SYSTEM AND METHOD FOR ELECTROCHEMICAL CO2 REDUCTION

Description

RELATED APPLICATION

This application claims priority from U.S. Provisional Application No. 63/652,715, filed May 29, 2024, the subject matter of which is incorporated herein by reference in its entirety.

GOVERNMENT FUNDING

This invention was made with government support under 2045111 awarded by the National Science Foundation. The government has certain rights in the invention.

BACKGROUND

The rise in atmospheric CO₂ has led to changes in temperature, rising sea levels, and ocean acidification. This rise in CO₂ is directly attributed to human activity. Continuing emissions as they are now will lead to adverse climate changes that will be essentially irreversible. Two-thirds of these emissions can be traced to 90 major industrial carbon producers. The Paris Agreement has established commitments, such as global net-zero greenhouse gas emissions. Even though these commitments were established, the global demand for energy is expected to increase by as much as 30% in the next two decades. Because of this demand for energy, producing a method that creates less polluting energy is imperative.

The only known reaction that can lower CO₂ emissions while yielding alternative fuel sources that give off net zero greenhouse gases is CO₂ reduction reaction (CO₂RR). CO₂RR is an electrochemical reaction that reduces CO₂ into various products such as formic acid, acetate, hydrocarbons (methane, ethane, ethene, etc.), and alcohols (methanol, ethanol, propanol, etc.). Though this is a promising reaction, it requires a catalyst due to high overpotentials and competing reactions.

Significant progress has been made in developing electrode materials, electrolyzer configurations, and various electrolytes to electrochemically convert CO₂ to commodity chemicals. However, the initial electron transfer to activate CO₂ that is otherwise linear and stable remains a critical challenge. This continues to be one of the primary obstacles in commercializing technologies for the electrochemical CO₂RR, preventing CO₂RR from being economically viable at a large scale, along with the related challenges of selectivity and catalyst stability.

SUMMARY

Embodiments described herein relate to an electrochemical CO₂ reduction system that includes a functionalized ionic liquid (IL) that generates ion-CO₂ adducts and a hydrogen bond donor (HBD) upon CO₂ absorption to modulate CO₂ reduction reaction (CO₂RR) on a Cu cathode in a non-aqueous electrolyte. As revealed by transient voltammetry, electrochemical impedance spectroscopy (EIS), and in situ surface-enhanced Raman spectroscopy (SERS) complemented with image charge augmented quantum-mechanical/molecular mechanics (IC-QM/MM) computations, the electrochemical CO₂ reduction system described herein provides a unique microenvironment where the catalytic activity of the CO₂RR is primarily governed by the IL and HBD concentrations in the non-aqueous electrolyte. The IL concentration controls the thickness of double-layer structures that can interact with reaction intermediaries of CO₂RR through IL-CO₂ adducts. The HBD concentration modulates the local proton availability. Modulation of the IL and HBD concentrations can provide a CO₂RR that has ample CO₂ availability, reduced overpotential, and suppressed hydrogen evolution reaction (HER) where C₄ products are obtained.

In some embodiments, the electrochemical CO₂ reduction system can include an electrochemical cell. The non-aqueous electrolyte and Cu cathode are provided in the electrochemical cell.

In some embodiments, the non-aqueous electrolyte includes the functionalized IL and HBD.

In some embodiments, the functionalized IL includes a bifunctional IL. The bifunctional IL can include a cation, which enhances an electric field to stabilize CO₂ between the cation and a Cu cathode surface, and a CO₂ chemisorbing anion, such as an aprotic heterocyclic anion or nucleophilic anion.

In some embodiments, the combination of the cation and anion produces the HBD.

In some embodiments, the HBD is formed in situ in the non-aqueous electrolyte by absorption of CO₂.

In some embodiments, the bifunctional IL includes an imidazolium-based cation and a pyrrolide-based anion.

In other embodiments, the bifunctional IL includes 1-ethyl-3-methylimidazolium pyrrole-2-carbonitrile ([EMIM][2-CNpyr]).

In some embodiments, the non-aqueous electrolyte includes a non-aqueous diluent in which the bifunctional IL is dissolved. The non-aqueous diluent can minimize mass transfer limitations of the bifunctional IL and increase the ionic conductivity of the non-aqueous electrolyte. The non-aqueous diluent can include, for example, acetonitrile or ethylene glycol.

In some embodiments, the non-aqueous electrolyte further includes a supporting electrolyte to maintain a stable ionic conductivity of the non-aqueous electrolyte. The supporting electrolyte can include a quaternary ammonium salt, such as tetraethylammonium perchlorate (TEAP).

In some embodiments, the electrochemical CO₂ reduction system can further include a voltage source configured to apply a voltage overpotential to the Cu cathode and the non-aqueous electrolyte to implement an electrochemical CO₂RR of CO₂ on the Cu cathode in the non-aqueous electrolyte. The applied voltage overpotential can be effective to reduce CO₂ in the non-aqueous electrolyte to at least one of CO, CH₄, C₂H₄, C₂H₆, formate, succinate, formaldehyde, or butane

Other embodiments described herein relate to a method for electrochemical CO₂ reduction. The method includes providing an electrochemical cell that includes a Cu cathode in contact with a non-aqueous electrolyte. The non-aqueous electrolyte includes a bifunctional ionic liquid (IL) which generates ion-CO₂ adducts and a hydrogen bond donor (HBD) upon CO₂ absorption. An overpotential can be applied to the Cu cathode and the non-aqueous electrolyte to implement an electrochemical CO₂ reduction reaction (CO₂RR) of CO₂ in the non-aqueous electrolyte.

In some embodiments, the bifunctional IL includes a cation, which enhances an electric field to stabilize CO₂ between the cation and Cu cathode surface, and a CO₂ chemisorbing anion, such as an aprotic heterocyclic anion or nucleophilic anion. The bifunctional IL can include, for example, 1-ethyl-3-methylimidazolium pyrrole-2-carbonitrile ([EMIM][2-CNpyr]).

In some embodiments, the non-aqueous electrolyte of the method includes a non-aqueous diluent in which the bifunctional IL is dissolved. The non-aqueous diluent can minimize the mass transfer limitations of the bifunctional IL and increase the ionic conductivity of the non-aqueous electrolyte. The non-aqueous diluent can include, for example, acetonitrile or ethylene glycol.

In some embodiments of the method, the non-aqueous electrolyte further includes a supporting electrolyte to maintain a stable ionic conductivity of the non-aqueous electrolyte. The supporting electrolyte can include a quaternary ammonium salt, such as tetraethylammonium perchlorate (TEAP).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic of a CO₂ reduction system in accordance with an embodiment.

FIG. 2 illustrates CO₂ absorption by [EMIM][2-CNpyr] as determined by NMR analysis (FIG. 7). The formation of 2-CNpyrH and carboxylate ([EMIM]⁺—CO₂⁻) through the CO complexation with carbene (deprotonated [EMIM]⁺ at C2 position) (top) and carbamate formation ([2-CNpyr]-CO₂⁻ (bottom)).

FIGS. 3(A-D) illustrate electrochemical and spectroscopic analysis of [EMIM][2-CNpyr] on Cu electrode. LSV of the IL under N₂ and CO₂ (A); Differential capacitance curve of neat IL (B); Potential-dependent in situ SERS of neat IL under N₂ (C) and CO₂ (D). The spectra are normalized with respect to the —C≡N stretching peak located at 2189 cm⁻¹ for N₂ and 2223 cm⁻¹ for CO₂.

FIGS. 4(A-B) illustrate (A) SERS peak intensity as a function of absolute applied potential (j E-IR_Ω j). Dotted line marks the potential of the change in the orientation of imidazolium species under N (black) and CO (red). Peaks: 1116 cm⁻¹ (pentagons) for δ(C C—H); 1347 cm⁻¹ for +ν(Im ring)+ν(CH₂(N)) (spheres); 1380 cm⁻¹ for ν(Im ring)+ν(CH₂(N))+ν(CH₃) (triangles). (B) Lowest energy geometries calculated for [EMIM] at −1.0 and −1.7 V on Cu (100) (top and side views) indicating the preference of the parallel orientation at a more negative potential. Atom color code: blue=N; cyan=C; white=H.

FIGS. 5(A-F) illustrate IL concentration and HBD effects on catalytic activity. Differential capacitance curves (A) and LSV measurements (B) for three different IL concentrations (0.1, 0.5, and 1.0 M) in acetonitrile with 0.1 M TEAP as the supporting salt (dashed line: N₂ and solid line: CO₂). The cyclic voltammograms (C) illustrate the effects of IL:HBD concentrations on CO₂RR. SERS spectra (D) of the 0.1 M IL and 0.5 M HBD containing electrolyte showing CO_ad peak (2088 cm⁻¹). The spectra are normalized with respect to the —C≡N stretching peak of acetonitrile at 2258 cm. Optimized geometries of [EMIM]+−CO²⁻ adduct with 2-CNpyrH demonstrating hydrogen bonding between —COO⁻ and the HBD (E) and [EMIM]+—CO²⁻ adduct with [2-CNpyr]⁻ (F) at −1.7 V. Atom color code: blue=N; cyan=C; white=H; red=O.

