Method for Producing a Catalyst Material for an Electrode of an Electrochemical Cell

Abstract

A method for producing a catalyst material for an electrode of an electrochemical cell includes doping a carbon material with nitrogen atoms, where the doping includes: bringing a carbon material into contact with urea at a temperature in a temperature range from 750° C. to 850° C.; bringing an oxidized carbon material into contact with cyanamide at a temperature in a temperature range from 550° C. to 650° C.; or bringing an oxidized carbon material into contact with melamine at a temperature in a temperature range from 550° C. to 650° C.

Claims

1-10. (canceled)

11. A process for producing a catalyst material for an electrode of an electrochemical cell, the process comprising: doping a carbon material with nitrogen atoms, the doping including: bringing the carbon material into contact with urea at a temperature in a temperature range of 750° C. to 850° C.; or bringing an oxidized carbon material into contact with cyanamide at a temperature in a temperature range of 550° C. to 650° C.; or bringing an oxidized carbon material into contact with melamine at a temperature in a temperature range of 550° C. to 650° C.

12. The process according to claim 11, wherein the doping of the carbon material with nitrogen atoms is carried out in a tubular oven.

13. The process according to claim 11, wherein the doping of the carbon material with nitrogen atoms is carried out for 1.5 to 12 hours.

14. The process according to claim 13, wherein the doping of the carbon material with nitrogen is carried out for 2 to 7 hours.

15. The process according to claim 14, wherein the doping of the carbon material with nitrogen is carried out for 2 to 4 hours.

16. The process according to claim 11, wherein the carbon material is selected from the group consisting of carbon black, graphite, and graphitized carbon.

17. The process according to claim 11, further comprising, after doping the carbon material with nitrogen atoms, washing the doped carbon material with water.

18. The process according to claim 11, further comprising oxidizing the carbon material to form the oxidized carbon material.

19. The process according to claim 18, wherein the oxidation of the carbon material is carried out by reacting the carbon material with a 70% by weight aqueous HNO.sub.3 solution under reflux.

20. The process according to claim 11, further comprising, after doping the carbon material with nitrogen atoms, adding platinum or a platinum-containing alloy to the carbon material doped with nitrogen atoms.

21. The process according to claim 20, wherein a platinum or platinum-containing alloy content, based on the carbon material doped with nitrogen atoms, is 5 to 50% by weight.

22. The process according to claim 11, wherein the doping of the carbon material with nitrogen atoms is carried out such that the carbon material is doped with 0.4% by weight to 2% by weight nitrogen.

23. The process according to claim 22, wherein the doping of the carbon material with nitrogen atoms is carried out such that the carbon material is doped with 0.8% by weight to 1.5% by weight nitrogen.

24. The process according to claim 11, wherein the electrode is formed as cathode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Further details, features and advantages of the invention are apparent from the following description and figures. Shown are:

[0021] FIG. 1 a diagram illustrating power densities of an electrochemical cell using carbon materials doped with various dopants at 600° C.,

[0022] FIG. 2 a diagram illustrating power densities of an electrochemical cell using carbon materials doped with various dopants at 800° C. and

[0023] FIG. 3 a diagram illustrating power densities of an electrochemical cell using oxidized carbon materials doped with various dopants at 600° C.

DETAILED DESCRIPTION OF THE DRAWINGS

[0024] The diagram in FIG. 1 shows in detail the power densities i of electrochemical cells, measured in A/cm.sup.2 against the voltage in volts. The temperature of the cell was 90° C. and the pressure was 170 kPa.sub.a,c. Unless explicitly defined otherwise, all % figures refer to % by weight (wt %).

[0025] The cell was produced as follows:

[0026] 1. Catalyst Production

[0027] Firstly, commercially available Ketjenblack EC-300J (manufacturer: Akzo Nobel) was stirred under reflux with 100 ml of aqueous 70% by weight HNO.sub.3 solution for 0.5 hours, wherein an oil bath heated at a temperature of 70° C. was used to produce the reflux. After the reaction time, the oxidized carbon was filtered and washed with hot water in order to remove all acid residues. The oxidized carbon was then dried in an oven.

