JP2002516472A - Membrane electrode unit for fuel cells - Google Patents

Abstract

(57) [Summary] A membrane electrode unit for a fuel cell, that is, a membrane electrode unit having an anode and a cathode and a proton conductor between the anode and the cathode, and the anode and the cathode are optionally coated with a catalyst . The proton conductor is formed by a microfibrous non-woven fabric impregnated with electrolyte to saturation, wherein the non-woven fabric is resistant to the electrolyte at temperatures up to + 200 ° C and also under oxidizing or reducing conditions. is chemically inert, the weight of the nonwoven fabric is 20 to 200 g / m ^2, the thickness of the nonwoven fabric is less than 1 mm, the pore volume is 65 to 92%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a membrane electrode unit for a fuel cell. More particularly, the invention comprises an anode, a cathode, a proton conductor between the anode and the cathode,
These anodes and cathodes are optionally coated with a catalyst,
The present invention relates to a membrane electrode unit for a fuel cell.

[0002]

[Prior art]

Such a membrane electrode unit is known. In this membrane electrode unit, in order to obtain electric energy from chemical energy in a direct manner, separation of an ionization process and an electrical process occurs when a reaction gas or a fluid component containing hydrogen and oxygen reacts in a fuel cell. .

[0003] The existence and mode of operation of various types of fuel cells is described in "Spektrum der Wissensch
aft "(July 1995), KDKreuer and J. Maier, 92-96.

[0004] These electrodes must be very good electronic conductors (electrical resistance around 0.1 Ωcm ⁻¹ ). This requires a catalytic reaction-in relation to the electrolyte surface. This electrolyte must have as low a conductivity as possible and a high ionic conductivity.
The electrolyte must in particular be as impermeable as possible to the starting gas.
All substances must be chemically inert with respect to one another and to those involved in the reaction. That is, both under conditions of strong oxidizing power at the cathode and under conditions of strong reducing power at the anode, undesired bonds must not form between them.

[0005] In order to connect a plurality of cells to form a battery stack, the solid components included in the cells must be given a sufficient mechanical load bearing capacity. Furthermore, the cost, life, and environmental compatibility of the materials and processes of the battery components are also important.

When the operating temperature is between 80 and 90 ° C., proton conductive polymer membranes have gained value in fuel cells. This membrane has both the property of a liquid, which gives free mobility to molecules and protons, and the property of a solid, which is stable in shape. This requirement is almost ideally met by a perfluoro-based ionomer membrane based on polytetrafluoroethylene with sulfonated perfluorovinyl ether side chains. This material consists of a hydrophobic part and a hydrophilic part, both parts separating in the presence of water, forming a gel-like but stable film. The hydrophobic backbone of this polymer is very stable against oxidation and reduction, and gives the membrane a stable backbone even in the swollen state. Very good proton conductivity is achieved with hydrophilic, liquid-like, sulfonic acid-containing side chains swollen in water. A pore with a size of several nanometers corresponds to a size of several water molecules. The presence of water results in a high degree of proton motility within the pathway and pores.

[0007] The disadvantage of this cation exchange device is that it is expensive, as described in the references already cited, due to the complexity of the production procedure.
Furthermore, its disposal or recycling also raises environmental concerns.

[0008] During operation of a fuel cell, such membranes tend to dry. The nature of the proton flow, which transports water molecules from the anode to the cathode, is also the case, especially when the combustion oxygen is supplied to the cell by an air flow.

The upper limit of the heat stability of a known film or its sulfonic acid group is 90 to 100.
° C. At higher temperatures the morphology begins to collapse.

