AU3704099A

AU3704099A - Membrane-electrode unit for a fuel cell

Info

Publication number: AU3704099A
Application number: AU37040/99A
Authority: AU
Inventors: Ulrich Stimming
Original assignee: Carl Freudenberg KG
Current assignee: Carl Freudenberg KG
Priority date: 1998-05-18
Filing date: 1999-04-01
Publication date: 1999-12-06
Anticipated expiration: 2019-04-01
Also published as: CA2327520A1; AU738679B2; DE19821978A1; JP2002516472A; EP1088361A1; ZA200001232B; KR100392921B1; BR9910535A; WO1999060650A1; DE19821978C2; CN1294762A; KR20010071286A

Description

1 Applicant: Carl Freudenberg, 69469 Weinheim 5 15 March 1999 Ho/sta 10 Membrane electrode unit for a fuel cell Description Technical field 15 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. 20 Current state-of-the-art A unit of this kind is well-known. It's function is to separate the ionic and electrical paths in the reaction between the reaction gases or fluid components containing hydrogen and oxygen in a fuel cell for the purpose of directly converting chemical 25 into electrical energy. The nature and functioning of various types of fuel cells has been described by K.-D. Kreuer and J. Maier in "Spektrum der Wissenschaft" (July 1995), 92-96. 30 The electrodes must be very good electron conductors (electrical resistance around 0.1 7 cm'). Their purpose - together with the electrolyte surface - is to catalyse the 2 desired reactions. The electrolyte must have a high ionic conductivity whilst having the lowest possible electron conductivity. Furthermore, it must be as impermeable as possible to the product gases. All materials should be chemically inert with respect to one another and the reaction components, that is, they should not form any undesired 5 compounds in the strongly oxidising conditions at the cathode or the strongly reducing conditions at the anode. To be able to interconnect several single cells into cell stacks, the solid components present within the single cells should have sufficient mechanical strength to do so. 10 Furthermore, material and process costs, service life and environmental compatibility of the cell components also play an important role. For operating temperatures of 80 to 90*C, proton conducting polymer membranes have proven the most successful for fuel cells. They combine the fluid-like ability to 15 provide free mobility for the molecules and protons with the solid-like ability to remain inherently stable. These requirements are met almost ideally by a perfluorinated ionomer membrane based on polytetrafluoroethylene with sulfonated perfluoro vinylether side chains. This material comprises both hydrophobic and hydrophilic regions which in the presence of water separate out to form a gel-like but 20 nevertheless inherently stable membrane. The hydrophobic backbone chain of the polymer is very resistant to oxidation and reduction and provides the membrane with an inherently stable framework even in the swelled state. It is the swelled hydrophilic, liquid-like, sulphonic acid containing side chains in the water that provide the very good proton conductivity. The pore size of a few nanometers corresponds to the 25 dimension of just a few water molecules. The presence of water facilitates the high mobility of the protons in the channels and pores. The disadvantage of this cation exchanger, as already described in the cited literature source, is its high price as a result of the expensive manufacturing process required. 30 Furthermore, its disposal or recycling poses ecological problems.

3 When operating these fuel cells such membranes have a tendency to dry out, in particular when the combustion oxygen is fed to the cell by means of an air current but also because of the particular property of the proton current to transport water 5 molecules from the anode to the cathode. The upper limit of the thermal stability of the known foil or its sulphonic acid groups is about 90 to 100 *C; at higher temperatures the morphological structure begins to break down. 10 The known perfluorinated ionomer membrane therefore deteriorates as an independent foil at higher operating temperatures thus making it unsuitable for the following applications: 15 a) when hydrogen from reformed methanol is used as the fuel at temperatures above 130 *C (this process is described by U. Benz et al,. "Spektrum der Wissenschaft" (July 1995) 97-104); b) when used at temperatures above 130 *C, typically 150 - 200 *C, for direct 20 oxidation of methanol at the anode. Description of the invention 25 The task of the present invention is to make available a membrane electrode unit for a fuel cell that complements the previously described advantageous properties of perfluorinated ionomer membranes with the following properties: 30 1. Reduction in production costs compared to the state-of-the-art polymer membrane 4 2. Reduction in pollutants when disposing of the unit 3. Temperature resistance up to 200*C in the interests of reducing the effect of catalyst poisons, being able to use hydrogen from reformed methanol as a fuel, the 5 internal reforming of methanol or direct oxidation of methanol. The present invention solves this task for a membrane electrode unit as per the preamble by means of the characterising features of claim 1. The subclaims refer to advantageous embodiments. 10 The invention provides for a proton conductor consisting 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/m 2 . 15 The thickness of the fleece is less than 1 mm and the pore volume is 65 to 92 %. The median pore radius of the microfibre fleece material needs to be between 20 nm and 10 im. 20 In the object in accordance with the present invention, the fleece supporting structure of the microfibre fleece material provides the mechanical stability of the membrane so that the electrolyte no longer needs to fulfill this task. This can reduce the material costs for the membrane by up to 90 %, compared for example to the costs required to manufacture an equivalently dimensioned, independent membrane made of 25 perfluorinated ionomer. The microfibre fleece material can be filled with perfluorinated ionomer where the perfluorinated ionomer could be a polytetrafluoroethylene with sulphonated perfluoro vinylether side chains. A possible alternative would be to impregnate the microfibre 30 fleece material with one to 5 molar, aqueous sulphuric acid or with concentrated 5 phosphoric acid. Furthermore, it would also be possible to use hydrated zirconium phosphate and and ammonium dihydrogen phosphate. The following examples serve to illustrate that the invention, in respect to the power 5 output of the fuel cell (ionic conductivity), compares favourably with a pure polymer membrane made of perfluorinated ionomer without the need to use the costly materials required up to now. 10 Implementation of the invention All examples use the same base materials which will now be described: 15 Fleece material: polysulphone fibres with a rectangular cross-section (width 6 to 13 pm, height 1.7 to 2.4 pm). Mechanical properties of the polysulphone material: melting range: 343 to 399 *C Tensile strength: 70 MPa 20 Fracture strain: 50 to 100% E modulus: 2.4 GPa Bending temperature at a load of 1.8 MPa: 174 *C Production of the fibres: spinning from a solution of polysulphone in methylene 25 chloride in an electrostatic field. This could for example be done using a device as per DE-OS 26 20 399. The fibres are collected on a linear, continuously moving, textile carrier. Fleece properties: 30 Weight: 150 g/m 2 Thickness (compressed): 0.05 mm 6 Thickness (impregnated with electrolyte): 0.18 mm Median pore radius in the uncompressed state: 8 jim Median pore radius in the compressed state: 4 pm Pore volume: 83 % 5 The temperature resistance of the membrane in accordance with the invention, other things being equal, is essentially determined by the fleece material and thus breaks down at about 174 *C for pure polysulphone fibre material. As a consequence of the mechanical linkage of the fibres in the fleece material the mechanical stability can be 10 extended even to temperatures of 250 *C. This allows the fuel cell to be operated at high temperatures which can significantly reduce the amount of contamination of the anode catalyst. 15 Example 1: The microfibre fleece material was covered with a layer of liquid Nafion, a commercial perfluorinated ionomer from the DuPont company, in a 16 mm diameter fitted glass. By applying a light partial vacuum, the liquid phase was drawn into the 20 pore structure of the fleece material. To remove all solvent, the membrane impregnated in this manner was treated at 60 *C in a drying chamber. The membrane can then be stored in distilled water until further processing. 25 30

