Method for continuously performing electrochemical nitrogen reduction

Abstract

The present invention provides a method for continuous electrochemical nitrogen reduction to produce ammonia. The method includes supplying nitrogen to an electrochemical cell containing an electrolyte in contact with at least a cathode, introducing protons into the electrolyte by anodic oxidation of a hydrogen-containing species, and cathodic reduction of the nitrogen to produce ammonia in the presence of a metal selected from lithium, magnesium, calcium, strontium, barium, zinc, aluminum, and vanadium. The electrolyte includes a cationic proton carrier capable of reversibly deprotonating to form a neutral proton acceptor. The neutral proton acceptor is a ylide.

Description

Technical Field

[0001] The present invention relates to a method for electrochemically and continuously reducing nitrogen to produce ammonia. This method includes supplying nitrogen to an electrochemical cell containing an electrolyte that at least contacts a cathode, introducing protons into the electrolyte by anodic oxidation of a hydrogen-containing species, and cathodically reducing nitrogen in the presence of a specific metal to produce ammonia. The electrolyte includes a cationic proton carrier capable of forming a neutral proton acceptor by reversible deprotonation. The present invention further relates to a liquid electrolyte for an electrochemical reduction reaction, the use of an electrolyte for a reduction reaction in an electrochemical cell, and a system for a continuous electrochemical reduction reaction including an electrolyte in an electrochemical cell.

Background Art

[0002] Supplying sufficient food and energy to meet the demands of the rapidly growing world population is an ongoing challenge for humanity. New technologies for fixing nitrogen (N2) to produce ammonia (NH3) offer potential solutions to these two challenges. Synthetic ammonia-based fertilizers are already essential for global food production, and the high energy density of NH3 offers great potential for use as a transportable fuel and a carrier for renewable energy.

[0003] The Haber-Bosch process, invented in the 20th century, provided the first industrial route for mass-producing synthetic ammonia. However, due to the very stable triple bond of nitrogen (N≡N, 942 kJ / mol), the Haber-Bosch process requires extreme reaction conditions of high pressure (150 - 350 atm) and high temperature (400 - 550 °C), and a supply of pure H2 usually supplied from a steam reforming process of natural gas. As a result, this process consumes about 2% of the world's energy supply and accounts for ~1.5% of the world's greenhouse gas emissions. Therefore, there is an urgent need for a conversion technology from N2 to NH3 that can be driven by sustainable resources.

[0004] If the development of an electrochemical nitrogen reduction reaction (NRR) process is successful, it will be possible to directly convert renewable power into NH3 using a simple electrolytic cell. The cathodic half-reaction of NRR is shown in Equation (1). N2+6H + +6e - →2NH3(1)

[0005] The protons required for NRR can be supplied not by relying on steam-reformed H2, but by H2 generated from the anodic oxidation of water (oxygen evolution reaction) or a sustainable water splitting process. Unfortunately, the NRR of 6e - and 6H + is kinetically slow, so it is electrochemically less favorable than the hydrogen evolution reaction (HER) of 2e - and 2H + shown in Equation (2). As a result of the competition with HER, in many reported electrochemical syntheses of NH3, very low Faradaic efficiency and low NH3 yield have been problems. 2H + +2e - →H2(2)

[0006] One approach to address this problem is disclosed in DE102018210304. Suitable metals including lithium, magnesium, calcium, strontium, barium, zinc, aluminum, and vanadium are used to form the corresponding nitrides. First, the metal in metallic form is formed, preferably in its liquid state, by electrolyzing a molten salt containing metal ions at high temperature, and then reacted with N2 to form a metal nitride. When the formation of the nitride is complete, the nitride is separated and introduced into the anodic compartment of the electrochemical cell to generate protons and finally ammonia. Since the metal nitride needs to be operated between separate process environments, it is a complex multi-step process and inefficient in terms of equipment investment and energy.

[0007] Another approach developed in the past is the continuous electrochemical ammonia synthesis via lithium, as reported by Tsuneto et al. in Chemistry Letters, 1993, 851 - 854. In a typical continuous electrochemical NH3 synthesis reaction via lithium, the electrolyte system contains a lithium salt such as lithium triflate (LiOtf), lithium perchlorate (LiClO4), or lithium tetrafluoroborate (LiBF4) in an organic solvent such as tetrahydrofuran, and a proton carrier (or proton donor). The proposed mechanism of this reaction is shown in Fig. 1 for the case where the proton source is the anodic oxidation of H2. At the cathode 102, lithium cations (Li + ) are reduced to metallic lithium (Li), which spontaneously reacts with nitrogen (N2) to form lithium nitride (Li3N). Subsequently, Li3N is protonated by the proton carrier (BH) present in the electrolyte to produce ammonia and the deprotonated proton carrier (B), and the lithium cations are regenerated. At the anode 104, protons (H + ) are generated by the anodic oxidation of H2. These protons protonate B in the electrolyte to regenerate the proton carrier (BH). In this way, the reaction cycle is completed. Since protons are only indirectly involved in the nitrogen reduction reaction, the competition with HER is expected to be minimized.

[0008] The proton carrier molecule (BH) needs to be highly reactive with Li3N to produce NH3, but ideally should be weakly acidic to reduce the rate of proton reduction to competing hydrogen and / or hydrides. Currently, the most common proton carriers used in this process are ethanol and similar alcohols (such as methanol, isopropyl alcohol, etc.). Thus, the proton carrier (BH) is usually a neutral species such as CH3CH2OH, and its deprotonated form B is an anionic species such as CH3CH2O - . Water is highly acidic and H + is at a more positive potential than the reduction of Li 2Since it is reduced, it is generally not suitable as a proton carrier.

[0009] A major problem in electrochemical synthesis reactions using lithium as a medium is to effectively regenerate alcohol-based proton carriers such as ethanol, which has not been experimentally verified so far. Generally, when alcohols, which are more expensive than ammonia, are not sufficiently recycled, they are consumed uneconomically as the proton source for synthesis. The anodic decomposition of alcohol is recognized as a parasitic process contributing to ethanol consumption in the cell.

[0010] Another problem is that after the proton carrier is deprotonated at the cathode, the anion species precipitates as an insoluble salt together with Li + and an insoluble by-product is formed. When these substances precipitate, the internal resistance of the cell rapidly increases, and the cell voltage required to drive the desired reaction may increase. The anions generated by the deprotonation of alcohols such as ethanol are very strong bases and strongly interact with Li + cations to form salts with low solubility.

[0011] Therefore, in continuous electrochemical nitrogen reduction for producing ammonia, there is a continuing need for new methods that at least partially solve one or more of the above problems or provide useful alternatives.

[0012] References to patent documents or other matters presented as prior art in this specification should not be construed as an admission that the document or matter was known, or that the information it contains was part of common general knowledge, at the priority date of any claim.

Summary of the Invention

[0013] According to a first aspect, the present invention provides a method for continuous electrochemical nitrogen reduction for producing ammonia. The method includes supplying nitrogen to an electrochemical cell including an electrolyte that at least contacts a cathode, introducing protons into the electrolyte by anodic oxidation of a hydrogen-containing species, and cathodically reducing nitrogen in the presence of a metal selected from lithium, magnesium, calcium, strontium, barium, zinc, aluminum, and vanadium to produce ammonia. The electrolyte includes a cationic proton carrier capable of forming a neutral proton acceptor by reversible deprotonation. The neutral proton acceptor is an iridate.

[0014] In some embodiments, cathodically reducing nitrogen includes reacting nitrogen with a cationic proton carrier to produce ammonia and form a neutral proton acceptor. The cationic proton carrier may be regenerated in the electrolyte by protonating the neutral proton acceptor with a proton.

[0015] Reacting nitrogen with a cationic proton carrier may include (i) reacting nitrogen with a metal to form a metal nitride, and (ii) reacting the metal nitride with a cationic proton carrier to produce ammonia and form a neutral proton acceptor.

[0016] In some embodiments, the metal exists as a metal cation in the electrolyte.

[0017] In some embodiments, the metal is lithium.

[0018] In some embodiments, the iridate includes a carbanion adjacent to a heteroatom selected from the group consisting of phosphorus, nitrogen, sulfur, and oxygen having a positive charge.

[0019] In some embodiments, the neutral proton acceptor is selected from phosphonium ylides and sulfonium ylides. In some embodiments, the neutral proton acceptor is a phosphonium ylide.

[0020] In some embodiments, the cationic proton carrier is selected from alkylphosphonium cations and alkylsulfonium cations. In some embodiments, the cationic proton carrier is an alkylphosphonium cation. The alkylphosphonium cation may be a tetraalkylphosphonium cation. The tetraalkylphosphonium cation may be in the form of, for example, [PR 6 R 7 R 8 R 9 + wherein R 6 R 7 R 8 R 9 are independently selected from C1-C 20 n-alkyl groups.

[0021] In some embodiments, the cationic proton carrier can be deprotonated upon contact with lithium nitride to form a neutral proton acceptor.

[0022] In some embodiments, the electrolyte is a non-aqueous liquid electrolyte. The non-aqueous liquid electrolyte may contain one or more molecular solvents. The molecular solvent may be selected from the group consisting of ethers, methylated polyethers, methylated glycol ethers, fluorinated ethers, fluorinated alkyls, fluorinated cycloalkyls, carbonates, sulfolanes, and dimethyl sulfoxide. In addition or alternatively, the non-aqueous liquid electrolyte may contain, for example, a room temperature ionic liquid solvent in an amount of at least 50% by weight of the total solvent in the electrolyte.

[0023] In some embodiments, the cationic proton carrier and the neutral proton acceptor are soluble in the electrolyte.

[0024] ​ In some embodiments, the cationic proton carrier is charge balanced in the electrolyte by one or more electrochemically stable anions. The one or more electrochemically stable anions may be selected from the group consisting of tetrafluoroborate ions, hexafluorophosphate ions, chloride ions, perchlorate ions, fluoroalkyl phosphate ions such as tris(pentafluoroethyl)trifluorophosphate ions, fluoroaryl borate ions such as tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ions and tetrakis(pentafluorophenyl)borate ions, fluoroalkyl borate ions such as tetrakis[hexafluoroisopropyl]borate ions, bis(fluorosulfonyl)imide, bis(trifluoromethanesulfonyl)imide, and fluorinated bis(sulfonyl)imides such as (fluorosulfonyl)-(trifluoromethanesulfonyl)imide, and fluorinated sulfonate ions such as trifluoromethanesulfonate ions and other perfluoroalkylsulfonate ions.

