Carbon-containing membrane for water and gas separation

Abstract

The invention relates to a multilayer metallic or ceramic membrane device, comprising a macroporous carrier layer including pores having a pore diameter of more than 50 nm, and at least one mesoporous intermediate layer disposed thereon, including pores having a pore diameter of 2 nm to 50 nm. The membrane device according to the invention furthermore comprises at least one microporous cover layer disposed on the mesoporous intermediate layer, including pores having an average pore diameter of 0.3 nm to 1.5 nm, comprising graphite oxide or few-layer graphene oxide or graphite or few-layer graphene. In an advantageous embodiment, the cover layer comprises between 5 and 1000 layers of graphene oxide. In an advantageous embodiment, the cover layer can comprise between 5 and 1000 layers of partially reduced graphene oxide or graphene as a result of the at least partial reduction of the graphene oxide. The multilayer, chemically and mechanically stable and temperature-resistant membrane device according to the invention, comprising the functional cover layer thereof including microporous graphene oxide or graphene, is advantageously suitable for use in water separation or purification, or for gas separation.

Claims

1. A method for producing a membrane device, wherein at least one mesoporous intermediate layer is applied onto a macroporous, ceramic carrier layer, and a microporous cover layer comprising graphite oxide or few-layer graphene oxide or graphite is in turn applied thereon, the microporous cover layer being applied by way of a colloidal dispersion using dip coating and subsequently being dried at temperatures of up to 200 C.

2. The method according to claim 1, wherein a colloidal dispersion comprising graphene oxide particles having particle sizes of between 10 nm and 5 m is used.

3. The method according to claim 1, wherein a diluted colloidal dispersion having a solids content of graphene oxide particles of between 20 mg and 2 g per liter of colloidal dispersion is used.

4. A method according to claim 1, wherein at least one mesoporous intermediate layer comprising yttria-stabilized zirconia is applied onto the macroporous, ceramic carrier layer.

5. A method according to claim 1, wherein thermal treatment at temperatures of between 200 and 1000 C. takes place after the drying step.

6. A method according to claim 1, wherein a microporous cover layer comprising 5 to 1000 layers of graphene oxide is produced.

7. A method according to claim 1, wherein a microporous cover layer at least partially comprising graphene is produced.

8. A method according to claim 1, wherein said step of said microporous cover layer being applied by way of said colloidal dispersion using dip coating and subsequently being dried at temperatures of up to 200 C. takes place under a reducing atmosphere or under inert gas or under vacuum, the graphene oxide of the cover layer being at least partially reduced to graphene.

9. A method according to claim 1, wherein, after said step of said microporous cover layer being applied by way of said colloidal dispersion using dip coating and subsequently being dried at temperatures of up to 200 C., chemical reduction of the cover layer takes place at temperatures up to 200 C., the graphene oxide of the cover layer being at least partially reduced to graphene.

10. A multilayer membrane device, producible according to claim 1, comprising: at least one macroporous, ceramic carrier layer including pores having a pore diameter of >50 nm; at least one mesoporous intermediate layer disposed thereon, including pores having an average pore diameter of 2 nm to 50 nm; and at least one microporous cover layer disposed on the mesoporous intermediate layer, including pores having an average pore diameter of <0.5 nm, wherein the microporous cover layer comprises graphite oxide, partially reduced graphite oxide or graphite, and the membrane has molecular sieve properties.

11. The membrane device according to claim 10, comprising at least two mesoporous intermediate layers, in which the particle size, roughness and pore size decrease in the direction of the cover layer.

12. The membrane device according to claim 10, wherein said at least one mesoporous intermediate layer comprises a first and a second mesoporous intermediate layer, of which the first mesoporous intermediate layer is in contact with the macroporous carrier layer, and the second mesoporous intermediate layer is in contact with the microporous cover layer, at least one intermediate layer of the first and second mesoporous intermediate layer including pores having an average pore diameter of less than 5 nm.

13. The membrane device according to claim 12, including a macroporous carrier layer comprising Al.sub.2O.sub.3 TiO.sub.2, ZrO.sub.2, YSZ, SiO.sub.2, CeO.sub.2, MgO, Y.sub.2O.sub.3, Gd.sub.2O.sub.3, mullite, cordierite, zeolite, BaTiO.sub.3, metallic components, carbon, SiC, Si.sub.3N.sub.4, SiOC, SiCN, AlN or a mixture of the aforementioned materials.

