METHODS FOR ELECTROCHEMICAL REDUCTION OF CARBON DIOXIDE

TECHNICAL FIELD

The present disclosure generally relates to the use of a copper composite in the electrochemical reduction of carbon dioxide to yield hydrocarbon products.

BACKGROUND

The fast-growing amount of CO₂in the global atmosphere poses a huge threat to the environment, which has motivated researchers in different fields to develop effective technologies to tackle CO₂emission problems. In this regard, electrochemical CO₂reduction reaction (CO₂RR) is considered one of the most promising techniques as it can upgrade CO₂to value-added fuels and chemical feedstocks by utilizing renewable electricity. Currently, Cu-based catalysts show very unique selectivity in the electrochemical CO₂RR, because they can facilitate the generation of multi-carbon (C₂₊) products (e.g., ethylene (C₂H₄), ethanol (C₂H₅OH) and n-propanol (n-C₃H₇₀H)) with considerable Faradaic efficiency (FE), which are much more valuable than the single-carbon (C₁) products (e.g., formic acid (HCOOH), and carbon monoxide (CO)). To enhance the CO₂RR performance, research has been devoted to developing novel Cu-based catalysts involving different strategies, including the control of morphology, composition, size, facet, crystal phase, oxidation state and defect, as well as the surface functionalization. In addition, reaction environment modulation (e.g., electrolytes, local pH, and mass transfer) and cell design (e.g., H-type cell, gaseous cell, and flow cell) were also extensively explored to improve the overall CO₂-reduction performance. However, although significant progresses have been achieved, Cu-based catalysts still suffer severely from their insufficient activity and limited selectivity toward electrochemical CO₂RR, especially for the formation of high-value C₂±products.

Recently, cation engineering has emerged as an efficient approach to boost the performance of electrochemical CO₂RR. In general, cations can suppress competitive hydrogen evolution reaction (HER), tune the selectivity, and adjust the reaction rate of CO₂RR. To date, several mechanisms have been proposed to rationalize the aforementioned cation effects, including tuning the structure of the electrochemical double layer (EDL), buffering the local pH, modulating the interfacial electric field, and interacting with the reaction intermediates. Since cations can interact with the intermediates and affect their electronic structures, it is theorized that the cations could also interact with the surface atoms of catalysts under negative bias and change their electronic structures if the cations are attracted close enough to the surface of the electrode. In particular, previous studies have revealed that specifically adsorbed cations on the catalysts can affect the electronic structure of their surface atoms, thereby modulating catalytic properties. Significantly, it was recently observed that the coverage of specifically adsorbed alkali metal cations on Au electrode during CO₂-to-CO reduction follows the order of Li⁺<Na⁺<K⁺<Cs⁺ under the same bulk concentration, and the rate of CO₂-to-CO conversion increases with the coverage increase of specifically adsorbed alkali metal cations. Nevertheless, current investigations are mostly restricted to the interactions between cations and the reaction environment or intermediates in CO₂RR, and the influence of cation-catalyst interaction on the CO₂RR performance has been almost neglected.

There thus exists a need to develop improved copper composite catalysts for use in electrochemical CO₂reduction.

SUMMARY

Disclosed herein is a copper composite comprising a Cu nanosheet (CuNS) array disposed on the surface of Cu substrates prepared by a straightforward two-step wet-chemical method. Significantly, the obtained CuNS arrays can promote the catalytic conversion of CO₂to high-value C₂₊ products in common electrolytes, such as 0.1 M KHCO₃, while the bare Cu foils mainly favors the generation of C₁products under the same conditions. Detailed studies show that the adsorption of K⁺ ions on the surface of CuNS is greatly enhanced compared to corresponding pristine Cu foils. Density functional theory (DFT) calculations demonstrate that the CuNS shows intrinsic uneven electron distributions, which facilitates the adsorption of K⁺ ions on the surface to optimize the surface electroactivity. The modified electronic structures of CuNS lead to improved C—C coupling for C₂₊ products, while the bare Cu foils show strong preferences for the C₁products.

Provided herein is a method of reducing carbon dioxide to form one or more hydrocarbon products, the method comprising: providing an electrochemical cell comprising: a working electrode comprising a copper composite comprising a copper nanosheet array comprising a plurality of copper nanosheets, wherein the plurality of copper nanosheets comprise copper(100) facets; a counter electrode; optionally a reference electrode; and an electrolyte solution comprising an electrolyte and CO₂, wherein the electrolyte solution is between and in contact with the working electrode, the counter electrode, and optionally the reference electrode; and applying an electric current between the working electrode and the counter electrode resulting in electrolytic reduction of the CO₂thereby forming the one or more hydrocarbon products.

In certain embodiments, the copper nanosheet array is disposed on a surface of a modified copper substrate and the copper nanosheet array is chemically bonded to the modified copper substrate.

In certain embodiments, the plurality of copper nanosheets have an average thickness of 20-30 nm.

In certain embodiments, the copper composite further comprises potassium ions disposed on at least one surface of the copper nanosheet array.

In certain embodiments, the electrolyte comprises a metal carbonate, a metal bicarbonate, a metal hydroxide, a metal oxide, or a mixture thereof.

In certain embodiments, the method further comprises electrochemically reducing an oxidized copper composite comprising a modified copper substrate and an oxidized copper nanosheet array, wherein the oxidized copper nanosheet array is disposed on a surface of the modified copper substrate and the oxidized copper nanosheet array is chemically bonded to the modified copper substrate, thereby forming the copper composite.

In certain embodiments, the step of reducing the oxidized copper composite is conducted in situ in the electrochemical cell.

In certain embodiments, the method further comprises contacting a copper substrate with an oxidizing agent thereby forming the oxidized copper composite.

In certain embodiments, the copper substrate is a copper foil, a copper foam, or a copper mesh.

In certain embodiments, the oxidizing agent is potassium persulfate.

In certain embodiments, the one or more hydrocarbon products comprise C₁-C₃hydrocarbons.

In certain embodiments, the one or more hydrocarbon products comprise formic acid, methanol, methane, ethylene, ethanol, or n-propanol.

In certain embodiments, the one or more hydrocarbon products comprise C_2-3hydrocarbons and C₁hydrocarbons in a molar ratio of 2:1 to 7.2:1, respectively.

In certain embodiments, the one or more hydrocarbon products comprise ethylene as a major product.

In certain embodiments, the plurality of copper nanosheets have an average thickness between 20-30 nm; the electrolyte comprises K₂CO₃, KHCO₃, KOH, or a mixture thereof; and the one or more hydrocarbon products comprise C_2-3hydrocarbons and C₁hydrocarbons in a molar ratio of 2:1 to 7.2:1, respectively.

