METHOD FOR ELECTROCHEMICAL REDUCTION OF CARBON DIOXIDE USING SILVER DEPOSITED CuO NANOSHEETS

Abstract

A method for electrochemical reduction of carbon dioxide in an H-type electrochemical cell includes applying a voltage to a solution including carbon dioxide in an H-type electrochemical cell, which includes a working electrode at least partially coated with a catalyst composition including CuO nanosheets doped with silver nanoparticles. The silver nanoparticles are disposed on the surface of the CuO nanosheets and the amount of silver is in a range from 15% to 60% of the combined weight of the CuO nanosheets and silver nanoparticles. During the applying the carbon dioxide is reduced to form one or more C2+ products and where the Faradaic efficiency for the formation of the C2+ products is greater than or equal to 35% at −1.05 V vs. RHE.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present disclosure claims the benefit of Saudi Patent Application No. 1020250445, filed Jan. 22, 2025, with the Saudi Authority for Intellectual Property Office, which is incorporated herein by reference in its entirety.BACKGROUNDTechnical Field

[0002] The present disclosure is directed towards carbon dioxide reduction techniques, and more particularly, relates to the use of silver nanoparticle doped copper oxide nanosheets for electrochemical reduction of carbon dioxide.Description of Related Art

[0003] The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

[0004] With the advent of civilization, global population has increased multi-fold. In order to meet various demand of the global population, there has been a huge and continuous rise in carbon dioxide (CO2) emissions, released into the atmosphere. Further, the declining fossil fuel reserves pose a challenge, considering the increasing demand for energy globally. The CO2 concentration has increased from around 280 ppm in the late 19th century up to 422 ppm in December 2023, (See: The NOAA global monitoring laboratory, trends in atmospheric carbon dioxide, https: / / gml.noaa.gov / ccgg / trends / global.html, accessed 12 / 1 / 2024). The increased CO2 is a factor for natural carbon cycle imbalance of the Earth, subsequently, leading to global warming, ocean acidification, and unstable weather conditions. To limit global to within 2° C., CO2 emissions must be reduced. Hence, as CO2 levels continue to rise, an efficient and sustainable carbon reduction technology is required.

[0005] Several conventional options for reducing the effects of climate change by carbon capture or converting CO2 to fuels are being considered. Carbon capture and storage (CCS) is geared towards alleviating climate change by capturing CO2 emitted from power plants and other industrial processes rather than releasing it into the atmosphere. The CO2 is then trapped and kept underground, preventing it from entering the atmosphere indefinitely. CCS is a bridge technology that allows fossil fuels to be used in power generation and industries until low-carbon substitutes become available. However, CCS requires a massive volume of CO2 to be captured and stored to reduce carbon emissions considerably Storage of CO2 has limitations of high cost, intensive energy requirements for separation and pumping, permanency of stored CO2 in sites, making this approach currently infeasible. Another approach to tackle CO2 emissions is electrocatalytic reduction. Further, techniques like photocatalytic reduction, and thermochemical catalytic conversion are somewhat popular for converting CO2. However, when compared with photocatalytic and thermochemical catalytic conversion methods, electrocatalytic CO2 reduction reaction (ECO2RR) has the advantages of providing green, easy-to-scale devices and moderate operating conditions. Hence, development of efficient and stable ECO2RR catalysts is desirable. In general, coupling ECO2RR with electricity generated from renewable energy resources may provide a neutral carbon cycle while producing valuable chemicals for the chemical industry.

[0006] Numerous electrocatalysts have been evaluated for ECO2RR including metals and metal oxides, metal complexes, metal nitrides, carbon-based catalysts, and metal organic framework (MOF). However, the current catalysts for ECO2RR have limited activity with high overpotential, low selectivity toward a particular product, and limited stability. Out of all transition metals, copper-based catalysts are the materials found capable of producing hydrocarbons such as methane and ethylene and oxygenates such as methanol and ethanol. Various copper-based catalysts have been synthesized for the ECO2RR, including Cu alloys, oxide-derived copper, and Cu nanoparticles, which have shown great potential for CO2 reduction into valuable chemicals. Previous research has attempted to optimize surface properties of Cu nanomaterials by changing the size, shape, and crystal facet of Cu nanomaterials, or alloying Cu with other metals to improve the intrinsic activity toward desired products. However, the other metals used, such as, gold, platinum, and zinc are expensive and economically taxing to use at industrial scale. Hence, despite the years of research on ECO2RR, the reduction of CO2 to C2+ products is still not viable at an industrial scale, with currently used catalysts suffering drawbacks such as, high overpotentials and low product selectivity. Hence, there is still a requirement for a better and more effective technique for curbing CO2 emissions and improving yield for CO2 to C2+ conversion.

[0007] Accordingly, it is one object of the present disclosure to provide a method for electrochemical reduction of CO2 using silver (Ag)-doped CuO nanosheets that may circumvent the drawbacks and limitations such as, high overpotential, high cost, low yield, and low product selectivity, of current methods known in the art.SUMMARY

[0008] In an exemplary embodiment, a method for electrochemical reduction of carbon dioxide in an H-type electrochemical cell is described. The method includes applying a voltage to a solution including carbon dioxide in an H-type electrochemical cell, which includes a working electrode at least partially coated with a catalyst composition including CuO nanosheets doped with silver nanoparticles. The silver nanoparticles are disposed on the surface of the CuO nanosheets and the amount of silver is in a range from 15% to 60% of the combined weight of the CuO nanosheets and silver nanoparticles. During the applying the carbon dioxide is reduced to form one or more C2+ products and where the Faradaic efficiency for the formation of the C2+ products is greater than or equal to 35% at −1.05 V vs. RHE.

[0009] In some embodiments, the amount of silver is in a range from 15% to 35% of the combined weight of the CuO nanosheets and silver nanoparticles.

[0010] In some embodiments, the amount of silver is in a range from 23% to 28% of the combined weight of the CuO nanosheets and silver nanoparticles.

[0011] In some embodiments, the Faradaic efficiency for the formation of the C2+ products is greater than or equal to 40% at −1.05 V vs. RHE.

[0012] In some embodiments, the Faradaic efficiency for C2+ products is greater than or equal to 45% at −1.05 V vs. RHE.

[0013] In some embodiments, the Faradaic efficiency for the formation of H2 is less than or equal to 45% at −1.05 V vs. RHE.

[0014] In some embodiments, the Faradaic efficiency for the formation of H2 is less than or equal to 40% at −1.05 V vs. RHE.

[0015] In some embodiments, the Faradaic efficiency for the formation of H2 is less than or equal to 35% at −1.05 V vs. RHE.

[0016] In some embodiments, the solution includes 0.1 M K2SO4.

[0017] In another exemplary embodiment, a method for electrochemical reduction of carbon dioxide in an electrochemical flow cell is described. The method includes supplying carbon dioxide to an electrochemical flow cell including an electrolyte solution, where the carbon dioxide flows over a cathode gas diffusion electrode at least partially coated with a catalyst composition including CuO nanosheets doped with silver nanoparticles. The silver nanoparticles are disposed on the surface of the CuO nanosheets and the amount of silver is in a range from 15% to 60% of the combined weight of the CuO nanosheets and the silver nanoparticles. The method further includes applying a voltage to the electrochemical flow cell, during the applying the carbon dioxide is reduced to form one or more C2+ products and the Faradaic efficiency for the formation of the C2+ products at a partial current density of 228 mA cm−2 is greater than 60%.

[0018] In some embodiments, the amount of silver is in a range from 15% to 35% of the combined weight of the CuO nanosheets and silver nanoparticles.

[0019] In some embodiments, the amount of silver is in a range from 23% to 28% of the combined weight of the CuO nanosheets and silver nanoparticles.

[0020] In some embodiments, the Faradaic efficiency for the formation of the C2+ products is greater than or equal to 65% at −1.5 V vs RHE.

