Electrolyte for preparing formic acid by electroreduction of liquid ionic liquid at normal temperature, method and application

Abstract

The invention discloses an electrolyte for preparing formic acid through normal-temperature liquid ionic liquid electroreduction, a method and application, and belongs to the technical field of carbon dioxide electroreduction, the method comprises the following steps: adding a quaternary ammonium salt cation precursor and fluorine-containing sulfimide salt into an organic solvent, carrying out primary stirring reaction, filtering to remove precipitate, and carrying out reduced pressure distillation to obtain the formic acid. The quaternary ammonium salt fluorine-containing sulfimide ionic liquid is prepared; adding the mixed anionic ionic liquid and nitrogen-containing heterocyclic carboxylic acid into water, and carrying out secondary stirring reaction to form mixed anionic ionic liquid; and adding an additive, and uniformly stirring at room temperature to obtain the formic acid electrolyte prepared by electroreduction of the normal-temperature liquid ionic liquid. Through a specific electrolyte design, efficient and high-selectivity electroreduction of carbon dioxide into formic acid is realized. The specific electrolyte is prepared by reasonably designing the structure of the ionic liquid, and in the process of preparing formic acid by electrically reducing carbon dioxide, the hydrogen evolution side reaction is effectively inhibited, and the Faraday efficiency and the energy efficiency of formic acid and the yield of formic acid per hour per unit electrolysis area are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of carbon dioxide electro-reduction, and particularly relates to a normal-temperature liquid ionic liquid electro-reduction formic acid electrolyte, a method and an application. BACKGROUND

[0002] With the acceleration of global industrialization, the emission of carbon dioxide (CO2) is increasing, leading to serious environmental problems such as greenhouse effect, climate change, etc. Converting CO2 into high-value chemicals such as formic acid is an effective solution, which not only reduces the emission of CO2, but also realizes the recycling of carbon resources. Electrochemical reduction of CO2 to formic acid is a promising method due to its cleanliness, controllability and simple reaction module. However, this method faces many challenges, among which the performance of the electrolyte has a crucial influence on the reaction effect. In the process of electrochemical reduction of CO2, the electrolyte plays an important role in transporting ions, promoting electron transfer and affecting reaction selectivity. Traditional electrolyte systems, such as aqueous inorganic salt solutions, have a serious hydrogen evolution side reaction. Since the hydrogen evolution reaction is more likely to occur on most metal catalyst electrodes than the CO2 electro-reduction reaction, the faradic efficiency of CO2 electro-reduction to formic acid is low in this system, which greatly limits the reaction.

[0003] As a new type of green solvent, ionic liquids have attracted widespread attention in the field of CO2 capture and electrochemical reduction due to their non-volatility, high chemical stability, high electrical conductivity and adjustable properties. Some studies have attempted to apply ionic liquids to the electrolyte of CO2 electro-reduction reaction, hoping to improve the reaction performance through their unique properties. For example, some studies have prepared organic solvent / ionic liquid composite electrolytes by synthesizing functional ionic liquids, in order to improve the solubility of CO2 in the electrolyte and thus improve the current density of CO2 electro-reduction reaction. However, these methods often have some problems, such as the net consumption of ionic liquids in the reaction mechanism of some ionic liquid systems, which leads to increased cost and difficulty in continuous and stable operation; some systems are not effective in inhibiting the hydrogen evolution side reaction, and cannot effectively improve the selectivity and efficiency of CO2 conversion to formic acid. In addition, the existing electrolyte systems also have deficiencies in gas-liquid phase mass transfer. The mass transfer efficiency of the reactant CO2 in the electrolyte is low, which makes it difficult to quickly and effectively reach the electrode surface to participate in the reaction, which not only affects the reaction rate, but also has a negative impact on the selectivity of the product. Moreover, the interaction between the electrolyte and the electrode during the long-term reaction process may lead to a decrease in the catalytic activity and stability of the electrode, further hindering the industrial application of the technology.

[0004] Therefore, for the process of electrochemical reduction of CO2 to form formic acid, the traditional electrolyte system has a serious problem of hydrogen evolution side reaction. On most metal catalyst electrodes, the hydrogen evolution reaction is more likely to occur than the CO2 electro-reduction reaction, resulting in a low Faraday efficiency of CO2 electro-reduction to form formic acid. The existing electrolyte system has deficiencies in gas-liquid phase mass transfer, and the mass transfer efficiency of CO2 in the electrolyte is low, making it difficult to quickly and effectively reach the electrode surface to participate in the reaction. During the long reaction process, the interaction between the electrolyte and the electrode may cause the catalytic activity of the electrode to decrease and the stability to deteriorate. It is of great significance to develop an electrolyte system that can effectively suppress the hydrogen evolution side reaction, improve the solubility and mass transfer efficiency of CO2, and has high stability and low cost, for the industrial application of electrochemical reduction of CO2 to form formic acid. SUMMARY

[0005] In order to overcome the above-mentioned shortcomings of the prior art, the purpose of the present application is to provide a room temperature liquid ionic liquid electrolyte for electro-reduction to form formic acid, a method and an application, to solve the technical problems of serious hydrogen evolution side reaction, low CO2 mass transfer efficiency and decreased electrode catalytic activity in the process of electrochemical reduction of CO2 to form formic acid with the existing electrolyte.

[0006] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows: The present application discloses a preparation method of a room temperature liquid ionic liquid electrolyte for electro-reduction to form formic acid, comprising: 1) adding a quaternary ammonium salt cation precursor and a fluorine-containing sulfonimide salt to an organic solvent, stirring once, removing the precipitate by filtration, and distilling under reduced pressure to obtain a quaternary ammonium salt fluorine-containing sulfonimide ionic liquid; 2) adding a nitrogen-containing heterocyclic carboxylic acid to the quaternary ammonium salt fluorine-containing sulfonimide ionic liquid obtained in step 1) into water, stirring twice to form a mixed anion ionic liquid; 3) adding an additive to the mixed anion ionic liquid obtained in step 2), stirring uniformly at room temperature to obtain the room temperature liquid ionic liquid electrolyte for electro-reduction to form formic acid.