14. A membrane device according to claim 10, comprising at least one ceramic mesoporous intermediate layer.

15. The membrane device according to claim 14, including a mesoporous intermediate layer comprising Al.sub.2O.sub.3, TiO.sub.2, ZrO.sub.2, YSZ, SiO.sub.2, CeO.sub.2, MgO, Y.sub.2O.sub.3, Gd.sub.2O.sub.3, ZnO, SnO.sub.2, mullite, cordierite, zeolite, metal organic framework (MOF) materials, clay, BaTiO.sub.3, carbon, SiC, Si.sub.3N.sub.4, AlN, SiOC, SiCN, or mixtures of the aforementioned materials.

16. A membrane device according to claim 10, wherein the first mesoporous intermediate layer has a layer thickness of between 1 m and 20 m.

17. A membrane device according to claim 10, wherein said at least one mesoporous intermediate layer comprises a first and a second mesoporous intermediate layer, and wherein the second mesoporous intermediate layer has a layer thickness of between 0.1 m and 2 m.

18. A membrane device according to claim 10, wherein the microporous cover layer has a layer thickness of between 3 nm and 2 m, and advantageously between 5 nm and 300 nm.

19. A membrane device according to claim 10, wherein the microporous cover layer comprises between 5 and 1000 layers of graphene oxide.

20. A membrane device according to claim 10, wherein the cover layer at least partially comprises graphene.

21. The membrane device according to claim 19, wherein the microporous cover layer comprises between 5 and 1000 layers of graphene oxide or graphene.

22. A membrane device according to claim 10, wherein the microporous cover layer has electrical conductivity of at least 15000 S/m after a heat treatment at 750 C. in Ar/3% H.sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) To further illustrate the invention, this will be described in greater detail based on several figures, diagrams and exemplary embodiments, without thereby limiting the invention. In the drawings:

(2) FIG. 1 shows a schematic illustration of the composition of the multilayer graphite or graphite oxide membrane device according to the invention;

(3) FIG. 2 shows a schematic illustration of the alternating method steps during the production of the membrane device according to the invention;

(4) FIG. 3 shows a schematic illustration of two alternative method steps for at least partial reduction of the graphite oxide into graphite;

(5) FIG. 4 shows test results of permporosimetry measurements on a membrane device according to the invention, comprising two different mesoporous intermediate layers;

(6) FIG. 5 shows a high resolution TEM image of the graphite cover layer;

(7) FIG. 6 shows test results of flux analyses for He, H.sub.2, CO.sub.2 and N.sub.2 on a membrane device according to the invention, based on two different cover layers; and

(8) FIG. 7 shows the Arrhenius plot of the flux results for He and H.sub.2, for ascertaining the corresponding activation energies.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) FIG. 1 shows a schematic cross-section of an embodiment of the invention. Building upon one another, the membrane comprises a metallic or ceramic porous carrier 1, a ceramic mesoporous intermediate layer 2 disposed on this macroporous carrier, and a microporous cover layer 3, which is disposed on this mesoporous intermediate layer and comprises graphite oxide or few-layer graphene oxide or graphite or few-layer graphene. The macroporous layer 1 can have a monolayer or multilayer graded design. The mesoporous intermediate layer 2 can also have a monolayer or multilayer graded design.

(10) FIG. 2 schematically shows the production processes for the membrane device according to the invention. On the left side (FIG. 2a), first, the application of the mesoporous intermediate layer 2 onto the macroporous carrier 1 is shown, while on the right side (FIG. 2b), the application of the functional cover layer 3 onto the mesoporous intermediate layer 2, in a subsequent method step, is shown.

(11) FIG. 3 shows the alternative methods for reducing the graphene oxide into graphene. Thermal reduction (left side), for example, under vacuum, inert gas or under a reducing atmosphere, (such as Ar/H.sub.2 mixture), necessitates higher temperatures and can only be used for membrane devices that comprise carrier and intermediate layers made of an appropriately temperature-resistant material. Alternatively, chemical reduction (right side) using a liquid or gaseous reducing agent is an obvious choice for materials that are not as temperature-resistant.

