EP2467889A1

EP2467889A1 - Inorganic and/or organic acid-containing catalyst ink and use thereof in the production of electrodes, catalyst-coated membranes, gas diffusion electrodes and membrane electrode units

Info

Publication number: EP2467889A1
Application number: EP10743147A
Authority: EP
Inventors: Ömer ÜNSAL; Sigmar BRÄUNINGER
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2009-08-21
Filing date: 2010-08-18
Publication date: 2012-06-27
Also published as: CN102742053A; JP2013502678A; WO2011020843A1; US20120148936A1; KR20120044384A

Abstract

Catalyst ink, comprising one or more catalyst materials, a solvent component, and at least one acid, selected from the group consisting of phosphoric acid, polyphosphoric acid, sulfuric acid, nitric acid, HCIO4, organic phosphonic acids, inorganic phosphonic acids, trifluoromethane sulfonic acids, or the mixtures thereof, an electrode comprising at least one catalyst ink according to the present invention, a fuel cell comprising at least one membrane electrode unit according to the invention, and a method for producing a membrane electrode unit according to the present invention.

Description

Inorganic and / or organic acid-containing catalyst ink and their use in the manufacture of electrodes, catalyst-coated membranes, gas diffusion electrodes and membrane-electrode assemblies Description

The present invention relates to a catalyst ink containing one or more catalyst materials, a solvent component and at least one acid, an electrode containing at least one catalyst ink according to the present invention, a membrane electrode assembly containing at least one electrode according to the invention or containing at least one catalyst ink according to the present invention Invention, a fuel cell containing at least one membrane electrode assembly according to the invention and a method for producing a membrane electrode assembly according to the present invention.

Polymer electrolyte membrane fuel cells (PEM fuel cells) are known in the art. In them, almost exclusively sulfonic acid-modified polymers are currently used as proton-conducting membranes. Here are predominantly perfluorinated polymers application. Prominent example of this is Nafion ^® from DuPont. Proton conduction requires a relatively high water content in the membrane, typically about 4 to 20 molecules of water per sulfonic acid group. The necessary water content, but also the stability of the polymer in conjunction with acidic water and the reaction gases hydrogen and oxygen, the operating temperature of the PEM fuel cell stacks usually limited to 80 to 100 ⁰ C. Under pressure, the operating temperature can be increased to> 120 ⁰ C. Otherwise, higher operating temperatures can not be realized without a power loss of the fuel cell.

For system technical reasons, however, higher operating temperatures than 100 ⁰ C in the fuel cell are desirable. The activity of the precious metal-based catalysts contained in the membrane-electrode assembly is much better at high operating temperatures. When using so-called formates from hydrocarbons, in particular, significant amounts of carbon monoxide are contained in the reformer gas, which usually have to be removed by expensive gas treatment or gas cleaning. At high operating temperatures, the tolerance of the catalysts to the CO impurities increases.

Furthermore, heat is generated during the operation of fuel cells. The cooling of this

Systems at below 80 ⁰ C, however, can be very expensive. Depending on the power output, the cooling devices can be made much simpler. That means, that in fuel cells, which are operated at temperatures above 100 ⁰ C, the waste heat made much better usable and thus the fuel cell system efficiency can be increased by electricity-heat coupling. In order to reach these temperatures, membranes with new conductivity mechanisms are generally used. A promising approach, such as working with no or very little humidification at operating temperatures of> 100 ⁰ C, generally 120 ⁰ C to 180 ⁰ C, fuel cell can be realized, relates to a fuel cell type in which the conductivity of the membrane on the Content of liquid, electrostatically bound to the polymer backbone of the membrane based acid, which takes over the proton conductivity even with almost complete dryness of the membrane above the boiling point of water without additional humidification of the operating gases. Such a fuel cell type as known in the art is generally referred to as a high temperature polymer electrolyte membrane (HTM) fuel cell. In particular, polybenzimidazole (PBI) is known as a material for such membranes, which are impregnated, for example, with phosphoric acid as a liquid electrolyte.

In order to obtain the highest possible efficiency of membranes impregnated with an acidic liquid electrolyte, the electrodes used in a membrane electrode assembly or in a fuel cell must be adapted to the conditions in the fuel cell membrane. Among other things, it is important that the dosage and distribution of the liquid electrolyte (the acid) in the membrane-electrode assembly is optimal to ensure good proton conductivity. M. Uchida et al., J. Electrochem. Soc, Vol. 142, no. 2, pages 463 to 468, relates to a method of making a catalyst layer in electrodes of polymer electrolyte fuel cells, comprising the preparation of a perfluorosulfonate ionomer (PFSI) colloid. Both a good network of PFSI and a uniform distribution of PFSI on Pt particles should be achieved. This is achieved by colloid formation of the PFSI chains in specific organic solvents. The PFSI colloids are selectively adsorbed on carbon agglomerates with highly dispersed Pt particles on their surface, and then a catalyst paste is prepared. According to the examples in M. Uchida PFSI solutions are first prepared by reacting commercially available Nafion ^® solutions in isopropanol or Flemi- on ^® solutions in ethanol to specific organic solvents, namely, esters, ethers, acetone, ketones, amines, Carboxylic acids, alcohols and non-polar solvents, are given. From the mixtures obtained, the mixtures are selected in which PFSI are in colloidal form. To these mixtures is added the catalytically active component Pt-C. Subsequently, a paste is produced from the mixtures by means of ultrasound treatment. The pastes will be used for the production of gas diffusion electrodes and further for the production of membrane electrode assemblies and for the production of fuel cells. In this case, have the membrane electrode assemblies or the fuel cell membranes in the form of Nafion ^®, Flemion ^® and so perfluorosulfonate ionomers.

WO 2005/076401 relates to membranes for fuel cells of at least one polymer containing nitrogen atoms, the nitrogen atoms of which are chemically bonded to central atoms of polybasic inorganic oxo acids or derivatives thereof. In a preferred embodiment, the polymer and the oxo acid derivative are crosslinked into a network which is capable of accepting dopants to form proton-conducting properties. A suitable dopant is z. B. phosphoric acid. Furthermore, WO 2005/076401 relates to a fuel cell in which the gas distribution electrodes of the fuel cell are loaded with the dopant such that they form a dopant reservoir for the membrane, wherein the membrane becomes proton conductive by receiving the dopant after exposure to pressure and temperature and proton-conducting to the Gas distribution electrodes is connected. According to WO 2005/076401 it is the object of WO 2005/076401 to provide membranes for fuel cells, which are characterized by a homogeneous uptake of dopants and their retention. According to the example, the loading of the electrodes with the doping agent takes place in the form that the finished electrodes are doped with the doping agent, preferably phosphoric acid.

