Method for electrochemical conversion of co2 captured by a deep eutectic solvent in the presence of a palladium catalyst

Abstract

The method comprises the steps of: a) providing an electrochemical device (100) comprising: - an anode compartment (1) comprising an anode (7) and an anolyte (6) comprising at least one aqueous electrolyte for the electrolysis of H2O according to equation (I) below: - a cathode compartment (2) for the reduction of CO2 according to equation (II) below: the cathode compartment (2) comprising a palladium-based cathode (5) and a catholyte (4) comprising a deep eutectic solvent (DES), and - a proton exchange membrane (3), b) introducing CO2 into the cathode compartment (2) until said cathode compartment (2) is saturated, c) applying a potential between the anode (7) and the cathode (5) so that the CO2 is reduced to carbon monoxide (CO) and H2O is oxidized to oxygen, and d) recovering the CO at the outlet of the cathode compartment (2), the deep eutectic solvent being chosen from a mixture of at least one ammonium salt and of at least one amine, a mixture of at least one ammonium salt and of at least one polyol, and a combination of these mixtures.

Description

[0001] DESCRIPTION

[0002] Electrochemical conversion process of CO2 captured by a deep eutectic solvent in the presence of a palladium catalyst

[0003] The present invention relates to the field of electrochemical conversion of CO2 into carbon monoxide. In particular, the invention relates to the electrochemical conversion of CO2 using a deep eutectic solvent capture technique in the presence of a palladium-based catalyst.

[0004] Recycling CO2 from industrial effluents poses a real environmental challenge. A known technique uses an initial CO2 trapping step with an amine (e.g., monoethanolamine and N-methyldiethanolamine) used as an aqueous solvent. The CO2 binds with the amine to generate a complexed form that can be easily stored and then transported to a reprocessing site. However, these steps are time-consuming and costly. The amine used in very large quantities requires reprocessing, and decomplexing between the amine and CO2 is not easy to achieve, making this method relatively effective.

[0005] Another method involves using the same solvent for CO2 capture and transformation. It involves the electrochemical transformation of carbon dioxide into carbon monoxide, which appears to be a sustainable solution for closing the carbon cycle. In this cycle, CO2 emitted by factories, power plants, or directly into the air (Direct Air Capture) can be absorbed by a capture solution before being transformed into various products in an electrochemical cell. In particular, it is possible to generate carbon monoxide (CO), which will in turn be transformed in a process for manufacturing valuable molecules, such as hydrocarbons.This technique for capturing CO2 and converting it electrochemically is based on the ability of deep eutectic solvents (also known by the acronym DES) to behave simultaneously as a solvent for capturing CO2 and as an electrolyte for its conversion into CO. Deep eutectic solvents, or DES, consist of a hydrogen bond donor and acceptor that behave as a pure substance, which allows their use as a solvent. DES are liquid, at ambient pressure and temperature, in most cases. Examples of such mixtures most often cited in the literature are choline chloride - ethylene glycol (ChCl : EG (1 : 2)) or choline chloride - urea (ChCl : urea (1 : 2)).They have several advantages for CO2 conversion: wide electrochemical windows, low volatility, good ionic conductivity and high CO2 absorption capacity, which allows high selectivity towards CO2, particularly from industrial effluents containing a large number of gaseous substances. The composition of DES can therefore be very varied. However, the DESs commonly used are viscous and the conversion results remain poor.

[0006] One of the aims of the invention is to improve the yields of processes coupling the capture and electrochemical conversion of CO2.

