DE102009028308A1 - Membrane electrode unit, useful in fuel cell, comprises a polymer electrolyte membrane made of a polymer, two electrodes sandwiching polymer electrolyte membrane, an electrolyte wetting polymer electrolyte membrane and silicate derivative - Google Patents

Abstract

Membrane electrode unit comprises a polymer electrolyte membrane (16) made of at least a polymer, two electrodes (18, 20) sandwiching the polymer electrolyte membrane, at least one electrolyte, which wets the polymer electrolyte membrane, and at least a silicate derivative that wets and incorporated into the polymer electrolyte membrane and/or the two electrodes, where silicate derivative comprises at least a hydrophobically substituted silicate group or at least a hydrophobically substituted silicate oligomer, and at least a polar acid group. An independent claim is included for a fuel cells comprising a single cell (12) containing the membrane electrode unit.

Description

The The invention relates to a membrane-electrode unit for Fuel cell and a fuel cell with at least one such.

Polymer electrolyte membranes are often used for electrochemical applications, their use in fuel cells being by far the largest field of application. Fuel cells use the chemical transformation of hydrogen and oxygen into water 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 sandwiched 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, a fuel, in particular hydrogen H ₂ or a hydrogen-containing gas mixture, is fed to the anode, where an electrochemical oxidation of H ₂ to H ⁺ takes place with emission 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 currently the most advanced fuel cell technology based on polymer electrolyte membranes (PEM) where the membrane itself consists of a polymer electrolyte. These are often acid-modified Polymers, in particular perfluorinated polymers used. The most common Representative of this class of polymer electrolytes is a membrane of a sulfonated polytetrafluoroethylene copolymer (trade name: Nafion; a 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. From this There are a number of disadvantages. So that's the maximum operating temperature this membrane fuel cell - also due to the lack of thermal stability of the membranes - at Standard pressure limited to below 100 ° C. At temperatures above 80 to 95 ° C, the performance deteriorates due to the fluid loss clearly and to maintain the conductivity of the membrane are due to the temperature dependence of the Water vapor pressure large amounts of water to moisten it required. In systems with above normal pressure working pressure, the temperature can only be at the expense of Efficiency, size and weight of the overall system increase. For operation above 100 ° C, the pressure required would be dramatic increase. If there is a failure of the humidification system, are Power losses and irreversible damage to the membrane-electrode assembly the episode.

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 the tolerance to contamination of the combustion gases, for example, with CO, with increasing temperature too. simultaneously decreases the viscosity of the electrolytic 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 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 mineral acids, which ensure the proton conductivity even with complete dryness of the membrane above the boiling point of water. The most promising approach is the use of acid-doped heterocyclic polymers, in particular polyazoles, where the proton conduction is based on the acid bound as a complex in the polymer. 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 proton transport takes place in the here typically basic polymers, such as PBI via three mechanisms that contribute to the total conductivity depending on the moisture content of the membrane to varying degrees, namely via the transmission paths polymer - electrolyte - polymer, electrolyte - electrolyte and electrolyte - water electrolyte.

Even though the property profile of these polymer electrolyte membranes for the use in HT-PEM fuel cells is already quite good a further optimization of the material properties desirable. For example, an increase in proton conductivity desirable, for use in mobile as well as in stationary Energetically improve the sector. Also, the mechanical properties this acid-doped Polybenzimidazolmembranen in need of improvement.

Critical In addition, with this HT membrane type, the operating temperature is lowered below the boiling point of water, such as at the start or at Shutting down the fuel cell is the case. The water-soluble Electrolyte can thus under condensation conditions in the liquid Product water is dissolved and removed from the membrane in the direction of Electrodes shifted and from these completely from the Cell be discharged. By the displacement of the electrolyte in the electrodes, there is a reduction of gas diffusion in the reaction zone with the result of a decrease in performance, in particular at low temperatures because of the reduced oxygen solubility and diffusion. In addition, the loss of charge carriers for electron transport to irreversible damage the MEA, which also leads to a reduced performance. The previous, based on this type of membrane cells must thus kept current until reaching the operating temperature to prevent electrolyte leakage. A performance requirement may only take place when temperatures are reached where excluding vapor product water is obtained. To the mechanical Loading of the components, in particular the sensitive electrolyte membrane has to wait a significant amount of time, before the system is ready.

