Catalyst ink usable with a doctor blade and having a high solids proportion

Abstract

The invention relates to a catalyst ink, to a method for coating substrates with said ink, to the coated substrates obtained using the method, and to the use thereof. It addresses the problem of providing a catalyst ink which has a high solids content and at the same time can still be easily used with a doctor blade. As far as possible, the catalyst ink should not contain any solvents which are known to be carcinogenic, mutagenic or toxic to reproduction. The coated substrates produced using the catalyst ink should be usable, in particular, in alkaline membrane water electrolysis as a catalytically active or activatable anion exchange membrane. This problem is solved by a solvent mixture containing alkanol and acetonitrile. The improvement consists in that both the proportion by weight of the acetonitrile and also the proportion by weight of the alkanol are each greater than the proportion by weight of the binder contained in the catalyst ink.

Description

[0001] Doctorable catalyst ink with high solids content

[0002] The invention relates to a catalyst ink, a process for coating substrates with this ink, the coated substrates obtained by the process and their use.

[0003] Catalyst ink is a term used in the field of manufacturing components for electrochemical cells. Electrochemical cells are devices used to conduct electrochemical reactions. In electrochemical reactions, substances are converted by absorbing or releasing electrical energy. To operate these reactions particularly efficiently, it is often advisable to conduct the reaction in the presence of so-called electrocatalysts. Electrocatalysts are substances that accelerate the desired electrochemical reaction or even enable it without being significantly consumed. Electrocatalysts are often particulate, inorganic materials, usually metals or metal oxides.

[0004] To conduct an electrochemical reaction in the presence of an electrocatalyst, the electrocatalyst must be incorporated into the electrochemical cell. In technical designs of electrochemical cells, this is often achieved by coating a component of the electrochemical cell with an electrocatalytically active or activatable coating containing the electrocatalyst.

[0005] Components that may be coated include separators, membranes, electrodes, or fluid power devices such as gas distributors. If a membrane is coated with a catalytically active or activatable coating, it is referred to as a catalyst coated membrane (CCM). If electrodes or other components of a cell are coated with an electrocatalyst, this is referred to as a catalyst coated substrate (CCS).

[0006] In this context, the term "substrate" is used generally to refer to the object to be coated. This can generally be a component of an electrochemical cell, such as a membrane or an electrode; however, the term also includes auxiliary materials that are not incorporated into the cell, such as transfer substrates used for the indirect coating of electrodes or membranes.

[0007] A catalyst ink is a mixture of at least one particulate electrocatalyst and at least one liquid binder solution in a solvent. The catalyst ink is designed to be applied to the substrate to be coated. Once applied, the catalyst ink is dried so that the solvent evaporates and the binder precipitates from the solution. What remains is the electrocatalyst and the binder, which together form the catalytically active or activatable coating of the substrate. The electrocatalyst imparts its catalytic activity to the coating, while the binder immobilizes the electrocatalyst on the substrate. The binder precipitated from the solution forms a solid matrix bound to the substrate, in which the particulate electrocatalyst is dispersed. The binder is typically a polymer.Polymers that exhibit a certain degree of ionic conductivity, so-called ionomers, are often used as binders. Ionomers are known to exhibit conductivity for anions and those that exhibit conductivity for cations.

[0008] Since ionomers do not have any significant conductivity for electrons, some catalyst inks also contain conductivity additives such as carbon black.

[0009] Furthermore, a catalyst ink can also contain processing aids such as surfactants or rheology aids.

[0010] Finally, a catalyst ink can also contain a liquid dispersing medium that reduces the ink's viscosity. The catalyst ink is diluted with the dispersing medium. Highly volatile substances such as water or ethanol are often used as dispersing media, which evaporate along with the solvent during the drying process.

[0011] As a result, a catalyst ink can be regarded as a precursor to the catalytically active or activatable coating of the substrate to be produced.

[0012] The choice of electrocatalyst, binder and any conductivity additives depends on the intended use of the coated component, more precisely on the electrochemical reaction to be carried out and on the location of the component, for example on the anode or cathode side of the electrochemical cell.

[0013] The choice of solvent and, if necessary, processing aids or dispersing media depends on the specifications of the desired coating technology: Both the solvent and the dispersing medium are removed during the drying process and thus do not form part of the subsequent catalytically active or activatable coating. Solvents and dispersing media are therefore selected to ensure the ink achieves the desired processability. If this cannot be achieved through the careful selection of solvent and dispersing medium alone, processing aids are required. However, since these are likely to remain in the coating and impair the efficiency of the electrochemical reaction, a good catalyst ink should ideally be designed without such processing aids.

[0014] The present invention specifically addresses the optimal selection of solvents and optional dispersing media. Its primary goal is to achieve a high solids content in the catalyst ink so that as much layer-forming material as possible can be applied to the substrate per unit of time during coating. The solids content describes the sum of all proportions of the ink components that remain on the substrate as solids after drying, i.e., the non-volatile components of the ink. These are essentially the precipitated binder and the electrocatalyst. Since solvents and any dispersing media are expelled during drying, they are not part of the solids content. On the contrary, in the interest of a high solids content, the proportion of volatile components such as solvents and any dispersing media must be reduced.

[0015] A further objective of the invention is to adjust the dynamic viscosity of the catalyst ink so that it can be processed quickly and with high quality. In particular, the catalyst ink should be applicable to the substrate by doctor blade, because doctor blade coating is a rational coating process that delivers high coating quality and is well-controlled. Unfortunately, there is a trade-off between high solids content and good doctorability, because excessively high solids content increases the viscosity of the catalyst ink so much that it can no longer be doctored.

