Process for preparing a composite metal membrane, the composite metal membrane prepared therewith and its use

Abstract

A process for preparing a composite metal membrane which contains a thin metal membrane with a desired thickness and a metallic membrane support with a porous structure, wherein metal membrane and membrane support consist of two different metals or metal alloys. The process is carried out by placing a precursor of the metal membrane on a non-porous precursor of the membrane support, the metal composite is then formed from the two precursors, the desired thickness of metal membrane is adjusted by mechanical working the metal composite and then the porous structure for the membrane support is produced.

Description

INTRODUCTION AND BACKGROUND

[0001] The present invention provides a process for preparing a composite metal membrane on a porous membrane support. Composite metal membranes of this type are used for separating gas mixtures, in particular for separating hydrogen from a reformate gas for supplying fuel cells with the required fuel gas.

[0002] For this purpose, palladium or palladium alloy membranes on either porous or non-porous supports are normally used, such as compact palladium or palladium alloy membranes. Foils made of hydrogen-permeable metals, inter alia, are used as non-porous supports The permeability of the membranes for hydrogen increases with temperature. Typical operating temperatures are therefore between 300 and 600° C.

[0003] T. S. Moss and R. C. Dye [Proc.-Natl. Hydrogen Assoc. Annu. U.S. Hydrogen Meet., 8th (1997), 357-365] and T. S. Moss, N. M. Peachey, R. C. Snow and R. C. Dye [Int. J. Hydrogen Energy 23(2), (1998), 99-106 ISSN: 0360-3199] describe the preparation and use of a membrane which is obtained by applying Pd or PdAg by cathode atomization to both faces of a foil of a metal from group 5B. The thickness of the layers applied to the two faces may be varied so that an asymmetric component is produced (for example: 0.1 &mgr;m Pd/40 &mgr;m V/0.5 &mgr;m Pd). Permeation trials demonstrate twenty-fold higher hydrogen permeation as compared with self-supported Pd membranes. Accordingly, the membrane described is suitable for use in a PEM fuel cell system instead of the traditional catalytic gas purification steps (water gas shift reaction and preferential oxidation of CO).

[0004] GB 1 292 025 describes the use of iron, vanadium, tantalum, nickel, niobium or alloys thereof as a non-porous support for a non-coherent, or porous, palladium (alloy) layer. The palladium layer is applied by a pressing, spraying or electrodeposition process in a thickness of about 0.6 mm to a support with a thickness of 12.7 mm. Then the thickness of the laminate produced in this way is reduced to 0.04 to 0.01 mm by rolling.

[0005] According to DE 197 38 513 C1, particularly thin hydrogen separation membranes (thickness of layer less than 20 &mgr;m) can be prepared by alternate electrodeposition of palladium and an alloy metal from group 1B or 8 of the periodic system of elements to a metallic support which is not specified in any more detail. To convert the alternating layers into a homogeneous alloy, appropriate thermal treatment may follow the electrodeposition process.

[0006] Either metallic or ceramic materials are suitable as porous supports for palladium (alloy) membranes. In accordance with JP 05078810 (WPIDS 1993-140642), palladium may be applied to a porous support by a plasma spray process for example.

[0007] According to Y. Lin, G. Lee and M. Rei [Catal. Today 44 (1998) 343-349 and Int. J. of Hydrogen Energy 25 (2000) 211-219] a defect-free palladium membrane (thickness of layer 20-25 &mgr;m) can be prepared on a tubular support made of porous stainless steel 316L in a electrolyses plating process and integrated as a component in a steam reforming reactor. At working temperatures of 300 to 400° C., a purified reformate containing 95 vol. % H2 is obtained. However, the optimum working temperature is very restricted because below 300° C. the palladium membrane starts to become brittle due to the presence of hydrogen, whereas above 400 to 450° C. the alloying constituents in the stainless steel support diffuse into the palladium layer and lead to impairment of the permeation properties.

[0008] Electrolyses plating processes are preferably used for coating ceramic supports. Thus, CVD coating of an asymmetric, porous ceramic with palladium is described by E. Kikuchi [Catal. Today 56 (2000) 97-101] and this is used in a methane steam reforming reactor for separating hydrogen from the reformate. The minimum layer thickness is 4.5 &mgr;m. If the layers are thinner, the gas-tightness of the layer can no longer be guaranteed. Apart from CVD coating with pure Pd, coating with palladium alloys is also possible, wherein the alloy with silver prevents embrittlement of the palladium membrane and increases the permeability to hydrogen.

