Reversible hydrogen storage

Abstract

The present disclosure provides for systems and methods for reversible storage of hydrogen using an electrolyte, including redox active molecules. Unlike other methods that employ a temperature or pressure swing, the present invention demonstrates hydrogen storage and release utilizing a voltage swing.

Description

[0001] REVERSIBLE HYDROGEN STORAGE

[0002] Background of Invention

[0003] Hydrogen production is the subject of enormous amounts of research and development due to its potential use as an energy carrier in a low-emissions economy. The combustion of hydrogen gas produces water, which has little environmental cost, as a combustion product. Additionally, hydrogen exhibits the highest combustion energy per mass among all chemical fuels at about 40 kWh / kgH2. This energy can either be released as heat via direct combustion or electricity via fuel cells. Apart from its high energy density, hydrogen gas has many other uses, such as electrochemically or thermochemically converting captured CO2 to valuable chemicals, such as hydrocarbon fuels. In the development of an emissions-free economy, hydrogen is so flexible that many envision its ubiquitous use in a so-called “hydrogen economy”.

[0004] One of the major obstacles a hydrogen economy must overcome is the safe and cost-effective storage of hydrogen. Safe containment of hydrogen gas is difficult due to its high diffusivity through plastics and many metals. Such diffusion can cause metal embrittlement, leading to tank leakage or material failure. On the top of that, even without ignition, high-pressure hydrogen leakage can cause spontaneous combustion and explosion. High-pressure hydrogen explosion-related disasters have involved fuel tanks, refueling stations, and power plants. Fatal explosions can happen during large-scale hydrogen storage and transportation, raising the safety concern of hydrogen usage.

[0005] Accordingly, there is a need to develop new systems and methods of storing hydrogen.

[0006] Summary of the Invention

[0007] The present invention relates to new systems and methods for reversible storage of hydrogen in an electrolyte including redox active molecules. The method requires a voltage swing - rather than a temperature or pressure swing - for hydrogen storage and release, enabling a low energy cost for the storage / release cycle.

[0008] In one aspect, the invention provides a system for storing and releasing hydrogen gas. The system includes an electrochemical cell including: (i) a first chamber including a first electrode and being in fluid communication with a hydrogen source; (ii) a second chamber including a second electrode and a liquid electrolyte including a redox active species that undergoes proton-coupled electron transfer; and (iii) an ion exchange membrane (IEM) separating the first and second chambers. In some embodiments, the system further includes a first reservoir in fluid communication with the first chamber and / or a second reservoir in fluid communication with the second chamber.

[0009] In some embodiments, the redox active species is a liquid or dissolved in a liquid. In some embodiments, the redox active species is organic. In some embodiments, the redox active species is selected from the group consisting of benzoquinones, naphthoquinones, anthraquinones, phenazines, alloxazines, isoalloxazines, phenoxazines, and phenothiazines. In some embodiments, the redox active species is inorganic. In some embodiments, the redox active species includes a metal ion that does not plate upon reduction, VO2+, a polyoxometalate, or nitrate / nitrite. In some embodiments, the ion exchange membrane is a cation exchange membrane. In some embodiments, the ion exchange membrane is an anion exchange membrane.

[0010] In some embodiments, the first chamber includes a liquid electrolyte. In some embodiments, the system includes a temperature control element.

[0011] In another aspect, the invention provides a method of storing hydrogen gas by (a) providing a system described herein; and (b) oxidizing hydrogen gas in the first chamber and reducing the redox active species in the second chamber, wherein protons or hydroxide ions traverse between the first chamber and the second chamber, thereby storing hydrogen in a liquid phase. In some embodiments, the method includes the release of stored hydrogen through by oxidizing the redox active species in the reduced state, wherein protons or water is reduced to produce hydrogen gas.

[0012] In some embodiments, the method includes storing the electrolyte in a second reservoir in fluid communication with the second chamber. In some embodiments, the method includes reducing the temperature in the second reservoir to precipitate reduced redox active species.

[0013] In some embodiments, the redox active species is organic. In some embodiments, the redox active species is selected from the group consisting of benzoquinones, naphthoquinones, anthraquinones, phenazines, alloxazines, isoalloxazines, phenoxazines, and phenothiazine. In some embodiments, the redox active species is inorganic. In some embodiments, the redox active species comprises a metal ion that does not plate upon reduction, VO2+, a polyoxometalate, or nitrate / nitrite.

[0014] Definitions

[0015] By “about” is meant ±10% of a recited value.

[0016] By “alkoxy” is meant a group of formula -OR, where R is an alkyl group, as defined herein.

[0017] 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.

[0018] By “alkylene” is meant a divalent alkyl group.

[0019] By “alkyl thio” is meant -SR, where R is an alkyl group, as defined herein.

[0020] By “alkyl ester” is meant -COOR, where R is an alkyl group, as defined herein.

[0021] 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.

[0022] 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. By “halo” is meant, fluoro, chloro, bromo, or iodo.

[0023] By “hydroxyl” is meant -OH. An exemplary ion of hydroxyl is -O'.

