Magnesium-mediated nitrogen electrochemical reduction synthesis ammonia system based on composite electrolyte and synthesis method

Abstract

The invention relates to the technical field of electrochemical ammonia synthesis, and particularly discloses a magnesium-mediated nitrogen electrochemical reduction ammonia synthesis system based on a composite electrolyte and a synthesis method. A magnesium-mediated nitrogen electrochemical reduction synthesis ammonia system comprises an electrolytic tank, an electrolyte in the electrolytic tank is a composite electrolyte, the composite electrolyte is composed of magnesium salt, cyclic ether and chain ether, the volume ratio of the cyclic ether to the chain ether is (1-4): 1, and the concentration of the magnesium salt is 0.1-1.0 mol.L. According to the composite electrolyte system constructed by the invention, the deposition uniformity and interface stability of magnesium ions are remarkably improved, a repeatable and controllable nitrogen electrochemical reduction process is realized, and the composite electrolyte system has the advantages of high efficiency, stability, low energy consumption and the like.

Description

Technical Field

[0001] This invention relates to the field of electrochemical ammonia synthesis technology, and specifically discloses a magnesium-mediated nitrogen electrochemical reduction ammonia synthesis system and synthesis method based on a composite electrolyte. Background Technology

[0002] Ammonia (NH3) is a crucial basic chemical in agriculture, chemical industry, and energy storage and transportation systems. The traditional Haber-Bosch process requires high temperature and pressure conditions and relies heavily on fossil fuels for hydrogen production, resulting in high energy consumption and significant carbon emissions. To achieve low-carbon or zero-carbon ammonia synthesis, electrochemical nitrogen reduction synthesis based on renewable electricity is considered a promising alternative route. Among these, metal-mediated nitrogen electrochemistry has become a hot research topic in recent years due to its high reaction selectivity and low kinetic barriers.

[0003] Among the reported metal-mediated systems, the lithium-mediated nitrogen reduction system (Li-NRR) was the earliest developed. Its typical reaction process involves three steps: first, lithium ions are reduced and deposited as metallic lithium on the cathode; second, active lithium reacts with nitrogen to generate lithium nitride (Li3N) intermediates; finally, Li3N reacts with a proton source in the electrolyte to generate ammonia and release lithium ions, completing one electrochemical cycle. In 2019, Anderson et al. reported a single-chamber lithium-mediated electrochemical nitrogen reduction system using 0.2 M LiClO4 / THF solution as the electrolyte, molybdenum as the anode, and ethanol as the proton source, achieving a Faraday efficiency of 10%. The ammonia generation was rigorously verified by nuclear magnetic resonance (¹H NMR) and indophenol blue colorimetry (Nature, 2019, 570(7762): 504–508). Although the Li-NRR system has made some progress, it still has shortcomings in terms of energy efficiency, system stability, and cycle life.

[0004] To overcome the low energy efficiency and instability of lithium systems, researchers have begun exploring safer, cheaper, and more abundant alternatives to gold. Magnesium, with a standard electrode potential of -2.37 V (vs SHE), slightly higher than lithium (-3.04 V), exhibits strong reducing power and a lower dendrite risk. In existing technology, patent CN 116102035 A discloses a method for synthesizing ammonia through a medium-cycle nitrogen fixation process based on metallic magnesium. This method involves reacting metallic magnesium with nitrogen at high temperature to generate magnesium nitride; the magnesium nitride then reacts with water to produce magnesium hydroxide precipitate and ammonia; subsequently, hydrogen chloride gas is introduced into the magnesium hydroxide solution to generate a magnesium chloride solution, which is then heated and dehydrated to obtain anhydrous magnesium chloride. Electrolysis of this solution produces chlorine and metallic magnesium; the chlorine is then burned in a hydrogen atmosphere to generate hydrogen chloride gas, thus forming a complete cycle system. Although this method achieves magnesium-mediated cyclic nitrogen fixation, the reaction conditions are harsh, the system involves reactive gases such as Cl2, the process is complex, and there are potential safety hazards. Melinda et al. verified the feasibility of magnesium reacting with nitrogen to produce ammonia under mild conditions, and proved through rigorous controlled experiments that the ammonia formation originates from a magnesium-mediated N2 reduction process, rather than from trace amounts of NO. x The reduction. In this proof-of-concept study, based on Mg 0 / 2+ The cyclic nitrogen reduction reaction can indeed produce ammonia, but the Faraday efficiency is less than 10%, indicating that the electrochemical efficiency and stability of the system still need to be significantly improved (Energy Environ. Sci., 2024, 17, 4481).

