Method of recovering catalyst material from a membrane electrode assembly from water electrolysis

Abstract

The invention relates to a method of recovering catalyst material from a membrane electrode assembly (10) from water electrolysis, comprising the steps of: – providing (1) a membrane electrode assembly (10) comprising a membrane (24) coated with a metallic catalyst material (20, 22), – comminuting (2) the membrane electrode assembly (10), – pyrolytically decomposing (3) the comminuted membrane electrode assembly (10) to obtain a solid pyrolysis product as residue, – dissolving (4) the solid pyrolysis product in a mixture of concentrated hydrochloric acid and concentrated nitric acid, – removing (5) the nitrates by heating the solution to 100°C to 110°C, – filtering (6) the insoluble residue, – drying (7) the insoluble residue at a drying temperature (TD) to recover the metallic catalyst material (20). The method is preferably employed for the recycling of a membrane electrode assembly (10) from PEM water electrolysis, wherein iridium as metallic catalyst material (20) is recovered.

Description

[0001] Description

[0002] Process for the recovery of catalyst material from a membrane electrode assembly in water electrolysis

[0003] The invention relates to a process for recovering catalyst material from a membrane electrode assembly for water electrolysis. A used membrane electrode assembly containing a membrane coated with a metallic catalyst material is provided.

[0004] Hydrogen is an important substance used in countless industrial and technological applications. Hydrogen typically occurs on Earth only in a bound state. One of these substances that contains hydrogen in a bound state is water. Hydrogen can also serve as an energy storage medium, particularly for storing electrical energy generated using renewable energy generation methods for later use.

[0005] An important process for producing hydrogen is the electrolysis of water, particularly using electrical energy. Hydrogen can serve, among other things, as an energy store, for example by being used as a fuel to stabilize the electrical energy supply, particularly from renewable energies such as wind power, photovoltaics, or the like. But hydrogen can also be used for other processes that require a fuel or a reducing agent. The hydrogen produced by electrolysis can therefore be used industrially, for example, or electrical energy can be generated again electrochemically using fuel cells.

[0006] The separation of water into its chemical components hydrogen and oxygen can be carried out using suitable electrolysis cells. These can be designed as so-called polymer electrolyte membrane electrolysis cells. In such an electrolysis cell, a membrane is generally provided which has a respective catalyst layer on surfaces facing away from one another. The catalyst layers are generally bordered by respective gas diffusion layers, which in turn are bordered by respective electrically conductive contact plates, occasionally also called bipolar plates, which serve, among other things, for electrical contact. The contact plates or the bipolar plates are preferably also designed in such a way that they can enable the necessary material transport during electrolysis in the electrolysis cell during normal operation.For this purpose, appropriate channels can be provided for supplying a suitable electrolyte and for removing the reaction products of the electrolysis, namely a hydrogen gas and an oxygen gas. The gas diffusion layer generally provides electrical conductivity to electrically couple the contact plates and the catalyst layers. This allows the desired electrochemical reaction to be realized in the region of the catalyst layers.

[0007] If the electrolysis reaction takes place in the alkaline range, an anion exchange membrane (AEM) is used as the membrane. If, however, the electrolysis reaction takes place in the acidic range, a proton exchange membrane (PEM) is used instead.

[0008] The production of hydrogen from water occurs through the process of electrolysis. This is an electrochemical process in which water is separated into its chemical components, oxygen and hydrogen. The electrochemical cell reactions can be described and differentiated as follows, depending on the operating mode: Alkaline electrolysis:

[0009] Acid electrolysis:

[0010] In polymer electrolyte membrane electrolysis, the two partial reactions are spatially separated by an ion-conductive membrane. For electrolysis in alkaline environments, an anion exchange membrane (AEM) is used, whereas for electrolysis in acidic environments, a proton exchange membrane (PEM) is used.

[0011] However, the structural design of the membrane electrode assembly (MEA) can be fundamentally comparable in both cases.

