DE102018222338A1 - Electrolyser for carbon dioxide reduction - Google Patents

Abstract

The invention relates to an electrolyzer for reducing carbon dioxide. The gas entry into the electrolyte can be reduced by means of degassing devices at various suitable points in the electrolyte circuit. Carbon dioxide removed from the electrolyte can be fed back into the feed gas stream.

Description

The invention relates to an electrolyzer for reducing carbon dioxide. Carbon dioxide is transported past a gas diffusion cathode of an electrolysis cell and catalytically reduced there to at least one product of higher energy value.

Introduction and state of the art

The burning of fossil fuels currently covers around 80% of the world's energy needs. In 2011, approximately 34,032.7 million tons of carbon dioxide were emitted into the atmosphere worldwide. This release of carbon dioxide is the easiest way to dispose of large amounts of carbon dioxide. Lignite-fired power plants, for example, generate over 50,000 tons a day. However, the discussion about the negative effects of the greenhouse gas carbon dioxide on the climate has led to the fact that recycling of carbon dioxide is desirable and is being worked on. Since carbon dioxide is thermodynamically very low, it can only be difficult to reduce it to usable products.

The carbon dioxide is naturally converted into carbohydrates by photosynthesis. This process, which is divided into many sub-steps in terms of time and at the molecular level, is very difficult to copy on an industrial scale. Compared to pure photo catalysis, electrochemical reduction of carbon dioxide is currently the more efficient way. A mixed form is light-assisted electrolysis or electrically assisted photo catalysis. Both terms are to be used synonymously, depending on the perspective of the viewer.

As with photosynthesis, this process uses photo-assisted electrical energy, which is preferably obtained from regenerative energy sources such as wind or sun, to convert carbon dioxide into a higher-energy product such as carbon monoxide, methane, ethene or other alcohols. The amount of energy required for this reduction ideally corresponds to the combustion energy of the fuel and should only come from renewable sources.

Some possible ways of producing energy sources and chemical raw materials based on renewable energies are currently being discussed. The direct electrochemical or photochemical conversion of carbon dioxide into hydrocarbons or their oxygen derivatives is particularly desirable. No industrial-grade catalysts are currently available for these direct routes. Therefore, multi-stage routes are under discussion, which promise a timely solution due to the higher level of technical maturity of the individual steps. The most important intermediate in these multi-level value chains is carbon monoxide. It is commonly considered the most important C1 building block in synthetic chemistry. As a synthesis gas mixture, i.e. hydrogen and carbon monoxide in a ratio greater than 2: 1, it can be used to build up hydrocarbons and to synthesize methanol via the fishing droplet process. Gas mixtures rich in carbon monoxide or pure carbon monoxide are also used for carbonylation reactions such as hydroformulation by carboxylic acid synthesis or alcohol carbonylation, in which the primary carbon chain is extended. The possibility of generating carbon monoxide from carbon dioxide using renewable energy sources opens up a multitude of possibilities for partially or completely replacing fossil raw materials as a carbon source for many chemical products.

One of these routes is the electrochemical decomposition of carbon dioxide into carbon monoxide and oxygen. It is a one-step process that does not require high temperatures or overpressure. However, it is a relatively complex electrolysis process in which carbon dioxide must be added as a substrate as a gaseous substrate. In addition, the gaseous carbon dioxide can react with the charge carriers generated in the electrolysis and is therefore chemically bound in the electrolytes used: CO ₂ + 2 e ^- + H ₂ O → CO + 2 OH ^- CO ₂ + 2 OH ^- → CO ₃ ^2- + H ₂ O

During the process, these carbonates are then broken down again as a result of proton generation at the anode: 2 H ₂ O → O ₂ + 4 H ⁺ + 4 e ^- 4 H ⁺ + 2CO ₂ ^3- → 2 CO ₂ + 2 H ₂ O

