Electrolyzer Column

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application claims the benefit of and priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 63/722,408, entitled “Electrolyzer Column”, filed Nov. 19, 2024, the disclosure of which is incorporated herein by reference in its entirety for all purposes.

FIELD OF THE INVENTION

The invention is generally directed to a CO₂ electrolyzer for conversion of CO₂ to liquid fuels and fine chemicals, units and systems comprising thereof, and methods of use thereof.

BACKGROUND OF THE INVENTION

Current designs of CO₂ electrolyzers typically rely on the same zero-gap and gas diffusion electrode-based electrode/membrane architectures as used in fuel cells and water electrolyzers. However, such architectures are not well-suited for CO₂ electrolyzers used to obtain high energy-density liquid fuels. In particular, such architectures are limited by low CO₂ one-pass conversion and, as such, produce diluted fuel streams, which, in turn, require energy intensive downstream separations. Accordingly, new CO₂ electrolyzer designs, with fundamentally new learning curves, are needed for efficient production of liquid fuels via CO₂ electrolysis.

SUMMARY OF THE INVENTION

Various embodiments are directed to a CO₂ electrolyzer for electrocatalytic conversion of CO₂ to a high value chemical including:

• • a column reactor characterized by a vertical axis having a bottom, a top, and a middle;
  • a gas inlet, situated at the bottom of the column reactor, for delivering a gas stream including CO₂ gas, wherein the gas stream enters at the bottom and flows upward the column reactor;
  • a liquid inlet situated at the top of the column reactor, for delivering a liquid stream including a liquid electrolyte, wherein the liquid stream enters at the top and flows downward the column reactor driven by gravity;
  • an effluent outlet, situated at the bottom of the column reactor for collecting a liquid effluent including electrolysis products including the high value chemical; and
  • any number of other inlets, outlets, and valves, as needed for efficient and safe operation of the CO₂ electrolyzer, wherein
  • the column reactor further includes a plurality of stages wherein
    • each stage includes:
      • a cathode compartment including a cathode characterized by a cathode area and functionalized with an electrolysis step-specific catalyst,
      • an anode compartment including an anode characterized by an anode area;
      • an ion conductive membrane separating the cathode and anode compartments;
      • a system of back-pressure regulators equilibrating the pressure between the cathode and anode compartments; and
      • a plurality of bubble caps, characterized by a number and a density of bubble caps, distributed throughout each stage; wherein
    • individual stages of the plurality of stages are stacked along the vertical axis of the column reactor, such that the cathode area and anode area of each stage extend perpendicularly to the vertical axis, and are in fluid communication with each other; and wherein
  • the plurality of stages further includes:
    • a first plurality of stages, including a first number of stages, situated at the bottom of the column reactor, wherein the electrolysis step-specific catalyst is a first catalyst for selective conversion of CO₂ to CO;
    • a second plurality of stages, including a second number of stages, situated at the top of the column reactor, wherein the electrolysis step-specific catalyst is a second catalyst for selective conversion of CO to acetaldehyde; and
    • a third plurality of stages, including a third number of stages, situated in the middle of the column reactor, immediately above and in fluid communication with the first plurality of stages and immediately below and in fluid communication with the second plurality of stages, wherein the electrolysis step-specific catalyst is a third catalyst for selective conversion of acetaldehyde to the high value chemical.

In various such embodiments, the high value chemical is ethanol.

In still various such embodiments, the first number of stages is 6 and the first catalyst is porous silver catalyst; the second number of stages is 11 and the second catalyst is porous copper-silver alloy; and the third number of stages is 3 and the third catalyst is gold nanoparticles supported on a porous copper.

Various other embodiments are directed to a method for converting CO₂ to a high value chemical including:

• • providing a CO₂ electrolyzer including:
  • a column reactor characterized by a vertical axis having a bottom, a top, and a middle;
  • a gas inlet, situated at the bottom of the column reactor, for delivering a gas stream including CO₂ gas, wherein the gas stream enters at the bottom and flows upward the column reactor;
  • a liquid inlet situated at the top of the column reactor, for delivering a liquid stream including a liquid electrolyte, wherein the liquid stream enters at the top and flows downward the column reactor driven by gravity;
  • an effluent outlet, situated at the bottom of the column reactor for collecting a liquid effluent including electrolysis products including the high value chemical; and
  • any number of other inlets, outlets, and valves, as needed for efficient and safe operation of the CO₂ electrolyzer, wherein
  • the column reactor further includes a plurality of stages, wherein
    • each stage includes:
      • a cathode compartment including a cathode characterized by a cathode area and functionalized with an electrolysis step-specific catalyst,
      • an anode compartment including an anode characterized by an anode area;
      • an ion conductive membrane separating the cathode and anode compartments;
      • a system of back-pressure regulators equilibrating the pressure between the cathode and anode compartments; and
      • a plurality of bubble caps, characterized by a number and a density of bubble caps, distributed throughout each stage, wherein
    • individual stages of the plurality of stages are stacked along the vertical axis of the column reactor, such that the cathode area and anode area of each stage extend perpendicularly to the vertical axis, and are in fluid communication with each other; and wherein
  • the plurality of stages further includes:
    • a first plurality of stages, including a first number of stages, situated at the bottom of the column reactor, wherein the electrolysis step-specific catalyst is a first catalyst for selective conversion of CO₂ to CO;
    • a second plurality of stages, including a second number of stages, situated at the top of the column reactor, wherein the electrolysis step-specific catalyst is a second catalyst for selective conversion of CO to acetaldehyde; and
    • a third plurality of stages, including a third number of stages, situated in the middle of the column reactor, immediately above and in fluid communication with the first plurality of stages and immediately below and in fluid communication with the second plurality of stages, wherein the electrolysis step-specific catalyst is a third catalyst for selective conversion of acetaldehyde to the high value chemical;
  • providing the gas stream, wherein the gas stream is a compressed gas stream, and allowing the gas stream to flow upward, along the vertical axis of the column reactor, passing the plurality of stages orthogonally to the cathode area and anode area;
  • providing the liquid stream and allowing the liquid stream to flow downward the column reactor along the vertical axis, driven by gravity, passing the plurality of stages orthogonally to the cathode area and anode area;
  • applying an operating potential to each stage of the plurality of stages and using a dynamic control method for adjusting the operating potential; and
  • operating the column reactor at a pressure
    to afford an electrolysis process producing the liquid effluent including a concentration of the high value chemical pooling at the bottom of the column reactor for facile collection through the effluent outlet.

In various such embodiments, the pressure is 5 to 100 bar.

In still various such embodiments, the high value chemical is ethanol.

In still yet various embodiments, wherein the high value chemical is ethanol, the first number of stages is 6 and the first catalyst is porous silver catalyst; the second number of stages is 11 and the second catalyst is porous copper-silver alloy; and the third number of stages is 3 and the third catalyst is gold nanoparticles supported on a porous copper.

In yet still various such embodiments, the dynamic control method leverages machine learning-based predictive control systems.

Still various other embodiments are directed to a unit for cost-efficient electrocatalytic conversion of CO₂ to the high value chemical including a plurality of the CO₂ electrolyzers bundled such as to optimally use a single cooling system and a single set of auxiliary pumps and compressors.

