METHODS FOR THE ELECTROREDUCTION OF CARBON DIOXIDE TO VALUE ADDED CHEMICALS

Abstract

The present disclosure provides a method of electroreducing carbon dioxide (CO2). The method of electroreducing carbon dioxide may include feeding a first stream comprising carbon dioxide into a chamber through a chamber inlet, the chamber containing a gas diffusion cathode and a gas diffusion anode; feeding a second stream comprising glycerol or glucose into the chamber, the second stream having a pH of 12 to 14; and applying an electrical potential between the gas diffusion anode and the gas diffusion cathode to reduce the carbon dioxide and oxidize the glycerol or glucose.

Description

RELATED APPLICATIONS

The present patent document claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/546,044, filed on Aug. 16, 2017, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure is related generally to the electroreduction of carbon dioxide and more particularly to a method of electrochemically reducing carbon dioxide and oxidizing glycerol or glucose.

BACKGROUND

The global atmospheric carbon dioxide (CO₂) concentrations have been on a constant rise in the past few decades, with the daily average value crossing and staying above the 400-ppm mark in 2016, for the first time in recorded human history. The rise in CO₂ levels has been correlated to the increase in mean global temperature anomalies (global warming). Thus, developing cost effective technologies that can mitigate, capture, or utilize the excess anthropogenic CO₂ emissions remains a grand challenge of the 21^st century. To limit excess anthropogenic carbon dioxide (CO₂) emissions (˜4GtC yr⁻¹), and achieve the 2° C. target set forth in the Paris climate change agreement, a portfolio of technologies, such as (i) transitioning from fossil fuels to renewable energy (wind, solar, biofuels, etc.); (ii) improving the energy efficiency of vehicles and buildings; and (iii) CO₂ capture and sequestration, need to be implemented together. However, for a majority of these solutions, the associated costs and impact on economic growth is high, resulting in slow global adoption. An alternative to mitigating CO₂ emissions could be the utilization of CO₂ as a resource to produce value added chemicals, such as formate/formic acid (HCOO⁻/HCOOH), carbon monoxide (CO), methane (CH₄), methanol (CH₃OH), ethylene (C₂H₄), and ethanol (C₂H₅OH) via an electrochemical (i.e., electroreduction) approach, that are currently manufactured on the industrial scale using carbon intensive fossil fuel methods.

The dominant design for state of the art electrochemical CO₂ conversion processes consists of a cathodic CO₂ reduction reaction (CO₂RR) coupled to an anodic oxygen evolution reaction (OER). The electrochemical system is characterized by a standard cell potential (E⁰_cell) that represents the minimum thermodynamic energy required to drive the reaction. Thermodynamic analysis of these two reactions shows that ˜90% of the overall energy (hence, cell potential) requirements comes from the OER. As a result, there is a need to think beyond OER and identify other oxidation reactions (with a lower thermodynamic energy barrier) that can lower E⁰_cell.

Lowering E⁰_cell represents one of the most important factors in making electrochemical conversion of CO₂ economically viable. Additionally, for a practical implementation of the process, it would be advantageous to drive the reaction using grid electricity (comprising mainly of fossil fuel resources) and remain carbon neutral or negative.

BRIEF SUMMARY

According to one embodiment, a method of electroreducing carbon dioxide comprises: feeding a first stream comprising carbon dioxide into a chamber through a chamber inlet, the chamber containing a gas diffusion cathode and a gas diffusion anode; feeding a second stream comprising glycerol or glucose into the chamber, the second stream having a pH of 12 to 14; and applying an electrical potential between the gas diffusion anode and the gas diffusion cathode to reduce the carbon dioxide and oxidize the glycerol or glucose.

According to another embodiment, a method of electroreducing carbon dioxide comprises: feeding a gas stream comprising carbon dioxide into chamber divided into a cathode compartment and an anode compartment by an ion permeable membrane, the anode compartment containing a gas diffusion anode and the cathode compartment containing a gas diffusion cathode, the gas diffusion cathode dividing the cathode compartment into a gaseous region and a liquid region, wherein the gas stream is fed into the gaseous region; feeding a catholyte stream into the liquid region of the cathode compartment; feeding an anolyte stream comprising glycerol into the anode compartment to contact the gas diffusion anode; and applying an electrical potential between the gas diffusion anode and the gas diffusion cathode to reduce the carbon dioxide to a reduction product and oxidize the glycerol to an oxidation product, wherein an onset cell potential for formation of the reduction product is from −1.5 V to −0.5 V.

