ELECTRODE, ELECTROCHEMICAL CELL, AND METHOD OF USE THEREOF

Description

TECHNICAL FIELD

The present disclosure generally relates to silver-based electrodes useful for electrocatalytic reduction of CO₂, electrochemical cells comprising the same and methods of use thereof.

BACKGROUND

The electrocatalytic reduction of CO₂ has found numerous industrial applications including, for example, formation of higher value chemicals and feedstock. Optimizing selectivity, i.e., Faradaic efficiency (FE), of catalysts for products such as CO, increasing their productivity (current density), and lowering overpotentials of the reduction reactions have become priorities. However, major problems still stand, such as system stability and efficiency, particularly in the pure H₂O system.

There thus exists a need for improved electrodes and electrochemical cells that address at least some of the challenges described above.

SUMMARY

In a first aspect, provided herein is an electrode comprising: a plurality of metallic silver nanoparticles, wherein the plurality metallic silver nanoparticles have an average coordination number between 8.9-11.6 and an average tensile strength strain of 0.14-0.81%.

In certain embodiments, each of the plurality metallic silver nanoparticles comprise a plurality of stacking faults and a plurality of grain boundaries.

In certain embodiments, the plurality metallic silver nanoparticles have an average diameter of 10-50 nm.

In certain embodiments, each of the plurality metallic silver nanoparticles have an average coordination number 9.2-9.8 and an average tensile strength strain of 0.56-0.81%.

In certain embodiments, each of the plurality metallic silver nanoparticles have an average coordination number about 9.2 and an average tensile strength strain of about 0.81%.

In certain embodiments, the plurality of metallic silver nanoparticles are prepared by a method comprising: contacting AgNO₃, octadecylamine, oleylamine, and trioctylphosphine thereby forming the plurality of metallic silver nanoparticles.

In certain embodiments, the plurality of metallic silver nanoparticles are prepared by a method comprising: contacting AgNO₃, octadecylamine, and an organic solvent at 60-100° C. thereby forming a silver-based stock solution; contacting the silver-based stock solution, oleylamine, and trioctylphosphine at 100-200° C. thereby forming the plurality of metallic silver nanoparticles.

In certain embodiments, the plurality of metallic silver nanoparticles are not annealed at a temperature greater than 250° C.

In certain embodiments, the electrode further comprises a base electrode or a substrate, wherein the plurality of metallic silver nanoparticles are disposed on a surface of the base electrode or the substrate.

In certain embodiments, the substrate comprises a gas permeable metal mesh.

In a second aspect, provided herein is a method of preparing the electrode described herein, the method comprising: depositing a solution comprising the plurality of metallic silver nanoparticles, a binder, and a solvent on the surface of the substrate thereby forming a coated substrate and calcining the coated substrate thereby forming the electrode.

In certain embodiments, the solvent comprises an organic solvent.

In certain embodiments, the solution is deposited by screen printing.

In certain embodiments, the method further comprises compressing the coated substrate prior to calcining the coated substrate.

In a second aspect, provided herein is an electrochemical cell comprising: the electrode described herein; a counter electrode; and an electrolyte solution comprising an electrolyte, wherein the electrolyte solution is between and in contact with the electrode and the counter electrode.

In certain embodiments, the electrode further comprises a base electrode or a substrate, wherein the plurality of metallic silver nanoparticles are disposed on a surface of the base electrode or the substrate.

In certain embodiments, the base electrode is selected from the group consisting of a glassy carbon electrode, a graphite electrode, an indium tin oxide (ITO) electrode, a fluorine doped tin oxide (FTO) electrode, a carbon paper electrode, a carbon fiber electrode, a polycarbonate track etch (PCTE)-based electrode, and a titanium-based electrode; and the substrate comprises a gas permeable metal mesh.

In certain embodiments, the electrolyte comprises water and optionally a metal hydroxide.

In certain embodiments, the electrochemical cell further comprises at least one ion exchange membrane disposed between the electrode and the counter electrode.

In certain embodiments, the electrochemical cell further comprises an anion exchange membrane and a proton exchange membrane, wherein the electrode is in contact with the anion exchange membrane, the anode is in contact with the proton exchange membrane, and the anion exchange membrane and proton exchange membrane are in contact with each other.

In certain embodiments, the electrochemical cell further comprises a CO₂ inlet in fluid communication with the electrode and a water inlet in fluid communication with the counter electrode.

In a third aspect, provided herein is a method for reducing carbon dioxide, the method comprising: providing the electrochemical cell described herein; contacting CO₂ and the electrode and contacting water and the counter electrode; and applying an electric current between the electrode and the counter electrode resulting in electrolytic reduction of the CO₂ thereby forming CO.

In certain embodiments, the electrochemical cell has a Faradaic efficiency (FE) of CO of 90-99%.

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. (A) HAADF-STEM image of stepped surface silver nanoparticles (SS-Ag). (B) False-colour dark-field four-dimensional STEM (4D-STEM) map, corresponding to (A). (C) An atomic-resolution HAADF-STEM image of SS-Ag showing stepped faces induced by a stacking fault and a twin boundary, both along {111} planes (dashed white lines).

FIG. 2. Fine structure characterization of SS-Ag. (A-C) Typical SEM (A) and TEM (B and C) images of SS-Ag. (D-G) HR-TEM images of SS-Ag. (E) and (G) are from the selected areas in (D) and (F), respectively.

