DE102022004678A1 - Process for the electrolysis of carbon dioxide with prereduction of a silver oxide-containing gas diffusion electrode - Google Patents

Abstract

The use of a pre-reduced gas diffusion electrode containing silver for the electrochemical reduction of carbon dioxide leads to a more selective formation of carbon monoxide with improved current efficiency if the pre-reduced gas diffusion electrode is obtained by at least the following process steps:Introducing a silver oxide-containing gas diffusion electrode into a reduction chamber of an electrochemical device,Providing a reduction environment in which at least one composition is introduced into the reduction chamber for the introduced gas diffusion electrode, guided to the gas diffusion electrode and contacted therewith, whereby the gas diffusion electrode is at least partially enveloped by this composition, with the proviso that this introduced composition contains, based on its total weight, an amount of 0 mg/kg to 1000 mg/kg of at least one compound selected from CO2, O2 or a mixture thereof;Reducing silver oxide of the introduced gas diffusion electrode to silver while maintaining the reduction environment and obtaining a pre-reduced gas diffusion electrode.

Description

The invention relates to a method for the electrochemical reduction of carbon dioxide at a pre-reduced gas diffusion electrode, as well as the use of a correspondingly pre-reduced gas diffusion electrode for the electrochemical reduction of carbon dioxide.

Silver-based gas diffusion electrodes (GDE) can be used for the electrochemical reduction of carbon dioxide to carbon monoxide. Under standard conditions, the cell potential is 1.33 V. The cathode reaction in alkaline medium compared to the standard hydrogen electrode (SHE) is: CO ₂ + H ₂ O + 2e ^- → CO + 2 OH ^- E vs. SHE (pH 7) = -0.52 V

The competitive reaction is: 2 H ₂ O + 2e ^- → 2 OH ^- + H ₂ E vs. SHE (pH 7) = -0.42 V

On the anode side, a common reaction is the formation of oxygen: 2 OH ^- → ½ O ₂ + H ₂ O + 2e ^-

However, other reactions are also possible on the anode side and are described in the literature, e.g. the production of chlorine: 2Cl ^- → ½Cl ₂ + H ₂ O + 2e ^-

If the anode and cathode reactions are described together, the example reaction equations are as follows: CO ₂ → CO + ½ O ₂ E vs. SHE (pH 7) = 0.82 V CO ₂ → CO + ½ Cl ₂

The cell voltage of an electrochemical process is composed of various factors: $$E=E^{\text{tn}}+{\eta }^{\text{act}}+{\eta }^{\text{ohm}}+{\eta }^{\text{mt}}$$

Etn

thermoneutrale Spannung (Spannung bei Standardbedingungen)

ηakt

kinetische Aktivierung

ηohm

Ohm'sche Verluste

ηmt

limitierender Massentransport

This includes

Etn

thermoneutral voltage (voltage at standard conditions)

ηact

kinetic activation

ηohm

Ohmic losses

ηmt

limiting mass transport

In research into carbon dioxide electrolysis, the majority of studies were carried out in H-cell electrolyzers. In this type of cell, the electrodes are immersed in the electrolyte while the carbon dioxide required for the reaction is dissolved in the electrolyte. Due to the limited solubility of the carbon dioxide, the conversion at the electrode is limited by mass transport and takes place at low current densities of around 60 mA/cm ² .

Gas diffusion electrodes, on the other hand, have become more and more of a focus of development in recent years and can work with gaseous carbon dioxide. Current densities of around 300 mA/cm ² According to current theory, the reaction takes place at the three-phase boundary, where carbon dioxide as reactant gas, liquid electrolyte and catalyst come into contact. The carbon dioxide diffuses from the gas space through the porous structure of the gas diffusion electrode.

Suitable gas diffusion electrodes are those based on silver and/or silver oxide, preferably silver particles, as electrocatalyst and with a powdered fluorine-containing polymer, in particular PTFE powder, as non-conductive binder compacted on a metallic or non-metallic, conductive or non-conductive carrier, as they are manufactured by the company Covestro Deutschland AG, for example in WO 2021/069470 A1 or WO 2020/239716 A1 or EP 1 728 896 A2 were described.

Increased current yields for the reduction of CO ₂ to carbon monoxide are particularly desirable for efficient application of CO ₂ electrolysis on an industrial scale. The aim of the developments is also to improve the selectivity of the cathodic reduction of CO ₂ to CO and in particular to minimize the formation of hydrogen at the gas diffusion electrode.

• Einbringen der Gasdiffusionselektrode in einen Reduktionsraum einer elektrochemischen Vorrichtung, bevorzugt in den Kathodenraum der Elektrolysezelle ;
• Bereitstellung einer Reduktionsumgebung, in dem zur eingebrachten Gasdiffusionselektrode mindestens eine Zusammensetzung in den Reduktionsraum eingebracht, zur Gasdiffusionselektrode geführt und damit kontaktiert wird, wodurch die Gasdiffusionselektrode von dieser Zusammensetzung zumindest teilweise eingehüllt wird, unter der Maßgabe, dass diese eingebrachte Zusammensetzung bezogen auf ihr Gesamtgewicht eine Menge von 0 mg/kg bis 1000 mg/kg mindestens einer Verbindung, ausgewählt aus CO₂, O₂ oder deren Gemisch enthält;
• Reduktion von Silberoxid der eingebrachten Gasdiffusionselektrode zu Silber unter Aufrechterhaltung der Reduktionsumgebung und unter Erhalt einer vorreduzierten Gasdiffusionselektrode;
• Anwendung besagter vorreduzierter Gasdiffusionselektrode als Silber enthaltende Gasdiffusionselektrode als Kathode in besagter Elektrolysezelle zur elektrochemischen Reduktion von Kohlendioxid durch Anlegen eines Elektrolysestromes, wobei dem Gasraum ein Kohlendioxid-haltiges Gas in Form eines Gasstroms zugeführt wird und die Zusammensetzung des zugeführten Gases bezogen auf dessen Volumen mehr als 80 Vol.-% CO₂ enthält.

It has now been found that the current efficiency and the selectivity in the CO ₂ reduction at a gas diffusion electrode can be significantly improved if the metal oxide-containing gas diffusion electrode is pre-reduced in an upstream step to reduce the CO ₂ to be reduced before the electrolysis voltage is applied, wherein the gas diffusion electrode is in contact with an inert gas atmosphere during the pre-reduction. The invention therefore relates to a method for the electrochemical reduction of carbon dioxide in at least one electrolysis cell with at least one anode chamber containing at least one anode in contact with anolyte and with at least one cathode chamber containing at least one silver-containing gas diffusion electrode as a cathode, wherein said gas diffusion electrode divides the cathode chamber into a gas chamber and a catholyte chamber and the gas chamber has at least one inlet for a gas stream and at least one outlet for a product gas stream, and wherein the method is carried out by carrying out at least the following steps in the following order of priority

• introducing the gas diffusion electrode into a reduction chamber of an electrochemical device, preferably into the cathode chamber of the electrolysis cell;
• Providing a reduction environment in which at least one composition is introduced into the reduction space for the introduced gas diffusion electrode, guided to the gas diffusion electrode and contacted therewith, whereby the gas diffusion electrode is at least partially enveloped by this composition, with the proviso that this introduced composition contains, based on its total weight, an amount of 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof;
• Reduction of silver oxide of the introduced gas diffusion electrode to silver while maintaining the reduction environment and obtaining a pre-reduced gas diffusion electrode;
• Application of said pre-reduced gas diffusion electrode as a silver-containing gas diffusion electrode as a cathode in said electrolysis cell for the electrochemical reduction of carbon dioxide by applying an electrolysis current, wherein a carbon dioxide-containing gas is supplied to the gas space in the form of a gas stream and the composition of the supplied gas contains more than 80 vol.% CO ₂ based on its volume.