In certain embodiments, the electrolyte comprises KHCO₃; and the one or more hydrocarbon products comprise C_2-3hydrocarbons and C₁hydrocarbons in a molar ratio of 6:1 to 7.2:1, respectively.

In a second aspect, provided herein is a method of reducing carbon dioxide to form one or more C_1-3hydrocarbon products, the method comprising: providing an electrochemical cell comprising: a working electrode comprising a copper composite, wherein the copper composite comprises a modified copper substrate and a copper nanosheet array comprising a plurality of copper nanosheets comprising copper(100) facets, wherein the plurality of copper nanosheets have an average thickness of 20-30 nm, wherein the copper nanosheet array is disposed on a surface of the modified copper substrate and the copper nanosheet array is chemically bonded to the modified copper substrate; a counter electrode; optionally a reference electrode; and an electrolyte solution comprising a KHCO₃and CO₂, wherein the electrolyte solution is between and in contact with the working electrode, the counter electrode, and optionally the reference electrode; and applying an electric current between the working electrode and the counter electrode resulting in electrolytic reduction of the CO₂thereby forming the one or more C_1-3hydrocarbon products, wherein the one or more hydrocarbon products comprise C_2-3hydrocarbons selected from the group consisting of ethylene, ethanol, and n-propanol and C₁hydrocarbons selected from the group consisting of formic acid, methanol, and methane in a molar ratio of C_2-3hydrocarbons to C₁hydrocarbons of 6:1 to 7.2:1, respectively.

In certain embodiments, the method further comprises contacting a copper substrate with potassium persulfate thereby forming an oxidized copper composite comprising a modified copper substrate and an oxidized copper nanosheet array, wherein the oxidized copper nanosheet array is disposed on a surface of the modified copper substrate and the oxidized copper nanosheet array is chemically bonded to the modified copper substrate; and electrochemically reducing the oxidized copper composite thereby forming the copper composite.

In certain embodiments, the step of reducing the oxidized copper composite is conducted in situ in the electrochemical cell.

In certain embodiments, the one or more hydrocarbon products comprise ethylene as a major product.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present disclosure will become apparent from the following description of the disclosure, when taken in conjunction with the accompanying drawings.

FIG. 1 depicts the synthesis and structure characterization of CuONS and CuNS-0.8 (A) The schematic illustration for the formation of CuNS from Cu foil. (B-G) SEM images of CuONS (B-D) and CuNS-0.8 (E-G) at different magnification levels (scale bare: 2 m in B and E; 500 nm in C and F; 100 nm in D and G). (H) TEM image of CuONS (scale bar: 100 nm). (I) HRTEM image of CuONS, with the corresponding fast Fourier transform (FFT) pattern as the inset (scale bar: 1 nm). (J) TEM image of CuNS-0.8 (scale bar: 100 nm). (K) HRTEM image of CuNS-0.8, with the corresponding FFT pattern as the inset (scale bar: 1 nm).

FIG. 2 depicts the electrochemical properties and performance of Cu foil and CuNS-0.8 (A) CV curves of CuNS after reducing the CuONS at −0.8 V (vs RHE). (B) The OH⁻ adsorption profiles of CuNS-0.8 (left panel) and Cu foil (right panel). (C) The charging current density differences plotted against the scan rates over CuNS-0.8 and Cu foil. (D) LSV curves of CuNS-0.8 and Cu foil in CO₂-saturated 0.1 M KHCO₃with a scan rate of 10 mV/s. (E,F) The product distributions and current densities of (E) Cu foil and (F) CuNS-0.8 at different potentials. The error bars represent the standard deviation around the mean of 3 measurements. (G) The partial current densities of C₂H₄and C₂₊ products on CuNS-0.8 and Cu foil at different potentials. (H) The Faradaic efficiency ratio of C₂H₄to CH₄on CuNS-0.8 and Cu foil at different potentials. (I) The ratio of C₂₊ products to C1 products on CuNS-0.8 and Cu foil at different potentials.

FIG. 3 depicts the cation-catalyst interaction effect on the CO2RR performance of CuNS-0.8 (A) The density of K⁺ ions adsorbed on CuNS-0.8 and Cu foil. (B-D) The CO2RR performance of CuNS-0.8 in CO₂-saturated (B) 0.1 M LiHCO₃, (C) 0.1 M NaHCO₃, and (D) 0.1 M CsHCO₃. The error bars represent the standard deviation around the mean of 3 measurements.

FIG. 4 depicts theoretical calculations of the CuNS and Cu foil for CO2RR (A) The atomic strain distributions of CuNS. (B) The electronic distributions of bonding and anti-bonding orbitals near the Fermi level (EF) of Cu NS with K+ ions on the surface. Brown ball, Cu; purple ball, K+; blue isosurface, bonding orbitals; green isosurface, anti-bonding orbitals. (C) The PDOSs of CuNS with adsorption of K+ ions. (D) The dependence of the d-band center on the K+ ion coverage. (E,F) The site-dependent PDOS of Cu-3d in (E) CuNS and (F) Cu foil. (G,H) The PDOS of key adsorbates of CO2RR for C1 and C2 products in (G) CuNS and (H) Cu foil. (I) The adsorption energies of different alkali cations on CuNS and Cu foil. (J,K) The reaction energies of (J) C₂₊ and (K) C1 pathways of CO2RR on CuNS and Cu foil, respectively.

FIG. 5 depicts XRD patterns of Cu foil (top), CuONS (middle curve), and CuNS-0.8 (down curve).

FIG. 6 depicts XPS spectra of Cu foil (A), CuONS (B), and CuNS-0.8 (C).

FIG. 7 depicts Raman spectrum of CuNS-0.2, CuNS-0.5, CuNS-0.8, CuNS-1.1 and Cu foil.

FIG. 8 depicts CV curves of CuNS-0.8 (A) and Cu foil (B) at different scanning rates in 1.0 M KOH.

FIG. 9 depicts the stability test of CuNS-0.8 at −1.1 V (vs RHE) in CO₂-saturated 0.1 M KHCO₃.

FIG. 10 depicts the morphology evolution of CuNS-0.8 at −1.1 V (vs RHE) at different times: (A) original CuONS; (B) 1 h; (C) 1.5 h; (D) 5 h.

FIG. 11 depicts the CO2RR performance of CuNS obtained at the reduction potential of −0.2 V (vs RHE).

FIG. 12 depicts the CO2RR performance of CuNS obtained at the reduction potential of −0.5 V (vs RHE).

FIG. 13 depicts the CO2RR performance of CuNS obtained at the reduction potential of −1.1 V (vs RHE).

FIG. 14 depicts CV curves of CuNS-0.2 (A), CuNS-0.5 (B), CuNS-1.1 (C) at different scanning rates in 1.0 M KOH and their ECSA profiles (D).

