Method for Producing a Carbon Material, Carbon Material, and Use of a Carbon Material in a Fuel Cell

Abstract

A method for producing a nitrogen-modified mesoporous and dendritic carbon material includes preparing a carbon precursor comprising a metal acetylide. The carbon precursor is mixed with a nitrogen precursor to form a starter mixture. Thereafter, a first heat treatment of the starter mixture is carried out at a temperature in the range of 40 to 80? C. under vacuum to form a metal inclusion compound. In a next step, a second heat treatment is carried out at a temperature in the range of 120 to 220? C. to produce an intermediate by decomposing the metal inclusion compound under a vacuum. The intermediate is treated to remove the metal, and finally consolidation of the treated intermediate is carried out by a third heat treatment at a temperature in the range of 200 to 1000? C. under vacuum or in an inert gas atmosphere to obtain the nitrogen-modified mesoporous and dendritic carbon material. Also described is a nitrogen-modified mesoporous and dendritic carbon material and a fuel cell comprising the carbon material.

Claims

1-11. (canceled)

12. A process for producing a nitrogen-modified mesoporous and dendritic carbon material, the process comprising: preparing a carbon precursor comprising a metal acetylide; mixing the carbon precursor with a nitrogen precursor to form a starter mixture; performing a first heat treatment of the starter mixture at a temperature in a range from 40 to 80? C. under reduced pressure to form a metal inclusion compound; performing a second heat treatment at a temperature in the range from 120 to 220? C. to break down the metal inclusion compound under the reduced pressure and produce an intermediate comprising a metal, the intermediate having a carbon lattice in which some carbon atoms are replaced by nitrogen atoms; treating the intermediate to remove the metal and form a treated intermediate, and consolidating the treated intermediate by performing a third heat treatment at a temperature in the range from 200 to 1000? C. under reduced pressure or in an inert gas atmosphere to obtain the nitrogen-modified mesoporous and dendritic carbon material.

13. The process according to claim 12, wherein the nitrogen precursor is selected from the group consisting of urea, cyanamide, melamine, and combinations thereof.

14. The process according to claim 12, wherein a molar ratio of nitrogen to carbon in the range from 0.05 to 1.5 is established in the starter mixture.

15. The process according to claim 14, wherein the molar ratio is in the range from 0.1 to 1.0.

16. The process according to claim 15, wherein the molar ratio is in the range from 0.3 to 0.92.

17. The process according to claim 12, wherein the third heat treatment is conducted at a temperature in the range from 600 to 900? C.

18. A nitrogen-modified mesoporous and dendritic carbon material formed by the process according to claim 12, wherein the carbon material has the carbon lattice in which some of the carbon atoms are replaced by nitrogen atoms, and the carbon material has a specific surface area in a range from 900 to 3000 m.sup.2/g determined by a BET method using a nitrogen adsorption isotherm.

19. The carbon material according to claim 18, wherein 0.5 to 1.5 percent by weight of the carbon atoms in the carbon lattice have been replaced by nitrogen atoms.

20. The carbon material according to claim 18, wherein the specific surface area is in the range from 1000 to 2150 m.sup.2/g.

21. The carbon material according to claim 20, wherein the specific surface area is in the range from 1300 to 2150 m.sup.2/g.

22. The carbon material according to claim 18, wherein the carbon material has a V.sub.meso/V.sub.total ratio in the range from 0.25 to 0.75, wherein V.sub.meso denotes a volume of all pores of the carbon material of a pore size in the range from 2.5 to 6.0 nm and V.sub.total denotes a total volume of all pores of the carbon material.

23. The carbon material according to claim 22, wherein the V.sub.meso/V.sub.total ratio is in the range from 0.30 to 0.65.

24. The carbon material according to claim 23, wherein the V.sub.meso/V.sub.total ratio is in the range from 0.30 to 0.6.

