EP3788183A1 - Gas diffusion electrode for carbon dioxide utilization, method for producing same, and electrolytic cell having a gas diffusion electrode - Google Patents

Abstract

The invention relates to a gas diffusion electrode (1) for carbon dioxide utilization, comprising a metal substrate (7) and an electrically conductive catalyst layer (13), which is applied to the metal substrate and has hydrophilic pores and/or channels and hydrophobic pores and/or channels, wherein: the catalyst layer (13) comprises metal particles (3) and a first polymeric binding material (5); and a porous gas diffusion layer (9) containing the first polymeric binding material (5) is formed on the surface (11) of the catalyst layer (13). The invention further relates to a method for producing a gas diffusion electrode (1) for CO2 utilization and an electrolytic cell having a corresponding gas diffusion electrode (1).

Description

description

Gas diffusion electrode for carbon dioxide utilization, process for its production and electrolysis cell with gas diffusion on electrode

The present invention relates to a Gasdiffusionselektro de for carbon dioxide utilization and a method for the manufacture of a gas diffusion electrode. The invention further relates to an electrolysis system with a corresponding gas diffusion electrode.

Around 80% of global energy requirements are currently met by burning fossil fuels. These combustion processes mean that around the world annually

34,000 million tons of the greenhouse gas carbon dioxide (carbon dioxide, CO2) emitted into the atmosphere. As a result of this release into the atmosphere, most of the carbon dioxide is disposed of (over 50,000 tonnes per day for large lignite-fired power plants).

Due to the growing scarcity of fossil resources

Fuels and the volatile availability of regenerative energy sources are increasingly turning the focus of research into the C0 ₂ reduction. This would reduce CO2 emissions and the CO2 could be used as an inexpensive source of carbon.

The discussion about the negative effects of CO2 on the climate has led to the consideration of CO2 recycling. However, from a thermodynamic point of view, CO2 is very low and can therefore only be reduced to usable products with difficulty.

Natural carbon dioxide degradation takes place, for example, through photosynthesis. In this process, carbon dioxide is converted into carbohydrates in a process that is divided into many sub-steps in terms of time and on a molecular level. However, this process is not readily transferable to an industrial scale. A copy of the natural photosynthesis process with large-scale photo-catalysis has not yet been sufficiently efficient.

Another method is the electrochemical reduction of carbon dioxide. Systematic studies of the electrochemical reduction of carbon dioxide are still a relatively young field of development. Only a few years ago there have been efforts to develop an electrochemical system that can reduce an acceptable amount of carbon dioxide.

Electrolysis systems with gas diffusion electrodes are now increasingly being used for this purpose. These usually consist of a cathode and an anode compartment. To achieve an effective implementation of the CO _{2 used} , the method is ideally carried out as a porous gas diffusion electrode. Gas diffusion electrodes (GDE) are porous electrodes in which liquid, solid and gaseous phases are present and the electrically conductive catalyst catalyzes the electrochemical reaction between the liquid and the gaseous phase.

A typical problem with gas diffusion electrodes used for CO 2 reduction in contact with an electrolyte (for example KHCO3, K2SO4, KOH or mixtures thereof) is to avoid undesirable side reactions in the electrolyte-side area of the gas diffusion electrode. The gas diffusion electrode must ensure a sufficient supply of the catalytically active centers with CO2 and the respective electrolyte through gas and ion transport.

Gas diffusion electrodes are often produced using a roll calendaring process, as is the case in

US 2013 010 190 6 Al is disclosed. Here are used for the formation of the gas diffusion electrode catalytic active metallic particles mixed with hydrophobic particles such as PTFE, the resulting mixture is applied to a metallic carrier and arranged between two PTFE films before it is calendered. The shear force allows the PTFE to flow and a network of PTFE binds the catalytically active particles together. The compression force and the particle size allow the porosity and to a certain extent also the pore size to be modified.