[0025] In some embodiments, the cationic proton carrier and the neutral proton acceptor are present in the electrolyte at a combined concentration higher than 0.001 mol / L, or higher than 0.01 mol / L, or in the range between 0.1 mol / L and 4 mol / L.

[0026] In some embodiments, the hydrogen-containing species are selected from hydrogen and water.

[0027] In some embodiments, the method includes supplying nitrogen to the electrochemical cell at a partial pressure higher than 1 bar, or higher than 5 bar, preferably higher than 10 bar.

[0028] In some embodiments, the cathode has a potential lower (more negative) than -2.0 V (vs Ag / Ag + ) when reducing nitrogen. This potential may be lower than the reduction potential of the metal, i.e., the potential for the reduction of its cation to the metallic form.

[0029] According to a second aspect, the present invention provides a method for continuous electrochemical nitrogen reduction for producing ammonia. The method includes supplying nitrogen to an electrochemical cell including an electrolyte that contacts at least a cathode, introducing protons into the electrolyte by anodic oxidation of a hydrogen-containing species, and reducing nitrogen at the cathode in the presence of lithium to produce ammonia. The electrolyte includes a cationic proton carrier capable of forming a neutral proton acceptor by reversible deprotonation. The cationic proton carrier is a phosphonium cation, and the neutral proton acceptor is a phosphonium ylide.

[0030] According to a third aspect, the present invention provides a method for continuous electrochemical nitrogen reduction for producing ammonia. The method includes supplying nitrogen to an electrochemical cell including an electrolyte that contacts at least a cathode, introducing protons into the electrolyte by anodic oxidation of a hydrogen-containing species, and reducing nitrogen at the cathode in the presence of a metal selected from lithium, magnesium, calcium, strontium, barium, zinc, aluminum, and vanadium to produce ammonia. The electrolyte includes at least one selected from alkylphosphonium cations and alkylsulfonium cations.

[0031] In some embodiments, the electrolyte includes an alkylphosphonium cation. The alkylphosphonium cation may be a tetraalkylphosphonium cation. The tetraalkylphosphonium cation may be, for example, in the form of [PR 6 R 7 R 8 R 9 + . Here, R 6 , R 7 , R 8 , R 9 are independently selected from C1 to C 20 n-alkyl groups. In some embodiments, the alkylphosphonium cation is capable of deprotonating upon contact with lithium nitride to form a ylide. ​

[0032] The features related to the first aspect of the present invention are also applicable to the second and third aspects.

[0033] According to a fourth aspect, the present invention provides a liquid electrolyte for an electrochemical reduction reaction. This liquid electrolyte comprises (i) a neutral proton acceptor that is an ylide, (ii) a cationic proton carrier capable of forming the neutral proton acceptor by reversible deprotonation, and (iii) a non-aqueous solvent.

[0034] In some embodiments, the liquid electrolyte further comprises (iv) a metal cation selected from lithium, magnesium, calcium, strontium, barium, zinc, aluminum, and vanadium.

[0035] In some embodiments, the ylide comprises a carbanion adjacent to a heteroatom selected from the group consisting of phosphorus, nitrogen, sulfur, and oxygen, which has a positive charge.

[0036] In some embodiments, the neutral proton acceptor is selected from phosphonium ylides and sulfonium ylides. In some embodiments, the neutral proton acceptor is a phosphonium ylide.

[0037] In some embodiments, the cationic proton carrier is selected from alkylphosphonium cations and alkylsulfonium cations. In some embodiments, the cationic proton carrier is an alkylphosphonium cation. The alkylphosphonium cation may be a tetraalkylphosphonium cation. The tetraalkylphosphonium cation may be in the form of, for example, [PR 6 R 7 R 8 R 9 + wherein R 6 , R 7 , R 8 , R 9 are C1 to C 20 ​It is independently selected from n-alkyl groups.

[0038] In some embodiments, the cationic proton carrier is charge balanced in the liquid electrolyte by one or more electrochemically stable anions. The one or more electrochemically stable anions may be selected from the group consisting of tetrafluoroborate ions, hexafluorophosphate ions, chloride ions, perchlorate ions, fluoroalkyl phosphate ions including tris(pentafluoroethyl)trifluorophosphate ions, fluoroarylborate ions including tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ions and tetrakis(pentafluorophenyl)borate ions, fluoroalkylborate ions including tetrakis[hexafluoroisopropyl]borate ions, bis(fluorosulfonyl)imide, bis(trifluoromethanesulfonyl)imide, and fluorinated bis(sulfonyl)imide including (fluorosulfonyl)-(trifluoromethanesulfonyl)imide, and fluorinated sulfonic acid ions including trifluoromethanesulfonate ions and other perfluoroalkylsulfonic acid ions.

[0039] In some embodiments, the liquid electrolyte is substantially free of water.

[0040] In some embodiments, the non-aqueous solvent includes one or more molecular solvents. The molecular solvent may be selected from the group consisting of ethers, methylated polyethers, methylated glycol ethers, fluorinated ethers, fluorinated alkyls, fluorinated cycloalkyls, carbonates, sulfolane, and dimethyl sulfoxide.

[0041] In some embodiments, the non-aqueous solvent includes an ambient temperature ionic liquid solvent. For example, the non-aqueous solvent may include an ambient temperature ionic liquid solvent in an amount of at least 20 wt%, or at least 50 wt% of the total solvent in the electrolyte.

[0042] In some embodiments, the cation proton carrier can be deprotonated upon contact with lithium nitride to form a neutral proton acceptor.

[0043] According to a fifth aspect, the present invention provides a liquid electrolyte for an electrochemical reduction reaction. The liquid electrolyte comprises (i) a neutral proton acceptor that is a phosphonium ylide, (ii) a cation proton carrier that is a phosphonium cation capable of forming a neutral proton acceptor by reversible deprotonation, and (iii) a non-aqueous solvent.

[0044] The features related to the fourth aspect of the present invention are also applicable to the fifth aspect.

[0045] According to a sixth aspect, the present invention provides the use of a liquid electrolyte according to any embodiment of the third or fourth aspect for a reduction reaction in an electrochemical cell.

[0046] In some embodiments, the neutral proton acceptor is protonated by protons generated by anodic oxidation of a hydrogen-containing species in the electrochemical cell to form a cation proton acceptor, and the cation proton acceptor is deprotonated in the reduction reaction to reform the neutral proton acceptor.

[0047] In some embodiments, the reduction reaction is a continuous electrochemical nitrogen reduction mediated by a metal selected from lithium, magnesium, calcium, strontium, barium, zinc, aluminum, and vanadium.

[0048] According to a seventh aspect, the present invention provides a system for continuously performing an electrochemical reduction reaction. The system comprises an electrochemical cell including a cathode, an anode, and a power source for applying a voltage between the cathode and the anode, and a liquid electrolyte according to any embodiment of the third or fourth aspect that is in contact with at least the cathode.

[0049] When the terms "comprising", "including", and "having" are used in this specification (including the claims), they are to be construed as identifying the stated features, integers, steps, or components, but not precluding the presence of one or more other features, integers, steps, components, or groups thereof.

[0050] A further aspect of the present invention will become apparent from the following detailed description of the invention.

Brief Description of the Drawings

[0051] Embodiments of the present invention are described herein for illustrative purposes only with reference to the accompanying drawings.

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Mode for Carrying Out the Invention

[0060] [Method for Producing Ammonia by Continuous Electrochemical Nitrogen Reduction] The present invention relates to a continuous electrochemical nitrogen reduction method for producing ammonia. This method is generally carried out in an electrochemical cell comprising a cathode, an anode, and a power source for applying a voltage between the cathode and the anode. The electrolyte in the electrochemical cell, which is in contact with at least the cathode, contains a cationic proton carrier. The cationic proton carrier can be reversibly deprotonated to form a neutral proton acceptor which is an iridium molecule. This method includes supplying nitrogen to the electrochemical cell for the reaction, introducing protons into the electrolyte by anodic oxidation of a hydrogen-containing species, and cathodically reducing nitrogen in the presence of a metal selected from lithium, magnesium, calcium, strontium, barium, zinc, aluminum, and vanadium to produce ammonia. The metal may be present in the electrolyte and / or on the cathode surface. The cationic proton carrier can provide protons for producing ammonia and can be deprotonated to form a neutral proton acceptor.

[0061] Without wishing to be bound by any theory, it is suggested that the ammonia synthesis reaction of the present invention proceeds by a mechanism generally understood for continuous ammonia synthesis via lithium. According to this suggestion, the metal cation is reduced on the cathode to form the metal in its corresponding metallic form, and nitrogen reacts with the metal in its metallic form to form the corresponding metal nitride. Subsequently, the metal nitride is protonated by the cationic proton carrier to produce ammonia and form a neutral proton acceptor to regenerate the metal cation. The cationic proton carrier is continuously regenerated in the electrolyte by protonating the neutral proton acceptor with the protons introduced into the electrolyte by oxidation of the hydrogen-containing species at the anode.

[0062] The main difference from conventional lithium-mediated ammonia synthesis is the use of a cationic proton carrier and its corresponding neutral proton acceptor, in particular a cationic proton carrier that deprotonates to form an iridium proton acceptor. This has been found to have many advantages compared to neutral proton carriers such as alcohols that deprotonate to form anionic proton acceptors.

[0063] These advantages can include a low initial cell resistance and improved cell stability. Furthermore, excellent Faradaic efficiency and high reaction rates may be obtained. This performance improvement is suggested to be due, at least in part, to the absence of anionic species in the reaction sequence. In fact, the anions that are necessarily present in the system during NH3 synthesis are only those intentionally introduced, particularly as counterions to metal cations and / or cationic proton donors. These anions can be selected for the stability of the cathode and anode under electrochemical reaction conditions and their solubility in the electrolyte, so that anion decomposition pathways can be avoided or minimized.

[0064] The performance improvement is also thought to be due to the tendency of cationic species to be attracted to the cathode by electrostatic forces. Thus, the cationic proton donor can be enriched in the electrolyte layer closest to the cathode, thereby potentially improving its availability for the protonation reaction at or near the cathode surface.

