CARBON DIOXIDE/CARBON MONOXIDE ELECTROREDUCTION SYSTEM WITH GAS-LIQUID RECIRCULATION

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 63/500,289, filed May 5, 2023, the contents of which are incorporated herein by reference in their entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under award number DE-EE0008501 awarded by the U.S. Department of Energy and award numbers CBET-1930013 and CBET-1803482 awarded by the National Science Foundation. The government has certain rights in the invention.

BACKGROUND

Carbon capture, utilization, and storage (CCUS) on a global scale can play a critical role in meeting the Paris Agreement target of warming well below 2° C. above pre-industrial levels, which may require an estimated carbon emission reduction of 14-20% by 2050. The carbon utilization process in CCUS, instead of storing carbon permanently, is re-using the captured carbon in industrial processes by converting it into, for example, plastics, concrete, or biofuel [Burkart, 2019; Rashidi, 2016; Ravikumar, 2021]. This strategy of alternative production can provide carbon neutrality to several difficult-to-decarbonize industry sectors including, but not limited to, cement production and commercial aviation.

Among carbon utilization technologies, renewable-electricity-driven electrochemical carbon dioxide (CO₂) reduction process are known to be promising for its diverse product list. Various chemicals, which are presently produced from fossil fuel or fossil fuel-derived feedstocks, are able to be produced directly from CO₂ including, but not limited to, syngas, olefins, alkyl alcohols, and oxygenates [Jouny, 2019; Raciti, 2018]. Global attention is presently focused on the displacement of petrochemical processes with renewably powered electrochemical processes using CO₂ [De Luna, 2019; Jordaan, 2021].

Sodium formate/formic acid are widely used in the textile, leather, and pharmaceutical industries. Beyond use in these established applications, the biosynthesis of chemicals and biofuels from these organic C-1 feedstocks through the formate metabolic pathway is an emerging and promising application for sustainable bioproduction [Ruttinger, 2022; Nguyen, 2016; Xiong, 2021]. Carbon-neutral and carbon-negative production of sodium formate/formic acid is important to the sustainability of the entire process. As early as 1985, it was reported that several metals and metal oxides predominantly favored the CO₂-to-formate pathway in electrochemical CO₂ conversion [Hori, 1985]. Rational-designed catalysts with higher activity and selectivity are reported in successive studies [Zhang, 2014; Jhong, 2021]. For example, Zhang et al. synthesized a nanostructured tin oxide nanoparticle with an average diameter of 5 nm using hydrothermal processes [Zhang, 2014]. The SnO₂ nanoparticles exhibited activity with a low overpotential of 340 mV and a dominant selectivity of >93% towards formate in 0.1 mol/L NaHCO₃ aqueous solutions. Over 10 mA/cm² current density was achieved with high surface area graphene supports. Advantageously, electrochemical systems for this reaction can use gas-fed electrolyzers with gas diffusion electrode and multifold higher yield, instead of the traditional H-type electrolyzers [Shen, 2019; Grigioni, 2020; Liu, 2020; Löwe, 2021]. Compared to the other products, the formation of formate (CO₂+H₂O+2e→HCOO+OH) has relatively fast kinetics due to the rather simple two-electron transfer process, and moreover, only releases one hydroxide anion per molecule of formate product, alleviating carbon loss due to the undesirable side reaction between CO₂ and the electrochemically generated OH on the gas diffusion electrode (GDE) [Rabinowitz, 2020].

Disadvantageously, the diverse product list from the electrochemical CO₂ reduction reaction has been a concern for the practical application of this technology. For example, in a widely-used flow-cell electrolyzer system, neither the gas-phase nor the liquid-phase products are produced as pure chemicals. With regards to the gas phase, the CO₂ cannot be 100% converted into desired products and the water will inevitably be reduced into hydrogen [Xie, 2022]. Without a separation process, such as membrane separation and pressure swing adsorption, the product cannot be removed and the CO₂ feedstock will be degraded by gaseous products and hydrogen byproducts. Moreover, the carbon conversion efficiency is limited by the left-over CO₂ and carbon-containing byproducts. With regards to the liquid phase, the product will be immediately dissolved into aqueous electrolyte [Gabardo, 2019] and the concentration is limited due to the membrane crossover [Ma, 2020; Zhang, 2020]. Notably, these previous works focused on highly selective catalysts as well as high single pass conversion and although a significant increase in product concentrations were observed, the accumulated gaseous byproducts such as hydrogen and carbon monoxide were still unremovable and required an independent separation process.

There continues to be a need for a system and process that can electrochemically convert carbon dioxide into value-added chemicals, e.g., formates, wherein no gas separation processes are needed.

SUMMARY

In some aspects, a system to convert carbon dioxide to target product with an anodic oxidation of water is described, said system comprising:

• • a flow-cell electrolyzer;
  • an anolyte recirculation system in fluid communication with the flow-cell electrolyzer;
  • a catholyte recirculation system in fluid communication with the flow-cell electrolyzer; and
  • a gas recirculation system in fluid communication with the flow-cell electrolyzer,
  • wherein the flow-cell electrolyzer comprises three compartments, wherein a first compartment and a second compartment are separated by a gas diffusion electrode (GDE) coated with cathodic electrocatalyst material and the second compartment and a third compartment are separated by an anion exchange membrane (AEM), wherein the first compartment is in fluid communication with the gas recirculation system, the second compartment is in fluid communication with the catholye recirculation system, and the third compartment comprises an anode and is in fluid communication with the anolyte recirculation system;
  • wherein carbon dioxide-containing gas in the gas recirculation system is directed to the first compartment for an electrochemical reaction at the GDE between carbon dioxide in the gas and catholyte in the second compartment, wherein a post-electrochemical reaction mixture of gas exits the first compartment into the gas recirculation system, is quantitatively compensated with additional carbon dioxide, and is reintroduced to the first compartment following carbon dioxide concentration adjustment;
  • wherein the catholyte recirculation system directs catholyte comprising catholyte electrolyte solution to the second compartment and electrolyte ions are reduced to target product at the GDE during the electrochemical reaction, wherein a target product-containing solution is removed from the second compartment; and
  • wherein water is produced at the anode in the third compartment during the electrochemical reaction.

In other aspects, a system to convert carbon dioxide to target product with an anodic oxidation of hydrogen gas and carbon monoxide gas is described, said system comprising:

• • a flow-cell electrolyzer;
  • a gas-liquid separator in fluid communication with the flow-cell electrolyzer;
  • an anolyte recirculation system in fluid communication with the flow-cell electrolyzer;
  • a catholyte recirculation system in fluid communication with the flow-cell electrolyzer; and
  • a gas recirculation system in fluid communication with the flow-cell electrolyzer,
  • wherein the flow-cell electrolyzer comprises three compartments, wherein a first compartment and a second compartment are separated by a gas diffusion electrode (GDE) coated with cathodic electrocatalyst material and the second compartment and a third compartment are separated by an anion exchange membrane (AEM), wherein the first compartment is in fluid communication with the gas recirculation system and the third compartment via the gas-liquid separator, the second compartment is in fluid communication with a catholyte recirculation system, and the third compartment comprises an anode and is in fluid communication with an anolyte recirculation system and the gas recirculation system;
  • wherein carbon dioxide-containing gas in the gas recirculation system is directed to the third compartment with the anolyte to oxidize, at the anode, any hydrogen gas and/or carbon monoxide gas present in the carbon dioxide-containing gas, wherein carbon dioxide-containing gas having a reduced concentration of hydrogen and carbon monoxide is separated from the anolyte using the gas-liquid separator and is directed to the first compartment for an electrochemical reaction at the GDE, wherein a post-electrochemical reaction mixture of gas exits the first compartment into the gas recirculation system, is quantitatively compensated with additional carbon dioxide, and is reintroduced to the third compartment following carbon dioxide concentration adjustment;
  • wherein the catholyte recirculation system directs catholyte comprising catholyte electrolyte solution to the second compartment and electrolyte ions are reduced to target product at the GDE during the electrochemical reaction, wherein a target product-containing solution is removed from the second compartment; and
  • wherein water is produced at the anode in the third compartment during the electrochemical reaction.

