SYSTEM FOR REDUCING CARBON DIOXIDE, AND ELECTROLYSIS CELL FOR SAME

Description

The invention relates to an electrolysis cell for reducing carbon dioxide, comprising at least one stack made of an anode space, a cathode space, and optionally a gas space adjoining the cathode space, as well as fluid supply lines and fluid discharge lines that are configured to supply the anode space with anolyte, the cathode space with catholyte and electively gas, or the optional gas space with gas, and a power line for applying a voltage between the cathode space and anode space.

The invention furthermore relates to a system for continuously reducing carbon dioxide using an electrolysis cell of this type.

The emissions of carbon dioxide (CO₂) into the atmosphere, which have increased in recent decades, is considered to be one of the main causes of global warming. As with other climate-relevant gases, efforts are being made to find solutions to this problem. A first approach is to reduce CO₂ emissions. However, this is difficult to achieve in a short period of time, especially at the international level. In addition, individual branches of industry cannot readily switch to CO₂-free production, or at least to production with reduced CO₂ emissions. For this reason, a second approach is to minimize the CO₂ emissions that arise, in that, at the site where it is produced, the carbon dioxide is reactively converted into substances which are not harmful to the climate and, advantageously, can be used again in other reactions. Since a reduction of CO₂ emissions may not be sufficient to stop global warming, great importance is attached to this second approach.

Among the various possibilities of immediately utilizing CO₂ at the location where it is produced, the electrochemical reduction of CO₂ into fuels represents a particularly interesting option. For decades, research has been performed in the prior art on converting CO₂ into compounds such as methane, methanol, and/or ethanol in what is referred to as a “dream reaction”. If this were to succeed with energy efficiency and in a manner that is clean for the environment, it would be possible to convert CO₂ that is harmful to the climate into usable substances, so that this would, in fact, be a reaction that is perfect for the environment.

CO₂ emissions arising from industry, for example flue gas, waste gas from heating systems, waste gas from biotechnology systems and the like could be exploited to form useful products that are either further converted or, where applicable, temporarily stored.

Even though a conversion of CO₂ as explained above offers many benefits and has therefore already been explored for decades, there has been no critical breakthrough in the electrochemical conversion of CO₂ to date. For an electrochemical conversion of this type, an electrolysis cell is necessary which allows the possibility of efficiently carrying out a corresponding reaction on an industrial scale. Even though isolated successes with an electrolysis cell of the type named at the outset on a laboratory scale can validate theoretical approaches, they do not permit an efficient implementation on a larger scale.

This is addressed by the invention. The object of the invention is to specify an electrolysis cell of the type named at the outset with which an electrochemical conversion of CO₂ is possible in an efficient manner and in a small space, as well as with high throughput.

Furthermore, an object is to create a system with an electrolysis cell of this type.

The object of the invention is attained if, in the case of an electrolysis cell of the type named at the outset, multiple stacks are provided and the fluid supply lines and the fluid discharge lines and the power line are configured for the simultaneous supply of fluid and application of current to multiple, in particular all, stacks.

An electrolysis cell according to the invention combines multiple advantages: Firstly, the electrolysis cell comprises multiple stacks that are embodied to rest against one another, which enables a space-saving construction. The stacks are thereby advantageously constructed as 3-chamber systems in that said stacks each comprise a gas space, a cathode space, and an anode space. The gas space is in operative connection with the cathode space such that supplied CO₂ can enter the cathode space via a membrane or film or the like, according to the principle of a gas diffusion electrode. Due to the arrangement of multiple stacks resting against one another, the electrolysis cell is highly efficient in terms of a CO₂ conversion. It thus succeeds in achieving the same level of efficiency, the same yield, the same selectivity, and the same final conversion as in the case of larger cells. In a single electrolysis cell, between 2 to 50, in particular 2 to 25, for example 3 to 10, stacks are typically provided. However, the number of stacks is not limited to 50 and, in principle, can be selected as desired, as long as a sufficient stability during operation is achieved. In a practical manner, the quantity of stacks or connected cells can be designed according to the desired productivity and/or the desired cell volume.

