Method for recovering catalytic material from membrane electrode assemblies for water electrolysis - Patents.com

Abstract

The present invention relates to a method for recovering catalytic materials from a membrane electrode device (10) for the electrolysis of water, comprising the following steps: -(1) providing a membrane electrode assembly (10) containing a membrane (24) coated with a metallic catalytic material (20, 22); -(2) crushing the membrane electrode assembly (10); - (3) pyrolyzing the pulverized membrane electrode assembly (10), thereby obtaining a solid pyrolysis product as a residue. -(4) dissolving the solid pyrolysis product in a mixture of concentrated hydrochloric acid and concentrated nitric acid; -(5) removing nitrates by heating the solution to 100°C to 110°C; - (6) filtering the insoluble residue; -(7) Drying temperature of insoluble residue (T D ) to recover the metal catalyst material (20). The method is preferably used to recycle PEM water electrolysis membrane electrode assemblies (10) whereby iridium is recovered as a metallic catalyst material (20).

Description

[Technical field]

[0001] The present invention relates to a method for recovering catalytic material from a water electrolysis membrane electrode assembly, wherein a used membrane electrode assembly is provided that includes a membrane coated with a metallic catalytic material. [Background technology]

[0002] Hydrogen is an important substance used in countless applications in industry and technology. Generally, hydrogen only occurs in a combined state on Earth. One of these substances that contains hydrogen in a combined state is water. Furthermore, hydrogen also serves as an energy store, and can store electrical energy, especially that generated using renewable energy generation methods, for later use.

[0003] An important method for obtaining hydrogen is the electrolysis of water, in particular with the aid of electrical energy. In this case, hydrogen can, inter alia, act as an energy store, in particular by being used as a fuel, in order to stabilize the supply of electrical energy, in particular from renewable energies, such as wind power, solar power, etc. However, hydrogen can also be used in other processes in which a fuel or reducing agent is required. The hydrogen obtained in electrolysis can thus, for example, be used industrially or to regenerate electrical energy electrochemically with the aid of fuel cells.

[0004] The separation of water into its chemical components hydrogen and oxygen can be carried out by suitable electrolysis cells. For this purpose, these can be designed as so-called polymer electrolyte membrane electrolysis cells. Electrolysis cells of this kind are usually provided with a membrane with a respective catalyst layer on each of their opposing surfaces. The catalyst layers are generally adjacent to respective gas diffusion layers, which are in turn adjacent to respective electrically conductive contact plates, sometimes also called bipolar plates, which serve inter alia as electrical contacts. At the same time, the contact or bipolar plates are preferably also designed in such a way that they can enable the necessary mass transport for the desired operation during electrolysis in the electrolysis cell. For this purpose, corresponding channels can be provided for the supply of a suitable electrolyte and for the discharge of the reaction products of the electrolysis, i.e. hydrogen gas and oxygen gas. The gas diffusion layers usually provide electrical conductivity for electrically connecting the contact plates and the catalyst layers to one another. Thereby, the desired electrochemical reactions can be realized in the region of the catalyst layers.

[0005] When the electrolysis reaction is in the alkaline range, the membrane is provided as an anion exchange membrane (AEM), whereas when the electrolysis reaction is in the acidic range, a proton exchange membrane (PEM) is provided instead.

[0006] Hydrogen is produced from water through the process of electrolysis, an electrochemical process in which water is separated 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] [Table 1]

[0008] In polymer electrolyte membrane electrolysis, each of the two partial reactions is spatially separated by an ion-conducting membrane. Here, for electrolysis in the alkaline region, an anion exchange membrane (AEM) is provided, whereas for electrolysis in an acidic environment, a proton exchange membrane (PEM) is provided, but the structure of the membrane electrode assembly (MEA) is essentially the same in both cases.