[0065] Continuous electrochemical nitrogen reduction can be distinguished from sequential electrochemical processes in which nitrogen is converted to ammonia in a series of process steps that are temporally and / or spatially separated, such as separate batch processes for lithium electrolysis, lithium nitride formation, and ammonia production. As described above, continuous reduction is suggested to involve a cycle between different forms of one or more species, including metal species and / or proton carrier species, in a single process step of the synthesis.

[0066] [Metal] Continuous electrochemical nitrogen reduction is carried out using a metal as a mediator or catalyst. This synthesis is suggested to involve metal nitride intermediates in the reaction cycle. Thus, the range of metals capable of forming metal nitrides from nitrogen under electrochemical reaction conditions can be used in the present invention. As used herein, "metal" means a metallic element and does not mean a specific reduced state or chemical species. When the metal in its zero oxidation state form of the metal is specified, for example, in the context of the proposed reaction mechanism, the term "metal form" or "metal in its metal form" is used.

[0067] According to the mechanism suggested above, electrochemical reduction of the metal cation generates the metal in its metal form at the cathode, which spontaneously reacts with N2 to produce the corresponding metal nitride. Thus, the latter reaction under electrochemical ammonia production conditions is thermodynamically favorable (the Gibbs energy of the reaction of the general formula nM + + mN2 ←→ M n N 2m should be negative). Based on the thermodynamic data summarized in tables (e.g., L.B. Pankratz et al., Thermodynamic Data for Mineral Technology, Washington D.C., 1984, John R. Rumble, CRC Handbook of Chemistry and Physics 101st Edition, 2020), published theoretical calculations (e.g., Norskov et al., in Energy Environ. Sci, 2017, 10, 1621 - 1630) and experimental reports (e.g., DE102018210304), suitable metals include lithium, magnesium, calcium, strontium, barium, zinc, aluminum, and vanadium.

[0068] In some embodiments, the metal comprises or consists of lithium. Lithium is considered particularly suitable due to its demonstrated ability to activate nitrogen at room temperature.

[0069] The metal is in the form of metal cations dissolved in the electrolyte during reduction (e.g., Li+ ) may exist. As described above, the reaction cycle is considered to include reducing metal cations from the electrolyte to form a metal on the cathode and regenerating the metal cations as the final stage of the cycle. However, it is also considered possible for the metal to circulate through a continuous reaction between solid species (e.g., metal nitrides and metals in metallic form) on the cathode surface without using soluble metal cations as intermediates. Thus, in the case of lithium, the cathodic reaction mechanism, in principle, is that (i) lithium in metallic form chemically reacts with nitrogen on the cathode to form lithium nitride, and (ii) in the presence of a cation proton donor, lithium nitride is directly electrochemically reduced and metallic lithium is directly regenerated to produce ammonia (i.e., Li3N + 3HB + 3e - = NH3 + 3Li(0) + 3B - ).

[0070] The metal cations may be present in the electrolyte at a concentration higher than 0.001 mol / L, higher than 0.01 mol / L, or higher than 0.1 mol / L. The upper limit of the concentration is considered to be limited only by the solubility of the precursor metal salt in the electrolyte medium. In some embodiments, the concentration of the metal cations in the electrolyte ranges from 0.1 mol / L to 4 mol / L, for example, about 0.2 mol / L.

[0071] Most conveniently, the metal is introduced into the electrochemical cell in the form of cations, for example, by dissolving a suitable metal salt in the electrolyte. However, it does not exclude the metal being introduced as a metal nitride or in metallic form. Assuming that the metal cations are involved in the reaction cycle, they may be generated in the electrolyte from those species.

[0072] [Cation Proton Carrier - Ylide Proton Donor] The electrolyte further includes a cationic proton carrier that can be reversibly deprotonated to form a neutral proton acceptor. The proton carrier and the proton acceptor are typically organic species. As used herein, reversible deprotonation means that the cationic proton carrier can be deprotonated to form a neutral proton acceptor and then reprotonated to regenerate the cationic proton carrier.

[0073] Similar to the suggested mechanism, the cationic proton carrier may preferably be deprotonated to a neutral proton acceptor by reacting with a metal nitride such as Li3N in a solution at room temperature. The neutral proton acceptor may preferably be protonated by reaction with free protons and / or an organic acid in a solution at room temperature to form a cationic proton carrier. The inventors have found that such reactions may be a convenient way to evaluate candidates for cationic proton carrier-neutral proton acceptor that can be used in the method of the present invention.

[0074] An ylide that is a neutral proton acceptor is a neutral dipolar molecule containing an atom with a negative formal charge directly bonded to a heteroatom with a positive formal charge and is a type of zwitterion.

[0075] While not intending to be bound by any theory, it is believed that suitable ylides can be reversibly interconverted with a cationic proton donor by protonation and deprotonation reactions as needed because the electrons of the negative charge are partially shared with the empty orbital of the positive center. This is thought to result in the acidity in the weakly acidic region required for a proton donor in the continuous ammonia synthesis via a metal in the protonated form.

[0076] In some embodiments, the ylide comprises a carbanion adjacent to a positively charged heteroatom. Thus, the proton carrier site on the molecule is a carbon atom and transitions between a carbanion in the deprotonated form and a C-H covalent bond in the protonated form. The positively charged (or cationic) heteroatom of the ylide may be selected from the group consisting of phosphorus, nitrogen, sulfur, and oxygen.

[0077] In some embodiments, the cationic proton donor is a phosphonium cation or a sulfonium cation, and the neutral proton acceptor is the corresponding phosphonium ylide or sulfonium ylide. In some embodiments, the cationic proton donor is a phosphonium cation, and the neutral proton acceptor is the corresponding phosphonium ylide.

[0078] In some embodiments, the cationic proton carrier is an alkylphosphonium cation or an alkylsulfonium cation, and the neutral proton acceptor is the corresponding phosphonium ylide or sulfonium ylide. As used herein, an alkylphosphonium cation or an alkylsulfonium cation refers to a phosphonium cation or a sulfonium cation containing at least one optionally substituted alkyl group. In some embodiments, the cationic proton donor is an alkylphosphonium cation, and the neutral proton acceptor is the corresponding phosphonium ylide. An alkylphosphonium cation may generally be any species capable of deprotonating to form a phosphonium-carbanion ylide (R’)3P + -C-(R”)2. Here, each organic group R’ and R” may be the same or different.

[0079] The generation of ylides by deprotonation of alkylphosphonium cations is known in the field of organic synthetic chemistry, and ylides are generally called Wittig reagents. Phosphonium-carbanion ylides are useful as nucleophiles in many synthetic reaction schemes. For example, in the Wittig reaction, phosphonium ylides undergo a [2+2] cycloaddition reaction with carbonyl groups to form oxaphosphetanes, which then eliminate to produce alkenes and phosphine oxides. Synthetic reactions with ylide reagents are generally driven by the irreversible conversion of reactive ylides to stable species such as phosphine oxides.

[0080] In contrast, in the embodiments of the present invention, phosphonium ylides are used as reversible proton shuttling agents to block protons in the electrolyte and transport them for the protonation reaction with nitrogen to produce ammonia.

[0081] A wide range of alkylphosphonium cations are considered suitable for the present invention, provided only that they meet the requirement of being capable of reversible deprotonation to an ylide proton acceptor, as shown, for example, in Scheme 1. Thus, the alkylphosphonium cation may have the structure of Formula I, and the corresponding ylide may have the structure of Formula II. [Chemical formula]

[0082] In some embodiments, R 1 , R 2 , R 3 are independently selected from alkyl (e.g., a C1-C 20 n-alkyl group) and aryl (e.g., a phenyl group), R 4 is hydrogen, alkyl (e.g., a C1-C 20 n-alkyl group) and aryl (e.g., a phenyl group), and R 5 is hydrogen, alkyl (e.g., a C1-C 19alkyl groups), cycloalkyl (e.g., C3-C6 cycloalkyl groups), halogen, ether, ester, acyl, amino, and alkyl substituted with nitrile functional groups (e.g., C1-C 19 alkyl groups) or cycloalkyl, aryl (e.g., phenyl groups containing -C6F5), esters (e.g., -C(=O)O(C1-C6 alkyl)), amides (e.g., C(=O)NHC6F5, C(=O)N(Me)OMe), nitriles (-CN), halogens, ethers (e.g., -O(C1-C6 alkyl)), thioethers (e.g., -S(C1-C6 alkyl), -SC6F5), -PR 10 R 11 , and -P(=O)R 12 R 13 selected from. Here, R 10 ~R 13 are, independently, alkyl (e.g., C1-C6 alkyl) and aryl (e.g., -C6F5). The alkyl groups and aryl groups in any of R 1 ~R 4 may be unsubstituted or substituted with substituents such as halogen, ether, hydroxy, ester, acyl, amino, nitrile functional groups, etc., and any two of R 1 ~R 4 may be linked to form a cyclic structure.

[0083] As will be understood by those skilled in the art, the R 1 ~R 5 groups, especially R 5 can be selected to control the acidity of the alkylphosphonium cation and thus the proton-donating ability.

[0084] In some embodiments, R 1 , R 2 , and R 3 are, independently, selected from n-alkyl of C1-C 20 and phenyl, R 4 is hydrogen, and R 5 is selected from hydrogen and n-alkyl of C1-C 19 ​

[0085] In some embodiments, the alkylphosphonium cation is an ionic liquid cation. This means that when paired with a suitable counterion, it is capable of forming an ionic liquid, such as a room temperature ionic liquid. As used herein, an ionic liquid is a salt having a melting point below 100 °C, and a room temperature ionic liquid has a melting point below 25 °C. Such cations are considered preferable because they have high solubility / miscibility with other solvents and salts in the electrolyte and high conductivity. Various phosphonium cations of general formula I containing tetraalkylphosphonium cations are BF4 - , PF6 - , fluoroalkyl phosphates containing tris(pentafluoroethyl) trifluorophosphate (eFAP), fluoroalkyl borates such as tetrakis[hexafluoroisopropyl] borate, bis(fluorosulfonyl)imide, bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)-(trifluoromethanesulfonyl)imide and other fluorinated bis(sulfonyl)imides, fluorinated sulfonates such as triflate and other perfluoroalkyl sulfonates, etc. to form ionic liquids in combination with counterions. This type of ionic liquid without dissolved metal cations has been used in the field of nitrogen reduction in the prior art (e.g., MacFarlane et al., WO2017 / 132721), but at the cathode potentials disclosed in the prior art, these ionic liquids do not show any tendency to deprotonate. At more significantly negative potentials required in this case, e.g., more negative than -2.0 V (vs Ag / Ag + ), these ionic liquids can act as active proton donors to achieve the formation of metal nitrides.