In addition, because the stacks are connected in series, the electrolysis cell is compact, so that a high efficiency is also obtained in terms of the space requirement or the volume. A. sequencing of the stacks is possible because the stacks are conductively connected to one another. For this purpose, bipolar electrodes are provided at the ends of the stacks, which electrodes allow a passage of current to the next stack. It is thus possible that the individual stacks rest against one another without spacing. The stacks can therefore be positioned directly against one another.

The corresponding advantages can be realized since, with the given arrangement of the stacks, the fluid supply lines and the fluid discharge lines and the power line are additionally configured for the simultaneous supply of fluid and application of current to multiple stacks. Via a single fluid supply for the anolyte, anolyte can be simultaneously applied to all anode spaces of the individual stacks. Analogously, the anolyte can be discharged from the anode spaces via a single fluid discharge line. The situation is similar for the cathode spaces, which likewise can be supplied using a single fluid supply line, and wherein a single fluid discharge line is provided for the discharge of catholyte from the cathode spaces. The same also applies analogously to the gas supply.

The power supply is also configured such that a voltage can be applied in all stacks via a single power line in order to carry out the electrochemical reduction of CO₂. Since the individual stacks are conductively connected to one another via bipolar electrodes, a voltage can be simultaneously applied in all stacks using a single bus bar. Similarly to the fluid supply lines and fluid discharge lines, it is also provided in this case that the current is supplied to the individual stacks via a first bus bar and a second bus bar is provided as a current collector.

The foregoing statements regarding the construction also apply if the stacks are embodied as 2-chamber systems, which is likewise possible within the scope of the invention. In this case, the gas is not fed into the gas space, but rather into the cathode space. The cathode space can also be constructed in a very simple manner in this case, in that said cathode space comprises a membrane that is merely kept moist so that the supplied moisture can be considered to be catholyte in this case. In principle, however, 3-chamber systems are preferred, since CO₂ can be supplied at virtually 100% in said systems, whereas the CO₂ solubility in the catholyte is limiting in the case of 2-chamber systems. 3-chamber systems with a separate supply of anolyte to the anode space, catholyte to the cathode space, and gas to the gas space are therefore preferred in terms of a high CO₂ conversion.

In principle, the stacks can be embodied in any desired manner. For a most space-saving construction possible and a simple supply and discharge of fluid, however, it is preferred if the stacks are respectively constructed in layer form. A stack then comprises a layered anode space, an adjoining layered cathode space, and a layered gas space via which the CO₂ supply takes place. If the stacks rest against one another, the stack also comprises an insulating layer in order to ensure an insulation against the next adjacent stack. The layered design of a stack or of all stacks is thereby advantageously chosen such that the maximum diameter of a layer is at least 5 times, preferably at least 10 times, in particular at least 20 times, a thickness of the layer. A shape of the respective layer can thereby be as desired. Advantageously, stacks constructed in layer form are roughly circular in a top view. This yields advantages in terms of production as well as for the construction of the electrolysis cell, since the electrolysis cell can then be constructed with a cylindrical shape, which proves beneficial for a high load on the electrolysis cell during use, especially since high pressures are present and the electrolysis cell needs to withstand said pressures with suitable pressing forces. This can be satisfactorily achieved if the stacks form a cylinder, wherein the stacks rest against one another. For example, a screw connection of the stacks can then take place between two end plates, via which end plates suitable pressing forces can be applied to the stacks.

A particularly advantageous variants results if the fluid supply lines are arranged perpendicularly to the stacks. In particular, a single fluid supply line for all anode spaces and a single fluid supply line for all cathode spaces and a single fluid supply line for the supply of gas containing CO₂ to the gas spaces can then be respectively provided. Especially if the stacks are embodied in layer form, the respective fluid supply can take place laterally. Particularly for these reasons, it is then also expedient that the fluid discharge lines are arranged perpendicularly to the stacks.

