Electrochemical conversion with humidification

Abstract

Process for electrochemical reduction of carbon dioxide in an electrochemical cell comprising a selective barrier and a cathode compartment comprising a carbon dioxide feed chamber separated from a catholyte chamber by a gas diffusion electrode which is located spatially apart from the selective barrier, wherein a carbon dioxide feed is introduced int the carbon dioxide feed chamber which carbon dioxide feed comprises carbon dioxide and water and has a temperature of from 50 to 100 °C.

Description

ACK346 FFElectrochemical conversion with humidificationField of the invention

[0001] The present invention relates to a process for electrochemical reduction of carbon dioxide.Background art

[0002] The electrochemical conversion of carbon dioxide into economically valuable materials such as fuels and industrial chemicals or intermediate products thereof is of interest in view of mitigating the emission of carbon dioxide into the atmosphere having damaging effects such as climate change, change in pH of seawater, melting of polar ice and sea level rise. A mechanism for mitigating emissions is to convert carbon dioxide into economically valuable materials such as fuels and industrial chemicals. Although many electrochemical reductions of carbon dioxide have been proposed in the art, it was found that further development and improvement is still needed to make carbon dioxide reduction economically viable.

[0003] T raditional three compartment electrochemical cells for the reduction of carbon dioxide comprise an anode compartment comprising an anode, a selective barrier, and a cathode compartment comprising a carbon dioxide feed chamber separated from a catholyte chamber by a gas diffusion electrode. During operation, liquid anolyte is fed to the anode compartment and liquid catholyte to the catholyte chamber. The advantage of such traditional cell over zero-gap electrochemical cells is that traditional cells are more simple and flexible in design and modular flexibility while their pH and ion concentration of the electrolytes are easier to control. Zero-gap devices can have increased acidification and therefore increased production of hydrogen. In traditional cells, electrolyte layers between the electrodes prevents this.

[0004] Furthermore, it is advantageous to use a gas diffusion electrode which does not need to be combined with a membrane, more especially an anion membrane. The absence of an anion membrane reduces complexity and allows more robust operation.

[0005] Unfortunately, it was found that salts tend to precipitate at the back-side of the gas diffusion electrode in traditional electrochemical cells. The “back-side” is the side of the gas diffusion electrode which receives carbon dioxide feed. The back-side also can be referred to as the downstream side when considering the flow of carbon dioxide through the gas diffusion electrode during normal operation.

[0006] The article “How alkali cations affect salt precipitation and CO2 electrolysis performance in membrane electrode assembly electrolyzers” by Sahil Garg et al., Energy Environ Sci., 2023, 16, 1631 , suggests to circumvent salt precipitation by using highly soluble alkali cation salts as the anolyte along with an optimal salt concentration. However, this would limit the process operating window to conditions directed at preventing salt formation. It is described on page 1640 under item (6) that CO2 electrolysis at elevated temperatures (60-80 °C) can reduce salt formation by increasing the solubility of carbonate salts. Humidification of the CO2 feed is not mentioned.

[0007] The article “Multilayer electrolyzer stack converts carbon dioxide to gas products at high pressure with high efficiency” by B. Endrbdi et al, ACS Energy Lett. 2019, 4, 1770-1777,ACK346 FF describes CO2 reduction in zero gap membrane electrolyzers which operate without catholyte. It mentions on page 1774 the problem of precipitation of K2CO3 at the cathode side thereby decreasing the performance of the electrolyzer by blocking the gas channels and the active catalyst sites. A solution which is proposed is to increase the temperature of the humidified CO2 inlet tot 85 °C and, thus, continuously feed more water vapor into the cell. The Faradic Efficiency to CO (FE CO) during the increased temperature operation was 65-70 % which is considerably lower compared than the values found in previous studies at lower inlet temperature. This trend can be attributed to the increased amount of water in the cell which increases the probability of hydrogen evolution reaction (HER).

[0008] Page 326 of the article “Zero-gap electrochemical CO2 reduction cells: challenges and operational strategies for prevention of salt precipitation”, ACS Energy Lett. 2023, 8, 321-331, notes that humidifying the gas feed of zero-gap membrane electrode assemblies lowers selectivity for CO2 reduction at higher current densities because of flooding of the cathode.