[0012] The core of such a polymer electrolyte membrane electrolysis cell is usually formed by a membrane electrode assembly (MEA). The membrane electrode assembly has at least the membrane, which is usually coated on two opposite surfaces on both the anode and cathode sides with a layer of a respective catalyst material. The respective cell reaction of the electrolysis takes place in the region of the layer formed by the respective catalyst material. During normal operation, electrons are conducted to the contact plates via the respective catalyst material and a support structure, which can be formed by the gas diffusion layer or can provide it. For this reason, a high electrical conductivity of the catalyst layers is desired.

[0013] During the reaction in the alkaline range, hydroxide anions 0H~ are released in addition to electrons, and during operation in the acidic range, protons H + which are moved as charge carriers through the respective membrane. Therefore, good conductivity of the structure of the layers of the respective catalyst materials is desired in order to promote the respective ions to the membrane or from the membrane to the respective catalyst materials. It is a technical challenge to simultaneously provide a good ionic bond between the catalyst material and the membrane and good electrical conductivity of the catalyst material.

[0014] In the prior art, membrane electrode assemblies or units are produced by providing the catalyst materials in the form of a paste, which is applied either directly to the corresponding surface of the membrane or to the corresponding surface of a substrate. The pasty or paste-like catalyst material consists of the catalyst material itself, an ionomer, a polymeric binder and a solvent. After application, the solvent is usually removed or thermally driven off, for which purpose the layers of catalyst material are pressed onto the membrane at a high temperature, which is usually greater than 100 °C. This is intended to ensure ionic contact between the ionomer and the catalyst material and the membrane.

[0015] A membrane electrode assembly comprises a membrane with a catalyst material on two opposite surfaces. In many applications, particularly in PEM water electrolysis, very expensive and therefore very rare precious metals are used as catalyst material. Therefore, in addition to ecological considerations in favor of a circular economy, there is a very strong economic interest in reprocessing used membrane electrode assemblies in order to recover the valuable metallic catalyst material and, if necessary, use it in a new membrane electrode assembly or supply it for other applications.

[0016] The invention is based on the object of providing a method for recovering catalyst material from a membrane electrode arrangement which enables a particularly simple and cost-effective recovery of catalyst material.

[0017] The object is achieved according to the invention by a process for the recovery of catalyst material from a membrane electrode assembly of water electrolysis, comprising the steps of: providing a membrane electrode assembly containing a membrane coated with a metallic catalyst material, comminuting the membrane electrode assembly, pyrolytically decomposing the comminuted membrane electrode assembly, a solid pyrolysis product being obtained as residue, dissolving the solid pyrolysis product in a mixture of concentrated hydrochloric acid and concentrated nitric acid, removing the nitrates by heating the solution to 100 ° C to 110 ° C, filtering the insoluble residue, drying the insoluble residue at a drying temperature, the metallic catalyst material being recovered.

[0018] The invention is based on the general principle that recycling makes a significant contribution to protecting the environment. Recycling means sustainability, because raw material resources do not last forever. And recycling is hard cash, because the metallic catalyst materials are often precious metals that are far too valuable not to be recycled. On the other hand, with the large-scale use of electrolyzers for water electrolysis, a rapidly increasing demand for membrane electrode assemblies is to be expected, so that the requirements for the economic viability of known recycling processes are increasingly being placed in terms of cost and efficiency aspects.

[0019] Although numerous laboratory-scale processing and separation processes are described in the scientific literature, they are too complex and, as a result, very costly for commercial and large-scale implementation. Therefore, the need for a simple and cost-effective recovery of metallic catalyst material has been recognized by this invention.

