CONVERSION OF LIQUID CO2

Abstract

A method for electrochemically reducing carbon dioxide includes the step of providing a liquid or supercritical carbon dioxide mixture comprising liquid or supercritical carbon dioxide, water, a phase transfer catalyst, and an ion conducting salt. A potential difference is applied across a portion of the liquid or supercritical carbon dioxide mixture to cause the reduction of carbon dioxide with protons from the water. A reactor for electrochemically reducing carbon dioxide is also disclosed.

Description

FIELD OF THE INVENTION

The present invention relates to the electrochemical reduction of carbon dioxide in a liquified or supercritical form, and electrodes for performing this reduction.

BACKGROUND OF THE INVENTION

Nearly all electrochemical approaches to CO₂ conversion rely on processes where CO₂ is bubbled through acidic or basic media and subsequently reduced. The resulting electrochemistry leads to excessive generation of H₂ over fractions of CO₂ conversion. This is due to the limited solubility of CO₂ (0.033 mol/cm³ at 25° C.), the inherent molecular stability of CO₂, non-favorable interfacial bonding, and the poor adsorption on surfaces, all of which make chemical transformations difficult under standard temperatures and pressures.

Increasing the pressure on CO₂ to 75 psi at room temperature results in the liquification of CO₂, which drastically changes the molecular environment and interactions of CO₂ compared to a gas. Under these conditions CO₂ becomes a solvating reagent with the ability to dissolve small molecules.

Increasing the CO₂ pressure beyond 1000 psi at temperatures >36° C. causes CO₂ to phase transition into a supercritical fluid, supercritical CO₂ (scCO₂), which drastically changes the molecular interactions and reaction conditions. Under these conditions CO₂ becomes a highly concentrated solvating reagent with the ability to dissolve small molecules at greater concentrations and also becomes more chemically active, opening pathways for instantaneous easy product separations.

It was demonstrated that high CO₂ pressures result in reduced H₂ production rates compared to the electroreduction of CO₂. For example, less than 50% H₂ production has been reported (compared to 36% CO₂ conversion to ethylene). It has been shown that using a supercritical (scCO₂) solution with acetonitrile resulted in a 4.6× increase in current density and less than 8% H₂ evolution. In contrast, aqueous-based CO₂ electroreduction shows poor efficiencies, rather these systems favor hydrogen evolution, which routinely hover around 90+% unless operated at extremely alkaline conditions. This type of chemical selectivity is rarely observed for neutral or acidic aqueous based catalysts.

Limited prior work explores copper, silver, and nickel as suitable electrocatalysts for CO₂ reduction. From the data in the literature (mostly bubbling through aqueous mixtures) catalyst systems focus on low surface area copper electrodes made from rods or mesh. Furthermore, it is clear that the copper acts as an electrocatalyst during the conversion as evidenced by the extensive pitting of the copper rods. There are few studies using traditional, high surface area catalysts due to the instability of binders and composite powders during the electrochemical reaction. This leads to questions regarding electrode design and structure revolving around the size, shape, alloying and bonding of the catalyst and how these change with cycling.

SUMMARY OF THE INVENTION

A method for electrochemically reducing carbon dioxide, includes the step of providing a liquid or supercritical carbon dioxide mixture comprising liquid or supercritical carbon dioxide, water, a phase transfer catalyst, and an ion conducting salt. A potential difference is applied across a portion of the liquid or supercritical carbon dioxide mixture to cause the reduction of carbon dioxide with protons from the water.

The potential difference can be applied by electrodes. The cathode can comprise at least one selected from the group consisting of molybdenum, carbon fiber, copper, nickel, metal carbides, metal borides, titanium and aluminum. Other cathode materials are possible. The anode can comprise platinum, iridium oxide, or a material capable of the oxygen evolution reaction (OER). Other anode materials are possible. Combinations of anode and cathode materials are possible.

The potential difference can be applied by electrodes comprising molybdenum, and such an electrode can include molybdenum and MoO₂. The electrode can comprise 5 atomic percent to 100% molybdenum.

The electrodes can have a porosity of from 15 to 90%. The electrodes can comprise a material that is electrochemically stable (−2.5 V to 0 V versus a standard hydrogen electrode (SHE)) and supercritical carbon dioxide stable.

The ion conducting salt can comprise a cation selected from the group consisting of tetraethylammonium, tetrabutylammonium, 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, and 1-hexyl-3-methylimidazolium, and an anion selected from the group consisting of iodide, hexafluorophosphate, tetrafluoroborate, bis(trifluoromethane) sulfonimide, and triflate (trifluoromethanesulfonate). Combinations of ion conducting salts are possible.

The method can further comprise at least one reaction promoting salt. The reaction promoting salt can comprise at least one selected from the group consisting of CsTFSI, CsOTf, CsOAc, CsI, Cs2CO3, CsF, CsCl, KTFSI, KOTf, KOAc, KI, K2CO3, KF, and KCl.

The phase transfer catalyst can comprise at least one selected from the group consisting of acetonitrile, succinonitrile, monoglyme, diglyme, triglyme, tetraglyme, tetrahydrofuran (THF), dioxolane, and 1-octanol.

The product of the electrochemical reduction of the carbon dioxide can be at least one selected from the group consisting of ethanol, propane, methanol, isopropanol, ethylene, acetone, and carbon monoxide. Other reaction products are possible.

The liquid or supercritical carbon dioxide mixture can comprise from 10 to 95 wt % carbon dioxide, 5 to 80 wt % water, 5 to 70 wt % phase transfer catalyst, 1 to 50 wt % ion conducting salt, and 1 to 50 wt % reaction promoting salt.

The pressure of the liquid carbon dioxide mixture can be from 100 to 1070 psi. The pressure of the supercritical carbon dioxide mixture can be from 1070 psi to 1500 psi. The temperature of the supercritical carbon dioxide mixture can be from 40 to 100° C. The potential applied to the electrodes can be from −2 to −0.5 V versus a Ag/AgCl reference.

