid and nitric acid are driven off and separated by heating, wherein an insoluble residue is obtained. By means of the dissolution process in aqua regia, the soluble components are present, for example, in the form of metal salts, in particular in the form of metal nitrates. They are driven off from the solid residue by heating the solution to 100° C. to 110° C., i.e., they escape from the residue by degassing or evaporation. This material is advantageously collected for a complete recycling process of the soluble material.

[0044] In a particularly preferred embodiment of the method, platinum is separated and recovered as a dissolved metal component. Platinum can be advantageously used as a metal catalyst material on the cathode side of the membrane of a membrane electrode unit. Since platinum is an expensive noble metal, recovery makes economic sense. Platinum-containing binary metals, such as platinum-nickel, can also be used as soluble metal components and recycled. In membrane electrode units, platinum or platinum-nickel are particularly advantageous as catalyst metals on the cathode side of the membrane, especially in applications in PEM electrolysis.

[0045] In a particularly preferred embodiment of the method, iridium is recovered as metallic catalyst material. In this case, the insoluble components in the residue are preferably iridium, which is not attacked by aqua regia. In this case, the iridium is advantageously already obtained in the residue in the form of metal or metal particles of good quality and purity, preferably after washing or rinsing the filter. The iridium obtained in this way can therefore be applied directly as catalyst material to the anode side of a new ion-conducting membrane and can be reused for electrolysis purposes in a new membrane electrode assembly.

[0046] The iridium is preferably recovered in the form of solid iridium black, wherein an iridium purity of 97% to 99.5%, in particular 98% to 99.3%, is achieved.

[0047] Iridium black is already available in the corresponding particle size, wherein a grinding process can optionally be provided to produce the particle size for catalyst use in membrane electrode assemblies. Here, the iridium is present in the form of high-purity iridium black, so that due to the prefabricated particle size it can be directly reused in the production of new metal electrode assemblies. Iridium black is a solid with a metallic black color.

[0048] Preferably, the yield of the recovered iridium black is greater than 80%, in particular between 92% and 96%, based on the original amount of iridium.

[0049] Due to the simplified process control, essentially only the separation process and the treatment of the soluble and insoluble metal components are required, with almost no material loss, but an almost closed and loss-free recovery process of the catalyst material is achieved. In particular, for the iridium catalyst material that is insoluble in aqua regia, no other technically complex conversion processes are required, which would inevitably lead to material losses of iridium in the precious metal separation process.

[0050] In a particularly preferred embodiment, the method is applied to a membrane electrode assembly for PEM water electrolysis. Proton exchange membranes made of PFSA "perfluorosulfonic acid" are used in PEM electrolysis. The PFSA membrane is a particularly important element for the functioning of the membrane electrode assembly and thus the electrolysis cell. The membrane is coated with a corresponding catalyst material on the anode side and the cathode side, preferably a thin iridium layer is applied on the anode side and a platinum layer is applied on the cathode side. The circulation method according to the invention can be advantageously applied to membrane electrode assemblies of other electrolysis systems, for example alkaline electrolysis.

[0051] The embodiments described below are preferred embodiments of the present invention. The features, feature combinations previously given in the specification, and the features and feature combinations mentioned in the following description of the embodiments and / or shown separately in the drawings can be used in combination with the corresponding given ones, and can be used in other combinations. Therefore, the present invention also includes or should be regarded as disclosing such embodiments, which are not clearly shown and described in the drawings, but can be derived and generated from the described embodiments by the individual combination of features. The features, functions and / or effects shown by the embodiments can all represent the respective features, functions and / or effects of the present invention, which can be viewed independently of each other and can also improve the present invention independently of each other. Therefore, the embodiments should also include combinations other than the combinations in the described embodiments. In addition, the described embodiments can also be supplemented by other features, functions and / or effects in the already described features, functions and / or effects of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In the accompanying drawings, the same reference numerals represent the same features or functions.

