Electrochemical conversion with polar solvent wash

Abstract

System for electrochemical reduction of carbon dioxide which system comprises an electrochemical cell which comprises a selective barrier and a cathode compartment comprising a carbon dioxide feed chamber (1) separated from a catholyte chamber (3) by a gas diffusion electrode (2) which is located spatially apart from the selective barrier, wherein the carbon dioxide feed chamber (1) comprises an inlet for polar solvent (15) and a distributing means capable of creating a polar solvent film (7, 17); and 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 film of polar solvent is fed to the back-side of the gas diffusion electrode thereby creating a polar solvent film.

Description

ACK339 FF-1-Electrochemical conversion with polar solvent washField of the invention

[0001] The present invention relates to a system and 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 cells over zero-gap electrochemical cells is that the 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 can prevent this.

[0004] Unfortunately, it was found that salts tend to precipitate at the back-side of the gas diffusion electrode in traditional three compartment electrochemical cells. The “back-side” of the gas diffusion electrode is the side of the electrode which receives the carbon dioxide feed. This back-side also is referred to as the downstream side considering the flow of carbon dioxide through the gas diffusion electrode during normal operation.

[0005] 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 (1 ) that periodic water flushing of cathode GDE prevents salt formation but CO2 reduction reactions shift toward the hydrogen evolution reaction over time due to high water content at the cathode GDE.

[0006] 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,ACK339 FF-2- 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 flushing with deionized water at the beginning of each hour for 10 s. While this washing step restored the current to its original value, it is unattractive to have to stop operation to allow for such flushing. Furthermore, page 325 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 , mentions that later work by the same group casts doubt on cathode rinsing as viable long-term technique for removing precipitates since significant pressure is necessary to penetrate the hydrophobic cathode and effectively clean out the precipitated salts. Current carbon-based gas diffusion electrodes commonly used for CO2 reduction are only mechanically robust enough to withstand pressure differences up to 100 mbar prior to flooding. Moreover, droplets that remain in the gas diffusion electrode after rinsing can promote electrochemical hydrogen evolution reaction and limit the free accessibility of CO2 to the catalyst.

[0007] EP4060091 describes an electrolysis cell comprising a zero gap electrolyzer. In order to remove precipitated salt from the carbon dioxide gas flow path, rinse material can flow through either the carbon dioxide gas supply flow path or an auxiliary flow path. By mixing carbon dioxide gas and the rinse material, the rinse operation can be performed while electrolysis continues.

[0008] The article “Direct water injection in catholyte-free zero-gap carbon dioxide electrolyzers” by De Mot et al., ChemElectroChem 2020, 7, 3839-3843 concerns a zero gap electrolyzer. The tin-coated gas diffusion electrode was combined with a membrane (“membrane electrode assembly” or“MEA”). Insufficient water supply in the electrolyzer was found to lead to salt precipitation while an excess of water would dilute the product.

[0009] The article “Continuous-flow electroreduction of carbon dioxide” by Endrodi et al., Progress in Energy and Combustion Science 62 (2017) 133-154, reviews development options together with the intrinsic limitations of the different approaches.

[0010] DE 102020207186 relates to electrolysis of CO2 and / or CO wherein salt formation in the cathode is avoided by using an electrolyte with which no salts are formed with the cathodic ionic electrolysis products OH HCOs', COs2-. Examples are ammonium ion, alkylammonium ion, dialkylammonium ion and trialkylammonium ion.

[0011] Surprisingly, it now has been found that salt formation can be prevented or salts can be removed from a gas diffusion electrode used in carbon dioxide reduction in a traditional electrolysis cell without requiring frequent flushing. This can be achieved by allowing solvent to flow along the back-side of the membrane. Such flow prevents exerting pressure on the membrane and reduces the risk of introducing water into the gas diffusion electrode.

[0012] Therefore, the present invention relates to a system for electrochemical reduction of carbon dioxide which system comprises an electrochemical cell which electrochemical cell comprises (a) an anode compartment (5) comprising an anode (6), (b) a selective barrier (4), and (c) a cathode compartment comprising a carbon dioxide feed chamber (1 ) separated from aACK339 FF-3- catholyte chamber (3) by a gas diffusion electrode (2) which is located spatially apart from the selective barrier, which carbon dioxide feed chamber (1) comprises an inlet for carbon dioxide feed (10) and which catholyte chamber (3) comprises an inlet for catholyte (11 ) and outlet for catholyte (12), wherein the carbon dioxide feed chamber (1 ) further comprises an inlet for polar solvent (15), an outlet for polar solvent (16) and a distributing means (7, 17) in fluid communication with the inlet for polar solvent (15) which distributing means (7, 17) is capable of creating a polar solvent film.