In another aspect, a process to convert carbon dioxide to target product with an anodic oxidation of water is described, said process comprising:

• • directing carbon dioxide-containing gas to a flow-cell electrolyzer comprising three compartments, wherein a first compartment and a second compartment are separated by a gas diffusion electrode (GDE) coated with cathodic electrocatalyst material and the second compartment and a third compartment are separated by an anion exchange membrane (AEM), wherein the first compartment is in fluid communication with a gas recirculation system comprising a carbon dioxide-containing gas, the second compartment is in fluid communication with a catholyte recirculation system comprising a catholyte, and the third compartment comprises an anode and is in fluid communication with an anolyte recirculation system comprising an anolyte;
  • applying a voltage across the GDE and the anode of the flow-cell electrolyzer, wherein carbon dioxide in the gas that is in contact with the GDE in the first compartment electrochemically reacts and produces target product in the catholyte, and water is produced at the anode in the anolyte;
  • removing a post-electrochemical reaction mixture of gas from the first compartment and passing same through the gas recirculation system, wherein the post-electrochemical reaction mixture of gas is quantitatively compensated with additional carbon dioxide, and is reintroduced to the first compartment following carbon dioxide concentration adjustment;
  • removing a target product-containing solution from the second compartment.

In still other aspects, a process to convert carbon dioxide to target product with an anodic oxidation of hydrogen gas and carbon monoxide gas is described, said system comprising:

• • directing carbon dioxide-containing gas to a flow-cell electrolyzer comprising three compartments, wherein a first compartment and a second compartment are separated by a gas diffusion electrode (GDE) coated with cathodic electrocatalyst material and the second compartment and a third compartment are separated by an anion exchange membrane (AEM), wherein the first compartment is in fluid communication with the gas recirculation system comprising a carbon-dioxide containing gas and the third compartment via a gas-liquid separator, the second compartment is in fluid communication with a catholyte recirculation system comprising a catholyte, and the third compartment comprises an anode and is in fluid communication with an anolyte recirculation system comprising an anolyte and the gas recirculation system;
  • oxidizing, at the anode, any hydrogen gas and/or carbon monoxide gas present in the carbon dioxide-containing gas;
  • separating the carbon dioxide-containing gas having a reduced concentration of hydrogen and carbon monoxide from the anolyte using the gas-liquid separator;
  • directing the carbon dioxide-containing gas having a reduced concentration of hydrogen and carbon monoxide to the first compartment for an electrochemical reaction at the GDE to produce target product in the catholyte;
  • removing a post-electrochemical reaction mixture of gas from the first compartment and passing same through the recirculation system for reintroduction to the first compartment, further comprising quantitatively compensating the post-electrochemical reaction mixture of gas with additional carbon dioxide prior to reintroduction;
  • removing a target product-containing solution from the second compartment.

In yet another aspect, a continuous gas-to-liquid conversion process to convert carbon dioxide to target product without requiring any separation processes to remove unreacted reactants or unwanted byproducts is described, said process comprising:

• • (a) producing target product from carbon dioxide at a cathode with anodic oxidation of water for a time T1 by:
    • directing carbon dioxide-containing gas to a flow-cell electrolyzer comprising three compartments, wherein a first compartment and a second compartment are separated by a gas diffusion electrode (GDE) coated with cathodic electrocatalyst material and the second compartment and a third compartment are separated by an anion exchange membrane (AEM), wherein the first compartment is in fluid communication with the gas recirculation system comprising a carbon-dioxide containing gas and the third compartment via a gas-liquid separator, the second compartment is in fluid communication with a catholyte recirculation system comprising a catholyte, and the third compartment comprises an anode and is in fluid communication with an anolyte recirculation system comprising an anolyte and the gas recirculation system;
    • adjusting valves so that carbon dioxide-containing gas is introduced to the first compartment for contact with the GDE;
    • applying a voltage across the GDE and the anode of the flow-cell electrolyzer, wherein carbon dioxide in the gas that is in contact with the GDE in the first compartment electrochemically reacts and produces target product in the catholyte, and water is produced at the anode in the anolyte;
    • removing a post-electrochemical reaction mixture of gas from the first compartment and passing same through the gas recirculation system, wherein the post-electrochemical reaction mixture of gas is quantitatively compensated with additional carbon dioxide, and is reintroduced to the first compartment following carbon dioxide concentration adjustment;
    • removing a target product-containing solution from the second compartment;
  • (b) switching valves and reducing the anodic potential to produce target product from carbon dioxide at a cathode with anodic oxidation of hydrogen and carbon monoxide for a time T2 by:
    • adjusting valves so that carbon dioxide-containing gas is introduced to the third compartment for contact with the anode;
    • oxidizing, at the anode, any hydrogen gas and/or carbon monoxide gas present in the carbon dioxide-containing gas;
    • separating the carbon dioxide-containing gas having a reduced concentration of hydrogen and carbon monoxide from the anolyte using the gas-liquid separator;
    • directing the carbon dioxide-containing gas having a reduced concentration of hydrogen and carbon monoxide to the first compartment for an electrochemical reaction at the GDE to produce target product in the catholyte;
    • removing a post-electrochemical reaction mixture of gas from the first compartment and passing same through the recirculation system for reintroduction to the first compartment, further comprising quantitatively compensating the post-electrochemical reaction mixture of gas with additional carbon dioxide prior to reintroduction;
    • removing a target product-containing solution from the second compartment; and
  • (c) optionally repeating (a), optionally repeating (b) after (a).

Other aspects, features and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A. Summary of the Faradaic Efficiencies (FEs) and geometric current densities measured for the electroreduction of CO₂ on commercial SnO₂.

FIG. 1B. Summary of the FEs and geometric current densities measured for the electroreduction of CO₂ on commercial Bi.

FIG. 1C. FEs towards sodium formate and hydrogen on commercial SnO₂ and commercial Bi catalysts at different CO₂ concentrations.

FIG. 1D. Single-pass carbon conversion efficiency on SnO₂ and Bi catalysts at different potentials.

FIG. 2A. Current density and formate Faradaic efficiency of the operation with cathodic CO₂ reduction and anodic water oxidation (Cell voltage=2.65 V).

FIG. 2B. The quantitative mass flow of CO₂, H₂, CO and sodium formate.

FIG. 2C. Gas composition in volume ratio of CO₂, H₂ and CO.

FIG. 2D. Carbon conversion efficiency of each sampling period.

FIG. 3A. Current density and formate Faradaic efficiency of the operation with cathodic CO₂ reduction and anodic H₂/CO oxidation (Cell voltage=1.94 V).

FIG. 3B. The quantitative mass flow of CO₂, H₂, CO and sodium formate.

FIG. 3C. Gas composition in volume ratio of CO₂, H₂ and CO.

FIG. 3D. Carbon conversion efficiency of each sampling period.

FIG. 4A. Current densities of the electrolyzer at different operation conditions in the continuous operation.

FIG. 4B. Gas composition in volume ratio of CO₂, H₂ and CO at the end of different operation periods.

FIG. 4C. Faradaic efficiency towards different products during each operation period.

FIG. 4D. Carbon conversion efficiency of each operation period.

FIG. 5. System configuration for the operation with cathodic CO₂ reduction and anodic water oxidation.

FIG. 6. System configuration for the operation with cathodic CO₂ reduction and anodic hydrogen/carbon monoxide oxidation.

DETAILED DESCRIPTION OF THE DISCLOSURE

Although the claimed subject matter will be described in terms of certain embodiments, other embodiments, including embodiments that do not provide all of the benefits and features set forth herein, are within the scope of this disclosure as well. Various structural and parameter changes may be made without departing from the scope of this disclosure.

Definitions

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. In case of conflict, the present document, including definitions, will control. Preferred methods and materials are described below, although methods and materials similar or equivalent to those described herein can be used in practice or testing of the present disclosure. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. The materials, methods, and examples disclosed herein are illustrative only and not intended to be limiting.

“About” and “approximately” are used to provide flexibility to a numerical range endpoint by providing that a given value may be “slightly above” or “slightly below” the endpoint without affecting the desired result, for example, +/−5%.

The phrase “in one embodiment” or “in some embodiments” as used herein does not necessarily refer to the same embodiment, though it may. Furthermore, the phrase “in another embodiment” as used herein does not necessarily refer to a different embodiment, although it may. Thus, as described below, various embodiments of the invention may be readily combined, without departing from the scope or spirit of the invention.

The terms “comprise(s),” “include(s),” “having,” “has,” “can,” “contain(s),” and variants thereof, as used herein, are intended to be open-ended transitional phrases, terms, or words that do not preclude the possibility of additional acts or structures. The singular forms “a,” “and” and “the” include plural references unless the context clearly dictates otherwise. The present disclosure also contemplates other embodiments “comprising,” “consisting of” and “consisting essentially of,” the embodiments or elements presented herein, whether explicitly set forth or not.

