Device and method for a bubble-free electrolysis

Abstract

The invention relates to a device and a method for a bubble-free electrolysis. The device has at least two gas-permeable electrodes (1, 2), between which a membrane (3) is embedded. In the device, an arrangement of a plurality of fibers which are distributed over the membrane (3) are integrated into the membrane (3) and / or are applied onto the membrane (3) on one side or on both sides, wherein at least 10 of the fibers are hollow fibers (4) which extend at least to the edge of the membrane (3) and are open at one end or both ends. The sheath of the hollow fibers (4) is designed to be partly permeable to a liquid medium which can be introduced into the hollow fibers (4) under pressure via the open ends. By virtue of the proposed device and the corresponding method, electrolysis cells for water electrolysis can be inexpensively provided and can be scaled for industrial applications.

Description

[0001] Device and method for bubble-free electrolysis Technical field of application The present invention relates to a device which can be used for electrolysis and / or synthesis, in particular electrochemical synthesis, or as a fuel cell and which has at least two electrodes between which a membrane is embedded. The invention also relates to a method for carrying out electrolysis using the proposed device. For electrolysis, for example of water to produce hydrogen and oxygen, electrolysis cells with at least two electrodes are used, between which a partially permeable membrane is arranged. Various methods can be used for water electrolysis, including PEM electrolysis (PEM: Proton Exchange Membrane), AEM electrolysis (AEM: Anion Exchange Membrane) and alkaline electrolysis.A particular challenge is preventing the formation of gas bubbles on the catalyst layers, which limit the efficiency of the electrolysis. In a sandwich construction in which the membrane is embedded between the two electrodes, also known as zero-gap electrolysis cells or electrolyzers, the formation of gas bubbles can be avoided with a suitable membrane design and water supply through the membrane. State of the art For example, WO 2022 / 056606 A1 describes a zero-gap electrolysis cell in which the membrane, in this case porous, is immersed in a reservoir containing the liquid electrolyte, which is sucked into the membrane between the two electrodes by capillary force. This technology is also known as capillary fed electrolysis (CFE).During water electrolysis, the hydrogen and oxygen escape through the gas-permeable electrodes without generating gas bubbles. However, with this technology, the capillary rise height in the porous membrane and thus the size of the electrolysis cells is limited. Particularly with large membrane pores, electrolyzers of sufficiently large dimensions, such as two meters in height, cannot be reliably operated. Small pores, in turn, limit the inflow of large quantities of water, which also limits the maximum size of the electrolyzers. A further challenge with this concept is thermal management due to reduced heat dissipation through the water. Furthermore, the concept is limited to wetting (hydrophilic) materials for the membrane, as otherwise only negative capillarity occurs.EP 2765 224 A1 also describes a method for operating an electrolysis cell in which the membrane is passively supplied with liquid water. This method uses a porous membrane that additionally has a channel structure through which the water drawn into the membrane via capillary forces is distributed within the membrane. EP 2463 407 A1 describes an electrolysis cell and an electrolysis method in which the liquid to be split is actively pumped into the membrane via a channel structure formed in the membrane. The liquid is distributed via this channel structure of micro- and / or nanochannels in the membrane. The object of the present invention is to provide a device and a method for bubble-free electrolysis, in particular of water, that can be operated cost-effectively and scaled.Description of the invention The object is achieved with the device and the method according to patent claims 1 and 11. Advantageous embodiments of the device and the method are the subject of the dependent patent claims or can be derived from the following description and the exemplary embodiments. The proposed device for electrolysis, which is also suitable for synthesis or as a fuel cell, has at least two gas-permeable electrodes, between which a membrane is embedded. It is therefore a so-called zero-gap electrolysis cell. One or more catalyst layers can also be embedded between the membrane and the electrodes in a known manner.In the proposed device, an arrangement of a plurality of fibers distributed across the membrane, preferably in the form of a fiber mesh, is integrated into the membrane and / or applied to one or both sides of the membrane. At least 10 of these fibers are hollow fibers that extend to the edge of the membrane and have open ends on one or both sides, through which a liquid medium can be introduced into the hollow fibers. The sheath of the hollow fibers is partially permeable to this liquid medium, which is to be subjected to electrolysis. The