[0024] By “amino” is meant -NH2. An exemplary ion of amino is -NHs+.

[0025] By “nitro” is meant -NO2.

[0026] By “carboxyl” is meant -COOH. An exemplary ion of carboxyl is -COO-.

[0027] By “phosphoryl” is meant -PO3H2. Exemplary ions of phosphoryl are -POsH- and -POs2-.

[0028] 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-.

[0029] By “oxo” is meant =0.

[0030] By “sulfonyl” is meant -SO3H. An exemplary ion of sulfonyl is -SOs-.

[0031] By “thiol” is meant -SH.

[0032] 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.

[0033] 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 groups can be substituted by oxo (=0). An exemplary heteroalkylene includes an amido group, e.g., -(CH2)nC(O)NH(CH2)m-, wherein n and m are independently 1-6.

[0034] 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.

[0035] 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). 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-butyldipheny 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.

[0036] 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).

[0037] 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. Exemplary substituents include alkyl or alkoxy groups substituted with carboxyl, phosphonyl, sulfonyl, or amino groups, e.g., -O-(CH2)3-COOH, -O-(CH2)3-PO3H2, -(CH2)-COOH, -(CH2)3-COOH, -(CMe2)-(CH2)-COOH, PEG, e.g., 2-(2-(2-hydroxyethoxy)ethoxy)ethoxy, and (dimethylamino)methyl.

[0038] 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 -PO3H2are -PO3H_and -PO32-; an exemplary ion of -PO3HRais — PO3Ra-, where Rais not H; exemplary ions of -PO4H2are -PC H and -PC2-; and an exemplary ion of — SO3H is — SO3_.

[0039] Brief Description of the Drawings

[0040] FIG. 1 shows a scheme for the hydrogen storage cell of the present invention. Black rods indicate porous carbon paper and spheres represent a dispersed metal (e.g., platinum) catalyst used to activate hydrogen gas.

[0041] FIGs. 2A-B show two schemes for storage and release of hydrogen gas using an electrochemical cell. FIG. 2A shows a scheme for the storage and release of hydrogen gas in acidic conditions (pH < 7) using a CEM. FIG. 2B shows a scheme for the storage and release of hydrogen gas in basic conditions (pH > 7) using an AEM.

[0042] FIG. 3 shows the cell voltage vs. charge / discharge capacity for a cell of the present invention. The upper family of curves occurs during storage (charging); the lower family of curves occurs during release (discharging).

[0043] FIG. 4 shows the open-circuit potential, high frequency impedance, and area specific resistance (ASR) under different state of charge (SOC) for an exemplary cell of the present invention.

[0044] Detailed Description of the Invention

[0045] The present invention relates to systems and methods for reversible storage of hydrogen in an electrolyte including redox active molecules. Unlike other methods that employ a temperature or pressure swing, the present invention demonstrates hydrogen storage and release utilizing a voltage swing.

[0046] System

[0047] Electrochemical cells of the present invention include two chambers - a first chamber and a second chamber, each containing an electrode, an ion exchange membrane (IEM) separating the chambers, a redox active species in the second chamber, and a source of hydrogen in fluid contact with the first chamber (FIG. 1).

[0048] The chambers may be made from any suitable material that is inert to the electrolyte and does not irreversibly degrade during the electrochemical process. Examples include stainless steel, plastics, glass, and ceramics. Other materials are known in the art. Each chamber may be of any suitable volume, e.g., from about 1 mL to about 100 L, from about 10 mL to about 100 L, from about 100 mL to about 100 L, from about 1 L to about 100 L, from about 10 L to about 100 L, from about 20 L to about 100 L, from about 30 L to about 100 L, from about 40 L to about 100 L, from about 50 L to about 100 L, from about 60 L to about 100 L, from about 70 L to about 100 L, from about 80 L to about 100 L, from about 90 L to about 100 L, or about 100 L. In addition to the electrode, the first chamber may hold hydrogen gas or a mixture of hydrogen and another gas. Alternatively, the first chamber may hold a liquid electrolyte, which may be the same or different from the electrolyte in the second chamber. Hydrogen gas may be bubbled into or otherwise dispersed in the liquid electrolyte for reaction.

[0049] Electrodes of the present invention 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)). The electrode may be a metal mesh (e.g., Ti mesh, Cu mesh, Ni Mesh, Mo mesh, or a mesh made of any appropriate metal material described herein). The electrode may be a porous electrode (e.g., porous graphene, Ti foam, Cu foam, Ni foam, Mo foam, or a porous electrode made of any appropriate metal material described herein). Chemical vapor deposition may be used for conformal coatings of complex 3D electrode geometries by ultra-thin electrocatalyst or protective films. Other electrodes are known in the art.

