WO2018001637A1

WO2018001637A1 - Arrangement and method for the electrolysis of carbon dioxide

Info

Publication number: WO2018001637A1
Application number: PCT/EP2017/061927
Authority: WO
Inventors: Savo Asanin; Van An DU; Maximilian Fleischer; Philippe Jeanty; Erhard Magori; Angelika Tawil; Kerstin Wiesner; Oliver von Sicard
Original assignee: Siemens Aktiengesellschaft
Priority date: 2016-06-30
Filing date: 2017-05-18
Publication date: 2018-01-04
Also published as: AU2017291062A1; AU2017291062B2; DE102016211819A1; EP3478879A1; CN109415830A; US20190233958A1

Abstract

The invention relates to an arrangement for the electrolysis of carbon dioxide and to a method for the operation thereof, comprising an electrolytic cell with an anode and a cathode, the anode and cathode being connected to a voltage supply, the cathode being a gas diffusion electrode and a gas chamber being connected to a first side of the cathode and a cathode chamber being connected to a second side of the cathode, also comprising an electrolytic circuit which is connected to the electrolytic cell, and a gas supply for supplying carbon dioxide-containing gas into the gas chamber, characterised in that the gas chamber comprises an outlet for the electrolyte and the electrolyte outlet comprising a blocking device which is designed such that the blocking device is opened when the difference in pressure between the gas chamber and the cathode chamber exceeds a threshold value.

Description

description

Apparatus and method for carbon dioxide electrolysis, the invention relates to an arrangement and a method for the carbon dioxide electrolysis according to the preamble of An ^¬ demanding. 1

The burning of fossil fuels currently covers about 80% of global energy needs. Through this

Combustion processes were in 2011 around the world about 34 billion tons of carbon dioxide (C02) in the atmosphere emit ^¬ advantage. This release is the easiest way to dispose of even large amounts of C02 (large lignite-fired power plants over 50,000 tons per day).

The discussion about the negative impact of the greenhouse gas C02 on the climate has meant that we think about a How To ^¬ derverwertung C02. C02 is a strongly bonded molecule and therefore it is difficult to reduce it back to useful products.

In nature, the C02 process of photosynthesis to carbohydrate ^¬ th. This complex process is very difficult to reproduce on an industrial scale. A currently technically feasible way is the electrochemical reduction of C02 dar. The carbon dioxide is converted with the supply of electrical energy in a higher energy product such as CO, CH4, C2H4 or C 1 -C 4 alcohols. The electrical energy, in turn, is preferably derived from renewable energy sources such as wind power or photovoltaics.

For the electrolysis of C02 usually metals are used as catalysts Kata ^¬. The type of metal influences the products of electrolysis. For example, CO 2 is reduced almost ^exclusively to CO, for example on Ag, Au, Zn, and with restrictions on Pd, Ga, whereas copper has a large number of hydrocarbons as reduction products. care is. In addition to pure metals and metal alloys and mixtures of metal and metal oxide which is catalytically effective co-, of interest because they may increase the Selekti ^¬ tivity of a specific hydrocarbon.

When C02-electrolysis, a gas diffusion electrode (GDE) are used as the cathode, similar to the chlor-alkali electrolysis to Zvi ^¬ rule the liquid electrolyte, the gaseous C02 and the solid silver particles to produce a three-phase boundary. In this case, an electrolytic cell, as known also from the fuel cell technology, used with two electrolyte compartments, the Elect ^¬ rolytkammern are separated by an ion exchange membrane. The working electrode is a porous gas diffusion electrode. It comprises a metal net on which a mixture of PTFE, activated carbon, a catalyst and other components is applied. It includes a pore system into which the

Reactants invade and react at the three-phase interfaces.

The counter electrode is a sheet applied with platinum or an iridium mixed oxide. The GDE is in contact with the electrolyte on one side. On the other hand, it is supplied with C02, which is forced through the GDE with overpressure (so-called convective mode of operation). The GDE can while holding various metals and metal compounds ent ^¬ that have a catalytic effect on the process. The functioning of a GDE is known, for example, from EP 297377 A2, EP 2444526 A2 and EP 2410079 A2.

