OXYGEN-CONSUMING ELECTRODE WHICH CONTAINS CARBON NANOTUBES AND METHOD FOR PRODUCING SAME

Abstract

The invention relates to an oxygen-consuming electrode, in particular for use in chloralkali electrolysis, comprising a catalyst coating based on carbon nanotubes, and to an electrolysis device. The invention further relates to a method for producing said oxygen-consuming electrode and to the use thereof in chloralkali electrolysis or fuel cell technology.

Claims

1.-21. (canceled)

22. A process for producing a gas diffusion electrode for the reduction of oxygen, where the gas diffusion electrode has at least one sheet-like electrically conductive support element and a gas diffusion layer applied to the support element and an electrocatalyst, wherein the gas diffusion layer is formed by a mixture of carbon nanotubes and a fluoropolymer, and wherein a mixture of carbon nanotubes and fluoropolymer is applied in powder form to the support element and compacted, with the carbon nanotubes forming the electrocatalyst and being substantially free of nitrogen constituents.

23. The process as claimed in claim 22, wherein the carbon nanotubes are in form of an agglomerate, where at least 95% by volume of the agglomerate particles have an external diameter in the range from 30 μm to 5000 μm.

24. The process as claimed in claim 22, wherein the fluoropolymer has an average particle size d50 in agglomerated form of from 100 μm to 1 mm.

25. The process as claimed in claim 22, wherein the mixture of carbon nanotubes and fluoropolymer is produced by dry mixing.

26. The process as claimed in claim 25, wherein the dry mixing is carried out in a first phase until a homogeneous premix is obtained, with the temperature of the material being mixed being not more than 25° C.

27. The process as claimed in claim 25, wherein the dry mixing is carried out in a second phase, after obtaining a homogeneous premix from the first phase, using mixing tools, with the temperature of the mixture being more than 30° C.

28. The process as claimed in claim 22, wherein compaction is carried out by means of rollers in a roller apparatus, with the linear pressing force exerted by the roller(s) used on the support element and the sprinkled-on powder mixture preferably being from 0.1 to 1 kN/cm.

29. The process as claimed in claim 22, wherein rolling is carried out at a constant ambient temperature of the manufacturing rooms, in particular at a temperature of not more than 20° C.

30. The process as claimed in claim 22, wherein the mixture of carbon nanotubes and fluoropolymer comprises from 1 to 70% by weight of PTFE and 99-30% of carbon nanotubes.

31. The process as claimed in claim 22, wherein the electrically conductive support element is a mesh, nonwoven, foam, woven fabric, braid or expanded metal.

32. The process as claimed in claim 22, wherein the support element consists of carbon fibers, nickel, silver or nickel coated with noble metal.

33. A gas diffusion electrode for the reduction of oxygen, where the gas diffusion electrode has at least one sheet-like electrically conductive support element and a gas diffusion layer and electrocatalyst applied to the support element, wherein the gas diffusion layer consists of a mixture of carbon nanotubes and PTFE, with the carbon nanotubes and fluoropolymer having been applied in powder form to the support element and compacted and the carbon nanotubes forming the electrocatalyst.

34. The gas diffusion electrode as claimed in claim 33, wherein the electrode has been produced by a process as claimed in claim 22.

35. The gas diffusion electrode as claimed in claim 33, wherein the mixture of carbon nanotubes and PTFE, contains from 1 to 70% of PTFE and 99-30% of carbon nanotubes.

36. The gas diffusion electrode as claimed in claim 33, wherein the electrode has a thickness of from 0.1 to 3 mm.

37. The gas diffusion electrode as claimed in claim 33, wherein the gas diffusion layer has been applied on one or both sides to the surfaces of the support element.

38. The gas diffusion electrode as claimed in claim 33, wherein the carbon nanotubes have a content of catalyst residues of the catalyst used for producing the carbon nanotubes of less than 1% by weight.

39. The gas diffusion electrode as claimed in claim 33, wherein the carbon nanotube powder is present as agglomerate, with at least 95% by weight of the agglomerate particles having an external diameter in the range from 30 μm to 5000 μm.

40. The gas diffusion electrode as claimed in claim 33, wherein the proportion of nitrogen in the form of nitrogen chemically bound to the carbon nanotubes is less than 0.5% by weight.

41. A method comprising utilizing the gas diffusion electrode as claimed in claim 33 as oxygen-depolarized electrode for the reduction of oxygen in an alkaline medium or as electrode in an alkaline fuel cell or as electrode in a metal/air battery.

42. An electrolysis apparatus comprising a gas diffusion electrode as claimed in claim 33 as oxygen-depolarized cathode.

Description

EXAMPLES

Example 1

[0063] The production of an electrode is described below.

