BIMETALLIC ZINC/COPPER OXIDE ELECTRODE FOR CO2 CONVERSION

Abstract

A method of forming an electrode including dissolving a copper salt and benzene-1,3,5-tricarboxylate in a solvent and heating to a temperature of 60° C. to 100° C. to form a framework. Further, the method includes mixing a zinc salt and the framework to form a zinc doped framework and heating the zinc doped framework to a temperature of 300° C. to 600° C. under air to form ZnCuO nanoparticles. Furthermore, the method includes mixing the ZnCuO nanoparticles, a binding compound, and a conductive carbon compound in a solvent to form a suspension. Moreover, the method includes coating a substrate with the suspension and drying to form the electrode and the ZnCuO nanoparticles have an oval shape with an average size of 50 nm to 200 nm.

Description

STATEMENT OF ACKNOWLEDGEMENT

Support provided by King Fahd University of Petroleum and Minerals (KFUPM) is gratefully acknowledged.

BACKGROUND

Technical Field

The present disclosure is directed to a method of synthesizing an electrode, more particularly directed to a bimetallic zinc/copper oxides derived from HKUST-1 metal-organic framework-based electrode for electrocatalytic reduction of carbon dioxide (CO₂) to ethane C₂H₆.

Description of Related Art

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

Burning of fossil fuels produces greenhouse gases such as CO₂, NO, and CH₄, which negatively impacts the environment. Therefore, CO₂ reduction is a positive approach for a robust environmental value-added chemical production and energy storage infrastructure. The electrochemical reduction of CO₂ requires improvements in regard to control of product selectivity, the enhancement of product conversion rate, and the minimal needed over-potential. Typically, this process includes numerous proton and electron transfers, thus producing various products, each with several likely reaction intermediates. This complexity poses significant drawbacks in the characterization of molecular-level reaction mechanisms, which are demanding in the design of selective, effective electrodes and stable electro-catalysts.

Materials employed in electrochemical reduction should be available in large quantities with significant efficiency while preserving low costs to ensure their economic viability. Furthermore, favorable properties include high CO₂ adsorption capacity, efficient charge transfer kinetics, and the ability to suppress competing reactions. The catalysts should exhibit long-term stability, allowing continuous CO₂ reduction under various operating conditions. To date, different electrode materials and designs have been investigated for the electrochemical reduction of CO₂, with materials such as metal nanoparticles, metal-organic frameworks, and single-atom catalysts.

Copper (Cu)-based catalysts have emerged as promising candidates for CO₂ electroreduction due to their high activity and selectivity towards producing C₂⁺ products. The presence of copper oxide species on the catalyst surface is helpful in facilitating the desired chemical transformations. However, challenges such as catalyst stability and competing hydrogen evolution reactions still need to be addressed to maximize the efficiency of Cu-based catalysts for CO₂ conversion.

Zinc (Zn) is also a potential electrocatalyst for CO₂ reduction. Zinc-based catalysts offer advantages such as low cost, earth abundance, and tunable catalytic properties. Zinc promotes electrochemical CO₂ reduction to carbon monoxide (CO). Active sites on the zinc catalyst surface facilitate the adsorption of CO₂, leading to the subsequent reduction of CO₂ to CO. Zinc-based catalysts possess electrochemical stability, which is attributed to the formation of protective surface layers or oxides that prevent the dissolution or deterioration of the catalyst.

Hence, efficient methods need to be developed, which may substantially reduce or eliminate the above limitations. Accordingly, an object of the present disclosure is to develop an electrode for CO₂ reduction including both Zn and Cu.

SUMMARY

In an exemplary embodiment, a method of making an electrode is described. The method includes dissolving a copper salt and benzene-1,3,5-tricarboxylate in a solvent and heating to a temperature of 60° C. to 100° C. to form a framework. Further, the method includes mixing a zinc salt and the framework to form a zinc doped framework and heating the zinc doped framework to a temperature of 300° C. to 600° C. under air to form ZnCuO nanoparticles. Furthermore, the method includes mixing the ZnCuO nanoparticles, a binding compound, and a conductive carbon compound in a solvent to form a suspension and coating a substrate with the suspension followed by drying process to form the electrode. Moreover, the ZnCuO nanoparticles have an oval shape with an average size of 50 nanometers (nm) to 200 nm.

In some embodiments, the ZnCuO nanoparticles comprise 5 mol percentage (mol %) to 50 mol % Zn, relative to a total number of Zn and Cu moles in the ZnCuO nanoparticles.

In some embodiments, the ZnCuO nanoparticles comprise Cu, C, Zn, and O.

In some embodiments, the ZnCuO nanoparticles comprise 30 weight percent (wt. %) to 40 wt. % Cu (Copper), 30 wt. % to 40 wt. % C (Carbon), 10 wt. % to 20 wt. % Zn (Zinc), and 10 wt. % to 20 wt. % O (Oxygen).

In some embodiments, the ZnCuO nanoparticles comprise CuO and ZnO, and the CuO has a monoclinic crystal structure, whereas, the ZnO has a hexagonal crystal structure.

In some embodiments, the CuO and ZnO are uniformly dispersed in the ZnCuO nanoparticles.

In some embodiments, during the mixing of the zinc salt, zinc is homogeneously dispersed in pores of the framework without distortion of the framework.

In some embodiments, the heating is to about 500° C.

In some embodiments, the copper salt is selected from the group consisting of copper (II) chloride, copper (II) sulfate, copper (II) nitrate, copper (II) acetate, copper (II) bromide, and hydrates thereof.

In some embodiments, the zinc salt is selected from the group consisting of zinc (II) chloride, zinc (II) sulfate, zinc (II) nitrate, zinc (II) acetate, zinc (II) bromide, and hydrates thereof.

In some embodiments, the conductive carbon compound is at least one selected from the group consisting of graphite, activated carbon, reduced graphene oxide, carbon nanotubes, carbon nanofibers, and carbon black.