FIG. 15 depicts CV curves of CuNS-0.8 electrodes that were used for LiHCO₃(A), NaHCO₃(B), and CsHCO₃(C) at different scanning rates in 1.0 M KOH, and their corresponding ECSA profiles (D).

FIG. 16 depicts the calibration curves of formate, methanol, ethanol, acetate and n-propanol.

FIG. 17 depicts Table 1 that shows the Comparison of the CO2RR performance of CuNS-0.8 in this work with the previously reported Cu-based catalysts in the H-type cell.

DETAILED DESCRIPTION
Definitions

Throughout the application, where compositions are described as having, including, or comprising specific components, or where processes are described as having, including, or comprising specific process steps, it is contemplated that compositions of the present teachings can also consist essentially of, or consist of, the recited components, and that the processes of the present teachings can also consist essentially of, or consist of, the recited process steps.

In the application, where an element or component is said to be included in and/or selected from a list of recited elements or components, it should be understood that the element or component can be any one of the recited elements or components, or the element or component can be selected from a group consisting of two or more of the recited elements or components. Further, it should be understood that elements and/or features of a composition or a method described herein can be combined in a variety of ways without departing from the spirit and scope of the present teachings, whether explicit or implicit herein.

It should be understood that the order of steps or order for performing certain actions is immaterial so long as the present teachings remain operable. Moreover, two or more steps or actions may be conducted simultaneously.

The use of the singular herein includes the plural (and vice versa) unless specifically stated otherwise. In addition, where the use of the term “about” is before a quantitative value, the present teachings also include the specific quantitative value itself, unless specifically stated otherwise. As used herein, the term “about” refers to a ±10%, ±7%, ±5%, ±3%, ±1%, or ±0% variation from the nominal value unless otherwise indicated or inferred.

The methods described herein can be carried out using one or more electrolytic cells comprising two or more electrodes, wherein the two or more electrodes can comprise the copper composite (i.e., the cathodic working electrode), a counter electrode (or counter/reference electrode) and optionally a reference electrode (e.g., in a three-electrode system).

The working electrode comprises the copper composite comprising a copper nanosheet array comprising a plurality of copper nanosheets, wherein the plurality of copper nanosheets comprise copper(100) facets. As illustrated in FIGS. 1E and 1F, the plurality of copper nanosheets can be substantially perpendicular (e.g., between 30-60, 35-55, or 40-50 degrees relative to the surface of the copper substrate) to the surface of a modified copper substrate. The plurality of copper nanosheets can have an average thickness of 20-30 nm, 20-29 nm, 20-28 nm, 20-27 nm, 21-27 nm, 22-26 nm, or 23-25 nm. In certain embodiments, the thickness of the plurality of copper nanosheets is about 24 nm.

At least 10% of the crystal facets in the plurality of copper nanosheets is copper (100) facet. In certain embodiments, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 9%, or greater of the crystal facets in the plurality of copper nanosheets is copper(100) facet. In certain embodiments, 10-95%, 10-90%, 10-80%, 10-70%, 20-70%, 30-70%, 40-60%, 40-50%, 40-70%, 40-80%, 40-90%, 50-70%, or 60-70% of the crystal facets in the plurality of copper nanosheets is copper(100) facet.

In certain embodiments, the copper composite further comprises cations disposed on at least one surface of the copper nanosheet array. The cations can be selected from lithium, sodium, potassium, or a mixture thereof.

In certain embodiments, the copper nanosheet array is disposed on a surface of a modified copper substrate, wherein the copper nanosheet array is chemically bonded to the modified copper substrate.

The copper composite comprising the copper nanosheet array disposed on the surface of the modified copper substrate can be prepared by first oxidizing a copper substrate (e.g., a copper foil) with an oxidizing agent that oxidizes the surface of the copper substrate thereby forming thereby forming an oxidized copper composite comprising the copper substrate and an oxidized copper nanosheet array, wherein the oxidized copper nanosheet array is disposed on a surface of the copper substrate; and then electrochemically reducing the oxidized copper composite thereby forming the copper composite.

The copper substrate can be any copper-containing metallic substance. In certain embodiments, the copper substrate comprises a copper foil, a copper foam, a copper mesh, or a copper layer (e.g., a copper thin film or copper particles) deposited on a non-copper substrate (e.g., carbon paper). The copper foil can comprise at least 90% wt/wt, at least 95% wt/wt, or greater (e.g., at least 96% wt/wt, at least 97% wt/wt, at least 98% wt/wt, at least 99% wt/wt, at least 99.5% wt/wt, at least 99.9% wt/wt or greater) copper. The thickness of the copper foil is not particularly limited. In certain embodiments, the copper foil can have a thickness between 0.05-5 mm, 0.05-4 mm, 0.05-3 mm, 0.05-2 mm, 0.05-1 mm, 0.05-0.75 mm, 0.05-0.50 mm, 0.05-0.25 mm, 0.05-0.2 mm, 0.05-0.15 mm, or 0.1-0.15 mm. In instances in which the copper substrate is a copper foam, the thickness of the copper foam can range from 0.1 to 1 mm. In instances in which the copper substrate is a copper layer deposited on a non-copper substrate, the thickness of the copper layer can range from 0.5 to 3 μm. In certain embodiments, the copper foil has a thickness of about 0.125 mm.

The oxidizing agent is not particularly limited. Any reagent that is capable of oxidizing the surface of the copper substrate can be used. Exemplary oxidizing agents include, but are not limited to, ozone, nitric acid (HNO₃), hydrogen peroxide (H₂O₂), oxone (potassium peroxymonosulfate, 2KHSO₅·KHSO₄·K₂SO₄), ammonium polyatomic salts (e.g., ammonium peroxomonosulfate, ammonium chlorite (NH₄CO₂), ammonium chlorate (NH₄ClO₃), ammonium iodate (NH₄IO₃), ammonium perborate (NH₄BO₃), ammonium perchlorate (NH₄CO₄), ammonium periodate (NH₄IO₃), ammonium persulfate ((NH₄)₂S₂O₈), ammonium hypochlorite (NH₄ClO)), sodium polyatomic salts (e.g., sodium persulfate (Na₂S₂O₈), sodium hypochlorite (NaClO)), potassium polyatomic salts (e.g., potassium iodate (KIO₃), potassium permanganate (KMnO₄), potassium persulfate, potassium persulfate (K₂S₂O₈), potassium hypochlorite (KClO)), tetramethylammonium polyatomic salts (e.g., tetramethylammonium chlorite ((N(CH₃)₄)ClO₂), tetramethylammonium chlorate ((N(CH₃)₄)ClO₃), tetramethylammonium iodate ((N(CH₃)₄)IO₃), tetramethylammoniumperborate ((N(CH₃)₄)BO₃), tetramethylammonium perchlorate ((N(CH₃)₄)ClO₄), tetramethylammonium periodate ((N(CH₃)₄)IO₄), tetramethylammonium persulfate ((N(CH₃)₄)S₂O₈)), tetrabutylammonium polyatomic salts (e.g., tetrabutylammonium peroxomonosulfate), peroxomonosulfuric acid, urea hydrogen peroxide, peracetic acid (CH₃(CO)OOH), sodium nitrate, potassium nitrate, ammonium nitrate, and combinations thereof. In certain embodiments, the oxidizing agent is potassium persulfate.