hollow fibers are thus designed as hollow-fiber membranes. This medium can be, for example, water or an aqueous solution. In the present patent application, a fiber mesh is understood to be an arrangement of fibers that regularly intersect or cross one another. This can be a two-dimensional or three-dimensional mesh.The mesh preferably extends at least over the entire or almost the entire extent (length and width) of the membrane region that lies between the two electrodes. If the membrane lies completely between the electrodes, the mesh preferably extends over the entire or almost the entire extent of the membrane. The hollow fibers are arranged in the arrangement of fibers or in this mesh in such a way that the introduction of the liquid medium results in an at least approximately uniform distribution of the medium over this membrane region or over the membrane. The membrane itself preferably has no through-pores. In the proposed method for carrying out electrolysis using such a device, an electrical voltage required for the electrolysis is applied between the two electrodes in a known manner and the medium to be split is introduced into the hollow fibers under pressure.Thus, in the proposed device and the associated method, the medium to be split, such as water, is not pumped by capillary forces, but by a suitable overpressure, for example a specific water pressure, through the hollow channels provided by the hollow fibers into the membrane or to the interfaces with the electrodes. Hollow fibers with larger diameters can be used here than would be the case with pores in capillary operation according to the state of the art. While with the state of the art CFE technology, starting with a capillary radius of 30 m, only rise heights of just 30 cm can be achieved, this limitation does not apply to the proposed device and the associated method. By adjusting the overpressure, significantly greater and higher rise heights and thus stack sizes in the range of 1 to 2 meters or more can be achieved.The proposed device can be used for PEM electrolysis, AEM electrolysis, or alkaline electrolysis, among other things. This requires only the appropriate selection of materials for the electrodes and membrane, and possibly also the addition of a liquid electrolyte via the hollow channels. Typical materials for membrane electrolysis, such as those that can be used for the membrane in the proposed device and process, are therefore the membrane materials already used for PEM and AEM, such as perfluorosulfonic acid. Of course, other materials suitable for membrane electrolysis can also be used. The device is also suitable for other electrochemical processes that require a membrane, such as solid-state ammonia synthesis (SSAS) processes, methanol synthesis, as a methanol fuel cell, chlor-alkali electrolysis, ODC processes, or RODOSAN® processes.The shape and thickness of the membranes depend on the desired cell or stack design and can be varied. For example, square or circular membranes can be used. The hollow fiber mesh can be incorporated into the membrane using the classic membrane manufacturing process, but can also be applied to the membrane or integrated into the membrane in a lamination process, e.g. laminated onto the surface of the membrane or as an intermediate layer between two thin membranes (with or without catalyst layers). When applied externally to the membrane, this is preferably done at the liquid-splitting electrode. The type of mesh orThe weave and spacing between the fibers can be flexibly selected for the proposed mesh and are adjusted to ensure sufficient proton or anion transport in the membrane matrix and uniform wetting of the membrane with the medium introduced into the hollow fibers. The fiber arrangement or mesh of fibers can consist entirely of hollow fibers or of hollow fiber-solid fiber mixtures, with the solid fibers supporting the stability of the membrane or simply ensuring the fabric bond. There can also be one or more preferred directions of the hollow fibers in the arrangement or mesh. The ends of the hollow fibers can end at the edge of the membrane, protrude from the membrane, or end inside the membrane, although the latter applies only to one of the ends of the hollow fibers. The fibers can be combined into a fiber bundle both inside and outside the membrane.The hollow fibers themselves can be used with inner diameters between 0.5 and 200 μm. The shape of the hollow fibers does not have to be circular, but can also be elliptical or square. For non-circular fibers, the inner diameter is the smallest diameter, for example, in the case of an elliptical inner cross-section, the diameter measured along the short semi-axis. The outer shell or cladding of the hollow fibers preferably has wall thicknesses between 0.2 and 50 μm. When used for reverse osmosis, the wall thicknesses are preferably in the range between 50 and 250 μm. In this case, the outer shell is preferably composed of a highly porous support layer approximately 50 to 250 μm thick and a denser barrier layer only approximately 50-250 nm thick, possibly also with one or more intermediate layers. The outer shell must be partially permeable to the introduced medium, for