In some embodiments, the binding compound is a fluorinated polymer.

In some embodiments, the substrate is made from at least one material selected from the group consisting of conductive carbon, stainless steel, aluminum, nickel, copper, platinum, zinc, tungsten, and titanium.

In some embodiments, the suspension comprises 70 wt. % to 90 wt. % of the ZnCuO nanoparticles and 10-30 wt. % of the conductive carbon compound, based on a total weight of the ZnCuO nanoparticles and the conductive carbon compound.

In another exemplary embodiment, the method includes applying a potential of less than 0 V to −2.0 V vs RHE to an electrochemical cell and the electrochemical cell is at least partially submerged in an aqueous solution comprising carbon dioxide. Further, on applying the potential, the carbon dioxide is reduced to a conversion product. Furthermore, the electrochemical cell includes an electrode and a counter electrode.

In some embodiments, the conversion product is selected from ethane and carbon monoxide.

In some embodiments, the aqueous solution further comprises a base selected from at least one of sodium bicarbonate and potassium bicarbonate.

In some embodiments, the method has a faradic efficiency for reducing carbon dioxide to ethane of 35% to 45%.

In some embodiments, the method has a faradic efficiency for reducing carbon dioxide to carbon monoxide of 50% to 60%.

In some embodiments, the aqueous solution is saturated with the carbon dioxide.

The foregoing general description of the illustrative present disclosure 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

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:

FIG. 1 is a flowchart illustrating a method for synthesizing an electrode, according to certain embodiments;

FIG. 2A is X-ray diffraction (XRD) pattern of HKUST-1, CuO and ZnCuO synthesized at 500° C. (designated as CuO-500 and ZnCuO-500), according to certain embodiments;

FIG. 2B is a XRD pattern for different concentrations of Zn in ZnCuO-500, namely 5%, 25%, and 50%, designated as 5% ZnCuO-500, 25% ZnCuO-500, and 50% ZnCuO-500, respectively, according to certain embodiments;

FIG. 3A is a scanning electron microscopy (SEM) image of CuO-500, according to certain embodiments;

FIG. 3B is a SEM image of ZnCuO-500, according to certain embodiments;

FIG. 3C is a high-resolution transmission electron microscopy (HRTEM) image of ZnCuO-500, according to certain embodiments;

FIG. 3D is a transmission electron microscopy (TEM) image of CuO-500, according to certain embodiments

FIG. 3E is a TEM image of ZnCuO-500, according to certain embodiments;

FIG. 3F is a HRTEM of ZnCuO-500, according to certain embodiments;

FIG. 3G is a SEM image of 5% ZnCuO, according to certain embodiments;

FIG. 3H shows energy dispersive X-ray (EDX) image of ZnCuO-500 depicting copper, according to certain embodiments;

FIG. 3I shows EDX image of ZnCuO-500 depicting zinc, according to certain embodiments;

FIG. 3J shows EDX image of ZnCuO-500 depicting oxygen, according to certain embodiments;

FIG. 3K shows Energy-Dispersive Spectroscopy (EDS) analysis of 5% ZnCuO depicting the presence of Cu, Zn, O, and C, according to certain embodiments;

FIG. 3L shows EDS analysis of 5% ZnCuO depicting the presence of Cu, Zn, O, and C, with weight percentages, according to certain embodiments;

FIG. 4A shows comparative polarization curves for HKUST-1, CuO-500, and ZnCuO-500, according to certain embodiments;

FIG. 4B shows comparative polarization curves for ZnCuO-500 in nitrogen and carbon dioxide, saturated with 0.5 M potassium bicarbonate, according to certain embodiments;

FIG. 4C shows comparative linear sweep voltammetry (LSV) analysis for HKUST-1 and ZnCuO samples prepared at different temperatures, according to certain embodiments;

FIG. 4D shows comparative LSV analysis for ZnO, CuO, ZnCuO, and a physically mixed (PM) sample of ZnCuO, according to certain embodiments;

FIG. 4E shows a comparative LSV analysis for different concentrations of zinc in ZnCuO, according to certain embodiments;

FIG. 4F is a graph illustrating comparative Tafel slopes for HKUST-1, CuO-500, and ZnCuO-500, according to certain embodiments;

FIG. 4G is a graph illustrating comparative Nyquist plots for HKUST-1, CuO-600, and ZnCuO-500, according to certain embodiments;

FIG. 5A is a bar graph illustrating faradic efficiency (FE) percentage of CuO-500, according to certain embodiments;

FIG. 5B is a bar graph illustrating FE percentage of ZnO-500, according to certain embodiments;

FIG. 5C is a bar graph illustrating FE percentage of ZnCuO-500, according to certain embodiments; and

FIG. 5D is a graph illustrating the long-term stability of ZnCuO-500, according to certain embodiments.

DETAILED DESCRIPTION

In the drawings, 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.

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.

Where a numerical limit or range is stated herein, the endpoints are included. Also, all values and subranges within a numerical limit or range are specifically included as if explicitly written out.

As used herein, “particle size” may be thought of as the length or longest dimension of a particle.

As used herein, the term “electrode” refers to an electrical conductor used to contact a non-metallic part of a circuit e.g., a semiconductor, an electrolyte, a vacuum, or air.

As used herein, the term “current density” refers to the amount of electric current traveling per unit cross-section area.

As used herein, the term “Tafel slope” refers to the relationship between the overpotential and the logarithmic current density.

As used herein, the term “electrochemical cell” refers to a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions.

As used herein, the term “overpotential” refers to the difference in potential which exists between a thermodynamically determined reduction potential of a half-reaction and the potential at which the redox event is experimentally observed. The term is directly associated to a cell's voltage efficacy. In an electrolytic cell, the occurrence of overpotential implies that the cell needs more energy as compared to that thermodynamically expected to drive a reaction. The quantity of overpotential is specific to each cell design and varies across cells and operational conditions, even for the same reaction. Overpotential is experimentally measured by determining the potential at which a given current density is reached.

