AU4542600A

AU4542600A - Method and device for obtaining hydrogen

Info

Publication number: AU4542600A
Application number: AU45426/00A
Authority: AU
Inventors: Klaus Rennebeck
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-04-25
Filing date: 2000-04-04
Publication date: 2000-11-10
Also published as: DE10016591C2; CA2371170A1; JP2002543285A; WO2000065129A1; EP1177332A1; DE10016591A1

Description

1 Method and device for obtaining hydrogen The invention concerns a method and a device to obtain hydrogen by electrolysis, in particular at atmospheric pressure or at a pressure of up to 30 bar. 5 A known device to obtain hydrogen by electrolysis, also known as electrolysers, are constructed with bipolar electrolysis cells. These electrolysis cells have bipolar film membranes that carry the cathode on the one side and on the other side, separated by the thickness of the membrane, the anode. The volumes of 10 the electrolytic fluid are the same on both sides of the membrane. When voltage is applied to the electrodes, hydrogen is released on the cathode side and oxygen on the anode side from the electrolyte. The use of electrolysers is of particular interest in hydrogen-driven vehicles for 15 the purpose of generating fuel on board of the vehicle, necessary for these vehicles. However, planar electrolysers of the state-of-the-art are not suitable for this purpose, since they require a large volume and turbulences occur in the electrolytic fluid during the movement of the vehicle and consequently of the built in electrolyser. In addition, the known electrolysers have the disadvantage that 20 due to their small thickness, bipolar flat films have to be stabilised at many positions, resulting in considerable construction expenses. Accordingly, the object of this present invention is to specify a method that enables the production of hydrogen by electrolysis requiring only little space that 25 can be particularly used on board of a motor vehicle. Furthermore, it is an object of the invention to specify a device to carry out the method. According to the invention this objective is achieved by a method according to claim 1 and a device according to claim 8. 30 In the case of the method according to the invention to produce hydrogen by electrolysis, a first electrolyte is provided in the interior of a hollow microfibre, that carries on the surfaces of its wall separately the anode and the cathode, while a second electrolyte is so provided outside of the hollow fibre, that it flows around 2 the outside wall of the hollow fibre and while a voltage is applied between the anode and cathode. By virtue of the arrangement of the electrolytes and electrodes in accordance with 5 the invention a very large reaction surface is achieved at small volumes. Furthermore, during the movement of the arrangement, what would take place, for example, if the arrangement is used in a moving vehicle, there is none or very little turbulence of the electrolytic fluid, making a trouble-free progress of the method feasible. 10 In conjunction with the method according to the invention the expression "flows around" is to be so understood, that the outside wall of the hollow fibre is in contact with the electrolyte fluid. An optimal management of the method is ensured if essentially the entire outside surface of the hollow microfibres is in 15 contact with the liquid electrolyte, so that an as large as possible reaction surface is available. The electrolytic fluids are preferably continuously supplied, so that the amount of electrolyte present in and around the hollow fibre is kept continuously constant. 20 The reaction products produced, in particular the hydrogen, can be supplied to a follow-up sequence for use or further processing. One of the follow-up sequences could be, for example, a fuel cell. Such fuel cells can be increasingly used for individual generation of current, e.g. in electric vehicles, or for the preparation of potable water, for example on space stations and the like. By means of the 25 electrolysis method according to the invention the fuel requirement for such fuel cells can be covered at least partially, this having the advantage that the storage of the hydrogen, necessary for the fuel cell, connected with the known problems, can be avoided or at least reduced. 30 Depending from the field of application, the supply of voltage, necessary for the method according to the invention, can originate from various sources. Thus, for example, the energy may be supplied by a photo-voltaic element, an electric dynamo, solar cells, as well as, when the method according to the invention is 3 being carried out in a vehicle, by a travel wind anemometer, an impeller drive or by partially recovering the braking power. In the case of the fibres used for the method according to the invention one deals 5 with hollow microfibres, meaning that their equivalent outside diameters are in the range of a few tenths of micrometres up to a few millimetres. In general linguistic usage the expression "nanofibres" has been increasingly accepted to describe fibres having a diameter below 10 pm. In conjunction with this present invention, under the term "hollow microfibres" hollow microfibres produced both by usual 10 methods, e.g. spinning, and capillary tubes wound from thin films or stalks with a corresponding diameter are to be understood. The manufacture of hollow microfibres of this type is described in EP-A-0 874 788 by the applicant. Such hollow microfibres can be produced by spinning with very 15 thin wall thicknesses of approx. 