[0009] US 10047446 describes the use of humidified carbon dioxide for hydration of the anion exchange membrane which is combined with the gas diffusion electrode. Anion membranes are to be hydrated to maintain their conductivity in flow-through electrochemical operation. The central flow compartment is separated from the cathode compartment by a combination of an anion exchange membrane and a gas diffusion electrode.

[0010] CN114540882 describes bismuth metal nanosheets having abundant active sites and a process for their manufacture. These are used as catalysts to reduce carbon dioxide and water in a flow-by process in which carbon dioxide is passed along a cathode. Water is present in the compartment to which electrolyte is added. This compartment is at the downstream side of the gas diffusion electrode.

[0011] Surprisingly, it now has been found that salt accumulation can be reduced at high Faradic Efficiencies in electrochemical reduction of CO2 in a so-called three compartment gas diffusion electrode cell by humidification of the carbon dioxide feed at elevated temperature.

[0012] Therefore, the present invention relates to a process for electrochemical reduction of carbon dioxide in an electrochemical cell which electrochemical cell comprises (a) an anode compartment comprising an anode, (b) a selective barrier, and (c) a cathode compartment comprising a carbon dioxide feed chamber separated from a catholyte chamber by a gas diffusion electrode which is located spatially apart from the selective barrier, wherein a carbon dioxide feed is introduced into the carbon dioxide feed chamber which carbon dioxide feed comprises carbon dioxide and water and has a temperature of from 50 to 100 °C upon introduction in the carbon dioxide feed chamber.

[0013] The process can further comprise (i) introducing anolyte in the anode compartment, (ii) introducing catholyte in the catholyte chamber, and (Hi) applying an electrical potential between the gas diffusion electrode and the anode such that the carbon dioxide is reduced.

[0014] In a traditional electrochemical cell as used in the present process, the downstream side of the gas diffusion electrode is in direct contact with liquid electrolyte. Direct contact means that there is no selective barrier such as a membrane between the gas diffusion electrode and theACK346 FF continuous volume of electrolyte, more specifically catholyte. The downstream side is with respect to the flow of carbon dioxide through the gas diffusion electrode. This set-up differs from so-called zero-gap electrochemical cells in which a combination of anode, selective barrier and gas diffusion electrode separates the anode compartment from the cathode compartment, more especially the catholyte chamber. It further differs from electrochemical cells in which is used a combination of a gas diffusion electrode which is in direct contact with a membrane. In the cell of the present invention, the gas diffusion electrode is located spatially apart from any selective barrier more specifically any membrane. Surprisingly, it was found that humidification in such setup is easier as the amount of water added is less critical which allows for more robust operation. This is especially the case if aqueous catholyte is used in which case excess water can be removed with the catholyte.

[0015] The anode preferably also is located spatially apart from and not in direct contact with a membrane. Preferably, liquid electrolyte, more specifically anolyte, separates the anode from any selective barrier, more specifically membrane.

[0016] The anode preferably also is located spatially apart from and not in direct contact with a selective barrier or selective barrier such as a membrane. Preferably, liquid electrolyte separates the anode from any selective barrier.

[0017] The present process operates in so-called “flow-through” mode in which the flow of carbon dioxide is substantially perpendicular to the gas diffusion electrode. This is contrary to so- called “flow by” operation in which the flow of carbon dioxide is substantially parallel to the gas diffusion electrode.Brief description of the drawingFig. 1 displays a schematic overview of a system for carrying out a process according to the present invention.Detailed description

[0018] It is preferred that the carbon dioxide feed contains only a limited amount of compounds other than carbon dioxide and water. The total amount of carbon dioxide and water is as high as possible such as above 90 wt%, preferably above 95 wt%, more preferably above 99% wt% or even above 99.9 wt%, based on total amount of feed. In addition to CO2 and water some other gaseous species may be present, such as inert gases (N2, Ar) and / or H2. The presence of oxygen in the gas fed to the electrode is preferably avoided. The feed introduced into the carbon dioxide feed chamber preferably consists of carbon dioxide and water.

[0019] Carbon dioxide feed comprising carbon dioxide and water is introduced into the carbon dioxide feed chamber. While water can be introduced separate from carbon dioxide, it is preferred to introduce a mixture of carbon dioxide and water into the carbon dioxide feed chamber. Most preferably, both carbon dioxide and water are gaseous when introduced into the carbon dioxide feed chamber. The feed can suitably be formed by introducing carbon dioxide into water under pressure for example by bubbling or injecting gaseous carbon dioxide into the water with the helpACK346 FF of injectors or diffusers preferably at a temperature equal to or higher than the temperature of the electrochemical cell more specifically the highest temperature at the surface of the gas diffusion electrode.