[0086] In some embodiments, the alkylphosphonium cation is a tetraalkylphosphonium cation. The tetraalkylphosphonium cation is [PR 6 R 7 R 8 R 9 + ​It may have the structure. Here, R 6 、R 7 、R 8 、and R 9 are independently selected from C1 - C 20 n - alkyl. In some embodiments, R 6 、R 7 、R 8 、and R 9 have a total of at least 7, or at least 13, or at least 16 carbon atoms. As will be understood by those skilled in the art, an increase in the bond chain length of the tetraalkylphosphonium cation will generally increase solubility in organic media, lower the melting point of the salt, and reduce the tendency to absorb or dissolve water. In some embodiments, R 6 、R 7 、and R 8 are independently selected from C4 - C 20 n - alkyl, and R 9 is C1 - C 20 n - alkyl.

[0087] During the continuous synthesis of NH3, especially under steady - state or near - steady - state operating conditions, the electrolyte will generally contain a mixture of both cationic proton carriers and ylide proton acceptors. A continuously operated cell will reach a steady state in the relative concentrations of both species when proton generation at the anode and consumption at the cathode (as NH3 of interest or H2 as a by - product, etc.) exactly match, provided that no other substances (e.g., Li, Li3N, LiH) are accumulating. In fact, it is understood that the presence of both species in the electrolyte results in the desired buffering action. Due to this buffering action, the proton carrier can absorb excess proton generation at the anode during startup and / or current fluctuations due to intermittent driving. Furthermore, by operating with a high concentration of ylide proton acceptor in the electrolyte, it is ensured that many protons are blocked and consumed by the ylide before participating in unwanted cathode reactions such as HER.

[0088] In some embodiments, the proton carrier system is introduced into the chemical cell in the form of its cations, for example, by dissolving a suitable salt of a cationic proton carrier species in an electrolyte. However, it will be understood that either the cationic proton donor or the corresponding ylide proton acceptor may be supplied to the electrochemical cell to promote the NH3 synthesis reaction. The two mixtures may be formed in situ in either case, as described above.

[0089] The cationic proton donor and its corresponding ylide proton acceptor may be present in the electrolyte together at a concentration higher than 0.001 mol / L, or higher than 0.01 mol / L, or higher than 0.05 mol / L. In some embodiments, for example, when the electrolyte solvent is mainly a molecular organic solvent, the combined concentration ranges between 0.05 mol / L and 1 mol / L, for example, about 0.1 mol / L. In other embodiments, for example, when a phosphonium-based ionic liquid occupies a significant proportion of the electrolyte solvent, the combined concentration of the cationic proton donor and its corresponding ylide proton acceptor may be substantially higher.

[0090] From the perspective of the proposed reaction mechanism, it is expected that the total concentration of the metal cation and the cationic proton carrier will be constant during electrochemical NH3 synthesis, but it will be understood that the ratio of these two species can vary under different steady-state conditions. In some embodiments, the metal cation and the cationic proton carrier are present in the electrolyte together at a concentration higher than 0.001 mol / L, or higher than 0.01 mol / L, or higher than 0.1 mol / L. The combined concentration may range between 0.1 mol / L and 4 mol / L, or between 0.1 mol / L and 1 mol / L, for example, about 0.3 mol / L.

[0091] [Electrochemically stable anion] The electrolyte contains one or more anions to balance the charge of the cationic species present in the electrolyte that includes metal cations and cationic proton carriers. The anion is preferably an electrochemically stable anion. As used herein, an electrochemically stable anion is an anion that is acceptably stable or significantly resistant to the cathode or anode reactions under continuous electrochemical nitrogen reduction conditions.

[0092] As already mentioned, it is considered that significant advantages are obtained in the method of the present invention because the anionic species do not participate in the reaction pathway. Thus, the anions present in the electrolyte may be selected for cathode and anode stability and solubility in the electrolyte under electrochemical reaction conditions, including those necessary to balance the charge of the metal cations and cationic proton carriers. Thereby, an anionic decomposition pathway can be avoided or minimized.

[0093] In the fields of electrochemical synthesis and ionic liquid technology, a wide range of electrochemically stable anions are known, and any such anion or combinations thereof are considered suitable in the method of the present invention. Non-limiting examples of suitable anions include tetrafluoroborate ions, hexafluorophosphate ions, chloride ions, perchlorate ions, fluoroalkyl phosphate ions such as tris(pentafluoroethyl)trifluorophosphate ions, fluoroarylborate ions such as tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ions and tetrakis(pentafluorophenyl)borate ions, fluoroalkylborate ions such as tetrakis[hexafluoroisopropyl]borate ions, bis(fluorosulfonyl)imide, bis(trifluoromethanesulfonyl)imide, and fluorinated bis(sulfonyl)imides such as (fluorosulfonyl)-(trifluoromethanesulfonyl)imide, and fluorinated sulfonate ions such as trifluoromethanesulfonate ions (triflates) and other perfluoroalkylsulfonate ions (such as perfluorohexanesulfonate ions). Chloride ions may be used as they are considered to be sufficiently stable for anodic oxidation in some embodiments, for example when the anodic reaction is H2.

[0094] Electrochemically stable anions can be introduced into the electrolyte together with a metal cation, i.e., as a metal salt, or together with a cationic proton donor, or both.

[0095] [Solvent] The electrolyte is typically a non-aqueous liquid electrolyte and may contain one or more non-aqueous solvents. Suitable non-aqueous solvents are generally aprotic solvents and may include both aprotic molecular solvents and ionic solvents such as room temperature ionic liquids. The solvent is preferably stable under the reaction conditions or at most decomposes slightly.

[0096] The non-aqueous solvent should ideally be a good solvent for the main species involved in the reaction cycle, including metal cations, cationic proton carriers, and iridium proton acceptors. The solubility of cationic species in a particular solvent is known to be increased by the judicious choice of counterions and the design of the cationic proton carrier systems disclosed herein. Accordingly, a wide range of aprotic solvents can be suitable in embodiments of the present invention.

[0097] In some embodiments, the non-aqueous liquid electrolyte is composed of one or more molecular solvents selected from the group consisting of ethers, methylated polyethers, methylated glycol ethers such as tetraglyme, fluorinated ethers, fluorinated alkyls, fluorinated cycloalkyls, carbonates, sulfolane, and dimethyl sulfoxide.

[0098] In some embodiments, the non-aqueous liquid electrolyte contains an ambient temperature ionic liquid solvent, for example, in an amount of at least 20 wt% or at least 50 wt% of the total non-aqueous solvent in the electrolyte. A wide range of ambient temperature ionic liquids are known as solvents suitable for electrochemical synthesis, such as those disclosed in WO2017 / 132721, and such solvents can generally be used in the methods of the present disclosure.

[0099] Accordingly, in some embodiments, the ambient temperature ionic liquid solvent includes the following. (i) PR 1~4 (phosphonium), NR 1~4 (tetraalkylammonium), C4H8NR2 (pyrrolidinium), at least one cation selected from the group consisting of: wherein each R group is independently linear, branched, or cyclic, preferably containing 1 to 18 carbon atoms, optionally partially or fully halogenated, optionally containing heteroatoms, optionally containing a functional group selected from ether, alcohol, carbonyl (acetate), thiol, sulfoxide, sulfonate, amine, azo, or nitrile, and two R groups may be linked to form a monocyclic or heterocyclic ring. (ii) (R’O) x PF 6-x (phosphate ion), (R’O) x BF 4-x (borate ion), R’SO2NSO2R’ (imide), R’SO2C(SO2R’)(SO2R’)(methide), FSO2NSO2F, C2O4BF2, C2O4PF4, RC2O4BF2, RC2O4PF4, CF3SO3 (triflate), R’SO3 (sulfonate ion), R’CO2 (carboxylate ion), CF3COO (trifluoroacetate ion), R’ x PF 6-x (FAP), R’ x BF 4-x at least one anion selected from the group consisting of: wherein each R’ group is independently linear, branched, or cyclic, preferably containing 1 to 18 carbon atoms, optionally partially or fully fluorinated, and optionally containing a functional group selected from ether, alcohol, carbonyl (acetate), thiol, sulfoxide, sulfonate, amine, azo, or nitrile, and two R’ groups may be linked to form a monocyclic or heterocyclic ring. In each case, x ranges from zero to the maximum number of covalent bonds of the central atom.

[0100] In some embodiments, the ionic liquid solvent is stable, i.e., substantially non-reactive, under the conditions of electrochemical NH3 synthesis.

[0101] In other embodiments, at least one cation of the ionic liquid solvent is a cationic proton carrier species capable of forming an iridoproton acceptor by reversible deprotonation. Examples of such ionic liquids are those having a tetraalkylphosphonium cation, as disclosed herein. It is envisioned that phosphonium-based ionic liquid solvents can promote the continuous electrochemical reduction of nitrogen to ammonia mediated by a metal according to the principles disclosed herein in the absence of water. The electrolyte solvent may be composed of an ionic liquid solvent having a reactive cation. Alternatively, an ionic liquid solvent having a mixture of a reactive cation and a stable cation may be used, or a mixture of a reactive ionic liquid and a stable molecular solvent may be used.

[0102] The electrolyte preferably substantially does not contain water. This means that the amount of water is zero or so small that it does not significantly interfere with the reaction cycle of the continuous metal-mediated electrochemical NH3 synthesis reaction as disclosed herein. For example, the electrolyte may contain water at 1000 ppm or less, preferably 100 ppm or less, and most preferably 20 ppm or less.

[0103] The electrolyte may have a sufficient viscosity such that limitations in mass transfer are avoided or are acceptably low. In some embodiments, the non-aqueous electrolyte has a viscosity of less than 50 MPa·s, or less than 20 MPa·s, or less than 15 MPa·s at 25°C. This viscosity may be measured according to ISO 12058 using a Lovis 2000M Anton Paar viscometer (Lovis angle 30°).

