DE102016211819A1 - Arrangement and method for carbon dioxide electrolysis - Google Patents

Abstract

The invention relates to an arrangement for the carbon dioxide electrolysis and a method for its operation, comprising an electrolysis cell with an anode and a cathode, wherein anode and cathode are connected to a power supply, wherein the cathode is designed as a gas diffusion electrode, on a first Side a gas chamber and on a second side of a cathode compartment, an adjoining the electrolytic cell electrolyte circuit and a gas supply for supplying carbon dioxide-containing gas into the gas space, wherein the gas space has an electrolyte outlet and the electrolyte outlet is provided with a shut-off device configured such that the shut-off device is opened when the pressure difference between the gas space and the cathode space exceeds a threshold value.

Description

The invention relates to an arrangement and a method for the carbon dioxide electrolysis according to the preamble of claim 1.

The burning of fossil fuels currently covers about 80% of global energy needs. In 2011, these combustion processes emitted around 34,000 million tonnes of carbon dioxide (CO2) into the atmosphere worldwide. This release is the easiest way to dispose of even large amounts of CO2 (large lignite-fired power plants over 50000 t per day).

The discussion about the negative effects of the greenhouse gas CO2 on the climate has led to a reflection on the reuse of CO2. CO2 is a strongly bound molecule and therefore can hardly be reduced back to useful products.

In nature, CO2 is converted into carbohydrates through photosynthesis. This complex process is very difficult to reproduce on an industrial scale. A currently technically feasible way is the electrochemical reduction of CO2. Here, the carbon dioxide is converted by supplying electrical energy in a higher energy product such as CO, CH4, C2H4 or C1-C4 alcohols. The electrical energy, in turn, preferably comes from renewable energy sources such as wind power or photovoltaic.

As a rule, metals are used as catalysts for the electrolysis of CO2. The type of metal influences the products of electrolysis. For example, CO2 on Ag, Au, Zn, and with restrictions on Pd, Ga, is reduced almost exclusively to CO, whereas copper has a large number of hydrocarbons as reduction products. In addition to pure metals, metal alloys as well as mixtures of metal and metal oxide, which is cocatalytically active, are of interest because they can increase the selectivity of a particular hydrocarbon.

In the case of CO2 electrolysis, a gas diffusion electrode (GDE) can be used as a cathode similar to the chlor-alkali electrolysis to establish a three-phase boundary between the liquid electrolyte, the gaseous CO2, and the solid silver particles. In this case, an electrolytic cell, as also known from the fuel cell technology, is used with two electrolyte chambers, wherein the electrolyte chambers 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 penetrate 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 CO2, which is forced through the GDE with overpressure (so-called convective mode of operation). The GDE may contain various metals and metal compounds that have a catalytic effect on the process. The functioning of a GDE is, for example, from the EP 297377 A2 , of the EP 2444526 A2 and the EP 2410079 A2 known.

In contrast to chlor-alkali electrolysis and fuel cell technology, the resulting product in the carbon dioxide electrolysis is gaseous and not liquid. Furthermore, the CO2 used forms salts with the alkali or alkaline earth metal hydroxide formed from the electrolyte. For example, when using potassium salts as electrolytes KOH is formed and the salts KHCO3 and K2CO3 are formed. Due to the operating conditions, there is a crystallization of the salts in and on the GDE from the gas side.

The electrochemical conversion of CO2 to silver electrodes is carried out according to the following equation: Cathode: CO2 + 2e- + H2O → CO + 2OH- with the backlash Anode: 6H2O → O2 + 4e- + 4H3O +

Due to the electrochemical conditions, the charge balance of the chemical equations is not uniform with H3O + or OH-. Despite acidic electrolyte, the GDE has local alkaline pH values. To operate an alkaline fuel cell technology, the introduced oxygen must be CO2-free, since otherwise KHCO / K2CO3 would form according to the following equations: CO2 + KOH → KHCO3 CO2 + 2KOH → K2CO3 + H2O

The same process can now also be observed with CO2 electrolysis, with the difference that the feed can not be CO2-free. As a result, after a finite period of 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 becomes strong loads and tears off 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 can be observed with other alkali / alkaline earth metals, for example cesium.

A stable long-term operation of the gas diffusion electrode in the range of more than 1000 h is not possible in the CO2 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 a 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. With regard to the method, there is a solution in the method with the features of claim 9. The subclaims relate to advantageous embodiments of the arrangement.

The arrangement for the carbon dioxide electrolysis according to the invention comprises an electrolytic cell with an anode and a cathode, both of which are 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 subsequent to the electrolytic cell electrolyte circuit and a gas supply for supplying 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. In this case, the arrangement is configured such that the shut-off device is opened when the pressure difference between the gas space and the cathode space exceeds a threshold value.

In the inventive method for operating an arrangement for the carbon dioxide electrolysis with an electrolytic cell having an anode and a cathode, wherein the anode and cathode are connected to a power supply, wherein the cathode is designed as a gas diffusion electrode, to which on a first side of a gas space and Connected on a second side of a cathode compartment, provided with a shut-off device electrolyte outlet 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, advantageously, the salts forming are removed by the electrolyte in situ. This enables a permanent operation of the electrolysis.

