Electrochemical CO2 Reduction to Methane

Abstract

Nation-modified electrodes for the CO2 reduction reaction (CO2RR) to hydrocarbon products. Depending on the thickness of the Nation membrane and its admixture with other polymers, CO2 reduction occurs principally at the electrode-polymer interface. A Nation overlayer of 15 μm on a Cu electrode enables an extraordinarily high yield of CH4 production (88% Faradaic efficiency) at a low overpotential (540 mV). Other embodiments directed to admixtures of Nation and other polymers and/or cocatalysts, various metal substrates and electrolyte solutions which comprise an aprotic solvent in addition to a bicarbonate solution show impact on the Faradaic efficiency, yield and carbon-based products produced by the present invention.

Description

FIELD OF THE INVENTION

This invention relates to the production of methane and other carbon-based chemical products in electrochemical reactions involving the reduction of carbon dioxide. This invention is also directed to polymer coated metal substrates (electrodes) which find use in reducing carbon dioxide/bicarbonate to hydrocarbons, organic acids and alcohols, among other carbon-based products.

BACKGROUND AND OVERVIEW OF THE INVENTION

The accelerated increase of CO₂ concentrations in the atmosphere due to anthropogenic activities is causing a host of economic and environmental issues such as coastal flooding, increased catastrophic weather events, shifting agricultural productivities, and decreased biodiversity. The global CO₂ concentration measured at the Mauna Loa Observatory in April 2021 was 418 ppm.¹ In the 1960s, CO₂ levels increased approximately 0.6 ppm per year, and this rate rose to approximate 2 ppm per year in the last decade.² To combat rising global CO₂ levels in a world with an economy heavily dependent upon fossil fuels, chemical carbon mitigation aims to capture atmospheric CO₂ and convert it to value-added products.³ Electrochemical reduction of CO₂ to synthetic fuels using renewable energy sources is a promising approach to store energy into chemical bonds for industrial applications⁴ and is a renewable and efficient method of reducing CO₂ to various products based on multiple electron transfer mechanisms.^5,6,7,8

Electrochemical CO₂ reduction has been of interest for many decades because it is a viable pathway to produce synthetic fuels in aqueous electrolytes and at room temperatures. This method presents a promising path towards establishing a carbon-neutral cycle.^9,10 However, there are still major drawbacks that limit the commercialization of CO₂ reduction catalysts. The main problems associated with electrochemical CO₂ reduction are the high overpotentials required to reduce CO₂, poor product selectivity, and low Faradaic efficiencies due to the hydrogen evolution reaction (HER) that occurs at similar reduction potentials as CO₂.^11,12 The high overpotentials and poor product selectivity are due to the adsorption energies of key reaction intermediates.^13,14,15 Therefore, novel electrocatalysts for CO₂ reduction need to be designed that are robust and selective while lowering overpotentials.

Of all the catalysts tested for electrochemical CO₂ reduction, Cu-based materials are the only class of catalysts that have demonstrated high activity toward more reduced hydrocarbons and alcohols.^{10,11,12,16,17} In 1985, electrochemical CO₂ reduction on metal electrodes was pioneered by Hori and colleagues. Hori's work found that electrochemical CO₂ reduction on a Cu electrode produced hydrocarbons, mainly methane (CH₄) and ethylene (C₂H₄).^18,19,20 Jaramillo and coworkers found that Cu electrodes produced 16 different products, out of which 12 are C2 or C3 species.²¹ In an attempt to understand product selectivity and to elucidate the mechanism of CO₂ reduction, it was found that CO is a key intermediate in the formation of CH₄ and C₂H₄,²² and that the products of CO₂ reduction reaction depend on the metal's binding energy to CO.²¹ Based on these findings, one strategy for efficient electrochemical CO₂ conversion is to separate the process into two steps: CO₂ reduction to CO, followed by CO reduction to oxygenates and hydrocarbons.²³

Nafion is a sulfonated fluoropolymer which has been used in proton exchange membrane fuel cells (PEMFCs) and electrochemical CO₂ reduction reactions to separate the working electrode from the counter electrode to prevent the re-oxidation of products. In a previous study by Kim and coworkers, a thin layer of Nafion overlayer was introduced onto Pd-deposited TiO₂ nanoparticles, which enhanced the photo-conversion of CO₂ to methane and ethane under UV and solar irradiation without the use of electron donor.²⁵

SUMMARY OF THE INVENTION

The present invention is directed to CO₂ reduction on polymeric, Nafion-modified electrodes and contemplates a mechanism in which CO₂ reduction occurs in the presence of Nafion and Nafion based polymers. Previous work has only mixed catalysts with Nafion²⁶ or used Nafion to separate the two sides of electrochemical devices.²⁷ The present invention steps beyond the prior art in controlling proton transport by the thickness and composition of the Nafion layer on top of an electrode, that is, on an electrode surface in contact with an effective solution, preferably, an aqueous biocarbonate solution, which may include an aprotic reductively stable solvent such as acetonitrile (MeCN), dimethylformamide (DMF), dimethylacetamide (DMA), dimethylsulfoxide (DMSO), tetrahydrofuran (THF), propylene carbonate (PC) and alkyl nitriles (such as propylnitrile, butyl nitrile, adiponitrile, benzonitrile), among others.

In an embodiment, with an optimal Nafion layer, a copper electrode produces a remarkably high yield of methane (CH₄), (Faradaic efficiency of 88.0%) at −0.38 V vs. RHE (reversible hydrogen electrode), which is evidently the highest yield for CH₄ production from a CO₂ reduction electrocatalyst and an unexpected result. It is hypothesized that the Nafion increases the CH₄ yield by stabilizing an intermediate in which CO* is bound to the electrode surface and allows reduction of the CO intermediate to methane. Additional experiments show that providing the Nafion in admixture with at least one additional polymer at varying weight percentages, such as polyvinylidene fluoride (PVDF), polyvinylpyrrolidine (PVP), polyethyleneglycol (PEG), polyvinylalcohol (PVA), polyethyleneimine (PEI), polytetrafluoroethylene (PTFE) and mixtures thereof, especially PVDF and PTFE, among others, enhances the production of alternative carbon products from CO₂ (bicarbonate solution) such as formic acid (HCOOH), ethanol, ethylene, propylene and 1-propanol, among others. Often, when a polymer is admixed with Nafion, the polymer has a CO₂ gas permeability ranging from 5×10⁻¹⁵ mol-cm/cm²-s-Pa to 5×10⁻¹⁸ mol-cm/cm²-s-Pa. Among these polymers are the highly permeable fluoropolymers polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), which are characterized as having CO₂ permeabilities of 2.16×10⁻¹⁷ mol-cm/cm²-s-Pa and 5.15×10⁻¹⁶ mol-cm/cm²-s-Pa, respectively. Nafion has a CO₂ permeability of 8.70×10⁻¹⁶ mol-cm/cm²-s-Pa. See, Flaconneche, et al., Oil Gas Sci. Technol.-Rev. IFP, 2001, 56(3), 261-278; Ren, et al., J. Electrochem. Soc., 2015, 162(10), F1221-F1230; and Giacobbe, et al., Matt. Lett., 1990, 9(4), 142-146. In still other embodiments, the polymer is admixed with nanoparticles or nanowires of cocatalysts such as copper (metallic), cuprous oxide (Cu₂O), cupric oxide (CuO), Zn, zinc oxide (ZnO) or silver (Ag) or other metals to influence the Faradaic efficiency and/or the product mixture obtained from practicing the present invention.

In an embodiment, the invention is directed to metal substrate electrodes which are uniformly coated with polymeric materials comprising of Nafion polymer, alone or in admixture with other polymers and/or cocatalysts as described herein, which facilitates the efficient reduction of carbon dioxide into reduced carbon-containing chemical compounds including hydrocarbons (e.g., methane, ethane, propane, ethylene and/or propylene), organic acids and alcohols such as methanol, ethanol and 1-propanol, among others. Polymers (principally as dispersions of Nafion or Nafion and another polymer as described herein ranging from 1% to 20-25% by weight polymer, often about 5-15% by weight polymer in aqueous solvent) are deposited onto metal substrates at uniform thicknesses ranging from 1 μm to 90-100 μm. Often the polymer coating has a uniform thickness of 1-30 μm, more often 1-20 μm or 2-15 μm (for Nafion polymers) and 20 to 90-100 μm, often 20-90 μm (for Nafion/other polymer admixtures) using methods which are well known in the art, such as drop-casting, spin coating, spray-coating and blade-containing, among others known in the art. After deposition, the polymer coating is dried (e.g. air-dried or dried using hot air dryer) to remove aqueous solvent and what remains is a uniform coating of desired thickness.

The polymer composition of the coating is often solely or principally Nafion (to produce methane gas efficiently, but the Nafion may be admixed with another polymer such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinylpyrrolidine (PVP), polyethyleneglycol (PEG), polyvinylalcohol (PVA), polyethyleneimine (PEI) or mixtures thereof, especially PVDF and PTFE). In polymer overcoatings, the Nafion comprises between 5% and 100% by weight of the polymer coating, often more than 40-50% by weight of the polymer coating, with the remaining portion of the polymer coating comprising one or more of the above described polymers and/or cocatalysts in admixture with the Nafion. In embodiments, a cocatalyst such as nanoparticles ranging from 1-500 nm in diameter or nanowires of copper (metallic), cuprous oxide (Cu₂O), cupric oxide (CuO), Zn, zinc oxide (ZnO) or silver (Ag) or other metals is added to the Nafion polymer or Nafion polymer admixture in an effective amount, preferably ranging from 0.5% to 50%, often 5% to 30% often 10-15%, most often approximately 10% by weight of the polymer coating. The inclusion of cocatalyst may assist in facilitating (increasing the Faradaic efficiency) the production of and/or influencing the type of carbon products produced by the CO₂ reduction reaction produced by the present invention. The cocatalysts are incorporated into the polymer coating by mixing the nanoparticles with the polymer(s) to provide a uniform suspension by stirring, sonication and/or heating and the suspension of polymer and cocatalyst nanoparticles and/or nanowires are deposited on the metal substrate by drop-casting, spin coating, spray-coating and blade-containing, followed by drying to a uniform coating.

In an embodiment, the invention is directed to metal substrates (electrodes) which are coated with a uniform polymer coating and which function as electrodes in a CO₂ reduction apparatus or cell as depicted in FIG. 1 hereof for electrochemically converting CO₂ to carbon-containing chemical compounds pursuant to the methods which are described herein. In embodiments, the metal substrate, which can vary in size and thickness over a wide range from a thin foil to a substrate of substantial thickness, comprises carbon or a transition metal or a transition metal alloy or an intermetallic (i.e., an admixture of two or more metals, at least one of which is a transition metal). Transition metals include metals which are found in the d-block of the periodic table, which includes groups 3-12 and periods 4-7 of the periodic table. These atoms have between 0 and 10 d-electrons. In embodiments, the metal substrate comprises a late transition metal of groups 8-12 of the periodic table or an alloy thereof. In embodiments, the substrate comprises carbon or a late transition metal of groups 10-12, often copper, zinc, silver, gold, cadmium, nickel, palladium, platinum or an alloy or intermetallic thereof, more often copper, nickel or zinc or an alloy or intermetallic thereof. In embodiments, the metal substrate most often comprises copper, or a copper alloy or intermetallic, often brass (copper and zinc), bronze/phosphor bronze (copper and tin), naval brass (copper, zinc and tin), aluminum bronze (copper and aluminum), berylliumcopper (copper and beryllium), cupronickel (copper and nickel, optionally iron and/or manganese), nickel silver (copper with nickel and zinc), copper silver (copper with silver) and copper gold (copper with gold). All of the above-described transition metals or alloys are useful as electrodes for conducting CO₂ reduction reactions.

