Electrochemical co 2 capture through PH-independent redox chemistry

Abstract

A system and method for an electrochemical capture and release of CO2 via a pH- independent, pH swing redox chemistry is described. More specifically, the system and method rely on the advantageously pH-independent redox chemistry of a carefully selected redox matter, wherein the redox matter is a molecule or material that undergoes reversible electrochemical redox reactions with the subsequent chemical interactions with H2O resulting in large pH changes without direct involvement of protons or hydroxide ions during the electrochemical step. The redox matter is stable in both its reduced and oxidized states, and is water soluble. The pH difference afforded by the redox matter is utilized to modulate the CO2 solubility in solution and capture and release CO2. This redox chemistry is pH-independent, wherein the equilibrium potential of the reaction is not directly dependent on the pH, which, in turn, enables highly efficient CO2 capture and release.

Description

ELECTROCHEMICAL CO2 CAPTURE THROUGH PH-INDEPENDENT REDOX CHEMISTRY CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The current application claims the benefit of and priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No.63 / 668,259, entitled “Electrochemical CO2 Capture through pH-Independent Redox Chemistry”, filed July 7, 2024, the disclosure of which is incorporated herein by reference in its entirety for all purposes. FIELD OF THE INVENTION

[0002] The current disclosure is directed to electrochemical capture of CO2 via a pH- independent pH swing redox chemistry. BACKGROUND OF THE INVENTION

[0003] Efficient removal of anthropogenic CO2, a greenhouse gas, from the atmosphere is critical for mitigating climate change and meeting climate goals (as discussed, for example, in J. Skea, et al., Mitigation of Climate Change Summary for Policymakers Climate Change 2022 Working Group III contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (2022); and S. Chu, Carbon Capture and Sequestration. Science 325, 1599 (2009), the disclosures of which are incorporated herein by reference). However, the practical deployment of currently available carbon capture technologies is limited primarily by their high energy costs. More specifically, most CO2capture and release methods require energy inputs on the order of 100 kJ / mol (see, for example, A. M. Zito, et al., Electrochemical Carbon Dioxide Capture and Concentration. Chem. Rev.123, 8069–8098 (2023), the disclosure of which is incorporated herein by reference), due to the irreversible energy loss in chemical bond forming / breaking, as well as heating of inactive components, such as aqueous solvent. By comparison, the minimum Gibbs free energy needed to increase the partial pressure of CO2is determined by the change in entropy and scales at ln(^^^^ / ^^^^), where ^^^^, ^^^^ are the initial capture and final release pressures of CO2. As such,concentrating from a 15 % CO2 stream at a point source to pure CO2 stream requires 4.7 kJ / mol, while direct air capture starting from 400 ppm requires 19.4 kJ / mol. As an even more specific example, some of today’s most common technologies for carbon capture at point sources utilize amine-functionalized sorbents and thermal stimuli to spontaneously capture CO2 from gas streams with sorbent’s amine functional groups, and then to controllably releases CO2upon sorbent heating. However, such an approach incurs high energy costs associated with the CO2 desorption from the sorbent, as well as with heating / evaporation of large amounts of non-active water solvent. Furthermore, major direct air capture (DAC) technologies, including technologies relying on alkaline water solutions and amine-functionalized solid matrices, which have amine functional groups decorated on porous support such as metal organic frameworks (MOFs), require even higher energy costs. Notably, emerging strategies, such as, for example, porous materials, do improve energy efficiencies of the CO2capture processes (as discussed, for example in: R. L. Siegelman, et al., Porous materials for carbon dioxide separations. Nat. Mater.20, 1060–1072 (2021); M. Ding, et al., Carbon capture and conversion using metal–organic frameworks and MOF-based materials. Chem. Soc. Rev.48, 2783–2828 (2019); and J.-B. Lin, et al., A scalable metal-organic framework as a durable physisorbent for carbon dioxide capture. Science 374, 1464–1469 (2021), the disclosures of which are incorporated herein by reference), however, the energy loss to heating inactive components remains difficult to avoid for thermochemical methods in general. Accordingly, there exists a great need for efficient and effective carbon capture technology. SUMMARY OF THE INVENTION

[0004] Various embodiments are directed to a system for electrochemical capture and release of carbon dioxide via a pH-swing mechanism, wherein the system is characterized by an operational pH swing range having a lower pH limit and an upper pH limit, and wherein the system includes: a flow cell, wherein the flow cell is an electrochemical flow cell, including: an anode, disposed within an anodic chamber,a cathode, disposed within a cathodic chamber, an ion exchange membrane for selectively transporting ions separating the anodic chamber and the cathodic chamber; and a plurality of endplates and gaskets to ensure tight electrical contact to the anode and cathode and sealing to prevent leakage; a capture tank, further including a gas inlet for delivering CO2to the system and a capture gas outlet; a release tank, further including a release gas outlet for outflow of released CO2, an electrolyte solution, at least including a redox matter in a redox matter concentration sufficient to generate the operational pH swing range optimized for thermodynamics and kinetics of CO2absorption; wherein the redox matter is a molecule or material that is redox active without exchanging protons or hydroxide ions, water soluble, and stable, including water-stable, in both its reduced and oxidized states, and wherein the electrochemical step is coupled with subsequent reversible chemical interactions with water to produce H+or OH- ions which have pH- dependance; and a supporting salt in a salt concentration optimized for CO2 capture performance; and any number of safety valves, pumps, inlets and outlets, as needed for safe and efficient operation; and wherein the capture tank, the flow cell, and the release tank are in fluid communication, such that the electrolyte solution is able to continuously circulate in sequence from the capture tank to the anodic chamber to the release tank to the cathodic chamber and back to the capture tank.

[0005] In various such embodiments, the redox matter is a molecule or material selected from the group consisting of: TEMPO; a TEMPO derivative; a soluble redox active material suitable for use in redox flow batteries.

[0006] In still various such embodiments, the redox matter is a TEMPO-derivative selected from the group consisting of: 4-hydroxy-TEMPO, amino-TEMPO, 4-oxo-TEMPO,trimethylammonium-TEMPO, 4-[3-(trimethylammonio)propoxy]-TEMPO, pyrrolidinium- TEMPO, riboflavin-TEMPO, a RSO3-(CH2)n-O-TEMPO radical, 4-carboxy-TEMPO, acetamido-TEMPO, 4-azido-TEMPO, 4-phosphonooxy-TEMPO, TEMPO functionalized with one or more functional group selected from: nitro, nitrile, sulfonic acid, sulfoxide, halide; and any combination thereof.

[0007] In still yet various embodiments, the anode and the cathode include porous graphite.

[0008] In yet still various such embodiments, the supporting salt is a salt selected from the group consisting of: KCl, NaCl, LiCl, K2SO4, Na2SO4, KH2PO3, NaH2PO3, Na2CO3, K2CO3, an organic amine, an amino acid salt, and any combination thereof.

[0009] In yet various such embodiments, the electrolyte solution further includes a base.

[0010] In various such embodiments, the base is selected from the group consisting of: KOH, NaOH, LiOH, K2CO3, Na2CO3, KHCO3, NaHCO3, NH4OH, a weak base, an amino acid salt, another zwitterion, and any combination thereof.

[0011] In still various such embodiments, the system is maintained within the operational pH swing range by one of the means selected from the group consisting of: a single pH buffer system; a dual pH buffer system; an active pH monitoring and electrochemical control without any buffer; and any combination thereof.

[0012] In yet still various such embodiments, the dual pH buffer system includes a first buffer, characterized by a first pKa, for maintaining the lower pH limit of the operational pH swing range; and a second buffer, characterized by a second pKa, higher than the first pKa, for maintaining the upper pH limit of the operational pH swing range.

[0013] In still yet various such embodiments, the first buffer is selected from the group consisting of: formic acid / formate, acetic acid / acetate, and any other buffer characterized by a pKavalue that is the same or near the first pKa value; and the second buffer is selected from the group consisting of: carbonate / bicarbonate system, phosphate species, an organic amine, ammonium ions, an amino acid,and another buffer characterized by a pKa value that is the same or near the second pKavalue.

[0014] In yet various such embodiments, the second buffer includes an organic amine, and the organic amine is monoethanolamine.

[0015] In various such embodiments, the second buffer includes the amino acid, and the amino acid is selected from the group consisting of: glycine, sarcosine, piperazine, and any salt derivative thereof.

[0016] In still various such embodiments, the single pH buffer system includes a buffer pair characterized by a single buffer pair pK^ of between about 6 and about 11, such as to maintain the system within the operational pH swing range.

[0017] In yet still various such embodiments, the single buffer pair is a buffer selected from the group consisting of: alkanolamines, amino acids, carbonate / bicarbonate, phosphate species, and another weak base.

[0018] In still yet various such embodiments, the system further includes a pH sensor and a controller configured to actively monitor and adjust pH of the electrolyte solution, such as to maintain the system within the operational pH swing range.

[0019] Various other embodiments are directed to a method for electrochemical capture and release of carbon dioxide via a pH-swing mechanism, including: providing a system characterized by an operational pH swing range having a lower pH limit and an upper pH limit including: a flow cell, wherein the flow cell is an electrochemical flow cell, including: an anode, disposed within an anodic chamber, a cathode, disposed within a cathodic chamber, an ion exchange membrane for selectively transporting ions separating the anodic chamber and the cathodic chamber; and a plurality of endplates and gaskets to ensure tight electrical contact to the anode and cathode and sealing to prevent leakage; a capture tank, further including a gas inlet for delivering CO2 to the system and a capture gas outlet; a release tank, further including a release gas outlet for outflow of released CO2,an electrolyte solution, at least including: a redox matter in a redox matter concentration sufficient to generate the operational pH swing range optimized for thermodynamics and kinetics of CO2 absorption; wherein the redox matter is a molecule or material that is redox active without exchanging protons or hydroxide ions, water soluble, and stable, including water-stable, in both its reduced and oxidized states, and wherein the electrochemical step is coupled with subsequent reversible chemical interactions with water to produce H+or OH- ions which have pH- dependance; and a supporting salt in a salt concentration optimized for CO2 capture performance; and any number of valves, pumps, inlets and outlets, as needed for efficient operation; and wherein the capture tank, the flow cell, and the release tank are in fluid communication, such that the electrolyte solution is able to continuously circulate in sequence from the capture tank to the anodic chamber to the release tank to the cathodic chamber and back to the capture tank; providing a gaseous stream including CO2; and delivering it to the capture tank; applying voltage to the flow cell and circulating the electrolyte solution through the system, to continuously and energy efficiently capture CO2 from the gaseous stream and controllably release CO2for further concentration and storage.

[0020] In various such embodiments, the method is characterized by an energy cost, and the energy cost is as low as 0 – 30 kJ mol-1.

[0021] In still various such embodiments, the redox matter is a molecule or material selected from the group consisting of: TEMPO; a TEMPO derivative; a soluble redox active material suitable for use in redox flow batteries.

[0022] In still yet various embodiments, the redox matter is a TEMPO-derivative selected from the group consisting of: 4-hydroxy-TEMPO, amino-TEMPO, 4-oxo-TEMPO, trimethylammonium-TEMPO, 4-[3-(trimethylammonio)propoxy]-TEMPO, pyrrolidinium-TEMPO, riboflavin-TEMPO, a RSO3-(CH2)n-O-TEMPO radical, 4-carboxy-TEMPO, acetamido-TEMPO, 4-azido-TEMPO, 4-phosphonooxy-TEMPO, TEMPO functionalized with one or more functional group selected from: nitro, nitrile, sulfonic acid, sulfoxide, halide; and any combination thereof.

[0023] In yet still various such embodiments, the anode and the cathode include porous graphite.

[0024] In yet various such embodiments, the supporting salt is a salt selected from the group consisting of: KCl, NaCl, LiCl, K2SO4, Na2SO4, KH2PO3, NaH2PO3, Na2CO3, K2CO3, an organic amine, an amino acid salt, and any combination thereof.

[0025] In various such embodiments, the electrolyte solution further includes a base.

[0026] In still various such embodiments, the base is selected from the group consisting of: KOH, NaOH, LiOH, K2CO3, Na2CO3, KHCO3, NaHCO3,NH4OH, a weak base, an amino acid salt, another zwitterion, and any combination thereof.

[0027] In yet still various such embodiments, the system is maintained within the operational pH swing range by one of the means selected from the group consisting of: a single pH buffer system; a dual pH buffer system; an active pH monitoring and electrochemical control without any buffer; and any combination thereof.

[0028] In still yet various such embodiments, the dual pH buffer system includes a first buffer, characterized by a first pKa, for maintaining the lower pH limit of the operational pH swing range; and a second buffer, characterized by a second pKa, higher than the first pKa, for maintaining the upper pH limit of the operational pH swing range.

[0029] In yet various such embodiments, the first buffer is selected from the group consisting of: formic acid / formate, acetic acid / acetate, and any other buffer characterized by a pKa value appropriate for establishing and maintaining the lower pH limit of the operational pH swing range; and the second buffer is selected from the group consisting of: carbonate / bicarbonate system, phosphate species, monoethanolamine, ammonium ions, sarcosine, and another buffer characterized by a pKavalue appropriate for establishing and maintaining the upper pH limit of the operational pH swing range.

[0030] In still various such embodiments, the single pH buffer system includes a buffer pair characterized by a single buffer pair pK^ of between about 6 and about 11, such as to maintain the system within the operational pH swing range.

[0031] In yet still various such embodiments, the single buffer pair is a buffer selected from the group consisting of: alkanolamines, amino acids, carbonate / bicarbonate, phosphate species, and another weak base.

[0032] In still yet various such embodiments, the system further includes a pH sensor and a controller configured to actively monitor and adjust pH of the electrolyte solution, such as to maintain the system within the operational pH swing range; and the method further includes utilizing the pH sensor and the controller.

[0033] 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 forms a part of this disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] 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:

[0035] FIG. 1 provides the Nernstian potential Δ^^, which gives the minimum electrochemical potential difference needed to increase the number of hydrogen ions, and sets the minimum in the electrochemical energy voltage needed to increase [H+] and release CO2 in a conventional pH swing approach, assuming the ratio between [Red-H] and [Ox] remains constant, wherein CO2 is captured at high pH and released at low pH, according to prior art.

[0036] FIGs.2A and 2B depict the reversible electrochemical redox chemistry of the redox matter, wherein, more specifically, FIG.2A depicts the reversible electrochemical redox chemistry of the redox matter, wherein the redox matter is, for example, a generic(2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) molecule, such that the reduced form of the redox matter induces a high pH and the oxidized form induces a low pH, thus enabling pH swing through the redox reaction; while FIG.2B depicts the observed pH swing cycles during the redox reaction using the as-prepared and titrated (to pH 8) (2,2,6,6- Tetramethylpiperidin-1-yl)oxyl TEMPO derivative (4-oxo-TEMPO), as an example, in accordance with embodiments of the application.

[0037] FIGs.3A and 3B show cyclic voltammetry of 5 mM of H-TEMPO with 100 mM of KCl, wherein FIG.3A shows the data collected under N2 or CO2 bubbling, wherein voltage is swept at 50 mV s-1, and the peak with the positive current corresponds to the oxidation reaction, while the peak with the negative current corresponds to the reduction reaction, which illustrates that, in both cases of N2 and CO2, TEMPO redox is reversible and the peak positions are almost identical, signifying that the equilibrium potential does not change upon CO2injection, and further suggesting that the thermodynamic energy penalty associated with CO2 capture is minimal; while FIG.3B shows the data collected over 1000 cycles at 50 mV s-1sweep rate, wherein it could be seen that the peak currents remain relatively steady over 1000 cycles; and even slightly increase over cycling, potentially due to improved wetting of porous electrodes, thus illustrating that H-TEMPO redox is highly stable, in accordance with embodiments of the application.