[0009] In addition to pure hydrogen separation membranes, membranes which are provided with a reactive layer in addition to the hydrogen separation layer (palladium) are also described for applications in fuel cell systems. Thus, the porous support for a palladium (alloy) membrane may be covered, for example on the face which is not coated with Pd, with a combustion catalyst. The heat released during combustion at the reactive face is then simultaneously used to maintain the operating temperature of the hydrogen separation membrane (EP 0924162 A1). Such a component may then be integrated in the reforming process downstream of a reformer or incorporated directly in the reformer (EP 0924161 A1, EP 0924163 A1).

[0010] In addition, not only palladium membranes can be used for hydrogen separation in the fuel cell sector. EP 0945174 A1 discloses a design for the use of universally constructed layered membranes which may contain both fine-pore, separation-selective plastics and/or several ceramic layers and/or layers made of a separation-selective metal (preferably from groups 4B, 5B or 8), wherein these layers are applied to a porous support (glass, ceramic, expanded metal, carbon or porous plastics).

[0011] The objective of developing metal membranes for the separation of hydrogen from gas mixtures is to obtain high rates of permeation for the hydrogen. For this purpose, the metal membrane must be designed to be as thin as possible while avoiding the occurrence of leakiness in the form of holes. Such membranes can be processed only in a supported form. In order for the membrane support to have as little effect as possible on the permeation of hydrogen, it must have a high porosity. Thus there is the difficulty, in the case of known processes for preparing supported membranes, of depositing a defect-free membrane on a porous support. There are two problems involved here. On the one hand, the methods described for depositing for example palladium or a palladium alloy can guarantee a relatively defect-free membrane layer only above a certain thickness of the layer. This minimum layer thickness is about 4 to 5 &mgr;m. On the other hand, the coating techniques used for applying the membrane layer to the porous membrane support means that the average pore diameter of the membrane support ought not exceed a certain value because otherwise it would be impossible to apply coherent and defect-free coatings. The maximum pore sizes of known membrane support materials, such as porous ceramics or porous metal supports, are therefore less than 0.1 &mgr;m. This means that the resistance to flow of the gas through the pores cannot be reduced to a desirable extent.

[0012] WO 89/04556 describes an electrochemical process for preparing a pore-free membrane based on palladium supported by a porous metal structure. In accordance with the process, a pore-free palladium(-silver) membrane on a porous, metallic support is produced by coating one face of a metal alloy foil (preferably brass) with palladium or palladium/silver (thickness of palladium layer: about 1 &mgr;m) using an electrodeposition process. The porosity of the support is produced later by dissolving the base metal components out of the brass foil. Dissolution is performed electrochemically, wherein, in a cyclic process, both metal support components are first taken into solution but the more base metal component is redeposited directly onto the palladium layer (electrochemical recrystallisation). The less base metal component in the foil-shaped alloy thus goes virtually quantitatively into solution so that a porous metal structure, preferably a porous copper structure, remains as a support for the palladium/silver membrane.

[0013] The process in accordance with WO 89/04556 has the disadvantage that the brass foil used as support is virtually completely dissolved and has to be built up again by electrochemical recrystallisation. This means that the composite formed between the palladium layer and the support foil is destroyed. The mechanical strength of the recrystallised foil is low and its porosity is undefined.

[0014] An object of the present invention is to provide a simple and cost-effective process for the preparation of a composite metal membrane for separating hydrogen from gas mixtures.

[0015] Another object of the invention is to obtain composite metal membranes, the membrane supports for which have a hitherto unrealizable, high porosity (average pore sizes and pore volumes).

[0016] A further object of the present invention is to obtain composite metal membranes in which the average pore size of the membrane support is greater than the thickness of the metal membranes.

