Unlocking new reaction pathways for electrocatalytic conversion using plasma activation

Abstract

A combined plasma-electrocatalysis CO2 / CO conversion system and method of using the same is described. The reactor platform is capable of delivering non-thermal plasma-activated CO2 / CO to a gas diffusion electrode (GDE) to enable electrocatalytic conversion of reactive plasma species while avoiding quenching in the aqueous electrolyte. Using a co-fed mixture of CO2 and CO to suppress O2 generation in plasma enhances C3+ productivity compared to electrocatalysis-only CO2 and CO conversion. The synergistic plasma-electrocatalytic CO2 / CO conversion process can be used for the electrosynthesis of a larger variety of high-value multi-carbon products at elevated production rates.

Description

UNLOCKING NEW REACTION PATHWAYS FOR ELECTROCATALYTIC CONVERSION USING PLASMA ACTIVATIONCROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present invention claims the benefit of U.S. Provisional Application Serial No. 63 / 651,432, the subject matter of which is herein incorporated by reference in its entirety.FIELD OF THE INVENTION

[0002] The present invention relates generally to an electrochemical flow cell containing a gas diffusion electrode with a dielectric barrier discharge non-thermal plasma component.BACKGROUND OF THE INVENTION

[0003] Utilization of CO2as a feedstock for producing chemicals and fuels represents a scalable and revenue-generating means of mitigating atmospheric CO2levels. For example, replacing less than 10% of annual U.S. motor gasoline with a fuel produced by renewable energy-powered CO2conversion, such as methanol, would accomplish sufficient net CO2reduction to achieve the Paris Agreement 2°C scenario capture target. However, the development of electrified processes that are compatible with renewable electricity is crucial to the development of CO2conversion processes that do not emit more CO2than they consume. Such processes must overcome the challenges of activating the strong / inert chemical bonds in CO2or its reduction intermediates while also controlling the reaction pathway to produce a high yield of valuable products.

[0004] Electrochemical conversion of CO2with water under mild conditions that are compatible with renewable energy could provide a low-carbon approach compared to alternatives that require H2or high temperature and pressure. For example, the modular scale and flexible operation of CO2reduction reaction (CO2RR) electrolyzers could enable upgrading of point source CO2emissions. However, electrocatalytic CO2reduction with H2O is rate-limited by the CO2or CO activation step. While CO2RR can generate C1products such as CO and formate, C2products such as ethylene and ethanol, and C3products such as propanol, only C1products have been generated with high selectivity and production rate. Multi-carbon products, which can provide significantly improved market value and energy density, can be generated using Cu electrocatalysts. However, other than ethylene, partial current density and faradaic efficiency (FE) of other multi-carbon products, particularly high-value C3+ products, remain low. The ability to convert CO2to C3+ products is significantly limited by the constraint of needing to design electrocatalysts that can both accomplish CO2 / CO activation and steer catalytic pathways toward high-value multi-carbon products.

[0005] Thus, producing value-added chemicals and fuels from electricity-driven CO2conversion could provide a means of achieving net-zero carbon emissions and long-term storage of intermittent renewable energy. Based on considerations for product value and separation costs associated with mixed outstream gases, CO2RR research has focused on improving selectivity and production rate, targeting C1products such as carbon monoxide (CO) and formate (HCOO-), C2products such as ethylene (C2H4) and ethanol (C2H5OH), and C3products such as propanol (C3H7OH). Although the production of longer chain hydrocarbons is preferred from an energy density and economic perspective, only C1products can be generated with sufficiently high cunent density (J) greater than 300 mA cm-2and Faradaic efficiency (FE) of 80-90%. Multi- carbon products have been produced by employing Cu as an electrocatalyst. However, other than C2H4, partial J (i.e., J contributing to generating a specific product) and FE fall far from practical targets, and formation of C3+ products is even more challenging. To date, FE and partial J for CO2-to-C3products conversion have generally been limited to below 20% and 30 mA cm-2.

[0006] Most efforts to improve the generation of multi-carbon products have focused on modifying catalytic active sites of Cu with regard to microstructure, alloying, and composites, as well as controlling reaction environment to facilitate C-C coupling. Although directly feeding CO and cascade approaches of CO2-to-CO followed by CO-to-C2H4or CO-to-C3H7OH have been reported, simply supplying CO as reactant still does not sufficiently address limitations for C3products formation and promotion of C-C coupling. Generation of C4products at significant rate has been only achieved when C2hydrocarbons such as acetylene (C2H2) or C2H4were used as feed gas. Studies on pre-activating CO2 / CO into reactive intermediates to directly generate multi-carbon products over Cu catalysts have been rarely conducted. Additionally, although recent studies have reported the production of C3and C4hydrocarbons over Ni catalysts, FE and partial J were lower than those over Cu catalysts, overwhelmed by the competitive hydrogen evolution reaction (HER).

[0007] Plasma represents an alternative electrified CO2conversion approach. In nonthermal plasma, electron-mediated collisions lead to formation of the ionized gas. Energetic electrons activate gases into higher energy states such as vibrationally excited species, ions, and radicals, while the bulk gas remains near room temperature. Nonthermal plasma has accomplished CO2conversion at atmospheric pressure using various reactor configurations and catalysts, leading to decomposition into CO and O2in pure CO2feeds or a variety of hydrocarbons and oxygenated hydrocarbons in the presence of H2or CH4. However, the non-selective nature of plasma makesit challenging to control the subsequent conversion to valuable products. Additionally, co- conversion with H2or CH4makes net process CO2reduction unlikely due to the reliance upon fossil fuels. A few studies have reported CO2conversion at a plasma-water interface. However, rapid quenching of plasma species in the dense liquid phase severely limit reactions with water, and no studies to date have employed an electrocatalyst to upgrade plasma-activated CO2in an aqueous electrolyte.

[0008] Due to participation of these reactive species in reactions, non-thermal plasma reactions can achieve relatively faster rates and unconventional reaction pathways compared to electrocatalytic reactions. CO2plasma has been utilized to decompose CO2into CO and O2using various reactor configurations and catalysts. Producing hydrocarbons from plasma- activated CO2typically requires the use of H2or short-chain hydrocarbons such as CH4as co-reactants. When CO2is converted with H2or light alkanes to valuable multi-carbon products, though, the result is large .product distributions and low selectivity to target products.

[0009] Few examples exist of plasma-activated CO2conversion with water. For example, Ar plasma has been used to convert dissolved CO2in aqueous electrolyte, in which solvated electrons reduced CO2to oxalate and HCOO-. While catalysts have been combined with plasma to improve product selectivity, incorporating electrocatalysts into plasma-water systems remains challenging due to quenching of short-lived reactive plasma species prior to reaching electrocatalytic surfaces submerged in the electrolyte, and there have been no reports of plasma electrocatalytic CO2reduction.

[0010] CO2RR can selectively convert CO2to C1products such as CO and formate. However, reducing overpotential and accessing multi-carbon products remain challenging in electrocatalytic CO2conversion. Both issues are often related to the activation of CO2or key intermediates such as CO. For example, the rate-limiting step for state-of-the-art CO2-to-CO electrocatalysts is typically CO2adsorption or CO2conversion to *COOH. The rate-limiting step for CO2reduction to C2+ products on Cu catalysts is believed to be CO activation / reduction.Thus far, ethylene is the only C2product that can be selectively generated from CO2RR, and Cu- based catalysts are the only effective materials that have been identified for this reaction. While C3+ products such as propanol can also be formed in the same reaction, the selectivity is low and a large overpotential is required.

[0011] Many researchers have investigated a wide range of catalytic materials, but high selectivity to C3+ products at moderate overpotentials has not been attained. For example, FE andpartial current density for CO2-to-C3products conversion is generally limited to below 20% and 8-30 mA cm-2, respectively. C4products have been rarely reported, and only with extremely low FE and partial current density. Several studies have adopted tandem approaches for CO2-to-CO followed by CO-to-w-propanol, but the reported C3FE still remains well below 20%.

[0012] There remains a need in the art for an improved method and reactor system for converting gaseous compounds and for generating multi-carbon products that overcomes the deficiencies of the prior art.SUMMARY OF THE INVENTION

[0013] It is an object of the present invention to provide a method and reactor system for converting a gaseous compound.

[0014] It is another object of the present invention to provide a method and reactor system for converting a gaseous compound in an efficient manner.

[0015] It is still another object of the present invention to provide a method and reactor system for the generation of multi-carbon products, especially high value C3+ products.

[0016] It is another object of the present invention to provide a method and reactor system for the generation of multi-carbon products with high current density and high faradaic efficiency.

[0017] To that end, in one embodiment, the present invention relates generally to a reactor system for converting one or more gaseous compounds, the reactor system comprising: a. a plasma tube comprising a dielectric barrier discharge plasma to activate the one or more gaseous compounds to a reactive plasma species, wherein the one or more gaseous compounds are fed into a gas inlet and the reactive plasma species exits the plasma tube coupled to an electrochemical flow cell reactor; and b. the electrochemical flow cell reactor, wherein the electrochemical flow cell reactor comprises: i. the plasma tube for generating and transporting the reactive plasma species from the dielectric barrier discharge plasma to the electrochemical flow cell reactor and delivering the reactive plasma species to electrocatalytic surfaces on the gas diffusion electrode; ii. the gas diffusion electrode, preferably wherein the gas diffusion electrode is a catalyst-coated gas diffusion electrode comprising a gas diffusion layer and a catalyst layer, andiii. an electrolyte, wherein the electrolyte is contained in a chamber and arranged on the catalyst layer side of the gas diffusion electrode; and wherein an interface is formed between the reactive plasma species, the catalyst-coated gas diffusion electrode, and the electrolyte to allow the reactive plasma species to participate in electrocatalysis and convert the reactive plasma species.BRIEF DESCRIPTION OF THE FIGURES

[0018] Figs. l(a)-(e) depict a concept of electrocatalytic conversion of plasma-activated CO2 / CO.

[0019] Figs. 2(a)-(f) depict an evaluation of plasma-electrocatalytic CO2conversion.

[0020] Figs. 3(a)-(e) depict the effect of CO2dissociation in plasma on coupled electrocatalytic conversion.

[0021] Figs. 4(a)-(e) depict an evaluation of plasma-electrocatalytic CO conversion.

[0022] Figs. 5(a)-(e) depict the plasma-electrocatalytic conversion of CO2+ CO feed gas mixture.

[0023] Figs. 6(a)-(f) depict kinetic simulations on vibrational excitation and ground state species in CO2+ He, CO + He, and CO2+ CO + He plasma.

[0024] Fig. 7 depicts electrocatalytic reduction of plasma pre-activated CO2overcomes separate plasma and electrochemical limitations to enable catalyst design toward multi-carbon product generation.

[0025] Fig. 8 depicts stepwise vibrational excitation or “ladder climbing” leading to dissociation in CO2plasma.

[0026] Fig. 9 depicts faradaic efficiency of selected CO2reduction products using Cu with (dashed boxes) and without plasma over a range of applied electrochemical potentials.

[0027] Fig. 10 depicts a comparison of partial current density (J) of new products for 12.0 W plasma-activated CO2and ground-state CO2 / CO / O2plasma effluent mixture at -1.0 V vs. RHE.

[0028] Fig. 11 depicts the fraction of electron energy lost to various excitation modes for CO2plasma under varying reduced electric filed strength calculated using Bolsig Plus.

[0029] Fig. 12 depicts linear sweep voltammetry curves for CO electrocatalytic reduction (plasma off) and 10.8 W plasma-activated CO reduction over Cu.

[0030] Fig. 13 depicts diffusion distance of plasma species with various lifetimes as affected by varying pore sizes in a catalyst-coated gas diffusion layer.

[0031] Fig. 14 depicts plasma-activated CO2reduction products using different Cu-based electrocatalysts.

[0032] Fig. 15(a) depicts a schematic illustration and Fig. 15(b) depicts a photograph of the plasma electrocatalysis combined flow reactor for CO2 / CO conversion.

[0033] Fig. 16 depicts XRD analysis of Cu phase in as-prepared gas diffusion electrode (GDE). Metallic Cu film with 400 run thickness was deposited at a rate of 0.2 nm sec-1.

[0034] Figs. 17(a) and 17(b) depict XPS analysis of Cu oxidation states in as-prepared GDE. Metallic Cu film with 400 nm thickness was deposited at a rate of 0.2 nm sec-1. Fig. 17(a) depicts Cu 2p and Fig. 17(b) depicts Cu EMM spectra.

[0035] Fig. 18(a) and 18(b) depict microstructures of as-prepared Cu GDE. Fig. 18(a) depicts low and Fig. 18(b) depicts high magnification images.

[0036] Figs. 19(a)-19(d) depict the product distribution of electrochemical conversion of ground state (plasma off) and plasma-activated CO2at different plasma powers.

[0037] Figs. 20(a) and 20(b) depict an evaluation of hydrogen evolution reaction (HER) during plasma off and plasma-activated CO2electrochemical reduction at different plasma powers. Fig. 20(a) depicts faradaic efficiency (FE) Fig. 20(b) depicts partial J versus applied potential. Each data point represents the mean and standard deviation of three independent measurements.

[0038] Figs. 21(a)-(d) depict partial / versus applied potential at different plasma powers and with plasma off for products that were exclusively detected with plasma coupling compared to electrocatalysis-only.

[0039] Figs. 22(a)-(d) depict partial / versus applied potential for products that showed an increase in generation rate with plasma coupling at different powers compared to electrocatalysis-only.

[0040] Fig. 23(a)-(e) depict Partial / and FE versus applied potential for carboxylic acids HCOO- (Figs. 23(a) and (c)) and CH3COO' (Figs. 23(b) and (d)), which showed similar FE with plasma coupling compared to electrocatalysis-only. Figs. 23 (a) and (b) show partial J and Figs. 23(c) and (d) show FE for ground-state CO2and plasma-activated CO2 electrochemical reduction at different powers. Fig. 23(e) shows the partial / of C3H6for plasma off and plasma- activated CO2reduction at different powers.

[0041] Figs. 24(a)-(d) depict a gas chromatogram of 42.3 W plasma-activated CO2.

[0042] Figs. 25(a) and (b) depict a gas chromatogram of CO2+CO+O2gas mixture (plasma off) for control experiment, including a gas feed consisting of 38 ml min-1CO2+ 10 ml min-1He + 1.7 ml min-1CO + 0.7 ml min-1O2. Fig. 25(a) shows FID and Fig. 25(b) shows TCD column.

[0043] Figs. 26(a) and (b) show the electrocatalytic conversion of the CO2+ CO + O2mixed gas (plasma off) control experiment. Fig. 26(a) shows partial J and Fig. 26(b) shows FE distribution versus applied potential. The flow rates of the gases in the mixed feed gas were 38 ml min-1CO2+ 10 ml min-1He + 1.7 ml min-1 CO + 0.7 ml min-1O2. Each data point represents the mean and standard deviation of three independent measurements.

[0044] Fig. 27 depicts sweep voltammetry (LSV) curves for ground state CO2(plasma off) and CO2+ CO + O2(plasma off) mixture gas. The potential was swept from 0 to -1.2 V at a rate of 1 mV sec-1in 1 M KOH electrolyte. The flow rates of gases in the mixed feed were 38 ml min-1CO2+ 10 ml min-1He + 1.7 ml min-1CO + 0.7 ml min-1O2. Each data point represents the mean of three independent measurements.

[0045] Figs 28(a)-(d) depict a partial J comparison between electrocatalytic conversion of CO2(plasma off) and CO2+ CO + O2mixed feed (plasma off; simulated plasma effluent) versus applied potential.

[0046] Figs. 29(a)-(d) depict a partial J comparison between electrocatalytic conversion of CO2(plasma off) and CO2+ CO + O2mixture gases (plasma off) versus applied potential.

