Water-soluble compounds in electrochemical systems for energy storage and carbon dioxide capture

Abstract

The present disclosure relates to a water-soluble and air-stable compound comprising a fluoflavine derivative, wherein the fluoflavine derivative comprises one or more water- solubilizing groups, which functions as a redox-active sorbent for electrochemical carbon dioxide capture from flue gas or ambient air.

Description

WATER-SOLUBLE COMPOUNDS IN ELECTROCHEMICAL SYSTEMS FOR ENERGY STORAGE AND CARBON DIOXIDE CAPTURECROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore Patent Application No.10202403797Y filed on December 4, 2024, and Singapore Patent Application No.10202500648U filed on March 13, 2025, the contents of both being hereby incorporated by reference in its entirety for all purposes.TECHNICAL FIELD

[0002] The present disclosure generally relates to a water-soluble compound for electrochemical carbon dioxide capture. In particular, the present disclosure relates to a water-soluble compound comprising a fluoflavine derivative having one or more water-solubilizing groups, for electrochemical carbon dioxide capture from flue gas or ambient air.BACKGROUND

[0003] The removal of carbon dioxide (CO2) from point sources and diffuse atmospheric sources is essential for mitigating climate change. Existing CO2 capture technologies, such as amine scrubbing and strongly alkaline liquid sorbents for direct air capture (DAC), typically rely on temperature-swing regeneration, which requires high energy input and may involve volatile, corrosive, or environmentally problematic materials. Alternative approaches such as direct ocean capture also face substantial challenges due to the need to process large volumes of water.

[0004] Electrochemically mediated CO2 capture has emerged as a promising alternative because it can be driven by clean electricity, operate at ambient conditions, and enable isothermal sorbent regeneration with significantly lower energy cost than thermochemical systems. In particular, aqueous organic redox-active sorbents have attracted considerableattention due to advantages such as non-flammability, scalability, modularity, and low material cost.

[0005] However, a key barrier to the practical deployment of redox-active sorbents, especially for DAC, is their inherent oxygen sensitivity. The reduced forms of these sorbents, generated during electrochemical regeneration, are readily oxidized by molecular oxygen rather than reacting selectively with CO2. This poor selectivity for CO2 over O2 severely limits capture efficiency under ambient air exposure and remains a fundamental challenge despite advances in molecular design, electrolyte formulation, and electrochemical engineering.

[0006] It is therefore desirable to provide a redox-active sorbent or compound that seeks to address at least one of the problems described hereinabove, or at least to provide an alternative solutionSUMMARY

[0007] In accordance with a first aspect of the present disclosure, a water-soluble compound for electrochemical carbon dioxide capture is provided. The water-soluble compound comprises: a fluoflavine derivative comprising one or more water-solubilizing groups, represented by the general formulae (I) to (IV):A IM AFtJlYI 1 N,1 Y1n-,HA (I)RyxA. N. A1 1 1 1 1RZ J-' N’ ‘N AR4(IDx.RHx,H<whereineach of R1to R8is independently H, OH, SO3H, COOH, PO3H2, NH2, N(CH3)3+, [OC2H4]nOH, or [OC2H4]nOCH3;at least one of each of R1to R8is OH, SO3H, COOH, PO3H2, NH2, N(CH3)3+, [OC2H4]nOH, or [OC2H4]nOCH3; andeach X is independently carbon, nitrogen, oxygen, sulfur, or phosphorus.

[0008] In accordance with a second aspect of the present disclosure, an electrochemical carbon dioxide capture system is provided. The system comprises: an inlet configured to receive a gas stream comprising flue gas or ambient air; an outlet configured to release carbon dioxide;

[0009] a membrane disposed between an anode compartment and a cathode compartment; and an electrolyte comprising a water-soluble compound capable of capturing carbon dioxide from the gas stream, wherein the water-soluble compound comprises: a fluoflavine derivative comprising one or more water-solubilizing groups, represented by the general formulae (I) to (IV):(i)(inR-x '„ R7v A y,- y y A '.yx6X y N N Y X R3(IV)whereineach of R1to R8is independently H, OH, SO3H, COOH, PO3H2, NH2, N(CH3)3+, [OC2H4]nOH, or [OC2H4]nOCH3;at least one of each of R1to R8is OH, SO3H, COOH, PO3H2, NH2, N(CH3)3+, [OC2H4]nOH, or [OC2H4]nOCH3; andeach X is independently carbon, nitrogen, oxygen, sulfur, or phosphorus.