[0020] It can be advantageous to introduce carbon dioxide at a rate higher than the conversion rate. Excess can be as high as up to 7 times the carbon dioxide conversion rate. Such excess has the advantage that it improves carbon dioxide mass transport. Introducing excess water has the advantage that the ensuing higher liquid flow rate allows for gas to be removed quicker which lowers the cell voltage.

[0021] The exact amount of water added to the carbon dioxide feed chamber depends on the amount of salt which tends to be formed. It is advantageous to ensure that the amount is at least sufficient to substantially remove the salt. The carbon dioxide feed preferably comprises at least 2 % by weight (%wt) of water, based on total amount of carbon dioxide and water when introduced into the carbon dioxide feed chamber, more preferably at least 4 % wt, more preferably at least 6 %wt. The amount of water in the carbon dioxide feed, based on total amount of carbon dioxide and water, preferably is at most 50 %wt, more preferably at most 40 %wt, more preferably at most 35 %wt. The carbon dioxide feed preferably is saturated with water at the operating conditions prevailing in the carbon dioxide feed chamber.

[0022] The temperature of the carbon dioxide feed upon introduction into the carbon dioxide feed chamber is of from 50 to 100 °C. The temperature is as measured upon introduction of the carbon dioxide and water in the carbon dioxide feed chamber. If the temperature of the carbon dioxide differs from the temperature of the water, the weight average temperature of the carbon dioxide and water is to be used. The temperature of the mixture of carbon dioxide and water preferably is at least 60 °C, more preferably at least 65 °C, more preferably at least 70 °C. The temperature of the mixture of carbon dioxide and water preferably is at most 95 °C at most 90 °C, more preferably at most 85 °C, more preferably at most 80 °C.

[0023] Preferably, the carbon dioxide feed chamber comprises a flow plate upstream of the gas diffusion electrode. A flow plate can contribute to guiding condensed water along the gas diffusion electrode. The flow plate suitably is a mesh, more specifically a mesh which is electrochemically inert. Preferably, the flow plate is a plastic mesh. The expression “upstream” is with respect to the flow of carbon dioxide feed during normal operation.

[0024] As is well known in the art, a pressure difference is the main cause for gas to flow through a gas diffusion electrode. In the current process, the pressure will be higher in the carbon dioxide feed chamber than in the catholyte chamber.

[0025] The gas diffusion electrode can be any gas diffusion electrode known in the art to be suitable for the reduction of CO2, preferably into formate. Such gas diffusion electrodes may be referred to as catalytic gas diffusion electrodes, and typically contain at least one metal selected from Pb, In, Sn, Bi and Hg. In a preferred embodiment, the gas diffusion electrode contains at least In. The gas diffusion electrode may be an alloy, containing at least two metals. In one embodiment, the gas diffusion electrode contains a first metal selected from the group consisting of Pb, In, Sn, Bi and Hg and a second element selected from the group consisting of In, C, Pt, Pd,ACK346 FFRh, Mo, Zr, Nb, Os, Au, Ag, Ti, Cu, Ir, Ru, Re, Hg, Pb, Ni, Co, Zn, Cd, Sn, Fe, Cr, Mn, Ga, Tl, Sb, Ga and Bi. In a more preferred embodiment, the gas diffusion electrode contains a first metal selected from the group consisting of Pb, In, Sn, Bi and Hg and a second element selected from the group consisting of Sn, Pb, Ga and Bi. In an even more preferred embodiment, the gas diffusion electrode contains In as first metal and a second element selected from the group consisting of Sn, Pb, Ga and Bi. The atoms are typically present in their metallic form, although metal oxides, metal phosphides, metal nitrides and metal sulfides have also been known to reduce carbon dioxide. The gas diffusion electrode may contain further components, such as ligands to stabilize the metal atoms and / or to catalyse the reduction of CO2, e.g. hydrides, halides, phosphines and porphyrins. Single metal gas diffusion electrodes may be used as well as alloys. Indium-containing alloys have been found particularly effective in the reduction of CO2. Especially preferred gas diffusion electrodes are selected from indium gas diffusion electrodes, indium-bismuth gas diffusion electrodes, indium-tin gas diffusion electrodes and indium-lead gas diffusion electrodes. In a preferred embodiment, the catalytic gas diffusion electrode is an indiumbismuth catalyst, indium-tin catalyst or an indium catalyst. Most preferably, the gas diffusion electrode comprises indium and bismuth.