However, gas diffusion electrodes manufactured in this way tend to have the limiting pore diameter only within the gas diffusion electrode, but not on the surface. The pore size diameter on the electrolyte side is comparatively large compared to gas diffusion electrodes with a gas diffusion layer, so that an undesirable flooding behavior of the pore system is observed. Furthermore, gas diffusion electrodes with large pore openings or low hydrophobicity show a strong electrolyte permeation through the respective gas diffusion electrode. The permeation is controlled by the electrical field gradient, i.e. electro-osmosis. This effect is followed by an undesirable decrease in Faraday's efficiency of the gaseous products CO or ethylene.

The invention is therefore based on the object of specifying a more efficient way of electrochemical CO ₂ recycling compared to the prior art.

This object is achieved according to the invention by the features of claims 1, 9 and 11. Advantageous refinements of the invention are set out in the subclaims and the description below.

The gas diffusion electrode according to the invention is used for carbon dioxide utilization and comprises a metallic carrier and an electrically conductive catalyst layer applied thereon with hydrophilic pores and / or channels and hydrophobic pores and / or channels. The catalytic converter The gate layer comprises metallic particles and a first polymeric binding material, a porous gas diffusion layer containing the first polymeric binding material being formed on the surface of the catalyst layer.

The polymer gas diffusion layer formed on the surface (the side of the catalyst layer or the gas electrode facing the electrolyte in an electrolysis cell) of the catalyst layer represents the reaction zone outside the gas diffusion electrode, which is in contact with the reactant, in particular CO2 and the electrolyte.

The gas diffusion layer represents the limiting Po diameter for the entire gas diffusion electrode. Through the targeted choice of the binding material used, the size of the metallic, catalytically active particles and the parameters in the manufacturing process of the gas diffusion electrode, both the degree of hydrophobicity and the pore size of the gas diffusion layer can be controlled.

The gas diffusion layer preferably has a porosity above 70%. The thickness of the gas diffusion layer is preferably in a range between 150 μm and 500 μm.

The thickness of the catalyst layer is preferably in a range between 5 nm and 500 nm. By means of a catalyst layer designed in this way, the differential pressure based on the passage of a fluid medium through the catalyst layer and the hydrostatic pressure based on the passage of a fluid medium can be influence or adjust by the boundary layer.

A fluoropolymer is preferably used as the first polymeric binding material. In particular, 3% by weight to 15% by weight of the first polymeric binding material is used. The use of polyvinylidene fluoride is particularly suitable here

(PVDF). This polymer allows the formation of the desired surface layer in the manufacture of the gas diffusion electrode. In addition, the first polymeric binder material is preferably partially embedded within the pores and / or channels of the catalyst layer. As a result, a hydrophobic “sub-network” is additionally formed within the pores of the catalyst layer, which increases the overall hydrophobicity of the catalyst layer and thus of the gas diffusion electrode.

With a gas diffusion layer designed in this way, the differential pressure based on the passage of a fluid medium through the gas diffusion layer and the hydrostatic pressure based on the passage of a fluid medium through the gas diffusion layer can be influenced or set. The differential pressure based on the passage of a fluid medium through the gas diffusion layer is in a range between 20 mbar and 220 mbar, in particular in a range between 60 mbar and 200 mbar. The hydrostatic pressure based on the passage of a fluid medium through the gas diffusion layer is preferably in a range between 20 mbar and 1000 mbar, and in particular in a range between 200 mbar and 1000 mbar.

The pore size of the catalyst layer is preferably in a range between 0.3 μm and 5 μm. In particular, the pore size of the catalyst layer is in a range between 2 μm and 3 μm. The particle size of the metallic particles is preferably in a range between 500 nm and 5 μm and particularly preferably in a range between 2 μm and 3 μm

The metallic particles are preferably at least partially coated with a second polymeric binding material. This further increases the hydrophobicity of the gas diffusion electrode. PTFE (polytetrafluoroethylene) is preferably used as the second polymeric binder material (binder polymer). Silver particles are preferably used as metallic particles. The use of copper particles or other catalytically active particles is also possible. The metallic particles used are preferably at least partially coated with the first polymeric binding material.

The metallic carrier is preferably formed as a metallic network (or a corresponding sheet of wire). The material of the carrier is expediently matched to the metallic particles used. A silver mesh is preferably used as the metallic support.