The foregoing has outlined rather broadly the features and technical advantages of the present disclosure in order that the detailed description that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter that form the subject of the claims of this application. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other embodiments for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent embodiments do not depart from the spirit and scope of the disclosure as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of the invention is hereafter described with specific reference being made to the drawings in which:

FIG. 1 is a block diagram of a system according to an embodiment of the present disclosure;

FIG. 2 is a block diagram of a system according to an embodiment of the present disclosure;

FIG. 3 is a block diagram of a system according to an embodiment of the present disclosure;

FIG. 4A shows partial current density for CO (j_CO) as a function of the cell potential for the electroreduction of CO₂ to CO coupled to O₂ evolution, glycerol electrooxidation, or glucose electrooxidation at the anode;

FIG. 4B shows individual electrode potential as a function of the total current density (j_Total) for the electroreduction of CO₂ to CO coupled to O₂ evolution, glycerol electrooxidation, or glucose electrooxidation at the anode;

FIG. 5 shows variation in the cell potential and Faradaic efficiency for CO production as a function of time; and

FIGS. 6A-6C show partial current density for 6A, HCOO⁻ (j_HCOO—) 6B, C₂H₄ (j_C2H4) and 6C, C₂H₅OH (j_C2H5OH) as a function of the cell potential for the electroreduction of CO₂.

DETAILED DESCRIPTION

Various embodiments are described below. The relationship and functioning of the various elements of the embodiments may better be understood by reference to the following detailed description. The embodiments, however, are not limited to those illustrated below. In certain instances details may have been omitted that are not necessary for an understanding of embodiments disclosed herein.

The present disclosure provides a method of electrochemically converting CO₂, glycerol, and glucose to value added carbon chemical feedstocks. A feature of this method entails the use of glycerol or glucose electrooxidation as the anodic reaction instead of the traditionally used OER for CO₂ electroreduction systems. According to the method disclosed herein, the required cell potentials can be lowered, thereby reducing the electrical energy requirements by up to 53%. Some benefits may include (i) the process lowers electrical energy requirement in comparison to the state of the art CO₂ electroreduction systems leading to lower operating cost; (ii) the process is close to economic viability; (iii) the process could potentially be carbon neutral/negative even when using grid electricity to drive the process; and (iv) the process produces value added carbon chemical feedstocks.

The methods disclosed herein can be performed in electrochemical cells having different configurations. For example, FIGS. 1-3 depict embodiments of an electrochemical cell having different positions of the ion permeable membrane.

Referring to FIG. 1, a method of electroreducing carbon dioxide is provided. The method can include feeding a first stream 112 comprising carbon dioxide into a chamber 102 through a chamber inlet. The chamber 102 contains a gas diffusion cathode 103 and a gas diffusion anode 104. The method includes feeding a second stream 113 into the chamber 102. The second stream 113 comprises glycerol or glucose and has a pH of 12 to 14. The method includes applying an electrical potential between the gas diffusion anode 104 and the gas diffusion cathode 103 to reduce the carbon dioxide and oxidize the glycerol or glucose. The gas diffusion cathode 103 may divide the chamber 102 into a gaseous region 106 and a liquid region 107.

The electrochemical cell 100 can include a CO₂ source 101 from which the first stream 112 is fed into a chamber 102. A glycerol or glucose source 110 can be fed into the chamber 102. An electricity source 109 is connected to the gas diffusion cathode 103 and the gas diffusion anode 104 for applying an electrical potential between the electrodes to reduce the carbon dioxide and oxidize the glycerol or glucose.

Gaseous reduction products 110 can be withdrawn from an outlet of the compartment in a gaseous product stream 114 to be purifed. Liquid phase reduction products and oxidation products 111 can be withdrawn from the chamber 102 in a liquid product stream 115 to be purified.

The liquid phase products 111 include certain reduction products of CO₂ such as, for example, ethanol or formate and oxidation products of glycerol or glucose. Without an ion permeable membrane the liquid products of the reactions at the cathode and anode mix and are withdrawn together for purification.

The method is not limited to the cell configuration shown in FIG. 1. Referring to FIG. 2, an electrochemical cell 200 may include a chamber 102 divided into a cathode compartment 202 and an anode compartment 206 by an ion permeable membrane 201. The ion permeable membrane 201 allows the passage of electrons from one compartment to the other while preventing transport of certain molecules.

Gaseous reduction products 110, liquid phase reduction products 2033, and oxidation products 204 can be withdrawn from the chamber 102 and purified. Liquid reduction products 203 can be withdrawn from the liquid region 107 in a liquid reduction product stream 205.

In another embodiment, a method of electroreducing carbon dioxide is provided. The method can include feeding a gas stream comprising carbon dioxide into chamber divided into a cathode compartment and an anode compartment by an ion permeable membrane. The anode compartment contains a gas diffusion anode and the cathode compartment contains a gas diffusion cathode. The gas diffusion cathode divides the cathode compartment into a gaseous region and a liquid region, where the gas stream is fed into the gaseous region. The method includes feeding a catholyte stream into the liquid region of the cathode compartment; feeding an anolyte stream comprising glycerol into the anode compartment to contact the gas diffusion anode; and applying an electrical potential between the gas diffusion anode and the gas diffusion cathode to reduce the carbon dioxide to a reduction product and oxidize the glycerol to an oxidation product. The onset cell potential for formation of the reduction product can be from −1.5 V to −0.5 V.

