WO2018103769A1 - Electrolysis cell and method for operating such an electrolysis cell - Google Patents

Abstract

According to the invention, it is provided to completely dispense with the media supply on the anode side during the electrolysis of oxygen and hydrogen from water. Since the proton-conductive membrane transports water in a diffusive manner, it has been found to be sufficient when the water provided for the decomposition is fed exclusively to the cathode side or the cathode space of the electrolysis cell. The water needed for decomposition is then directed to the anode in a purely diffusive manner by the PEM electrolyte membrane in order to react there to oxygen under proton delivery. The protons formed there diffuse again to the cathode side, where they recombine to form hydrogen. The invention is based on the concept that, during operation of a PEM electrolysis cell or a PEM electrolysis stack, the use of an anodic media supply, i.e. the supply of water, should be completely dispensed with and only the cathodic supply should be used. The water needed for decomposition is fed exclusively to the cathode.

Description

 Description

 Electrolysis cell and method for operating such

The invention relates to an electrolysis cell, in particular an electrolysis cell for generating hydrogen and oxygen from water, and preferably an electrolysis cell with a proton-conducting membrane (PEM).

State of the art

 Numerous electrolysis cells for the production of hydrogen are known from the literature. The supplied water is anodically split into oxygen, protons and electrons with the aid of electric current, while the hydrogen is produced cathodically. A distinction can be made between two technical systems, alkaline electrolysis and PEM (proton exchange membrane) electrolysis.

An electrolytic cell is basically always the same. The electrolysis cell comprises two electrodes, between which an electrolyte is arranged. In a PEM electrolysis cell, the electrolyte consists of a proton-conducting membrane. The unit of the two electrodes and the electrolyte is also referred to as MEA (Membrane Electrode Assembly). In the prior art, media supplies in the form of flow distributor structures and corresponding manifolds are generally integrated into cells and stacks both on the anode side and on the cathode side. This regularly increases manufacturing costs and increases the cost of stacks.

The power is conducted to the individual electrolysis cells via the bipolar plates. In particular embodiments, suitably structured bipolar plates simultaneously take over the functions of the electrical contacting and the media supply to the electrodes.

The tasks of the system components of the anodic region of an electrolytic cell consist in the supply of the electrolyzer with water and in the removal of the oxygen produced, as well as the phase separation of oxygen and entrained liquid. In the cathodic area, the structure looks similar. On a water supply can be omitted. The removal of the hydrogen produced also takes place via flow distributor structures. Since, during operation, water is normally drawn from the anode side through the membrane to the cathode side, a hydrogen / water mixture always forms on the cathode side, which is removed as a mixture. In a downstream separation this water is then usually separated from the hydrogen produced and optionally forwarded again for disassembly to the anode.

During operation, the hydrogen is generated on the anode side (negative pole) and the oxygen on the cathode side (positive pole).

As a rule, several electrolysis cells are assembled into a stack. An electrolysis stack is usually arranged between two end plates, which on the one hand ensure the supply of the electrical current to the individual cells and on the other hand ensure the necessary contact pressure for sealing the individual cells. For this purpose, the end plates are usually braced against each other.

In an electrolysis stack gas diffusion layers are arranged on both sides of the MEAs, which are each separated by bipolar plates, which can be configured structured, for the separation of the individual electrolysis cells. A gas diffusion layer is porous and serves as a distributor structure of the supply and discharge of the operating materials, or the product gases generated, hydrogen and oxygen, to or from an electrode. Normally, the mass transport in the gas diffusion layers diffusely takes place perpendicular to the membrane plane while in the medium supply it takes place as a flow parallel to the membrane plane. Depending on the embodiment, the gas diffusion layer can simultaneously assume the function of an electrode in the areas close to the membrane and is then called a gas diffusion electrode.

