USE OF ACIDIC ELECTROLYTE CATIONS FOR SELECTIVE ELECTROCHEMICAL CO2 AND CO CONVERSIONS TO METHANOL

Abstract

Described is a method is to improve catalytic activity and selectivity for electrochemical CO2-to-methanol and CO-to-methanol conversions by employing acidic electrolyte cations that can facilitate proton transfer during the electrocatalytic conversion reactions.

Description

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application No. 63/499,221, filed Apr. 29, 2023.

BACKGROUND

It has recently been reported that commercially available cobalt phthalocyanine (CoPc) catalyst, which is loaded on a carbon support (e.g., carbon nanotube), can electrochemically convert CO₂ and CO into methanol at a reductive potential in aqueous electrolyte condition (i.e., CO₂-to-methanol and CO-to-methanol conversion). For such reactions, aqueous electrolyte with potassium cation (K⁺) is typically used, for example, 0.1 M potassium bicarbonate (KHCO₃), potassium phosphate buffer (KH₂PO₄ and K₂HPO₄ mix), or potassium hydroxide (KOH). Although having been tested for CO₂— and CO-to-other reaction product conversions (e.g., carbon monoxide, ethylene), different cations such as Li⁺, Na⁺, and Cs⁺, to the best of our knowledge, have never been tested for the CO₂— and CO-to-methanol conversions.

SUMMARY OF THE INVENTION

The crucial role of electrolyte cations in CO₂ electroreduction has received intensive attention. One prevailing theory is that through electrostatic interactions or direct coordination, larger cations such as Cs⁺ can better stabilize the key intermediate species for CO and multi-carbon (C₂₊) product generation, for example, on silver and copper, respectively. Herein, we show that smaller, more acidic alkali metal cations greatly enhance CO₂-to-methanol conversion kinetics (Li⁺>Na⁺>K⁺>Cs⁺) on an immobilized molecular cobalt catalyst, unlike the trend observed for CO and C₂₊. Through electrokinetic analyses and kinetic isotope effect studies along with computational investigations, we show that hydration shell of a cation serves as a proton donor in the rate-determining protonation step of adsorbed CHO where acidic cations promote the proton-coupled electron transfer. This disclosure reveals the promotional effect of cation solvation environment on CO₂ electroreduction beyond the widely acknowledged stabilizing effect of cations.

In one aspect, the present disclosure provides a method of producing methanol, comprising:

• • contacting an aqueous solution comprising a cation and an anion with CO₂ or CO, thereby forming an aqueous solution comprising a cation, an anion, and CO₂ or CO;
  • contacting the aqueous solution comprising the cation, the anion, and CO₂ or CO with an electrode; and
  • applying a voltage to the aqueous solution comprising the cation, the anion, and CO₂ or CO via the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows methanol selectivity as a function of applied potential (vs. reversible hydrogen electrode, RHE) for CO and CO₂ reduction reaction (CORR and CO₂RR, respectively) in an exemplary system. For CORR, 0.1 M alkali metal hydroxide electrolyte (pH 13) was used, while 0.1 M alkali metal bicarbonate electrolyte (pH 6.8) was used for CO₂RR.

FIGS. 2A-2D show electrocatalytic CO₂-to-methanol conversion by cobalt phthalocyanine catalyst supported on carbon nanotube (CoPc/CNT) and its cation dependence. FIG. 2A shows a schematic of CoPc/CNT with an electrolyte cation in the vicinity of the cobalt active site. FIG. 2B shows a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of CoPc/CNT (scale bar: 5 nm). The inset shows an electron energy loss spectrum obtained from the dashed area. FIG. 2C shows Faradaic efficiencies (FEs) for carbon monoxide (CO, top) and methanol (MeOH, bottom). The inset in c shows a proton nuclear magnetic resonance (¹H NMR) spectrum after ¹³CO₂ reduction experiment. FIG. 2D shows partial current densities for CO (top) and MeOH (bottom). The dashed line indicates the experimental onset potential for methanol production. Electrochemical testing was conducted in an H-cell where cathodic and anodic compartments are separated by an anion exchange membrane. Gas and liquid reaction products were quantified using gas chromatography (GC) and NMR, respectively. Error bars are one standard deviation of at least three independent measurements.

FIGS. 3A-3D show the effect of electrolyte cation on CO and MeOH production rate and an energy profile of the reaction pathway for methanol production. FIG. 3A shows Tafel plots with different cations for CO production from CO₂ reduction reaction (CO₂RR) on the reversible hydrogen electrode (RHE) scale. Linear lines represent fitted lines to obtain Tafel slopes. FIG. 3B shows Tafel plots with different cations for MeOH production from CO reduction reaction (CORR, left) and CO₂RR (right) on the RHE scale. For CO₂RR, bicarbonate electrolytes (pH 6.8) were used, while hydroxide electrolytes (pH 13) were used for CORR. Linear lines represent fitted lines to obtain Tafel slopes. FIG. 3C shows Tafel plots for MeOH production replotted on the standard hydrogen electrode (SHE) scale. FIG. 3D shows density functional theory (DFT) free energy diagram of the proposed reaction pathway from CO₂ to methanol at 0.02 V_RHE (equilibrium potential for CO₂-to-methanol reduction) and −0.7 V_RHE.

FIGS. 4A & 4B show the kinetic isotope effect (KIE) on CO and methanol generation. Comparison in partial current densities for CO (FIG. 4A) and MeOH (FIG. 4B) measured in 0.1 M LiHCO₃/H₂O and 0.1 M LiDHCO₃/D₂O. The gray dashed line in a indicates the onset potential for methanol production. Error bars are one standard deviation of at least three independent measurements.

FIGS. 5A-5E show a proposed mechanistic pathways for electrochemical CO₂-to-methanol conversions catalyzed by CoPc and the role of electrolyte cation. FIG. 5A shows the overall catalytic cycle of CoPc catalyst during CO₂ electro-conversion. The RDS for CO₂-to-CO conversion at small overpotentials (n) is an electron transfer (ET) to CO₂ molecule, while the rate-determining step RDS for MeOH production at large overpotentials is the protonation of *CHO intermediate through a proton-coupled electron transfer (PCET). FIG. 5B shows representative snapshots from molecular dynamics simulations showing different solvation structures near the cation (Li⁺ and Cs⁺, left and right, respectively) and the adsorbed CHO intermediate. FIG. 5C shows radial distribution function (RDF, g) of M⁺-O_*CHO with different cations. FIG. 5D shows cation-dependent average number of hydrogen bonds within 6 Å of *CHO (left y-axis) and average coordination number of M⁺-O_*CHO at the first peak in the RDF plot in c (right y-axis). FIG. 5E shows RDF of H_water—C_*CHO (depicted in the inset) for Li⁺ and Cs⁺.

DETAILED DESCRIPTION OF THE INVENTION

It is generally known that larger cations such as Cs⁺ promote CO₂ and CO reduction reactions (i.e., Li⁺<Na⁺<K⁺<Cs⁺). However, in stark contrast to the commonly observed cationic trend, we discovered that for CO₂— and CO-to-methanol conversions, the smaller the cation size, the better the catalytic outcome (activity and selectivity), namely Li⁺>Na⁺>K⁺>Cs⁺. This unexpected discovery led us to find that smaller, acidic cations enable facile proton transfer, which is a critical reaction step during electrochemical methanol production, consequently improving the reaction rates. This cationic trend could be generally applicable to other types of catalyst that produce methanol from CO₂ or CO reduction reaction.

When smaller cations are used, CO₂— and CO-to-methanol conversion rates are significantly enhanced, leading to a higher methanol selectivity at a certain applied potential. For instance, at around-0.80 V vs. reversible hydrogen electrode (RHE), CoPc catalyst loaded on carbon nanotube (CoPc/CNT) shows 10.2% methanol selectivity for CO₂ conversion in 0.1 M KHCO₃ electrolyte (FIG. 1). Meanwhile, it shows 26.9% methanol selectivity in 0.1 M LiHCO₃ electrolyte, exhibiting nearly three-fold enhancement in product selectivity. On the other hand, when 0.1 M CsHCO₃ is used, methanol selectivity is only 5% (i.e., Li⁺>Na⁺>K⁺>Cs⁺). In addition, in 0.1 M KHCO₃, ˜27% methanol selectivity can only be achieved at −0.88 V, which is ˜100 mV more negative (that is, an additional energy input) than what is needed in 0.1 M LiHCO₃. This translates to ˜10% reduction in energy input at the cathode where the CO₂-to-methanol conversion occurs, when LiHCO₃ is used for electrochemical CO₂-to-methanol conversion.