FIGS. 6(A-B) illustrate faradaic efficiencies (FE) for C₁ products and H₂ (A), and partial currents (B) for C₂₊ products identified through GC and NMR after electrolysis for 3 hours at −2.1 V vs. Ag/Ag⁺.

FIG. 7 illustrates ¹³C NMR of neat [EMIM][2-CNpyr]before and after CO2 exposure, highlighting formation of carboxylate (marked with #) and carbamate (marked with $) with CO₂ absorption.

FIG. 8 illustrates ¹³C NMR of electrolytes containing 1.0 M (A), 0.5 M(B), and 0.1 M (C) [EMIM][2-CNpyr] in acetonitrile after CO₂ exposure, highlighting formation of carboxylate (marked with #) and carbamate (marked with $) with CO₂ absorption. For comparison, 0.1 M IL (D) before CO₂ exposure is also provided. The carbamate signal is within noise for the 0.1 M IL containing electrolyte after exposure to CO₂ due to its low concentration; however the peak signal becomes discernible at higher concentrations. The acetonitrile peak is labeled with a star.

FIG. 9 illustrates concentration-dependent viscosity and conductivity measurements of [EMIM][2-CNpyr] in acetonitrile with viscosity represented by black color and conductivity by red color. Square symbol for measurements under N₂; sphere symbol for measurements under CO₂ saturation.

FIGS. 10(A-C) illustrate potential dependent in-situ SERS of (A) 0.1 M, (B) 0.5 M, and (C) 1.0 M [EMIM][2-CNpyr]under CO₂ in acetonitrile with 0.1 M TEAP on the roughened Cu electrode.

FIG. 11 illustrate Potential dependent in-situ SERS of 0.1 M [EMIM][2-CNpyr] and 0.5 M [2-CNpyrH] on the roughened copper electrode.

FIG. 12 illustrates a schematic of the experimental 2-compartment electrolysis setup. The counter electrode is a Pt foil, the working electrode is Cu foil, and the reference electrode consists of an Ag wire immersed in acetonitrile with 10 mM Ag/AgNO₃ with 0.1 M TEAP, as indicated in the experimental section. The anolyte consists of a 0.1 M H₂SO₄ aqueous solution, while the catholyte is neat IL or electrolytes containing different concentrations of IL in acetonitrile and 0.1 M TEAP. A Nafion 115 separator separates the anolyte and catholyte. The head space of the catholyte compartment connect to GC for gaseous sample analysis. Liquid samples are analyzed by sampling from the chamber before and after electrolysis experiments.

FIGS. 13(A-C) illustrate an example GC results demonstrating the identified and quantified peaks for the gaseous products obtained from the FID (A) and TCD detector (B and C). The injection interval for gas sampling was 20 minutes.

FIG. 14 illustrates ¹H-NMR of electrolytes after 3 hours of electrolysis; normalized to the peak height of TEAP (CH3 at 1.2 ppm). For comparison, the CO₂-saturated 0.1 M IL electrolyte (before electrolysis) is provided at the bottom. Top panel specifically shows the zoomed-in TEAP, succinate, and formate peaks in 0.1 M IL sample. The middle panel shows the regions of interest, with identifiable CO₂RR products highlighted. Only formate and succinate were quantifiable for FE calculation purposes. Unidentified peaks are marked with a red asterisk. Black: 0.1 M IL, Red: 0.1 M IL+0.5 M HBD, Blue: 0.5 M IL, Orange: 1 M IL.

FIG. 15 illustrates the distinctive peak of isobutane revealed by Gas Chromatography without quantification. Gas Chromatography analysis showcases a prominent iso-butane peak; however, a quantitative determination is not possible due to the absence of a calibration curve. Nevertheless, the detected peak signifies the presence of butane in the reaction products, providing preliminary insights into the C4 product formation ability of the microenvironment enabled by [EMIM][2-CNpyr] in addition to succinate.

FIG. 16 illustrates confirmation of the aldehyde product at 5.5 ppm by ¹H NMR. Comparison of the electrolyte (0.1, 0.5 and 1 M IL) after 3 hr. electrolysis to that of CO₂ saturated 0.1 M IL dosed with formaldehyde. Peaks arising from formaldehyde complexes with nucleophiles in the electrolyte are highlighted in pink. Since it is difficult to determine which protons correspond to each peak, quantification of formaldehyde is not possible.

FIGS. 17(A-C) illustrate surface preparations for Cu electrode. (A) Cyclic voltammogram of the copper electrode subjected to 50 cycles in an 85% o-phosphoric acid solution. The scan rate was 100 mV·s⁻¹ in a two-electrode setup with a Pt counter electrode. This step was performed to polish the copper surface, preparing it for transient voltammetry and electrolysis experiments. (B) The potential regime applied for the oxidative-reductive cycle (5 cycles) of Cu surface roughening for SERS measurements. (C) The current response after applying the potential regime indicates successful Cu surface roughening. The surface roughening of Cu was conducted in a three-electrode setup, with a Pt counter electrode and a saturated calomel electrode serving as the reference electrode in a 0.1 M KCl solution.

FIGS. 18(A-B) illustrates schematics of the in-situ SERS cell. (A) Isometric view of the custom-designed cell. Openings for laser (10 mm) and reference electrode (4 mm) are marked with arrows. This configuration enables precise positioning of the reference electrode and ensures an unobstructed laser path for accurate and controlled SERS measurements. (B) Side view of the SERS cell showing the electrode arrangement and liquid volume. The liquid volume is carefully controlled to ensure proper electrode immersion and to minimize background interference.

FIG. 19 illustrates schematics of the electrochemical impedance spectroscopy (EIS) experimental setup. The figure depicts a cross-sectional view of the experimental setup utilized for EIS involving Pt as the counter electrode, Cu rod with a diameter of 3 mm, coated and insulated with epoxy as the working electrode, and Ag/Ag⁺ reference electrode (described previously). The insulation around the Cu electrode ensures selective contact of the electrode with the solution, enabling precise measurements.

FIG. 20 illustrates Nyquist plots of the neat IL under N2 and CO2 environments. AC voltage with a 10 mV amplitude was applied at a frequency range of 200 kHz to 500 Hz at potentials ranging from −1.1 V to −2.3 V vs. Ag/Ag⁺.

FIG. 21 illustrates Nyquist plots of IL electrolytes (0.1, 0.5, and 1.0 M in acetonitrile with 0.1 M TEAP). The left panels shows measurements conducted under N2 environment and the right panels represent measurements under CO₂. AC voltage with a 10 mV amplitude was applied at a frequency range of 200 kHz to 500 Hz at potentials ranging from 900 mV to −2.3 V vs. Ag/Ag⁺.

FIG. 22 illustrates gas chromatography calibration curves for CO, CH₄, C₂H₄, C₂H₆, and H₂.

FIG. 23 illustrates atom types for adsorbates in IC-QM/MM simulations.

DETAILED DESCRIPTION

All scientific and technical terms used in this application have meanings commonly used in the art unless otherwise specified. The definitions provided herein are to facilitate understanding of certain terms used frequently herein and are not meant to limit the scope of the application.

The articles “a” and “an” are used herein to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element.

As used herein, the verb “comprise” as is used in this description and in the claims and its conjugations are used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. The present invention may suitably “comprise”, “consist of”, or “consist essentially of”, the steps, elements, and/or reagents described in the claims.

Throughout the description, where compositions are described as having, including, or comprising, specific components, it is contemplated that compositions also consist essentially of, or consist of, the recited components. Similarly, where methods or processes are described as having, including, or comprising specific process steps, the processes also consist essentially of, or consist of, the recited processing steps. Further, it should be understood that the order of steps or order for performing certain actions is immaterial so long as the compositions and methods described herein remains operable. Moreover, two or more steps or actions can be conducted simultaneously.

The term “A and/or B” means “A or B, or A and B”.

As used herein, the term “about” or “approximately” refers to a quantity, level, value, number, frequency, percentage, dimension, size, amount, weight or length that varies by as much as 15%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2% or 1% to a reference quantity, level, value, number, frequency, percentage, dimension, size, amount, weight or length. In one embodiment, the term “about” or “approximately” refers a range of quantity, level, value, number, frequency, percentage, dimension, size, amount, weight or length ±15%, ±10%, ±9%, ±8%, ±7%, ±6%, ±5%, ±4%, ±3%, ±2%, or ±1% about a reference quantity, level, value, number, frequency, percentage, dimension, size, amount, weight or length.

