CN111315921A

CN111315921A - Device for carbon dioxide electrolysis

Info

Publication number: CN111315921A
Application number: CN201880071882.7A
Authority: CN
Inventors: P·让蒂; E·玛格里; R·帕斯图夏克; C·舍雷尔; K·维斯纳-弗莱舍尔
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2017-11-07
Filing date: 2018-10-15
Publication date: 2020-06-19
Also published as: EP3655565A1; WO2019091703A1; US20200291537A1; DE102017219766A1; AU2018363551A1

Abstract

The invention relates to a device for carbon dioxide electrolysis, comprising: an electrolytic cell with an anode and a cathode, wherein the anode and the cathode are connected to a power supply, wherein the cathode is designed as a gas diffusion electrode, which is connected on a first side to a gas chamber and on a second side to a cathode chamber; -an electrolyte circuit connected to the electrolytic cell; -a gas supply for supplying a gas containing carbon dioxide into the gas chamber; in the device, one or more grooves are arranged in the gas chamber, wherein the grooves at least partially abut against the gas diffusion electrode and are designed for transporting electrolyte permeating through the gas diffusion electrode to a side region of the gas chamber.

Description

Device for carbon dioxide electrolysis

Technical Field

The present invention relates to a device for carbon dioxide electrolysis according to the preamble of claim 1.

Background

Currently, approximately 80% of the global energy demand is covered by burning fossil fuels. With these combustion processes, about 340 hundred million tons of carbon dioxide were emitted into the atmosphere worldwide in 2011.

The discussion of the negative impact of the greenhouse gas CO2 on climate led to thinking about recycling CO 2. CO2 is a strongly bonded molecule and therefore difficult to reduce again to a usable product.

In nature, CO2 is converted into carbohydrates through photosynthesis. This complex process is difficult to replicate on a large scale. One currently technically feasible method is electrochemical reduction of CO 2. Wherein the carbon dioxide is converted into higher energy products, such as CO, CH4, C2H4 or C1-C4 alcohols, with the supply of electrical energy. The electrical energy is in turn preferably from renewable energy sources, such as wind or photovoltaic.

For the electrolysis of CO2, metals are typically used as catalysts. The type of metal can affect the electrolysis product. For example, CO2 is reduced almost only to CO on Ag, Au, Zn and to a very limited extent to CO on Pd, Ga; on copper, however, a large amount of hydrocarbons can be observed as reduction products. In addition to pure metals, metal alloys and mixtures made of metals and promoted metal oxides are also of interest because they can improve the selectivity for specific hydrocarbons.

In CO2 electrolysis, a gas diffusion electrode may be used as a cathode, similar to that in chlor-alkali electrolysis, in order to establish a three-phase boundary between the liquid electrolyte, gaseous CO2 and the solid silver particles. As is known in the fuel cell art, an electrolysis cell is used having two electrolyte chambers, wherein the electrolyte chambers are separated by an ion exchange membrane.

The working electrode is a porous Gas Diffusion Electrode (GDE). The gas diffusion electrode generally comprises a metal mesh on which a mixture made of PTFE, activated carbon, catalyst and other components is applied. The gas diffusion electrode has a pore system into which the reactants penetrate and react at the three-phase interface.

The counter electrode is, for example, a metal plate coated with platinum or iridium mixed oxide. The GDE is in contact with the electrode on one side. The GDE is fed CO2 on the other side. The functional mode of GDEs is known, for example, from EP 297377 a2, EP 2444526 a2 and EP2410079 a 2.

For a continuous process, the cells may be operated in flow-by-Modus. In this mode, the reactant gas diffuses into the pores of the GDE. Reaction in GDE pores to form OH^-Ions of these OH^-The ions result in a locally high pH. If the reaction gas CO2 encounters this basic liquid which also contains alkali metal cations (e.g. potassium), a sparingly soluble carbonate is formed which precipitates out as a salt and blocks the pores.

There are several possible approaches to avoid this, including the use of electrolyte transpiration through the GDE. The electrolyte flushes the pores in situ, flows off downwards on the gas side of the GDE and is conducted out of the cell at the bottom of the cell together with unconverted reaction gas and product gas.