[0028] Subsequently, the oxidized carbon (see the table below for details) was mixed with various nitrogen sources (i.e. with melamine, urea or cyanamide). Also shown in the table below are the ratios of carbon to nitrogen source.

[0029] The mixture was then heated in a tubular oven under constant nitrogen flow at the respective specified temperatures (see T.sub.set° C.) at a heating rate of 400 K/h and maintained for 2.5 hours.

[0030] Subsequently, a catalyst was produced using the oxidized and doped carbon by depositing platinum on the respective carbon by means of a polyol process. The polyol process comprised mixing 300 mg of functionalized carbon with 200 ml of ethylene glycol, 100 ml of deionized water and 1.35 ml of a 0.25 mol/L concentrated H.sub.2PtCl.sub.6 solution. The resulting dispersion was first stirred at 25° C. for 18 hours and then at 120° C. under reflux for 2 hours. After completion of the reaction, the catalyst was filtered off and washed with hot water in order to remove reaction residues and traces of chloride. The resulting catalyst powder was then dried in a vacuum oven.

TABLE-US-00001 The tables below give an overview of the tests carried out: Nitrogen source Melamine Cyanamide Urea Temperature 600 600 800 600 600 800 600 600 800 [° C.] Oxidation with No Yes No No Yes No No Yes No HNO.sub.3 % by weight 0.93 1.13 0.73 0.95 1.29 0.78 0.90 1.29 0.60 nitrogen Mol of nitrogen 6.9 3.4 43.0 6.0 2.9 30.4 56.2 29.6 154.8 source/mol of carbon

[0031] 2. Cell Production

[0032] All membrane electrode assemblies produced were produced using a decal transfer process. For this purpose, catalyst inks were produced by mixing a catalyst powder as prepared above with water followed by 1-propanol and at least one ionomer dispersion containing water (725 EW 3M dry powder dispersed in 40% H.sub.2O/60% 1-propanol, which resulted in an 18% by weight ionomer solution). The catalyst inks were then coated on PTFE using a Mayer rod. The coated decal was then dried.

[0033] For both electrodes, the ionomer carbon ratio by weight (I/C) was adjusted to 0.65. For the cathode catalyst, the catalyst with the modified nitrogen was used (melamine, urea, cyanamide), with 20% by weight Pt deposited on the carbon support. The % by weight nitrogen of all carbons ranged in a range between 0.8 to 1.2% by weight nitrogen.

[0034] All anode electrodes comprised 30% by weight Pt on graphitized Ketjenblack (TEC10EA30E, sold by Tanaka Kikinzoco).

[0035] The Pt loading of the cathode electrodes was 0.11 mg.sub.Pt/cm.sup.2 with a nominal layer thickness of about 14 μm when 20% by weight PUC-modified (=N-doped carbon) was used. All anodes were loaded with about 0.1 mg.sub.Pt/cm.sup.2 when 30% by weight Pt/C (non-modified carbon=non N-doped carbon) was used.

[0036] 5 cm.sup.2 MEAs were produced by hot-pressing a 10 μm thick membrane (Gore MX20.10) disposed between the anode decals and cathode decals as produced above at 155° C. for 3 minutes and subsequent application of force of 0.24 kN/cm.sup.2. The MEAs were then laminated between two seals (200×200 mm, 25 μm PEN film, CMC 61325 coated with a heat-activatable adhesive (approximate thickness: 15 μm) from CMC Klebetechnik, Germany), resulting in an active MEA surface area of 5 cm.sup.2 (50×10 mm).

[0037] All tests were conducted using a single cell test station using modified single cell hardware (Tandem Technologies Ltd.) equipped with 14 graphite channel composite flow fields containing serpentine channels. The contact pressure of the cell was set to 9 bar and the compression of the gas diffusion layers (SGL 29BC in both electrodes, anode and cathode) was set to 20% using non-compressible glass fibre PTFE seals (Fiberflon).

[0038] The polarization curves of FIGS. 1 to 3 were recorded at a cell temperature of 90° and a relative humidity (RH) at 30% RH.

[0039] The flow remained constant at 1000 nccm H.sub.2 on the anode side and 2000 nccm air on the cathode side, while the outlet pressure was set to 170 kPa on both sides. The very small amount of water produced and the excess of reaction gases ensured that the inlet humidity (RH) corresponded to the outlet humidity (RH), also taking into account a pressure drop across the active area of the MEA of about 1 kPa.