Throughout this disclosure, various aspects of this invention can be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual and partial numbers within that range, for example, 1, 2, 3, 4, 5, 5.5 and 6. This applies regardless of the breadth of the range.

Embodiments described herein relate to an electrochemical CO₂ reduction system that includes a functionalized ionic liquid (IL) that generates ion-CO₂ adducts and a hydrogen bond donor (HBD) upon CO₂ absorption to modulate CO₂ reduction reaction (CO₂RR) on a Cu cathode in a non-aqueous electrolyte. As revealed by transient voltammetry, electrochemical impedance spectroscopy (EIS), and in situ surface-enhanced Raman spectroscopy (SERS) complemented with image charge augmented quantum-mechanical/molecular mechanics (IC-QM/MM) computations, the electrochemical CO₂ reduction system described herein provides a unique microenvironment where the catalytic activity of the CO₂RR is primarily governed by the IL and HBD concentrations in the non-aqueous electrolyte. The IL concentration controls the thickness of double-layer structures that can interact with reaction intermediaries of CO₂RR through IL-CO₂ adducts. The HBD concentration modulates the local proton availability. Modulation of the IL and HBD concentrations can provide a CO₂RR that has ample CO₂ availability, reduced overpotential, and suppressed hydrogen evolution reaction (HER) where C₄ products are obtained.

FIG. 1 schematically illustrates an electrochemical CO₂ reduction system 10 in accordance with an embodiment described herein. The CO₂ reduction system includes an electrochemical cell 12 for electrochemical CO₂RR of CO₂. The electrochemical cell 12 includes a catholyte chamber 14, which contains a cathode 16 or working electrode and a reference electrode (not shown) immersed in a catholyte 20, and a separate anolyte chamber 22, which contains an anode 24 or counter electrode immersed in an anolyte 26. The cathode 16 and anode 24 can be electrically connected to an electrical source or voltage source 30 that can apply a voltage potential difference between the cathode 16 and the anode 24. The catholyte 20 and cathode 16 contained in the catholyte chamber 14 are configured for CO₂ reduction of CO₂ in the catholyte 20 upon application of a voltage overpotential to the cathode 16 and catholyte 20. The anode 24 and anolyte 26 are configured for oxidation, and the reference electrode is configured to measure the potential of the cathode 16 or the working electrode.

The cathode-side CO₂ reduction reaction depends on the material used to form the cathode 16 or coat an outer surface of the cathode 16. In some embodiments, cathode materials or surface coatings can include, for example, copper, gold, silver, zinc, palladium, gallium, bismuth, and mixtures or alloys thereof. Advantageously, the cathode or surface coating of the cathode can include copper. Copper can electrochemically produce higher carbon products than CO in the non-aqueous electrolyte than other metal catalysts. In some embodiments, the cathode 16 or surface coating of the cathode 16 can include about 10 wt. % to about 100 wt. % copper or a copper alloy. In other embodiments, the cathode or surface coating of the cathode consists of or consists essentially of copper or a copper alloy, preferably at more than 90 wt. %, more preferably at more than 99 wt. % of copper.

The anode material of the anode 24 is not subject to any special restrictions and can include any anode material capable of being used in an anolyte of an electrochemical cell. For example, the anode can be formed from platinum, ruthenium, or graphite.

The catholyte 20 or cathode-side electrolyte includes a non-aqueous electrolyte that can absorb CO₂ from a CO₂ source, such as circulating CO₂ gas supplied by a gas inlet (not shown) to the catholyte chamber. The non-aqueous electrolyte includes a functionalized ionic liquid (IL) that generates ion-CO₂ adducts and a hydrogen bond donor (HBD) upon CO₂ absorption to modulate CO₂ reduction reaction (CO₂RR) on the Cu cathode 16 in the non-aqueous electrolyte.

In some embodiments, the functionalized IL includes a bifunctional IL. The bifunctional IL can include a cation, which enhances an electric field to stabilize CO₂ between the cation and the Cu cathode surface, and a CO₂ chemisorbing anion. The HBD can be formed in situ in the non-aqueous electrolyte by absorption of CO₂.

In some embodiments, the cation can include an imidazolium-based cation. For example, the imidazolium-based cation can have the following general formula (I):

• • where R¹, R², R³, R⁴, and R⁵ each independently are selected from the group of linear, branched or cyclic C₁-C₂₅ alkyl, C₆-C₂₅ aryl, C₇-C₂₅ alkylaryl, C₇-C₂₅ arylalkyl, C₁-C₂₀ heteroalkyl or C₂-C₂₅ heteroaryl, C₃-C₂₅ alkylheteroaryl, C₃-C₂₅ heteroarylalkyl, C₇-C₂₅ heteroalkylaryl, C₇-C₂₅ arylheteroalkyl, C₃-C₂₅ heteroalkylheteroaryl, C₃-C₂₅ heteroarylheteroalkyl moieties or hydrogen.

Examples of imidazolium-based cations having the general formula (I) include 1-ethyl-3-methylimidazolium [EMIM]+, 1-butyl-3-methylimidazolium [BMIM]+, 1-hexyl-3-methylimidazolium [HMIM]+, 1-octyl-3-methylimidazolium [OMIM]+, 1-decyl-3-methylimidazolium [DMIM]+, 1-propyl-3-methylimidazolium [PMIM]+, 1-allyl-3-methylimidazolium [AMIM]+, 1-benzyl-3-methylimidazolium [BnMIM]+, 1-ethyl-3-propylimidazolium [EPIM]+, 1,3-deimethylimidazolium [MMIM]+, or mixtures thereof.

In some embodiments, the imidazolium-based cation is 1-Ethyl-3-methylimidazolium [EMIM]+.

In some embodiments, the CO₂ chemisorbing anion can include an aprotic heterocyclic anion or nucleophilic anion. The CO₂ chemisorbing aprotic heterocyclic anion or nucleophilic anion can include a pyrrolide-based anion. The pyrrolide-based anion can include, for example, pyrrole-2-carbonitrile.

In some embodiments, a bifunctional IL, which can produce the HBD in situ in the non-aqueous electrolyte upon CO₂ absorption, can include an imidazolium-based cation and a pyrrolide-based anion. Advantageously, imidazolium with negative polarization can trap CO₂ saturation products including the protonated anion that functions as a native HBD, thus resulting in a unique microenvironment to drive CO₂RR at lower overpotentials. Furthermore, the HBD component can form in situ by the absorption of CO₂ and contribute to CO₂RR at high reaction rates with reduced overpotentials.

An example of an imidazolium-based cation and a pyrrolide-based anion that can produce the HBD in situ upon CO₂ absorption is 1-ethyl-3-methylimidazolium pyrrole-2-carbonitrile ([EMIM][2-CNpyr]). [EMIM][2-CNpyr]can be synthesized using 1-ethyl,3-methylimidazolium chloride ([EMIM][Chloride]) and pyrrole-2-carbonitrile (2-CNpyrH) as starting materials. The halide salt, [EMIM][Chloride] can be transformed to [EMIM][hydroxide] using an anion exchange resin (e.g., Amberlite IRN-87, Alfa Aesar) in methanol, followed by acid-base neutralization reaction between [EMIM][hydroxide] and 2-CNpyrH to yield IL with water as a side product. The IL can then be dried to remove the water.

In some embodiments, the non-aqueous electrolyte can further include a non-aqueous diluent in which the bifunctional IL is dissolved. The non-aqueous diluent is used to adjust properties such as viscosity, ionic conductivity, solubility, and mass transport of the non-aqueous electrolyte. These diluents are molecular solvents that are immiscible or only partially miscible with water, and they do not disrupt the ionic nature of the IL. In some embodiments, the non-aqueous diluent can minimize mass transfer limitations of the bifunctional IL and increase the ionic conductivity of the non-aqueous electrolyte.

Examples of non-aqueous diluents can include acetonitrile, ethylene glycol, dimethyl carbonate, tetrahydrofuran, dimethyl sulfoxide, propylene carbonate, N, N-dimethylformamide, butyrolactone, or dioxane. Preferably, the non-aqueous diluent is acetonitrile or ethylene glycol.

The IL can be provided in the non-aqueous diluent at a concentration effective to minimize mass transfer limitations of the bifunctional IL and increase the ionic conductivity of the non-aqueous electrolyte. In some embodiments, where the non-aqueous diluent is acetonitrile, the IL can be provided in the non-aqueous electrolyte at a concentration of about 0.1 M IL to about 5.0 M IL, for example, about 0.1 M IL to about 4.0 M IL, about 0.1 M IL to about 3.0 M IL, about 0.1 M IL to about 2.0 M IL, or about 0.1 M IL to about 1.0 M IL.