[0047] Fig. 30(a)-(h) depict a microstructure analysis of Cu GDE after electrocatalytic conversion of plasma off CO2(Figs. 30(a) and (b)), 12.0 W plasma-activated CO2(Figs. 30(c) and (d)), 28.7 W plasma-activated CO2(Figs. 30(e) and (f)) and 42.3 W plasma-activated CO2(Figs. 30(g) and (h)) The reactions were conducted for 1 hour at -1.7 V (vs RHE, before IR compensation) in 1 M KOH electrolyte. Figs. 30(a), (c), (e) and (g) show low magnification and Figs. 30(b), (d), (f), and (h) show high magnification. A mixed gas feed consisting of 40 ml min-1CO2+ 10 ml min-1He was used.

[0048] Figs 31(a) and (b) depict an XPS analysis of Cu oxidation states after electrocatalytic conversion of ground state CO2(plasma off) and plasma-activated CO2at different powers. Fig. 31(a) depicts Cu 2p and Fig. 31(b) depicts Cu EMM spectra. The reactions were conducted for 1 hour at -1.7 V (vs RHE, before IR compensation) in 1 M KOH electrolyte. A mixed gas feed consisting of 40 ml min-1CO2+ 10 ml min-1He was used.

[0049] Fig. 32 depicts an XRD pattern of Cu GDE after electrocatalytic conversion of ground state CO2(plasma off) and plasma-activated CO2at different powers. The experiments were conducted for 1 hour at -1.7 V (vs RHE, before IR compensation) in 1 M KOH electrolyte.

[0050] Figs. 33(a)-(e) depict an analysis on microstructure, oxidation state, and phase of Cu GDE after electrocatalytic CO2+ CO + O2(plasma off) conversion. Figs. 33(a) and (b) depict low and high magnification SEM images. Fig. 33(c) depicts an XPS Cu 2p spectrum, Fig. 33(d) depicts an XPS Cu EMM spectrum, and Fig. 33(e) depicts an XRD pattern. The flow rates of the mixed feed gases were 38 ml min-1CO2, 10 ml min-1He, 1.7 ml min-1CO, and 0.7 ml min-1O2. The experiment was conducted for 1 hour at -1.7 V (vs RHE, before IR compensation) in 1 M KOH electrolyte.

[0051] Figs. 34(a)-(c) depict a gas chromatogram of 10.8 W plasma-activated CO. Fig. 34(a) depicts FID and Fig. 34(b) depicts TCD columns. Fig. 34(c) depicts a magnified TCD column around O2peak. O2peaks were compared before and during coupling with electrocatalytic-1 conversion. O2was not detected. A mixed gas feed consisting of 30 ml min-1CO and 20 ml min He was used.

[0052] Figs. 35(a)-(c) depict the electrocatalytic conversion of CO with and without plasma. Product distribution of electrochemical conversion of ground state CO (plasma off) (Fig. 35(a)) and 10.8 W plasma activated CO (Fig. 35(b)) versus applied potential are shown. Fig. 35(c) shows a total J comparison. A mixed gas feed consisting of 30 ml min-1CO and 20 ml min-1He was supplied. Each data point represents the mean and standard deviation of three independent measurements.

[0053] Figs. 36(a) and (b) show an of HER during ground state (plasma off) CO and 10.8 W plasma-activated CO electroreduction. A comparison of FE (Fig. 36(a)) and partial J (Fig.36(b)) versus applied potential is shown. A mixed gas feed consisting of 30 ml min-1CO and 20 ml min-1He was supplied. Each data point represents the mean and standard deviation of three independent measurements.

[0054] Figs. 37(a)-(c) provide a comparison of partial J for Cl products between electrocatalytic conversion of ground state CO (plasma off) and 10.8 W plasma-activated CO.

[0055] Figs. 38(a)-(j) provide a comparison of partial J for C2products between electrocatalytic conversion of ground state CO (plasma off) and 10.8 W plasma-activated CO.

[0056] Figs. 39(a)-(e) provide a comparison of partial J for C3+ products between electrocatalytic conversion of ground state CO (plasma off) and 10.8 W plasma-activated CO.

[0057] Fig. 40(a) is a photograph of filamentary CO plasma at 10.8 W power and Fig. 40(b) is a photograph of C deposition on the quartz tube wall after plasma operation. The flow rates of the gases in the mixed feed were 30 ml min-1CO and 20 ml min-1He.

[0058] Fig. 41 provides a as chromatogram of plasma-activated CO2+CO with different feed compositions. The TCD chromatogram was magnified around the O2peak.

[0059] Figs. 42(a)-(d) provide a gas chromatogram of 46.9 W plasma-activated CO2+CO. Fig. 42(a) depicts the FID and Fig. 42(b) depicts the TCD columns. Fig. 42(c) depicts a magnified TCD column around the O2peak. The O2peak remained unchanged before and during coupling with electrocatalytic conversion, which suggested the absence of oxygen reduction. Fig. 42(d) depicts an image of the quartz tube after plasma operation. C deposition was mitigated compared to CO plasma at 10.8 W. The flow rates of gases in the mixed feed were 30 ml min-1CO2+ 10 ml min-1He + 10 ml min-1CO.

[0060] Figs 43(a) and (b) provide an evaluation of HER during electroreduction of ground state (plasma off) CO2, 42.3 W plasma-activated CO2, and 46.9 W plasma-activated CO2+CO. A comparison of FE (Fig. 43(a)) and partial J (Fig. 43(b)) versus applied potential is shown. A mixed gas feed consisting of 30 ml min-1CO2, 10 ml min-1CO, and 10 ml min-1He was used. Each data point represents the mean and standard deviation of three independent measurements.

[0061] Figs. 44(a) and (b) show the production rates during electrocatalytic conversion of ground state CO2, ground state CO, and 46.9 W plasma-activated CO2+ CO for C3+ products (Fig. 44(a)) and new products (Fig. 44(b)). C3+ products include C3H6, C3H7OH, C3H8, and CH3COCH3, and C4H10. New products include CH3OH, C2H2, C2H6, C3H8, and C4H10. Each data point represents the mean and the standard deviation of three independent measurements.

[0062] Fig. 45 shows an FID gas chromatogram of electroreduction of 46.9 W plasma-activated CO2+CO at -1 .1 V, magnified around 1, 2 -butanol peaks (top). Standard chemicals were analyzed (2-butanol, middle, and 1 -butanol, bottom). The flow rates of gases in the mixed feed were 30 ml min-1CO2+ 10 ml min-1He + 10 ml min-1CO. The reaction was conducted in 1 M KOH electrolyte.

[0063] Figs. 46(a)-(c) provide measurements of power consumption and microdischarge current of DBD plasma.

[0064] Figs. 47(a)-(c) depict measurements and characterization of microdischarge current of CO2plasma.

[0065] Fig. 48 depicts a Lissajous plot of 10.8 W plasma-activated CO.

[0066] Fig. 49 depicts measurements and characterization of microdischarge current of CO plasma at 10.8 W.

[0067] Fig. 50 depicts a Lissajous plot of 46.9 W plasma-activated CO2+CO.

[0068] Fig. 51 depicts measurements and characterization of microdischarge cunent of CO2+CO plasma at 46.9 W.

[0069] Fig. 52 depicts modeling conditions for plasma simulation and shows a sketch of the plasma zone, with simulation volume outlined by dashed boundaries.

[0070] Figs. 53(a)-(c) depict a calculated reduced electric field (E / N, i.e., ratio of electric field over gas number density), cunent (I) and electron temperature (Te) as a function of time for the first pulse for 80% CO2+ 20% He (Fig. 53(a)), 60% CO + 40% He (Fig. 53(b)), and 60% CO2+ 20% CO + 20% He (Fig. 53(c)). The end of the plasma pulse and the beginning of the afterglow region are marked by a vertical dashed line.

[0071] Fig. 54 depicts the effect of CO2addition on vibrational mode of CO (TCO). A simulated time evolution of TCO during the first pulse when CO2was added to 60% CO + 40% He is shown. When CO2was added, the same amount of CO was removed from the feed gas mixture.

[0072] Fig. 55 depicts a relaxation of vibrational mode of CO. Time evolution of TCO in the afterglow after the 10th consecutive pulse for 60% CO + 40% He is shown.

[0073] Figs. 56(a)-(f) depict simulated molar fractions of the main electronically excited species as a function of time for 80% CO2+ 20% He (Figs. 56(a) and (b)), 60% CO + 40% He (Figs.56(c) and (d)), and 60% CO2+ 20% CO + 20% He (Figs. 56(e) and (f)). Figs. 56(a), (c) and (e) show the evolution of electronically excited species during the first pulse and Figs. 56(b), (d), and (f) show the evolution of electronically excited species over 10 consecutive pulses.CO(A1P), CO(a3P), CO(a3Su), O(1D), and O2(a1) stand for CO(A1π ), CO(a3π ), CO(a’3∑+), O(1D), and O2(a1Δg), respectively.

[0074] Figs. 57(a)-(d) depict simulated ground state species concentrations. Temporal evolution of the molar fractions of the ground state species formed upon CO and CO2dissociation and recombination processes are shown during the first pulse for 80% CO2+ 20% He (Fig. 57(a)) and 60% CO2+ 20% CO + 20% He (Fig. 57(b)), and over 10 consecutive pulses for 80% CO2+ 20% He (Fig. 57(c)) and 60% CO2+ 20% CO + 20% He (Fig. 57(d)).

[0075] Figs. 58(a) and (b) show optical emission spectra (OES) of plasma-activated CO2at different plasma powers. Spectra were acquired in the wavelength range of 250 to 500 nm (Fig.58(a)) and 500 to 800 nm (Fig. 58(b)) for 500 ms. A mixed gas feed consisting of 40 ml min-1CO2and 10 ml min-1He was supplied.

[0076] Figs. 59(a) and (b) depict optical emission spectra (OES) of plasma-activated CO2at different plasma powers. Spectra were acquired in the wavelength range of 50 to 500 nm (Fig. 59(a)) and 500 to 800 nm (Fig. 59(b)) for 500 ms. A mixed gas feed consisting of 40 ml min-1CO2and 10 ml min-1He was supplied.

[0077] Figs. 60(a) and (b) depict OES of plasma-activated CO2+ CO at 46.9 W plasma power. Spectra were acquired in the wavelength range of 250 to 500 nm (Fig. 60(a)) and 500 to 800 nm (Fig. 60(b)) for 500 ms. A mixed gas feed consisting of 30 ml min-1CO2, 10 ml min-1CO, and 10 ml min-1 He was supplied.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0078] The present invention is directed to an electrochemical flow cell containing a gas diffusion electrode with a dielectric barrier discharge non-thermal plasma component. The new cell design enables delivery of excited plasma species to electrocatalysts at a three-phase plasma- electrocatalyst-electrolyte interface. The flow cell design allows for continuous operation and industrially-relevant high current densities.

[0079] As described herein, the inventors of the present invention have conceived a unique reaction platform to couple nonthermal plasma with selective electrocatalysts. The process utilizes plasma to overcome the rate-limiting CO2 / CO activation step in conventional CO2electrochemical reduction, and electrocatalysts enable control of product formation to address the non-selective nature of plasma. A key element of the process is the use of a gas diffusion electrode to establish a multi-phase plasma-electrocatalyst-electrolyte interface, which allows for efficient delivery of short-lived excited plasma species to electrocatalysts while avoiding quenching in the electrolyte.

[0080] Additionally, the co-feeding of CO2and CO represents an additional key innovation in this system. CO2plasma alone generates O2, which results in competitive O2reduction reaction occurring at the electrocatalyst and lowers the faradaic efficiency for CO2conversion. Feeding CO alone leads to coke formation in the plasma zone, leading to an unstable plasma and limiting the range of plasma power that can be applied. By co-feeding CO2and CO, generated O2and C can react to re-form CO2, essentially eliminating delivery of O2to the electrocatalyst as well as coke formation. Thus, the invention described herein allows for significantly higher overall CO2reduction faradaic efficiency, enhanced conversion rate of CO2 / CO, as well as significantly improved generation rates of multi-carbon (C3+) products.

[0081] The inventors of the present invention discovered that the plasma pre-activation of CO2can overcome known rate-limiting steps for generation of longer-chain hydrocarbons in Cu electrocatalysis. Further, coupling electrocatalysis with plasma conversion of CO2across a three- phase plasma-electrocatalyst-electrolyte interface can enable H2O to serve as the hydrogen source for forming hydrocarbons through proton-coupled electron transfer while avoiding liquid quenching. The synergistic plasma- electrocatalytic conversion can also provide a means of controlling product selectivity following the non-selective plasma activation step.

[0082] Although the production of longer chain hydrocarbons is preferred from an energy density perspective, only C1products can be converted with adequate performance. Using Cu as an electrocatalyst, which has been uniquely known to produce multi-carbon products in significant amounts, considerable advancement is accomplished especially in production of C2H4. However, other than C2H4, partial current density (J) and Faradaic efficiency (FE) fall far from requirements, and formation of C3+ products is more challenging. FE and partial J for CO2- to-C3products conversion has been limited below 20% and 50 mA cm-2so far. Although recent studies have shown that the production of C3and C4hydrocarbons over Ni catalysts, FE and partial J was lower than that achieved over Cu, overwhelmed by competitive hydrogen evolution reaction (HER). While development for performance enhancement was mainly dedicated to modifying catalytic active sites with regard to microstructure, alloying, composites, and reaction environment, study on designing conversion process has been rarely conducted except for cascade reaction of CO2-to-CO and CO-to-C2H4.

[0083] In addition to electrocatalysis, plasma represents an electrified conversion which can activate gases into higher energy states such as vibrationally excited species, ionized gases, and activated radicals with relatively faster rate and higher energy efficiency than those of electrocatalysis. CO2plasma has been utilized to decompose CO2to CO and O2using various reactor configurations and catalysts. Ar plasma has been used to convert dissolved CO2in aqueous electrolyte in which solvated electron reduced CO2to oxalate and HCOO-. Among plasma configurations, dielectric barrier discharge (DBD) enabled activation at atmospheric pressure and near-ambient temperature, making it compatible with electrocatalytic conversion module. However, plasma has limitations that short lifetimes of activated species and unselective nature. Producing longer hydrocarbons necessitated H2or short-chain hydrocarbons as reactants,and even then, the productivity is still low. Based thereon, the inventors of the present invention discovered that combining plasma gas activation with electrocatalysis, especially copper electrocatalysis, could create synergies for efficient formation of multi-carbon products. Plasma activation could facilitate electrocatalysis and proton coupled electron transfer in electrocatalysis could waive H2 / hydrocar bon supply.

[0084] As used herein, “a,” “an,” and “the” refer to both singular and plural referents unless the context clearly dictates otherwise.

[0085] As used herein, the term “about” refers to a measurable value such as a parameter, an amount, a temporal duration, and the like and is meant to include variations of + / -15% or less, preferably variations of + / - 10% or less, more preferably variations of + / -5% or less, even more preferably variations of + / -!% or less, and still more preferably variations of + / -0.1% or less of and from the particularly recited value, in so far as such variations are appropriate to perform in the invention described herein. Furthermore, it is also to be understood that the value to which the modifier “about” refers is itself specifically disclosed herein.

[0086] As used herein, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” “front,” “back,” and the like, are used for ease of description to describe one element or feature's relationship to another element(s) or feature(s). It is further understood that the terms “front” and “back” are not intended to be limiting and are intended to be interchangeable where appropriate.

[0087] As used herein, the terms “comprises” and / or “comprising,” specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0088] The present invention is directed to new electrocatalyst designs for C3+ product generation using CO2 / CO pre-activation with nonthermal plasma across a three-phase plasma- electrocatalyst-electrolyte interface.