Referring to FIG. 3, an electrochemical cell 300 may include a chamber 102 where an ion permeable membrane 301 may be in contact with the gas diffusion cathode 103. The ion permeable membrane 301 is positioned adjacent to and in contact with the gas diffusion cathode 103. Gaseous reduction products 110 and liquid phase products 302 can be withdrawn from the chamber 102 and purified.

The electrochemical cell of the present disclosure can be flow electrolyzer that is a modified version of the electrochemical cell disclosed in U.S. Pat. No. 7,635,530, which is incorporated by reference in its entirety. In some embodiments, the chamber is defined inside a flow electrolyzer.

The methods disclosed herein can include withdrawing a liquid product stream from an outlet of the chamber. The liquid product stream can include oxidation products of glycerol or glucose or reduction products of carbon dioxide. The method can include withdrawing a gaseous product stream that includes a reduction products of carbon dioxide. Gaseous reduction products of carbon dioxide can be selected from carbon monoxide, ethylene, methane, and any combination thereof. Table 1 shows major products of the reduction of CO₂ associated with metal catalysts.

TABLE 1
Cathode
Reaction	Catalyst	Major Product
CO2 electroreduction	Au, Ag, Zn, Pd	Carbon monoxide (gas)
	Sn, Pb, Hg, In	Formate (liquid)
	Cu	Carbon monoxide (gas),
		ethylene (gas), ethanol
		(liquid)

In some embodiments, the carbon dioxide is reduced to carbon monoxide. The partial current density for carbon monoxide can be anywhere from 15 mA cm⁻² to 350 mA cm⁻² at a cell potential of −1.0 V to −2.5 V. In some embodiments, the partial current density for carbon monoxide can be anywhere from 15 mA cm⁻² to 100 mA cm⁻² at a cell potential of −1.0 V to −1.5 V.

In some embodiments, the carbon dioxide is reduced to ethanol, formate, or any combination thereof.

In some embodiments, the carbon dioxide is reduced to a reduction product of carbon dioxide and an onset cell potential for formation of the reduction product of carbon dioxide is from −1.5 V to −0.5 V.

Table 2 shows major products of the oxidation of glycerol and glucose.

TABLE 2
Anode
	Reaction	Catalyst	Major Product
	Glycerol	Pt black	Glyceraldehyde (liquid),
	electrooxidation		formate (liquid), lactate
			(liquid)
	Glucose	Pt black	Gluconate (liquid)
	electrooxidation

In one embodiment, the method can include additional purification steps to produce substantially pure streams of the reduction and oxidation products. The method can include purifying the liquid product stream to obtain a substantially pure stream of the oxidation product of glycerol or glucose or the reduction product of carbon dioxide. The method can also include purifying the gaseous product stream to obtain a substantially pure stream of the reduction product of carbon dioxide.

The oxidation and reduction products can be purified using known techniques. For example, carbon monoxide and other gaseous products can be separated from the gaseous product stream using pressure swing adsorption. Ethanol can be separated from the liquid product stream using distillation.

The second stream, according to the disclosure, can be an electrolyte that contacts both the anode and the cathode when an ion permeable membrane is not present. When an ion permeable membrane is present that divides the chamber into an anode compartment and a cathode compartment, the second stream can include an anolyte stream and a catholyte stream. The anolyte stream is fed into the anode compartment and the catholyte stream is fed into the cathode compartment.

The second stream can include an alkali metal hydroxide or carbonate salts of Na⁺, K⁺, Rb⁺, or Cs⁺. The concentration of the alkali metal hydroxide or carbonate salt in the second stream can be from 0.5 M to 2.5 M. In some embodiments, the concentration of the alkali metal hydroxide or carbonate salt in the second stream is 2.0 M. Preferably, the second stream contains 2.0 M of potassium hydroxide.

The pH of the second stream can range from 12 to 14. For example, the pH can be from 12.5 to 14, 13 to 14, or greater than 12. In some embodiments, the pH of the catholyte and the anolyte can be from 12 to 14, 12.5 to 14, 13 to 14, or greater than 12.

Operating the electrochemical reactions at a pH above 12 enables greater selectivity and selectivity while also lowering overpotential for CO₂ reduction. The overpotential is the difference between the theoretical cell potential and the observed cell potential. In some embodiments, the overpotential for CO₂ reduction can be from −0.1 V to −1.0 V.

