Method for Converting Carbon Dioxide (CO2) into Syngas by an Electrolysis Reaction

Abstract

The present invention relates to a method for CO.sub.2 electroreduction to syngas, a mixture of carbon monoxide (CO) and hydrogen (H.sub.2), using a cathode comprising an electrically conductive support of which at least a part of the surface is covered by a metal deposit of zinc and of a second metal selected from copper, gold and mixtures thereof, and being preferably copper, said metal deposit comprising at least 1 wt % of one or several phases of an alloy of zinc and of the second metal.

The present invention relates also to an electrode useful for performing this method, a process for preparing such an electrode and an electrolysis device comprising such an electrode.

Claims

1. A method for converting carbon dioxide (CO.sub.2) and water (H.sub.2O) into syngas, which is a mixture of carbon monoxide (CO) and hydrogen (H.sub.2), comprising the following steps: a) providing an electrolysis device comprising an anode and a cathode, wherein said cathode comprises an electrically conductive support of which at least a part of the surface is covered by a metal deposit of zinc and of a second metal selected from copper, gold and mixtures thereof, said metal deposit comprising at least 1 wt % of one or several phases of an alloy of zinc and of the second metal; b) exposing the cathode of said electrolysis device to a CO.sub.2-containing aqueous catholyte solution; c) applying an electrical current between the anode and the cathode in order to reduce the carbon dioxide into syngas.

2. The method according to claim 1, wherein the catholyte solution comprises a salt of hydrogen carbonate, which is optionally formed in situ by reaction of a hydroxide salt with CO.sub.2 contained in the catholyte solution.

3. The method according to claim 1, wherein the metal deposit has a specific surface area of at least 0.1 m.sup.2.Math.g.sup.−1; and/or wherein the metal deposit comprises at least 5 wt %, of one or several phases of an alloy of zinc and of the second metal; and/or wherein the metal deposit has a thickness comprised between 1 μm and 250 μm; and/or wherein the metal deposit has a porous structure with an average pore size of between 1 μm and 500 μm.

4. The method according to claim 1, wherein the weight ratio zinc/second metal in the metal deposit is comprised between 99/1 and 35/65.

5. The method according to claim 1, wherein the weight ratio zinc/second metal in the metal deposit is less than 35/65.

6. The method according to claim 1, wherein the obtained syngas is converted into saturated or unsaturated hydrocarbons, alcohols and/or aldehydes.

7. An electrode comprising an electrically conductive support of which at least a part of the surface is covered by a metal deposit of zinc and of a second metal selected from copper, gold and mixtures thereof, wherein said metal deposit comprises at least 1 wt % of one or several phases of an alloy of zinc and of the second metal and has a specific surface area greater than or equal to 0.1 m.sup.2.Math.g.sup.−1.

8. The electrode according to claim 7, 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.

9. The electrode according to claim 7, wherein the metal deposit has a specific surface area between 0.1 and 500 m.sup.2.Math.g.sup.−1; and wherein the metal deposit comprises at least 5 wt % of one or several phases of an alloy of zinc and of the second metal.

10. The electrode according to claim 7, wherein the metal deposit has a thickness comprised between 1 μm and 250 μm; and/or wherein the metal deposit has a porous structure with an average pore size of between 1 μm and 500 μm.

11. The electrode according to claim 7, wherein the weight ratio zinc/second metal in the metal deposit is comprised between 99/1 and 35/65.

12. A process for preparing an electrode according to claim 7 comprising the following successive steps: (i) providing an electrically conductive support; (ii) immersing said electrically conductive support at least partially in an acidic aqueous solution containing ions of zinc and ions of the second metal; and (iii) applying a current between the electrically conductive support and a second electrode, said current having a density comprised between −0.5 A.Math.cm.sup.−2 and −0.1 A.Math.cm.sup.−2 and being applied for a duration comprised between 30 s and 200 s.

13. The process according to claim 12, wherein the acidic aqueous solution containing ions of zinc and ions of the second metal is an acidic aqueous solution containing: a salt of zinc; an oxidised zinc species; a Zn(OH).sup.3−-based salt; a Zn(OH).sub.4.sup.2−-based salt; or a ZnO.sub.2.sup.2−-based salt; a salt of the second metal; an oxidised species of the second metal; or a mixture thereof.

