Method for reducing dinitrogen to ammonia and cell for reducing dinitrogen to ammonia

Abstract

The present invention provides a method for reducing dinitrogen to produce ammonia, comprising contacting a cathode of an electrochemical cell with an electrolyte, supplying dinitrogen to the electrochemical cell for cathodic reduction, and applying a potential to the cathode sufficient to reduce dinitrogen to produce ammonia, the electrolyte comprising (i) a metal cation selected from the group consisting of lithium, magnesium, calcium, strontium, barium, zinc, aluminum, vanadium, and combinations thereof, (ii) one or more anions comprising at least one anion selected from the group consisting of fluorinated sulfonylimides, fluorinated sulfonylmides, and combinations thereof, (iii) a proton carrier, and (iv) optionally at least one phosphonium cation, the metal cation being present in the electrolyte at a concentration greater than 0.5 mol / L, and the total amount of the metal cation and any at least one phosphonium cation in the electrolyte being greater than 1 mol / L.

Description

[Technical field]

[0001] The present invention relates to a method for reducing dinitrogen to produce ammonia, the method comprising contacting a cathode of an electrochemical cell with an electrolyte comprising a high concentration of a metal cation (e.g., lithium), at least one anion selected from the group consisting of fluorinated sulfonyl imides, fluorinated sulfonyl methides, and combinations thereof, and a proton carrier, providing dinitrogen to the electrochemical cell for cathodic reduction, and applying a potential to the cathode sufficient to reduce the dinitrogen to form ammonia. The present invention further relates to an electrochemical cell for reducing dinitrogen to produce ammonia. [Background technology]

[0002] Providing enough food and energy to meet the demands of a burgeoning world population remains an ongoing challenge for humanity. New technologies for dinitrogen (N2) fixation to form ammonia (NH3) offer a potential solution to both of these challenges. Synthetic ammonia-based fertilizers are already essential for global food production, and the high energy density of NH3 offers great prospects for its use as a transportable fuel or carrier of renewable energy.

[0003] The invention of the Haber-Bosch process in the 20th century provided the first route to industrially produce large amounts of synthetic ammonia. However, the dinitrogen triple bond (N≡N, 942 kJmol -1 Due to the extraordinary stability of N2, 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, typically sourced from a natural gas steam reforming process. As a result, the process consumes about 2% of the world's energy supply and accounts for about 1.5% of global greenhouse gas emissions. Therefore, there is an urgent need for N2 conversion technologies to NH3 powered by renewable resources.

[0004] If successful, the electrochemical nitrogen reduction reaction (NRR) process could enable the direct conversion of renewable electricity to NH3 in a simple electrolysis cell. The cathodic half-reaction of NRR is shown in equation (1): [ka]

[0005] Instead of relying on steam reforming H2, the protons required for NRR can be supplied by H2 generated from the anodic oxidation of water (oxygen evolution reaction) or a sustainable water splitting process. - and 6H + Since the NRR of is kinetically slow, the easier 2e - and 2H + The hydrogen evolution reaction (HER) is electrochemically less favorable than the hydrogen evolution reaction (HER). As a result of competition with the HER, many reported electrochemical syntheses of NH3 suffer from very low faradaic efficiency and / or low NH3 yields. [ka]

[0006] Suitable metals, including lithium, magnesium, calcium, strontium, barium, zinc, aluminum, and vanadium, are used to form the corresponding nitrides. The metals, preferably in liquid form, are first formed by electrolyzing molten salts containing the metal ions at high temperatures, and then reacted with nitrogen to form the metal nitrides. Once the nitride formation is complete, it is separated and introduced into the anode compartment of an electrochemical cell, where protons are produced and ultimately ammonia is produced. The need to manipulate the metal nitride compounds between separate process environments requires complex, multi-step processes that are capital intensive and energy inefficient.

[0007] Another approach that has been developed previously is continuous lithium-mediated electrochemical ammonia synthesis, as reported for example by Tsuneto et al, Chemistry Letters 1993, 851-854. In a typical lithium-mediated continuous electrochemical NH3 synthesis reaction, the electrolyte system contains a lithium salt, such as lithium triflate (LiOTf), lithium perchlorate (LiClO4) or lithium tetrafluoroborate (LiBF4), and a proton carrier (proton donor) in an organic solvent, such as tetrahydrofuran. The reaction mechanism is shown in Figure 1, where the source of protons is the oxidation of the anode H2. At the cathode 102, lithium cations (Li + ) is reduced to metallic lithium (Li), which spontaneously reacts with dinitrogen (N2) to form lithium nitride (Li3N). Li3N is then protonated by a proton carrier (BH) present in the electrolyte to produce ammonia and a deprotonated proton carrier (proton acceptor B), regenerating lithium cations. At the anode 104, protons (H + ) is produced. These protons protonate B in the electrolyte to regenerate the proton carrier (BH), completing the reaction cycle. Because protons only indirectly participate in the nitrogen reduction reaction, competition with HER is expected to be minimal.

[0008] Despite the great progress to date, the development of a commercially viable lithium-mediated NRR process requires further improvements in yield and selectivity (faradaic efficiency). Moreover, this performance must be sustainable for extended reactions, e.g., for many days, without interrupting the reaction. A key challenge for lithium-mediated NRR is the formation of insoluble by-products on the cathode. The deposition of these electrolyte decomposition materials causes a rapid increase in the cell's internal resistance and thus in the cell voltage required to drive the desired reaction, ultimately leading to unstable and deteriorated electrochemical performance and premature termination of the reaction. Furthermore, parasitic decomposition reactions consume some of the charge of the electrochemical process, thus reducing the faradaic efficiency and gradually and irreversibly destroying the solvent, electrolyte and / or proton carriers.

[0009] Thus, there is a need for new methods of reducing dinitrogen to produce ammonia that at least partially address one or more of the above-mentioned shortcomings or provide a useful alternative.

[0010] The reference herein to a patent document or other matter stated as prior art is not to be construed as an admission that the document or matter was publicly known or that the information it contains was part of the common general knowledge at the priority date of the claim. Summary of the Invention

[0011] In a first aspect, the present invention provides a method for reducing dinitrogen to produce ammonia, the method comprising contacting a cathode of an electrochemical cell with an electrolyte, supplying dinitrogen to the electrochemical cell for cathodic reduction, and applying a potential to the cathode sufficient to reduce dinitrogen to produce ammonia, the electrolyte comprising (i) a metal cation selected from the group consisting of lithium, magnesium, calcium, strontium, barium, zinc, aluminum, vanadium, and combinations thereof, (ii) one or more anions comprising at least one anion selected from the group consisting of fluorinated sulfonylimides, fluorinated sulfonylmides, and combinations thereof, (iii) a proton carrier, and (iv) optionally at least one phosphonium cation, the metal cation being present in the electrolyte at a concentration greater than 0.5 mol / L, and the combined amount of the metal cation and any at least one phosphonium cation in the electrolyte being greater than 1 mol / L.

[0012] Surprisingly, it has been found that the use of fluorinated sulfonylimides or methide anions in combination with high concentrations of suitable cations in the electrolyte results in very significant improvements in yield and / or faradaic efficiency in continuous metal-mediated electrochemical dinitrogen reduction compared to electrolytes previously used in this process. The reduction performance is particularly significant at metal cation concentrations above 1 mol / L, or (b) at metal concentrations above 0.5 mol / L when phosphonium cations are added such that the combined metal and phosphonium concentration is above 1 mol / L. The metal cation (and phosphonium cation, if present) is typically the most abundant cation in the electrolyte, and it is preferred that interfering non-metallic cations, such as imidazolium and pyrrolidinium cations, are absent or present only in sufficiently low amounts that they do not unacceptably affect the dinitrogen reduction reaction. More preferably, the fluorinated sulfonylimides or methide anions are the major or only anions present in the electrolyte. Up to 500 nmol s -1 cm -2 Overall yields of 0.1 nm (normalized to the cathode surface area) and nearly quantitative faradaic efficiencies (>98%) were obtained during 24 or 96 h of reaction.

[0013] Without wishing to be limited by any theory, it has been proposed that metal cations and bulky, electrochemically stable fluorinated sulfonylimides or methide anions form a protective ionic aggregate at the electrolyte-electrode interfacial layer on the cathode surface during electrochemical reduction. This protective interface, enhanced by high ion concentrations, allows for high rates of dinitrogen reduction while suppressing the decomposition of solution components (anions, solvent molecules, proton carriers, etc.) and excessive precipitation of products of reduction processes involving metal mediators (metals, metal nitrides, metal hydrides, etc.). The result is a highly productive and selective reduction of dinitrogen to ammonia that can be sustained over long reaction times.

[0014] The protective effect of the electrolyte composition described in this disclosure can be discerned by analysis of the cathode surface after electrochemical reaction. After prolonged reaction in an electrolyte containing a high concentration of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), the cathode surface was visually pristine. Microscopy and spectroscopic analysis showed the presence of a thin (<10 nm) but coherent solid layer consisting of LiF, SO species, and intact LiTFSI. In contrast, the use of other weakly coordinating anions or lower lithium concentrations in the electrolyte resulted in the deposition of significant amounts of insoluble products on the cathode.

[0015] Fluorinated sulfonyl imides and methide anions have been used previously in lithium-based electrolytes for secondary batteries to promote cycling stability through the formation of a stable solid-electrolyte interface (SEI) layer at the electrode. The inventors have recognized that there is an analogy between the electrolyte-electrode interface present during cycling of lithium batteries and the lithium-mediated dinitrogen reduction reaction. Thus, a range of fluorinated sulfonyl imides and methide anions suitable for lithium battery applications can be used in the methods of the present disclosure.

[0016] In a second aspect, the present invention provides a method for reducing dinitrogen to produce ammonia, the method comprising contacting a cathode of an electrochemical cell with an electrolyte, supplying dinitrogen to the electrochemical cell for cathodic reduction, and applying a potential to the cathode sufficient to reduce dinitrogen to produce ammonia, the electrolyte comprising (i) a metal cation selected from the group consisting of lithium, magnesium, calcium, strontium, barium, zinc, aluminum, vanadium, and combinations thereof, (ii) one or more anions comprising at least one anion selected from the group consisting of fluorinated sulfonylimides, fluorinated sulfonylmethides, and combinations thereof, and (iii) a proton carrier, the metal cation being present in the electrolyte at a concentration greater than 1 mol / L.

[0017] In a third aspect, the present invention provides an electrochemical cell for reducing dinitrogen to produce ammonia, the electrochemical cell comprising a cathode, an anode, an electrolyte in contact with at least the cathode, a dinitrogen source supplying dinitrogen to the electrochemical cell for cathodic reduction, and a power source connected to the cathode and the anode and capable of applying a sufficient potential to the cathode to reduce dinitrogen to produce ammonia, the electrolyte comprising: i) a metal cation selected from the group consisting of lithium, magnesium, calcium, strontium, barium, zinc, aluminum, vanadium, and combinations thereof; (ii) one or more anions comprising at least one anion selected from the group consisting of fluorinated sulfonylimides, fluorinated sulfonylmides, and combinations thereof; (iii) a proton carrier; and (iv) optionally, at least one phosphonium cation, the metal cation being present in the electrolyte at a concentration greater than 0.5 mol / L, and the total amount of the metal cation and any at least one phosphonium cation in the electrolyte being greater than 1 mol / L.

[0018] In a fourth aspect, the present invention provides an electrochemical cell for reducing dinitrogen to produce ammonia, the electrochemical cell comprising a cathode, an anode, an electrolyte in contact with at least the cathode, a dinitrogen source supplying dinitrogen to the electrochemical cell for cathodic reduction, and a power source connected to the cathode and the anode and capable of applying a potential to the cathode sufficient to reduce dinitrogen to produce ammonia, the electrolyte comprising: i) a metal cation selected from the group consisting of lithium, magnesium, calcium, strontium, barium, zinc, aluminum, vanadium, and combinations thereof, (ii) one or more anions comprising at least one anion selected from the group consisting of fluorinated sulfonylimides, fluorinated sulfonylmethides, and combinations thereof, and (iii) a proton carrier, the metal cation being present in the electrolyte at a concentration greater than 1 mol / L.

[0019] In some embodiments of the first and third aspects, the metal cations are present in the electrolyte at a concentration of greater than 0.75 mol / L, or greater than 1 mol / L.

[0020] In some embodiments of the first and third aspects, the combined amount of metal cations and any at least one phosphonium cation in the electrolyte is greater than 1.5 mol / L.

[0021] In some embodiments of the first through fourth aspects, the metal cation is present in the electrolyte at a concentration of greater than 1.25 mol / L, or greater than 1.5 mol / L, or greater than 1.75M.

[0022] In some embodiments of the first through fourth aspects, at least one anion is selected from the group consisting of fluorinated sulfonylimides.

[0023] In some embodiments of the first through fourth aspects, the fluorinated sulfonylimide is represented by Formula 1: [ka] In the formula, R f1 and R f2 are independently -F, C1-C 12 selected from the group consisting of perfluoroalkyl and fluoroaryl, or R f1 and R f2 are linked to form a perfluoroalkylene linker. In some such embodiments, R f1 and R f2 is independently selected from the group consisting of -F and C1-C6 perfluoroalkyl.

[0024] In some embodiments of the first through fourth aspects, at least one anion is selected from the group consisting of bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)imide (FSI), (trifluoromethanesulfonyl)-(fluorosulfonyl)-imide (FTFSI), tris(trifluoromethanesulfonyl)methide, and combinations thereof. In some embodiments, at least one anion consists of TFSI.

[0025] In some embodiments of the first through fourth aspects, at least one anion is present in the electrolyte at a concentration of greater than 1 mol / L, or greater than 1.25 mol / L, or greater than 1.5 mol / L.

[0026] In some embodiments of the first and third aspects, the at least one anion is present in a concentration substantially equal to or greater than the combined amount of the metal cations and any at least one phosphonium cation.

[0027] In some embodiments of the first through fourth aspects, the at least one anion comprises at least 50 mol %, or at least 80 mol %, or at least 90 mol % of the one or more anions.

[0028] In some embodiments of the first through fourth aspects, the at least one anion comprises at least 90 mol %, or substantially 100 mol %, of the total amount of weakly coordinating anions in the electrolyte.

[0029] In some embodiments of the first to fourth aspects, the metal cation is lithium.

[0030] In some embodiments of the first to fourth aspects, the electrolyte comprises at least one phosphonium cation, which may be present in the electrolyte at a concentration of greater than 0.2 mol / L, or greater than 0.4 mol / L, for example, from 0.2 mol / L to 1.5 mol / L, for example, from 0.4 mol / L to 1.2 mol / L.

[0031] In some embodiments of the first to fourth aspects, the electrolyte is substantially free of organic nitrogen cations or contains any organic nitrogen cations in a total amount of less than 0.2 mol / L, such as less than 0.1 mol / L.

[0032] In some embodiments of the first through fourth aspects, the cathode comprises a metal selected from the group consisting of Ni, Nb, Ti, Mo, Fe, Cu, Ag, Zn, and alloys thereof, hi some such embodiments, the cathode comprises metallic nickel or niobium.

[0033] In some embodiments of the first through fourth aspects, the metal surface of the cathode is uncoated or is coated with a solid interface layer comprising LiF after 6 hours of ammonia production, and the thickness of the solid interface layer is 100 nm or less, or 50 nm or less, or 10 nm or less.

