WO2012052945A1

WO2012052945A1 - Catalyst support material comprising polyazole, electrochemical catalyst, and preparation of gas diffusion electrode and of membrane-electrode assembly thereof

Info

Publication number: WO2012052945A1
Application number: PCT/IB2011/054668
Authority: WO
Inventors: Ömer ÜNSAL; Sigmar BRÄUNINGER; Claudia Querner; Ekkehard Schwab
Original assignee: Basf Se; Basf (China) Company Limited
Priority date: 2010-10-21
Filing date: 2011-10-19
Publication date: 2012-04-26
Also published as: DE112011103533A5

Abstract

A process for preparing a catalyst material, a catalyst material, a catalyst ink, a catalyst-coated membrane, a gas diffusion electrode, a membrane-electrode assembly thereof are provided. The process for preparing a catalyst material comprises: (a) at least one electrically conductive support material, (b) at least one proton-conducting, polyazole-based polymer, (c) at least one catalytically active material.

Description

Polyazole containing catalyst support material, electrochemical catalyst and the preparation of a gas diffusion electrode and a membrane electrode assembly thereof Description

The present invention relates to a process for the preparation of a catalyst material comprising at least one electrically conductive support material, at least one polyazole-based proton-conducting polymer and at least one catalytically active material, a catalyst material preparable by the process according to the invention, a catalyst ink containing at least one inventive catalyst material and at least one Solvent, a catalyst-coated membrane (CCM) comprising a polymer electrolyte membrane and catalytically active layers comprising at least one catalyst material according to the present invention, a gas diffusion electrode (GDE) comprising a gas diffusion layer and a catalytically active layer containing at least one catalyst material according to the invention, a membrane electrode Unit (MEA) comprising a polymer electrolyte membrane, catalytically active layers containing at least one inventive catalysis atormaterial and gas diffusion layers and a fuel cell containing at least one membrane electrode assembly according to the present invention.

Proton conducting, i. acid-doped polyazole membranes for use in PEM fuel cells are already known in the art. The basic polyazole films are generally doped with concentrated sulfuric acid or phosphoric acid and then act as proton conductors and separators in so-called polymer electrolyte membrane fuel cells (PEM fuel cells). Due to the outstanding properties of the polyazole polymer, such polymer electrolyte membranes can be processed into membrane-electrode assemblies (MEA) and used at continuous operating temperatures above 100 ° C., in particular above 120 ° C., in fuel cells. These high steady-state operating temperatures allow the activity of the noble metal-based catalysts contained in the membrane-electrode assembly to be increased. In particular, when using so-called reformates of hydrocarbons in the reformer gas significantly higher amounts of carbon monoxide contained, which usually must be removed by a complex gas treatment or gas cleaning. The possibility of increasing the operating temperature means that significantly higher concentrations of carbon monoxide contaminants can be permanently tolerated.

The use of polymer electrolyte membranes based on polyazole polymers makes it possible, first, to carry out complex gas treatment or gas purification partly dispensed with and on the other hand, the catalyst loading in the membrane electrode assembly can be reduced. Both are indispensable prerequisites for the mass use of PEM fuel cells, since otherwise the costs for a PEM fuel cell system are too high.

A key factor in the performance and long-term stability of a membrane-electrode assembly containing an acid-doped proton-conducting polyazole membrane is the amount of acid present in the catalyst layer of the membrane-electrode assembly. Frequently, the catalyst layer constituting the electrode of the membrane-electrode assembly contains a nonpolar polymer such as polytetrafluoroethylene (PTFE), so that the amount of acid in the catalyst layer is small and can cause problems by the hydrophobizing property of the catalyst layer in long-term operation. The prior art therefore describes membrane-electrode assemblies and catalyst layers in which the amount of acid in the catalyst layer can be increased and maintained during operation of the fuel cell.

Thus, DE 10 2004 063 457 A1 relates to a membrane electrode assembly with a fuel cell membrane, which is arranged between two gas diffusion layers, wherein the fuel cell membrane is formed on the basis of an acid-impregnated polymer, wherein between the fuel cell membrane and the gas diffusion layers in each case at least one catalyst-containing Layer with a polymer additive is arranged so that water is held in the membrane-electrode assembly and / or the fuel cell membrane and / or acid is stored. In this case, the polymer additive of the catalyst-containing layer is selected from the group of polyazoles, in particular at least one component from the group of polybenzimidazoles, poly (pyridine), polybenzoxazole or mixtures thereof. The preparation of the catalyst layer or of electrodes containing this catalyst layer is effected by adding at least one preferably pulverulent catalyst material with a solvent, a pore-forming material and a polymer solution and processing it into an electrode paste in a substantially homogeneous mixed state. By means of screen printing, the catalyst material, which is contained in the electrode paste, applied to the gas diffusion layer of a membrane-electrode assembly.