[0010] In accordance with a third aspect of the present disclosure, a method for capturing carbon dioxide is provided. The method comprises introducing a gas stream comprising carbon dioxide into an electrochemical carbon dioxide capture system of the present disclosure; contacting the gas stream with the electrolyte comprising the water-soluble compound to capture carbon dioxide; and electrochemically regenerating the water-soluble compound to release carbon dioxide.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale; emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.FIG. 1 shows the synthetic routes and the ¹H NMR of fluoflavine sulfonate (FFS) and fluoflavine disulfonate (FFDS) in D2O at pH 14. The peaks in aromatic region are from the target molecules and the peaks in aliphatic region are from trace solvents. Because of the H-D exchange in D2O, the protons from -SO3H and -NH- groups are not showing up in the spectra. FIG. 2A shows the structure and synthesis of fluoflavine disulfonate (FFDS) from fluoflavine (FF).FIG. 2B shows the structure and synthesis of an oxidized FFDS (ox. FFDS) and the electrochemical proton-coupled electron transfer (PCET) between the FFDS and the ox. FFDS. FIG. 2C shows the1H and13C NMR spectra of the ox. FFDS and FFDS in DMSO-ri, FIG. 2D shows the ¹H NMR spectrum of FFDS of FIG. 2A.FIG. 2E shows the 'H NMR spectrum of the ox. FFDS of FIG. 2B.FIG. 3A illustrates the cyclic voltammogram of FFS at pH 7 and 14, at a scan rate of 100 mV / s. The supporting salts are 1M KCl at pH 7 and 1 M KOH at pH 14.FIG. 3B illustrates the cyclic voltammogram of FFDS at pH 7 and 14, at a scan rate of 100 mV / s. The supporting salts are IM KC1 at pH 7 and 1 M KOH at pH 14.FIG. 3C illustrates the cyclic voltammograms of FFDS in buffered electrolytes at different pH ranging from 5.7 to 13.6.FIG. 3D illustrates the Pourbaix diagram of FFDS.FIG. 3E shows the experimental and DFT- calculated reduction potential of FFDS vs pH. FIG. 4 illustrates a schematic representation of an electrochemical CO2 capture setup.FIG. 5A illustrates a graph showing the preliminary data of FFDS sorbent-based electrochemical CO2 capture. The graph shows the representative voltage profiles of the FFDS, Fe(CN)6 flow cell in an atmosphere of 100% N2; 20% CO2, 80% N2 (red); and 17.8% CO2, 10.7% O2, and 72.5% N2.FIG. 5B illustrates variation of CO2 readings in percentage during electrochemical chargedischarge induced CO2 capture-release process while charging the feed gas composition from 20% CO2, 80% N2to 17.8% CO2, 10.7% O2; 72.5% N2. The flow cell configuration is 10 mb, saturated FFDS, 1 M KC1, pH 9 | 100 mL 30 mM K4Fe(CN)6, 10 mM K3Fe(CN)6, 1 M KC1, pH 7. The cell is charged / discharged at a constant current density of 20 mA / cm2followed with potential holds until current density drops to 2 mA / cm2.FIG. 6A shows the potential and pH profiles of FFDS electrolyte over time in nitrogen (N2), in the absence of CO2.FIG. 6B shows the potential and pH profiles of FFDS electrolyte over time when carbon dioxide (CO2) is introduced.FIG. 6C shows the potential vs. capacity profiles with and without captured CO2.FIG. 6D shows the energy cost for CO2 release and sorbent regenerationFIG. 7A illustrates a UV-Vis absorbance spectra a varied state of charge (SOC) from 100% to 0%.FIG. 7B illustrates a UV-Vis absorbance spectra at varied state of charge (SOC) from 0% to 100%.FIG. 7C shows a simulated UV-Vis spectra of FFDS and oxidized FFDS.FIG. 7D shows a UV-Vis absorbance spectra of FFDS at charge state (100% SOC) after CO2 capture or under 2 eq. KHCO3, compared with those at varied state of charge (SOC).FIG. 7E shows a zoomed-in view of FIG. 7D at wavelength from 200 to 320 nm, where the first UV-Vis absorbance peak of FFDS was.FIG. 7F shows a zoomed-in view of FIG. 7D at wavelength from 320 to 600 nm, where the second UV-Vis absorbance peak of FFDS was.FIG. 7G shows a stacked in-situ IR transmittance spectra of FFDS over an electrochemical reduction process (bottom to top).FIG. 8A shows the fluorescence emission spectra of FFDS at varied SOC at 0%, 50% and 100%.FIG. 8B shows the fluorescence emission spectra of FFDS at varied SOC ranging from 0% to 100%, at interval of 10%.FIG. 8C shows a graph illustrating the fluorescence peak intensity vs. SOC.FIG. 9A illustrates a one-cycle and 11-cycle CO2 capture and release at 20 mA / cm2. Top to bottom: voltage profile, variation of pH, O2 level, and CO2 flow rate.FIG. 9B shows a graph illustrating volume released CO2, molar ratio of CO2 / T over 38 cycles and 200 hours.FIG. 9C shows a graph illustrating fraction distribution of carbonate species over pH.FIG. 9D shows a five-cycle CO2 capture and release at varied current densities of 40, 60, 80 and 100 mA / cm2.FIG. 9E illustrates FFDS-based CO2 capture and release in simulated flue gas at varied current densities of 20, 40, 60, 80 and 100 mA / cm2. FIG. 9F illustrates a graph showing cumulative CO2 capture over time at 20, 40, 60, 80, 100 mA / cm2.FIG. 9E illustrates a graph showing cumulative CO2 released over time at 20, 40, 60, 80 and 100 mA / cm2.FIG. 9G illustrates a graph showing a FFDS-based five-cycle CO2 capture and release at varied current densities from 20, 40, 60, 80 to 100 mA / cm2.FIG. 9H illustrates a graph showing an averaged Coulombic efficiency and energy cost over five cycles at 20, 40, 60, 80 and 100 mA / cm2.FIG. 10A illustrates a graph showing a normal discharge-charge voltage profile of a FFDS sorbent-based electrochemically induced DAC and CO2 release from both indoor and outdoor ambient air in a tropical country. 10 mL, 0.1 M FFDS and 0.4 M KOH was used as the sorbent. The posolyte is composed of 100 mL of 0.03 M K4Fe(CN)e and 0.07 M K3Fe(CN)e. The flow cell is charged-discharged at 20 mA / cm2with potential holds until the current density decreases to 2 mA / cm2.FIG. 10B illustrates a graph showing 10 cycles of normal discharge induced CO2 release after three-day indoor DAC over 40 days.FIG. 10C illustrates a graph showing the CO2 release from 20 mL 50 mM FFDS after 12th three-day indoor DAC. Pure N2 at a flow rate of 28 mL / min served as the carrier gas. 10.49 mL of CO2 was released after the normal discharge-induced CO2 release. 1 M HC1 aliquots was added to the sorbent to release the accumulated DIC in the solution. 42.54 mL of CO2 was released after the addition of total 4.0 mL 1 M HC1 over four times.FIG. 10D illustrates a graph showing 16 cycles of deep discharge induced CO2 release after accelerated overnight indoor DAC.FIG. 10E illustrates graphs showing indoor and outdoor air monitoring including temperature, relative humidity, O and CO2 levels for 20 days.FIG. 10F illustrates a graph showing a comparison of three-day indoor and outdoor DAC with the 10 mL, 0.2 M FFDS sorbent. The posolyte is composed of 200 mL of 0.03 M K4Fe(CN)e and 0.07 M KsFefCNXFIG. 10G illustrates the 10 cycles of CO2 release after three-day indoor DAC. The results show that the volumes of released CO2 across the 10 cycles over 40 days are comparable.FIG. 11A illustrates the pH, the concentration of CCh2', HCCh', and the dissolved inorganic carbon (DIC) changes over time when the freshly regenerated FFDS sorbent is exposed to indoor air and stirred for three-day DAC.FIG. 11B illustrates the pH variation during the discharge-induced CO2 release and the subsequent charge-induced sorbent regeneration.FIG. 11C illustrates the CO2 flow rate variation during the first CO2 release process.FIG. 11D illustrates the volumes of the released CO2 and the Coulombic efficiencies of the FFDS electrolyte over the 10 cycles of three-day indoor DAC -release processes.FIG. 11E illustrates the charge-discharge capacity of FFDS over 40 days, 10 cycles of three-day indoor DAC-release processes.FIG. 12 illustrates a graph showing a 14-cycle CO2 capture and release performance in a one-bar simulated flue gas comprising 10% CO2, 10% O2 and 80% N2 at a flow rate of 30 mL / min. FIG. 13 illustrates a graph showing a 5-cycle CO2 capture and release performance in the same simulated flue gas.FIG. 14 illustrates a graph showing an 8-cycle CO2 capture and release performance in the same simulated flue gas.FIG. 15 illustrates a graph showing a FFDS-based CO2 capture and release performance in the same simulated flue gas at varied current densities of 20, 40, 60, 80, and 100 mA / cm2.FIG. 16A illustrates the 10thcycle of normal discharge, deep discharge and normal charge profiles of FFDS sorbent.FIG. 16B illustrates the pH profile of the FFDS sorbent over timeFIG. 16C illustrates the CO2 release from 10 mL 100 mM FFDS sorbent after the 10ththree-day indoor DAC.FIG. 17A illustrates the results obtained after FFDS sorbent was bubbled with indoor air at a stabilized flow rate of 375 mL / min.FIG. 17B illustrates the downstream CO2 level variation during the overnight indoor airbubbling DAC and the integrated volume of captured CO2.FIG. 17C illustrates the variation of pH, the concentrations of CCh2’, HCOf, and DIC in a fresh FFDS sorbent solution during the overnight air-bubbling DAC.FIG. 18A illustrates the charge-discharge voltage profile of FFDS sorbent.FIG. 18B illustrates the pH profile of the FFDS sorbent.FIG. 18C illustrate the CO2 release from 10 mL 100 mM FFDS sorbent.FIG. 19A illustrates the charge-deep discharge voltage profile of FFDS sorbentFIG. 19B illustrates the pH profile of the FFDS sorbent.FIG. 19C illustrates the CO2 release from 10 mL 100 mM FFDS sorbent.FIG. 20A illustrates the discharge voltage profile of the FFDS sorbent.FIG. 20B illustrates the deep discharge voltage profile of the FFDS sorbent.FIG. 20C illustrates the charge voltage profile for sorbent regeneration.FIG. 20D illustrate the CO2 release from 10 mL 100 mM FFDS sorbent.FIG. 20E illustrate the CO2 release from 10 mL 100 mM FFDS sorbent.FIG. 20F illustrates the pH profile of the FFDS sorbentFIG. 21A illustrates the charge-deep discharge voltage profile of the FFDS sorbent.FIG. 21B illustrates the pH monitoring of the FFDS sorbent.FIG. 21C illustrates the CO2 release from 10 mL 100 mM FFDS sorbent.FIG. 22A illustrates the discharge voltage profile of the FFDS sorbent.FIG. 22B illustrates the deep discharge voltage profile of the FFDS sorbent.FIG. 22C illustrates the charge voltage profile for sorbent regeneration.FIG. 22D illustrate the corresponding CO2 release during the normal discharge from 10 mL 100 mM FFDS sorbent.FIG. 22E illustrate the corresponding CO2 release during the deep discharge from 10 mL 100 mM FFDS sorbent.FIG. 23A illustrates a three-day indoor DAC voltage profile of FFDS sorbent.FIG. 23B illustrates the CO2 release performance following the protocol GP2.FIG. 23C illustrates the pH profile of the FFDS sorbent over time.DESCRIPTION

[0012] The following description sets forth exemplary methods, parameters, and the like. The embodiments are described in sufficient detail to enable those skilled in the art to practise the invention. Other embodiments may be utilized, and structural and logical changes may be made without departing from the scope of the invention. The various embodiments are notnecessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0013] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and / or combinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0014] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0015] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance, e.g. within 10% of the specified value.

[0016] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0017] By “comprising” it is meant including, but not limited to, whatever follows the word “comprising”. Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0018] By “consisting of’ is meant including, and limited to, whatever follows the phrase “consisting of’. Thus, the phrase “consisting of’ indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0019] The present disclosure relates to a water-soluble compound for electrochemical carbon dioxide capture, and an electrochemical carbon dioxide capture system comprising the water-soluble compound of the present disclosure.

[0020] The water-soluble compound is an air-stable redox species that can be electrochemically driven to capture carbon dioxide (CO2) from flue gas or air. The water-soluble compound is insensitive to oxygen and can undergo either proton-coupled electron transfer and pH-swing induced CO2 capture, or proton-decoupled electron transfer and nucleophilicity-swing induced CO2 capture. The oxygen insensitivity of the compound enables CO2 to be captured in the presence of oxygen, making the present disclosure a practical approach for carbon capture from flue gas or air.

[0021] The electrochemical carbon dioxide capture system is an oxygen-tolerant aqueous organic flow-driven electrochemical direct air capture system that is scalable, modular, low-cost and operates at ambient temperature and pressure.

[0022] According to a first aspect, a water-soluble compound for electrochemical carbon dioxide capture is provided. The water-soluble compound comprises a fluoflavine derivative comprising one or more water-solubilizing groups, represented by the general formulae (I) to (IV):Rvs A R8. N. A X. R,Ar 'I YI 'NI' N' O f 'Rs Fh ft)n,, A N,, N, A R,Rr ■' n n J ■RSR+(II)RsRa (IV)

[0023] wherein each of R1to R8is independently H, OH, SO3H, COOH, PO3H2, NH2, N(CH3)3+, [OC2H4]nOH, or [OC2H4]nOCH3;at least one of each of R1to R8is OH, SO3H, COOH, PO3H2, NH2, N(CH3)3+, [OC2H4]nOH, or [OC2H4]nOCH3; andeach X is independently carbon, nitrogen, oxygen, sulfur, or phosphorus.

[0024] In various embodiments, the water-soluble compound disclosed herein may itself function as a redox-active sorbent or may be incorporated into a sorbent composition. As such, references to a “sorbent” may include the compound alone or in combination with one or more additional components, unless the context clearly indicates otherwise.

[0025] Tn various embodiments, the water-soluble compound is redox -active and has a redox potential close to oxygen reduction reaction potential. In some embodiments, the redox potential is close to or comparable with that of ferro- / ferricyanide (0.45 V vs. SHE) at pH 7. 4.

[0026] The decomposition rate of the water-soluble compound is around 0.1% per day. The oxygen induced oxidation rate of the water-soluble compound is around 5% per day.

