DE102019205316A1 - Energy-efficient hydrogen production - Google Patents

Abstract

The present invention relates to a method for energy-efficient hydrogen production and a device for this. A carbon dioxide electrolysis (3) in which an intermediate containing formate and / or formic acid is produced is combined with a subsequent thermal decomposition (12) of the intermediate in which hydrogen (H2) is released. For example, the waste heat (37) from the carbon dioxide electrolysis (3) can be used for the thermal decomposition (12) of the intermediate.

Description

The present invention relates to a method for energy-efficient hydrogen production and a device for this.

State of the art

Fossil fuels cause carbon dioxide emissions that are not in line with global climate protection goals. For this reason, for example, the expansion of renewable energies is being strongly promoted and promoted in Germany. In regions of the world with very good weather conditions for solar and wind power plants, electricity generation costs of less than 3 cents per kilowatt hour are possible today. However, the generation of renewable electricity is subject to local and temporal fluctuations. In addition, locations with very good weather conditions, such as deserts and unpopulated coastlines, are not in the immediate vicinity of electrical load centers such as cities and industrial plants. Attempts are currently being made to make sensible use of this inexpensive electricity and, for example, to manufacture chemical products of value. One possibility is the electrochemical conversion of water into hydrogen and oxygen.

Hydrogen production processes used up to now require a very high expenditure of energy. As a first approximation, the practical energy expenditure for generating hydrogen is proportional to the cell voltage of the hydrogen electrolysis. The cell efficiency of a water electrolyser is the ratio of theoretical to actual energy expenditure. The lower limit at which voltage electrolysis can take place is the so-called ΔG ⁰ voltage, which is around 1.23 V in the electrolytic splitting of liquid water. The thermo-neutral equilibrium voltage, i.e. the voltage at which the temperature remains constant during the electrolysis reaction, is around 1.48 V. This means that from a thermodynamic point of view it is possible to use part of the chemical energy of the product hydrogen thermally, namely from the ambient heat. For technical and, above all, economic reasons, low-temperature electrolysers, i.e. PEM and alkaline electrolysers, are not operated with cell voltages in the range from 1.23 V to 1.48 V, since this would only be possible with very low current densities. As a consequence, all energy in these technologies has so far been applied in electrical form.

In the high-temperature electrolysis (SOE), which takes place at operating temperatures between approx. 850 ° C and 1,000 ° C, water vapor is split electrochemically on ceramic electrodes made of nickel / zirconium oxide for the cathode and LaMnO ₃ for the anode. The SOE is seen by many as particularly promising. The reason for this is that the cell voltage is significantly lower than with the technologies of PEM electrolysis and alkaline electrolysis. This is due to the fact that with SOE the energy for evaporating water, for example, does not come from the electrical current of the electrolysis, but is applied thermally. Even if the SOE technology was not able to establish itself on a large scale, mainly because of the high material requirements, at temperatures of 850 ° C to 1,000 ° C, theoretically, in particular in connection with an exothermic, chemical synthesis reaction, in which the enthalpy of vaporization from the waste heat can be used, very high efficiencies of over 80% are possible.

So far, mainly alkaline and PEM electrolysers and, to a lesser extent, high-temperature electrolysers have been used to produce hydrogen. These currently achieve a stack efficiency of around 70% and an overall efficiency of around 60%. The thermo-neutral equilibrium voltage of hydrogen when liquid water is fed into the electrolysis is 1.48 V and represents the minimum achievable in practice. In the case of SOE, waste heat with at least one evaporation temperature of the water is dependent on a good efficiency greater than 80 % to achieve, which limits the area of application.

Consequently, it is technically necessary to propose an improved solution which avoids the disadvantages known from the prior art. In particular, the proposed solution should enable energy-efficient hydrogen production, which advantageously uses thermal energy input.

These objects on which the present invention is based are achieved by a method according to patent claim 1 and a device according to patent claim 10. Advantageous embodiments of the invention are the subject of the subclaims.