Depending on the structure of the electrolytic cell, the release takes place either in the electrolyte, on a membrane contact surface or directly on the anode. In the first two cases, gas bubbles are released in the ionic current path, which can lead to greatly increased cell voltages and thus to massive losses in energy efficiency. In the latter case, a mixture of carbon dioxide and oxygen would be formed at the anode. There are currently no possible uses for such mixtures and separation would be necessary but very costly. Classic carbon dioxide separation processes such as amine or methanol washes can Security reasons are not applied. Purified carbon dioxide is also used for such electrochemical cells for decomposing carbon dioxide into carbon monoxide and oxygen. Accordingly, it is a significant loss of resources if carbon dioxide is significantly lost in the amount of oxygen and carbon dioxide generated at the anode. In addition, with the predominant release of carbon dioxide, the technology loses its character as green technology. Returning all of the gas to the feed gas stream is inefficient, since the electrochemically generated oxygen in the feed gas stream would be reduced to water again and would thus reduce the efficiency of the electrolysis system and process.

There is therefore a need for a carbon dioxide electrolyzer, by means of which electrochemical decomposition of carbon dioxide into carbon monoxide and oxygen can be carried out, while at the same time the carbon dioxide losses are minimized via the gas mixture formed at the anode and entries of the carbon dioxide into the electrolyte can be separated off as completely as possible.

The object is achieved by an electrolyzer according to the invention according to claim 1 and by a use according to claim 12. Advantageous embodiments of the invention are the subject of the dependent claims.

The electrolyzer for carbon dioxide reduction according to the invention comprises an electrolysis cell, an electrolyte line which splits into the catholyte supply line and the anolyte supply line, an electrolyte, at least one pump, a cathode which is designed as a gas diffusion electrode, in a cathode space which comprises a cathode gas space and a catholyte space, furthermore one Anode in an anode compartment and a separator which separates the cathode compartment and the anode compartment. The electrolyte has a pH value less than 7 but greater than 1. The electrolyzer comprises an electrolyte reservoir in which the catholyte line and the anolyte line are brought together and to which the electrolyte line is connected, the electrolyte line or the electrolyte reservoir having a gas separation device, to which a third gas outlet is connected, for separating the gas bubbles occurring in the electrolyte reservoir. In the electrolyzer according to the invention, a first gas inlet for a carbon dioxide-containing feed gas and a first gas outlet are connected to the cathode gas space and on the anode side, the electrolyzer comprises at least one second gas outlet, which is designed, in particular in combination with an active degassing device or a phase separator, which is formed on the anode To discharge gases.

Carbon dioxide is used to describe gas bubbles but also physically dissolved gas and chemically bound gas in the electrolyte. On the anode side means that the at least second gas outlet is connected to the anode space and / or the anolyte line leading away from the cell. The anolyte and catholyte supply lead the mixed and accordingly balanced electrolyte from the electrolyte reservoir into the electrolytic cell. The anolyte and catholyte lines lead anolyte and catholyte, which have different compositions and pH values due to the chemical reactions occurring in the cell, away from the electrolytic cell and back into the electrolyte reservoir. The electrolyte creates an ionic connection between the anode and cathode in the cell.

The electrolyzer according to the invention has the advantage of minimizing the carbon dioxide losses via the anode gas, since it enables the carbon dioxide stored or physically dissolved in carbonates to be released separately from the oxygen.

The electrolyzer expediently comprises a separator which is designed to prevent convective mass transport between the anode compartment and the cathode compartment. The separator is, for example, a membrane or a diaphragm, particularly preferably made of a porous film made of polyphenylene sulfone (PPSU) and ZrO ₂ particles.

In an advantageous embodiment of the invention, the electrolyzer comprises a degassing device which is connected to the anolyte feed line and which is designed to remove dissolved gas from the anolyte and to feed it to a fourth gas outlet. The anolyte feed line is located upstream of the electrolytic cell in the direction of electrolyte flow, that is to say that gas dissolved in the fourth gas outlet is removed from the anolyte before it flows into the electrolytic cell. This has the advantage that less carbon dioxide is released by dissolving anodically formed oxygen or by neutralizing carbonates in the anolyte compartment.