[0026] In a preferred embodiment, the gas diffusion electrode is an indium-bismuth gas diffusion electrode, wherein the amount of bismuth is in the range of 5 - 94 wt.% based on the total amount of bismuth and indium, preferably in the range of 10 - 90 wt.%, more preferably 30 - 90 wt.%, such as 35 - 90 wt.%, most preferably in the range of 40 - 60 wt.%, such as 45 - 55 wt.%. Such ratios have shown to provide improved catalytic properties regarding carbon dioxide to formate conversion, see e.g. WO 2019 / 141827. The catalyst can comprise a combination of bismuth and indium in different thermodynamic phases.

[0027] The gas diffusion electrode may be structured as a foam, felt and / or mesh. The gas diffusion electrode can consist of the catalytic material, but the catalytic material may also be deposited on a support, such as a carbon support. Preferably, the catalyst is applied in combination with an electrically conductive support. As a conductive support a particulate material, in particular carbon particles, may be used. Preferably the conductive support comprises a porous structure of carbon particles bonded together. A preferred binding material is a hydrophobic binder, such as a fluorinated binder. The catalyst is deposited onto or adhered to the conductive material. The weight ratio of metal, such as indium and / or bismuth, to carbon can advantageously be in the range of 0.10 - 1.50, preferably 0.2 - 0.8.

[0028] A gas-diffusion electrode provides a high surface area or interface for solid-liquid-gas contact. Such a gas-diffusion electrode typically comprises an electrically conductive substrate, which may serve as a supporting structure for a gas-diffusion layer. The gas-diffusion layer provides a thin porous structure or network, e.g. made from carbon, for passing a gas like carbon dioxide from one side to the other. T y pi cal ly the structure is hydrophobic to distract water. The gas diffusion layer preferably comprises catalytically active material. By diffusion of gaseous CO2 through the pores of the gas diffusion electrode, the area that is available for reducing CO2 is maximized, as such increasing the overall efficacy of the process.ACK346 FF

[0029] The gas diffusion electrode typically contains an indium-containing catalytic system embedded in the gas-diffusion layer or provided as one or more additional separate layers thereof. Examples of suitable substrates include metal structures like expanded or woven metals, metal foams, and carbon structures including wovens, cloth and paper. As explained above, the conductive support for the catalyst is preferably formed by particulate carbon. The catalyst system is preferably bonded to the electrically conductive substrate using a hydrophobic fluorinated binder. The gas diffusion electrode for use in the present process tends to consist of (i) an electrically conductive carbon support, (ii) one or more catalytic metals, (Hi) fluorinated binder, especially poly(tetra fluoro ethylene) and (iv) optionally at most a limited amount of auxiliary additives, specifically at most 2 %wt, more specifically less than 1 %wt of auxiliary additives, based on total amount of gas diffusion electrode.

[0030] Especially preferred gas diffusion electrodes have been described in WO patent application PCT / EP2025 / 062006 and WO patent application PCT / EP2025 / 062009. Such electrodes preferably comprise a catalyst composition comprising indium and bismuth, preferably as an alloy, and optionally further comprise zinc preferably an amount of zinc which is 1-50 wt.% based on the total amount of indium and bismuth and zinc. Such electrodes can be prepared by (a) combining at least an indium containing salt, a bismuth containing salt and an oxalate salt in a liquid to obtain a precursor mixture; (b) combining the precursor mixture with a reducing agent to obtain a suspension comprising the catalyst composition; and (c) separating the catalyst composition from the suspension, wherein a support material is either added to the precursor mixture or to the reducing agent.

[0031] Any anode known to be suitable can be used in the present system and process. Preferably, the anode comprises so-called mixed metal oxides. One oxide is usually ruthenium dioxide, iridium dioxide or platinum dioxide which conduct electricity and catalyze the desired reaction. The other metal oxide is typically titanium dioxide which does not conduct or catalyze the reaction, but is cheaper and prevents corrosion.

