METHOD FOR ELECTROCHEMICAL OXYGEN REDUCTION IN ALKALINE MEDIA

Abstract

The invention relates to a method for electrochemical reduction of oxygen in alkaline media, a catalyst comprising nitrogen-doped carbon nanotubes (NCNTs) having nanoparticles located on their surface being used.

Claims

1-10. (canceled)

11. A process for the electrochemical reduction of oxygen in alkaline media having a pH of more than 10, comprising carrying out in the presence of a catalyst comprising nitrogen-doped carbon nanotubes comprising metal nanoparticles having an average particle size in the range from 1 to 15 nm present in a proportion of from 2 to 60% by weight on a surface thereof, wherein at least 40 mol % of the nitrogen is pyridinic nitrogen and said metal nanoparticles comprise silver Ag.

12. The process as claimed in claim 11, wherein the nitrogen-doped carbon nanotubes have a proportion of nitrogen of at least 0.5% by weight.

13. The process as claimed in claim 11, wherein the metal nanoparticles consist of silver (Ag).

14. A process as claimed in claim 11, wherein the catalyst is produced by a process comprising at least a) providing nitrogen-doped carbon nanotubes (NCNTs) having a proportion of at least 0.5% by weight of nitrogen as suspension (A) in a first solvent, b) providing a suspension (B) of metal nanoparticles in a second solvent, c) mixing of the suspensions (A) and (B) to give a suspension (C) and d) separating the nitrogen-doped carbon nanotubes (NCNTs) now loaded with metal nanoparticles from the suspension (C).

15. The process as claimed in claim 14, wherein the suspension (B) is obtained by b1) a solvent (A) containing a metal salt is provided and in a step b2) the metal salt in the solvent (A) is subsequently reduced to metal nanoparticles to give a suspension (B).

16. The process as claimed in claim 14, wherein the first solvent and the second solvent as per steps a) and b) of the process are selected independently from the group consisting of water, alcohols, toluene, cyclohexane, pentane, hexane, heptane, octane, benzene, xylenes and mixtures thereof.

17. A nitrogen-doped carbon nanotubes (NCNTs) loaded with metal nanoparticles on a surface thereof and capable of being used for the electrochemical reduction of oxygen in alkaline media having a pH of more than 10.

18. The process as claimed in claim 12, wherein the proportion of nitrogen is 0.5 to 18% by weight.

19. The process as claimed in claim 11, wherein at least 50 mol % of the nitrogen is pyridinic nitrogen.

20. The process as claimed in claim 18, wherein the proportion of nitrogen is 1 to 16% by weight, wherein at least 50 mol % of the nitrogen is pyridinic nitrogen and wherein the metal nanoparticles consist of silver (Ag).

Description

[0075] In addition, the invention is illustrated with the aid of figures, without being restricted thereto.

[0076] FIG. 1 shows the result of the TEM examination of the catalyst as per example 1 from example 6.

[0077] FIG. 2 shows the result of the TEM examination of the catalyst as per example 2 from example 6.

[0078] FIG. 3 shows the result of the TEM examination of the catalyst as per example 3 from example 6.

[0079] FIG. 4 shows the results of the measurements of the process using the catalysts from examples 1 to 3 as per examples 7 and 8. The curve of the reduction current (1) versus the applied voltage (U) relative to an Ag/AgCl reference electrode is shown, and also the voltage on the x axis at a reduction current of in each case −10.sup.−4 A.

[0080] FIG. 5 shows a comparison of the overvoltages (U) for the reduction of one mole of oxygen for the processes (1-2) according to the invention using the catalysts as per examples 1 and 2 compared to the processes (3-4) which are not according to the invention using the catalysts as per examples 3 and 4.

EXAMPLES

Example 1: Production of a Catalyst which can be Used in the Process of the Invention

[0081] Nitrogen-doped carbon nanotubes were produced as described in example 5 of WO 2009/080204 with the only differences that pyridine was used as starting material, the reaction was carried out as a reaction temperature of 700° C. and the reaction time was restricted to 30 minutes.

[0082] Residual amounts of the catalyst used (a catalyst was prepared as described in example 1 of WO 2009/080204 and used) were removed by washing the nitrogen-doped carbon nanotubes obtained in 2 molar hydrochloric acid for 3 hours under reflux.

[0083] Some of the nitrogen-doped carbon nanotubes obtained were subjected to the examination as per example 5. An amount of 800 mg of the nitrogen-doped carbon nanotubes were introduced into 100 ml of cyclohexane and treated with ultrasound for 15 minutes (ultrasonic bath, 35 kHz).

