ELECTROCHEMICAL CO2 CONVERSION

Abstract

The present invention is related to the electrochemical conversion of CO.sub.2 and provides the use of Gas Diffusion Electrode with an aprotic solvent in such conversion of CO.sub.2 as well as an electrochemical cell for use in such conversion. The application and electrochemical cell as herein provided are particularly useful in the conversion of CO2 into oxalate/oxalic acid.

Claims

1-12. (canceled)

13. An electrochemical cell for the electrochemical conversion of CO.sub.2, said electrochemical cell comprising: (a) a gas diffusion electrode as a cathode; (b) a gaseous CO.sub.2 inlet to the gas diffusion electrode; (c) a supporting electrolyte in an aprotic solvent as a catholyte; and (d) an anode, wherein the electrochemical cell is a single-chamber electrochemical cell.

14. The electrochemical cell according to claim 13, wherein the supporting electrolyte is selected from the group consisting of tetraalkylammonium salts of tetrafluoroborates, tetraalkylammonium salts of perchlorates, and tetraalkylammonium salts of hexafluorophosphates, the supporting electrolyte being dissolved in an aprotic solvent.

15. The electrochemical cell according to claim 14, wherein the tetraalkylammonium salts are tetraethylammonium salts or tetrabutylammonium salts.

16. The electrochemical cell according to claim 13, wherein the aprotic solvent is selected from the group consisting of acetonitrile, dimethyl sulfoxide, dimethylformamide, and propylene carbonate.

17. The electrochemical cell according to claim 16, wherein the aprotic solvent is acetonitrile.

18. The electrochemical cell according to claim 13, wherein the anode in the electrochemical cell is a sacrificial anode.

19. The electrochemical cell according to claim 18, wherein the anode in the electrochemical cell is a sacrificial zinc anode or a sacrificial aluminum anode.

20. The electrochemical cell according to claim 13, further comprising a catholyte inlet and a catholyte outlet.

21. The electrochemical cell according to claim 13, wherein the gas diffusion electrode comprises a metal or metal oxide catalyst.

22. The electrochemical cell according to claim 21, wherein the gas diffusion electrode comprises metal or metal oxide catalyst nanoparticles.

23. The electrochemical cell according to claim 21, wherein the metal or metal oxide catalyst comprises a metal catalyst selected from Pb, Ti, Fe, Mo, or combinations thereof.

24. The electrochemical cell according to claim 21, wherein the metal or metal oxide catalyst is provided on a porous support.

25. The electrochemical cell according to claim 24, wherein the metal or metal oxide catalyst is provided as finely dispersed nanoparticles on the porous support.

26. The electrochemical cell according to claim 24, wherein the porous support comprises hydrophobic carbon black agglomerates.

27. The electrochemical cell according to claim 13, wherein the gas diffusion electrode is a double-layer gas diffusion electrode comprising: a catalyst layer comprising a metal or metal oxide catalyst; and a gas diffusion layer consisting of a hydrophobic porous material.

28. The electrochemical cell according to claim 27, wherein the hydrophobic porous material is polytetrafluoroethylene mixed with a pore former.

29. The electrochemical cell according to claim 27, wherein the gas diffusion electrode comprises a current collector.

30. The electrochemical cell according to claim 29, wherein the current collector consists of a layer of an electrochemically inert but conductive material.

31. The electrochemical cell according to claim 29, wherein the current collector consists of a layer of a graphite or a layer of stainless steel mesh.

32. A method for the electrochemical conversion of CO.sub.2, the method comprising: supplying CO.sub.2 in gaseous form to the cathode of the electrochemical cell according to claim 13 through the catholyte.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0017] Aspects of the invention will now be described in more detail with reference to the appended drawings, wherein same reference numerals illustrate same features and wherein:

[0018] FIG. 1—Provides a schematic representation of a single layer GDE cathode for use in the methods and electrochemical cell according to the invention, comprising as a Catalyst Layer (CL) Metal Catalyst Nanoparticles supported on (hydrophobic) Carbon Black Agglomerates. The CL is exposed to the electrolyte and a supply of gaseous CO.sub.2;

[0019] FIG. 2—Provides a schematic representation of a double layer GDE cathode for use in the methods and electrochemical cell according to the invention, comprising the CL as shown in FIG. 1, and a second Gas Diffusion Layer (GDL). The CL is exposed to the electrolyte and the GDL to a supply of gaseous CO.sub.2;