[0032] The anode may contain further elements, such as one or more elements selected from the group consisting of S, O, P, N, C, Si, Fe and Mo, preferably from the group consisting of S, O, P, N, C and Si. The atoms may be present in their metallic form, or in any other suitable form known in the art. In a preferred embodiment, the nickel is present in metallic form or as sulphide, oxide and / or hydroxide. The anode may contain further components, such as ligands to stabilize the metal atoms e.g. hydrides, halides, phosphines and porphyrins. Single metal nickel anodes may be used as well as alloys. Preferably, the anode contains nickel sulphide or nickel- molybdenum-nitride. Especially promising results have been obtained met nickel sulphide based anodes.

[0033] The anode may be structured as a foam, felt and / or mesh. Preferably, the anode contains nano-structured catalyst on nickel foam or on copper foam. These nano-structured anodes enable high Faraday efficiencies at high current densities. The anode can consist of the catalytic material, but the catalytic material may also be deposited on a support, such as a carbon or nickel support. Preferably, the catalyst is applied in combination with an electrically conductive support.ACK346 FFAs a conductive support a particulate material, in particular nickel particles, may be used. Preferably, the conductive support comprises a porous structure, such as particles bound together or a foam. A preferred binding material is a hydrophobic binder, such as a fluorinated binder. The catalyst is deposited onto or adhered to the conductive material. In case carbon is used in the support, the weight ratio of metal, including nickel, to carbon can advantageously be in the range of 0.10 - 1.50, preferably 0.2 - 0.8.

[0034] The selective barrier preferably contains an ion exchange membrane. The membrane may be made from porous glass frit, microporous material, ion exchanging membrane or ion conducting bridge, and allows ionic species to travel from one compartment to the other, such as protons generated at the anode to the cathode compartment. Preferably, the membrane allows the passage of protons from the anode compartment to the cathode compartment. In a preferred embodiment, the membrane is a bipolar membrane. Protons are released at the gas diffusion electrode side of the bipolar membrane, and hydroxide anions are released at the anode side of the bipolar membrane. The protons can be used for the reduction of CO2 into formate at the gas diffusion electrode. As such, a net flow of protons from the anode to the gas diffusion electrode is provided. An especially preferred membrane is a sulfonated tetra- fluoroethylene based fluoropolymer-copolymer also known as Nation.

[0035] The process according to the invention may be a continuous process, preferably wherein a plurality of electrochemical cells are connected in parallel and wherein some of the cells are being subjected to regeneration while other cells are simultaneously used for operation.

[0036] In a preferred embodiment, the process is performed in an electrochemical cell assembly, comprising a plurality of electrochemical cells. Each cell contains an inlet for receiving anolyte to the anode compartment and an outlet for anolyte, an inlet for receiving catholyte to the cathode compartment and an outlet for discharging product such as formate.

[0037] The electrochemical cell assembly may contain a plurality of electrochemical cells arranged in blocks, wherein each block typically contains an equal number of electrochemical cells, preferably 1 - 25 electrochemical cells, most preferably 1 or 10 electrochemical cells. During operation, each block alternates between a first position wherein it is used for conversion of CO2 preferably to formic acid or a salt thereof, i.e. step (c) of the process according to the present invention, and a second position wherein it is regenerated.

[0038] The process according to the invention generally involves the regular operation of an electrochemical cell. During this operation, carbon dioxide is converted into formate at the gas diffusion electrode. Regular operation of an electrochemical cell may further involve a regeneration step, wherein the gas diffusion electrode, the anode, or both, are regenerated in order to improve the yields obtained at the electrode(s) during operation and / or to improve the lifetime of the electrode(s). Such regeneration is known in the art.

[0039] During operation, anolyte is fed to the anode compartment and catholyte is fed to the cathode compartment. Suitable electrolytes are well known in the art. The electrolyte for use in the present invention can be an aqueous or non-aqueous solutions and may include buffers such as bicarbonates and / or phosphates. Non-aqueous electrolytes can be beneficial in the reduction ofACK346 FFCO2 as the side-reaction at higher potentials wherein H2 is formed (due to reduction of protons in solution) is reduced.

[0040] It can be preferred for the anode compartment and / or the catholyte chamber to contain a spacer.