0.01 to 15 pm and equivalent outside diameters of as low as 0.5 to 35 pm. Due to the small dimensions these hollow microfibres have textile properties, i.e. they can be particularly easily bent without breaking. With the manufacturing method described in EP-A-0 874 788 hollow microfibres with very accurate dimensions can be produced, while the fluctuation of the wall 20 thickness and of the equivalent outside diameter is not more than ± 6 %. The accuracy of adherence to the dimensions of the diameter and particularly to the wall thickness ensure a homogeneous progress of the method over the entire length of the hollow fibre. 25 As an alternative, it is possible to produce the required hollow fibres from planar, smooth or structured, plastic or bipolar films, which are rolled into stalks or into helical or spiral capillary tubes. Particularly hollow fibres with an equivalent outside diameter of approx. 0.28 to 10 mm can be produced in this manner. In conjunction with this for capillary tubes made from structured films under the 30 expression "equivalent outside diameter" that diameter is understood which corresponds to a circumferential area equalling the actual circumferential area of the structured capillary tube. In this conjunction the method of rolling the film into a stalk is similar to that known, for example, from the cigarette production. The length of the stalks or helical capillary tubes produced in this manner is generally 4 between approx. 0.03 m and 3.00 m, while for the stalk used in the method according to the invention to obtain hydrogen by electrolysis a length of approx. 0.5 to approx. 1.0 m is preferred. Any desirable and technically sensible length/diameter ratio can be realised. After the forming of the stalks or helical 5 capillary tubes they can be vitrified. The films can be extruded together with the electrode material prior to their further processing into stalks or helical capillary tubes. In this conjunction particularly the sol-gel method can be used for the production of the films. 10 As starting material for the hollow microfibres, used in the method according to the invention, all proven membrane materials from the bipolar membrane technology can be used, like, for example, carbon hollow nanofibres, polyetheretherketone (PEEK), polyetherketone ketone, polymethyl pentene (TPX), zirconium oxide, PTFE, polymer proteins, mixed oxides, spinels as well as 15 zeolites. The membrane films and polymer films are coated on both sides with the electrode material. Metals, like, for example, magnesium, aluminium or spinels, etc. are suitable as electrode material. Vacuum plasma spraying method has proven itself as a very suitable method for the production of electrodes. In this thermal spraying process the sprayed material is injected in a powdery form into 20 a plasma beam, melted and carried away by this, and then deposited on the film as a coating. By optimising the spraying parameters targeted coatings can be realised with the electrode materials having various surface morphologies, by virtue of this the voltage losses during the electrolysis of the water can be considerably reduced. As an example the vacuum plasma spraying of gas 25 pulverised NiAIMo powder is to be mentioned, whereby by an extensive leaching out of the aluminium contents of a highly structured surface, a so called Ranay nickel, is produced. Such a highly structured surface is desirable to enable an effective depositing and embedding of catalysts for the method according to the invention. A further possibility to produce such a highly structured or graphitised 30 surface of the electrode material is a method known under the expression "anodic oxidation". A catalytic material, e.g. TiO 2 , WOS, V0 5 , Pt, Ru is deposited onto the electrode coating or the graphitised surface, that material being suitable in the reaction 5 used in the method according to the invention. The catalyst is deposited in the form of clusters. It has to be present in a porous state, so that not to prevent the passing of ions through the hollow microfibre membrane. 5 By using structured, e.g. pleated or undulating or wavy films for the manufacture of stalks and helical capillary tubes, the surface, available for the reaction, of the stalks or capillary tubes, is further increased. A further advantage of using structured films is in the increased flexural strength exhibited by the stalks and capillary tubes produced from them. 10 The density of the wall of the hollow fibre should be so designed for the method according to the invention, that the ions of the electrolytes can diffuse through it, but the reaction products obtained can not. In this manner it will be assured that the reaction products are separated and consequently each can be conveyed for 15 its further use. The composition of the first electrolyte and of the second electrolyte is preferably the same. The electrolytic fluids can be, for example, any of the known, suitable liquid electrolytes. Due to its complete utilisation pure water as well as potassium 20 hydroxide lye can be used as electrolyte. Thus by using very pure water and KHO lye the longest possible service life of the hollow microfibre used is ensured, since the water can be completely decomposed into its components, hydrogen and oxygen. However, according to an alternative it is also possible to use waste water for one or both electrolytic