In some embodiments, the non-aqueous electrolyte further includes a supporting electrolyte to maintain a stable ionic conductivity of non-aqueous electrolyte. The supporting electrolyte can include, for example, a simple organic salt, such as LiPF₆, NaTFSI, or NH₄PF₆, or an organic salt, such as a quaternary ammonium salt. Examples of quaternary ammonium salts that can be used as a supporting electrolyte in the nonaqueous electrolyte include tetraethylammonium tetrafluoroborate, tetraethylammonium perchlorate. Tetrabutylammonium hexafluorophosphate, tetrabutylammonium tetrafluoroborate, tetrabutylammonium bromide, or tetrahexylammonium chloride.

The concentration of the supporting electrolyte in the non-aqueous electrolyte can be about 0.01 M to about 1 M, for example, about 0.02 M to about 0.9 M, 0.03 M to about 0.8 M, 0.04 M to about 0.7 M, 0.05 M to about 0.6 M, 0.06 M to about 0.5 M, 0.07 M to about 0.4 M, 0.08 M to about 0.3 M, 0.09 M to about 0.2 M, or about 0.1M.

In some embodiments, the anolyte 26 provided in the anolyte chamber 22 can include an acidic aqueous solution, such as an about 0.1 M to about 1 M H₂SO₄ aqueous solution.

The catholyte chamber 14 of the electrochemical cell 12 containing the catholyte 20 and cathode 16 is separated from the anolyte chamber 22 containing the anode 24 and anolyte 26 with a membrane 40, which prevents any mixing of the electrolytes 20 and 26. The membrane 40 is not subject to any special restrictions provided it separates the catholyte chamber 14 and the anolyte chamber 22. In particular, the membrane prevents essentially any crossover of CO₂ and/or its dissolved form to the anode 24. The membrane 40 can include an ion exchange membrane, for example a polymer-based ion exchange membrane. A preferred material for an ion exchange membrane is a sulfonated tetrafluoroethylene polymer such as Nafion®, for example Nafion® 115. Ceramic membranes, for example, are useful as well as polymer membranes.

During operation of the CO₂ reduction system 10, the voltage source 30 can apply a voltage overpotential to the Cu cathode 16 and the non-aqueous electrolyte 20 to implement an electrochemical CO₂RR of CO₂ on the Cu cathode 16 in the non-aqueous electrolyte 20. Suitable potentials levels include, but are not limited to, levels between −0.5 V and −3.0 vs. the reference electrode. The potential can be applied by the voltage source 30. The applied voltage overpotential can be effective to reduce CO₂ in the non-aqueous electrolyte to at least one of CO, CH₄, C₂H₄, C₂H₆, formate, succinate, formaldehyde, or butane.

Optionally, when it is desirable for the CO₂ reduction system 10 to operate at or near steady state, the CO₂ reduction system 10 can include a CO₂ source (not shown). The CO₂ source can be configured to maintain the concentration of CO₂ in the catholyte. For instance, the CO₂ source can be configured to bubble CO₂ through the catholyte. Additionally, or alternately, the CO₂ source can maintain a CO₂ atmosphere over the catholyte. Other mechanisms for providing CO₂ in the catholyte include, but are not limited to, high pressure electrochemical cells and gas diffusion electrodes.

Other embodiments described herein relate to a method for electrochemical CO₂ reduction. The method includes providing an electrochemical cell that includes a Cu cathode in contact with a non-aqueous electrolyte. The non-aqueous electrolyte includes a bifunctional ionic liquid (IL) which generates ion-CO₂ adducts and a hydrogen bond donor (HBD) upon CO₂ absorption. An overpotential can be applied to the Cu cathode and the non-aqueous electrolyte to implement an electrochemical CO₂ reduction reaction (CO₂RR) of CO₂ in the non-aqueous electrolyte.

In some embodiments, the bifunctional IL includes a cation, which enhances an electric field to stabilize CO₂ between the cation and Cu cathode surface, and a CO₂ chemisorbing anion, such as an aprotic heterocyclic anion or nucleophilic anion. The bifunctional IL can include, for example, 1-ethyl-3-methylimidazolium pyrrole-2-carbonitrile ([EMIM][2-CNpyr]).

In some embodiments, the non-aqueous electrolyte of the method includes a non-aqueous diluent in which the bifunctional IL is dissolved. The non-aqueous diluent can minimize the mass transfer limitations of the bifunctional IL and increase the ionic conductivity of the non-aqueous electrolyte. For example, the non-aqueous diluent can include acetonitrile.

In some embodiments of the method, the non-aqueous electrolyte further includes a supporting electrolyte to maintain a stable ionic conductivity of non-aqueous electrolyte. The supporting electrolyte can include a quaternary ammonium salt, such as tetraethylammonium perchlorate (TEAP).

The invention is further illustrated by the following example, which is not intended to limit the scope of the claims

Example

In this example, we selected the bifunctional ionic liquid (IL), [EMIM][2-CNpyr], to further understand its role in modulating the CO₂RR reaction pathways and energetics over a Cu electrode, focusing specifically on its voltage-driven behavior and modification of the interface. The reaction of this IL with CO₂ in acetonitrile based supporting electrolyte follows the known reaction Scheme (NMR in FIG. 7). Cu was explicitly selected in this study as it is the only known metal catalyst that can produce high carbon products other than CO in aqueous electrolytes. With in situ surface-enhanced Raman spectroscopy (SERS) and electrochemical impedance spectroscopy (EIS), both the surface species and the interfacial changes were determined. EIS is a powerful technique to probe the electrode-electrolyte interfaces including potential-dependent capacitance, surface adsorption, and charge transfer reactions. Specifically, careful analysis of the impedance data as a function of frequency through a circuit model fit can be used to determine capacitance due to excess charge accumulation. In complement, SERS provides chemical information in relation to the surface species and molecular orientations. We have shown the simultaneous use of ETS and SERS as an effective approach for examining the structuring of ILs, deep eutectic solvents, and similarly concentrated electrolytes at electrode surfaces.

This study captures the orientational change of [EMIM]⁺ on the Cu surface upon negative polarization and links it to the observed decay in capacitance revealing double-layer thickening. These results motivated the hypothesis that interface enrichment of Im species with negative polarization traps the CO₂ saturation products (FIG. 2), including the protonated anion that functions as a native HBD, thus resulting in a unique microenvironment to drive CO₂RR at lower overpotentials. Even though the concentration-dependent investigations of 0.1, 0.5, and 1.0 M IL in acetonitrile revealed that the diluted IL can form interfacial microenvironment similar to the neat IL, the surface coverage by [EMIM]⁺ at high concentrations (1.0 M) and thickening of the double layer decreases the catalytic activity. Furthermore, the HBD component that forms in situ by the absorption of CO₂ contributes to CO₂RR at high reaction rates with reduced overpotentials. Through gas chromatography (GC) and nuclear magnetic resonance (NMR) spectroscopy analysis, the formation of CO, CH₄, C₂H₄, C₂H₆, H₂, formate, succinate, formaldehyde and butane were identified with prolonged electrolysis experiments conducted at −2.1 V vs. Ag/Ag⁺.

Materials, Synthesis, and Characterization

[EMIM][2-CNpyr]was synthesized using 1-ethyl,3-methylimidazolium chloride ([EMIM][Chloride]; >99%, Sigma-Aldrich®) and pyrrole-2-carbonitrile (2-CNpyrH; 97%, SynQuest Laboratories, Inc.) as starting materials. Briefly, the halide salt, [EMIM][Chloride], was transformed to [EMIM][hydroxide] using an anion exchange resin (Amberlite IRN-87, Alfa Aesar) in methanol (HPLC grade, Fisher Chemical™), followed by acid-base neutralization reaction between [EMIM][hydroxide] and 2-CNpyrH that yielded the IL with water as a side product. After drying the synthesis product, the IL was confirmed by ¹H- and ¹³C-Nuclear Magnetic Resonance (NMR) spectroscopy (Bruker Ascend 500 MHz) in DMSO-d6 (99.9%, Thermo Scientific™). The elemental analysis by combustion ion chromatography (Atlantic Labs) further confirmed that the halide content was less than 0.25% (detection limit).