[0027] The structural stability of the water-soluble compound is improved by introducing a variety of water-solubilizing groups to the fluoflavine through C-C or C-heteroatom bonds.

[0028] In various embodiments, the water-solubilizing group is selected from the group consisting of hydroxyl, sulfonate, carboxylic acid, carboxylate, polyether and quaternary ammonium groups.

[0029] Examples of such fluoflavine derivatives include, but are not limited to, the following:Fluoflavine molecules with C-C linked water-solubilizing groups

[0030] The direct functionalization of the C-H bond is a promising way to establish C-C bonds. The C-C bond linked fluoflavine derivatives can be synthesized through an electrocatalytic hydrogen atom transfer strategy. With C-C bond linked fluoflavine derivatives, we expect to avoid nucleophilic attack induced hydrolysis.

[0031] Tn various embodiments, the water-soluble compound may comprise two or more water-solubilizing groups. In some embodiments, the two or more water-solubilizing groups are identical. In other embodiments, the two or more water-solubilizing groups are different.

[0032] In some embodiments, due to the superior redox activity, high pKa, and high oxygen tolerance of fluoflavine, water-solubilizing group such as sulfonate groups are introduced to fluoflavine to increase its aqueous solubility Through introducing water-solubilizing sulfonate groups to fluoflavine, an oxygen-tolerant aqueous soluble fluoflavine disulfonate sorbent is developed that can be electrochemically induced to capture CO2 from simulated flue gas, indoor and outdoor ambient air with low energy cost, excellent reversibility, and high stability.

[0033] FIG. 1 shows the synthetic routes and the NMR spectra of mono-sulfonated (FFS) and di-sulfonated fluoflavine (FFDS), with the following structures:Mono-sulfonated (FFS)H HO,5 '' NvN SO-. HHDi-sulfonated fluoflavine (FFDS)

[0034] Fluoflavine (FF) may be synthesized via condensation reaction. After a one-step sulfonation of FF, the fluoflavine disulfonic acid (FFDS) is obtained with an outstanding yield of 94%. Oxidized FFDS (ox. FFDS) can be quantitatively obtained through a one-step electrochemical oxidation. Disulfonation of FF is validated by the mass to charge ratio obtained from high-resolution mass spectrometry data.

[0035] Through analysing the splitting pattern and the ratio of peak integrals in the aromatic region of the two1H NMR spectra, the structure of FFDS was determined as shown in FIG. 2A, where two sulfonate groups are each attached to a phenyl ring at the ortho- or para-positions. The FFDS illustrated in FIG. 2A was prepared by subjecting fluoflavine (FF) to sulfonation under a reaction condition comprising adding concentrated sulphuric acid to a solution containing fluoflavine (FF) and heating the solution at approximately 150 °C for about 6 hours.

[0036] FIG. 2B shows the synthesis of an oxidized FFDS (ox. FFDS) from FFDS and the electrochemical proton-coupled electron transfer (PCET) between the FFDS and the ox. FFDS.

[0037] FIG. 2C shows the1H and13C NMR spectra of the ox. FFDS and FFDS in DMSO-d6. The number of peaks in the13C NMR spectra further confirm the proposed structures of FFDS and the oxidized FFDS.

[0038] FIG. 2D shows the1H NMR spectrum of FFDS of FIG. 2A.

[0039] FIG. 2E shows the1H NMR spectrum of the ox. FFDS of FIG. 2B.

[0040] FIG. 3A depicts the cyclic voltammogram of FFS, and FIG. 3B depicts the cyclic voltammogram of FFDS, both at pH 7 and pH 14, at a scan rate of 100 mV / s. The redox potentials of both FFS and FFDS are pH-dependant. When the pH increases from 7 to 14, the redox potential shifts to more negative values, suggesting its proton-coupled electron transfer. Both FFS and FFDS show reversible electrochemical activity at high redox potentials at pH 7 and 14. In various embodiments, the fluoflavine derivative has a water-solubility higher than 0.1 M when between a pH of 7 and 14.

[0041] FFS and FFDS possess one pyrazine and one dihydro-pyrazine, so the reduced states of FFS and FFDS are oxygen-insensitive, which is validated by the electrochemical chargedischarge and CO2 capture-release preliminary results. In some embodiments, the results show that both FFS and FFDS demonstrated excellent CO2 capture-release reversibility with over 95% CO2 capture efficiency when fed with a gas mixture of 10.7% O2, 17.8% CO2, and 71.5% N2. In exemplary embodiments, the FFDS sorbent demonstrates a reversible CO2 capture and release with an average CCh / e molar ratio of 0.88 when exposed to flue gas containing about 10% O2.

[0042] The FFDS exhibits stable cycling performance over 40 days of a three-day direct air capture (DAC) under the nonnal discharge-charge mode in both indoor and outdoor DAC. The FFDS delivers an improved CO2 capture capacity over an accelerated overnight DAC and a Coulombic efficiency of more than 99% under the deep discharge-charge mode. The CO2 capture / release capacity increases twofold when the concentration of FFDS is doubled, regardless of whether the DAC is indoor or outdoor. The energy cost for CO2 release and sorbent regeneration varies from 60 to 150 kJ / mol CO2, depending on the applied current densities as well as the charge-discharge modes.

[0043] Tn various embodiments, the water-soluble compound of the present disclosure is redox-active and undergoes reversible two-proton, two-electron redox transfer process for oxygen tolerance The water-soluble compound exhibits enhanced water solubility and oxygen tolerance.

[0044] In various embodiments, the water-soluble compound is capable of reversibly capture and release carbon dioxide via pH-swing of the water-soluble compound in a solution in presence of oxygen.

[0045] According to a second aspect, an electrochemical carbon dioxide capture system is provided. The electrochemical carbon dioxide capture system comprises an inlet configured to receive a gas stream comprising flue gas or ambient air; an outlet configured to release carbon dioxide; a membrane disposed between an anode compartment and a cathode compartment; and an electrolyte comprising a water-soluble compound capable of capturing carbon dioxide from the gas stream, wherein the water-soluble compound comprises a fluoflavine derivative comprising one or more water-solubilizing groups, represented by the general formulae (I) to (IV):H N N ' " RaRsHFUx,„ Rs x.„ Rawherein each of R1to R8is independently H, OH, SO3H, COOH, PO3H2, NH2, N(CH3)3+, [OC2H4]nOH, or [OC2H4]nOCH3;at least one of each of R1to R8is OH, SO3H, COOH, PO3H2, NH2, N(CH3)3+, [OC2H4]nOH, or [OC2H4]nOCH3; andeach X is independently carbon, nitrogen, oxygen, sulfur, or phosphorus.

[0046] In various embodiments, the gas stream comprises at least carbon dioxide (CO2), nitrogen (N2) and oxygen (O2).

[0047] In various embodiments, the electrochemical carbon dioxide capture system is configured to be operated at ambient temperature and pressure, and the system is a continuous-flow system.

[0048] According to a third aspect, a method for capturing carbon dioxide is provided. The method comprises introducing a gas stream comprising carbon dioxide into an electrochemical carbon dioxide capture system of the present disclosure; contacting the gas stream with the electrolyte comprising the water-soluble compound to capture carbon dioxide, and electrochemically regenerating the water-soluble compound to release carbon dioxide.

[0049] In various embodiments, the carbon dioxide capture is performed by direct air capture (DAC). The water-soluble compound of the present disclosure is capable of reversibly capturing and releasing carbon dioxide under indoor and outdoor conditions by direct air capture (DAC).

[0050] In various embodiments, the method is capable of achieving a Coulombic efficiency of close to 99% over 16 cycles. In various embodiments, the method may further comprise performing deep discharge to trigger oxygen evolution for removing accumulated or excess ions.

[0051] To facilitate a better understanding of the present disclosure, the following examplesof specific embodiments are given. In no way should the following examples be read to limit or define the entire scope of the disclosure. One skilled in the art will recognize that the examples set out below are not an exhaustive list of the embodiments of this disclosure.EXAMPLESExample 1

[0052] Synthesis of fluoflavine

[0053] The fluoflavine was synthesized via a thermal condensation between benzene-1,2-diamine and 2,3- dichloroquinoxaline. Typically, 2,3-dichloroquinoxaline (20 mmol, 4.0 g) and benzene-1,2-diamine (30 mmol, 3.2 g) were added into a 200 mL flask with 120 mL ethylene glycol and then refluxed at 160 °C with stirring overnight. After cooling to room temperature, the products were filtered and washed with an amount of ethanol and petroleum ether. Yellow powder was obtained in an isolated yield of 86%.Example 2

[0054] Synthesis of fluoflavine disulfonic acid (FFDS)H H88% H2SO41 MJ ']vN'' I'Sr )x'* J’' - 15C* -C, 3 n — * HO3S'' J J % H 4SO?HH H

[0055] The fluoflavine (4.7 g, 20 mmol) was added into 98% H2SO4 to afford a 0.5 M fluoflavine solution. The reaction mixture was stirred at 150 °C for 3 h. After cooling down, the acid solution was slowly added into an ice water mixture (40 mL). The resulting precipitate was then vacuum -filtered, washed with ice water to remove H2SO4 residues, and vacuum-dried at 60°C overnight to afford the desired pale yellow product, fluoflavine disulfonic acid (FFDS), in an isolated yield of 94%.

[0056] FIG.2D shows the ’H XMR spectrum of FFDS.

[0057] 1H NMR (400 MHz, DMSO-d6) δ 8.46 (dd, J = 4.4, 1.7 Hz, 2H), 8.36 (dd, J = 9.2, 2.0 Hz, 2H), 8.28-8.22 (m, 2H).