As used herein “metal-organic frameworks” or MOFs are compounds having a lattice structure made from (i) a cluster of metal ions as vertices (“cornerstones”)(“secondary building units” or SBUs) which are metal-based inorganic groups, for example metal oxides and/or hydroxides, linked together by (ii) organic linkers. The linkers are usually at least bidentate ligands which coordinate to the metal-based inorganic groups via functional groups such as carboxylates and/or amines. MOFs are considered coordination polymers made up of (i) the metal ion clusters and (ii) linker building blocks.

The present disclosure is intended to include all hydration states of a given compound or formula, unless otherwise noted or when heating a material.

Aspects of the present disclosure are directed towards an electrode including bimetallic zinc/copper oxides, derived from HKUST-1 metal-organic framework, for electrocatalytic reduction of carbon dioxide (CO₂) to ethane (C₂H₆) and other environmentally friendly compounds. The results indicate that the electrode of the present disclosure demonstrates a higher selectivity to CO₂ reduction products compared to a Cu oxide-based electrode.

Referring to FIG. 1, a flowchart 100 depicting a method for synthesizing an electrode is illustrated, according to certain embodiments. The order in which the method 100 is described is not intended to be construed as a limitation, and any number of the described method steps may be combined in any order to implement the method 100. Additionally, individual steps may be removed or skipped from the method 100 without departing from the spirit and scope of the present disclosure.

At step 102, the method 100 include dissolving a copper salt and benzene-1,3,5-tricarboxylate (BTC) in a solvent and heating to a temperature of 60-100° C. to form a framework. In some embodiments, benzene-1,3,5-tricarboxylate is replaced by any aromatic compound substituted with at least 2 carboxyl groups, preferably 2-4, or 3 carboxyl groups. For example, a benzene, naphthalene, anthracene, toluene substituted with at least 2 carboxyl groups. The copper salt may be copper chloride (CuCl₂), however, in some embodiments, the copper salt is selected from a group including, but may not be limited to, copper (II) sulfate, copper (II) nitrate, copper (II) acetate, copper (II) bromide, and hydrates thereof. The solvent may be an organic solvent or an inorganic solvent. Suitable examples of the organic solvent include tetrahydrofuran, ethyl acetate, dimethylformamide (DMF), acetonitrile, acetone, dimethyl sulfoxide, nitromethane, propylene carbonate, ethanol, formic acid, n-butanol, methanol, benzene, cyclohexane, ethyl acetate, dichloromethane, toluene, and diethyl ether, or any combination thereof. In some embodiments, the solvent is water. In some embodiments, the solvent is a mixture of organic and inorganic solvents. In a preferred embodiment, the solvent includes a mixture of DMF, ethanol, and water. The v/v ratio of DMF to ethanol is in a range of 1:1 to 1:10, preferably 1:1 to 1:5, preferably 1:1 to 1:4, preferably 1:1 to 1:3, preferably 1:1 to 1:2, preferably 1:1. The v/v/v ratio of DMF to ethanol to water is 1:1:1. In an embodiment, the heating is carried to a temperature of 60° C. to 100° C., preferably 65-95° C., preferably 70-90° C., preferably 75-85° C., preferably 80° C. to form a framework for 18-30 hours, preferably 20-28 hours, preferably 22 to 26 hours, preferably 24 hours to form the framework. In a specific embodiment, the metal-organic framework is synthesized by dissolving copper chloride and BTC in a solvent (including a mixture of DMF, ethanol, and water) and heating to a temperature of 80° C. to form the framework. In some embodiments, the framework is a metal-organic framework. In some embodiments, the metal-organic framework is HKUST-1. The HKUST-1 (Hong Kong University of Science and Technology) framework is built up of dimeric metal units, which are connected by benzene-1,3,5-tricarboxylate linker molecules. A paddlewheel unit is the structural motif to describe the coordination environment of the SBU of the HKUST-1 structure. The paddlewheel is built up of four benzene-1,3,5-tricarboxylate linkers molecules, which bridge two metal centers. One water molecule is coordinated to each of the two metal centers at the axial position of the paddlewheel unit in the hydrated state, which is usually found if the material is handled in air. After an activation process (heating, vacuum), these water molecules can be removed (dehydrated state) and the coordination site at the metal atoms is left unoccupied. This unoccupied coordination site is called a coordinatively unsaturated site (CUS) and can be accessed by other molecules. In this step of the synthesis, the metal center is Cu²⁺.

At step 104, the method 100 includes mixing a zinc salt and the framework to form a zinc doped framework. The zinc salt may be one or more of zinc (II) chloride, zinc (II) sulfate, zinc (II) nitrate, zinc (II) acetate, zinc (II) bromide, and hydrates thereof. In an embodiment, the zine salt is zinc (II) acetate. The weight ratio of the zinc salt to the framework is in a range of 1:1 to 1:10, preferably 1:2 to 1:9, preferably 1:3 to 1:8, preferably 1:4 to 1:7, preferably 1:5 to 1:6, preferably 1:5. On mixing the zinc salt, Zn is homogeneously dispersed in pores of the framework without distortion of the framework. For complete dispersion of the Zn⁺² within the pores of the framework, a suspension including the zinc salt, an organic solvent (ethanol), and the framework are sonicated for 5-15 minutes, preferably 6-14 minutes, preferably 7-13 minutes, preferably 8-12 minutes, preferably 9-11 minutes, preferably 10 minutes to form the zinc doped framework.