The electrochemical reduction of the oxidized copper composite can be performed prior to the step of applying the electric current between the working electrode and the counter electrode. Alternatively, the electrochemical reduction of the oxidized copper composite can be performed in situ, i.e., a working electrode comprising the oxidized copper composite can be used in place of the working electrode comprising a copper composite and during the step of applying the electric current between the working electrode and the counter electrode the oxidized copper composite is reduced in situ thereby forming the copper composite. Thus, in certain embodiments, the working electrode comprises the copper composite or the oxidized copper composite.

The electrochemical reduction of the oxidized copper composite can be conducted at a voltage of −0.1 to −2V (vs RHE). In certain embodiments, the electrochemical reduction of the oxidized copper composite is conducted at −0.2 to −2V, −0.3 to −2V, −0.4 to −2V, −0.5 to −2V, −0.6 to −2V, −0.7 to −2V, −0.7 to −1.9V, −0.7 to −1.8V, −0.7 to −1.7V, −0.7 to −1.6V, −0.7 to −1.5V, −0.7 to −1.4V, −0.7 to −1.3V, −0.7 to −1.2V, −0.7 to −1.2V, −0.8 to −1.2V, −0.9 to −1.2V, −1.0 to −1.2V, or −1.1 to −1.2V (vs RHE). In certain embodiments, the electrochemical reduction of the oxidized copper composite is conducted at about −1.1V (vs RHE). Advantageously, the yield of C₂₊ hydrocarbon products can be optimized when the electrochemical reduction of the oxidized copper composite is conducted between −1.0 to −1.2V (vs RHE).

A counter electrode refers to an electrode paired with the working electrode, through which passes a current equal in magnitude and opposite in sign to the current passing through the working electrode. The counter electrode can include counter electrodes which also function as reference electrodes (i.e., a counter/reference electrode). Any suitable counter electrode known in the art can be used in connection with the methods described herein. For example, the counter electrode can comprise carbon (e.g., highly oriented pyrolytic graphite), a metal [e.g., Al, Au, Ag, Bi, C, Cd, Co, Cr, Cu, Cu alloys (e.g., brass and bronze), Ga, Hg, In, Mo, Nb, Ni, NiCo₂O₄, Ni alloys, Ni—Fe alloys, Pb, Pd alloys, Pt, Pt alloys, Rh, Sn, Sn alloys, Ti, V, W, Zn, or stainless steel], glassy carbon, a conductive polymer, or the like.

The reference electrode can be selected from a standard hydrogen electrode, calomel electrode, copper-copper (II) sulfate electrode, silver chloride electrode, palladium-hydrogen electrode, mercury-mercurous sulfate electrode, and the like.

The electrolyte solution can comprise a saturated solution of carbon dioxide or any concentration below saturation.

The carbon dioxide can be added to the electrolyte prior to the step of applying an electric current between the working electrode and the counter electrode and/or introduced into the electrolyte continuously during the electrochemical reduction, e.g., by bubbling CO₂into the electrolyte solution.

The electrolyte solution can comprise an aqueous solvent, a nonaqueous solvent, such as methanol and acetonitrile, or mixtures thereof and an electrolyte. Suitable electrolytes, include salts comprising one or more cations selected from the group consisting of lithium, sodium, potassium, cesium, calcium, magnesium, and tetraalkylammonium; and one or more anions selected from the group consisting of halide, carbonate, bicarbonate, perchlorate, silicate, borate, phosphate, sulfate, polyphosphate, and nitrate. Exemplary electrolytes include, but are not limited to M₂SO₄, M₂CO₃, MHCO₃, MF, MCl, MClO₄, MClO₄, M′SO₄, M′CO₃, M′(HCO₃)₂, M′F₂, M′Cl₂, M′(ClO₄)₂, M′(ClO₄)₂, wherein M for each instance is independently lithium, sodium, potassium, cesium, or tetra(C₁-C₄)alkylammonium and M′ is calcium or magnesium. In certain embodiments, electrolyte salt is K₂CO₃, KHCO₃, KOH, Na₂CO₃, NaHCO₃, NaOH, or mixtures thereof.

The concentration of the electrolyte salt in the electrolyte solution can range from 0.01 to 1M, 0.01 to 0.75M, 0.01 to 0.5M, 0.01 to 0.4M, 0.01 to 0.3M, 0.05 to 0.3M, 0.05 to 0.2M, or 0.05 to 0.15M. In certain embodiments, the electrolyte salt is present in the electrolyte solution at a concentration of about 0.1M.

The electric current between the working electrode and the counter electrode can be applied at a voltage of −0.1 to −2V (vs RHE). In certain embodiments, the electric current between the working electrode and the counter electrode is applied at −0.2 to −2V, −0.3 to −2V, −0.4 to −2V, −0.5 to −2V, −0.6 to −2V, −0.7 to −2V, −0.7 to −1.9V, −0.7 to −1.8V, −0.7 to −1.7V, −0.7 to −1.6V, −0.7 to −1.5V, −0.7 to −1.4V, −0.7 to −1.3V, −0.7 to −1.2V, −0.7 to −1.2V, −0.8 to −1.2V, −0.9 to −1.2V, −1.0 to −1.2V, or −1.1 to −1.2V (vs RHE). In certain embodiments, the electric current between the working electrode and the counter electrode is applied at about −1.1V (vs RHE). Advantageously, the yield of C₂₊ hydrocarbon products can be optimized when the electric current between the working electrode and the counter electrode is applied at −1.0 to −1.2V (vs RHE).

The one or more hydrocarbon products can comprise C_2-3hydrocarbons selected from the group consisting of ethylene, ethanol, and n-propanol and C₁hydrocarbons selected from the group consisting of formic acid, methanol, and methane. The one or more hydrocarbon products can comprise C_2-3hydrocarbons and C₁hydrocarbons in a molar ratio of 1:1 to 7.2:1; 3:2 to 7.2:1; 7:3 to 7.2:1; 4:1 to 7.2:1; or 17:3 to 7.2:1, respectively.