In some embodiments, the catholyte can include an alkali metal hydroxide salt of Na⁺, K⁺, Rb⁺, or Cs⁺. The catholyte can include 0.5 to 2.5 M of an alkali metal hydroxide salt. Preferably, the concentration of the alkali metal hydroxide salt in the catholyte is 2.0 M.

In some embodiments, the anolyte can include an alkali metal hydroxide or carbonate salt of Na⁺, K⁺, Rb⁺, or Cs⁺. The anolyte can include 0.5 to 2.5 M of an alkali metal hydroxide or carbonate salt. Preferably, the concentration of the alkali metal hydroxide salt in the anolyte is 2.0 M.

The second stream or anolyte stream can contain from 0.5 M to 3.0 M of glycerol or glucose. In some embodiments, the concentration of glycerol or glucose is 2.0 M.

The gas diffusion cathode, according to the present disclosure, can include an electrocatalytic cathode coating configured for reducing carbon dioxide. The electrocatalytic cathode coating can include a metal selected from silver, gold, zinc, palladium, tin, lead, mercury, indium, copper, and any combination thereof.

The gas diffusion anode, according to the present disclosure, can include an electrocatalytic anode coating configured for oxidizing glycerol. The electrocatalytic anode coating comprises a platinum black catalyst.

The methods electrochemically reducing CO₂ to platform chemicals such as carbon monoxide, formate, ethylene, and ethanol on a silver, gold, zinc, palladium, tin, lead, mercury, indium, or copper catalyst, while simultaneously performing an electrochemical oxidation of glycerol, glucose to value added products such as glyceraldehyde, formate, lactate, gluconate on a platinum black catalyst. The CO₂ electroreduction catalysts may be in a nanoparticle form. In a related embodiment of the method, the electroreduction of carbon monoxide to ethylene and ethanol on copper nanoparticle catalyst can be coupled to the electrochemical oxidation of glycerol or glucose to glyceraldehyde, formate, lactate, gluconate on a platinum black catalyst. All the electrochemical reactions can be carried out in a gas diffusion electrode based flow electrolyzer.

Rationale for Method

The electroreduction of CO₂ could become carbon neutral and/or negative even with grid electricity. The current share of low carbon renewables in the U.S. electricity grid is low (13%), and projected not to exceed 30% by 2040. Being able to drive CO₂ electroreduction using grid electricity instead of pure renewables and still be carbon neutral and/or negative could be an ideal scenario, as the process can be integrated into the existing infrastructure.

Thus, utilizing anode reactions with energy requirements lower than OER could be a strategy for radically lowering the energy requirements for CO₂ electroreduction. The anodic oxidation of glycerol, a cheap byproduct of industrial biodiesel and soap production, coupled to the cathodic reduction of CO₂ (i.e., co-electrolysis of CO₂ and glycerol) can lower the energy requirements compared to OER. Co-electrolysis of CO₂ and glycerol lowers the CO₂ electroreduction cell potential by about 0.85 V, resulting in a reduction in electricity consumption by up to 53%.

Glycerol can be obtained on an industrial scale as a waste byproduct either from the biodiesel industry or the soap industry. Due to the increase in the global biodiesel production, glycerol supply (2.4 MT in 2007) far exceeds demand (1 MT in 2007), and has resulted in a drop in the glycerol prices to values as low as $0.05 lb⁻¹. On the other hand, glucose can be obtained in large amounts as a renewable energy source from waste agricultural biomass or energy crops such as corn.

The electrooxidation of high volume building block chemicals such as glycerol, biomass derived glucose, or even CH₄ (large natural gas reserves, otherwise flared off-gas at oil fields), could satisfy the process design rules for suitable anode reactions. Table 3 shows the calculated ΔG⁰_reaction and |E⁰_cell| values for select combinations of CO₂ electroreduction with glycerol, glucose, and CH₄ electrooxidation. The values suggest that a significant lowering of |E⁰_cell| and hence, electricity requirements can be realized by moving away from the anodic OER.

In certain cases, the electroreduction of CO₂ to CH₃OH, C₂H₄, or C₂H₅OH on the cathode with the electrooxidation of glucose to gluconic acid on the anode the process becomes spontaneous (ΔG⁰_reaction<0), i.e., behaves like a fuel cell, and can thus in principle, be used for the simultaneous production of electricity and carbon chemicals.

The Gibb's free energy of reaction can be calculated using the following formula:

ΔG⁰_reaction=Σv_product*ΔG_fproduct⁰−Σv_reactant*ΔG_freactant⁰

where v=stoichiometric coefficient and ΔG⁰_f=Gibb's free energy of formation. |E⁰_cell|=|−ΔG⁰/z*F| where z=number of electrons transferred and F=Faraday's constant=96485 C mol⁻¹. All thermodynamic properties are reported under standard conditions (1 bar and 298 K).