fluids, which, however, has to be pre-filtered to 25 prevent a possible clogging up of the hollow microfibres. If the method according to the invention is used, for example, to obtain hydrogen or clean potable water in manned space crafts, then human urine can be used as electrolytic fluid. By means of the method according to the invention the hydrogen and oxygen components, contained in the urine, can be eliminated as gases and synthetised 30 to pure water in a subsequent synthesis process, for example in a fuel cell unit. Those components contained in waste water or urine, which cannot be regenerated by this method, with the passing of time become deposited on the inside and outside walls of the hollow microfibres, so that they have to be counter-flushed. In the case of hollow microfibres, through which, according to 6 the method according to the invention, the urine passes as first and second electrolyte, the service life is approx. 20,000 operating hours, depending from the diameter of the fibres. 5 The method according to the invention is preferably so carried out, that the first electrolyte is divided into a plurality of part-streams, that are introduced into a plurality of hollow microfibres lying in parallel planes, and that the second electrolyte is so provided that it flows around the outside walls of the hollow microfibres lying in parallel planes. Thus the hollow microfibres used in the 10 method are lying superposed in a stack, while the exact arrangement will be explained in detail in conjunction with the explanations of the device according to the invention. The electrodes of the single hollow microfibres are connected parallel in this arrangement, so that the same voltage is applied to every fibre. In this manner the same quantities of reaction products will be obtained from every 15 fibre. This type of management of the method is characterised by a particularly high efficiency with regard to both the simplicity and saving of space. The method according to the invention is preferably carried out using an arrangement containing approx. 100.000 to 900.000 hollow microfibres. 20 According to a particularly advantageous embodiment the method is carried out at a temperature below approx. 100*C and in particular below approx. 950C. The lower temperatures are particularly advantageous when used in a vehicle, since in this case the danger that the gases produced would ignite, is reduced, In addition, the method according to the invention can be carried out in this manner 25 at atmospheric pressure, simplifying the construction of the device required to conduct the method. By means of the catalysts present on the electrodes the method according to the invention can be conducted also at the lower temperatures mentioned. The catalysts can be, however, replaced by so called "split capillaries". In this case one deals with a molecular sieve. 30 Prior to being introduced into the interior or prior to providing it around the outside wall of the hollow microfibre(s), preferably at least one of the electrolytes is stored in the interior of one or several storage hollow microfibres. Consequently these storage fibres function as a tank, in which the liquid electrolytes are stored until 7 their use. Carbon hollow nanofibres PTFE, PEEK, PEEKK, TPX have proved themselves as particularly suitable materials for these storage hollow fibres. The storage hollow fibres have preferably an equivalent outside diameter from approx. 3 pm to 25 pm, in particular from approx. 10 pm to 25 pm, and a wall thickness 5 from approx. 1 pm to 3 pm. According to a particularly preferred embodiment the walls of the storage hollow fibres are executed in a para-permeable or semi permeable manner, so that the liquid to be stored is stored also in the interior of the fibre wall. Thus the storage volumes can be increased in a simple manner. By virtue of the described type of storage of the electrolytic fluids, required for the 10 method according to the invention, a back and forth sloshing of the liquids in the moving state, e.g. during acceleration, are prevented and the danger of leakage is reduced. Thus this type of storage is particularly suitable for use in vehicles. The device suitable for the carrying out of the method according to the invention 15 has a plurality of stacked hollow microfibres, the inside and outside surfaces of which carry the anodes and cathodes, while the ends of the hollow microfibres are bundled in a frame in a dimensionally stable manner. Thus the stacked hollow microfibres form a disc with a final thickness that is bound by the frame. The binding of the hollow fibres in the frame can be carried out in any manner, for 20 example by cementing the ends of the hollow fibres with the frame. The ends of the hollow fibres are exposed on the outer circumference of the frame, so that access to the interior of the hollow fibres, i.e. to the inside diameter of the hollow fibres, is ensured. 25 According to a particularly preferred embodiment the hollow microfibres are arranged inside the stack parallel to one another and the frame has a rectangular or square shape. Thus all the hollow microfibres of a stack have essentially the same length. 30 According to an alternative embodiment the frame can be constructed also as an annular flange, wherein the hollow microfibres are irregularly stacked and held. This arrangement has the advantage that it requires only a little time for its manufacture and the scrap rate is very low.