• – Die Absperreinrichtung kann insbesondere ein Absperrschieber, eine Absperrklappe oder Kugelhahn sein. Die Absperreinrichtung kann ein Sicherheitsventil (Überdruckventil) oder ein Proportionalventil sein. Vorteilhaft benötigt ein Überdruckventil keine Steuerung, sondern öffnet automatisch bei Überschreiten des Schwellwerts für die Druckdifferenz zwischen Gasraum und Kathodenraum.
• – Der Elektrolyt-Auslass im Gasraum ist bevorzugt bodenseitig angeordnet, so dass ein Ausfließen des Elektrolyten ermöglicht wird.
• – Ein erster Drucksensor kann im Gasraum vorhanden sein. Dieser gibt ein Drucksignal beispielsweise an eine Steuerungseinrichtung zur Ansteuerung der Absperreinrichtung. Ein zweiter Drucksensor kann im Kathodenraum angeordnet sein. Dieser kann ebenfalls ein Drucksignal an die Steuerungseinrichtung geben. Aus den beiden Drucksignalen kann die Steuerungseinrichtung die Druckdifferenz bestimmen und eine Steuerung der Absperreinrichtung vornehmen.
• – Alternativ kann ein Differenzdrucksensor für Gasraum und Kathodenraum vorhanden sein. Dieser gibt direkt ein Signal für die Druckdifferenz an eine Steuerungseinrichtung oder direkt an die Absperreinrichtung.
• – Der Elektrolyt-Auslass kann mit dem Elektrolytkreislauf verbunden sein. Vorteilhaft kann damit der über die Absperreinrichtung abgelassene Elektrolyt anschließend dem System wieder zugeführt werden. Somit wird der Elektrolyt auch nicht verbraucht. Der CO₂-Feedgasstrom wird hierbei nicht beeinflusst und es wird somit eine ausreichende CO₂-Versorgung des Prozesses gewährleistet.
• – Eine Steuerungseinrichtung kann vorhanden sein, ausgestaltet, die Absperreinrichtung in Abhängigkeit von der Druckdifferenz zu steuern.
• – Die Absperreinrichtung kann so betrieben werden, dass die Druckdifferenz zwischen Gasraum und Kathodenraum in einem festlegbaren Intervall bleibt. Dabei bleibt bevorzugt stets im Gasraum ein höherer Druck als im Kathodenraum. Das Intervall kann schmal gewählt werden, so dass beispielsweise die Druckdifferenz um nicht mehr 10% oder nicht mehr als 5% schwankt.

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 in particular be a gate valve, a butterfly valve or ball valve. The shut-off device can be a safety valve (pressure relief valve) or a proportional valve. Advantageously, a pressure relief 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 is made possible.
• - A first pressure sensor may be present in the gas space. This gives 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 can 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 CO ₂ feed gas flow is not influenced in this case and thus a sufficient CO ₂ supply of the process is ensured.
• A control device may be provided, designed to control the shut-off device as a function of the pressure difference.
• - 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 limiting embodiment of the invention will now be explained in more detail with reference to the figures of the drawing. The features are shown schematically.

The in 1 schematically illustrated construction of an electrolytic cell 11 is typically suitable to carry out a carbon dioxide electrolysis. In this case, the embodiment of the electrolysis cell 11 at least one anode 13 with adjoining anode compartment 12 and a cathode 15 and an adjacent cathode compartment 14 , anode chamber 12 and cathode compartment 14 are through a membrane 21 separated from each other. Depending on the electrolyte solution used is also a structure without a membrane 21 conceivable, in which then a pH balance over that of the membrane 21 goes.

anode 13 and cathode 15 are electrical with a power supply 22 connected by the control unit 23 is controlled. The control unit 23 may be a protective voltage or an operating voltage to the electrodes 13 . 15 So the anode 13 and the cathode 15 , invest. The anode compartment 12 the electrolysis cell shown 11 is equipped with an electrolyte inlet. Likewise, the depicted anode compartment comprises 12 an outlet for electrolyte as well as, for example, oxygen O ₂ or another gaseous by-product formed in the carbon dioxide electrolysis at the anode 13 is formed. In the case of a chloride-containing anolyte, for example, chlorine gas is formed. The cathode compartment 14 also each has at least one product and electrolyte outlet. In this case, the total electrolysis product can be composed of a large number of electrolysis products.

The electrolytic cell 11 is further carried out in a three-chamber design in which the carbon dioxide CO ₂ via the designed as a gas diffusion electrode cathode 15 in the cathode compartment 14 is flowed. Gas diffusion electrodes make it possible to bring a solid catalyst, a liquid electrolyte and a gaseous electrolysis product into contact with each other. 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 can be present in it or on its electrically accessible surface simultaneously. An example of a gas diffusion electrode is an oxygen-consuming electrode.

