A method of using an ionic liquid electrolyte for the electrolysis of ammonia to produce hydrogen

Abstract

The application discloses a method for electrolysis of ammonia to hydrogen by using an ionic liquid electrolyte, and belongs to the field of hydrogen production. The method is characterized in that the ionic liquid is directly used as an electrolyte, and the electrolysis of ammonia is performed in a constant potential mode. The ionic liquid as the electrolyte can effectively reduce the internal resistance of the solution of the ammonia decomposition reaction, reduce the reaction overpotential, and improve the current density. Compared with the reported ammonium salt electrolyte, the ionic liquid can not only be better miscible with anhydrous solvents, but also can effectively avoid the problem that the high concentration of ammonium ions leads to the inhibition of the self-coupling ionization of ammonia molecules. The electrolysis of ammonia to hydrogen in an anhydrous environment can avoid the occurrence of side reactions such as oxygen evolution reaction, and improve the Faraday efficiency of the ammonia decomposition method. Compared with the traditional industrial thermal decomposition process, the electrolysis of ammonia to hydrogen has a more moderate reaction condition, and can effectively reduce the energy consumption and the maintenance cost of the equipment.

Description

Technical Field

[0001] This invention belongs to the field of hydrogen production and relates to a method for producing hydrogen by electrolyzing ammonia using an ionic liquid electrolyte. Background Technology

[0002] The promotion of hydrogen energy is crucial for achieving dual-carbon goals, but hydrogen's low density, difficulty in compression, and flammability and explosiveness severely limit its storage and transportation. Chemical hydrogen storage, which converts hydrogen into hydrogen storage compounds, can avoid the leakage and explosion risks associated with direct hydrogen transportation. Ammonia, due to its high hydrogen storage density and good safety, is considered an ideal chemical hydrogen storage energy carrier, and online ammonia decomposition for hydrogen production can effectively solve the hydrogen storage and transportation problems. However, the mainstream industrial ammonia pyrolysis hydrogen production technology requires conditions of 700℃ and 0.5 MPa, facing challenges such as high energy consumption and significant equipment wear and tear during long-term continuous operation. Therefore, researching milder ammonia decomposition hydrogen production technologies can effectively reduce energy consumption and equipment maintenance costs, further promoting the development of "carbon-free ammonia-hydrogen energy storage."

[0003] Electrolysis of ammonia is a method for producing hydrogen by decomposing ammonia at room temperature. The theoretical decomposition potential of ammonia is only 0.077 V. Ammonia oxidation (AOR) occurs at the anode to produce nitrogen, and hydrogen evolution reaction (HER) occurs at the cathode to produce hydrogen. In 2005, Gerardine G. Botte et al. (On the use of ammonia electrolysis for hydrogen production, Journal of Power Sources, 142(2005), 18–26) proposed the concept of using an alkaline aqueous ammonia electrolyzer for hydrogen production. Because this method uses an alkaline aqueous solution containing ammonia, and the theoretical decomposition potential of water is 1.23 V, when the applied potential is too high, oxygen evolution reaction (OER) will occur on the anode surface in addition to AOR, leading to a decrease in the Faraday efficiency of ammonia decomposition. If aqueous ammonia electrolysis for hydrogen production is only operated at low potentials, the reaction kinetics are relatively slow, failing to fully utilize the high hydrogen storage density of ammonia. Nobuko Hanada et al. (Hydrogen generation by electrolysis of liquid ammonia, Chem. Commun., 2010, 46, 7775–7777) proposed a method for directly electrolyzing liquid ammonia to produce hydrogen in 2010. Since liquid ammonia itself is non-conductive, 1 MkNH2 needs to be added as an electrolyte to connect the circuit. A hydrogen production rate of 7.2 mA / cm² was obtained at a potential of 2 V. 2The current density and 85% Faraday efficiency of ammonia decomposition were achieved, and the H2 / N2 ratio during electrolysis was 2.96, confirming the feasibility of electrolyzing liquid ammonia. Subsequently, Bao-Xia Dong et al. (Improved electrolysis of liquid ammonia for hydrogen generation via ammonium salt electrolyte and Pt / Rh / Ir electrocatalysts, International Journal of Hydrogen Energy, 41(2016), 14507-14518) studied the electrolyte and anode materials of this system, using ammonium salt electrolyte (NH4Br / NH4NO3 / NH4Cl) at 120 mA / cm². 2 Electrolysis of liquid ammonia at the prepared current density for 3 hours consistently yielded a Faraday efficiency exceeding 80%. Furthermore, using the prepared RhPtIr alloy catalyst as the anode, the efficiency reached 46.9 mA / cm² at 2.0 V. 2 The current density. Meanwhile, Chen Haijun et al. (Hydrogen production system and hydrogen production method and power system using the hydrogen production system, application publication number CN114430057A) mentioned that the electrolyte composition used can increase the Faraday efficiency of ammonia electrolysis to a maximum of 90%. The electrolyte composition contains ammonium salt, solvent, and trace amounts of ionic liquid used as a solvent to dissolve the ammonium salt. The main conductive component is an organic / inorganic ammonium salt containing ammonium ions. Therefore, reducing the overpotential and internal resistance of the ammonia electrolysis hydrogen production reaction can effectively improve the Faraday efficiency of ammonia decomposition, which is key to achieving a mild and low-energy-consumption ammonia-hydrogen conversion.