The implementation of a pre-reduction of silver oxide-containing gas diffusion electrodes has so far been described in advance of a chlor-alkali electrolysis in the publication EP 2824218 A1 The implementation of a pre-reduction as well as the special execution conditions of a pre-reduction described below in connection with the application of gas diffusion electrodes in a process for electrochemical CO ₂ reduction are not yet known.

Z

Elektrolysezelle

1

Kathodenhalbschale

2

Anodenhalbschale

3

Separator (Diaphragma, Ionenaustauschermembran)

4

Gasraum (Kathode)

5

erste Gaszuleitung für Kohlendioxid (Kathodenraum)

6

erste Gasableitung für gasförmige Reaktionsprodukte (Kathodenraum)

7

zweite Gasableitung für Anodenreaktionsprodukt

8

Anolytzulauf

9

Anolytablauf

10

Anode

11

Kathode (Silber enthaltende Gasdiffusionselektrode)

12

Katholytraum (Elektrolytraum)

12a

Katholytspalt (Elektrolytraum)

13

Katholytzulauf

14

Katholytablauf

15

Anodenraum

15a

Anolyt

16

Kathodenraum

17

Katholyt

31

Stromleitung Kathode

32

Stromleitung Anode

34

Verteilerkanal

35

elektrisch leitende Auflagestruktur

36

Elastische elektrisch leitende Verbindung zwischen Kathode 11 und Auflagestruktur 35

55

Kohlendioxidgasquelle

90

elektrisch leitende Verbindung Anode mit Anodenhalbschale

91

elektrisch leitende Verbindung von GDE 11, elastischer Struktur 36, elektrisch leitender Auflagestruktur 35 mit der Kathodenhalbschale 1

100

Elektrolyseur mit Elektrolysezellen Z (n), n = Anzahl der Elektrolysezellen Z

According to the invention, the electrochemical reduction of carbon dioxide takes place in an electrolysis cell. An example of such an electrolysis cell is shown in 1 shown. According to 1 the reference symbols mentioned therein have the following meaning:

Z

Electrolysis cell

1

Cathode half shell

2

Anode half shell

3

Separator (diaphragm, ion exchange membrane)

4

Gas space (cathode)

5

first gas supply line for carbon dioxide (cathode chamber)

6

first gas discharge for gaseous reaction products (cathode compartment)

7

second gas discharge for anode reaction product

8th

Anolyte feed

9

Anolyte drain

10

anode

11

Cathode (silver-containing gas diffusion electrode)

12

Catholyte dream (electrolyte dream)

12a

Catholyte gap (electrolyte space)

13

Catholyte inlet

14

Catholyte drain

15

Anode compartment

15a

Anolyte

16

Cathode compartment

17

Catholyte

31

Power line cathode

32

Power line anode

34

Distribution channel

35

electrically conductive support structure

36

Elastic electrically conductive connection between cathode 11 and support structure 35

55

Carbon dioxide gas source

90

electrically conductive connection anode with anode half-shell

91

electrically conductive connection of GDE 11, elastic structure 36, electrically conductive support structure 35 with the cathode half-shell 1

100

Electrolyzer with electrolysis cells Z (n), n = number of electrolysis cells Z

The cathode half-shell of the electrolysis cell according to 1 has an electrolyte inlet 13 and an electrolyte outlet 14 as well as a gas inlet 5 and a gas outlet 6. The electrolyte inlet 13 to the cathode half-shell 1 is from above and the supplied catholyte 17 flows from top to bottom along the gas diffusion electrode (GDE) 11. The catholyte 17 flows downwards in the gap 12 between the ion exchange membrane 3 and the GDE 11. In order to avoid excessive electrolyte flows through the gap 12 or 12a, a flow brake 24 is mounted in the gap 12. The flow brake 24 is made of a porous fabric made of PTFE as in the WO2003/042430A2 described. The gap is supplied with catholyte 17 by feeding it through a distribution channel 34. The contact between the GDE 11 and the power supply line 31 in the cathode chamber is made via an elastically mounted electrically conductive structure 35. This elastic structure 35 rests on a rigid nickel-metal structure in the form of an expanded metal, on which (in 1 not shown). The expanded metal is electrically connected to the cathode half-shell 1 by a rigid connection 91. The GDE 11 is mounted on the elastic structure 35 and is electrically contacted from the side of the gas chamber 4. The cathode half-shell 1 is either in the bipolar connection of several electrolysis cells Z1, in electrical contact with an anode half-shell 2 of another adjacent electrolysis cell Z2 or with a power supply plate. The outermost anode half-shell 2 is correspondingly electrically connected to the power discharge plate. The gas supply 5 for the introduction of carbon dioxide to the gas chamber 4 takes place in the lower part of the gas chamber 4, the discharge line 6 for gaseous reaction products from the cathode chamber 16 is attached to the top of the gas chamber 4. In order to prevent gas with the electrolyte from leaving the cathode chamber 16 via the catholyte outlet 14, the outlet 14 is expediently arranged in one variant so that it flows into an external collecting pipe dip for catholyte. This dip is basically known (see e.g. DE102005027735A1 ). The drain lines 14 and the feed lines 13 for the catholyte are brought together in an electrolyzer E with a plurality (n) of electrolysis cells Z1, Z2, ...Z(n) via external connecting pipes in which a flow connection exists. The gas outlet lines 6 and gas feed lines 5 are also brought together via external connecting pipes. The gas outlet lines 7 for the anode gas are collected, which can also be connected via a collecting pipe (not in 1 drawn) and fed to a second gas separation unit or the anode gas is discharged together with the anolyte from the anode chamber 15 via common lines. The gap 12 or 12a between the GDE 11 and the separator (ion exchange membrane 3) has a gap width of at least 0.1 mm and is at least 30 cm high and 10 cm wide. Catholyte 17 can be discharged from the cathode half-shell 1 via the cathode outlet 14.

In general, anolyte is contained in the anode compartment of an electrolysis cell used in the process according to the invention for CC ₂ reduction. In a preferred embodiment, the anolyte is an aqueous solution of an electrolyte, it being particularly preferred if the anolyte for the CO ₂ reduction is an aqueous solution containing alkali hydrogen carbonate, more preferably potassium hydrogen carbonate, cesium hydrogen carbonate or sodium hydrogen carbonate, very particularly preferably potassium hydrogen carbonate.

For example, a commercially available dimensionally stable anode equipped with a platinum coating from Umicore, Platinode®, for oxygen development, or an iridium dioxide anode can be used as an anode in the electrolysis cell.

The anode chamber and cathode chamber are preferably separated from one another by a common separating device, the separator. In a preferred embodiment of the invention, the separator is an ion exchange membrane or a diaphragm; the separator is particularly preferably an ion exchange membrane.

Membranes that are particularly suitable as ion exchange membranes are those that are designed as cation exchange membranes and can transport cations from the anode compartment to the cathode compartment. These are generally known from the state of the art. Alternatively, anion exchange membranes that transport anions from the cathode compartment to the anode compartment can also be used. Cation exchange membranes are preferably used. In conventional ion exchange membranes, the ion transport is also associated with water transport, which depends on the selected concentrations of the anolyte and catholyte, temperature and operating conditions.

In particular, all known diaphragms that separate the anode chamber from the cathode chamber in a gas-tight manner are suitable as diaphragms. The diaphragm should in particular have a gas-tightness (bubble point) of more than 10 mbar, preferably more than 300 mbar, particularly preferably more than 1000 mbar. In particular, the diaphragm should also be inert to the electrolyte and the reaction gases and resistant to the operating temperatures. Diaphragms for electrolysis are basically known from the state of the art.

Catholyte is contained in the cathode compartment of the electrolysis cell. In a preferred embodiment, the catholyte is an aqueous solution of an electrolyte, and it has proven particularly preferred for CO ₂ reduction if the catholyte is an aqueous solution of alkali hydrogen carbonate, more preferably potassium hydrogen carbonate, cesium hydrogen carbonate or sodium hydrogen carbonate, very particularly preferably potassium hydrogen carbonate.