[0007] To this end, the present invention provides a method for the electrochemical conversion of CC into carbon monoxide (CO) comprising the steps of: a) providing an electrochemical device comprising:

[0008] - an anode compartment comprising an anode and an anolyte comprising at least one aqueous electrolyte intended for the electrolysis of H2O according to the following equation (I):

[0009] - a cathode compartment intended for the reduction of CO2 according to the following equation (II): the cathode compartment comprising a palladium (Pd)-based cathode and a catholyte comprising a deep eutectic solvent (DES), and

[0010] - a proton exchange membrane between the anode compartment and the cathode compartment, b) introducing CO2 into the cathode compartment until saturation of said cathode compartment, part of the introduced CO2 being captured by the deep eutectic solvent, c) applying a voltage between the anode and the cathode so that the CO2 captured by the deep eutectic solvent is reduced to carbon monoxide (CO) in the cathode compartment according to equation (II) and H2O is oxidized to oxygen in the anode compartment according to equation (I), the protons H +migrating from the anode compartment to the cathode compartment through the proton exchange membrane, d) recovery of CO at the outlet of the cathode compartment, the deep eutectic solvent being chosen from a mixture of at least one ammonium salt and at least one amine, a mixture of at least one ammonium salt and at least one polyol such as glycerol, and a combination of these mixtures.

[0011] This process thus allows a co-catalysis of the electrochemical conversion of CO2, the DES allowing the capture of CO2 with a good absorption capacity while promoting the reduction reaction on the palladium of the cathode. This capture process corresponds to the physical absorption of CO2 in the medium. No reaction takes place between CO2 and the solvent. Only weak forces (such as hydrogen bonds, Van Der Waals...) are at the origin of this process. This process does not require the use of toxic amines and does not pose the problem of decomplexation with an amine which is difficult to obtain because the bonds formed between DES and CO2 are weaker than those of the amines of the prior art with CO2 so that the latter can be easily and very quickly converted on the surface of the cathode. The process is selective for CO2, it gives a very good faradaic efficiency (FE) as can be seen below.

[0012] One possibility is that the cathode is made of palladium deposited on carbon (Pd / C). This cathode, or working electrode, ensures the efficient production of carbon monoxide CO on its surface.

[0013] According to one arrangement, the cathode comprises palladium particles having an average particle size between the size of a Pd atom and 1.5 pm, in particular between the size of a Pd atom and 100 nm and in particular between the size of a Pd atom and 4 nm, this average particle size being typically measured by transmission electron microscopy (TEM). It has indeed been found that Pd deposited in the form of nanoparticle powder makes it possible to achieve better faradaic efficiency.

[0014] The deposition of Pd / C catalyst is carried out on GDL (Gas Diffusion Layer). This deposition known in the literature includes the following main steps: dispersion of a Pd powder in a solvent and airbrushing of the dispersion onto a support formed of carbon paper (GDL).

[0015] According to an advantageous embodiment of the invention, the catholyte comprises, in addition to a deep eutectic solvent, a buffer solution having a basic pH. This arrangement makes it possible to fix the pH of the catholyte at a basic pH. This limits the known parasitic reaction of reduction of water molecules on the Pd-based cathode, which is favored in an acidic medium. In addition, this buffer solution is an aqueous solution which has the advantage of reducing the viscosity of the deep eutectic solvent and also increasing the ionic character of the electrolyte. This facilitates the movement of species in the catholyte and the transport of CO2 captured by the deep eutectic solvent to the surface of the Pd-based cathode.

[0016] Furthermore, experience shows that the implementation of a palladium-based cathode immersed in a catholyte comprising a DES makes it possible to obtain a faradic efficiency much higher than that obtained with the implementation of a silver-based cathode immersed in the same catholyte.

[0017] Alternatively, the basic pH buffer solution of the catholyte consists of an aqueous solution of KHCO3, KCl, or KOH, and preferably an aqueous solution of KHCO3.

[0018] In fact, an aqueous solution of KHCO3 allows the acid-base balance between CO2 and HCOs to be maintained. 2- .

[0019] According to one embodiment of the invention, the concentration of the KHCO3 buffer solution in the catholyte is approximately 0.1 M. According to one possibility, the buffer solution of the catholyte, and in particular the aqueous KHCO3 solution, has a pH of 8.

[0020] As for the anolyte, it comprises at least one aqueous electrolyte. This aqueous electrolyte can be a solution chosen from H2O, KHCO3 and H2SO4.