• • Verwendung eines Elektrolyten mit höherer Anbindungstendenz an das Polymermaterial der Membran (z. B. Alkyl- oder Phenylphosphorsäure als Elektrolyt US 6124060 )
• • Verwendung eines Polymermaterials für die Membran mit höherer Anbindungstendenz an den Elektrolyten (kovalente oder nicht-kovalente Einbindung eines anorganischen, silikatischen Polymers in das basische Basismaterial der Membran, DE 10 2006 010 705 A1 )
• • Aufbringen einer ionisch leitenden, wasserundurchlässigen aber größere Elektrolytmoleküle zurückhaltende Sperrschicht auf die Membran ( DE 10 2004 044 760 A1 )
• • Oberflächenvernetzung der Membran im Anschluss an die Imprägnierung mit dem Elektrolyten, so dass sein Austrag durch die an der Oberfläche erzeugten Netzstruktur unterbunden wird
• • Ausstattung der Membranoberfläche mit negativ geladenen Substituenten, die zu einer Abstoßung des Säureelektrolyten führen
• • Veränderung des Aufbaus der Elektroden zur Erzeilung eines Rückhalteeffektes für flüssige Elektrolyte bei Betrieb unter Kondensationsbedingungen ( DE 10 2004 063 457 A1 , DE 10 2005 023 897 A1 ; DE 10 2006 061 779 A1 )
• • Einführung eines Flüssigkeitsreservoirs in die MEA, aus welchem der dort angereicherte Elektrolyt periodisch zurückgeführt wird ( WO 00/59060 A1 )

Various strategies for limiting the risk of electrolyte discharge and thus the development of operating temperatures below the boiling point of water are conceivable and have been described in part:

• Use of an electrolyte with a higher binding tendency to the polymer material of the membrane (for example, alkyl or phenylphosphoric acid as the electrolyte US 6124060 )
• Use of a polymer material for the membrane with higher attachment tendency to the electrolyte (covalent or noncovalent incorporation of an inorganic, silicate polymer into the basic base material of the membrane, DE 10 2006 010 705 A1 )
• Applying an ionically conductive, water-impermeable but larger electrolyte molecules retaining barrier layer on the membrane ( DE 10 2004 044 760 A1 )
• • Surface crosslinking of the membrane following impregnation with the electrolyte, so that its discharge is prevented by the network structure generated on the surface
• • Equipment of the membrane surface with negatively charged substituents, which lead to a rejection of the acid electrolyte
• Altering the structure of the electrodes to produce a retention effect for liquid electrolytes when operating under condensation conditions ( DE 10 2004 063 457 A1 . DE 10 2005 023 897 A1 ; DE 10 2006 061 779 A1 )
• Introduction of a liquid reservoir into the MEA, from which the electrolyte enriched there is periodically returned ( WO 00/59060 A1 )

The The present invention relates exclusively to the former Approach, d. H. the improvement of the connection of the electrolyte the membrane-forming polymer and / or to the electrode or to a in this existing polymeric binder. When looking for alternatives Electrolytes that meet this requirement are more To consider claims to the electrolyte. In particular, the electrolyte should have a boiling temperature above have the desired operating temperature and a decomposition temperature far above this. It is also a high acidity to aim at simultaneous amphoteric of the electrolyte to the Proton conduction via the Grothius mechanism to ensure. Another advantage is the lowest possible Temperature dependence of the conductivity over the entire operating temperature range. Finally, should the oxygen solubility and diffusion comparable to Be sure to provide good water, even at low temperatures ensure the active centers of the electrodes with the reaction gases (Diffusion limitation).