[0084] A suspension of silver nanoparticles was obtained by firstly dissolving 22.7 g (131.4 mmol) of decanoic acid (>99%, Acros Organics) and 30 g (131.4 mmol) of myristic acid (>98%, Fluka) in 500 ml of toluene (>99.9%, Merck). 22.3 g (131.4 mmol) of silver nitrate (>99%, Roth) dissolved in 25 ml of deionized water were added thereto. After the addition, the mixture was stirred for 5 minutes.

[0085] 19.2 g (263 mmol) of n-butylamine (>99.5%, Sigma-Aldrich) were subsequently added dropwise over a period of three minutes while stirring. 1.24 g (32.9 mmol) of sodium borohydride (>98%, Acros Organics) which had previously been dissolved in 40 ml of ice-cooled deionized water were then added to the reaction mixture over a period of 15 minutes while stirring.

[0086] After continuing to stir at room temperature for four hours, a 7-fold excess of acetone (>99.9%, Kraemer & Martin GmbH) based on the volume of toluene used was added, resulting in precipitation of a solid from the solution, with the solid subsequently being filtered off.

[0087] The moist, filtered solid obtained was washed with acetone and dried at about 50° C. in a vacuum drying oven (pressure ˜10 mbar) for two hours.

[0088] About 200 mg of the dry solid were dispersed in 30 ml of cyclohexane and this dispersion was then combined with 100 ml of the above-described dispersion of the nitrogen-doped carbon nanotubes.

[0089] The mixture formed was stirred until the dispersion medium had become completely decolorized (<2 h).

[0090] The mixture was subsequently filtered (Blue Band round filter, Schleicher&Schüll) and the catalyst obtained as filter cake was washed with acetone and again dried at 50° C. in a vacuum drying oven (pressure ˜10 mbar) for two hours.

[0091] The quantitative elemental analysis (inductively coupled plasma optical emission spectroscopy “ICP-OES”, instrument: Spectroflame D5140, from Spectro, method according to the manufacturer's instructions) subsequently carried out to determine the silver content indicated a loading of 19.0% by weight.

[0092] The catalyst obtained was subsequently partly passed to the examination as per example 6 and partly to example 7.

Example 2: Production of a Catalyst which can be Used in the Process of the Invention

[0093] Nitrogen-doped carbon nanotubes were produced in a manner analogous to example 1 with the sole difference that the reaction was now carried out for 60 minutes.

[0094] The nitrogen-doped carbon nanotubes were likewise partly passed to the examination as described in example 5 before mixing with a silver nanoparticle dispersion.

[0095] This was once again followed by a treatment the same as that in example 1 for applying silver nanoparticles. Quantitative elemental analysis (ICP-OES) for silver indicated a loading of 20.6% by weight.

[0096] The catalyst obtained was subsequently passed both partly to the examination as per example 6 and partly to example 7.

Example 3: Production of a Catalyst which Cannot be Used in the Process of the Invention

[0097] An experiment as described in example 1 was carried out with the sole difference that commercial carbon nanotubes (BayTubes®, from BayTubes) were now used instead of the nitrogen-doped carbon nanotubes used there.

[0098] An examination as described in example 5 was not carried out due to the lack of nitrogen constituents in the commercial carbon nanotubes.

[0099] Quantitative elemental analysis (ICP-OES) for silver indicated a loading of 20.6% by weight.

[0100] The catalyst obtained was subsequently passed both partly to the examination as per example 6 and partly to example 8.

Example 4: Production of a Further Catalyst which Cannot be Used in the Process of the Invention

[0101] Nitrogen-doped carbon nanotubes were produced in a manner analogous to example 1. In contrast to example 1, these were not subsequently loaded with silver nanoparticles. The catalyst obtained in this way was passed to example 8.

Example 5: X-Ray Photoelectron Spectroscopic Analysis (ESCA) of the Catalysts from Example 1 and Example 2

[0102] The proportion by mass of nitrogen in the nitrogen-doped carbon nanotubes and also the molar proportion of various nitrogen species in the proportion by mass of nitrogen found in the nitrogen-doped carbon nanotubes were determined for the nitrogen-doped carbon nanotubes as obtained in the course of example 1 and of example 2 by means of X-ray photoelectron spectroscopic analysis (ESCA; instrument: ThermoFisher, ESCALab 220iXL; method: according to the manufacturer's instructions). The values determined are summarized in table 1.

TABLE-US-00001 Measured values as per example 5 Pyridine N content Pyridine Amine Pyrrole Quaternary Quaternary (oxidized) [% by N N N N N N.sup.+—O NO.sub.x Sample weight] [mol %] [mol %] [mol %] [mol %] [mol %] [mol %] [mol %] Ex. 1 6.1 51.4 0 21.8 11.3 8.4 4.4 2.7 Ex. 2 9.9 42.6 0 13.5 27.2 6.7 6.6 3.4

Example 6: Transmission Electron Microscopic (TEM) Examination of the Catalysts as Per Example 1, Example 2 and Example 3

[0103] The catalysts obtained as described in examples 1 to 3 were subsequently optically examined for their loading with silver under a transmission electron microscope (TEM. Philips TECNAI 20, with 200 kV acceleration voltage).