14. An electrolysis device comprising an electrode according to claim 7.

15. The electrolysis device according to claim 14, coupled to a source of an electrical energy.

16. The method according to claim 1, wherein the second metal is copper.

17. The method according to claim 1, wherein the metal deposit comprises at least 20 wt % of one or several phases of an alloy of zinc and of the second metal.

18. The method according to claim 1, wherein the metal deposit has a specific surface area between 1 and 100 m.sup.2.Math.g.sup.−1.

19. The electrode according to claim 7, wherein the second metal is copper.

20. The electrode according to claim 7, wherein the metal deposit comprises at least 20 wt % of one or several phases of an alloy of zinc and of the second metal.

Description

FIGURES

[0127] FIG. 1: SEM images of Zn|ZnSO.sub.4.sup.(100-X) %CuSO.sub.4.sup.X %, where percentage of CuSO.sub.4 (X) is 0, 1, 5, 10, 20, 25, 30 and 35 as indicated in the top-left corner of corresponding images.

[0128] FIG. 2: Powder X-ray diffraction patterns of Zn|ZnSO.sub.4.sup.(100-X) %CuSO.sub.4.sup.X % where percentage of CuSO.sub.4 (X) is varied between 0 and 100.

[0129] FIG. 3: STEM-XEDS analysis of a Zn|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10% fern-shaped structure before and after 3 h of electrolysis at 1.0 V vs. RHE in CO.sub.2-saturated 0.1 M CsHCO.sub.3. The far-right image shows the Cu and Zn overlay.

[0130] FIG. 4: XPS analysis of Zn|ZnSO.sub.4.sup.95%CuSO.sub.4.sup.5% (X=5) and Zn|ZnSO.sub.4.sup.70%CuSO.sub.4.sup.30% (X=30) before and after 3 h of electrolysis and corresponding surface Cu and Zn content. For the samples after electrolysis, the sum of Zn and Cu content is lower than 100% as Cs was detected and accounted for the remaining part.

[0131] FIG. 5: Thicknesses of the Zn|ZnSO.sub.4.sup.(100-X) %CuSO.sub.4.sup.X % edge, X being equal to 1, 5, 10, 20 or 30 as indicated in the top-left corner of the corresponding images. The images were obtained by vertical scratching and further SEM imaging using a 45′ tilt of the electrodes. The indicated distances must be multiplied by √2 to correct the angular bias of the set-up and obtain the actual thicknesses.

[0132] FIG. 6: Deposition of a 0.5 M H.sub.2SO.sub.4 aqueous solution of 0.1 M total metal salts distributed between 10% CuSO.sub.4 and 90% ZnSO.sub.4 onto either (a) a 1 cm.sup.2 Zn plate or (b) a 1 cm.sup.2 Cu plate or (c) a CDL leading to the generation of Zn|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10%, Cu|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10% and GDL|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10% respectively.

[0133] FIG. 7: Homogeneously-deposited 8 cm.sup.2 Zn|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10%.

[0134] FIG. 8: (a) Molar H.sub.2:CO ratios and (b) corresponding total partial current densities for syngas production obtained at various potentials using Zn|ZnSO.sub.4.sup.(100-X) %CuSO.sub.4.sup.X %, where percentage of CuSO.sub.4 (X) is 0, 5, 10, 20, 25 or 30, in CO.sub.2-saturated 0.1 M CsHCO.sub.3 at room temperature.

[0135] FIG. 9: (a) H.sub.2:CO ratios and (b) corresponding partial currents for syngas production (j.sub.syngas) obtained at different potentials using Zn|ZnSO.sub.4.sup.65%CuSO.sub.4.sup.35% in CO.sub.2-saturated 0.1 M CsHCO.sub.3 at room temperature.

[0136] FIG. 10: Stability of the activity and selectivity of Zn|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10% operated for 3 h at 1.0 V vs. RHE in CO.sub.2-saturated 0.1 M CsHCO.sub.3 at room temperature. Symbols represent the H.sub.2:CO ratio (Right axis). Minor variation after 120 min can be attributed to electrolyte composition modification, change in pH notably, justifying the change in electrolyte for longer experiment such as done in FIG. 11.