DE 103 01 810 A1 relates to a membrane electrode assembly for polymer electrolyte fuel cells with an operating temperature of up to 250 ⁰ C, which consists of at least two planar gas distribution electrodes and a polymer membrane disposed therebetween, with at least one basic polymer and a dopant, with which the gas distribution electrodes are loaded so that they represent a dopant reservoir for the polymer membrane, wherein the polymer membrane is bound via the dopant after exposure to pressure and temperature proton-conducting and fixed to the gas distribution electrodes. The proton-conducting connection between electrode and electrolyte is usually ensured by phosphoric acid. For this purpose, the electrodes are impregnated with phosphoric acid before assembling the cell. According to the examples, a commercially available electrode is vacuum-impregnated at room temperature with concentrated phosphoric acid.

WO 2006/005466 A1 discloses gas diffusion electrodes with an improved proton conduction between an electrocatalyst located in the catalyst layer and an adjacent polymer electrolyte membrane which operate at operating temperatures can be used above the boiling point of water and ensure a permanently high gas permeability, and the corresponding manufacturing process. According to WO 2006/005466, the gas diffusion electrodes are loaded with dopants in such a way that they represent a dopant reservoir for the membrane. As doping agent according to WO 2006/005466 phosphoric acid is preferably used. According to the examples in WO 2006/005466, the preparation of a membrane electrode assembly based on gas diffusion electrodes is carried out in such a way that the gas diffusion electrodes are impregnated with concentrated phosphoric acid. DE 101 55 543 A1 discloses proton-conducting polymer electrolyte membranes which comprise at least one base material and at least one doping agent which is the reaction product of an at least dibasic inorganic acid with an organic compound, the reaction product having an acidic hydroxyl group, or the condensation product of this compound with a polybasic acid. Phosphoric acid itself is not contained in the proton-conducting electrolyte membrane according to DE 101 55 543 A1. The preparation of a membrane-electrode assembly is carried out according to the examples in DE 101 55 543 A1, characterized in that commercially available electrodes are impregnated in vacuo with concentrated phosphoric acid at room temperature.

With regard to the production of acid-loaded gas diffusion electrodes, a subsequent acid treatment of the gas diffusion electrodes already loaded with catalyst material and subsequent compression of a suitable polymer electrolyte membrane with the obtained gas diffusion electrodes to form a membrane-electrode unit thus takes place according to the prior art. In this case, the amount and the distribution of the acid (dopant) in the electrode are disadvantageous. How much acid in the compression in the membrane and how much acid goes out of the gas diffusion electrode is not definable and uncontrollable. The distribution of the acid in the catalyst layer is highly dependent on the nature of the catalyst layer.

It is therefore an object of the present invention to provide a catalyst ink which is suitable for the production of gas diffusion electrodes, catalyst-coated membranes, membrane-electrode units or fuel cells, which on the one hand has good processing properties, an outstanding distribution which is improved over the prior art having acid (dopant) in the catalyst layer, allowing controlled metering of the amount of acid (dopant) into the catalyst layer, and further enabling a reproducible and reliable manufacturing process of gas diffusion electrodes, catalyst coated membranes, membrane electrode assemblies, and fuel cells, respectively. This object is achieved by a catalyst ink containing:

(a) one or more catalyst materials, as component A;

(b) a solvent component, as component B; and

(C) at least one acid selected from the group consisting of phosphoric acid, polyphosphoric acid, sulfuric acid, nitric acid, HCIO ₄ , organic phosphonic acids (eg vinylphosphonic acid), inorganic phosphonic acid, trifluoromethanesulfonic acid or mixtures thereof. For the purposes of the present application, the term "catalyst ink" means both catalyst inks and catalyst pastes.

The catalyst ink according to the invention has numerous advantages over the catalyst inks of the prior art or over subsequently acid-doped electrodes. On the one hand, introduction and distribution of a controlled and suitable amount of acid into the electrode is possible.

Furthermore, a novel pore structure is generated in the catalyst layer by the presence of the acid in the catalyst ink. Since the drying temperatures of the gas diffusion electrodes are generally below the boiling point of the acid, the acid molecules are interposed between the catalyst particles.

Furthermore, the presence of acid can result in improved processability of the catalyst inks. Since the acids used according to the invention are difficult to evaporate, the catalyst ink dries more slowly during processing. As a result, exact loading and reproducibility of the electrode production is possible, and mass production is facilitated by the use of larger catalyst ink volumes. Further, the acids adsorbed in the catalyst layers may contribute to proton conductivity in a membrane-electrode assembly made using the catalyst ink of the present invention.

The catalyst ink of the invention can be prepared by known standard methods, for. Screen printing, knife coating, other printing or spray coating can be applied to gas diffusion layers or membranes.

The catalyst ink according to the invention is particularly suitable for high-temperature fuel cells in which the conductivity of the membrane is based on the content of liquid, electrostatically bound to the polymer backbone of the membrane. Siert, wherein the membrane is based in particular on polyazoles and is used as a liquid electrolyte, for example phosphoric acid.

Component A: Catalyst Materials

According to the present invention, the catalyst ink contains one or more catalyst materials as component A. These catalyst materials serve as a catalytically active component. Suitable catalyst materials which can be used as catalyst materials for the anode or for the cathode of a membrane electrode assembly or a fuel cell are known to the person skilled in the art. For example, suitable catalyst materials are those which contain at least one noble metal as the catalytically active component, the noble metal in particular being platinum, palladium, rhodium, iridium, gold and / or ruthenium. These substances can also be used in the form of alloys with one another. Furthermore, the catalytically active component may contain one or more base metals as alloying additives, these being selected from the group consisting of chromium, zirconium, nickel, cobalt, titanium, tungsten, molybdenum, vanadium, iron and copper. In addition, the oxides of the aforementioned noble metals and / or base metals can be used as catalyst materials.

The catalyst material may be in the form of supported catalysts or supported catalysts, with supported catalysts being preferred. The carrier materials used are preferably electrically conductive carbon, particularly preferably selected from carbon blacks, graphite and activated carbons.

The catalyst materials are generally used in the form of particles. In the case when the catalyst materials are present as carrier-free catalysts, the particles (eg noble metal crystallites) may have average particle sizes of <5 nm, eg. B. 1 to 1000 nm, determined by XRD measurements. In the case where the catalyst material is used in the form of supported catalysts, the particle size (catalytically active component + support material) is generally from 0.01 to 100 .mu.m, preferably from 0.01 to 50 .mu.m, particularly preferably from 0.01 to 30 .mu.m. In general, the catalyst ink according to the present invention contains such a content of noble metals that the noble metal content in the catalyst layer of the electrode or membrane electrode assembly prepared by the catalyst ink is 0.1 to 10.0 mg / cm ² , preferably 0.2 to 6.0 mg / cm ² , more preferably 0.2 to 3.0 mg / cm ² . These values can be determined by elemental analysis of a flat sample. When producing a membrane-electrode assembly using the catalyst ink of the invention, a weight ratio of a membrane polymer for producing the membrane present in the membrane-electrode assembly generally to the catalyst material used in the catalyst ink comprising at least one noble metal and optionally one or more Support materials of> 0.05, preferably 0.1 to 0.6, selected.