In a further advantageous embodiment of the invention, the electrolyzer comprises a degassing device which comprises a membrane and a pump. A vacuum can be generated by means of the pump, as a result of which gas dissolved in the electrolyte is actively released.

Alternatively, the degassing device comprises an ultrasound source and a phase separator. Through the ultrasound source in Electrolyte dissolved gases are extracted and removed from the electrolyte circuit via the subsequent phase separation.

Both embodiments lead to degassing of the electrolyte before it flows into the anode compartment. This reduces the amount of carbon dioxide dissolved. At the same time, this causes less carbon dioxide to be expelled at the anode when oxygen gas is dissolved in the anolyte.

The gas separated at this point in the electrolyte circuit has a high carbon dioxide content due to the high solubility of carbon dioxide and is suitable for recycling. In a particularly advantageous embodiment of the invention, the third gas outlet and / or the fourth gas outlet is connected to the first gas inlet via additional gas lines. In this way, the gas separated from the anolyte can be fed back into the feed gas stream and thus be recycled.

The carbon dioxide or hydrogen carbonate equilibrium is not completely on the educt side even at a pH of less than 7, i.e. as long as the electrolyte is in contact with a carbon dioxide-containing gas phase, it also contains significant amounts of dissolved hydrogen carbonate at pH less than 7, which is produced by anodization Protons can be neutralized to carbon dioxide. Since the degassing removes the carbon dioxide-containing gas phase, the equilibrium is shifted to the educt side and, in addition to the physically dissolved carbon dioxide, also dissolved hydrogen carbonate is removed. Due to these effects, the amount of carbon dioxide gas bubbles in the anolyte decreases and the gas generated in the anolyte has a high oxygen content. This also reduces the total amount of gas in the anolyte.

Reaction equation to the carbon dioxide-hydrogen carbonate equilibrium: CO ₂ + H ₂ O → HCO ₃ ^- + H ⁺

In a further advantageous embodiment of the invention, the electrolyzer comprises an anode, which is designed as a gas diffusion electrode, and the anode compartment comprises an anolyte compartment and an anode gas compartment. The anode gas space is connected to the second gas outlet, so that the gases transported into the anode gas space can be removed.

In a particularly preferred embodiment thereof, the anode gas space has a second gas inlet and is connected to the second gas outlet in such a way that a gas stream can be passed through the anode gas space in order to remove gases formed at the anode. The anode gas space can be flushed with a gas stream, so to speak, in order to remove the gases which are produced at the anode. With such a configuration of the cell, the phase separator in the anolyte line can be dispensed with.

The gas diffusion electrode between the liquid and gaseous phases in the anode compartment absorbs gas bubbles on the side with the liquid phase and transports them to the gaseous phase. In cell operation, oxygen is formed on the anode surface. The oxygen formed is therefore transported to the gas side of the gas diffusion anode and thus does not form any gas bubbles in the liquid anolyte. Due to the significantly reduced contact time between the oxygen gas phase and the electrolyte, the oxygen can dissolve less efficiently in the electrolyte and thereby displace already dissolved carbon dioxide. Thus, less carbon dioxide is displaced from the liquid phase in the anode compartment due to the gas diffusion electrode used.

At low concentrations of carbonates, the carbonates in the immediate vicinity of the electrode are not sufficient to neutralize the H ⁺ ions generated. The neutralization reaction between HCO ₃ ^- and H ⁺ is therefore not limited to the electrode surface. The carbon dioxide released in this reaction therefore does not necessarily have to come into contact with the anode surface. Therefore, only a part of the carbon dioxide generated during neutralization is transported into the anode gas space. This separates the resulting carbon dioxide and the oxygen gas, and the gas in the anode compartment mainly consists of oxygen gas. In addition, the total amount of gas bubbles in the electrolyte is reduced. Since less oxygen is dissolved in the anolyte, less oxygen can also be released in the electrolyte reservoir or during degassing, which enables or improves the recycling of the gas streams which are withdrawn from the third and fourth gas outlet.