[0089] In one embodiment, the present invention relates generally to a reactor system for converting one or more gaseous compounds, the reactor system comprising: a. a plasma tube comprising a dielectric barrier discharge plasma to activate the one or more gaseous compounds to a reactive plasma species, wherein the one or more gaseous compounds are fed into a gas inlet and the reactive plasma species exits the plasma tube coupled to an electrochemical flow cell reactor; andb. the electrochemical flow cell reactor, wherein the electrochemical flow cell reactor comprises:1. the plasma tube for generating and transporting the reactive plasma species from the dielectric barrier discharge plasma to the electrochemical flow cell reactor and delivering the reactive plasma species to electrocatalytic surfaces on a gas diffusion electrode;11. the gas diffusion electrode, preferably wherein the gas diffusion electrode is a catalyst-coated gas diffusion electrode comprising a gas diffusion layer and a catalyst layer, and iii. an electrolyte, wherein the electrolyte is contained in a chamber and arranged on the catalyst layer side of the gas diffusion electrode; and wherein an interface is formed between the reactive plasma species, the catalyst- coated gas diffusion electrode, and the electrolyte to allow the reactive plasma species to participate in electrocatalysis and convert the reactive plasma species.

[0090] In one embodiment, the electrolyte is selected from the group consisting of potassium based-electrolytes, cesium-based electrolytes, lithium-based electrolytes, and sodium based electrolytes, including, for example, potassium hydroxide, potassium chloride, and potassium bicarbonate. Electrolytes based on cesium, lithium, and sodium can also be used, including, but not limited to hydroxides, chlorides, and bicarbonates. The choice of electrolyte affects the reaction mechanism, conversion rate, and product selectivity. In one embodiment, the electrolyte comprises potassium hydroxide or potassium bicarbonate, both of which have been used as electrolytes in CO2conversion research, considering their conversion efficiency, solubility in water, pH, and ionic conductivity.

[0091] In one embodiment, the catalyst-coated gas diffusion electrode comprises metallic species deposited on a porous membrane. In some embodiments, the metallic species are transition metals, optionally wherein the transition metals are selected from the group consisting of copper, nickel, iron, silver, gold, palladium, platinum, ruthenium, zinc, tin cobalt, bismuth, indium, and combinations of the foregoing.

[0092] In one embodiment, the catalyst is a bimetallic copper-based catalyst comprising one or more of silver, palladium, ruthenium, gold, zinc, cobalt, nickel, tin, bismuth and indium. In one embodiment, the inventors of the present invention discovered that a copper-silver alloy provides CO2 / CO conversion at a higher rate as compared with metallic copper. It also demonstratedsignificant improvement in terms of C3+ products formation, including the formation of up to C8hydrocarbon products from CO2.

[0093] In one embodiment, the porous membrane comprises microfibers. In some embodiments, the microfibers have a diameter of about 100 to about 600 μm, more preferably about 200 to about 500 μm, more preferably about 300 to about 350 μm.

[0094] In one embodiment, the porous membrane is a hydrophobic membrane. For example, the porous membrane may be selected from the group consisting of polytetrafluoroethylene, carbon- fiber, metal fiber, including titanium-based membranes with hydrophobic additives.

[0095] In one embodiment, the catalyst layer comprises the catalyst material (i.e., metallic species) and an ionomer, preferably an ionomer that is proton / hydroxide conductive for ion transport. In one embodiment, the ionomer may be a perfluorosulfonic acid polymer (a commercial product of which is available from Chemours under the tradename Nation™) or a Sustanion™ polymer. Other similar materials can also be used in the practice of the invention.

[0096] In one preferred embodiment, the porous membrane comprises polytetrafluoroethylene. For example, the porous membrane may comprise a catalyst layer comprising the catalyst material and the ionomer deposited on a polytetrafluoroethylene membrane.

[0097] In one embodiment, the porous membrane has a thickness within the range of about 2 to about 20 μm, more preferably about 3 to about 15 μm, more preferably about 4 to about 10 μm, more preferably about 5 to about 9 μm. The porous membrane may also have a porosity within the range of about 100 to about 600 μm, more preferably about 200 to about 500 μm, more preferably about 350 to about 450 μm.

[0098] In one embodiment, the reactive plasma species diffuse through the gas diffusion electrode and contact the electrocatalytic surfaces on the gas diffusion electrode prior to liquid quenching of the reactive plasma species.

[0099] In one embodiment, the dielectric barrier discharge plasma operates at a temperature within the range of about 15 to about 130°C, more preferably about 20 to about 80°C, more preferably about room temperature to about 60°C. In addition, the dielectric barrier discharge plasma preferably operates at or near atmospheric pressure.

[0100] In one embodiment, the dielectric barrier discharge plasma and the electrochemical flow cell reactor are preferably on independent electrical circuits, wherein operation of the dielectric barrier discharge plasma and the electrochemical flow cell reactor can be separately controlled.

[0101] In one embodiment, the activation of the gaseous compound is accomplished at a power in the range of about 1 to about 80 W, more preferably about 5 to about 50 W, more preferably about 10 to about 40 W. Power was controlled in this range because too much increase of temperature after plasma activation could have a negative influence on electrochemical reactions and the integrity of the gas diffusion electrode.

[0102] By coupling plasma CO2activation with selective electrocatalytic upgrading, plasma may accomplish pre-activation of CO2 / CO to overcome the rate-limiting activation step in conventional CO2RR, and electrocatalysts enable control of product formation to address the non-selective nature of plasma as illustrated in Fig. 7. Plasma activation may enable lower overpotentials due to the lower relative energy barrier to activate CO2on the electrocatalyst surface, as shown in Fig. 1(c), and reactive plasma species may provide unconventional reaction pathways to expand the range of possible products.

[0103] In addition, while the present invention is described with respect to CO and CO2, the gaseous compound may contain, for example, one or more of a carbon-containing gas, a nitrogen-containing gas, or a sulfur-containing gas.

[0104] The invention described herein comprises a coupled plasma-electrocatalytic conversion system in which CO2 / CO is pre-activated by dielectric barrier discharge (DBD) plasma and then electrochemically converted to hydrocarbons and oxygenates over the catalyst-coated gas diffusion electrode, which in some embodiments may comprise Cu. By introducing a gas diffusion electrode (GDE), plasma-activated CO2 / CO can be delivered to electrocatalytic surfaces, while avoiding quenching in the electrolyte, thereby directly involving reactive plasma species in electrocatalytic reactions. Plasma activation of CO2 / CO not only improves conversion efficiency and boosts reaction rates of multi-carbon products, but also unlocks new electrocatalytic reaction pathways to widen the viable product range.

[0105] As described herein, electrocatalytic conversion of activated CO2exhibited a decrease in production of alkenes (C2H4and C3H6) while increasing production of alkanes (methane (CH4), ethane (C2H6), and propane (C3H8)), acetaldehyde (CH3CHO), and alcohols (methanol (CH3OH), ethanol (C2H5OH), and n-propanol (C3H7OH)) compared to electrocatalysis alone. Similarly, plasma-electrocatalytic CO conversion shows an overall increase in partial J of all hydrocarbons except C2H4, promoting C2+ hydrocarbons formation including butane (C4H10). New products such as CH3OH, C2H2, C2H6, C3H8, and C4H10can be detected only when plasma-activated CO2 / CO is used compared to electrocatalysis alone.

[0106] To optimize the plasma coupling, it is critical to suppress competitive electrocatalytic oxygen reduction reactions (ORR) and solid carbonaceous product deposition caused by CO2 / CO splitting in plasma. Therefore, in one embodiment, a mixture of CO2+ CO is co-fed to the reactor to obtain an 02-free effluent gas to feed to the electrolyzer. Plasma-activated CO2+ CO substantially enhances C3+ productivity and promotes the new reaction pathways compared to electrochemical-only and CO2- or CO-only plasma-electrochemical controls.

[0107] Using the system described herein, short-lived plasma species can be delivered to electrocatalysts in aqueous electrolyte across a gas diffusion electrode (GDE) and plasma- activated CO2reduction selectivity can be tuned toward high-value C3+ products.

[0108] CO2electroreduction to C2+ products catalyzed by Cu usually proceeds through CO as a reaction intermediate. The overall reaction can therefore be broken down into two parts for investigation and optimization purposes, i.e., CO2reduction to CO followed by CO reduction. Studies have shown that direct CO electroreduction can yield C2products such as ethanol and acetate more effectively than indirect CO2reduction. Several catalysts such as Au, Ag, and metal-nitrogen coordination structures (MNCs) are selective for CO2-to-CO conversion, whereas Cu is one of the very few that can reduce CO. Mechanistic studies have shown that both reactions are limited by reactant activation. In other words, their potential-limiting steps are CO2activation for Au, Ag, and MNCs, and CO activation for Cu. Therefore, catalysts for generating multi-carbon products from CO2 / CO electroreduction must possess high activity for CO2 / CO activation. As a result, catalyst design for multi-carbon product generation is severely constrained by CO2 / CO activation ability, limiting the design space for tuning steps further along the reaction pathway that control final product selectivity. It is believed that pre-activating CO2 / CO prior to adsorption on an electrocatalyst may eliminate this constraint to enable catalyst design focused on tuning multi-carbon product formation pathways with lower overpotentials.

[0109] In one embodiment, the present invention also relates generally to a method of converting one or more gaseous compounds, the method comprising the steps of: a. subjecting the one or more gaseous compounds to plasma forming conditions to activate the gaseous compound to a reactive plasma species; b. delivering the reactive plasma species to electrocatalytic surfaces on a gas diffusion electrode;c. forming an interface between the reactive plasma species, the gas diffusion electrode and an electrolyte to allow the reactive plasma species to engage in electrocatalysis and convert the reactive plasma species.

[0110] In one embodiment, the one or more gaseous compounds comprise one or more of a carbon-containing gas, a nitrogen-containing gas, or a sulfur-containing gas. In one embodiment, the one or more gaseous compounds comprise a carbon-containing gas, which may comprise flue gas.

[0111] In one embodiment, the carbon-containing gas comprises carbon dioxide, carbon monoxide, or a mixture of carbon monoxide and carbon dioxide. For example, the carbon- containing gas may comprise a mixture of carbon monoxide and carbon dioxide and the ratio of carbon monoxide to carbon dioxide in the carbon-containing gas may be in the range of greater than 0 to about 5, more preferably about 0.001 to about 4, more preferably about 1 to about 3. The ratio between carbon monoxide and carbon dioxide was determined to suppress oxygen byproducts formation, and suppress subsequent oxygen reduction reaction in the electrochemical reactor. The ratio can be tuned depending on plasma activation conditions.

[0112] In one embodiment, the converted reactive plasma species comprise liquid and / or gaseous products and the liquid or gaseous products may comprise one or more of C3+ products, hydrocarbons, aldehydes, acids, and alcohols.

[0113] In one embodiment, the converted reactive plasma species comprise one or more alcohols selected from the group consisting of CH3OH, C2H5OH, C3H7OH, CH3(CH2)3OH, CH3(CH2)3CH2OH, and other higher alcohols, more preferably wherein the one or more alcohols comprise CH3OH, C2H5OH, and CH3H7OH. The partial cunent density for alcohol conversion is preferably greater than about 30 mA / cm2, more preferably greater than about 40 mA / cm2, more preferably greater than about 50 mA / cm2. In addition, the faradaic efficiency for alcohol conversion is preferably greater than about 30%, more preferably greater than about 40%, more preferably greater than about 45%, more preferably greater than about 50%.

[0114] In one embodiment, the gaseous reduced compounds comprise hydrocarbons selected from the group consisting of C3H4, C3H6, C3H8, CH3COCH3, C4H10, C5H12, and other hydrocarbons up to at least C8 hydrocarbons, more preferably wherein the one or more compounds comprise C3H6, C3H8, CH3COCH3, C3H7OH, and C4H10. The partial current density for CO2to saturated hydrocarbons conversion is preferably greater than about 30 mA / cm2, more preferably greater than about 40 mA / cm2, more preferably greater than about 50 mA / cm2. Inaddition, the faradaic efficiency for CO2to saturated hydrocarbons is preferably greater than about 10%, more preferably greater than about 15%.

[0115] In one embodiment, the plasma forming conditions take place at a temperature in the range of 15 to about 130°C, more preferably about 20 to about 80°C, more preferably about room temperature to about 40°C and / or the plasma forming conditions take place at or near atmospheric pressure.

[0116] The reactive plasma species preferably diffuse through the gas diffusion electrode and contact the electrocatalytic surfaces on the gas diffusion electrode prior to liquid quenching of the reactive plasma species.

[0117] In one embodiment, the plasma forming conditions comprise using a dielectric barrier discharge plasma reactor to activate the gaseous compound to the reactive plasma species. For example, the gaseous compound may be activated to the reactive plasma species at a power within the range of about 1 to about 80 W, preferably about 5 to 50 W, more preferably about 10 to about 40 W.

[0118] Most (if not all) studies of plasma-activated CO2conversions to hydrogenated products require the use of H2or CH4as a co-reactant. This requirement results in either dependence upon fossil fuels to provide CH4, or a preliminary energy intensive H2generation step (i.e., from water electrolysis or steam methane reforming), significantly increasing the net CO2emissions associated with the overall CO2conversion process. Direct conversion of CO2with H2O could eliminate fossil fuel materials inputs and the energy intensive H2generation step, while also providing process simplification. However, plasma-activated CO2conversion directly with water has not been widely investigated, and coupling with electrocatalysts to control product selectivity has also not been explored. If plasma-activated CO2conversion with water to high-value multi- carbon products can be achieved, this green synthesis route may offer possibilities for net reduction of CO2.

[0119] Based on the ability to form plasma-activated species at gas-liquid interfaces, reactive systems involving plasma-liquid activation have been suggested for coupling plasma gas conversion with reactions in water. At the plasma-liquid interface, gas-phase plasma species may diffuse into the liquid, but the more reactive species rapidly become quenched. Several reactive plasma species may be generated at the surface (<1 μm) such as solvated electrons, H2O+,.OH,.O, and.H. Quasi-stable species such as H2O2, H3O+, OH-, O3, HO2, O2-, and H2may diffuse into the bulk. For example, estimated diffusion lengths in a liquid for H3O4+, O2+, and O- are 3.3 x 10-5m, 2.1 x 10-6m, and 1.4 x IO-7m, respectively. Reactions at a plasma- water interface enable conversion of gases with water, but the extremely limited ability of plasma-excited gases to transport within the liquid prevents the delivery of activated gases to electrocatalysts submerged at least several hundred micrometers below the electrolyte surface.

[0120] Due to the low solubility of CO2in water, gas diffusion electrodes have been employed for CO2RR to provide a multi-phase gas-electrocatalyst-electrolyte interface. Particularly when used in membrane-electrode assemblies (MEAs), these systems can provide efficient delivery of CO2to electrocatalysts while also enabling high cunent density.

[0121] Plasma-activated species may diffuse into small pores within GDEs. For example, theoretical simulations have predicted significant penetration of electrons and ions into pores greater than 100 μm and 30 μm, respectively. The relatively large pore radii in gas diffusion layers, which typically range from 0.2 to 20 μm, are believed to allow substantial transport of plasma species. Especially under convective gas flow conditions as in MEA flow cells, substantial concentrations of excited plasma gas species are capable of transport across the GDE to reach electrocatalyst surfaces. Therefore, the combination of plasma-activated CO2with electrocatalytic reduction can be accomplished across a GDE, enabling substantial delivery of short-lived plasmas species to catalyst surfaces as further described herein.

[0122] Previous plasma-electrochemical systems employed a plasma- water interface, which leads to quenching of reactive plasma species in the dense liquid phase. In contrast, the present invention is directed to a new reactor to efficiently couple plasma, electrocatalyst, and electrolyte interfaces. The plasma membrane electrode assembly (PMEA) comprises an electrochemical flow cell equipped with a GDE and dielectric barrier discharge plasma. A schematic of a reactor system in accordance with one embodiment of the present invention is shown in Figs. 15(a) and (b).