25. The carbon material according to claim 18, wherein the carbon material has a nitrogen uptake volume V.sub.N:0.4-0.8 in the range from 80 to 220 cm.sup.3 (STP)/g, where V.sub.N:0.4-0.8 denotes volume of nitrogen in a relative pressure range p/p.sub.0 from 0.4 to 0.8 of a nitrogen adsorption isotherm.

26. The carbon material according to claim 25, wherein the nitrogen uptake volume V.sub.N:0.4-0.8 is in the range from 100 to 200 cm.sup.3 (STP)/g.

27. The carbon material according to claim 26, wherein the nitrogen uptake volume V.sub.N:0.4-0.8 is in the range from 110 to 190 cm.sup.3 (STP)/g.

28. The carbon material according to claim 18, wherein the carbon material has a dV.sub.2.5-6 nm value in a range from 0.5 to 0.9 cm.sup.3/g, where the dV.sub.2.5-6 nm value describes a cumulative volume of pores having a pore diameter in a range from 2.5 to 6.0 nm based on a unit of weight, obtainable by derivation of cumulative pore volume based on pore diameter, followed by integration over respective pore diameter range.

29. The carbon material according to claim 28, wherein the dV.sub.2.5-6 nm value is in the range from 0.53 to 0.86 cm.sup.3/g.

30. The carbon material according to claim 29, wherein the dV.sub.2.5-6 nm value is in the range from 0.55 to 0.574 cm.sup.3/g.

31. A fuel cell comprising the nitrogen-modified mesoporous and dendritic carbon material according to claim 18.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0055] The figures show:

[0056] FIG. 1: a block diagram of the process of the disclosure for producing a nitrogen-modified mesoporous and dendritic carbon material,

[0057] FIG. 2: an SEM image of an intermediate obtained in the process of the disclosure as per FIG. 1,

[0058] FIG. 3: a detail of the SEM image from FIG. 2 in greater magnification,

[0059] FIG. 4: a TEM image of an intermediate obtained in the process of the disclosure as per FIG. 1,

[0060] FIG. 5: the cumulative specific surface area of the carbon materials of examples 1 to 5 as a function of pore diameter,

[0061] FIG. 6: the cumulative specific surface area of the carbon materials of examples 6 to 9 as a function of pore diameter,

[0062] FIG. 7: the cumulative specific surface area of the carbon materials of examples 10 to 13 as a function of pore diameter,

[0063] FIG. 8: the derivative of the pore volume with respect to the pore diameter as a function of pore diameter for examples 1 to 5,

[0064] FIG. 9: the derivative of the pore volume with respect to the pore diameter as a function of pore diameter for examples 6 to 9,

[0065] FIG. 10: the derivative of the pore volume with respect to the pore diameter as a function of pore diameter for examples 10 to 13,

[0066] FIG. 11: the relative weight of the intermediate from examples 1 to 5 as a function of temperature,

[0067] FIG. 12: the relative weight of the intermediate from examples 6 and 7 as a function of temperature, and

[0068] FIG. 13: the relative weight of the intermediate from examples 10 and 13 as a function of temperature.

DETAILED DESCRIPTION OF THE FIGURES

[0069] A commercially available porous and three-dimensional carbon material from AkzoNobel was used as a comparative example, which is sold under the name KETJENBLACK EC300j and is used for fuel cells.

[0070] A process of the disclosure is elucidated in detail hereinafter together with examples 2 to 13.

Step S1: Producing a Metal Acetylide as Carbon Precursor

[0071] First of all, a metal acetylide is produced as carbon precursor (step S1 in FIG. 1).

[0072] For this purpose, an aqueous ammoniacal silver nitrate solution is prepared by adding 20.34 mL of an aqueous ammonia solution (20 percent by weight of ammonia) to 414 mL of an aqueous silver nitrate solution (1.275 mg of silver nitrate), so as to establish a molar ratio of ammonia:silver of about 29:1.

[0073] Subsequently, the solution is purged with nitrogen or argon for 10 to 20 minutes in order to displace oxygen present in the solution.

[0074] Then gaseous acetylene is passed through the solution for about 5 minutes under treatment with ultrasound until the solution turns yellow.