In embodiments, the substrate/electrode may be any size or thickness that is appropriate for the apparatus or cell, including experimental cells of relatively small size and commercial embodiments of great size for industrial applications. The size and thickness of the substrate does not impact the rate (current density) or extent of product and is otherwise not a critical feature for the process of the present invention and the electrochemical reaction to reduce CO₂ produces the same result because the reaction takes place on the electrode at the polymer-electrode interface. The current of the reaction scales linearly with the electrode area, so the reaction can work with any size substrate.

In embodiments, the electrolyte solution is a bicarbonate solution ranging from 0.01 M to 1.1 M bicarbonate (the solubility of bicarbonate in water at room temperature), although solutions of 0.05 M to 0.2 M are often used and 0.1M bicarbonate is most often used. In embodiments, an aprotic solvent is added to the electrolyte solution (at a volume percent ranging from 1% to 95% of the electrolyte solution, often 20-80% by volume or 40-60% by volume and most often approximately 50% by volume of the electrolyte solution to influence the organic products produced from the CO₂ reduction reaction. It was determined experimentally that the inclusion of an effective amount of an aprotic solvent such as acetonitrile (MeCN), dimethylformamide (DMF), dimethylacetamide (DMA), dimethylsulfoxide (DMSO), tetrahydrofuran (THF), propylene carbonate (PC) and alkyl nitriles (such as propylnitrile, butyl nitrile, adiponitrile, benzonitrile), among others tends to inhibit the reduction reaction to some extent (because fewer protons are available) resulting in products such as ethylene, methanol, ethanol, propanol and formic acid as well as carbon products of higher carbon number because of the promotion of CO* intermediate dimerization or trimerization at the electrode surface and the reduced proton concentration of the reduction environment.

In embodiments, to provide electrolyte solutions, CO₂ is often bubbled through a solution which may be buffered to maintain high local concentrations of bicarbonate within the ranges specified above. The pH of the electrolyte solution generally reflects the concentration of the bicarbonate in solution with solvent and/or buffer effects influencing the pH of the solution. At equilibrium solution concentration, the pH of the solution is approximately 6.8, although the pH may range substantially depending on the concentration of the biocarbonate and other components (other solvents/buffering agents) in solution.

In embodiments, the metal substrate/electrode comprises a uniform polymer layer on the surface of the substrate having a thickness ranging from 1 μm to 90-100 μm, with a polymer which contains Nafion as its sole polymeric component ranging from 1 μm to 30 μm, often 2 μm to 20 μm or 2 μm to 15 μm. In the case of admixtures of Nafion and other polymers, often fluoropolymers such as polyvinylidene fluoride (PVDF) and/or polytetrafluoroethylene (PTFE) or other polymers such as polyethyleneglycol (PEG), polyvinylalcohol (PVA) or polyethyleneimine (PEI) as described herein, the thickness of the coating on the metal substrate will often range from 20-100 μm and above, often 20-90 μm.

Generally, the CO₂ reduction reactions of the present invention are conducted within the apparatus or cell using a voltage ranging from −0.2 V to −2 V vs. RHE (reversible hydrogen electrode). The current (expressed as current density) which is used in the electrolytic processes to reduce CO₂ to carbon-based products as described herein ranges from 1-100 milliamps per cm², often 10-100 milliamps per cm².

In embodiments, a high amount of methane gas (CH₄) is produced using a uniform Nafion polymer (alone) overcoating ranging from 2 to 15 μm on a copper electrode (Faradaic efficiency of 50+%) at an effective voltage (very negative reduction potentials). In embodiments, methane gas (CH₄) is produced using a uniform Nafion polymer (alone) overcoating of approximately 15 μm on a copper electrode (Faradaic efficiency of 88.0%) at −0.38 V vs. RHE (reversible hydrogen electrode).

In embodiments, the inclusion of effective amounts of an additional polymer in admixture with Nafion (in embodiments, the polymer is polyvinylidene fluoride (PVDF), polyvinylpyrrolidine (PVP), polyethyleneglycol (PEG), polyvinylalcohol (PVA), polyethyleneimine (PEI), polytetrafluoroethylene (PTFE) or mixtures thereof) favors the production of formate at less negative reduction potentials.

In embodiments, the production of ethylene gas is favored when copper alloys are used, when the alloy electrode has a hydrophobic coating comprising an effective amount of PVDF in admixture with Nafion and when aprotic solvents as otherwise described herein (often acetonitrile) are used in effective amounts in combination with a bicarbonate in the electrolyte solution. Thus, the invention provides that methane gas formulation is favored using Nafion copolymer (in the absence of any other copolymer) of uniform thickness between 2 and 15 μm or 10 and 15 μm, more often approximately 15 μm at an effective voltage between −0.2 V and −2.0 V vs. RHE. In embodiments, the production of formate is favored in a hydrophobic polymer environment comprising a uniform overlayer of Nafion in combination with an effective amount of copolymer, especially PVDF, as described herein above. In embodiments, ethylene production is favored by the use of hydrophobic fluoropolymer (PVFD and/or PTFE) in admixture with Nafion on an alloy (often copper alloy) electrode. In embodiments, the inclusion of a nanoparticulate, nanowire cocatalyst or covalently bonded cocatalyst into the Nafion polymer or additional polymer may enhance the formation of CO* intermediates and methane and/or ethylene products, especially on copper electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a CO₂ reduction apparatus having a three electrode configuration for carrying out reduction of CO₂ to various carbon-based products pursuant to the present invention as otherwise disclosed herein. As illustrated in FIG. 1, an electrochemical apparatus or cell for the production of a gas such as methane from carbon dioxide comprises a body member or housing 10 that defines a chamber 12. A reference electrode 30 extends from a cap or cover member 16 through an insulating seal 18 axially down into the chamber 12. A distal end portion 20 of reference electrode 30 is disposed in a cavity or chamber extension 22 at the bottom of chamber 12. A working electrode 24 as described in detail herein is disposed at a lower end of cavity 22, sandwiched between a shoulder (not designated) of housing 10 and a base plate 28. A counter electrode 14, co-functioning with working electrode 24 extends into chamber 12 from cover member 16 and through insulator-seal 18. Electrically conductive structures 32 and 34 are provided in cover member 16 for operatively connecting reference electrode 30 and counter electrode 14 to a voltage source 36. Working electrode 24 is connected to voltage source 36 via a copper foil 42 disposed adjacent working electrode 24 for electrical conduction. Two port members or fittings 38 and 40 are fixed to housing 10 on opposite sides thereof and communicate with chamber 12. Carbon dioxide gas is fed into chamber 12 via port member or fitting 38, while gas containing electrochemical product such as methane is conveyed out of the cell housing 10 via port member or fitting 40. Working electrode 24 is a cathode for purposes of the voltage of −0.2 to −2 V. Counter electrode 14 serves as an anode. Direct current is principally used, reference electrode 30 serving to maintain a constant voltage between −0.2 and −2 volts. Alternatively, oscillating current (AC) could be applied.

The apparatus shown in FIG. 1 is presented as a three-electrode configuration comprising a working electrode (where reduction of CO₂ to carbon-based products pursuant to the present invention takes place), a reference electrode (which is used to maintain a constant voltage applied to the working electrode) and a counter electrode (which is used to as the counter electrode to the working electrode—in preferred aspects of the present invention as an anode counter to the working electrode, which is a cathode). In embodiments, the apparatus is a two-electrode configured cell with the reference electrode being eliminated from the apparatus.

FIG. 2A is a graph, specifically a linear sweep voltammogram of carbon without Nafion (top curve), with a 2 μm Nafion overlayer (middle curve, right side of figure), and with a 15 μm Nafion overlayer (lower curve, right side of figure), each in CO₂-saturated 0.1 M NaHCO₃ electrolyte carbon in CO₂-saturated 0.1 M NaHCO₃ electrolyte at a scan rate of 10 mV/s.

FIG. 2B is a graph, specifically a linear sweep voltammogram of copper foil without Nafion (top curve), with a 2 μm Nafion overlayer (middle curve, right side of figure), and with a 15 μm Nafion overlayer (lower curve, right side of figure), each in CO₂-saturated 0.1 M NaHCO₃ electrolyte carbon in CO₂-saturated 0.1 M NaHCO₃ electrolyte at a scan rate of 10 mV/s.

FIG. 3A is a graph of electrochemical impedance spectroscopy (EIS) of Nafion-modified carbon taken using a three-electrode configuration at −0.89 V vs. RHE in 0.1 M NaHCO₃ electrolyte saturated with CO₂.

FIG. 3B is a graph of electrochemical impedance spectroscopy (EIS) of Nafion-modified copper taken using a three-electrode configuration at −0.89 V vs. RHE in 0.1 M NaHCO₃ electrolyte saturated with CO₂.

FIG. 4A shows graphs, specifically linear sweep voltammograms of carbon and Cu foil (next to lowest and uppermost curves at extreme left of figure) and carbon and Cu foil modified with 15 μm of PVDF (next to highest and lowest curves at extreme left of figure) in CO₂-saturated 0.1 M NaHCO₃ electrolyte at a scan rate of 10 mV/s.

FIG. 4B is a pair of electrochemical impedance spectroscopy (EIS) plots of 15 μm of PVDF respectively on carbon and Cu, taken at −0.89 V vs. RHE.

FIG. 5 is a graph, showing linear sweep voltammograms of a carbon mesh electrode in 0.1 M NaHCO₃ saturated with CO₂ (lower line on left side) and 0.1 M NaHCO₃ adjusted to pH of 2.6 with HCl and saturated with CO₂ (upper line on left side) at a scan rate of 10 mV/s.

FIGS. 6A and 6C-6F are images, while FIG. 6B is a graph, showing surface characteristics of Nafion-modified electrodes. FIG. 6A is a scanning electrode microscopy (SEM) image, while FIG. 6B is an EDS spectrum. FIGS. 6C-F show EDS mapping of a Cu electrode modified with a 2 μm thick layer of Nafion. FIGS. 6D, 6E and 6F shows elemental mapping of the Cu electrode for fluorine (FIG. 6D), oxygen (FIG. 6E) and sulfur (FIG. 6F).

FIG. 7 is a cross-sectional scanning electron microscope (SEM) image of a 2 μm thick layer of Nafion on Cu foil.

FIGS. 8A, 8C, and 8D are images, w % bile FIG. 8B is a graph, also showing surface characteristics of Nafion-modified electrodes. FIG. 8A is a SEM image. FIG. 8B shows an EDS spectrum. FIGS. 8C and 8D show EDS mapping of Cu electrode modified with 8 μm of Nafion.