[0038] FIG.4 schematically shows reversible oxidation and reduction reactions of 4- amino-TEMPO (A-TEMPO), wherein A-TEMPO is a stable free radical in its reduced form, stabilized by delocalized electrons between nitroxyl nitrogen and oxygen, and also stable in its oxidized, the positively charged, oxoammonium form (top); and also provides cyclic voltammetry of 5 mM A-TEMPO 100 mM KCl solution under N2and CO2bubbling, showing similar peak positions and equilibrium potential, which suggests minimal thermodynamic energy cost of CO2capture and release (bottom), in accordance with embodiments of the application.

[0039] FIGs.5A and 5B illustrate pH change with oxidation of H-TEMPO, wherein FIG. 5A provides data illustrating that pH of an H-TEMPO solution decreases with oxidation, wherein, notably, pH is higher than -log[TEMPO+], signifying that TEMPO+acts as a weaker acid than H+; while FIG. 5B shows pH before and after oxidation for variousconcentrations of H-TEMPO solutions, wherein increasing H-TEMPO concentration leads to higher pH when reduced and lower pH when oxidized, allowing for larger pH swings at higher concentrations of oxidized TEMPO, in accordance with embodiments of the application.

[0040] FIG.6 shows titration data for oxidized (top) and reduced (bottom) H-TEMPO, illustrating that oxidized H-TEMPO acts as a weak acid, wherein a solution 10 mL of 0.4 M of either oxidized (top) or reduced (bottom) H-TEMPO in 1.2 M KCl is titrated with 1 M KOH or 1 M HCl titrant respectively, in accordance with embodiments of the application.

[0041] FIG.7 shows pH before and after oxidation of a 100 mM A-TEMPO 400 mM KCl solution, showing that A-TEMPO oxidation allows for pH swings similar to that of H- TEMPO, in accordance with embodiments of the application

[0042] FIG.8 illustrates and provides data for experiments, wherein data was collected from 5 mM H-TEMPO 100 mM KCl electrolyte solution at different pHs, illustrating that the redox potential of H-TEMPO is largely independent of pH; in accordance with embodiments of the application.

[0043] FIG. 9 provides cyclic voltammetry (top) and the corresponding equilibrium redox potential data (bottom, as obtained from the midpoint of the oxidative and reductive peaks in cyclic voltammetry plots) collected from a 5 mM A-TEMPO 100 mM KCl solution at various pHs (top), wherein A-TEMPO is shown to have a pH-independent equilibrium potential, similarly to H-TEMPO, in accordance with embodiments of the application.

[0044] FIG. 10 shows molecular dynamics (MD) simulations of 0.4 M reduced H- TEMPO with 1.2 M KCl 0.2 M KOH electrolyte solution (top), and 0.4 M oxidized H- TEMPO with 1.2 M K+1.6 M Cl- 0.2 M KOH electrolyte solution (bottom), in accordance with embodiments of the application.

[0045] FIG.11A shows MD simulations of the radial distribution functions, defined as ^^(^^)=〈^^(^^)〉 / ^^^^^^^^, wherein ^^ is the density of the oxygen atom on OH- and H2O (abbreviated OH- O and H2O O, respectively) , K+and Cl- around the nitroxyl O on H-TEMPO (top); and of the cumulative distribution functions, defined as 4^^^^^^^^^^∫^^2^^(^^)^^^^, for the nitroxyl O on H-TEMPO in 0.4 M reduced H-TEMPO with 1.2 M KCl 0.2 M KOH electrolyte; FIG. 11B shows MD simulations of the radial distribution functions (top) and the cumulativedistribution functions, defined as 4^^^^^^^^^^∫^^2^^(^^)^^^^ (bottom), which are essentially the coordination numbers for the nitroxyl N on H-TEMPO in 0.4 M reduced H-TEMPO with 1.2 M KCl 0.2 M KOH electrolyte; FIG.11C shows MD simulations of the radial distribution functions (top) and the cumulative distribution functions (bottom), which are essentially the coordination numbers for the nitroxyl O on H-TEMPO in 0.4 M oxidized H-TEMPO with 1.2 M K+1.6 M Cl- 0.2 M KOH electrolyte; while FIG.11D shows MD simulations of the radial distribution functions (top) and the coordination function between the oxidized TEMPO of the oxygen atoms in water s (bottom), which are essentially the coordination numbers for the nitroxyl N on H-TEMPO in 0.4 M oxidized H-TEMPO with 1.2 M K+1.6 M Cl- 0.2 M KOH electrolyte, in accordance with embodiments of the application.

[0046] FIG. 12 shows MD simulation of the radial distribution function, defined as ^^(^^)≡〈^^(^^)〉 / ^^^^^^^^ where ^^ is the density of the oxygen atom attached to water (abbreviated H2O O) around the nitroxyl O on H-TEMPO, indicating that the TEMPO-water distance depends on the oxidation state, in accordance with embodiments of the application.

[0047] FIG.13A shows MD simulation of hydration structures of reduced (left) and oxidized (right) H-TEMPO, showing different water polarization; while FIGs.13B and 13C provide snapshots of reduced (FIG.13B) and oxidized (FIG.13C) H-TEMPO and water interactions simulated with molecular dynamics, wherein reduced TEMPO closely interacts with H2O H and oxidized TEMPO closely interacts with H2O O, due to the partial charges on the nitroxyl groups depending on the oxidation state, in accordance with embodiments of the application.

[0048] FIG. 14 provides density functional theory (DFT) simulation of the partial charges on nitroxyl N and O on reduced and oxidized H-TEMPO, showing more positive polarization for oxidized TEMPO compared to its reduced state, in accordance with embodiments of the application.

[0049] FIG.15A and Fig.15B show DFT simulation of electrostatic potential surfaces of reduced (left) and oxidized (right) H-TEMPO and surrounding water simulated with DFT, wherein it can be seen that the water molecules surrounding reduced H-TEMPO are negatively polarized (left), whereas the water molecules surrounding oxidized H-TEMPO are more positively polarized (right), wherein the water polarization impacts the activity ofH+, including through secondary chemical equilibrium which changes pH, in accordance with embodiments of the application.

[0050] FIG.16 schematically illustrates the pH-independent redox reaction changes the activity coefficient ^^, wherein because [H+] is not directly modulated by the redox reaction, Δ^^ needed to cycle TEMPO between its reduced and oxidized state greatly reduced and is close to 0, assuming the ratio between [Red-H] and [Ox] remains constant, in accordance with embodiments of the application.

[0051] FIG. 17 schematically illustrates the configuration and flow of the electrochemical CO2capture system comprising an electrochemical flow cell with symmetrical redox reactions connected by a flow system to CO2 capture and release tanks for continuous capture and release of CO2, wherein, first, CO2from a flue gas or air is captured in a basic electrolyte solution to form bicarbonate ions (bottom); then, the resulting solution flows into the anodic chamber of the electrochemical cell, where reduced redox matter is oxidized to decrease the pH of the solution (middle right); followed by the oxidized solution releasing the captured CO2in high partial pressure of CO2 (top); and circulation of the solution into the cathodic chamber of the electrochemical cell, where oxidized redox matter is reduced for a new capture cycle (middle left), in accordance with embodiments of the application.

[0052] FIG.18A provides a schematic illustration of a flow cell system, comprised of four distinct steps: 1) CO2 capture; 2) redox matter (TEMPO) oxidation; 3) CO2 release; 4) redox matter (TEMPO) reduction, while FIG.18B shows exploded view drawing of the same electrochemical cell design, composed of a graphite felt electrode, gasket, flow- patterned graphite plate, polypropylene spacer and stainless steel plates, in accordance with embodiments of the application.

[0053] FIG.19 provides data plots for CO2concentration, current, and voltage profiles obtained during CO2 capture and release with various currents, wherein an operando cell voltage as low as 0.022 V and an energy input as low as 2.6 kJ mol-1is shown, and the equilibrium voltage is ~ 0 V, a result that stems from the pH-independence of the redox chemistry, in accordance with embodiments of the application.

[0054] FIG.20 shows flow cell CO2 capture and release performance using 100 mM A-TEMPO 400 mM KCl solution, illustrating that at a current density of 1.25 mA cm-2, only a small voltage of ~ 0.01 V is needed for TEMPO redox and CO2capture / release, in accordance with embodiments of the application.

[0055] FIG.21 provides a summary of CO2 capture and release performance including cell voltage, Faradaic efficiency and energy cost, wherein an operando cell voltage as low as 0.022 V and an energy input as low as 2.6 kJ mol-1are shown, and the dependence on current density is shown, in accordance with embodiments of the application.

[0056] FIG.22 provides electrochemical impedance spectroscopy of the cell, wherein ohmic and diffusion resistance are shown as the major contributors to the overall cell impedance, in accordance with embodiments of the application.

[0057] FIG.23 shows differential pressure (left) and pump power (right) across one side of the electrochemical cell with flow rate, wherein the power required for electrolyte flow is very small and orders of magnitude lower than the electrochemical power for the redox reaction, in accordance with embodiments of the application.

[0058] FIG.24 provides data for a stability test of the electrochemical CO2capture system, wherein no significant increase in cell voltage and CO2 capture / release performance is seen over 30 hours of continuous operation, in accordance with embodiments of the application.

[0059] FIG.25 shows a schematic illustration of the H-cell setup for CO2 capture and release of H-TEMPO, wherein an electrolyte solution containing 0.1 M of H-TEMPO was initially bubbled with 20% CO2and 80% N2for 1 hour for CO2capture then oxidative current was flowed and inert Ar gas was flowed at 5 sccm as a carrier gas to monitor the CO2 concentration change during CO2 release, in accordance with embodiments of the application.

[0060] FIG. 26 provides electrochemical and CO2concentration measurements showing release of CO2 with oxidative current, showing the capture and release of CO2 in an H-cell demonstration, in accordance with embodiments of the application.

[0061] FIG.27 provides a schematic illustration of the in-situ FT-IR experimental setup, where solution from the H-cell is flowed through the spectrometer for measurement, in accordance with embodiments of the application.

[0062] FIG. 28 provides in-situ FT-IR spectra showing bicarbonate peak emerging upon CO2 injection and subsiding with oxidation, showing the formation of bicarbonate with CO2injection at the reduced state and the decomposition of bicarbonate into CO2and H2O as oxidation proceeds, in accordance with embodiments of the application.

[0063] FIG.29A provides ex-situ FT-IR of 100 mM A-TEMPO 400 mM KCl under Ar, CO2and oxidized A-TEMPO with CO2, showing that bicarbonate peak around 1360 cm-1appears for CO2 bubbled reduced solution but disappears after oxidation, which illustrates that A-TEMPO can capture CO2 in the form of bicarbonate, but oxidation of A-TEMPO converts bicarbonate into CO2that is released; FIG.29B shows ex-situ FT-IR of 400 mM KCl electrolyte without A-TEMPO and 100 mM A-TEMPO 400 mM KCl solution with and without CO2 injection, illustrating that bicarbonate peak around 1360 cm-1emerges only with TEMPO and CO2 injection; while FIG.29C provides in-situ FT-IR of 5 mM A-TEMPO 100 mM KCl solution, which starts out saturated with CO2and is oxidized over time, showing that the bicarbonate peak around 1360 cm-1subsides over time with oxidation, in accordance with embodiments of the application.

[0064] FIG. 30 shows ultraviolet-visible (UV-Vis) absorption spectra of the reduced state, the reduced state with CO2 injected, and the oxidized state of 100 mM H-TEMPO in 400 mM KCl, wherein the reduced sample, both before and after CO2 injection, have absorption peaks around 510 nm, which gives rise to its orange color, and wherein the absorption spectrum redshifts after oxidation, which is also reflected in the color change, in accordance with embodiments of the application.

[0065] FIG.31 shows UV-Vis spectra of reduced, CO2bubbled in when TEMPO is in the reduced state, and oxidized H-TEMPO at different concentrations: 50 mM (top), 25 mM (middle), and 12.5 mM (bottom), wherein it can be seen that with decreased concentration absorption intensities decrease, but the peak positions remain the same, in accordance with embodiments of the application.

[0066] FIG. 32 provides extracted-ion chromatograms for the reduced state, the reduced state with CO2injected, and the oxidized state of 50 μM H-TEMPO samples for 173.14, 174.13 and 217.13 m / z ions, showing the negligible presence of 217.13 m / z ions for Red+CO2 sample, demonstrating that CO2 is not directly bound to the H-TEMPO, in accordance with embodiments of the application.

[0067] FIG.33 shows mass spectra for the reduced state, the reduced state with CO2injected and the oxidized state of 50 μM H-TEMPO samples for a retention time of 1.14 min, showing the absence of signal at higher m / z than the parent peaks after CO2 bubbling, in accordance with embodiments of the application.

[0068] FIG.34 provides mass spectra for the reduced state, the reduced state with CO2 injected and the oxidized state of 50 μM H-TEMPO samples for a retention time of 1.39 min, showing the absence of signal at higher m / z than the parent peaks after CO2bubbling, in accordance with embodiments of the application.

[0069] FIG.35 shows total ion chromatograms for the reduced state, the reduced state with CO2 injected and the oxidized state of 50 μM H-TEMPO samples, in accordance with embodiments of the application.

[0070] FIG. 36 provides extracted-ion chromatograms for the reduced and CO2 bubbled 50 μM A-TEMPO samples for 172.16 and 173.16 m / z ions, both of which represent the parent A-TEMPO, in accordance with embodiments of the application.

[0071] FIG.37 shows mass spectra for reduced and CO2 bubbled 50 μM A-TEMPO samples for a retention time of 3.84 min, showing the absence of signal at higher m / z than the parent peaks after CO2bubbling, in accordance with embodiments of the application. DETAILED DISCLOSURE

[0072] 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.

[0073] Turning to the drawings, schemes, and data, embodiments of a system and a method for electrochemical capture and release of carbon dioxide via a pH-swing mechanism is provided. In many embodiments, the system comprises: a flow cell, wherein the flow cell is an electrochemical flow cell; a capture tank, further comprising a gas inlet for delivering CO2 to the system and a capture gas outlet; a release tank, further comprising a release gas outlet for outflow of released CO2; an electrolyte solution, at least comprising a redox matter and a supporting electrolyte; and any number of pumps, inlets and outlets, as needed for efficient operation. In many embodiments, the redox matter is a molecule or material that is redox active without directly exchanging protons or hydroxide ions, water soluble, and stable at room temperature and in open air, including water-stable, in both its reduced and oxidized states. In some embodiments, the electrolyte solution further comprises a base. In many embodiments, the system operates within an operational pH-swing range defined by a lower and an upper pH limits to modulate CO2 solubility. In many embodiments, the flow cell further comprises: an anode, disposed within an anodic chamber, a cathode, disposed within the cathodic chamber, an ion exchange membrane for selectively transporting ions separating the anodic and cathodic chambers; and any number of endplates and gaskets to ensure tight electrical contact to the electrodes and sealing to prevent leakage. In many embodiments the ion exchange membrane is an anion exchange membrane (AEM) or a cation exchange membrane (CEM) for selectively transporting anions or cations, respectively. In particular, in many embodiments, wherein the redox matter is a neutral or cationic species, such as, for example, a neutral or a cationic TEMPO / TEMPO-derivative molecule, such as, as a more specific example, 4-hydroxy-TEMPO / 4-oxo-TEMPO, AEM is employed to prevent crossover; while, on the other hand, in some other embodiments, wherein the redox matter is an anionic species, such as, for example, an anionic TEMPO / TEMPO-derivative molecule, such as, as a more specific example, a sulfonated-TEMPO, CEM is employed to prevent crossover. Furthermore, in many embodiments, the capture tank, the flow cell, and the release tank are in fluid communication, such that the electrolyte solution is able to continuously circulate in sequence from the capture tank to the anodic chamber, to the release tank, to the cathodic chamber, and back to the capture tank. It is important tonote that this configuration for continuous capture of CO2 is only one example, and, in some other embodiments, other system configurations include implements such as: batch-mode, multiple stacks of electrochemical cells, addition of components such as trays and packed beds for improved gas exchange, and any combination thereof.