For the recitation of numeric ranges herein, each intervening number there between with the same degree of precision is explicitly contemplated. For example, for the range of 6-9, the numbers 7 and 8 are contemplated in addition to 6 and 9, and for the range 6.0-7.0, the number 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, and 7.0 are explicitly contemplated.

As used herein, a “system” refers to a plurality of real and/or abstract elements operating together for a common purpose. In some embodiments, a “system” is an integrated assemblage of hardware and/or software elements. In some embodiments, each component of the system interacts with one or more other elements and/or is related to one or more other elements. In some embodiments, a system refers to a combination of components and software for controlling and directing methods.

As used herein, “substantially pure” corresponds to a measure of a target product produced when the faradaic efficiency is greater than about 90%, preferably greater than about 92%, and more preferably greater than about 94%.

As defined herein, a description that “no gas separation processes are needed” means that the target chemical products produced are substantially pure without requiring any separation processes to remove unreacted reactants or unwanted byproducts.

As defined herein, a “target product” includes, but is not limited to, formaldehyde formate ions (e.g., formate salts and/or formic acid), acetaldehyde, alcohols (e.g., methanol, ethanol, n-propanol, isopropanol, butanol, etc.), acetate ions, and acetone.

System and Process for CO₂ to Target Product Conversion

Broadly, an electrochemical gas- and liquid-recirculated system and process for CO₂-to-target product conversion are described, wherein said system produces the desired chemical products that are substantially pure without requiring any separation processes to remove unreacted reactants or unwanted byproducts.

In a first aspect, a system and a process are described that converts CO₂ to target product with anodic oxidation of water. Referring to FIG. 5, a flow-cell electrolyzer 100 comprising powder as cathodic electrocatalysts (e.g., SnO₂ or Bi) can be used to efficiently reduce CO₂ into target product in neutral electrolyte and any unconverted CO₂, including gaseous byproducts H₂ and CO, can be directly recirculated back into the electrolyzer 100, optionally after retention in a pressure-buffering reservoir. In this system, the CO₂ feed stream can be driven by a vacuum pump and is a precisely controlled closed loop with multiple pressure controllers and mass flow controllers. In some embodiments, the gas recirculation system can be maintained in a pressure-static state of 1-10 bar pressure. In some embodiments, the reacted CO₂ can be quantitatively compensated by a CO₂ cylinder. In this system, the liquid-recirculation part can be driven by peristaltic pumps and the electrolyte with designated target product concentration (>100 mmol/L) can be removed as final product. In some embodiments, the reduced electrolyte volume can be compensated by fresh bicarbonate solution. In some embodiments, the liquid recirculation system can be maintained in a constant flow state with 1-100 mL/min/cm².

In some embodiments, the flow-cell electrolyzer 100 is a three-compartment cell, wherein a first compartment 102 is separated from a second compartment 104 by a gas-diffusion electrode (GDE) coated with cathodic electrocatalyst material, for example, SnO₂ or Bi. In some embodiments, the cathodic electrocatalyst material is coated at an amount of about 0.5 to about 5.0 mg/cm², or about 0.5 to about 3.0 mg/cm², or about 0.5 to about 1.5 mg/cm², per geometric area of the electrode. The first compartment 102 is arranged for introduction of a gas to, and withdrawal of gas from, said compartment. The second compartment 104 is separated from a third compartment 106 by an anion exchange membrane (AEM). The second compartment 104 is arranged for introduction of a liquid to, and withdrawal of liquid from, said compartment. As shown in FIG. 5, the second compartment 104 is in fluid communication with a catholyte comprising electrolyte solution at entry and a solution comprising target product at egress from the second compartment 104. The third compartment 106 is arranged for introduction of a liquid to, and withdrawal of liquid from, said compartment. As shown in FIG. 5, the third compartment 106 is in fluid communication with an anolyte comprising electrolyte solution. Anodes include dimensionally stable anodes (DSA) such as platinized titanium anodes and mixed metal oxide (MMO) coated anodes, e.g., Ir-Ru-Ox, as readily understood by the person skilled in the art. In some embodiments, the catholyte and the anolyte comprise the same electrolytic component. In some embodiments, the catholyte and the anolyte comprise different electrolytic components. In some embodiments, both the catholyte and the anolyte comprise a bicarbonate solution. In some embodiments, the anolyte further comprises Na₂CO₃ to increase the buffering strength for the anodic reaction and offset the potential gradient-driven migration of carbonate anions across the AEM [Weng, 2019]. In some embodiments, the concentration of electrolytic component in the anolyte is in a range from about 0.1 mol/L to about 2 mol/L, preferably about 0.5 mol/L to about 1.5 mol/L and even more preferably about 0.7 mol/L to about 1.3 mol/L. In some embodiments, the concentration of electrolytic component in the catholyte is in a range from about 0.1 mol/L to about 2 mol/L, preferably about 0.5 mol/L to about 1.5 mol/L and even more preferably about 0.7 mol/L to about 1.3 mol/L.

Anion exchange membranes (AEM) are conventionally known in the art. In some embodiments, the polymeric anion-exchange membranes comprise —NH₃⁺, —NRH₂⁺, —NR₂H⁺, —NR₃⁺, or —SR₂⁻ anion exchange functional groups. The polymers for the preparation of anion-exchange membranes can be perfluorinated ionomers such as NAFION (a perfluorosulfonic-based membrane), FLEMION, and NEOSEPTA-F, partially fluorinated polymers, non-fluorinated hydrocarbon polymers, non-fluorinated polymers with aromatic backbone, or acid-base blends. It will be appreciated that in some embodiments, depending on the need to restrict or allow migration of a specific cation or an anion species between the electrolytes, an anion exchange membrane that is more restrictive and thus allows migration of one species of anions while restricting the migration of another species of anions may be used. Such restrictive anion exchange membranes are commercially available and can be selected by one ordinarily skilled in the art.

Electrolyte solutions are well known in the art and can comprise at least one “electrolytic component” selected from a bicarbonate, a carbonate, and/or a sulfate including, but not limited to, sodium bicarbonate, potassium bicarbonate, ammonium bicarbonate, cesium bicarbonate, lithium bicarbonate, sodium carbonate, potassium carbonate, lithium carbonate, calcium carbonate, ammonium carbonate, sodium sulfate, potassium sulfate, lithium sulfate, and ammonium sulfate.

In some embodiments, the systems described herein comprise a gas recirculation system, a catholyte recirculation system, and an anolyte recirculation system. In some embodiments, the gas recirculation system comprises at least one of at least one mass flow controller, at least one pressure controller, a source of carbon dioxide for quantitative compensation, a pressure-buffering reservoir for storage, a gas pump, a moisture trap, at least one valve, and a vent. In some embodiments, the gas recirculation system is communicatively connected to a GC-MS for sampling and analysis. In some embodiments, the anolyte recirculation system comprises a container comprising electrolyte solution and at least one pumping means, optionally a gas-liquid separator. In some embodiments, the catholyte recirculation system comprises a container comprising electrolyte solution, at least one pumping means, and a product container for target product produced during the electrochemical reaction (not shown). In some embodiments, the catholyte comprising target product is not recirculated (i.e., single pass) and is sent directly to the product container for downstream production. In some embodiments, the catholyte comprising target product is recirculated through the catholyte recirculation system (i.e., accumulated) until reaching a targeted concentration, at which point it will be sent to the product container for downstream production.

In one embodiment of the first aspect, a system to convert carbon dioxide to target product with an anodic oxidation of water is described, said system comprising:

• • a flow-cell electrolyzer comprising a gas diffusion electrode (GDE) coated with cathodic electrocatalyst material and an anode; and
  • a gas recirculation system in fluid communication with the flow-cell electrolyzer,
  • wherein an electrochemical reaction at the GDE reduces carbon dioxide to produce target product in a catholyte, and water at the anode in an anolyte, when voltage is applied; and
  • wherein a post-electrochemical reaction mixture of gas is in fluid communication with the gas recirculation system for reintroduction to the flow-cell electrolyzer.