[0058] 13C NMR (101 MHz, DMSO-d6) δ 145.4, 145.2, 144.5, 144.3, 144.1, 131.9, 131.4, 130.2, 129.6, 123.5, 123.3, 118.4, 116.1, 115.9.Example 3

[0059] Synthesis of oxidized fluoflavine disulfonic acid (FFDS)

[0060] Oxidized FFDS was prepared by electrochemical oxidation of FFDS. The oxidation of FFDS was performed in an electrochemical flow cell. Electrolytes comprising 10 mL 100 mM FFDS in 0.4 M KOH (negolyte, initial pH at 13.3) and 100 mL 70 mM KsFe(CN)6 and 30 mM K4Fe(CN)e (posolyte) were used. Five pieces of unbaked carbon cloth electrodes (geographical area of 5 cm2) were used as electrodes on each side. The cell was discharged galvanostatically at 20 mA / cm2with a voltage cutoff of -0.2 V, followed by a potentiostatic hold until the magnitude of the current density fell below 2 mA / cm2under N2. After the discharge, the pH of FFDS negolyte was adjusted to 2, and 100 mL of acetone was added to the solution. The resulting suspension was filtered, and the filter cake was washed with acetone three times to afford oxidized FFDS in quantitative yield.

[0061] FIG. 2E shows the1H NMR spectrum of the oxidized FFDS.

[0062] 1H NMR (400 MHz, DMSO-d6) δ 8.50 - 8.41 (m, 2H), 8.40 - 8.32 (m, 2H), 8.29-8.22 (m, 2H).

[0063] 13C NMR (101 MHz, DMSO-d6) δ 151.9, 151.9, 147.3, 147.2, 147.2, 144.7, 144.5, 144.4, 132.6, 130.3, 125.1.Example 4

[0064] Solubility test of FFDS

[0065] Solubility test was carried out on FFDS at pH 7 and pH 14. The results are as shown in Table 1.

[0066] Table 1: Solubility of FFDS in different stages in different supporting salts at pH 7 and 14.Compound Solubility at pH = 7 (M) Solubility at pH = 14 (M) Oxi-FFDS 0.21±0.01 (K+ form)Oxi-FFDS 0.26±0.01 (Na+ form)FFDS 1.5 (K+ form) 2.0 M (K+ form)

[0067] FFDS solubility test procedure at pH=7

[0068] 39.4 mg (0.1 mmol) of FFDS was charged to an empty centrifugal tube. 30 pL aqueous KOH_(0.2 mmol) solution was then added to the tube. Subsequently, 10 pL H2O was added to the tube every time until FFDS was observed fully dissolved by eye. The concentration of the resulting solution was considered the solubility of FFDS.

[0069] FFDS solubility test procedure at pH = 14

[0070] 30 pL of 1 M KOH solution was added to a centrifugal tube charged with 39.4 mg (0.1 mmol) FFDS. Subsequently, 10 pL 1 M KOH solution was added to the tube every time until FFDS was observed fully dissolved by eye. The concentration of the resulting solution was considered the solubility of FFDS.

[0071] Oxidized FFDS solubility test procedure at pH = 7 with K+ as the counter ion

[0072] 39.2 mg (0.1 mmol) of the oxidized FFDS was charged to an empty centrifugal tube.30 pL H2O aqueous KOH (0.2 mmol) solution was then added to the tube. Subsequently, 10 pL H2O was added to the tube every time until the oxidized FFDS was observed fully dissolved by eye. The concentration of the resulting solution was considered the solubility of oxidized FFDS.

[0073] Oxidized FFDS Solubility test procedure at pH = 7 with Na+ as the counter ion

[0074] 39.2 mg (0.1 mmol) of the oxidized FFDS was charged to an empty centrifugal tube.30 μL aqueous NaOH (0.2 mmol) solution was then added to the tube. Subsequently, 10 μL H2O was added to the tube every time until the oxidized FFDS was observed fully dissolved by eye. The concentration of the resulting solution was considered the solubility of oxidized FFDS.Example 5

[0075] Cyclic voltammograms of FFDS

[0076] Cyclic voltammetry (CV) measurements were carried out with a CHI1140D electrochemical workstation. The three-electrode system includes a glassy carbon as the working electrode, a platinum wire as the counter electrode, and an Ag / AgCl (3 M KC1) as the reference electrode.

[0077] The cyclic voltammograms of FFDS in buffered aqueous media exhibit reversible redox behaviour over a pH range of 5.7 to 13.9 (FIG. 3C). The Pourbaix diagram in FIG. 3D indicates that the redox potential of FFDS is negatively proportional to pH with a fitted slope of 59 mV / pH throughout the pH range from 0 to 14. The overlap between the experimental reduction potentials of FFDS and the theoretical values (FIG. 3E) further confirms that FFDS undergoes two-proton, two-electron transfer over the pH range of 0 to 14. It is worth noting from FIG. 3D that the oxidation potentials of FFDS fall between the potentials of two-electron transfer oxygen reduction reaction (ORR) and four-electron transfer ORR. Given that the four-electron transfer ORR pathway is significantly disfavoured in the absence of a catalyst, FFDS could potentially tolerate ambient oxygen, thus enabling FFDS sorbent-based electrochemically induced direct air capture (DAC).Example 6

[0078] Electrochemical CO2 capture

[0079] FIG. 4 is a schematic of an electrochemical CO2 capture setup 400.1

[0080] In this setup, three mass flow controllers 401, 402 and 403 were used to adjust gas flow rates for CO2, N2 and O2, respectively. The three gases were merged as feed gas 404 to flow into the sorbent electrolyte 405, then flow through a drying tube 406 before detected by a flow meter 407, a CO2 sensor 408 and an O2 sensor 409 in a sensor box 410 for CO2 capturerelease quantification. A syringe 411 filled with water was mounted to a syringe pump 412 and connected to the electrolyte sorbent to offset water loss due to the constant gas flow. A counter ferro- / ferricyanide electrolyte was used in the flow cell to balance the charges. Commercial carbon paper 413 and Nafion 117 414 were used as the electrode and the membrane, respectively. The entire set up must be well sealed to be free of liquid- and gas-leakages, so that the number of electrons transferred can be correlated with the number of CO2 molecules captured and released.

[0081] During the charge-discharge process, feed gas was gradually changed from pure N2 to 80% N2, 20% CO2, and eventually to 72.5% N2, 17.8% CO2, 10.7% O2; meanwhile, the CO2 sensor kept monitoring the gas composition variations in real time. The reversibility in chargedischarge capacity (FIG. 5A) and CO2 capture, and release volume (FIG. 5B) indicates the excellent oxygen tolerance of FFDS.

[0082] FIG. 5A shows the representative voltage profiles of FFDS in a Fe(CN)6 flow cell under an atmosphere of 100% N2; 20% CO2, 80% N2; and 17.8% CO2, 10.7% O2; 72.5% N2.

[0083] FIG. 5B shows the variation of CO2 readings in percentage during electrochemical charge-discharge induced CO2 capture-release process while charging the feed gas composition from 20% CO2, 80% N2to 17.8% CO2, 10.7% O2; 72.5% N2. The flow cell configuration is 10 ml, saturated FFDS, 1 M KC1, pH 9 | 100 mL 30 mM K4Fe(CN)6, 10 mM K3Fe(CN)6, 1 M KCl, pH 7. The cell is charged / discharged at a constant current density of 20 mA / cm2, followed by potential holds until current density drops to 2 mA / cm2.Example 7

[0084] Electrochemical behaviour of FFDS

[0085] The electrochemical behaviour of FFDS was first examined in the absence of oxygen.

[0086] FFDS shows a reversible swing in both pH and potential while operating in N2 (FIG.6A). FIG. 6B shows the potential and pH profiles of FFDS electrolyte over time when CO2 is introduction The introduction of CO2 to the reduced electrolyte leads to a steep decline in pH and a sharp rise in potential, which are attributed to the CO2 buffering capability. A slight pH increase and potential drop are observed after CO2 is replaced by N2. Subsequent oxidation of FFDS lowers the electrolyte pH from 9.5 to 4, accompanied with the CO2 release. The replotted voltage profiles in FIG. 6C clearly show that the oxidation potential of FFDS shifts positively after CO2 is captured. The integrated loop area in FIG. 6D represents the energy consumed to release two equivalents of CO2 and regenerate one equivalent of FFDS sorbent. The molar ratio of CO2 / e is 1 in one-bar pure CO2, and the corresponding energy cost is 31.75 kJ / mol CO2.Example 8

[0087] Spectroscopic studies on FFDS

[0088] In light of distinctive colours of azaacene family, spectroscopic studies on FFDS were carried out at varied state of charge (SOC). FIG. 7A and FIG. 7B show that UV-Vis absorbance spectrum of FFDS evolves reversibly between 0% and 100% SOC, confirming the reversible redox behaviour. Oxidation of FFDS affords an extended conjugation, thus leading to a red shift in the UV-Vis absorbance spectrum, which is also validated by the simulated result (FIG. 7C) The FFDS solutions with and without captured CO2 show the same UV-Vis spectrum (FIGs. 7D, 7E, 7F), indicating that CO2 is captured as dissolved inorganic carbon (DIC) species and does not affect the structure of FFDS, further confirming that FFDS captures / releases CO2 via pH-swing rather than nucleophilicity-swing.

