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

Abstract

The present invention relates to an electrode comprising a metal deposit of zinc and silver, a process for preparing such an electrode, an electrolysis device comprising such an electrode and a method for CO.sub.2 electroreduction to CO using such an electrode as a cathode.

Claims

1. 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 silver, wherein said metal deposit has a specific surface area greater than or equal to 0.1 m.sup.2.Math.g.sup.−1.

2. The electrode according to claim 1, wherein the electrically conductive support comprises an electrically conductive material selected from the group consisting of a metal; a metal oxide; a metal sulfide; carbon; a polymer intrinsically electrically conductive or made conductive by a coating with a film of conductive material; a semiconductor; and a mixture thereof; and optionally wherein the electrode has been submitted to a treatment to modify its conductivity; a treatment to modify its hydrophobicity, or a combination thereof.

3. The electrode according to claim 1, wherein the metal deposit has a specific surface area between 0.1 and 500 m.sup.2.Math.g.sup.−1.

4. The electrode according to claim 1, wherein the metal deposit comprises at least 1 wt % of one or several phases of an alloy of zinc and silver.

5. The electrode according to claim 1, wherein the metal deposit has a thickness comprised between 1 μm and 500 μm.

6. The electrode according to claim 1, wherein the metal deposit has a porous structure with an average pore size of between 1 μm and 500 μm.

7. A process for preparing an electrode according to claim 1 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 silver; and (iii) applying a current or a potential between the electrically conductive support and a second electrode in order to form a metal deposit of zinc and silver on the electrically conductive support, so as to have a current density equal to or less than −0.1 A.Math.cm.sup.−2 between the electrically conductive support and a second electrode.

8. The process according to claim 7, wherein the acidic aqueous solution containing ions of zinc and ions of silver 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; a ZnO.sub.2.sup.2−-based salt; or a mixture thereof; a salt of silver; an oxidised species of silver; or a mixture thereof.

9. The process according to claim 7, wherein the metal deposit of zinc and silver is removed from the electrically conductive support and applied on a second electrically conductive support.

10. An electrolysis device comprising an electrode according to claim 1.

11. The electrolysis device according to claim 10, coupled to a source of an electrical energy.

12. A method for converting carbon dioxide (CO.sub.2) into CO comprising the following steps: a) providing an electrolysis device comprising an anode and a cathode, wherein said cathode is an electrode according to claim 1; b) exposing the cathode of said electrolysis device to a gaseous or liquid CO.sub.2-containing composition; c) applying an electrical current or a potential between the anode and the cathode in order to reduce the carbon dioxide into CO.

13. The method according to claim 12, being performed under a CO.sub.2 pressure of from 100 to 100000 kPa.

14. The method according to claim 12, being performed at a temperature from 10 to 100° C.

15. The method according to claim 12, wherein the gaseous or liquid CO.sub.2-containing composition is a CO.sub.2-containing aqueous catholyte solution or a gaseous CO.sub.2-containing composition.

16. The electrode according to claim 2, wherein the carbon is in the form of carbon felt, graphite, vitreous carbon, carbon nanofibers, carbon nanotubes, carbon black, boron-doped diamond, any form of gas diffusion layer (GDL) with or without microporous layer.

17. The electrode according to claim 2, wherein the metal deposit has a specific surface area between 1 and 25 m.sup.2.Math.g.sup.−1.

18. The electrode according to claim 4, wherein the one or several phases of an alloy of zinc and silver is a phase Ag.sub.0.13Zn.sub.0.87.

19. The process according to claim 7, wherein the current density between the electrically conductive support and the second electrode is between −5 A.Math.cm.sup.−2 and −0.1 A.Math.cm.sup.−2.

20. The method according to claim 12, being performed under a CO.sub.2 pressure of from 100 to 1000 kPa.

Description

FIGURES

[0166] FIG. 1: SEM images of the Y %-Ag-doped Zn electrodes prepared with Y % being 0, 1.0, 1.9, 5.6, 9.4 or 20.1% at low (left column) and high (right column) magnifications.