[0013] The present invention also relates to a process for electrochemical reduction of carbon dioxide wherein the process is performed in an electrochemical cell comprising a cathode compartment comprising a carbon dioxide feed chamber separated from a catholyte chamber by a gas diffusion electrode, an anode compartment comprising an anolyte chamber and a selective barrier separating the anode compartment from the cathode compartment wherein the gas diffusion electrode is located spatially apart from the selective barrier which process comprises (i) feeding anolyte to the anode compartment, (ii) feeding carbon dioxide to the carbon dioxide feed chamber, (iii) applying an electrical potential between the gas diffusion electrode and the anode such that the cathode carbon dioxide is reduced, and (iv) flowing polar solvent along the backside of the gas diffusion electrode thereby creating a polar solvent film. A flow “along” the gas diffusion electrode means that the flow of fluid is substantially parallel to this electrode.

[0014] In a traditional electrochemical cell as used in the present system and 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 the continuous volume of electrolyte, more specifically catholyte. The downstream side is with respect to the flow of carbon dioxide through the gas diffusion electrode during normal operation. 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. 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 solvent wash in such setup is easier as the amount of water added is less critical and 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 present system and process preferably are operated 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 with the feed inlet in a wall perpendicular to the gas diffusion electrode. For the present system, the inlet for carbon dioxide feed preferably is in a wall opposite the gas diffusion electrode.ACK339 FF-4-Brief description of the drawingFig. 1 displays a schematic overview of a system according to the present invention.Fig. 2 displays a preferred means for creating a thin film of polar solvent in fluid contact with the gas diffusion electrode.Detailed description

[0017] The present system and process allow to remove salt which may be formed during electrochemical reduction of carbon dioxide in a traditional three compartment cell. The salt removal can take place during normal operation of the cell.

[0018] It will be clear to the person skilled in the art that it is possible to apply any solvent in which salt dissolves under the conditions at which the gas diffusion electrode feed chamber is being operated. Preferably, the polar solvent consists of water more specifically deionised water.

[0019] It was surprisingly found that a film of water which flows along the back-side of a gas diffusion electrode in such traditional three compartment cell allows to dissolve salt which is formed without interrupting the diffusion of carbon dioxide into the gas diffusion electrode. The process according to the invention can be operated at high activity for a considerable time without requiring to shut down.

[0020] The film of polar solvent for use in the present invention preferably has a thickness of at most 0.5 mm, more specifically at most 0.4 mm, more specifically at most 0.3 mm, more specifically at most 0.2 mm, most preferably at most 0.1 mm. The thickness is on average over the catalytically active surface of the gas diffusion electrode in contact with carbon dioxide feed and on average during the time that the system is in operation. If salt accumulates faster, it is possible to increase the flow of water to allow for more intense washing. The film can be formed by allowing liquid to flow along the gas diffusion electrode.

[0021] A person skilled in the art will know how to ensure that a thin film of polar solvent is present on the back-side of the gas diffusion electrode. A suitable distributing means is a perforated pipe in fluid connection with the inlet for the polar solvent. The openings of the pipe can be such that the fluid is directed at the gas diffusion electrode or for the fluid to flow along the gas diffusion electrode depending on the distance between the pipe and the electrode. An alternative is a pipe containing a single slit. Another option is to spray the polar solvent on the electrode. Any distributing means is considered suitable provided that a film of polar solvent is obtained preferably having the desired thickness. Preferably, the distributing means is a tube (17) comprising one or more openings (18) directing fluid at the gas diffusion electrode (2).

[0022] Preferably, the distributing means (7, 17) is in fluid contact with both the inlet for polar solvent (15) and the gas diffusion electrode (2).

[0023] Preferably, the distributing means (7, 17) is located in the carbon dioxide feed chamber (1 ) upstream of the gas diffusion electrode (2). Preferably, the gas diffusion electrode is covered by a film of polar solvent during normal operation. In the process, the polar solvent preferably isACK339 FF-5- added upstream of the gas diffusion electrode. In this context, upstream is with respect to the flow of polar solvent during normal operation.

[0024] 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. The counter ion depends on the base used in the catholyte, if any. In the absence of base, formic acid may be formed. The process allows to produce for example sodium formate and potassium formate.

[0025] 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.

