Electrochemical co 2 capture with air stable redox species

Abstract

The invention features solutions of water-soluble, oxygen-resistant redox-active species circulated in flow cells to electrochemically capture CO2 from air or flue gas and release pure CO2, wherein the redox-active species includes a substituent that forms an intramolecular hydrogen bond in the reduced state. The method is safe, scalable, and potentially inexpensive, as it utilizes non-volatile and potentially low-cost redox organic and inexpensive inorganic species and can operate at ambient temperature and pressure and can operate at high current densities.

Description

[0001] ELECTROCHEMICAL CO2CAPTURE WITH AIR STABLE REDOX SPECIES

[0002] BACKGROUND OF THE INVENTION

[0003] CO2 can be captured from air or flue gas, and pure CO2 released, through an electrochemical cycle. Molecules that undergo proton-coupled electron transfers (PCET) change the solution pH from near neutral to highly basic during charging, and from highly basic to near neutral during discharging. In other words, hydroxide is reversibly generated during charging and consumed during discharging. Since hydroxide binds with CO2 to form bicarbonate and then carbonate, the electrolyte captures CO2 upon charging and releases CO2 upon discharging. The energy cost for capturing from a 0.1 bar CO2 stream and releasing into a 1 bar stream at 40 mA / cm2costs 86 kJ / molCO2, which is highly competitive compared to traditional amine scrubbing processes that typically cost more than 100 kJ / molCO2.

[0004] Electrochemical CO2 capture systems typically require an oxygen-free environment. This is because when the redox-active organic molecules are reduced (e.g., when the systems are charged), they may be susceptible to oxidation by molecular oxygen, thereby stopping the discharge process from happening, causing an incomplete cycle and, e.g., resulting in accumulation of hydroxide and dissolved carbon species in the electrolyte. Eventually the device will be fully oxidized on both sides and no longer functional. In real life application, a perfectly sealed flow device is difficult to achieve. Even if initially perfectly sealed, external factors such as aging of cell components or the sealing material, extreme weather, geological changes or even vandalism may cause molecular oxygen to come into contact with the electrolyte. For carbon capture, molecular oxygen is almost always unavoidable, as factory flue gas is 1 to 3% of oxygen and air is -20% oxygen. Therefore, mitigating the adverse effect of oxygen is essential to electrochemical carbon capture flow cell.

[0005] SUMMARY OF THE INVENTION

[0006] The invention features solutions of water-soluble, oxygen-resistant redox-active species circulated in flow cells to electrochemically capture CO2, e.g., from air or flue gas, and release pure CO2. The method is safe, scalable, and potentially inexpensive, as it utilizes non-volatile and potentially low-cost redox organic and inexpensive inorganic species and can operate at ambient temperature and pressure and at high current densities. The present invention further features redox-active species capable of intramolecular hydrogen bonding and their use in methods for CO2 capture.

[0007] In an aspect, the invention provides a method for capturing CO2. The method includes providing an electrochemical cell including a redox-active species dissolved or dispersed in aqueous solution, electrochemically reducing the redox-active species to a reduced state, and contacting a mixture of gases including CO2 with the aqueous solution. The redox-active species may include a redox core having at least one aromatic ring including a heteroatom or having the heteroatom bound thereto and one or more substituents positioned to form an intramolecular hydrogen bond with the heteroatom.

[0008] In some embodiments, the intramolecular hydrogen bond is between a hydroxyl or amino group and the heteroatom. In some embodiments, the one or more substituents and the heteroatom, together with the atoms to which they are attached, form a 5-member, 6-member, or 7-member ring. In some embodiments, the redox active species includes one or more electron withdrawing groups. In some embodiments, the aromatic redox core is para or ortho benzoquinone, naphthoquinone, anthraquinone, phenanthrenequinone, fluorenone, benzophenone, anthrone, xanthone, thioxanthone, acridone, phenazine, viologen, alloxazine, isoalloxazine, azobenzene, phthalimide, phenothiazine, naphthalimide, pyromellitic diimide, 1 ,4,5,8-naphthalenetetracarbodiimide, or benzo(c)cinnoline.

[0009] In some embodiments, the redox-active species has the formula: wherein X, Y, and Z are independently O, S, N, or NE1. E1, E2, E3, E4, E5, and E6are independently selected from -REWG; -RQA, -H, -ORX, -RX1ORX, -N(RX)(RY), or -RX1N(RX)(RY). At least one of E1, E2, E3, E4, E5, and E6is -ORX, -RX1ORX, -N(RX)(RY), or -RX1N(RX)(RY). Rxand RYare independently selected from H; optionally substituted C1-6 alkyl, provided that the C1-6 alkyl is not substituted with quaternary amine; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; and a nitrogen protecting group. RX1is optionally substituted C1-6 alkylene.

[0010] R1, R2, R3, R4, R5, R6, R7and R8are independently selected from REWG; RQA; H; halo; optionally substituted C1-6 alkyl; oxo; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -P(=O)Ra2; and -P(=O)(ORa)2; or any two adjacent groups selected from R1, R2, R3, R4, R5, R6, R7and R8are joined to form an optionally substituted 3-6 membered ring, or an ion thereof, wherein each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; and a nitrogen protecting group. Each REWGis independently selected from -NO2, -CN, -SO3Ra, -CHO, -C(=O)Ra, -C(=O)O C(=O)Ra, - C(=O)ORa, (e.g., -C(=O)OH), -CONH2, -COO", -NRa3+, -CF3, -SO2Ra, -F, -Cl, -Br, -I, -ORa, - SRa, -P(=O)(ORa)2-(CH2)I-3NO2, -(CH2)I-3CN, -(CH2)i-3SO3Ra, -(CH2)I-3CHO, -(CH2)i-3C(=O)Ra, - (CH2)i-3C(=O)OCORa, -(CH2)i-3COORa, -(CH2)I-3C(=O)OH, -(CH2)I-3CONH2, -(CH2)I-3C(=O)O", -(CH2)I- 3NRa3+,-(CH2)i-3SO2Ra, -CH2CF3, -CH2CCI3, -CH2CBr3, -CH2CI3, -CH2CHF2, -CH2CHCI2, -CH2CHBr2, - CH2CHI2, -CH2(CF2)i-6CF3,-(CH2)i-3ORa, and -(CH2)i-3SRa, wherein each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; or optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; each RQAis independently selected from -(CH2)i-eNRa3+, -O(CH2)i-eNRa3+, -S(CH2)i-eNRa3+, -NRa(CH2)i-eNRa3+, and -N((CH2)i-6NRa3+)2, wherein each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; or optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; or a salt, free base, or ion thereof.

[0011] In some embodiments, the redox-active species is selected from formulas I to XIII or a salt, free base, or ion thereof.

[0012] In some embodiments, the redox-active species is selected from:

[0013] E1O X O

[0014] E2O (HA), e.g., where E4is -ORX, -RX1OR\ -N(RX)(RY), or -RX1N(RX)(RY);

[0015] In some embodiments, the electrochemical reduction forms hydroxide ions, and the contacting results in forming inorganic carbonates. In some embodiments, the reduced state reacts with one or more CO2 molecules to form a CO2 adduct. In some embodiments, the method includes electrochemically oxidizing the reduced state and / or CO2 adduct to release CO2.

[0016] In some embodiments, the redox-active species is selected from:

[0017]

[0018] In some embodiments, the redox-active species is: thereof.

[0019] 10 In some embodiments, the redox-active species is not

[0020] In some embodiments, the aqueous solution includes a catalyst or enzyme to reduce an overpotential of reduction of the redox-active species and / or oxidation of the reduced state.

[0021] In some embodiments, the catalyst includes Pt, Ir, Pd, Ni, Rh, Ru, Zn, Cu, Fe, or Co, or a combination thereof; or the catalyst includes an aminoxyl radical (e.g., (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPOL), 1 -hydroxy-2, 2,6,6- tetramethylpiperidine (TEMPOH), 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy, 4-acetamido-2, 2,6,6- tetramethylpiperidine 1 -oxyl, 4-carboxy-2,2,6,6-tetramethylpiperidine 1 -oxyl, 4-Amino-2, 2,6,6- tetramethylpiperidine-1-oxyl, etc.), or a phthalimido-N-oxyl radical (e.g., N-phthalimido-N-oxyl (PINO)). In some embodiments, the enzyme is an alcohol dehydrogenase.