[0167] FIG. 2: From left to right: (a) STEM dark-field image and elemental XEDS maps of (b) Zn and (c) Ag based on their Zn-Kα and Ag-Lα signals of typical dendritic structures from a 5.6%-Ag-doped Zn electrode showing the homogeneous repartition of Ag and Zn at the nanoscale.

[0168] FIG. 3: Representative portion of the PXRD pattern of the Y %-Ag-doped Zn electrodes prepared with Y % being 1.0, 1.9, 5.6, 9.4 or 20.1%. The peaks assigned to pure Zn are highlighted in light grey. The peaks assigned to Ag.sub.0.13Zn.sub.0.87 are highlighted in dark grey.

[0169] FIG. 4: XPS spectra of the Y %-Ag-doped Zn electrodes prepared with Y % being 1.0, 1.9, 5.6, 9.4 or 20.1% in the Ag 3d and Zn 2p regions.

[0170] FIG. 5: (a) Faradaic efficiencies and (b) overall catalytic current density and partial current densities for CO, H.sub.2 and HCOOH formation on the 1.9%-Ag-doped Zn electrode. All experiments were carried out in a two-compartment H-cell containing 0.1 M CsHCO.sub.3 with a flow of CO.sub.2 of 20 ml min.sup.−1. Average values and error bars are calculated on at least three data points.

[0171] FIG. 6: Long-term electrolysis data for the 1.9%-Ag-doped Zn electrode while passing a controlled current density of −10 mA.Math.cm.sup.−2. The recorded potential (solid line) is indicated on the left axis whereas the faradaic efficiencies for CO (filled square symbols) and H.sub.2 (hollow round symbols) are reported on the right axis. Stars indicate times at which the electrolyte was changed. All experiments were carried out in a two-compartment H-cell containing 0.1 M CsHCO.sub.3 with a flow of CO.sub.2 of 20 ml.Math.min.sup.−1.

[0172] FIG. 7: Faradaic efficiencies for CO, H.sub.2 and HCOOH formation on 1.0 to 20.1%-Ag-doped Zn electrodes. All experiments were carried out in a two-compartment H-cell containing 0.1 M CsHCO.sub.3 with a flow of CO.sub.2 of 20 ml min-.

[0173] FIG. 8: Overall catalytic current densities (solid line) and partial current densities for CO formation (dashed line) on the Y %-Ag-doped Zn electrodes with increasing % Ag content (from 1.0% to 20.1%) and potential. All experiments were carried out in a two-compartment H-cell containing 0.1 M CsHCO.sub.3 with a flow of CO.sub.2 of 20 ml min-.

[0174] FIG. 9: (a) Faradaic efficiencies for CO production using the 1.9%-Ag-doped Zn electrode deposited for increasing deposition durations and (b) the corresponding partial current densities. The j.sub.CO-1 bar threshold (at around −21 mA.Math.cm.sup.−2) indicates the limit of partial current density for CO formation (j.sub.CO) that cannot be overcome due to limited CO.sub.2 dissolution in aqueous media at 1 bar. In all cases, electrolysis was carried out in 0.1 M CsHCO.sub.3 at a CO.sub.2 flow rate of 20 mL.Math.min.sup.−1.

[0175] FIG. 10: Constant current electrolyses at (a) −200 mA.Math.cm.sup.−2 and (b) −400 mA.Math.cm.sup.−2 using a 9.4%-Ag-doped Zn electrode in a single-compartment reaction vessel in 0.1 M CsHCO.sub.3 at various CO.sub.2 pressures. Current densities (left axis) and faradaic efficiencies (right axis) for CO, H.sub.2 and HCOOH formation are reported. When displayed, error bars are based on the standard deviation of at least 3 individual experiments.