In the process according to the invention, said pre-reduced gas diffusion electrode is used as a silver-containing gas diffusion electrode as a cathode in said electrolysis cell for the electrochemical reduction of carbon dioxide by applying an electrolysis current, wherein a carbon dioxide-containing gas is supplied to the gas space in the form of a gas stream and the composition of the supplied gas contains more than 80 vol.% CO ₂ , preferably more than 95 vol.% CO ₂ , based on its volume.

The gas diffusion electrode in question divides the cathode space into a gas space and a catholyte space. The catholyte space is preferably designed as a gap (also called cathode gap) between the separator and the gas diffusion electrode, whereby this gap is arranged to pass the catholyte according to the principle of a falling liquid film.

In another preferred embodiment of the new electrolysis cell, a means for flow braking of the catholyte flow, hereinafter referred to as flow brake, is provided in the gap between the membrane and the GDE. This can be used to control the residence time of the catholyte in the gap in front of the cathode. The flow brake is particularly preferably designed as an electrically non-conductive, inert textile fabric.

The flow brake can in particular consist of a porous textile fabric, particularly preferably a woven, knitted or warp-knitted fabric, which is arranged in the gap. As an alternative, mechanical fittings in the gap are also conceivable, which enable the electrolyte to be guided horizontally or at a slight angle to the horizontal, resulting in a meandering flow of the electrolyte.

Further embodiments of a cathode operating according to the principle of the falling liquid film are described in the publication WO 2020/057998 A1 which are expressly and fully incorporated herein by reference.

In a first step of the method according to the invention, a gas diffusion electrode is provided which contains silver oxide. The gas diffusion electrode preferably contains silver-I-oxide, silver-II-oxide or mixtures thereof as silver oxide. The gas diffusion electrode provided particularly preferably contains silver-I-oxide, silver-II-oxide or mixtures thereof in combination with metallic silver.

In a further embodiment, the gas diffusion electrode provided contains at least 50% by weight; preferably at least 75% by weight; particularly preferably at least 80% by weight of silver-I-oxide, in each case based on the total weight of the gas diffusion electrode, before the reduction of the silver oxide.

Particularly preferred are gas diffusion electrodes which contain 70 to 95 wt.% silver-I-oxide, 0 to 15 wt.% metallic silver and 3 to 15 wt.% fluorine-containing polymer (preferably polytetrafluoroethylene (PTFE)).

A preferred embodiment of the method is characterized in that the gas diffusion electrode containing silver oxide comprises at least one compacted mixture applied to a metallic or non-metallic, conductive or non-conductive carrier, containing silver oxide particles in a mixture with particles of at least one powdered, fluorine-containing polymer, in particular polytetrafluoroethylene (PTFE) powder, as a binder.

The porosity of the compacted mixture, calculated from the material densities of the raw materials used, is more than 10% but less than 80% in a preferred embodiment of the gas diffusion electrode.

• (a) Herstellen einer Pulvermischung, welche wenigstens einen Katalysator und ein Bindemittel enthält (b) Aufbringen der Pulvermischung auf ein, bevorzugt elektrisch leitendes, Trägerelement (c) Verpressen der Pulvermischung mit dem elektrisch leitenden Trägerelement.

A gas diffusion electrode which can preferably be used in the process according to the invention is particularly preferably produced by a process which comprises the following steps:

• (a) Producing a powder mixture which contains at least one catalyst and one binder (b) Applying the powder mixture to a, preferably electrically conductive, carrier element (c) Pressing the powder mixture with the electrically conductive carrier element.

The powder mixture, which contains the catalyst and the binder as well as other components if necessary, is applied directly to the carrier element and then pressed onto the carrier element. This produces the compacted mixture mentioned.

The powder mixture contains at least one catalyst and at least one binder. The catalyst used is silver, a silver compound or a mixture thereof, with at least silver oxide being present as the silver compound. The silver oxide is preferably silver-I-oxide, silver-II-oxide or mixtures thereof.

The binder is preferably a fluorinated polymer, particularly preferably polytetrafluoroethylene (PTFE). Powder mixtures containing 70 to 95% by weight of silver-I-oxide, 0 to 15% by weight of silver metal powder and 3 to 15% by weight of PTFE are particularly preferred. A mixture such as that made from DE 101 30 441 A The catalyst, e.g. silver, is precipitated on a PTFE substrate.

The powder mixture can contain additional components, e.g. fillers containing nickel metal, Raney nickel, Raney silver powder or mixtures thereof. The powder mixture containing at least one catalyst and at least one binder forms an electrochemically active layer of the gas diffusion electrode in the form of a compacted mixture after being applied to the carrier element and pressed onto the carrier element.

The powder mixture is produced by mixing the powders of the catalyst and the binder and, if necessary, other components. Mixing is preferably carried out in a mixing device which has rapidly rotating mixing elements, such as impact knives. To mix the components of the powder mixture, the mixing elements preferably rotate at a speed of 10 to 30 m/s or at a speed of 4000 to 8000 rpm. If the catalyst, e.g. silver-I-oxide, is mixed with PTFE as a binder in such a mixing device, the PTFE is stretched to form a thread-like structure and in this way acts as a binder for the catalyst. After mixing, the powder mixture is preferably sieved. Sieving is preferably carried out using a sieving device which is equipped with nets or the like, the mesh size of which is 0.1 to 1.5 mm, particularly preferably 0.2 to 1.2 mm.

The powder mixture is applied to the electrically conductive carrier element in particular by scattering. The powder mixture can be scattered onto the carrier element using a sieve, for example.

The powder mixture can be pressed onto the carrier element in particular by means of rollers. Preferably, a pair of rollers is used. However, a roller can also be used on a substantially flat base, with either the roller or the base being moved. Furthermore, the pressing can be carried out using a press stamp. The forces during pressing are preferably 0.01 to 7 kN/cm, more preferably 0.1 to 1 kN/cm.

• 3,5 kg einer Pulvermischung, bestehend aus 5 Gew.% PTFE-Pulver, 88 Gew.% Silber-I-Oxid und 7 Gew.% Silberpulver (z.B. Typ 331 der Fa. Ferro), wurden in einem Mischer der Fa. Eirich, Typ R02, ausgerüstet mit einem Sternwirbler als Mischelement, bei einer Drehzahl von 6000 U/min so gemischt, dass die Temperatur der Pulvermischung 55 °C nicht überstieg. Insgesamt wurde das Mischen dreimal bei einer Mischzeit von 50 Sekunden und dreimal bei einer Mischzeit von 60 Sekunden durchgeführt. Nach dem Mischen wurde die Pulvermischung mit einem Sieb mit einer Maschenweite von 1,0 mm gesiebt. Die gesiebte Pulvermischung wurde anschließend auf ein elektrisch leitfähiges Trägerelement aufgebracht. Das Trägerelement war ein Drahtnetz aus Silber mit einer Drahtdicke von 0,14 mm und einer Maschenweite von 0,5 mm. Das Aufbringen erfolgte mit Hilfe einer 2 mm dicken Schablone, wobei das Pulver mit einem Sieb mit einer Maschenweite von 1,0 mm aufgebracht wurde. Überschüssiges Pulver, das über die Dicke der Schablone hinausragte, wurde mittels eines Abstreifers entfernt. Nach Entfernung der Schablone wird der Träger mit der aufgebrachten Pulvermischung mittels einer Walzenpresse mit einer Presskraft von 0,45 kN/cm verpresst. Der Walzenpresse wurde die Gasdiffusionselektrode entnommen. Die resultierende Gasdiffusionselektrode hatte eine Porosität von etwa 50 %.