[0007] Preferably, in step 1), the quaternary ammonium salt cation precursor is a tetraalkyl quaternary ammonium halide. The cation of the tetraalkyl quaternary ammonium halide has a general formula of [N(R1)(R2)(R3)(R4)] + ; wherein R1 is any one of C1-C8 alkyl; R2 is any one of C1-C8 alkyl; R3 is any one of C1-C8 alkyl; and R4 is any one of C1-C8 alkyl.

[0008] Preferably, in step 1), the mass ratio of the quaternary ammonium salt cation precursor, the fluorine-containing sulfonimide salt and the organic solvent is (5-30) g:(7-52) g:(50-400) mL. The molar ratio of the quaternary ammonium salt cation precursor to the fluorine-containing sulfonimide salt is (1-1.1):(1-1.2). The organic solvent is acetone or acetonitrile. The conditions of the first stirring reaction include stirring at 40-60℃ for 6-12h.

[0009] Preferably, in step 2), the mass ratio of the total mass of the nitrogen-containing heterocyclic carboxylic acid and the quaternary ammonium salt fluorine-containing sulfonimide ionic liquid to the mass of water is (1-5):(1-5). The conditions of the second stirring reaction include stirring at 25-40℃ for 3-5h.

[0010] Preferably, in step 3), the mass of the additive is 15%-25% of the mass of the room-temperature liquid ionic liquid electrolyte for electro-reduction of formic acid; the additive includes a small-molecule organic solvent and a proton buffer.

[0011] Further preferably, in step 3), the small-molecule organic solvent is acetonitrile or N,N-dimethylformamide; the proton buffer is sodium dihydrogen phosphate or sodium citrate; and the mass ratio of the small-molecule organic solvent to the proton buffer is (2-4):1.

[0012] The application also discloses a room-temperature liquid ionic liquid electrolyte for electro-reduction of formic acid, which is prepared by the above method and comprises quaternary ammonium salt cations, mixed anions and an additive. The quaternary ammonium salt cation is a tetramethyl quaternary ammonium cation, a tetraethyl quaternary ammonium cation, a tetrapropyl quaternary ammonium cation or a tetrabutyl quaternary ammonium cation. The mixed anion comprises an anion of a fluorine-containing sulfonimide salt and a nitrogen-containing heterocyclic carboxylic acid root; and the molar ratio of the anion of the fluorine-containing sulfonimide salt to the nitrogen-containing heterocyclic carboxylic acid root is (1-3):(1-3).

[0013] Preferably, the anion of the fluorine-containing sulfonimide salt is a bis(trifluoromethanesulfonyl)imide anion or a bis(pentafluoroethanesulfonyl)imide anion; and the nitrogen-containing heterocyclic carboxylic acid root anion is an imidazole-2-carboxylic acid root or a pyridine-3-carboxylic acid root.

[0014] The application also discloses application of the normal-temperature liquid ionic liquid electrolyte for electro-reduction of formic acid in preparation of formic acid from carbon dioxide by electro-reduction, and the application adopts a three-electrode system, uses carbon paper loaded bismuth oxide catalyst as a working electrode, uses iridium-plated titanium felt as a counter electrode, uses a saturated mercury-mercury electrode as a reference electrode, and uses the normal-temperature liquid ionic liquid electrolyte for electro-reduction of formic acid as an electrolyte; the reaction potential is-1.8 to-2.2 V, the temperature is 20 to 30 DEG C, and the reaction time is 1 to 3 hours.

[0015] Preferably, the normal-temperature liquid ionic liquid electrolyte for electro-reduction of formic acid has hydrogen Faraday efficiency of less than or equal to 0.07% for inhibiting a hydrogen evolution side reaction; formic acid Faraday efficiency is 99.1% to 99.8%, formic acid energy efficiency is 63% to 77%, and formic acid production per hour per unit electrolysis area is 0.35 to 0.42 mmol / (cm 2 ·h).

[0016] Compared with the prior art, the application has the following beneficial effects: The application discloses a preparation method of a normal-temperature liquid ionic liquid electrolyte for electro-reduction of formic acid, a quaternary ammonium salt cation precursor forms a closely arranged cation adsorption layer on an electrode surface through a steric hindrance effect, physically shields a migration channel of protons to the electrode surface, and improves an activation energy barrier of a hydrogen evolution reaction. + A fluorosulfonylimide salt reduces free H + concentration in a solution through a strong electron-withdrawing effect, and a nitrogen-containing heterocyclic carboxylic acid forms a stable intermediate with a CO2 molecule through a hydrogen bond donor of a carboxylate, competitively inhibits H - from combining with an electrode active site. A mixed anion system activates the CO2 molecule through a Lewis acid-base pair mechanism, and reduces a generation energy barrier of a key intermediate HCOO + Through specific electrolyte design, carbon dioxide is efficiently and selectively electro-reduced into formic acid. Through reasonable design of the structure of the ionic liquid, a specific electrolyte is prepared, hydrogen evolution side reactions are effectively inhibited in the process of electro-reduction of carbon dioxide into formic acid, formic acid Faraday efficiency, energy efficiency and formic acid production per hour per unit electrolysis area are improved.

[0017] Further, the structure of the quaternary ammonium salt cation precursor is designed to be able to enhance adsorption density on the electrode surface and improve electrostatic shielding effect. The migration channel of H + is physically blocked through the steric hindrance effect, and the activation energy barrier of the HER is improved. Different electrode interface requirements are flexibly adapted, and hydrogen evolution inhibition universality is realized.

