DE102007031280A1 - Gas diffusion electrode with membrane-electrode-unit for high temperature fuel cells based on membrane for traction system, has gas diffusion layer and catalyst layer is impregnated with another electrolyte and has hydrophobic material - Google Patents

Abstract

The gas diffusion electrode (18a,18b) has a gas diffusion layer (22a,22b) and a porous catalyst layer (20a,20b) arranged on the gas diffusion layer. The gas diffusion layer has an electrical conducting carrier material and a catalyst material applied on the carrier material. The catalyst layer is impregnated with another electrolyte (28) and has a hydrophobic material (30). An independent claim is also included for a membrane electrode unit has a polymer electrolyte membrane.

Description

The The invention relates to a gas diffusion electrode and to a gas diffusion electrode Membrane electrode unit (MEA) for a fuel cell, in particular a high-temperature fuel cell.

Fuel cells use the chemical conversion of hydrogen H ₂ and oxygen O ₂ to water H ₂ O to generate electrical energy. For this purpose, fuel cells contain as a core component, the so-called membrane electrode assembly (MEA for membrane electrode assembly), which is a composite of a proton-conducting membrane and each on both sides of the membrane arranged gas diffusion electrode (anode and cathode). As a rule, the fuel cell is formed by a multiplicity of stacked MEAs whose electrical powers add up. During operation of the fuel cell, hydrogen or a hydrogen-containing gas mixture is supplied to the anode, where an electrochemical oxidation of H ₂ to H ⁺ takes place with release of electrons. Via the membrane, which separates the reaction spaces gas-tight from each other and electrically isolated, takes place (water-bound or anhydrous) transport of protons H ⁺ from the anode compartment in the cathode compartment. The electrons provided at the anode are supplied to the cathode via an electrical line. The cathode is further supplied with oxygen or an oxygen-containing gas mixture, so that a reduction of O ₂ to O ² takes place with absorption of the electrons. At the same time, these oxygen anions in the cathode compartment react with the protons transported via the membrane to form water. The direct conversion of chemical to electrical energy fuel cells achieve over other electricity generators due to the circumvention of the Carnot factor improved efficiency.

The Each electrode has a catalyst layer facing the membrane on top of a gas-permeable substrate, the so-called gas diffusion layer (GDL for gas diffusion layer), for the homogeneous supply of Reaction gases is applied. The catalyst layer contains reactive centers, which are usually made of platinum as catalytically active Component which is on an electrically conductive porous Support material, such as carbon particles, supported is present. For the efficient conversion of the chemical energy of the Reaction components, the reaction centers need three conditions at the same time. First, an electric conductive connection of the reaction centers of the electrodes be present with an external circuit. Secondly The reaction centers must be ionically conductive be connected to the membrane, with a high transport rate to be supplied with protons or To be able to dissipate protons. Third, must the reaction centers have good access to the reaction gases.

The currently the most advanced fuel cell technology based on polymer electrolyte membranes (PEM) where the membrane itself consists of a polyelectrolyte. The most common PEM is a membrane of a sulfonated polytetrafluoroethylene copolymer (Trade name: Nafion; copolymer of tetrafluoroethylene and a Sulfonyl fluoride derivative of a perfluoroalkyl vinyl ether). The electrolytic line takes place via hydrated Protons take place, which is why for the proton conductivity the presence of liquid water is condition. hereby There are a number of disadvantages. Such is the operation of the PEM fuel cell moistening the operating gases required, resulting in a high overhead means. If there is a failure of the humidification system, are Power losses and irreversible damage to the membrane-electrode assembly the episode. Furthermore, the maximum operating temperature of these Nafion membrane fuel cells - too due to the lack of thermal stability of the membranes - at Normal pressure limited to below 100 ° C. For However, the mobile as well as the stationary use are Operating temperatures above 100 ° C for many reasons desirable. This increases the heat transfer with increasing difference to the ambient temperature and allows a better cooling of the fuel cell stack. Further take the catalytic activity of the electrodes as well as the tolerance against contamination of the combustion gases with rising Temperature too. At the same time, the viscosity of the electrolytic decreases Substances with increasing temperature and improves the mass transfer to the reactive centers of the electrodes. Finally falls at temperatures above 100 ° C, the resulting product water gaseous and can better dissipated from the reaction zone be such that in the gas diffusion layer existing gas transport paths (Pores and mesh) are kept free and also a washout the electrolytes and electrolyte additives is prevented.

To overcome these problems, high temperature polymer electrolyte membrane fuel cells (HT-PEM or short HTM fuel cells) have been developed which operate at operating temperatures of 120 to 180 ° C and which require little or no humidification. The electrolytic conductivity of the membranes used here is based on liquid, bound by electrostatic complex binding to the polymer backbone electrolyte, in particular acids or bases that ensure the proton conductivity even with complete dryness of the membrane above the boiling point of water. For example, high temperature polybenzimidazole (PBI) membranes complexed with acids, such as phosphoric acid, sulfuric acid or others, in US 5,525,436 . US 5,716,727 . US 5,599,639 . WO 01/18894 A . WO 99/04445 A . EP 0 983 134 B and EP 0 954 544 B described.

The Although HTM fuel cells have the advantage of relatively high operating temperatures, However, they have the problem that lowering the temperature below the boiling point of water, such as when starting the fuel cell or when switching off the system occurs to irreversible damage to the MEA. This is due to the liquid product water, which washes the membrane bound to the electrolyte and discharges, so that no longer enough charge carriers be available for proton transport. The optimum operating temperature of today's HTM fuel cells therefore at 160 ° C and the manufacturers recommend the operating temperatures Always keep above 120 ° C and lower the cells Keep temperatures de-energized. Especially for mobile applications in motor vehicles, however, a broad temperature window is starting at room temperature or below, up to temperatures significantly above 100 ° C desirable.

Out DE 10 2004 024 844 A and DE 10 2004 024 845 A Gas diffusion electrodes for HTM fuel cells are known whose catalyst layers are made of an electrode paste having a pore-forming agent and a binding polymer material. The fuel cells with these electrodes have a significantly improved cycle stability compared to standard electrodes. With cyclic temperature variation between 40 and 160 ° C in the two-hour rhythm, no loss of power at a reference temperature of 160 ° C was observed for more than 800 h.

For the system PBI / H ₃ PO ₄ describes DE 102 46 459 A a performance improvement at an optimum operating temperature of 160 ° C due to an increase in the oxygen solubility and diffusion by additions of perfluorinated sulfonic acid to the electrolyte membrane, in particular trifluoromethanesulfonic acid or perfluorohexanesulfonic in proportions of 0.1 to 20 wt .-%.