For the corresponding introduction and removal of the fluids, it is expedient if the cathode space of a stack comprises a cathode space inlet, which is connected to the fluid supply line for the cathode space, and a cathode space outlet, which is connected to the fluid discharge line from the cathode space. In this context, it is particularly expedient, especially in the case of a layered construction of the stacks, that the cathode space outlet lies opposite from the cathode space inlet, in particular is offset by 180°. Via a single fluid supply line, all cathode spaces in the respective layer or stratum can be laterally supplied in this embodiment. A removal then takes place at the opposite side of the respective stratum, so that the layer or stratum can be fully utilized.

For analogous reasons, it is particularly expedient if the anode space of a stack comprises an anode space inlet, which is connected to the fluid supply line for the anode space, and an anode space outlet, which is connected to the fluid discharge line from the anode space. In this case, it is also particularly expedient that the anode space outlet lies opposite from the anode space inlet, in particular is offset by 180°. This arrangement is particularly expedient if the stacks are respectively embodied in layer form.

Finally, it also holds true for the gas space of a stack that said gas space advantageously comprises a gas space inlet, which is connected to the fluid supply line for the gas space, and comprises a gas space outlet, which is connected to the fluid discharge line from the gas space.

In this case, the gas space outlet can also lie opposite from the gas space inlet, in particular can be offset by 180°, which proves to be an advantage, especially in the case of a layered construction of a stack. Gas can be fed, for example, at a pressure of 1 bar to 20 bar, in particular 1 bar to 10 bar.

In order for it to be possible to respectively feed all stacks using a single fluid supply line, and also for a removal of outflowing fluids to be possible using a single fluid discharge line each, the individual fluid supply lines are offset from one another in a top view of the stacks. This then also applies to the fluid discharge lines.

If, as explained in the foregoing, a fluid supply and a fluid discharge are designed such that, in each stack, the fluids enter at one edge via the fluid supply and exit into the fluid discharge at another edge, the fluids flow through each layer parallel to one another and perpendicular to a longitudinal axis of the electrolysis cell. However, it is also possible that the inlets and outlets are designed such that the fluids run in series through all stacks, before said fluids finally exit. For this purpose, it is merely necessary, with an otherwise unaltered construction, to close individual inlets and outlets. The fluids can then flow through the individual stacks in a loop or in a meandering pattern.

Particularly in the case of a layered construction of the stacks, which can respectively be separately present in a cylindrical shape and are positioned such that they rest against one another, it is advisable that the stacks are clamped between end plates. Since the cathode spaces and the anode spaces are provided with corresponding fittings, which are surrounded by clamping rings, a particularly high pressing force is expedient in order to ensure a leak-tightness of the electrolysis cell as a whole. A supply of the individual fluids then preferably takes place such that the fluid supply lines and the fluid discharge lines run through the end plates. A roughly cylindrical embodiment of the electrolysis cell then results, with two end plates that overhang in a frontal view, through which end plates the fluid supply lines and the fluid discharge lines are guided. The individual fluid supply lines and fluid discharge lines are offset from one another. Firstly, the fluid supply line for a medium, for example the catholyte or even the anolyte or the gas, is preferably offset by 180° relative to the corresponding fluid discharge line for the catholyte, the anolyte, or the gas. In addition, the individual fluid supply lines, and therefore also with a defined offset of the corresponding fluid discharge lines, are then offset from one another, so that there is an angle of, for example, 10°to 90°between the individual supply lines and discharge lines. For example, a fluid supply line for the anolyte can be offset by 60°relative to a fluid supply line for the catholyte. The same then applies to the corresponding fluid discharge lines. A fluid supply line for the gas can also be offset by 60°relative to the fluid supply line for the anolyte. Since the same then once again applies to the fluid discharge line for the gas, the individual fluid supply lines and fluid discharge lines can be arranged in a highly symmetrical manner in the end plates, so that a balanced load profile also results in terms of occurring forces. Apart from this, corresponding bolts are provided which connect the end plates using corresponding nuts such that the electrolysis cell as a whole withstands the pressure during operation. Since the bolts are not intended to penetrate the stacks, the end plates are preferably produced to be wider than the stacks. Like the stacks, the end plates can also be embodied to be circular.