Description of the invention

The method according to the invention for producing hydrogen comprises a carbon dioxide electrolysis in which an intermediate which comprises formate and / or formic acid is produced and a subsequent thermal decomposition of this intermediate in which hydrogen is released. In particular, the waste heat from the carbon dioxide electrolysis is used for the thermal decomposition of the intermediate. This has the benefit of a particularly efficient generation of hydrogen. The intermediate formate is a particularly cheap chemical substance that can be efficiently generated using electricity from renewable energy sources and can be thermally decomposed at low cost with the release of hydrogen. Formates are the salts or esters of formic acid. Formates have the semi-structural formula (HCOO) _n M, where n corresponds to the valence of the metal ion. Alternatively, formic acid can also be produced as an intermediate instead of a formate, or a mixture of both. For example, formic acid and formates exist side by side in an aqueous solution in a dynamic equilibrium and, depending on the pH value, merge. In the context of the application, formate is always to be understood as formate and / or formic acid. It is particularly advantageous that low-temperature waste heat, for example from the electrolysis reaction, is sufficient for the thermal decomposition. In addition, the intermediate formate “green” can be produced: For the production of the formate, a carbon dioxide electrolysis is carried out, the main educt of which carbon dioxide is currently available in excess. In addition, this process offers the possibility of using or avoiding carbon dioxide.

In a further advantageous embodiment of the method, the endothermic decomposition reaction of the intermediate is used to cool the electrolyte in the electrolyzer. This has the advantage that it can be used to cool the electrolyser very efficiently, since the decomposition reaction is so strongly endothermic that the return of the cooled catholyte to the electrolyser is sufficient to cool it, provided that the electrolysis and decomposition take place at the same time or are operated at the same time .

In the process, an aqueous hydrogen carbonate and / or carbonate-containing electrolyte is expediently flowed into the electrolyzer on the cathode side as the catholyte.

The hydrogen carbonate is broken down in the electrolyser, for example, by protons that pass through a membrane from the anode side to the cathode side. The decomposition of the hydrogen carbonate takes place according to the following reaction: HCO ₃ ^- (aq) + H ⁺ (aq) → H ₂ O (1) + CO ₂

The resulting carbon dioxide (CO ₂ ) can be in gaseous or dissolved form and depends on the pressure and pH conditions in the electrolyzer. From this carbon dioxide, formate and / or formic acid is then produced at the cathode: 2 CO ₂ + H ₂ O (1) + 2 e ^- → HCOO ^- (aq) + HCO ₃ ^-

With the simultaneous anode reaction in the electrolyser, not only oxygen but also protons (H ⁺ ) are generated, which pass through the membrane and, as just described, decompose hydrogen carbonate into carbon dioxide in the cathode gap. The anode reaction looks like this: H ₂ O → 2 H ⁺ + ½ O ₂ + 2e-

Accordingly, the anolyte, that is to say the electrolyte which is flowed through the electrolysis cell on the anode side, can also consist of pure water (H ₂ O), for example. During operation of the electrolytic cell, oxygen is produced at the anode, which is led out of the electrolytic cell together with the anolyte and, in particular, fed to a phase separator in which the oxygen is separated off and removed from the system. The remaining anolyte is conveyed through the lines with a pump, for example, and fed back into the anode compartment of the electrolytic cell. In an exemplary embodiment, water that has been separated off from the catholyte can be fed to the anolyte's water supply line. The fresh water supply line into the anode compartment ensures, in particular, that the water consumed in the electrolysis reaction is compensated for, and the water removed from the catholyte can, for example, compensate for the transfer of water through the membrane in the electrolysis cell, which is caused by proton transport (H ⁺ ).