8 According to a further alternative embodiment the frame is constructed as an annular flange and the hollow microfibres are arranged in several parallel lying planes, wherein the hollow microfibres lying in one plane are offset relative to the hollow microfibres of the next plane by 150. According to this embodiment the 5 hollow microfibres of one plane are parallel to one another. This version has the advantage that the open ends of the hollow fibre spaces of the hollow microfibres are not exposed over the entire circumference on the outside of the annular flange, but that on two opposing sides there is an angular region, each 1500 of the annular flange, which does not have any open ends of the hollow fibres. This 10 free region can be used, for example, to produce the connection of the electrodes situated on the wall surfaces of the hollow microfibres. To prevent any damaging, the hollow microfibres of a stack can be provided with a protective fabric. This protective fabric is preferably mounted on the inner edge 15 of the frame, so that it extends over the entire surface on which the hollow microfibres are exposed within the frame. Such a protective fabric is preferably applied to both the top and the bottom sides of the frame. The protective fabric can be constructed, for example, as a spun fabric or plastic gauze or another suitable material. 20 The hollow microfibres of the device according to the invention have an equivalent outside diameter from approx. 1 pm to 1000 pm, in particular from approx. 50 pm to 280 pm. Hollow fibres of these dimensions can be easily arranged in stacks and are easily to handle. In addition, they offer an excellent 25 surface/volume ratio. Within the scope of this present invention such hollow microfibres are particularly used which have a wall thickness from approx. 0.01 pm to 15 pm. Despite the small wall thickness hollow microfibres of this type do not need support, unlike 30 the planar, thin bipolar films according to the state-of-the-art, since the geometrical shape assures an adequate stability. To produce a tank for the storing of the electrolytic fluid, the device has preferably a further stack of hollow microfibres, the ends of which are joined with one of the 9 ends of each hollow microfibre of the first stack. In this case the electrolytic fluid can be stored in the further stack of hollow microfibres until it is used in the method according to the invention. The stack of hollow fibres required for the reaction and the stack of hollow fibres used for storage, the latter made, for 5 example, from carbon nanofibres, PTFE PEEKK, TPX, are preferably arranged as stacked fibre layers. When joining the two stacks with one another, one could deal, for example, with a simple tube or hose connection, by means of which the liquid stored in the interior of the hollow fibres as well as, possibly in their walls, is connected with the hollow spaces and/or the outer space of the hollow fibres 10 required for the reaction. A valve is built into this hose or tube connection to prevent the backflow of the electrolytic fluid into the stack of storage hollow fibres. A supply regulator is preferably connected between the two staples that controls the supply of the electrolytic fluid from the stack of storage hollow fibres to the stack of hollow fibres required for the reaction. In this manner the electrolytic fluid 15 consumed in the method is continuously replaced. At an energy uptake of 5 kWh on the anodes between 8 and 10 L water is consumed. The various stacks can be joined with one another, framed and superposed. The hollow microfibres of the further stack are preferably so constructed, that the 20 electrolytic fluid is stored both in the inner hollow space of the fibres and in their walls to achieve an as large as possible storage volume. It is also feasible to accommodate the frame with the storage hollow fibres in an enclosed container, so that the electrolytic fluid is stored both in the inside hollow space or inside diameter of the fibres, possibly in their wall and outside of the fibres within the 25 container. Due to the small intermediate space between the fibres an excessive back and forth sloshing of the stored liquid will be prevented also in the outside space of the fibres. According to a preferred embodiment the device has a storage medium in the 30 form of deposited material storage to store the hydrogen gas produced and/or a molecular sieve to store the oxygen produced. By virtue of this the gases produced can be securely stored until their use.