FIGS. 9A, 9B, and 9C are diagrams depicting three possibilities of CO₂ reduction occurring at a polymer-electrolyte interface (FIG. 9A), a polymer-electrode interface (FIG. 9B), or an electrode-electrolyte interface (FIG. 9C).

FIG. 10A is a graph showing Faradaic efficiencies for formate, CH₄, and CO for all catalysts at −0.89 V, while FIG. 10B is a graph showing partial charge densities, and FIG. 10C is a graph showing rates of product formation on bare substrates, 15 μm Nafion-modified substrates, and 15 μm PVDF-modified substrates.

FIG. 11A is a graph showing Faradaic efficiencies as a function of Nafion thickness on Cu foil substrate at −0.89 V vs. RHE, while FIG. 11B is a graph showing partial charge densities, and FIG. 11C is a graph showing rates of product formation as a function of Nafion thickness on substrate.

FIG. 12A is a graph showing Faradaic efficiencies of CO, CH₄, and HCOOH as a function of voltage for a 15 μm thick Nafion overlayer on Cu foil. FIG. 12A is a graph also showing Faradaic efficiencies of CO, CH₄, and HCOOH as a function of partial current density for a 15 μm thick Nafion overlayer on Cu foil. FIG. 12A is a graph also showing Faradaic efficiencies of CO, CH₄, and HCOOH as a function of rates of product formation for a 15 μm thick Nafion overlayer on Cu foil.

FIG. 13 is a diagram showing a proposed mechanism of CO₂ reduction to CO and CH₄. CO₂ adsorbs onto the electrode surface, with the addition of 2 H⁺ and 2 e⁻ is reduced to a CO intermediate with two possible resonance structures (shown in dotted box). Both structures are capable of either being released as gaseous CO or further reduction to CH₄.

FIG. 14 is a diagram showing proposed mechanism of CO₂ reduction to CH₄ using a polymer-modified Cu electrode. CO₂ is reduced to CO at the polymer-electrode interface. CO that is not bound to the electrode surface is released as a product, and CO that is bound to the electrode surface is denoted as a ═C═O* intermediate. Nafion helps stabilize this intermediate allowing for the subsequent reduction to CH₄ while preventing CO release.

FIGS. 15A and B are cross-sectional SEM images of a Cu electrode modified with a PVDF-Nafion polymer overlayer. FIG. 15A shows a 100 μm thick polymer layer and FIG. 15 B shows a 20 μm thick polymer layer.

FIG. 16A shows a cross-sectional SEM image of a Cu electrode modified with a PVDF-Nafion polymer overlayer. FIG. B-D show EDS elemental mapping of F (FIG. 16B), O (FIG. 16C) and Cu (D) of the same Cu electrode modified with a PVDF-Nafion polymer overlayer.

FIG. 17A shows photographic image of the contact angle of a water droplet on a bare Cu electrode. FIGS. 17B-D show photographic images of the contact angle of a water droplet on a Cu electrode modified with Nafion-PVDF overlayers containing 30 wt. % PVDF (FIG. 17B), 52 wt. % PVDF (FIG. 17C), and 100 wt. % PVDF (FIG. 17D).

FIG. 18 shows linear sweep voltammograms (LSV) of bare Cu (black), Cu modified with 15 μm Nafion (red), Cu modified with 52, 60, and 100 wt. % PVDF in Nafion overlayer (blue, green, and purple) in CO₂-saturated 0.1 M NaHCO₃ electrolyte at a scan rate of 10 mV/s.

FIG. 19A-C shows the Faradaic efficiencies (FIG. 19A), the partial charge density over the 1 hour experiment (FIG. 19B), and rate of formation (FIG. 19C) for formate, CO, and CH₄ produced from 20-90 μm PVDF-Nafion-modified Cu at −0.89 V vs. RHE. The PVDF-Nafion overlayer becomes increasingly thick as the weight percentage of PVDF increases.

FIG. 20A-C shows the Faradaic efficiencies (FIG. 20A), the partial charge density over the 1 hour experiment (FIG. 20B), and rate of formation (FIG. 20C) for formate, CO, and CH₄ produced from 52 wt. % PVDF in Nafion modified Cu at different voltages.

FIG. 21A-C shows the Faradaic efficiencies (FIG. 21A), the partial charge density over the 1 hour experiment (FIG. 21B), and the rate of formation (FIG. 21C) for gas products produced from unmodified Cu in acetonitrile/bicarbonate electrolyte at −0.89 V vs. RHE.

FIG. 22A-C shows the Faradaic efficiencies (FIG. 21A), the partial charge density over the 1 hour experiment (FIG. 21B), and rate of formation (FIG. 21C) for liquid products produced from unmodified Cu in acetonitrile/bicarbonate electrolyte at −0.89 V vs. RHE.

FIG. 23A-C shows the Faradaic efficiencies (FIG. 23A), the partial charge density over the 1 hour experiment (FIG. 23B), and rate of formation (FIG. 23C) for gas products produced from Cu modified with 15 μm Nafion overlayer in acetonitrile/bicarbonate electrolyte at −0.89 V vs. RHE.

FIG. 24A-C shows the Faradaic efficiencies (FIG. 24A), the partial charge density over the 1 hour experiment (FIG. 24B), and rate of formation (FIG. 24C) for liquid products produced from Cu modified with 15 μm Nafion overlayer in acetonitrile/bicarbonate electrolyte at −0.89 V vs. RHE.

FIG. 25 shows the total carbon-containing products produced from an unmodified Cu electrode (FIG. 25A) and a Cu electrode modified with 15 μm Nafion overlayer (FIG. 25B) in varying concentrations of acetonitrile in the bicarbonate electrolyte at −0.89 V vs. RHE.

FIG. 26 shows a proposed mechanism of high formate production using Cu modified by 52 wt. % PVDF in Nafion polymer overlayer (FIG. 26A). The blue curved lines represent Nafion and the grey spheres represent PVDF. In this hydrophobic environment, formate is the preferred product because formate is the only CO₂ reduction product that does not generate water, and generating water is unfavorable in a hydrophobic environment. Proposed mechanism of ethylene formation on a Cu electrode in an acetonitrile/bicarbonate electrolyte (FIG. 26B). Adding an aprotic solvent decreases the total proton concentration, which subsequently decreases the rate of M-CO protonation. This aprotic environment promotes M-CO and M-CO coupling to generate C2+ products instead of protonating M-CO to generate CH₄.

FIG. 27 shows a proposed mechanism of CO₂ reduction to C₂H₄ using a Nafion-modified Cu electrode. The black dots embedded in the Nafion represents a hydrophobic polymer that slows proton transfer. CO₂ is reduced to CO at the polymer-electrode interface, and without rapid proton transfer to protonate the CO* intermediate, the stabilized CO* intermediates are allowed to dimerize which eventually produces C₂H₄. Trimerization (mechanism not shown) to selectively produce C3 products is also hypothesized at even slower proton transfer rates.

FIG. 28 shows the CO Faradaic efficiency as a function of Nafion thickness on brass foil substrate at −0.89 V vs. RHE over 1 hour experiment. Brass composition: 62% Cu, 37% Zn, trace amounts of Fe (<0.15%), Pb (<0.08%), and Sn (<0.005%).

FIG. 29 shows the Faradaic efficiencies of CO and CH₄ as a function of voltage on brass foil over a 1 hour experiment.

FIG. 30 shows the Faradaic efficiencies over a 1 hour experiment for CO and C₂H₄ produced from 20-90 μm PVDF-Nafion-modified brass at −0.89 V vs. RHE. The PVDF-Nafion overlayer becomes increasingly thick as the weight % PVDF increases.

FIG. 31 shows Faradaic efficiencies over a 1 hour experiment for CO and C₂H₄ produced from unmodified brass foil in acetonitrile/bicarbonate electrolyte at −0.89 V vs. RHE.

FIG. 32 shows Faradaic efficiencies over a 1 hour experiment for CO, CH₄, and C₂H₄ produced from brass foil modified with 15 μm Nafion in acetonitrile/bicarbonate electrolyte at −0.89 V vs. RHE.

FIG. 33 shows Faradaic efficiencies over the 1 hour experiment for CO, CH₄, HCOOH, and CH₃OH produced from Zn foil modified with 15 μm Nafion in acetonitrile/bicarbonate electrolyte at −0.89 V vs. RHE.

FIG. 34 shows CO Faradaic efficiency as a function of Nafion thickness on Zn foil substrate at −0.89 V vs. RHE over 1 hour experiment.

FIG. 35 shows Faradaic efficiencies over the 1 hour experiment of CO produced from 20-90 μm PVDF-Nafion-modified Zn at −0.89 V vs. RHE. The PVDF-Nafion overlayer becomes increasingly thick as the weight % PVDF increases.

FIG. 36 shows Faradaic efficiencies over a 1 hour experiment for CO and C₂H₄ produced from 52 wt. % of polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polyvinyl alcohol (PVA) and polyethylenimine (PEI) in Nafion on a Cu substrate at −0.89 V vs. RHE.

FIG. 37 shows Faradaic efficiencies over a 1 hour experiment for CO and C₂H₄ produced from various polymer blends in Nafion on a Cu substrate at −0.89 V vs. RHE. (A=100 wt. % Teflon on Cu, B=50 wt. % each of Teflon and PVDF on Cu, C=52 wt. % Teflon in Nafion on Cu, D=40 wt. % each of Teflon and PVDF in Nafion on Cu, E=64 wt. % Teflon and 30 wt. % PVDF in Nafion on Cu, F=30 wt. % Teflon and 64 wt. % PVDF in Nafion on Cu.).

FIG. 38 shows Faradaic efficiencies over the 1 hour experiment for CO, CH₄, C₂H₄, and HCOOH produced from nanoparticulate Cu₂O on various metal substrates at −0.89 V vs. RHE. (A=10 wt. % Cu₂O dispersed in Nafion on Cu, B=10 wt. % Cu₂O dispersed in Nafion on Zn, C, D, E=Cu₂O thin film on Zn. Cu, and Ni metal substrates, respectively.)

FIG. 39 shows electrocatalysis at a polymer electrode interface with an embedded cocatalyst.

DETAILED DESCRIPTION OF THE INVENTION

It is noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the,” include plural referents unless expressly and unequivocally limited to one referent. Thus, for example, reference to “a compound” can include two or more different compounds depending on the context of the use of the term. As used herein, the term “include” and its grammatical variants are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that can be substituted or other items that can be added to the listed items.

The term “effective” is used to describe an amount of a component, an element, an energy source, a reactant, precursor or product which is used in or produced by the present invention to produce an intended result.

The term “uniform” is used to describe the polymer overcoating which is used to coat the metal substrate pursuant to the present invention. As used, a uniform overcoating is a coating on a metal substrate pursuant to the present invention which has a measured thickness at all areas of the coating within 10%± of the designated thickness.