[0104] [Supply of Nitrogen for Reduction to Ammonia] The method of the present invention includes supplying nitrogen to an electrochemical cell containing an electrolyte that contacts at least the cathode and reducing the nitrogen at the cathode in the presence of a metal to produce ammonia.

[0105] As used herein, "cathodic reduction" does not indicate a specific mechanism, does not identify intermediate species involved in the reaction cycle, nor does it suggest the location where these species react (e.g., on the cathode surface or in the bulk electrolyte). However, without intending to be bound by any theory, it is suggested that nitrogen is cathodically reduced to ammonia according to the mechanism disclosed herein with reference to FIG. 1. As generally disclosed herein, in the present invention, the proton donor (BH in FIG. 1) is a cationic proton carrier (Y-H + ), and the proton acceptor (B in FIG. 1) is the corresponding neutral iridium molecule (Y). Therefore, the overall cathodic nitrogen reduction reaction is considered to be as shown in Equation (3). N2 + 6Y-H + + 6e - → 2NH3 + 6Y (3)

[0106] Therefore, the cathode may be at a potential suitable for driving this cathodic reaction. This potential may be lower (more negative) than, in particular, the reduction potential of the corresponding reduced form of the metal cation, such as the metal form and / or metal nitride (e.g., the Li + / Li reduction potential). This reduction potential may be determined as the apparent reduction potential of the metal cation in the non-aqueous electrolyte under nitrogen reduction conditions, measured by the crossover point in cyclic voltammetry.

[0107] In some embodiments, the cathode potential is -0.2 V, or 0.4 V lower (more negative) than the apparent reduction potential of the metal cation in the non-aqueous electrolyte. At such a potential, excellent yields and Faradaic efficiencies may be obtained. However, in some embodiments, the cathode potential is -1 V, or 0.8 V higher (more positive) than the apparent reduction potential of the metal cation in the non-aqueous electrolyte.

[0108] Nitrogen may be supplied to the electrochemical cell at a partial pressure greater than 1 bar, or greater than 5 bar, or greater than 10 bar. In some embodiments, nitrogen is supplied at a partial pressure ranging from 0.7 bar to 100 bar, or from 2 bar to 30 bar, or from 5 bar to 20 bar. Increasing the partial pressure of N2 in the cell can improve the faradaic efficiency of ammonia synthesis by increasing the concentration of N2 dissolved in the electrolyte. This is believed to promote the desired reaction between N2 and the metal to form the metal nitride.

[0109] Nitrogen may be supplied to the electrochemical cell for cathodic reduction by contacting the electrolyte with the nitrogen, thereby solubilizing the nitrogen in the electrolyte. In some embodiments, the nitrogen is primarily or exclusively in the solution phase when exposed to the cathode. Alternatively, nitrogen gas may be passed over the cathode. In this case, a three-phase boundary is formed between the electrode, the gas, and the electrolyte.

[0110] The electrolyte may be maintained at a suitable temperature to facilitate ammonia synthesis, which may range from -35°C to 200°C, for example from 15°C to 100°C.

[0111] Since the ammonium cation is a stronger acid than the cationic proton carrier of the present invention (such as alkylphosphonium), the generated ammonia is converted to NH4 in the electrolyte. + However, if an excess of protons is produced, some ammonia will be converted to NH4 + The possibility of it being generated in the form of

[0112] [Anodization] The method of the present invention involves introducing protons into the electrolyte by anodic oxidation of hydrogen-containing species. According to the principles disclosed herein, the protons are expected to react with the iridium proton acceptor in the electrolyte to regenerate the cationic proton carrier. It is considered that the latter species, rather than the proton itself, is the main protonating agent involved in the nitrogen reduction reaction.

[0113] Protons can be generated at the anode of an electrochemical cell by oxidation of any suitable hydrogen-containing species, such as hydrogen (H2) or water (H2O). The anodic half-reactions of these reactants are shown in Equations (4) and (5) respectively. H2 → 2H + + 2e - (4) 2H2O → O2 + 4H + + 4e - (5)

[0114] Therefore, the overall anodic regeneration process of the proton carrier is represented by either Equation (6) or Equation (7), in equilibrium with Equation (3). Here, Y is the iridium proton acceptor, and Y-H + is the cationic proton carrier. 3H2 + 6Y → 6Y-H + + 6e - (6) 3H2O + 6Y → (3 / 2)O2 + 6Y-H + + 6e - (7)

[0115] When oxidizing H2 to generate protons, an electrolyte containing the cationic proton carrier may be brought into contact with both the cathode and the anode. In this case, the protons are directly introduced into the electrolyte as soon as they are generated at the anode. H2 may be obtained from any source, including electrolysis of water by renewable power. Optionally, an electrolysis cell for water can be integrated with a nitrogen reduction cell to directly supply H2 from electrolysis to nitrogen reduction.

[0116] Water is inexpensive and thus particularly preferred as a source of protons in electrochemical synthesis, but is known to interfere with continuous electrochemical nitrogen reduction via metals. Therefore, when oxidizing H2O to generate protons, it is considered preferable to indirectly transfer the protons from the anode to an electrolyte containing a cationic proton carrier. This may help to limit or avoid the presence of water in the electrolyte where nitrogen reduction occurs to an acceptable extent. For example, the electrochemical cell may comprise two electrolytes: a catholyte containing a cationic proton carrier (generally disclosed herein), and an anolyte (either a liquid, a solid, or a mixture of a solid and a liquid) in contact with the anode where water is oxidized. The electrochemical cell is configured to allow proton transfer from the anolyte to the catholyte, but substantially limit or avoid water transfer. Various configurations for achieving this are known in the field of electrochemical synthesis and are described in more detail below.

[0117] H2 is typically an important byproduct of the nitrogen reduction reaction due to competition with HER at the cathode. The H2 byproduct can optionally be recycled for oxidation at the cathode and can supplement the supply of selected hydrogen-containing species. Since there are no other energy-containing byproducts, this can reduce the energy consumption per unit production of ammonia.

[0118] [Electrochemical cell] The method of the present invention is generally carried out in an electrochemical cell comprising a cathode, an anode, and a power source connected to the cathode and the anode. The power source is configured to apply a voltage sufficient to drive electrochemical ammonia synthesis between the cathode and the anode.

[0119] The cathode may generally be any conductive electrode that is stable at the required reduction potential, for example, a metal electrode used in previously reported lithium-mediated continuous electrochemical synthesis (e.g., Tsuneto et al., Chemistry Letters, 1993, 851-854) or other processes involving the reduction of metal cations to their metallic form. Non-limiting examples of suitable metals may include Ni, Nb, Ti, Mo, Fe, Cu, Ag, and Zn, as well as alloys thereof. In some embodiments, a copper (Cu) cathode is used. In other embodiments, the metal of the cathode consists of, or comprises, the metallic form of a metal that mediates ammonia synthesis.

[0120] The cathode can be in the shape of a cylinder, disk, plate, or other shape suitable for the cell design. The cathode may further be porous, such as achieved by, for example, etching, or may be constructed as a foam, or a mass of compressed particles, or by an inverse opal structure. The desired mediating metal may be coated on the underlying structure that provides the optimal roughness and porosity, for example, by electrodeposition or chemical vapor deposition. The cathode can also be formed by depositing metal nanoparticles on an inert structure.

[0121] Anodes suitable for oxidizing hydrogen-containing species such as H2O or H2 to form protons are well known in the field of electrochemistry. In some embodiments, the anode is a platinum electrode.

[0122] The power source may be any conventional power source for an electrolysis system, such as a direct current power source. Optionally, the power source may include a solar power generation cell. It is considered a particularly advantageous aspect of the present invention that ammonia can be produced from electrical power, especially renewable electrical power. For example, it is envisioned that, according to the present invention, ammonia-based fertilizers can be produced when needed using solar- or wind-generated electrical power. This would be particularly useful for high-value agricultural applications such as hydroponics, or would likely minimize the logistics issues associated with transporting fertilizers to remote locations.

[0123] An example of an electrochemical cell for carrying out an embodiment of the present invention is schematically shown in FIG. 2. Cell 200 includes a copper cathode 210 within cell chamber 211. Cell 200 further includes a platinum anode 212 and optionally includes a conventional reference electrode 213 such as Ag / Ag + . These three electrodes are immersed in the same non-aqueous liquid electrolyte 214 containing lithium cations and cationic proton carriers generally disclosed herein. Optionally, a stirrer or other means for mixing or circulating the electrolyte may be included to enhance mass transport in cell chamber 211. The electrodes are connected to a power source (not shown) capable of applying a voltage between cathode 210 and anode 212, and the reduction potential of the cathode is controlled (or measured) relative to the reference electrode.

[0124] Cell 200 further includes a gas inlet 215 for introducing a mixed gas 218 containing nitrogen (N2) and hydrogen (H2) into chamber 211. The cell may include a gas outlet 216 for removing gas 219 from the headspace of the chamber, an electrolyte inlet 220 for replenishing the electrolyte with electrolyte feed 222, and an electrolyte outlet 221 for recovering electrolyte 214. The cell is preferably configured to operate at high pressure.

[0125] In use, the mixed gas 218 is pressurized into chamber 211 via gas inlet 215, and a voltage sufficient to achieve a reduction potential lower (more negative) than the Li + / Li reduction potential is applied between the cathode and the anode. The partial pressure of nitrogen in cell chamber 211 may be higher than 10 bar and the partial pressure of hydrogen may be higher than 1 bar. As a result, the current flowing through the cell results in the electrochemical reduction of nitrogen to ammonia according to the principles disclosed herein. Ammonia products may be continuously or periodically removed from cell chamber 211 from gas 219 via gas outlet 216 and / or from electrolyte 214 recovered via electrolyte outlet 221.

[0126] In some embodiments, the cell is operated in a steady state by continuously recovering one or both of these streams and, optionally, introducing gas mixture 218 and / or electrolyte feed 222 to continuously replenish the gas reactants (N2 and H2) and / or the electrolyte. Ammonia may be separated from the recovered stream of gas 219 and / or electrolyte 214, and the residual gas and electrolyte may be returned to cell chamber 211 as part of gas mixture 218 and electrolyte feed 222, respectively. A portion of the electrolyte 214 recovered via electrolyte outlet 221 may be discarded (or regenerated) and exchanged with fresh electrolyte in feed 222. Thereby, the target electrolyte residence time within the cell is maintained.