EXAMPLES

Electrode Preparation

[0176] Unless stated otherwise, electrodes were prepared on 1 cm.sup.2 Zn foil (GoodFellow, 99.99+%, 1 mm) successively polished by P1200, and P2400 emery paper followed by sonication in water before deposition. Each electrode was then immersed in a 1.5 M H.sub.2SO.sub.4 aqueous solution of 0.2 M metal salts apportioned between X % AgNO.sub.3 and (100−X) % ZnSO.sub.4 with X % varying between 0% and 10% depending on the targeted Ag content and exposed to −4 A.Math.cm.sup.−2 for 30 s (unless stated otherwise) using a three-electrode set-up with an Ag/AgCl (KC sat.) reference and Pt counter. In each case the electrode was immediately rinsed with milliQ water and air-dried after deposition. AgNO.sub.3 (99.9999%) 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.

Structure Characterisation

[0177] Imaging and Energy dispersive X-Ray spectrometry (EDX) 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 analysed 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 mode with the same microscope, equipped with Jeol system for X-ray detection and cartography. The elemental composition of the metallic electrodes was probed with ICP-AES in a ThermoFisher iCAP 6000 device after gently scratching the deposited powders from their Zn-plate support with a plastic blade and subsequently dissolving the metallic structures in 20% HNO.sub.3 (Sigma-Aldrich, 65%).

[0178] 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 model (Kr cross-sectional area 0.210 nm.sup.2). The specific surface area value derived from BET measurement, reported in m.sup.2.Math.g.sup.−1 was converted, for convenience, to a physical surface area in cm.sup.2.sub.phys.Math.cm.sup.−2.sub.geo by multiplying it by the mass of deposited electrode onto the 1 cm.sup.2 flat Zn support.

[0179] Powder X-ray diffraction (PXRD) 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.

[0180] The electrochemically active (‘echem’) surface area available per cm.sup.2 of flat (‘geo’) electrode was determined using a double layer capacitance measurement technique. This capacitance is determined as the slope of the linear relationship between the widths of cyclic voltammograms obtained at a potential at which no faradaic phenomenon occurs and the scan rates used to perform the cyclic voltammogram. Such experiments were led in CO.sub.2-saturated 0.1 M CsHCO.sub.3 to which an 85%-iR-correction was applied, just after electrolysis in order to get the most realistic value of the operando electrochemically active surface area.

Electrochemical Performance Testing

[0181] Electrocatalytic measurements were carried out using a Bio-logic SP300 potentiostat. A H-type cell was used with the two compartments being separated by a bipolar 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 0.5 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 reach 6.8. During the electrolysis, CO.sub.2 was constantly bubbled at 20 mL.Math.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. 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.

Products Characterisation

[0182] H.sub.2 and gaseous CO.sub.2 reduction products were analysed by gas chromatography (GC, Multi-Gas Analyser #5 SRI Instruments), equipped with Haysep D and MoleSieve 5A columns, thermal conductivity detector (TCD) and flame ionisation detector (FID) with methaniser 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 exchange chromatography (for oxalate −883 Basic IC, Metrohm) and NMR spectroscopy (Bruker AVANCE III 300 spectrometer).

Example 1: Preparation and Characterization of a Range of Ag-Doped Zn Electrodes

[0183] The general conditions mentioned above for electrode preparation were used to generate a range of Ag-doped Zn electrodes fabricated by varying the precursor Ag.sup.+ concentration. The so-generated Ag—Zn electrodes will be referred to, hereafter, as Y %-Ag-doped Zn electrodes where Y % is the incorporated atomic % Ag determined by ICP-AES (rounded to the decimal) taken equal to 1.0%, 1.9%, 5.6%, 9.4% or 20.1% (Table 1).