[0020] FIG. 3—Is a schematic representation of the double layer GDE as shown in FIG. 2, further comprising a Current Collector (CC) on the side exposed to the electrolyte;

[0021] FIG. 4—Provides a schematic representation of a single chamber electrochemical cell for use in the electrochemical conversion of CO.sub.2 in an aprotic solvent using a GDE as cathode;

[0022] FIG. 5—Shows a schematic representation of an electrochemical setup used to test the conversion of CO.sub.2 to oxalate in an aprotic solvent using different metal catalysts and with increasing concentrations of water in the electrolyte solution.

[0023] FIG. 6—Linear sweep voltammetry (LSV) experiment (left, dV.Math.dt.sup.−1=5 mV.Math.s.sup.−1) and galvanostatic experiments (right, I=10 mA)—Employing a Mo wire Working Electrode (WE), continuously supplied CO.sub.2 ({dot over (V)}=10 mL.Math.min.sup.−1) for various concentrations of water c(H.sub.2O). Left: Measured current I/mA plotted over WE potential E.sub.WE/V. N.sub.2 purged reactor (red plot) as blank experiment. Right: WE potential E.sub.WE/V plotted over reaction time t/min.

[0024] FIG. 7—Faradaic Efficiency (FE) to Oxalate (right) for the respective experiments, at various water concentrations in solution.

[0025] FIG. 8—LSV (dV.Math.dt.sup.−1=5 mV.Math.s.sup.−1) experiments employed in microflow cell setup (see FIG. 4). Measured current density i/mA.Math.cm.sup.−2 plotted over WE potential E.sub.WE/V. N.sub.2 purged reactor (red plot) as blank experiment. Left: Non-porous Pb/PTFE electrode. Right: two-layer GDE with 5 wt. % Pb catalyst layer.

[0026] FIG. 9—Potentiostatic experiments at an applied working electrode potential E.sub.WE=−2 V. Measured current density i/mA.Math.cm.sup.−2 plotted over the reaction time t/min. Results of non-porous Pb/PTFE electrode (circle) compared to two-layer GDE with 5 wt. % Pb catalyst layer (cross).

[0027] FIG. 10—Galvanostatic experiments in a two-electrode configuration employing a porous lead electrode Full-Pb. Measured cell voltage U/V (left y axis) and applied current density i/mA.Math.cm.sup.−2 (right y axis) plotted over the reaction time t/min.

[0028] FIG. 11—Comparison of results displayed in FIG. 10 with highest reported literature value in terms of achieved current density without the use of a GDE in CO.sub.2 to oxalate reduction. Results of galvanostatic experiments in a two-electrode configuration employing a porous Pb electrode Full-Pb. Applied current density i/mA.Math.cm.sup.−2 plotted over the measured cell voltage U/V.

DESCRIPTION OF THE INVENTION

[0029] The electrochemical reduction of CO.sub.2 in general is an emerging technology as a means to utilize CO.sub.2 from waste streams and electrical energy from renewable sources to produce value-added chemicals or fuels. The reaction at submerged electrodes in a liquid electrolyte at standard conditions is limited by the low solubility of CO.sub.2 in the electrolyte. Consequently, the application of GDEs can alleviate this challenge by using gaseous CO.sub.2 as a feedstock, the CO.sub.2 is dissolved in the applied solvent inside the electrode (SoA aqueous CO.sub.2 reduction). The application of aprotic solvents allows the CO.sub.2 reduction to oxalate with high faradaic efficiencies. Aprotic solvents additionally increase the CO.sub.2 solubility, allowing the reduction at reduced CO.sub.2 concentrations (CO.sub.2 reduction without purification of e.g. flue gas feedstock is possible). This further provides a more selective reduction [17]. The application of GDE for the electrochemical CO.sub.2 reduction to oxalate has not been reported. For CO.sub.2 reduction in aqueous solution, one important parameter towards commercialization of an electrochemical CO.sub.2 reduction is a sufficient reaction rate (per geometrical electrode surface area), expressed by the current density i in mA.Math.cm.sup.−2. As the CO.sub.2 solubility in aqueous solution is rather low, around 35 mmol/L, the bottleneck in electrochemical CO.sub.2 reduction is often the mass transport limitation of CO.sub.2 to the electrocatalytically active site. This is especially true for alkaline reaction media, which have often shown to be the preferred reaction conditions as a suppression of the HER enhances the selectivity (FE) to the desired CO.sub.2 reduction product. In alkaline media the CO.sub.2 concentration is even more reduced due to the formation of HCO.sub.3.sup.−. It is generally accepted that the commercialization of the aqueous electrochemical reduction of CO.sub.2 to products requires the use of gas diffusion electrodes to reach commercially relevant current densities in the range from 100 to 500 and above mA.Math.cm.sup.−2.