In the catalyst ink according to the invention, the catalyst materials (component A) are generally present in an amount of from 2 to 30% by weight, preferably from 2 to 25% by weight, particularly preferably from 3 to 20% by weight, based on the components A, B and C of the catalyst ink, before.

In the event that the catalyst materials used according to the invention contain a carrier material, the proportion of carrier material in the catalyst materials used according to the invention is generally from 40 to 90% by weight, preferably from 60 to 90% by weight. The proportion of noble metal in the catalyst materials used according to the invention is generally from 10 to 60% by weight, preferably from 10 to 40% by weight. If, in addition to the precious metal, a base metal is additionally used as an alloying additive, the proportion of noble metal is reduced by the corresponding amount of the base metal. The proportion of base metal as alloying additive, based on the total amount of metal present in the catalyst material, is usually from 0.5 to 15% by weight, preferably from 1 to 10% by weight. If the corresponding oxides are used instead of the metals, the quantities indicated for the metals apply.

Component B: solvent component

In general, the catalyst ink according to the invention contains from 2 to 30% by weight, preferably from 2 to 25% by weight, particularly preferably from 3 to 20% by weight of component A and from 0.1 to 6% by weight, preferably 0.2 to 4 wt .-%, particularly preferably 0.2 to 3 wt .-% of component C. That is, the catalyst ink according to the invention generally contains 64 to 97.9 wt .-%, preferably 71 to 97.8 wt. %, more preferably 77 to 96.8 wt .-% of the solvent component, based on the total amount of components A, B and C.

As the solvent component, a single solvent or a mixture comprising two or more solvents can be used in the catalyst ink of the present invention. In general, an aqueous medium is used in the catalyst ink according to the invention, preferably water. In addition or as an alternative to water, the solvent component may be alcohols or polyalcohols such as glycerol or Ethylene glycol, or organic solvents such as dimethylacetamide (DMAc), N-methylpyrrolidone (NMP) or dimethylformamide (DMF). The water, alcohol or polyalcohol content and / or content of organic solvent can be selected in the catalyst ink in order to adjust the rheological properties of the catalyst ink. In general, the catalyst ink according to the invention contains, in addition to water, 0 to 50% by weight of alcohol and / or 0 to 20% by weight of polyalcohol and / or 0 to 50% by weight of at least one organic solvent.

Component C: at least one acid

As component C, the catalyst ink according to the invention contains at least one acid selected from the group consisting of phosphoric acid, polyphosphoric acid, sulfuric acid, nitric acid, HCIO ₄ , organic phosphonic acids (eg vinylphosphonic acid), inorganic phosphonic acid, trifluoromethanesulfonic acid or mixtures thereof.

Preferably, the at least one acid present in the catalyst ink according to the present invention is at least one acid used as a liquid electrolyte (dopant) in polymer electrolyte membranes for fuel cells. Suitable acids are known in principle to those skilled in the art, the acids preferably being selected from the group consisting of phosphoric acid, sulfuric acid, polyphosphoric acid, vinylphosphonic acid. Phosphoric acid is particularly preferably used as the acid. Suitable acids present in a polymer electrolyte membrane of a membrane electrode assembly or catalyst-coated membrane or fuel cell produced by means of the catalyst ink according to the invention are mentioned below.

The acid is generally in the catalyst ink according to the invention in an amount of 0.1 to 6 wt .-%, preferably 0.2 to 4 wt .-%, particularly preferably 0.2 to 3 wt .-%, based on the sum the components A, B and C, which gives 100 wt .-%, used.

The catalyst ink of the invention may optionally additionally contain at least one dispersant as component D. The dispersant is generally present in an amount of 0.1 to 4 wt .-%, preferably 0.1 to 3 wt .-%, based on the total amount of components A, B and C. Suitable dispersants are known to those skilled in principle. A particularly preferably used as component D dispersant is at least one perfluorinated polymer, for. At least one tetrafluoroethylene polymer, preferably at least one perfluorinated sulfonic acid. repolymer, z. B. at least one sulfonated tetrafluoroethylene polymer, particularly preferably Nation ^® from DuPont, fumion ^® from Fumatech or ligion ^® from lonpower.

In a further preferred embodiment, the present invention therefore relates to a catalyst ink according to the invention, wherein the catalyst ink further comprises a component D as a dispersant:

(d) at least one perfluorinated polymer, e.g. At least one tetrafluoroethylene polymer, preferably at least one perfluorinated sulfonic acid polymer, e.g. B. control for at least a sulfonated tetrafluoroethylene polymer, particularly preferably Nafion ^® by DuPont ^® fumion of Fumatech or ligion ^® from lonpower.

Other suitable perfluorinated polymers are, for. For example, tetrafluoroethylene polymer (PTFE), polyvinylidene fluoride (PVdF), perfluoropropyl vinyl ether (PFA) and / or perfluoromethylvinyl nylether (MFA).

In addition, the catalyst ink according to the invention may further comprise at least one surfactant as component E. Suitable surfactants are known to the person skilled in the art. These may be surfactants which, after application of the catalyst ink, are either washed out or decompose pyrolytically, eg. B. when the electrode prepared after application of the catalyst ink z. B. is heated to temperatures of <200 ⁰ C. Preferred surfactants are selected from the group consisting of anionic surfactants and nonionic surfactants, e.g. B. Fluorosurfactants such as surfactants of the general formula CF ₃ - (CF ₂ ) pX, where p = 3 to 12 and X is selected from the group consisting of -SO ₃ H, -PO ₃ H ₂ and -COOH, z. A tetraethylammonium salt of heptadecafluorooctanoic acid. Further suitable surfactants are octylphenolpoly (ethylene glycol ethers) _x , where x is z. B. may be 10, z. ^Triton® X-100 from Roche Diagnostics GmbH, nonylphenol ethoxylates, e.g. As nonylphenol ethoxylates of the Tergitol ^® series of Dow Chemical Company, sodium salts of naphthalene sulfonic acid condensates such. B. sodium salts of naphthalenesulfonic acid condensates of the series Tamol ^® BASF SE, fluorinated surfactants, eg. B. Zonyl ^® fluorinated surfactants from DuPont, alkoxylation products predominantly linear fatty alcohols, eg. B. Alkoxylierungsproduk- te predominantly linear fatty alcohols of the series Plurafac ^® , z. B. Plurafac ^® LF 71 1 BASF SE, alkoxylates of ethylene oxide or propylene oxide, eg. B. alkoxylates of ethylene oxide or propylene oxide of the series Pluriol ^® BASF SE, in particular polyethylene glycols of the formula HO (CH ₂ CH ₂ O) _n H, z. B. the Pluriol ^® E series of BASF SE, z. B. Pluriol ^® E300 and ß-Naphtholethoxylat, z. B. Lugalvan ^® BNO12 BASF SE. The at least one surfactant is usually used in an amount of from 0.1 to 4% by weight, preferably from 0.1 to 3% by weight, particularly preferably from 0.1 to 2.5% by weight, if surfactant is used. , based on the components A, B and C used. A further subject of the present invention is therefore a catalyst ink according to the invention, wherein the catalyst ink further contains a component E:

