Electrolyte for lithium-mediated chemical synthesis of ammonia and application of electrolyte

Abstract

The invention belongs to the technical field of electrochemical synthesis and energy chemical industry, and particularly relates to an electrolyte for lithium-mediated conductive chemical synthesis of ammonia and application of the electrolyte, namely application in synthesis of ammonia by electrochemical conversion of nitrogen under mild conditions. By innovatively designing the components of the eutectic solvent, the bottleneck of the traditional organic electrolyte in the aspects of nitrogen solubility, stability and proton source regulation and control is solved, so that efficient, stable and safe electrochemical ammonia synthesis is realized at normal temperature and normal pressure. The device is simple in structure, safe to operate and low in energy consumption, and a brand new solution is provided for solving the problems of high energy consumption and high emission of the traditional ammonia synthesis technology.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical synthesis and energy chemical technology, specifically relating to an electrolyte for lithium-dielectric electrochemical synthesis of ammonia, and its application in the electrochemical conversion of nitrogen to ammonia under mild conditions. Background Technology

[0002] Ammonia, as a fundamental chemical raw material and a highly promising carbon-free energy carrier, requires green synthesis technology for sustainable development. Currently, global ammonia production almost entirely relies on the traditional Haber-Bosch process, which requires harsh conditions of high temperature (400-500°C) and high pressure (15-25 MPa). This process not only consumes enormous amounts of energy but also generates hundreds of millions of tons of carbon dioxide annually. Therefore, developing novel ammonia synthesis technologies that operate under milder conditions is urgently needed. Among numerous alternative routes, lithium-mediated electrochemical nitrogen reduction has attracted widespread attention due to its ability to directly convert nitrogen and water (or a proton source) into ammonia at ambient temperature and pressure. The reaction mechanism of this method typically involves the electrochemical generation of active lithium metal in an electrolyte. The active lithium reacts with dissolved nitrogen to form a lithium nitride intermediate, which is then protonated by a proton source (such as ethanol) to release the target product, ammonia.

[0003] However, the practical application of this technical route still faces a series of severe challenges, one of the core bottlenecks being the lack of an efficient and stable electrolyte system. Current research generally employs organic electrolytes based on ethers such as tetrahydrofuran, dissolving lithium salts (such as lithium perchlorate) as the lithium-ion source. This traditional system has many inherent defects: First, its limited solubility for nitrogen leads to insufficient nitrogen concentration at the reaction interface, limiting the reaction rate and ammonia yield; second, these organic solvents are usually volatile and flammable, posing safety hazards, and have poor compatibility with highly reactive lithium metal anodes, easily causing side reactions that lead to continuous electrolyte decomposition and decreased coulombic efficiency; third, the introduction of the proton source in the system is crude, usually involving the direct addition of small-molecule alcohols such as ethanol, which easily triggers a fierce competitive hydrogen evolution reaction. Furthermore, excessive proton sources can react directly with active lithium, wasting reactants and potentially causing incomplete protonation of lithium nitride intermediates, ultimately making it difficult to improve the Faraday efficiency and selectivity of ammonia synthesis.

[0004] In recent years, eutectic solvents, as a novel type of green solvent composed of hydrogen bond donors and acceptors, have shown great application potential in the field of electrochemistry due to their unique advantages such as simple preparation, low cost, low vapor pressure, non-flammability, and good thermal and chemical stability. Despite the excellent properties of dissolved oxygen (DES), its direct application in the demanding lithium-mediated ammonia synthesis system remains an underexplored and challenging area. Designing a DES that can meet the specific requirements of this reaction—requiring not only efficient dissolution of nitrogen and conduction of lithium ions, but also stable coexistence with the lithium metal electrode and precise control of the proton transfer process to suppress side reactions—constitutes a key technical challenge that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides an electrolyte specifically for lithium-dielectric conductive chemical synthesis of ammonia and its application. The aim is to create a stable, efficient, and safe reaction environment, significantly improving the Faraday efficiency and yield of ammonia synthesis.

[0006] The purpose of this invention is to provide an electrolyte for lithium-dielectric electrochemical synthesis of ammonia. This technical solution, through innovative design of the eutectic solvent composition, overcomes the bottlenecks of traditional organic electrolytes in terms of nitrogen solubility, stability, and proton source control, thereby achieving efficient, stable, and safe electrochemical synthesis of ammonia at room temperature and pressure.