[0040] FIG. 1 shows a diagram illustrating the power densities of an electrochemical cell using carbon materials doped at 600° C. with different dopants.

[0041] Curve 1 shows the test results of an electrochemical cell in which a carbon material was used which had been doped with urea at a temperature of 600° C.

[0042] Curve 2 shows the test results of an electrochemical cell in which a carbon material was used which had been doped with melamine at a temperature of 600° C.

[0043] Curve 3 shows the test results of an electrochemical cell in which a carbon material was used which had been doped with NH.sub.3 at a temperature of 600° C.

[0044] Curve 4 shows the test results of an electrochemical cell in which the same carbon material was used as in the electrochemical cells above but without nitrogen doping.

[0045] Curve 5 shows the test results of an electrochemical cell in which a carbon material was used which had been doped with cyanamide at a temperature of 600° C.

[0046] From the diagram in FIG. 1, it can be deduced that the use of carbon material doped with urea at 600° C. achieves significantly better and thus higher power densities than all other doped carbon materials and in particular also the carbon material doped with NH.sub.3, which serves as a comparative example.

[0047] FIG. 2 shows a diagram in which the power densities of electrochemical cells i, measured in A/cm.sup.2, is plotted against the voltage in volts. The electrochemical cells were produced and measured analogously to those in FIG. 1.

[0048] Curve 1 shows the test results of an electrochemical cell in which a carbon material was used which had been doped with cyanamide at a temperature of 800° C.

[0049] Curve 2 shows the test results of an electrochemical cell in which a carbon material was used which had been doped with urea at a temperature of 800° C.

[0050] Curve 3 shows the test results of an electrochemical cell in which a carbon material was used which had been doped with NH.sub.3 at a temperature of 800° C.

[0051] Curve 4 shows the test results of an electrochemical cell in which a carbon material was used which had been doped with melamine at a temperature of 800° C.

[0052] Curve 5 shows the test results of an electrochemical cell in which the same carbon material was used as in the electrochemical cells above but without nitrogen doping.

[0053] From the diagram in FIG. 2, it can be deduced that the use of carbon material doped with urea and cyanamide at 800° C. achieves significantly better and thus higher power densities than all other doped carbon materials and in particular also the carbon material doped with NH.sub.3, which serves as a comparative example. In addition, carbon materials from all doping materials showed advantageous improvements in power densities compared to the undoped carbon material.

[0054] FIG. 3 shows a diagram in which the power densities of electrochemical cells i, measured in A/cm.sup.2, is plotted against the voltage in volts. The electrochemical cells were produced and measured analogously to those in FIG. 1, but with the difference that the carbon material was oxidized prior to doping, namely with a 70% by weight aqueous HNO.sub.3 solution under reflux for 30 minutes, in which the oxidized carbon material was washed prior to doping in order to remove residues of the oxidation reagents.

[0055] Curve 1 shows the test results of an electrochemical cell in which an oxidized carbon material was used which had been doped with cyanamide at a temperature of 600° C.

[0056] Curve 2 shows the test results of an electrochemical cell in which an oxidized carbon material was used which had been doped with melamine at a temperature of 600° C.

[0057] Curve 3 shows the test results of an electrochemical cell in which an oxidized carbon material was used which had been doped with NH.sub.3 at a temperature of 600° C.

[0058] Curve 4 shows the test results of an electrochemical cell in which an oxidized carbon material was used which had been doped with melamine at a temperature of 600° C.

[0059] Curve 5 shows the test results of an electrochemical cell in which the same oxidized carbon material was used as in the electrochemical cells above but without nitrogen doping.

[0060] From the diagram in FIG. 3, it can be deduced that the use of oxidized carbon material doped with cyanamide or melamine at 600° C. achieves significantly better and thus higher power densities than all other oxidized and doped carbon materials and in particular also the carbon material doped with NH.sub.3, which serves as a comparative example. In addition, oxidized carbon materials from all doping materials showed advantageous improvements in power densities compared to oxidized but non-doped carbon material.