(E) at least one surfactant, preferably selected from the group consisting of anionic surfactants, for. B. Fluorosurfactants such as surfactants of the general formula CF ₃ - (CF ₂ ) _P -X, where p = 3 to 12 and X is selected from the group consisting of -SO ₃ H, -PO ₃ H ₂ and -COOH, z. B. a tetraethylammonium salt of hepta- decafluorooctanoic acid. Further suitable surfactants are octylphenol poly (ethylene glycol ethers) _x , where x is z. B. may be 10, z. ^Triton® X-100 from Roche Diagnostics GmbH, nonylphenol ethoxylates, e.g. As nonylphenol ethoxylates of Se rie Tergitol ^® from Dow Chemical Company, sodium salts of naphthalene sulfonic acid condensates such. B. sodium salts of naphthalenesulfonic acid condensates of the series Tamol ^® BASF SE, fluorinated surfactants, eg. B. Fluoro surfactants Zonyl ^{® series} of DuPont, alkoxylation products predominantly linear fatty alcohols, eg. As alkoxylation predominantly linear fatty alcohols series Plurafac ^® , z. B. Plurafac ^® LF 71 1 BASF SE, alkoxylates of ethylene oxide or propylene oxide, z. B. alkoxylates of ethylene oxide or propylene oxide of the series Pluriol ^® BASF SE, in particular polyethylene glycols of the formula HO (CH ₂ CH ₂ O) _n H, z. B. the Pluriol ^® E series of BASF SE, z. B. Pluriol ^® E300 and ß-Naphtholethoxylat, z. B. Lugalvan ^® BNO12 BASF SE.

In addition, the catalyst ink according to the invention may further comprise polymer particles comprising one or more proton-conducting polymers as component F.

The polymer particles are not present in solution in the catalyst ink in a preferred embodiment of the present invention, but are preferably dispersed in the liquid medium of the catalyst ink.

The catalyst ink of the invention is - as mentioned above - particularly suitable for high-temperature fuel cells in which the conductivity of the membrane based on the content of liquid, electrostatically bound to the polymer backbone of the membrane acid, the membrane is based in particular on polyazoles and as a liquid electrolyte, for example Phosphoric acid is used.

The polymer particles, which are finely dispersed in the catalyst layer, allow the acid, in particular phosphoric acid, to be taken up and added to the catalyst particles present in the catalyst layer. ing polymer particles are bound. This can increase the three-phase interface (catalyst, ionomer and gas). It has been found that a membrane electrode assembly based on a catalyst ink of the present invention has lower resistances as compared to a membrane electrode assembly based on a catalyst ink containing no finely dispersed polymer.

In this context, proton-conducting polymers are understood to mean that the polymers used together with a liquid as the electrolyte, which comprises acids or acidic compounds, can conduct protons.

Suitable proton-conducting polymers are the polymers mentioned below as polymers of the polymer electrolyte membrane.

The polymer particles generally have an average particle size of <100 .mu.m, preferably <50 .mu.m. The particle size and particle size distribution is determined by laser diffraction with a Malvern Master Sizer ^® instrument.

Usually, the catalyst ink of the invention contains - if the component F is present in the catalyst ink according to the invention - from 1 to 50 wt .-%, preferably 1 to 30 wt .-%, particularly preferably 1 to 15 wt .-% of the at least one used as component F proton-conducting Polymer, based on the amount of catalyst used in the ink.

A further subject of the present invention is therefore a catalyst ink according to the invention, wherein the catalyst ink further comprises a component F: polymer particles comprising one or more proton-conducting polymers. Suitable proton-conducting polymers are mentioned above.

The preparation of the catalyst ink according to the invention is carried out by simply mixing see the components A, B and C and optionally the components D, E and optionally F. The mixing can be carried out in conventional mixing devices, wherein conventional mixing devices are known in the art. This mixing can be carried out by all methods known to the person skilled in the art, e.g. B. in stirred reactors, Kugelschüttelmischern or continuous mixing devices, optionally using ultrasound. Usually, the components of the catalyst ink are mixed at room temperature. However, it is possible to mix the components of the catalyst ink in a temperature range of 0 to 70 ⁰ C, preferably 10 to 50 ⁰ C. The catalyst ink according to the invention is distinguished by improved processing properties which enable exact loading and reproducibility of the electrode production. Furthermore, a controlled and suitable amount of acid can be introduced into the electrode and the acid adsorbed in the catalyst layers made from the catalyst ink can contribute to proton conductivity.

The catalyst ink according to the invention serves to form catalyst layers, in particular catalyst layers in catalyst-coated membranes (CCM), gas diffusion electrodes (GDE), membrane electrode assemblies (MEA) and fuel cells.

In this case, the catalyst layer is generally not self-supporting, but is usually applied to the gas diffusion layer (GDL) and / or the proton-conducting polymer electrolyte membrane. In this case, part of the catalyst layer can diffuse into the gas diffusion layer and / or the membrane, for example, whereby transition layers form. This can be z. B. also lead to the fact that the catalyst layer can be considered as part of the gas diffusion layer. The thickness of the catalyst layer built up from the catalyst ink according to the invention in a catalyst-coated membrane (CCM), gas diffusion electrode (GDE), membrane electrode assembly (MEA) or fuel cell is generally from 1 to 1000 μm, preferably from 5 to 500 μm preferably 10 to 300 microns. This value represents an average value that can be determined by measuring the layer thickness in the cross-section of images that can be obtained with a scanning electron microscope (SEM).

Another object of the present invention is the use of the catalyst ink according to the invention for producing a catalyst-coated membrane (CCM), a gas diffusion electrode (GDE), a membrane electrode assembly (MEA) or a fuel cell, wherein the above-mentioned catalyst-coated membranes, gas diffusion electrodes and membrane-electrode assemblies are preferably used in polymer electrolyte fuel cells or in PEM electrolysis.