[0123] As shown in Fig. 15(a), the reactor system of the invention comprises a dielectric barrier discharge plasma coupled to an electrochemical reactor. The dielectric barrier discharge plasma comprises a tube, such as a quartz tube, used in combination with coaxial electrodes. In one embodiment, a high voltage rod, which may be a stainless steel rod, is fixed in the center of the tube and a coil, which may be a steel coil, is wrapped around the outside of the quartz tube as the ground electrode. In one embodiment, the length of the plasma generated may be in the range of about 12 cm, more preferably about 10 to about 15 cm, most preferably about 12 to about 13 cm.The gaseous compounds are fed into a gas inlet at one end of the tube and the reactive plasma species exits the plasma tube and enters the electrochemical flow reactor.

[0124] As described herein, the electrochemical flow reactor receives the reactive plasma species form the dielectric barrier discharge plasma. The electrochemical flow reactor comprises a gas diffusion electrode and an electrolyte. An interface is formed between the reactive plasma species, the gas diffusion electrode and the electrolyte to allow the reactive plasma species to participate in electrocatalysis and convert the reactive plasma species. as further described herein.

[0125] Critically, plasma-activated gases diffuse through the GDE, contacting the catalytic layer prior to liquid quenching. This configuration improves interactions between short-lived plasma species and electrocatalysts to enhance synergistic plasma-electrocatalytic reactions. The room- temperature conditions allow for aqueous electrolytes, and the novel plasma flow cell design is more relevant for applications because of the enhanced mass transport and high current.

[0126] The present invention demonstrates that plasma pre-activation can modify CO2R reaction pathways (e.g., doubling n-propanol FE at certain applied electrochemical potentials) and enable formation of products that cannot be attained with electrocatalysis alone (e.g., acetone and acetylene) as shown in Fig. 9. These results illustrate the potential for plasma-activated CO2electrocatalytic reduction (pCO2R) to unlock novel reaction pathways and significantly enhance selectivity to multi-carbon products. They also demonstate that plasma species can be delivered effectively to the electrocatalyst prior to relaxing to ground state.

[0127] To further demonstrate that plasma-activated CO2species could be transported to the electrocatalyst without becoming quenched, control experiments were conducted with a CO2 / CO / O2feed mixture (no plasma) fed to the electrolyzer, mimicking the stable plasma product mix. As shown in Fig. 10, the new products that were observed with CO2plasma were not observed for the CO2 / CO / O2control, further illustrating the involvement of short-lived plasma-activated species such as excited CO2in electrocatalytic reactions during pCO2R. In other words, the presence of CO and O2in the plasma-generated feed to the electrolyzer does not explain the modified reaction trends obtained for pCC^R.

[0128] It was expected that modifying the plasma power would influence both the distribution of different types of excited CO2species as well as the total density of excited species in the gas. Although higher powers are likely to increase the overall density of excited species, modifying the electric field strength based on the plasma power will also change the fraction of electron energy lost to different excitation modes. As a result, different plasma powers resulted invariations in the distribution of excited CO2species, as shown in Fig. 8. For example, greater efficiency of electron energy loss to vibrational excitation may be obtained for lower reduced electric field strengths. As a result, more efficient CO2conversion with moderate plasma powers (e.g., <100 Td) may be observed that correspond to relatively greater plasma electron energy loss to vibrational excitation (which in turn can enable vibrational ladder climbing to higher excitation levels without additional initial energy input). In situ optical emission spectroscopy was employed to identify the activated CO2 / CO species, and to correlate which of these species are associated with electrochemical reduction to novel products or increased selectivity to desirable products.

[0129] Additionally, the effect of electrochemical potential on product formation at constant plasma power was evaluated to provide insight into the electrocatalytic reactions involving reactive plasma species. In the absence of plasma activation, the electrochemical potential influences the reaction pathways and modifies the FE and partial current densities. Therefore, it is expected that the electrochemical potential will also influence the electrocatalytic reaction pathways involved in plasma-activated CO2reduction, although the effects of potential on the reactions may differ with and without plasma. Comparing the influence of applied potential on the FE may reveal the ways in which the plasma-activated CO2reduction pathways differ from the ground-state CO2reduction. Coupled with reactor and electrochemical analysis, ex situ catalyst materials characterization (e.g., XPS, XRD, SEM, and TEM) and electrochemical characterization (e.g., linear sweep voltammetry, cyclic voltammetry, and electrochemical impedance spectroscopy) of fresh and spent catalysts reveal changes in catalyst structure and electrocatalytic activity after plasma-activated and electrochemical-only reactions.

[0130] Using plasma to activate CO2results in a mixed feed to the electrolyzer containing both ground-state and excited CO2, CO, and O2species, due to CO2splitting being accomplished by homogeneous gas-phase plasma reactions. Although conversion in the CO2plasma remains relatively low, resulting in relatively small CO and O2gas fractions, these species may still influence subsequent electrocatalytic reactions. Therefore, the influence of CO and O2on PCO2R was investigated.

[0131] To assess the contribution of plasma-generated CO to the reactions, CO was used in plasma-only, electrocatalytic-only, and plasma-electrocatalytic reactions to provide insight into the key species involved in electrocatalytic CO2reduction reactions. For example, if CO2reduction to CO is the rate-limiting step on a given electrocatalyst, then similar productformation trends may be observed in CO electrocatalytic-only reactions and CO2plasma- electrocatalytic reactions. Such results would suggest that a primary role of plasma is in pre- activating CO2or converting it to CO to lower the energy barrier for initial reactant activation on the catalyst. However, it is expected that the influence of plasma activation may be somewhat more complex.

[0132] A preliminary linear sweep voltammetry experiment of CO reduction with plasma and in the absence of plasma was conducted as shown in Fig. 12. As applied potential became more negative, the plasma-activated CO reduction showed a higher reaction rate, suggesting that plasma-activated species modify the kinetics of the electrocatalytic reactions.

[0133] In comparison to CO generation, it is expected that the generation of O2in CO2plasma may lead to undesirable competition with the O2reduction reaction (O2RR) on electrocatalysts. The competitive O2reduction may also lead to lower total FE of carbon-containing products compared to electrocatalysis alone, as observed in Fig. 9. The generation of O2in a CO2plasma as well as consumption of the generated O2were measured when an electrocatalytic reaction with a Cu catalyst was run using the effluent of the plasma reactor as shown in Fig. 25(c). These results suggest that generated O2competes with CO2since it is consumed by the electrolyzer.

[0134] It was discovered through preliminary experiments that Cu alloying with Ag, Pd, and Ru shifts product trends during pCO2R, as shown in Fig. 14. Employing bimetallic Cu-based catalysts containing Ag, Pd, and Ru enhances the FE of ethanol detected in the gas phase, but only Cu-Pd was found to improve the FE of ethylene. In contrast to these p CO2R trends, Cu-Pd electrocatalysts typically favor C1generation such as CO. Further, alloying Pd with Cu has been shown to increase CH4generation with respect to Cu alone during CO2RR, whereas preliminary pCO2R results show similar FE of CH4for both Cu and Cu-Pd catalysts, with enhancement of C2product generation with Cu-Pd. These results suggest that pre-activating CO2may significantly modify the subsequent electrocatalytic reactions. Investigations focused on understanding the differences in reaction trends on bimetallic catalysts for CO2RR versus pCO2R may provide further insight into key steps in the pCO2R pathways, where the potential-determining step may be different in the plasma-activated and conventional electrocatalytic reactions.

[0135] The present invention describes a coupled conversion system in which CO2 / CO is pre- activated by DBD plasma and electrochemically converted to hydrocarbons over a suitable catalyst such as Cu, initiating catalysis from higher and diversified energy states. By introducing a gas diffusion electrode (GDE), the inventors of the present invention discovered that it waspossible to involve plasma-activated CO2 / CO in electrocatalytic reaction avoiding quenching in electrolyte, thereby directly converting them in excited states. Plasma activation of CO2 / CO not only improved conversion efficiency and boosted reaction rates of multi-carbon products, but also unlocked new electrocatalytic reaction pathways widening viable product range. Electrocatalysis of activated CO2exhibited a decrease in production of C2H4and carboxylic acids (HCOO- and acetate (CH3COO-)), while increasing production of alkane (methane (CH4), ethane (C2H6), and propane (C3H8)), acetaldehyde (CH3CHO), and alcohols (methanol (CH3OH), C2H5OH, and C3H7OH) compared to ground state counterpart. Similarly, activated CO showed overall increase in partial J of each hydrocarbon except C2H4, promoting of C2+ hydrocarbons formation including butane (C4H10). Only when plasma activated CO2 / CO, hydrocarbons were produced such as CH3OH, acetylene (C2H2), C2H6, CH3CHO, C3H8, and C4H10.

[0136] To optimize the plasma coupling, it was determined that it was critical to suppress competitive oxygen reduction reactions (ORR) and C deposition, and byproducts-free effluent gas was obtained using CO2+ CO mixture in light of plasma simulation. Plasma-activated CO2+ CO substantially enhanced C3+ productivity and promoted the new reaction pathways. The present invention demonstrates not only electrocatalysis of pre-activated reactants for efficient conversion of gaseous compounds, including multi-carbon products formed from CO2 / CO, but also synergistic convergence of plasma and electrocatalysis.

[0137] The invention will now be described in reference to the following non-limiting examples: Examples:

[0138] As seen in the Examples below, a series of experiments was performed which were coupled with kinetic modeling to identify the mechanisms responsible for the enhancements upon plasma activation. Kinetic modeling enabled identification of potential reactive chemical species, including vibrational and electronic excitation modes and ground-state reactive species. The experimental setup and modeling were linked via a plug flow reactor approximation, with microdischarge filaments representing as a series of temporal pulses. This approach simplified the generation and relaxation of species, providing qualitative analysis and hypothesizing the effects of plasma activation on electrocatalysis. The invention describes herein demonstrates high generation rates for electrosynthesis of multi-carbon products from CO2 / CO and water under mild conditions, as well as opportunities for accessing new products through coupling with plasma, presenting a new paradigm for synergistic convergence of plasma and electrocatalysis.

[0139] By feeding CO2to the system and using an aqueous electrolyte and a suitable electrocatalyst, reduction of CO2to a variety of products can be achieved, including generation of new products that could not be obtained with electrocatalysis alone such as methanol, acetaldehyde, and butane.

[0140] Enhanced performance for generating multi-carbon products was achieved by feeding CO, where the total partial current density for C3+ products was increased from ~5 to ~35 mA cm-1. By co-feeding CO2and CO (which enables scavenging of O2and C generated in CO2and CO plasmas, respectively), the partial current density for C3+ products was further enhanced to ~55 mA cm-1.Example 1. - Design of electrocatalytic conversion of plasma activated CO2 / CO

[0141] Without wishing to be bound by theory, the inventors of the present invention theorized that plasma activation of CO2 / CO to a higher energy state could enable subsequent reaction intermediates to undergo different electrocatalytic pathways compared to conventional electrocatalytic reduction of ground state CO2 / CO. As a result, new products that are thermodynamically or kinetically unattainable from ground state reactants can potentially be formed.

[0142] Figs. l(a)-(e) depicts a concept of electrocatalytic conversion of plasma-activated CO2 / CO. Theoretical reaction energy diagram comparing activation energy barrier for CO2 / CO to CxHyOz / Cx’Hy’Oz’ pathways starting from ground state (plasma off) or plasma-activated states of COXare shown in Figs. 1(a) and 1(b). Plasma-activated CO2 / CO could result in different products (Fig. 1(a)) and reduced energy barrier (Fig. 1(b)). Fig. 1(c) depicts a schematic illustration of plasma activation of CO2 / CO prior to electrocatalytic conversion towards value- added products over a Cu-coated gas diffusion electrode. The * denotes short-lived plasma- generated species such as C3O2(see modeling results in Figs. 6(a)-(f)). Fig. 1(d) depicts an enlargement of the image highlighted in the dashed box in Fig. 1(c) and shows the three-phase interface between plasma-activated CO2 / CO gases, the electrocatalyst layer, and the aqueous electrolyte. Fig. 1 (e) depicts scanning electron microscopy image of Cu film deposited on polytetrafluoroethylene gas diffusion layer.

[0143] The schematic illustration in Fig. 1(c) depicts the combined plasma-electrocatalytic conversion process. Plasma activation was incorporated to elevate the energy state of the reactants prior to electrocatalytic CO2 / CO conversion using a GDE-based electrochemical flow reactor, as depicted in Figs. 15(a) and (b). The GDE enables formation of an interface betweenCO2 / CO plasma, the electrocatalyst, and the electrolyte, allowing activated reactants to participate in electrocatalysis, as shown in Fig. 1(d). The use of the GDE not only addressed the limited solubility of CO2in the aqueous electrolyte but also prevented the quenching of plasma species observed during previously reported electrochemical systems involving direct plasma- water interfaces.Example 2. - Methods a. Plasma-electrocatalysis combined flow reactor.

[0144] The gas feed tube of an electrochemical flow reactor was outfitted with dielectric barrier discharge (DBD) plasma. Specifically, a quartz tube with inner and outer diameters of 12.7 and 10 mm, respectively, was used with coaxial electrodes. A stainless steel rod with diameter of 4 mm fixed in the center of the tube was used as the high voltage electrode, and a steel coil wrapped outside of the quartz tube was the ground electrode. The length of the generated plasma was 12.0 cm, and the distance between the tip of the rod to the back side of the GDE was 2.0 cm. All electrochemical flow reactor parts were manufactured using polyetheretherketone for chemical resistance, low electrical conductivity, and thermal stability. b. Cu gas diffusion electrode preparation

[0145] The electrochemical cathode was fabricated by depositing metallic Cu on a PTFE membrane (STERLITECH, 0.45 μm, polypropylene backer, Aspire laminated). Cu deposition was conducted at a thickness of 400 nm using a Denton E-beam evaporator with a deposition rate of 0.2 nm sec-1. The base pressure in the vacuum chamber was maintained below 1.0x10-6Torr. A 99.999% Cu pellet was used as the evaporation material. Mesoporous carbon powder was loaded onto the Cu-deposited membrane by airbrushing an ink, which was a homogeneous mixture of 9 ml of isopropyl alcohol (Sigma Aldrich, W292907), 200 μl of Nafion™ 117 solution (Sigma Aldrich, 70160), and 40 mg of mesoporous carbon (Sigma Aldrich, 699624). The loading amount was 0.1 mg cm-2. The electrode was dried at room temperature after air brushing. The geometric surface area of the cathode was 1 cm2.

[0146] To fabricate the GDE, a metallic Cu film with a thickness of 400 nm was deposited on a polytetrafluoroethylene (PTFE) membrane at a rate of 0.2 nm sec-1using an e-beam evaporator. The phase was identified as metallic Cu with native oxides via X-ray diffraction (XRD), as shown in Fig. 16 and X-ray photoelectron spectroscopy (XPS), as shown in Figs. 17(a) and (b). Scanning electron microscopy (SEM) analysis revealed the porous microstructure of the electrode as shown in Fig. 1(e) and Figs. 18(a) and (b). Scanning electron microscopy (SEM)analysis revealed porous microstructure of electrode (as shown in Fig. 1(f) and Figs. 18(a) and (b)) The electrode was composed of microfibers with diameter of -300 nm containing pores at size of about 450 nm, with a thickness ranging from about 5 to about 9 μm.