In some embodiments, the gas recirculation system comprises at least one of at least one mass flow controller, at least one pressure controller, a source of carbon dioxide for quantitative compensation, a pressure-buffering reservoir for storage, a gas pump, a moisture trap, at least one valve, and a vent. In some embodiments, the system further comprises an anolyte recirculation system and a catholyte recirculation system. During operation, enough voltage is applied for cathodic carbon dioxide reduction at the GDE and anodic water oxidation. Theoretical cell voltage is 1.5V and practical cell voltages are in a range of about 1.5 V to about 10V. In some embodiments, the applied voltage is about 1.6V to about 5 V. In some embodiments, the applied voltage is about 2 V to about 3V. A current density of the system during operation is about 1 mA/cm² to about 5 A/cm². In some embodiment, the current density is about 1 mA/cm² to about 3 A/cm². In some embodiment, the current density is about 1 mA/cm² to about 1 A/cm².

In another embodiment of the first aspect, a system to convert carbon dioxide to target product with an anodic oxidation of water is described, said system comprising:

• • a flow-cell electrolyzer;
  • an anolyte recirculation system in fluid communication with the flow-cell electrolyzer;
  • a catholyte recirculation system in fluid communication with the flow-cell electrolyzer; and
  • a gas recirculation system in fluid communication with the flow-cell electrolyzer,
  • wherein the flow-cell electrolyzer comprises three compartments, wherein a first compartment and a second compartment are separated by a gas diffusion electrode (GDE) coated with cathodic electrocatalyst material and the second compartment and a third compartment are separated by an anion exchange membrane (AEM), wherein the first compartment is in fluid communication with the gas recirculation system, the second compartment is in fluid communication with the catholye recirculation system, and the third compartment comprises an anode and is in fluid communication with the anolyte recirculation system;
  • wherein carbon dioxide-containing gas in the gas recirculation system is directed to the first compartment for an electrochemical reaction at the GDE between carbon dioxide in the gas and catholyte in the second compartment, wherein a post-electrochemical reaction mixture of gas exits the first compartment into the gas recirculation system, is quantitatively compensated with additional carbon dioxide, and is reintroduced to the first compartment following carbon dioxide concentration adjustment;
  • wherein the catholyte recirculation system directs catholyte comprising electrolyte solution to the second compartment and electrolyte ions are reduced to target product at the GDE during the electrochemical reaction, wherein a target product-containing solution is removed from the second compartment; and
  • wherein water is produced at the anode in the third compartment during the electrochemical reaction.

In some embodiments, the gas recirculation system comprises at least one of at least one mass flow controller, at least one pressure controller, a source of carbon dioxide for quantitative compensation, a pressure-buffering reservoir for storage, a gas pump, a moisture trap, at least one valve, and a vent. In some embodiments, the catholyte recirculation system comprises a container comprising electrolyte solution, at least one pumping means, and a product container for target product produced during the electrochemical reaction (not shown). In some embodiments, the anolyte recirculation system comprises a container comprising electrolyte solution and at least one pumping means. During operation, enough voltage is applied for cathodic carbon dioxide reduction at the GDE and anodic water oxidation. Theoretical cell voltage is 1.5V and practical cell voltages are in a range of about 1.5 V to about 10V. In some embodiments, the applied voltage is about 1.6V to about 5 V. In some embodiments, the applied voltage is about 2 V to about 3V. A current density of the system during operation is about 1 mA/cm² to about 5 A/cm². In some embodiment, the current density is about 1 mA/cm² to about 3 A/cm². In some embodiment, the current density is about 1 mA/cm² to about 1 A/cm².

In still another embodiment of the first aspect, a system to convert carbon dioxide to target product with an anodic oxidation of water is described, said system comprising:

• • a flow-cell electrolyzer;
  • an anolyte recirculation system in fluid communication with the flow-cell electrolyzer;
  • a catholyte recirculation system in fluid communication with the flow-cell electrolyzer; and
  • a gas recirculation system in fluid communication with the flow-cell electrolyzer, wherein the gas recirculation system comprises a pressure-buffering reservoir, at least one mass flow controller, at least one pressure controller, at least one valve, a moisture trap, a gas pump, a vent and a CO₂ gas cylinder,
  • wherein the flow-cell electrolyzer comprises three compartments, wherein a first compartment and a second compartment are separated by a gas diffusion electrode (GDE) coated with cathodic electrocatalyst material and the second compartment and a third compartment are separated by an anion exchange membrane (AEM), wherein the first compartment is in fluid communication with the gas recirculation system, the second compartment is in fluid communication with the catholyte recirculation system, and the third compartment comprises an anode and is in fluid communication with the anolyte recirculation system;
  • wherein carbon dioxide-containing gas in the gas recirculation system is directed to the first compartment for an electrochemical reaction at the GDE between carbon dioxide in the gas and catholyte in the second compartment, wherein a post-electrochemical reaction mixture of gas exits the first compartment into the gas recirculation system, is quantitatively compensated with additional carbon dioxide from the gas cylinder therein, and is reintroduced to the first compartment following carbon dioxide concentration adjustment;
  • wherein the catholyte recirculation system directs catholyte comprising electrolyte solution to the second compartment and electrolytic ions are reduced to target product at the GDE during the electrochemical reaction, wherein a target product-containing solution is removed from the second compartment; and
  • wherein water is produced at the anode in the third compartment during the electrochemical reaction.
    In some embodiments, the gas cylinder comprises carbon dioxide. It should be appreciated by the person skilled in the art that the source and/or purity of carbon dioxide is not relevant to the practice of the system. In some embodiments, the catholyte recirculation system comprises a container comprising electrolyte solution, at least one pumping means, and a product container for target product produced during the electrochemical reaction (not shown). In some embodiments, the anolyte recirculation system comprises a container comprising electrolyte solution and at least one pumping means. During operation, enough voltage is applied for cathodic carbon dioxide reduction at the GDE and anodic water oxidation. Theoretical cell voltage is 1.5V and practical cell voltages are in a range of about 1.5 V to about 10V. In some embodiments, the applied voltage is about 1.6V to about 5 V. In some embodiments, the applied voltage is about 2 V to about 3V. A current density of the system during operation is about 1 mA/cm² to about 5 A/cm². In some embodiment, the current density is about 1 mA/cm² to about 3 A/cm². In some embodiment, the current density is about 1 mA/cm² to about 1 A/cm².

In another embodiment of the first aspect, a process to convert carbon dioxide to target product with an anodic oxidation of water is described, said process comprising:

• • directing carbon dioxide-containing gas to a flow-cell electrolyzer, wherein the flow-cell electrolyzer comprises a gas diffusion electrode (GDE) coated cathodic electrocatalyst material and an anode;
  • applying a voltage across the GDE and the anode of the flow-cell electrolyzer, wherein carbon dioxide in the gas comes in contact with the GDE for an electrochemical reaction that produces target product in a catholyte, and water is produced at the anode in an anolyte; and
  • directing a post-electrochemical reaction mixture of gas through a gas recirculation system for reintroduction to the flow-cell electrolyzer.

In some embodiments, the gas recirculation system comprises at least one of at least one mass flow controller, at least one pressure controller, a source of carbon dioxide for quantitative compensation, a pressure-buffering reservoir for storage, a gas pump, a moisture trap, at least one valve, and a vent. In some embodiments, the anolyte is introduced to the flow-cell electrolyzer using an anolyte recirculation system. In some embodiments, the catholyte is introduced to the flow-cell electrolyzer using a catholyte recirculation system. During operation, enough voltage is applied for cathodic carbon dioxide reduction at the GDE and anodic water oxidation. Theoretical cell voltage is 1.5V and practical cell voltages are in a range of about 1.5 V to about 10V. In some embodiments, the applied voltage is about 1.6V to about 5 V. In some embodiments, the applied voltage is about 2 V to about 3V. A current density of the system during operation is about 1 mA/cm² to about 5 A/cm². In some embodiment, the current density is about 1 mA/cm² to about 3 A/cm². In some embodiment, the current density is about 1 mA/cm² to about 1 A/cm².