In various such embodiments, the unit includes twelve 36 kW CO₂ electrolyzers.

Yet various other embodiments are directed to a system for energy storage as the high value chemical including a plurality of the units for cost-efficient electrocatalytic conversion of CO₂ to the high value chemical.

In various such embodiments, the system includes twenty two 432 kW units and is characterized by an ability to respond to 9.5 MW energy availability.

Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the disclosed subject matter. A further understanding of the nature and advantages of the present disclosure may be realized by reference to the remaining portions of the specification and the drawings, which form a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in conjunction with the accompanying data and figures, wherein:

FIG. 1 shows a typical architecture and functioning of a gas-diffusion electrode (GDE)-based CO₂ electrolyzer, according to prior art.

FIG. 2 shows a typical architecture of a distillation column comprising trays (top) and a single typical tray (bottom), according to prior art.

FIGS. 3A through 3C schematically illustrate the design architecture and operation of the overall column reactor of the CO₂ electrolyzer (FIGS. 3A and 3C), a single stage (FIGS. 3A, inset, and 3B), and electrolysis steps-specific sections (FIG. 3C), in accordance with embodiments of the application.

FIG. 4 schematically illustrates an energy storage system with a capacity to respond to an intermittent energy availability of up to 9.5 MW, wherein the energy storage system is built using 22 units of twelve 36 kW CO₂ electrolyzers each (a total of 264 CO₂ electrolyzers), in accordance with embodiments of the application.

FIGS. 5A through 5C provide computer generated data obtained for a CO₂ electrolyzer design prototype via AVEVA PRO/II SIMULATION using kinetic experimental data acquired in a bench-scale rotating cylinder electrode cell, wherein FIG. 5A provides simulated distribution of Faradaic efficiencies on the cathode at each stage of the column reactor; FIG. 5B provides cathode gas flow rates for reactants and products at each stage of the column reactor; and FIG. 5C provides cathode liquid flow rates for reactants and products at each stage of the column reactor, in accordance with embodiments of the application.

FIGS. 6A through 6C show a mode of operation of an energy storage system comprising 264 36 kW CO₂ electrolyzers grouped in 22 units of 12 CO₂ electrolyzers, wherein FIG. 6A shows a representative energy availability scenario; FIG. 6B shows numbers of CO₂ electrolyzers active at given times of such scenario; and FIG. 6C shows average power per unit of CO₂ electrolyzers, in accordance with embodiments of the application.

DETAILED DISCLOSURE

Turning now to the schemes, images, and data, a CO₂ electrolyzer is described, as well as units and systems comprising thereof, and methods of using the same, for converting CO₂ to a high value chemical. In many embodiments, the high value chemical is a high energy-density liquid fuel or another valuable/fine chemical. In some embodiments, the production of the high value chemical by the CO₂ electrolyzer serves as means of energy storage, including when the energy supply is intermittent, wherein, when available, excess energy is used to power the CO₂ electrolyzer to produce, for example, a fuel, which can, in turn, be used to power another process as needed at a different time of the day or a different day of the year. However, in some other embodiments, the CO₂ electrolyzer is used to store excess electrical energy from other, non-intermittent, sources, when economics dictate that the production of the fuel or another valuable chemical is more profitable. In particular, in some embodiments, the CO₂ electrolyzer produces ethanol (CH₃CH₂OH) from CO₂, water, and electricity via a step-wise process generating CO and acetaldehyde (CH₃CHO) as the two main reaction intermediates. However, in other embodiments, CO₂ electrolyzer converts CO₂ to ethanol via a different catalytic pathway involving different reaction intermediates, including a proton-donating source other than water. Furthermore, in still other embodiments, the CO₂ electrolyzer relies on a similar step-wise reduction of CO₂ to access other high value chemical products, such as, for example: liquid alcohols other than ethanol, including propanol, butanol, and pentanol; long chain hydrocarbons and olefins, including ethylene and propylene, synthetic gasoline, and jet fuel hydrocarbons. In yet other embodiments, the CO₂ electrolyzer and the method of use thereof described herein are employed to convert starting materials other than CO₂ and water to the high value chemical, including the high value chemical comprising one or more heteroatoms.

To this end, in many embodiments, the CO₂ electrolyzer comprises a column reactor with a distillation column-like architecture comprising distillation tray-like stages, wherein each stage comprises an electrocatalytically active cell functionalized with an electrolysis step-specific catalyst, and wherein pluralities of such stages are organized into sections for optimally performing individual intermediate electrolysis reaction steps of the CO₂ (or another starting material) conversion to the high value chemical. Furthermore, in many embodiments, the flow of the electrolysis reactants is perpendicular to the electrocatalytic surfaces of the CO₂ electrolyzer's stages/electrodes. As such, in many embodiments, the CO₂ electrolyzer demonstrates high selectivity, excellent Faradaic efficiencies (F.E.s), and high one pass starting material conversion, especially as compared to conventional electrolyzers. In many embodiments, the CO₂ electrolyzer is amenable to facile dynamical control of the electrolysis process via operating potential, thus allowing for further optimization and scale up of the electrocatalytic reduction of CO₂ (or another starting material) to the high value chemical of choice. In many embodiments, the dynamic control of the electrolysis process leverages machine learning-based predictive control systems.

Accordingly, in many embodiments, the CO₂ electrolyzer comprises a column reactor, wherein the column reactor is a multi-stage vertical column (i.e., a column characterized by a vertical axis) comprising a plurality of electrocatalytically active stages/trays stacked along the vertical axis of the column reactor. Furthermore, in many embodiments, each stage of the plurality of stages comprises a cathode compartment comprising a cathode and an anode compartment comprising an anode; wherein the cathode and anode compartments are separated by an ion conductive membrane, and wherein the pressure across the compartments is controlled via a system of back-pressure regulators. Furthermore, in many embodiments, each stage comprises an electrolysis step-specific catalyst, disposed over the electron conductive components of the cathode, and a plurality of bubble caps (or another implement for optimizing gas-liquid contacting, such as, for example, gas spurgers), characterized by a number and a density of bubble caps, distributed throughout each stage, such as to optimize gas/liquid contacting between the reagents passing through the stage. In many embodiments, the column reactor further comprises: a gas inlet situated at the bottom of the column reactor for delivering a gas stream comprising CO₂ (or another starting material of choice) as compressed gas, such that the gas stream enters at the bottom and flows upward the reactor column; a liquid inlet situated at the top of the column reactor for delivering a liquid stream comprising a liquid electrolyte, such that the liquid stream enters at the top and flows downward the column reactor driven by gravity; an effluent outlet situated at the bottom of the column reactor for collecting a liquid effluent comprising liquid electrolysis products including the high value chemical; and any number of other inlets, outlets and valves, as needed for efficient and safe operation of the CO₂ electrolyzer. In addition, in many embodiments, any number of other liquid and or gas inlets and or outlets are added at any stage, section of stages, and or position of the column reactor to introduce or remove electrolysis step-specific reagents, intermediates, and or products, such as to control activity and selectivity of that stage or section, and, as such, to improve performance of the CO₂ electrolyzer towards the production of the high value chemical and or efficiency of the energy storage. In many embodiments, the stages of the plurality of stages, wherein each stage comprises the cathode and the anode, each comprising an electrocatalytically active surface area, are stacked along the vertical axis of the column reactor, such that the electrocatalytically active surface areas of the electrodes extend orthogonally to the flow of the gas and liquid streams, and are in fluid communication with each other. Furthermore, in many embodiments, the applied potential on each stage (i.e., that stage's cathode and anode) is individually controlled, allowing for variability, such as to optimize the electrocatalytic activity and selectivity of each stage.