For the embodiment as a gas diffusion electrode, the cathode comprises 15 in this example, a metal net on which a mixture of PTFE, activated carbon and a catalyst is applied. For introducing the carbon dioxide CO2 into the catholyte cycle, the electrolysis cell comprises 11 a carbon dioxide inlet 24 in the gas space 16 , The carbon dioxide reaches the gas space 16 the cathode 15 and can there in the porous structure of the cathode 15 penetrate and come to the reaction.

Furthermore, the arrangement comprises 10 an electrolyte circuit 20 over which the anode space 12 and the cathode compartment 14 is supplied with a liquid electrolyte, for example K2SO4, KHCO3, KOH, Cs2SO4 and the electrolyte in a reservoir 19 is returned. The circulation of the electrolyte in the electrolyte circuit 20 done by a pump 18 ,

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

The control device 23 takes the measuring signals of the pressure sensors 30 . 31 and determines the pressure difference between the cathode compartment 14 and the gas space 16 , If the pressure difference exceeds a definable threshold, the valve becomes 32 opened, so accumulated electrolyte from the gas space 16 can expire. The electrolyte is returned to the reservoir 19 directed. If the pressure difference falls below the threshold value or a second threshold value, the valve is closed.

When starting the electrolysis is despite an overpressure on the gas side, ie in the gas space 16 due to the applied voltage at the cathode 15 Electrolyte from the catholyte compartment 14 through the gas diffusion electrode, so the cathode 15 , in the direction of gas space 16 "Pumped". It arises on the side of the gas space 16 Drops on the surface of the cathode 15 that coalesce and settle at the bottom of the cathode 15 collect in the form of a movie.

The accumulating electrolyte thereby causes a pressure increase in the gas space 16 , whereupon, after a short time (about 30 min), the voltage-electrolyte pumping effect through the cathode 15 comes to a halt. 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 the operation of the valve 32 However, the electrolysis is constant in a certain differential pressure range between the gas space 16 and the electrolyte operated. This turns the "electrolyte pump" through the cathode 15 maintained and salinisation of the gas diffusion electrode avoided. At the same time it is ensured that the electrolyte in the gas space 16 can be drained again.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been 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 assumes no liability for any errors or omissions.

Cited patent literature

• EP 297377 A2 [0008]
• EP 2444526 A2 [0008]
• EP 2410079 A2 [0008]

Claims (10)

Arrangement for carbon dioxide electrolysis, comprising - an electrolytic cell ( 11 ) with an anode ( 13 ) and a cathode ( 15 ), where anode ( 13 ) and cathode ( 15 ) with a power supply ( 22 ), wherein the cathode ( 15 ) is designed as a gas diffusion electrode, to which on a first side a gas space ( 16 ) and on a second side a cathode compartment ( 14 ), - one to the electrolytic cell ( 11 ) subsequent electrolyte cycle ( 20 ), - a gas supply ( 17 ) for supplying carbon dioxide-containing gas into the gas space ( 16 ), characterized in that the gas space ( 16 ) an electrolyte outlet ( 25 ) and the electrolyte outlet ( 25 ) with a shut-off device ( 32 ), designed such that the shut-off device ( 32 ) is opened when the pressure difference between gas space ( 16 ) and cathode space ( 14 ) exceeds a threshold. Arrangement according to Claim 1, in which the shut-off device ( 32 ) a pressure relief valve ( 32 ). Arrangement according to claim 1 or 2, wherein the electrolyte outlet ( 25 ) in the gas space ( 16 ) is arranged on the bottom side. Arrangement according to one of the preceding claims with a first pressure sensor ( 31 ) for the gas space ( 16 ). Arrangement according to one of the preceding claims with a second pressure sensor ( 30 ) for the cathode compartment ( 14 ). Arrangement according to one of claims 1 to 3 with a differential pressure sensor for gas space ( 16 ) and cathode space ( 14 ). Arrangement according to one of the preceding claims, in which the electrolyte outlet ( 25 ) with the electrolyte circuit ( 20 ) connected is. Arrangement according to one of the preceding claims with a control device ( 23 ), which is designed, the shut-off device ( 32 ) as a function of the pressure difference. Method for operating an arrangement for the carbon dioxide electrolysis with an electrolysis cell ( 11 ) with an anode ( 13 ) and a cathode ( 15 ), where anode ( 13 ) and cathode ( 15 ) with a power supply ( 22 ), the cathode ( 15 ) is designed as a gas diffusion electrode, to which on a first side a gas space ( 16 ) and on a second side a cathode compartment ( 14 ), wherein carbon dioxide-containing gas in the gas space ( 16 ), characterized in that - in the gas space ( 16 ) with a shut-off device ( 32 ) provided electrolyte outlet ( 25 ), - a pressure difference between gas space ( 16 ) and cathode space ( 14 ), - the shut-off device ( 32 ) is opened when the pressure difference exceeds a threshold value. Method according to Claim 9, in which the shut-off device ( 32 ) is operated so that the pressure difference between gas space ( 16 ) and cathode space ( 14 ) remains within a definable interval.

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