[0018] Further, the molar ratio of the quaternary ammonium salt cation precursor to the fluorine-containing sulfonimide salt is (1~1.1):(1~1.2); complete anion exchange is ensured, and residual halogen ions are avoided; the organic solvent is acetone or acetonitrile, which has high solubility and low boiling point, facilitating distillation removal and ensuring the purity of the ionic liquid; stirring at 40~60℃ for 6~12h can balance the reaction rate and energy consumption, and avoid decomposition of the quaternary ammonium salt caused by high temperature.

[0019] Further, stirring at 25~40℃ for 3~5h can protect the heat-sensitive structure of the nitrogen-containing heterocyclic carboxylic acid, avoid ring-opening side reactions, ensure the integrity of the mixed anion, and maintain the ability of the "Lewis acid-base pair" to activate CO2.

[0020] Further, the small-molecule organic solvent in the additive improves the CO2 solubility through solvation effect and reduces the ionic liquid viscosity, thereby improving the CO2 mass transfer efficiency. The proton buffer in the additive maintains the local pH in an appropriate range, avoiding the decrease of CO2 solubility under acidic conditions.

[0021] The application also discloses a room-temperature liquid ionic liquid electrolyte for electro-reduction of formic acid prepared by the preparation method, which realizes the following three synergistic mechanisms: complete inhibition of hydrogen evolution; the quaternary ammonium salt cation forms a dense adsorption layer on the electrode surface by virtue of the long-chain alkyl group, which physically blocks the migration channel of H + + and the strong electron-attracting effect of the fluorine-containing sulfonimide anion and the hydrogen bond capture of the nitrogen-containing heterocyclic carboxylate on CO2, so that the hydrogen evolution side reaction is suppressed to a very low level, and the hydrogen Faraday efficiency is ≤0.07%. Efficient activation of CO2; the mixed anion polarizes the CO2 molecule through the Lewis acid-base pair mechanism, reduces the HCOO ⁻ formation energy barrier; under the synergistic effect of the additive, the formic acid Faraday efficiency is 99.1%~99.8%, the energy efficiency is 63%~77%, the unit area yield is 0.35~0.42 mmol / (cm 2 ·h), and the like. Industrialization adaptation; the room-temperature liquid characteristics eliminate the need for temperature control equipment, low-cost raw materials such as tetrabutyl quaternary ammonium salt are combined with a simple room-temperature synthesis process, and the by-products can be recycled, thereby providing a core electrolyte solution for a CO2 electro-reduction formic acid factory, which has high performance, low energy consumption and green.

[0022] The application also discloses application of the normal-temperature liquid ionic liquid electrolyte prepared by the preparation method in electro-reduction of carbon dioxide to form formic acid, and the application realizes formic acid Faraday efficiency of 99.1-99.8% (hydrogen evolution side reaction is less than or equal to 0.07%) and energy efficiency of 63-77% under the condition of a low overpotential of-1.8 to-2.2 V and normal temperature of 20-30 DEG C, only 1-3 hours of reaction is needed, the selectivity and energy efficiency are improved compared with traditional electrolyte (formic acid efficiency is 80-95%, hydrogen evolution is more than 10%); the formic acid yield per unit area on a bismuth-based catalyst is 0.35-0.42 mmol / (cm 2 ·h), the electrode material cost is greatly reduced; the operation under normal temperature and pressure directly matches an industrial electrolytic cell, no additional energy consumption equipment is needed, and the electrolyte has excellent stability. DETAILED DESCRIPTION

[0023] The technical solutions of the present application will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0024] In the present application, all the embodiments and preferred embodiments mentioned in the present application can be combined to form new technical solutions if no special instructions are given.

[0025] In the present application, all the technical features and preferred features mentioned in the present application can be combined to form new technical solutions if no special instructions are given.

[0026] In the present application, percentage (%) or part refers to the weight percentage or weight part of the composition if no special instructions are given.

[0027] In the present application, all the components or preferred components involved can be combined to form new technical solutions if no special instructions are given.

[0028] In the present application, unless otherwise specified, the numerical range "a-b" represents a shorthand notation for any real number combination between a and b, wherein a and b are both real numbers. For example, the numerical range "6-22" represents that all the real numbers between "6-22" have been listed in the present application, and "6-22" is only a shorthand notation for these numerical combinations.

[0029] The lower limit and upper limit of the range disclosed in the present application can be one or more lower limits and one or more upper limits, respectively.

[0030] In the present application, the term "and / or" used in the present application means any combination of one or more of the associated listed items and all possible combinations, and includes these combinations.

[0031] In the present application, unless otherwise specified, each reaction or operation step can be carried out sequentially or in sequence. Preferably, the reaction method herein is carried out sequentially.

[0032] Unless otherwise specified, the professional and scientific terms used herein have the same meaning as those familiar to those skilled in the art. In addition, any method or material similar or equivalent to that described can also be applied in the present application.

[0033] The present application provides a preparation method of a normal temperature liquid ionic liquid electrolyte for inhibiting the hydrogen evolution side reaction of electro-reduction of formic acid, comprising the following steps: 1) Tetraalkyl quaternary ammonium halide is added as a quaternary ammonium salt cation precursor and a fluorine-containing sulfonimide salt in a molar ratio of (1-1.1):(1-1.2) to an organic solvent such as acetone or acetonitrile, and the amount ratio of the tetraalkyl quaternary ammonium halide, the fluorine-containing sulfonimide salt and the organic solvent is (5-30) g:(7-52) g:(50-400) mL, and the mixture is stirred at 40-60°C for 6-12 h. After the reaction is completed, the generated lithium halide or sodium halide precipitate is removed by filtration, and the organic solvent is removed from the filtrate by reduced pressure distillation to obtain a quaternary ammonium salt fluorine-containing sulfonimide ionic liquid. In the tetraalkyl quaternary ammonium halide, the quaternary ammonium salt cation is a tetraalkyl quaternary ammonium cation, and the general formula is [N(R1)(R2)(R3)(R4)] + , R1, R2, R3 or R4 is independently any one of C1-C8 alkyl; the anion of the fluorine-containing sulfonimide salt corresponding to the fluorine-containing sulfonimide salt is a bis(trifluoromethanesulfonyl)imide anion ([NTf2] - ) or a bis(pentafluoroethanesulfonyl)imide anion ([N(SO2C2F5)2] - ).

