Method for preparing partially sulphonated polyphenylene sulphone and PEM membrane arrangement having a catalyst-coated membrane

Abstract

The invention relates to a method (100) for preparing a reactant for partial sulphonation of polyphenylene sulphone, the method having the following steps: S1: introducing polyphenylene sulphone into sulphuric acid at least up to the solubility limit and S2: separating undissolved constituents to obtain the reactant. The invention also relates to methods for preparing partially sulphonated polyphenylene sulphone (1), to a method (400) for producing a catalyst-coated membrane (10) for an electrochemical cell and to a PEM membrane arrangement having a catalyst-coated membrane (10).

Description

[0001] Description

[0002] Process for the preparation of partially sulfonated polyphenylene sulfone and PEM membrane assembly with a catalyst-coated membrane

[0003] The invention relates to a process for producing a reactant for a partial sulfonation of polyphenylenesulfone, a process for producing partially sulfonated polyphenylenesulfone, a process for producing a catalyst-coated membrane for an electrochemical cell, a PEM membrane arrangement with a catalyst-coated membrane, an electrolysis cell, a cell stack and an electrolysis plant.

[0004] Hydrogen can be produced by electrolysis of water, using various technologies. One technology already industrially established is polymer electrolyte membrane water electrolysis (PEMWE). The core element here is the polymer electrolyte membrane, which is currently based on perfluorinated tonomers. Tonomers are polymers with ionic groups, which give them the property of proton conductivity. In addition to the membrane, components of the electrodes are also currently made of fluorinated polymers.

[0005] The fluorinated polymers used to date for PEMWE all belong to the group of PEAS (per- and polyfluoroalkyl substances), which will be completely banned by the European Chemicals Agency ECHA from 2025 for health and environmental reasons or can only be used with strict exemptions for a currently undefined transitional period. Therefore, in order to improve environmental and health protection and to comply with the changed legal framework, it is necessary to develop fluorine-free toners that can be used as substitute materials for the current fluoropolymers in electrodes and / or membranes. For example, DI VONA, ML, SGRECCIA, E., TAMILVANAN, M., KHADHRAOUI, M., CHASSIGNEUX, C . , KNAUTH, P. High ionic exchange capacity Polyphenylenesulfone (SPPSU) and polyethersulfone (SPES) cross-linked by annealing treatment: Thermal stability, hydration level and mechanical properties. J. Mem. Sei.2010, Vol. 354, pp. 134 - 141, membranes made of sulfonated polyphenylene sulfone (SPPSU) are known. A production process for SPPSU is disclosed in which polyphenylene sulfone (PPSU) was dissolved in 96% sulfuric acid at 50 °C. The solution was then transferred to ice-cold water, whereupon a white solid precipitated, which was filtered off and washed with cold water to neutralize it. The resulting SPPSU was then dried under vacuum at room temperature. To obtain a membrane, the dried SPPSU was dissolved in dimethyl sulfoxide, poured into a Petri dish, and dried for 16 hours at 120 °C. The resulting membrane was detached from the Petri dish, dried under vacuum at 80 °C for a further 24 hours, and then heated to 170 °C for crosslinking.

[0006] However, the inventors of the present invention found that the yield of partially functionalized SPPSU in the preparation according to this process is low and the described process is therefore unsuitable for large-scale use.

[0007] There are also known experiments in which the water-soluble portion of the reaction product of PPSU with concentrated sulfuric acid—i.e., the SPPSU contained in the liquid phase after separation of the insoluble portion—was used to produce membranes. This is presumably highly functionalized SPPSU. However, this is readily water-soluble and highly sensitive to hydrolysis. Although highly functionalized SPPSU can be further processed into membranes, subsequent cross-linking is essential for stabilization to reduce water solubility and hydrolysis sensitivity. However, investigations by the inventors of the present invention have shown that even with subsequent cross-linking, if at all possible, sufficient long-term stability cannot be achieved, particularly at elevated temperatures.

[0008] Against this background, it is the object of the present invention to provide possibilities by which partially sulfonated PPSU can be obtained with high yield, which can be used, for example, for the production of membranes and / or electrodes for water electrolysis.

[0009] This problem is solved by the subject matter of the independent claims. The dependent claims relate to embodiments.

[0010] A first aspect of the invention relates to a process for preparing a reactant for the partial sulfonation of polyphenylene sulfone (GAS No. 25608-64-4). Partially sulfonated polyphenylene sulfone (partially sulfonated PPSU) can be understood in particular as a sulfonated PPSU with a ion exchange capacity of less than 1 mmol / g, preferably less than 0.5 mmol / g, more preferably less than 0.2 mmol / g, for example less than 0.1 mmol / g or even less than 0.05 mmol / g. The ion exchange capacity can be determined by acid-base titration.

[0011] The proposed process comprises the following steps: introducing polyphenylene sulfone (PPSU) into sulfuric acid, preferably concentrated sulfuric acid, for example with a proportion of 96 wt% to 98 wt% in aqueous solution, at least up to the solubility limit and separating undissolved components to obtain the reactant.

[0012] The resulting reactant can subsequently be used to produce partially sulfonated PPSU, as explained in more detail below.