[0137] FIG. 11: Long-term electrolysis of a high surface area Zn foam|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10% in CO.sub.2-saturated 0.1 M CsHCO.sub.3 varying the applied potential to mimic the foreseeable voltage fluctuations delivered by an intermittent renewable power source. Catalytic current is reported on the left axis and corresponding H.sub.2:CO ratio on the right axis (bulk symbols). Electrolyte was changed between each potential and roughly every hour beyond 260 minutes, causing slight signal variations.

[0138] FIG. 12: SEM images of Zn foam|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10% before and after over 9 h of electrolysis at various potentials in CO.sub.2-saturated 0.1 M CsHCO.sub.3 at room temperature.

[0139] FIG. 13: Faradaic efficiencies of formate production with respect to CuSO.sub.4 content ‘X’ of the Zn|ZnSO.sub.4.sup.(100-X) %CuSO.sub.4.sup.X %. For each electrode tested, formate was quantified after 2 hours of controlled potential electrolysis in CO.sub.2-saturated 0.1 M CsHCO.sub.3 at room temperature, where potentials of −0.8, −0.9, −1.0 and −1.2 V vs. RHE were applied for 30 minutes each.

[0140] FIG. 14: H.sub.2:CO ratios reachable using fossil-fuel-based techniques versus the CO.sub.2 electroreduction (CO.sub.2eR) process presented in this work. Data for fossil-fuel-based processes are taken from Foit, S. R., I. C. Vinke, et al. (2017): “Power-to-Syngas: An Enabling Technology for the Transition of the Energy System?” Angewandte Chemie International Edition 2017, 56(20): 5402-5411. Primary syngas ratio refers to the obtained products without additional gas-shift reactions to readjust the ratio.

[0141] FIG. 15: SEM images of Zn|ZnSO.sub.4.sup.(100-X) %AgNO.sub.3.sup.X % where X is taken equal to 0 showing pure Zn deposit (Left column) or X is taken equal to 10 (Right column).

[0142] FIG. 16: SEM image and corresponding elemental mapping showing the Zn and Ag repartition of Zn|ZnSO.sub.4.sup.90%AgNO.sub.3.sup.10%

[0143] FIG. 17: SEM images at different magnifications of an electrode prepared by immersing a 1 cm.sup.2 Zn plate in a 1.5 M H.sub.2SO.sub.4 aqueous solution of total 0.2 M metal precursor solution apportioned between 1% of AgNO.sub.3 [0.002 M] and 99% ZnSO.sub.4 [0.198 M] and further subjected to −4 A.Math.cm.sup.−2 for 30 s.

[0144] FIG. 18: SEM image and corresponding elemental mapping showing the Zn and Ag repartition of an electrode prepared by immersing a 1 cm.sup.2 Zn plate in a 1.5 M H.sub.2SO.sub.4 aqueous solution of total 0.2 M metal precursor solution apportioned between 1% of AgNO.sub.3 [0.002 M] and 99% ZnSO.sub.4 [0.198 M] and further subjected to −4 A.Math.cm.sup.−2 for 30 s.

EXAMPLES

[0145] Structure Characterization

[0146] Scanning Electron Microscopy (SEM) Imaging and EDX (Energy dispersive X-Ray spectrometry) were performed on a SU-70 Hitachi FEGSEM fitted with an X-Max 50 mm.sup.2 Oxford EDX spectrometer. The imaging setup was 5 kV in order to observe surface features. Setup for quantitative analysis and mapping was 15 kV. Standards used as a reference for this voltage were purchased at Geller microanalytical laboratory (Boston, Mass.). Volume analyzed at this voltage is approximatively a sphere with diameter of ˜700 nm. This value was calculated with Single Scattering Monte Carlo Simulation. Transmission electron microscopy images and chemical maps were acquired with a Jeol 2100F microscope operated at 200 kV. Chemical maps were acquired in STEM (Scanning Transmission Electron Microscopy) mode with the same microscope, equipped with Jeol system for X-ray detection and cartography. The elemental compositions of metallic electrodes were verified with ICP-AES (Inductively Coupled Plasma—Atomic Emission Spectroscopy) in a ThermoFisher iCAP 6000 device after dissolution of the metallic structures in 20% HNO.sub.3 (Sigma-Aldrich, 65%) and ICP-derived values were converted to moles.