(12) Exemplary Embodiment 1:

(13) Production of a graded membrane comprising a flat or tubular macroporous carrier made of -Al.sub.2O.sub.3, a mesoporous intermediate layer made of zirconia stabilized with 8 mol % yttria (8YSZ) and a microporous graphite oxide cover layer.

(14) The flat carriers are composed of a monolayer and have pore sizes around approximately 80 nm (ascertained by way of mercury porosimetry). The tubular carriers are designed as graded layers. The -Al.sub.2O.sub.3 layers, which thereafter are coated with a mesoporous layer and a microporous layer, have a similar pore size in the range of approximately 70 nm.

(15) The production of the flat carriers included the following steps:

(16) 1) tape casting an -Al.sub.2O.sub.3 powder suspension (Sumitomo AKP-30 powder) under vacuum, in tapes having a layer thickness of approximately 3 mm and the desired size;

(17) 2) sintering the green sheets at temperatures around 1100 C.;

(18) 3) grinding; and

(19) 4) polishing one side of the tape with a diamond paste (Struers, DP paste 6 m and 3 m).

(20) The final layer thickness of the porous carrier thus produced is approximately 2 mm. Commercially available tubular, porous -Al.sub.2O.sub.3 membranes (Inopor), having a pore size in a range suitable for ultrafiltration, are used as the tubular carriers. These carriers were graded, which is to say the porous tubular carrier material was coated on the inside with three additional layers of -Al.sub.2O.sub.3.

(21) The uppermost -Al.sub.2O.sub.3 layer has a pore size around 70 nm. All carriers included glass seals at the ends. The cover layer was applied onto 105 mm and 250 mm long carriers having an outside diameter of 10 mm and an inside diameter of 7 mm.

(22) A graded mesoporous layer was applied by way of dip coating, wherein two different kinds of 8YSZ-containing liquids were used. The first liquid includes an aqueous dispersion comprising particles having a particle size around 60 nm (ascertained by way of dynamic light scattering). The second liquid includes a sol having a particle size of between 30 and 40 nm. Polyvinyl alcohol (PVA, 60,000 g/mol) was added to prevent extensive penetration of the 8YSZ particles during the coating step.

(23) All coating steps were carried out in a clean room. So as to protect the liquids from the influence of dust, these were cleaned prior to use with 0.8 m syringe filters (Whatman FP 30/0 8CA).

(24) So as to generate the coating liquid of the first mesoporous layer, 4 g of a commercially available 8% Y.sub.2O.sub.3-doped ZrO.sub.2 (8YSZ) nano powder (Sigma Aldrich catalog no. 572349) was dispersed in 100 ml of a 0.05 M (mol/L) solution of HNO.sub.3 in water with the aid of ultrasound. After 15 minutes of ultrasonic treatment, the dispersion was transferred into small centrifuge tubes and centrifuged at 6500 rpm for 4 minutes. This results in two phases comprising a solid sediment of the larger particles and a supernatant liquid, which contains the desired nanoparticles for coating the mesoporous layer. The supernatant is removed using a pipette.

(25) In a second step, a solution was prepared from polyvinyl alcohol (PVA) in aqueous HNO.sub.3 (0.05 M). For this purpose, 3 g PVA was added to 100 ml HNO.sub.3 solution, and the mixture was subsequently heated at 98 C. under reflux overnight (approximately 16 hours).

(26) The coating liquid for the first mesoporous intermediate layer is then generated by mixing the supernatant with the PVA solution at a ratio of 3 to 2.

(27) The flat carriers were coated with the liquid comprising the 8YSZ particles using spin coating on the one hand, and dip coating on the other. The tubular carriers were coated on the inside using special devices. These comprise a holder for the carrier to be coated and a mount for a bottle containing the coating liquid, which are connected to one another via a flexible line. While the carrier is fixed, the coating liquid is transferred into the carrier by raising the bottle containing the liquid. For example, the liquid level was thus raised at a speed of 10 mm/s until the carrier was completely filled. After 30 seconds, the liquid level was lowered again at a speed of 10 mm/s. The carrier thus coated was air dried for approximately 1 hour and then thermally treated. In a conventional oven, the layers were calcined under air at 500 C. for 2 hours. The heating and cooling steps took place at 1 C./min. In a typical synthesis pathway, the aforementioned steps involving coating, drying and heating were each carried out twice. With the aid of scanning electron microscope (SEM) analyses, the layer thickness of the produced mesoporous layer was determined to be 1 to 2 m.

