Catalyst ink usable with a doctor blade and having a high solids proportion
Patent Information
- Application Number
- AU2025227269
- Authority / Receiving Office
- AU · AU
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2024-02-26
- Filing Date
- 2025-02-14
- Publication Date
- 2026-10-08
Smart Images
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Abstract
Description
The invention relates to a catalyst ink, to a process for coating substrates with this ink, to the coated substrates obtained by the process and to their use. "Catalyst ink" is a term from the field of the production of components of electrochemical cells. Electrochemical cells are apparatuses with which electrochemical reactions are conducted. In electrochemical reactions, substances are converted with the absorption or release of electrical energy. In order to operate these reactions particularly efficiently, it is appropriate in many cases to conduct the reaction in the presence of what are known as electrocatalysts. Electrocatalysts are substances which accelerate the desired electrochemical reaction, or even make it possible in the first place, without being substantially consumed in the process. Electrocatalysts are often particulate, inorganic material, usually metals or metal oxides. In order to be able to conduct an electrochemical reaction in the presence of an electrocatalyst, the electrocatalyst must be incorporated in the electrochemical cell. This is often done in industrial designs of electrochemical cells by providing a component of the electrochemical cell with an electrocatalytically active or activatable coating containing the electrocatalyst. Suitable components to be coated are in particular separators or membranes or electrodes or fluid -technical devices such as gas distributors. Where a membrane has been provided with a catalytically active or activatable coating, this is referred to in the field as a catalyst coated membrane (CCM). When electrodes or other constituents of a cell have been coated with an electrocatalyst, this is referred to as a catalyst coated substrate (CCS). In this context, the term "substrate" is used in general for the object to be coated. This may generally be a component of an electrochemical cell, such as a membrane or an electrode; however, the term also encompasses auxiliary elements that are not incorporated within the cell, such as transfer substrates, which are used for the indirect coating of electrodes or membranes. A catalyst ink is a mixture of at least one particulate electrocatalyst and at least one liquid solution of a binder in a solvent. The catalyst ink is intended to be applied to the substrate to be coated. Applied there, the catalyst ink is dried so that the solvent volatilizes and the binder precipitates from the solution. The electrocatalyst and the binder remain behind, and together form the catalytically active or activatable coating of the substrate. The electrocatalyst gives the coating its catalytic activity, while the binder immobilizes the electrocatalyst on the substrate. The binder that has precipitated from the solution namely forms a solid matrix bonded to the substrate, within which matrix the particulate electrocatalyst is dispersed. The binder is usually polymers. Extensively used as binders are those polymers which have a certain ion conductivity, namely what are known as ionomers. Ionomers are known which exhibit a conductivity for anions, as are those which exhibit a conductivity for cations. Since ionomers do not exhibit any appreciable conductivity for electrons, some catalyst inks also contain conductivity additives such as carbon black. Furthermore, a catalyst ink may also contain processing aids such as surfactants or rheology aids. Lastly, a catalyst ink may also contain a liquid dispersing medium, which reduces the viscosity of the ink. The catalyst ink is diluted with the dispersing medium. Volatile substances such as water or ethanol are extensively used as dispersing media, evaporating off together with the solvent during the drying process. As a result, a catalyst ink can be considered to be a precursor to the catalytically active or activatable coating, that is to be produced, of the substrate. The choice of the electrocatalyst, the binder and any conductivity additives is guided by the intended purpose of the coated component, more precisely by the electrochemical reaction to be operated and by the location in which the component is used, for instance on the anode or cathode side of the electrochemical cell. In contrast, the choice of solvent and optionally of processing aids or dispersing media is guided by the specifications of the desired coating technology: Both the solvent and the dispersing medium are removed in the course of drying and thus do not form part of the later catalytically active or activatable coating. Therefore, solvents and dispersing media are selected such that the ink has the desired processability. If this is not possible solely by the skilful selection of solvent and dispersing medium, processing aids are required. However, since these, if in doubt, remain in the coating and there negatively impact the efficiency of the electrochemical reaction, a good catalyst ink should in the best case work without such processing aids. The present invention is concerned specifically with the optimal selection of solvents and optional dispersing media. Its primary aim is that of achieving 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 namely describes the sum of all fractions of the components of the ink that remain behind as solid on the substrate after drying, that is to