In order to produce a catalyst-coated membrane (CCM), a gas diffusion electrode (GDE) or a membrane electrode assembly (MEA), the catalyst ink is generally applied in homogeneously dispersed form to the catalyst-coated membrane (CCM) ion-conducting polymer electrolyte membrane or gas diffusion layer (GDL) applied to a gas diffusion electrode. The production of a homo- The dispersed ink can be carried out by means known to the person skilled in the art, for example by means of high-speed stirrers, ultrasound or ball mills.

The application of the homogeneously dispersed catalyst ink to the polymer electrolyte membrane or the gas diffusion layer can be effected by means of various techniques known to the person skilled in the art. Suitable techniques are for. As printing, spraying, knife coating, rolling, brushing, brushing, Decal, screen printing or inkjet printing.

In general, the catalyst layer obtained is prepared by applying the catalyst ink according to the invention dried after application. Suitable drying methods are known to the person skilled in the art. Examples are hot air drying, infrared drying, microwave drying, plasma processes and combinations of these processes. A further subject of the present invention is a catalyst-coated membrane (CCM) comprising a polymer electrolyte membrane having a top and a bottom, wherein on both the top and on the bottom of a catalytically active layer is applied, prepared by applying the catalyst ink of the invention on the polymer electrolyte membrane.

The CCM according to the invention is characterized in particular by the special distribution of the acid (component C of the catalyst ink according to the invention) in the catalytically active layer, due to the use of the catalyst ink according to the invention.

Suitable polymer electrolyte membranes for the catalyst-coated membrane are known in principle to the person skilled in the art. Particularly suitable are proton-conducting polymer electrolyte membranes based on proton-conducting polymers. In this context, proton-conducting polymers are understood to mean that the polymers used together with a liquid as the electrolyte, which comprises acids or acidic compounds, can conduct protons.

Suitable polymers capable of conducting protons as electrolytes in the presence of acids or acidic compounds are, for example, selected from the group consisting of poly (phenylene), poly (p-xylylene), polyarylmethylene, polystyrene, polymethylstyrene, polyvinyl alcohol, polyvinyl acetate, Polyvinyl ether, polyvinylamine, poly (N-vinylacetamide), polyvinylimidazole, polyvinylcarbazole, polyvinylpyrrolidine, polyvinylpyridine; Polymers with CO bonds in the main chain, for example polyacetal, polyoxymethylene, polyether, polypropylene oxide, polyether ketone, polyester, in particular polyhydroxyacetic acid, polyethylene terephthalate, polybutylene terephthalate, polyhydroxybenzoate, polyhydroxypropionic acid, polypivalolactone, polycaprolactone, polymalonic acid, polycarbonate;

Polymers with C-S bonds in the main chain, for example polysulfide ethers, polyphenylene sulfide, polysulfones, polyethersulfone; Polymers with C-N bonds in the main chain, for example polyimines, polyisocyanides, polyetherimine, polyetherimides, polyaniline, polyaramides, polyamides, polyhydrazides, polyurethanes, polyimides, polyazoles, polyazole ether ketone, polyazines;

Liquid-crystalline polymers, in particular ^Vectra® from Ticona GmbH, and also inorganic polymers, for example polysilanes, polycarbosilanes, polysiloxanes, polysilicic acid, polysilicates, silicones, polyphosphazenes and polythiazyl.

In this case, basic polymers are preferred, and basically all basic polymers are suitable with which - after acid doping - protons can be transported. Preferred acids used are those which contain protons without additional water, e.g. B. by means of the so-called Grotthos mechanism transport. For the purposes of the present invention, a basic polymer having at least one nitrogen, oxygen or sulfur atom, preferably having at least one nitrogen atom, in a repeat unit is preferably used as the basic polymer. Furthermore, basic polymers comprising at least one heteroaryl group are preferred.

The repeating unit in the basic polymer according to a preferred embodiment contains an aromatic ring having at least one nitrogen atom. The aromatic ring is preferably a 5- or 6-membered ring having from 1 to 3 nitrogen atoms which may be fused to another ring, especially another aromatic one.

According to a preferred embodiment, high-temperature-stable polymers are used which contain at least one nitrogen, oxygen and / or sulfur atom in one or in different repeat units. High temperature stability in the context of the present invention is a polymer which can be operated as a polymeric electrolyte in a fuel cell at temperatures above 120 ⁰ C permanently. In this case, permanent means that a membrane of this polymer can generally be operated for at least 100 hours, preferably for at least 500 hours, at at least 80 ^° C., preferably at least 120 ^° C., particularly preferably at least 160 ^° C., without the power, which can be measured according to the method described in WO 01/18894 A2, by more than 50%, based on the initial power decreases. In the context of the present invention, all the abovementioned polymers can be used individually or as a mixture (blend). Blends which contain polyazoles and / or polysulfones are particularly preferred. The preferred blend components are polyether sulfone, polyether ketone and polymers modified with sulfonic acid groups, as described in DE 100 522 42 and DE 102 464 61.

Furthermore, polymer blends which comprise at least one basic polymer and at least one acidic polymer, preferably in a weight ratio of 1:99 to 99: 1 (so-called acid-base polymer blends), have also proven suitable for the purposes of the present invention. Particularly suitable acidic polymers in this context include polymers having sulfonic acid and / or phosphoric acid groups. Very particularly suitable acid-base polymer blends according to the invention are described, for example, in EP 1 073 690 A1. Most preferably, the proton-conducting polymers are polyazoles or mixtures of polyazoles which are proton-conductive doped with acid, preferably phosphoric acid.

A basic polymer based on polyazole particularly preferably contains recurring azole units of the general formula (I) and / or (II) and / or (III) and / or (IV) and / or (V) and / or (VI) and / or (VII) and / or (VIII) and / or (IX) and / or (X) and / or (XI) and / or (XII) and / or (XIII) and / or (XIV) and / or or (XV) and / or (XVI) and / or (XVII) and / or (XVIII) and / or (XIX) and / or (XX) and / or (XXI) and / or (XXII):

10

(XIII)

(XVlIl)

10

wherein

Ar are the same or different and represent a four-membered aromatic or heteroaromatic group which may be mononuclear or polynuclear,

Ar ^{1 are the} same or different and are a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{2 are the} same or different and represent a di- or trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear .

Ar ^{3 are the} same or different and represent a trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,

Ar ^{4 are the} same or different and represent a trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,

Ar ^{5 are the} same or different and represent a four-membered aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{6 are the} same or different and represent a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,

Ar ^{7 are the} same or different and represent a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,

Ar ^{8 are the} same or different and are a trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{9 are the} same or different and represent a di- or tri- or tetravalent aromatic or heteroaromatic group, the on or can be polynuclear,

Ar ^{10 are the} same or different and represent a divalent or trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,

Ar ^{11 are the} same or different and represent a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, X is the same or different and represents oxygen, sulfur or an amino group which represents a hydrogen atom, a 1-20 carbon atom group , preferably a branched or unbranched alkyl or alkoxy group, or carries an aryl group as a further radical, R is the same or different than hydrogen, an alkyl group or an aromatic group and in formula (XX) is an alkylene group or an aromatic group with the proviso that R in formula (XX) is other than hydrogen, and n, m is an integer ≥ 10, preferably ≥ 100.