FIG. 3. Fine structure characterization of SS-Ag. (A and D) Typical HAADF-STEM images of SS-Ag. (B and C) Atomic-resolution HAADF-STEM images of the selected areas in (A) and (B), respectively. (E-G) Atomic-resolution HAADF-STEM images showing the stepped surface of SS-Ag caused by the abundant SFs and GBs in SS-Ag. (E) is from the selected area in (D). The yellow and white lines in the images highlight GBs and SFs. The arrows in (F and E) highlight steps. The arrows in (G) highlight GBs in SS-Ag.

FIG. 4. XRD patterns of SS-Ag, Ag-250, Ag-350 and Ag-450 on the carbon paper, and bare carbon paper.

FIG. 5. In-situ heating TEM characterization. (A and B) TEM and HRTEM images of the pristine SS-Ag. (C and B) TEM and HRTEM images of the pristine SS-Ag heated at 250° C. for 10 min. (E and F) TEM and HRTEM images of the sample that was continuously heated at 350° C. for 10 min. (G and H) TEM and HRTEM images of the sample that was continuously heated at 450° C. for 10 min. (B, D, F and H) are from the selected areas in (A, C, E and G), respectively. (I-L) In-situ heating TEM images of the SS-Ag and the SS-Ag directly heated at 450° C. for 10 min.

FIG. 6. XAS measurements of SS-Ag, Ag-250, Ag-350 and Ag-450, and the standard Ag foil reference. (A) Ag K-edge XANES spectra. (B) Fourier transform of Ag K-edge EXAFS spectra.

FIG. 7. Two-dimensional plots of wavelet transform EXAFS (2D WT EXAFS). (A) The standard Ag foil reference. (B) SS-Ag. (C) Ag-250. (D) Ag-350. (E) Ag-450.

FIG. 8. Ag K-edge EXAFS fitting curves at R and q space, respectively. (A and B) Standard Ag foil reference. (C and D) SS-Ag. (E and F) Ag-250. (G and H) Ag-350. (I and J) Ag-450.

FIG. 9. The tensile strain and coordination number of SS-Ag, Ag-250, Ag-350, Ag-450 and Ag foil reference. Values are means and error bars indicate the highest and lowest ones.

FIG. 10. ECO₂R performance on SS-Ag under different applied potentials in a flow cell with 1 M KOH as the electrolyte. (A) Faradaic efficiency (FEs) toward ECO₂R products. (B) Total and partial current densities. Values are means and error bars indicate SD (n=3 replicates).

FIG. 11. ECO₂R performance on Ag-250 under different applied potentials in a flow cell with 1 M KOH as the electrolyte. (A) FEs toward ECO₂R products. (B) Total and partial current densities. Values are means and error bars indicate SD (n=3 replicates).

FIG. 12. ECO₂R performance on Ag-350 under different applied potentials in a flow cell with 1 M KOH as the electrolyte. (A) FEs toward ECO₂R products. (B) Total and partial current densities. Values are means and error bars indicate SD (n=3 replicates).

FIG. 13. ECO₂R performance on Ag-450 under different applied potentials in a flow cell with 1 M KOH as the electrolyte. (A) FEs toward ECO₂R products. (B) Total and partial current densities. Values are means and error bars indicate SD (n=3 replicates).

FIG. 14. Comparisons of ECO₂R performance on different samples in the flow cell with 1 M KOH as the electrolyte under a range of applied potentials. (A) Total current densities. (B) FE towards CO. (C) The partial current density of CO. (D) FE towards H₂. (E) The partial current density of H₂. Values are means and error bars indicate SD (n=3 replicates).

FIG. 15. Relationships between strain, CN and the peak j_CO (A) and j_H2 (B) for ECO₂R in a flow cell with 1 M KOH as the electrolyte. Values are means and error bars indicate the highest and lowest ones for CN. Values are means and error bars indicate SD for the partial current densities (n=3 replicates).

FIG. 16. The schematic of the pure-H₂O-fed anion-and proton-exchange membranes (AEM and PEM assembly: APMA) membrane-electrode-assembly APMA MEA system architecture for ECO₂R.

FIG. 17. The FEs of products and corresponding cell voltages of the pure-H₂O-fed APMA MEA system at a total current density of 100 mA/cm² and different reaction temperatures. The low temperature could cause the high overpotential and too high temperature would suppress ECO₂R and meanwhile increase HER. Values are means and error bars indicate SD (n=3 replicates).

FIG. 18. ECO₂R performance of SS-Ag in the pure-H₂O-fed APMA MEA cell. (A) The FEs toward ECO₂R products on SS-Ag under a range of applied current densities and at 60° C., and the corresponding cell voltages. (B) FEs and cell voltages for ECO₂R on SS-Ag in the pure-H₂O-fed APMA MEA cell at 150 mA/cm², 60° C. and different CO₂ inlet flow rates. (C) The CO₂ conversions and cell voltage for ECO₂R-to-CO on SS-Ag under different CO₂ inlet flow rates. (D and E) FEs (G) and corresponding partial current densities (H) towards products under different CO₂ inlet flow rates for ECO₂R. Values are means and error bars indicate SD (n=3 replicates).

FIG. 19. A schematic of an exemplary free high-diffusion-flux gas diffusion electrode HDF-GDE in accordance with certain embodiments described herein.

FIG. 20. ECO₂R performance in the pure-H₂O-fed membrane-electrode-assembly (APMA MEA) system with different HDF-GDEs. (A) Digital photograph of SS-Ag HDF-GDE, showing its hydrophobicity. (B) An SEM image of SS-Ag HDF-GDE. (C) The FEs towards ECO₂R products on SS-Ag HDF-GDE under a range of applied current densities, and the corresponding cell voltages. (D-F) Comparisons of CO FEs (D), H₂ FEs (E) and cell voltages (F) at different current densities for ECO₂R on SS-Ag HDF-GDE and SS-Ag carbon paper (traditional GDE). (G) Comparison of the CO₂ conversion of SS-Ag traditional GDE and SS-Ag HDF-GDEs. (H) In-situ Raman spectra of ECO₂R on SS-Ag carbon paper GDE and SS-Ag HDF-GDE in the pure-H₂O-fed APMA MEA cell at 400 mA/cm².