[0022] In a further aspect, the invention provides a method for capturing CO2 including providing an electrochemical cell including a redox-active species dissolved or dispersed in aqueous solution, electrochemically reducing the redox-active species to a reduced state, and contacting a mixture of gases including CO2 with the aqueous solution, wherein the redox active species is selected from the group consisting of formula I to formula XX and any salt or ion thereof.

[0023] In some embodiments, the redox-active species is selected from:

[0024] E1O x o

[0025] E2O (HA), e.g., where E4is -ORX, -RX1ORX, -N(RX)(RY), or -RX1N(RX)(RY);

[0026] In some embodiments, the redox-active species is selected from:

[0027]

[0028] In some embodiments, the aqueous solution includes a catalyst or enzyme. In some embodiments, the catalyst includes Pt, Ir, Pd, Ni, Rh, Ru, Zn, Cu, Fe, or Co, or a combination thereof. In some embodiments, the catalyst includes an aminoxyl radical or a phthalimido-N-oxyl radical. In some embodiments, the aminoxyl radical includes (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), (4-hydroxy-

[0029] 2.2.6.6-tetramethylpiperidin-1-yl)oxyl (TEMPOL), 1 -hydroxy-2, 2, 6, 6-tetramethylpiperidine (TEMPOH), 4- oxo-2,2,6,6-tetramethyl-1 -piperidinyloxy , 4-acetamido-2, 2, 6, 6-tetramethylpiperidine 1 -oxyl, 4-carboxy-

[0030] 2.2.6.6-tetramethylpiperidine 1-oxyl, or 4-Amino-2,2,6,6-tetramethylpiperidine-1-oxyl; and wherein the a phthalimido-N-oxyl radical includes.

[0031] Definitions

[0032] By “about” is meant ±10% of a recited value.

[0033] By “alkoxy” is meant a group of formula -OR, where R is an alkyl group, as defined herein.

[0034] By “alkyl” is meant straight chain or branched saturated groups from 1 to 6 carbons. Alkyl groups are exemplified by methyl, ethyl, n- and iso-propyl, n-, sec-, iso- and tert-butyl, neopentyl, and the like, and may be optionally substituted with one or more, substituents.

[0035] By “alkylene” is meant a divalent alkyl group.

[0036] By “alkyl thio” is meant -SR, where R is an alkyl group, as defined herein.

[0037] By “alkyl ester” is meant -COOR, where R is an alkyl group, as defined herein.

[0038] By “amino” is meant -NH2. An exemplary ion of amino is -NHa+. By “aryl” is meant an aromatic cyclic group in which the ring atoms are all carbon. Exemplary aryl groups include phenyl, naphthyl, and anthracenyl. Aryl groups may be optionally substituted with one or more substituents.

[0039] By “carbocyclyl” is meant a non-aromatic cyclic group in which the ring atoms are all carbon. Exemplary carbocyclyl groups include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl. Carbocyclyl groups may be optionally substituted with one or more substituents.

[0040] By “carboxyl” is meant -COOH. An exemplary ion of carboxyl is -COO-.

[0041] By “halo” is meant, fluoro, chloro, bromo, or iodo.

[0042] By “heteroalkylene” is meant an alkylene group in which one or more CH2 units are replaced with one or more heteroatoms selected from O, N, and S. Heteroalkylene can be substituted by oxo (=0). An exemplary heteroalkylene includes an amido group, e.g., -(CH2)nC(O)NH(CH2)m-, where n and m are independently 1 -6.

[0043] By “heteroaryl” is meant an aromatic cyclic group in which the ring atoms include at least one carbon and at least one O, N, or S atom, provided that at least three ring atoms are present. Exemplary heteroaryl groups include oxazolyl, isoxazolyl, tetrazolyl, pyridyl, thienyl, furyl, pyrrolyl, imidazolyl, pyrimidinyl, thiazolyl, indolyl, quinolinyl, isoquinolinyl, benzofuryl, benzothienyl, pyrazolyl, pyrazinyl, pyridazinyl, isothiazolyl, benzimidazolyl, benzothiazolyl, benzoxazolyl, oxadiazolyl, thiadiazolyl, and triazolyl. Heteroaryl groups may be optionally substituted with one or more substituents.

[0044] By “heterocyclyl” is meant a non-aromatic cyclic group in which the ring atoms include at least one carbon and at least one O, N, or S atom, provided that at least three ring atoms are present. Exemplary heterocyclyl groups include epoxide, thiiranyl, aziridinyl, azetidinyl, thietanyl, dioxetanyl, morpholinyl, thiomorpholinyl, piperazinyl, piperidinyl, pyrrolidinyl, tetrahydropyranyl, tetra hydrofuranyl, di hydrofuranyl, tetrahydrothienyl, dihydrothienyl, dihydroindolyl, tetrahydroquinolyl, tetrahydroisoquinolyl, pyranyl, pyrazolinyl, pyrazolidinyl, dihydropyranyl, tetrahydroquinolyl, imidazolinyl, imidazolidinyl, pyrrolinyl, oxazolidinyl, isoxazolidinyl, thiazolidinyl, isothiazolidinyl, dithiazolyl, and 1 ,3-dioxanyl. Heterocyclyl groups may be optionally substituted with one or more substituents.

[0045] By “hydrocarbyl” is meant a branched, unbranched, cyclic, or acyclic group including the elements C and H. Hydrocarbyl groups may be monovalent, e.g., alkyl, or divalent, e.g., alkylene. Hydrocarbyl groups may be substituted with groups including oxo (=0).

[0046] By “hydroxyl” is meant -OH. An exemplary ion of hydroxyl is -O'.

[0047] By “nitro” is meant -NO2.

[0048] By a “nitrogen protecting group” is meant those groups intended to protect an amino group against undesirable reactions during synthetic procedures. Commonly used nitrogen protecting groups are disclosed in Greene, “Protective Groups in Organic Synthesis,” 3rdEdition (John Wiley & Sons, New York, 1999), which is incorporated herein by reference. Nitrogen protecting groups include acyl, aryloyl, or carbamyl groups such as formyl, acetyl, propionyl, pivaloyl, t-butylacetyl, 2-chloroacetyl, 2-bromoacetyl, trifluoroacetyl, trichloroacetyl, phthalyl, o-nitrophenoxyacetyl, a-chlorobutyryl, benzoyl, 4-chlorobenzoyl, 4-bromobenzoyl, 4-nitrobenzoyl, and amino acids such as alanine, leucine, and phenylalanine; sulfonyl- containing groups such as benzenesulfonyl, and p-toluenesulfonyl; carbamate forming groups such as benzyloxycarbonyl, p-chlorobenzyloxycarbonyl, p-methoxybenzyloxycarbonyl, p-nitrobenzyloxycarbonyl, 2-nitrobenzyloxycarbonyl, p-bromobenzyloxycarbonyl, 3,4-dimethoxybenzyloxycarbonyl, 3,5-dimethoxybenzyloxycarbonyl, 2,4-dimethoxybenzyloxycarbonyl, 4-methoxybenzyloxycarbonyl, 2-nitro-4,5-dimethoxybenzyloxycarbonyl, 3,4,5-trimethoxybenzyloxycarbonyl, 1 -(p-bi ph e ny ly I)- 1 - methylethoxycarbonyl, a,a-dimethyl-3,5-dimethoxybenzyloxycarbonyl, benzhydryloxy carbonyl, t-butyloxycarbonyl, diisopropylmethoxycarbonyl, isopropyloxycarbonyl, ethoxycarbonyl, methoxycarbonyl, allyloxycarbonyl, 2, 2, 2, -trichloroethoxycarbonyl, phenoxycarbonyl, 4-nitrophenoxy carbonyl, fluorenyl-9- methoxycarbonyl, cyclopentyloxycarbonyl, adamantyloxycarbonyl, cyclohexyloxycarbonyl, and phenylthiocarbonyl, alkaryl groups such as benzyl, triphenylmethyl, and benzyloxymethyl, and silyl groups, such as trimethylsilyl. Preferred nitrogen protecting groups are alloc, formyl, acetyl, benzoyl, pivaloyl, t-butylacetyl, alanyl, phenylsulfonyl, benzyl, t-butyloxycarbonyl (Boc), and benzyloxycarbonyl (Cbz).