The term “Faradaic efficiency” (synonymously faradaic yield, coulombic efficiency or current efficiency) is used to describe the efficiency with which charge (electrons) is transferred in a system facilitating an electrochemical reaction, in the present invention, the reduction of CO₂ to one or more carbon-containing products. In other words, Faradaic efficiency is the percent yield of product based on the number of electrons transferred during the reaction. A higher percentage yield of product using a lower number of transferred electrons provides higher Faradaic efficiency. In the present invention, the number of electrons is the limiting reactant, not carbon dioxide and a higher Faradaic efficiency is the desired outcome. Many CO₂ reduction catalysts have low Faradaic efficiencies for carbon products in aqueous electrolytes because (fast) electron transfer can also occur to protons in water to create hydrogen gas, reducing the yield of the desired carbon product. The regulated proton transfer rates with the polymer overcoatings using in the present invention very often increases the Faradaic efficiency of the CO₂ reduction reaction(s), a particularly favorable and unexpected result.

The inventors found that there was a relationship between the Faradaic efficiency of the CO₂ reduction reaction and the selectivity of the carbon-based products which are produced reflective of the polymeric overcoating and electrolyte solution used. For example, the inventors found that an extraordinarily high amount of CH₄ (at 88% Faradaic efficiency) is generated using a Cu electrode modified with a 15 μm Nafion overlayer at −0.4 V vs. RHE. In contrast to the present invention, on unmodified metal electrodes, a more negative voltage is required to give rise to higher Faradaic efficiencies of CH₄. As shown in the examples section hereof, high formate Faradaic efficiencies can be achieved by using PVDF-Nafion overlayers at a less negative voltage. Formate is favored in a hydrophobic environment because producing water as a CO₂ reduction product is unfavorable. Since formate is the only CO₂ reduction product in which water is not produced concomitantly, a hydrophobic electrode favors formate production pursuant to the present invention.

Thus, as shown in the examples section, a copper electrode modified with 52 wt. % PVDF in Nafion at −0.14 V vs. RHE gives reasonably high formate yield (58%). This yield of formate is fairly high for a Cu-based catalyst, and most previous works used other metals to produce high formate yields such as 81% and 98%. There is some literature precedent, however, for Cu-based catalysts that achieve high formate yields including a Cu—Au catalyst that produces formate at a 81% Faradaic efficiency at −0.4 V vs. RHE.⁵ Cu₂O nanoparticle films also generated formate at 98% Faradaic efficiency under high pressure (≥45 atm) at −0.64 V vs. RHE. The authors of this work also found that at more negative potentials formate decreased.⁶ Comparing the present invention to previous studies it seems that formate production is favored at lower voltages, especially around from −0.4 to −0.6 V vs. RHE.

Reasonably high yields of C₂H₄ (75%) is generated when an alloy substrate is modified by PVDF-Nafion overlayers on a Cu—Zn alloy (brass, 62% Cu and 37% Zn). In addition, C₂H₄ is produced in the presence of acetonitrile in the bicarbonate electrolyte (higher volume percent, ie. 75% of acetonitrile generates more C₂H₄ than lower volume percent). Lastly, Cu electrodes modified with Teflon-Nafion overlayers favor the production C₂H₄ while simultaneous hindering CO production. Chen and coworkers fabricated Cu, Cu—Ag, and Cu—Sn alloy films that exhibited high Faradaic efficiencies (60%) for C₂H₄ production.⁷ The origin of the high C₂H₄ production is attributed to the presence of alloys, which leads to the increased CO density on the electrode surface. In addition, higher local pH near the electrode surface also contributes to C₂H₄ production because CO* dimerization and C₂H₄ formation.

In addition, alcohols such as methanol, ethanol, and 1-propanol are generated in the presence of acetonitrile/bicarbonate electrolyte on unmodified Cu electrodes or Cu electrodes modified with 15 μm Nafion overlayer.

The term “Nafion” is used to describe Nafion (CAS Name Perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid-tetrafluoroethylene copolymer, also IUPAC name 1,1,2,2-tetrafluoroethene; 1,1,2,2-tetrafluoro-2-[1,1,1,2,3,3-hexafluoro-3-(1,2,2-trifluoroethenoxy)propan-2-yl]oxyethanesulfonic acid), which is a sulfonated fluoropolymer which has a hydrophobic perfluorinated polytetrafluoroethylene (PTFE) backbone with side chains terminated by strongly acidic hydrophilic sulfonic acid groups. The protons on the sulfonic acid groups are responsible for providing proton conductivity. Nafion can be formulated as a dispersion in water/alcohol (ethanol/I-propanol) in the acidic form. A preferred dispersion of Nafion, Nafion D520, D521, D2020 and D2021 (with Nafion polymer in the dispersion ranging from 5% by weight up to 20% by weight) can be purchased from the Chemours Company, Wilmngton Del., USA. Nafion may be admixed with other polymers to form admixtures which are used as overcoatings of the metal substrate in the present invention.

While not being limited by way of theory, it appears that Nafion has provided enhanced efficiency of CO₂ reduction in the present invention for at least the following three reasons, among others. First, Nafion is a gas permeable superacid and an excellent proton conductor, and it is believed that the Nafion layer enhances the local activity of protons on the surface of the metal substrate which are necessary for increased Faradaic efficiency of CO₂ reduction. Second, CO* is believed to be stabilized between the substrate/polymer interface, which would favor electron transfer to the intermediates to form more highly reduced products, especially when considering the enhanced local activity of protons by Nafion. Third, Nafion is stable against photocatalytic oxidation and is inert toward photoinduced redox reactions, thus forcing the equilibrium reactions toward reduction products rather than back to oxidized precursors.

The term “overpotential” is used to describe the difference in potential which exists between a thermodynamically determined reduction potential of a half-reaction and the potential at which the redox event is experimentally observed. The term is directly related to a cell's voltage efficiency. In an electrolytic cell the existence of overpotential indicates that the cell requires more energy than is thermodynamically expected to drive a reaction. The quantity of overpotential is specific to each cell design and varies across cells and operational conditions, even for the same reaction. Overpotential is experimentally determined by measuring the potential at which a given current density is achieved.

Mechanism of Action

A following description of proposed mechanisms for CO₂ reduction pursuant to the present invention provides a basis for the formation of methane, formate, ethylene and other carbon-based produced according to the present invention. FIGS. 14, 26 and 27 provide proposed mechanisms for methane, formate and ethylene formation. The Nafion overcoat stabilizes the M-CO intermediate, which allows for subsequent protonation to methane. Formate is produced when the electrode is hydrophobic (facilitated by higher concentrations of Teflon and/or PVDF in admixture with Nafion) because CO₂ reduction to formate does not require the production of water. Ethylene is favored as a carbon-based product when the electrolyte solution (bicarbonate source) comprises substantial quantities of acetonitrile by volume. The rate determining step (RDS) of ethylene formation is the dimerization of the CO* intermediate.

FIG. 14 shows the proposed mechanism of CO₂ reduction to methane (CH₄) using a polymer modified copper electrode. As shown, CO₂ is reduced to CO at the electrode-polymer interface. CO that is not bound to the electrode surface is released as a product. Nafion stabilizes this intermediate allowing for the subsequent reduction to methane while preventing CO release from the surface of the electrode.

FIGS. 26A and 26B show the proposed mechanism of high formate production using Cu modified by 52 wt. % PVDF in Nafion polymer overlayer (FIG. 26A). The blue curved lines represent Nafion and the grey spheres represent PVDF. In this hydrophobic environment, formate is the preferred product because formate is the only CO₂ reduction product that does not generate water, and generating water is unfavorable in a hydrophobic environment. Proposed mechanism of ethylene formation on a Cu electrode in an acetonitrile/bicarbonate electrolyte (FIG. 26B). Adding an aprotic solvent decreases the total proton concentration, which subsequently decreases the rate of M-CO protonation and favoring the production of higher carbon products, especially ethylene and alcohols such as methanol, ethanol and 1-propanol. This aprotic environment promotes M-CO and M-CO coupling to favor the production of C2+ products instead of protonating M-CO to generate CH₄.

FIG. 27 shows a proposed mechanism of CO₂ reduction to C₂H₄ using a Nafion-modified Cu electrode. The black dots embedded in the Nafion represents a hydrophobic polymer that slows proton transfer. CO₂ is reduced to CO at the polymer-electrode interface, and without rapid proton transfer to protonate the CO* intermediate, the stabilized CO* intermediates are allowed to dimerize which eventually produces C₂H₄. Trimerization (mechanism not shown) to selectively produce C3 products is also believed to occur at even slower proton transfer rates.

The above-described mechanisms are useful in predicting carbon-based products that can be produced pursuant to features of the present invention. For example, methane (CH₄) production is favored when electrodes are modified with a Nafion overlayer and on unmodified electrodes at a very negative reduction potentials. The Nafion overlayer or coating provides unexpectedly high Faradaic efficiency for the production of methane. Formate is favored with hydrophobic electrodes (PVDF-Nafion overlayer) and at less negative reduction potentials. Ethylene (C₂H₄) is favored when Cu alloys are used, when the alloy electrode is hydrophobic (PVDF-Nafion overlayer), and when aprotic solvents are used in conjunction with the bicarbonate electrolyte. The inventors have concluded that formate can be further enhanced by creating a hydrophobic environment. C₂H₄ production can be enhanced by hydrophobic fluoropolymer-Nafion overlayers on an alloy electrode.

The following non-limiting examples further describe and support embodiments and further aspects of the present invention.

EXAMPLES

First Set of Experiments

The first set of experiments presented herein are directed to a study of Nafion-modified electrodes for the CO₂ reduction reaction (CO₂RR) to hydrocarbon products. Nafion, described herein above, is a sulfonated polymer possessing high proton conductivity. By varying the thickness, substrates, and voltage, the inventors performed a detailed study of the effect of Nafion on metal and carbon mesh electrodes for CO₂ reduction. These studies allowed for the elucidation of the mechanism in which CO₂ reduction occurs on these Nafion-modified electrodes. Depending on the thickness of the polymeric membrane surface, CO₂ reduction occurs at either the polymer-electrolyte interface or electrode-polymer interface. It was determined that a Nafion overlayer of 15 μm on Cu electrode enables extraordinary high yield of CH₄ production (88% Faradaic efficiency) at a low overpotential (540 mV). To the best of our knowledge, this yield is the highest reported for electrocatalytic CO₂ reduction to CH₄ production at room temperature reported thus far. Other products detected include formate, CO, ethanol and methanol.

Experimental Procedure

Materials and Electrode Preparation.

Nafion D520 dispersion and carbon paper (AvCarb EP40T) were purchased from Fuel Cell Store. Cu and Zn foil were purchased from All-Foils, Inc, and Ni foil was purchased from Goodfellow, Inc. Sodium bicarbonate was purchased from Sigma Aldrich. CO₂ and CO were purchased from Airgas. Nafion-modified electrodes were fabricated by drop-casting Nafion (D520 Dispersion) directly onto the substrate.

Electrochemical Measurements and Material Characterization.