At step 106, the method 100 includes heating the zinc doped framework to a temperature of 300-600° C., preferably 350-550° C., 400-500° C., or about 450° C. under air to form ZnCuO nanoparticles. In a preferred embodiment, the heating is carried out at 500° C. to form the ZnCuO nanoparticles. In some embodiments, the heating is carried out in a furnace at a heating rate of 1-5 degrees Celsius per minute (° C. min⁻¹), preferably 1, preferably 2, preferably 3, preferably 4, preferably 5° C. min⁻¹, under air, until the temperature of the furnace reaches a temperature of about 500° C. to form ZnCuO nanoparticles.

In general, the ZnCuO nanoparticles can be any shape known to one of ordinary skill in the art. Examples of suitable shapes the spinel ferrite may take include spheres, spheroids, lentoids, ovoids, solid polyhedra such as tetrahedra, cubes, octahedra, icosahedra, dodecahedra, hollow polyhedral (also known as nanocages), stellated polyhedral (both regular and irregular, also known as nanostars), triangular prisms (also known as nanotriangles), hollow spherical shells (also known as nanoshells), tubes (also known as nanotubes), nanosheets, nanoplates, nanodisks, rods (also known as nanorods), and mixtures thereof. In some embodiments, the ZnCuO nanoparticles are oval shaped and have an average size in a longest length dimension of 50-200 nm, preferably 75-175 nm, 100-150 nm, or about 125 nm. In some embodiments, a width of the oval shape has an average size of 50-200 nm, preferably 75-175 nm, 100-150 nm, or about 125 nm. In some embodiments, the ZnCuO nanoparticles have a semispherical crystal form. In some embodiments, the semispherical crystals have a size of less than 200 nm, preferably 150 nm, 100 nm, or about 50 nm.

The ZnCuO nanoparticles include Cu, C, Zn, and O. In some embodiments, the ZnCuO nanoparticles include 30-40 wt. %, preferably 31-39 wt. %, preferably 32-38 wt. %, preferably 33-37.5 wt. %, preferably 34-37.1 wt. %, preferably 37.1 wt. % Cu; 30-40 wt. %, preferably 32-38 wt. %, preferably 33-37 wt. %, preferably 34-36 wt. %, preferably 35-36 wt. % C; 10-20 wt. %, preferably 11-19 wt. %, preferably 12-18 wt. %, preferably 13-17 wt. %, preferably 14-16 wt. %, preferably 14.6 wt. % Zn; and 10-20 wt. %, preferably 11-19 wt. %, preferably 12-18 wt. %, preferably 13-17 wt. %, preferably 13 wt. % O.

In some embodiments, the ZnCuO nanoparticles include 5-50 mol % Zn, preferably 10-45 mol % Zn, 15-40 mol % Zn, 20-35 mol % Zn, or 25-30 mol % Zn relative to a total number of Zn and Cu moles in the ZnCuO nanoparticles. In some embodiments, the ZnCuO nanoparticles include CuO and ZnO. In some embodiments, the CuO and ZnO are uniformly dispersed in the ZnCuO nanoparticles. The CuO has a monoclinic or cubic crystal structure, preferably monoclinic. The ZnO has a hexagonal, cubic or monoclinic crystal structure, preferably hexagonal.

At step 108, the method 100 includes mixing the ZnCuO nanoparticles, a binding compound, and a conductive carbon compound in a solvent to form a suspension. In an embodiment, the conductive carbon compound is one or more selected from the group of graphite, activated carbon, reduced graphene oxide, carbon nanotubes, carbon nanofibers, and carbon black. The binding compound is a fluorinated polymer. Suitable examples of fluorinated polymer include polytetrafluoroethylene (PTFE), polyethylene chlorotrifluoroethylene (ECTFE), polyethylene tetrafluoroethylene (ETFE), fluorinated-ethylene-propylene (FEP), perfluoro-alkoxy (PFA), polychlorotrifluoroethylene (PCTFE), polyvinylidene-fluoride (PVDF), sulfonated tetrafluoroethylene (Nafion®), and combinations thereof. The substrate is made from at least one material selected from conductive carbon, stainless steel, aluminum, nickel, copper, platinum, zinc, tungsten, and titanium. In a preferred embodiment, the substrate is conductive carbon. The solvent is an organic solvent. Suitable examples of the organic solvent include tetrahydrofuran, ethyl acetate, dimethylformamide (DMF), acetonitrile, acetone, isopropanol, dimethyl sulfoxide, nitromethane, propylene carbonate, ethanol, formic acid, n-butanol, methanol, benzene, cyclohexane, ethyl acetate, dichloromethane, toluene, and diethyl ether, or any combination thereof. In a preferred embodiment, the solvent is isopropanol. The suspension comprises 70-90 wt. %, preferably 75-85 wt. % or about 80 wt. % of the ZnCuO nanoparticles and 10 to 30 wt. %, preferably 15-25 wt. %, or about 20 wt. % of the conductive carbon compound, based on the total weight of the ZnCuO nanoparticles and the conductive carbon compound.

At step 110, the method 100 includes coating a substrate with the suspension and drying to form the electrode. The substrate may be coated by any conventional techniques known in the art, such as drop-casting, spin coating, or using an automatic coating machine. In an embodiment, the substrate surface is 50% coated with the suspension, preferably 60%, 70%, 80%, 90%, or 100% coated. In some embodiments, the coating has a thickness of 1-100 μm, preferably 10-90 μm, 20-80 μm, 30-70 μm, 40-60 μm, or about 50 μm. The substrate is further dried at 25-37° C., preferably 27-35° C., 29-33° C., or about 31° C. for 6 to 12 hours, preferably 7-11, or about 8-10 hours to form the electrode.