FIG. 21. Schematic of different GDEs. (A) Traditional carbon paper GDE. (B) HDF-GDE.

FIG. 22. Digital photographs of the SS-Ag HDF-GDEs with different sizes.

FIG. 23. Scaled-up kW-scale pure-H₂O-fed APMA MEA cell stack assembled with HDF-GDEs for ECO₂R. (A-C) Digital photographs of the flow channel plate in the cell stack system (A), the assembled cell stack in side view (B) and the cell stack in front view (C). (D and E) The system stability performance of ECO₂R on the SS-Ag HDF-GDE (D) and SS-Cu HDF-GDE (E).

FIG. 24. Structure characterization of SS-Ag after experiencing ECO₂R for 1000 h. (A and B) Typical HAADF-STEM images. (B) is from the selected area in (A).

FIG. 25. Crystal Structure parameters of SS-Ag, Ag-250, Ag-350, Ag-450 and Ag foil reference from the EXAFS fitting.

FIG. 26. Schematic of an exemplary electrochemical cell in accordance with certain embodiments described herein.

DETAILED DESCRIPTION

Definitions

The following terms shall be used to describe the present invention. In the absence of a specific definition set forth herein, the terms used to describe the present invention shall be given their common meaning as understood by those of ordinary skill in the art.

Throughout the present disclosure, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. It is also noted that in this disclosure and particularly in the claims and/or paragraphs, terms such as “comprises”, “comprised”, “comprising” and the like can have the meaning attributed to it in U.S. Patent law; e.g., they can mean “includes”, “included”, “including”, and the like; and that terms such as “consisting essentially of” and “consists essentially of” have the meaning ascribed to them in U.S. Patent law, e.g., they allow for elements not explicitly recited, but exclude elements that are found in the prior art or that affect a basic or novel characteristic of the present invention.

Furthermore, throughout the present disclosure and claims, unless the context requires otherwise, the word “include” or variations such as “includes” or “including”, will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

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.

Provided herein is an electrode comprising: a plurality of metallic silver nanoparticles, wherein the plurality metallic silver nanoparticles have an average coordination number between 8.9-11.6 and an average tensile strength strain of 0.14-0.81%.

The average coordination number of the plurality of metallic silver nanoparticles can range from 8.9-11.6, 9.2-11.2, 9.2-10.5, 9.2-9.8, or 8.9-9.5. In certain embodiments, the average coordination number of the plurality of metallic silver nanoparticles is about 9.2.

The average tensile strength strain of the plurality of metallic silver nanoparticles can range from 0.14-0.81%, 0.35-0.81%, 0.56-0.81%,0.61-0.81%, 0.66-0.81%, 0.71-0.81%, or 0.76-0.81%. In certain embodiments, the average tensile strength strain of the plurality of metallic silver nanoparticles is about 0.81%.

Each of the plurality metallic silver nanoparticles comprise a plurality of stacking faults and/or a plurality of grain boundaries.

The plurality metallic silver nanoparticles can have an average diameter of 10-50 nm, 10-40 nm, 10-30 nm, 10-20 nm, 20-50 nm, 30-50 nm, 40-50 nm, 20-40 nm, 20-30 nm, 30-40 nm, or 15-25 nm. In certain embodiments, the plurality metallic silver nanoparticles can have an average diameter of about 20 nm.

The plurality metallic silver nanoparticles can be readily prepared from readily available commercially available materials. In certain embodiments, the plurality metallic silver nanoparticles are prepared by contacting a silver salt with an aliphatic primary amine thereby forming the plurality metallic silver nanoparticles.

The silver salt can be AgOAc, AgNO₃, AgOTf, AgOTFA, AgBF₄, AgOSO₂Me, Ag₂SO₄, AgPF₆, and mixtures thereof. In certain embodiments, the silver salt is AgNO₃.

In certain embodiments, the aliphatic primary amine is a C₈-C₂₄ alkyl primary amine, a C₁₀-C₂₄ alkyl primary amine, a C₁₂-C₂₄ alkyl primary amine, a C₁₄-C₂₄ alkyl primary amine, a C₁₆-C₂₄ alkyl primary amine, a C₁₈-C₂₄ alkyl primary amine, a C₁₈-C₂₂ alkyl primary amine, a C₁₆-C₂ alkyl primary amine, a C₈-C₂₄ alkenyl primary amine, a C₁₀-C₂₄ alkenyl primary amine, a C₁₂-C₂₄ alkenyl primary amine, a C₁₄-C₂₄ alkenyl primary amine, a C₁₆-C₂₄ alkenyl primary amine, a C₁₈-C₂₄ alkenyl primary amine, a C₁₈-C₂₂ alkenyl primary amine, a C₁₆-C₂ alkenyl primary amine, or mixtures thereof. In certain embodiments, the aliphatic primary amine comprises octadecylamine, oleylamine, or a mixture thereof.

In certain embodiments, the method of preparing the plurality of metallic silver nanoparticles further comprises contacting the silver salt and the aliphatic primary amine with a trialkylposphine. Exemplary trialkylposphines include, but are not limited to, tri(C₆-C₁₂alkyl)phosphine, tri(C₆-C₁₀alkyl)phosphine, tri(C₆-C₈alkyl)phosphine, tri(C₈-C₁₂alkyl)phosphine, tri(C₈-C₁₀alkyl)phosphine, or mixtures thereof. In certain embodiments, the trialkylposphine is trioctylphosphine.