[0053] Figure 1 shows a schematic cross-sectional view of an electrolysis cell having a membrane electrode assembly for electrolyzing water;

[0054] Figure 2 A membrane electrode assembly is shown;

[0055] Figure 3 A schematic diagram of a method for recovering catalyst material from a membrane electrode assembly is shown. DETAILED DESCRIPTION

[0056] Figure 1 A schematic cross-sectional view of an electrolysis cell 12 with a membrane electrode assembly 10 is shown. In this case, the membrane electrode assembly 10 has a membrane 24, which has two surfaces 16, 18 facing away from each other, on which the corresponding catalyst materials 20, 22 are arranged. The catalyst materials 20, 22 themselves contact the corresponding gas diffusion layers, which are formed by the corresponding nonwoven materials 26, 28 and themselves contact the opposite corresponding contact plates 34, 36. This forms an anode region 30 and a cathode region 32. The contact plates 34, 36 and the gas diffusion layers are designed to be electrically conductive, so that an electrical contact is established between the corresponding catalyst materials 20, 22 and the corresponding contact plates 34, 36. The contact plates 34, 36 also have flow channels, not shown, through which water or electrolytes can be supplied to the electrolysis cell 12 on the one hand and electrolysis products, namely hydrogen and oxygen, can be drawn off on the other hand.

[0057] The membrane electrode assembly 10 is Figure 2In addition to the membrane 26, the membrane electrode assembly 10 also includes catalyst material layers 20, 22. The membrane electrode assembly 10 can be manufactured as a separately processable component, so that the membrane electrode assembly 10 can be processed in a simple manner during the manufacturing process of the electrolysis cell 12.

[0058] The membrane electrode assembly 10 comprises at least catalyst layers on the anode side and the cathode side, which are usually connected to the membrane 24 to form a component. The corresponding chemical reaction takes place in the catalyst layers, wherein the electrons can be conducted to the contact plates 34, 36 via the catalyst and a possible conductive support structure. It is therefore advantageous that the corresponding layers of catalyst material 20, 22 have the best possible conductivity and catalytic capacity. In addition, hydroxide ions OH are generated in an alkaline environment. - , in an acidic environment, protons H are generated + , which migrate through the respective membranes as charge carriers. Therefore, it is also desirable to have catalyst materials 20, 22 that have respectively good conductivity for the respective ions so that they can be easily transported to the membrane 24 and from the membrane 24 to the respective catalytic centers. Therefore, it is desirable to provide good ionic bonding of the respective catalyst materials 20, 22 to the respective surfaces 16, 18 of the membrane 24 and at the same time provide good electrical conductivity of the catalyst materials 20, 22.

[0059] The membrane electrode assembly 10 is designed specifically for PEM water electrolysis for use in an acidic environment. For this purpose, the membrane 24 is arranged as a proton-conducting membrane 24. The catalyst layer on the anode side has iridium as a first catalyst material 20. The catalyst layer on the cathode side has platinum as a second catalyst material 22. Here, iridium is applied to a first surface 16 of the membrane 24, and platinum is applied to the opposite second surface 18 of the membrane 24. Here, the membrane 24 serves as a substrate 14.

[0060] When the membrane electrode assembly 10 is newly manufactured or recycled or reprocessed, a membrane 24 is first provided as a substrate 14 to produce a coated membrane electrode assembly 10. The membrane 24 has a surface 16 and a second surface 18 facing away from the first surface 16. In order to coat with the corresponding catalyst materials 20, 22, a coating tool not further shown is used. Then, the catalyst materials 20, 22 are provided in the form of a paste, in particular as a slurry, so that they can be applied well and tightly to the corresponding surfaces 16, 18 using a coating tool. Here, it can be provided that the particles of the corresponding catalyst materials 20, 22 are dissolved and mixed only with the ionomer in a highly viscous solvent. The production of the catalyst paste is not further shown in the figure. However, conventional methods for mixing substances can be used for this purpose. However, compared with the prior art, this method does not require the addition of a polymer binder or a non-ionic binder. The viscosity of the paste can be adjusted by the ionomer content in the solvent, so that conventional industrial coating methods for electrode pastes can be used. Therefore, for example, scraping or dipping can also be provided alternatively.