WO 2006/005466 relates to gas diffusion electrodes having a plurality of gas-permeable, electrically conductive layers which are composed of at least one gas diffusion layer and a catalyst layer, wherein the catalyst layer comprises at least particles of an electrically conductive support material, and at least a part of the Particles carries an electrocatalyst and / or at least partially loaded with at least one porous proton-conducting polymer and this proton-conducting polymer is used at temperatures above the boiling point of water. The loading with the proton-conducting polymer and formation of the porous structure is carried out according to WO 2006/005466 by means of the phase inversion method. This includes, inter alia, the step of providing at least one particulate, electrically conductive carrier material for the catalyst layer, in which at least part of the particles carries an electrocatalyst and / or is at least partially loaded with at least the porous proton-conducting polymer. According to the examples in WO 2006/005466, particles of an electrically conductive carrier material for the catalyst layer loaded with at least one porous proton-conducting polymer are prepared by adding a suspension of catalyst black to a solution of polybenzimidazole. The catalyst black is loaded with 20% platinum. S. Seland et al., Journal of Power Sources 160 (2006) 27-36, discloses improving the performance of high temperature PEM fuel cells based on polybenzimidazole electrolytes. In order to find out the optimal structures for anodes and cathodes and the platinum content in electrodes suitable for PEM fuel cells, both the platinum content in the Pt / C catalyst used and the catalyst loading as well as the loading with the polybenzimidazole electrolyte dispersed in the catalyst layer were dispersed is, varies. In order to obtain a catalyst layer in which polybenzimidazole is dispersed in the catalyst layer, polybenzimidazole is dissolved in dimethylacetamide, and the catalyst particles (Pt / C) are dispersed in this solution.

O.E. Kongstein et al., Energy 32 (2007) 418-422, relates to polymer electrolyte fuel cells based on polybenzimidazole membranes doped with phosphoric acid. The most suitable electrodes are prepared according to O.E. Kongstein et al. by spraying a dispersion of a catalyst into a solution of polybenzimidazole in dimethylacetamide.

All of the abovementioned processes for producing a catalyst layer for use in polymer electrolyte membrane fuel cells have in common that the finished catalyst containing Pt on carbon black is treated with a solution of polybenzimidazole. That is, the polybenzimidazole polymer is deposited on the Pt / C catalyst. However, there is a risk that the catalytically active surface is covered with polymer and the activity of the catalyst is thereby lowered.

It is therefore an object of the present invention over the aforementioned prior art to provide a catalyst material which has good accessibility allows the reaction components to the catalytically active species, for example Pt, so that the catalyst material has a high activity.

Furthermore, it is an object of the present invention to improve the following points in polyazole-based polymer electrolyte membrane fuel cells:

(i) increasing the three-phase interface between catalyst, ionomer and gas;

(ii) Better and more homogeneous distribution of the acid in the catalyst layer than in the fuel cells or membrane electrode assemblies according to the prior art;

(iii) reducing or avoiding acid loss during cell operation; (iv) increase in long-term stability.

This object is achieved by a method for producing a catalyst material containing

(a) at least one electrically conductive carrier material,

(b) at least one polyazole-based proton-conducting polymer,

(c) at least one catalytically active material,

comprising the steps:

(i) contacting the at least one electrically conductive carrier material with the at least one proton-conducting polymer based on polyazole,

(ii) applying at least one precursor compound of the at least one catalytically active material or applying the at least one catalytically active material itself to the support material obtained according to step (i),

(iii) in the case where in step (ii) at least one precursor compound of the at least one catalytically active material is applied, converting the at least one precursor compound into the at least one catalytically active material by reduction. With the aid of the method according to the invention, better use of the catalyst surface is possible by increasing the three-phase interface and improving the long-term stability of membrane electrode assemblies or fuel cells containing the catalyst material produced according to the invention. Electrically conductive carrier material

In principle, any electrically conductive carrier material known to those skilled in the art can be used as the electrically conductive carrier material. The electrically conductive carrier material is preferably a carbon-containing carrier material, preferably selected from carbon black, graphite, carbon fibers, activated carbon, Carbon nanomers (nanotypes) and carbon foams. Very particular preference is given to using carbon black.