A reactor for electrochemically reducing carbon dioxide can include a reactor vessel for containing a supercritical carbon dioxide mixture comprising liquid or supercritical carbon dioxide, water, a phase transfer catalyst, and an ion conducting salt. The reactor can have electrodes for applying a potential difference across a portion of the liquid or supercritical carbon dioxide mixture. The electrodes can comprise an electrode composition comprising from 5 atomic percent to 100 atomic percent molybdenum. The electrode can be comprised of molybdenum and MoO₂. The electrode can be the cathode and can comprise 5 to 100 atomic % molybdenum.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings embodiments that are presently preferred it being understood that the invention is not limited to the arrangements and instrumentalities shown, wherein:

FIG. 1 is a schematic diagram of reactor to reduce liquid and supercritical CO₂ to hydrocarbon and oxygenated molecules.

FIG. 2 is a plot of current versus voltage (Current (mA) vs. potential vs. Ag/AgCl)) for a control experiment with a copper working electrode using cesium iodide as a salt and no polar cosolvent and water as a proton source at 200 psi CO₂.

FIG. 3 is a plot of current versus voltage (Current (mA) vs. potential vs. Ag/AgCl)) for a control experiment with a copper working electrode, cesium iodide as a salt and acetonitrile as a cosolvent and water as a proton source at 200 psi CO₂.

FIG. 4 is a plot of current versus voltage (Current (mA) vs. potential vs. Ag/AgCl)) for a control experiment with a copper working electrode, cesium acetate (CsOAc) as a salt and acetonitrile as a cosolvent and water as a proton source at 200 psi CO₂.

FIG. 5 is a plot of current versus voltage (Current (mA) vs. potential vs. Ag/AgCl)) for a control experiment with a molybdenum working electrode, cesium carbonate as a salt and acetonitrile as a cosolvent and water as a proton source at 200 psi CO₂.

FIG. 6 is a comparison plot of current versus voltage (Current (mA) vs. potential vs. Ag/AgCl)) for a control experiment with copper and molybdenum working electrodes in various salts and cosolvents and water as a proton source at 200 psi CO₂.

FIG. 7 is a plot of current versus voltage data (l/mA vs. Ewe/V) measured at 250 psi for a copper working electrode with cesium iodide salt and acetonitrile cosolvent and water as a proton source.

FIG. 8 is a plot of current versus voltage data (l/mA vs. Ewe/V) measured at 250 psi for a molybdenum working electrode with cesium iodide salt and acetonitrile cosolvent and water as a proton source.

FIG. 9 is a plot of current versus voltage data (l/mA vs. Ewe/V) measured at 750 psi for a copper working electrode with cesium carbonate salt and acetonitrile cosolvent and water as a proton source.

FIG. 10 is a plot of current versus voltage data (l/mA vs. Ewe/V) measured at 750 psi for a molybdenum working electrode with cesium carbonate salt and acetonitrile cosolvent and water as a proton source.

FIG. 11 is a plot of a comparison of current versus voltage data (l/mA vs. Ewe/V) measured at 250 psi and 750 psi for copper and molybdenum working electrodes with cesium carbonate or cesium iodide salt and acetonitrile cosolvent and water as a proton source.

FIG. 12 is a plot of current versus voltage data (l/mA vs. Ewe/V) measured at 100 psi for a molybdenum working electrode with cesium acetate (CsOAc) salt and acetonitrile cosolvent and water as a proton source.

FIG. 13 is a plot of current versus voltage data (l/mA vs. Ewe/V) measured at 500 psi for a molybdenum working electrode with cesium acetate (CsOAc) salt and acetonitrile cosolvent and water as a proton source.

FIG. 14 is a plot of current versus voltage data (l/mA vs. Ewe/V) measured at 750 psi for a molybdenum working electrode with cesium acetate (CsOAc) salt and acetonitrile cosolvent and water as a proton source.

FIG. 15 is a plot of current versus voltage data (l/mA vs. Ewe/V) measured at 1100 psi for a molybdenum working electrode with cesium acetate (CsOAc) salt and acetonitrile cosolvent and water as a proton source.

FIG. 16 is a plot of a comparison of current versus voltage data (l/mA vs. Ewe/V) measured at various CO₂ pressures for molybdenum working electrode with cesium acetate (CsOAc) salt and acetonitrile cosolvent and water as a proton source.

FIG. 17 is a plot of electrochemical impedance spectroscopy data (−lm (Z)/Ohm vs. Re (Z)/Ohm) measured at 100 psi CO₂ pressures for a molybdenum working electrode with cesium acetate (CsOAc) salt and acetonitrile cosolvent and water as a proton source.

FIG. 18 is a plot of an electrochemical impedance spectroscopy data (−lm (Z)/Ohm vs. Re (Z)/Ohm) measured at 500 psi CO₂ pressures for a molybdenum working electrode with cesium acetate (CsOAc) salt and acetonitrile cosolvent and water as a proton source.

FIG. 19 is a plot of electrochemical impedance spectroscopy data (−lm (Z)/Ohm vs. Re (Z)/Ohm) measured at 750 psi CO₂ pressures for a molybdenum working electrode with cesium acetate (CsOAc) salt and acetonitrile cosolvent and water as a proton source.

FIG. 20 is a plot of electrochemical impedance spectroscopy data (−lm (Z)/Ohm vs. Re (Z)/Ohm) measured at 1100 psi CO₂ pressures for a molybdenum working electrode with cesium acetate (CsOAc) salt and acetonitrile cosolvent and water as a proton source.

FIG. 21 is a plot of a comparison of electrochemical impedance spectroscopy data (−lm (Z)/Ohm vs. Re (Z)/Ohm) measured at various CO₂ pressures for molybdenum working electrode with cesium acetate (CsOAc) salt and acetonitrile cosolvent and water as a proton source.

FIG. 22 is a plot of current versus voltage data (l/mA vs. Ewe/V) measured at 750 psi for a molybdenum working electrode with cesium acetate (CsOAc) salt and acetonitrile cosolvent and water as a proton source.