In yet another embodiment of the first aspect, a process to convert carbon dioxide to target product with an anodic oxidation of water is described, said process comprising:

• • directing carbon dioxide-containing gas to a flow-cell electrolyzer comprising three compartments, wherein a first compartment and a second compartment are separated by a gas diffusion electrode (GDE) coated with cathodic electrocatalyst material and the second compartment and a third compartment are separated by an anion exchange membrane (AEM), wherein the first compartment is in fluid communication with a gas recirculation system comprising a carbon dioxide-containing gas, the second compartment is in fluid communication with a catholyte recirculation system comprising a catholyte, and the third compartment comprises an anode and is in fluid communication with an anolyte recirculation system comprising an anolyte;
  • applying a voltage across the GDE and the anode of the flow-cell electrolyzer, wherein carbon dioxide in the gas that is in contact with the GDE in the first compartment electrochemically reacts and produces target product in the catholyte, and water is produced at the anode in the anolyte;
  • removing a post-electrochemical reaction mixture of gas from the first compartment and passing same through the gas recirculation system, wherein the post-electrochemical reaction mixture of gas is quantitatively compensated with additional carbon dioxide, and is reintroduced to the first compartment following carbon dioxide concentration adjustment;
  • removing a target product-containing solution from the second compartment.
    In some embodiments, the gas recirculation system comprises at least one of at least one mass flow controller, at least one pressure controller, a source of carbon dioxide for quantitative compensation, a pressure-buffering reservoir for storage, a gas pump, a moisture trap, at least one valve, and a vent. In some embodiments, the catholyte recirculation system comprises a container comprising electrolyte solution, at least one pumping means, and a product container for target product produced during the electrochemical reaction (not shown). In some embodiments, the anolyte recirculation system comprises a container comprising electrolyte solution and at least one pumping means. During operation, enough voltage is applied for cathodic carbon dioxide reduction at the GDE and anodic water oxidation. Theoretical cell voltage is 1.5V and practical cell voltages are in a range of about 1.5 V to about 10V. In some embodiments, the applied voltage is about 1.6V to about 5 V. In some embodiments, the applied voltage is about 2 V to about 3V. A current density of the system during operation is about 1 mA/cm² to about 5 A/cm². In some embodiment, the current density is about 1 mA/cm² to about 3 A/cm². In some embodiment, the current density is about 1 mA/cm² to about 1 A/cm².

Over time, the system and process of the first aspect starts to build up hydrogen gas and carbon monoxide gas in the carbon dioxide-containing gas, which is not favored. Accordingly, in a second aspect, a system is described that converts CO₂ to target product with anodic oxidation of hydrogen and carbon monoxide that can reduce the H₂ and CO levels in the carbon dioxide-containing gas. To remove the slowly accumulated hydrogen and carbon monoxide without energy or carbon loss, a hydrogen and carbon monoxide oxidation reaction as an anodic reaction can be arranged by switching valves in the gas recirculation system of the first aspect to introduce the carbon dioxide-containing gas to the compartment containing the anode, prior to reintroduction into the compartment containing the GDE. For example, referring to FIG. 6, to remove the H₂ in the CO₂ stream and restore CO from the “dead end” back into CO₂, the mixture of carbon dioxide-containing gas (e.g., comprising CO₂, H₂ and CO) was introduced to the anodic chamber before being fed into the gas chamber of the GDE cathode. This can be accomplished by valve switching within the system. The carbon dioxide-containing gas was contacted with a dimensionally stable anode (DSA) at a controlled anodic potential in a range above 0V to about 1.35 V. In some embodiments, the controlled potential is in a range from about 0V to below the theoretical OER onset potential of 1.23V. At this potential, the hydrogen electro-oxidation reaction and carbon monoxide electro-oxidation are performed simultaneously at the anode, removing the H₂ (g) and converting the CO(g) to CO₂.

As illustrated in FIG. 6, after contact with the anode, the carbon dioxide-containing gas, which has a reduced amount of CO(g) and H₂ (g), is separated from the anolyte prior to contact with the GDE, as readily understood by the person skilled in the art. In some embodiments, a gas-liquid separator can be used to separate gases from liquids following egress from the third compartment. In some embodiments, the third compartment can comprise an egress that is in fluid communication with the first compartment such that the gas that collects in the headspace of the third compartment will be directed to the first compartment. Similarly, the third compartment can comprise a separate egress that is in fluid communication with the anolyte recirculation system such that the liquid of the third compartment can recirculate in the anolyte recirculation system.

In one embodiment of the second aspect, a system to convert carbon dioxide to target product with an anodic oxidation of hydrogen gas and carbon monoxide gas is described, said system comprising:

• • a flow-cell electrolyzer comprising a gas diffusion electrode (GDE) coated with cathodic electrocatalyst material and an anode;
  • a gas-liquid separator in fluid communication with the flow-cell electrolyzer; and
  • a gas recirculation system in fluid communication with the flow-cell electrolyzer,
  • wherein an electrochemical reaction at the GDE reduces carbon dioxide to produce target product in a catholyte, and carbon monoxide gas and hydrogen gas are oxidized at the anode in an anolyte, when voltage is applied; and
  • wherein a post-electrochemical reaction mixture of gas is in fluid communication with the gas recirculation system for reintroduction to the flow-cell electrolyzer.

In some embodiments, the gas recirculation system comprises at least one of at least one mass flow controller, at least one pressure controller, a source of carbon dioxide for quantitative compensation, a pressure-buffering reservoir for storage, a gas pump, a moisture trap, at least one valve, and a vent. In some embodiments, the system further comprises an anolyte recirculation system and a catholyte recirculation system. During operation, carbon dioxide is reduced at the GDE and H₂ (g) and/or CO(g) is oxidized at the anode at a cell voltage of about 0.5V to about 2.3V, an anodic potential not greater than 1.23 V, and a stabilized current density of about 1 mA/cm² to about 3 A/cm², depending on the voltage applied.

In another embodiment of the second aspect, a system to convert carbon dioxide to target product with an anodic oxidation of hydrogen gas and carbon monoxide gas is described, said system comprising:

• • a flow-cell electrolyzer;
  • a gas-liquid separator in fluid communication with the flow-cell electrolyzer;
  • an anolyte recirculation system in fluid communication with the flow-cell electrolyzer;
  • a catholyte recirculation system in fluid communication with the flow-cell electrolyzer; and
  • a gas recirculation system in fluid communication with the flow-cell electrolyzer,
  • wherein the flow-cell electrolyzer comprises three compartments, wherein a first compartment and a second compartment are separated by a gas diffusion electrode (GDE) coated with cathodic electrocatalyst material and the second compartment and a third compartment are separated by an anion exchange membrane (AEM), wherein the first compartment is in fluid communication with the gas recirculation system and the third compartment via the gas-liquid separator, the second compartment is in fluid communication with a catholyte recirculation system, and the third compartment comprises an anode and is in fluid communication with an anolyte recirculation system and the gas recirculation system;
  • wherein carbon dioxide-containing gas in the gas recirculation system is directed to the third compartment with the anolyte to oxidize, at the anode, any hydrogen gas and/or carbon monoxide gas present in the carbon dioxide-containing gas, wherein carbon dioxide-containing gas having a reduced concentration of hydrogen and carbon monoxide is separated from the anolyte using the gas-liquid separator and is directed to the first compartment for an electrochemical reaction at the GDE, wherein a post-electrochemical reaction mixture of gas exits the first compartment into the gas recirculation system, is quantitatively compensated with additional carbon dioxide, and is reintroduced to the third compartment following carbon dioxide concentration adjustment;
  • wherein the catholyte recirculation system directs catholyte comprising electrolyte solution to the second compartment and electrolyte ions are reduced to target product at the GDE during the electrochemical reaction, wherein a target product-containing solution is removed from the second compartment; and
  • wherein water is produced at the anode in the third compartment during the electrochemical reaction.

In some embodiments, the gas recirculation system comprises at least one of at least one mass flow controller, at least one pressure controller, a source of carbon dioxide for quantitative compensation, a pressure-buffering reservoir for storage, a gas pump, a moisture trap, at least one valve, and a vent. In some embodiments, the catholyte recirculation system comprises a container comprising electrolyte solution, at least one pumping means, and a product container for target product produced during the electrochemical reaction. In some embodiments, the anolyte recirculation system comprises a container comprising electrolyte solution and at least one pumping means. During operation, carbon dioxide is reduced at the GDE and H₂ (g) and/or CO(g) is oxidized at the anode at a cell voltage of about 0.5V to about 2.3V, an anodic potential not greater than 1.23 V, and a stabilized current density of about 1 mA/cm² to about 3 A/cm², depending on the voltage applied. It should be appreciated by the person skilled in the art that the third compartment can be directly or indirectly in fluid communication with the gas recirculation system. For example, in some embodiments, the gas recirculation system is in direct fluid communication with the anolyte recirculation system. In some embodiments, the gas recirculation system is in direct fluid communication with the third compartment.