The mean particle size of the primary particles of the electrically conductive carrier material, preferably carbon black, is generally from 10 to 300 nm, preferably from 10 to 200 nm, particularly preferably from 10 to 150 nm.

Polyazole-based proton-conducting polymer In the context of the present application, proton-conducting polymers are to be understood as meaning polymers which are per se proton-conducting or are capable of proton conduction, for example by incorporating a doping agent, eg. As a strong organic or inorganic acid. Suitable organic or inorganic acids are phosphoric acid, sulfuric acid, with phosphoric acid being particularly preferred.

Polyazoles which are preferably used are polyazoles which contain 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 (XV ) and / or (XVI) and / or (XVII) and / or (XVIII) and / or (XIX) and / or (XX) and / or (XXI) and / or (XXII):



10 

are the same or different and represent a tetravalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,

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

Ar ^{2 are the} same or different and are a bivalent or trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{3 are the} same or different and are 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 are a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{8 are the} same or different and represent 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 which may be mononuclear or 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 identical or different and represents oxygen, sulfur or an amino group which bears a hydrogen atom, a group having 1 to 20 carbon atoms, preferably a branched or unbranched alkyl or alkoxy group, or an aryl group as a further radical,

R is the same or different hydrogen, an alkyl group or an aromatic group and in formula (XX) is an alkylene group or an aromatic group, provided 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, which may optionally also be substituted , from.

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. 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 CrC 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 of the present invention, the polyazole comprises recurring azole units of the formula (I) and / or (II).

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

Other 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, polyazoles containing benzimidazole recurring units have been used. The following are suitable polyazoles having recurring benzimidazole units:







where n and m are integers> 10, preferably> 100.

Particularly preferably, the polyazole repeating units of the following mel

where n is an integer> 10, preferably> 100.

The polyazoles, preferably the polybenzimidazoles, are characterized by a high molecular weight. Measured as intrinsic viscosity, the molecular weight is at least 0.2 dl / g, preferably 0.8 to 10 dl / g, particularly 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 using a capillary viscometer from the viscosity of the solution at 25 ° C, the relative viscosity of the corrected times for solvent tO and solution t1 calculated according to the following equation eta rel = t1 / t0. The conversion to eta i is carried out according to the above relationship, as described 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.

Catalytically active material

The catalytically active material used may be any of the catalytically active materials conventionally used in fuel cells. Preferably, the catalytically active material is a material selected from metals of the Pt group (Pt, Pd, Rh, Ir, Os and Ru), silver and gold. Particularly preferably, the catalytically active material is selected from the group consisting of Pt, Pd, Rh, Ir and Ru. These substances can also be used in the form of alloys with one another. Furthermore, these substances can also be used in alloys with non-precious metal len, preferably selected from Cr, Zr, Ni, Co and Ti, can be used. Particular preference is given to using Pt as the catalytically active material.

The catalytically active materials are preferably used in the form of particles, which particularly preferably have a weight-average particle size of 0.5 to 100 nm, very particularly preferably 0.75 to 20 nm and particularly preferably 1 to 10 nm, the mean particle size being Particle diameter is to be understood. The noble metal content of the catalyst material according to the invention is generally 0.1 to 10 mg / cm ² , preferably 0.2 to 6.0 mg / cm ² , particularly preferably 0.2 to 3.0 mg / cm ² , after application of the catalyst material according to the invention in the form of a catalyst layer. These values can be determined by elemental analysis of a flat sample.

According to the invention, the catalyst material is produced by means of the abovementioned steps (i), (ii) and (iii).

(i) contacting the at least one electrically conductive carrier material with the at least one proton-conducting polymer based on polyazole

The contacting of the at least one electrically conductive carrier material with the at least one polyazole-based proton-conducting polymer generally takes place in the presence of a solvent. This is usually a solvent in which the at least one polyazole-based proton-conducting polymer is soluble, while the at least one electrically conductive support material is not soluble therein, but in the form of a suspension. Suitable solvents are for example selected from the group consisting of alcohols, for. As methanol, ethanol, n-propanol, iso-propanol, n-butanol, sec-butanol, tert-butanol, iso-butanol, nitrogen-containing solvents, eg. For example, dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and mixtures thereof or mixtures of one or more of the aforementioned solvents with water. Nitrogen-containing solvents are particularly preferably used as solvents, with dimethylacetamide and N-methylpyrrolidone being very particularly preferred.