[0016] Based on these general technical specifications, the technical task is condensed in the light of the following state of the art:

[0017] A special form of electrochemical reaction is alkaline membrane water electrolysis (AEMWE). It is used to produce hydrogen (H2) and oxygen (O2) from water (H2O) using electrical energy. AEMWE is considered a technology for the sustainable production of "green" hydrogen, provided the required electrical energy is generated from renewable sources.

[0018] An overview of the structure and materials of the electrochemical cells currently used in AEMWE is provided by:

[0019] Miller, Hamish Andrew et al: Green hydrogen from anion exchange membrane water electrolysis: a review of recent developments in critical materials and operating conditions. Sustainable Energy Fuels, 2020, 4, 2114 DOI: 10.1039 / c9se01240k

[0020] In particular, for the production of the membranes used in AEMWE, polymers with conductivity for anions, more precisely for hydroxide ions (OH ), are required.

[0021] Polymers with anion conductivity are known from patent applications EP3770201A1, EP4032934A1, and EP4059988A1. They are suitable for the production of anion exchange membranes (AEMs). These, in turn, can be used in alkaline water electrolysis. None of the AEMs disclosed therein exhibit catalytic activity.

[0022] During the production of AEMs, the ionomers disclosed in EP3770201A1 and EP4032934A1 are dissolved in N,N-dimethylacetamide (DMAC) and / or N,N-dimethylformamide (DMF). Since DMAC and DMF are known to be carcinogenic, mutagenic, or toxic to reproduction (CMR), they are each classified as CMR substances under the REACH regulation, requiring costly safety measures during industrial processing.

[0023] The synthesis of the ionomer precursors described in EP4059988A1 uses dimethyl sulfoxide (DMSO) and acetonitrile (AON). Both solvents are not classified as CMR substances. However, this advantage is not leveraged by EP4059988A1 because DMAC is still used as the solvent during membrane casting. Thus, a CMR substance must still be handled during the production of the anion exchange membrane, which increases production costs.

[0024] The production of a catalyst ink from the ionomers disclosed in the aforementioned documents is also disclosed in WO2023088714A1. The catalyst ink contains DMSO as a solvent and an ethanol / water mixture as a dispersing medium. This ink is used to produce CCM or CCS, which can be used in AEMWE. Although the description of WO2023088714A1 suggests that this ink could be applied by a doctor blade, an ultrasonic spray coater was used in the experimental section to apply the catalyst ink to various substrates. In fact, the tests presented here show that the catalyst ink disclosed in WO2023088714A1 cannot be applied by a doctor blade. The catalyst content of this ink is calculated to be approximately 11 mg / ml, which is good for a spray ink, but still quite low for a doctor blade ink.

[0025] European patent application 24152389.3, still unpublished at the time of filing, by the same applicant describes a catalyst ink whose binder was prepared according to EP3770201A1. The catalyst ink also contains silica, DMSO, ethanol, and water. The description also mentions that DMSO, ethanol, and acetonitrile can be used as solvents, either alone or in mixtures. A mixing ratio is not specified. Furthermore, the experiments presented here show that neither ethanol nor acetonitrile alone are capable of dissolving the polymer disclosed in EP3770201A1.

[0026] An alternative, doctor blade-applicable catalyst ink for coating anion exchange membranes was developed by Koch et al.:

[0027] Koch, S., Metzler, L., Kilian, SK, Heizmann, PA, Lombeck, F., Breitwieser, M., Vierrath, S., Toward Scalable Production: Catalyst-Coated Membranes (CCMs) for Anion-Exchange Membrane Water Electrolysis via Direct Bar Coating. Adv. Sustainable Syst. 2023, 7, 2200332. DOI: 10.1002 / adsu.202200332

[0028] Unfortunately, this ink also contains a CMR substance as a solvent, namely methanol. Industrial production within the scope of the REACH regulation is also associated with high costs.

[0029] Against this background, the technical task was to develop a catalyst ink with a high solids content and, at the same time, easy to stretch. The catalyst ink should, if possible, contain no solvents known to be carcinogenic, mutagenic, or reproductively toxic. The coated substrates produced with the catalyst ink should be suitable for use as catalytically active or activatable anion exchange membranes, particularly in alkaline membrane water electrolysis.