Electrochemical CO₂ conversion is a highly promising technology in that it can utilize affordable renewable electricity and anthropogenic CO₂ captured from large-emission point sources or directly from the atmosphere to produce various value-added products such as methanol under ambient conditions (i.e., room temperature and ambient atmosphere). Furthermore, in this technology, water is conveniently used as a proton source for the CO₂ conversion reaction, in contrast to conventional Fischer-Tropsch process where gas-phase hydrogen (H₂) is used to hydrogenate CO (g). Given large CO₂ emissions and high costs related to H₂ production/storage/transport, direct electrochemical CO₂ conversion can be an alternative carbon-neutral technology to conventional chemical manufacturing industry.

Among the possible reaction products from the electrochemical CO₂ conversion, methanol is an industrially important and versatile chemical as it has wide industrial applications (e.g., a liquid energy carrier, a chemical feedstock for further upgrading) with a high annual demand (>100 million metric tons per year).

In one aspect, the present disclosure provides method for producing methanol, comprising:

• • contacting an aqueous solution comprising a cation and an anion with CO₂ or CO, thereby forming an aqueous solution comprising a cation, an anion, and CO₂ or CO;
  • contacting the aqueous solution comprising the cation, the anion, and CO₂ or CO with an electrode; and
  • applying a voltage to the aqueous solution comprising the cation, the anion, and CO₂ or CO via the electrode.

In certain embodiments, the method comprises:

• • contacting an aqueous solution comprising a metal cation and an anion with CO₂, thereby forming an aqueous solution comprising a cation, an anion, and CO₂;
  • contacting the aqueous solution comprising the cation, the anion, and CO₂ with an electrode; and
  • applying a voltage to the aqueous solution comprising the cation, the anion, and CO₂ via the electrode.

In certain embodiments, the method comprises:

• • contacting an aqueous solution comprising a metal cation and an anion with CO, thereby forming an aqueous solution comprising a cation, an anion, and CO;
  • contacting the aqueous solution comprising the cation, the anion, and CO with an electrode; and
  • applying a voltage to the aqueous solution comprising the cation, the anion, and CO via the electrode.

In certain embodiments, the cation is a metal cation. In certain embodiments, the metal cation is an alkali metal or an alkaline-earth metal. In certain embodiments, the metal cation is selected from the group consisting of Li⁺, Na⁺, K⁺, Cs⁺, Be²⁺, Mg²⁺, Ca²⁺, and Ba²⁺; or a combination thereof. In certain embodiments, the metal cation is Li⁺, Na⁺, K⁺, or Cs⁺. In certain embodiments the metal cation is Li⁺. In certain the metal cation is Be²⁺, Mg²⁺, Ca²⁺, or Ba²⁺.

In certain embodiments, the cation is an organic cation. In certain embodiments, the cation is an ammonium cation. In certain embodiments, the nitrogen cation is an alkylammonium cation. In certain embodiments, the alkylammonium cation is trimethylammonium, triethylammonium, tripropylammonium, or tributylammonium.

In certain embodiments, the anion is bicarbonate or hydroxide. In certain embodiments, the anion is phosphate, boronate, or sulphate.

In certain embodiments, the method is performed at about pH 2, about pH 3, about pH 4, about pH 5, about pH 6, about pH 7, about pH 8, about pH 9, about pH 10, about pH 11, about pH 12, or about pH 13. In certain embodiments, the method is performed at about pH 6.8 or pH 13. In certain embodiments, the method is performed at about pH 6.8. In certain embodiments, the method is performed at about pH 13.

In certain embodiments, the voltage is applied vs. a reference hydrogen electrode (RHE) (i.e., V_RHE). In certain embodiments, the voltage is applied at about −0.5 V_RHE, about −0.6 V_RHE, about −0.7 V_RHE, about −0.8 V_RHE, about −0.9 V_RHE, or about −1.0 V_RHE. In certain embodiments, the voltage is applied at about −0.5 V_RHE. In certain embodiments, the voltage is applied at about-0.6 V_RHE. In certain embodiments, the voltage is applied at about −0.7 V_RHE. In certain embodiments, the voltage is applied at about −0.8 V_RHE. In certain embodiments, the voltage is applied at greater than −0.5 V_RHE. In certain embodiments, the voltage is applied at greater than-0.6 V_RHE. In certain embodiments, the voltage is applied at greater than −0.7 V_RHE. In certain embodiments, the voltage is applied at greater than −0.8 V_RHE.

In certain embodiments, the electrode comprises CoPc. In certain embodiments, the CoPc is ligated to a ligand comprising amino, nitrogen, alkyl (e.g., methyl or t-butyl), carboxyl, or halo (e.g., fluoro). In certain embodiments, the electrode comprises CoPc on a carbon support. In certain embodiments, the carbon support is carbon black, graphene, single-walled carbon nanotubes, or multi-walled carbon nanotubes. In certain embodiments, the carbon support is graphene or a carbon nanotube.

In certain embodiments, the method has a selectivity of methanol vs. carbon monoxide at greater than 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90% in favor of methanol. In certain embodiments, the method has a selectivity of methanol vs. carbon monoxide at greater 30% in favor of methanol. In certain embodiments, the method has a selectivity of methanol vs. carbon monoxide of about 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90% in favor of methanol. In certain embodiments, the method has a selectivity of methanol vs. carbon monoxide of about 30% in favor of methanol.

In another aspect, the provided herein are methods converting CO₂ to-methanol, the method comprising

• • a. preparing an aqueous 0.1 M alkali metal bicarbonate electrolyte solution, preferentially at pH 6.8;
  • b. applying an electrochemical potential against a reversible hydrogen electrode (RHE) to reduce the CO₂ to methanol.

In certain embodiments, the alkali metal is lithium.

In certain embodiments, the electrode comprises a CoPc catalyst loaded on carbon nanotube.

In certain embodiments, a voltage of −0.8 V vs. RHE is applied.

In another aspect, the provided herein are methods converting and CO to methanol, the method comprising

• • a. preparing an aqueous 0.1 M alkali metal hydroxide electrolyte solution comprising CO, preferentially at pH 13;
  • b. applying a potential using a reversible hydrogen electrode (RHE) to reduce the CO to methanol.

In certain embodiments, the alkali metal is lithium.

In certain embodiments, the reversible hydrogen electrode comprises a CoPc catalyst loaded on carbon nanotube.

In certain embodiments, a voltage of −0.5V vs. RHE is applied.

Definitions

Unless otherwise defined herein, scientific and technical terms used in this application shall have the meanings that are commonly understood by those of ordinary skill in the art. Generally, nomenclature used in connection with, and techniques of chemistry described herein, are those well known and commonly used in the art.

The methods and techniques of the present disclosure are generally performed, unless otherwise indicated, according to conventional methods well known in the art and as described in various general and more specific references that are cited and discussed throughout this specification. See, e.g. “Principles of Neural Science”, McGraw-Hill Medical, New York, N.Y. (2000).

Chemistry terms used herein, unless otherwise defined herein, are used according to conventional usage in the art, as exemplified by “The McGraw-Hill Dictionary of Chemical Terms”, Parker S., Ed., McGraw-Hill, San Francisco, C.A. (1985).

All of the above, and any other publications, patents and published patent applications referred to in this application are specifically incorporated by reference herein. In case of conflict, the present specification, including its specific definitions, will control.

As used herein, the terms “optional” or “optionally” mean that the subsequently described event or circumstance may occur or may not occur, and that the description includes instances where the event or circumstance occurs as well as instances in which it does not. For example, “optionally substituted alkyl” refers to the alkyl may be substituted as well as where the alkyl is not substituted.

