DIFUNCTIONAL ELECTRODE AND ELECTROLYSIS DEVICE FOR CHLOR-ALKALI ELECTROLYSIS

Abstract

The invention relates to an oxygen-consuming electrode for use in chlor-alkali electrolysis which, as required, can either evolve hydrogen or can also consume oxygen, on the basis of a silver-based catalyst and an additional electrocatalyst based on ruthenium and/or iridium. The invention further relates to an electrolysis device consisting thereof. When said electrode is used in the chlor-alkali electrolysis, a correspondingly equipped chlor-alkali electrolysis system can be used for example for network stabilization of power supply networks.

Claims

1.-16. (canceled)

17. A bifunctional electrode for operation as cathode in a chlor-alkali electrolysis, in which either hydrogen is generated at the cathode or, when oxygen is being supplied to the cathode, oxygen is consumed at the cathode, having at least one two-dimensional, electrically conductive carrier and a gas diffusion layer and electrocatalyst based on silver and/or silver oxide (silver catalyst) that has been applied to the carrier, wherein the additional electrocatalyst that has been provided is a ruthenium catalyst based on ruthenium and/or ruthenium oxide and/or an iridium catalyst based on iridium and/or iridium oxide, preferably ruthenium catalyst, where the carrier has a catalytic coating with additional electrocatalyst and/or the additional electrocatalyst is in a mixture with the silver catalyst.

18. The electrode as claimed in claim 17, wherein the gas diffusion layer consists at least of a mixture of fluoropolymer and silver catalyst and optionally ruthenium catalyst.

19. The electrode as claimed in claim 17, wherein the catalytic coating of the carrier with ruthenium and/or optionally iridium catalyst is present in an amount of 0.05% to 2.5% by weight, preferably 0.1% to 1.5% by weight, based on the total content of silver catalyst, ruthenium catalyst and fluoropolymer.

20. The electrode as claimed in claim 17, wherein a mixture of fluoropolymer and silver catalyst and optionally ruthenium catalyst has been applied to the carrier in powder form and compacted.

21. The electrode as claimed in claim 17, wherein the content of fluoropolymer in the electrode, especially PTFE as fluoropolymer, is 1% to 15% by weight, preferably 2% to 13% by weight, more preferably 3% to 12% by weight, of fluoropolymer and 99-85% by weight, preferably 98-87% by weight, more preferably 97% to 88% by weight, of silver catalyst, based on the sum total of the contents of fluoropolymer and silver catalyst.

22. The electrode as claimed in claim 17, wherein the electrode has a thickness of 0.2 to 3 mm, preferably 0.2 to 2 mm, more preferably 0.2 to 1 mm.

23. The electrode as claimed in claim 17, wherein the silver catalyst consists of silver, silver oxide or a mixture of silver and silver oxide, where the silver oxide is preferably silver(I) oxide, and the silver catalyst preferably consists of silver.

24. The electrode as claimed in claim 17, wherein the gas diffusion layer has been applied to the outer faces of the carrier on one or two sides, preferably to the carrier on one side.

25. The electrode as claimed in claim 17, wherein the weight ratio of ruthenium catalyst and iridium catalyst to the silver catalyst is from 0.05:100 to 3:100, especially from 0.06:100 to 0.9:100.

26. The electrode as claimed in claim 17, wherein the electrically conductive carrier takes the form of a mesh, nonwoven, foam, weave, braid or expanded metal.

27. The electrode as claimed in claim 17, wherein the electrically conductive carrier consists of carbon fibers, nickel or silver, preferably of nickel.

28. The electrode as claimed in claim 17, wherein the area loading of ruthenium catalyst, calculated as ruthenium metal, is 1 to 55 g/m.sup.2.

29. An electrolysis apparatus for bifunctional operation of a chlor-alkali electrolysis having a cathode at which either hydrogen is generated or, in a gas diffusion layer of the cathode, oxygen is consumed, at least comprising an electrolysis cell for chlor-alkali electrolysis having an anode half-cell, a cathode half-cell and a cationic exchange membrane that separates the anode half-cell and the cathode half-cell from one another, an anode disposed in the anode half-cell for evolution of chlorine, a cathode disposed in the cathode half-cell, and an inlet for optional supply of an oxygen-containing gas to a gas space of the cathode half-cell, and inlets and outlets for the reactant streams and product streams, wherein the cathode used is an electrode as claimed in claim 17.