FIG. 23 is a plot of current versus voltage data (l/mA vs. Ewe/V) measured at 750 psi for a molybdenum working electrode with cesium iodide salt and acetonitrile cosolvent and water as a proton source.

FIG. 24 is a plot of current versus voltage data (l/mA vs. Ewe/V) measured at 750 psi for a molybdenum working electrode with cesium carbonate salt and acetonitrile cosolvent and water as a proton source.

FIG. 25 is a plot of a comparison of current versus voltage data (l/mA vs. Ewe/V) measured at 750 psi CO₂ pressure for molybdenum working electrode with cesium acetate (CsOAc), cesium iodide, and cesium carbonate salt and acetonitrile cosolvent and water as a proton source.

FIG. 26 is a plot of electrochemical impedance spectroscopy data (−lm (Z)/Ohm vs. Re (Z)/Ohm) measured at 750 psi CO₂ pressures for a molybdenum working electrode with cesium acetate (CsOAc) salt and acetonitrile cosolvent and water as a proton source.

FIG. 27 is a plot of electrochemical impedance spectroscopy data (−lm (Z)/Ohm vs. Re (Z)/Ohm) measured at 750 psi CO₂ pressures for a molybdenum working electrode with cesium iodide salt and acetonitrile cosolvent and water as a proton source.

FIG. 28 is a plot of electrochemical impedance spectroscopy data (−lm (Z)/Ohm vs. Re (Z)/Ohm) measured at 750 psi CO₂ pressures for a molybdenum working electrode with cesium carbonate salt and acetonitrile cosolvent and water as a proton source.

FIG. 29 is a plot of a comparison (−lm (Z)/Ohm vs. Re (Z)/Ohm) of electrochemical impedance spectroscopy data measured at 750 psi CO₂ pressures for a molybdenum working electrode with cesium acetate (CsOAc), cesium iodide, and cesium carbonate salt and acetonitrile cosolvent and water as a proton source.

FIG. 30 is a plot of current (Current at −1.5 V (vs. Ag/AgCl) vs. CO₂ pressure (psi)) at −1.5 V (vs. Ag/AgCl reference electrode) as a function of pressure for molybdenum (star symbol) and copper (circle symbol) measured with Cs₂CO₃ salt with acetonitrile cosolvent and water as a proton source.

FIG. 31 is a plot of mass spectrometry data (Signal Mass Spectrometer vs. AMU) of gas phase products measured from products produced with Cs₂CO₃ salt with acetonitrile cosolvent and water as a proton source at 750 psi CO₂.

FIG. 32 is a plot of x-ray photoelectron spectroscopy data (Intensity vs. Binding Energy (eV)) collected for a molybdenum electrode surface post electrochemical reduction of CO₂ performed in Cs₂CO₃ salt with acetonitrile cosolvent and water as a proton source. The dashed lines indicate the characteristic binding energies for Mo⁶⁺, Mo⁴⁺, and molybdenum metal (Moº).

FIG. 33 is a plot of x-ray photoelectron spectroscopy data (Intensity vs. Binding Energy (eV)) collected for a copper electrode surface post electrochemical reduction of CO₂ performed in Cs₂CO₃ salt with acetonitrile cosolvent and water as a proton source. The dashed lines indicate the characteristic binding energies for Cu²⁺, copper metal (Cuº).

FIG. 34 is a legend for the illustration of the components of a reactor system depicted in FIGS. 35-38.

FIG. 35A is a schematic depiction of a low-pressure reactor system with a low starting CO₂ concentration in a water solution; FIG. 35B is a schematic depiction of the system of FIG. 35A after mixing; FIG. 35C is a schematic depiction of the system after electrolysis.

FIG. 36A is a schematic depiction of a reactor system with liquid or supercritical CO₂ (no water); FIG. 36B shows the system after electrolysis.

FIG. 37A is a schematic depiction of a reactor system with liquid or supercritical CO₂ and water; FIG. 37B is a depiction of the system of FIG. 37A after mixing; FIG. 37C is a depiction of the system of FIG. 37A after electrolysis.

FIG. 38A is a schematic depiction of a reactor system with liquid or supercritical CO₂ and water; FIG. 38B is a depiction of the system of FIG. 38B after mixing; FIG. 38C is a depiction of the system of FIG. 38A after electrolysis.

DETAILED DESCRIPTION OF THE INVENTION

A method for electrochemically reducing carbon dioxide includes the step of providing a liquid or supercritical carbon dioxide mixture comprising supercritical carbon dioxide, water, a phase transfer catalyst, and an ion conducting salt. A potential difference is applied across a portion of the liquid or supercritical carbon dioxide mixture to cause the reduction of carbon dioxide with protons from the water. The potential difference can be applied by electrodes.

The advantage of this approach is the ability to tune the molecular interactions of CO₂ and other reagents through electrochemical potential, pressure, and temperature. These methods provide a pathway to active synthetic control. For example, an activated CO₂ molecule may be formed electrochemically then the pressure can be increased to promote the competitive bonding of a second reactant resulting in a preferential pathway to addition in a sorption/desorption type non-equilibrium process. Furthermore, electrochemistry in supercritical CO₂ is a relatively unexplored field of research, whereas using CO₂ as a reagent in organic synthesis at liquid or supercritical conditions has been extensively investigated. Indeed, the heterogeneous or homogenously catalyzed addition of CO₂ to an organic molecule is explored extensively.

The electrodes can comprise a material that is electrochemically stable (−2.5 V to 0 V versus a standard hydrogen electrode (SHE)) and supercritical carbon dioxide stable. The cathode can comprise at least one selected from the group consisting of molybdenum, carbon fiber, copper, nickel, metal carbides, metal borides and aluminum. The anode can comprise platinum, iridium oxide, or another material capable of the oxygen evolution reaction (OER). Other materials are possible.