[0034] 2) The quaternary ammonium salt fluorine-containing sulfonimide ionic liquid prepared in step 1) is added to water in a molar ratio of the anion of the fluorine-containing sulfonimide salt to the nitrogen-containing heterocyclic carboxylic acid root anion (1-3):(1-3), and the mixture anion ionic liquid and water are mixed in a mass ratio of (1-5):(1-5), and the mixture is stirred at 25-40°C for 3-5 h to partially replace the anion of the fluorine-containing sulfonimide salt with the nitrogen-containing heterocyclic carboxylic acid root anion to form a mixed anion ionic liquid. The nitrogen-containing heterocyclic carboxylic acid root anion is an imidazole-2-carboxylic acid root anion or a pyridine-3-carboxylic acid root anion. Preferably, the molar ratio of the nitrogen-containing heterocyclic carboxylic acid root anion to the anion of the fluorine-containing sulfonimide salt is 1:1-1:2.

[0035] 3) adding 15%~25% of the total mass of the electrolyte of additives (mass ratio of small molecule organic solvent to proton buffer is (2~4):1) into the mixed anion ionic liquid, wherein the small molecule organic solvent is acetonitrile or N,N-dimethylformamide, and the proton buffer is sodium dihydrogen phosphate or sodium citrate, stirring uniformly at room temperature, thereby obtaining the room temperature liquid ionic liquid electrolyte for electro-reduction of formic acid.

[0036] The room temperature liquid ionic liquid electrolyte for electro-reduction of formic acid of the present application is composed of quaternary ammonium salt cations, mixed anions containing fluorine-containing sulfonimide anions and nitrogen-containing heterocyclic carboxylate anions, and small molecule organic solvents and proton buffers. The quaternary ammonium salt cations inhibit hydrogen evolution through steric hindrance and electrostatic interaction, the mixed anions activate CO2 through Lewis acid-base pair mechanism, and the additives optimize CO2 solubility and interfacial mass transfer, thereby finally realizing high selectivity and high efficiency of formic acid synthesis.

[0037] The prepared electrolyte is applied to the reaction of electro-reduction of carbon dioxide to formic acid, and a three-electrode system is adopted: a carbon paper loaded bismuth oxide catalyst is used as a working electrode, an iridium-plated titanium felt is used as a counter electrode, and a saturated calomel electrode is used as a reference electrode, the reaction is carried out at-1.8~-2.2V (relative to the saturated calomel electrode) potential and at 20~30℃ for 1~3h. During the reaction, the electrolyte inhibits the hydrogen evolution side reaction (hydrogen Faraday efficiency ≤0.07%) through the synergistic effect of quaternary ammonium salt cations and mixed anions, and simultaneously efficiently activates CO2 to generate formic acid, the formic acid Faraday efficiency reaches 99.1%~99.8%, the formic acid energy efficiency is 63%~77%, and the formic acid production per hour per unit electrolysis area is 0.35~0.42mmol / (cm 2 ·h).

[0038] The effects of the room temperature liquid ionic liquid electrolyte for electro-reduction of formic acid of the present application include: 1) efficient inhibition of hydrogen evolution side reaction; the present application significantly changes the electrode-electrolyte interface properties by carefully designing quaternary ammonium salt cations, mixed anion systems and additive formulations. The unique combination of anions and cations reduces the hydrogen evolution reaction active sites, the proton buffer in the additives maintains the acid-base balance of the system, and the small molecule organic solvent optimizes the mass transfer, which effectively inhibits the hydrogen evolution side reaction and greatly improves the reaction selectivity. 2) significantly improving the performance of formic acid preparation; the mixed anion containing fluorine-containing sulfonimide salt and nitrogen-containing heterocyclic carboxylate anion can efficiently activate carbon dioxide molecules, stabilize reaction intermediates and promote the conversion of carbon dioxide to formic acid. The additives improve the solubility and mass transfer rate of carbon dioxide in the electrolyte, and optimize the reaction kinetics. It has been verified through experiments that, when the electrolyte of the present application is used for the reaction of electro-reduction of carbon dioxide to formic acid, the formic acid Faraday efficiency can reach 99.1%~99.8%, the energy efficiency can reach 63%~77%, and the formic acid production per hour per unit electrolysis area can reach 0.35~0.42mmol / (cm 2·h), which is far superior to traditional electrolytes. 3) Good process applicability; the electrolyte is in a liquid state at room temperature, has good stability and flowability, does not require complex temperature control equipment, and reduces production energy consumption and cost. The preparation method is simple, the raw materials are easy to obtain, the reaction conditions are mild, and it is suitable for large-scale industrial production, which provides reliable technical support for the industrial application of carbon dioxide electro-reduction to formic acid.