[0089] The in-situ ATR-FTIR spectra of FFDS over the reduction process in FIG. 7G show gradual transmittance variations at 1150-1200, 1600, 2900, and 3200-3300 cm'1, which correspond to the stretching vibrations of C-N, C=N, N-H, and O-H bonds, respectively. During the reduction, the C=N bonds in one of the two pyrazine rings are converted to C-N and N-H bonds, intensifying the absorption bands at 1150, 1200, and 2900 cm'1, weakening the C=N absorption band at 1600 cm'1. The O-H vibration absorption in the range of 3200-3300 cm'1diminishes, as water molecules are consumed over reduction. Interestingly, the signal from CO2 vibration was detected at 2300-2400 cm'1even though CO2 was not introduced intentionally. As expected, the CO2 vibration absorption / transmittance decreases / increases over the electrochemical reduction of FFDS, as the residual CO2 is converted to inorganic carbonate (CO32-).

[0090] As implied by the name, fluorescence is an inherent feature of fluoflavine derivatives. Similar to the UV-Vis spectra, the fluorescence spectra demonstrate negligible change for the 100% SOC FFDS with and without captured CO2 (FIG. 8A). The fluorescence of FFDS linearly diminishes with the decrease of SOC (FIGs. 8B, 8C). Such a linear correlation was also observed from the mixture of FFDS and ox. FFDS in different ratios Surprisingly, the ox. FFDS is almost non-fluorescent. It is anticipated that m-situ spectroscopic techniques could serve as non-invasive tools for quantitative analysis.Example 9

[0091] Oxygen tolerance of FFDS

[0092] To investigate the oxygen tolerance of FFDS, the CO2 capture performance of the sorbent-based FFDS was first evaluated in a simulated flue gas comprising 10% CO2, 10% O2, and 80% N2 at one bar. Serving as a feed gas, the simulated flue gas was introduced into a FFDS electrolyte at a flow rate of 30 mL / min. The flow cell was charged to electrochemically reduce the oxidized FFDS (ox. FFDS), thus regenerating the sorbent. The pH of the sorbent firstincreases from 7.25 to 9.5, then decreases and stabilizes at 7.7, as a result of the acid-base reaction between the CO2 in the feed gas and the OH- generated via the PCET process. Correspondingly, the CO2 flow rate becomes lower than 3 mL / min, while the oxygen level becomes higher than 10%. The acid-base reaction-induced CO2 capture continues after the charge process and completes during the rest period. During the electrochemical oxidation of FFDS, the pH of the sorbent decreases from 7.7 to 6.6, then increases and stabilizes at 7.25. The corresponding CO2 release leads to the increase of CO2 flow rate and the decrease of O2 level. The 11-cycle reversible alterations in voltage, pH, O2 level, and CO2 capture flow rate indicate the excellent oxygen tolerance of FFDS in 0.1 bar 02 (FIG. 9A).

[0093] Throughout the 38-cycle CO2 capture and release under 200-hour exposure of the simulated flue gas, the FFDS sorbent steadily releases CO2 at an average volume of 36.7 mL per cycle with an average Coulombic efficiency of 99.02% (FIG. 9B), indicating that FFDS possesses exclusive selectivity of CO2 over O2, superior oxygen-tolerance, and excellent reversibility. The molar ratio of CO2 / e approaching 0.88 suggests that CO2 was primarily absorbed via the acid-base reaction of CO2 + OH⁻ → HCO3⁻ during the capture process, which is also reflected by the stabilized pH at 7.7 after the completion of CO2 capture As shown in FIG. 9C, the dissolved inorganic carbon in water at pH 7.7 is primarily in the form of bicarbonate.Example 10

[0094] CO2 capture-release behaviour

[0095] The CO2 capture-release behaviour of the FFDS sorbent was assessed using simulated flue gas at current densities of 20, 40, 60, 80, and 100 mA / cm2. The corresponding voltage cutoffs are 0.85 and -0.5 V at 20 mA / cm2; 1.0 and -0.7 V at 40 mA / cm2; 1.1 and -1.1 V at 60 mA / cm2; 1.2 and -1.2 V at 80 mA / cm2; 1.5 and -1.0 V at 100 mA / cm2. FIG. 15 illustrates the charge-discharge voltage profdes of FFDS at 20, 40, 60, 80, and 100 mA / cm2.

[0096] The aqueous electrochemical flow system, which is capable of operating at high current densities, can accelerate coupled CO2 release and sorbent regeneration, thus boosting the overall CO2 removal rate. FIG 9D illustrates the CO2 capture-release cycles at different current densities. The time required to complete CO2 capture decreases markedly from 132 to 92 mins as the applied current density increases from 20 to 40 mA / cm2(FIG. 9E). Further increasing the current density does not shorten the CO2 capture time, indicating that the ratelimiting step is the acid-base reaction, which is independent of current density. The CO2 release peak intensifies with increasing current density, thereby shortening the duration of each CO2 capture-release cycle. As shown in FIG. 9F, the time required to complete CO2 release decreases from 50 minutes to 20 minutes as the applied current density increases from 20 to 100 mA / cm2The volumes of CO2 captured and released by the FFDS sorbent remain comparable across the tested current densities (FIG. 9G). Because the discharge voltage becomes increasingly negative at higher current densities, the oxygen evolution reaction is triggered at 80 and 100 mA / cm2, resulting in a Coulombic efficiency slightly exceeding 100% (FIG. 9H). The energy cost for CO2 release and sorbent regeneration increases from 58 to 190 kJ / mol CO2 as the applied current density is increased from 20 to 100 mA / cm2.Example 11

[0097] Direct Air Control (DAC)

[0098] FFDS-based electrochemically induced direct air control (DAC) was investigated.

[0099] A 10 mL portion of 0.1 M FFDS sorbent was stirred under indoor air for three days. The outcomes of two competing reactions of carbon capture vs oxidative deactivation of FFDS under prolonged air exposure were evaluated. Instead of releasing CO2 in a mixed gas stream containing 10% CO2 at ambient pressure, high-purity N2 (99.9995%) as employed as the carrier gas to avoid any unintended CO2 uptake. The sorbent immediately began to capture CO2 upon1transitioning from 0.4 mbar in air to a 0.1 bar CO2 environment, which ultimately leads to overestimations in DAC capacity and the molar ratio of CCh / e.

[0100] A Coulombic efficiency of about 87% suggests that most FFDS remained intact after the three-day indoor air exposure (FIG. 10A). The FFDS sorbent demonstrated a CO2 / e molar ratio of 0.51 and released 20.16 mL of CO2, which is 55% of CO2 volume released from the simulated flue gas capture.

[0101] To assess the cycling stability of the FFDS sorbent-based DAC and release process, 10 cycles of normal discharge-induced CO2 release were conducted following continuous three-day indoor DAC over a period of 40 days, as shown in FIG. 10B and FIG. 10G. During the 40 days operation, the FFDS sorbent was stirred in air for three days, then discharged for CO2 release. The results in FIG. 10G show that the volumes of released CO2 across the 10 cycles over 40 days are comparable. The FFDS sorbent released 16.5 to 20.0 mL of CO2 per cycle, with a Coulombic efficiency of 85 to 95% and at an energy cost of 80 to 110 kj / mol for CO2 release and sorbent regeneration. The fluctuation in the released CO2 volume is partly due to imperfect sorbent transfer, resulting in sorbent leftover or loss.

[0102] Although the significantly improved oxygen tolerance enables FFDS-based DAC and the subsequent CO2 release, it is noted that approximately 5 to 15% of FFDS was oxidized by molecular oxygen during the three-day DAC process. As a result, a portion of the captured CO2 became retained within the sorbent solution and accumulated over successive cycles. This incomplete CO2 release led to the formation of a buffer solution, which stabilizes the pH at approximately 9.3 after CO2 release and at 12.6 after the sorbent regeneration

[0103] To verify whether there were accumulated dissolved inorganic carbon (DIC) in the sorbent, 1 M hydrochloric acid (HC1) was added into the sorbent after 12 cycles of three-day indoor DAC (FIG. 10C), and a total of 36 mL of CO2 was released as anticipated. However,the addition of HCI introduces excess ions into the sorbent, compromising the aqueous solubility of FFDS and leaving charges unbalanced.

[0104] To eliminate the influence of excess ions and rebalance charges within the system, the discharge voltage was lowered to trigger the oxygen evolution reaction (OER), herein referred to as a deep discharge. With the implementation of deep discharge, the Coulombic efficiency increased to over 97% and the initial CO2 release volume increased to 23.31 mL. Meanwhile, the pH decreased to 4.0 after CO2 release, indicating that almost no HCO3-, CO32-remained in the sorbent solution (FIG. 9C). The subsequent charge process restored the sorbent pH to 13.3.

[0105] It is further noted that, to accelerate the DAC-release cycle, the sorbent solution was bubbled with indoor air at a flow rate of 375 mL / min overnight, instead of stirring the sorbent in indoor air for three days. Bubbling was stopped when the sorbent pH decreased to approximately 10, enabling completion of a DAC-release cycle on a daily basis. FIG. 10D shows the cycling over 16 days with an average CO2 release volume of 21.6 mL. The fluctuation in volume of released CO2 was mainly due to the unavoidable liquid splashes caused by air bubbling Compared to the normal discharge, the deep discharge method exhibits improved Coulombic efficiency, higher volume of released CO2, and the rebalanced charges with a higher energy cost (FIG. 10D)

[0106] The concentration of FFDS was further increased to 0.2 M and evaluated for both indoor and outdoor DAC. The experiments were conducted in Singapore, which is located near the equator and has a tropical climate characterized by high humidity and relatively consistent temperatures throughout the year. FIG. 10E presents the continuous 20-day indoor and outdoor temperature, relative humidity, oxygen level and CO2 concentration measured in Singapore in Feb. 2025. The outdoor air exhibits recurring oscillations in all the monitored parameters. In contrast, due to air conditioning, the indoor environment maintains consistent temperature,humidity, and O2 levels. Variations in indoor CO2 concentrations are primarily influenced by the activities of group members within the indoor space.