Current electrolysis stacks usually have an anodic and cathodic media space for the supply of water and the removal of the reaction products. An electrolytic cell or an electrolysis stack also has at least one supply line for water, as well as regularly via at least two discharge lines for the generated hydrogen and oxygen. EP 2957659 A1 discloses a PEM electrolysis cell in which the positive pole (the cathode) is fed with demineralized water, which is decomposed at the negative pole (anode) into oxygen gas and protons (H ⁺ ). The protons migrate through the proton-conducting membrane (electrolyte) to the cathode, where they recombine to form hydrogen gas. The gas diffusion layers arranged adjacent to the electrodes ensure, in addition to contacting the electrodes, the optimum distribution of water and thus the wetting of the proton-conducting membrane as well as the removal of the gases produced. The gas diffusion layers comprise an electrically conductive material and are made porous. In the electrolysis various variants of the operation management are conceivable. There are the classic concepts with a liquid water supply at the anode or the anode and cathode. In addition, variants with a vaporous supply of the anode are possible, and depending on the pressure level on the cathode side results there liquid or gaseous water. In the case of a cathodic overpressure, the liquid water can also be used to cool the system. In any case, sufficient water is provided at the anode so that water transport takes place in the membrane from the anode to the cathode side. In addition, there is the chloralkali electrolysis in which the anode is fed with NaCl solution and on the cathode side liquid water or in the case of the oxygen-consuming cathode air or oxygen are fed.

Task and solution

 The object of the invention is to provide an alternative electrolysis system which, compared to the prior art, on the one hand requires less production effort and at the same time saves installation space, so that the system as a whole is more cost-effective than hitherto.

Another object of the invention is to provide a method of operating such an alternative electrolysis system.

The objects of the invention are achieved by a method for operating a PEM electrolysis cell with the features of the skin claim, by a PEM electrolysis cell according to the first additional claim and by a PEM electrolysis stack according to further independent claim.

Advantageous embodiments of the PEM electrolysis cell, the PEM electrolysis stack or the method emerge from the respective back claims.

Subject of the invention

So far, the operation of an electrolytic cell for the production of hydrogen usually the anode side water is supplied, which then reacts electrochemically to hydrogen with elimination of protons. The oxygen formed at the anode during the electrolysis is removed together with excess water from the anode space and fed to a gas / liquid separation. There, the oxygen is separated and the excess water usually fed back to the anode or the anode compartment.

At the same time hydrogen is formed at the cathode. Part of the water supplied to the anode is transported due to diffusion processes through the proton-conducting membrane to the cathode, where it is discharged together with the hydrogen generated there from the cathode space and also fed to a gas / liquid separation. The previously generated hydrogen is separated off and the excess water is optionally fed back to the anode or the anode chamber in a closed circuit.

So far developed electrolytic cells therefore have a separate media supply both on the anode and the cathode side and are very complex, which nachteii- lig increases the manufacturing cost in the production of electrolysis cells. In addition, in addition to the individual electrolysis cells or electrolysis stacks (stacks), as a rule, electrolysis systems generally also have corresponding separating devices for the generated product gases on the anode or cathode side, in order to free them from excess water.

In the present invention, a distinction is made between a porous diffusion structure, in which mass transfer is generally diffusive, and a media supply, in which the transport takes place predominantly by a flow parallel to the membrane. According to the invention, it is now provided to completely dispense with the media supply on the anode side and to supply the water intended for decomposition exclusively to the cathode.

In the context of the invention, it has been found that it is not absolutely necessary for the operation of a PEM electrolysis cell to supply the water intended for decomposition to the anode side or the anode chamber of an electrolysis cell. Since the proton-conducting membrane diffusively transports water, it has proven to be sufficient if the water intended for decomposition is supplied exclusively to the cathode side or the cathode space of the electrolysis cell. The water required for the decomposition is passed purely diffusely through the PEM electrolyte membrane to the anode, where it reacts with oxygen to release protons. The protons formed there diffuse again to the cathode side, where they recombine to form hydrogen. If the water transported through the membrane to the anode is not completely decomposed into oxygen and protons, this excess, unreacted water on the anode side can easily be discharged with the generated oxygen stream. The transport path of the water through the proton-conducting membrane to the anode is a kind of transport limitation in comparison with the direct supply to the anode according to the prior art and preferably results in sufficient water reaching the anode, where it is split up into oxygen and protoners but on the other hand there remains only so little excess and unreacted water that the oxygen generated and derived at the anode has significantly less water than has hitherto been customary. As a rule, the entrained water discharged with the oxygen does not exceed the saturation point, so that as a further advantage of the method according to the invention a subsequent gas / liquid separation unit can be dispensed with. In addition, owing to the lower volume flows, gas transport in the plane of the porous structure can now be sufficient for removing the generated oxygen.