Object of the present invention is therefore a membrane-electrode unit for high temperature to provide fuel cells available, in which under condensation conditions, a discharge of the electrolyte is reduced or prevented by the resulting product water and thus has in addition to good mechanical and electrochemical properties improved long-term stability.

These Tasks are performed by a membrane-electrode unit (MEA) as well by a fuel cell with the characteristics of independent Claims solved.

The inventive membrane-electrode unit for Fuel cells include a polymer electrolyte membrane made of at least one polymer is formed; two, the polymer electrolyte membrane Sandwich-contacting electrodes (a cathode and an anode) and at least one electrolyte containing at least the polymer electrolyte membrane and optionally wetting the electrodes. According to the invention the MEA further comprises at least one silicate derivative comprising the polymer electrolyte membrane and / or the electrodes wetted and / or incorporated into this is. It is with the term "wetted" one superficial adhesion of the silicate derivative to the polymer material the membrane or to the electrode due to non-covalent interactions Understood. Is on the other hand, the silicate derivative in the Membrane and / or electrode "incorporated", means this is a substantially homogeneous mixture at the molecular level, in which the silicate derivative penetrates the membrane or the electrode. The silicate derivative has at least one hydrophobically substituted Silicate group or at least one hydrophobically substituted silicate oligomer and at least one polar acid group, preferably is arranged terminally.

by virtue of the at least one polar acid group, in particular a protic acid group with at least one acidic Is hydrogen, the silicate derivative has an electrolytic proton conductivity, which increases the overall conductivity of the MEA.

The polar acid group thus acts as an additional intrinsic covalently bound proton conductor, on the one hand directly increased the proton conductivity of the MEA and on the other hand, to obtain a desired conductivity required amount of the first electrolyte reduced. The Silicate derivative thus represents an electrolyte itself. In addition the silicate backbone has a high chemical and mechanical stability in the range of the desired operating temperature, by which in particular the membrane is stabilized. by virtue of the hydrophobicity of the silicate group takes place simultaneously an optimal attachment or involvement in the polymer material of the membrane and / or in the optional polymeric binder material of Electrode instead. Particularly noteworthy, however, is the on the Silikatderivat attributable higher Electrolyte uptake of the membrane or the MEA and the stronger Connection to the membrane, through which a washing out of the electrolyte from the membrane by liquid accumulating product water is significantly reduced. While known hybrid membranes Although based on purely hydrophilic silicates, an improved mechanical and have thermal stability, but have this a deteriorated electrolyte uptake and affinity, for example, to phosphoric acid. this Disadvantage shows a with the invention hydrophilic modified silicate derivative not treated membrane.

worin R₁, R₂, R₃, R₄, R₅ und R₆ der beiden Silikatgruppen bzw. Silikatoligomeren unabhängig voneinander eine C1- bis C5-Alkylgruppe oder C1- bis C5-Alkoxygruppe bedeuten, insbesondere eine C1- bis C3-Alkylgruppe oder C1- bis C3-Alkoxygruppe, besonders bevorzugt eine C2-Alkyl- oder C2-Alkoxygruppe;
A eine Bindung oder ein Sauerstoffatom bedeutet;
n, n1 und n2 unabhängig voneinander eine ganze Zahl von 1 bis 20 bedeuten, insbesondere eine ganze Zahl von 1 bis 10, besonders bevorzugt von 1 bis 5;
HC eine Bindung oder eine substituierte oder unsubstituierte, verzweigte oder unverzweigte C1- bis C20-Kohlenwasserstoffgruppe bedeutet, insbesondere eine C1- bis C15-Kohlenwasserstoffgruppe, besonders bevorzugt eine C5- bis C10-Kohlenwasserstoffgruppe;
X eine polare Säuregruppe bedeutet und
R eine substituierte oder nicht substituierte homo- oder heteroaromatische Gruppe bedeutet oder die Bedeutung der Reste R₁, R₂, R₃ und R₄, der Kohlenwasserstoffgruppe HC oder der polaren Säuregruppe X hat.According to a preferred embodiment of the invention, the silicate derivative has two hydrophobically substituted silicate groups or silicate oligomers, at least one terminal polar acid group and a substituted or unsubstituted hydrocarbon group linking together the silicate groups or oligomers. In particular, the silicate derivative corresponds to the general formula (I) or (II)