[0013] In the first process step, PPSU, e.g., in the form of PPSU granules or powder, is added to the sulfuric acid. At least enough PPSU is added to the sulfuric acid until the solubility limit of the PPSU is reached, which is indicated by the formation of a sediment, turbidity, etc.

[0014] The PPSU can preferably be introduced into the sulfuric acid with stirring at a temperature between 20 °C and 120 °C, preferably between 40 °C and 60 °C. A preferred reaction time can be between 3 h and 24 h, preferably between 5 h and 7 h, more preferably 6 h. The pressure can, for example, be in a range from 0.1 bar to 100 bar and in particular correspond to atmospheric pressure. Optionally, the PPSU can be introduced into the sulfuric acid under a protective gas atmosphere, e.g., in an argon atmosphere.

[0015] In a further process step, the undissolved components are separated from the reaction mixture, e.g. by filtration, in order to obtain the desired reactant as a liquid phase.

[0016] Investigations by the present inventors have shown that a portion of the PPSU introduced into the sulfuric acid solution is highly functionalized with sulfonic acid groups and is therefore soluble. Consequently, the resulting reactant contains sulfuric acid and dissolved PPSU highly functionalized with sulfonic acid groups. The separated residue essentially comprises non- and partially functionalized PPSU.

[0017] The resulting reactant enables partial sulfonation of PPSU with high yield, i.e. subsequent sulfonation of PPSU using the resulting reactant occurs with high selectivity with respect to partially functionalized products.

[0018] A further aspect of the invention relates to a process for the partial sulfonation of polyphenylene sulfone or for the production of partially sulfonated polyphenylene sulfone. The proposed process comprises the following steps: adding polyphenylene sulfone to a reactant prepared according to a process as described above and separating the partially sulfonated polyphenylene sulfone as an insoluble reaction product.

[0019] After adding the polyphenylene sulfone to the reactant, the polyphenylene sulfone is functionalized with sulfonic acid groups. The use of the proposed reactant ensures that this functionalization preferentially leads to partial functionalization, resulting in partially sulfonated PPSU.

[0020] The partially sulfonated PPSU is then separated from the liquid phase as an insoluble reaction product, for example, by centrifugation or filtration. After optional washing with water and / or ethanol, the partially sulfonated PPSU can be dried, for example, at 60 °C for 12 hours. The result is powdered partially sulfonated PPSU.

[0021] The liquid phase can be reused as a reactant for the partial sulfonation of polyphenylene sulfone and, for example, recycled. This reduces the required quantities of sulfuric acid, which is advantageous both for environmental and cost reasons.

[0022] The PPSU can preferably be introduced into the reactant with stirring at a temperature between 20 °C and 120 °C, preferably between 40 °C and 60 °C. A preferred reaction time can be between 3 h and 24 h, preferably between 5 h and 7 h, more preferably 6 h. The pressure can, for example, be in a range from 0.1 bar to 100 bar and in particular correspond to atmospheric pressure. Optionally, the reaction can take place under a protective gas atmosphere, e.g. in an argon atmosphere. The proportion of PPSU can preferably be between 5 wt% and 30 wt%, preferably between 10 wt% and 20 wt%. In other words, the PPSU can be added to the reactant in such a way that a composition with the stated weight proportion of PPSU results immediately after addition of the PPSU.

[0023] The proposed process enables the production of partially sulfonated PPSU with high yield, which can be used to produce membranes and / or electrodes for water electrolysis, which have high chemical stability and thermal resistance up to 160 ° C and are long-term stable.

[0024] The resulting partially sulfonated PPSU, for example, is soluble up to 25 wt% in l-methyl-2-pyrrolidone and can be used in particular as a binder ionomer for PEM water electrolysis. Compared to sulfonated polyetheretherketone (PEEK), the material exhibits particularly high stability against nucleophilic attack. It has been shown that the resulting partially sulfonated PPSU is stable in O2 and H2 atmospheres at temperatures above 80 °C for a period of more than 4 weeks, as well as against OH radical attack triggered by storage in 10 wt% H2O2 under the same conditions.

[0025] In addition to improved environmental compatibility, the partially sulfonated PPSU as an ionomer offers the possibility of improving thermal, mechanical, and / or chemical stability. Due to its modified chemical structure compared to conventional PEAS, it also has the potential to solve other problems regarding long-term stability and gas transfer (gas crossover).

[0026] A further aspect of the invention relates to another process for the preparation of partially sulfonated polyphenylenesulfone. This process provides for a total synthesis of partially sulfonated polyphenylenesulfone.

[0027] Total synthesis, also known as full synthesis, means that the partially sulfonated polyphenylene sulfone is produced entirely from simple basic substances.

[0028] The process can, for example, comprise reacting at least one hydroxy component, for example a dihydroxy component, with at least one halogen component, wherein at least one of the two components, preferably both components, has one or more sulfonic acid groups and / or corresponding salts. The hydroxy component can, for example, be 4,4'-dihydroxy-(1,1'-biphenyl)-3-sulfonic acid or 4,4'-dihydroxydiphenylsulfone. The halogen component can, for example, be a bis(halophenyl)sulfone, e.g. 2-fluoro-5-((4-fluorophenyl)sulfonyl)benzenesulfonic acid or 2-chloro-5-((4-chlorophenyl)sulfonyl)benzenesulfonic acid, or disodium bis(4-chloro-3-sulfophenyl)sulfone (GAS No. 51698-33-0).