In certain embodiments, the plurality of metallic silver are prepared according to a method comprising: contacting the silver salt, a first aliphatic primary amine, and a solvent thereby forming a silver-based stock solution; and contacting the silver based stock solution with a second aliphatic primary amine and a trialkyl phosphine, wherein the each of the first aliphatic primary ameind and the second aliphatic primary amine are independently is a C₈-C₂₄ alkyl primary amine, a C₁₀-C₂₄ alkyl primary amine, a C₁₂-C₂₄ alkyl primary amine, a C₁₄-C₂₄ alkyl primary amine, a C₁₆-C₂₄ alkyl primary amine, a C₁₈-C₂₄ alkyl primary amine, a C₁₈-C₂₂ alkyl primary amine, a C₁₆-C₂ alkyl primary amine, a C₈-C₂₄ alkenyl primary amine, a C₁₀-C₂₄ alkenyl primary amine, a C₁₂-C₂₄ alkenyl primary amine, a C₁₄-C₂₄ alkenyl primary amine, a C₁₆-C₂₄ alkenyl primary amine, a C₁₈-C₂₄ alkenyl primary amine, a C₁₈-C₂₂ alkenyl primary amine, a C₁₆-C₂ alkenyl primary amine, or mixtures thereof, and each of the silver salt and the trialkyl phosphine are independently as define in any embodiment described herein.

The solvent can be any high boiling point solvent in which the starting materials are at least partially soluble. In certain embodiments, the solvent comprises a C₁₀-C₃₀ alkane, C₁₅-C₃₀ alkane, C₁₅-C₂₅ alkane, C_15-C₂₀ alkane, C₁₀-C₃₀ alkene, C₁₅-C₃₀ alkene, C₁₅-C₂₅ alkene, C₁₅-C₂₀ alkene, and C₁₀-C₁₄ aryl, or mixtures thereof. Exemplary solvents include, but are not limited to 1-octadecene, diisopropylbiphenyl, diisopropylnaphthalene, dibenzyl ether, paraffins, alkyl stearates, alkyl oleates, squalane, and the like. In certain embodiments, the solvent comprises squalane.

The step of contacting the silver salt, the first aliphatic primary amine, and the solvent can be conducted at 60-100° C., 70-90° C., or 75-85° C. In certain embodiments, the step of of contacting the silver salt, the first aliphatic primary amine, and the solvent is conducted at about 80° C.

The step of contacting the silver based stock solution with the second aliphatic primary amine and the trialkyl phosphine can be conducted at at 100-220° C., 120-220° C., 140-220° C., 160-220° C., 180-220° C., 150-190° C., 160-180° C., or 165-175° C. In certain embodiments, contacting the silver based stock solution with the second aliphatic primary amine and the trialkyl phosphine is conducted at about 170° C.

In certain embodiments, the first salt is AgNO₃, the first aliphatic primary amine is octadecylamine, the second aliphatic primary amine is oleylamine, the trialkylphosphine is trioctylphosphine, and the solvent is squalane.

The plurality of metallic silver nanoparticles can be isolated from the reaction mixture, e.g., by centriguaiton, filtration, decanting, or the like and dried. Drying can be conducted using conventional methods, such as under reduced pressure optionally a temperature between 40-100° C. As described in detail herein, the average coordination number and average tensile strength of the plurality of metallic silver nanoparticles can change at elevated temperatures. Accordingly, in certain embodiments, the method for preparing the plurality of metallic silver nanoparticles does not include annealing the plurality of metallic silver nanoparticles or subjecting the plurality of metallic silver nanoparticles to a temperature above 80° C., 100° C., 150° C., 200° C., 150° C., 200° C., 250° C., 300° C., 350° C., 400° C., or 450° C.

In certain embodiments, the electrode is gas and liquid permeable. In certain embodiments, the electrode is a cathode.

In certain embodiments, the electrode further comprises a base electrode or a substrate. In certain embodiments, the plurality of metallic silver nanoparticles are disposed on a surface of the base electrode or the substrate.

The base electrode can be an inert electrode such as a glassy carbon electrode, a graphite electrode, an indium tin oxide (ITO) electrode, a fluorine doped tin oxide (FTO) electrode, carbon paper electrode, carbon fiber electrode, a polycarbonate track etch (PCTE)-based electrode, or a titanium-based electrode.

The electrode can optionally comprise a binder. The binder may optionally be cured to further bind the SS-Ag particle or a plurality of the SS-Ag particles with the base electrode and can increase the conductivity of electrode. Typical binders include, for example polyvinylidene fluoride (PVDF), alkaline ionomers (such as Sustainion® XA-9, Sustainion® XC-2 and Sustainion® XB-7), Nafion™ polymer dispersion (such as D520CS, D521CS, D2020CS, and D2021CS), polyvinyl alcohol (PVA), starch, sodium alginate, hydroxypropyl cellulose, carboxymethyl cellulose (CMC), regenerated cellulose, polyvinylpyrrolidone, polyimide, polyamideimide, polyethylene, polypropylene, an ethylene-propylene-diene terpolymer (EPDM), an acrylic resin, a sulfonated EPDM, a styrene-butadiene rubber, polytetrafluoroethylene (PTFE), a polyacrylic polymer, and combinations thereof. In certain embodiments, the binder is Sustainion® XA-9.