The intermediate formed at the cathode is discharged from the electrolysis cell as an aqueous solution and fed to the decomposition step in a decomposition reactor. In this, the formate and / or formic acid is decomposed to hydrogen carbonate and hydrogen. The hydrogen is discharged from the system and represents the product of value of the device and the method. The aqueous hydrogen carbonate solution that remains in addition to the hydrogen is returned to the electrolysis cell, in particular the cathode compartment of the electrolysis cell, with the catholyte pump through a catholyte line. The aqueous intermediate solution generated in the carbon dioxide electrolyser is continuously discharged from the cell. The formate and / or the formic acid contained in the solution is then catalytically decomposed in the decomposition reactor, a separate apparatus, with hydrogen (H ₂ ) also forming an aqueous hydrogen carbonate solution. This reaction proceeds according to the following reaction equation, where M stands for a monovalent cation, depending on which hydrogen carbonates are used: HCOOM (aq) + H ₂ O (1) → H ₂ (g) + MHCO ₃ (aq)

This decomposition can take place at low temperatures in the range from 50 ° C to 70 ° C and accordingly be provided by the waste heat of the electrolysis step.

In a particularly advantageous embodiment of the invention, in the process, carbon dioxide gas flows into the electrolyzer via a gas diffusion cathode, in particular carbon dioxide gas which is recovered from the formate-containing aqueous electrolyte via phase separation. This has the advantage of minimizing CO ₂ losses via the released hydrogen. It is generally difficult to convert all of the carbon dioxide released in the cathode compartment to formate and / or formic acid in a single electrolysis pass at the cathode. Therefore, carbon dioxide gets into the hydrogen and the catholyte would be depleted of hydrogen carbonate, so that not the intermediate but hydrogen would be formed at the cathode. In the method according to the invention, however, a carbon dioxide electrolyzer with a gas diffusion electrode is used as the cathode, so that the cathode space is divided into a cathode gas space and a cathode electrolyte space, which is designed in particular as a cathode gap. The gaseous educt carbon dioxide is fed to the cathode via the cathode gas space of the gas diffusion cathode and the electrolysis product is then predominantly present on the catholyte side. If there is still gaseous carbon dioxide in the aqueous formate stream leaving the electrolysis cell, it is separated from the catholyte stream before it reaches the decomposition reactor, in particular by means of a separation unit, for example a phase separator, and then back to the carbon dioxide feed via a return line with a fan the gas diffusion cathode supplied.

In order to achieve the most complete possible separation of the carbon dioxide by means of the separation unit, in particular a part of the separated carbon dioxide could be passed through the liquid phase. In this way a solution oversaturated with carbon dioxide would be avoided. The carbon dioxide circulated in this way can be transported with the aid of a fan. For example, carbon dioxide which flowed along the gas diffusion electrode could be fed into the separation unit. However, part of the carbon dioxide conveyed by the fan can also be diverted.

In the method according to the invention, in particular the intermediate produced is fed to the thermal decomposition in the form of an aqueous solution. The formate decomposition with hydrogen formation advantageously takes place in a temperature range from 50.degree. C. to 90.degree. The decomposition reactor can be heated in particular by means of the waste heat from the electrolysis cell. In the decomposition reactor there is, for example, a catalyst which contains at least one metal from the group consisting of ruthenium, rhodium, palladium or platinum. During the decomposition, in addition to hydrogen, an aqueous hydrogen carbonate is produced, which can be fed back into the electrolysis cell. The catalyst in the decomposition reactor can, for example, have a solid support material on which the active material is located, or it can have an organometallic compound that is dissolved in an organic phase.

Formate and hydrogen carbonate are in particular in the form of aqueous solutions, specifically particularly advantageously as aqueous solutions of an alkali metal. A particularly suitable example of an alkali metal is potassium. Carbonate can also be present in some cases in the aqueous hydrogen carbonate solution, the characteristics of the catholyte described here applying analogously.

A particular advantage lies in the combination of carbon dioxide electrolysis for the production of the intermediate with the concept of catalytic decomposition of the intermediate at low temperatures. In the production of hydrogen via the intermediate product formate and / or formic acid, part of the chemical energy of the hydrogen is applied with waste heat from the formate electrolysis, which leads to high energy efficiencies. The decomposition reaction of the formate is again an endothermic reaction. The resulting cooling causes at least a partial input of energy in the form of chemical energy into the product hydrogen.