All electrochemical measurements were performed using a VSP-300 Biologic Potentiostat. All electrochemical data were collected versus a Ag/AgCl reference electrode and converted to the reversible hydrogen electrode (RHE) scale by V_{(vs. RHE)}=V_{(measured vs. Ag/AgCb}+0.21+0.059*6.8 (where 6.8 is the pH of solution). All values are reported versus RHE. To evaluate the CO₂ reduction activity of the thin films, the working electrodes were studied in 0.1 M sodium bicarbonate buffer sparged with CO₂ gas for at least 30 min using a one-compartment, three-electrode configuration (as set forth in FIG. 1 hereof). The thin films on carbon paper served as the working electrode, a Pt wire was used as the counter electrode, and a Ag/AgCl electrode was used as the reference electrode. Electrochemical impedance spectroscopy (EIS) was performed in 0.1 M sodium bicarbonate buffer sparged with CO₂ gas using a three-electrode configuration cell at −0.89 V vs. RHE. The frequency was varied from 200 kHz to 100 mHz sinusoidally with amplitude of 10 mV. Scanning electron microscope (SEM) images and energy-dispersive X-ray (EDS) analysis were obtained for each sample using a JEOL JSM-6010LA analytical SEM or a JEOL JSM-7100F field emission SEM operated using an accelerating voltage of 15 kV. Onset potentials were calculated by determining the voltage at which the current density reached 15% of the maximum current density for each linear sweep voltammogram.

Product Determination. Electrochemical reactions were performed chronoamperometrically at −0.89 V vs. RHE (and at −0.38 V, −0.13 V, and 0.12 V vs. RHE for voltage-dependent experiments) for one hour using carbon as a counter electrode in a beaker for determining liquid and solid products and Pt wire as a counter electrode in a custom-made cell for determining gas products. During chronoamperometry, CO₂ was continuously sparged through the solution at a rate of 5 cm³/min. Liquid products were quantified using a Varian 400 MHz NMR Spectrometer using DMF as an internal standard. The water in the reaction solution was evaporated under reduced pressure, and sodium formate along with other residual solids from the electrolyte were collected and dissolved in D₂O. Liquid products were extracted from the reaction solution using deuterated chloroform. Gas products were quantified using a SRI 8610C gas chromatograph equipped with a flame ionization detector (FID) and a methanizer. The limits of detection for formate, liquid products, and gas products were determined to be 11 μM, 85 μM, and 1 ppm, respectively.

FIGS. 2A and 2B show linear sweep voltammograms of carbon mesh and Cu substrates with and without Nafion overlayers in CO₂-saturated bicarbonate electrolyte undergoing electrochemical CO₂ reduction. Unmodified carbon mesh (FIG. 2A, uppermost curve) reaches a maximum current density of about −4 mA/cm² at −1.5 V vs. RHE and exhibits an onset potential of −0.29 V vs. RHE. In contrast, carbon mesh modified with a 2 μm and 15 μm thick Nafion overlayer both exhibit a decreased onset potential of −0.20 V and +0.25 V, respectively (FIG. 2A, middle and lower curves on the left side of the graph). This positive shift in onset potential upon addition of a Nafion overlayer is also observed with a Cu substrate. Cu electrodes modified with 2 and 15 μm of Nafion both showed decreased onset potentials of +0.10 V for Cu electrode modified with 2 μm of Nafion, and +0.40 V for Cu electrode modified with 15 μm of Nafion as compared to −0.19 V for the Cu electrode without Nafion. This consistent positive shift of onset potential signifies that the electrodes with increasingly thick Nafion layers are more efficient at reducing CO₂.

In addition to the positive shift of onset potential, the shapes of the LSVs for the carbon mesh and Cu substrates modified with 15 μm of Nafion are both relatively linear compared to the corresponding LSVs without Nafion, signifying that the electrochemical behavior of these electrodes are resistive. It is hypothesized herein that this increase in electrochemical resistance arises from impeded electron transfer through the thick Nafion layers. FIGS. 3A and 3B present the electrochemical impedance spectroscopy (EIS) data for Nafion-modified carbon and Cu electrodes, respectively, measured at −0.89 V vs. RHE. Impedance data was fitted using a Randles circuit (Equation 1), in which R_f and R_ct refer to solution resistance and charge-transfer resistance, respectively. The equation also contains C_dl the double-layer capacitance, and an electrochemical element of diffusion. Z_W. When the impedance data is fitted using Randles equation, R_s and R_ct values, as well as the diameter of the semicircle, provides information regarding the resistivity of the electrocatalyst. Nafion-modified Cu has a much larger R_ct value than bare Cu (Table 1), and R_ct increases as the Nafion overlayer increases, demonstrating increased resistivity of Nafion-modified Cu. This trend correlates well with the LSV from FIG. 2B (lowermost curve) because the resistor-like behavior of thick Nafion-modified Cu possesses high resistivity. In contrast, the diameter of the semicircle (FIG. 3A, curve through triangle-marked points) and Rn decreases (Table 1) for 15 μm of Nafion on carbon when compared to bare carbon. This decrease in resistivity with increasing Nafion thickness may be attributed to the porous nature of carbon, in which the Nafion is embedded within the pores of the carbon rather than acting as an overlayer.

$\begin{array}{cc}{R}_s+\frac{C_{\mathrm{dl}}}{\left[R_{\mathrm{ct}}+Z_w\right]}& \left(1\right)\end{array}$

TABLE 1
Summary of solution resistance (Rs) and charge transfer
resistance (Rct) obtained from electrochemical impedance
spectroscopy data fitted to a Randles circuit.
	Catalyst	Rs (Ω)	Rct (Ω)
	Carbon	144	1961
	2 μm Nafion on carbon	67	1978
	15 μm Nafion on carbon	384	601
	Cu	638	402
	2 μm Nafion on Cu	646	587
	15 μm Nafion on Cu	644	896
	PVDF on carbon	8	25
	PVDF on Cu	12	36

Based on the observation that electron transfer is impeded by thick Nafion layers, contrasting experiments were performed with a hydrophobic polymer to block proton transfer to the CO₂ reduction electrodes. Electrodes with a hydrophobic polymer were created by modifying carbon and Cu substrates with a 15 μm thick overlayer of polyvinylidene fluoride (PVDF). FIG. 4A presents the LSVs of carbon and Cu in CO₂-saturated electrolyte. Interestingly, the opposite effect is observed with PVDF overlayers in which the onset potential is shifted more negative in the presence of PVDF. The LSV for an unmodified carbon electrode possesses an onset potential of −0.29 V (next to lowest curve, at left margin of figure), while the LSV for a PVDF-modified carbon electrode possesses an onset potential of −0.6 V (lowest curve at left margin). Similarly, the LSV for an unmodified Cu electrode exhibits an onset potential of −0.19 V (uppermost line at left margin), while a PVDF-modified Cu electrode exhibits an increased onset potential of −0.55 V (second highest line at left margin). This negative shift in onset potential for PVDF-modified electrodes is attributed to the hindered proton transfer from electrode to CO₂, therefore increasing the driving force needed to reduce CO₂. This effect is further confirmed by EIS (FIG. 4B) and R_ct values. PVDF on carbon and Cu exhibited R_ct values of 25Ω and 36Ω, respectively, which signifies that protons are blocked. Furthermore, thick PVDF overlayer on electrodes does not exhibit resistor-like behavior as seen in thick Nafion overlayer on electrodes, illustrating the differences in the impedance of electrons (Nafion) and protons (PVDF).

Taken together, the data in FIGS. 2A, 2B, 3A. 3B, 4A, and 4B demonstrate that while Nafion overlayers decrease the overpotential of CO₂ reduction for both carbon and Cu electrodes, PVDF overlayers increase the overpotential due to their hydrophobicity and blocking of protons. Pursuant to an initial hypothesis, the positive shifts in CO₂ reduction with Nafion overlayers are simply due to Nernstian changes in the pH at the polymer-electrode interface (Nafion is a superacid with pK_a˜−6).²⁸ To evaluate this hypothesis, CO₂ reduction was conducted under different pH values. FIG. 5 shows LSVs of a carbon electrode in CO₂-sparged 0.1 M NaHCO₃ buffer at pH 6.8 (lower line at left) and pH 2.6 (upper line at left). CO₂ reduction at pH 6.8 has a much earlier onset potential (−0.5 V vs. RHE) and higher current density than CO₂ reduction in an acidic medium (−0.9 V vs. RHE). The observation that the increase in acidity causes the onset potential to shift negative is the opposite of what would be predicted if Nafion's effect on onset potential were caused by interfacial pH effects because Nafion is acidic. Instead, Nafion elicits a positive shift in onset potential, and therefore we conclude that this positive shift is not simply due to pH changes at the polymer-electrode interface.

Based on the LSV results presented, further investigation was undertaken to discern whether CO₂ reduction occurs at the polymer-electrolyte interface (FIG. 9A), the electrode-polymer interface (FIG. 9B), or the electrode-electrolyte interface (FIG. 9C). Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDS) analysis were employed to show the uniformity of Nafion on the electrode. FIGS. 6-F respectively show the SEM image (6A), EDS spectrum (6B), and EDS mapping of a 2 μm thick layer of Nafion on Cu (6C-F). EDS mapping shows that F, O, and S, all elements present in Nafion, are uniformly present on top of the Cu electrode. The uniform nature of the Nafion overlayer suggests that CO₂ reduction is not occurring at the electrode-electrolyte interfaces as might occur with a nonuniform overlayer (FIG. 9C). FIG. 8 presents a representative cross-sectional SEM image of a 2 μm thick layer of Nafion on Cu. EDS mapping of the cross-sectional view clearly shows Cu (FIG. 8C) and F (FIG. 8D) from Nafion and also is evidence of the uniformity of the Nafion layer.

CO₂ Reduction on Nafion-Modified Electrodes

To elucidate whether CO₂ reduction is occurring at the polymer-electrolyte interface or at the electrode-polymer interface. CO₂ reduction products were quantified using nuclear magnetic resonance (NMR) spectroscopy (for liquid products) and gas chromatography (GC) (for gaseous products). FIG. 10A summarizes the Faradaic efficiencies (FE) of three detected products (CO, CH₄, and HCOOH) at −0.89 V vs. RHE. Four different substrates (carbon, Cu, Ni, and Zn) were tested to evaluate the effect of the Nafion overlayer. These four substrates were tested without any modification, modified with 15 μm of Nafion, or modified with 15 μm of PVDF. Unmodified Cu produced CO and HCOOH, while Nafion-modified Cu showed a significantly enhanced CH₄ production of 68% FE, while the Faradaic efficiencies for CO and HCOOH decreased. The decrease in CO and concomitant increase in CH₄ strongly suggest that CO₂ is reduced by the substrate and is subsequently trapped and stabilized as a CO intermediate by Nafion, which is then further reduced to CH₄. Cu modified by proton-blocking PVDF only produced trace amounts of CO. Carbon modified with Nafion compared to bare carbon shows an increase in CO and HCOOH, while no CH₄ was made. Similar to PVDF-modified Cu, PVDF-modified carbon produced no carbon-containing products. In response to the differences in the results between Cu and carbon, two additional substrates were tested. Compared to unmodified Ni, Nafion-modified Ni showed a decrease in CO production and a slight increase in HCOOH production. Unmodified Zn produced CO, CH₄, and HCOOH, while Nafion-modified Zn only produced increased formate and CO. Because each substrate yields its own unique set of Faradaic efficiencies for each product, it is concluded that CO₂ reduction occurs at the electrode-polymer interface with a 15 μm thick Nafion overlayer. In contrast, if the Faradaic efficiencies for each product were similar regardless of substrate, the conclusion would have been that CO₂ reduction occurs at the polymer-electrode interface because the nature of this interface does not depend strongly on the substrate.