[0007] An electrolyte for lithium-mediated electrochemical synthesis of ammonia comprises a eutectic solvent and an electrochemical promoter, wherein the eutectic solvent is composed of a hydrogen bond acceptor and a hydrogen bond donor; the hydrogen bond acceptor is lithium halide LiX, wherein X is selected from Cl, Br or I; the hydrogen bond donor is selected from at least one of acetamide, urea, ethylene glycol, glycerol, 1,3-propanediol, 1,4-butanediol, benzamide, citric acid, and fructose; the molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 1:1 to 1:4.

[0008] The electrochemical promoter is a crown ether, cryptane, imidazole ionic liquid, quaternary ammonium salt ionic liquid, or pyrrolidine ionic liquid. Specifically, the crown ether is 12-crown-4, 15-crown-5, 18-crown-6, dibenzo-18-crown-6, or aza-18-crown-6; the imidazole ionic liquid is 1-butyl-3-methylimidazolium tetrafluoroborate, 1-alkyl-3-methylimidazolium tetrafluoroborate, 1-alkyl-3-methylimidazolium hexafluorophosphate, or 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, wherein the alkyl group is a C1-C12 straight-chain or branched alkyl group, allyl, or benzyl.

[0009] The concentration of the electrochemical accelerator in the electrolyte is from 0.005 M to 2.0 M.

[0010] In addition, the present invention also provides a system for electrochemical synthesis of ammonia.

[0011] A system for electrochemical synthesis of ammonia includes an electrolytic cell, electrodes, and an electrolyte; The electrode includes a working electrode, a counter electrode, and a reference electrode; the working electrode is a copper, nickel, stainless steel, iron, platinum, gold, glassy carbon, graphite, carbon felt, carbon cloth, or a composite material of the above; the counter electrode and the reference electrode are lithium metal, lithium aluminum alloy, or lithium tin alloy electrodes.

[0012] The electrolyte comprises a eutectic solvent and an electrochemical promoter. The eutectic solvent is composed of a hydrogen bond acceptor and a hydrogen bond donor. The hydrogen bond acceptor is lithium halide (LiX), wherein X is selected from Cl, Br, or I. The hydrogen bond donor is selected from at least one of acetamide, urea, ethylene glycol, glycerol, 1,3-propanediol, 1,4-butanediol, benzamide, citric acid, and fructose. The molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 1:1 to 1:4.

[0013] The electrochemical promoter is a crown ether, cryptane, imidazole ionic liquid, quaternary ammonium salt ionic liquid, or pyrrolidine ionic liquid. Specifically, the crown ether is 12-crown-4, 15-crown-5, 18-crown-6, dibenzo-18-crown-6, or aza-18-crown-6; the imidazole ionic liquid is 1-butyl-3-methylimidazolium tetrafluoroborate, 1-alkyl-3-methylimidazolium tetrafluoroborate, 1-alkyl-3-methylimidazolium hexafluorophosphate, or 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, wherein the alkyl group is a C1-C12 straight-chain or branched alkyl group, allyl, or benzyl.

[0014] The concentration of the electrochemical accelerator in the electrolyte is from 0.005 M to 2.0 M.

[0015] The present invention also provides a method for electrochemical synthesis of ammonia.

[0016] An electrochemical method for synthesizing ammonia includes the following steps: a) Under an inert atmosphere, the electrolyte of any one of claims 1-4 is placed in an electrolytic cell and nitrogen gas is introduced into it; b) An electrochemical reduction reaction is carried out under conditions of 0–60 °C and 0.8–1.2 atm, with the application of current or voltage, to produce ammonia. The applied current density is -0.05 mA / cm². 2 ~ -20 mA / cm 2 Alternatively, the applied potential may be -0.3 V to -4.0 V relative to the lithium reference electrode.