say the nonvolatile components of the ink. These are essentially the precipitated binder and the electrocatalyst. Since solvents and any dispersing medium are driven out during drying, they are not part of the solids content. In contrast, in the interests of a high solids content, the proportion of volatile constituents such as solvents and any dispersing media must be reduced. A further aim of the invention is that of adjusting the dynamic viscosity of the catalyst ink so that it can be processed rapidly and with good quality. In particular, the catalyst ink should be able to be applied to the substrate by a bar coater because bar coating is an efficient coating process which provides a high coating quality and is readily mastered. Unfortunately, there is a trade-off between high solids content and good bar coatability because an excessively high solids content increases the viscosity of the catalyst ink so much that it can no longer be bar coated. Proceeding from these general technical specifications, the technical object is consolidated in the light of the following state of the art: A special form of an electrochemical reaction is alkaline membrane water electrolysis (AEMWE). It is used for the production of hydrogen H2 and oxygen O2 from water H2O with the absorption of electrical energy. AEMWE is considered to be a technology for the sustainable production of "green" hydrogen, provided that the electrical energy required is obtained from renewable sources. An overview of the construction and materials of the electrochemical cells currently in use in AEMWE is given by: 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 Particularly for the production of the membranes used in AEMWE, polymers exhibiting conductivity for anions, more precisely for hydroxide ions (OH-), are required. Polymers exhibiting conductivity for anions are known from patent applications EP3770201A1, EP4032934A1 and EP4059988A1. They are suitable for the production of anion exchange membranes (AEMs). These can in turn be used in alkaline water electrolysis. None of the AEMs disclosed in that document has a catalytic activity. In the course of the production of the AEMs, the ionomers disclosed in EP3770201A1 and EP4032934A1 are dissolved in N,N-dimethylacetamide (DMAC) and / or in N,N-dimethylformamide (DMF). Since DMAC and DMF are known to be carcinogenic, mutagenic or reprotoxic (CMR), they are classified as CMR substances in accordance with the REACH Regulation, requiring costly safety measures in industrial processing. In the ionomer precursor synthesis described in EP4059988A1, dimethyl sulfoxide (DMSO) and acetonitrile (ACN) are used. Neither solvent is classified as a CMR substance. However, this advantage is not exploited by EP4059988A1 because DMAC is still used as a solvent when casting the membrane: Thus, the production of the anion exchange membrane still requires handling with a CMR substance, which makes production more expensive. The preparation of a catalyst ink from the ionomers disclosed in the above-cited documents is in turn disclosed in WO2023088714A1. The catalyst ink contains DMSO as solvent and additionally an ethanol / water mixture as dispersing medium. This ink is used to produce CCMs or CCSs that can be used in AEMWE. Although the description of WO2023088714A1 suggests that this ink could be bar coated, an ultrasonic spray coater was used in the experimental part to apply the catalyst ink to various substrates. In fact, experiments presented here show that the catalyst ink disclosed in WO2023088714A1 cannot be bar coated. In addition, the catalyst content of this ink is calculated to be about 11 mg / ml, which is already good for a spray ink, but is still quite low for a bar-coating ink. European patent application 24152389.3, which had not yet been published at the time of the application, from the same applicant describes a catalyst ink the binder of which was prepared in accordance with EP3770201A1. The catalyst ink further contains silica, DMSO, ethanol and water. The description also mentions that DMSO, ethanol and acetonitrile could be used as solvents, namely alone or in mixtures with one other. A mixing ratio is not specified. Moreover, experiments presented here show that both ethanol and acetonitrile alone are not capable of dissolving the polymer disclosed in EP3770201A1. An alternative, bar-coatable catalyst ink for the coating of anion exchange membranes has been developed by Koch et. al: Koch, S., Metzler, L., Kilian, S. K., Heizmann, P. A., 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 Unfortunately, this ink also contains a CMR substance as solvent, namely methanol. Industrial manufacture in the area in which the REACH Regulation is effective is also associated with high costs here. Against this background, the technical object was that of specifying a catalyst ink that has a high solids content and can at the same time still be bar coated satisfactorily. The catalyst ink should as far as possible not contain any solvents known to be carcinogenic, mutagenic or reprotoxic. The coated substrates produced with the catalyst ink are intended in particular to be usable in alkaline membrane water electrolysis as catalytically active or activatable anion exchange membrane. This object is achieved by a catalyst ink according to Claim 1. The invention therefore provides 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 dissolved in the solvent mixture, the binder being an anion-conducting polymer containing at least one structure corresponding to one of the following formulae (I), (II), (III): where, in (I), X is 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 