[0050] The electrode in the first chamber includes a material capable of supporting the oxidation of hydrogen gas. In some embodiments, the first chamber electrode may further include a catalyst, e.g., a metal or metal alloy (e.g., platinum, palladium, ruthenium, nickel, molybdenum, copper, cobalt, and alloys thereof), or be a mixed material including a metal (e.g., 10% Pd / C). The catalyst may be loaded onto the first chamber electrode at a concentration from about 0.01 mg / cm2to about 10 mg / cm2, e.g., from about 0.1 to about 9.5 mg / cm2, from about 0.5 mg / cm2to about 9 mg / cm2, from about 1 mg / cm2to about 8 mg / cm2, from about 2 mg / cm2to about 7 mg / cm2, from about 3 mg / cm2to about 6 mg / cm2, about 4 mg / cm2, or about 5 mg / cm2.

[0051] Systems of the present invention may also include reservoirs of redox -active species (e.g., a secondary storage container which allows for a large volume of electrolyte to be available for uptake and release of hydrogen) and hydrogen gas (e.g., a pressurized tank of hydrogen gas as a source and / or pipeline for storage of released hydrogen gas). Multiple reservoirs may be used, e.g., more than one reservoir is in fluid communication with the second chamber by means of a manifold or system of valves. The presence of multiple reservoirs allows for one reservoir to be cleaned or serviced while others are still in operation. The reservoir volume may range from about 1 m3to about 100000 m3, e.g., from about 5 m3to about 10000 m3, from about 10 m3to about 1000 m3, from about 100 m3to about 500 m3, about 200 m3, about 300 m3, about 400 m3, etc. The system may include temperature control elements, e.g., heaters or coolers (such as Peltier, water baths, resistive heaters, air conditioners, heat pumps, etc.). Such elements may be used to maintain a desired temperature in the first and / or second chambers and / or in any external reservoirs. Temperature control elements may be used to modify the solubility of a redox active species (or other component of the electrolyte) to result in precipitation and resuspension of the redox active species (or other component of the electrolyte) in the liquid. For example, the redox active species (or other proton acceptor species) may be precipitated by a reduction in temperature, increasing the amount of stored hydrogen, and then resuspended by an increase in temperature when hydrogen release is desired.

[0052] A system may further include one or more pumps to pump electrolyte past one or both electrodes. Pumps may also be employed to move liquid to and from an external reservoir and a chamber. Alternatively, the chamber may be stirred or have liquids recirculated by any other method, e.g., convection, sonication, etc. A system may also include graphite flow plates and corrosion-resistant metal current collectors. The balance of the system around the cell includes electrical connections and fluid handling and storage; voltage and round-trip energy efficiency measurements can be made. Systems configured for measurement of flows and pH, pressure, temperature, current density and cell voltage may be included and used to evaluate the system. Fluid sample ports can be provided to permit sampling, which will allow for the evaluation of parasitic losses due to, e.g., reactant crossover or side reactions. Electrolytes can be sampled and analyzed with standard techniques. Suitable cells, electrodes, membranes, and pumps for redox flow batteries are known in the art (see, e.g., WO 2014 / 052682, WO 2015 / 048550, WO 2016 / 144909, and WO 2020 / 072406, the battery components of which are hereby incorporated by reference).

[0053] The electrolyte may include any suitable solvent or liquid carrier. In some embodiments, the solvent or liquid carrier is aqueous or other nonflammable liquid, e.g., an ionic liquid. Alternatively, if the redox active species is a liquid at the temperature of operation, no or minimal solvent or liquid carrier (e.g., 0 to 50% v / v, such as 0-25%, 0-10%, or 0-5%) may be employed. The electrolyte includes a redox active species that is reduced when hydrogen gas is oxidized and vice versa. Mixtures of redox active species may also be employed. The reduction of the redox active species may or may not include reaction with protons released upon oxidation of hydrogen gas. If the protons do not react with the redox active species, then the pH of the electrolyte is decreased during storage and increased during release. The half-cell potential of the redox active species determines whether the storage or the release process is thermodynamically spontaneous.

[0054] The electrolyte may include other solutes, e.g., acids (e.g., HCI), bases (e.g., LiOH, NH4OH, NaOH, or KOH), or 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. The redox active species will be present in amounts suitable to operate the system, for example, from 0.1-15 M, or from 0.1-10 M or at the limit of solubility. In some embodiments, the solution is at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, or 80% water, by mass.

[0055] The systems provide for storing hydrogen gas under acidic or alkaline conditions. For acidic aqueous H2 storage, a system as shown in FIG. 2A may be employed. In this system, the first and second chambers are separated by a cation exchange membrane that allows protons to pass. During storage, hydrogen is oxidized to two protons in the first chamber releasing two electrons, and the redox active species is reduced with one or both of the electrons. In certain embodiments, the redox active species also gains the two protons. This process can be reversed for the release of hydrogen. For alkaline aqueous H2 storage, an exemplary system shown in FIG. 2B may be employed. In FIG. 2B, the first and second chambers both include an aqueous electrolyte and are separated by an anion exchange membrane. During storage, hydrogen is oxidized in a process involving hydroxide ions, and the redox active species is reduced. The process can be reversed for the release of hydrogen.