[0009] The core of such a polymer electrolyte membrane electrolysis cell is generally formed by a membrane electrode assembly (MEA). The membrane electrode assembly generally has at least a membrane, on both the anode and cathode sides, which is coated on two oppositely facing surfaces with a layer of the respective catalytic material. In the area of ​​the layer formed by the respective catalytic material, the respective cell reaction of the electrolysis takes place. Here, during normal operation, electrons are conducted to the contact plate via the respective catalytic material and a support structure which may be formed by or provide a gas diffusion layer. For this reason, the catalytic layer is required to have a high electrical conductivity.

[0010] In alkaline reactions, in addition to electrons, hydroxide anions (OH - In the acidic region, protons H + are generated, which are transported through the respective membranes as charge carriers. In this regard, therefore, good electrical conductivity is desired for the structure of the respective catalytic material layers in order to transport the respective ions to and from the membrane to the respective catalytic material. Thus, it is a technical challenge to simultaneously provide good ionic bonding of the catalytic material to the membrane and good electrical conductivity of the catalytic material.

[0011] In the prior art, membrane electrode assemblies or membrane electrode units are manufactured by preparing a catalyst material in the form of a paste and applying it directly to either the corresponding surface of the membrane or the corresponding surface of the substrate. Here, the paste-form or pasty catalyst material consists of the catalyst material itself, an ionomer, a polymeric binder and a solvent. After application, the solvent is generally thermally removed or thermally expelled, usually by pressing a layer from the catalyst material onto the membrane at high temperatures, usually above 100° C. This is aimed at ensuring ionic contact between the ion exchange resin, the catalyst material and the membrane.

[0012] A membrane electrode assembly contains a membrane with two opposing surfaces with respective catalytic materials. In many applications, particularly in PEM water electrolysis, precious metals are used as catalytic materials in membrane electrode assemblies, which are very expensive and therefore very rare. In addition to the ecological aspect of supporting a circular economy, there is therefore a great economic interest in regenerating used membrane electrode assemblies in order to recover the expensive metallic catalytic materials and, if necessary, use them in new membrane electrode assemblies or supply them to other applications. Summary of the Invention [Problem to be solved by the invention]

[0013] The invention is based on the problem of identifying a method for recovering catalytic material from a membrane electrode assembly, which allows a particularly simple and cost-effective recovery of the catalytic material. [Means for solving the problem]

[0014] This object is achieved according to the invention by a method for recovering catalytic materials from a membrane electrode assembly for water electrolysis, comprising the following steps: - providing a membrane electrode assembly containing a membrane coated with a metallic catalytic material; - pulverizing the membrane electrode assembly; - pyrolyzing the pulverized membrane electrode assembly to obtain a solid pyrolysis product as a residue; - dissolving said solid pyrolysis product in a mixture of concentrated hydrochloric acid and concentrated nitric acid; - removing nitrates by heating the solution to 100°C to 110°C; - filtering the insoluble residue; and - drying the insoluble residue at a drying temperature to recover a metal catalyst material.

[0015] The present invention already presupposes the general basic approach that recycling is a major contribution to environmental protection. Recycling means sustainability, since raw material resources do not last forever. Also, metal catalyst materials are often precious metals, which are too valuable not to recover, so recycling makes money. Furthermore, the large-scale industrial use of electrolyzers for water electrolysis is expected to rapidly increase the demand for membrane electrode assemblies, which will increasingly demand the economic feasibility of known recycling processes, both in terms of cost and efficiency.

[0016] For example, although many laboratory scale reprocessing and separation methods are described in the scientific literature, their economical and industrial implementation is overly complicated in terms of process procedures and, as a result, very costly. Thus, the present invention has recognized the need for a simple and cost-effective recovery of metal catalyst materials. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The procedure of the invention allows for a highly efficient recovery of catalytic metal materials and especially precious metals. The catalytic material on the membrane of the membrane electrode assembly may, for example, on the anode side, contain one or more of the following substances: nickel-aluminum, nickel-zinc, cobalt-aluminum, cobalt-iron, nickel-iron, nickel-iron-vanadium, nickel-cobalt, nickel-molybdenum, nickel-iron double layer hydroxide, nickel-iron-cobalt, iridium, ruthenium oxide, nickel hydroxide, nickel oxide and nickel.