Electrochemical Measurements and Product Analysis

Transient Voltammetry

Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) experiments were performed to examine CO₂ reduction using Bio-Logic VSP-300 electrochemical workstation. While the LSV method was used to investigate the CO₂RR onset, CV was preferred to study if the electron-coupled proton transfer from HBD is reversible. LSVs and CVs were performed in a two-compartment H-type electrochemical cell separated by Nafion 115 (Fuel Cell Store) proton exchange membrane (FIG. 12). The Cu foil (99.9%, Sigma-Aldrich) working electrode and the Ag/Ag⁺ reference electrode were placed in the catholyte chamber, while the Pt counter electrode (4 cm² foil, 99.9%, BASi) was placed in the anolyte chamber. The reference electrode was prepared by immersing Ag wire in 10 mM AgNO₃ dissolved in 0.1 M tetraethylammonium perchlorate (TEAP, 99%, Sigma-Aldrich) in acetonitrile. Before every measurement, the working electrode (Cu foil) was polished mechanically with a polishing paper of 5000 and 8000 grids, then washed with 0.1 M H₂SO₄ (Thermo Scientific Chemicals) solution ultrasonically to remove any residual surface oxides. The electrode was then immersed in o-phosphoric acid (85%, Fisher Chemical™) and cycled between 0 V and 1.5 V vs. a Pt pseudo-reference electrode in a two-electrode setup for 50 cycles (FIG. 15A) for electrochemical polishing. Cu foil (working electrode) was further electrochemically roughened (FIG. 7B and C) and etched with 1.0 M HNO³ solution for SERS activation.

The uncompensated solution resistance in the CO₂RR cell was measured before each set of measurements and automatically corrected for iR drop at 85% through positive feedback using the EC-Lab software (V11.42, Bio-Logic). The scan rate for reported LSVs and CVs was 20 mV·s⁻¹. Before the experiment, the electrolyte solution was sparged with N₂ (99.99%, Airgas) or CO₂ (99.995%, Airgas) at a 10 mL/min flow rate. During the measurement, the headspace in the H-cell was blanketed with the same gas to avoid electrolyte evaporation or exposure to the atmosphere.

The catholyte for IL concentration-dependent study was prepared by mixing the 0.1, 0.5, or 1 M H₂SO₄ (aq.) solution. The water content of the catholyte was measured with Karl-Fischer (Metrohm 899 Coulometer) before each experiment and found to be less than 500 ppm.

Spectro-Electrochemical Measurements

In-situ surface-enhanced Raman spectroscopy (SERS) measurements, probing the surface species, were performed using a Raman system (Renishaw inVia Raman Microscope) attached to Leica DM 2500 M microscope with 785 nm excitation laser, 0.50 mW laser power, and 10% laser focus. The spectra were recorded with the Olympus LMPLFLN 20× objective with a 12 mm working distance. The custom-made SERS cell (FIG. 18) was used to collect potential dependent SERS spectra. SERS activated Cu was used as the working electrode. The non-aqueous Ag/Ag⁺ and Pt wire were used as the reference and the counter electrodes, respectively.

Electrochemical Impedance Spectroscopy (EIS)

A single-compartment three-electrode cell (FIG. 19) was used for EIS measurements to examine the interfacial charge density. A cleaned Cu rod (3 mm dia.) sealed with epoxy was used as the working electrode in EIS measurements. A mirror-polished Pt disk (1 cm² area) and non-aqueous Ag/Ag⁺ were used as counter and reference electrodes, respectively. AC voltage with a 10 mV amplitude was applied at a frequency range of 200 kHz to 500 Hz at potentials ranging from 900 mV to −2.3 V vs. Ag/Ag⁺. The obtained EIS data was validated with Kramers-Kronig (K-K) transformation. The electrochemical window EIS data collected includes regions of only capacitive behavior (first 600 mV starting from Eoc) and capacitive-faradaic behavior together (where electrochemical reduction reactions occur). Nyquist plots were converted into Cole-Cole plots using equation (1):

C ′ + ì ⁢ C ″ = 1 i ⁢ ωZ = - i ⁢ Z ′ ω ⁡ ( Z ′ 2 + Z ″ 2 ) - Z ″ ω ⁡ ( Z ′2 + Z ″2 ) ( 1 )

• • where ω is the frequency (Hz), C′ and C″ correspond to the real and imaginary components of capacitance, respectively, whereas Z′ and Z″ are the real and imaginary components of the impedance. To avoid circuit element selection-based errors over the electrochemical window studied, the real part of the capacitance obtained from equation 1 was averaged over the frequency range to obtain Cdiff plots. Nyquist plots for neat IL and diluted electrolytes are given in FIGS. 20 and 21, respectively.

Constant Potential Electrolysis (CPE) and Product Analysis

The CPE experiments were conducted at −2.1 V vs. Ag/Ag⁺ for 3 hours in the H-cell configuration described above. During CPE, CO₂ (10 mL/min) was passed through the headspace of the cathode chamber. To analyze the gaseous electrolysis products, the headspace of the cathode chamber was directly connected to a Gas Chromatography (GC-7890B, Agilent Technologies) equipped with a flame ionization detector (FID) and thermal conductivity detector (TCD). The calibration curves for quantifying CO, CH₄, C₂H₄, C₂H₆, and H² are shown in FIG. 22. For liquid product analysis, ¹H NMR was performed (Bruker Ascend 500 MHz) in DMSO-d6. Calculation details for the Faradaic efficiency are provided in SI, following the procedure reported in the literature.

Image Charge Augmented QM/MM Computations

The polarization effects are accounted for using the image charge augmented QM/MM (IC-QM/MM) methodology. This approach is employed to study the orientation change and hydrogen bonding in the following systems under applied potential within −1.0 to −2.0 V potential window: [EMIM]⁺, [EMIM]⁺—CO₂− and [2-CNpyrH], and [EMIM]⁺—CO₂− and [2-CNpyr]⁻ pairs on Cu (100) surface. In this study, the adsorbed molecules are treated using the Kohn-Sham (KS) density functional theory (DFT) at a mixed Gaussian and plane wave (GPW) level. The Cu substrate is modeled using the embedded atom model (EAM). To describe the interactions between the QM and MM regions, Lennard Jones potentials are used, with the repulsion and dispersion parameters obtained from the OPLS-AA force field for the adsorbates, and optimized parameters calculated specifically for Cu. Table 5 lists the sigma values used in calculations for adsorbates; FIG. 23 shows the atom types for adsorbates on their molecular structures. The charge distribution in the metal p_m is modeled by a set of Gaussian charges centered at the metal atoms shown in equation 2.

p m ( r ) = ∑ a ⁢ c a ⁢ g a ( r , R a ) ( 2 )

Here, R_α is the position of the metal atom α and g₋ is the spherical Gaussian function located at α. The expansion coefficients ca are determined self consistently at each step using a constant-potential condition throughout the metal (V₀) described in equation 3.

V H ( r ) + V m ( r ) = ∫ p ⁡ ( r ) + p m ( r ) ❘ "\[LeftBracketingBar]" r ′ - r ❘ "\[RightBracketingBar]" ⁢ dr ′ = V 0 ( 3 )

V_H(r) is the Hartree potential that extends beyond the metal surface into the adsorbate and generates a response to the electrostatic potential of the metal surface V_m(r). V₀ can be set to a non-zero number, and has been set in the range between −1.0 and −2.0 in this study.

TABLE 5
List of sigma values for adsorbates used for IC-
QM/MM simulations. See FIG. 21 for atom types
labeled on the molecular structures studied

	Atom Type	sigma

	C1	3.55
	C2	3.50
	C3	3.30
	C4	3.75
	H1	2.42
	H2	2.50
	H3	0.00
	N1	3.25
	N2	3.20
	O1	2.96
	Cu	2.338

For all systems, a polarization (TZVP) basis set of Molopt-type is used to represent the valence electrons while the core electrons are represented by the Goedecker, Teter, Hutter psuedopotentials. The exchange and correlation potentials are described using the Perdew-Burke-Ernzerhof (PBE) functional and the Grimme D3 dispersion correction was additionally added. The energy cutoff for the plane wave expansion is set to 600 Ry with a multigrid number 4 and REL_CUTFOFF of 40 using the smoothing for electron density (NN10_SMOOTH) and its derivative (NN10). Geometry optimization calculations were performed using the IC-QM/MM methodology within the CP2K package and final structures were analyzed using the VMD software package.

Faradaic Efficiency Calculations

The following set of equations and definitions were used for calculating the Faradaic efficiencies reported.