[0147] Specific surface areas were obtained from the analysis of Kr sorption isotherms measured on a BelSorp Max set-up at 77 K. Prior to the measurement, samples were treated under vacuum at 130° C. during at least 7 h. Surface areas were estimated using the BET (Brunauer, Emmett et Teller) model (Kr cross-sectional area 0.210 nm.sup.2). The value derived from BET measurement, reported in m.sup.2.Math.g.sup.−1, was also converted to cm.sup.2.Math.cm.sup.−2.sub.geometric by multiplying it by the mass of deposited metal onto the 1 cm.sup.2 flat Zn support. This provided a roughness factor (RF), as defined by the IUPAC GoldBook.

[0148] Powder X-ray diffraction measurements were performed in Bragg-Brentano geometry using a BRUKER D8 Advance diffractometer with Cu Kα radiation (λKα1=1.54056 Å, λKα2=1.54439 Å) and a Lynxeye XE detector.

[0149] XPS characterization was performed using a Thermo ESCALAB 250 X-Ray photoelectron spectrometer with a monochromatic Al-Kα X-ray source (hv=1486.6 eV) operating at a pressure around 2×10.sup.−9 mbar. The analyzer pass energy was 50 eV for survey spectra and 20 eV for high-resolution spectra. The spectrometer was calibrated using Au 4f7/2 at 84.1 eV. Charging effects were not compensated during analysis. Spectra were recorded and analyzed using Thermo Avantage software version 5.966.

[0150] Electrochemical Performance Testing

[0151] Electrocatalytic measurements and constant potential electrolysis were carried out using a Bio-logic SP300 potentiostat. A H-type cell was used with the two compartments being separated by an anion exchange membrane (AMV Selemion™, ACG Engineering) with an inter-electrode distance of 6 cm between the working and Pt counter and an Ag/AgCl reference (saturated KCl) placed at 0.7 cm from the working. 0.1 M CsHCO.sub.3 (Sigma-Aldrich, 99.9%) aqueous solution was used as both anolyte and catholyte, the latter being CO.sub.2-saturated preceding the experiment (CO.sub.2, Linde, HiQ 5.2) until the catholyte pH reached 6.8. During the electrolysis, CO.sub.2 was constantly bubbled at 20 mL min.sup.−1 through a frit at the bottom of the cathodic chamber and generated gaseous products and excess CO.sub.2 were flowed to the gaseous inlet of a gas chromatograph for online measurement. All electrochemical testing experiments were carried out at room temperature.

[0152] Potentials are reported against the Reversible Hydrogen Electrode (RHE) according to the relationship E vs. RHE=E vs. Ag/AgCl+0.197+0.059*pH.

[0153] Products Characterization

[0154] H.sub.2 and gaseous CO.sub.2 reduction products were analyzed by a gas chromatography set-up (GC, Multi-Gas Analyzer #5 SRI Instruments) equipped with Haysep D and MoleSieve 5A columns, thermal conductivity detector (TCD) and flame ionization detector (FID) with methanizer using Argon as a carrier gas. GC was calibrated 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 exchange chromatography (for oxalate—883 Basic IC, Metrohm) and Nuclear Magnetic Resonance (NMR) spectroscopy (Bruker AVANCE III 300 spectrometer).

[0155] Faradaic efficiencies (FE) were calculated according to the following formula:

[00001]$F{E}_{\mathrm{product}}=\frac{n_e*n_{\mathrm{product}}*F}{Q}$

[0156] Where n.sub.product [mol] is the quantity of analyzed product, n.sub.e [no unit] is the number of electrons involved in the formation of this product, F is the Faraday's constant equal to 96485 C.Math.mol.sup.−1 and Q is the corresponding passed charge. Partial current density for syngas production was calculated as follows:

j.sub.syngas=(FE.sub.H2+FE.sub.CO)*j

[0157] where j refers to the total current density [mA.Math.cm.sup.−2].