[0021] In some embodiments, the Faradaic efficiency for the formation of the C2+ products is greater than or equal to 70% at −1.5 V vs RHE.

[0022] In some embodiments, the Faradaic efficiency for the formation of H2 is less than or equal to 28% at −1.5 V vs RHE.

[0023] In some embodiments, the Faradaic efficiency for the formation of H2 is greater than or equal to 23% at −1.5 V vs RHE.

[0024] In some embodiments, the Faradaic efficiency for the formation of H2 is less than or equal to 23% at −1.5 V vs RHE.

[0025] In some embodiments, the electrolyte solution includes 1.0 M KOH.

[0026] In some embodiments, the Faradaic efficiency for the formation of the C2+ products remains above 70% for at least 5 hours of operation at a current density of 300 mA cm−2 in galvanostatic conditions.

[0027] In some embodiments, the Faradaic efficiency for the formation of the C2+ products remains above 70% for at least 10 hours of operation at a current density of 300 mA cm−2 in galvanostatic conditions.

[0028] The foregoing general description of the illustrative embodiments and the following detailed description thereof are merely exemplary aspects of the teachings of this disclosure and are not restrictive.BRIEF DESCRIPTION OF THE DRAWINGS

[0029] A more complete appreciation of this disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

[0030] FIG. 1A is a schematic illustration depicting a method for electrochemical reduction of carbon dioxide in an H-type electrochemical cell, according to certain embodiments.

[0031] FIG. 1B is a schematic diagram depicting an exemplary scheme for synthesis of silver (Ag)-doped copper oxide (CuO) nanosheets (CuONS), according to certain embodiments.

[0032] FIG. 2A shows X-ray diffraction (XRD) pattern of as-synthesized Ag-doped CuONS at various concentration of Ag doping, according to certain embodiments.

[0033] FIG. 2B shows XRD spectrum of CuONS on carbon paper (CP) substrate after removing background noise and CP peaks, according to certain embodiments.

[0034] FIG. 2C shows an optical image of carbon paper casted with CuONS before electrolysis, according to certain embodiments.

[0035] FIG. 2D shows an optical image of carbon paper casted with CuONS during electrolysis, according to certain embodiments.

[0036] FIG. 2E shows an optical image of carbon paper casted with CuONS after electrolysis, according to certain embodiments.

[0037] FIG. 2F is a transmission electron microscopy (TEM) image of pristine CuO nanosheets showing a morphology of Cu nanosheets, according to certain embodiments.

[0038] FIG. 2G is a TEM image of 50% Ag-doped CuO nanosheet showing spherical AgNP on the surface of Cu nanosheets, according to certain embodiments.

[0039] FIG. 2H is an enlarged image at 2 nanometers (nm) of magnification, of the pristine CuONS depicting the d-spacings, according to certain embodiments.

[0040] FIG. 2I is an enlarged image of FIG. 2G, magnified at 10 nm magnification, according to certain embodiments.

[0041] FIG. 2J shows TEM images at different scales for Ag-doped CuO nanosheets, according to certain embodiments.

[0042] FIG. 3A is a scanning electron microscopy (SEM) image of Ag-doped CuONS, depicting morphology and chemical composition of the Ag-doped CuONS at 2 micrometer (μm) magnification, according to certain embodiments.

[0043] FIG. 3B is a SEM image of Ag-doped CuONS, depicting morphology and chemical composition of the Ag-doped CuONS at 500 nm magnification, according to certain embodiments.

[0044] FIG. 3C is a SEM image of Ag-doped CuONS, depicting morphology and chemical composition of the Ag-doped CuONS at 5 μm magnification, according to certain embodiments.

[0045] FIG. 3D is a SEM image of Ag-doped CuONS, depicting morphology and chemical composition of the Ag-doped CuONS at 1 μm magnification, according to certain embodiments.

[0046] FIG. 3E is an elemental map obtained from scanning electron microscopy—energy dispersive X-ray (SEM-EDX) analysis depicting a homogeneous distribution of silver nanoparticles (AgNP) on CuO substrate, according to certain embodiments.

[0047] FIG. 4A shows a linear sweep voltammetry (LSV) curves for an electrochemical CO2 reduction reaction (CO2RR) performance of various Ag-doped CuONS samples in H-Cell using 0.1 M K2SO4 as the electrolyte, according to certain embodiments.

[0048] FIG. 4B shows faradaic efficiencies (FE) for CO2RR on CuONS between potentials of −0.95 V vs. RHE to 1.35 V vs. RHE, according to certain embodiments.

[0049] FIG. 4C shows FE for CO2RR on 25% Ag-doped CuONS between −0.95 V vs. RHE to 1.35 V vs. RHE, according to certain embodiments.

[0050] FIG. 4D shows ethylene and hydrogen gas (H2) FE as a function of the amount of Ag doping, according to certain embodiments.

[0051] FIG. 5A shows LSV curves for electrochemical CO2 reduction performance of various Ag-doped CuONS samples in flow-cell configuration, and 1 M KOH electrolyte, according to certain embodiments.

[0052] FIG. 5B shows FE for CO2RR on Ag-doped CuONS at current densities of 100 mA cm2, 200 mA cm−2, and 300 mA cm−2, according to certain embodiments.

[0053] FIG. 5C shows a graph depicting results of a stability test for 25% Ag-doped CuONS at 300 mA cm−2 in 1 M KOH electrolyte, according to certain embodiments.

[0054] FIG. 5D is an image of a gas diffusion electrode sprayed with CuONS before electrolysis, according to certain embodiments.

[0055] FIG. 5E is an image of a gas diffusion electrode sprayed with CuONS during electrolysis, according to certain embodiments.

[0056] FIG. 5F is an image of a gas diffusion electrode sprayed with CuONS after electrolysis showing the color change of the catalyst, according to certain embodiments.

[0057] FIG. 6A is a free energy diagram of H2O splitting to H2 on Cu-111 and Ag-111 surfaces, according to certain embodiments.

[0058] FIG. 6B shows top and side views of optimized binding geometries of intermediates in the formation of H2 and CO on the Cu-Ill surface, according to certain embodiments.

[0059] FIG. 6C is a free energy diagram of CO2 reduction to CO on Cu-111 and Ag-111 surfaces, according to certain embodiments.

[0060] FIG. 6D shows top and side views of optimized binding geometries of intermediates in the formation of H2 and CO on the Ag-111 surface, according to certain embodiments.DETAILED DESCRIPTION

[0061] In the drawings, like reference numerals designate identical or corresponding parts throughout the several views. Further, as used herein, the words “a,”“an” and the like generally carry a meaning of “one or more,” unless stated otherwise.

[0062] Furthermore, the terms “approximately,”“approximate,”“about,” and similar terms generally refer to ranges that include the identified value within a margin of 20%, 10%, or preferably 5%, and any values therebetween.

[0063] As used herein, the term “electrochemical reduction” refers to a process in which an electrochemical reaction leads to the gain of electrons by a species, resulting in its conversion to a lower oxidation state. This process typically occurs at the cathode of an electrochemical cell, where a reduction reaction is driven by an applied electric current. In the context of carbon dioxide (CO2) conversion, electrochemical reduction involves the transformation of CO2 into various valuable products, such as hydrocarbons or alcohols, through the transfer of electrons and protons.

[0064] As used herein, the term “H-type electrochemical cell” refers to a specific design of electrochemical cell that features a divided chamber, typically separated by a proton-conducting membrane, such as Nafion. This design allows for the separation of the anode and cathode compartments, facilitating distinct electrochemical reactions at each electrode while preventing the mixing of the reactants and products. In an H-type cell, the electrolyte solutions in each compartment can be independently controlled, which is advantageous for optimizing reaction conditions. The configuration enhances the efficiency of electrochemical processes, such as the reduction of carbon dioxide, by minimizing undesired side reactions and allowing for effective ion transport.