[0020] The process of the invention enables, in particular, the recovery of metallic catalyst material, in particular precious metals, to be carried out very efficiently. The catalyst material on the membrane of a membrane electrode assembly can, for example, comprise one or more of the following substances on the anode side: nickel-aluminum, nickel-zinc, cobalt-aluminum, cobalt-iron, nickel-iron, nickel-iron-vanadium, nickel-cobalt, nickel-molybdenum, nickel-iron double-layered hydroxide, nickel-iron-cobalt, iridium, ruthenium oxide, nickel hydroxide, nickel oxide, nickel.

[0021] The catalyst material on the membrane can, for example, comprise one or more of the following substances on the cathode side, namely nickel, nickel-molybdenum on carbon black, nickel-molybdenum, nickel-platinum, platinum, nickel on carbon black, nickel-phosphate, nickel-vanadium.

[0022] This process initially involves mechanically comminuting a used membrane electrode assembly to a predetermined size. The economic and technical service life of the ion-conductive membrane is limited by various 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 can always occur during water electrolysis, small amounts of H2O2 or OH radicals can form within the electrolyzer, for example in the area of ​​the cells. It is generally known that such species can chemically attack the membrane material of PEM electrolyzers, with fluorides being released as degradation products during such degradation.

[0023] This is because the membrane contains a fluorine component and is made of, for example, PFSA “Perf luorosul f icacid”.

[0024] The membrane is a particularly important element for the functionality of the electrolysis cells in PEM electrolysis, so its service life and its limitations due to degradation effects are of great importance, especially from an economic perspective. In contrast, the metallic catalyst material is practically unused and is not prone to degradation, which is why reprocessing and recycling is very advantageous.

[0025] The invention is characterized by a considerably simplified process which requires only a few separation steps for the separation of precious metals from high-quality catalyst material, as has surprisingly been shown.

[0026] After the membrane electrode assembly has been crushed, pyrolysis takes place. Pyrolysis or pyrolytic decomposition describes various thermochemical conversion processes in which organic compounds are broken down at high temperatures and largely in the absence of oxygen. The high temperatures break some chemical bonds in the starting materials, and the lack of oxygen prevents complete combustion. The resulting products are diverse. The residue obtained in the process of the invention is a solid pyrolysis product, an ash, which has the appropriate granularity or grain size and contains metallic catalyst material. The ash contains a mixture of the catalyst material used on the anode side and the catalyst material used on the cathode side.In general, pyrolysis produces complex product mixtures of solid, liquid, and gaseous products, with the exact proportions depending on the specific conditions and the starting material. In principle, it can be said that higher temperatures and longer pyrolysis times result in more gaseous products, while lower temperatures and shorter times result in more liquid products. When polymers are pyrolyzed, the corresponding monomers are often formed as a product. The products can be used both as secondary energy sources because of their high energy content and for further material use by extracting individual chemicals from them. In the present process, higher temperatures are preferred in order to pyrolytically break down the comminuted membrane electrode assembly.

[0027] Preferably, the membrane is completely converted into the gas state during pyrolysis so that no carbon residues remain in the solid pyrolysis product, the ash.

[0028] The solid pyrolysis products, the ash residue, are now 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 according to the invention makes a fundamental distinction between catalyst material that is insoluble in aqua regia and catalyst material that is soluble in aqua regia, and separates these successively in just two subsequent steps. First, the solution is heated to 100°C to 110°C, and the nitrates are thermally driven out of the solution. This step thus involves the separation of precious metal nitrates and processing of the catalyst material dissolved in the aqua regia, which was preferably used on the cathode side of the membrane electrode arrangement and was applied to the intact membrane.These are precious metals soluble in aqua regia, such as platinum or binary platinum alloys, such as nickel-platinum, which are used as catalysts. Thus, after this process step, the metallic catalyst material insoluble in aqua regia, such as iridium, which was preferably used on the anode side of the membrane electrode assembly, advantageously remains in the residual solution. This solid component can now be removed very easily and advantageously in the filtration step. This results in an insoluble residue that predominantly already contains high-quality metallic catalyst material.