In still another embodiment of the second aspect, a system to convert carbon dioxide to target product with an anodic oxidation of hydrogen gas and carbon monoxide gas is described, said system comprising:

• • a flow-cell electrolyzer;
  • a gas-liquid separator in fluid communication with the flow-cell electrolyzer;
  • an anolyte recirculation system in fluid communication with the flow-cell electrolyzer;
  • a catholyte recirculation system in fluid communication with the flow-cell electrolyzer; and
  • a gas recirculation system in fluid communication with the flow-cell electrolyzer, wherein the gas recirculation system comprises a pressure-buffering reservoir, at least one mass flow controller, at least one pressure controller, at least one valve, a moisture trap, a vent, a gas pump, and a CO₂ gas cylinder,
  • wherein the flow-cell electrolyzer comprises three compartments, wherein a first compartment and a second compartment are separated by a gas diffusion electrode (GDE) coated with cathodic electrocatalyst material and the second compartment and a third compartment are separated by an anion exchange membrane (AEM), wherein the first compartment is in fluid communication with the gas recirculation system and the third compartment via the gas-liquid separator, the second compartment is in fluid communication with a catholyte recirculation system, and the third compartment comprises an anode and is in fluid communication with an anolyte recirculation system and the gas recirculation system;
  • wherein carbon dioxide-containing gas in the gas recirculation system is directed to the third compartment with the anolyte to oxidize, at the anode, any hydrogen gas and/or carbon monoxide gas present in the carbon dioxide-containing gas, wherein carbon dioxide-containing gas having a reduced concentration of hydrogen and carbon monoxide is separated from the anolyte using the gas-liquid separator and is directed to the first compartment for an electrochemical reaction at the GDE, wherein a post-electrochemical reaction mixture of gas exits the first compartment into the gas recirculation system, is quantitatively compensated with additional carbon dioxide from the gas cylinder, and is reintroduced to the third compartment following carbon dioxide concentration adjustment;
  • wherein the catholyte recirculation system directs catholyte comprising electrolyte solution to the second compartment and electrolyte ions are reduced to target product at the GDE during the electrochemical reaction, wherein a target product-containing solution is removed from the second compartment; and
  • wherein water is produced at the anode in the third compartment during the electrochemical reaction.

In some embodiments, the gas cylinder comprises carbon dioxide. It should be appreciated by the person skilled in the art that the source and/or purity of carbon dioxide is not relevant to the practice of the system. In some embodiments, the catholyte recirculation system comprises a container comprising electrolyte solution, at least one pumping means, and a product container for target product produced during the electrochemical reaction (not shown). In some embodiments, the anolyte recirculation system comprises a container comprising electrolyte solution and at least one pumping means. During operation, carbon dioxide is reduced at the GDE and H₂ (g) and/or CO(g) is oxidized at the anode at a cell voltage of about 0.5V to about 2.3V, an anodic potential not greater than 1.23 V, and a stabilized current density of about 1 mA/cm² to about 3 A/cm², depending on the voltage applied. It should be appreciated by the person skilled in the art that the third compartment can be directly or indirectly in fluid communication with the gas recirculation system. For example, in some embodiments, the gas recirculation system is in direct fluid communication with the anolyte recirculation system. In some embodiments, the gas recirculation system is in direct fluid communication with the third compartment.

In yet another embodiment of the second aspect, a process to convert carbon dioxide to target product with an anodic oxidation of hydrogen gas and carbon monoxide gas is described, said process comprising:

• • directing carbon dioxide-containing gas to a flow-cell electrolyzer, wherein the flow-cell electrolyzer comprises a gas diffusion electrode (GDE) coated cathodic electrocatalyst material and an anode;
  • applying a voltage across the GDE and the anode of the flow-cell electrolyzer, wherein carbon monoxide and hydrogen gas in contact with the anode in an anolyte are oxidized and carbon dioxide in contact with the GDE is reduced to target product in a catholyte; and
  • directing a post-electrochemical reaction mixture of gas through a gas recirculation system for reintroduction to the flow-cell electrolyzer in proximity to the anode.

In some embodiments, the gas recirculation system comprises at least one of at least one mass flow controller, at least one pressure controller, a source of carbon dioxide for quantitative compensation, a pressure-buffering reservoir for storage, a gas pump, a moisture trap, at least one valve, and a vent. In some embodiments, the system further comprises an anolyte recirculation system and a catholyte recirculation system. During operation, carbon dioxide is reduced at the GDE and H₂ (g) and/or CO(g) is oxidized at the anode at a cell voltage of about 0.5V to about 2.3V, an anodic potential not greater than 1.23 V, and a stabilized current density of about 1 mA/cm² to about 3 A/cm², depending on the voltage applied.

In another embodiment of the second aspect, a process to convert carbon dioxide to target product with an anodic oxidation of hydrogen gas and carbon monoxide gas is described, said system comprising:

• • directing carbon dioxide-containing gas to a flow-cell electrolyzer comprising three compartments, wherein a first compartment and a second compartment are separated by a gas diffusion electrode (GDE) coated with cathodic electrocatalyst material and the second compartment and a third compartment are separated by an anion exchange membrane (AEM), wherein the first compartment is in fluid communication with the gas recirculation system comprising a carbon-dioxide containing gas and the third compartment via a gas-liquid separator, the second compartment is in fluid communication with a catholyte recirculation system comprising a catholyte, and the third compartment comprises an anode and is in fluid communication with an anolyte recirculation system comprising an anolyte and the gas recirculation system;
  • oxidizing, at the anode, any hydrogen gas and/or carbon monoxide gas present in the carbon dioxide-containing gas;
  • separating the carbon dioxide-containing gas having a reduced concentration of hydrogen and carbon monoxide from the anolyte using the gas-liquid separator;
  • directing the carbon dioxide-containing gas having a reduced concentration of hydrogen and carbon monoxide to the first compartment for an electrochemical reaction at the GDE to produce target product in the catholyte;
  • removing a post-electrochemical reaction mixture of gas from the first compartment and passing same through the recirculation system for reintroduction to the first compartment, further comprising quantitatively compensating the post-electrochemical reaction mixture of gas with additional carbon dioxide prior to reintroduction;
  • removing a target product-containing solution from the second compartment.

In some embodiments, the gas recirculation system comprises at least one of at least one mass flow controller, at least one pressure controller, a source of carbon dioxide for quantitative compensation, a pressure-buffering reservoir for storage, a gas pump, a moisture trap, at least one valve, and a vent. In some embodiments, the catholyte recirculation system comprises a container comprising electrolyte solution, at least one pumping means, and a product container for target product produced during the electrochemical reaction (not shown). In some embodiments, the anolyte recirculation system comprises a container comprising electrolyte solution and at least one pumping means. During operation, carbon dioxide is reduced at the GDE and H₂ (g) and/or CO(g) is oxidized at the anode at a cell voltage of about 0.5V to about 2.3V, an anodic potential not greater than 1.23 V, and a stabilized current density of about 1 mA/cm² to about 3 A/cm², depending on the voltage applied. It should be appreciated by the person skilled in the art that the third compartment can be directly or indirectly in fluid communication with the gas recirculation system. For example, in some embodiments, the gas recirculation system is in direct fluid communication with the catholyte recirculation system. In some embodiments, the gas recirculation system is in direct fluid communication with the third compartment.

Continuous Operation

Using the electrochemical systems (and processes) of the first and second aspect with different anodic oxidation reactions, a continuous separation-free electrochemical CO₂ reduction system (and process) is disclosed with the capability of (1) changing the cell voltage with limited anodic potential and (2) switching valves for recirculated gas stream passing anode.

In a third aspect, a continuous process to convert carbon dioxide to target product without requiring any separation processes to remove unreacted reactants or unwanted byproducts is described, said process comprising:

• • (a) producing target product from carbon dioxide at a cathode with anodic oxidation of water for a time T1 by:
    • directing carbon dioxide-containing gas to a flow-cell electrolyzer, wherein the flow-cell electrolyzer comprises a gas diffusion electrode (GDE) coated cathodic electrocatalyst material and an anode;
    • applying a voltage across the GDE and the anode of the flow-cell electrolyzer, wherein carbon dioxide in the gas comes in contact with the GDE for an electrochemical reaction that produces target product in a catholyte, and water is produced at the anode in an anolyte; and
    • directing a post-electrochemical reaction mixture of gas through a gas recirculation system for reintroduction to the flow-cell electrolyzer in proximity of the GDE;
  • (b) switching valves and reducing the anodic potential to produce target product from carbon dioxide at a cathode with anodic oxidation of hydrogen and carbon monoxide for a time T2 by:
    • directing carbon dioxide-containing gas to the flow-cell electrolyzer;
    • applying a voltage across the GDE and the anode of the flow-cell electrolyzer, wherein carbon monoxide and hydrogen gas in contact with the anode in an anolyte are oxidized and carbon dioxide in contact with the GDE is reduced to target product in a catholyte; and directing a post-electrochemical reaction mixture of gas through a gas recirculation system for reintroduction to the flow-cell electrolyzer in proximity to the anode; and
  • (c) optionally repeating (a), optionally repeating (b) after (a).