[0074] In many embodiments, the redox matter is a molecule or material selected from the group comprising, but not limited to: (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO), a TEMPO-derivative, a soluble, redox active material suitable for use in redox flow batteries, either as a catholyte or an anolyte. In some embodiments, the soluble, redox active material is a material selected from the groups comprising, but not limited to: a hexacyanometallate couple, such as ferrocyanide / ferricyanide; a sulfonated or phosphonated metallocene (e.g., ferrocene / ferrocenium analogues); a halogen / halide couple, such as iodide / iodine / triiodide (I⁻ / I2 / I3- ); a transition-metal aqua or amine complex, exemplified by Cr(III) / Cr(II); a stable organic radical, including nitroxides, such as TEMPO and its 4-oxo or 4-sulfonate derivatives, a stable derivative of triphenylmethyl (Gomberg) radicals, and related carbon- or nitrogen-centered species, whose initial redox step is proton-decoupled, even though slower follow-on chemistry may show pH sensitivity (although not to be bound by any theory); and any combination thereof. In many embodiments, wherein the redox matter is a TEMPO-derivative, the redox matter is a molecule selected from the group comprising, but not limited to: 4-hydroxy-TEMPO; amino-TEMPO; 4-oxo-TEMPO; trimethylammonium-TEMPO; 4-[3- (trimethylammonio)propoxy]-TEMPO; pyrrolidinium-TEMPO; riboflavin-TEMPO; a RSO3- (CH2)n-O-TEMPO radical; 4-carboxy-TEMPO; acetamido-TEMPO; 4-azido-TEMPO, 4- phosphonooxy-TEMPO; another TEMPO molecule functionalized with one or more functional groups selected from the group comprising: nitro, nitrile, sulfonic acid, sulfoxide, and halide; and any combination thereof.

[0075] In many embodiments the supporting electrolyte is an inexpensive and highly soluble salt. In many embodiments, the supporting electrolyte is a salt selected from the group comprising, but not limited to: KCl, NaCl, LiCl, K2SO4, Na2SO4, KH2PO3, NaH2PO3, Na2CO3, K2CO3, an organic amine salt, anamino acid salt (such as, for example, potassium sarcosinate), and any combination thereof. In many embodiments, a base isused in addition to the supporting electrolyte to further enhance the CO2 carrying capacity of the capture solution. In many such embodiments, the base is selected from the group comprising, but not limited to: KOH, NaOH, LiOH, NH4OH, K2CO3, Na2CO3, KHCO3, NaHCO3, an amine, another weak base (such as, for example, monoethanolamine), an amino acid salt, another zwitterion (such as, for example, potassium sarcosinate), and any combination thereof.

[0076] In many embodiments, some or all components of the electrolyte solution (e.g., the redox matter, the supporting electrolyte, and the base) are present in a concentration that affords sufficient ionic conductivity for low-resistance cell operation; maintains the bulk ionic strength below a level that would materially impair CO₂ solubility, absorption kinetics, or mass-transfer rates of the system; mitigates viscosity-induced pumping losses, corrosion, and precipitation; and otherwise serves to handle constraints of the system, thereby providing an optimized balance of electrochemical performance and operability of the system.

[0077] In many embodiments, the system operates within the operational pH-swing range defined by the lower and the upper pH limits to modulate CO2 solubility, and, thus, to drive the CO2capture and release process. In some embodiments, the operational pH-swing range is controlled such as to remain in a regime wherein the redox matter remains chemically stable. In some embodiments, the electrolyte solution further comprises either a single or a dual-buffer system configured to keep the electrolyte solution within the operational pH swing range, thereby achieving a practical balance between minimizing pH-induced degradation of the redox matter and maintaining effective CO₂-capture performance. In many embodiments, the operational pH-swing range is a pH range selected from the group comprising any of the following illustrative intervals: pH 2-10, pH 4–8, pH 5.5–8.5, pH 5–10, pH 7.5–11, pH 4–11, pH 5-10, or pH 8– 12. However, in some other embodiments, the operational pH swing range is another, empirically or practically determined range that satisfies the stability and performance criteria described herein (i.e., a range that simultaneously maintains the CO2capture performance of the system and pH-dependent stability of the redox matter).

[0078] In some embodiments, the single pH buffer system comprises a single buffer with a singular intermediate pKato define both the upper and the lower limits of the operational pH swing range. In some embodiments, the dual pH buffer system comprises a first buffer, characterized by a first pKa, for maintaining the lower limit of the operational pH swing range; and a second buffer, characterized by a second pKa higher than the first pKa, for maintaining the upper limit of the operational pH swing range. In many embodiments, the first buffer is selected from the group comprising: formic acid / formate, acetic acid / acetate, and any other buffer characterized by a pKa value appropriate for establishing and maintaining the lower pH limit of the operational pH swing range. In many embodiments, the second buffer is selected from the group comprising of: carbonate, bicarbonate, HPO₄²⁻ / PO₄³⁻, a weak base (e.g., monoethanolamine), an amino acid salt (e.g. potassium sarcosinate), and another buffer characterized by a pKavalue appropriate for establishing and / or limiting the upper pH limit of the operational pH swing range. In many embodiments, the first pKais roughly 3-5, which is 1-3 units below the apparent pKa1≈ 6.3 of carbonic acid (at ~25 °C), and the second pKais roughly 11- 14, which is 1-3 units above pKa2 ≈ 10.3 of carbonic acid (at ~25 °C), adjusted for operating conditions (i.e. temperature) accordingly.

[0079] In many embodiments, the anode and the cathode comprise electrochemically stable, high surface area, electronically conductive material. In many such embodiments, the anode and the cathode comprise porous graphite.

[0080] In many embodiments the method for electrochemical capture and release of carbon dioxide via a pH-swing mechanism comprises providing a CO2 -contaminated gas stream, applying voltage to the flow cell of the system, and circulating the electrolyte solution carrying the redox matter through the system in sequence: from the capture tank to the anodic chamber, to the release tank, to the cathodic chamber, and back to the capture tank, to continuously and energy efficiently capture CO2 from the gas stream and controllably release CO2at a set of operational conditions, for further concentration and storage. In many embodiments, the set of operational conditions include room / atmospheric temperature and atmospheric environment.

[0081] High energy cost is one of the bottlenecks limiting practical deployment of carbon capture technologies. To this end, electrochemical carbon capture has recently garnered attention as an approach that can potentially allow to circumvent these energy resource limitations (as reported by, for example: S. E. Renfrew, et al, Electrochemical Approaches toward CO2 Capture and Concentration (2020), doi:10.1021 / acscatal.0c03639; R. Sharifian, et al., Electrochemical carbon dioxide capture to close the carbon cycle. Energy Environ. Sci.14, 781–814 (2021); and K. M. Diederichsen, et al., Electrochemical methods for carbon dioxide separations. Nat. Rev. Methods Prim.2 (2022), doi:10.1038 / s43586-022-00148-0, the disclosures of which are incorporated herein by reference). More specifically, since electrochemical activation specifically targets active materials, electrochemical approaches to CO2 capture do not necessitate expenditure of energy for water and / or substrate heating, thus circumventing energy loss to substrate heating, and allowing to achieve higher process efficiencies. To this end, several different electrochemical carbon capture approaches have emerged, including pH swing (see, for example: P. Zhu, et al., Continuous carbon capture in an electrochemical solid-electrolyte reactor. Nature.618, 959–966 (2023); S. Pang, et al., A phenazine-based high-capacity and high-stability electrochemical CO2 capture cell with coupled electricity storage (2023), doi:10.1038 / s41560-023-01347-z; M. D. Eisaman, et al., CO2separation using bipolar membrane electrodialysis. Energy Environ. Sci. 4, 1319–1328 (2011); and H. Seo, et al., Electrochemical Carbon Dioxide Capture and Release with a Redox-Active Amine. J. Am. Chem. Soc.144, 2164–2170 (2022), the disclosures of which are incorporated herein by reference), redox-active sorbents (see, for example: X. Li, et al., Redox-tunable Lewis bases for electrochemical carbon dioxide capture. Nat. Energy (2022), doi:10.1038 / s41560-022-01137-z.; K. M. Diederichsen, et al., Toward solvent-free continuous-flow electrochemically mediated carbon capture with high-concentration liquid quinone chemistry. Joule.6, 221–239 (2022); and S. Voskian, T. A. Hatton, Faradaic electro-swing reactive adsorption for CO2 capture. Energy Environ. Sci.12, 3530–3547 (2019), the disclosures of which are incorporated herein by reference), and electrochemically-mediated amine regeneration (EMAR) (see, for example, M. C. Stern, et al., Post-combustion carbon dioxide capture using electrochemically mediatedamine regeneration. Energy Environ. Sci.6, 2505–2517 (2013); and M. Wang, et al., CO2 Capture Using Electrochemically Mediated Amine Regeneration. Ind. Eng. Chem. Res. 59, 7087–7096 (2020), the disclosures of which are incorporated herein by reference). More specifically, as one such example, CO2 capture with redox-active sorbents utilizes the redox of molecules and materials, such as quinones and sp2N, to directly capture and release CO2. However, forming and breaking covalent bonds of CO2with such sorbents still incur large energy costs. As another more specific example, the pH swing mechanism leverages the pH dependence of the solubility of inorganic carbon (i.e., carbonates, bicarbonates and carbonic acid), wherein, at high pH, the high solubility and reaction rates of inorganic carbon enables efficient CO2 capture, while at low pH CO2 is rapidly released. Notably, the pH swing approach has been shown to be a particularly effective method for CO2capture and release (X. Wang, et al., Comprehensive Study of the Hydration and Dehydration Reactions of Carbon Dioxide in Aqueous Solution. J. Phys. Chem. A. 114, 1734–1740 (2010), the disclosure of which is incorporated herein by reference)

[0082] To this end, several different methods have been explored for electrochemical pH swing. For example, bipolar membrane electrodialysis (BPMED) dissociates water into OH- and H+that migrate to opposite sides of an electrochemical cell, creating a pH gradient. As another example, proton-coupled electron transfer (PCET) utilizes redox- active molecules that uptake or release H+upon reduction or oxidation to modulate the pH (see, for example, H. Seo, T. A. Hatton, Electrochemical direct air capture of CO2 using neutral red as reversible redox-active material. Nat. Commun.14, 313 (2023); H. Xie, et al., Low-Energy Electrochemical Carbon Dioxide Capture Based on a Biological Redox Proton Carrier. Cell Reports Phys. Sci.1, 100046 (2020); and S. Jin, et al., pH swing cycle for CO2 capture electrochemically driven through proton-coupled electron transfer. Energy Environ. Sci. 13, 3706–3722 (2020), the disclosures of which are incorporated herein by reference). As yet another example, in gas (H2, O2, etc.) looping, a pH gradient can be created by coupling oxygen evolution and oxygen reduction reactions (as discussed, for example, in L. Yan, et al., An Electrochemical Hydrogen- Looping System for Low-Cost CO2 Capture from Seawater. ACS Energy Lett.7, 1947–1952 (2022), the disclosure of which is incorporated herein by reference). However, the fundamental property shared by all these pH swing mechanism approaches is direct involvement of H+or OH- in the corresponding redox reaction. In other words, whether the redox molecule for PCET, or H2 for BPMED, or hydrogen looping, the reduced redox molecule releases a proton or consumes a hydroxide ion upon oxidation. As such, the electrochemical energy needed to overcome the entropic penalty of directly modulating the concentration of H+is given by the Nernst equation provided in FIG.1, wherein the minimum voltage needed to change the ^^^^ by changing the hydrogen concentration [H+] is −0.059 V per unit change in pH (Δ^^^^ = 1), assuming the ratio between [Red-H] and [Ox] remains constant. Therefore, the electrochemical pH modulation still results in significant energy inefficiencies, wherein the pH difference of the electrochemically induced pH swings leads to voltage differences between the CO2storage and release processes, which, in turn, add to the energy cost. Accordingly, the efficiency of electrochemical methods for CO2 capture also remains lacking.

[0083] This application is directed to embodiments of a system and method for electrochemical CO2 capture. In particular, the application is directed to embodiments of a system and method for energy efficient, electrochemical CO2 capture via a pH swing mechanism, wherein the system and the method rely on a pH-independent redox chemistry of a redox matter. In many embodiments, the redox matter is a redox-active, stable and water-soluble molecule, ion, particle, or another material, that is stable in both its oxidized and reduced states in water, at room temperature and in open air. In particular, in many embodiments, the redox matter undergoes redox reaction without direct involvement of protons or hydroxide ions. In many embodiments, this attribute is key to enabling the pH-independent redox electrochemistry and the improvement in energy input. In many embodiments, the redox matter is characterized by a solubility in water of 0.1 - 5 M. In many embodiments, the redox matter is inexpensive. In some embodiments, the redox matter being inexpensive signifies that it is commercially available on a large scale, wherein it can be used in very large production volumes, and that it is already being used in demonstration projects for utility scale energy storage. In many embodiments, the redox matter is a redox-active molecule or material selected from the group comprising,but not limited to: TEMPO; a TEMPO derivative; a redox active material suitable for use in redox flow batteries, as either anolyte and / or catholyte. In many embodiments, the redox-active material suitable for use in redox flow batteries is a material selected from the group comprising, but not limited to: a hexacyanometallate couple, such as, for example, ferrocyanide / ferricyanide; a sulfonated or phosphonated metallocene, such as, for example, ferrocene / ferrocenium analogues; a halogen / halide couple, such as, for example, iodide / iodine / triiodide (I⁻ / I2 / I3- ); a transition-metal aqua or amine complex, exemplified by Cr(III) / Cr(II); a stable organic radical, including nitroxides, such as, for example, TEMPO and its 4-oxo or 4-sulfonate derivatives; a stable derivative of triphenylmethyl (Gomberg) radicals and related stable carbon- or nitrogen-centered species, whose initial redox step is proton-decoupled, even though slower follow-on chemistry may show pH sensitivity (although not to be bound by any theory); and any combination thereof. In many embodiments, the system and method are pH-independent, wherein the equilibrium potential of the system and method’s redox chemistry does not change with pH, which is the key characteristic of the above redox matter. Accordingly, in many embodiments, the method circumvents the energy costs associated with a pH gradient in a working electrochemical cell, achieving energy costs that approach the theoretical limit of entropic penalty of CO2 concentration. In many embodiments, the energy costs are in the range of 0 – 30 kJ mol-1, depending on the partial pressures of CO2at capture and release, current density, system design, flow rate, and other factors.

