System and Method for Electrochemical Co2 Reduction

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.

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

The rise in atmospheric COhas led to changes in temperature, rising sea levels, and ocean acidification. This rise in COis 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 COemissions while yielding alternative fuel sources that give off net zero greenhouse gases is COreduction reaction (CORR). CORR is an electrochemical reaction that reduces COinto 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 COto commodity chemicals. However, the initial electron transfer to activate COthat is otherwise linear and stable remains a critical challenge. This continues to be one of the primary obstacles in commercializing technologies for the electrochemical CORR, preventing CORR from being economically viable at a large scale, along with the related challenges of selectivity and catalyst stability.

Embodiments described herein relate to an electrochemical COreduction system that includes a functionalized ionic liquid (IL) that generates ion-COadducts and a hydrogen bond donor (HBD) upon COabsorption to modulate COreduction reaction (CORR) 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 COreduction system described herein provides a unique microenvironment where the catalytic activity of the CORR 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 CORR through IL-COadducts. The HBD concentration modulates the local proton availability. Modulation of the IL and HBD concentrations can provide a CORR that has ample COavailability, reduced overpotential, and suppressed hydrogen evolution reaction (HER) where Cproducts are obtained.

In some embodiments, the electrochemical COreduction 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 CObetween the cation and a Cu cathode surface, and a COchemisorbing 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 CO.

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 COreduction 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 CORR of COon the Cu cathode in the non-aqueous electrolyte. The applied voltage overpotential can be effective to reduce COin the non-aqueous electrolyte to at least one of CO, CH, CH, CH, formate, succinate, formaldehyde, or butane

Other embodiments described herein relate to a method for electrochemical COreduction. 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-COadducts and a hydrogen bond donor (HBD) upon COabsorption. An overpotential can be applied to the Cu cathode and the non-aqueous electrolyte to implement an electrochemical COreduction reaction (CORR) of COin the non-aqueous electrolyte.

In some embodiments, the bifunctional IL includes a cation, which enhances an electric field to stabilize CObetween the cation and Cu cathode surface, and a COchemisorbing 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).

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 COreduction system that includes a functionalized ionic liquid (IL) that generates ion-COadducts and a hydrogen bond donor (HBD) upon COabsorption to modulate COreduction reaction (CORR) 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 COreduction system described herein provides a unique microenvironment where the catalytic activity of the CORR 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 CORR through IL-COadducts. The HBD concentration modulates the local proton availability. Modulation of the IL and HBD concentrations can provide a CORR that has ample COavailability, reduced overpotential, and suppressed hydrogen evolution reaction (HER) where Cproducts are obtained.

schematically illustrates an electrochemical COreduction systemin accordance with an embodiment described herein. The COreduction system includes an electrochemical cellfor electrochemical CORR of CO. The electrochemical cellincludes a catholyte chamber, which contains a cathodeor working electrode and a reference electrode (not shown) immersed in a catholyte, and a separate anolyte chamber, which contains an anodeor counter electrode immersed in an anolyte. The cathodeand anodecan be electrically connected to an electrical source or voltage sourcethat can apply a voltage potential difference between the cathodeand the anode. The catholyteand cathodecontained in the catholyte chamberare configured for COreduction of COin the catholyteupon application of a voltage overpotential to the cathodeand catholyte. The anodeand anolyteare configured for oxidation, and the reference electrode is configured to measure the potential of the cathodeor the working electrode.

The cathode-side COreduction reaction depends on the material used to form the cathodeor coat an outer surface of the cathode. 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 cathodeor surface coating of the cathodecan 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 anodeis 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 catholyteor cathode-side electrolyte includes a non-aqueous electrolyte that can absorb COfrom a COsource, such as circulating COgas supplied by a gas inlet (not shown) to the catholyte chamber. The non-aqueous electrolyte includes a functionalized ionic liquid (IL) that generates ion-COadducts and a hydrogen bond donor (HBD) upon COabsorption to modulate COreduction reaction (CORR) on the Cu cathodein 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 CObetween the cation and the Cu cathode surface, and a COchemisorbing anion. The HBD can be formed in situ in the non-aqueous electrolyte by absorption of CO.

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

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 COchemisorbing anion can include an aprotic heterocyclic anion or nucleophilic anion. The COchemisorbing 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 COabsorption, can include an imidazolium-based cation and a pyrrolide-based anion. Advantageously, imidazolium with negative polarization can trap COsaturation products including the protonated anion that functions as a native HBD, thus resulting in a unique microenvironment to drive CORR at lower overpotentials. Furthermore, the HBD component can form in situ by the absorption of COand contribute to CORR 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 COabsorption 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 LiPF, NaTFSI, or NHPF, 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 anolyteprovided in the anolyte chambercan include an acidic aqueous solution, such as an about 0.1 M to about 1 M HSOaqueous solution.

The catholyte chamberof the electrochemical cellcontaining the catholyteand cathodeis separated from the anolyte chambercontaining the anodeand anolytewith a membrane, which prevents any mixing of the electrolytesand. The membraneis not subject to any special restrictions provided it separates the catholyte chamberand the anolyte chamber. In particular, the membrane prevents essentially any crossover of COand/or its dissolved form to the anode. The membranecan 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 COreduction system, the voltage sourcecan apply a voltage overpotential to the Cu cathodeand the non-aqueous electrolyteto implement an electrochemical CORR of COon the Cu cathodein the non-aqueous electrolyte. 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. The applied voltage overpotential can be effective to reduce COin the non-aqueous electrolyte to at least one of CO, CH, CH, CH, formate, succinate, formaldehyde, or butane.

Optionally, when it is desirable for the COreduction systemto operate at or near steady state, the COreduction systemcan include a COsource (not shown). The COsource can be configured to maintain the concentration of COin the catholyte. For instance, the COsource can be configured to bubble COthrough the catholyte. Additionally, or alternately, the COsource can maintain a COatmosphere over the catholyte. Other mechanisms for providing COin 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 COreduction. 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-COadducts and a hydrogen bond donor (HBD) upon COabsorption. An overpotential can be applied to the Cu cathode and the non-aqueous electrolyte to implement an electrochemical COreduction reaction (CORR) of COin the non-aqueous electrolyte.