[0034] In some embodiments of the first and second aspects, producing ammonia comprises deprotonating a proton carrier to form a proton acceptor, and the method further comprises regenerating a proton carrier in the electrolyte by protonating the proton acceptor with protons introduced to the electrolyte by anodic oxidation of a hydrogen-containing species in the electrochemical cell. The hydrogen-containing species can optionally be dihydrogen or water.

[0035] In some embodiments of the first and second aspects, the proton carrier and the proton acceptor (formed by deprotonation of the proton carrier) are present in the electrolyte at a combined concentration greater than 0.001 mol / L, or greater than 0.01 mol / L, or in the range of 0.05 mol / L to 0.5 mol / L. In some embodiments of the second aspect, the proton carrier and any proton acceptor formed by deprotonation of the proton carrier are present in the electrolyte at a combined concentration greater than 0.001 mol / L, or greater than 0.01 mol / L, or in the range of 0.05 mol / L to 0.5 mol / L.

[0036] In some embodiments of the first and second aspects, producing ammonia comprises reducing a metal cation and dinitride at a cathode to form a metal nitride, and reacting the metal nitride with a proton carrier to produce ammonia.

[0037] In some embodiments of the first through fourth aspects, the proton carrier is selected from the group consisting of (a) a neutral proton carrier capable of reversible deprotonation to form an anionic proton acceptor, and (b) a cationic proton carrier capable of reversible deprotonation to form a neutral proton acceptor, wherein the neutral proton acceptor is an ylide.

[0038] In some such embodiments, the proton carrier is (a) a neutral proton carrier, and the neutral proton carrier is selected from the group consisting of an alcohol and an acid, hi some embodiments, the neutral proton carrier is an alcohol, optionally selected from the group consisting of methanol, ethanol, propanol, and butanol.

[0039] In other embodiments, the proton carrier is (b) a cationic proton carrier. In some such embodiments, the cationic proton carrier is selected from the group consisting of alkylphosphonium cations and alkylsulfonium cations, and the neutral proton acceptor is selected from the group consisting of phosphonium ylides and sulfonium ylides. The cationic proton carrier may be a tetraalkylphosphonium cation, optionally with [PR 6 R 7 R 8 R 9 ] + (In the formula, R 6 , R 7 , R 8 and R 9 is C1-C 20 n-alkyl).

[0040] In some embodiments of the first to fourth aspects, the electrolyte further comprises (iv) one or more molecular solvents selected from the group consisting of ethers, polyethers, glycol ethers, fluorinated ethers, fluorinated alkyls, fluorinated cycloalkyls, carbonates, sulfolane, and dimethyl sulfoxide.

[0041] In some embodiments of the first through fourth aspects, the electrolyte comprises an aprotic donor solvent capable of solvating metal cations.

[0042] In some embodiments of the first and second aspects, dinitrogen is supplied to the electrochemical cell for cathodic reduction by contacting the electrolyte with dinitrogen at a dinitrogen partial pressure of greater than 1 bar, or greater than 5 bar, or greater than 10 bar. In some embodiments of the third and fourth aspects, a dinitrogen source supplies dinitrogen to the electrochemical cell for cathodic reduction by contacting the electrolyte with dinitrogen at a dinitrogen partial pressure of greater than 1 bar, or greater than 5 bar, or greater than 10 bar.

[0043] In some embodiments of the first and second aspects, the cathode is not in contact with gaseous dinitrogen at the meniscus between the static gas and the electrolyte when ammonia is produced. In some embodiments of the third and fourth aspects, the electrochemical cell is configured such that the cathode is not in contact with gaseous dinitrogen at the meniscus between the static gas and the electrolyte when ammonia is produced.

[0044] In some embodiments of the first and second aspects, the potential of the cathode is below (more negative) than the apparent reduction potential of the metal cation in the electrolyte, preferably below (more negative) than -0.4 V relative to the apparent reduction potential of the metal cation in the electrolyte.

[0045] In some embodiments of the first and second aspects, the potential of the cathode is above (more positive than) −0.8 V with respect to the apparent reduction potential of the metal cation in the electrolyte.

[0046] In some embodiments of the first to fourth aspects, the viscosity of the electrolyte at 25° C. is less than 20 MPa·s.

[0047] When the terms "comprises" are used in this specification (including the claims), these terms are to be interpreted as specifying stated features, integers, steps or components, but not excluding the presence of one or more other features, integers, steps or components, or groups thereof.

[0048] Further aspects of the present invention are described in the following detailed description of the invention. [Brief description of the drawings]

[0049] Hereinafter, an embodiment of the present invention will be described by way of example with reference to the accompanying drawings. [Figure 1] FIG. 1 shows a schematic of a proposed mechanism for continuous electrochemical dinitrogen reduction to produce ammonia. [Diagram 2] FIG. 1 is a schematic diagram of a single-compartment electrochemical cell for continuous electrochemical dinitrogen reduction in accordance with an embodiment of the present invention, using H2 as the hydrogen-containing species. [Diagram 3] FIG. 1 is a schematic diagram of a membrane-separated, dual-compartment electrochemical cell for continuous electrochemical dinitrogen reduction in accordance with an embodiment of the present invention, using HO as the hydrogen-containing species. [Figure 4] FIG. 13 depicts (a) the ionic conductivity and viscosity of electrolytes containing different concentrations of LiTFSI (0.1 mol / L-3 mol / L) and ethanol (EtOH) (0.1 mol / L) in tetrahydrofuran (THF), and (b) the ammonia yield and faradaic efficiency obtained in a series of chronoamperometric electrochemical experiments with these electrolytes carried out on the nickel cathode of Example 1 at −0.55 V vs Li / Li+ and 15 bar N2 pressure. [Diagram 5]Figure 1 shows plots of ammonia yields (diamonds) and faradaic efficiencies (triangles) from a series of chronoamperometric electrochemical experiments on the nickel cathode of Example 2 carried out at -0.55 V vs Li / Li+ under 15 bar N2 pressure using electrolytes containing different lithium salts (1 mol / L or 2 mol / L) and EtOH (0.1 mol / L) in THF. Results are plotted against (a) electrolyte viscosity and (b) electrolyte ionic conductivity. [Figure 6] Figure 1 shows X-ray photoelectron (XP) spectra (S 2p region) showing sulfur species present at the cathode surface after chronoamperometric electrochemical experiments carried out on the nickel cathode of Example 1 at -0.55 V vs Li / Li+ under 15 bar N2 pressure with an electrolyte containing LiTFSI (2 mol / L) and EtOH (0.1 mol / L) in THF. NRR region B is the portion of the cathode that is fully immersed in electrolyte. NRR region A is the portion of the cathode where the electrochemical reaction took place at the static gas-electrolyte meniscus. [Figure 7] FIG. 1 shows an XP spectrum (N 1s region) showing nitrogen species present at the cathode surface after a chronoamperometric electrochemical experiment performed on the nickel cathode of Example 1 at −0.55 V vs Li / Li+ and 15 bar N2 pressure with an electrolyte containing LiTFSI (2 mol / L) and EtOH (0.1 mol / L) in THF. [Figure 8] FIG. 1 shows XP spectra (F 1s region) showing the fluorine species present at the cathode surface after chronoamperometric electrochemical experiments performed on the nickel cathode of Example 1 at −0.55 V vs Li / Li+ and 15 bar N2 pressure with an electrolyte containing LiTFSI (2 mol / L) and EtOH (0.1 mol / L) in THF. [Figure 9]FIG. 1 shows an XP spectrum (Ni 2p region) showing nickel species present at the cathode surface after chronoamperometric electrochemical experiments performed on the nickel cathode of Example 1 at −0.55 V vs Li / Li+ and 15 bar N2 pressure with an electrolyte containing LiTFSI (2 mol / L) and EtOH (0.1 mol / L) in THF. [Figure 10] 1A and 1B are plots of current density, total charge passed and overall cell potential as a function of time for a series of chronoamperometric electrochemical experiments using an electrolyte containing LiTFSI (2 mol / L) and EtOH (0.1 mol / L) in THF, carried out at −0.55 V vs Li / Li+ under 15 bar N2 pressure on a single nickel cathode of Example 5. [Figure 11] FIG. 13 depicts current density as a function of time for a series of chronoamperometric electrochemical experiments using an electrolyte containing LiTFSI (2 mol / L) and EtOH (0.1 mol / L) in THF carried out on the nickel cathode of Example 6 at different cathode potentials (-0.4 V to -1 V vs Li / Li+) and under 15 bar N2 pressure. [Figure 12] FIG. 13 shows a series of 31P NMR spectra obtained in Example 9 demonstrating the reversible deprotonation of the [P666,14]+ cation in a 0.2 M [P666,14][eFAP] solution in THF by successive deprotonation reactions with lithium nitride and reprotonation reactions with weak acid. [Figure 13] FIG. 1 shows a plot of (a) ammonia yield and (b) faradaic efficiency as a function of proton carrier concentration from a series of chronoamperometric electrochemical experiments using electrolytes containing LiTFSI (2 mol / L) and different types and concentrations of proton carriers, carried out on the nickel cathode of Example 8 at −0.55 V vs Li / Li+ under 15 bar N2 pressure. [Figure 14]FIG. 13 plots ammonia yields (diamonds) and faradaic efficiencies (bars) obtained in a series of chronoamperometric electrochemical experiments using electrolytes containing LiTFSI (2 mol / L) and different alcohol proton carriers (0.1 M) carried out at −0.55 V vs Li / Li+ under 15 bar N2 pressure on the nickel cathode of Example 8. [Figure 15] FIG. 13 shows plots of ammonia yields (diamonds) and faradaic efficiencies (bars) from a series of chronoamperometric electrochemical experiments using an electrolyte containing LiTFSI (2 mol / L) and EtOH (0.1 M) at −0.55 V vs Li / Li+ and varying partial pressures of N2 and H2 at 15 bar total pressure on the nickel cathode of Example 10. [Figure 16] FIG. 13 shows plots of ammonia yields (diamonds) and faradaic efficiencies (bars) from a series of chronoamperometric electrochemical experiments using electrolytes containing LiTFSI (1-2 mol / L) and EtOH (0.1 M) with various ionic liquid additives, carried out under 15 bar N2 pressure on the nickel cathode of Example 11. [Figure 17] FIG. 13 is a plot of ammonia yields (diamonds) and faradaic efficiencies (bars) from a series of chronoamperometric electrochemical experiments using electrolytes containing LiTFSI (1.5-2 mol / L) and EtOH (0.1 M) with different amounts of phosphonium-based ionic liquid additive, performed at 1 bar N2 pressure on the nickel cathode of Example 12. [Figure 18] FIG. 13 depicts current density as a function of time for a series of chronoamperometric electrochemical experiments using electrolytes containing LiTFSI (0.2-1.5 mol / L) and EtOH (0.1 M) with different amounts of phosphonium-based ionic liquid additive, performed at 15 bar N2 pressure on the nickel cathode of Example 13. [Figure 19]FIG. 13 is a plot of ammonia yields (diamonds) and faradaic efficiencies (bars) from a series of chronoamperometric electrochemical experiments using electrolytes containing LiTFSI (0.2-1.5 mol / L) and EtOH (0.1 M) with different amounts of phosphonium-based ionic liquid additive, performed at 15 bar N2 pressure on the nickel cathode of Example 13. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] Method for producing ammonia by reducing dinitrogen The present invention relates to a method for reducing dinitrogen to produce ammonia, comprising contacting a cathode of an electrochemical cell with an electrolyte, the electrolyte comprising (i) a metal cation selected from the group consisting of lithium, magnesium, calcium, strontium, barium, zinc, aluminum, vanadium, and combinations thereof, at a concentration of greater than 0.5 mol / L in the electrolyte, (ii) one or more anions including at least one anion selected from the group consisting of fluorinated sulfonyl imides, fluorinated sulfonyl methides, and combinations thereof, (iii) a proton carrier, and (iv) optionally at least one phosphonium cation. The total amount of the metal cation and any phosphonium cation components is greater than 1 mol / L in the electrolyte. Thus, in the absence of a phosphonium cation, the metal cation is present in the electrolyte at a concentration of greater than 1 mol / L. Dinitrogen is fed to an electrochemical cell for cathodic reduction, and a potential is applied to the cathode that is sufficiently negative to reduce dinitrogen to produce ammonia. To produce ammonia, the proton carrier provides a proton, which can be deprotonated to form a proton acceptor.

[0051] Generally, electrochemical cells also include an anode, where an anodic oxidation reaction occurs during electrochemical ammonia synthesis to maintain charge neutrality and allow current to flow through the cell. The anodic oxidation of hydrogen-containing species, such as dihydrogen or water, at the anode introduces protons into the electrolyte. These protons can at least partially regenerate proton carriers in the electrolyte by reprotonating proton acceptors.

[0052] Thus, the present disclosure relates to a continuous metal-mediated (e.g., lithium-mediated) electrochemical dinitrogen reduction process. Such a process can be distinguished from sequential electrochemical processes in which dinitrogen is converted to ammonia in a series of temporally and / or spatially separated process steps, such as separate batch processes for lithium electrolysis, lithium nitride formation, and ammonia production. As explained above, it is proposed that the continuous reduction involves cycling one or more species, including metal species and / or proton carrier species, between different forms in one process step of the synthesis.

[0053] Metals and any phosphonium cation The electrolyte contains at least one metal cation that mediates or catalyzes the continuous electrochemical dinitrogen reduction. The synthesis is proposed to involve a metal nitride intermediate in the reaction cycle. Thus, a range of metals capable of forming metal nitrides from dinitrogen under electrochemical reaction conditions may be used in the present invention. In this disclosure, "metal" refers to the metal element and does not imply a specific reduction state or species. For example, if a metal form in the zero oxidation state is specifically identified in the proposed reaction mechanism, this will be referred to as that "metal form," "metal," or "metal(0)."

[0054] According to the proposed mechanism, electrochemical reduction of metal cations produces metals on the cathode, which react spontaneously with N2 to produce the corresponding metal nitrides. This latter reaction is thermodynamically favorable under the conditions of ammonia electrosynthesis (general reaction [ka] According to tabular thermodynamic data (e.g. LB Pankratz, et al, Thermodynamic Data for Mineral Technology, Washington DC, 1984 John R. Rumble, CRC Handbook of Chemistry and Physics 101st Edition, 2020), published theoretical calculations (e.g. NΦrskov et al, in Energy Environ. Sci., 2017, 10, 1621-1630) and experimental reports (e.g. DE 102018210304 A1), suitable metals include lithium, magnesium, calcium, strontium, barium, zinc, aluminum, and vanadium.

[0055] In some embodiments, the metal comprises or consists of lithium, which is believed to be particularly suitable due to its demonstrated ability to activate dinitrogen at ambient temperatures.

[0056] The metal is dissolved in the electrolyte by metal cations, e.g. Li + As mentioned above, the reaction cycle is believed to involve the reduction of metal cations from the electrolyte to form metal on the cathode, followed by regeneration of the metal cation as the final step of the cycle. However, it is believed possible to carry out the metal cycle without the use of soluble metal cations as intermediates, while the solid species on the cathode surface (e.g. metal nitride and metal) react continuously. Thus, for lithium, the cathodic reaction mechanism could, in principle, involve (i) the chemical reaction of metallic lithium on the cathode with dinitrogen to form lithium nitride, followed by (ii) the direct electrochemical reduction of lithium nitride in the presence of a proton donor to directly regenerate metallic lithium and produce ammonia (i.e. Li3N+3HB+3e - =NH3+3Li(0)+3B - ).