[0010] The known perfluoro-based ionomer membranes are therefore closed off at higher operating temperatures as stand-alone foils and are therefore unsuitable for: a) hydrogen obtained from modified methanol As fuel at a temperature of 130 ° C. or higher (this method is described in U.S. Pat.
Benz et al.). b) use at temperatures above 130 ° C., typically 150-200 ° C., for direct oxidation of methanol at the anode;

[0011]

[Problems to be solved by the invention]

It is an object of the present invention to produce a membrane electrode unit for a fuel cell which adds the following properties to the above-mentioned advantageous properties of a perfluoro-based ionomer membrane. 1. Lower manufacturing costs than prior art polymer membranes. 2. Reduction of pollutants during disposal. 3. To reduce the effect of catalyst poison, apply reformed methanol to hydrogen as fuel, and reform or directly oxidize methanol inside the device, so that temperature stability up to 200 ° C is obtained.

[0012]

[Means for Solving the Problems]

The object is achieved by a membrane electrode unit according to the general concept of the invention. Advantageous embodiments are set out in the subclaims.

[0013] The present invention contemplates forming a proton conductor from a microfibrous nonwoven fabric impregnated with an electrolyte until saturated. This nonwoven is chemically inert to the electrolyte up to a temperature of + 200 ° C. and also under conditions of oxidizing and reducing power, the weight of the nonwoven is 20-200 g / m ² and the thickness of the nonwoven is Is up to 1 mm and the pore volume is 65-92%.

The micropore nonwoven fabric has an average pore radius of 20 nm to 10 μm.

In the case of the object according to the invention, the electrolyte does not need to provide this mechanical stability, since the nonwoven fabric skeleton of the microfiber nonwoven fabric ensures the mechanical stability of the membrane. This can reduce the material cost of the membrane by up to 90%, for example, as compared to the cost of producing independent membranes of comparable dimensions, for example perfluoroionomers.

[0016] The microfiber nonwoven fabric can be filled with a perfluoro-based ionomer. The perfluoro-based ionomer can be polytetrafluoroethylene with sulfonated perfluorovinylethylene side chains. As an alternative method, it is conceivable to impregnate the microfiber nonwoven fabric with a 1 to 5 mol aqueous sulfuric acid solution or concentrated phosphoric acid. In addition, zirconium phosphate hydrate and ammonium dihydrogen phosphate hydrate can also be used.

The present invention shows that the output (ion conductivity) of a fuel cell is equivalent to a pure polymer membrane composed of a perfluoro ionomer, and that there is no need to use a conventional expensive material. Let me clarify with an example.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION

 The basic materials are common in each example and are as follows.

Nonwoven fabric material: a polysulfone fiber having a square cross section (width: 6 to 13 μm, height: 1.7 to 2.4 μm).

Mechanical properties of polysulfone material: Melting range: 343 to 399 ° C. Tensile strength: 70 Mpa Elongation at break: 50 to 100% Young's modulus: 2.4 GPa Bending temperature under 1.8 MPa load: 174 ° C.

Production of fiber: A solution of polysulfone in methylene chloride is fibrillated in an electrostatic field. For example, the device described in DE-OS 26 20 399 can be used for that purpose. The fibers are collected in a linearly moving fiber carrier.

Characteristics of non-woven fabric: Weight: 150 g / m ² Thickness (when compressed): 0.05 mm Thickness (when impregnated with electrolyte): 0.18 mm Average pore radius in non-compressed state: 8 μm Average pore radius in compressed state: 4 μm pore volume: 83%.

The temperature stability of the membranes of the present invention is determined primarily by the nonwoven material, unless there is a different reason to the contrary, so that the temperature stability is lost to about 174 ° C. for the pure fiber material polysulfone. It is after that. Since the fibers are mechanically bonded to each other in the nonwoven fabric, the mechanical stability is further increased to a temperature of 250 ° C. This allows the fuel cell to operate at high temperatures, thereby significantly reducing anode catalyst poisoning.

Example 1 Liquid Nafion, a perfluoro-based ionomer available from DuPont, is coated on a nonwoven microfiber in a glass filter 16 mm in diameter. Applying a slight negative pressure, the liquid phase material is sucked into the pore structure of the nonwoven fabric. The membrane impregnated in this way is treated at 60 ° C. in a drying cabinet to separate off the solvent. Thereafter, it can be stored in distilled water until reprocessing.

