Method for producing a catalyst-coated membrane

Abstract

The present invention relates to a method for producing a membrane for a fuel cell or electrolytic cell, in which (i) a liquid coating composition, which contains a supported catalyst containing precious metal and also contains an ionomer, is applied to a polymer electrolyte membrane which contains an ionomer, the ionomer of the liquid coating composition and the ionomer of the polymer electrolyte membrane each being a copolymer which contains as monomer a fluoroethylene and a fluorovinyl ether containing a sulfonic acid group, (ii) the coated polymer electrolyte membrane is heated to a temperature in the range from 178? C. to 250? C.

Claims

1. A method for producing a membrane for a fuel or electrolytic cell, comprising: (i) applying a liquid coating composition containing a supported precious metal-containing catalyst and an ionomer both to a front side and a back side of a polymer electrolyte membrane containing an ionomer, or a first liquid coating composition containing a supported precious metal-containing catalyst and an ionomer is applied only on a front side of a polymer electrolyte membrane containing an ionomer, and a second liquid coating composition containing a supported precious metal-containing catalyst and an ionomer is applied on a back side of the polymer electrolyte membrane, wherein the first and second liquid coating compositions are different, so that a coated polymer electrolyte membrane with a catalyst-containing layer on its front and back side is obtained, wherein the ionomer of the liquid coating composition and the ionomer of the polymer electrolyte membrane are each a copolymer which, as monomers, contain a fluoroethylene and a fluorovinyl ether containing a sulfonic acid group, wherein the fluorovinyl ether containing a sulfonic acid group has the formula (I):
CF(OR)CF.sub.2(I) wherein R has the following formula (II):
(CF.sub.2CF(CF.sub.3)O).sub.x(CF.sub.2).sub.ySO.sub.3H(II) where x=0-3 and y=1-5, and (ii) thermally treating the coated polymer electrolyte membrane by heating it to a temperature in a range of 178? C. to 250? C. without exposure to external pressure.

2. The method according to claim 1, wherein the fluoroethylene is a tetrafluoroethylene.

3. The method according to claim 1, wherein the polymer electrolyte membrane does not contain any additional ionomer; or the polymer electrolyte membrane does not contain any inorganic oxoacid of the phosphorus or sulfur.

4. The method according to claim 1, wherein no polymer containing a basic monomer is present in the polymer electrolyte membrane.

5. The method according to claim 1, wherein the precious metal is a platinum metal and the supported catalyst contains, as carrier material, a carbon material or an oxide.

6. The method according to claim 1, wherein the liquid coating composition is an aqueous coating composition.

7. The method according to claim 6, wherein the aqueous coating composition contains the supported precious metal-containing catalyst in an amount of 5-20 wt %, the ionomer in an amount of 2-8 wt % and water in an amount of at least 60 wt %, and a C.sub.1-4 alcohol in an amount of 10-20 wt %.

8. The method according to claim 1, wherein the liquid coating composition is applied to the polymer electrolyte membrane with a nozzle, scraper, roller, rod, spraying device, screen printing, offset printing, stencil printing, or halftone printing, or a combination of at least two of these coating methods.

9. The method according to claim 1, wherein the coated polymer electrolyte membrane is subjected to a drying step prior to the thermal treatment in step (ii).

10. The method according to claim 1, wherein the coated polymer electrolyte membrane heated to 178? C. to 250? C. in step (ii) is maintained in this temperature range for a period of from 1 second to 5 minutes.

11. The method according to claim 1, wherein the fuel cell is a hydrogen fuel cell or a methanol fuel cell; or wherein the electrolytic cell is a water electrolytic cell.

Description

EXAMPLES

(1) In all examples, a supported platinum catalyst (carbon as carrier material) was used.

Example 1

(2) A polymer electrolyte membrane was used whose ionomer is a copolymer of tetrafluoroethylene and a vinyl ether containing a perfluorinated sulfonic acid group. The membrane is commercially available from Gore under the designation MX820.15.