[0021] Alternatively, the anolyte comprises, or consists solely of, a buffer solution having a basic pH, for example a preferred pH of 8.

[0022] Such a buffer solution makes it possible to fix the pH of the anolyte to a basic pH and thus to increase the ionic conductivity of the system.

[0023] According to an advantageous variant, the anolyte buffer solution is a KHCO3 solution. Indeed, a KHCO3-based anolyte allows better pH regulation.

[0024] According to a preferred variant, the concentration of the KHCO3 buffer solution in the anolyte is approximately 0.1 M, and / or the KHCO3 buffer solution has a pH of 8.

[0025] As previously indicated, the DES of the catholyte is chosen from a mixture of at least one ammonium salt and at least one amine, a mixture of at least one ammonium salt and at least one polyol, and a combination of these mixtures. This type of mixture is, in fact, a hydrogen bond acceptor and also allows a large quantity of CO2 to be captured.

[0026] According to one possibility, the ammonium salt is selected from tetraethylammonium chloride, choline chloride and diethylammonium chloride.

[0027] According to one possibility, the amine is selected from monoethanolamine, N-methyldiethanolamine and diethanolamine.

[0028] Alternatively, the amine is a ternary amine, for example, N-methyldiethanolamine (MDEA).

[0029] According to an advantageous variant, the polyol is glycerol.

[0030] Experience shows, in fact, that the implementation of a palladium-based cathode immersed in a catholyte comprising, like DES, an ammonium salt and glycerol, makes it possible to obtain a faradic efficiency higher than that obtained with the implementation of this same cathode immersed in a catholyte comprising, like DES, an ammonium salt and ethylene glycol.

[0031] According to an advantageous embodiment of the invention, the catholyte comprises a deep eutectic solvent consisting of a mixture of an ammonium salt and an amine, for example chosen from:

[0032] ® a mixture of tetraethylammonium chloride (TEACI) and monoethanolamine (MEA) in respective molar proportions of 1 to 4;

[0033] “a mixture of tetraethylammonium chloride (TEACI) and N-methyldiethanolamine (MDEA) in respective molar proportions of 1 to 2;

[0034] * a mixture of tetraethylammonium chloride (TEACI) and diethanolamine (DEA) in respective molar proportions of 1 to 2;

[0035] ® a mixture of choline chloride and diethanolamine (DEA) in respective molar proportions of 1 to 2 or 1 to 4;

[0036] ® a mixture of diethylammonium chloride (DEACI) and monoethanolamine (MEA) in respective molar proportions of 1 to 2 or 1 to 4;

[0037] ® a mixture of diethylammonium (DEACI) and N-methyldiethanolamine (MDEA) in respective molar proportions of 1 to 2 or 1 to 4; and

[0038] "a mixture of diethylammonium (DEACI) and diethanolamine (DEA) in respective molar proportions of 1 to 2 or 1 to 4. According to another advantageous embodiment of the invention, the catholyte comprises a deep eutectic solvent consisting of a mixture of an ammonium salt and a polyol, for example chosen from: ® a mixture of tetraethylammonium chloride (TEACI) and glycerol (Gly) in respective molar proportions of 1 to 2, 1 to 3 or 1 to 4;

[0039] ® a mixture of diethanolamine chloride (DEACI) and glycerol (Gly) in respective molar proportions of 1 to 2, 1 to 3 or 1 to 4.

[0040] According to yet another embodiment of the invention, the catholyte comprises a deep eutectic solvent consisting of an ammonium salt, a polyol and an amine such as a mixture of tetraethylammonium chloride (TEACI), glycerol (Gly) and monodiethanolamine (MEA), in respective molar proportions of 1:2:2 or 1:2:4.

[0041] In another advantageous embodiment, the catholyte comprises the deep eutectic solvent, consisting of the mixture TEACI: Gly (in molar proportions 1:2) and a buffer solution of KHCO3 at 0.1 M with a mass ratio of deep eutectic solvent / buffer solution of 1 / 1 and the cathode comprises Pd on carbon particles, the average size of said Pd particles being less than 4 nm.