TABLE 3
		ΔG0reaction	|E0cell|
Cathode reaction	Possible anode reactions	[kJ mol−1]	[V]
Carbon dioxide →	Water → Oxygen	257.20	1.33
Carbon monoxide	2OH− → H2O + 0.5O2 + 2e−
CO2 + H2O + 2e− → CO +	Overall: CO2 → CO + 0.5O2
2OH−	Glycerol → Glyceraldehyde	97.48	0.51
	C3H8O3 + 2OH− → C3H6O3 + 2H2O + 2e−
	CO2 + C3H8O3 → CO + C3H6O3 + H2O
	Glycerol → Lactic acid	68.08	0.35
	C3H8O3 + 2OH− → C3H6O3 + 2H2O + 2e−
	Overall: CO2 + C3H8O3 → CO + C3H6O3 + H2O
	Glycerol → Formic acid	46.53	0.24
	C3H8O3 + 8OH− → 3HCOOH + 5H2O + 8e−
	Overall: CO2 + 0.25C3H8O3 → CO + 0.75HCOOH +
	0.25H2O
	Glucose → Gluconic acid	6.20	0.03
	C6H12O6 + 2OH− → C6H12O7 + H2O + 2e−
	CO2 + C6H12O6 → CO + C6H12O7
	Methane → Methanol	141.10	0.73
	CH4 + 2OH− → CH3OH + H2O + 2e−
	Overall: CO2 + CH4 → CO + CH3OH
	Methane → Carbon monoxide	52.68	0.36
	CH4 + 6OH− → 5H2O + CO + 6e−
	Overall: 0.75CO2 + 0.25CH4 → CO + 0.5H2O
Carbon dioxide →	Water → Oxygen	1331.40	1.15
Ethylene	2OH− → H2O + 0.5O2 + 2e−
2CO2 + 8H2O + 12e− →	Overall: 2CO2 + 2H2O → C2H4 + 3O2
C2H4 + 12OH−	Glycerol → Glyceraldehyde	373.08	0.32
	C3H8O3 + 2OH− → C3H6O3 + 2H2O + 2e−
	Overall: 2CO2 + 6C3H8O3 → C2H4 + 6C3H6O3 +
	4H2O
	Glycerol → Lactic acid	196.68	0.17
	C3H8O3 + 2OH− → C3H6O3 + 2H2O + 2e−
	Overall: 2CO2 + 6C3H8O3 → C2H4 + 6C3H6O3 +
	4H2O
	Glycerol → Formic acid	67.35	0.06
	C3H8O3 + 8OH− → 3HCOOH + 5H2O + 8e−
	Overall: 2CO2 + 1.5C3H8O3 + 0.5H2O → C2H4 +
	4.5HCOOH
	Glucose → Gluconic acid	−174.60	0.15
	C6H12O6 + 2OH− → C6H12O7 + H2O + 2e−
	Overall: 2CO2 + 6C6H12O6 + 2H2O → C2H4 +
	6C6H12O7
	Methane → Methanol	634.80	0.55
	CH4 + 2OH− → CH3OH + H2O + 2e−
	Overall: 2CO2 + 6CH4 + 2H2O → C2H4 + 6CH3OH
	Methane → Carbon monoxide	209.60	0.18
	CH4 + 6OH− → 5H2O + CO + 6e−
	Overall: 2CO2 + 2CH4 → C2H4 + 2CO + 2H2O

EXAMPLES

Example 1. Electrochemical Performance for the Electroreduction of CO₂

Unless stated otherwise, all experiments were performed under ambient conditions of 1 atm and 293 K, all commercially available materials were used as received, and >18.0 MO cm deionized (DI) water was used when required.

The electrolytes used herein were prepared by dissolving the appropriate amount of the salt and/or chemical in DI water. The salts and chemicals used were: potassium hydroxide (Fisher Chemical, product number: P250), glycerol (Alfa Aesar, product number: 38988), D-(+)-glucose (Sigma Life Science, product number: 49139). The pH and conductivity of the different electrolytes were measured using an Orion 4-star pH conductivity meter.