[0127] In one embodiment in terms of the use of the fertilizer production cell, the outlet gas stream 219 is passed through an aqueous solution of sulfuric acid or phosphoric acid to absorb ammonia as ammonium (NH4 + ). The product of this process is a solution of the ammonium salt of the acid used, for example ammonium sulfate solution, which can be applied directly as a fertilizer solution. When used in hydroponics or commercial greenhouses, the cell can be controlled to continuously supply fertilizer in-line to the water supply to the plants.

[0128] Another example of an electrochemical cell for practicing embodiments of the present invention is schematically shown in FIG. 3. Cell 300 includes a cathode chamber 311 and an anode chamber 331 separated by a membrane separator 333 permeable to protons, such as a membrane composed of a sulfonated poly(tetrafluoroethylene) ionomer such as Nafion. A copper cathode 310 is disposed in cathode chamber 311. A conventional reference electrode 313 is also within the cathode chamber. A platinum anode 312 is disposed in anode chamber 331. The electrodes are connected to a power source (not shown) capable of applying a voltage between cathode 310 and anode 312, and the reduction potential of the cathode is controlled (or measured) relative to the reference electrode.

[0129] The cathode 310 and the reference electrode 313 are immersed in the catholyte 314, and the anode 312 is immersed in the anolyte 334. The catholyte 314 is a non-aqueous liquid electrolyte containing lithium cations and cation proton carriers as generally disclosed herein and substantially free of water. The anolyte 334 contains water for oxidation at the anode, but otherwise may have the same or different composition compared to the catholyte 314. The membrane separator 333 suppresses or substantially prevents the transfer of species other than protons between the cathode reaction chamber and the anode reaction chamber.

[0130] The cell 300 further includes a gas feed inlet 315 for introducing a nitrogen feed 318 into the cathode chamber 311. The cell may include a cathode gas outlet 316 for removing gas 319 from the headspace of the cathode chamber, a catholyte inlet 324 for replenishing the catholyte with the catholyte feed 325, and a cathode outlet 321 for recovering the catholyte 314. The cell 300 may include an anode inlet 340 for introducing or replenishing the anolyte 334 and, optionally, introducing one or more hydrogen-containing species (e.g., H2O and / or H2 as either a liquid or a vapor) into the anode chamber 331. The anolyte outlet 341 is provided for recovering the anolyte 334, and the anode gas outlet 344 is provided for recovering gas 345 from the headspace of the anode chamber.

[0131] In use, nitrogen 318 is pressurized into the cathode chamber 311 via the gas supply inlet 315. The partial pressure of nitrogen in the cathode chamber 311 may be higher than 10 bar. Water may initially be present in the anolyte 344 and / or may be supplied to the anode chamber via the anode inlet 340. A voltage sufficient to achieve at the cathode 310 a reduction potential lower (more negative) than the Li + / Li reduction potential is applied. As a result, the current flowing through the cell can electrochemically reduce nitrogen to ammonia in the cathode chamber 311 according to the principles disclosed herein.

[0132] In cell 300, different from cell 200, water is oxidized at cathode 312, and protons are generated in anolyte 334 according to formula (5). The protons move through membrane separator 333 to maintain charge neutrality within the cell, enter catholyte 314, protonate neutral proton receptors, and regenerate cation proton carriers. However, water and other undesirable species are excluded or suppressed from moving from anolyte 334 to catholyte 314 by membrane separator 333.

[0133] The ammonia product may be continuously or periodically removed from cathode chamber 311 from gas 319 through gas outlet 316 and / or from catholyte 314 recovered through electrolyte outlet 221. The cell may be operated continuously, and the electrolyte and gas removed from the cell may be returned after removing ammonia and other by-products in the same manner as described for cell 200. Hydrogen generated as a by-product in cathode chamber 311 may be recovered and reused in anode chamber 331 for oxidation.

[0134] In a variant, hydrogen may be introduced into cell 300 as the only hydrogen-containing species for oxidation at anode 312. In this case, both catholyte 314 and anolyte 334 may be substantially free of water.

[0135] The configuration shown in cell 300 is merely an example of an electrochemical cell configured to enable the oxidation of water at the cathode and the subsequent selective transfer of protons to an electrolyte that substantially excludes the water participating in the water-sensitive cathode reaction. In another reported approach, the separator is placed in proximity to a porous anode (e.g., a gas diffusion electrode). A gas stream containing water (e.g., humid air) is directed to pass over the outer surface of the anode while being prevented from convective mixing by the separator. Since the electrolyte is sufficiently hydrophobic, little moisture is absorbed from the gas stream. If the hydrophobicity of the electrolyte and the humidity of the gas stream are appropriately adjusted, the separator may not require proton selectivity to keep the moisture content in the bulk electrolyte low.

[0136] In another reported approach, a hydrophobic organic catholyte is immiscible with a polar (such as an aqueous solution) anolyte, and proton transfer is achieved at the phase interface between the two. This configuration can also enable sufficient proton transfer while appropriately suppressing the migration of moisture into the catholyte. To stably maintain the phase interface, a separator may be used as the positioning of the phase interface.

[0137] [Liquid Electrolytes, Use of Electrolytes, and Systems Containing Electrolytes] The present invention relates to a liquid electrolyte for an electrochemical reduction reaction. The electrolyte includes (i) a neutral proton acceptor that is an iridium, (ii) a cationic proton carrier capable of forming the neutral proton acceptor by reversible deprotonation, and (iii) a non-aqueous solvent. The liquid electrolyte preferably substantially excludes water. For example, the liquid electrolyte may contain less than 1000 ppm, preferably less than 100 ppm, and most preferably less than 20 ppm of water.

[0138] The present invention further relates to the use of this electrolyte in a reduction reaction in an electrochemical cell.

[0139] A cation proton carrier, its corresponding neutral iridium proton acceptor, an electrochemically stable anion that balances the charge of the cationic species in the electrolyte, the concentration of the above species, and a non-aqueous solvent are generally as disclosed herein in the context of a continuous electrochemical nitrogen reduction process that produces ammonia.

[0140] The inventors recognized that an electrolyte containing both a cation proton donor and an iridium proton acceptor during use may offer advantages in the field of electrochemical reduction. Of particular interest is cathodic reduction that requires protons generated by anodic oxidation, but (a) direct reduction of protons or water to H2 at the cathode competes with the desired reduction process, (b) non-aqueous conditions are desirable in the electrolyte, and / or (c) when the anionic organic intermediates in the reaction sequence are prone to precipitation, decomposition, or other degradation pathways.

[0141] According to the principles disclosed herein, the cation proton donor provides protons to the cathodic reduction reaction, and the corresponding iridium proton acceptor blocks the protons introduced into the electrolyte by anodic oxidation of a hydrogen-containing species (preferably H2 or H2O) at the anode. Certain iridiums containing phosphonium-carbanion irid can be repeatedly cycled between the protonated form and the deprotonated form without substantial degradation or precipitation, and it has been found that a high turnover number can be achieved. Furthermore, the presence of both species in the electrolyte buffers the overall reduction reaction, absorbs excess reaction intermediates (including protons) generated or accumulated during startup or online intermittency, and ensures that many anodic protons are blocked and consumed before participating in an undesired cathodic reaction such as HER.

[0142] In some embodiments, the reduction reaction in the electrochemical cell is a continuous electrochemical nitrogen reduction mediated by a metal selected from lithium, magnesium, calcium, strontium, barium, zinc, aluminum, and vanadium, as detailed in the present disclosure. In such embodiments, the electrolyte may contain these metal cations. However, it will be understood that metal cations are a specific requirement for this particular reduction reaction, and the metal cation component may not be present in other embodiments accordingly.

[0143] The present invention further relates to a system for a continuous electrochemical reduction reaction. The system comprises an electrochemical cell including a cathode, an anode, and a power source for applying a voltage between the cathode and the anode, and a liquid electrolyte as described herein that contacts at least the cathode. The electrochemical cell may generally be as described in the context of a method for continuous electrochemical nitrogen reduction to produce ammonia.

[0144] [Examples] The present invention is described with reference to the following examples. It should be understood that the examples are illustrative of the invention described herein and not limiting.

[0145] [Materials and Methods] Tetrahydrofuran (THF) stabilized with butylated hydroxytoluene (BHT) was purchased from Chem-Supply. Before preparing the electrolyte, the received THF was dried on 4 Å molecular sieves for 24 hours or until the detected water content (by Karl Fischer titration) was less than 5 ppm. Lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiOTf), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) were purchased from commercial suppliers such as ACROS Organics (anhydrous LiBF4 with a purity of 98%). Before use in electrolyte preparation, the salts were further dried under vacuum at 120 °C for 12 hours. Phosphonium and sulfonium salts, trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate ([P 666,14 [eFAP]), tributyl(octyl)phosphonium tris(pentafluoroethyl)trifluorophosphate ([P 444,8 [eFAP]), triethyl(methyl)phosphonium tris(pentafluoroethyl)trifluorophosphate ([P 1222 [eFAP]), trihexyl(tetradecyl)phosphonium perfluorohexanesulfonate ([P 666,14 [PFHS]), and triethylsulfonium bis(trifluoromethanesulfonyl)imide (Et3S-TFSI) were synthesized according to known methods.

[0146] Electrochemical measurements were carried out in a one-compartment three-electrode configuration pressure cell with an internal volume of 75 mL. The standard amount of electrolyte used in each experiment was 5 mL. The working electrode (WE) used in the experiment was a Cu disk electrode with a diameter of 1.25 mm. The Pt sheet counter electrode (CE, geometric surface area GSA = 1 cm 2) and an Ag quasi-reference electrode (RE) separated by glass frit was used. Before each experiment, the working electrode (WE) was polished using alumina slurry mixtures (0.3 μm and 0.05 μm) on an electrode polishing pad (Buehler). Subsequently, the polished WE was rinsed with deionized water (DI) to remove residual alumina particles, further polished with a clean polishing pad (without alumina) to obtain a clean surface, and then further rinsed with acetone. For the counter electrode (CE), the electrode was heated in a butane flame until a bright red glow was observed and then rinsed with acetone. After the cleaning process, all electrodes and pressure cell components were dried under vacuum at 80 °C for at least 4 hours. Then, the electrochemical cell was assembled and filled with electrolyte inside an Ar-glove box. Next, the cell was pressurized to the target pressure with ultra-high purity N2 (purity 99.999%) outside the Ar-glove box.