[0030] Within the context of the present invention several configurations are conceivable: [0031] a single-layer-GDE (e.g. shown in FIG. 1): In this configuration the cathode is a GDE that consists of a single porous layer of a hydrophobic material (either the electrocatalyst itself or the electrocatalyst on a porous support (e.g. carbon)). In a particular embodiment the single porous layer is composed of an electrocatalyst on a porous support; more in particular metal catalyst nanoparticles supported on (hydrophobic) carbon black agglomerates. [0032] a double-layer-GDE (e.g. shown in FIGS. 2 and 3): In this configuration the GDE consists of two porous layers, which are fixed e.g. by calendaring them together. The first layer, the catalyst layer (CL) includes a finely dispersed electrocatalyst metal, e.g. on a porous carbon support. The second layer (Gas Diffusion Layer, GDL) consists of a hydrophobic porous material (e.g. made from PTFE mixed with a pore former such as NH.sub.4HCO.sub.3 ammonium bicarbonate). The described three-phase boundary forms between the intersection of CL (electrolyte filled) and the GDL (gas side). The GDE can further be equipped with a current collector (CC) (FIG. 3), an electrochemically inert, but highly conductive material (e.g. graphite, stainless steel mesh) providing a uniform current distribution along the surface of the GDE.

[0033] Where the GDE cathode comprises a gas diffusion layer consisting of a hydrophobic porous material, the current density can be enhanced by increasing the surface area of the electrode. Further, the supply of gaseous CO.sub.2 to the backside of this layer of porous material allows for a more efficient mass transport of CO.sub.2 to the active site. In contrast, it is believed that with a flat electrode the reachable current density is limited by the relatively slow charge transfer of electrons to CO.sub.2. The GDEs comprising a layer of porous material according to an embodiment of the present invention, further provide the advantage of allowing to be configured between charge-transfer and mass-transport controlled regime. This maximizes the achievable current densities at low local CO.sub.2 concentrations, favoring the formation of oxalate and increasing the selectivity to the desired product. Maximizing the achievable current densities at low local CO.sub.2 concentrations is of great importance for the application of the electrochemical cells according to the present invention in the reduction of CO.sub.2 in diluted CO.sub.2 waste gas streams (e.g. flue gases, waste gas streams from a biorefinery) without or with a less extensive gas washing and purification process step, reducing overall process costs. With a flat electrode, which therefore does not comprise said layer of porous material, the reachable current density is limited by the relatively slow charge transfer of electrons to CO.sub.2 therefore providing a worse solution to the reduction of CO.sub.2 at low concentrations. n a particular embodiment the GDEs used in the context of the present invention comprise a CL wherein the electrocatalyst is fixed on a porous support, e.g. by physically mixing with a binder (e.g. PTFE), precipitation and/or electrodeposition. More in particular the CL comprises metal catalyst nanoparticles supported on (hydrophobic) carbon black agglomerates.

[0034] A schematic representation of an electrochemical cell for the electrochemical conversion of CO.sub.2 in an aprotic solvent using a GDE as cathode is shown in FIG. 4. CO.sub.2 gas is continuously supplied to the GDL side of the GDE. The electrochemical cell can either be operated in a continuous mode, meaning both CO.sub.2 and the electrolyte in the cathode chamber (catholyte) are continuously supplied, liquid (e.g. oxalic acid) or precipitated (e.g. zinc oxalate) products are taken out of the reactor with the catholyte stream. This catholyte can also be recycled, directly or after the reaction products have been separated from the electrolyte. Another mode of operation is a semi-batch mode, where, while the CO.sub.2 is continuously supplied through the GDL of the gas diffusion electrode, the catholyte is kept in the cathode chamber in a batch-operated mode. The cell is operated by applying an external voltage (supplied by an potentiostat) between the two electrodes, at the cathode CO.sub.2 is reduced to oxalate (CO and carbonate CO.sub.3.sup.2−, formate and/or hydrogen may be produced as side products).