The gas diffusion electrode is specifically manufactured as follows:

• 3.5 kg of a powder mixture consisting of 5 wt.% PTFE powder, 88 wt.% silver-I-oxide and 7 wt.% silver powder (e.g. type 331 from Ferro) were mixed in an Eirich mixer, type R02, equipped with a star vortex as a mixing element, at a speed of 6000 rpm so that the temperature of the powder mixture did not exceed 55 °C. In total, mixing was carried out three times with a mixing time of 50 seconds and three times with a mixing time of 60 seconds. After mixing, the powder mixture was sieved with a sieve with a mesh size of 1.0 mm. The sieved powder mixture was then applied to an electrically conductive carrier element. The carrier element was a wire mesh made of silver with a wire thickness of 0.14 mm and a mesh size of 0.5 mm. The powder was applied using a 2 mm thick stencil, with the powder being applied using a sieve with a mesh size of 1.0 mm. Excess powder that protruded beyond the thickness of the stencil was removed using a scraper. After the stencil was removed, the carrier with the applied powder mixture was pressed using a roller press with a pressing force of 0.45 kN/cm. The gas diffusion electrode was removed from the roller press. The resulting gas diffusion electrode had a porosity of about 50%.

In general, in the process according to the invention, before the electrochemical reduction of the carbon dioxide, the silver oxide of the gas diffusion electrode provided is pre-reduced in the presence of the reduction environment to obtain the gas diffusion electrode containing silver. The gas diffusion electrode provided can be pre-reduced in the electrolysis cell in which the electrochemical reduction of carbon dioxide also takes place following the pre-reduction. For this purpose, the gas diffusion electrode containing silver oxide is introduced into the cathode compartment of the electrolysis cell used for the electrochemical reduction of carbon dioxide before the reduction of the silver oxide and, to provide the reduction environment, such a composition is introduced into the cathode compartment, ie the gas compartment and the catholyte compartment, so that the gas diffusion electrode is at least partially enveloped by this composition and is in contact with this composition, the introduced composition containing, based on its total weight, an amount of 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof.

It is essential for the method according to the invention that the silver oxide of the gas diffusion electrode provided is reduced in a reduction environment to obtain the pre-reduced gas diffusion electrode. It is particularly preferred according to the invention if the pre-reduced gas diffusion electrode obtained in this way has a weight ratio of metallic silver to silver oxides of at least 100,000.

Preferably, the reduction of the silver oxide is carried out electrochemically by applying an electric current and the gas diffusion electrode is in contact at least with catholyte and the said composition.

Carrying out the reduction of the silver oxide in the electrolysis cell used for the reduction of carbon dioxide is a preferred embodiment of the method according to the invention. It is also possible to reduce the gas diffusion electrode provided in a separate electrochemical device for pre-reduction, in which the gas diffusion electrode provided acts as a cathode and an anode is provided accordingly and the composition is also introduced accordingly. The pre-reduced gas diffusion electrode resulting according to this embodiment is then removed from this separate device and introduced into the electrolysis cell for the reduction of carbon dioxide before the pre-reduced gas diffusion electrode is used as a silver-containing gas diffusion electrode in the electrolysis cell for the electrochemical reduction of carbon dioxide by applying an electrolysis current while supplying a gas stream containing carbon dioxide.

The said composition at least partially envelops the gas diffusion electrode, i.e. at least those parts of the gas diffusion electrode which are in contact with the catholyte and the carbon dioxide during the subsequent reduction of carbon dioxide are in contact with the gas diffusion electrode during the reduction conditions.

The composition in question can be present, for example, as a gas and/or as a liquid. It is preferred if the gas is present in the form of a gas phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one gas selected from CO ₂ , O ₂ or a mixture thereof. Such a gas phase is also referred to below as an inert gas or inert gas atmosphere. It is also preferred if the liquid is present in the form of a water-containing, liquid phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof in solution.

According to the invention, a "phase" is understood to mean a substance or a mixture of substances that is in contact with another substance or mixture of substances and forms a phase boundary. A phase boundary is a term for surfaces that separate two phases that are not mixed with each other; for example, the separating surfaces between the liquid-solid, liquid-liquid, solid-solid, solid-gaseous or liquid-gaseous phases.

A gas phase is understood to be a substance or a mixture of substances, whereby the said substance or mixture of substances is in gaseous form at 20°C and 1013 mbar and forms a phase boundary upon contact with the gas diffusion electrode.

A liquid phase is understood to be a substance or a mixture of substances, whereby the said substance or mixture of substances is in gaseous form at 20°C and 1013 mbar and forms a phase boundary upon contact with the gas diffusion electrode.

A person skilled in the art understands an inert gas to be a substance which is gaseous at 20°C and 1013 mbar and which, in particular under the conditions of pre-reduction, does not enter into any noticeable chemical reaction with the gas diffusion electrode and which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one gas selected from CO ₂ , O ₂ or a mixture thereof. Nitrogen serves as a reference value for an inert effect. An inert gas atmosphere is a gas phase which is inert to the extent described above, containing at least one gaseous substance and, based on the total weight of the inert gas phase, 0 mg/kg to 1000 mg/kg of at least one gas selected from CO ₂ , O ₂ or a mixture thereof. A preferred embodiment of the method is characterized in that the inert gas atmosphere contains nitrogen, argon or mixtures thereof and, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one gas selected from CO ₂ , O ₂ or a mixture thereof.

The inert gas atmosphere can preferably penetrate a provided gas diffusion electrode. This can be achieved by applying an inert gas stream passed through the gas diffusion electrode before and/or during the reduction of the silver oxide, preferably with a flow rate in a range from 5 L/h to 21 L/h. It is particularly preferred if the inert gas of the inert gas atmosphere is passed through the gas diffusion electrode containing silver oxide before the reduction, preferably with a flow rate in a range from 5 L/h to 21 L/h.

If the reduction of the silver oxide of the gas diffusion electrode is carried out in an electrochemical device for pre-reduction in which the cathode space has a catholyte space and a gas space, then in a preferred embodiment of the method for providing the reduction environment, the said composition in the form of a gas phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one gas selected from CO ₂ , O ₂ or a mixture thereof, can be introduced into the gas space of the cathode space and a further composition in the form of a water-containing, liquid phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof in solution, can be introduced into the catholyte space as a catholyte.

It is particularly preferred if the device for pre-reduction is the electrolysis cell used for the electrochemical reduction of carbon dioxide.

A further preferred embodiment of the method is characterized in that the device for reduction has at least one cathode space containing at least one catholyte space and at least one gas space and the gas diffusion electrode containing silver oxide is introduced into the cathode space of the device for reduction (in particular the electrolysis cell used for the electrochemical reduction of carbon dioxide) before the reduction of the silver oxide, and to provide the reduction environment, said composition in the form of a water-containing, liquid phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or their mixture in solution, is introduced both as a catholyte into a catholyte space of the device for reduction and into the gas space of said device.

It is also possible for the reduction device to have at least one cathode chamber containing at least one catholyte chamber and for the gas diffusion electrode containing silver oxide to be introduced into the cathode chamber of the reduction device (in particular the electrolysis cell used for the electrochemical reduction of carbon dioxide) before the reduction of the silver oxide, and for the provision of the reduction environment, said composition in the form of a water-containing, liquid phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof in solution is introduced as a catholyte into a catholyte chamber of the reduction device.

The reduction of silver oxide of the gas diffusion electrode provided to the silver-containing pre-reduced gas diffusion electrode in said reduction environment is preferably carried out electrochemically by applying an electric current. It has again proven to be particularly preferred if the reduction of silver oxide of the gas diffusion electrode to silver is carried out by applying an electric current at a current density of 0.5 to 4 kA/m ² , preferably 0.5 to 2.5 kA/m ² .