In some embodiments, the gas recirculation system comprises at least one of at least one mass flow controller, at least one pressure controller, a source of carbon dioxide for quantitative compensation, a pressure-buffering reservoir for storage, a gas pump, a moisture trap, at least one valve, and a vent. In some embodiments, the anolyte is introduced to the flow-cell electrolyzer using an anolyte recirculation system. In some embodiments, the catholyte is introduced to the flow-cell electrolyzer using a catholyte recirculation system. During step (a), practical cell voltages are in a range of about 1.5 V to about 10V and current densities during operation are about 1 mA/cm² to about 5 A/cm². During step (b), carbon dioxide is reduced at the GDE and H₂ (g) and/or CO(g) is oxidized at the anode at a cell voltage of about 0.5V to about 2.3V, the anodic potential is not greater than 1.23 V, and a stabilized current density is about 1 mA/cm² to about 3 A/cm², depending on the voltage applied. It should be appreciated by the person skilled in the art that T1 and T2 are values that are dependent on parameter settings. In some embodiments, T1 is greater than T2. In some embodiments, T1 is approximately the same as T2. In some embodiments, T1 is less than T2 if more CO/H₂ is to be removed.

In another embodiment of the third aspect, a continuous process to convert carbon dioxide to target product without requiring any separation processes to remove unreacted reactants or unwanted byproducts is described, said process comprising:

• • (a) producing target product from carbon dioxide at a cathode with anodic oxidation of water for a time T1 by:
    • directing carbon dioxide-containing gas to a flow-cell electrolyzer comprising three compartments, wherein a first compartment and a second compartment are separated by a gas diffusion electrode (GDE) coated with cathodic electrocatalyst material and the second compartment and a third compartment are separated by an anion exchange membrane (AEM), wherein the first compartment is in fluid communication with the gas recirculation system comprising a carbon-dioxide containing gas and the third compartment via a gas-liquid separator, the second compartment is in fluid communication with a catholyte recirculation system comprising a catholyte, and the third compartment comprises an anode and is in fluid communication with an anolyte recirculation system comprising an anolyte and the gas recirculation system;
    • adjusting valves so that carbon dioxide-containing gas is introduced to the first compartment for contact with the GDE;
    • applying a voltage across the GDE and the anode of the flow-cell electrolyzer, wherein carbon dioxide in the gas that is in contact with the GDE in the first compartment electrochemically reacts and produces target product in the catholyte, and water is produced at the anode in the anolyte;
    • removing a post-electrochemical reaction mixture of gas from the first compartment and passing same through the gas recirculation system, wherein the post-electrochemical reaction mixture of gas is quantitatively compensated with additional carbon dioxide, and is reintroduced to the first compartment following carbon dioxide concentration adjustment;
    • removing a target product-containing solution from the second compartment;
  • (b) switching valves and reducing the anodic potential to produce target product from carbon dioxide at a cathode with anodic oxidation of hydrogen and carbon monoxide for a time T2 by:
    • adjusting valves so that carbon dioxide-containing gas is introduced to the third compartment for contact with the anode;
    • oxidizing, at the anode, any hydrogen gas and/or carbon monoxide gas present in the carbon dioxide-containing gas;
    • separating the carbon dioxide-containing gas having a reduced concentration of hydrogen and carbon monoxide from the anolyte using the gas-liquid separator;
    • directing the carbon dioxide-containing gas having a reduced concentration of hydrogen and carbon monoxide to the first compartment for an electrochemical reaction at the GDE to produce target product in the catholyte;
    • removing a post-electrochemical reaction mixture of gas from the first compartment and passing same through the recirculation system for reintroduction to the first compartment, further comprising quantitatively compensating the post-electrochemical reaction mixture of gas with additional carbon dioxide prior to reintroduction;
    • removing a target product-containing solution from the second compartment; and
  • (c) optionally repeating (a), optionally repeating (b) after (a).

In some embodiments, the gas recirculation system comprises at least one of at least one mass flow controller, at least one pressure controller, a source of carbon dioxide for quantitative compensation, a pressure-buffering reservoir for storage, a gas pump, a moisture trap, at least one valve, and a vent. In some embodiments, the catholyte recirculation system comprises a container comprising electrolyte solution, at least one pumping means, and a product container for target product produced during the electrochemical reaction (not shown). In some embodiments, the anolyte recirculation system comprises a container comprising electrolyte solution and at least one pumping means. During step (a), practical cell voltages are in a range of about 1.5 V to about 10V and current densities during operation are about 1 mA/cm² to about 5 A/cm². During step (b), carbon dioxide is reduced at the GDE and H₂ (g) and/or CO(g) is oxidized at the anode at a cell voltage of about 0.5V to about 2.3V, the anodic potential is not greater than 1.23 V, and a stabilized current density is about 1 mA/cm² to about 3 A/cm², depending on the voltage applied. It should be appreciated by the person skilled in the art that T1 and T2 are values that are dependent on parameter settings. In some embodiments, T1 is greater than T2. In some embodiments, T1 is approximately the same as T2. In some embodiments, T1 is less than T2 if more CO/H₂ is to be removed.

Continuous CO₂-to-formate conversion at high carbon conversion efficiencies is disclosed. The integration of CO₂ electrolyzer with gas and liquid circulation enabled multi-pass conversions and enrichment of liquid products, respectively. The utilization of anodic oxidation built in the electrochemical cell to remove H₂ and CO byproducts and restore the CO₂ feeding addressed the gas separation issue without relying on external facilities. The process engineering with alternative operations of electroreduction and regeneration demonstrated continuous operation of gas-to-liquid conversion at >90% CCE.

Computer Program Product

The present subject matter described in the first, second or third aspect may be a system, a method, and/or a computer program product. In some embodiments, the computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present subject matter.

In some embodiments, the computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a RAM, a ROM, an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire.

In some embodiments, computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network, or Near Field Communication. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device.

In some embodiments, computer readable program instructions for carrying out operations of the present subject matter may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++, Javascript or the like, and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present subject matter.

In some embodiments, the computer readable program instructions may be provided to a processor of a computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. In some embodiments, the computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks.

In some embodiments, the computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks.

In light of the present disclosure and the general level of skill in the art, those of skill can appreciate that the following examples are intended to be exemplary only and that numerous changes, modifications, and alterations can be employed without departing from the scope of the presently disclosed subject matter. The synthetic descriptions and specific examples that follow are only intended for the purposes of illustration, and are not to be construed as limiting in any manner to make compounds of the disclosure by other methods.

EXAMPLE

Catalyst Selection

Commercially purchased tin oxide (SnO₂) and bismuth (Bi) nanopowders were used as candidates of cathode electrocatalysts. Both are based on earth-abundant elements and commercially available at relatively low cost. Electrocatalytic performance of the commercial SnO₂ and Bi for CO₂ reduction was evaluated using a three-compartment cell 100 comprising a GDE [Wang, 2019] (see, e.g., FIGS. 5 and 6). The coated electrodes delivered similar current densities, for example, 105 and 101 mA/cm² at −1.0 V (versus reversible hydrogen electrode (RHE); the same potential scale is used in following example unless otherwise specified) for SnO₂ and Bi, respectively (FIGS. 1A and 1B). Sodium formate was the primary product obtained with both catalysts. Hydrogen (H₂) and carbon monoxide (CO) were detected as two byproducts throughout the investigated potential region. At −0.6V, the hydrogen evolution reaction (HER) was the major faradaic process on the cathode and occupied >60% FE for both SnO₂ and Bi. On the SnO₂-coated electrode, CO₂-to-formate conversion became dominant at the potential of −0.7 V and the corresponding FE was found to be consistently above 50% from −0.7 V to −1.2 V. At −1.0 V, 95% FE toward formate was achieved, while H₂ and CO only took up 2.8% and 0.6% of the total FE. Similar to the SnO₂-coated electrode, the Bi-coated electrode exhibited the highest CO₂-to-formate selectivity at −1.0 V with 91% FE. However, more than 6.6% FE towards H₂ and 2.1% FE towards CO was detected at this potential. From the electrocatalytic performance, the SnO₂ seems to be a better electrocatalyst than Bi for both higher FE towards sodium formate and less H₂ and CO formation. However, it is noted that the above discussion and comparison of electrocatalytic performance is based on the scenario of pure CO₂ feeding, which is unlikely in a recirculated system as described herein.