[0057] The one or more metal cations are present in the electrolyte at a concentration of more than 0.5 mol / L. In some preferred embodiments, the one or more metal cations are present in the electrolyte at a concentration of more than 0.75 mol / L, or more than 1 mol / L, or more than 1.5 mol / L. Metal cation concentrations in such ranges should be understood to mean that the electrolyte contains a very high mass fraction of metal salt. For example, 1.5 mol / L LiTFSI corresponds to 430 g / L, or more than 37 mass%. Surprisingly, it has been found that such high concentrations of metal cations in the metal-mediated NRR lead to improved electrochemical performance (faradaic efficiency and / or yield) and suppress the formation of electrolyte decomposition products on the cathode. Without wishing to be limited by any theory, it is proposed that the high ion concentration increases the protective ion aggregation of the metal cations and the fluorinated sulfonylimide or methide at the cathode surface. The upper range of the metal cation concentration may be limited by the viscosity of the electrolyte, which may limit mass transport and ionic conductivity at high concentrations of electrolyte salt. Such limitations may depend on factors such as the presence and choice of any solvent, and reaction temperature. In some embodiments, the metal cation or cations may thus be present at a concentration of less than 3 mol / L, e.g., in the range of 1 mol / L to 3 mol / L, e.g., 1.5 mol / L to 2.5 mol / L. In this disclosure, "mol / L" and "M" are used interchangeably as units of molar concentration (mol / L).

[0058] The metals are most conveniently introduced into the electrochemical cell in cationic form, for example by dissolving a suitable metal salt in the electrolyte. However, it is not excluded that the metals are introduced as metal nitrides or in metallic form. Metal cations may be generated in situ in the electrolyte from such species.

[0059] Metal cations, such as lithium cations, may be the most abundant (i.e., greater than 50% of all cations present) or the only cationic species present in significant concentrations in the electrolyte during ammonia synthesis. In some embodiments, the electrolyte is thus substantially free of non-metallic cations or contains any non-metallic cations in a total amount of less than 0.2 mol / L, e.g., less than 0.1 mol / L.

[0060] However, other cations may be present in some embodiments, provided they do not unacceptably affect electrochemical performance. In particular, phosphonium cations are suitable for complementing metal cations and have been found to increase the faradaic efficiency of dinitrogen reduction reactions in some cases. Thus, in some embodiments, the electrolyte comprises at least one phosphonium cation in a sufficient amount such that the total amount of metal and phosphonium cations is greater than 1 mol / L in the electrolyte. In some embodiments, the phosphonium cation is present in the electrolyte in an amount greater than 0.2 mol / L (e.g., 0.2 mol / L to 1.5 mol / L), or greater than 0.4 mol / L (e.g., 0.4 mol / L to 1.2 mol / L).

[0061] However, certain other non-metallic cations, such as imidazolium cations and pyrrolidinium cations, have been found by experiment to have a detrimental effect on the faradaic efficiency and / or yield of metal-mediated NRR.Thus, in some embodiments, the electrolyte is substantially free of imidazolium cations and pyrrolidinium cations, or contains any imidazolium cations and pyrrolidinium cations in a total amount of less than 0.5 mol / L, or less than 0.2 mol / L, for example less than 0.1 mol / L.In some embodiments, the electrolyte is substantially free of organic nitrogen cations, or contains any organic nitrogen cations in a total amount of less than 0.5 mol / L, or less than 0.2 mol / L, for example less than 0.1 mol / L.In the present disclosure, organic nitrogen cations refer to organic cations that contain cationic nitrogen centers, including imidazolium, pyrrolidinium, ammonium, etc. In some embodiments, the electrolyte is substantially free of non-phosphonium non-metal cations, or contains any non-phosphonium non-metal cations in a total amount of less than 0.5 mol / L, or less than 0.2 mol / L, for example less than 0.1 mol / L.

[0062] The phosphonium cation, if present, may suitably be an alkylphosphonium cation, such as a tetraalkylphosphonium cation. A tetraalkylphosphonium cation has the structure [PR 6 R 7 R 8 R 9 ] + (In the formula, R 6 , R 7 , R 8 and R 9 is C1-C 20 n-alkyl). In some embodiments, R 6 , R 7 , R 8 and R 9The total number of carbon atoms in R is at least 7, or at least 13, or at least 16. As will be appreciated by those of skill in the art, increasing the total chain length of the tetraalkylphosphonium cation generally increases its solubility in organic media, decreases the melting point of the salt, and decreases its tendency to absorb or dissolve water. 6 , R 7 and R 8 is C4-C 20 n-alkyl; R 9 is C1-C 20 It is n-alkyl.

[0063] In some embodiments, the alkylphosphonium cation is an ionic liquid cation, meaning that it can form an ionic liquid, e.g., a room temperature ionic liquid, when paired with a suitable counterion (e.g., at least one anion selected from the group consisting of fluorinated sulfonylimides, fluorinated sulfonylmethides, and combinations thereof). In this disclosure, an ionic liquid is a salt with a melting point below 100° C., and a room temperature ionic liquid has a melting point below 25° C. Such cations are believed to be preferred due to their high solubility / miscibility with other solvents and salts in the electrolyte, high electrical conductivity, and ability to dissolve high concentrations of N2 (in ionic liquids).

[0064] Anion The electrolyte comprises one or more anions, including at least one anion selected from the group consisting of fluorinated sulfonyl imides, fluorinated sulfonyl methides, and combinations thereof. The one or more anions are present to balance the charge of the cationic species present in the electrolyte, including the metal cation and any other cationic species present. In this disclosure, anion and anion have the same meaning.

[0065] The anions, especially those that are purposely incorporated into the electrolyte prior to initiation of the electrochemical reaction, are preferably weakly coordinating anions. A wide range of weakly coordinating anions, sometimes referred to as non-coordinating anions, are known in the field of electrochemical synthesis and ionic liquid technology. Non-limiting examples of weakly coordinating anions include tetrafluoroborate, hexafluorophosphate, perchlorate, fluoroalkyl phosphates such as tris(pentafluoroethyl)trifluorophosphate, fluoroaryl borates such as tetrakis[3,5-bis(trifluoromethyl)phenyl]borate and tetrakis(pentafluorophenyl)borate, fluoroalkyl borates such as tetrakis[hexafluoroisopropyl]borate, fluorinated sulfonates such as trifluoromethanesulfonate (triflate) and other perfluoroalkylsulfonates (e.g., perfluorohexanesulfonate), fluorinated sulfonylimides such as bis(fluorosulfonyl)imide, bis(trifluoromethanesulfonyl)imide and (fluorosulfonyl)-(trifluoromethanesulfonyl)imide, and fluorinated sulfonyl methides such as tris(trifluoromethanesulfonyl)methide.

[0066] Although weakly coordinating anions are preferred for electrochemical applications, it is not precluded that other anions are present in the electrolyte in combination with the fluorinated sulfonylimide or methide anion(s). For example, chloride is believed to be sufficiently stable to anodic oxidation in some embodiments, such as when the anodic reaction is H2 oxidation, and therefore may be used.

[0067] The one or more anions present in the electrolyte include at least one anion selected from the group consisting of fluorinated sulfonylimides, fluorinated sulfonyl methides, and combinations thereof. In the present disclosure, a fluorinated sulfonylimide is defined as an anion having one or two fluorinated sulfonyl groups (i.e., -SO2-R f , where R f is a fluorine-substituted organyl group) is a monovalent anion containing a covalently bonded negative carbon atom.

[0068] Fluorinated sulfonyl imides and methides are electrochemically stable weakly coordinating anions due to high charge delocalization from the formally negative nitrogen or carbon to the electron-withdrawing fluorinated sulfonyl group. At least some fluorinated sulfonyl imides and methides are also sterically bulky anions. Fluorinated sulfonyl imides and methide anions can promote cycling stability by forming a stable solid-electrolyte interface (SEI) layer on the electrode and have been used in lithium-based electrolytes for secondary batteries. Thus, for example, their use in lithium metal battery electrolytes can suppress the formation of lithium dendrites and electrolyte decomposition at the lithium metal anode during battery charging. The inventors have recognized the similarity between such battery processes and the electrochemical nitrogen reduction reaction via the lithium cathode. Thus, the range of fluorinated sulfonyl imides and methide anions previously demonstrated or proposed for lithium battery applications are suitable for the methods of the present disclosure. It should be noted that the nomenclature of these structures is unsystematic in the literature and the IUPAC naming conventions are unclear. For example, the anion [CF3SO2-N-SO2CF3] - are referred to as imides or amides, although the term imide is used herein to refer to the negatively charged nitrogen.

[0069] In some embodiments, the fluorinated sulfonylimide has a structure according to Formula 2: [ka]

[0070] In formula 2, R f is a fluorinated organyl group, optionally selected from the group consisting of -F, fluoroalkyl (e.g., perfluoroalkyl) and fluoroaryl (e.g., perfluoroaryl). R EWG is an electron withdrawing group, optionally a sulfonyl (e.g., a fluorinated sulfonyl, -SO2-R f), cyano (-CN), and acyl groups (e.g., fluorinated acyl, -C(=O)-R f ), and nitroso (-N=O). Optionally, R f and R EWG are linked to form a ring structure.

[0071] Exemplary compounds according to Formula 2 that have been demonstrated or proposed for use in lithium batteries include: (i) methyl carbonate (trifluoromethanesulfonyl)imide (R f =-CF3;R EWG =-C(=O)-CH3) (Gunderson-Briggs et al, Angew. Chem. Int. Ed. 2019, 58, 4390), (ii) cyano(trifluoromethanesulfonyl)imide and cyano(perfluorobutanesulfonyl)imide (R f =-CF3,-C4F9;R EWG =-CN) (U.S. Pat. No. 6,294,289), (iii) bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)imide (FSI), and (trifluoromethanesulfonyl)-(fluorosulfonyl)imide (FTFSI) (R f =-CF3,-F;R EWG =-SO2-CF3, -SO2-F), (iv) symmetrical and asymmetrical bis(sulfonyl)imide salts containing various fluoroaryl groups (R f =-CF 3、 Ar f ;R EWG =-SO2-Ar f , where Ar f = partially fluorinated aryl or perfluoroaryl groups) (Huang et al, Energy Environ. Sci., 2018, 11, 1326), and (v) cyclo-difluoromethane-1,1-bis(sulfonyl)imide (R f is R EWG to form -CF2-) (Murmann et al, Phys. Chem. Chem. Phys., 2015, 17, 9352).

[0072] In some embodiments, the fluorinated sulfonyl methide has a structure according to Formula 3: [ka]

[0073] In formula 3, R f is a fluorinated organyl group, optionally selected from the group consisting of -F, fluoroalkyl (e.g., perfluoroalkyl) and fluoroaryl (e.g., perfluoroaryl). R EWG are each independently an electron-withdrawing group, optionally including a sulfonyl (e.g., a fluorinated sulfonyl, -SO-R f ), cyano (-CN), and acyl groups (e.g., fluorinated acyl, -C(=O)-R f ), and nitroso (-N=O). Optionally, R f and R EWG are linked to form a ring structure.

[0074] Exemplary compounds according to Formula 3 that have been demonstrated or proposed for lithium battery applications include: (i) bis(cyano)(trifluoromethanesulfonyl)methide and bis(cyano)(perfluorobutanesulfonyl)methide (R f =-CF3,-C4F9;R EWG =-CN) (U.S. Pat. No. 6,294,289), and (ii) tris(trifluoromethanesulfonyl)methide (R f =-CF3;R EWG =-SO2-CF3) (Walker et al, J. Electrochem. Soc., Vol. 143, 1996).

[0075] In some embodiments, at least one anion is optionally selected from the group consisting of fluorinated sulfonylimides having a structure according to Formula 2. In some embodiments, one or more fluorinated sulfonylimides have the structure of Formula 1: [ka]

[0076] In formula 1, R f1 and R f2 are independently -F, C1-C 12 selected from the group consisting of perfluoroalkyl and fluoroaryl (optionally perfluoroaryl), or R f1 and R f2 are linked to form a perfluoroalkylene linker. In some embodiments, R f1 and R f2 is independently selected from the group consisting of F and C1-C6 perfluoroalkyl. In some embodiments, at least one anion is selected from the group consisting of bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)imide (FSI), (trifluoromethanesulfonyl)(fluorosulfonyl)imide (FTFSI), and combinations thereof. In some embodiments, at least one anion is a single fluorinated sulfonylimide as described herein. In some such embodiments, at least one anion is TFSI (also referred to as NTf2).

[0077] One or more anions selected from fluorinated sulfonyl imides, fluorinated sulfonyl methides and combinations thereof may be present in the electrolyte at a concentration of more than 1 mol / L, preferably more than 1.25 mol / L, more preferably more than 1.5 mol / L. In some embodiments, the concentration is substantially the same as the metal cation concentration. This is the case when the electrolyte is formulated only with one or more salts of metal cations and fluorinated sulfonyl imides and / or one or more fluorinated sulfonyl methide anions. When the electrolyte includes one or more phosphonium cations, the concentration may be substantially the same as the total concentration of metal cations and one or more phosphonium cations. This is the case when the electrolyte is formulated in combination with (i) one or more salts of metal cations and one or more anions of fluorinated sulfonyl imides and / or fluorinated sulfonyl methides, and (ii) one or more salts of one or more phosphonium cations and one or more anions of fluorinated sulfonyl imides and / or fluorinated sulfonyl methides.

[0078] The anion selected from fluorinated sulfonylimides, fluorinated sulfonylimides, and combinations thereof is preferably the most abundant (i.e., greater than 50%) or the only weakly coordinating anion present in the electrolyte. In some embodiments, the anion comprises at least 50 mol%, or at least 80 mol%, or at least 90 mol%, or substantially 100 mol% of the total amount of weakly coordinating anions present in the electrolyte. Without wishing to be limited by any theory, it is proposed that the minimization or elimination of other anions promotes protective ion assembly of the metal cation and the fluorinated sulfonylimide (or methide) anion at the cathode surface.

[0079] In some embodiments, the anion is the most abundant (i.e., greater than 50%) anion in the electrolyte or is the only anion present in the electrolyte, excluding any anionic reaction intermediates (such as deprotonated proton carriers). In some embodiments, the at least one anion comprises at least 50 mol%, or at least 80 mol%, or at least 90 mol% of the total amount of anions present in the electrolyte.

[0080] The fluorinated sulfonylimides and / or methides are usually introduced into the electrolyte together with the metal and any phosphonium cations, e.g. as metal salts or phosphonium ionic liquid additives, but it is not excluded that they may also be introduced as counterions of other cationic species in the electrolyte, e.g. cationic proton carriers.