In the process, the aqueous solution remaining after the hydrogen has been released is expediently returned to the electrolyzer as catholyte solution. In particular, the catholyte is fed into a catholyte circuit, which runs through the cathode compartment of the electrolyzer, is transported at the catholyte outlet as an intermediate flow to the decomposition reactor, where hydrogen is separated off by the thermal decomposition reaction and aqueous hydrogen carbonate remains as electrolyte, which is then returned to the catholyte inlet in particular via an electrolyte return line Electrolyser is transported. The cathode compartment of the electrolyzer is designed in particular as a catholyte gap.

Additional devices can be provided in the catholyte circuit in order to avoid carbon dioxide carryover. A further advantageous measure for the efficient management of a catholyte circuit is the positioning of a water separator in the catholyte circuit, in particular in the catholyte return line, by means of which from the remaining aqueous solution which the Leaves decomposition reactor, water is separated so that the catholyte concentration suitable for the electrolysis is restored before it flows back into the electrolyser. The water thus separated from the catholyte stream can advantageously be fed to the water stream which flows into the electrolysis cell on the anode side. An aqueous alkali salt solution is particularly preferably used as the catholyte, particularly preferably potassium carbonate or potassium hydrogen carbonate. Water is sufficient as an anolyte. One or more pumps can be provided for transporting the catholyte through the catholyte circulation lines.

As an alternative to the process in which the carbon dioxide electrolysis and thermal formate decomposition for hydrogen production take place simultaneously and also locally combined in one device, the intermediate can alternatively be used as a cost-effective hydrogen storage device, which would mean that electrolysis and decomposition are operated separately. The formate and / or the formic acid or the aqueous formate solution generated by the carbon dioxide electrolysis can be fed, for example, to a reservoir. Alternatively, it can also be transported to another location. It is true that the efficient cooling of the catholyte stream would then no longer be possible, but a temporal decoupling of thermal decomposition and electrolysis can be useful, since the aqueous formate solution represents a very inexpensive energy store. In addition, it is possible to produce a formate solution completely regeneratively. The thermal decomposition also only requires temperatures which can be easily generated by solar collectors, for example. This is an advantage over commercial hydrogen storage systems; if the storage option is only used to compensate for day-night fluctuations, at least the synergistic use of waste heat would be possible.

The device for hydrogen production which is particularly suitable for the method comprises a carbon dioxide electrolyser, a thermal decomposition reactor and an intermediate line which connects the catholyte outlet of the carbon dioxide electrolyser to the decomposition reactor. Catholyte inlet and catholyte outlet are understood to mean the electrolyte inlet and outlet of the cathode compartment of the electrolyzer, with the inlet and outlet describing the direction of flow of the electrolyte. The carbon dioxide electrolyzer used can have a design of known carbon dioxide electrolyzers which are based on aqueous electrolytes containing a dissolved conductive salt. In particular, the carbon dioxide electrolyser has a membrane, for example an ion-selective or porous membrane, by means of which the anode compartment and cathode compartment are separated from one another. The membrane prevents the different gases that are produced at the anode and the cathode from being mixed together. The cathode space is designed in particular as a cathode gap through which the liquid catholyte is transported. The dissolved conductive salt is, for example, an alkali salt, in particular a potassium salt. The required charge transport through the electrolyte liquid is ensured by potassium ions. In particular, in practice a number of electrolysis cells are coupled to form a stack of a number of electrolysis cells.

In an advantageous embodiment of the invention of the device, the carbon dioxide electrolyser and the decomposition reactor are arranged so that the waste heat generated during the carbon dioxide electrolysis in the electrolyser can be fed to the thermal decomposition of the intermediate in the decomposition reactor. This can be ensured in particular through a structural proximity, alternatively through a heat conduction system.