[0039] The preparation method of the room-temperature liquid ionic liquid electrolyte for electro-reduction to formic acid disclosed in the application, compared with the prior art, through the triple innovation mechanism of ionic liquid structure design-interface microenvironment regulation-additive function synergy, a high-performance hydrogen evolution side reaction inhibiting electrolyte system is constructed, and the specific advantages are as follows: 1) Breakthrough of hydrogen evolution side reaction inhibition mechanism; the present application suppresses the hydrogen evolution path from the root by the synergistic effect of the directional adsorption of quaternary ammonium salt cations and mixed anions. The quaternary ammonium salt cation (general formula [N(R1-R4)4] + ) forms a close-arranged cation adsorption layer on the electrode surface by virtue of the steric hindrance effect of the long-chain alkyl group, physically shields the migration channel of protons (H + ) to the electrode surface, and increases the activation energy barrier of the hydrogen evolution reaction (HER) to more than 1.2 eV. At the same time, the anion of the fluorosulfonylimide salt ([NTf2] - / [N(SO2C2F5)2] - ) and the nitrogen-containing heterocyclic carboxylate anion (imidazole-2-carboxylate / pyridine-3-carboxylate) form an "electron trap-hydrogen bond network" composite structure: the former reduces the concentration of free H + ([H + ]<10 -5 M) in the solution by strong electron-withdrawing effect, and the latter forms a stable intermediate with CO2 molecules by the hydrogen bond donor of the carboxylate, competitively inhibits the combination of H + with the electrode active sites, and finally makes the Faraday efficiency of the hydrogen evolution side reaction ≤0.07%, which is significantly lower than that of the traditional aqueous solution system (the Faraday efficiency of the hydrogen evolution side reaction >10%).

[0040] 2) Multi-dimensional improvement of formic acid generation efficiency; CO2 activation and intermediate stability: the mixed anion system activates CO2 molecules through the "Lewis acid-base pair" mechanism, the strong electron-withdrawing property of the fluorosulfonylimide anion polarizes CO2 molecules, and the imidazole / pyridine ring of the nitrogen-containing heterocyclic carboxylate anion forms a complex with CO2 through π-π conjugation, which reduces the concentration of the key intermediate HCOO -The production energy barrier. Mass transfer and kinetics optimization: small molecule organic solvents (acetonitrile / N,N-dimethylformamide) in additives can increase the CO2 solubility from 0.03 mol / L in water to 1.2 mol / L through the "solvation effect", and reduce the ionic liquid viscosity (from 800 mPa·s to 300 mPa·s), so that the CO2 mass transfer coefficient is increased by 2.5 times; the proton buffer (sodium dihydrogen phosphate / sodium citrate) maintains the local pH in the range of 6.5-7.5, avoiding the decrease of carbon dioxide solubility under acidic conditions. Performance parameters breakthrough; in the potential range of-1.8~-2.2 V, the formic acid faradic efficiency can reach 99.1%~99.8% (the traditional ionic liquid system is 80%~95%), and the energy efficiency is increased to 63%~77% (the traditional system is <50%), realizing the dual improvement of selectivity and energy efficiency.

[0041] 3) Process adaptability and stability advantage; the electrolyte is in a low viscosity liquid state at 20~30℃, without the need for heating or cooling equipment, and can be directly adapted to existing industrial electrolytic cells. The synthesis process adopts a "solution mixing-room temperature reaction" process, without the need for high temperature and high pressure or special equipment, and the raw material cost is reduced by more than 60% compared with the noble metal-based electrolyte. Taking tetrabutyl quaternary ammonium salt as an example, its preparation cost is only 1 / 3 of that of the traditional imidazole ionic liquid, and the by-product halide can be recovered by simple filtration, in line with the concept of green chemistry. Moreover, the catalytic system has good compatibility, and the electrolyte has good compatibility with non-noble metal electrodes such as bismuth, and can achieve a formic acid faradic efficiency of 99% on a lead electrode, further reducing the industrialization cost.

[0042] The present application constructs an ionic liquid electrolyte system with "high selectivity-high stability-low cost" through the synergistic innovation of molecular engineering and interface engineering, and provides a solution with both scientific breakthrough and engineering value for the industrial application of CO2 electro-reduction to formic acid.

[0043] In order to make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the technical scheme in the embodiments of the present application will be described clearly and completely below in combination with the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor are within the scope of protection of the present application.

[0044] Embodiment 1 5g of tetraethylammonium bromide and 7g of lithium bis(trifluoromethanesulfonyl)imide were added to 50mL of acetonitrile at a molar ratio of 1:1. The mixture was stirred at 50℃ for 8h. The generated lithium bromide precipitate was removed by filtration, and the acetonitrile was removed by vacuum distillation to obtain tetraethylbis(trifluoromethanesulfonyl)imide ionic liquid.

[0045] The above ionic liquid was reacted with imidazole-2-carboxylic acid according to [NTf2]. - An imidazole-2-carboxylate anion was added to water at a molar ratio of 1:1, resulting in a 1:1 mass ratio of the mixed anionic liquid to water. The mixture was stirred at 30°C for 4 hours to obtain a mixed anionic liquid. Acetonitrile (10% by mass) and sodium dihydrogen phosphate (5% by mass, 2:1 by mass) were then added to the mixed anionic liquid, and the mixture was stirred at room temperature until homogeneous to obtain the electrolyte.

[0046] Using a carbon paper-supported bismuth oxide catalyst as the working electrode, an iridium-plated titanium felt as the counter electrode, and a saturated calomel electrode as the reference electrode, the electrolyte was placed in an electrolytic cell. The electroreduction of carbon dioxide was carried out at -2.0 V (relative to the saturated calomel electrode) at 25 °C for 2 h. The results showed a Faradaic efficiency of 0.07% for hydrogen, 99.3% for formic acid, a formic acid energy efficiency of 69%, and a formic acid yield of 0.35 mmol / (cm²) per unit electrolytic area per hour. 2 ·h).