In principle, in all such MEA systems also a reversible power dip at temperature drop is observed, which disappears again with increasing temperature. This drop in temperature at the time of temperature reduction is due, on the one hand, to the formation of diffusion inhibition, which leads to insufficient gas transport to the electrodes, in particular oxygen transport to the cathode. A corresponding behavior is known from phosphoric acid fuel cells (PAFC) (eg. EP 0 520 468 A ). The cause of the decrease in power occurring during temperature reduction lies in a clogging of the gas transport channels due to the incorporation of product water or electrolyte liquid into the gas diffusion layer (GDL for gas diffusion layer) or into the catalyst layer or catalyst particles. In low temperature fuel cells, similar behavior occurs with excessive humidification or heavy product water formation.

Of the The invention is therefore based on the object, a gas diffusion electrode and a membrane electrode assembly for fuel cells provide better than known systems Have low temperature performance, especially at temperatures below 100 ° C, and also no irreversible degradation phenomena due to operating temperatures below 100 ° C show. All in all So the MEA should have a wider temperature window be usable.

These The object is achieved by a gas diffusion electrode and a gas diffusion electrode containing Membrane Electrode Unit (MEA) with the characteristics of independent Claims solved. The inventive Gas diffusion electrode, which is advantageously based in a high-temperature fuel cell on membranes which impregnated with a first electrolyte can be used, comprises a gas diffusion layer (GDL) and a porous catalyst layer disposed on the gas diffusion layer, the an electrically conductive substrate and at least one, applied to the substrate comprising catalytic material. According to the invention provided that the catalyst layer with at least a second Electrolyte is impregnated and at least one hydrophobic Contains material.

Through the use of at least one second electrolyte in the electrode, which may be in liquid, dissolved or solid form, for example in the form of phosphoric acid with a further addition of electrolyte, it is possible to reduce the diffusion inhibition, in particular of oxygen, at low temperatures, so that an equipped with the electrodes according to the invention MEA provides high power densities even at low temperatures. In addition, only a very small irreversible degradation is observed in long-term operation, which is caused by washing out of the first electrolyte from the membrane. These properties can be attributed on the one hand to the high acid strength caused by the second electrolyte and thus to the high proton concentration in the introduction zone of the protons and, on the other hand, to low intermolecular distances of the electrolyte molecules in the electrode leading to high proton transport rates. However, the decisive factor seems to be the two th electrolyte to be high solubility and diffusion rate for the mass transfer of the reactants, in particular of oxygen, so that the catalytic centers of the electrodes have a good supply situation even at low temperatures. In addition, the electrolyte in the electrode has a blocking function or a function as "acid scavenger" for the first electrolyte of the membrane and prevents outflow and subsequent flow of the electrolyte of the membrane, so that it shows a slowed degradation behavior. The second electrolyte may be the same as or different than the first electrolyte contained in the membrane.

By the introduction of the hydrophobic material into the catalyst layer on the other hand, the removal of the at low temperatures liquid product water through the GDL to the outside accelerates before the water releases the electrolyte from the membrane or the catalyst layer can discharge. By the improved The product water is also removed by the gas diffusion paths kept free, whereby the supply of the electrodes with the Reaction gases is improved. This effect is especially important the cathode to carry, since the diffusion of oxygen to the slowest and thus speed-determining steps of the fuel cell processes belongs. There is a reduction in the diffusion inhibition of Reaction gases and the protons in the catalyst layer instead. As a result, an increased electric power of the MEA according to the invention or one of these containing fuel cell over the entire temperature range, especially at low temperatures below 100 ° C, achieved.

While the use of the hydrophobic material in the electrode without their Impregnation with the second electrolyte to a certain Decrease in power density at higher operating temperatures due to the insulating and water-repellent and thus drying out Effect of the hydrophobic material leads, this disadvantage by the addition of the second invention Electrolytes compensated, allowing the combination of both components shows a synergistic effect.

in the Within the scope of the present invention, the term "hydrophobicity" understood the property of a material, polar substances, especially water, repel, that is the tendency, one as possible low contact surface between a surface of material and water. The hydrophobicity increases with decreasing polarity of the material. Exactly in the Under the present invention, the term "hydrophobicity" by a contact angle of at least 90 ° of a water drop defined on a planar surface of the material. As the contact angle here the angle is called, which is a drop of water formed on the surface of the material, the contact angle between a straight tangent to the drop edge touching and the contact area is measured. Here is the contact angle the smaller, the less hydrophobic (i.e., the more hydrophilic and polar) the Material is, and decreases with increasing hydrophobicity of the material too. Even more advantageous are in the context of the present Invention hydrophobic materials that have a hydrophobicity according to a contact angle with water of at least 100 °, preferably at least 110 °.

With Advantage the hydrophobic material is chosen among materials, which are chemically and thermally stable and one in the sense the invention have sufficient hydrophobicity. These Properties have graphites and unsubstituted, complete or partially halide-substituted polyolefins, in particular Polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE), Polyvinylidene fluoride (PVDF), on which the at least one hydrophobic Material is preferably selected. Of these, PTFE is special prefers.

Regarding the mass fraction of the at least one hydrophobic material has a range of 0.5 to 50 wt .-%, in particular from 5 to 35 Wt .-%, preferably from 10 to 20 wt .-%, based on the sum the mass of the catalytic material (precious metal) and the catalyst support (for example Pt / C) proved to be advantageous. This information refers to - in Trap of several hydrophobic materials - on the sum all existing hydrophobic materials. Lies the mass fraction of the hydrophobic material below 0.5 wt .-%, is the inventive Transport effect of the water too low. If it is above 50% by weight, the performance of the MEA deteriorates at higher temperatures, due to an increase in the electrical resistance of the electrodes and cause the membrane to dry out is because the accumulated product water due to its increased Abtransportrate can not diffuse into the membrane and most hydrophobic materials due to their insulating properties reduce the conductivity to the catalyst particles.

According to a preferred embodiment of the invention, the at least one hydrophobic material within the catalyst layer has a gradual or gradual concentration gradient, the concentration increasing from the membrane side in the direction of the GDL side of the catalyst layer. In this way, an optimal connection to the (hydrophilic) membrane is ensured on the membrane side due to the lower hydrophobicity of the catalyst layer, while on the GDL side due to the higher hydrophobicity an optimal Removal of product water is ensured. It is also conceivable to design the two electrodes (anode and cathode) within a membrane-electrode assembly with different gradients of the hydrophobic material.

As well can be provided with advantage that also the at least one second Electrolyte of the electrode, which may be polymer bound with advantage (see below), a gradual or gradually varying content within the catalyst layer or within the GDL, wherein in this case, the electrolyte content from the membrane side of the catalyst layer preferably decreases in the direction of the GDL side. Thus, the membrane side a particularly high proton conductivity of the catalyst layer achieved.