[0005] In summary, current metal-mediated nitrogen reduction systems still suffer from the following problems: Due to the high charge density of metal ions, they readily form stable complexes with solvents and anions, leading to high solvation energies and slow deposition kinetics. Simultaneously, commonly used ether solvents exhibit low solubility of metal salts and limited ion transport efficiency, and passivation layers easily form on the electrode surface, causing uneven deposition and interfacial instability. Furthermore, significant side reactions such as hydrogen evolution occur during the reaction, competing with the nitrogen reduction process and significantly reducing the Faraday efficiency of ammonia. Therefore, this invention proposes a magnesium-mediated electrochemical reduction system for nitrogen, centered on a composite electrolyte. By designing a weakly coordinating anionic salt combined with a cyclic-chain ether composite solvent, the solvation structure of magnesium ions is optimized, promoting uniform deposition and improving ion transport efficiency, thereby enhancing the interfacial stability and catalytic activity of the system. Summary of the Invention

[0006] The present invention aims to provide a magnesium-mediated electrochemical reduction system for ammonia synthesis using a composite electrolyte as the core, in order to solve the problems of high solvation energy, uneven deposition, unstable interface and serious side reactions in the existing magnesium system, and to achieve a high-efficiency and stable electrochemical reduction process of nitrogen.

[0007] To solve the above-mentioned technical problems, the present invention provides a composite electrolyte for the synthesis of ammonia by the electrochemical reduction of nitrogen gas mediated by magnesium.

[0008] A composite electrolyte for the electrochemical reduction of nitrogen to synthesize ammonia mediated by magnesium is composed of magnesium salt, cyclic ether and chain ether, with a volume ratio of cyclic ether to chain ether of 1 to 4:1 and a magnesium salt concentration of 0.1 to 1.0 mol·L⁻¹.

[0009] The magnesium salt is one or more of Mg(TFSI)2, Mg(FSI)2, Mg(BF4)2, Mg(ClO4)2, Mg(PF6)2, and Mg(CF3SO3)2; the cyclic ether is selected from one or more of tetrahydrofuran (THF), oxetane (THB), 1,3-dioxolane, and 1,4-dioxane (DOL); the chain ether is selected from one or more of diethylene glycol dimethyl ether (diglyme), dimethoxyethane (DME), and triethylene glycol dimethyl ether (TEGDME).

[0010] It also contains 0.1 to 5 vol% of an alcohol proton modifier. The alcohol proton modifier is selected from ethanol, isopropanol, methanol, or combinations thereof.

[0011] This invention provides a magnesium-mediated electrochemical reduction system for ammonia synthesis via nitrogen gas through the synergistic effect of a weakly coordinating anionic salt and a cyclic-chain ether complex solvent, which can effectively improve the solvation structure of magnesium ions and the stability of electrode reactions.

[0012] A magnesium-mediated electrochemical reduction system for ammonia synthesis using nitrogen includes an electrolytic cell. The electrolyte in the electrolytic cell is a composite electrolyte composed of magnesium salt, cyclic ether, and chain ether. The volume ratio of cyclic ether to chain ether is 1 to 4:1, and the concentration of magnesium salt is 0.1 to 1.0 mol·L⁻¹.