[0029] The use of bis(halophenyl)sulfone 2-fluoro-5-((4-fluorophenyl)sulfonyl)benzenesulfonic acid enables a particularly high reaction rate. The use of several of the above-mentioned hydroxy or halogen components is possible, allowing the properties of the reaction product to be specifically influenced.

[0030] Depending on the specific reactants, the reaction can be carried out under alkaline conditions, for example by adding sodium hydroxide, potassium hydroxide, potassium carbonate and / or sodium carbonate.

[0031] This is a polycondensation reaction. In the reaction of 4,4'-dihydroxy-(1,1'-biphenyl)-3-sulfonic acid with bis(halophenyl)sulfone 2-fluoro-5-((4-fluorophenyl)sulfonyl)benzenesulfonic acid, according to a non-binding reaction mechanism, the corresponding sodium salt of the hydroxy component is formed in situ in a first step by reaction of the 4,4'-dihydroxy-(1,1'-biphenyl)-3-sulfonic acid with sodium hydroxide. In a second step, the actual polycondensation of the sodium salt of the dihydroxy component with the bis(halophenyl)sulfone takes place, giving partially sulfonated PPSU, in which a sulfonic acid group is formed on the diphenylsulfone unit and a further sulfonic acid group on the biphenyl unit.

[0032] The resulting partially sulfonated PPSU precipitates and can be separated, e.g., by filtration, centrifugation, etc. After optional washing with water and / or ethanol, the partially sulfonated PPSU can be dried, e.g., at 60 °C for 12 h. The result is powdered partially sulfonated PPSU.

[0033] The polycondensation reaction can be carried out, for example, at a temperature in a range from 0°C to 285°C. The pressure can, for example, be in a range from 0.1 bar to 100 bar and, in particular, correspond to atmospheric pressure. The ratio of the molar amount of the hydroxy component to the molar amount of the halogen component can, for example, be in a range from 0.5 to 2 and, in particular, be 1. It is conceivable to initially charge demineralized water and to prepare a solution of the hydroxy component, in particular in a sulfolane-NaOH solution containing 10 wt% to 50 wt% NaOH, and a solution of the halogen component, in particular in a sulfolane-NaOH solution containing 10 wt% to 50 wt% NaOH. The solution of the hydroxy component and the solution of the halogen component can be added to the water simultaneously while stirring.The concentration of the hydroxy component in the solution of the hydroxy component and the concentration of the halogen component in the solution of the halogen component can, for example, be in a range of 0.5 mol / l to 5 mol / l.

[0034] According to various embodiments, the hydroxy component and the halogen component, for example, 4,4'-dihydroxy-(1,1'-biphenyl)-3-sulfonic acid and bis(halophenyl)sulfone, can each be dissolved in sulfolane (tetrahydrothiophene-1,1-dioxide, CAS No. 126-33-0) and used as sulfolane solutions. Sulfolane is characterized by aprotic properties and a high boiling point.

[0035] Optionally, the process can involve terminating the reaction by adding a chain terminator, e.g., chloromethane. This allows the chain length to be regulated to a range that allows for industrial melt processing.

[0036] The proposed further process for the production of partially sulfonated polyphenylene sulfone also enables the production of partially sulfonated PPSU with high yield, which can be used to produce membranes and / or electrodes for water electrolysis that have high chemical stability and thermal resistance and are long-term stable.

[0037] The resulting partially sulfonated PPSU is soluble up to 25 wt% in l-methyl-2-pyrrolidone and can be used particularly as a binder ionomer and for the production of membranes for PEM water electrolysis. Compared to sulfonated polyetheretherketone (PEEK), the material exhibits particularly high stability against nucleophilic attack. It was shown that the resulting partially sulfonated PPSU is stable in O2 and H2 atmospheres at temperatures above 80 °C for a period of more than 4 weeks, as well as against OH radical attack triggered by storage in 10 wt% H2O2 under the same conditions.

[0038] A further aspect of the invention relates to the use of partially sulfonated polyphenylene sulfone produced by one of the processes described above for producing an electrode or membrane for carrying out water electrolysis.

[0039] The partially sulfonated PPSU produced has, due to its

[0040] Sulfonic acid groups have ionic conductivity and can therefore be used as an ionomer. For example, it can be used as a so-called binder ionomer. The term "binder ionomer" refers to the ionomer's ability to act as a binding agent between the membrane and the catalyst and to enable stable attachment of the catalyst in the vicinity of the membrane. For example, the anode of a membrane electrode assembly (MEA) can be formed by applying a catalyst dispersed in a so-called binder ionomer to the membrane surface. The membrane itself can also comprise partially sulfonated PPSU or consist of partially sulfonated PPSU produced according to the proposed process. Alternatively or additionally, another, preferably fluorine-free ionomer can be used to produce the membrane.

