Electrolyser for CO2 Reduction into Hydrocarbons

Abstract

The present invention relates to an electrolysis device comprising an anode and a cathode, wherein the anode and the cathode each are an electrode comprising an electrically conductive support of which at least a part of the surface is covered by a metal deposit of copper, wherein the surface of the metal deposit is in an oxidized, sulfurated, selenated and/or tellurized form and the metal deposit has a specific surface area greater than or equal to 1 m.sup.2/g. The present invention relates also to a method for reducing CO.sub.2 into hydrocarbons using an electrolysis device according to the invention. The method according to the invention comprises: a) providing an electrolysis device according to the invention; b) exposing the cathode of said electrolysis device to a CO.sub.2-containing aqueous catholyte solution; c) exposing the anode of said electrolysis device to an aqueous anolyte solution; and d) applying an electrical current between the anode and the cathode in order to reduce the carbon dioxide into hydrocarbons.

Claims

1. An electrolysis device comprising an anode and a cathode, wherein the anode and the cathode each are an electrode comprising an electrically conductive support of which at least a part of the surface is covered by a metal deposit of copper, wherein the metal deposit may comprise other metals than copper selected from iron, nickel, zinc, cobalt, manganese, titanium, gold, silver, lead, ruthenium, iridium and a mixture thereof, said other metals representing no more than 50% by weight of the metal deposit, wherein the surface of the metal deposit is in an oxidized form and the metal deposit has a specific surface area greater than or equal to 1 m.sup.2/g, the specific surface area being determined by the Brunauer, Emmett and Teller (BET) method.

2. The electrolysis device according to claim 1, wherein said other metals represent no more than 30% by weight of the metal deposit.

3. The electrolysis device according to claim 1, wherein the electrically conductive support comprises an electrically conductive material selected from a metal; a metal oxide; a metal sulphide; carbon; a semiconductor; and a mixture thereof.

4. The electrolysis device according to claim 1, wherein the metal deposit is dendritic.

5. The electrolysis device according to claim 1, wherein the metal deposit has a thickness comprised between 10 μm and 2 mm.

6. The electrolysis device according to claim 1, wherein the metal deposit has a specific surface area comprised between 1 m.sup.2/g and 500 m.sup.2/g.

7. The electrolysis device according to claim 1, wherein the metal deposit has a porous structure with an average pore size of between 10 μm and 500 μm, the average pore size being determined by means of photographs obtained by Scanning Electron Microscopy (SEM).

8. The electrolysis device according to claim 1, wherein the distance between the anode and the cathode is comprised between 15 and 0.1 cm.

9. The electrolysis device according to claim 1, comprising an anodic compartment and a cathodic compartment separated by a membrane.

10. The electrolysis device according to claim 9, wherein the anodic compartment and the cathodic compartment each comprise an inlet and an outlet intended to allow the circulation of an anolyte solution through the anodic compartment and a catholyte solution through the cathodic compartment respectively.

11. The electrolysis device according to claim 10, wherein the anodic compartment and the cathodic compartment each comprise a flow spacer linked to the inlet and to the outlet of the anodic or cathodic compartment respectively, the flow spacer being a system that guides the flow of the anolyte or catholyte solution from the inlet to the outlet of the anodic or cathodic compartment respectively.

12. The electrolysis device according to claim 11, wherein the flow spacer is separated from the anode or the cathode and from the membrane by a sealing ring.

13. The electrolysis device according to claim 1, coupled to a source of an electrical energy.

14. A method for reducing carbon dioxide (CO.sub.2) into hydrocarbons comprising the following steps: a) providing an electrolysis device according to claim 1; b) exposing the cathode of said electrolysis device to a CO.sub.2-containing aqueous catholyte solution; c) exposing the anode of said electrolysis device to an aqueous anolyte solution; and d) applying an electrical current between the anode and the cathode in order to reduce the carbon dioxide into hydrocarbons.

15. The method according to claim 14, wherein the catholyte solution comprises a salt of hydrogen carbonate, and wherein the anolyte solution comprises a salt of carbonate.

16. The method according to claim 14, wherein the electrical current applied between the anode and the cathode has a potential difference comprised between 10 and 1.5 V.

17. The electrolysis device according to claim 3, wherein the metal is copper, steel, aluminum, zinc or titanium; the metal oxide is Fluorine-doped Titanium Oxide (FTO) or Indium Tin Oxide (ITO); the metal sulphide is cadmium sulphide or zinc sulphide; the carbon is in the form of carbon felt, graphite, vitreous carbon, or boron-doped diamond; the semiconductor is silicon.