[0030] This object is achieved by a catalyst ink according to claim 1. The invention therefore relates to a catalyst ink having the following features: a) the catalyst ink comprises a solid electrocatalyst in particulate form; b) the catalyst ink comprises a liquid solvent mixture containing at least acetonitrile and at least one alkanol; c) the catalyst ink comprises a binder which is dissolved in the solvent mixture, wherein the binder is an anion-conducting polymer which contains at least one structure corresponding to one of the following formulas (I), (II), (III): wherein in (I) X stands for a structural element comprising a positively charged nitrogen atom which is bonded to C1 and C2 and which is bonded via two bonds to one or two hydrocarbon radicals comprising 1 to 12, preferably 1 to 6, particularly preferably 1 or 5 carbon atoms and wherein in (I) Z stands for a structural element which comprises a carbon atom which is bonded to C3 and C4 and which comprises at least one aromatic six-membered ring which is bonded directly to one of the oxygen atoms, wherein the aromatic six-membered rings may be substituted by one or more halogen and / or one or more C1 to C4 alkyl radicals; wherein in (II) X stands for a structural element comprising a positively charged nitrogen atom which is bonded to C1 and C2 and which is bonded via two bonds to one or two hydrocarbon radicals comprising 1 to 12, preferably 1 to 6, particularly preferably 1 or 5 carbon atoms, and wherein in (II) Z stands for a structural element which comprises a carbon atom which is bonded to C3 and C4 and which comprises at least one aromatic six-membered ring which is bonded directly to one of the oxygen atoms, wherein the aromatic six-membered ring may be substituted in positions 3 and 5 with the same or different C1 to C4 alkyl radicals, in particular with a methyl, isopropyl or tert-butyl group, wherein the methyl group is preferred; where in (III) X stands for a ketone or sulfone group; where in (III) Z stands for a structural element which comprises at least one tertiary carbon atom and at least one aromatic six-membered ring, where the aromatic six-membered ring is directly bonded to one of the two oxygen atoms; and where in (III) Y stands for a structural element which comprises at least one nitrogen atom with a positive charge, where this nitrogen atom is bonded to the structural element Z; where the improvement of the catalyst ink consists in that both the weight fraction of acetonitrile and the weight fraction of alkanol are each greater than the weight fraction of the binder. The said weight fractions each relate to the total weight of the catalyst ink.

[0031] The present invention is based on the surprising discovery that the binders of interest of formula (I), (II), or (III) can be dissolved with a combination of an alkanol (ROH) and acetonitrile (ACN). This discovery is surprising because CMR substances such as DMAC or DMF have always been described as suitable solvents for these ionomers. Furthermore, these ionomers cannot be dissolved in either pure ethanol or pure acetonitrile: only a mixture of acetonitrile with an alkanol is capable of dissolving the relevant binders. To obtain a readily processable ink, the proportions of alkanol and acetonitrile must each be greater than the proportion of the binder. Here, too, these are the weight proportions relative to the total weight of the ink.

[0032] Another unexpected effect is that the use of the solvent mixture of ACN and ROH allows the solid content of the ink to be increased significantly.

[0033] Fortunately, a potentially CMR-free solvent cocktail is used in all of this, which imposes lower safety requirements on production than is the case, for example, with the processing of DMAC-based inks. "Potentially CMR-free" in this context means that it is possible to formulate the ink according to the invention without solvents known to be carcinogenic, mutagenic, or toxic to reproduction. Nevertheless, it is possible to use an alkanol in the formulation that is suspected of having such a toxic effect, such as methanol. Under certain circumstances, it may be advantageous for the processability of the ink to use a CMR-relevant alkanol as the solvent, thereby accepting higher safety precautions during the coating process.If this is not desired, the formulation according to the invention also allows the use of alkanols that are not classified as CMR substances, such as 2-propanol.

[0034] Within the solvent mixture, ACN and ROH can be present in varying ratios. It is important that both substances are present and that their individual weight fractions are greater than the weight fraction of the binder. For example, the ratio of the weight fraction of acetonitrile to the weight fraction of the alkanol can range between 1:10 and 17:1.

[0035] Particularly good doctorability and a particularly high solids content can be achieved by adding dimethyl sulfoxide (DMSO) to the ink as a solvent. DMSO is not CMR-relevant.

[0036] If the viscosity of the catalyst ink is too high, it is possible to dilute the ink with a dispersing medium. Unlike the solvent mixture, the dispersing medium does not dissolve the binder. However, the particulate electrocatalyst can be dispersed in the dispersing medium. Water is most simply used as the dispersing medium because it is highly volatile and inexpensive. Furthermore, it is not a CMR substance. A particular embodiment of the invention therefore provides that the catalyst ink additionally comprises water. The weight fraction of water can be greater than any other weight fraction of the other components of the catalyst ink. Nevertheless, the catalyst ink should not contain significantly more water than the other components.

[0037] A concrete recipe suggestion for an advantageous catalyst ink with high solids content is set out in Table 1.

[0038] Table 1: Advantageous formulation of a catalyst ink according to the invention It goes without saying that the sum of the weight proportions of all components listed in Table 1 does not exceed 100 wt.%. However, it is possible that the sum does not reach 100 wt.%, namely if the catalyst ink comprises another component not listed in Table 1. Advantageously, the catalyst ink consists exclusively of the components listed in Table 1. In this case, the sum of the weight proportions of all components listed in Table 1 is exactly 100 wt.%.

[0039] In principle, any alkanol can be used as alkanol raw material. Examples include: methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-nonanol, 1-decanol, and 2-propanol. Secondary or tertiary alkanols, such as 1,2-propanediol, can also be used. Alkanols derived from cycloalkanes, such as cyclohexanol, can also be used. Ethanol (EtOH) is preferably used as the alkanol because EtOH is not classified as a CMR substance. 1-Propanol or 2-Propanol, which are also not CMR-relevant, can also be used. This also applies to 1,2-propanediol. Of course, mixtures of these alkanols can also be used. In the event that a mixture of several alkanols is used, the numerical values ​​for the minimum and maximum proportions refer to the weight proportion of the mixture, i.e. the sum of the weight proportions of all alkanols.

[0040] Preferably, the catalyst ink is formulated so that the binder is completely dissolved in the solvent mixture. If the binder partially precipitates from the solvent mixture, the ink becomes lumpy and difficult to process. Preferably, the ink contains no other binder besides the mandatory binder, which contains a structure according to formula (I), (II), or (III).