[0184] Scanning electron microscopy (SEM) revealed that even at the lowest % Ag (1.0%), high-surface-area microporous dendritic structures were attained, offering greatly improved structuration over the stacked configuration of pure Zn (FIG. 1). As the % Ag was increased, both the density of the dendritic structure and electrode thickness increased further (Table 1), leading to high physical surface area (BET SA) as established by Kr-adsorption measurements and subsequent BET analysis (Table 1). Values ranged between 176 and 3133 cm.sup.2.sub.phys.Math.cm.sup.−2.sub.geo, from 1.0% to 20.1%-Ag-doped Zn electrode, respectively outranging previously reported Zn-based catalysts surface areas [13].

TABLE-US-00001 TABLE 1 Complete characterisation of the Ag-doped Zn electrodes deposited with increasing % Ag Pre- Incor- BET cursor porated specific % % surface Deposited BET SA ECSA Thick- [Ag.sup.+] Ag area mass [cm.sup.2.sub.phys .Math. [cm.sup.2.sub.echem .Math. ness [%] [%].sup.a) [m.sup.2 .Math. g.sup.−1].sup.b) [mg .Math. g.sup.−2].sup.c) cm.sup.−2.sub.geo].sup.d) cm.sup.−2.sub.geo].sup.e) [μm].sup.f) 0.5 1.0 NA 10.4 NA 17.17 22 1 1.9 1.57 11.2 176 38.16 77 3 5.6 6.13 13 797 84.90 128 5 9.4 9.07 13.1 1188 121.7 126 10 20.1 22.7 13.8 3133 349.0 203 .sup.a)determined by Inductively coupled plasma-atomic emission spectroscopy (ICP-AES). .sup.b)specific surface area determined by Kr-adsorption measurements and BET analysis. .sup.c)determined by weighing the electrode before and after deposition. .sup.d)BET SA corresponds to the physical (‘phys’) surface area deposited per cm.sup.2 of flat (‘geo’) electrode: it is calculated by multiplying the BET specific surface area by the mass of deposited electrode. .sup.e)electrochemically active (‘echenn’) surface area available per cm.sup.2 of flat (‘geo’) electrode determined by double layer capacitance measurements. .sup.f)thickness determined using 45°-tilted SEM of the electrode cross section.

[0185] The alloyed nature of the Ag-doped Zn electrodes was proven by High Resolution-Transmission Electron Microscopy (HR-TEM) combined with Scanning TEM—Energy-Dispersive X-ray Spectroscopy (STEM-EDXS) elemental mapping, which showed a homogeneous distribution of Ag and Zn within the structures at the nanoscale (FIG. 2).

[0186] Powder X-ray Diffraction (PXRD) on the powder recovered from the electrodes, revealed the presence of two sets of peaks that can be indexed in the hexagonal P6.sub.3/mmc space group (FIG. 3). The first set of peaks (marked with light grey domains) can be indexed with lattice parameters a=2.67 Å and c=4.92 Å and corresponds to pure Zn. The intensity of the second set of peaks (dark grey domains) increases at the expense of the Zn peaks when the incorporated % Ag increases, and can be indexed with a=2.82 Å and c=4.39 Å, corresponding to the Ag.sub.0.13Zn.sub.0.87 phase [16, 17]. For a % Ag of 20.1%, only Ag.sub.0.13Zn.sub.0.87 peaks are observed in the PXRD pattern.

[0187] The surface and near-surface composition (up to a depth of 526 nm) of each electrode was investigated by X-ray photoelectron spectroscopy (XPS) and 15 kV SEM-XEDS respectively (FIG. 4). Both experiments revealed the presence of Ag and Zn, even at the lowest % Ag (1.0%), as well as low amounts of S from namuwite-like zinc-sulfate species after electrodeposition. Equivalent measurements after application of negative potentials in aqueous media proved the stability of such Ag—Zn alloyed structures while the namuwite phases were removed, as verified by the disappearance of their spectroscopic signals.