18. The electrolysis device according to claim 1, wherein the metal deposit has a specific surface area comprised between between 3 m.sup.2/g and 50 m.sup.2/g; a porous structure with an average pore size of between 30 μm and 70 μm, the average pore size being determined by means of photographs obtained by Scanning Electron Microscopy (SEM); and a thickness comprised between 70 μm and 300 μm.

19. The electrolysis device according to claim 9, wherein the membrane is an anion exchange membrane.

20. The electrolysis device according to claim 13, wherein the electrical energy is a photovoltaic panel or a wind turbine.

21. The method according to claim 15, wherein the salt of hydrogen carbonate is an alkali metal salt or a quaternary ammonium salt of hydrogen carbonate, and the salt of carbonate is an alkali metal salt or a quaternary ammonium salt of carbonate.

Description

LEGENDS OF THE FIGURES

[0159] FIG. 1: Schematic electrolyzer cell according to the invention with:

[0160] (1) anion exchange membrane (e.g. thickness: 0.2 mm)

[0161] (2) sealing O-ring (e.g. fiberglass reinforced silicone, thickness: 0.2 mm)

[0162] (3) flowing spacer (e.g. PTFE, thickness: 3 mm)

[0163] (4) anode

[0164] (5) cathode

[0165] (6) electrode area (cathode or anode) (e.g. about 1 cm.sup.2)

[0166] (a) interelectrode distance (e.g. 7 mm)

[0167] R.E. Reference electrode

[0168] the arrows representing the flows of the electrolyte solutions.

[0169] This electrolyzer cell has been used in all the examples, except otherwise mentioned, with the features indicated in parenthesis.

[0170] FIG. 2: a) Linear sweep voltammetry (LSV) of DN—CuO cathode (light grey) and DN-CuO anode (black), using a scan rate of 10 mV.Math.s.sup.−1 (currents are uncorrected for resistive losses incurred within the electrolyte, all current densities are based on projected geometric area). b) J-E curve of the electrolyzer cell using DN—CuO electrodes. c) Faradaic efficiencies for CO.sub.2 reduction products using DN—CuO cathode at different potentials. All measurements were carried out using the electrolyzer cell described in the examples below and illustrated on FIG. 1 using an anion exchange membrane separating the cathodic (CO.sub.2 saturated 0.1 M CsHCO.sub.3) and anodic (0.2 M Cs.sub.2CO.sub.3) compartments. Constant CO.sub.2 saturation was ensured by constant sparging of the cathodic electrolyte with CO.sub.2 at 2.5 mL.Math.min.sup.−1.

[0171] FIG. 3: a) LSV of DN—CuO electrode for CO.sub.2 reduction in 0.1M NaHCO.sub.3 at different flow rates of electrolyte. b) Total faradaic efficiency (FE) of ethylene and ethane at −0.95V vs RHE (reversible hydrogen electrode) at different flow rates of electrolyte.

[0172] FIG. 4: a) J-E curve of the electrolyzer cell using DN—CuO electrodes as both cathode and anode in different electrolytes: (black-solid line) cathodic solution—CO.sub.2 saturated 0.1 M NaHCO.sub.3 and anodic solution—0.2 M Na.sub.2CO.sub.3, (black-dash line) cathodic solution —CO.sub.2 saturated 0.1 M KHCO.sub.3 and anodic solution—0.2 M K.sub.2CO.sub.3, (light grey-solid line) cathodic solution—CO.sub.2 saturated 0.1 M CsHCO.sub.3 and anodic solution—0.2 M Cs.sub.2CO.sub.3. b) FE of ethylene and ethane when cations are Na.sup.+ and Cs.sup.+.

[0173] FIG. 5: a) Long-term (3 h) electrolysis for splitting CO.sub.2 using flow cell with DN—CuO electrode as both cathode and anode and b) corresponding Faradaic efficiency for C.sub.2H.sub.4+C.sub.2H.sub.6 during 3 h CO.sub.2 reduction electrolysis.

[0174] FIG. 6: a) Comparison of cell potentials (E.sub.cell) as a function of current density between the flow electrochemical cell (solid line) and a H-type electrochemical cell (dash line). In both setup DN—CuO electrodes were employed as both cathode and anode, using a solution of CO.sub.2-saturated 0.1 M CsHCO.sub.3 (pH 6.8) as catholyte, a solution of 0.2 M Cs.sub.2CO.sub.3 (pH 11) as anolyte, separated by a Selemion AEM. b) Faradaic efficiencies of CO.sub.2 reduction in 0.1 M CsHCO.sub.3-saturated CO.sub.2 using DN—CuO in H-type electrochemical cell.