[0035] The Anode reaction can be a sacrificial anode (e.g. zinc, aluminium), producing zinc oxalate or aluminium oxalate as end products (hardly soluble, precipitates in solution). Alternatively, other established oxidation reactions such as oxygen evolution reaction OER, hydrogen oxidation reaction (HOR, possibly also at GDE or in a membrane) can be applied, producing oxalic acid as the end product. The oxidation and reduction reaction at respectively the anode and cathode can either be performed in a single chamber (such as shown in FIG. 4) where the anode electrolyte (anolyte)—and cathode electrolyte (catholyte) are the same, or can be separated by a conducting membrane.

[0036] As mentioned herein before, one of the characteristics of the method according to the invention is the use of an aprotic solvent at the cathode reaction. Catholytes could for example be selected from 0.1 M tetraalkylammonium salts as cations, e.g. tetraethylammonium NEt.sub.4.sup.+ or tetrabutylammonium NBu.sub.4.sup.+ and e.g. tetrafluoroborates BF.sub.4.sup.−, perchlorates ClO.sub.4.sup.− or hexafluorophosphates PF.sub.6.sup.− as anions in aprotic solvents (e.g. AN, DMF, PC, DMSO). Aprotic meaning no acidic hydrogen bond such as O—H, N—H. In a particular embodiment the catholyte used in the method according to the invention consists of a tetraalkylammonium tetrafluoroborate salt as supporting electrolyte, e.g. tetraethylammonium tetrafluoroborate NEt.sub.4BF.sub.4 or tetrabutylammonium tetrafluoroborate NBu.sub.4BF.sub.4 in an aprotic solvent (e.g. AN, DMF, PC, DMSO). In a more particular embodiment 0.1 M tetraethylammonium tetrafluoroborate NEt.sub.4BF.sub.4 in AN.

[0037] If a conducting membrane is applied in a multi-chamber reactor, the anolyte can differ from the catholyte, e.g. an aqueous electrolyte for the OER (water oxidation) can be applied. Established electrolytes are e.g. aqueous solutions of alkali metal (bi-)carbonates, (hydrogen-) sulfates, (bihydrogen-, hydrogen-) phosphates or halide salts. As shown below, the best results are however achieved in a single chamber reaction as it is difficult to fully prevent water cross-over from the anode to the cathode chamber, and it has been found that the presence of water at the cathode side has a negative effect on the CO.sub.2 to the FE to oxalate at the cathode.

[0038] The cathode catalyst layer as used herein preferably comprise metal or metal oxide catalysts selected from the group consisting of Pb, Ti, Fe, Mo or combinations thereof; more in particular metal nanoparticles selected from Pb, Ti, Fe, Mo or combinations thereof. In one embodiment the metal catalysts are selected from the group consisting of Pb, Fe, Mo or combinations thereof; more in particular metal nanoparticles selected from Pb, Fe, Mo or combinations thereof. In another embodiment the metal catalysts are selected from the group consisting of Pb, Mo or combinations thereof; more in particular metal nanoparticles selected from Pb, Mo or combinations thereof. In a preferred embodiment the cathode catalyst layer comprises Pb as metal catalyst, in particular Pb nanoparticles.

RESULTS

Metal Patalyst Screening and Effect of c(H.SUB.2.O) on Catalyst Performance

[0039] FIG. 5 shows a schematic representation of an electrochemical setup used to test the conversion of CO.sub.2 to oxalate in an aprotic solvent using different metal catalysts and with increasing concentrations of water in the electrolyte solution. The cell is a H-type cell with a electrolyte volume of V.sub.E=20 mL of 0.1 M tetraethylammonium tetrafluoroborate (Et.sub.4NBF.sub.4) in AN with a Ag/AgNO.sub.3 reference electrode. The applied AN was dried over 3 Å molecular sieves for at least 48 h, the tetraethylammonium tetrafluoroborate was recrystallized from methanol and dried under vacuum. In experiments with increased c(H.sub.2O), demineralized water was added pre-experiment.