It is understood that substituents and substitution patterns on the compounds of the present invention can be selected by one of ordinary skilled person in the art to result chemically stable compounds which can be readily synthesized by techniques known in the art, as well as those methods set forth below, from readily available starting materials. If a substituent is itself substituted with more than one group, it is understood that these multiple groups may be on the same carbon or on different carbons, so long as a stable structure results.

As used herein, the term “optionally substituted” refers to the replacement of one to six hydrogen radicals in a given structure with the radical of a specified substituent including, but not limited to: hydroxyl, hydroxyalkyl, alkoxy, halogen, alkyl, nitro, silyl, acyl, acyloxy, aryl, cycloalkyl, heterocyclyl, amino, aminoalkyl, cyano, haloalkyl, haloalkoxy, —OCO—CH₂—O-alkyl, —OP(O)(O-alkyl)₂ or —CH₂—OP(O)(O-alkyl)₂. Preferably, “optionally substituted” refers to the replacement of one to four hydrogen radicals in a given structure with the substituents mentioned above. More preferably, one to three hydrogen radicals are replaced by the substituents as mentioned above. It is understood that the substituent can be further substituted.

The term “acyl” is art-recognized and refers to a group represented by the general formula hydrocarbylC(O)—, preferably alkylC(O)—.

The term “acylamino” is art-recognized and refers to an amino group substituted with an acyl group and may be represented, for example, by the formula hydrocarbylC(O) NH—.

The term “acyloxy” is art-recognized and refers to a group represented by the general formula hydrocarbylC(O)O—, preferably alkylC(O)O—.

The term “alkoxy” refers to an alkyl group having an oxygen attached thereto. Representative alkoxy groups include methoxy, ethoxy, propoxy, tert-butoxy and the like.

The term “alkoxyalkyl” refers to an alkyl group substituted with an alkoxy group and may be represented by the general formula alkyl-O-alkyl.

The term “alkyl” refers to saturated aliphatic groups, including straight-chain alkyl groups, branched-chain alkyl groups, cycloalkyl (alicyclic) groups, alkyl-substituted cycloalkyl groups, and cycloalkyl-substituted alkyl groups. In preferred embodiments, a straight chain or branched chain alkyl has 30 or fewer carbon atoms in its backbone (e.g., C_1-30 for straight chains, C_3-30 for branched chains), and more preferably 20 or fewer.

Moreover, the term “alkyl” as used throughout the specification, examples, and claims is intended to include both unsubstituted and substituted alkyl groups, the latter of which refers to alkyl moieties having substituents replacing a hydrogen on one or more carbons of the hydrocarbon backbone, including haloalkyl groups such as trifluoromethyl and 2,2,2-trifluoroethyl, etc.

The term “alkylamino”, as used herein, refers to an amino group substituted with at least one alkyl group.

The term “alkylthio”, as used herein, refers to a thiol group substituted with an alkyl group and may be represented by the general formula alkylS-.

The term “amido”, as used herein, refers to a group

• • wherein R⁹ and R¹⁰ each independently represent a hydrogen or hydrocarbyl group, or R⁹ and R¹⁰ taken together with the N atom to which they are attached complete a heterocycle having from 4 to 8 atoms in the ring structure.

The terms “amine” and “amino” are art-recognized and refer to both unsubstituted and substituted amines and salts thereof, e.g., a moiety that can be represented by

• • wherein R⁹, R¹⁰, and R¹⁰′ each independently represent a hydrogen or a hydrocarbyl group, or R⁹ and R¹⁰ taken together with the N atom to which they are attached complete a heterocycle having from 4 to 8 atoms in the ring structure.

The term “aminoalkyl”, as used herein, refers to an alkyl group substituted with an amino group.

The term “aralkyl”, as used herein, refers to an alkyl group substituted with an aryl group.

The term “aryl” as used herein include substituted or unsubstituted single-ring aromatic groups in which each atom of the ring is carbon. Preferably the ring is a 5- to 7-membered ring, more preferably a 6-membered ring. The term “aryl” also includes polycyclic ring systems having two or more cyclic rings in which two or more carbons are common to two adjoining rings wherein at least one of the rings is aromatic, e.g., the other cyclic rings can be cycloalkyls, cycloalkenyls, cycloalkynyls, aryls, heteroaryls, and/or heterocyclyls. Aryl groups include benzene, naphthalene, phenanthrene, phenol, aniline, and the like.

The term “carbamate” is art-recognized and refers to a group

• • wherein R⁹ and R¹⁰ independently represent hydrogen or a hydrocarbyl group.

The term “carbocyclylalkyl”, as used herein, refers to an alkyl group substituted with a carbocycle group.

The term “carbocycle” includes 5-7 membered monocyclic and 8-12 membered bicyclic rings. Each ring of a bicyclic carbocycle may be selected from saturated, unsaturated and aromatic rings. Carbocycle includes bicyclic molecules in which one, two or three or more atoms are shared between the two rings. The term “fused carbocycle” refers to a bicyclic carbocycle in which each of the rings shares two adjacent atoms with the other ring. Each ring of a fused carbocycle may be selected from saturated, unsaturated and aromatic rings. In an exemplary embodiment, an aromatic ring, e.g., phenyl, may be fused to a saturated or unsaturated ring, e.g., cyclohexane, cyclopentane, or cyclohexene. Any combination of saturated, unsaturated and aromatic bicyclic rings, as valence permits, is included in the definition of carbocyclic. Exemplary “carbocycles” include cyclopentane, cyclohexane, bicyclo[2.2.1]heptane, 1,5-cyclooctadiene, 1,2,3,4-tetrahydronaphthalene, bicyclo[4.2.0]oct-3-ene, naphthalene and adamantane. Exemplary fused carbocycles include decalin, naphthalene, 1,2,3,4-tetrahydronaphthalene, bicyclo[4.2.0]octane, 4,5,6,7-tetrahydro-1H-indene and bicyclo[4.1.0]hept-3-ene. “Carbocycles” may be substituted at any one or more positions capable of bearing a hydrogen atom.

The term “carbocyclylalkyl”, as used herein, refers to an alkyl group substituted with a carbocycle group.

The term “carbonate” is art-recognized and refers to a group —OCO₂—.

The term “carboxy”, as used herein, refers to a group represented by the formula —CO₂H.

The term “cycloalkyl” includes substituted or unsubstituted non-aromatic single ring structures, preferably 4- to 8-membered rings, more preferably 4- to 6-membered rings. The term “cycloalkyl” also includes polycyclic ring systems having two or more cyclic rings in which two or more carbons are common to two adjoining rings wherein at least one of the rings is cycloalkyl and the substituent (e.g., R¹⁰⁰) is attached to the cycloalkyl ring, e.g., the other cyclic rings can be cycloalkyls, cycloalkenyls, cycloalkynyls, aryls, heteroaryls, and/or heterocyclyls. Heteroaryl groups include, for example, pyrrole, furan, thiophene, imidazole, oxazole, thiazole, pyrazole, pyridine, pyrazine, pyridazine, pyrimidine, denzodioxane, tetrahydroquinoline, and the like.

The term “ester”, as used herein, refers to a group —C(O)OR⁹ wherein R⁹ represents a hydrocarbyl group.

The term “ether”, as used herein, refers to a hydrocarbyl group linked through an oxygen to another hydrocarbyl group. Accordingly, an ether substituent of a hydrocarbyl group may be hydrocarbyl-O—. Ethers may be either symmetrical or unsymmetrical. Examples of ethers include, but are not limited to, heterocycle-O-heterocycle and aryl-O-heterocycle. Ethers include “alkoxyalkyl” groups, which may be represented by the general formula alkyl-O-alkyl.

The terms “halo” and “halogen” as used herein means halogen and includes chloro, fluoro, bromo, and iodo.

The terms “hetaralkyl” and “heteroaralkyl”, as used herein, refers to an alkyl group substituted with a hetaryl group.