[0061] In the case of a used membrane electrode assembly 10, it is advantageous from an economic and ecological point of view to recover the valuable metal catalyst materials 20, 22 in a recycling process. To this end, the membrane electrode assembly 10 is removed from the electrolytic cell 12 and provided to the recycling process proposed in method step 1. Figure 3 The step 1 of providing a membrane electrode assembly and the further method steps for recovering catalyst materials 20 , 22 from the membrane electrode assembly are schematically shown in the flow chart of .

[0062] In this case, the used membrane electrode assembly 10 is first mechanically comminuted 2 to a predetermined degree of comminution in method step 2. The economic and technical service life of the ion-conducting membrane 24 is limited by various influencing factors. Thus, for example, degradation effects on the membrane 24 occur, which damage the membrane material and impair its function.

[0063] After the fragmentation, pyrolysis is carried out in method step 3. The thermal treatment of the fragmented membrane electrode assembly 10 is carried out in a corresponding pyrolysis reactor at a pyrolysis temperature T of 600° C. to 1000° C. P In this process, various thermochemical transformation processes take place, in which organic compounds are cracked at high temperatures and largely in the absence of oxygen. The high temperatures crack some of the chemical bonds in the starting materials, wherein the lack of oxygen prevents complete combustion. The resulting products are diverse. The residue obtained in method step 3 is a solid pyrolysis product, ash, which is present in a suitable particle size or particle size and contains metal catalyst materials 20, 22. In this case, a mixture of catalyst material 20 used on the anode side and catalyst material 22 used on the cathode side is present in the ash. In the example described here, this is iridium as anode-side catalyst material 20 and platinum as cathode-side catalyst material 22.

[0064] In the present method, it is preferred to use a higher pyrolysis temperature T in the range of 600°C to 1000°C. P , so as to pyrolyze and crack the fragmented membrane electrode unit 10. During pyrolysis, the membrane 24 is completely converted into a gaseous state, so that no carbon residues remain in the solid pyrolysis product, the ash, but essentially the metal components of the catalyst material 20, 22.

[0065] In method step 4, the solid pyrolysis products, the ash residue, are now dissolved in aqua regia, thereby forming a solution. Aqua regia is formed by mixing hydrochloric acid HCl and nitric acid HNO3 in a ratio of 3:1. In this case, the process control according to the invention primarily distinguishes between catalyst material 20 iridium, which is insoluble in aqua regia, and catalyst material 22 platinum, which is soluble in aqua regia, and in only two subsequent steps, these components are separated continuously. First, in method step 5, the solution is heated to 100° C. to 110° C., and the nitrates are thermally driven out of the solution. Thus, in step 5, the noble metal nitrates, in particular platinum nitrate, are separated and the catalyst material 22 dissolved in aqua regia is processed, which is used on the cathode side of the membrane electrode assembly 10 and is applied to the initially intact membrane 24. These are noble metals soluble in aqua regia, such as preferably platinum, or binary platinum alloys, such as nickel-platinum, which serve as second catalyst material 22. Thus, in the residual solution after this process step 5, the first metal catalyst material 20, such as iridium, which is insoluble in aqua regia and used on the anode side of the membrane electrode assembly is advantageously retained.

[0066] This solid component with a high iridium content is now very advantageously and easily separated and thus separated by filtering the insoluble residue in method step 6 (filtration step). Thus, there is an insoluble residue, which mainly contains the already high-value metallic catalyst material 20, namely iridium.

[0067] As a further method step 7, finally at a drying temperature T D The residue is dried for a period of time such that all liquid and possibly still gaseous components in the residue are removed or driven out thermally. D It can be 60°C to 80°C. In order to shorten the drying time, a higher drying temperature T above 80°C can also be used. D. A solid residue is now advantageously recovered, which contains a high-purity insoluble metallic catalyst material 20, i.e. iridium, or already consists essentially of it. The iridium can be present in the form of iridium black with a purity of more than 90%, in particular 97% to 99.5%. The iridium as the first catalyst material 20 of the precious metal can be processed directly and can be supplied to reuse in the membrane electrode assembly 10, for example to be applied to a new membrane 24 as anode-side catalyst material 20. If necessary, a grinding process for the recovered iridium black as the first catalyst material 20 can be provided in a further method step 8 in order to achieve the desired particle size of the iridium for use and application to the provided membrane 24. Here, the degree of grinding or the average particle size of the recovered iridium is set between 20 μm and 50 μm. The method shows that the yield of the recovered iridium is significantly higher than 80%. Based on the original amount of iridium, a yield of more than 90%, for example between 92% and 96%, is achieved. Therefore, the method is particularly and advantageously useful for the efficient recovery of iridium and platinum from spent membrane electrode assemblies 10 of PEM water electrolysis having a proton exchange membrane made of PFSA "perfluorosulfuric acid".