The at least one hydrophobic material may be in different forms in the catalyst layer. According to a preferred embodiment of the invention, the hydrophobic material is distributed substantially "dispersed" within the catalyst layer, that is in a more or less homogeneous mixture with the other components of the catalyst layer, in particular the support material and the catalytic material fixed on this. For example, the hydrophobic material can be present in the form of homogeneously distributed micro- or nanoparticles or as polymer fibers which pass through the catalyst layer. The representation of such a dispersed catalyst layer can easily consist of an electrode paste, as in DE 10 2004 024 844 A1 and DE 10 2004 024 845 A1 described, done. In this case, an electrode paste is produced, which contains a catalytic material immobilized on a carrier, which contains at least one hydrophobic material, a pore former and optionally a polymer binder in a suitable solvent. This electrode paste is applied to a GDL and cured at elevated temperature to form the solid porous catalyst layer. In this case, if appropriate, the above-described concentration profile of the hydrophobic material can be adjusted.

Alternatively, the at least one hydrophobic material can pass through the catalyst layer in layers, wherein it is in the form of at least one layer, preferably several layers, which are aligned in particular substantially plane-parallel to the gas diffusion layer. In this case, the catalyst layer has a layered construction in which thin layers of the hydrophobic material and layers containing the support material and the catalytic material (and optionally a polymeric binder component) alternate. In this case, preferably in the layers of the hydrophobic material, in addition to the hydrophobic material, a conductive component (for example, graphite or a particular catalytic metal) to improve the conductivity of these layers. The representation of such layer structures can take place, for example, by means of screen-printing, spraying or rolling processes or combinations of these, the porous catalyst layers in turn correspondingly DE 10 2004 024 844 A1 and DE 10 2004 024 845 A1 can be prepared as described above. A concentration gradient of the hydrophobic material is also advantageous in connection with a layered structure, in particular in such a way that layers with a higher content of hydrophobic material are present in the region of the GDL in order to allow improved removal of the product water into the GDL, while the Membrane due to the lower hydrophobicity an optimal connection to the membrane and a good catalyst activity is ensured.

The Introduction of the second electrolyte into the gas diffusion electrode can either be done later by the example Catalyst layer prepared as described above contains the hydrophobic material, or the gas diffusion electrode subsequently impregnated with the electrolyte becomes. Alternatively, the second electrolyte may already be in the first place be introduced during the manufacturing process of the catalyst layer, for example, by placing the electrolyte next to the others, incorporated above ingredients in the electrode paste becomes.

A Further advantageous embodiment of the invention provides that - in addition to the catalyst layer - also the gas diffusion layer at least one hydrophobic material may have. Is between GDL and catalyst layer an optional microporous layer If present, this may alternatively or in addition to the GDL also have a hydrophobic finish. It can do that hydrophobic material of the GDL and / or the microporous layer identical or different from the hydrophobic material of the catalyst layer be elected.

The Invention further relates to an MEA with a polymer electrolyte membrane, which is impregnated with at least a first electrolyte Polymer material comprises, and with two on both sides of the polymer electrolyte membrane subsequent gas diffusion electrodes, of which at least one of the electrodes of an inventive invention described above Gas diffusion electrode corresponds. In addition, concerns the invention is a fuel cell stack constructed from a plurality of such MEAs.

By improving the level of performance at temperatures below the boiling point of water, that is in the vehicle-relevant temperature range, as well as by avoiding mistaken The MEA according to the invention or a fuel cell containing the same can be used particularly advantageously in mobile applications, such as in traction systems of motor vehicles or for additional energy supply as a so-called APU (for auxiliary power unit) in motor vehicles. Of course, it is also applicable for stationary applications, especially for small power plants or domestic energy supply facilities.

Further preferred embodiments of the invention will become apparent from the others, in the subclaims mentioned features.

The Invention will be described below in embodiments the accompanying drawings explained. Show it:

1A a highly schematic fuel cell;

1B a section from 1A with a membrane-electrode assembly;

1C a more detailed view of a membrane electrode assembly according to the present invention;

2 Course of the power density of a MEA according to the prior art in the periodic temperature cycle experiment;

3 Course of power density of a prior art advanced MEA in the periodic temperature cycle experiment; and

4 Course of the power density of an MEA according to the present invention in the periodic temperature cycle experiment.

1A shows a highly schematic representation of a fuel cell 10 with a fuel cell stack 12 , which consists of a variety of membrane electrode assemblies 14 (MEA), one of which is in 1B is shown in an enlarged sectional view. A more detailed representation of a section of the membrane-electrode assembly 14 shows 1C also in sectional view.

Like from the 1B and 1C can be seen, includes the MEA 14 a proton-conducting (essentially anhydrous) polymer electrolyte membrane 16 made of a suitable polymer material 24 formed and with at least a first electrolyte 26 impregnated. For example, the polymer material 24 be a polymer from the group of polyazoles and polyphosphazenes. In particular, polybenzimidazoles, polypyridines, polypyrimidines, polyimidazoles, polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines, polythiadiazoles, poly (tetrazapyrene), polyvinylpyridines, polyvinylimidazoles can be mentioned here.

The anhydrous proton conduction of the polymer membrane 16 or the polymer material 24 based on the first electrolyte 26 which is in particular a solution of a high-boiling temperature-resistant electrolyte. It is preferably an acid, such as phosphoric acid, phosphinic acid, phosphoric acid, nitric acid, hydrochloric acid, formic acid, acetic acid, trifluoroacetic acid, sulfuric acid, sulfonic acid, in particular (per) halogenated alkyl- or arylsulfonic acid or (per) halogenated alkyl- or arylphosphonic acid, in particular Methanesulfonic acid or phenylsulfonic acid. Also suitable are phosphoric acid alkyl or aryl esters, heteropolyacids, such as hexafluoroglutaric acid (HFGA) or squarric acid (SA). Alternatively, the first electrolyte 26 a base, in particular an alkali or alkaline earth metal hydroxide, such as potassium hydroxide, sodium hydroxide or lithium hydroxide. Polysiloxanes or nitrogen-containing heterocycles can also be used as the electrolyte 26 or addition of electrolytes, for example imides, imidazoles, triazoles and derivatives of these, in particular perfluorosulfonimides. Likewise, ionic liquids, such as 1-butyl-3-methylimidazolium trifluoromethanesulfonite, are suitable as the electrolyte. All of the aforementioned electrolytes can also be used as a derivative or salt. It is also conceivable to use a mixture of various of the abovementioned electrolytes for impregnating the polymer material 24 use, for example, a mixture of phosphoric acid and another electrolyte.

Preference is given to using proton exchange membranes which are formed by impregnating a temperature-resistant basic polymer with an acid, the content of the electrolyte solution being from 60 to a maximum of 99% by weight, based on the polymer material 24 is. In the present example, a membrane of polybenzimidazole (PBI) is used as an anhydrous polymeric material 24 used, on the phosphoric acid as an electrolyte 26 is bound.