[0013] The electrolytic cell has a single-chamber structure with a diaphragm; the cathode is a copper, molybdenum, or stainless steel foil that has undergone surface roughening or porous treatment; the anode is a platinum, iridium, or other inert metal that has undergone argon plasma treatment or light chemical etching to form a rough and porous surface.

[0014] The distance between the anode and cathode is 5~15 mm, the electrochemical reaction adopts constant potential or constant current mode, the temperature is 20~100 ℃, and the nitrogen pressure is 0~20 bar.

[0015] In addition, this application also provides a method for synthesizing ammonia by magnesium-mediated electrochemical reduction of nitrogen.

[0016] A method for synthesizing ammonia by magnesium-mediated electrochemical reduction of nitrogen gas includes the following steps: (1) Add magnesium salt to a mixed solvent in which cyclic ether and chain ether are mixed at a volume ratio of 1-4:1, with a magnesium salt concentration of 0.1~1.0 mol·L⁻¹, and then add anhydrous ethanol as a proton regulator. After fully dissolving the solution, filter to remove impurities, and degas under vacuum to obtain a transparent composite electrolyte. (2) The cathode is made of copper, molybdenum or stainless steel foil, which is ultrasonically cleaned in acetone, ethanol and deionized water for 10 minutes each, dried and then treated with argon plasma for 5 minutes to obtain a rough and porous surface to enhance gas adsorption and reaction activity; the anode is made of inert metal of the same size, which is ultrasonically cleaned in acetone, ethanol and deionized water for 10 minutes each, dried and ready for use; the electrolytic cell adopts a single-cavity structure, which is composed of a stainless steel shell and a polytetrafluoroethylene liner, with a glass fiber membrane as the separator, and the distance between the anode and cathode is 5~15 mm; after adding the composite electrolyte of step (1) to the battery, the system is purged with high-purity nitrogen gas at 60 sccm for 30 minutes, and the nitrogen pressure is further adjusted to 0~20 bar to start the electrolysis reaction; The electrochemical reactions were carried out at 20–100 °C using a constant potential or constant current mode, with an applied voltage of 4.0 V, a reaction time of 4 hours, and a current density stabilized at 3.5 mA·cm⁻¹. -2 .

[0017] Specifically, the present invention includes the following steps: (1) First, weigh 1.00 g of anhydrous magnesium bis(trifluoromethanesulfonyl)imide salt (Mg(TFSI)2, purity ≥99.9%) in an argon-protected glove box, and add it to 10.0 mL of a solvent composed of tetrahydrofuran (THF) and diethylene glycol dimethyl ether (diglyme) at a volume ratio of 3:1. Then add 0.10 mL of anhydrous ethanol as a proton modifier, and stir magnetically for 8 hours to fully dissolve. After filtering the resulting solution through a 0.2 μm polytetrafluoroethylene filter membrane to remove impurities, degas it under vacuum for 12 hours to obtain a transparent composite electrolyte.

[0018] (2) The cathode uses a 0.1 mm thick copper foil (1 cm × 1 cm), which is ultrasonically cleaned for 10 minutes each in acetone, ethanol, and deionized water, and then dried and placed in an argon glove box for later use. The copper electrode is treated with argon plasma for 5 minutes to obtain a rough and porous surface, which enhances gas adsorption and reaction activity. The anode uses a platinum foil of the same size and is cleaned and dried in the same way. The electrolytic cell adopts a single-chamber structure, consisting of a stainless steel shell and a polytetrafluoroethylene liner, with a glass fiber membrane as the separator, and the distance between the anode and cathode is 8 mm. After adding 10 mL of the prepared composite electrolyte to the battery, the system is purged with high-purity nitrogen gas (99.999% purity) at 60 sccm for 30 minutes, and the nitrogen pressure is further adjusted to 10 bar to start the electrolysis reaction.