To further determine the more superior electrocatalysts for the system, the partial-pressure-dependent electrocatalytic behavior of two catalysts with co-fed CO₂ and Ar at different ratios at −1.0V were investigated. At −1.0 V, both electrocatalysts exhibited a two-stage behavior, with a sharp rise of FE_HCOONa as the CO₂ concentration increased from 0 to 10% and then a plateau at >10% CO₂. In the latter region, the FE_HCOONa derived from SnO₂ is in the range of 92-95%, in comparison to 85-91% for Bi (FIG. 1C). The dependence of partial current density towards formate (J_formate) on the CO₂ partial pressure (P_CO2) consistently exhibited a linear regime at lower CO₂ partial pressures and a plateau at higher CO₂ partial pressures. The plateau was reached at 10.3 kPa for both catalysts (not shown). It is noted that the calcium carbonate equivalent (CCE) was quite low in the single-pass measurements. Even in the case of SnO₂ with relatively high CCEs, the single-pass conversion of CO₂ was measured to be only 1.4% at −1.0 V at the given flow rate of 50 mL/min for CO₂ feeding (FIG. 1D).

CO₂-to-Formate Conversion with Anodic Oxidation of Water

Referring to FIG. 5, a continuous gas- and liquid-recirculated reaction system for electrochemical CO₂-to-formate conversion with flow-cell electrolyzer was established. The gas-recirculation portion of the system was driven by a piston vacuum pump and controlled by flow controllers and pressure controllers. The CO₂ was fed into the electrolyzer 100 and the post-electrochemical reaction mixture after a single pass was re-fed into the system, optionally after retention in a pressure-buffering reservoir. The reacted CO₂ was quantitatively compensated for using a CO₂ cylinder. The liquid-recirculation portion of the system was driven by peristaltic pumps and the electrolyte with designated product concentration (>100 mmol/L sodium formate), as determined using for example nuclear magnetic resonance (NMR) analysis, was removed as final product from the second compartment 104. The reduced electrolyte volume in the second compartment 104 was compensated for with fresh NaHCO₃ solution. During the experiment, a cell voltage of 2.65 V was maintained and the current density was maintained around 100 mA/cm². The formate faradaic efficiency was 94% during the beginning 2 hours and gradually decreased to 89% at the end of a 70-hour operation. (FIG. 2A) The CO₂ inlet was metered quantitatively with a gas flow meter and the overall inlet amount was 281 mmol (including a dead volume of gas pipeline system). The formate yield was 219 mmol/L. The majority of the CO₂ remained unreacted in gas pipelines while a small amount of CO₂ was converted into carbon monoxide (CO), which cannot be converted to formate electrochemically (FIG. 2B). The CO₂ volume ratio was reduced from 100% to 57% during the reaction and the hydrogen accumulated to over 41% (FIG. 2C). The carbon conversion efficiency was over 93%, with a 5-6% of CO₂ loss and 0.6-1.0% of CO₂ into CO conversion (FIG. 2D). The formate Faradaic efficiency was as high as 88% at the end of the operation. From the pressure-dependence study, the accumulated H₂ and CO at this concentration should have a negligible effect on the formate selectivity. As such, the carbon conversion efficiency was affected by the CO production since it is a “dead end” for formate production.

CO₂-to-Formate Conversion with Anodic Oxidation of Hydrogen and Carbon Monoxide

To remove the H₂ in the CO₂ stream and convert the CO from the “dead end” back into CO₂, the system can be switched to a “regeneration” mode whereby the mixture of CO₂, H₂ and CO from the CO₂ electrolysis system was directed to an anodic chamber 106 before being introduced into the gas chamber of the cathode 102 via the vent attached to the anolyte container (see, FIG. 6). This can be accomplished by valve switching inside the system. The CO₂, H₂ and CO gas mixture was contacted with the dimensionally stable anode (DSA) at a controlled potential of <0.9 V versus reversible hydrogen electrode (RHE). At this potential, the hydrogen electro-oxidation reaction to water and carbon monoxide electro-oxidation to CO₂ were performed simultaneously on the DSA, while avoiding the onset (1.23V) of oxygen evolution. The residual gas mixture, including CO₂ and remaining H₂ and CO, was further fed to the cathode for reduction and then circulated back to the gas feeding loop. This allows for purification of the feeding gas mixture without using external separation processes (e.g., additional membranes or TSA/PSA processes), and the separated CO₂ can be subjected to further conversion. Due to the activity of anodic oxidation, the cathodic CO₂ reduction was limited at 25 mA/cm² while the cell voltage was stabilized at 1.94 V. The formate faradaic efficiency was 90% during the beginning 2 hours and gradually decreased to 87% at the end of 12-hour operation (FIG. 3A). The CO₂ inlet was metered quantitatively with a gas flow meter and the overall inlet amount was 8.5 mmol. The formate yield was 7.8 mmol. The majority of the rest of the CO₂ was unreacted in gas pipelines while a small amount of CO₂ was in a close balance between cathodic CO₂-to-CO reduction and anodic CO-to-CO₂ oxidation (FIG. 3B). The CO₂ volume ratio was restored from 53% to 72% during the reaction and the hydrogen volume ratio dropped from 44% to 26% (FIG. 3C). The carbon conversion efficiency was over 92% and there was around 8% CO₂ loss in the system (FIG. 3D).

Continuous Operation

Using the experimentally-observed results from the two electrochemical setups (i.e., FIGS. 5 and 6) with different anodic oxidation reactions, a continuous separation-free electrochemical CO₂ reduction system was built with the capability of (1) changing the cell voltage with limited anodic potential and (2) switching valves for recirculated gas stream anode passing. The system was tested for over 312 hours (nearly 2 weeks) without any interruption. The system operation method was programmed to first use a typical setup to reduce CO₂ at a cell voltage of 2.65 V for 70 hours, then reciprocating for a 12.5-hour anodic H₂/CO oxidation at cell voltage of 1.94 V (anodic potential not above 1.0 V throughout the process), and a 45-hour anodic water oxidation at cell voltage of 2.65 V. At the periods of CO₂ reduction with anodic oxidation of water, the current density was around 100 mA/cm². The current density reduced and stabilized at 24 mA/cm² when the system was switched into CO₂ reduction with anodic oxidation of hydrogen and carbon monoxide. These current densities measured in the continuously operated system were consistent with the separately measured results (FIG. 4A). The gas composition inside the gas pipeline was monitored by GC-MS throughout the experiment. During the “electroreduction” operations, the current density was found to be in the range of ca. 100-120 mA/cm². In the “regeneration” mode, the current density of the electrolyzer reduced to ˜24 mA/cm². Low CO₂ concentrations were reached at the end of each period of CO₂ reduction with anodic oxidation of water for 50-55% CO₂, 42-45% H₂ and 2-2.5% CO. The restoration of CO₂ and removal of H₂ and CO were successfully accomplished by five operation periods of CO₂ reduction with anodic oxidation of hydrogen and carbon monoxide. The restored feeding gas comprised >75% CO₂, 20-23% H₂ and <2% CO after those operation periods (FIG. 4B). The Faradaic efficiency was measured at the end of each operation period before system switching. When the system was running with CO₂ reduction and anodic oxidation of water, the formate FE was over 94% for all five segmental periods. The formate FE dropped to 88% when the CO₂ reduction was driven by anodic oxidation of hydrogen and carbon monoxide (FIG. 4C). The CCE towards sodium formate of each segment were calculated by the formate yield and CO₂ consumption. The CCEs of CO₂ reduction and anodic oxidation of water were always above 85% while the CCEs of CO₂ reduction and anodic oxidation of hydrogen and carbon monoxide were consistently over 90% (FIG. 4D). In the end, over 8.74 liters of sodium formate solution was produced with designated concentrations of >100 mmol/L with a 1-cm² catalyst-coated electrode. Note that the SnO₂ electrocatalyst and the GDE structures were well preserved after the continuous-operation study, indicating great potential of the developed electrochemical system for scalable CO₂-to-formate conversion. The as-synthesized product solution was directly used in biosynthesis as C-1 feedstock.

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