Each of the two outer membrane surfaces is followed by a gas diffusion electrode 18a and 18b on, namely an electrode connected as a cathode 18a on the cathode side of the membrane 16 and an electrode connected as an anode 18b on anode side. The gas diffusion electrodes 18a and 18b each comprise a microporous catalyst layer 20a and 20b containing the polymer electrolyte membrane 16 contact on both sides. The catalyst layers 20a . 20b contain as actually reactive centers of the electrodes, a catalytic material, which is usually a noble metal as a catalytically active substance, such as platinum, Iridi or ruthenium or transition metals such as chromium, cobalt, nickel, iron, vanadium or tin or mixtures or alloys of these. The catalytic substance is preferably fixed on a porous, electrically conductive carrier material. For the carrier material gas-permeable electrically conductive carbon materials, such as gas-permeable particles, fabrics and felts carbon-based in question. About the support material of the catalyst layers 20a and 20b is an electrically conductive connection of the reaction centers of the electrodes with an external circuit (not shown) realized. In the present example, the reactive centers of the catalyst layers exist 20a . 20b of platinum supported on carbon particles (Pt / C), whereby the particles are joined to form a porous and thus gas-permeable composite.

The gas diffusion electrodes 18a and 18b each additionally comprise a gas diffusion layer (GDL) 22a and 22b attached to the outer, from the polymer membrane 16 opposite surfaces of the catalyst layer 20a respectively 20b connect. Function of the GDL 22a . 22b it is, a uniform flow of the catalyst layers 20a . 20b to ensure with the reaction gases oxygen or air on the cathode side and hydrogen on the anode side. Furthermore, the GDL 18a . 18b adjacent to the catalyst layer 20a . 20b still have a thin microporous layer, for example based on carbon (not shown). Not shown in the 1B and 1C are also so-called bipolar plates (BP), which connect to both sides of the MEA composite and provide for the supply of process gases and the discharge of product water and also the individual MEA 14 in the fuel cell stack 12 separate gas-tight from each other.

According to the invention, the catalyst layers 20a . 20b the gas diffusion electrode 18a and 18b at least a second electrolyte 28 on ( 1C ), which is a liquid or solid electrolyte or an electrolyte solution or mixtures thereof. Ideally, one or more electrolytes are selected which have the following properties. The boiling and the decomposition temperature of the electrolyte (s) 28 are well above the desired operating temperature of the fuel cell 10 , ie above 150 ° C, while the melting temperature of liquid electrolytes and the freezing point of electrolyte solutions is below -30 ° C. The conductivity in the anhydrous state should be at least 0.01 S / cm at -20 ° C, at least 0.05 S / cm at 20 ° C and at temperatures of 80 to 130 ° C at least 0.1 S / cm. Furthermore, the electrolyte has 28 a high stability against oxidation by oxygen and radicals and a high stability against reduction, in particular by hydrogen. Furthermore, the lowest possible water solubility of the electrolyte 28 aimed at over the entire operating temperature range and a high affinity for the catalyst layer 20a . 20b or to materials which can be added to this, to wash out the electrolyte 28 to avoid.

In principle, the second electrolyte 28 from the same, already above for the first electrolyte 26 selected group. These electrolytes meet the requirements outlined above and show in the MEA according to the invention 14 In addition, the following advantageous effects. First, they are characterized by a high acid strength and thus allow the representation of a high proton concentration in the injection zone of the protons. Secondly, the electrolytes advantageously have a high density of functional groups or atoms with lone pairs of electrons, for example OH, SH, O, N or others, which leads to small molecule spacings among one another and thus to a good proton transfer function. In this way a fast proton transport is ensured according to the Grothus mechanism. Third, they have high oxygen solubility (eg> 1.0 x 10 ^-3 mol / L at 298 K) and high diffusion coefficients for oxygen through the liquid phase of the electrolyte (eg> 1.4 x 10 ^-2 cm ² / s at 298 K). As a result, the proton conduction and also the diffusion of the reactants (H ₂ and O ₂ ) are thus accelerated and thus the chemical partial reactions at the electrodes 18a . 18b , in particular on the Katalysatorschichen 20a . 20b , This is especially true for the rate-limiting cathode reaction, which is limited by the slower diffusion of oxygen. By the addition of the electrolyte according to the invention 28 to the catalyst layers 20a . 20b is achieved in a particularly advantageous manner that at low temperatures, a significantly lower power loss of the MEA 14 as in conventional, non-electrolyte-impregnated electrodes is observed. In addition, the impregnation of the electrodes inhibits 18a . 18b or their catalyst layers 20a . 20b the washing out of the electrolyte 26 from the membrane 16 , which is chosen according to an optimal proton conductivity. In this way, the MEA according to the invention is 14 or a fuel cell 10 containing a plurality of such in a fuel cell stack 12 contains, particularly suitable for applications in the traction system of motor vehicles, but also for stationary applications, for example for small power plants or domestic energy supplies.

Basically, the second electrolyte 28 the electrodes 18a . 18b be the same as the ers te electrolyte 26 the membrane 16 , Already in this case, the advantages of the electrolyte-doped electrode described above are achieved. However, it is preferably provided that different electrolytes 26 . 28 for the membrane 16 on the one hand and the catalyst layer 20a . 20b on the other hand. In this way, targeted combinations of electrolytes 26 . 28 which are optimized to the respective requirements and complement each other. For example, while the first electrolyte 26 the membrane corresponding to a particularly high proton conductivity and high affinity for the polymer material 24 can be selected, the second electrolyte 28 the electrode can be selected according to a good diffusion and solubility of the reactant H ₂ and / or O ₂ , in particular a high oxygen diffusion and solubility. Particularly advantageous among these aspects is the use of phosphoric acid as a membrane electrolyte 26 and the use of partially or perfluorinated alkyl or aryl phosphonic acids or partially or perfluorinated alkyl or aryl sulfonic acids as the electrode electrolyte 28 proved. The use of the latter electrolytes in the electrode is advantageous because they have a higher oxygen solubility and oxygen transport rate than phosphoric acid. Thus, the oxygen solubility in 85% phosphoric acid at room temperature is 3.3 × 10 ^-3 mol / l and the diffusion coefficient of oxygen 1.2 × 10 ^-6 cm ² / s and increases to 8.7 × 10 in the fluorinated phosphoric acid derivatives ^{. 3} mol / l or 1.6 × 10 ^-6 cm ² / s.