In addition to Faradaic efficiencies, product formation can also be expressed in terms of partial charge density and rates. Partial charge densities (FIG. 10B) of product formation follow the same general trends as those of the Faradaic efficiencies, while the rates of product formation (in units of nmol/cm²-s) show disproportionately slower CH₄ production rates because CH₄ production requires 8 e⁻/mol as opposed to CO and HCOOH production, which both only require 2 e⁻/mol.

As previously demonstrated by linear sweep voltammetry (FIG. 3), the hydrophobic polymer PVDF completely blocks proton transfer in the CO₂ reduction reaction. When CO₂ reduction is attempted with an electrode that is modified by PVDF, no carbon-containing products are made (FIG. 10). This has been tested on two substrates (a Cu electrode and a carbon electrode), and only very low yields of carbon-containing products were made on either substrate modified with PVDF. In other words, with PVDF-modified substrates, H₂ is the only product. These findings demonstrate that product selectivity is based on proton availability and proton transfer rates. Nafion is a highly proton-conductive polymer that can rapidly shuttle protons, which has a significant effect on product selectivity. With Nafion, the CO* intermediate can be rapidly protonated, which subsequently leads to CH₄ formation.

To further verify that CO₂ reduction is occurring at the electrode-polymer interface with a 15 μm overlayer, the thickness of Nafion was varied. Faradaic efficiencies as a function of Nafion thickness on a Cu electrode is presented in FIGS. 11A and 11B. Varying the thickness of Nafion on a Cu electrode (FIG. 11A) results in different Faradaic efficiencies of CO, CH₄, and HCOOH. Without Nafion, Cu produces mostly CO and HCOOH under these experimental conditions. When modified with a 2 μm thick layer of Nafion, CH₄ production is greatly increased and reaches a maximum Faradaic efficiency of 68.4% when modified with 15 μm overlayer of Nafion. Thicker layers of Nafion cause a decrease in products, which indicates that the hydrogen evolution reaction (HER) occurs on very thick Nafion membranes. Based on these results, it is posited that CO₂ is reduced at the electrode-polymer interface to produce CO when thinner membranes are used and that the HER occurs at the polymer-electrolyte interface with thicker membranes.

Partial charge densities (FIG. 11B) for CH₄ follow the same general trends as Faradaic efficiencies. However, 2 μm of a Nafion overlayer inhibits HCOOH formation. Rates of product formation (FIG. 11C) show that on unmodified Cu electrodes, CO and HCOOH formation is similar (4.4 nmol/cm² s for CO and 4.3 nmol/cm² s for HCOOH), while no CH₄ is produced. Cu electrode modified with 2 μm of Nafion exhibits an increased CO production rate (6.3 nmol/cm² s) and a decreased HCOOH production rate (0.8 nmol/cm² s) and still no CH₄ is produced. When the Cu electrode is modified with 8 μm thick or 15 μm thick layers of Nafion, CH₄ formation is observed. The rate of CH₄ production is faster with a 15 μm Nafion overlayer (6.12 nmol/cm² s) as compared to a 8 μm thick overlayer (6.1 nmol/cm² s). These results signify that Nafion should be thick enough to trap CO generated from the Cu electrode and that 15 μm is the optimal Nafion thickness for enhanced CH₄ formation. With thicker Nafion overlayers (30 μm), the CO production rate is slowed, and no CH₄ formation is observed. Furthermore, with extremely thick Nafion overlayers (183 μm), all product rates are significantly decreased, and no CH₄ is produced. This result suggests that on very thick Nafion overlayers, only the HER is occurring on top of the polymer at the polymer-electrolyte interface.

Voltage-dependent experiments are presented in FIG. 12. At −0.38 V vs. RHE. CH₄ production reaches 88.0% Faradaic efficiency on a Cu electrode modified with 15 μm of Nafion. By comparison, previous literature reports on unmodified Cu electrode at −0.9 V vs. RHE give only 0-1% CH₄.²² These results indicated that a Nafion-modified Cu electrode not only enhances CH₄ production but also produces it at a significantly decreased overpotential. Notably, to the best of the inventors' knowledge, the 88% Faradaic efficiency for CH₄ obtained is higher than any previous literature reports under any experimental conditions. See Table 2, herein below. The currently reported best CO₂ to CH₄ catalysts have a Faradaic efficiency of ˜70%.²⁹ In addition to CO, CH₄, and HCOOH, this polymer-modified Cu electrode also produces ethanol and methanol at Faradaic efficiencies of 0.2% and 0.06%, respectively, at −0.98 V vs. RHE. Both partial charge density and rate of CO and HCOOH formation plots (FIGS. 12B, 12C) follow the same general trends as those of the Faradaic efficiencies. However, at −0.38 V, the partial charge density and rate of CH₄ formation is lower than at −0.9 V due to the lower driving force at this decreased overpotential.

Given the remarkably high Faradaic efficiency for CH₄ production by Nafion-modified Cu electrodes, several experiments were performed to gain insight into the mechanism of CO₂ reduction under these conditions. First was an experiment in which sodium formate was added to the bicarbonate buffer in the absence of dissolved CO₂. This experiment resulted in trace amounts of CO and no CH₄ production, indicating that CO₂ reduction to CH₄ does not occur via a formate intermediate. Secondly, a CO reduction experiment was performed using a Cu electrode with a 15 μm thick Nafion overlayer at −0.38 V vs. RHE (the electrode with the highest Faradaic efficiency for CH₄ production). This experiment yielded 38% Faradaic efficiency of CH₄. The relatively high Faradaic efficiency for CH₄ production using CO-sparged electrolyte indicates that a good portion of the formed CH₄ in the CO₂ reduction case originates from a CO intermediate. However, the observation that the Faradaic efficiency for CO reduction to CH₄ is still significantly lower than the Faradaic efficiency for CO₂ reduction to CH₄ (88%) under the same experimental conditions suggests that additional factors need to be considered. In the pathway leading to CH₄ formation, the protonation of CO to CHO on the electrode surface is the rate-determining step.¹⁴ Furthermore, previous studies suggest that CH₄ formation is pH dependent and that CH₄ formation is favored at lower pH values.^30,31,32,33 CO₂-saturated 0.1 M NaHCO₃ electrolyte has a pH of 6.8 while the pH of CO-saturated electrolyte has a pH closer to 9. The abundance of H⁺ in a more acidic CO₂-saturated electrolyte implies rapid protonation of the CO intermediate, favoring CH₄ formation. The higher pH of the CO-saturated electrolyte yields less CH₄ due to less H⁺ present in the electrolyte.

FIGS. 12A-12C present a proposed mechanism of CO₂ reduction to CO and CH₄. With the addition of 2 H⁺ and 2 e⁻ a CO intermediate is formed with two possible resonance structures (FIGS. 12A-12C, dotted box). Each of the two structures can either be released as CO or proceed to be further reduced to CH₄. FIG. 12 proposes a mechanism to explain the extremely enhanced CH₄ production on a Nafion-modified electrode at −0.38 V. First, CO₂ is reduced to CO at the polymer-electrode interface. CO that is not bound to the electrode surface is released as a product, and CO that is bound to the electrode surface (denoted as a ═C═O* intermediate) is stabilized by Nafion, allowing for the subsequent reduction of CO to CH₄ while preventing CO release. CO₂ reduction to CH₄ also may occur through an alternative pathway that does not proceed through a CO intermediate.

As described herein novel Nafion-modified electrodes have been fabricated that exhibit significantly enhanced CH₄ production (up to 88% Faradaic efficiency) as a CO₂ reduction product. With variation of the thickness, voltage, and substrate. CO₂ reduction occurs at the electrode-polymer interface under the conditions that produced enhanced yields of CH₄. It is posited that CO₂ reduction to CH₄ is significantly enhanced because Nafion helps to stabilize the Cu—CO* intermediate, which allows for the stabilized CO to be protonated and further reduced to CH₄. In addition, the hydrophobic polymer PVDF hinders proton transfer, which results in increased hydrogen production and very inhibited carbon product formation. Future studies include tuning the hydrophilicity of Nafion to further modulate proton transfer rates by utilizing different polymer overlayer structures.

TABLE 2
Summary of various electrocatalysts for electrochemical
CO2 reduction to CH4 reported in literature.
	Voltage		CH4 Faradaic	References
Catalyst	(vs. RHE)	Electrolyte	Efficiency (%)	First Set
Cu foil	−2.4	V	NaClO4/MeOH	70.5	1
Cu-Co electrode	−1.19	V	0.1M KHCO3	47.5	2
Cu foil	−1.2	V	0.1M KHCO3	40	3
Cu electrode	−1.04	V	0.1M KHCO3	33.3	4
Pd electrode	−0.80	V	0.1M KHCO3	2.9	4
Cd electrode	−1.23	V	0.1M KHCO3	1.3	4
Ni electrode	−1.08	V	0.1M KHCO3	1.8	4
N-doped graphene	−0.86	V	1M KOH	15	5
quantum dots
Ti-phthalocyanine	−1.58	V	—	28.1	6
Cu-phthalocyanine	−1.23	V	—	28.0	6
Cu nanofoam	−1.5	V	0.1M KHCO3	<2	7
Pt GDE	−1.32	V	0.5M KHCO3	38.8	8
Pt/C	0.0	V	—	6.8	9
Cu single crystal	−1.01	V	0.1M KHCO3	6	10
Cu mesh	−1.2	V	2M KBr	28.8	11
Cu nanocubes (24 nm)	−1.1	V	0.1M KHCO3	15	12
Cu nanocubes (44 nm)	−1.1	V	0.1M KHCO3	22	12
Cu nanocubes (63 nm)	−1.1	V	0.1M KHCO3	10	12
Cu foil	−1.1	V	0.1M KHCO3	18	12
Polycrystalline Cu	−1.0	V	0.1M KHCO3	4.6	13
Cu nanoparticles	−1.35	V	0.1M NaHCO3	76	14
Cu2O/Zn	−1.9	V	0.3M KOH in	7.5	15
			MeOH
Nanoporous carbon	−1.6	V	0.1M KHCO3	0.18	16
Cu electrode	−1.05	V	0.1M LiHCO3	32.2	17
Cu electrode	−1.05	V	0.1M NaHCO3	55.1	17
Cu electrode	−0.99	V	0.1M KHCO3	32.0	17
Cu electrode	−0.98	V	0.1M CsHCO3	16.3	17
Cu2O films	−1.1	V	0.1M KHCO3	5	18
Cu2O films	−0.99	V	0.1M KHCO3	<1	19
Cu electrode	−1.01	V	0.1M KHCO3	29.4	20
Cu electrode	−1.04	V	0.1M KCl	11.5	20
Cu electrode	−0.99	V	0.5M KCl	14.5	20
Cu electrode	−1.00	V	0.1M KClO4	10.2	20
Cu electrode	−1.00	V	0.1M K2SO4	12.3	20
Cu electrode	−0.83	V	0.5M K2HPO4	17.0	20
Cu electrode	−0.77	V	0.1M K2HPO4	6.6	20
Cu electrode	−0.96	V	0.1M KHCO3	22.3	21
Fe electrode	−0.98	V	0.1M KHCO3	1.1	21
Ni electrode	−1.09	V	0.1M KHCO3	1.1	21
Cu electrode	−1.0	V	0.1M KHCO3	3	22
Cu—Ni electrode	−1.3	V	0.5M KHCO3	20.2	23
Ag electrode	−1.4	V	0.1M KHCO3	0.09	24
Ni electrode	−1.00	V	0.1M KHCO3	0.6	25
Ni electrode	−1.08	V	0.1M KHCO3	1.8	25
Ni electrode	−1.02	V	0.1M KHCO3	2.4	25
Cu sheet	−1.00	V	0.1M KHCO3	16.3	26
Cu2O/carbon black	−1.3	V	NaCl/MeOH	26.9	27
Cu sheet	−1.35	V	0.1M KHCO3	44	28
Cu porphyrin	−1.0	V	0.5M KHCO3	26	29
Cu electrode	−1.35	V	0.5M KHCO3	5.3	30
Cu electrode	−1.6	V	1.1M KHCO3	44	31
Cu electrode	−2.4	V	0.5M LiClO4/MeOH	71.8	32
Cu wire electrode	−3.35	V	Tetraethylammonium	28.1	33
			perchlorate methanol
Cu nanoparticles	−1.3	V	0.1M KHCO3	50	34
Cu2Pd	−1.2	V	0.1M TBAPF6/CH3CN	55	35
Co protoporphyrin	−0.8	V	0.1M HClO4	2.5	36
Polycrystalline Cu	−1.4	V	0.5M KHCO3	42	37