A cathode comprising, consisting essentially of, or consisting of molybdenum has been found to be particularly desirable. The cathode electrode can comprise molybdenum and MoO₂. The amount of molybdenum in the cathode electrode can vary. The cathode electrode can comprise 5 atomic percent to 100% molybdenum. The amount of molybdenum in the cathode, in atomic percent, can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or 100%, and can be within a range of any high value and low value selected from these values.

The ion conducting salt can comprise a cation selected from tetraethylammonium, tetrabutylammonium, 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, and 1-hexyl-3-methylimidazolium, and an anion selected from iodide, hexafluorophosphate, tetrafluoroborate, bis(trifluoromethane) sulfonimide, and triflate (trifluoromethanesulfonate). Other ion conducting salts are possible.

At least one reaction promoting salt can be provided. The reaction promoting salt can comprise at least one selected from the group consisting of CsTFSI, CsOTf, CsOAc, CsI, Cs₂CO₃, CsF, CsCl, KTFSI, KOTf, KOAc, KI, K₂CO₃, KF, and KCl. Other reaction promoting salts are possible, such as imidazolium-based ionic liquids whose cations include: 1-Ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, and 1-hexyl-3-methylimidazolium, and their mixtures, being charge balanced with the following anions: iodide, tetrafluoroborate, acetate, hexafluorophosphate, bis(trifluoromethylsulfonyl)imide, and their mixtures.

The phase transfer catalyst can comprise at least one selected from the group consisting of acetonitrile, succinonitrile, monoglyme, diglyme, triglyme, tetraglyme, tetrahydrofuran (THF), and dioxolane. Other phase transfer catalysts are possible, such as 1-octanol.

The product of the electrochemical reduction of the carbon dioxide can be at least one selected from the group consisting of ethanol, propane, methanol, isopropanol, ethylene, acetone, and carbon monoxide. The reactions can be complex, but can be generally represented by the following equations:

$\begin{array}{cc}{\mathrm{CO}}_2+2H^{+}+2e^{-}\to \mathrm{CO}+H_{2}O& \left(1\right)\end{array}$ $\begin{array}{cc}{x\mathrm{CO}}_2+\left(y+4x-1\right)H^{+}+\left(y+4x-1\right)e^{-}\to C_x{H}_y\mathrm{OH}+\left(2x-1\right)H_{2}O& \left(2\right)\end{array}$ $\begin{array}{cc}{x\mathrm{CO}}_2+\left(y+4x\right)H^{+}+\left(y+4x\right)e^{-}\to C_x{H}_y+2x{H}_{2}O& \left(3\right)\end{array}$

Other reactions are possible.

The supercritical carbon dioxide mixture comprises from 10 to 95 wt % carbon dioxide, 5 to 80 wt % water, 5 to 70 wt % phase transfer catalyst, 1 to 50 wt % ion conducting salt, and 1 to 50 wt % reaction promoting salt. The weight % of carbon dioxide can be 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, or 95 wt. %, and can be within a range of any high value and low value selected from these values. The weight % of water can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, or 80 wt. %, and can be within a range of any high value and low value selected from these values. The weight % phase transfer catalyst can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, or 70 wt. %, and can be within a range of any high value and low value selected from these values. The weight % of the ion conducting salt can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50 wt. %, and can be within a range of any high value and low value selected from these values. The weight % of the reaction promoting salt can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50 wt. %, and can be within a range of any high value and low value selected from these values.

The pressure of the liquid carbon dioxide mixture can be from 100 to 1070 psi. The pressure of the liquid carbon dioxide mixture can be 100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 375, 400, 425, 450, 475, 500, 525, 550, 575, 600, 625, 650, 675, 700, 725, 750, 775, 800, 825, 850, 875, 900, 925, 950, 975, 1000, 1025, 0150, or 1070 psi. The pressure of the liquid carbon dioxide mixture can be within a range of any high value and low value selected from these values.

The pressure of the supercritical carbon dioxide mixture can be from 1070 psi to 1500 psi. The pressure of the supercritical carbon dioxide mixture can be 1070, 1080, 1090, 1100, 1110, 1120, 1130, 1140, 1150, 1160, 1170, 1180, 1190, 1200, 1210, 1220, 1230, 1240, 1250, 1260, 1270, 1280, 1290, 1300, 1310, 1320, 1330, 1340, 1350, 1360, 1370, 1380, 1390, 1400, 1410, 1420, 1430, 1440, 1450, 1460, 1470, 1480, 1490, or 1500 psi. The pressure of the supercritical carbon dioxide mixture can be within a range of any high value and low value selected from these values.

The temperature of the supercritical carbon dioxide mixture can be from 40 to 100° C. The temperature of the supercritical carbon dioxide mixture can be 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 or 100° C. The temperature of the supercritical carbon dioxide mixture can be within a range of any high value and low value selected from these values.

The potential applied to the electrodes can be from −2 to −0.5 V versus a Ag/AgCl reference. The potential applied to the electrodes versus a Ag/AgCl reference can be −2, −1.9, −1.8, −1.7, −1.6, −1.5, −1.4, −1.3, −1.2, −1.1, −1.0, −0.9, −0.8, −0.7, −0.6, or −0.5 V. The potential applied to the electrodes versus a Ag/AgCl reference can be within a range of any high value and low value selected from these values.

A reactor for electrochemically reducing carbon dioxide can include a reactor vessel for containing a supercritical carbon dioxide mixture comprising liquid or supercritical carbon dioxide, water, a phase transfer catalyst, and an ion conducting salt. The reactor has electrodes for applying a potential difference across a portion of the liquid or supercritical carbon dioxide mixture. The cathode electrode can comprise an electrode composition comprising from 5 atomic percent to 100 atomic percent molybdenum. The electrode composition can comprise molybdenum and MoO₂. The electrode can consist of 100 atomic % molybdenum.