[0084] To this end, in many embodiments, the redox matter is a redox-active molecule or material that can change pH of a solution through a redox reaction, without direct involvement of protons or hydroxide ions in the redox reaction, as illustrated, for example, in FIGs. 2A. More specifically, as illustrated in FIG. 2A, and according to many embodiments, the redox matter modulates the pH through polarizing surrounding water molecules, rather than directly producing or consuming H+or OH-. Polarization of the water molecules changes the activity of H+or OH-, that can shift the equilibrium between CO2, carbonic acid (H2CO3), bicarbonate ion (HCO3-), and carbonate ion (CO32-). By decreasing pH, the equilibrium is shifted towards carbonic acid and CO2, resulting in CO2 release. By increasing the pH, CO2is captured in the form of bicarbonates or carbonates.In addition, in many embodiments, the redox matter is characterized by 0.1 - 5 M solubility in water, such as to maximize CO2capture per volume of solution.

[0085] In some embodiments, the redox matter is (2,2,6,6-tetramethylpiperidin-1- yl)oxyl and (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO), or any derivative thereof. In some embodiments, the redox matter is 4-hydroxy-TEMPO (H-TEMPO). However, in some other embodiments, the redox matter is a molecule or materials selected from the group comprising of (but not limited to): amino-TEMPO (A-TEMPO), 4-oxo-TEMPO, trimethylammonium-TEMPO, 4-[3-(trimethylammonio)propoxy]-TEMPO, pyrrolidinium- TEMPO, riboflavin-TEMPO, a RSO3-(CH2)n-O-TEMPO radical, 4-carboxy-TEMPO, acetamido-TEMPO, 4-azido-TEMPO, 4-phosphonooxy-TEMPO, a TEMPO molecule functionalized with one or more functional groups selected from the group comprising: nitro, nitrile, sulfonic acid, sulfoxide, halide; and any combination thereof. In many such embodiments, the choice of the TEMPO-derivative is carefully considered to ensure that the chosen TEMPO substituent or substituents enhance the redox activity, electrochemical reversibility, chemical stability, and or other characteristics of the redox matter, such as to enhance the performance of the present system and method for electrochemical CO2 capture. For example, in some embodiments, use of 4-hydroxy- TEMPO or certain related species as the redox matter leads to undesirable self-oxidation reactions, resulting in the direct formation of hydrochloric acid (HCl), which, in turn, contributes to irreversible degradation of the redox couple and limits long-term cycling stability of the redox matter and, ultimately, the whole system. However, alternative TEMPO derivatives — such as, for example, 4-oxo-TEMPO and sulfonated TEMPO analogs — have been developed, wherein, such variants exhibit greater resistance to self-discharge and support more reversible pH swing over extended lifetimes and pH ranges (FIG.2B). Accordingly, in many embodiments, the redox matter is TEMPO or a TEMPO derivative characterized by an exceptional robustness towards self-discharge or oxidation, maximizing electrochemical reversibility, cycling stability, chemical stability at room temperature and in open air, and overall lifetime.

[0086] To this end, TEMPO or TEMPO derivatives are example molecules possessing all the properties critically required of the redox matter of the instant application accordingto many embodiments. More specifically, TEMPO and applicable TEMPO-derivatives are stable free radicals, stable in both their native (reduced) aminoxyl state, as well as their oxidized oxoammonium state (FIG.2A) (and as also described in, for example: Y. Liu, et al., A Long-Lifetime All-Organic Aqueous Flow Battery Utilizing TMAP-TEMPO Radical. Chem.360 5, 1861–1870 (2019); and W. Zhou, et al., Fundamental properties of TEMPO- based catholytes for aqueous redox flow batteries: effects of substituent groups and electrolytes on electrochemical properties, solubilities and battery performance. RSC Adv. 10, 21839–21844 (2020), the disclosures of which are incorporated herein by reference). Furthermore, the delocalized electron between the N and O of TEMPO that forms a three electron πN-O bond stabilizes the molecule, further protected by the steric screening provided by the four adjacent methyl groups (see G. I. Likhtenshtein, Nitroxides (Springer Cham, ed.1, 2020), the disclosure of which is incorporated herein by reference). As such, the free radical of TEMPO is readily oxidized, and possesses favorable electrochemical properties. Notably, TEMPO’s oxidation is highly reversible within the thermodynamic stability window of water and has rapid charge-transfer kinetics, as shown for H-TEMPO in FIG. 3A, and shows no molecular degradation even after 1000 cycles in cyclic voltammogram experiments (FIG. 3B). Furthermore, FIG. 4 illustrates similarly advantageous properties of many embodiments for 4-amino-TEMPO (A-TEMPO). In addition, TEMPO and TEMPO-derivatives have very high (>1 M) solubilities in water, making them highly compatible with aqueous solutions. As such, TEMPO-derivatives have already been deployed in redox flow batteries (see, for example: T. Liu, et al., A Total Organic Aqueous Redox Flow Battery Employing a Low Cost and Sustainable Methyl Viologen Anolyte and 4-HO-TEMPO Catholyte. Adv. Energy Mater.6, 1501449 (2016); X.-L. Lv, et al., Modular dimerization of organic radicals for stable and dense flow battery catholyte. Nat. Energy.8, 1109–1118 (2023); and W. Zhou, et al., Fundamental properties of TEMPO-based catholytes for aqueous redox flow batteries: effects of substituent groups and electrolytes on electrochemical properties, solubilities and battery performance. RSC Adv. 10, 21839–21844 (2020), the disclosures of which are incorporated herein by reference), and as redox mediators in lithium-air batteries (see B. J. Bergner, et al., TEMPO: A mobile catalyst for rechargeable Li-O2 batteries. J. Am.Chem. Soc.136, 15054–15064 (2014); D. J. Lee, et al., Sustainable Redox Mediation for Lithium–Oxygen Batteries by a Composite Protective Layer on the Lithium-Metal Anode. Adv. Mater.28, 857–863 (2016); H. D. Lim, et al., Rational design of redox mediators for advanced Li-O2 batteries. Nat. Energy. 1 (2016), doi:10.1038 / nenergy.2016.66; the disclosures of which are incorporated herein by reference). However, no attempts to use TEMPO, TEMPO-derivatives, or other pH-independent redox matter in electrochemical capture of CO2 have been reported to date. Notably, although these materials have been considered for applications such as redox flow batteries, they have not been considered for application that require modulating the pH, such as CO2capture. Furthermore, while modulating the pH is not a crucial factor in redox flow batteries applications, it is central to CO2 capture through pH swing. It is also important to note here, that TEMPO is one example of pH-independent redox matter of many embodiments that satisfy the important criteria of stability in both oxidized and reduced states, pH-independent redox properties, and high solubility in water.

[0087] Moreover, FIG.5A provides an illustrative example of the pH swing capabilities of the redox matter of many embodiments, wherein the pH of a solution comprising H- TEMPO as the redox matter is measured after varying degrees of oxidation. As seen from FIG. 5A, H-TEMPO in its reduced state affords high pH of approximately 9.4, however, upon oxidation of H-TEMPO, the solution’s pH drops, and reaches pH of 1.8 at complete oxidation of H-TEMPO (FIG.5B). Accordingly, in many embodiments, a pH gap between reduced and oxidized states of the redox matter spans 2—10 or any sufficient range useful for pH swing applications. In some embodiments, the pH gap between reduced and oxidized states of the redox matter spans pH 2-10, pH 4–8, pH 5.5–8.5, pH 5–10, pH 7.5–11, pH 4–11, pH 5–10, pH 8–12, or any combination thereof. In addition, titration of TEMPO shows that oxidized H-TEMPO acts as a weak acid (FIG. 6). Moreover, as another example of the suitable redox matter, A-TEMPO affords similar pH changes as seen from the data provided in FIG.7. Notably, in both examples, the pH approximately follows the −log10[^^^^^^^^^^+], signifying that pH changes with the amount of TEMPO+produced, however, TEMPO+acts as a weaker acid than H+(FIG. 5A). Accordingly, although protons or hydroxide ions do not appear to be directly consumedor produced in the redox of TEMPO, the pH of TEMPO’s redox chemistry still changes in the course of this reaction – an observation that has been alluded to in existing redox flow battery literature (see, for example, H. Fan, et al., Mitigating Ring-Opening to Develop Stable TEMPO Catholytes for pH-Neutral All-Organic Redox Flow Batteries. Adv. Funct. Mater.32, 2203032 (2022), the disclosure of which is incorporated herein by reference).

[0088] As another illustrative example of the critical electrochemical properties of the redox matter of many embodiments, FIG.8 shows that the thermodynamics of the redox matter (represented here by H-TEMPO) is independent of pH. More specifically, FIG.8 shows that, according to many embodiments, the equilibrium potential, which is the midpoint between the peaks of the cyclic voltammograms in the inset, remains essentially constant with pH. As another example, a similar pH-independent nature of the redox chemistry of many embodiments is also observed for A-TEMPO (FIG.9). Notably, this pH-independent redox behavior of the redox matter of many embodiments has also been previously observed in flow battery applications (see, for example, J. B. Gerken, et al., Structural Effects on the pH-Dependent Redox Properties of Organic Nitroxyls: Pourbaix Diagrams for TEMPO, ABNO, and Three TEMPO Analogs. J. Org. Chem.83, 7323–7330 (2018); the disclosure of which is incorporated herein by reference).

[0089] Furthermore, the pH-independent redox behavior of the redox matter of many embodiments is in stark contrast to conventional redox chemistries for pH swing that directly involve protons or hydroxide ions. More specifically, when protons or hydroxide ions are involved in a redox reaction, pH directly affects the equilibrium potential of such a reaction. However, when pH swing mechanisms are employed for electrochemical capture of CO2, the processes for CO2capture and CO2release intrinsically have different pH values. As such, the pH-dependency of pH swing chemistries relying on protons / hydroxide ions leads to energy penalties that hamper the energy efficiency of CO2capture. On the other hand, and according to many embodiments, the redox chemistries that do not involve protons / hydroxide ions are independent of pH, and, as such, have a constant equilibrium potential across all pH values, resulting in minimal thermodynamic energy penalties of the CO2capture and release via a pH swing approach.

[0090] In other words, the thermodynamics of the conventional redox reactions for pH swing changes with pH, as it is a direct measure of the chemical potential of H+. Therefore, a high pH for CO2capture leads to a low redox potential, while a low pH for CO2release leads to a high redox potential, inducing a thermodynamic energy penalty. However, because the pH-independent redox chemistry of the redox matter of many embodiments can swing the pH without directly producing or consuming H+, its electrochemical equilibrium potential is independent of pH. Therefore, it can be argued, as demonstrated in the Experimental section below, that if the voltage needed to oxidize the redox matter, such as, for example, TEMPO, i.e.: ^^^^^^^^^^^^^^^^⇌^^^^^^^^^^++^^−, is pH independent, then the thermodynamic energy penalty of ^^^^^^^^^^^^^^^^→ ^^^^^^^^^^+→^^^^^^^^^^^^^^^^ is zero. Accordingly, the pH-independent nature of the redox chemistry of the redox matter of many embodiments is critically important to the pH modulation of the electrochemical CO2capture via the energy efficient system and method of the instant application.

[0091] Moreover, FIG.10 through 13C shows molecular dynamics (MD) simulations, while FIGs. 14 through 15B show density functional theory (DFT) simulations and analysis for H-TEMPO molecule (as a representative redox matter of many embodiments), that provide additional insight into the molecular-level mechanism behind the pH- independent modulation of pH by the system and method of the instant application. To this end, first, it is observed that water molecules dominate the first hydration layer of both reduced and oxidized H-TEMPO molecules (FIGs.11A through 11D). Next, although K+is attracted to reduced H-TEMPO and Cl- is attracted to oxidized H-TEMPO, the large number of water molecules (as compared to other species) and the bulky nature of TEMPO+cations result in weaker electrostatic interactions, with water populating the primary hydration shell. In addition, OH- ions are not attracted to either reduced or oxidized H-TEMPO. Furthermore, FIG.12 shows that the radial distance of water oxygen to nitroxyl oxygen depends on the oxidation state of H-TEMPO. In addition, FIGs.13A through 13C show that hydrogen atoms of nearest water molecules interact closely with the oxygen on reduced H-TEMPO, whereas oxygen atoms interact closely with the nitrogen of oxidized H-TEMPO. Moreover, notably, DFT simulations of H-TEMPO in the reduced and oxidized states provided in FIG.14 show that the partial charges on thenitroxyl oxygen and nitrogen atoms shift positively with oxidation. Still further, the data provided in FIGs. 15A and 15B indicate that the oxidized H-TEMPO accepts partial electron density from surrounding water molecules, suggesting (although not to be bound by any theory) notable orbital interactions that lead to positive polarization of the water molecules.

[0092] In other words, the important consideration of many embodiments is that the pH modulation mechanism circumvents direct production or consumption of H+, but acts through orbital interactions with water, or via reversible chemical equilibrium that indirectly modulates H+concentration . More specifically, water hydrated around the redox matter of many embodiments will become polarized, depending on the charge state of the redox matter. For example, when the redox matter is negatively charged, the water molecules also become negatively polarized, and vice versa. Moreover, polarized water molecules not only modulate the activity of H+or OH-, but also act as buffers. As such, in the presence of bicarbonates, preexisting H+will bind with bicarbonates to form carbonic acid, that subsequently decomposes into CO2 and H2O. Once H+is used up, positively polarized water molecules can provide additional H+to the electrolyte, which occurs dueto the chemical equilibrium between ^^^^^^^^^^ା ^ ^^ଶ^^ ↔ ^^^^^^^^^^ା ∙ ^^^^ି ^ ^^ା , whereTEMPO+and OH- are weakly interacting. Therefore, positively polarized water can act as a buffer to provide H+without directly increasing H+concentration. On the contrary, negatively polarized water molecules can provide additional OH- to the electrolyte, enabling the capture of CO2in the form of bicarbonates and carbonates.

[0093] To this end, in view of the data presented herein, and especially the DFT simulations, polarization of water molecules alters the activity of protons (as discussed, for example, by W. B. Jensen, The Lewis acid-base definitions: a status report. Chem. Rev. 78, 1–22 (1978), the disclosure of which is incorporated herein by reference). Notably, Lewis acid sites on solid catalytic materials are known to polarize interacting water molecules (as discussed in Y. Wang, et al., Heterogeneous Ceria Catalyst with Water-Tolerant Lewis Acidic Sites for One-Pot Synthesis of 1,3-Diols via Prins Condensation and Hydrolysis Reactions. J. Am. Chem. Soc.135, 1506–1515 (2013); and J. Li, et al., Insights into the Interfacial Lewis Acid–Base Pairs in CeO2-Loaded CoS2Electrocatalysts for Alkaline Hydrogen Evolution. Small. 17, 2103018 (2021), the disclosures of which are incorporated herein by reference), and redox of Lewis acids, such as, for example, zinc ions, are also known to be pH-independent (see X. Guo, G. He, Opportunities and challenges of zinc anodes in rechargeable aqueous batteries. J. Mater. Chem. A.11, 11987–12001 (2023), the disclosure of which is incorporated herein by reference). Therefore, although, again, not to be bound by any theory, it appears that the redox matter of many embodiments, such as TEMPO, modulates pH through orbital interactions with water, which, in turn, changes the electrolyte environment, and, thus, modulates the H+activity without direct production of H+. Furthermore, since the concentration of the redox matter in any given electrolyte solution is much higher than the concentration of protons, the subtle changes in the interactions between the redox matter and the aqueous electrolyte solution have a profound influence on the pH. As such, and since the redox chemistry does not directly involve H+, the thermodynamics of this electrochemical redox is independent of pH, while the subsequent reversible chemical interactions with H2O will produce H+or OH- ions, demonstrating pH-dependance. Accordingly, in many embodiments, the system and method of the instant application rely on the pH-independent redox reaction of the redox matter described herein, as schematically shown in FIG. 16. In general, in many embodiments, energy efficient electrochemical pH swing capture of CO2requires electrochemical redox chemistry wherein the voltage needed to oxidize or reduce the redox matter is pH-independent.

