MXPA00010143A - Membrane-electrode unit for a fuel cell - Google Patents

Abstract

The invention relates to a membrane electrode unit for a fuel cell, comprising an optionally catalyst-coated anode, an optionally catalyst-coated cathode and a proton conductor located between said anode and said cathode. The proton conductor consists of a microfibre-fleece material which has been impregnated with an electrolyte to the point of saturation. The fleece material is chemically inert in relation to the electrolyte at temperatures of up to +200°C and in oxidising and reducing conditions and weighs 20 to 200 g/m2. The thickness of the fleece is less than 1 mm and the pore volume is 65 to 92%.

Description

MEMBRANE ELECTRODE UNIT FOR A FUEL CELL Technical Field The present invention relates to a membrane electrode unit for a fuel cell, which includes an anode optionally coated with catalyst, a cathode optionally coated with catalyst, a proton conductor located between the anode and the cathode.

State of the art This unit is known. It causes a separation of ionic and electrical form with the reaction of reaction gases containing oxygen and hydrogen or usual components in a fuel cell by means of a direct chemical conversion in electrical energy. The nature and operation of the different types of fuel cells is described by K.-D. Kreuer und J. Maier in "Spe trum. Der issenschaft" (July 1995), 92-96. The electrodes must be very good elect roconductors (electrical resistance of approximately 0.1 O_1). They must catalyze the Necessary reaction at the junction with the electrolyte surface. The electrolyte must have a high ionic conductivity, preferably with less electronic conductivity where possible. In addition, they should preferably be as insensitive as possible to the exhaust gases. All materials must be chemically inert among themselves, as well as with the other reagents, undesired contacts should not be allowed under the strongly oxidizing conditions at the cathode as well as the strongly reducing conditions at the anode. In order to interconnect several individual cells to the batches of cells, an adequate mechanical load capacity of the fixed constituents contained in the individual cells must be given. The material, process costs, life span and environmental compatibility of the cell components play an important role.

For operating temperatures of 80 to 90 ° C, the polymeric membranes of the proton conductor have generally become accepted with the fuel cells. They unite in themselves the capacity of the liquids to give the molecules and protons a free mobility and the capacity of the solids to be stable. Ideally, this This requirement is fulfilled by a monomeric membrane based on polytetrafluoroethylene with per-fluorovinyl ether side chains. This material consists of hydrophobic and hydrophilic areas, however, it decomposes in the presence of water forming a gel-like membrane, stably. The hydrophobic main chain of the polymer is very stable to oxidation and reduction and gives the membrane in a poured condition a structure in a stable manner. The side chains that contain sulfoacid, liquid type, hydrophilic, poured in water, allow very good proton conductivity. The pore size of few nonameters corresponds to the measurement of a few water molecules. The presence of water allows a high mobility of protons in channels and pores. In this cation exchanger, as described in the literature, its high price is unfavorable, due to the complex manufacturing process. In addition, its elimination or rejection also presents ecological problems. In the operation of the fuel cell, the membrane is bent to drain, in particular if combustion air is supplied by means of air flow from the cell, however due to the characteristic of the proton stream to transport water molecules from the anode to the cathode.

The upper limit of the thermal stability of the known sheet as well as its sulfonic acid groups is 90 to 100 ° C, the morphological structure begins to collapse at higher temperatures. Therefore the known membrane of fluorinated monomer is blocked at higher operating temperatures than the independent sheet, so that it is unsuitable for the following operations: a) Application of hydrogen from methanol reformed with a temperature of more than 130 ° as fuel (this procedure is described in U. Benz et al., "Spektrum der Wissenschaft" (July 1995) 97-104); b) Use at temperatures of more than 130 ° C typically 150-200 ° C for direct oxidation of methanol at the anode.

DESCRIPTION OF THE INVENTION The task of the invention is to prepare a membrane electrode unit for a fuel cell, having the mentioned favorable characteristics of the monomer membrane fluorinated with the following characteristics: 1. Decrease in process costs opposite to the polymer membrane of the state of the art. 2. Decrease of contaminants with poisoning. 3. Stability at a temperature of up to 200 ° C in order to reduce the effect of catalyst pollutants, the ability to use hydrogen from methanol reformed as fuel, the internal reformation of methanol as well as the direct oxidation of methanol.

This function is solved according to the invention with a membrane electrode unit as set forth with the features of claim 1. The dependent rei indications refer to advantageous arrangements. According to the invention, it is proposed that the proton conductor is formed by a microfiber wool material, which is stamped with an electrolyte until saturation, whereby the wool material is chemically inert at temperatures up to 200 ° C. as well as under opposite oxidizing and reducing conditions, at electrolytes, so that the wool material accounts for a weight of 20 to 200 g / cm2, whereby the thickness of wool accounts for a thickness of 1 mm maximum and thus accounts for a pore volume of 65 to 92%. The average pore radius of the microfiber wool material accounts for 20 nm to 10 μm. With the appropriate article of the invention, the structure of the microfiber wool material guarantees the mechanical stability of the membrane, so that the electrolyte must no longer comply with this task. Therefore, the costs of the material for the membrane can be reduced by up to 90% compared, for example, with the costs for the production of an independent, dimensioned membrane of a perfluorinated monomer. The microfiber wool material can be filled with perfluorinated monomer, the perfluorinated monomer can be polytetrafluoroethylene with sulphonated perf luorvinyl ether side chains. As an alternative, it is offered to wet the microfiber wool material with a solution of aqueous sulfuric acid of one to 5 molar or concentrated phosphoric acid. It is also possible to use hydrated zirconium phosphate and diacid ammonium phosphate.