[0042] According to another characteristic, the method of the invention is carried out at a temperature between 20°C and 50°C, in particular between 20°C and 30°C, for example a temperature of approximately 25°C.

[0043] According to another arrangement, the method of the invention is carried out under a pressure between atmospheric pressure and 1 bar relative, in particular between atmospheric pressure and 100 mbar relative, for example at atmospheric pressure.

[0044] According to yet another arrangement, the method of the invention is carried out by applying a potential V of between -1.7 V and -1.4 V relative to a reference electrode.

[0045] According to other characteristics, the electrochemical conversion method of the invention comprises one or more of the following optional characteristics considered alone or in combination: - The Pd-based cathode comprises or consists of Pd nanoparticles on a support having suitable electronic conductivity.

[0046] - The Pd-on-carbon based cathode comprises Pd with a content of 20% by weight and C with a content of 80% by weight.

[0047] - The Pd of the cathode comprises or consists of Pd nanoparticles.

[0048] - Pd nanoparticles have an average particle size between the size of a Pd atom and 1.5 pm, in particular between the size of a Pd atom and 100 nm and in particular between the size of a Pd atom and 4 nm.

[0049] - The anode is made of platinum.

[0050] - The anode consists of a platinum wire surrounded by a platinum grid.

[0051] - Step d) of the process also includes the recovery of oxygen O2 at the outlet of the anode compartment.

[0052] - The anode acts as a counter-electrode (CE) on which the oxidation of the water takes place.

[0053] - The proton exchange membrane is Nation™. It is available from the supplier Dupont de Nemours™.

[0054] - The proton exchange membrane is Nation™ 117.

[0055] - The reference electrode is made of Ag / AgCI.

[0056] - The reference electrode is placed in the catholyte.

[0057] - The cathode compartment includes the catholyte and a gaseous canopy and the CO2 introduction step includes the saturation of the catholyte and the gaseous canopy.

[0058] - The voltage is applied using a potentiostat.

[0059] Other aspects, aims and advantages of the present invention will appear better on reading the following description of an embodiment thereof, given by way of non-limiting example and with reference to the appended drawings. The figures do not necessarily respect the scale of all the elements represented so as to improve their readability and in which:

[0060] Figure 1 represents a schematic sectional view of an electrochemical device for the conversion of CO2 according to an embodiment of the method of the invention.

[0061] Figure 2 represents a schematic sectional view of another electrochemical device for the conversion of CO2 according to an alternative embodiment of the method of the invention.

[0062] Figure 3 represents a diagram illustrating the variation of the faradaic efficiency of CO2 conversion with different applied potentials.

[0063] Figure 4 represents a diagram illustrating the variation of the faradaic efficiency of H2 conversion with different applied potentials.

[0064] Figure 5 represents a diagram illustrating the variation of the faradaic efficiency of CO2 conversion over time.

[0065] Figure 6 represents a diagram illustrating the variation of the faradaic efficiency of H2 conversion over time.

[0066] Figure 7 represents an NMR spectrum of DES before and after the CO2 conversion reaction.

[0067] Figure 8 represents an IR-ATR spectrum of DES before and after the CO2 conversion reaction.

[0068] Figure 9 represents a diagram illustrating the variation of the faradaic efficiency of CC conversion with different average Pd particle sizes and different potentials.

[0069] Figure 10 represents a diagram illustrating the variation of the faradaic efficiency of H2 conversion with different average Pd particle sizes and different potentials.

[0070] With reference to Figures 1 and 2, the method for electrochemical conversion of CO2 according to the invention comprises the provision of an electrochemical device 100 comprising an anode compartment 1 and a cathode compartment 2 separated by a proton exchange membrane 3 (step a)). The experiments described below are carried out in an electrochemical cell called an H cell referenced 100 in Figure 1, but the method can be implemented with any other suitable electrochemical device, such as a proton exchange membrane cell, known under the terminology "PEM cell" (an acronym for Proton Exchange Membrane) illustrated in Figure 2 under the reference 100'.