• • m_gaseous [mol]: The measured amount of gaseous product from GC using the area under the peak and the calibration curves
  • m_liquid [mol]: The measured amount of liquid product

m liquid [ mol ] = Area ⁢ of ⁢ the ⁢ products ⁢ related ⁢ ⁢ NMR ⁢ peak × λ V liquid × ( proton ⁢ number ) ⁢ λ : conversion ⁢ factor = 12 / ( Area ⁢ of ⁢ TEAP ⁢ peak ) ( 1 )

• • I [A]˜(Current at the injection time

r [ sccm ] : Gas ⁢ flow ⁢ rate = r 60 [ cm 3 · s - 1 ]

• • V [cm³]: GC Sam ping loop volume

F [ C . mol - 1 ] : Faraday ’ ⁢ s ⁢ constant = 96485 ⁢ t [ s ] : the ⁢ time ⁢ required ⁢ for ⁢ gaseous ⁢ products ⁢ to ⁢ fill ⁢ the ⁢ sampling ⁢ loop = V r 60 ⁢ Q [ C ] : Charge = I × t

• • e_required: Number of electrons needed for a particular product of CO₂RR
    Required amount of electrons for the formation of each product detected

	CO	H2	Formate	CH4	C2H4	C2H6	Succinate

erequired	2	2	2	8	12	14	14

The number of moles of electrons required for a particular product:

e required = m × e required ( 2 )

The total number of moles of electrons injected:

e input = Q F ( 3 )

Faradaic efficiency for gaseous products:

FE = e required e input × 100 ( 4 )

Results and Discussion

Constructing of an Interfacial Microenvironment Through CO₂— Reactive IL

Linear sweep voltammetry (LSV) was performed with neat [EMIM][2-CNpyr] to investigate the electrochemical behavior of the IL under N₂ and CO₂ on the Cu surface, as shown in FIG. 3A. Under CO₂ environment, a reduction potential at −1.8 V vs. Ag/Ag⁺ is observed and attributed to CO₂RR since it was absent under N₂. In addition, the open circuit potential (E_oc) shifts from 1.0 to 1.11 V. This is associated with the changes in the pH and composition of the electrolyte due to CO₂ absorption. The surface species are [EMIM]⁺, 2-CNpyrH, carbamate, and carboxylate, as shown in FIG. 2, formed upon CO₂ chemisorption. Specifically, [EMIM]⁺ adsorption is evident as more clearly seen under the N₂ environment where the current gradually increases in the potential range of 1.0 V to 2.1 V. Beyond 2.1 V, current increases sharply due to the reduction of [EMIM]⁺ to an adsorbed radical or carbene. Other general observations are the lower current under CO₂ compared to N₂ and the shallower slope in current beyond −1.8 V. This is due to the complexation with CO₂ that results in increased viscosity and decreased conductivity; consequently, species diffuse slower to the electrode surface, slowing charge transport rate.

Differential capacitance (C_diff) measurements were performed to probe the electrode-electrolyte interface, as shown in FIG. 3B. The capacitance curve starts with a plateau at −1.1 V vs. Ag/Ag⁺ followed by a decay in capacitance with negative polarization up to −2.0 V and another plateau at potentials more negative than −2.0 V where a minimum of 0.25 μF·cm⁻² is reached for the Cu| [EMIM][2-CNpyr]interface under both N₂ and CO₂. Initial capacitance of the interface is lower under CO₂, suggesting a difference in the interface, consistent with the shift in E_oc. As increasing negative polarization causes more and more cations to accumulate on the electrode surface, [EMIM]⁺ cations crowd the interface, as interpreted from the Goodwin-Kornyshev model which treats the interface through the mean field theory. Accordingly, the charge density at the interface increases with increased applied potential until the ions feel their excluded volumes. Since the neat IL has no solvent molecules to displace, it can only admit voids upon increased polarization. However, when the cations admit all the voids, further increase in potential results in the increased double layer thickness, known as crowding, which leads to decrease in C_diff as seen in FIG. 3B.

To further investigate the nature of C_diff and the interfacial species, in situ SERS was performed with N₂ and CO₂-saturated electrolytes (FIGS. 3C and 3D). Table 2 summarizes the observed peaks and vibrational assignments. The prominent peaks are consistent with both N₂ (FIG. 3C) and CO₂ (FIG. 3D), and the spectra point to the co-existence of both the anion and the cation on the electrode surface. However, as discussed below, there are notable changes related to the [EMIM]⁺ orientation with increased polarization and compositional changes with CO₂ absorption.

TABLE 2
SERS peaks for [EMIM][2-CNpyr] on
Cu and their vibrational assignments

Raman Shift (cm−1)

Vibrations	N2	CO2	References
d(In-plane Im ring)	1032	1027	[16-18]
u(C4-C5)	1080	1042	[17]
Pyr ring breathing mode	1090	1095	[19]
d(C4-C5—H) bending	1116	1117	[17]
d(In-plane asym. Im ring)	1183	1183	[16-18]
d(C2—H)	1254	1254	[20]
d(In-plane Pyr ring)	1266	1266	[21]
d(Out-plane Pyr ring)	1290	—	[21]
u(Im ring) + u(CH2(N))	1347	1348	[17-18]
u(Im ring) + u(CH2(N)) + u(CH3)	1380	1383	[17-18]
u(Pyr ring)	1398	1405	[22]
u(CH3(N))	1423	1432	[18]
u(Pyr ring) + u(CH3(N))	1460	1456	[18, 22]
u(Im ring) + u(CH2(N)) + u(CH3(N))	1568	1568	[17-18]
u(Im ring) + u(CH2(N)) + u(CH3(N))	1596	1597	[17-18]
u(Im —COO)	—	1613	[22-23]
u(Pyr—C≡N)	2189	2194	[21]
u(Pyr—C≡N)	2217	2223	[21]

Spectral peaks related to [EMIM]⁺ ring located at 1347, 1380, and 1423 cm⁻¹ under N₂ (FIG. 3C) intensify with applied polarization, indicating a stronger interaction with the electrode surface and further blueshift to 1348, 1383, and 1432 cm⁻¹, respectively, under CO₂ (FIG. 3D)), confirming the change in its environment due to CO₂ absorption. In parallel, [2-CNpyr]⁻ ring stretching at 1398 cm⁻¹ (FIG. 3C) that is weakly seen under N₂ intensifies and shifts to 1405 cm⁻¹ (FIG. 3D). This is due to the closer approach of the neutral 2-CNpyrH specie that forms upon CO₂ saturation to the surface. The presence of 2-CNpyrH upon CO₂ exposure is also evidenced by the shift in the C N stretching vibration at 2189 cm⁻¹ under N₂ to 2223 cm⁻¹ under CO₂. The peak at 1423 cm⁻¹ under N₂ and 1432 cm⁻¹ under CO₂ are attributed to the symmetric stretching of the CH₃ group. Since it presents very low enhancement with negative polarization, CH₃ groups are interpreted to be away from the surface, pointing towards the bulk electrolyte. On the contrary, the peaks at 1116, 1342, and 1380 cm⁻¹ associated with C₄ and C₅ of [EMIM]⁺ and N CH₂ moiety of the ethyl group increase in intensity with negative polarization, as seen in FIG. 4A, indicating a change in orientation of the cation. This behavior is persistent with N₂ and CO₂, although it is better seen under N₂ without interference from complexation with CO₂. Since the surface is already crowded by [EMIM]⁺, the intensification of these specific ring modes is interpreted to be due to the changes in the orientation of [EMIM]⁺ from tilted (C₂ position facing the electrode surface) to parallel configuration with respect to the electrode surface.

The potential at which [EMIM]⁺ transitions from tilted to parallel can be easily seen from the recorded intensities of the associated peaks plotted in FIG. 4A. Specifically, the intensities of the 1342 and 1380 cm⁻¹ peaks associated with N—CH₂ stretching increase sharply beyond 1.5 V vs. Ag/Ag⁺, which also coincides with the potential where the decrease in capacitance observed in FIG. 3B during the negative potential sweep in EIS. When compared between N₂ and CO₂, the relative enhancement in the peak intensities used to identify the orientation change is lower under CO₂ as the [EMIM]⁺ CO₂ adduct forms. The change in the orientation of Im-species is further supported by IC-QM/MM calculations of the optimized geometries as shown in FIG. 4B. At −1.0 V, the lowest energy structure shows a tilted orientation, while at 1.7 V the lowest energy structure changes to a parallel orientation. IC-QM/MM calculations at 1.5 V still show an energetic preference for the tilted orientation, however there is a clear transition to the parallel preference at more negative potentials, marking 1.5 V vs. Ag/Ag⁺ as the potential of transition. This trend continues into 2.0 V where the preferred orientation is parallel as shown in the relative energies given in Table 3. In the light of SERS analysis, the interfacial liquid structure can be considered a cation-rich layer infiltrated with anions under N₂. Under CO₂, the surface is dominated by [EMIM]⁺, [EMIM]⁺ CO₂—, 2-CNpyrH, and fewer [2-CNpyr]−. The crowding and thickening of the interface with [EMIM]⁺ upon negative polarization accompanied by reorientation are believed to impair the ability of the anion to escape from the interface, especially when the neutral form of the anion (2-CNpyrH) gets reduced, the effect of which is discussed later. Therefore, we observe features of the anion in SERS at 2189 cm⁻¹ under N₂ and 2194 cm⁻¹ under CO₂. The existence of the pyrrole species contributes to the formation of a unique microenvironment for CO₂RR since the anion also complexes with CO₂ ([2-CNpyr]-CO₂⁻) and can serve as a CO₂ carrier to the electrode surface. In addition, the protonated anion, 2-CNpyrH, can act as HBD.