An electrochemical pre-reduction can generally be carried out without much expenditure of time. Methods of this embodiment are preferred which are characterized in that the reduction of silver oxide of the gas diffusion electrode to silver is carried out by applying an electric current over a period of at least 20 minutes, preferably at least 30 minutes. It has also been shown to be particularly preferred that the reduction of silver oxide of the gas diffusion electrode to silver can be carried out by applying an electric current over a period of at most 120 minutes, particularly preferably at most 60 minutes. The reduction of silver oxide of the gas diffusion electrode to silver is very particularly preferably carried out by applying an electric current over a period of 20 to 120 minutes, more preferably from 20 to 60 minutes, more preferably from 20 to 120 minutes, most preferably from 30 to 60 minutes.

Before the step of reducing silver oxide, it is preferred if the gas diffusion electrode is in contact with electrolyte and an anode is also in contact with electrolyte. For this purpose, it is preferred if, before the step of reducing silver oxide, the anolyte space of the device for pre-reduction is filled with anolyte and the catholyte space is filled with catholyte. Within the scope of a particularly preferred embodiment of the invention, before the step of reducing silver oxide, the anolyte space of the electrochemical device for pre-reduction is filled with anolyte and the catholyte space is filled with catholyte and the catholyte space is introduced into and removed from the catholyte space in the form of a catholyte stream at least during the reduction.

In a further embodiment, it has again been found to be preferred if the said composition acts as a catholyte in the reduction environment and an aqueous solution of alkali hydrogen carbonate, preferably potassium hydrogen carbonate, caesium hydrogen carbonate or sodium hydrogen carbonate, particularly preferably potassium hydrogen carbonate.

In addition, a preferred embodiment of the process is characterized in that the anolyte in the device for reduction is an aqueous solution of alkali hydrogen carbonate, preferably potassium hydrogen carbonate, cesium hydrogen carbonate or sodium hydrogen carbonate, particularly preferably potassium hydrogen carbonate.

It has further been found to be preferred if, during the reduction step, the pH of the electrolyte, in particular of the catholyte, has a value of < pH 9.0, particularly preferably < pH 8.5.

If the reduction of silver oxide of the provided gas diffusion electrode to the silver-containing pre-reduced gas diffusion electrode is carried out electrochemically by applying electric current using the reduction environment, it has been found to be effective that the operating temperature of the anolyte and catholyte is independently in a range of 10°C to 60°C during the reduction.

• Einbringen der einer Silberoxid-haltigen Gasdiffusionselektrode in einen Reduktionsraum einer elektrochemischen Vorrichtung,
• Bereitstellung einer Reduktionsumgebung, in dem zur eingebrachten Gasdiffusionselektrode mindestens eine Zusammensetzung in den Reduktionsraum eingebracht, zur Gasdiffusionselektrode geführt und damit kontaktiert wird, wodurch die Gasdiffusionselektrode von dieser Zusammensetzung zumindest teilweise eingehüllt wird, unter der Maßgabe, dass diese eingebrachte Zusammensetzung bezogen auf ihr Gesamtgewicht eine Menge von 0 mg/kg bis 1000 mg/kg mindestens einer Verbindung, ausgewählt aus CO₂, O₂ oder deren Gemisch enthält;
• Reduktion von Silberoxid der eingebrachten Gasdiffusionselektrode zu Silber unter Aufrechterhaltung der Reduktionsumgebung und unter Erhalt einer vorreduzierten Gasdiffusionselektrode.

The use of a gas diffusion electrode pre-reduced according to the method according to the invention for the electrochemical reduction of carbon dioxide has proven to be advantageous for solving the technical problems according to the invention. A further subject of the invention is therefore the use of a pre-reduced gas diffusion electrode containing silver for the electrochemical reduction of carbon dioxide, wherein the pre-reduced gas diffusion electrode is obtained by reducing silver oxide of a silver oxide-containing gas diffusion electrode to silver by at least the following process steps:

• Introducing a silver oxide-containing gas diffusion electrode into a reduction chamber of an electrochemical device,
• Providing a reduction environment in which at least one composition is introduced into the reduction space for the introduced gas diffusion electrode, guided to the gas diffusion electrode and contacted therewith, whereby the gas diffusion electrode is at least partially enveloped by this composition, with the proviso that this introduced composition contains, based on its total weight, an amount of 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof;
• Reduction of silver oxide of the introduced gas diffusion electrode to silver while maintaining the reduction environment and obtaining a pre-reduced gas diffusion electrode.

All relevant preferred features of the method according to the invention (in particular preferred features of the gas diffusion electrode, the provision of the reduction environment, the reduction of silver oxide) are also considered to be preferred for use mutatis mutandis.