In a further advantageous embodiment of the invention, the anolyte line of the electrolyzer comprises a second degassing device, which is designed in particular as a phase separator for separating gas bubbles from the electrolyte and which is connected in particular to the second gas outlet. The phase separator can in particular also be arranged in the anolyte compartment or connected directly to it.

In a further advantageous embodiment of the invention, the electrolyte is in the electrolyzer an aqueous electrolyte, especially an aqueous solution of a fully dissociating salt. An example of this is a potassium sulfate solution. An aqueous electrolyte is particularly preferably used, the ions of which are innermost in relation to the electrochemical processes on the anode and cathode. Advantageously, there is an electrolyte in the electrolyzer which has concentrations of hydrogen carbonate, bicarbonate and carbonate below 50 mM.

Carbon dioxide is reduced at the interface between the cathode and the electrolyte. The representation is shown based on the formation of carbon monoxide, but can also be applied to other carbon dioxide reduction products: CO ₂ + 2 e ^- + H ₂ O → CO + 2 OH ^-

As a side reaction, the water can also be reduced, for example: 2 H ₂ O + 2 e ^- → H ₂ + 2 OH ^-

Due to the gas permeability of the cathode, the electrolyte is also saturated with carbon dioxide. The dissolved carbon dioxide in the electrolyte then reacts with the formed OH ^- ions continue to bicarbonate or carbonate: CO ₂ + OH ^- → HCO ₃ ^- CO ₂ + 2 OH ^- → CO ₃ ^2- + H ₂ O

Since the pH value of the electrolyte at the beginning is less than 7, the formation of bicarbonate HCO ₃ ^- preferred. For the cathode reaction, the reactions can therefore be summarized as a net equation as follows: 3 CO ₂ + 2 e ^- + H ₂ O → CO + 2 HCO ₃ ^-

• - HCO₃ ^-
• - Gelöstes CO₂
• - Gelöstes Produktgas, was im beschriebenen Fall eine Mischung aus Kohlenstoffmonoxid und Wasserstoff ist
• - Eventuell Gasblasen von Produktgasen sowie Kohlenstoffdioxidgasblasen

The catholyte emerging from the electrolytic cell therefore contains, in addition to the electrolyte salts originally dissolved, also:

• - HCO ₃ ^-
• - Dissolved CO ₂
• - Dissolved product gas, which in the case described is a mixture of carbon monoxide and hydrogen
• - Possibly gas bubbles from product gases as well as carbon dioxide gas bubbles

The electrolyser described has the particular advantage that the gases dissolved in the anolyte can be removed and thus there is no saturation of the anolyte with carbon dioxide or even supersaturation.

The water of the electrolyte is oxidized to oxygen at the anode: 2 H ₂ O → O ₂ + 4 H ⁺ + 4 e ^-

The resulting oxygen O ₂ is released in the form of gas bubbles or can dissolve in the electrolyte. If it dissolves, it displaces gases that have already dissolved in the electrolyte, for example dissolved carbon dioxide. Thus, carbon dioxide can escape from the electrolyte in the anode compartment without carbon dioxide being formed in the anode reaction and without carbon dioxide being passed to the anode, since the solubility of oxygen and carbon dioxide in the aqueous electrolyte differ. The amount of gas released in this way can be determined via the ratio of the maximum solubilities and the actual concentrations. The solubility of carbon dioxide is many times greater than that of oxygen gas. This means that even a small amount of oxygen can release a non-negligible amount of carbon dioxide: at a temperature of 25 ° C and a carbon dioxide-saturated electrolyte, 28 times the amount of carbon dioxide is released compared to the amount of oxygen that is newly bound in the electrolyte.

In the electrolyte there is a balance between dissolved carbon dioxide gas and HCO ₃ ^- : CO ₂ (aq) + H ₂ O ↔ H ₂ CO ₃ ↔ H ⁺ + HCO ₃ ^- ↔ 2 H ⁺ + CO ₃ ^2-

The anodically formed protons H ⁺ shift the equilibrium and the carbonates dissolved in the electrolyte, eg hydrogen carbonate ions HCO ₃ ^- , can react to CO ₂ : H ⁺ + HCO ₃ ^- → H ₂ O + CO ₂

• - Sauerstoff O₂ in gelöster Form oder als Gasbläschen
• - CO₂ in gelöster Form oder als Gasbläschen, wobei die Menge mit der ursprünglich im Anolyten gelösten Kohlenstoffdioxidmenge ansteigt,
• - H⁺, wobei die Menge mit der ursprünglich im Anolyten gelösten CO₂-Menge sinkt.