10 The invention is described in the following based on preferred embodiments by referring to the attached drawing. The drawings show in: Fig.1a - a longitudinal section through a hollow microfibre used in the method 5 according to the invention Fig.1b - a cross-section through the hollow microfibre of Fig.1a, Fig.1c - a schematic illustration of a stack of hollow microfibres, 10 Fig.2 - an alternative embodiment of a stack of hollow fibres, Fig.3a - the embodiment of Fig.2 that, however, has a protective fabric, 15 Fig.3b - the embodiment of Fig.3a in side view, Fig.3c - a perspective view of the embodiment of Fig.3a, Fig.4a - a schematic illustration of a frame without hollow microfibres, in which 20 especially the conducting connections to the electrodes are shown, and Fig.4b - a cross-section of Fig.3a in elevation. Figs.1a and lb show a hollow microfibre suitable for the carrying out of the 25 method according to the invention, that in toto is designated by 1. The hollow microfibre is semi-permeable, i.e. can be passed through by protons produced during the electrolysis, yet is impermeable for the gaseous reaction products. The hollow fibre can be produced by spinning as well as by extruding a film and subsequent winding or twisting of the film to form a capillary tube. The proton 30 conductivity of the fibres or of the film used for the production of the capillary tube can be increased by sulphonation, i.e. by bathing in sulphonic acid. The hollow microfibre carries on its wall surfaces the two electrodes, separated by the thickness of the wall, in this case the anode 2 on the outside and the cathode 3 on the inside. The electrodes can be made, for example, from carbon paper. In 11 the case of this embodiment the carbon paper is applied to the membrane foil prior to winding or twisting. Another possibility is to metallise the films, for example by anodic oxidation of metals on it, like, for example, aluminium. On their outside surfaces the electrodes can be coated with a catalyst each, that is 5 constructed, for example, as a spun fabric. Fig.1c shows schematically the arrangement of hollow microfibres in a frame 4, in which the ends of the hollow microfibres are firmly joined, for example, by cementing. So that not to jeopardise the clarity of the illustration, only four hollow 10 microfibres are shown in this figure, enlarged and at a greater distance from one another. In practice the individual hollow microfibres 1 are tightly packed and are stacked over the entire height of the frame at right angle to the plane of the figure. The frame 4 is preferably rectangular or square, while the hollow microfibres 1 are provided parallel to one another. 15 A further possibility of arranging a hollow fibre stack is shown in Fig.2. In this case the frame is constructed as an annular flange, in which the hollow microfibres are bound in a dimensionally stable manner. The position of the hollow microfibres is indicated here schematically in full lines. The ends of the 20 hollow microfibres themselves are flush with the outer edge of the frame 4. The individual layers of the hollow microfibres are offset relative to one another by 150, so that two opposite situated flange ends 5 are produced, that are flattened in this case and on which no ends of the hollow microfibres are exposed. Therefore on these ends 5 there is room to introduce the conducting connections 25 to the electrodes and connect them to a voltage source. Figs.3a to 3c show a frame 4 in the form of an annular flange, that corresponds to that of the embodiment of Fig.2. On the top and bottom sides of the frame 4 a protective fabric 6 is fastened, covering the circular surface limited by the frame. 30 The purpose of this protective fabric 6 is to prevent damaging of the hollow microfibres mounted in the frame. On the lateral edge surfaces of the frame, situated between the ends 5, the open ends 7 of the hollow fibres open outwards (for the sake of clarity of the illustration only some ends of the hollow microfibres 12 are shown enlarged). In the following an example of the dimensions of such a frame are shown: Inside diameter Di 9 cm Outside diameter Da 11.2 cm 5 Distance a between the two ends 5 10.5 cm Thickness d 0.3 cm Depending from the number and diameter of the enclosed hollow microfibres, a free surface of 3-6 m 2 can be achieved for the hollow fibres. Thus a large reaction 10 surface is available in a very small space. All other suitable dimensions and shapes can, of course, be used for the frame. As far as the dimensions are concerned, the range, wherein the individual sizes deviate by ±50 % from the above mentioned data, has proved itself. Since the 15 device according to the invention in cross-section is constructed as a flat disc, what is best shown in Fig.3b, several devices of this type can be stacked superposed to achieve a higher yield of hydrogen. When the method according to the invention is to be carried out under pressure, 20 the individual stacks are to be provided in a suitable pressure housing. Such housings are known from the state-of-the-art and consequently are not explained in detail. Figs.4a and 4b show a frame 4 of the device according to the invention, that is 25 provided with terminals 8, 9 for both electrodes. The frame 4 itself is made from a dielectric material. According to the arrangement shown in Fig.4a, by stacking several frames superposed a series connection of the electrodes of the hollow microfibres enclosed in the frame 4 can be accomplished. 30 Fig.4b shows the frame 4 of Fig.4a in cross-section, wherein for the sake of clarity of the illustration only one hollow microfibre 2 is shown, that extends into the frame 4. On the outside surface of the hollow microfibre 1 there is one of the two electrodes 10 of the hollow microfibre, that can be used as anode or cathode. The electrode 10 of the hollow microfibre 1 is in direct contact with the 13 corresponding terminal 9, that is placed on the circumference of the frame. From the other electrode (not illustrated) of the hollow microfibre, that is placed on the inside surface of the hollow microfibre, a lead goes from the frame to the second terminal 8 of the frame 4. 5 When the cathodes are provided on the inside surfaces of the hollow microfibres 1, the hydrogen of the electrolytic method occurs on the outside surface of the fibres, which form the anodes. In this case the anode contact is directly established to the housing. 10 The hollow microfibres designated for the storage can be analogously arranged in analogously constructed frames, the electrical leads, however, become redundant.