The flow grid is used to achieve good mixing of the gases in the gas chamber and thereby a fast diffusion of carbon dioxide to the GDE pores. This ensures vortex and cross-mixing, thereby improving mass transfer of the reactant and product gases. The flow grid also supports the GDE so that it does not bend.

Although this enables stable long-term operation of the gas diffusion electrode (>1000h) in CO2 electrolysis, the high liquid content in the gas chamber reduces the process efficiency. On the one hand, the liquid film on the GDE makes it difficult for the reaction gas to diffuse into the pores of the GDE. On the other hand, known flow grates in the form of a grate structure or a braid make it difficult for the transpiration liquid to flow out, as it adheres to and accumulates at the flow grate.

Disclosure of Invention

The object of the present invention is to provide an improved device for carbon dioxide electrolysis, with which a stable long-term operation can be achieved, while avoiding the above-mentioned disadvantages.

This object is achieved by a device having the features of claim 1. The dependent claims relate to advantageous embodiments of the device.

The device according to the invention for the electrolysis of carbon dioxide comprises an electrolytic cell with an anode and a cathode, wherein the anode and the cathode are connected to a power supply, wherein the cathode is designed as a gas diffusion electrode, which is connected on a first side to a gas chamber and on a second side to a cathode chamber. The apparatus for carbon dioxide electrolysis further comprises an electrolyte circuit connected to the electrolytic cell and a gas supply for supplying a gas containing carbon dioxide into the gas chamber. Finally, the device for carbon dioxide electrolysis comprises one or more grooves in the gas chamber, wherein the grooves at least partially abut against the gas diffusion electrode and are designed for conveying the electrolyte permeating through the gas diffusion electrode to a side region of the gas chamber.

The grooves advantageously enable the discharge of the electrolyte, which permeates through the cathode as transpiration liquid and wets the surface of the gas diffusion electrode. If the electrolyte layer on the surface of the gas diffusion electrode is sufficiently thick, the electrolyte starts to flow out. The trenches discharge the electrolyte to the sides and thereby reduce the electrolyte layer thickness in the region below the respective trenches. This allows carbon dioxide to reach the surface of the gas diffusion electrode well, thereby achieving improvement in electrolysis efficiency.

Furthermore, the grooves also provide, by their location and arrangement, a flow resistance to carbon dioxide flowing over the surface of the gas diffusion electrode. Thus, the laminar flow of the gas is interrupted, and a vortex is generated. This also achieves a better utilization of the carbon dioxide in the gas.

Advantageous embodiments of the device for the electrolysis of carbon dioxide according to the invention emerge from the dependent claims of claim 1. Embodiments according to claim 1 can be combined here with the features of one of the dependent claims or preferably also with the features from the dependent claims. Therefore, the following features can additionally be provided for the device:

the grooves may be designed substantially straight and arranged inclined and at an angle of between 1 ° and 30 °, in particular between 1 ° and 10 °, to the horizontal.

There may be one groove per a substantial length of the vertical extent of the gas diffusion electrode, wherein the substantial length is between 3cm and 10 cm. In other words, the grooves are at a distance of between 3cm and 10cm from each other.

The trenches are preferably connected by a common support structure, wherein the support structure is spaced apart from the gas diffusion electrode. Thus, the groove can be anchored and mechanically supported independently of the cathode, since the groove is supported by the support structure. However, the support structure does not prevent the gas from reaching the surface of the gas diffusion electrode.

The support structure has one or more support pins which are in mechanical contact with the surface of the gas diffusion electrode. Thereby giving the support structure better mechanical strength.

The supporting pins preferably have a diameter of less than 1 mm. It is hereby achieved that the surface coverage of the support pins with the gas diffusion electrodes is small and therefore the support pins have only a small influence on the electrolysis.

It is particularly advantageous if at least a part of the support pins have swirl elements which are arranged at a distance of at least 1mm (in particular at least 2mm) from the surface of the gas diffusion electrode. This causes a swirl of the gas flow which leads to a significantly better access of the carbon dioxide to the cathode, which otherwise flows over the surface of the gas diffusion electrode predominantly in laminar flow. At the same time, the distance from the cathode surface ensures that the electrolyte does not accumulate on the surface of the swirl element, so that the electrolyte outflow is not impaired. The swirl element can be designed, for example, as an undulated shape.