[0081] Proton Carrier The electrolyte contains a proton carrier. As explained in this disclosure, the role of the proton carrier (BH in FIG. 1) is to provide protons for the cathodic reduction of dinitrogen to ammonia. In at least some embodiments, this is proposed to occur by reaction with a metal nitride intermediate formed on the cathode. The resulting deprotonated proton carrier (B in FIG. 1) becomes a proton acceptor and can be regenerated in the electrolyte by reprotonation with protons introduced into the electrolyte by the anodic reaction. In the entire continuous process, the proton carrier transports or shuttles protons generated at the anode to react at the cathode. Protons are not reduced to H2 via HER because they are intercepted by the deprotonated proton carrier before reaching the cathode.

[0082] As a result, the proton carrier should be reactive with Li3N or other metal nitride to produce NH3, but in such embodiments, only weakly acidic to reduce the rate of competitive proton reduction to dihydrogen and / or hydride. Also, the proton carrier is preferably capable of cycling through multiple deprotonation / regeneration cycles with minimal side reactions. In this way, the proton carrier plays a catalytic role in the process, minimizing consumption of the proton carrier reagent and undesired decomposition reactions at the cathode or anode. Thus, a weakly acidic proton carrier capable of shuttling between protonated and deprotonated forms is preferred, but alternatively, the proton carrier may be capable of transferring protons (H + ) or hydronium ion (HO + ) is not excluded.

[0083] During continuous NH3 synthesis, especially at or near steady-state operating conditions, the electrolyte can contain a mixture of both proton carriers and their corresponding deprotonated forms (proton acceptors). In the absence of accumulation of other materials (e.g., Li, Li3N, LiH), a continuously operated cell reaches a steady-state relative concentration of both species when the production of protons at the anode exactly matches the consumption at the cathode (desired NH3 or by-products such as H2). In fact, it has been recognized that the presence of both species in the electrolyte creates a desirable buffering effect. This buffering effect allows the proton carriers to absorb excess proton production at the anode during start-up and / or current fluctuations due to intermittent operation. Furthermore, operating with a fairly high concentration of proton acceptors in the electrolyte ensures that a high proportion of protons are blocked and consumed before they can participate in undesirable cathode reactions such as HER.

[0084] It is understood that the suitable concentration of the proton carrier will depend on the specifications of the overall electrochemical system, including the choice of proton carrier molecule. In some embodiments, the proton carrier and proton acceptor are present in the electrolyte at a combined concentration of greater than 0.001 mol / L, or greater than 0.01 mol / L, or in the range of 0.05 mol / L to 0.5 mol / L.

[0085] The proton carrier may be neutral or cationic and may be capable of reversible deprotonation to form the corresponding anionic or neutral proton acceptor. In this disclosure, reversible deprotonation means that the proton carrier can be deprotonated to form the proton acceptor and then reprotonated to regenerate the proton carrier. Consistent with the proposed mechanism, the proton carrier can be deprotonated to the proton acceptor by reaction with a metal nitride such as Li3N, preferably in solution at room temperature. The proton acceptor can be protonated to form the proton carrier by reaction with free protons and / or organic acids, preferably in solution at room temperature. The inventors have found that such reactions provide a convenient method of evaluating potential proton carriers for use in the methods of the invention.

[0086] In some embodiments, the proton carrier is a neutral proton carrier capable of reversible deprotonation to form an anionic proton acceptor. A wide range of neutral proton carriers are effective, including alcohols, ethers, and acids. In some embodiments, the neutral proton carrier is an alcohol, such as methanol, ethanol, propanol, or butanol. In some embodiments, the acid is the conjugate acid (protonated form) of a weakly coordinating anion, such as a fluorinated sulfonylimide and / or methide present in the electrolyte.

[0087] In some embodiments, the proton carrier is a cationic proton carrier that can be reversibly deprotonated to form a neutral proton acceptor that is an ylide. The cationic proton carrier and its corresponding proton acceptor are typically organic species. The neutral proton acceptor molecule is an ylide, which is a neutral dipolar molecule in which a formally negatively charged atom is directly bonded to a formally positively charged heteroatom. Thus, an ylide is a type of zwitterion.

[0088] Without wishing to be bound by any theory, it is believed that suitable ylides can be reversibly interconverted into cationic proton donors by protonation and deprotonation reactions as required, since some of the electrons of the negative charge are shared with the vacant orbitals of the positive charge center, which is believed to provide the protonated form with the acidity in the weak acidity range required for the proton donor in the metal-mediated continuous ammonia synthesis.

[0089] In some embodiments, the ylide contains a carbanion adjacent to a positively charged heteroatom, and thus the proton carrier site on the molecule is a carbon atom, which transitions from a carbanion in the deprotonated form to a C-H covalent bond in the protonated form.

[0090] In some embodiments, the cationic proton carrier is an alkyl phosphonium cation or an alkyl sulfonium cation and the neutral proton acceptor is the corresponding phosphonium ylide or sulfonium ylide.

[0091] In some embodiments, the cationic proton carrier is a phosphonium cation and the neutral proton acceptor is the corresponding phosphonium ylide. The phosphonium cation can be an alkylphosphonium cation. In the present disclosure, an alkylphosphonium cation refers to a phosphonium cation that includes at least one optionally substituted alkyl group. An alkylphosphonium cation is generally a phosphonium-carbanion ylide (R')3P + -C -(R″) can be any such species capable of deprotonation to (R″), where each of the R′ and R″ organyl groups can be the same or different.

[0092] The formation of ylides by deprotonation of alkylphosphonium cations is known in the field of organic synthesis, and ylides are commonly referred to as Wittig reagents. Phosphonium-carbanion ylides are useful as nucleophiles in many synthetic reaction schemes. For example, in the Wittig reaction, phosphonium ylides react with carbonyl groups via a [2+2] cycloaddition reaction to form oxaphosphetane, which subsequently eliminates to produce an alkene and a phosphine oxide. Synthetic reactions using ylide reagents generally proceed via the irreversible conversion of a reactive ylide to a stable species such as a phosphine oxide.

[0093] In contrast, embodiments of the present invention use phosphonium ylides as reversible proton shuttle agents to intercept and transport protons in the electrolyte for a protonation reaction with nitrogen to form ammonia.

[0094] A wide range of alkylphosphonium cations are believed to be suitable for the present invention, subject only to the requirement that they be susceptible to reversible deprotonation to an ylide proton acceptor, for example as shown in Scheme 1. Thus, the alkylphosphonium cation has the structure of Formula 4, and the corresponding ylide has the structure of Formula 5. [ka]

[0095] In some embodiments, R 1 , R 2 , R 3 are independently alkyl (e.g., C1-C 20 n-alkyl groups) and aryl (e.g., phenyl groups); R 4 is hydrogen, alkyl (e.g. C1-C 20 n-alkyl groups) and aryl (e.g., phenyl groups); R5 is hydrogen, alkyl (e.g. C1-C 19 alkyl groups), cycloalkyl groups (e.g., C3-C6 cycloalkyl groups), alkyl groups (e.g., C1-C 19 alkyl), cycloalkyl, aryl (e.g., -C6F5 containing a phenyl group), ester (e.g., -C(=O)O(C1-C6 alkyl), amide (e.g., C(=O)NHC6F5, C(=O)N(Me)OMe), nitrile (-CN), halogen, ether (e.g., -O(C1-C6 alkyl), thioether (e.g., -S(C1-C6 alkyl), -SC6F5), -PR 10 R 11 and -P(=O)R 12 R 13 where R 10 ~R 13 are independently alkyl (e.g., C1-C6) alkyl and aryl (e.g., -C6F5). 1 ~R 4 Any of the alkyl and aryl groups in R may be unsubstituted or substituted with substituents such as halogen, ether, hydroxy, ester, acyl, amino and nitrile functional groups; 1 ~R 4 Any two of these may be linked to form a cyclic structure.

[0096] As will be appreciated by those skilled in the art, the group R 1 ~R 5 , especially R 5 can be selected to control the acidity, and therefore the proton donating ability, of the alkylphosphonium cation.

[0097] In some embodiments, R 1 , R 2 and R 3 is C1-C 20 R is independently selected from n-alkyl and phenyl; 4 is hydrogen, and R 5 is hydrogen and C1-C 19 n-alkyl.

[0098] In some embodiments, the alkylphosphonium cation is an ionic liquid cation, meaning that when paired with a suitable counterion, it can form an ionic liquid, e.g., a room temperature ionic liquid. Phosphonium cations of general formula 4, including tetraalkylphosphonium cations, are BF4 - , PF6 - , fluoroalkyl phosphates including tris(pentafluoroethyl)trifluorophosphate (eFAP); fluoroalkyl borates such as tetrakis[hexafluoroisopropyl]borate; fluorinated bis(sulfonyl)imides including bis(fluorosulfonyl)imide bis(trifluoromethanesulfonyl)imide and (fluorosulfonyl)-(trifluoromethanesulfonyl)imide; fluorinated sulfonates including triflates as well as other perfluoroalkyl sulfonates to form ionic liquids. Ionic liquids of this type have been used without dissolved metal cations in the context of nitrogen reduction in the prior art (e.g. MacFarlane et al., WO 2017 / 132721). However, at the cathodic potentials disclosed in the prior art, these ionic liquids do not show a tendency to deprotonate. To achieve the formation of metal nitrides, for example at -2.0 V vs Ag / Ag + At significantly more negative potentials than required in this case, such as more negative than 0.1 V, these ionic liquids can become active proton donors (especially in the absence of more reactive neutral proton carriers).

[0099] In some embodiments, the alkylphosphonium cation is a tetraalkylphosphonium cation. The tetraalkylphosphonium cation is represented by the formula [PR 6 R 7 R 8 R 9 ] + (In the formula, R 6 , R 7 , R 8 and R 9 is C1-C 20n-alkyl). In some embodiments, R 6 , R 7 , R 8 and R 9 The total number of carbon atoms in R is at least 7, or at least 13, or at least 16. As will be appreciated by those of skill in the art, increasing the bond chain length of the tetraalkylphosphonium cation generally increases its solubility in organic media, decreases the melting point of the salt, and decreases the tendency to absorb or dissolve water. In some embodiments, R 6 , R 7 and R 8 is C4-C 20 n-alkyl; R 9 is C1-C 20 It is n-alkyl.

[0100] The proton carrier system may be introduced into the chemical cell in protonated form, for example, by dissolving an appropriate salt of a neutral proton carrier or cationic proton carrier species in the electrolyte. However, it will be understood that either a protonated proton carrier or a corresponding proton acceptor may be initially provided to the electrochemical cell to facilitate the NH synthesis reaction.

[0101] solvent The electrolyte is typically a liquid, and preferably has a viscosity as low as possible to avoid or tolerate mass transfer limitations. In some embodiments, the 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. The viscosity can be measured according to ISO 12058 using a Lovis 2000M Anton Paar viscometer (Lovis angle 30°).

[0102] The electrolyte can thus include one or more non-aqueous solvents. Suitable non-aqueous solvents are generally aprotic solvents, such as aprotic molecular solvents. The solvent should preferably be stable under the reaction conditions or at most decompose only to a small extent.

[0103] In some embodiments, the electrolyte comprises one or more molecular solvents selected from the group consisting of ethers, polyethers (e.g., methylated polyethers), glycol ethers (e.g., methylated glycol ethers such as tetraglyme), fluorinated ethers, fluorinated alkyls, fluorinated cycloalkyls, carbonates, sulfolane, and dimethylsulfoxide. An example of a suitable ether solvent is tetrahydrofuran (THF).

[0104] In some embodiments, the electrolyte comprises an aprotic donor solvent capable of solvating metal cations. Examples of suitable solvents include THF, cyclopentyl methyl ether (CPME), carbonate, dimethoxyethane, glyme, and dioxolane. Without wishing to be limited by any theory, it is proposed that such solvents may facilitate the necessary high concentration of metal cations in the electrolyte by coordinating to the metal cations. The presence of the aprotic donor solvent may also advantageously increase the conductivity of high ion concentration electrolyte liquids by promoting charge separation between the metal cations and the anions. Thus, the aprotic donor solvent may be present in sufficient concentration such that each metal cation may be solvated by multiple aprotic donor solvent molecules.

[0105] In some embodiments, the liquid electrolyte comprises a room 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. Without wishing to be limited by any theory, certain ionic liquid solvents may be useful to aid in the solvation of metal cations and / or increase the solubility of N2 in the electrolyte. Optionally, the ionic liquid comprises, as an anion, at least one anion selected from the group consisting of fluorinated sulfonylimides, fluorinated sulfonylmethides, and combinations thereof.

[0106] However, some ionic liquid cations have been found experimentally to be detrimental to the dinitrogen reduction reaction, presumably because they decompose irreversibly at the cathodic potentials required to drive metal-mediated NRR. Thus, in some embodiments, the ionic liquid is not an imidazolium- or pyrrolidinium-based ionic liquid. In some embodiments, the cation of the ionic liquid is not an organic nitrogen cation. In contrast, phosphonium-based ionic liquid additives have been found to be capable of improving the faradaic efficiency and yield, especially at low N2 pressures. Such ionic liquids are sufficiently stable or exhibit desirable reactivity (as cationic proton carriers) under the conditions of metal-mediated NRR.

[0107] The electrolyte of the present disclosure may be a non-aqueous electrolyte, so that there is no water present as a solvent or proton carrier.The non-aqueous electrolyte is preferably substantially free of water, meaning that the amount of water is zero or so low that it does not interfere to a significant extent with the reaction cycle of the continuous metal-mediated electrochemical NH3 synthesis reaction as disclosed herein.For example, it may contain 1000 ppm or less of water, preferably 100 ppm or less, and most preferably 20 ppm or less of water.

[0108] Cathode, Anode and Power Supply The methods of the present disclosure are generally carried out in an electrochemical cell that includes a cathode, an anode, and a power source connected to the cathode and the anode, the power source configured to apply a voltage between the cathode and the anode sufficient to drive electrochemical ammonia synthesis.

[0109] The cathode can be any conductive electrode stable at the required reduction potential, such as the metal electrodes 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 metallic form. Non-limiting examples of suitable metals include Ni, Nb, Ti, Mo, Fe, Cu, Ag, and Zn and their alloys. In other embodiments, the metal of the cathode is made of or consists of the metal that mediates ammonia synthesis (e.g., metallic lithium).

[0110] Although the commonly understood mechanism of the lithium-mediated nitrogen reduction reaction does not suggest an electrocatalytic role of the cathode, it has nevertheless been surprisingly found that the choice of cathode material influences the yield and faradaic efficiency. Without wishing to be limited by any theory, it is proposed that protective ionic aggregates, including bulky, electrochemically stable anions and metal cations in the electrolyte-electrode interfacial layer, are formed on the cathode surface during synthesis. The electrolyte composition as disclosed herein is believed to be the primary contributor to the improved ammonia synthesis results, although the cathode surface composition may play a secondary role in promoting the formation of the desired ionic aggregates. In some embodiments, the cathode comprises metallic nickel, niobium or copper, preferably nickel or niobium, most preferably nickel.

[0111] The cathode can be a cylinder, disc, plate, or other shape suitable for the cell design. The cathode can also be made porous, for example by etching, as a foam or mass of compressed particles, or in an inverse opal structure. The desired mediator metal can also be coated, for example by electrodeposition or chemical vapor deposition, onto an underlying structure that provides optimal roughness and porosity. The cathode can also be formed by depositing metal nanoparticles onto an inert structure.