The electrochemical characterization of the different combinations of CO₂ electroreduction at the cathode with the O₂ evolution reaction and glycerol, glucose, or CH₄ electrooxidation at the anode was performed in a gas diffusion electrode based dual electrolyte channel flow electrolyzer with a precisely machined active geometric area of 1 cm², as described previously. The catholyte and the anolyte chamber was separated by a Fumapem FAA-3-PK-75 anion exchange membrane to prevent crossover of the liquid products from the cathode to the anode and vice versa. The catholyte for all experiments was 2.0 M KOH. The anolyte for studying the OER and CH₄ electrooxidation was 2.0 M KOH whereas the anolyte for studying the electrooxidation of glycerol and glucose was 2.0 M KOH+2.0 M glycerol and 2.0 M KOH+2.0 M glucose, respectively. Electrochemical experiments were performed by maintaining a constant cell potential using a potentiostat (Autolab PGSTAT-30, EcoChemie). The individual cathode and anode potentials were measured with a multimeter (AMPROBE 15XP-B) connected between the appropriate electrode and an Ag/AgCl reference electrode (3 mol kg⁻¹, RE-5B BASi). The individual electrode potentials (vs. Ag/AgCl) were then converted to the RHE scale using the Nernst equation: E_RHE=E_Ag/AgCl+0.210+0.058×pH. All cell, cathode, and anode potentials are reported as measured without any iR corrections. The CO₂ (Airgas) feed for the reaction was provided as a continuous stream over the teflonized side of the cathode gas diffusion layer (GDL) using a flow controller (Smart Trak 2, Sierra Instruments). A CO₂ flow rate of 17 sccm was maintained for cell potentials at which the total current density (j_Total) was >5 mA cm⁻² and lowered to 5 sccm for cell potentials at which j_Total was <5 mA cm⁻², to enable a gas product analysis with high sensitivity. A pressure controller (Cole Parmer, 00268TC) was used in the electrolyzer downstream to maintain a low pressure of 14.20 psi and thus facilitate an easy transfer of the gas products from the cathode GDL to the effluent gas stream. A low downstream pressure also minimized the dissolution of the reacting CO₂ and the gas products into the electrolyte stream. Both the catholyte and the anolyte stream was circulated through the electrolyte channels of the electrolyzer using a syringe pump (PHD 2000, Harvard Apparatus) at flow rate of 0.5 mL min⁻¹ for cell potentials at which j_Total was >5 mA cm⁻² and lowered to 0.2 mL min⁻¹ for cell potentials at which j_Total was <5 mA cm⁻², to enable a liquid product analysis with high sensitivity. For all electrochemical experiments, after a particular cell potential was switched on, the resulting current was allowed to stabilize for at least 180 seconds before the product analysis was initiated.

For a particular cell potential, the gas products of CO₂ electroreduction were analyzed for a total time period of 180 seconds by diverting 1 mL of the effluent gas stream, thrice, at regular intervals of 90 seconds to an on-line gas chromatograph (Thermo Finnigan Trace GC with a Carboxen 1000 column from Supelco). The GC was equipped with both the thermal conductivity detector (TCD) and the flame ionization detector (FID). Helium with a flow rate of 20 sccm was used as the carrier gas. The concentration of the gas products was quantified by averaging the peak areas over the three sample injections and using the appropriate calibration curves. Meanwhile, the liquid products were analyzed for the same 180 second time period by collecting both the catholyte and the anolyte streams followed by ex situ ¹H NMR (UI500NB, Varian) analysis (16 scans with solvent suppression). The liquid samples for the ¹H NMR analysis were prepared by mixing 100 μL of the collected electrolyte with 400 μL of D₂O (Aldrich, product number: 151882) and 100 μL of an internal standard comprising of 1.25 mM DMSO in D₂O. The concentration of the liquid products was quantified using the appropriate calibration curves. The total current density (=the total current as the electrolyzer area is 1 cm²) was quantified by averaging the data obtained during the same 180 second time period when the CO₂ electroreduction products were being analyzed. The Faradaic efficiency for the different CO₂ electroreduction products was calculated per the following equation:

$\mathrm{FE}\left(\%\right)=\frac{\mathrm{znF}}{Q}\times 100$

where z is the number of electrons exchanged to form a particular CO₂ electroreduction product, n is the number of moles of the product formed, F is the Faraday's constant (96485 C mol⁻¹), and Q is the amount of charge passed. The partial current density for a particular product was calculated by multiplying j_Total with the Faradaic efficiency for that product. The onset cell potential for a specific CO₂ electroreduction product defined in this work refers to the lowest (least negative) cell potential at which the product is first observed in the GC (for gas products) or ¹H NMR analysis (for liquid products).

The cathode was a 1±0.1 mg cm⁻² Ag nanoparticle coated gas diffusion layer (GDL) electrode. The anode was a 1±0.1 mg cm⁻² IrO² coated GDL electrode for O₂ evolution, and a 1±0.1 mg cm⁻² Pt black coated GDL electrode for glycerol and glucose electrooxidation. All data collected under ambient conditions of 1 atm and 293 K.