The weight ratio between the electrically conductive carrier material (in the dry state) and the polyazole-based proton-conducting polymer (also in the dry state) is generally from 70:30 to 99: 1, preferably from 80:20 to 95: 5. The contacting in step (i) of the process according to the invention can be carried out according to all methods known to the person skilled in the art. Usually, a solution of the at least one polyazole-based proton-conducting polymer in at least one of the aforementioned solvents is mixed with the above-mentioned electrically conductive carrier material. The mixing can be done in any known to the expert device, for. B. using a ball mill and / or under the action of ultrasound. Step (i) of the process according to the invention is generally carried out at temperatures of 20 to 200 ° C, preferably 20 to 150 ° C, particularly preferably 20 to 100 ° C. In this case, step (i) at any printing, z. As atmospheric pressure or elevated pressure, are performed. Preferably, step (i) of the process according to the invention is carried out at atmospheric pressure.

It is possible that the solution or suspension of the electrically conductive carrier material and of the proton-conducting polymer based on polyazole obtained according to step (i) is used directly without further workup in step (ii) of the process according to the invention. However, it is also possible to completely or at least partially remove solvent from the mixture obtained in step (i) before carrying out step (ii). In this case, the partial or complete removal of the solvent or solvent mixture by any method known in the art, for. Example, by removal of solvent at elevated temperature and / or reduced pressure (a pressure below atmospheric pressure), for. B. in a rotary evaporator, falling film evaporator or by spray drying.

Step (ii) applying at least one precursor compound of the at least one catalytically active material or applying the at least one catalytically active material itself to the carrier material obtained according to step (i)

According to step (ii) of the process according to the invention, the electrically conductive carrier material treated with the polyazole-based proton-conducting polymer is mixed with a precursor compound of the at least one catalytically active material or the catalytically active material itself.

According to one embodiment of step (ii) of the process according to the invention, a precursor compound of catalytically active material is mixed with the carrier material treated according to step (i). Suitable methods for mixing are the Specialist known. Usually, the mixing takes place in the presence of a solvent. In this case, if the solvent was not or only partially removed after step (i), it may be the same solvent used in step (i) of the process according to the invention. However, if the solvent has been completely removed in step (i) of the process according to the invention, it is also possible to use a different solvent or solvent mixture for the mixture in step (ii) than in step (i).

In principle, in step (ii) of the process according to the invention, the same solvents are suitable which have already been mentioned above with respect to step (i) of the process according to the invention.

As already mentioned above, the precursor compound of the at least one catalytically active material is generally applied by mixing the at least one precursor compound of the at least one catalytically active material with the electrically conductive carrier material treated according to step (i). The mixing can be carried out according to all methods known to those skilled in the art, eg. B. by stirring and / or the application of ultrasound, or the use of ball mills or dispersing, z. B. according to the rotor / stator principle. As precursor compounds of the at least one catalytically active material, at least one salt or complex of the metal of the at least one catalytically active material are suitable, suitable metals already mentioned above. It is preferably at least one halogen-free salt or at least one halogen-free complex, wherein the at least one salt or the at least one complex are particularly preferably selected from the group consisting of ammonium salts, nitrates, Nitrosylnitraten, nitrite complexes, amine complexes and mixtures thereof. Very particular preference is given to using Pt (II) nitrate as the precursor compound of the at least one catalytically active material. The weight ratio of the precursor compound of the at least one catalytically active material to the electrically conductive carrier material used as carrier material according to step (i) is arbitrary. In general, catalysts are prepared with 10-80 wt .-% noble metal loading on carbon. According to a further embodiment, instead of at least one precursor compound of the at least one catalytically active material, the catalytically active material itself can be applied. In this case, the abovementioned solvents and the above-mentioned mixing methods can be used. The weight ratio of electrically conductive carrier material used in step (i) and Catalytically active material is arbitrary. In general, catalysts are prepared with 10-80 wt .-% noble metal loading on carbon.