Advantageously, the FE of C₂₊ hydrocarbon products reaches a maximum value within a range of −1.0 V to −1.2V (vs RHE) with a peak of 64.0% at −1.1 V (vs RHE) and a maximum FE_C2H4value of 53.6%.

The electrical energy used for the electrochemical reduction of carbon dioxide may come from any energy source, including nuclear energy, alternative energy (e.g., hydroelectric, wind, solar power, geothermal, etc.), solar energy, coal, gas, or other sources of electricity.

The carbon dioxide may be obtained from any source, such as an exhaust stream from fossil-fuel burning power or industrial plants, from geothermal or natural gas wells, natural gas streams, flue gases of fossil fuel, cement factory exhaust, or the atmosphere itself.

Synthesis and Characterization of CuNS Arrays

The CuNS arrays were prepared through a two-step method involving the surface oxidation treatment of Cu foils and the subsequent in situ electroreduction of CuO nanosheet (CuONS), as shown in FIG. 1A (see more details in Supplemental Information). Briefly, the pristine Cu foils were first immersed into an aqueous solution containing K₂S₂O₈and NaOH at 60° C. After 30 mins, the color of Cu foils turned black, which indicates the formation of CuONS on the surface of Cu foils. Then the CuONS was reduced to CuNS at a given potential in the specific electrolyte for 10 mins before the CO₂RR test, along with a color change from black to red.

The crystal structure of the Cu foil, CuONS, and CuNS was characterized by powder X-ray diffraction (XRD). As shown in FIG. 5, the pristine Cu foil mainly shows three peaks at 43.3°, 50.4°, and 74.10, which correspond to the (111), (200), and (220) facets of Cu (PDF #04-0836), respectively. The peak at 50.4° is the dominant peak and has the strongest intensity, while the other peaks are almost neglectable, suggesting that the Cu foil is mostly (001) oriented (FIG. 5B). The CuONS demonstrates two sets of XRD peaks. Peaks located at 35.5° and 38.7° correspond to the (002) and (111) facets of CuO (PDF #45-0937) (FIG. 5A). The peak at 50.4° is attributed to the (200) facet of the Cu substrate, implying that the CuONS is formed on the surface of Cu foil and the inner part of Cu foil is not completely oxidized. After reduction, the obtained CuNS at the reduction potential of −0.8 V (vs reversible hydrogen electrode (RHE), denoted as CuNS-0.8) shows a similar XRD pattern to the pristine Cu foil, indicating that the CuO is completely reduced to the metallic Cu. Besides, the chemical states of all the aforementioned materials were further verified by the X-ray photoelectron spectroscopy (XPS, FIG. 6). For Cu foil, there are two peaks at 932.5 eV and 953.5 eV, and no satellite peaks can be found in the XPS spectrum, which matches well with the standard Cu spectrum (FIG. 6A). For CuONS, the strong satellite peaks between 940 eV and 945 eV are the typical characteristics corresponding to CuO (FIG. 6B). The obtained CuNS shows a similar XPS spectrum to Cu foil along with very weak satellite peaks (FIG. 6C), which is commonly believed to result from the oxidation of Cu nanostructures by air during the sample preparation. In addition, Raman spectroscopy was further used to evaluate the oxidation state of CuNS prepared at the reduction potentials of −0.2, −0.5, −0.8, and −1.1 V (vs RHE), as shown in FIG. 7. Both CuNS-0.2 and CuNS-0.5 show a CuO peak at 628 cm⁻¹and two Cu₂O peaks at 218 cm⁻¹and 146 cm¹, while no obvious CuxO peaks can be observed on CuNS-0.8 and CuNS-1.1, indicating the complete reduction of CuONS to metallic CuNS.

Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) were combined to identify the morphology and structure of both CuONS and CuNS-0.8. As shown in FIG. 1B,C, vertically aligned CuONS arrays are formed on the surface of the Cu foil after the oxidation step. The thickness of individual CuO nanosheets is around 28 nm (FIG. 1D). The typical SEM images of the CuNS-0.8 array are shown in FIG. 1E,F. The thickness of individual Cu nanosheets is about 24 nm, and the surface of Cu nanosheets becomes uneven (FIG. 1G). The nanosheet structure of CuONS was also confirmed by TEM, as shown in FIG. 1H. The HRTEM image in FIG. 11 shows clear lattice fringes with an interplanar spacing of 2.51 Å, corresponding to the (002) facet of CuO. The nanosheet structure of CuNS can also be seen in FIG. 1J, which displays an uneven surface. The HRTEM image of CuNS-0.8 shows lattice fringes with an interplanar spacing of 1.81 Å, which is attributed to the (200) facet of Cu (FIG. 1K), consistent well with the XRD result (FIG. 5).

Electrochemical Tests of CuNS Arrays

The electrochemical tests were conducted in a gas-tight H-type cell under ambient conditions to study the electrochemical properties and CO₂RR performance of CuNS arrays. The cyclic voltammetry (CV) scan was adopted to identify the chemical states of CuNS-0.8 in 0.1 M KHCO₃before CO₂RR (FIG. 2A). There are two typical reduction peaks at 0.48 and 0.18 V (vs RHE) in the CV curve of CuONS arrays, which corresponds to the reduction of cu²⁺ to Cu⁺ and Cu⁺ to cu, respectively. After the reduction of cuons arrays at −0.8 V (vs RHE) in 0.1 M KHCO₃for 10 mins, no reduction peaks can be found in the CV curve, indicating the pure metallic state of CuNS-0.8. This is well consistent with the XRD result (FIG. 5). To further investigate the surface structure of the Cu foil and CuNS-0.8, the OH⁻adsorption experiment and electrochemically active surface area (ECSA) measurement were conducted in 1.0 M KOH (FIG. 2B,C and FIG. 8). As can be seen in FIG. 2B, there are no obvious anodic or cathodic peaks in the CV curve of Cu foil, suggesting that the Cu foil shows very weak specific adsorption of OH—. While for the CuNS-0.8, the sharp and strong anodic and cathodic peaks at 0.35 and 0.33 V (vs RHE), respectively, are attributed to the adsorption and desorption of OH-on the Cu(100) facet, which reveals that the CuNS is mainly enclosed by the {100} facet, agreeing well with the HRTEM result (FIG. 1K). In addition, the double layer capacitances of CuNS-0.8 and Cu foil were measured to be 1.67 and 0.56 mF/cm², which indicates the ECSA of CuNS-0.8 is almost 3 times that of the Cu foil, as shown in FIG. 2C.