The terms “heteroaryl” and “hetaryl” include substituted or unsubstituted aromatic single ring structures, preferably 5- to 7-membered rings, more preferably 5- to 6-membered rings, whose ring structures include at least one heteroatom, preferably one to four heteroatoms, more preferably one or two heteroatoms. The terms “heteroaryl” and “hetaryl” also include polycyclic ring systems having two or more cyclic rings in which two or more carbons are common to two adjoining rings wherein at least one of the rings is heteroaromatic, e.g., the other cyclic rings can be cycloalkyls, cycloalkenyls, cycloalkynyls, aryls, heteroaryls, and/or heterocyclyls. Heteroaryl groups include, for example, pyrrole, furan, thiophene, imidazole, oxazole, thiazole, pyrazole, pyridine, pyrazine, pyridazine, and pyrimidine, and the like.

The term “heteroatom” as used herein means an atom of any element other than carbon or hydrogen. Preferred heteroatoms are nitrogen, oxygen, and sulfur.

The term “heterocyclylalkyl”, as used herein, refers to an alkyl group substituted with a heterocycle group.

The terms “heterocyclyl”, “heterocycle”, and “heterocyclic” refer to substituted or unsubstituted non-aromatic ring structures, preferably 3- to 10-membered rings, more preferably 3- to 7-membered rings, whose ring structures include at least one heteroatom, preferably one to four heteroatoms, more preferably one or two heteroatoms. The terms “heterocyclyl” and “heterocyclic” also include polycyclic ring systems having two or more cyclic rings in which two or more carbons are common to two adjoining rings wherein at least one of the rings is heterocyclic, e.g., the other cyclic rings can be cycloalkyls, cycloalkenyls, cycloalkynyls, aryls, heteroaryls, and/or heterocyclyls. Heterocyclyl groups include, for example, piperidine, piperazine, pyrrolidine, morpholine, lactones, lactams, and the like.

The term “hydrocarbyl”, as used herein, refers to a group that is bonded through a carbon atom that does not have a ═O or ═S substituent, and typically has at least one carbon-hydrogen bond and a primarily carbon backbone, but may optionally include heteroatoms. Thus, groups like methyl, ethoxyethyl, 2-pyridyl, and even trifluoromethyl are considered to be hydrocarbyl for the purposes of this application, but substituents such as acetyl (which has a ═O substituent on the linking carbon) and ethoxy (which is linked through oxygen, not carbon) are not. Hydrocarbyl groups include, but are not limited to aryl, heteroaryl, carbocycle, heterocycle, alkyl, alkenyl, alkynyl, and combinations thereof.

The term “hydroxyalkyl”, as used herein, refers to an alkyl group substituted with a hydroxy group.

The term “lower” when used in conjunction with a chemical moiety, such as, acyl, acyloxy, alkyl, alkenyl, alkynyl, or alkoxy is meant to include groups where there are ten or fewer atoms in the substituent, preferably six or fewer. A “lower alkyl”, for example, refers to an alkyl group that contains ten or fewer carbon atoms, preferably six or fewer. In certain embodiments, acyl, acyloxy, alkyl, alkenyl, alkynyl, or alkoxy substituents defined herein are respectively lower acyl, lower acyloxy, lower alkyl, lower alkenyl, lower alkynyl, or lower alkoxy, whether they appear alone or in combination with other substituents, such as in the recitations hydroxyalkyl and aralkyl (in which case, for example, the atoms within the aryl group are not counted when counting the carbon atoms in the alkyl substituent).

The terms “polycyclyl”, “polycycle”, and “polycyclic” refer to two or more rings (e.g., cycloalkyls, cycloalkenyls, cycloalkynyls, aryls, heteroaryls, and/or heterocyclyls) in which two or more atoms are common to two adjoining rings, e.g., the rings are “fused rings”. Each of the rings of the polycycle can be substituted or unsubstituted. In certain embodiments, each ring of the polycycle contains from 3 to 10 atoms in the ring, preferably from 5 to 7.

The term “sulfate” is art-recognized and refers to the group —OSO₃H, or a pharmaceutically acceptable salt thereof.

The term “sulfonamido” is art-recognized and refers to the group represented by the general formulae

• • wherein R⁹ and R¹⁰ independently represents hydrogen or hydrocarbyl.

The term “sulfoxide” is art-recognized and refers to the group —S(O)—.

The term “sulfonate” is art-recognized and refers to the group SO₃H, or a pharmaceutically acceptable salt thereof.

The term “sulfone” is art-recognized and refers to the group —S(O)₂—.

The term “substituted” refers to moieties having substituents replacing a hydrogen on one or more carbons of the backbone. It will be understood that “substitution” or “substituted with” includes the implicit proviso that such substitution is in accordance with permitted valence of the substituted atom and the substituent, and that the substitution results in a stable compound, e.g., which does not spontaneously undergo transformation such as by rearrangement, cyclization, elimination, etc. As used herein, the term “substituted” is contemplated to include all permissible substituents of organic compounds. In a broad aspect, the permissible substituents include acyclic and cyclic, branched and unbranched, carbocyclic and heterocyclic, aromatic and non-aromatic substituents of organic compounds. The permissible substituents can be one or more and the same or different for appropriate organic compounds. For purposes of this invention, the heteroatoms such as nitrogen may have hydrogen substituents and/or any permissible substituents of organic compounds described herein which satisfy the valences of the heteroatoms. Substituents can include any substituents described herein, for example, a halogen, a hydroxyl, a carbonyl (such as a carboxyl, an alkoxycarbonyl, a formyl, or an acyl), a thiocarbonyl (such as a thioester, a thioacetate, or a thioformate), an alkoxyl, a phosphoryl, a phosphate, a phosphonate, a phosphinate, an amino, an amido, an amidine, an imine, a cyano, a nitro, an azido, a sulfhydryl, an alkylthio, a sulfate, a sulfonate, a sulfamoyl, a sulfonamido, a sulfonyl, a heterocyclyl, an aralkyl, or an aromatic or heteroaromatic moiety. It will be understood by those skilled in the art that the moieties substituted on the hydrocarbon chain can themselves be substituted, if appropriate.

The term “thioalkyl”, as used herein, refers to an alkyl group substituted with a thiol group.

The term “thioester”, as used herein, refers to a group —C(O)SR⁹ or —SC(O)R⁹ wherein R⁹ represents a hydrocarbyl.

The term “thioether”, as used herein, is equivalent to an ether, wherein the oxygen is replaced with a sulfur.

The term “urea” is art-recognized and may be represented by the general formula

• • wherein R⁹ and R¹⁰ independently represent hydrogen or a hydrocarbyl.

EXAMPLES

The invention now being generally described, it will be more readily understood by reference to the following examples which are included merely for purposes of illustration of certain aspects and embodiments of the present invention and are not intended to limit the invention.

Example 1: Exemplary CO₂ to Methanol Conversion

Non-innocent role of electrolyte cations in electrocatalytic reactions has recently attracted tremendous research interests, spanning from hydrogen evolution/oxidation reactions (HER/HOR), simple two-electron/proton transfer reactions, to complex electrochemical CO₂ reduction reaction (CO₂RR) involving multiple electron/proton transfers. Non-covalent interactions between cations, interfacial water molecules, and reactants/intermediate species in the catalytic microenvironment have shown to greatly affect intrinsic activity and selectivity of catalytic sites beyond what is typically determined by covalent bonding between the reaction site and an adsorbate. For instance, Hori et al. have reported in their seminal CO₂RR study that ethylene selectivity on a polycrystalline copper surface increases with the increase of alkali metal cation size in bicarbonate electrolyte (Li⁺<Na⁺<K⁺<Cs⁺), which has been further supported by recent studies. If a rate-determining step (RDS) involves a proton-coupled electron transfer (PCET), cations and their solvation environments can have non-negligible influence on the PCET kinetics, since a proton transfer should occur from the interfacial water molecules, which non-covalently interact with a cation nearby, to an adsorbed intermediate species. While CO₂RR generally involves PCET steps, the origin of its cation-dependent catalytic results remains elusive.