1. 1. Verfahren zur elektrochemischen Reduktion von Kohlendioxid in mindestens einer Elektrolysezelle (Z) mit mindestens einem Anodenraum (15), der mindestens eine mit Anolyt (15a) in Kontakt stehende Anode (10) enthält, und mit mindestens einem Kathodenraum (16), der mindestens eine Silber enthaltende Gasdiffusionselektrode als Kathode (11) enthält, wobei besagte Gasdiffusionselektrode den Kathodenraum (16) in einen Gasraum (4) und einem Katholytraum (12) unterteilt und der Gasraum (4) mindestens einen Einlass für einen Gasstrom und mindestens einen Auslass für einen Produktgasstrom aufweist, und wobei das Verfahren unter Ausführung mindestens folgender Schritte in nachfolgender Rangfolge durchgeführt wird Bereitstellung mindestens einer Gasdiffusionselektrode, enthaltend Silberoxid; Einbringen der Gasdiffusionselektrode in einen Reduktionsraum einer elektrochemischen Vorrichtung, bevorzugt in den Kathodenraum (14) der Elektrolysezelle (Z); Bereitstellung einer Reduktionsumgebung, in dem zur eingebrachten Gasdiffusionselektrode mindestens eine Zusammensetzung in den Reduktionsraum eingebracht, zur Gasdiffusionselektrode geführt und damit kontaktiert wird, wodurch die Gasdiffusionselektrode von dieser Zusammensetzung zumindest teilweise eingehüllt wird, unter der Maßgabe, dass diese eingebrachte Zusammensetzung bezogen auf ihr Gesamtgewicht eine Menge von 0 mg/kg bis 1000 mg/kg mindestens einer Verbindung, ausgewählt aus CO₂, O₂ oder deren Gemisch enthält; Reduktion von Silberoxid der eingebrachten Gasdiffusionselektrode zu Silber unter Aufrechterhaltung der Reduktionsumgebung und unter Erhalt einer vorreduzierten Gasdiffusionselektrode; Anwendung besagter vorreduzierter Gasdiffusionselektrode als Silber enthaltende Gasdiffusionselektrode als Kathode (11) in besagter Elektrolysezelle (Z) zur elektrochemischen Reduktion von Kohlendioxid durch Anlegen eines Elektrolysestromes, wobei dem Gasraum (4) ein Kohlendioxid-haltiges Gas in Form eines Gasstroms zugeführt wird und das zugeführte Kohlendioxid-haltige Gas bezogen auf dessen Volumen mehr als 80 Vol.-% CO₂ enthält.
2. 2. Verfahren nach Aspekt 1, dadurch gekennzeichnet, dass besagte Zusammensetzung in Form einer Gasphase vorliegt, die bezogen auf ihr Gesamtgewicht 0 mg/kg bis 1000 mg/kg mindestens eines Gases, ausgewählt aus CO₂, O₂ oder deren Gemisch enthält.
3. 3. Verfahren nach Aspekt 1 oder 2, dadurch gekennzeichnet, dass besagte Zusammensetzung in Form einer Gasphase vorliegt, die bezogen auf ihr Gesamtgewicht 0 mg/kg bis 1000 mg/kg mindestens eines Gases, ausgewählt aus CO₂, O₂ oder deren Gemisch enthält, und die Zusammensetzung vor der Reduktion durch die Silberoxid enthaltende Gasdiffusionselektrode hindurch geleitet wird, bevorzugt mit einer Durchflussrate in einem Bereich von 5 L/h bis 21 L/h.
4. 4. Verfahren nach Aspekt 1, dadurch gekennzeichnet, dass besagte Zusammensetzung in Form einer wasserhaltigen, flüssigen Phase vorliegt, die bezogen auf ihr Gesamtgewicht 0 mg/kg bis 1000 mg/kg mindestens einer Verbindung, ausgewählt aus CO₂, O₂ oder deren Gemisch gelöst enthält.
5. 5. Verfahren nach einem der vorangehenden Aspekte, dadurch gekennzeichnet, dass die bereitgestellte Gasdiffusionselektrode mindestens 50 Gew.%; bevorzugt mindestens 75 Gew.-%; besonders bevorzugt mindestens 80 Gew.-%, Silber-I-Oxid enthält, bezogen auf das Gesamtgewicht der Gasdiffusionselektrode.
6. 6. Verfahren nach einem der vorangehenden Aspekte, dadurch gekennzeichnet, dass die Gasdiffusionselektrode, enthaltend Silberoxid, mindestens eine auf einem metallischen oder nichtmetallischen, leitfähigen oder nichtleitfähigen Träger aufgebrachte, kompaktierte Mischung umfasst, enthaltend Silberoxidpartikel im Gemisch mit Partikeln mindestens eines pulverförmigen, Fluor-haltigen Polymers, insbesondere Polytetrafluoroethylen Pulver, als Bindemittel.
7. 7. Verfahren nach einem der vorangehenden Aspekte, dadurch gekennzeichnet, dass die Reduktion von Silberoxid der Gasdiffusionselektrode zu Silber durch Anlegen von elektrischem Strom bei einer Stromdichte von 0,5 bis 4 kA/m², bevorzugt von 0,5 bis 2,5 kA/m² durchgeführt wird.
8. 8. Verfahren nach einem der vorangehenden Aspekte, dadurch gekennzeichnet, dass die Reduktion von Silberoxid der Gasdiffusionselektrode zu Silber durch Anlegen von elektrischem Strom über einen Zeitraum von mindestens 20 Minuten, bevorzugt mindestens 30 Minuten, durchgeführt wird.
9. 9. Verfahren nach einem der vorangehenden Aspekte, dadurch gekennzeichnet, dass während der Reduktion von Silberoxid der pH-Wert des Elektrolyts, insbesondere des Katholyts, einen Wert von < pH 9,0 , besonders bevorzugt von < pH 8,5 , aufweist.
10. 10. Verfahren nach einem der vorangehenden Aspekte, dadurch gekennzeichnet, dass die vorreduzierte Gasdiffusionselektrode ein Gewichtsverhältnis von metallischem Silber zu Silberoxiden von wenigstens 100000 aufweist.
11. 11. Verfahren nach einem der vorangehenden Aspekte, dadurch gekennzeichnet, dass die Gasdiffusionselektrode, enthaltend Silberoxid, vor der Reduktion des Silberoxids in den Reduktionsraum, insbesondere in den Kathodenraum (14) der für die elektrochemische Reduktion von Kohlendioxid genutzten Elektrolysezelle (Z), eingebracht wird, wobei der Reduktionsraum durch die Gasdiffusionselektrode in einen Gasraum und einen davon getrennten Katholytraum aufgeteilt wird und zur Bereitstellung der Reduktionsumgebung die besagte Zusammensetzung in Form einer Gasphase, die bezogen auf ihr Gesamtgewicht 0 mg/kg bis 1000 mg/kg mindestens eines Gases, ausgewählt aus CO₂, O₂ oder deren Gemisch enthält, in den Gasraum des Reduktionsraumes eingebracht wird und in den Katholytraum eine weitere Zusammensetzung in Form einer wasserhaltigen, flüssigen Phase, die bezogen auf ihr Gesamtgewicht 0 mg/kg bis 1000 mg/kg mindestens einer Verbindung, ausgewählt aus CO₂, O₂ oder deren Gemisch gelöst enthält, als Katholyt eingebracht wird.
12. 12. Verfahren nach einem der Aspekte 1 bis 10, dadurch gekennzeichnet, dass die Gasdiffusionselektrode, enthaltend Silberoxid, vor der Reduktion des Silberoxids als Kathode in den Reduktionsraum eingebracht wird, und zur Bereitstellung der Reduktionsumgebung eine besagte Zusammensetzung in Form einer wasserhaltigen, flüssigen Phase, die bezogen auf ihr Gesamtgewicht 0 mg/kg bis 1000 mg/kg mindestens einer Verbindung, ausgewählt aus CO₂, O₂ oder deren Gemisch gelöst enthält, in den Reaktionsraum als Katholyt eingebracht, zur Gasdiffusionselektrode geführt und damit kontaktiert wird.
13. 13. Verfahren nach einem der Aspekte 11 oder 12, dadurch gekennzeichnet, dass vor der Reduktion der Anolytraum mit Anolyt befüllt wird.
14. 14. Verfahren nach einem der Aspekte 11 bis 13, dadurch gekennzeichnet, dass vor der Reduktion der Anolytraum mit Anolyt befüllt wird und der Katholyt in Form eines Katholytstromes zumindest während der Reduktion in den Katholytraum ein- und wieder ausgebracht wird.
15. 15. Verfahren nach einem der Aspekte 11 bis 14, dadurch gekennzeichnet, dass der Katholyt eine wässrige Lösung enthaltend Alkalihydrogencarbonat, bevorzugt Kaliumhydrogencarbonat, Caesiumhydrogencarbonat oder Natriumhydrogencarbonat ist, besonders bevorzugt Kaliumhydrogencarbonat ist.
16. 16. Verfahren nach einem der Aspekte 11 bis 15, dadurch gekennzeichnet, dass der Anolyt eine wässrige Lösung enthaltend Alkalihydrogencarbonat, bevorzugt Kaliumhydrogencarbonat, Caesiumhydrogencarbonat oder Natriumhydrogencarbonat ist, besonders bevorzugt Kaliumhydrogencarbonat ist.
17. 17. Verfahren nach einem der Aspekte 11 bis 16, dadurch gekennzeichnet, dass die Betriebstemperatur von Anolyt und Katholyt unabhängig voneinander in einem Bereich von 10°C bis 60°C liegt.
18. 18. Verwendung einer vorreduzierten Gasdiffusionselektrode, enthaltend Silber, zur elektrochemischen Reduktion von Kohlendioxid, wobei die vorreduzierte Gasdiffusionselektrode erhalten wird durch Reduktion von Silberoxid einer Silberoxid-haltigen Gasdiffusionselektrode zu Silber dadurch gekennzeichnet, dass die vorreduzierte Gasdiffusionselektrode durch zumindest folgende Verfahrensschritte erhalten wird:
    • Einbringen der einer Silberoxid-haltigen Gasdiffusionselektrode in einen Reduktionsraum einer elektrochemischen Vorrichtung,
    • Bereitstellung einer Reduktionsumgebung, in dem zur eingebrachten Gasdiffusionselektrode mindestens eine Zusammensetzung in den Reduktionsraum eingebracht, zur Gasdiffusionselektrode geführt und damit kontaktiert wird, wodurch die Gasdiffusionselektrode von dieser Zusammensetzung zumindest teilweise eingehüllt wird, unter der Maßgabe, dass diese eingebrachte Zusammensetzung bezogen auf ihr Gesamtgewicht eine Menge von 0 mg/kg bis 1000 mg/kg mindestens einer Verbindung, ausgewählt aus CO₂, O₂ oder deren Gemisch enthält;
    • Reduktion von Silberoxid der eingebrachten Gasdiffusionselektrode zu Silber unter Aufrechterhaltung der Reduktionsumgebung und unter Erhalt einer vorreduzierten Gasdiffusionselektrode.