[0019] (3) The electrochemical reaction was carried out at 25 °C, controlled by a constant potential mode, with an applied voltage of 4.0 V, a reaction time of 4 hours, and a current density that stabilized at 3.5 mA·cm. -2 After the reaction was completed, 1.0 mL of electrolyte sample was taken, diluted 20 times with 0.1 M H₂SO₄ solution, and the ammonia production was determined by the indophenol blue colorimetric method. The cathode after the reaction was cleaned with anhydrous THF and dried. Its surface morphology was observed using a scanning electron microscope. The results showed that the magnesium deposition layer was dense and uniform, without dendrite structure.

[0020] In summary, the composite electrolyte system constructed in this invention significantly improves the deposition uniformity and interfacial stability of magnesium ions, and realizes a repeatable and controllable nitrogen electrochemical reduction process, which has the advantages of high efficiency, stability and low energy consumption. Attached Figure Description

[0021] Figure 1 These are electrode characterization images of the present invention. Detailed Implementation

[0022] 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 magnesium-mediated electrochemical reduction system and method for ammonia synthesis using nitrogen, the specific operation of which is as follows: In an argon-protected glove box (O2 and H2O content both <0.1 ppm), 1.00 g of anhydrous magnesium bis(trifluoromethanesulfonyl)imide salt (Mg(TFSI)2, purity ≥99.9%, purchased from Sigma-Aldrich) was weighed and added to 10.0 mL of anhydrous solvent prepared by mixing tetrahydrofuran (THF) and diethylene glycol dimethyl ether (diglyme) at a volume ratio of 3:1. Then, 0.10 mL of anhydrous ethanol was added as a proton modifier. The mixture was stirred at 25 °C with a magnetic stirrer at 600 rpm for 8 hours to completely dissolve the magnesium salt. The resulting transparent solution was filtered through a 0.2 μm polytetrafluoroethylene (PTFE) membrane to remove impurities, and then subjected to vacuum conditions (10... -3 Degassing was performed at 12 hours under 1000 Pa to obtain a transparent composite electrolyte.

[0023] The cathode used 0.1 mm thick copper foil (1.0 cm × 1.0 cm, purity ≥99.9%), which was ultrasonically cleaned sequentially for 10 minutes each in acetone, anhydrous ethanol, and deionized water. After cleaning, the copper foil was dried in a vacuum oven at 60 °C for 1 hour and then placed in a glove box for later use. To enhance reactivity, the copper electrode was treated with 50 W argon plasma for 5 minutes to form a rough, porous surface. The anode used platinum foil of the same size (1 cm × 1 cm), which was ultrasonically cleaned sequentially in acetone, ethanol, and deionized water, and then dried for later use. The electrolytic cell was a single-chamber structure, consisting of a stainless steel shell and a polytetrafluoroethylene liner. The electrode spacing was set at 8 mm, and a glass fiber membrane (Whatman GF / A) was used as the diaphragm. After injecting 10 mL of the prepared composite electrolyte into the electrolytic cell, the system was purged with high-purity nitrogen gas (99.999% purity) at a flow rate of 60 sccm for 30 minutes to completely remove residual air. The system was then sealed and the nitrogen pressure was increased to 10 bar.

[0024] The electrochemical reaction was carried out under isothermal conditions at 25 °C, controlled by a potentiostatic mode (potential 4.0 V), for 4 hours. The current density stabilized at approximately 3.5 mA·cm⁻¹. -2After the reaction, 1.0 mL of electrolyte sample was taken out and diluted 20 times with 0.1 M H2SO4 solution. The ammonia content generated was determined by the indophenol blue colorimetric method. The absorbance of the colorimetric reaction was measured at a wavelength of 655 nm, and the ammonia yield was calculated according to the standard curve. The ammonia production was measured to be 11.8 μmol, corresponding to a Faraday efficiency of 36.5%. After the reaction, the cathode was removed and thoroughly cleaned with anhydrous THF to remove residual salts and electrolyte, and then dried in a vacuum drying oven for 4 hours. The morphology of the electrode surface was observed, and the results showed that the magnesium deposition layer was dense, continuous, and uniformly distributed, and no dendrite or needle-like structures were observed. At the same time, obvious Mg-N bond signals were detected by X-ray photoelectron spectroscopy (XPS), indicating that nitrogen underwent effective chemical adsorption and reduction reactions on the electrode surface. Figure 1 These are electrode characterization images of the present invention.