The process according to the invention can be carried out under atmospheric conditions at temperatures between 10 ° C. and not more than 95 ° C. Operating temperatures between 60 ° C and 90 ° C and in particular operating temperatures around 80 ° C have proven to be advantageous. Above 95 ° C, an increased degradation of the polymers used is to be expected, so that this temperature range is permanently not meaningful. In addition, in unpressurized operation and at such high temperatures, a high heat output due to the evaporation of water is to be expected, which requires an additional heating, which reduces the efficiency.

The inventive method is also suitable to be carried out under pressure. For this purpose, the water intended for decomposition can be fed to the cathode side of the electrolysis cell at pressures of from 1,000 hPa to 50,000 hPa, preferably between 2,000 hPa and 20,000 hPa bar. It can be distinguished between a constant pressure method and a differential pressure method. In Gleichdruckverfahren anode and cathode are kept at the same pressure level, whereas in the differential pressure operation, only the cathode side is operated under pressure. In the case of pressure-charged operation, the liquid water is brought to the cathode side by means of a pump to the desired pressure. This is advantageous under energetic considerations, since water is an incompressible medium and no volume change work is required. In the case of constant pressure operation, the generated oxygen is the anode side via a pressure control valve. The method according to the invention can be operated in both modes.

A suitable for implementing the method according to the invention electrolytic cell differs from the prior art in that is dispensed with a separate media supply on the anode side. This means that the PEM electrolysis cell according to the invention has no supply line for water to the anode side. In addition, the electrolysis cell in its interior next to a porous anode and no further separate distribution structure as a media supply.

In the new system variant of a PEM electrolysis cell proposed here, the media supply thus takes place exclusively via the cathode side. Here, the diffusive water transport in the membrane is used to supply the catalyst layer lying on the anode with water. A separate media supply of the anode with water is no longer necessary. Separate distributor structures and manifolds for supplying and distributing the supplied water to the anode can therefore advantageously be dispensed with. Instead of previously structured as a distributor bipolar plates simple thin unstructured bipolar plates can now be used advantageously. For the electrolysis cell according to the invention, all hitherto customary polymer membranes can be used as PEM membrane. In particular NAFION ^® membranes have proven to be very suitable for electrolysis cells and also for the novel process. Commercially available NAFION ^® membranes are already μΐη with different layer thicknesses in the range of, for example, 22 to 183 offered μΐη. For the PEM electrolysis cell according to the invention, a PEM membrane with a layer thickness between 5 and 200 μΐη, in particular between 10 and 100 μΐη and particularly preferably between 15 and 50 μΐη proposed.

Depending on the mode of operation (for example pressure and temperature), the required suitable layer thickness of the PEM membrane can easily be determined by a person skilled in the art and the electrolysis cell designed accordingly.

The greater the thickness of the PEM membrane used, the more limiting the diffusive transport of water through the membrane. On the other hand, for reasons of stability, however, the PEM membrane should not be designed as thin as desired. This is to be considered in particular in the case of the differential pressure operation, since the Membrane must withstand the mechanical stresses resulting from the differential pressure permanently.

As material for the anode of the PEM electrolysis cell according to the invention, all conventional anode materials can be used. For example, a porous titanium sintered ceramic is particularly suitable for this purpose. Conventional anodes of an electrolytic cell have layer thicknesses between 1 mm and 1.5 mm. Conceivable and suitable would be thinner anodes with thicknesses in the range of 0.2 to 1 mm. The porous anode of the PEM electrolysis cell according to the invention is suitable according to the invention to dissipate the generated there during operation oxygen flow directly in the layer plane to the outside. The required sufficient porosity and layer thickness of the anode can easily be determined by a person skilled in the art and the electrolysis cell can be designed accordingly.