Furthermore, in many embodiments, the plurality of stages comprises different types of stages, corresponding to different sections of the column reactor, wherein each section is optimized for a different step of the electrolysis process conducted by the CO₂ electrolyzer and comprises the electrolysis step-specific catalyst. In other words, in many embodiments, the electrolysis process conducted by the CO₂ electrolyzer is broken down into two or more steps and, as such, the plurality of stages comprises step-specific pluralities of stages (i.e., sections of stages), wherein each such plurality/set/section of stages is optimized, including with the catalyst choice, for the step of the electrolysis it is meant to conduct. To this end, for example, in some embodiments, wherein the CO₂ electrolyzer is used for conducting the CO₂ to ethanol electrocatalytical conversion described herein, the plurality of stages comprises three electrocatalysis step-specific pluralities of stages, i.e.: a first plurality of stages (a first section), comprising a first number of stages situated at the bottom of the column reactor, wherein the electrolysis step-specific catalyst is a first catalyst for selective conversion of CO₂ to CO; a second plurality of stages (a second section), comprising a second number of stages situated at the top of the column reactor, wherein the electrolysis-step specific catalyst is a second catalyst for selective conversion of CO to acetaldehyde; and a third plurality of stages (a third section), comprising a third number of stages situated in the middle of the column reactor, immediately above and in fluid communication with the first plurality of stages and immediately below and in fluid communication with the second plurality of stages, wherein the electrolysis-step specific catalyst is a third catalyst for selective conversion of acetaldehyde to the fuel. As a more specific example, in some such embodiments, wherein the CO₂ electrolyzer is used to convert CO₂ to ethanol in three steps described herein, the first number of stages is 6 and the first catalyst is porous silver catalyst; the second number of stages is 11 and the second catalyst is porous copper-silver alloy; and the third number of stages is 3 and the third catalyst is gold nanoparticles supported on a porous copper. However, it should be noted here that, in many embodiments, the numbers of stages designated to different sections of the column reactor, as well as the number of the sections and the identities of the section-specific catalysts, are all tailored for optimization of each electrolysis step's electrochemistry, such as to afford the highest conversion of the starting material (e.g., CO₂) to the high value product of choice, overall energy efficiency of the CO₂ electrolyzer, and the highest concentration of the high value product in the liquid effluent. For example, in some embodiments, a more complex transformation of a starting material to the high value chemical necessitates more sections and or more stages within each section. Moreover, in many embodiments, a number of stages within a section or the overall number of stages within the column reactor is increased to enhance separation of process components and or intermediates.

Furthermore, in many embodiments, the CO₂ electrolyzer is operated by: 1) supplying the gas stream as compressed gas comprising CO₂ at the bottom of the column reactor, and allowing the gas stream to flow upward the column reactor driven by a pressure gradient within the column reactor, such that the gas stream passes the plurality of stages from the bottom to the top stages; 2) supplying the liquid stream comprising the liquid electrolyte, and relying on gravity to flow the liquid stream downward the column reactor, such that it also passes the plurality of stages but from top to bottom stages; while 3) applying an operating potential to each stage of the plurality of stages under a dynamic control method for adjusting the operating potential, to perform the electrolysis process producing the high value chemical from, for example, CO₂. In addition, in many embodiments, the CO₂ electrolyzer is operated at a high pressure of 5 to 100 bar to optimize gas mass flow rates and to improve the dissolution of gaseous reactants in the liquid phase of the liquid stream. In many such embodiments, the liquid effluent comprising the high value chemical in a product concentration is pooling at the bottom of the column reactor during the electrolysis process, where it is collected through the effluent outlet. In many embodiments, the product concentration is 0.1 to 50% by volume of the liquid effluent, or higher. In many embodiments, a plurality of CO₂ electrolyzers is bundled together into a unit to optimize use of cooling systems and auxiliary pumps and compressors, and, therefore, for a more cost-efficient electrocatalytic conversion of CO₂ (or another starting material) to the high value chemical. In some embodiments, a plurality of such units is further bundled into systems or plants, such as, for example, customized energy storage systems or plants.

It will be understood that the embodiments of the invention described herein are not intended to be exhaustive or to limit the invention to precise forms disclosed. Rather, the embodiments selected for description have been chosen to enable one skilled in the art to practice the invention.

Typically, electrocatalytic methods for transformation of CO₂ into liquid fuels require large scale electrolyzers with high conversion of CO₂ and high Faradaic efficiency. In particular, current CO₂ electrolyzer reactors (such as, for example, described in Perry, S., et al., Developments on carbon dioxide reduction: Their promise, achievements, and challenges. Current Opinion in Electrochemistry 2020, 20, 88-98, the disclosure of which is incorporated herein by reference) predominantly rely on gas-diffusion electrodes (GDEs) technology, and are, as such, characterized by a limitingly low CO₂ one-pass conversion, resulting in generation of diluted fuel streams, which, in turn, require energy intensive downstream separations. Furthermore, since scale-up of the current electrolyzers is very limited, the efficiency of production of liquid fuels from CO₂ electrolysis is also severely lacking.

More specifically, GDEs typically used in laboratory bench-scale CO₂ electrolyzers are fundamentally flawed towards production of liquid fuels at industrial (i.e., large) scale. In particular, as illustrated in FIG. 1, the macroscopic transport of the reactants (here, CO₂ and CO) in the GDE-based electrolyzers is orthogonal to the direction of the microscale delivery of the reactants to the electrodes' catalyst. In other words, the flow of reactants in conventional electrolyzers is parallel to the electrocatalytic surfaces of their electrodes. Accordingly, the electrode surface area in the GDEs-based electrolyzers extends parallel (and in the same or similar direction) to the flow of CO₂, thus leading to reactant, product, and intermediate product concentration gradients and a limit to the maximum attainable CO₂ conversion (FIG. 1). Moreover, GDEs use millimeter size channels for gas transport, which leads to high pressure drops in the flow cells employing such technology. Accordingly, alternative approaches and architectures are needed to improve efficiency of electrolyzers for conversion of CO₂ to valuable products at industrial scales.