[0004] In the process of hydrogen production from ammonia by electrolysis, we found that the introduction of ionic liquids can significantly reduce the internal resistance of the solution and improve the conductivity of the electrolyte. Simultaneously, there are interactions between ionic liquids and ammonia molecules, forming numerous hydrogen bond networks or coordination complexes, or enhancing ammonia absorption through electrostatic interactions, thus activating ammonia molecules. Furthermore, ionic liquids also possess good chemical stability and a wide electrochemical window. Unlike ammonium salts, ionic liquids are well miscible with most anhydrous solvents and do not inhibit the self-ionization of ammonia molecules due to the presence of ammonium ions. Therefore, we propose directly using ionic liquids as electrolytes for hydrogen production from ammonia by electrolysis. By reducing the internal resistance of the solution and activating ammonia molecules, this effectively promotes the ammonia decomposition reaction, improves its Faraday efficiency, and reduces the total energy consumption of the reaction process. Summary of the Invention

[0006] The purpose of this invention is to propose a method for producing hydrogen by electrolysis of ammonia using an ionic liquid electrolyte, in order to solve the above-mentioned problems and achieve efficient hydrogen production by ammonia decomposition under mild conditions.

[0007] This invention is based on the unique physicochemical properties of ionic liquids, using one or more ionic liquids composed of cations and anions as electrolytes (the cation is any one of monosubstituted imidazole, disubstituted imidazole, or hydroxyl-functionalized monosubstituted imidazole, and the anion is NTf2). - BF4 - OTf - NO3 - TFA - Cl - (Any of the following methods) Electrolysis of ammonia to produce hydrogen is performed under a constant potential in an anhydrous environment at a specific temperature and pressure, decomposing ammonia into hydrogen and nitrogen under mild conditions. This method requires no addition of any organic / inorganic ammonium salts and can achieve a flow rate of 85 mA / cm². 2 It can stably produce hydrogen at current density, and the Faraday efficiency of ammonia decomposition is as high as 97%.

[0008] To achieve the above objectives, the present invention provides the following technical solution: 1) Electrolysis of ammonia to produce hydrogen In this invention, a certain amount of ammonia gas is introduced into an anhydrous electrolyte in a pressure-resistant electrolytic cell, and hydrogen is produced by electrolysis of ammonia under certain temperature and pressure through a constant potential method.

[0009] The aforementioned pressure-resistant electrolytic cell structure includes a platinum working electrode, a platinum auxiliary electrode, a silver / silver oxide reference electrode, an anhydrous electrolyte, two pressure sensors, an ammonia inlet pipe, a temperature sensor, and two sapphire visualization windows. The pressure-resistant electrolytic cell can withstand 5 MPa. The cell body is made of stainless steel, and the surface is coated with polytetrafluoroethylene for insulation and corrosion protection. The anhydrous electrolyte is prepared by compounding an ionic liquid with an anhydrous solvent, or by directly compounding an ionic liquid with liquid ammonia formed by pressurizing ammonia gas. The anhydrous solvent is one or more of N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), anhydrous acetonitrile (ACN), and N-methylpyrrolidone (NMP). The ionic liquid is composed of different cations and anions. The cation is any one of monosubstituted imidazole, disubstituted imidazole, or hydroxyl-functionalized monosubstituted imidazole, and the anion is NTf2. - BF4 - OTf - NO3 - TFA - Cl - Any one of them, with a concentration of 0.1-1.0 mol / L.

[0010] The aforementioned constant potential method uses the chronoamperometry test procedure in an electrochemical workstation, with the applied constant potential range being 0-2 V.

[0011] 2) Collect and analyze the product gas The product gas is collected through a pre-vacuumed gas sampling bag, and the gas in the bag is qualitatively and quantitatively analyzed by gas chromatography.