As cathode for the PEM electrolysis cell according to the invention also all conventional cathode materials can be used. The thickness of the cathodes is in the range of 0.1 to 0.5 mm, typically 0.2 mm. The catalysts used for the electrolysis according to the invention at the anode and the cathode also correspond to the hitherto conventional catalysts. For example, platinum is particularly suitable as a catalyst for the cathode and iridium as a catalyst for the anode. This results in a significant simplification for the inventive PEM electrolysis cell, which advantageously reduces the investment costs. There are no other materials needed than hitherto. Both on the anode side and on the cathode side previously customary catalysts can be used. Another advantage of the PEM electrolysis cell according to the invention is that can be significantly reduced by the invention, the space required for an electrolysis cell or an electrolysis stack. Thus, for example, instead of a structured, 1 to 2 mm thick bipolar plate with distribution structures, which also takes over the contact simultaneously also the function of a media supply and as it is known from the prior art, now an unstructured thinner bipolar plate of preferably less than 1 mm used which is only responsible for the electrical contact. This can be According to the invention have a thickness of even less than 0.8 mm and advantageously even be configured only between 0.3 to 0.6 mm thick.

In addition to the system costs, the costs and the complexity of the stack can be significantly reduced, since anodic media supply (flow distributor) can be dispensed with.

Optionally, in an electrolysis system comprising one or more electrolysis cells according to the invention also advantageously be dispensed with a hitherto conventional gas / liquid separator for the discharged oxygen, since this usually has only up to its saturation limit water.

In summary, the invention is based on the idea, when operating a PEM electrolysis cell or a PEM electrolysis stack completely on the anodic media supply, i. to dispense with the supply of water to the anode, and to use only the cathodic supply. The water intended for decomposition is fed exclusively to the cathode.

In a PEM electrolysis cell according to the invention, therefore, a supply of water is provided exclusively to the cathode side of the electrolysis cell. In addition, the hitherto customary separate anodic media distribution (distributor structure) is eliminated, which significantly reduces the installation space of a single electrolysis cell and in particular of an electrolysis stack. The oxygen generated at the anode can be directly discharged to the outside via a sufficiently porous anode.

In a particular embodiment of the PEM electrolysis cell according to the invention not only the supply of water is provided on the cathode side, but also the electrical contacting of both the cathode and the anode. For this purpose, the cell has a PEM membrane which is arranged in a U-shape, wherein the porous cathode and the associated media supply are arranged on one side of the membrane between the arms of the membrane and the porous anode is arranged on the other side of the membrane , The contacting of the anode via at least one bipolar plate, which now extends from the anode to the cathode side and is arranged outside the arms of the PEM membrane.

Such a special embodiment of the invention is very advantageous, for example, for use in combination with photovoltaic cells. The electrolysis cell or cells could on the back of a photovoltaic cell are arranged in such a way that the cathode side is arranged with the two contacts directed against this. In this case, a contacting of both electrodes on one side of the cell is advantageous because then the current generated in the solar cell can be passed directly into the electrolysis cell without detours. At the same time, the opposite side (anode side) of the electrolysis cell would be free, for example, to be able to dissipate the oxygen generated there directly into the ambient air.

Special description part

 In addition, the invention of embodiments and some figures erläu tert closer without this should lead to a limitation of the broad scope of protection.

Showing:

 Figure 1: Schematic representation of an electrolytic cell according to the prior art.

Figure 2: Schematic representation of the operation of an electrolytic cell according to

 State of the art.

 Figure 3: Schematic representations of the operation of the invention:

 a) when using an electrolytic cell according to the prior art

 b) when using an embodiment of the electrolytic cell according to the invention. Figure 4: Different space requirements for a conventional Elektrolysestack and an embodiment of the invention

 Figure 5: Schematic representation of an overall system with an electrolytic cell according to the prior art.

 Figure 6: Schematic representation of an overall system with an embodiment of the

 Invention.