[0107] FIG. 10F presents the continuous cycling of 10 mL, 0.2 M FFDS sorbent for two cycles of three-day indoor DAC, and another two cycles of three-day outdoor DAC. The 0.2 M FFDS sorbent releases over 46 mL CO2 after the three-day indoor and outdoor DAC, doubling the DAC capacity with the same air exposure time compared to the 0.1 M FFDS sorbent. The sorbent when used for the indoor and outdoor DAC exhibits comparable released CO2 volumes, Coulombic efficiency, and energy cost, indicating that the fluctuations in temperature and humidity of the outdoor air have insignificant influence on the FFDS sorbent. It is noticed that the water loss from the FFDS sorbent during the three-day outdoor DAC is 40% to 50% less than that during the three-day indoor DAC The relative humidity of outdoor air is, on average, 10% higher than that of indoor air, which slows down the water evaporation of FFDS sorbent during the three-day outdoor DAC (FIG. 10E).

[0108] The FFDS-sorbent based electrochemically induced DAC process exhibits an estimated energy cost of 3.15 to 6.14 GJ / tonne of CO2 when air contact, CO2 release, sorbent regeneration, and pressurization are taken into consideration Among the evaluated materials, the FFDS sorbent demonstrates the lowest energy cost associated with CO2 release and sorbent regeneration.Example 12

[0109] The fresh 100% SOC 50 mM FFDS solution was prepared with the initial pH at 13.0. During the indoor DAC, the sorbent experienced a sharp drop in pH from 13.0 to 10.5 in the first 20 hours, then slowly decreased to 10.0 after 72-hour DAC (FIG. 11A). The pH further decreased 9.8 when the DAC duration was extended to 95 hours. The pH decrease is the result of CO2 in air gradually being captured and stored in the solution as the dissolved inorganic carbon (DIC) species including CO32-and HCO3-Further extending DAC duration wouldincrease the CO2 capture capacity, we decided to conduct three-day DAC for all of the stirring-assisted DAC experiments.

[0110] When the sorbent composed of 20 mL of 50 mM FFDS and 0.2 M KOH with an initial pH of 13.0, is stirred to air for DAC, the pH decreases. At a given pH, the concentration of [DIC], [CO32’], and [HCO3'] can be deduced with the known pKa1 and pKa2 of H2CO3and the law of charge conservation. After the three-day DAC, the DIC in the FFDS sorbent reaches 0.071 M, which indicates that the FFDS sorbent captured 1.42 mmol CO2, i.e., 31.8 mL CO2 when considered as an ideal gas. As shown in FIG. 11B and FIG. 11C, the pH of the sorbent solution stabilizes at 9.1 after 16.5 mL of CO2 is released, suggesting that an appreciable portion of the DIC (HCO3‘) remains in the solution. Because of the leftover DIC in the sorbent, the regenerated FFDS sorbent elevates to 12.2 instead of the initial pH at 13, which is attributed to the buffering effect of DIC.

[0111] The average Coulombic efficiency is around 91% across the 10 cycles (FIG. 11D), indicating only 9% of FFDS is chemically oxidized by O2 while being stirred in the air for three days at ambient pressure, suggesting the extraordinary oxygen-tolerance of FFDS. FIG. 11E shows the charge-discharge capacity of FFDS over 40 days, 10 cycles of three-day indoor DAC-release processes.Example 13

[0112] General procedure 1 (GP1) for FFDS-based carbon capture and release in a simulated flue gas

[0113] The simulated flue gas is composed of 10% CO2, 10% O2, and 80% N2. The total pressure is one bar, and the flow rate is 30 mL / min. The negative electrolyte (negolyte) comprises 10 mL of 0.1 M FFDS and 0.4 M KOH. The positive electrolyte (posolyte) comprises 100 mL of 0.03 M K4Fe(CN)6and 0.07 M K3Fe(CN)6. The two electrolytes are separated with a cation exchange membrane Nafion 117. The electrodes are 5 pieces of baked carbon cloth oneach side. The flow cell is galvanostatically charged and discharged at constant current densities followed with the potential holds until the current density decreases to 2 mA / cm2The charge process is followed by a 180-minute rest to complete CO2 capture. The discharge process is followed by a 90-minute rest to complete CO2 release. CO2 data recorded by 100% CO2. O2 data recorded by 25% Oxygen Flow Through Sensor.Example 14

[0114] General procedure 2 (GP2) for stirring the FFDS sorbent in ambient air for three-day indoor / outdoor DAC, CO2release and regeneration

[0115] The negolyte is composed of 10 mL 0.1 M FFDS and 0.4 M KOH with an initial pH of 13.3. Alternatively, the negolyte is composed of 20 mL 0.05 M FFDS and 0.2 M KOH with an initial pH of 13.0. The posolyte is composed of 100 mL 0.07 M ferricyanide and 0.03 M ferrocyanide. The electrodes are 5 pieces of baked carbon cloth on each side. The membrane is Nafion 117 cation exchange membrane.

[0116] The 100% state of charge (SOC) FFDS sorbent is transferred to a 50 mL PTFE beaker equipped with a cylindrical stir bar and being stirred in air at a speed of 500 rpm for three-day direct air capture (DAC), either indoor or outdoor After the three-day DAC, approximately 5 to 7 mL of water is added to compensate for the water evaporation losses and the pH of the FFDS sorbent decreases from the initial 13.3 to ~10.2. The CCb-captured FFDS sorbent is then transferred back to the negolyte reservoir, and the flow cell is reconnected for discharge-induced CO2 release.

[0117] Pure N2 serves as the carrier gas at a flow rate of 28 mL / min. To minimize the evaporation induced water loss in the negolyte, N2 is humidified via flowing through a deionized water reservoir before flowing into the FFDS negolyte. The N2 gas outlet was positioned at the bottom of the FFDS negolyte to ensure that all of the released CO2 during the discharge can be bubbled out from the negolyte and being detected by the CO2 sensor. To betterintegrate the volume of released CO2, the baseline of CO2 sensor in pure N2 is produced via simply flowing N2 through a CO2 sensor before initiating CO2 release and after the completion of CO2 release.

[0118] Flow cell discharge-induced CO2 release

[0119] The flow cell is discharged at 20 mA / cm2followed with potential holds. -0.5 V was applied for normal discharge until current density decreases to 2 mA / cm2. -1.2 V was applied for deep discharge until the pH of negolyte drops to 4. The CO2 release process is considered complete when the 5% CO2 sensor reading stabilizes at approximately 200 ppm.

[0120] After the completion of CO2 release, the flow cell is charged in N2 to regenerate the FFDS sorbent at 20 mA / cm2followed by a potential hold at 1.0 V until the current density decreases to 2 mA / cm2.Example 15

[0121] General procedure 3 (GP3) for bubbling air to FFDS sorbent for accelerated overnight indoor / outdoor DAC. CO2 release and regeneration

[0122] The negolyte is composed of 10 mL 0.1 M FFDS and 0.4 M KOH with an initial pH of 13.3. The posolyte is composed of 100 mL of 0.07 M ferricyanide and 0.03 M ferrocyanide. The electrodes are 5 pieces of baked carbon cloth on each side. The membrane is Nafion 117 cation exchange membrane.

[0123] Powered by an electric air pump, the indoor or outdoor air is first pumped through a water reservoir then introduced into the FFDS sorbent through a PTFE tube (Æ3.2 mm). The air outlet is positioned at the bottom of the FFDS sorbent to increase the air-liquid interface for sufficient CO2 capture. The air flow rate is regulated at 375 mL / min with a mass flow controller (Siargo, Ltd. MFC-4200) With such a high flow rate, the whole DAC process is greatly accelerated. The pH of the FFDS sorbent decreases from 13.3 to around 10.0 after overnight air bubbling CO2 data recorded by 1% CO2 sensor.

[0124] After the accelerated overnight DAC, the air is replaced by pure N2 at a flow rate of 28 mL / min. Serving as the carrier gas, N2 flows through the FFDS negolyte for 30 minutes to produce a baseline of CO2 level at approximatly 200 ppm before discharge.

[0125] The flow cell is discharged at 20 mA / cm2until the voltage approaches the pre-set cutoffs. A potential hold at -0.5 V is applied for normal discharge until the current density decreases to 2 mA / cm2. A potential hold at -1.2 V is applied for deep discharge until the pH of the negolyte drops to 4. The CO2 release is considered complete when the CO2 sensor reading stabilizes at approximaely 200 ppm. The flow cell is charged in N2 to regenerate the FFDS sorbent at 20 mA / cm2followed with a potential hold at 1.0 V until the current density decreases to 2 mA / cm2.Example 16

[0126] FFDS-based carbon capture and release in a simulated flue gas

[0127] After the 11 cycles shown in FIG. 9A, the same FFDS sorbent was subjected to an additional 14 cycles following the same protocol described in Example 12 (GP1). The corresponding results are presented in FIG. 12. The 14-cycle CO2 capture and release was performed under a one-bar simulated flue gas comprising 10% CO2, 10% O2, and 80% N2 at a flow rate of 30 mL / min.