[0018] The catalytic material on the membrane may include, for example, on the cathode side, one or more of the following materials: nickel, nickel-molybdenum on carbon black, nickel-molybdenum, nickel-platinum, platinum, nickel on carbon black, nickel phosphate, and nickel-vanadium.

[0019] Here, firstly, mechanical comminution of the used membrane electrode assembly to a predefined degree of comminution is performed. The economic and technical life of ion-conducting membranes is limited by various influencing factors. For example, degradation effects on the membrane have been reported, which damage the membrane material and impair its function. Due to the fact that harsh electrochemical conditions can always occur during the electrolysis of water, small amounts of H2O2 or OH radicals can be formed in the electrolyzer, for example in the region of the cell. It is well known that such species chemically attack the membrane material of the PEM electrolyzer, and that such decomposition releases fluorides as decomposition products. This is because the membrane has a fluorine component and consists, for example, of PFSA (perfluorosulfuric acid). The membrane is a particularly important element for the functionality of the electrolysis cell in PEM electrolysis, so that its service life and its limitations due to degradation effects are of great interest, especially from an economic point of view. Metal catalyst materials, on the other hand, are practically not consumed and do not show any tendency to degradation, so that their regeneration and recovery are very favorable.

[0020] The present invention surprisingly features a significantly simplified process that, as shown, requires only a few separation steps to separate the precious metals from the high value catalytic material.

[0021] After the comminution of the membrane electrode assembly, pyrolysis first takes place. Pyrolysis or thermal degradation refers to a variety of thermochemical conversion processes in which organic compounds are cleaved at high temperatures and almost without oxygen. The high temperatures cleave some chemical bonds in the starting materials, while the lack of oxygen prevents complete combustion. The products obtained are diverse. The residue obtained in the process of the present invention is a solid pyrolysis product, present in a suitable granular or particle size, and an ash containing the metal catalytic material. Here, in the ash, there is a mixture of catalytic material used on the anode side and catalytic material used on the cathode side.

[0022] Generally, pyrolysis produces a complex mixture of solid, liquid and gaseous products, the exact proportions depending on the specific conditions and starting materials. It can be said that, in principle, higher temperatures and longer pyrolysis times give more gaseous products, while lower temperatures and shorter times give more liquid products. When polymers are pyrolyzed, the corresponding monomers are often formed as products. These products have a high energy content and can be energetically utilized as secondary energy sources or can be further utilized as materials by obtaining individual chemicals therefrom. In the method of the present invention, higher temperatures are preferred to pyrolytically cleave the pulverized membrane electrode units.

[0023] Upon pyrolysis, the membrane is preferably converted completely to a gaseous state so that no carbon residue remains in the solid pyrolysis product, ash.

[0024] Here, the solid pyrolysis product, the ash residue, is dissolved in aqua regia, so that a solution is formed. Aqua regia consists of a mixture of hydrochloric acid HCl and nitric acid HNO3 in a ratio of 3:1. The process control of the invention makes a fundamental distinction between catalytic materials that are insoluble in aqua regia and catalytic materials that are soluble in aqua regia, which are successively separated in only two subsequent steps. First, the solution is heated to 100°C-110°C, so that the nitrates are thermally driven out of the solution. This step therefore involves the separation of the noble metal nitrates and the regeneration of the catalytic materials that were dissolved in aqua regia, preferably used on the cathode side of the membrane electrode assembly and that were applied to the intact membrane. These are noble metals that are soluble in aqua regia, such as, for example, platinum or binary platinum alloys, such as nickel-platinum, that are used as catalysts. Metallic catalytic materials that are insoluble in aqua regia, such as, for example, iridium, that were preferably used on the anode side of the membrane electrode assembly, therefore advantageously remain in the residual solution after this process step. This solid component can be very advantageously and easily removed and thus isolated in a filtration step, giving an insoluble residue which is already dominated by the valuable metal catalyst material.