The gas diffusion electrode is particularly preferably manufactured by means of an extraction process. This method enables the growth of the desired thin gas diffusion layer on the surface of the catalyst layer of the gas diffusion electrode.

The inventive method is used to produce a gas diffusion electrode for C0 _{2 ~} utilization. The method comprises the mixing of metallic particles with a first binding material with the formation of a suspension, the application of the suspension onto a metallic carrier, as well as the introduction of the metallic carrier loaded with the suspension into a precipitation bath with the formation of a

electrically conductive catalyst layer. A porous gas diffusion layer containing the first polymeric binding material forms on the surface of the catalyst layer within the precipitation bath.

The process described above is an extraction process ("inversion casting" process, phase inversion). By manufacturing the gas diffusion electrode by means of this process, a thin gas diffusion layer can be produced on the surface of the catalyst layer, the pore diameter of which is limiting for the The entire gas diffusion electrode has an influence on the degree of hydrophobicity as well as the pore size of the gas diffusion layer here in the first binding material, the size of the metallic, ca talytically active particles and the parameters in the manufacturing process of the gas diffusion electrode.

Furthermore, an intensive connection between the metallic particles and the binding materials used is achieved by the extraction process, whereby the mechanical stability of the gas diffusion electrode is improved compared to common gas diffusion electrodes.

The production of “tailor-made” gas diffusion layer on the surface of the catalyst layer of gas diffusion electrodes is particularly advantageous because the Faraday 'see efficiency is improved due to the reduced risk of flooding the gas diffusion electrode and thus two-phase side reactions such as hydrogen development are reduced ,

Furthermore, the gas diffusion electrode can be made smaller, since a larger differential pressure by the gas diffusion electrode is less sensitive to the hydrostatic pressure of the electrolyte.

In addition, the production of the gas diffusion electrode is associated with less effort, since one process step, namely the activation of the electrode (oxidation of additional metal oxides) can be omitted. The metallic particles can be used directly.

It is also advantageous that the gas diffusion electrode is easier to integrate into an electrolysis system due to the lower passage of electrolyte compared to common electrodes.

A mixture of water and isopropanol is expediently used as the precipitation bath. This mixture represents a so-called "non-solvent" for the polymeric binding materials and, due to diffusion, causes an exchange of solvent and non-solvent and thus a phase separation. The first polymeric binding material solidifies and forms the gas diffusion layer on the surface of the catalyst layer.

The advantages and preferred embodiments described for the gas diffusion electrode according to the invention apply equally to the method according to the invention and can be applied accordingly to this.

The electrolytic cell according to the invention comprises a gas diffusion electrode according to one of the above-described embodiments. The gas diffusion electrode is preferably used here as a method. The electrolysis cell is expediently designed on the cathode side for reducing carbon dioxide.

The other components of the electrolytic cell, such as the anode, possibly one or more membranes, supply lines) and discharge line (s), the voltage source and further optional devices such as cooling or heating devices are fundamentally variable according to the invention. The same applies to the anolytes and / or catholytes which are used in such an electrolytic cell.

Overall, the use of a gas diffusion electrode according to the invention in a corresponding electrolysis system or in an electrolysis cell leads to better efficiency of the electrochemical system and thus makes the end product more competitive. This is additionally supported by the manufacturing method of the gas diffusion sensor according to the invention.

Exemplary embodiments of the invention are explained in more detail below with reference to a drawing. Show:

1 shows a schematic representation of a section of a gas diffusion electrode manufactured by means of an extraction process, 2 shows a schematic representation of a section of a gas diffusion electrode manufactured by means of a calendaring process,

3 shows a section of the gas diffusion electrode according to

 FIG 2, and

4 shows a further section of the gas diffusion electrode according to FIG. 2.

1 shows a schematic representation of a section of a gas diffusion electrode 1 produced by means of an extraction process. For this purpose, a suspension with metallic particles 3 and a first polymeric binder material 5 is applied to a metallic carrier 7 (only indicated by an arrow). Phase inversion (immersion of the coated metallic carrier 7 in a “non” solvent) solidifies the first binding material 5 and forms a gas diffusion layer 9 on the upper surface 11 of the catalyst layer 13 of the gas diffusion electrode 1.