[0017] Specifically, the present invention includes the following operational steps: 1) Electrolyte Synthesis: In a glove box filled with an inert atmosphere (H2O and O2 concentrations both below 0.1 ppm), lithium halides (such as lithium chloride) and hydrogen bond donors (such as acetamide) are accurately weighed according to a preset molar ratio (1:1~1:4) and placed in a reaction vessel. The mixture is continuously heated and stirred at 80°C~150°C until a homogeneous, transparent, colorless liquid is formed, thus preparing the eutectic solvent described in this invention. Subsequently, a crown ether (such as 18-crown-6) or an ionic liquid (such as 1-butyl-3-methylimidazolium tetrafluoroborate) at a concentration of 0.01 M~1.0 M is added to the solvent as an electrochemical promoter, and the mixture is stirred until completely dissolved to form a homogeneous electrolyte.

[0018] 2) Ammonia Synthesis Reaction: Transfer the prepared electrolyte to an electrochemical electrolytic cell. Assemble a three-electrode system using lithium metal sheets or lithium strips as the counter and reference electrodes, and copper foil, nickel mesh, or glassy carbon electrodes as the working electrodes. Then, continuously purge the electrolyte with high-purity nitrogen gas (≥99.999%) for at least 20 minutes to ensure complete removal of oxygen and nitrogen saturation of the electrolyte. Maintain the electrolytic cell at room temperature (15-35℃) and atmospheric pressure (one standard atmosphere), connect to an electrochemical workstation, and apply a constant current or potential to the working electrodes. The current density is controlled within the range of -0.1 mA / cm² to -10 mA / cm² to carry out the electrochemical reduction reaction. The reaction time can be sustained from 1 to 10 hours as needed.

[0019] 3) Product Acquisition and Detection: After the reaction, the total ammonia generated in the system was quantitatively analyzed. The total ammonia yield consists of three parts: liquid phase, gas phase, and solid phase. First, the electrolyte after the reaction was purged with nitrogen at 60-80℃ to drive out the ammonia, which was then absorbed by dilute sulfuric acid to obtain liquid ammonia. Second, the dilute sulfuric acid absorbent that captured the outflowing gas during the reaction was combined to obtain gaseous ammonia. Finally, all electrodes and electrolytic cell components were ultrasonically cleaned with dilute sulfuric acid (15-30 minutes) to convert the nitrogen-containing species adsorbed or deposited on the surface into ammonium salts, obtaining solid ammonia. The three ammonium-containing solutions were quantitatively analyzed using indophenol blue spectrophotometry to calculate the total ammonia yield, and the Faraday efficiency of ammonia synthesis was calculated in conjunction with the total charge consumption.

[0020] In summary, the eutectic solvent electrolyte for lithium-mediated ammonia synthesis in this invention not only provides a stable lithium source and reaction medium through an "integrated" design, but also precisely controls the key protonation step using an "in-situ proton source" mechanism, ultimately achieving highly efficient electrocatalytic reduction of nitrogen under mild conditions. The device of this invention has a simple structure, is safe to operate, and has low energy consumption, providing a novel solution to the high energy consumption and high emission problems of traditional ammonia synthesis technology. Detailed Implementation

[0021] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto: Example 1: A eutectic solvent, electrolyte, and application for lithium-dielectric conductive chemical synthesis of ammonia. In an argon-filled glove box (H₂O and O₂ < 0.1 ppm), accurately weigh 0.1 mol of lithium chloride (LiCl) and 0.30 mol of acetamide into a round-bottom flask. Seal the flask and remove it from the glove box. Stir in an 85°C oil bath for 2 hours to form a homogeneous, transparent eutectic solvent. After cooling, add 0.10 mmol of 18-crown-6 ether to the solution in the glove box and stir to dissolve, obtaining an electrolyte containing 0.01 M of promoter. Take 5 mL of this electrolyte in an H-type electrolytic cell, using a copper sheet as the working electrode and a lithium sheet as the counter / reference electrode. After purging with high-purity nitrogen (20 sccm, 30 min), apply an external pressure of -1.0 mA / cm at 25°C and atmospheric pressure. 2 Electrolysis was performed at a current density for 2 hours. After the reaction, liquid ammonia was obtained by nitrogen purging (60°C, 40 min), and solid ammonia was obtained by ultrasonic cleaning (150 W, 20 min). The gaseous ammonia absorption liquid from the reaction process was combined. The total ammonia content was determined by the indophenol blue method, and the Faraday efficiency was calculated to be 35.2%.