[0049] By “oxo” is meant =0.

[0050] By an “oxygen protecting group” is meant those groups intended to protect an oxygen containing (e.g., phenol, hydroxyl, or carbonyl) group against undesirable reactions during synthetic procedures. Commonly used oxygen protecting groups are disclosed in Greene, “Protective Groups in Organic Synthesis,” 3rd Edition (John Wiley & Sons, New York, 1999), which is incorporated herein by reference. Exemplary oxygen protecting groups include acyl, aryloyl, or carbamyl groups, such as formyl, acetyl, propionyl, pivaloyl, t-butylacetyl, 2-chloroacetyl, 2-bromoacetyl, trifluoroacetyl, trichloroacetyl, phthalyl, o- nitrophenoxyacetyl, a-chlorobutyryl, benzoyl, 4-chlorobenzoyl, 4-bromobenzoyl, t-butyldimethy Isilyl, tri- iso-propylsilyloxymethyl, 4,4'-dimethoxytrityl, isobutyryl, phenoxyacetyl, 4-isopropylpehenoxyacetyl, dimethylformamidino, and 4-nitrobenzoyl; alkylcarbonyl groups, such as acyl, acetyl, propionyl, and pivaloyl; optionally substituted arylcarbonyl groups, such as benzoyl; silyl groups, such as trimethylsilyl (TMS), tert-butyldimethy Isilyl (TBDMS), tri-iso-propylsilyloxy methyl (TOM), and triisopropylsilyl (TIPS); ether-forming groups with the hydroxyl, such methyl, methoxy methyl, tetrahydropyranyl, benzyl, p- methoxybenzyl, and trityl; alkoxycarbonyls, such as methoxycarbonyl, ethoxycarbonyl, isopropoxycarbonyl, n-isopropoxycarbonyl, n-butyloxycarbonyl, isobutyloxycarbonyl, secbutyloxycarbonyl, t-butyloxycarbonyl, 2-ethylhexyloxycarbonyl, cyclohexyloxycarbonyl, and methyloxycarbonyl; alkoxyalkoxycarbonyl groups, such as methoxymethoxycarbonyl, ethoxymethoxycarbonyl, 2-methoxyethoxycarbonyl, 2-ethoxyethoxycarbonyl, 2-butoxyethoxycarbonyl, 2- methoxyethoxymethoxycarbonyl, allyloxycarbonyl, propargyloxycarbonyl, 2-butenoxycarbonyl, and 3- methyl-2-butenoxycarbonyl; haloalkoxycarbonyls, such as 2-chloroethoxycarbonyl, 2- chloroethoxycarbonyl, and 2,2,2-trichloroethoxycarbonyl; optionally substituted arylalkoxycarbonyl groups, such as benzyloxycarbonyl, p-methylbenzyloxycarbonyl, p-methoxybenzyloxycarbonyl, p- nitrobenzyloxycarbonyl, 2,4-dinitrobenzyloxycarbonyl, 3,5-dimethylbenzyloxycarbonyl, p- chlorobenzyloxycarbonyl, p-bromobenzyloxy-carbonyl, and fluorenylmethyloxycarbonyl; and optionally substituted aryloxycarbonyl groups, such as phenoxycarbonyl, p-nitrophenoxycarbonyl, o- nitrophenoxycarbonyl, 2,4-dinitrophenoxycarbonyl, p-methyl-phenoxycarbonyl, m- methylphenoxycarbonyl, o-bromophenoxycarbonyl, 3,5-dimethylphenoxycarbonyl, p- chlorophenoxycarbonyl, and 2-chloro-4-nitrophenoxy-carbonyl); substituted alkyl, aryl, and alkaryl ethers (e.g., trityl; methylthiomethyl; methoxymethyl; benzyloxymethyl; siloxymethyl; 2,2,2,- trichloroethoxymethyl; tetrahydropyranyl; tetra hydrofuranyl; ethoxyethyl; 1 -[2-(trimethylsilyl)ethoxy]ethyl; 2-trimethylsilylethyl; t-butyl ether; p-chlorophenyl, p-methoxyphenyl, p-nitrophenyl, benzyl, p- methoxybenzyl, and nitrobenzyl); silyl ethers (e.g., trimethylsilyl; triethylsilyl; triisopropylsilyl; dimethylisopropylsilyl; t-butyldimethy Isilyl ; t-buty Idipheny Isilyl; tribenzylsilyl; triphenylsilyl; and diphenymethylsilyl); carbonates (e.g., methyl, methoxy methyl, 9-fluorenylmethyl; ethyl; 2,2,2- trichloroethy I; 2-(trimethy Isily l)ethy I; vinyl, allyl, nitrophenyl; benzyl; methoxybenzyl; 3,4-dimethoxybenzyl; and nitrobenzyl); carbonyl-protecting groups (e.g., acetal and ketal groups, such as dimethyl acetal, and 1 ,3-dioxolane; acylal groups; and dithiane groups, such as 1 ,3-dithianes, and 1 ,3-dithiolane); carboxylic acid-protecting groups (e.g., ester groups, such as methyl ester, benzyl ester, t-butyl ester, and orthoesters; and oxazoline groups.

[0051] By “phosphoryl” is meant -PO3H2. Exemplary ions of phosphoryl are -POsH- and -PCs2-.

[0052] By “phosphonyl” is meant -PO3R2, where each R is H or alkyl, provided at least one R is alkyl, as defined herein. An exemplary ion of phosphoryl is -POsR-.

[0053] By “quaternary amino” is meant -NRa3+, wherein each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; or optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S.

[0054] For the purposes of this invention, the term “quinone” includes a compound having one or more conjugated, C3-10 carbocyclic, fused rings, substituted, in oxidized form, with two or more oxo groups, which are in conjugation with the one or more conjugated rings. Preferably, the number of rings is from one to ten, e.g., one, two, or three, and each ring has 6 members.

[0055] By “sulfonyl” is meant -SO3H. An exemplary ion of sulfonyl is -SOs-.

[0056] By “thiol” is meant -SH.

[0057] As noted, substituents may be optionally substituted with halo, optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -CN; -NO2; -ORa; -N(Ra)2; -C(=O)Ra; -C(=O)ORa; - S(=O)2Ra; -S(=O)2ORa; -P(=O)Ra2; -O-P(=O)(ORa)2, or -P(=O)(ORa)2, or an ion thereof; where each Rais independently H, C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; or a nitrogen protecting group. Cyclic substituents may also be substituted with optionally substituted C1-6 alkyl. In specific embodiments, substituents may include optionally substituted with halo, optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -NO2; -ORa; -N(Ra)2; -C(=O)Ra; -C(=O)ORa; -S(=O)2Ra; -S(=O)2ORa; - P(=O)Ra2; -O-P(=O)(ORa)2, or -P(=O)(ORa)2, or an ion thereof; where each Rais independently H, C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; or a nitrogen protecting group, and cyclic substituents may also be substituted with C1-6 alkyl. In specific embodiments, alkyl groups may be optionally substituted with one, two, three, or, in the case of alkyl groups of two carbons or more, four substituents independently selected from the group consisting of halo, hydroxyl, C1-6 alkoxy, SO3H, amino, nitro, carboxyl, phosphoryl, phosphonyl, thiol, C1-6 alkyl ester, optionally substituted C1-6 alkyl thio, and oxo, or an ion thereof. Substituents may also be selected from the electron withdrawing groups described herein.