[0041] In addition, a membrane comprising partially sulfonated PPSU, which has been produced according to the proposed method, or consisting of such partially sulfonated PPSU can be combined with another, preferably fluorine-free, ionomer as electrode material.

[0042] The electrodes and membranes made from partially sulfonated PPSU for water electrolysis are characterized by improved environmental compatibility. They therefore represent an effective alternative in the event of a potential ban on PEAS. Due to the improved temperature stability, higher process temperatures can be selected for electrolysis. Furthermore, the membrane is characterized by lower gas transfer, resulting in electrolysis with higher efficiency.

[0043] A further aspect of the invention relates to a process for producing a catalyst-coated membrane for an electrochemical cell, in particular an electrolysis cell. The proposed process comprises the following steps: providing solid partially sulfonated polyphenylene sulfone, dispersing the partially sulfonated polyphenylene sulfone in an anhydrous dispersant to obtain a dispersion, mixing a catalyst material with the dispersion to form a catalyst paste, and applying the catalyst paste to a membrane substrate.

[0044] As solid partially sulfonated PPSU, partially sulfonated polyphenylene sulfone produced by a process as described above can be used in solid form, e.g. as powder or granules.

[0045] The catalyst-coated membrane (CCM) has at least one porous electrode, which is realized by applying and bonding a catalytically active porous layer. The catalyst-coated membrane can therefore also be understood as a membrane electrode assembly (MEA).

[0046] The method comprises, in a first step, the provision of a solid, for example powdered, i.e. preferably undissolved or in solution, partially sulfonated PPSU.

[0047] The process comprises, in a further step, dispersing the partially sulfonated PPSU in an anhydrous dispersant to obtain a dispersion.

[0048] The proportion of partially sulfonated polyphenylenesulfone in the dispersion can, for example, be between 2 wt% and 15 wt%. Such a proportion of partially sulfonated polyphenylenesulfone has resulted in particularly good processability of the dispersion in the subsequent process steps.

[0049] The anhydrous dispersant can, for example, be selected from a group comprising: dimethyl sulfoxide, N,N-dimethylformamide, N,N-diethylformamide, 2-pyrrolidone, N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, a mixture of N-methyl-2-pyrrolidone and a portion selected from N-ethyl-2-pyrrolidone, γ-butyrolactone or diethylene glycol monoethyl ether.

[0050] It has been shown that dispersants from the above-mentioned group, in combination with the partially sulfonated PPSU, promote the formation of an intermediate layer ("interlayer") as a functional layer for good and long-term stable adhesion of the catalyst to the membrane substrate, i.e., for example, a material-tight bond can be established. This is achieved by a targeted and increased dissolution of the surface of the membrane substrate compared to water-based dispersants. This primarily improves the electrochemical stability of the catalyst-coated membrane and high long-term stability when used in an electrochemical cell.By using a mixture of dispersants, a further increased possibility of adaptation in the production of the catalyst paste is provided, so that, depending on the coating system of the catalyst layer, a sufficiently high dissolution of the surface of the membrane substrate and an intimate material bond in the area of ​​the intermediate layer are made possible.

[0051] A dispersion of the partially sulfonated PPSU in l-methyl-2-pyrrolidone has proven particularly stable over time. No gel formation was observed.

[0052] The process comprises, in a further step, the intimate mixing of a catalyst material, for example in solid form or in powder form, with the dispersion to form a catalyst paste.

[0053] The combined proportion of the catalyst material and the partially sulfonated polyphenylene sulfone in the catalyst paste can be 35 wt% to 60 wt%, in particular 45 wt% to 55 wt%. This allows for particularly good and uniform application of the catalyst paste to the membrane substrate in the subsequent process step. The catalyst material can comprise, for example, iridium, in particular solid or powdered iridium black, and / or platinum, in particular solid or powdered platinum black.

[0054] An iridium-containing catalyst material can be used in particular as an OER catalyst for an anode-side oxygen evolution reaction. Alternatively or additionally, the anode-side catalyst material can contain IrOOH, IrCt, IrRuCt and / or TiCf-, NbCp- or SnCp-supported variants of the aforementioned catalysts or corresponding material systems.

[0055] A platinum-containing catalyst material can be used, in particular, as a HER catalyst for a hydrogen evolution reaction at the cathode. Alternatively or additionally, palladium, ruthenium, and / or carbon-supported variants / mixtures of the aforementioned catalysts can be included.

[0056] When formulating the catalyst paste, the choice of dispersant is of great importance. Compared to conventional coating methods, the use of an anhydrous dispersant for the catalyst paste is preferred. In contrast to the previously common processing of fluorine-free ionomers using water / alcohol mixtures based on methanol, ethanol, or isopropanol, the electrode compositions described here create a more stable "interlayer," i.e., an intermediate layer or bonding layer between the membrane substrate and the electrode, which leads to increased electrochemical stability and thus to longer service life when used in water electrolysis.