In the cathode space and in the anode space, a highest possible efficiency for the desired reactions is to be achieved, which relates to a conversion of CO₂ taken as a whole. For this reason, it is advantageously provided that the stacks are embodied with, in particular static, mixing elements in the cathode space and/or anode space. Static mixing elements can be realized in various ways. For example, it is possible that the corresponding fittings in the cathode space and/or in the anode space are embodied as metal foam or metal mesh. It has proven particularly beneficial if the mixing elements comprise helical fluid paths. Especially if a lateral supply of the respective fluid is envisaged, a helical conducting of the respective fluid results in a comparatively long retention time before the fluid is discharged again via the corresponding fluid discharge line. This long retention time in the actual reaction space benefits a high efficiency during the conversion of CO₂.

The other object of the invention is attained if a system for continuously reducing carbon dioxide comprises an electrolysis cell according to the invention.

In a corresponding system, the advantages explained for the electrolysis cell come into full effect due to the additionally provided peripheral equipment, in particular an anolyte container for the supply of anolyte to the anode spaces of the stacks and a cathode container for the supply of catholyte to the cathode spaces, including the corresponding discharges.

Advantageously, a circuit for carbon dioxide is thereby provided, via which circuit unconverted carbon dioxide can be returned to the stacks for further conversion. It can thus be ensured that supplied carbon dioxide is converted to a large extent.

Within the scope of the invention, as explained in the foregoing, stacks can be used which are constructed either in a 3-chamber system or in a 2-chamber system. If multiple electrolysis cells are used, it is also conceivable to realize combinations of 3-chamber systems and 2-chamber systems in a system. Generally, 3-chamber systems are exclusively preferred, however, since they result in a better product yield.

Additional features, advantages, and effects of the invention follow from the exemplary embodiments described below. In the drawings which are thereby referenced:

FIG. 1 shows an electrolysis cell with a stack in a 3-chamber construction;

FIG. 2 shows an electrolysis cell with multiple stacks in a frontal view;

FIG. 3 shows the electrolysis cell from FIG. 2 in a top view;

FIG. 4 shows the electrolysis cell from FIG. 2 and FIG. 3 in a perspective illustration;

FIG. 5 through FIG. 7 show the electrolysis cell from FIG. 2 through FIG. 4 with an illustration of the fluid supply lines and fluid discharge lines for gas (FIG. 5), for the catholyte (FIG. 6) and the anolyte (FIG. 7);

FIG. 8 shows a schematic illustration of a current flow;

FIG. 9 shows a top view of a fitting for a cathode space and/or an anode space;

FIG. 10 shows a variant of a fitting;

FIG. 11 shows a further variant of a fitting;

FIG. 12 shows an electrolysis cell with a stack in a 2-chamber construction;

FIG. 13 shows a system with an electrolysis cell according to the invention, for reducing CO_2.

An electrolysis cell 1 is illustrated in FIG. 1. The electrolysis cell 1 comprises three stacks 2 with one 3-chamber cell each. The illustration in FIG. 1 is used only for an explanation by way of example. An electrolysis cell 1 according to the invention can, in principle, comprise as many stacks 2 as desired, in particular 3 to 15 stacks 2.

The electrolysis cell 1 according to FIG. 1 is constructed such that said cell comprises two end plates 9. The end plates 9 are embodied to be circular. The two end plates 9 have a plurality of openings for purposes that will be explained below.