[0071] The H-cell comprises a cathode compartment 2, intended for the CO2 conversion reaction on the surface of a cathode 5 made of Pd / C, an anodic compartment 1, intended for the water hydrolysis reaction generating protons and gaseous oxygen, and a proton exchange membrane 3 connecting the two compartments 1 and 2 and making it possible to supply the catholyte 4 with protons. These protons will then be used in the CO2 conversion reaction into carbon monoxide.

[0072] The cathode compartment 2 comprises a catholyte 4 consisting of a deep eutectic solvent DES, in particular a mixture of tetraethylammonium chloride and glycerol: TEACI: Gly (1: 2). It is possible to add an aqueous basic buffer solution, for example 0.1 M KHCO3 in a mass proportion of 50 / 50 so as to set the pH of the catholyte 4 at 8. The addition of this buffer solution makes it possible to avoid an acid pH which promotes a well-known parasitic reaction of production of H? from HzO on the surface of the Pd cathode 5.

[0073] According to other variants not illustrated, the DES is chosen from a mixture of at least one ammonium salt and at least one amine, a mixture of at least one ammonium salt and at least one polyol, for example glycerol, and a combination of these mixtures.

[0074] The choice of the nature of Pd is also an important parameter on the faradic efficiency of CO2 reduction. As will be seen below, a cathode 5 having Pd particles with an average particle size of less than 100 nm, or even less than 10 nm and even less than 4 nm is preferred.

[0075] The anode compartment 1 comprises an aqueous anolyte 6, for example the aqueous basic buffer solution of pH 8 already used for the catholyte 4. The anode 7 used is made of platinum Pt and the system allows the oxidation of water into O2 and

[0076] H + .

[0077] The proton exchange membrane 3 used is in particular in Nation™ 117.

[0078] A reference electrode 8 made of Ag / AgCl is arranged in the catholyte 4 to control the potential difference applied between the cathode 5 and the anode 7.

[0079] Agitation is maintained in catholyte 4 and anolyte 6 to facilitate the circulation of species, in particular the movement of CO2 captured by the DES towards cathode 5, especially if the DES is very viscous. As illustrated in Figures 1 and 2, agitation is obtained by magnetic bars 9 in the experiments carried out in the laboratory.

[0080] An outlet orifice 11, 12, communicating with the gaseous ceiling 13, is provided at the top of each of the compartments 1, 2 in order to recover respectively the gaseous oxygen formed during the hydrolysis of the water and the carbon monoxide resulting from the CO2 conversion reaction for subsequent use.

[0081] A bubbler 14, or any other bubbling device, is provided in order to carry out the bubbling of the CC in the cathode compartment 2 and saturate the catholyte 4 (step b)). The bubbler 14 is in particular configured to be movable in vertical translation so as to allow bubbling of the CO2 directly into the catholyte 4, with a view to optimal capture by the DES, and also to allow bubbling in the gaseous ceiling 13, with a view in particular to preventing the entry of air into the cathode compartment 2. Although not illustrated, this step b) of saturating the cathode compartment 2 with CO2 can be carried out by any other suitable methods.

[0082] A potential difference, for example of approximately -1.4 V, is applied between the working electrode 5 in Pd / C and the counter-electrode 7 in Pt allowing the reduction of CO2 into carbon monoxide (step c)). This potential difference can be advantageous because it limits in parallel the production of H2 which can be significant when applying other potentials. The CO obtained is gaseous, it is collected, recovered, at the outlet orifice 12 (step d)) with a view to its subsequent use, in particular for the manufacture of valuable molecules such as hydrocarbons.

[0083] The process of the invention is advantageously carried out at room temperature although a heat treatment up to 80°C is possible, particularly if the viscosity of the DES used is very high. Similarly, the process of the invention is carried out at atmospheric pressure. However, it is possible to increase the pressure to affect the solubility and capture of CO2 in the DES. For example, the maximum pressure possible in the H-cell illustrated in Figure 1 is 100 mbar relative and that in the PEM 100' cell is 1 bar relative.