[0029] The next step involves drying the residue at a drying temperature for a drying time such that all liquid and, if present, gaseous components in the residue are removed or thermally expelled. A solid residue is then advantageously recovered, which contains the insoluble metallic catalyst material or already consists predominantly of it. The metallic catalyst material can be immediately processed and reused, for example, as anode-side catalyst material applied to a new membrane.

[0030] The process described here enables the particularly advantageous direct recovery of valuable anodic precious metal catalysts (OER catalysts) from the metal electrode arrangement of PEM water electrolysis, without dissolving, melting, or reprocessing them. The process described here significantly reduces the number of process steps and avoids loss paths in a large number of separation steps. From an economic perspective, further and complex reprocessing costs during separation (removal process) and the manufacturing costs for the precipitation of the new catalyst material are eliminated. The cathodic catalyst material can also be advantageously reused without major expenditure.The process merely distinguishes between catalyst material that is insoluble in aqua regia and catalyst material that is soluble in aqua regia, and separates each of these materials efficiently in a separate process step. Compared to previously described technical approaches, the process of the invention is superior both technically and economically. These previous approaches are based, for example, on pyrolysis of the membrane, dissolution of the catalysts or removal by complete dissolution of the catalyst components in a solvent. If insoluble metallic catalyst material is used, the option of dissolving it in aqua regia is eliminated, since the insoluble anodic precious metal catalyst material is not soluble in aqua regia. The subsequent separation steps required for the precious metal separation would therefore involve complex and loss-prone processes such as Na2O2 oxidation melts, etc.to convert metallic catalyst material, for example, into a soluble oxide of the metallic catalyst material. The invention specifically avoids the loss-prone path of reprocessing the catalyst material via conversion processes using soluble intermediates. From a process engineering perspective, this results in high process costs due to the numerous process steps involved in recovering the purest possible metallic catalyst material. Furthermore, expensive reactor linings are generally required due to the corrosiveness of the chemicals used. This is avoided by the invention.

[0031] It has been advantageously shown that the pyrolysis step did not result in any significant increase in particle size compared to a new catalyst. Furthermore, it has surprisingly been found that the metallic catalyst material is already of sufficient purity and good quality after the drying step, so that it can be used directly for the production of a new membrane electrode assembly. The process proposed here therefore enables a considerable reduction in the established process steps, thereby achieving significant cost and time savings in process control. Thus, in particular, the precious metal recovery can be increased thanks to the proposed process.

[0032] In a preferred embodiment of the process, the insoluble residue is ground so that an average grain size of 10 pm to 80 pm, in particular of 20 gm to 50 gm, is achieved.

[0033] After drying the insoluble residue at the drying temperature, the metallic catalyst material is already present in the form of solid particles. It can be advantageous to subject this residue to a grinding process until a desired grain size distribution, in particular a mean particle size, is achieved for the metallic catalyst material. The grinding process allows a more favorable surface / volume ratio of the catalyst particles to be achieved, which is advantageous for a subsequent coating process of a new membrane with the recovered catalyst material and for high catalytic efficiency with the lowest possible material usage in a membrane electrode assembly.

[0034] The pyrolysis step is preferably carried out at a pyrolysis temperature of 600 ° C to 1000 ° C, in particular of 700 ° C to 900 ° C. Thus, a high-temperature pyrolysis is carried out, so that the components of the membrane are converted into gaseous products by the pyrolysis and do not remain in the solid pyrolysis product, or rather in the residue, but are expelled.

[0035] In a particularly preferred embodiment of the process, 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, the temperature being maintained for between 3 hours and 5 hours, in particular 4 hours, during dissolution. It has been shown that a pyrolysis temperature of 80°C is particularly advantageous with an exposure time of 4 hours, so that a good dissolution result of the soluble components in the solid pyrolysis product is achieved. However, depending on the batch, this can be deviated from within certain limits, with lower temperatures requiring a longer exposure time and vice versa.