[0047] Example 2 10 g of tetrabutylammonium chloride and 15 g of sodium bis(pentafluoroethanesulfonyl)imide were added to 150 mL of acetone at a molar ratio of 1:1. The mixture was stirred at 45 °C for 10 h. The resulting sodium chloride precipitate was removed by filtration, and the acetone was removed by vacuum distillation to obtain a tetrabutylbis(pentafluoroethanesulfonyl)imide ionic liquid. This ionic liquid was then reacted with pyridine-3-carboxylic acid in a reaction [N(SO₂C₂F₅)₂]. - A mixture of pyridine-3-carboxylate anion and water was added at a molar ratio of 2:1, resulting in a mixed anionic ionic liquid with a mass ratio of 2:3 to water. The mixture was stirred at 25°C for 5 hours to obtain a mixed anionic ionic liquid. 12% N,N-dimethylformamide and 6% sodium citrate (mass ratio 2:1) were then added to the mixed anionic ionic liquid, and the mixture was stirred at room temperature until homogeneous to obtain an electrolyte.

[0048] Using the same electrodes and reaction apparatus as in Example 1, the electroreduction of carbon dioxide was carried out at a potential of -1.9V (relative to a saturated calomel electrode) and 22°C for 2.5 h. The results showed that the Faraday efficiency for hydrogen was 0.05%, the Faraday efficiency for formic acid was 99.2%, the energy efficiency for formic acid was 73%, and the formic acid yield per unit electrolysis area per hour was 0.38 mmol / (cm²).2 ·h)。

[0049] Example 3 Into 200 mL of acetonitrile, 15 g of tetramethylammonium bromide and 32 g of potassium bis(trifluoromethanesulfonyl)imide were added in a molar ratio of 1:1, the reaction was stirred at 60°C for 6 h, the generated potassium bromide precipitate was removed by filtration, and the acetonitrile was removed by distillation under reduced pressure to obtain the tetramethylbis(trifluoromethanesulfonyl)imide ionic liquid. The ionic liquid was added to water in a molar ratio of 1:1 with the imidazole-2-carboxylic acid anion, the mass ratio of the mixed anion ionic liquid to water was 3:3, the reaction was stirred at 40°C for 3 h to obtain the mixed anion ionic liquid. Into the mixed anion ionic liquid, acetonitrile and sodium dihydrogen phosphate were added in a mass ratio of 3:1, the mass fraction of acetonitrile was 15%, and the mass fraction of sodium dihydrogen phosphate was 5%, and the mixture was stirred uniformly at room temperature to obtain the electrolyte. - Into 200 mL of acetonitrile, 15 g of tetramethylammonium bromide and 32 g of potassium bis(trifluoromethanesulfonyl)imide were added in a molar ratio of 1:1, the reaction was stirred at 60°C for 6 h, the generated potassium bromide precipitate was removed by filtration, and the acetonitrile was removed by distillation under reduced pressure to obtain the tetramethylbis(trifluoromethanesulfonyl)imide ionic liquid. The ionic liquid was added to water in a molar ratio of 1:1 with the imidazole-2-carboxylic acid anion, the mass ratio of the mixed anion ionic liquid to water was 3:3, the reaction was stirred at 40°C for 3 h to obtain the mixed anion ionic liquid. Into the mixed anion ionic liquid, acetonitrile and sodium dihydrogen phosphate were added in a mass ratio of 3:1, the mass fraction of acetonitrile was 15%, and the mass fraction of sodium dihydrogen phosphate was 5%, and the mixture was stirred uniformly at room temperature to obtain the electrolyte.

[0050] The same electrode and reaction device as in Example 1 were used, the carbon dioxide electro-reduction reaction was carried out at a potential of -2.1 V (relative to the saturated calomel electrode) at 28°C, and the reaction time was 1.5 h. It was detected that the faradic efficiency of hydrogen was 0.04%, the faradic efficiency of formic acid was 99.7%, the energy efficiency of formic acid was 66%, and the formic acid production per hour per unit electrolysis area was 0.39 mmol / (cm 2 ·h). Example 4 Into 250 mL of acetone, 20 g of tetrapropylammonium chloride and 37 g of lithium bis(pentafluoroethanesulfonyl)imide were added in a molar ratio of 1:1.1, the reaction was stirred at 55°C for 9 h, the generated lithium chloride precipitate was removed by filtration, and the acetone was removed by distillation under reduced pressure to obtain the tetrapropylbis(pentafluoroethanesulfonyl)imide ionic liquid. The ionic liquid was added to water in a molar ratio of 1:2 with the pyridine-3-carboxylic acid anion, the mass ratio of the mixed anion ionic liquid to water was 5:4, the reaction was stirred at 35°C for 4 h to obtain the mixed anion ionic liquid. Into the mixed anion ionic liquid, N,N-dimethylformamide and sodium citrate were added in a mass ratio of 3:1, the mass fraction of N,N-dimethylformamide was 18%, and the mass fraction of sodium citrate was 6%, and the mixture was stirred uniformly at room temperature to obtain the electrolyte. - Into 250 mL of acetone, 20 g of tetrapropylammonium chloride and 37 g of lithium bis(pentafluoroethanesulfonyl)imide were added in a molar ratio of 1:1.1, the reaction was stirred at 55°C for 9 h, the generated lithium chloride precipitate was removed by filtration, and the acetone was removed by distillation under reduced pressure to obtain the tetrapropylbis(pentafluoroethanesulfonyl)imide ionic liquid. The ionic liquid was added to water in a molar ratio of 1:2 with the pyridine-3-carboxylic acid anion, the mass ratio of the mixed anion ionic liquid to water was 5:4, the reaction was stirred at 35°C for 4 h to obtain the mixed anion ionic liquid. Into the mixed anion ionic liquid, N,N-dimethylformamide and sodium citrate were added in a mass ratio of 3:1, the mass fraction of N,N-dimethylformamide was 18%, and the mass fraction of sodium citrate was 6%, and the mixture was stirred uniformly at room temperature to obtain the electrolyte.

[0051] The same electrode and reaction device as in Example 1 were used, the carbon dioxide electro-reduction reaction was carried out at a potential of -1.8 V (relative to the saturated calomel electrode) at 20°C, and the reaction time was 3 h. It was detected that the faradic efficiency of hydrogen was 0.03%, the faradic efficiency of formic acid was 99.8%, the energy efficiency of formic acid was 77%, and the formic acid production per hour per unit electrolysis area was 0.42 mmol / (cm 2 ·h).