[0025] As a further step, the residue is finally dried for a drying time at a drying temperature, so that all liquid and possibly still gaseous components in the residue are removed or thermally expelled. This advantageously recovers a solid residue that contains or is largely composed of insoluble metal catalyst material. The metal catalyst material can be immediately processed and reused, for example, applied to a new membrane as an anode side catalyst material.

[0026] The method described herein particularly advantageously allows the recovery of expensive anode precious metal catalysts (OER catalysts) directly from the metal electrode devices of PEM water electrolysis without dissolving, melting or reprocessing them. The method described herein significantly reduces the number of process steps and avoids loss pathways associated with numerous separation steps. From an economic point of view, further complex processing costs for separation (separation steps) and production costs for precipitation of new catalyst materials are not necessary. Advantageously, the cathode catalyst materials can also be reused without much effort. Here, the process control only distinguishes between aqua regia-insoluble and aqua regia-soluble catalyst materials and efficiently separates these materials in individual process steps.

[0027] In comparison with the technical approaches described so far, the process control of the present invention is technically and economically superior. These previous approaches are based, for example, on the pyrolysis of the membrane, on the dissolution of the catalyst or on removal by complete dissolution of the catalyst components in a solvent. Here, if an insoluble metal catalyst material is used, the possibility of dissolution in aqua regia is eliminated, since the insoluble anode precious metal catalyst material is not soluble in aqua regia. The subsequent separation steps required for the separation of the precious metal therefore include a complex and lossy Na2O2 oxidation melting process to convert the metal catalyst material, for example, into a soluble oxide of the metal catalyst material. The present invention specifically avoids the lossy route of processing the catalyst material by way of a conversion process with soluble intermediate products. From the point of view of process engineering, the process costs are high, since a large number of process steps are required to recover the purest possible metal catalyst material. Furthermore, due to the corrosive nature of the chemicals used, expensive reactor linings are generally required, which are avoided by the present invention.

[0028] Advantageously, it has been shown that the pyrolysis step does not allow for any significant coarsening of the particle size compared to fresh catalysts. Moreover, it has been surprisingly shown that the metal catalyst material is already present after the drying step in sufficient purity and good quality to be directly used for the manufacture of fresh membrane electrode assemblies. The method proposed here therefore allows a significant reduction in established process steps, thereby achieving significant cost and time savings in established process procedures. Thanks to the proposed process, in particular the precious metal recovery rate can therefore be increased.

[0029] In a preferred embodiment of this method, the insoluble residue is ground to achieve an average particle size of between 10 μm and 80 μm, in particular between 20 μm and 50 μm.

[0030] After drying of the insoluble residue at the drying temperature, the metal catalyst material is already present in the form of solid particles. It may be advantageous to subject this residue to a grinding process until the desired particle size distribution, especially the average particle size, of the metal catalyst material is achieved. The grinding process allows a more favorable surface area / volume ratio of the catalyst particles to be adjusted, which is advantageous for the subsequent coating of new membranes with the recovered catalyst material and for obtaining high catalytic efficiency while minimizing the use of materials in the membrane electrode unit.

[0031] The pyrolysis step is preferably carried out at a pyrolysis temperature of 600° C. to 1000° C., in particular 700° C. to 900° C. Thus, high-temperature pyrolysis is carried out, so that the components of the film are converted into gaseous products as a result of pyrolysis and are driven off without remaining in the solid pyrolysis products or in the residue.

[0032] In a particularly preferred configuration of the process, the solid pyrolysis product is dissolved in aqua regia at a temperature between 70° C. and 90° C., in particular at a temperature of 80° C., and this temperature is maintained during dissolution for 3 to 5 hours, in particular for 4 hours. A contact time of 4 hours at a pyrolysis temperature of 80° C. has been shown to be particularly advantageous for good dissolution of the soluble components in the solid pyrolysis product. However, this can deviate from this within certain limits depending on the batch, the lower the temperature, the longer the contact time will be required and vice versa.