Third Set of Experiments

In this third set of experiments, the concepts which were established in the first two sets of experiments were extended to other substrates. A number of experiments were run as described in FIG. 28-38, to determine the impact that changing the metal substrate as the electrode for conducting the reduction of CO₂ would have on the Faradaic efficiency of the reaction as a function of the use of an overcoating with a change in voltage. In experiments which utilized a brass substrate, the brass was a mixture of 62% Cu, 37% Zn and trace amounts of Fe (<0.15%), Pb (<0.08%) and Sn (<0.005%) by weight. Zinc substrates were also used in this third set of experiments. The experiments conducted here are analogous to the copper experiments which are described herein above and were generally run for a one hour period.

As set forth in FIG. 28, the Faradaic efficiencies on brass were impacted dramatically by the thickness of the Nafion overcoat (FIG. 28) at −089V vs. RHE over the 1 hour experiment which was conducted, with the greatest Faradaic efficiency occurring with a Nafion overcoating thickness ranging from 2-15 μm.

FIG. 29 shows the Faradaic efficiencies of CO and methane gas as a function of the voltage on brass foil no overcoating (see FIG. 28 above) over the period of the experiment (I hour). The graph presented in FIG. 29 evidences that the voltage used for the reduction reaction also significantly impacted the production of methane from CO₂ with a voltage vs. RHE ranging from −0.2 to approximately −2.0 V being effective and a voltage within the range of −1.0 to −1.7 V being particularly effective for generating methane gas.

FIG. 30 shows that the Faradaic efficiencies of CO and ethylene gas production for the 1 hour experiment conducted at −0.89V vs. RHE produced using brass electrodes with an overcoating of an admixture of Nafion/PVDF ranging from 0% by weight PVDF to 100% by weight PDVF and a thickness of 20-90 μm, showed high Faradaic efficiency for ethylene production at 20-90 μm, by weight PVDF in the Nafion/PVDF admixture. The inventors note that the PVDF-Nafion overlayer becomes increasingly thick as the weight percentage of the PVDF in the admixture increases. This is an unexpected and commercially relevant result inasmuch as ethylene is a particularly valuable commercial product.

FIG. 31 shows the Faradaic efficiencies for CO and ethylene production over a one hour experiment using a brass foil electrode (no overcoating) in acetonitrile/bicarbonate electrolyte solution using a voltage of −0.89V vs. RHE. As evidenced by the data presented in FIG. 31, the Faradaic efficiency for ethylene gas production was greatest between 50% and 80% by volume of acetonitrile.

FIG. 32 shows that a Nafion coating (15 μm) on a brass electrode using an acetonitrile/bicarbonate electrolyte solution at −0.89V vs. RHE as indicated dramatically influences the Faradaic efficiency of CO and ethylene production and has little impact on methane gas production. An acetonitrile/bicarbonate solution ranging from 10-60% by volume acetonitrile provided the highest Faradaic efficiencies in the experiment.

FIG. 33 shows the impact of acetonitrile on Faradaic efficiency for the production of CO, methane, ethylene and formic acid on a zinc foil coated with 15 μm thick Nafion performed at −0.89 V vs. RHE. As the acetonitrile volume % increased, much more ethylene was produced, little formic acid was produced at any level of acetonitrile and methane was most efficiently produced (high Faradaic efficiency) at approximately 10-40 volume % acetonitrile in the electrolyte solution.

FIG. 34 shows the CO Faradaic efficiency as a function of the thickness of Nafion coating on a zinc substrate at −0.89 V vs. RHE over the one hour period of the experiment. Noted is that the CO Faradaic efficiency is highest at 2 μm to 15 μm coating thickness and dissipates as the thickness of the coating increases to 90-100 μm.

FIG. 35 shows the Faradaic efficiencies of CO produced from 20-90 μm PVDF and Nafion admixture coating on zinc substrate at −0.89 V vs. RHE over the one hour period of the experiment. Note that the Faradaic efficiency of CO production is highest at low PVDF content and at approximately 40-60% by weight PVDF. Above 60% PVDF by weight of the admixture, the Faradaic efficiency is reduced to close to zero.

FIG. 36 shows the Faradaic efficiencies over the 1 hour experiment for CO and ethylene gas produced using a copper substrate overcoated with 52 weight % of several different polymers (polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polyvinyl alcohol (PVA) and polyethylenimine (PEI) in admixture with Nafion at −0.89 V vs. RHE. An admixture of polyvinyl alcohol and Nafion provided the highest faradaic efficiency for the production of ethylene gas among the polymer admixtures tested.

FIG. 37 shows the Faradaic efficiencies for CO and ethylene gas produced using various polymer blends in Nafion on a copper substrate at −0.89 V vs. RHE. A represents 100% polytetrafluoroethylene (Teflon). B represents 50 weight % each of Teflon and PVDF in admixture. C represents 52 weight % Teflon and Nafion in admixture. D represents 40 weight % each of Teflon and PVDF in Nafion admixture. E represents 64 weight percent Teflon and 30 weight % PVDF in Nafion admixture and F represents 30 weight % and 64 weight % PVDF in Nafion. The results evidence that a fluoropolymer which excludes Nafion does not produce ethylene gas (or even significant concentrations of CO) and the inclusion of a fluoropolymer (Teflon) with Nafion at approximately 50% by weight (52:48) produced a larger concentration of ethylene gas as did polymer admixtures with greater percentages of fluoropolymer (PVDF) in Nafion.

FIG. 38 shows the Faradaic efficiencies for the production of CO, methane, ethylene and formic acid producing using 10 weight % nanoparticulate cuprous oxide (Cu₂O) in admixture with Nafion polymer coated on metal substrates (A and B) or a Cu₂O nanoparticulate coating (a thin film of Cu₂O nanoparticulates without Nafion coated onto metal substrates by drop casting from a dispersion of Cu₂O nanoparticulates) on metal substrates (C. D and E) over the one hour experiment at −0.89 V vs. RHE. A represents the results for the Nafion admixture overcoating on copper substrate, B represents the results for the Nafion admixture overcoating on zinc, C represents the results for the Cu₂O thin film on zinc (C), Copper (D) and Nickel (D) substrates. As indicated, the inclusion of Cu₂O in the Nafion overcoating had a significant impact on CO and formic acid production with high Faradaic efficiency for CO. Given the other experiments using fluoropolymers it is anticipated that the inclusion of Teflon and/or PVDF in the Nafion polymer (often at weight % great than 50 weight %) is expected to have a substantial impact in producing methane and ethylene products during CO₂ reduction.

FIG. 39 shows a mechanism for electrocatalysis at a polymer-substrate (catalyst) interface with an embedded cocatalyst in admixture with the polymer to provide tandem catalysis. Cocatalysts can be small nanoparticulates (having a diameter ranging from 1 to 500 nm) or nanowires which are dispersed in the polymer overcoating. Alternatively, a molecular species which functions as a cocatalyst may be covalently attached to the polymer backbone. By coupling an electrode catalyst that is selective for a partially reduced intermediate and with a membrane/coating bound catalysis which facilitates further reduction, reference cells can be provided for reducing CO₂ to selectively desired products with high Faradaic efficiencies.

CONCLUSIONS DRAWN FROM THE EXAMPLES

The experiments evidence that the use of a uniform Nafion overcoating ranging from 2 to 15 μm (often 10-15 μm, most often 15 μm) on a copper electrode at an effective voltage using a bicarbonate solution (with no additional aprotic solvent in the solution) provides high Faradaic efficiency and dramatically high yield of methane gas.

Also evidenced by the experiments described herein, this work demonstrates that controlling the hydrophobicity of the electrode and proton availability of the electrolyte strongly dictates the production of different CO₂ reduction products. Formate production is favored by a hydrophobic electrode, however, too hydrophobic causes mass transport issues because hydrophobic PVDF is less permeable to CO₂. The decrease in proton concentration slows down the protonation of the M-CO intermediate to generate CH₄, but promotes M-CO and M-CO coupling chemistry to produce C2+ products. This control of hydrophobicity by using polymer blends and mixed aprotic-protic solvent systems is a facile and effective method to tune the selectivity of CO₂ reduction catalysts.

A skilled practitioner can predict carbon-based product selectivity from CO₂ electrolysis reduction reactions by the design of the electrode, the electrode's polymer coating (including the thickness of the polymer coating) and the composition of the bicarbonate electrolyte solution used as the CO₂ source. CH₄ production is favored when electrodes are modified with a Nafion overlayer and on unmodified electrodes at a very negative reduction potentials. Formate is favored with hydrophobic electrodes (e.g. PVDF-Nafion overlayer) and at less negative reduction potentials. C₂H₄ is favored when Cu alloys are used, when the alloy electrode is hydrophobic (e.g. PVDF-Nafion overlayer), and when aprotic solvents are used in conjunction with the bicarbonate electrolyte. Further, the inventors have surmised that formate can be further enhanced by creating a hydrophobic environment on the electrode and/or in the electrolysis solution. C₂H₄ production can be enhanced by hydrophobic fluoropolymer-Nafion overlayers on an alloy electrode.