The CO₂RR performance of Cu foils and CuNS-0.8 was firstly evaluated in 0.1 M KHCO₃. The linear sweep voltammetry (LSV) curves show that the CuNS-0.8 has a higher current density than Cu foils, which might be resulting from the higher ECSA of CuNS-0.8 (FIG. 2D). FIG. 2E shows the product distribution of the Cu foil. Specifically, at the potentials from −0.6 to −0.8 V (vs RHE), the major gaseous products are H₂and CO, and the liquid product is HCOOH. The C₂H₄can be detected at −0.9 V (vs RHE) with a FE of 3.3%. The maximum FE_C2H4and FE_C2+ are 19.9% and 28.8% on the Cu foil, respectively. As the applied potential shifts to more negative values, CH₄gradually becomes the dominant CO₂-reduction product, and the FE_CH4reaches 54.8% at −1.2 V (vs RHE). In sharp contrast, CuNS-0.8 shows a very different product distribution from the Cu foil (FIG. 2F). In particular, at −0.6 V (vs RHE), the detected products are H₂, HCOOH and CO, which are similar to the Cu foil. Notably, C₂H₄can be detected at −0.7 V (vs RHE), and becomes the major product as the applied potential changes to more negative values. The FE_C2H4shows a volcano trend, which first increases to the maximum value of 53.6% at −1.1 V (vs RHE) and then decreases. The typical liquid products at this potential are HCOO—, C₂H₅OH, n-C₃H₇₀H, and a very trace amount of acetate. And the FE of C₂₊ products also reaches a maximum value of 64.0% at −1.1 V (vs RHE). Note that the FE of CH₄on CuNS-0.8 is below 10%, implying that the production of CH₄is greatly suppressed in comparison with the Cu foil. The CuNS-0.8 also demonstrates a much larger current density than the Cu foil in the tested potential window (FIG. 2E,F). The partial current densities of both C₂H₄and C₂₊ products on CuNS-0.8 are much higher than those of Cu foils (FIG. 2G). At −1.1 V (vs RHE), the partial current densities of C₂H₄and C₂₊ products on CuNS-0.8 are −23.6 and −28.2 mA/cm², respectively, which are 7.4 and 6.1 times those of Cu foils. At −1.1 V (vs RHE), the C₂H₄/CH₄ratio on CuNS-0.8 is 11.5, while it is only 0.6 on Cu foils (FIG. 2H). Significantly, the ratio of C₂₊ to C₁products (C₂₊/C₁) on CuNS-0.8 is as high as 7.2 at −1.1 V (vs RHE), which is 18 times that on Cu foils (FIG. 2I).

The stability of CuNS-0.8 was also evaluated at −1.1 V (vs RHE). As shown in FIG. 9, the selectivity of CuNS-0.8 toward C₂H₄production can be well maintained above 50% in the first 1 h (FIG. 10A). After about 1.5 h, the FE_C2H4decreases to around 35% and keeps this level in the following 3.5 h. In the early stage, the nanosheet array structure can be well preserved (FIG. 10A,B), so it shows a relatively high FE_C2H4. However, when the electrolysis continued for a long time, the nanosheet array structure of CuNS-0.8 collapsed (FIG. 10C), resulting in the change of their surface structure and the corresponding sharp decrease of FE_C2H4. These results further indicate that the nanosheet structure plays a vital role in promoting the selective formation of C₂±products. Besides, it should be mentioned that the CO₂RR performance of CuNS has been optimized by systematically tuning the reduction potentials of CuONS from −0.2 to −1.1 V (vs RHE), as shown in FIG. 2F and FIGS. 13-16.

The sharp difference in CO₂RR selectivity of CuNS-0.8 and Cu foils is attributed to the uneven nanosheet structure of CuNS-0.8 as well as the cation-catalyst interaction effect. On the one hand, uneven two-dimensional nanosheets can lead to electron perturbation and change in the electronic structure of surface atoms. On the other hand, cations are essential to initiate CO₂RR as mainly HER occurs in the absence of cations. The modified electron distribution of uneven nanosheets could affect the specific adsorption of cations and thus modulate the CO₂RR performance. To validate this hypothesis, the density of K⁺ ions adsorbed on both CuNS-0.8 and Cu foil was measured at −1.1 V (vs RHE). As shown in FIG. 3A, the densities of adsorbed K⁺ ions on CuNS-0.8 are as high as 11.69 and 3.92 μmol/cm²based on the geometric area and ECSA, respectively, while the values are only 0.80 μmol/cm²on Cu foils. These observations suggest that the cation-catalyst interaction on CuNS-0.8 has been significantly enhanced compared to the Cu foil.

To further investigate the cation-catalyst interaction effects, we have also evaluated the CO₂RR performance of CuNS-0.8 in 0.1 M LiHCO₃, 0.1 M NaHCO₃, and 0.1 M CsHCO₃(FIG. 3B-D). Three independent CuNS-0.8 electrodes were adopted, and their ECSA profiles are shown in FIG. 15. In 0.1 M LiHCO₃, H₂is the dominant product at all tested potentials (FIG. 3B). At low applied potentials, CO and HCOOH are the dominant CO₂-reduction products. At high applied potentials (i.e., from −1.0 to −1.2 V (vs RHE)), CH₄gradually becomes the major CO₂-reduction product. The maximum FE_C2H4is merely 12.7% at −1.2 V (vs RHE), which accounts for the whole part of C₂₊ products. In 0.1 M NaHCO₃, the production of C₂H₄and C₂H₅OH is greatly enhanced between −1.0 and −1.2 V (vs RHE), as can be seen in FIG. 3C. The maximum FE_C2H4and FE_C2+ are 28.9% and 46.7% at −1.1 V (vs RHE), respectively. In 0.1 M KHCO₃, the production of C₂H₄is further enhanced, with a maximum FE_C2H4of 53.6% and maximum FE_C2+ of 64.0% at −1.1 V (vs RHE), as discussed before (FIG. 2F). In 0.1 M CsHCO₃, C₂H₄can also be detected at the low potential of −0.7 V (vs RHE), and the maximum FE_C2H4is 29.2% at −1.1 V (vs RHE), as shown in FIG. 3D. Although the FE_C2H4is lower than that in 0.1 M KHCO₃, the production of alcohols (including C₂H₅OH and n-C₃H₇OH) is significantly boosted, with a high FE of 35.5%. The maximum FE_C2+ is 64.7% in 0.1 M CsHCO₃at −1.1 V (vs RHE), which is almost equal to that in 0.1 M KHCO₃. Notably, both the FE_C2H4of CuNS-0.8 in 0.1 M KHCO₃and the FE_alcoholsof CuNS-0.8 in 0.1 M CsHCO₃are much higher than most of the previously reported Cu-based catalysts (FIG. 17, Table 1). As the cations of the electrolyte change from Li⁺ to Cs⁺, the CuNS-0.8 shows a volcano trend in FE_C2H4and a sharp increase in the selectivity of C₂₊ alcohols, suggesting that the catalytic selectivity of CuNS-0.8 is closely related to the cations. According to the previous studies, the cations with larger crystal radii demonstrate smaller hydrated radii and thus show higher densities on the surface of the electrode, which indicates that the density of alkali cations specifically adsorbed on Cu should follow the order of Li⁺<Na⁺<K⁺<Cs⁺. Bearing this in mind, it is anticipated that Li⁺ and Na⁺ can only modulate the electronic structure of CuNS to a slight degree because of their relatively lower densities on the surface of the catalyst, while K⁺ and Cs⁺ can tune the electronic structure of CuNS to a larger degree and thereby greatly improve the selectivity of C₂₊ products.