[0033] Preferably, the metal components dissolved in the mixture of hydrochloric and nitric acids are released and separated by heating to obtain an insoluble residue. Due to 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. These are eliminated from the solid residue by heating the solution to 100°C-110°C, i.e. they leave the residue by outgassing or evaporation. Advantageously, these species are also collected for a complete recycling process of the soluble species.

[0034] 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 metallic catalytic material on the cathode side of the membrane of a membrane electrode unit. Since platinum is an expensive precious metal, recovery is economically rational. It is also possible to use platinum-containing bimetallic compounds, such as platinum-nickel, as a soluble metallic component and subject them to a recycling process. Platinum or platinum-nickel are particularly advantageous as catalytic materials on the cathode side of the membrane in a membrane electrode unit, especially in applications in PEM electrolysis.

[0035] In a particularly preferred embodiment of this method, iridium is recovered as a metal catalytic material. In this case, the insoluble component in the residue is preferably iridium, which is not attacked by aqua regia. The iridium is advantageously already obtained in the residue as metal or metal particles of good quality and purity, preferably after cleaning or rinsing the filter. The iridium thus obtained can therefore be directly applied as catalytic material to the anode side of a new ion-conducting membrane and can be reused for electrolysis purposes in a new membrane electrode assembly.

[0036] Preferably, the iridium is recovered as solid iridium black, in which an iridium purity of 97% to 99.5%, in particular 98% to 99.3%, is achieved.

[0037] Iridium black is already present in the corresponding particle size, and if desired, a grinding step can be provided to adjust the particle size suitable for catalytic applications in membrane electrode assemblies. Here, the iridium is present as high purity iridium black, and therefore can be directly reused in the manufacture of new membrane electrode assemblies by prior particle size adjustment. Iridium black is a metallic black solid.

[0038] Preferably, a yield of recovered iridium black is achieved of more than 80%, in particular between 92% and 96%, based on the original amount of iridium.

[0039] The simplified process sequence, which essentially involves only one separation process and one treatment of the soluble and insoluble metal components, achieves a nearly closed, loss-free recycling process for the catalytic material with almost no material loss. In particular, the iridium catalytic material, which is insoluble in aqua regia, does not require any further complicated conversion process, which may inevitably result in material loss of iridium in the precious metal separation process.

[0040] In a particularly preferred embodiment, the method is applied to a membrane electrode assembly of PEM water electrolysis. In PEM electrolysis, a proton-permeable membrane (proton exchange membrane) made of PFSA (perfluorosulfuric acid) is used. The PFSA membrane is a particularly important element for the functionality of the membrane electrode assembly and thus the electrolysis cell. The membrane is coated on the anode and cathode sides with the respective catalytic materials, preferably a thin layer of iridium is applied to the anode side and a thin layer of platinum to the cathode side. The recycling process of the present invention can also be advantageously applied to the membrane electrode assemblies of other electrolysis systems, such as alkaline electrolysis. EXAMPLES

[0041] The examples described below are preferred embodiments of the present invention. The features and combinations of features described herein above, as well as the features and combinations described in the following description of the examples and / or shown alone in the drawings, can be used not only in the respective described combinations, but also in other combinations. Therefore, implementations that are not explicitly shown or described in the drawings, but are evident and can be produced from the described embodiments by separate combinations of features, are also considered to be included or disclosed in the present invention. The features, functions and / or advantages described based on the examples may represent individual inventive features, functions and / or actions that are themselves considered to be independent of each other and also develop the invention independently of each other. Therefore, the examples are intended to include combinations other than the described embodiments. Moreover, the described embodiments may be supplemented by further features, functions and / or advantages of the present invention already described. [Brief description of the drawings]

[0042] In the drawings, the same reference symbols denote the same features or functions. [Figure 1] FIG. 1 shows a schematic cross-sectional view of an electrolysis cell having a membrane electrode assembly for the electrolysis of water. [Diagram 2] 1 shows a membrane electrode assembly. [Diagram 3]FIG. 2 shows a schematic diagram of a method sequence for recovering catalytic material of a membrane electrode assembly.