where, in (I), Z is a structural element comprising a carbon atom which is bonded to C3 and C4 and comprising at least one aromatic six-membered ring bonded directly to one of the oxygen atoms, where the aromatic six-membered rings may be substituted by one or more halogen radicals and / or one or more C1- to C4-alkyl radicals; (II) where, in (II), X is 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 where, in (II), Z is a structural element comprising a carbon atom which is bonded to C3 and C4 and comprising at least one aromatic six-membered ring bonded directly to one of the oxygen atoms, where the aromatic six-membered ring may be substituted in positions 3 and 5 by the same or different C1- to C4-alkyl radicals, in particular by a methyl, isopropyl or tert-butyl group, preferably the methyl group; (III) where, in (III), X is a ketone or sulfone group; where, in (III), Z is a structural element comprising at least one tertiary carbon atom and at least one aromatic six-membered ring, where the aromatic six-membered ring is joined directly to one of the two oxygen atoms; and where, in (III), Y is a structural element comprising at least one nitrogen atom having a positive charge, this nitrogen atom being joined to the structural element Z; wherein the improvement in the catalyst ink resides in the fact that both the proportion by weight of the acetonitrile and the proportion by weight of the alkanol are each greater than the proportion by weight of the binder. Said proportions by weight are in each case based on the total weight of the catalyst ink. The present invention is based on the surprising finding 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 finding is therefore surprising because to date it has always been CMR substances such as DMAC or DMF that have been described as solvents suitable for these ionomers. In addition, these ionomers cannot be dissolved in pure ethanol or in pure acetonitrile: Only a mixture of acetonitrile with an alkanol is capable of dissolving the relevant binders. In order to obtain a readily processable ink as a result, the proportion of alkanol and acetonitrile must each be greater than the proportion of the binder. This also means the proportions by weight based on the total weight of the ink. A further unexpected effect is that the use of the solvent mixture of ACN and ROH makes it possible to considerably increase the solids content of the ink. Fortunately, all of this involves the use of a potentially CMR-free solvent cocktail, which imposes lower safety requirements on the 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 reprotoxic. Nevertheless, it is possible to use in the formulation an alkanol which is subject to corresponding suspicion, such as for example methanol. In certain circumstances, it may be advantageous for the processability of the ink to use a CMR-relevant alkanol as a solvent and to accept higher safety precautions during coating in return. If this is not desired, the formulation according to the invention also allows the use of those alkanols that are not classified as CMR substances, such as 2-propanol. Within the solvent mixture, ACN and ROH may be present in different ratios. What is important is that both substances are present and that their individual proportion by weight is greater than the proportion by weight of the binder. For example, the ratio of the proportion by weight of the acetonitrile to the proportion by weight of the alkanol may be in the range between 1:10 and 17:1. A particularly good bar-coatability and a particularly high solids content can be formulated by additionally adding dimethyl sulfoxide (DMSO) as solvent to the ink. DMSO is not CMR relevant. Should the viscosity of the catalyst ink be 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. The particulate electrocatalyst can, however, be dispersed in the dispersing medium. It is simplest to use water as dispersing medium because it is volatile and inexpensive. In addition, it is not a CMR substance. In a particular embodiment of the invention, therefore, the catalyst ink additionally comprises water. The proportion by weight of water may be greater than any other proportion by weight of the other components of the catalyst ink. Nevertheless, the catalyst ink should not contain substantially more water than other components. A specific proposed recipe for an advantageous catalyst ink with a high solids content is set out in Table 1. Table 1: Advantageous recipe for a catalyst ink according to the invention Component Minimum proportion (wt.%) Maximum proportion (wt.%) Dimethyl sulfoxide (DMSO) 0 40 Alkanol (ROH) 2 75 Acetonitrile (ACN) 2 85 Water (H2O) 0 60 Binder (I), (II), (III) 0.8 46 Electrocatalyst (eCat) 8 35 It goes without saying that the sum total of the proportions by weight of all components listed in Table 1 does not exceed 100 wt.%. However, it is possible for the sum total not to reach 100 wt.%, namely when the catalyst ink comprises a further component not listed in Table 1. Advantageously, the catalyst ink consists exclusively of the components listed in Table 1. The sum total of the proportions by weight of all components listed in Table 1 in this case is exactly 100 wt.%. As alkanol ROH, it is possible in principle to use any alkanol. Examples of these are: methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-nonanol, 1-decanol, 2-propanol. Secondary or tertiary alkanols may also be used , such as, for example, propane-1,2-diol. Alkanols derived from cycloalkanes may also be used, such as, for example, cyclohexanol . Preference is given to using ethanol EtOH as alkanol, because EtOH is not classified as a CMR substance. 