SUMMARY OF THE INVENTION

[0017] The above and other objects of the invention can be achieved by a process for preparing a composite metal membrane which contains a thin metal membrane with a desired thickness and a metallic membrane support with a porous structure, wherein the metal membrane and the membrane support comprises two different metals or metal alloys. The process is characterized in that a precursor of the metal membrane is placed on a non-porous precursor of the membrane support, the metal composite is produced between the two precursors, the desired thickness of the metal membrane is obtained by mechanically working the metal composite and then the porous structure for the membrane support is produced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention will be further understood with reference to the accompanying drawings, wherein:

[0019] FIG. 1 is a schematic cross section of an asymmetric composite metal membrane according to the invention;

[0020] FIG. 2 is a schematic cross section of a composite metal membrane of the invention after electrochemical action;

[0021] FIG. 3 is a schematic cross section of a composite metal membrane of the invention with a temporary covering membrane;

[0022] FIG. 4 is a schematic cross section of a symmetric composite metal membrane of the invention before dissolution;

[0023] FIG. 5 is a schematic cross section of a completed composite metal membrane of the invention;

[0024] FIG. 6 is an electron micrograph of the composite metal membrane of the invention; and

[0025] FIG. 7 is an enlarged view of the cross section of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

[0026] FIGS. 1 and 2 illustrate the production of an asymmetric composite metal membrane (1) as described in example 2. The composite metal membrane (1) comprises a metal membrane (2) (e.g. made from PdAg23) and a membrane support (3) comprising an eutectic alloy (e.g. AgCu28). The eutectic alloy comprises two phase regions (5) and (6). Phase regions (5) are the more noble regions which are destined for forming the membrane support after dissolution of the more electronegative material from phase regions (6). There is an interface (4) between the metal membrane (2) and the membrane support (3).

[0027] Phase regions (5) are of less electronegative phase, e.g. the Ag-rich phase of AgCu28 Phase regions (c) are of more electronegative phase, e.g. the Cu-rich phase of AgCu28.

[0028] FIG. 1 shows the situation after metal working (e.g. rolling) the laminate down to the desired thickness but before dissolution of the material from phase regions (6).

[0029] There are areas (7) of contact of the less electronegative phase regions (5) with the metal membrane in the plane of the interface (4).

[0030] In addition, there are areas (8) of contact of the more electronegative phase regions (6) with the metal membrane in the plane of the interface (4).

[0031] FIG. 2 shows the cross-section of FIG. 1 after electrochemical dissolution of the more electronegative material from phase regions (6). Due to this electrochemical dissolution, the phase regions (6) are transformed into the pores of the completed membrane support. The resulting pore structure is an open pore structure providing unobstructed flow paths for the gases from the interface (4) to the opposite face of the membrane support (3).

[0032] FIG. 3 shows a cross-section through a composite metal membrane (1) with a temporary covering membrane (10) (e.g. made of copper) before the temporary membrane and the phase regions (6) have been dissolved. There is an interface between temporary covering membrane (10) and the metal membrane (2). After dissolution of the more electronegative material from phase regions (6) and of the temporary covering membrane (10) the same composite metal membrane as shown in FIG. 2 results.

[0033] This method is used in example 2 to produce an asymmetric composite metal membrane.

[0034] FIGS. 4 and 5 illustrate the production of a symmetric composite metal membrane. FIG. 4 shows the situation before dissolution of the more electronegative material from phase regions (6) while FIG. 5 shows the completed composite metal membrane after dissolution of the more electronegative material.

[0035] The second membrane support (11) is comprised of a eutectic alloy of two phase regions (5) and (6). The eutectic alloy can be AgCu28.

[0036] This method of producing a symmetric composite metal membrane is described in example 1.

[0037] FIGS. 6 and 7 show experimentally obtained composite metal membranes of the symmetric type. These cross sections were taken with a scanning electron microscope. FIG. 7 is an enlarged view of the cross section of FIG. 6. The pore structure of the membrane support can clearly be seen. The thickness of the membrane is approximately 5 &mgr;m as can be derived by comparing with the ruler in the lower region of the photographs.

[0038] Throughout this invention the terms “mechanical working”, “metal working” and “forming” are interchangeably used to denote chipless metal forming techniques such as rolling, pressing, flow moulding and deep-drawing.