Examples 2 to 4 Similar to Example 1, the microfiber nonwoven fabric is impregnated with three different moles of an aqueous sulfuric acid solution. The sulfuric acid is heated to about 70 ° C. in order to reduce the viscosity. The nonwoven can be boiled for several minutes in an acid heated to 70 ° C. without any abnormalities.

The storage of the membrane thus obtained is expediently carried out in the same impregnating medium.

The specific conductivity of the thus prepared film was determined according to the method described in DIN 53779, dated May 1979. The results are shown in the table below.

[0028]

[Table 1]

Example 5 in the table is a comparative example in which the same measurement was performed on a polymer film according to the prior art, and was composed of a perfluoro ionomer (Nafion-117, Dupont),
A polymer film having a thickness of 125 μm and capable of self-supporting without requiring other supports was used.

The numerical values of the specific conductivity S / cm indicate the following. In other words, the membrane of the present invention, which has a very advantageous cost compared to pure Nafion, has a simpler structure, and is mechanically stable, can operate a fuel cell having an output equivalent to that of the prior art. That's what it means. When employed at a temperature of 100 ° C. or higher, concentrated phosphoric acid can be used as the ion conductor.

For example, when compared with a swollen Nafion membrane having a thickness of 125 μm, the non-woven fabric impregnated with the electrolyte used in Examples 1 to 4 has twice the thickness.

The output of the fuel cell, which is obtained from the product of the voltage and the current, can be obtained not only by increasing the acid concentration, that is, by increasing the specific conductivity S / cm, but also by using a thinner nonwoven fabric to prevent the diffusion obstacle. It can also be obtained by reducing.

As an example, FIG. 1 shows current / voltage curves at room temperature corresponding to Examples 1, 3, and 5, respectively. In comparison with the prior art (Example 5), it can be seen that comparable characteristic curves are obtained with the membrane according to the invention. The above effect of increasing the acid concentration or the thinning of the non-woven fabric material to increase the battery output is obtained by moving each curve in the figure in the positive direction of the vertical axis.

[0034] Due to the high temperature stability of the nonwoven fabric, concentrated phosphoric acid can also be used as an electrolyte when applied to temperatures above 100 ° C.

Claims (7)

[Claims] 1. A membrane electrode unit for a fuel cell comprising an anode coatable with a catalyst, a cathode coatable with a catalyst, and a proton conductor between the anode and the cathode, wherein the proton conductor is Formed by a microfibrous nonwoven fabric impregnated with electrolyte until saturated, said microfibrous nonwoven fabric being chemically inert to said electrolyte at temperatures up to + 200 ° C. and under conditions of oxidizing and reducing power Wherein the microfiber nonwoven fabric has a weight of 20 to 200 g / m ² , a thickness of less than 1 mm, and a pore volume of 65 to 92%. 2. The membrane electrode unit for a fuel cell according to claim 1, wherein the micropore nonwoven fabric has an average pore radius of 20 nm to 10 μm. 3. The fuel cell membrane electrode unit according to claim 1, wherein the microfiber nonwoven fabric is filled with a perfluoro ionomer. 4. The membrane electrode unit for a fuel cell according to claim 3, wherein the perfluoro ionomer is polytetrafluoroethylene having a sulfonated perfluorovinyl ether side chain. 5. The membrane electrode unit for a fuel cell according to claim 1, wherein the microfiber nonwoven fabric is impregnated with a 1 to 5 mol aqueous sulfuric acid solution. 6. The membrane electrode unit for a fuel cell according to claim 1, wherein the microfiber nonwoven fabric is impregnated with concentrated phosphoric acid. 7. The membrane electrode unit for a fuel cell according to claim 1, wherein the microfiber nonwoven fabric is impregnated with zirconium phosphate hydrate or ammonium dihydrogen phosphate hydrate.