The electrode of the present disclosure can be used for electrochemical carbon dioxide (CO₂) reduction. The electrochemical reaction is carried out in an electrochemical cell. The electrochemical cell includes a working electrode, a counter electrode, and a reference electrode. The electrode, as described in FIG. 1, is the working electrode. The reference electrode is an electrode that has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant (buffered or saturated) concentrations of each relevant species of the redox reaction. A reference electrode may enable a potentiostat to deliver a stable voltage to the working or counter electrodes. The reference electrode may be a standard hydrogen electrode (SHE), a normal hydrogen electrode (NHE), a reversible hydrogen electrode (RHE), a saturated calomel electrode (SCE), a copper-copper (II) sulfate electrode (CSE), a silver chloride electrode (Ag/AgCl), a pH-electrode, a palladium-hydrogen electrode, a dynamic hydrogen electrode (DHE), a mercury-mercurous sulfate electrode, or some other type of electrode. In a preferred embodiment, a reference electrode is present and is a silver chloride electrode (Ag/AgCl).

In some embodiments, where the counter electrode includes platinum, the counter electrode is a rod or wire. The rod or wire may have straight sides and a circular cross-section, similar to a cylinder. A ratio of the length of the rod or wire to its width may be 1,500:1-1:1, preferably 500:1-2:1, more preferably 300:1-3:1, even more preferably 200:1-4:1. The length of the rod or wire maybe 0.5-50 cm, preferably 1-30 cm, more preferably 3-20 cm, and a long wire may be coiled or bent into a shape that allows the entire wire to fit into an electrochemical cell. The diameter of the rod or wire maybe 0.5-20 mm, preferably 0.8-8 mm, more preferably 1-3 mm. In some embodiments, a rod may have an elongated cross-section, similar to a ribbon or strip of metal. Alternatively, the counter electrode may comprise some other electrically-conductive material such as platinum-iridium alloy, iridium, titanium, titanium alloy, stainless steel, gold, cobalt alloy, and some other electrically-conductive material, where an “electrically-conductive material” as defined here is a substance with an electrical resistivity of at most 10⁻⁶ Ω·m, preferably at most 10⁻⁷ Ω·m, more preferably at most 10⁻⁸ Ω·m at a temperature of 20-25° C. In a preferred embodiment, the electrochemical cell is a H-cell. An H-cell is a divided electrochemical cell, having of two compartments connected through a diaphragm. In a preferred embodiment, the diaphragm is a proton exchange membrane.

The electrochemical cell is at least partially submerged in an aqueous solution. During the electrochemical process, the electrochemical cell is at least partially submerged preferably 50%, preferably at least 60%, 70%, 80%, 90%, or fully submerged in the aqueous solution. Preferably, to maintain uniform concentrations and/or temperatures of the aqueous solution, the aqueous solution may be stirred or agitated during the step of the subjecting. The stirring or agitating may be done intermittently or continuously. This stirring or agitating may be done by a magnetic stir bar, a stirring rod, an impeller, a shaking platform, a pump, a sonicator, a gas bubbler, or some other device. Preferably, the stirring is done by an impeller or a magnetic stir bar. The working electrode and the counter-electrode may be arranged as obvious to a person of ordinary skill in the art. In a specific embodiment, the Pt wire and Ag/AgCl (saturated with KCl) were used as counter electrode and reference electrode, respectively.

The aqueous solution may include water and an inorganic base. The water may be tap water, distilled water, bi-distilled water, deionized water, deionized distilled water, reverse osmosis water, and/or some other water. In one embodiment, the water is bi-distilled to eliminate trace metals. Preferably, the water is bi-distilled, deionized, deionized, distilled, or reverse osmosis water at 25° C. The aqueous solution has a salt concentration of 0.05 to 2 M, preferably 0.1 to 1 M, preferably 0.1 M. In a preferred embodiment, the salt includes potassium bicarbonate (KHCO₃). In some embodiments, the salt may include ammonium bicarbonate, barium carbonate, calcium carbonate, magnesite, sodium percarbonates, and sodium carbonate. In some embodiments, the aqueous solution includes CO₂, preferably it is saturated with CO₂.

During operation, a potential of less than 0 V to −2.0 V vs. RHE preferably −0.2 to −1.8 V, −0.4 to −1.6 V, −0.6 to −1.4 V, −0.8 to −1.2 V, or about −1 V, is applied between the working electrode and the counter electrode in the electrochemical cell via the aquoues solution. In one embodiment, the potential may be applied to the electrodes by a battery, such as a battery including one or more electrochemical cells of alkaline, lithium, lithium-ion, nickel-cadmium, nickel metal hydride, zinc-air, silver oxide, and/or carbon-zinc. In another embodiment, the potential may be applied through a potentiostat or some other source of direct current, such as a photovoltaic cell. In one embodiment, a potentiostat may be powered by an AC adaptor, which is plugged into a standard building or home electric utility line. In one embodiment, the potentiostat may connect with a reference electrode in the electrolyte solution. Preferably the potentiostat is able to supply a relatively stable voltage or potential. For example, in one embodiment, the electrochemical cell is subjected to a voltage that does not vary by more than 5%, preferably by no more than 3%, preferably by no more than 1.5% of an average value throughout the subjecting. In another embodiment, the voltage may be modulated, such as being increased or decreased linearly, being applied as pulses, or being applied with an alternating current.

Applying the potential reduces the carbon dioxide to a conversion product. The conversion product is ethane and/or carbon dioxide. The faradaic efficiency (FE) for reducing carbon dioxide to ethane is about 35% to 45% at −0.7 V_RHE, and the FE for reducing carbon dioxide to carbon monoxide is about 50% to 60% at −1.2 V_RHE. As used herein, the term ‘faradaic efficiency (FE)’ refers to the overall selectivity of an electrochemical process and is defined as the amount (moles) of collected product relative to the amount that can be produced from the total charge passed, expressed as a fraction or a percent.

While not wishing to be bound to a single theory, it is thought that in the synthetic method, the matrix of HKUST-1 helps in the dispersion of Zn⁺² in the pores of the HKSUT-1 and leads to a uniform mixture of the oxides of Zn and Cu in the ZnCuO nanoparticles. Further, the small size of the nanoparticles and presence of Cu—O and Zn—O bonds improves the charge transfer rate and conductivity.