Example 2: Electrocatalytic Performance of the 1.9%-Ag-Doped Zn Electrode

[0188] Electrochemical studies were undertaken in a two-compartment H-cell separated by a bipolar membrane using 0.1 M CsHCO.sub.3 as an electrolyte. The cathodic compartment was CO.sub.2-saturated beforehand and CO.sub.2 was continuously flowed at 20 mL.Math.min.sup.−1 throughout the electrolysis. Products were analysed by online gas chromatography (GC) and .sup.1H-NMR after each controlled potential electrolysis (CPE).

[0189] The potential-dependent activity of the 1.9%-Ag-doped Zn electrode was first investigated (FIG. 5). Product analysis during CPE showed remarkable selectivity for CO evolution, particularly between −0.9 V and −1.1 V vs. RHE, where FE.sub.CO was >90% and parasitic side-reactions were suppressed (FE.sub.H2<7% and FE.sub.HCOOH<2.5%, FIG. 5). The electrode was also remarkably robust, as an average FE.sub.CO above 90% could be attained at a controlled current density of −10 mA.Math.cm.sup.−2 for 40 h of continuous operation (FIG. 6). A slight decrease in selectivity was seen between 40 h and 100 h, resulting in an average FE.sub.CO of 85%, with a FE.sub.HCOOH of 5.3% and FE.sub.H2<5% over the 100 h of operation.

Example 3: Electrocatalytic Performance of the Ag-Doped Zn Electrodes—Influence of Ag Content

[0190] Further electrochemical analyses were performed to establish the influence of Ag content on the corresponding Ag-doped Zn electrodes. Analysis of the product distribution showed that all electrodes generated CO as the major product (FIG. 7) and the required overpotential to reach optimal FE.sub.CO decreased with the % Ag: 1.0%- and 1.9%-Ag-doped Zn electrodes showed maximum FE.sub.CO of respectively 93% and 91% at −1.0V vs. RHE; 5.6%- and 9.4%-Ag-doped Zn electrodes attained highest FE.sub.CO of 90% and 97% at −0.9 V vs. RHE whereas the 20.1%-Ag-doped Zn electrode reached maximum FE.sub.CO of 85% at −0.8 V vs. RHE.

[0191] FIG. 8 shows the catalytic current density (j.sub.total) increases with % Ag, which correlates with the enhancement of available physical surface area of the electrodes (Table 1). The corresponding partial current densities for CO formation (j.sub.CO, dashed lines in FIG. 8) comprised mostly of j.sub.total and followed a similar trend at low overpotentials. However, at high j.sub.total (>−20 mA cm.sup.−2), discordance between j.sub.total and j.sub.CO was observed as j.sub.CO plateaus at ˜−21 mA cm.sup.−2, while j.sub.t continued to increase. This plateauing effect is particularly noticeable for Ag—Zn electrodes with the largest surface areas (namely 9.4%- and 20.1%-Ag-doped Zn electrodes), since high currents were attained at lower overpotentials. Upon reaching this j.sub.CO plateau, FE.sub.CO decayed in favour of a surge in FE.sub.H2, as most clearly exemplified by the 20.1%-Ag-doped Zn electrode (FIG. 7). Rather than an intrinsic limitation of the electrode, this is assigned to a CO.sub.2-mass-transport limitation in aqueous solution due to its low solubility.

Example 4: Electrocatalytic Performance of Ag-Doped Zn Electrodes—Influence of Thickness

[0192] 1.9%-Ag-doped Zn electrodes were prepared with varying thicknesses between 43 μm and 288 μm with otherwise identical nanostructures (confirmed by specific surface area analysis, Table 2).