A structure for an electrochemical reduction of CO₂ is located between the two end plates 9. Starting on the left side of the exploded view in FIG. 1, a first power connection 83 of a power line 8 is first provided. A separating plate 10 connects thereto, which separating plate insulates the end plate 9 against the power connection 83. The power connection 83 is followed by multiple stacks 2, wherein one stack 2 is shown in an exploded view, whereas the other two stacks are shown in an assembled view. For the stack 2 shown in the exploded view, there is first the construction for an anode space 3, starting with a bipolar plate 58 or bipolar electrode. The anode space 3 comprises a central anode fitting 33, which is surrounded by an anolyte chamber plate 34 and a seal 35. The seal 35 can be formed from polytetrafluoroethylene (PTFE). The anode fitting 33 is held by the anolyte chamber plate 34 and the seal 35. This connects to a membrane 36, which produces a connection to a cathode space 4. A cathode fitting 43 is provided in the cathode space 4. The cathode fitting 43 can, analogously to the anode fitting 33, be designed as is depicted in FIG. 9 through FIG. 11. In particular, both the anode fitting 33 and the cathode fitting 43 can be provided with helical fluid guides, as is illustrated in FIG. 9. The cathode space 4 comprises a cathode chamber plate 44 and a further seal 45 made of PTFE, as well as a conductive seal 46. Said conductive seal 46 enables a current transfer from otherwise insulated components, so that current can flow through the cathode space 4 and the anode space 3 via the conductive seal 46. The current is, as it were, thereby supplied externally, and then enters the interior of the fittings for the cathode space 4 and the anode space 3 before the current is collected again. The conductive seal 46 can in particular be composed of graphite.

A gas space 5 connects to the cathode space 4. The gas space 5 first comprises a conductive support plate 53. Said support plate 53 connects to a gas diffusion electrode 54. This, in turn, first connects to a further seal made of a conductive material such as graphite, in particular a graphite sealing ring 55. A gas space plate 56 and a gas space fitting 57 and a bipolar plate 58 conclude the gas space 5. The gas space plate 56 can also be omitted in order to reduce a current resistance with the accompanying reduction in gas space volume; in this case, a fluid supply and discharge take place via the seal 55. This is followed by a second power connection 84 and a further separating plate 10 before the second end plate 9 follows.

As mentioned, the two end plates 9 respectively have a plurality of openings or bores. A portion of said bores is intended, with corresponding nut/bolt combinations, to compressively hold together the electrolysis cell 1 and the individual fittings, as well as the surrounding rings, as can be seen in FIG. 2 and FIG. 4. The remaining openings are used to form fluid supply lines 61, 62, 63 and fluid discharge lines 71, 72, 73 and, finally, a power line 8, likewise equipped with a supply line and discharge line.

The corresponding supply lines and discharge lines are depicted according to the counterflow principle with the aid of FIG. 2 through FIG. 4, and in particular FIG. 5 through FIG. 7. However, the electrolysis cell can also be designed such that the fluids are conducted in a parallel flow. In the case of such an electrolysis cell according to FIG. 2 through FIG. 7, multiple stacks 2 are provided, namely three. The quantity of stacks 2 can, however, certainly be higher. The individual stacks 2 are constructed according to the illustration in FIG. 1 and are connected such that they rest against one another and conduct current via the bipolar plates 58. With a corresponding layered construction of the stacks 2, a compact arrangement can be obtained. Said compact arrangement is facilitated by fluid supply lines 61, 62, 63 that run perpendicular to the individual stacks 2. One fluid supply line 61, 62, 63 each is provided for the anode space 3, the cathode space 4, and the gas space 5 of each stack 2. Due to the corresponding perpendicular arrangement, a supply to the respective spaces such as anode space 3 or cathode space 4 and gas space 5 can take place from the side. For this purpose, the individual spaces can respectively comprise suitable inlets and suitable outlets for the discharges in the form of the fluid discharge lines 71, 72, 73. Said inlets can be seen in FIG. 1, namely the anode inlets 31, the anode outlets 32, the cathode inlets 41, the cathode outlets 42, and the gas inlets 51 and gas outlets 52. Thus, due to the perpendicularly running supply lines and discharge lines, a fluid supply can take place to each space from the side and a fluid discharge can likewise take place from the side. To this end, for a highest possible level of efficiency, the corresponding supply lines and discharge lines and inlets and outlets are respectively arranged in 180°opposition, so that a retention time within a single layer can be optimized as far as possible. As can be seen in FIG. 3 in particular, a corresponding arrangement requires that the inlets and outlets are offset from one another, so that the individual fluid supply lines 61, 62, 63 and the fluid discharge lines 71, 72, 73 do not collide with one another. An arrangement according to FIG. 3 proves expedient in this regard. If the individual supply lines and discharge lines are offset from one another at a corresponding angle of, for example, 45°, the available area can be well utilized. It should also be noted in this context that the current supply and current discharge also occur in a corresponding manner and space should likewise be provided therefor in the chosen configuration.