Preferred aromatic or heteroaromatic groups are derived from benzene, naphthalene, biphenyl, diphenyl ether, diphenylmethane, diphenyldimethylmethane, bisphenone, diphenylsulfone, quinoline, pyridine, bipyridine, pyridazine, pyrimidine, pyrazine, triazine, tetrazine, pyrol, pyrazole, anthracene, benzopyrrole, benzotriazole, Benzooxathiadiazole, benzooxadiazole, benzopyridine, benzopyrazine, benzopyrazidine, benzopyrimidine, benzotriazine, indolizine, quinolizine, pyridopyridine, imidazolepyrimidine, pyrazinopyrimidine, carbazole, azeridine, phenazine, benzoquinoline, phenoxazine, phenotiazine, aziridizine, benzopteridine, phenanthroline and phenanthrene, optionally may also be substituted. In this case, the substitution pattern of Ar ¹ , Ar ⁴ , Ar ⁶ , Ar ⁷ , Ar ⁸ , Ar ⁹ , Ar ¹⁰ and Ar ^{11 is} arbitrary, in the case of phenylene, for example, Ar ¹ , Ar ⁴ , Ar ⁶ , Ar ⁷ , Ar ⁸ , Ar ⁹ , Ar ¹⁰ and Ar ^{11 are} independently ortho, meta and para-phenylene. Particularly preferred groups are derived from benzene and biphenylene, which may be optionally substituted.

Preferred alkyl groups are alkyl groups having 1 to 4 carbon atoms, e.g. For example, methyl, ethyl, n-propyl, i-propyl and t-butyl groups. Preferred aromatic groups are phenyl or naphthyl groups. The alkyl groups and the aromatic groups may be monosubstituted or polysubstituted.

Preferred substituents are halogen atoms, e.g. For example, fluorine, amino groups, hydroxy groups or C 1 -C ₄ -alkyl groups, for. For example, methyl or ethyl groups.

The polyazoles can in principle have different recurring units, which differ, for example, in their radical X. However, the respective polyazoles preferably have only the same radicals X in a recurring unit.

In a particularly preferred embodiment, the polyazoles contain recurring azole units of the formula (I) and / or (II).

The polyazoles in one embodiment are polyazoles containing recurring azole units in the form of a copolymer or a blend containing at least two units of the formulas (I) to (XXII) which differ from each other. The polymers can be present as block copolymers (diblock, triblock), random copolymers, periodic copolymers and / or alternating polymers. The number of repeating azole units in the polymer is preferably an integer ≥ 10, more preferably 100 100.

In a further preferred embodiment, polyazoles are used which contain repeating units of the formula (I) in which the radicals X within the repeating units are identical.

Further preferred polyazoles are selected from the group consisting of polybenzimidazole, poly (pyridine), poly (pyrimidine), polyimidazole, polybenzothiazole, polybenzoxazole, polyoxadiazole, polyquinoxaline, polythiadiazole and poly (tetrazapyrene). In a particularly preferred embodiment, the polyazoles contain benzimidazole recurring units. The following are suitable polyazoles having recurring benzimidazole units:

10

where n and m are integers ≥ 10, preferably ≥ 100;

wherein the present in the aforementioned benzimidazole units phenylene or heteroarylene units may be substituted with one or more F atoms.

The polyazole particularly preferably has repeating units of the following formula

or

where n is an integer ≥ 10, preferably ≥ 100, and o is 1, 2, 3 or 4. The polyazoles, preferably the polybenzimidazoles, are generally characterized by a high molecular weight. Measured as intrinsic viscosity, the molecular weight is preferably at least 0.2 dl / g, more preferably 0.8 to 10 dl / g, most preferably 1 to 10 dl / g. The viscosity eta i - also called intrinsic viscosity - is calculated from the relative viscosity eta rel according to the following equation eta i = (2.303 x log eta rel) / concentration. The concentration is given in g / 100 ml. The relative viscosity of the polyazoles is determined with the aid of a capillary viscometer from the viscosity of the solution at 25.degree. C., the relative viscosity being calculated from the corrected throughput times for solvent t.sub.θ and solution t.sub.1 in accordance with the following equation eta rel = t.sub.1 / t.sub.θ , The conversion to eta i is carried out according to the above relationship based on the data in "Methods in Carbohydrate Chemistry", Volume IV, Starch, Academic Press, New York and London, 1964, page 127.

Preferred polybenzimidazoles are, for. , Under the trade name Celazol ^® PBI (PBI Performance Products Inc.) commercially available.

In a very particularly preferred embodiment, the proton conductive polymer is pPBI (poly-2,2'-p- (phenylene) -5,5'-dibenzimidazole and / or F-pPBI (poly-2,2 '). -p- (perfluorophenylene) -5,5'-dibenzimidazole), which is proton conductive after doping with acid.

The polymer electrolyte membranes are generally prepared by methods known to those skilled in the art, e.g. Example, by casting, spraying or knife coating a solution or dispersion, which used to prepare the polymer electrolyte membrane

Contains components on a support. Suitable carriers are all customary carrier materials known to the person skilled in the art, eg. As plastic films such as polyethylene terephthalate (PET) films or polyethersulfone films, or metal strip, wherein the membrane can then be detached from the metal strip.

The polymer electrolyte membrane used in the catalyst-coated membranes (CCM) according to the invention generally has a layer thickness of from 20 to 2000 .mu.m, preferably from 30 to 1500 .mu.m, particularly preferably from 50 to 1000 .mu.m.

Another object of the present invention is a gas diffusion electrode (GDE), comprising a gas diffusion layer (GDL) and a catalytically active layer, prepared by applying the catalyst ink of the invention to the gas diffusion layer (GDL). As in the case of the CCM according to the invention, the GDE according to the invention is also distinguished in particular by the specific distribution of the acid (component C of the catalyst ink according to the invention) in the catalytically active layer, due to the use of the catalyst ink according to the invention.

Flat, electrically conductive and acid-resistant structures are usually used as gas diffusion layers. These include, for example, graphite fiber papers, carbon fiber papers, graphite fabrics and / or papers made conductive by the addition of carbon black. Through these layers, a fine distribution of the gas or liquid flows is achieved.