The application of the catalytically active material can be carried out by all methods known to the person skilled in the art, eg. B. by vapor deposition or electrochemical deposition.

Step (ii) of the process according to the invention is generally carried out at temperatures of 40 to 100 ° C, preferably 50 to 90 ° C, particularly preferably 60 to 85 ° C. Step (ii) may be performed at any pressure, e.g. B. at atmospheric pressure or elevated pressure. Preferably, step (ii) is carried out at atmospheric pressure.

Subsequent to step (ii) of the process according to the invention, the preferred solvent used can be completely or at least partially removed. However, it is also possible in principle that the solvent-containing mixture obtained in step (ii) is used directly in step (iii). A complete or partial removal of the solvent can be carried out by any method known to the skilled person, for. B. at elevated temperature and / or a pressure below normal pressure, for example in a rotary evaporator, falling film evaporator or by spray drying.

Step (iii) converting the at least one precursor compound of the at least one catalytically active material into the catalytically active material by reduction

Step (iii) of the process according to the invention is only used if in step (ii) at least one precursor compound of the at least one catalytically active material is used. If the catalytically active material itself is used in step (ii), step (iii) is omitted.

The reduction in step (iii) can be carried out by all methods known to the person skilled in the art. Usually, the reduction takes place electrochemically or chemically. The reduction ensures that the at least one catalytically active material is deposited on the electrically conductive carrier material treated according to step (i) with at least one polyazole-based proton-conducting polymer.

In a preferred embodiment, the reduction in step (iii) is carried out by chemical reduction of the precursor compounds of the at least one catalytically active material. If the precursor compounds of at least one catalytically active Materials as complexes, it can be done in one embodiment in step (iii) before the reduction of the corresponding metal cations, a cleavage of these complexes. Suitable methods and / or reagents are known to the person skilled in the art. As reducing agent, in step (iii) of the process according to the invention, it is generally possible to use all suitable reducing agents known to the person skilled in the art, which are capable of converting the precursor compounds of the catalytically active material into the catalytically active material itself. Suitable reducing agents may, for. B. be liquid or gaseous.

If the solvent in step (ii) of the process according to the invention has not been removed prior to the reduction in step (iii) of the process according to the invention, or has not been completely removed (which is preferred), a reducing agent is used in step (iii) of the process according to the invention which is miscible with the solvents used in step (ii). At least one alcohol is preferably used as the reducing agent. Ethanol is particularly preferably used as a reducing agent. Other suitable reducing agents are for. As hydrogen, NaBH ₄ , formic acid or formate. In a preferred embodiment, step (iii) is carried out such that the mixture obtained in step (ii) is mixed either with at least one of the solvents already mentioned above in step (i), if in step (ii) the solvent has been completely removed, or the mixture obtained in step (ii) is used directly in the solvent or solvent mixture used in step (ii) and the reducing agent is brought into contact in gaseous, in dispersion or in solution, with the precursor compound to be reduced.

The contacting takes place in general by mixing. Suitable mixing methods are known to the person skilled in the art.

Usually, step (iii) of the process according to the invention is carried out at temperatures of generally from 40 to 100.degree. C., preferably from 50 to 90.degree. C., particularly preferably from 60 to 85.degree. The pressure in step (iii) of the process according to the invention is generally arbitrary, preferably step (iii) is carried out at atmospheric pressure or elevated pressure, more preferably at atmospheric pressure.

The reducing agent is generally used in excess relative to the precursor compound to be reduced. Subsequent to step (iii) of the process according to the invention, the solvent used in step (iii) is completely removed. The removal of the solvent can be carried out by all methods known to those skilled in the art, for example at elevated temperature and / or a pressure below atmospheric pressure, for. B. in a rotary evaporator, falling film evaporator or by spray drying. In this case, the desired catalyst material is obtained.

If the catalytically active material itself is used in step (ii), step (iii) is omitted and the material obtained after step (ii) is completely freed of the solvent used in step (ii).

By means of the process according to the invention, a catalyst material according to the invention is obtained, which is characterized in that the catalytically active material is readily accessible to the reactants and is not covered by the polymer based on polyazoles. As a result, a high activity of the catalytically active material is achieved. At the same time, however, the treatment of the support material with the polyazole-based proton-conducting polymer ensures that the acid used as the dopant of the polyazole-based, proton-conducting membrane, preferably sulfuric acid or phosphoric acid, particularly preferably phosphoric acid, is homogeneously distributed in the catalyst layer and Acid loss during cell operation is avoided or reduced.