[0040] Pb, Ti, Fe, and Mo wires with a diameter of ø=0.5 mm and a length of I=25 cm were employed as working electrodes, while a Zn wire with a diameter of ø=0.5 mm and a length of I=50 cm was used as the counter electrode. The measurements were performed in a one compartment setup, without the use of a membrane. After galvanostatic measurements, the AN was evaporated and the solid residue of Et.sub.4NBF.sub.4, ZnC.sub.2O.sub.4 and Zn(HCOO).sub.2 is picked up in 1 M H.sub.2SO.sub.4 and the produced oxalate and formate is determined via HPLC. The water concentration of the employed electrolyte was assessed using Karl-Fischer Titration.

[0041] Linear sweep voltammetry (LSV) experiments were employed to investigate the activity of the employed metal catalysts. Galvanostatic measurements were utilized to assess the product distribution in aprotic conditions and with added water impurities.

[0042] FIG. 6 shows that the onset potential is shifted towards less negative potentials with increasing c(H.sub.2O), indicating an increased activity of the metal catalyst at higher water concentrations. This effect is related to an increasingly predominant side reaction (such as HER, formate production) with increasing c(H.sub.2O).

[0043] To investigate the shift in potential with increasing water concentrations, galvanostatic measurements were performed, collecting the dissolved and precipitated reaction products, quantifying them using HPLC. FIG. 7 shows the FE(Oxalate) for the four different metal catalysts plotted over log(c(H.sub.2O)). The results show that indeed a decrease of FE(Oxalate) towards other reaction products is the cause for the increased activity of the applied catalyst metal wires at higher c(H.sub.2O). For the Pb wire, the decrease in FE(Oxalate) corresponded to an increase in c(Formate). For Mo, Fe and Ti, no formate was detected. These results can be explained with the activities of the respective metals in aqueous solution. As stated in the introduction, CO.sub.2 reduction catalysts are categorized into groups including CO forming catalysts, hydrocarbon forming catalysts, formate forming catalysts and catalysts which show no activity towards CO.sub.2 reduction. Pb is part of the group forming formate in aqueous solutions, explaining the shift from oxalate towards formate with an increasing c(H.sub.2O) in the electrolyte. In contrast, Mo, Fe and Ti are metal catalysts which show no activity towards CO.sub.2 reduction in aqueous solution, as the overpotential required for the HER is too low. The observed reaction in water is therefore the formation of H.sub.2 via water reduction.

[0044] These experiments accordingly show that Mo and Fe show intermediate FE(Oxalate), Ti shows the lowest and Pb the highest FE(Oxalate) at low c(H.sub.2O). The present results clearly show that the activity of Mo, Fe and Ti metal catalysts towards CO.sub.2 reduction drops with increasing c(H.sub.2O) in the electrolyte.

Pb Metal Catalyst Comparison of a Non-Porous Electrode with a Two-Layered GDE Electrode

[0045] Having identified Pb as the metal catalyst with the highest activity towards CO.sub.2 reduction in an aprotic environment two types of electrodes were prepared to validate the cell setup and to compare the applicability in an industrial electrochemical conversion of CO.sub.2.

[0046] NPPb100: A non-porous Pb/PTFE electrode was prepared by mixing Pb powder with PTFE powder in a knife mill with a mass ratio of Pb:PTFE of 94:6. The mixed powder was consequently pressed to a cake at a pressure of 5 bar. The cake was then rolled down in 0.05 mm steps using a roll down to a final thickness of 0.5 mm.

[0047] Pb5 GDE: A porous, two-layered GDE was prepared based on the production procedures of the patented VITO CORE® GDEs. The gas diffusion layer (GDL) was prepared by sieving NH.sub.4HCO.sub.3 (pore former) to achieve a uniform particle size. Consequently, NH.sub.4HCO.sub.3 and PTFE are pressed to form flakes in rolling cylinders filled with metal balls of different weight. The flakes are mixed and cut with graphite in a knife mill afterwards to achieve a mass ratio of NH.sub.4HCO.sub.3:PTFE:Graphite of 66:29:5. The mix is pressed to a cake with a pressure of 5 bar and the cake is rolled down to a thickness of 1 mm. The catalyst layer was produced by mixing Norit Activated Carbon, PTFE and Pb metal powder in a knife mill in a ratio of Norit:PTFE:Pb of 75:20:5. Likewise to the GDL, the mixed powder was pressed to a cake and rolled down to a size of 1 mm. Finally, GDL and catalyst layer were rolled down together to a final thickness of 0.5 mm. The prepared electrode was kept at 70° C. overnight.