There are different schools of thought explaining the cation effect on catalytic CO₂RR performance, which include electric field effect and cation-intermediate coordination. For the electric field effect, cations have been proposed to stabilize key intermediate species such as anionic CO₂ adsorbate (*CO₂⁻) through electric field-dipole interactions for CO production on silver surface. Similarly, adsorbed anionic CO dimer (*OCCO⁻) can be stabilized by cations after symmetric coupling of adsorbed CO on copper surface for multicarbon (C₂₊) product generation. More recently, short-range electrostatic interactions via direct cation-intermediate species coordination have been shown to be crucial in stabilizing intermediate species such as adsorbed CO₂ (*CO₂⁻) for CO production. Furthermore, larger cations such as Cs⁺ can accumulate at the electrified interface owing to their weakly bound hydration shell and more effectively stabilize the negatively charged key intermediates (e.g., *CO₂⁻ and *OCCO⁻ on silver and copper surface, respectively) than smaller cations with a strongly bound hydration shell.

Beyond metal surfaces catalyzing CO₂RR, a molecular cobalt catalyst has been shown to selectively produce methanol, which involves total six electron transfers via CO₂+6e⁻+5H₂O→CH₃OH+6OH⁻. Theoretical studies on molecular cobalt catalysts have suggested that a protonation step after CO adsorbate formation could be the RDS for methanol generation. Given the potential impact of cations on proton transfer during CO₂RR, we reasoned that one can control the cation-water molecule interactions and surrounding hydrogen bonding network to promote the proton transfer kinetics, achieving improved catalytic results for methanol production.

In this study, we investigated the role of electrolyte cation for electrochemical CO₂-to-methanol conversion on a molecular cobalt catalyst. Cobalt phthalocyanine (CoPc) was employed as a model catalyst (FIG. 2A), which has a well-defined structure (i.e., a cobalt single atom coordinated by four nitrogen atoms). CoPc monomers were well dispersed on carbon nanotubes (CoPc/CNT), confirmed by electron microscopy and spectroscopy (FIG. 2B). Such molecular-level dispersion of CoPc on a CNT support has been shown to be necessary for the production of methanol, while other carbon supports are not as conducive. In a recent study, it was proposed that the curvature of CNT induces molecular distortion of CoPc, leading to strong *CO adsorption for its further reduction. This catalyst-support interaction was found to be important in facilitating methanol production.

CoPc/CNT loaded on a carbon support was tested for 1 hour in an H-cell for electrochemical CO₂RR in a wide range of potentials with alkali metal cations in bicarbonate solution (FIG. 2C). Gas and liquid products were analyzed using gas chromatography (GC) and proton nuclear magnetic resonance (¹H NMR) spectroscopy, respectively. A series of experiments including an open-circuit potential experiment, an isotope-labeling experiment (¹³CO₂RR), and a CO₂RR experiment using polycrystalline cobalt foil validated that methanol detected by NMR was generated from electrochemical CO₂RR by CoPc (FIG. 2C).

CO was found to be the major product from CO₂RR in the bicarbonate electrolyte, achieving up to ˜90% CO selectivity, at small overpotentials—potentials greater than −0.7 V versus reversible hydrogen electrode (V_RHE) (FIG. 2C)—in agreement with previous works. The partial current density of CO in this region grows with potential with an average Tafel slope of 139±5 mV/dec (FIG. 3A). Similar Tafel slopes (e.g., 120 mV/dec by assuming a symmetry factor of 0.5) have been reported for CoPc, which can be attributed to the first electron transfer to CO₂ as the RDS.

At greater overpotentials (i.e., potentials more negative than −0.7 V_RHE), the CO partial current density reached a maximum and decreased with increasing overpotential (FIG. 2D), which can result from subsequent reduction of CO generated from CO₂RR to methanol. While no apparent cation dependence was noted for CO production at potentials higher than −0.7 V_RHE compared to silver foil, more CO was detected for larger cations (Li⁺<Na⁺<K⁺<Cs⁺) at potentials lower than −0.7 V_RHE, which can be ascribed to faster CO-to-methanol conversion kinetics with smaller cations. This hypothesis is supported by the following observations. First, methanol was detected from −0.7 V_RHE, and the selectivity of methanol increased up to ˜30% with decreasing potential at the expense of CO selectivity (FIG. 2C, bottom). Second, smaller cations were found to promote the kinetics of CO reduction reaction (CORR) to methanol (FIG. 3B). As *CO is an important intermediate for methanol production, we therefore conducted CORR. A similar cation dependence (Li⁺>Na⁺>K⁺>Cs⁺) for CORR kinetics in alkaline electrolyte (pH 13) was observed to that of CO₂RR (pH 6.8). The lower limiting partial current for methanol from CORR than CO₂RR can be attributed to lower solubility of CO (1 mM) than that of CO₂ (34 mM) in aqueous electrolytes. The cation-dependent catalytic activity signifies that electrolyte cations play an important role in methanol production. Below we discuss our hypothesis for the cation dependence of CO₂RR and CORR, and further experimental and computational findings to provide mechanistic insights.

We propose that the increasing kinetics of CO₂RR and CORR to methanol with smaller cations are attributed to strong interactions between smaller acidic cations and their surrounding water molecules, and resultant hydrogen bonding network near the catalytic center, facilitating water dissociation before RDS, proton transfer and/or PCET steps. Our proposition is supported by the following observations. First of all, smaller, water-structure-making cations (e.g., Li⁺) are known to promote HER kinetics owing to the aforementioned reason. In our experiments at large overpotentials (<−0.7 V_RHE) where CoPc begins to produce methanol from CO₂RR, a cation-dependent trend similar to that for methanol production kinetics was observed for HER kinetics (Li⁺>Na⁺>K⁺>Cs⁺). Considering neutral conditions were used for electrochemical testing, water should be the major proton source. Hence, the stronger interactions between smaller acidic cations and water molecules (i.e., M⁺-O_water) can lead to strong polarization of water and permit more facile water dissociation, increasing proton transfer rate and consequently HER activity. Also, the water-structure-making cations (e.g., Li⁺) promote hydrogen bonding network near the catalytic site and enhance proton transfer kinetics. In contrast, larger cations disturb the interfacial hydrogen bonding network, increasing an entropic kinetic barrier of proton transfer for reactions such as HER. Therefore, smaller acidic cations can facilitate the proton transfer kinetics associated with the RDS of methanol production on CoPc catalyst.

To identify the RDS for the methanol production, density functional theory (DFT) computation was performed. DFT computation of the free energy for methanol production from CO₂RR and CORR showed that the PCET step from *CHO to the adsorbed formaldehyde (*CH₂O), a previously reported reaction intermediate, is the RDS (FIG. 3D). Our DFT computation considered five steps for CORR, consisting of the adsorption of CO on the Co center and four subsequent PCET steps (*CO+H⁺+e⁻→*CHO, *CHO+H⁺+e⁻→*CH₂O, *CH₂O+H⁺+e⁻→*CH₂OH, and *CH₂OH+H⁺+e⁻→*+CH₃OH). The formation of *CH₂O was found to have the largest thermodynamic free energy barrier of 0.42 eV (FIG. 3D). Further support came from crystal orbital Hamilton populations (COHP) analysis to compare the interaction between CoPc and intermediates through the Co—C bond, revealing that CoPc-CH₂O has a very weak bonding state compared to CoPc-CO and —CHO, and a large energy gap for the *CHO protonation step. At −0.7 V_RHE, the experimental onset potential, methanol production becomes energetically downhill (FIG. 3E), allowing methanol production. In addition, our electrokinetic analysis of this reaction mechanism, which yielded a Tafel slope of 40 mV/dec (assuming a symmetry factor of 0.5), is in good agreement with our experimentally measured Tafel slopes of 47±5.5 mV/dec (FIG. 3B). A recent study reported Tafel slopes larger than ˜130 mV/dec for CO-to-methanol conversion, but this discrepancy may have resulted from a different choice of potential region to obtain the Tafel slope. For accurate Tafel analysis, we chose a smaller potential window near the experimental onset potential (i.e., a low-current, kinetic-control region), compared to the literature, to avoid any mass transport related impact in the Tafel analysis. For methanol production from CO₂RR, the experimental value of 40±4 mV/dec agrees well with a predicted Tafel slope of 40 mV/dec. Recent in situ infrared spectroscopy studies observed *CO and *CHO on the cobalt site of CoPc, supporting our proposed reaction pathway and the assumptions that we made in our kinetic model (e.g., the steady-state approximation).