[0026] 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, Rh, 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 sulphides 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 gas diffusion electrode comprises an indium-bismuth catalyst, indiumtin catalyst or an indium catalyst. Most preferably, the gas diffusion electrode comprises indium and bismuth.

[0027] In a preferred embodiment, the gas diffusion electrode is an indium-bismuth gas diffusion electrode, wherein he 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.ACK339 FF-6-

[0028] 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.

[0029] It was found to be possible to use in the present process and system 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 for more robust operation.

[0030] 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. Typically 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.

[0031] 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 system and 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.

[0032] 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 catalystACK339 FF-7- composition from the suspension, wherein a support material is either added to the precursor mixture or to the reducing agent.

[0033] 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.

[0034] 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.

[0035] 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. As 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.

[0036] The selective barrier in the system according to the invention 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 tetrafluoro-ethylene based fluoropolymer-copolymer also known as Nation.

[0037] 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.ACK339 FF-8-

[0038] 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, an outlet for anolyte, an inlet for receiving catholyte to the cathode compartment and an outlet for discharging product such as formate.

[0039] 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.

[0040] 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.

[0041] 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 of CO2 as the side-reaction at higher potentials wherein H2 is formed (due to reduction of protons in solution) is reduced.

[0042] It can be preferred for the anode compartment and / or the catholyte chamber to contain a spacer.

[0043] 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. Preferably, 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. A suitable anolyte for use in the present process is an aqueous solution of sulphuric acid.

[0044] Preferably, the system further contains a flow plate. The presence of such flow plate in the carbon dioxide feed chamber (1) allows to guide polar solvent along the gas diffusion electrode. The catholyte chamber (3) can contain a flow plate to improve the flow of catholyte. A flow plate suitably is a mesh, more specifically a mesh which is electrochemically inert. A flow plate can be a plastic mesh.

[0045] Depending on the operational conditions, it can be preferred for the anode compartment and / or the catholyte chamber to contain a spacer.

[0046] 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.ACK339 FF-9-

[0047] In an especially preferred embodiment, the distributing means (7, 17) is integrated with a flow plate. This combination allows to distribute the polar solvent thereby creating a thin film which subsequently is guided along the gas diffusion electrode by the flow plate.

[0048] It will be clear for the person skilled in the art that a distributing means can cause stagnant zones of polar solvent which can hamper optimal operation of the gas diffusion electrode. In order to prevent this, it can be preferred that the gas diffusion electrode (2) is embedded in a distributing plate containing the distributing means (7, 17) upstream of the gas diffusion electrode (2) and openings to remove fluid downstream of the gas diffusion electrode, wherein the expressions “upstream” and “downstream” are with respect to the flow of polar solvent during normal operation in other words the direction in which the polar solvent flows if the system is in normal operation.

[0049] While CO2 can be fed in liquid form, it is preferred to be introduced into the carbon dioxide feed chamber as a gas. It can be preferred that the carbon dioxide feed chamber contains a means for evenly distributing the CO2 over the gas diffusion electrode. CO2 preferably is fed as a gas to flow through 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. It is preferred that the concentration of CO2 in the feed is as high as possible. Therefore, the feed to the carbon dioxide feed chamber contains at least 90 wt%, preferably at least 95 wt%, more preferably at least 99% wt% or even at least 99.9 wt% based on total feed. The amount of feed excludes the polar solvent which is added to the carbon dioxide feed chamber. In addition to CO2, some other gaseous species may be present, such as inert gases (N2, Ar) and / or H2. The presence of O2 in the gas fed to the electrode is preferably avoided.

[0050] 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.

[0051] 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 phase of an electrochemical cell, in order to find the optimal voltage for the desired reaction, while duringACK339 FF-10- 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.

[0052] 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.

[0053] The process according to the invention is preferably performed at or near ambient pressure and temperature, although deviation from these conditions is possible without significantly affecting the process. In one embodiment, the temperature during the operation of the electrochemical cell is in the range of 10 - 50 °C, preferably 15 - 40 °C.

[0054] The process according to the invention allows to produce formate, such as sodium formate or potassium 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.