[0058] Exemplary ions of substituent groups are as follows: an exemplary ion of hydroxyl is -O-; an exemplary ion of -COOH is -COO-; exemplary ions of -PO3H2 are -POsH- and -POs2-; an exemplary ion of -PO3HRais -POsRe , where Rais not H; exemplary ions of -PO H2 are -PO4I+ and -PO42-; an exemplary ion of - SO3H is -SO3-; and an exemplary ion of NRa2 is NRa2H+.

[0059] BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Fig. 1 shows a process schematic for CO2 capture / release via the pH swing and the nucleophilicity swing mechanisms. The processes between numbered states are electrochemical oxidation for reduced RO (3^1), CO2 outgassing (1 ^2), electrochemical reduction of RO (2^4), and CO2 invasion (4^3).

[0061] Fig. 2 shows electrochemical redox reactions involved during the CO2 capture / release processes. The processes are illustrated with an anthraquinone (AQ) core. AQ can be reversibly reduced to anthraquinone dianion (AQ2) via two-electron transfer. AQ2-can react with two CO2 molecules and form an adduct of organic carbonate: AQ(CO2)22-. In addition, depending on the pKa values of anthrahydroquinone (AQH2), AQ2-can react with water to become half-protonated (AQH ) or fully protonated (AQH2) as well as produce hydroxyl anions, which can react with CO2 to form the inorganic (bi)carbonate. Other redox cores described herein may undergo similar processes.

[0062] Fig. 3 shows the 02-induced side reactions of reduced AQ species include: AQ(CO2)22can be oxidized by oxygen to AQ. AQ2-, AQH-, and AQH2 can also be oxidized by oxygen to AQ.

[0063] Fig. 4 shows the cycling of 5 mL 0.1 M 1 ,4-BTMAPAQ, 1 M KCI under 10% CO2, 90% N2 against 40 mL 0.1 M FcNCI, 1 M KCI under 100% N2. The membrane is DSV-N anion exchange membrane. Electrode: 1 piece of unbaked HCBA carbon cloth on each side. Current density: 40 mA / cm2followed with potential holds. Voltage cut offs: 0.05 V for discharge; 1 .45 V for charge. Every charge is followed with a 2-hour rest; every discharge is followed with a 1 -hour rest. Fig. 5 shows the cycling of 5 mL 0.1 M 1 ,5-BTMAPAQ, 1 M KCI under 10% CO2, 90% N2 against 40 mL 0.1 M FcNCI, 1 M KCI under 100% N2. The membrane is DSV-N anion exchange membrane. Electrode: 1 piece of unbaked HCBA carbon cloth on each side. Current density: 40 mA / cm2followed with potential holds. Voltage cut offs: 0.05 V for discharge; 1 .45 V for charge. Every charge is followed with a 2-hour rest; every discharge is followed with a 1 -hour rest.

[0064] Fig. 6 shows the cycling of 5 mL 0.1 M 1 ,5-BTMAPAQ, 1 M KCI under 88% N2, 9% CO2, 3% O2 against 40 mL 0.1 M FcNCI, 1 M KCI under 100% N2.

[0065] DETAILED DESCRIPTION OF THE INVENTION

[0066] The invention provides electrochemically cycled methods of capturing CO2, e.g., from air or flue gas, and releasing pure CO2, using water-soluble oxygen-resistant redox-active species circulated in flow cells. The method is safe, scalable, and potentially inexpensive, as it utilizes non-volatile and potentially low- cost redox organic (RO) and inexpensive redox inorganic (Rl) species, can operate at ambient temperature and pressure, and can operate at high current densities.

[0067] Electrochemical flow cell capture systems such as those described herein rely solely on electrical work, while amine scrubbing requires high thermal input, and the amines and related decomposition products are volatile and environmental hazards. For air capture, redox flow methods such as described herein are projected to cost 110 kJ / molCO2 at 40 mA / cm2, which is again very competitive compared with existing technologies costing more than 150 kJ / molCO2.

[0068] The capture method involves a pH-swing cycle and / or a nucleophilicity-swing cycle of dissolved redoxactive species, electrochemically driven through the proton-coupled electron transfer (PCET) or CO2- coupled electron transfer. In pH-swing cycle capture, redox actives species (RO, e.g., an anthraquinone) react with water, e.g., to become, e.g., ROH2 (e.g., an anthrahydroquinone) during the electrochemical reduction and produce hydroxides which react with CO2 and form inorganic (bi)carbonates (e.g., HCOs-, CO32-). In nucleophilicity-swing capture, ROs become nucleophiles after reduction which can react with CO2 directly to form organic carbonates (e.g., RO(CO2)22-, e.g., where RO is a quinone, e.g., an anthraquinone, core). During the electrochemical oxidation, ROs can release protons that react with inorganic (bi)carbonates and release CO2; or ROs lose affinity for CO2 and release pure CO2. .

[0069] In both mechanisms of capture involving ROs, a reduced state of the RO is required for CO2 capture. However, reduced ROs are oxygen sensitive, and atmospheric O2 can chemically oxidize a reduced RO back to the oxidized state of the RO, which does not have CO2 affinity (Fig. 3). Therefore, the instability of reduced ROs in air has prevented their application in direct air capture (DAC) or CO2 capture from flue gas until now. Our newly designed ROs have hydrogen bonding groups that make them air-stable in their reduced states. Our ROs exhibit good air stability, allowing use in DAC or CO2 capture from flue gas where it would undergo multiple oxidation / reduction cycles per day. Redox-active Species

[0070] The present invention features enhanced air-stability of reduced redox-active species, e.g., through inclusion of hydrogen bonding functional groups on the redox core. The functional group may be attached to the redox core to form hydrogen bonds with a heteroatom within or bound to the redox core. The introduced functional groups could be the same, or different, e.g., both hydrogen bonding and electron withdrawing and / or water solubilizing functional groups may be present. Redox cores include, but are not limited to, para or ortho benzoquinone, naphthoquinone, anthraquinone, phenanthrenequinone, fluorenone, benzophenone, anthrone, xanthone, thioxanthone, acridone, phenazine, viologen, alloxazine, isoalloxazine, azobenzene, phthalimide, phenothiazine, naphthalimide, pyromellitic diimide, 1 , 4,5,8- naphthalenetetracarbodiimide, or benzo(c)cinnoline. Specific formulas are provided in formulas I to XX. Redox active species may be present in a mixture.

[0071] ROs of the present invention may include electron-withdrawing groups to elevate redox potentials to be comparable to or higher than the O2 reduction potential at the pH of interest, so that the oxidation of the reduced molecules by O2 is thermodynamically less favored, or even disfavored. The electronwithdrawing groups can be directly connected to the redox cores, or indirectly connected to the redox cores via C, N, O, or S atoms, with zero or multiple, e.g., one, two, three, etc., methylene spacing groups.

[0072] Alternatively, or in addition, redox-active species of the invention may include groups which frustrate the oxygenation of the reduced state kinetically, e.g., by intramolecular hydrogen bonding. The intramolecular interactions may be introduced by covalently attaching functional groups to the redox cores.

[0073] Exemplary electron withdrawing groups include -NO2, -CN, -SO3Ra, -CHO, -C(=O)Ra, -C(=O)ORa, (e.g., -C(=O)OH), -CONH2, -COO", -NRa3+, -CF3, -SO2Ra, -F, -Cl, -Br, -I, -ORa, -SRa, -P(=O)(ORa)2, -(CH2)I-3NO2, -(CH2)I-3CN, -(CH2)i-3SO3Ra, -(CH2)I-3CHO, -(CH2)i-3C(=O)Ra, -(CH2)i-3C(=O)OCORa, - (CH2)i-3COORa, -(CH2)I-3C(=O)OH, -(CH2)I-3CONH2, -(CH2)I-3C(=O)O-, -(CH2)i-3NRa3+,-(CH2)i-3SO2Ra, -CH2CF3, -CH2CCI3, -CH2CBr3, -CH2CI3, -CH2CHF2, -CH2CHCI2, -CH2CHBr2, -CH2CHI2, -CH2(CF2)I- 6CF3,-(CH2)i-3ORa, and -(CH2)i-3SRa, where each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-io carbocyclyl; or optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S, e.g., -SO3H, -PO3H2, -COOH, -OH, -NH2, - N(CH3)3+, -NH3+, and ethyleneglycol (-OCH2CH2OCH2CH2OH).