[0041] To prevent the introduction of impurities into the catalyst layer that could impair electrolysis, the catalyst ink should, if possible, consist exclusively of the components explicitly mentioned above. However, under certain circumstances, it may still be necessary to add another component to the catalyst ink that differs from those mentioned above. This can be organic or inorganic additives, such as a dispersing agent, a rheology aid, a surfactant, or a conductivity additive. Specific examples of additional components are silica or carbon black. Carbon black is a conductivity additive, while silica is suitable as a rheology aid.

[0042] The dynamic viscosity of the catalyst ink should be between 10 1 mPas and 10 4mPas. The dynamic viscosity is measured at a temperature of 25°C using a rotational rheometer with plate-on-plate geometry. The diameter of the plates is 40 mm, the distance between the plates is 1 mm. The dynamic viscosity is measured at increasing shear rates between 0.1 s -1 and 1000 s -1 measured, the characteristic value is the dynamic viscosity at 1 s -1A suitable rotational rheometer with plate-on-plate geometry is available, for example, from Malvern Kinexus. The catalyst ink according to the invention was developed for the production of catalytically coated substrates. A corresponding coating process utilizing the ink according to the invention comprises at least the following steps: i) providing the substrate; ii) providing a catalyst ink according to the invention; iii) applying the catalyst ink to the substrate; iv) drying the catalyst ink applied to the substrate.

[0043] Such a manufacturing process is therefore a further subject of the invention.

[0044] The ink is characterized by its good doctorability. Therefore, the catalyst ink is preferably applied to the substrate by doctoring.

[0045] An anion-conducting membrane is preferably used as the substrate. Coating it with the catalyst ink creates a catalytically active or activatable AEM.

[0046] Particularly preferred is an AEM containing at least one structure according to formula (I), (II), or (III) as the substrate. This means that the membrane and the binder consist essentially of the same ionomer. This ensures particularly good bonding of the catalyst layer to the membrane.

[0047] The advantages of the catalyst ink according to the invention not only impact production costs but also lead to a better product: Experiments show that the ink according to the invention leads to improved efficiency of the substrate coated with it, which is likely due to improved coating quality. A catalytically coated substrate obtainable by the coating process according to the invention with the ink according to the invention is therefore also a subject of the invention.

[0048] Since the advantages of the substrate according to the invention are particularly evident in AEMWE, namely in an improvement in efficiency, the use of a catalytically coated substrate according to the invention in alkaline water electrolysis is a further subject of the invention. The use occurs by conducting alkaline water electrolysis in the presence of the catalytically coated substrate. The electrocatalyst contained in the catalyst layer catalyzes the electrochemical reaction. The catalyst layer can also serve as an electrode. If an AEM was used as the substrate, it functions as the membrane of the AEMWE and transports hydroxide ions OH- from the cathodic compartment of the electrochemical cell to the anodic compartment.

[0049] According to current research, the following elements can be processed in pure form, as oxide, hydroxide, oxide-hydroxide, or phosphide in the catalyst ink according to the invention: iridium (Ir), nickel (Ni), cobalt (Co), chromium (Cr), iron (Fe), ruthenium (Ru), copper (Cu), molybdenum (Mo), zinc (Zn), lead (Pb), manganese (Mn), tungsten (W), platinum (Pt), sulfur (S), tin (Sn), gold (Au), silver (Ag), palladium (Pd), rhenium (Re), rhodium (Rh), and cerium (Ce). Therefore, the elements can be considered as electrocatalysts in pure form, as oxide, hydroxide, or oxide-hydroxide, individually or in combination. The electrocatalyst is formulated into the ink in particulate form, i.e., as a powder or granulate.

[0050] Most preferably, the catalyst ink is formulated in such a way that it contains platinum or platinum alloy supported on carbon (Pt / C) or a nickel-iron (oxide) hydroxide (NiFe a ObH c) or a nickel-iron phosphide (NiFe a Pb). The indices a, b, and c represent real numbers in the interval from 0 to 8. The indices a, b, and c can be the same or different. The nickel-iron ratio in the nickel-iron (oxide) hydroxide should ideally be in the range of 2:1 to 10:1.

[0051] Examples

[0052] In the following, the solubility behavior of an ionomer of interest is first investigated. Then, nine different catalyst inks are produced from this ionomer. The inks differ in the solvents used. These nine inks are then used to create catalyst layers on anion exchange membranes. The catalytically coated AEMs are then compared with regard to their performance in water electrolysis. In this way, the effects achieved with the invention are experimentally verified.

[0053] 1 . Preparation of an ionomer (not part of the invention)

[0054] An anion-conducting, cationic polymer was synthesized according to Example 3 of EP3770201A1. This polymer was initially in solution and then dried into a powder. Subsequently, to minimize the residual solvent content, the dried polymer was further dried for 48 h in a vacuum drying oven at 80 °C.

[0055] 2. Test on the solubility of the ionomer in ethanol, 1-propanol and 2-propanol

[0056] For this purpose, three vessels were prepared: the first containing 27 g of ethanol, the second containing 27 g of 1-propanol, and the third containing 27 g of 2-propanol. 3 g of the cationic polymer synthesized from Example 1 were added to each of these vessels, sealed, and shaken at 60 °C for 4 h. The result was a slightly swollen, softened lump of polymer at the bottom of the vessel, which had not significantly dissolved.