in which R ₁ , R ₂ , R ₃ , R ₄ , R ₅ and R _{6 of} the two silicate groups or silicate oligomers independently of one another denote a C 1 - to C 5 -alkyl group or C 1 - to C 5 -alkoxy group, in particular a C 1 - to C 3 -alkyl group or C1 to C3 alkoxy group, more preferably a C2-alkyl or C2-alkoxy group;
A represents a bond or an oxygen atom;
n, n1 and n2 independently of one another denote an integer from 1 to 20, in particular an integer from 1 to 10, particularly preferably from 1 to 5;
HC represents a bond or a substituted or unsubstituted, branched or unbranched C1 to C20 hydrocarbon group, in particular a C1 to C15 hydrocarbon group, particularly preferably a C5 to C10 hydrocarbon group;
X means a polar acid group and
R represents a substituted or unsubstituted homo- or heteroaromatic group, or the meaning of the radicals R _1, R _2, R ₃ and R _4, the hydrocarbon group or the HC acid polar group X has.

in the Case of formula (II) is assumed that when HC a Bond is, at least one of the two A also a bond is.

by virtue of The relatively short-chain silicate oligomer groups become one in long-chain Silicates observed demixing tendency of the membrane polymer and the silicate counteracted, leading to deterioration the mechanical stability of the membrane leads. Are the chain lengths of silicate derivatives compared those of the organic membrane-forming polymer are kept short, in particular the Kettelängen n or n1 and n2 in the above frame chosen, so will the positive properties of the electrolyte storage associated with those of improved mechanical stability.

As already mentioned, the acid group (X in formula I or II) is a protic acid, especially a phosphonic acid, sulfonic acid, carboxylic acid or hydroxide group. Preferably, the acid group is a carboxylic acid group -CO ₂ H or a phosphonic acid group PO ₃ H ₂ .

To A preferred embodiment of the invention means in formula I or II the radical R is a substituted or unsubstituted N-heteroaromatic group, in particular a substituted or unsubstituted benzimidazole group. In this way, the non-covalent attachment of the silicate derivative to typical membrane materials, which are often N-heteroaromatic polymers, z. As polyazoles and polyphosphazenes, is amplified. In addition, R is preferably substituted by an acid group, in which it is like the acid group X in particular to a phosphonic acid, sulfonic acid, carboxylic acid or hydroxide group, preferably a carboxylic acid or phosphonic acid group.

The radicals R ₁ , R ₂ , R ₃ , R ₄ , R ₅ and R ₆ in formula I and II are preferably each an alkoxy group, in particular in each case an ethoxy group.

The at least one polymer of the membrane is advantageous Form the invention an N-heteroaromatic polymer, in particular is selected from the group of polyazoles and polyphosphazenes. Particularly suitable representatives of these polymer classes are polybenzimidazoles, Polypyridines, Polypyrimidines, Polyimidazoles, Polybenzothiazoles, Polybenzoxazoles, Polyoxadiazoles, polyquinoxalines, polythiadiazoles, poly (tetrazapyrene), Polyvinylpyridines and polyvinylimidazoles. All These polymers are characterized by their suitability, a large Amount of acid, especially by hydrogen bonding, stable at the nitrogen atoms present in the polymer chain tie.