[0055] Fig. 1 shows a general concept of a process line-up of the present invention containing an anode compartment 5 and a cathode compartment 3. Voltage is applied to gas diffusion electrode 2 and anode 6. Gaseous carbon dioxide can be fed via line 10 to the carbon dioxide feed chamber 1. Carbon dioxide will flow through the gas diffusion electrode 2 to catholyte chamber 3. At least part of the carbon dioxide will be converted into formate. Polar solvent is added via line 15 such that the gas diffusion electrode 2 facing the cathode feed compartment is covered with a thin film of solvent. In this case, the polar solvent is added from line 15 to a distributing means integrated with a flow plate 7. The flow plate consists of a plastic mesh. Polar solvent which has flown down along the gas diffusion electrode is removed via line 16. Catholyte is added to catholyte chamber 3 via line 11 and removed via line 12. The catholyte which is removed will also contain any formate which has been produced. The cathode compartment is separated from the anode compartment by selective barrier 4. 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 is removed via line 14.

[0056] Fig. 2 shows a preferred means for adding polar solvent. This distributing means is present upstream of the gas diffusion electrode for which there is left the opening 8. Carbon dioxide is added via line 10 (not shown). Polar solvent is added via line 15 which is in fluid connection with distributing means 17 which is a perforated tube having openings. This distributing means 17 is connected with pipes 18 which ensure further diversion and distribution of the polar solvent. This set-up creates aACK339 FF thin film of polar solvent flowing downward thereby covering the back-side of gas diffusion electrode2. Polar solvent is removed via line 16.

Claims

ACK339 FF-12-CLAIMS1. System for electrochemical reduction of carbon dioxide which system comprises an electrochemical cell which electrochemical cell comprises(a) an anode compartment (5) comprising an anode (6),(b) a selective barrier (4), and(c) a cathode compartment comprising a carbon dioxide feed chamber (1) separated from a catholyte chamber (3) by a gas diffusion electrode (2) which is located spatially apart from the selective barrier, which carbon dioxide feed chamber (1) comprises an inlet for carbon dioxide feed (10) and which catholyte chamber (3) comprises an inlet for catholyte (11) and an outlet for catholyte (12), wherein the carbon dioxide feed chamber (1 ) further comprises an inlet for polar solvent (15), an outlet for polar solvent (16) and a distributing means (7, 17) in fluid communication with the inlet for polar solvent (15) which distributing means (7, 17) is capable of creating a polar solvent film.

2. System according to claim 1 wherein the distributing means (7, 17) is in fluid contact with both the inlet for polar solvent (15) and the gas diffusion electrode (2).

3. System according to claim 1 or 2 wherein the distributing means is a tube (17) comprising one or more openings (18) directing fluid at the gas diffusion electrode (2).

4. System according to any one of the preceding claims wherein the distributing means (7, 17) is located in the carbon dioxide feed chamber (1 ) upstream of the gas diffusion electrode (2) wherein upstream is with respect to the flow of polar solvent during normal operation.

5. System according to any one of the preceding claims wherein the carbon dioxide feed chamber (1) comprises a flow plate.

6. System according to any one of the preceding claims wherein the gas diffusion electrode (2) is embedded in a distributing plate containing the distributing means (7, 17) upstream of the gas diffusion electrode (2) and openings to remove fluid are downstream of the gas diffusion electrode (2), wherein upstream and downstream are with respect to the flow of polar solvent during normal operation.

7. System according to claim 5 and / or 6 in which the flow plate is an electrochemically inert mesh.

8. Process for electrochemical reduction of carbon dioxide wherein the process is performed in an electrochemical cell comprising a cathode compartment comprising a carbon dioxide feed chamber separated from a catholyte chamber by a gas diffusion electrode, an anode compartment comprising an anolyte chamber and a selective barrier separating the anode compartment from the cathode compartment wherein the gas diffusion electrode is located spatially apart from the selective barrier which process comprises(i) feeding anolyte to the anode compartment,(ii) feeding carbon dioxide to the carbon dioxide feed chamber,ACK339 FF-13-(iii) applying an electrical potential between the gas diffusion electrode and the anode such that the cathode carbon dioxide is reduced, and(iv) flowing polar solvent along the back-side of the gas diffusion electrode thereby creating a polar solvent film.

9. Process according to claim 8 wherein the polar solvent is added upstream of the gas diffusion electrode and is removed downstream of the gas diffusion electrode, wherein upstream and downstream are with respect to the flow of polar solvent during normal operation.

10. Process according to claim 8 or 9 wherein the gas diffusion electrode is covered by a film of polar solvent.

11. Process according to any of claims 8-10 wherein the carbon dioxide is reduced to formate.

12. Process according to any of claims 8-11 wherein the polar solvent consists of water.

13. Process according to any one of claims 8-12 wherein the polar solvent film has a thickness of at most 0.5 mm.

14. Process according to any of claims 8-13 the wherein the catholyte is an aqueous solution of potassium bicarbonate.

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