[0147] Electrochemical measurements were carried out using a Biologic VMP XXX device. Chronoamperometry (CA) measurements were performed following a series of electrochemical measurements. Following the pressurization of the cell, electrochemical impedance spectroscopy (EIS) was carried out, and then open circuit potential (OCP) measurements were performed until a stable potential measurement value was observed. Following the OCP, cyclic voltammetry experiments were carried out, and the crossover potential corresponding to the Li / Li + potential was determined. This potential value was used as an internal standard for determining the working electrode (WE) potential (vs Li / Li + ) in the subsequent CA experiment.

[0148] The ammonia generated during the CA experiment was measured with a Berthelot (indophenol blue) spectrophotometer by the internal standard addition method according to the procedure of ACS Energy Lett., 2020, 5, 736 - 741. After the electrochemical measurement, the electrolyte was collected and diluted with an aqueous solution of 50 mM H2SO4. Then, 100 μL aliquots of the diluted sample were placed into four 1.5 mL sample tubes, and 400 μL of deionized water and 400 μL of internal standards (10, 20, 50 μM) were added to each tube to prepare four different calibration points. Additionally, a tube containing 0.5 ml of deionized water was prepared as a blank. To each of the five tubes, 0.4 mL of a solution containing 1 M NaOH (containing 5 wt% salicylic acid and 5 wt% sodium citrate), 100 μL of 10 wt% NaClO solution, and 30 μL of 1 wt% C5FeN6Na2O (sodium nitroprusside) were added. This mixture was incubated for 2 hours to develop color. After incubation, the five sample solutions were measured with an ultraviolet - visible spectrophotometer, the absorbance at 655 nm was taken and plotted to determine the ammonia amount in the original electrolyte.

[0149] [Example 1. Preparation of Electrolyte] In the preparation of a typical electrolyte, to minimize the accumulation of contaminants, a 25 mL batch of electrolyte was prepared inside an Ar - glove box. One electrolyte contains 0.2 M LiBF4 and 0.1 M 666,14 [eFAP] in THF. To prepare this electrolyte, 2.3 ± 0.1 g of 666,14 [eFAP] was added to a 25 mL volumetric flask, then 0.47 ± 0.05 g of LiBF4 was added, and it was dissolved with THF to make 25 mL. The water content of the final electrolyte was further tested by Karl Fischer titration, and only batches with a water content of less than 20 ppm were used for electrochemical tests.

[0150] A similar electrolyte was prepared as follows. · 0.2 M lithium triflate (LiOtf) / 0.1 M 666,14 [eFAP] / THF; ·0.2M LiBF4 / 0.1M [P 444,8 [eFAP] / THF; ·0.2M LiBF4 / 0.1M [P 666,14 [PFHS] / THF; ·0.2M LiOTf / 0.1M [P 444,8 Cl / THF; ·0.2M LiBF4 / 0.1M [P 4444 [BF4] / THF; ·2.0M LiTFSI / 0.1M Et3S-TFSI / THF; ·0.2M LiBF4 / 0.1M [P 4444 [BF4] / THF; ·0.2M LiOTf / 0.1M [P 666,14 [eFAP] / dimethoxyethane (DME); ·2.0M LiTFSI / 0.1M [P 666,14 [eFAP] / sulfolane ·0.2M LiOTf / 0.1M [P 666,14 [TFSI] / THF

[0151] [Example 2. Cyclic Voltammetry Test] To investigate the electrochemical properties of the phosphonium cation as a proton carrier, first, cyclic voltammetry experiments were performed using a 0.2M LiBF4 / 0.1M [P 666,14 [eFAP] / THF electrolyte to analyze the affinity between this molecule and the electrochemistry of Li. The comparison was with a typical electrolyte used in previously reported continuous electrochemical systems via Li, consisting of 0.2M LiOtf / 0.17M ethanol in THF. The cyclic voltammetry experiments were carried out while varying the pressure of N2 up to 20 bar.

[0152] Figure 4 shows the results of cyclic voltammetry for both proton carrier systems. [P 666,14 It is clear that the presence of [eFAP] does not interfere with the electrochemistry of Li. Instead, [P 666,14[eFAP] improved the ionic conductivity of the electrolyte. As a result, a significantly higher electrochemical reaction rate can be inferred from a higher current density. For example, 666,14 In a system containing [eFAP], the current at the Li oxidation peak of 0.22 V (vs Li / Li + ) was observed to be 8.5 mA·cm -2 , and this value corresponds to 24 times the value (0.35 mA·cm -2 ) observed in a standard ethanol system.

[0153] [Example 3. Ammonia production amount by chronoamperometry (CA)] To further evaluate the ability of the phosphonium cation / phosphonium ylide system as a proton carrier in ammonia production by reduction of nitrogen via lithium, a series of CA experiments were performed by applying a potential in the range of -0.15 V to -1.05 V (vs Li / Li 666,14 ) using a 0.2 M LiBF4 / 0.1 M [P + [eFAP] / THF electrolyte. The experiment was carried out for 2 hours under the condition of 20 bar of N2. Hydrogen was not introduced into the cell so that the reduction of H2 to LiH did not contribute to the cathodic process. In the absence of H2, the anodic reaction that generates protons is the oxidation of THF. The results shown in Figure 5 indicated that ammonia production was successfully achieved with a high Faradaic efficiency (FE). The maximum FE for ammonia production, 82 ± 12%, was achieved when the applied potential was -0.75 V (vs Li / Li + ). At this potential, an ammonia yield of 77 ± 16 nmol·cm -2 ·s -1 was obtained. In the tested potential range, it was confirmed that the ammonia production yield increased as the overpotential applied increased. The error bar for 0.75 V represents the standard deviation of the results of n = 4 (4 repeated experiments), and the other error bars represent the standard deviation of the results of n = 2.

[0154] [Example 4. Cell stability test by chronopotentiometry] In the chronopotentiometer (CP) experiment, the cathodic potential stability of the cationic phosphonium proton carrier system was compared with that of the ethanol proton carrier system. 0.2 M lithium triflate (LiOTf) / 0.1 M 666,14 [eFAP] / THF electrolyte was compared with 0.2 M lithium triflate (LiOTf) / 0.17 M ethanol / THF electrolyte at a constant current of 27 mA·cm -2 respectively. The experiments were carried out under the conditions of 19.5 bar of N2 and 0.5 bar of H2 for phosphonium, and 20 bar of N2 for ethanol (the latter result is the same whether H2 is present or not). The results are shown in Fig. 6. The inset is an enlarged view of the initial few minutes of the experiment. From these results, it was shown that in the ethanol-based system, there was a problem that the required cathodic voltage increased immediately after the start of the CP experiment. The working electrode potential exceeded -6 V (vs Li / Li + ) within only 20 minutes, leading to an over-negative potential load on the electrochemical system. In contrast, the phosphonium-based proton carrier system showed significantly higher stability and maintained voltage stability for more than 20 hours. The cathodic potential of the phosphonium proton carrier remained below -2 V (vs Li / Li + ) during the experiment. This also results in higher energy efficiency in the phosphonium proton carrier system.

[0155] Over the entire 20-hour experiment, a FE of 40% and a yield of 5.5 nmol·cm -2 ·s -1 were achieved. In a similar experiment for only 1.5 hours, a FE of 67% and a yield of 9.0 nmol·cm -2 ·s -1 were achieved. The performance degradation after 6 hours was suggested to be at least partially contributed by the decomposition of the THF solvent.

[0156] After each CP experiment, the working electrode (WE) was inspected. After a 20-hour experiment using a phosphonium proton carrier system, the Cu surface maintained its luster. On the other hand, in the ethanol proton carrier system, white deposits strongly adhering to the WE surface were confirmed after only 8 hours of experiment. This is thought to be due to the insolubility and / or instability of the deprotonated carrier (i.e., EtOLi), resulting in the accumulation of insoluble substances and the gradual passivation of the WE. This is consistent with the previous suggestion that the increase in cell potential during the operation of ammonia synthesis using lithium is the result of the decomposition of the alcohol electrolyte component.

[0157] [Example 5. Investigation of Iridium Regeneration] To investigate the role of alkylphosphonium species as renewable proton carriers in electrochemical lithium-mediated ammonia synthesis, a series of experiments were conducted with [P666,14][eFAP] and, as shown in Figure 7, 31 monitored by P-NMR spectroscopy. All reactions were carried out using dry materials under an inert atmosphere in an argon glove box (O2 and H2O < 0.5 ppm). 31 The P-NMR spectra were calibrated with the axis set to 0 ppm using PPh3 as a reference with an external capillary and recorded in THF.

[0158] In the first step, a THF solution of 0.2 M [P 666,14 [eFAP] was prepared and 31 the P-NMR spectrum was recorded. As shown in Figure 7, this spectrum contains one 666,14 P-NMR signal corresponding to the phosphonium cation [P 31 at 39.3 ppm and a signal group corresponding to the [eFAP] anion at -131 to -151 ppm. In the second step, an excess amount of Li3N was added to [P 666,14It was added to a 0.2 M solution of [eFAP], and the mixture was stirred for 24 hours. No change in the mixture was observed visually; it remained colorless and transparent, and there was no visible precipitate. In the 31P-NMR spectrum recorded after 24 hours (the central spectrum in Figure 7), it was confirmed that the peak at 39.3 ppm completely disappeared and a new peak appeared at 15.7 ppm. This peak corresponds to an amphoteric ion species formed in almost quantitative yield by the deprotonation of the phosphonium cation by reaction with Li3N. This NMR data is consistent with the formation of phosphonium ylide. In the third step, 0.2 ml of 0.1 M acetic acid solution was added to 0.5 ml of the ylide-containing solution, and the 31P-NMR spectrum was recorded (the lower spectrum in Figure 7). From this spectrum, quantitative recovery of the phosphonium cation (peak at 39.3 ppm) was confirmed.

[0159] The recovery of the phosphonium cation was confirmed by mass spectrometry (MS). The mass spectra of the first and third steps were identical, showing only one signal (m / z = 483) corresponding to the [P 666,14 cation.