[0094] In many embodiments, the instant system and method for efficient electrochemical capture of CO2rely on a pH-independent redox reaction of the redox matter in conjunction with various configurations of the system of many embodiments. One example of the system of many embodiments is an electrochemical system for continuous capture and controlled release of CO2schematically shown in FIG.17. More specifically, FIG.17 illustrates the system of many embodiments comprising a flow cell (middle); a capture tank for capture of CO2 (bottom); and a release tank for release of CO2 (top) – all in continuous fluid communication. In many embodiments, the flow cell further comprises an anode, disposed within an anodic chamber of the flow cell, and a cathode, disposed within a cathodic chamber of the flow cell, wherein the anodic chamberand the cathodic chamber are separated by an ion exchange membrane selectively transporting ions. In many embodiments, the ion exchange membrane is an anion exchange membrane selectively transporting anions. However, in some other embodiments, the ion exchange membrane is a cation exchange membrane that selectively transports cations. In particular, in many embodiments, wherein the redox matter is a neutral or cationic species, such as, for example, a neutral or a cationic TEMPO / TEMPO-derivative molecule, such as, as a more specific example, 4-hydroxy- TEMPO / 4-oxo-TEMPO, AEM is employed to prevent crossover; while, on the other hand, in some other embodiments, wherein the redox matter is an anionic species, such as, for example, an anionic TEMPO / TEMPO-derivative molecule, such as, as a more specific example, a sulfonated-TEMPO, CEM is employed to prevent crossover. In many embodiments, the anode and the cathode, each, comprises a high surface area, electronically conductive material that is resistant to chemical corrosion under a wide range of pH values. In many such embodiments, the anode and the cathode comprise porous graphite. Furthermore, in many embodiments, the system comprises an electrolyte solution further comprising the redox matter in a redox matter concentration. In many embodiments, the redox matter concentration is > 0.1 M concentration. In many embodiments, the redox reaction (such as, for example, shown in FIGs.2 and 4) takes place in the flow cell, wherein oxidation of the redox matter occurs at the anode and reduction of the redox matter at the cathode. Furthermore, in many embodiments, the capture tank comprises a gas inlet for delivering CO2 to the system and a capture gas outlet; while the release tank comprises a release gas outlet for outflow of released CO2, and the system further comprises any number of safety valves, as well as inlets and outlets for process safety and monitoring as necessary.

[0095] In many embodiments, the electrolyte solution further comprises a supporting salt in a supporting salt concentration to enhance the electrolyte conductivity and enable charge-balancing. In many such embodiments, the supporting salt provides a plurality of ions that crosses the ion exchange membrane to balance the charge during the redox reaction of the embodiments. For example, in some embodiments, the electrolyte solution comprises 0.4 M TEMPO (as the redox matter) and 1.2 M KCl in water. However,in many other embodiments, the redox matter concentration, as well as the supporting salt concentration, and the identity of the supporting salt are varied as needed to optimize the redox chemistry, although, in some embodiments, within the limitation of > 0.1 M concentrations of the redox matter and supporting salt, and complete dissolution of all components in water. In some embodiments, a base, such as, for example, but not limited to: KOH, NaOH, K2CO3, Na2CO3, KHCO3, NaHCO3, LiOH, NH4OH, an organic weak base (such as monoethanolamine), an amino acid or its salt (such as potassium sarcosinate), and any combination thereof, is initially added to the electrolyte solution as a source of alkalinity to increase the pH of the electrolyte solution and, as such, CO2solubility, while, in some embodiments, also ensuring the pH remains in a range where the redox active matter is stable. However, while KCl and KOH, each, represent one example of the supporting salt and the base of many embodiments, respectively, in many other embodiments, other supporting salt-base combinations are employed, such as, for example, NaCl, NaOH, and other alternatives listed herein.

[0096] In addition, in some embodiments, the system and method for electrochemical CO2capture are characterized by an operational pH swing range. In many embodiments, the operational pH swing range is the interval between a more acidic lower-bound pH (at which CO₂ is released from the electrolyte solution) and a less acidic upper-bound pH (at which CO₂ is absorbed). To this end, in many embodiments, the magnitude of this interval / swing is intentionally modulated such that it provides sufficient thermodynamic driving force and acceptable mass-transfer kinetics for CO2 capture and release, while simultaneously avoiding pH extremes known to accelerate chemical or electrochemical degradation of the redox matter. Furthermore, as disclosed herein, since the concentration of the redox matter in the electrolyte solution of many embodiments is much higher than the concentration of protons, the subtle changes in the interactions between the redox matter and the aqueous electrolyte solution have a profound influence on the pH. Accordingly, in some embodiments, especially wherein the electrolyte solution is unbuffered, the oxidation and reduction of the redox matter results in highly acidic or basic pH regimes (FIG 5A and 5B), which may risk degradation of the redox matter. For example, experimental studies with 4-hydroxy-TEMPO and related nitroxyl derivativeshave shown that strongly acidic (≲^pH^3) and strongly basic (≳^pH^12) environments promote proton- or hydroxide-driven self-discharge, as well as acid- / base-catalysed disproportionation or ring-opening reactions, which, in turn, irreversibly deactivate the molecule (as discussed, for example, in A.^Orita^et^al., J. Power Sources 321, 126-134 (2016); and J.R. Fish^et^al. J. Phys. Chem.92, 3745-3751 (1988), the disclosure of which is incorporated herein by reference).

[0097] Accordingly, in some embodiments, molecular design strategies such as derivatization of TEMPO molecule and use of other redox-active molecules are employed to broaden the operational pH swing range. However, in many other embodiments, the operational pH swing range relied upon by the system and method is sufficient in its magnitude to exploit the pH-dependent speciation of dissolved inorganic carbon (DIC), yet avoids the extreme acidic or alkaline conditions that lead to accelerated degradation of the redox matter. Furthermore, in many embodiments, the operational pH swing range is controlled via either passive means (such as, for example, by relying on a single pH- buffering solution, or an optimized mixture of multiple buffers); or active means (such as, for example, active monitoring of pH of the electrolyte solution using an electronic pH meter, and transmitting active feedback to the electrochemical system or power supply to terminate or modify electrochemical cycling once a certain pre-determined pH value is reached).

[0098] As such, in some embodiments, the electrolyte solution further comprises a single or a dual pH buffer for maintaining the system and method within the operational pH swing range. More specifically, in many embodiments, the pH buffer is a dual buffer system comprising a first buffer, characterized by a first pKa, for maintaining the lower limit of the operational pH swing range; and a second buffer, characterized by a second pKa, higher than the first pKa, for maintaining the upper limit of the operational pH swing range. To this end, in many embodiments, the first buffer with the first, low, pKamitigates excursions to excessively low pH values (i.e. maintains pH > 3-4) during electrochemical cycling of the system; while the second buffer with the second, high, pKa prevents exposure of the system to excessively high pH values (i.e. maintains pH < 12-14). In many embodiments, the operational pH swing range is selected such as to balanceefficiency with stability for the system and method. Notably, the highest per-proton efficiency for the system and method is obtained when the operational pH swing range spans only one carbonate-system dissociation — either near the carbonic-acid / bicarbonate pK^1 (≈^6.3 at ~25 °C) or near the bicarbonate / carbonate pK^2 (≈^10.3 at ~25 °C) — so that a single equivalent of alkalinity is consumed per mole of CO₂ captured. However, also notably, in systems lacking catalysts, CO₂ hydration / dehydration is often slow near neutral pH, which motivates wider swings to higher pH in conventional carbonate processes. By contrast, the system and method of many embodiments described herein limit the operational pH swing range to a certain window — between about pH^4 and about pH^12, according to some embodiments; and, between pH 2-10, pH 4–8, pH 5.5–8.5, pH 5–10, pH 7.5–11, pH 4–11, pH 5-10, or pH 8– 12, according to some other embodiments; or, in yet other embodiments, any other range empirically or practically determined to be useful in maintaining the stability of the redox matter, while delivering acceptable CO2 capture kinetics. In addition, in many embodiments, operating conditions (including temperature, ionic strength, catalyst loading, current density) are further adjusted to fine-tune the optimum limits for the operational pH swing range. In some embodiments, the operating conditions include room / atmospheric temperature.

[0099] Still more specifically, in some embodiments, wherein the pH buffer is a dual buffer system, suitable buffer options for the first buffer with the low first pKainclude buffers selected from the group comprising (but not limited to): formate, acetate, citrate, and another buffer characterized by a pKa value appropriate for establishing and maintaining the lower pH limit of the operational pH swing range. In many such embodiments, the first buffer restricts acid excursions, while still facilitating CO₂ desorption (i.e., pH can still fall several units below carbonic acid’s pK^1≈^6.3, but not so low that the redox matter begins to degrade, hence a preferred pKa of 3—4). In turn, in some embodiments, suitable buffer options for the second buffer with the high second pKa(of roughly 10—12) include buffers selected from the group comprising (but not limited to): carbonate / bicarbonate, HPO₄²⁻ / PO₄³⁻ , select amines or other weak bases (such as monoethanolamine), an amino acid salt (e.g. potassium sarcosinate), andanother buffer characterized by a pKa value appropriate for establishing and maintaining the upper pH limit of the operational pH swing range. In many such embodiments, the second buffer restricts base excursions (i.e. all the way to pH 14), while still enabling a high pH (9—13) for sufficiently rapid CO₂ absorption (as the pK^ of the secondary buffer is targeted to be near or above the carbonate / bicarbonate pK^2 value of ^10.3 (at ~25 °C). In many embodiments, total buffer concentration, ionic strength and temperature are selected such that buffering capacity does not unduly suppress the desired pH swing, nor interferes with redox kinetics of the system and method,

[0100] In addition, in some embodiments, the pH buffer is the single buffer system, wherein the buffering and pH control strategy utilizes a single buffer characterized by a single buffer pK^, wherein the single buffer pKais within, or straddles, the operational pH swing range. For example, in some embodiments, wherein the operational pH swing range is centered near neutral-to-mildly basic conditions — e.g., a lower bound of about pH^5–7 and an upper bound of about pH^9-13 — some preferred buffers include alkanolamines with pK^ values of approximately 8–9,for example, monoethanolamine (MEA, pK^ ≈^9.5). As another example, in some embodiments, amino acids characterized by second acid dissociation constants that are in a similar range — such as, for example, glycine (pK^₂ ≈^9.6) or sarcosine (pK^₂ ≈^10.3 at ~25°C) — provide an alternative buffering strategy that combines low volatility with minimal long-term fouling. Notably, according to many embodiments, these buffers sustain rapid CO₂ absorption, while mitigating degradation of the redox matter, by maximizing bicarbonate concentration, yet avoiding strongly alkaline conditions (>^ pH ^12). On the other hand, in some embodiments, for higher-pH-bounded operation (upper bound >^pH^10), carbonate / bicarbonate (pK^₂ ≈^10.3 at~25°C) or phosphate (HPO₄²⁻ / PO₄3⁻, pK^ ≈^12.3) buffers are more appropriate. In many embodiments, the benefits of relying on the single-buffer approach include: simplified formulation and potentially reduced ionic strength and viscosity of the electrolyte solution. Furthermore, such a single-buffer system may be coupled with active electrochemical feedback to arrest any residual pH drift.

[0101] Accordingly, imposing the operational pH swing range on the present system and method for capture and release of CO2, as described herein, exploits the well-characterized pH-dependent speciation of dissolved inorganic carbon (DIC) in water (although not to be bound by any theory), as discussed, for example, in Zeebe & Wolf- Gladrow, CO₂ in Seawater: Equilibrium, Kinetics, Isotopes (Elsevier Oceanography Series 65, 2001), the disclosure of which is incorporated herein by reference. More specifically, at alkaline pH values exceeding the second pKaof carbonic acid (i.e., pKa₂ ≈ 10.3 at ~25°C), CO2 dissolving as carbonic acid (H2CO3) is progressively deprotonated to bicarbonate (HCO₃⁻) and then to carbonate (CO₃²⁻); wherein, above about pH 10.6, carbonate becomes the dominant species (while CO2can also be directly absorbed as carbonate via reaction with OH-), enabling robust chemisorption of CO₂ even from dilute gas streams. Furthermore, between roughly pH 6 and pH 10, bicarbonate is the principal form and provides the bulk of the solution’s DIC capacity. Conversely, as the pH is lowered below the apparent first pK (pK₁* ≈ 6.3 at~25°C), and especially below pH~5.8, the equilibrium shifts toward dissolved molecular CO₂(aq), thereby promoting desorption of CO₂ from the liquid phase. Thus, traversing a pH swing that moves the electrolyte from a CO₂(aq)-rich regime (< 6) to a bicarbonate / carbonate-rich regime (> 10) increases the DIC content by orders of magnitude and underpins efficient capture-and-release cycling. As such, by confining the electrochemical operation to this tailored pH window, maintained either passively by buffer(s) or actively by electrochemical pH control, the redox matter is protected from excessively acidic or alkaline extremes, while still enabling high-capacity CO₂ uptake and facile regeneration.

[0102] In addition, although not to be bound by any theory, it should be noted that, due to the apparent pK values of the carbonic-acid system and the Henry constant for CO₂ shift with temperature, with CO₂ partial pressure and with salinity (ionic strength), in some embodiments, the upper and the lower pH set-points of the swing are adjusted (either passively by buffer(s) or actively by electrochemical pH control) to track changes in speciation and, thus, preserve optimal speciation and energy efficiency across a broad range of process conditions. For instance, higher temperatures or lower pCO₂ tilt the equilibria toward CO₂(aq) and may warrant a modestly more acidic desorption endpoint, whereas lower temperatures or elevated pCO₂ favor bicarbonate / carbonate formation, and allow effective capture at less-alkaline pH.

[0103] In many embodiments, the flow cell is in fluid communication with the capture tank and the release tank, and the electrolyte solution is transported / circulated in and out of the electrochemical cell and between the capture and release tank in the sequence described herein. In many embodiments, the capture tank is connected to a flue gas or air source via the gas inlet. In many embodiments, the release tank is connected to an inert carrier gas source, and any number of safety valves and detection and measurement devices as needed, such as, for example, infrared CO2 sensor. In many embodiments, the inert carrier gas is a gas selected from the group comprising, but not limited to: Ar, N2, and any combination thereof.