[0012] The gas chromatography method described above is an analytical method that uses gas chromatography to determine the components and concentrations of a mixed gas. It uses the retention time of the mixed gas in the chromatographic column to qualitatively identify the gas components, and uses the external standard method to establish standard curves for each component gas, thereby determining the concentration of each component gas in the mixed gas.

[0013] Compared with the prior art, the present invention has the following advantages. 1) By using electrical energy to decompose ammonia into hydrogen and nitrogen under mild conditions, hydrogen is produced by electroammonolysis in an anhydrous environment, avoiding side reactions such as oxygen evolution and making full use of the advantages of ammonia's high hydrogen storage density and low decomposition potential.

[0014] 2) Using ionic liquids with high conductivity and ammonia affinity as electrolytes overcomes the problem that ammonium ions inhibit the self-ionization of ammonia molecules caused by using ammonium salts as electrolytes, thus improving current utilization.

[0015] 3) An anhydrous electrolyte prepared from [EtOHIm][NTf2] ionic liquid and liquid ammonia can achieve a 1V electrolyte at room temperature and pressure below 1.1 MPa. vs Maintain 85 mA / cm at Ag potential 2 The current density is used for stable and efficient electrolysis of ammonia to produce hydrogen, with an ammonia decomposition Faraday efficiency of 97%. Detailed Implementation

[0016] The present invention will be further described below with reference to embodiments. The specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0017] Example 1 Using [EtOHIm][NTf2] ionic liquid as the electrolyte, anhydrous DMF solvent with a molar ratio of 22.8 to the ionic liquid was added to prepare 100 ml of a 0.5 M [EtOHIm][NTf2] / DMF electrolyte. This electrolyte was placed in the aforementioned pressure-resistant electrolytic cell, and ammonia gas was introduced at a flow rate of 100 ml / min for 20 minutes until saturation was achieved. Electrolysis of ammonia to hydrogen was performed using a three-electrode system at a pressure of 0.1 MPa and a temperature of 25°C. At 1.3 V... vs At a constant potential for Ag, the internal resistance of the electrolyte solution is only 1.249 Ω, maintaining a current of 125 mA / cm². 2At a current density of 1 hour, the hydrogen production was stably produced by electrolyzing ammonia. The H2 / N2 ratio was 2.8:1 as determined by gas chromatography, and the Faraday efficiency was 94%.

[0018] Example 2 Using [BIm][TFA] ionic liquid as the electrolyte, anhydrous ACN solvent with a molar ratio of 34.6 to the ionic liquid was added to prepare 100 ml of a 0.5 M [BIm][TFA] / ACN electrolyte. This electrolyte was placed in the aforementioned pressure-resistant electrolytic cell, and ammonia gas was introduced at a flow rate of 100 ml / min for 20 minutes until saturation was achieved. Electrolysis of ammonia to hydrogen was performed using a three-electrode system at a pressure of 0.1 MPa and a temperature of 20°C. At 1.0 V... vs At a constant potential for Ag, the internal resistance of the electrolyte solution is only 1.616 Ω, maintaining a resistance of 78 mA / cm². 2 At a current density of 1 hour, the hydrogen production was stably produced by electrolyzing ammonia. The H2 / N2 ratio was 2.5:1 as determined by gas chromatography, and the Faraday efficiency was 60%.

[0019] Example 3 Using [BMIm][BF4] ionic liquid as the electrolyte, 100 ml of anhydrous THF solvent with a molar ratio of 121.0 to the ionic liquid was added to prepare an electrolyte with a concentration of 0.1 M [BMIm][BF4] / THF. This electrolyte was placed in the aforementioned pressure-resistant electrolytic cell, and ammonia gas was introduced at a flow rate of 100 ml / min for 20 minutes until saturation was achieved. Electrolysis of ammonia to hydrogen was performed using a three-electrode system at a pressure of 0.1 MPa and a temperature of 25°C. At 1.5V... vs At a constant potential for Ag, the internal resistance of the electrolyte solution is only 7.353 Ω, exceeding 80 mA / cm². -2 At the given current density, hydrogen was produced by electrolysis of ammonia for 1 hour. The H2 / N2 ratio was determined to be 2.5:1 by gas chromatography, and the Faraday efficiency was 58%.

[0020] Example 4 Using [EMIm][Cl] ionic liquid as the electrolyte, 100 ml of anhydrous NMP solvent with a molar ratio of 20.8 to the ionic liquid was added to prepare a 0.5 M [EMIm][Cl] / NMP electrolyte. This electrolyte was placed in the aforementioned pressure-resistant electrolytic cell, and ammonia gas was introduced at a flow rate of 100 ml / min for 20 minutes until saturation was achieved. Electrolysis of ammonia to hydrogen was then performed using a three-electrode system at a pressure of 0.1 MPa and a temperature of 40°C. At 1.5 V... vs At a constant potential for Ag, the internal resistance of the electrolyte solution is only 1.957 Ω, exceeding 45 mA / cm². 2 At the given current density, after 1 hour of ammonia electrolysis to produce hydrogen, the H2 / N2 ratio determined by gas chromatography was 2.2:1, and the Faraday efficiency was 88%.