[0057] 2 bis . Test on the solubility of the ionomer in 1,2-propanediol, 1-butanol, cyclohexanol, 1-heptanol and 1-octanol

[0058] For this purpose, five vessels were prepared: the first containing 9 g of 1,2-propanediol, the second containing 9 g of 1-butanol, the third containing 9 g of cyclohexanol, the fourth containing 9 g of 1-heptanol, and the fifth containing 9 g of 1-octanol. 1 g of the cationic polymer synthesized from Example 1 was added to each of these vessels, sealed, and shaken at 60 °C for 4 h. The result was a partially slightly swollen lump of polymer at the bottom of the vessel, which had not significantly dissolved.

[0059] 3. Test on the solubility of the ionomer in acetonitrile

[0060] For this purpose, 3 g of the cationic polymer synthesized from Example 1 was added to a vessel containing 27 g of acetonitrile and shaken at 60 °C for 4 h. The result was that no significant dissolution of the polymer was visible.

[0061] 4. Test on the solubility of the ionomer in a mixture of ethanol or 1-propanol or 2-propanol in combination with acetonitrile

[0062] For this purpose, three vessels were prepared: the first containing 13.5 g of ethanol and 13.5 g of acetonitrile, the second containing 13.5 g of 1-propanol and 13.5 g of acetonitrile, and the third containing 13.5 g of 2-propanol and 13.5 g of acetonitrile. 3 g of the cationic polymer synthesized from Example 1 was added to each of these vessels and shaken at 60 °C for 4 h. The result was a nearly clear solution. The polymer had dissolved in all three vessels.

[0063] 4 bis . Test on the solubility of the ionomer in a mixture of 1,2-propanediol, 1-butanol, cyclohexanol, 1-heptanol and 1-octanol in combination with acetonitrile

[0064] For this purpose, five vessels were prepared: the first containing 4.5 g of 1,2-propanediol, the second containing 4.5 g of 1-butanol, the third containing 4.5 g of cyclohexanol, the fourth containing 4.5 g of 1-heptanol, and the fifth containing 4.5 g of 1-octanol. 4.5 g of acetonitrile and 1 g of the cationic polymer synthesized from Example 1 were added to each of these vessels. The vessels were sealed and shaken at 60 °C for 4 h. The result was a nearly clear solution with a yellowish tinge. The polymer had dissolved in all three vessels.

[0065] 5. Interim result

[0066] The ionomer tested can be dissolved in a solvent mixture of ethanol and acetonitrile, but not in ethanol or acetonitrile alone. The same applies to the mixture of 1-propanol with acetonitrile and the mixture of 2-propanol with acetonitrile. This also applies analogously to all other alkanols tested here in combination with acetonitrile.

[0067] 6. Preparation of an anion exchange membrane (not part of the invention)

[0068] An anion-conducting membrane was prepared from the cationic polymer synthesized in Example 1, as described in Example 4 of EP3770201A1. The polymer solution was used for this purpose before drying. 7. Preparation of a conventional catalyst ink (not part of the invention)

[0069] A catalyst-containing dispersion was prepared from the cationic polymer synthesized in Example 1 as described in Example 12 of WO2023088714A1:

[0070] The ionomer was first dissolved in dimethyl sulfoxide with stirring and at temperature (60 °C) for 16 h. The platinum on carbon (Pt / C, ~50 wt%) catalyst was then dispersed in the dispersion medium, consisting of equal volumes of water and ethanol, using ultrasound (BRANSONIC™ B-1200 E2 from Branson Ultrasonics Corporation, Brookfield, CT, US) for 30 min in an ice bath at a power of 30 W. After addition of the ionomer solution (50 mg / ml in DMSO), further dispersion was carried out using ultrasound in an ice bath for 1 min at a power of 30 W and dispersion with a shaker (MS1 Minishaker from IKA, Staufen, DE) for 10 s at 2500 rpm. The proportions were selected according to Table 1, Example 12 of WO2023088714A1. Therefore, the catalyst loading was set at 11 mg / ml, with the Pt / C catalyst particles accounting for three mass fractions and the ionomer solids accounting for one mass fraction. This resulted in a low-viscosity catalyst ink.

[0071] 8. _ Producing a catalyst ink with increased solids content (not part of the invention)

[0072] Since the catalyst ink prepared in Example 7 has a relatively low catalyst content of only 11 mg / ml and its viscosity does not appear optimal for doctor blade coating, the solids content was maximized while maintaining the solvent ratio and the ionomer to catalyst ratio. If the mass ratio ethanol:water:DMSO = 0.405:0.512:0.083 and the catalyst:ionomer ratio = 3:1 are to remain constant, the ionomer solution is a limiting factor, since an ionomer content of over 20 wt.% in DMSO is no longer easy to disperse. Thus, an ink with a catalyst solids content of 5.73 wt.% was prepared using the same procedure as described in Example 7. The platinum on carbon (Pt / C, ~50 wt.-%) Catalyst was dispersed in the dispersion medium, consisting of equal volume parts of water and ethanol, under ultrasound (BRANSONIC™ B-1200 E2 from Branson Ultrasonics Corporation, Brookfield, CT, US) for 30 min in an ice bath and a power of 30 W. After addition of the ionomer solution (20 wt.% in DMSO), further dispersion was carried out using ultrasound in an ice bath for 1 min at a power of 30 W and dispersion with a shaker (MS1 Minishaker from IKA, Staufen, DE) for 10 s at 2500 rpm. This resulted in a catalyst ink with increased viscosity compared to the catalyst ink prepared in Example 7.