TABLE 3
Relative energies differences in units of kcal/mol for [EMIM]+ on Cu
with a range of applied potentials between −1.0 and −2.0 V

Applied Potential (V)	−1.0	−1.3	−1.5	−1.7	−2.0

Tilted	0.00	0.00	0.00	0.19	0.86
Parallel	0.26	0.42	0.65	0.00	0.00

CO₂RR Catalytic Behavior Modulated by the Microenvironment

Neat [EMIM][2-CNpyr]has a viscosity of 68 cP at 25° C., which increases to 247 cP upon saturation with CO₂; these high viscosities is a limiting factor for CO₂RR. Therefore, acetonitrile was used as a diluent to minimize the mass transfer limitation and increase the ionic conductivity in CO₂RR. The concentration-dependent ionic conductivity and the viscosity of diluted [EMIM][2-CNpyr](0.0 to 3.0 M) are provided in FIG. 9. A maximum ionic conductivity at 1.5 M and a consistent increase in viscosity with increasing IL concentration was observed. Because the measured ionic conductivity of [EMIM][2-CNpyr]/acetonitrile decreased after CO₂ saturation, 0.1 M tetraethylammonium perchlorate (TEAP) was also added as the supporting electrolyte to maintain a stable ionic conductivity. Therefore, further analysis of the interface and CO₂RR with IL/acetonitrile electrolytes was performed with 0.1, 0.5, and 1.0 M concentrations of IL and 0.1 M TEAP supporting salt.

Interestingly, the general behavior of the C_diff curves is nearly identical between the IL electrolytes in acetonitrile (FIG. 5A) and with the neat IL (FIG. 3B), which suggests that the ionic microhabitat and the structure of the double-layer formed on the electrode surface are similar. With 0.1 and 0.5 M IL electrolytes, the decay in capacitance with a negative potential sweep gradually starts at about 1.5 V vs. Ag/Ag⁺. The decline in capacitance occurs at a notably faster rate with 1.0 M IL. This decay further confirms the higher [EMIM]⁺ concentration covering the electrode surface, thickening the double-layer faster than the relatively diluted electrolytes (0.1 and 0.5 M IL), and impeding the diffusion of new IL CO₂ adducts towards the electrode surface, i.e., increased mass transfer overpotential. In addition to the similarities in C_diff, the comparison of the SERS spectra obtained from the neat (FIGS. 3C and 3D) and diluted IL electrolytes (FIG. 10) further confirm the similar microenvironment for CO₂RR where Im-species (i.e., [EMIM]⁺ and [EMIM]⁺—CO₂) dominate the interface with entrapped [2-CNpyr]⁻ and HBDs (2-CNpyrH). This microenvironment is unique compared to prior literature involving Im-based ILs since more active species exist in the interface along with native HBD (in situ generated due to CO₂ absorption).

TABLE 1
Comparison of CO2RR Faradaic efficiencies according to the reported
products in IL electrolytes on Cu-based electrodes.

Faradaic Efficiency

Specifications	Electrolyte	H2	C1	C2-3	C4
Cu foil 2-C	[EMIM][2-CNpyr]	7.6%	70% CO +	1.5% C2H4 + C2H6	7.3% C4H6O4
H-cell CA at −2.1	in ACN*		COOH− + CH4
V vs. Ag/Ag+	0.1M [EMIM][2-CNpyr] +	39%	33% CO +	~6% C2H4 + C2H6	6.5% C4H6O4
	0.5M 2-CNpyrH		COOH + CH4
Cu foil 2-C	0.3M [BMIM][BF4] +	N/A	99.5% CO	N/A	N/A
H-cell CA at −2.5	0.2M [C12mim][BF4]
V vs. Ag/Ag+	in ACN
Poly(IL)- Cu 3-C	1M KOH (aq.)	8.1%	18.3% CO + CH4 +	76.1% C2H4 +	N/A
Flow Cell CA at −0.85			HCOOH + CH3OH	C2H5OH +
V vs. RHE				CH3COOH + C3H7OH
Cu2O/IL-graphite 2-C	0.1M KHCO3 (aq.)	20%	25% CH4 +	52.4% C2H4 +	N/A
H-Cell CA at −1			CO + COOH−	C2H5OH
V vs. RHE
Cu foil 2-C	0.1M KHCO3 + 10 mM	17%	52% CO +	20% C2H4 +	N/A
H-Cell CA at −1.12	[BMIM][TFSI] (aq.)		COOH− + CH4	C2H5OH
V vs. RHE
BaO/Cu 3-C	1M KOH	~10%	N/A	~81% C3H7OH +	N/A
Flow Cell CP at 500		H2		CH3COO +
mA · cm−2				C2H5OH + C3H7OH
CA: Chronoamperometry, CP: Chronopotentiometry, 2-C: Two Compartment, 3-C: Three Compartment.
*FEs calculated for [EMIM][2-CNpyr] in CAN reflect the average values obtained for the 0.1-1M IL concentration range

The LSV measurements under CO₂ suggest the onset potential for CO₂RR is at about −1.9 V vs. Ag/Ag⁺ for all the IL electrolytes studied (FIG. 5B). Consistent onset potential implies the activation energy for the charge trans-fer does not depend on IL concentration. However, the IL concentration significantly impacts CO₂RR current densities, similar to our previous report with the Ag electrode. Increasing the IL concentration from 0.1 to 0.5 M results in a more than two-fold increase in CO₂RR current due to the increased availability of CO₂ at the electrode surface. However, the catalytic current decreases with further increase in IL concentration to 1.0 M. [EMIM]⁺to cover the entire surface even at low concentrations. Therefore, the decrease in CO₂RR activity is attributed to the surface layer formation by the cation and thickening of the double layer which prevents easy access of reactants to the electrode surface, thus increased mass transfer overpotential and lowered catalytic activity.

With respect to the role of the IL in lowering the overpotential needed for CO₂RR, one widely acknowledged explanation involves the formation of the IL-CO₂ adducts, i.e., carboxylate formation at the interface with CO₂ binding to the C₂ position of the Im ring. The facilitation of CO₂RR by the carboxylate results in a decreased overpotential compared to its absence; however, after a particular concentration, the reaction becomes impaired, and the rate slows down, as seen with 1.0 M IL in FIG. 5C, due to the double layer dynamics discussed above. Another explanation of the role of the IL focuses on the hydrogen bonding that also modulates the electrical field and interaction with the intermediates at the interface, especially with CO₂⁺⁻. To probe specifically the impact of the hydrogen bonding network on CO₂RR at the interface, we performed CV measurements with additional 2-CNpyrH (0.1 and 0.5 M) to vary HBD concentration independently in the IL electrolyte (FIG. 5C).

We hypothesized that incorporating HBDs could enhance the CO₂ reduction activity by increasing the —COOH intermediate formation. As seen in FIG. 5C, additional 2-CNpyrH at 0.1 M does not influence the CO₂RR onset potential whereas it increases the catalytic current, compared to 0.1 M IL electrolyte without the HBD addition. When the 2-CNpyrH concentration was increased to 0.5 M, the catalytic activity also increased along with a positive shift in the onset potential by 120 mV. These results suggest a substantial modification to the energetics of the first proton-coupled electron transfer step in CO₂RR (i.e., formation of —COOH) as a function of HBD concentration.

The enhanced catalysis by HBD is also evident from the SERS spectra shown in FIG. 5D, where the surface adsorbed CO peak (*CO_ads in 2046-2088 cm⁻¹ range) gains intensity at potentials more negative than 1.5 V vs. Ag/Ag⁺ compared to that without HBD (full spectra is given in FIG. 11). The observed broad peaks indicate the presence of intramolecular C≡O stretching due to the binding of CO to the Cu electrode surface, consistent with multiple binding configurations. Furthermore, the results suggest that an optimal ratio of [EMIM]⁺ (hence carboxylate) and HBD may exist, highlighting the significance of the ionic microenvironment on the electrode surface for CO₂RR and demonstrating a new approach to tune the microenvironment in non-aqueous electrolytes.