[0147] Typical GDEs with pore radii ranging from 0.4 to 20 μm provide relatively large pores that should not significantly hinder transport of plasma species. However, it is believed that GDE pore size and thickness may be influenced. This was evaluated based on the use of commercial gas diffusion layers with varying pore sizes and thicknesses, or by manipulating sputtering parameters to control pore sizes (nanometers to micrometers) and layer thicknesses. By implementing a range of GDE pore sizes, a range of plasma species penetration abilities may be obtained, resulting in varied electrocatalyst contributions to plasma chemistry reflected in the product generation rates and selectivities. c. Materials characterization

[0148] A scanning electron microscope (Hitachi SU-70) was used to investigate the microstructure of the Cu GDE before and after the conversion reaction. The phase and oxidation states of the Cu catalyst were studied using X-ray photoelectron spectroscopy (PHI VersaProbe II ) with an Al Ka target (1486.7 eV) and X-ray diffraction (Rigaku Smartlab) using a Cu Ka radiation source. d. Evaluation of CO2 / CO conversion reactions

[0149] Biologic (VSP potentiostat) and CHI (CHI 660E) electrochemical workstations equipped with 4 and 5 A boosters, respectively, were employed to operate all electrochemical measurements. Sustanion X37-50 (Dioxide Materials) was used as the anion exchange membrane to separate cathodic and anodic electrolyte chambers. Ni foil (Sigma Aldrich, 357588- 2.2G) was used as the anode and an Ag / AgCl electrode (CHI instrument, CHI 111) was used as a reference. The electrolyte was 1 M KOH (Sigma Aldrich, 221473) aqueous solution. All potentials were converted to RHE scale using the equation below.ERHE= EAg / AgCl+ 0.197 V + 0.0592 x pH

[0150] IR correction was manually conducted at 50% after electrochemical reaction. The solution resistance of the electrolyte was 2.38 Ω measured by electrochemical impedance spectroscopy. Volumes of 80 ml and 40 ml KOH solution were used as the cathodic and anodic electrolytes for all experiments. Each electrolyte was circulated at a rate of 10 ml min-1using a peristaltic pump. During CO2conversion experiments with and without plasma, 40 ml min-1CO2and 10 ml min-1He were fed to the reactor. He was included to lower the power input required toignite plasma and to increase the plasma uniformity. Because of the instability of CO plasma, the He ratio was increased for CO conversion experiments, using a feed consisting of 30 ml min-1CO and 20 ml min-1He. The flow rate of gases was controlled using mass flow controllers (Masterflex, MFLX32907-59). All CO2 / CO conversion experiments were conducted for 1 hour at each applied potential and plasma power. GC (Agilent 8890) was used to analyze the gas products. The effluent gas was sampled three times during the experiment following a period of 10 minutes to allow the system to reach steady state, and averaged. The circulated electrolyte was analyzed by nuclear magnetic resonance (NMR, Broker 400 MHz) using dimethyl sulfone (Sigma Aldrich, 41867) and deuterium oxide (Tokyo Chemical Industry, 4JTCA-QD) mixture as an internal standard. The measurements were repeated three times using independent GDE samples to obtain averages and standard deviations. The FE of each product was calculated by the equation below,where z is the number of electrons transferred for reduction of the given product, n is the moles of product detected, F is the Faraday constant (96485 C mol-1), and Q is the charge consumed. e. CO2 / CO plasma generation and characterization

[0151] The electrical circuit for the DBD plasma and characterization are described in Fig. 46(a). A PVM 500 plasma generator was used to apply an alternating voltage to the stainless- steel rod. The plasma frequency was fixed at 19.2 kHz and voltage was controlled manually. The voltages were determined by considering plasma ignition and temperature increase due to Joule heating. The power consumption was measured by Lissajous plots obtained using a 10 nF capacitor (Lumimax Optoelectronic Tech.), and the microdischarge currents were investigated using a Rogowski coil (PEARSON Electronics, 2877, as shown in Figs. 46(a)-(d), Figs. 47(a)- (c), Fig. 48, Fig. 49, Fig. 50, and Fig. 51. Voltage was monitored using an oscilloscope (Tektronix, TBS 100C) with a 1000:1 attenuator (FLUKE, 80K-40). Microdischarge currents were processed in order to obtain the true plasma current. Measured current / was corrected by subtracting the capacitive displacement cunent (Idisplacement), Idisplacementwas calculated using the following equation:

[0152] The Ccellvalue was extracted from the Lissajous plots by fitting a straight line to the beginning and end of the rising side of the curve. Optical emission spectroscopy (OES, Avantes,AvaSpec-ULS4096CL-EVO) was used to identify the electronically excited states of CO2 / CO as shown in Fig. 58, Figs. Figs. 59(a)-(b), and Figs. 60(a)-(b). Spectra were calibrated using an Ar lamp prior to experimental measurements.

[0153] The schematic illustration in Fig. 1(a) depicts the combined conversion process. To elevate energy state of the reactants, plasma activation was incorporated prior to electrocatalytic CO2 / CO conversion using the GDE-based flow reactor. Plasma-activated CO2 / CO was delivered to GDE before relaxation to ground state. The GDE enabled formation of an interface between excited CO2 / CO in gas phase, electrocatalysts, and electrolyte, allowing reactants in activated state to engage in electrocatalysis. On the electrocatalyst surface, adsorbed CO2 / CO was converted into hydrocarbons through multiple steps of proton-coupled electron transport. GDE not only overcame limited solubility of CO2but also avoided quenching which could occur during CO2dissolution and mass transport to electrocatalysts in H-type reactor.

[0154] By comparing reaction coordination of CO2 / CO catalysis starting from ground and activated state (as shown in Fig. 1(d) and Fig. 16), reaction pathways and effects of the plasma coupling were studied. Activated CO2 / CO possessed a higher energy and their intermediate could undergo different reaction pathways, converting to a new product which was thermodynamically or kinetically infeasible from ground state. It was previously reported that distinguished coordination of CO2on Cu surface resulted in different product. Furthermore, even to form the same product, reactants in a higher energy state could benefit from a reduced activation barrier, making the reaction kinetically favorable. It is believed that pre-activation of the reactant could enhance production rate and form diverse hydrocarbons.Example 3. - Plasma-activated CO2electrocatalysis

[0155] Figs. 2(a)-(f) depict an evaluation of plasma-electrocatalytic CO2conversion. Fig. 2(a) provides linear sweep voltammetry (LSV) curves collected without (plasma off) and with plasma activation at different plasma powers. The potential was swept from 0.4 to -1.2 V vs RHE with a scan rate of 1 mV sec- 1- in 1 M KOH electrolyte and a mixed gas feed consisting of 40 ml min- 1 CO2 and 10 ml min-1 He. Each data point represents the mean of three independent measurements. Fig. 2(b) depicts the faradaic efficiency (FE) of carbon-containing products for electrocatalytic CO2RR without plasma (left) and 28.7 W plasma coupled CO2RR (right). Figs 2(c)-(e) depict partial current density J without (plasma off) and with plasma at different plasma powers for electrocatalytic CO2conversion to CH3OH (Fig. 2(c)), CH3CHO (Fig. 2(d)), and C2H4(Fig. 2(e)). Each data point represents the mean and the standard deviation of threeindependent measurements. Fig. 2(f) provides semi-quantitative summary of changes in product generation rates (partial J) based on employing plasma activation (x-axis) and increasing plasma power (y-axis). Products to the right of the y-axis increase with plasma coupling compared to electrocatalysis alone, and products to the left of the y-axis decrease with plasma coupling compared to electrocatalysis alone. Products above the x-axis increase with plasma power, and products below the x-axis decrease with increasing plasma power. Products shown in dashed square boxes were detected only during plasma-activated electrocatalysis.

[0156] To investigate electrocatalytic activity of excited CO2compared to ground, linear sweep voltammetry (LSV) was conducted at a scan rate of 1 mV sec-1in 1 M KOH electrolyte (see Fig. 2(a)). Because the DBD plasma and electrochemical flow reactor were independent, the plasma power could be varied from 12.0 to 42.3 W (during LSV analysis. Collecting LSVs by sweeping in the negative direction from 0.4 to -1.2 V (vs. RHE, all applied potentials were in RHE scale below) revealed a significantly more positive onset potential around 0.2 V It was a significant positive shift compared to onset around -0.2 V for ground state CO2. In addition, as the plasma power increased, so did electrocatalytic activity. This trend became more pronounced in applied potential range of large conversion rates. CO2activated by 42.3 W plasma showed a higher reaction rate compared to ground state CO2.

[0157] With regard to products (see Fig. 2(b) and Figs. 19(a)-(e)), electrocatalytic conversion of ground state CO2(plasma off) produced CO, C2H4, and C2H5OH as the main products, with minor amounts of CH4, HCOO-, CH3COO-, and C3H7OH. As applied potential became negative, FE for CO decreased and C-C coupling was promoted, significantly increasing the selectivity for C2H4. At -1.0 V, FE for C2H4and C2H5OH reached their maximum of 41.2±1.6% and 31.3±1.3%, respectively, while FE for C3H7OH was only 1.4±0.2%. Further increase in applied potential led to insufficient suppression of HER, resulting in decreased total FE for multi-carbon products. The product distribution and selectivity trends according to applied potential were consistent with previously reported research on CO2electrocatalysis using Cu GDE.

[0158] Activating CO2by plasma at powers of 12.0, 28.7, and 42.3 W resulted in an overall decrease in total FE for CO2-to-C products and in HER activity, with a notable decrease in C2H4FE. For example, at -1.0 V in 28.7 W plasma-coupled CO2electrocatalysis, the FE for C2H4was only 26.0±4.3%. Despite the overall FE decrease in 28.7 W plasma-activated CO2electrocatalysis, the FE for C2H5OH (30.4±0.5%) and C3H7OH (3.5±0.6%) was maintained and the partial J slightly increased. The CO FE in the plasma-coupled process was excluded from thisanalysis because CO generation from electrocatalysis and gas-phase plasma reactions was not distinguishable. Additionally, the enhanced activity of plasma-activated CO2may have resulted in the observed positive shift of applied potential and increased reaction rate, which were consistent with LSV curves. Fig 19(a) depicts plasma off, Fig. 19(b) depicts 12.0 W plasma, Fig. 19(c) depicts 28.7 W plasma, and Fig. 19(d) depicts 42.3 W plasma. Fig. 19(e) depicts a comparison of cunent density (J) versus applied potential. A mixed gas feed of 40 ml min-1CO2and 10 ml min-1He was supplied. Each data point represents the mean and standard deviation of three independent measurements.

[0159] In addition to generating typical CO2reduction products, the plasma-activated electrocatalytic reaction produced several new chemicals that were not detected in ground state CO2reduction, as shown in Fig. 2(c) and Figs. 21(a)-(d)). Hydrocarbons and oxygenates including CH3OH, C2H2, C2H6, C3H8, and C4H10were detected exclusively in plasma-activated CO2electrocatalysis, and the generation rates of these products exhibited different plasma power dependencies.

[0160] Figs. 21(a)-(d) depict partial J versus applied potential at different plasma powers and with plasma off for products that were exclusively detected with plasma coupling compared to electrocatalysis-only. Partial J is shown for C2H2(Fig. 21(a)), C2H6(Fig. 21(b)), C3H8(Fig. 21(c)), and (C4H10(Fig. 21(d)). Each data point represents the mean and standard deviation of three independent measurements.

[0161] For example, C2H2showed the highest partial J at the lowest plasma power (at 12.0 W), whereas C3H8and C4H10exhibited higher partial J as the plasma power increased (highest partial J at 42.3 W). CH3OH achieved its highest partial J at an intermediate power of 28.7 W, reaching 8.8±1.2 mA cm-2. Plasma coupling thus promoted the formation of several hydrocarbons and oxygenates. Compared to ground state CO2, plasma-activated CO2exhibited a higher partial J for CH3CHO which further increased with higher plasma power, as shown in Fig. 2(d). The partial J increased from 1.2±0.1 mA cm-2for electrocatalysis without plasma to 2.5±0.7 mA cm-2for electrocatalysis with CO2activated in a 28.7 W plasma. In addition, CH4, C2H5OH, CH3COCH3, and C3H7OH showed increased generation rates with plasma activation, as shown in Figs. 22(a)- (d).

[0162] Figs. 22(a)-(d) depict partial J versus applied potential for products that showed an increase in generation rate with plasma coupling at different powers compared to electrocatalysis-only. Partial Jis shown for CH4(Fig. 22(a)), C2H5OH (Fig. 22(b)), CH3COCH3(Fig. 22(c)), and C3H7OH (Fig. 22(d)). Each data point represents the mean and standard deviation of three independent measurements. The partial J for CH4, CH3COCH3, and C3H7OH showed their maximum at 42.3 W plasma at most of the evaluated applied potentials, while the maximum partial J for C2H5OH was achieved at a plasma power of 12.0 W. In contrast, the partial J of C2H4decreased with plasma coupling, and a greater decrease was observed as the plasma power increased, as shown in Fig. 2(e). The partial J decreased from 144.4±43.0 mA cm-2with plasma off to 58.4±14.4 mA cm-2with 42.3 W plasma-activated CO2.

[0163] Plasma coupling also reduced the generation rate for C3H6compared to electrocatalysis only, as shown in Fig. 23(a)-(e). Fig. 23(a)-(e) depict Partial J and FE versus applied potential for carboxylic acids HCOO- (Figs. 23(a) and (c)) and CH3COO- (Figs. 23(b) and (d)), which showed similar FE with plasma coupling compared to electrocatalysis-only. Figs. 23 (a) and (b) show partial J and Figs. 23(c) and (d) show FE for ground-state CO2and plasma-activated CO2 electrochemical reduction at different powers. Fig. 23(e) shows the partial J of C3H6for plasma off and plasma-activated CO2reduction at different powers. Each data point represents the mean and standard deviation of three independent measurements.

[0164] To summarize the effect of the plasma coupling on CO2RR product generation, Fig. 2(f) presents a qualitative chart in which the x-axis indicates whether the partial J of a given product increased or decreased with plasma coupling compared to electrocatalysis alone, and the y-axis indicates whether the partial J of a given product generated during coupled plasma- electrocatalysis increased or decreased with plasma power. For example, products in the top right quadrant are those for which plasma activation enhanced generation compared to ground state CO2 electrocatalysis and showed a positive correlation of partial J with plasma power. This plot reveals that hydrocarbons and oxygenates exhibited different trends based on their functional groups. Saturated hydrocarbons such as CH4, C2H6, and C3H8showed improved productivity with higher plasma power, while unsaturated hydrocarbons such as C2H2, C2H4, and C3H6generally exhibited a decrease in partial J as the power increased.

[0165] The formation of alcohols such as CH3OH, C2H5OH, and C3H7OH was promoted with plasma coupling. In contrast, the generation of carboxylic acids such as HCOO- and CH3COO- remained relatively unchanged with and without plasma coupling. Despite the similar FE with and without plasma, the partial J for the carboxylic acids at 42.3 W plasma slightly increased due to total activity increase. Ketones (CH3COCH3) and aldehydes (CH3CHO) (for which only one chemical was detected per functional group) showed an increase in partial J with plasma 'coupling and a positive correlation with plasma power. Five products (CH3OH, C2H2, C2H6,C3H8, and C4H10) were newly detected only in plasma-coupled conversion.Example 4. - Influence of plasma on electrolyzer feed gas composition and catalyst structure

[0166] In order to understand why the total CO2-to-C products FE decreased with plasma coupling (see e.g., Fig. 2(b)), the effluent from the CO2plasma was analyzed using gas chromatography (GC). Plasma activation resulted in dissociation of CO2(3.3±1.0 to 4.2±1.5% of input CO2) into CO and O2through electron-impact collisions, e + CO2→ CO + O, and subsequent recombination of O atoms as shown in Fig. 3(a) and Figs. 24(a)-(d).