[0065] As used herein, the term “electrochemical flow cell” refers to a type of electrochemical cell designed for continuous operation, where reactants are supplied in a flowing manner through the cell's compartments. This setup typically includes an anode and a cathode, separated by an electrolyte solution that facilitates ion transport. In an electrochemical flow cell, reactants-such as gases like carbon dioxide—are continuously introduced into the cell, allowing for real-time electrochemical reactions.

[0066] As used herein, the term “Faradaic efficiency” refers to the measure of the effectiveness of an electrochemical reaction in converting charge (electrons) into a specific product. It is defined as the ratio of the actual amount of a desired product generated during an electrochemical reaction to the theoretical amount that could be produced based on the total electric charge passed through the system.

[0067] As used herein, the term “galvanostatic conditions” refers to a mode of operation in electrochemical experiments where a constant current is applied to the electrochemical cell throughout the reaction. Under galvanostatic conditions, the current density remains fixed, and the cell voltage may vary in response to changes in the system, such as concentration changes or resistance fluctuations. This approach is commonly used to study electrochemical reactions because it allows for controlled and consistent electron transfer, facilitating the analysis of reaction kinetics, product formation, and other performance metrics. Galvanostatic operation is particularly useful in applications like battery charging and discharging, electroplating, and electrochemical synthesis, as it provides insights into the efficiency and stability of the process under steady-state conditions.

[0068] As used herein, the term “current density” refers to the amount of electric current flowing per unit area of an electrode surface in an electrochemical cell. It is typically expressed in units of milliamperes per square centimeter (mA / cm2) or amperes per square meter (A / m2). Current density is a parameter in electrochemical processes as it influences the rate of electrochemical reactions, mass transport, and overall system performance. Higher current densities can lead to increased reaction rates but may also result inside reactions or increased resistance, affecting the efficiency and stability of the system. Thus, optimizing current density achieves desired outcomes in various applications, such as fuel cells, electrolysis, and battery technologies.

[0069] Aspects of this disclosure pertain to a method for the electrochemical reduction of carbon dioxide in both H-type electrochemical cells and electrochemical flow cells. This method plays a role in energy conversion and storage, providing a means to utilize CO2 as a feedstock for sustainable chemical production while addressing environmental concerns related to greenhouse gas emissions. The method of the present disclosure enhances mass transport and improves reaction efficiency, potentially leading to higher product yields compared to traditional static cells. This method is particularly advantageous for applications such as energy conversion, CO2 reduction, and fuel cell technologies, as it allows for better control over reaction conditions and facilitates the collection of products for analysis.

[0070] In one or more embodiments, a CO2 reduction reaction occurs on the catalyst surface when CO2 and water meet on the catalyst surface. Chemical reactions, with the aid of the catalyst, may break the CO2 and water molecules into smaller components that may rearrange into useful C2+ products. Hydrogen gas may also form, which limits CO2 reduction into C2+ products, therefore, ideal catalyst performance will lead to a system that favors the production of C2+ products, having a high Faradaic efficiency for C2+ products and a low Faradaic efficiency for H2.

[0071] A method for electrochemical reduction of carbon dioxide in an H-type electrochemical cell is described. The method includes applying a voltage to a solution including carbon dioxide in an H-type electrochemical cell. The electrochemical cell includes a working electrode at least partially coated with a catalyst composition including CuO nanosheets doped with silver nanoparticles.

[0072] In some embodiments, voltage applied to a solution may range from −0.5 V to −3.0 V, −1.0 V to −2.0V, −1.2V to −2.0V, −1.4V to −2.0V, −1.6V to −2.0V, −1.8 V to −2.0V versus a reference electrode, such as Ag / AgCl. In a preferred embodiment, voltage applied to a solution is −1.5 V and −1.9 V versus Ag / AgCl.

[0073] In some embodiments, the reference electrode may include but is not limited to, saturated calomel electrode (SCE), silver / silver sulfate electrode (Ag / Ag2SO4), mercury / mercury oxide electrode (Hg / HgO), copper / copper sulfate electrode (Cu / CuSO4), pH-sensitive glass electrode, saturated potassium chloride electrode, platinum wire electrode, carbon paste electrode, ion-selective electrode, metal oxide electrode, vanadium pentoxide electrode, lithium / lithium ion electrode, gold / gold oxide electrode, nickel / nickel hydroxide electrode, indium / inidum oxide electrode, graphite electrode. In a preferred embodiment, reference electrode is Ag / AgCl.

[0074] In some embodiments, the amount of silver deposited on CuO may range from 10 to 70%, 15 to 60%, preferably 15 to 35%, preferably 23 to 28% of the combined weight of the CuO nanosheets and silver nanoparticles. In a preferred embodiment, the amount of silver is about 25% of the combined weight of the CuO nanosheets and silver nanoparticles.

[0075] In some embodiments, the electrolyte solution may include but is not limited to sodium sulfate (Na2SO4), potassium chloride (KCl), sodium bicarbonate (NaHCO3), lithium chloride (LiCl), calcium chloride (CaCl2), magnesium sulfate (MgSO4), ammonium sulfate ((NH4)2SO4), sodium nitrate (NaNO3), potassium phosphate (K3PO4), lithium sulfate (Li2SO4), sodium acetate (NaC2H3O2), potassium hydroxide (KOH), lithium carbonate (Li2CO3), sulfuric acid (H2SO4), hydrochloric acid (HCl), phosphoric acid (H3PO4), sodium citrate (Na3C6H5O7), potassium acetate (KC2H3O2), ammonium chloride (NH4Cl), and boric acid (H3BO3). In a preferred embodiment, the electrolyte solution is K2SO4

[0076] In some embodiments, the concentration of the electrolyte, preferably K2SO4, may range from 0.01-4.0 M, 0.05-2.0 M, 0.1-2.0 M, 0.12-2.0 M, 0.14-2.0 M, 0.16-2.0 M, 0.18-2.0 M, 0.04-2.0 M, 0.08-2.0 M. In a preferred embodiment, the concentration of K2SO4 is 0.1 M.

[0077] During the applying of voltage to the solution, the carbon dioxide is reduced to form one or more C2+ products. This process involves the transfer of electrons to the CO2 molecules, facilitating their conversion into higher-order carbon compounds. The applied voltage serves to drive the reaction, overcoming the thermodynamic barriers associated with the reduction of CO2, which is generally stable and inert.

[0078] In some embodiments, the products formed may include but are not limited to, acetic acid, propanol, propylene, butanol, butyric acid, 1-butene, 2-butene, isobutanol, diethyl ether, methanol, formic acid, methanethiol, dimethyl ether, isopropanol, pentane, hexane, heptane, octanol, nonane, decanol, 1,3-butadiene, 2-pentanone, 1,2-propylene glycol, 1,3-propanediol, lactic acid, furfural, acetone, benzene, toluene, and cyclohexane. In preferred embodiments, during the applying of voltage to the solution, the carbon dioxide is reduced to form one or more C2+ products such as ethylene and ethanol.

[0079] In some embodiments, the Faradaic efficiency for the formation of the C2+ products may be greater than or equal to 35%, 40%, 45% at −1.05 V vs. RHE. In a preferred embodiment, the Faradaic efficiency for the formation of the C2+ products is 50% at −1.05 V vs. RHE. In some embodiments, the Faradaic efficiency for the formation of H2 may be, less than or equal to 45%, 40%, 35% at −1.05 V vs. RHE. In a preferred embodiment, the Faradaic efficiency for the formation of H2 is less than or equal to 30-40% at −1.05 V vs. RHE.