[0043] FIG. 1 shows in a schematic cross-sectional view an electrolysis cell 12 with a membrane electrode assembly 10. Here, the membrane electrode assembly 10 has a membrane 24 with two oppositely facing surfaces 16, 18 on which catalytic materials 20, 22, respectively, are arranged. The catalytic materials 20, 22 are also in contact with respective gas diffusion layers formed by respective nonwoven materials 26, 28 and, on their opposite sides, with respective contact plates 34, 36. Thereby, an anode region 30 and a cathode region 32 are formed. The contact plates 34, 36 and the gas diffusion layers are also designed to be electrically conductive, so that an electrical contact is created between each catalytic material 20, 22 and the corresponding contact plate 34, 36. Furthermore, the contact plates 34, 36 have unfilled flow paths via which, on the one hand, water or electrolyte can be supplied to the electrolysis cell 12 and, on the other hand, the electrolysis products, i.e. hydrogen and oxygen, can be discharged.

[0044] The membrane electrode assembly 10 is shown in Figure 2 as a separate component. The membrane electrode assembly 10 contains layers 20, 22 of catalytic material in addition to the membrane 26. The membrane electrode assembly 10 may be manufactured as individually handleable components, so that the membrane electrode assembly 10 can be easily handled during the fabrication process of the electrolysis cell 12.

[0045] The membrane electrode assembly 10 contains at least an anode-side catalyst layer and a cathode-side catalyst layer, which are usually connected with the membrane 24 to form a single component. Within the catalyst layers, the respective chemical reactions take place and electrons can be conducted to the contact plates 34, 36 via the catalyst and any conductive support structures. It is therefore advantageous for the respective layers of catalytic material 20, 22 to have as good electrical conductivity and catalytic ability as possible. Furthermore, hydroxide ions OH - However, in an acidic environment, proton H +are generated, which migrate as charge carriers through the respective membranes. It is therefore desirable for the catalytic materials 20, 22 to have correspondingly good conductivity for the respective ions, so that the ions can be well transported to and from the membrane 24 to the respective catalytic centers. It is therefore desirable to provide good ionic bonding of the corresponding catalytic materials 20, 22 to the respective surfaces 16, 18 of the membrane 24, while at the same time providing good electrical conductivity of the catalytic materials 20, 22.

[0046] For use in acidic environments, the membrane electrode arrangement 10 is specially designed for PEM water electrolysis. To that end, a membrane 24 is prepared as a proton conducting membrane 24. The anode side catalyst layer contains iridium as a first catalytic material 20. The cathode side catalyst layer contains platinum as a second catalytic material 22. Here, the iridium is coated on a first surface 16 of the membrane 24, and the platinum is coated on an opposite, second surface 18 of the membrane 24. Here, the membrane 24 serves as a substrate 14.

[0047] When the membrane electrode device 10 is newly manufactured, recycled or reprocessed, firstly a membrane 24 is prepared as substrate 14 for the manufacture of the membrane electrode device 10, which is to be coated. The membrane 24 has a surface 16 and a second surface 18 facing away from the first surface 16. For the purpose of coating with the respective catalytic materials 20, 22, a coating tool, not shown in detail, is used. The catalytic materials 20, 22 are then prepared in paste form, in particular as a pasty mass, so that they can be well and closely applied to the respective surfaces 16, 18 by means of the coating tool. Here, the particles of the respective catalytic materials 20, 22 may simply be dissolved and mixed with an ionomer in a highly viscous solvent. The production of the catalytic paste is not shown in detail in the figures. However, conventional methods for mixing substances can be used for this purpose. However, in contrast to the prior art, this method does not require the addition of polymeric or non-ionic binders. Conventional industrial coating methods for electrode pastes can be used, since the viscosity of the paste can be adjusted by the ionomer content in the solvent, so for example knife coating or dip coating can also be envisaged as alternatives.