[0041] Generally, fluid will be present in the catholyte chamber. In one embodiment, gaseous catholyte is added. In an alternative preferred embodiment, liquid catholyte is added. Preferably, the catholyte is aqueous. Suitably, the catholyte is an aqueous solution of a metal salt selected from the group consisting of sodium oxide, sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium oxide, potassium hydroxide, potassium carbonate and potassium bicarbonate. The catholyte preferably is an aqueous solution of potassium bicarbonate. It is well- known to the skilled person to select specific electrochemical conditions such as the voltage applied and catholyte composition, in order to optimize the formation of formate. A suitable anolyte for use in the present process is an aqueous solution of sulphuric acid.

[0042] CO2 is fed as a gas to flowthrough the gas diffusion electrode of the gas diffusion electrode and thereby be converted. In a preferred embodiment, the CO2 originates from exhaust gases, flue gases or air. Typically, the CO2 originates from industrial flue gases, such as from power plants or the chemical industry. CO2 can be captured from exhaust gases, flue gases and air by methods known in the art.

[0043] In one embodiment, gaseous CO2 is led through the gas diffusion electrode and a liquid catholyte comprising a base is fed to the catholyte chamber containing the product side of the gas diffusion electrode.

[0044] In a preferred process according to the invention, a voltage difference is applied between the gas diffusion electrode and the anode such that at the gas diffusion electrode CO2 is reduced to formate and water is oxidized at the anode to oxygen.

[0045] An electrical potential is applied between the anode and the gas diffusion electrode. The anode is positively charged and the gas diffusion electrode negatively. In other words, an electrical potential to the electrochemical cell so that the anode is at a higher potential than the gas diffusion electrode. Cations, typically protons, will thus flow from the anode towards the gas diffusion electrode where they combine with a molecule of CO2 to form a formate molecule. Electrons, liberated at the anode by the anodic reaction, are taken up by the anode The electrical potential may be a direct current voltage. In preferred embodiments, the applied electrical potential is generally between about 1 V and about 6 V, preferably from about 1 V to about 5 V, such as in the range of 3 V to 5 V and more preferably from about 1.5 V to about 4 V.

[0046] It is noted that applying an electrical potential is considered synonymous with creating a voltage difference between the gas diffusion electrode and the anode, so that the anode is at a higher potential than the gas diffusion electrode. The process may be controlled by setting a certain voltage (galvanostatic) or by setting a certain current (potentiostatic). If the voltage is set, the current will automatically follow from the reactions that occur in the cell. If the current is set, the voltage will automatically follow from the reactions that occur in the cell. The process according to the invention is equally workable in both operation modes. Typically, the current is controlled in the start-up phaseACK346 FF of an electrochemical cell, in order to find the optimal voltage for the desired reaction, while during standard operation of the electrochemical cell, the voltage will be controlled. The process according to the invention operates with such a voltage difference and / or such a current that carbon dioxide is reduced at the gas diffusion electrode.

[0047] Preferably, the current density of the electrochemical cell during operation is at least 10 mA / cm2, such as in the range of 10 mA / cm2- 5 A / cm2, more preferably at least 100 mA / cm2, such as in the range 100 mA / cm2- 3 A / cm2. A certain minimal current, typically at least 10 mA / cm2, preferably at least 100 mA / cm2, is preferred in terms of process economics, as below these values too little product is formed for an economically viable process. The upper limit of the current at which the process can operate is determined by safety issues. For example, it the current is too high, the cell may heat up too much. Other than that, higher currents are preferred since it will result in more product formation. Excellent results have been obtained with a current density in the range of 50 - 200 mA / cm2. Herein, the currents are defined based on the projected area of the electrode. The optimal current for the process according to the invention may differ based on the exact conditions that are applicable in the electrochemical cell, and the skilled person is able to determine the optimal current in terms of product conversions.

[0048] The process according to the invention is preferably performed at or near ambient pressure, although deviation is possible without significantly affecting the process.

[0049] The present process has been found to be especially suitable for reduction of carbon dioxide to formate. The formate may be formed with any counter ion, which depends on the base used in the catholyte. In the absence of base, formic acid may be formed. The process allows to produce for example sodium formate and potassium formate.