The support structures, support pins and/or grooves are preferably of a material with low hydrophobicity, such as PE. The material preferably constitutes the surface of the respective element, for example as a coating. The support structures, support pins and/or channels may also be made primarily or entirely of this material. Therefore, the liquid is distributed more easily on the surface of the material, and the liquid is made to flow out easily.

The support structure, support pins and/or grooves may further comprise an electrically conductive material for contacting the gas diffusion electrode. When joining a plurality of electrolytic cells together to form a so-called Stack (Stack), one cell unit can be electrically connected to the next by means of a flow grid formed by support structures, support pins, turbulence elements and grooves.

Drawings

Preferred, but not limiting, embodiments of the invention are now further explained with the aid of the figures. The features are shown schematically in the drawings. Wherein:

FIG. 1 shows an electrolysis apparatus for the electrolysis of CO 2; and is

Fig. 2 and 3 show side and plan views of the flow grid.

Detailed Description

The construction of the electrolytic cell 11 schematically shown in fig. 1 is generally suitable for carrying out carbon dioxide electrolysis. This embodiment of the electrolytic cell 11 comprises at least one anode 13 with an adjoining anode compartment 12 and a cathode 15 and an adjoining cathode compartment 14. The anode chamber 12 and cathode chamber 14 are separated from each other by a membrane 21. The membrane 21 is typically made of a PTFE-based material. Depending on the electrolyte solution used, a structure without the membrane 21 in which the pH balance exceeds that with the membrane 21 can also be considered.

The anode 13 and the cathode 15 are electrically connected to a power source 22, which is controlled by a control unit 23. The control unit 23 may apply a protection voltage or an operating voltage to the

electrodes

13, 15, i.e. the anode 13 and the cathode 15. The anode chamber 12 of the illustrated electrolytic cell 11 is equipped with an electrolyte inlet. The illustrated anode chamber 12 also includes outlets for electrolyte and, for example, oxygen O2 or other gaseous byproducts that are formed at the anode 13 during electrolysis of carbon dioxide. Likewise, cathode compartment 14 has at least one product outlet and an electrolyte outlet, respectively. The total electrolysis product may consist of a plurality of electrolysis products.

The cell 11 is also designed as a three-chamber structure in which carbon dioxide CO2 is introduced into the cathode chamber 14 via the cathode 15, the cathode 15 being in the form of a gas diffusion electrode. The gas diffusion electrode allows for contact between the solid catalyst, the liquid electrolyte, and the gaseous electrolysis reactants. For this purpose, the catalyst can be designed, for example, to be porous and to assume the function of an electrode, or the porous electrode can assume the function of a catalyst. The pore system of the electrode is designed such that the liquid and gas phases can equally penetrate into the pore system and can be present both in the pore system and at the electrically accessible surface of the pore system. One example of a gas diffusion electrode is an oxygen consuming electrode used in chlor-alkali electrolysis.

For the design as a gas diffusion electrode, the cathode 15 comprises in this example a metal mesh on which a mixture made of PTFE, activated carbon and catalyst is applied. To introduce carbon dioxide CO2 into the catholyte loop, the electrolytic cell 11 includes a carbon dioxide inlet 24 into the gas chamber 16. The carbon dioxide reaches the cathode 15 in the gas chamber 16 and can penetrate into the porous structure of the cathode 15 at the cathode and react.

The device 10 also comprises an electrolyte circuit 20 by means of which liquid electrolyte, for example K2SO4, KHCO3, KOH, Cs2SO4, is fed to the anode chamber 12 and to the cathode chamber 14 and the electrolyte is recirculated into the reservoir 19. Circulation of electrolyte in the electrolyte circuit 20 is performed by an electrolyte pump 18.

In the present example, the gas chamber 16 comprises an outlet 25, which is arranged in the bottom region. The outlet 25 is designed as an opening with a sufficient cross section so that both the electrolyte and the carbon dioxide and product gases passing through the cathode 15 can pass through the outlet into the connected pipe. The outlet 25 leads to an overflow reservoir 26. The liquid electrolyte is collected and accumulated in the overflow reservoir 26. Carbon dioxide and product gases from the gas chamber 16 are separated from the electrolyte and accumulate above the electrolyte.