[0158] Electrode General Preparation

[0159] For the construction of the highly-porous alloyed metallic foams, a solution of total 0.1 M metal salts apportioned between X % ‘doping’ metal salt (second metal salt) and (100−X) % ‘main’ metal salt (zinc salt) in 0.5 M H.sub.2SO.sub.4, was prepared. Then, a conductive support (S) was immersed into the so-prepared precursor solution and a −0.5 A.Math.cm.sup.−2 current density was applied for 160 s using a three-electrode set-up with an Ag/AgCl (KCl sat.) reference and a 1 cm.sup.2 Pt-mesh counter facing the electrodepositing electrode, with a corresponding interelectrode distance of 1 cm. In each case, the electrode was immediately rinsed with milliQ water and air-dried after deposition. These electrodes are labelled S|Main.sup.(100-X) % doping.sup.X % in the following sections.

[0160] Unless stated otherwise, the support (S) employed for electrode preparation was a 1 cm.sup.2 Zn foil (GoodFellow, 99,99+%, 1 mm) successively polished by P1200, P2400 emery paper and Al-powder followed by sonication in water before deposition. When the support employed was Cu, the same mechanical polishing procedure was applied on a 1 cm.sup.2 Cu foil (GoodFellow, 99,999%, 1 mm). When the support employed was a Gas diffusion Layer (GDL—AVCarbGDS 3250, FuelCellStore), it was used as commercially available without any pre-treatment before deposition. The Zn foam (Mesh 4, Zn003811, Good Fellow) was used without any treatment previous to electrodeposition as well.

[0161] CuSO.sub.4.5H.sub.2O (99.9%) and H.sub.2SO.sub.4 (99.8%), were purchased from Sigma-Aldrich and used without further purification. ZnSO.sub.4.7H.sub.2O (99.5%) was purchased from Roth chemicals.

Example 1: Preparation of a Range of Zn—Cu Foams

[0162] The preparation method described in the previous section was applied to generate a range of Zn—Cu foams using ZnSO.sub.4 as main metal salt and CuSO.sub.4 as doping metal salt. A 1 cm.sup.2 Zn plate—successively polished using P1200/P2400 grade emery paper and Al-powder before electrodeposition—was employed as the ‘support’ (S). The percentage of doping salt, X, was varied between 0, 1, 5, 10, 20, 25, 30 and 35 leading to the generation of 8 different electrodes labelled as follows: Zn|ZnSO.sub.4.sup.100%CuSO.sub.4.sup.0%, Zn|ZnSO.sub.4.sup.99%CuSO.sub.4.sup.1%, Zn|ZnSO.sub.4.sup.95%CuSO.sub.4.sup.5%, Zn|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10%, Zn|ZnSO.sub.4.sup.80%CuSO.sub.4.sup.20%, Zn|ZnSO.sub.4.sup.75%CuSO.sub.4.sup.25%, Zn|ZnSO.sub.4.sup.70%CuSO.sub.4.sup.30% and Zn|ZnSO.sub.4.sup.65%CuSO.sub.4.sup.35%.

Example 2: Structure Characterisation of the Zn—Cu Foams

[0163] Zn|ZnSO.sub.4.sup.(100-X) %CuSO.sub.4.sup.X % were prepared as described in Example 1 with X being equal to 0, 1, 5, 10, 20, 25, 30 and 35 and their structure observed by SEM. The as-prepared electrode materials display an increasing structuration with the amount of CuSO.sub.4 doping (FIG. 1).

Example 3: Elemental Characterisation of the Zn—Cu Foams

[0164] Elemental composition of the electrodeposited alloys was probed by ICP-AES measurements and is presented in Table 1 below.

TABLE-US-00001 TABLE 1 Relationship between the CuSO.sub.4 percentage in the precursor solution, the subsequent percentage of incorporated Cu either in the electrode bulk (determined by ICP-AES) or on the electrode surface (determined by XPS) and resulting electrode specific surface area (BET) and corresponding Roughness Factor (RF). % Cu % Cu BET Electrode %CuSO.sub.4 (bulk) (surface) [m.sup.2 .Math. g.sup.−1] RF Zn|ZnSO.sub.4.sup.95%CuSO.sub.4.sup.5%  5  6.5 ± 0.5  1  1.3  130 Zn|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10% 10 13 ± 1  2  3.8  350 Zn|ZnSO.sub.4.sup.80%CuSO.sub.4.sup.20% 20 32 ± 3 n/d 16.9 1500 Zn|ZnSO.sub.4.sup.75%CuSO.sub.4.sup.25% 25 39.5 ± 4   n/d 14.6 1300 Zn|ZnSO.sub.4.sup.30%CuSO.sub.4.sup.30% 30 59.3 ± 6   12 27.4 2500

[0165] The incorporation of Cu in the materials was confirmed at each loading investigated, and ICP-AES measurements revealed that the Cu:Zn ratio in the material is typically 1.5 times higher than the CuSO.sub.4:ZnSO.sub.4 ratio present in the metal sulfate precursor solution, in agreement with the thermodynamically preferential reduction of Cu.sup.2+ vs. Zn.sup.2+.