example water.To achieve this, it must possess good permeation properties for this medium. The permeation value (permeation coefficient / wall thickness) is preferably in the range of 1x10, depending on the design (fiber density per unit area, fiber volume / membrane volume, fiber diameter, differential pressure). -4 up to 1x10 5 cm³ / h / bar / cm² (corresponds to 360 to 3.6x10 11m / s / Pa), where cm³ refers to the amount of water penetrating the wall and cm² refers to the surface area of ​​the hollow fibers. The fiber density of the hollow fibers is preferably in the range of 1 to 1000 fibers per cm (along the cross-section in one direction of the membrane). The outer shell can also have membrane properties (hollow fiber membrane). If the material of the outer shell does not have suitable permeation properties for the medium, the outer shell must be perforated and / or micro- or nanoporous to allow a sufficient amount of medium to escape. Both organic and inorganic materials can be used for the hollow fibers (and solid fibers, if present). Examples are polysulfone, polyethersulfone, carbon, silica-based materials, or the same materials used for the membranes themselves, e.g., Nafion.In principle, a variety of synthetic and natural polymers, as well as inorganic materials and hybrid materials (inorganic-organic hybrid polymers) can be used. These include, in addition to the materials already mentioned, polyamide, polyacrylonitrile, cellulose acetate, cuprammonium rayon, elastin, ORMOCER® compounds, polyvinyl (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene (TFE), polyimides, polyamides, silicone-based materials, polyvinyl chloride (PVC), cellulose acetate, polyolefins, sulfonated polyphenylenes, polyvinyl fluoride, or polyphenylene sulfide. The material of the hollow fibers must be tailored to the respective environment (PEM / AEM environment; acidic or alkaline) and the corresponding mechanical and chemical stability within the temperature range of the device's operation.The properties of the hollow fibers should be designed such that they are resistant to the solvents and temperatures used, and also dense enough with respect to the components in membrane production so that they are not accidentally filled during the membrane production process. The hollow fibers should be stable enough so that they are not completely compressed during pressing during the production of the catalyst-coated membrane or in the assembled cell or stack. The expansion coefficients of the fibers should be similar to the base material to prevent detachment. In a particularly advantageous embodiment, the hollow fibers are selected so that they have a specific filter function, through which catalyst poisons and other contaminants for the membrane cannot escape from the hollow fiber.The pore size in the hollow fiber outer shell (hollow fiber membrane) determines the extent to which micro- (>100 nm), ultra- (2-100 nm), and nanofiltration (<2 nm) are effective. Micro-, ultra-, and nanofiltration can retain undissolved materials / microparticles and macromolecular substances / materials with high molecular weight (see also Nominal Molecular Weight Cut-Off). In the preferred nanofiltration area, even dissolved molecules and heavy metal ions can be filtered out. This prevents potentially critical materials from entering the membrane or electrochemically active surfaces / as far as the catalyst layers. For example, fewer heavy metal ions reach the active areas, thus preventing degradation processes catalyzed by these ions. An overview of the production and application of hollow fiber membranes for nanofiltration can be found, for example, in Sewerin, T. et al., J.Advances and Applications of Hollow Fiber Nanofiltration Membranes: A Review. Membranes 2021, 11, 890. The filter function is particularly advantageously selected via the pore size in the hollow fiber outer shell so that metal ions are retained by the hollow fiber shell, especially trivalent metal ions such as iron Fe3+, as these are crucial for membrane degradation processes, as they can catalyze certain damaging reactions. In electrolysis, these metal ions come from, among other sources, the water cycle. Here, the ultrapure water or alkali can attack the periphery (piping, etc.) through leaching processes. In the present embodiment, the appropriate design of the hollow fiber membranes prevents these metal ions from being introduced into the (PEM) membrane or the catalyst layer via the water cycle. This significantly extends the service life of the device.The porosity of the hollow fibers can therefore be selected according to the potential water contaminants to be filtered. At high pressures, a process similar to reverse osmosis is also possible, in which the osmosis membranes do not have continuous pores. The transport of ions and molecules occurs via diffusion (solution-diffusion model), leaving contaminant molecules behind. In this way, even direct seawater electrolysis could be possible without the need for an additional desalination plant. The use of a mesh of fibers, at least some of which are hollow fibers, can be achieved using existing techniques for producing fiber fabrics. For example, membranes reinforced