To gain more insights into the RDS, pH-dependent potential shifts on the RHE and SHE scales were examined. Since both CO₂RR (pH 6.8) and CORR (pH 13) involve *CO as a reaction intermediate, one can construct a rate expression, including a kinetic barrier term based on the potential versus SHE (i.e., U_SHE), and a thermodynamic term related to *CO coverage based on the potential versus RHE (i.e., U_RHE) and the number of PCET steps (n) prior to an RDS. A potential shift on the SHE scale (ΔU_SHE) induced by a change in pH (ΔpH) is then ΔU_SHE=−60 mV×ΔpH×(nln+β). If the first protonation of *CO to *CHO is the RDS (or n=0), there should be no potential shift on the SHE scale (ΔU_SHE=0) and no pH-dependent partial current mismatch on the SHE scale (i.e., Δlog(j_MeOH)=0). However, in our experimental results, the methanol partial currents from CO₂RR and CORR and their onset potentials do not overlap with one another on the SHE scale (ΔU_SHE≠0, FIG. 2c), which rules out the possibility of proton-electron transfer to *CO to form an adsorbed formyl (*CHO) as the RDS. In addition, CORR under neutral and alkaline conditions (pH 6.8 and 13) shows not only the potential shift but also a large reduction in methanol partial current density under the alkaline condition, supporting our conclusion. Similarly, a potential shift on the RHE scale and potential mismatch on the SHE scale was previously observed for methane production on polycrystalline copper catalysts, which was absent for CO and C₂₊. This observation was also attributed to a rate-determining proton-electron transfer step, occurring later than the protonation of CO (n≥1). Likewise, our results showing the pH-dependent potential mismatch on the SHE scale (FIG. 3C) offer further support that protonation of *CHO is the RDS.

To confirm the involvement of proton in the RDS during methanol production and gain mechanistic understanding, we conducted kinetic isotope effect (KIE) studies. KIE measurements were performed using 0.1 M LiDCO₃ in D₂O, where the kinetics of CO, D₂, and CD₃OD formation were compared with that of CO, H₂, and CH₃OH in LiHCO₃ in H₂O. HER current decreased with increasing fraction of LiDCO₃/D₂O, which is due to slower dissociation of D₂O and deuteron transfer than that of H₂O. In contrast, CO partial current (j_CO) remains comparable among pure deuterated and protic, and mixed LiDCO₃/D₂O—LiHCO₃/H₂O electrolytes when CO is the major CO₂RR product at potentials greater than −0.7 V_RHE, further supporting that proton is not involved in the RDS of CO₂-to-CO conversion (FIG. 4A). On the other hand, at potentials lower than −0.7 V_RHE, although the overall CO intermediate generation rates, derived from both CO and methanol partial currents, are not affected by the proton donor (LiDCO₃/D₂O and LiHCO₃/H₂O), methanol partial current density in deuterated electrolyte decreased by about 6.5 times compared to that in the protic electrolyte (j_CD3OD^D<j_CH3OH^H) (FIG. 4B), while CO partial current density increased (j_CO^D)>j_CO^H) (FIG. 4A). Hence, this notable decrease of the methanol production rate in the deuterated electrolyte is attributed to sluggish water dissociation and associated deuteron transfer, confirming that proton is indeed involved in the RDS of methanol production on the CoPc catalyst.

A complete catalytic cycle of CoPc catalyst is described in FIG. 5A. For CO₂-to-CO conversion, CO₂ activation with an electron transfer was found to be the RDS, which is supported by the Tafel analysis and the absence of KIE. On the other hand, for CO₂-to-methanol conversion, a proton-electron transfer to *CHO is the RDS, based on the Tafel analyses from CO₂RR and CORR, the DFT computed energy profile, pH-dependent potential shift, and the notable KIE observed. The cation dependence of the methanol production kinetics signifies that promoting cation-water interactions and hydrogen bonding network in the hydration shell of a cation in the vicinity of a cobalt single site facilitates proton transfer in the RDS. To study the local catalytic environment involving cations, water molecules, and the key intermediate (*CHO), we performed molecular dynamics (MD) simulations. While a strongly bound hydration shell of smaller cations (e.g., Li⁺) prevents the cations from coming closer to the *CHO intermediate (e.g., M⁺-O_*CHO) compared to larger cations (e.g., Cs⁺) with water-structure-breaking property (FIGS. 4B-4D), smaller acidic cations provide a greater availability of interfacial water molecules through the solvation shell (FIG. 5E) and a higher number of interfacial hydrogen bonds near the *CHO (FIG. 5D), which can lower the kinetic barrier for the proton transfer, as our previous study suggested, enhancing proton transfer kinetics. Furthermore, the smaller acidic cations (e.g., Li⁺) with a large hydration energy (namely, strong cation-water interactions and a weaker O—H bond) can polarize water molecules, promoting the proton transfer to the key intermediate (*CHO). As a result, smaller and more acidic cations enhance the methanol production kinetics.

Electrode Preparation

Catalyst ink and electrode were prepared following the procedure described in a previous report with some modifications. As-received multi-walled carbon nanotubes (CNTs, >98% carbon basis, Sigma-Aldrich) was calcined at 500° C. for 1 hour under ambient atmosphere. The calcined CNTs were transferred to a centrifuge tube containing 5 wt. % HCl solution and sonicated for 30 mins, after which it was stirred at 700 rpm overnight. The purified CNTs were repeatedly washed with deionized water and centrifuged (12,000 rpm for 10 mins) multiples times until pH becomes neutral. The CNTs were dried in a freeze dryer (Labconco FreeZone 2.5 plus).

For catalyst preparation (CoPc/CNT), 30 mg of the purified CNTs was dispersed in 20 ml of dimethylformamide (DMF, HPLC grade, ≥99.9%, Sigma-Aldrich), while 1.5 mg of cobalt phthalocyanine (CoPc, Thermo Fisher Scientific) was dispersed in 10 ml of DMF. After 30 mins of sonication, two solutions were mixed, and its mixture was sonicated for another 30 mins. The mixed solution was stirred at 700 rpm for 24 hours and centrifuged twice at 12,000 rpm for 10 mins each. The precipitate was washed with DMF and ethanol, and centrifuged, respectively, and dispersed in 10 ml of deionized water before freeze drying. For bare CNT control sample, identical procedure was used without adding CoPc.

To make catalyst ink, 8 mg of CoPc/CNT was added to 2 ml of absolute ethanol along with 24 μl of Nafion solution (5 wt. %, Sigma-Aldrich). For complete dispersion, the ink was sonicated for at least 2 hours. To prepare electrodes for electrochemical testing, total 100 μl of the catalyst ink was drop casted using a pipette on a carbon paper (Sigracet 29AA, Fuel Cell Store) with 1 cm² geometric active area (catalyst mass loading: 0.4 mg/cm²_geo).

For polycrystalline foil testing (0.1 mm thick, Thermo Fisher Scientific), as-received silver foil (99.998%) and cobalt foil (99.995%) were cut into pieces with 1 and 0.3 cm² geometric active area, respectively, and mechanically polished using a sandpaper (400 grit, 3M), and thoroughly rinsed with ethanol and deionized water before each experiment.

Electrochemical Testing

All electrochemical testing was conducted in an H-cell type electrochemical cell where two compartments were separated by an anion exchange membrane (Selemion, DSVN, Bellex International Corp.). Platinum foil was used as a counter electrode while Ag/AgCl 3M KCl was used as a reference electrode.