[0048] In the case of a used membrane electrode assembly 10, it is advantageous from an economic and environmental protection point of view to recover the expensive metallic catalyst material 20, 22 in a recycling process. For this purpose, the membrane electrode assembly 10 is removed from the electrolysis cell 12 and prepared for the here proposed recycling process in method step 1. The preparation step 1 and further steps for recovering the catalyst material 20, 22 from the membrane electrode assembly are shown diagrammatically in the flow chart of FIG.

[0049] Here, first, in method step 2, a mechanical comminution 2 of the used membrane electrode assembly 10 to a predefined degree of comminution is performed. The economic and technical life span of the ion-conducting membrane 24 is limited by various influencing factors. For example, degrading effects on the membrane 24 are described, which damage the membrane material and impair its function.

[0050] After comminution, pyrolysis is carried out in method step 3. The heat treatment of the pulverized membrane electrode assembly 10 is carried out in a corresponding pyrolysis reactor at a pyrolysis temperature T P In the process, various thermochemical conversion processes take place, in which organic compounds are cleaved at high temperatures, mainly with the exclusion of oxygen. The high temperatures cleave some chemical bonds in the starting materials, and the lack of oxygen prevents complete combustion. The products obtained are manifold. In method step 3, as residue, a solid pyrolysis product, ash, is obtained, which is present in a corresponding granular or particle size and contains the metallic catalytic materials 20, 22. In the ash, there is now present a mixture of catalytic materials 20 used on the anode side and 22 used on the cathode side. In the example described here, this is iridium as the anode side catalytic material 20 and platinum as the cathode side catalytic material 22.

[0051] In the present method, in order to cleave the pulverized membrane electrode unit 10 by pyrolysis, a higher pyrolysis temperature T P applies. During pyrolysis, the membrane 24 is completely converted to a gaseous state, so that the solid pyrolysis product, ash, leaves no carbon residue and essentially consists of the metallic components of the catalytic material 20, 22.

[0052] Here, in method step 4, the ash residue, which is a solid pyrolysis product, is dissolved in aqua regia to form a solution. Aqua regia consists of a mixture of hydrochloric acid HCl and nitric acid HNO3 in a ratio of 3:1. The process of the invention essentially distinguishes between catalytic materials 20, which are insoluble in aqua regia, i.e. iridium, and catalytic materials 22, which are soluble in aqua regia, i.e. platinum, and successively separates these components in only two subsequent steps. In method step 5, the solution is first heated to 100°C-110°C, so that the nitrates are thermally driven out of solution. This step 5 thus involves the separation of the noble metal nitrates, in particular platinum nitrate, and the regeneration of the catalytic materials 22, dissolved in aqua regia, which were used on the cathode side of the membrane electrode assembly 10 and which were originally applied to the intact membrane 24. These are noble metals, preferably platinum or binary platinum alloys, such as nickel-platinum, which are soluble in aqua regia, which are used as the second catalytic material 22. Thus, advantageously, the remaining solution after this treatment step 5 contains the aqua regia insoluble metallic first catalyst material 20, for example iridium, that was used on the anode side of the membrane electrode assembly.

[0053] This solid component with a high iridium content is now very advantageously and easily separated and isolated by filtration of the insoluble residue in process step 6, a filtration step, so that an insoluble residue is present which is already dominated by the high-value metal catalyst material 20, i.e. iridium.