[0057] The process comprises, in a further step, the preferably direct application (direct membrane coating) of the catalyst paste onto a corresponding membrane substrate. A previously described catalyst paste can be used very advantageously in a direct membrane coating process. This offers significantly greater application potential compared to a decal process, in which the catalyst layer is first deposited onto a thermally stable carrier film, the dispersant is subsequently removed thermally, and the catalyst layer is transferred to the membrane in a further process step under the influence of pressure and temperature.

[0058] The catalyst paste can be applied to the membrane substrate, for example, by means of a doctor blade application, a roll-to-roll application process, a slot die coating, a spraying process, a screen printing process and / or via an application roller. Optionally, a vacuum plate can be used to hold the membrane substrate. The pressure during application can, for example, be in a range from 0.1 bar to 100 bar and, in particular, correspond to atmospheric pressure. The temperature during application can, for example, be in a range from 0°C to 100°C and, in particular, be room temperature.

[0059] Optionally, the process can include a further process step involving thermal post-treatment of the membrane substrate with the applied catalyst paste, for example in an oven. The thermal post-treatment can be carried out, for example, at a temperature between 110 °C and 160 °C for a period of between 1 and 15 minutes. The thermal post-treatment can improve the bonding of the catalyst layer to the membrane substrate and thus contribute to even greater long-term stability. In addition, the thermal post-treatment influences the formation of the electrode structure by influencing the evaporation rate of the dispersant and the volume reduction.

[0060] The proposed process advantageously enables cost-effective and easily scalable production of large-area catalyst-coated membranes for PEM water electrolysis. In particular, the application of the catalysts, whether as a so-called HER (hydrogen evolution reaction) for the hydrogen evolution reaction at the cathode or as an OER (oxygen evolution reaction) for an anode-side oxygen evolution reaction, can be significantly simplified and improved, thus enabling the use of fluorine-free, catalyst-coated membranes (COM) based on partially sulfonated PPSU for HER and OER in an electrolysis cell.

[0061] Furthermore, the proposed process enables the production of fluorine-free membranes (CCMs) coated with catalyst material on an industrial scale, e.g. using a roll-to-roll application technique, and thus advantageously faster throughput times for corresponding electrolyzer components. The presented process also makes it possible to significantly improve both the ionic and mechanical bonding of the catalyst material to the membrane substrate for fluorine-free membrane electrode assemblies (MEAs). Furthermore, the advantageously high material compatibility and very good paste stability of the catalyst paste as well as the improved sedimentation behavior of this paste formulation and application are worth highlighting. From a technical and economic point of view, complex pressing processes and post-treatment processes are also no longer necessary.

[0062] A further aspect of the invention relates to a catalyst-coated PEM membrane arrangement which is manufactured or can be manufactured by the described method for producing a catalyst-coated membrane.

[0063] Another aspect of the present invention relates to an electrolysis cell with a PEM membrane arrangement.

[0064] A further aspect of the invention relates to a cell stack with a plurality of such electrolysis cells, i.e., comprising a plurality of such electrolysis cells stacked and electrically connected in series. These form a cell stack or electrolysis stack that is scalable for large electrolysis capacities.

[0065] A further aspect of the invention relates to an electrolysis plant with such a cell stack.

[0066] Embodiments, features and / or advantages which relate to the methods in the present case also relate to the manufactured product itself or the membrane arrangement as well as the electrolysis cell, the cell stack, the electrolysis plant and / or an entire Power-to-X power plant, and vice versa.

[0067] The term "and / or" as used herein, when used in a series of two or more elements, means that any one of the listed elements may be used alone, or any combination of two or more of the listed elements may be used.

[0068] The above-described properties, features and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more clearly understandable in connection with the following description of the embodiments, which are explained in more detail in connection with the figures, wherein the figures show the following:

[0069] Fig. 1 shows a reaction scheme for the sulfonation of PPSU;

[0070] Fig. 2 is a flow diagram of a first exemplary process for producing partially sulfonated PPSU;

[0071] Fig. 3 is a flow diagram of a second exemplary process for producing partially sulfonated PPSU;

[0072] Fig. 4 shows a reaction scheme for the production of partially sulfonated PPSU by polycondensation; Fig. 5 shows a flow diagram of a process for producing a catalyst-coated membrane;

[0073] Fig. 6 shows a catalyst-coated membrane in a simplified schematic representation;

[0074] Fig. 7 Polarization curves of a membrane electrode assembly with partially sulfonated PPSU as the anode ionomer; and

[0075] Fig. 8 shows a representation of the long-term stability of a membrane electrode assembly with partially sulfonated PPSU as the anode ionomer.

[0076] Figure 1 shows a reaction scheme for the sulfonation of PPSU. If PPSU is reacted with sulfuric acid, e.g. at 60°C for 6 h, sulfonation occurs, resulting in a mixture of partially functionalized SPPSU or partially sulfonated PPSU 1 and highly functionalized SPPSU. The partially sulfonated PPSU 1 can, for example, have two sulfonic acid groups per repeat unit, as shown in Fig. 1 on the left. Partially sulfonated PPSU 1 is not water-soluble. Highly functionalized SPPSU, on the other hand, has a higher degree of sulfonation, as shown in Fig. 1 on the right. Highly functionalized SPPSU is easily soluble in water and is therefore not suitable for further processing into membranes or use as a binder ionomer. Highly functionalized SPPSU is also highly sensitive to hydrolysis.