Electrolytes for CO₂ reduction reaction (CO₂RR) were prepared by purging 0.05 M carbonate solution with CO₂ gas (Airgas, Research Grade) overnight and pH was measured (˜6.8). Carbonate salts used in this study are Li₂CO₃ (99.999%, Sigma-Aldrich), Na₂CO₃ (99.998%, Thermo Fisher Scientific), K₂CO₃ (99.997%, Thermo Fisher Scientific), and Cs₂CO₃ (99.995%, Sigma-Aldrich). For bicarbonate concentration dependence experiment, Li₂SO₄ (99.99%, Sigma-Aldrich) was used as a supporting electrolyte. For organic cations, tetramethylammonium hydroxide (TMAOH, 25 wt. %, 99.9999%, Thermo Fisher Scientific) was first converted to bicarbonate solution by purging with CO₂ gas and diluted to make 0.1 M bicarbonate solution. For deuterated electrolyte preparation, deuterium oxide (99.9 at. % D, Sigma-Aldrich) was used instead of deionized water (18.2 MΩ·cm, Milli-Q Direct Water Purification System). Prior to each electrocatalytic testing, electrolyte was saturated with CO₂ gas, and the gas flow was kept at 20 sccm (standard cubic centimeters per minute) with continuous stirring during CO₂ electrocatalysis.

For CO reduction reaction (CORR) experiment, hydroxide electrolytes were used for testing and prepared by using LiOH (99.995%, Thermo Fisher Scientific), NaOH (99.99%, Sigma-Aldrich), KOH (99.99%, Sigma-Aldrich), and CsOH·H₂O (99.95%, Sigma-Aldrich). For neutral pH testing, KH₂PO₄ (99.99%, Sigma Aldrich) and K₂HPO₄ (99.95%, Sigma Aldrich) were used. Before testing, electrolyte was purged with CO (Airgas, ultra-high purity) for 30 mins at 20 sccm and kept bubbled during the electrolysis. For electrocatalytic testing, potentiostatic experiments were performed for typically 65 mins, and electrode potentials were converted to the reversible hydrogen electrode (RHE) scale using the following equation: E (versus RHE)=E (versus Ag/AgCl 3 M KCl)+0.210+0.0591×pH. For ohmic loss compensation, solution resistance was measured at the end of each testing and iR drop was manually compensated.

In the kinetic isotope effect (KIE) experiments where deuterated electrolyte was used, all potentials were referenced to the reversible hydrogen electrode scale for catalytic activity comparison. Hence, a correction constant (25 mV, as pD=pH×1.062 due to the difference in pK_W(H) and pK_W(D) (14 vs. 14.87)) was additionally added to the potential in the potential conversion calculation described above.

Product Quantification

Gas products were analyzed by using an on-line gas chromatograph (GC, SRI Instruments, 8610C) with a thermal conductivity detector and a flame ionization detector. Gas samples were injected into GC every 20 mins and average values over 20, 40, 60 min marks are reported in this study.

For liquid product quantification after electrocatalytic testing in aqueous electrolyte, proton nuclear magnetic resonance (¹H NMR, Bruker Avance Neo 500) was used with water suppression. Dimethylsulfoxide (DMSO) with a known concentration was typically used as an internal standard. For deuterated liquid product quantification, deuterium NMR (²H NMR, Bruker Avance Neo 600) was used instead and deuterated DMSO (DMSO-d6) was used as an internal standard.

Faradaic efficiency (FE) of a product was calculated by dividing the amount of charge consumed to produce each product by the total charge passed during CO₂ electrolysis. Reaction products are H₂, CO, formate, and methanol. However, FE for formate after CO₂ electroreduction was trivial (typically less than 0.2%) and thus it is not reported in this work. Total faradaic efficiency normally ranges between 95-100%.

Generation rate of a product (g_product) was calculated by dividing the product partial current by the Faraday constant and the number of electrons required for product formation from CO₂ (e.g., six for methanol). CO_int-to-MeOH conversion (%) was obtained by taking the ratio of MeOH generation rate to CO_int generation rate (i.e., g_MeOH/g_COint=g_MeOH/(g_CO+g_MeOH)), while CO_int selectivity was calculated by dividing the CO_int generation rate by total generation rate of all reaction products (CO, MeOH, and H₂) (i.e., g_COint/(g_CO+g_MeOH+g_H2)).

¹³CO2 Electroreduction Experiment

¹³CO₂ (99 at. % ¹³C, 1 L, Sigma-Aldrich) was used to saturate 0.1 M NaH¹³CO₃ (98 at. % ¹³C, Sigma-Aldrich) electrolyte at 20 sccm for 15 mins prior to ¹³CO₂ electrolysis, and the electrolyte was continuously bubbled at 20 sccm during the electrolysis. Due to its limited volume and low pressure (˜20 psig), 15-min electrolysis was performed.

Structural Characterization

Scanning transmission electron microscopy (STEM) analysis was performed using a probe-aberration corrected Thermo Fisher Scientific Themis Z G3 60-300 kV. High angle annular dark-field (HAADF) STEM images were acquired with a convergence semi-angle of 18.9 mrad operated at 200 kV. Energy dispersive spectroscopy (EDS) elemental maps were collected using a Thermo Fisher Scientific Super-X detector (>0.7 strad collection solid angle). Electron energy loss (EEL) spectra were collected using a Gatan Imaging Filter 1066 HR with 0.3 eV/ch energy dispersion. Scanning electron microscopy (SEM) images were obtained using Zeiss Merlin with the in-lens detector.

Density Functional Theory (DFT) Calculation

For a model system, adopted from the literature⁴ one CoPc molecule was placed above a graphene layer (a total of 98 carbon atoms) at a vertical distance of 15 Å with vacuum between the two structures. All the DFT calculations were conducted using the Vienna Ab initio Simulation Package (VASP) with the projector augmented-wave (PAW) approach for the interaction between the ionic core and valence electrons. Electron exchange and correlation were addressed using generalized gradient approximation in the form of the Perdew-Burke-Ernzerhof (PBE) functional, and the D3 method was used for van der Waals dispersion energy-correction. The ferromagnetic initial state was used in geometry optimization for a consistent and tractable set of magnetic structures. The plane-wave basis set with energy cutoff of 520 eV and a Γ-centered single k-point were used. The energy and force convergence criteria were set to be 10⁻⁵ CV and 0.05 eV/Å, respectively. We fixed the atomic positions of the graphene layer and the benzene rings of CoPc for better convergence in geometry optimization. We also performed Crystal Orbital Hamilton Populations (COHP) analysis on the Co—C bonding between the cobalt of CoPc and the carbon of adsorbed intermediate species using LOBSTER software (local orbital basis suite towards electronic-structure reconstruction)⁴⁹.

The Gibbs free energy (G) of all adsorbed states at 298 K and 1 atm was calculated as follows:

$G=H-T\Delta S=E_{\mathrm{DFT}}+E_{\mathrm{ZPE}}+E_{\mathrm{solv}}+{\int }_0^{298}{C}_v\mathrm{dT}-T\Delta S$

• • where E_DFT is the electronic total energy calculated from the VASP, E_ZPE is the zero-point vibrational energy, and E_solv is the solvation energy. The enthalpy (∫₀²⁹⁸ C_vdT) and entropy (ΔS) contributions at room temperature were calculated from the vibrational modes of the system. To determine the solvation corrections (E_solv), single-point calculations using the VASPsol solvent model⁵⁰ were performed to the relaxed structures. We also applied an energy correction of 0.15 cV per C═O double-bond in the DFT calculations using GGA-PBE functionals. To check whether the DFT results are dependent on the choice of exchange-correlation functionals, the RPBE functional⁵² and the BEEF-vdW functional, which provides a reliable description of van der Waals interactions, were further used to reproduce the energy diagram. The Bayesian error estimation was also performed using an ensemble of functionals around BEEF-vdW. The computational hydrogen electrode (CHE) model was used to determine the reaction thermodynamics.