[0075] Subsequently, acetylene is passed through the solution without ultrasound treatment for a further 5 minutes. The solution then turns gray to colourless, with precipitation of a white-grayish solid.

[0076] As soon as the solid precipitates out, the acetylene flow is stopped and the precipitate is obtained by filtering the solution through a membrane filter. The precipitate is washed with methanol and filtered again. This keeps the precipitate moist in order to prevent detonation of the precipitate.

Step S2: Mixing the Carbon Precursor with a Nitrogen Precursor to Give a Starter Mixture

[0077] The required amount of nitrogen precursor is weighed out and dissolved completely in methanol. The nitrogen precursor solution is initially charged in a Teflon reactor.

[0078] The filtrate of the carbon precursor is then added and hence mixed with the nitrogen precursor (cf. step S2 in FIG. 2).

[0079] Table 1 lists the nitrogen precursors used in examples 2 to 13 and the molar ratio of nitrogen to carbon used in the starter mixture.

[0080] In all examples, silver acetylide was used as the carbon precursor.

TABLE-US-00001 TABLE 1 Overview of examples Temperature in the Molar third heat Nitrogen nitrogen:carbon treatment in Designation precursor ratio ? C. Example 1* KETJENBLACK EC 300j Example 2 M2 cyanamide 0.0507 Example 3 M3 cyanamide 0.0634 200 Example 4 M4 cyanamide 0.0951 600 Example 5 M5 cyanamide 0.0951 900 Example 6 M6 melamine 0.1188 Example 7 M7 melamine 0.1585 200 Example 8 M8 melamine 0.5071 600 Example 9 M9 melamine 0.5071 900 Example 10 M10 urea 0.3097 Example 11 M11 urea 0.4954 200 Example 12 M12 urea 0.6193 600 Example 13 M13 urea 0.9290 900 *comparative example

Step S3: First Heat Treatment

[0081] The Teflon reactor containing silver acetylide as carbon precursor and the particular nitrogen precursor used is placed in a stainless steel cylinder having a diameter of 50 mm and a height of 70 mm, which is closed with a stainless steel lid. The lid has two valves, one of the valves being connected to a vacuum pump and the other of the valves being set up to release the reduced pressure within the stainless steel cylinder and flood it with air. As a safety measure, there is an additional pressure relief valve disposed between the stainless steel cylinder and the valve connected to the vacuum pump.

[0082] The stainless steel cylinder is closed tight, and the vacuum pump is switched on in order to dry the starter mixture and to establish an airless atmosphere within the stainless steel cylinder.

[0083] The stainless steel cylinder is heated under reduced pressure to a temperature of 80? C. by means of a heating mantle or heating bath overnight for a total of about 20 hours. Alternatively, an oven can also be used for heating.

[0084] In this way, the first heat treatment creates a metal inclusion compound in the Teflon reactor (step S3 in FIG. 1).

Step S4: Second Heat Treatment

[0085] Immediately after the first heat treatment, i.e. without removing the metal inclusion compound from the stainless steel cylinder, without removing the stainless steel cylinder from the oven, and still under reduced pressure, a second heat treatment is conducted to produce an intermediate with breakdown of the metal inclusion compound (step S4 in FIG. 1).

[0086] For this purpose, the stainless steel cylinder is heated under reduced pressure to a temperature of 220? C. in the oven for 15 minutes.

[0087] This heating results in onset of a spontaneous and explosive breakdown reaction of the silver acetylide, which gives the carbon material in the intermediate with a carbon lattice in which some of the carbon atoms have been replaced by nitrogen from the nitrogen precursor.

Step S5: Treatment of the Intermediate

[0088] The intermediate obtained from step S4 is removed from the Teflon reactor and immersed into a 65% solution of concentrated nitric acid at a temperature of 25? C. for 30 minutes.

[0089] In this way, silver present in the intermediate and carbon compounds present on the surface of the carbon material are washed out (step S5 in FIG. 1).