The electrode further comprising the base electrode or substrate can be prepared using methods well known in the art. In certain embodiments, the electrode further comprising the base electrode or substrate is prepared by depositing a solution comprising the plurality of metallic silver nanoparticles, a binder, and a solvent on at least one surface of the substrate thereby forming a coated substrate and calcining the coated substrate thereby forming the electrode.

The solvent can be any organic solvent in which the binder is at least partially soluble. Exemplary organic solvents include, but are not limited to, ethers, alcohols, alkanes, ketones, aromatic solvents, formaimdes, esters, sulfoxides, and mixtures thereof. In certain embodiments, the organic solvent comprises α-terpineol and 2-(2-butoxyethoxy)ethanol.

The step of depositing the solution on the at least one surface of the substrate can be accomplished using any method known to those of skill in the art. Exemplary methods for depositing the solution on the substrate include, but are not limited to, spin coating, printing, screen printing, spraying, painting, doctor-blading, inkjet printing, roller printing, or flexo textile printing, and dip coating. In certain embodiments, the solution is deposited by screen printing.

The solvent can optionally be removed from the coated substrate prior to calcining, e.g., by subjecting the coated substrate to reduced pressure (vacuum) and/or heat. Attachment of the plurality of silver nanoparticles to the substrate can advantageously be further improved by compressing the coated substrate prior to calcining (e.g., by direct compression of the coated substrate or compression of the coated substrate after solvent removal). Compression of the coated substrate can comprise applying a pressure, e.g., of up to 10,000, 5,000, 4,000, 3,000, 2,000, or 1,000 pounds pressure to the coated substrate for a period of time, e.g., 1-5 hrs, 1-4 hrs, 1-3 hrs, or 1-2 hrs.

Calcining the coated substrate can comprise heating the coated substrate at a temperature between 100-400° C., 150-400° C., 200-400° C., 250-400° C., 300-400° C., or 325-375° C. The step of calining can be conducted for 15-120 minutes, 15-90 minutes, 15-60 minutes, or 15-45 minutes. In certain embodiments, the coated substrate is calcined at a temperature of about 350° C. for about 30 minutes.

The present disclosure also provides an electrochemical cell comprising: the electrode described herein; a counter electrode (or counter/reference electrode); optionally a reference electrode (e.g., in a three-electrode system); and an electrolyte solution between and in contact with the electrode, the counter electrode, and optionally the reference electrode. In certain embodiments, the electrolyte solution comprises an aqueous solution.

A counter electrode refers to an electrode paired with the electrode described herein, through which passes a current equal in magnitude and opposite in sign to the current passing through the 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. In certain embodiments, the counter electrode is IrO_x, RuO_x, Ir, Pt, or a mixture of IrO_x and RuO_x. In certain embodiments, the counter electrode is Ir.

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 or pure H₂O.

The carbon dioxide can be added to the electrolyte prior to the step of applying an electric current between the electrode and the counter electrode and/or introduced into the electrolyte continuously during the electrochemical reduction, and/or introduced into the back side of the gas diffusion electrode, e.g., by bubbling CO₂ into 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, MCl, M′SO₄, M′CO₃, M′(HCO₃)₂, and M′Cl₂, 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. In certain embodiments, the electrolyte consists of water or is an aqueous solution comprising KOH.

The concentration of the electrolyte salt in the electrolyte solution can range from 0.1 to 5M, 0.1 to 4.5M, 0.1 to 4M, 0.1 to 3.5M, 0.1 to 3M, 0.1 to 2.5M, 0.1 to 2M, 0.5 to 1.5M, 0.6 to 1.4M, 0.7 to 1.3M, 0.8 to 1.2M, or 0.9 to 1.1M. In certain embodiments, the electrolyte salt is present in the electrolyte solution at a concentration of about 1M.

In certain embodiments, the electrochemical cell further comprises an anion exchange membrane and a proton exchange membrane, wherein the electrode is in contact with the anion exchange membrane, the anode is in contact with the proton exchange membrane, and the anion exchange membrane and proton exchange membrane are in contact with each other.

FIG. 26 illustrates an exemplary electrochemical cell (100) in accordance with certain embodiments described herein comprising: an electrode described herein (101), a counter electrode (102), an optional proton exchange membrane (104), an optional anion exchange membrane (103), an optional CO₂ inlet (105), an optional water inlet (106), a CO outlet (107), and an optional product outlet (108).

The present disclosure also provides an electrochemical cell comprising: a plurality of cells, wherein each cell comprises an electrode described herein; a counter electrode (or counter/reference electrode), an anion exchange membrane, and a proton exchange membrane, wherein the electrode is in contact with the anion exchange membrane, the counter electrode is in contact with the proton exchange membrane, and the anion exchange membrane and proton exchange membrane are in contact with each other; and an electrolyte solution between and in contact with the plurality of cells, wherein each electrode, each counter electrode, and the electrolyte are each independently as defined in any embodiment described herein.

The proton exchange membrane can include, but is not limited to, proton exchange membranes sold under the tradenames Nafion™ NR 50, Nafion™ 117, Nafion™ 211, and Nafion™ 212. In certain embodiments, the proton exchange membrane is Nafion™ 117.

In certain embodiments, the anion exchange membrane include be, but is not limited to, anion exchange membranes sold under the tradenames Sustainion® X-37-50 Grade RT, Sustainion® X-37-50 Grade 60, Sustainion® X-37 Grade FA, Sustainion® X-37 Grade T, Sustainion E-30 Grade T, Sustainion® E-28 Grade T, Sustainion® B-22 Grade T, MTCP-50 (described in Wanjie Song, et al., Alkaline Membranes toward Electrochemical Energy Devices: Recent Development and Future Perspectives ACS Cent. Sci. 2023, 9, 8, 1538-1557 herein incorporated by reference in its entirety), quaternary ammonia poly(N-methyl-piperidine-co-p-terphenyl (QAPPT), and quaternary ammonia poly(ether ether ketone) (QAPEEK). In certain embodiments, the anion exchange membrane is Sustainion® X-37-50 Grade 60.