It it is particularly preferred that the membrane-forming polymer [2,2 '- (m-phenylene) -5,5'-benzimidazole] (PBI) according to Formal (III) and / or Poly (2,5-benzimidazole) (ABPBI) according to formula (IV) or a derivative of this includes.

Across from On the one hand, these polymers have the advantage over many other polyazoles a high conductivity in the acid-doped state and also allow a very good and fast membrane production. suitable Derivatives of PBI and ABPBI include, for example, sulfonic acid, Sulfonate, phosphonic acid or phosphonate derivatives, wherein these groups are attached at any position of the benzene ring could be. In particular, by these functional groups achieved an improvement in proton conductivity. Preferably These groups can be modified by subsequent modification of ABPBI, for example by sulfonation.

The at least one (first) electrolyte is in particular selected from the group comprising acids and their salts, in particular phosphoric acid, phosphinic acid, phosphonic acid, alkyl or aryl phosphates, sulfuric acid, sulfonic acid, alkyl- and arylsulfonic acids, such as methanesulfonic acid and phenylsulfonic acid, alkyl- and arylphosphonic acids , Nitric, hydrochloric, formic, acetic, trifluoro acetic acid, perchloric acid, heteropolyacids, hexafluoro- glutaric acid (HFGA), squarric acid (SA); and bases, especially alkali and alkaline earth hydroxides such as potassium hydroxide, sodium hydroxide, lithium hydroxide; nitrogen-containing heterocycles, in particular imides, imidazoles, triazoles, perfluorosulfonimides, ionic liquids, in particular 1-butyl-3-methylimidazolium trifluoromethanesulfonite; and derivatives of these. Of these, the use of phosphoric acid, sulfuric acid, sulfonic acid, phosphonic acid and perchloric acid is preferred, in particular of phosphoric acid.

The At least one silicate derivative can be made according to a particular embodiment in a mixture with the at least one electrolyte, wherein the silicate derivative in particular a mass fraction of 1 to 60 Wt .-%, preferably from 2 to 40 wt .-%, particularly preferably from 5 to 30 wt .-% based on this (electrolyte) mixture.

With Advantage has a polymer electrolyte membrane according to the Invention a content of 1 to 90 wt .-%, in particular from 2 to 80 wt .-%, preferably from 5 to 75 wt .-%, of the at least one Silicate derivatives based on the sum of the masses of the dry Membrane and the silicate derivative.

The Introduction of the silicate derivative, in particular the mixture of Electrolyte and the silicate derivative, in the membrane can either subsequently for membrane production by impregnation be done, the membrane about as by pouring, Dipping, spraying or the like in contact with the Silicate derivative or the electrolyte mixture is brought, wherein the Electrolytes by non-covalent interactions with the Bind polymer material of the membrane. Result of the subsequent Impregnation is essentially superficial Connection of the Silkate derivative to the membrane.

Prefers However, it is envisaged that the Silkatderivat already the so-called Ziehlösung added for the preparation of the membrane and intense is mixed with this and then the production of the membrane, about by casting and / or knife. In this way is a more even distribution of the silicate in the membrane and thus a higher proton conductivity and achieved mechanical stability.

In Both of these processes, the silicate derivative undiluted or in solution, the (first) electrolyte, For example, phosphoric acid, in each case be added can.

There for the function of a fuel cell MEA the presence of electrolyte in the region of the three-phase boundary (catalyst - reaction gases - electrolyte) crucial for the removal of the resulting at the anode Protons is, it is preferably provided that the inventive Silicate derivative as an electrolyte or as an electrolyte additive to a other electrolyte also in the area of the electrodes of the MEA available is by wetting them and / or incorporated into them. In the construction of the electrodes while a maximum utilization of To seek catalyst activity by, if possible the entire electrode volume with electrolyte or the electrolyte mixture developed. The introduction of the silicate derivative or the electrolyte mixture in the electrode can already in the catalyst-containing paste at the preparation of the electrode carried out or subsequently by impregnation of the electrode outside Assembly of the MEA. Equally possible is the processing the electrolyte-free electrodes with an already electrolyte-laden Membrane, wherein the transfer of the electrolyte from the Membrane into the electrode by means of a hot pressing step, with which the MEA structure is generated, takes place or during a run-in cycle of the fuel cell after assembly of the MEA.