[0104] The catalysts as per example 1 and example 2 are shown in FIGS. 1 and 2, respectively. FIG. 3 shows a transmission electron micrograph of a catalyst as per example 3. All three figures confirm the high silver loading of about 20% by weight determined by means of elemental analysis.

[0105] As a result of the use of additives for stabilizing the silver nanoparticles during the synthesis, agglomeration of the silver was generally prevented and the average silver particle size is less than 10 nm after application to the nitrogen-doped and undoped carbon nanoparticles for all three examples.

[0106] The differences in the activity for the electrochemical reduction of oxygen determined in examples 7 and 8 and shown in FIG. 4 can thus be attributed to synergistic effects between the carbon support material and the silver nanoparticles. The silver loading or silver particle size, on the other hand, can thus be ruled out as cause of the differing activity.

Example 7: Process According to the Invention Using Catalysts from Examples 1 and 2

[0107] 80 mg of the catalysts from example 1 or example 2 were firstly dispersed in 50 ml of acetone and 20 μl of this dispersion were in each case dripped onto a polished electrode surface of a rotating annular disk electrode (from Jaissle Elektronik GmbH).

[0108] After evaporation of the acetone, about 10 μl of a saturated polyvinyl alcohol solution was dripped on to fix the solid.

[0109] The rotating annular disk electrode, now comprising the catalysts as per example 1 or example 2 was then used as working electrode in a laboratory cell containing 3 electrodes (working electrode, counterelectrode and reference electrode).

[0110] The arrangement used is generally known as a three-electrode arrangement to those skilled in the art. A 1 molar NaOH solution in water which had been saturated with oxygen beforehand by passing a gas stream of pure oxygen through it was used as electrolyte surrounding the working electrode.

[0111] A commercial Ag/AgCl electrode (from Mettler-Toledo) was used as reference electrode.

[0112] The electrolyte was maintained at 25° C. The reduction of the oxygen dissolved molecularly in the electrolyte was likewise carried out at this temperature, which was controlled.

[0113] A potential difference between the working electrode and the reference electrode in the range from +0.14 V to −0.96 V was then set and the reduction current curve was then measured. The abovementioned range from +0.14 V to −0.96 V was measured at a speed of 5 mV/s.

[0114] The speed of rotation of the annular disk electrode was 3600 rpm.

[0115] To determine the advantageous nature of a process carried out according to the invention for reducing oxygen in alkaline media, the potential difference between working electrode and reference electrode at a current of 10.sup.−4 A was in each case read off from the graphs recorded by means of the above measurement. A graph for the measurements relating to the process according to the invention using the catalysts from examples 1, 2 and 3 is shown in FIG. 4.

[0116] It can be seen that the potential difference between reference electrode and working electrode when using the catalyst as per example 1 is about −0.116 V and that when using the catalyst as per example 2 is about −0.137 V and in the case of the process which is not according to the invention using the catalyst as per example 3 is about −0.208 V when the respective potential difference is read off at a current of 10.sup.−4 A.

[0117] All results obtained in this way for the experiments of examples 7 and 8 are summarized once more in FIG. 5 as overvoltages relative to an Ag/AgCl reference electrode in 1 N NaOH under standard conditions (23° C., 1013 hPa) to be overcome in the respective processes.

[0118] It can be seen in FIG. 5 that a potential difference of (−) 0.116 V or (−) 0.137 V prevails in the process according to the invention. This means that, in the process of the invention, only an overvoltage of 0.316 V or 0.337 V (assuming a redox potential of oxygen of 0.2 V relative to an Ag/AgCl reference electrode) prevails, so that only this has to be overcome in order to achieve flow of current and thus reduction of oxygen. In contrast, an overvoltage of 0.408 V or 0.36 V has to be overcome in the processes which are not according to the invention. Merely a reduced energy consumption per mole of reduced oxygen is therefore required in the processes according to the invention.

Example 8: Process which is not According to the Invention Using Catalysts from Examples 3 and 4

[0119] An experiment identical to that in example 7 was carried out with the sole difference that the catalysts as per examples 3 and 4 were used. The result of the process which is not according to the invention is, as described above for example 7, also shown for the catalyst as per example 3 in FIG. 4. The result from the process which is not according to the invention using the catalyst as per example 4 is, for reasons of clarity, no longer shown in FIG. 4. The summarized experimental data for examples 7 and 8 are shown in FIG. 5. It can be seen that, as shown above in the course of the discussion of example 7, that the processes which are not according to the invention have a higher energy consumption for the reduction of oxygen as a consequence.