[0036] Preferably, by heating, metallic components dissolved in the mixture of hydrochloric acid and nitric acid are driven off and separated, with the insoluble residue being recovered. 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 driven off the solid residue by heating the solution to 100°C to 110°C, i.e., they escape from the residue by outgassing or evaporation. Advantageously, these species are also collected for a complete recycling process for the soluble species.

[0037] In a particularly preferred embodiment of the process, platinum is separated off as a dissolved metallic component and recovered. Platinum can advantageously be used as a metallic catalyst material on the cathode side of the membrane of a membrane electrode unit. Since platinum is an expensive precious metal, recovery makes economic sense. It is also possible for a binary metal containing platinum, such as platinum-nickel, to be used as a soluble metallic component and subjected to the recycling process. Platinum or platinum-nickel is particularly advantageous in a membrane electrode unit as a catalyst metal on the cathodic side of the membrane, particularly for applications in PEM electrolysis.

[0038] In a particularly preferred embodiment of the process, iridium is recovered as the metallic catalyst material. The insoluble component in the residue is then preferably iridium, which is not attacked by aqua regia. The iridium is advantageously of good quality and purity, recovered already as metal or metal particles in the residue, preferably after washing or rinsing the filter. The iridium recovered in this way can thus be applied directly as catalyst material to the anode side of a new ion-conducting membrane and is reusable for electrolysis purposes in new membrane electrode assemblies.

[0039] Preferably, the iridium is recovered as solid iridium black, achieving a purity of 97% to 99.5%, in particular 98% to 99.3% of iridium.

[0040] The iridium black is already available in the appropriate particle size, with the option of a grinding process to adjust the particle size for catalyst application in a membrane electrode assembly. The iridium is present as iridium black in high purity, so that, thanks to its pre-adjusted particle size, it can be directly reused for the production of new metal electrode assemblies. Iridium black is a solid with a metallic black color.

[0041] Preferably, a yield of recovered iridium black greater than 80%, in particular between 92% and 96%, based on the original amount of iridium is achieved.

[0042] Due to the simplified process, with essentially only one separation and treatment process for the soluble and insoluble metallic components, there is hardly any material loss, and a virtually closed and loss-free recycling process for the catalyst materials is achieved. In particular, the iridium catalyst material, which is insoluble in aqua regia, eliminates the need for additional and technically complex conversion processes, which would inevitably lead to iridium material losses in the precious metal separation process.

[0043] In a particularly preferred embodiment, the method is applied to a membrane electrode assembly for PEM water electrolysis. PEM electrolysis utilizes a proton-permeable membrane (proton-exchange membrane) made of PFSA (perfluorosulphonic acid). The PFSA membrane is a particularly important element for the functionality of the membrane electrode assembly and thus of an electrolysis cell. The membrane is coated on the anode and cathode sides with a respective catalyst material; preferably, a thin layer of iridium is applied to the anode side and platinum to the cathode side. Applications of the recycling method of the invention to membrane electrode assemblies of other electrolysis systems, such as alkaline electrolysis, are advantageously possible.

[0044] The exemplary embodiments explained below are preferred embodiments of the invention. The features and combinations of features specified above in the description, as well as the features and combinations of features mentioned in the following description of exemplary embodiments and / or shown alone in the figures, can be used not only in the respective combination specified, but also in other combinations. Thus, embodiments are also encompassed by the invention or are to be regarded as disclosed which are not explicitly shown and explained in the figures, but which arise from and can be produced by separate combinations of features from the explained embodiments.The features, functions, and / or effects illustrated by the exemplary embodiments may, in and of themselves, represent individual features, functions, and / or effects of the invention that can be viewed independently of one another, and which also further develop the invention independently of one another. Therefore, the exemplary embodiments are intended to encompass combinations other than those in the embodiments explained. Furthermore, the described embodiments may also be supplemented by further features, functions, and / or effects of the invention already described.