[0025] Example 2: Compared to Example 1, the following changes are made: The solvent system was changed to THF / dimethoxyethane (DME) = 2:1 (volume ratio), while other conditions remained unchanged. After 4 hours of electrolysis, the ammonia yield was 10.2 μmol, the Faraday efficiency was 34.8%, the magnesium deposition on the cathode surface was uniform and dense, and the Mg-N signal was obvious.

[0026] Example 3: Compared to Example 1, the following changes are made: When the magnesium salt was replaced with Mg(FSI)2, and the solvent system was kept at THF / diglyme = 3:1, with other conditions remaining unchanged, the ammonia yield was 13.1 μmol after 4 h of electrolysis, the Faraday efficiency was 39.2%, the cathode surface was smooth, and the Mg-N signal was obvious, indicating that the anion modification improved the magnesium deposition behavior.

[0027] Example 4: Compared to Example 1, the following changes are made: Compared to Example 1, the amount of ethanol added was increased to 0.20 mL, while the solvent system and magnesium salt remained unchanged, and other conditions were also unchanged. After 4 h of electrolysis, the ammonia yield was 12.4 μmol, the Faraday efficiency was 38.1%, the proportion of by-product hydrogen decreased, and the magnesium deposition was uniform and the interface was stable.

[0028] Example 5: Compared to Example 1, the following changes are made: The nitrogen pressure was increased to 15 bar, while other conditions remained unchanged. After 4 hours of electrolysis, the ammonia yield was 15.6 μmol, the Faraday efficiency was 41.7%, the deposited layer was continuous and dense, and the system showed good stability.

[0029] Comparative Example 1, compared to Example 1, the following changes were made: Using only pure THF solvent (without diglyme co-solvent), all other conditions remained unchanged. The solubility of magnesium salts in the solution decreased, the ammonia yield was 2.4 μmol, the Faraday efficiency was 4.2%, and obvious dendrites appeared at the cathode.

[0030] Comparative Example 2, compared to Example 1, makes the following changes: Using only MgCl2 as the magnesium salt, with all other conditions unchanged, the ammonia yield was only 1.5 μmol after 4 h of electrolysis, with a Faraday efficiency of 5.6%. No obvious Mg–N signal was detected at the cathode, indicating limited formation of nitride intermediates.

[0031] 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. A composite electrolyte for the electrochemical reduction of nitrogen gas to synthesize ammonia mediated by magnesium, characterized in that: The electrolyte is composed of magnesium salt, cyclic ether and chain ether, with a volume ratio of cyclic ether to chain ether of 1 to 4:1 and a magnesium salt concentration of 0.1 to 1.0 mol·L⁻¹.

2. The composite electrolyte according to claim 1, characterized in that: The magnesium salt is one or more of Mg(TFSI)2, Mg(FSI)2, Mg(BF4)2, Mg(ClO4)2, Mg(PF6)2, and Mg(CF3SO3)2; the cyclic ether is selected from one or more of tetrahydrofuran (THF), oxetane (THB), 1,3-dioxolane, and 1,4-dioxane (DOL); the chain ether is selected from one or more of diethylene glycol dimethyl ether (diglyme), dimethoxyethane (DME), and triethylene glycol dimethyl ether (TEGDME).

3. The composite electrolyte according to claim 1, characterized in that: It also contains 0.1 to 5 vol% of alcohol proton modifiers.

4. The composite electrolyte according to claim 3, characterized in that: The alcohol proton modifier is selected from ethanol, isopropanol, methanol, or combinations thereof.

5. A magnesium-mediated electrochemical reduction system for ammonia synthesis using nitrogen, characterized in that: It includes an electrolytic cell, the contents of which are the composite electrolyte as described in any one of claims 1-6.