[0084] Examples of implementation of the process and experimental measurements

[0085] In order to test and improve the efficiency of the electrochemical conversion of CO2, the influence of different factors was studied from an H-type electrochemical cell in the laboratory, including the value of the applied potential difference, the nature of the DES, the pH of the catholyte 4, and the nature of the Pd used at the cathode 5.

[0086] During these reactions, carbon monoxide and hydrogen were detected by micro-gas chromatography, notably with the Agilent MicroGC 990 model. In the micro-GC, the gases, CO and H2, are detected and separated using the MolSieve 5 A (MS-5A) column with helium as the carrier gas. The PoraPLOT U column, with helium as the carrier gas, allows the detection of CO2. The thermal conductivity detector (TCD) of the micro-GC is installed for both columns.

[0087] The gases at the cell outlet (i.e. at outlet 12 of cathode compartment 2) are measured using the micro-GC (previously calibrated with CO, H2 and CO2) and the faradic efficiencies (FE %) corresponding to the conversion of CO2 into CO and H2O into H2 are calculated using Faraday's law (see equations (3) and (4) where m = mass (g), I = current (A), t = time (sec), M = molar mass (g mol -1 ), n e- is the number of electrons, and F is the Faraday constant (C in mol -1 ) :

[0088] Influence of DES on CO2 conversion in the presence of a Pd cathode 5 as a function of the applied potential

[0089] CO2 conversion experiments on Pd are carried out in the absence and presence of DES, by varying the applied potential difference. The experimental conditions are as follows: in the absence of DES (control experiments): o Catholyte 4 for CO2 capture consisting of a 0.1 M aqueous KHCO3 solution o Anolyte 6: 0.1 M KHCO3 solution o Membrane 3: Nafion 117 ® o Room temperature ~ 20°C o Atmospheric pressure: 1 bar o Electrodes:

[0090] ■ Working electrode 5: Pd / C, palladium particles having an average particle size of 7-8 nm

[0091] ■ Counter electrode 7: Pt

[0092] ■ Reference electrode 8: Ag / AgCI in the presence of DES: o Catholyte 4 for CC capture is made up for DES of the mixture TEACI (tetraethylammonium chloride) and Gly (glycerol) (1:2), and a buffer solution of 0.1 M KHCO3 in a proportion of 50% by weight. The other elements are identical to the control experiment, without DES.

[0093] Process steps: o Bubbling of CO2 into cathode compartment 2 for at least 30 minutes to saturate catholyte 4 before reduction (step b)), o New bubbling of CO2 for 10 minutes before each potential change, o Sampling of gaseous products (CO and H2) at the cathode outlet 12 at each potential difference applied (step d)).

[0094] The faradaic efficiencies obtained for the production of CO and H2 (parasitic reaction on cathode 5) in the presence of DES and in the absence of DES (control) are recorded in the diagrams of figures 3 and 4 respectively.

[0095] The abscissa axis corresponds to the potential applied with respect to the reference electrode 8 and the ordinate axis corresponds to the faradaic efficiency, in Figure 3 the conversion of CO2 and in Figure 4 the parasitic conversion of H2O (production of H2). Curves a and a' represent the values ​​obtained with DES and curves b and b' represent the values ​​obtained for the experiments carried out without DES.

[0096] As can be seen in these figures 3 and 4, the presence of the deep eutectic solvent in catholyte 4 shows a faradaic efficiency twice as high for CO production as that obtained with the aqueous solution of KHCO3 without DES. Similarly, the parasitic reaction of H2 production is significantly reduced in the presence of DES. Thus, the presence of DES coupled to a Pd electrode 5 is effective for CO2 reduction. The beneficial effect of DES is also proven for the reduction of the parasitic reaction of H2 production at cathode 5.