(28) The pore size of this mesoporous 8YSZ layer was ascertained with the aid of permporosimetry. Permporosimetry is an established method for characterizing mesoporous layers. This measures the permeance of a gas, for example of N.sub.2, which is conducted through the porous layer. If the gas is moistened with a condensable liquid, such as water, the pores of the layer become increasingly blocked as the moisture content rises as a result of capillary condensation of the liquid inside the pores.

(29) Based on the Kelvin equation, it is possible to determine the so-called Kelvin radius (and the Kelvin diameter). FIG. 4 shows the curve of the N.sub.2 permeance of the mesoporous 8YSZ layer as a function of the Kelvin diameter. The measurements were carried out at room temperature using water as the condensable liquid. The membrane is located on the inner side of a 250 mm long tubular -Al.sub.2O.sub.3 carrier structure. The measurement points are plotted as squares.

(30) The Kelvin diameter, which corresponds to 50% of the original permeance, can be defined as the average pore diameter. The 8YSZ layer thus has an average pore diameter (d.sub.50) of approximately 4 to 4.5 nm. No pores were found that had a diameter of more than 7 nm. Moreover, a very narrow pore size distribution is apparent since more than 95% of pores have a diameter of smaller than 5 nm.

(31) The second mesoporous 8YSZ layer having a smaller pore size was applied with the aid of an aqueous sol. In a typical method step, 50 ml 2-propanol (Merck, SeccoSolv, catalog no. 100994) was added to 11.6 g of a 70 wt. % zirconium(IV) propoxide solution (Sigma-Aldrich, catalog no. 333972) in an Erlenmeyer flask. The mixture was stirred for 15 minutes and then mixed with 125 ml of a 1 M aqueous HNO.sub.3 solution while stirring. During heating, the mixture initially transforms into a clear solution, and then into a sol. The particle size of the sol depends on the heating time. A coating liquid that is suitable for the second mesoporous layer is obtained after approximately 18 hours.

(32) The finished sol is mixed with 1.52 g Y(NO.sub.3).sub.36H.sub.2O (Sigma Aldrich, catalog no. 237957) in 175 ml of an aqueous 0.05 M HNO.sub.3 solution while stirring, so as to obtain zirconia stabilized with 8 mol % yttria (8YSZ).

(33) In a second step, a solution was prepared from polyvinyl alcohol (PVA) in aqueous HNO.sub.3 (0.05 M). For this purpose, 3 g PVA was added to 100 ml HNO.sub.3 solution, and the mixture was subsequently heated at 98 C. under reflux overnight (approximately 16 hours).

(34) The coating liquid for the second mesoporous intermediate layer is then created by mixing 5 ml of the sol with 20 ml of an aqueous 0.05 M HNO.sub.3 solution and 20 ml of the PVA solution.

(35) The aforementioned first mesoporous 8YSZ layer having an average pore diameter of 4 to 4.5 nm was subsequently treated with the second coating liquid. The same device that was also used to carry out the first coating was used. The coated carriers were likewise initially air dried for 1 hour and subsequently calcined as previously described. With the aid of SEM analyses, the layer thickness of the produced second mesoporous layer was determined to be 200 to 400 m.

(36) FIG. 4 also shows the curve of the N.sub.2 permeance of the second mesoporous 8YSZ layer on the inner side of a 250 mm long tubular -Al.sub.2O.sub.3 carrier structure, ascertained by way of permporosimetry. The measurement points are identified as triangles. The second 8YSZ layer thus has an average pore diameter (d.sub.50) of approximately 2 to 2.5 nm. No pores were found that had a pore diameter of more than 5 nm. Moreover, again a very narrow pore size distribution is apparent since more than 95% of pores have a size of smaller than 3.5 nm.