The following aspects 1 to 18 represent a further embodiment of the invention, wherein the reference numerals in brackets are merely for clarification purposes and the embodiment does not refer to the subject matter of the 1 restricts:

1. 1. Process for the electrochemical reduction of carbon dioxide in at least one electrolysis cell (Z) with at least one anode chamber (15) which contains at least one anode (10) in contact with anolyte (15a), and with at least one cathode chamber (16) which contains at least one silver-containing gas diffusion electrode as a cathode (11), wherein said gas diffusion electrode divides the cathode chamber (16) into a gas chamber (4) and a catholyte chamber (12) and the gas chamber (4) has at least one inlet for a gas stream and at least one outlet for a product gas stream, and wherein the process is carried out by carrying out at least the following steps in the following order of priority: providing at least one gas diffusion electrode containing silver oxide; introducing the gas diffusion electrode into a reduction chamber of an electrochemical device, preferably into the cathode chamber (14) of the electrolysis cell (Z); Providing a reduction environment in which at least one composition is introduced into the reduction space for the introduced gas diffusion electrode, guided to the gas diffusion electrode and contacted therewith, whereby the gas diffusion electrode is at least partially enveloped by this composition, with the proviso that this introduced composition contains, based on its total weight, an amount of 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof; reduction of silver oxide of the introduced gas diffusion electrode to silver while maintaining the reduction environment and obtaining a pre-reduced gas diffusion electrode; use of said pre-reduced gas diffusion electrode as a silver-containing gas diffusion electrode as a cathode (11) in said electrolysis cell (Z) for the electrochemical reduction of carbon dioxide by applying an electrolysis current, wherein a carbon dioxide-containing gas is supplied to the gas space (4) in the form of a gas stream and the supplied carbon dioxide-containing gas contains more than 80 vol.% CO ₂ based on its volume.
2. 2. Process according to aspect 1, characterized in that said composition is in the form of a gas phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one gas selected from CO ₂ , O ₂ or a mixture thereof.
3. 3. Process according to aspect 1 or 2, characterized in that said composition is in the form of a gas phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one gas selected from CO ₂ , O ₂ or a mixture thereof, and the composition is passed through the silver oxide-containing gas diffusion electrode before reduction, preferably at a flow rate in a range of 5 L/h to 21 L/h.
4. 4. Process according to aspect 1, characterized in that said composition is in the form of a water-containing liquid phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof in solution.
5. 5. Method according to one of the preceding aspects, characterized in that the gas diffusion electrode provided contains at least 50 wt.%; preferably at least 75 wt.%; particularly preferably at least 80 wt.%, silver-I-oxide, based on the total weight of the gas diffusion electrode.
6. 6. Method according to one of the preceding aspects, characterized in that the gas diffusion electrode containing silver oxide comprises at least one compacted mixture applied to a metallic or non-metallic, conductive or non-conductive carrier, containing silver oxide particles in a mixture with particles of at least one powdered, fluorine-containing polymer, in particular polytetrafluoroethylene powder, as a binder.
7. 7. Method according to one of the preceding aspects, characterized in that the reduction of silver oxide of the gas diffusion electrode to silver is carried out by applying electric current at a current density of 0.5 to 4 kA/m ² , preferably of 0.5 to 2.5 kA/m ² .
8. 8. Method according to one of the preceding aspects, characterized in that the reduction of silver oxide of the gas diffusion electrode to silver is carried out by applying electric current over a period of at least 20 minutes, preferably at least 30 minutes.
9. 9. Process according to one of the preceding aspects, characterized in that during the reduction of silver oxide the pH of the electrolyte, in particular of the catholyte, has a value of < pH 9.0, particularly preferably of < pH 8.5.
10. 10. Method according to one of the preceding aspects, characterized in that the pre-reduced gas diffusion electrode has a weight ratio of metallic silver to silver oxides of at least 100,000.
11. 11. Method according to one of the preceding aspects, characterized in that the gas diffusion electrode containing silver oxide is introduced into the reduction chamber, in particular into the cathode chamber (14) of the electrolysis cell (Z) used for the electrochemical reduction of carbon dioxide, before the reduction of the silver oxide, wherein the reduction chamber is divided by the gas diffusion electrode into a gas chamber and a catholyte chamber separate therefrom and to provide the reduction environment, said composition in the form of a gas phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one gas selected from CO ₂ , O ₂ or a mixture thereof, is introduced into the gas chamber of the reduction chamber and a further composition in the form of a water-containing, liquid phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof in solution, is introduced into the catholyte chamber as a catholyte.
12. 12. Method according to one of aspects 1 to 10, characterized in that the gas diffusion electrode containing silver oxide is introduced into the reduction chamber as a cathode before the reduction of the silver oxide, and to provide the reduction environment, said composition in Form of a water-containing, liquid phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof in dissolved form, is introduced into the reaction chamber as a catholyte, led to the gas diffusion electrode and contacted therewith.
13. 13. Method according to one of aspects 11 or 12, characterized in that the anolyte chamber is filled with anolyte before the reduction.
14. 14. Method according to one of aspects 11 to 13, characterized in that before the reduction the anolyte space is filled with anolyte and the catholyte is introduced into and removed from the catholyte space in the form of a catholyte stream at least during the reduction.
15. 15. Process according to one of aspects 11 to 14, characterized in that the catholyte is an aqueous solution containing alkali hydrogen carbonate, preferably potassium hydrogen carbonate, cesium hydrogen carbonate or sodium hydrogen carbonate, particularly preferably potassium hydrogen carbonate.
16. 16. Process according to one of aspects 11 to 15, characterized in that the anolyte is an aqueous solution containing alkali hydrogen carbonate, preferably potassium hydrogen carbonate, cesium hydrogen carbonate or sodium hydrogen carbonate, particularly preferably potassium hydrogen carbonate.
17. 17. Method according to one of aspects 11 to 16, characterized in that the operating temperature of anolyte and catholyte independently of one another is in a range from 10°C to 60°C.
18. 18. Use of a pre-reduced gas diffusion electrode containing silver for the electrochemical reduction of carbon dioxide, wherein the pre-reduced gas diffusion electrode is obtained by reducing silver oxide of a silver oxide-containing gas diffusion electrode to silver, characterized in that the pre-reduced gas diffusion electrode is obtained by at least the following process steps:
    • Introducing a silver oxide-containing gas diffusion electrode into a reduction chamber of an electrochemical device,
    • Providing a reduction environment in which at least one composition is introduced into the reduction space for the introduced gas diffusion electrode, guided to the gas diffusion electrode and contacted therewith, whereby the gas diffusion electrode is at least partially enveloped by this composition, with the proviso that this introduced composition contains, based on its total weight, an amount of 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof;
    • Reduction of silver oxide of the introduced gas diffusion electrode to silver while maintaining the reduction environment and obtaining a pre-reduced gas diffusion electrode.

Examples

The experiments were carried out in an electrochemical cell "Micro Flow Cell" from ElectroCell with an active geometric area of 10cm ² . The cell had three flow chambers, each 2 mm wide: anolyte chamber, catholyte chamber and gas chamber. The anode used was a solid metal electrode made of IrO ₂ . A cation exchange membrane type F133 separated the electrolyte chambers. Viton seals were used between the frame, electrodes and membrane.

1.33M KHCO ₃ (Bernd Kraft) was used as anolyte and catholyte, so the pH was 7.9 at the start of the experiment. Anolyte and catholyte were each fed through containers with double-walled jackets via radial pumps at 20°C. The electrolyte tanks were permanently blanketed with N ₂ . The oxygen generated during the anode reaction was removed from the cell with the anolyte reflux. The nitrogen blanket of the catholyte tank was combined with the outlet stream of the gas chamber, freed of water in a cold trap and then analyzed in a commercially available gas analyzer.