1 -propanol or 2-propanol, which are also not CMR-relevant, may be used as well. This also applies to propane-1,2-diol. Of course, mixtures of these alkanols may also be used. For the case where a mixture of two or more alkanols is used, the numerical values for the minimum proportion and the maximum proportion relate to the proportion by weight of the mixture, i.e. the sum total of the proportions by weight of all alkanols. Preferably, the catalyst ink is formulated such that the binder is completely dissolved in the solvent mixture. This is because, if the binder partially precipitates from the solvent mixture, the ink becomes lumpy and can no longer be processed well. Preferably, the ink does not contain any further binder besides the obligatory binder containing a structure according to formula (I), (II) or (III). In order not to introduce into the catalyst layer any disruptive substances that could impair the electrolysis, the catalyst ink should consist as exclusively as possible of the components explicitly mentioned above. However, in some circumstances it may still be necessary to add to the catalyst ink a further component which differs from the aforementioned components. This may be organic or inorganic additives, such as a dispersing aid or a rheology aid or a surfactant or a conductivity additive. Specific examples of further components are silica or carbon black. Carbon black is a conductivity additive, and silica is suitable as a rheology aid. The characteristic value of the dynamic viscosity n of the catalyst ink should be between 101 mPas and 104 mPas. The dynamic viscosity n is measured at a temperature of 25°C with a rotational rheometer with plate-plate geometry. The diameter of the plates is 40 mm, and the gap between the plates is 1 mm. The dynamic viscosity is measured at increasing shear rates between 0.1 s-1 and 1000 s-1, the dynamic viscosity at 1 s-1 being used as the characteristic value. A suitable rotational rheometer with plate-plate geometry is available, for example, from Malvern Kinexus. The catalyst ink according to the invention has been developed for the production of catalyst coated substrates. A corresponding coating process using 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. The invention therefore further provides such a production process. A notable feature of the ink is its good bar-coatability. Therefore, the catalyst ink is preferably applied to the substrate by bar coating. As substrate, an anion-conducting membrane is preferably used. The coating with the catalyst ink creates a catalytically active or activatable AEM. Particularly preferably, the substrate used is an AEM containing at least one structure according to formula (I), (II) or (III). This means that the membrane and the binder consist essentially of the same ionomer. This creates a particularly good attachment of the catalyst layer to the membrane. The advantages of the catalyst ink according to the invention not only affect the production costs, but also lead to a better product: Experiments show that the ink according to the invention leads to a better efficiency of the substrate coated therewith, which can probably be attributed to a better coating quality. A catalyst coated substrate, obtainable by the coating process according to the invention with the ink according to the invention, is therefore also provided by the invention. 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 catalyst coated substrate according to the invention in alkaline water electrolysis is further provided for by the invention. This use is made by conducting the alkaline water electrolysis in the presence of the catalyst coated substrate. The electrocatalyst present in the catalyst layer catalyses the electrochemical reaction. The catalyst layer can at the same time serve as an electrode. If an AEM was used as substrate, it acts as a membrane for the AEMWE and transports hydroxide ions OH- from the cathodic compartment of the electrochemical cell into the anodic compartment. According to the current state of research, the following elements may be used in the catalyst ink according to the invention, in pure form or as oxide, as hydroxide, as oxide hydroxide or as phosphide: 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). The elements are consequently useful as electrocatalyst in pure form or as oxide or as hydroxide or as oxide hydroxide individually in each case or in combinations. The electrocatalyst is formulated into the ink in particulate form, i.e. as powder or granules. Very particular preference is given to formulating the catalyst ink such that it contains as electrocatalyst platinum or platinum alloy supported on carbon (Pt / C) or a nickel-iron (oxide) hydroxide (NiFeaObHc) or a nickel-iron phosphide (NiFeaPb). The indices a, b, and c are real numbers in the interval from 0 to 8. The indices a, b, and c may be the same or different. The nickel-iron ratio in the nickel-iron (oxide) hydroxide should ideally be in the range from 2:1 to 10:1. Examples Hereinafter, the dissolution behavior of an ionomer of interest is first investigated. Nine different catalyst inks are then prepared from this ionomer. The inks differ in the solvents used. These nine inks are then used to produce catalyst layers on anion exchange membranes. Finally, the catalyst coated AEMs are compared in terms of their performance in water electrolysis. The effects achieved with the invention are in this way supported experimentally. 