The Invention relates according to a further aspect a fuel cell having at least one, a membrane electrode assembly according to the present invention Single cell.

The Invention will be described below in embodiments the accompanying figures explained in more detail. Show it:

1 a fuel cell and

2 a sectional view of a single cell of the fuel cell 1 with a membrane-electrode assembly;

In 1 is a fuel cell 10 represented a plurality of series-connected single cells 12 includes, of which a single in 2 is shown in more detail. Every single cell 12 has a membrane-electrode unit 14 (MEA), each containing a proton-conducting polymer electrolyte membrane 16 includes, and two adjacent to the two outer membrane surfaces electrodes 18 . 20 namely an anode 18 and a cathode 20 , Furthermore, the single cells include 12 between every two MEAs 14 arranged bipolar plates 22 , the two-sided the MEA composite 14 Contact electrically and ensure the supply of the process gases and the discharge of the product water. In addition, they separate the individual MEA 14 in the fuel cell stack 10 gas-tight from each other. The bipolar plates 22 have a plurality of inner transport channels, which serve to supply the reaction gases (in the case of the anode hydrogen and in the case of the cathode oxygen or air) and the cathode side also the removal of the product water. Materials for sealing and stabilizing the MEA 14 are not shown.

The fuel cell 10 also has hydrogen supply lines 24 on which the bipolar plates 22 Feed hydrogen gas. An inner anode-side channel system of the bipolar plates 22 directs the supplied hydrogen H _{2 to} the anodes 18 the membrane electrode units 14 to where it is oxidized to protons H ⁺ . About hydrogen discharges 26 connected to another anode-side internal channel system of the bipolar plates 22 In connection, the unused residual hydrogen (and through the membrane 16 diffused product water) and recycled. Furthermore, air supply lines 28 provided with which air and thus oxygen to the bipolar plates 22 and from there via a cathode-side channel system of the same the cathode 20 is forwarded. Via another cathode-side channel system of the bipolar plates 22 and connected air discharges 30 the discharge of the remaining air and the product water takes place. The stack of the single cells 12 becomes laterally from end plates 32 limited. Not shown are other components of the fuel cell 10 For example, a cooling system, control electronics, pumps, valves and the like.

For commissioning, the fuel cell 10 and with it the single cells 12 subjected to the operating gases and connected to an electrical consumer. From the presence of a current of 0.2 A / cm ² , the operating gases are tracked in one of the current requirement stoichiometric ratio.

How out 2 As can be seen, the two electrodes comprise 18 . 20 each a microporous catalyst layer 34 containing the polymer electrolyte membrane 16 contacted on both sides. The catalyst layers 34 contain as reactive centers of the electrodes, a catalytic material, which is usually a noble metal, such as platinum, iridium 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. In the example shown, the electrodes 18 . 20 designed as gas diffusion electrodes, which also each have a gas diffusion layer (GDL for gas diffusion layer) 36 include, at the respective outer, of the polymer membrane 16 opposite surfaces of the catalyst layer 34 connect. Function of the GDL 36 it is, a uniform flow of the catalyst layers 34 to ensure with the reaction gases oxygen or air on the cathode side and hydrogen on the anode side.

The polymer electrolyte membrane 16 has - as usual in high-temperature fuel cells - a polymer component and a complexed bound to this present electrolyte component. The latter imparts the polymer electrolyte membrane 16 their proton conductivity. According to the invention, the polymer component contains at least one membrane-forming polymer, in particular an N-heteroaromatic polymer, which in a preferred embodiment comprises PBI or a blend of ABPBI and PBI.