The following examples are to clarify the invention with respect to the performance of the fuel cell (ionic conductivity) of a pure polymer membrane to one of fluorinated monomer, without using more expensive materials.

EXECUTION OF THE INVENTION The base materials described in the following are the most common examples: Wool material: polysulfone fibers with rectangular cross section (width from 6 to 13 μm, height from 17 to 2.4 μm). Mechanical characteristics of the polysulfone material: melting range: 343 to 399 ° C. Tensile strength: 70 MPa Elongation at break: 50 to 100% Module E; 2.4 GPa Bending temperature under a load of 1.8 MPa: 174"C Manufacture of fibers: mix a solution of polysulfone in methylene chloride in the electrostatic field For example, an apparatus according to DE-OS 26 20 399 can be used. The fibers are collected in a textile collector, linear moved continuously.

Characteristics of the wool material: Weight: 150 g / m2 Thickness (compressed): 0.05 mm Thickness (soaked with electrolyte): 0.18 mm Pore average radius in uncompressed condition: 8 μm Pore average radius in the compressed condition: 4 μm Pore volume: 83% The temperature stability of the membrane according to the invention becomes essentially, nor is there anything to be opposed, that of the wool material and therefore has its limit only at about 174 ° C of the pure polysulfone fiber material . Due to the mechanical connections between the fibers in the wool material the mechanical stability is further increased up to temperatures of 250 ° C. In this way a high operating temperature of the fuel line is possible, which can clearly reduce the contamination of the catalyst.

Example 1 The microfiber wool material is overlaminated with liquid Nafion, a commercial brand of perfluorinated monomer from DuPont, in a glass frit with a diameter of 16 mm. The liquid phase is sucked through the negative pressure in the pore structure of the wool material. For the removal of the solvents in the membrane wetted in this way, it is treated at 60 ° C in a drying oven. Storage until subsequent treatment is possible in Fairy water.

Elos 2 to 4. The microfiber wool material is moistened with aqueous solutions of sulfuric acid of three different molarities similar to Example 1, so however for the reduction of the necessary viscosity the sulfuric acid is heated up to about 70 ° C. Without obtaining another result, the wool material can be boiled directly in the acid heated at 70 ° C for a few minutes.The storage of this received membrane takes place in the appropriate humidification medium.

For the membranes prepared in this way with a method according to DIN 53 779 of March 1979, the following specific conductivities were determined: Example 5 of the table represents an example of comparison of appropriate measurements of a self-supporting polymer membrane 125 μm thick representing the state of the art of perfluorinated monomer (Nafion-117, DuPont). The specific conductivity values clearly show that the membrane according to the invention of a simpler and mechanically more stable construction, essentially more advantageous than pure Nafion, it is possible the performance of the fuel cell according to the state of The technique . For example, in comparison to a Nafion membrane poured of 125 μm thick, the wool materials moistened with the electrolyte, used in Examples 1 to 4, are twice as thick. The performance of the fuel cell, which results from the voltage and amperage, can be achieved not only by a higher concentration of acid, that is, higher specific conductivity S / cm, but also by the degradation of the diffusion inhibition by the use of thinner wool materials. For example, the appropriate curves of room temperature / ambient temperature are shown in the Figure, which correspond to Examples 1, 3 and 5. It turns out that, compared to the state of the art (Example 5), strokes are achieved comparable curve through the membranes according to the invention. The effects mentioned above with respect to high cell performance by a high concentration of acid or by thinner wool material during will have an effect on this representation by a change of the curves in the positive direction of the ordinates. One reason for the high temperature stability of the wool material is also to be useful for applications over 100 ° C with concentrated phosphoric acid electrolytes.

Claims (7)

1. CLAIMS 1. Membrane electrode unit for a fuel cell, comprising an anode optionally coated with catalyst, a cathode optionally coated with catalyst and a proton conductor located between the anode and the cathode, the proton conductor consists of a material of microfiber wool that has been impregnated with an electrolyte at the saturation point, the wool material is chemically inert in relation to the electrolyte at temperatures up to + 200 ° C and in oxidizing and reducing conditions and weighs 20 to 200 g / m2. The thickness of the wool is less than 1 mm and the pore volume is 65 to 92%.
2. 2. The membrane electrode unit according to claim 1, characterized in that the microfiber wool material shows an average pore radius of 20 nm to 10 μm.
3. 3. The membrane electrode assembly according to claim 1 or 2, characterized in that the icrofiber wool material is filled with perfluorinated monomer.
4. 4. The membrane electrode unit according to claim 3, characterized in that the perfluorinated monomer is polytetraf luoret i leno sul furado. The membrane electrode unit according to claim 1 or 2, characterized in that the microfiber wool material is moistened with an aqueous solution of sulfuric acid 1 to
5. 5 Molar.
6. 6. The membrane electrode unit according to claim 1 or 2, characterized in that the microfiber wool material is moistened with concentrated phosphoric acid. The membrane electrode unit according to claim 1 or 2, characterized in that the microfiber wool material is moistened with hydrated zirconium phosphate or diacid ammonium phosphate.