[0039] Thus, according to the present invention, solid, pore-free metal foils are initially used to produce the composite metal membrane. For the gas separation membrane, a metal foil with a thickness from 50 to 100 &mgr;m is used as precursor. A foil of this thickness can be produced virtually pore-free in outstanding quality via a metal-processing route. This foil is placed on a thicker metal foil (or sheet) which later forms the membrane support. Thereafter the composite is formed between the two metal foils. This is preferably achieved by roll-bonding, explosive plating or diffusion welding. The result is a two-layered composite. Before bonding the metal foils, it is recommended that the contact areas be carefully cleaned and roughened in a known manner.

[0040] When producing the metal composite by this process, a certain reduction in thickness takes place. After this, further mechanical working procedures by means of rolling, pressing, flow moulding, deep-drawing or combinations of these forming techniques take place until the desired thickness of the metal membrane is achieved. The measures required for this such as, for example, thermal treatments between the individual forming steps, are known to a person skilled in the art of metals. The shape of the final composite metal membrane is not restricted to flat membranes. Rather, the composite metal membrane may be shaped to give various types of geometric structures which also have the advantage that their mechanical stability is substantially better than that of a flat membrane with the same wall thickness. The techniques which can be used for this are, for example, rolling, pressing, flow moulding or deepdrawing. Mechanical working the composite metal membrane to give thin tubules by means of a drawing process is mentioned in particular here.

[0041] The ratio of thicknesses between metal membrane and membrane support in the final composite metal membrane is preferably between 1:5 and 1:20 and corresponds to the ratio of the thicknesses of the initial foils before metal-processing has been performed.

[0042] The metal-processing production of the metal membrane described has the essential advantage over known coating processes that a pore-free metal foil of high quality can be used initially and its freedom from pores can also be guaranteed after the reforming procedures.

[0043] Only after completing the reforming procedures is the porous structure of the support foil produced. The porous structure may be either a regular perforated structure, which can be produced, for example, by chemical, electrochemical or physical etching processes, or else an open-pore structure with a statistical distribution of pore sizes and pore arrangements. The latter structure is preferably used. It can be produced when the precursor for the membrane support contains a two-phase or multi-phase metal alloy and, after producing and reforming the metal composite, one or more alloy phases are electrochemically dissolved out of the membrane support.

[0044] The membrane support preferably contains a eutectic alloy, wherein the porous structure is formed by electrochemical dissolution of the more base (more electronegative) phase. The eutectic alloy AgCu which contains an Ag-rich and a Cu-rich phase, for example, is especially suitable. The Cu-rich phase can be very easily dissolved out via an electrochemical route. The Ag-rich phase then remains almost untouched. Whereas the membrane support in accordance with WO 89/04556 is completely dissolved and then rebuilt, a rigid structure consisting of the Ag-rich alloy phase is retained in accordance with the present process, with corresponding positive effects on the stability of the membrane support.

[0045] Another advantage of the process according to the invention comprises the fact that the domain structure of the two-phase or multi-phase metallic membrane support can be altered or adjusted within certain limits by choosing the alloy composition and by thermal treatment so that deliberate control of the porosity of the membrane support is possible. The pore diameter can be varied by the present process to a much greater extent than when using the traditional process. Thus it is also possible in particular to design the average pore diameter in the membrane support to be greater than the thickness of the metal membrane. Average pore diameters in the membrane support greater than 0.5 and less than 10 &mgr;m are preferably striven for.

[0046] The copper content of the eutectic alloy is preferably between 20 and 80 wt. %, with respect to the total weight of alloy. Before dissolving the Cu-rich alloy phase out of the membrane support, the composite metal membrane is subjected to a thermal treatment at 400 to 750° C. On the one hand this reverses any structural changes resulting from the metal working process and on the other hand affects the structural characteristics of the membrane support, and thus its subsequent porosity in a desirable manner.

[0047] The proposed process is suitable for the preparation of supported metal membranes from a variety of materials. However, the metal membranes preferably contain palladium or palladium alloys which have especially advantageous properties as gas separation membranes. Suitable palladium alloys are, for example, PdAg23, PdCu40 or a PdY alloy.

[0048] Another characteristic of the process is the fact that the structure of the boundary surface of the metal membrane is provided by the surface structure of the metal foil used in the preparation and thus can be relatively smooth. Subsequent production of porosity in the membrane support affects the surface structure of the metal membrane to only an insubstantial extent. The final metal membrane therefore has a very uniform thickness and is substantially smooth.