[0056] The systems of the present invention may be further included within a larger infrastructure, e.g., gridscale hydrogen storage. Such infrastructure may include more than one hydrogen storage cell, e.g., more than 2 hydrogen storage cells, more than 3 hydrogen storage cells, more than 4 hydrogen storage cells, more than 5 hydrogen storage cells, more than 10 hydrogen storage cells, more than 20 hydrogen storage cells, more than 30 hydrogen storage cells, more than 40 hydrogen storage cells, more than 50 hydrogen storage cells, more than 60 hydrogen storage cells, more than 70 hydrogen storage cells, more than 80 hydrogen storage cells, more than 90 hydrogen storage cells, more than 100 hydrogen storage cells, more than 1000 hydrogen storage cells, etc. The more than one hydrogen storage cells may be included in one or more electrical circuits. Hydrogen fuel cells in the same electrical circuit may be connected to a source of applied voltage (i.e., a battery) in parallel or in series.

[0057] Typically, the systems of the present invention are designed to be stationary. In some embodiments, the systems of the present invention may be further connected to an electrochemical cell (or combustion source) designed to use hydrogen gas as a fuel source in the generation of electrical power.

[0058] An ion exchange membrane (I EM) is a porous material designed to prevent the passage of non-ionic species through the membrane. An IEM may be a cation exchange membrane (CEM) or an anion exchange membrane (AEM). lEMs are commonly made from polymeric materials with hydrophobic and hydrophilic domains. The hydrophilic domains are typically characterized by acidic groups (e.g., -SO3H, - COOH, PO3H2, etc.) alkyl- or arylammonium groups (e.g., piperidinium), alkyl- or arylphosphonium groups (e.g., triphenylphosphonium), or alkyl- or aryl sulfonium groups (e.g., sulfone) which allow for the shuttling of ions through the hydrophilic domains of the polymer. lEMs of the present invention may have an area of from about 1 cm2to about 1000000 cm2, from about 10 cm2to about 500000 cm2, from about 100 cm2to about 100000 cm2, from about 1000 cm2to about 50000 cm2, from about 5000 cm2to about 10000 cm2, about 6000 cm2, about 7000 cm2, about 8000 cm2, or about 9000 cm2, facing the electrolyte.

[0059] Exemplary CEMs include Nation (i.e., a sulfonated tetrafluoroethylene fluoropolymer), Ultrex CMI 7000 (polystyrene crosslinked with divinyl benzene with numerous sulfonic acid groups), Zirfon (a polysulfone matrix mixed with ZrO2), Hyflon (a copolymer of tetrafluoroethylene and 2,2,4,-trifluoro-5-trifluoromethoxy- 1 ,3-dioxole), heteropolyacids (e.g., polymolybdates, polytungstates, etc.), and gelled acid electrolytes (e.g., borophosphates, borosulfonic acids, etc.). In some embodiments, the cation exchange membrane is soaked in an acid solution (e.g., H2SO4, H3PO4, acetic acid, etc.) prior to use.

[0060] In some embodiments, the CEM is a Nation material. Nation materials are specified by a combination of their equivalent weight (the number of grams of dry Nation material per moles of sulfonic acid functional groups, given as one hundredth of the equivalent weight) and material thickness (in thousandths of an inch). For example, Nation 117 is a Nation material with an equivalent weight of 1100 g / mol and a thickness of 0.007 in, and Nation 212 is a Nation material with an equivalent weight of 2100 g / mol and a thickness of 0.002 in.

[0061] Exemplary AEMs include polymers with cationic functional groups such as quaternary ammoniums (e.g., poly(fluorenyl aryl piperidinium)), imidazolium and benzimidazoliums (e.g., benzimidazolium poly(phenylene oxide)), guanidiniums (e.g., poly(aryl ether sulfone) hexaalkylguanidinium), pyridines (e.g., (poly(vinyl alcohol))-co-(4-formyl-1-methyl-pyridinium benzenesulphonate)), phosphoniums (e.g., tris(2,4,6-trimethoxyphenyl)polysulfone-methylene quaternary phosphonium hydroxides), sulfoniums (e.g., poly(ether sulfone) hydroxide), and metal ions (e.g., dicyclopentadiene-co-bis(4’-(2-oxo-noborene)- terpyridine ruthenium(ll)).

[0062] Any suitable redox active species may be employed in the systems of the invention. Organic and inorganic redox active species may be employed. The redox active species may undergo proton-coupled electron transfer (PCET) during oxidation or reduction. The redox active species may be selected to have a reduction potential near the reduction potential of hydrogen ions (0 V, relative to the standard hydrogen electrode). This allows for a low voltage required to operate.

[0063] Organic species amenable to use as redox active species in the systems of the present invention include, but are not limited to, benzoquinones, naphthoquinones, anthraquinones, phenazines (e.g., 7,8- dihydroxyphenazine-2-sulfonic acid), alloxazines, isoalloxazines, phenoxazines, phenothiazine, or any water soluble organic molecule that undergoes PCET.

[0064] In some embodiments, the redox active species is a quinone. Exemplary quinones are neutral molecules of general formulas (la), (lb), (Ic), (Id), (le), (If), (Ig), or (Ih).