[0167] Figs. 24(a)-(d) depict a gas chromatogram of 42.3 W plasma-activated CO2. Gas chromatogram of flame ionization detector (FID) (Fig. 24(a)) and thermal conductivity detector (TCD) columns (Fig. 24(b)) are shown. Fig. 24(c) shows the O2peak before and during coupling with electrocatalytic reaction. Fig. 24(d) is a photograph of condensed H2O at gas outlet generated from oxygen reduction reaction on Cu GDE. A mixed gas feed consisting of 40 ml min-1CO2and 10 ml min-1He was used.

[0168] Figs. 3(a)-(e) depict the effect of CO2dissociation in plasma on coupled electrocatalytic conversion. Fig. 3(a) is a schematic illustration of CO2dissociation through DBD plasma and competitive ORR on Cu catalyst. Fig. 3(b) shows the concentration of the gas-phase homogeneous plasma reaction products (CO and O2) depending on plasma power. Fig. 3(c) shows the estimated FE for ORR during plasma-activated CO2RR at different plasma powers. Fig. 3(d) shows the total CO2-to-C products FE for electrocatalytic conversion of 12.0, 28.7, and 42.3 W plasma-coupled CO2electrocatalysis and for electrocatalytic (plasma off) conversion of pure CO2and of a mixed CO2+ CO + O2feed. Fig. 3(e) provides a comparison of partial J of new products between 12.0 W plasma-activated CO2and electrocatalytic (plasma off) conversion of a mixed CO2+ CO + O2feed at -1.0 V. Flow rates of the gases in the mixed feed were 38 ml min-1CO2, 10 ml min-1He, 1.7 ml min-1CO, and 0.7 ml min-1O2. Each data point represents the mean and the standard deviation of three independent experiments.

[0169] The composition of the plasma effluent depend on the plasma power, consisting of 5.2 to 3.1 mol. % CO and 1.2 to 0.6 mol. % O2for plasma powers ranging from 12.0 to 42.3 W, as shown in Fig. 3(b). The decrease in CO and O2generation with increasing plasma power may be attributed to more filamentary plasma characteristics, such that lower fractions of the feed gas are treated by plasma. Hydrocarbons and oxygenates are not observed in the plasma effluent.

[0170] The plasma reaction products can affect the electrocatalytic reaction, particularly since the reduction of O2(equation (1)) is thermodynamically favored compared to CO2RR as shown in equations (2-5). Given that metallic Cu also has O2reduction reaction (ORR) activity, the reduction of O2to H2O competes with CO2RR.O2+ 4H++ 4e- = 2H2O (Eo = 1.230 V) (1) CO2+ 2H++ 2e- = CO + H2O (Eo = -0.106 V) (2) CO2+ 8H++ 8e- = CH4+ 2H2O (EO = 0.169 V) (3) 2CO2+ 12H++ 12e- = C2H4+ 4H2O (Eo = 0.064 V) (4) 2CO2+ 12H++ 12e- = C2H5OH + 3H2O (Eo = 0.084 V) (5)Based on the detection of CO and O2in the plasma effluent, the influence of these gases on the electrocatalytic reaction and electrocatalyst properties was further investigated.

[0171] In the electrocatalytic conversion of the plasma effluent, the undesired charge consumption due to ORR was estimated by comparing the amount of O2in the CO2plasma effluent with that in the coupled plasma-electrocatalysis effluent, as shown in Fig. 24(c), assuming that all O2was electrocatalytically reduced to H2O and not H2O2(see Fig. 3(c)). The estimated ORR FE ranged from 13.1±4.7% to 39.2±9.9%. The small amount of O2in the plasma effluent consumed a substantial fraction of electrons, decreasing the FE of CO2electrocatalysis, as shown in Fig. 2(b). At lower overpotentials where CO2RR was not sufficiently promoted, a larger portion of the charge was consumed by ORR. These results revealed that competitive reduction of O2generated by plasma was the main cause of the lower CO2-to-C products FE in plasma-coupled CO2electrocatalysis, indicating that suppressing competitive ORR would be crucial for maximizing the plasma activation effect.

[0172] To exclude possible effects of the stable plasma reaction products (i.e., CO and O2) on the performance enhancement and the new products observed in coupled plasma-electrocatalytic CO2reduction, a control experiment was conducted using a CO2+ CO + O2mixed feed (plasma off) in the same composition as that measured in the CO2plasma effluent, as shown in Figs.25(a) and (b). The CO2RR in the control was inhibited due to competitive ORR, resulting in similar CO2-to-C product conversion FE and J as in plasma-activated electrocatalysis (see Fig. 3(d) and Fig. 26(a) and (b). The early onset potential around 0.2 V that had been observed for plasma-electrocatalysis (see Fig. 2(a)) was also observed for electrocatalytic reduction of the CO2+ CO + O2mixed feed (see e.g., Fig. 27), suggesting that the early onset potential can beattributed to ORR in both cases. However, the activity increase at negative applied potential that was observed in plasma-activated CO2was not apparent for the CO2+ CO + O2control.

[0173] Additionally, the products that were exclusively generated with plasma activation were either not detected (CH3OH, C2H2, and C2H6) or showed substantially reduced production rates (CH3CHO and CH3COCH3), as shown in Fig. 3(e). The increase in total J and in partial J of several products (e.g., CH4and C2H5OH) that was observed for plasma-electrocatalysis compared to electrocatalytic-only CO2RR, as shown in Figs. 22(a)-(d), was not observed in the control. Instead, the range of products narrowed and the total Jas well as the partial J for several products decreased for the control compared to electrocatalytic reduction of the pure CO2feed at most evaluated applied potentials as shown in Figs. 28(a)-(d) and 29(a)-(d). Similar performance degradation due to O2impurities in a CO2feed have previously been reported.

[0174] Figs 28(a)-(d) depict a partial J comparison between electrocatalytic conversion of CO2(plasma off) and CO2+ CO + O2mixed feed (plasma off; simulated plasma effluent) versus applied potential. Partial J is shown for CH4(Fig. 28(a)), HCOO- (Fig. 28(b)), C2H4(Fig. 28(c)) and CH3COO- (Fig. 28(d)). For the CO2conversion, a mixture of 40 ml min-1CO2+ 10 ml min-1He was fed. For the conversion of the simulated plasma effluent, a mixture of 38 ml min-1CO2+ 10 ml min-1He + 1.7 ml min-1CO + 0.7 ml min-1O2was fed. Each data point represents the mean and standard deviation of three independent measurements.

[0175] Figs. 29(a)-(d) depict a partial J comparison between electrocatalytic conversion of CO2(plasma off) and CO2+ CO + O2mixture gases (plasma off) versus applied potential. Partial Jis shown for C2H5OH (Fig. 29(a)), C3H6(Fig. 29(b)) and C3H7OH (Fig. 29(c)). Total J was compared between CO2(plasma off) and CO2+ CO + O2mixture gases (plasma off) versus applied potential and the results are shown in Fig. 29(d). For the CO2conversion, mixed gas of 40 ml min-1CO2+ 10 ml min-1He was fed. For the mixture gases conversion, mixed gas of 38 ml min-1CO2+ 10 ml min-1He + 1.7 ml min-1CO + 0.7 ml min-1O2was fed. Each data point represents the mean and standard deviation of three independent measurements.

[0176] Importantly, these results show that changes in the gas composition fed to the electrolyzer due to the formation of CO and O2plasma reaction products were not responsible for the formation of new products, increase in total J, or changes in partial J of several products in plasma-electrocatalytic CO2RR. Rather, effects related to plasma activation — not simply due to the presence of stable plasma reaction product gases — markedly influenced the electrocatalytic reaction.

[0177] Furthermore, ex-situ characterization of the Cu GDE was conducted to assess the influence of plasma on the electrocatalyst properties, and to evaluate whether these influences could account for the observed changes in reaction performance due to plasma. SEM (see Figs. 30(a)-(h) and XPS (see Figs. 31(a) and (b)) analyses revealed that plasma activation did not result in significant changes to the microstructure and oxidation state of Cu, respectively, compared to those after electrocatalysis-only reactions. The Cu GDE showed similar surface reconstruction involving the formation of adparticles following electrocatalysis of both ground state and plasma-activated CO2. After the reactions, the surface became slightly oxidized to Cu2+and the bulk crystal structure remained metallic, as shown in Fig. 32. In addition, the phase, oxidation state, and microstructure of Cu after the CO2+ CO + O2control showed analogous trends, as shown in Fig. 33(a)-(e)).

[0178] Therefore, it was determined that plasma and plasma reaction products did not induce substantial changes in the Cu GDE. Overall, the control experiments and the ex-situ characterization results reveal that the enhancements (e.g., new products and increased partial J of several products) in plasma-coupled CO2electrocatalysis were derived from activated CO2species, rather than due to participation of CO and O2plasma products in the reaction or modification of catalyst properties by plasma. Additionally, potential contributions of increased gas temperature to multi-carbon product formation were considered to be negligible since heated CO2 / CO has been reported to show decreased multi-carbon products selectivity in electrochemical CO2reduction.Example 5. - Plasma-activated CO electrocatalysis

[0179] As the presence of O2generated from CO2splitting in plasma led to competitive ORR and negatively impacted CO2RR FE, it was theorized that eliminating O2in the plasma effluent would promote the formation of CO2reduction products. To achieve this, CO was used as the reactant instead of CO2. GC analysis showed that CO plasma formed only a small amount of CO2(0.15±0.04 mol%) through the reaction of 2CO → C + CO2, as shown in Fig. 34(a)-(c); no O2was detected.

[0180] Figs. 4(a)-(e) depict an evaluation of plasma-electrocatalytic CO conversion. Fig. 4(a) shows LSV curves collected without (plasma off) and with 10.8 W plasma activation of CO at a scan rate of 1 mV sec-1in 1 M KOH electrolyte with a mixed gas feed consisting of 30 ml min-1and 20 ml min-1He. Each data point represents the mean of three independent measurements. Fig. 4(b) depicts FE of carbon-containing products for electrocatalytic CORR with plasma off(left) and 10.8 W plasma-activated CORR. Figs. 4(c) and (d) depict partial J without (plasma oft) and with plasma for electrocatalytic CO conversion to C2products (Fig. 4(c)) and C3+products (Fig. 4(d)). Each data point represents the mean and standard deviation of three independent experiments. Fig. 4(e) provides a semi-quantitative summary of changes in product generation rates (partial current density J) based on employing plasma activation. Products to the right of the small vertical line on the x-axis increase with plasma coupling compared to electrocatalysis alone, and products to the left of origin of the line decrease with plasma coupling compared to electrocatalysis alone. Products shown in dashed square boxes were detected only during plasma-activated electrocatalysis.

[0181] The effect of plasma activation on CO electrocatalysis was investigated by linearly sweeping the applied potential at a scan rate of 1 mV sec-1for both ground state and 10.8 W plasma-activated CO, as shown in Fig. 4(a). The early onset potential observed in the plasma- activated CO2case due to ORR was not observed for CO, where both ground state and plasma- activated CO reduction exhibited similar onsets around -0.4 V; this is consistent with the absence of O2in CO plasma. As the applied potential became more negative, the plasma- activated CO electroreduction showed a higher reaction rate. The increased J suggested that plasma elevated the energy level of the reactant and in turn lowered the relative activation energy barrier for electrocatalytic reduction, thus making electrocatalysis more kinetically favorable.

[0182] The product distribution was compared between ground state and 10.8 W plasma-coupled CO electroreduction, as shown in Fig. 4(b) and Figs. 35(a)-(c). For electrocatalytic CO reduction (plasma oft), the main products were C2H4, CH3COO-, and C2H5OH, with a decrease in C2H5OH FE and an increase in CH3COO- FE compared to electrocatalytic CO2RR. C3H7OH was also produced with a FE of 4.8±0.8% at -0.7 V; no other C3products were detected in substantial amounts from electrocatalytic CO2 / CO reduction. As the applied potential became more negative, the FE for C2H4decreased, and the FE for CH4increased, reaching 18.6±5.7% at -1.2 V. HER was not efficiently suppressed at potentials more negative than -1.0 V, as shown in Fig. 36, resulting in a decrease in CO-to-C products selectivity (see Figs. 37(a)-(c), Figs. 38(a)- (f), and Figs. 39(a)-(e)). The product generation trends with applied potential matched those reported for CO reduction reaction (CORR) on Cu GDE.

[0183] Figs. 37(a)-(c) provide a comparison of partial J for Cl products between electrocatalytic conversion of ground state CO (plasma off) and 10.8 W plasma-activated CO. The partial J of CH3OH (Fig. 37(a)), HCOO- (Fig. 37(b)), and CH4(Fig. 37(c)) is shown. A mixed gas feedconsisting of 30 ml min-1CO and 20 ml min-1He was supplied. Each data point represents the mean and standard deviation of three independent measurements.

[0184] Figs. 38(a)-(f) provide a comparison of partial J for C2products between electrocatalytic conversion of ground state CO (plasma off) and 10.8 W plasma-activated CO. Partial Jis shown for C H3COO- (Fig. 38(a)), C2H5OH (Fig. 38(b)), CH3CHO (Fig. 38(c)), C2H2(Fig. 38(d)), C2H4(Fig. 38(e)), and C2H6(Fig. 38(f)). A mixed gas feed consisting of 30 ml min-1CO and 20 ml min-1He was supplied. Each data point represents the mean and standard deviation of three independent measurements.

[0185] Figs. 39(a)-(e) provide a comparison of partial J for C3+ products between electrocatalytic conversion of ground state CO (plasma off) and 10.8 W plasma-activated CO. The partial J of C3H8(Fig. 39(a)), C3H7OH (Fig. 39(b)), CH3COCH3(Fig. 39(c)), C3H6(Fig. 39(d)), and C4H10(Fig. 39(e)) is shown A mixed gas feed consisting of 30 ml min-1CO and 20 ml min-1He was supplied. Each data point represents the mean and standard deviation of three independent measurement.

[0186] In plasma-coupled CORR, it was determined that the overall CO-to-C products conversion efficiency improved and competitive HER was suppressed compared to electrochemical-only CORR. Unlike in the case of CO2RR, plasma activation of CO did not generate O2or change significantly the composition of the gas fed to the electrolyzer, revealing that the increase in CO-to-C products conversion with plasma was likely due to the increased activity of excited CO or to short-lived plasma intermediates. Notably, the FE for the CO2RR products that were detected only after coupling with plasma significantly increased during plasma-coupled CORR. Hydrocarbons and oxygenates such as C2H2(2.6±0.5%), C2H6(4.5±2.6%), CH3CHO (1.6±0.4%), and acetone (CH3COCH3, 4.6±1.7%)) were produced in substantial quantities (FE values represent the maximum for each product), demonstrating diversification of viable products.

[0187] Compared to ground state CORR, plasma-coupled CORR resulted in increased partial J (see Figs. 37(a)-(c), Figs. 38(a)-(f), and Figs. 39(a)-(e)) for saturated hydrocarbons (CH4, C2H6, C3H8, and C4H10) and alcohols (CH3OH, C2H5OH, C3H7OH) as well as for CH3CHO and CH3COCH3. Especially for alcohols, which are highly desirable products from CO2 / CO, the maximum reaction rate was almost doubled from 36.7±16.1 mA cm'2for ground state CORR to 70.6±8.2 mA cm-2for plasma-coupled CORR. For carboxylic acids (HCOO- and CH3COO-), the selectivity was similar with and without plasma coupling, although the increase in overallactivity resulted in partial J increase. Unlike the CO2case, the partial ,J of unsaturated hydrocarbons such as C2H2and propylene (C3H6) also increased. C2H4was the only detected product that showed a decrease with plasma coupling.

[0188] Considering multi-carbon products formation, plasma activation of CO facilitated C-C coupling. Comparing C2productivity, as shown in Fig. 4(c), plasma coupling resulted in a higher conversion rate at lower overpotentials compared to ground state CORR. Enhanced C-C coupling was more pronounced for C3+ products, as shown in Fig. 4(d). Whereas electrocatalytic-only CO-to-C3+ reduction was observed to have a limited reaction rate (maximum 5.1±2.0 mA cm-2), plasma-coupled CO was converted to C3+ products at a maximum partial J of 35.2±11.9 mA cm'2. In addition, whereas C3H7OH was the primary C3+ product in electrocatalysis-only conversion, this class of products broadened to include C3H6, C3H8, CH3COCH3, and C4H10with plasma activation.