[0049] When preparing the gas separation membrane, the smallest possible membrane thickness is striven for in order to endow the membrane with a high hydrogen permeability. Gas separation membranes of palladium or palladium alloys with a thickness of more than 20 &mgr;m are of only little interest for separating hydrogen from gas mixtures due to the high cost of the noble metal and its low permeability. Membranes with a thickness of less than 1 &mgr;m on the other hand are still very difficult to obtain with the proposed process and may have a number of defects. In addition, the permeability to undesired gases also increases at these small thicknesses. As a result of these two effects, the separating power of a membrane with a membrane thickness of less than 1 &mgr;m drops to values which are no longer tolerable. Therefore, gas separation membranes with a thickness between 5 and 1 &mgr;m are preferably prepared with the aid of the process.

[0050] The porous, metallic membrane support is used to support the thin metal membrane, wherein the membrane support should impair the permeability of the laminated membrane as little as possible, as compared with a freely suspended metal membrane of the same thickness. On the other hand, a certain minimum thickness of membrane support is required in order to ensure the requisite mechanical stability of the laminated membrane. The thickness of the membrane support should therefore be less than 100 &mgr;m and should not be less than 20 &mgr;m. Membrane support thicknesses between 50 and 20 &mgr;m are preferably striven for.

[0051] The process described so far produces the composite metal membrane by metal working of a two-layered arrangement of a precursor of the metal membrane and a precursor of the membrane support. For certain material combinations of metal membrane and membrane support, it may be expedient to also provide a temporary covering membrane for the metal membrane in order to improve the processability during the metal working process. A base metal alloy or a metal alloy which can be readily removed in a chemical way, without the metal membrane or the membrane support being attacked, is chosen as the material for the covering membrane. The covering membrane may be removed before, at the same time as or after the production of porosity in the membrane support.

[0052] Naturally, in this process variant, the specifications already mentioned with regard to the choice of materials for the metal membrane and the membrane support and also for their thicknesses in the final metal composite and for the porosity of the membrane support still apply.

[0053] In another process variant, a second membrane support is used instead of the temporary covering membrane. In this case the metal membrane is located between two membrane supports. After completing the forming process, the requisite porosity is produced in both membrane supports. Therefore, the second membrane support advantageously consists of the same material as the first membrane support.

[0054] The process products of this process variant are thus symmetric, three-layered composite metal membranes, wherein both faces of the gas separation membrane are covered by porous metallic membrane supports. For certain material combinations, this process variant also has better processability during the metal working process than is the case when preparing the two-layered composite metal membrane. For this process variant, the specifications already mentioned with regard to the choice of materials for the metal membrane and the membrane support and also for their thicknesses in the final metal composite and for the porosity of the membrane support also still apply.

[0055] The composite metal membranes prepared by the process according to the invention are preferably used for the separation of hydrogen from gas mixtures, in particular from reformate gas. The various process variants enable the preparation of composite metal membranes in which the membrane supports have a previously unrealisable, high, porosity (average pore sizes and pore volumes). With thicknesses of gas separation membrane of 1 to 20, preferably 1 to 5 &mgr;m, the membrane support(s) have an average pore size greater than 0.5 and less than 10 &mgr;m. Thus, it is possible for the first time, using the process described above, to produce composite metal membranes in which the average pore size in the membrane support(s) is greater than the thickness of the gas separation membrane. These composite metal membranes therefore have outstanding hydrogen permeability.

[0056] The invention is explained in more detail by means of the following examples.

EXAMPLE 1

[0057] A foil of PdAg23 (dimensions: 30×0.07×500 mm) was placed between two foils of AgCu28 (dimensions: 30×1.0×500 mm). The contact areas were carefully cleaned and mechanically roughened beforehand. The three foils were welded together at a front face and then bonded to each other by metal-processing in a hot roll-bonding procedure. For this purpose, the foils were annealed in a tubular furnace at 600° C. for a period of 20 min under an inert gas (argon) and then rolled out on preheated roll faces (200° C.) with a deformation aspect of 45% to form one composite foil.