[0080] A method for electrochemical reduction of carbon dioxide in an electrochemical flow cell is described. The method includes supplying carbon dioxide to an electrochemical flow cell comprising an electrolyte solution. As the CO2 is supplied, it diffuses into the electrolyte, where it interacts with the catalyst present on the working electrode. This catalyst promotes the electrochemical reduction of CO2 into valuable products, such as hydrocarbons or alcohols, through a series of electron transfer reactions.

[0081] In some embodiments, the electrolyte solution may include but is not limited to sodium sulfate (Na2SO4), potassium chloride (KCl), sodium bicarbonate (NaHCO3), lithium chloride (LiCl), calcium chloride (CaCl2), magnesium sulfate (MgSO4), ammonium sulfate ((NH4)2SO4), sodium nitrate (NaNO3), potassium phosphate (K3PO4), lithium sulfate (Li2SO4), sodium acetate (NaC2H3O2), potassium hydroxide (KOH), lithium carbonate (Li2CO3), sulfuric acid (H2SO4), hydrochloric acid (HCl), phosphoric acid (H3PO4), sodium citrate (Na3C6H5O7), potassium acetate (KC2H3O2), ammonium chloride (NH4Cl), and boric acid (H3BO3). In a preferred embodiment, the electrolyte solution is KOH.

[0082] In some embodiments, the concentration of K2SO4 may range from 0.01-4.0 M, 0.05-2.0 M, 0.1-2.0 M, 0.12-2.0 M, 0.14-2.0 M, 0.16-2.0 M, 0.18-2.0 M, 0.04-2.0 M, 0.08-2.0 M. In a preferred embodiment, the concentration of KOH is 1 M.

[0083] The carbon dioxide flows over a cathode gas diffusion electrode at least partially coated with a catalyst composition comprising CuO nanosheets doped with silver nanoparticles. In some embodiments, the cathode gas diffusion electrode may include but is not limited to materials such as stainless steel mesh, nickel foam, conductive carbon cloth, graphene oxide membranes, titanium felt, activated carbon fabrics, metal-organic frameworks (MOFs), carbon nanotube membranes, porous ceramic electrodes, gold-coated substrates, copper mesh, conductive polymer films, silicone rubber composites, woven carbon fibers, alumina-based substrates, carbon aerogels, titanium dioxide-coated electrodes, hydrophobic Teflon sheets, perovskite-based materials, and composite conductive material. In a preferred embodiment, carbon paper is used as cathode gas diffusion electrode.

[0084] In some embodiments, the working electrode is coated with a catalyst composition of copper oxide (CuO) nanosheets doped with silver nanoparticles. In some embodiments, the amount of silver deposited on CuO may range from 10 to 70%, 15 to 60%, 15 to 35%, 23 to 28% of the combined weight of the CuO nanosheets and silver nanoparticles. In a preferred embodiment, the amount of silver is 25% of the combined weight of the CuO nanosheets and silver nanoparticles.

[0085] The method further includes applying a voltage to the electrochemical flow cell. In some embodiments, voltage applied to a solution may range from −0.5 V to −3.0 V −1.0 V to −2.0 V, −1.2 V to −2.0 V, −1.4 V to −2.0 V, −1.6 V to −2.0 V, −1.8 V to −2.0 V versus a reference electrode, such as Ag / AgCl. In a preferred embodiment, voltage applied to a solution is −1.5 V and −1.9 V versus Ag / AgCl.

[0086] In some embodiments, the reference electrode may include but is not limited to, saturated calomel electrode (SCE), silver / silver sulfate electrode (Ag / Ag2SO4), mercury / mercury oxide electrode (Hg / HgO), copper / copper sulfate electrode (Cu / CuSO4), pH-sensitive glass electrode, saturated potassium chloride electrode, platinum wire electrode, carbon paste electrode, ion-selective electrode, metal oxide electrode, vanadium pentoxide electrode, lithium / lithium ion electrode, gold / gold oxide electrode, nickel / nickel hydroxide electrode, indium / inidum oxide electrode, graphite electrode. In a preferred embodiment, reference electrode is Ag / AgCl.

[0087] During the applying of voltage to the solution, the carbon dioxide is reduced to form one or more C2+ products. This process involves the transfer of electrons to the CO2 molecules, facilitating their conversion into higher-order carbon compounds. The applied voltage serves to drive the reaction, overcoming the thermodynamic barriers associated with the reduction of CO2, which is generally stable and inert.

[0088] In some embodiments, the products formed may include but are not limited to, acetic acid, propanol, propylene, butanol, butyric acid, 1-butene, 2-butene, isobutanol, diethyl ether, methanol, formic acid, methanethiol, dimethyl ether, isopropanol, pentane, hexane, heptane, octanol, nonane, decanol, 1,3-butadiene, 2-pentanone, 1,2-propylene glycol, 1,3-propanediol, lactic acid, furfural, acetone, benzene, toluene, and cyclohexane. In preferred embodiments, during the applying of voltage to the solution, the carbon dioxide is reduced to form one or more C2+ products such as g ethylene, ethanol, n-propanol, and acetate.

[0089] In some embodiments, the flow cell may operate under galvanostatic conditions with current densities ranging from 50-500 mA cm−2, 100-500 mA cm−2, 150-500 mA cm−2, 200-500 mA cm−2, 250-500 mA cm−2, 300-500 mA cm−2, 350-500 mA cm−2, 400-500 mA cm−2, 450-500 mA cm−2. In a preferred embodiment, the flow cell operates at galvanostatic conditions specifically at current densities of 100, 200, and 300 mA cm−2.

[0090] In some embodiments, the Faradaic efficiency for the formation of the C2+ products at a partial current density of 228 mA cm−2 may be greater than 60%, 65%, 70%, 75%, 80%. In a preferred embodiment, the Faradaic efficiency for the formation of the C2+ products at a partial current density of 228 mA cm−2 is 76%.

[0091] In some embodiments, the Faradaic efficiency for the formation of the C2+ products may be greater than or equal to 65%, 70%, 75%, 80% at −1.5 V vs RHE. In a preferred embodiment, the Faradaic efficiency for the formation of the C2+ products is 76% at −1.5 V vs RHE.

[0092] In some embodiments, the Faradaic efficiency for the formation of H2 is less than or equal to 28%, 23% at −1.5 V vs RHE.

[0093] In some embodiments, the Faradaic efficiency for the formation of the C2+ products may remain above 70%, 75%, 80% for at least 5 hours 7 hours, 9 hours, 11 hours, 13 hours, 15 hours, 17 hours, 19 hours of operation at a current density of 300 mA cm−2 in galvanostatic conditions. In a preferred embodiment, the Faradaic efficiency for the formation of the C2+ products is 80% for at least 12 hours of operation at a current density of 300 mA cm−2 in galvanostatic conditions.EXAMPLES

[0094] The following examples demonstrate a method for electrochemical reduction of carbon dioxide in an H-type electrochemical cell. The examples are provided solely for illustration and are not to be construed as limitations of the present disclosure, as many variations thereof are possible without departing from the spirit and scope of the present disclosure.Example 1: Catalyst Synthesis—CuO Nanosheet (CuONS)