Theoretical Calculations

We have further introduced theoretical calculations to study the CO₂RR performance of CuNS-0.8. Due to the ultrathin thickness, CuNS shows evident atomic strain without even distributions in the range from −0.7% to 0.7% (FIG. 4A). This leads to the intrinsically uneven electronic distributions of the CuNS, which is more likely to bind with the K⁺ ions in the electrolyte and further modify the surface electronic structures. Notably, after the adsorption of K⁺ ions on the surface, the surface electronic structures are strongly perturbed (FIG. 4B). The electron-rich surfaces are able to guarantee a highly efficient electron transfer from CuNS to the intermediates for deep reduction of CO₂toward C₂₊ products. Detailed electronic structures of CuNS have been demonstrated through the partial density of states (PDOSs), as shown in FIG. 4C. Importantly, Cu-3d shows the dominant peak at E_V-2.30 eV (E_V=0 eV) while the s,p orbitals of surface K⁺ ions mostly locate above the E_F. It is noted that K-4s orbitals overlap with the Cu-3d orbitals, which alleviates the electron transfer barrier. The d-band center in Cu foil is overall higher than that of the CuNS, which potentially results in the overbinding of the CO* and causes the transition to CHO* and C₁products (FIG. 4D). As the K⁺ ion coverage increases, the electronic structures in both CuNS and Cu foil are affected in a similar trend, where the sensitivity in CuNS is much stronger than that of Cu foil. This indicates that K⁺ ions more actively interact with the CuNS surface, which induces the strong electric field to modulate surface electronic structures and benefit the stabilization of key intermediates towards the C₂₊ products. Within the CuNS, the Cu-3d orbitals display a relatively stable electronic structure with a subtle upshifting trend (FIG. 4E). The appropriate d-band center of Cu-3d orbitals ensures the suitable binding strength of CO*, which promotes the C—C couplings for C₂₊ reaction pathways. Meanwhile, the Cu-3d orbitals in Cu foil show an evident upshifting from the bulk to the surface (FIG. 4F). The K⁺ ion coverage on the surface shows a downshifting trend of the Cu-3d, supporting that the electrolyte cations modulate the surface electroactivity. Due to the stronger binding trends with key intermediates towards the C₂₊ products induced by the surface cation effects, the CuNS shows much higher C₂₊ selectivity than that of the Cu foil. For the selectivity of the C₁or C₂₊ pathways, the PDOS of the key adsorbates on the CuNS demonstrates that the conversion from CO₂* to OCCO* shows much smooth electron transfer while the formation of CHO* shows an evident deviation (FIG. 4G). This supports that the formation of C₁products meets higher energy barriers than that of the C₂₊ products. The Cu foil shows an opposite trend (FIG. 4H). Note that the formation of CHO* shows a linear correlation while the C—C couplings exhibit deviation from the correlation, leading to a strong preference for the C1 pathways.

As a comparison, the adsorption of other alkali cations (Li⁺, Na⁺ and Cs⁺) was also investigated together with K⁺ (FIG. 4I). Li⁺ has shown the highest adsorption energies due to its lightest weight, which is difficult to be stabilized. Remarkably, at the lower coverage, Cs⁺ has displayed a stronger adsorption preference than other alkali cations, which is attributed to the much larger atomic mass and smaller hydrated radius. However, the adsorption energies of K⁺ on CuNS surface show a sudden drop and display negative binding energy at the high coverage, leading to a strong preference for higher coverage (FIG. 3A). In contrast, the Cu foil shows even higher binding energies than CuNS with a continuously increasing trend as K⁺ ion coverage increases, supporting the low possibility of adsorption (FIG. 3A). The coverage trends of different alkali cations are consistent well with the corresponding CO₂RR performances (FIG. 2F and FIG. 3B-D).

For the C₂₊ reaction pathways, the CuNS displays a much smaller energy barrier for the C—C couplings and less energy barriers than that of Cu foil towards the formation of C₂H₄(FIG. 4J). In addition, the formation of CH₃CH₂OH delivers a high energy barrier of 1.02 eV, supporting the low FE in experiments (FIG. 2F). For the formation of C₂H₄, the conversion from CH₂CHO is more preferred over the CH₂CH₂OH due to lower energy barriers. For the C₁pathways, the formation of HCOO⁻and CO is more favored for CuNS and Cu foil, respectively (FIG. 4K). For the CH₄pathway, the formation of CHO* shows an energy barrier of 1.50 and 0.64 eV for CuNS and Cu foil, respectively. The much higher energy barrier of the C₁pathways and stronger reaction trend of the C₂₊ pathways lead to the higher selectivity of C₂₊ products on CuNS, agreeing well with experimental observation (FIG. 2F). In the meantime, the formation of CH₄is also favored for Cu foil due to the much lower energy barriers than the C₂₊ pathways (FIG. 2E). For both CuNS and Cu foil, the formation of CH₃OH is not preferred, which is consistent well with the experimental results (FIG. 2E,F).

In summary, we have successfully prepared a self-supported CuNS array electrode with an uneven surface. Compared with the bare Cu foil, the CuNS demonstrated remarkably enhanced selectivity toward C₂₊ products in CO₂RR. At −1.1 V (vs RHE), a maximum FE of C₂₊ products of 64.0% was achieved on CuNS in 0.1 M KHCO₃, which is over two times that on the Cu foil. Detailed studies revealed that the adsorption of K⁺ ions on the surface of CuNS is significantly enhanced than the Cu foil. DFT calculations have revealed that the high selectivity towards the C₂₊ reaction pathway of the CuNS is attributed to the energetically favored K⁺ adsorption on the surface, which not only modify the electronic structures of Cu sites but also promote the C—C couplings. The CuNS displays much lower energy barriers and higher reaction trends for the generation of C₂₊ products. It is believed that this work will open up new opportunities for boosting the CO₂RR performance via rationally tuning the cation-catalyst interaction.