[0128] When FFDS is electrochemically reduced, the pH of the negolyte increases as a result of the proton-coupled electron transfer (PCET) process. The pH increase subsequently drives the acid-base reaction between CO2 and OH", lowering the CO2 flow rate due to the CO2 capture. The acid-base chemical reaction continues after the completion of the electrochemical reduction step and reaches completion during the rest period. The oxygen level in the downstream gas correspondingly increases because the CO2 in the feed gas is captured by the sorbent. Similarly, when CO2 is released, the O2 level decreases. The reversibility in the charge-discharge capacity and the CO2 capture / release volumes demonstrates the excellent oxygen tolerance of FFDS.

[0129] After the 14 cycles shown in FIG 12, the same FFDS sorbent was subjected to five cycles following the same protocol described in Example 12 (GP1). The corresponding results are presented in FIG 13

[0130] After the 5 cycles shown in FIG. 13, the same FFDS sorbent was subjected to eight cycles following the same protocol described in Example 12 (GP1). The corresponding results are presented in FIG 14Example 17

[0131] Normal discharge, deep discharge, and normal charge

[0132] FIG. 16A illustrates the 10thcycle of normal discharge, deep discharge and normal charge. FIG. 16B shows the pH of FFDS, and FIG. 16C shows CO2release from 10 mL of 100 mM FFDS after the 10ththree-day indoor DAC.

[0133] The negolyte is composed of 10 mL 100 mM FFDS and 0.4 M KOH. The posolyte is composed of 100 mL 0.03 M K4Fe(CN)6 and 0.07 M K3Fe(CN)6.

[0134] Instead of adding 1 M HC1 to the sorbent, a more negative voltage was applied to deep discharge the flow cell.

[0135] Sequence 1: a normal discharge was conducted galvanostatically at 20 m A / cm2with a subsequent voltage hold at -0.5 V until the current density was below 2 mA / cm2.

[0136] Sequence 2: a deep discharge was conducted galvanostatically at 20 mA / cm2with a subsequent voltage hold at -1.2 V until the pH of the negolyte dropped to 4.

[0137] Sequence 3: a normal charge was conducted galvanostatically at 20 mA / cm2with a subsequent potential hold at 1.0 V until the current density was below 2 mA / cm2. Pure N2 at a flow rate of 28 mL / min was used as the carrier gas.

[0138] According to FIG. 9C, the fractions of CO32-and HCO3-would approximately be 0 when the sorbent pH approaches 4. Hence, the deep discharge to -1.2 V was applied to trigger OER and produce protons until the pH of the negolyte dropped from 7.5 to 4.0. 35.45 mL ofCO2was released after 154.12 C of deep discharge capacity The molar ratio of released CO2 versus the transferred electrons during the deep discharge is equals to 1, indicating that HCOf is the dominant species left in the sorbent. When the sorbent was regenerated through a normal charge, the pH of the sorbent solution exceeded 13, indicating the removal of CO32-and HCO3-in the regenerated sorbent. Note that the deep discharge capacity equals to the capacity difference between charge and discharge capacity over 10 cycles, ∑110(charge capacity - discharge capacity). With the implementation of deep discharge, the accumulated DIC could be converted to release CO2 and also rebalance the system.Example 18

[0139] Overnight indoor air-bubbling DAC

[0140] A 10 mL 100 mM FFDS sorbent was employed for evaluating overnight indoor airbubbling DAC. The detailed protocol can be seen in the GP3.

[0141] FIG. 17A illustrates the results obtained for bubbling FFDS sorbent with indoor air at a stabilized flow rate of 375 mL / min. FIG. 17B shows the downstream CO2 level variation during the overnight indoor air-bubbling DAC and the integrated volume of captured CO2. FIG.17C shows the variation of pH, the concentrations of CO32-, HCO3-, and DIC in a fresh FFDS sorbent solution during the overnight air-bubbling DAC. The initial pH of the FFDS sorbent is 13.3 The pH lowered to 10.0 after the overnight air-bubbling corresponds 0.106 M [DIC], From the [DIC], we calculated the volume of CO2 being captured is 23.74 mL, which is consistent with the integrated volume of the captured CO2 shown in FIG. 17B.Example 19

[0142] Overnight indoor air-bubbling DAC-related charge-discharge voltage profile. pH monitoring and CO2 release from 10 mL 100 mM FFDS sorbent

[0143] FIG. 18A shows the charge-discharge voltage profile of FFDS sorbent. FIG. 18B shows the pH of the FFDS sorbent, and FIG. 18C shows the CO2release from 10 mL 100 mM FFDS sorbent.

[0144] Step 1: overnight indoor air-bubbling. The FFDS sorbent was constantly bubbled with indoor air at a flow rate of 375 mL / min overnight. The pH decreased from 13.3 to 9.9.

[0145] Step 2: a normal discharge step was applied for CO2 release at 20 mA / cm2until the voltage approached -0.5 V. The voltage was held at -0.5 V until current density decreased to 2 mA / cm2. The sorbent released 12.39 mL CO2.

[0146] Step 3: a charge step was applied to regenerate the sorbent at 20 mA / cm2until the voltage approached 1.0 V. The voltage was then held at 1.0 V until current density decreased to 2 mA / cm2. The sorbent pH was elevated to 12.9.

[0147] The total energy cost for CO2 release and sorbent regeneration is 156.3 kJ / mol CO2. Compared to the three-day indoor DAC, the overnight air-bubbling DAC results in a lower Coulometric efficiency of 66%, implying an accelerated oxidation rate of FFDS within a shorter air exposure duration. The vigorous air bubbling significantly increases the air-liquid contact area, leading to more FFDS being oxidized by O2.Example 20

[0148] This Example illustrates the overnight indoor air-bubbling DAC -related chargeconstant voltage deep discharge voltage profile, pH monitoring and CO2 release from 10 mL 100 mM FFDS sorbent.

[0149] FIG. 19A shows the charge-deep discharge voltage profile of the FFDS sorbent FIG.19B shows the pH of the FFDS sorbent, and FIG. 19C shows the CO2 release from 10 mL 100 mM FFDS sorbent.

[0150] Step 1: overnight indoor air-bubbling. The FFDS sorbent was constantly bubbled with indoor air at a flow rate of 375 mL / min overnight, the pH decreased from 13.3 to 9.9.

[0151] Step 2: a constant voltage deep discharge step was applied for CO2 release at -1.0 V until the capacity approached 145 C. The sorbent released 21.01 mL CO2.

[0152] Step 3: a charge step was applied to regenerate the sorbent at 20 mA / cm2until the voltage approached 1.0 V. The voltage was held at 1.0 V until current density decreased to 2 mA / cm2. The sorbent pH was elevated to 12.9.

[0153] The total energy cost for CO2 release and sorbent regeneration is 236.6 kJ / mol CO2. To compensate the discharge capacity loss incurred by O2-induced FFDS oxidation, deep discharge is employed to trigger OER and increase the volume of released CO2 at the expense of increased energy cost. The high energy cost is mainly due to the constant voltage deep discharge, which can be further optimized.Example 21

[0154] This Example illustrates the overnight indoor air-bubbling DAC -related chargedischarge, stepwise deep discharge voltage profile, pH monitoring, and CO2 release from 10 mL 100 mM FFDS sorbent.

[0155] FIG. 20A shows the discharge voltage profile of the FFDS sorbent. FIG. 20B shows the deep discharge voltage profile of the FFDS sorbent. FIG. 20C shows the charge voltage profile for sorbent regeneration. FIGs. 20D and 20E shows the CO2 release from 10 mL 100 mM FFDS sorbent, and FIG. 20F shows the pH profile of the FFDS sorbent.

[0156] Step 1: overnight indoor air-bubbling. The FFDS sorbent was constantly bubbled with indoor air at a flow rate of 375 mL / min overnight, the pH decreased from 13.3 to 10.1.

[0157] Step 2: a normal discharge step was applied for CO2 release at 20 mA / cm2until the voltage approached -0.5 V. The voltage was held at -0.5 V until current density decreased to 2 mA / cm2. Meanwhile, the sorbent released 11.88 mL CO2 with a stabilized pH at 9.5.

[0158] Step 3: a deep discharge step was applied for CO2 release at two constant voltages of -1.2 and -1.4 V until extra 57.6 C of discharge capacity was released. Meanwhile, the sorbent pH was further lowered to 3.5 Another 10.06 mb CO2 was released

[0159] Step 4: a charge step was applied to regenerate the sorbent at 20 mA / cm2until the voltage approached 1.0 V, The voltage was held at 1.0 V until current density decreased to 2 mA / cm2, The sorbent pH was elevated to 13.0

[0160] The total energy cost for CO2 release and sorbent regeneration is 175.0 kJ / mol CO2. Compared to the direct deep discharge at a constant voltage, the combination of a normal discharge and a deep charge in series largely lowers the energy cost. The total volume of released CO2reaches 21.94 mL. A total molar ratio of CO2 / e approaches 0.57.Example 22

[0161] This Example illustrates the overnight indoor air-bubbling DAC -related charge-deep discharge voltage profile, pH monitoring, and CO2 release from 10 mL 100 mM FFDS sorbent.

[0162] FIG. 21 A shows the charge-deep discharge voltage profile of the FFDS sorbent, FIG.21B shows the pH monitoring of the FFDS sorbent, and FIG. 21C shows the CO2 release from 10 mL 100 mM FFDS sorbent.

[0163] Step 1: overnight indoor air-bubbling. The FFDS sorbent was constantly bubbled with indoor air at a flow rate of 375 mL / min overnight. The pH decreased from 13.3 to 10.4.

[0164] Step 2: a deep discharge step was applied for CO2 release at 20 mA / cm2until the voltage approached -1.1 V. The voltage was held at -1.1 V until the discharge capacity reached 171.3 C.

[0165] Meanwhile, the sorbent released 21.52 mL CO2 with a resulting pH of 2. The negolyte pH gradually increases to 5 over 1600-min rest with continuous circulation.

