High-performance membrane electrode unit and the use thereof in fuel cells

Abstract

The present invention relates to a membrane electrode unit comprising a polymer membrane doped with a mineral acid as well as two electrodes, characterized in that the polymer membrane comprises at least one polymer with at least one nitrogen atom and at least one electrode comprises a catalyst which is formed from at least one precious metal and at least one metal less precious according to the electrochemical series.

Claims

1. A membrane electrode unit consisting essentially of A) at least one polymer membrane which consists essentially of at least one alkaline polyazole polymer with at least one nitrogen atom, the polymer membrane including at least one mineral acid which is phosphoric acid, B) at least two electrodes one which is a cathode and one is an anode, wherein i) the cathode side consists of a Pt/Ni catalyst the Pt/Ni catalyst being on a carbon support and ii) the catalyst on the anode side consists of a Pt catalyst on a carbon support, iii) the alkaline polyazole polymer is a polybenzimidazole, iv) said catalyst on the anode side is loaded with 0.6 to 1 g/m.sup.2 Pt based on the surface area of the polymer membrane, v) said polymer membrane has proton conductivity of at least 0.1 S/cm @ 120° C. and vi) the membrane electrode unit providing for a current density of at least 0.3 A/cm.sup.3 (cell voltage of 0.6) when air is used on the cathode side.

2. The membrane electrode unit according to claim 1, wherein a mixture of one or more polybenzimidazole polymers with another polymer is employed.

3. The membrane electrode unit according to claim 1, wherein the polymer membrane comprises para-polybenzimidazoles.

4. The membrane electrode unit according to claim 1, wherein the catalyst is applied to the polymer membrane.

5. The membrane electrode unit according to claim 1, wherein the catalyst layer has a thickness in the range of from 0.1 to 50 μm.

6. The membrane electrode unit according to claim 1, wherein the catalyst comprises catalytically active particles on a carbon support, the size of the catalyst particles being in the range of from 1 to 20 nm.

7. A fuel cell containing one or more membrane electrode units according to claim 1.

8. The membrane electrode unit according to claim 1, wherein the ratio of Pt to Ni is 1:100 to 100:1.

9. The membrane electrode unit according to claim 1, wherein said polymer membrane has proton conductivity of at least 0.12 S/cm @ 120° C.

10. The membrane electrode unit according to claim 1, wherein the catalyst is on a carbon black support, graphite support or graphitized carbon black support.

11. The membrane electrode unit according to claim 1, wherein the ratio of Pt to Ni is 1:100 to 100:1.

12. The membrane electrode unit according to claim 1, wherein the ratio of Pt to Ni is 1:1.

13. The membrane electrode unit according to claim 1, wherein the Pt/Ni is on the carbon catalyst and the Pt/Ni has a particle size of 1 to 20 nm and the carbon particles have a particle size of 20 to 100 nm.

14. The membrane electrode unit according to claim 1, wherein the Pt/Ni is on the carbon catalyst and the Pt/Ni has a particle size of 2 to 6 nm and the carbon particles have a particle size of 30 to 60 nm.

15. The membrane electrode unit according to claim 1, wherein the polymer membrane has a thickness from 20 to 1500 m and the membrane is doped with phosphoric acid and the degree of doping expressed as mole per acid per repeat unit is from 15 to 80.

16. The membrane electrode unit according to claim 1, wherein hydrogen gas is fed on the anode side.

17. The membrane electrode unit according to claim 1, wherein the membrane electrode unit providing for a current density of at least 0.4 A/cm.sup.3 (cell voltage of 0.6) when air is used on the cathode side.

Description

EXAMPLES

(1) Table 1 shows the cell voltages of 4 different membrane electrode units at current densities of 0.2 A/cm.sup.2 and 0.5 A/cm.sup.2, respectively. The values were recorded in a single fuel cell with an active area of 50 cm.sup.2 at 160° C. Pure hydrogen served as the anode gas (with a stoichiometry of 1.2 and a pressure of 1 bara), air served as the cathode gas (with a stoichiometry of 2 and a pressure of 1 bara). The composition of the individual specimens is described below:

(2) Specimen 1:

(3) Anode A: The anode catalyst is Pt on a carbon support. The electrode loading is 1 mg.sub.Pt/cm.sup.2.

(4) Cathode B: The cathode catalyst is Pt on a carbon support. The electrode loading is 1 mg.sub.Pt/cm.sup.2.

(5) Membrane A: A polymer membrane doped with phosphoric acid, the polymer of which consists of poly-((2,2′-m-phenylene)-5,5′-bisbenzimidazole)-co-poly-((2,5-pyridine)-5,5′-bisbenzimidazole), serves as the membrane.

(6) Specimen 2:

(7) Anode A: The anode catalyst is Pt on a carbon support. The electrode loading is 1 mg.sub.Pt/cm.sup.2.

(8) Cathode B: The cathode catalyst is Pt on a carbon support. The electrode loading is 1 mg.sub.Pt/cm.sup.2.

(9) Membrane B: A polymer membrane doped with phosphoric acid, the polymer of which consists of poly-((2,2′-m-phenylene)-5,5′-bisbenzimidazole), serves as the membrane.

(10) Specimen 3:

(11) Anode A: The anode catalyst is Pt on a carbon support. The electrode loading is 1 mg.sub.Pt/cm.sup.2.

(12) Cathode B: The cathode catalyst is a PtNi alloy on a carbon support. The electrode loading is 1 mg.sub.Pt/cm.sup.2. The ratio of Pt to Ni is 1:1.

(13) Membrane A: A polymer membrane doped with phosphoric acid, the polymer of which consists of poly-((2,2′-m-phenylene)-5,5′-bisbenzimidazole)-co-poly-((2,5-pyridine)-5,5′-bisbenzimidazole), serves as the membrane.

(14) Specimen 4:

(15) Anode A: The anode catalyst is Pt on a carbon support. The electrode loading is 1 mg.sub.Pt/cm.sup.2.

(16) Cathode B: The cathode catalyst is a PtNi alloy on a carbon support. The electrode loading is 1 mg.sub.Pt/cm.sup.2. The ratio of Pt to Ni is 1:1.

(17) Membrane B: A polymer membrane doped with phosphoric acid, the polymer of which consists of poly-((2,2′-m-phenylene)-5,5′-bisbenzimidazole), serves as the membrane.

(18) By comparing specimen 1 and 2, which include the same electrodes and Pt catalysts but the differing membranes A and B, it can be seen that, by using membrane B, an only very slight increase of the cell voltage by 3 and 4 mV at 0.2 and 0.5 A/cm.sup.2, respectively, can be achieved. If the cathode (specimen 3 with membrane A and specimen 4 with membrane B) is changed to an alloy catalyst, as is described in this specification, the anode with Pt catalyst remaining the same, the cell voltage can be increased substantially. In this way, the cell voltage increases by 22 mV at 0.2 A/cm.sup.2 with membrane A if said alloy catalyst is employed. Using membrane B, the cell voltage increases by 37 mV at 0.2 A/cm.sup.2 if an alloy catalyst is employed.

(19) TABLE-US-00001 TABLE 1 Cell voltage in [V] at No. Cathode catalyst Membrane 0.2 A/cm.sup.2 0.5 A/cm.sup.2 1 Pt/C, 1 mg.sub.Pt/cm.sup.2 A 0.634 0.555 2 Pt/C, 1 mg.sub.Pt/cm.sup.2 B 0.637 0.559 3 PtNi/C, 1 mg.sub.metal/cm.sup.2 A 0.656 0.571 4 PtNi/C, 1 mg.sub.metal/cm.sup.2 B 0.674 0.598