To account for an electric field induced by electrolyte cations that can lead to notable stabilization of intermediate species, we applied external electrostatic field in the z-direction to the adsorbates in vacuum with the solvent model removed³⁹. The interaction energy between an adsorbate and an electric field at the interface is obtained by using the following equation:

$\Delta E=\mu \epsilon -\frac{1}{2}\alpha {\epsilon }^2$

• • where ΔE is the change in binding energy, which is calculated by subtracting an adsorption energy in the absence of an electric field (E₀) from a corresponding adsorption energy in the presence of an applied field (E)¹⁴. ε is the electric field strength, and μ and α are the intrinsic dipole moment and polarizability of the adsorbate, respectively. We used an electric field strength of −1 V/Å to approximate the cation-induced electric field.

Classical Molecular Dynamics (MD) Simulations

To elucidate how cations and their vicinal water molecules (i.e., hydration shell) interact with the key reaction intermediate (*CHO) adsorbed on CoPc for electrocatalytic CO₂-to-methanol conversion, we conducted a series of MD simulations of an aqueous electrolyte with alkali cations (Li⁺, Na⁺, K⁺, and Cs⁺) confined between two graphene electrodes. Note that due to limited validation of the force field parameters of bicarbonate anions, an equivalent number of chloride anions was considered instead, which does not influence our conclusions on the role of cations in methanol production. One CoPc molecule with a CHO intermediate adsorbed on the cobalt single site was placed on top of the bottom graphene layer, and a constant potential of −1.2 V relative to the point of zero charge (PZC) for graphene (0.10 V vs. SHE in pH 6.8 aqueous electrolyte) 56, which corresponds to −0.7 V vs. RHE, was applied. All the simulations were performed using LAMMPS (large-scale atomic/molecular massively parallel simulator). We first prepared a simulation box with dimensions of 29.9 Å×34.5 Å×24.0 Å, and periodically replicated in the x, y directions while keeping the fixed boundary conditions for the z-direction, containing the graphene electrodes. Each simulation box contained 784 carbon atoms (the graphene electrodes), 800 water molecules, and 18 alkali ions with the equivalent number of chloride anions to achieve charge neutrality. This condition corresponds to ˜1 M concentration of cations. The CoPc-CHO structure was constructed by adding the CHO on the cobalt site of the CoPc molecule and relaxing the structure using DFT calculations (VASP using GGA-PBE functionals). We used the universal force field (UFF) parameters for the CoPc-CHO complex and built the initial configuration LAMMPS data file for the CoPc-CHO on the graphene layer using the LAMMPS-interface software package. The charges and the optimized geometries for the CoPc-CHO complex were obtained using the DFT computations using Gaussian16 software package. After that, we added water molecules, cations, and anions to the system using the PACKMOL software package. Lorentz-Berthelot mixing rules were employed to derive the mixed Lennard-Jones (LJ) parameters. We used SPC/E (extended simple point-charge model) force field parameters for water molecules, while the graphene electrode atoms were modelled using the LJ force field from UFF parameters. The charges on each graphene atom was calculated at each time step to satisfy the imposed voltage across the cell (−0.7 V vs. RHE) by the constant potential fix in LAMMPS.

After preparing the initial configurations of the systems, the system was then energy minimized using the steepest descent method for 500 steps followed by 500 steps of conjugate gradient minimization to get rid of any unphysical/bad contacts. The system was then equilibrated for 20 ns under an NVT ensemble before performing the final production runs of 5 ns for analysis with time step of 1 fs. The long-range electrostatic interactions were calculated by using a Particle-mesh Ewald algorithm with a real-space cut off value of 9 Å. An NVT ensemble with Langevin thermostat was employed to keep the system at 300 K. Langevin thermostat was used here as we were only concerned with the statics properties of the system at equilibrium. Also, the Shake algorithm was used to constrain the bonds and angles of SPC/E water.

The number density for different species of the simulation box was computed using the LAMMPS module, while the radial distribution functions (RDFs) were computed using the VMD (visual molecular dynamics) software package. The average coordination number values were calculated by integrating the RDF (obtained using the last 5 ns of MD-simulated trajectories) up to the position of the first peak. The number of hydrogen bonds (H-bonds) within 6 Å of CoPc-CHO intermediate was computed using VMD during the last 2 ns of the MD simulated trajectories. A H-bond was defined to exist between a hydrogen atom of water molecule and an electronegative acceptor (A), in this case oxygen atom of another water molecule, with a distance cutoff of 3.5 Å and a D-H . . . A angle cutoff of 30°, where D represents the donor oxygen.

INCORPORATION BY REFERENCE

All publications and patents mentioned herein are hereby incorporated by reference in their entirety as if each individual publication or patent was specifically and individually indicated to be incorporated by reference. In case of conflict, the present application, including any definitions herein, will control.

EQUIVALENTS

While specific embodiments of the subject invention have been discussed, the above specification is illustrative and not restrictive. Many variations of the invention will become apparent to those skilled in the art upon review of this specification and the claims below. The full scope of the invention should be determined by reference to the claims, along with their full scope of equivalents, and the specification, along with such variations.

Claims

1. A method of producing methanol, comprising:

contacting an aqueous solution comprising a cation and an anion with CO2 or CO, thereby forming an aqueous solution comprising a cation, an anion, and CO2 or CO;

contacting the aqueous solution comprising the cation, the anion, and CO2 or CO with an electrode (RHE); and

applying a voltage to the aqueous solution comprising the cation, the anion, and CO2 or CO via the electrode.

2. The method of claim 1, wherein the cation is a metal cation.

3. The method of claim 2, wherein the metal cation is an alkali metal or an alkaline-earth metal.

4. The method of claim 2, wherein the metal cation is selected from the group consisting of Li+, Nat, K+, Cs+, Be2+, Mg2+, Ca2+, and Ba2+; or a combination thereof.

5-8. (canceled)

9. The method of claim 1, wherein the cation is an ammonium cation.

10. The method of claim 1, wherein the cation is an alkylammonium cation.

11. (canceled)

12. The method of claim 1, wherein the anion is bicarbonate or hydroxide.

13. The method of claim 1, wherein the anion is phosphate, boronate, or sulphate.

14. The method of claim 1, wherein the method is performed at about pH 2, about pH 3, about pH 4, about pH 5, about pH 6, about pH 7, about pH 8, about pH 9, about pH 10, about pH 11, about pH 12, or about pH 13.

15. The method of claim 1, wherein the method is performed at about pH 6.8 or about pH 13.

16. (canceled)

17. (canceled)

18. The method of claim 1, wherein the voltage is applied vs. a reference hydrogen electrode (RHE) (i.e., VRHE).

19. The method of claim 1, wherein the voltage is applied at about −0.5 VRHE, about −0.6 VRHE, about −0.7 VRHE, about −0.8 VRHE, about −0.9 VRHE, or about −1.0 VRHE.

20-27. (canceled)

28. The method of claim 1, wherein the electrode comprises CoPc.

29-31. (canceled)

32. The method of claim 28, wherein the CoPc is ligated to a ligand comprising amino, nitrogen, alkyl, carboxyl, or halo.

33. The method of claim 1, wherein the method has a selectivity of methanol vs. carbon monoxide at greater than 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90% in favor of methanol.

34. The method of claim 1, wherein the method has a selectivity of methanol vs. carbon monoxide at greater than 30% in favor of methanol.

35. The method of claim 1, wherein the method has a selectivity of methanol vs. carbon monoxide of about 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90% in favor of methanol.

36. The method of claim 1, wherein the method has a selectivity of methanol vs. carbon monoxide of about 30% in favor of methanol.

37. The method of claim 1, wherein the method comprises:

contacting an aqueous solution comprising a metal cation and an anion with CO2, thereby forming an aqueous solution comprising a cation, an anion, and CO2;

contacting the aqueous solution comprising the cation, the anion, and CO2 with an electrode; and

applying a voltage to the aqueous solution comprising the cation, the anion, and CO2 via the electrode.

38. The method of claim 1, wherein the method comprises:

contacting an aqueous solution comprising a metal cation and an anion with CO, thereby forming an aqueous solution comprising a cation, an anion, and CO;

contacting the aqueous solution comprising the cation, the anion, and CO with an electrode; and

applying a voltage to the aqueous solution comprising the cation, the anion, and CO via the RHE.

39-46. (canceled)