In certain embodiments, the electrochemical cell further comprises a CO₂ inlet in fluid communication with the electrode for introducing CO₂ to the electrochemical cell; an electrolyte inlet in fluid communication with the counter electrode for introducing the electrolyte to the counter electrode; a product outlet in fluid for communication with the electrode for removing carbon monoxide from the electrochemical cell; and a product outlet in fluid communication with the counter electrode for removing products (e.g., O₂) from the electrochemical cell.

The electric current between the electrode and the counter electrode can be applied at a voltage of −0.1 to −1V, −0.1 to −0.9V, −0.1 to −0.8V, −0.1 to −0.7V, −0.1 to −0.6V, −0.1 to −0.5V, −0.1 to −0.4V, −0.1 to −0.3V, −0.1 to −0.2V, −0.2 to −0.9V, −0.3 to −0.9V, 0.4 to −0.9V, −0.5 to −0.9V, −0.6 to −0.9V, −0.7 to −0.9V, −0.8 to −0.9V, −0.2 to −0.7V, or −0.32 to −0.65 (vs RHE). In certain embodiments, the electric current between the electrode and the counter electrode is applied at about −0.32 V to about −0.65 V (vs RHE). Advantageously, the yield of CO can be optimized when the electric current between the electrode and the counter electrode is applied at −0.32 to −0.65 (vs RHE).

In other embodiments, the electric current between the electrode and the counter electrode is applied at a voltage of 0.1-10 V, 0.1-9 V, 0.1-8 V, 0.1-7 V, 0.1-6 V, 0.1-5 V, 0.1-4 V, 0.1-3 V, 0.1-2 V, 0.1-1 V, 0.1-0.5 V, 0.5-10 V, 1-10 V, 2-10 V, 3-10 V, 4-10 V, 5-10 V, 6-10 V, 7- 10 V, 8-10 V, 9-10 V, 1-5 V, 2-5 V, 3-5 V, 3-4.5 V, 3.5-4.5 V, 4-4.5 V, or 3.5-4 V.

In instances in which the electrochemical cell comprises a plurality of cells, the electric current between each of the electrodes and each of the counter electrodes is applied at a voltage of 1-100 V, 10-100 V, 20-100 V, 20-90 V, 20-80 V, 20-70 V, 20-60 V, 20-50 V, 20-40 V, 25-40 V, 30-40 V, 35-40 V, 20-35 V, 20-30 V, 20-25 V, 25-35 V, 30-35 V, or 25-30 V.

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.

Also provided is a method for reducing carbon dioxide, the method comprising: providing the electrochemical cell described herein; contacting CO₂ and the electrode and contacting water and the counter electrode; and applying an electric current between the electrode and the counter electrode resulting in electrolytic reduction of the CO₂ thereby forming CO.

In certain embodiments, the electrochemical cell has a Faradaic efficiency (FE) of CO of 90-99% at −0.32 V to −0.65 V vs reversible hydrogen electrode (RHE).

In a typical synthesis, 0.07 g of AgNO₃ and 0.02 g of octadecylamine were dissolved in 0.8 mL of squalane and this solution was heated for 0.5 h at 80° C. under the inert gas (Ar) to form an Ag-based stock solution. Meanwhile, the mixed solution of 10 mL of oleylamine and 0.04 mL of trioctylphosphine was heated to 170° C. under an Ar atmosphere with intense magnetic agitation. Subsequently, the Ag-based stock solution was injected into the above-mixed solution and kept at 170° C. for 5 h. After natural cooling, the resulting sample was collected by centrifugation and washed several times with n-hexane. Finally, the sample was dried at 80° C. in a vacuum before use. The sample was denoted as SS-Ag.

The SS-Ag samples were annealed at various temperatures (250, 350, and 450° C.) in the tube furnace for 2 h under a mixed gas (H₂/Ar: 5 v/v%; 200 sccm (standard cubic centimeters per minute)), denoted as Ag-250, Ag-350 and Ag-450, respectively.

The model catalyst SS-Ag (˜20 nm) with abundant stacking faults (SFs) and grain boundaries (GBs) was prepared using a facile oil bath method (FIG. 1). Four-dimensional aberration-corrected high-angle annular dark-field (HAADF) scanning TEM (STEM) (4D-STEM) revealed that abundant SFs and GBs in SS-Ag intersect with each other and result in the stepped surface formation (FIG. 1). High-resolution transmission electron microscopy (HR-TEM) and HAADF-STEM showed multitudinous SFs and GBs in SS-Ag, and GBs contained Σ3 coincident site lattice boundaries (twin boundaries) and some of them formed some typical five-fold twinning structures (FIGS. 2 and 3). SFs and GBs caused the stepped surface formation at the surface exits of GBs and SFs (FIGS. 1C and 3). According to the powder X-ray diffraction (XRD) pattern, the as-synthesized SS-Ag is in the cubic phase (JCPDS No. 04-0783) and stays as metallic Ag (FIG. 4).