In addition to the salts dissolved in the electrolyte, the anolyte leaving the electrolytic cell also contains:

• - Oxygen O ₂ in dissolved form or as gas bubbles
• CO ₂ in dissolved form or as gas bubbles, the amount increasing with the amount of carbon dioxide originally dissolved in the anolyte,
• - H ⁺ , the amount decreasing with the amount of CO ₂ originally dissolved in the anolyte.

The gas bubbles carried in the anolyte are oxygen-rich gas. Therefore, it must be separated before it reaches the electrolyte reservoir. The separation takes place e.g. via phase separator and second gas outlet.

Undissolved gases are separated again from the electrolyte before entering the electrolytic cell, for example through the third or fourth gas outlet.

In a further advantageous embodiment of the invention, an electrolyzer is used, with an electrolyte based on a completely dissociating salt, the salt not being a carbonic acid salt. This ensures that there is no neutralization by means of which carbon dioxide would be released. This effect is detectable even with small admixtures of carbonate salts.
An electrolyzer is advantageously used in which gas which can be taken from the third and / or fourth gas outlet is mixed into the feed gas stream, provided that it has a suitable composition or has been subjected to a treatment step.

Explanation of how the electrolyte reservoir works:

Leaving anolyte and catholyte flows are mixed again in the electrolyte reservoir. Since the same number of charges are converted at the cathode and the anode, the part of the protons and hydrogen carbonate ions that have not yet neutralized in the anolyte is neutralized in the electrolyte reservoir: H ⁺ + HCO ₃ ^- → CO ₂ + H ₂ O

During the operation of the electrolyzer, a balance is established for the gases dissolved in the electrolyte. Mainly carbon dioxide CO _{2 is dissolved} in the electrolyte. The gas bubbles present in the electrolyte are separated from the system via the third gas outlet before they pass through the electrolysis cell again. The gas bubbles mainly contain carbon dioxide and oxygen gas. With a suitable composition, this gas can be fed back into the feed gas stream via the first gas inlet. In particular, a preparation step can be provided beforehand.

The advantages of the electrolyser described can be summarized as follows: The degassing of the electrolyte reduces the amount of gas in the anode compartment and thus the carbon dioxide emissions at the anode. It is not necessary to use ion-selective membranes. If the anode is additionally designed as a gas diffusion electrode, this further reduces the amount of gas in the anolyte compartment, the oxygen input into the electrolyte and, as a result, a reduction in carbon dioxide emissions in the anode gas compartment. By using an electrolyte based on a completely dissociating salt, which is not a salt of carbonic acid, and by the fact that the electrolyte has a pH value between 1 and 7, the amount of carbon dioxide gas generated in the anolyte space can be further reduced.

Figure list

• The 1 shows a structure of an electrochemical cell with two gas diffusion electrodes and a degassing device of the electrolyte before entering the anolyte space.
• 2nd shows the structure of an electrochemical cell with a gas-impermeable anode and three chambers.
• 3rd shows a structure of an electrochemical cell with two gas diffusion electrodes and a degassing device with a membrane and a pump for generating a negative pressure for degassing the electrolyte.
• 4th shows a structure of an electrochemical cell with two gas diffusion electrodes and a degassing device, by means of ultrasound and subsequent phase separation.
• 5 shows a structure of an electrochemical cell with two gas diffusion electrodes and a degassing device for degassing the electrolyte in front of the anolyte chamber and return devices CO ₂ -rich gases in the feed gas stream.
• 6 shows a structure of an electrochemical cell, which is used to measure the gas compositions when performing different degassing measures.
• The 7 , 8th and 9 show the corresponding measurement results for the gas composition.