[0045] In the figures, like reference symbols denote like features or functions. They show:

[0046] FIG 1 shows a schematic sectional view through an electrolysis cell with a membrane electrode arrangement for the electrolysis of water;

[0047] FIG 2 shows a membrane electrode arrangement;

[0048] FIG 3 is a schematic representation of a process for the recovery of catalyst material from the membrane electrode assembly.

[0049] FIG 1 shows a schematic sectional view of an electrolysis cell 12 with a membrane electrode assembly 10. The membrane electrode assembly 10 in this case has a membrane 24 which has two opposite surfaces 16, 18 on which a respective catalyst material 20, 22 is arranged. The catalyst material 20, 22 in turn contacts a respective gas diffusion layer, which is formed by a respective nonwoven material 26, 28 and which in turn contacts a respective contact plate 34, 36 opposite it. This forms an anode region 30 and a cathode region 32. The contact plates 34, 36 as well as the gas diffusion layers are electrically conductive, so that an electrical contact is established between the respective catalyst materials 20, 22 and the respective contact plate 34, 36.The contact plates 34, 36 also have flow channels (not designated) through which water or electrolyte can be supplied to the electrolysis cell 12 on the one hand and electrolysis products, namely hydrogen and oxygen, can be removed on the other hand.

[0050] The membrane electrode assembly 10 is shown as a separate component in FIG. 2. The membrane electrode assembly 10 comprises, in addition to a membrane 26, the layers of catalyst material 20, 22. The membrane electrode assembly 10 can be manufactured as a separately handleable component, so that the membrane electrode assembly 10 can be easily handled during the manufacturing process of the electrolysis cell 12.

[0051] The membrane electrode assembly 10 comprises at least the anode-side and the cathode-side catalyst layer, which are generally connected to the membrane 24 to form one component. The respective chemical reactions take place in the catalyst layers, wherein electrons can be conducted via the catalyst and any support structure that is electrically conductive to the contact plates 34, 36. Therefore, it is advantageous if the respective layer of catalyst material 20, 22 has the best possible electrical conductivity and a catalytic capacity. In addition, hydroxide ions OH~ are formed in an alkaline medium and protons H in an acidic medium. +generated, which migrate as charge carriers through the respective membrane. It is therefore also desired for the catalyst materials 20, 22 to have a correspondingly good conductivity for the respective ions, so that these can be easily transported to the membrane 24 or from the membrane 24 to the respective catalytic centers. It is therefore desired that a good ionic bond of the respective catalyst material 20, 22 to the respective surface 16, 18 of the membrane 24 and, at the same time, a good electrical conductivity of the catalyst material 20, 22 can be provided.

[0052] The membrane electrode assembly 10 is specially designed for use in an acidic environment for PEM water electrolysis. For this purpose, the membrane 24 is prepared as a proton-conductive membrane 24. The anode-side catalyst layer has iridium as the first catalyst material 20. The cathode-side catalyst layer has platinum as the second catalyst material 22. The iridium is applied to the first surface 16 of the membrane 24 and the platinum to the opposite second surface 18 of the membrane 24. The membrane 24 acts as a substrate 14. When a membrane electrode assembly 10 is manufactured from scratch or is recycled or reprocessed, a membrane 24 is first provided as the substrate 14 to produce the membrane electrode assembly 10, and this membrane 24 is coated. The membrane 24 has the surface 16 and the second surface 18 facing away from the first surface 16.For the purpose of coating with the respective catalyst materials 20, 22, a coating tool (not shown in detail) is used. The catalyst materials 20, 22 are then provided in paste form, in particular as a pasty mass, so that they can be applied well and intimately to the respective surfaces 16, 18 using the coating tool. In this case, it can be provided that particles of the respective catalyst material 20, 22 are simply dissolved and mixed with an ionomer in a highly viscous solvent. The production of the catalyst paste is not shown in detail in the figures. However, conventional methods for mixing substances can be used for this purpose. In contrast to the prior art, however, no polymeric binder or non-ionic binder needs to be added in this method.The viscosity of the paste can be adjusted by adjusting the ionomer content in the solvent, allowing conventional industrial coating processes for electrode pastes to be used. Alternatively, doctor blade or dip coating can also be used.