In the context of the present inventions are also such embodiments in which - in contrast to the in 1C illustrated embodiment - the two electrodes 18a . 18b have a different electrolyte equipment. For example, the electrode connected as a cathode 18a with another electrolyte 28 be doped as the anode 18b , that the cathode 18a is optimized for a particularly high oxygen diffusion and solubility and the anode 18b in terms of a particularly high hydrogen diffusion and solubility. An advantageous example here is the doping of the anode with phosphoric acid and the cathode with a partially fluorinated or perfluorinated alkyl or arylphosphonic acid or sulfonic acid (see above). Alternatively, it may be advantageous to use only one of the two gas diffusion electrodes 18a or 18b with an electrolyte 28 equip, in particular the cathode 18a since the cathode reaction usually represents the rate-limiting step, while the diffusion and conversion of hydrogen on the anode side is not critical.

In test series, a content of the second electrolyte or a mixture of various such has at least 0.5 mmol, in particular of at least 1.0 mmol in each catalyst layer 20a . 20b proved effective. Based on the area of the catalyst layer 20a . 20b 0.005 to 2 mmol per cm ² , in particular 0.01 to 1 mmol / cm ^{2 of} the second electrolyte or of the electrolyte mixture are preferred.

According to a particularly preferred embodiment, the catalyst layer contains 20a . 20b in addition to the at least one electrolyte 28 at least one polymeric binder component, preferably corresponding to a particularly high affinity for the electrolyte 28 is selected. By this measure, an additional fixation of the electrolyte 28 in the catalyst layer 20a . 20b whereby a washing out of the same is prevented by the resulting product water, especially under operating conditions in which the product water is obtained in liquid form. In this way, the system is particularly long-term stability. The presence of the polymeric binder component associated with the electrolyte in the electrode enhances performance in both the optimum operating temperature range of 160 to 180 ° C and in the range of low temperatures to cold start conditions. As a polymeric binder component in particular materials have been found to be suitable, which also as polymer materials 24 the polymer electrolyte membrane 16 Can be used, in particular those that have already been listed above in this context. Advantageously, for both components, the polymer material 24 the polymer electrolyte membrane 16 and the polymeric binder component of the catalyst layer 20a . 20b that uses the same material. The polymeric binder component preferably has a porous or fibrous structure, with the polymer fibers distributed throughout the catalyst layer 20a . 20b exist, so to speak enforce them. Suitable polymeric binder components are in DE 10 2004 024 844 A and DE 10 2004 024 845 A described.

According to the invention, the catalyst layers 20a . 20b the gas diffusion electrodes 18a and 18b furthermore, at least one hydrophobic material 30 which has a contact angle with water of at least 90 °, in particular of at least 100 °, preferably of at least 110 °. For the hydrophobic material 30 in particular it is graphite and / or an unsubstituted, fully or partially halide-substituted polyolefin, in particular polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) or a mixture of these, PTFE being particularly is preferred.

Due to the presence of the hydrophobic material 30 becomes a low wetting tendency of the catalyst layers 20a . 20b achieved with liquid product water. This leads to a high Abtransportrate of at the cathode 18a educated pro water by the GDL 22a as well as the through the membrane 16 diffused water on anode side. The good removal of the product water on the one hand ensures that the gas diffusion channels in the catalyst layers 20a . 20b be kept free, whereby an optimal supply of process gases oxygen or air on the side of the cathode 18a and hydrogen on the anode side 18b is guaranteed. Furthermore, the rapid removal of water prevents due to the very short contact times with the membrane 16 a triggering and removal of the electrolyte 24 , which extends the life of the membrane 16 and therefore the MEA 14 extended especially under condensation conditions.

The hydrophobic material according to the invention 30 can - as in 1C indicated - in the form of disperse distributed micro or nanoparticles in the catalyst layer 20a . 20b available. In this case, the concentration of the hydrophobic material 30 and thus the hydrophobicity of the catalyst layer 20a . 20b advantageous in the direction of the GDL 22a . 22b increase, whereby the discharge of liquid product water is still improved. The preparation of the catalyst layer 20a . 20b with a concentration gradient of hydrophobic material 30 can be easily done by applying an electrode paste to the GDL 22a . 22b take place, during which a gradual addition of the hydrophobic material 30 he follows.

Alternatively, the hydrophobic material 30 also in layers within the catalyst layers 20a . 20b wherein one or more layers of the hydrophobic material 30 , which may also contain an addition of a conductive component, such as a catalytic metal and / or graphite to improve the conductivity, alternating with layers of the catalytic material. In this case, an increasing concentration profile of the hydrophobic material 30 in the direction of the GDL 22a . 22b be generated by the GDL 22a . 22b Layers with decreasing content of hydrophobic material 30 be applied alternately with the catalytic layers. The hydrophobic layers can be produced, for example, by spraying a suspension of the hydrophobic substance onto the GDL. Alternatively, in particular soluble hydrophobic materials (eg PVDF) can be introduced directly into the electrode paste (see below).

The introduction of the second electrolyte according to the invention 28 or a mixture of several electrolytes 28 and the hydrophobic material 30 in the catalyst layer 20a . 20b the electrodes 18a . 18b is particularly advantageous for electrodes according to DE 10 2004 024 844 and DE 10 2004 024 845 whose disclosure contents are expressly included by the present invention. In particular, in analogy to these publications, the porous catalyst layer 20a . 20b prepared from an electrode paste containing a carbon-supported catalyst material, a solution or suspension of the binding polymer, a pore-forming material and the hydrophobic material according to the invention in a suitable solvent. This electrode paste is applied to a substrate, in particular a GDL, as a layer and preferably heated under reduced pressure until the solvent has evaporated and the pore-forming material has decomposed. The result is thus a microporous gas-permeable catalyst layer 20a . 20b containing the carbon supported catalyst material and the polymer present as the polymeric binder component for the electrolyte or electrolytes 28 serves, contains. In this case, the second electrolyte 28 in two principal ways in the catalyst layer 20a . 20b be introduced. First, first, the catalyst layer 20a . 20b or the corresponding gas diffusion electrode 18a . 18b be prepared and the at least one second electrolyte 28 subsequently in the finished catalyst layer 20a . 20b be introduced. Alternatively, the at least one electrolyte 28 directly into the electrode paste, in particular according to DE 10 2004 024 844 are incorporated and from this the electrolyte-doped catalyst layer 20a . 20b be generated.

• (a) Bereitstellen einer porösen, elektrisch leitfähigen Katalysatorschicht 20a, 20b, die zumindest ein katalytisches Material sowie zumindest ein hydrophobes Material 30 aufweist,
• (b) nachträgliches Einbringen zumindest eines zweiten Elektrolyten 28 in die Katalysatorschicht 20a, 20b und
• (c) Zusammenfügen einer Polymerelektrolytmembran 16, die ein mit einem ersten Elektrolyten 26 imprägniertes Polymermaterial 24 umfasst, mit jeweils einer beidseitig an die Polymerelektrolytmembran 16 anschließenden Katalysatorschicht 20a, 20b.