[0189] Without wishing to be bound by theory, it is believed that plasma activation of CO to higher and diverse energy states facilitates multi-carbon product formation and expands the number of multi-carbon products that could be formed. The effects of plasma activation on electroreduction were more dramatic for CO than for CO2. The effect on CO conversion is qualitatively illustrated in Fig. 4(e). Although plasma coupling improved the generation rate of several valuable products in CORR, the filamentary characteristics of CO plasma caused electric field localization and solid carbonaceous depositions (from CO dissociation in plasma) on the reactor wall, as shown in Figs. 40(a) and (b). Carbonaceous deposition posed issues for power control and long-term operation and therefore required amelioration to improve the viability of high-value multi-carbon product generation using plasma-electrocatalytic CO2 / CO conversion.Example 6. - Optimized plasma coupling using CO2+CO feed gas mixture

[0190] To enable efficient and stable plasma coupling with electrocatalysis, suppression of O2formation in CO2plasma and carbonaceous deposition in CO plasma was pursued via feeding a mixed CO2+ CO gas stream. Specifically, it was theorized that O2generated from CO2dissociation could react with C from CO dissociation to regenerate CO2in the plasma, leading to an O2- and C-free plasma effluent even at elevated plasma power. To determine the optimal gas composition for enabling these plasma reactions, plasma effluent gases were investigated at different feed ratios, as shown in Fig. 41. GC analysis of plasma-activated CO2+ CO in a volumetric ratio of 3 CO2to 1 CO revealed complete suppression of O2generation at 46.9 W plasma power, as shown in Figs. 42(a)-(d).

[0191] Whereas in the CO2-only case the O2peak intensity of the plasma effluent decreased when plasma was coupled with electrocatalysis (suggesting consumption of plasma-generated O2by ORR), as shown in Fig. 24(c), the O2peak intensity for the CO2+ CO case remained unchanged when coupled with electrocatalysis (i.e., minimal trace peak for O2with plasma on and off). This result was consistent with a lack of O2in the plasma effluent, since no electrocatalytic O2consumption was detected. In addition, carbonaceous product deposition on the plasma reactor walls was significantly mitigated, even at 46.9 W, as shown in Fig. 42(d).

[0192] Previously, electrocatalytic reduction of a mixed CO2+ CO feed that simulated flue gas was reported to outperform pure CO2conversion to multi-carbon products. However, these studies focused on enhancing the generation of C2H4, not opening new reaction pathways or producing high-value C3+ products. Therefore, since electrocatalytic reduction of co-fed CO2+ CO has been shown only to enhance C2H4 generation, the performance enhancements in the present invention, such as improved generation of alcohols and C3+ products, are attributed to plasma coupling.

[0193] Figs. 5(a)-(e) depict the plasma-electrocatalytic conversion of CO2+ CO feed gas mixture. Fig. 5(a) depicts a comparison of total production rate for C-containing products between ground state CO2, 42.3 W plasma-coupled CO2, and 46.9 W plasma-coupled CO2+ CO conversion. Fig. 5(b) provides production rates of hydrocarbon products from 46.9 W plasma- activated CO2+ CO conversion. Figs. 5(c)-(e) depict production rates during electrocatalytic conversion of ground state CO2, 42.3 W plasma-activated CO2, and 46.9 W plasma-activated CO2+ CO for new products including CH3OH, C2H2, C2H6, C3H8, and C4H10(Fig. 5(c)), C3products including C3H6, C3H7OH, C3H8, and CH3COCH3(Fig. 5(d)), and C4H10(Fig. 5(e)). Each data point represents the mean and the standard deviation of three independent measurements.

[0194] Plasma-coupled CO2+ CO electrocatalysis exhibited an increase in the total production rate of C products (maximum of 11.0±0.5 μmol cm-2s-1), which can be attributed to the absence of ORR compared to CO2plasma (7.4 ±1.6 μmol cm-2s-1of at 42.3 W) and increased intrinsic activity of the reactants compared to ground state CO2electroreduction (7.6 ±1.1 μmol cm-2s-1) (Fig. 5(a)). Production rate is used instead of FE and partial J, since the two reactants could both potentially be converted into the products. Compared to electrocatalysis without plasma, plasma activation of CO2+ CO promoted conversion and suppressed HER, as shown in Figs. 43(a) and (b). In addition, conversion of the combined CO2and CO mixed feed led to generation of newproducts that were not generated with electrocatalysis alone, as in the CO2-only and CO-only reactions, as well as increasing the generation of C3and C4hydrocarbons and oxygenates, as shown in Fig. 5b and Figs. 44(a) and (b). The production rate of new products including CH3OH, C2H2, C2H6, C3H8, and C4H10increased substantially for plasma-coupled CO2+ CO electrocatalysis (520.6±244.8 nmol cm’2s’1at -1.1 V) compared to plasma-coupled CO2electrocatalysis (261.2±56.8 nmol cm-2s’1, as shown in Fig. 5(c)).

[0195] Furthermore, plasma activation boosted C3product generation (as shown in Fig. 5(d)). Without plasma, CO2was converted to C3products (exclusively C3H7OH) at a reaction rate of 208.1±70.9 nmol cm-2s-1at -1.0 V. Plasma coupling with a pure CO2feed slightly enhanced the C3products generation rate, reaching 277.0±108.5 nmol cm-2s-1at -1.1V. However, plasma activation of in CO2+ CO electroreduction not only increased the production rate for C3products to 708.3±51.4 nmol cm-2s-1at -1.1 V (including a C3H7OH generation rate of 536.2±36.4 nmol cm -'2s-1) but also expanded the range of possible C3products to C3H6, C3H8, and CH3COCH3.

[0196] Another noteworthy result was the production of C4H10at a production rate of 48.5±3.8 nmol cm-2s’1(as shown in Fig. 5(e)). Extended GC analysis also revealed the formation of 1, 2- butanol at -1.1 V, as shown in Fig. 45. Although the amount of these products was too small for quantification, generation of these C4products can be further improved at higher conversion rates and with modified catalysts. Direct and substantial C4production over Cu catalysts from CO2or CO electroreduction has been rarely reported, to the best of our knowledge. C3+ product and alcohol production in this work was compared to other reported electrocatalysis systems as shown in Tables 1 and 2.

[0197] Table 1 provides a comparison between the present invention and previous studies on electrocatalytic conversion of CO2 / CO to C3+ products. FE and partial / were calculated assuming products were generated from CO2.Table 1.

[0198] Table 2 provides a comparison between the present invention and previous studies on electrocatalytic conversion of CO2to alcohols. FE and partial J were calculated assuming products were generated from CO2.Table 2.

[0199] Coupled plasma electrocatalysis achieved the highest reported generation rate of C2H5OH (3.1±0.03 μmol cm-2s-1) and C3H7OH (536.2±36.4 nmol cm-2s-1) without the use of precious metals. Compared to electrocatalysis-only conversion, the cathodic conversion efficiency to C2H5OH (18.1%) and C3H7OH (5.3%) was enhanced, as shown in Table 3.

[0200] Table 3 shows the maximum cathodic conversion efficiency of ground state CO2(plasma off), plasma-activated CO2at different powers, and 46.9 W plasma-activated CO2+CO for C2H5OH and C3H7OH production. Energy efficiency was calculated assuming products were generated from CO2.

[0201] Therefore, altering the initial energy state of CO2 / CO by plasma can provide a compelling strategy to improving production rates for C3+ products and alcohols, rather than feeding pure CO or modifying catalytic active sites through high-cost strategies such as alloying with platinum group metals, surface engineering, and composites.Table 3.Example 7. - Plasma diagnostics and kinetic simulations to investigate reaction mechanisms

[0202] To identify the underlying mechanisms of plasma-electrocatalytic coupled conversion of CO2and CO, qualitative kinetic modeling of potential reactive species in plasma was conducted using COMSOL Multiphysics. For the simulations, the plasma characteristics such as powerconsumption, microdischarge frequency, and intensity of voltage and cunent required were investigated, as shown in Figs. 46(a)-(d), Figs. 47(a)-(c), Fig. 48, Fig. 49, Fig. 50, and Fig. 51.

[0203] Figs. 46(a)-(c) provide measurements of power consumption and microdischarge current of DBD plasma. Fig. 46(a) depicts the electrical circuit for the measurements, including a capacitor for power measurements and a Rogowski coil for microdischarge analysis. Fig. 46(b) shows the Lissajous plots of CO2plasma at different powers. Fig. 46(c) shows power consumption as measured using the internal area of the Lissajous plots. A mixed gas feed consisting of 40 ml min-1CO2and 10 ml min-1He was supplied.

[0204] Figs. 47(a)-(c) depict measurements and characterization of microdischarge current of CO2plasma at 12.0 W (Fig. 47(a)), 28.7 W (Fig. 47(b)), and 42.3 W (Fig. 47(c)). A mixed gas feed consisting of 40 ml min-1CO2and 10 ml min-1He was supplied. Measurements were processed based on previous work. The lines show voltage and microdischarge current, respectively. The dashed line represents displacement current.

[0205] Fig. 48 depicts a Lissajous plot of 10.8 W plasma-activated CO. A mixed gas feed consisting of 30 ml min-1CO and 20 ml min-1He was supplied.

[0206] Fig. 49 depicts measurements and characterization of microdischarge current of CO plasma at 10.8 W. A mixed gas feed consisting of 30 ml min-1CO and 20 ml min-1He was supplied. Measurements were processed based on previous work. 1 The lines show voltage and microdischarge current. The dashed line represents displacement current.

[0207] Fig. 50 depicts a Lissajous plot of 46.9 W plasma-activated CO2+CO. A mixed gas feed consisting of 30 ml min-1CO2, 10 ml min-1CO, and 10 ml min-1He was supplied.

[0208] Fig. 51 depicts measurements and characterization of microdischarge current of CO2+CO plasma at 46.9 W. A mixed gas feed consisting of 30 ml min-1CO2, 10 ml min-1CO, and 10 ml min-1He was supplied. Measurements were processed based on previous work.l The lines show voltage and microdischarge current. The dashed line represents displacement cunent.

[0209] Excitation-relaxation kinetics of electrically charged ions and vibrational modes in plasma were modeled under atmospheric pressure with CO2, CO, and mixed CO2+CO gases, considering the reactor dimensions, gas flow conditions, and plasma power per microdischarge in the experiments. Fig. 52 depicts modeling conditions for plasma simulation and shows a sketch of the plasma zone, with simulation volume outlined by dashed boundaries. Simulation conditions are described.

[0210] Plasma parameters such as reduced electric field (E / N), current, and electron temperature were computed by the model shown in Figs. 53(a)-(c). These parameters govern electron kinetics and the consequent excitation of heavy particles. Vibrational excitation was studied in this context because it facilitated dissociation of CO2via lower energy barriers compared to other pathways. Vibrational excitation, which leads to vibrational-translational (V-T) non-equilibrium, where vibrational temperatures exceed gas (translational-rotational) temperatures, can also enhance surface reaction rates. For example, while excitation of the lowest-energy vibrational states may be insufficient to trigger homogeneous gas-phase reactions, such excited molecules may encounter significantly reduced dissociation barriers on catalytic surfaces.

[0211] To evaluate the potential role of vibrational states in promoting catalytic pathways toward new products, the temporal evolution of the vibrational temperatures of the asymmetric and symmetric stretching modes of CO2, as well as that of CO stretching was analyzed as shown in Fig. 6(a)-(f).

[0212] Fig. 6(a)-(f) depict kinetic simulations on vibrational excitation and ground state species in CO2+ He, CO + He, and CO2+ CO + He plasma. Figs. 6(a)-(c) depict temporal evolution during the first pulse for 80% CO2+ 20% He (Fig. 6(a)), 60% CO2+ 20% CO + 20% He (Fig. 6(b)), and 60% CO + 40% He (Fig. 6(c)). Fig. 6(d) depicts a time evolution of vibrational temperature of CO over 10 subsequent pulses for 60% CO + 40% He. The dashed line represents the gas temperature, which is fixed at 400 K. Figs. 6(e)-(f) depict temporal evolution of the molar fractions of the ground state species formed upon CO and CO2dissociation and recombination processes during the first pulse (Fig. 6(e)), and over 10 consecutive pulses for 60% CO + 40% He (Fig. 6(f)). Dotted vertical lines in the plots distinguish the plasma discharge regime from the afterglow.

[0213] Vibrational excitation was strongly influenced by the power input, with higher power levels leading to increased electron density and, consequently, greater electron-impact vibrational excitation. In the presence of CO2, all vibrational temperatures equilibrated with the gas temperature within 10 μs after the pulse and before the subsequent pulse. Notably, in the CO and He mixture, vibrational relaxation occurred more slowly, with most excitation retained until the next pulse. In the CO2, CO, and He mixture, full equilibration of asymmetric stretching of CO2, symmetric stretching of CO2, and CO stretching was observed before the next pulse. The vibrational CO temperature according to the addition of 1 to 10% CO2to the CO and He mixture was also investigated as shown in Fig. 54. The addition of CO2rapidly relaxed the vibrationallyexcited CO. Only vibrationally excited CO in CO and He mixture showed accumulative increase with successive pulses. After 10 pulses, vibrational temperature approached a steady state of approximately 2200 K, as shown in Fig. 6(d).

[0214] To determine the time required for equilibration between vibrational excitation and gas temperature, the simulation time after the 10th pulse with the plasma off was extended. Complete relaxation of the vibrational excitation to gas temperature occurred approximately 5 ms after the plasma was turned off as shown in Fig. 55.

[0215] In addition, electron-impact excitation can result in the formation of electronically excited species, which cany significant amounts of energy. Electronic excitation in DBDs is a highly probable process due to the high electron energies achievable. However, at atmospheric pressure, simulations revealed that collisional quenching predominated, reducing the lifetime of electronic states to that of the microdischarge. An exception was O2(a1Δg), which was a relatively stable state that accumulated pulse after pulse (see Figs. 56(a)-(f). While it is likely that ORR would more favorably consume O2(a1Δg) at the electrocatalyst, it is possible that this electronically excited species could influence CO2 / CO electroreduction pathways as well, possibly contributing to enhancements in the formation of oxygenates such as alcohols.

[0216] Ground state species were also investigated in the plasma kinetic modeling. High fractions of C and O atoms were produced inside the microdischarges due to the efficient direct electron-impact dissociation of CO2and CO, as shown in Figs. 6(e) and 6(f) and in Figs. 57(a)- (d). C atoms rapidly recombined to form Cnspecies (where n = 1, 2, 3, or 4), which subsequently aggregated into solid carbon (C(S)) that was later oxidized to solid oxygen complexes, CO(S). Additionally, C atoms reacted with CO to produce C2O, which served as a precursor of carbon suboxide (C3O2). In contrast, O atoms recombined more slowly, leading to the formation of O3, particularly in the presence of CO2. Among the species observed, C3O2was accumulated with increasing CO feed fraction. C3O2has been reported to be a precursor to C3H6and C3H8through catalytic hydrogenation. Solid carbon, C(S), was also a product suggested by the model, particularly in the CO + He mixture, but it predominantly existed in an oxidized form (CO(S)) and at molar fractions several orders of magnitude lower than C3O2. Based on the kinetic studies, it is possible that the effects of plasma observed in plasma-coupled electrocatalysis can be attributed to either vibrationally excited states of CO or ground state C3O2.Plasma simulation

[0217] The kinetic model developed for use in the present invention was based on a computational work, which focused on the charged particle and vibrational kinetics in a low- pressure, CO2pulsed glow discharge. The model was implemented in COMSOL Multiphysics by coupling the plasma module with the vibrational energy balance equations for the symmetric and asymmetric modes of CO2, as well as the vibrational manifolds of CO and O2. To avoid inaccuracies associated with approximations required for energy transfer to reactor walls and the specific reactor geometry, the energy balance equation for the gas temperature (Tg) was not considered. Instead, Tgwas fixed at 400 K, a value considered reasonable for the average Tg, as higher temperatures would lead to melting of the polymer GDE.