[0064] 15 g of a powder mixture, consisting of 40% by weight of PTFE powder Dyneon grade TF2053Z and 60% by weight of CNT powder (produced as described in WO 2009/036877A2, example 2), average agglomerate diameter about 450 μm (d50 by means of laser light scattering), bulk density about 200 g/l, content on residual catalyst (Co and Mn) about 0.64% by weight and a nitrogen content of 0.18% by weight, were premixed in a first phase at a temperature of about 19° C. to give a homogeneous mixture and then heated to 50° C. in a drying oven and introduced into a mixer from IKA which had been preheated to 50° C. The IKA mixer was equipped with a star whirler as mixing element and was operated at a speed of rotation of 15 000 rpm. The mixing time in the second phase of the mixing process was 60 seconds, with mixing being interrupted after every 15 seconds to detach mixed material on the wall. The temperature of the powder mixture after the second mixing phase was 49.6° C. Heating of the powder during the mixing process was not observed. The powder mixture was cooled to room temperature. After cooling, the powder mixture was sieved using a sieve having a mesh opening of 1.0 mm. The powder mixture had a bulk density of 0.0975 g/cm.sup.3.

[0065] The sieved powder mixture was subsequently applied to a mesh made of gilded nickel wires having a wire thickness of 0.14 mm and a mesh opening of 0.5 mm. Application was carried out with the aid of a 4 mm thick template, with the powder being applied using a sieve having a mesh opening of 1 mm. Excess powder projecting above the thickness of the template was removed by means of a scraper. After removal of the template, the support element with the applied powder mixture was pressed by means of a roller press at a pressing force of 0.45 kN/cm. The gas diffusion electrode was taken from the roller press. The density of the electrode without the support element was 0.5 g/cm.sup.3, giving a compaction ratio of 5.28. The thickness of the finished electrode was 0.6 mm.

[0066] The oxygen-depolarized cathode (ODC) produced in this way was installed in a laboratory electrolysis cell with an active area of 100 cm.sup.2 and operated under the conditions of chloralkali electrolysis. An ion-exchange membrane from DuPONT, type N982WX, was used here. The sodium hydroxide gap between ODC and membrane was 3 mm. A titanium anode consisting of an expanded metal having a commercial DSA® Coating for chlorine production from Denora was used as anode. The cell voltage at a current density of 4 kA/m.sup.3, an electrolyte temperature of 90° C., a sodium chloride concentration of 210 g/l and a sodium hydroxide concentration of 32% by weight was on average 2.20 V. The experiment could be operated for 120 days without an increase in voltage.

Example 2 Comparative Example—Carban Black—Support Element Silver Mesh

[0067] The production of the electrode was carried out as described in example 1, but Vulcan carbon black grade XC72R from Cabot was used instead of the CNTs.

[0068] The cell voltage was 2.20V at the beginning of the experiment and remained constant for 7 days. After the 7.sup.th day, the cell voltage increased continuously by 16 mV every day. On the 19.sup.th day of operation, the cell voltage was 2.40V. The used electrode displayed mechanical deformation resulting from swelling of the electrode coating. This means that this material does not have long-term stability.

Example 3 Comparative Example—Use of Nitrogen-Doped Carbon Nanotubes

[0069] Nitrogen-modified carbon nanotubes NCNTs were produced by means of a catalyst as described in WO2007/093337A2 (example 1, catalyst 1), which was introduced into a fluidized-bed reactor (diameter 100 mm). 60 g of the catalyst and 200 g of NCNTs (from a preliminary experiment) were firstly introduced into the reactor and reduced at 750° C. in a stream of 27 liters/minute of hydrogen and 3 liters/minute of nitrogen for 30 minutes, before the hydrogen stream was switched off, the nitrogen stream was increased to 21.5 liters/minute and the introduction of pyridine at a feed rate of 30 g per minute was commenced at the same time and carried on for a time of 30 minutes, likewise at 750° C. After cooling, about 400 g of NCNTs having a nitrogen content of 5.1% by weight were obtained. Further NCNT materials were produced analogously, and a mixture of at least 2 NCNT production batches was subsequently produced and then used for electrode production.

[0070] These NCNTs having a nitrogen content of 5.1% by weight were processed instead of the CNTs by the process described above in example 1 to give an electrode. The potential of the half cell was 387 mV relative to the RILE. The potential of the electrode based on NCNT is obviously significantly lower than the potential of the corresponding electrode based on CNT (example 1).

Example 4

[0071] For this example, use was made of a CNT material which had been produced in a similar way to the CNT material of example 1, with the difference that the material used for example 4 was specially cleaned in order to remove the residual content of catalyst from the fluidized-bed production. The purified CNT material had a residual content of CNT catalyst (Co and Mn) of 0.02% by weight. The CNTs used had an N content of 0.15% by weight. In a laboratory cell test, the ODC displayed an average cell voltage over 16 days of 2.18 V and the cell voltage was thus 20 mV below the cell voltage of an ODE produced from unpurified CNT material (example 1).