The electrodes were supplied with direct current via a rectifier. An Ag/AgCl reference electrode was used in the catholyte chamber to measure the potential of the GDE.

Before the start of the CO ₂ reduction, the GDE was reduced in the CO ₂ electrolyzer using the method according to the invention. For this purpose, an electric current of 1 A (current density 1 kA/m ² ) was applied galvanostatically. On the gas side, 21 l/h of N ₂ flowed instead of CO ₂ . The running time was 1 hour at 20°C to ensure complete reduction of the GDE. The GDE changed its color from black-gray to white-gold. The end of the reduction could be determined by visual observation of the discoloration. The application of the GDE can also be documented by measuring the electrode potential. These are 2 illustrated. The electrode potential E vs. Ag/AgCl [V] decreased noticeably during the operating period. In addition, the production of hydrogen began. The hydrogen formation φ _H2 [vol.%] observed in the product gas stream began before the Ag ₂ O reduction process had finished, since no externally supplied gas flow was required with the electrolyte present and the decreasing potential exceeded the activation potential of the hydrogen formation reaction.

1. a) Indem N₂ unter Anlegen eines Stromes wie oben beschrieben in die Zelle eingeleitet wurde und
2. b) Indem der Ausgang des Gasraumes verschlossen wurde und der Stickstoff dadurch zum Durchtritt durch die Gasdiffusionselektrode gezwungen wurde. Dabei variierte der Gasdurchtritt zwischen 5 und 21 l/h. Eine dementsprechende in situ Reduktion des Silberoxids wurde bei folgenden Betriebsparametern durchgeführt:

Temperatur in °C Differenzdruck zwischen Katholytraum und Gasraum in mbar Stromdichte in kA/m² 20 20 1,0 30 20 1,0 40 20 1,0 20 40 1,0 20 60 1,0 20 20 0,5 20 20 2,0 20 20 3,0 20 20 4,0 The in-situ reduction could take place in two different ways and under different operating parameters:

1. a) By introducing N ₂ into the cell under a current as described above and
2. b) By closing the outlet of the gas chamber and forcing the nitrogen to pass through the gas diffusion electrode. The gas passage varied between 5 and 21 l/h. A corresponding in situ reduction of the silver oxide was carried out under the following operating parameters:

Temperature in °C Differential pressure between catholyte chamber and gas chamber in mbar Current density in kA/m ² 20 20 1.0 30 20 1.0 40 20 1.0 20 40 1.0 20 60 1.0 20 20 0.5 20 20 2.0 20 20 3.0 20 20 4.0

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• WO 2021/069470 A1 [0011]
• WO 2020/239716 A1 [0011]
• EP1728896A2 [0011]
• EP 2824218 A1 [0014]
• WO 2003/042430 A2 [0016]
• DE 102005027735 A1 [0016]
• WO 2020/057998 A1 [0027]
• DE 10130441 A [0036]

Claims (10)

Method for the electrochemical reduction of carbon dioxide in at least one electrolysis cell (Z) with at least one anode chamber (15) which contains at least one anode (10) in contact with anolyte (15a), and with at least one cathode chamber (16) which contains at least one silver-containing gas diffusion electrode as cathode (11), wherein said gas diffusion electrode divides the cathode chamber (16) into a gas chamber (4) and a catholyte chamber (12) and the gas chamber (4) has at least one inlet for a gas stream and at least one outlet for a product gas stream, and wherein the method is carried out by carrying out at least the following steps in the following order of priority: providing at least one gas diffusion electrode containing silver oxide; introducing the gas diffusion electrode into a reduction chamber of an electrochemical device, preferably into the cathode chamber (14) of the electrolysis cell (Z); Providing a reduction environment in which at least one composition is introduced into the reduction space for the introduced gas diffusion electrode, guided to the gas diffusion electrode and contacted therewith, whereby the gas diffusion electrode is at least partially enveloped by this composition, with the proviso that this introduced composition contains, based on its total weight, an amount of 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof; Reduction of silver oxide of the introduced gas diffusion electrode to silver while maintaining the reduction environment and obtaining a pre-reduced gas diffusion electrode; Use of said pre-reduced gas diffusion electrode as a silver-containing gas diffusion electrode as a cathode (11) in said electrolysis cell (Z) for the electrochemical reduction of carbon dioxide by applying an electrolysis current, wherein a carbon dioxide-containing gas is supplied to the gas space (4) in the form of a gas stream and the supplied carbon dioxide-containing gas contains more than 80 vol.% CO ₂ based on its volume. Procedure according to Claim 1 , characterized in that said composition is in the form of a gas phase which contains, based on its total weight, 0 mg/kg to 1000 mg/kg of at least one gas selected from CO ₂ , O ₂ or a mixture thereof. Method according to one of the preceding claims, characterized in that said composition is in the form of a gas phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one gas selected from CO ₂ , O ₂ or a mixture thereof, and the composition is passed through the gas diffusion electrode containing silver oxide before reduction, preferably at a flow rate in a range of 5 L/h to 21 L/h. Procedure according to Claim 1 , characterized in that said composition is in the form of a water-containing liquid phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof in solution. Method according to one of the preceding claims, characterized in that the gas diffusion electrode provided contains at least 50 wt.%; preferably at least 75 wt.%; particularly preferably at least 80 wt.%, silver-I-oxide, based on the total weight of the gas diffusion electrode. Method according to one of the preceding claims, characterized in that the reduction of silver oxide of the gas diffusion electrode to silver is carried out by applying electric current at a current density of 0.5 to 4 kA/m ² , preferably 0.5 to 2.5 kA/m ² . Method according to one of the preceding claims, characterized in that the gas diffusion electrode containing silver oxide is introduced into the reduction chamber, in particular into the cathode chamber (14) of the electrolysis cell (Z) used for the electrochemical reduction of carbon dioxide, before the reduction of the silver oxide, the reduction chamber being divided by the gas diffusion electrode into a gas chamber and a catholyte chamber separate therefrom, and to provide the reduction environment, said composition in the form of a gas phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one gas selected from CO ₂ , O ₂ or a mixture thereof, is introduced into the gas chamber of the reduction chamber, and a further composition in the form of a water-containing, liquid phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof in solution, is introduced into the catholyte chamber as a catholyte. Method according to one of the Claims 1 until 7 , characterized in that the gas diffusion electrode containing silver oxide is introduced into the reduction chamber as a cathode before the reduction of the silver oxide, and to provide the reduction environment, a said composition in the form of a water-containing, liquid phase which, based on its total weight, contains 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof in solution, is introduced into the reaction chamber as a catholyte, led to the gas diffusion electrode and contacted therewith. Method according to one of the Claims 7 or 8th , characterized in that the catholyte and/or the anolyte is an aqueous solution containing alkali hydrogen carbonate, preferably potassium hydrogen carbonate, caesium hydrogen carbonate or sodium hydrogen carbonate, particularly preferably potassium hydrogen carbonate. Use of a pre-reduced gas diffusion electrode containing silver for the electrochemical reduction of carbon dioxide, wherein the pre-reduced gas diffusion electrode is obtained by reducing silver oxide of a silver oxide-containing gas diffusion electrode to silver, characterized in that the pre-reduced gas diffusion electrode is obtained by at least the following method steps: introducing a silver oxide-containing gas diffusion electrode into a reduction chamber of an electrochemical device, providing a reduction environment in which at least one composition is introduced into the reduction chamber for the introduced gas diffusion electrode, guided to the gas diffusion electrode and contacted therewith, whereby the gas diffusion electrode is at least partially enveloped by this composition, with the proviso that this introduced composition contains, based on its total weight, an amount of 0 mg/kg to 1000 mg/kg of at least one compound selected from CO ₂ , O ₂ or a mixture thereof; Reduction of silver oxide of the introduced gas diffusion electrode to silver while maintaining the reduction environment and obtaining a pre-reduced gas diffusion electrode.

Images