[0095] The CuONS was synthesized by slightly modifying previously reported method, included as reference herein: (Zhang, B. et al., Highly Electrocatalytic Ethylene Production from CO2 on Nanodefective Cu Nanosheets, J. Am. Chem. Soc., 2020, 142, 13606-13613, and Ma, Z. et al., CO2 electroreduction to multicarbon products in strongly acidic electrolyte via synergistically modulating the local microenvironment, Nature Communications, 2022, 13, 7596, incorporated herein by reference in its entirety). Initially, 10 milliliters (mL) of 6 molar (M) sodium hydroxide (NaOH) solution was prepared by dissolving 2.4 g (60 mmol) of NaOH in 10 mL deionized (DI) water. Further, 10 mL of 0.1 M copper chloride (CuCl2) solution was prepared by dissolving 170.5 mg of copper (II) chloride dihydrate in 10 mL of DI water. Subsequently, the NaOH solution was added dropwise to the CuCl2 solution under magnetic stirring, at room temperature and stirred continuously for 30 minutes. Furthermore, the solution was transferred to a 50 mL Teflon-lined autoclave and kept in an oven at 100° C. for 12 hours. After cooling to room temperature, the product was collected by centrifugation at 4000 revolutions per minute (rpm) for 5 mins. The product was then washed with water and ethanol several times before drying in a vacuum oven at 50° C. overnight. The Cu nanosheet catalyst was prepared by reducing the CuO nanosheets under CO2 reduction conditions at −1.6 V vs. Ag / AgCl (or −1.06 V vs. RHE) for at least 30 mins. The following formulas depicts the chemical reactions undergoing in this process.Example 2: Synthesis of Ag-doped CuO Nanosheet

[0096] According to the present disclosure, for 25% Ag / CuONS synthesis, around 20 mg of CuONS powder at a concentration of about 0.2514 mmol CuO, was dissolved in 10 mL of ethanol under sonication. Further, around 14.24 mg of silver nitrate (AgNO3), about 0.08383 mmol concentration, was dissolved in 10 mL of DI water. The AgNO3 solution was quickly added to the CuONS solution before keeping the solution inside a UV transilluminator. The solution was then irradiated with UV light at 365 nm for 30 mins under magnetic stirring, at around 1200 rpm. Furthermore, the product was collected by centrifugation at 4000 rpm for 5 mins. The product was further washed with water and ethanol several times before drying in a vacuum oven at 50° C. overnight. The other Ag-doped samples were synthesized using the same method by varying the amount of AgNO3 while keeping the amount of CuONS constant. Referring to FIG. 1B, a schematic flow of synthesis scheme for the Ag-doped CuO nanosheet catalyst is illustrated. The following formula depicts the photoreduction of Ag+ ions into Ag nanoparticles.Example 3: Catalyst Characterization

[0097] The crystal structure of all synthesized samples was analyzed using the powder X-ray diffraction (XRD) (Rigaku, MiniFlex) at a wavelength (λ) of about 1.54 angstrom (Å). The XRD measurements were taken for 20 between 3° and 80°. To understand the catalyst morphology and surface features, transition electron microscope (TEM) (Tecnai TF20) was used to analyze pristine and Ag-doped CuO nanosheet samples. Further, scanning electron microscopy (SEM) (Tescan Lyra-3) and energy dispersive X-ray spectrometry (EDS) were performed to understand the structure morphology and elemental composition, the. The samples were pre-coated with gold (Au) to enhance their conductivity.Example 4: Preparation of Working Electrode

[0098] The working electrode was prepared by drop casting a catalyst ink on a conductive carbon paper. Initially, the carbon paper was rinsed with water and ethanol under sonication. The catalyst ink was then prepared by mixing 10 mg of catalyst, 1 mg of carbon black, 750 microliters (μL) of iso-propanol, 200 μL of deionized water (DI), and 50 μL of 5 wt. % Nafion solution (a total of 1000 L solution). The prepared ink was sonicated for at least 30 mins before further usage. Using a micropipette, 100 μL of the prepared ink was painted on a 1 cm2 area of the carbon paper, giving a catalyst loading of 1 mg / cm2. The carbon paper was then left to dry at room temperature.Example 5: Electrochemical CO2 Reduction in H-Cell

[0099] The electrochemical CO2 reduction was carried out in a gas-tight electrochemical H-cell separated by Nafion 117 membrane to prevent reoxidation of reduced products. The spacing between the working and reference electrodes were kept at about 0.5 cm to minimize uncompensated solution resistance. Platinum (Pt) was used as the counter electrode, and the saturated Ag / AgCl as the reference electrode. Each side of the H-cell was filled with 11 mL of 0.1 M potassium sulfate (K2SO4) aqueous electrolyte. The CO2 was then purged through the electrolyte in the cathode compartment for 15 mins until saturated with CO2, providing a pH of 5.8. The CO2 was set at 20 mL / min using a flow controller. All potentiostatic electrolysis was performed by Gamry potentiostat. The potentials were converted to RHE using the following equation:ER⁢H⁢E=EAg / AgCl+0.197+0.0⁢5⁢91×pH(1)

[0100] Further, a constant potential electrolysis (chronoamperometry) was performed by applying a constant potential across the working electrode in the range from −1.5 V to −1.9 V vs. Ag / AgCl reference electrode. Each potential was maintained for 80 mins. Three gas samples were collected for gas chromatography (GC) analysis at around 10 min, 45 min, 80 min. Around 1 mL of electrolyte was also collected from the cathode compartment for nuclear magnetic resonance (NMR) analysis to identify any liquid products. After each electrolysis, the cell was cleaned with DI water and then refilled with fresh electrolyte.Example 6: Electrochemical CO2 Reduction in Flow-Cell

[0101] The electrochemical CO2 reduction was carried out in a flow cell configuration (GAOSS UNION) using an anion exchange member (Fumasep FAA-3-PK-130), Pt plate as the anode, and carbon paper (AvCarb GDS3215) as the cathode gas diffusion electrode. The microporous side of the GDE was sprayed with 50 μL of catalyst using a spray gun to give a catalyst loading of 0.5 mg / cm2. The Ag / AgCl reference electrode was used to measure potential at the cathode side. The catholyte and anolyte were circulated through the flow cell using peristaltic pumps at flow rate of 5 mL / min. Both catholyte and anolyte reservoirs were filled with 30 mL of 1 M KOH. The CO2 flow at the gas chamber was kept constant at 40 mL / min using a flow meter.

[0102] Further, the flow cell was tested at galvanostatic conditions (chronopotentiometry) by varying applied voltage to maintaining constant currents. The tests were performed for current densities of 100 mA / cm2, 200 mA / cm2, and 300 mA / cm2 for a period of one hour. Two GC measurements were made for each run and the 1 mL from the catholyte liquid was collected for NMR analysis.Example 7: Product Analysis