 FIG. 7: Graphical representation of the current / voltage curve of a device according to the invention

 Electrolysis cell with cathodic water supply in the inventive mode of operation as a function of the thickness of the PEM membrane used.

FIG. 8: Special embodiment of the electrolytic cell according to the invention with contacting on the cathode side.

 Figure 9: Schematic representation of classical modes of operation for an electrolysis (a) and a chloralkali electrolysis (b).

In all figures: 1 = bipolar plate, 2 = anode, 3 = membrane, 4 = cathode, 5 media channel, 6 = flow distributor structure. 1 shows a schematic representation of an electrolytic cell according to the prior art. The center forms the PEM membrane 3 with the electrodes adjoining on both sides (anode 2 and cathode 4). On both sides of the membrane-electrode assembly (2 + 3 + 4) are devices for supplying media with flow distributor structures 6 and thus formed separate media channels 5 for anodic and cathodic supply of water, as well as for the discharge of an oxygen / water mixture on the anode side and a hydrogen / water mixture on the cathode side. The flow distributor structures 6 are formed here simultaneously by the bipolar plates 1, which ensure the electrical contacting of the electrolysis cell.

The operation of an electrolytic cell according to the prior art is shown in FIG. The intended for decomposition water is fed mainly to the anode. However, some of the water is also supplied to the cathode, wherein the water supplied to the cathode is usually that separated from the discharged hydrogen / water mixture water. Excess water from the anode side to the cathode side is transported through the membrane in addition to the protons. Via the media supply on the anode and cathode side, the discharge of the generated product gases hydrogen and oxygen takes place. In contrast to the electrolysis cell from FIG. 2, the electrolysis cell shown in FIG. 3 a) also has flow distributor structures on both sides of the membrane-electrode unit, but during operation the water intended for decomposition is supplied exclusively to the cathode side. In this mode of operation according to the invention, water passes exclusively via diffusion processes from the cathode side through the PEM membrane to the anode side, where it is decomposed into oxygen, protons and electrons. In this respect, FIG. 3 b) shows an electrolysis cell optimized for the operation according to the invention, in which separate media channels are dispensed with on the anode side and a simple, unstructured thin bipolar plate is now provided instead of a correspondingly structured bipolar plate (prior art).

FIG. 4 illustrates the difference in installation space between a conventional electrolysis stack and an embodiment of the invention. By omitting or saving the separate media channels each on the anode sides of the space can be significantly reduced in an electrolysis stack according to the invention. This advantage is maintained even if the porous anode structure would each be made slightly thicker than in the prior art. The overall system of an electrolysis system according to the prior art with subsequent separation and purification stages is shown schematically in FIG. Separating devices for gas / liquid separation of the product gases are provided both on the anode side and on the cathode side. On the cathode side, purification and drying steps optionally additionally follow for the treatment of the generated hydrogen, before the hydrogen, if appropriate still compressed, is removed for further use.

In contrast to Figure 5, in a particularly advantageous embodiment of the invention, not only space in the individual electrolysis cells but also additionally by the saving of a gas / liquid separation for the product gas oxygen can be reduced. Since the discharged oxygen in the erfindungdgemäßen operation regularly only little water, can be dispensed with an additional separation of the water from the oxygen in the rule. This also contributes to further Kostereduzierung.

Embodiment:

To test the operation of the electrolysis cells according to the invention, electrolysis cells according to the invention were constructed and current-voltage investigations were carried out. In the electrolytic cells of different PEM membranes were thick comprising Nafion ^® used with an area of 17.84 cm, respectively.

The different materials and layer thicknesses of the membranes used can be taken from the table below.

The anode used was a catalyst layer based on an iridium catalyst in combination with a 0.2 mm thick diffusion structure. Alternatively, the use of titanium sintered metals with a porosity of about 20 vol .-% is possible. The cathode used was a catalyst layer based on a platinum catalyst. As a diffusion substrate, a carbon fiber fleece with a layer thickness of 0.2 mm was also used here. The results of the investigations are shown in FIG.