The in 1 shown electrolyzer comprises an electrolytic cell 10 with four chambers: cathode gas space I. , Catholyte space II , Anolyte space III and anode gas space IV . The gas rooms I. and IV each have a gas inlet E1 and E2 and gas outlet A1 and A2 on. In the cathode compartment I / II and anode compartment III / IV . are each gas diffusion electrodes as a gas diffusion cathode GDK and gas diffusion anode GDA Installed. Cathode compartment I / II and anode compartment III / IV are through a separator SEP Cut. Gas diffusion cathode GDK and gas diffusion anode GDA are electrically connected to each other via a voltage supply U, via the electrolyte that is in the catholyte compartment II as well as in the anolyte room III located ionically linked. Catholyte space II and anolyte space III each have a catholyte supply 111 or anolyte feed 121 on, via which the electrolyte enters the electrolytic cell 10 is initiated. The transportation of the Electrolytes are pumped around the electrolyte in the electrolyte lines 111 , 121 for example through the pumps P1 and P2 . The discharge of the electrolyte from the electrolytic cell 10 happens via separate catholyte 112 and anolyte lines 122 which is then in the electrolyte reservoir RES be brought together. By mixing the electrolyte, it is electrically neutralized again: H ⁺ + HCO ₃ ^- → CO ₂ + H ₂ O.

In the electrolyte there is a balance between dissolved gases and gas bubbles and a charge balance between those on the electrodes GDK , GDA converted and formed anions and cations.

From the electrolyte reservoir RES leads an electrolyte line 13 to the electrolytic cell 10 which before entering the electrolytic cell 10 back into the catholyte supply 111 and anolyte feed 121 is split up. At this point in the electrolysis system, unwanted gas is removed from the electrolyte circuit. This is in the electrolyte line 13 a phase separator 23 and a gas outlet A3 intended. At this point, mainly gas bubbles, i.e. non-chemically bound and not physically dissolved gas components, are separated.

In addition is in the anolyte feed 121 a degassing device DEG with another gas outlet A4 provided, by means of which physically dissolved gases can also be separated from the electrolyte. This degassing can take place, for example, via a membrane and a vacuum applied to it by means of a pump P4 be done as it is in the 3rd is shown: The degassing device DEG has a phase separator 24th as well as a pump P4 on. Alternatively, gases can be extracted from the electrolyte using ultrasound US respectively. To this end, an alternative degassing device has DEG , as in 4th shown an ultrasound source US and a subsequent phase separator 24th and the gas outlet A4 on.

As an alternative to the embodiment with the gas diffusion anode GDA can also be an electrolytic cell 10 with three chambers I. to III , as in 2nd shown, be provided: In this case, the gas bubbles floating in the anolyte after passing through the electrolysis cell 10 by means of a phase separator 22 in another degassing device DEG2 separated and via a gas outlet A2a removed from the electrolyte circuit.

In 5 are additional gas lines 14 , 33 shown about what gas, what the gas outlets A3 and A4 is taken from the feed gas stream via the gas inlet E1 is fed again. This enables particularly advantageous gas recycling. For example, before the gas inlet E1 gas conditioning may be provided.

In 6 there is also shown an experimental setup for measuring the gas compositions, which demonstrate the effectiveness of the proposed measures. The measurements made to prove the effectiveness of the proposed improvement measures of an electrolyser are shown using four possible combinations of the use of a gas diffusion anode and a degassing device. In all measurements, 0.5 molar potassium sulfate solution was used as the electrolyte (0.5 MK ₂ SO ₄ ). The electrolyte was removed using the pumps P1 , P2 through the electrolytic cell 10 pumped. The cathode was used as the starting gas GDK about the gas inlet E1 a gas stream of 100 sccm carbon dioxide CO _{2 is} supplied. Between the electrodes GDK , GDA a constant current flow of two amperes was set. As a cathode GDK a silver-based gas diffusion electrode with at least 90% selectivity was used for the reduction of carbon dioxide to carbon monoxide. As a separator SEP between the catholyte space II and the anolyte space III a zirconium oxide-PPSU composite diaphragm was used (ZrO ₂ / PPSU composite). This has a special ion selectivity.