The former variant (subsequent introduction of the second electrolyte 28 ) thus comprises the following process steps:

• (a) providing a porous, electrically conductive catalyst layer 20a . 20b containing at least one catalytic material and at least one hydrophobic material 30 having,
• (B) subsequent introduction of at least one second electrolyte 28 in the catalyst layer 20a . 20b and
• (c) assembling a polymer electrolyte membrane 16 that one with a first electrolyte 26 impregnated polymer material 24 comprising, on both sides of the polymer electrolyte membrane 16 subsequent catalyst layer 20a . 20b ,

• – Einpressen des zumindest einen flüssigen oder gelösten Elektrolyten 28 durch Anwendung eines äußeren Überdrucks, der insbesondere mindestens 0,5 MPa beträgt, bei Raumtemperatur oder oberhalb von dieser für eine Dauer von mindestens 5 Minuten.
• – Einbringen des zumindest einen flüssigen oder gelösten Elektrolyten 28 durch Anwendung eines äußeren Unterdrucks, der insbesondere zwischen 0,5 mbar und 1 bar liegt, bei Raumtemperatur oder oberhalb von dieser für eine Dauer von mindestens 5 Minuten. In Abwandlung dieser Methode kann der Elektrolyt 28 auch durch Anlegen eines Vakuums von unten durch die Katalysatorschicht 20a, 20b unter Verwendung einer Filternutsche eingesogen werden.
• – Betropfen oder Beschichten der Katalysatorschicht 20a, 20b mit dem flüssigen oder gelösten Elektrolyten 28.
• – Tauchen der Katalysatorschicht 20a, 20b in den zumindest einen flüssigen oder gelösten Elektrolyten 28.

In this case, the subsequent introduction of the at least one second electrolyte 28 in the catalyst layer 20a . 20b by one of the following methods:

• - Pressing the at least one liquid or dissolved electrolyte 28 by applying an external overpressure, in particular at least 0.5 MPa, at or above room temperature for a period of at least 5 minutes.
• - Introducing the at least one liquid or dissolved electrolyte 28 by applying a external negative pressure, which is in particular between 0.5 mbar and 1 bar, at room temperature or above this for a period of at least 5 minutes. In a modification of this method, the electrolyte 28 also by applying a vacuum from below through the catalyst layer 20a . 20b be sucked in using a suction filter.
• - Dripping or coating of the catalyst layer 20a . 20b with the liquid or dissolved electrolyte 28 ,
• - Dipping the catalyst layer 20a . 20b in the at least one liquid or dissolved electrolyte 28 ,

• (a) Herstellen einer Elektrodenpaste, welche das zumindest eine katalytische Material, einen Porenbildner, das zumindest eine hydrophobe Material 30 sowie den zumindest einen zweiten Elektrolyten 28 und ein Lösungsmittel enthält, insbesondere analog DE 10 2004 024 844 ,
• (b) Erwärmung der Elektrodenpaste solange, bis ein der Porenbildner und das Lösungsmittel zersetzt oder verdampft sind, wobei die poröse Katalysatorschicht 20a, 20b entsteht, und
• (c) Zusammenfügen einer Polymerelektrolytmembran 16, die ein mit einem ersten Elektrolyten 26 imprägniertes Polymermaterial 24 umfasst, mit jeweils einer beidseitig an die Polymerelektrolytmembran 16 anschließenden Katalysatorschicht 20a, 20b.

In contrast, the second-mentioned method (introduction of the electrolyte 28 during the manufacturing process of the catalyst layer 20a . 20b ) the following process steps:

• (a) preparing an electrode paste which comprises the at least one catalytic material, a pore-forming agent comprising at least one hydrophobic material 30 and the at least one second electrolyte 28 and a solvent, in particular analogously DE 10 2004 024 844 .
• (b) heating the electrode paste until one of the pore formers and the solvent is decomposed or vaporized, the porous catalyst layer 20a . 20b arises, and
• (c) assembling a polymer electrolyte membrane 16 that one with a first electrolyte 26 impregnated polymer material 24 comprising, on both sides of the polymer electrolyte membrane 16 subsequent catalyst layer 20a . 20b ,

According to a particularly advantageous embodiment, the catalyst layer contains 20a . 20b a gradually or gradually varying content of the at least one second electrolyte 28 in particular in the direction of the polymer electrolyte membrane 16 increases. This varying electrolyte content can be represented in a particularly simple manner by the second production method, during the production process of the catalyst layer 20a . 20b a gradual addition of the electrolyte to the electrode paste takes place and this is applied in layers. The same effect can be achieved if the polymer provided as a binder itself has a concentration gradient or a gradient of functional groups, whereby also a concentration gradient of the electrolyte 28 is realized.

Alternatively, in the context of the present invention, membranes 16 be used, in which the catalyst layer 20a . 20b directly on the outer surface of electrolyte-impregnated membrane 16 is applied (so-called CCM for catalyst coated membrane). Here is the second electrolyte 28 preferably subsequently from the outside into the catalyst layers 20a . 20b introduced before the GDL 22a . 22b be added.

• – Einbringen des mindestens einen ersten Elektrolyten 26 in eine, das Polymermaterial 24 enthaltene Ziehlösung zur Herstellung der Polymermembran 16 mit einem Massenanteil des Elektrolyten von 5 bis 95% bezogen auf den Endgehalt in der Membran 16. Anschließendes Gießen der Lösung und Verdampfen des Lösungsmittels unter Ausbildung der imprägnierten Membran 16.
• – Einbringen des mindestens einen ersten Elektrolyten 26 in ein, das Polymermaterial 24 enthaltenes Polymergranulat und Herstellung einer Ziehlösung mit einem Massenanteil des Elektrolyten in der Lösung von 5 bis 95%. Anschließendes Gießen der Lösung insbesondere und Verdampfen des Lösungsmittels unter Ausbildung der imprägnierten Membran 16.
• – Behandlung der Membran 16 durch Tauchen oder Betropfen mit einer Lösung mit 5 bis 95 Gew.-% des Elektrolyten 26 bei Temperaturen oberhalb der Raumtemperatur für mindestens 5 Minuten.

In both manufacturing approaches described above, the polymer membrane 16 before joining with the catalyst layers 20a . 20b or with the gas diffusion electrodes 18a . 18b with the at least one first electrolyte 26 impregnated. This can - analogous to the impregnation of the catalyst layer 20a . 20b - also done by the electrolyte 26 already added to a manufacturing solution or composition for the preparation of the membrane or by the membrane 16 subsequently with the electrolyte 26 is impregnated. Thus, the preparation of the impregnated polymer membrane 16 for example, by one of the following methods:

• - introducing the at least one first electrolyte 26 into one, the polymer material 24 included Ziehlösung for the preparation of the polymer membrane 16 with a mass fraction of the electrolyte of 5 to 95% based on the final content in the membrane 16 , Subsequent casting of the solution and evaporation of the solvent to form the impregnated membrane 16 ,
• - introducing the at least one first electrolyte 26 in, the polymer material 24 contained polymer granules and preparation of a Ziehlösung with a mass fraction of the electrolyte in the solution of 5 to 95%. Subsequent casting of the solution in particular and evaporation of the solvent to form the impregnated membrane 16 ,
• - Treatment of the membrane 16 by dipping or dripping with a solution with 5 to 95 wt .-% of the electrolyte 26 at temperatures above room temperature for at least 5 minutes.