[0065] (la) (lb) (Ic) where each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from 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; -CN; -NO2; -ORa(e.g., hydroxyl or C1-6 alkoxy); -SRa(e.g., thiol or C1-6 alkyl thio); -N(Ra)2 (e.g., amino); -C(=O)Ra; -C(=O)ORa(e.g., carboxyl); -S(=O)2Ra; -S(=O)2ORa(e.g., SO3H); -P(=O)Ra2; and -P(=O)(ORa)2 (e.g., phosphonyl or phosphoryl); or any two adjacent groups selected from R1, R2, R3, and R4are joined to form an optionally substituted 3-6 membered ring, 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.

[0066] In certain embodiments, each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from H, optionally substituted C1-6 alkyl, halo, hydroxyl, optionally substituted C1-6 alkoxy, SO3H, amino, nitro, carboxyl, phosphoryl, phosphonyl, and oxo, or an ion thereof. In particular embodiments, each of R1, R2, R3, R4, R5, R6, R7, and R8is independently selected from H, hydroxyl, optionally substituted C1-4 alkyl, carboxyl, and SO3H, such as each of R1, R2, R3, R4, R5, R6, R7and R8being independently selected from H, hydroxyl, optionally substituted C1-4 alkyl (e.g., methyl), and oxo. In some embodiments, at least one, e.g., at least two, of R1, R2, R3, R4, R5, R6, R7, and R8is not H.

[0067] In some embodiments, the redox active species is a naphthoquinone. Exemplary naphthoquinones are e.g., of formula (II):

[0068] or a salt thereof, wherein the dashed bonds are single or double bonds; where either W and X, W and Z, or Z and Y are C=O, and where the two of W, X, Y, or Z that are not C=O are independently selected from C-R, where R is H; halo; 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; -CN; -NO2; -ORa(e.g., hydroxyl or C1-6 alkoxy); -SRa(e.g., thiol or C1-6 alkyl thio); -N(Ra)2 (e.g., amino); -C(=O)Ra; -C(=O)ORa(e.g., carboxyl); -S(=O)2Ra; -S(=O)2ORa(e.g., SO3H); - P(=O)Ra2; and -P(=O)(ORa)2 (e.g., phosphonyl or phosphoryl); or any two adjacent R groups are joined to form an optionally substituted non-aromatic 3-6 membered ring, 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; where each of R1, R2, R3, and R4is independently selected from 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; -CN; -NO2; -ORa(e.g., hydroxyl or C1-6 alkoxy); -SRa(e.g., thiol or C1-6 alkyl thio); -N(Ra)2 (e.g., amino); -C(=O)Ra; -C(=O)ORa(e.g., carboxyl); - S(=O)2Ra; -S(=O)2ORa(e.g., SO3H); -P(=O)Ra2; and -P(=O)(ORa)2 (e.g., phosphonyl or phosphoryl); or any two adjacent groups selected from R1, R2, R3, and R4are joined to form an optionally substituted 3-6 membered ring, 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.

[0069] In certain embodiments, each of R1, R2, R3, and R4is independently selected from H, optionally substituted C1-6 alkyl, halo, hydroxyl, optionally substituted C1-6 alkoxy, SO3H, amino, nitro, carboxyl, phosphoryl, phosphonyl, and oxo, or an ion thereof. In particular embodiments, each of R1, R2, R3, and R4 is independently selected from H, hydroxyl, optionally substituted C1-4 alkyl, carboxyl, and SO3H, such as each of R1, R2, R3, and R4being independently selected from H, hydroxyl, optionally substituted C1-4 alkyl (e.g., methyl), and oxo. In embodiments, at least one, e.g., at least two, of R1, R2, R3, and R4is not H. In some embodiments, at least one of R1-R4is a substituted alkyl or substituted alkoxy. Ions and reduced species thereof are also contemplated.

[0070] In other embodiments, the quinone, such as a 9,10-anthraquinone, is substituted with at least one hydroxyl group and optionally further substituted with an optionally substituted C1-4 alkyl, such as methyl. Exemplary quinones include 2,6-dihydroxy-9,10-anthraquinone (2,6-DHAQ), 1 ,5-dimethyl-2,6-dihydroxy- 9,10-anthraquinone, 2,3,6,7-tetrahydroxy-9,10-anthraquinone, 1 ,3,5,7-tetrahydroxy-2,4,6,8-tetramethyl- 9,10-anthraquinone, and 2, 7-dihydroxy-1 ,8-dimethyl-9,10-anthraquinone. Ions and reduced species thereof are also contemplated. Quinones having one or more hydroxyl groups may include at least one carboxy C1-4 alkyl (e.g., carboxymethyl) group. An exemplary quinone for use in the system and methods of the invention is 1 ,8-dihydroxy-2,7-dicarboxymethyl-9,10-anthraquinone or a salt thereof.

[0071] Other exemplary quinones are:

[0072] Further exemplary quinones are known in the art (see, e.g., Baclig et al, Large Decrease in Melting Point of Benzoquinones via High-n Eutectic Mixing Predicted by a Regular Solution Model, ChemRxiv.