This application is directed to embodiments of a CO₂ electrolyzer, units and systems comprising thereof, and methods of using the same for electroreduction of CO₂ or another starting material to obtain high value chemicals. In many embodiments, the CO₂ electrolyzer possesses a modular, multi-stage column architecture inspired by distillation column architectures and processes (such as shown, for example, in FIG. 2). In many embodiments, the high value chemical is a high energy density liquid fuel. In some embodiments, the high value chemical is a molecule used as a chemical precursor for a base, intermediate, and or a fine chemical. Accordingly, in many embodiments, the high value chemical is any oxygenated hydrocarbon fuel or fine chemical. More specifically, in many embodiments, the high value chemical is a chemical selected from the group comprising (but not limited to): liquid alcohols, including ethanol, propanol, butanol, and pentanol; long chain hydrocarbons and olefins, including ethylene and propylene; synthetic gasoline; jet fuel hydrocarbons, and any combination thereof. In many such embodiments, the high value chemical is ethanol (CH₃CH₂OH). In particular, in many embodiments, the CO₂ electrolyzer produces ethanol from CO₂, water, and electricity via a step-wise, yet continuous, process generating CO and acetaldehyde (CH₃CHO) as the two main reaction intermediates. However, in some other embodiments, the CO₂ electrolyzer is employed to produce ethanol via the same method approach as described herein, but a chemically distinct route, including from different starting materials and or via different intermediates. For example, in some embodiments, a different proton source, such as, for example, a sacrificial alcohol, ammonia, a protonated amine, an inorganic acid (e.g., bicarbonate), or organic acid, is used instead of water.

In still yet other embodiments, the CO₂ electrolyzer relies on the process and method described herein to produce the high value chemical from a source molecule or molecules other than CO₂ or in combination with CO₂. For example, in some embodiments the CO₂ electrolyzer is employed to convert nitrogen gas to a nitrogen containing fuel or chemical. As yet another example, in some embodiments, the CO₂ electrolyzer is employed according to the process and method described herein to convert NO or NO₂ as the only components, or in combination, or in any combination with CO and CO₂, to carbon and nitrogen containing chemical products of value. Furthermore, in some embodiments, the starting materials fed into the CO₂ electrolyzer comprise dissolved nitrates or nitrites which, in turn, serve as a source of nitrogen atoms for the high value chemical. In some embodiments, the CO₂ electrolyzer is employed to convert SOx compounds to sulfur containing high value chemicals.

In many embodiments, the CO₂ electrolyzer comprises a column reactor further comprising vertically stacked and continuously connected sections of electrocatalytic stages, wherein each section's stages are functionalized with the electrolysis step-specific catalyst, for performing the individual intermediate reaction steps of CO₂ (or another starting material) conversion to the high value chemical, such as to optimize the overall conversion of CO₂ (or another starting materials) to the high value chemical.

More specifically, in many embodiments, each section comprises two or more stages for conducting the section-specific electrolysis step, as illustrated in FIGS. 3A through 3C. In many such embodiments, a stage comprises a cathode compartment comprising a cathode functionalized with the electrolysis step-specific catalyst and an anode compartment comprising an anode, separated by an ion conductive membrane. In addition, in many embodiments, a system of back-pressure regulators is used to balance the pressure between the cathode and anode compartments. Moreover, in many embodiments, the stage comprises a plurality of bubble caps (or gas spurgers), characterized by a number and a density of bubble caps, distributed throughout the stage (FIG. 3A, inset), wherein the number of bubble caps determines the total gas flow through the stage (from the stage below), while the density of bubble caps determines the degree of gas and liquid contact at each stage. In many embodiments, the architecture, porosity, morphology, thickness, and composition of the cathode and anode, as well as the distance of the cathode from the ion conductive membrane and from the gas/liquid interface in each stage are all rationally optimized to improve the transport of the gas and liquid reactants to the electrocatalytic surface of the cathode, and, thus, to optimize the reaction occurring on the electrocatalytic surface of the cathode of a given stage. In particular, although not to be bound by any theory, transport at the mesoscale determines the electrocatalyst's selectivity (as described, for example, in Carlos Morales-Guio, et al. Electrochemical CO₂ Reduction Mechanism on Copper: Relation between Mesoscopic Mass Transport and Intrinsic Kinetics, 1 Apr. 2024, PREPRINT (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs-4189647/v1]; the disclosure of which is incorporated herein by reference) by modifying the microscopic environment, as well as the residence time, of reactants, intermediates and products. Accordingly, in many embodiments, various physical and chemical properties of each electrocatalytic stage (including its cathode and anode) within the multi-stage column architecture of the CO₂ electrolyzer are individually addressed and adjusted to optimize the performance of each stage in the CO₂ electrolysis step it is intended to perform.

Furthermore, in many embodiments, the choice of the electrolysis step-specific catalyst for each step of the electrolysis process to obtain the high value chemical is dictated by the results of simulation studies of the CO₂ electrolyzer as described herein. In many embodiments, the electrolysis step-specific catalyst is a catalyst selected from the group comprising (but not limited to): transition metals, transition metal oxides, carbon-based materials, transition metal nanoparticles, monodispersed catalytically active sites, metal alloys, and any combination thereof. In many embodiments, the electrolysis step-specific catalyst is applied to the cathode via one of the methods selected from the group comprising (but not limited to): sprayed, grown, electrodeposited, painted, deposited, and any combination thereof. In many embodiments, the electrolysis step-specific catalyst is optimized to perform the intended electrolysis step of the overall electrochemical transformation to obtain the high value chemical.

In many embodiments, the stages (and, therefore, sections) are stacked within the column reactor in a vertical, bottom to top fashion, such that the stages (and, thus, the electrocatalytic surfaces of the cathode and anode) are perpendicular to the flow of the electrolysis reactants through the column reactor of the CO₂ electrolyzer (FIGS. 3A through 3C). In many embodiments, a minimum of 2 stages are functionalized with each electrolysis step-specific catalyst (i.e., resulting in at least 6 stages for the CO₂ electrolyzer used to perform the CO₂ electroreduction to ethanol in the three step process described herein). In many embodiments, the overall number of stages is 6 to 100. In many such embodiments, more stages allow for more detailed tenability of the transformation. However, it should also be noted that, according to many embodiments, too many stages make it more difficult to control the overall electrolysis process, and result in a more expensive to build and operate CO₂ electrolyzer.

In many embodiments, the cross section area of each stage is optimized (e.g., increased) by optimizing (e.g., increasing) the number of bubble caps on that stage, the area of the cathode covered by the electrolysis step-specific catalyst, and or the cross section area of the ion conductive membrane, such as to maximize reactant conversions, and, hence, to obtain high current and power. In many embodiments, the total area of the electrode in one stage can be adjusted to within 100 to 20,000 cm² range, thus affording the corresponding current densities of 10 to 2,000 mA/cm² (wherein mA is a milliamp). In some embodiments, wherein the electrode area is 0.001 to 2 m² the corresponding current densities range from 100 to 20,000 A/m². It should be noted here that, although not to be bound by any theory, electrode cross section areas much larger than 20,000 cm² may result in challenges associated with distributing gases and liquids over the stage area. On the other hand, electrode cross section areas much smaller than 100 cm² may result in high capex with low overall capacity for production in one column. Moreover, low current densities of less than 100 A/m² may result in high capex; while high current densities of higher than 20,000 A/m² may result in low selectivities and high demands for gas distribution, further resulting in potentially more expensive bubblers.

In many embodiments, the column reactor further comprises a gas inlet for delivering a gas stream comprising CO₂ gas to the bottom of the column reactor, a liquid inlet for delivering a liquid stream comprising a liquid electrolyte to the top of the column reactor, an effluent outlet for collecting a liquid effluent comprising liquid electrolysis products including the high value chemical at the bottom of the column, and any number of other inlets, outlets and valves, as needed for efficient and safe operation of the CO₂ electrolyzer. In many embodiments, the gas stream is a compressed gas stream. In addition, in many embodiments, any number of other liquid and or gas inlets and or outlets are added at any stage, section, or position of the column reactor to introduce or remove electrolysis step-specific reagents, intermediates, or products, such as to control activity and selectivity of that stage or section, and, as such, to improve performance of the CO₂ electrolyzer towards the production of the high value chemical and or efficiency of the energy storage.