[0050] Fig. 1 shows a general concept of a process line-up of the present invention. The carbon dioxide is fed via line 10 to the carbon dioxide feed chamber 1 . The feed can be formed by bubbling carbon dioxide through water under pressure. This mixture of carbon dioxide and water will come into contact with the gas diffusion electrode 2. Carbon dioxide will flow through the gas diffusion electrode 2 to catholyte chamber 3. Water tends to condense upon contact with the gas diffusion electrode and can be removed via line 15 together with any dissolved salts. Catholyte is added to catholyte chamber 3 via line 11 and removed via line 12. The cathode compartment is separated from the anode compartment by selective barrier 4. Anode 6 forms a further wall of the anode compartment 5. If selective barrier 4 is an ion exchange membrane, ions can pass this selective barrier 4. Anolyte is added to the anode compartment 5 via line 13 and removed via line 14.

[0051] The anode compartment contained an iridium ruthenium oxide dimensionally stable anode. The gas diffusion electrode comprised indium and bismuth and was prepared as described in WO 2019 / 141827. Indium and bismuth nanoparticles were obtained by chemical reduction of their salts. After purification of the nanoparticles, a suspension of these nanoparticles was sprayed on a carbon-based gas diffusion layer.ACK346 FF

[0052] The anode and cathode (both 9.25 cm2) were placed in a three-compartment electrochemical cell comprising an anode compartment comprising the anode, a selective barrier and a cathode compartment comprising a carbon dioxide feed chamber separated from a catholyte chamber by a gas diffusion electrode. A flow of carbon dioxide was introduced into the carbon dioxide feed chamber (50 mL / min) in combination with 50 mL / min of water. Through the catholyte chamber flowed a solution of 0.1 M KHCO3 (the catholyte) at a rate of 50 mL / min. Through the anolyte chamber flowed an aqueous solution of 0.5 M sulphuric acid (50 mL / min). The catholyte chamber and the anolyte chamber were separated by an ion-exchange membrane (Fumasep biopolar membrane). In the anolyte compartment a plastic mesh was placed to fill the gap between the anode and the ion exchange membrane.

[0053] Constant current electrolysis (100 mA / cm2) was applied. The catholyte was periodically sampled and analysed for formate. The formate concentration was analysed with ion-exchange chromatography. T able 1 describes the Faraday efficiency at room temperature and at 80 °C which temperatures were measured at the introduction of the mixture of carbon dioxide and water into the catholyte feed chamber. The indicated number of hours are after start of electrolysis.Table 1It will be clear from the above that the use of a water containing carbon dioxide feed at a temperature of from 70 to 120 °C allows to achieve high activity in the reduction of CO2 to formate for a long period of time in a three compartment electrochemical cell.

Claims

ACK346 FFCLAIMS1. Process for electrochemical reduction of carbon dioxide in an electrochemical cell comprising (a) an anode compartment comprising an anode, (b) a selective barrier, and (c) a cathode compartment comprising a carbon dioxide feed chamber separated from a catholyte chamber by a gas diffusion electrode which is located spatially apart from the selective barrier, wherein a carbon dioxide feed is introduced into the carbon dioxide feed chamber which carbon dioxide feed comprises carbon dioxide and water and has a temperature of from 50 to 100 °C upon introduction in the carbon dioxide feed chamber.

2. Process according to claim 1 wherein the carbon dioxide feed contains at least 2 % by weight of water, based on total amount of carbon dioxide and water, when introduced into the carbon dioxide feed chamber.

3. Process according to claim 1 or 2 wherein the temperature of the mixture of carbon dioxide and water is of from 60 to 90 °C, preferably of from 65 to 85 °C.

4. Process according to any of the preceding claims wherein the carbon dioxide feed chamber comprises a flow plate upstream of the gas diffusion electrode.

5. Process according to any of the preceding claims wherein the gas diffusion electrode comprises indium and bismuth.

6. Process according to any of the preceding claims wherein the catholyte is an aqueous solution of a metal salt selected from the group consisting of sodium oxide, sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium oxide, potassium hydroxide, potassium carbonate and potassium bicarbonate.

7. Process according to any of the preceding claims wherein the process comprises(i) introducing anolyte in the anode compartment,(ii) introducing catholyte in the catholyte chamber, and(iii) applying an electrical potential between the gas diffusion electrode and the anode such that the carbon dioxide is reduced.

8. Process according to any of the preceding claims wherein the carbon dioxide is reduced to formate.

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