According to the invention, the electrolyte component comprises at least one silicate derivative and at least one further electrolyte, preferably polyphosphoric acid. The silicate derivative is characterized by one or two hydrophobically substituted silicate groups or by one or two hydrophobically substituted silicate oligomers having a high affinity for the membrane-forming polymer, as a result of which, on the one hand, a good anchoring on and in the membrane 16 and on the other hand, stabilization thereof is achieved. In the case of two silicate groups or oligomers, these may be linked to one another by a hydrocarbon radical. This may optionally be substituted with suitable groups which further improve, for example, the anchoring of the silicate derivative in the membrane polymer. Furthermore, the silicate derivative has at least one protic acid group, by which the proton conductivity of the membrane 16 is directly increased and the required content of the other electrolyte, in particular phosphoric acid, is lowered. Preferred structures of the silicate derivative according to the invention have already been explained above with reference to the general formulas (I) and (II). Particularly preferred examples of suitable silicate derivatives are shown in the following formulas (V) to (VIII):

The Compound (V) is characterized by a hydrophilic sulfonic acid group and on the other side by a hydrophobic C5 to C10 alkyl group out. This compound is believed to be hydrophilic proton-conducting channels within the hydrophobic polymeric material forms the membrane on which they with their hydrophobic hydrocarbon chains does not bind covalently. This will be a particularly good one Proton conductivity of the polymer electrolyte membrane obtained. In contrast, the silicate derivatives (VI) and (VIII) by a total of relatively hydrophilic compounds that have a strong interaction enter with the electrolyte and even make a significant contribution to form proton conductivity. The advantage of the connection (VII) can be seen in the benzimidazole group, which is particularly intense interacts with the membrane polymer and thus very stable to this binds. This will be a particularly strong backing of the Electrolyte, in turn, with the phosphonic acid group interacts, in the polymer electrolyte membrane also under condensation conditions achieved.

Preferably, the at least one silicate derivative is also in the region of the electrodes 18 . 20 , in particular their catalyst layers 34 in front.

The MEA according to the invention 14 or the fuel cell according to the invention 10 is characterized by the fact that the silicate derivative and the other electrolyte component extremely stable in the membrane 16 or the electrodes 18 . 20 are bound. Thus, even under condensation conditions, in which the product water is obtained in liquid form, there is no leaching of the electrolyte on the membrane 16 or the electrodes 18 . 20 or even a complete discharge from the single cell 12 , As a result, the MEA 14 a very good long-term stability.

LIST OF REFERENCE NUMBERS

10

fuel cell

12

single cell

14

Membrane-electrode assembly (MEA)

16

Polymer electrolyte membrane

18

electrode (Anode)

20

electrode (Cathode)

22

bipolar

24

Hydrogen supply

26

Hydrogen discharge

28

air supply

30

air discharge

32

endplate

34

catalyst layer

36

Gas diffusion layer (GDL)

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]
• - US 6124060 [0008]
• - DE 102006010705 A1 [0008]
• DE 102004044760 A1 [0008]
• DE 102004063457 A1 [0008]
• - DE 102005023897 A1 [0008]
• - DE 102006061779 A1 [0008]
• WO 00/59060 A1 [0008]

Claims (14)

Membrane electrode unit ( 14 ) for fuel cells ( 10 ) comprising a polymer electrolyte membrane ( 16 ) formed of at least one polymer, two electrodes sandwiching the polymer electrolyte membrane ( 18 . 20 ) and at least one electrolyte containing at least the polymer electrolyte membrane ( 16 ), characterized in that the membrane-electrode assembly (MEA) further comprises at least one silicate derivative comprising the polymer electrolyte membrane ( 16 ) and / or the electrodes ( 18 . 20 ) is wetted and / or incorporated into this, which has at least one hydrophobically substituted silicate group or at least one hydrophobically substituted silicate oligomer and at least one polar acid group. Membrane electrode unit ( 14 ) according to claim 1, wherein the silicate derivative has two hydrophobically substituted silicate groups and / or two hydrophobically substituted silicate oligomers and a substituted or unsubstituted hydrocarbon group linking together the silicate groups or oligomers. Membrane electrode unit ( 14 ) according to claim 1 or 2, wherein the silicate derivative of the general formula (I) or (II)