with a fabric made of solid fibers to better withstand mechanical loads are currently in use in PEM electrolysis.These fabric-supported membranes made of solid fibers are already being manufactured on an industrial scale and are used in both fuel cell applications and electrolysis. Accordingly, in such a fabric, the solid fibers can be replaced by hollow fibers, or at least partially by hollow fibers, during production without having to change the current manufacturing processes. Hollow fibers themselves are also already in large-scale use, for example, as hollow fiber membranes for water purification. The hollow fibers and the mesh of the membrane used in the proposed device can therefore be manufactured cost-effectively and are sufficiently durable. The liquid medium, in particular water, can be supplied to the membrane of the proposed device from one or more edge sections or sides of the membrane, or even comprehensively across the entire edge of the membrane.The supply can be achieved by individually connecting hollow fibers or by a concentrated flow. For the hollow fiber connection, an inward membrane seal can be provided, for example, so that the membrane edge with open hollow fibers extends into a reservoir that can be pressurized. In a stack of electrolysis cells, this can be a cavity embossed into the bipolar plates, which is sealed to the outside (stack exterior) and the inside (active area of ​​the membrane electrode assembly). The hollow fibers can be constantly filled (refill quantity corresponding to the consumption of the medium to be split) or flowed through continuously, the latter leading to better heat dissipation. The liquid pressure can be set statically or controlled dynamically so that no (local) overhydration of any adjacent PTL layers (PTL: Porous Transport Layer) or (local) drying out of the membrane occurs.In an advantageous embodiment of the method, pressure control takes place in order to avoid over- or undersupply of the membrane with the liquid to be split. For this purpose, the pressure can, for example, be varied cyclically, e.g. at 0.1 Hz or 0.01 Hz or an even lower frequency, and always regulated within the narrowest possible limits around the optimum. The cell voltage or the stack voltage can preferably be used as an indicator for this. The pressure is then slowly increased until the cell voltage has increased significantly again, and then reduced again until the cell voltage has once again increased significantly after passing through a minimum. This takes advantage of the fact that at the optimum operating point the cell voltage (or the added cell voltages of all individual cells, i.e. the stack voltage) is at a minimum. If this is considered as a function of the water pressure, there are two phenomena which affect the cell or stack voltage.Increase stack voltage: 1) If the pressure is too low, too little water reaches the surface, meaning more H2 could be produced than water is supplied. This reduces the efficiency of the entire system and (at the same current) automatically leads to an increase in voltage (loss of active area, increased ohmic losses, activation of less efficient areas). 2) If the water pressure is too high, significantly more water is made available than is consumed. As a result, the membrane "waters out," and gas bubbles inevitably form in the water layer, which, by predominantly masking efficient catalyst areas, in turn leads to lower efficiency and a higher cell / stack voltage. This type of pressure control can be implemented either for each cell individually or for the entire cell stack comprising several electrolysis cells.If the cell or stack is controlled by current rather than voltage, the pressure can be regulated using current instead of voltage, with the optimum operating point then being defined by the maximum current. The overpressure inside the hollow fibers set during liquid supply is preferably at least a multiple of the pure capillary pressure (= hydrostatic pressure at the lower end of the hollow fiber) so that liquid permeation out of the fiber is homogeneous and largely independent of the position in the liquid column. The proposed device and the associated method enable bubble-free, i.e. inherently extremely efficient, water electrolysis. This can be realized with available materials in such a way that temporal changes in material properties do not lead to excessive degradation or premature failure of the electrolysis system.The device and process can be implemented and scaled at comparable or even lower costs than conventional electrolysis systems. The application or integration of a fiber mesh achieves greater mechanical stability of the membrane. The flow through the hollow fibers enables active thermal management. Compared to the bubble-free techniques mentioned above, the elimination of continuous pores in the membrane results in better proton and anion conductivity due to the larger active transport volume within the membrane. This elimination of continuous pores also increases the reliability of the membrane. The active supply of the medium, especially water, also enables improved process control / controllability of this supply.When using hollow fiber membranes