A further conduit 28 leads from a point in the upper part of the overflow container 26 to a pump 27 and further to the gas supply 17, the pump 27 being a membrane pump in this example. The pump 27 may also be a piston pump, a reciprocating pump, an extrusion pump or a gear pump. Thus, a part of the gas supply 17, the gas chamber 16, the pipe 18 and the overflow container 26 and the connection of the overflow container 26 with the outlet 25 together form a circuit. From the overflow vessel 26, the carbon dioxide and the product gas present are recirculated to the gas supply by means of a pump 27, so that the gas is partly recirculated. The volumetric flow rate of pump 27 is significantly higher than the volumetric flow rate of fresh carbon dioxide. The unconsumed reaction gas is therefore advantageously guided past the cathode 15 again and has one or more further opportunities to be reduced. The product gas is also partly recycled. By directing the carbon dioxide through the cathode 15 multiple times, the conversion efficiency is increased.

There is another connection leading from the overflow vessel 26 back to the electrolyte circuit 20. The connection starts at an outlet 29, which outlet 29 is arranged on the side wall of the overflow container 26, preferably close to the bottom, but not in the bottom. The outlet 29 is connected to a throttle 30, which throttle 30 is designed as a vertical pipe section having a length of, for example, 90 cm. The diameter of this pipe section is considerably larger than the diameter of the feed line to the throttle valve 30. The inlet conduit has an internal diameter of, for example, 4mm and the pipe sections have an internal diameter of 20 mm. The throttle 30 is connected to the electrolyte circuit 20 on the output side, i.e. at the upper end of the pipe section.

During operation, a pressure difference between the electrolyte circuit 20 connected on the upper side and the overflow container 26, and thus between the cathode chamber 14 and the gas chamber 16, is established and maintained on the one hand by the throttle 30. This pressure difference is between 10hPa and 100hPa, i.e. the gas chamber 16 is kept at only a slight overpressure relative to the cathode chamber 14.

At the start of electrolysis, despite a slight overpressure on the gas side (i.e. in the gas chamber 16), the electrolyte is "pumped" from the cathode chamber 14 through the gas diffusion electrode (i.e. the cathode 15) in the direction of the gas chamber 16 due to the voltage applied to the cathode 15. On the gas chamber 16 side, droplets occur at the surface of the cathode 15, which coalesce and accumulate in the lower region of the cathode 15.

Albeit through the OH of the cathode 15^-The ions form salts with the carbon dioxide and alkali metal cations from the electrolyte, but the pressure differential at the cathode 15 is small so that there is enough liquid to flush through the cathode 15 and cause the formed salts to become a solution, permanently wash them away and drain them from the gas chamber 16 into the overflow container 26. The throttle valve 30 prevents further pressure rise which could lead to salt crystals being formed.

A flow grid 40 is arranged on the gas diffusion electrode. The flow grid 40 is arranged such that the gas flow between the carbon dioxide inlet 24 and the outlet 25 is between the surface of the gas diffusion electrode and the support structure 41 of the flow grid 40. The specific structure of the flow grid 40 is shown in fig. 2 and 3.

Fig. 2 shows an enlarged side view of the flow grid 40. The flow grid 40 abuts the cathode 15 on the right in fig. 2. Fig. 3 shows a plan view of the flow grid 40 from the cathode 15 side.

The flow grid 40 comprises a support structure made of struts or plates, which mechanically connect further elements. The flow grid 40 is closed off to the outside by a substantially rectangular frame 46, which frame 46 allows gas entry and gas exit only at

openings

47 and 48. In the region of the

openings

47, 48, the flow grate 40 comprises parallel webs 50 oriented in line with the direction of the gas flow and one or more baffles 49 for shaping the gas flow.

A plurality of support pins 42 are disposed in a central region of the flow grid 40. The support pins 42 contribute to the mechanical strength of the flow grid and achieve a fixed minimum distance of the support structure 41 from the gas diffusion electrode surface. In this example, there are 8 horizontal rows with alternately 9 or 10 support pins 42. The support pins 42 are spaced from each other by a distance of about 6 mm. Thus, in other embodiments of the flow grid 40, there may also be more support pins 42 or fewer support pins 42 depending on the size of the flow grid 40. The distance of the support pins 42 is preferably between 3mm and 12 mm. The support pins should cover up to 10% of the plane of the gas diffusion electrode, wherein the coverage is preferably less than 5%.