EXAMPLES
Chemicals and Materials

The Cu foil (99.9%, 0.125 mm in thickness) was purchased from Alfa Aesar. K₂S₂O₈(99.5%) and NaOH (95%) were purchased from Macklin. Acetone (98%), HCl (37%), LiHCO₃(99.99%), NaHCO₃(99.99%), KHCO₃(99.99%), CsHCO₃(99.99%), D₂O, and dimethyl sulfoxide (DMSO) were purchased from Aladdin. All the chemical reagents were used as received without further purification. The CO₂gas (99.999%) was purchased from South China Special Gas.

Material Characterization

Scanning electron microscopy (SEM) measurements of all samples were conducted on a QUATTRO S scanning electron microscope. X-ray diffraction (XRD) analysis was performed on Rigaku SmartLab SE X-ray powder diffractometer with Cu—K_α radiation. X-ray photoelectron spectroscopy (XPS) spectra were obtained on an ESCALAB-MKII spectrometer with an Al K_α X-ray source by using C is (284.5 eV) as the reference. The transmission electron microscopy (TEM) images and high-resolution TEM (HRTEM) images were collected on a JEOL JEM-2100F transmission electron microscope.

Example 1—Electrode Preparation

The CuNS array was prepared by in situ electroreduction of CuONS. Firstly, the CuONS was obtained by a chemical oxidation method. In specific, a large piece of Cu foil was cut into small pieces with a size of 1.0 cm×0.5 cm, followed by washing with 3 M HCl, acetone, and de-ionized water (18.2 MΩ) sequentially under sonication to remove the impurities. In parallel, 0.7 g of K₂S₂O₈and 2.7 g of NaOH were dissolved into 50 mL of de-ionized water (18.2 MΩ). The obtained solution was transferred into a 100 mL beaker and heated to 60° C. Then the Cu foils were immersed in the aforementioned solution and maintained for 30 mins. The color of the Cu foils turned from yellow to black, which indicates the formation of CuONS. After that, the resultant samples were collected and rinsed with ethanol and water for 5 times, and later dried at 60° C. in a vacuum oven overnight. Finally, the CuNS electrodes were obtained by in situ reducing the CuONS at −0.2V, −0.5 V, −0.8V, and −1.1V (vs RHE).

Example 2—Electrochemical CO₂RR Test

The electrochemical CO₂RR measurements were conducted in a gas-tight H-type cell (Gaoss Union), which is separated by an ion exchange membrane (Nafion 117, Dupont). The 0.1 M KHCO₃aqueous solution was used as the electrolyte, and was saturated by CO₂before the electrochemical test. During CO₂RR, CO₂gas was purged into the cathodic chamber at a rate of 30 standard cubic centimeters per minute (sccm) and the electrolyte in cathodic chamber was stirred at 400 rpm. The electrochemical measurements were carried out on a CHI 760E workstation. The platinum mesh and Ag/AgCl (saturated with KCl) were used as the counter electrode and reference electrode, respectively. The obtained CuNS served as the working electrode, which was prepared via the in situ electroreduction of CuONS before the CO₂RR test. During the electrolysis test, the chronoamperometry measurement was adopted. At each potential, the time of electrolysis was 45 mins so as to accumulate an adequate amount of liquid products for quantification analysis. All the potentials were compensated and converted to RHE scale with ohmic compensation according to the following equation:

$E (vs RHE) = E (vs Ag / AgCl) + 0.197 + 0.0591 \times pH + 0.8 \times iR$

All gas products were quantified by an on-line gas chromatograph (GC, Agilent 7890B). The GC system is equipped with three detectors in three channels. One channel is equipped with a thermal conductivity detector (TCD) to quantify H₂. Another channel is equipped with a methanator and a flame ionization detector (FID) to analyze CO. And the third channel is equipped with another FID to analyze other gas products, such as CH₄, C₂H₄, and C₂H₆. Liquid products were quantified by a nuclear magnetic resonance spectroscopy (NMR 300 MHz, Bruker AVANCE III BBO Probe). After getting the liquid sample, a 600 L aliquot of the electrolyte was mixed with 30 L D₂0 and 16.7 ppm (m/m) dimethyl sulfoxide (DMSO) as the internal standard. The ¹H spectrum was acquired with water suppression using a pre-saturation method.

Example 3—K⁺ Density Measurement

The density of K⁺ was determined by using an inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 8000). After getting the Cu foil and CuNS-0.8, the electrodes were set in the same electrolysis system as CO₂RR. The potential was set at −1.1 V (vs RHE) and held for 120 s. Then the electrode was carefully pulled out of the electrolyte, followed by the careful rinsing with 5 mL of de-ionized water. After that, the density of K⁺ in the water was tested by ICP-OES.

Faradaic Efficiency Calculation

The FE of gas and liquid products was calculated based on the following two equations:

${FE}_{gas} = \frac{C_{gas} \times v \times z \times F}{i_{total} \times 22400} \times 100 %$

${FE}_{liquid} = \frac{C_{liquid} \times V \times z \times F}{Q} \times 100 %$

where C_gas(ppm) is the volume concentration of gas products read from the GC; v (sccm) is the flow rate of CO₂; z is the number of charges transferred when forming one molecule of the products; F is the Faradaic constant (96485 C/mol); i_total(mA) is the measured current read from the electrochemical workstation; 22400 is the molar volume of gas at standard conditions (mL/mol); C_liquid(mol/L) is the concentration of liquid products obtained from NMR measurement; V is the amount of electrolyte in the cathode chamber (8 mL); Q (C) is the total amount of charges passed through the electrode, which can be read from the electrochemical workstation.

The partial current density (j_partial) of the products was calculated by the following equation,

$j_{partial} = j_{total} \times FE$

where j_totalis the total geometric current density.

Example 4—Calculation Setup

To investigate the CO₂RR performance of CuNS and Cu foil, we have introduced the DFT calculations within the CASTEP packages. For the functionals, the generalized gradient approximation (GGA) and Perdew-Burke-Ernzerhof (PBE) were both chosen to achieve accurate descriptions of exchange-correlation interactions. Meanwhile, we have applied the ultrasoft pseudopotentials for all the geometry optimizations and set the plane-wave basis cutoff energy to 380 eV. In addition, the Broyden-Fletcher-Goldfarb-Shannon (BFGS) algorithm was selected with the coarse quality setting of k-points for all the energy minimizations in this work. For both CuNS and Cu foil, the (100) surfaces were chosen according to the experimental characterizations with 20 Å vacuum space introduced in the z-axis to guarantee the geometry relaxations. The following convergence criteria were applied including the Hellmann-Feynman forces, the total energy difference and the inter-ionic displacement should not exceed 0.001 eV/A, 5×10⁻⁵eV/atom and 0.005 Å, respectively.

METHODS FOR ELECTROCHEMICAL REDUCTION OF CARBON DIOXIDE

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