The recorded amount of electrolyte can be determined for example by gravimetric methods. The so represented, with the at least one electrolyte 26 impregnated membrane 16 can be further processed immediately after impregnation or stored in an inert container, such as aluminum-coated PE or PTFE film, in vacuo or under inert gas. It is possible that such an impregnated membrane is installed with dry electrodes, which are then impregnated.

Example:

The preparation of the gas diffusion electrode according to the invention 18a . 18b , in particular the catalyst layer 20a . 20b takes place accordingly DE 10 2004 024 844 A1 using an electrode paste. To prepare the electrode paste is a solvent, for example N, N-dimethylformamide or N, N-dimethylacetamide, mixed with a pore former and a hydrophobic material, in particular PTFE, and PBI as a polymer binder. The pore former used is preferably a material which can be thermally completely or partially converted into the gaseous state and thus, when heated, a pore structure or gas passages within the catalyst layer 20a . 20b generated. For example, an inorganic carbonate such as ammonium carbonate and / or an azide such as sodium or calcium azide is used as the pore former. The PTFE and the PBI can be added as an emulsion or dispersion or first emulsified or dispersed in the electrode paste. The paste is further provided with a catalyst powder containing a catalytic material, for example Pt, supported on a support material, for example carbon particles. In particular, Pt / C with a platinum loading of 10-70% by weight (eg E-TEK HP-II) is used as the catalyst powder. The amount of the hydrophobic material PTFE is between 0.5 to 50 wt .-% based on the Pt / C used. After mixing the components, the electrode paste is homogenized for 30 to 90 minutes in an ultrasonic bath. Subsequently, the electrode paste is applied to a gas diffusion layer 22a . 22b , which contains a microporous carbon-based layer, applied and heated under reduced pressure and dried until it comes to decomposition of the pore-forming agent and volatiles of it have evaporated. The result is thus a microporous gas-permeable catalyst layer 20a . 20b containing the carbon supported catalyst material as well as the hydrophobic material 30 contains in largely homogeneous distribution. Subsequently, this catalyst layer 20a . 20b by one of the methods described above with the second electrolyte 28 , here perfluorinated alkylphosphonic, impregnated, wherein a surface-related content of the electrolyte 28 from 0.01 to 2 mmol / cm ² . Two gas diffusion electrodes produced in this way 18a . 18b be with a PBI membrane 24 , previously using one of the previously described methods with phosphoric acid H ₃ PO ₄ as the electrolyte 26 was impregnated to an MEA 14 pressed.

Comparative Example 1

It was an MEA as in the previous example including the polymer binder PBI prepared, but without the addition of the hydrophobic material PTFE and the electrolyte perfluorinated alkylphosphonic acid.

Comparative Example 2:

It an MEA was prepared as in the previous example, however without the addition of the polymer binder PBI, the hydrophobic material PTFE and the electrolyte perfluorinated alkylphosphonic acid.

The power density of an MEA prepared according to the example of the invention was investigated in a temperature cycle experiment, with the temperature being periodically varied between 160 and 40 ° C. The result is in 4 shown. A corresponding MEA without the addition according to the invention of hydrophobic material and of electrolyte was also investigated as Comparative Example 1 ( 3 ). The MEA according to the prior art without hydrophobic material, without electrolyte and without polymer binder (Comparative Example 2) was also investigated, in which case the temperature was varied between 160 ° C. and a variable, progressively decreasing lower temperature ( 2 ).

Out 2 It can be seen that the conventional MEA without polymer binder, without hydrophobic material and without an additional second electrolyte in the electrode shows a significant performance drop at low temperatures. Even after a temperature reduction to 100 ° C, ie below the boiling point of water (at 3 bar operating pressure), there is an irreversible power dip, so that the original performance at the optimum operating temperature of 160 ° C is no longer achieved. This degradation phenomenon is due to a washing out of the first electrolyte (phosphoric acid) from the PBI membrane by the product water arising in liquid form.

In contrast, shows 3 that even the addition of the polymer binder PBI in the catalyst layer (Comparative Example 1) causes a significant improvement in the cycle stability of the MEA, that is, at a temperature variation between 160 and 40 ° C, no irreversible power dip was observed even after 800 h of continuous operation. However, when the temperature is lowered to 40 ° C, the MEA shows a reversible performance drop to about 0.02 W / cm ² , which limits the use of MEA in motor vehicles. This burglary is based on a diffusion inhibition of the reaction gases.

On the other hand, the MEA according to the invention shows in FIG 4 Although also a decrease in performance at lowered temperature, but at the lower temperature of 40 ° C, a power of about 0.05-0.06 W / cm ^{2 is} observed without the according to 3 reached cycle stability is affected. In summary, it has been shown that the functionality of gas diffusion electrodes for membrane-electrode assemblies based on liquid electrolyte-impregnated membrane systems, such as PBI / H ₃ PO ₄ , via the incorporation of hydrophobic materials and electrolytes in the Ka can be influenced positively. The result is a significant decrease in the diffusion inhibition over the entire temperature range and in particular at low temperatures by improving the removal of the liquid product water and by keeping the gas channels free. In particular, the hydrophobic material and the second electrolyte to increase the fuel cell performance in the entire temperature range and especially in the range of lower temperatures below 100 ° C with very good cyclic stability.

10

fuel cell

12

fuel cell stack

14

Membrane-electrode assembly (MEA)

16

Polymer electrolyte membrane

18a

first Gas diffusion electrode (cathode)

18b

second Gas diffusion electrode (anode)

20a

cathode side catalyst layer

20b

anode side catalyst layer

22a

cathode side Gas diffusion layer (GDL)

22b

anode side Gas diffusion layer (GDL)

24

polymer material

26

first electrolyte

28

second electrolyte

30

hydrophobic material

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• US 5525436 [0005]
• US 5716727 [0005]
• US 5599639 [0005]
• WO 01/18894 A [0005]
• - WO 99/04445 A [0005]
• EP 0983134 B [0005]
• EP 0954544 B [0005]
• - DE 102004024844 A [0007, 0045]
• - DE 102004024845 A [0007, 0045]
• - DE 10246459 A [0008]
• - EP 0520468 A [0009]
• - DE 102004024844 A1 [0020, 0021, 0058]
• DE 102004024845 A1 [0020, 0021]
• - DE 102004024844 [0050, 0050, 0053]
• - DE 102004024845 [0050]

Claims (22)