In FIG. 5 through FIG. 7, it is illustrated by way of example how a corresponding arrangement responds to the supply of the individual components (gas in FIG. 5, catholyte in FIG. 6 and anolyte in FIG. 7), wherein reference is made to the sections indicated in FIG. 3. As can be seen, through the corresponding guides, all spaces of all stacks 2 are supplied simultaneously, and a substance outflow can, in turn, simultaneously take place from all spaces.

In FIG. 8, a current flow is illustrated which, similar to the fluid supply lines 61, 62, 63 and the fluid discharge lines 71, 72, 73, is configured to simultaneously apply current to all stacks 2 via two bus bars 81, 82 and the first power connection 83 and the second power connection 84.

In FIG. 9 through FIG. 11, various fittings are illustrated which can be used for the anode space 3 and/or the cathode space 4 and/or the gas space 5. It is expedient that a most thorough possible exchange be possible. For this purpose, a helical embodiment according to FIG. 8 is preferably provided, so that supplied fluid must travel a longest possible, and equally long, distance before said fluid can be discharged again via a fluid discharge line 71, 72, 73. Alternatives are illustrated in FIG. 9 (metal foam) and FIG. 10 (metal netting fitting). An efficiency is also maximized in this case. In the case of a metal foam, the flow channel can be controlled via the pore size of the sponge, and a very uniform flow distribution results.

In FIG. 12, an alternative electrolysis cell 1 is illustrated. This alternative variant is, in principle, embodied identically to the previously explained electrolysis cell 1 with a 3-chamber system, wherein the same reference symbols correspond to the same parts. In FIG. 12, however, the gas space 5 is omitted, as well as the fittings necessary therefor, so that the electrolysis cell I is embodied, with an otherwise essentially analogous construction, with a stack of 2-chamber systems. An application of current and a supply of the fluids, including the gas, occurs to a large extent analogously to the electrolysis cell 1 with a 3-chamber system, as it is explained with the aid of FIG. 1 through FIG. 11. The embodiment of the anode space 3 and of the cathode space 4 can also occur as explained for FIG. 1 through FIG. 11.

In FIG. 13, a system 11 with an electrolysis cell I according to the invention is illustrated, preferably in the design of a 3-chamber system. In addition to the electrolysis cell 1, the system 11 comprises an anolyte container 12 and a catholyte container 13, as well as a product container 14 from which methanol and/or formic acid, for example, can be withdrawn. The corresponding containers are connected to the electrolysis cell 1. In particular, suitable lines are provided which are configured to introduce anolyte into the electrolysis cell 1 and remove it again from said cell. The same is provided in terms of a circulation of the catholyte. In addition, for the gas supply, a corresponding line is provided which is configured to supply carbon dioxide, or possibly a gas containing carbon dioxide, to the respective gas spaces 5 of the electrolyte cell 1. Here, the gas is supplied at a pressure in the range of 1.5 bar or more, for example. The gas can thereby be guided in a circuit, as can be seen in FIG. 11. As a result, CO₂ can be guided in the circuit so that it can be converted to the best possible extent. Gaseous products can be collected in a container 15. For this purpose, the converted product is separated by a corresponding separating device and stored in the container 15, for example. Unconverted CO₂ is supplied to the circuit again.