[0166] X-ray diffraction analysis (XRD) was also performed to gain insight in the phase composition of the electrodes. Diffractograms for Zn|ZnSO.sub.4.sup.(100-X) %CuSO.sub.4.sup.X % electrodes confirmed the strong correlation of alloy content and the relative metal stoichiometry of the soluble precursors (FIG. 2). At low loadings (X≤10), the XRD patterns of the corresponding Zn|ZnSO.sub.4.sup.(100-X) %CuSO.sub.4.sup.X % structures show hexagonal-Zn phases with slight variations of lattice parameters, suggesting a solid solution of Cu in these materials, as well as additional hexagonal phases attributed to a Cu.sub.0.2Zn.sub.0.8 alloy. With increasing Cu content (10<X≤30), diffraction patterns attributed to cubic Cu.sub.5Zn.sub.8 and Cu.sub.3Zn are observed.

[0167] High Resolution TEM (HRTEM) combined with elemental mapping with Scanning Transmission Electron Microscopy-Energy-Dispersive X-ray Spectroscopy (STEM-EDXS) confirmed that homogeneous distribution of Cu and Zn was present even at the nanoscale features of the dendrite as illustrated in the typical ‘fern-shaped’ structure (FIG. 3). Comparison of the microstructures before and after use in electrocatalytic conditions shows no change in the atomic distribution of the metal sites, demonstrating high structural stability of this surface.

[0168] X-ray Photoelectron Spectroscopy (XPS) of Zn|ZnSO.sub.4.sup.(100-X) %CuSO.sub.4.sup.X % electrodes confirms that both Cu and Zn are present at the surface of the electrode at low and high Cu loading (Table 1). The quantification of the relative Cu:Zn surface ratio indicates that the surface Cu content is around 6 times lower than the bulk Cu content determined by ICP-AES. This Cu:Zn surface ratio is unchanged before and after electrolysis (FIG. 4).

[0169] Also, rough quantification of the alloy content could be obtained by analysis of the XRD spectra and are summarized in the following Table 2 according to the CuSO.sub.4 content in the precursor solution.

TABLE-US-00002 TABLE 2 Relationship between the CuSO.sub.4 percentage in the precursor solution and the alloy content and composition in the deposited metal. X (CuSO.sub.4%) Cu.sub.3Zn (%) Cu.sub.5Zn.sub.8 (%) Cu.sub.2Zn.sub.8 (%) Total alloy (%) 30 35    47 — 82   20 7   61 22 90   10 2.5 — 69 71.5  5 — — 38 38  

Example 4: Thickness and Specific Surface Area Characterisation of the Zn—Cu Foams

[0170] Zn|ZnSO.sub.4.sup.99%CuSO.sub.4.sup.1%, Zn|ZnSO.sub.4.sup.95%CuSO.sub.4.sup.5%, Zn|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10%, Zn|ZnSO.sub.4.sup.80%CuSO.sub.4.sup.20%, Zn|ZnSO.sub.4.sup.70%CuSO.sub.4.sup.30% were prepared as described in Example 1 and their surface vertically scratched with a wooden tip. An estimation of the thicknesses of the resulting surfaces was provided by SEM in each case (FIG. 5). The thickness increases by about ten-fold between 1% CuSO.sub.4 doping and 5% CuSO.sub.4 and by twelve-fold between 1% and 10%. Beyond 10%, the thickness only increases marginally (<5% between 10% and 20%).

[0171] Higher structuration with Cu content was also confirmed by BET measurements allowing to precisely measure the Zn—Cu nanofoam specific surface areas (Table 1). Surface areas as high as 27.4 m.sup.2.Math.g.sup.−1 were reached for the highest Cu content tested.