[0048] Full-Pb-GDE: A porous, two-layered GDE was prepared based on the production procedures of the patented VITO CORE® GDEs. The gas diffusion layer (GDL) was prepared by sieving NH.sub.4HCO.sub.3 (pore former) to achieve a uniform particle size. Consequently, NH.sub.4HCO.sub.3 and PTFE are pressed to form flakes in rolling cylinders filled with metal balls of different weight. The flakes are mixed and cut in a knife mill afterwards to achieve a mass ratio of NH.sub.4HCO.sub.3:PTFE of 3:7. The mix is pressed to a cake with a pressure of 5 bar and the cake is rolled down to a thickness of 0.5 mm. The catalyst layer was produced by mixing NH.sub.4HCO.sub.3, PTFE and Pb metal powder in a knife mill in a ratio of NH.sub.4HCO.sub.3:PTFE:Pb of 36:18:195. Likewise to the GDL, the mixed powder was pressed to a cake and rolled down to a size of 0.5 mm. Finally, GDL and catalyst layer were rolled down together to a final thickness of 0.6 mm. The prepared electrode was kept at 70° C. overnight.

[0049] FIG. 8 shows the first LSV experiments performed in the microflow cell. Both LSVs show an enhanced activity comparing CO.sub.2 to N.sub.2, which is related to the electrochemical reduction of CO.sub.2. A distinct reduction peak for CO.sub.2 is visible for both electrodes at −1.8V vs. Ref. All in all, a drastic increase in measured current densities is achieved compared to experiments employing a metal wire. This can be explained by the optimized geometry of both the electrode as well as the applied electrochemical cell, reducing the overall required cell voltage. A difference in the slope of the increasing current density between NPPb100 and Pb5 GDE is visible. A steeper slope for the NPPb100 indicates that an increased Pb surface area is improving the reaction rate of the charge transfer controlled reaction. A regime where the reaction becomes mass transport limited is not reached yet, the compliance voltage limit of the potentiostat is reached for both electrodes at around a current density of i=40 mA.Math.cm.sup.−2, corresponding to a current of I=400 mA. The shifted baseline for the Pb5 GDE (FIG. 8, right) is related to the applied carbon material, as the current flow below −1.8 V (for both CO.sub.2 and N.sub.2 purged experiments) is caused by capacitive currents. Preliminary experiments have shown that the measured current density strongly depends on the applied scan rate during the LSV experiment and is reduced with reduced scan rates.

[0050] Remarkable however is how the performance of the GDE compares to the non-porous electrode. Compared to the later the GDE comprises 20 times less of the metal catalysts still shows only a slightly lower current density. In addition, when looking at potentiostatic experiments over time (see FIG. 9) current densities in the GDE or stable compared to the fluctuations in the non-porous electrode. This could be related to the reduced current density, and therefore gas evolution, or due to the hydrophobic nature of the GDL.

[0051] FIG. 10 indicates the achievable current densities when applying a GDE. FIG. 10 shows the corresponding cell voltages of a two electrode configuration applying a Full-Pb-GDE. It shows that current densities, depicted in the graph in black, of above 100 mA.Math.cm.sup.−2 can be achieved, which has not been reported in literature. In FIG. 10 the cell voltages are instead depicted in grey. FIG. 11 shows the comparison of these results to the highest current densities reported in literature for CO.sub.2 to oxalate electroreduction under ambient conditions [11]. The results indicate that higher current densities can be achieved at lower cell voltages compared to literature. At a cell voltage of 4 V, a current density of 100 mA.Math.cm.sup.−2 could be reached, an increase by a factor 3 compared to the results reported in literature.

[0052] In order to achieve a stable CO.sub.2 reduction at high current densities, it will be imperative to optimize the hydrophobicity of the GDL even further, not only to provide a sufficient CO.sub.2 supply to the active site, but also to prohibit evolving gas from exiting the reactor through the catalyst layer and the electrolyte.