[0073] 8 bis . Providing a catalyst ink according to the invention based on EtOH-ACN

[0074] As an inventive composition, 13.85 g of a catalyst-containing ink were prepared. For this purpose, 3 g of Pt / C (Pt ~ 50 wt%) were first weighed into a shatter-proof container (volume 100 ml) under oxygen exclusion, then 8.5 g of acetonitrile were added and shaken. 2.35 g of an ionomer solution consisting of 15 wt% ionomer solids from Example 1, 42.5 wt% ethanol, and 42.5 wt% acetonitrile were added to the catalyst-containing composition. The mixture was dispersed in a shaker mixer (Lau, Germany) for 2 hours using grinding balls (total amount 30 g, diameter 3 mm) for 120 min. The solids content of this catalyst ink was therefore 24.2 wt%.

[0075] 8 ter Providing an inventive catalyst ink based on EtOH-ACN-H2O

[0076] As a further inventive composition, 14.32 g of catalyst-containing ink was prepared. For this purpose, 3 g of Pt / C (Pt ~ 50 wt.%) and 6 g of ultrapure water were first weighed into a shatter-proof container (volume 100 ml) under oxygen exclusion, then 0.5 g of acetonitrile and 0.5 g of ethanol were added and shaken. 4.32 g of an ionomer solution consisting of 10 wt.% ionomer solids from Example 1, 45 wt.% ethanol, and 45 wt.% acetonitrile were added to the catalyst-containing composition. The mixture was dispersed in a shaker mixer (Lau, Germany) using grinding balls (total amount 30 g, diameter 3 mm) for 120 min. The solids content of this catalyst ink was therefore 24.0 wt.%.

[0077] 9. Providing a catalyst ink according to the invention based on H2O-DMSO-EtOH-ACN

[0078] As a further inventive composition, 14.6 g of catalyst-containing ink was prepared. For this purpose, 3 g of Pt / C (Pt ~ 50 wt.%) were first weighed into a shatter-proof container (volume 100 ml) under oxygen exclusion, then 6 g of ultrapure water and 2 g of DMSO were added and shaken. 3.6 g of an ionomer solution consisting of 12 wt.% ionomer solids from Example 1, 20 wt.% ethanol, 20 wt.% acetonitrile, and 48 wt.% DMSO were added to the catalyst-containing composition. The mixture was dispersed in a shaker mixer (Lau, Germany) using grinding balls (total amount 30 g, diameter 3 mm) for 120 min. The solids content of this catalyst ink was therefore 23.5 wt.%.

[0079] 9 bis . Providing an inventive catalyst ink based on EtOH-ACN-H2O-C

[0080] As yet another inventive composition, 15.1 g of catalyst-containing ink was prepared. For this purpose, 3 g of Pt / C (Pt ~ 50 wt%) and 0.1 g of carbon black (Ketjenblack® 600JD) were first weighed into a shatter-proof container (volume 100 ml) under oxygen exclusion. 6.4 g of ultrapure water and 2 g of DMSO were then added and shaken. 3.6 g of an ionomer solution consisting of 12 wt% ionomer solids from Example 1, 20 wt% ethanol, 20 wt% acetonitrile, and 48 wt% DMSO were added to the catalyst-containing composition. The mixture was dispersed in a shaker mixer (Lau, Germany) using grinding balls (total amount 30 g, diameter 3 mm) for 120 min. The solids content of this catalyst ink (ionomer dry mass, electrocatalyst + carbon black) was 23.4 wt%.

[0081] 9 ter . Providing an inventive catalyst ink based on EtOH-ACN-H2O-SiO2

[0082] As yet another inventive composition, 15.1 g of catalyst-containing ink was prepared. For this purpose, 3 g of Pt / C (Pt ~ 50 wt%) were first weighed into a shatter-proof container (volume 100 ml) under oxygen exclusion. 6 g of silica dispersion (5% in H2O), 0.5 g of water, and 2 g of DMSO were then added and shaken. 3.6 g of an ionomer solution consisting of 12 wt% ionomer solids from Example 1, 20 wt% ethanol, 20 wt% acetonitrile, and 48 wt% DMSO were added to the catalyst-containing composition. The mixture was dispersed in a shaker mixer (Lau, Germany) using grinding balls (total amount 30 g, diameter 3 mm) for 120 min. The solids content of this catalyst ink (ionomer dry mass, electrocatalyst + silica) was therefore 24.7 wt.%. Providing a catalyst ink according to the invention based on H2O-DMSO-EtOH-ACN

[0083] As a further example of an inventive composition, 10.2 g of catalyst-containing ink was prepared. For this purpose, 1 g of Pt / C (Pt ~ 50 wt%) was first weighed into a shatter-proof container (volume 100 ml) under oxygen exclusion, then 6 g of ultrapure water and 2 g of DMSO were added and shaken. 1.2 g of an ionomer solution consisting of 12 wt% ionomer solids from Example 1, 20 wt% ethanol, 20 wt% acetonitrile, and 48 wt% DMSO were added to the catalyst-containing composition. The mixture was dispersed in a shaker mixer (Lau, Germany) using grinding balls (total amount 30 g, diameter 3 mm) for 120 min. The solids content of this catalyst ink was therefore 11.2 wt%. gguinguies Providing an inventive catalyst ink based on 2-propanol-ACN

[0084] As an inventive composition, 14.85 g of a catalyst-containing ink were prepared. For this purpose, 3 g of Pt / C (Pt ~ 50 wt%) were first weighed into a shatter-proof container (volume 100 ml) under oxygen exclusion, then 1.0 g of acetonitrile and 8.5 g of 2-propanol were added and shaken. 2.35 g of an ionomer solution consisting of 10 wt% ionomer solids from Example 1, 45 wt% 2-propanol, and 45 wt% acetonitrile were added to the catalyst-containing composition. The mixture was dispersed in a shaker mixer (Lau, Germany) for 2 hours using grinding balls (total amount 30 g, diameter 3 mm) for 120 min. The solids content of this catalyst ink was therefore 21.78 wt%.