[0022] Example 2: Compared to Example 1, the following changes are made: The hydrogen bond donor was replaced with an equimolar amount of urea (0.30 mol) to prepare a LiCl / urea eutectic solvent. The remaining steps and conditions were identical to those in Example 1. The Faraday efficiency for ammonia synthesis using this system was tested to be 28.7%. This example demonstrates that urea is also effective as an "in-situ proton source," but its efficiency is slightly lower than that of acetamide.

[0023] Example 3: Compared to Example 1, the following changes are made: The LiBr / acetamide eutectic solvent was prepared by replacing the hydrogen bond acceptor with an equimolar amount of lithium bromide (LiBr, 0.10 mol). The remaining steps and conditions were identical to those in Example 1. The Faraday efficiency for ammonia synthesis using this system was tested to be 32.5%. This example demonstrates that different halide anions affect the reaction efficiency, and the bromide ion system maintains high reactivity.

[0024] Example 4: Compared to Example 1, the following changes are made: The electrochemical promoter was replaced by an equimolar (0.10 mmol) amount of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]). The remaining steps and conditions were identical to those in Example 1. The Faraday efficiency for ammonia synthesis in this system was tested to be 38.9%. This example demonstrates that ionic liquids, as promoters, may achieve superior performance by synergistically modulating the physicochemical properties of the electrolyte.

[0025] Example 5: Compared to Example 1, the following changes are made: The current density of the electrolysis reaction was increased from -1.0 mA / cm² to -2.5 mA / cm², while the reaction time was correspondingly shortened to 1 hour to maintain a substantially consistent total charge. The remaining steps and conditions were identical to those in Example 1. Testing showed that the Faraday efficiency for ammonia synthesis under these conditions was 30.1%. This example demonstrates that the system of this invention can maintain high reaction efficiency even at higher current densities, exhibiting good reaction rate adaptability.

[0026] Comparative Example 1, compared to Example 1, the following changes were made: A conventional organic electrolyte system was used, specifically a 0.5 M lithium perchlorate (LiClO4) tetrahydrofuran solution as the electrolyte, with 5 vol% ethanol added as the proton source. The remaining steps and conditions (including electrodes, nitrogen treatment, and testing methods) were identical to those in Example 1. Testing revealed that the Faraday efficiency of this conventional system was only 9.5%. This result strongly demonstrates the significant superiority of the eutectic solvent electrolyte system described in this invention over the conventional system in terms of efficiency and selectivity.

[0027] Comparative Example 2, compared to Example 1, makes the following changes: No electrochemical promoter (18-crown-6 ether) was added. The remaining steps and conditions were identical to those in Example 1. The Faraday efficiency for ammonia synthesis was tested to be 22.1%. This result, compared to Example 1 (35.2%), demonstrates the crucial role of the electrochemical promoter in improving reaction kinetics and efficiency.

[0028] Comparative Example 3: Compared to Example 1, the following changes were made: The hydrogen bond donor was replaced by an equimolar amount of ethylene glycol, which does not possess the potential to act as an "in-situ proton source." The remaining steps and conditions were identical to those in Example 1. Testing revealed almost no detectable ammonia formation in this system (Faraday efficiency <1%). This comparison clearly demonstrates that selecting specific hydrogen bond donors (such as acetamide or urea) capable of slowly decomposing and releasing protons is essential for achieving efficient lithium-mediated ammonia synthesis.

[0029] Finally, it should be noted that the above examples are merely some specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments and many variations are possible. All variations that can be directly derived or conceived by those skilled in the art from the disclosure of the present invention should be considered within the scope of protection of the present invention.

Claims

1. An electrolyte for lithium-dielectric conductive chemical synthesis of ammonia, characterized in that, The mixture includes a eutectic solvent and an electrochemical promoter. The eutectic solvent is composed of a hydrogen bond acceptor and a hydrogen bond donor. The hydrogen bond acceptor is lithium halide (LiX), where X is selected from Cl, Br, or I. The hydrogen bond donor is selected from at least one of acetamide, urea, ethylene glycol, glycerol, 1,3-propanediol, 1,4-butanediol, benzamide, citric acid, and fructose. The molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 1:1 to 1:

4.

2. The electrolyte according to claim 1, characterized in that, The electrochemical accelerator is a crown ether, crypt ether, imidazole ionic liquid, quaternary ammonium salt ionic liquid, or pyrrolidine ionic liquid.