[0053] If a used membrane electrode assembly 10 is involved, it is advantageous from an economic and ecological point of view to recover the valuable 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 made available for the recycling process proposed here in a method step 1. The making available 1 and the further method steps for recovering catalyst material 20, 22 from the membrane electrode assembly are shown schematically in the flow diagram in FIG 3. In this case, a used membrane electrode assembly 10 is first mechanically crushed to a predetermined degree of comminution in a method step 2. The economic and technical service life of the ion-conductive membrane 24 is limited by various influencing factors.For example, degradation effects on the membrane 24 are described, which damage the membrane material and impair its function.

[0054] After comminution, pyrolysis is carried out in a process step 3. The thermal treatment of the crushed membrane electrode assembly 10 is carried out in a corresponding pyrolysis reactor at a pyrolysis temperature T Pfrom 600 ° C to 1000 ° C . This involves various thermochemical conversion processes in which organic compounds are split at high temperatures and largely in the absence of oxygen. The high temperatures break down some chemical bonds in the starting materials, and the lack of oxygen prevents complete combustion. The products formed are diverse. The residue obtained in process step 3 is a solid pyrolysis product, an ash which has the appropriate granularity or grain size and which contains metallic catalyst material 20, 22. The ash contains a mixture of the catalyst material 20 used on the anode side and the catalyst material 22 used on the cathode side. In the example described here, this is iridium as the anode-side catalyst material 20 and platinum as the cathode-side catalyst material 22.

[0055] In the present process, higher pyrolysis temperatures T P in a range between 600 ° C and 1000 ° C is preferably used to pyrolytically decompose the crushed membrane electrode assembly 10. The membrane 24 is completely converted into the gas state during pyrolysis, so that no carbon residues remain in the solid pyrolysis product, the ash, but essentially the metallic components of the catalyst material 20, 22.

[0056] Next, in process step 4, the solid pyrolysis products, the ash residue, are 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 according to the invention fundamentally distinguishes between catalyst material 20, iridium, which is insoluble in aqua regia, and catalyst material 22, platinum, which is soluble in aqua regia, and separates these components successively in just two subsequent steps. First, in process step 5, the solution is heated to 100°C to 110°C, and the nitrates are thermally expelled from the solution.In this step 5, noble metal nitrates, in particular platinum nitrates, are separated off, and the catalyst material 22 dissolved in the aqua regia, which was used on the cathode side of the membrane electrode assembly 10 and applied to the originally intact membrane 24, is processed. These are noble metals that are soluble in aqua regia, such as preferably platinum or binary platinum alloys, such as nickel-platinum, which are used as the second catalyst material 22. Thus, after this process step 5, the metallic first catalyst material 20 that is insoluble in aqua regia and was used on the anode side of the membrane electrode assembly, such as iridium, advantageously remains in the residual solution.

[0057] This solid component with a high iridium content is then very advantageously and simply separated in a process step 6, a filtration step, by filtering the insoluble residue. This results in an insoluble residue that predominantly already contains high-quality metallic catalyst material 20, namely iridium.