(3) An aqueous coating composition was prepared. This contained the supported platinum catalyst in an amount of 7.19 wt. % and an ionomer in an amount of 4.05 wt. %. The ionomer was a copolymer of tetrafluoroethylene and a vinyl ether containing a perfluorinated sulfonic acid group represented by the following formula:
CF(OR)CF.sub.2 wherein R has the formula:
(CF.sub.2).sub.4SO.sub.3H

(4) The aqueous coating composition was applied successively at 40? C. to the front and rear sides of the membrane by a slit scraper having a slit height of 175 ?m, and then dried.

(5) In this way, several coated membranes were produced. Each of these catalyst-coated membranes was then subjected to a thermal treatment, wherein the membranes were heated to different temperatures. Membrane 1.1: Heated to 150? C. Membrane 1.2: Heated to 160? C. Membrane 1.3: Heated to 170? C. Membrane 1.4: Heated to 180? C.

(6) All membranes were heated in an oven for a period of 4 minutes.

(7) For each of these 4 membranes, the current density was then determined as a function of time in order to observe the break-in behavior. For this purpose, a respective commercially available gas diffusion layer (28BC by SGL Carbon) was applied on each side of the coated membrane and this arrangement was installed in a test cell with 20% compression. The test cell comprises heatable end plates as well as gold-coated current collectors and gas distribution plates made of graphite. After installation in a test plant (G40 by Greenlight Innovation), the following conditions were set: Cell temperature 60? C.; relative input humidity of the gases on the anode and cathode 100%; input pressure on the anode and cathode 150 kPa (abs.); gas flow at the anode 1394 standard cubic centimeters of hydrogen; gas flow at the cathode 3323 standard cubic centimeters of air. The break-in, as can be seen in FIGS. 1 and 3, consists of 8 identical voltage-guided load cycles with the following profile: 45 min at 0.6 V, 5 min at 0.95 V and 10 min at 0.85 V.

(8) FIG. 1 shows the results of these measurements.

(9) Thermal treatment at 180? C. leads to a significant increase in performance under moist operating conditions and to performance stabilization, which starts very early rather than after a plurality of cycles. A so-called pre-conditioning or break-in step for activating the membrane can therefore be omitted.

(10) For each of the membranes 1.1 to 1.4, the polarization curve (cell voltage as a function of current density) under SAE conditions (cell temperature 80? C.; relative inlet humidity of the gases on anode and cathode 66%; inlet pressure on anode and cathode 170 kPa (abs.); gas flow at anode 1000 standard cubic centimeter of hydrogen; gas flow at cathode 5000 standard cubic centimeter of air) was also determined. The results are shown in FIG. 2. A significant increase in the high current range (>0.6 A/cm.sup.2) is shown for the membrane that was heated to 180? C., compared to the membranes treated at a lower temperature.

Example 2

(11) The polymer electrolyte membrane in Example 2 corresponded to the polymer electrolyte membrane used in Example 1.

(12) An aqueous coating composition was prepared. This contained the supported platinum catalyst in an amount of 5 wt % and an ionomer in an amount of 2.24 wt %. The ionomer was a copolymer of tetrafluoroethylene and a vinyl ether containing a perfluorinated sulfonic acid group represented by the following formula:
CF(OR)CF.sub.2 wherein R has the formula:
(CF.sub.2).sub.2SO.sub.3H

(13) The aqueous coating composition was applied at 40? C. with a slit scraper having a slit height of 100 ?m on the front and 300 ?m on the rear side of the membrane, and then dried.

(14) In this way, several coated membranes were produced. Each of these catalyst-coated membranes was then subjected to a thermal treatment, wherein the membranes were heated to different temperatures. Membrane 2.1: Heated to 155? C. Membrane 2.2: Heated to 175? C. Membrane 2.3: Heated to 205? C.

(15) All membranes were heated in an oven for a period of 4 minutes.

(16) For each of these 4 membranes, the current density was subsequently determined as a function of time.

(17) FIG. 3 shows the results of these measurements.

(18) Thermal treatment at 205? C. leads to a significant increase in performance under moist operating conditions and to performance stabilization, which starts very early rather than after a plurality of cycles. A so-called pre-conditioning or break-in step for activating the membrane can therefore be omitted.