Gas diffusion electrodes and process for production thereof

Abstract

A gas diffusion electrode is described, especially for use in chloralkali electrolysis, said gas diffusion electrode having finely divided components on the liquid side. The electrode is notable for a low perviosity to gases and a lower operating voltage.

Claims

1. An oxygen-consuming electrode comprising (1) at least one carrier in the form of a flat structure, (2) a coating with a gas diffusion layer, and (3) a catalytically active component comprising silver, silver(I) oxide, silver(II) oxide, or mixtures thereof, wherein the oxygen-consuming electrode further comprises hydrophilic pores with an entry, and a finely divided hydrophilic component having a specific surface area in the range of from 8 to 12 m.sup.2/g, having a mean particle diameter in the range from 20 to 100 nm and in an amount of from 100 mg to 10 g/m.sup.2 of electrode area, on a side which faces towards liquid or an ion exchanger membrane when in operation.

2. The oxygen-consuming electrode of claim 1, wherein the finely divided hydrophilic component catalyses the reduction of oxygen.

3. The oxygen-consuming electrode of claim 1, wherein the finely divided hydrophilic component is silver.

4. The oxygen-consuming electrode of claim 1, wherein the finely divided hydrophilic component covers from 5 to 80% of the entry area of the hydrophilic pores in the oxygen-consuming electrode.

5. The oxygen-consuming electrode of claim 1, wherein the oxygen-consuming electrode, apart from the finely divided hydrophilic component, comprises mixtures which contain, as a catalytically active component, from 70 to 95% by weight of silver oxide, from 0 to 15% by weight of silver metal, and from 3 to 15% by weight of an insoluble fluorinated polymer.

6. The oxygen-consuming electrode of claim 5, wherein the silver oxide is silver(I) oxide and the insoluble fluorinated polymer is PTFE.

7. The oxygen-consuming electrode of claim 1, wherein the oxygen-consuming electrode has, as a carrier element, an electrically conductive flat structure.

8. The oxygen-consuming electrode of claim 7, wherein the electrically conductive flat structure is based on nickel or silver-coated nickel.

9. A fuel cell or a metal/air battery comprising the oxygen-consuming electrode of claim 1.

10. The oxygen-consuming electrode of claim 1, wherein the finely divided hydrophilic component having a mean particle diameter in the range from 40 to 80 nm.

11. The oxygen-consuming electrode of claim 1, wherein the finely divided hydrophilic component having a mean particle diameter in the range from 50 to 70 nm.

12. The oxygen-consuming electrode of claim 1, comprising the finely divided hydrophilic component in an amount of from 1 g to 5 g/m.sup.2 of electrode area.

13. A process for producing the oxygen-consuming electrode of claim 1, comprising (1) applying or spraying a finely divided, hydrophilic component in a suspension with a concentration of from 0.1 to 50% by weight to a flat base electrode comprising at least one carrier in the form of a flat structure, a coating with a gas diffusion layer, and a catalytically active component, and (2) subsequently removing the suspension medium by evaporation.

14. The process of claim 13, wherein the suspension medium has a boiling point of from 50 to 150 C.

15. The process of claim 13, wherein the suspension medium is an alcohol.

16. The process of claim 13, wherein the amount of the finely divided component applied is 100 mg to 10 g per square meter of electrode area.

17. The process of claim 13, wherein the base electrode used is a spent or leaky oxygen-consuming electrode.

18. The process of claim 13, wherein the finely divided, hydrophilic component is in a suspension with a concentration of from 1 to 20% by weight.

19. The process of claim 13, wherein the suspension medium has a boiling point of from 60 to 100 C.

20. The process of claim 13, wherein the suspension medium is i-propanol.

21. The process of claim 13, wherein the amount of the finely divided component applied is 1 to 5 g per square meter of electrode area.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 The surface of the oxygen-consuming electrode before the treatment by the process according to the invention

(2) FIG. 2 The surface of the oxygen-consuming electrode after the treatment by the process according to the invention

(3) In the figures, the reference symbols are defined as follows:

(4) A: PTFE

(5) B: Silver crystals

(6) C: Nanoscale silver

(7) D: Oversize hydrophilic pore

EXAMPLES

(8) An oxygen-consuming electrode manufactured by the wet process was incorporated into an electrolysis half-cell. The electrode had, at 4 kA/m.sup.2, a potential of 400 mV (measured against an Ag/AgCl electrode). The electrode is permeable to visible amounts of oxygen (formation of small gas bubbles) at a pressure differential of 20 mbar between gas side and liquid side.

(9) The oxygen-consuming electrode was deinstalled, rinsed with deionized water and dried on the outside. 100 g/m.sup.2 of a suspension of 1.4 g of nanoscale silver powder of the SP-7000-95 type from Ferro Corporation, Cleveland, USA, (mean particle diameter 60 nm) in 100 g of i-propanol were sprayed on. The isopropanol was vaporized, then the electrode was dried at 80 C. in a drying cabinet for 30 min, and then installed back into the electrolysis half-cell. At 4 kA/m.sup.2, the electrode had a potential of 320 mV and was impervious at a pressure differential of 40 mbar between gas side and liquid side.

(10) Thus, the treatment of the surface with nanoscale silver powder improved the potential by 80 mV, and generated clear imperviosity to gas and liquid.

(11) FIG. 1 shows a schematic of the surface of the oxygen-consuming electrode facing the liquid in microscopic view before the treatment with a suspension comprising fine silver particles. The catalyst particles B are held by the filamentous PTFE matrix A; oxygen can break through to the liquid side through an oversize pore D.

(12) FIG. 2 shows the surface of the same oxygen-consuming electrode after treatment with a suspension comprising fine silver particles. The fine silver particles C have been deposited between the catalyst particles; the clear area of the pore opening is reduced. The pore can hold the liquid better; passage of oxygen into the liquid phase is more difficult. In addition, the number of catalytically active sites in the phase interface has increased, which results in increased conversion.