(37) The hydrothermal stability of the graded mesoporous 8YSZ layer was examined. The coated carriers were aged at 300 C. in a water vapor atmosphere at 30 bar in an autoclave for one week. No cracks or enlarged pores whatsoever were found subsequent to that. The curves of the permporosimetry measurements showed no significant difference over the coated carriers prior to aging.

(38) A comparison of the SEM analyses based on fresh and aged samples confirmed that the layers were not influenced by the aging at high temperatures.

(39) So as to examine thermal stability, the coated carriers were heated at 600 C., 700 C. and 800 C., each for 2 hours. The curves of the permporosimetry measurements here likewise did not show any significant difference over the coated carriers that were calcined at 500 C. for 2 hours.

(40) Subsequently, the cover layer comprising graphite oxide was applied to the thus created 2-layer (graded) mesoporous 8YSZ intermediate layer. Using known methods, such as the Hummer method, it is possible to use graphite to produce the required graphene oxide dispersion comprising monolayer graphene oxide. Alternatively, commercially available monolayer graphene oxide particles may be used. For example, a diluted monolayer graphene oxide particle dispersion can be created from one part of a commercially available monolayer graphene oxide particle dispersion having a concentration of 4 mg/L (Sigma Aldrich, catalog no. 777676) and 20 parts of demineralized water. Thereafter, this dispersion was purified using a 5 m syringe filter (Whatman FP 30/5.0 CN). The average particle size of the graphene oxide particles was approximately 500 nm to 1 m, measured by way of dynamic laser light scattering.

(41) With the aid of this coating dispersion, it was possible to apply a homogeneous and defect-free graphite oxide cover layer onto the graded mesoporous 8YSZ intermediate layer. On planar substrates, the coating dispersion was applied by way of spin coating or by way of dip coating. The same device that was already used to apply the mesoporous intermediate layers onto the carrier was used to coat the inside surfaces of tubular carriers, including the mesoporous 8YSZ intermediate layers.

(42) The liquid level of the diluted monolayer graphene oxide particle dispersion was raised at a speed of 10 mm/s until the carrier was completely filled with the mesoporous intermediate layer. After 30 seconds, the liquid level was lowered again at a speed of 10 mm/s. The carrier thus coated, comprising the intermediate layer, was air dried for approximately 1 hour and then thermally treated. In a conventional oven, the coated carriers were heated under air at approximately 300 C. for 1 hour. The heating and cooling steps took place at 1 C./min. In a typical synthesis pathway, the aforementioned steps involving coating, drying and heating were each carried out twice. With the aid of SEM analyses, the layer thickness of the cover layer thus produced was determined to be 10 to 20 m.

(43) The graded membrane thus produced, including a cover layer comprising predominantly graphite oxide, is very hydrophilic and exhibits very high permeability for water vapor. Pervaporation analyses were conducted to determine the selectivity of the membrane for water as compared to a second, larger molecule. For this purpose, isopropanol (IPA), which is frequently used in pervaporation analyses, was used as a further larger test molecule. The examinations were carried out on tubular membranes having a length of 250 mm. The test solution comprised 95 wt. % IPA and 5 wt. % water. The analyses were carried out at 70 C. The membrane exhibited very high water permeability, while the diffusion for the larger IPA molecule was blocked. The result, in the form of the separation factor of the gas flux, is considerably better than for previously known membranes. In a representative examination, separation factors of more than 800 were measured, in combination with a flux of 5 kg/m.sup.2h, at a temperature of 70 C. In a dehydration test, likewise at 70 C., complete dehydration of the IPA solution containing 5 wt. % water was achieved after 3 hours.

(44) Exemplary Embodiment 2:

(45) First, a graded membrane comprising a graphite oxide cover layer according to Exemplary Embodiment 1 was produced. This was subjected to thermal treatment in a conventional oven under air at 300 C. for 1 hour. Subsequently, the membrane was exposed to a further thermal treatment at 750 C. in a mixture of 3% H.sub.2 and 97% Ar, which resulted in reduction of the cover layer into a graphite layer. This reduction took place in an oven specifically adapted for this treatment, in which a vacuum of approximately 10.sup.4 mbar (0.01 Pa) was initially applied, and thereafter the mixture of H.sub.2 and Ar was added.

(46) After reduction, the extremely thin graphite cover layer exhibited no cracks or other defects, neither macroscopically nor under the scanning electron microscope (SEM).