[0103] The gas products were detected by an online gas chromatography (Shimadzu GC-2014) equipped with the barrier ionization discharge detector (BID) and using helium as the carrier gas. The gas products concentrations were calculated based on the calibration curves made for each gas product. The Faradaic efficiencies of gas products were calculated as follows:F⁢El,gas=zi⁢FniQtotal=zi⁢Fxi⁢P⁢V⁢tRTI×t=0.7⁢1⁢7⁢4⁢8⁢9×1⁢0-4⁢zi⁢xi⁢VI(2)where “zi” is the number of electrons consumed for 1 mol of product “i”, “F” is the Faraday constant (96,485 C / mol e−), “xi” is product concentration in parts per million (PPM), “V” is the volumetric flow rate in SCCM, and “I” is the total current in mA.The liquid products were analyzed by 1H NMR on a BrukerAVANCE III400 MHz NMR) spectrometer. The NMR samples were prepared by mixing 30 μL of 50 mM phenol, 70 μL of D2O (deuterated water), and 600 μL of products solution. The Faradaic efficiencies of liquid products were calculated as follows:F⁢Ei,liquid=zi⁢F⁢niQtotal=(zi⁢FI×t)⁢(AiAS⁢T⁢D)⁢(NS⁢T⁢DNi)⁢(30⁢ μL600⁢ μL)⁢(50⁢ mM)⁢(Vcell)(3)where “Ai” and “ASTD” are the NMR peak areas for the product and NMR standard, respectively, “Ni” and “NSTD” are the number of probed equivalent “H1” atoms in the product and NMR standard, respectively, and “Vcell” in mL, is the volume of cathode reservoir.Example 8: Computational DetailsElectronic structure calculations were performed using density functional theory (DFT) to find the lowest free energy pathways for the reduction of H2O and CO2 to H2 and CO, respectively. The DFT simulations were conducted using the GPAW software, which is based on the projector augment wave (PAW) method and the atomic simulation environment (ASE). The Perdew-Burke-Ernzerhof (RBE) exchange correlation functional along with plane-wave pseudopotentials were used to describe the electronic wave functions. The Cu-111 and Ag-111 surfaces were modelled using 2×2×3 supercells with 3.64 Å lattice constant and 10 Å vacuum space. The first two layers were relaxed in all directions, while the bottom layer was fixed at the bulk equilibrium positions. The plane-wave energy cutoff was set to 500 electron volts (eV) with a (8,8,1) k-point sampling mesh for each lattice. The relaxation calculations were performed until the maximal force on any atom was below 0.05 eV / Å. All adsorption sites on the 111 facet, including fcc, hcp, ontop, and bridge binding sites, were investigated and the lowest energy binding site was used.Thermodynamic quantities for gas species were obtained based on the ideal-gas limit by assuming that all spatial degrees of freedom are independent and separable into translational, rotational, and vibrational degrees of freedom. Vibration frequencies of adsorbates were obtained using the normal mode analysis on adsorbate atoms, assuming the harmonic approximation and considering only the vibration of adsorbate atoms. The reaction free energies were calculated using the following relations:Δ⁢G=Δ⁢H-T⁢Δ⁢S(4)Δ⁢H=Δ⁢U+PV(5)Δ⁢U≈Δ⁢EDFT+Δ⁢EZPE+Δ⁢EC⁢v+Δ⁢Es⁢o⁢l⁢v(6)where “EDFT” is the potential energy obtained from DFT calculation, “EZPE” is the zero-point energy, “Esolv” is the solvation correction term.The computational hydrogen electrode (CHE) technique was used to model the potential dependence of the free energy pathway by defining zero voltage based on the reversible hydrogen electrode (RHE). This model avoids the explicitly treatment of solvated protons, and thus a pH correction is not needed. Using this model, the total chemical potential of the proton-electron pair may be expressed as a function of applied potential at all temperatures and pH values, using the following relation:μ⁡(H+)+μ⁡(e-)=[1 / 2⁢μ⁡(H2)]-e⁢U(7)where “μ” is the chemical potential, “e” is the electron charge, and “U” is the applied potential. The calculation details for adsorbate free energies, as well as the optimized geometric coordinates for all adsorbates, can be found in Tables 1 to Table 8.TABLE 1Free energy calculation details of gas-phase molecules.FugacityEDFTEelecZPE∫Cpdt−TSGSpecies(Pa)(eV)(eV)(eV)(eV)(eV)(eV)H230296−6.7680.0000.2680.090−0.434−0.076H2O3534−14.3260.0000.5690.103−0.6700.002CO2101325−23.251−0.7810.3480.092−0.655−0.996CO5562−14.9130.0000.1340.090−0.685−0.461H2(ref)101325−6.7680.0000.2680.090−0.403−0.045TABLE 2Free energy of adsorption and its contributionsfor adsorbates on Cu-111 surface.AdsorptionEDFTEelecZPECV.harm−TSGSpeciesSite(eV)(eV)(eV)(eV)(eV)(eV)*HFCC43.9090.1960.1630.0050.0060.035*COFCC56.1310.8890.2040.0440.0730.715*COOHTOP66.1910.0080.7110.0660.1110.658TABLE 3Free energy of adsorption and its contributions for adsorbates on Ag-111 surface.AdsorptionEDFTEelecZPECV.harm−TSGSpeciesSite(eV)(eV)(eV)(eV)(eV)(eV)*HFCC−34.9030.1690.1360.008−0.0110.303*COFCC−46.763−0.1620.2220.042−0.0720.030*COOHTOP−56.9650.5780.6500.049−0.0781.200TABLE 4Free energy change (ΔG) for HER on Cu-111 and Ag-111 surfaces.ReactionStepCu-111 (eV)Ag-111 (eV)1002−0.0120.3263−0.031−0.031TABLE 5Free energy change (ΔG) forCO2RR to CO on Cu-111 and Ag-111 surfaces.ReactionStepCu-111 (eV)Ag-111 (eV)10.0000.00020.8461.3883−0.2220.52240.1310.131TABLE 6Solvent stabilization corrections for surface adsorbates.Solvent StabilizationAdsorbate(eV)*CO−0.1*COOH−0.38TABLE 7Gas corrections.MoleculeGas correction (eV)CO2+0.45TABLE 8Calculated reference atoms energy.AtomEDFT (eV)H−3.384O−7.558C−7.354Cu−3.361Ag−2.641H−3.384The CuO nanosheets were synthesized from CuCl2 and NaOH by a precipitation method to form Cu(OH)2, which further decomposes to CuO, at a temperature of about at 100° C. The formed CuO nanosheets were then irradiated with UV light in AgNO3 solution to deposit the Ag nanoparticles on the CuO nanosheets. Furthermore, the Ag-doped CuO nanosheets were electrochemically reduced to Cu under CO2RR conditions.The crystallinity of CuO nanosheets were examined by the XRD crystallography. The XRD spectrum confirmed that synthesized samples were indeed polycrystalline CuO. FIG. 2A shows the XRD spectrums of CuO nanosheets samples at several Ag doping percentages. In addition to CuO peaks, the Ag peaks were observed for Ag-doped samples, confirming the deposition of Ag nanoparticles on the CuO nanosheets. The XRD spectrum was also obtained after the CO2 electrolysis to understand catalyst changes during CO2RR. Further, FIG. 2B shows the XRD spectrum of electrochemically reduced CuO nanosheet (CuONS) coated on CP substrate after fitting and removing background noise and carbon paper (CP) peaks. The XRD confirmed the reduction of CuO to metallic Cu under CO2RR, showing the peaks for metallic Cu only (Cu-111, Cu-200, and Cu-220). This change was also observed from the color change of the catalyst from from black to reddish-brown, which is a character of metallic Cu (FIGS. 2C-2D). However, it is worth noting that the metallic copper quickly oxidizes in air, which was seen clearly from the gradual change of the catalyst color from reddish brown into black within few mins of exposure to air, as shown in FIG. 2E. Therefore, the catalyst under CO2RR conditions, “CuONS” may be interchangeably referred to as “CuNS” hereinafter, for the sake of brevity in explanation.In addition, the morphology of CuO nanosheets were studied using transmission electron microscope (TEM). FIG. 2F shows the high-resolution transmission electron microscopy (HR-TEM) image for pristine CuONS. The HR-TEM images confirm the catalyst morphology of nanosheets, with a lateral size in the order of 100 nm. The HR-TEM images also show that the CuO nanosheets are piling up above each other with a minuscule thickness for each layer. The HR-TEM images further confirm the polycrystalline nature of synthesized catalysts. Three grains with well-defined d-spacings were identified which may be attributed to the (002), (111), and (110) facets, as shown in FIGS. 2G-2I. The d-spacings for all identified facets matched the diffraction angles obtained from XRD, confirming the identities of observed facets. Furthermore, the HR-TEM images for Ag-doped CuONS confirmed the formation of Ag nanoparticles on the surface of CuO nanosheets. FIG. 2G shows the HR-TEM image of 50% Ag-doped CuONS. FIG. 2H shows an enlarged image at 2 nm magnification of the pristine CuONS. A magnified image for FIG. 2G is shown in FIG. 2I where a spherical Ag nanoparticle is clearly shown on the surface of CuO nanosheet. The d-spacing for this particle (0.232 nm) matches the diffraction angle for Ag-111 facets, confirming that it is a AgNP. The diameter of AgNP was found to be around 16 nm. The d-spacing of the underlying CuO nanosheet was found to be 0.274 nm, which matches the diffraction angle of CuO-110 facet. The results from HR-TEM images corroborated XRD spectrums. The TEM images at different scales for Ag-doped CuO nanosheets is also depicted in FIG. 2J.Further, the catalyst morphology was investigated using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM / EDX). FIGS. 3A-3D show the SEM images of as-synthesized Ag-doped CuO nanosheet catalyst. As can be seen from FIGS. 3A-3D, the CuO nanosheet has a smooth jigsaw puzzle-like surface structure with a lateral size of around 1 μm and a thickness of around 25 nm. Due to their low thickness, the CuO nanosheets pack on top of each other, creating a dense catalyst packing. Furthermore, FIG. 3B shows small nanoparticles on the surface of CuO nanosheet which may be the AgNPs. Moreover, FIG. 3E depicts the elemental composition of the catalyst as obtained from the EDX analysis. The Ag doping is shown to be homogenous across the surface of the catalyst, which further confirms the deposition of AgNP on the surface of CuO nanosheets.The electrochemical CO2 reduction performance of the new synthesized materials was tested in a small gas-tight electrochemical H-cell in CO2 saturated 0.1 M K2SO4 solution at potentials ranging from −0.4 vs. RHE to −1.4 V vs. RHE. FIG. 4A shows the LSVs for CuO nanosheets at various amounts of Ag loading. The total current density slightly decreased as the Ag doping increased, with the highest activity recorded for the pristine CuO nanosheet sample. The above mentioned trend may be due to higher activity of Cu in CO2RR compared to Ag, which requires slightly higher overpotential to activate. However, this drop-in activity is largely neglected when considering at the influence of Ag for tuning catalyst selectivity in CO2RR. The product distribution of CO2RR was analyzed for all samples using GC and NMR for gas and liquid products, respectively. The results are depicted in Table 9.TABLE 9Product identification based on NMR peaks for CO2RR productsChemical1HPeak #ShiftSplittingProbed NucleusProduct Name18.26sHCOO−Formate27.13tC6H5OH(α)Phenol36.8tC6H5OH (β)Phenol46.73dC6H5OH (γ)Phenol55.8mCH2CHCH2OHAllyl Alcohol65.1dCH2CHCH2OHAllyl Alcohol75dCH2CHCH2OHAllyl Alcohol83.92dCH2CHCH2OHAllyl Alcohol93.46tCH3CH2OHEthanol103.37tCH2CH2CH2 OHPropanol113.16sCH3OHMethanol122.04sCH3C═OCH3Acetone131.72sCH3C(═O) O−Acetate141.35sextetCH3CH2CH2OHPropanol150.99tCH3CH2OHEthanol160.7tCH3CH2CH2OHPropanolFIGS. 4B-4C show the calculated Faradaic efficiencies (FE) as a function of applied potential for the pristine and 25% Ag-doped CuO Nanosheet samples, respectively. The synthesized samples showed a diverse product distribution, with C2+ products (ethylene, ethanol, propanol, acetate) and Cl products (carbon monoxide, formate, methane, and methanol). The main products were in the following order: C2H4>H2>EtOH>HCOOH>CO>PrOH, with the other products having FE less than 1%. As shown in FIGS. 4B-4C, the 25% Ag-doped CuNS had a higher C2+ selectivity compared to pristine Cu Nanosheets, reaching FEC2 around 50% at −1.05 V vs. RHE, compared to a FEC2 of only 30% for the pristine CuNS at the same potential. The H2 selectivity was also suppressed in the Ag-doped sample, averaging around 30% to 40% in contrast to 40% to 50% of the pristine CuNS. The highest FE for C2H4, at around 43%, was achieved with the 25% Ag—CuNS sample at −1.15 V vs RHE and a partial current density of 5.9 mA cm−2. The FE for ethylene, however, dropped at higher potentials, which may be due to the rapid consumption of *CO adsorbates on the CuNS surface.The results indicate that Ag doping lowers the FE for H2 while increases the selectivity for ethylene. FIG. 4D shows the FE of ethylene plotted against the amount of Ag doping for CuNSs at −1.05 V vs. RHE. The 25% Ag CuNS was found to be the desirable Ag-doped sample for maximizing C2+ products while minimizing H2 formation in CO2RR. It may be noted that ethanol selectivity increased with the applied potential, while propanol selectivity decreased at higher potentials for the 25% Ag-doped CuNS sample. The inverse relation may be due to the reduction of C2 to C3 step being favored at low overpotentials, such that propanol is produced instead of ethanol. This pathway is then disfavored at high potentials, producing ethanol. Further, the above results suggest that AgNPs on CuNS surface plays a role in enhancing the selectivity of CO2RR toward C2+ products while suppressing H2 selectivity.According to the present disclosure, the doping of Ag on CuNS enhanced the C2+ selectivity in CO2RR in the H-Cell. However, both the current density and FEs for C2+ products are still limited by the low solubility of CO2 in aqueous electrolytes. The low solubility of CO2 in aqueous electrolytes was largely overcome by testing the synthesized catalysts in a flow cell configuration, with the CO2 being fed at the cathode side using a carbon-based gas diffusion electrode (GDE). The flow cell was run at galvanostatic conditions at current densities of 100 mA cm−2, 200 mA cm−2, 300 mA cm−2, and 1 M KOH was used as both the catholyte and anolyte. As can be seen from FIG. 5A, the linear sweep voltammetry (LSV) for synthesized samples showed the same trend, as the H-cell, with the pristine CuNS providing the highest current density while the activity of Ag-doped CuNS drops with increasing Ag doping.FIG. 5B shows the FEs for synthesized samples at various current densities. The FE for ethylene increases as the current density increases while the FE of CO decreases. The aforementioned trend may be attributed to the increase of local pH around the catalyst at high current density, which facilitates the CO—CO coupling step, consuming the CO gas while increasing the formation of C2+ products such as ethylene. Additionally, similar to the H-cell, the 25% Ag-doped CuNS showed enhanced performance compared to pristine CuNS, with a C2+ FE of 76% at a partial current density of 228 mA cm−2 for C2+ products (including ethylene, ethanol, n-propanol, and acetate) and at a potential of −1.5 V vs RHE, compared to a C2+ FE of 62% for pristine CuNS. The 50% Ag-doped CuNS had an intermediate performance with a C2+ FE of 66% at the same conditions.The long-term catalytic stability was evaluated by operating the flow cell at a current density of 300 mA cm−2 in galvanostatic conditions for 18 hours. FIG. 5C shows the FE of gas products for the stability test of 25% Ag-doped CuNS at 300 mA cm−2 in 1 M KOH electrolyte. The FE of ethylene remained constant around 80% for 12 hours before the decay of gas diffusion electrode, causing the H2 FE to increase while the H2 FE of ethylene and CO decreased, respectively. The decay in performance was observed clearly from the wetting of GDE as well as the precipitation of KOH salt on its surface (FIG. 5D-FIG. 5F)In order to better evaluate the mechanisms behind the enhanced performance of the synthesized Ag-doped CuNS, density functional theory (DFT) calculations were performed on the modelled system. It is more thermodynamically favorable for Cu and Ag to coexist as two separated phases in the catalyst described herein. This notion is supported from the TEM results which showed the formation of AgNP on the surface of CuNS, as depicted in FIG. 2I. The TEM results also showed the presence of both Cu-111 and Ag-111 facets in the synthesized catalysts. Therefore, the calculations were conducted on Cu-111 since it is the dominant facet in the synthesized CuNS, as shown by the XRD results of FIG. 2B. The influence of Ag nanoparticles on CuNS surface was modelled as an Ag-111 facet. Both of these surfaces were modelled separately assuming negligible change of the lattice constant of AgNP on the surface of CuNSs.In order to evaluate the H2 suppression and high C2+ selectivity of Ag-doped CuNS, the mechanisms for water splitting into H2, and CO2 reduction to CO were examined. In general, H2 is produced by the reduction of H+ ions in the solution, forming the *H intermediate, which further reduces the solution into H2 gas, with another H+ ion. FIG. 6A shows the free energy diagram constructed for H2 formation on both Cu-111 and Ag-111 surfaces. Due to the higher energy of *H on Ag-111 surface (0.33 eV) compared to Cu-111 surface (−0.01 eV), H2 formation is suppressed in Ag-doped Cu catalysts compared to their pristine Cu counterparts. This was seen in the synthesized Ag-doped CuNS samples, with roughly 10% drop in the FEH2 compared to the pristine CuNS sample, as shown in FIG. 4B and FIG. 2F. FIG. 6B shows top and side views of optimized binding geometries of intermediates in the formation of H2 and CO on the Cu-111 surface.