 A further advantageous embodiment of the invention can be seen in FIG. In this embodiment, the electrolyte is arranged in a U-shape with the cathode having the cathode supply between the arms of the U-shaped electrolyte and the anode on the opposite side of the PEM membrane. In this way, on one side of the cell (cathode side) the contacting can advantageously take place for both electrodes, while on the other side of the cell, the oxygen can be transported away directly via the porous anode.

For example, such an arrangement can be ideally coupled to a photovoltaic cell in which the generated electricity can be used directly for the electrolysis and there especially for the production of hydrogen. Here, the aforementioned embodiment of the cell with the possibility of contacting on only one side of the electrolysis cell would be particularly advantageous. The oxygen produced can be removed separately (not shown in FIG. 8) or else released directly to the environment via the porous anode.

In the electrolysis various variants of the operation management are conceivable. There are the classic concepts with a liquid water supply at the anode or the anode and cathode. In addition, variants with a vaporous supply of the anode are possible, and depending on the pressure level on the cathode side results there liquid or gaseous water. In the case of a cathodic overpressure, the liquid water can also be used to cool the system. (FIG. 9a) In any case, sufficient water is made available at the anode so that water transport takes place in the membrane from the anode to the cathode side.

In addition, there is the chloralkali electrolysis in which the anode is fed with NaCl solution and on the cathode side liquid water or in the case of Sauerstoffverzehr- cathode air or oxygen are fed. (Figure 9b)

Claims

Patent claims A process for producing hydrogen and oxygen from water by means of an electrolytic cell comprising an anode, a cathode and a proton-conducting membrane as the electrolyte, in which oxygen is generated at the anode and hydrogen at the cathode,  characterized,  the water intended for decomposition is supplied exclusively to the cathode side, and this is transported by diffusion through the proton-conducting membrane to the anode where it is decomposed into oxygen and protons. 2. The method according to claim 1,  in which the gaseous oxygen generated at the anode side is discharged directly from the electrolysis cell through the porous anode. 3. The method according to any one of claims 1 to 2,  in which a proton-conducting membrane having a thickness between 5 and 200 μΐτι, in particular between 10 and 100 μηη and particularly preferably between 15 and 50 μΐη is used. 4. The method according to any one of claims 1 to 3, in which a proton conductive membrane is used comprising NAFION ^®. 5. The method according to any one of claims 1 to 4,  in which the hydrogen and the oxygen are produced from water at temperatures between 10 ° C and at most 95 ° C, advantageously at temperatures between 60 ° C and 90 ° C and in particular at temperatures around 80 ° C. 6. The method according to any one of claims 1 to 5, in which the water is supplied to the cathode side at an operating pressure between 1,000 hPa and 50,000 hPa, preferably between 2,000 hPa and 20,000 hPa. 7. electrolytic cell for generating hydrogen and oxygen from water, comprising two electrodes, a proton-conducting membrane disposed between the electrodes as electrolyte, at least two bipolar plates for electrical contacting of the electrolytic cell and a media supply for water  characterized,  that the media supply for water is arranged exclusively on the cathode side. 8. electrolytic cell according to the preceding claim 7,  wherein the proton conductive membrane has a thickness between 5 and 200 μΐη, in particular between 10 and 100 μιη and particularly preferably between 15 and 50 μηι. 9. electrolysis cell according to one of the preceding claims 7 to 8,  with a proton-conducting membrane comprising NAFION®. 10. electrolysis cell according to one of the preceding claims 7 to 9,  in which the bipolar plate on the anode side has no structuring and has a maximum thickness of 1 mm, preferably less than 0.8 mm and particularly advantageously between 0.3 and 0.6 mm. 11. electrolytic cell according to one of claims 7 to 10,  with a U-shaped electrolyte,  wherein the cathode and the media supply for the water are disposed on one side of the proton conductive membrane between the arms of the U-shaped electrolyte and the anode is disposed on the other side of the proton conductive membrane, and  wherein the bipolar plates are arranged for contacting both electrodes on the cathode side. 12. electrolysis stack comprising at least two electrolysis cells,  characterized,  in that the electrolysis stack has at least one electrolytic cell according to one of Claims 7 to 11.