[0160] This stepwise reaction process was repeated for the following other phosphonium salts. [P 1222 [eFAP], [P 4448 [eFAP], and triphenylmethylphosphonium tetrafluoroborate ([PPh3Me][BF4]. In all 31 P-NMR spectra, it was confirmed that ylide species were generated upon reaction with Li3N and phosphonium cations were regenerated upon addition of acetic acid. This indicates that various alkylphosphonium cations are suitable as cationic proton carriers and that this stepwise reaction test can be used as a screening method for potential proton carriers.

[0161] [Example 6. Evaluation of Other Alkylphosphonium Salts] In a series of CA experiments, other alkylphosphonium salts were evaluated. The CA experiment was (a) 0.2 M LiBF4 / 0.1 M [P444,8 [eFAP] / THF electrolyte, (b) 0.2 M LiBF4 / 0.1 M [P 666,14 [PFHS] / THF electrolyte, (c) 0.2 M LiOTf / 0.1 M [P 666,14 Cl / THF electrolyte, when used, the applied potential was -0.45 V (vs Li / Li + ) for 8 hours, (d) 0.2 M LiBF4 / 0.1 M [P 4444 [BF4] / THF electrolyte, (e) 0.2 M LiOTf / 0.1 M [P 666,14 [TFSI] / THF electrolyte, when used, the applied potential was -0.75 V (vs Li / Li + ) for 2 hours. The experiments were carried out under the condition of 20 bar of N2 (without H2). The results are shown in Table 1 below.

Table 1

[0162] [P 444,8 [eFAP] The productivity and selectivity obtained were equivalent to those of [P 666,14 [eFAP] proton carriers at the same potential (see Example 3), indicating that various alkylphosphonium structures are suitable. [P 666,14 [TFSI] gave excellent performance, but [P 666,14 [PFHS] and [P 666,14 Cl had slightly lower productivity. This is thought to be due to the low conductivity of the electrolytes containing these salts. However, this result shows that various counterions that are electrochemically stable are suitable. Among these, chloride has sufficient stability for use in the anodic reaction of H2.

[0163] [Example 7. Evaluation of H2 as a hydrogen-containing species to be anodically oxidized] H2 was used as a proton source for the electrolyte and continuous ammonia synthesis was carried out. The CA experiment was carried out using an electrolyte of 0.2 M LiBF4 / 0.1 M [P 666,14 [eFAP] / THF at an applied potential of -0.45 V (vs Li / Li +) was carried out. Hydrogen gas (H2) was supplied to a pressurized electrochemical cell. Four measurements were performed at a total cell pressure of 20 bar consisting of 15 bar of N2 and 5 bar of H2. The average yield of NH3 was 22 ± 11 nmol·cm -2 ·s -1 , and a Faradaic efficiency (FE) of 65 ± 22% was obtained.

[0164] [Example 8. Comparative Example Using Ar Instead of N2] To prove that the NH3 produced in Examples 2 - 7 was derived from the supplied N2 gas, a CA experiment was carried out using an electrolyte of 0.2M LiBF4 / 0.1M [P 666,14 [eFAP] / THF under an Ar pressure of 20 bar with an applied potential of -0.45V (vs Li / Li + ). After a charge of 2.5C had flowed, a total of only 6 nmol of NH3 was detected. This was in contrast to the 3479 nmol (when a charge of 1.5C had flowed) detected in an equivalent experiment using N2, indicating that the produced NH3 was derived from the electrochemical reduction of N2.

[0165] [Example 9. Influence of N2 Pressure] To investigate the influence of N2 pressure, a series of CA experiments were carried out using an electrolyte of 0.2M LiBF4 / 0.1M [P 666,14 [eFAP] / THF at an applied potential of -0.75V (vs Li / Li + ). The experiments were carried out for 2 hours at N2 pressures in the range of 2 - 20 bar. The results shown in Figure 8 indicated that the yield and Faradaic efficiency improved with increasing pressure.

[0166] [Example 10. Evaluation of Other Solvents] Since polymerization by electrodeposited Li is not desirable in THF, a solubility investigation was carried out to identify other non-aqueous solvents suitable for ammonia electrosynthesis via Li. Solubility was evaluated at a concentration of 0.2M salt in the specified solvent. The results are shown in Table 2.

Table 2

[0167] 0.2 M LiOTf / 0.1 M [P 666,14 [eFAP] / DME electrolyte was used, and dimethoxyethane (DME) was evaluated as the solvent in a CA experiment at an applied potential of - 0.45 V (vs Li / Li + ). The experiment was carried out for 1.3 hours under the condition of 20 bar of N2. The results are shown in Table 3 below.

[0168] 2.0 M LiTFSI / 0.1 M [P 666,14 [eFAP] / sulfolane electrolyte was used, and sulfolane was evaluated as the solvent in a CA experiment at an applied potential of - 0.25 V (vs Li / Li + ). The experiment was carried out for 2 hours under the conditions of 20 bar of N2 and 60 °C. The results are shown in Table 3 below.

Table 3

[0169] [Example 11. Evaluation of Alkylsulfonium Salts as Cation Proton Carriers] An electrolyte containing 2 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and 0.1 M triethylsulfonium bis(trifluoromethanesulfonyl)imide (Et3S-TFSI) in THF was used, and a nickel wire cathode (surface area 0.15 cm 2 ) was used in a single compartment cell. An applied potential of 0.55 V (vs Li / Li + ) was set, and a series of CA experiments were carried out to evaluate the alkylsulfonium salts. The experiments were carried out for 6 hours with the electrolyte stirred at 600 rpm under the condition of 15 bar of N2 (without H2). The yield of NH3 was 71 nmol·s -1 ·cm -2 , and the Faraday efficiency was 30%.

[0170] These results are equivalent to those obtained when using the [P 666,14 [eFAP] proton carrier at the same potential (see Example 3). This result demonstrates the compatibility of the sulfonium-type cation as a proton carrier in the NRR via lithium.

[0171] It is understood by those skilled in the art that the invention described herein may be affected by variations and modifications other than those explicitly described. The invention is understood to include all such variations and modifications that fall within the spirit and scope of the invention.

Claims

1. A method for continuous electrochemical nitrogen (N ２ ) reduction for producing ammonia, comprising The method comprises: supplying nitrogen to an electrochemical cell comprising an electrolyte that contacts at least a cathode; introducing protons into the electrolyte by anodic oxidation of a hydrogen-containing species; cathodically reducing nitrogen in the presence of a metal selected from lithium, magnesium, calcium, strontium, barium, zinc, aluminum, and vanadium to produce ammonia; wherein the electrolyte comprises a cationic proton carrier capable of forming a neutral proton acceptor by reversible deprotonation; the neutral proton acceptor is an ylide; method.

2. Cathodically reducing nitrogen comprises reacting nitrogen with the cationic proton carrier to produce ammonia and form the neutral proton acceptor; The method according to claim 1.

3. The cationic proton carrier is regenerated in the electrolyte by protonating the neutral proton acceptor with a proton; The method according to claim 2.

4. Reacting nitrogen with the cationic proton carrier comprises: (i) reacting nitrogen with the metal to form a metal nitride; (ii) reacting the metal nitride with the cationic proton carrier to produce ammonia and form the neutral proton acceptor; wherein The method according to claim 2 or 3.

5. The metal exists as a metal cation in the electrolyte; The method according to any one of claims 1 to 4.

6. The metal is lithium; The method according to any one of claims 1 to 5.

7. The ylide comprises a carbanion adjacent to a heteroatom selected from the group consisting of phosphorus, nitrogen, sulfur, and oxygen having a positive charge; The method according to any one of claims 1 to 6.

8. The neutral proton acceptor is selected from phosphonium ylides and sulfonium ylides; The method according to any one of claims 1 to 7.

9. The cationic proton carrier is selected from alkylphosphonium cations and alkylsulfonium cations; The method according to any one of claims 1 to 8.

10. The cationic proton carrier is a tetraalkylphosphonium cation; The method according to any one of claims 1 to 9.

11. The electrolyte is a non-aqueous liquid electrolyte The method according to any one of claims 1 to 10.

12. The non-aqueous liquid electrolyte contains one or more molecular solvents selected from the group consisting of ether, methylated polyether, methylated glycol ether, fluorinated ether, fluorinated alkyl, fluorinated cycloalkyl, carbonate, sulfolane, and dimethyl sulfoxide The method according to claim 11.

13. The cationic proton carrier is charge-balanced in the electrolyte by one or more electrochemically stable anions selected from the group consisting of tetrafluoroborate ion, hexafluorophosphate ion, chloride ion, perchlorate ion, fluoroalkyl phosphate ion, fluoroallyl borate ion, fluoroalkyl borate ion, bis(sulfonyl)imide fluoride, and sulfonate fluoride ion The method according to any one of claims 1 to 12.

14. The cationic proton carrier and the neutral proton acceptor are present in the electrolyte at a combined concentration higher than 0.001 mol / L The method according to any one of claims 1 to 13.

15. The hydrogen-containing species is selected from hydrogen (H ２ ) and water The method according to any one of claims 1 to 14.

16. When reducing nitrogen at the cathode, the cathode has a potential lower (more negative) than -2.0 V (vs Ag / Ag ＋ ). The method according to any one of claims 1 to 15.

17. A method for continuous electrochemical nitrogen (N ２ ) reduction for producing ammonia, comprising The method comprises supplying nitrogen to an electrochemical cell containing an electrolyte that contacts at least the cathode, introducing protons into the electrolyte by anodic oxidation of a hydrogen-containing species, cathodically reducing nitrogen in the presence of a metal selected from lithium, magnesium, calcium, strontium, barium, zinc, aluminum, and vanadium to produce ammonia, and the electrolyte contains at least one selected from alkylphosphonium cations and alkylsulfonium cations method.

18. A system for continuously performing an electrochemical reduction reaction, comprising an electrochemical cell including a cathode, an anode, and a power source for applying a voltage between the cathode and the anode, a liquid electrolyte that contacts at least the cathode, and the liquid electrolyte (i) a neutral proton acceptor that is an ylide, (ii) a cationic proton carrier capable of forming the neutral proton acceptor by reversible deprotonation, (iii) a non-aqueous solvent, and The liquid electrolyte further contains a metal cation selected from (iv) lithium, magnesium, calcium, strontium, barium, zinc, aluminum, and vanadium system.

19. The cation proton carrier is selected from an alkylphosphonium cation and an alkylsulfonium cation The system according to claim 18.

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