[0073] Cambridge: Cambridge Open Engage; 2022, hereby incorporated by reference). In some embodiments, the redox active species is a phenazine. Exemplary phenazines are of formula (HI): or a salt thereof, where each of R1, R2, R3, R4, R5, R6, R7, and R8is independently selected from 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; -CN; -NO2; -ORa(e.g., hydroxyl or C1-6 alkoxy); -SRa(e.g., thiol or C1-6 alkyl thio); -N(Ra)2 (e.g., amino); -C(=O)Ra; -C(=O)ORa(e.g., carboxyl); -S(=O)2Ra; -S(=O)2ORa(e.g., SO3H); -P(=O)Ra2; and -P(=O)(ORa)2 (e.g., phosphonyl or phosphoryl); or any two adjacent groups selected from R1, R2, R3, and R4are joined to form an optionally substituted 3-6 membered ring, 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.

[0074] In certain embodiments, each of R1, R2, R3, R4, R5, R6, R7, and R8is independently selected from H, optionally substituted C1-6 alkyl, halo, hydroxyl, optionally substituted C1-6 alkoxy, SO3H, amino, nitro, carboxyl, phosphoryl, phosphonyl, and oxo, or an ion thereof. In particular embodiments, each of R1, R2, R3, R4, R5, R6, R7, and R8is independently selected from H, hydroxyl, optionally substituted C1-4 alkyl, carboxyl, and SO3H, such as each of R1, R2, R3, R4, R5, R6, R7and R8being independently selected from H, hydroxyl, optionally substituted C1-4 alkyl (e.g., methyl), and oxo. In embodiments, at least one, e.g., at least two, of R1, R2, R3, R4, R5, R6, R7, and R8is not H. In some embodiments, at least one of Ri-Ra is a substituted alky or substituted alkoxy.

[0075] Exemplary phenazines include, e.g., 2,3-dihydroxy-phenazine-6-sulfonic acid, 3,3'-(phenazine-2,3- diylbis(oxy))bis(propane-1 -sulfonic acid), 2,3-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenazine, and 7,8-dihydroxyphenazine-2-sulfonic acid. Ions and reduced species thereof are also contemplated. In some embodiments, the redox active species is a phenoxazine or a phenothiazine. Exemplary phenoxazines and phenothiazines are of, e.g., formula (IV): or a salt thereof, where dashed bonds are single or double bonds; where X is N or NRX, Y is O or S, and Z is CR6, C=O, C=S, C=NRZ, or C=NH+Rz; where Rxis selected from 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; or a nitrogen protecting group, where Rzis selected from 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; or a nitrogen protecting group, where each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from 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; -CN; -NO2; -ORa(e.g., hydroxyl or C1-6 alkoxy); -SRa(e.g., thiol or C1-6 alkyl thio); -N(Ra)2 (e.g., amino); -C(=O)Ra; -C(=O)ORa(e.g., carboxyl); -S(=O)2Ra; -S(=O)2ORa(e.g., SO3H); - P(=O)Ra2; and -P(=O)(ORa)2 (e.g., phosphonyl or phosphoryl); or any two adjacent groups selected from R1, R2, R3, and R4are joined to form an optionally substituted 3-6 membered ring, 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.

[0076] In certain embodiments, each of R1, R2, R3, R4, R5, R6, R7and R8is independently selected from H, optionally substituted C1-6 alkyl, halo, hydroxyl, optionally substituted C1-6 alkoxy, SO3H, amino, nitro, carboxyl, phosphoryl, phosphonyl, and oxo, or an ion thereof. In particular embodiments, each of R1, R2, R3, R4, R5, R6, R7, and R8is independently selected from H, hydroxyl, optionally substituted C1-4 alkyl, carboxyl, and SO3H, such as each of R1, R2, R3, R4, R5, R6, R7and R8being independently selected from H, hydroxyl, optionally substituted C1-4 alkyl (e.g., methyl), and oxo. In embodiments, at least one, e.g., at least two, of R1, R2, R3, R4, R5, R6, R7, and R8is not H. In some embodiments, at least one of R1-R8is a substituted alky or substituted alkoxy.

[0077] In some embodiments, the compound is an alloxazine of formula (V) or isoalloxane of formula (VI): wherein each of R9and R10is independently H; optionally substituted C1-10 alkyl (e.g., C1-6 alkyl, unsubstituted C1-10 alkyl, or unsubstituted 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; -C(=O)Ra; and -C(=O)ORa; and each of R1, R2, R3, and R4is independently H; C1-10 alkyl (e.g., 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; -NO2; -ORa; -SRa; -N(Ra)2; -C(=O)Ra; - C(=O)ORa; -S(=O)2Ra; -S(=O)2ORa; -P(=O)Ra2; and -P(=O)(ORa)2; or any two adjacent groups selected from R1, R2, R3, and R4are joined to form an optionally substituted 3-6 membered ring, or an ion thereof; wherein each Rais independently H; C1-10 alkyl (e.g., 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.