FIG. 3A though 3C generally illustrate the operation of the CO₂ electrolyzer, including at the single stage level (FIGS. 3A, inset, and 3B), according to many embodiments. In many embodiments, the design and operation of the instant CO₂ electrolyzer is inspired by the design and operation of industrial scale distillation columns and processes, wherein the distillation columns and processes are optimized for contacting and handling of gas and liquid streams, such as to maximize mass and energy transfer between gas, solid, and liquid phases at industrially relevant scales.

To this end, in many embodiments, a gas stream rich in CO₂ is supplied to the column reactor of the CO₂ electrolyzer via the gas inlet situated at the bottom of the column reactor and travels upwards from stage to stage, perpendicularly to the plane of each stage/cathode it passes. In many embodiments, the gas stream comprises 1 to 100% CO₂ by volume. In some embodiments, the gas stream also comprises additional components, such as, for example (but not limited to): CO, H₂, O₂, CH₄, ethane, an aldehyde, an olefin, NH₃, NO_x, SO_x, N₂, a halogen (including Cl₂, I₂, Br₂), an inert gas (including Ar and He), and any combination thereof; as needed to promote the flow and conversion of the reagents, and, thus, to optimize the operation of the CO₂ electrolyzer. In many embodiments, the gas stream entering at the bottom of the column reactor is compressed and the gas stream travels upward the column reactor due to a pressure gradient between the bottom and the top of the column. In addition, in many embodiments, the gas stream is promoted upwards due to an upward decrease in pressure between stages of the column reactor, wherein the gas stream's flow is promoted upwards from higher pressure at the lower stages to lower pressure at the higher stages. In addition, in many embodiments, the CO₂ electrolyzer is operated at a high pressure of 5 to 100 bar to optimize gas mass flow rates and to improve the dissolution of gaseous reactants in the liquid phase of the liquid stream. As such, in many embodiments, the residence time of gaseous reactants (e.g., CO₂) of the gas stream at each stage comprising the electrolysis step-specific catalyst is increased (as compared to conventional electrolyzers), resulting in improved single pass conversion values for the reactants.

Furthermore, in many embodiments, as the gas stream travels upwards through the column reactor, it is met by the liquid stream delivered through the liquid inlet situated at the top of the column reactor and travelling downward. In many embodiments, the liquid stream comprises a liquid electrolyte. In many embodiments, the liquid electrolyte is an aqueous, a non-aqueous, or a two-phase mixture (including an immiscible aqueous/non-aqueous phase) ionic conductive liquid capable of liquid-like flow and conducting charges. In many embodiments, the liquid electrolyte is an aqueous electrolyte. In some embodiments, a pump situated at the top of the column reactor is used to pump the liquid stream into the column reactor, towards the top-most stage. However, in some embodiments one or more additional pumps are used to direct the liquid stream to any given stage within the column reactor as a pump-around feature. Accordingly, in many embodiments, each stage of the column reactor receives the liquid stream from the stage immediately above (or directly from the liquid inlet for the top-most stage, or from the pump-around feature) and the gas stream from the stage immediately below (or directly from the gas inlet for the lowest stage). In many such embodiments, the gas stream enters the stage comprising the liquid electrolyte by being bubbled directly into or above the liquid volume in the stage through the plurality of the bubble caps and, as such, gets distributed homogeneously over the stage/cathode and saturates the liquid electrolyte with the gaseous reactants (e.g., CO₂ and or CO), thus enabling efficient electrolysis of the gaseous reactants. Next, in many embodiments, the incoming liquid stream washes over the stage/cathode collecting all liquid products of the electrolysis, and spills over to the stage immediately below driven by gravity, similarly to the processes occurring in a distillation column. Accordingly, in many embodiments, the process described herein affords the liquid effluent, collected at the bottom of the column reactor, comprising the high value chemical in a product concentration, wherein the product concentration is 0.1 to 50% by volume of the liquid effluent.

In many embodiments, the product selectivity of the CO₂ electrolyzer is optimized by applying dynamic multi-variable control to the applied potential, temperature, and kinetics of the gas-liquid contact at each stage of the column reactor. In many such embodiments dynamic control ensures that the composition of the gas and liquid streams traveling from one stage to another is tuned to maximize the production of the high value chemical. In many embodiments, the dynamic control of the CO₂ electrolyzer leverages machine learning-based predictive control systems. In some embodiments, multivariable dynamic feedback control, such as described, for example, in: Çitmaci, B., et al., Machine learning-based ethylene and carbon monoxide estimation, real-time optimization, and multivariable feedback control of an experimental electrochemical reactor. Chemical Engineering Research and Design 2023, 191, 658-681; Richard, D., et al., Smart manufacturing inspired approach to research, development, and scale-up of electrified chemical manufacturing systems. iScience 2023, 26 (6), 106966; Luo, J. W., et al., Machine Learning-Based Operational Modeling of an Electrochemical Reactor: Handling Data Variability and Improving Empirical Models. Industrial & Engineering Chemistry Research 2022, 61 (24), 8399-8410; Çitmaci, B., et al., Machine Learning-Based Ethylene Concentration Estimation, Real-Time Optimization and Feedback Control of an Experimental Electrochemical Reactor. Industrial & Engineering Chemistry Research 2022, 185, 87-107; and Çitmaci, B., et al., Digitalization of an experimental electrochemical reactor via the smart manufacturing innovation platform. Digital Chemical Engineering 2022, 5, 100050 (the disclosures of which are incorporated herein by reference) is utilized in the control of the CO₂ electrolyzer to optimize the cost of the high value chemical production.

Accordingly, as one illustrative example of many embodiments, the CO₂ electrolyzer achieves CO₂ reduction to ethanol in three discrete electrochemical transformation steps conducted in the corresponding sections of the column reactor comprising stages bearing the step/section specific catalysts. In many such embodiments, the chemical transformation steps and the corresponding sections comprise:

• • 1) CO₂→CO, conducted in a first section comprising the plurality of stages located at the bottom of the column reactor;
  • 2) CO→CH₃CHO (acetaldehyde), conducted in a second section comprising the plurality of stages located at the top of the column reactor; and
  • 3) CH₃CHO→CH₃CH₂OH (ethanol), conducted in a third section comprising the plurality of stages located in the middle of the column reactor, immediately above and in fluid communication with the first section and immediately below and in fluid communication with the second section,
    as schematically shown in FIG. 3C.