The device expediently has a catholyte return line which connects the decomposition reactor to the catholyte inlet of the carbon dioxide electrolyzer, the carbon dioxide electrolyzer and decomposition reactor being arranged so that the endothermic decomposition reaction of the intermediate in the decomposition reactor can be used to cool the catholyte in the catholyte return line. It can be sufficient for the catholyte or the aqueous hydrogen carbonate remaining after the decomposition reaction of the intermediate to leave the decomposition reactor cooled by the endothermic decomposition reaction and to be fed back into the electrolysis cell in a correspondingly cooled manner. To avoid unnecessary heating, the catholyte return line can be designed to be thermally insulated.

In particular, the catholyte return line also has a water separation, by means of which the catholyte concentration can be regulated upstream of the catholyte inlet. Separated water can, for example, be fed to the anolyte stream, which in particular can only consist of water without addition of conductive salt. In a further advantageous embodiment of the device, the cathode of the carbon dioxide electrolyzer is designed as a gas diffusion cathode and a phase separator is provided in the intermediate line, the gas outlet of which is in turn connected to the carbon dioxide feed of the gas diffusion cathode. This has the particular advantage of minimizing CO2 losses via the released hydrogen. A carbon dioxide carryover is reduced by the fact that the carbon dioxide necessary for the formation of the intermediate is in gaseous form via the Gas diffusion cathode is connected. Before unreacted carbon dioxide reaches the decomposition reactor via the intermediate line, the gaseous carbon dioxide is removed from the intermediate flow by means of a separation unit, in particular a phase separator, and is again conveyed back into the electrolysis cell via a fan. In order to achieve the most complete possible separation of the carbon dioxide from the intermediate line, part of the separated carbon dioxide can be passed through the liquid phase. In this way, a solution oversaturated with carbon dioxide can be avoided. The carbon dioxide circulated in this way can be conveyed with the help of a fan, for example. One possible embodiment is to lead carbon dioxide which has flowed along the gas diffusion cathode back into the separation unit for this purpose. However, part of the carbon dioxide conveyed by the fan can also be diverted.

In a further advantageous embodiment of the device, it has a throttle valve and at least one pump for generating an overpressure in the cathode compartment of the electrolyzer, which cathode compartment is designed in particular as a cathode gap. This embodiment has the advantage, for example, that gas bubbles in the cathode space are largely avoided. This variant manages, for example, without a gas diffusion electrode and physically dissolved carbon dioxide is converted to formate and / or formic acid at the cathode. In order to minimize the carryover of carbon dioxide into the hydrogen produced in this embodiment, the liquid catholyte is circulated by means of a pump in the catholyte circuit and, in combination with the catholyte gap, a standardization of dissolved carbon dioxide across the cell surface is ensured.

For the electrolysis operation, the pump and the built-in throttle valve generate a certain overpressure in the electrolyser. This has the effect that hardly any gas bubbles arise within the cathode gap or that these are kept very small. With a correspondingly high volume flow of the catholyte in the circulation of the catholyte, gas bubbles in the cathode space can be completely avoided. Before the flow into the decomposition reactor and correspondingly before the release of the hydrogen, this overpressure is lowered again via a throttle valve, and the catholyte flow is relaxed. For the electrolysis, the pressure is built up again by means of the pump.

A combination of the exemplary embodiments presented can also be useful, in which a phase separator, which circulates the carbon dioxide, is additionally provided between the throttle valve and the decomposition reactor. Here the carbon dioxide losses are minimized even more and at the same time the amount of physically dissolved carbon dioxide that gets into the decomposition reactor is reduced.

• 1 zeigt eine erfindungsgemäße Anordnung der Vorrichtung für die elektrolytische Produktion von Wasserstoff über das Intermediat Formiat,
• 2 zeigt eine beispielhafte Ausgestaltung der Vorrichtung für die elektrolytische Produktion von Wasserstoff über das Intermediat Formiat mit erhöhter Wasserstoffreinheit,
• 3 zeigt eine weitere vorteilhafte Ausführungsform der erfindungsgemäßen Vorrichtung für die elektrolytische Produktion von Wasserstoff über das Intermediat Formiat mit Überdruck.