[0218] Although the model was validated for low-pressure conditions, its applicability was extended to atmospheric pressure in this study by including three-body reactions between neutral species, interactions with charged particles, and three-body electron-ion recombination. The inclusion of electron-ion recombination was particularly crucial for the stability of the simulations, as their absence would result in an unreasonably high ionization degree (approaching 10%) under the studied conditions. Moreover, despite the significant pressure difference between a previous study (6.7 mbar) and the present invention (1 bar), both discharges operated in a non-thermal regime where thermal chemistry held limited influence. Consequently, while thermal processes were included in the kinetic scheme, the lack of model validation for conditions involving substantial gas heating was not a limiting factor. In addition, both discharges exhibited only mild vibrational-translational non-equilibrium, ensuring the validity of the assumptions and scaling laws used previously.

[0219] The set of vibrational relaxation rates used in the vibrational energy balance equations was adopted from previous work. Moreover, vibrational deactivation of CO by collisions with helium atoms was included based on the rates reported by Billing, G. D. & Cacciatore, M. Semiclassical calculation of vibrational relaxation of CO colliding with 4He and 3He. Chemical -24e(35.48 - 157*Tg1 / 3+ 193*Tg2 / 3)Physics Letters 86, 20-25 (1982), with a rate coefficient of 1.66 x 10 cm3 / s. Transitions involving higher vibrational levels (up to level 5) were scaled according to the Schwartz-Slawsky-Herzfeld (SSH) theory as described in Kozak, T. & Bogaerts, A. Splitting of CO2by vibrational excitation in non-equilibrium plasmas: a reaction kinetics model. Plasma Sources Science and Technology 23, 045004 (2014).

[0220] The detailed chemical kinetics were modified and added considering electron-impact processes as shown in Table 4. The formation of solid carbon in the gas-phase was included,while the carbon surface reaction scheme was taken from previous research. Reverse rates were computed from the detailed balance principle, with enthalpy and entropy as function of temperature for each species taken from the NASA polynomials.

[0221] As shown below, Table 4 depicts rate coefficients added or modified in the present invention as compared to previous research. The electron-impact processes, described by energy- dependent cross sections and the electron energy distribution function (EEDF), are taken from specific databases within the LxCat database, as detailed in the reference column. M is a colliding partner, which includes all neutral species unless specified differently.Table 4.a Includes ionization to electronically excited CO2+, with instantaneous relaxation to ground state CO2b Excitation to triplet (T) and singlet (S) states are lumped together c M - all neutrals except He

[0222] The simulation domain was obtained by dividing the reactor into slices, each corresponding to the width of a single microdischarge (2 mm), based on estimates from similar conditions. The input flow rate for the simulations was calculated using:where Qsliceis the flow rate for a single slice, Q is the total flow rate, and Vsliceand Vtotrepresent the volume of a single slice and the total reactor volume, respectively.The time between consecutive microdischarges within the same reactor slice was derived from the microdischarge frequency fMDand the gas residence time (tres) inside the plasma zone. The total number of microdischarges a gas volume experiences was given by:NMD, total= fMDX tplasma onwhere tplasma onis the total plasma-on time per second. The number of microdischarges per slice is:where Nshcesis the number of slices in the reactor. The time between two consecutive microdischarges within a slice is then:

[0223] Using the reactor geometry (12 cm plasma zone, 1 cm tube inner diameter), Nslices= 31.4, and based on the gas residence time (tres= 9.5 s), the calculated time between microdischargeswas approximately 1x10-4s. These numbers are kept the same for all conditions investigated, as the microdischarge frequency remains nearly constant.

[0224] The power input during tp|asma onwas calculated from the charge per microdischarge qMDand the applied voltage UMD:PMD = (qMDX UMD) / tMD, lifetimewhere IMD, lifetime was the microdischarge duration, which was set to 15 ns based on estimates from for similar conditions. The power density was obtained by dividing the power input by the microdischarge volume, calculated assuming a cylindrical shape, and was then used as input parameter for the model.

[0225] To mimic plug flow behavior, a continuous stirred-tank reactor (CSTR) model without inflow and outflow terms was employed. This approximation captured the effects of plasma- induced reactions within the reactor slices.

[0226] Thus, it can be seen that the present invention describes plasma-electrocatalytic CO2 / CO reduction and introduces a GDE to enable efficient delivery of short-lived plasma-activated reactants to an electrocatalyst in contact with aqueous electrolyte. Compared to electrocatalysis alone, coupling with plasma activation resulted in the generation of new products including methanol, acetylene, ethane, propane, and butane, and enhanced the generation of multi-carbon products such as propanol. Co-feeding of CO2and CO addressed issues of competitive ORR and solid carbonaceous product deposition that arose due to plasma activation of CO2, further enhancing FE and partial J for the new products, C3+ hydrocarbons, and alcohols.

[0227] Kinetic modeling suggests that vibrationally excited CO and C3O2from plasma could be responsible for the enhancement of multi-carbon products and the formation of new products. Coupling plasma with electrocatalysis across a GDE may open new directions for electrocatalytic conversion of CO2 / CO, since plasma allows us to access new reaction pathways and longer-chain hydrocarbons, such as C4H10and 1- and 2- butanol. Future investigations may tune the reactions toward specific desired products and gain additional insight into the coupling mechanisms through catalyst design, modification of the porous characteristics of the GDE, and manipulating the plasma chemistry and physics.

[0228] While processes for electrochemical CO2reduction and plasma CO2reduction exist, the present invention is believed to provide the first coupled plasma-electrocatalytic process for CO2conversion. It also represents the first demonstration of using a gas diffusion electrode or an electrochemical flow cell for plasma-activated reactions. The novel architecture unlocks newreaction pathways for CO2reduction, leading to formation of products (including, i.e., methanol, acetaldehyde, and butane) that are not detected in the absence of plasma activation. Additionally, plasma pre-activation enables significantly higher production rates of high-value C3+ products compared to electrochemical-only operation. Thus, the system described herein provides one-pot, low-temperature, and electrified CO2conversion to high-value products that are currently challenging to make using conventional CO2electrocatalysis. Such a process is also compatible with intermittent renewable energy, modular, and scalable, facilitating retrofitting of point-source emitters to accomplish CO2utilization and storage in liquid fuels.

[0229] Producing value-added chemicals and fuels from electricity-driven CO2conversion offers potential to achieve net-zero carbon emission and store intermittent renewable energy over the long-term. Electrocatalytic CO2reduction reaction (CO2RR) is of particular interest due to its modular scale and flexible operation, which can remove point source of CO2emissions.Considering product value and separation process of outstream gases, a research focus is the achievement of sufficient selectivity and production rate, targeting to C1products such as carbon monoxide (CO) and formate (HCOO-), C2products such as ethylene (C2H4) and ethanol (C2H5OH), and C3products such as propanol (C3H7OH).

Claims

WHAT IS CLAIMED IS:

1. A reactor system for converting one or more gaseous compounds, the reactor system comprising: a. a plasma tube comprising a dielectric barrier discharge plasma to activate the one or more gaseous compounds to a reactive plasma species, wherein the one or more gaseous compounds are fed into a gas inlet and the reactive plasma species exits the plasma tube coupled to an electrochemical flow cell reactor; and b. the electrochemical flow cell reactor, wherein the electrochemical flow cell reactor comprises: i. the plasma tube for generating and transporting the reactive plasma species from the dielectric barrier discharge plasma to the electrochemical flow cell reactor and delivering the reactive plasma species to electrocatalytic surfaces on a gas diffusion electrode; ii. the gas diffusion electrode, preferably wherein the gas diffusion electrode is a catalyst-coated gas diffusion electrode comprising a gas diffusion layer and a catalyst layer, and m. an electrolyte, wherein the electrolyte is contained in a chamber and arranged on the catalyst layer side of the gas diffusion electrode; and wherein an interface is formed between the reactive plasma species, the catalyst- coated gas diffusion electrode, and the electrolyte to allow the reactive plasma species to participate in electrocatalysis and convert the reactive plasma species.

2. The reactor system according to claim 1, wherein the electrolyte is selected from the group consisting of potassium based-electrolytes, cesium-based electrolytes, lithium- based electrolytes, and sodium based electrolytes, including hydroxides, chlorides, and bicarbonates of any of the foregoing, preferably wherein the electrolyte comprises potassium hydroxide or potassium bicarbonate.

3. The reactor system according to claim 1 , wherein the catalyst-coated gas diffusion electrode comprises metallic species deposited on a porous membrane.

4. The reactor system according to claim 3, wherein the metallic species are transition metals, optionally wherein the transition metals are selected from the group consisting ofcopper, nickel, iron, silver, gold, palladium, platinum, ruthenium, zinc, tin, cobalt, bismuth, indium, and combinations of the foregoing.

5. The reactor system according to any of claims 4, wherein the catalyst is a bimetallic copper-based catalyst comprising one or more of silver, palladium, ruthenium, gold, zinc, cobalt, nickel, tin, bismuth, and indium.

6. The reactor system according to claim 3, wherein the porous membrane comprises microfibers.

7. The reactor system according to claim 3, wherein the microfibers have a diameter of about 100 to about 600 μm, more preferably about 200 to about 500 μm, more preferably about 300 to about 350 μm.

8. The reactor system according to claim 3, wherein the porous membrane is a hydrophobic membrane.

9. The reactor system according to claim 6, wherein the porous membrane is selected from the group consisting of polytetrafluoroethylene, carbon-fiber, metal fiber, including titanium-based membranes with hydrophobic additives, preferably wherein the porous membrane comprises polytetrafluoroethylene.

10. The reactor system according to claim 3, wherein the porous membrane comprises a copper catalyst or a copper alloy catalyst, preferably a copper-silver catalyst deposited on a polytetrafluoroethylene membrane.

11. The reactor system according to claim 3, wherein the porous membrane has a thickness within the range of about 2 to about 20 μm, more preferably about 3 to about 15 μm, more preferably about 4 to about 10 μm, more preferably about 5 to about 9 μm.

12. The reactor system according to claim any of claims 6 to 11, wherein the porous membrane has a porosity within the range of about 100 to about 600 μm, more preferably about 200 to about 500 μm, more preferably about 350 to about 450 μm.

13. The reactor system according to claim 1, wherein the reactive plasma species diffuse through the gas diffusion electrode and contact the electrocatalytic surfaces on the gas diffusion electrode prior to liquid quenching of the reactive plasma species.

14. The reactor system according to claim 1, wherein the dielectric barrier discharge plasma operates at a temperature within the range of about 15 to about 130°C, more preferably about 20 to about 80°C, more preferably about room temperature to about 40°C.

15. The reactor system according to claim 1, wherein the dielectric barrier discharge plasma operates at or near atmospheric pressure.

16. The reactor system according to claim 1, wherein the dielectric barrier discharge plasma and the electrochemical flow cell reactor are on independent electrical circuits, wherein operation of the dielectric barrier discharge plasma and the electrochemical flow cell reactor are separately controlled.

17. The reactor system according to claim 16, wherein the activation of the gaseous compound is accomplished at a power in the range of about 1 to about 80 W, more preferably about 5 to about 50 W, more preferably about 10 to about 40 W.

18. A method of converting one or more gaseous compounds, the method comprising the steps of: a. subjecting the one or more gaseous compounds to plasma forming conditions to activate the gaseous compound to a reactive plasma species; b. delivering the reactive plasma species to electrocatalytic surfaces on a gas diffusion electrode; c. forming an interface between the reactive plasma species, the gas diffusion electrode and an electrolyte to allow the reactive plasma species to engage in electrocatalysis and convert the reactive plasma species.

19. The method according to claim 18, wherein the one or more gaseous compounds comprise one or more of a carbon- containing gas, a nitrogen-containing gas, or a sulfur- containing gas.

20. The method according to claim 19, wherein the gaseous compound is a carbon-containing gas.

21. The method according to claim 20, wherein carbon-containing gas comprises flue gas.

22. The method according to claim 21, wherein the carbon-containing gas comprises carbon dioxide, carbon monoxide, or a mixture of carbon monoxide and carbon dioxide.

23. The method according to claim 20, wherein the carbon-containing gas comprises a mixture of carbon monoxide and carbon dioxide and the ratio of carbon monoxide to carbon dioxide in the carbon-containing gas is in the range of greater than 0 to about 5, more preferably about 0.001 to about 4, more preferably about 1 to about 3.

24. The method according to claim 23, wherein the converted reactive plasma species comprise liquid and / or gaseous products, wherein the liquid or gaseous products comprise one or more of C3+ products, hydrocarbons, aldehydes, acids, and alcohols.

25. The method according to claim 24, wherein the converted reactive plasma species comprise one or more alcohols selected from the group consisting of CH3OH, C2H5OH, C3H7OH, CH3(CH2)3OH, CH3(CH2)3CH2OH, and other higher alcohols, more preferably wherein the one or more alcohols comprise CH3OH, C2H5OH, and C3H7OH.

26. The method according to claim 25, wherein partial current density for alcohol conversion is greater than about 30 mA / cm2, more preferably greater than about 40 mA / cm2, more preferably greater than about 50 mA / cm2.

27. The method according to claim 25, wherein faradaic efficiency for alcohol conversion is greater than about 30%, more preferably greater than about 40%, more preferably greater than about 45%, more preferably greater than about 50%.

28. The method according to claim 24, wherein the gaseous reduced compounds comprise hydrocarbons selected from the group consisting of C3H4, C3H6, C3H8, CH3COCH3, C4H10, C5H12, and other hydrocarbons up to at least C8hydrocarbons, more preferably wherein the one or more compounds comprise C3H6, C3H8, CH3COCH3, and C4H10.

29. The method according to claim 28, wherein partial cunent density for CO2to saturated hydrocarbons conversion is greater than about 30 mA / cm2, more preferably greater than about 40 mA / cm2, more preferably greater than about 50 mA / cm2.

30. The method according to claim 28, wherein faradaic efficiency for CO2to saturated hydrocarbons is greater than about 10%, more preferably greater than about 15%.

31. The method according to claim 18, wherein the plasma forming conditions take place at a temperature in the range of 15 to about 130°C, more preferably about 20 to about 80°C, more preferably about room temperature to about 40°C.

32. The method according to claim 18, wherein the plasma forming conditions take place at or near atmospheric pressure.

33. The method according to claim 18, wherein the reactive plasma species diffuse through the gas diffusion electrode and contact the electrocatalytic surfaces on the gas diffusion electrode prior to liquid quenching of the reactive plasma species.

34. The method according to claim 18, wherein the plasma forming conditions comprise using a dielectric barrier discharge plasma reactor to activate the gaseous compound to the reactive plasma species.

35. The method according to claim 18, wherein the gaseous compound is activated to the reactive plasma species at a power within the range of about 1 to about 80 W, preferably about 5 to 50 W, more preferably about 10 to about 40 W.

36. The method according to claim 18, wherein the gas diffusion electrode comprises metallic species deposited on a porous membrane.

37. The method according to claim 18, wherein the electrolyte is selected from the group consisting of potassium based-electrolytes, cesium-based electrolytes, lithium-based electrolytes, and sodium based electrolytes, including hydroxides, chlorides, and bicarbonates of any of the foregoing, preferably wherein the electrolyte comprises potassium hydroxide or potassium bicarbonate.

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