Claims

1. A method to obtain hydrogen by electrolysis, characterised in that a first electrolyte is provided in the interior of a hollow microfibre (1), that carries on 5 the surfaces of its wall separately the anode (2) and the cathode (3), that a second electrolyte is so provided outside of the hollow fibre (1), that it flows around the outside wall of the hollow fibre, and that a voltage is applied between the anode (2) and cathode (3). 10

2. A method according to claim 1, characterised in that the first electrolyte and the second electrolyte have essentially the same composition.

3. A method according to claim 1 or 2, characterised in that at least one of the electrolytes is pre-filtered waste water. 15

4. A method according to any one of claims 1 to 3, characterised in that the first electrolyte is divided into a plurality of part-streams, that are introduced into a plurality of hollow microfibres (1) lying in parallel planes, and that the second electrolyte is so provided that it flows around the outside walls of the hollow 20 microfibres lying in parallel planes.

5. A method according to any one of claims 1 to 4, characterised in that it is carried out at a temperature below approx. 100C. 25

6. A method according to claim 5, characterised in that it is carried out at a temperature below approx. 95*C.

7. A method according to any one of the preceding claims 1 to 6, characterised in that prior to being introduced into the interior or prior to providing it around 30 the outside wall of the hollow microfibre(s) (1) at least one of the electrolytes is stored in the interior of one or several storage hollow microfibres. 15

8. A device to obtain hydrogen by electrolysis, characterised in that it has a plurality of stacked hollow microfibres (1), the inside and outside surfaces of which carry the anodes (2) and cathodes (3), while the ends of the hollow microfibres (1) are bundled in a frame (4) in a dimensionally stable manner. 5

9. A device according to claim 8, characterised in that the hollow microfibres (1) are arranged inside the stack parallel to one another and that the frame (4) has a rectangular or square shape.

10 10. A device according to claim 8, characterised in that the frame is constructed as an annular flange, wherein the hollow microfibres (1) are irregularly provided in planes lying parallel to one another.

11. A device according to claim 8, characterised in that the frame (4) is 15 constructed as an annular flange, wherein the hollow microfibres are provided in parallel lying planes and the hollow microfibres lying of each plane are offset relative to the hollow microfibres of the adjacent plane by 150*.

12. A device according to any one of claims 8 to 11, characterised in that the 20 hollow microfibres (1) have an equivalent outside diameter from approx. 1 pm to 1000 pm, in particular from approx. 50 pm to 280 pm.

13. A device according to any one of claims 8 to 12, characterised in that the hollow microfibres (1) have a wall thickness from approx. 0.01 pm to 15 pm. 25

14. A device according to any one of claims 8 to 13, characterised in that the device has a further stack of hollow microfibres, the ends of which are joined with one of the ends of each hollow microfibre of the first stack. 30

15. A device according to any one of claims 8 to 14, characterised in that it has a storage medium in the form of deposited material storage to store the hydrogen gas produced. 16

16. A device according to any one of claims 8 to 13, characterised in that it has a molecular sieve to store the oxygen produced.