[0058] Finally, in process step 7, the residue is dried at a drying temperature T D over a drying time so that all liquid and possibly still gaseous components in the residue are removed or thermally expelled. The drying temperature T D can be 60 ° C to 80 ° C. To shorten the drying time, higher drying temperatures T Dof over 80 ° C. A solid residue is now advantageously recovered which contains the insoluble metallic catalyst material 20, namely iridium in high purity or already consists predominantly thereof. The iridium is present as iridium black with a purity of over 90%, in particular from 97% to 99.5%. The iridium as noble metallic first catalyst material 20 can be processed directly and fed for reuse in a membrane electrode arrangement 10, for example applied as anode-side catalyst material 20 to a new membrane 24. A grinding process for the recovered iridium black as first catalyst material 20 can be provided in a further method step 8 if required in order to achieve a desired particle size for the application and application of the iridium to a provided membrane 24. In this case, a degree of grinding orAn average particle size of the recovered iridium of 20 pm to 50 pm was set. The process has shown that the yield of recovered iridium is significantly greater than 80%. Yields of over 90%, for example, between 92% and 96%, based on the original amount of iridium, have been achieved. Thus, the process is particularly and advantageously applicable for the efficient recovery of iridium and platinum from used membrane electrode assemblies 10 from PEM water electrolysis with a proton-permeable membrane (proton-exchange membrane) made of PFSA "perfluorosulphuric acid".

[0059] The method according to the invention proves to be particularly suitable, as can be seen in particular from the exemplary embodiment according to FIG. 3, for recovering the valuable noble metallic catalyst material 20, 22 from a used membrane electrode assembly 10 in a recycling process with high purity and recovery yield. The end result is that in this way, a new membrane electrode assembly 10 can be produced using the separation method of the invention, the iridium and platinum being reused. From a process engineering point of view, the manageable number of process steps in the recovery of pure iridium results in significantly lower costs. In particular, iridium black is recovered directly, so that complicated and expensive conversion and processing steps are no longer necessary.

[0060] The present invention enables a simplified and very efficient recovery of iridium black catalysts of high quality and purity from used materials of a membrane electrode assembly 10 from PEM water electrolysis. Significant advantages can be seen in the cost savings in the separation process, cost savings in catalyst synthesis, energy savings within the entire process chain of the method, shorter process times and high availability of the method. The invention enables a cost-effective and easily scalable recovery of iridium black on an industrial scale. The precious metals iridium and platinum can thus be made available again in the production process for new membrane electrode assemblies 10, with maximum yield and quality.

[0061] The embodiments of the invention serve solely to explain the invention and are not intended to limit it.

Claims

Patent claims 1. Method for recovering catalyst material from a membrane electrode arrangement (10) of water electrolysis, comprising the steps: - Providing (1) a membrane electrode arrangement (10) comprising a membrane (24) coated with a metallic catalyst material (20, 22) , - Comminution (2) of the membrane electrode arrangement (10) , - Pyrolytic decomposition (3) of the comminuted membrane electrode arrangement (10) , whereby a solid pyrolysis product is obtained as residue, - Dissolving (4) the solid pyrolysis product in a mixture of concentrated hydrochloric acid and concentrated nitric acid, - Removal (5) of the nitrates by heating the solution to 100°C to 110°C, - Filtering (6) the insoluble residue, - Drying (7) the insoluble residue at a drying temperature (T D) , whereby the metallic catalyst material (20) is recovered.

2. Method according to one of the preceding claims, wherein the insoluble residue is ground in a grinding process (8) such that an average particle size of 10 pm to 80 pm, in particular of 20 pm to 50 pm, is obtained.

3. The method of claim 1 or 2, wherein the pyrolysis is carried out at a pyrolysis temperature (T P ) from 600°C to 1000°C, especially from 700°C to 900°C.

4. Method according to 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 a temperature maintenance of between 3h and 5h, in particular of 4h hours, is applied during dissolution.

5. A method according to any of the preceding claims, wherein metallic components dissolved in the mixture are driven off and separated by heating, the insoluble residue being obtained.

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

7. A method according to any of the preceding claims, wherein iridium (Ir) is recovered as the metallic catalyst material.

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

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

10. Method according to one of the preceding claims, applied to a membrane electrode arrangement (10) of PEM water electrolysis .

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