[0078] In some embodiments, each of R9and R10is independently H, optionally substituted C1-10 alkyl (e.g., C1-6 alkyl), or -C(=O)ORa; and each of R1, R2, R3, and R4is independently H, halo, optionally substituted C1-10 alkyl (e.g., C1-6 alkyl), -NO2, -ORa, -SRa; -N(Ra)2, -C(=O)ORa, -S(=O)2ORa, -P(=O)Ra2or -P(=O)(ORa)2; wherein each Rais independently H or optionally substituted C1-10 alkyl (e.g., C1-6 alkyl). In some embodiments, none of, any two of, any three of, any four of, any five of, or any six of R1, R2, R3, R4, R9, and R10are H.

[0079] In some embodiments, the compound is a polymer e.g., dimer or trimer, of formula (VII) or (VIII): wherein n is an integer from 2 to 40; wherein R is a substituent that increase the aqueous solubility of the polymer, e.g., -OH, -COOH, -SO3H, -N(Ra)2, and -P(=O)(ORa)2, where at least one Rais H and other groups known in the art; and wherein other groups are as described herein. In preferred embodiments, one or both of R9and R10are H.

[0080] Organic species, e.g., quinones, may be present in a mixture. An organic species of the invention is a source of electrons during release and not merely a charge transfer agent. In embodiments, the organic species is water soluble. In some embodiments, the redox active species is a polymer, e.g., a polymer including a redox active species described herein, e.g., a polyquinone.

[0081] Organic species may be, e.g., those described in WO 2019 / 157437, WO 2018 / 032003, WO 2019 / 136374, WO2016 / 144909, WO2014 / 052682, and WO 2015 / 048550, which are hereby incorporated by reference.

[0082] In some embodiments, the redox active species is inorganic, e.g., a metal ion that does not plate during reduction, e.g., a molecular metal oxide (e.g., VO22+ / VO2+and salts thereof), a polyoxometalate (POM, e.g., [SiWi204o]4-,[PVi4042]9-, SiVv3Wvl904o7- / SiVlv3Wvl904o1°- and SiVlv3Wvl9O4010- / SiVlv3Wv3Wvl6O4013-), or nitrate (NO3) / nitrite (NO2 ).

[0083] In some embodiments, the first chamber is pressurized with hydrogen. In some embodiments, the first chamber is pressurized to 1 or more atm of hydrogen, e.g., 1 atm, 5 atm, 10 atm, 20 atm, 30 atm, 40 atm, 50 atm, 100 atm, 150 atm, 200 atm, 250 atm, etc.

[0084] In some embodiments, the source of hydrogen gas is a compressed form, e.g., a gas cylinder or gas tank. A gas cylinder or tank may be compressed to a pressure from about 50 atm to about 700 atm. Hydrogen may also be delivered to the first chamber directly from a source of synthesis, e.g., from electrolysis of water or chemical reaction.

[0085] In some embodiments hydrogen may be provided in a mixture of hydrogen gas and an inert gas (e.g., nitrogen, helium, krypton, argon, etc.). In some embodiments, the mixture is at least 5% of the inert gas. Methods of Storing and Releasing Hydrogen Gas

[0086] The invention provides hydrogen storage and release. Methods of the invention may include providing a source of hydrogen gas to a system as described herein. Under acidic conditions, during storage, the hydrogen gas is oxidized in the first chamber into hydrogen ions, and the redox active species is reduced in the second chamber. Under basic conditions, during storage, the hydrogen gas is oxidized in the first chamber into hydrogen ions, which combine with hydroxide ions to form water, and the redox active species is reduced in the second chamber. Depending on the nature of the redox active species and the electrolyte, hydrogen or hydroxide ions cross the I EM between the first and the second chamber for storage (e.g., via reaction with the redox active species or by increase or decrease in electrolyte pH). For example, under acidic conditions, hydrogen ions pass from the first chamber to the second chamber during storage, and, under basic conditions, hydroxide ions pass from the second chamber to the first chamber during storage. During release of hydrogen gas, the process is reversed, resulting in the migration of hydrogen or hydroxide ions across the IEM between the first and second chambers, with subsequent production of hydrogen gas. This process may be repeated over multiple cycles. The release of hydrogen may occur on demand.

[0087] Depending on the potential of the redox active species relative to hydrogen, a voltage may be applied during storage and / or release. Such a voltage may be applied as an overpotential. In some embodiments, the absolute value of the potential is less than 2 V per cell, e.g., less than 1 .9 V per cell, less than 1 .8 V per cell, less than 1 .7 V per cell, less than 1 .6 V per cell, less than 1 .5 V per cell, less than 1 .4 V per cell, less than 1.3 V per cell, less than 1 .2 V per cell, less than 1.1 V per cell, less than 1 .0 V per cell, less than 0.9 V per cell, less than 0.8 V per cell, less than 0.7 V per cell, less than 0.6 V per cell, less than 0.5 V per cell, less than 0.4 V per cell, less than 0.3 V per cell, etc. In some embodiments an overpotential may be applied to the cell to increase the rate of hydrogen storage or release. In some embodiments, the rate of hydrogen storage and / or release is greater than 0.0001 mol / cm2 / h, greater than 0.001 mol / cm2 / h, greater than 0.01 mol / cm2 / h, greater than 0.1 mol / cm2 / h, greater than, 0.2 mol / cm2 / h etc.