In some embodiments, the electrode may be used for other purposes such as but not limited to hydrogen or oxygen evolution reaction in water splitting, or it can be incorporated into a supercapacitor.

EXAMPLES

The following examples demonstrate the synthesis and use of bimetallic zinc/copper oxides derived from Zn-HKUST-1 for CO₂ conversion to ethane. 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: Synthesis of HKUST-1

100 mg of CuCl₂·2H₂O was dissolved in 30 mL of (1:1:1) mixture of dimethyl formamide (DMF):ethanol:de-ionized water (DI) H₂O to form a solution. Then 200 mg of benzene-1,3,5-tricarboxylate (BTC) was added to the solution. The solution was then heated at 80° C. for 24 h, forming blue crystals. The blue crystals were separated by centrifugation, washed several times with DMF, DI H₂O, and ethanol, and dried under vacuum at 120° C. overnight.

Example 2: Synthesis of ZnCuO

20 mg of Zn(Ac)₂ was dissolved in 20 mL ethanol, and then 100 mg of HKUST-1 (as synthesized in Example 1) was added to the Zn⁺² solution. The solution was sonicated for 10 min and stirred for 12 h, resulting in the formation of solid crystals. After that, the solid crystals were separated and washed three times with ethanol and dried at 50° C. under vacuum. Then, the crystals were heated at 500° C. with a 5° C./min heating rate for 5 h. The CuO, ZnCuO prepared at 500° C. are designated as CuO-500 and ZnCuO-500, respectively. ZnCuO was also prepared by similar methods at different temperatures of 300, 400, and 600° C., which are labeled as ZnCuO-300, ZnCuO-400, and ZnCuO-600 respectively. Further, a plurality of Cu:Zn ratios are investigated with 5%, 10%, and 50% Zn ratios, labeled as X % ZnCuO, where X is 5, 10 or 50%. The samples with the varying Cu:Zn ratios were prepared at 500° C.

Example 3: Preparation of Working Electrode

10 mg of ZnCuO and 2 mg carbon were dispersed in 750 μL isopropanol, 200 μL DI H₂O, and 50 μL Nafion (5%) to obtain a suspension. The suspension was ultrasonicated for 20 min. 100 μL of this ink was drop-casted onto 1 cm² of conductive carbon paper and dried at room temperature.

Example 4: Electrochemical Characterization Techniques

A linear sweep voltammetry (LSV) study was performed in the aforementioned three-electrode electrochemical cell configuration with a potentiostat (Gammry) electrochemical workstation. A platinum (Pt) wire and a silver/silver chloride (Ag/AgCl) (saturated with potassium chloride) were used as the counter electrode and reference electrode, respectively. The catalyst-coated carbon compound was used as the working electrode. The LSVs were recorded in N₂ and CO₂-saturated 0.1 M KHCO₃ aqueous solutions from 0 V to −1.6 V (vs. RHE) at a scan rate of 20 mV s⁻¹. CO₂ and N₂ saturated solutions were prepared by bubbling high-purity CO₂ and N₂ in the 0.1 M KHCO₃ electrolyte for 30 minutes. All the potentials recorded with reference to Ag/AgCl are converted RHE. The formula used is described:

E(RHE)=E(Ag/AgCl)+0.197V+(0.059V×pH)

Bulk CO₂ electrolysis experiments were performed using the chronoamperometric (CA) technique. All the electrochemical experiments were carried out in the customized H-type cell. The cathode and the anode compartments of the electrochemical cell were separated by a pretreated proton exchange membrane.

Example 5: Chemical and Morphological Characterization

Referring to FIG. 2A, a graph depicting the X-ray diffraction (XRD) analysis of a comparison between the HKUST-1, CuO, and ZnCuO is illustrated. When Cu was largely replaced by Zn, XRD analysis confirmed that the primary structure of original Cu-HKUST-1 (JCPDS, 00-064-0936) remained stable (FIG. 2A). All synthesized samples (with varied Zn/Cu mole ratios) showed very identical peak patterns and intensities, indicating a homogeneous addition of Zn in Cu-HKUST-1 pores without HKUST-1 framework distortion. The samples calculated cell characteristics matched those of Cu-HKUST-1. Since, the sample ZnCuO was derived from HKUST-1, it revealed dominant CuO peaks on ZnO. Peaks at 2θ of 35.4, 38.6, 48.5, 58.0, and 61.2° correspond to monoclinic CuO (−111), (111), (−202), (202), and (−113) facets, while lower intensity peaks at 2θ of 31.78, 47.55, 56.61, and 67.87° are indexed to hexagonal ZnO (100), (102), (110), and (112) facets, respectively. Increasing the ZnO ratio from 5% to 25% to 50% (designated as 5% ZnCuO-500, 25% ZnCuO-500, and 50% ZnCuO-500, respectively) showed an increase in the characteristic reflection peaks of ZnO (FIG. 2B).

Referring to FIGS. 3A-3G, morphological structures of CuO-500 and ZnCuO-500 are illustrated. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) were utilized to analyze the morphological structure as described in FIG. 3A-3G. The SEM revealed that both CuO-500 (FIG. 3A) and ZnCuO-500 (FIG. 3B) are composed of discrete semispherical nanoparticles with crystal sizes less than 200 nm. It is anticipated that the electrochemical performance will be affected by the relatively tiny size of these crystals.

The TEM images of CuO-500 (FIG. 3D) and ZnCuO-500 (FIG. 3E) have confirmed the creation of a morphology similar to a sphere. The HRTEM of CuO-500 and ZnCuO-500 are depicted in FIG. 3C and FIG. 3F, respectively. The HRTEM reveals the fringe planes (interplanar distances) between ZnO-500 and CuO-500 (FIG. 3F). The SEM image of 5% ZnCuO is depicted in FIG. 3G.