[0053] Where the current experiments successfully show the application and design of a GDE cathode in the electrochemical conversion of CO.sub.2, experiments to further improve the GDE in terms of long-term stability, FE, CE and concerning the whole electrochemical system (solvent, anode reaction, supporting electrolyte) are currently being conducted

[0054] This technology can be valorized in the electrochemical reduction of CO.sub.2, in particular to produce oxalate/oxalic acid in a sustainable manner (utilizing waste CO.sub.2) and has the potential to be cheaper compared to existing technologies with main cost factors being the capital cost of the electrode, the electrode lifetime/stability and most importantly the electricity tariff. Oxalic acid, either as a bulk chemical or as an intermediate to produce other value added chemicals (e.g. through further reduction to produce ethylene glycol, which is applied as a precursor to produce polymers such as PET. The total PET demand for plastics in Europe (EU28+NO/CH) was 4 Mt in 2018 [18])

[0055] Compared to existing technology the methods and devices used in the present invention show the following distinguishing characteristics. [0056] a) Thermal CO.sub.2 Reduction/Production of Oxalic Acid in General [0057] Production under ambient conditions without pressure/increased temperature [0058] No additional oxidizing agent/co-catalyst (such as nitric acid) required [0059] b) Electrochemical Production of Oxalic Acid (using conventional electrodes, through the dimerization through formate) [0060] Application of GDE for increased mass transport properties in solution, allows application at reduced CO.sub.2 concentrations, and accordingly provides improved CO.sub.2 conversion. [0061] High selectivity with the application of aprotic solvent (only gaseous side product: CO), as opposed to aqueous CO.sub.2 reduction. [0062] CO.sub.2 Reduction to oxalate can be done in a one-step process as opposed to two-step process through formate with expensive downstream processing of diluted formate/formic acid solutions in between (e.g. through rectification of solution).

[0063] The aforementioned functional characteristics of the present invention are based on the application of an electrochemical cell of which the technical characteristics can be summarized in the following numbered embodiments or any combinations thereof.

[0064] Embodiment 1—Use of an electrochemical cell for the electrochemical conversion of CO.sub.2 characterized in that said electrochemical cell comprises a Gas Diffusion Electrode (GDE) as cathode and that the CO.sub.2 is supplied in gaseous form to the cathode where it is dissolved in a catholyte ion solution comprising an aprotic solvent.

[0065] Embodiment 2—Use according to embodiment 1, wherein the electrochemical cell is operated in a continuous mode wherein the catholyte ion solution and the CO.sub.2 are continuously supplied to the cathode chamber.

[0066] Embodiment 3—Use according to embodiment 1, wherein the electrochemical cell is operated in a semi-batch mode, wherein the CO.sub.2 is continuously supplied to the cathode chamber and the catholyte ion solution is kept in the cathode chamber in a batch-operated mode.

[0067] Embodiment 4—Use according to any one of the previous embodiments wherein the-supporting electrolyte in the catholyte ion solution is selected from the group consisting of tetraalkylammonium salts of tetrafluoroborates, perchlorates or hexafluorophosphates as supporting electrolytes in aprotic solvents; in particular tetraalkylammonium salts such as tetraethylammonium or tetrabutylammonium. It would be clear to the skilled in the art that the supporting electrolyte salt comprises at least one cation and one anion.

[0068] Embodiment 5—Use according to any one of the previous embodiments wherein the aprotic solvent is selected from the group consisting of Acetonitrile, Dimethyl Sulfoxide, Dimethylformamide, and Propylene Carbonate; in particular Acetonitrile.

[0069] Embodiment 6—Use according to any one of the previous embodiments wherein the anode in the electrochemical cell is a sacrificial anode, such as a sacrificial Zinc or Aluminium anode.

[0070] Embodiment 7—Use according to any one of the previous embodiments wherein the electrochemical cell is a single chamber electrochemical cell.

[0071] Embodiment 8—Use according to embodiment 1 wherein the GDE comprises a metal or metal oxide catalyst; in particular metal or metal oxide catalyst nanoparticles.

[0072] Embodiment 9—Use according to embodiment 8, wherein the metal or metal oxide catalysts are selected from Pb, Ti, Fe, Mo or combinations thereof.

[0073] Embodiment 10—Use according to embodiments 8 or 9, wherein the metal or metal oxide catalyst is provided on a porous support; in particular finely dispersed as nanoparticles on a porous support.