Furthermore, gas diffusion layers may also be used which contain a mechanically stable support material which is coated with at least one electrically conductive material, e.g. Carbon (such as soot) is impregnated. For this purpose, particularly suitable support materials include fibers, for example in the form of nonwovens, papers or fabrics, in particular carbon fibers, glass fibers or fibers containing organic polymers, for example propylene, polyester (polyethylene terephthalate), polyphenylene sulfide or polyether ketones. Further details on such diffusion layers can be found, for example, WO 97/20358.

The gas diffusion layers preferably have a thickness in the range from 80 μm to 2000 μm, particularly preferably 100 μm to 1000 μm, very particularly 150 μm to 500 μm. Furthermore, the gas diffusion layers favorably have a high porosity. This is preferably in the range of 20% to 80%.

The gas diffusion layers may contain conventional additives. These include u. a. Fluoropolymers, for example polytetrafluoroethylene (PTFE) and surface-active substances.

According to one embodiment, the gas diffusion layer may be constructed of a compressible material. In the context of the present invention, a compressible material is characterized by the property that the gas diffusion layer can be pressed by pressure to at least half, preferably to at least one third of its original thickness without loss of its integrity. This property generally includes gas diffusion layers of graphite fabric and / or paper rendered conductive by carbon black addition. The catalytically active layer in the gas diffusion electrode according to the invention is based on the catalyst ink according to the invention.

In this case, the catalytically active layer is applied to the gas diffusion electrode by means of the abovementioned catalyst ink according to the invention. Incidentally, the method of applying the catalyst ink to the gas diffusion electrode corresponds to the method of applying the catalyst ink to the catalyst-coated membrane described in detail above. Another object of the present invention is a membrane-electrode assembly comprising a polymer electrolyte membrane having a top and a bottom, both on the top, and on the bottom of a catalytically active layer is applied, prepared based on the catalyst ink according to the invention , and in each case a gas diffusion layer is applied to the respective catalytically active layer.

Suitable polymer electrolyte membranes are the polymer electrolyte membranes mentioned above with respect to the catalyst-coated membrane. Suitable gas diffusion layers are the gas diffusion layers mentioned above with respect to the gas diffusion electrode according to the invention. The catalytically active layer is characterized by the features mentioned with regard to the CCM and the GDL.

In principle, the preparation of the membrane-electrode units according to the invention is known to the person skilled in the art. Usually, the various constituents of the membrane-electrode assembly are superimposed and interconnected by pressure and temperature, usually at a temperature of 10 to 300 ⁰ C, preferably 20 to 200 ⁰ C, and at a pressure of generally 1 to 1000 bar, preferably 3 to 300 bar, is laminated. The membrane-electrode unit can, for. Example, be prepared by first two gas diffusion electrodes (GDE) are prepared, with suitable GDE's are mentioned above, and the gas diffusion electrodes are pressed with the polymer electrolyte membrane at the above temperatures and pressures.

Alternatively, a catalyst-coated membrane (CCM) may first be prepared, with suitable CCMs mentioned above, and this may be compressed at the aforementioned pressures and temperatures with two gas diffusion layers. An advantage of the membrane-electrode assemblies according to the invention according to the present invention is that they allow the operation of a fuel cell at temperatures above 120 ⁰ C. This applies to gaseous and liquid fuels such as hydrogen-containing gases, which are prepared for example in an upstream reforming of hydrocarbons. For example, oxygen or air can be used as the oxidant.

Another advantage of the membrane-electrode assemblies according to the invention is that they have a high tolerance to carbon monoxide in operation above 120 ⁰ C even with pure platinum catalysts, ie without a further alloying ingredient. At temperatures of 160 ^° C., for example, more than 1% carbon monoxide can be contained in the fuel gas, without this leading to a noticeable reduction in the power of the fuel cell. Furthermore, it is a significant advantage of the membrane electrode units according to the invention that a good and homogeneous distribution of acid in the catalyst layer is achieved by the use of the catalyst ink according to the invention in the preparation of the catalytically active layer of the membrane electrode assembly. This is achieved in particular by the fact that the catalyst ink of the invention contains as component C at least one acid selected from phosphoric acid, polyphosphoric acid, sulfuric acid, nitric acid, HCIO ₄ , organic phosphonic acids (eg vinylphosphonic acid), inorganic phosphonic acid, trifluoromethanesulfonic acid or mixtures thereof. The membrane-electrode assemblies according to the invention can be operated in fuel cells, without the fuel gases and the oxidants having to be moistened despite the possible high operating temperatures. The fuel cell is still stable and the membrane does not lose its conductivity. This simplifies the entire fuel cell system and brings additional cost savings, as the management of the water cycle is simplified. Furthermore, this also improves the process at temperatures below 0 ^° C. of the fuel cell system.

The membrane-electrode assemblies according to the invention furthermore make it possible for the fuel cell to be cooled down to room temperature and below without problems and then to be put back into operation without losing its power.

Furthermore, as already mentioned above, the membrane-electrode assemblies according to the present invention show a high long-term stability. This fuel cells can be provided, which also has a high long-term stability exhibit. Furthermore, the membrane electrode assemblies according to the invention have excellent temperature and corrosion resistance and a comparatively low gas permeability, especially at high temperatures. A decrease in the mechanical stability and the structural integrity, in particular at high temperatures, is reduced or avoided in the membrane-electrode assemblies according to the invention.

In addition, the membrane-electrode assemblies according to the invention can be produced inexpensively and easily.

Another object of the present invention is a fuel cell containing at least one membrane-electrode unit according to the invention. Suitable fuel cells and their components are known in the art. Since the performance of a single fuel cell is often too low for many applications, in the context of the present invention preferably a plurality of single fuel cells are combined via separator plates to form a fuel cell stack. In this case, the separator plates, if appropriate in conjunction with other sealing materials, seal the fit of the cathode and the anode to the outside and between the gas spaces of the cathode and the anode. For this purpose, the separator plates are preferably applied sealingly to the membrane-electrode assembly. The sealing effect can be increased further by pressing the composite of separator plates and membrane-electrode assembly. The separator plates preferably each have at least one gas channel for reaction gases, which are conveniently arranged on the sides facing the electrodes. The gas channels are to allow the distribution of reactant fluids.

Due to the high long-term stability of the membrane-electrode assemblies according to the present invention, the fuel cell according to the invention also has a high

Long-term stability. Usually, the fuel cell according to the invention can be operated continuously for long times, eg more than 5000 hours, at temperatures of more than 120 ⁰ C with dry reaction gases, without a noticeable

Performance degradation is detected. The achievable power densities are high even after such a long time.