[0112] An advantage of some embodiments described in the present disclosure is that harmful fouling of the cathode may be avoided or minimized during the ongoing electrochemical reduction of dinitrogen. Instead, the formation of a very thin (<10 nm) and coherent solid interfacial layer comprising solid LiF, SO species and intact fluorinated sulfonylimide anions was observed. Without wishing to be limited to any theory, this layer may simply be benign or may beneficially mediate the reduction process occurring at the cathode surface. Thus, in some embodiments, the cathode comprises a surface covered with a thin solid interfacial layer comprising LiF (and optionally also SO species and / or intact fluorinated sulfonylimide or methide anions) under electrochemical reduction conditions, for example after generating ammonia for 6 hours. This layer may be generated in situ by electrolytic reduction of the electrolyte or by another suitable technique, for example electrolytic coating or other synthesis of the solid interfacial layer on the cathode surface in a preliminary step.

[0113] Suitable anodes for oxidizing hydrogen-containing species, such as H2O or H2, to form protons are well known in the art of electrochemistry. In some embodiments, the anode is a platinum electrode.

[0114] The power source can be any conventional power source for an electrolysis system, such as a DC power source. Optionally, the power source can include a photovoltaic solar cell. It is believed that the ability to produce ammonia from electricity, particularly renewable electricity, is a particular advantage of the present invention. For example, it is envisioned that the present invention will allow ammonia-based fertilizer to be produced at the point of need using solar or wind-generated electricity. This could be particularly valuable in high-value agricultural applications such as hydroponics, or could minimize the logistical challenges associated with transporting fertilizer to remote locations.

[0115] Cathodic reduction of dinitrogen to ammonia. The disclosed method includes providing dinitrogen to an electrochemical cell for cathodic reduction and applying a potential to the cathode sufficient to reduce the dinitrogen to produce ammonia. The resulting current flow from the cathode to the anode through the electrolyte increases the yield of ammonia over time.

[0116] It is understood that in this disclosure "cathodic reduction" does not denote any particular mechanism, does not identify intermediate species involved in the reaction cycle, or suggest where these species react (e.g., at the cathode surface or in the bulk electrolyte). However, without wishing to be bound by any theory, it is proposed that dinitrogen is cathodically reduced to ammonia according to the mechanism described in this disclosure with reference to FIG. 1. Thus, the overall cathodic nitrogen reduction reaction involves the reaction of neutral [BH] and cationic [BH] + The proton carriers are believed to be as shown in formulas (3) and (4), respectively. [ka]

[0117] Dinitrogen can be supplied to the electrochemical cell for cathodic reduction at dinitrogen partial pressures greater than 1 bar, or greater than 5 bar, or greater than 10 bar. In some embodiments, the dinitrogen partial pressure is in the range of 0.7 bar to 100 bar, or 2 bar to 30 bar, or 5 bar to 20 bar, or 10 bar to 15 bar. Increasing the N2 partial pressure in the cell can improve the yield and 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.

[0118] Dinitrogen may be provided to an electrochemical cell for cathodic reduction by contacting the electrolyte with dinitrogen, thereby solubilizing the dinitrogen in the electrolyte. Preferably, the dinitrogen is primarily or exclusively in the solution phase when exposed to the cathode. It has been found that in the static gas-electrolyte meniscus (cathode / electrolyte / N2 gas), electrolyte decomposition is accelerated in the region of the cathode exposed to gaseous dinitrogen. Without wishing to be bound by any theory, this undesirable process is attributed to a very high concentration gradient of dinitrogen and a depletion of proton carriers across the static gas-electrolyte meniscus, which leads to an excessively fast rate of formation of metal nitrides and / or metal(0) on the cathode, inducing uncontrollable electroreduction conversion of the electrolyte. Accelerated electrolyte decomposition in the electrode-electrolyte-gas-static gas-electrolyte meniscus consumes a significant portion of the charge, thereby reducing the faradaic efficiency, and the resulting insoluble deposits can inhibit mass transport to the cathode, thereby destabilizing the reaction system. Thus, in some embodiments, when producing ammonia, the cathode does not contact gaseous dinitrogen at a static gas-electrolyte meniscus. To accomplish this, the electrochemically active portion of the cathode may be fully immersed in the electrolyte.

[0119] It is understood that the absolute cathodic potential sufficient to reduce dinitrogen and the proton carrier depends on a variety of factors, including the choice of metal cation. When lithium cations are used, the cathodic potential is -2.0 V vs Ag / Ag + It can be less than (more negative).

[0120] In some embodiments, the cathodic potential is increased by increasing the concentration of the corresponding reduced forms, such as metal forms and / or metal nitrides (nominally Li +In the present disclosure, the apparent reduction potential is the reduction potential of the metal cation in the electrolyte under dinitrogen reduction conditions measured by the crossover point in cyclic voltammetry. In some embodiments, the cathodic potential is below (more negative) than -0.2V or -0.4V with respect to the apparent reduction potential of the metal cation in the electrolyte. At such negative potentials, excellent yields and faradaic efficiencies can be obtained. However, in some embodiments, the cathodic potential is above (more positive) than -1V or -0.8V with respect to the apparent reduction potential of the metal cation in the electrolyte. Potentials more negative than this can promote undesirable electrolyte decomposition reactions.

[0121] The electrolyte may be maintained at a temperature suitable to promote ammonia synthesis. The temperature may be in the range of -35°C to 200°C, for example 15°C to 100°C.

[0122] Dinitrogen may be reduced to ammonia with a faradaic efficiency of at least 70%, or at least 80%, such as at least 30%, or at least 40%, or at least 50%, or at least 60%, etc. Such high faradaic efficiency is highly desirable to minimize by-products and energy losses per unit of ammonia produced.

[0123] The product ammonia is NH4 + However, if excess protons are generated, they are expected to exist in the electrolyte as NH3 rather than NH4 + The possibility of ammonia being produced in the form of ammonia cannot be denied. It is also expected that the ammonia produced will be released into the gas phase.

[0124] Anodizing The method of the present disclosure may include introducing protons into the electrolyte by anodic oxidation of hydrogen-containing species at the anode of an electrochemical cell. In accordance with the principles described in this disclosure, the protons are expected to react with proton acceptors in the electrolyte to regenerate cationic proton carriers. It is the latter species, rather than the protons themselves, that is believed to be the primary protonating agent involved in the nitrogen reduction reaction.

[0125] Protons can be generated at the anode of an electrochemical cell by the oxidation of any suitable hydrogen-containing species, including dihydrogen (H2) and water (HO). In the case of H2 oxidation, the entire anodic regeneration process of proton carriers is represented by Equation (4) for neutral proton carriers [BH], and Equation (5) for cationic proton carriers [BH]. + The corresponding reactions for the anodic oxidation of H2O are shown in equations (6) and (7). [ka]

[0126] When H2 is oxidized to produce protons, the electrolyte can be in contact with both the cathode and the anode. Thus, when formed at the anode, the protons are directly introduced into the electrolyte. H2 can be obtained from any source, including water electrolysis with renewable energy. Optionally, the water electrolysis cell can be integrated with the nitrogen reduction cell to directly supply H2 from electrolysis to nitrogen reduction.

[0127] Water is a particularly desirable source of protons in electrochemical synthesis because it is inexpensive, but is known to interfere with continuous NRR through metals. Thus, when oxidizing HO to produce protons, indirect transfer of protons from the anode to the electrolyte may be preferred. This may help to limit or avoid the presence of water in the electrolyte where nitrogen reduction takes place to an acceptable level. For example, an electrochemical cell may include two electrolytes: a catholyte containing a metal cation, a fluorinated sulfonylimide or methide anion, and a proton carrier (generally as described in this disclosure), and an anolyte (liquid, or solid, or mixture of liquid and solid) in contact with the anode where water is oxidized. The electrochemical cell is configured to allow the transfer of protons from the anolyte to the catholyte, but substantially limit or avoid the transfer of water. Various arrangements for achieving this are known in the field of electrochemical synthesis and are described in more detail below.

[0128] Although the methods described herein generally result in high faradaic efficiency, in some embodiments, H2 may be formed as a significant by-product of the nitrogen reduction reaction due to competition with HER at the cathode. The H2 by-product may optionally be recycled for oxidation at the cathode to supplement the supply of selected hydrogen-containing species. This results in reduced energy consumption per unit of ammonia produced due to the absence of other energy-containing by-products.

[0129] Electrochemical Cell An example of an electrochemical cell for implementing embodiments of the present invention is shown generally in Figure 2. Cell 200 includes a nickel cathode 210 within a cell chamber 211. Cell 200 includes a platinum anode 212 and, optionally, a Ag / Ag anode. +The three electrodes further include a conventional reference electrode 213 such as a ZnSe ...

[0130] The cell 200 further includes a gas inlet 215 for introducing a gas mixture 218 including dinitrogen (N2) and dihydrogen (H2) into the 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 an electrolyte supply 222, and an electrolyte outlet 221 for withdrawing the electrolyte 214. The cell is preferably configured to operate at high pressure.

[0131] In use, a gas mixture 218 is pressurized into the chamber 211 via the supply port 215, producing an apparent Li + A voltage sufficient to establish a reduction potential at the cathode 210 that is below (more negative) the / Li reduction potential is applied between the cathode and the anode. The partial pressure of dinitrogen in the cell chamber 211 may be greater than 10 bar and the dihydrogen partial pressure may be greater than 1 bar. The resulting flow of current through the cell electrochemically reduces the dinitrogen to ammonia according to the principles described in this disclosure. The ammonia product may be continuously or periodically removed from the cell chamber 211 into gas 219 via gas outlet 216 and / or into recovered electrolyte 214 via electrolyte outlet 221.

[0132] In some embodiments, the cell is operated at steady state by continuously withdrawing one or both of these streams and continuously replenishing the gas reactants (N2 and H2) and / or electrolyte as needed by introducing gas mixture 218 and / or electrolyte feed 222. Ammonia may be separated from the withdrawn streams of gas 219 and / or electrolyte 214, and the remaining gas and electrolyte may be recycled to the cell chamber 211 as part of gas mixture 218 and electrolyte feed 222, respectively. A portion of the electrolyte 214 withdrawn via electrolyte outlet 221 may be discarded (or regenerated) and replaced with fresh electrolyte in feed 222, thus maintaining a target electrolyte residence time in the cell.

[0133] In one embodiment of the point-of-use fertilizer production cell, the outlet gas stream 219 converts ammonia into ammonium (NH + The fertilizer is passed through a solution of sulfuric or phosphoric acid in water for absorption as fertilizer. The product of this process is a solution of the ammonium salt of the acid used, e.g., ammonium sulfate solution, which can be applied directly as a fertilizer solution. For use in hydroponics or commercial greenhouses, the cell can be controlled to deliver fertilizer continuously in-line during the water supply to the plants.

[0134] Another example of an electrochemical cell for practicing embodiments of the present invention is shown diagrammatically in Figure 3. The cell 300 includes a cathode chamber 311 and an anode chamber 331 separated by a proton-permeable membrane separator 333, e.g., a membrane made of a sulfonated poly(tetrafluoroethylene) ionomer such as Nafion. A nickel cathode 310 is disposed in the cathode chamber 311. A conventional reference electrode 313 is also located in the cathode chamber. A platinum anode 312 is disposed in the cathode chamber 331. The electrodes are connected to a power supply (not shown) capable of applying a voltage between the cathode 310 and anode 312, such that the reduction potential of the cathode is controlled (or measured) relative to the reference electrode.

[0135] The cathode 310 and reference electrode 313 are immersed in catholyte 314, and the anode 312 is immersed in anolyte 334. Catholyte 314 is a liquid electrolyte containing lithium cations (>1 mol / L, or >0.5 mol / L with phosphonium cations at a total concentration of >1 mol / L), fluorinated sulfonylimide or methide anions, and a proton carrier as generally described in this disclosure. Anolyte 334 contains water for oxidation at the anode, but may otherwise be identical or different in composition compared to catholyte 314. Membrane separator 333 inhibits or substantially prevents permeation of species other than protons between the cathode and anode reaction chambers.

[0136] The cell 300 further includes a gas inlet 315 for introducing a dinitrogen 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 a catholyte feed 325, and a catholyte outlet 321 for withdrawing the catholyte 314. The cell 300 may include an anode inlet 340 for introducing or replenishing anolyte 334, and optionally an anode inlet 340 for introducing one or more hydrogen-containing species (e.g., H2O and / or H2 as liquid or vapor) into the anode chamber 331. An anolyte outlet 341 is provided for withdrawing the anolyte 334, and an anode gas outlet 344 is provided for withdrawing gas 345 from the headspace of the anode chamber.

[0137] In use, dinitrogen 318 is pressurized into the cathode chamber 311 via the gas inlet 315. The partial pressure of dinitrogen in the cathode chamber 311 can be greater than 10 bar. Water is initially present in the anolyte 344 and / or is fed into the anode chamber via the anode inlet 340. The apparent Li +A voltage sufficient to establish a reduction potential at the cathode 310 that is below (more negative) than the / Li reduction potential is applied between the cathode and anode. The resulting current flow through the cell electrochemically reduces dinitrogen to ammonia in the cathode chamber 311 according to principles described in this disclosure.

[0138] In cell 300, unlike cell 200, water is oxidized at anode 312 to produce protons in anolyte 334. The protons migrate through membrane separator 333 to maintain charge neutrality in the cell, and they enter catholyte 314 where they regenerate proton carriers by protonating proton acceptors. However, water and other undesirable species are excluded or inhibited from migrating from anolyte 334 to catholyte 314 by membrane separator 333.

[0139] The ammonia product may be continuously or periodically removed from the cathode chamber 311 in gas 319 via gas outlet 316 and / or in catholyte 314 recovered via electrolyte outlet 221. The cell may be operated continuously and the electrolyte and gas recovered from the cell may be recycled after removal of ammonia and other by-products in a manner similar to that described for cell 200. Any dihydrogen produced as a by-product in the cathode chamber 311 may be recovered and recycled to the anode chamber 331 for oxidation.

[0140] In the alternative, dihydrogen may be introduced into the cell 300 as the only hydrogen-containing species for oxidation at the anode 312. In this case, both the catholyte 314 and the anolyte 334 are substantially free of water.

[0141] The arrangement depicted in cell 300 is just one example of an electrochemical cell configured to oxidize water at the cathode and selectively transmit the resulting protons to a substantially water-free electrolyte to participate in the water-sensitive cathodic reaction. In another reported approach, a separator is placed in close proximity to a porous anode (e.g., a gas diffusion electrode). A water-laden gas stream (e.g., humid air) passes across the outside of the anode, and the separator prevents convective mixing. The electrolyte is sufficiently hydrophobic that little water is absorbed from the gas stream. If the hydrophobicity of the electrolyte and the humidity of the gas stream are properly adjusted, the separator does not need to be proton selective to maintain a low water content in the bulk electrolyte.

[0142] In another approach that has been reported, the hydrophobic organic catholyte and the polar (e.g., aqueous) anolyte are immiscible and proton transfer occurs across the phase boundary between the two. This arrangement may allow satisfactory proton transfer while sufficiently suppressing the permeation of water into the catholyte. To maintain a stable phase boundary, a separator may be used as the location of the phase boundary. EXAMPLES

[0143] The invention will now be described with reference to the following examples, which are understood to be illustrative and not limiting of the invention described herein.