[0074] In some embodiments, the substituents are optionally substituted C1-6 alkyl groups including a quaternary ammonium group, optionally substituted C1-6 alkoxy groups including a quaternary ammonium group, optionally substituted C1-6 alkyl thio groups including a quaternary ammonium group. In some embodiments, the quaternary amine group is attached to a terminal carbon. In some embodiments, the one or more substituents include at least one -(CH2)nNRa3+group, where n = 2-11 , where each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-io carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; or a nitrogen protecting group. ROs of the present invention may include functional groups that allow intramolecular hydrogen bonding between a heteroatom present in the redox core (e.g., phenazine) or bound directly to the redox core (e.g., quinone). Redox-active species, once reduced, may be stabilized via hydrogen bonding interactions. The redox-active species of the present invention may include one or more intramolecular hydrogen bonds (e.g., two intramolecular hydrogen bonds, three intramolecular hydrogen bonds, four intramolecular hydrogen bonds, five intramolecular hydrogen bonds, six intramolecular hydrogen bonds, etc.). In some embodiments, the hydrogen bonding functional group and the heteroatom bound to or part of the redox core form a ring. In some embodiments, the ring is a five-member ring, a six-member ring, or a seven-member ring. Redox-active species including intramolecular hydrogen bonds stabilize the redox- active species in the reduced form to side reactions with molecular oxygen. Exemplary redox cores include quinones including anthraquinones, phenanthraquinones, 1 ,2(4)-naphthoquinones, 1 ,2(4)- benzoquinones, phenazines, benzo[c]cinnolines, and fluorenones. Exemplary formulas of ROs, highlighting intramolecular-hydrogen bonding, are shown below.

[0075] (E2) (F1) Formulas A1 , B1 , C1 , D1 , E1 , and F1 are redox-active species capable of intramolecular hydrogen bonding and include an amino or hydroxyl group proximal to a heteroatom included in or bound to the redox-active core (anthraquinone for A1 and B1 , phenazine for C1 and E1 , and fluorenone for D1 and F1). Formulas A2, A3, B2, B3, C2, D2, E2, and F2 are the reduced forms of formulas A1 , B1 , C1 , D1 , E1 , and F1 with the hydrogen bonds between heteroatoms highlighted via dashed lines. Formulas A4 and B4 show the redox-active species adduct with CO2. The formula highlights the stabilization of the negatively charged deprotonated carboxyl group formed after reacting with CO2.

[0076] In some embodiments, the redox-active species has a formula selected from formulas I to XX.

[0077] wherein X, Y, and Z are independently O, S, N, or NE1. E1, E2, E3, E4, E5, and E6are independently selected from -REWG; -RQA, -H, -ORX, -RX1ORX, -N(RX)(RY), or -RX1N(RX)(RY). At least one of E1, E2, E3, E4, E5, and E6is -ORX, -RX1ORX, -N(RX)(RY), or -RX1N(RX)(RY). Rxand RYare independently selected from H; optionally substituted C1-6 alkyl, provided that the C1-6 alkyl is not substituted with quaternary amine; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; and a nitrogen protecting group. RX1is optionally substituted C1-6 alkylene.

[0078] R1, R2, R3, R4, R5, R6, R7and R8are independently selected from REWG; RQA; H; halo; optionally substituted C1-6 alkyl; oxo; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -P(=O)Ra2; and -P(=O)(ORa)2; or any two adjacent groups selected from R1, R2, R3, R4, R5, R6, R7and R8are joined to form an optionally substituted 3-6 membered ring, or an ion thereof, wherein each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; and a nitrogen protecting group. Each REWGis independently selected from -NO2, -CN, -SO3Ra, -CHO, -C(=O)Ra, -C(=O)O C(=O)Ra, - C(=O)ORa, (e.g., -C(=O)OH), -CONH2, -COO", -NRa3+, -CF3, -SO2Ra, -F, -Cl, -Br, -I, -ORa, - SRa, -P(=O)(ORa)2-(CH2)I-3NO2, -(CH2)I-3CN, -(CH2)i-3SO3Ra, -(CH2)I-3CHO, -(CH2)i-3C(=O)Ra, - (CH2)i-3C(=O)OCORa, -(CH2)i-3COORa, -(CH2)I-3C(=O)OH, -(CH2)I-3CONH2, -(CH2)I-3C(=O)O", -(CH2)I- 3NRa3+,-(CH2)i-3SO2Ra, -CH2CF3, -CH2CCI3, -CH2CBr3, -CH2CI3, -CH2CHF2, -CH2CHCI2, -CH2CHBr2, - CH2CHI2, -CH2(CF2)i-6CF3,-(CH2)i-3ORa, and -(CH2)i-3SRa, wherein each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; or optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; each RQAis independently selected from -(CH2)i-eNRa3+, -O(CH2)i-eNRa3+, -S(CH2)i-eNRa3+, -NRa(CH2)i-eNRa3+, and -N((CH2)i-6NRa3+)2, wherein each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; or optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; or any salt, free base, or ion thereof. In some embodiments, the redox-active species is selected from formulas I - XX or a salt, free base, or ion thereof. In some embodiments, the redox-active species is selected from formulas I to XIII, or a salt, free base, or ion thereof.

[0079] In some embodiments, the redox-active species is selected from: In some embodiments, the redox-active species is selected from:

[0080]

[0081] In some embodiments, the redox-active species is selected from:

[0082]

[0083] Examples include bis-(trimethylammonio)propyl anthraquinone (BTMAPAQ)-based isomers and derivatives. Our results indicate that the reduced states of BTMAPAQ isomers (with and without captured CO2) show excellent air-resistance, which is the key to be utilized for CO2 capture in air or flue gas. In some embodiments, the redox-active species is not a BTMAPAQ, e.g., is not 1 ,8-BTMAPAQ.

[0084] In some embodiments, at least 10%, e.g., at least 20, 30, 40, 50, 60, 70, 80, 90, 95, or 99%, of the reduced state is present after exposure to air for at least one day, e.g., at least 2, 3, 4, 5, 6, or 7 days or at least 1 , 2, 3, or 4 weeks, or at least 1 , 2, 3, 4, 5, or 6 months, e.g., the redox species is stable up to 6 months or one year.

[0085] Capture Devices

[0086] Capture devices may include a negolyte (negative electrolyte) that includes, e.g., a redox-active species dissolved or suspended in aqueous solution; a posolyte (positive electrolyte) that includes, e.g., a redoxactive species; and a barrier separating the two. The redox-active species in the negolyte and the posolyte could be either the same molecule or different molecules. The device further includes at least two electrodes, one in contact with the negolyte and one in contact with the posolyte. The electrodes may be carbon-based materials, pure metals, or alloys. Electrodes may be doped or decorated with catalysts. Fig. 1 provides a device configured to capture and then release CO2, e.g., from either flue gas or air. Gas containing CO2 contacts the negolyte containing a proton-coupled redox-active species at a high pH or a nucleophilic reduced redox-active species. This region also includes a gas outlet to allow the carrier source gas, e.g., flue gas or air, to exit the device after being depleted of CO2. Nitrogen or another inert gas may be used to purge the negolyte of dissolved gases, e.g., oxygen or CO2. The device may further include a port connected to storage container for CO2.

[0087] During invasion, the high-pH liquid may be sprayed down through a solid lattice, providing a liquid / gas interface for CO2 in the gas to enter the liquid. A similar lattice may be employed when CO2 gas is released from the liquid.