As indicated by the Gibb's free energy analysis, many different anode reactions other than OER can be utilized to lower |E⁰_cell|, and hence the overall electricity requirements for CO₂ electroreduction. To assess the practicality of such processes, we performed an experimental electroanalytical evaluation of the different combinations proposed in Table 3, using a gas diffusion layer (GDL) electrode based dual electrolyte channel flow electrolyzer under ambient conditions. The catholyte was chosen as 2.0 M KOH, previously demonstrated by us to lower overpotentials and improve activity for CO₂ electroreduction. The anolyte was chosen as a mixture of 2.0 M KOH and 2.0 M glycerol, a mixture of 2.0 M KOH and 2.0 M glucose, and 2.0 M KOH for the electrooxidation of glycerol, glucose, and CH₄ respectively.

The electrooxidation of glycerol or glucose on a Pt black coated GDL anode coupled to the electroreduction of CO₂ on a Ag coated GDL cathode resulted in a significant lowering (i.e., less negative value) of the onset cell potential for CO formation, with a value of −0.75 and −0.95 V being observed, respectively, in comparison to the state of the art value of −1.6 V with OER at the anode (FIG. 4A). However, the activity (partial current density for CO, j_CO) with glucose electrooxidation (j_CO=12.47 mA cm⁻² or production rate=0.065 kg_CO m⁻² h⁻¹ at a cell potential of −1.5 V) was much lower than with glycerol electrooxidation (j_CO=88.44 mA cm⁻² or production rate=0.462 kg_CO m⁻² h⁻¹ at a cell potential of −1.5 V) at the anode. These results indicate that the electroreduction of CO₂ to CO could indeed become carbon neutral and/or negative even when using the present-day grid electricity mix to drive the process. Depending on the j_CO value, the electrooxidation of glycerol at the anode instead of OER results in a 37 to 53% reduction in electricity requirements, thus improving the process economics. Single electrode plot suggests the major improvement to be at the anode with the cathodic CO₂ electroreduction remaining unaffected (FIG. 4B). The anodic glycerol electrooxidation results in the formation of value-added chemicals such as formate and lactate that further improves the economics of the overall process. Further, we also evaluated the durability of CO₂-glycerol co-electrolysis with respect to CO production (FIG. 5). The results indicate the cell potential and FE_CO to be fairly stable over a 1.5 h time period. However, flooding of the electrolyte through the cathode GDL was observed at −1.5 h (similar to earlier observations in the literature) indicating the need to develop more durable GDLs to improve the prospects of this process.

A similar lowering in onset cell potentials for the electroreduction of CO₂ to HCOO⁻, C₂H₄, and C₂H₅OH was observed when utilizing the electrooxidation of glycerol at the anode instead of OER (FIG. 6A). For example, the onset cell potential for the electroreduction of CO₂ to HCOO⁻ on a Sn coated GDL cathode and C₂H₄, C₂H₅OH on a Cu coated GDL cathode was −0.9, −0.95, and −1.3 V, respectively, with the anodic electrooxidation of glycerol, in comparison to −1.75, −1.8, and −2.1 V with the anodic OER (FIG. 6B and FIG. 6C). Preliminary experiments with the electrooxidation of CH₄ on a Pt black, Cu, Pd, IrO₂, and Pt—Ru black coated GDL anode coupled to the electroreduction of CO₂ on a Ag coated GDL cathode did not result in a change in the onset cell potentials for CO production, in comparison to OER at the anode. This is of course expected due to the high dissociation enthalpy of the C—H bond in CH₄ (435 kJ mol⁻¹). For these experiments, the anode was a 1±0.1 mg cm⁻² IrO₂ coated GDL electrode for 02 evolution and 1±0.1 mg cm⁻² Pt black coated GDL electrode for glycerol electrooxidation. The catholyte included 2.0 M KOH. The anolyte included 2.0 M KOH for 02 evolution, and 2.0 M KOH+2.0 M glycerol for glycerol electrooxidation. All data collected under ambient conditions of 1 atm and 293 K.

In summary, we have shown that the prospects of CO₂ electroreduction, in terms of both cradle-to-gate CO₂ emissions and economics can be drastically improved by looking beyond the conventionally used OER at the anode, which essentially acts as an energy sink. The indicate that several different anodic reactions are available to replace the OER, thereby yielding superior thermodynamic processes with a lower |E⁰_cell|. Of the alternatives, the electrooxidation of glycerol (a cheap industrial waste) seems particularly promising with the resulting process (co-electrolysis of CO₂ and glycerol) lowering the electricity requirements for conventional CO₂ electroreduction approaches by up to 53%. The new process offers avenues for integrating two different CO₂ mitigation approaches i.e., CO₂ electroreduction and biodiesel production as well. Furthermore, with the future development of more active and selective catalysts (particularly for glycerol electrooxidation), co-electrolysis of CO₂ and glycerol can be improved even further, resulting in low energy pathways for the production of carbon chemicals from waste CO₂.