1 Preparation of an ionomer (not part of the invention) An anion-conducting cationic polymer was synthesized in accordance with Example 3 of EP3770201A1. This polymer was initially provided as a solution and was processed into a powder by drying. Subsequently, in order to minimize the residual solvent content, the dried polymer was dried further at 80°C in a vacuum drying cabinet for 48 h. 2.Experiment concerning the solubility of the ionomer in ethanol, 1-propanol and 2-propanol Three vessels were prepared for this, the first with 27 g of ethanol, the second with 27 g of 1-propanol and the third with 27 g of 2-propanol. Added to these vessels were in each case 3 g of the cationic polymer synthesized from Example 1, and the vessels were closed and shaken at 60°C for 4 h. The result in each case was a slightly swollen, softened lump of the polymer, which had not significantly dissolved, at the bottom of the vessel. 2bis. Experiment concerning the solubility of the ionomer in propane-1,2-diol, 1-butanol, cyclohexanol, 1-heptanol and 1-octanol Five vessels were prepared for this, the first with 9 g of propane-1,2-diol, the second with 9 g of 1-butanol, the third with 9 g of cyclohexanol, the fourth with 9 g of 1-heptanol and the fifth with 9 g of 1-octanol. Added to these vessels was in each case 1 g of the cationic polymer synthesized from Example 1, and the vessels were closed and shaken at 60°C for 4 h. The result in each case was a partially slightly swollen lump of the polymer, which had not significantly dissolved, at the bottom of the vessel. Experiment concerning the solubility of the ionomer in acetonitrile For this, 3 g of the cationic polymer synthesized from Example 1 was added to a vessel containing 27 g of acetonitrile and this was shaken at 60°C for 4 h. No significant dissolution of the polymer was visible here as a result either. 4.Experiment concerning the solubility of the ionomer in a mixture of ethanol or 1 -propanol or 2-propanol in combination with acetonitrile Three vessels were prepared for this, the first with 13.5 g of ethanol and 13.5 g of acetonitrile, the second with 13.5 g of 1-propanol and 13.5 g of acetonitrile, and the third with 13.5 g of 2-propanol and 13.5 g of acetonitrile. Added to these vessels were in each case 3 g of the cationic polymer synthesized from Example 1, and these were shaken at 60°C for 4 h. The result was a virtually clear solution. The polymer had dissolved in all three vessels. 4bis. Experiment concerning the solubility of the ionomer in a mixture of propane-1,2-diol, 1-butanol, cyclohexanol, 1-heptanol and 1-octanol in combination with acetonitrile Five vessels were prepared for this, the first with 4.5 g of propane-1,2-diol, the second with 4.5 g of 1-butanol, the third with 4.5 g of cyclohexanol, the fourth with 4.5 g of 1-heptanol and the fifth with 4.5 g of 1-octanol. Added to these vessels were in each case 4.5 g of acetonitrile and also 1 g of the cationic polymer synthesized from Example 1, and the vessels were closed and shaken at 60°C for 4 h. The result was a virtually clear solution with a yellowish tint. The polymer had dissolved in all three vessels. 5 Interim findings The investigated ionomer 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 further alkanols tested here in combination with acetonitrile. 6 Production of an anion exchange membrane (not part of the invention) The cationic polymer synthesized in Example 1 was used to produce an anion-conducting membrane, as described in Example 4 of EP3770201A1. Used for this was the polymer solution prior to drying. 7.Preparation of a conventional catalyst ink (not part of the invention) The cationic polymer synthesized in Example 1 was used to produce a catalyst-containing dispersion, as described in Example 12 of WO2023088714A1. The ionomer was initially dissolved in dimethyl sulfoxide with stirring and temperature (60°C) for 16 h. Subsequently, the platinum on carbon (Pt / C, ~50 wt.%) catalyst was dispersed in the dispersing medium, consisting of equal parts by volume of water and ethanol, under ultrasound (BRANSONICTM B-1200 E2 from Branson Ultrasonics Corporation, Brookfield, CT, US) in an ice bath and at a power of 30 W for 30 min. Addition of the ionomer solution (50 mg / ml in DMSO), is followed by further dispersion using ultrasound in an ice bath for 1 min at a power of 30 W and dispersion using a shaker (MS1 Minishaker from IKA, Staufen, DE) for 10 s at 2500 rpm. The proportions were selected in accordance with Table 1, Example 12 of WO2023088714A1. Thus, the catalyst content was set to 11 mg / ml, with the Pt / C catalyst particles being three parts by mass and the ionomer solid being one part by mass. This resulted in a low-viscosity catalyst ink. 8.Preparation of a catalyst ink with an increased solids content (not part of the invention) 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 to be optimal for bar coating, the solids content was maximized while maintaining the solvent ratio and the ionomer-to-catalyst ratio. If the intention is to keep the ethanol : water : DMSO = 0.405 : 0.512 : 0.083 mass ratio and the catalyst : ionomer = 3 : 1 ratio constant, then the ionomer solution is a limiting factor since an ionomer proportion of 20 wt.% in DMSO can no longer be easily dispersed. An ink with a catalyst solids content of 5.73 wt.% was thus prepared according to the same procedure as described in Example 7. The platinum on carbon (Pt / C, ~50 wt.%) catalyst was dispersed in the dispersing medium, consisting of equal parts by volume of water and ethanol, under ultrasound (BRANSONICTM B-1200 E2 from Branson Ultrasonics Corporation, Brookfield, CT, US) in an ice bath and at a power of 30 W for 30 min. Addition of the ionomer solution (20 wt.