In one aspect, the invention provides an cathode electrode composition to electrochemically reduce liquified CO₂ or supercritical CO₂, wherein the electrode composition comprises 5 atomic percent to 100% molybdenum. The electrode composition can, for example, comprise a solid metal rod or plate or supported nanoparticles on an electrochemically stable and liquified or supercritical CO₂ stable material. The electrode composition can have a high surface area to increase available reaction sites. Suitable geometries include, for example, nanoparticles (2 nm minimum size) to mesoparticles approaching single crystal sizes on the orders of centimeters or larger. The electrode composition can be decorated on an electrochemically stable and liquified or supercritical CO₂ stable material forming nanoparticles or surface decoration resulting in the formation of an electrocatalyst.

The electrode composition can be decorated on an electrically conductive material. Suitable electrically conductive material includes, for example, carbon fiber, copper, nickel, metal carbides, metal borides, titanium and aluminum. The electrically conductive material should be stable at the voltages the electrode operates and avoid side reactions with products or salts/species. The electrical interconnections can be maintained with the addition of a conductive additive like carbon nanotubes or graphene sheets.

The molybdenum can remain as metallic molybdenum metal or oxidize to Mo⁴⁺ resulting in a compound like MoO₂ that remains electrically conductive without extensive surface passivation resulting in an insulating surface thereby blocking reactivity and transport.

The electrode composition can be porous. Porosity introduces unique molecular interactions which can mediate reaction processes by directing molecular interactions. Such molecular interactions are exemplified by, for example, the well-known hydrocracking of large hydrocarbons in porous materials like zeolites and their derivatives. A similar pore structure would be valuable with the incorporation of an electrically conductive porous structure that is also simultaneously stable under electrochemical reduction conditions. Example materials include so called Mxene phases, carbons, metal foams, and the like, provided they maintain stability under liquified or supercritical CO₂ conditions.

The electrodes can have a porosity of from 15 to 90%. The electrodes, either independently or together, can have a porosity of 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, or 90%. The porosity of the electrodes can be within a range of any high value and low value selected from these values.

Traditional approaches to make porous electrodes involve the addition of added solvent, freeze tape casting, laser drilling, electrodeposition of materials, reactive ion etching, and catalytically promoted nanowire growth. These processes can be used to form the porous electrode compositions described herein decorated with molybdenum and/or molybdenum containing alloy catalysts where molybdenum makes up 100% of the alloy down to 5% of the alloy.

Alternatively, the electrode compositions described herein can be prepared as discussed above where MoO₂ is incorporated into or on the electrically conductive, redox stable, electrode surface resulting in the electrode. The MoO₂ is electrically conductive and suitable for liquid or supercritical CO₂ reduction.

The electrode composition described herein can be immersed in liquified or supercritical CO₂ along with an ion conducting salt such as, for example, tetrabutylamine iodide, tetrabutlyamine hexafluorophosphate, which will dissolve in the liquified or supercritical CO₂ promoting ion conduction.

The electrode composition described herein can be immersed in liquified or supercritical CO₂ along with an ion conducting salt (e.g, tetrabutylamine iodide, tetrabutlyamine hexafluorophosphate, etc) along with a co-salt made of cesium (or potassium) which interacts with the liquified or supercritical CO₂ to form a reaction complex suitable for reduction. Ideal co-salts will from a complex with CO₂ that is free of other organic species or moieties. Examples include CsI and Cs₂CO₃ where the anion is large. Smaller anion salts like CsF and CsCl are less preferable due to limited molecular interactions. Species like cesium acetate (CsOAc) are an intermediate material with a large anion structure but contain protons which restrict reactivity and change molecular interactions with the liquid or supercritical CO₂.

Typically, a cosolvent is needed in the above reactions that will be stable with the electrocatalyst and facilitate diffusion between non-polar CO₂ and water. The water is the proton source to reduce the liquified or supercritical CO₂. Candidate co-solvents need to be electrochemically stable to −2.2 V vs. H₂ as measured with a silver/silver chloride reference electrode which has a potential of 0.2 volts versus hydrogen. A more preferable solvent would have an electrochemically stability window to −2.7 V vs. H₂ as measured with a silver/silver chloride reference electrode which has a potential of 0.2 volts versus hydrogen to avoid the reduction of cesium ions to cesium metal which has a standard reduction potential of −2.92 volts versus hydrogen. Example materials include acetonitrile, succinonitrile, ethylene carbonate, dimethyl carbonate, propylene carbonate, dioxolane, and known ionic liquids like 1-butyl-3-methyl imidazolium tetrafluoroborate (BMIM-BF₄) and 1-ethyl-3-methyl imidazolium tetrafluoroborate (EMIM-BF₄). Non-polar molecules stable to low electrochemical potentials like hexane are generally not suitable due to the poor transport ability between non-polar CO₂ and aqueous water.

A reactor can be provided where CO₂ gas is liquified or turned into supercritical CO₂. The reactor can comprise a molybdenum-containing electrode composition as described herein, a promoting salt, an electrochemically stable co-solvent, and a supporting salt. The molybdenum electrode composition works in conjunction with the promoting salt which can be, for example, cesium salt. The electrochemically stable co-solvent can be, for example, acetonitrile, succinonitrile, diglyme, or dioxolane. The supporting salt can be, for example, tetrabutylamine iodide or tetrabutlyamine hexafluorophosphate. The reaction is electrochemical in nature, and thus the co-salt supports ion transport needed for electrochemistry, co-solvent promotes electrochemistry and ion transport/solvation, promoting salt based on cesium ions, water as a proton source, and the electrode composition as the electrocatalyst.

An example of a suitable reactor is schematically represented in FIG. 1. The reactor 10 includes a reactor vessel 14 with an open interior 18. Within open interior 18 are electrodes 22, 23 and a mixing device 24. The cathode electrode 22 can comprise a molybdenum electrode composition described herein, and is immersed in the liquid or supercritical CO₂ with cesium salt, electrochemically stable co-solvent, a supporting salt, and water as a proton source. The mixture is stirred with a stirring or flow apparatus to avoid stratification, segregation or other causes limiting diffusion to the electrode surface where the CO₂ is reduced to form hydrocarbons or oxygenated hydrocarbons like ethanol, methanol, ether or glymes. From a reactor design point of view, the use of scCO₂ provides an opportunity to separate reagents from products without the use of complex distillations of hydrocarbons from water/salt mixtures through simple evaporation.