To this end, the first section and the corresponding stages comprised within (e.g., stages 15-20 in FIG. 3C) are optimized for selective transformation of CO₂ to CO. Accordingly, the section specific catalyst in the first section is a first catalyst selective for the production of CO and enabling the electroreduction of CO₂ to CO. In many such embodiments, the first catalyst is porous silver catalyst, selected on the principle that CO binds weakly to porous silver, thus resulting in high turnover rates for the production of CO from CO₂ electrolysis (as discussed, for example, in Wakerley, D., et al., Gas diffusion electrodes, reactor designs and key metrics of low-temperature CO2electrolysers. Nature Energy 2022, 7 (2), 130-143, the disclosure of which is incorporated herein by reference). However, in some other embodiments, the first catalyst is a catalyst selected from the group comprising: porous gold, silver and gold nanoparticles on a conductive support, a single-site transition metal on a nitrogen-doped conductive carbon support, an ionic-liquid modified and unmodified metal sulfide, a carbide, a phosphide, a boride, and any combination thereof. In many embodiments, Faradaic efficiency of the CO₂ electrolyzer (the first section) for the production of CO from CO₂ is as high as 96% or higher.

Furthermore, the second section and the corresponding stages comprised within (e.g., stages 1-10 in FIG. 3C) are optimized for selective transformation of CO to acetaldehyde. Accordingly, the section specific catalyst in the second section is a second catalyst selective for the production of acetaldehyde from CO. In many such embodiments, the second catalyst is a porous copper-silver alloy catalyst, which has been shown to be highly selective for the production of acetaldehyde starting from streams enriched in CO (see, for example: Wang, L., et al., Selective reduction of CO to acetaldehyde with CuAg electrocatalysts. Proc. Natl. Acad. Sci. USA 2020, 117 (23), 12572-12575; and Wang, et al., Electrochemically converting carbon monoxide to liquid fuels by directing selectivity with electrode surface area. Nature Catalysis 2019, 2 (8), 702-708; the disclosures of which are incorporated herein by reference). Here, silver is added to copper in the second catalyst on the principle that Ag changes the selectivity of copper for CO reduction by lowering the binding energy of acetaldehyde to the surface of the second catalyst (see, for example: Bertheussen, E.; et al., Acetaldehyde as an Intermediate in the Electroreduction of Carbon Monoxide to Ethanol on Oxide-Derived Copper. Angew. Chem. Int. Ed. 2016, 55 (4), 1450-1454; Clark, E. L.; et al., Electrochemical CO₂ Reduction over Compressively Strained CuAg Surface Alloys with Enhanced Multi-Carbon Oxygenate Selectivity. J. Am. Chem. Soc. 2017, 139 (44), 15848-15857; and Higgins, D., et al., Guiding Electrochemical Carbon Dioxide Reduction toward Carbonyls Using Copper Silver Thin Films with Interphase Miscibility. ACS Energy Lett. 2018, 3 (12), 2947-2955, the disclosures of which are incorporated herein by reference), such that, by reducing the number of electrons transferred in this transformation to each carbon atom, the rate of conversion accelerates, allowing to achieve Faradaic efficiencies conversion to liquid products of up to 100%. However, in some other embodiments, the second catalyst is a catalyst selected from the group comprising: another copper-based material, an alloy of Ni, Fe, Au, Zn, Sn, Ag, and In without inclusion of Cu, and any combination thereof. In addition, in many embodiments, the selectivity of the second catalyst for acetaldehyde is tuned via the rational control of the catalyst's composition and porosity, as well as the hydrodynamic conditions at each stage. Accordingly, in many embodiments, Faradaic efficiency of the CO₂ electrolyzer (the second section) for the production of acetaldehyde from CO is as high as 80%, or higher.

It should be noted here, that some gas volume contraction and accompanying pressure drop are expected in the second section of the CO₂ electrolyzer as CO gas is converted to liquid acetaldehyde. Accordingly, in some embodiments, the column reactor is tapered at the upper portion (corresponding to the second section) to a narrower diameter than the rest of the column reactor to compensate for the contraction of the gas volume as CO is consumed, such as a small pressure drop is maintained as CO is transformed into liquid CH₃CHO.

Next, the third section and the corresponding stages comprised within (e.g., stages 11-14 in FIG. 3C) are optimized for selective transformation of acetaldehyde to ethanol in the presence of the CO enriched gas stream. Accordingly, the section specific catalyst in the third section is a third catalyst selective for the production of ethanol from acetaldehyde in the presence of CO and CO₂. In many embodiments, the third catalyst is porous copper or gold nanoparticles supported on porous copper. Here, the addition of gold nanoparticles further increases the conversion of CO₂ to ethanol through a previously elucidated tandem reaction mechanism (as discussed in Morales-Guio, C., et al., Improved CO₂ reduction activity towards C2+ alcohols on a tandem gold on copper electrocatalyst. Nature Catalysis 2018, 1 (10), 764-771, the disclosure of which is incorporated herein by reference). In addition, copper is known to strongly bind acetaldehyde and, as such, is expected to reduce it to ethanol selectively. However, in some embodiments, the third catalyst is a catalyst selected from the group comprising: a transition metal, including Pd, and any catalyst capable of the electrochemical hydrogenation of carbonyl compounds.

Accordingly, in many embodiments, as illustrated in FIG. 3C, the CO₂-rich stream enters the CO₂ electrolyzer at the bottom of the column reactor and is directed to the first section, wherein CO₂ is electrocatalytically and selectively converted to CO, thus depleting the CO₂-rich stream of CO₂ and enriching it with CO to afford a CO-enriched gas stream. In many embodiments, next, the CO-enriched gas stream continues to travel upward the column reactor to the second section at the top of the column reactor, wherein the CO gas generated in the first section of the column reactor is selectively transformed into liquid acetaldehyde. Furthermore, in many embodiments, the liquid stream entering the CO₂ electrolyzer at the top of the column reactor, washes the acetaldehyde producing stages of the second section and carries the acetaldehyde down as the liquid stream travels towards the third section in the middle of the column rector. In many such embodiments, next, the acetaldehyde delivered with the liquid stream to the third section is further electrocatalytically reduced to ethanol, which, in turn, continues to travel down the column reactor with the liquid stream, and then leaves the CO₂ electrolyzer through the effluent outlet for collection.

In some embodiments, the CO₂ electrolyzer is used as a standalone reactor, while in other embodiments, pluralities of CO₂ electrolyzers are bundled into units, as shown, for example, in FIG. 4, in order to optimize their operation. In many such embodiments, bundling of CO₂ electrolyzers into units that share cooling systems and require a lesser number of auxiliary pumps and compressors allows for cost-effective operations. For example, in some embodiments, a unit of CO₂ electrolyzers comprises 12 individual CO₂ electrolyzers of many embodiments. For example, twelve 36 kW CO₂ electrolyzers may be operated as a single 432 kW unit according to some embodiments, as schematically shown in FIG. 4 (top). However, in other embodiments, a unit comprises another number of CO₂ electrolyzers, optimized for the individual size of each column reactor and the need for cooling by a heat exchanger.