[0175] FIG. 7: a) Current vs. cell voltage obtained in the flow cell: (light grey) DN—CuO was used as both cathode and anode, (black) Cu oxide plate electrode was used as both cathode and anode electrode. b) Faradaic efficiency of CO.sub.2 reduction using Cu oxide plate electrode.

[0176] FIG. 8: a) Current-potential characteristic of the perovskite mini-module under 1 sun, AM1.5G illumination (squares) and measured operating current of the electrolyzer cell (geometric areas of cathode=0.35 cm.sup.2 and anode=0.85 cm.sup.2, current measured after 5 min electrolysis) at various potentials (dots). b) Electrolyzer cell current as a function of photoelectrolysis time using the perovskite mini-module as the sole energy source.

[0177] FIG. 9: Faradaic efficiencies for CO.sub.2 reduction products using crystalline Cu dendrites electrode in electrolyzer cell using DN—CuO as anode (comparative example).

EXAMPLES

General Considerations

[0178] Electrocatalytic measurements and electrolysis experiments in the flow electrochemical cell were carried out using a Bio-logic SP300 potentiostat. H.sub.2 and gaseous CO.sub.2 reduction products were analyzed by gas chromatography (GC) (SRI Instruments), Multi-Gas Analyzer #5 equipped with a HayeSep® D column and MoleSieve 5A column, thermal Conductivity Detector (TCD) and Flame Ionization Detector (FID) with methanizer using Argon as a carrier gas. GC was calibrated by using a standard gas mixture containing 2500 ppm of H.sub.2, CO, CH.sub.4, C.sub.2H.sub.4, C.sub.2H.sub.6, C.sub.3H.sub.6, C.sub.3H.sub.8, C.sub.4H.sub.8 and C.sub.4H.sub.10 in CO.sub.2 (Messer). The liquid phase products were quantified using Ionic chromatography and Nuclear Magnetic Resonance (NMR) spectroscopy. Formate and oxalate were analyzed by ionic exchange chromatography (883 Basic IC, Metrohm). Ethanol was analyzed by .sup.1H NMR spectroscopy using a Bruker AVANCE III 300 spectrometer. SEM images were acquired using a Hitachi S-4800 scanning electron microscope. TEM images were obtained on a JEM-2010F transmission electron microscope (JEOL) with an accelerating voltage of 200 kV.

[0179] The most relevant criteria to evaluate the performance of the electrical to chemical energy conversion device is the energy efficiency (EE). The thermoneutral potential (E.sub.th) must then be introduced so that thermal losses are taken into account into the calculations avoiding any energy efficiency overestimation. The calculations are detailed below in the particular cases of ethane (C.sub.2H.sub.6) and ethene (C.sub.2H.sub.4), which are the targeted products herein.

2CO.sub.2+2H.sub.2O.fwdarw.C.sub.2H.sub.4+3O.sub.2  (2)

ΔH(C.sub.2H.sub.4)=−2ΔH(CO.sub.2 (aq))−2ΔH(H.sub.2O)+3ΔH(O.sub.2)=1600.41 kJ.Math.mol.sup.−1  (3)

2CO.sub.2+3H.sub.2O.fwdarw.C.sub.2H.sub.6+7/2O.sub.2  (4)

ΔH(C.sub.2H.sub.6)=−2ΔH(CO.sub.2(aq))−2ΔH(H.sub.2O)+7/2ΔH(O.sub.2)=1451.52 kJ.Math.mol.sup.−1  (5)

E.sub.th=ΔH/n (n is the number of electron)  (6)

EE(C.sub.2H.sub.4+C.sub.2H.sub.6)=(FY(C.sub.2H.sub.4)×E.sub.th(C.sub.2H.sub.4)+FY(C.sub.2H.sub.6)×E.sub.th(C.sub.2H.sub.6))/Ucell  (7)

ΔH=standard enthalpy of reaction; FY=faradaic yield of the catalytic reaction

General Preparation of the Electrolyser

[0180] Preparation of the electrodes

[0181] Electrodes were prepared as described in Angew. Chem. Int. Ed. 56, 4792 (2017) and are named DN—CuO (dendritic nanostructured copper oxide material) in the following sections. [0182] Flow electrochemical cell