For example, in a 1 L high pressure reactor, 0.8 g of Cs-salt, 2.5 g of deionized water, 15 mL acetonitrile (cosolvent), 0.2 g of tetrabutylammonium iodide were added and mixed well. Then 88 g of dried ice was added to the reactor and allowed to pressurize and displaced air from laboratory. Pressure is controlled by venting gas till desired pressure is achieved.

In a typical reaction copper would be employed as an electrocatalyst along with a mixture of salts, solvents, and water. FIG. 2 demonstrates the electrochemical activity of a copper working electrode without co-solvent at 200 psi CO₂ using CsI as a cesium salt to coordinate with the liquified CO₂. This plot demonstrates that there is little reduction activity as evidenced by the low current density.

FIG. 3 demonstrates the electrochemical activity of a copper working electrode with acetonitrile co-solvent at 200 psi CO₂ using CsI as a cesium salt to coordinate with the liquified CO₂. This plot demonstrates that there is an increase in reduction activity as evidenced by the increase in current density to about 2 mA/cm².

FIG. 4 demonstrates the electrochemical activity of a copper working electrode with acetonitrile co-solvent at 200 psi CO₂ using cesium acetate as a cesium salt to coordinate with the liquified CO₂. This plot demonstrates that there is a further increase reduction activity as evident by the increase in current density to about 8 mA/cm² over the base line activity without co-solvent and a 6 mA/cm² increase over copper electrodes with cesium acetate. Also, FIG. 4 indicates that the choice of salt makes a significant difference in the resulting activity as evidenced by the increase in current density.

FIG. 5 shows the electrochemical activity of a molybdenum working electrode with acetonitrile co-solvent at 200 psi CO₂ using cesium carbonate as a cesium salt to coordinate with the liquified CO₂. This plot demonstrates that there is a further increase in reduction activity as evidenced by the increase in current density to about 55 mA/cm² over the base line activity without co-solvent and an 8 mA/cm² increase over copper electrodes with cesium iodide. This plot further indicates that the addition of molybdenum results in a significant increase in activity. Further the choice of salt makes a significant difference in the resulting activity as evidenced by the increase in current density. This is more apparent in FIG. 6 which plots the data for FIGS. 2-5 on the same plot.

FIG. 7 shows the current as a function of potential measured at 250 psi liquid CO₂ in a CsI salt with acetonitrile co-solvent and copper electrode. FIG. 8 shows the current as a function of potential measured at 250 psi liquid CO₂ in a CsI salt with acetonitrile co-solvent and molybdenum electrode. FIG. 9 shows the current as a function of potential measured at 750 psi liquid CO₂ in a CsI salt with acetonitrile co-solvent and copper electrode. FIG. 10 shows the current as a function of potential measured at 750 psi liquid CO₂ in a CsI salt with acetonitrile co-solvent and molybdenum electrode. Increasing the CO₂ results in an increase in electroreduction currents. The nature of the catalyst has a controlling effect on the reduction currents, namely at these high pressures the Mo electrodes show significantly more activity than the copper electrodes.

FIG. 11 compares the current as a function of potential measured at 250 and 750 psi liquid CO₂ in a CsI and Cs₂CO₃ salt with acetonitrile co-solvent and copper or molybdenum electrodes. This plot demonstrates that the choice of electrocatalyst results in a significant increase in current density. Specifically, the molybdenum with CsI is nearly twice as electrochemically active as the copper analog at the same potentials.

Further, for the molybdenum electrode the addition of Cs₂CO₃ results in a further enhancement of over 12 times the activity of the copper electrode (as measured at −1.5 V versus Ag/AgCl reference electrode). This increase in activity is evidence for the important role of molybdenum for the reduction of liquid CO₂. Further, the ability to reduce the reduction potential to −1.5V (versus Ag/AgCl reference electrode) significantly reduces the probability of side reactions degrading the ion conducting supporting salt.

A series of studies were performed with molybdenum electrodes and cesium acetate salt to determine the effect of reaction pressure. FIG. 12 shows current versus reduction potential data measured at 100 psi liquified CO₂ with acetonitrile as a co-solvent and water as a proton source. FIG. 13 shows current versus reduction potential data measured at 500 psi liquified CO₂ with acetonitrile as a co-solvent and water as a proton source. FIG. 14 shows current versus reduction potential data measured at 750 psi liquified CO₂ with acetonitrile as a co-solvent and water as a proton source. FIG. 15 shows current versus reduction potential data measured at 1100 psi supercritical CO₂ with acetonitrile as a co-solvent and water as a proton source. FIG. 16 compares current versus reduction potential data measured at 100, 500, 750 psi liquid CO₂ and 1100 psi supercritical CO₂ with acetonitrile as a co-solvent and water as a proton source. This data demonstrates that the current density increases significantly as a function of pressure. This increase in current is attributed to an increase in carbon dioxide reduction activity. This data shows that the Mo electrode works better at higher pressures than lower pressures.

FIG. 17 shows electrochemical impedance spectroscopy data measured for 100 psi liquified CO₂ with acetonitrile as a co-solvent and water as a proton source. FIG. 18 shows electrochemical impedance spectroscopy data measured for 500 psi liquified CO₂ with acetonitrile as a co-solvent and water as a proton source. FIG. 19 shows electrochemical impedance spectroscopy data measured for 750 psi liquified CO₂ with acetonitrile as a co-solvent and water as a proton source. FIG. 20 shows electrochemical impedance spectroscopy data measured for 1100 psi supercritical CO₂ with acetonitrile as a co-solvent and water as a proton source. FIG. 21 compares electrochemical impedance spectroscopy data measured at 100, 500, 750 psi liquid CO₂ and 1100 psi supercritical CO₂ with acetonitrile as a co-solvent and water as a proton source. This data demonstrates that the resistance of the cell decreases significantly as a function of pressure. Specially at 750 psi the resistance decreases to almost 1000 Ohms versus an extrapolated resistance of 7000 Ohms for 500 psi. This decrease in resistance indicates an improved ionic conductivity, and resulting CO₂ reduction, over the molybdenum electrode. Further, upon transitioning to supercritical conditions the resistance increases slightly to 6000 ohms, likely due to the change in state of the CO₂ from liquid to supercritical.