The reaction between a gaseous reactant 15, in the present case CO 2, and the electrolyte 17 on the metallic particles 3 then takes place in this gas diffusion layer 9. This is a 3-phase reaction in which a conversion of CO2 to CO takes place at the phase boundary 19 between the metallic particles 3 in the gas diffusion layer 9, the CO2 and the electrolyte 17 (see also FIG. 4).

2 shows a schematic representation of a section of a gas diffusion electrode 21 produced by means of a calendaring process. Here, the reaction of the CO2 also takes place in the electrolyte 17, which leads to undesirable side reactions. The gas diffusion electrode 21 has larger pores in the upper surface due to the manufacturing process, so that there is a risk of undesired flooding of the gas diffusion electrode 21. 3 and 4 each show corresponding sections 25, 27 of the 2-phase reactions (section 25) and the 3-phase reactions (section 27) according to FIG. 2. A 2-phase reaction is shown in FIG. This takes place within the electrolyte 17 and, as already described, leads to undesired by-products. 3 shows a 3-phase reaction in which a reaction of CO2 takes place within the gas diffusion layer 9 of the gas diffusion electrode 1.

The advantages described for the gas diffusion electrode according to the invention and the method according to the invention and before preferred embodiments apply equally to the electrolysis cell according to the invention and can accordingly be correspondingly transferred to it.

LIST OF REFERENCE NUMBERS

1 gas diffusion electrode

 3 metallic particles

 5 polymeric binding material

 7 metallic carrier

 9 surface layer

 11 catalyst surface

 13 catalyst layer

15 reactant

 17 electrolyte

 18 surface layer

19 phase boundary

21 gas diffusion electrode

25 Section of gas diffusion electrode

27 Section of gas diffusion electrode

Claims

claims 1. Gas diffusion electrode (1) for carbon dioxide utilization, comprising a metallic support (7) and an electrically conductive catalyst layer (13) applied thereon with hydrophilic pores and / or channels and hydrophobic pores and / or channels, the catalyst layer (13 ) comprises metallic particles (3) and a first polymeric binding material (5), and wherein a porous gas diffusion layer (9) containing the first polymeric binding material (5) is formed on the surface (11) of the catalyst layer (13). 2. Gas diffusion electrode (1) according to claim 1, wherein the gas diffusion layer (9) has a porosity above 70%. 3. Gas diffusion electrode (1) according to claim 1 or 2, wherein a fluoropolymer is used as the first polymeric binding material (5). 4. Gas diffusion electrode (1) according to one of the preceding claims, wherein the thickness of the catalyst layer (13) is in a range between 5 nm and 500 nm. 5. gas diffusion electrode (1) according to any one of the preceding claims, wherein the first polymeric binder material (5) antei lig within the pores and / or channels of the catalyst layer (13) is embedded. 6. Gas diffusion electrode (1) according to one of the preceding claims, wherein the differential pressure based on the passage of a fluid medium through the gas diffusion layer (9) in a range between 20 mbar and 220 mbar, in particular in a range between 60 mbar and 200 mbar lies. 7. Gas diffusion electrode (1) according to one of the preceding claims, wherein the hydrostatic pressure based on the passage of a fluid medium through the gas diffusion Layer (9) lies in a range between 20 mbar and 1000 mbar, in particular in a range between 200 mbar and 1000 mbar. 8. Gas diffusion electrode (1) according to one of the preceding claims, wherein the metallic particles (3) are coated at least in some areas with a second polymeric binding material. 9. A method for producing a gas diffusion electrode (1) for C0 _{2 ~} utilization, comprising  Mixing metallic particles (3) with a first binding material (5) to form a suspension,  Applying the suspension to a metallic carrier (7), as well  Introducing the metallic carrier (7) loaded with the suspension into a precipitation bath to form an electrically conductive catalyst layer (13), wherein a porous gas diffusion layer (9) containing the first polymeric binding material (5) forms on the surface (11) of the catalyst layer (13) within the precipitation bath. 10. The method according to claim 9, wherein a Mi mixture of water and isopropanol is used as the precipitation bath. 11. Electrolysis cell, comprising a gas diffusion electrode (1) according to one of claims 1 to 8.