All of the methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While this invention may be embodied in many different forms, there are described in detail herein specific preferred embodiments of the invention. The present disclosure is an exemplification of the principles of the invention and is not intended to limit the invention to the particular embodiments illustrated. In addition, unless expressly stated to the contrary, use of the term “a” is intended to include “at least one” or “one or more.” For example, “a device” is intended to include “at least one device” or “one or more devices.”

Any ranges given either in absolute terms or in approximate terms are intended to encompass both, and any definitions used herein are intended to be clarifying and not limiting. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encompass any and all subranges (including all fractional and whole values) subsumed therein.

The invention encompasses any and all possible combinations of some or all of the various embodiments described herein. It should also be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the invention and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.

Furthermore, the advantages described above are not necessarily the only advantages of the invention, and it is not necessarily expected that all of the described advantages will be achieved with every embodiment of the invention.

Claims

1. A method of electroreducing carbon dioxide, comprising:

feeding a first stream comprising carbon dioxide into a chamber through a chamber inlet, the chamber containing a gas diffusion cathode and a gas diffusion anode;

feeding a second stream comprising glycerol or glucose into the chamber, the second stream having a pH of 12 to 14; and

applying an electrical potential between the gas diffusion anode and the gas diffusion cathode to reduce the carbon dioxide and oxidize the glycerol or glucose.

2. The method of claim 1, further comprising withdrawing a liquid product stream from an outlet of the chamber, the liquid product stream comprising an oxidation product of glycerol or glucose or a reduction product of carbon dioxide and withdrawing a gaseous product stream comprising a reduction product of carbon dioxide selected from carbon monoxide, ethylene, methane, and any combination thereof.

3. The method of claim 2, further comprising:

a) purifying the liquid product stream to obtain a substantially pure stream of the oxidation product of glycerol or glucose or the reduction product of carbon dioxide; or

b) purifying the gaseous product stream to obtain a substantially pure stream of the reduction product of carbon dioxide.

4. The method of claim 1, wherein the gas diffusion cathode divides the chamber into a gaseous region and a liquid region.

5. The method of claim 1, wherein the second stream comprises from 0.5 M to 2.5 M of an alkali metal hydroxide.

6. The method of claim 1, wherein the carbon dioxide is reduced to carbon monoxide, and wherein a partial current density for carbon monoxide is from 15 mA cm−2 to 350 mA cm−2 at a cell potential of −1.0 V to −2.5 V.

7. The method of claim 1, wherein the carbon dioxide is reduced to a reduction product of carbon dioxide and an onset cell potential for formation of the reduction product of carbon dioxide is from −1.5 V to −0.5 V.

8. The method of claim 1, wherein the chamber is divided into a cathode compartment and an anode compartment by an ion permeable membrane.

9. The method of claim 8, wherein the second stream comprises an anolyte stream and a catholyte stream, wherein the anolyte stream is fed into the anode compartment and the catholyte stream is fed into the cathode compartment.

10. The method of claim 9, wherein the anolyte stream comprises an alkali metal hydroxide or an alkali metal carbonate.

11. The method of claim 1, wherein the gas diffusion cathode comprises an electrocatalytic cathode coating configured for reducing carbon dioxide.

12. The method of claim 11, wherein the electrocatalytic cathode coating comprises a metal selected from silver, gold, zinc, palladium, tin, lead, mercury, indium, copper, and any combination thereof.

13. The method of claim 1, wherein the gas diffusion anode comprises an electrocatalytic anode coating configured for oxidizing glycerol.

14. The method of claim 13, wherein the electrocatalytic anode coating comprises a platinum black catalyst.

15. The method of claim 1, wherein the chamber further comprises an ion permeable membrane in contact with the gas diffusion cathode.

16. The method of claim 1, further comprising reducing the carbon dioxide to ethanol, formate, or any combination thereof.

17. The method of claim 1, wherein the chamber is defined inside a flow electrolyzer.

18. The method of claim 1, wherein the pH of the second stream is 12.5 to 14.

19. A method of electroreducing carbon dioxide, comprising:

feeding a gas stream comprising carbon dioxide into chamber divided into a cathode compartment and an anode compartment by an ion permeable membrane, the anode compartment containing a gas diffusion anode and the cathode compartment containing a gas diffusion cathode, the gas diffusion cathode dividing the cathode compartment into a gaseous region and a liquid region, wherein the gas stream is fed into the gaseous region;

feeding a catholyte stream into the liquid region of the cathode compartment;

feeding an anolyte stream comprising glycerol into the anode compartment to contact the gas diffusion anode; and

applying an electrical potential between the gas diffusion anode and the gas diffusion cathode to reduce the carbon dioxide to a reduction product and oxidize the glycerol to an oxidation product,

wherein an onset cell potential for formation of the reduction product is from −1.5 V to −0.5 V.

20. The method of claim 19, wherein the catholyte stream or the anolyte stream has a pH from 13 to 14.