Example 5: Generalization to Other Supports

[0172] Cu|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10% and GDL|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10% were prepared using thoroughly polished 1 cm.sup.2 Cu plate (successively polished using P1200/P2400 emery paper and Al-powder) and a Gas Diffusion Layer (AVCarb GDS 3250, FuelCellStore) respectively to support the growth. As displayed in FIG. 6, similar morphologies were obtained regardless of the employed conducting deposition support.

Example 6: Scaling-Up to a Homogeneous 8 cm.SUP.2 .Zn—Cu Foam

[0173] A Zn|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10% nanofoam was deposited onto a 8 cm.sup.2 (2.5 cm×3.2 cm) flat Zn support using the same deposition procedure as described in Example 1 and proved to be homogeneous (FIG. 7).

Example 7: Catalytic Activity of the Zn—Cu Foams Used as Cathode for CO.SUB.2.eR

[0174] Catalytic activity of the Zn|ZnSO.sub.4.sup.100%CuSO.sub.4.sup.0%, Zn|ZnSO.sub.4.sup.95%CuSO.sub.4.sup.5%, Zn|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10%, Zn|ZnSO.sub.4.sup.80%CuSO.sub.4.sup.20%, Zn|ZnSO.sub.4.sup.75%CuSO.sub.4.sup.25% and Zn|ZnSO.sub.4.sup.70%CuSO.sub.4.sup.30% materials used as cathodes for CO.sub.2eR was assessed in the H-type cell using a 0.1 M CsHCO.sub.3 supporting electrolyte in both cathodic and anodic compartments. Before experiment, the catholyte was CO.sub.2-saturated and CO.sub.2 was continuously bubbled at 20 mL.Math.min.sup.−1 during experiment. In terms of selectivity, the produced syngas mixture displayed an increasing H.sub.2:CO ratio as the CuSO.sub.4 content in the precursor solution and resulting electrode-incorporated Cu content increased (FIG. 8a). H.sub.2:CO ratios ranging from 0.2 to 1.6 could be achieved, extreme values being obtained using Zn|ZnSO.sub.4.sup.95%CuSO.sub.4.sup.5% and Zn|ZnSO.sub.4.sup.70%CuSO.sub.4.sup.30% respectively. These ratios were stable regardless of the electrolysis potential varied between −1.2 V and −0.8 V vs. RHE. Higher H.sub.2:CO ratios (up to 3.65), in the range of those required for the methanation reaction could also be obtained when using electrodes with higher Cu loading (X>30), yet losing the invariance of the ratio with applied potential observed at lower loadings (FIG. 9).

[0175] Also, catalytic activity increased with the CuSO.sub.4 doping as revealed by the measurement of catalytic current density for syngas production, j.sub.syngas, (FIG. 8b) performed in CO.sub.2-saturated 0.1 M CsHCO.sub.3 with continuous 20 mL.Math.min.sup.−1 CO.sub.2 bubbling in the catholyte: for example at 1.0 V vs. RHE, j.sub.syngas obtained with Zn|ZnSO.sub.4.sup.100%CuSO.sub.4.sup.0% was equal to 0.94 mA.Math.cm.sup.−2 in comparison with 9.8 mA.Math.cm.sup.−2 obtained using Zn|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10%, −11.5 mA.Math.cm.sup.−2 obtained using Zn|ZnSO.sub.4.sup.80%CuSO.sub.4.sup.20% and −15.6 mA.Math.cm.sup.−2 obtained using Zn|ZnSO.sub.4.sup.70%CuSO.sub.4.sup.30%.

[0176] Stability of the system was investigated over a three-hour period in static electrolyte conditions on Zn|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10% (FIG. 10). Current density proved to be stable during the 3-hour electrolysis despite a slight decrease after 2 hours between −11 mA.Math.cm.sup.−2 and −10 mA.Math.cm.sup.−2, that might be attributed to pH gradient formation between cathode and anode and that should be solved by circulating electrolyte (an issue which is not at stake, here). The FE.sub.CO also remained steady with an average value of 70.1% during the first two hours before a slight decreased to 65.3% in average for the last hour that might also be attributed to changes in the electrolyte composition. This resulted in a slight increase of the H.sub.2:CO ratio from 0.12 to 0.22 over the course of the electrolysis.