To investigate the effects of SFs and GBs on the Ag catalyst, SS-Ag was treated with different high temperatures (250, 350 and 450° C.; corresponding Ag-250, Ag-350 and Ag-450) to decrease the SF and GB density or eliminate SFs and GBs in SS-Ag, because atoms could rearrange at the high temperature to reach a more thermodynamically favorable state, minimizing the surface energy. In-situ heating TEM showed that SFs and GBs decreased or even disappeared at high temperatures, the higher the temperature, the lower the density of SF and GB, and there was no appreciable change in sample size after the high-temperature treatment (FIG. 5).

We used X-ray absorption spectroscopy (XAS) to study the samples'fine structure (FIGS. 6-8). Ag K-edge X-ray absorption near-edge structure (XANES) spectra and the two-dimensional plots of wavelet transform EXAFS (2D WT EXAFS) of samples presented a near-edge feature characteristic of the metallic Ag phase (FIGS. 6A and 7). Fourier-transformed χ(R) functions of extended X-ray absorption fine structure (EXAFS) data in the frequency domain (R) revealed an increase in coordination number (CN) as increasing the treatment temperature (FIG. 6B). The fine structure information from EXAFS fitting results indicated that the average CN of Ag increased gradually (from ˜9.2 to 11.2), meanwhile, the average tensile strain decreased gradually (from ˜0.81% to 0.14%) as increasing the treatment temperature (FIGS. 8 and 25). The tensile strain and CN showed a strong linear correlation with the change in the annealing temperature due to the thermodynamic influence (high-temperature treatment) (FIG. 9).

For ECO₂R, SS-Ag delivered that FE towards CO reached ˜99% under a wide potential window [−0.32 V to −0.65 V versus a reversible hydrogen electrode (RHE)] in a flow cell with the conventional electrolyte (1 M KOH) (FIG. 10). We also found that low CN and high tensile strain boosted ECO₂R performance (FIGS. 10-14). With the increase of CN and the decrease of the tensile strain, the ECO₂R-to-CO performance of samples showed a noticeable decline (FIG. 15). More importantly, the CO partial current density (j_CO) emerged with a monotonic increase with the decline of CN and increase of the tensile strain of Ag (FIG. 15A), meaning that j_CO showed a strong linear correlation with the function of the tensile strain and CN. This strong linear correlation could reflect that the underlying reason why SF and GB could improve ECO₂R activity might be derived from the low CN and high tensile strain. Additionally, the HER activity seems to be independent of SF and GB (FIG. 15B).

To suppress carbonate formation during ECO₂R and improve the stability of the electrolysis system, we adopted a pure-H₂O-fed MEA architecture with the anion-and proton-exchange membranes (AEM and PEM assembly: APMA) to evaluate the ECO₂R performance of SS-Ag (FIG. 16). Due to using pure H₂O as the anolyte, we first increased the reaction temperature (from ˜25°C. to 80° C.) to decrease the reaction overpotential. The optimized reaction temperature was 60° C. with FE_CO of ˜91%, FE_H2 of ˜8% and cell voltage of ˜3.61 V at a total current density of 100 mA/cm² (FIG. 17), which indicated that the excessive temperature would suppress ECO₂R and make the HER dominant. Thus, the following ECO₂R tests were conducted at 60° C. (unless otherwise noted).

In the pure-H₂O-fed APMA MEA system, at 150 mA/cm², SS-Ag achieved CO and H₂ FEs of ˜93% and 6%, respectively (FIG. 18A). The cell voltage was ˜3.88 V (without iR compensation for all tests in the MEA cell throughout the text). Although this system could effectively suppress carbonate formation, the low current density (150 mA/cm²) resulted in a low CO₂ conversion, drastically limiting its viability.

To increase CO₂ conversion, we first decreased the CO₂ inlet flow rate from 30 to 2 sccm (FIG. 18B). As expected, CO₂ conversion gradually increased from 3.23% to 42.97% (FIG. 18C). However, the CO FE and partial current density gradually decreased as decreasing CO₂ inlet flow rate (FE_CO: from ˜92.70% to 82.26%; j_CO: from ˜139.04 to 123.40 mA/cm²) and those of H₂ gradually increased (FE_H2: from ˜6.49% to 17.29%; j_H2: from 9.74 to 25.94 mA/cm²) (FIGS. 18D and 18E). These indicated that the side product (H₂) FE increased at the expense of that of the product (CO) in this CO₂ mass transport limited regime (i.e., low CO₂ flow rate). The cell voltage also slightly decreased (from ˜3.88 to 3.49 V) due to the favorable thermodynamic driving force for HER (FIGS. 18B and 18C). These showed that exceedingly decreasing CO₂ inlet flow rate could decelerate CO₂ mass transfer kinetics, causing the low CO₂/*CO coverage on the catalyst surface and thus reducing ECO₂R activity and increasing HER.

For the ECO₂R system, CO₂ mass transfer needs to be further revolutionized for a high CO₂ conversion. Since CO₂ and the key intermediate *CO coverage on the catalyst surface is determined by the overall flux of species both to and from the GDE surface, we re-designed and assembled HDF-GDE by screen printing to increase the CO₂/*CO coverage, thereby increasing ECO₂R current density and ultimately improving CO₂ conversion efficiency (FIG. 19), rather than based on the traditional GDE (just like carbon paper or porous polytetrafluoroethylene (PTFE)).

To strengthen the mechanical strength of HDF-GDE, the stainless-steel mesh (SSM) was used as the supporting skeleton of the catalyst sheet (FIGS. 20A, 21 and 22). The porous SS-Ag HDF-GDE showed hydrophobicity (FIGS. 20A and 20B) and its thickness was controlled to ˜300 μm. SS-Ag HDF-GDE only contains the catalyst and an SSM skeleton located in the middle layer of the catalyst, which can allow CO₂ to directly contact the catalyst without passing through the substrate (such as carbon paper and PTFE). This can increase the overall flux of CO₂/*CO both to and from the HDF-GDE surface, and thus, accelerate CO₂ mass transfer kinetics.