Examples and embodiments of the present invention will be further exemplified with reference to FIG 1 to 3 described in the attached drawing:

• 1 shows an arrangement according to the invention of the device for the electrolytic production of hydrogen via the intermediate formate,
• 2 shows an exemplary embodiment of the device for the electrolytic production of hydrogen via the intermediate formate with increased hydrogen purity,
• 3 shows a further advantageous embodiment of the device according to the invention for the electrolytic production of hydrogen via the intermediate formate with excess pressure.

Shown in each case are schematic representations of the devices, in each of which an electrolyser 3 for the conversion of carbon dioxide to an intermediate, which has formate and / or formic acid, with a decomposition reactor 12 for the intermediate is combined, from which the hydrogen H ₂ generated by thermal decomposition of the intermediate can then be removed 13 . The electrolytic cell can have different embodiments, as are known from the prior art for electrolytic cells. The design of the electrolytic cell is particularly advantageous for the method according to the invention 3 with anode compartment 5 and cathode compartment 4th , which is particularly advantageously designed as a cathode gap. A membrane 8th ensures in particular that the gases produced at the two electrodes do not mix during operation. In the anolyte circle 20th , 21st , 25th In particular, water (H2O) is used as an anolyte. This can be done via a fresh water inlet 29 are constantly replenished in the electrolyser. The anolyte supply line 20th can also use water from the water separation device 16 be fed, which excess water from the catholyte circuit or the catholyte return line 15th removes. At the outlet of the anode compartment 5 In addition to water, there are also formed oxygen gas O2 in the anolyte. This can be done via a phase separator 22nd and its gas outlet 23 removed from the system. The remaining water is via the anolyte return line 25th and a pump provided therein 24 back in the electrolyser 3 promoted. On the cathode side there is also a cycle 11 , 15th , 10 provided, by means of which the aqueous catholyte solution, in particular an aqueous conductive salt solution, based on an alkali salt, in particular a potassium salt, expediently based on a hydrogen carbonate or carbonate. An integrated decomposition reactor 12 removes the hydrogen generated by the thermal decomposition of formate from the electrolyte solution and routes it through the gas outlet 13 out of the system. The remaining aqueous electrolyte solution can be fed via the electrolyte or catholyte return line 15th and a pump provided therein 14th back into the cathode compartment 4th be promoted. A water separation can also be installed in the catholyte return line 16 be provided. This compensates for the passage of water through the membrane. The cathode can be designed as a gas diffusion cathode. In this case, a carbon dioxide feed, that is to say a gas inlet into the gas space of the cathode, is provided; the carbon dioxide is, for example, recirculated by means of a phase separation 32 is taken from the electrolyte flow emerging from the electrolytic cell and via a fan 33 back to the gas diffusion cathode 2 is promoted. The catholyte outlet opens into the intermediate line 11 , by means of which that in the electrolytic cell 3 generated intermediate, the formate and / or the formic acid, to the thermal decomposition reactor 12 is transported. Pumps can be used to achieve the overpressure release 14th , 31 be provided and a throttle valve 36 before joining the intermediary management 11 into the decomposition reactor 12 .

The membrane provided in the electrolyzer 8th is in particular a proton-permeable membrane, because this is necessary for the desired cathode reaction. The anode is advantageously located directly on this membrane, as is known, for example, in PEM electrolysers for the production of hydrogen. However, the cathode is not attached to the membrane in order to avoid the formation of hydrogen from protons that have passed over. The cathode is spatially separated from the membrane in particular by a gap through which the catholyte is passed. The cathode is expediently made of a catalytically active material which favors the formation of formate and / or formic acid. Examples such as lead can be used for this. In this case, the intermediate is formed from carbon dioxide, which is dissolved in the catholyte. The dissolved carbon dioxide is formed on site through a neutralization of hydrogen carbonate by overflowing protons. Alternatively, the carbon dioxide can be supplied to the system in gaseous form if it is operated with a gas diffusion cathode.