[0120] In addition, the reduction of CO2 to CO via the *COOH intermediate was evaluated on Cu-111 and Ag-111 surfaces. FIG. 6C shows the free energy diagram for CO2 reduction to CO on those surfaces. The mechanism starts with a concerted proton-electron transfer step for CO2 to form the *COOH intermediate, which is then reduced into a *CO adsorbate and a water molecule. The *CO may further be released as a gaseous CO molecule or reacted to produce more reduced products. The first step in the mechanism may be the rate limiting step as it has the highest thermodynamic barrier as shown in FIG. 6C. Ag has slightly higher *COOH energy at around 1.39 eV when compared to Cu at around 0.85 eV, which explains the higher overpotential required for Ag in CO2RR. The aforementioned trend is also seen with the *CO intermediate, with the Cu having stronger adsorption with *CO than Ag. Moreover, on the Cu-111 surface, the desorption of *CO to CO (g) is not favorable since it has a barrier of +0.35 eV, thus *CO may tend to further reduce to more reduced products. In contrast, the *CO is weakly adsorbed on the Ag-111 surface, so desorption of *CO is more favorable than further reduction. As can be seen from FIG. 6B, the desorption of *CO to CO (g) stabilizes *CO by about 0.39 eV, while a migration of *CO to an adjacent Cu-111 surface stabilizes *CO by about 0.75 eV. According to the present disclosure, the kinetic barrier for *CO migration from the Ag cluster to Cu surface is only 0.17 eV, which is easily overcome at room temperature. Hence, the synergy between Cu nanosheets and Ag nanoparticles may be favorable. As such, *CO intermediate produced at Ag sites may migrate to adjacent Cu surfaces where it may be further reduced to C2+ products such as ethylene and ethanol. FIG. 6D shows top and side views of optimized binding geometries of intermediates in the formation of H2 and CO on the Ag-111 surface