[0097] Influence of DES on CO2 conversion in the presence of a Pd cathode over time

[0098] CO2 conversion experiments on Pd with an average particle size of 7-8 nm are carried out in the absence and presence of DES by applying the optimal potential difference for CO production (step c)). The faradaic efficiencies (y-axis of Figures 5 and 6) are calculated over time expressed in hours (h) (x-axis of Figures 5 and 6). The electrochemical device 100 and the experimental conditions are unchanged from those previously described.The sequence of steps b) and d), in both cases, is carried out as follows: o Bubbling of CO2 into cathode compartment 2 for 15 min before the reaction, o Chronoamperometry (CA) for 5 h at a potential leading to the best faradaic yields in CO production, i.e. a potential of -1.3 V vs Ag / AgCI in the absence of DES and a potential of -1.5 V vs Ag / AgCI in the presence of DES, o Sampling of gaseous products (CO and H2) during chronoamperometry and calculation of faradaic yields (see eq. 3 and 4).

[0099] The effect of the co-catalysis of CO2 into CO is illustrated in Figure 5: in the presence of DES (curve c), CO production is twice as high as in buffered medium alone (curve d) and remains stable for the duration of the experiment (3 - 4 h). Moreover, this experiment, carried out with a much shorter bubbling time, shows that CO2 capture by DES is very efficient and very fast. It should be noted that the pure TEACI:Gly (1:2) mixture solution is able to capture three times more CO2 (measured value: 0.09 mol CO2 / 1 kg TEACI:Gly (1:2)) than the aqueous solution (literature value: 0.03 mol CO2 / 1 kg H2O). Even with the addition of the buffer solution, DES retains a higher CO2 absorption capacity (~ 0.045 mol / kg TEACI:Gly (1:2)).

[0100] The production of H2 in the presence of DES (curve c') is also reduced compared to the aqueous solution (curve d') when the production of CO remains high (see figure 6).

[0101] Thus, the presence of DES in catholyte 4 has a positive effect on the reduction of CO2 to CO on the Pd electrode surface 5 over time. This shows the effect of co-catalysis of DES with the palladium-based electrode 5. Furthermore, it is important to note that the DES structure remains intact (i.e., undegraded) throughout the experiment, which was confirmed by NMR and IR analyses (see Figures 7 and 8) before and after chronoamperometry. The NMR and IR spectra are identical before and after the reaction (the curves are superimposed on Figures 7 and 8).

[0102] This justifies the use of DES for both CO2 capture and reduction on the Pd cathode 5.

[0103] Influence of Pd shape on CO2 conversion in the presence or absence of DES in catholyte 4 for several applied potentials

[0104] The experimental conditions and the reduction process are identical to those used for the study of the influence of DES on CO2 conversion (Figure 3). The faradaic efficiencies are calculated at different potentials and, in a first case (curve e), the cathode 5 used is made of Pd / C comprising particles with an average particle size between 7 and 8 nm and, in a second case (curve f), the cathode 5 comprises Pd particles with an average particle size less than 4 nm.

[0105] At the same time, the parasitic reaction of H2 production from water is studied (Figure 10) and the evolution of the faradaic yields is illustrated in the case of Pd particles with an average particle size of 7 to 8 nm (curve e') and Pd particles with an average particle size of less than 4 nm (curve f').

[0106] As illustrated in curves e and f, the efficiency of CO2 conversion to CO is higher in the presence of Pd particles with an average particle size of less than 4 nm. In parallel, it is observed that the parasitic reaction of H2 production is also reduced when using a cathode 5 of Pd particles with an average particle size of less than 4 nm.

[0107] Thus, the invention proposes a method that controls the generation of CO by the choice of the DES constituting the catholyte 4, the nature of the material of the cathode 5 and its shape (Pd nanoparticles) as well as the potential applied during electrolysis. This method operating at room temperature and atmospheric pressure, it is entirely compatible with an electricity supply produced from renewable energies. In addition, the DES used for the conversion is not degraded and can be reused. Note that, unlike some authors of research in this field, the calculations of the faradaic efficiencies carried out in the invention take into account the carbon reduction products (CO, CH4, etc.) and consider the production of H? (competitive reaction of the reduction of the HzO molecule) for more precise results.