that are suitable as filters against membrane and / or catalyst poisons, a reduction in membrane and / or catalyst poisoning is advantageously achieved through this additional filter function of the hollow fibers and thus an increase in long-term stability. Brief description of the drawings The proposed device and the associated method are briefly explained again below using exemplary embodiments in conjunction with the drawings. Herein: Fig. 1 is a schematic representation of the structure and operation of the proposed device; Fig. 2 is a side or cross-sectional view of a configuration of the membrane in the proposed device; Fig. 3 is a side or cross-sectional view of a further configuration of the membrane in the proposed device; Fig. 4 is a side or cross-sectional view of a further configuration of the membrane in the proposed device; Fig.5A-E show a plan view of different exemplary embodiments of the membrane of the proposed device with different water supply options; and Fig. 6 shows a side or cross-sectional view of a further embodiment of the membrane in the proposed device. Ways of carrying out the invention The proposed device or electrolysis cell has two electrodes with a membrane embedded between them and thus represents a so-called zero-gap electrolysis cell. An exemplary schematic representation of this electrolysis cell is shown in Figure 1 together with the operating mode of this cell using the example of the splitting of water. The figure shows the cathode 1 and the anode 2, between which the membrane 3 is embedded. Cathode 1, anode 2 and membrane 3 accordingly form a sandwich structure. In this example, a mesh of hollow fibers 4 is integrated into the membrane 3.The water to be split is introduced into these hollow fibers 4 under excess pressure via a pump (not shown), so that in this example the water flows through the hollow fibers. A voltage suitable for electrolysis between anode 2 and cathode 1 starts the electrolysis process, whereby the water is split into hydrogen and oxygen. The hydrogen exits the electrolysis cell via the gas-permeable cathode 1, the oxygen via the gas-permeable anode 2, as indicated in the figure. Any catalyst layers and porous transport layers (PTL) that may still be present are not shown in this figure. As a rule, several of these electrolysis cells are connected together to form a stack via bipolar plates. This cannot be seen from the figure either, but is known to the person skilled in the art. Figures 2 to 4 show side and rear views.Cross-sectional views of an exemplary membrane 3 of the proposed device in different designs. The three examples differ in the arrangement of the mesh of hollow fibers in or on the membrane. In the design shown in Figure 2, the hollow fiber mesh is integrated directly into the membrane. The membrane material 5, the applied catalyst layers 6 (including ionomer), and the hollow fibers 4 in their different orientations can be seen. Figure 3 shows a design in which the hollow fiber mesh is applied to the membrane surface. In this design, as in the following Figure 4, only the hollow fibers 4 of the mesh running in one direction can be seen. Figure 4 shows a design in which the hollow fiber mesh with the hollow fibers 4 is laminated between two membrane layers. Fig.Figure 6 finally shows a design in which the hollow fibers are not integrated as a mesh, but next to one another in the membrane. In this example, the hollow fibers 4 have a large diameter that corresponds approximately to the thickness of the membrane. Figures 5A-E show different options for supplying water to the hollow fibers of the hollow fiber mesh. Each figure shows a plan view of the membranes 3, which in this example are square, in which the hollow fibers 4 can be seen. In the example in Figure 5A, the water is supplied to the hollow fibers 4 from only one side of the membrane. In the example in Figure 5B, the hollow fibers 4 protrude significantly beyond the edge of the membrane 3 on two opposite sides and are each combined to form a bundle. The water is supplied to the bundle from one side, with a water outlet provided on the opposite side.In the example of Figure 5C, the hollow fiber ends on two opposite sides of the membrane 3 are each connected to a common supply line and a common outlet line, respectively, which run parallel to the edges of the membrane 3. The water is supplied via the supply line, the water is outlet via the outlet line, as indicated in Figure 5C. This occurs in the same way in the example of Figure 5D, although in this example the same arrangement is also used for the other two sides of the membrane. In the example of Figure 5E, in contrast to Figure 5D, for example, there is no passage, but rather only an introduction of the water into the fibers. However, these are only examples of possible water supplies and, if applicable, water outlets. Any other configurations of water supply or water passage can be used.Furthermore, not every fiber shown in Figures 2 to 5 needs to be a hollow fiber. Solid fibers can also be used as support fibers between the hollow fibers. In the example of Figures 5A, 5B, or 5C, for example, all fibers running horizontally in the figure can also be solid fibers.