[0058] Further metal working to give a composite metal membrane with a total thickness of 0.1 mm was performed by conventional strip milling in the cold state with deformation aspects of about 15% and intermediate annealing at 600° C. for 15 min in a tubular furnace under an inert gas after a total deformation aspect of about 70%.

[0059] After completing the rolling process, the composite metal foil was subjected to thermal treatment under an inert gas (argon) at 600° C. for a period of 30 min and cleaned by cathodic degreasing. The Cu-rich phase in the AgCu28 alloy was then anodically dissolved out in a sulfuric acid electrolyte with 10% strength sulfuric acid operated potentiostatically at 40° C. and with a constant bath voltage of 220 mV over the course of 16 hours. This produced an open-pore structure in the membrane support foils.

[0060] Metallographic examination and images produced by a scanning electron microscope over the cross-section of the finally produced composite metal membrane showed a firmly adhering, dense PdAg membrane with a membrane thickness of 3 to 5 &mgr;m and, on both faces, the porous AgCu support layers with open porosity and a pore size of 1 to 2 &mgr;m.

EXAMPLE 2

[0061] In this example, a gas separation membrane supported on one face was prepared. The advantage of this one-faced arrangement is the larger exposed access area on the gas supply face and the associated lower resistance to diffusion of the composite membrane.

[0062] A foil of PdAg23 (dimensions 30×0.07×500 mm) was placed on a foil of AgCu28 (dimensions 30×1.0×500 mm). The two foils were welded together at a front face. The contact areas of the foils had been cleaned and roughened beforehand, as described in example 1.

[0063] The metal laminate was produced by hot roll-bonding as in example 1. Further processing was also performed as described in example 1.

EXAMPLE 3

[0064] In this example, a gas separation membrane supported on one face was also prepared. To facilitate the laminating process, the foil of PdAg23 (dimensions: 30×0.07×500 mm) was placed between two strips, one of which consisted of an AgCu28 alloy and subsequently formed the membrane support, whereas the second foil consisted of copper. The copper foil was used only as a temporary support foil and was completely removed during the electrolytic treatment to form the pores in the membrane support foil.

EXAMPLE 4

[0065] In this example, a tubular composite metal membrane was produced.

[0066] A round plate of PdAg23 (diameter 60 mm; thickness 1 mm) was placed between a lower round plate of AgCu28 (diameter 60 mm; thickness 12 mm) and an upper round plate of copper (diameter 60 mm, thickness 8 mm). The contact areas had been carefully cleaned and mechanically roughened beforehand.

[0067] The round plates were inserted in a hydraulic press and pressed together with a compression force of 2000 kg/cm2 to produce the metal composite. This produced a reduction in thickness of about 10%. Cylindrical pellets with a diameter of 12 mm were cut out of the laminated plate produced in this way and moulded into tubular blanks, the walls of which consisted, from the inside to the outside, of a layer of copper, a layer of PdAg23 and a layer of AgCu28, in an inverted flow-moulding process using a hydraulic press. The tubular blanks were drawn out by conventional tube drawing, after thermal treatment at 600° C. for 20 min under an inert gas and rapid cooling, in several steps to form tubes with an external diameter of 2 mm and a total wall thickness of 0.2 mm. Between the individual drawing steps, appropriate intermediate annealing was performed to provide sufficient softening for further forming.

[0068] The layer of copper found on the internal face of the tube wall was completely removed and the Cu-rich alloy phase was dissolved out of the AgCu alloy on the external face by electrochemical treatment so that thin-walled PdAg23 tubules with porous support structures of silver on the external faces were obtained.

EXAMPLE 5

[0069] Example 4 was repeated, but this time the round plate of copper used only for temporary support purposes was omitted so that the PdAg surface was present directly on the internal face of the tubes prior to the final electrochemical treatment. As a result, difficult dissolution of the internal coating, in particular in the case of small tube diameters, was not required.

[0070] Further variation and modification of the foregoing will be apparent to those skilled in the art and are intended to be encompassed by the claims appended hereto.

[0071] Germany priority application 100 39 595.3 is relied on and incorporated herein by reference.

