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 10⁻⁶Ω·m, preferably at most 10⁻⁷Ω·m, more preferably at most 10⁻⁸Ω·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⁺²in 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 CO₂conversion 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⁺²solution. 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 cm²of 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 N₂and CO₂-saturated 0.1 M KHCO₃aqueous solutions from 0 V to −1.6 V (vs. RHE) at a scan rate of 20 mV s⁻¹. CO₂and N₂saturated solutions were prepared by bubbling high-purity CO₂and N₂in the 0.1 M KHCO₃electrolyte 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

)

+

0.197

V

+

(

0.59

V
×
pH

)

Bulk CO₂electrolysis 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.