Unlike the chlor-alkali electrolysis and fuel cell technology ^¬ material, the resulting product is gaseous and not liquid at the Koh ^¬ dioxide electrolysis. Furthermore, with the from the electrolyte ent ^¬ standing alkali or alkaline earth metal hydroxide used the C02 forming salts. Beispielswei ^¬ se is formed with the use of potassium salts as the electrolyte KOH and there are the salts of KHC03 and K2C03. by virtue of The operating conditions lead to a crystallization of the salts in and on the GDE from the gas side.

The electrochemical conversion of CO 2 to silver electrodes takes place according to the following equation:

Cathode: C02 + 2e- + H20 -> CO + 20H- with the backreaction

Anode: 6H20 -> 02 + 4e- + 4H30 +

Due to the electrochemical conditions, the charge balance of the chemical equations is not uniform with H30 + or OH-. Despite acidic electrolyte, the GDE has local alkaline pH values. For operating a see alkaline fuel cell technology, the introduced oxygen must be C02-free, because otherwise KHCO would form K2C03 as fol ^¬ constricting equations /:

C02 + KOH -> KHCO3

C02 + 2KOH -> K2C03 + H20

The same process is now also observed in the C02 electrolysis, with the difference that the fed can not be C02-free. As a result, after a finite time (depending on the current density), salt in and on the GDE crystallizes from the gas side and clogs the pores of the GDE. The gas pressure rises, the GDE is heavily loaded and tears from a certain pressure. In addition, the potassium ions required for the process are removed from the process and the gas space is gradually filled with salt. An analogous process is observed with ande ren ^¬ alkali / alkaline earth metals, such as cesium.

A stable long-term operation of the gas diffusion electrode in the range of more than 1000 h is not possible in the C02 electrolysis, since the resulting salt clogs the pores of the GDE and thus this gas-impermeable. It is an object of the present invention to provide an improved arrangement for the carbon dioxide electrolysis and a method for operating an arrangement for the carbon dioxide electrolysis, with the stable long-term operation while avoiding the aforementioned disadvantages is made possible.

This object is achieved by an arrangement having the features of claim 1. As for the method, there is a Lö ^¬ solution in the method having the features of claim 9. The subclaims relate to advantageous embodiments of the arrangement.

The arrangement according to the invention for the carbon dioxide electrolysis comprises an electrolysis cell with an anode and a cathode, which are both connected to a power supply. In this case, the cathode is designed as a gas diffusion electrode, to which a gas space is connected on a first side and a cathode space on a second side. Furthermore, the arrangement comprises a sequent to the electrolysis cell subse- electrolyte circulation and a gas supply for Zufüh ^¬ tion of carbon dioxide-containing gas into the gas space.

The gas space has an electrolyte outlet and the electrolyte outlet is provided with a shut-off device. DA in the arrangement is configured such that the shut ^¬ device is opened when the pressure difference between the gas space and cathode space exceeds a threshold.

In the method according to the invention for operating an arrangement for the carbon dioxide electrolysis with an electrolysis ^¬ cell having an anode and a cathode, wherein the anode and Ka ^¬ method are connected to a power supply, wherein the cathode is designed as a gas diffusion electrode to the on a gas space is connected to a first side and a cathode space to a second side, an electrolyte outlet provided with a shut-off device is provided in the gas space, a pressure difference between the gas space and the cathode space is determined and the shut-off device is opened when the pressure difference exceeds a threshold value.

For the invention, it was recognized that the voltage-driven electrolyte pumping effect by the gas diffusion electrode allows a structurally simple solution in order to avoid the salification at the gas diffusion electrode in the CO 2 electrolysis. The shut-off ensures that the pressure difference is not too high and thus the electrolyte flow through the gas diffusion electrode permanently stops. As a result, the salts which form are removed by the electrolyte in situ before ^¬ geous. This enables a permanent operation of the electrolysis. Advantageous embodiments of the device according to the invention will become apparent from the dependent of claim 1 claims. In this case, the embodiment can be combined according to claim 1 with the features of one of the subclaims or preferably also with those of several subclaims. Accordingly, the following features can additionally be provided for the arrangement:

- The shut-off device may be in particular a Absperrschie ^¬ ber, a butterfly valve or ball valve. The shut-off device can be a safety valve (pressure relief valve) or a proportional valve. Advantageously, an over ^¬ pressure valve requires no control, but opens automatically when the threshold value for the pressure difference between the gas space and the cathode space is exceeded.