V/

7 Examples 2 to 4: 5 The microfibre fleece material was impregnated with three different molar, aqueous sulphuric acid solutions analogous to example 1 however to reduce the viscosity, the sulphuric acid was heated to about 70*C. The fleece material can be boiled in the 70 "C hot acid for several minutes without affecting the results. 10 It is advantageous to store the membrane obtained in this way in the actual impregnation medium used. Using the method described in DIN 53 779, dated March 1979, the following specific 15 conductivities were obtained for the membranes prepared in this way: Example measurement temperature "C specific conductivity S/cm 1 23 0.016 2 18 0.031 1 M H 2 S0 4 3 18 0.041 3 M H 2 SO4 4 18 0.080 5 M H 2 S0 4 5 25 0.070 (reference) Example 5 in the table represents a reference example for corresponding measurements on a 125 pm thick, state-of-the-art self-supporting polymer membrane *RA4,, 0 made of perfluorinated ionomer (Nafion-1 17, DuPont).

8 The specific conductivity values (S/cm) clearly show that the membrane in 5 accordance with the invention, which is considerably less expensive, of simpler construction and mechanically more robust than pure Nafion, can be used to operate a fuel cell at the typical power output for a state-of-the-art fuel cell. If used at temperatures above 100*C, concentrated phosphoric acid would be suitable as the ion conductor. 10 In comparison to other swollen Nafion membranes of for example 125 pm thickness, the fleece material impregnated with electrolyte used in examples 1 to 4 is twice as thick. 15 The power output of the fuel cell, which is given by the product of the voltage and the current intensity, can not only be achieved by using higher acid concentrations, i.e. higher specific conductivities S/cm, but also by decreasing the diffusion resistance by using thinner fleece material. 20 To illustrate this, Figure 1 shows the respective current/voltage curves at room temperature corresponding to the the examples 1, 3 and 5. It can be seen that, compared to the state-of-the-art membrane (example 5), a comparable curve can be obtained for the membrane in accordance with the invention. The above described effects, of achieving a higher cell output by increasing the acid concentration or by 25 using a thinner fleece material, would have the effect of shifting the curve in the positive direction on the Y axis on this graph. Due to the high temperature resistance of the fleece, concentrated phosphoric acid could be used as the electrolyte for applications at temperatures above 100*C. 30 1

Claims

1. Membrane electrode unit for a fuel cell comprising an optionally catalyst-coated anode, an optionally catalyst-coated cathode and a proton conductor located 5 between said anode and said cathode, characterised in that the proton conductor consists of a microfibre fleece material which has been impregnated with an electrolyte to the point of saturation; wherein 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/m 2 ; wherein the 10 thickness of the fleece is less than 1 mm and the pore volume is 65 to 92 %.

2. Membrane electrode unit as per claim 1 characterised in that the microfibre fleece material has a median pore radius between 20 nm and 10 pm. 15

3. Membrane electrode unit as per claims 1 or 2 characterised in that the microfibre fleece material is filled with perfluorinated ionomer.

4. Membrane electrode unit as per claim 3 characterised in that the perfluorinated ionomer is a polytetrafluoroethylene with sulphonated perfluoro vinylether side 20 chains.

5. Membrane electrode unit as per claims 1 or 2 characterised in that the microfibre fleece material has been impregnated with a one to 5 molar, aqueous sulphuric acid solution. 25

6. Membrane electrode unit as per claims 1 or 2 characterised in that the microfibre fleece material has been impregnated with concentrated phosphoric acid.

7. Membrane electrode unit as per claims 1 or 2 characterised in that the microfibre 30 fleece material has been impregnated with hydrated zirconium phosophate or ammonium dihydrogen phosphate. R