(47) The thickness of the cover layer and the number of stacked graphene layers were analyzed by way of transmission electron microscopy (TEM). FIG. 5 shows a high resolution TEM image. In the TEM image, an approximately 10 nm thick graphite cover layer (2) is apparent on the uppermost 8YSZ intermediate layer (1). In addition to the membrane layers, a gold coating (3) and a platinum coating (4) are apparent in the image, which were vapor deposited onto the actual membrane sample during the preparation of the samples. The analysis of the high resolution TEM images shows that the cover layer comprises approximately 20 graphene layers stacked on top of one another.

(48) Thereafter, the distances between the individual graphene layers were analyzed by way of transmission electron diffraction (TED). The analyses showed a distance between layers in the range of 0.3 nm to 0.4 nm. These values vary around 0.35 nm, which corresponds to the distance between the individual graphene layers in graphite (0.335 nm).

(49) In several select flux analyses, the selectivity of these membranes according to the first and second exemplary embodiments for smaller gases was examined. The tests were carried out at temperatures of between 50 and 200 C. using different gases, such as He, H.sub.2, CO.sub.2 or N.sub.2.

(50) FIG. 6 shows the permeance for several gases at 200 C. (473 K) and a pressure of 4 bar (4*10.sup.5 Pa) for the two membranes (graphite oxide and graphite cover layer). Good values for He and H.sub.2 permeance are yielded for the two membranes, while CO.sub.2 and N.sub.2 permeate very little or not at all. The two cover layers, both the graphite oxide cover layer and the graphite-based cover layer, thus exhibit excellent selectivity for the following gas pairs: He/N.sub.2: selectivity graphite oxide cover layer >100, graphite cover layer >100; H.sub.2/N.sub.2: selectivity graphite oxide cover layer >50, graphite cover layer >150; H.sub.2/CO.sub.2: selectivity graphite oxide cover layer >50, graphite cover layer >80.

(51) The literature cites 0.26 nm for He, 0.289 nm for H.sub.2, 0.33 for CO.sub.2 and 0.364 nm for N.sub.2 as kinetic diameters for the corresponding gases. Since the membranes are not permeable by CO.sub.2, it can be assumed that the layer distances of the individual layers of the graphite oxide and graphite cover layers range between 0.289 and 0.33 nm. These values are comparable to the distances between individual graphene layers (0.335 nm) in graphite. Based on this, it can be concluded that a graphite-like structure is present as the cover layer in the two membranes.

(52) Moreover, it was possible to show that the measured permeance for He and H.sub.2 is 2 to 3 times greater for membranes comprising a graphite cover layer. This effect can be explained by the removal of the oxygen-containing groups during the conversion of graphite oxide into graphite, which otherwise impede the transport of the gas molecules. The membranes according to the invention developed here, comprising a macroporous ceramic carrier, a mesoporous 8YSZ-comprising intermediate layer adapted thereto, and a thermally treated cover layer, exhibit considerably improved properties over the previously known graphene oxide membranes. Similarly to the previously known graphene oxide membranes, the membranes introduced here can be used in the lower temperature range below 150 C. However, the graphite oxide membranes according to the invention can moreover also be used at higher temperatures up to 400 C., and the graphite membranes according to the invention even up to 900 C., wherein in particular the high temperature range is more advantageous since the permeance of He and H.sub.2 increases exponentially with the rising operating temperature.

(53) Exemplary Embodiment 3:

(54) FIG. 7 shows the temperature dependence of the He and H.sub.2 permeance of the graphite-based membrane according to Exemplary Embodiment 2. The permeation was measured in the temperature range of between 50 and 200 C. Both He permeance and H.sub.2 permeance increase exponentially with the temperature and thus follow the Arrhenius equation. The activation energy E.sub.act can be determined from the gradient of the line. The experimentally determined value for E.sub.act, on average, was 18 kJ mol.sup.1 for He and 17.3 kJ mol.sup.1 for H.sub.2. Both activation energies are considerably above the generally recognized value of 10 kJ mol.sup.1 for high-quality membranes and confirm the successful production of high-quality microporous gas separation membranes having a thermally activated transport mechanism.