3. The electrolyte according to claim 2, characterized in that, The crown ether is 12-crown-4, 15-crown-5, 18-crown-6, dibenzo-18-crown-6, or aza-18-crown-6; the imidazole ionic liquid is 1-butyl-3-methylimidazolium tetrafluoroborate, 1-alkyl-3-methylimidazolium tetrafluoroborate, 1-alkyl-3-methylimidazolium hexafluorophosphate, or 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, wherein the alkyl group is a C1-C12 straight-chain or branched alkyl group, allyl, or benzyl.

4. The electrolyte according to claim 2 or 3, characterized in that, The concentration of the electrochemical accelerator in the electrolyte is from 0.005 M to 2.0 M.

5. A system for electrochemical synthesis of ammonia, characterized in that, It includes an electrolytic cell, electrodes, and an electrolyte as described in any one of claims 1-4; the electrodes include a working electrode, a counter electrode, and a reference electrode; the working electrode is a copper, nickel, stainless steel, iron, platinum, gold, glassy carbon, graphite, carbon felt, carbon cloth, or composite material of the above; the counter electrode and the reference electrode are lithium metal, lithium aluminum alloy, or lithium tin alloy electrodes.

6. A method for electrochemical synthesis of ammonia, characterized in that, Includes the following steps: a) Under an inert atmosphere, the electrolyte of any one of claims 1-4 is placed in an electrolytic cell and nitrogen gas is introduced into it; b) Under conditions of 0~60℃ and 0.8~1.2 standard atmospheres, an electrochemical reduction reaction is carried out by applying current or voltage to generate ammonia.

7. The method according to claim 6, characterized in that, Step b) the current density applied is -0.05 mA / cm 2 -20 mA / cm 2 or the potential applied is -0.3 V - 4.0 V vs. a lithium reference electrode.

8. The method according to claim 6, characterized in that, Specifically, the following steps are included: 1) Electrolyte synthesis: In a glove box filled with an inert atmosphere, lithium halide and hydrogen bond donor are accurately weighed in a molar ratio of 1:1 to 1:4 and placed in a reaction vessel; the mixture is continuously heated and stirred at 80°C to 150°C until a homogeneous, transparent, colorless liquid is formed, thus preparing the eutectic solvent; subsequently, a crown ether or ionic liquid with a concentration of 0.01 M to 1.0 M is added to the solvent as an electrochemical promoter, and stirred until completely dissolved to form a homogeneous electrolyte; 2) Ammonia synthesis reaction: Transfer the electrolyte prepared in step 1) to an electrochemical electrolytic cell; A three-electrode system is assembled using lithium metal sheets or lithium strips as counter and reference electrodes, and copper foil, nickel mesh, or glassy carbon electrodes as working electrodes. High-purity nitrogen is then continuously introduced into the electrolyte for at least 20 minutes to ensure complete removal of oxygen from the system and saturation of the electrolyte with nitrogen. The electrolytic cell is maintained at room temperature and pressure. An electrochemical workstation is connected, and a constant current or potential is applied to the working electrode. The current density is controlled within the range of -0.1 mA / cm² to -10 mA / cm² to carry out the electrochemical reduction reaction. The reaction time can be sustained from 1 to 10 hours depending on the requirements. 3) Product Acquisition and Detection: After the reaction, the total ammonia generated in the system was quantitatively analyzed. The total ammonia yield consists of three parts: liquid phase, gas phase, and solid phase. First, the electrolyte after the reaction was purged with nitrogen at 60-80℃ to drive out the ammonia, which was then absorbed by dilute sulfuric acid to obtain liquid ammonia. Second, the dilute sulfuric acid absorbent that captured the outflowing gas during the reaction was combined to obtain gas phase ammonia. Finally, all electrodes and electrolytic cell components were ultrasonically cleaned with dilute sulfuric acid to convert the nitrogen-containing species adsorbed or deposited on the surface into ammonium salts to obtain solid phase ammonia. The three ammonium-containing solutions were quantitatively analyzed using indophenol blue spectrophotometry to calculate the total ammonia yield, and the Faraday efficiency of ammonia synthesis was calculated in conjunction with the total charge consumption.