% in DMSO), is followed by further dispersion using ultrasound in an ice bath for 1 min at a power of 30 W and dispersion using 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 produced in Example 7. 8bis. Provision of a catalyst ink according to the invention based on EtOH-ACN As an inventive composition, 13.85 g of a catalyst-containing ink were prepared. For this, 3 g of Pt / C (Pt ~50 wt.%) was initially weighed into a shatterproof vessel (volume 100 ml) with the exclusion of oxygen, followed by addition of 8.5 g of acetonitrile and shaking. 2.35 g of an ionomer solution consisting of 15 wt.% of ionomer solids from Example 1, 42.5 wt.% of ethanol and 42.5 wt.% of acetonitrile were added to the catalyst-containing composition. The mixture was dispersed in an agitated mixer (Lau, Germany) for 2 hours with the aid of grinding balls (total amount 30 g, diameter 3 mm) for 120 min. The solids content of this catalyst ink was accordingly 24.2 wt.%. 8ter. Provision of a catalyst ink according to the invention based on EtOH-ACN-H2O As a further inventive composition, 14.32 g of catalyst-containing ink was prepared. For this, 3 g of Pt / C (Pt ~50 wt.%) and 6 g of ultrapure water were initially weighed into a shatterproof vessel (volume 100 ml) with the exclusion of oxygen, followed by addition of 0.5 g of acetonitrile and 0.5 g of ethanol and shaking. 4.32 g of an ionomer solution consisting of 10 wt.% of ionomer solids from Example 1, 45 wt.% of ethanol and 45 wt.% of acetonitrile were added to the catalyst-containing composition. The mixture was dispersed in an agitated mixer (Lau, Germany) with the aid of grinding balls (total amount 30 g, diameter 3 mm) for 120 min. The solids content of this catalyst ink was accordingly 24.0 wt.%. 9 Provision of a catalyst ink according to the invention based on H2O-DMSO-EtOH-ACN As a further inventive composition, 14.6 g of catalyst-containing ink was prepared. For this, 3 g of Pt / C (Pt ~50 wt.%) was initially weighed into a shatterproof vessel (volume 100 ml) with the exclusion of oxygen, followed by addition of 6 g of ultrapure water and 2 g of DMSO and shaking. 3.6 g of an ionomer solution consisting of 12 wt.% of ionomer solids from Example 1, 20 wt.% of ethanol, 20 wt.% of acetonitrile and 48 wt.% of DMSO were added to the catalyst-containing composition. The mixture was dispersed in an agitated mixer (Lau, Germany) with the aid of grinding balls (total amount 30 g, diameter 3 mm) for 120 min. The solids content of this catalyst ink was accordingly 23.5 wt.%. 9bis. Provision of a catalyst ink according to the invention based on EtOH-ACN-H2O-C As yet a further inventive composition, 15.1 g of catalyst-containing ink was prepared. For this, 3 g of Pt / C (Pt ~50 wt.%) and 0.1 g of carbon black (Ketjenblack® 600JD) were initially weighed into a shatterproof vessel (volume 100 ml) with the exclusion of oxygen, followed by addition of 6.4 g of ultrapure water and 2 g of DMSO and shaking. 3.6 g of an ionomer solution consisting of 12 wt.% of ionomer solids from Example 1, 20 wt.% of ethanol, 20 wt.% of acetonitrile and 48 wt.% of DMSO were added to the catalystcontaining composition. The mixture was dispersed in an agitated mixer (Lau, Germany) with the aid of 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 accordingly 23.4 wt.%. 9ter. Provision of a catalyst ink according to the invention based on EtOH-ACN-H2O-SiO2 As yet a further inventive composition, 15.1 g of catalyst-containing ink was prepared. For this, 3 g of Pt / C (Pt ~50 wt.%) was initially weighed into a shatterproof vessel (volume 100 ml) with the exclusion of oxygen, followed by addition of 6 g of silica dispersion (5% in H2O), 0.5 g of water and 2 g of DMSO and shaking. 3.6 g of an ionomer solution consisting of 12 wt.% of ionomer solids from Example 1, 20 wt.% of ethanol, 20 wt.% of acetonitrile and 48 wt.% of DMSO were added to the catalyst-containing composition. The mixture was dispersed in an agitated mixer (Lau, Germany) with the aid of 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 accordingly 24.7 wt.%. 9quater. Provision of a catalyst ink according to the invention based on H2O-DMSO-EtOH-ACN As a further example of an inventive composition, 10.2 g of catalyst-containing ink was prepared. For this, 1 g of Pt / C (Pt ~50 wt.%) was initially weighed into a shatterproof vessel (volume 100 ml) with the exclusion of oxygen, followed by addition of 6 g of ultrapure water and 2 g of DMSO and shaking. 1.2 g of an ionomer solution consisting of 12 wt.% of ionomer solids from Example 1, 20 wt.% of ethanol, 20 wt.% of acetonitrile and 48 wt.% of DMSO were added to the catalyst-containing composition. The mixture was dispersed in an agitated mixer (Lau, Germany) with the aid of grinding balls (total amount 30 g, diameter 3 mm) for 120 min. The solids content of this catalyst ink was accordingly 11.2 wt.%. 9quinquies. Provision of a catalyst ink according to the invention based on 2-propanol-ACN As an inventive composition, 14.85 g of a catalyst-containing ink were prepared. For this, 3 g of Pt / C (Pt ~50 wt.%) was initially weighed into a shatterproof vessel (volume 100 ml) with the exclusion of oxygen, followed by addition of 1.0 g of acetonitrile and 8.5 g of 2-propanol and shaking. 2.35 g of an ionomer solution consisting of 10 wt.% of ionomer solids from Example 1, 45 wt.% of 2-propanol and 45 wt.% of acetonitrile were added to the catalyst-containing composition. The mixture was dispersed in an agitated mixer (Lau, Germany) for 2 hours with the aid of grinding balls (total amount 30 g, diameter 3 mm) for 120 min. The solids content of this catalyst ink was accordingly 21.78 wt.