[0090] Subsequently, the intermediate is washed with water in order to remove all traces of nitric acid and further silver.

[0091] Finally, the intermediate thus treated is dried at 80? C. under reduced pressure for at least 12 hours.

[0092] For all of examples 2 to 13, the same residence time in the nitric acid and the same drying time were used.

[0093] FIGS. 2 and 3 show SEM images of the intermediate from example 2 after step S5, i.e. after the wash with nitric acid; FIG. 3 shows a detail from FIG. 2 with greater magnification.

[0094] The three-dimensional mesoporous and dendritic structure created in the intermediate is clearly apparent from these images. It should be noted that, because of the available resolution in FIGS. 2 and 3, what can be seen are essentially what are called primary aggregates composed of primary particles, and secondary agglomerates comprising a multitude of primary aggregates.

[0095] The primary particles themselves have the desired mesopores having a pore diameter in the range from 2.5 to 6.0 nm, which are of crucial significance for later use as catalyst support in fuel cells.

[0096] The porous structure of the primary aggregates is illustrated by the TEM image of the intermediate from Example 2 shown in FIG. 4.

Step S6: Consolidating the Intermediate/Third Heat Treatment

[0097] The washed intermediate is transferred into a tubular furnace and purged with argon for at least 10 minutes.

[0098] Subsequently, the intermediate is heated in an inert gas atmosphere using argon as inert gas for 20 hours, using the temperature listed in Table 1 for each of examples 2 to 13 (step S6 in FIG. 1).

[0099] A heating rate of 400 K/h is used for heating and an argon flow rate of 50 mL/min to maintain the inert gas atmosphere.

[0100] A nitrogen-modified mesoporous and dendritic carbon material is obtained. Tables 2 and 3 show the properties of the carbon materials obtained in examples 2 to 13 and of comparative example 1.

TABLE-US-00002 TABLE 2 Properties of the carbon materials Proportion of nitrogen in the carbon dV.sub.2.5-6 nm V.sub.N: 0.4-0.8 lattice (% by S.sub.BET in V.sub.meso/ in in Designation wt.) m.sup.2/g V.sub.total cm.sup.3/g cm.sup.3(STP)/g Example 1* 0 843 0.22 0.29 135 Example 2 0.68 1730 0.55 0.73 169 Example 3 0.33 1519 0.42 0.53 131 Example 4 0.71 2101 0.54 0.86 184 Example 5 0.72 1689 0.40 0.53 144 Example 6 1.07 1634 0.39 0.51 127 Example 7 0.89 1706 0.37 0.55 145 Example 8 1.1 1517 0.48 0.59 133 Example 9 0.11 1679 0.49 0.64 147 Example 10 1 1777 0.50 0.66 148 Example 11 1.47 1754 0.54 0.74 171 Example 12 1.33 1816 0.46 0.67 173 Example 13 0.85 1916 0.47 0.67 147 *comparative example

Elemental Analysis

[0101] The proportion of nitrogen in the carbon lattice of the carbon material was determined by elemental analysis. For this purpose, a Thermo Flash 1112 elemental analyzer from THERMO FINNIGAN was used to determine the proportions of carbon, hydrogen, nitrogen and sulfur.

[0102] The samples were burnt in the presence of V.sub.2O.sub.5 as oxidizing agent at a combustion temperature of 1020? C. by dynamic flash combustion (modified Dumas method). The breakdown took place in a manually layered reactor having WO.sub.3/Cu/Al.sub.2O.sub.3 layers. The gases formed were determined and quantified by gas chromatography (GC).

Microstructure Analysis

[0103] In order to examine the micro- and mesoporous structure of the samples and to determine the specific surface area, nitrogen isotherms (physisorption isotherms) were determined at a temperature of 77 K by means of an Autosorb-1 analysis instrument from QUANTACHROME.

[0104] The samples were each transferred into a glass tube having a diameter of 4 mm, which had additionally been filled with layers of glass wool and a glass rod in order to minimize the dead volume.