Claims

CLAIMS 1. A method for the electrochemical conversion of CO2 into carbon monoxide (CO) comprising the steps of: a) providing an electrochemical device (100) comprising: - an anode compartment (1) comprising an anode (7) and an anolyte (6) comprising at least one aqueous electrolyte intended for the electrolysis of HzO according to the following equation (I): - a cathode compartment (2) intended for the reduction of CO2 according to the following equation (II): the cathode compartment (2) comprising a palladium-based cathode (5) and a catholyte (4) comprising a deep eutectic solvent (DES), and - a proton exchange membrane (3) between the anode compartment (1) and the cathode compartment (2), b) introducing CO2 into the cathode compartment (2) until said cathode compartment (2) is saturated, a portion of the introduced CO2 being captured by the DES, c) applying a potential between the anode (7) and the cathode (5) so that the CO2 captured by the DES is reduced to carbon monoxide (CO) in the cathode compartment (2) according to equation (II) and H2O is oxidized to oxygen in the anode compartment (1) according to equation (I), the protons H +migrating from the anode compartment (1) to the cathode compartment (2) through the proton exchange membrane (3), and d) recovery of CO at the outlet of the cathode compartment (2), the deep eutectic solvent being chosen from a mixture of at least one ammonium salt and at least one amine, a mixture of at least one ammonium salt and at least one polyol such as glycerol, and a combination of these mixtures.

2. Electrochemical conversion method according to claim 1, in which the cathode (5) consists of palladium deposited on carbon (Pd / C).

3. Electrochemical conversion method according to claim 1 or 2, wherein the cathode (5) comprises Pd particles, said Pd particles having an average particle size between the size of a Pd atom and 1.5 pm, in particular between the size of a Pd atom and 100 nm, and in particular between the size of a Pd atom and 4 nm.

4. Electrochemical conversion method according to any one of claims 1 to 3, which is carried out by applying a potential V between - 1.7 V and - 1.4 V relative to a reference electrode (8).

5. Electrochemical conversion method according to any one of claims 1 to 4, wherein the catholyte (4) further comprises a buffer solution of basic pH.

6. Electrochemical conversion process according to claim 5, wherein the basic pH buffer solution of the catholyte (4) consists of an aqueous solution of KHCO3, KCl or KOH and, preferably, of an aqueous solution of KHCO3.

7. Electrochemical conversion method according to one of claims 1 to 6, in which the anolyte (6) comprises a solution chosen from H2O, KHCO3 and H2SO4.

8. Electrochemical conversion process according to any one of claims 1 to 6, in which the anolyte (6) comprises, or consists solely of, a buffer solution of basic pH, preferably a KHCO3 solution.

9. Electrochemical conversion process according to any one of claims 1 to 8, wherein the ammonium salt is chosen from tetraethylammonium chloride, choline chloride and diethylammonium chloride, the amine is chosen from monoethanolamine, N-methyldiethanolamine and diethanolamine, and / or the polyol is glycerol.

10. Electrochemical conversion process according to any one of claims 1 to 9, wherein the catholyte (4) comprises a deep eutectic solvent DES consisting of the mixture of tetraethylammonium chloride and glycerol, preferably in the molar proportion TEACI:Gly of 1:2, and a buffer solution of KHCO3, preferably at 0.1 M with a DES / buffer solution mass ratio of 1 / 1 and the cathode (5) comprises Pd on carbon particles, the average size of said particles being less than 4 nm.

11. Electrochemical conversion process according to any one of claims 1 to 10, which is carried out at a temperature between 20°C and 50°C, in particular between 20°C and 30°C, for example a temperature of approximately 25°C.

12. Electrochemical conversion process according to any one of claims 1 to 11, which is carried out under a pressure between atmospheric pressure and 1 bar relative, in particular between atmospheric pressure and 100 mbar relative, for example at atmospheric pressure.

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