[0085] 10. Coating a membrane with conventional ink using a doctor blade (not part of the invention)

[0086] For the reference test using the formulation from WO2023088714 (Example 12), the catalyst ink provided in Example 7 was applied to the anion-conducting membrane prepared in Example 6 using an automatic doctor blade (Elcometer 4340, feed rate 5 mm Is, spiral blade, 60 pm). The coated AEM was then dried for 10 min at 80 °C in a laboratory oven. The result was designated Membrane A and is shown in Figure 1.

[0087] Fig. 1 : Photo of a membrane coated with conventional ink A

[0088] As can be seen in Figure 1, the membrane became heavily corrugated during the coating process. This resulted in an inhomogeneous coating. Nevertheless, the resulting catalytically coated AEM is, in principle, suitable for water electrolysis. Figure 11. Coating of a membrane with an ink not according to the invention with an increased solids content using a doctor blade (not part of the invention).

[0089] For the second reference test, the composition provided in Example 8 was applied to the membrane prepared in Example 6 using an automatic doctor blade (Elcometer 4340, feed rate 5 mm ls, spiral blade, 60 μm). The coated membrane was then dried for 10 min at 80 °C in a laboratory oven. The result was designated Membrane B and is shown in Figure 2.

[0090] Fig. 2: Photo of a membrane B coated with ink not according to the invention

[0091] As can be seen in Figure 2, the membrane had slightly rippled during the coating process. The coating exhibited numerous areas with a relatively thin layer thickness, which are visible as white spots in Figure 2. In principle, the resulting coating is suitable as an electrode for water electrolysis, but the coating is not optimal.

[0092] 12. Coating of membranes with catalyst inks according to the invention (part of the invention)

[0093] For the inventive coating using ACN / ROH-containing formulation, the coating composition described in Examples 8 bis to 9 quinquis The inks provided were applied to a membrane prepared according to Example 6 using an automatic doctor blade (Elcometer 4340, feed rate 5 mm / s, spiral blade, 60 pm). The coated membranes were then dried for 10 min at 80°C in a laboratory oven. For the membranes prepared using the formulation from Example 8 bis The coated membrane was stretched a second time over the same area using the same parameters. The resulting membranes were designated Membranes C to I and are shown in Figures 3 to 8.

[0094] Fig. 3: Photo of the example 8 bis coated membrane C

[0095] Fig. 4: Photo of the example 8 ter coated membrane D

[0096] Fig. 5: Photo of the membrane E coated in Example 9

[0097] Fig. 6: Photo of the example 9 biscoated membrane F

[0098] Fig. 7: Photo of the example 9 ter coated membrane G

[0099] Fig. 8: Photo of the example 9 quater coated membrane H

[0100] Fig. 9: Photo of the example 9 quinquies coated membrane I

[0101] In the coatings, the membrane was only slightly corrugated, resulting in a relatively homogeneous coating that is suitable as an electrode for water electrolysis.

[0102] 13. Testing of a catalyst-coated membrane in an electrolysis test cell

[0103] The performance of the catalytically coated membranes in alkaline water electrolysis (AEMWE) was compared. The catalytically coated membranes from examples 10, 11, and 12 were used in an electrolysis cell with an active area of ​​25 cm 2 tested. The catalyst layer served as a cathode catalyst.

[0104] A dimensionally stable, porous stainless steel electrode was used on the anode side. The measuring cell was maintained at 60 °C during the electrolysis experiments and flushed with 1 M KOH solution on both the anode and cathode sides. The characteristic current-voltage curves are shown in Figure 10; the legend is shown in Table 2.

[0105] Fig. 10: Current-voltage characteristics of the tested CCMs

[0106] Table 2: Legend for Figure 10

[0107] The graphs show the required voltage versus the externally applied current. At a constant current (e.g. 1000 mA / cm 2 ), a low voltage is preferable, as this reduces the electrical energy required to produce the same amount of hydrogen at the same production rate, thereby increasing efficiency. Currents above 500 mA / cm are particularly relevant for industrial applications. 2It can be seen that the current-voltage characteristic curve is lower at higher current densities for the CCM produced with the inventive catalyst ink than for the reference CCM. The lower current density results in lower power consumption at the same voltage, so the specific energy requirement of the CCM produced with the inventive ink is lower.

[0108] 14. Conclusion

[0109] A comparison of the current-voltage characteristics in Figure 10 shows that the CCMs produced with a catalyst ink according to the invention achieve higher efficiency in alkaline water splitting than both CCMs based on a different solvent mixture. The higher efficiency can be explained by the fact that the coating was more homogeneous and that the increased viscosity allowed for a somewhat thicker catalyst layer to be applied.