[0104] To this end, in many embodiments, the electrolyte solution flows and circulates through the system of many embodiments as a single solution. In particular, in many embodiments, a CO2-containing gas, such as, for example, flue gas or air comprising CO2, first enters the capture tank via the gas inlet, wherein CO2from the CO2-containing gas is chemically captured as bicarbonate ions by the electrolyte solution having high pH due to the electrolyte solution comprising the fully reduced redox matter of embodiments. In many embodiments, any number of implements and strategies are employed to increase the surface area between the CO2-containing gas and the electrolyte solution. For example, in many embodiments, engineering strategies such as structured packing and trays are used to improve the gas exchange. Next, in many embodiments, the electrolyte solution comprising the bicarbonate ions flows from the capture tank to the anodic chamber of the flow cell, wherein the neutral redox matter is oxidized, reducing the pH of the solution. As such, in many embodiments, the electrolyte solution flowing through the anodic chamber and the bicarbonate ions dissolved therewithin are converted into carbonic acid, which is then decomposed into CO2 and H2O. To this end, since the electrolyte solution is supersaturated with CO2, CO2is released to be collected for storage or utilization via the release gas outlet. As such, the collected CO2can be next sequestered in underground reservoirs or utilized to produce sustainable fuels, polymers, concrete, and other products of value. Next, in many embodiments, the acidified electrolyte solution flows into the cathodic chamber of the flow cell, wherein the oxidized acidic form of the redox matter (e.g., TEMPO+) is electrochemically reduced to its neutralform (e.g., TEMPO for TEMPO+), and the electrolyte solution is returned to the capture tank for a new capture / release cycle. It is important to note here, that the system configuration described herein is only one example, wherein, in many other embodiments, the redox matter is used in conjunction with a different system, such as, as additional examples, a system selected from the group comprising, but not limited to: a non-flow systems for batch operation, cyclic operation system, a multistack systems for pH- independent CO2 capture and release, and any combination thereof.

[0105] FIGs.18A and 18B provide a more specific example, accompanied by various analytical data, of the system of many embodiments for continuous capture and release of CO2. In this particular example, and according to many embodiments, the flow cell consists of serpentine flow channels patterned into graphite plates, porous graphite felts as the electrodes, separated by an anion exchange membrane. In these exemplary experiments, the CO2capture and release process proceeds through the same sequential steps as described above, according to many embodiments, i.e.: 1) a flue gas comprising CO2 (here, simulated flue gas comprising 20 % CO2 and 80 % N2) is bubbled into the electrolyte solution comprising the redox matter in its reduced state (here, TEMPO) disposed within the CO2 capture tank; 2) the electrolyte solution is pumped into the anodic chamber, wherein the redox matter becomes oxidized and becomes acidic (here, TEMPO to TEMPO+); 3) the electrolyte solution is flown into the release tank, wherein CO2 is released upon concentration, and carried by an inert gas out of the release gas outlet for further concentration and quantification; 4) the electrolyte solution after CO2release is pumped into the cathodic chamber, wherein the redox matter is reduced (TEMPO+back to TEMPO in this particular example) and returned to the capture tank, therefore becoming regenerated for subsequent cycles.

[0106] As such, the system of many embodiments possesses some attractive properties, including, but not limited to: (1) Since the electrochemical cell conducts symmetrical redox reactions, it minimizes the equilibrium cell voltage and, thus, the energy input. The energy used to strip anelectron from the neutral form of the redox matter on side (or half-cell) of the electrochemical cell is recovered at the other half cell. (2) Utilizing both half cells to conduct critical steps (i.e., the redox matter’s oxidation and regeneration), instead of conducting charge-balancing reactions, reduces materials and capital costs. In addition, the ion membrane allows ions to neutralize the oxidized and reduced forms of the redox matter to maintain charge balance. (3) The continuous capture and release of CO2 maximizes utilization rate and allows for a compact electrochemical CO2 capture system.

[0107] To this end, FIGs.19 and 20 provide the CO2concentration, current, and cell voltage profiles for the system of many embodiments, wherein the system comprises H- TEMPO as the redox matter, at various current conditions. For these particular experiments (but according to many embodiments), the electrolyte solution comprised 50% reduced TEMPO and 50% oxidized TEMPO, and the initial pH of the electrolyte solution was tuned with KOH. However, it should be noted here that, in many embodiments, a number of initial conditions, including, but not limited to, the redox matter concentration, the ratio of the reduced / oxidized redox matter, initial pH, the composition and concentration of the supporting salt, additives, and any combination thereof, are optimized for the most efficient operation of the system. In addition, CO2 was flowed for capture prior to applying electrical current as described in Methods, “CO2capture and release with flow cell” section below. Accordingly, for all conditions, it was observed that, without any current, the equilibrium cell voltage was close to 0 V, which signified a thermodynamic energy penalty close to zero. With applied current, cell voltage and CO2concentration increased. At a relatively low current density of 1.25 mA cm-2, the operando cell voltage was 0.022 V. By coupling Faradaic efficiency (assuming 1 mol CO2 / mol e-) and the cell voltage, the energy cost of the oxidation / reduction roundtrip cycle was deduced to be 2.6 kJ mol-1CO2(also see Y. Liu, et al., Electrochemically mediated carbon dioxide separation with quinone chemistry in salt-concentrated aqueous media. Nat. Commun. 11, 1–11 (2020), the disclosure of which is incorporated herein by reference). For comparison, a typical pH swing mechanism requires thermodynamic potentials of 0.18-0.65 V, which corresponds to pH differences of 3—11. In contrast, the instant pH-independent redox chemistry-based method minimizes equilibrium cell voltages and energy input. Accordingly, although the instant system and method are subject to the Nernst equation, changes in the redox matter oxidation states (e.g., TEMPO / TEMPO+) are typically small, allowing for a minimal equilibrium cell voltage.

[0108] Furthermore, as summarized in FIG.21, the overpotential increases linearly with current, which signifies that the system of the instant application has primarily an ohmic impedance response. In addition, FIG. 22 also corroborates this insight via electrochemical impedance spectroscopy. However, in many embodiments, the system decreases the conductive resistances and increases the electrode surfaces relative to the liquid volumes even further, in order to increase the energy efficiency and boost the reaction rates.

[0109] It should be noted here, that, according to many embodiments, the pump power requirement for the electrolyte solution transport is small, compared to the electrochemical process. For example, FIG.23 shows the pressure drop across one side of the flow cell of many embodiments for different flow rates (left), as well as the corresponding power requirements, calculated using the Hagen-Poiseuille equation (right). To this end, for 6.25 mA cm-2operation of the flow cell of the system of many embodiments, electrochemical power requirement is 890 μW cm-2, whereas the pump power requirement for both sides of the flow cell is 6.9 μW cm-2, which is 2 orders of magnitude lower. In addition, FIG.24 shows continuous capture and release of CO2 by the system of many embodiments for 30 hours, confirming that the system can be operated at steady state stably, without decrease in Faradaic efficiency, and increase in overpotential. Moreover, a turnover ratio, defined as the total moles of released CO2over the amount of the redox matter present in the electrolyte solution (H-TEMPO in these particular experiments), of ~ 1.2 was observed in these experiments. Therefore, this finding confirms that the redox matter of many embodiments is being reused as a capture and release agent.

[0110] In some embodiments, the CO2 capture performance by the instant system and method are further improved, beyond the metrics presented herein, wherein the metrics include: cell voltage, Faradaic efficiency (CO2 / electron ratio), long-term operationalstability, current density, CO2 capacity, and cost. For example, in many embodiments, the overpotential of the system is lower than the value shown herein. In many embodiments, and according to the data presented in FIGs.21, 22, the electrochemical flow cell exhibits an ohmic behavior and the charge-transfer kinetics of the instant redox chemistry are not limiting. Accordingly, in many embodiments, decreasing the ohmic resistance of the electrochemical flow cell through reductions in the contact resistance, bulk electrolyte resistance, electrode and membrane resistances affords a smaller cell impedance and a lower overall energy cost. It should be noted here that, indeed, an order of magnitude lower cell resistances have been previously demonstrated in redox flow batteries (see, for example, K. Lin, et al., A redox-flow battery with an alloxazine-based organic electrolyte. Nat. Energy.1, 16102 (2016), the disclosure of which is incorporated herein by reference). However, ensuring fast reaction kinetics at all pH is important for achieving low overpotentials and high energy efficiency, as well as higher CO2capacity. Accordingly, in many embodiments, the composition of the electrolyte solution and the molecular structure of the redox matter are adjusted to improve the reduction kinetics of the redox matter and, as such the overall performance of the system and method of the instant application. Moreover, in many embodiments, the design of the capture and release tanks is adjusted to increase gas-liquid interface, while minimizing liquid and gas flow energy, and, as such to improve the CO2capture and release efficiency of the instant system and method. In many embodiments, structured packing and tray engineering are utilized in the design of the instant system to further improve the system’s CO2 capture and release efficiency by improving liquid and gas mass transfer properties of the system (for relevant discussions, see, for example: M. Afkhamipour, M. Mofarahi, Review on the mass transfer performance of CO2 absorption by amine-based solvents in low- and high- pressure absorption packed columns. RSC Adv.7, 17857–17872 (2017); S. van Loo, et al., Mass Transfer Characteristics in Structured Packing for CO2 Emission Reduction Processes. Ind. Eng. Chem. Res. 46, 3035–3040 (2007); and N. Hüser, et al., Experimental and numerical characterization of a new structured packing for CO2 capture. AIChE J. 64, 4053–4065 (2018), the disclosures of which are incorporated herein by reference).

[0111] Although not to be bound by any particular theory, it is believed that the pH- independent redox chemistry underlying the present system and method enables both the CO₂-capture step and the regeneration (desorption) step to proceed under or near ambient operating conditions (i.e. at a temperature of 25 °C and a pressure of ~1 bar). In many embodiments, the electrolyte solution and contacting gas streams are maintained at a temperature within about 5 °C to about 60 °C, optionally limited to any sub-range between (e.g., about 15 °C – 35 °C or about 20 °C – 40 °C), thereby avoiding or limiting the quantity of steam reboilers, vacuum equipment, or thick-walled pressure vessels commonly required in thermal-swing or pressure-swing processes. In some embodiments, the operating temperature is deliberately biased anywhere within a broader span, where compatible with materials selection or waste-heat integration, or down to sub-freezing temperatures to facilitate capture from cold ambient air. Likewise, in some embodiments, the regeneration step is, when advantageous, performed under a modest vacuum — for example, about 0.3 bar to about 0.8 bar absolute—to enhance CO₂ release, or under a slight over-pressure, for example, up to about 5 bar absolute, to accelerate CO₂ absorption. In many such embodiments, such pressure variations are implemented without modification of the fundamental electrochemical stack, because the requisite chemical driving force is supplied internally by the redox couple. In many embodiments, these temperature- and pressure-adjusted modifications are deemed to fall within the spirit and scope of the present disclosure, which is intended to encompass any operating regime that leverages the described pH-independent redox mechanism to capture and release CO₂ efficiently.

[0112] In many embodiments, the system and all the materials it comprises are highly scalable and low cost. More specifically, in many embodiments, the electrochemical system comprises: highly abundant carbonaceous electrodes, water as the electrolyte solution’s solvent, the redox matter that is commercially available and inexpensive, and one of commercially available flow cell configurations. In addition, in many embodiments, the system and method are energy efficient, because the energy cost of moving incompressible fluids (i.e., water) is far lower than the cost of moving gases, as evidenced by the round-trip efficiency of pumped hydro-energy storage. Furthermore, since,according to many embodiments, the redox matter is highly soluble in water (e.g., TEMPO’s solubility is ~ 1 M in water), the instant system and method allow for capture of significant amounts of CO2. For example, the system and method employing TEMPO or a TEMPO derivative as the redox matter can capture up to 22.4 L of CO2 gas per 1 L of the electrolyte solution. Furthermore, the instant electrochemical system and method advantageously circumvents energy costs associated with a pH gradient in a working electrochemical cell. As such, electrochemical system and method of many embodiments allow to achieve energy costs of as low as in the ranges of 0 – 30 kJ mol-1, depending on factors such as partial pressure of CO2at capture and release, current density, flow rate, electrochemical and flow system design and redox matter and electrolyte concentrations, which is an order of magnitude lower than other electrochemical CO2 capture technologies commercially available to date.

[0113] Accordingly, in many embodiments, the instant system and method enable an energy-efficient capture and release of CO2 via the pH-independent redox chemistry. In many embodiments, the redox chemistry is pH-independent by circumventing the direct involvement of H+, which is in contrast to the conventional pH swing mechanisms, wherein equilibrium redox potentials depend on the pH. As such, the pH-independence of the redox chemistry employed in the instant system and method has profound implications for the thermodynamic energy cost of the CO2capture and release according to many embodiments, since potential gradients that arise from pH swings can be minimized. In many embodiments, the oxidized redox matter acts as a Lewis acid that accepts electrons from surrounding water molecules (without directly producing H+), wherein this acid dramatically changes the activity coefficient ^^ of H+, and, hence, changes the ^^^^≡−log10(^^[^^+]). For example, in many embodiments, the water polarization impacts the activity of H+or through secondary chemical equilibrium, which changes pH, in accordance with embodiments of the application. In many embodiments, the redox matter is a redox-active molecule or material, that is stable at both reduced and oxidized states, wherein neither reduced nor oxidized states directly involve H+, and exhibits pH- independence. Owing to this property, in many embodiments, equilibrium cell voltage of the system is negligible and the theoretical limit of CO2 capture and release can beapproached. In many embodiments, the materials costs for the instant system and method for electrochemical CO2capture are low, and the capture capital expenditures for the same are lower than many currently deployed comparable systems. Accordingly, the electrochemical system and method of the instant application offer a very attractive, practical, and cost-effective approach for the electrochemical capture CO2. In many embodiments, the system and method of the instant application are employed for CO2capture at point sources, such as, for example, cement and power plants. However, in other embodiments, the system and method of the instant application are employed for direct air capture. EXEMPLARY EMBODIMENTS

[0114] 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 number average molecular weight, temperature is in degrees Celsius, and pressure is at or near atmospheric.

[0115] As one illustrative example, the CO2 capture capability of the system and method of many embodiments were tested using an H-cell schematically shown in FIG. 25. To this end, the electrolyte solution containing 0.1 M of H-TEMPO as the redox molecule was initially bubbled with 20 % CO2 and 80 % N2 for 1 hour. Subsequently, inert Ar gas was flowed at 5 sccm as a carrier gas to monitor the CO2concentration. The CO2concentration monitoring showed minimal CO2concentration at the onset, which increased with current flow (FIG.26). After the current was stopped, the CO2 release stalled, and the CO2 concentration dropped. As such, the instant H-cell experiment demonstrates that, according to many embodiments, the instant system and methodemploying H-TEMPO as the redox molecule to spontaneously capture CO2 and release it through oxidation.