%. 10. Coating of a membrane with conventional ink by means of a bar coater (not part of the invention) For the reference experiment with the formulation from WO2023088714 (Example 12), the catalyst ink provided in Example 7 was applied by means of an automatic bar coater (Elcometer 4340, advance rate 5 mm / s, spiral coating bar, 60 pm) to the anion-conducting membrane produced in Example 6. The coated AEM was then dried in a laboratory oven at 80°C for 10 min. The result was called membrane A and can be seen in Figure 1. Fig. 1: Photo of a membrane A coated with conventional ink As can be seen in Figure 1, the membrane has become intensely curled during the coating. This has resulted in an inhomogeneous coating. Nevertheless, the catalyst coated AEM obtained in this way is in principle suitable for water electrolysis. 11. Coating of a membrane with an ink not in accordance with the invention having an increased solids content by means of a bar coater (not part of the invention) For the second reference experiment, the composition provided in Example 8 was applied by means of an automatic bar coater (Elcometer 4340, advance rate 5 mm / s, spiral coating bar, 60 pm) to the membrane produced in Example 6. The coated membrane was then dried in a laboratory oven at 80°C for 10 min. The result was called membrane B and can be seen in Figure 2. Fig. 2: Photo of a membrane B coated with ink not in accordance with the invention As can be seen in Figure 2, the membrane has become slightly curled during the coating. The coating exhibited numerous sites with a relatively thin layer thickness, which are visible in Figure 2 as white spots. In principle, the coating obtained is suitable as an electrode for water electrolysis, but the coating is not optimal. 12. Coating of membranes with catalyst inks according to the invention (part of the invention) For the inventive coating using the ACN / ROH-containing formulation, the inks provided in Examples 8bis to 9quinquis were in each case applied by means of an automatic bar coater (Elcometer 4340, advance rate 5 mm / s, spiral coating bar, 60 pm) to a membrane produced in accordance with Example 6. The coated membranes were then dried in a laboratory oven at 80°C for 10 min. For the membrane coated using the formulation from Example 8bis, bar coating was performed a second time over the same location using the same parameters. The results were respectively called membranes C to I and can be seen in Figures 3 to 8. Fig. 3: Photo of membrane C coated in Example 8bis Fig. 4: Photo of membrane D coated in Example 8ter Fig. 5: Photo of membrane E coated in Example 9 Fig. 6: Photo of membrane F coated in Example 9bis Fig. 7: Photo of membrane G coated in Example 9ter Fig. 8: Photo of membrane H coated in Example 9quater Fig. 9: Photo of membrane I coated in Example 9quinquies In the coatings, the membrane has curved only slightly, resulting in a relatively homogeneous coating that is suitable as an electrode for water electrolysis. 13. Testing of a catalyst-coated membrane in an electrolysis test cell The performance of the catalyst coated membranes in alkaline water electrolysis AEMWE was compared. The catalyst coated membranes from Examples 10, 11 and 12 were tested in an electrolysis cell with an active area of 25 cm2. The catalyst layer served as a cathode catalyst. On each anode side, a dimensionally stable, porous stainless steel electrode was used. During the electrolysis experiments, the measurement cell was heated to 60°C and flushed with 1 M KOH solution on the anode and cathode sides. The characteristic current-voltage characteristic curves are presented in Figure 10; Table 2 shows the key. Fig. 10: Current-voltage characteristic curves of the tested CCMs Table 2: Key to Figure 10 Ink: Qualification Membrane Label Curve 7. not inventive A solid diamonds dashed 8. not inventive B solid squares dotted 8bis inventive C hollow diamonds dashed 8ter inventive D hollow circles solid 9 inventive E crosses, rotated 45° dotted 9bis inventive F hollow diamonds thin solid 9ter inventive G crosses with a vertical centre line thin dashed 9quater inventive H hollow squares thin dotted In the graphs, the voltage required has been plotted against the externally applied current density. At a constant current density (e.g. 1000 mA / cm2), a low voltage is preferable, since the electrical energy required for the production of the same amount of hydrogen, at the same production rate, decreases and as a result the efficiency increases. Of particular relevance for industrial use are current densities that exceed 500 mA / cm2. It can be seen that the current-voltage characteristic curve at the higher current densities in the case of the CCM produced with inventive catalyst ink is lower than the CCM produced for reference. The lower current density results in lower power consumption at the same voltage, and so the specific energy requirement of the CCM produced with ink according to the invention is lower. 14. Conclusion From the comparison of the current-voltage characteristic curves of Figure 10, it is apparent that the CCMs produced with a catalyst ink according to the invention achieve a 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 effected more homogeneously and that a somewhat thicker catalyst layer could be applied as a result of the increased viscosity. It has thus been demonstrated that, thanks to the solvent mixture selected according to the invention, better results can be achieved than would be possible solely by increasing the solids content of conventional catalyst inks.