[0077] In contrast, studies by the inventors of the present invention have shown that partially sulfonated PPSU 1 is hardly or not at all water-soluble, but soluble up to 25 wt% in l-methyl-2-pyrrolidone and is suitable for processing into membranes or use as a binder ionomer. Even though the number of sulfonic acid groups that influence the ion exchange capacity is lower in partially functionalized PPSU than in highly functionalized PPSU and consequently a lower ion exchange capacity would be expected, experiments by the inventors have shown that, on the one hand, a sufficient ion exchange capacity can be achieved and, on the other hand, a significant improvement in temperature and chemical resistance combined with greater long-term stability can be achieved. As a result, partially sulfonated PPSU 1 can be advantageously used, particularly for the large-scale production of membranes and electrodes for water electrolysis.

[0078] In addition to the reaction products shown in Figure 1, further reaction products with a different number of sulfonic acid groups per repeating unit and / or with a different position of the sulfonic acid groups can be formed.

[0079] Investigations by the inventors of the present invention showed that when the sulfonation reaction shown schematically in Fig. 1 is carried out in concentrated sulfuric acid according to the prior art, only approximately 80% partially sulfonated PPSU and approximately 20% highly functionalized SPPSU are obtained. The aim of the invention was therefore to provide ways to increase the yield of partially functionalized PPSU and / or to provide alternative synthesis routes.

[0080] Figure 2 shows a flow diagram of a first process for producing partially sulfonated PPSU. The overall process is divided into two sub-processes 100, 200, which can be carried out directly one after the other or at different times and / or locations.

[0081] The first sub-process 100 serves to produce a reactant for the partial sulfonation of PPSU. In a first process step S1, 25 g of PPSU granules (Radel R-5000 NT) are introduced into 100 ml of 96% sulfuric acid with stirring and reflux cooling at 50°C under an argon atmosphere to obtain a saturated sulfuric acid solution. After a reaction time of 6 hours, all undissolved components are separated and discarded in process step S2. The resulting liquid phase is subsequently used in the second sub-process 200 as a reactant for the production of partially sulfonated PPSU 1.

[0082] In process step S3, 100 ml of the resulting reactant is placed in a 250 ml three-neck round-bottom flask equipped with a precision glass stirrer and reflux condenser. 25 g of PPSU granules (Radel R-5000 NT) are added to the reactant under argon atmosphere while stirring and refluxing, and the mixture is functionalized for 6 h at 50 °C.

[0083] In process step S4, the partially sulfonated PPSU 1 is separated as an insoluble reaction product by filtration.

[0084] In process step S5, the separated reaction product is quickly washed neutral on a suction filter and then dried in process step S6 at 60 °C for 12 h. By using the previously generated reactant, the yield of partially sulfonated PPSU 1 could be increased to over 90% compared to functionalization without reactants.

[0085] The partially functionalized PPSU can then be further processed as explained below with reference to Figure 5.

[0086] Figure 3 shows a flow chart of an embodiment of a further method 300 for producing partially sulfonated PPSU.

[0087] In process step S 10, 2-fluoro-5-((4-fluorophenyl)sulfonyl)benzenesulfonic acid is dissolved in a sulfolane sodium hydroxide solution containing 30 wt% sodium hydroxide at room temperature while stirring.

[0088] In process step S11, 4,4'-dihydroxy-(1,1'-biphenyl)-3-sulfonic acid is dissolved in a sulfonane sodium hydroxide solution containing 30 wt% sodium hydroxide at room temperature with stirring. In process step S12, 100 ml of distilled water are placed in a 2 l three-neck round-bottom flask with argon purge and a KPG stirrer and heated to T = 90 °C.

[0089] In process step S13, both sulfolane solutions are added at a rate of 100 ml / min using a peristaltic pump. After complete addition of both solutions, the mixture is stirred at 90°C for 1.5 h under argon purge to allow all reactants to react in process step S14. The flask is then cooled to room temperature in process step S15 under argon purge and with stirring.

[0090] Subsequently, in process step S16, the material is heated again to 90 °C for 5 minutes and then cooled to room temperature (process step S17). The cooling in process step S15, together with the reheating in process step S16, leads to a higher yield, as a type of nucleation can be observed, leading to almost complete precipitation.

[0091] In process step S18, the precipitate formed is separated by centrifugation and then washed three times with 500 ml of water and twice with 200 ml of ethanol (process step S19). The residue, which is partially sulfonated PPSU 1, is dried at 60°C for 12 h (process step S20).

[0092] Figure 4 shows a simplified representation of the reaction scheme of the polycondensation reaction underlying process 300. The reaction product is partially sulfonated PPSU 1, wherein the reaction product in particular has one sulfonic acid group on the diphenyl sulfone unit and another sulfonic acid group on the biphenyl unit. The proposed process 300 enables the introduction of a defined number of active groups, i.e. chemical groups that contribute to the ion exchange capacity. The product, which is easily definable due to the total synthesis, enables increased chemical stability, since an unwanted or statistical distribution of sulfonic acid groups could possibly lead to an undesirably increased solubility in water. The partially functionalized PPSU can then be further processed as explained below with reference to Figure 5.