[0121] The aspects of the present disclosure provide a method for electrochemical reduction of carbon dioxide. The photo-deposition of Ag on CuO nanosheets using UV light is an effective method for steering the CO2RR selectivity toward multi-carbon products. DFT calculations of the present disclosure confirmed the synergy between Cu nanosheets and Ag nanoparticles, where Ag with its higher thermodynamic barrier for *H suppresses H2 production while producing the CO needed for the CO—CO coupling reaction on Cu nanosheets. The exergonic desorption of *CO on Ag also increases local gaseous CO concentration near the surface, while the endergonic desorption of *CO on Cu increases the surface concentration of *CO, favoring the cross coupling reaction. The results derived herein show that Cu nanosheet catalyst coated with Ag nanoparticles may achieve about 50% FE toward C2+ products at 14 mA cm−2 in an H-cell. In a flow reactor, the catalyst achieved a C2+ selectivity of about 80% FE at an industrial-relevant current density of 300 mA cm−2 for at least 12 hours. To summarize, the present disclosure synthesis provides an effective method to introduce dopant materials into the surface of metal nanosheets, which may be used in development of active catalyst for CO2RR.

[0122] Numerous modifications and variations of the present disclosure are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described herein.

Claims

1. A method for electrochemical reduction of carbon dioxide in an H-type electrochemical cell, the method comprising:applying a voltage to a solution comprising carbon dioxide and water in an H-type electrochemical cell comprising a working electrode at least partially coated with a catalyst composition comprising CuO nanosheets doped with silver nanoparticles, where the silver nanoparticles are disposed on the surface of the CuO nanosheets and the amount of silver is in a range from 15 to 60 wt. % of the combined weight of the CuO nanosheets and silver nanoparticles,wherein during the applying the carbon dioxide is reduced to form one or more C2+ products, andwherein the Faradaic efficiency for the formation of the C2+ products is greater than or equal to 35% at −1.05 V vs. RHE.

2. The method of claim 1, wherein the amount of silver is in a range from 15 to 35 wt. % of the combined weight of the CuO nanosheets and silver nanoparticles.

3. The method of claim 2, wherein the amount of silver is in a range from 23 to 28 wt. % of the combined weight of the CuO nanosheets and silver nanoparticles.

4. The method of claim 1, wherein the Faradaic efficiency for the formation of the C2+ products is greater than or equal to 40% at −1.05 V vs. RHE.

5. The method of claim 4, wherein the Faradaic efficiency for the formation of C2+ products is greater than or equal to 45% at −1.05 V vs. RHE.

6. The method of claim 1, wherein the Faradaic efficiency for the formation of H2 is less than or equal to 45% at −1.05 V vs. RHE.

7. The method of claim 6, wherein the Faradaic efficiency for the formation of H2 is less than or equal to 40% at −1.05 V vs. RHE.

8. The method of claim 7, wherein the Faradaic efficiency for the formation of H2 is less than or equal to 35% at −1.05 V vs. RHE.

9. The method of claim 1, wherein the solution comprises 0.1 M K2SO4.

10. A method for electrochemical reduction of carbon dioxide in an electrochemical flow cell, the method comprising:supplying carbon dioxide to an electrochemical flow cell comprising an electrolyte solution comprising water, wherein the carbon dioxide flows over a cathode gas diffusion electrode at least partially coated with a catalyst composition comprising CuO nanosheets doped with silver nanoparticles, where the silver nanoparticles are disposed on the surface of the CuO nanosheets and the amount of silver is in a range from 15 to 60% of the combined weight of the CuO nanosheets and silver nanoparticles; andapplying a voltage to the electrochemical flow cell,wherein during the applying the carbon dioxide is reduced to form one or more C2+ products,wherein the Faradaic efficiency for the formation of the C2+ products at a partial current density of 228 mA cm−2 is greater than 60%.

11. The method of claim 10, wherein the amount of silver is in a range from 15 to 35% of the combined weight of the CuO nanosheets and silver nanoparticles.

12. The method of claim 11, wherein the amount of silver is in a range from 23 to 28% of the combined weight of the CuO nanosheets and silver nanoparticles.

13. The method of claim 10, wherein the Faradaic efficiency for the formation of the C2+ products is greater than or equal to 65% at −1.5 V vs RHE.

14. The method of claim 13, wherein the Faradaic efficiency for the formation of the C2+ products is greater than or equal to 70% at −1.5 V vs RHE.

15. The method of claim 10, wherein the Faradaic efficiency for the formation of H2 is less than or equal to 28% at −1.5 V vs RHE.

16. The method of claim 15, wherein the Faradaic efficiency for the formation of H2 is less than or equal to 23% at −1.5 V vs RHE.

17. The method of claim 16, wherein the Faradaic efficiency for the formation of H2 is less than or equal to 23% at −1.5 V vs RHE.

18. The method of claim 10, wherein the electrolyte solution comprises 1.0 M KOH.

19. The method of claim 14, wherein the Faradaic efficiency for the formation of the C2+ products remains above 70% for at least 5 hours of operation at a current density of 300 mA cm−2 in galvanostatic conditions.

20. The method of claim 19, wherein the Faradaic efficiency for the formation of the C2+ products remains above 70% for at least 10 hours of operation at a current density of 300 mA cm−2 in galvanostatic conditions.

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