[0144] material and method Copper metal wire (1.3 mm diameter; 99.99%) was purchased from Fisher Scientific. Nickel wire (0.5 and 2 mm diameter, ≥99.9% trace metals standard) was purchased from Sigma-Aldrich. Niobium foil (0.127 mm thick, 99.8% trace metals standard) was purchased from Alfa Aesar.

[0145] Lithium perchlorate (LiClO4, 99.99%) and lithium trifluoromethanesulfonate (LiOTf, 96%) were supplied by Sigma-Aldrich. Lithium tetrafluoroborate (LiBF 4、98%) was purchased from Acr·s Organics. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI; ≥ 99%; HQ-115, LOT10197) was purchased from 3M Fluorad. Lithium bis(fluorosulfonyl)imide (LiFSI) was obtained from Nippon Shokubai (Japan).

[0146] Ethanol (anhydrous, ≥95%) and triethylsulfonium bis(trifluoromethanesulfonyl)imide (Et3S-TFSI) were purchased from Sigma-Aldrich. Tetrahydrofuran (stabilized with BHT, analytical grade), soluble starch and phosphoric acid (85 wt.%) were supplied by Chem-Supply. Dimethylsulfoxide-d6 (DMSO-d6; D, 99%) was obtained from Cambridge Isotope Laboratories Inc. (UK). Sulfuric acid (98%) and acetone were supplied by Univar Solution. NH4Cl (>99%), NaOH (pellets, analytical grade), salicylic acid (≥99%), sodium tricitrate dihydrate (analytical grade), sodium hypochlorite (10–15 wt.% chlorine), sodium nitroprusside dihydrate (≥99%) and maleic acid (≥99%) were purchased from Sigma-Aldrich and Merck. High-purity deionized water (Sartorius Arium Comfort I ultrapure water system H2O-I-1-UV-T; resistivity 18.2 MΩ cm measured at 23 ± 2 °C) was used in all procedures requiring water. High-purity grade N2 (99.999%; CO2 < 1 ppm, O2 < 2 ppm, H2O < 2 ppm) and Ar (99.999%; CO2 < 1 ppm, O2 < 2 ppm, H2O < 2 ppm) gases were purchased from BOC Australia.

[0147] Phosphonium and trihexylsulfonium salts (tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate; ([P 666,14 [eFAP]), tributyl(octyl)phosphonium tris(pentafluoroethyl)trifluorophosphate; ([P 444,8][eFAP]), trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)imide ([P 666,14 [TFSI]), triethyl(methyl)phosphonium tris(pentafluoroethyl)trifluorophosphate; ([P 1222 [eFAP]) and triethylsulfonium bis(trifluoromethanesulfonyl)imide [EtS][TFSI] were synthesized according to previously reported methods.

[0148] The electrolyte solution was prepared using tetrahydrofuran dried over activated zeolite "molecular sieve" (3A, Sigma-Aldrich) for 1 day and then stored over another fresh piece of activated zeolite in an Ar-filled glove box (Korea Kiyon; continuously monitored for O2 ≤ 0.6 ppm and H2O ≈ 0.0 ppm levels). All lithium salts were dried at elevated temperatures under vacuum in the glove box antichamber and then transferred into the glove box without exposure to the ambient environment. Specifically, LiBF4 and LiFSI were dried at 80 °C for 12 h, and LiClO4, LiOTf, and LiTFSI were dried at 120 °C for 24 h.

[0149] The electrolyte solutions were prepared by dissolving dry lithium salts at the required concentrations in dry THF using volumetric flasks in an Ar-filled glove box. Ethanol was kept over activated zeolite for at least 5 days in an Ar-filled glove box before use as a proton carrier. All chemicals required for the preparation of the electrolyte solutions, the prepared electrolyte solutions, and the used electrolyte solutions were kept inside the Ar-filled glove box at all times.

[0150] Electrochemical experiments were carried out at ambient temperature (23 ± 2 °C) in a gas-tight polyetheretherketone (PEEK) autoclave cell in a three-electrode configuration using a Biologic VSP electrochemical workstation. The working and reference electrodes were placed at the center of the auxiliary electrode's helical coil (d = 16 mm).

[0151] The working electrode (cathode) used was a copper foil (electroactive area 0.62 cm 2 ), copper wire (0.74cm 2 ), niobium foil (0.18 cm 2 ), bare nickel wire (Φ0.5mm; 0.15cm 2 ), isolated nickel wire (Φ0.5mm; 0.05cm 2 ) or nickel disc (Φ2mm; 0.031cm 2 ). Copper wire, niobium foil, and nickel wire were used as obtained from commercial suppliers. Copper foil was rolled from copper wire into a sheet with a thickness of 0.39 mm. The current collector for the copper foil was copper wire. The nickel disk electrode was custom-made with nickel wire (Φ2 mm) encased in a Teflon sheath with only one flat end exposed to the electrolyte. The insulated nickel wire electrode was fabricated by glassblowing nickel wire (Φ0.5 mm) into a glass sheath leaving 3 mm exposed to the electrolyte. Prior to use, all working electrodes except the nickel disk and niobium foil were directly electropolished for 2 min with an applied voltage of 5 V using a DC power supply (Powertech, MP-3091) in phosphoric acid (85% aqueous solution) containing soluble starch (1:1000 w / v) with continuous stirring (Teflon-lined magnetic stirrer; 1000 rpm). The electropolished electrodes were washed with absolute ethanol and dried with a compressed nitrogen blow gun. Prior to electropolishing for 1 min following the procedure above, the nickel disk electrodes were mechanically polished (to a mirror finish of at least 150 digits "8") using an abrasive pad cloth and a slurry of alumina (0.3 μm, Buehler) in ethanol. The niobium foil electrodes could not be electropolished using the procedure above and were only rinsed with ethanol before use.

[0152] The auxiliary electrode for the electrochemical experiments was a platinum wire, which was ultrasonically cleaned (40KHz, 120W) in absolute ethanol for 1 h, dried under compressed nitrogen flow, and then flame annealed in a propane-butane burner. A silver wire encased in a fritted glass tube filled with the same electrolyte solution as the main compartment was used as a quasi-reference electrode. Before each experiment, the fritted tube was cleaned with absolute ethanol under ultrasonic treatment (40KHz, 120W) for 30 min, and ethanol was further forced into the frit under nitrogen gas pressure. After repeating these procedures three times, the fritted tube was dried in an oven at 120 °C for 1 h and further dried at 80 °C under vacuum for 20 min. The potential of the employed silver wire quasi-reference electrode was calibrated against the apparent potential of the lithium (0 / +) process estimated from the crossover point of cyclic voltammetry. The potential measured by this method was calculated using the Li 0 It is affected by chemical reactions with N2, ethanol, and sometimes tetrahydrofuran. [ka] It is not the true potential of the redox couple. Therefore, in this disclosure, the apparent lithium (0 / +) potential (Li / Li + ) is called

[0153] Before being introduced into the glovebox for assembly, the cell was diluted with 0.1 M KOH. (aq.) , then 0.05M HSO 4(aq.) The samples were immersed in each solution for several hours and then vigorously washed with water and absolute ethanol. This cleaning procedure was performed to remove any residual ammonia and oxidized nitrogen (NO) that may interfere with NRR. x All volumetric flasks, containers, vials and other labware used for preparation and storage of solutions and chemicals before and after the electrochemical experiments were washed with water and absolute ethanol and dried following the same procedure used for the cells.

[0154] After the cleaning procedure was completed, all parts of the electrochemical cell and the necessary labware were dried in the glovebox anteroom under vacuum at 80 °C for at least 15 min. The cell was introduced into the Ar-filled glovebox for assembly and, if necessary, for filling with electrolyte solution. The cell was then sealed, removed from the glovebox, and pressurized with N2 in a manner that prevented the ingress of air into the cell interior. Unless otherwise noted, hydrogen was not introduced into the cell to avoid any contribution from the reduction of H2 to LiH in the cathodic process. In the absence of H2, the proton-forming anodic reaction is the oxidation of THF.

[0155] The system was allowed to equilibrate for approximately 30 min while stirring the electrolyte solution in the main chamber at 600 rpm using a Teflon-lined magnetic Stirling bar (l = 10 mm, d = 3 mm). Afterwards, the electrochemical reduction reaction was performed and the main experiment was reproduced at least three times, and the corresponding data are presented as the mean ± 1 standard deviation.

[0156] After the experiment was completed, the pressurized gas was slowly released (approximately 10 mL min ) through a trap filled with 15 mL of 0.05 M H2SO4 aqueous solution. -1 ), which trapped gaseous ammonia. The amount of NH3 in the trap was always at least two orders of magnitude lower than that found in the working electrolyte solution. Therefore, all yield and faradaic efficiency data are based solely on the amount of ammonia produced in the working electrolyte solution.

[0157] Because a large amount of ammonia was produced in the experiment, it was necessary to dilute the electrolyte solution after the reaction 10 to 4000 times with water. + The accumulated concentration was quite low, but it was still possible to obtain a solution by diluting it up to 10 times (0.05 M HSO 4(aq.) To ensure reliable quantification, at least two significantly different dilutions of the same sample were applied in the main experiment.

[0158] For routine quantification of ammonia, the spectrophotometric Berthelot analysis (Analyst 109, 549-568 (1984); ACS Energy Letters 5, 736-741 (2020)) was employed. For this purpose, 500 μl of sample was added to a 2 ml Axygen microtube and diluted with 400 μl of 1 M NaOH containing 5 wt.% salicylic acid and 5 wt.% trisodium citrate. (aq.) Then, 0.05M NaClO (aq.) 100 μl of the solution and 30 μl of 1 wt.% aqueous sodium nitroprusside were added sequentially. The resulting homogenous mixture was incubated in the dark at ambient temperature for exactly 2 h, then immediately transferred to a polystyrol / polystyrene 10 mm cuvette (Sarstedt) and UV-vis spectra (Cary spectrophotometer) were recorded in the range of 500–1000 nm at 10 nm s -1 The electrolyte and trapping solutions were analyzed using Berthelot's reagent in water and 0.05 M HSO, respectively. 4(aq.) A background spectrum was recorded for each sample using the FTIR spectrophotometer. All absorbance data were background corrected.

[0159] The quantification of ammonium in the acid trap was based on a calibration curve constructed using 5-100 μM NH4Cl standard solutions in 0.05 M H2SO4 (aq.). + Concentration (C NH4+ ) is A=0.0091C NH4+ / μM+0.019(R 2 =0.99), it was linear.

[0160] Reliable quantitative Berthelot analysis of electrolyzed tetrahydrofuran solutions could not be achieved by the usual calibration approach and required the implementation of a standard addition method that takes into account the interference effects of the environment specific to each sample. In a typical procedure, 1 mL of the diluted sample was added to six Axygen microtubes (2 mL) to which 1 mL of 0, 10, 20, 30, 40 and 50 μM NH4Cl in H2O was added. The six resulting mixtures were further analyzed according to the standard Berthelot spectrophotometric method described above. A plot of the absorbance at 655 nm against the added NH4Cl concentration was fitted with a linear dependence and the Y-intercept was divided by the slope to obtain the negative reciprocal of the X-intercept. The latter corresponds to the ammonia concentration in the analyzed diluted sample.

[0161] Ammonia 1 HNMR spectroscopy was performed on a Bruker Avance III 600 MHz (14.1 Tesla magnet) instrument equipped with a 5 mm CPTCI 1H, 13C, 15N, 2D autotunable cryoprobe with Z gradient and a 600.27 MHz 1 The measurements were performed using a BACS 60 tube autosampler set to H. The measurement procedure followed the lc1pncwps pulse sequence (512 scans, d1 1.5 s, p12 0.08 s), a 1D version of noesyprph with presaturation during the relaxation delay and mixing time, a shaped pulse for off-resonance presaturation, and cw decoupling on the f2 channel during the measurement.

[0162] Samples were prepared by sequentially mixing 50 μL of 4 M H2SO4 in DMSO-d6, 125 μL of 4.3 mM maleic acid in DMSO-d6, 740 μL DMSO-d6, 125 μL of analyte solution, and 10 μL H2O. Ammonia signal integrals were normalized to the integral of the maleic acid internal standard peak (set at 2.0). Standards 0–1 M were also included, including 2 M LiTFSI and 0.1 M C2H5OH. 14 NH4Cl and 15A calibration plot of NH4Cl in tetrahydrofuran was made. The normalized ammonia integral (I NH4+ )NH4 + The concentration dependence is I NH4+ =144[ 14 NH4 + ]-0.3077(R 2 = 0.994) and I NH4+ =100[ 15 NH4 + ]+0.2745(R 2 = 0.999), the relationship was essentially linear.

[0163] X-ray diffraction (XRD) analysis showed that Cu K α The diffraction patterns were measured using a Bruker D8 Advance diffractometer equipped with an X-ray source. The 2θ range investigated was 25°–110°, and the scanning speed was 0.014° s -1 The electrodes were rotated at 10 rpm during the measurements. After electrochemical testing, the electrodes were removed from the cell in an Ar-filled glove box, dipped in THF several times to remove electrolyte residues, and then left to dry overnight. The dried electrodes were loaded into a custom-made gas-tight dome holder, removed from the glove box, transferred to the XRD instrument, and analyzed without contact with the ambient environment at any stage.

[0164] Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy were performed using a JEOL 50mm microscope with the Gatan EDX DigitalMicrograph plug-in. 2 The characterization was performed using a JEOL JSM-7001F FEGSEM microscope equipped with a Si(Li) EDX detector. The instrument was operated at an accelerating voltage of 15 kV, a probe current of 50 pA, and a field emission gun. The electrodes were pretreated following the same procedure as for the XRD characterization, firmly attached to SEM stubs with conductive double-sided carbon tape, and transported to the instrument in a sealed container, except that they were exposed to ambient conditions for the short period required to load the samples into the microscope.

[0165] X-ray photoelectron spectroscopy (XPS) was performed using a ThermoFisher Scientific Instrument Nexsa Surface Analysis System with a monochromatic Al Kα source (1486.6 eV). The X-ray spot size was set to 400 μm. The analysis chamber was 1.0 × 10 -8 The pressure was maintained at less than 1 bar. Survey scans were recorded with a pass energy of 200 eV and a step size of 1 eV, and high-resolution data were acquired with a pass energy of 50 eV and a step size of 0.1 eV. The sample was loaded into a holder in an Ar-filled glove box and left under vacuum in the glove box anteroom for 10 min before being transferred to the instrument without contacting the ambient environment at any stage. The sample was kept in ultra-high vacuum overnight before XPS measurements were performed. There was no electrical contact between the sample and the instrument earth, and the sample was charge neutralized before analysis. The collected spectral data was energy corrected by adjusting the maximum of the aliphatic C-C peak in the C 1s spectrum to 284.8 eV.

[0166] Viscosity measurements were performed at a controlled temperature of 25 °C using a Lovis 2000M Anton Paar viscosimeter (Lovis angle 30°). Each sample was analyzed in triplicate with a standard deviation of 0.001 g cm -3 and was less than MPa·s.