[0116] As another illustrative example, an in-situ attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) experiment to capture CO2 utilizing the system and method of many embodiments was conducted as illustrated in FIG.27. Here, the electrochemical setup was similar to the one depicted in FIG.25, with the addition of a CO2 gas microbubbler and a peristaltic pump for transport of the electrolyte solution comprising H-TEMPO as the redox molecule of many embodiments. To this end, the electrolyte solution was continuously circulated between the H-cell and a custom designed ATR-FTIR flow cell at 50 mL min-1(for a relevant discussion see J. E. Avilés Acosta, et al., Electrochemical Flow Reactor Design Allows Tunable Mass Transport Conditions for Operando Surface Enhanced Infrared Absorption Spectroscopy. ChemCatChem. 15, e202300520 (2023), the disclosure of which is incorporated herein by reference). Within the flow cell, the electrolyte solution flowed over a Ge ATR crystal, allowing for continuous infrared measurements of the electrolyte solution at the Ge-liquid interface. Here, as seen from FIG.28 (and FIGs.29A through 29C for A-TEMPO), a peak around 1360 cm-1that corresponds to the bicarbonate anion (for a relevant discussion, see A. Stefánsson, et al., Magnesium bicarbonate and carbonate interactions in aqueous solutions: An infrared spectroscopic and quantum chemical study. Geochim. Cosmochim. Acta.198, 271–284 (2017), the disclosure of which is incorporated herein by reference) emerged upon CO2 injection, signaling bicarbonate formation. Next, after a steady state was reached, an oxidative current was turned on, under continuous bubbling of CO2, which resulted in the bicarbonate peak decreasing with time, consistent with the conversion of bicarbonate into CO2. In addition, the CO2 capture by H-TEMPO was further evidenced by a measured decrease in the pH of the electrolyte solution, ultraviolet-visible (UV-Vis) absorption spectroscopy provided in FIGs. 30 and 31, and liquid chromatography-mass spectrometry (LC-MS) shown in FIGs.32 through 37, which, together, rule out the alternative mechanism of direct adsorption of CO2 onto TEMPO.Methods

[0117] Cyclic voltammetry experiments. For all experiments, A-TEMPO (TCI Chemicals) and H-TEMPO (Sigma-Aldrich), potassium chloride (KCl, Sigma-Aldrich) and potassium hydroxide (KOH, Sigma-Aldrich) were used as received. 5 mM of A-TEMPO or H-TEMPO with 100 mM of KCl in deionized water were used for CV experiments. Graphite felt (AvCarb G280A, Fuel Cell Store) was used as working and counter electrodes, while Ag / AgCl reference electrode (Gamry Instruments) was used as the reference electrode in a H-cell (Adams & Chittenden). An anion exchange membrane (FAS-30, Fuel Cell Store) was used between the two half-cells of the H-cell. BioLogic potentiostats were used and a scan rate of 50 mV / s was used.

[0118] CO2 capture with H-cell. The graphite felt, Ag / AgCl reference electrode, and anion exchange membrane are identical to the CV experiments. An electrolyte solution of 100 mM H-TEMPO, 300 mM KCl and 80 mM KOH was used on the working electrode side, while 300 mM KCl was used on the counter electrode half cell. Initially the electrolyte solution was bubbled with 20% CO280% N2 gas for 1 hour while stirring. A cap sealed the half-cell to prevent gas leakage while allowing for Ar carrier gas into the cell and Ar+CO2 gas out of the cell. Ar carrier gas was flowed using a mass flow controller (Alicat) at 5 sccm into the half-cell, and the gas in the half cell flowed into the CO2 sensor (CO2Meter) where CO2concentration was measured. After initial resting of 5 minutes, an oxidative current of 6 mA was applied to the working electrode, and CO2 concentration was measured. After 55 minutes of oxidation, current was stopped, while CO2 concentration measurement continued to record the changes in CO2concentration.

[0119] LC-MS analysis. Samples were analyzed on two different LC-MS instrument setups: (1) an Agilent 1260 high-performance liquid chromatography (HPLC) instrument paired with an Agilent 6520 accurate-mass quadrupole time-of-flight (Q-TOF) mass spectrometer (6520 LC–MS) or (2) an Agilent 1290 Infinity II UHPLC paired with a coupled Agilent 6546 Q-TOF mass spectrometer (6546 LC–MS). Electrospray ionization (ESI) in positive ionization mode was used for both instruments. For detection of molecules of interest, HILIC analysis was performed using a Poroshell 120 HILIC-Z column (Agilent, 2.7μm, 2.1 x 100 mm) with water and 9:1 ACN:water, each with 0.1% formic acid and 10mM ammonium formate, as mobile phases. For 6520 LC-MS, an injection volume of 2 μL with a flow rate of 0.25 mL / min was used with the following 22 min gradient method: 0-3 min, 100% B; 3-13 min 100-60% B; 13-14 min 60-100% B; 14-22 min, 100% B. For 6546 LC-MS, an injection volume of 1 μL with a flow rate of 0.25 mL / min was used with the following 12.5 min gradient method: 0-3 min, 100% B; 3-8 min 100-60% B; 8-9 min 60-100% B; 9-12.5 min, 100% B.

[0120] For mass spectrometer parameters, the following parameters were used for 6520 LC-MS to collect MS data in positive ion mode: mass range of 50-1000 m / z; drying gas temperature of 300oC; drying gas flow rate of 10 L / min; nebulizer of 35 psi; fragmentor at 150 V; skimmer at 65 V; Oct 1 RF Vpp at 750 V; VCap at 3500 V; 1000 ms per spectrum). For 6546 LC-MS, samples were run in positive mode (mass range: 30- 1700 m / z; drying gas temperature: 325oC; drying gas flow rate: 10 L / min; nebulizer: 35 psi; sheath gas temperature: 350oC; sheath gas flow: 12 L / min; fragmentor: 135 V; skimmer: 45 V; Oct 1 RF Vpp: 750 V; VCap: 4000 V; 1000 ms per spectrum. For both instruments, first 0.5 min of each run was discarded to avoid salt contamination of MS apparatus. For LC-MS data analyses, Agilent MassHunter Qualitative Analysis software was used in general. Extracted ion chromatograms (EICs) shown in figures were generated by extracting for the exact m / z for the target ion of interest with a 20 to 100 ppm mass tolerance.

[0121] In-situ FT-IR characterization. ATR-FTIR measurements were taken using a Nicolet IS30 spectrometer and an MCT (mercury cadmium telluride)-A detector. The spectrometer was coupled with the VeeMAX III (Pike Technologies) attachment at a 60 degree angle of incidence and 128 scans were added to form each measured spectrum. The custom ATR-FTIR flow cell was constructed to allow liquid flow over a Ge face-angled ATR crystal (as described, for example, in J. E. Avilés Acosta, et al., Electrochemical Flow Reactor Design Allows Tunable Mass Transport Conditions for Operando Surface Enhanced Infrared Absorption Spectroscopy. ChemCatChem.15, e202300520 (2023), the disclosure of which is incorporated herein by reference). A 100 mM H-TEMPO, 300 mM KCl and 80 mM KOH solution was prepared. A Masterflex peristaltic pump (Avantor) was used to pump electrolyte solution at 50 mL min-1. First, an initial FT-IR spectrum ofthe pristine solution was collected, and spectra were collected every 5 minutes. Subsequently, CO2was bubbled in using a disperser until CO2saturation. After CO2saturation, an oxidation current of 3 mA was flowed while the CO2bubbling continued. Throughout the duration of the experiment, the TEMPO solution was circulated between the ATR-FTIR flow cell and H-cell at 50 mL min-1.

[0122] Flow cell assembly. The electrochemical flow cell set-up consists of custom machined parts. Stainless steel end plates were used to contain polypropylene frames that served as insulators and hose adaptors to insert the liquid feed tubing. Serpentine flow patterns (1.25 mm for both channel width and depth, 1.25 mm spacing between channels, 8 cm2of active surface area) were carved on graphite plates (8x8 cm area, 0.25” thickness, McMaster). A protuberance (1x1 cm area) of the graphite plate was used for the connection to the external circuit. Viton gaskets were used to seal the space around 3.1-mm-thick graphite felts (4x4 cm area, Rayon Graphite Felt, Fuel cell store), which were baked in air for 6 h at 400oC before use. An anion exchange membrane (FAS-30, Fuel Cell Store) was used between the two half-cells of the flow cell. Polyvinylidene fluoride compression fittings were used to connect the cell with Teflon tubing. Two peristaltic pumps (BW100, Chonry) were used to circulate the liquid among the CO2 capture and release tanks. Mass flow controllers (MKS Instruments and Omega Engineering) were used to flow simulated flue gas and argon to the CO2capture and release tanks, respectively. The gas was dehydrated by passing through a Nafion tubing (CO2Meter) before entering a non-dispersive infrared absorption CO2 sensor (CO2Meter).

[0123] CO2capture and release with flow cell. The solution consisted of 0.4 M H- TEMPO, 1.2 M KCl, and 0.2 M KOH solution. To create a 50% oxidized and 50% reduced mixture, half of the total solution amount was completely oxidized through a constant current oxidation and combined with the unoxidized solution. The oxidized and unoxidized solutions were mixed and stabilized until equilibrium is reached for 30 minutes. CO2 was then bubbled into the solution for ≥ 1 hour. Equal amounts of the solution were allocated to capture and release tanks. The tanks were sealed properly to prevent gas leakages or mixtures with the atmosphere. An argon carrier gas was flowed at 30 sccm into the release tank, which carried the gas into the CO2 sensor for quantification. Onceinitial conditions are stabilized, liquid solution was flowed at 2 sccm and current were flowed at different current rates for CO2release. After release, current and liquid solution flow were stopped, while continuing to flow and measure gases.

[0124] Molecular Simulations. Classical, fixed-charge MD was conducted using LAMMPS from initial amorphous configurations. For the two systems that were simulated, boxes containing 4450 water molecules, 32 reduced / oxidized H-TEMPO, 16 OH-, 109 K+, and either 125 or 93 Cl- anions for the oxidized and reduced H-TEMPO states, respectively. The water molecules were described using the TIP3P forcefield (as described, for example, by W. L. Jorgensen, et al., Comparison of simple potential functions for simulating liquid water. J. Chem. Phys.79, 926–935 (1983), the disclosure of which is incorporated herein by reference), K+, Cl- and both oxidation states of H- TEMPO were described using the General Amber forcefield with partial charges generated with the AM1-BCC method in ANTECHAMBER (see, for example, A. Jakalian, et al., Fast, efficient generation of high-quality atomic charges. AM1-BCC model: I. Method. J. Comput. Chem.21, 132–146 (2000), the disclosure of which is incorporated herein by reference). OH- ions were described using the madrid forcefield (as seen in, for example, P. Habibi, et al. A New Force Field for OH–for Computing Thermodynamic and Transport Properties of H2 and O2 in Aqueous NaOH and KOH Solutions. J. Phys. Chem. B. 126, 9376–9387 (2022), the disclosure of which is incorporated herein by reference). Non-bonded interactions not explicitly specified in the forcefields were generated using Lorentz-Berthelot mixing rules. In all cases the charges of the ionic species were scaled to 0.75. A 10 Å cutoff for Van der Waals and real space coulomb were applied. Long-range Coulomb interactions were computed with a particle-particle- particle-mesh solver, with an error tolerance of 10-3. Periodic boundary conditions were also applied in all directions.

[0125] For each system, an initial energy minimization at 0 K was performed to obtain the ground-state structure with energy and force tolerances of 10-4. Then, the system was slowly heated from to 298 K at constant volume over 0.01 ns using a Langevin thermostat, with a damping parameter of 100 ps. 5 cycles of quench-annealing dynamics was then applied in an attempt to eliminate any meta-stable solvation states, where thesystem temperature was cycled between 298 K and 894 K with a ramp period 0.025 ns followed by 0.1 ns of dynamics at either temperature extreme at constant volume. Next, a constant temperature, constant pressure ensemble was applied using the Andersen barostat for 1.5 ns with a pressure relaxation constant of 1 ps to allow for equilibration of the system density. Finally, 10 ns of constant volume, constant temperature (NVT) production dynamics at 298 K was performed. Radial distribution functions were calculated during this period. Snapshots of the H-TEMPO solvation shells were obtained using Visual Molecular Dynamics (VMD) software using trajectories generated from the production dynamics.

[0126] Quantum chemistry simulations were performed using the Q-Chem 5.1 quantum chemistry package on cells comprised of 1 H-TEMPO molecule with 5 waters of random dipole orientation placed around the redox-active nitroxyl moiety. Calculations were conducted at the B3LYP / / 6-31+G(d,p) level of theory for geometry optimization and the B3LYP / / 6-311++G** level of theory for single-point energy calculations. No implicit solvent field is applied to either system. Partial charges for each atom was calculated using the CHELPG method native to Q-CHEM (as described, for example, in J. M. Herbert, et al., Rapid computation of intermolecular interactions in molecular and ionic clusters: self-consistent polarization plus symmetry-adapted perturbation theory. Phys. Chem. Chem. Phys.14, 7679–7699 (2012)., the disclosure of which is incorporated herein by reference). Electrostatic potential maps were generated using the IQ-mol software, also based on the CHELPG method. Supplementary Note

[0127] Considering the chemical reaction ^^^^^^^^^^ା ^ ^^^^ି ⇌ ^^^^^^^^^^^^ௗ, where ^^ arethe number of electrons transferred in the reaction, theFree energy is Δ^^ ൌ Δ^^^ ^ ^^^^ ln^^^^^ௗ⁄ ^^^௫^ , (1),where ^^ is the gas constant, ^^ ൌ 8.31^^ ⋅ ^^^^^^ ⋅ ^^ି^, ^^^^ௗ , ^^^௫ are the chemical activities,^^^௫ ൌ ^^^௫^^^^^^ and ^^^^ௗ ൌ ^^^^ௗ^^^^^^^^ . The activity coefficients ^^^௫, ^^^^ௗ are the activitycoefficients. At dilute concentrations of ^^^^^^ and ^^^^^^^^, ^^^௫, ^^^^ௗ → 1.At chemical equilibrium, the equilibrium constant ^^ is ^^ ൌ ^^^^ௗ⁄ ^^^௫.The free energy change of the electrochemical reaction (Δ^^^ is equal to the negative ofthe electrochemical potential (^^^ multiplied by Faraday constant (^^ ൌ 96,500 ^^ ⋅ ^^^^^^ି^ )and the number of charges (^^) Δ^^ ൌ െ^^^^^^, (2).Thus, the voltage of the half-cell equals to Δ^^ ൌ ^^ ^^^ௗ െ ^^^^^⁄ ^^^^^ ln ^^^^ௗ⁄ ^^^௫ , (3),where ^^^^^ௗis the voltage of some standard reference cell. In the reaction ^^^^ ^ ^^^^ ⇌ ^^^^ ^ ^^^^ െ ℎ^^ା െ ^^^^ି, (4)^^^ ൌ ^^^^ௗ ൌ ^^ ^^^ௗ െ బ.బఱవభల^୪୭^൬^^^^^ವ^^^ಲ^ೌ^ಳ^್^ ି ^.^ହଽ^^^^^^^ு, (5),where the curlyused. The third term in the right side of Equation 4 is separated out as the െ^^^ା^^part.The ^^^^ of water is defined as ^^^^ ≡ െ log ^^ శ ൌ െ ା^^ ு log^^ ^^^^^ ^ , where the protonchemical activity is ^^ுశ ൌ ^^^^^ା^. If the ^^^^ is changed by directly modulating^^^ା^, thenΔ^^ ൌ ln ൬^ுశ^^^^ ^ಹ^ுశ^^^^^^, since the act of creating a proton concentration gradient, i.e. creating [H+] and+low [H ] from two reservoirs of equal concentration, decreases the entropy of the system, and by the 2ndLaw of Thermodynamics, this minimum energy has to be supplied externally. Numerically, the voltage needed to change the ^^^^ is -0.0592 V per unit of ^^^^.

[0128] In many embodiments of the instant application, the ^^^^ is changed bychanging the activity coefficient ^^. In the instant model, a few water molecules in the vicinity of ^^^^^^^^^^ାare polarized so that the hydrogen atoms attached to ^^ଶ^^ are pointing away from ^^^^^^^^^^ା, and are very slightly easier to detach in the reaction ^^^^^^^^^^ା^^^ଶ^^ → ^^^^^^^^^^ା ⋅ ^^^^ି ^ ^^ା . Thus, ^^^^^^^^^^ା behaves as a weak acid. Similarly,^^^^^^^^^^^^ௗhas slightly electronegative charges and the hydrogen atoms are pulled intowards the N-O group ^^^^^^^^^^ ^ ^^ ^^ ⇌ ^^^^^^^^^^ ⋅ ^^ା ^ ^^ ି^^ௗ ଶ ^^ௗ ^^ .