Moreover, in many embodiments, units of pluralities of CO₂ electrolyzers are further bundled into even larger systems, as needed, for optimization of a desired application. For example, FIG. 4 (bottom) shows an energy storage system with a capacity to respond to an intermittent energy availability, wherein the energy storage system is built using 22 units of 12 ethanol producing CO₂ electrolyzers each (a total of 264 CO₂ electrolyzers), according to many embodiments. In this example of an application, the number of CO₂ electrolyzer units that are actively producing ethanol at a given time depends on energy availability at that time, wherein when more energy is available, more units of the energy storage system of many embodiments are actively producing ethanol (i.e., in this particular example, storing the excess energy as a fuel). However, as energy availability declines, an appropriate number of units is put into a standby mode until further need. Accordingly, in many embodiments, the CO₂ electrolyzers and systems comprising pluralities thereof are extremely well suited for applications related to storage of intermittent renewable energy (such as, for example, wind and solar energy) as liquid fuels (or other fine chemicals of value) due to the modular design and dynamic operation capabilities of the CO₂ electrolyzer. As such, in many embodiments, the CO₂ electrolyzer and systems comprising pluralities thereof are employed for long-and medium-term storage of energy from intermittent renewable energy sources, wherein the energy is stored in high value chemicals, such as, for example, liquid fuels, produced by the CO₂ electrolyzer. In many such embodiments, the dynamic operation of the CO₂ electrolyzer is optimized by machine learning based process control. In many embodiments, the CO₂ electrolyzer increases the deployment of renewable energy and allows to bypass the wait time for interconnection to the electric grid.

Accordingly, in many embodiments, the distillation column-inspired, multi-stage CO₂ electrolyzer is employed to complete a multi-electron electrochemical transformation in a step-wise manner, wherein reactants, intermediates, and products are transported up and down the vertical column geometry of the column reactor to generate the high value chemical; and wherein each step of the electrochemical transformation is independently controlled by electrical and chemical means, including via control over the applied potential, the current, and the catalyst choice at each step. Furthermore, the column reactor design of the CO₂ electrolyzer of many embodiments ensures homogenous distribution of the gas components of the transformation over large electrode areas in a manner that is distinct from that of conventional gas diffusion and flow cell electrolyzers. As such, the CO₂ electrolyzer allows for dramatically increased (as compared to the current state of the art electrolyzers) single pass efficiency and enables high CO₂ conversion—a key metric for the CO₂ electroreduction process. In many embodiments, the instant CO₂ electrolyzer is characterized by 80% or higher single pass conversion of CO₂. In addition, in many embodiments, and in contrast to conventional electrolyzers, the modular nature of the CO₂ electrolyzer of the instant disclosure allows for facile scale up of the CO₂ electrolysis process to high power. In many embodiments, the CO₂ electrolyzer follows a principally new learning curve that differs from that of conventional electrolyzers based on gas-diffusion electrode architectures, but rather resembles that of commercial chemical manufacturing facilities at large scale. In some embodiments, the CO₂ electrolyzer is adapted for other (than CO₂ to ethanol) electrocatalytic transformation requiring high gas conversion.

More specifically, in many embodiments, the column reactor design of the CO₂ electrolyzer comprising vertically stacked, plate-like electrocatalytic stages eliminates concentration gradients of reactants at each stage, thus enabling long residence times for the reactants (e.g., CO₂) and a high single pass conversion. These features and enhancements are in stark contrast to conventional fuel cells and water electrolyzers, which are inherently limited to a low one pass CO₂ conversion. Furthermore, in contrast to conventional electrolyzers (such as, for example, shown in FIG. 1), wherein the reactants are flown parallel to and along the electrocatalytic surface of the cathode, the electrode area of the CO₂ electrolyzer can be increased in the direction perpendicular to the flow of the reactants (both gaseous, e.g., CO₂ and CO, and liquid, e.g., acetaldehyde) by simply increasing the cross-section area of the stage and the number of bubble caps (FIG. 3B).

In addition, in many embodiments, operating the CO₂ electrolyzer at a high pressure not only allows to process high gas mass flow rates in a reasonable capex range, but also improves the dissolution of gaseous reactants (i.e., CO₂ and CO) in the liquid phase of the liquid stream and, as such, the delivery efficiency of the reactants to the catalyst, further improving their single pass conversion. In many embodiments, the high pressure is in the range of 5 to 100 bar. Moreover, in many embodiments, the column pressure within the column reactor and the stage pressure at each stage are optimized for the most efficient delivery of reactants to the electrocatalytic surfaces of the stages, as well as for operating within a range of pressures optimal for selectivity of the corresponding catalysts. In addition, in many embodiments, the operating pressure and pressure drops across stages are also adjusted for optimal bubble caps performance, similarly to optimizations conducted when operating gas/liquid contacting systems, such as, for example, distillation and reactive distillation columns. Accordingly, in many embodiments, the CO₂ electrolyzer decouples macroscale transport of gases from reactant transport at the micro-scale, and, thus, allows for enhanced production and facile collection of concentrated liquid fuels.

Furthermore, in many embodiments, the CO₂ electrolyzer decomposes the electroreduction of CO₂ (or another starting material) to the high value chemical into several distinct electrochemical steps conducted in different sections of the column reactor, wherein each section comprises a differently optimized catalyst specific to each electrochemical step/transformation. As such, in many embodiments, the CO₂ electrolyzer enables very high Faradaic efficiency and product selectivity at each electrochemical step of the overall electrolysis process (thus improving the overall efficiency and selectivity), in stark contrast to conventional one-pot reactions approaches, which suffer from low selectivity and, thus, low Faradaic efficiencies. Furthermore, in contrast to conventional electrolyzers, optimization of the CO₂ electrolyzer of many embodiments allows for continuous production processes at large scale.

Moreover, in many embodiments, the CO₂ electrolyzer is amenable to facile dynamical control of the electrolysis process via operating potential. In many embodiments, the dynamic control enables tuning of the gas-liquid composition at each stage within the column reactor. In many embodiments, the dynamic control of the electrolysis process within the CO₂ electrolyzer is greatly simplified, as compared to conventional fuel cells and electrolyzers, due to elimination of concentration gradients across large electrode surface areas at each stage.

EXEMPLARY EMBODIMENTS

The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to make and use the present invention, and are not intended to limit the scope of what the inventors regard as their invention nor are they intended to represent that the experiments below are all or the only experiments performed. Efforts have been made to ensure accuracy with respect to numbers used (e.g., amounts, temperature, etc.) but some experimental errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, molecular weight is weight average molecular weight, and temperature is in degrees Celsius. Standard abbreviations may be used, e.g., s or sec, second(s); min, minute(s); h or hr, hour(s); and the like.

Example 1—A Prototype 36 kW CO₂ Electrolyzer With 20 Stages

FIG. 3C schematically illustrates an exemplary embodiment, wherein the CO₂ electrolyzer is a 36 kW electrolyzer comprising 20 stacked electrochemical cell stages (stage 1 through stage 20), wherein the first section (at the bottom of the column reactor) comprises stages 15 through 20 (functionalized with the first catalyst, selective for conversion of CO₂ to CO), the second section (at the top of the column reactor) comprises stages 1 through 10 (functionalized with the second catalyst, selective for conversion of CO to acetaldehyde), and the third section (in the middle of the column reactor) comprises stages 11 through 14 (functionalized with the third catalyst, selective for conversion of acetaldehyde to ethanol). However, it should be noted here that, in many embodiments, the CO₂ electrolyzer is up to 100 kW, wherein the CO₂ electrolyzer's electrode surface area and or the current density applied are increased correspondingly. Furthermore, in many embodiments, the overall number of stages, (as well as the number of stages within a section) are adjusted as needed for optimization of steps of a desired electrolysis process. Accordingly, in many embodiments, the number of stages is at least 2 stages per electrolysis step (i.e., at least total of 6 stages for electrolysis of CO₂ to ethanol described herein). In some embodiments the number of stages is 20 to 100 stages. As illustrated here, and discussed throughout the instant disclosure, the gas stream comprising CO₂ enters the column reactor at the bottom and flows upwards, while the liquid stream enters the column reactor at the top and flows downwards driven by gravity.