In complement, computational analysis of the simplified interface shown in FIGS. 5E and 5F using IC-QM/MM optimized geometries of [EMIM]⁺—CO₂ with 2-CNpyrH and [EMIM]⁺ CO₂ with 2-CNpyr, respectively, at −1.7 V demonstrate the hydrogen bonding in the presence of the native HBD. The HBD (CNpyrH) prefers to orient itself towards the carboxylate moiety interacting through hydrogen bonding and stays close to the surface while [EMIM]⁺—CO₂ prefers a tilted orientation, consistent with the lower increase in intensities under CO₂ (FIG. 4A). Upon deprotonation and the formation of the anion, the [CNpyr]⁻ prefers to move away from the negative Cu surface, however, gets trapped in the microenvironment due to the double layer thickening as seen from 2194 cm⁻¹ feature in FIG. 3D. From the computed anion-cation configurations, it is also seen that there is still hydrogen bonding between [CNpyr]− and [EMIM]⁺ CO₂³¹ through the C4, C5 protons of Im. This causes the cation to move more towards a parallel orientation as in FIG. 5F.

To distinguish between the microenvironments formed by different electrolyte concentrations, constant potential electrolysis experiments were conducted at 2.1 V vs. Ag/Ag⁺ for 3 hours in a two-compartment cell (FIG. 12) coupled with in-line GC analysis. The gaseous products were identified and quantified by in-line GC (example in FIG. 13) while the liquidus products were analyzed after electrolysis by NMR (example in FIG. 14). Table 4 summarizes the quantified products from electrolysis. The catholyte was the IL electrolyte of interest and the anolyte was aq. 0.1 M H₂SO₄ serving as the proton source. The calculated Faradaic efficiencies (FE) for H₂ and C₁ products, such as CO, formate, and CH₄, are given in FIG. 6A, and that of the C₂⁺ products, such as succinate, C₂H₄, and C₂H₆, are shown in FIG. 6B. Selectivity for CO (C₁, 2e⁻) increases from 23% to 43% as the concentration of IL increases from 0.1 M to 0.5 M, as seen in FIG. 6A. Further increase in the concentration slightly decreases the selectivity for CO to 37%. High local CO₂ concentrations cause premature CO desorption from the Cu surface and lead to high CO selectivity. Increasing IL concentration increases CO₂ concentration on the electrode surface, leading to CO desorption. Furthermore, [EMIM]⁺ adsorption may also replace the CO_ads on the electrode surface, causing high CO selectivity.

TABLE 4
The identified electrolysis products and their quantities as determined from NMR and GC analysis in terms of Faradaic efficiency (FE)

IL
concentration		Faradaic Efficiency %
in 0.1M TEAP		Time [min]	Average FE

acetonitrile	0	20	40	60	80	100	120	140	160	180

0.1M	H2	0.00	0.0556	4.2119	4.1971	5.112	5.9779	9.0753	14.518	20.651	28.029	10.203
	CO	0.00	4.68	42.519	23.522	25.368	16.989	27.801	23.0308	20.599	21.458	22.885
	CH4	0.00	N/D	N/D	N/D	N/D	N/D	N/D	2.94	2.82	3.08	2.946
	C2H4	0.00	N/D	N/D	N/D	N/D	N/D	N/D	N/D	N/D	N/D	N/D
	C2H6	0.00	1.578	1.6162	1.6184	1.6220	1.5921	1.6383	1.6066	1.689	1.583	1.616

	Formate	0.00	44.729	44.729
	Succinate	0.00	6.629	6.629

TOTAL

89.0080

0.5M	H2	0.00	0.8530	0.902	1.608	2.951	5.508	9.553	18.628	22.001	23.575	7.1973
	CO	0.00	0.01	50.952	48.655	49.570	56.879	52.105	43.823	33.740	47.749	43.809
	CH4	0.00	N/D	N/D	N/D	N/D	N/D	N/D	N/D	0.862	0.790	0.826
	C2H4	0.00	N/D	N/D	N/D	N/D	N/D	0.495	0.553	0.575	0.576	0.549
	C2H6	0.00	0.551	0.530	0.529	0.544	0.520	0.538	0.521	0.541	0.528	0.533

	Formate	0.00	26.0695	26.0695
	Succinate	0.00	11.242	11.252

TOTAL

90.2358

1.0M	H2	0.00	N/D	1.172	1.641	2.639	4.377	6.891	8.978	14.371	21.564	7.704
	CO	0.00	5.42098	21.08895	39.03585	38.346095	42.52047	47.9607	43.19646	53.067716	56.08117	37.405
	CH4	0.00	N/D	N/D	N/D	N/D	N/D	N/D	N/D	N/D	N/D	N/D
	C2H4	0.00	N/D	N/D	N/D	0.524	0.574	0.591	0.618	0.718	0.954	0.6631
	C2H6	0.00	0.572	0.611	0.597	0.611	0.579	0.600	0.602	0.599	0.591	0.595

	Formate	0.00	36.025	36.025
	Succinate	0.00	4.082	4.082

TOTAL

86.4741

0.1M IL	H2	0.00	N/D	24.485	41.142	48.973	51.238	50.456	49.765	45.804	42.485	39.668
with	CO	0.00	2.726	25.87	17.895	14.581	11.723	10.235	11.187	10.376	9.389	14.247
0.5M HBD	CH4	0.00	4.888	5.510	5.320	5.619	5.358	5.398	5.043	5.856	5.043	5.993
	C2H4	0.00	N/D	2.965	2.659	2.791	2.863	3.157	3.505	3.737	3.958	3.204
	C2H6	0.00	2.831	2.793	2.707	2.834	2.697	2.564	2.658	2.613	2.408	2.679

	Formate	0.00	18.104	18.104
	Succinate	0.00	6.405	6.405

TOTAL	90.3007

The second major product is formate (C₁, 2e⁻), with 45% FE for 0.1 M IL, 26% FE for 0.5 M IL, and 36% FE for 1.0 M IL. The spectral evidence to —COOH in complement to significant formate presence in products strengthen the argument for the role of the proton dynamics between the Im species and HBD in controlling the mechanism leading up to CO formation and beyond in these non-aqueous electrolytes. As seen in FIG. 6A, the selectivity of formate decreases from 45% to 26% when the electrolyte concentration is increased from 0.1 M to 0.5 M IL. This decrease is accompanied by an increase in the selectivity towards succinate (C₄, 14e⁻), as seen in FIG. 6B. Succinate has a relatively low selectivity with 6%, 11%, and 4% FE for 0.1 M, 0.5 M, and 1.0 M concentrations, respectively. The initial increase in selectivity towards succinate may be due to the COOH coupling that proceeds it; however, further increase in the concentration of COOH does not result in increased FE for succinate, likely due to the over saturation of the electrode with [EMIM]⁺ as discussed earlier, thus preventing the C₂-C₂ coupling. FE for H₂ is small, less than 10%, since HER is suppressed in IL containing electrolytes, as generally reported in the literature and most of the observed HER is from water cross-over from the anolyte. However, HER enhances when HBD is introduced at 0.5 M (39% FE) suggesting a high surface coverage by H³⁰, which further explains the positive potential shift observed in FIG. 5C. This finding underlines the role of HBDs in CO₂RR selectivity and further suggests that selectivity can be tuned by IL:HBD composition and by replacing the aqueous anolyte with a non-aqueous system as the proton source. Increased H⁺ availability due to HBD also promotes the formation of CH₄, C₂H₄, and C₂H₆, as seen in FIG. 6B. Faradaic efficiencies in FIG. 6 add up to 90% of the total efficiency. Apart from the reported products in FIG. 6, butane and formaldehyde were also observed in GC (FIG. 15) and NMR (FIG. 16), respectively, in negligible quantities. Table 1 compares the obtained Faradaic efficiencies with previous reports utilizing IL electrolytes in CO₂RR on Cu-based electrodes. Uniquely, C₄ products are obtained in this study owing to the constructed microenvironment presented.

The dynamics of reactive IL at the electrode-electrolyte interface was demonstrated by identifying the orientational change of [EMIM]⁺ through spectro-electrochemical studies complemented by IC-QM/MM calculations. The potential at which the orientation changes correspond to the start of the decline of double-layer capacitance, indicating the increase in double-layer thickness. The dynamics and the concentration of the IL were shown to influence the electrocatalytic behavior of CO₂RR through transient voltammetry. By examining the in situ generated HBDs by SERS and simultaneously probing the independent HBD additions in electrolysis, the kinetics, the onset potential, and the reaction products of CO₂RR are found to be dependent on the HBD composition in the electrolyte. The concentration of IL and HBD can switch the reaction pathways by preventing C C coupling and increasing the hydrogenation of carbon products. Constant potential electrolysis revealed the formation of a broad spectrum of products (C₂₊ up to C₄) enabled by the constructed microenvironment. The examination of the reaction mechanisms leading to C₂₊ products in these non-aqueous electrolytes by ultrafast in situ spectroscopy complemented by energetic calculations of the possible reaction steps by theory can guide synergistic electrode-electrolyte design for further tuning of product selectivity in CO₂RR.

From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims. All references, publications, and patents cited in the present application are herein incorporated by reference in their entirety.