[0183] The scheme of the flow electrochemical cell is presented in FIG. 1. The distance between cathode and anode is 0.7 cm. A Selemion™ AEM (Anion Exchange Membrane) separates the cathodic and anodic compartments. Each half-cell comprises, in sequence, the electrode, a 0.2 mm-thick sealing O-ring made from fiberglass-reinforced silicone, a 3 mm PTFE (polytetrafluoroethylene) flowing spacer and another O-ring before the membrane. The geometrical surface area of the working electrodes was chosen to be 1 cm.sup.2 for all this study, unless otherwise specified. The PTFE spacers were designed using a trapezoidal shape for the electrolyte inlet/outlet and the connection with the tubing. Ag wire was used as pseudo-reference electrode and placed in both compartments and was calibrated with an aqueous Ag/AgCl reference electrode before each experiment. The electrode potentials were referred to RHE according to the following formula:

E(vs RHE)=E(vs Ag wire)+ΔE+0.2+0.059×pH

where the potential difference (ΔE) between the Ag wire and the Ag/AgCl electrode was determined using the E.sub.1/2 potential of K.sub.3Fe(CN).sub.6 in 0.1M CsHCO.sub.3 solution as a reference. Unless otherwise stated, catalytic activity was investigated in this setup using 0.1 M CsHCO.sub.3 saturated with CO.sub.2 (pH 6.8) at the cathode and 0.2 M Cs.sub.2CO.sub.3 (pH 11.0) at the anode, flowed through the two compartments at a constant flow of 1.0 mL.Math.min.sup.−1.

Example 1: Electrical to Chemical Energy Conversion According to the Invention

[0184] The catalytic activity of a full electrochemical cell comprising two identical 1 cm.sup.2 DN—CuO electrodes for cathode and anode, a CO.sub.2-saturated 0.1 M CsHCO.sub.3 aqueous solution in the cathodic compartment and a 0.2 M Cs.sub.2CO.sub.3 aqueous solution in the anodic compartment flowing at 1 mL.Math.min.sup.−1 was assessed. A stable current density of 25 mA.Math.cm.sup.−2 at a cell potential of 2.95 V was obtained leading to an energy efficiency conversion of CO.sub.2 towards ethene and ethane equal to 21%. The detail of the reduction products' faradaic yield is displayed in FIG. 2. Among the CO.sub.2 reduction products, C.sub.2H.sub.4 accounted for 37% FY, C.sub.2H.sub.6 for 12.8% FY, HCOOH for 7% FY and CO for 5%.

Example 2: Influence of the Electrolyte Flow Rate

[0185] The influence of the electrolyte flow rate was tested, maintaining all other operating conditions described in Example 1. Increasing the electrolyte flow led to an increase of the cathodic activity of the DN—CuO as shown by linear sweep voltammetry studies (LSVs, FIG. 3): at −1.0 V vs. RHE, −20 mA.Math.cm.sup.−2 were reached using a CO.sub.2 flow of 4 mL.Math.min.sup.−1, in comparison with −15.5 mA.Math.cm.sup.−2 at 0.25 mL.Math.min.sup.−1.

[0186] The FE selectivity of CO.sub.2 reduction to hydrocarbons was also varied at different electrolyte flow rates. The highest FE of ethylene and ethane was obtained at the flow rates of 1.0 mL.Math.min.sup.−1 and justifies the choice of this flow rate in further studies (FIG. 3.b).

Example 3: Influence of the Carbonate-Based Electrolyte Cation

[0187] The influence of the carbonate-based electrolyte cation “X+” was tested, maintaining all other operating conditions described in Example 1 (FIG. 4). In each case, 0.1 M XHCO.sub.3 was used along with 0.2 M X.sub.2CO.sub.3 as the catholyte and anolyte respectively. At 3.0 V cell potential, −25 mA.Math.cm.sup.−2 were reached using Cs.sup.+ as a cation, in comparison with −17 mA.Math.cm.sup.−2 and −13 mA.Math.cm.sup.−2 using K.sup.+ and Na.sup.+ respectively. The FE selectivity of CO.sub.2 reduction to hydrocarbons was also varied depending on the cation. The highest FE of 47% for ethylene and ethane was obtained using Cs.sup.+ in comparison with 33% using Na.sup.+ for instance and justifies the choice of this cation in further studies (FIG. 4.b).