Owing to the sequence of the method steps of the method according to the invention, the catalyst material obtained according to the method of the invention differs from the catalyst material obtained according to the prior art, wherein the catalyst material comprising carrier material and catalytically active species is treated with a polyazole-based polymer.

A further subject of the present invention is therefore a catalyst material producible by the process according to the invention.

Compared with the catalyst materials known from the prior art, this catalyst material has the advantage that the catalytically active surface of the catalyst material is not covered by a polyazole-based proton-conducting polymer and thus ensures a high catalytic activity of the catalyst material.

The catalyst material according to the invention or produced according to the invention serves for the formation of catalyst layers, in particular catalyst layers in catalytics. lysor-coated membranes (CCM), gas diffusion electrodes (GDE), and membrane-electrode assemblies (MEA).

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, for example, into the gas diffusion layer and / or the membrane, 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 constructed from the catalyst material according to the invention in a catalyst-coated membrane (CCM), gas diffusion electrode (GDE) or membrane electrode assembly (MEA) is generally 1 to 1000 μm, preferably 5 to 500 μm, particularly preferably 10 to 300 μm , 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 light microscope or a thickness gauge.

The application of the catalyst material according to the invention or prepared according to the invention to a polymer electrolyte membrane for producing a catalyst-coated membrane (CCM) or to a gas diffusion layer (GDL) for producing a gas diffusion electrode (GDE) can be carried out by all methods known to the person skilled in the art. Usually, the application of the catalyst material by means of a catalyst ink, which contains at least one inventive catalyst material and at least one solvent.

Another object of the present invention is therefore a catalyst ink containing at least one inventive catalyst material or at least one inventively prepared catalyst material and at least one solvent, wherein the solvent is preferably selected from the group consisting of water, monohydric and polyhydric alcohols, N-containing Solvents, glycols, glycol ether alcohols and glycol ethers, more preferably selected from propylene glycol, dipropylene glycol, glycerol, ethylene glycol, hexylene glycol, dimethylacetamide (DMAc), N-methylpyrrolidone (NMP) and water.

In addition, the catalyst ink according to the invention may contain further additives. These may be wetting agents, leveling agents, defoamers, pore formers, stabilizers, rheology additives, pH modifiers and other substances. In general, the catalyst ink according to the invention contains from 1 to 30% by weight, preferably from 2 to 25% by weight, particularly preferably from 5 to 20% by weight, of the catalyst material according to the invention or of the catalyst material prepared according to the invention and from 70 to 99% by weight , preferably 75 to 98 wt .-%, particularly preferably 80 to 95 wt .-% of the at least one solvent. The total amount of catalyst material and solvent gives 100 wt .-%.

The additives are - based on the total amount of the catalyst material - generally in an amount of 0.5 to 15 parts by weight, preferably 0.5 to 12.5 parts by weight, more preferably 0.5 to 10 parts by weight. Parts used.

The at least one catalyst material is generally dispersed in the catalyst ink of the invention in the at least one solvent. The preparation of the catalyst ink is generally carried out by contacting the at least one catalyst material with the at least one solvent and optionally the abovementioned additives.

The present invention furthermore relates to the use of the catalyst material according to the invention for producing a catalyst ink comprising at least one catalyst material and at least one solvent. Suitable catalyst materials and solvents and optionally suitable additives have already been mentioned above. A further subject of the present invention is the use of the catalyst ink according to the invention for the production of a catalyst-coated membrane (CCM), a gas diffusion electrode (GDE) or a membrane electrode assembly (MEA), wherein the above-mentioned catalyst-coated membranes, gas diffusion electrodes and membrane electrodes Units are preferably used in polymer electrolyte fuel cells or in the PEM electrolysis.

For producing 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 preparation of a homogeneously dispersed ink can by known to those skilled auxiliaries, for. Example by means of high-speed stirrer, 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 and brushing.

5

In general, the resulting catalyst layer containing the catalyst material according to the invention or prepared by applying the catalyst ink according to the invention is 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 which has a top and a bottom, wherein a catalytically active layer is applied to both the top and the bottom, containing at least a catalyst material according to the invention or at least one catalyst material prepared according to the process of the invention.