[0105] The mass of sample was chosen so as to achieve an absolute surface area of more than 10 m.sup.2, in order to reduce measurement errors.

[0106] The samples were degassed at 90? C. under reduced pressure for at least 24 hours in order to remove any adsorbent present, for example water or gas, before the measurement. The degassing temperature chosen was no higher in order to avoid breakdown of the nitrogen-containing groups of the carbon material.

[0107] The adsorption isotherms and the desorption isotherms of the nitrogen isotherms were recorded in the range of 10.sup.?5? p/p.sub.0? 0.995, where p.sub.0 indicates the saturation pressure and p the actual gas pressure.

[0108] The specific surface area S.sub.BET was determined by the BET method.

[0109] The V.sub.N:0.4-0.8 value indicates the difference in volume of adsorbed nitrogen at a p/p.sub.0 value of 0.8 and at a p/p.sub.0 value of 0.4 in cm.sup.3 (STP)/g or cc (STP)/g.

[0110] The characterization of the micro- and mesopores present was undertaken by numerical analysis using a DFT model, using the QSDFT kernel (QSDFT: Quenched Solid Density Functional Theory) with a model for slot-shaped (diameter <2 nm) and cylindrical pores (diameter >2 nm), based on the adsorption branch of the nitrogen isotherms.

[0111] Using the numerical analysis, the volume V.sub.meso of all pores having a diameter in the range from 2.5 to 6.0 nm and the total volume V.sub.total of all pores were ascertained.

[0112] The derivative of pore volume with respect to diameter, dV(d), was calculated using the values obtained from the QSDFT analysis.

[0113] It becomes clear from Table 2 that the carbon materials obtainable by the process of the disclosure have a high specific surface area S.sub.BET, especially a specific surface area several times higher than comparative example 1.

[0114] It also becomes clear from the V.sub.meso/V.sub.total ratio ascertained for the examples that the carbon materials obtainable by the process of the invention have a high proportion of mesopores having a pore diameter in the range from 2.5 to 6.0 nm.

[0115] This is also apparent from the plots shown in FIGS. 5 to 7, which indicate the cumulative specific surface area as a function of pore diameter. It is apparent that a significant proportion of the specific surface area S.sub.BET is generated by pores having a pore diameter in the range from 2.5 to 6.0 nm.

[0116] FIGS. 8 to 10 show the derivative of volume with respect to pore diameter as a function of pore diameter. The pore size distribution is particularly clear in this diagram. In particular, it is apparent that there is a narrow distribution of pores having a very small diameter in all examples. In addition, however, the carbon materials obtainable by the process of the invention have a significant proportion of pores having a pore diameter in the range from 2.5 to 6.0 nm, especially a greater proportion than is the case for comparative example 1 (cf. FIG. 8).

[0117] FIGS. 11 to 13 show the relative weight of the intermediate from examples 1 to 5, 6 and 7 and 10 and 13 as a function of temperature, as determined by thermogravimetry analysis by means of a STA-8000 from PerkinElmer. For the thermogravimetry analysis, a ceramic crucible was charged with the sample to be analyzed and the loss of mass was recorded at a heating rate of 5 K/min under argon atmosphere. After calibration at a gas pressure of 1.1 bar, the temperature was increased within a temperature range from 23 to 1100? C.

[0118] The third heat treatment is necessary in order to obtain sufficient stability of the carbon material. In particular, in the third heat treatment, organic residues that have an adverse effect on the performance of the carbon material of the invention are removed from the intermediate. At the same time, however, increasing degradation of the carbon material also takes place at high temperatures, and so a temperature of 1000? C. in the third heat treatment step should not be exceeded.

[0119] Thus, by means of the process of the disclosure, carbon materials that are firstly nitrogen-modified, and in this way show improved interaction with ionomers, and secondly have a mesoporous and dendritic structure with high surface area are producible in a simple manner, such that mass transfer in fuel cell applications is promoted, combined with a high specific surface area for high performance in such applications.