[0002] List of reference symbols 1 Cathode 2 Anode 3 Membrane 4 Hollow fiber 5 Membrane material 6 Catalyst layer

Claims

Patent Claims 1. A device which can be used for electrolysis and / or synthesis or as a fuel cell and which has at least two gas-permeable electrodes (1, 2) between which a membrane (3) is embedded, characterized in that an arrangement of a plurality of fibers distributed over the membrane is integrated into the membrane (3) and / or applied to one or both sides of the membrane (3), of which at least 10 fibers are hollow fibers (4) which extend at least to one edge of the membrane (3) and have open ends on one or both sides, wherein a sheath of the hollow fibers (4) is partially permeable to a liquid medium which can be introduced into the hollow fibers (4) via the open ends.

2. A device according to claim 1, characterized in that the arrangement of a plurality of fibers distributed over the membrane is a fiber mesh. 3.Device according to claim 1 or 2, characterized in that the membrane (3) is an anion- or proton-conducting polymer electrolyte membrane. is formed.

4. Device according to one of claims 1 to 3, characterized in that the hollow fibers (4) have an inner diameter in the range of 0.5-200 μm.

5. Device according to one of claims 1 to 4, characterized in that the sheath of the hollow fibers (4) has a wall thickness in the range between 0.2 and 50 μm.

6. Device according to one of claims 1 to 4, characterized in that the sheath of the hollow fibers (4) has a wall thickness in the range between 50 and 250 μm.

7. Device according to one of claims 1 to 6, characterized in that the hollow fibers (4) are formed from polysulfone, polyethersulfone, Nafion, sulfonated polyphenylene, polyetheretherketone, polyvinyl (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene (TFE), polyimide, polyamide, polyvinyl chloride (PVC), cellulose acetate, polyolefin, polyvinyl fluoride, polyphenylene sulfide, carbon, or a silicone- or silica-based material. 8.Device according to one of claims 1 to 7, characterized in that the jacket of the hollow fibers (4) is designed such that it has a filter function for catalyst. Poisons and / or other harmful substances for the membrane.

9. Device according to one of claims 1 to 8, characterized in that the arrangement of a plurality of fibers distributed over the membrane is formed by a mixture of solid fibers and hollow fibers (4).

10. Device according to one of claims 1 to 9, characterized in that the device is designed to carry out PEM electrolysis, AEM electrolysis, or alkaline electrolysis, or as a PEM fuel cell.

11. Method for carrying out electrolysis with a device according to one or more of the preceding claims, in which an electrical voltage required for the electrolysis is applied between the two electrodes (1, 2), and a liquid medium to be split is introduced under pressure into the hollow fibers (4).Method according to claim 11, characterized in that the liquid medium is supplied from one or more sides of the membrane (3) into the hollow fibers (4).

13. Method according to claim 11 or 12, characterized in that the liquid medium is supplied into the. Hollow fibers (4) are controlled via a pressure control that prevents an oversupply or undersupply of the liquid medium to the membrane (3).

14. The method according to claim 13, characterized in that the pressure control is carried out such that the electrical voltage between the two electrodes (1, 2) or an electrical current flowing between the two electrodes (1, 2) does not exceed a predeterminable value, which characterizes an oversupply or undersupply of the membrane (3), in the case of the voltage, and does not fall below this value in the case of the current.

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