[0093] Figure 5 shows a flow chart of an exemplary method 400 for producing a catalyst-coated membrane 10 for an electrochemical cell, in this case an electrolysis cell for water electrolysis.

[0094] After starting, the method 400 first comprises, in step S50, providing partially sulfonated polyphenylenesulfone 1 as starting material in solid form. In the exemplary embodiment, partially sulfonated PPSU 1 is used for this purpose, which was obtained by means of the method 100, 200 explained above with reference to Figure 2 for producing partially sulfonated polyphenylenesulfone 1. Alternatively, partially sulfonated PPSU 1 could also be used for this purpose, which was obtained by means of the method 300 explained above with reference to Figure 3 for producing partially sulfonated polyphenylenesulfone 1.

[0095] The method 400 further comprises, in step S51, dispersing the partially sulfonated PPSU 1 in an anhydrous dispersant to form a dispersion. In the exemplary embodiment, N-methyl-2-pyrrolidone is used as the anhydrous dispersant. The proportion of the partially sulfonated PPSU 1 in the dispersion is 15 wt%. Alternatively, other anhydrous dispersants can be used. The proportion of the partially sulfonated PPSU 1 can be adapted to the dispersant, for example, its viscosity.

[0096] In process step S52, the dispersion is intimately mixed with a catalyst material 2. In the exemplary embodiment, metallic iridium (black) was used as the catalyst material 2. From this process step S52, a pasty, generally viscous or highly viscous coating material is obtained, which forms the catalyst paste. The catalyst paste is adjusted such that it has a high overall solids content of catalyst material 2 and partially sulfonated PPSU 1. In the exemplary embodiment, this solids content was between 40 wt% and 60 wt%, preferably 50 wt%.

[0097] In process step S53, the catalyst paste is applied directly to a membrane substrate 4. Membrane substrates 4 made of fluorine-free polymers can preferably be used. A direct coating method is used to apply the catalyst paste to the membrane substrate 4. In the exemplary embodiment, application is carried out using a doctor blade, although other direct coating methods can also be used. The use of the direct coating method enables cost-effective and scalable production of a large-format fluorine-free catalyst-coated membrane 10.

[0098] After the catalyst paste has been applied to the membrane substrate 4, a thermal post-treatment follows (process step S54). In the exemplary embodiment, the thermal post-treatment comprises pre-drying at 80°C for 4 hours and subsequent heating to 140°C for 5 to 10 minutes, or even up to 20 minutes, for curing. This achieves a good surface structure of the catalytically active porous layer and long-term stable and uniform bonding to the membrane substrate.

[0099] The proposed method creates in particular a characteristic intermediate layer 5 (see Figure 6) as an important functional layer ("interlayer") by targeted and sufficient dissolution of the membrane substrate 4 in the boundary region to the applied layer with the catalyst material 2. The partially sulfonated PPSU 1 therefore acts not only as an ionomer, but advantageously also as a binder or adhesive for the catalyst material 2. It has been shown that the selection of an anhydrous dispersant for the production of the catalyst paste promotes this dissolution process in the boundary layer and the formation of the intermediate layer 5.

[0100] Figure 6 shows a simplified representation of a catalyst-coated membrane 10. The selected composition of the catalyst paste and the application method specifically initiate a dissolution process which, through the choice of dispersant and the properties of the partially sulfonated PPSU 1, leads to the formation of an intermediate layer 5 as an important functional layer in the manufacturing process. This intermediate layer 5 ensures good adhesion and long-term stable bonding of the catalyst layer 3 comprising the catalyst material 2 and the partially sulfonated PPSU 1 to the membrane substrate 4.

[0101] The intermediate layer 5 has a layer thickness D of approximately 3-10 pm, so that a firm and long-term stable material bond is formed between the catalyst layer 3 and the membrane substrate 4 by dissolving the similar ionomer.

[0102] An electrode is provided on the membrane substrate 4 by the catalyst layer 3; in the enlarged detail in Figure 8, this is an anode electrode with, for example, iridium (Ir) as the catalyst material 2. In a similar way, a cathode electrode is formed on the opposite side of the membrane substrate 4, which cathode electrode has platinum (Pt) as the catalyst material 2. The microstructure of the electrode is a three-dimensional network made up of the ionomer, i.e. the partially sulfonated PPSU 1, the catalyst material 2, and pores. Due to the porous structure, the electrode is permeable to diffusion for the transport of the reactants and the electrolysis products.

[0103] For the ionomer in the anode electrode, an ion exchange capacity of 0.031 mmol / g was determined in the exemplary embodiment. The present invention enables the production of fluorine-free catalyst-coated membranes for PEM water electrolysis using partially sulfonated PPSU as a binder ionomer on an industrial scale. This can counteract a threatening Europe-wide ban on PFSA. The catalyst-coated membrane 10 proves to be long-term stable and efficient with regard to the water electrolysis that can be carried out therewith, as demonstrated by Figures 7 and 8 and the associated explanation below.