[0052] Example 5 The 25 g of tetraethylammonium chloride and 37 g of sodium bis(trifluoromethanesulfonyl)imide were added to 300 mL of acetonitrile in a molar ratio of 1.1:1, stirred at 40 °C for 12 h, the generated sodium chloride precipitate was removed by filtration, and the acetonitrile was removed by distillation under reduced pressure to obtain the tetraethylbis(trifluoromethanesulfonyl)imide ionic liquid. The ionic liquid was added to the imidazole-2-carboxylic acid in a molar ratio of 1:3, the mass ratio of the mixed anion ionic liquid to water was 4:3, and the reaction was stirred at 30 °C for 5 h to obtain the mixed anion ionic liquid. The mixed anion ionic liquid was added to acetonitrile with a mass ratio of 20% and sodium dihydrogen phosphate with a mass ratio of 5% (the mass ratio of the two was 4:1), and stirred uniformly at room temperature to obtain the electrolyte. - The mixed anion ionic liquid was added to water in a molar ratio of 1:3, the mass ratio of the mixed anion ionic liquid to water was 5:5, and the reaction was stirred at 28 °C for 4 h to obtain the mixed anion ionic liquid. The mixed anion ionic liquid was added to N,N-dimethylformamide with a mass ratio of 16% and sodium citrate with a mass ratio of 4% (the mass ratio of the two was 4:1), and stirred uniformly at room temperature to obtain the electrolyte.

[0053] The same electrode and reaction device as in Example 1 were used, the carbon dioxide electro-reduction reaction was carried out at a potential of -2.2 V (relative to saturated calomel electrode) at 30 °C, and the reaction time was 1 h. It was detected that the faradic efficiency of hydrogen was 0.05%, the faradic efficiency of formic acid was 99.1%, the energy efficiency of formic acid was 63%, and the formic acid production per hour per unit electrolysis area was 0.36 mmol / (cm 2 ·h).

[0054] Example 6 The 30 g of tetrabutylammonium bromide and 52 g of potassium bis(pentafluoroethanesulfonyl)imide were added to 400 mL of acetone in a molar ratio of 1:1.2, stirred at 50 °C for 7 h, the generated potassium bromide precipitate was removed by filtration, and the acetone was removed by distillation under reduced pressure to obtain the tetrabutylbis(pentafluoroethanesulfonyl)imide ionic liquid. The ionic liquid was added to the pyridine-3-carboxylic acid in a molar ratio of 3:3, the mass ratio of the mixed anion ionic liquid to water was 5:5, and the reaction was stirred at 28 °C for 4 h to obtain the mixed anion ionic liquid. The mixed anion ionic liquid was added to N,N-dimethylformamide with a mass ratio of 16% and sodium citrate with a mass ratio of 4% (the mass ratio of the two was 4:1), and stirred uniformly at room temperature to obtain the electrolyte. - The mixed anion ionic liquid was added to water in a molar ratio of 1:3, the mass ratio of the mixed anion ionic liquid to water was 5:5, and the reaction was stirred at 28 °C for 4 h to obtain the mixed anion ionic liquid. The mixed anion ionic liquid was added to N,N-dimethylformamide with a mass ratio of 16% and sodium citrate with a mass ratio of 4% (the mass ratio of the two was 4:1), and stirred uniformly at room temperature to obtain the electrolyte.

[0055] The same electrode and reaction device as in Example 1 were used, the carbon dioxide electro-reduction reaction was carried out at a potential of -1.9 V (relative to saturated calomel electrode) at 23 °C, and the reaction time was 2 h. It was detected that the faradic efficiency of hydrogen was 0.03%, the faradic efficiency of formic acid was 99.4%, the energy efficiency of formic acid was 73%, and the formic acid production per hour per unit electrolysis area was 0.40 mmol / (cm 2 ·h).

[0056] As can be seen from the above examples, the normal-temperature liquid ionic electrolyte prepared by the present application for inhibiting the hydrogen evolution side reaction exhibits significant advantages in the electro-reduction of carbon dioxide to form formic acid, realizing high selectivity and high efficiency of formic acid synthesis. Different quaternary ammonium salt cations, mixed anion proportions and additive compositions have significant effects on the electrolyte performance, and the performance optimization can be realized by adjusting the components and reaction conditions.

[0057] In summary, the normal-temperature liquid ionic electrolyte for electro-reduction to form formic acid, the preparation method and application of the present application, the preparation method of the electrolyte comprises: reacting a quaternary ammonium halide and a fluorine-containing sulfonimide salt in an organic solvent to obtain a quaternary ammonium salt fluorine-containing sulfonimide ionic liquid; then reacting with a nitrogen-containing heterocyclic carboxylic acid to form a mixed anion ionic liquid; and finally adding a small molecule organic solvent and a proton buffer to obtain the electrolyte. The electrolyte is composed of quaternary ammonium salt cations, mixed anions and additives, can effectively inhibit the hydrogen evolution side reaction, and efficiently activate carbon dioxide. In the reaction of electro-reduction of carbon dioxide to form formic acid, a three-electrode system is used, the Faraday efficiency of the hydrogen evolution side reaction is ≤0.07%, the Faraday efficiency of formic acid is 99.1% to 99.8%, the energy efficiency is 63% to 77%, and the formic acid production per hour per unit electrolysis area is 0.35 to 0.42 mmol / (cm 2 ·h), which is suitable for efficient conversion of carbon dioxide to formic acid. The electrolyte effectively inhibits the hydrogen evolution side reaction and efficiently activates carbon dioxide molecules through the synergistic effect of quaternary ammonium salt cations, mixed anions of fluorine-containing sulfonimide salt and nitrogen-containing heterocyclic carboxylic acid, and small molecule organic solvents and proton buffers, optimizes mass transfer and reaction kinetics. The preparation method adopts a solution mixing-room temperature reaction process, which is simple and low in cost, and is suitable for large-scale production. The specific examples show that under different preparation conditions and material combinations, the electrolyte performance is different, but all exhibit high Faraday efficiency, formic acid energy efficiency and formic acid production per unit electrolysis area, which is suitable for efficient conversion and utilization of CO2.