[0110] This proves that thanks to the solvent mixture selected according to the invention, better results can be achieved than would be possible by simply increasing the solids content of conventional catalyst inks.

Claims

Patent claims 1 . A catalyst ink comprising the following components: a) a solid electrocatalyst in particulate form; b) a liquid solvent mixture containing at least acetonitrile and at least one alkanol; c) a binder dissolved in the solvent mixture, wherein the binder is an anion-conducting polymer containing at least one structure corresponding to one of the following formulas (I), (II), (III): where in (I) X represents a structural element comprising a positively charged nitrogen atom which is attached to C 1 and C 2 and which is bonded via two bonds to one or two hydrocarbon radicals comprising 1 to 12, preferably 1 to 6, particularly preferably 1 or 5 carbon atoms and wherein in (I) Z stands for a structural element comprising a carbon atom which is bonded to C 3 and C 4and which comprises at least one aromatic six-membered ring which is directly bonded to one of the oxygen atoms, wherein the aromatic six-membered rings may be substituted by one or more halogen and / or one or more Ci- to C4-alkyl radicals; where in (II) X represents a structural element comprising a positively charged nitrogen atom which is attached to C 1 and C 2 and which is bonded via two bonds to one or two hydrocarbon radicals comprising 1 to 12, preferably 1 to 6, particularly preferably 1 or 5 carbon atoms, and wherein in (II) Z stands for a structural element which comprises a carbon atom which to C 3 and C 4and which comprises at least one aromatic six-membered ring which is directly bonded to one of the oxygen atoms, wherein the aromatic six-membered ring may be substituted in positions 3 and 5 with the same or different Ci to C4 alkyl radicals, in particular with a methyl, isopropyl or tert-butyl group, the methyl group being preferred; wherein in (III) X stands for a ketone or sulfone group; wherein in (III) Z stands for a structural element which comprises at least one tertiary carbon atom and at least one aromatic six-membered ring, wherein the aromatic six-membered ring is directly bonded to one of the two oxygen atoms; and wherein in (III) Y stands for a structural element which comprises at least one nitrogen atom with a positive charge, wherein this nitrogen atom is bonded to the structural element Z; characterized in that the weight fraction of the acetonitrile is greater than the weight fraction of the binder, and in that the weight fraction of the alkanol is greater than the weight fraction of the binder, wherein the weight fractions are each based on the total weight of the catalyst ink.

2. Catalyst ink according to claim 1, wherein the ratio of the weight fraction of acetonitrile to the weight fraction of alkanol is between 1:10 and 17:

1.

3. Catalyst ink according to claim 1 or 2, characterized in that the catalyst ink additionally comprises water.

4. Catalyst ink according to claim 1 or 2 or 3, characterized in that the catalyst ink additionally comprises dimethyl sulfoxide.

5. Catalyst ink according to any one of claims 1 to 4, characterized in that the weight proportions of the components based on the total weight of the catalyst ink are in the following ranges, provided that the sum of the weight proportions of all components listed here does not exceed 100% by weight:

6. Catalyst ink according to one of claims 1 to 5, characterized in that the alkanol is ethanol or 2-propanol or 1-propanol or a mixture thereof.

7. Catalyst ink according to one of claims 1 to 5, characterized in that the alkanol is 1,2-propanediol or 1-butanol or cyclohexanol or 1-heptanol or 1-octanol or a mixture thereof.

8. Catalyst ink according to one of the preceding claims 1 to 7, characterized in that the binder is completely dissolved in the solvent mixture.

9. Catalyst ink according to one of the preceding claims 1 to 8, additionally containing at least one further component which differs from the aforementioned components.

10. Catalyst ink according to claim 9, characterized in that the further component is a dispersing aid or a rheology aid or a surfactant or a conductivity additive.

11. Catalyst ink according to one of claims 1 to 10, having a dynamic viscosity for which a characteristic value is determined according to the method defined in the description, characterized in that the characteristic value of the dynamic viscosity is between 10 mPas and 10 4 mPas.

12. Catalyst ink according to one of the preceding claims 1 to 11, characterized in that the electrocatalyst contains at least one element selected from the group consisting of the following elements: iridium (Ir), nickel (Ni), cobalt (Co), chromium (Cr), iron (Fe), ruthenium (Ru), copper (Cu), molybdenum (Mo), zinc (Zn), lead (Pb), manganese (Mn), tungsten (W), platinum (Pt), sulfur (S), tin (Sn), gold (Au), silver (Ag), palladium (Rd), rhenium (Re), rhodium (Rh), cerium (Ce), where the element is present in its pure form or as oxide or as hydroxide or as oxide hydroxide or as phosphide.

13. A method for producing a catalytically coated substrate, comprising at least the following steps: i) providing the substrate; ii) providing a catalyst ink according to any one of claims 1 to 12; iii) applying the catalyst ink to the substrate; iv) drying the catalyst ink applied to the substrate.

14. The method according to claim 13, characterized in that the catalyst ink is applied to the substrate by doctor blade coating.

15. The method according to claim 13 or 14, characterized in that the substrate is an anion-conducting membrane.

16. The method according to claim 15, characterized in that the anion-conducting membrane contains at least one structure corresponding to one of the formulas (I), (II), (III) defined in claim 1.

17. A catalytically coated substrate obtainable by a process according to any one of claims 13 to 16.

18. Use of a catalytically coated substrate according to claim 17 in an electrochemical water splitting in a basic medium.

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