When building with a solid electrode A there is no anode gas space IV and accordingly no gas flow through the gas outlet A2 . That on the anode A emerging gas is in these measurements via the gas outlet A2a separated and referred to as anolyte gas. During tests with degassing devices DEG / DEG2 becomes the pump P4 put into operation and this creates a negative pressure on the membrane cartridge 24th created. At the gas outlet A4 a gas flow was measured ( M4 ), at the same time at the gas outlet A3 no gas flow can be measured after the electrolyte reservoir ( M3 ). In the tests without degassing devices DEG / DEG2 the pump remains P4 switched off, resulting in no gas flow at the gas outlet A4 occurs. But then a gas flow at the gas outlet can then A3 observed and measured ( M3 ) become. The gas flows at the gas outlets A3 , A4 are each volumetric using a gas meter M3 , M4 and the composition using a gas chromatograph M3 / M4 . In all measurements, either an anode gas emerges from the gas outlet A2 or an anolyte gas from the gas outlet A2a off, or either an electrolyte reservoir gas from the gas outlet A3 or a pump gas from the gas outlet A4 . The gas flows from the anode gas or anolyte gas are shown below as anode gas, the gas flows from the gas outlets A3 as A4 are summarized as electrolyte gas. To solve the problem, the smallest possible amount of carbon dioxide in the anode gas and the smallest possible amount of oxygen gas in the electrolyte gas should be achieved. The associated measured values are in the diagrams of the 7 , the composition of the anode gas in the diagram of the 8th as well as that of the electrolyte gas in the diagram of 9 shown.

Both the use of a gas diffusion anode GDA as well as the degassing device DEG effect as a single measure a reduction of the carbon dioxide content in the anode or anolyte gas by at least 8 sccm, which corresponds to a reduction by 80%. When using a solid electrode as an anode A the amount of gas released in the anolyte is reduced by 50% by degassing the electrolyte beforehand. When using a gas diffusion anode GDA no gas bubbles were observed in the exiting anolyte. By using a gas diffusion anode GDA can in the electrolyte reservoir RES and accordingly the amount of oxygen occurring in the pump gas by a factor 3rd to 4th compared to the respective measurement with a solid electrode A can also be reduced. When using a gas diffusion anode GDA the amount of gas released into the anode gas space IV conducted instead of being carried as gas bubbles by the anolyte, which reduces the electrical resistance of the anolyte. The implementation of both measures, i.e. the use of a gas diffusion anode GDA and the degassing device DEG before introducing the anolyte into the anode compartment III / IV reduced compared to the case without degassing and without gas diffusion anode GDA the amount of carbon dioxide released in the anolyte by a factor of more than 40. The amount of oxygen released in the pump gas or electrolyte reservoir is reduced by 30% from 0.58 sccm to 0.38 sccm by implementing all measures. With a calculated total amount of 7.6 sccm oxygen and a return of the pump or electrolyte reservoir gas into the feed gas stream via the gas inlet E1 this corresponds to a reduction in oxygen loss from 7.6% to 5.0%.

This in 7 The diagram shows the carbon dioxide flow CO ₂ ( M4 ), which is released from the anolyte and anode gas space together. The measurements of the oxygen flow O ₂ (hatched ₎ M3 ) in the electrolyte gas. Both values were given for the constellations of solid anode and no further degassing device ( A ), solid anode with degassing device ( A _DEG ), Gas diffusion anode but no degassing device ( GDA ) and the combination of both gas diffusion anode measures and the use of a degassing device ( GDA _DEG ) measured. In the 8th and 9 are the compositions of the anode gas M4 and the electrolyte gas M3 shown or the volume fraction of the carbon dioxide gas and the oxygen gas.

Reference list

10th

Electrolytic cell

13

Electrolyte line

111

Catholyte supply

121

Anolyte feed

P1, P2

pump

RES

Electrolyte reservoir

112

Catholyte line

122

Anolyte line

K

cathode

A

anode

GDK

Gas diffusion cathode

GDA

Gas diffusion anode

I / II

Cathode compartment

I.