At a distance of 1.5mm (or in another example 2.5mm) from the surface of the cathode 15, on the support pins 42, respectively, eddy current elements 43 are arranged. In the present example, the turbulence elements 43 have the shape of a flat, substantially rectangular piece of material, but curved into a wave-like shape. The swirl elements 43 are arranged substantially transversely to the main flow direction of the gas. By the shape of the swirl elements 43 and the remaining flow area between the swirl elements 43, the gas flow is significantly turbulent, i.e. a laminar flow over the gas diffusion electrode is eliminated.

Likewise, in the middle region of the flow grate 40, the flow grate 40 also has two grooves 44. The grooves 44 are fixed on the plurality of support pins 42, respectively, and are arranged such that the grooves abut against the surface of the gas diffusion electrode. The grooves are arranged at a small angle (e.g. 10 °) to the horizontal, i.e. not completely horizontal. By means of the arrangement of the grooves on the surface of the cathode 15, the grooves receive transpiration liquid, i.e. electrolyte, which flows down through the cathode 15 at the surface of the cathode 15, and the grooves discharge the liquid to the side by their inclination. On the side of the flow grate 40, the grooves 44 merge into outflow channels 45 in the frame 41, which outflow channels let the liquid out to the openings 48. It is achieved thereby that the transpiration liquid wets the surface of the cathode 15 to a lesser extent, so that the gas is less hindered from entering the pores of the gas diffusion electrode.

As in the case of the support pins 42, the number of grooves 44 depends on the overall size of the flow grid 40 and therefore on the size of the cathode 15. The grooves are preferably arranged at a distance of between 3cm and 10cm from each other.

In this example, the flow grid 40 is made primarily of polyethylene. In other embodiment variants, other materials, preferably materials with a low hydrophobicity, can be selected. The flow grid 40 may be made entirely or predominantly of this material, or this material may be used as a surface coating. Due to the low hydrophobicity, the contact angle between the flow grid 40 and the material is minimized, so that the liquid is distributed over the material surface and it is ensured that the liquid flows away as well as possible.

Claims

1. An apparatus (10) for carbon dioxide electrolysis, comprising:

-an electrolytic cell (11) having an anode (13) and a cathode (15), wherein the anode (13) and the cathode (15) are connected to a power source (22), wherein the cathode (15) is designed as a gas diffusion electrode to which a gas chamber (16) is connected on a first side and a cathode chamber (14) is connected on a second side;

-an electrolyte circuit (20) connected to said electrolytic cell (11);

-a gas supply (17) for supplying a gas containing carbon dioxide into the gas chamber (16);

it is characterized in that the preparation method is characterized in that,

one or more grooves (44) are arranged in the gas chamber (16), wherein the grooves (44) at least partially abut against the gas diffusion electrode and are designed for conveying electrolyte permeating through the gas diffusion electrode to a side region (45) of the gas chamber (16).

2. Device (10) according to claim 1, wherein the groove (44) is designed substantially straight and is arranged inclined and at an angle of between 1 ° and 30 ° to the horizontal.

3. The device (10) according to claim 1, wherein the grooves (44) are at a distance of between 3cm and 10cm from each other.

4. The device (10) of claim 1, wherein the grooves (44) are connected by a common support structure (40), wherein the support structure (40) is spaced apart from the gas diffusion electrode.

5. The apparatus (10) of claim 1, wherein the support structure (40) has one or more support pins (42) in mechanical contact with a surface of the gas diffusion electrode.

6. The apparatus (10) of claim 5 wherein the support pin (42) has a diameter of less than 1 mm.

7. Device (10) according to claim 1, wherein at least a part of the support pins (42) have swirl elements (43) arranged at a distance of at least 1mm, in particular at least 2mm, from the surface of the gas diffusion electrode.

8. Device (10) according to claim 7, wherein the swirl element (43) is designed to be undulated.

9. Device (10) according to claim 1, wherein the support structure (40), the support pins (42) and/or the grooves (44) are of a material with low hydrophobicity, such as PE.

10. The device (10) of claim 1 having an electrically conductive material for contacting the gas diffusion electrode.