[0167] The conductivity of the electrolyte solution was measured in a two-electrode (Pt wire) dip cell using a Solartron 1296 dielectric interface connected to a Biologic MTZ-35 frequency response analyzer at a controlled temperature of 25 °C. 7 It was measured by electrochemical impedance spectroscopy in the frequency range of ~1 Hz. The recorded conductivity was 1408 μS cm at 25 °C. -1 Standard 0.01M KCl (aq.) The cell constant measured using the solution was 1.19 cm -1 Three measurements were performed for each sample, with the standard deviation being 0.001 mS cm -1 It was less than.

[0168] Example 1. Chronoamperometry (CA) ammonia production at different LiTFSI concentrations Seven electrolytes were prepared containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and ethanol (EtOH) in tetrahydrofuran (THF), with EtOH concentrations of 0.1 M and LiTFSI concentrations of 0.1 M, 0.5 M, 1 M, 1.5 M, 2 M, 2.5 M, and 3 M. The ionic conductivity and viscosity of these electrolytes were measured. The results are shown in Table 1 and Figure 4(a).

[0169] A series of CA experiments were performed at -0.55V vs Li / Li + At an applied voltage of 0.15 cm, the nickel wire cathode 2 ) with the electrolyte. The experiment was carried out for 6 h under 15 bar N2 (static pressure) with the electrolyte stirred at 600 rpm. The results are shown in Table 1 and Figure 4(b). [Table 1] a,b n= b 3 and c Means and standard deviations of 7 independent experimental replicates.

[0170] The current through the system remained fairly stable over a 6-h period, regardless of the lithium salt concentration used. Increasing the LiTFSI concentration from 0.1 to 0.5M increased the NH yield (YR) by 20 nmol s -1 cm -2 The YR increased significantly from 160 nmol s−1 to 160 nmol s−1 in both experiments, although the faradaic efficiency (FE) in both experiments was in the range of 10–20%. Further increasing the LiTFSI concentration to 1 M increased the YR to 160 nmol s−1. -1 cm -2 , FE increased significantly to 45%.

[0171] Further increasing the LiTFSI concentration gradually suppressed the ionic conductivity and resulted in a more viscous solution. Surprisingly, this did not immediately deteriorate the NRR performance. In fact, the ionic conductivity was 250 ± 20 and 230 ± 20 nmol s for 1.5 and 2 M LiTFSI electrolytes, respectively. -1 cm -2 Very high NH3 yields of 0.1% were achieved, with FE >80% in the latter case. A decrease in YR was only observed for the very viscous 2.5 and 3 M solutions, where the viscosity exceeded 20 MPa s and mass transport became the limiting factor. However, FE remained high in these electrolytes. The results show that increasing the LiTFSI salt concentration progressively increases the faradaic efficiency of N2 reduction, approaching 90% for [LiTFSI] ≥ 2 M.

[0172] Example 2. Chronoamperometric (CA) ammonia production using different lithium salts at low and high concentrations The electrochemical nitrogen reduction results obtained with the TFSI anion at low lithium concentrations, as obtained in Example 1, were compared with the results of CA experiments carried out with the trifluoromethanesulfonate (OTf) anion. Using an electrolyte containing 0.2 M lithium trifluoromethanesulfonate (LiOTf) and 0.18 M ethanol in THF, a single compartment cell with a copper wire cathode showed a negative charge of -0.55 V vs Li / Li + The CA experiment was carried out at an applied voltage of 0.001 ppm. The experiment was carried out for 12 h under 15 bar N2 (static pressure) with the electrolyte stirred at 600 rpm. The experiment showed a faradaic efficiency of only 7% and a yield of 0.049 nmol s -1 cm -2 This demonstrated the superior performance of the TFSI anion.

[0173] Perchlorate (OCl4 - ), tetrafluoroborate (BF4 - ), triflate (OTf -) Using various weakly coordinating anions such as bis(fluorosulfonyl)imide (FSI) and bis(trifluoromethanesulfonyl)imide, the effect of anion selection was investigated by increasing the lithium salt concentration. Electrolytes containing lithium salts of these coordinating anions and ethanol (EtOH) in tetrahydrofuran (THF) were prepared as shown in Table 2 with an EtOH concentration of 0.1 M and a lithium salt concentration of 2 M. The ionic conductivities and viscosities of these electrolytes were measured, and the results are shown in Table 2.

[0174] A series of CA experiments were performed at an applied voltage of -0.55 V vs Li / Li + using the electrolyte in a single-compartment cell equipped with a nickel wire cathode (0.15 cm 2 surface area). The experiments were carried out for 6 hours while stirring the electrolyte at 600 rpm under 15 bar of N2 (static pressure). The dinitrogen reduction results are also shown in Table 2.

Table 2

[0175] The dinitrogen reduction results (including the 1M LiTFSI results of Example 1) are compared in Figure 5 where the electrolytes are arranged based on viscosity or ionic conductivity. The viscosity of the solution increases in the order of BF4 - <ClO4 - <OTf - <FSI - <TFSI, and the conductivity is in the order of OTf - <ClO4 - <BF4 - <TFSI < FSI -The NH3 yield and faradaic efficiency are somewhat correlated with the ionic conductivity (Figure 5b). However, despite the significant difference in ionic conductivity, both 2M LiTFSI and 2M LiFSI electrolytes showed exceptionally high yield (TFSI comparable to FSI) and faradaic efficiency (TFSI superior to FSI). Also, despite the lower conductivity, the electrolyte containing 1M LiTFSI showed improved results compared to other lithium salts (LiBF4, LiOCl4, LiOTF). Thus, fluorinated sulfonylimide anions (e.g., TFSI and FSI) offer a clear advantage over other weakly coordinating anions, independent of ionic conductivity or viscosity effects.

[0176] 1M LiTFSI and 1M LiBF4 (total 2MLi + ) also showed good dinitrogen reduction performance, but with lower yields than electrolytes containing 2M LiTFSI or 1M LiTFSI, demonstrating improved results independent of lithium concentration when a fluorinated sulfonylimide is the only weakly coordinating anion present.

[0177] Example 3. Chronoamperometric (CA) ammonia production with different cathode materials The effect of cathode composition was investigated using a variety of different metals as the working electrode in the electrochemical cell, as shown in Table 3. -0.55V vs Li / Li + CA experiments were carried out in a single compartment cell with an electrolyte containing LiTFSI (2 M) in THF and EtOH (0.18 M or 0.10 M) at an applied voltage of 0.2 V. The experiments were carried out for 6 h under 15 bar N2 (static pressure) with the electrolyte stirred at 600 rpm. The results are shown in Table 3. [Table 3]

[0178] Although all cathode materials exhibited good ammonia synthesis performance, the chemical properties of the cathode affected the reaction rate, and the Faraday efficiency and NH3 yield increased in the order of Cu < Nb < Ni. Without wishing to be bound by any theory, it is proposed that the cathode composition may play a secondary role in establishing a desirable electrode-electrolyte interface layer on the cathode surface in the Li-mediated NRR process.

[0179] Example 4. Characterization of the cathode after reaction The nickel wire cathode used in the dinitrogen reduction reaction with different electrolytes was analyzed after the reaction. The cathode used in the electrolyte containing 2M LiTFSI and 0.1M EtOH in THF (Example 1) was visually clean in the part of the wire immersed in the electrolyte (Region B), but visible deposits became apparent along the part of the wire near the stirred electrolyte surface (Region A) where an electrochemical reaction occurred at the meniscus (cathode / electrolyte / gas phase) between the static gas and the electrolyte. Regions A and B were characterized by electron microscopy and XPS. The selected XPS spectra are shown in FIGS. 6 (S 2p), 7 (N 1s), 8 (F 1s), and 9 (Ni 2p).

[0180] The main components of the deposit in Region A were identified as LiF (see FIG. 8), Li3N (see FIG. 7), and sulfur-based compounds including lithium sulfide and polysulfides (see FIG. 6). In contrast, the characterization of the cathode part in Region B showed the presence of only a very thin, coherent layer of electrolyte (containing intact TFSI anions), solid LiF, and S-O species (see FIG. 8). In contrast to Region A, the thickness of this solid interface layer was less than 10 nm as evidenced by the detectable Ni 2p signal in XPS (see FIG. 9).

[0181] Without wishing to be bound by any theory, it is proposed that the formation of deposits in region A is, at least in part, related to the very high concentration gradient of N2 and the depletion of ethanol proton carriers across the gas-liquid boundary, which leads to an excessively high Li3N formation rate on the cathode near the three-phase interface. 0 It promotes deposition and uncontrollable electroreduction conversion of the electrolyte. The formation of deposits at the electrode-electrolyte-gas three-phase boundary is expected to consume a significant portion of the charge, thereby reducing the faradaic efficiency and inhibiting mass transport to the cathode, promoting instability of the reaction system over time.

[0182] The cathode used in an electrolyte containing 0.5 M LiTFSI and 0.1 M EtOH in THF (Example 1) was covered with a visible gray deposit after the reaction, including the portion of the cathode that was fully immersed in the electrolyte. The major Li-based component of the deposit was identified by XPS as LiF, which was electrodeposited uncontrollably during the experiment.

[0183] The cathode using 0.5M LiTFSI electrolyte showed significantly more electrolyte decomposition than the cathode using 2M LiTFSI electrolyte. It was suggested that the higher the concentration of fluorinated sulfonylimide lithium salts (e.g., LiTFSI) in the electrolyte, the easier it is for protective ion aggregates containing bulky, electrochemically stable anions and Li cations to form in the electrolyte-electrode interfacial layer on the cathode surface. This electrolyte-electrode interface suppresses electrolyte decomposition while enabling a high rate of dinitrogen reduction. As a result, high productivity and selective reduction of dinitrogen to ammonia can be obtained on the cathode, and the reaction can be sustained for a long time.

[0184] The cathode used in an electrolyte containing 2M LiBF4 and 0.1M EtOH in THF (Example 1) was covered with a thick gray deposit after the reaction, including the portion of the cathode that was fully immersed in the electrolyte. The major Li-based component of the deposit was again LiF. Without wishing to be bound by any theory, this experiment highlights (i) the superior ability of fluorinated sulfonylimide lithium salts (e.g., LiTFSI) to form a protective electrolyte-electrode interface compared to other lithium salts of weakly coordinating anions, and / or (ii) the superior electrochemical stability of the fluorinated sulfonylimide anion.

[0185] Example 5. Chronoamperometric (CA) ammonia production without a static gas-electrolyte meniscus at the cathode For longer CA experiments, -0.55V vs Li / Li + An electrolyte containing LiTFSI (2 M) and EtOH (0.10 M) in THF was passed through an insulated nickel wire cathode (0.05 cm 2 The experiments were carried out using a single compartment cell with a 1000 sq. m. (surface area) of 1000 sq. m. The nickel wire, except for a 3 mm end, was enclosed in glass so that the entire exposed nickel surface area was fully immersed in the electrolyte during the reaction (i.e., there was no static gas-electrolyte meniscus). The experiments were carried out for 24 or 96 h under 15 bar N2 (static pressure) with the electrolyte stirred at 600 rpm. The results are shown in Table 4 and Figure 10, showing the current density, charge passed and overall cell potential over time. [Table 4]

[0186] With the new cathode configuration, the nitrogen reduction reaction was carried out with a faradaic efficiency of 99 ± 1% (Table 4). During the first activation period of about 24 h, the average ammonia yield was about 500 nmol s -1 cm -2 After this period, the system stabilized, resulting in stable performance over 96 h, with an average ammonia yield of 170–200 nmol s -1 cm -2 It was.

[0187] At the end of the reaction, the cathode was visually clean. The improved faradaic efficiency in these experiments (compared to previous examples using the same electrolyte) is likely due to the elimination of the three-phase boundary at the electrolyte surface region. By exposing the cathode only to dinitrogen dissolved in the electrolyte, electrochemically induced electrolyte decomposition reactions were significantly suppressed.

[0188] These results indicate that when an electrolyte containing a fluorinated sulfonylimide anion and a lithium salt is used at a high lithium concentration, dinitrogen can be electrochemically reduced to ammonia with nearly quantitative selectivity and a high reaction rate, even over a long reaction time.

[0189] Example 6. Voltage Dependence Nickel wire cathode (0.15cm 2 The effect of reduction potential was investigated in a series of CA experiments carried out in a single compartment cell equipped with a 1000 sq. m. surface area (SAR) and an electrolyte containing LiTFSI (2 M) in THF and EtOH (0.10 M). The experiments were carried out for 6 h under 15 bar N2 (static pressure) with the electrolyte stirred at 600 rpm. + The potential was examined in the range of −0.2 to −1 V with respect to the potential, and the results are shown in Table 5 and FIG. [Table 5] b,c n= b 3 and c Means and standard deviations of 7 independent experimental replicates.

[0190] Li / Li + As the potential went from -0.2 V to -0.8 V, both the reduction rate (Figure 11) and the ammonia yield (Table 8) increased. The faradaic efficiency of over 80% was observed at -0.50 to -0.80 V vs Li / Li + However, the values ​​were maintained in the range of -0.7, -0.8 and -1.0 V vs Li / Li + However, performance deteriorated after a few hours of operation (Figure 11).

[0191] -1.0V vs Li / Li + After reaction at 1000 K, the nickel cathode was covered with a significant amount of visible deposits, including on the parts of the cathode that were fully immersed in the electrolyte. The major Li-based component of the deposit was identified by XPS as LiF, which had been electrodeposited uncontrollably at the very negative potentials of the experiment. Analysis by X-ray diffraction (XRD) detected a series of peaks associated with lithium amides, oxides, sulfides and fluorides, the latter two being decomposition products of electrolyte anions. The decrease in reduction performance over time was likely due to the accumulation of these decomposition products on the cathode surface.

[0192] Example 7. Pressure dependence -0.55V vs Li / Li + At an applied voltage of 0.15 cm, the nickel wire cathode 2 The effect of dinitrogen pressure was investigated in a series of CA experiments carried out with an electrolyte containing LiTFSI (2 M) and EtOH (0.10 M) in THF in a single compartment cell with a 1000 sq. m (surface area) pressure of 1000 s. The experiments were carried out for 6 h under 15 bar of N2 (static pressure) with the electrolyte stirred at 600 rpm. Pressures ranging from 1 bar to 20 bar were investigated and the results are shown in Table 6. [Table 6] b,c n= b 3 and c Means and standard deviations of 7 independent experimental replicates.

[0193] During the experiment, P N2 At P = 20 bar, the electrolytic reduction rate decreased after about 4 hours, but in all other cases, relatively stable current densities were recorded. N2 The results at = 20 bar were attributed to excess Li3N accumulation on the cathode surface. Both the yield and faradaic efficiency were positively correlated with dinitrogen pressure over the 6 hour experiment.

[0194] Example 8. Different proton carriers and proton carrier concentrations -0.55V vs Li / Li + At an applied voltage of 0.15 cm, the nickel wire cathode 2 The use of different proton carriers at different concentrations was investigated in a series of CA experiments carried out with an electrolyte containing LiTFSI (2 M) in THF in a single compartment cell with a 1000 sq. m (surface area) sample. The experiments were carried out for 6 h while the electrolyte was stirred at 600 rpm. The results are shown in Table 8 and summarized in Figures 13 and 14. [Table 7] a No proton carrier added. b-C n= b 7 and c Means and standard deviations of three independent experimental replicates. HTFSI = bis(trifluoromethanesulfonyl)amine; (CF3SO2)2NH.