[0058] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for preparing a room temperature ionic liquid electro-reduction formic acid electrolyte, characterized in that, include: 1) The quaternary ammonium salt cation precursor and the fluorosulfonyl imide salt were added to an organic solvent, reacted with stirring once, the precipitate was removed by filtration, and the mixture was distilled under reduced pressure to obtain the quaternary ammonium salt fluorosulfonyl imide ionic liquid. 2) Add the nitrogen-containing heterocyclic carboxylic acid and the quaternary ammonium salt fluorosulfonyl imide ionic liquid obtained in step 1) to water, and react with stirring twice to form a mixed anionic ionic liquid; 3) Add additives to the mixed anionic liquid obtained in step 2), stir evenly at room temperature to obtain room temperature liquid ionic liquid electroreduction to produce formic acid electrolyte.

2. The method for preparing formic acid electrolyte by electroreduction of room-temperature liquid ionic liquid according to claim 1, characterized in that, In step 1), the quaternary ammonium salt cation precursor is a tetraalkyl quaternary ammonium halide; The cation of the tetraalkyl quaternary ammonium halide has a general formula of [N(R1)(R2)(R3)(R4)] + ; wherein R1 is any one of C1-C8 alkyl; R2 is any one of C1-C8 alkyl; R3 is any one of C1-C8 alkyl; and R4 is any one of C1-C8 alkyl.

3. The method for preparing formic acid electrolyte by electroreduction of room-temperature liquid ionic liquid according to claim 1, characterized in that, In step 1), the ratio of the quaternary ammonium salt cation precursor, the fluorosulfonyl imide salt, and the organic solvent is (5~30) g: (7~52) g: (50~400) mL; The molar ratio of the quaternary ammonium salt cationic precursor to the fluorosulfonyl imide salt is (1~1.1):(1~1.2). The organic solvent is acetone or acetonitrile; The conditions for the first stirring reaction include: stirring at 40~60℃ for 6~12 hours.

4. The method for preparing formic acid electrolyte by electroreduction of room-temperature liquid ionic liquid according to claim 1, characterized in that, In step 2), the total mass ratio of the nitrogen-containing heterocyclic carboxylic acid and the quaternary ammonium salt fluorosulfonyl imide ionic liquid to water is (1~5):(1~5). The conditions for the secondary stirring reaction include stirring at 25~40℃ for 3~5 hours.

5. The method for preparing formic acid electrolyte by electroreduction of room-temperature liquid ionic liquid according to claim 1, characterized in that, In step 3), the mass of the additive is 15% to 25% of the mass of the formic acid electrolyte produced by electroreduction of room temperature liquid ionic liquid; the additive includes: small molecule organic solvent and proton buffer.

6. The method for preparing formic acid electrolyte by electroreduction of room-temperature liquid ionic liquid according to claim 5, characterized in that, In step 3), the small molecule organic solvent is acetonitrile or N,N-dimethylformamide; the proton buffer is sodium dihydrogen phosphate or sodium citrate; and the mass ratio of the small molecule organic solvent to the proton buffer is (2~4):

1.

7. A method for preparing formic acid electrolyte by electroreduction of a room-temperature liquid ionic liquid, characterized in that, The formic acid electrolyte, prepared by the preparation method according to any one of claims 1 to 6, comprises: quaternary ammonium salt cations, mixed anions, and additives; The quaternary ammonium salt cation is a tetramethyl quaternary ammonium cation, a tetraethyl quaternary ammonium cation, a tetrapropyl quaternary ammonium cation, or a tetrabutyl quaternary ammonium cation; The mixed anions include: anions of fluorosulfonyl imide salts and nitrogen-containing heterocyclic carboxylate groups; the molar ratio of the anions of fluorosulfonyl imide salts and nitrogen-containing heterocyclic carboxylate groups is (1~3):(1~3).

8. The method for preparing formic acid electrolyte by electroreduction of room-temperature liquid ionic liquid according to claim 7, characterized in that, The anion of the fluorosulfonyl imide salt is bis(trifluoromethanesulfonyl)imide anion or bis(pentafluoroethanesulfonyl)imide anion; the nitrogen-containing heterocyclic carboxylate anion is imidazole-2-carboxylate or pyridine-3-carboxylate.

9. The application of the room-temperature liquid ionic liquid electrolyte for the electroreduction of formic acid prepared by the preparation method according to any one of claims 1 to 6 in the electroreduction of carbon dioxide to formic acid, characterized in that, A three-electrode system was adopted, with carbon paper-supported bismuth oxide catalyst as the working electrode, iridium-plated titanium felt as the counter electrode, saturated calomel electrode as the reference electrode, and formic acid electrolyte prepared by electroreduction of room temperature liquid ionic liquid as the electrolyte; the reaction potential was -1.8~-2.2V, the temperature was 20~30℃, and the reaction time was 1~3h.

10. The application of the room-temperature liquid ionic liquid electrolyte for the electroreduction of formic acid according to claim 9 in the electroreduction of carbon dioxide to formic acid, characterized in that, The hydrogen Faraday efficiency of the normal-temperature liquid ionic liquid electrolyte for electro-reduction of formic acid is ≤0.07%; the formic acid Faraday efficiency is 99.1%~99.8%, the formic acid energy efficiency is 63%~77%, the formic acid production per hour per unit electrolysis area is 0.35~0.42mmol / (cm 2 ·h). h).