Example 4: Long-Run Operation of the System

[0188] Stability of the system was investigated over a 3 h period in the flow cell maintaining all other operating conditions described in Example 1. Stable current density of 22 mA.Math.cm.sup.−2 was observed along with a stable total FY of 47% for ethane and ethene (FIG. 5) preserving an energy efficiency of 21%.

Example 5: Advantages of the Invention Compared to an Electrochemical H-Cell Set-Up

[0189] The electrodes of the present invention were tested in a H-type electrolyzer maintaining all other operating conditions described in Example 1. The H-cell set-up is used in the literature (Nature Catalysis 2018, 1, 421-428) for catalytic performances testing, made of two glass half-cells separated by a defined membrane (anion exchange membrane) with an interelectrode distance of 6 cm.

[0190] To obtain a stable current of 25 mA.Math.cm.sup.−2, 4.8 V was needed in such a H-type electrolyzer. This should be compared with the electrolyzer of the present invention, for which a cell potential of only 2.95 V was sufficient to reach this current (FIG. 6). Using the H-type set-up, lower faradaic efficiencies were reached (˜25% total FY for ethene and ethane) at about −0.90 V vs. RHE compared to the 50% obtained at −0.95 V vs. RHE obtained in the flow conditions. The resulting energy efficiency for ethylene and ethane using this H-cell set-up is only 5.4%, which is significantly lower than when using the flow cell of the present invention (49.8%).

Example 6: Advantage of the Invention Compared to Non-Nanostructured Electrodes

[0191] The electrolyzer of the present invention was tested using non-nanostructured Cu-based electrodes while maintaining all other operating conditions described in Example 1. These Cu oxide plate electrodes were fabricated by annealing flat Cu foil (1.0 cm.sup.2) under air condition at 300° C. for 30 min before depositing a Cu oxide nanoparticle layer (following DN—CuO synthesis). These steps are the equivalent of steps (ii) and (iii) described above and have been performed in the same conditions as for preparing DN—CuO in example 1 (using Cu(imidazole).sub.4Cl.sub.2 as copper precursor in step (iii)). At a cell potential of 3.0 V, only 6.0 mA was reached (FIG. 7) in comparison with the 25 mA obtained using the synergetic combination of electrolyzer and DN—CuO electrodes according to the present invention. As such, the energy efficiency for ethylene and ethane formation with non-nanostructured electrodes is only 1.3% in comparison with the record 21% presented in the reported invention.

Example 7: Engineering of a Fully Integrated Solar-to-Fuel Conversion Device

[0192] The electrolysis cell (EC) according to the invention was coupled with high performing perovskite photovoltaic cell (PV) (Science, 2016, 360, 6392) mini-module made of two series of three perovskite solar cells connected in parallel as an electrical power source. The full PV-EC system demonstrated a 2.3% solar-to-hydrocarbons (η.sub.S-H) efficiency calculated as follows:

η.sub.S-H=η.sub.S-E×EE(C.sub.2H.sub.4+C.sub.2H.sub.6)

[0193] with η.sub.S-E being the solar-to-electricity efficiency of the perovskite mini-module obtained experimentally as displayed in FIG. 8.

[0194] This performance sets a new benchmark for an inexpensive all earth-abundant PV-EC system.

Comparative Example 8

[0195] A 1 cm.sup.2 dendritic Cu electrode free of copper oxide surface was prepared by immersing 1 cm.sup.2 of a freshly cleaned Cu plate in a 0.2 M CuSO.sub.4, 1.5 M H.sub.2SO.sub.4 solution (20 ml) and applying a current of −0.5 A using a galvanostatic method for a duration of 80 s, followed by a rinsing with copious amounts of distilled water before being dried in air at room temperature. The catalytic activity of a full electrochemical cell comprising such a 1 cm.sup.2 dendritic Cu electrode and a 1 cm.sup.2 DN—CuO electrode for anode, a CO.sub.2-saturated 0.1 M CsHCO.sub.3 aqueous solution in the cathodic compartment and a 0.2 M Cs.sub.2CO.sub.3 aqueous solution in the anodic compartment flowing at 1 mL.Math.min.sup.−1 was assessed. A stable current density of 21 mA.Math.cm.sup.−2 at a cell potential of 2.9 V was obtained leading to an energy efficiency conversion of CO.sub.2 towards ethene and ethane equal to 6.5%. The detail of the reduction products' faradaic yield is displayed on FIG. 9. Among the CO.sub.2 reduction products, C.sub.2H.sub.4 accounted for 11% FY, C.sub.2H.sub.6 for 3.6% FY, HCOOH for 8% FY and CO for 6.5%.