6. The magnesium-mediated nitrogen electrochemical reduction ammonia synthesis system according to claim 5, characterized in that: The electrolytic cell has a single-chamber structure with a diaphragm; the cathode is a copper, molybdenum, or stainless steel foil that has undergone surface roughening or porous treatment; the anode is a platinum, iridium, or other inert metal that has undergone argon plasma treatment or light chemical etching to form a rough and porous surface.

7. The magnesium-mediated nitrogen electrochemical reduction ammonia synthesis system according to claim 6, characterized in that: The distance between the anode and cathode is 5~15 mm, the electrochemical reaction adopts constant potential or constant current mode, the temperature is 20~100 ℃, and the nitrogen pressure is 0~20 bar.

8. A method for synthesizing ammonia by magnesium-mediated electrochemical reduction of nitrogen, characterized in that: Includes the following steps: (1) Add magnesium salt to a mixed solvent in which cyclic ether and chain ether are mixed at a volume ratio of 1-4:1, with a magnesium salt concentration of 0.1~1.0 mol·L⁻¹, and then add anhydrous ethanol as a proton regulator. After fully dissolving the solution, filter to remove impurities, and degas under vacuum to obtain a transparent composite electrolyte. (2) The cathode is made of copper, molybdenum or stainless steel foil, which is ultrasonically cleaned in acetone, ethanol and deionized water for 10 minutes each, dried and then treated with argon plasma for 5 minutes to obtain a rough and porous surface to enhance gas adsorption and reaction activity; the anode is made of inert metal of the same size, which is ultrasonically cleaned in acetone, ethanol and deionized water for 10 minutes each, dried and ready for use; the electrolytic cell adopts a single-cavity structure, which is composed of a stainless steel shell and a polytetrafluoroethylene liner, with a glass fiber membrane as the separator, and the distance between the anode and cathode is 5~15 mm; after adding the composite electrolyte of step (1) to the battery, the system is purged with high-purity nitrogen gas at 60 sccm for 30 minutes, and the nitrogen pressure is further adjusted to 0~20 bar to start the electrolysis reaction; The electrochemical reactions were carried out at 20–100 °C using a constant potential or constant current mode, with an applied voltage of 4.0 V, a reaction time of 4 hours, and a current density stabilized at 3.5 mA·cm⁻¹. -2 .

9. The method according to claim 8, characterized in that: The specific steps are as follows: (1) First, weigh 1.00 g of anhydrous magnesium bis(trifluoromethanesulfonyl)imide salt in an argon-protected glove box and add it to 10.0 mL of a solvent composed of tetrahydrofuran and diethylene glycol dimethyl ether in a volume ratio of 3:

1. Then add 0.10 mL of anhydrous ethanol as a proton modifier and stir magnetically for 8 hours to fully dissolve the solution. After filtering the resulting solution through a 0.2 μm polytetrafluoroethylene filter membrane to remove impurities, degas it under vacuum for 12 hours to obtain a transparent composite electrolyte; (2) The cathode uses 0.1 mm thick copper foil, which is ultrasonically cleaned for 10 minutes each in acetone, ethanol and deionized water, and then dried and placed in an argon glove box for later use; the copper electrode is treated with argon plasma for 5 minutes to obtain a rough and porous surface to enhance gas adsorption and reaction activity. The anode uses platinum foil of the same size and is cleaned and dried in the same way. The electrolytic cell adopts a single-chamber structure, consisting of a stainless steel shell and a polytetrafluoroethylene liner, with a glass fiber membrane as the separator, and the distance between the anode and cathode is 8 mm. After adding 10 mL of the prepared composite electrolyte to the battery, the system is purged with high-purity nitrogen at 60 sccm for 30 minutes, and the nitrogen pressure is further adjusted to 10 bar to start the electrolysis reaction; The electrochemical reaction was carried out at 25 °C using a constant potential mode, with an applied voltage of 4.0 V, a reaction time of 4 hours, and a current density that remained stable at 3.5 mA·cm⁻¹. -2 .

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