Example 8: Potential-Independence of the H.SUB.2.:CO Ratio

[0177] The stability and scalability of the electrode were finally investigated on a higher surface area support to target industrially relevant currents. Depositing ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10% onto commercially-available Zn foam (Mesh 4, 1 cm.sup.3, Zn003811, Good Fellow) applying 1 A for 160 s generated a highly-structured surface referred to as Zn foam|ZnSO.sub.4.sup.90%CuSO.sub.4.sup.10%, similar to those deposited on flat Zn (FIG. 12), and afforded stable currents in the range of −50 to −60 mA (FIG. 11).

[0178] As proof of the electrode's amenability to versatile syngas production in real-world conditions, we tested its response to the potential variations commonly observed while coupling the electrolyzer to an intermittent source of energy, such as solar panels. The applied potential was varied during electrolysis with the aforementioned foam over a 300 mV range, over which time the electrode maintained a stable H.sub.2:CO ratio (FIG. 11). In total, the system was operated for more than 9 h without any decline in activity or selectivity, nor evidence of structural degradation as shown by the comparison of SEM images before and after electrolysis (FIG. 12).

Example 9: Turning-Off the Formic Acid Production with Increasing CuSO.SUB.4 .Doping

[0179] The faradaic efficiency for formate production (typical product CO.sub.2eR catalysed by monometallic Zn) was “turned-off” when increasing the CuSO.sub.4 content of the precursor solution and the resulting increasing Cu content of the resulting Zn—Cu electrodes prepared as described in Example 1 (FIG. 13).

Example 10: Relevancy of the Syngas Production Process Presented Herein Compared to Benchmarking Fossil-Fuel-Based Techniques

[0180] The electrode materials presented herein for CO.sub.2 conversion to syngas are particularly relevant from two perspectives compared to existing industrial syngas production devices. First, they allow reaching a wide range of H.sub.2:CO ratios ranging from 0.2 to 3.65 depending on the alloy stoichiometry which cannot be achieved by the current fossil-fuel based processes which ensure most of syngas production (FIG. 14).

[0181] Also, they are particularly relevant in the context of renewable electricity conversion, which requires the design of electrolytic devices tolerant to the significant variations of power provided by intermittent energy sources, such as photovoltaic panels. To the best of our knowledge, all the electrocatalytic systems for syngas generation developed up-to-date, present a significant variation of the H.sub.2:CO ratio with applied potential, preventing an efficient coupling with such sources of electricity. Herein, the developed electrodes present the unique ability to maintain a constant H.sub.2:CO ratio over a broad range of applied potentials (of at least 300 mV) providing a new practical system to convert CO.sub.2 to industrially relevant products using intermittent, renewable energy sources.

Example 11: Generalisation to Other Alloys

[0182] A Zn|ZnSO.sub.4.sup.90%AgNO.sub.3.sup.10% was prepared using ZnSO.sub.4 as main metal salt and AgNO.sub.3 as doping metal salt, according to the procedure described in the ‘Electrode General preparation’ section. The as-prepared electrode exhibits a hierarchical porosity that cannot be attained with the pure Zn equivalent, also referred as Zn|ZnSO.sub.4.sup.100%AgNO.sub.3.sup.0% (FIG. 15). The presence of both Zn and Ag in the electrodeposited structure could be confirmed by EDX elemental mapping (FIG. 16).

[0183] Another set of deposition conditions was tested. A 0.2 M total metal salt precursor solution was used apportioned between 1% AgNO.sub.3 (i.e. 0.002 M) and 99% ZnSO.sub.4 (i.e. 0.198 M) in 1.5 M H.sub.2SO.sub.4 aqueous solution. A 1 cm.sup.2 Zn plate support was immersed in this solution and subjected to −4 A.Math.cm.sup.−2 during 30 s. The electrode was thoroughly rinsed and air-dried immediately after deposition. The as-prepared electrode showed highly nanostructured architecture as confirmed by SEM imaging (FIG. 17). The presence of both Zn and Ag in the electrodeposited structure could be confirmed by EDX elemental mapping (FIG. 18). These preliminary results exemplify the possible generalisation of the procedure developed herein to other alloys.