List of reference symbols

1

Arrangement for the production of hydrogen

2

Gas diffusion electrode (GDE)

3

Electrolysis cell for the production of formate and / or formic acid

4th

Cathode gap

5

Anode compartment

6th

Proton-permeable membrane with anode (1/2 MEA)

7th

Connection for cathode

8th

Connection for anode

10

Feed for catholyte (aqueous hydrogen carbonate solution)

11

Forwarding for aqueous formate solution

12

Decomposition reactor for the release of hydrogen

13

Derivation for generated hydrogen

14th

Catholyte pump

15th

Return catholyte (aqueous hydrogen carbonate solution)

16

Water separation (e.g. reverse osmosis or thermal)

20th

Anolyte feed (e.g. water)

21st

Forwarding anolyte (e.g. water) and oxygen

22nd

Phase separator for the separation of oxygen from the anolyte

23

Discharge for generated oxygen

24

Anolyte circulation pump

25th

return

29

Adding water (feed)

31

Pump for recirculation of dissolved CO2

32

Phase separator for recovery of CO2

33

Fan for recirculation of CO2

36

Throttle / relaxation device

Claims (15)

A method for producing hydrogen comprising a carbon dioxide electrolysis (3) in which an intermediate comprising formate and / or formic acid is produced and a subsequent thermal decomposition (12) of the intermediate in which hydrogen (H ₂ ) is released. Procedure according to Claim 1 , in which the waste heat (37) from the carbon dioxide electrolysis (3) is used for the thermal decomposition (12) of the intermediate. Method according to one of the preceding claims, in which the endothermic decomposition reaction (12) of the intermediate is used to cool (38) the electrolyte in the electrolyzer (3). Method according to one of the preceding claims, in which an aqueous electrolyte containing hydrogen carbonate and / or carbonate is flowed into an electrolyzer (3) on the cathode side (7). Process according to one of the preceding claims, in which the intermediate produced is fed to the thermal decomposition in the form of an aqueous solution. Method according to one of the preceding claims, in which carbon dioxide gas (CO2) flows into an electrolyzer (3) via a gas diffusion cathode (2), in particular carbon dioxide gas (CO2), which is extracted from the formate-containing aqueous electrolyte via a phase separation (32) is recovered. Method according to one of the preceding claims, in which the aqueous solution (15) remaining after the hydrogen release is returned to the electrolyser (3) as catholyte solution. Procedure according to Claim 7 , in which the suitable catholyte concentration is restored by means of a water separation (16) from the remaining aqueous solution (15) before it flows back into the electrolyzer (2). Procedure according to Claim 1 , in which the intermediate produced is fed to a storage facility before it is fed, if necessary, to the thermal decomposition (12) in which hydrogen (H2) is released. Device for hydrogen production comprising a carbon dioxide electrolyser (3), a thermal decomposition reactor (12) and an intermediate line (11) which connects the catholyte outlet of the carbon dioxide electrolyser (3) with the decomposition reactor (12). Device according to Claim 10 , in which the carbon dioxide electrolyser (3) and the decomposition reactor (12) are arranged so that the waste heat (37) of the (3) generated during the carbon dioxide electrolysis in the electrolyser (3) can be used for the thermal decomposition of the intermediate in the decomposition reactor (12). Device according to Claim 10 or 11 , with a catholyte return line (15) which connects the decomposition reactor (12) with the catholyte inlet of the carbon dioxide electrolyzer (3), the carbon dioxide electrolyzer (3) and decomposition reactor (12) being arranged so that the endothermic decomposition reaction of the intermediate in the decomposition reactor (12) is used Cooling (38) of the catholyte in the catholyte return line (15) can be used. Device according to one of the Claims 10 to 12 , with a water separation (16) in the catholyte return line (15) to regulate the catholyte concentration upstream of the catholyte inlet. Device according to one of the Claims 10 to 13 , with a gas diffusion cathode (2) and a phase separator (32) in the intermediate line (11), the gas outlet of which is connected to the carbon dioxide feed of the gas diffusion cathode (2). Device according to one of the Claims 10 to 13 , with a throttle valve (36) and at least one pump (14, 31) for generating an overpressure in the cathode space (4) of the electrolyzer (3), which is designed in particular as a cathode gap (4).

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