The energy dispersive X-ray (EDX) analysis of ZnCuO-500 shows the presence of copper and zinc, which are dispersed quite evenly. Elemental mapping of ZnCuO-500 reveals the presence of elements copper (FIG. 3H), zinc (FIG. 3I), and oxygen (FIG. 3J). The EDS spectra of 5% ZnCuO depicts the presence of Cu, Zn, O, and C (FIG. 3K). In particular, the Cu has a wt. % of 37.1, the C has a wt. % of 35.6, the Zn has a wt. % of 14.3, and the O has a wt. % of 13.0 (FIG. 3L).

Example 6: Electrochemical Characterization

The electrochemical performance of the electrodes (HKUST-1, CuO, and CuZnO-500) was investigated by performing the LSV in CO₂-saturated 0.5 M KHCO₃. Comparative LSVs for three electrodes HKUST-1, CuO, and CuZnO-500 were performed in negative potential from 0 to −1.6 V vs RHE, and the results are noted as illustrated in FIG. 4A. The HKUST-1 showed the lowest performance; this could be attributed to the regular and oxygen bonds from the linker, which reduces the charge transfer rate. After the thermal treatment of the HKUST-1 sample, the yielded CuO-500 showed higher current density due to the enhancement of the conductivity of the sample. Further, upon introducing ZnO to the HKUST-1 and the thermal treatment, the ZnCuO-500 electrode showed the highest current density. The LSV of ZnCuO-500 was recorded in CO₂ and N₂ saturated 0.5 M KHCO₃. A higher current is observed in the case of the CO₂ curve reaching −60 mA cm⁻² at 1.6 negative potential (FIG. 4B).

Further, the effect of preparation temperature was investigated, and the results of this study are depicted in FIG. 4C. A graph depicting comparative LSVs for HKUST-1, ZnCuO-300, ZnCuO-400, ZnCuO-500, and ZnCuO-600, respectively. Herein, the 300, 400, 500, and 600 were the temperatures at which the ZnCuO samples are synthesized. As can be observed from the FIG. 4C, the ZnCuO-500 showed a high current density at the lowest applied voltage, and was used in the following studies.

Referring to FIG. 4D, a graph depicting comparative LSV analysis for the samples CuO, ZnO, ZnCuO, and the ZnCuO-PM (which is a counterpart of ZnCuO synthesized by physical mixing) is illustrated. The results indicated that the ZnCuO LSV showed the best performance in comparison to the pure ZnO, pure CuO, and ZnCuO samples prepared by physical mixing. Further, a plurality of Cu:Zn ratios are investigated with 5%, 10%, and 50% Zn ratios. A graph depicting comparative LSV analysis for these samples is depicted in FIG. 4E. The graph shows a variation in current density variation with a change in the applied potential for different concentrations of Zn²⁺ ions in the ZnCuO suspension. The electrode with the 5% ZnCuO exhibited the highest current density. This implies that the matrix of HKUST-1 helps in the dispersion of Zn⁺² in the pores of the HKSUT-1 and leads to a uniform mixture of oxide of Zn and Cu.

The kinetics of ECO₂R were investigated through the Tafel slopes calculated from the polarization curves (FIG. 4F). The electrodes HKUST-1, CuO-500, and ZnCuO-500 showed Tafel slope values of 120, 80, and 51 mV dec⁻¹, respectively. The lower Tafel value for ZnCuO-500 indicates faster reaction kinetics than HKUST-1 and CuO-500.

The conductivity and charge transfer rate of the electrodes were tested for the three electrodes with the aid of electrochemical impedance spectroscopy (EIS), and the results of this study are shown in FIG. 4G. The semicircle obtained from the EIS expresses the charge transfer resistance (Rct). The experiment was carried out under identical conditions to the LSV (electrolyte and electrodes) and by applying AC current with a frequency (105 to 0.1 Hz) and DC potential of −1.2V_RHE. The MOF showed high resistance, followed by the CuO-500, while the ZnCuO-500 showed a high charge transfer rate (smallest semicircle). The EIS concluded that the thermal treatment for the HKUST-1 and the introduction of ZnO increased the charge transfer rate for the ECO₂R.

The CO₂ conversion efficiency was investigated by calculating the faradic efficiency (FE %). A hydrogen cell (H-Cell) connected to a barrier ionization discharge detector (GC-BID) online system was used to evaluate the gaseous products by running chronoamperometric at different potentials (−0.7, −0.8, −0.9, −1.0, −1.1, −1.2 and −1.3 V). Only two products (CO and H₂) were observed by the electrodes CuO-500 and ZnO-500. The reduction reaction was observed to be a predominant hydrogen evolution reaction (HER). The CO FE % increased with potential increment until reaching the maximum FE of 25% at a potential of −1.2 V_RHE in the case of CuO-500 (FIG. 5A). In comparison, it showed a FE % of 37% in the case of ZnO-500 at the same potential (FIG. 5B). In case of 5% ZnCuO a third gaseous product (ethane) started emerging with FE % of 7% (FIG. 5C). Ethane is produced with a FE of 40% at −0.7 VRHE. The ethane FE began to decrease with the potential, while the CO is obtained with a maximum FE of 55% at −1.2 V_RHE (FIG. 5C). The long-term stability of the electrode ZnCuO-500 was investigated and plotted as a current-time curve. A steady current for 12 hours with no significant loss in the current is observed (FIG. 5D).

A ZnCuO electrocatalyst was prepared by dispersing the ZnO nanoparticles and pyrolyzed at 500° C. under air. The ECO₂R products were evaluated, and the catalyst exhibited excellent conversion efficiency with ethane FE of 40% at −0.7 V_RHE, and CO production with FE of 55% at −1.2 V_RHE. A comparative catalytic CO₂ reduction performance to ethane is summarized in Table 1.