[0166] Step 3: a charge step was applied to regenerate the sorbent at 20 mA / cm2until the voltage approached 1.0 V. The voltage was held at 1.0 V until current density decreased to 2 mA / cm2. The sorbent pH was elevated to 13.3.

[0167] The total energy cost for CO2 release and sorbent regeneration is further lowered to 150.0 kJ / mol CO2.Example 23

[0168] A three-day indoor DAC with normal discharge (FIG. 22A), deep discharge (FIG.22B), and normal charge (FIG. 22C) of 10 mL 100 mM FFDS sorbent for sorbent regeneration were performed. The corresponding CO2 releases during the normal discharge and deep discharge are illustrated in FIG. 22D and FIG. 22E, respectively.Example 24

[0169] A three-day indoor DAC and release performance following the protocol described in the GP2 was carried out, and the results are as shown in FIG. 23A, FIG. 23B, and FIG. 23C.The negolyte is composed of 10 mL of 200 mM FFDS, 0.4 M KOH and 0.4 M NaOH. The posolyte is composed of 200 mL of 0.03 M K4Fe(CN)6and 0.07 M K3Fe(CN)6.Example 25

[0170] Table 2 shows a comparison of different discharge approaches used for CO2 release after DAC in accordance with various embodiments of the present disclosure.

[0171] Table 2: A comparison of different discharge approaches used for CO2 release after DAC.

[0172] It is evident that a normal discharge could not fully release the CO2 captured by the sorbent. This unreleased DIC accumulates, preventing the pH of the regenerated sorbent from reaching its initial level, limiting the following CO2 capture capacity. To fully release the captured CO2, deep discharge is applied. Lowering the pH to 4 further converts HCO3-to H2CO3. The deep discharge could also compensate the capacity loss resulting from the O2-induced FFDS oxidation during the DAC process. 32.6% increase in energy cost leads to 30% increase in the volume of released CO2.

[0173] Conclusion

[0174] Compared to the conventional thermal-swing based amine scrubbing or strongly alkaline solutions used for CO2 capture, the electrochemical CO2 capture system induced by oxygen-tolerant aqueous organic flow chemistry offers significant advantages, including substantially lower energetic cost, the ability to operate using renewable electricity, operation at ambient temperature and pressure, and improved environmental compatibility.

[0175] In contrast to emerging electrochemically mediated CO2 capture systems that operate in non-aqueous media and require expensive electrolytes or supporting salts such as ionic liquids (e.g. [BMIM][TFSI] or tetrabutylammonium hexafluorophosphate (NBu4l’F<>). the present approach employs water as the solvent together with low-cost supporting salts such as NaCl or KC1. Importantly, the high ionic conductivity of aqueous media enables charge anddischarge at current densities that are two orders of magnitude higher than those achievable in non-aqueous systems. The ability to operate at high current densities shortens the duration of each electrochemical CO2 capture-release cycle and substantially reduces the capital cost associated with the cell stack. Furthermore, the high ionic conductivity helps reduce resistive losses and associated heat generation, thereby improving overall energy efficiency.

[0176] Compared to emerging metal-organic framework (MOF)-based pressure- or temperature-swing CO2 capture approaches, the electrochemical CO2 capture system based on aqueous organic flow chemistry provides substantially higher CO2 capture capacity and eliminates the moisture-sensitivity issues commonly encountered with MOFs materials.

[0177] In summary, the present disclosure demonstrates an aqueous fluoflavine sorbentbased electrochemically induced CO2 capture system capable of operating with simulated flue gas, as well as indoor and outdoor ambient air. The water-soluble fluoflavine disulfonate (FFDS) can be synthesized in a one-step sulfonation with excellent yield. Electrochemical characterization, theoretical calculations, and spectroscopic studies indicate that FFDS undergoes a reversible two-proton, two-electron redox process and exhibits an oxidation potential significantly higher than that of the two-electron oxygen reduction reaction over the entire pH range of 0 to 14. These features suggest that FFDS can reversibly capture and release CO2 via a pH-swing mechanism even in the presence of oxygen In a simulated flue gas stream at 1 bar comprising 10% CO2, 10% O2, and 80% N2, the FFDS sorbent consistently enables reversible CO2 capture and release, achieving an average CO2 / e molar ratio of 0.88 and a Coulombic efficiency exceeding 99% over 38 cycles and 200 hours of operation. In addition, increasing the applied current density substantially accelerates the discharge-induced CO2 release rate. Throughout the 10 continuous three-day indoor DAC cycles conducted over a period of 40 days, FFDS exhibited an average Coulombic efficiency of approximately 90% under a normal discharge-charge process and released 16.5 to 20.0 mL of CO2 per cycle. Toaccelerate DC, ambient air was continuously passed through the sorbent solution overnight instead of stirring the sorbent in air for three days, and a deep-discharge step was applied to fully release the captured CO2. Under these conditions, a steady DAC-CO2 release performance was achieved with an improved Coulombic efficiency of approximately 99% over 16 cycles. FFDS exhibits negligible difference in CO2 capture and release capacities between the indoor and outdoor DAC operations, despite the elevated and fluctuating outdoor temperature and humidity. Furthermore, the CO2 capture capacity increases approximately twofold when the concentration of FFDS is doubled, irrespective of whether DAC is performed indoors or outdoors.

[0178] Although embodiments of the invention have been shown and described, the invention is not limited to the described embodiments. Instead, it would be appreciated by those skilled in the art that various modifications and variations can be made to the embodiments of the invention without departing from the scope of the invention, the scoop of which is set forth in the following claims.

Claims

CLAIMS1. A water-soluble compound for electrochemical carbon dioxide capture, the watersoluble compound comprising:a fluoflavine derivative comprising one or more water-solubilizing groups, represented by the general formulae (I) to (IV):RsHHiR,. X, N, N, AR.<' n r Nn'XN' Ar A3RsHFh (I)RaR,Rfs XRSA / 'R3RSR4(II)X x.„R5R4(IV)whereineach of R1to R8is independently H, OH, SO3H, COOH, PO3H2, NH2, N(CH3)3+, [OC2H4]nOH, or [OC2H4]aOCH3;at least one of each of R1to R8is OH, SO3H, COOH, PO3H2, NH2, N(CH3)3+, [OC2H4]nOH, or [OC2H4]nOCH3; andeach X is independently carbon, nitrogen, oxygen, sulfur, or phosphorus.

2. The compound of claim 1, wherein the compound has a water solubility higher than 0.1 M between pH 7 and 14.

3. The compound of claim 1, wherein the compound comprises two or more water solubilizing groups.

4. The compound of claim 3, wherein the two or more water solubilizing groups are identical.

5. The compound of claim 3, wherein the two or more water solubilizing groups are different.

6. The compound of any one of claims 1 to 5, wherein the water solubilizing groups are selected from the group consisting of hydroxyl, sulfonate, carboxylic acid, carboxylate, polyether and quaternary ammonium groups.

7. The compound of claim 1, wherein the water-soluble compound is redox-active and has a redox potential close to oxygen reduction reaction potential.

8. The compound of claim 1, wherein the water-soluble compound is capable of undergoing a reversible two-proton, two-electron redox process.

9. The compound of claim 1, wherein the water-soluble compound is capable of reversibly capture and release carbon dioxide via pH-swing of the water-soluble compound in a solution in presence of oxygen.

10. The compound of claim 1, wherein the water-soluble compound is capable of reversibly capturing and releasing carbon dioxide under indoor and outdoor conditions by direct air capture (DAC).

11. The compound of claim 1, wherein the water-soluble compound functions as a redoxactive sorbent.

12. An electrochemical carbon dioxide capture system comprising:an inlet configured to receive a gas stream comprising flue gas or ambient air;an outlet configured to release carbon dioxide;a membrane disposed between an anode compartment and a cathode compartment; and an electrolyte comprising a water-soluble compound capable of capturing carbon dioxide from the gas stream, wherein the water-soluble compound comprises:a fluoflavine derivative comprising one or more water-solubilizing groups, represented by the general formulae (I) to (IV):(it)(in)(IV)whereineach of R1to R8is independently H, OH, SO3H, COOH, PO3H2, NH2, N(CH3)3+, [OC2H4]nOH, or [OC2H4]nOCH3;at least one of each of R1to R8is OH, SO3H, COOH, PO3H2, NH2, N(CH3)3+, [OC2H4]nOH, or [OC2H4]nOCH3; andeach X is independently carbon, nitrogen, oxygen, sulfur, or phosphorus.

13. The system of claim 12, wherein the gas stream comprises at least carbon dioxide (CO2), nitrogen (N2) and oxygen (O2).

14. The system of claim 12, wherein the electrochemical carbon dioxide capture system is configured to be operated at ambient temperature and pressure.

15. The system of any one of claims 12 to 14, wherein the electrochemical carbon dioxide capture system is a continuous-flow system.

16. A method for capturing carbon dioxide comprising:introducing a gas stream comprising carbon dioxide into an electrochemical carbon dioxide capture system according to any one of claims 12 to 15;contacting the gas stream with the electrolyte comprising the water-soluble compound to capture carbon dioxide; andelectrochemically regenerating the water-soluble compound to release carbon dioxide.

17. The method of claim 16, wherein the carbon dioxide capture is performed by direct air capture (DAC).

18. The method of claim 16, wherein the electrochemical regeneration achieves a Coulombic efficiency of 99% over 16 cycles.

19. The method of claim 16, wherein the method further comprises performing deep discharge to trigger oxygen evolution for removing accumulated ions.

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