[0074] Embodiment 11—Use according to any one of the previous embodiments wherein the GDE cathode is a single-layer-GDE cathode.

[0075] Embodiment 12—Use according to any one of the previous embodiments wherein the GDE cathode is double-layer-GDE cathode.

[0076] Embodiment 13—Use according to embodiment 12, wherein the double-layer-GDE cathode comprises a catalyst layer comprising the metal or metal oxide catalyst according to any one of claims 8 to 10; and a gas diffusion layer consisting of a hydrophobic porous material; in particular polytetrafluoroethylene (PTFE) mixed with a pore former such as ammonium bicarbonate.

[0077] Embodiment 14—Use according to any one of embodiments 8 to 13, wherein the GDE comprises a current collector, in particular a current collector consisting of a layer of an electrochemically inert but highly conductive material such as a graphite or stainless steel mesh.

[0078] Embodiment 15—Use according to embodiment 10, wherein the metal or metal oxide catalyst is provided on a support of hydrophobic carbon black agglomerates.

[0079] Further to the foregoing embodiments related to the use of a GDE Cathode with an aprotic solvent in the electrochemical conversion of CO.sub.2, it is also an object of the present invention to provide GDE's for use in such electrochemical cell and the cell thus obtainable as summarized in the following numbered embodiments or combinations thereof.

[0080] Embodiment 16—An electrochemical cell for use in the electrochemical conversion of CO.sub.2, said electrochemical cell comprising; [0081] a. GDE cathode, [0082] b. a gaseous CO.sub.2 inlet to the GDE cathode, and [0083] c. a supporting electrolyte in an aprotic solvent.

[0084] Embodiment 17—The electrochemical cell according to embodiment 16, further comprising a sacrificial anode, such as a sacrificial Zinc or Aluminium anode.

[0085] Embodiment 18—The electrochemical cell according to claim 16, further comprising a catholyte inlet and outlet.

[0086] Embodiment 19—The electrochemical cell according to embodiments 16 to 18, wherein said cell is a single-chamber electrochemical cell.

[0087] Embodiment 20—The electrochemical cell according to embodiment 16, wherein the supporting electrolyte is selected from the group consisting of tetraalkylammonium salts of tetrafluoroborates, perchlorates or hexafluorophosphates as cation and anion, respectively; in particular tetraalkylammonium salts such as tetraethylammonium or tetrabutylammonium.

[0088] Embodiment 21—The electrochemical cell according to embodiment 16, wherein the aprotic solvent is selected from the group consisting of Acetonitrile, Dimethyl Sulfoxide, Dimethylformamide, and Propylene Carbonate; in particular Acetonitrile.

[0089] Embodiment 22—The electrochemical cell according to embodiment 16, wherein the GDE cathode comprises a metal or metal oxide catalyst; in particular metal or metal oxide catalyst nanoparticles.

[0090] Embodiment 23—The electrochemical cell according to embodiment 22, wherein the metal or metal oxide catalysts are selected from Pb, Ti, Fe, Mo or combinations thereof.

[0091] Embodiment 24—The electrochemical cell according to embodiment 22, wherein the metal or metal oxide catalyst is provided on a porous support; in particular finely dispersed as nanoparticles on a porous support.

[0092] Embodiment 25—The electrochemical cell according to any one of embodiments 16 to 24, wherein the GDE cathode is a single-layer-GDE cathode.

[0093] Embodiment 26—The electrochemical cell according to any one of embodiments 16 to 24, wherein the GDE cathode is double-layer-GDE cathode.

[0094] Embodiment 27—The electrochemical cell according to embodiment 26, wherein, wherein the double-layer-GDE cathode comprises a catalyst layer comprising the metal or metal oxide catalyst according to any one of embodiments 22 to 24; and a gas diffusion layer consisting of a hydrophobic porous material; in particular polytetrafluoroethylene (PTFE) mixed with a pore former such as ammonium bicarbonate.

[0095] Embodiment 28—The electrochemical cell according to any one of embodiments 16 to 24, wherein the GDE cathode comprises a current collector, in particular a current collector consisting of a layer of an electrochemically inert but highly conductive material such as a graphite or stainless steel mesh.

[0096] Embodiment 29—The electrochemical cell according to embodiment 24, wherein the metal or metal oxide catalyst is provided on a support of hydrophobic carbon black agglomerates.

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