[0054] As a further method step 7, the residue is finally dried at a constant drying temperature T D The residue is dried at a drying temperature T D The drying temperature T may be 60°C to 80°C. In order to shorten the drying time, a higher drying temperature T above 80°C may be used. Dmay be applied. Here, it is advantageous to recover a solid residue that contains or already consists largely of an insoluble metal catalytic material 20, i.e. iridium, in high purity. The iridium is present as iridium black with a purity of more than 90%, in particular 97% to 99.5%. The iridium as the noble metal first catalytic material 20 can be immediately regenerated and used for reuse in the membrane electrode assembly 10, for example, as the anode side catalytic material 20, applied to a new membrane 24. In order to achieve the desired particle size for the application and application of iridium to the prepared membrane 24, a grinding process of the recovered iridium black as the first catalytic material 20 may be provided in a further method step 8, if necessary. Here, the grinding degree or the average particle size of the recovered iridium is adjusted to ≧20 μm and ≦50 μm. In this method, it has been shown that the yield of recovered iridium is significantly higher than 80%. Based on the original iridium amount, a yield of more than 90%, for example 92%-96%, was achieved, making this method particularly and advantageously applicable for efficient recovery of iridium and platinum from spent membrane electrode assemblies 10 of PEM water electrolysis using proton permeable membranes (proton exchange membranes) made of PFSA (perfluorosulfuric acid).

[0055] As can be seen especially from the embodiment according to FIG. 3, the method according to the invention has proven to be particularly suitable for recovering valuable precious metal catalyst materials 20, 22 with high purity and recovery yield from used membrane electrode assemblies 10 in a recycling process.

[0056] Finally, in this way, the separation method of the present invention allows the production of new membrane electrode assemblies 10, and the iridium and platinum are reused. From a process engineering point of view, the cost of recovering pure iridium is significantly lower, due to the manageable number of processing steps. In particular, iridium black is immediately recovered, eliminating the need for complex and laborious conversion and regeneration steps.

[0057] The present invention allows for an easy and very efficient recovery of high quality and high purity iridium black catalyst from spent materials of membrane electrode assemblies 10 from PEM water electrolysis. Important advantages are seen in the cost reduction of the separation process, the cost reduction of the catalyst synthesis, the energy reduction over the whole process chain of the method, the reduction of the process duration and the high availability of the method. The present invention allows for a cost-effective and easily scalable recovery of iridium black on an industrial scale. Thereby, the precious metals iridium and platinum are again available in the manufacturing process of new membrane electrode assemblies 10 with maximum yield and quality.

[0058] The examples of the present invention mainly serve to illustrate the present invention and are not intended to limit it.

Claims

1. A method for recovering catalytic material from a membrane electrode assembly (10) for water electrolysis, comprising the steps of: - providing (1) a membrane electrode assembly (10) containing a membrane (24) coated with a metallic catalytic material (20, 22); - a step (2) of crushing the membrane electrode assembly (10); - pyrolysis of the pulverized membrane electrode assemblies (10) to obtain a solid pyrolysis product as a residue (3); - dissolving said solid pyrolysis product in a mixture of concentrated hydrochloric acid and concentrated nitric acid (4); - removing nitrates by heating the solution to 100°C to 110°C (5); - a step (6) of filtering the insoluble residue, - the drying temperature of the insoluble residue (T Ｄ ) to recover the metal catalyst material (20) (7).

2. 2. The method according to claim 1, wherein the insoluble residue is ground in a grinding step (8) to achieve an average particle size of from 10 μm to 80 μm, in particular from 20 μm to 50 μm.

3. The pyrolysis is carried out at a pyrolysis temperature (T Ｐ 3. The method according to claim 1 or 2, wherein the reaction is carried out at

4. 4. The method according to any one of claims 1 to 3, wherein the solid pyrolysis product is dissolved at a temperature of 70°C to 90°C, in particular 80°C, and the temperature is maintained during the dissolution for a period of 3 hours to 5 hours, in particular 4 hours.

5. The method according to any one of claims 1 to 4, wherein the metal components dissolved in the mixture are removed and separated by the heating, and the insoluble residue is recovered.

6. 6. The method of claim 5, wherein platinum (Pt) is separated and recovered as a dissolved metal component.

7. The method according to any one of claims 1 to 6, wherein iridium (Ir) is recovered as the metal catalyst material.

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

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

10. The method according to any one of claims 1 to 9, applied to a membrane electrode assembly (10) for PEM water electrolysis.

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