Claims

1. A process for preparing a composite metal membrane containing a thin metal membrane with a desired thickness and a metallic membrane support with a porous structure, wherein metal membrane and membrane support consist of two different metals or metal alloys, comprising placing a precursor of the metal membrane on a non-porous precursor of the membrane support, forming the metal composite from the two precursors, the desired thickness of the resulting metal membrane is adjusted by working the metal composite and then forming a porous structure in the membrane support is produced.

2. The process according to claim 1, wherein the forming of the metal composite is produced by roll-bonding, explosive plating or diffusion welding.

3. The process according to claim 2, wherein working the metal composite is achieved by rolling, pressing, flow moulding, deep drawing or combinations of these forming techniques.

4. The process according to claim 3, wherein the composite metal membrane is formed into tubules by means of a drawing process.

5. The process according to claim 1, wherein the metal membrane contains palladium or a palladium alloy and the precursor of the membrane support contains a two-phase or multi-phase metal alloy and the porous structure of the membrane support is produced by electrochemical dissolution of one or more alloy phases in the membrane support after preparation and working of the metal composite.

6. The process according to claim 5, wherein the precursor of the membrane support contains a eutectic alloy and the porous structure is produced by electrochemical dissolution of the more electronegative) phase.

7. The process according to claim 6, wherein characterised in that the precursor of the membrane support contains the eutectic alloy AgCu and the porous structure is produced by electrochemical dissolution of the Cu-rich phase.

8. The process according to claim 7, wherein the copper content of the eutectic alloy is between 20 and 80 wt. %, with respect to the total weight of alloy.

9. The process according to claim 8, wherein dissolution of the Cu-rich alloy phase is performed after thermal treatment at 400 to 750° C.

10. The process according to claim 9, wherein the metal membrane contains a PdAg23, PdCu40 or a PdY alloy.

11. The process according to claim 1, wherein the porous, metallic membrane support has a thickness of less than 100 and more than 20.

12. The process according to claim 1, wherein the porous, metallic membrane support has a thickness between 50 and 20 &mgr;m.

13. The process according to claim 11, wherein the metal membrane has a thickness of less than 20 &mgr;m and more than 1 &mgr;m.

14. The process according to claim 11, wherein the metal membrane has a thickness 6 between 5 and 1 &mgr;m.

15. The process according to claim 1, further comprising providing a precursor for a temporary covering membrane of a base metal alloy or a metal alloy in addition to the precursors for the metal membrane and the membrane support, placing the precursor for the metal membrane between the precursor for the membrane support and the precursor for the covering membrane, producing the metal composite from the three precursors, forming the desired thickness of metal membrane by working the metal composite and then producing the porous structure for the membrane support, dissolving the temporary covering membrane completely away before, at the same time as or after production of the porous structure in the membrane support.

16. The process according to claim 1, further comprising providing a further non-porous precursor for a second membrane support in addition to the precursors for the metal membrane and the membrane support, placing the precursor for the metal membrane between the precursor for the membrane support and the precursor for the second membrane support, producing the metal composite from the three precursors, forming the desired thickness of the metal membrane by working the metal composite and then producing the porous structure for the membrane supports.

17. A composite metal membrane comprising a metal membrane on a metallic membrane support with a porous structure, wherein the metal membrane has a thickness of 1 to 20 &mgr;m and the average pore size of the membrane support is greater than 0.5 and less than 10 &mgr;m.

18. The composite metal membrane according to claim 17, wherein the average pore size of the membrane support is greater than the thickness of the metal membrane.

19. The composite metal membrane according to claim 17, wherein the metal membrane contains palladium or a palladium alloy and the porous membrane support contains a two-phase or multi-phase metal alloy.

20. The composite metal membrane according to claim 18, wherein the metal membrane contains palladium or a palladium alloy and the porous membrane support contains a two-phase or multi-phase metal alloy.

21. The composite metal membrane according to claim 19, wherein the membrane support contains an eutectic alloy.

22. The composite metal membrane according to claim 21, wherein the membrane support contains the eutectic alloy AgCu.

23. The composite metal membrane according to claim 22, wherein the copper content of the eutectic alloy is between 20 and 80 wt. %, with respect to the total weight of the alloy.

24. The composite metal membrane according to claim 23, wherein the metal membrane contains PdAg23, PdCu40 or a PdY alloy.