[0195] A variety of different types of proton carriers were shown to be effective, including (i) neutral proton carriers including alcohols (such as methanol, ethanol, n-propanol, isopropanol and n-butanol), Bronsted acid bis(trifluoromethanesulfonyl)amine and THF itself (an ether), and (ii) cationic proton carriers such as phosphonium salts. Figure 13 plots the yield and faradaic efficiency results for different types of proton carriers, showing that the optimum concentration may vary depending on the type of proton carrier. The best results were obtained with alcohols. Figure 14 compares the yield and faradaic efficiency obtained with various alcohols, all at a concentration of 0.1 M, which was found to be optimal for ethanol. Excellent results were obtained with all C2-C4 alcohols.

[0196] Example 9. Ylide regeneration study To investigate the role of alkylphosphonium species as renewable proton carriers during electrochemical Li-mediated ammonia synthesis, a series of experiments were carried out to investigate the role of alkylphosphonium species as renewable proton carriers during electrochemical Li-mediated ammonia synthesis using [P 666,14 [eFAP], as shown in Figure 12.31 The reactions were monitored by P NMR spectroscopy. All reactions were carried out in an argon glove box under an inert atmosphere (O2 and H2O < 0.5 ppm) with dry materials. 31 P-NMR spectra were recorded in THF using an external capillary with PPh3 as the reference and the axis was calibrated at 0 ppm.

[0197] In the first step, [P 666,14 Prepare a 0.2 M solution of [eFAP] 31 The P NMR spectrum was recorded. As can be seen in FIG. 12, the spectrum shows the presence of the phosphonium cation [P 666,14 ] corresponding to one 31 The first step is characterized by a P NMR signal and a group of signals between -131 and -151 ppm corresponding to the [eFAP] anion. In the second step, excess Li3N is reacted with [P 666,14 [eFAP] was added to a 0.2 M solution of 1,2-dichlorophenyl phosphate (pH 7.0) and the mixture was stirred for 24 hours. Visually, no change in the mixture was observed, it remained colorless and transparent with no visible precipitate. 31 The P NMR spectrum (middle spectrum in Figure 12) shows that the peak at 39.3 ppm has completely disappeared and a new peak has appeared at 15.7 ppm. This peak corresponds to a zwitterionic species formed in nearly quantitative yield via deprotonation of the phosphonium cation by reaction with Li3N. The NMR data are consistent with the formation of a phosphonium ylide. In the third step, 0.2 ml of a 0.1 M solution of acetic acid was added to 0.5 ml of the ylide-containing solution, 31 A P NMR spectrum was recorded (bottom spectrum in Figure 12), which shows quantitative recovery of the phosphonium cation (peak at 39.3 ppm).

[0198] The recovery of the phosphonium cation was confirmed using mass spectrometry (MS). The mass spectra for steps 1 to 3 were identical, indicating that [P 666,14 Only one signal (m / z=483) corresponding to the cation is shown.

[0199] This stepwise reaction process can be carried out with other phosphonium salts, [P 1222 [eFAP], [P 4448 Repeated for [eFAP] and triphenylmethylphosphonium tetrafluoroborate ([PPh3Me][BF4]). All 31 P-NMR spectra showed the generation of ylide species when reacted with Li3N, and the regeneration of the phosphonium cation after the addition of acetic acid, indicating that various alkylphosphonium cations are suitable cationic proton carriers and that the stepwise reaction test can be used as a screening method for potential proton carriers.

[0200] Example 10. H2 as a proton source for the anode Experiments similar to Example 1 (electrolyte: 2M LiTFSI in THF, 0.1M EtOH) were carried out using a N2 gas supply at a total pressure of 15 bar mixed with dihydrogen (H2) at 2 or 4 bar. The results are shown in Figure 15, which shows that high yields and faradaic efficiencies are maintained in the presence of H2.

[0201] Example 11. Ionic liquid additives -0.55V vs Li / Li + At an applied voltage of 0.15 cm, a non-insulated nickel wire cathode 2 CA experiments were carried out in a single compartment cell with a 1000 sq. m. surface area; used in Example 1) using an electrolyte containing LiTFSI (1 M, 1.5 M or 2 M) in THF, EtOH (0.10 M) and different amounts of ionic liquid additive. The experiments were carried out at room temperature for 6 h under 15 bar N2 (static pressure) with stirring the electrolyte at 600 rpm. The results are shown in Figure 16 and Table 9. [Table 8] a,b n=3 and b Means and standard deviations of 7 independent experimental replicates.

[0202] The extremely high faradaic efficiency and yield at lithium concentrations above 1M, as well as the increase in faradaic efficiency when the lithium concentration is increased from 1M to 2M, are also evident in experiments without an ionic liquid additive. The presence of 1-butyl-1-methylpyrrolidinium bis-(trifluoromethylsulfonyl)imide ([C4mpyr][TFSI]) as an ionic liquid additive (0.5M or 1M to keep the total salt concentration in the electrolyte constant at 2M) was found to be detrimental to both the faradaic efficiency and the yield. In contrast, the presence of trihexyl(tetradecyl)phosphonium bis-(trifluoromethylsulfonyl)imide ([P 6,6,6,14 The presence of 0.5 M TFSI (to keep the total salt concentration in the electrolyte constant at 2 M) improved the faradaic efficiency (although the yield was reduced).

[0203] The results indicate that pyrrolidinium cations (and non-metallic cations that are not electrochemically stable, such as imidazolium) should preferably be avoided, whereas phosphonium cations are tolerated or even beneficial. Without being limited by theory, it is proposed that the tetraalkylphosphonium cations are either unreactive (in the presence of ethanol as a proton carrier) or react reversibly under reducing conditions to form proton acceptors for the phosphonium ylide, thus avoiding deleterious decomposition reactions.

[0204] Example 12. Phosphonium ionic liquid additives at low N2 pressure -0.55V vs Li / Li + At an applied voltage of 0.05 cm, the cathode of an insulated nickel wire 2 The CA experiments were carried out in a single compartment cell equipped with a 100-μL ... 6,6,6,14The experiment was carried out using an electrolyte containing [TFSI] additive. The nickel wire, except for a 3 mm end, was encapsulated in glass so that the entire surface area of ​​the exposed nickel was fully immersed in the electrolyte during the reaction (i.e., there was no static gas-electrolyte meniscus). The experiment was carried out at room temperature for 6 hours under 1 bar N2 (static pressure) with the electrolyte stirred at 600 rpm. The results are shown in Figure 17 and Table 10. [Table 9]

[0205] With 2M LiTFSI, and no phosphonium ionic liquid additive, the faradaic efficiency was only 16% due to the low N2 pressure (1 bar) (see Example 17). 6,6,6,14 [TFSI] (total salt concentration in the electrolyte is 2M, Li + By keeping the concentration constant at 1.5M, the faradaic efficiency increased to about 50% and the yield also increased significantly. [P 6,6,6,14 If we increase the amount of TFSI to 0.65M (Li + At still 1.5 M, the faradaic efficiency increased even more dramatically, approaching 90%. This faradaic efficiency is similar to that obtained without the use of an ionic liquid additive at high N2 pressure (see Example 7). 6,6,6,14 ][TFSI] or [P 6,6,6,14 [eFAP]) while maintaining the concentration of Li + Reducing the concentration to 1M caused a decrease in the faradaic efficiency at 1 bar.

[0206] Without wishing to be limited by any theory, it is believed that the phosphonium-based ionic liquids enhance the solubility of N in the electrolyte while maintaining high ionic concentration and conductivity of the electrolyte, favoring the rate and selectivity of NRR.

[0207] Example 13. Phosphonium ionic liquid additives at high N2 pressure -0.55V vs Li / Li +At an applied voltage of 0.05 cm, the cathode of an insulated nickel wire 2 A single-compartment cell with a 100-μL surface area was filled with LiTFSI (0.2–1.5 M), EtOH (0.10 M) and different amounts of [P 6,6,6,14 CA experiments were carried out using electrolyte containing the [TFSI] additive. The nickel wire, except for the 3 mm end, was encapsulated in glass so that the entire surface area of ​​the exposed nickel was fully immersed in the electrolyte during the reaction (i.e., there was no static gas-electrolyte meniscus). The experiments were carried out at room temperature for 6 hours under 15 bar N2 (static pressure) with the electrolyte stirred at 600 rpm. The results are shown in Figure 18, Figure 19 and Table 11. [Table 10]

[0208] The results again show the surprising effect of increasing the lithium concentration on the faradaic efficiency of nitrogen reduction. + ], [P 6,6,6,14 Despite the presence of the [TFSI] ionic liquid additive and the high ion concentration in the electrolyte (1.5 M total lithium and phosphonium cations; 1.5 M TFSI anion), the faradaic efficiency was only about 10%. + ] and a total ion concentration of 1.7M, the faradaic efficiency increased significantly (about 25%). + ], the faradaic efficiency exceeded 90% at a total ion concentration of 2M. In this case, [P 6,6,6,14 Although the yield decreased with the addition of [TFSI] (see Table 1 for the results without the phosphonium cation), the faradaic efficiency was 1.5 M[Li + ] is equivalent to that obtained alone.

[0209] Example 14. Fluorinated sulfonylmethide anions -0.55V vs Li / Li + At an applied voltage of 0.05 cm, the cathode of an insulated nickel wire 2CA experiments were carried out in a single compartment cell with a 1000 sq. m. (surface area) and an electrolyte containing lithium tris(trifluoromethanesulfonyl)methide [Li(CF3SO2)3C; ≥ 99%; Fujifilm Wako Pure Chemical Industries, Ltd.] in THF (1.1 M) and EtOH (0.10 M). The nickel wire, except for a 3 mm end, was enclosed in glass so that the entire surface area of ​​the exposed nickel was fully immersed in the electrolyte during the reaction (i.e., there was no static gas-electrolyte meniscus). The experiments were carried out at room temperature for 6 h under 15 bar N2 (static pressure) with the electrolyte stirred at 600 rpm. The ammonia yield was 301 nmol / s / cm with a faradaic efficiency of 70%. 2 It was.

[0210] Those skilled in the art will appreciate that the invention described herein may be susceptible to variations and modifications other than those specifically described, and it is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the invention.

Claims

1. A method for producing ammonia by reducing dinitrogen, comprising the steps of: contacting a cathode of an electrochemical cell with an electrolyte; providing dinitrogen to the electrochemical cell for cathodic reduction; applying a potential to the cathode sufficient to reduce the dinitrogen to produce ammonia; The electrolyte is (i) a metal cation selected from the group consisting of lithium, magnesium, calcium, strontium, barium, zinc, aluminum, vanadium, and combinations thereof; (ii) one or more anions comprising at least one anion selected from the group consisting of fluorinated sulfonylimides, fluorinated sulfonylmethides, and combinations thereof; and (iii) a proton carrier. (iv) optionally, at least one phosphonium cation; The method for producing ammonia, wherein the metal cations are present in the electrolyte at a concentration greater than 0.5 mol / L, and the combined amount of the metal cations and any of the at least one phosphonium cations in the electrolyte is greater than 1 mol / L.

2. 2. The method for producing ammonia according to claim 1, wherein the metal cations are present in the electrolyte at a concentration of greater than 1 mol / L.

3. 2. The method for producing ammonia according to claim 1, wherein the metal cations are present in the electrolyte at a concentration of greater than 1.5 mol / L.

4. The at least one anion is selected from the group consisting of fluorinated sulfonylimides, and the fluorinated sulfonylimides have the structure of Formula 1: 【Chemistry 1】 (In the formula, R ｆ１ and R ｆ２ are independently -F, C １ -C １２ or R is selected from the group consisting of perfluoroalkyl and fluoroaryl; ｆ１ and R ｆ２ The method for producing ammonia according to any one of claims 1 to 3, wherein

5. The method for producing ammonia according to any one of claims 1 to 3, wherein the at least one anion is selected from the group consisting of bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)imide (FSI), (trifluoromethanesulfonyl)-(fluorosulfonyl)-imide (FTFSI), tris(trifluoromethanesulfonyl)methide, and combinations thereof.

6. The method for producing ammonia according to any one of claims 1 to 3, wherein the at least one anion is present in the electrolyte at a concentration of more than 1 mol / L.

7. The method for producing ammonia according to any one of claims 1 to 3, wherein the at least one anion comprises at least 80 mol% of the one or more anions.

8. The method for producing ammonia according to any one of claims 1 to 3, wherein the metal cation is lithium.

9. The method for producing ammonia according to any one of claims 1 to 3, wherein the at least one phosphonium cation is present in the electrolyte at a concentration of more than 0.2 mol / L.

10. The method for producing ammonia according to any one of claims 1 to 3, wherein the electrolyte is substantially free of organic nitrogen cations or contains any organic nitrogen cations in a total amount of less than 0.1 mol / L.

11. 4. The method for producing ammonia according to claim 1, wherein the proton carrier is a neutral proton carrier selected from the group consisting of alcohols and acids.

12. 4. The method for producing ammonia according to claim 1, wherein the electrolyte comprises an aprotic donor solvent capable of solvating the metal cations.

13. 4. The method for producing ammonia according to claim 1, wherein the potential of the cathode is lower (more negative) than −0.4 V with respect to the apparent reduction potential of the metal cation in the electrolyte.

14. 1. An electrochemical cell for producing ammonia by reduction of dinitrogen, comprising: A cathode; An anode; an electrolyte in contact with at least the cathode; a dinitrogen source that provides dinitrogen to the electrochemical cell for cathodic reduction; a power source connected to the cathode and the anode and capable of applying a potential to the cathode sufficient to reduce dinitrogen to produce ammonia; The electrolyte is (i) a metal cation selected from the group consisting of lithium, magnesium, calcium, strontium, barium, zinc, aluminum, vanadium, and combinations thereof; (ii) one or more anions comprising at least one anion selected from the group consisting of fluorinated sulfonylimides, fluorinated sulfonyl methides, and combinations thereof; (iii) a proton carrier; and (iv) optionally, a phosphonium cation; An electrochemical cell wherein said metal cations are present in said electrolyte at a concentration greater than 0.5 mol / L, and the combined amount of said metal cations and any said phosphonium cations is greater than 1 mol / L in said electrolyte.

15. 15. The electrochemical cell of claim 14, wherein the metal cations are present in the electrolyte at a concentration greater than 1 mol / L.

16. The at least one anion is selected from the group consisting of fluorinated sulfonylimides, the fluorinated sulfonylimides having the structure of Formula 1: 【Chemistry 2】 (In the formula, R ｆ１ and R ｆ２ are independently selected from the group consisting of -F, C １ -C １２ perfluoroalkyl and fluoroaryl, or R ｆ１ and R ｆ２ 16. The electrochemical cell of claim 14 or 15, comprising:

17. 16. The electrochemical cell of claim 14 or 15, wherein the at least one anion comprises at least 80 mol % of the one or more anions.

18. 16. An electrochemical cell according to claim 14 or 15, wherein the metal cation is lithium.

19. 16. The electrochemical cell of claim 14 or 15, wherein the electrolyte is substantially free of organic nitrogen cations or contains any organic nitrogen cations in a total amount of less than 0.1 mol / L.

20. 16. An electrochemical cell according to claim 14 or 15, wherein the electrolyte comprises an aprotic donor solvent capable of solvating the metal cations.

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