- The electrolyte outlet in the gas space is preferably arranged on the bottom side, so that an outflow of the electrolyte made ^¬ light. - A first pressure sensor may be present in the gas space. This is a pressure signal, for example, to a control ^¬ device for controlling the shut-off device. A second pressure sensor may be arranged in the cathode compartment. This may also give a pressure signal to the controller. From the two pressure signals, the control device can determine the pressure difference and make a control of the shut-off device.

- Alternatively, a differential pressure sensor for gas space and cathode space may be present. This gives directly a signal for the pressure difference to a control device or directly to the shut-off device.

- The electrolyte outlet can be connected to the electrolyte circuit. Advantageously, the electrolyte discharged via the shut-off device can subsequently be returned to the system. Thus, the electrolyte is not consumed. The C0 ₂ feed gas flow is not influenced in this case and thus a sufficient C0 ₂ supply of the process is ensured.

- A controller may be provided ausgestal- tet to control the shut-off function of the Druckdif ^¬ ference.

- The shut-off device can be operated so that the pressure difference between the gas space and the cathode space remains within a definable interval. It remains preferred always in the gas space, a higher pressure than in the cathode compartment. The interval can be narrow, so that, for example, the pressure difference does not fluctuate by more than 10% or not more than 5%.

A preferred, but by no means limitative exporting ^¬ approximately example of the invention will now be further explained with reference to Figu ^¬ ren the drawing. The features are shown schematically.

The construction of an electrolysis cell 11, shown schematically in Figure 1 is typically adapted to carry out a carbon dioxide ^¬ electrolysis. In this case, the guide shape of the electrolytic cell 11 at least one anode 13 with adjacent anode space 12 and a cathode 15 and egg ^¬ NEN adjacent cathode space 14. Anodenraum 12 and cathode space 14 are separated by a membrane 21 from each other. Depending on the electrolyte solution used, a construction without membrane 21 is also conceivable in which a pH compensation then exceeds that of the membrane 21.

Anode 13 and cathode 15 are electrically connected to a voltage supply 22, which is controlled by the control unit 23. The control unit 23 can ^apply a protective voltage or an operating voltage to the electrodes 13, 15, that is to say the anode 13 and the cathode 15. The anode compartment 12 of the electrolysis cell 11 shown is equipped with an electrolyte inlet. Likewise, the illustrated Ano ^¬ denraum 12 includes an outlet for electrolyte and, for example, oxygen O ₂ or other gaseous by-product, which is gebil ^¬ det in the carbon dioxide electrolysis at the anode 13. In the case of a chloride-containing anolyte, for example, chlorine gas is formed. The cathode compartment 14 also has depending ^¬ weils at least one product and electrolyte outlet on. Since ^¬ the total electrolysis ^product can be composed of a variety of electrolysis products. The electrolytic cell 11 is further designed in a three-chamber structure, in which the carbon dioxide CO _{2 is} flowed through the cathode 15 designed as a gas diffusion electrode in the Katho ^¬ denraum 14. Gas diffusion electrodes make it possible to bring a solid catalyst th a liquid electrolytically, and a gaseous Elektrolyseedukt in contact MITEI ^¬ Nander. For this purpose, for example, the catalyst can be made porous and take over the electrode function, or a porous electrode takes over the catalyst function. The pore system of the electrode is designed so that the liquid and the gaseous phase can equally penetrate into the pore system and present therein or at its electrically accessible surface at the same time. can. An example of a gas diffusion electrode is an oxygen discharge electrode.

For the embodiment as a gas diffusion electrode comprising the Ka Thode 15 in this example, a metal mesh on which a Mi ^¬ research made of PTFE, activated carbon and a catalyst is applied. For the introduction of the carbon dioxide C02 in the

Catholyte includes the electrolytic cell 11 is a Koh ^¬ lenstoffdioxideinlass 24 16 into the gas space, the carbon dioxide reaches the gas chamber 16, the cathode 15 and may there penetrate into the porous structure of the cathode 15 and arrive to the reaction.