In addition, from the description of the present invention, the polymer overlayers as hosts for tandem catalysis. Cocatalysts can be nanoparticles and/or nanowires dispersed in the polymer overlayers or molecular species covalently attached to the polymer backbone. By coupling an electrode-bound catalyst that is selective for a partially reduced intermediate and with a membrane-bound catalysis that facilitates further reduction, one can envision the ready design of electrolysis systems utilizing CO₂ reduction that selectively form desired products.

Supplemental Information

Mass Transport Calculations

Effect of Mass Transport on CO₂ Electrocatalysis on Nafion/PVDF-Modified Electrodes

The permeability of CO₂ in PVDF and Nafion were taken to be 2.16×10⁻¹⁷ mol-cm/cm²-s-Pa and 8.70×10⁻⁶ mol-cm/cm²-s-Pa, two values obtained from Flaconneche, et al., Oil Gas Sci. Technol.-Rev. IFP. 2001, 56(3) and 261-278; Ren, et al., J. Electrochem. Soc., 2015, 162(10), F1221-F1230. The permeability of CO₂ in PVDF-Nafion mixtures were calculated based on the weight percent of PVDF in Nafion multiplied by the permeability of CO2 in PVDF added to the weight percent of Nafion multiplied by the permeability of CO2 in Nafion. The thickness of the PVDF-Nafion overlayer was determined by cross-sectional SEM. Using the thickness of the PVDF-Nafion mixture (18 μm for 4 weight % PVDF in Nafion overlayer) and the pressure of CO2 is 1 atm. the flux of CO₂ through the membrane is calculated to be 4.7×10⁻⁸ mol/cm²-s. This flux value is then compared to the maximum theoretical rate of consumption of CO₂ at the electrode-polymer interface. The maximum CO₂ consumption rate is determined from the steady state current of the chronoamperometry, assuming all CO₂ is reduced to either CO or HCOOH. Because these products require only 2 e/mol, they consume CO₂ faster than more highly reduced products such as CH₄. Therefore, assuming a 100% yield of CO or HCOOH is an upper bound for the CO₂ consumption rate. For the Cu electrode modified with 4 weight percent PVDF in Nafion overlayer, the steady state current density is −0.21 mA/cm². From this value, the upper bound for the CO₂ consumption rate is 1.1×10⁻⁹ mol/cm²-s, a value less than the calculated CO₂ flux. Therefore, these calculations suggest that CO₂ mass transport is not a limiting factor for this electrode.

However, for the Cu electrodes modified with 56, 60, and 64 weight percent PVDF in Nafion, the CO2 flux is less than the maximum theoretical CO₂ consumption. This means at these higher weight percentages of PVDF in Nafion, CO₂ mass transport does become a limiting factor and the availability of CO2 at the Cu-polymer interface is an issue.

TABLE S1
Contact angle measurements on PVDF-
Nafion-modified Cu electrodes.
Electrode
(Weight % PVDF in Nafion) Angle (degrees)
Bare Cu                   28.9 ± 2
0                         43.7 ± 2
4                         38.0 ± 0.8
8                         43.9 ± 0.7
15                        49.7 ± 2
18                        70.3 ± 1
30                        74.9 ± 3
52                        78.7 ± 3
56                        84.8 ± 4
60                        86.5 ± 2
64                        95.1 ± 1
100                       124.0 ± 0.4

TABLE S2
Mass transport calculations.
Electrode		Max theoretical CO2
(Weight % PVDF in Nafion)	CO2 flux	consumption
4	4.7 × 10−8	1.1 × 10−9
8	4.1 × 10−8	5.7 × 10−9
15	3.0 × 10−8	4.5 × 10−9
30	1.8 × 10−8	6.2 × 10−10
52	8.6 × 10−9	1.6 × 10−9
56	6.0 × 10−9	6.7 × 10−9
60	4.5 × 10−9	5.7 × 10−9
64	3.5 × 10−9	5.7 × 10−9

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Claims

1. A method for CO2 reduction, comprising: providing an electrode having a layer of a predetermined uniform thickness of a polymeric composition; andplacing the electrode with the layer of polymeric composition in contact with a solution effective for CO2 reduction, wherein said polymeric composition consists essentially of Nafion polymer or an admixture of Nafion in combination with another polymer and/or a cocatalyst.

2. The method defined in claim 1 wherein the polymeric composition is Nafion or Nafion in combination with at least one additional polymer selected from the group consisting of polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP), polyethyleneglycol (PEG), polyvinylalcohol (PVA), polyethyleneimine (PEI), polytetrafluoroethylene (PTFE) and mixtures thereof.

3. The method defined in claim 1 wherein the polymeric composition includes a fluoropolymer.

4. The method defined in claim 3 wherein the fluoropolymer is polyvinylidene fluoride (PVDF, polytetrafluoroethylene (PTFE) or a mixture thereof.

5. The method defined in claim 2 wherein the at least one additional polymer is PDVF.

6. The method defined in claim 1 wherein the layer of the polymeric composition is Nafion having a thickness between approximately 2 μm and approximately 15 μm.

7. The method defined in claim 1 wherein the layer of the polymeric composition has a thickness effective to stabilize an intermediate in which CO is bound to the electrode coated with the layer of the polymeric composition.

8. The method defined in claim 2 wherein the layer of the polymeric composition has a thickness between approximately 20 μm and approximately 90 μm.

9. The method defined in claim 1, wherein the electrode is made of a material selected from the group consisting of carbon, copper, nickel and zinc and mixtures and alloys thereof.

10. The method defined in claim 1, wherein the electrode is made of a transition metal or transition metal alloy.

11. The method defined in claim 10 wherein the electrode is made of copper, zinc, silver, gold, cadmium, nickel, palladium, platinum or an alloy thereof.

12. The method of claim 10 wherein the electrode is made of copper or a copper alloy.

13. The method according to claim 12 wherein the copper alloy is brass (copper and zinc), bronze/phosphor bronze (copper and tin), naval brass (copper, zinc and tin), aluminum bronze (copper and aluminum), berylliumcopper (copper and beryllium), cupronickel (copper and nickel, and optionally iron and/or manganese), nickel silver (copper with nickel and zinc), copper silver (copper with silver) or copper gold (copper with gold).

14. The method defined in claim 13 wherein the copper alloy is brass.

15. The method defined in claim 10 wherein the layer of the polymeric composition has a thickness effective to stabilize an intermediate in which CO is bound to the electrode coated with the layer of polymeric composition.

16. The method defined in claim 1 wherein the solution is a bicarbonate solution or a bicarbonate solution further comprising an effective amount of an aprotic solvent.

17. The method according to claim 16 wherein said aprotic solvent is selected from the group consisting of acetonitrile (MeCN), dimethylformamide (DMF), dimethylacetamide DMA), dimethylsulfoxide (DMSO), tetrahydrofuran (THF), propylene carbonate (PC), or an alkyl nitrile (such as propylnitrile, butyl nitrile, adiponitrile, benzonitrile) or a mixture thereof.

18. The method according to claim 16 wherein said aprotic solvent is acetonitrile.

19. The method defined in claim 1, further comprising conducting an electrical current through said solution to said electrode at least in part through the layer of the polymeric composition.

20. The method defined in claim 1 wherein said polymeric composition further comprises a cocatalyst.

21. The method defined in claim 20 wherein said cocatalyst is in the form of a nanoparticle or a nanowire.

22. The method defined in claim 20 wherein said cocatalyst is made of copper (metallic), cuprous oxide (Cu2O), cupric oxide (CuO), Zn, zinc oxide (ZnO) or silver (Ag).

23. The method according to claim 1 wherein said polymeric composition is Nafion.

24. An electrode for CO2 reduction, comprising: a base or body of electrically conductive material; anda layer of a polymeric composition of a predetermined uniform thickness ranging from 1 μm to 100 μm on a surface of said base or body, wherein said polymeric composition consists essentially of Nafion polymer or an admixture of Nafion in combination with another polymer and/or a cocatalyst.

25. The electrode defined in claim 24 wherein the polymeric composition is Nafion polymer in the absence of an additional polymer or cocatalyst.

26. The electrode defined in claim 24 wherein the polymeric composition further includes polyvinylidene fluoride and mixtures thereof with Nafion polymer.

27. The electrode defined in claim 24 wherein said polymeric composition comprises at least one additional polymer selected from the group consisting of polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP), polyethyleneglycol (PEG), polyvinylalcohol (PVA), polyethyleneimine (PEI), polytetrafluoroethylene (PTFE) and mixtures thereof.

28. The electrode defined in claim 24 wherein the polymeric composition includes a fluoropolymer.

29. The electrode defined in claim 28 wherein the fluoropolymer is polyvinylidene fluoride (PVDF, polytetrafluoroethylene (PTFE) or a mixture thereof.

30. The electrode defined in claim 27 wherein the at least one additional polymer is PVDF.

31. The electrode defined in claim 24 wherein the layer of the polymeric composition is Nafion having a thickness between approximately 2 μm and approximately 15 μm.

32. The electrode defined in claim 24 wherein the layer of the polymeric composition has a thickness effective to stabilize an intermediate in which CO is bound to the electrode coated with the layer of the polymeric composition.

33. The electrode defined in claim 24 wherein the layer of the polymeric composition has a thickness between approximately 20 μm and approximately 90 μm.

34. The electrode defined in claim 24, wherein the electrode is made of a material selected from the group consisting of carbon, copper, nickel and zinc and mixtures and alloys thereof.

35. The electrode defined in claim 2424-33, wherein the electrode is made of a transition metal or transition metal alloy.

36. The electrode defined n claims 24 and 35 wherein the electrode is made of copper, zinc, silver, gold, cadmium, nickel, palladium, platinum or an alloy thereof.

37. The electrode according to claim 35 wherein the electrode is made of copper or a copper alloy.

38. The electrode defined in claim 37 wherein the copper alloy is brass (copper and zinc), bronze/phosphor bronze (copper and tin), naval brass (copper, zinc and tin), aluminum bronze (copper and aluminum), berylliumcopper (copper and beryllium), cupronickel (copper and nickel, and optionally iron and/or manganese), nickel silver (copper with nickel and zinc), copper silver (copper with silver) or copper gold (copper with gold).

39. The electrode defined in claim 38 wherein the copper alloy is brass.

40. The electrode defined in claim 24 wherein said polymeric composition further comprises a cocatalyst.

41. The electrode defined in claim 40 wherein said cocatalyst is in the form of a nanoparticle or a nanowire.

42. The electrode defined in claim 40 wherein said cocatalyst is made of copper (metallic), cuprous oxide (Cu2O), cupric oxide (CuO), Zn, zinc oxide (ZnO) or silver (Ag).

43. An electrolysis apparatus comprising: a housing defining a chamber; at least two electrodes disposed in part in said chamber and operatively connectable to a voltage source, said two electrodes including a working electrode; a first port member or fitting fixed to housing and communicating with said chamber for directing fluid into said chamber; and a second port member or fitting fixed to housing and communicating with said chamber for conveying fluid out of said chamber, said working electrode including an electrically conductive base member and a coating layer of a predetermined thickness of a polymeric composition disposed on said base member, wherein said polymeric composition comprises Nafion alone or in combination with an additional polymer and/or a cocatalyst.

44-61. (canceled)