For HDF-GDEs, the cathode catalyst (SS-Ag or SS-Cu) was printed on the SSM skeleton by screen printing, followed by drying in a vacuum overnight. After that, the SSM-skeleton-supported catalyst was pressed at 1000 pounds of pressure for 2 h and the thickness of the HDF-GDE was controlled to ˜300 μm. Finally, it was calcined at 350° C. for 30 min. The printing ink was prepared by mixing the prepared catalyst and PTFE solution in an organic solution consisting of α-terpineol, 2-(2-butoxyethoxy)ethanol and acrylic resin with an approximate weight ratio of 65:15:20. The loading of the catalyst in HDF-GDE was controlled to about 10 mg/cm².

Upon acquiring the expected SS-Ag HDF-GDE, we evaluated its ECO₂R performance in the pure-H₂O-fed APMA MEA cell including an Ir counter electrode. The SS-Ag HDF-GDE still maintained high CO FE (˜90%) (FIG. 20C). More importantly, the peak current density was significantly increased to 400 mA/cm² with a cell voltage of ˜3.91 V. Compared with SS-Ag assembled on the traditional carbon paper GDE under identical test conditions (e.g., 30 sccm CO₂ inlet flow rate), although the SS-Ag HDF-GDE showed a negligible decline in CO FE (from ˜93% to ˜90%) and a slight increase in cell voltage (from ˜3.88 to ˜3.91 V) (FIGS. 20D and 20F), the peak current density was increased from 150 to 400 mA/cm². And H₂ FE was effectively suppressed, especially at the high current density (from ˜70% to ˜14% at 500 mA/cm²) (FIG. 20E). These indicated that more CO₂/*CO could occupy active sites in SS-Ag HDF-GDE while decreasing *H coverage on the catalyst surface, which improved the ECO₂R activity and suppressed HER. As a result, the CO₂ conversion efficiency in SS-Ag HDF-GDE was significantly increased by ˜2.6 times (from ˜3.2% to ˜8.5%) at the peak CO FE without decreasing CO₂ inlet flow rate (FIG. 20G), compared with that of the traditional GDE (SS-Ag carbon paper GDE).

Additionally, we carried out in-situ Raman experiments to study ECO₂R product (CO/*CO) concentration on different GDEs (FIG. 20H) because if HDF-GDE could facilitate CO₂ mass transfer and diffusion, the CO₂ concentration on the HDF-GDE surface would be higher than that on the carbon paper GDE, which can cause the higher CO/*CO concentration during ECO₂R. For SS-Ag HDF-GDE, the peak intensity of CO/*CO located at ˜237 cm⁻¹ is significantly higher than that based on the SS-Ag carbon paper GDE, which directly proved the overall flux of species both to and from the HDF-GDE was increased (HDF-GDE significantly accelerated CO₂ mass transfer kinetics). The high-efficiency CO₂ mass transfer could suppress the accumulation of *H through the transfer of H₂O on the catalyst surface, resulting in high CO₂/*CO coverage on the catalyst surface, improving the peak current density, and significantly increasing CO₂ conversion.

After demonstrating ECO₂R-to-CO in the pure-H₂O-fed APMA MEA cell with an SS-Ag HDF-GDE area of 1×1 cm², we designed and customized a cell stack containing six MEA cells (each MEA cell comprising the electrode described herein, Ir counter elextrode, and APMA with an area of 150 cm²) stacked in a series circuit structure to validate the practical application possibilities of HDF-GDE combined the pure-H₂O-fed APMA MEA system (FIG. 23A-23C). The scale-up demonstration and stability measurement can reflect its practical viability, further advancing the progress of the ECO₂R technology toward industrial deployment.

In the process of scaling up, a variety of engineering problems happen. For example, under high-power operation, the temperature at the MEA core can be significantly increased due to resistive heating, and the uniform voltage distribution throughout the stack is necessary for the series circuit. Since the power of the scaled-up pure-H₂O-fed APMA MEA cell stack assembled with SS-Ag HDF-GDE was scaled up to a kilowatt level, the initial temperature of pure H₂O anolyte was reduced to 30° C. to balance the temperature at the MEA core and the CO₂ inlet flow rate was increased to 3000 sccm.

To examine the stack's durability, the scaled-up pure-H₂O-fed APMA MEA cell stack assembled with SS-Ag HDF-GDEs was operated for 1000 h at a total current of 60 A (FIG. 23D). The full-cell-stack voltage remained remarkably stable between 21 and 22 V and the operation power was up to ˜1.29 kW. Six SS-Ag HDF-GDEs gave a relatively stable CO FE of ˜89-80% throughout the stability test. The slight voltage increase and CO FE decrease could be attributed to the mild GDE flooding under the high power.

The CO₂ conversion in this kW-scale cell stack assembled with SS-Ag HDF-GDEs was up to ˜81% at a CO₂ inlet flow rate of 3000 sccm, which was ˜25 times more than that of the SS-Ag carbon paper GDE (˜3.2%) in single pure-H₂O-fed APMA MEA cell (FIG. 20G). Moreover, we also investigated the stability of SS-Ag with SFs and GBs. HAADF-STEM images showed that the SS-Ag retained its structural integrity (GBs and SFs) after 1000 h of ECO₂R (FIG. 24). The stable SS-Ag catalyst, durable pure-H₂O-fed MEA system, and effective HDF-GDE synergistically boosted ECO₂R performance.