[0088] Storage and release may occur at ambient temperature or under temperature control. The electrochemical reactions may also take place at one temperature, while external reservoirs are held at different temperatures. For example, an external reservoir may be held at a lower temperature than the second chamber to precipitate redox active species (or other proton reactive species) from the electrolyte. For example, the temperature may reduce the solubility of the species by at least 10%, e.g., about 10 %, about 20%, about 30%, about 40%, about 50%, about 60%, about 70%, about 80%, or about 90%.

[0089] Examples

[0090] Hydrogen storage and release under various current densities is shown in FIG. 3. These experiments were performed with an electrochemical cell containing: (1) an second chamber which includes 0.1 M 9,10-anthraquinone-2,6-disulfonic acid (2,6-AQDS), further supported by 0.5 M sulfuric acid; (2) a Nation 212 cation exchange membrane treated with boiling in 0.1 M sulfuric acid for 1 h, followed by boiling for 1 h in a 3% hydrogen peroxide solution; and (3) a first chamber which includes a hydrophilic carbon paper electrode coated with 5 mg / cm2of 10% Pt / C and 1 atm of hydrogen gas. The experiments were performed at room temperature under 1 atm of pressure.

[0091] Only small voltages need to be applied during both steps. Even with the highest current density at 200 mA / cm2, the energy cost for the whole cycle is < 7 kWh / kgH2 (< 18% of the energy content per kg of H2 released during combustion).

[0092] The impedance of a 5 cm2cell is shown in FIG. 4. The open circuit potential of the hydrogen storage process ranges from -0.28 V to -0.17 V, from 0 % to 90 % state of charge (SOC). The polarization area specific resistance (ASR) measurements show an impedance of ~ 1.5 fi cm2and high frequency ASR at ~ 1 .0 Q cm2. These low impedance values, coupled with the high reaction reversibility, guarantee the low energy cost for this system.

[0093] In this example, the hydrogen storage density is limited by the solubility of quinone species. Even if a mixture of saturated 2,6-AQDS and 2,7-AQDS is used, the volumetric density of hydrogen stored is equivalent to 50 atm at room temperature. However, if the redox-active species can be precipitated out from the solution, which can likely be achieved by a temperature difference between cell operation area and storage tanks, about 250-atm-equivalent hydrogen storage density can be achieved.

[0094] Other embodiments are in the claims.

Claims

What is claimed is:Claims1 . A system for storing and releasing hydrogen gas comprising an electrochemical cell comprising:(i) a first chamber comprising a first electrode and being in fluid communication with a hydrogen source;(ii) a second chamber comprising a second electrode and a liquid electrolyte comprising a redox active species that undergoes proton-coupled electron transfer; and(iii) an ion exchange membrane (IEM) separating the first and second chambers.

2. The system of claim 1 , further comprising a first reservoir in fluid communication with the first chamber and / or a second reservoir in fluid communication with the second chamber.

3. The system of claim 1 , wherein the redox active species is a liquid or dissolved in a liquid.

4. The system of claim 1 , wherein the redox active species is organic.

5. The system of claim 4, wherein the redox active species is selected from the group consisting of benzoquinones, naphthoquinones, anthraquinones, phenazines, alloxazines, isoalloxazines, phenoxazines, and phenothiazines.

6. The system of claim 1 , wherein the redox active species is inorganic.

7. The system of claim 6, wherein the redox active species comprises a metal ion that does not plate upon reduction, VO2+, a polyoxometalate, or nitrate / nitrite.

8. The system of claim 1 , wherein the ion exchange membrane is a cation exchange membrane.

9. The system of claim 1 , wherein the ion exchange membrane is an anion exchange membrane.

10. The system of claim 1 , wherein the first chamber further comprises a liquid electrolyte.11 . The system of claim 1 , further comprising a temperature control element.

12. A method of storing hydrogen gas comprising (a) providing a system of any one of claims 1-11 ; and (b) oxidizing hydrogen gas in the first chamber and reducing the redox active species in the second chamber, wherein protons or hydroxide ions traverse between the first chamber and the second chamber, thereby storing hydrogen in a liquid phase.

13. The method of claim 12, further comprising the release of stored hydrogen through the steps of oxidizing the redox active species in the reduced state, wherein protons or water is reduced to produce hydrogen gas.

14. The method of claim 12, further comprising storing the electrolyte in a second reservoir in fluid communication with the second chamber.

15. The method of claim 14, further comprising reducing the temperature in the second reservoir to precipitate reduced redox active species.

16. The method of claim 12, wherein the redox active species is organic.

17. The method of claim 16, wherein the redox active species is selected from the group consisting of benzoquinones, naphthoquinones, anthraquinones, phenazines, alloxazines, isoalloxazines, phenoxazines, and phenothiazine.

18. The method of claim 12, wherein the redox active species is inorganic.

19. The method of claim 18, wherein the redox active species comprises a metal ion that does not plate upon reduction, VO2+, a polyoxometalate, or nitrate / nitrite.

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