Cathode gas space

II

Catholyte space

III / IV

Anode compartment

III

Anolyte space

IV

Anode gas space

SEP

separator

E1

first gas inlet

E2

Second gas inlet

A1

first gas outlet

A2, A2a

second gas outlet

A3

third gas outlet

A4

fourth gas outlet

DEG

Degassing device

23

Gas separation device

P4

pump

US

Ultrasound source

24th

Phase separator

DEG2

Second degassing device

33, 14

More gas lines

Claims (13)

Electrolyser for carbon dioxide reduction comprising an electrolysis cell (10), an electrolyte line (13) which splits into the catholyte supply line (111) and anolyte supply line (121), an electrolyte, at least one pump (P1, P2), a cathode (GDK), which is designed as a gas diffusion electrode, in a cathode space (I / II), which comprises a cathode gas space (I) and a catholyte space (II), further comprising an anode (A, GDA) in an anode space (III / IV ), and a separator (SEP), which separates the cathode compartment (I / II) and anode compartment (III / IV) from one another, a first gas inlet (E1) for a carbon dioxide-containing feed gas and a first gas outlet (A1) on the cathode gas compartment (I) and at least one second gas outlet (A2, A2a) is provided on the anode side, which is configured to discharge the gases formed on the anode, and wherein the electrolyzer comprises an electrolyte reservoir (RES) in which catholyte line (112) and anolyte line ( 122) are brought together and to which the electrolyte line (13) is connected, the electrolyte line (13) or the electrolyte reservoir (RES) having a gas separation device (23) to which a third gas outlet (A3) connects t, for the separation of the gas bubbles occurring in the electrolyte reservoir (RES) and the electrolyte has a pH value less than 7 and greater than 1. Electrolyser after Claim 1 , wherein the anolyte feed line (121) has at least one degassing device (DEG) to which a fourth gas outlet (A4) is connected, which is designed to remove dissolved gas from the anolyte and to feed the fourth gas outlet (A4). Electrolyser after Claim 1 or 2nd , wherein the degassing device (DEG) comprises a membrane and a pump (P4). Electrolyser according to one of the preceding claims, wherein the anode (GDA) is designed as a gas diffusion electrode and the anode space comprises an anolyte space (III) and an anode gas space (IV). Electrolyser after Claim 4 , wherein the anode gas space (IV) is connected to a second gas inlet (E2) and the second gas outlet (A2) in such a way that a gas flow, for removal of gases arising at the anode (GDA), can be passed through the anode gas space (IV). Electrolyser according to one of the preceding claims, wherein the anolyte line (122) has a second degassing device (DEG2), which is designed in particular as a phase separator (22) for separating gas bubbles from the electrolyte and which is connected to the second gas outlet (A2a). Electrolyser according to one of the preceding claims, wherein the anolyte line (122) has a second degassing device (DEG2) which is designed to remove actively dissolved gases from the electrolyte and to feed them to the second gas outlet (A2a). Electrolyser according to one of the preceding claims, wherein the electrolyte is an aqueous electrolyte, in particular an aqueous solution of a fully dissociating salt. Electrolyser according to one of the preceding claims, wherein the ions of the electrolyte are inert to the electrochemical processes on the anode (A, GDA) and cathode (GDK). Electrolyser according to one of the preceding claims, wherein the electrolyte has concentrations of hydrogen carbonate, bicarbonate and carbonate below 50 mM. Electrolyser according to one of the preceding claims, wherein the third gas outlet (A3) and / or the fourth gas outlet (A4) are connected to the first gas inlet (E1) via gas lines (33, 14). Use of an electrolyzer according to one of the preceding claims, in which an electrolyte based on a completely dissociating salt is used, which is not a salt of carbonic acid and the electrolyte has a pH value less than 7 and greater than 1. Use of an electrolyser after Claim 11 , In the gas, which can be removed from the third and / or fourth gas outlet (A3 / A4), is mixed into the feed gas stream, in particular after a treatment step.

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