Suitable polymer electrolyte membranes for the catalyst-coated membrane are generally known to those skilled in the art. Particularly suitable are proton-conducting polymer electrolyte membranes based on polyazole. Suitable polyazole polymers for the preparation of the corresponding membranes are the polyazole-based proton-conducting polymers mentioned above with regard to the catalyst material. These polymer electrolyte membranes are generally prepared by adding an acid, e.g. B. 25 sulfuric acid or phosphoric acid, more preferably phosphoric acid, made proton conductive.

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 30 or dispersion containing the components used to prepare the polymer electrolyte membrane, 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 carrier material.

35

The polymer electrolyte membrane used in the catalyst-coated membranes (CCM) according to the invention generally has a layer thickness of 20 to 2000 μm, preferably 30 to 1500 μm, particularly preferably 50 to 1000 μm. Another object of the present invention is a gas diffusion electrode (GDE), comprising a gas diffusion layer (GDL) and a catalytically active layer containing at least one inventive catalyst material or a catalyst material prepared 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 can be used which contain a mechanically stable support material, which with at least one electrically conductive material, for. As carbon (for example carbon black) 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.

In 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 without loss of its integrity by pressure to at least half, preferably to at least one third of its original thickness. This property generally includes gas diffusion layers of graphite fabric and / or paper made conductive by carbon black addition. The catalytically active layer in the gas diffusion electrode according to the invention contains the catalyst material according to the invention or the catalyst material prepared according to the invention. Usually, the catalytically active layer is applied to the gas diffusion electrode by means of the catalyst ink according to the invention mentioned above. 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.

A further subject of the present invention is a membrane electrode unit comprising a polymer electrolyte membrane which has an upper side and a lower side, wherein a catalytically active layer is applied to both the upper side and the lower side, comprising at least one according to the invention or According to prepared catalyst material, and on the respective catalytically active layer in each case a gas diffusion layer is applied.

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 regard to the gas diffusion electrode according to the invention.

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 ° C, preferably 20 to 200 ° C, and at a pressure of generally 1 to 1000 bar, preferably 3 to 300 bar, is laminated. An advantage of the membrane-electrode assemblies according to the present invention is that they allow operation of a fuel cell at temperatures above 120 ° C. This applies to gaseous and liquid fuels such as hydrogen-containing gases, the z. B. in an upstream reforming of hydrocarbons are produced. As oxidant can be z. As oxygen or air can be used.

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 ° C even with pure platinum catalysts, ie without a further alloying ingredient. At temperatures of 160 ° C z. B. more than 1% carbon monoxide in Fuel gas may be included, without resulting in a significant reduction in the performance of the fuel cell.

Furthermore, it is a significant advantage of the membrane electrode units according to the invention that by using the catalyst material according to the invention in the catalytically active layer of the membrane-electrode assembly an increase in the three-phase interfaces, a good and homogeneous distribution of acid in the catalyst layer, a reduction or avoidance of acid loss during cell operation and increased long-term stability can be achieved. This will u. a. achieved in that due to the order in the inventive method for producing the catalyst material according to the invention, first the electrically conductive carrier material of the catalyst material is treated with the proton-conducting polymer based on polyazole and only then the active component is applied. This avoids masking of the catalytically active surface with the polyazole-based proton-conducting polymer and increases the activity towards catalyst materials prepared according to other processes.

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, since 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 of the present invention further allow the fuel cell to be easily cooled to room temperature and below, and then put back into service without sacrificing performance.

Furthermore, as already mentioned above, the membrane-electrode assemblies according to the present invention show a high long-term stability. As a result, fuel cells can be provided, which also have a high long-term stability. 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 placed sealingly against the membrane-electrode unit. 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 favorably 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 over long periods, for. B. more than 5000 hours, are operated continuously at temperatures of more than 120 ° 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 is operated with a hydrogen 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 the quiescent voltage apply for a temperature of 160 ° C. In addition, the fuel cell preferably shows a low gas penetration after this time (gas cross-over). To measure the crossover, the anode side of the fuel cell is operated with hydrogen (5 L / h). ben, 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 is transported in a mass-limited manner oxidized. 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 crossover apply to a temperature of 160 ° C.

Another object of the present invention is the use of the inventive catalyst material or the catalyst material according to the invention for the preparation of the catalytically active layers of a membrane electrode assembly.