In this case, the fuel cells according to the invention show a high quiescent voltage, even after a long time, for example more than 5000 hours, which is preferably at least 900 mV after this time. To measure the quiescent voltage, the fuel cell with a water flow on the anode and an air flow on the Cathode de-energized. The measurement is made by the fuel cell is switched from a current of 0.2 A / cm ² to the de-energized state and then recorded there for 5 minutes, the quiescent voltage. The value after 5 minutes is the corresponding resting potential. The measured values of open circuit voltage apply for a temperature of 160 ⁰ C. Moreover, the fuel cell after this time is preferably a small gas passage (gas cross-over). To measure the crossover, the anode side of the fuel cell is operated with hydrogen (5 L / h) and the cathode with nitrogen (5 L / h). The anode serves as a reference and counter electrode, the cathode as a working electrode. The cathode is set to a potential of 0.5 V and the hydrogen diffusing through the membrane at the cathode mass-transported oxidized limited. The resulting current is a measure of the hydrogen permeation rate. The current is <3 mA / cm ² , preferably <2 mA / cm ² , more preferably <1 mA / cm ² in a 50 cm ² cell. The measured values of the H ₂ cross over apply for a temperature of 160 ⁰ C.

Another object of the present invention is the use of the catalyst ink according to the invention for the production of catalytically active layers of a membrane electrode assembly. The following examples further illustrate the invention.

Example: 2 parts of Nafion ionomer in H ₂ O (10% by weight) EW1100 (DuPont), 3.5 parts of H ₂ O and 0.25 parts of phosphoric acid (85%) were placed in a glass bottle and stirred with the magnetic stirrer. Then a portion of catalyst Pt / C is weighed and slowly added to the batch with stirring. The batch was stirred for about 5-10 minutes at room temperature with the magnetic stirrer. The sample was then treated with ultrasound until the value of the energy input was 0.015 KWh. This value referred to a batch size of 20g.

The catalyst-coated gas diffusion electrode (GDE) was prepared by screen printing from the anode side and the cathode side. The catalyst powder-containing catalyst ink was used only for cathode GDEs.

For the cell tests, the MEA (Membrane Electrode Assembly) was made from

GDEs and Celtec-P membrane with a spacer to 75% of the initial thickness at 140 ⁰ C for 30 seconds pressed. The active area of MEA was 45cm ² . Subsequently, the samples were incorporated into the cell block and then at 160 ⁰ C, with H ₂ (Anode stoichiometry 1, 2), air (cathode stoichiometry 2) tested. The performance of the samples at 1 A / cm ² is given below.

Table: Performance of the sample at 1A / cm ²

Claims

claims

A catalyst ink comprising: (a) one or more catalyst materials as component A;

(b) a solvent component, as component B; and

(C) at least one acid selected from the group consisting of phosphoric acid, polyphosphoric acid, sulfuric acid, nitric acid, HCIO ₄ , organic phosphonic acids, inorganic phosphonic acids, trifluoromethanesulfonic acid or mixtures thereof.

2. catalyst ink according to claim 1, characterized in that the catalyst material contains at least one noble metal as a catalytically active component, in particular platinum, palladium, rhodium, iridium and / or ruthenium and alloys thereof, wherein the catalytically active component one or more base metals may contain as alloying additives, wherein the base metals are preferably selected from the group consisting of chromium, zirconium, nickel, cobalt, titanium, tungsten, molybdenum, vanadium, iron and copper, further including the oxides of the aforementioned noble metals and / or base metal can be used as catalyst materials, and wherein the catalytically active component may be present in the form of supported catalysts or supported catalysts, wherein in the case of supported catalysts preferably electrically conductive carbon is used as a support, more preferably selected from carbon black, graphite and activated carbon.

3. Catalyst ink according to claim 1 or 2, characterized in that the solvent component is an aqueous medium, preferably water.

4. catalyst ink according to one of claims 1 to 3, characterized in that the at least one acid is phosphoric acid.

5. Catalyst ink according to one of claims 1 to 4, characterized in that the catalyst ink (a) 2 to 30 wt .-%, preferably 2 to 25 wt .-%, particularly preferably 3 to 20 wt .-% of the component A, (b) 64 to 97.9% by weight of component B, and

(C) 0.1 to 6 wt .-% of component C, wherein the sum of components A, B and C is 100 wt .-%.

6. catalyst ink according to one of claims 1 to 5, characterized in that the catalyst ink further contains a component D:

(D) at least one perfluorinated polymer, preferably at least one perfluorinated sulfonic acid polymer.

7. catalyst ink according to claim 6, characterized in that the catalyst ink, the component D in an amount of 0.1 to 4 wt .-%, preferably 0.1 to

3 wt .-%, based on the total amount of components A, B and C in the catalyst ink contains.

8. catalyst ink according to one of claims 1 to 7, characterized in that the catalyst ink further contains a component E:

(E) at least one surfactant, preferably selected from the group consisting of anionic surfactants and nonionic surfactants, more preferably fluorosurfactants such as surfactants of the general formula CF ₃ - (CF ₂ ) pX, where p = 3 to 12 and X is selected from the group consisting of

-SO ₃ H, -PO ₃ H ₂ and -COOH, e.g. A tetraethylammonium salt of hepta- decafluorooctanoic acid; Octylphenolpoly (ethylene glycol ethers) _x , where x is z. B. can be 10; nonylphenol ethoxylates; Sodium salts of naphthalenesulfonic acid condensates; Alkoxylation products of predominantly linear fatty alcohols; Alkoxylates of ethylene oxide and propylene oxide, in particular

Polyethylene glycols of the formula HO (CH ₂ CH ₂ O) _n H; and β-naphthol ethoxylate.

9. A process for preparing a catalyst ink according to any one of claims 1 to 8 by mixing the components A, B, C, optionally D and optionally E.

10. Use of a catalyst ink according to any one of claims 1 to 8, or prepared by a process according to claim 9 for the preparation of a catalyst-coated membrane, a gas diffusion electrode, a membrane electrode assembly or a fuel cell.

1 1. A catalyst-coated membrane comprising a polymer electrolyte membrane having a top and a bottom, wherein on both the top and on the bottom of a catalytically active layer is applied, prepared by applying a catalyst ink according to any one of claims 1 to 8 or prepared according to claim 9 on the polymer electrolyte membrane.

12. A gas diffusion electrode comprising a gas diffusion layer and a catalytically active layer prepared by applying a catalyst ink according to any one of claims 1 to 8, or prepared according to claim 9, to the gas diffusion layer.

13. membrane electrode assembly comprising a polymer electrolyte membrane having a top and a bottom, wherein both on the top and on the bottom of a catalytically active layer is applied, prepared based on a catalyst ink according to any one of claims 1 to 8 or manufactured according to claim 9, and on the respective catalytically active layer in each case a gas diffusion layer is applied.

14. Fuel cell, comprising at least one membrane-electrode assembly according to claim 13.

15. Use of the catalyst ink according to any one of claims 1 to 8 or prepared according to claim 9, for the preparation of catalytically active layers of a membrane electrode assembly.