30. The apparatus as claimed in claim 29, wherein it has at least one inlet for purging of the gas space of the cathode half-cell with inert gas.

31. A bifunctional method of chlor-alkali membrane electrolysis, wherein either, in the case of low supply of electrical power from the power grid connected to the electrolysis apparatus, the cathode is supplied with oxygen-containing gas to the gas space of the cathode half-cell and oxygen is reduced at the cathode at a first cell voltage, or, in the case of high supply of electrical power from the power grid connected to the electrolysis cell, the cathode is not supplied with any oxygen-containing gas and hydrogen is generated at the cathode at a second cell voltage higher than the first cell voltage, wherein the electrolysis apparatus used is an electrolysis apparatus as claimed in claim 29.

32. The method as claimed in claim 31, wherein, in the operation of the cathode for generation of hydrogen, the pressure differential between the gas space of the cathode half-cell and the pressure on the side of the gas diffusion electrode facing the alkali is adjusted such that the hydrogen formed at the cathode is led away exclusively into the gas space of the cathode half-cell.

Description

EXAMPLES

Example 1Operation of an Inventive Electrode

[0053] For the experiments, a 3-chamber laboratory cell having an ion exchange membrane and electrode area of 100 cm.sup.2 was used. The first chamber one consisted of the anode chamber that was charged with a sodium hydroxide solution, the charge volume having been chosen such that the effluxing concentration of NaCl was about 210 g/L and the temperature about 85 C. The anode used consisted of an expanded metal that had been provided with a commercial ruthenium oxide-based anode coating for evolution of chlorine from DENORA. The membrane used was a Nafion N982. The second chamber was defined via the distance of the membrane from the bifunctional electrode of 3 mm, and there was a flow of sodium hydroxide solution through the second chamber such that the temperature of the sodium hydroxide solution leaving the chamber was 85 C. and the concentration 31.5% by weight. The third chamber serves for supply and removal of gas. In the case of operation of the bifunctional electrode in ORR mode, O.sub.2 was introduced into the chamber.

[0054] In the case of HER mode, the hydrogen escaped on the side of the of the bifunctional electrode that faced the gas space, given a sufficiently selective pressure level and pressure differential, and did not get into the second chamber.

[0055] The electrode of the invention was operated at different pressure differentials. The pressure differential reported is the differential that results from the pressure on the side of the electrode directed to the liquid and the pressure on the side of the electrode directed to the gas side. The amount of hydrogen that could be withdrawn from the second chamber was in each case as specified below:

TABLE-US-00001 Amount of H.sub.2 from 2nd chamber Liquid Gas Pressure as percentage of the total pressure pressure differential amount of H.sub.2 formed [mbar] [mbar] [mbar] [%] 28 0 28 0.8 28 30 2 7.9 28 59 31 33.2 28 70 42 43.9 46 0 46 0.0 46 30 16 0.0 46 59 13 23.1 46 70 24 30.9

[0056] At an alkali pressure of 46 mbar and up to a gas pressure of 30 mbar, all the hydrogen is led off via the third chamber. This is not possible at a lower alkali pressure, in spite of the same pressure differential.

Example 2Comparative ExampleHER Mode(Prior Art)

[0057] As described above under Description of the cell construction and test method, an RuO2-coated nickel weave is produced and used. This will be used as reference for HER mode. A nickel weave of size 7 cm3 cm (wire thickness: 0.15 mm; mesh size: 0.5 mm) was coated with RuO.sub.2. The amount of RuO.sub.2 applied was 8.2 g/m.sup.2 (where the area in m.sup.2 is the geometrically projected area found as the area when the product of electrode length and width is calculated, where the area corresponds to that opposite the anode). This cathode was examined by the principle described above in a half-cell; see Description of the cell construction and test method section. The potential for HER mode corrected by the R3 resistance was 169 mV vs. RHE (measured at 1.5 kA/m.sup.2, sodium hydroxide solution temperature: 63 C., NaOH conc.: 32% by weight). This type of electrode fundamentally cannot be operated in ORR mode.