(55) Exemplary Embodiment 4:

(56) Another alternative production method for a membrane according to the invention starts with a colloidal dispersion of reduced graphene particles. Prior to depositing the cover layer, graphene oxide particles can generally be reduced to graphene particles. Such a reduction can be achieved by way of all previously known methods, such as by way of chemical methods, or by way of microwaves.

(57) Alternatively, the colloidal dispersion comprising reduced graphene particles can also be generated directly from graphene particles. However, since the graphene particles are hydrophobic, such a dispersion will generally be obtained with an organic solvent.

(58) Proceeding from a mesoporous 8YSZ intermediate layer, the coating can be applied by way of dip coating, which was already described elsewhere. As a result, a structure composed of stacked graphene layers is obtained as the cover layer. This cover layer can subsequently be thermally treated at temperatures up to 1000 C. under vacuum, under inert gas or under reducing conditions (such as Ar/H.sub.2 mixture).

(59) Exemplary Embodiment 5:

(60) In a further embodiment of the invention, a doped colloidal dispersion made of graphene oxide particles is used as the starting material. Prior to depositing the cover layer, the dispersion is mixed with a doping agent, such as Ca.sup.2+ or Mg.sup.2+. The application of the cover layer and thermal treatment are carried out analogously to Exemplary Embodiment 1. As a result, a structure composed of stacked graphene oxide layers comprising doping agents that are predominantly disposed in the interstices between the layers is obtained as the cover layer. If a conventional cross-linking agent is used as the doping agent, the individual layers can be joined in this way. This cover layer can subsequently be reduced, as described in Exemplary Embodiment 2, wherein a structure comprising cross-linked graphene layers is obtained.

(61) Exemplary Embodiment 6:

(62) Due to the large content of sp.sup.3 carbon atoms comprising oxygen-containing groups, graphite oxide is known to be a relatively poor electrical conductor. However, it is known that this can be converted into a graphite-like material by removing such oxygen-containing groups from the graphene oxide layers and forming sp.sup.2 carbon atoms. Measuring the conductivity of the cover layer subsequent to a thermal treatment is thus a suitable means for proving the formation of a graphite-like material.

(63) A graphite oxide cover layer according to Exemplary Embodiment 1 was therefore applied onto a graded, mesoporous 8YSZ intermediate layer. Analogously to Exemplary Embodiment 2, this was converted into an extremely thin graphite cover layer by way of thermal treatment. In an anhydrous atmosphere (<1 ppm H.sub.2O), the conductivity of the cover layer was ascertained at approximately 25 C. so as to exclude a contribution by adsorbed water. Following the thermal treatment under air at 300 C., the cover layer exhibited the behavior of a semiconductor having electrical conductivity of approximately 300 S/m. Following thermal treatment at 750 C. in a mixture of 3% H.sub.2 and 97% Ar, the cover layer exhibited electrical conductivity of approximately 15000 S/m. Such a figure is comparable to solid graphite material and thus points to the formation of a graphitic cover layer.

(64) In summary, the membranes according to the invention exhibit, in particular, the following advantages:

(65) a) The membrane device is thermally stable at temperatures between 25 and 800 C.

(66) b) The membrane device is suitable for separating He and H.sub.2 from other larger molecules in the manner of a molecular sieve.

(67) c) The gas permeability of the membrane device can be altered by varying the thermal treatment during production.

(68) d) The hydrophilic/hydrophobic properties of the membrane device can be altered by varying the thermal treatment during production.

(69) e) The hydrophilic membrane device is suitable for separating H.sub.2O from other solvents.

(70) f) In the membrane device, the distance between the individual graphene oxide layers of the cover layer can be altered by varying the moisture level.

(71) g) In the membrane device, the distance between the individual graphene oxide layers of the cover layer can additionally or alternatively be altered by using a cross-linking agent as a doping agent.

(72) h) The membrane device comprises a microporous cover layer, which after heat treatment at 750 C. in Ar/3% H.sub.2, is sufficiently electrically conductive and, in particular, has electrical conductivity of at least 15000 S/m.

(73) i) The membrane device according to the invention is advantageously able to separate small gas molecules such as He, H.sub.2, CO.sub.2, N.sub.2 or water from among one another and/or from other larger molecules in solid, liquid or gaseous form.