wherein R ₁ , R ₂ , R ₃ , R ₄ , R ₅ and R ₆ independently represent a C 1 to C ₅ alkyl group or C 1 to C 5 alkoxy group; A represents a bond or an oxygen atom; n, n1 and n2 independently represent an integer from 1 to 20; HC represents a bond or a substituted or unsubstituted, branched or unbranched C1 to C20 hydrocarbon group; X is a polar acid group and R is a substituted or unsubstituted homo- or heteroaromatic group or has the meaning of the radicals R ₁ to R ₆ , the hydrocarbon group HC or the polar acid group X has. Membrane electrode unit ( 14 ) according to any one of the preceding claims, wherein the polar acid group is a phosphonic acid, sulfonic acid, carboxylic acid or hydroxide group, in particular a phosphonic acid group. Membrane electrode unit ( 14 ) according to one of claims 3 or 4, wherein R represents a substituted or unsubstituted N-heteroaromatic group, in particular a substituted or unsubstituted benzimidazole group. Membrane electrode unit ( 14 ) according to claim 5, wherein R is substituted with an acid group, in particular with a phosphonic acid, sulfonic acid, carboxylic acid or hydroxide group. Membrane electrode unit ( 14 ) according to one of claims 3 to 6, wherein R ₁ , R ₂ , R ₃ , R ₄ , R ₅ and R _{6 are} each an alkoxy group, in particular in each case an ethoxy group. Membrane electrode unit ( 14 ) according to any one of claims 3 to 7, wherein the hydrocarbon group is a C1 to C20 alkyl group, in particular a straight-chain C1 to C15 alkyl group. Membrane electrode unit ( 14 ) according to one of the preceding claims, wherein the polymer is an N-heteroaromatic polymer and in particular is selected from the group of polyazoles and polyphosphazenes; preferably the polybenzimidazoles, polypyridines, polypyrimidines, polyimidazoles, polybenz hiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines, polythiadiazoles, poly (tetrazapyrene), polyvinylpyridines, polyvinylimidazoles. Membrane electrode unit ( 14 ) according to claim 9, wherein said at least one further N-heteroaromatic polymer is poly [2,2 '- (m-phenylene) -5,5'-benzimidazole] (PBI) and / or poly (2,5-benzimidazole) (ABPBI ). Membrane electrode unit ( 14 ) according to one of the preceding claims, wherein the at least one electrolyte is selected from the group comprising acids and their salts, in particular phosphoric acid, phosphinic acid, phosphonic acid, alkyl or aryl phosphates, sulfuric acid, sulfonic acid, alkyl and aryl sulfonic acids, such as methanesulfonic acid and phenylsulfonic acid, Alkyl and arylphosphonic acids, nitric acid, hydrochloric acid, formic acid, acetic acid, trifluoroacetic acid, perchloric 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, in particular imides, imidazoles, triazoles, perfluorosulfonimides, ionic liquids, in particular 1-butyl-3-methyl-imidazolium trifluoromethanesulfonite, and derivatives of these. Membrane electrode unit ( 14 ) according to one of the preceding claims, wherein the at least one silicate derivative is in a mixture with the at least one electrolyte. Membrane electrode unit ( 14 ) according to claim 12, wherein at least one silicate derivative has a mass fraction of 1 to 60 wt .-% in a mixture, in particular from 2 to 50 wt .-%, preferably from 5 to 30 wt .-%. Fuel cell ( 10 ) with at least one, a membrane electrode assembly ( 14 ) according to one of claims 1 to 12 having single cell ( 12 ),

Images