Furthermore, the arrangement 10 comprises an electrolyte circuit 20, via which the anode space 12 and the cathode space 14 are supplied with a liquid electrolyte, for example K 2 SO 4, KHCO 3, KOH, Cs 2 SO 4, and the electrolyte is returned to a reservoir 19. The circulation of the electrolyte in the electrolyte ^¬ rolytkreislauf 20 is effected by a pump 18th

The gas space 16 in the present example comprises an electrolyte outlet 25, which is arranged in the bottom area. The electrolyte outlet 25 leads via a pressure-controlled proportional valve 32 to the reservoir 19. Furthermore, a first pressure sensor 31 is present which measures the pressure in the gas space 16 and a second pressure sensor 30 for measuring the pressure in the cathode space 14.

The control device 23 receives the measurement signals of the pressure sensors 30, 31 and determines the pressure difference between the cathode chamber 14 and the gas chamber 16. If the pressure difference exceeds a definable threshold, the valve 32 is opened so that accumulated electrolyte from the gas space 16 from ^¬ run can. The electrolyte is returned to the reservoir 19. If the pressure difference falls below the threshold value or a second threshold value, the valve is closed. When starting the electrolysis, despite an overpressure on the gas side, ie in the gas space 16 due to the applied electrical voltage at the cathode 15 electrolyte from the Katholytraum 14 through the gas diffusion electrode, so the cathode 15, "pumped" in the direction of the gas space 16. It arises 16 drops on the surface of the cathode 15 which coalesce and collect in the form of a film in the lower portion of the cathode 15. The accumulating electrolyte thereby causes a drop

Pressure increase in the gas space 16, whereupon after a short time (about 30 min), the voltage-electrolyte pumping effect by the cathode 15 comes to a standstill. There is no electrolyte replenished, the gas space 16 dries out, the transported salt crystallizes out and thus clogs the pores of the cathode 15.

By operating the valve 32 the electrolysis of all ^¬ recently is constantly operated in a certain differential pressure area between gas space 16 and the electrolyte. This will maintain the "electrolyte pump" through the cathode 15 and avoid salinisation of the gas diffusion electrode

Gas space 16 can be drained again.

Claims

claims

1. Arrangement for the carbon dioxide electrolysis, comprising

- An electrolytic cell (11) having an anode (13) and a cathode (15), said anode (13) and cathode (15) having a

Voltage supply (22) are connected, wherein the cathode (15) is designed as a gas diffusion electrode to which on ei ^¬ ner first side, a gas space (16) and on a second side of a cathode space (14),

- one of the electrolytic cell (11) subsequent electric ^¬ lyte circuit (20)

a gas feed (17) for feeding carbon dioxide-containing gas into the gas space (16),

characterized in that

the gas space (16) has an electrolyte outlet (25) and the electrolyte outlet (25) with a shut-off device (32) is provided, configured such that the Absperreinrich ^¬ device (32) is opened when the pressure difference between the gas space ( 16) and cathode space (14) exceeds a threshold value.

2. Arrangement according to claim 1, wherein the shut-off device (32) is a pressure relief valve (32).

3. Arrangement according to claim 1 or 2, wherein the electrolyte outlet (25) in the gas space (16) is arranged on the bottom side.

4. Arrangement according to one of the preceding claims with a first pressure sensor (31) for the gas space (16).

5. Arrangement according to one of the preceding claims with a second pressure sensor (30) for the cathode space (14).

6. Arrangement according to one of claims 1 to 3 with a differential pressure sensor for gas space (16) and cathode space (14).

7. Arrangement according to one of the preceding claims, wherein the electrolyte outlet (25) with the electrolyte circuit (20) is connected.

8. Arrangement according to one of the preceding claims with a control device (23) which is designed to control the Ab ^¬ blocking device (32) in dependence on the pressure difference.

9. A method for operating an arrangement for the carbon dioxide electrolysis with an electrolytic cell (11) having an anode (13) and a cathode (15), said anode (13) and Ka ^¬ method (15) connected to a power supply (22) in which the cathode (15) is designed as a gas diffusion electrode, to which a gas space (16) adjoins on a first side and a cathode space (14) on a second side, wherein carbon dioxide-containing gas is conducted into the gas space (16), characterized in that

in the gas space (16) is provided an electrolyte outlet (25) which is provided with a shut-off device (32),

a pressure difference between gas space (16) and cathode space (14) is determined,

- The shut-off device (32) is opened when the pressure difference exceeds a threshold value.

10. The method of claim 9, wherein the shut-off device (32) is operated so that the pressure difference between the gas space (16) and the cathode space (14) remains within a definable interval.