Example 3Comparative ExampleORR Mode (Prior Art)

[0058] For ORR with an oxygen-depolarized cathode (ODC), an ODC was produced according to the example of DE 10 2005 023 615 A1 and characterized as above. The potential for ORR mode, corrected by the R3 resistance, was +740 mV vs. RHE (4.0 kA/m.sup.2, sodium hydroxide solution temperature: 80 C., NaOH conc.: 32% by weight).

Example 4Comparative Example: ODC According to Prior Art (from Example 3) Operated in HER Mode (Hydrogen Evolution Mode)

[0059] Since there has not yet been any description of a bifunctional electrode and it is not possible to operate a hydrogen-evolving electrode in oxygen reduction mode, the ODC known according to the example from the prior art according to DE 10 2005 023 615 A1 was operated in hydrogen evolution mode.

[0060] For this purpose, the electrode as operated in HER mode in example 2 was characterized. The potential for HER mode, corrected by the R3 resistance, was 413 mV vs. RHE (1.5 kA/m.sup.2, sodium hydroxide solution temperature: 63 C., NaOH: 32% by weight).

[0061] Hydrogen was evolved at a worse potential by 244 mV by comparison with the hydrogen evolution electrode known from the prior art (example 2).

Example 5Inventive Bifunctional CathodeUse of an RuO2-Coated Ni Weave as Carrier and Current Distributor in the Gas Diffusion Layer

[0062] For the bifunctional cathode according to the invention, the carrier of the electrode from example 3 was replaced by an RuO.sub.2-coated Ni weave. The weave was produced as described in example 2. This carrier was used as carrier for the gas diffusion layer analogously to the example of DE 10 2005 023 615 A1 described. This electrode was installed into the half-cell and characterized as described above.

[0063] The potential for ORR mode, corrected by the R3 resistance, is +785 mV vs. RHE (4.0 kA/m.sup.2, sodium hydroxide solution temperature: 80 C., NaOH: 32% by weight).

[0064] Thus, the potential for the ORR is 45 mV better than that of the ODC known according to the prior art from DE 10 2005 023 615 A1.

[0065] The potential for HER mode, corrected by the R3 resistance, was 277 mV vs. RHE (1.5 kA/m.sup.2, sodium hydroxide solution temperature: 63 C., NaOH: 32% by weight).

[0066] Thus, the electrode is only 108 mV worse than the electrode from the prior art according to example 2 that has been optimized for the evolution of hydrogen (HER mode) and simultaneously better in operation in ORR mode.

Example 6Bifunctional Electrode (Inventive): Silver Oxide (Ag.SUB.2.O)-Based Gas Diffusion Layer with 1% by Weight of Added RuO.SUB.2 .Powder

[0067] For this electrode, an electrode was manufactured analogously to the example of DE 10 2005 023 615 A1. However, the composition of the catalyst mixture was different, as follows: 5% by weight of PTFE, 7% by weight of Ag, 87% by weight of Ag.sub.2O and 1% by weight of RuO.sub.2 (ACROS: 99.5% anhydride). The potential of the bifunctional electrode for HER, at 109 mV vs RHE, was 60 mV better than that of the standard electrode (RuO.sub.2) for the evolution of hydrogen.

[0068] The potential of the bifunctional electrode in ORR mode was 794 mV vs. RHE by 54 mV better than the ODC known from the prior art (see example 3) in ORR mode.

Example 7Bifunctional Electrode with 3% by Weight of Added RuO.SUB.2 .Powder (Inventive)

[0069] For this electrode, an electrode was manufactured according to DE 10 2005 023 615 A1. However, the composition of the catalyst mixture was as follows: 5% by weight of PTFE, 7% by weight of Ag, 85% by weight of Ag.sub.2O and 3% by weight of RuO.sub.2 (ACROS: 99.5% anhydride).

[0070] The potential of the bifunctional electrode for HER, at 146 mV vs RHE, was slightly poorer by 37 mV than that of the electrode with 1% by weight of RuO.sub.2 powder.

[0071] The potential of the bifunctional electrode in ORR operation, at 702 mV vs. RHE, was slightly poorer by 92 mV than that of the electrode with 1% by weight of RuO.sub.2.

[0072] This electrode in HER operation is also comparatively better than the known hydrogen-evolving electrode (example 2).

[0073] The electrodes of the invention thus achieve an unknown synergism in relation to bifunctional use in chloralkali electrolysis under hydrogen production conditions and oxygen-depolarized conditions.