TABLE-US-00002 TABLE 2 Complete characterisation of the 1.9%-Ag-doped Zn electrodes deposited with increasing deposition time BET specific Deposited Deposition surface mass BET SA ECSA time area [mg .Math. [cm.sup.2.sub.phys .Math. [cm.sup.2.sub.echem .Math. Thickness [s].sup.a) [m.sup.2 .Math. g.sup.−1].sup.b) cm.sup.−2].sup.c) cm.sup.−2.sub.geo].sup.d) cm.sup.−2.sub.geo].sup.e) [μm].sup.f) 15 NA 6.2 NA 24.3 43 30 1.57 12.3 193 38.9 77 60 1.85 24.1 445 58.6 156 90 2.44 33.2 810 57.5 268 .sup.a)deposition carried out at −4 A cm.sup.−2. .sup.b)specific surface area determined by Kr-adsorption measurements and BET analysis. .sup.c)determined by weighing the electrode before and after deposition. .sup.d)BET SA corresponds to the physical (‘phys’) surface area deposited per cm.sup.2 of flat (‘geo’) electrode: it is calculated by multiplying the BET specific surface area by the mass of deposited electrode. .sup.e)electrochemically active (‘echem’) surface area available per cm.sup.2 of flat (‘geo’) electrode determined by double layer capacitance measurements. .sup.f)thickness determined using 45°-tilted SEM of the electrode cross section.

[0193] This was achieved by varying the electrode deposition time from 15 to 90 s in identical electrodeposition conditions. Analysis of their electrocatalytic activity revealed that little increase in j.sub.CO was seen, indicating that electrodes above 43 μm-thick contain extra material that does not significantly add to the overall activity (FIG. 9). On the other hand, the electrochemically-active surface area of the aforementioned electrodes continues to increase with thicknesses between 43 and 150 μm, suggesting electrolyte penetration is not the limit of catalytic activity (Table 2). The j.sub.CO limitation can tentatively be assigned to the CO.sub.2 mass transport, which does not exceed 43 μm within the electrode.

Example 5: Electrocatalytic Performance of Ag-Doped Zn Electrodes—Influence of Co.SUB.2 .Pressure

[0194] The most restrictive parameter of CO.sub.2 mass transport is its aqueous solubility posing a significant strain on the electrocatalytic performance of the Ag-doped Zn electrodes presented herein. This was confirmed by performing CO.sub.2-electrocatalytic reduction at increased CO.sub.2 pressure. The 9.4%-Ag-doped Zn electrode was chosen for this experiment, since it exhibited the ‘j.sub.CO-1 bar’ plateau at a low overpotential. The experiment was carried out in a one-compartment high-pressure reactor with a graphite counter electrode in order to avoid the production of O.sub.2, otherwise preferentially reduced on the cathode at the expense of CO.sub.2-reduction efficiency. Three CO.sub.2 pressures were tested (1, 3 and 6 bars) while passing a constant current density (j.sub.total) of −200 mA.Math.cm.sup.−2. At 1 bar, the applied −200 mA.Math.cm.sup.−2 of current was mostly expended on H.sub.2 evolution (FIG. 10a FE.sub.H2 of 69%) as the quantity of dissolved CO.sub.2 at 1 bar was limiting the rate of the CO.sub.2 reduction reaction. As the amount of dissolved CO.sub.2 increased (with increasing CO.sub.2 pressures), j.sub.CO values far beyond the −21 mA.Math.cm.sup.−2 plateau were achieved: At 3 bar and 6 bar, j.sub.CO increased dramatically to −131 mA.Math.cm.sup.−2 and −188 mA.Math.cm.sup.−2, respectively, the latter corresponding to a FE.sub.CO of 94%, which lies in the range of the intrinsic best performance recorded in the absence of CO.sub.2 mass-transport limitation discussed previously. Given that the high-pressure cell required anode and cathode to operate in the same compartment, control experiments were used to confirm all CO was derived from CO.sub.2 reduction. Analysis of the anodic graphite oxidation reaction in 0.1 M CsHCO.sub.3 under Ar with a Pt cathode at a current density of −200 mA.Math.cm.sup.−2 showed that a small amount of CO.sub.2 and a trace of CO were produced (FE.sub.CO<1.6%), alongside large amounts of H.sub.2 from the cathode. The anodic reaction was therefore predominantly oxidation of the graphite surface functionality, which may produce some CO.sub.2 but very little CO. Further to this control, the dependency of j.sub.CO on the CO.sub.2 pressure and the observed 100% total FE were conclusive evidence of purely cathodic CO evolution. At higher set current densities and pressure (−400 mA.Math.cm.sup.−2) showed in FIG. 10b a j.sub.CO as high as −297 mA.Math.cm.sup.−2 (i.e. an FE.sub.CO of 86%) was achieved, which sets a new record for a predominantly Zn-based electrocatalyst.