Claims
1. 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, the binder being an anion-conducting polymercontaining at least one structure corresponding to one of the following formulae (I), (II), (III):where, in (I), X is 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 where, in (I), Z is a structural element comprising a carbon atom which is bonded to C3 and C4 and comprising at least one aromatic six-membered ring bonded directly to one of the oxygen atoms, where the aromatic six-membered rings may be substituted by one or more halogen radicals and / or one or more C1- to C4-alkyl radicals;(II)where, in (II), X is 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 where, in (II), Z is a structural element comprising a carbon atom which is bonded to C3 and C4 and comprising at least one aromatic six-membered ring bonded directly to one of the oxygen atoms, where the aromatic six-membered ring may be substituted in positions 3 and 5by the same or different C1- to C4-alkyl radicals, in particular by a methyl, isopropyl or tertbutyl group, preferably the methyl group;(III)where, in (III), X is a ketone or sulfone group; where, in (III), Z is a structural element comprising at least one tertiary carbon atom and at least one aromatic six-membered ring, where the aromatic six-membered ring is joined directly to one of the two oxygen atoms; and where, in (III), Y is a structural element comprising at least one nitrogen atom having a positive charge, this nitrogen atom being joined to the structural element Z;characterizedin that the proportion by weight of the acetonitrile is greater than the proportion by weight of the binder,and in that the proportion by weight of the alkanol is greater than the proportion by weight of the binder,wherein the proportions by weight are in each case based on the total weight of the catalyst ink.
2. Catalyst ink according to Claim 1, wherein the ratio of the proportion by weight of the acetonitrileto the proportion by weight of the alkanol is between 1:10 and 17:1.
3. Catalyst ink according to Claim 1 or 2, characterized in that the catalyst ink additionallycomprises water.
4. Catalyst ink according to Claim 1 or 2 or 3, characterized in that the catalyst ink additionallycomprises dimethyl sulfoxide.
5. Catalyst ink according to any of Claims 1 to 4, characterized in that the proportions by weight ofthe components, based in each case on the total weight of the catalyst ink, lie within the following ranges, with the proviso that the sum total of the proportions by weight of all components listed here does not exceed 100 wt.%:Component Minimum proportion (wt.%) Maximum proportion (wt.%) Dimethyl sulfoxide (DMSO) 0 40 Alkanol (ROH) 2 75 Acetonitrile (ACN) 2 85 Water (H2O) 0 60 Binder (I), (II), (III) 0.8 46 Electrocatalyst (eCat) 8 356. 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 propane-1,2-diol 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 iscompletely dissolved in the solvent mixture.
9. Catalyst ink according to one of the preceding Claims 1 to 8, additionally also containing at leastone further component which differs from the aforementioned components.
10. Catalyst ink according to Claim 9, characterized in that the further component is a dispersing aidor 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 n, for which acharacteristic value is determined in accordance with the method defined in the description, characterized in that the characteristic value of the dynamic viscosity n is between 10 mPas and 104 mPas.
12. Catalyst ink according to one of the preceding Claims 1 to 11, characterized in that theelectrocatalyst 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 (Pd), rhenium (Re), rhodium (Rh), cerium (Ce), wherein the element is present in its pure form or as oxide or as hydroxide or as oxide hydroxide or as phosphide.
13. Process for producing a catalyst coated substrate, comprising at least the following steps:i) providing the substrate;ii) providing a catalyst ink according to one of Claims 1 to 12,iii) applying the catalyst ink to the substrate;iv) drying the catalyst ink applied to the substrate.5 14. Process according to Claim 13, characterized in that the catalyst ink is applied to the substrate bybar coating.
15. Process according to Claim 13 or 14, characterized in that the substrate is an anion-conductingmembrane.1016. Process according to Claim 15, characterized in that the anion-conducting membrane contains atleast one structure corresponding to one of the formulae (I), (II), (III) defined in Claim 1.
17. Catalyst coated substrate obtainable by a process according to one of Claims 13 to 16.1518. Use of a catalyst coated substrate according to Claim 17 in electrochemical water splitting in abasic medium.