Claims

claims

1 . A process for producing a catalyst material comprising at least one electrically conductive carrier material, at least one polyazole-based proton-conducting polymer, at least one catalytically active material, comprising the steps

Contacting the at least one electrically conductive carrier material with the at least one proton-conducting polymer based on polyazole,

Applying at least one precursor compound of the at least one catalytically active material or applying the at least one catalytically active material itself to the carrier material obtained according to step (i),

in the case where in step (ii) at least one precursor compound of the at least one catalytically active material is applied, converting the at least one precursor compound into the at least one catalytically active material by reduction.

A method according to claim 1, characterized in that the at least one electrically conductive carrier material is a carbonaceous carrier material, preferably selected from carbon black, graphite, carbon fibers, activated carbon, carbon nanomers and carbon foams, preferably carbon black, particularly preferably carbon black having a weight-average particle size of the primary particles from 10 to 50 nm.

Process according to Claim 1 or 2, characterized in that the at least one polyazole-based proton-conducting polymer is based on a polyazole which contains recurring azole units of the general formula (I) and / or (II):

wherein

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

Ar ^{2 are the} same or different and are a bivalent or trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,

X is identical or different and represents oxygen, sulfur or an amino group which has a hydrogen atom, a group having 1 to 20 carbon atoms, preferably a branched or unbranched alkyl or alkoxy group, or an aryl group as a further radical, n is an integer greater than or equal to 10, preferably greater than or equal to 100.

Method according to one of claims 1 to 3, characterized in that the at least one catalytically active material is selected from metals of the Pt group, silver and gold, preferably selected from Pt, Pd, Rh, Ir and Ru, the metals also in Form of alloys with one another and / or in the form of alloys with base metals, preferably selected from Cr, Zr, Ni, Co and Ti.

Method according to one of claims 1 to 4, characterized in that the contacting in step (i) in the presence of a solvent, preferably selected from the group consisting of alcohols, preferably methanol, ethanol, n-propanol, iso-propanol, n-butanol , sec-butanol, tert-butanol, iso-butanol, nitrogen-containing solvents, preferably dimethylacetamide, N-methylpyrolidone, dimethylformamide; Dimethyl sulfoxide and mixtures thereof, and mixtures of one or more of the aforementioned solvents with water.

A method according to claim 5, characterized in that the solvent is completely or at least partially removed before applying according to step (ii). Method according to one of claims 1 to 6, characterized in that the at least one precursor compound in step (ii) is at least one salt or complex of the metal of the at least one catalytically active material, preferably at least one halogen-free salt or at least one halogen-free complex, particularly preferred at least one salt or complex selected from ammonium salts, nitrates, nitrosyl nitrates, nitrite complexes, amine complexes and mixtures thereof, most preferably Pt (II) nitrate.

Method according to one of claims 1 to 7, characterized in that the application of the at least one precursor compound or of the at least one catalytically active material is carried out on the carrier material obtained according to step (i) in the presence of a solvent, preferably selected from the group consisting of alcohols, preferably methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol, isobutanol, nitrogen-containing solvents, preferably dimethylacetamide, N-methylpyrrolidone, dimethylformamide; Dimethyl sulfoxide and mixtures thereof, and mixtures of one or more of the aforementioned solvents with water.

A method according to claim 8, characterized in that the solvent is completely or at least partially removed after the application of the at least one precursor compound or the at least one catalytically active material.

Method according to one of claims 1 to 9, characterized in that the reduction in step (iii) takes place electrochemically or chemically.

Catalyst material producible by a process according to one of claims 1 to 10.

Catalyst ink containing at least one catalyst material according to claim 11 or prepared according to one of claims 1 to 10 and at least one solvent, preferably selected from water, mono- and polyhydric alcohols, nitrogen-containing solvents, glycols, glycol ether alcohols and glycol ethers, more preferably selected from Propylene glycol, dipropylene glycol, glycerol, ethylene glycol, hexylene glycol, dimethylacetamide, N-methylpyrrolidone and water. 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, comprising at least one catalyst material according to claim 1 1 or prepared according to any one of claims 1 to 10th

A gas diffusion electrode comprising a gas diffusion layer and a catalytically active layer containing at least one catalyst material according to claim 11, or prepared according to any one of claims 1 to 10.

A membrane-electrode assembly 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 containing at least one catalyst material according to claim 1 1 or prepared according to any one of claims 1 to 10, and on the respective catalytically active layer in each case a gas diffusion layer is applied.