REFERENCES

[0195] [1] P. De Luna, C. Hahn, D. Higgins, S. A. Jaffer, T. F. Jaramillo, E. H. Sargent, Science 2019, 364, eaav3506. [0196] [2] H. A. Hansen, J. B. Varley, A. A. Peterson, J. K. Norskov, J. Phys. Chem. Lett. 2013, 4, 388-392. [0197] [3]W. Zhang, Y. Hu, L. Ma, G. Zhu, Y. Wang, X. Xue, R. Chen, S. Yang, Z. Jin, Adv. Sci. 2018, 5, 1700275. [0198] [4] Y. Hori, K. Kikuchi, S. Suzuki, Chem. Lett. 1985, 14, 1695-1698. [0199] [5] S. R. Foit, I. C. Vinke, L. G. J. de Haart, R.-A. Eichel, Angew. Chemie Int. Ed. 2017, 56, 5402-5411. [0200] [6] A. Bagger, W. Ju, A. S. Varela, P. Strasser, J. Rossmeisl, Chem Phys Chem 2017, 18, 3266-3273. [0201] [7] S. Ma, R. Luo, J. I. Gold, A. Z. Yu, B. Kim, P. J. A. Kenis, J. Mater. Chem. A 2016, 4, 8573-8578. [0202] [8] E. J. Dufek, T. E. Lister, S. G. Stone, M. E. Mcllwain, J. Electrochem. Soc. 2012, 159, F514-F517. [0203] [9] S. Verma, Y. Hamasaki, C. Kim, W. Huang, S. Lu, H.-R. M. Jhong, A. A. Gewirth, T. Fujigaya, N. Nakashima, P. J. A. Kenis, ACS Energy Lett. 2018, 3, 193-198. [0204] [10] A. Dutta, C. E. Morstein, M. Rahaman, A. Cedeño López, P. Broekmann, ACS Catal. 2018, 8, 8357-8368. [0205] [11] D. L. T. Nguyen, M. S. Jee, D. H. Won, H. Jung, H.-S. Oh, B. K. Min, Y. J. Hwang, ACS Sustain. Chem. Eng. 2017, 5, 11377-11386. [0206] [12] F. Quan, D. Zhong, H. Song, F. Jia, L. Zhang, J. Mater. Chem. A 2015, 3, 16409-16413. [0207] [13] J. Rosen, G. S. Hutchings, Q. Lu, R. V Forest, A. Moore, F. Jiao, ACS Catal. 2015, 5, 4586-4591. [0208] [14] D. H. Won, H. Shin, J. Koh, J. Chung, H. S. Lee, H. Kim, S. I. Woo, Angew. Chem. Int. Ed. Engl. 2016, 55, 9297-9300. [0209] [15] K. P. Kuhl, T. Hatsukade, E. R. Cave, D. N. Abram, J. Kibsgaard, T. F. Jaramillo, J. Am. Chem. Soc. 2014, 136, 14107-14113. [0210] [16] S. Lee, G. Park, J. Lee, ACS Catal. 2017, 7, 8594-8604. [0211] [17] T. Burdyny, P. J. Graham, Y. Pang, C.-T. Dinh, M. Liu, E. H. Sargent, D. Sinton, ACS Sustain. Chem. Eng. 2017, 5, 4031-4040.