[0104] Figure 7 shows, by way of example, an electrochemical characterization of the catalyst-coated membrane 10 or membrane-electrode unit of the exemplary embodiment using polarization curves (U-I characteristics with a cell voltage U plotted against the respective current density I). The polarization curves were recorded after 24 h, 120 h, and 192 h of operation at a temperature of 60 °C and ambient pressure.

[0105] All three curves are almost identical and show a uniform increase in voltage with increasing current density. The membrane electrode assembly of the exemplary embodiment therefore exhibits good electrochemical performance.

[0106] Figure 8 shows the change in cell voltage U with increasing time for the membrane electrode assembly of the exemplary embodiment as evidence of its long-term stability. It should be noted that from the start of the measurement until approximately 24 hours after the start of the measurement, a current density of 1 A / cm 2 was applied, resulting in a voltage of approximately 1.8 V. After 24 hours, a first polarization curve was recorded (see Figure 7, curve 24 h), for which the measurement had to be interrupted, which can be seen in Figure 8 by an abrupt voltage drop.

[0107] After recording the first polarization curve, an increased current density of 2 A / cm 2 applied, resulting in a voltage of approximately 2.0 A. After 120 h, the measurement was interrupted again to record another polarization curve (see Figure 7, curve 120 h). Subsequently, a current density of 2 A / cm 2applied. The measured voltage was again approximately 2 V. Another interruption was made after 192 h to record a third polarization curve (Figure 7, curve 192 h). After recording a third polarization curve, the measurement was continued with a current density of 2 A / cm 2 continued. The constant voltage throughout the entire time is evidence of the excellent long-term stability of the membrane electrode assembly. In contrast, degradation would be detectable by a voltage increase at constant current density, which can be attributed to various causes of degradation, such as catalyst passivation, loss of contact between the components, and ionomer degradation.

[0108] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited to the disclosed examples and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

[0109] The invention relates to a process 100 for producing a reactant for a partial sulfonation of polyphenylenesulfone, comprising the following steps: S1: introducing polyphenylenesulfone into sulfuric acid at least up to the solubility limit, and S2: separating undissolved components to obtain the reactant. The invention further relates to processes for producing partially sulfonated polyphenylenesulfone 1, a process 400 for producing a catalyst-coated membrane 10 for an electrochemical cell, and a PEM membrane arrangement with a catalyst-coated membrane 10.

Claims

Patent claims 1. A process (100) for preparing a reactant for partial sulfonation of polyphenylene sulfone, the process (100) comprising the following steps: - Sl: Introducing polyphenylene sulfone into sulfuric acid at least up to the solubility limit and - S2 : Separation of undissolved components to obtain the reactant.

2. Process (200) for producing partially sulfonated polyphenylene sulfone (1), the process (200) comprising the following steps: - S3: adding polyphenylene sulfone to a reactant prepared according to a process (100) according to claim 1 and - S4: Separation of the partially sulfonated polyphenylene sulfone (1) as an insoluble reaction product.

3. The process (200) of claim 2, wherein the proportion of polyphenylene sulfone in the reactant is between 5 wt% and 30 wt%.

4. A process (300) for the preparation of partially sulfonated polyphenylene sulfone (1) by total synthesis, comprising: - S14: reacting at least one hydroxy component with at least one halogen component, wherein the hydroxy component is 4,4'-dihydroxy-(1,1'-biphenyl)-3-sulfonic acid or 4,4'-dihydroxydiphenylsulfone.

5. The process according to claim 4, wherein the halogen component is a bis(halophenyl)sulfone or disodium bis(4-chloro-3-sulfophenyl)sulfone.

6. Method (300) according to claim 4 or 5, comprising: - S10, Sil: Dissolving the hydroxy component and the halogen component in sulfolane.

7. Use of partially sulfonated polyphenylene sulfone (1) produced according to a process (200, 300) according to one of claims 2 to 6 for producing an electrode or membrane for carrying out water electrolysis.

8. A method (400) for producing a catalyst-coated membrane (10) for an electrochemical cell, the method (400) comprising the following steps: - S50: Providing solid partially sulfonated polyphenylene sulfone ( 1 ), - S51 dispersing the partially sulfonated polyphenylene sulfone (1) in an anhydrous dispersant to obtain a dispersion, - S52: Mixing a catalyst material (2) with the dispersion to form a catalyst paste, and - S53: Applying the catalyst paste to a membrane substrate (4) .

9. The method (400) according to claim 8, comprising: - S54: thermal post-treatment of the membrane substrate (4) with the applied catalyst paste.

10. The process (400) according to claim 8 or 9, wherein the partially sulfonated polyphenylene sulfone (1) used is a partially sulfonated polyphenylene sulfone (1) produced according to a process (200, 300) according to one of claims 2 to 7.

11. PEM membrane assembly comprising a catalyst-coated membrane (10) produced according to a method (400) according to any one of claims 8 to 10.

12. An electrolytic cell comprising a PEM membrane assembly according to claim 11.

13. A cell stack comprising a plurality of electrolysis cells according to claim 12. 14 . Electrolysis plant with a cell stack according to claim

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