Furthermore, FIGS. 5A through 5C provide data obtained for the CO₂ electrolyzer design shown in FIG. 3C from AVEVA PRO/II SIMULATION using kinetic experimental data acquired in a bench-scale rotating cylinder electrode cell. It should be noted here that, although AVEVA PRO/II SIMULATION was used in these particular exemplary experiments, any other suitable chemical process simulator may be used as needed. Accordingly, in some embodiments, another chemical process simulator is used in the design and modeling of the CO₂ electrolyzer. Here, the 36 kW CO₂ electrolyzer of many embodiments comprises 20 stages to afford a total electrode surface area of 10 m²; while the target average current density is 2,000 A/m², and the target overall Faradaic Efficiency (F.E) for ethanol is 91%. It should be noted here that, although not to be bound by any theory, the only by-products of the electrocatalytic conversion of CO₂ to ethanol by the CO₂ electrolyzer of many embodiments are hydrogen (F.E. 5%) and unconverted CO (F.E.=4%). (Notably, in some embodiments, even higher F.E. values are achieved, yet to the detriment of the economics of the electrolysis process.) In addition, in some embodiments, the ethanol effluent leaving the column reactor comprises trace amounts of acetaldehyde.

To this end, FIGS. 5A through 5C provide PRO/II simulation generated parameters for the section-specific catalyst selectivity at each stage (FIG. 5A), as well as the flow rates of gas (FIG. 5B) and liquid (FIG. 5C) reactants and products of the CO₂ electrolyzer of many embodiments. Furthermore, such simulation in PRO/II has been used to determine the mass and energy balance around the operating prototype of the CO₂ electrolyzer shown in FIG. 3C using activity coefficient models regressed from experimental data.

More specifically, for example, and according to many embodiments, as seen from FIG. 5A, according to PRO/II the simulation, the first section of the column reactor (comprising stages 15-20 functionalized with porous silver catalyst) selectively transforms CO₂ to CO, achieving >96% Faradaic efficiency. Furthermore, according to the data provided in FIG. 5B, the gas stream flowing upward through the column reactor becomes enriched with CO in the first section.

Moreover, to further illustrate the functionality and capabilities of the CO₂ electrolyzer, the data in FIG. 5A shows that, according to many embodiments, the CO generated in the first section at the bottom of the column reactor is selectively transformed into acetaldehyde in the second section (comprising stages 1-11 functionalized with a porous copper-silver alloy catalyst) with >80% Faradaic efficiency. In addition, FIG. 5C provides simulation data showing that the acetaldehyde produced in the second section travels with the liquid stream down the column reactor towards the third section (situated in the middle of the column reactor), where it is further reduced to ethanol.

As yet further illustrative example, the data in FIG. 5A also shows that, according to many embodiments, the acetaldehyde generated in the second section at the top of the column reactor, in the presence of the CO enriched gas stream coming from the first section at the bottom of the column reactor, is selectively transformed into ethanol in the second section (comprising stages 12-14 functionalized with gold nanoparticles supported on a porous copper as the stage-specific catalyst). In addition, FIG. 5C provides simulation data showing that the thus produced in the third section ethanol travels with the liquid stream down the column reactor towards the exit at the bottom of the column reactor, past stage 20.

Example 2—An Energy Storage System Comprising Multiple CO₂ Electrolyzers

FIGS. 6A through 6C show a mode of operation of an energy storage system according to some embodiments, wherein the energy storage system comprises 264 36 kW CO₂ electrolyzers of many embodiments grouped in 22 units of 12 CO₂ electrolyzers each (as schematically shown in FIG. 4). Here, and according to many embodiments, grouping CO₂ electrolyzers into units is a cost-effective measure, as it allows for sharing of cooling systems, as well as reduces the need for auxiliary pumps and compressors. Accordingly, twelve 36 kW CO₂ electrolyzers operate as a one-unit assembly of 432 kW, equipped with a shared heat exchanger, water tank, and other necessary infrastructure (FIG. 4). Furthermore, in many embodiments, such an energy storage system comprising 264 36 kW CO₂ electrolyzers has the capacity to respond to energy availability of up to 9.5 MW. In many embodiments, increasing the number of repeating units comprising CO₂ electrolyzers increases the energy storage capacity of such energy storage systems. In many such embodiments, the energy storage capacity is up to 10 times larger than the 9.5 MW system described herein (close to 100 MW), or even larger.

However, it should be noted here that, in many embodiments, the number of the CO₂ electrolyzers in a unit or system is dictated by the balance between capital expenditures (CAPEX) and operating expenses (OPEX). More specifically, in many embodiments, the number of the CO₂ electrolyzers in a unit or system is determined by the balance between the cost of building and operating a single CO₂ electrolyzer and its size/capacity. For example, in some embodiments, wherein it is cheaper (for example, per kW entering the system or per unit of ethanol produced) to build and operate a bigger individual CO₂ electrolyzer than a smaller one, there exists an incentive to build larger CO₂ electrolyzers for larger overall plants. Accordingly, in many such embodiments, wherein, as an illustrative example, the peak power of electricity entering the system is 100 MW, it is more sensible to increase the size of each individual CO₂ electrolyzer in an energy storage system (for example, from 36 kW to 360 kW), but maintain or even reduce the overall number of the CO₂ electrolyzers in such a system.

To this end, as one example of many embodiments, if the energy input changes over time as shown in, for example, FIG. 6A (according to Wind Data from Iowa State University Automated Surface Observation System January 2022 https://www.energy.gov/sites/default/files/2023-08/land-based-wind-market-report-2023-edition.pdf, the disclosure of which is incorporated herein by reference), the number of CO₂ electrolyzers/units that are active at a given time changes accordingly, as shown in FIG. 6B. As such, the average power per unit fluctuates within a small range of 18 to 36 kW (FIG. 6C). In many embodiments, utilization capacity for a single CO₂ electrolyzer column is 50 to 100%, wherein lower powers result in changes in the performance of the column reactor and challenges with the control of gas and liquid transport in the column reactor. Furthermore, when energy availability requires that the number of active units decreases, the CO₂ electrolyzers of many embodiments are easily set in a pressurized standby mode. Notably, in many embodiments, during such standbys, potentials are applied across the cathode and anode to prevent corrosion of the catalysts, which is, otherwise, common under open circuit potentials (see, for example, Choi, J., et al., Direct Electrochemical Reduction of Ammonium Carbamate on Transition Metal Surfaces: Finding Activity and Stability Descriptors beyond those for CO₂ Reduction. 2024, the disclosure of which is incorporated herein by reference). In addition, in many embodiments, small currents are passed through the CO₂ electrolyzers when changes in energy input activity are detected to regenerate the catalysts.

DOCTRINE OF EQUIVALENTS

This description of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications. This description will enable others skilled in the art to best utilize and practice the invention in various embodiments and with various modifications as are suited to a particular use. The scope of the invention is defined by the following claims.