[0129] In the cyclic voltammetry data in FIG.8, between a pH change of 4-10, the center voltage position where the reduced and oxidize states are in equilibrium is ~ 0.62 V.

[0130] A mechanism alternative to the activity coefficient modulation is direct modulation of H+through our electrochemical reaction. The Nernst Equation can be used to set an upper limit on the protons generated in the reaction ^^^^^^^^^^ା ⇌ ^^^^^^^^^^ା ⋅ ^^^^ି ^ ^^ା. (6).The electrochemical conversion ^^^^^^^^^^ ⇌ ^^ ା ି^^ௗ ^^^^^^^^ ^ ^^ is either independent ordependent on the pH of the solution. If it is independent on the pH, we cannot deduce anything about the voltage needed to oxidize ^^^^^^^^^^^^ௗ.

[0131] The reaction in Equation 6 is not plausible, because it would have a pHdependence of െ0.0592 ^^ / ^^^^ , which is 300 σ away from our slope of ^0.55 േ1.97 ൈ 10ିସ ^^ / ^^^^. However, it is perhaps more plausible to assume that a fraction of H+is formed in the coupled reaction. ^^^^^^^^^^ ⇌ ^1 െ ^^^^^^^^^^^^^ା ^ ^^^^^^^^^^^^^ା ⋅ ^^^^ି^ ^ ^ ^ ା^ ି^^ௗ ^ ^^ ^ ^^ . (7)If this coupling is correct, and it is assumed that for each ^^^^^^^^^^ା, a small number ^^ ≪ 1of ^^^^^^^^^^ାproduces a proton ^^ା(in this approximation, the activity factor is ~ 1 sincethe number of ^^^^^^^^^^ା ⋅ ^^^^ି and ^^ା is small.Δ^^ ൌ ^.^ହଽ^^ log ^^^^^^ವ^^^ಹశ^^ ^ൌ ^.^ହଽ^^ log^^^^^^^^^^^^ା ⋅ ^^^^ି^^^^^ା^^௭^ಲ^ೌ^ಳ^್௭ / ^^^^^^^^^^^^^ௗ^^ (8)the reaction of Equation (7) to the left, thereby increasing ^^^^^^^^^^ା. At equilibrium^^^^^^^^^^^ା ⋅ ^^^^ି^^ ൌ ^^^ା^^ and log ^^^^^^^^^^^^ା^^ି^⁄ ^^^^^^^^^^^^^ௗ^^ ~0 as changes inTEMPO concentrations are orders of magnitude smaller than changes in [H+], and Δ^^ ൌ 2nlog^^^ା^~ ^.^ହଽ^^௭^2^^^ log^^^ା^ ൌ 0.118 ^^ ା௭^ log^^^ ^. (9)Note needed to oxidize ା ି^^^^^^^^^^^^ௗ → ^^^^^^^^^^ ^ ^^ is^0.55 േ 1.97^ ൈ 10ିସ per Δ^^^^ ൌ 1 and per mole of TEMPO, whereas the slope shouldhave been negative. The ^^^^ uncertainty of having the needed negative slope is^0.55 െ 1.97^ ൈ 10ିସ ൌ െ1.42 ൈ 10ିସ per Δ^^^^ ൌ 1. The ^^^^ uncertainty is 5.36 ൈ 10ିସper Δ^^^^ ൌ 1.

[0132] Solving for ^^ , 0.118^^ ൌ 1.42 ൈ 10ିସ , ^^ ൌ 1.203 ൈ 10ିଷ mole H+ per mole ofTEMPO. This is a small change in ^^ା compared to the change in the Δ^^^^ ൌ 9.4 → 1.8 ൌ7.6 when go from fully reduced TEMPO to fully oxidized TEMPO at 0.4 M concentration.A pH change of 9.4 → 1.8 means that 0.0158 mole of H+ were produced from 0.4 mole ofTEMPO, which is a ratio is 0.0395. This is much larger than what is possible by ourresults, which is a ratio of 1.203 ൈ 10ିଷ mole H+ per mole of TEMPO. Thus, the pHchange of 9.4 → 1.8 cannot be accounted for by the direct H+ formation through thereaction shown in Equation 7. The conclusion is that the number of hydrogen atoms attached to the water molecules polarized by TEMPO remain that detach is negligible compared to the change in Δ^^^^, and the entropy penalty of changing ^^^ା^ is negligible.

[0133] By default, Δ^^^^ ൌ െΔ^log ^^^^^ା^^ means the activity factor ^^ has changed.The more easily detachable hydrogen atoms serve as a buffer source of protons, but they remain attached to water. These water molecules at as a buffer that maintains the proton concentration and the conversion of bicarbonate to neutral carbonic acid and then eventually to ^^^^ଶ,^^^in the reactions: ^^^^^^^^^^ା ^ ^^ଶ^^ ^ ^^^^^^ିଷ ^ ^^ା ⇌ ^^^^^^^^^^ା ⋅ ^^^^ି ^ ^^ା ^ ^^ଶ^^^^ଷ⇌ ^^^^^^^^^^ା ⋅ ^^^^ି ^ ^^ା ^ ^^^^ଶ,^^ ^ ^^ଶ^^⇌ ^^^^^^^^^^ା ⋅ ^^^^ି ^ ^^ା ^ ^^^^ଶ,^^^ ^ ^^ଶ^^. (10).DOCTRINE OF EQUIVALENTS

[0134] 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.

Claims

CLAIMS:

1. A system for electrochemical capture and release of carbon dioxide via a pH-swing mechanism, wherein the system is characterized by an operational pH swing range having a lower pH limit and an upper pH limit, and wherein the system comprises: a flow cell, wherein the flow cell is an electrochemical flow cell, comprising: an anode, disposed within an anodic chamber, a cathode, disposed within a cathodic chamber, an ion exchange membrane for selectively transporting ions separating the anodic chamber and the cathodic chamber; and a plurality of endplates and gaskets to ensure tight electrical contact to the anode and cathode and sealing to prevent leakage; a capture tank, further comprising a gas inlet for delivering CO2to the system and a capture gas outlet; a release tank, further comprising a release gas outlet for outflow of released CO2, an electrolyte solution, at least comprising a redox matter in a redox matter concentration sufficient to generate the operational pH swing range optimized for thermodynamics and kinetics of CO2 absorption; wherein the redox matter is a molecule or material that is redox active without exchanging protons or hydroxide ions, water soluble, and stable, including water-stable, in both its reduced and oxidized states, and wherein the electrochemical step is coupled with subsequent reversible chemical interactions with water to produce H+or OH- ions which have pH- dependance; and a supporting salt in a salt concentration optimized for CO2 capture performance; and any number of safety valves, pumps, inlets and outlets, as needed for safe and efficient operation; and wherein the capture tank, the flow cell, and the release tank are in fluid communication, such that the electrolyte solution is able to continuously circulate in sequence from thecapture tank to the anodic chamber to the release tank to the cathodic chamber and back to the capture tank.

2. The system of claim 1, wherein the redox matter is a molecule or material selected from the group consisting of: TEMPO; a TEMPO derivative; a soluble redox active material suitable for use in redox flow batteries.

3. The system of claim 2, wherein the redox matter is a TEMPO-derivative selected from the group consisting of: 4-hydroxy-TEMPO, amino-TEMPO, 4-oxo-TEMPO, trimethylammonium-TEMPO, 4-[3-(trimethylammonio)propoxy]-TEMPO, pyrrolidinium-TEMPO, riboflavin-TEMPO, a RSO3-(CH2)n-O-TEMPO radical, 4- carboxy-TEMPO, acetamido-TEMPO, 4-azido-TEMPO, 4-phosphonooxy-TEMPO, TEMPO functionalized with one or more functional group selected from: nitro, nitrile, sulfonic acid, sulfoxide, halide; and any combination thereof.

4. The system of claim 1, wherein the anode and the cathode comprise porous graphite.

5. The system of claim 1, wherein the supporting salt is a salt selected from the group consisting of: KCl, NaCl, LiCl, K2SO4, Na2SO4, KH2PO3, NaH2PO3, Na2CO3, K2CO3, an organic amine, an amino acid salt, and any combination thereof.

6. The system of claim 1, wherein the electrolyte solution further comprises a base.

7. The system of claim 6, wherein the base is selected from the group consisting of: KOH, NaOH, LiOH, K2CO3, Na2CO3, KHCO3, NaHCO3,NH4OH, a weak base, an amino acid salt, another zwitterion, and any combination thereof.

8. The system of claim 1, wherein the system is maintained within the operational pH swing range by one of the means selected from the group consisting of: a single pHbuffer system; a dual pH buffer system; an active pH monitoring and electrochemical control without any buffer; and any combination thereof.

9. The system of claim 8, wherein the dual pH buffer system comprises a first buffer, characterized by a first pKa, for maintaining the lower pH limit of the operational pH swing range; and a second buffer, characterized by a second pKa, higher than the first pKa, for maintaining the upper pH limit of the operational pH swing range.

10. The system of claim 9, wherein the first buffer is selected from the group consisting of: formic acid / formate, acetic acid / acetate, and any other buffer characterized by a pKa value that is the same or near the first pKavalue; and the second buffer is selected from the group consisting of: carbonate / bicarbonate system, phosphate species, an organic amine, ammonium ions, an amino acid, and another buffer characterized by a pKa value that is the same or near the second pKavalue.

11. The system of claim 10, wherein the second buffer comprises an organic amine, and the organic amine is monoethanolamine.

12. The system of claim 10, wherein the second buffer comprises the amino acid, and the the amino acid is selected from the group consisting of: glycine, sarcosine, piperazine, and any salt derivative thereof.

13. The system of claim 8, wherein the single pH buffer system comprises a buffer pair characterized by a single buffer pair pK^ of between about 6 and about 11, such as to maintain the system within the operational pH swing range.

14. The system of claim 13, wherein the single buffer pair is a buffer selected from the group consisting of: alkanolamines, amino acids, carbonate / bicarbonate, phosphate species, and another weak base.

15. The system of claim 8, wherein the system further comprises a pH sensor and a controller configured to actively monitor and adjust pH of the electrolyte solution, such as to maintain the system within the operational pH swing range.

16. A method for electrochemical capture and release of carbon dioxide via a pH-swing mechanism comprising: providing a system, characterized by an operational pH swing range having a lower pH limit and an upper pH limit, comprising: a flow cell, wherein the flow cell is an electrochemical flow cell, comprising: an anode, disposed within an anodic chamber, a cathode, disposed within a cathodic chamber, an ion exchange membrane for selectively transporting ions separating the anodic chamber and the cathodic chamber; and a plurality of endplates and gaskets to ensure tight electrical contact to the anode and cathode and sealing to prevent leakage; a capture tank, further comprising a gas inlet for delivering CO2 to the system and a capture gas outlet; a release tank, further comprising a release gas outlet for outflow of released CO2, an electrolyte solution, at least comprising: a redox matter in a redox matter concentration sufficient to generate the operational pH swing range optimized for thermodynamics and kinetics of CO2absorption; wherein the redox matter is a molecule or material that is redox active without exchanging protons or hydroxide ions, water soluble, and stable, including water-stable, in both its reduced and oxidized states, and wherein the electrochemical step is coupled with subsequent reversiblechemical interactions with water to produce H+or OH- ions which have pH- dependance; and a supporting salt in a salt concentration optimized for CO2capture performance; and any number of valves, pumps, inlets and outlets, as needed for efficient operation; and wherein the capture tank, the flow cell, and the release tank are in fluid communication, such that the electrolyte solution is able to continuously circulate in sequence from the capture tank to the anodic chamber to the release tank to the cathodic chamber and back to the capture tank; providing a gaseous stream comprising CO2; and delivering it to the capture tank; applying voltage to the flow cell and circulating the electrolyte solution through the system, to continuously and energy efficiently capture CO2 from the gaseous stream and controllably release CO2 for further concentration and storage.

17. The method of claim 16, wherein the method is characterized by an energy cost, and the energy cost is as low as 0 – 30 kJ mol-1.

18. The method of claim 16, wherein the redox matter is a molecule or material selected from the group consisting of: TEMPO; a TEMPO derivative; a soluble redox active material suitable for use in redox flow batteries.

19. The method of claim 18, wherein the redox matter is a TEMPO-derivative selected from the group consisting of: 4-hydroxy-TEMPO, amino-TEMPO, 4-oxo-TEMPO, trimethylammonium-TEMPO, 4-[3-(trimethylammonio)propoxy]-TEMPO, pyrrolidinium-TEMPO, riboflavin-TEMPO, a RSO3-(CH2)n-O-TEMPO radical, 4- carboxy-TEMPO, acetamido-TEMPO, 4-azido-TEMPO, 4-phosphonooxy-TEMPO, TEMPO functionalized with one or more functional group selected from: nitro, nitrile, sulfonic acid, sulfoxide, halide; and any combination thereof.

20. The method of claim 16, wherein the anode and the cathode comprise porous graphite.

21. The method of claim 16, wherein the supporting salt is a salt selected from the group consisting of: KCl, NaCl, LiCl, K2SO4, Na2SO4, KH2PO3, NaH2PO3, Na2CO3, K2CO3, an organic amine, an amino acid salt, and any combination thereof.

22. The method of claim 16, wherein the electrolyte solution further comprises a base.

23. The method of claim 22, wherein the base is selected from the group consisting of: KOH, NaOH, LiOH, K2CO3, Na2CO3, KHCO3, NaHCO3, NH4OH, a weak base, an amino acid salt, another zwitterion, and any combination thereof.

24. The method of claim 19, wherein the system is maintained within the operational pH swing range by one of the means selected from the group consisting of: a single pH buffer system; a dual pH buffer system; an active pH monitoring and electrochemical control without any buffer; and any combination thereof.

25. The method of claim 24, wherein the dual pH buffer system comprises a first buffer, characterized by a first pKa, for maintaining the lower pH limit of the operational pH swing range; and a second buffer, characterized by a second pKa, higher than the first pKa, for maintaining the upper pH limit of the operational pH swing range.

26. The method of claim 25, wherein: the first buffer is selected from the group consisting of: formic acid / formate, acetic acid / acetate, and any other buffer characterized by a pKavalue appropriate for establishing and maintaining the lower pH limit of the operational pH swing range; and the second buffer is selected from the group consisting of: carbonate / bicarbonate system, phosphate species, monoethanolamine, ammonium ions, sarcosine, andanother buffer characterized by a pKa value appropriate for establishing and maintaining the upper pH limit of the operational pH swing range.

27. The method of claim 24, wherein the single pH buffer system comprises a buffer pair characterized by a single buffer pair pK^ of between about 6 and about 11, such as to maintain the system within the operational pH swing range.

28. The method of claim 27, wherein the single buffer pair is a buffer selected from the group consisting of: alkanolamines, amino acids, carbonate / bicarbonate, phosphate species, and another weak base.

29. The method of claim 24, wherein the system further comprises a pH sensor and a controller configured to actively monitor and adjust pH of the electrolyte solution, such as to maintain the system within the operational pH swing range; and the method further comprises utilizing the pH sensor and the controller.

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