[0088] The devices may also employ redox species having an aminoxyl radical group, e.g., 2, 2,6,6- tetramethylpiperidine-N-oxyl (TEMPO) or substituted versions thereof (e.g., substituted like a heterocycle as described herein). Devices may also include aminoxyl radical species as catalysts / charge mediators, e.g., compounds such as (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPOL), 1 -hydroxy-2, 2,6,6- tetramethylpiperidine (TEMPOH), 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy, 4-acetamido-2, 2,6,6- tetramethylpiperidine 1 -oxyl, 4-carboxy-2,2,6,6-tetramethylpiperidine 1 -oxyl, 4-Amino-2, 2,6,6- tetramethylpiperidine-1-oxyl, etc.). Compounds having a phthalimido-N-oxyl radical group (e.g., N- phthalimido-N-oxyl (PINO)) may also be used. Such species may be employed to capture CO2 on the positive or negative side of the device.

[0089] Examples of redox-active species for the posolyte include bromine, chlorine, iodine, vanadium, chromium, cobalt, iron (e.g., ferricyanide / ferrocyanide or a ferrocene derivative, e.g., as described in WO 2018 / 032003), aluminum, e.g., aluminum(lll) biscitrate monocatecholate, manganese, cobalt, nickel, copper, or lead, e.g., a manganese oxide, a cobalt oxide, or a lead oxide. A benzoquinone may also be used as the second redox-active species. Other redox-active species suitable for use are described in WO 2014 / 052682, WO 2015 / 048550, WO 2016 / 144909, and WO 2020 / 072406, the redox-active species of which are incorporated by reference. The redox-active species may be dissolved or suspended in solution (such as aqueous solution) or be in the solid state. The posolyte and the negolyte may include the same redox species but with the opposite states, e.g., one is the reduced state, and the other one is the oxidized state. One side is to capture CO2, e.g., from flue gas or air, and the other side is to release CO2.

[0090] In some embodiments, the electrolytes are both aqueous, where the negolyte and the posolyte are aqueous. In addition, the electrolyte may include other solutes, e.g., acids (e.g., HCI) or bases (e.g., LiOH, NH4OH, NaOH, or KOH). In some embodiments, the solute may include alcohols (e.g., methyl, ethyl, or propyl) and other co-solvents to increase the solubility of a particular species, e.g., quinone / hydroquinone. Counter ions, such as cations, e.g., NH4+, Li+, Na+, K+, or a mixture thereof, may also be present. In certain embodiments, the pH of one or both of the electrolytes may be >7, e.g., at least s, 9, 10, 11 , 12, 13, or 14, 8-14, 9-14, 10-14, 11-14, 12-14, 13-14, or about 14. In certain embodiments, the pH of one or both of the electrolytes may be <7, e.g., at less than 7, 6, 5, 4, 3, 2, or 1 , e.g., 7-1 , 7-5, 6-4, 5-2, 3-1 , 2-1 , or about 1 . The pH may be less than 1 . The pH may be a negative pH. The electrolyte may or may not be buffered to maintain a specified pH. In methods and systems using nitrite ions, the pH may be modulated to remain above pH 7 to prevent unwanted side reactions of the protonated nitrite ion. The concentration of the negolyte and the posolyte will be suitable to operate the device, e.g., carbon capture device, for example, from 0.1-15 M, or from 0.1-10 M. In some embodiments, the solution is at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, or 80% water, by mass. The electrolytes may contain one or more redox-active species (e.g., ROs or RIs) that act as redox mediators.

[0091] In addition to water, solutions or suspensions may include alcohols (e.g., methyl, ethyl, or propyl) and other co-solvents to increase the solubility of a particular species. In some embodiments, the solution or suspension is at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, or 80% water, by mass. Alcohol or other co-solvents may be present in an amount required to result in a particular concentration of species. The pH of the aqueous solution or suspension may also be adjusted by addition of acid or base, e.g., to aid in solubilizing a species.

[0092] The barrier allows the passage of ions, such as sodium or potassium, but not a significant amount of the redox-active species. Examples of ion conducting barriers are NAFION®, i.e., sulfonated tetrafluoroethylene based fluoropolymer-copolymer, FUMASEP®, i.e., non-fluorinated, sulfonated polyaryletherketone-copolymer, e.g., FUMASEP® E-620(K), hydrocarbons, e.g., polyethylene, and size exclusion barriers, e.g., ultrafiltration or dialysis membranes with a molecular weight cut off of 100, 250, 500, or 1 ,000 Da. Examples of membranes include Selemion DSV and Selemion AMV. For size exclusion membranes, the required molecular weight cut off is determined based on the molecular weight of the negolytes and posolytes employed. Porous physical barriers may also be included, e.g., when the passage of redox-active species is tolerable. A redox flow cell may have two barriers and a central compartment disposed therebetween.

[0093] Electrodes for use with CO2 capture devices may include any carbon electrode, e.g., glassy carbon electrodes, carbon paper electrodes, carbon felt electrodes, or carbon nanotube electrodes. Titanium electrodes may also be employed. Electrodes can also be made of a high specific surface area conducting material, such as a nanoporous metal sponge (T. Wada, A.D. Setyawan, K. Yubuta, and H. Kato, Scripta Materialia 65, 532 (2011)), which has been synthesized previously by electrochemical dealloying (J.D. Erlebacher, M.J. Aziz, A. Karma, N. Dmitrov, and K. Sieradzki, Nature 410, 450 (2001)), or a conducting metal oxide, which has been synthesized by wet chemical methods (B.T. Huskinson, J.S. Rugolo, S.K. Mondal, and M.J. Aziz, arXiv: 1206.2883 [cond-mat.mtrl-sci]; Energy & Environmental Science 5, 8690 (2012); S.K. Mondal, J.S. Rugolo, and M.J. Aziz, Mater. Res. Soc. Symp. Proc. 1311 , GG10.9 (2010)). Chemical vapor deposition can be used for conformal coatings of complex 3D electrode geometries by ultra-thin electrocatalyst or protective films. Other electrodes are known in the art.

[0094] A carbon capture device may include additional components as is known in the art. The device may include one or more suitable reservoirs for housing the negolyte and the posolyte. A carbon capture device may further include one or more pumps to pump aqueous solutions or suspensions past one or both electrodes. Alternatively, the electrodes may be placed in a reservoir that is stirred or in which the solution or suspension is recirculated by any other method, e.g., convection, sonication, etc. A carbon capture device may also include graphite flow plates and corrosion-resistant metal current collectors.

[0095] The balance of the system around the cell includes fluid handling and storage, and voltage and round-trip energy efficiency measurements can be made. Systems configured for measurement of negolyte and posolyte flows and pH, pressure, temperature, current density and cell voltage may be included and used to evaluate cells, e.g., to determine when to apply the electrical pulse. Fluid sample ports can be provided to permit sampling of both electrolytes, which will allow for the evaluation of parasitic losses due to reactant crossover or side reactions. Electrolytes can be sampled and analyzed with standard techniques.

[0096] Suitable cells, electrodes, membranes, and pumps are known in the art, e.g., WO 2014 / 052682, WO 2015 / 048550, WO 2016 / 144909, and WO 2020 / 072406, the components of which are hereby incorporated by reference.

[0097] Methods

[0098] Solutions of oxygen-resistant redox-active species are used to capture CO2, e.g., from air or flue gas and release pure CO2.

[0099] The methods include providing an electrochemical cell including a redox-active species dissolved or dispersed in aqueous solution. The redox-active species is electrochemically reduced to a reduced state. The reduced state is then contacted with a mixture of gases including CO2, e.g., by bubbling air or flue gas through an aqueous solution. Air is present in the mixture of gases, which would typically oxidize a reduced state of the redox-active species back to the oxidized state, but these undesired reactions are prevented or slowed using redox-active species that are resistant to oxidation in their reduced states. The reduction of the redox-active species may generate hydroxide ions which react with CO2 to form inorganic carbonates (PCET), and / or the reduced state may itself react (e.g., by a nucleophilic addition reaction) with CO2 to form a CO2 adduct (Figs. 2-3). Both the inorganic carbonates and CO2 adducts may be disproportionated to release CO2 by exposing the solution to an oxidizing electrochemical potential, while also regenerating the redox-active species for further capture cycles.