[0021] Example 5 Using [BIm][NO3] ionic liquid as the electrolyte, anhydrous DMSO solvent with a molar ratio of 11.9 to the ionic liquid was added to prepare 100 ml of a 1.0 M [BIm][NO3] / DMSO electrolyte. This electrolyte was placed in the aforementioned pressure-resistant electrolytic cell, and ammonia gas was introduced at a flow rate of 100 ml / min for 20 minutes until saturation was achieved. Electrolysis of ammonia to hydrogen was then performed using a three-electrode system at a pressure of 0.1 MPa and a temperature of 25°C. At 1V... vs At a constant potential for Ag, the internal resistance of the electrolyte solution is only 5.018 Ω, maintaining a resistance of 48 mA / cm. 2 At the current density, after 1 hour of stable electrolysis of ammonia to produce hydrogen, the H2 / N2 ratio was determined by gas chromatography to be 2.19:1, and the Faraday efficiency was 85.3%.

[0022] Example 6 Using [BIm][OTf] ionic liquid as the electrolyte, anhydrous DMF solvent with a molar ratio of 23.4 to the ionic liquid was added to prepare 100 ml of a 0.5 M [BIm][OTf] / DMF electrolyte. This electrolyte was placed in the aforementioned pressure-resistant electrolytic cell, and ammonia gas was introduced at a flow rate of 100 ml / min for 20 minutes until saturation was achieved. Electrolysis of ammonia to hydrogen was performed using a three-electrode system at a pressure of 0.1 MPa and a temperature of 25°C. At 1.3 V... vs At a constant potential, the internal resistance of the electrolyte is only 1.308 Ω, exceeding 160 mA / cm². 2 The current density was measured at 2.8:1 by gas chromatography for hydrogen production from ammonia over 1 hour, and the Faraday efficiency was 96%.

[0023] Example 7 Using [EtOHIm][NTf2] ionic liquid as the electrolyte, without adding any anhydrous solvent (molar ratio with the ionic liquid being 0), ammonia gas was directly introduced and continuously pressurized to 0.8 MPa to form liquid ammonia. 100 ml of an electrolyte solution with a concentration of 0.5 M [EtOHIm][NTf2] / liquid ammonia was prepared. Electrolysis of ammonia to hydrogen was performed in the aforementioned pressure-resistant electrolytic cell using a three-electrode system at 0.8 MPa pressure and 20°C. At 1.0 V... vs At a constant potential for Ag, the internal resistance of the electrolyte solution is only 2.959 Ω, maintaining a resistance of 85 mA / cm². 2 At the current density, after stabilizing ammonia decomposition to produce hydrogen for 3 hours, the electrolytic cell pressure increased from 0.8 MPa to 1.1 MPa. The H2 / N2 ratio was determined to be 2.9:1 by gas chromatography, and the Faraday efficiency was 97%.

[0024] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various improvements and modifications to this invention without departing from its principles, and these improvements and modifications also fall within the protection scope of the claims.

Claims

1. A method for using an ionic liquid electrolyte to produce hydrogen through electrolysis of ammonia, characterized in that... The process includes the following steps: using an ionic liquid as the electrolyte, adding a certain amount of anhydrous solvent to prepare an electrolyte solution, introducing ammonia into the electrolyte solution, and then applying a certain potential at a certain temperature and pressure to electrolyze ammonia to produce hydrogen.

2. The method according to claim 1, characterized in that, The cation of the ionic liquid is any one of monosubstituted imidazole, disubstituted imidazole, or hydroxyl-functionalized monosubstituted imidazole, and the anion of the ionic liquid is NTf2. - BF4 - OTf - NO3 - TFA - Cl - Any one of them.

3. The method according to claim 1, characterized in that, The anhydrous solvent is one or more of N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), anhydrous acetonitrile (ACN), and N-methylpyrrolidone (NMP).

4. The method according to claim 1, characterized in that, The electrolyte is prepared by combining an ionic liquid and an anhydrous solvent, with a molar ratio of 0-200 between the anhydrous solvent and the ionic liquid.

5. The method according to claim 1, characterized in that, The temperature is 20℃-40℃.

6. The method according to claim 1, characterized in that, The pressure is 0.1-2 MPa.

7. The method according to claim 1, characterized in that, The potential range is 0-2V.