25. The composite metal membrane according to claim 17, wherein the porous metallic membrane support has a thickness of less than 100 and more than 20.

26. The composite metal membrane according to claim 17, wherein the porous metallic membrane support has a thickness between 50 and 20 &mgr;m.

27. A composite metal membrane comprising a metal membrane between two metallic membrane supports with a porous structure, wherein the metal membrane has a thickness of 1 to 20 &mgr;m and the average pore size of the membrane support is greater than 0.5 and less than 10 &mgr;m.

28. The composite metal membrane according to claim 27, wherein the average pore size of the membrane supports is greater than the thickness of the metal membrane.

29. The composite metal membrane according to claim 27, wherein the metal membrane contains palladium or a palladium alloy and the porous membrane supports contain a two-phase or multi-phase metal alloy.

30. The composite metal membrane according to claim 28, wherein the metal membrane contains palladium or a palladium alloy and the porous membrane supports contain a two-phase or multi-phase metal alloy.

31. The composite metal membrane according to claim 29, wherein the membrane supports contain a eutectic alloy.

32. The composite metal membrane according to claim 31, wherein the membrane supports contain the eutectic alloy AgCu.

33. The composite metal membrane according to claim 32, wherein the copper content of the eutectic alloy is between 20 and 80 wt. %, with respect to the total weight of alloy.

34. The composite metal membrane according to claim 33, wherein the metal membrane contains PdAg23, PdCu40 or a PdY alloy.

35. The composite metal membrane according to claim 27, wherein the porous metallic membrane supports have a thickness of less than 100 and more than 20, preferably between 50 and 20 &mgr;m.

36. A fuel cell containing the composite metal membrane according to claim 17 as a gas separation membrane.

37. A fuel cell containing the composite metal membrane according to claim 27 as a gas separation membrane.

38. A process for separating hydrogen from a gas mixture in a fuel cell comprising providing a gas mixture containing hydrogen through the gas separation membrane according to claim 17.

39. A process for separating hydrogen from a gas mixture in a fuel cell comprising providing a gas mixture containing hydrogen through the gas separation membrane according to claim 27.

40. A process for preparing a composite metal membrane containing a thin metal membrane with a desired thickness and a metallic membrane support with a porous structure, wherein metal membrane and membrane support consist of two different metals or metal alloys, comprising placing a metal foil having a first thickness as placing a precursor of the metal membrane on a non-porous metal foil having a second thickness as a precursor of the membrane support, wherein said first thickness is less than said second thickness, producing the metal composite by bonding the two precursors together, the desired thickness of the resulting metal membrane being reduced by working the metal composite and then dissolving out an alloy phase from the membrane support to thereby produce the porous structure in the membrane support.

41. The process according to claim 40, wherein bonding of the metal composite is produced by roll-bonding, explosive plating or diffusion welding.

42. The process according to claim 42, wherein working the metal composite is achieved by at least one of rolling, pressing, flow moulding, or deep drawing.

43. The process according to claim 42, wherein the composite metal membrane is formed into tubules by means of a drawing process.

44. A composite metal membrane produced by the method according to claim 40.

45. The process according to claim 40, further comprising providing a precursor for a temporary covering membrane of a base metal alloy or a metal alloy in addition to the precursors for the metal membrane and the membrane support, placing the precursor for the metal membrane between the precursor for the membrane support and the precursor for the covering membrane, producing the metal composite by bonding all three precursors, forming the desired thickness of metal membrane by working the metal composite and then forming the porous structure for the membrane support by dissolving out at least one alloy phase, wherein the temporary covering membrane is completely dissolved away before, at the same time as or after production of the porous structure in the membrane support.

46. The process according to claim 1, further comprising providing a further non-porous precursor for a second membrane support in addition to the precursors for the metal membrane and the membrane support, placing the precursor for the metal membrane between the precursor for the membrane support and the precursor for the second membrane support, forming the metal composite by bonding together all three precursors, forming the desired thickness of the metal membrane by working the metal composite to reduce the thickness thereof and then dissolving out an alloy phase from the membrane support to thereby form the porous structure for the membrane supports.

47. A composite metal membrane containing a metal membrane on a metallic membrane support with a porous structure, produced by the method according to claim 45.

48. A composite metal membrane produced by the process according to claim 45.