[0100] Fig. 1 shows a process schematic for applying methods of the invention to capture and release CO2 via the pH swing and the nucleophilicity swing mechanism. “Q” refers to a PCET organic or inorganic species. Processes between numbered states are electrochemical oxidation of reduced RO (3->1), CO2 outgassing (1 ->2), electrochemical reduction of RO (2->4), and CO2 invasion (4->3).

[0101] The invention can be used as an electrochemical CO2 capture system, e.g., employing proton-coupled redox-active species whose protonation and deprotonation can be controlled electrochemically to modify the pH of an aqueous solution or aqueous suspension. This change in pH can be used to sequester and release CO2. The CO2 capture device can be used to sequester gaseous CO2 from a point source, such as flue gas, or from ambient air. The total possible amount of sequestered carbon, the Dissolved Inorganic Carbon (DIC), depends on the partial pressure of CO2 above the aqueous solution or aqueous suspension, and the pH determines the form of the carbon, e.g., dissolved CO2, HCOs- or COs2-. CO2 can be captured from a gaseous source, e.g., point sources or ambient air, by dissolving into an aqueous solution. More CO2 can be dissolved as the pH of the aqueous solution or aqueous suspension increases, resulting in the conversion of CO2 into HCOs- or COs2-ions. More CO2 can be dissolved in an aqueous solution or aqueous suspension as HCOs- or COs2-than CO2, resulting in supersaturation of CO2 in the aqueous solution or aqueous suspension. Once captured, the CO2 can be released by acidifying the aqueous solution or aqueous suspension, e.g., by electrochemical oxidation. Alternatively, the reduced state may react directly with CO2to produce an adduct, and the CO2 can subsequently be released by oxidizing the adduct. In principle, the pure CO2 obtained after separation can be converted back into useful chemical fuels and feedstocks with carbon-free energy, thus providing fuels and feedstocks without added CO2 emissions.

[0102] EXAMPLES

[0103] The invention will be further described by the following non-limiting examples.

[0104] It has been well known that reduced quinones are susceptible to chemical oxidation in the presence of O2, donating two electrons to molecular oxygen and turning themselves back to quinones. Therefore, it is indispensable to interrogate the oxygen sensitivity of reduced quinone species including QH2, QH_, and Q2-; as well as QH(CO2)- and Q(CO2)22-.

[0105] Given that oxygen can chemically oxidize the reduced anthraquinones, charge / discharge capacity of anthraquinone electrolytes would become higher / lower than theoretical capacity during charge / discharge processes in the presence of oxygen, lowering Coulombic efficiency. Therefore, oxygen sensitivity of the reduced BTMAPAQs can also be evaluated by comparing the Coulombic efficiency with varying the ratio of O2 over N2 while keeping the percentage of CO2 constant in feed gas.

[0106] Coulombic efficiency of 1 ,4-, 1 ,5-, and 1 ,8-BTMAPAQ flow cells in the presence of varying compositional feed gas was examined. The cells were set to rest for 68 mins under the feed gas exposure once they were fully charged, (see Table 1 and Figs. 4-6).

[0107] Table 1 . Coulombic Efficiency at Varying O2 : N2 : CO2

[0108] The amount of CO2 released in every cycle is less than that captured due to the side reactions caused by molecular oxygen. Oxygen can oxidize some of the reduced molecules back to their oxidized form, and oxygen molecules are converted to hydroxide ions, which can irreversibly capture CO2 that will not be released under normal cycling.

[0109] Redox cores, once reduced, bearing one or multiple intramolecular hydrogen bonds, preferably forming five-, six, or even seven-membered rings, would stabilize their reduced forms against molecular oxygen to prevent them from being oxidized back to initial oxidized forms.

[0110] A compound of formula II was investigated. The electrochemical properties of an exemplary compound are summarized in Table 2. Table 2. Electrochemical Properties of an Exemplary Compound

[0111] Other embodiments are in the claims.

Claims

What is claimed is:CLAIMS1 . A method for capturing CO2 comprising: a) providing an electrochemical cell comprising a redox-active species dissolved or dispersed in aqueous solution; b) electrochemically reducing the redox-active species to a reduced state; and c) contacting a mixture of gases comprising CO2 with the aqueous solution; wherein the redox-active species comprises a redox core comprising at least one aromatic ring comprising a heteroatom or having the heteroatom bound thereto and one or more substituents positioned to form an intramolecular hydrogen bond with the heteroatom.

2. The method of claim 1 , wherein the one or more substituents comprise hydroxyl or amino.

3. The method of claim 1 , wherein the one or more substituents and the heteroatom, together with the atoms to which they are attached, form a 5-member, 6-member, or 7-member ring.

4. The method of claim 1 , wherein the redox active species further comprises one or more electron withdrawing groups.

5. The method of claim 4, wherein the electron withdrawing groups are separated from the redox core by -(CH2)I-6-, -0(CH2)O-6-, -S(CH2)O-6-, -NH(CH2)O-6-, or -NRa(CH2)o-6-.

6. The method of claim 4, wherein the one or more electron withdrawing groups comprise one or more groups selected from -NO2, -CN, -SO3Ra, -CHO, -C(=O)Ra, -C(=O)OC(=O)Ra, -C(=O)ORa, -CONH2, - COO-, -NRa3+, -CF3, -SO2Ra, -F, -Cl, -Br, -I, -ORa, -SRa, -P(=O)(ORa)2, -(CH2)I-3NO2, -(CH2)I-3CN, - (CH2)i-3SO3Ra, -(CH2)I-3CHO, -(CH2)i-3C(=O)Ra, -(CH2)i-3C(=O)OCORa, -(CH2)i-3COORa, -(CH2)I-3C(=O)OH, -(CH2)1-3CONH2, -(CH2)I-3C(=O)O-, -(CH2)i-3NRa3+,-(CH2)i-3SO2Ra, -CH2CF3, -CH2CCI3, - CH2CBr3, -CH2CI3, -CH2CHF2, -CH2CHCI2, -CH2CHBr2, -CH2CHI2, -CH2(CF2)i-6CF3,-(CH2)i-3ORa, and -(CH2)i-3SRa, wherein each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; or optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S.

7. The method of claim 1 , wherein the redox active species further comprises an optionally substituted C1- e alkyl group comprising a quaternary ammonium group, optionally substituted C1-6 alkoxy groups comprising a quaternary ammonium group, or optionally substituted C1-6 alkyl thio groups comprising a quaternary ammonium group.

8. The method of any one of claims 1-7, wherein the aromatic redox core is para or ortho benzoquinone, naphthoquinone, anthraquinone, phenanthrenequinone, fluorenone, benzophenone, anthrone, xanthone, thioxanthone, acridone, phenazine, viologen, alloxazine, isoalloxazine, azobenzene, phthalimide, phenothiazine, naphthalimide, pyromellitic diimide, 1 ,4,5,8-naphthalenetetracarbodiimide, or benzo(c)cinnoline.