Gas diffusion electrode ( 18a . 18b ) for high-temperature fuel cells ( 10 ) based on membranes, which are impregnated with a first electrolyte, with a gas diffusion layer ( 22a . 22b ) and one on the gas diffusion layer ( 22a . 22b ) arranged porous catalyst layer ( 20a . 20b ), which comprises an electrically conductive carrier material and at least one catalytic material applied to the carrier material, characterized in that the catalyst layer ( 20a . 20b ) with at least one second electrolyte ( 28 ) and at least one hydrophobic material ( 30 ) contains. Gas diffusion electrode according to claim 1, characterized in that the catalyst layer ( 20a . 20b ) at least 0.5 mmol, in particular at least 1.0 mmol, of the at least one second electrolyte ( 28 ) or a mixture of such. Gas diffusion electrode according to claim 1 or 2, characterized in that the catalyst layer ( 20a . 20b ) at least 0.005 mmol per cm ² surface, in particular at least 0.01 mmol / cm ² , of the at least one second electrolyte ( 28 ) or a mixture of such. Gas diffusion electrode according to one of the preceding claims, characterized in that the catalyst layer ( 20a . 20b ) a gradually or gradually varying content of the at least one second electrolyte ( 28 ) having. Gas diffusion electrode according to one of the preceding claims, characterized in that the at least one second electrolyte ( 28 ) is selected from the group comprising acids, in particular phosphinic acid, phosphonic acid, alkyl and aryl phosphonic acids, phosphoric acid, alkyl or aryl phosphates, sulfuric acid, sulfonic acid, alkyl and aryl sulfonic acids, nitric acid, hydrochloric acid, formic acid, acetic acid, trifluoroacetic acid, heteropolyacids, hexafluoroglutaric acid ( HFGA), squarric acid (SA); and bases, especially alkali and alkaline earth hydroxides such as potassium hydroxide, sodium hydroxide, lithium hydroxide; nitrogen-containing heterocycles, especially imides, imidazoles, triazoles, perfluorosulfonimides, ionic liquids, especially and 1-butyl-3-methyl-imidazolium trifluoromethanesulfonite; Polysiloxanes, and derivatives and salts of these. Gas diffusion electrode according to one of the preceding claims, characterized in that the at least one hydrophobic material ( 30 ) has a hydrophobicity corresponding to a contact angle with water of at least 90 °, in particular of at least 100 °, preferably of at least 110 °. Gas diffusion electrode according to one of the preceding claims, characterized in that the at least one hydrophobic material ( 30 ) comprises at least one substance selected from the group comprising graphites and unsubstituted, fully or partially halide-substituted polyolefins, in particular polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF). Gas diffusion electrode according to one of the preceding claims, characterized in that the at least one hydrophobic material ( 30 ) has a mass fraction in the range of 0.5 to 50 wt .-%, in particular from 5 to 35 wt .-%, preferably from 10 to 20 wt .-%, based on the sum of the masses of the catalytic material and the carrier material. Gas diffusion electrode according to one of claims 1 to 8, characterized in that the at least one hydrophobic material ( 30 ) homogeneously distributed within the catalyst layer ( 20a . 20b ) is present. Gas diffusion electrode according to one of the preceding claims, characterized in that the at least one hydrophobic material ( 30 ) one in the direction of the gas diffusion layer ( 22a . 22b ) stepwise or gradually increasing concentration within the catalyst layer ( 20a . 20b ) having. Gas diffusion electrode according to one of claims 1 to 8, characterized in that the at least one hydrophobic material ( 30 ) in the form of at least one substantially plane-parallel to the gas diffusion layer ( 22a . 22b ) is present, in particular in the form of at least one of the at least one hydrophobic material ( 28 ) and a layer containing an electrically conductive component. Gas diffusion electrode according to claim 11, characterized in that the at least one hydrophobic material ( 30 ) in the form of several, the catalyst layer ( 20a . 20b ), wherein the layers in particular have a concentration gradient of hydrophobic material ( 28 ) exhibit. Gas diffusion electrode according to one of the preceding claims, characterized in that the gas diffusion layer ( 22a . 22b ) and / or between the gas diffusion layer ( 22a . 22b ) and the catalyst layer ( 20a . 20b ) optional existing microporous layer also has at least one hydrophobic material and / or an electrolyte. Gas diffusion electrode according to one of the preceding claims, characterized in that the catalyst layer ( 20a . 20b ) at least one polymeric binder component with high affinity for the second electrolyte ( 28 ) contains. Membrane electrode unit ( 14 ) with a polymer electrolyte membrane ( 16 ), one with a first electrolyte ( 26 ) impregnated polymer material ( 24 ) and two on both sides of the polymer electrolyte membrane ( 16 ) subsequent gas diffusion electrodes ( 18a . 18b ), of which at least one is a gas diffusion electrode ( 18a . 18b ) according to one of claims 1 to 13. Membrane electrode unit ( 14 ) according to claim 15, characterized in that the at least one second electrolyte ( 28 ) of the gas diffusion electrode ( 18a . 18b ) different from the first electrolyte ( 26 ) of the polymer electrolyte membrane ( 16 ). Membrane electrode unit ( 14 ) according to claim 15, characterized in that 2 the at least one second electrolyte ( 28 ) of the gas diffusion electrode ( 18a . 18b ) identical to the first electrolyte ( 26 ) of the polymer electrolyte membrane ( 16 ). Membrane electrode unit ( 14 ) according to one of claims 15 to 17, characterized in that only one of the gas diffusion electrodes ( 18a . 18b ), in particular a gas diffusion electrode connected as cathode ( 18a ), the at least one second electrolyte ( 28 ) and / or the at least one hydrophobic material ( 28 ). Membrane electrode unit ( 14 ) according to one of claims 15 to 17, characterized in that a gas diffusion electrode connected as cathode ( 18a ) at least one different electrolyte ( 28 ) as an anode connected gas diffusion electrode ( 18b ). Membrane-electrode assembly according to one of claims 15 to 19, characterized in that the polymer material ( 24 ) of the polymer electrolyte membrane ( 16 ) and optionally the polymeric binder component of the catalyst layer ( 20a . 20b ) are independently selected from the group comprising polyazoles and polyphosphazenes, in particular polybenzimidazoles, polypyridines, polypyrimidines, polyimidazoles, polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines, polythiadiazoles, poly (tetrazapyrene), polyvinylpyridines, polyvinylimidazolemes. Use of a membrane-electrode unit ( 14 ) according to one of claims 15 to 20 for fuel cells ( 10 ) for mobile applications, in particular for traction systems or for the auxiliary power supply of motor vehicles. Use of a membrane-electrode unit ( 14 ) according to one of claims 15 to 20 for a fuel cell ( 10 ) for stationary applications, in particular for small power plants or domestic energy supply facilities.