The role of the salt on the molybdenum electrodes was tested by a series of experiments using molybdenum electrodes, which were evaluated for activity in CsOAc, CsI, and Cs₂CO₃. FIG. 22 demonstrates the current as a function of potential measured at 750 psi liquid CO₂ in a CsOAc salt with acetonitrile co-solvent and molybdenum electrode. FIG. 23 demonstrates the current as a function of potential measured at 750 psi liquid CO₂ in a CsI salt with acetonitrile co-solvent and molybdenum electrode. FIG. 24 demonstrates the current as a function of potential measured at 750 psi liquid CO₂ in a Cs₂CO₃ salt with acetonitrile co-solvent and molybdenum electrode. FIG. 25 compares current as a function of potential measured at 750 psi liquid CO₂ the CsOAc, CsI, and Cs₂CO₃ with acetonitrile co-solvent and molybdenum electrode. This data demonstrates that the Cs salt results in a significant modification to the observed current densities with larger current densities resulting in more excessive CO₂ reduction activity.

FIG. 26 shows electrochemical impedance spectroscopy data measured for 750 psi liquified CO₂ with CsOAc, acetonitrile as a co-solvent and water as a proton source. FIG. 27 shows electrochemical impedance spectroscopy data measured for 750 psi liquified CO₂ with CsI, acetonitrile as a co-solvent and water as a proton source. FIG. 28 shows electrochemical impedance spectroscopy data measured for 750 psi liquified CO₂ with Cs₂CO₃, acetonitrile as a co-solvent and water as a proton source. FIG. 29 compares electrochemical impedance spectroscopy data measured for 750 psi liquified CO₂ with Cs₂CO₃, CsOAc and CsI, with acetonitrile as a co-solvent and water as a proton source. This data demonstrates that the choice of Cs₂CO₃ as a co-salt results in a significantly reduced cell impedance and correspondingly higher observed current density. Therefore the choice of salt in conjunction with molybdenum is critical to the observed electrocatalytic activity.

FIG. 30 shows measured currents at −1.5V (vs. Ag/AgCl reference electrode) as a function of pressure for molybdenum (star symbol) and copper (circle symbol) measured with Cs₂CO₃ salt with acetonitrile cosolvent and water as a proton source. This data demonstrates the benefit of molybdenum containing electrodes over traditional copper electrodes as evidenced by the increased current density measured. It is also noted that the current density decreases upon shifting to the supercritical CO₂ regime as consistent with the impedance and activity data presented in FIG. 21 due to changes in the CO₂ environment. Finally, it is noted that qualitatively more liquid products are collected at supercritical conditions over liquid CO₂ reaction conditions.

FIG. 31 demonstrates mass spectrometer data measured for the reduction performed at 750 psi of liquified CO₂. The data shows a low hydrogen signal as evident from the low signal at 2 atomic mass units (AMU) from H₂. In contrast there is a large signal at 46 AMU consistent with the formation of ethanol. There is a large signal from the produced O₂ gas at 32 AMU that overlaps with produced methanol (32 AMU). There are various other fragments from the decomposition of ethanol in the mass spectrometer. There is evidence for higher molecular weight hydrocarbons (above 50 amu) consistent with C4 oxygenates. This data demonstrates a complex reaction product distribution from the activity over molybdenum.

FIG. 32 shows representative X-ray photoelectron spectroscopy data collected for the molybdenum electrode surface post electrochemical reduction of CO₂ performed in Cs₂CO₃ salt with acetonitrile cosolvent and water as a proton source. XPS binding energies provide evidence for distinctive chemical speciation on the top 5 nm of the electrode surface. For comparison purposes dashed lines indicate the characteristic binding energies for Mo⁶⁺, Mo⁴⁺, and molybdenum metal (Moº). This data demonstrates that the surface is terminated with Mo⁴⁺ species. Typical Mo⁴⁺ materials are MoO₂ an electrically conductive oxide with a typical conductivity of 6000 S/cm at room temperature.

FIG. 33 shows X-ray photoelectron spectroscopy data collected for copper electrode surface post electrochemical reduction of CO₂ performed in Cs₂CO₃ salt with acetonitrile cosolvent and water as a proton source. The dashed lines indicate the characteristic binding energies for Cu²⁺, copper metal (Cuº). This data indicates that the surface of the copper electrode is terminated with Cu²⁺ species like CuO, as CuO is an insulator with a band gap of 1.2 eV. This passivation behavior likely reduces electrocatalytic activity in liquid or supercritical CO₂.

There is shown in FIGS. 34-38 a series of schematic depictions of possible reactor systems for the conversion of CO₂. FIG. 34 is a legend for the illustration of the components of the reactor systems depicted in FIGS. 35-38. These components include CO₂ 110, CO₃²⁻ 114, CO 116, O₂ 118, Salt ion 112, water solution 108, liquid or supercritical CO₂ 128, CO₂/H₂O mixture with phase transfer catalyst 168, and CO₂ conversion reaction product C_xH_yO_z 170.

FIG. 35A is a schematic depiction of a reactor system 100 with cathode electrode 102, anode electrode 104, and separator 106. FIG. 35A depicts a low-pressure system with a low dissolved starting CO₂ 110 concentration in a water solution 108, and no liquid or supercritical CO₂. The system contains dissolved CO₂ 110 and salt ion 112. FIG. 35A depicts the system at an initial state of operation, and little or no carbonate 114 is present. FIG. 35B is a schematic depiction of the system of FIG. 35A after mixing. Some carbonate 114 has formed. FIG. 35C is a schematic depiction of the system after electrolysis. Some CO 116 has formed. A small amount of C_xH_yO_z product 170 that represents the conversion of CO₂ has formed on the anode 104 side of the separator 106.