9. The method of claim 1 , wherein the redox-active species has the formula:wherein X, Y, and Z are independently O, S, N, or NE1; E1, E2, E3, E4, E5, and E6are independently selected from -REWG; -RQA, -H, -ORX, -RX1ORX, -N(RX)(RY), or -RX1N(RX)(RY), wherein at least one of E1, E2, E3, E4, E5, and E6is -ORX, -RX1ORX, -N(RX)(RY), or -RX1N(RX)(RY);Rxand RYare independently selected from H; optionally substituted C1-6 alkyl, provided that the C1-6 alkyl is not substituted with quaternary amine; optionally substituted C3-10 carbocyclyl; optionally substituted C1- 9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted Ce-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; and a nitrogen protecting group; and RX1is optionally substituted C1-6 alkylene;R1, R2, R3, R4, R5, R6, R7and R8are independently selected from REWG; RQA- |_|- halo; optionally substituted C1-6 alkyl; oxo; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted Ce 20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -P(=O)Ra2; and -P(=O)(ORa)2; or any two adjacent groups selected from R1, R2, R3, R4, R5, R6, R7and R8are joined to form an optionally substituted 3-6 membered ring, or an ion thereof, wherein each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; and a nitrogen protecting group; each REWGis independently selected from -NO2, -CN, -SO3Ra, -CHO, -C(=O)Ra, -C(=O)O C(=O)Ra, - C(=O)ORa, -CONH2, -COO’, -NRa3+, -CF3, — SO2Ra, -F, -Cl, -Br, -I, -ORa, -SRa, -P(=O)(ORa)2- (CH2)I-3NO2, -(CH2)I-3CN, -(CH2)i-3SO3Ra, -(CH2)I-3CHO, -(CH2)i-3C(=O)Ra, -(CH2)i-3C(=O)OCORa, - (CH2)i-3COORa, -(CH2)I-3C(=O)OH, -(CH2)I-3CONH2, -(CH2)I-3C(=O)O-, -(CH2)i.3NRa3+,-(CH2)i.3SO2Ra, -CH2CF3, -CH2CCI3, -CH2CBr3, -CH2CI3, -CH2CHF2, -CH2CHCI2, -CH2CHBr2, -CH2CHI2, -CH2(CF2)i-eCFs,- (CH2)i-3ORa, and -(CH2)i-3SRa, wherein each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted Cs-io carbocyclyl; or optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; each RQAis independently selected from -(CH2)I- eNRa3+, — O(CH2)i-eNRa3+, — S(CH2)i-eNRa3+, — NRa(CH2)i-eNRa3+, and — N((CH2)i-eNRa3+)2, wherein each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted Cs-io carbocyclyl; or optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; or a salt, free base, or ion thereof.

10. The method of any one of claims 1 -9, wherein the redox-active species is not:1 1. The method of any one of claims 1-10, wherein step b) produces hydroxide ions and step c) comprises forming inorganic carbonates.

12. The method of any one of claims 1-10, wherein the reduced state reacts with one or more CO2 molecules to form a CO2 adduct.

13. The method of any one of claims 1-12, further comprising electrochemically oxidizing the reduced state to release CO2.

14. The method of claim 1 , wherein the redox-active species is selected from:

15. The method of any one of claims 1 -14, wherein the aqueous solution further comprises a catalyst or enzyme.

16. The method of claim 15, wherein the catalyst comprises Pt, Ir, Pd, Ni, Rh, Ru, Zn, Cu, Fe, or Co, or a combination thereof.

17. The method of claim 15, wherein the catalyst comprises an aminoxyl radical or a phthalimido-N- oxyl radical.

18. The method of claim 17, wherein the aminoxyl radical comprises (2,2,6,6-tetramethylpiperidin-1- yl)oxyl (TEMPO), (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPOL), 1 -hydroxy-2, 2,6,6- tetramethylpiperidine (TEMPOH), 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy, 4-acetamido-2, 2,6,6- tetramethylpiperidine 1-oxyl, 4-carboxy-2,2,6,6-tetramethylpiperidine 1-oxyl, or 4-Amino-2, 2,6,6- tetramethylpiperidine-1-oxyl; and wherein the a phthalimido-N-oxyl radical comprises.

19. The method of claim 15, wherein the catalyst comprises an enzyme, wherein the enzyme is an alcohol dehydrogenase.

20. A method for capturing CO2 comprising: a) providing an electrochemical cell comprising a redox-active species dissolved or dispersed in aqueous solution; b) electrochemically reducing the redox-active species to a reduced state; and c) contacting a mixture of gases comprising CO2 with the aqueous solution; wherein the redox-active species is selected from:wherein X, Y, and Z are independently O, S, N, or NE1; E1, E2, E3, E4, E5, and E6are independently selected from -REWG; -RQA, -H, -ORX, -RX1ORX, -N(RX)(RY), or -RX1N(RX)(RY), wherein at least one of E1, E2, E3, E4, E5, and E6is -ORX, -RX1OR*, -N(RX)(RY), or -RX1N(RX)(RY);Rxand RYare independently selected from H; optionally substituted C1-6 alkyl, provided that the C1-6 alkyl is not substituted with quaternary amine; optionally substituted C3-10 carbocyclyl; optionally substituted C1- 9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; and a nitrogen protecting group; and RX1is optionally substituted C1-6 alkylene;R1, R2, R3, R4, R5, R6, R7and R8are independently selected from REWG; RQA; H; halo; optionally substituted C1-6 alkyl; oxo; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted C6-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; -P(=O)Ra2; and -P(=O)(ORa)2; or any two adjacent groups selected from R1, R2, R3, R4, R5, R6, R7and R8are joined to form an optionally substituted 3-6 membered ring, or an ion thereof, wherein each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted C3-10 carbocyclyl; optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; optionally substituted Ce-20 aryl; optionally substituted C1-9 heteroaryl having one to four heteroatoms independently selected from O, N, and S; an oxygen protecting group; and a nitrogen protecting group; each REWGis independently selected from -NO2, -CN, -SO3Ra, -CHO, -C(=O)Ra, -C(=O)O C(=O)Ra, - C(=O)ORa, -CONH2, -COO-, -NRa3+, -CF3, — SO2Ra, -F, -Cl, -Br, -I, -ORa, -SRa, -P(=O)(ORa)2(CH2)I-3NO2, -(CH2)I-3CN, -(CH2)i-3SO3Ra, -(CH2)I-3CHO, -(CH2)i-3C(=O)Ra, -(CH2)i-3C(=O)OCORa, - (CH2)i-3COORa, -(CH2)I-3C(=O)OH, -(CH2)I-3CONH2, -(CH2)I-3C(=O)O-, -(CH2)i-3NRa3+,-(CH2)i-3SO2Ra, -CH2CF3, -CH2CCI3, -CH2CBr3, -CH2CI3, -CH2CHF2, -CH2CHCI2, -CH2CHBr2, -CH2CHI2, -CH2(CF2)I-eCFs,- (CH2)i-3ORa, and -(CH2)i-3SRa, wherein each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted Cs-io carbocyclyl; or optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; each RQAis independently selected from -(CH2)I- eNRa3+, — O(CH2)i-eNRa3+, — S(CH2)i-eNRa3+, — NRa(CH2)i-eNRa3+, and — N((CH2)i-eNRa3+)2, wherein each Rais independently H; optionally substituted C1-6 alkyl; optionally substituted Cs-io carbocyclyl; or optionally substituted C1-9 heterocyclyl having one to four heteroatoms independently selected from O, N, and S; or a salt, free base, or ion thereof.21 . The method of claim 20, wherein the redox -active species is not:

22. The method of any one of claims 20-21 , wherein step b) produces hydroxide ions and step c) comprises forming inorganic carbonates.

23. The method of any one of claims 20-22, wherein the reduced state reacts with one or more CO2 molecules to form a CO2 adduct.

24. The method of any one of claims 20-23, further comprising electrochemically oxidizing the reduced state to release CO2.

25. The method of any one of claims 20-24, wherein the redox-active species is selected from:any salt or ion thereof.

26. The method of any one of claims 20-25, wherein the aqueous solution further comprises a catalyst or enzyme.

27. The method of claim 26, wherein the catalyst comprises Pt, Ir, Pd, Ni, Rh, Ru, Zn, Cu, Fe, or Co, or a combination thereof.

28. The method of claim 26, wherein the catalyst comprises an aminoxyl radical or a phthalimido-N- oxyl radical.

29. The method of claim 28, wherein the aminoxyl radical comprises (2,2,6,6-tetramethylpiperidin-1- yl)oxyl (TEMPO), (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPOL), 1 -hydroxy-2, 2,6,6- tetramethylpiperidine (TEMPOH), 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy, 4-acetamido-2, 2,6,6- tetramethylpiperidine 1-oxyl, 4-carboxy-2,2,6,6-tetramethylpiperidine 1-oxyl, or 4-Amino-2, 2,6,6- tetramethylpiperidine-1-oxyl; and wherein the a phthalimido-N-oxyl radical comprises.

30. The method of claim 26, wherein the catalyst comprises an enzyme, wherein the enzyme is an alcohol dehydrogenase.

Images