[0068] Especially from the basis Figure 3 As can be seen from the exemplary embodiments, the method according to the invention has proven to be particularly suitable for recovering valuable precious metal catalyst materials 20, 22 from used membrane electrode assemblies 10 in a recycling process with high purity and recovery yield.

[0069] As a result, a new membrane electrode assembly 10 can be produced in this way by the separation method of the invention, wherein iridium and platinum are reused. From a process technology point of view, the cost of recovering pure iridium is significantly reduced due to the controllable number of process steps. In particular, iridium black is directly recovered, thereby eliminating the need for cumbersome and time-consuming conversion and processing steps.

[0070] The present invention enables simple and very efficient recovery of high-quality and pure iridium black catalyst from the spent materials of the membrane electrode assembly 10 of PEM water electrolysis. Significant advantages are reflected in cost savings in the separation process, cost savings in the catalyst synthesis, energy savings in the entire process chain of the method, shorter process duration and high availability of the method. The present invention enables cost-effective and easily scalable iridium black recovery on an industrial scale. As a result, the precious metals iridium and platinum can be used again in the production process of new membrane electrode assemblies 10 with maximum yield and quality.

[0071] The embodiments of the present invention are only used to illustrate the present invention and are not intended to limit the present invention.

Claims

1. A method for recovering catalyst material from a membrane electrode assembly (10) for water electrolysis, comprising the following steps: - a step (1) of providing a membrane electrode assembly (10) comprising a membrane (24) coated with a metal catalyst material (20, 22), - a step (2) of fragmenting the membrane electrode assembly (10), - a step (3) of pyrolytically decomposing the fragmented membrane electrode assembly (10), wherein: obtaining a solid pyrolysis product as a residue, - a step (4) of dissolving the solid pyrolysis product in a mixture containing concentrated hydrochloric acid and concentrated nitric acid, - a step (5) of removing nitrates by heating the solution to 100° C. to 110° C., - a step (6) of filtering the insoluble residue, - At drying temperature (T D ) a step (7) of drying the insoluble residue, wherein the metal catalyst material (20) is recovered.

2. The method according to the preceding claim, wherein: In a grinding step (8), the insoluble residue is ground such that an average particle size of 10 μm to 80 μm, in particular 20 μm to 50 μm, is achieved.

3. The method according to claim 1 or 2, wherein: At a pyrolysis temperature (T P ) was subjected to pyrolysis.

4. A method according to any one of the preceding claims, wherein: The solid pyrolysis product is dissolved at a temperature of 70° C. to 90° C., in particular at a temperature of 80° C., wherein during the dissolution, the temperature is maintained for 3 h to 5 h, in particular for 4 h.

5. A method according to any one of the preceding claims, wherein: The metal components dissolved in the mixture are driven out and separated by heating, wherein an insoluble residue is obtained.

6. The method according to claim 5, wherein: Platinum (Pt) as the dissolved metal component is separated and recovered.

7. A method according to any one of the preceding claims, wherein: Iridium (Ir) is recovered as a metal catalyst material.

8. The method according to claim 7, wherein: The iridium (Ir) is recovered as solid iridium black, wherein an iridium purity of 97% to 99.5%, in particular 98% to 99.3%, is achieved.

9. The method according to claim 8, wherein: A yield of recovered iridium black (Ir) of greater than 80%, in particular between 92% and 96%, based on the original amount of iridium, is achieved.

10. A method according to any one of the preceding claims, wherein: The invention is applied to a membrane electrode assembly (10) for PEM water electrolysis.

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