FIG. 36A is a schematic depiction of a reactor system 120 with cathode 122, anode 124, and separator 126. The system includes liquid or supercritical CO₂ 128 and no water. Water is a proton source for the CO₂ conversion reaction. Dissolved CO₂ 110 and salt ion 112 are present. No mixing is performed. FIG. 36B shows the system after electrolysis. There is no conversion of CO₂ because there is no ion transport, and only the dissolved CO₂ 110 and salt ion 112 is present.

FIG. 37A is a schematic depiction of a reactor system 140 with a cathode 142, anode 144, and separator 146. The system includes water 108 and liquid or supercritical CO₂ 128. Dissolved CO₂ 110 and salt ion 112 are present. FIG. 37B is a depiction of the system of FIG. 37A after mixing. Some carbonate 114 has formed. FIG. 37C is a depiction of the system of FIG. 37A after electrolysis. There is no CO₂ conversion reaction due to little ion transport.

FIG. 38A is a schematic depiction of a reactor system 160 with cathode 162, anode 164, and separator 166. The cathode electrode 162 can comprise molybdenum. Dissolved CO₂ 110 and salt ion 112 are present. The system includes liquid or supercritical CO₂ 128, and a CO₂/H₂O and phase transfer catalyst mixture 168. FIG. 38B is a depiction of the system of FIG. 38B after mixing. Some carbonate 114 has formed. FIG. 38C is a depiction of the system of FIG. 38A after electrolysis. Fast kinetics due to high concentration and species mobility result in the vigorous production of CO₂ conversion product C_xH_yO_z 170. Some CO 116 and O₂ 118 has also formed.

The invention as shown in the drawings and described in detail herein disclose arrangements of elements of particular construction and configuration for illustrating preferred embodiments of structure and method of operation of the present invention. It is to be understood however, that elements of different construction and configuration and other arrangements thereof, other than those illustrated and described may be employed in accordance with the spirit of the invention, and such changes, alternations and modifications as would occur to those skilled in the art are considered to be within the scope of this invention as broadly defined in the appended claims. In addition, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

Claims

1. A method for electrochemically reducing carbon dioxide, comprising the steps of: providing a liquid or supercritical carbon dioxide mixture comprising liquid or supercritical carbon dioxide, water, a phase transfer catalyst, and an ion conducting salt; and,applying a potential difference across a portion of the liquid or supercritical carbon dioxide mixture to cause the reduction of carbon dioxide with protons from the water.

2. The method of claim 1, wherein the potential difference is applied by electrodes, the cathode comprising at least one selected from the group consisting of molybdenum, carbon fiber, copper, nickel, metal carbides, metal borides, titanium and aluminum, and the anode comprising platinum, iridium oxide, or a material capable of the oxygen evolution reaction (OER).

3. The method of claim 1, wherein the potential difference is applied by electrodes comprising molybdenum.

4. The method of claim 3, wherein the electrodes comprise molybdenum and MoO2.

5. The method of claim 3, wherein the electrodes comprise 5 atomic percent to 100% molybdenum.

6. The method of claim 1, wherein the electrodes have a porosity of from 15 to 90%.

7. The method of claim 1, wherein the electrodes comprise a material that is electrochemically stable (−2.5 V to 0 V versus a standard hydrogen electrode (SHE)) and supercritical carbon dioxide stable.

8. The method of claim 1, wherein the ion conducting salt comprises a cation selected from the group consisting of tetraethylammonium, tetrabutylammonium, 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, and 1-hexyl-3-methylimidazolium, and an anion selected from the group consisting of iodide, hexafluorophosphate, tetrafluoroborate, bis(trifluoromethane) sulfonimide, and triflate (trifluoromethanesulfonate).

9. The method of claim 1, further comprising at least one reaction promoting salt.

10. The method of claim 9, wherein the reaction promoting salt comprises at least one selected from the group consisting of CsTFSI, CsOTf, CsOAc, CsI, Cs2CO3, CsF, CsCl, KTFSI, KOTf, KOAc, KI, K2CO3, KF, and KCl.

11. The method of claim 1, wherein the phase transfer catalyst comprises at least one selected from the group consisting of acetonitrile, succinonitrile, monoglyme, diglyme, triglyme, tetraglyme, tetrahydrofuran (THF), dioxolane, and 1-octanol.

12. The method of claim 1, wherein the product of the electrochemical reduction of the carbon dioxide is at least one selected from the group consisting of ethanol, propane, methanol, isopropanol, ethylene, acetone, and carbon monoxide.

13. The method of claim 1, wherein the supercritical carbon dioxide mixture comprises from 10 to 95 wt % carbon dioxide, 5 to 80 wt % water, 5 to 70 wt % phase transfer catalyst, 1 to 50 wt % ion conducting salt, and 1 to 50 wt % reaction promoting salt.

14. The method of claim 1, wherein the pressure of the liquid carbon dioxide mixture is from 100 to 1070 psi, and the pressure of the supercritical carbon dioxide mixture is from 1070 psi to 1500 psi.

15. The method of claim 1, wherein the temperature of the supercritical carbon dioxide mixture is from 40 to 100° C.

16. The method of claim 1, wherein the potential applied to the electrodes is from −2 to −0.5 V versus an Ag/AgCl reference.

17. A reactor for electrochemically reducing carbon dioxide, comprising a reactor vessel for containing a supercritical carbon dioxide mixture comprising liquid or supercritical carbon dioxide, water, a phase transfer catalyst, and an ion conducting salt, the reactor having electrodes for applying a potential difference across a portion of the liquid or supercritical carbon dioxide mixture, the electrodes comprising an electrode composition comprising from 5 atomic percent to 100 atomic percent molybdenum.

18. The reactor of claim 17, wherein the electrode is comprised of molybdenum and MoO2.

19. The reactor of claim 17, wherein the electrode is the cathode and comprises 5 to 100 atomic % molybdenum.