TABLE 1
Comparative catalytic CO2 reduction performance to ethane.
Catalyst	Products	FE %	References
N-doped γ-Fe2O3	Ethane	42	P. Chen, P. Zhang, X. Kang, L. Zheng, G. Mo, R. Wu, J.
			Tai, B. Han, Efficient electrocatalytic reduction of CO2 to
			ethane over nitrogen-doped Fe2O3, J Am Chem Soc. 144
			(2022) 14769-14777.
OD—Cu	Ethane	27	C. S. Chen, J. H. Wan, B. S. Yeo, Electrochemical
			reduction of carbon dioxide to ethane using
			nanostructured Cu2O-derived copper catalyst and
			palladium (II) chloride, The Journal of Physical
			Chemistry C. 119 (2015) 26875-26882.
Cu—PdClx	Ethane	30	C. S. Chen, J. H. Wan, B. S. Yeo, Electrochemical
			reduction of carbon dioxide to ethane using
			nanostructured Cu2O-derived copper catalyst and
			palladium (II) chloride, The Journal of Physical
			Chemistry C. 119 (2015) 26875-26882.
Cu NW—NH2	Ethane	24	M. S. Xie, B. Y. Xia, Y. Li, Y. Yan, Y. Yang, Q. Sun, S. H.
			Chan, A. Fisher, X. Wang, Amino acid modified copper
			electrodes for the enhanced selective electroreduction of
			carbon dioxide towards hydrocarbons, Energy Environ
			Sci. 9 (2016) 1687-1695.
Porous OD—Cu	Ethane	37	A. Dutta, A. Kuzume, M. Rahaman, S. Vesztergom, P.
			Broekmann, A. C. S. Catal, 5, 7498-7502; g) A, Dutta, M.
			Rahaman, NC Luedi, M. Mohos, P. Broekmann, ACS
			Catal. 6 (2016) 3804-3814.
5% Zn—CuO	Ethane	40	Present disclosure

Numerous modifications and variations of the present disclosure are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims

1: A method of making an electrode, comprising:

dissolving a copper salt and benzene-1,3,5-tricarboxylate in a solvent and heating to a temperature of 60-100° C. to form a framework;

mixing a zinc salt and the framework to form a zinc doped framework;

heating the zinc doped framework to a temperature of 300° C. to 600° C. under air to form ZnCuO nanoparticles;

mixing the ZnCuO nanoparticles, a binding compound, and a conductive carbon compound in a solvent to form a suspension; and

coating a substrate with the suspension and drying to form the electrode,

wherein the ZnCuO nanoparticles have an oval shape with an average size of 50 nm to 200 nm.

2: The method of claim 1, wherein the ZnCuO nanoparticles comprise 5 mol % to 50 mol % Zn, relative to a total number of Zn and Cu moles in the ZnCuO nanoparticles.

3: The method of claim 1, wherein the ZnCuO nanoparticles comprise Cu, C, Zn, and O.

4: The method of claim 1, wherein the ZnCuO nanoparticles comprise 30 wt. % to 40 wt. % Cu, 30 wt. % to 40 wt. % C, 10 wt. % to 20 wt. % Zn, and 10 wt. % to 20 wt. % O.

5: The method of claim 1, wherein the ZnCuO nanoparticles comprise CuO and ZnO and wherein the CuO has a monoclinic crystal structure and the ZnO has a hexagonal crystal structure.

6: The method of claim 5, wherein the CuO and ZnO are uniformly dispersed in the ZnCuO nanoparticles.

7: The method of claim 1, wherein in the mixing the zinc salt, Zn is homogeneously dispersed in pores of the framework without distortion of the framework.

8: The method of claim 1, wherein the heating is to about 500° C.

9: The method of claim 1, wherein the copper salt is selected from the group consisting of copper (II) chloride, copper (II) sulfate, copper (II) nitrate, copper (II) acetate, copper (II) bromide, and hydrates thereof.

10: The method of claim 1, wherein the zinc salt is selected from the group consisting of zinc (II) chloride, zinc (II) sulfate, zinc (II) nitrate, zinc (II) acetate, zinc (II) bromide, and hydrates thereof.

11: The method of claim 1, wherein the conductive carbon compound is at least one selected from the group consisting of graphite, activated carbon, reduced graphene oxide, carbon nanotubes, carbon nanofibers, and carbon black.

12: The method of claim 1, wherein the binding compound is a fluorinated polymer.

13: The method of claim 1, wherein the substrate is made from at least one material selected from the group consisting of conductive carbon, stainless steel, aluminum, nickel, copper, platinum, zinc, tungsten, and titanium.

14: The method of claim 1, wherein the suspension comprises 70 wt. % to 90 wt. % of the ZnCuO nanoparticles and 10 wt. % to 30 wt. % of the conductive carbon compound, based on a total weight of the ZnCuO nanoparticles and the conductive carbon compound.

15: The method of claim 1, further comprising:

applying a potential of less than 0 V to −2.0 V vs RHE to an electrochemical cell,

wherein the electrochemical cell is at least partially submerged in an aqueous solution comprising carbon dioxide,

wherein on applying the potential the carbon dioxide is reduced to a conversion product,

wherein the electrochemical cell comprises:

the electrode; and

a counter electrode.

16: The method of claim 15, wherein the conversion product is selected from ethane and carbon monoxide.

17: The method of claim 15, wherein the aqueous solution further comprises a base selected from at least one of sodium bicarbonate and potassium bicarbonate.

18: The method of claim 16, having a faradic efficiency for reducing carbon dioxide to ethane of 35% to 45%.

19: The method of claim 16, having a faradic efficiency for reducing carbon dioxide to carbon monoxide of 50% to 60%.

20: The method of claim 15, wherein the aqueous solution is saturated with the carbon dioxide.